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Herein, we report a new finding that 3Mg/Mg2Sn alloy can be a promising solution to the aforementioned issues in Sn-based alloy-type MIB anodes. It is shown that 3Mg/Mg2Sn, which is composed of ternary phases (crystalline Mg-rich (c-Mg), amorphous Mg-rich (a-Mg), and intermetallic Mg2Sn phases), reversibly magnesiates/de-magnesiates a significant amount of Mg2+ ions in a non-Grignard and Lewis acid-free electrolyte even under high rates of C/D. In Table 1, the unprecedented electrochemical performance of 3Mg/Mg2Sn is compared with that of other popular alloy-type anodes. The origin of high capacities and excellent rate-capabilities is also discussed in the context of structural features. Finally, we describe the compatibility of 3Mg/Mg2Sn with versatile conventional electrolytes and the optimality of using 3Mg/Mg2Sn with Mg2+-trapping cathodes (e.g., Mo6S8).

What's the anode?

alloy-type

To elucidate the practical applicability, HSABs were assembled using metallic Na as the anode immersed in 1 M NaClO4 in TEGDME organic electrolyte, NASICON solid electrolyte as the separator, O2-saturated 0.1 M NaOH aqueous as the aqueous electrolyte, Pt3Ni1/NixFe LDHs, 20% Pt/C, and RuO2 loaded on Ni foam as the cathode (Fig. 1a). During the discharge process, the Na metal on the anode side is oxidized to Na+ and moves into the NaOH aqueous through the NASICON separator, and the ORR simultaneously occurs at the cathode. On charging, the Na+ ions were transported from the aqueous electrolyte into the anode chamber and reduced to metallic Na, together with the concurrent OER at the cathode side.

What's the cathode?

RuO2 loaded on Ni foam

To elucidate the practical applicability, HSABs were assembled using metallic Na as the anode immersed in 1 M NaClO4 in TEGDME organic electrolyte, NASICON solid electrolyte as the separator, O2-saturated 0.1 M NaOH aqueous as the aqueous electrolyte, Pt3Ni1/NixFe LDHs, 20% Pt/C, and RuO2 loaded on Ni foam as the cathode (Fig. 1a). During the discharge process, the Na metal on the anode side is oxidized to Na+ and moves into the NaOH aqueous through the NASICON separator, and the ORR simultaneously occurs at the cathode. On charging, the Na+ ions were transported from the aqueous electrolyte into the anode chamber and reduced to metallic Na, together with the concurrent OER at the cathode side.

What's the anode?

Na

To elucidate the practical applicability, HSABs were assembled using metallic Na as the anode immersed in 1 M NaClO4 in TEGDME organic electrolyte, NASICON solid electrolyte as the separator, O2-saturated 0.1 M NaOH aqueous as the aqueous electrolyte, Pt3Ni1/NixFe LDHs, 20% Pt/C, and RuO2 loaded on Ni foam as the cathode (Fig. 1a). During the discharge process, the Na metal on the anode side is oxidized to Na+ and moves into the NaOH aqueous through the NASICON separator, and the ORR simultaneously occurs at the cathode. On charging, the Na+ ions were transported from the aqueous electrolyte into the anode chamber and reduced to metallic Na, together with the concurrent OER at the cathode side.

What's the electrolyte?

1 M NaClO4 in TEGDME organic electrolyte

To elucidate the practical applicability, HSABs were assembled using metallic Na as the anode immersed in 1 M NaClO4 in TEGDME organic electrolyte, NASICON solid electrolyte as the separator, O2-saturated 0.1 M NaOH aqueous as the aqueous electrolyte, Pt3Ni1/NixFe LDHs, 20% Pt/C, and RuO2 loaded on Ni foam as the cathode (Fig. 1a). During the discharge process, the Na metal on the anode side is oxidized to Na+ and moves into the NaOH aqueous through the NASICON separator, and the ORR simultaneously occurs at the cathode. On charging, the Na+ ions were transported from the aqueous electrolyte into the anode chamber and reduced to metallic Na, together with the concurrent OER at the cathode side.

What's the anode?

Na metal

To elucidate the practical applicability, HSABs were assembled using metallic Na as the anode immersed in 1 M NaClO4 in TEGDME organic electrolyte, NASICON solid electrolyte as the separator, O2-saturated 0.1 M NaOH aqueous as the aqueous electrolyte, Pt3Ni1/NixFe LDHs, 20% Pt/C, and RuO2 loaded on Ni foam as the cathode (Fig. 1a). During the discharge process, the Na metal on the anode side is oxidized to Na+ and moves into the NaOH aqueous through the NASICON separator, and the ORR simultaneously occurs at the cathode. On charging, the Na+ ions were transported from the aqueous electrolyte into the anode chamber and reduced to metallic Na, together with the concurrent OER at the cathode side.

What's the electrolyte?

NASICON solid electrolyte

In the current study, we investigate composite cathodes that combine a NMC cathode (LiNi0.5Mn0.3Co0.2O2) with amorphous 75Li2S–25P2S5 (LPS) solid electrolyte. The cathode material was coated with an amorphous Li–Zr-oxide (LZO) layer to reduce the reaction between NMC and LPS and to minimize the effect of the chemical instability on the electrochemical performance. SSB cells were constructed with In metal as the anode, a composite of NMC/LPS/carbon nanofiber (CNF) in a weight ratio of 60:35:5 as the cathode, and LPS as the solid electrolyte. All the cells were cycled between 1.4 and 3.7 V vs. In (between 2 and 4.3 V vs. Li/Li+) using constant-current constant-voltage (CCCV) charging and constant-current discharging at a rate of 0.05 mA cm−2. The details of the cell fabrication have been previously reported and are provided in the ESI.†

What's the cathode?

NMC

In the current study, we investigate composite cathodes that combine a NMC cathode (LiNi0.5Mn0.3Co0.2O2) with amorphous 75Li2S–25P2S5 (LPS) solid electrolyte. The cathode material was coated with an amorphous Li–Zr-oxide (LZO) layer to reduce the reaction between NMC and LPS and to minimize the effect of the chemical instability on the electrochemical performance. SSB cells were constructed with In metal as the anode, a composite of NMC/LPS/carbon nanofiber (CNF) in a weight ratio of 60:35:5 as the cathode, and LPS as the solid electrolyte. All the cells were cycled between 1.4 and 3.7 V vs. In (between 2 and 4.3 V vs. Li/Li+) using constant-current constant-voltage (CCCV) charging and constant-current discharging at a rate of 0.05 mA cm−2. The details of the cell fabrication have been previously reported and are provided in the ESI.†

What's the anode?

In metal

In the current study, we investigate composite cathodes that combine a NMC cathode (LiNi0.5Mn0.3Co0.2O2) with amorphous 75Li2S–25P2S5 (LPS) solid electrolyte. The cathode material was coated with an amorphous Li–Zr-oxide (LZO) layer to reduce the reaction between NMC and LPS and to minimize the effect of the chemical instability on the electrochemical performance. SSB cells were constructed with In metal as the anode, a composite of NMC/LPS/carbon nanofiber (CNF) in a weight ratio of 60:35:5 as the cathode, and LPS as the solid electrolyte. All the cells were cycled between 1.4 and 3.7 V vs. In (between 2 and 4.3 V vs. Li/Li+) using constant-current constant-voltage (CCCV) charging and constant-current discharging at a rate of 0.05 mA cm−2. The details of the cell fabrication have been previously reported and are provided in the ESI.†

What's the electrolyte?

75Li2S–25P2S5 (LPS)

In the current study, we investigate composite cathodes that combine a NMC cathode (LiNi0.5Mn0.3Co0.2O2) with amorphous 75Li2S–25P2S5 (LPS) solid electrolyte. The cathode material was coated with an amorphous Li–Zr-oxide (LZO) layer to reduce the reaction between NMC and LPS and to minimize the effect of the chemical instability on the electrochemical performance. SSB cells were constructed with In metal as the anode, a composite of NMC/LPS/carbon nanofiber (CNF) in a weight ratio of 60:35:5 as the cathode, and LPS as the solid electrolyte. All the cells were cycled between 1.4 and 3.7 V vs. In (between 2 and 4.3 V vs. Li/Li+) using constant-current constant-voltage (CCCV) charging and constant-current discharging at a rate of 0.05 mA cm−2. The details of the cell fabrication have been previously reported and are provided in the ESI.†

What's the cathode?

LiNi0.5Mn0.3Co0.2O2

The fast advancement in portable and wearable electronics has aroused growing demand for energy storage devices not only with high electrochemical performance but also having extra appealing characteristics, such as high safety, low cost, environmental friendliness, and so on. Supercapacitors are a kind of energy storage device demonstrating distinct advantages of simplicity in fabrication and high power density. However, the insufficient energy density has indeed hampered their application. In contrast, lithium-ion batteries possess a much higher energy density, but suffer considerably from cost and safety issues especially under some extreme conditions. Recently, rechargeable Zn–MnO2 batteries have attracted significant research interest due to their unique features, including high safety, light weight, nontoxicity and ease of fabrication. Solid-state rechargeable Zn–MnO2 batteries coupled with hydrogel electrolytes show excellent cycling stability, making them a promising alternative to lithium-ion batteries. Nevertheless, most electrodes for rechargeable Zn–MnO2 batteries are made from synthetic and expensive materials like graphene, carbon nanotubes, acetylene black, etc., which encounter obstacles, such as complex synthesis process, low yield, insufficient capacity, and environmental incompatibility. It is also important to note that most of the recent studies on rechargeable Zn–MnO2 batteries were focused on the optimization of electrochemically active substances, but ignored the structural design of the bulk electrode materials. As is well recognized, the hierarchical microstructure of electrodes may affect their electrochemical properties profoundly, in particular when the electrode is thick or the load of active substances is high.

What's the electrolyte?

hydrogel

OECT device fabrication. OECT devices were oxygen plasma ashed before spin-coating the polymer solutions at 1000 rpm for 60 s. This was followed by baking the devices at 60 °C for 60 s before removing the sacrificial parylene layer. Following the parylene peel-off the devices were further baked at 140 °C for 30 min. A polydimethylsiloxane (PDMS) well was fabricated using Sylgard-184 base and a curing agent in a 10:1 ratio. The PDMS well contained the electrolyte with a total volume of 300 μl. TBA PF6 electrolytes and TBA PF6:DBSA mixed electrolytes (in both ACN and DCM) had a concentration of 0.1 M. The ratio of TBA PF6:DBSA was kept constant at 1:1 for all mixed electrolyte measurements.

What's the electrolyte?

TBA PF6 electrolytes and TBA PF6:DBSA mixed electrolytes

As the initial SEI formed on copper modifies the nucleation and growth of lithium metal in a way that gives rise to a columnar microstructure, understanding the formation of this layer and its resulting properties is imperative. XPS results show that the initial SEI formed by galvanostatically bringing copper working electrodes to 0 V vs. Li/Li+ at 0.5 mA cm−2 in LP30 electrolyte with and without added HF are quite similar in chemical composition (Table S1†). Both SEIs contain primarily LiF, LiOH, and organic species (Fig. 4), indicating that the nanostructure of this layer rather than its chemistry is likely key to modifying lithium nucleation and growth. The thickness of this initial SEI layer is roughly 2–3 nm based both on the charge passed (∼3 nm, assuming fully dense and uniform LiF (Fig. 3a)) and the Ar ion sputter rate (∼2 nm, calibrated using SiO2, Fig. S3†). The presence of a smaller peak around 687.6 eV in the F1s spectrum for the as-received sample in Fig. 4a corresponds to PF6− and could be due to a more porous SEI that traps electrolyte. This is another indication of different SEI structures with and without an HF additive. Alternatively, this could be residue from incomplete electrolyte removal during rinsing.

What's the electrolyte?

LP30

The NASICON solid electrolyte material characterization was performed through an XRD analysis using a Bruker D8 Advance X-ray diffractometer with a Cu Kα X-ray source. Measurements were performed with a 2θ range of 10°–80°. Scanning electron microscope (SEM) images were taken and an energy dispersive spectrometer (EDS) analysis of the films was performed using a Verios 460, FEI and XFlash 6130, Bruker, respectively, operated at an accelerating voltage of 10 kV.

What's the electrolyte?

NASICON

Lithium conducting garnets are attractive solid electrolytes for solid-state lithium batteries but are difficult to process, generally requiring high reaction and sintering temperatures with long durations. In this work, we demonstrate a synthetic route to obtain Ta-doped garnet (Li6.4La3Zr1.4Ta0.6O12) utilizing La- and Ta-doped lanthanum zirconate (La2.4Zr1.12Ta0.48O7.04) pyrochlore nanocrystals as quasi-single-source precursors. Via molten salt synthesis (MSS) in a highly basic flux, the pyrochlore nanocrystals transform to Li-garnet at reaction temperatures as low as 400 °C. We also show that the pyrochlore-to-garnet conversion can take place in one step using reactive sintering, resulting in densified garnet ceramics with high ionic conductivity (0.53 mS cm−1 at 21 °C) and relative density (up to 94.7%). This approach opens new avenues for lower temperature synthesis of lithium garnets using a quasi-single-source precursor and provides an alternative route to highly dense garnet solid electrolytes without requiring advanced sintering processes.

What's the electrolyte?

Lithium conducting garnets

Lithium conducting garnets are attractive solid electrolytes for solid-state lithium batteries but are difficult to process, generally requiring high reaction and sintering temperatures with long durations. In this work, we demonstrate a synthetic route to obtain Ta-doped garnet (Li6.4La3Zr1.4Ta0.6O12) utilizing La- and Ta-doped lanthanum zirconate (La2.4Zr1.12Ta0.48O7.04) pyrochlore nanocrystals as quasi-single-source precursors. Via molten salt synthesis (MSS) in a highly basic flux, the pyrochlore nanocrystals transform to Li-garnet at reaction temperatures as low as 400 °C. We also show that the pyrochlore-to-garnet conversion can take place in one step using reactive sintering, resulting in densified garnet ceramics with high ionic conductivity (0.53 mS cm−1 at 21 °C) and relative density (up to 94.7%). This approach opens new avenues for lower temperature synthesis of lithium garnets using a quasi-single-source precursor and provides an alternative route to highly dense garnet solid electrolytes without requiring advanced sintering processes.

What's the electrolyte?

garnet

In this work, a low upper cut-off voltage of 4.5 V after an initial activation at 4.6 V is proposed and the effects of upper cut-off voltages (4.8 V and 4.5 V) on the stability of cationic/anionic redox chemistries are also studied. We demonstrate the significantly improved reversibility of cationic/anionic redox processes at the low cut-off voltage and the mitigated structural transitions from layered to rock-salt phases along with a valence decrease of Mn ions. Consequently, the LLOs present outstanding capacity/voltage stability over long-term cycling (capacity/voltage retention of 95.2%/92.2% after 200 cycles at 0.5C). The assembled mesocarbon microbead (MCMB)|LLO full cells deliver a high energy density of above 300 W h kg−1 and superior cycling/voltage stability. Compared with the previously proposed methods, this strategy is simpler and enables the use of conventional electrolytes rather than high-voltage ones that have not yet been widely commercialized up to now, which will promote the practical application of LLOs in high energy-density LIBs.

What's the electrolyte?

In summary, a compressible and elastic N-doped porous carbon nanofiber aerogel was prepared with rGO-wrapped crossed PCNFs and coiled PCNFs as mechanically reinforced structures to support the frameworks. Meanwhile, a hierarchical and porous architecture with open-cell structures existed in N-PCNFAs, which provided plenty of short paths for electrolyte diffusion and enough area for electrochemical double-layer capacitance when N-PCNFAs served as self-supporting and binder-free electrodes. Moreover, N atoms were also introduced into PCNFs as active sites for faradaic redox reactions. Thus, as supercapacitor electrodes, the N-PCNFAs exhibited a high specific capacitance of 279 F g−1 at 0.5 A g−1, with a rate performance of 59% at 20 A g−1. They even reached a capacitance retention of 122% after 10000 cycles because of the N-induced electro-activation, steadily improved wettability of the electrodes during fast charging and discharging, and stable frameworks of N-PCNFAs. Finally, this work presents a wise and simple strategy to prepare CNF-based aerogels with hierarchical structures and heteroatom doping, which also exhibit excellent compressibility and elasticity. These characteristics make them good candidates for energy storage, sensors, adsorption and catalytic materials.

