1. Field of the Invention
The present invention relates to a cylindrical alkaline storage battery suitable for achieving a high capacity.
2. Description of the Related Art
Alkaline batteries available include, for example, a nickel cadmium secondary battery and a nickel hydrogen secondary battery depending on the types of active materials contained in the batteries. Some of those alkaline batteries are of a cylindrical type which has a cylindrical case. The case is sealed with a lid having a relief valve and accommodates an electrode assembly together with an alkaline electrolyte. The electrode assembly is formed by winding a belt-like negative plate and a belt-like positive plate spirally with a separator in between and are contained in the case while that part of the negative plate which is wound around the outermost one of the electrode assembly contacts the inner wall of the case.
The positive plate, which is called a nickel electrode, is formed by filling a positive mixture in a nickel substrate having a three-dimensional mesh structure. The positive mixture includes nickel hydroxide particles as a positive active material, additive particles and a binder which binds those particles. The negative plate is formed by covering both sides of a metal sheet as a negative substrate with a hydrogen absorbing alloy layer as a negative active material layer. The metal sheet has through holes in which the negative active material is filled. The hydrogen absorbing alloy layer is comprised of hydrogen absorbing alloy particles which can absorb and desorb hydrogen as a negative active material, and a binder which binds the hydrogen absorbing alloy particles. While the capacity of each of the positive plate and the negative plate is defined by the amount of the active material or the amount of the hydrogen absorbing alloy contained therein, the battery capacity is defined by the capacity of the positive electrode. This is because the capacity of the negative electrode in this type of cylindrical alkaline storage battery is set greater than the capacity of the positive electrode in order to prevent the inner pressure from rising by reducing an oxygen gas produced in the positive plate with the negative plate when the battery is overcharged.
Recently has been increasing the demand for cylindrical alkaline storage batteries of this type, particularly, cylindrical alkaline storage batteries of AA size compatible with AA-size dry cells, as electronic and electric devices, such as a digital camera, which use the batteries as power supplies become popular. A cylindrical alkaline storage battery of AA size is strongly demanded of higher capacity or an improvement on the volume energy density in order to enable continuous use of the devices for extended time. To increase the battery capacity, the capacity of the positive electrode that defines the battery capacity should be increased. Specifically, the amount of the positive active material should be increased or the ratio of usage of the positive active material should be improved. To increase the amount of the positive active material, it is known to increase the length, thickness and area of the positive plate and the filling density of the positive mixture in the positive substrate. In case of increasing the thickness of the positive plate or enhancing the filling density of the positive mixture in the positive substrate, it is unnecessary to make the separator and the negative plate longer, thus ensuring an efficient increase in the amount of the positive active material. For example, Japanese Patent Laid-Open Publication No. Hei 10-199520 discloses a cylindrical alkaline storage battery which achieves a high capacity by setting the thickness of a nickel electrode equal to or greater than 0.8 mm.
The following problems would however occur when the cylindrical alkaline storage battery described in the publication is adapted to an AA-size cylindrical alkaline storage battery whose case has an outside diameter of 13.5 mm to as large as 14.5 mm and whose positive plate is made thick enough so that the volume energy density becomes 340 Wh/l or higher.
First, the battery life becomes shorter with an increase in the thickness of the positive plate.
As the thickness of the positive plate increases, the distortion of a spiral shape drawn by the positive plate and the negative plate in view of the lateral cross-sectional area of the electrode assembly increases, making the gap between the positive plate and the negative plate nonuniform in the lengthwise direction of the positive plate and the negative plate. When the thickness of the positive plate becomes 0.95 mm or thicker, particularly, a fluctuation in the gap between both plates becomes greater. As the fluctuation in the gap between both plates becomes greater, an oxygen gas is locally produced when charging, increasing the inner pressure of the battery. This actuates the relief valve so that the alkaline electrolyte leaks out, thus shortening the battery life.
Secondly, when the inner end portion of the negative plate extends over the inner end portion of the positive plate in the circumferential direction of the electrode assembly on the outer surface side of the positive plate, a negative mixture comes off from that portion of the negative plate which extends over the inner end portion of the positive plate, thus decreasing the capacity of the negative electrode.
A clearance is defined in front of the inner end surface of the positive plate in view of the circumferential direction of the electrode assembly by the inner end surface of the positive plate and that portion of the separator which extends from the inner surface and outer surface of the positive plate over the inner end surface of the positive plate. The size of the clearance formed accords with the thickness of the positive plate. In the cylindrical alkaline storage battery which uses electrode assembly having such a clearance, after the initial charge/discharge, that portion of the negative plate which is positioned outside the clearance in the radial direction of the electrode assembly via the separator is bent toward the inner end surface of the positive plate in such a way as to reduce the clearance. When the thickness of the positive plate becomes 0.95 mm or greater, particularly, the bending of the negative plate in front of the inner end surface of the positive plate becomes larger, so that the negative mixture comes off from the bent portion of the negative plate, thus reducing the capacity of the negative electrode.
When the outer end portion of the negative plate extends over the outer end portion of the positive plate in the circumferential direction of the electrode assembly on the outer surface of the positive plate, the negative substrate may be broken at that portion of the negative plate which is overlaid on the outer surface edge of the outer end portion of the positive plate via the separator, thereby increasing the inner resistance, or the negative mixture may come off from that portion, thereby reducing the capacity of the negative electrode.
