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. image