What's the electrolyte?

The Li+ transport kinetics at the solid–solid electrode|electrolyte interfaces are crucial for the stable and durable performance of solid-state batteries (SSBs). A poor interface due to mechanical problems and/or (electro-)chemical instabilities will curtail the performance of such batteries. Herein, we present a detailed study on the interfaces of a lithium–sulfur (Li–S) SSB with a Li anode, Li–garnet (LLZO) solid electrolyte (SE), and a sulfur–carbon composite as the cathode. Interlayer gels based on ionic liquids were introduced to improve the interfacial properties of the system. For Li symmetric cells, the strategy resulted in a decrease in cell resistance by about a factor of five and stable voltage profiles with low overpotentials (∼300 mV at 0.4 mA cm−2 after >450 hours). Furthermore, the LLZO SE efficiently blocked the polysulfide shuttle to the Li anode. Due to the advantageous features of the design, good electrochemical performance was obtained, where the Li–S SSB delivered an initial discharge capacity of ca. 1360 mA h gsulfur−1 and a discharge capacity of ca. 570 mA h gsulfur−1 after 100 cycles. Detailed electrochemical and compositional characterization of the interphase layers was performed at the Li anode and sulfur cathode interfaces through X-ray photoelectron spectroscopy (XPS), applying depth-profiling techniques, and scanning transmission electron microscopy (STEM). The results revealed the presence of interphase nano-layers with varying thicknesses on the LLZO surface which contained organic and inorganic species.

What's the anode?

Li

The Li+ transport kinetics at the solid–solid electrode|electrolyte interfaces are crucial for the stable and durable performance of solid-state batteries (SSBs). A poor interface due to mechanical problems and/or (electro-)chemical instabilities will curtail the performance of such batteries. Herein, we present a detailed study on the interfaces of a lithium–sulfur (Li–S) SSB with a Li anode, Li–garnet (LLZO) solid electrolyte (SE), and a sulfur–carbon composite as the cathode. Interlayer gels based on ionic liquids were introduced to improve the interfacial properties of the system. For Li symmetric cells, the strategy resulted in a decrease in cell resistance by about a factor of five and stable voltage profiles with low overpotentials (∼300 mV at 0.4 mA cm−2 after >450 hours). Furthermore, the LLZO SE efficiently blocked the polysulfide shuttle to the Li anode. Due to the advantageous features of the design, good electrochemical performance was obtained, where the Li–S SSB delivered an initial discharge capacity of ca. 1360 mA h gsulfur−1 and a discharge capacity of ca. 570 mA h gsulfur−1 after 100 cycles. Detailed electrochemical and compositional characterization of the interphase layers was performed at the Li anode and sulfur cathode interfaces through X-ray photoelectron spectroscopy (XPS), applying depth-profiling techniques, and scanning transmission electron microscopy (STEM). The results revealed the presence of interphase nano-layers with varying thicknesses on the LLZO surface which contained organic and inorganic species.

What's the anode?

Li

CV measurements were conducted on an electrochemical workstation (CHI660A, USA) with glassy carbon as the working electrode (CHI104) and lithium metal as both the reference and counter electrode. A Fc/Fc+ redox couple serves as an internal reference (3.2 V vs. Li+/Li). Standard CR2032 coin cells were assembled inside an Ar-filled glovebox with a 2035 Celgard separator. 30 μL electrolyte was added to each coin cell with barely any electrolyte spilled out during cell crimping. Galvanostatic charge–discharge studies were conducted on a battery cycler (CT2001D, LAND Electronics Co., China). Before assembling full cells, the Si–C anodes were assembled into half-cells for pre-lithiation, which can reduce the initial irreversible capacity. All cells went through three formation cycles at 0.05C, 0.1C and 0.2C, respectively, until the current dropped below 0.05C.

What's the anode?

Si–C

CV measurements were conducted on an electrochemical workstation (CHI660A, USA) with glassy carbon as the working electrode (CHI104) and lithium metal as both the reference and counter electrode. A Fc/Fc+ redox couple serves as an internal reference (3.2 V vs. Li+/Li). Standard CR2032 coin cells were assembled inside an Ar-filled glovebox with a 2035 Celgard separator. 30 μL electrolyte was added to each coin cell with barely any electrolyte spilled out during cell crimping. Galvanostatic charge–discharge studies were conducted on a battery cycler (CT2001D, LAND Electronics Co., China). Before assembling full cells, the Si–C anodes were assembled into half-cells for pre-lithiation, which can reduce the initial irreversible capacity. All cells went through three formation cycles at 0.05C, 0.1C and 0.2C, respectively, until the current dropped below 0.05C.

What's the electrolyte?

Electrides are a unique class of materials, where the electron density is neither localised at an atomic orbital nor fully delocalised like in metals. Instead, their electrons occupy interstitial regions formed by cavities in the crystal structure, where they act as anions. Materials with anionic electrons offer versatile functionalities, such as high electrical conductivity, ultra-low work function, and non-linear optical responses, ranging their applications as electron emitters, battery anodes, and agent catalysts.

What's the anode?

Electrides are a unique class of materials, where the electron density is neither localised at an atomic orbital nor fully delocalised like in metals. Instead, their electrons occupy interstitial regions formed by cavities in the crystal structure, where they act as anions. Materials with anionic electrons offer versatile functionalities, such as high electrical conductivity, ultra-low work function, and non-linear optical responses, ranging their applications as electron emitters, battery anodes, and agent catalysts.

What's the electrolyte?

Wide Angle X-ray Scattering (WAXS) experiments were performed by a Rigaku three pinholes camera, coupled to an Fr-E+ superbright rotating anode microsource (Cu Kα, λ = 0.15405 nm) through a focusing Confocal Max Flux optics (CMF 15-105). Beam diameter was 0.2 mm. Sample-to-detector distance was 28 mm, calibrated by LaB6 powder standard. The sample was raster scanned with a 0.2 mm lateral step, in order to obtain an average pattern from a large crystal volume. 2D WAXS data were centered and calibrated using LaB6 standard and the corresponding 1D WAXS profiles were derived by SUNBIM software.

What's the anode?

Wide Angle X-ray Scattering (WAXS) experiments were performed by a Rigaku three pinholes camera, coupled to an Fr-E+ superbright rotating anode microsource (Cu Kα, λ = 0.15405 nm) through a focusing Confocal Max Flux optics (CMF 15-105). Beam diameter was 0.2 mm. Sample-to-detector distance was 28 mm, calibrated by LaB6 powder standard. The sample was raster scanned with a 0.2 mm lateral step, in order to obtain an average pattern from a large crystal volume. 2D WAXS data were centered and calibrated using LaB6 standard and the corresponding 1D WAXS profiles were derived by SUNBIM software.

What's the electrolyte?

(4) PEC: a typical PEC process (Fig. 9D) could be regarded as a combination of electrocatalysis, photocatalysis and their synergistic effects from mechanism. On one hand, electrocatalysis could generate functional radicals (e.g. ˙OH) to degrade oxidizable contaminants at the anode and reduce others at the cathode. On the other hand, the photocatalysis subprocess excites e−/h+ to degrade the existing pollutants. In particular, the electrons generated by photocatalysis could migrate along the electric field to the counter electrode by the utilized positive potential (Fig. 9D), which contributes to the reduction of photo-generated holes' and electrons' recombination and thus enhancement of photocatalytic performance. In other words, the increase of bias potential would not only enhance the separation of excitons but also elevate the degree of the electrochemical reaction, which would contribute to the acceleration of PEC performance.

What's the cathode?

(4) PEC: a typical PEC process (Fig. 9D) could be regarded as a combination of electrocatalysis, photocatalysis and their synergistic effects from mechanism. On one hand, electrocatalysis could generate functional radicals (e.g. ˙OH) to degrade oxidizable contaminants at the anode and reduce others at the cathode. On the other hand, the photocatalysis subprocess excites e−/h+ to degrade the existing pollutants. In particular, the electrons generated by photocatalysis could migrate along the electric field to the counter electrode by the utilized positive potential (Fig. 9D), which contributes to the reduction of photo-generated holes' and electrons' recombination and thus enhancement of photocatalytic performance. In other words, the increase of bias potential would not only enhance the separation of excitons but also elevate the degree of the electrochemical reaction, which would contribute to the acceleration of PEC performance.

What's the anode?

(4) PEC: a typical PEC process (Fig. 9D) could be regarded as a combination of electrocatalysis, photocatalysis and their synergistic effects from mechanism. On one hand, electrocatalysis could generate functional radicals (e.g. ˙OH) to degrade oxidizable contaminants at the anode and reduce others at the cathode. On the other hand, the photocatalysis subprocess excites e−/h+ to degrade the existing pollutants. In particular, the electrons generated by photocatalysis could migrate along the electric field to the counter electrode by the utilized positive potential (Fig. 9D), which contributes to the reduction of photo-generated holes' and electrons' recombination and thus enhancement of photocatalytic performance. In other words, the increase of bias potential would not only enhance the separation of excitons but also elevate the degree of the electrochemical reaction, which would contribute to the acceleration of PEC performance.

What's the electrolyte?

The potential of the respective working electrode Ewe and the electrolyte redox potential Esol as function of charge/discharge cycles are shown in Fig. 8a and c. Averaged overpotentials (, see eqn (3)) and CE per cycle for [FeII/III-racEDDHA] and [FeII/III(CN)6] are shown in Fig. 8b and d. In case of [FeII/III(CN)6] a time stable value for was obtained. In case of the [Fe-racEDDHA] complex even a slight decrease in was observed. Thus, no deactivation of the carbon electrode surface was measured during repetitive cycling. Excellent CE was observed for both redox systems, indicating the absence of parasitic side reactions and suggesting a good electrochemical stability of all electrochemically active species. The applied procedure of desizing and anodic activation of the CRLE lead to formation of a stable and active carbon surface, which is not degraded or passivated during extended electrolysis. Independent on electrode potential, passivation was not observed in the potential window between +800 mV and −1200 mV vs. Ag/AgCl, which demonstrates the high chemical stability of the surface towards electrochemical degradation processes at the chosen experimental conditions.

What's the electrolyte?

• Challenge: reality often deviates from theory. Because most metal sulfides are electrochemically active, lithium-inserted products are LiyMSx rather than pristine MSx. These materials strongly influence the behaviors of Li–S batteries but have long been overlooked in theoretical calculations. Solvation also plays an important role in triggering the ionization of LiPSs into solvated Li+ (electrolyte) cations and Sn2− (electrolyte) anions. Therefore, relying on the calculated binding energies of Li2Sx species rather than solvated Li+ (electrolyte) cations might lead to misleading results. There remains a need to optimize theoretical models to enhance the rationality and reliability of computation results.

What's the electrolyte?

solvated Li+ (electrolyte) cations and Sn2− (electrolyte) anions

• Challenge: reality often deviates from theory. Because most metal sulfides are electrochemically active, lithium-inserted products are LiyMSx rather than pristine MSx. These materials strongly influence the behaviors of Li–S batteries but have long been overlooked in theoretical calculations. Solvation also plays an important role in triggering the ionization of LiPSs into solvated Li+ (electrolyte) cations and Sn2− (electrolyte) anions. Therefore, relying on the calculated binding energies of Li2Sx species rather than solvated Li+ (electrolyte) cations might lead to misleading results. There remains a need to optimize theoretical models to enhance the rationality and reliability of computation results.

What's the electrolyte?

solvated Li+ (electrolyte)

In contrast, for the higher constant current of 2 mA cm−2, a close to constant intensity of the metal peak is observed for both electrolyte systems, with a slight increase occurring after passing 5 coulombs (3.25 mA h cm−2, Fig. 3e). Now mNMR is only slightly lower than mechem for both electrolytes, indicating a higher current efficiency for the LP30 electrolyte at 2 mA cm−2. This is tentatively ascribed to the competing reactions of SEI formation and Li deposition where at higher overpotentials, electrodeposition of Li metal occurs more rapidly than the kinetically-limited degradation reaction involving the electrolyte species. The morphology of the lithium deposits for the two electrodes is now very similar (Fig. S7†).

What's the electrolyte?

LP30

Implementability of Mg metal as an anode in magnesium-ion batteries (MIBs) mostly results from the dendrite-free nature of Mg during electrochemical plating/stripping, although it has been recently reported that the dendritic growth of Mg is dependent on the electrolyte nature and current densities. This feature provides potentially significant advantages to battery fabrication and/or electrochemical performance. For example, Mg metal delivers a higher volumetric capacity (3833 mA h cm−3) than either graphite (841 mA h cm−3) or Li metal (2047 mA h cm−3) in lithium-ion batteries, which enables cell designs that are more compact and flexible. The natural abundance of Mg (29k ppm in the Earth’s crust) is an additional benefit, in comparison with the limits of Li as a natural resource. The reversibility of Mg plating/stripping, however, is quite sensitive to the electrolyte system. The passivation of the Mg surface in conventional electrolytes restricts the versatility of utilizable electrolytes. To overcome this problem, various types of specially designed Mg-compatible electrolytes have been introduced during the past few decades: ‘all phenyl complex’ (APC); ‘magnesium aluminum chloride complex’ (MACC); [(DTBP)MgCl + MgCl2] (DTBP = 2,6-di-tert-butylphenolate); and [PhMgCl + MgCl2]. These electrolytes have shown reasonable levels of reversibility for Mg plating/stripping, but the relatively low anodic stability (≤ca. 3.0 V vs. Mg/Mg2+) and/or the difficulty in reaching high coulombic efficiencies remain problematic if MIBs are to move one step closer to commercialization.

What's the anode?

Mg metal

Implementability of Mg metal as an anode in magnesium-ion batteries (MIBs) mostly results from the dendrite-free nature of Mg during electrochemical plating/stripping, although it has been recently reported that the dendritic growth of Mg is dependent on the electrolyte nature and current densities. This feature provides potentially significant advantages to battery fabrication and/or electrochemical performance. For example, Mg metal delivers a higher volumetric capacity (3833 mA h cm−3) than either graphite (841 mA h cm−3) or Li metal (2047 mA h cm−3) in lithium-ion batteries, which enables cell designs that are more compact and flexible. The natural abundance of Mg (29k ppm in the Earth’s crust) is an additional benefit, in comparison with the limits of Li as a natural resource. The reversibility of Mg plating/stripping, however, is quite sensitive to the electrolyte system. The passivation of the Mg surface in conventional electrolytes restricts the versatility of utilizable electrolytes. To overcome this problem, various types of specially designed Mg-compatible electrolytes have been introduced during the past few decades: ‘all phenyl complex’ (APC); ‘magnesium aluminum chloride complex’ (MACC); [(DTBP)MgCl + MgCl2] (DTBP = 2,6-di-tert-butylphenolate); and [PhMgCl + MgCl2]. These electrolytes have shown reasonable levels of reversibility for Mg plating/stripping, but the relatively low anodic stability (≤ca. 3.0 V vs. Mg/Mg2+) and/or the difficulty in reaching high coulombic efficiencies remain problematic if MIBs are to move one step closer to commercialization.

What's the electrolyte?

The electrochemical data were collected using assembled 2032-coin cells. The cathode electrode was prepared by mixing the as-prepared MVO microspheres or V2O5·nH2O, carbon black and polytetrafluoroethylene (PTFE) binder in a weight ratio of 6:3:1. The coin cells were assembled using the prepared cathode, zinc metal as the anode, and the prepared PAM–CNF film as the electrolyte and separator. Galvanostatic charge/discharge electrochemical tests were performed on an eight-channel LAND battery analyzer (CT3001A, LAND Electronics Corporation, Wuhan, China) in the voltage range of 0.2–1.6 V. Cyclic voltammetry (CV) measurements were carried out on an electrochemical workstation (CHI 760e) in the potential range of 0.2–1.6 V. Electrochemical impedance spectroscopy (EIS) was carried out by applying an AC potential of 5 mV amplitude in the frequency range of 0.01–100 kHz. A freeze-resistance test was carried out at −18 °C in a freezer. Heat-resistance properties were tested in an EQ-DHG-9015 oven (MTI Inc., Richmond, CA, USA).

