Role of Surface Acidity in the Surface Stabilization of the High-Voltage Cathode LiNi_0.6Mn_0.2Co_0.2O₂

Metal oxide coatings have been reported to be an effective approach for stabilizing cathode interfaces, but the associated chemistry is unclear. In this work, thin films of TiO₂, ZnO, and Cr₂O₃, which have different surface acidities/basicities, were used to modify the surface chemistry of LiNi_0.6Mn_0.2Co_0.2O₂ and study the acidity’s role in the cathode/electrolyte interphase composition and impedance under high-voltage cycling (4.5 V vs Li/Li⁺). Cathodes with more acidic surfaces provided higher initial specific capacity and capacity retention with cycling. More basic surfaces had higher initial impedance and greater impedance growth with cycling. These differences appeared to depend on the degree of LiPF₆ salt decomposition at the interface, which was related to acidity, with more neutral surfaces having a LiF/Li_xPO_yF_z ratio close to unity, but basic surfaces had substantially more LiF. This chemistry was more significant than the cathode electrolyte interphase (CEI) thickness as the more acidic surfaces formed a thicker CEI than the basic surface, resulting in better capacity retention. These results suggest that the Brønsted acidity of cathodes directly influences electrolyte degradation, ion transport, and thus, cell lifetime.