Hybridizing carbonate and ether at  molecular scales for high-energy and high- safety lithium metal batteries.pdf
Article https://doi.org/10.1038/s41467-024-47448-5 Hybridizing carbonate and ether at molecular scales for high-energy and high- safety lithium metal batteries Jiawei Chen1, Daoming Zhang2, Lei Zhu1, Mingzhu Liu3, Tianle Zheng 4, Jie Xu 1, Jun Li2, Fei Wang 1, Yonggang Wang 1, Xiaoli Dong 1 & Yongyao Xia 1 Commonly-used ether and carbonate electrolytes show distinct advantages in active lithium-metal anode and high-voltage cathode, respectively. While these complementary characteristics hold promise for energy-dense lithium metal batteries, such synergy cannot be realized solely through physical blending. Herein, a linear functionalized solvent, bis(2-methoxyethyl) carbonate (BMC), is conceived by intramolecularly hybridizing ethers and carbonates. The integration of the electron-donating ether group with the electron- withdrawing carbonate group can rationalizes the charge distribution, imparting BMC with notable oxidative/reductive stability and relatively weak solvation ability. Furthermore, BMC also offers advantages including the ability to slightly dissolve LiNO3, excellent thermostability and nonﬂamm- ability. Consequently, the optimized BMC-based electrolyte, even with typical concentrations in the single solvent, demonstrates high-voltage tolerance (4.4 V) and impressive Li plating/stripping Coulombic efﬁciency (99.4%). Moreover, it fulﬁlls practical lithium metal batteries with satisfactory cycling performance and exceptional tolerance towards thermal/mechanical abuse, showcasing its suitability for safe high-energy lithium metal batteries. The ambitious goal of achieving carbon neutrality has been driving the advancement of energy-dense battery chemistry, particularly in the realm of high-voltage lithium metal batteries (LMBs)1–4. However, their practical implementation poses demanding requirements for the electrolytes, which must simultaneously satisfy the aggressive high- voltage cathode, the hyperactive Li anode and safety performance5–7 As the most successful electrolyte system in commercial lithium-ion batteries (LIBs), carbonate-based electrolytes have demonstrated excellent oxidative stability of ~4.3 V vs. Li+/Li, accommodating the well operation of high-voltage cathode8. Nevertheless, the conjugated electron-withdrawing effect of the carbonyl oxygen results in a pro- nounced positive charge on the carbonyl carbon. The electron- deﬁcient feature of carbonyl carbon makes carbonates tend to gain electrons and thus exhibit poor reductive stability9. Consequently, continuous severe parasitic reactions will occur when coupling carbonate-based electrolytes with hyperactive Li anode, generating organic-rich solid-electrolyte-interphase (SEI) that is non-uniform, unstable and mechanically fragile5. Additionally, the formation of nasty Li dendrites is easily triggered by inhomogeneous distributions of Li+ ﬂux and further fuels the parasitic reactions10,11. Ultimately, the resul- tant accumulation of resistive ‘dead Li’ and decreased Coulombic efﬁciency (CE) upon repeated Li plating/stripping processes would lead to the poor cycle life of LMBs using carbonate-based electrolytes5,12. By contrast, electrolytes based on ether solvents have long stood out for their exceptional reductive stability in supporting Li anode Received: 20 August 2023 Accepted: 27 March 2024 Check for updates 1Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Fudan University, Shanghai 200433, China. 2Sinopec Shanghai Research Institute of Petrochemical Technology Co., Ltd., Shanghai 201208, China. 3School of Chemistry, South China Normal University, Guangzhou 510006, China. 