What's the cathode?

Both chemical and mechanical instabilities play a role in the impedance build-up and capacity fade during SSB cycling. Chemical reactivity issues between conductor and cathode have been well-studied theoretically and experimentally. In this work, we focus on the mechanical failure modes of an SSB composite cathode. The latter consists of an intimate mixture of the solid electrolyte and active cathode particles so that a transport path exists for the Li ions to/from the cathode particles. Good Li-ion transport across the solid electrolyte/cathode particle interface is necessary to minimize the cell resistance and overpotential of the composite cathode. Chemical reactivity at the solid electrolyte/cathode interface can lead to the formation of a reaction layer, which increases cathode impedance. For example, sulfide-based electrolytes have been shown to react with oxide-based cathodes, resulting in overpotential growth and capacity fade. Coating the cathode particles has been shown to mitigate this issue to some extent and improve cycling stability.

What's the cathode?

SSB composite

Herein, we designed a type of NiAs-type structured V2S3 nanoparticle by a one-step vulcanization reaction under Ar. The results showed the resultant carbon nanofiber (CNF)@V2S3 composite films show an improved conductance and a high flexibility, which is due to the high conductivity of V2S3 and the catalytic effect of V atoms on the carbonization treatment of CNFs. And the sulfur loaded carbon nanofiber@V2S3 (CNF@V2S3/S) composite cathodes showed a high specific capacitance of 1200 mA h g−1 at 0.1C, an excellent rate capability (retain 78.9% at 2.0C), and an ultralow decay rate per cycle (0.0071% at 2.0C for 1000 cycles). This high rate capability as well as high cycling stability could be due to the high polarity and catalytic activity, and improved conductivity of V2S3 nanocrystals. Furthermore, the enhancement mechanisms of energy storage and reaction kinetics of the composite cathodes via V2S3 have also been discussed.

What's the cathode?

carbon nanofiber@V2S3 (CNF@V2S3/S) composite

The lithium-ion battery, which can convert chemical energy to electric energy, has been widely used in portable devices, such as the laptops and mobile phones. However, the pressing demand for high capacity and stability is yet to be solved, especially the cathode, which has been the bottleneck of battery capacity. The Li-rich layered oxide has gained much attention due to the extremely high capacity of over 280 mA h g−1. Recent research has revealed that the extra capacity in the Li-rich oxide comes from the participation in oxygen redox. The complex chemical and structural evolution of the Li-rich cathode has attracted extensive efforts, however, the critical problem of voltage decay remains unsolved with its elusive mechanism.

What's the cathode?

Li-rich

Combining the insights derived from the systematic set of experiments detailed in this report along with complimentary results from literature, we have uncovered a new understanding of the mechanisms underlying the growth of columnar lithium metal. A schematic representation of the proposed mechanism linking electrolyte additive concentration, initial SEI formation, and the resulting growth of lithium columns is shown in Fig. 13. The initially pristine, oxide-free copper surface enables the electrocatalytic reduction of HF at ∼2 V vs. Li/Li+ which forms (111) textured LiF deposits a few nanometers in size on the surface. Two morphologically distinct yet functionally equivalent LiF deposit morphologies are possible – uniform yet discontinuous LiF particles decorating the copper surface or a continuous polycrystalline film – with the former being more probable in this system when considering the substrate roughness and LiF crystallite size. This HF reduction and LiF deposition process slows significantly when HF becomes depleted near the interface. Subsequently, as the potential drops below 1 V, solvent molecules are decomposed on the remaining unpassivated surfaces between and/or on top of LiF particles, forming organic reduction products and slowing further reduction of electrolyte species. While the precise morphology of the LiF particle layer could not be directly imaged, both candidate morphologies shown schematically in Fig. 13 would result in an SEI with similar properties and identical effects on lithium nucleation and growth. Interfaces between individual crystalline LiF particles and/or between LiF particles and the amorphous matrix of solvent reduction products in the initial SEI act as fast lithium-ion diffusion pathways. These pathways serve to homogenize the electronic and ionic properties near the copper surface, enabling a high nucleation density of lithium metal as the potential of the working electrode is brought below 0 V vs. Li/Li+ and electrodeposition of the active material begins via an instantaneous nucleation event. High lithium-ion diffusion within the SEI created by these interfaces, which allows for easier movement of lithium ions laterally across the electrode surface, along with the layer being very thin, allow for uniform, lateral growth of the lithium metal deposits until they bump into one another. After lateral growth is inhibited the deposits are restricted to growing vertically, normal to the working electrode, resulting in the highly monodisperse (110) textured columnar morphology observed. The diameter of these lithium columns can be controlled by varying the current density during electrodeposition (Fig. S8†). Indeed, a columnar morphology may be the intrinsically preferred growth mode of lithium metal. However, such growth may be inhibited by the SEI formed in conventional electrolytes without additives due to non-uniform lithium-ion diffusivity and the presence of “hot spots” where preferential deposition occurs, whereas the nanostructured SEI formed in electrolytes with added HF would allow columnar growth to proceed uninhibited.

What's the electrolyte?

In comparison with MnOx, NiO and Co3O4, TiO2 with lower theoretical capacitance has also been tested as an electrode in SCs. For instance, by using TiO2 nanofibers, activated carbon and a PVA–H3PO4 membrane, a solid-state SC was fabricated, which delivered a specific capacitance of 310 F g−1. Recently, Ta-doped TiO2 nanofibers were synthesized through the electrospinning technique. In 1 M H2SO4, the 2% Ta-doped TiO2 nanofibers could deliver nearly two times higher specific capacitance than the undoped TiO2 counterpart (111 F g−1), which was attributed to the enhanced electronic conductivity of TiO2 stemming from the replacement of Ti4+ by Ta5+ in the lattice after doping. In addition to the most studied metal oxides mentioned above, Fe2O3, V2O5 and CuO have also been applied to SCs as electrodes. The first report on a pure iron oxide electrode composed of particles with sizes of 53 nm has demonstrated a specific capacitance of 256 F g−1 in 1 M LiOH. Recently, both interconnected Fe2O3 and V2O5 nanofibers with rich meso-/macro-pores were obtained via electrospinning and subsequent heat treatment. Owing to the hierarchical porous structure that acts as an excellent conductive highway for efficient electronic transfer and ion permeation, the resultant binder-free Fe2O3 and V2O5 electrodes gave specific capacitances of 255 F g−1 and 256 F g−1, respectively, at 2 mV s−1 in 1 M Na2SO4. Moreover, an all-solid-state ASC based on an Fe2O3 negative electrode and V2O5 positive electrode could be operated at up to 1.8 V and displayed a high energy density of 32.2 W h kg−1 at an average power density of 128.7 W kg−1. In 2012, pure electrospun V2O5 nanofibers with different structures have been synthesized and tested in two studies. These pure electrospun V2O5 nanofibers displayed a capacitance of around 200 F g−1 in both the studies. A high specific capacitance of 620 F g−1 (accounting for ∼35% of their theoretical capacitance) at 2 A g−1 in 6 M KOH was reported by Jose et al. for electrospun CuO nanowires in 2014. In another reference, an ASC device based on electrospun CuO nanowires as the anode and AC as the cathode was constructed and provided an enhanced voltage window (1.6 V), specific capacitance (83 F g−1), and threefold higher energy density (29.5 W h kg−1) than the AC-based symmetric capacitor (1.4 V, 33 F g−1, 11 W h kg−1).

What's the cathode?

AC

In comparison with MnOx, NiO and Co3O4, TiO2 with lower theoretical capacitance has also been tested as an electrode in SCs. For instance, by using TiO2 nanofibers, activated carbon and a PVA–H3PO4 membrane, a solid-state SC was fabricated, which delivered a specific capacitance of 310 F g−1. Recently, Ta-doped TiO2 nanofibers were synthesized through the electrospinning technique. In 1 M H2SO4, the 2% Ta-doped TiO2 nanofibers could deliver nearly two times higher specific capacitance than the undoped TiO2 counterpart (111 F g−1), which was attributed to the enhanced electronic conductivity of TiO2 stemming from the replacement of Ti4+ by Ta5+ in the lattice after doping. In addition to the most studied metal oxides mentioned above, Fe2O3, V2O5 and CuO have also been applied to SCs as electrodes. The first report on a pure iron oxide electrode composed of particles with sizes of 53 nm has demonstrated a specific capacitance of 256 F g−1 in 1 M LiOH. Recently, both interconnected Fe2O3 and V2O5 nanofibers with rich meso-/macro-pores were obtained via electrospinning and subsequent heat treatment. Owing to the hierarchical porous structure that acts as an excellent conductive highway for efficient electronic transfer and ion permeation, the resultant binder-free Fe2O3 and V2O5 electrodes gave specific capacitances of 255 F g−1 and 256 F g−1, respectively, at 2 mV s−1 in 1 M Na2SO4. Moreover, an all-solid-state ASC based on an Fe2O3 negative electrode and V2O5 positive electrode could be operated at up to 1.8 V and displayed a high energy density of 32.2 W h kg−1 at an average power density of 128.7 W kg−1. In 2012, pure electrospun V2O5 nanofibers with different structures have been synthesized and tested in two studies. These pure electrospun V2O5 nanofibers displayed a capacitance of around 200 F g−1 in both the studies. A high specific capacitance of 620 F g−1 (accounting for ∼35% of their theoretical capacitance) at 2 A g−1 in 6 M KOH was reported by Jose et al. for electrospun CuO nanowires in 2014. In another reference, an ASC device based on electrospun CuO nanowires as the anode and AC as the cathode was constructed and provided an enhanced voltage window (1.6 V), specific capacitance (83 F g−1), and threefold higher energy density (29.5 W h kg−1) than the AC-based symmetric capacitor (1.4 V, 33 F g−1, 11 W h kg−1).

What's the anode?

CuO nanowires

In order to demonstrate their remarkable performance for electrochemical energy storage in portable and wearable electronics, we assembled the flexible quasi-solid-state ARZIBs using the NV NSs@ACC as the cathode together with the Zn NSs@CC anode (deposited Zn nanosheets on CC). Fig. S7a† depicts the crystal structure of Zn NSs@CC, in which all the peaks were in good accordance with those of hexagonal Zn (JCPDS no. 87-0713). Furthermore, the SEM images of Zn NSs@CC are illustrated in Fig. S7b.† It is clearly observed that the Zn nanosheets are uniformly grown on CC. The cycling performance of the as-assembled flexible ZIB at 0.2 A g−1 is depicted in Fig. 6a. The device achieves an initial discharge capacity of 121 mA h g−1 and a reversible capacity of 91 mA h g−1 can be maintained after 50 cycles. Furthermore, it can deliver a specific capacity of 60 mA h g−1 and maintain a stable capacity of 52 mA h g−1 after 1000 cycles with excellent coulombic efficiency (∼100%) at a high current density of 1.0 A g−1 (Fig. 6b), which can be ascribed to the stable 2D ultrathin nanosheets structure and the free-standing feature. The flexible quasi-solid-state ARZIB device was bent into three mechanical states to further evaluate its flexibility (Fig. 6c). As presented in Fig. 6d and e, there are no evident changes in the EIS spectra and capacity decline (>92% retention) under different bending states, demonstrating the extraordinary structural durability of our flexible quasi-solid-state ARZIB devices. As a demonstration of the practical application, two series-connected devices were shown to successfully illuminate a red light-emitting diode (LED, 1.8 V) (inset of Fig. 6e). These significant results indicate that this flexible quasi-solid-state ARZIB device holds potential promise for powering future portable and wearable electronics.

What's the cathode?

NV NSs@ACC

In order to demonstrate their remarkable performance for electrochemical energy storage in portable and wearable electronics, we assembled the flexible quasi-solid-state ARZIBs using the NV NSs@ACC as the cathode together with the Zn NSs@CC anode (deposited Zn nanosheets on CC). Fig. S7a† depicts the crystal structure of Zn NSs@CC, in which all the peaks were in good accordance with those of hexagonal Zn (JCPDS no. 87-0713). Furthermore, the SEM images of Zn NSs@CC are illustrated in Fig. S7b.† It is clearly observed that the Zn nanosheets are uniformly grown on CC. The cycling performance of the as-assembled flexible ZIB at 0.2 A g−1 is depicted in Fig. 6a. The device achieves an initial discharge capacity of 121 mA h g−1 and a reversible capacity of 91 mA h g−1 can be maintained after 50 cycles. Furthermore, it can deliver a specific capacity of 60 mA h g−1 and maintain a stable capacity of 52 mA h g−1 after 1000 cycles with excellent coulombic efficiency (∼100%) at a high current density of 1.0 A g−1 (Fig. 6b), which can be ascribed to the stable 2D ultrathin nanosheets structure and the free-standing feature. The flexible quasi-solid-state ARZIB device was bent into three mechanical states to further evaluate its flexibility (Fig. 6c). As presented in Fig. 6d and e, there are no evident changes in the EIS spectra and capacity decline (>92% retention) under different bending states, demonstrating the extraordinary structural durability of our flexible quasi-solid-state ARZIB devices. As a demonstration of the practical application, two series-connected devices were shown to successfully illuminate a red light-emitting diode (LED, 1.8 V) (inset of Fig. 6e). These significant results indicate that this flexible quasi-solid-state ARZIB device holds potential promise for powering future portable and wearable electronics.

What's the anode?

Zn NSs@CC

In this contribution, we propose the use of a small amount of metallic 1T MoS2 nanodots (NDs, 3 wt% of the electrode) as a robust catalyst for advanced LSBs. Electrochemical tests and synchrotron in situ X-ray diffraction (XRD) characterizations demonstrated the strong anchoring and catalytic capability of MoS2 NDs. When MoS2 NDs were integrated with a porous carbon/catholyte, the new cathodes exhibited remarkable battery performance, including a high rate capacity of 8.5 mA h cm−2 at 1C and an impressive capacity retention of 9.3 mA h cm−2 after 300 cycles under a high sulfur loading of 12.9 mg cm−2, an extremely low E/S ratio of 4.6 μL mg−1, and a remarkable sulfur content of 81 wt%. Such high-energy densities delivered under high sulfur loading and lean electrolyte conditions are among the best reported so far for LSBs. Based on DFT calculations, we found that the phase and edge sites were the key to governing the catalytic capability for MoS2. Li2S anchored preferentially at Mo-terminated edges of MoS2 and the electrochemical dissociation occurred toward the surface of the monolayer where Li ions could diffuse faster. Indeed, the metallic 1T MoS2 showed a stronger affinity to polysulfides and a lower activation energy for Li2S than 2H MoS2 at most of the adsorption sites. These findings suggest that the catalytic activity in MoS2 can be maximized by downsizing 2H MoS2 flakes to 1T MoS2 NDs.

What's the cathode?

MoS2 NDs

(4) Photoelectrocatalysis (PEC): PEC is a special electricity-driven photocatalytic process which could be regarded as a combination of electrocatalysis and photocatalysis. In a typical PEC cell, the photocatalysts are usually immobilized on a conductive substrate as the anode and a bias potential as well as continuous illumination are then applied to the prepared anode to generate functional radicals to remove the contaminants. In particular, it should be mentioned that the separation of photo-induced charge carriers could be significantly enhanced by the utilized bias potential since the electrons generated by photocatalysis follow the electric field to the cathode, which contributes to the reduction of the recombination rate. As a result, the photoelectrocatalytic method would significantly promote the degradation efficiency of the titanate materials. For example, Chang et al. synthesized a Cu2O/titanate composite for photocatalytic and photoelectrocatalytic degradation of ibuprofen. The results indicated that the pseudo-first rate constant could be increased from 0.804 h−1 for photocatalysis to 2.28 h−1 for PEC. Finally, the morphology modulation, trap doping and compositing strategies were also extensively applied in PEC to further enhance the separation of charge carriers, for example, layer stacking (morphology modulation), Fe-modification (trap doping) and ZnO compositing (compositing) could significantly improve the PEC performance compared to that of pristine titanate characterized by the increased photocurrent under illumination.