When the electrode assembly is wound, the portion of the negative plate which is overlaid on the outer surface edge of the outer end portion of the positive plate via the separator is bent. In addition, the outside diameter of the electrode assembly increases at the bent portion of the negative plate, so that the bent portion of the negative plate slides against the electrode winding machine or the case at the time the electrode assembly is wound or are inserted into the case. When the thickness of the positive plate becomes 0.95 mm or greater, particularly, the bending of that portion of the negative plate which is overlaid on the outer surface edge of the outer end portion of the positive plate and sliding of the bent portion of the negative plate against the winding machine or the case becomes intense. Therefore, the negative substrate may be broken at the bent portion of the negative plate, thereby increasing the inner resistance, or the negative mixture may come off from that portion, thereby reducing the capacity of the negative electrode.
Further, when the thickness of the positive plate becomes 0.95 mm or greater, the positive plate is broken and the broken portion breaks through the separator to contact the negative plate at the time the electrode assembly is wound, causing short-circuiting.
When the positive plate is made thick enough so that the volume energy density becomes 340 Wh/l or higher, the volumes of the negative plate, the alkaline electrolyte and the separator decrease and the excess space in the battery excluding those volumes and the volume of the positive plate decreases as well. This would raise the following problems. Before going into the detailed description of the problems, terms are defined as follows.
Capacity ratio: the ratio of the capacity of the entire negative plate to the capacity of the positive electrode
Non-overlapping portion: the portion of the negative active material layer which does not overlap the positive plate via the separator
Overlapping portion: the portion of the negative active material layer which overlaps the adjoining positive plate via the separator
Ratio of the non-opposing portion of the negative plate: the occupying ratio of the amount of the negative active material contained in the non-overlapping portion of the negative active material layer to the total amount of the negative active material
Opposing capacity ratio: the ratio of the capacity of the overlapping portion of the active material layer of the negative plate to the capacity of the positive electrode
Capacity-electrolyte ratio: the ratio of the volume of the alkaline electrolyte to 0.2 C capacity
To begin with, a decrease in the volume of the negative plate or a decrease in the amount of the negative active material reduces the battery life.
As the capacity of the positive electrode is increased by making the positive plate thicker, the capacity ratio decreases, thereby reducing the amount of the negative active material contained in the overlapping portion of the negative active material layer (hydrogen absorbing alloy layer). As the battery reaction mainly progresses between the positive active material and the overlapping portion of the negative active material layer at the time of charge/discharge, the battery reaction does not progress smoothly if the amount of the negative active material contained in the overlapping portion is small.
In a battery with the ratio of the non-opposing portion of the negative plate of 29%, for example, when the capacity ratio drops to 1.4 or lower, the opposing capacity ratio becomes 1.00 or less, so that the capacity of the negative electrode substantially becomes smaller than the capacity of the positive electrode.
When the opposing capacity ratio becomes 1.00 or less, it becomes impossible to exchange protons at the shortest distance at the time the battery reaction occurs. So the reaction is made non-uniform, thereby lowering the discharge characteristics. Further, the time for the oxygen gas that has been produced in the positive plate at the time of charging to pass through the separator, reach the negative plate and be reduced becomes longer, thus increasing the inner pressure of the battery. This actuates the relief valve, so that the alkaline electrolyte leaks out. When charge/discharge is repeated, therefore, the battery life becomes shorter due to two factors: early and local deterioration of the active material caused by the non-uniform reaction and the leakage of the alkaline electrolyte caused by an increase in the inner pressure of the battery.
Secondly, when the amount of the alkaline electrolyte decreases, the capacity-electrolyte ratio decreases. When the capacity-electrolyte ratio becomes 0.85 ml/Ah or less, the amount of the electrolyte becomes short at the portion where the positive plate and the negative plate overlap each other via the separator. This increases the electric resistance, lowering the discharge characteristic.
Because the alkaline electrolyte mainly exist in the form of being contained in the positive plate, the negative plate and the separator in entirety in the battery, some of the alkaline electrolyte is contained in the non-overlapping portion of the negative plate which does not directly contribute to the battery reaction and the separator adjoining to the non-overlapping portion. Accordingly, the amount of the alkaline electrolyte which is contained in the positive plate, the overlapping portion of the negative plate where the battery reaction takes place, and the separator sandwiched therebetween is what is obtained by subtracting the amount of the alkaline electrolyte contained in the non-overlapping portion of the negative plate from the total amount of the alkaline electrolyte. In the case where the capacity-electrolyte ratio becomes 0.85 ml/Ah or less, if a part of the alkaline electrolyte is contained in the non-overlapping portion of the negative plate, the amount of the alkaline electrolyte that is present in the place of the battery reaction becomes short. This increases the electric resistance between the positive plate and the negative plate, thus lowering the discharge characteristic.
When continuous charging takes place at a low temperature, the positive plate is expanded to absorb the electrolyte. In a battery which has a small amount of the electrolyte, therefore, the discharge performance after continuous charging drops, causing a significant voltage drop at the initial discharge stage. This low-temperature continuous charge characteristic also considerably decreases when the capacity-electrolyte ratio becomes 0.85 ml/Ah or less.
When a separator with a small thickness or a light weight per area is used to reduce the volume of the separator, short-circuiting is likely to occur between the positive plate and the negative plate, resulting in lower quality. Short-circuiting is particularly apt to occur at a circumferential position where the outer end portion of the positive plate is positioned. This is originated from the outside diameter of the electrode assembly being maximized at the circumferential position and a burr is present at the edge of the outer end portion of the positive plate. That is, as the outside diameter is the maximum, the outer end portion of the positive plate is compressed by the inner wall of the case from both radial sides. At this time, the burr at the outer end portion of the positive plate is compressed and breaks through the separator, thereby increasing the frequency of occurrence of short-circuiting.
When the excess space is reduced, the space for temporarily storing the oxygen gas produced in the positive plate is gone. When the oxygen gas is produced in the positive plate at the time of charging, therefore, the inner pressure rises immediately to actuate the relief valve, thereby causing the alkaline electrolyte to leak out.