Résumé
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.