Coincident formation of trapped molecular O₂ in oxygen-redox-active archetypical Li 3d oxide cathodes unveiled by EPR spectroscopy

Li-excess 3d transition metal oxides with additional capacity contribution via oxygen redox are promising high-energy-density cathodes for next-generation Li-ion batteries. However, the chemical state of oxidized oxygen in the bulk of charged materials has been manifested very challenging to clarify and remains elusive. We herein apply the electron paramagnetic resonance (EPR) spectroscopy to uncover the oxidized oxygen states formed in the bulk of archetypical Li 3d oxide cathodes on oxygen redox, including O3-Li_1.2Ni_0.2Mn_0.6O₂ and Li_1.2Ni_0.13Co_0.13Mn_0.54O₂, O2-Li_1.033Ni_0.2Mn_0.6O₂, and disordered rocksalt Li_1.2Ti_0.4Mn_0.4O₂. The results substantiate the coincident formation of molecular O₂ trapped in the bulk of charged cathodes, which can be reduced back to O²⁻ on discharging. The implication is that in contrary to the conventional wisdom, the suppression of out-of-plane cation migration does not refrain the formation of molecular O₂. Moreover, the NMR study suggests that the local structural reversibility on oxygen redox depends on the inhibition of cation disorder rather than the formation of specific oxidized oxygen. This study advances our basic understanding of oxygen redox in Li-excess 3d transition metal oxide cathodes.