1. Field of the Invention
The present invention relates to an anode including an anode active material layer on an anode current collector, and a secondary battery including the anode.
2. Description of the Related Art
In recent years, portable electronic devices such as video cameras, cellular phones, or notebook computers are widely used, and size and weight reduction in the portable electronic devices and an increase in longevity of the portable electronic devices have been strongly demanded. Accordingly, as power sources for the portable electronic devices, the development of batteries, specifically lightweight secondary batteries capable of obtaining a high energy density have been promoted.
Among them, a secondary battery (a so-called lithium-ion secondary battery) using insertion and extraction of lithium for charge-discharge reaction holds great promise, because the secondary battery is capable of obtaining a larger energy density, compared to a lead-acid battery or a nickel-cadmium battery.
The lithium-ion secondary battery includes a cathode, an anode and an electrolytic solution. The anode includes an anode active material layer on an anode current collector, and the anode active material layer includes an anode active material contributing to charge-discharge reaction.
As the anode active material, a carbon material such as graphite is widely used; however, with enhancement of performance and expansion of functions in portable electronic devices, a further improvement in battery capacity is desired recently, so it is considered to use a material having a higher capacity such as silicon instead of a carbon material, because the theoretical capacity of silicon (4199 mAh/g) is much larger than the theoretical capacity of graphite (372 mAh/g), so a large increase in battery capacity is expected.
As a method of forming the anode active material layer, various methods such as a vapor-phase method, a liquid-phase method, a sintering method, a spraying method and a coating method are used; however, to stably improve the performance of the secondary battery, the vapor-phase method or the like is more preferable than the coating method, because the anode active material is prevented from being finely divided, and electronic conductivity is improved by strongly integrating the anode current collector and the anode active material layer. Thereby, a superior battery capacity and superior cycle characteristics are obtained as described in, for example, Japanese Unexamined Patent Application Publication No. H11-135115.
However, in the case where the material having a higher capacity such as silicon is used as the anode active material, there is concern that the anode active material layer is easily swelled or shrunk during charge and discharge. Thereby, when the anode active material layer is swelled and shrunk largely, even if the anode current collector and the anode active material layer are strongly integrated, the anode active material layer may be broken into pieces, thereby sufficient characteristics may not be obtained.
To prevent the anode active material layer from being broken into pieces, various studies have been conducted. More specifically, there are proposed a technique in which first layers and second layers having different oxygen contents are included, the first layers and the second layers are alternately laminated has been proposed as described in, for example, Japanese Unexamined Patent Application Publication No. 2004-349162. Moreover, there is proposed a technique in which a metal element (a ferromagnetic metal element such as iron, nickel or cobalt) is included in an anode active material layer as described in, for example, Japanese Unexamined Patent Application Publication No. 2007-257866. In this case, a high-concentration metal layer containing a high concentration of a ferromagnetic metal element and a low-concentration metal layer containing a low concentration of a ferromagnetic metal element are included. In these techniques, as the swelling and shrinkage of the anode active material layer are reduced, the anode active material layer is less prone to being broken into pieces during charge and discharge.