Oxygen Dimerization as a Defect-Driven Process in Ni-Rich Cathode Materials

Recent research points to significant oxygen involvement during the charging of nickel-based lithium-ion cathode materials [1, 2, 3]. Such activity is evidenced by incomplete oxidation from Ni³⁺ to Ni⁴⁺ in LiNiO2 at the top of charge [1], alongside resonant inelastic X-ray scattering (RIXS) spectra that indicate oxygen-redox activity and oxygen dimer formation [1, 3]. To explore the possibility of oxygen dimerization — particularly the formation of molecular oxygen-like species — in the bulk of LiNiO₂ lithium-ion cathodes materials at high states of charge, we conduct a redox-product structure search inspired by recent methodological developments for point defect structure prediction [4, 5, 6]. Utilising this novel methodology, we find that delithiated Li_{1 –x}NiO₂ (x = 1) has good kinetic stability towards decomposition into molecular oxygen and reduced transition metal oxides, but that point and extended defects can act as nucleation sites for oxygen dimerization. Certain defects leave local oxygen ions more susceptible to oxidation, these oxidised oxygen species then passivate their excess charge by rebonding and forming oxygen dimers. In this study we draw analogy to behaviours observed in lithium-rich cathode materials and explain how significant differences in observed electrochemistry of the stochiometric and Li-rich materials can be explained despite this highly related bulk reactivity. These results help reconcile conflicting reports on the formation of bulk molecular oxygen in LiNiO₂ and other nickel-rich cathode materials, highlighting the role of defect chemistry in driving the bulk degradation of these compounds. We will also discuss the acceleration of this structure search via fine-tuned machine-learned interatomic potentials.

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