Temperature-Dependent Resonant Inelastic X-ray Scattering at Ni L3-Edge for NaNiO2 and LiNiO2

Abstract

LiNiO₂ is a promising cathode material for Li-ion batteries, but its atomic and electronic structure is under debate. Indeed, two sets of Ni–O distances are identified from local structural probes that are related to either Jahn–Teller distortion or bond disproportionation of NiO₆ octahedra. Moreover, LiNiO₂ undergoes a monoclinic to rhombohedral transition at 200 K, the origin of which is still unclear. On the other hand, isostructural NaNiO₂ shows differences from LiNiO₂, as it is a well-known Jahn–Teller distorted system, and it undergoes a monoclinic to rhombohedral transition at 500 K associated with the loss of the Jahn–Teller distortion. To understand better these differences, we report herein Ni L₃-edge resonant inelastic X-ray scattering experiments on LiNiO₂ and NaNiO₂ at different temperatures (25–520 K) and follow the spectral changes below and above the phase transition temperatures. The observed RIXS spectra and the mapping indicate strong spectral changes for NaNiO₂, confirming the disappearance of Jahn–Teller distortion during phase transition, while the changes are minor for LiNiO₂, suggesting very few modifications in the local structure. Theoretical simulations of RIXS spectra are required for further understanding; however, we believe that the reported data set can be a crucial resource for developing advanced simulations that are essential for deepening our understanding of the atomic and electronic structure of these nickelates.