Fig. 5: Partial reduction of O₂ trapped in voids to form Li-coordinated O²⁻ on the 100th discharge.

a,b, ¹⁷O NMR spectra isolating fast (a) and slow (b) relaxing ¹⁷O environments. The sharp peaks in a are assigned to trapped molecular O₂, which decrease in intensity on discharge. There is still evidence of some residual molecular O₂ in the discharged sample, δ_cg(¹⁷O₂) = 2,770 ppm. In b the slow relaxation ¹⁷O is dominated by oxide environments coordinated to paramagnetic TM ions (TM–O²⁻), δ_cg = 2,100–2,300 ppm. After discharge, a new ¹⁷O environment is formed corresponding to oxide surrounded by Li (that is, Li–O²⁻) created by the reduction of O₂ in the voids and reinsertion of Li⁺ into the voids coordinated by the O²⁻, centred at δ_cg = 0 ppm. D₁, relaxation delay. c, Large voids accommodating O₂ are partially repopulated by Li⁺ on discharge. Most O₂ is reduced to O²⁻ but some residual O₂ remains.