The cathode materials Li1−x [Ni0.5Mn1.5]O4 prepared by coprecipitation from acetate solution by oxalic acid and annealing at 900°C in air had the preferred disordered Ni and Mn on the 16d octahedral sites of a spinel F d 3 ¯ m structure. The coprecipitation method provides better crystallinity than the F d 3 ¯ m phase previously obtained by quenching from the melt. Polycrystalline octahedral-shaped particles with smooth surfaces contained trace amounts of a Li y Ni1−y O impurity that introduced some Mn(III) into the spinel phase. Half-cells cycled at 0.2C rate between 3.5 and 4.8V versus Li exhibited a flat voltage V ≈4.7V with a small step at x ≈0.5 and a capacity at room temperature of 130mAhg−1 that showed no fade after 50 cycles. A small capacity fade was initiated with a cut-off voltage ≥4.9V; a significant capacity loss between 2 and 5C cycling rates was reversible to 134mAhg−1 on returning to 0.1C after 50 cycles at 10C between 3.5 and 5.0V.