Ethyl 2-(2-fluoroethoxy)ethyl carbonate as a new electrolyte additive for high-voltage Li-metal batteries

The combination of a high-Ni LiNi_xMn_yCo_1−x−yO₂ (NMC) cathode and Li metal anode is currently considered the most promising candidate for high-energy-density Li-metal batteries. However, undesired parasitic reactions in LiPF₆-based carbonate electrolytes hinder their further application. Herein, we report a new fluorinated linear carbonate additive, ethyl 2-(2-fluoroethoxy)ethyl carbonate) (EFEEC), to a lithium bis(trifluoromethanesulfonyl) imide–lithium bis(oxalate)borate-based dual-salt electrolyte system which is compatible with high-Ni NMC and Li metal electrodes. We showed that in our electrolyte with well-controlled lowest unoccupied molecular orbital/highest unoccupied molecular orbital levels, robust and stable solid electrolyte interphases are formed on both the cathode and anode synergistically, which enables neat and dense lithium metal deposition as well as the structural stability of NMC622 operated under a 4.5 V cut-off voltage. The use of the dual-salt-based electrolyte with the EFEEC additive improved cycle performance with high-capacity retention (76.0% at 200 cycles) even when operated under high voltage.