Depth-Resolving the Charge Compensation Mechanism from LiNiO2 to NiO2

The performances of lithium-ion batteries depend on the capability of the electrode materials to exchange lithium ions and electrons faster and reversibly. LiNiO₂ is a promising electrode candidate for achieving high voltage and capacity. However, its industrialization is hindered by surface and bulk instabilities. These instabilities are due to redox processes involving charge transfer between the cations and anions. Therefore, a fundamental understanding based on further experimental evidence is required to resolve the charge transfer between the cation and anion from the surface to the bulk in LiNiO₂. Herein, we resolve the roles of nickel and oxygen in the charge compensation process in Li_xNiO₂ electrodes from the extreme surface down to 30 nm by energy dependent core-level HAXPES supported by an ab initio simulation. We emphasize the central role of oxygen in the bulk charge compensation mechanism from LiNiO₂ to NiO₂ due to the negative charge transfer and bond/charge disproportionation characters of LiNiO₂.