4Department of Chemistry, College of Sciences, Shanghai University, Shanghai 200444, China. e-mail: xldong@fudan.edu.cn; yyxia@fudan.edu.cn Nature Communications|        (2024) 15:3217  1 1234567890():,; 1234567890():,; among various types of solvents5,6,9. Nevertheless, the lone pair elec- trons on ether oxygen have a strong donating tendency, which renders routine ether-based electrolytes exhibit limited oxidative stability beyond ~4.0 V vs. Li+/Li13, thereby failing to support state-of-the-art high-voltage LMBs. Although common strategies such as high/loca- lized high concentration electrolytes (HCEs/LHCEs) can be imple- mented to alter solvation structures and establish a superior cathode- electrolyte-interphase (CEI) to compensate the shortcomings of ethers14–16, the practical application of these approaches is still hin- dered by their associated costs and environmental burdens17. Recently, several researches also demonstrated the utilization of ether-based electrolytes in high-voltage LMBs through the incorporation of ﬂuorinated functional groups into ethers18,19. In fact, the upgraded oxidative stability can be regarded as the beneﬁts from the reduced electron cloud density on ether oxygen owing to the electron- withdrawing ﬂuorinated groups. Regrettably, above strategies rarely take into account that the modiﬁed solvent molecules are still volatile and ﬂammable with low ﬂash points, which can easily intertwine with short circuits induced by Li dendrites, thus exposing LMBs to thermal runaway and even catastrophic safety accidents20,21. Therefore, how to design a solvent simultaneously possessing high redox stability and high safety through molecular engineering presents a signiﬁcant challenge. Herein, we revisit ethers and carbonates from the molecule level, both of which are sort of like Yin and Yang with opposite properties in Chinese culture who can complement each other to compose integral Tai Chi. By hybridizing the electron-withdrawing carbonate bond and the electron-donating ether bond within a single molecule, it is pos- sible to precisely adjust their respective charge distributions, an achievement that cannot be attained through mere physical blending of ethers and carbonates. In this way, an intramolecularly hybridized linear solvent, bis(2-methoxyethyl) carbonate (BMC), was conceived and synthesized successfully, exhibiting considerable redox stability and relatively weak solvation power. Beyond that, the deliberate elongation of the molecular chain for increasing the intermolecular van der Waals forces brings about an elevated thermal stability and a heightened ﬂash point for BMC as expect. Despite the recent report on the use of BMC in safe graphite-based LIBs22, we initiated a funda- mental understanding on the molecular hybridization and a compre- hensive investigation of BMC for hyperactive Li metal anode and high- voltage LMBs. Paired with the general concentration of lithium bis(- ﬂuorosulfonyl)imide (LiFSI), the optimized single-solvent BMC-based electrolyte fulﬁlls a high average dendrite-free Li plating/stripping CE of 99.4%, surpassing those obtained in routine ether-based and carbonate-based electrolytes. Moreover, it adapts to the well opera- tion of LiNi0.8Co0.1Mn0.1O2 (NCM811) under high voltage of 4.4 V. These merits thus enable 92% capacity retention in Li | |high-loading- NCM811 (4.8 mAh cm−2) full cell after 150 cycles. Respectable cycling performance with BMC-based electrolyte can also be attained even with the more demanding anode-free Cu | |NCM811 coin and pouch cells. More importantly, Li metal pouch cells come through the nail penetration test and exhibit improved thermal safety temperature up to 155 °C, strongly manifesting the critical property of BMC as a safe solvent. The molecular design concept put forward in this work can offer an additional train of thought towards the development of pro- mising electrolytes for future commercial LMBs. Results Molecular design and physicochemical property In order to bolster the performance and ensure the safety of high- voltage LMBs, it is a pressing necessity to employ an electrolyte showcasing commendable redox stability and intrinsic safety. Never- theless, the prevailing ether-based and carbonate-based electrolytes, presently