Insights on the role of the covalent Ni-O bonds in LiNiO2 positive electrodes: A combined hard X-ray spectroscopy study

Abstract: The interest in Ni-rich layered oxide positive electrode materials has been increasing due to its wide applicability particularly in electric vehicles as high capacity and high energy density electrode materials. However, the Ni-O bond array which builds the overall framework and plays a critical role in the charge compensation mechanism of the material requires deeper understanding. This work presents a correlative approach elucidating the role of the local highly covalent Ni-O bonds in LiNiO2 (LNO) model material. Pristine and electrochemically obtained LNO positive electrodes are analyzed using ex situ X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) to compare the average and local structural evolution upon Li+ ion de-intercalation. Insights from Ni K-edge X-ray absorption near-edge structure (XANES) and non-resonant Ni Kbeta X-ray emission spectroscopy (XES) spectra are combined to track the electronic environment of Ni. X-ray Raman scattering (XRS) spectra at the Ni L2,3-edges and O K-edge provide direct bulk electronic information with regards to the interplay between Ni 3d and O 2p states. The overall findings imply that O plays a significant role in the charge compensation process, contributing to the substantial negative charge transfer from the O 2p orbitals, because of the covalency in the Ni-O bonds inside the NiO2 framework within the edge-sharing NiO6 octahedra. The utilization of complementary X-ray spectroscopy techniques clarifies the intricate electronic environment of LNO, which is helpful in understanding Ni-rich positive electrode materials and offering new insights into their covalent nature.