Monofluorinated Cyclic Ethers Enable Fast Ion Transport in Low-Temperature Lithium Metal Batteries.

Lithium metal batteries (LMBs) are promising next-generation batteries because of their high energy density. To enable high lithium plating-stripping reversibility, electrolyte design is critical. State-of-the-art solvents are fluorinated linear ethers, which demonstrate good lithium metal compatibility due to the ether component and improved cathode compatibility due to fluorination. However, the fluorinated cyclic ether family, especially 3-position-functionalized tetrahydrofuran (THF), has yet to be investigated. Herein, we modified the structure of 3-methyl THF (3MTHF) through fluorination, designing electrolytes with improved lithium compatibility. Electrolytes using 3FTHF (3-fluorotetrahydrofuran) as the solvent enable 20 μm Li||LiFePO (LFP) full cell cycling with 80% capacity retention after 175 cycles. Moreover, the fast ion transport of this electrolyte enables it to maintain a higher capacity and better lithium plating/stripping reversibility at a current density of C/3 and -20 °C compared to 1,1,1-trifluoro-2-(2-(2,2-difluoroethoxy)ethoxy)ethane (F5DEE), one of the leading state-of-the-art electrolytes. By optimizing the physicochemical properties such as conductivity, solvation behavior, and oxidative stability, we designed a single-solvent, single-salt electrolyte with high lithium compatibility and conductivity for LMBs at ambient and low-temperature conditions.