Co-Intercalation-Free Fluorinated Ether Electrolytes for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are one of the most promising power sources for electric vehicles because of their high energy density and long lifetime. Graphite is widely used as anode electrode material in the state-of-the-art LIBs. However, a graphite anode is usually only compatible with an ethylene carbonate (EC)-based electrolyte. When contacting with a propylene carbonate (PC)-based electrolyte, which is advantageous over the EC-based one for its wider temperature range and higher low-temperature conductivity, the graphite anode may suffer from substantial exfoliation problem during the initial lithium intercalation step. Consequently, a PC-based electrolyte cannot be used in LIBs with graphite as the anode unless some solid electrolyte interphase (SEI) film-formation additives are introduced into the electrolytes. Usually this type of additives is reduced predominantly on the graphite electrode surface before PC is intercalated and reduced. These additives mainly contain some functional groups such as vinylene or cyclic unsaturated compounds. For example, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) have been widely studied and used as additives for PC-based electrolytes. They play an important role in the protection of the structure of the graphitic anode from destruction by PC, but they also have some shortcomings. Usually these additives result in a thick SEI protective layer, which could significantly reduce the rate capability and limit low-temperature performance and cycling stability at elevated temperatures because of its additional impedance and poor thermal stability. Recently, we developed a novel electrolyte additive that can suppress the PC reductive decomposition on the graphite anode without a thick and resistant SEI film. The additive may provide a novel protective mechanism for the graphite anode in the PC-containing electrolytes without or with suppressed SEI film formation. The SEI-film-suppression additive can not only significantly improve the compatibility between graphite anode and PC-containing electrolyte, but also exhibit superior rate capability and excellent cycling stability at elevated temperatures.We believe that this new electrolyte additive can be applied to LIBs and other electrochemical systems using PC-containing electrolytes and graphite anodes. The details of the results will be reported. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technology of the U.S. Department of Energy (DOE). The electron microscopy and XRD measurements were performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL.

Lithium–sulfur (Li–S) batteries with high energy density are promising for next-generation energy storage but suffer from poor cycle life due to instability of the lithium anode. Commercial graphite anodes used in lithium-ion batteries could be viable for replacement of lithium metal as anode in Li–S batteries. However, graphite anodes are incompatible with the commonly used ether-based electrolytes in Li–S batteries due to the Li+ ion and solvent co-intercalation into graphite interlayers, which leads to exfoliation. Here, we report the stabilisation of graphite anode by in-situ formation of quasi-solid-state electrolyte (QSSE) in Li–S batteries. The QSSE is formed by the ring-opening of molecular ethers induced by metallic molybdenum disulfide sulfur cathode host to produce long-chain polymers. The resulting gel polymer matrix exhibits ionic conductivity (1.51 mS cm−1) comparable to that of liquid electrolyte and provides sufficient redox chemistry for the sulfur cathode without co-intercalation into the graphite anode. Li–S pouch cells based on in-situ formed QSSE and graphite anode show a specific capacity of ~ 1200 mAh g−1 and 90.3% capacity retention after 200 cycles. Our design addresses the electrolyte limitation associated with graphite anode, enabling graphite to be used in ether-based solvents for safe, cost-effective and stable Li–S batteries.Graphical abstractSupplementary InformationThe online version contains supplementary material available at 10.1557/s43581-025-00139-0.

