Molecular anchoring of free solvents for high-voltage and high-safety lithium metal batteries.pdf
Article https://doi.org/10.1038/s41467-024-46186-y Molecular anchoring of free solvents for high-voltage and high-safety lithium metal batteries Zhuangzhuang Cui1, Zhuangzhuang Jia2, Digen Ruan1, Qingshun Nian 1, Jiajia Fan1, Shunqiang Chen1, Zixu He1, Dazhuang Wang1, Jinyu Jiang1, Jun Ma1, Xing Ou 3, Shuhong Jiao 1, Qingsong Wang 2 & Xiaodi Ren 1 Constraining the electrochemical reactivity of free solvent molecules is pivotal for developing high-voltage lithium metal batteries, especially for ether sol- vents with high Li metal compatibility but low oxidation stability ( <4.0 V vs Li+/ Li). The typical high concentration electrolyte approach relies on nearly saturated Li+ coordination to ether molecules, which is confronted with severe side reactions under high voltages ( >4.4 V) and extensive exothermic reac- tions between Li metal and reactive anions. Herein, we propose a molecular anchoring approach to restrict the interfacial reactivity of free ether solvents in diluted electrolytes. The hydrogen-bonding interactions from the anchoring solvent effectively suppress excessive ether side reactions and enhances the stability of nickel rich cathodes at 4.7 V, despite the extremely low Li+/ether molar ratio (1:9) and the absence of typical anion-derived interphase. Fur- thermore, the exothermic processes under thermal abuse conditions are mitigated due to the reduced reactivity of anions, which effectively postpones the battery thermal runaway. Lithium (Li) metal is an ideal anode material with an extremely high speciﬁc capacity (3860 mAh g−1), and the lowest electrochemical potential (−3.04 V vs reversible hydrogen electrode)1–3. However, the lack of high-efﬁciency electrolytes has hindered the development of high-voltage Li metal batteries (>4.0 V, vs Li/Li+, same hereinafter)4–7. Conventional carbonate electrolytes are highly reactive to Li anode, and the derived porous and heterogeneous solid-electrolyte inter- phase (SEI) cannot prevent further side reactions of the electrolyte, which would lead to continuous Li consumption and dendrite growth2,8–10. In contrast, ether electrolytes stand out as a promising solution because of their superior reduction stability, lower viscosity, and faster Li+ transport dynamics11–13. Unfortunately, the oxidation stability of conventional ether electrolytes (~1 M) is very limited (<4.0 V), thus severely restricting their application with high-voltage cathodes, especially nickel (Ni)-rich cathodes with highly reactive surface sites (such as Ni3+/4+)14–17. Recently, extensive efforts have been made to address the com- patibility issue between ether-based electrolytes and high-voltage cathodes18–21. Apart from artiﬁcial cathode-electrolyte interphases (CEIs) that prevent direct contact between high-voltage cathodes and free ether molecules by physical isolation or nanopore-based de- solvation22,23, the fundamental challenge lies in reducing the reactivity of free solvents at the electrode-electrolyte interface. High- concentration electrolytes (HCE) and localized high-concentration electrolytes (LHCE) with high salt/solvent ratios minimize the popu- lation of labile-free ether molecules by using the Li+ coordination Received: 31 July 2023 Accepted: 18 February 2024 Check for updates 1Hefei National Research Center for Physical Sciences at the Microscale, CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China. 2State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, China. 3Engineering Research Center of the Ministry of Education for Advanced Battery Materials, School of Metallurgy and Environment, Central South University, No.932 South Lushan Road, Changsha, Hunan 410083, PR China. e-mail: pinew@ustc.edu.cn; xdren@ustc.edu.cn Nature Communications|        (2024) 15:2033  1 1234567890():,; 1234567890():,; strategy. More importantly, the enrichment of anions in the inner solvation sheath promotes the formation of inorganic-rich electrode- electrolyte interphases, which was found to be essential as the kinetic barrier for side reactions between reactive anodes/cathodes and ether- based electrolytes1,24. For example, Li ﬂuoride (LiF) has been identiﬁed as a crucial component of CEI due to its excellent electron-insulating properties as well as physical and electrochemical stability25–28. Despite the encouraging developments of ether-based HCEs or LHCEs, they are still restricted