What's the cathode?

(4) Photoelectrocatalysis (PEC): PEC is a special electricity-driven photocatalytic process which could be regarded as a combination of electrocatalysis and photocatalysis. In a typical PEC cell, the photocatalysts are usually immobilized on a conductive substrate as the anode and a bias potential as well as continuous illumination are then applied to the prepared anode to generate functional radicals to remove the contaminants. In particular, it should be mentioned that the separation of photo-induced charge carriers could be significantly enhanced by the utilized bias potential since the electrons generated by photocatalysis follow the electric field to the cathode, which contributes to the reduction of the recombination rate. As a result, the photoelectrocatalytic method would significantly promote the degradation efficiency of the titanate materials. For example, Chang et al. synthesized a Cu2O/titanate composite for photocatalytic and photoelectrocatalytic degradation of ibuprofen. The results indicated that the pseudo-first rate constant could be increased from 0.804 h−1 for photocatalysis to 2.28 h−1 for PEC. Finally, the morphology modulation, trap doping and compositing strategies were also extensively applied in PEC to further enhance the separation of charge carriers, for example, layer stacking (morphology modulation), Fe-modification (trap doping) and ZnO compositing (compositing) could significantly improve the PEC performance compared to that of pristine titanate characterized by the increased photocurrent under illumination.

What's the anode?

photocatalysts

Here, RGO serves as a 2D template and the carbon source to form the SiC/RGO nanocomposite by an in situ gas–solid reaction. The introduction of RGO not only reduces the recombination of photogenerated electron–hole pairs, but also promotes the transfer of photoexcited electrons between SiC and Li anode taking advantage of the high conductivity of RGO, which plays a crucial role in improving the performance of Li–CO2 batteries. The synthesis strategy of the SiC/RGO nanocomposite is displayed in Fig. 2a. By the sublimation of the solid Si at a high temperature, a new nucleus is generated on the surface of RGO by a gas–solid reaction (C(s) + Si(g) → SiC(s)), and then spreads around RGO to form a tightly connected SiC nanosheet on RGO. The morphology and the structure of RGO and SiC/RGO nanocomposite were studied by SEM and TEM. As shown in Fig. S1 (ESI†), the bare RGO exhibits a 2D nanosheet morphology. After the carbothermal process, SiC is uniformly distributed on the RGO substrate, presenting a distinct island-in-sea-like morphology (Fig. 2b and c). Furthermore, the elemental distribution of the material was studied by EDS. As shown in Fig. S2,† Si is present only in the island-like region and C exists throughout the test area, which directly proves that the island-like nanosheets observed in SEM and TEM images are SiC. The partial conversion of 2D RGO to the SiC/RGO nanocomposite facilitates the transfer of high energy electrons. The high resolution TEM (HRTEM) images (Fig. 2d) also demonstrate the tight junction between SiC and RGO. Clear lattice fringes with a d-spacing of 0.25 nm on the overlay correspond well to the (111) plane of SiC. It is also possible to distinguish between carbon layers due to the pleated RGO layer in the substrate with an interplanar distance of 0.34 nm. Apart from the enhanced electronic conductivity for the SiC/RGO composites, RGO also plays an important role in enhancing the CO2 adsorption capacity (Fig. S3†), which is beneficial for a higher actual discharge capacity of the cathode.

What's the cathode?

In an another set of experiments, the DPP dyes 1 and 2 were cross-linked with 3 with the classical copper-catalyzed protocol by heating the photoelectrode for 10 minutes at 30 °C in a DMF solution with Cu(SO4)·5H2O, sodium ascorbate in presence of the tris(benzyltriazolylmethyl)amine (TBTA) ligand. The completion of the reaction was confirmed by the ATR-IR spectroscopy (Fig. S25 and S26†). The results of the photovoltaic performances of the solar cells made with the I3−/I− redox couple are gathered in Table S3.† Clearly, the conditions of copper catalyzed Huisgen cycloaddition, although conducted in mild conditions, negatively impact the performances of the solar cells since the PCEs are notably lower than those measured with the thermal crosslinking activation. For TiO2 based solar cells, both Jsc and Voc were diminished upon cross-linking. Interestingly, the performances of photocathodes with NiO are less degraded than those with the photoanodes based on TiO2, because only the Jsc was decreased after copper-catalyzed cross-linking. This result is consistent with the upward shift of the valence band of NiO upon insertion of copper cation into the lattice as it was previously reported. Another possibility, is the presence of remaining Cu(II) salt, which is a paramagnetic metal that could quench the excited state of the dye and consequently diminishes the charge injection quantum yield as shown by Hanson and co-workers.

What's the cathode?

Utilization of ceramic SE based separators is also an interesting strategy that can potentially solve problems associated with the S cathode and Li anode. Thanks to their high shear modulus, both the polysulfide shuttle and the Li dendrite growth can be mechanically suppressed. Among various inorganic ceramic SEs, Li–garnet (LLZO) is of particular interest since it offers a relatively high (electro)-chemical stability with Li metal, a much wider potential window compared to other SEs, and a high shear modulus (61 GPa for LLZO; 4.2 GPa for Li). However, the surface impurities/roughness and brittle nature of LLZO SE restrict its intimate contact with Li and other solid-state cathode materials. With S as the cathode material, the electrode/electrolyte integration will be even more challenging considering the enormous volumetric expansion during the conversion reaction with Li. In this regard, surface modification strategies including the use of thin nanometric layer coatings, and 3D interfacial architectures, or the implementation of polymer, gel and liquid interlayers have been used to keep the electrode|SE contacts intact and stabilize the interfacial ionic transport. Using Li ion conducting gel/liquid interlayers instead of using solid interlayers may prove to be an even better strategy due to their inherent tendency to penetrate through the porous structures and hence to access a greater surface area of the composite cathode.

What's the cathode?

S

Utilization of ceramic SE based separators is also an interesting strategy that can potentially solve problems associated with the S cathode and Li anode. Thanks to their high shear modulus, both the polysulfide shuttle and the Li dendrite growth can be mechanically suppressed. Among various inorganic ceramic SEs, Li–garnet (LLZO) is of particular interest since it offers a relatively high (electro)-chemical stability with Li metal, a much wider potential window compared to other SEs, and a high shear modulus (61 GPa for LLZO; 4.2 GPa for Li). However, the surface impurities/roughness and brittle nature of LLZO SE restrict its intimate contact with Li and other solid-state cathode materials. With S as the cathode material, the electrode/electrolyte integration will be even more challenging considering the enormous volumetric expansion during the conversion reaction with Li. In this regard, surface modification strategies including the use of thin nanometric layer coatings, and 3D interfacial architectures, or the implementation of polymer, gel and liquid interlayers have been used to keep the electrode|SE contacts intact and stabilize the interfacial ionic transport. Using Li ion conducting gel/liquid interlayers instead of using solid interlayers may prove to be an even better strategy due to their inherent tendency to penetrate through the porous structures and hence to access a greater surface area of the composite cathode.

What's the anode?

Li

Utilization of ceramic SE based separators is also an interesting strategy that can potentially solve problems associated with the S cathode and Li anode. Thanks to their high shear modulus, both the polysulfide shuttle and the Li dendrite growth can be mechanically suppressed. Among various inorganic ceramic SEs, Li–garnet (LLZO) is of particular interest since it offers a relatively high (electro)-chemical stability with Li metal, a much wider potential window compared to other SEs, and a high shear modulus (61 GPa for LLZO; 4.2 GPa for Li). However, the surface impurities/roughness and brittle nature of LLZO SE restrict its intimate contact with Li and other solid-state cathode materials. With S as the cathode material, the electrode/electrolyte integration will be even more challenging considering the enormous volumetric expansion during the conversion reaction with Li. In this regard, surface modification strategies including the use of thin nanometric layer coatings, and 3D interfacial architectures, or the implementation of polymer, gel and liquid interlayers have been used to keep the electrode|SE contacts intact and stabilize the interfacial ionic transport. Using Li ion conducting gel/liquid interlayers instead of using solid interlayers may prove to be an even better strategy due to their inherent tendency to penetrate through the porous structures and hence to access a greater surface area of the composite cathode.

What's the cathode?

S

For the emerging potassium-ion energy storage technology, the major challenge is seeking suitable electrode materials with a robust structure and fast kinetics for the reversible insertion/desertion of potassium ions. Here, a pseudocapacitive core–shell heterostructure of titanium oxide/carbon confined into N, P, and S co-doped carbon (TiO2/C@NPSC) is obtained by pyrolyzing a metal–organic framework (MOF) precursor of MIL-125 (Ti) modified by poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) polymer. The distinctive structure of TiO2/C@NPSC can effectively buffer the volume variation of TiO2 nano-grains during the charge/discharge process, increase the electron/charge transfer, provide abundant active sites, and boost the pseudocapacitive-dominated K+-storage. Consequently, the TiO2/C@NPSC anode displays superior cyclability and fast kinetics behavior. Upon integrating it with a high capacitance activated carbon cathode derived from another MOF precursor, the as-built potassium-ion hybrid capacitor achieves a high-energy density of 114 W h kg−1 and a power output of 21 kW kg−1. Moreover, in a wide working potential window of 0–4.2 V, the device also maintains over 91.6% of its initial capacity after 10000 cycles, showing a superior cycle stability. Our results are conducive to understanding the importance of anode-engineering for designing advanced PIHCs.

What's the cathode?

carbon

For the emerging potassium-ion energy storage technology, the major challenge is seeking suitable electrode materials with a robust structure and fast kinetics for the reversible insertion/desertion of potassium ions. Here, a pseudocapacitive core–shell heterostructure of titanium oxide/carbon confined into N, P, and S co-doped carbon (TiO2/C@NPSC) is obtained by pyrolyzing a metal–organic framework (MOF) precursor of MIL-125 (Ti) modified by poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) polymer. The distinctive structure of TiO2/C@NPSC can effectively buffer the volume variation of TiO2 nano-grains during the charge/discharge process, increase the electron/charge transfer, provide abundant active sites, and boost the pseudocapacitive-dominated K+-storage. Consequently, the TiO2/C@NPSC anode displays superior cyclability and fast kinetics behavior. Upon integrating it with a high capacitance activated carbon cathode derived from another MOF precursor, the as-built potassium-ion hybrid capacitor achieves a high-energy density of 114 W h kg−1 and a power output of 21 kW kg−1. Moreover, in a wide working potential window of 0–4.2 V, the device also maintains over 91.6% of its initial capacity after 10000 cycles, showing a superior cycle stability. Our results are conducive to understanding the importance of anode-engineering for designing advanced PIHCs.

What's the anode?

TiO2/C@NPSC

The ATR-IR spectra (Fig. 2 for TiO2 based photoanode and Fig. S3† for NiO based photocathode) show that the intensity of the azido stretching bands has completely disappeared after heating, while that of the alkyne is visible meaning that there are some unreacted alkyne groups in the film. On the other hand, the stretching bands of the ester groups of the crosslinking agent 3 and that of the lactam of the DPP 1 or 2 (both around 1700 cm−1) are still clearly visible, proving that these moieties are present on the surface of the film. This result is consistent with the presence of four alkyne groups in 3 relative to only two azides on the dyes 1 or 2. The ATR-IR spectrum of the DPP 2 grafted on the electrode was not modified after heating in the crosslinking conditions (15 min at 140 °C in orthodichlorobenzene) in absence of the cross-linker 3 (Fig. S4†). This indicates that the azido groups are stable in these conditions and do not degrade upon heating at 140 °C for 15 min.

What's the cathode?

NiO

For the former, metal Ni and Cu are the common conductivity improvers and can be introduced into the sulfur cathode through a straightforward addition and/or interfusion. For example, Manthiram's group presented a pie-like electrode that consists of nickel foam as a “filling” and an outer carbon shell as a “crust” for facilitating the utilization of sulfur cathodes. Copper powder was introduced into the sulfur cathode by Jia et al. through partially replacing the sulfur contained active material with Cu during the manufacture procedure of the working electrode. Very similarly, Guo and coworkers recently reported the preparation of a CNT/graphene–S–Al3Ni2 cathode with a 3D network structure using 10 wt% Al3Ni2 powder to displace the CNT/graphene–S active material, where the Al in the Al3Ni2 provides an efficient channel for fast electron and ion transfer in the three dimensions, especially along the vertical direction of the cathode. Furthermore, Zheng et al. demonstrated a copper-stabilized sulfur–microporous carbon composite synthesized by uniformly dispersing 10% highly electronically conductive Cu nanoparticles into microporous carbon (MC), followed by wet-impregnation of S. To obtain highly dispersed Cu, commercial MC with a high surface area is first selected as the matrix and carrier. An ultrasonic-assisted multiple wetness impregnation and synchro-dry method is then applied to load 50% Cu(NO3)2 ethanol alcohol solution in MC and evaporate the solvent. This process is repeated until a certain amount of Cu(NO3)2 is added to MC. Finally, Cu nanoparticles anchored in the MC are gained by the reduction of the dry precursor at 200 °C for 1 h under an argon mixed with 5% hydrogen environment. Interestingly, the Cu content in MC can be easily adjusted by controlling the amount of Cu(NO3)2 addition via impregnation times.

What's the cathode?

CNT/graphene–S–Al3Ni2

For the former, metal Ni and Cu are the common conductivity improvers and can be introduced into the sulfur cathode through a straightforward addition and/or interfusion. For example, Manthiram's group presented a pie-like electrode that consists of nickel foam as a “filling” and an outer carbon shell as a “crust” for facilitating the utilization of sulfur cathodes. Copper powder was introduced into the sulfur cathode by Jia et al. through partially replacing the sulfur contained active material with Cu during the manufacture procedure of the working electrode. Very similarly, Guo and coworkers recently reported the preparation of a CNT/graphene–S–Al3Ni2 cathode with a 3D network structure using 10 wt% Al3Ni2 powder to displace the CNT/graphene–S active material, where the Al in the Al3Ni2 provides an efficient channel for fast electron and ion transfer in the three dimensions, especially along the vertical direction of the cathode. Furthermore, Zheng et al. demonstrated a copper-stabilized sulfur–microporous carbon composite synthesized by uniformly dispersing 10% highly electronically conductive Cu nanoparticles into microporous carbon (MC), followed by wet-impregnation of S. To obtain highly dispersed Cu, commercial MC with a high surface area is first selected as the matrix and carrier. An ultrasonic-assisted multiple wetness impregnation and synchro-dry method is then applied to load 50% Cu(NO3)2 ethanol alcohol solution in MC and evaporate the solvent. This process is repeated until a certain amount of Cu(NO3)2 is added to MC. Finally, Cu nanoparticles anchored in the MC are gained by the reduction of the dry precursor at 200 °C for 1 h under an argon mixed with 5% hydrogen environment. Interestingly, the Cu content in MC can be easily adjusted by controlling the amount of Cu(NO3)2 addition via impregnation times.

What's the cathode?

sulfur

At a volumetric current density of 268 mA cm−3, the volumetric capacity of ∼215 mA h cm−3 of LixMnO2/nanomesh is much larger than the capacity of many 3D core–shell cathodes at similar vol. current densities, such as MnO2-coated carbon nanotubes (∼53 mA h cm−3), LiCoO2-coated carbon fiber mats (∼70 mA h cm−3), LiCoO2-coated aluminium nanowires (∼140 mA h cm−3) or V2O5-coated CNT/PAN sheets (∼155 mA h cm−3). At a vol. current density of ∼270 mA cm−3, the volumetric capacity of the nanomesh cathodes is only significantly inferior to that of LiCoO2-coated carbon foam (∼370 mA h cm−3) and Li2MnSiO4 doubly-loaded on graphene inverse opals (∼520 mA h cm−3) – both electrodes having a very high volume fraction of the active materials of 57% and 90%, respectively. Furthermore, also at high current densities above 1000 mA cm−3, the nanomesh cathode retains superior volumetric capacity to most of the electrodes, being only inferior to the two aforementioned electrodes and the V2O5-coated graphene inverse opals with a highly regular structure and larger pores. At high currents densities, however, the comparison may be additionally affected by the varying thicknesses of the electrodes (Table S3†), since thinner electrodes (such as ours) typically show better high-rate performance.

