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

Abstract

• Molecular O 2 trapped in the bulk is the main oxidized oxygen species in Li 3 d oxide cathodes • The suppression of out-of-plane cation migration does not refrain the formation of molecular O 2 • The local structural reversibility on oxygen redox depends on the inhibition of cation disorder Li-excess 3 d 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 3 d oxide cathodes on oxygen redox, including O3-Li 1.2 Ni 0.2 Mn 0.6 O 2 and Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 , O2-Li 1.033 Ni 0.2 Mn 0.6 O 2 , and disordered rocksalt Li 1.2 Ti 0.4 Mn 0.4 O 2 . The results substantiate the coincident formation of molecular O 2 trapped in the bulk of charged cathodes, which can be reduced back to O 2− 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 O 2 . 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 3 d transition metal oxide cathodes. The coincident formation of trapped molecular O 2 on charging in four archetypical oxygen-redox-active Li 3 d oxide cathodes was certified by advanced EPR spectroscopy.