Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
1999-06-10
2001-07-24
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S164000, C429S229000, C429S223000, C429S321000, C429S238000
Reexamination Certificate
active
06265105
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sealed alkaline-zinc storage battery capable of starting with discharge, more particularly to a sealed alkaline-zinc storage battery which exhibits excellent storage characteristics, such as a low degree of deterioration in discharge capacity even after a long-term storage and the reduced occurrence of leakage of electrolyte. The storage battery capable of starting with discharge, as used herein, refers to the storage battery which can start initially with discharging without a need to be charged prior to use.
2. Related Art
In Japanese Patent Publication No. 45-16653 (1970), an alkaline-manganese secondary battery is disclosed which includes manganese dioxide as a depolarizer, zinc as negative active material and an aqueous caustic solution as an electrolyte, and which specifies a total water content of the alkaline electrolyte as being within the range of 0.75-1.10 g relative to 1 g of zinc as the negative active material. In this secondary battery, the water content of the alkaline electrolyte is in the range of 0.92-1.34 g for each theoretical capacity of the negative electrode.
The inventors of the present application have found that the water content range of the alkaline electrolyte specified in the above-identified reference, if applied to a sealed alkaline-zinc storage battery capable of starting with discharge and including nickel hydroxide as a positive active material and zinc as a negative active material, adversely affects the long-term storage characteristics thereof. Specifically, such a storage battery exhibits appreciable deterioration in discharge capacity. With the increased number of charge-discharge cycles, the increasing amount of oxygen gas evolution occurs at the positive electrode to raise an internal pressure of the battery, ultimately causing the electrolyte to leak.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a sealed alkaline-zinc storage battery which exhibits excellent storage characteristics, i.e., a low degree of deterioration in discharge capacity and reduced occurrence of electrolyte leakage even after a long-term storage.
A sealed alkaline-zinc storage battery of the present invention includes a battery can, a hollow positive electrode located within the battery can for electrical connection thereto and containing nickle hydroxide as a positive active material, a negative electrode located inwardly of the positive electrode and containing zinc as a negative active material, a separator interposed between the positive and negative electrodes, a negative current collector located for insertion into the negative electrode, and an electrolyte impregnated into the positive electrode, negative electrode and separator. The positive electrode, negative electrode, separator, negative current collector and electrolyte account for at least 75% of an internal volume of the battery can. The alkaline electrolyte is in the 30-45 mass % concentration range. A total water content of the alkaline electrolyte is in the range of 0.5-0.9 g for each theoretical capacity (expressed as 1 Ah (ampere-hour)) of the negative electrode.
For prior art sealed alkaline-zinc storage batteries, the charge-discharge cycling following a long-term storage causes deterioration in discharge capacity of a zinc negative electrode. This results from a reaction, Zn+2OH
−
→Zn(OH)
2
+2e
−
, at the negative electrode during storage. That is, water contained in the alkaline electrolyte is transferred to the zinc electrode through this reaction to cause depletion of dry-out of the alkaline electrolyte, leading to the increase in internal resistance of the battery. If a total water content of the alkaline electrolyte falls below 0.5 g for each theoretical capacity of the negative electrode, the shortage in absolute quantity of the electrolyte may result in the deterioration in discharge capacity of the battery after a long-term storage, i.e., result in the deteriorated storage characteristics. On the other hand, if a total water content of the alkaline electrolyte exceeds 0.9 g for each theoretical capacity of the negative electrode, the abundance in absolute quantity of the electrolyte may result in the increased oxygen gas evolution at the positive electrode on charge-discharge cycling of the battery after a long-term storage, which increases an internal battery pressure to finally cause leakage of the electrolyte. In the sealed alkaline-zinc storage battery of the present invention, a total water content of the alkaline electrolyte is accordingly specified as being within the range of 0.5-0.9 g for each theoretical capacity of the negative electrode.
Also in the present invention, the alkaline electrolyte is in the 30-45 mass % concentration range. If the concentration of the alkaline electrolyte is below 30 mass %, the solubility of zinc into the electrolyte will be lowered to result in the passivation of the zinc electrode which limits the battery performance. The battery, if rendered negative-limiting, fails to withdraw a full discharge capacity, resulting in the deteriorated cycle characteristics. On the other hand, if it exceeds 45 mass %, an absolute quantity of water in the electrolyte decreases to result in deteriorating storage characteristics of the battery. The preferred alkaline electrolyte consists principally of an aqueous solution of potassium hydroxide (KOH). Lithium hydroxide (LiOH) or sodium hydroxide (NaOH) may be added to the aqueous potassium hydroxide solution. In the sealed alkaline-zinc storage battery of the present invention, the positive electrode, negative electrode, separator, negative current collector and electrolyte account for at least 75% of an internal volume of the battery can. This permits the high-density loading of active materials in the battery can, so that the storage battery is allowed to have a high energy density. While an increase in internal pressure is highly expected for such sealed alkaline storage batteries having high loadings of active materials in a battery can, the battery of the present invention is able to effectively prevent the electrolyte from leaking to outside after repetitive charge-discharge cycles.
In the present invention, preferably utilized for the negative active material is zinc active material consisting principally of a mixture of metallic zinc and zinc oxide. It is further preferred that the zinc active material contains 5-40 mass % of zinc oxide, relative to the weight of metallic zinc. If the amount of zinc oxide falls below 5 mass %, relative to the weight of the metallic zinc in the zinc active material, the zinc oxide may become insufficient in amount to compensate for deterioration of zinc (discharge reserve), when subjected to a charge-discharge test after a long-term storage. This results in the deterioration in discharge capacity. On the other hand, if the amount of the zinc oxide exceeds 40 mass %, the deterioration in amount of the metallic zinc in the zinc active material necessitates the increased loading of the negative electrode, which in turn reduces a volumetric space left within the battery. This may result in the increased occurrence of electrolyte leakage.
In the present invention, the negative electrode is preferably in the form of gel containing negative active materials. Such a gelled negative electrode can be prepared from a combination of a negative active material(s), a gelling agent such as water-soluble polymeric material, and an alkaline electrolyte.
In the present invention, the positive active material is nickel hydroxide. Prior to a first discharge, the nickel hydroxide is preferably present in the higher-valent form, &ggr;-nickel oxyhydroxide. A valence number of nickel atoms in the &ggr;-nickel oxyhydroxide is preferably in the range of 3.4-3.8. If the valence number is below 3.4, the battery capacity may become insufficient. For &ggr;-nickel oxyhydroxide, the nickel valence number does not exceed 3.8. That is, once the battery is fully
Fujitani Shin
Nishio Koji
Nogami Mitsuzou
Tokuda Mitsunori
Yano Mutsumi
Chaney Carol
Fasse W. F.
Fasse W. G.
Martin Angela J.
Sanyo Electric Co,. Ltd.
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