Possessing from several times to more than ten times the theoretical capacity of graphite and offering the advantage of stable supply, silicon-based negative electrodes are considered the strongest candidates to be employed as the negative electrodes in next-generation lithium-ion secondary batteries. Among these, SiO is a promising active material, due to its theoretical capacity of 2007 mAh/g, as compared to 372 mAh/g for graphite. SiO is easily vaporized under conditions of high temperature and reduced pressure. It offers the advantage of allowing nanometer-scale SiO thin films to be formed on substrates via vapor deposition. However, due to its low electrical conductivity, there has previously been no concept of directly using a thin film formed by vapor deposition as an electrode.

For the developed negative electrode for lithium-ion secondary batteries, a nanometer-scale thin SiO film was formed on a conductive substrate using vapor deposition and a conductive additive was then layered on this thin film. The new electrode achieves 2000 mAh/g, approximately five times 372 mAh/g for a graphite negative electrode, corresponding almost exactly to the theoretical capacity of SiO (2007 mAh/g). In addition, the developed electrode maintained its capacity even after more than 500 charge-discharge cycles, clearly demonstrating that it displays long life at high capacity. This can be expected to increase the energy density of negative electrodes, and advance the realization of increased capacity and reduced size in lithium-ion secondary batteries.

This electrode requires a large capacity in the 1st charging process. If it is used as is, the lithium of the positive electrode will be consumed and the performance will immediately drop. To avoid this problem, the electrode must be pre-doped with lithium before a battery is fabricated by combining a positive electrode and this one. The performance verification is underway with the objective of practical application.