Enhancing Cycling Stability of Lithium Metal Batteries by a Bifunctional Fluorinated Ether

The lifespan of lithium (Li) metal batteries (LMBs) can be greatly improved by the formation of inorganic‐rich electrode‐electrolyte interphases (EEIs) (including solid‐electrolyte interphase on anode and cathode‐electrolyte interphase on cathode). In this work, a localized high‐concentration electrolyte containing lithium bis(fluorosulfonyl)imide (LiFSI) salt, 1,2‐dimethoxyethane (DME) solvent and 1,2‐bis(1,1,2,2‐tetrafluoroethoxy)ethane (BTFEE) diluent is optimized. BTFEE is a fluorinated ether with weakly‐solvating ability for LiFSI so it also acts as a co‐solvent in this electrolyte. It can facilitate anion decomposition at electrode surfaces and promote the formation of more inorganic‐rich EEI layers. With an optimized molar ratio of LiFSI:DME:BTFEE = 1:1.15:3, LMBs with a high loading (4 mAh cm−2) lithium nickel manganese cobalt oxide (LiNi0.8 Mn0.1 Co0.1) cathode can retain 80% capacity in 470 cycles when cycled in a voltage range of 2.8–4.4 V. The fundamental understanding on the functionality of BTFEE revealed in this work provides new perspectives on the design of practical high‐energy density battery systems. A localized high‐concentration electrolyte contains a fluorinated ether (1,2‐bis(1,1,2,2‐tetrafluoroethoxy)ethane (BTFEE)) with weakly‐solvating ability is optimized. BTFEE acts as a diluent for the salt‐solvent clusters and enhances anion decomposition kinetics, promoting the formation of inorganic‐rich electrode–electrolyte interphase (EEI) layers. It also partially decomposes and forms part of EEI layers, therefore improves the cycling stability of lithium metal batteries.