predominant in lithium battery chemistry, encounter obstacles in harmoniously integrating these coveted characteristics. Speciﬁcally, ether solvents, exempliﬁed by the commonly used linear 1,2-dimethoxyethane (DME, Fig. 1a), typically exhibit favorable reduc- tive stability. Coupled with its merit to dissolve LiNO3—a critical addi- tive for stabilizing the Li interface but scarcely soluble in carbonates— DME demonstrates its applicability for Li anode10,23,24. Nevertheless, the lone pair electrons of charge-concentrated ether-oxygen (see elec- trostatic potential (ESP) map) impart DME with electron-donating characteristics and make it prone to oxidation. Conversely, carbonate solvents like linear dimethyl carbonate (DMC, Fig. 1b) exhibit opposite behavior, highlighting their suitability for high-voltage cathodes with commendable oxidative stability. However, conjugate electron- withdrawing effect of the carbonyl oxygen results in the low electron density at carbonyl carbon, rendering it electropositive (as observed in the ESP map) and susceptible to gain electron (poor reductive stabi- lity). Given the complementary merits of ethers and carbonates, it is δ- + δ- Physical blending  Poor oxidative and reductive stability  Volatile and flammable δ+ c DME DMC Molecular hybridization: BMC vs 0.03 eV 0.02 eV 0.01 eV 0.01 eV 0.02 eV 0.03 eV d δ+ δ- δ-  Considerable oxidative and reductive stability  Relatively weak solvating power   Non-volatile and non-flammable Suitable for safe and high-voltage Li metal battery a  Good reductive stability  LiNO3-solubility  Poor oxidative stability  Volatile and flammable Electron-rich δ- δ- Ether: DME  Better for Li anode b Carbonate: DMC  Good oxidative stability  Poor reductive stability  Volatile and flammable Better for high-voltage cathode Electron-deficient  δ+  Fig. 1 | Schematic illustration of BMC molecule design. Molecular structures, electrostatic potential (ESP) maps and characteristics of (a) DME (representative linear ether) and (b) DMC (representative linear carbonate). c The intuitively phy- sical blending of DME and DMC. d Well-designed BMC solvent by molecularly hybridizing ether oxygen and ester carbonyl groups into a linear molecule. The meticulously engineered molecular hybridization results in a solvent capable of satisfying the demands for high-voltage and high-safety LMBs, a feat unattainable through mere physical blending of ethers and carbonates. Article https://doi.org/10.1038/s41467-024-47448-5 Nature Communications|        (2024) 15:3217  2 step of 1.0 femtosecond (fs). The systems were equilibrated for at least 20 picoseconds (ps) in the NVT ensemble, followed by production runs conducted in an NPT ensemble for 200 ps using the Berendsen baro- stat to maintain a pressure of 0.1 GPa with a decay constant of 0.1 ps. All steps were conducted under a Nosé thermostat with target tem- peratures set at 298 K. The simulation time was sufﬁcient to ensure that the electrolyte system reached equilibrium. The reliability of the MD simulation results was conﬁrmed by comparing the calculated densities of electrolytes from simulation with their experimentally measured counterparts as follows: 1.2 m DME(MD) = 0.964 g cm-3 com- pared to 1.2 m DME(Exp) = 0.956 g cm-3, 1.2 m DMC(MD) = 1.148 g cm-3 compared to 1.2 m DMC(Exp) = 1.137 g cm-3, and 1.2 m BMC(MD) = 1.210 g cm-3 compared to 1.2 m BMC(Exp) = 1.202 g cm-3, respectively, with discrepancies remaining within a margin of less than 1%. Material characterizations 1H and 13C NMR spectra were acquired using a 400 MHz NMR spec- trometer (AVANCE NEO, Bruker) and 7Li NMR spectra were obtained on a 600 MHz NMR spectrometer (AVANCE NEO, Bruker) at 25 °C. Raman spectra were carried out on an automated confocal Raman microscopy system (WITec Apyron, WITec) with an excitation wave- length of 532 nm and a laser power of 50 mW. The spectral data were analyzed using the WITec Project Plus software. To investigate the morphology of deposited Li, cycled