JPH0795443B2 - Sealed alkaline storage battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a sealed alkaline storage battery in which a hydrogen storage alloy or hydride is used as a negative electrode and a metal oxide is used as a positive electrode, and particularly to improvement of a negative electrode.

2. Description of the Related Art Conventionally, in a sealed metal oxide-hydrogen storage battery in which a hydrogen storage alloy or hydride of this kind is used as a negative electrode, oxygen gas generated in the positive electrode is reacted with hydrogen stored in the negative electrode to form water. According to the above, a method of maintaining a sealed state is considered. In this case, the oxygen gas needs to undergo a reduction reaction or an ionization reaction on the surface of the negative electrode to become water, but the surface of the hydrogen storage alloy particles is oxidized by the oxygen gas generated at the positive electrode, so that the oxygen gas is efficiently transferred. The rate of reduction is slow. Therefore, if a reaction to consume is delayed from a reaction to generate oxygen gas, oxygen gas accumulates in the battery and the internal pressure of the battery rises. This phenomenon is particularly noticeable in rapid charging.

Problems to be Solved by the Invention In such a conventional configuration, when the sealed metal oxide-hydrogen storage battery enters the overcharge region, oxygen gas is generated from the positive electrode. Oxidation of the surface of the hydrogen storage alloy by this oxygen gas cannot be completely suppressed. Therefore, the rate at which oxygen gas is generated from the positive electrode is higher than the rate at which it is absorbed on the surface of the negative electrode, and excessive oxygen gas is accumulated in the battery, leading to an increase in the internal pressure of the battery, and there is a problem that the integrity is reduced.

The present invention solves such a problem, and suppresses the oxidation of the hydrogen storage alloy constituting the negative electrode by oxygen gas and efficiently performs the oxygen absorption or the oxygen reduction reaction on the negative electrode surface, thereby reducing the internal pressure of the battery. The purpose is to suppress the rise, extend the charge / discharge cycle, and reduce the battery cost.

Means for Solving the Problems In order to solve this problem, the present invention comprises a metal oxide positive electrode, a negative electrode made of a hydrogen storage alloy or a hydride, a separator and an alkaline electrolyte, and is in contact with the positive electrode side. An oxygen catalyst layer made of conductive whiskers was provided on the surface of the negative electrode, and the conductive whiskers made potassium titanate (K ₂ O ・ n
TiO ₂ ), and a small amount of catalyst is supported on the surface of the negative electrode, and the negative electrode has a catalytic action and an oxidation suppressing function.

Action With this structure, the oxygen gas that reaches the negative electrode surface does not directly contact the oxygen gas generated from the positive electrode during overcharge with the hydrogen storage alloy that is storing hydrogen, and the oxygen gas that reaches the surface of the negative electrode is a conductive whisker or a conductive whisker carrying a catalyst. And is ionized by a reduction reaction in the oxygen catalyst layer. Further, before the oxygen gas reaches the vicinity of the particles of the hydrogen storage alloy, the preceding reaction takes precedence, so that the direct oxidation reaction of the hydrogen storage alloy is suppressed. Therefore, during overcharging, oxygen gas generated from the positive electrode can be reduced and consumed on the surface of the negative electrode, which suppresses the rise in internal pressure of the battery, prevents oxidation of the hydrogen storage alloy, and extends the cycle life of the battery. It will be.

Hereinafter, the details will be described by way of examples.

Example A sample composed of commercially available Mm (Misch metal), La, Ni, and Co was weighed and mixed so as to have a constant composition ratio, and heated and melted by an arc melting method. As an example, the alloy composition was selected to be Mm _0.5 La _0.5 Ni _3.5 Co _1.5 to obtain a hydrogen storage alloy for the negative electrode. This hydrogen-absorbing alloy was made into a fine powder of 38 μm or less by a ball mill or the like, kneaded well with an appropriate amount of an aqueous polyvinyl alcohol resin solution, and the paste-like alloy was filled into a foamed nickel porous body cut into a certain size. After coating the conductive whiskers with corrosion resistance on both sides in a paste state together with a binder such as a fluororesin dispersion, pressurization and drying were performed to form an oxygen catalyst layer, and a lead plate was attached to serve as a negative electrode. . If necessary, the alloy can be used in the form of a hydride. In this embodiment, the conductive whiskers are trade name Dentor BK-200,
Fibrous potassium titanate (K ₂ O
· NTiO ₂₎ was used. The average length of this fiber is 10 ~ 20μm
And the average diameter is 0.2 to 0.5 μm. The structure and cross section of this negative electrode are shown in FIG. In FIG. 1A, a hydrogen-occlusion alloy 2 is built in the pores of a foamed nickel porous body 1 that also serves as a current collector, and an oxygen catalyst layer 3 is formed on both surfaces thereof to form a negative electrode 4. B represents a cross section of the negative electrode 4.

