Electron Delocalization Enables Sulfone-based Single-solvent Electrolyte for Lithium Metal Batteries.

Abstract

Li-metal batteries (LMB), although providing high energy density, face the grand challenge of identifying good electrolyte solvents for cycling. Common solvents are either stable against lithium metal anode or stable against LiNixMnyCo1-x-yO2 (NMC) cathode. There is significant effort trying to increase the cathode stability for ether electrolytes, which are in general stable against lithium metal anode. In comparison, there is much less effort trying to increase the anode stability of electrolytes that are stable against NMC cathode. An example is the sulfone solvent. It has good cathode stability but is hindered from practical application because of (1) high viscosity and poor wetting capability and (2) poor anode stability. Here, we solve these issues by modifying the sulfone molecules using resonance and electron withdrawing effect. The viscosity is significantly reduced by delocalizing the solvent electrons by introducing additional oxygen on the molecular backbone and applying appropriate fluorination. The resulting molecule 2,2,2-trifluoroethyl mesylate (TFEM) has decreased Lewis basicity and less reactivity toward Li+. The electrolyte based on TFEM as single solvent enables cycling of LMBs under harsh conditions of low N/P ratio (21 mg/cm2 NMC811 and 50 μm Li) with 90% capacity retention after 160 cycles at C/3 discharge rate.