NCM811 cathode and Al collectors, SEM images were obtained through Hitachi S4800 (Hitachi) with the accelerating voltage of 1 kV. In order to investigate the behavior divergence of Li plating, in-situ optical observation was conducted using an electro-chemical reaction visualizing confocal system (ECCS B310, Lasertec). The in-situ cells were assembled using Cu foil (working electrode), Li foil (counter and reference electrode) and Celgard 2400 separator in the clamp with a certain pressure. The cells were then cut into cross-sections with special cutting tools for observation. The current of Li plating/stripping was ﬁxed at 0.5 mA, while the Li plating capacity was ﬁxed at 0.25 mAh. For XPS and TOF-SIMS mea- surements, Cu electrodes retrieved from Li | |Cu cells after one cycle of plating/stripping at 0.5 mA cm-2 and 5 mA cm-2 were transferred to instrumental chambers from an Ar-ﬁlled glove box without exposure to air. XPS depth proﬁles were collected on a Nexsa instrument (Thermo Fisher Scientiﬁc) using a monochromatic Al Ka X-ray source (excitation energy = 1468.6 eV) and Ar+ sputtering at 1 kV for 0, 10, 30 and 60 s. TOF-SIMS analyses were carried out on an IONTOF M6 (IONTOF) instrument in high mass resolution mode, utilizing a 30 keV Bi3+ ion beam for the acquisition phase and a 2 keV Cs+ ion beam for the sputter phase. The typical areas analyzed and sputtered were 100 μm × 100 μm and 300 μm × 300 μm, respectively. Prior to con- ducting morphology and composition characterizations, all retrieved electrodes underwent a triple rinse with DME in order to eliminate any residual electrolyte. Subsequently, the electrodes were dried for 12 h at room temperature. The viscosity tests were conducted on a rotational rheometer (HAAKE MARS III). TG curves of the electrolytes (~10 mg per sample) were recorded using a heating rate of 10 °C min-1 from 30 to 130 °C on TG 201 F1 Libra (Netzsch). DSC measurements were con- ducted using a DSC 200 F3 Maia (Netzsch) instrument with N2 as the method gas. The DSC curve of pure BMC was recorded ata heating rate of 5 °C min-1 from -50 to 250 °C. Prior to testing, ~10 mg of the sample was sealed in a stainless-steel sample pan and weighed, followed by cooling down to -50 °C and maintaining for 5 min with liquid nitrogen as coolant. DSC curves of cycled NCM811 powders (around 10 mg) mixed with different electrolytes (around 10 mg) were recorded at a heating rate of 5 °C min-1 from 25 to 300 °C. Before test, the NCM811 powders were retrieved from 100 μm Li | |NCM811 full coin cells that were fully charged to 4.3 V after 2 formation cycles and then sealed in a stainless-steel sample pan with the electrolyte. The ﬂash points of electrolytes were investigated on ﬂash point analyzers (MINIFLASH FPH VISION (10 ~ 400 °C) and MINIFLASH FP VISION (−45 ~ 120 °C), Grabner Instruments) according to ASTM D6450 standard. All mea- surements were conducted using a sample volume of 1 mL and repe- ated for three times to ensure the reliability. The initial temperature was typically set at 18 °C below the anticipated ﬂash point, and the heating rate was maintained at 5.5 ± 0.5 °C per minute. An electric arc with high voltage was applied as the ignition source, operating at a frequency of 1 °C per minute. Following each ﬂash test, a short air pulse from a small membrane compressor introduced ~1.5 ± 0.5 mL of air into the test chamber. Flash detection in the chamber relied on detecting an increase in pressure after ignition occurs. The limit for successful ﬂash point detection was set at a pressure increase of 20 kPa, which corresponded to approximately a ﬂame volume of 1.5 mL above barometric pressure. Further detailed testing process can be referred in previous literature40. Accelerating rate calorimetry (ARC, THT) was utilized to monitor the real-time temperature changes of pouch cells. The test temperature range was set at 30–300 °C. Before ARC test, industrial Li | |NCM811 (500 mAh, N/P = 1.33/1, Electrolyte/ Cathode = 2.5 g