Alkaline batteries, particularly alkaline batteries of the discharge-start type and alkaline primary batteries, have an inside-out type structure, in which cylindrical tablets of a positive electrode material mixture are disposed in a positive electrode case serving as a positive electrode terminal so as to closely adhere to the positive electrode case, and a gelled zinc negative electrode is disposed in the center thereof with a separator interposed therebetween.
With the recent proliferation of digital devices, there is a gradual increase in the load power of such battery-powered devices. Therefore, there is a demand for batteries having excellent heavy-load discharge performance.
In order to obtain alkaline batteries having excellent heavy-load discharge characteristics, for example, mixing nickel oxyhydroxide into the positive electrode material mixture has been proposed (see Japanese Laid-Open Patent Publication No. Sho 57-72266). Also, such batteries have recently been put into practical use.
Nickel oxyhydroxide used in alkaline batteries is obtained, for example, by oxidizing spherical or hen's egg-shaped nickel hydroxide by means of an oxidant, such as an aqueous solution of sodium hypochlorite. The spherical nickel hydroxide which is used as the raw material for such oxidation reaction is β-type nickel hydroxide having a high bulk density (tap density). When the β-type nickel hydroxide is oxidized, spherical nickel oxyhydroxide with a main structure of β-type is generated. The high-tap-density spherical nickel oxyhydroxide with a main structure of β-type thus obtained can be filled into the positive electrode at high densities.
Also, spherical nickel hydroxide with adequately controlled crystallinity is used in alkaline storage batteries to improve their high-temperature charge characteristics. The use of such spherical nickel hydroxide as the raw material makes it possible to obtain spherical nickel oxyhydroxide having a high oxidation number, i.e., a high discharge capacity.
The recent higher functionality of digital devices has created a demand for a further improvement in the discharge characteristics of alkaline batteries. With respect to alkaline batteries including the above-mentioned nickel oxyhydroxide, in particular, it is demanded to (1) improve their heavy-load discharge characteristics so as to be capable of a pulse discharge unique to digital devices and (2) enhance their capacity to allow for an increase in the power consumed by such devices.
To meet the former demand, the use of nickel oxyhydroxide coated with a cobalt oxide, cobalt oxyhydroxide, or the like has been proposed (see Japanese Laid-Open Patent Publication No. 2002-338252 and Japanese Laid-Open Patent Publication No. 2003-17079). The use of such nickel oxyhydroxide leads not only to an improvement in heavy-load discharge characteristics but also to an improvement in capacity.
With respect to the latter demand, improving the nickel oxyhydroxide material itself contained in the positive electrode material mixture has been under examination. For example, there has been a proposal of providing alkali metal ions between the crystals of β-type nickel oxyhydroxide to improve capacity (see Japanese Laid-Open Patent Publication No. 2001-325954).
Also, in order to enhance the energy density of the alkaline batteries containing nickel oxyhydroxide in their positive electrodes, the conditions of chemical oxidization of nickel hydroxide may be strengthened to heighten the oxidation number of nickel contained in nickel oxyhydroxide.
However, when the conditions of chemical oxidization are simply strengthened, for example, by increasing the concentration and amount of the oxidant used for the oxidation reaction and by heightening the temperature of the atmosphere, the crystallites having a β-type structure become finer, resulting in a decrease in the crystallinity of nickel oxyhydroxide.
Low-crystalline β-nickel oxyhydroxide has a large number of grain boundaries, which are a kind of defect. Such nickel oxyhydroxide locally includes γ-type structure crystal that is inactive and does not contribute to the discharge reaction. Therefore, even if the mean valence of nickel is seemingly high, sufficient discharge capacity cannot be obtained. In this way, the energy density of alkaline batteries cannot be heightened by merely heightening the mean valence of nickel contained in nickel oxyhydroxide
Therefore, an object of the present invention is to provide an alkaline battery having excellent heavy-load discharge characteristics and improved energy densities by optimizing the physical properties of nickel oxyhydroxide.