JPH0763005B2 - Sealed alkaline storage battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electrode of an alloy or hydride that reversibly stores and releases hydrogen, that is, a hydrogen storage electrode as a negative electrode and a metal oxide electrode as a positive electrode. The present invention relates to alkaline storage batteries, and more particularly to improvements in negative electrodes.

2. Description of the Related Art Conventionally, in an alkaline storage battery using this type of hydrogen storage electrode as a negative electrode, the hydrogen storage alloy or hydride constituting the negative electrode becomes fine molecules due to repeated charge / discharge cycles, or cracks due to expansion occur, and the electrode The performance of the battery deteriorates because it falls off the support. This phenomenon is particularly noticeable in open-type alkaline storage batteries. Therefore, an attempt has been proposed to solve the above problems by coating the surface of the hydrogen storage alloy particles with copper (Japanese Patent Laid-Open No. 50-111546). That is, a negative electrode is proposed in which the surface of a hydrogen storage alloy or hydrogenated particles is coated with copper / nickel by electroless plating to provide a coating film to improve the mechanical strength and electrical conductivity of the electrode itself. ing.

The use of this negative electrode improves the mechanical strength and conductivity of the electrode itself and improves the battery performance. On the other hand, since the metal coating the surface of the hydrogen storage alloy particles is inert to hydrogen, it is irrelevant to the energy storage capacity regulated by the hydrogen storage amount. Therefore, if the coated metal portion is large, the capacity per unit weight is reduced by that amount. Also, in this configuration, oxygen gas generated at the positive electrode needs to be converted to water by a reduction reaction on the surface of the negative electrode, but the reaction that consumes the oxygen gas generation is delayed, and oxygen gas accumulates in the battery, increasing the battery internal pressure. Let In particular, this phenomenon is manifested during rapid charging, which poses a problem in terms of safety.

Therefore, in order to solve these problems, a hydrogen storage alloy electrode using a hydrogen storage alloy powder whose surface is coated with a carbonaceous material has been proposed (JP-A-61-185863).

Problems to be solved by the invention When the negative electrode made of the hydrogen storage alloy powder coated with the carbonaceous material described above is used, the oxidation resistance of the electrode itself is improved, and it is useful for suppressing the oxidation of the hydrogen storage alloy and improving the electrode life. ,On the other hand,
Carbonaceous materials have a relatively high resistance as compared with metals and the like, and greatly reduce the conductivity of the hydrogen storage alloy electrode. Therefore, when a battery is constructed, the voltage drop becomes large due to the resistance polarization of the electrodes at high current discharge. Further, when the carbonaceous portion is large, the capacity per unit weight and unit volume is reduced by the amount.

Therefore, the present invention is to solve such a problem, to prevent the oxidation of the hydrogen storage alloy by oxygen gas and oxygen absorption or oxygen ionization on the negative electrode surface to proceed in a well-balanced manner even when the charging current is relatively large. The purpose of the present invention is to suppress the rise in internal pressure of the battery, to extend the charge / discharge cycle life, and to obtain excellent characteristics for high rate discharge.

Means for Solving the Problems In order to solve this problem, the present invention comprises a metal oxide positive electrode, a negative electrode composed of a hydrogen storage alloy or a hydride, a separator and an alkaline electrolyte, and carbon on the surface of the negative electrode. An oxidation inhibiting layer comprising a hydrogen storage alloy or hydride powder (mother particles) coated with particles (child particles) and a binder is provided. More preferably, the average particle size of the child particles coating the surface of the mother particles is about 1/10 to 1/100 smaller than the average particle size of the mother particles, the particle range of the mother particles is 0.1 to 50 μm, and the particle range of the child particles is It is set to 0.01 to 5 μm.

