Defect chemical studies on oxygen release from the Li-rich cathode material Li1.2Mn0.6Ni0.2O2−δ

Oxygen release from oxide-based cathode materials is a key phenomenon for the realization of high performance and highly reliable next-generation batteries, because it can be a trigger for a thermal runaway and closely related to electrochemical performances. In this study, the mechanism of oxygen release from Li1.2Mn0.6Ni0.2O2−δ and the corresponding electronic and crystal structural changes were studied. Li1.2Mn0.6Ni0.2O2−δ showed oxygen deficient nonstoichiometry until δ ≈ 0.042, and further oxygen extraction resulted in the reductive decomposition to MnNi6O8 and Li–Mn enriched Li(Li,Mn,Ni)O2−δ′. The oxygen vacancy formation mechanism was investigated by the defect chemical and thermodynamic analyses, and the oxygen vacancy formation energy was calculated from the nonstoichiometric data (ca. 2.03 eV). It was clearly confirmed that the lattice parameters and the distances of Mn–O and Ni–O were increased by the oxygen vacancy formation, which is known as the reduction expansion in nonstoichiometric compounds. Cooperative reduction of Ni, Mn and O due to the oxygen vacancy formation was observed from Ni-L, Mn-L and O-K edge X-ray absorption spectra. The charge compensation of the oxygen vacancy formation was maintained mainly by the reduction of Ni3+ to Ni2+ and mildly by the reduction of Mn4+ and O2−.