Methyl exchange facilitates the collective dissolution of electrolyte additives for better lithium metal batteries

Abstract

The development of advanced lithium metal batteries (LMBs) is strongly dependent on the progress of electrolyte chemistry. Herein, we investigate the molecular-level interactions of 1-ethyl-3-methylimidazole dimethyl phosphate (EMDP) and methyltrifluoromethane-sulfonic acid (MeOTf) within carbonate electrolytes to enhance the electrochemical performances of LMBs. The electron-rich dimethyl phosphate anion displays a significant proclivity to react with the electron-deficient methyl group in MeOTf, thereby leading to rapid methyl exchange reaction (MER). This enables the collective dissolution of EMDP and MeOTf in the carbonate solvents. The in situ generated products not only function as film-forming additives, but also facilitate the dissolution of LiNO₃ due to the highly electronegative trifluoromethanesulfonate anion and the high donor number of trimethyl phosphate. Consequently, the trace addition-optimized electrolyte promotes the formation of a gradient heterogeneous electrode-electrolyte interphase (EEI), comprising an inner layer rich in inorganic compounds (such as Li₃N, Li₃P, Li₂S). The nitrogen/phosphorus-rich EEI not only renders the formation of a dendrite-free lithium anode with a high Coulombic efficiency of 99.19 %, but also improves the voltage tolerance of the carbonate electrolyte to 4.4 V with a capacity retention of 82.5 % after 250 cycles. The proposed MER represents a novel and practical strategy to diversify the electrolyte chemistry for LMBs.