Sealed alkaline-zinc storage battery

#This application claims the priority of Japanese Patent Application Nos. 10-96866 filed on Mar. 24, 1998 and 11-36497/1999 filed on Feb. 15, 1999, which are incorporated herein by reference.

#The present invention relates to a sealed alkaline-zinc storage battery comprising a tubular positive electrode, a separator, a negative electrode disposed within the tubular positive electrode with the separator sandwiched therebetween and an alkaline electrolyte, in which the positive electrode has a capacity smaller than a capacity of the negative electrode at least in initial charge-discharge cycles, namely, the battery capacity is controlled by the capacity of the positive electrode. More particularly, it relates to improvement of the negative electrode for the purpose of improving the charge-discharge cycle performance of such a battery.

#A conventional sealed alkaline-zinc storage battery uses zinc as the negative electrode active material. Since zinc has a small electrochemical equivalent and has a base potential, an alkaline storage battery with a high energy density can be obtained by using zinc as the negative electrode active material. When zinc is used, dendrite (electrodeposited crystal with a branching treelike appearance) is grown during charge. When the dendrite is grown to penetrate through the separator, an internal short-circuit is caused. Accordingly, in order to avoid this problem, it is necessary to use a separator with a large mechanical strength, such as a laminated separator obtained by laminating plural separators, in a practical battery.

#A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

#FIG. 1 is a sectional view of a sealed alkaline-zinc storage battery manufactured in an experiment of the invention.

#A sealed alkaline-zinc storage battery according to the invention comprises a tubular positive electrode including, as an active material, a material having reversibility in a charge-discharge reaction; a separator; a negative electrode disposed within the tubular positive electrode with the separator sandwiched therebetween; and an alkaline electrolyte, and the positive electrode has a capacity smaller than a capacity of the negative electrode at least in initial charge-discharge cycles, and the amounts of an uncharged active material and zinc to be packed in the negative electrode in manufacture of the battery are set so that the theoretical capacity P of the uncharged active material existing in the negative electrode can be 0.3 through 1.8 times as large as a battery capacity in a completely charged state in the initial charge-discharge cycles, and that the theoretical capacity Q of zinc existing in the negative electrode can be 0.6 through 2.5 times as large as the battery capacity in a completely discharged state in the initial charge-discharge cycles.

#The present battery is a battery in which the capacity of the positive electrode is smaller than that of the negative electrode at least in initial charge-discharge cycles, namely, the battery capacity is controlled by the capacity of the positive electrode (hereinafter referred to as the “positive electrode control type” battery). The restriction of “at least in initial charge-discharge cycles” is herein placed considering that the charge-discharge efficiency of an alkaline-zinc storage battery is 100% in initial charge-discharge cycles, and that since zinc is generally more easily degraded than a positive electrode active material, the capacity of the positive electrode can become larger than that of the negative electrode after repeating charge-discharge cycles even though the capacity of the positive electrode is smaller than that of the negative electrode in the initial charge-discharge cycles. A conventional sealed alkaline-zinc storage battery uses, as the positive electrode active material, MnO2 which can suppress generation of oxygen from the positive electrode when the battery is charged with controlling the charge voltage for the positive electrode. Owing to the use of MnO2, the conventional sealed alkaline-zinc storage battery unavoidably adopts capacity design of the negative electrode control type which requires a large amount of positive electrode active material for attaining a reversibly reactive electric capacity. However, by using, as the positive electrode active material, a material that has reversibility in the charge-discharge reaction and can suppress the generation of oxygen by controlling the charge potential for the positive electrode, the generation of oxygen from the positive electrode during charge can be prevented even when capacity design of the positive electrode control type is adopted as in this invention.

#An example of the material that has reversibility in the charge-discharge reaction and can suppress the generation of oxygen by controlling the charge potential for the positive electrode includes a hydroxide of nickel including, as a solid-solution element, manganese (Mn), which has an α-Ni(OH)2 crystal structure in a discharged state and has a γ-NiOOH crystal structure in a charged state. The α-Ni(OH)2 in a discharged state has excellent features that oxygen overpotential (oxygen generation potential-charge potential) is high and that it can be charged without a side reaction. The α-Ni(OH)2 can be obtained by adding manganese as a solid-solution element. Manganese dioxide used as a positive electrode active material for an alkaline dry battery cannot be used as the positive electrode active material for a storage battery because it has so poor reversibility in the charge-discharge reaction that it cannot completely return to original manganese dioxide by charge after the first discharge. A hydroxide of nickel in which manganese (Mn) and at least one element selected from the group consisting of aluminum (Al), cobalt (Co), yttrium (Y), ytterbium (Yb), erbium (Er) and gadolinium (Gd) are added as solid-solution elements can be used. The oxygen overpotential of the positive electrode active material can be further increased by adding, as the solid-solution elements, any of these elements in addition to manganese. The hydroxide of nickel preferably has a valence of nickel of 3.4 through 3.8 in a completely charged state. When the hydroxide of nickel has a valence of nickel smaller than 3.4, a sufficient battery capacity is difficult to obtain. No hydroxide of nickel has a valence of nickel exceeding 3.8 in the completely charged state. The negative electrode can include an additive for increasing hydrogen overpotential (a material having large hydrogen overpotential) in order to suppress the generation of a hydrogen gas during storage. Examples of the additive include indium (In), bismuth (Bi), tin (Sn), gallium (Ga), diindium trioxide (In2O3), dibismuth trioxide (Bi2O3), tin oxide (SnO) and digallium trioxide (Ga2O3). One of these additives can be singly used or a combination of two or more of them can be used if necessary. An example of the alkaline electrolyte to be used includes an aqueous solution including at least one alkali selected from the group consisting of potassium hydroxide, sodium hydroxide and lithium hydroxide. An aqueous solution of potassium hydroxide is preferred. The present invention is particularly significant in application to a sealed alkaline-zinc storage battery of a high density packing type in which an energy generating unit consisting of a positive electrode, a negative electrode, an alkaline electrolyte, a separator and a negative current collector occupies 75% by volume or more of the content volume inside of an insulating packing of the battery can.