Crucial role of Ni-doping to interfacial Li2MnO3 layer of High-performance Ni-rich layered cathode in Lithium-Ion batteries

The surface instability of Ni3+ in Ni-rich layered oxide cathode materials is recognized as an obstacle in high-energy–density lithium-ion batteries. Researchers have previously attempted to solve this issue using a protective layer with a stable substance. Despite the popularity Ni-rich layered oxides, their exceptionally unstable surface has not been investigated comprehensively. Ni-rich layered oxides include lithium impurities and have a fragile surface, forming a NiO bi-phase. In this study, we perform Li2MnO3 coating to enable Ni doping via simple stirring and heat treatment combined, while considering the surface states of Ni-rich layered oxide, where lithium impurities are inevitable and a NiO bi-phase may exist. It is discovered that the tailoring interface consuming surface NiO is critical for mitigating the surface resistance. Among the samples with Li2MnO3 coating, only the sample prepared via 800 °C heating indicates the presence of Ni-doped Li2MnO3 based on electrochemical de-lithiation at 4.65 V vs. Li/Li+. It is effective in reducing NiO and stabilizing the surface for a high cycle life of 88.3% at the 100th cycle and a high rate capability of 76.9% at 5C, whereas a Li2MnO3-coated sample exhibits a cycle life of 70.4% at the 100th cycle and a rate capability of 29.1% at 5C. The surface is investigated via X-ray photoelectron spectroscopy, scanning transmission electron microscopy, and time-of-flight secondary ion mass spectroscopy analyses.