To meet demands on lithium ion batteries for high power applications, the rate capability of cathode materials has to be greatly improved. In this study, a significant improvement of rate capability in LiNi0.5Mn1.5O4−x was obtained by the aid of elevated oxygen non-stoichiometries. Various oxygen non-stoichiometries were achieved through calcining LiNi0.5Mn1.5O4 above the onset temperature of oxygen loss at 700°C. No obvious second phases were observed in XRD patterns of as-synthesized LiNi0.5Mn1.5O4−x powders. However, the crystallographic structure transformation of LiNi0.5Mn1.5O4−x was observed in FTIR analysis as x increased from 0.015 to 0.025, accompanying the expansion in cell parameters. Mixed phases, composing of both cation-ordered phase (P4332) and cation-disordered phases (Fd-3m) at 700°C, transformed into the single phase (Fd-3m) at 730°C. Electrochemical test shows that LiNi0.5Mn1.5O4−x with lower oxygen non-stoichiometries displayed the deteriorated performance at elevated discharge rate. In contrast, LiNi0.5Mn1.5O4−x with higher oxygen non-stoichiometries demonstrated a significantly enhanced rate capability, attributing to the disordered structure (Fd-3m) and elevated Mn3+ contents. With respect to LiNi0.5Mn1.5O4−x (x =0.015), the discharge capacity of both LiNi0.5Mn1.5O4−x (x =0.025 and 0.033) cathodes greatly improved about 33% and 47% at 3C and 5C, respectively.