1. Field of the Invention
The present invention relates to a negative electrode for a non-aqueous secondary battery containing an intermetallic compound capable of occluding/desorbing lithium as an active material, and a non-aqueous secondary battery using the negative electrode.
2. Description of the Related Art
Recently, there is a rapid increase in demand for portable terminal equipment such as a mobile telephone, a notebook personal computer, and a personal digital assistant (PDA). Along with the miniaturization, weight reduction, and increase in functionality of such equipment, there also is a demand for an increase in energy density in a non-aqueous secondary battery used as a power source. However, the capacity of a currently commercialized carbon negative electrode has reached a value dose to a theoretical value, so that it is necessary to develop a negative electrode material with a higher capacity.
A non-aqueous secondary battery using a negative electrode containing Al, Si, Sn, and the like that are alloyed with Li during charging as active materials has been reported (see Solid State Ionics, 113-115, p. 57 (1998)). These active materials are considered as prospects as negative electrode materials due to their very high density of mass capacity and volume capacity, compared with those of a carbon negative electrode.
Among the above-mentioned metal, particularly, Sn has an electron conductivity, so that it is not necessary to add a conductive assistant. Therefore, Sn enables a thin film electrode to be produced by electroless plating, electroplating, sputtering, or the like, in addition to a conventional coated electrode. As a result, Sn is expected to achieve the remarkable enhancement of a battery capacity, improvement of cycle characteristics, simplification of a production process, and the like.
In the case of using pure Sn, pure Si, and the like as negative active materials, Li is inserted/desorbed efficiently with respect to the negative active materials during charging in an initial cycle period, whereby a high capacity is achieved. However, as a charging/discharging cycle proceeds, the capacity is decreased remarkably. The reason for this is as follows. The volume of the active material particles is greatly changed along with the insertion/desorption of Li, so that the active material particles are pulverized due to the expansion and shrinkage, whereby the electron conductivity in the electrode becomes insufficient. Thus, in order to enhance cycle characteristics, this problem needs to be solved.
As means for solving the above-mentioned problem, JP2001-256968A shows that a copper foil is plated with an alloy such as Sn-Ni, and this plating is used as an active material.
Furthermore, Journal of Power Sources (107, p. 48-55 (2002)) shows that a Sn thin film formed on a Cu foil by electroplating is heat-treated at a temperature in the vicinity of the melting point of Sn, whereby a thin film having a gradient structure is obtained in which Cu atoms and Sn atoms interdiffuse at an interface between Cu and Sn. More specifically, a collector reacts with a Sn thin film to form a Cu—Sn alloy having a laminated configuration of Cu/Cu3Sn/Cu6Sn5/Sn, etc., and the intermetallic compound is used as an active material. Cu6Sn5 occludes Li to form Cu and Li4.4Sn that are electron conductors during charging, and desorbs Li to return to Cu6Sn5 during discharging. Therefore, Cu6Sn5 enables repetitive charging/discharging (see Journal of Electrochemical Society, 147, p. 1658-1662 (2000)).
However, a Cu3Sn phase does not desorb Li after occluding Li. Therefore, Li to be occluded is not discharged, and consequently, reversible charging/discharging cannot be performed. Furthermore, a Sn phase has the following problem. The Sn phase is pulverized due to the repetitive charging/discharging to decrease cycle characteristics, and also functions as a catalyst for decomposing an electrolyte solution. Furthermore, it has been clarified that, in the case of the Sn phase, depending upon the selection of a material for a collector, an active material layer and a collector react with each other gradually along with the progress of a charging/discharging cycle, which degrades the characteristics of an electrode. In order to further enhance the characteristics of an electrode, it is important to minimize an intermetallic compound phase having poor reversibility with respect to occlusion/desorption of lithium and an unreacted phase, such as the Cu3Sn phase and the Sn phase, and to efficiently form an intermetallic compound capable of occluding/desorbing lithium as in the Cu6Sn5 phase. Furthermore, it also is important to suppress the reaction between the active material layer and the collector during charging/discharging.