Demethylation of Fluorine-Free Ethers to Reconcile Li⁺ Transport Kinetics and Oxidation Stability

Li⁺ transport kinetics and oxidation stability are often mutually constrained in fast-charging and low-temperature electrolyte design. Existing strategies rely heavily on fluorinated solvents, diluents, and additives to form inorganic fluorine-rich interphases that facilitate interfacial Li⁺ desolvation and suppress electrode and electrolyte degradation. However, these approaches often raise environmental and cost concerns while limiting the salt dissociation and Li⁺ conduction in bulk electrolytes. Herein, we report that demethylation of fluorine-free ethers simultaneously reduces steric hindrance and weakens hyperconjugative radical stabilization, thereby enabling fast Li⁺ conduction and desolvation while preserving graphite structural integrity and enhancing oxidation stability. This strategy is the inverse of the widely adopted methylation approach, where methyl substitution reduces reactive α-H sites to achieve weakly solvating electrolytes and enhanced oxidation resistance. As a proof of concept, stepwise demethylation from 2,5-dimethyltetrahydrofuran (2,5-THF) to 2-methyltetrahydrofuran (2-THF) and ultimately to tetrahydrofuran (THF) facilitates the dissociation of lithium bis(fluorosulfonyl)imide (LiFSI) and improves the inherent molecular stability. Moreover, the effective inorganic fluorine-rich interphase formed at both negative and positive electrode surfaces ensures fast Li⁺ desolvation and electrochemical stability. The 1 Ah graphite∥LiNi_0.8Co_0.1Mn_0.1O₂ pouch cell with the formulated electrolyte retains 85.14% capacity at 1C (1000 cycles) under room temperature and 80.31% at 0.2C (200 cycles) under -20 °C. Our demethylation strategy expands molecular design principles of electrolyte solvents, advancing the development of robust lithium-ion batteries capable of operating under extreme conditions.