What's the cathode?

LixMnO2/nanomesh

Up to now, owing to the tremendous work done by different research groups, there have been several new types of electrolysers proposed for decoupled H2/O2 generation through the utilization of redox mediators, and a list of these new electrolysers has been provided in Table S1 in the ESI.† For example, a new PEM electrolyser is proposed with polyoxometalates (POMs) used as the redox relay, and the H2/O2 generation can be decoupled both in time and in space (Scheme 1b). Such a design requires three electrodes (OER electrode, carbon electrode for reduction of the soluble POM redox mediator, and Pt catalyst for spontaneous hydrogen evolution), one soluble redox mediator (POMs), one membrane (Nafion membrane) and two cells, which potentially increases the cost for the electrolysers. A solid redox mediator, such as Ni(OH)2, has also been successfully used as the relay for decoupled water splitting (Scheme 1c). This kind of design needs no separator between the cathode and the anode; however, an auxiliary relay electrode has been used and the whole cell is still a three-component system, and the overall cost for the electrolyser may not be effectively reduced. Also, among all these designs, since the HER and OER are decoupled in time, there is always an electrode (either the HER or the OER electrode) in an idle state when the cell is in operation. For decoupled water splitting in time, the integration of the HER and OER in one electrode can effectively reduce the cost and space for the electrolyser, which is more practical for scale-up applications; however, a rational design toward this direction has not been proposed.

What's the cathode?

Up to now, owing to the tremendous work done by different research groups, there have been several new types of electrolysers proposed for decoupled H2/O2 generation through the utilization of redox mediators, and a list of these new electrolysers has been provided in Table S1 in the ESI.† For example, a new PEM electrolyser is proposed with polyoxometalates (POMs) used as the redox relay, and the H2/O2 generation can be decoupled both in time and in space (Scheme 1b). Such a design requires three electrodes (OER electrode, carbon electrode for reduction of the soluble POM redox mediator, and Pt catalyst for spontaneous hydrogen evolution), one soluble redox mediator (POMs), one membrane (Nafion membrane) and two cells, which potentially increases the cost for the electrolysers. A solid redox mediator, such as Ni(OH)2, has also been successfully used as the relay for decoupled water splitting (Scheme 1c). This kind of design needs no separator between the cathode and the anode; however, an auxiliary relay electrode has been used and the whole cell is still a three-component system, and the overall cost for the electrolyser may not be effectively reduced. Also, among all these designs, since the HER and OER are decoupled in time, there is always an electrode (either the HER or the OER electrode) in an idle state when the cell is in operation. For decoupled water splitting in time, the integration of the HER and OER in one electrode can effectively reduce the cost and space for the electrolyser, which is more practical for scale-up applications; however, a rational design toward this direction has not been proposed.

What's the anode?

Commercial sublimated sulfur (S) was mixed with AB (C) in a mass ratio of 8:2, and the mixture (C/S) was fully ground and heat-treated at 155 °C for 6 h in an argon atmosphere. Then, the as-obtained composite was mixed with AB and PVDF (8:1:1 by weight), followed by dispersing in NMP solvent and stirred for 12 h. Next, the slurry was uniformly cast onto an Al foil substrate and dried in a vacuum for 12 h at 60 °C. Afterwards, the coated-Al foil was cut into 12 mm wafers with a sulfur loading of ∼2.0 mg cm−2. Moreover, a cathode wafer with a high sulfur loading of ∼5.6 mg cm−2 was also prepared in the same procedure by controlling the thickness of the coating. The as-prepared C/S disc, above separators (BFO/GO/AB@PP, GO/AB@PP, and PP) and lithium plate (diameter of 15.6 mm, thickness of 250 µm) were applied to assemble a 2025-type button battery, in which the electrolyte was 1 M LiTFSI in DOL/DME (v/v = 1:1) with 1 wt% LiNO3. The electrolyte/sulfur (E/S) ratio was fixed as 12:1 (µL mg−1). The battery was charged and discharged on a Neware battery test station (5V20 mA). The cyclic voltammetry (CV) curves were operated in a voltage window between 1.7 and 2.8 V with a scanning rate of 0.1 mV s−1. The electrochemical impedance spectroscopy (EIS) plots were obtained in the frequency range from 0.1 to 100 kHz with an amplitude of 5 mV. Both CV and EIS were carried out on an electrochemical workstation (Shanghai, Chenhua, CHI660D).

What's the cathode?

sulfur

Numerous studies reported core–shell 3D-nanostructured Li-ion electrodes with active components such as Si, Sn, TiO2, Nb2O5, Co3O4 or iron oxides, mostly applicable as negative electrodes (anodes). On the other hand, nanostructured 3D positive electrodes (cathodes) based on active materials with a desirable high redox potential vs. Li+/Li (e.g. lithiated manganese oxides, V2O5 or LiCoO2 have been reported in comparatively fewer publications. For such cathodes, a big difficulty lies in inherent oxidation of the 3D current collector during the synthesis of high voltage cathode materials – the latter typically requiring temperatures above 700 °C. By lowering the synthesis temperature to 300 °C, Zhang et al. and Pikul et al. successfully demonstrated both core–shell cathodes and full 3D Li-ion microbatteries which employed lithiated manganese oxides coated on inverse nickel opal current collectors. Impressively, the devices were operational at C-rates above 1000 (where the rate of 1C corresponds to the current required for 1 hour of a complete charge or discharge). The same group also demonstrated other high-performance 3D cathodes, such as a carbon foam coated with dense LiCoO2 or inverted opals doubly-filled with V2O5 and Li2MnSiO4. Currently, however, the electrode design based on regular inverse opal current collectors is difficult to envision for large scale battery manufacturing, as the fabrication of these current collectors is time consuming (e.g. >24 h for a 20 μm-thick structure), involves difficult to handle colloidal templates and is limited to small surfaces (∼1 cm2). Importantly, the application of a current collector with yet a higher volumetric surface area may allow further reducing the thickness of the active material and gaining access to its full theoretical capacity.

What's the cathode?

a carbon foam coated with dense LiCoO2

Numerous studies reported core–shell 3D-nanostructured Li-ion electrodes with active components such as Si, Sn, TiO2, Nb2O5, Co3O4 or iron oxides, mostly applicable as negative electrodes (anodes). On the other hand, nanostructured 3D positive electrodes (cathodes) based on active materials with a desirable high redox potential vs. Li+/Li (e.g. lithiated manganese oxides, V2O5 or LiCoO2 have been reported in comparatively fewer publications. For such cathodes, a big difficulty lies in inherent oxidation of the 3D current collector during the synthesis of high voltage cathode materials – the latter typically requiring temperatures above 700 °C. By lowering the synthesis temperature to 300 °C, Zhang et al. and Pikul et al. successfully demonstrated both core–shell cathodes and full 3D Li-ion microbatteries which employed lithiated manganese oxides coated on inverse nickel opal current collectors. Impressively, the devices were operational at C-rates above 1000 (where the rate of 1C corresponds to the current required for 1 hour of a complete charge or discharge). The same group also demonstrated other high-performance 3D cathodes, such as a carbon foam coated with dense LiCoO2 or inverted opals doubly-filled with V2O5 and Li2MnSiO4. Currently, however, the electrode design based on regular inverse opal current collectors is difficult to envision for large scale battery manufacturing, as the fabrication of these current collectors is time consuming (e.g. >24 h for a 20 μm-thick structure), involves difficult to handle colloidal templates and is limited to small surfaces (∼1 cm2). Importantly, the application of a current collector with yet a higher volumetric surface area may allow further reducing the thickness of the active material and gaining access to its full theoretical capacity.

What's the cathode?

inverted opals doubly-filled with V2O5 and Li2MnSiO4

In summary, we report a highly effective approach to prevent Li dendrite formation by an in situ formed self-repairing alloy layer (LixGa). During lithiation, evenly distributed Ga nanoparticles serve as lithiophilic sites to induce a homogeneous Li nucleation. Stress generated during Li plating can also be eliminated by the soft Ga nanoparticles. During the delithiation, metallic liquid Ga partially recovered from the dealloying process by timely filling the small gaps, which effectively avoided the generation of microcracks. A long-term cycling life of over 1800 h and 1400 h was obtained for the alloy-protected Li anodes at the high current densities of 2 mA cm−2 and 5 mA cm−2, respectively. Even increasing the current density and deposition capacity to 15 mA cm−2 and 15 mA h cm−2, respectively, the alloy-protected Li anodes still showed a stable cycling performance. Functionalized full cells coupled with LiFePO4 cathodes are finally cycled up to 600 cycles at 3C rate with the discharge capacity maintained at 128 mA h g−1. This simple and facile modification method is expected to realize the high-rate and long-life cycling of the Li metal anode and paves the way for the industrialization of Li metal batteries.

What's the cathode?

LiFePO4

The rate performance of the nanomesh cathode was tested at C-rates between 1.2 and 24C (corresponding to the volumetric currents of 269 mA cm−3 and 5387 mA cm−3, respectively). For each C-rate, five discharge–charge cycles were recorded, starting from the fully charged state. Therefore, we distinguish the “initial” discharge capacities (recorded after fully charging the cathode at 4.0 V for 1 hour) from the “continuous” discharge capacities recorded in the four consecutive cycles, which also depend on the charging kinetics at each C-rate. The discharge–charge profiles recorded in the first discharge–charge cycle at each C-rate are presented in Fig. 5c. The profiles have a similar shape at all the C-rates, showing that the higher currents did not affect the mechanism of Li storage in the cathode. The initial discharge capacity at 6C was 70% of the initial discharge capacity at 1.2C. When the cathode was discharged from the fully charged state at high rates of 18C and 24C (the current required for a theoretical full discharge in ∼3 min and 2.5 min, respectively), the initial capacity was still 50% and 45% of the initial capacity at 1.2C, demonstrating good rate performance of the cathode. The rate performance is, however, much inferior to that of LixMnO2-coated inverse Ni opals synthesisedby Zhang et al. (∼55% of 1.1C capacity at 743C). This is to be expected, as the mean pore size in the LixMnO2/nanomesh is about 40 nm which is significantly smaller than that in the LixMnO2/Ni opals (pore size of a few hundred nanometers). The small pore size lowers the diffusion rate of Li+ in the electrolyte within the electrode, which gradually becomes the rate-limiting factor during fast cycling of nanoporous electrodes. The differences in relative capacities of the nanomesh cathode are also amplified by the increased loss of active capacity after high rate cycling and prolonged charging at 4 V, as the initial discharge capacity at 1.2C in the 31st cycle was 73% of the initial discharge capacity at 1.2C in the first cycle (Fig. 5d).

What's the cathode?

nanomesh

In summary, we report novel free-standing DBHF nanofibers as a highly efficient and high-performance cathode for hybrid Zn batteries. Their unique structure with a fiber-in-tube hierarchical configuration, defect-rich crystals, carbon based network and hollow spherical basic units facilitates fast electron/ion transport and ORR/OER reaction kinetics. Benefitting from the advantages of the DBHF structure and the self-supporting character, the fabricated flexible hybrid Zn battery with the DBHF cathode achieves high operating voltage, high rate capability, high energy/power density and good adaptability under different working conditions. Furthermore, its outstanding stability and high efficiency are demonstrated by five-thousand high-rate cycling. More impressively, the excellent air charging capability makes it an outstanding power source for uninterrupted power supply while altering the working environment. Therefore, this work not only provides a novel strategy to synergistically improve the efficiency and performance of hybrid Zn batteries with both Zn-ion and Zn–air modes, but also opens a new avenue to fabricate high-performance flexible batteries for different electronics under various working conditions.

What's the cathode?

DBHF nanofibers

In summary, we report novel free-standing DBHF nanofibers as a highly efficient and high-performance cathode for hybrid Zn batteries. Their unique structure with a fiber-in-tube hierarchical configuration, defect-rich crystals, carbon based network and hollow spherical basic units facilitates fast electron/ion transport and ORR/OER reaction kinetics. Benefitting from the advantages of the DBHF structure and the self-supporting character, the fabricated flexible hybrid Zn battery with the DBHF cathode achieves high operating voltage, high rate capability, high energy/power density and good adaptability under different working conditions. Furthermore, its outstanding stability and high efficiency are demonstrated by five-thousand high-rate cycling. More impressively, the excellent air charging capability makes it an outstanding power source for uninterrupted power supply while altering the working environment. Therefore, this work not only provides a novel strategy to synergistically improve the efficiency and performance of hybrid Zn batteries with both Zn-ion and Zn–air modes, but also opens a new avenue to fabricate high-performance flexible batteries for different electronics under various working conditions.

What's the cathode?

DBHF

With the new development of electronic products and electric vehicles, state-of-the-art lithium-ion batteries are facing a grand challenge in meeting the rapidly expanding energy demand due to their inherently limited theoretical energy density. Lithium–sulfur (Li–S) batteries, on the other hand, shown to have five times higher theoretical specific capacity and energy density, have demonstrated tremendous potential for next-generation energy storage equipment applications. In addition, the environmental benignity and economic benefits of elemental sulfur further buttress its large-scale commercial manufacture. Despite these merits, the application of Li–S batteries is still hindered by some intractable challenges: firstly, the poor resistivity of elemental sulfur will reduce the utilization of raw materials and the reaction kinetics; secondly, the excessive volume expansion (∼80%) during charging/discharging may result in inevitable evolution of structures and strain; and thirdly, lithium polysulfides (LiPSs) dissolved from the cathode can penetrate the separator and deposit on the anode surface, leading to the “shuttle effect” and anode corrosion. These issues have been the bottlenecks in the development of high-performance Li–S batteries.

What's the cathode?

lithium polysulfides (LiPSs)

In this work, we developed lithium-ion cathodes based on 3D-interconnected Ni nanowire current collectors and a lithiated manganese oxide active material, which combine high volumetric capacity and good rate performance. The 3D-nanowire current collector exhibits high porosity and a very high surface-to-volume ratio. Consequently, upon conformally coating it with the active material, the energy-storing component is distributed over a few nanometer-thick layer. Since the Ni nanowire network provides access of electrons to the entire volume of the active material and the ionic transport in the thin active layer is unimpeded, the cathode reached very high utilization of the active material, which is typically inaccessible in bulk cathodes. As a result, the electrode exhibits a high rechargeable volumetric capacity of about 200 mA h cm−3, which is more than the capacity of most 3D-nanostructured cathodes reported in the literature. Additionally, thanks to the combination of the small thickness of the active material and its high contact area with the current collector and electrolyte, the cathode can deliver significantly high capacity during high rate charging and discharging, demonstrating its potential for use in high volumetric capacity and fast charging Li-ion batteries.

What's the cathode?

LLZO pellets were sputter-coated (LEICA EM ACE600) with Au on both sides to form contacts that acted as Li ion quasi-blocking electrodes. In order to fabricate Li symmetric cells, LLZO pellets were sandwiched between two Li metal discs (Sigma Aldrich) either with or without interposition of IL interlayers. 5 μL of the gel electrolytes were introduced at each interface. Cells without the interlayer were heated till 180 °C (melting temperature of Li) to promote Li–SE mutual contact. Both Au|LLZO|Au and Li|LLZO|Li were assembled in Swagelok©-type cell holders. Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (ZAHNER-Elektrik GmbH) with an applied sinusoidal excitation voltage of 10 mV in the frequency range from 5 MHz to 0.01 Hz. EIS data were fitted with ZView software. A Bio-Logic VMP-3 potentiostat was used to conduct the Li stripping and plating tests at current densities ranging from 0.05 to 0.4 mA cm−2. The cathode slurry was prepared by mixing S/NC powders (90 wt%) and the PVDF binder (10 wt%, Kynar) by magnetic stirring using NMP as solvent. The obtained slurry was cast onto aluminum foil by doctor blade techniques and thereafter dried at 60 °C for 24 h. The sulfur loading at the electrode was ∼0.5 mgsulfur cm−2. The electrochemical performance of the full cells was tested by galvanostatic cycling with 0.1C (1C = 1672 mA gsulfur−1) in the voltage range between 3 and 1.5 V, using an Arbin BT-2000 battery tester at room temperature. The preparation of the cells and all electrochemical tests were performed in an argon-filled glove-box.

