A Novel High-Performing Ammonium Cation-Based Ionic Liquid Electrolyte for Advanced Lithium Metal Batteries

The continued development of ionic liquid electrolytes is a promising pathway toward enabling the safe operation of high-energy-density lithium metal batteries (LMBs), which incorporate a high-voltage cathode such as nickel manganese cobalt oxide (NMC). The physicochemical properties and LMB performance of an ionic liquid electrolyte using an ammonium-based organic ionic plastic crystal (OIPC), N-trimethyl-N-propylammonium bis­(fluorosulfonyl)­imide (N1113FSI), mixed with lithium bis­(fluorosulfonyl)­imide (LiFSI) salt are herein reported. Existing as an OIPC at room temperature, the melting temperature of N1113FSI is depressed by the addition of LiFSI. Interestingly, the resulting ionic liquid electrolyte possesses high ionic conductivity, relatively low viscosity, and faster lithium-ion diffusivity, as measured by pulsed-field gradient nuclear magnetic resonance spectroscopy, than other reported benchmark phosphonium and pyrrolidinium-based room-temperature ionic liquid electrolytes. The Li+ solvation structure was probed using molecular dynamics simulations, where the N1113FSI was found to occupy a higher fraction of the monodentate coordination environment than the pyrrolidinium-based IL, namely N-methyl-N-propylpyrrolidinium bis­(fluorosulfonyl)­imide (C3mpyrFSI), which is the suggested basis for the higher conductivity and relative lithium-ion diffusivity. Similarly, the high salt concentration ionic liquid electrolyte formed upon the addition of 50 mol % LiFSI to the N1113FSI shows promising behavior toward LMB operation. Lithium metal cycling in symmetrical cell configuration was performed at 2 mA/cm2|2 mAh/cm2 at 50 °C and was shown to be stable for 100 cycles. Full cell cycling was performed in coin cells using an NMC811 cathode cycled up to 4.3 V (versus Li+|Li) at 50 °C at C/2 (1 mA/cm2), showing 100 cycles with high cathode active material (∼12 mg/cm2) utilization.