by limited anodic stability (<4.5 V), low ion transport kinetics arising from strong Li+-anion interactions, high salt cost, and unsatisfactory low-temperature performance29,30. Furthermore, the safety issue of Li metal batteries (LMBs) using concentrated electrolytes with reactive anions has rarely been dis- cussed. Several recent studies have concluded that the safety of elec- trolytes cannot be justiﬁed solely on the basis of their non- ﬂammability31–33. Exothermic reactions between the electrolyte and electrode materials are crucial for inducing battery thermal runaway reactions34,35. Ouyang and co-workers reported that battery thermal runaway was triggered by the huge amount of heat released by the reaction between the graphite anode and Li salt in concentrated electrolytes, even though a ﬂame-retarding phosphate ester solvent was used36. Similarly, Xu and co-workers found that the heat released by the reaction between the electrolyte and the charged cathode (Li iron phosphate after Li+ removal) or anode (graphite after Li insertion) is positively correlated with the Li salt concentration37. Despite the LHCE reduces the nominal concentration of Li salts (to 1–1.5 M) with the addition of diluents, the temperature of the battery rises sharply during the puncture test to damage the battery structure due to the enhanced reactivity of anions in LHCE. The reaction between Li salts and Li metal anode in concentrated electrolytes could potentially release a large amount of heat to induce the thermal runaway ava- lanche reaction due to the low melting point (180 °C) and highly reactive nature of Li metal. Therefore, it is urgent to develop effective strategies to design ether-based electrolytes with high Li metal Cou- lombic efﬁciency (CE), high-voltage stability, and, last but not least, suppressed exothermic reactions with Li metal to improve the safety of LMBs. Herein, we propose a molecular anchoring diluent electrolyte (MADE, down to 0.19 M) with a wide electrochemical stability window (>4.7 V on NMC811 cathode) and greatly increased thermal runaway temperature for LMBs. In contrast to the widely-used approach of restricting free solvents through Li+ coordination in concentrated electrolytes, we exploit the strong non-conventional hydrogen bond interactions with oxidation-resistant hydroﬂuoroether and ether molecules to improve the oxidation resistivity of ether solvents. The MADE design with decreased anion reactivity not only demonstrates enhanced oxidation stability (>4.7 V) compared to LHCE, which rely on extensive anion sacriﬁcial decomposition, but also fosters a more ﬂexible SEI layer to accommodate volume changes of Li metal during cycling. Furthermore, the thermal runaway temperature of LMBs is greatly increased (from 141 to 209 °C) due to suppressed exothermic side reactions with Li metal. This work sheds light on a promising approach to expanding the electrochemical stability window and enhancing the safety of electrolytes for LMB applications. Results and discussion The dilute electrolyte design with molecular anchoring To investigate the impact of hydroﬂuoroether molecular “anchor” (e.g., 1,1,2,2-tetraﬂuoroethyl-2,2,3,3-tetraﬂuoropropylether, TTE) on the electrochemical stability of the ether solvent (1,2-dimethox- yethane, DME), we formulated the electrolytes with a high TTE to DME molar ratio of 3:1, while the ratio of the Li bis(ﬂuorosulfonyl)imide (LiFSI) salt was kept low to minimize the ion-solvent interactions. Therefore, electrolytes with low molar ratios of LiFSI: DME: TTE, 1:9:27 (~0.19 M), 2:9:27(~0.38 M), and 3:9:27 (~0.56 M) were prepared and labeled as MADE-1, MADE-2, and MADE-3, respectively. To gain insight into the thermodynamic interaction between DME and TTE molecules, the heat of mixing was measured using isothermal titration calorimetry (ITC). Upon the addition of DME into TTE, distinct exothermic peaks were detected, with each drop estimated to release 706.1 cal mo1−1 of heat (Fig. 1a and Supplementary Fig. 1). By contrast, signiﬁcantly reduced heat release (185.9 cal mol−1) was observed when HCE (LiFSI- 1.2DME in molar ratio) was added into TTE. This suggests that the DME- TTE interaction is greatly eliminated when most DME molecules are strongly conﬁned in the inner solvation sheath (Li+-O(DME) coordination). Meanwhile, the dominant interaction between DME and TTE in MADEs is not from the pseudo hydrogen bond between H(DME) and F(TTE). As shown in Fig. 1b, despite the 1H-19F