What's the cathode?

For more than a decade, researchers have known that layered oxide cathodes, particularly NMC and NCA materials, have fragile surfaces. Thus, it allows various chemical/electrochemical reactions with gases (e.g., storing environments) or liquids (e.g., organic electrolytes) at the surfaces. As the urgent demand for commercialization of the Co-free, Ni-rich layered cathodes, the shelf life of active powders is considered as an important characteristic due to the uncertain delay from powder production to electrode processing and to battery manufacturing. In addition, with the nickel concentration approaching 100%, and the pursuit towards Co-free materials, surface stability against the organic electrolyte is vital in the commercialization. LiNiO2-based materials are more unstable than their cobalt-containing analogs because of the increased magnetic frustration of Ni3+ than Co3+. Therefore, surface degradation is more severe in Ni-rich oxide cathodes due to the high oxidation state of nickel. We previously discovered that the electrochemical performance of LiNiO2 cathode can be improved by incorporating dual dopants through the surface and bulk properties enhancement. Herein, we exploit the platform of LiNiO2-based materials to study the surface fragility under various practical conditions. Moreover, we will further discuss advantages of doping chemistry through the post-mortem characterizations. Lastly, we are aiming to provide insights into developing low-cost, high-energy Co-free Ni-rich cathode materials.

What's the cathode?

LiNiO2

For more than a decade, researchers have known that layered oxide cathodes, particularly NMC and NCA materials, have fragile surfaces. Thus, it allows various chemical/electrochemical reactions with gases (e.g., storing environments) or liquids (e.g., organic electrolytes) at the surfaces. As the urgent demand for commercialization of the Co-free, Ni-rich layered cathodes, the shelf life of active powders is considered as an important characteristic due to the uncertain delay from powder production to electrode processing and to battery manufacturing. In addition, with the nickel concentration approaching 100%, and the pursuit towards Co-free materials, surface stability against the organic electrolyte is vital in the commercialization. LiNiO2-based materials are more unstable than their cobalt-containing analogs because of the increased magnetic frustration of Ni3+ than Co3+. Therefore, surface degradation is more severe in Ni-rich oxide cathodes due to the high oxidation state of nickel. We previously discovered that the electrochemical performance of LiNiO2 cathode can be improved by incorporating dual dopants through the surface and bulk properties enhancement. Herein, we exploit the platform of LiNiO2-based materials to study the surface fragility under various practical conditions. Moreover, we will further discuss advantages of doping chemistry through the post-mortem characterizations. Lastly, we are aiming to provide insights into developing low-cost, high-energy Co-free Ni-rich cathode materials.

What's the cathode?

Ni-rich oxide

Although decomposition is unavoidable, lithium residuals do not negatively impact capacity decay, resulting in similar capacity retentions of 73.7%, 70.1%, and 68.7% for the samples aged in the Ar glovebox, in the dry box, and in the fresh state, respectively (Fig. 2b). According to many studies, surface contaminants increase the ion diffusion energy barrier and promote increased amounts of gas generation (i.e. CO2), which ultimately contributes to battery volume expansion. Degassing process after formation cycles is commonly adopted before the battery sealing, which adds to the battery cost. Thus, washing cathode powders using water, alcohol, or other solvents, or a simple high temperature annealing under oxygen flow can be utilized to remove the lithium residues. We also noticed that upon extensive storage (three months in the dry box), the large amount of carbonates can be visible in the SEM (scanning electron microscopy) images, which would more negatively impact the battery performance (Fig. S3†). Given that these materials are sensitive to storage environment and duration, it is recommended that scientific publications should specify these details when the performance is reported.

What's the cathode?

The electrode pastes were composed of active materials/carbon black/gel electrolyte precursor whose composition ratios were 60/8.5/31.5 (w/w/w) for the LiCoO2 (LCO) cathode and 52/7.5/40.5 (w/w/w) for the Li4Ti5O12 (LTO) anode, respectively. The gel polymer electrolyte precursor was prepared by mixing a UV-curable ethoxylated trimethylolpropane tri-acrylate (ETPTA) monomer (Aldrich) (including 1.0 wt% 2-hydroxy-2-methl-1-phenyl-1-1propanone (HMPP) as a photo-initiator) and high boiling point electrolyte (1 M LiPF6 in ethylene carbonate(EC)/propylene carbonate(PC) 1/1 (v/v)) in a composition ratio of 15/85 (w/w). To fabricate the bQSSB with cSiPV, the LTO anode paste was directly stencil-printed onto an Al current collector of the cSiPV module without any processing solvents and then exposed to UV irradiation. UV irradiation was performed using an Hg UV-lamp (Lichtzen) with an irradiation peak intensity of approximately 1260 mW cm−2. Subsequently, on the LTO anode (thickness = 50 μm and mass loading = 5.9 mg cm−2), a composite polymer electrolyte (CPE, gel electrolyte precursor/Al2O3 nanoparticles (average particle size ∼300 nm) = 56/44 (w/w)) was introduced through the UV curing-assisted printing process. Then, the LCO cathode paste was stencil-printed directly on the CPE/LTO anode and solidified by UV irradiation, yielding the printed LCO cathode (thickness = 30 μm and mass loading = 6.2 mg cm−2). After placing an Al current collector on top of the printed LCO cathode/printed CPE/printed LTO anode, the QSSB unit cell was obtained. Another QSSB unit cell was introduced, using the same printing process, on the top of the pre-fabricated QSSB unit cell, leading to the bQSSB with a bipolar structure. The number of QSSB unit cells on the cSiPV module was determined from the Vmax of the module concerning the voltage matching between the cSiPV and the bQSSB. Finally, the encapsulation of the bQSSB was conducted with Al pouch packaging substances and a UV-curable hydrophobic polymer (PRO-001, Novacentrix). All assembly steps of the cells were performed in a dry room.

What's the cathode?

LiCoO2 (LCO)

The electrode pastes were composed of active materials/carbon black/gel electrolyte precursor whose composition ratios were 60/8.5/31.5 (w/w/w) for the LiCoO2 (LCO) cathode and 52/7.5/40.5 (w/w/w) for the Li4Ti5O12 (LTO) anode, respectively. The gel polymer electrolyte precursor was prepared by mixing a UV-curable ethoxylated trimethylolpropane tri-acrylate (ETPTA) monomer (Aldrich) (including 1.0 wt% 2-hydroxy-2-methl-1-phenyl-1-1propanone (HMPP) as a photo-initiator) and high boiling point electrolyte (1 M LiPF6 in ethylene carbonate(EC)/propylene carbonate(PC) 1/1 (v/v)) in a composition ratio of 15/85 (w/w). To fabricate the bQSSB with cSiPV, the LTO anode paste was directly stencil-printed onto an Al current collector of the cSiPV module without any processing solvents and then exposed to UV irradiation. UV irradiation was performed using an Hg UV-lamp (Lichtzen) with an irradiation peak intensity of approximately 1260 mW cm−2. Subsequently, on the LTO anode (thickness = 50 μm and mass loading = 5.9 mg cm−2), a composite polymer electrolyte (CPE, gel electrolyte precursor/Al2O3 nanoparticles (average particle size ∼300 nm) = 56/44 (w/w)) was introduced through the UV curing-assisted printing process. Then, the LCO cathode paste was stencil-printed directly on the CPE/LTO anode and solidified by UV irradiation, yielding the printed LCO cathode (thickness = 30 μm and mass loading = 6.2 mg cm−2). After placing an Al current collector on top of the printed LCO cathode/printed CPE/printed LTO anode, the QSSB unit cell was obtained. Another QSSB unit cell was introduced, using the same printing process, on the top of the pre-fabricated QSSB unit cell, leading to the bQSSB with a bipolar structure. The number of QSSB unit cells on the cSiPV module was determined from the Vmax of the module concerning the voltage matching between the cSiPV and the bQSSB. Finally, the encapsulation of the bQSSB was conducted with Al pouch packaging substances and a UV-curable hydrophobic polymer (PRO-001, Novacentrix). All assembly steps of the cells were performed in a dry room.

What's the cathode?

LCO

In comparison with well-protected rigid batteries with liquid electrolytes, solid-state batteries (ssBs) are more beneficial, offering high flexibility, high wearability and leakage prevention. Currently, ssBs with the capability of bending and twisting have been extensively studied. However, it remains a challenge to develop a highly stretchable ssB with the maintenance of high performance. Herein, we report a stable solid-state zinc ion battery (ssZIB) based on a cellulose nanofiber (CNF)–polyacrylamide (PAM) hydrogel electrolyte and a Mg0.23V2O5·1.0H2O cathode. The designed CNF–PAM hydrogel shows high stretchability and robust mechanical stability. Moreover, the porous CNF–PAM hydrogel electrolyte provides efficient pathways for the transportation of zinc ions. And the robust layered structure of V2O5·1.0H2O pillared with Mg2+ ions and water supports the fast insertion/extraction of zinc ions in the lattice. Therefore, the designed ssZIB shows unprecedented high capacity at high current with durable cycling life. At a current density of 5 A g−1 (charging time of around 3 minutes), the ssZIBs can deliver a high reversible capacity of 216 mA h g−1 after 2000 cycles and retain 98.6% of the initial capacity, showing a high capacity and long-life durability at high currents. Furthermore, the designed spring ssZIBs can work under stretching with the strain reaching 650%. And the designed ssZIBs are still operational even under repeated bending, freezing, and heating conditions. The ssZIBs show robust mechanical stability, high stretchability and impressive electrochemical performance, providing a potential pathway to expand the application of ZIBs to a broad range of practical energy storage devices.

What's the cathode?

Mg0.23V2O5·1.0H2O

To investigate the electrochemical Na ion storage mechanism to predict the theoretical properties of Nax[Ni0.5Mn0.5]O2, we performed first-principles calculations based on the structural information verified through the Rietveld refinement analysis. At this stage, even though 0.01 mol of Ca2+ ions was incorporated into the bulk Na sites, Na0.98Ca0.01[Ni0.5Mn0.5]O2 showed a nearly identical crystal structure with Na[Ni0.5Mn0.5]O2; hence, we assumed that Na0.98Ca0.01[Ni0.5Mn0.5]O2 experienced the same O3–P3 phase transformation during the Na+ extraction/insertion process. Various Na/vacancy configurations on O3-Nax[Ni0.5Mn0.5]O2 compositions (0 ≤ x ≤ 1) were generated through the cluster assisted statistical mechanics (CASM) code and then, we predicted the theoretical formation energies of the O3-Nax[Ni0.5Mn0.5]O2 composition. Based on information of the formation energies, we calculated the redox potentials of O3-Nax[Ni0.5Mn0.5]O2 during Na+ extraction/insertion using the following equation where E(x) indicates the formation energy on the most stable Na/vacancy configuration of x species. By considering the typical O3–P3 phase transition in the O3-type layered oxide cathode in SIBs, we calculated the formation energies of P3-Nax[Ni0.5Mn0.5]O2 and then applied this information to compare the thermodynamic stability between O3- and P3-phases. As presented in Fig. 4a, we predicted that the O3–P3 phase transition occurred after extraction of 0.25 mol of Na ions from O3-Na1[Ni0.5Mn0.5]O2, and then, when the Na content in P3-Nax[Ni0.5Mn0.5]O2 was reduced to less than 0.25 mol, the O3–P3 phase transition re-occurred. Moreover, we verified that in the case of O3-/P3-Nax[Ni0.5Mn0.5]O2, the redox potentials required for Na+ extraction/insertion were less than ∼4.11 V (vs. Na+/Na) and more than ∼2.48 V, which indicates that 1 mol of Na ions can be reversibly extracted/inserted from/into the O3-Na1[Ni0.5Mn0.5]O2 structure despite the O3–P3 phase transition. Fig. 4b illustrates the predicted redox potential range of the O3/P3-Nax[Ni0.5Mn0.5]O2 cathode as a function of the Na content (0 ≤ x ≤ 1) with experimentally measured galvanostatic intermittent titration technique (GITT) profiles in the voltage range of 2.0–4.3 V. To confirm our hypothesis that the Ca-substituted cathode would undergo a similar O3–P3 phase transformation during the sodiation/de-sodiation process, we checked the GITT profiles of the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode and overlaid the results with the predicted theoretical redox potential of the Nax[Ni0.5Mn0.5]O2 cathode. Interestingly, we confirmed that the slope of the charge/discharge curve of O3-Na0.98Ca0.01[Ni0.5Mn0.5]O2 matched the predicted redox potential of the Nax[Ni0.5Mn0.5]O2 cathode well within the wide voltage window of 2.0–4.3 V; this clearly verified our hypothesis and confirmed that ∼1.0 mol of Na+ ions could be extracted/inserted from/into Na0.98Ca0.01[Ni0.5Mn0.5]O2.

What's the cathode?

O3-type layered oxide

To investigate the electrochemical Na ion storage mechanism to predict the theoretical properties of Nax[Ni0.5Mn0.5]O2, we performed first-principles calculations based on the structural information verified through the Rietveld refinement analysis. At this stage, even though 0.01 mol of Ca2+ ions was incorporated into the bulk Na sites, Na0.98Ca0.01[Ni0.5Mn0.5]O2 showed a nearly identical crystal structure with Na[Ni0.5Mn0.5]O2; hence, we assumed that Na0.98Ca0.01[Ni0.5Mn0.5]O2 experienced the same O3–P3 phase transformation during the Na+ extraction/insertion process. Various Na/vacancy configurations on O3-Nax[Ni0.5Mn0.5]O2 compositions (0 ≤ x ≤ 1) were generated through the cluster assisted statistical mechanics (CASM) code and then, we predicted the theoretical formation energies of the O3-Nax[Ni0.5Mn0.5]O2 composition. Based on information of the formation energies, we calculated the redox potentials of O3-Nax[Ni0.5Mn0.5]O2 during Na+ extraction/insertion using the following equation where E(x) indicates the formation energy on the most stable Na/vacancy configuration of x species. By considering the typical O3–P3 phase transition in the O3-type layered oxide cathode in SIBs, we calculated the formation energies of P3-Nax[Ni0.5Mn0.5]O2 and then applied this information to compare the thermodynamic stability between O3- and P3-phases. As presented in Fig. 4a, we predicted that the O3–P3 phase transition occurred after extraction of 0.25 mol of Na ions from O3-Na1[Ni0.5Mn0.5]O2, and then, when the Na content in P3-Nax[Ni0.5Mn0.5]O2 was reduced to less than 0.25 mol, the O3–P3 phase transition re-occurred. Moreover, we verified that in the case of O3-/P3-Nax[Ni0.5Mn0.5]O2, the redox potentials required for Na+ extraction/insertion were less than ∼4.11 V (vs. Na+/Na) and more than ∼2.48 V, which indicates that 1 mol of Na ions can be reversibly extracted/inserted from/into the O3-Na1[Ni0.5Mn0.5]O2 structure despite the O3–P3 phase transition. Fig. 4b illustrates the predicted redox potential range of the O3/P3-Nax[Ni0.5Mn0.5]O2 cathode as a function of the Na content (0 ≤ x ≤ 1) with experimentally measured galvanostatic intermittent titration technique (GITT) profiles in the voltage range of 2.0–4.3 V. To confirm our hypothesis that the Ca-substituted cathode would undergo a similar O3–P3 phase transformation during the sodiation/de-sodiation process, we checked the GITT profiles of the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode and overlaid the results with the predicted theoretical redox potential of the Nax[Ni0.5Mn0.5]O2 cathode. Interestingly, we confirmed that the slope of the charge/discharge curve of O3-Na0.98Ca0.01[Ni0.5Mn0.5]O2 matched the predicted redox potential of the Nax[Ni0.5Mn0.5]O2 cathode well within the wide voltage window of 2.0–4.3 V; this clearly verified our hypothesis and confirmed that ∼1.0 mol of Na+ ions could be extracted/inserted from/into Na0.98Ca0.01[Ni0.5Mn0.5]O2.

