Charge-compensation in 3d-transition-metal-oxide intercalation cathodes through the generation of localized electron holes on oxygen

During the charging and discharging of lithium-ion-battery cathodes through the de-and reintercalation of lithium ions, electroneutrality is maintained by transition-metal redox chemistry, which limits the charge that can be stored. However, for some transition-metal oxides this limit can be broken and oxygen loss and/or oxygen redox reactions have been proposed to explain the phenomenon. We present operando mass spectrometry of O-18-labelled Li-1.2[Ni0.132+Co0.133+Mn0.544+]O-2, which demonstrates that oxygen is extracted from the lattice on charging a Li-1.2[Ni0.132+Co0.133+Mn0.544+]O-2 cathode, although we detected no O-2 evolution. Combined soft X-ray absorption spectroscopy, resonant inelastic X-ray scattering spectroscopy, X-ray absorption near edge structure spectroscopy and Raman spectroscopy demonstrates that, in addition to oxygen loss, Li+ removal is charge compensated by the formation of localized electron holes on O atoms coordinated by Mn4+ and Li+ ions, which serve to promote the localization, and not the formation, of true O-2(2-)( peroxide, O-O similar to 1.45 angstrom) species. The quantity of charge compensated by oxygen removal and by the formation of electron holes on the O atoms is estimated, and for the case described here the latter dominates.