A universal surface reduction passivation strategy towards stabilized high-voltage high-nickel layered oxide cathodes for lithium-ion batteries

High-voltage high-nickel lithium layered oxide cathodes have garnered extensive research interest and commercial adoption owing to their exceptional energy density. Unfortunately, the deep oxidation–reduction reaction caused by high-voltage high-nickel will produce a quantity of highly active and unstable Ni⁴⁺, which will aggravate interface side reactions such as oxygen evolution, phase transition, and electrolyte decomposition, thereby increasing interface impedance and reducing battery performance. Here, an H₂/Ar reducing atmosphere is used to form a thin rock salt passivation layer on the surface of LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) high-nickel cathode materials. The time-of-flight secondary ion mass spectrometry (TOF-SIMS) results indicate that electrolyte decomposition and metal ions dissolution are restrained. The battery in-situ differential electrochemical mass spectrometry (DEMS) results display that the production of CO₂ and O₂ gases is repressed. The density functional theory (DFT) calculation results also confirm that the interface lattice oxygen loss is suppressed. These results fully demonstrate that the surface passivation strategy greatly improves the electrode–electrolyte interface stability. At a high voltage of 4.5 V, the surface passivated NCM622 exhibits superior cycling stability (capacity retention rate for 100 cycles: 92.2% vs. 85.0%) and rate performance (output specific capacity at 5 C high current density: 148 mAh·g⁻¹ vs. 127 mAh·g⁻¹) compared to the pristine NCM622. Consequently, the surface passivation strategy treated with reducing substances is recommended to improve the electrode–electrolyte interface stability and further enhance the lithium storage performance of high-voltage high-nickel layered oxide cathodes.