What's the cathode?

O3/P3-Nax[Ni0.5Mn0.5]O2

Another critical property of an electrode material toward practical application is the ease of handling against a moist environment. Most of the O3-type transition metal oxide cathodes reported so far are moisture sensitive because of the low redox potential associated with 3d-metals. Once the cathode is exposed to air or moisture, water oxidizes the transition metal ions with the concomitant removal of Na+ ions, which further react with CO2 and H2O in air to form Na2CO3 or NaOH. Such undesirable reactions usually cause structural degradation and poor electrochemical performances and thus make them difficult to handle. As revealed in Fig. 7, after exposure to air with a relative humidity of ≈55%, the Na[Ni0.5Mn0.5]O2 exhibited an apparent structural change to a Na-deficient monoclinic O′3 phase together with formation of Na2CO3·nH2O on the surface. In contrast, Na0.98Ca0.01[Ni0.5Mn0.5]O2 remarkably retarded spontaneous phase transition and retained the original O3 structure. To confirm in detail the structural stability against a humid atmosphere, a comparative study of the XRD evolution of the Na0.98Ca0.01[Ni0.5Mn0.5]O2 and Na[Ni0.5Mn0.5]O2 cathodes was conducted as a function of exposure time (1, 3, and 5 days and 1 week) (Fig. S16†). After 1 day, there were no significant structural changes in either cathode. However, after three days, the Na[Ni0.5Mn0.5]O2 cathode was readily oxidized in the presence of moisture, and subsequently, the intensity of (003)hex. and (104)hex. peaks decreased in the XRD patterns. After 5 days, splitting of the (003)hex. and (006)hex. diffraction lines was observed, indicating a slightly enlarged interslab distance for the new monoclinic O3 phase. Finally, after 1 week, the phase transformation from the hexagonal O3 to the monoclinic O′3 phase occurred in the Na[Ni0.5Mn0.5]O2 cathode. This monoclinic O′3 phase transformation can be observed in the sodium deficient structure when the sodium ions are extracted down to 20 mol% in the original O3 structure. In contrast, throughout the week, the intensified peaks, (003)hex., (006)hex., (101)hex., (102)hex., and (104)hex., in the O3 phase were well retained persistently for the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode, indicating less reactivity against moisture. These results suggest that the Ca2+ ions on the Na sites with strong Ca–O bonding prevent the removal of Na+ ions from the structure when exposed to air. As expected, in Fig. S17,† the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode demonstrated higher reversible capacity with lower voltage polarization and better cycling stability after prolonged exposure, while the Na[Ni0.5Mn0.5]O2 cathode showed poor electrochemical performance and even exhibited an irreversible reaction during charging, due to the decomposition of Na2CO3 which is a by-product of removed Na+ ions and CO2 in air. Such a comparison of the thermal properties and air-stability strengthens the merits of Ca-substituted cathodes in practical application.

What's the cathode?

Na0.98Ca0.01[Ni0.5Mn0.5]O2 and Na[Ni0.5Mn0.5]O2

Another critical property of an electrode material toward practical application is the ease of handling against a moist environment. Most of the O3-type transition metal oxide cathodes reported so far are moisture sensitive because of the low redox potential associated with 3d-metals. Once the cathode is exposed to air or moisture, water oxidizes the transition metal ions with the concomitant removal of Na+ ions, which further react with CO2 and H2O in air to form Na2CO3 or NaOH. Such undesirable reactions usually cause structural degradation and poor electrochemical performances and thus make them difficult to handle. As revealed in Fig. 7, after exposure to air with a relative humidity of ≈55%, the Na[Ni0.5Mn0.5]O2 exhibited an apparent structural change to a Na-deficient monoclinic O′3 phase together with formation of Na2CO3·nH2O on the surface. In contrast, Na0.98Ca0.01[Ni0.5Mn0.5]O2 remarkably retarded spontaneous phase transition and retained the original O3 structure. To confirm in detail the structural stability against a humid atmosphere, a comparative study of the XRD evolution of the Na0.98Ca0.01[Ni0.5Mn0.5]O2 and Na[Ni0.5Mn0.5]O2 cathodes was conducted as a function of exposure time (1, 3, and 5 days and 1 week) (Fig. S16†). After 1 day, there were no significant structural changes in either cathode. However, after three days, the Na[Ni0.5Mn0.5]O2 cathode was readily oxidized in the presence of moisture, and subsequently, the intensity of (003)hex. and (104)hex. peaks decreased in the XRD patterns. After 5 days, splitting of the (003)hex. and (006)hex. diffraction lines was observed, indicating a slightly enlarged interslab distance for the new monoclinic O3 phase. Finally, after 1 week, the phase transformation from the hexagonal O3 to the monoclinic O′3 phase occurred in the Na[Ni0.5Mn0.5]O2 cathode. This monoclinic O′3 phase transformation can be observed in the sodium deficient structure when the sodium ions are extracted down to 20 mol% in the original O3 structure. In contrast, throughout the week, the intensified peaks, (003)hex., (006)hex., (101)hex., (102)hex., and (104)hex., in the O3 phase were well retained persistently for the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode, indicating less reactivity against moisture. These results suggest that the Ca2+ ions on the Na sites with strong Ca–O bonding prevent the removal of Na+ ions from the structure when exposed to air. As expected, in Fig. S17,† the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode demonstrated higher reversible capacity with lower voltage polarization and better cycling stability after prolonged exposure, while the Na[Ni0.5Mn0.5]O2 cathode showed poor electrochemical performance and even exhibited an irreversible reaction during charging, due to the decomposition of Na2CO3 which is a by-product of removed Na+ ions and CO2 in air. Such a comparison of the thermal properties and air-stability strengthens the merits of Ca-substituted cathodes in practical application.

What's the cathode?

Na[Ni0.5Mn0.5]O2

Unlike DC voltage exfoliation, square-wave voltage uses bulk antimony, bismuth and their compounds as the anode and platinum wire or foil as the cathode, and changes the voltage direction to perform electrochemical exfoliation in an electrolytic solution of Na2SO4 or Li2SO4. This process mainly uses oxygen-containing free radicals produced by electrolysed water to assault large pieces of antimony or bismuth or their compounds in the anode, and SO42− intercalates into the interlayer. Subsequently, the gas generated between the layers of the bulk layered material further expands, and finally 2D antimony or bismuth or its compound is obtained in the electrolyte. In 2020, Marzo et al. elucidated the possible mechanism of changing the voltage exfoliation of 2D Sb and Sb2Te3. As shown in Fig. 4b, this exfoliation process comprises four different stages. In the first stage, oxygen-containing free radicals produced by the electrolysis of water on the anode bulk 2D material react with the edges and boundaries, creating bumps, cracks and holes in the bulk 2D material. Next, the small cations (H+, Li+ and Na+) intercalate into the gaps and holes formed in the previous bulk 2D material, initializing a slight and preliminary expansion of the 2D sandwich. When the conversion voltage is −5 V, the SO42− intercalation layer enters the interlayer of the bulk 2D material. Finally, the gases (H2, O2 and SO2 formed in these redox processes) generated by the insertion of ions between the 2D bulk material layers promote the exfoliation of the 2D material and disperses it into the electrolyte (Fig. 4b).

What's the cathode?

platinum wire or foil

Unlike DC voltage exfoliation, square-wave voltage uses bulk antimony, bismuth and their compounds as the anode and platinum wire or foil as the cathode, and changes the voltage direction to perform electrochemical exfoliation in an electrolytic solution of Na2SO4 or Li2SO4. This process mainly uses oxygen-containing free radicals produced by electrolysed water to assault large pieces of antimony or bismuth or their compounds in the anode, and SO42− intercalates into the interlayer. Subsequently, the gas generated between the layers of the bulk layered material further expands, and finally 2D antimony or bismuth or its compound is obtained in the electrolyte. In 2020, Marzo et al. elucidated the possible mechanism of changing the voltage exfoliation of 2D Sb and Sb2Te3. As shown in Fig. 4b, this exfoliation process comprises four different stages. In the first stage, oxygen-containing free radicals produced by the electrolysis of water on the anode bulk 2D material react with the edges and boundaries, creating bumps, cracks and holes in the bulk 2D material. Next, the small cations (H+, Li+ and Na+) intercalate into the gaps and holes formed in the previous bulk 2D material, initializing a slight and preliminary expansion of the 2D sandwich. When the conversion voltage is −5 V, the SO42− intercalation layer enters the interlayer of the bulk 2D material. Finally, the gases (H2, O2 and SO2 formed in these redox processes) generated by the insertion of ions between the 2D bulk material layers promote the exfoliation of the 2D material and disperses it into the electrolyte (Fig. 4b).

What's the anode?

bulk antimony, bismuth and their compounds

The development of high-capacity rechargeable lithium–sulfur batteries is an ongoing challenge because of the shuttle effect, which causes rapid capacity fade, limited rate capability, and slow kinetics. Herein, conductive hollow cobalt nitride/carbon (Co5.47Nx–C) spheres with nitrogen vacancies are developed as a high-performance cathode material for use in lithium–sulfur batteries. Nitrogen vacancies are formed by annealing a zeolite imidazole framework (ZIF-67) precursor in ammonia. Benefiting from its Co–N bonds and nitrogen-vacancy sites, the Co5.47Nx–C composite achieves strong anchoring of polysulfides, fast polysulfide conversion, and accelerated lithium-ion transport. The strong anchoring effect of Co5.47Nx is confirmed by experimental measurements and density functional theory (DFT) calculations. Because of its high conductivity and nitrogen vacancies, the Co5.47Nx cathode exhibits faster redox reaction kinetics and lower polarization than a Co5.47N cathode without nitrogen vacancies, thus realizing promising rate and cycling performance. The optimized Co5.47Nx–C electrode delivers a capacity of 850 mA h g−1 at 0.5C and a rate performance of 320 mA h g−1 at 10C. This high-performance, high-rate lithium–sulfur battery is promising for widespread application in electric vehicles and intelligent devices.

What's the cathode?

Co5.47Nx

The development of high-capacity rechargeable lithium–sulfur batteries is an ongoing challenge because of the shuttle effect, which causes rapid capacity fade, limited rate capability, and slow kinetics. Herein, conductive hollow cobalt nitride/carbon (Co5.47Nx–C) spheres with nitrogen vacancies are developed as a high-performance cathode material for use in lithium–sulfur batteries. Nitrogen vacancies are formed by annealing a zeolite imidazole framework (ZIF-67) precursor in ammonia. Benefiting from its Co–N bonds and nitrogen-vacancy sites, the Co5.47Nx–C composite achieves strong anchoring of polysulfides, fast polysulfide conversion, and accelerated lithium-ion transport. The strong anchoring effect of Co5.47Nx is confirmed by experimental measurements and density functional theory (DFT) calculations. Because of its high conductivity and nitrogen vacancies, the Co5.47Nx cathode exhibits faster redox reaction kinetics and lower polarization than a Co5.47N cathode without nitrogen vacancies, thus realizing promising rate and cycling performance. The optimized Co5.47Nx–C electrode delivers a capacity of 850 mA h g−1 at 0.5C and a rate performance of 320 mA h g−1 at 10C. This high-performance, high-rate lithium–sulfur battery is promising for widespread application in electric vehicles and intelligent devices.

What's the cathode?

Co5.47N

The carefully designed combination of TM-ions and non-TM-ions suppressed the occurrences of phase transformations during electrochemical cycling in Na ‘half’ cells. More importantly, in addition to air-cum-water stability, Ti-substitution for Mn-ions was found to drastically improve the cyclic stability, with the completely Ti-substituted Na-TM-oxide exhibiting an excellent first cycle reversible capacity of ∼140 mA h g−1 (within 2.0–4.0 V, at C/5), negligible voltage hysteresis and a capacity retention of ∼80% after 100 cycles. In the presence of Mn-ions, formation of Mn3+ at the particle surfaces, concomitant dissolution of Mn-ions (upon disproportionation) in electrolyte, steep rise in cathode/cell impedance and associated rapid increase in voltage hysteresis upon cycling were found to be the major causes for rapid capacity fading, which were suppressed/eliminated upon partial/complete Ti-substitution. The incorporation of functionalized MWCNTs, via simple physical mixing with the cathode-active particles, further improved the capacity retentions to ∼87% after 100 cycles and ∼75% after 200 cycles, along with bestowing excellent rate capability, viz., capacities at 2C and 5C being ∼65% and ∼50% of capacity at C/10, respectively.

What's the cathode?

MWCNTs

Sodium-ion batteries (SIBs) have recently attracted increasing attention as an alternative to lithium-ion batteries, especially for large-scale energy storage in light of the low cost and high abundance of Na resources. Layer structured sodium transition metal oxides are an important group of cathode materials for SIBs due to their high theoretical capacities. However, the poor cycling stability at high voltage hinders their practical applications, which is due to the multiple phase transition induced structural instability. Our recent work reveals that phase transition induced cracking is a major cause of the performance decay for layered cathodes. Bulk elemental doping has been demonstrated to be an effective approach to improve the cycling performance, because doping can effectively suppress phase transition induced volume changes and even realize a zero-strain cathode or phase transition free cathode. Furthermore, dopants can act as pillars to prevent the layered structure from collapsing and they can tailor the plane spacing to achieve superior diffusion kinetics of alkaline ions.

What's the cathode?

Layer structured sodium transition metal oxides

Polymers. Using polymeric analogs of small molecules might be an effective strategy to circumvent the solubility issue of organic redox-active materials. The first polymeric cathode material for potassium batteries was poly(anthraquinonyl sulfide) (PAQS, P1). With an ether-based electrolyte (0.5 M KTFSI in DME:DOL) it showed a reversible Qm of 200 mA h g−1 at 20 mA g−1. A capacity fading of 25% was observed after 50 cycles, while the cyclability with a carbonate-based electrolyte was inferior.

What's the cathode?

polymeric

Polymers. Using polymeric analogs of small molecules might be an effective strategy to circumvent the solubility issue of organic redox-active materials. The first polymeric cathode material for potassium batteries was poly(anthraquinonyl sulfide) (PAQS, P1). With an ether-based electrolyte (0.5 M KTFSI in DME:DOL) it showed a reversible Qm of 200 mA h g−1 at 20 mA g−1. A capacity fading of 25% was observed after 50 cycles, while the cyclability with a carbonate-based electrolyte was inferior.

What's the electrolyte?

(0.5 M KTFSI in DME:DOL)

Electrospun CNFs/Nb2O5 composite nanofibers were prepared by electrospinning a solution containing the precursors of Nb(C2O4H)5, PAN and TEOS, followed by carbonization and SiO2-etching.Fig. 7f and g show the structural features of the prepared CNFs/Nb2O5 nanocomposites, which reveal a continuous mesoporous network architecture. Compared with solid CNFs/Nb2O5 composites without using TEOS in spinnable solution and bulk C/Nb2O5 prepared via directly annealing the Nb(C2O4H)5, the mesoporous CNFs/Nb2O5 composites revealed much superior electrochemical reactivity. In detail, the mesoporous Nb2O5/CNF showed outstanding cyclability (94% retention after 10000 cycles at 100C, 1C = 200 mA g−1), and superior rate capability with specific capacities of 287 mA h g−1 at 0.5C and 172 mA h g−1 at 150C, which was much higher than that of pure CNFs (159.1 mA h g−1 at 0.5C and 64.9 mA h g−1 at 100C). In addition, a hybrid Na-ion capacitor was fabricated by employing mesoporous CNFs/Nb2O5 as the anode and CNFs/graphene as the cathode. This device showed both large energy densities (124 W h kg−1, 11.2 mW h cm−3) and impressive power densities (60 kW kg−1, 5.4 W h cm−3), indicating that both the introducing of Nb2O5 and the unique continuous mesoporous network architecture played a crucial role in the enhancement of the electrochemical properties.

What's the cathode?

