Achieving Long-Term Stability in High-Voltage LIBs with Fluoroaromatic Co-Solvent and Organosulfur Electrolytes

Lithium-ion batteries (LIBs) are widely used in consumer electronics and electric vehicles due to their long cycle life and high energy density. [1-2] Recent efforts to enhance energy density include increasing cell voltage, developing high-lithium-content materials, and optimizing active material loading. [3-4] Among high-voltage cathodes, LiNi₀.₈Mn₀.₁Co₀.₁O₂ (NMC811) is promising but suffers from surface reduction and structural changes at voltages above 4.3 V vs. Li/Li⁺. [5-6] To improve performance under demanding conditions, alternative electrolyte compositions are explored. Sulfone-based solvents show oxidative stability and high dielectric constants but suffer from high viscosity and poor passivation on graphite anodes. [7-8] Addressing these issues requires thick separators and co-solvents like FEC for SEI formation. However, diluting solvents are still needed to reduce viscosity without compromising lithium solvation.[9-10] In this study, we focus on clarifying the structure-activity relationships and evaluating alternative non-solvating co-solvents for sulfone-based electrolyte systems. Fluorinated ethers with fluoroalkyl groups attached to just one side of the molecule, such as 1,1,2,2-tetrafluoroethyl n -propyl ether (TPE), are inadequate for high-voltage applications (> 4.6V vs Li/Li + ) because the unprotected alkyl group is prone to unwanted reactions with reactive oxygen species produced by the NMC cathode at higher voltages. As a result, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTPE) and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TTEE) stand out as more effective co-solvents for sulfone electrolytes under high-voltage conditions. Interestingly, our studies show that fluorobenzene (FB), though not a fluorinated ether, outperforms both TTEE and TTPE as a co-solvent. FB is more accessible, less toxic than fluorinated ethers, and provides substantial improvements in electrochemical performance. The electrolyte incorporating FB exhibits the lowest viscosity and highest ionic conductivity of all tested formulations, along with enhanced lithium mobility, making it highly compatible with high-loading systems. These properties align closely with cycling performance, where the FB-based electrolyte delivers low polarization in the first cycle, excellent capacity utilization, and improved capacity retention, while electrolytes with fluorinated ether diluents underperform. Additionally, FB demonstrates superior chemical stability compared to ethers when exposed to reactive singlet dioxygen—a byproduct of layered metal oxide breakdown at high voltages—while maintaining adequate stability against the high-voltage cathode. Thus, employing FB as a non-solvating dilutant enhances the electrolyte’s physical characteristics and helps mitigate side reactions, ultimately boosting electrochemical performance. Reference: Ji, H.; Wu, J.; Cai, Z.; Liu, J.; Kwon, D.-H.; Kim, H.; Urban, A.; Papp, J.K.; Foley, E.; Tian, Y.; Balasubramanian, M.; Kim, H.; Clément, R.J.; McCloskey, B.D.; Yang, W.; Ceder, G. Energy 2020, 5 (3), 213-221. Hu, G.; Huang, P.; Bai, Z.; Wang, Q.; Qi, K. eTransportation 2021, 10 , 100140. Shen, Y.; Wang, L.; Jiang, J.; Wang, D.; Zhang, D.; Yin, D.; Wang, L.; Zhang, X.; Huang, G.; Cheng, Y. Chem. Eng. 2023, 454 , 140249. Heubner, C.; Nikolowski, K.; Reuber, S.; Schneider, M.; Wolter, M.; Michaelis, A. Batteries & Supercaps 2020, 4 (2), 268-285. Oswald, S.; Gasteiger, H.A. Electrochem. Soc. 2023, 170 (3), 030506. Jung, R.; Metzger, M.; Maglia, F.; Stinner, C.; Gasteiger, H.A. Electrochem. Soc. 2017, 164 (7), A1361-A1377. Ugata, Y.; Chen, Y.; Miyazaki, S.; Sasagawa, S.; Ueno, K.; Watanabe, M.; Dokko, K. Chem. Chem. Phys. 2023, 25 (43), 29566-29575. Jia, H.; Xu, Y.; Zou, L.; Gao, P.; Zhang, X.; Taing, B.; Matthews, B.E.; Engelhard, M.H.; Burton, S.D.; Han, K.S.; Zhong, L.; Wang, C.; Xu, W. Power Sources 2022, 527 , 231171. Ren, X.; Chen, S.; Lee, H.; Mei, D.; Engelhard, M.H.; Burton, S.D.; Zhao, W.; Zheng, J.; Li, Q.; Ding, M.S.; Schroeder, M.; Alvarado, J.; Xu, K.; Meng, Y.S.; Liu, J.; Zhang, J.-G.; Xu, W. 2018, 4 (8), 1877-1892. Moon, J.; Kim, D.O.; Bekaert, L.; Song, M.; Chung, J.; Lee, D.; Hubin, A.; Lim, J. Commun. 2022, 13 (1), 4538.