Lithium/sodium-selenium (Li/Na-Se) batteries are promising energy storage systems due to their relatively low cost and high volumetric energy density of Se cathode. Rechargeable lithium-ion batteries (LIBs) play a critical role in portable devices, electric vehicles and large-scale stationary energy storage, due to their relatively high energy density and low cost among available technologies [1]. Sodium-selenium (Na-Se) batteries have also drawn much attention, because of the large abundance and low cost of sodium and decent energy density of Se [2]. As a cathode material, Se provides a moderate gravimetric capacity of 678 mAh g−1, a high volumetric capacity of 3270 mAh cm−3, and possesses a high electronic conductivity of 1 ×10 −3 S m−1 [3, 4]. These advantages of Se have stimulated growing research in developing Li-Se and Na-Se batteries over the past years. However, the practical application of these batteries has been hindered by the polyselenides dissolution (or shuttle effect phenomena), which could cause low Coulombic efficiency and poor cycling lifetime. Microporous carbon (MPC) has been fabricated through a facile carbonization method from polyvinylidene fluoride (PVDF) and used without any further activation as selenium host to produce MPC/Se composite employed as cathode in rechargeable lithium/sodium-selenium (Li/Na-Se) batteries and the electrochemical properties of the prepared cell have been investigated in Li-Se batteries. (figures 1 and 4).Basically, in linear carbonate-containing electrolytes, the metal readily decomposes at the anode side, causing capacity drop over cycling. To address this problem, electrolyte design should accompany manifested cathode structure. Hence, fluoroethylene carbonate (FEC) has been used as an additive in the electrolyte solution for stabilizing the capacity in Na-Se batteries. Our investigation showed that exceptional cell stability could be reached by using the FEC in carbonate-based electrolyte. In lithium/sodium-selenium batteries, diethyl carbonate (DEC) electrolyte decomposes in contact with metal, producing some radicals and metal alkyl carbonates. These radicals start a chain reaction, causing the destruction of over cycles. By adding the FEC to the electrolyte solution, this side reaction will be prohibited. [5] FEC readily decomposes on the surface of the metal prior to any reduction of linear carbonates, making the protective and stable surface film on the metal. This layer makes the capacity much more stable for the cell contains FEC in the electrolyte. For the cell without FEC in the electrolyte, the capacity starts to fade from initial cycles and continue fading during the cycling. In sodium/selenium batteries, at the current rate of 0.1 C, the cell contains no FEC in the electrolyte, showed a capacity of 317 and 245 mAh g-1 after 100 and 200 cycles, respectively. However, the cell with an electrolyte solution containing 3% FEC, showing excellent cyclability by way of delivering 382 and 350 mAh g-1 capacity at those above-mentioned cycles, respectively (figures 2 and 5). Another important finding was that the use of FEC additive could prevent the corrosion of metal anode from soluble polyselenides, and preserve the structural integrity of metal anode, contributing to the enhanced cycling stability and specific capacity of Na-Se batteries.The utilization of nanoscale Al2O3 surface coating by atomic layer deposition (ALD) to protect a MPC/Se cathode and reduce polyselenide dissolution was also investigated in another stage. It was found that Al2O3 surface coating effectively suppressed the polyselenide dissolution from the MPC/Se cathode, thus reducing the loss of Se active material and improving the overall performance of Na-Se batteries. Compared to the pristine MPC/Se, Al2O3-coated MPC/Se cathode exhibited improved discharge capacity, cycling stability, and rate capability in Na-Se batteries (figures 3 and 6). Keywords: Fluoroethylene carbonate; lithium/sodium−selenium batteries; ALD coating; carbonate-based electrolyte; microporous carbon

Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are widely used as electrolyte additives in lithium ion batteries. Here we analyze the solid electrolyte interphase (SEI) formed on binder-free silicon nanowire (SiNW) electrodes in pure FEC or VC electrolytes containing 1 M LiPF6 by solid-state NMR with and without dynamic nuclear polarization (DNP) enhancement. We find that the polymeric SEIs formed in pure FEC or VC electrolytes consist mainly of cross-linked poly(ethylene oxide) (PEO) and aliphatic chain functionalities along with additional carbonate and carboxylate species. The formation of branched fragments is further confirmed by 13C-13C correlation NMR experiments. The presence of cross-linked PEO-type polymers in FEC and VC correlates with good capacity retention and high Coulombic efficiencies of the SiNWs. Using 29Si DNP NMR, we are able to probe the interfacial region between SEI and the Si surface for the first time with NMR spectroscopy. Organosiloxanes form upon cycling, confirming that some of the organic SEI is covalently bonded to the Si surface. We suggest that both the polymeric structure of the SEI and the nature of its adhesion to the redox-active materials are important for electrochemical performance.

The energy density of lithium-ion cells can be significantly increased by the use of silicon-containing negative electrodes. However, the long-term performance of these cells is limited by the stability of the silicon electrode-electrolyte interface, which is continually disrupted during electrochemical cycling. Therefore, the development of electrolyte systems that enhance the stability of this interface is a critical need. In this article, we examine the cycling of ∼20 mAh pouch cells with lithium bis(fluorosulfonyl)imide (LiFSI)-containing carbonate-based electrolytes, silicon-graphite negative electrodes, and Li1.03(Ni0.5Co0.2Mn0.3)0.97O2 based positive electrodes. The effect of fluoroethylene carbonate (FEC) and vinylene carbonate (VC) addition on cell performance is also examined and compared to the performance of our baseline LiPF6-containing cells. Our data show that cells containing only LiFSI show rapid loss of capacity, whereas additions of FEC and VC significantly improve cell capacity retention. Furthermore, the performance of LiFSI-FEC and LiPF6-FEC cells are very similar indicating that the electrolyte salts play a much smaller role in performance degradation than the electrolyte solvent. Future efforts to enhance longevity of cells with silicon-graphite negative electrodes will thereby focus on developing alternative solvent systems.