Ah-1) pouch cells were charged to 4.3 V after 2 forma- tion cycles. The nail penetration test for pouch cells was conducted at room temperature in an open-air environment using a M01-005 nail penetration testing system (Dongguan Saice). Before nail penetration test, 5 Ah Li | |NCM811 (4 mAh cm-2, N/P = 2.5/1, Electrolyte/Cathode = 2 g Ah-1) pouch cells were charged to 3.8 V after 2 formation cycles.The cell was vertically penetrated by a steel nail with a diameter of 3 mm at a speed of 25 mm s-1 during the test. The ionic conductivity of elec- trolytes at different temperatures was measured and calculated via electrochemical impedance spectroscopy (EIS) using two stainless- steel sheets (1 cm2) symmetrically placed in the electrolytes. Electrochemical measurements The anodic stability of electrolytes was determined by analyzing the LSV curves of Al and Pt in a three-electrode device with lithium metal serving as both counter and reference electrodes, using a scan rate of 1 mV s-1. To acquire the Li+ transference number of electrolytes, a constant voltage bias of 10 mV was applied to Li | |Li symmetric cells. For Al corrosion testing, Li | |Al half-cells were subjected to various voltages for 22 h, with each voltage being maintained for 2 h. The Li electrode potentials (ELi) in various electrolytes were revealed with the CV curves of ferrocene (Fc), which were obtained using three- electrode devices consisting of Pt as a working electrode and lithium metal as both counter and reference electrodes, at a scan rate of 5 mV s-1. It was assumed that the potential of Fc+/Fc remained constant according to IUPAC recommendations30,31. The LSV, CV, EIS and CA measurements were all conducted on VMP3 potentiostats (BioLogic). For CE tests of Li plating/stripping, Li | |Cu (CIVEN Metal, 10 μm) cells were assembled, and tested following a similar protocol as previously reported5,6. Speciﬁcally, an initial formation cycle was conducted with a plating/stripping capacity of 5 mAh cm-2 at 0.5 mA cm-2, followed by the plating of 5 mAh cm-2 Li (Qp) on Cu at 0.5 mA cm-2 as a reservoir. A ﬁxed capacity (Qc = 1.0 mAh cm-2) of Li was then repeatedly stripped/ plated at 0.5 mA cm-2 for n cycles, and the residual Li (Qr) was subse- quently stripped to 1 V at the same current density. The average CE can be calculated using the following equation: CE = nQc + Qr nQc + Qp For calendar ageing test, after 10 cycles of normal Li plating/ stripping in Li | |Cu cells at 0.5 mA cm-2 and 1 mAh cm-2, different ageing intervals (10 h, 24 h, 48 h and a week) were introduced in every alter- nate cycle when 1 mAh cm-2 Li had been plated. For Li | |NCM811 (thickness: 24 μm, areal capacity: 1.2 ~ 1.3 mAh cm-2) half cells, 100 μm Li | |NCM811 (thickness: 82 μm, areal capacity: 4.8 mAh cm-2) full coin cells, and anode-free Cu | |NCM811 (thickness: 36 μm, areal capacity: Article https://doi.org/10.1038/s41467-024-47448-5 Nature Communications|        (2024) 15:3217  9 2.0 mAh cm-2) coin cells, the cathode (diameter: 12 mm, area: 1.13 cm2) was prepared by mixing NCM811 powder (Shenzhen Kejing Star Technology Co. Ltd.), acetylene black, and polyvinylidene diﬂuoride (PVDF) in a ratio of 96:2:2 using N-methyl-2-pyrrolidone (NMP) as the solvent. All NCM811 coin cells were cycled at a charge rate of 0.2 C and discharge rate of 0.3 C (1 C = 200 mAh g-1) between voltage ranges of 3 V to 4.3 V or up to 4.4 V. For the coin cell conﬁgurations, each CR- 2032 type coin cell was fabricated using Celgard 2400 (polypropylene, thickness: 25 μm, diameter: 18 mm) as the separator and 50 μL elec- trolyte (unless otherwise speciﬁed). To suppress corrosion from the electrolyte, Al-Clad coin cell case (Hohsen) was used for NCM811 cathode. Both thick (1000 μm) and thin (100 μm) Li foils were pur- chased from China Energy Lithium Co., Ltd. Industrial Li | |NCM811 and anode-free Cu | |NCM811 pouch cells were purchased from Li-Fun Technology Co. Ltd. Following two formation cycles at a 0.1 C charge/ discharge, pouch cells underwent cycling at a 0.2 C charge/1 C dis- charge between 3 and 4.3 V. Additionally, pouch cell cycling was con- ducted using a constant-current-constant-voltage protocol (cells were charged to the maximum voltage and held until current dropped below 0.1 C) together with a pressure of 200 kPa. The CE measure- ments of Li plating/stripping and cycling performance of cells were carried out on a standard battery tester (CT4008, Neware). Reporting summary Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article. 