coupling signals observed in two-dimensional nuclear magnetic resonance (NMR), the pseudo H bonds between C-H (DME)…F-C (TTE) are weaker than those between H(TTE) and O(DME) 38,39. Further 1H NMR results indicate the H bond between O(DME) and H(TTE) accounts for the apparent heat release observed in the ITC (Supplementary Fig. 2). When DME was mixed with TTE, the chemical shifts of H(TTE) increased while those of H(DME) decreased, indicating that H(TTE) acts as the H bond donor with O(DME) as the acceptor40. Although the spatial proximity of H(DME) and F(TTE) induces relatively weak interactions, which would potentially cause the 1H chemical shift of DME to a higher value (H(DME) as the H bond donor instead), the dominant force is the H bond between H(TTE) and O(DME) and the resulting 1H chemical shift of DME is toward a lower value. Further geometry optimizations of DME-TTE com- plexes by density-functional theory (DFT) also suggest that the H(TTE)…O(DME) interactions are favored compared to H(DME)…F(TTE) interactions (Supplementary Fig. 3), which is consistent with pre- vious studies39. To further verify the hydrogen-bonding between TTE and ether molecules, we carried out Fourier-transform infrared spectroscopy (FT-IR) analysis in CCl4 solutions. To study the effect of hydrogen- bonding on the C-H vibration in TTE while avoiding the interference of C-H bonds in ether (overlapping signals), deuterated tetrahydrofuran (THF-d8) was selected because of its characteristic ethereal moiety and easy accessibility. As shown in Fig. 1c, after adding THF-d8 to the TTE solution, the CF2-H vibration peak shifts noticeably from 3000 to 3010 cm−1, which aligns with the featured blue-shifting vibration of CF2-H after H bond formation reported in the literature38,41,42. In addi- tion, we study the interaction between TTE and DMSO, which is often selected to study the C-H…O hydrogen bond as it contains no hydrogen donor43. Similarly, upon adding DMSO-d6, a noticeable blue shift of the CF2-H signal in TTE was also observed, which can demon- strate the hydrogen bond donating ability of TTE. Moreover, with the content of TTE increasing, the 17O-NMR signal of DME shifts to a lower chemical shift (Supplementary Fig. 4), which agrees with the previous study that hydrogen-bonding induces an upﬁeld chemical shift of oxygen atoms44. In contrast, there is no apparent shift of the 17O-NMR signal for the oxygen atom in TTE. These ﬁndings support the con- clusion that TTE forms hydrogen bonds with DME through interactions between the CF2-H(TTE) and O(DME). Furthermore, DFT calculations were employed to obtain more detailed coordination conﬁgurations (Supplementary Fig. 5a). From the deformation electron density, the interaction for DME-TTE, or DME-DME complexes could be identiﬁed with the overlapping defor- mation electron density, while the interaction between the C-H(TTE) and C-F(TTE) is not observed on the 0.03 a.u. isovalue map for the TTE- TTE complex. Meanwhile, as shown in the electrostatic potential mapping (ESP) (Supplementary Fig. 5b), the minimum electrostatic potential energy in the vicinity of oxygen of DME increases notably after its coordination with TTE (from −42.75 to −17.22 kcal mol−1), which has substantial effects on the oxidative stability of the molecules45. Article https://doi.org/10.1038/s41467-024-46186-y Nature Communications|        (2024) 15:2033  2 LiFSI, 100 DME, and 300 TTE molecules. For LHCE, the box contains 100 LiFSI, 120 DME, and 300 TTE molecules. The electrolyte systems were ﬁrst simulated annealing between 600 and 300 K using an NTV ensemble for 15 cycles and optimized the structure after each annealing. The structure with the lowest energy is selected for the next dynamics simulations. The electrolyte systems were equilibrated in the isothermal-isobaric ensemble (constant NPT) using the Berendsen barostat to maintain the pressure of 0.0001 GPa for 500 ps, aiming to make the systems have proper density. Then, the last frame from the NPT simulation ran for 800 ps in the canonical ensemble (NVT) at 298 K. Thereafter, the radial distribution function was obtained by dynamic simulation of the optimized model and calculated the mole- cular coordination number by the following formula: n r0 ð Þ = 4Πρ Z r0 0 gðrÞr2dr ð1Þ ρ is the Number density of certain types of atoms. The atomic coordinates of the initial and ﬁnal conﬁgurations of the MD simulation could be found in the source data. Density-functional