높은 에너지 밀도를 지닌 리튬 이온 전지는 현재 리튬 이온 전지에 상용화된 음극 활물질인 천연 흑연의 보다 높은 율 별 특성과 안정한 장수 명 특성을 요구하고 있다. 천연 흑연계 음극 활물질을 이용하여 리튬 전지 음극을 제작하여, SEI 피막의 형성 및 제어의 대표적인 전해질 첨가제인 VC (vinylene carbonate), VEC (vinyl ethylene carbonate), FEC (fluoroethylene carbonate)등의 다양한 첨가제를 사용하여 초기 반응에 의해 생성되는 SEI 피막을 분석하고 이에 따른 전기 화학 특성 변화를 측정하기 위하여 SEM, EVS (electorochemical voltage spectroscopy), 피막 분석, EIS (electrochemical impedance spectroscopy), FT-IR (Fourier transform infrared spectroscopy)등을 측정하여, 고온 수명 평가, 용량 유지율 및 성능 평가를 실시하여, <TEX>$0^{\circ}C$</TEX> 수명특성 이후의 음극에 대한 분석을 비교 및 분석 평가 하였다. 초기 충전 시 profile에서 SEI의 형성에 의한 변화를 나타냈으며, EVS를 통하여 No-Additive가 약 0.9 V에서 SEI의 형성이 이루어지지만, VC, VEC, FEC의 경우 1 V 이상에서 형성반응이 이루어졌다. <TEX>$60^{\circ}C$</TEX> 수명특성평가에서 초기 효율은 No-Additive가 가장 높게 나타나며 용량 유지율이 높게 나타났으나, cycle이 진행 될수록 충전 시 용량과 효율이 감소하여 VC, FEC보다 용량 유지율이 낮아졌고, VEC는 효율 및 용량 유지율 모두 성능이 가장 낮게 나타났다. SEM을 통하여 SEI의 변화를 확인할 수 없었지만, FT-IR을 통하여 SEI의 성분이 cycle이 진행이 될수록 첨가제에 의해 <TEX>$2850-2900cm^{-1}$</TEX>영역의 Alkyl carbonate (<TEX>$RCO_2Li$</TEX>) 계열의 성분이 더욱 견고하게 유지되는 것을 확인하였으며, EIS를 통하여 cycle이 진행될수록 저항은 증가하는 것으로 나타났고, 특히 No-Additive 및 VEC의 SEI에 의한 저항이 매우 커졌다는 것을 알 수 있었다. Lithium ion battery with high energy density is expanding its application area to electric automobile and electricity storage field beyond existing portable electric devices. Such expansion of an application field is demanding higher characteristic and stable long life characteristic of an anode material, the natural graphite that became commercialized in lithium ion battery. This thesis produced cathode by using natural graphite anode material, analyzed creation of the cathode SEI film created due to initial reaction by using electrolyte additives, VC (vinylene carbonate), VEC (vinyl ethylene carbonate), and FEC (fluoroethylene carbonate), and considered correlation with the accompanying electrochemical transformation. This study compared and analyzed the SEI film variation of natural graphite cathode according to the electrolyte additive with SEI that is formed at the time of initial filling and cathode of <TEX>$60^{\circ}C$</TEX> life characteristic. At the time of initial filling, the profile showed changes due to the SEI formation, and SEI was formed in No-Additive in approximately 0.9 V through EVS, but for VC, VEC, and FEC, the formation reaction was created above 1 V. In <TEX>$60^{\circ}C$</TEX> lifespan characteristic evaluation, the initial efficiency was highest in No-Additive and showed high contents percentage, but when cycle was progressed, the capacity maintenance rate decreased more than VC and FEC as the capacity and efficiency at the time of filling decreased, and VEC showed lowest performance in efficiency and capacity maintenance rate. Changes of SEI could not be verified through SEM, but it was identified that as the cycle of SEI ingredients was progressed through FT-IR, ingredients of Alkyl carbonate (<TEX>$RCO_2Li$</TEX>) affiliation of the <TEX>$2850-2900cm^{-1}$</TEX> was maintained more solidly and the resistance increased as cycle was progressed through EIS, and specially, it was identified that the resistance due to No-Additive and SEI of VEC became very significant. Continuous loss of additives was verified through GC-MS, and the loss of additives from partial decomposition and remodeling of SEI formed the non-uniform surface of SEI and is judged to be the increase of resistance.

