Morphological Evolution and Solid–Electrolyte Interphase Formation on LiNi_0.6Mn_0.2Co_0.2O₂ 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 (C3mpyrFSI) 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 (RSEI) 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 (RCT) throughout cycling.