Action As described above, a mixture of a hydrogen storage alloy or hydride powder in which the surface of the hydrogen storage alloy or hydride particle is coated with carbon particles having electrical conductivity, catalytic action, and oxidation resistance and a binder such as a fluororesin is used. By forming only on the surface of the electrode substrate made of a hydrogen storage alloy or a hydride, it has an oxygen catalytic action and an oxidation inhibiting action on the surface of the negative electrode, and leads to improvement of the discharge capacity per unit volume and weight. It also has excellent high-rate charging characteristics. This is because the hydrogen storage alloy coated with the carbon particles formed on the surface of the negative electrode also contributes to the discharge capacity.

Further, since the surface layer composed of the mixture of hydrogen storage alloy or hydride powder coated with carbon particles and the binder is porous and has a large surface area, it has an oxygen catalytic action and an oxidation suppressing action due to the oxidation resistance of carbon particles. This can be promoted to suppress the increase in battery internal pressure and improve the durability.

The details will be described in the following examples.

Example A commercially available sample composed of Mm (Misch metal), La, Ni, and Co was weighed and mixed at a certain composition ratio, and heated and melted by an arc melting method. As an example, the alloy composition is Mm _0.5 L
a _0.5 Ni _3.5 Co _1.5 was used as the hydrogen storage alloy for the negative electrode. This alloy was crushed to 37 μm or less by a crusher, filled in a foam metal together with a binder, and then pressed and dried to obtain a negative electrode sample a. Next, the fine particles of carbon are firmly bonded to the surface of the alloy particles crushed by a crusher, and a suitable amount of a dispersion liquid of a fluororesin having a concentration of several% is added to the sample powder that has been subjected to the surface modification of the alloy particles to form a paste. None, use this paste as the negative electrode sample a
The surface-modified electrode applied to the surface of, and dried and heat-treated was used as a negative electrode sample b. Further, an electrode manufactured by the same method as the negative electrode sample a by modifying the surface of the alloy particles with carbon and using only the sample powder was designated as the negative electrode sample c.

Here, as the method used for the surface modification method, the hybridization system described in "Chemical apparatus" September 1986 (P.19) was applied as an example. In this surface modification method, carbon particles (child particles) are electrostatically charged on the surface of alloy particles (mother particles).
Attach. In this state, the bonding force between the mother particles and the carbon particles is weak, and the carbon particles fall off.Therefore, the carbon-coated alloy powder is rotated in a rotating drum so that the carbon particles are driven into the surface of the mother particles. It is possible to give an impact and to make an alloy powder with a strong carbon coating.

A lead was attached to each of the negative electrode samples a, b, and c to serve as an electrode.
A total of 15 g of the untreated hydrogen storage alloy powder and the modified hydrogen storage alloy powder formed on the electrode surface were used. A publicly known foamed nickel positive electrode is used to form a sealed alkaline storage battery of nominal 2 Ah through a separator, and the batteries are designated as A, B, and C, respectively.
FIG. 1 shows the structure of a hydrogen storage alloy modified with carbon, FIG. 2 shows the constitution of the negative electrode, and FIG. 3 shows a sealed alkaline storage battery.

In FIG. 1, (I) is a hydrogen storage alloy particle (mother particle) 1
The carbon particles (child particles) 2 are attached to the surface of the
(II) shows a state in which carbon particles (child particles) 2 partially invade the surface of the hydrogen storage alloy particles (mother particles) 1 and the carbon particles are firmly bonded. Second
In the figure, (I) shows a negative electrode plate 5 in which an oxidation suppressing layer 4 is formed on both surfaces of a substrate made of a hydrogen storage alloy 3. (II) is a cross section of the negative electrode 5, (III) is an enlarged view of a part of the cross section of (II), and the surface-modified alloy particles 7 are arranged on the hydrogen storage alloy particles 6 to improve the surface modification. The fluororesin 8 is present in the binder of the refined alloy particles 7 and the hydrogen storage alloy particles 6.

