TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The oxide is cobalt oxide. The element included in the oxide is reduced to have a valence of less than or equal to 1 by electrochemical oxidation reduction reaction with lithium. The silicon is bonded in a state of silicon dioxide (SiO2). The carbon is uniformly distributed on the surface of the oxide. The element ratio of the silicon on the surface of the oxide is 3-90%. The average particle size of the substance is 0.005-1 mu m. EXAMPLE - Cobalt oxide (II, III) having an average particle size of 1 mu m and cobalt oxide having an average particle size of 0.01 mu m was used for making test substance. Thereafter, the cobalt oxide powder was treated with an silicon (Si) compound namely hexyltrimethoxysilane ((CH3O)3SiC6H13). The treatment with the Si compound was conducted by a wet process. Specifically, the cobalt oxide powder was stirred in a 1.5 wt.% aqueous solution of hexyltrimethoxysilane at room temperature for 10 hours, filtered, and then dried at 110 degrees C. Thus the test substance composed of cobalt oxide and Si chemically bonded to the surface of the oxide was obtained. The bonding state on the particle surfaces of the substance was Co-O-Si-R. A comparative substance was obtained in the same manner, except that a cobalt oxide having an average particle size of 0.01 mu m was used, and the Si compound treatment was not conducted. The bonding states on the particle surfaces of the substance were Co-O and Si. The ratio of Si on the surface was less than or equal to 1%, which was an impurity level. Batteries were made using the test and comparative substances, and tested for charge/discharge test. The test battery showed initial discharge capacity of 762 mAh/g, and discharge capacity retention of 93%, whereas the comparative battery showed initial discharge capacity of 776 mAh/g, and discharge capacity retention of 87%. Thus, test substance treated with the silicon (Si) compound showed better discharge capacity retentions than Comparative untreated with the Si compound.