Cathode materials with both high energy density and high power capability are in great demand to improve the performance of lithium ion batteries and expand the driving range of electric vehicles. Of particular interest are transition metal oxide-based cathodes. However, electrochemical performance of these cathode materials is very sensitive to the synthetic conditions and, particularly, the oxygen non-stoichiometry that is induced during high temperature synthesis and post treatment. This review highlights the critical roles of oxygen non-stoichiometry in high-energy-density cathode materials including high voltage spinet LiNi0.5Mn1.5O4, Ni-rich layered LiNixMnyCozO2 (NMC, x >0.5), and the Li-rich, Mn-rich layered cathode, with the aim to provide a fundamental understanding on the effects of the oxygen non-stoichiometry governing the crystalline structure, electrochemical performance, and thermal stability of different cathode materials. This review also offers perspectives and directions on how to best utilize oxygen non-stoichiometry in the future development of high-energy-density cathode materials for lithium ion batteries. (C) 2016 Elsevier Ltd. All rights reserved.