As an initial test of the generality of the mechanistic hypothesis proposed in this work and the structure search methodology, and also to aid in understanding the potential degradation mechanisms in LiNiO 2 , we note that any defect structure containing undercoordinated oxygen at high states of charge (ONi 3 -x V 3+x where x ≥ 1) should also spontaneously form oxygen dimers. To test this hypothesis, we apply our defect structure search methodology to a twin-boundary structure in NiO 2 . Twin boundary structures were recently proposed to be present in LiNiO 2 at a concentration high enough to contribute to the capacity . In the model proposed for these defect structures, when deilithiated, the oxygens at the bound- ary are left undercoordinated, ONi 2 V 4 , and as such, should form interesting test cases for the hypothesis that: (i) oxide ions that are undercoordinated to Ni are oxidised by the top of charge, and (ii) these O -species can then stabilize by dimerizing.