Quantification of solvent-mediated host-ion interaction in graphite intercalation compounds for extreme-condition Li-ion batteries

Electrochemical solvent co-intercalation into graphite, i.e., where instead of a bare ion a solvated ion is intercalated, is broadly seen as detrimental as it often leads to extreme volume expansion and delamination of the active material. It is also, however, accepted as the initial step in solid electrolyte interphase (SEI) formation.[1] Moreover, in 2014 it was shown that, surprisingly, the reaction can be extremely reversible and fast when glyme based electrolytes are used.[2] In addition, the solvent co-intercalation reaction allowed for storage of sodium in graphite, previously not thought possible due to a lack of thermodynamically stable high capacity sodium-graphite binary compounds, showing that solvent co-intercalation opens up for novel electrolyte-electrode combinations. This sparked additional interest in the phenomenon, where many electrolyte solutions have now been explored for several metal-ions (Li+, Na+, K+ as well as Mg2+ and Ca2+) showing that when electrochemical solvent co-intercalation occurs the redox reaction, both capacity (stoichiometry) and potential, becomes dependent on the exact electrolyte formulation. Hence systems with solvent co-intercalation offers an extremely diverse chemistry.[3,4] Thus, the process of electrochemical solvent co-intercalation seems ubiquitous when graphite anodes are used – leading to either successful SEI formation, destruction of the active material, or in rare cases to reversible solvent co-intercalation – Yet, the underlying mechanism is poorly understood and has received very little attention.Here we show that the simply model of reversible solvent co-intercalation often shown schematically in the literature is flawed and inconsistent with several experimental facts. Based on electrode weight change measurements, impedance spectroscopy and operando microscopy we present a new consistent model – showing that electrochemical solvent co-intercalation does not happen through a steady continuous reaction but is driven by i) flooding events where large amounts of solvated ions and free solvents enter the graphite and ii) preferentially replacement of free solvents by solvated ions. Armed with this new model of the underlying mechanism of solvent co-intercalation we show how solvents which have previously been thought to not electrochemically co-intercalate can be made to reversibly co-intercalate. Thus, with these new insights, we greatly expanded the possible electrolytes where the reaction occurs reversibly. We also discuss typical properties of systems where reversible co-intercalation occurs (the redox reaction being tunable by the electrolyte composition, extremely fast kinetics) and how to detect solvents co-intercalation as it is sometimes overlooked.[5] Finally, we discuss the use of solvent co-intercalation in full cells, along with our own data on full cells and on a system where both anode and cathode operate via a solvent co-intercalation mechanism – the solvent co-intercalation battery (CoIB).[5] References [1] K. Xu, Chemical reviews 104 (2004), 4303-4418[2] B. Jache, P. Adelhelm, Angewandte Chemie, 126(2014) 10333-10337.[3] J. Park, Z-L. Xu, K. Kang, Frontiers in Chemistry, 8 (2020), 2296-2646[4] B. Jache, J. O. Binder, T. Abe, P. Adelhelm, Phys. Chem. Chem. Phys., 18 (2016) 14299-14316[5] G. A. Ferrero, G. Åvall, K. A. Mazzio, Y. Son, K. Janßen, S. Risse, P. Adelhelm, Adv. Energy Mater. 12 (2022)2202377