In FIG. 3, a negative electrode 9 made of a hydrogen storage alloy and a nickel positive electrode 10 are spirally wound via a separator 11,
It is inserted into the case 12 which also serves as the negative electrode terminal. The insulating plates 13 and 14 are applied to the upper and lower sides of the electrode plate group, and the opening of the case 12 is sealed by a sealing plate 16 having a safety valve 15. Reference numeral 17 is a cap-shaped positive electrode terminal connected to the positive electrode lead 18 via the sealing plate 16. In order to suppress the generation of hydrogen 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. The battery was charged / discharged at 0.3 C (667 ma) for 5 hours (150% charge) and discharged at 0.2 C (400 ma). The temperature of the charge / discharge cycle test was set to 25 ° C, and the internal pressure of each battery was measured at 150% charge. The battery internal pressure was measured after 50 cycles. In addition, the capacity at 0.2C after 200 cycles was compared with the initial capacity. The measurement results are shown in Table 1 comparing the conventional batteries A and C with the battery B of the present invention.

As can be seen from Table 1, the internal pressure of Battery A reaches 10 kg / cm ² ,
A partial liquid leakage phenomenon was observed, and the discharge capacity was greatly reduced due to the increase in internal resistance caused by the decrease in the electrolyte solution. Discharge capacity after 200 cycles shows 1.2Ah, 40% of initial capacity
It is decreasing. This is also due to the fact that the hydrogen storage alloy is oxidized by the oxygen gas generated from the nickel positive electrode during overcharge, and the ability to store hydrogen is reduced.
On the other hand, the internal pressure of Battery C is 3 kg / cm ^2, which is not much different from that of Battery B, but the discharge capacity is 1.5 Ah, which is 25% of the initial capacity, but the discharge capacity of Battery B is 1.8 Ah. , The initial capacity has been reduced by 10%. It is considered that the decrease rate of the discharge capacity of the battery C is larger than that of the battery B because the internal resistance of the electrode itself of the battery C increases with charge / discharge cycles. That is, the bonding force between the carbon-coated alloy particles in the electrode is weakened, the contact resistance is increased, and the discharge capacity is reduced. Next, measure the current-voltage characteristics of each battery,
It is shown in FIG. This current-voltage curve shows the initial characteristics, and although there is almost no difference between the batteries A and B, it can be seen that the characteristics of the battery C are not as good as those of the batteries A and B. As described above, in the battery C, even in the initial state of discharge, the surface of the hydrogen storage alloy particles is covered with the oxidation resistant carbon particles,
Since all carbon particles having a resistance value much larger than the resistance value of the metal are interposed between the hydrogen storage alloy particles, the contact resistance between the hydrogen storage alloy particles is large, and naturally the polarization of the electrode itself is also large. The voltage drop increases when high-rate discharge is performed. When discharging current of 4A (equivalent to 2C), batteries A and B
In comparison with the battery C, the difference is 0.05V.

On the other hand, in the battery B of the present invention, as shown in (II) of FIG. 1, carbon particles having a particle size of 0.01 to 5 μm are implanted into the surface of hydrogen storage alloy particles (mother particles) having a particle size of 0.1 to 50 μm. By disposing the surface-modified hydrogen-absorbing alloy powder bonded in a mold only on the surface of the negative electrode made of hydrogen-absorbing alloy as shown in FIG. 2, without increasing the resistance of the negative electrode itself and during overcharging. Carbon, which is oxidation resistant to the oxygen gas generated from the nickel positive electrode, prevents the hydrogen storage alloy from being oxidized, and further, carbon particles having a large specific surface area of 100 m ² / g or more act as an oxygen catalyst. Suppresses the rise in battery internal pressure during charging,
Achieved longer battery life. At the same time, the decrease in discharge capacity is reduced and the high rate discharge characteristics are excellent, which is of high practical value as compared with conventional batteries. In such high-rate charging, the reason why the increase in battery internal pressure and the increase in capacity are obtained is that the oxygen catalyst works efficiently on the surface of the negative electrode plate to absorb the oxygen gas generated from the positive electrode. Oxidation of the surface of the hydrogen storage alloy due to gas is suppressed, and the hydrogen storage alloy coated with carbon particles also contributes to the discharge capacity. Further, since the carbon particles are used for the oxidation inhibiting layer, it is considered that the surface layer of the oxidation inhibiting layer has a large surface area and the oxygen catalyst becomes very active, thereby increasing the oxygen absorption rate.

