Insights into the role of the covalent Ni–O bonds in LiNiO2 positive electrodes: a correlative hard X-ray spectroscopy study

Abstract

The interest in Ni-rich layered oxide positive electrode materials has been increasing due to their 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 a 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 Kβ X-ray emission spectroscopy (XES) spectra are combined to track the electronic environment of Ni. X-ray Raman scattering (XRS) spectra at the Ni L_2,3-edges and O K-edge provide direct bulk electronic information with regard 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.