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 ¹⁸O^-labelled Li_1.2[Ni_0.13²⁺Co_0.13³⁺Mn_0.54⁴⁺]O₂, which demonstrates that oxygen is extracted from the lattice on charging a Li_1.2[Ni_0.13²⁺Co_0.13³⁺Mn_0.54⁴⁺]O₂ cathode, although we detected no O₂ 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 O₂^2- (peroxide, O朞 ~1.45 ? 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.

Nature Chemistry (2016) doi:10.1038/nchem.2471

http://www.nature.com/nchem/journal/vaop/ncurrent/full/nchem.2471.html