Morphological Evolution and Solid-Electrolyte Interphase Formation on LiNi 0.6 Mn 0.2 Co 0.2 O 2 Cathodes Using Highly Concentrated Ionic Liquid Electrolytes.

Employing high-voltage Ni-rich cathodes in Li metal batteries (LMBs) requires stabilization of the electrode/electrolyte interfaces at both electrodes. A stable solid-electrolyte interphase (SEI) and suppression of active material pulverization remain the greatest challenges to achieving efficient long-term cycling. Herein, studies of NMC622 (1 mAh cm -2 ) cathodes were performed using highly concentrated N -methyl- N -propylpyrrolidinium bis(fluorosulfonyl)imide (C 3 mpyrFSI) 50 mol % lithium bis(fluorosulfonyl)imide (LiFSI) ionic liquid electrolyte (ILE). The resulting SEI formed at the cathode enabled promising cycling performance (98.13% capacity retention after 100 cycles), and a low degree of ion mixing and lattice expansion was observed, even at an elevated temperature of 50 °C. Fitting of acquired impedance spectra indicated that the SEI resistivity ( R SEI ) had a low and stable contribution to the internal resistivity of the system, whereas active material pulverization and secondary grain isolation significantly increased the charge transfer resistance ( R CT ) throughout cycling.