CNFs/graphene

Electrospun CNFs/Nb2O5 composite nanofibers were prepared by electrospinning a solution containing the precursors of Nb(C2O4H)5, PAN and TEOS, followed by carbonization and SiO2-etching.Fig. 7f and g show the structural features of the prepared CNFs/Nb2O5 nanocomposites, which reveal a continuous mesoporous network architecture. Compared with solid CNFs/Nb2O5 composites without using TEOS in spinnable solution and bulk C/Nb2O5 prepared via directly annealing the Nb(C2O4H)5, the mesoporous CNFs/Nb2O5 composites revealed much superior electrochemical reactivity. In detail, the mesoporous Nb2O5/CNF showed outstanding cyclability (94% retention after 10000 cycles at 100C, 1C = 200 mA g−1), and superior rate capability with specific capacities of 287 mA h g−1 at 0.5C and 172 mA h g−1 at 150C, which was much higher than that of pure CNFs (159.1 mA h g−1 at 0.5C and 64.9 mA h g−1 at 100C). In addition, a hybrid Na-ion capacitor was fabricated by employing mesoporous CNFs/Nb2O5 as the anode and CNFs/graphene as the cathode. This device showed both large energy densities (124 W h kg−1, 11.2 mW h cm−3) and impressive power densities (60 kW kg−1, 5.4 W h cm−3), indicating that both the introducing of Nb2O5 and the unique continuous mesoporous network architecture played a crucial role in the enhancement of the electrochemical properties.

What's the anode?

CNFs/Nb2O5

A good dopant must endow the cathode material with superior cyclability. According to Fig. 1(a), among Cu, Ti, Mg and Zn, Mg and Zn are better dopants. From structural view, a good dopant should be able to segregate into precipitates during cycling, offering better material integration. However, when cycled at a lower upper cutoff voltage of 4.3 V, Mg dopant segregation cannot be activated. Therefore, Mg-doped P2-NMM10 shows comparable cyclability to Cu-doped P2-NMC10, which is shown in Fig. S10.† Intriguingly, when cycled at 4.3 V, the Zn-doped sample shows even better cycling stability (91% retention after 100 cycles). Fig. 5(a) shows the capacity retentions of the P2-NMZ10 electrodes at different upper cutoff voltages, from which we can see that Zn-doped P2-NMZ10 shows superior capacity retentions in all voltage windows. The corresponding TEM analysis on the cycled P2-NMZ10 samples shows that the Zn-dopant can form precipitates under all cycling conditions as shown in Fig. 5(b–i), which explains why P2-NMZ10 shows much higher cycling stability. Although increasing the upper cutoff voltage can accelerate the precipitate formation process, material degradation is also aggravated during high voltage cycling. Therefore, a good dopant should enable dopant precipitate formation easily, especially at low cycling voltage.

What's the cathode?

Supercapacitors have high power density and a long lifespan but poor energy density in contrast with rechargeable batteries, restricting their widespread applications. Adding soluble redox-active ingredients into electrolyte is an effective strategy to increase specific energy. However, an ion-selective membrane is generally needed in such supercapacitors to avoid the mixing of anolyte and catholyte, which significantly increases the cost. Here we report a supercapacitor that consists of a modified solid Ti3C2Tx anode and an active catholyte containing Mn2+, where the conversion between soluble Mn2+ and solid MnO2 occurs at the cathode, and the redox of Ti–O with the bonding/de-bonding of H3O+ occurs at the anode. Impressively, this hybrid supercapacitor displays a gratifying specific energy of 43.4 W h kg−1, without using any ion-selective membrane, and excellent cycling stability over 20000 cycles. Moreover, we also demonstrate its superior low-temperature performance even though the electrolyte has been frozen at −70 °C.

What's the cathode?

Although STEM-HAADF imaging has become the most widely used imaging mode in STEM, the strong atomic number dependency of STEM-HAADF contrast also means that the imaging contrast of light atoms is easily submerged by the adjacent heavy atoms. In contrast, STEM-ABF phase-contrast imaging based on wave interference provides better visualization of light elements with the presence of heavy elements, such as oxygen in the SrTiO3 crystal, lithium in LiCoO2 and even hydrogen in metal hydrides. An elegant example was reported by Gu et al., in which aberration-corrected STEM-ABF imaging was applied to directly visualize lithium columns in LiFePO4 and identify an intriguing lithium staging structure in the partially delithiated LixFePO4 (x ≈ 0.5) crystal. Fig. 6(a–c) present the atomic-resolution STEM-ABF images of the LiFePO4 cathode at different charging states. Compared with the pristine and fully-charged structures of LiFePO4 in Fig. 6(a and b), half-delithiated LiFePO4 in Fig. 6(c) demonstrated that part of the lithium remains in the lattice (yellow circles) at every other row of Li-extracted sites (orange circles). This ordered structure contradicted the previous model of LiFePO4/FePO4 two-phase separation, but was analogous to the stage-II phase in some layer compounds, and contributed to minimizing the Li–Li repulsive interaction to stabilize this intermediate phase. This result indicated that a continuous phase transition occurred in partially delithiated LixFePO4 through a metastable phase and helped to understand the high-rate capability of LiFePO4 cathode.

What's the cathode?

LiFePO4

Although STEM-HAADF imaging has become the most widely used imaging mode in STEM, the strong atomic number dependency of STEM-HAADF contrast also means that the imaging contrast of light atoms is easily submerged by the adjacent heavy atoms. In contrast, STEM-ABF phase-contrast imaging based on wave interference provides better visualization of light elements with the presence of heavy elements, such as oxygen in the SrTiO3 crystal, lithium in LiCoO2 and even hydrogen in metal hydrides. An elegant example was reported by Gu et al., in which aberration-corrected STEM-ABF imaging was applied to directly visualize lithium columns in LiFePO4 and identify an intriguing lithium staging structure in the partially delithiated LixFePO4 (x ≈ 0.5) crystal. Fig. 6(a–c) present the atomic-resolution STEM-ABF images of the LiFePO4 cathode at different charging states. Compared with the pristine and fully-charged structures of LiFePO4 in Fig. 6(a and b), half-delithiated LiFePO4 in Fig. 6(c) demonstrated that part of the lithium remains in the lattice (yellow circles) at every other row of Li-extracted sites (orange circles). This ordered structure contradicted the previous model of LiFePO4/FePO4 two-phase separation, but was analogous to the stage-II phase in some layer compounds, and contributed to minimizing the Li–Li repulsive interaction to stabilize this intermediate phase. This result indicated that a continuous phase transition occurred in partially delithiated LixFePO4 through a metastable phase and helped to understand the high-rate capability of LiFePO4 cathode.

What's the cathode?

LiFePO4

By comparing the C 1s, Li 1s, and O 1s XPS spectra and the corresponding area proportions, we found corroborative results. Firstly, a great amount of residual carbonate on the pristine LiNiO2 surface illustrates the continued challenge in avoiding carbonate formation during calcination and quick transferal. There was a clear increase in inorganic carbonates (C 1s, Li 1s, and O 1s XPS spectra) concurrent with the decrease of lattice Li and O proportions after exposure to ambient environment, which was amplified under continuous human exhalation (lattice Li and O are almost negligible on the surface) (Fig. 1). This is indicative of the continuous formation of inorganic carbonates, which strongly attenuates the photoelectron signals from underneath LiNiO2 layers. Our observation is consistent with previous studies suggesting that dense concentration of water and CO2 on the oxide cathode surface facilitates chemical reactions to yield increased amounts of surface inorganic carbonates. Previous studies have shown the surface chemical bonding and structure of NMC materials are readily transformed by certain environments, including CO2, H2O, O2, and other reactive gases in a glove box. These gases react with layered oxides by extracting lithium from the lattice, further generating LiOH and/or Li2CO3 as well as oxygen vacancies. Lithium residuals on NMC and NCA materials also have been confirmed by infrared spectroscopy, Raman spectroscopy, and TEM. It is a consensus that surface instability is one of the common challenges for many battery materials. Other materials, such as sodium-containing layered cathode materials and Li-rich materials, also encounter surface-air instability, which leads to Na/Li residual formation and elevated pH on the particle surfaces. For sodium-containing layered cathode materials, water intercalation or Na+/H+ exchange also takes place, which can disrupt the layered structure. These changes would trigger slurry alkalization, degassing problem, and electrolyte decomposition. Based on our results and previous findings, we recommend that the details about sample handling should be specified in the journal publications, especially those involving delicate surface chemical analysis.

What's the cathode?

sodium-containing layered

By comparing the C 1s, Li 1s, and O 1s XPS spectra and the corresponding area proportions, we found corroborative results. Firstly, a great amount of residual carbonate on the pristine LiNiO2 surface illustrates the continued challenge in avoiding carbonate formation during calcination and quick transferal. There was a clear increase in inorganic carbonates (C 1s, Li 1s, and O 1s XPS spectra) concurrent with the decrease of lattice Li and O proportions after exposure to ambient environment, which was amplified under continuous human exhalation (lattice Li and O are almost negligible on the surface) (Fig. 1). This is indicative of the continuous formation of inorganic carbonates, which strongly attenuates the photoelectron signals from underneath LiNiO2 layers. Our observation is consistent with previous studies suggesting that dense concentration of water and CO2 on the oxide cathode surface facilitates chemical reactions to yield increased amounts of surface inorganic carbonates. Previous studies have shown the surface chemical bonding and structure of NMC materials are readily transformed by certain environments, including CO2, H2O, O2, and other reactive gases in a glove box. These gases react with layered oxides by extracting lithium from the lattice, further generating LiOH and/or Li2CO3 as well as oxygen vacancies. Lithium residuals on NMC and NCA materials also have been confirmed by infrared spectroscopy, Raman spectroscopy, and TEM. It is a consensus that surface instability is one of the common challenges for many battery materials. Other materials, such as sodium-containing layered cathode materials and Li-rich materials, also encounter surface-air instability, which leads to Na/Li residual formation and elevated pH on the particle surfaces. For sodium-containing layered cathode materials, water intercalation or Na+/H+ exchange also takes place, which can disrupt the layered structure. These changes would trigger slurry alkalization, degassing problem, and electrolyte decomposition. Based on our results and previous findings, we recommend that the details about sample handling should be specified in the journal publications, especially those involving delicate surface chemical analysis.

What's the cathode?

oxide

Similarly to TiS2, NbS2 also shows a high electrochemical activity upon Li+ intercalation/deintercalation at potentials between 1.5 and 3.0 V, presenting a specific capacity of about 100 mA h g−1. Xiao et al. systematically investigated the reaction kinetics and electrochemical performance of a ternary hybrid sulfur cathode consisting of sandwich-type NbS2@S enveloped by iodine-doped graphene (IG). As an exhilarating discovery, the active sulfur species can be intercalated in the interlayers of NbS2, which further enhances the intrinsic conductivity and polarity of NbS2. Therefore, the introduction of NbS2 and IG into a Li–S cell cathode shows a synergistic effect for LiPS fixation and utilization. Adsorption experiments indicate that NbS2 and NbS2–IG visibly decolorize the Li2S6 solution, while G and IG only change the solution color from tawny to yellow, suggesting that NbS2 possesses much stronger affinity to LiPSs than G and IG. CV tests using symmetrical cells that are fabricated by sandwiching a Li2S6-containing electrolyte between two identical electrodes reveal that the G and IG electrodes only show a small current density, while the current density in NbS2 is significantly increased, and further improvement is observed in the NbS2–IG electrode. These results suggest that NbS2 is conducive to providing access for electric charge to the NbS2–LiPS interface and synchronously triggers the redox reactions of LiPSs. Consequently, reversible capacities of 195, 107, and 74 mA h g−1 (1.05 mg cm−2) are achieved after 2000 cycles at ultrahigh rates of 20, 30, and 40C, respectively, and a high discharge capacity of 405 mA h g−1 (3.25 mg cm−2) is maintained after 600 cycles at 1C. This work sheds light on the promising application of 2D layered metal sulfides in high-energy, high-power, and long-life Li–S batteries.

What's the cathode?

sulfur

The crystal structure of the current collectors was characterized using a Rigaku D/max-2200/PC X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.154 nm). The surface morphology of the samples was observed via field-emission scanning electron microscopy (FE-SEM, JEOL, JSM-6700F). Specifically, to characterize the morphology of the lithium metal anodes (LMAs), the coin cells were initially disassembled, the LMAs were extracted and rinsed with DOL/DME cosolvent to remove the residual electrolyte, and finally dried in a glove box. The LMAs were sealed in an air-tight plastic sample box before transferring them from the glove box to the SEM chamber. The coin cells were assembled with the abovementioned protocol in an argon-filled glove box. Coulombic efficiencies (CEs) were tested under different current densities and capacities at room temperature using a LAND battery test instrument. A certain amount of Li metal was initially electrodeposited on an NPAuLi3@NF or NF current collector, and then the cell was charged to 0.1 V (vs. Li+/Li) to strip the Li metal. The cut-off voltage of 0.1 V was set to prevent dealloying of the AuLi alloy. For long-term cycling and full cell tests, 3 mA h cm−2 of Li metal was firstly deposited on the current collectors, and then the cells were subjected to charge/discharge cycling at different current densities and capacities. LiFePO4 with an areal mass loading of 1.72 mg cm−2 was used as the cathode material in the full cell tests. The electrochemical impedance spectroscopy (EIS) study was conducted using a ZIVE SP1 electrochemical workstation in the frequency range of 100 kHz to 10 MHz with an AC potential amplitude of 5 mV.

What's the cathode?

LiFePO4

To directly observe and verify the possible contact loss between the cathode and solid electrolyte, FIB-SEM tomography was used to characterize the microstructural evolution in the composite cathode during cycling. Three SSB cells (before cycling, after 10 cycles, and after 50 cycles) were extracted in the discharged state and evaluated using FIB-SEM tomography. The workflow for the FIB-SEM tomography experiment is presented in Fig. 2. The SSB full cell was tilted at a 52° angle with the cathode composite perpendicular to the ion beam. The ion beam was first used to mill trenches surrounding the area of interest, exposing the cross-section to the top-mounted electron beam at a 52° angle. A backscattered electron (BSE) image was acquired to provide 2D morphology information on the cathode composite. Then, a thin slice (50 nm) was milled away from the cross-section with the ion beam, and another BSE image was taken. After repeating this process several hundred times, the image stack was aligned, cropped, and combined into a 2D image stack (Fig. 2(c)) (see ESI Fig. S2 and S3† for the detailed image process procedures). The 2D image stack and corresponding four-phase segmentation image are presented in Fig. 2(c) and (d), respectively, whereby the NMC cathode (blue), LPS solid electrolyte (yellow), carbon (black), and void (red) can be accurately distinguished. The segmented image stacks were then reconstructed into a 3D volume. The reconstructed structures of the composite cathodes before and after electrochemical cycling are presented in Fig. 3.

What's the cathode?

NMC

The development of primary batteries with high energy density, long shelf life, and stable discharge voltage is essential for civilization and military applications. Primary batteries with high power output and excellent low-temperature performance can further broaden their application. Herein, a Li/LiV2(PO4)3 primary battery was proposed and investigated for the first time. In order to improve the shelf life of the Li/LiV2(PO4)3 primary battery, the mechanism and corresponding inhibition strategy of self-discharge were studied in detail. It was found that the electrolyte composition is a key factor affecting the shelf life of Li/LiV2(PO4)3 primary batteries, where the corrosion of aluminum (Al) current collector triggered by the organic radical cations generated from electrochemical oxidation of the ethylene carbonate (EC) at high potential; and the detrimental reaction between LiV2(PO4)3 and electrolyte lead to the self-discharge of the Li/LiV2(PO4)3 primary battery. When the EC solvent was replaced by the propylene carbonate (PC) solvent, the corrosion of Al foil was alleviated. Moreover, the addition of lithium bis(oxalato)borate (LiBOB) to the electrolyte could improve the stability of cathode/electrolyte interface and enhance the shelf life of the Li/LiV2(PO4)3 primary battery. As a result, 100% capacity could be maintained after over one-month storage, 86% energy could be maintained at 50C, and 63% energy could be maintained at −40 °C at a current density of 0.1C. In addition, the Li/LiV2(PO4)3 primary battery showed great potential for all-weather applications.

What's the cathode?