Fluorinated Cyclic Ether Co-solvents for Ultra-high-Voltage Practical Lithium-Metal Batteries

Abstract

The advancement of liquid electrolytes in Li-metal batteries is an important strategy to realize a robust and uniform solid electrolyte interphase (SEI) on the Li metal anode while simultaneously achieving high oxidative stability and addressing the shortcomings of traditional carbonate and ether-based electrolytes suffering from side reactions and high-voltage decomposition, respectively. To circumvent these challenges, here, a fluorinated dioxolane-based cyclic co-solvent, that is 2-(2,2,2-trifluoroethoxy)-4-(trifluoromethyl)-1,3-dioxolane (TTD), is developed. After pairing 1.5 M lithium bis(fluorosulfonyl)imide (LiFSI) with TTD and 1,2-dimethoxyethane (DME), the 1.5 M LiFSI-8TTD-2DME electrolyte exhibits remarkable oxidation stability up to 6 V and a Coulombic efficiency of 99.4% over 210 cycles at 3 mA cm^–2 with a cut-off capacity of 3 mA h cm^–2 in the Li|Cu half-cell originating from efficient regulation of the electrolyte solvation structure and consequent anion-derived inorganic SEI layer formation. Full cells with advanced electrolytes, using 20 μm of Li foil paired with the NCM811 cathode by a negative and positive capacity ratio (N/P) of 2.5, achieve 75% capacity retention after 160 cycles at 0.5 C. Furthermore, even at an ultra-high charge cut-off voltage of 4.7 V, the Li|NCM811 full cell still realizes 80% retention at 0.5 C after 100 cycles.