New NixMg6-xMnO8 mixed oxides as active materials for the negative electrode of lithium-ion cells

New mixed oxides with an NixMg6-xMnO8 (x = 0, 2, 4, 5 and 6) stoichiometry and NaCl-related structure have been prepared by heating mixed oxalate precursors at 600degreesC. The existence of a solid solution was confirmed by the decrease of the cubic unit-cell parameter when magnesium was substituted by nickel. The solids were used as electrode materials in lithium cells, which were potentiostatically cycled. The structural transformations during reaction with lithium have been studied by ex-situ X-ray diffraction. A maximum value of ca. 20 F mol(-1) is found during the first discharge of Ni6MnO8 electrodes. Annealing the noncrystalline discharged electrodes at 600degreesC allows to detect metallic nickel. While Ni5MgMnO8 improves capacity retention as compared with pure nickel, the extended substitution of nickel by magnesium leads to a progressive decrease in cell capacity. The reversible capacity values of < 640 mA h g(-1) are explained in terms of the formation of Ni oxide that reacts with lithium reversibly. The potential in which the cell current peaks occur decreases on increasing the Mg content. (C) 2002 Elsevier Science (USA).