The pursuit of higher energy density in lithium-ion batteries has led to the development of new cathode materials that can operate at elevated voltages and provide increased specific capacities. [1-2] One such class of materials is the nickel-rich layered oxide cathodes known as LiNixMnyCozO2 (NMC). These cathodes offer high specific capacities and demonstrate electrochemical stability at high cutoff voltages, making them promising candidates for advanced lithium-ion batteries. [3-4] However, NMC cathodes face a significant challenge in the form of capacity fading during cycling. This issue primarily arises from the voltage instability of the conventional carbonate-based electrolytes used in lithium-ion batteries, which are typically designed for the 4-V lithium-ion chemistry. [5-6] As a result, researchers and scientists have directed extensive efforts toward developing new and improved electrolyte systems that can withstand the high voltage requirements of NMC cathodes and other high-energy applications. [7-8]Designing and synthesizing new molecules for use as electrolyte solvents in lithium-ion batteries is indeed a complex and challenging task. Researchers often focus on optimizing one specific property, which can lead to the development of molecules that excel in that aspect while potentially overlooking other crucial features that are necessary for stable battery cycling. For example, the development of α-fluorinated sulfones as electrolyte solvents was driven by their exceptional anodic stability. [9] This stability is achieved by introducing strong electron-withdrawing trifluoromethyl groups directly attached to the sulfonyl group. However, while this electron-withdrawing effect enhances anodic stability, it can also significantly increase the reduction potential of α-fluorinated sulfones. This heightened reduction potential renders them unstable when used with graphite anodes, highlighting the delicate balance required in designing electrolyte solvents that perform well across various aspects of battery operation.In this presentation, we embraced the concept of a "golden middle way" when designing and synthesizing new electrolyte solvents. The recently developed β-fluorinated sulfone, TFPMS, doesn't claim to be the best in any single property, but it strikes a balance across various key characteristics. This equilibrium has proven to be highly effective in ensuring the long-term stability of high-voltage graphite||NMC622 full cells. Positioned at the β site of the sulfone molecule, the strong electron-withdrawing trifluoromethyl group renders β-fluorinated sulfone sufficiently stable against the high-voltage NMC622 cathode, even if it possesses a slightly lower oxidation potential compared to its α-fluorinated counterparts. Furthermore, its reduction potential is lower than that of α-fluorinated sulfone, making it considerably more stable when paired with the graphite anode. While it may not possess the same high lithium solvating power as the typical sulfone (EMS), the lithium solvating capacity of β-fluorinated sulfone falls somewhere in between, mitigating transition metal dissolution and deposition in the graphite||NMC622 full cell that utilizes a β-fluorinated sulfone-based electrolyte. As a result, the full cell equipped with TFPMS-based electrolyte demonstrates superior cycling performance, with a significantly higher average capacity than cells using regular sulfone or α-fluorinated sulfone-based electrolytes. In summary, our approach exemplifies the successful application of the "golden middle way" in designing and synthesizing new electrolyte solvents.Reference: Santhanam, R.; Rambabu, B., Power Sources 2010, 195, 5442.Lin, F.; Markus, I. M.; Nordlund, D.; Weng, T.-C.; Asta, M. D.; Xin, H. L.; Doeff, M. M., Commun. 2014, 5, 3529.Li, W.; Song, B.; Manthiram, A., Chemical Society Reviews 2017, 46 (10), 3006.Xu, J.; Lin, F.; Doeff, M. M.; Tong, W., Journal of Materials Chemistry A 2017, 5 (3), 874.Xia, J.; Petibon, R.; Xiong, D.; Ma, L.; Dahn, J., Journal of Power Sources 2016, 328, 124.Chen, S.; Wen, K.; Fan, J.; Bando, Y.; Golberg, D., Journal of Materials Chemistry A 2018, 6 (25), 11631.Zheng, J.; Lochala, J. A.; Kwok, A.; Deng, Z. D.; Xiao, J., Advanced Science 2017, 4 (8), 1700032.Yamada, Y.; Wang, J.; Ko, S.; Watanabe, E.; Yamada, A., Nature Energy 2019, 4 (4), 269.Su, C.-C.; He, M.; Redfern, P. C.; Curtiss, L. A.; Shkrob, I. A.; Zhang, Z., Environ. Sci. 2017, 10 (4), 900.

The development of energy storage devices that can efficiently operate at temperatures below 0 °C is necessary for both terrestrial and space-based applications. NASA has previously carried out pioneering work that has enabled Li-ion batteries to operate at temperatures down to -30 °C through the development of new carbonate electrolyte formulations. [1-2] However, secondary batteries that utilize lithium metal anodes instead of conventional graphite anodes could significantly boost energy density and specific energy, but the behavior of lithium electrodes at low temperatures are largely unknown. In this work, we investigate the effect of sub-zero temperatures on electrochemical behavior, morphological evolution, and solid-electrolyte interphase (SEI) formation of Li metal anodes in carbonate and ether solvents. Electrolytes made with solvents such as ethylene carbonate and ethyl methyl carbonate with ester co-solvents (e.g., ethyl propionate) exhibit sufficient conductivity and show effective Li deposition/stripping down to -30 °C, but with relatively low average Coulombic efficiencies (CEs) of ~70 % (similar to the room-temperature CE of ~73 %). The introduction of SEI-stabilizing additives such as fluoroethylene carbonate (FEC) boosts the average Coulombic efficiency across a range of temperatures (CE = ~94 %). Ether-based electrolytes, on the other hand, enable Li deposition/stripping down to -70 °C with high CEs (~95 %). Interestingly, investigation of the Li morphology after cycling revealed that distinct Li morphologies form at different temperatures, which may be related to differences in SEI chemistry and structure. These results provide important new information for engineering Li metal anodes for efficient operation under a variety of conditions.

