Ta₂O₅ Coating as an HF Barrier for Improving the Electrochemical Cycling Performance of High-Voltage Spinel LiNi_0.5Mn_1.5O₄ at Elevated Temperatures

The high-voltage spinel LiNi0.5Mn1.5O4 cathode material suffers from the rapid degradation of electrochemical cycling performance at elevated temperatures, which prevents its successful commercialization. Herein, we show that coating the surface of this material with Ta2O5, which has high resistance against hydrofluoric acid (HF) attack, is an effective way to improve its electrochemical cycling performance. A Ta2O5-coated LiNi0.5Mn1.5O4 half-cell shows a capacity retention of ∼93% and a Coulombic efficiency of ∼98% after 100 cycles at 55 °C, compared to the corresponding values of ∼76% and ∼95% measured for the bare LiNi0.5Mn1.5O4 half-cell. The detailed structural analysis of the Ta2O5-coated LiNi0.5Mn1.5O4 shows that a small amount of Ta5+ ions diffuse into the 16c site on the cathode surface during the coating process, as directly observed by Cs corrected scanning transmission electron microscopy. The modification of the LiNi0.5Mn1.5O4 surface with Ta5+, together with the residual Ta2O5 coating, stabilizes the surface structure during cycling, leading to reduced Ni and Mn dissolution as well as formation of the solid electrolyte interface (SEI). In contrast, LiNi0.5Mn1.5O4 coated with HF scavengers, such as Al2O3, shows only limited improvement in cycling performance after prolonged cycling at 55 °C, due to the consumption of the surface coating by reaction with HF, which leaves LiNi0.5Mn1.5MnO4 unprotected against HF attack.