Non-aqueous electrolyte secondary batteries represented by lithium secondary batteries have high electromotive force and high energy density. For this reason, non-aqueous electrolyte secondary batteries are used as the main power supplies of mobile communications equipment and portable electronic equipment. Further, demand for non-aqueous electrolyte secondary batteries as a memory backup power supply is increasing year by year. Non-aqueous electrolyte secondary batteries with higher energy density are strongly demanded so that equipment using those batteries has a smaller size and higher performance, and achieves being maintenance-free.
To meet this demand, silicon materials having a larger theoretical capacity than carbon material are drawing attention. The theoretical capacity of silicon that stores and emits lithium ions is larger than that of graphite, lithium metals, aluminum, or the like.
However, when crystalline silicon stores lithium ions during charging, it expands to have a volume four times as large as the original volume at the maximum. For this reason, when silicon is used as a negative electrode material, it is deformed by the volumetric change and pulverized, and thus the electrode structure is destroyed. To address this problem, Patent Document 1, for example, proposes a method of making a phase including silicon into an amorphous state, and using composite particles made of a crystalline alloy phase of silicon and a transition metal and amorphous phase of silicon as a negative electrode so that pulverization of silicon is inhibited. Patent Document 2, for example, proposes a method of forming thick oxide films on the surfaces of particles by heat treatment and improving the high temperature storage characteristics.
However, Patent Document 1 does not discuss the thickness of the oxide films on the surface thereof sufficiently. Patent Document 2 sufficiently discusses the thickness of the silicon oxide film, but does not discuss the thickness of the oxide film of the transition metal. Thus, these techniques cannot provide a negative electrode material with excellent overcharge-resistant characteristics, which is one of important characteristics of a memory backup power supply.
Patent Document 1: Japanese Patent Unexamined Publication No. 2004-335272
Patent Document 2: Japanese Patent Unexamined Publication No. 2004-319469