By using a fluororesin when forming the oxidation suppressing layer as in this example, both powders can be combined without lowering the activity of the oxygen catalyst. The hydrogen-occlusion alloy powder coated with metal, the carbon powder, and the fluororesin powder are independent of each other, and the fluororesin intervenes between the bonds to strengthen the bond between both particles. Moreover, a great effect can be seen where the surface area is not reduced.

In this embodiment, a powder obtained by mechanically crushing a hydrogen storage alloy was used, but a hydride obtained by hydrogenating the hydrogen storage alloy and subdividing the hydrogen storage alloy may also be used. Making a negative electrode in the state of dehydrogenating the hydrogenated powder, to form a sealed alkaline storage battery,
When the characteristics were measured, the same value as that of the battery B was obtained.

In this embodiment, the carbon particles are used as a simple substance. However, when a catalytic metal is supported on the carbon particles, the oxygen catalytic action is improved, so that the oxygen gas absorption rate is increased. However, since it causes a problem in terms of price, it is desirable to add a small amount. Like the battery of the present invention, since the surface-modified hydrogen storage alloy particles having a large surface area are arranged on the surface of the negative electrode, a large effect can be expected even if a small amount of catalyst metal is carried.

The average particle size of the carbon particles (child particles) for modifying the surface of the hydrogen storage alloy or hydride powder (mother particles) is preferably 1/10 to 1/100 of the average particle size of the mother particles, Outside this range, the child particles do not adhere and bond uniformly to the surface of the mother particles, especially when the child particles become large, they easily fall off, the modification effect is thin, and the particle size is also large in the surface modification effect. Affects, the mother particles are 0.1-50 μm, and the child particles are
The range of 0.01 to 5 μm is efficient and the surface modification proceeds uniformly.

If each particle is too fine, the binding effect will be lost and surface modification will be difficult. On the other hand, if it is too large, the mother particles are further crushed during the surface modification, and the surface which is not surface modified is exposed, so that uniform surface modification cannot be performed.

Therefore, the optimum range of the mother particles is 0.1 to 50 μm, the daughter particles are 0.01 to 50 μm, and the daughter particles are in the range of 1/10 to 1/100 of the mother particles.

EFFECTS OF THE INVENTION As described above, according to the present invention, it is possible to provide a high-capacity sealed alkaline storage battery having high safety during overcharge, excellent efficiency discharge characteristics, and long cycle life.

[Brief description of drawings]

1 and 2 are schematic diagrams showing the structure of surface-modified hydrogen storage alloy particles used in the present invention, and FIGS. 2 I, II, and III are schematic diagrams showing the structure of the negative electrode plate in the present invention, and FIG. Is a perspective view showing a configuration of a sealed alkaline storage battery used in an embodiment of the present invention,
FIG. 4 is a diagram showing a current-voltage curve. 1 ... Hydrogen storage alloy particles, hydride particles (mother particles), 2
...... Carbon particles (child particles), 3 ... Hydrogen storage alloy, hydride, 4 ... Oxidation suppressing layer, 5 ... Negative electrode plate.

Claims (3)

[Claims] 1. A hydrogen storage alloy or hydride powder comprising a metal oxide positive electrode, a negative electrode composed of a hydrogen storage alloy or a hydride, a separator and an alkaline electrolyte, and the surface of the negative electrode coated with carbon particles (child particles). A sealed alkaline storage battery, characterized in that an oxidation-inhibiting layer composed of (mother particles) and a binder is provided. 2. Hydrogen storage alloy or hydride powder (mother particles)
The closed type according to claim 1, wherein the average particle size of the carbon particles (child particles) coating the surface of the is in the range of 1/10 to 1/100 of the average particle size of the mother particles. Alkaline storage battery. 3. Hydrogen storage alloy or hydride powder (mother particles)
2. The sealed alkaline storage battery according to claim 1, wherein the particle range of 0.1 to 50 μm and the particle range of the carbon particles (child particles) coating the mother particles are 0.01 to 5 μm.