Eutectic electrolytes composed of trifluoroacetamides and lithium bis(fluorosulfonyl)amide

Abstract

Eutectic electrolytes with high Li-salt concentrations are promising candidates for next-generation batteries. Amide-based eutectic electrolytes have insufficient oxidative stability for use alongside high-voltage positive electrode materials in Li-ion batteries, and their solution structures remain unknown. In this study, we investigate the solution structures and physicochemical properties of eutectic electrolytes with different molar ratios of lithium bis(fluorosulfonyl)amide (LiFSA) and different trifluoroacetamide (TFAA) derivatives and demonstrate their electrochemical stabilities. The crystal structure analysis of LiFSA:TFAA = 1:1, performed for the first time in literature, reveals a hydrogen-bonding network. A LiFSA:N-methyltrifluoroacetamide (MTFAA) = 1:1.5 eutectic electrolyte exhibits reversible deposition and dissolution of Li and high electrochemical oxidative stability. A cell with LiNi_0.5Mn_1.5O₄ and graphite electrodes and a LiFSA:MTFAA = 1:1.5 eutectic electrolyte is stable for 100 charge/discharge cycles, that is the first example that charge and discharge of a high-voltage cathode material has been achieved in an electrolyte with hydrogen bonds without any additives. The findings will promote the development of eutectic electrolytes for applications in next-generation energy storage devices.