Résumé
The intimate correlation between the local atomic arrangement and electronic
states in Li-ion battery cathode materials plays a crucial role in determining
their electrochemical properties, including capacity, cycling stability, and
rate capability. Despite almost 30 years of research efforts on high
performance cathodes based on Ni rich layered oxides, there is still no
consensus on LiNiO2 local atomic and electronic structure. Ni sites could be
either Jahn-Teller distorted or bond disproportionated and the role of Ni and
oxygen in the charge compensation mechanism remains unclear. In this study, we
compare the local and electronic structure of LiNiO2 and NaNiO2, a long-range
Jahn-Teller system, using a novel approach which aims at correlating the
results from bulk spectroscopy techniques, particularly under operando
conditions, obtained on standard samples to ensure sample interoperability and
enhance the reliability and robustness of our results. Despite being a
site-selective and local technique, XAS is unable to discriminate between the
proposed scenarios, as confirmed also by theoretical calculations. On the
contrary, Raman spectroscopy show local structural differences between
monoclinic distorted NaNiO2 and rhombohedral LiNiO2. Additionally, HAXPES
confirms the presence of multiple formal oxidation states for Ni, and RIXS data
provides evidence of 3d8 states, confirming the negative charge transfer
character of Ni and some degree of bond disproportionation in LiNiO2. Regarding
the charge compensation mechanism, XRS and RIXS support the participation of
oxygen holes in the redox activity, while Raman spectroscopy does not detect
molecular oxygen. By combing several high-fidelity spectroscopy datasets, this
study shows the value of correlative characterization workflows to provide
insights into complex structural-electrochemical relationships.