Due to their high capacity, Silicon (Si) anodes are very promising for the next generation of lithium-ion batteries. However, Si materials suffer from a high volume expansion, which cause cracking and accumulated growth of the solid electrolyte interphase (SEI), resulting in low coulombic efficiencies and lifetime of the Si anodes. Applying additives in the electrolyte is a facile and straightforward approach to improve the composition of the SEI and reduce cracking. For Si-containing anodes, fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are considered the most effective electrolyte additives. The salt lithium difluorophosphate (LiDFP) is known to improve the interphases of cathode materials and graphite when applied as additive, but has been poorly investigated for Si anodes. In our study, we combine VC, FEC, and a variation of LiDFP concentrations to investigate the effect on Si-dominant anodes in detail. We conducted detailed electrochemical testing in half coin cells and Si/NCM pouch cells, including long-term cycling, rate capability tests, potential monitoring of the anode and electrochemical impedance spectroscopy. For the latter two, we applied a novel perforated reference electrode concept, which allows for broad and precise measurement in pouch cells, while avoiding a blocking effect of the lithium ions. Furthermore, post-mortem analysis was conducted to gain detailed insights into the SEI formation and microstructure influenced by the different additive concentrations.

Machine learning technique-based data-driven model of exploring effects of electrolyte additives on LiNi0.6Mn0.2Co0.2O2/graphite cell

As the lithium-ion battery (LIB) market for electric vehicles and grid-based energy storage systems is rapidly growing, the upcoming urgent challenge is to charge faster the LIB than ever. Thus, battery chemistry and reaction kinetics are being evolved toward the development of quickly charged LIBs in minutes scale, not in hours scale. In the LIB electrolyte perspective, commercial carbonate-based liquid electrolyte has multiple limitations in attaining high rate performance, due to the limited ionic conductivity and viscosity, sluggish de-solvation kinetics on graphite anode, low thermal stability, etc. To overcome those limitations, extensive research on new electrolyte systems (e.g., highly concentrated salt, aqueous system, etc.) have been reported. Herein, we present the improved rate capability and cycling performance of nickel-rich layered oxide-based lithium cell with our newly designed electrolytes. We are going to discuss electrode-electrolyte interface chemistry and its correlation to performance in this meeting. Acknowledgements This research was supported by the National Research Foundation grant funded by the Ministry of Science and ICT (No. 2019R1A2C1084024) and Creative Human Resource Development Consortium for Fusion Technology of Functional Chemical/Bio Materials of BK Plus program by Ministry of Education of Korea.

Graphite anode suffers from great capacity loss and larger cell polarization under low-temperature conditions in lithium-ion batteries (LIBs), which are mainly caused by the high energy barrier for the Li+ desolvation process and sluggish Li+ transfer rate across the solid electrolyte interface (SEI). Regulating an electrolyte with an anion-dominated solvation structure could synchronously stabilize the interface and boost the reaction kinetics of the graphite anode. Herein, a highly ionic conductive electrolyte consisting of a fully methylated cyclic ether solvent of 2,2,4,4,5,5-hexamethyl-1,3-dioxolane (HMD) and fluoroethylene carbonate (FEC) cosolvent was designed. The high electron-donating effect and steric hindrance of -(CH3)2 in HMD endow the HMD-based electrolyte with high ionic conductivity but lower coordination numbers with Li+, and an anion-dominated solvation structure was formed. Such configuration can accelerate the desolvation process and induce the forming of a LiF-rich SEI film on the anode, avoiding the solvent coembedding into graphite and enhancing the ion migration rate under low temperatures. The assembled Li||graphite cell with the tame electrolyte outperformed the conventional carbonates-based cell, showing 93.8% capacity retention after 227 cycles for the DF-based cell compared to 64.7% after 150 cycles. It also exhibited a prolonged cycle life for 200 rounds with 81% capacity retention under -20 °C. Therefore, this work offers a valuable thought for solvent design and provides approaches to electrolyte design for low-temperature LIBs.

Structural regulation chemistry of lithium-ion solvation in nonflammable phosphate-based electrolytes for high interfacial compatibility with graphite anode