Combined data of powder XRD and XANES spectroscopy demonstrate that spinel solid solutions of general formula Li2Co1+XMn3-XO8 form in air at 600 degrees C, over the range -1 less than or equal to X less than or equal to 1, with the structural formula, Li-8a(+)[Co1/2+X/23+Mn1/2-X/23+Mn4+](16d)O-4, in space group Fd3m. Impedance measurements on pellets of Li2Co1+XMn3-XO8 with blocking electrodes show fast electronic conduction which may mainly occur between Mn3+/Mn4+ situated in adjacent octahedral 16d sites. Electrochemical extraction of lithium that resides in tetrahedral 8a sites takes place reversibly in cells, Li/LiPF6, propylene carbonate/Li2Co1+XMn3-XO8, with moderate cycling stability, initially at the plateau centred on ca. 4.0 V, along with the oxidation of Mn3+ to Mn4+, then at the second plateau centred on ca. 5.1 V, along with the oxidation of Co3+ to Co4+. When LiCo0.8Mn1.2O4 and LiCoMnO4 are incorporated as the cathode, the plateau centred on 5.0 V initially shows a discharge capacity of ca. 95 mA h g(-1), and thus has higher energy density than that of LiMn2O4, the cathode material used in the state-of-the-art cells.