Fig. 2 shows the constitution of a size AA alkaline storage battery used in the test by constructing a negative electrode using 15 g of hydrogen storage alloy powder and combining the foamed nickel positive electrode produced by a known method via a separator. Shown in. In FIG. 2, a negative electrode plate 4 made of a hydrogen storage alloy and a nickel oxide positive electrode 5 are spirally wound via a separator 6 and inserted into a case 7 which also serves as a negative electrode terminal. Insulating plates 8 and 9 are applied to the upper and lower sides of the electrode plate group, and the opening of the case 7 is sealed with a sealing plate 11 having a safety valve 10. Reference numeral 12 is a cap-shaped positive electrode terminal connected to the positive electrode lead 13 via the sealing plate 11. In addition, in order to suppress hydrogen generation from the negative electrode during charging, the negative electrode capacity was made larger than the positive electrode capacity, and the positive electrode law was adopted.
As a battery charging / discharging condition, the battery was charged (150% charge) for 7.5 hours at 0.2 C (current 400 mA) and discharged at 0.2 C (current 400 mA).
All the test temperatures were 20 ° C., and the battery internal pressure was measured when the battery was overcharged to 150%. Especially, the behavior of the battery internal pressure after 10 cycles was compared. Here, as a conventional battery, a battery A is constructed using a negative electrode provided with no oxygen catalyst layer. On the other hand, as the battery of the present invention, a battery B was prepared by forming fine fibrous potassium titanate as an electrically conductive whisker on an oxygen catalyst on the surface of a hydrogen storage alloy negative electrode. Further, a battery C was obtained by forming an oxidation catalyst layer having a palladium catalyst supported on the surface of a conductive whisker at about 0.1 wt% on the surface of the hydrogen storage alloy negative electrode. Further, FIG. 3 shows the battery internal pressures during charging in the conventional battery A and the present invention batteries B and C. As can be seen from FIG. 3, the battery internal pressure of the conventional battery A rises from before the charging rate of 100%, and when the charging rate reaches 150%, the battery internal pressure reaches 6 kg / cm ² . From the viewpoint of practical safety, it is said that the battery internal pressure is preferably 5 kg / cm ² or less, so this value becomes a problem from the viewpoint of safety. The reason for this phenomenon is that hydrogen gas is generated from the negative electrode before the charging rate is 100% and the oxygen gas generated from the positive electrode during charging cannot be completely absorbed by the negative electrode, and some hydrogen gas is added to the oxygen gas. Gas gradually accumulates in the battery and the internal pressure of the battery is increasing. That is, it is considered that the surface of the negative electrode lacks the function of absorbing hydrogen and the function of the oxygen catalyst.

On the other hand, the battery internal pressure of the batteries B and C of the present invention rises near the charging rate of 100%, and even when the charging rate reaches 150%, the battery internal pressure is 2.5 kg / cm ^{2 for} the battery B and 1.5 for the battery C. It is kg / cm ² . This is because hydrogen gas is hardly generated from the negative electrode until the charging rate is close to 100%, and oxygen gas generated from the positive electrode is almost absorbed by the negative electrode surface catalytic action even during overcharge. The accumulation of hydrogen and oxygen gas in the battery is low, and the rise in battery internal pressure is suppressed to a low level.

The conductive whiskers that act as oxygen catalysts have a fiber length of 10 to 20 μm and a diameter of 0.2 to 0.5 μm, and are very fine fibers. Therefore, the surface area is large and the resistance of carbon powder ( Since it has a value (0.1 to 0.5 Ω · cm) smaller than 1 Ω · cm), it has a low resistance on the surface of the negative electrode, is active with respect to oxygen gas, and has a function of reducing oxygen gas with high efficiency. It is thought that it is doing. It can be understood from the fact that the effect is further promoted by supporting the catalyst on the surface of the conductive whiskers.