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Flash point prediction for the binary mixture of phosphatic solvents and n-dodecane from UNIFAC group contribution model. J. Loss Prev. Proc. 33, 183–187 (2015). Acknowledgements This work is supported by the National Key Research and Development Program of China (2022YFB2402200 X.D.), National Natural Science Foundation of China (21935003 Y.X., 22109028 X.D., 22379028 X.D.), Natural Science Foundation of Shanghai (22ZR1404400 X.D.), Chen- guang Program sponsored by Shanghai Education Development Foun- dation and Shanghai Municipal Education Commission (19CG01 X.D.). Author contributions X.D. and Y.X. conceived the idea and designed the experiments. J.C. and J.X. synthesized the molecule. J.C. carried out material characterizations and electrochemical measurements. D.Z. performed in situ ECCS opti- cal observations. L.Z. conducted ARC and nail penetration tests. M.L. conducted DFT calculations. T.Z. performed MD simulations. All authors engaged in result discussions. J.C., J.L., F.W., Y.W., X.D. and Y.X. co- wrote the manuscript with input from all authors. Competing interests A patent related to the work has been submitted (application number CN202210168155.0) by Fudan University. The inventors are X.D., J.C., J.X. and Y.X. The patent refers to the methodology described in this paper, but it offers a greater variety of analogous solvents compared to this work. Other authors declare no competing interests. Additional information Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41467-024-47448-5. Correspondence and requests for materials should be addressed to Xiaoli Dong or Yongyao Xia. Peer review information Nature Communications thanks the anon- ymous reviewers for their contribution to the peer review of this work. A peer review ﬁle is available. 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Title: Hybridizing carbonate and ether at molecular scales for high-energy and high-safety lithium metal batteries ​
Authors: Jiawei Chen, Daoming Zhang, Lei Zhu, Mingzhu Liu, Tianle Zheng, Jie Xu, Jun Li, Fei Wang, Yonggang Wang, Xiaoli Dong, Yongyao Xia ​
Publisher: Nature Communications
Date: 15 April 2024 ​
Abstract: Commonly-used ether and carbonate electrolytes show distinct advantages in active lithium-metal anode and high-voltage cathode, respectively. ​ While these complementary characteristics hold promise for energy-dense lithium metal batteries, such synergy cannot be realized solely through physical blending. ​ Herein, a linear functionalized solvent, bis(2-methoxyethyl) carbonate (BMC), is conceived by intramolecularly hybridizing ethers and carbonates. ​ The integration of the electron-donating ether group with the electron-withdrawing carbonate group rationalizes the charge distribution, imparting BMC with notable oxidative/reductive stability and relatively weak solvation ability. ​ Furthermore, BMC also offers advantages including the ability to slightly dissolve LiNO3, excellent thermostability, and nonflammability. ​ Consequently, the optimized BMC-based electrolyte, even with typical concentrations in the single solvent, demonstrates high-voltage tolerance (4.4 V) and impressive Li plating/stripping Coulombic efficiency (99.4%). ​ Moreover, it fulfills practical lithium metal batteries with satisfactory cycling performance and exceptional tolerance towards thermal/mechanical abuse, showcasing its suitability for safe high-energy lithium metal batteries. ​