We demonstrate the effects of reduced cation ordering on the electrochemical properties of LiNi0.5Mn1.5O4. LiNi0.5Mn1.5O4 is treated with nitric acid to reduce the degree of cation ordering. Nitric acid treatment induces partial oxidation of Ni2+ to Ni3+ as confirmed by X-ray photoelectron spectroscopy (XPS). Thanks to this oxidation, the 16d octahedral site in LiNi0.5Mn1.5O4 is quite void of cation ordering and its structure is partially changed to more disordered spinel engaged in Fd3m. This structural change is accompanied by significantly enhanced electrochemical performances in rate capability as well as cyclic retention. Actually, nitric acid treated LiNi0.5Mn1.5O4 can deliver about 87% of the initial capacity (129 mAh g−1) even after 100 cycles even at 55 °C.