Does Trapped Oxygen Form in the Bulk of LiNiO2 during Charging?
Authors: Juelsholt Mikkel , House Robert , Bruce Peter 
Published: December 2024

LiNiO₂ remains a critical archetypal material for high energy density Li-ion batteries, forming the basis of Ni-rich cathodes in use today.1-4 Nevertheless, there are still uncertainties surrounding the charging mechanism at high states of charge and the potential role of oxygen redox.5-11 Substantial efforts have been made to understand these phenomena and the structural transitions that take place when Li is extracted from LiNiO₂, but there remains considerable debate over the extent of Ni oxidation and O-redox in LiNiO₂.4-11 Recent research into O oxidation in Li-rich cathodes, such as Li1.2Ni0.13Co0.13Mn0.54O2, has indicated that oxidised oxygen takes the form of molecular O2, which is trapped within vacancy clusters in the cathode structure.12 However, in the case of stoichiometric materials like LiNiO2, it has been argued that this same mechanism cannot apply due to the lack of transition metal vacancies in the fully dense transition metal layers (in the Li-rich materials the Li in the transition metal layers are removed on charge and the remaining vacancies cluster to accommodate the O2.11

In this study we show that oxidation of the oxide ions forms O₂ trapped in the particles and is accompanied by the formation of 8% Ni vacancies on the transition metal sites of previously fully dense transition metal layers. High resolution resonant inelastic X-ray scattering (RIXS) at the O K-edge confirms the presence of trapped molecular O₂, corresponding to approximately 2% of the O in the material. We employ a combination of neutron and synchrotron X-ray powder diffraction analysis that takes account of the stacking faults prevalent in these charged materials, showing that 8% Ni vacancies form in the originally fully dense transition metal layer. Such Ni vacancy formation on charging activates O-redox by generating non-bonding O 2p orbitals and is necessary to form vacancy clusters to accommodate O₂ in the particles. Ni accumulates at and near the surface of the particles on charging, forming a Ni-rich shell approximately 5 nm thick; enhanced by loss of O₂ from the surface. Our study shows the mechanism of O-redox involving the formation of trapped O₂ in the bulk is general to both Li-rich materials and LiNiO₂ despite the latter possessing fully dense transition metal layers in the pristine state.
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Figure 1