Asymmetric Ether Solvents for High-Rate Lithium Metal Batteries

Recent electrolyte solvent design based on weakening lithium-ion solvation have shown promise in enhancing cycling performance of Li-metal batteries (LMBs). However, they often face slow redox kinetics and poor cycling reversibility at high rate. Here, we report using asymmetric solvent molecules significantly accelerates Li redox kinetics. Asymmetric ethers (1-ethoxy-2-methoxyethane, 1-methoxy-2-propoxyethane) showed higher exchange current densities and enhanced high-rate Li⁰ plating/stripping reversibility compared to symmetric ethers. Adjusting fluorination levels further improved oxidative stability and Li⁰ reversibility. The asymmetric 1-(2,2,2-trifluoro)-ethoxy-2-methoxyethane, with 2 M lithium bis(fluorosulfonyl)imide, exhibited high exchange current density, oxidative stability, compact solid-electrolyte interphase (~10 nm). This electrolyte exhibited superior performance among state-of-the-art electrolytes, enabling over 220 cycles in high-rate Li (50 μm)||NMC811 (4.9 mAh cm^-2) cells and for the first time over 600 cycles in anode-free Cu||Ni95 pouch cells (200 mAh) under electric vertical take-off and landing (eVTOL) cycling protocols. Our findings on asymmetric molecular design strategy points to a new pathway towards achieving fast redox kinetics for high-power LMBs.