1. Field of the Invention
The present invention relates to alkaline storage batteries. It relates, for example, to nickel metal-hydride batteries using a hydrogen absorbing alloy.
2. Description of Related Art
In recent years, alkaline storage batteries have attracted wide attention as power sources of portable equipment, cellular phones or the like and those of electric cars, hybrid electric vehicles or the like. Accordingly, higher performance of the alkaline storage batteries has been requested. In particular, nickel metal-hydride batteries rapidly have become widespread as a secondary battery with a high energy density and an excellent reliability. The nickel metal-hydride batteries include a positive electrode using an active material mainly containing nickel hydroxide and a negative electrode using a hydrogen absorbing alloy as the main material.
In the nickel metal-hydride batteries mentioned above, cobalt is added to the positive electrode in order to raise the electric conductivity of the active material. For an active material for the negative electrode, the hydrogen absorbing alloy containing cobalt generally is used. The positive electrode and the negative electrode are insulated by a separator made of nonwoven fabric.
However, the nickel metal-hydride batteries described above have had a problem that self-discharge characteristics deteriorate after charge-discharge cycles are repeated. The inventors carried out an examination and newly found that metal ions eluted from the positive electrode and the negative electrode are deposited on the separator and form a conductive path, causing the deterioration of the self-discharge characteristics. Furthermore, this phenomenon was examined more in detail to find that (1) when the separator retains a sufficient amount of electrolyte, the metal ion such as cobalt that has been eluted into the electrolyte is deposited on the positive electrode, whereas (2) when the electrolyte retained in the separator decreases, the metal ion that has been eluted into the electrolyte is more likely to be deposited on the separator. Accordingly, the reason why the self-discharge characteristics deteriorate after many charge-discharge cycles is considered to be because the decrease in the electrolyte retained in the separator leads to a formation of the conductive path on the separator.
With the foregoing in mind, it is an object of the present invention to provide an alkaline storage battery that has excellent self-discharge characteristics even after a number of charge-discharge cycles.
In order to achieve the object mentioned above, a first alkaline storage battery of the present invention includes a case, and a positive electrode, a negative electrode, a separator and an electrolyte that are provided in the case. An amount of the electrolyte retained in the separator is equal to or more than 15 mg/cm2 (that is, equal to or more than 15 mg per cm2 of the separator) at least in a period P (that is, in a period after assembling the battery, from the time the separator is impregnated with the electrolyte to the time the battery is activated). In the above-described first alkaline storage battery, since a large amount of the electrolyte is retained in the separator, the electrolyte is not exhausted in the separator even when the charge-discharge cycles are repeated. Thus, according to the above-described first alkaline storage battery, the deposition of a conductive material on a surface of the separator can be prevented, making it possible to obtain the alkaline storage battery that has excellent self-discharge characteristics even after a number of charge-discharge cycles.
Also, a second alkaline storage battery of the present invention includes a case, and a positive electrode, a negative electrode, a separator and an electrolyte that are provided in the case. A total area X (cm2) of the separator and an amount Y (mg) of the electrolyte satisfy a relationship of Y/X greater than 20 at least in a period, after assembling the battery, from the time the separator is impregnated with the electrolyte to that the battery is activated. In the above-described second alkaline storage battery, since there is a large amount of the electrolyte, the electrolyte is not exhausted in the separator even when the charge-discharge cycles are repeated. Thus, according to the above-described second alkaline storage battery, it is possible to obtain an alkaline storage battery that has excellent self-discharge characteristics even after a number of charge-discharge cycles.
In the first and second alkaline storage batteries described above, the separator is formed of sulfonated polypropylene, and sulfur atoms and carbon atoms in the separator may satisfy a relationship of (the number of the sulfur atoms)/(the number of the carbon atoms)=A, where 2.0xc3x9710xe2x88x923xe2x89xa6Axe2x89xa65.5xc3x9710xe2x88x923. With the above structure, since an electrolyte retention of the separator becomes particularly high, it is possible to obtain an alkaline storage battery that has even better self-discharge characteristics after charge-discharge cycles.
In the first and second alkaline storage batteries described above, the electrolyte may be poured into the case in a vacuum atmosphere. This pouring method is referred to as a vacuum pouring method in this specification. With the above structure, since a larger amount of the electrolyte is retained in the separator in a uniform manner, it is possible to obtain an alkaline storage battery that has even better self-discharge characteristics. The vacuum pouring method includes (1) a method of pouring the electrolyte into the battery in which air has been removed from the space between fibers of the separator by evacuating a battery case in advance, and (2) a method of pouring the electrolyte into the battery case, then removing air present between fibers of the separator by creating a vacuum in an atmosphere in which the battery case is placed, so that the separator is impregnated sufficiently with the electrolyte upon exposure to the atmosphere.
In the first and second alkaline storage batteries described above, the separator may have a specific surface area ranging from 0.6 m2/g to 0.9 m2/g.
In the first and second alkaline storage batteries described above, the separator may have a median pore diameter on a volume basis of not larger than 30 xcexcm when pores are measured in a range of 0.1 xcexcm to 360 xcexcm with a mercury porosimeter. Also, in the first and second alkaline storage batteries described above, the separator may have a weight per unit area ranging from 60 g/m2 to 85 g/m2. With the above structure, since a path between the positive electrode and the negative electrode that is formed of the fibers of the separator becomes longer, it is possible to prevent a conductive deposit from forming a conductive path continuing from the positive electrode to the negative electrode.
Furthermore, a third alkaline storage battery of the present invention includes a case, and a positive electrode, a negative electrode, a separator and an electrolyte that are provided in the case. A chemical compound containing manganese is deposited on a surface of the separator. In the third alkaline storage battery described above, when cobalt is deposited on the surface of the separator, the cobalt forms a chemical compound with manganese so as to form a deposit with a low electric conductivity. Thus, it is possible to obtain an alkaline storage battery that has excellent self-discharge characteristics even after charge-discharge cycles.
In the third alkaline storage battery described above, the negative electrode may contain a hydrogen absorbing alloy as a main component, and the hydrogen absorbing alloy may contain misch metal and manganese in a composition ratio of 1:B, where 0.2xe2x89xa6Bxe2x89xa60.5. With the above structure, since the deposition of a highly conductive material such as cobalt oxyhydroxide on the surface of the separator can be prevented, it is possible to obtain an alkaline storage battery that has excellent self-discharge characteristics after a number of charge-discharge cycles.
In the third alkaline storage battery described above, the electrolyte may contain a manganese ion. With the above structure, since the deposition of a highly conductive material such as cobalt oxyhydroxide on the surface of the separator can be prevented, it is possible to obtain an alkaline storage battery that has excellent self-discharge characteristics after a number of charge-discharge cycles.