Two compositional factors that control the lithium (de)intercalation capacity of nickel-doped Li2MnO3 have been identified. First is the Ni2+ content and (de)intercalation associated with the Ni2+/4+ redox-active couple. Second is the amount of oxygen deficiency achieved by preparing samples at different temperatures, together with creation of Mn3+ ions for charge balance and (de)intercalation associated with the Mn3+/4+ redox-active couple. Results demonstrating this effect are presented for composition Li1.93Mn 0.97Ni0.10O3-δ. The creation of an oxygen deficiency provides a novel method for enhancing the electrochemical performance of layered, rock-salt structured oxides.