On the other hand, when a cycle life test was performed on these batteries, the conventional battery A had a decrease of 30% or more of the initial capacity after 100 charge / discharge cycles. In this battery, the internal pressure of the battery rises too much, causing the safety valve to operate, and a leakage phenomenon from the safety valve is seen. It is also conceivable that the capacity of the negative electrode is reduced by the oxidation of the hydrogen storage alloy. Ten
When the battery subjected to the 0-cycle test was disassembled and the negative electrode capacity was examined, it was confirmed that the capacity was reduced by about 30% compared with the initial capacity.

On the other hand, the batteries B and C of the present invention remained almost unchanged from the initial capacity even after 100 charge / discharge cycles. No liquid leakage from the safety valve is observed. As a reminder, the battery after a 100-cycle test was disassembled and the negative electrode capacity was examined, but the capacity decreased only by a few percent compared to the initial capacity. From this fact, it is understood that the battery of the present invention has not only the function of the oxygen catalyst but also the function of suppressing the oxidation by the oxygen gas, and effectively works for the long life of the negative electrode.

The conductive whisker used here is characterized by being an oxide with conductivity such as potassium titanate (K ₂ O ・ nTiO ₂ ), which is a material that is extremely stable chemically and physically. Is. Thus, the conductive whiskers are stable against oxygen gas, and after adsorbing and holding the oxygen gas on the surface of the conductive whisker once before directly contacting with the hydrogen storage alloy, the surface of the hydrogen storage alloy surface is reduced for reduction. It is considered to prevent oxidation.

Since the conductive whiskers are fine fibrous, when a fibrous fluororesin is used as a binder, each fiber is entangled with each other to maintain mechanical strength and improve the adhesion with the hydrogen storage alloy particles. It is also possible to increase the strength of the electrode plate itself. Since it is difficult to uniformly produce fibers having a length of less than 5 μm, the cost becomes high. Therefore, it is preferably 5 μm or more, which can be manufactured at low cost. Further, if the thickness is 40 μm or more, a minute short circuit may occur via the separator, so the range of 5 to 40 μm is optimal. On the other hand, a diameter of 0.1 μm or less is still difficult in the manufacturing process, and the cost is high to guarantee quality, which is not practical. It is practical to use 0.1 μm or more, which can be manufactured at low cost. However, since the surface area becomes smaller when it becomes 1 μm or more,
Oxygen catalysis is reduced. Therefore, considering the oxygen catalytic action and cost reduction, the range of 0.1 to 1 μm is optimal.

Here, the palladium catalyst is used on the surface of the conductive whiskers, but other noble metal catalysts or metal oxide catalysts may be used. Although potassium titanate is used as an example of the oxygen catalyst layer, conductive whiskers having other compositions may be used, and it may be used by mixing with other inorganic / metal powders.

EFFECTS OF THE INVENTION As described above, according to the present invention, a sealed alkaline storage battery having a small increase in pressure in the battery during overcharging and excellent safety, a long charge / discharge cycle life, low cost, and stable quality can be obtained. The effect of being able to be obtained is obtained.

[Brief description of drawings]

1A and 1B are a side view and a sectional view showing the constitution of a negative electrode in the present invention, FIG. 2 is a sectional view showing the structure of a sealed alkaline storage battery used in an embodiment of the present invention, and FIG. FIG. 6 is a diagram showing changes in battery internal pressure at a charging rate of a sealed alkaline storage battery using the negative electrode of FIG. 1 ... Foamed nickel porous body, 2 ... Hydrogen storage alloy, 3
...... Oxygen catalyst layer, 4 …… Negative electrode plate.

Claims (3)

[Claims] 1. A metal oxide positive electrode, a negative electrode made of a hydrogen storage alloy or a hydride, a separator and an alkaline electrolyte, and potassium titanate (K ₂ O.nTiO ₂ ) on the surface of the negative electrode in contact with the positive electrode side. A sealed alkaline storage battery, characterized in that an oxygen catalyst layer of a conductive whisker consisting of is provided. 2. The sealed alkaline storage battery according to claim 1, wherein the average length and diameter of the conductive whiskers are 5 to 40 μm and 0.1 to 1 μm, respectively. 3. The sealed alkaline storage battery according to claim 1, wherein a catalyst is supported on the surface of the conductive whiskers forming the oxygen catalyst layer.