A fundamental correlative spectroscopic study on Li${}_{1-x}$NiO${}_2$ and NaNiO${}_2$

The intricate relationship between local atomic arrangements and electronic states significantly influences the electrochemical properties of Li-ion battery cathode materials. Despite decades of investigation, a consensus regarding the local atomic and electronic structure of LiNiO${}_2$ remains elusive. This ambiguity stems from the potential distortion of Ni sites, either via Jahn-Teller (JT) distortion or bond disproportionation (BD), complicating the understanding of the charge compensation mechanism involving Ni and O. This study compares the structures of LiNiO${}_2$ and NaNiO${}_2$, a JT system, using an innovative approach that integrates bulk spectroscopy techniques on standardized interoperable samples for enhanced reliability. While X-ray absorption spectroscopy and theoretical calculations fail to differentiate between the proposed scenarios, Raman spectroscopy highlights local structural distinctions between monoclinic NaNiO${}_2$ and rhombohedral LiNiO${}_2$. HAXPES confirms various formal oxidation states for Ni, supported by RIXS data indicating 3d${}^8$ states, emphasizing negative charge transfer from Ni and some bond disproportionation in LiNiO${}_2$. Regarding charge compensation, XRS and RIXS suggest oxygen hole involvement in redox activity, whereas Raman spectroscopy does not detect molecular oxygen. This comprehensive spectroscopic analysis highlights the importance of correlative characterization workflows in elucidating complex structural-electrochemical relationships. (© 2024 Wiley‐VCH GmbH)