Low temperature synthesis of the lithium-rich cubic spinel Li4Mn5O12 with an oxidation state very close to +4 is described. The effect of doping this compound with cobalt according to the solid solution mechanism Li4−x Mn5−2x Co3x O12 up to the solid solution limit, LiMnCoO4, has also been explored. The evolution of the structure and electrochemistry with increasing cobalt doping is described. With increasing x there is increasing population of the tetrahedral 8a site by transition metal ions reaching 25% occupancy by Co at LiMnCoO4 with corresponding displacement of Li to 16d sites. The optimum performance was obtained for the composition Li3.75Mn4.5Co0.75O12 which yields a capacity of 150mAg−1 at a rate of 25mAg−1 (equivalent to C/6) with no perceptible capacity loss, in sharp contrast to the pure Mn materials.