Morphological Evolution and Solid–Electrolyte Interphase Formation on LiNi<sub>0.6</sub>Mn<sub>0.2</sub>Co<sub>0.2</sub>O<sub>2</sub> Cathodes Using Highly Concentrated Ionic Liquid Electrolytes
Meisam Hasanpoor, Damien Saurel, Rosalía Cid, Kilian S. Fraysse, María Echeverria, María Jáuregui, Francisco Bonilla, George W. Greene, Robert Kerr, Maria Forsyth, Patrick C. Howlett
Abstract
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.