Probing Oxygen Redox and Charging Mechanism of LiNiO₂

The demand of high-energy lithium-ion batteries boosts the interest in increasing Ni content in cathodes and/or raising charge cutoff voltage. In the drive to minimize the Co content from these cathodes, it is critical to understand the behaviour of the ultimate Ni-rich archetype, LiNiO₂. Substantial efforts have been made to understand the structural transitions of LiNiO₂when Li is extracted, but there remains considerable debate over the extent of Ni oxidation and O-redox in LiNiO₂, particularly across the voltage plateau at 4.2 V. Herein, we perform a structural and spectroscopic study of LiNiO₂ to probe the extent of O-redox and determine its mechanism. We employ combined neutron and synchrotron X-ray powder diffraction analysis that takes account of the stacking faults, and quantified the Ni vacancies formed when the material is charged across the voltage plateau. Chemical analysis shows that the Ni absent from the bulk does not leave the particles. While in principle there is sufficient Ni^3+/4+ oxidation capacity to account for all the Li⁺ removed on charge without O^2- oxidation, high resolution resonant inelastic X-ray scattering (RIXS) at the O K-edge confirms the presence of trapped molecular O₂, on charging across the plateau, which is accommodated Ni vacancy clusters. The atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) demonstrates the core-shell nature of the charged particles, with a Ni-rich Rocksalt like shell approx. 5 nm thick. Ni L-edge XAS measurements in fluorescence yield mode implies the average oxidation state of Ni of less than +4, coexisting with the presence of O-redox at the top of charge whilst it does not necessarily imply the latter commences on charging before Ni is oxidised to +4. This offers comprehensive insights of structural and chemical behaviours of highly charge LiNiO₂ in lithium-ion batteries.