In this study we have highlighted the role of point and extended defects in driving oxygen dimerization and molecular oxygen formation in the bulk of nickel-rich cathode materials. Uncoordinated oxygen ions at vacancies in the nickel layer at high states of charge are easily oxidized, and then these species will stabilize by dimerizing, passivating the excess charge. This mechanism appears to be applicable to extended defects, as shown via the calculations on the twin boundary structure shown in this work. Indeed other work simulating oxygen formation on the surface of NiO 2 has shown that it is the undercoordinated oxide ions on the surface layer that first oxidize, driving peroxide and then molecular oxygen formation that then evolves from the electrode . These results help reconcile conflicting reports of molecular oxygen formation in the bulk of nickel-based cathode materials while charging , and highlight the role of defect engineering in minimizing this behavior.