theory (DFT) calculation The DFT calculation was conducted on the Gaussian16 package61. The initial geometry structures are derived from molecular dynamics simulations. All molecules and complexes were optimized by the B3LYP method at 6–311 + + G (d,p) basis set. Frequency analysis was performed to guarantee that all the structures were the minimum points on the potential energy surface. Calculation of oxidation potential were performed under an implicit solvation model with dielectric properties of Aniline (dielectric constant = 7.3). The oxida- tion potential Eox vs. Li/Li+ was calculated as follows: EOX = G M +    G M ð Þ F  1:4V ð2Þ where G(M) and G(M+) are the free energy of species M, its oxidized forms at 298.15 K, respectively. F is the Faraday constant. The defor- mation charge density was calculated using Dmol3 in Materials Studio software. The extreme points of ESP are achieved through Multiwfn software45 combined with VMD programming. The atomic coordinates of the optimized geometries could be found in the source data. Reporting summary Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article. 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Commun. 14, 2655 (2023). 61. Frisch, M. J. et al. Gaussian 16, Revision C.01 (Gaussian, Inc., 2016). Acknowledgements This study was supported by the National Natural Science Foundation of China, grant Nos. 22179124 (X.R.), 21905265 (X.R.), CAS Project for Young Scientists in Basic Research, grant No. YSBR-098 (Q.W.), the Fundamental Research Funds for the Central Universities, grant No. WK3430000007 (X.R.), and 2021 Anhui Energy Internet Joint Fund Project, grant No. 2108085UD04 (Q.W. and X.R.). XAS tests were per- formed at beamlines BL12B-a and BL12B-b in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. The numerical calculations in this paper have been done at Hefei Advanced Computing Center. In addition, the authors are grateful for resources from the High Perfor- mance Computing Center of Central South University and the Center for Micro and Nanoscale Research and Fabrication at USTC. We are also thankful for the helpful discussion about NMR tests and analysis with Dr. Ke Gong from the Instruments Center for Physical Science at USTC. Author contributions Z.C. and X.R. conceived the idea and designed the experiments. Z.C. performed the main research work with the help of D.R., Q.N., J.F., and S.C. Z.J. and Q.W. conducted the ARC tests. Z.C., D.W., and X.O. per- formed the theoretical calculations. Z.H. and S.J. conducted the AFM test. J.J. performed the Raman test. J.M. carried out the DSC test. Z.C. and X.R. wrote the manuscript, and all authors discussed the results and revised the manuscript. Competing interests The authors declare no competing interests. Additional information Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41467-024-46186-y. Correspondence and requests for materials should be addressed to Qingsong Wang or Xiaodi Ren. 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Title: Molecular anchoring of free solvents for high-voltage and high-safety lithium metal batteries ​
Authors: Zhuangzhuang Cui, Zhuangzhuang Jia, Digen Ruan, Qingshun Nian, Jiajia Fan, Shunqiang Chen, Zixu He, Dazhuang Wang, Jinyu Jiang, Jun Ma, Xing Ou, Shuhong Jiao, Qingsong Wang, Xiaodi Ren 
Publisher: Nature Communications
Date: 2024-03-06 00:00:00
Abstract: Constraining the electrochemical reactivity of free solvent molecules is pivotal for developing high-voltage lithium metal batteries, especially for ether solvents with high Li metal compatibility but low oxidation stability (<4.0 V vs Li+/Li). ​ The typical high concentration electrolyte approach relies on nearly saturated Li+ coordination to ether molecules, which is confronted with severe side reactions under high voltages (>4.4 V) and extensive exothermic reactions between Li metal and reactive anions. ​ Herein, we propose a molecular anchoring approach to restrict the interfacial reactivity of free ether solvents in diluted electrolytes. ​ The hydrogen-bonding interactions from the anchoring solvent effectively suppress excessive ether side reactions and enhance the stability of nickel-rich cathodes at 4.7 V, despite the extremely low Li+/ether molar ratio (1:9) and the absence of typical anion-derived interphase. ​ Furthermore, the exothermic processes under thermal abuse conditions are mitigated due to the reduced reactivity of anions, which effectively postpones the battery thermal runaway.