A key technology to improve the cyclic performances of carbonaceous materials for lithium secondary battery anodes

Any carbonaceous material does not necessarily provide a favorable surface fora smooth electrochemical lithium doping/undoping reaction because the carbon surface is covered with a very thin inhibiting surface layer. Especially, carbonaceous materials prepared at lower temperatures exhibit very poor charge/discharge cycleability. Removing such a surface layer was found quite effective to provide very favorable electrode surfaces for the doping/undoping reaction. Simple heating of the pristine sample in vacuum was effective, but a more effective and versatile method was a mild oxidation treatment at a moderate temperature only for several minutes. Upon being treated with the mild oxidation, the cycleability as well as high rate capacity of carbonaceous materials were improved to a great extent. A trace amount of water contained in nonaqueous electrolyte was also found to deteriorate the electrochemical properties, which was ascribed to the formation of retarding surface film such as LiOH through the reduction reaction with lithium cation and water during charging.