In recent years, mobile devices such as mobile phones, laptop computers or camcorders have formed a large market. As a power source for use in such a mobile device, there has been a strong demand for a lightweight, small-sized secondary battery having a high energy density. In particular, a lithium-ion secondary battery has superiority in these required characteristics as compared with other secondary batteries, and its adoption to the mobile device is in progress. In the lithium-ion secondary battery during discharge, lithium existing in a negative electrode is oxidized into lithium ions and released, whereas lithium ions are reduced into a lithium compound and stored into a positive electrode. Further, during charge, the lithium ions are reduced into lithium and stored into the negative electrode, whereas the lithium compound existing in the positive electrode is oxidized into lithium ions and released. As thus described, in the lithium-ion secondary battery, lithium ions move between the positive electrode and the negative electrode, and stored as lithium or a lithium compound into either electrode.
As a negative-electrode material for such a secondary battery, a carbon material such as graphite is used. For example, when graphite is used as the negative-electrode material, lithium is stored between graphite layers, thereby generating charge/discharge. However, graphite has a drawback of being unable to obtain a charge/discharge capacity of 372 mAh/g or larger by calculation from LiC6 as the maximum lithium-introduced compound since intercalation of lithium ions into a graphite crystal is a principle of charge/discharge of graphite.
Meanwhile, there has also been conducted an extensive research on using a metal material, which is alloyed with lithium, for the negative electrode. It has then been reported that in one using such a metal material for the negative electrode, there can be obtained a capacity larger than 372 mAh/g which is the charge/discharge capacity of graphite. However, in the negative electrode using these materials, large volume expansion occurs at the time of forming an alloy with lithium, to generate stress on its inside, and hence the alloy is pulverized by repetition of charge/discharge, causing a problem of not being able to obtain excellent charge/discharge cycle characteristics (i.e., electrode life).
For this reason, Patent Document 1 proposes a negative electrode for a lithium secondary battery which is made of a metal capable of being alloyed with lithium and has a porous structure. Using such a negative electrode can alleviate internal stress associated with a volume change at the time of the negative-electrode material forming an alloy with lithium, so as to seek improvement in charge/discharge cycle characteristics. Further, Non-patent Document 1 proposes that the porous electrode described in Patent Document 1 is divided and formed into a plurality of columnar structures apart from each other on the surface of a collector. Using such a negative electrode can further alleviate internal stress of the negative electrode in charge/discharge, so as to further seek improvement in charge/discharge cycle characteristics.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2006-260886    Non-patent Document 1: Woo, Okada, Munekata, Kanemura, “Production and characteristic evaluation of porous Sn—Ni alloy negative electrode having domain structure for lithium-ion secondary battery”, Proceedings of the 50th Battery Symposium in Japan 3A01 (2009), THE COMMITTEE OF BATTERY TECHNOLOGY
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