Role of Li concentration and the SEI layer in enabling high performance Li metal electrodes using a phosphonium bis(fluorosulfonyl)imide ionic liquid

In this study the performance of the lithium (Li) anode is characterized in two alternative ionic liquid electrolytes: (i) a solution of 0.5 mol·kg^-1 of lithium bis- (fluorosulfonyl)imide (LiFSI) in trimethyl(isobutyl)- phosphonium FSI (P_111i4FSI) and (ii) an equimolar mixture of these two salts, effectively an inorganic-organic mixture IL. We have investigated the formation of the solid electrolyte interphase (SEI) at the lithium electrode and its influence on the polarization potential, the electrode surface impedance and deposition morphologies. Lithium metal cycling is revealed to be significantly more stable in the electrolyte with high lithium salt concentration due to the creation of a more uniform SEI. Stable and effective cycling was demonstrated at high applied currents (up to 12 mA·cm^-2) with large areal capacities being transferred with each polarization cycle (up to 6 mAh·cm^-2 at 50 °C). An average Coulombic efficiency of not less than 99.2% was demonstrated under these conditions and SEM observations of the cycled electrode surfaces show a uniform and compact deposit. Combined with spectroscopic characterization of the electrolyte and electrode surface, these observations indicate a role for the speciation and transport properties of these high concentration ionic liquid electrolytes in modifiying the physicochemical properties of the SEI which result in enhanced cycling performance of the Li metal electrode.