JPH0763004B2 - 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 an alkaline storage battery, and particularly to improvement of a negative electrode.

2. Description of the Related Art Conventionally, in an alkaline storage battery using a hydrogen storage electrode of this type as a negative electrode, the surface of particles of a hydrogen storage alloy or hydride constituting the negative electrode is oxidized by repeated charging / discharging cycles to deteriorate the performance. Normally, about 50% of the theoretical capacity of the positive electrode is overcharged due to the positive electrode utilization rate, so excess oxygen gas is generated from the positive electrode, which accelerates the rate of oxidation of the hydrogen storage alloy by the oxygen gas. . In order to suppress this oxidation reaction, a negative electrode has been proposed in which an electrode is formed using only hydrogen-absorbing alloy powder whose surface is covered with carbonaceous material (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 electrode itself made of a hydrogen storage alloy. 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 solves such problems,
Even when the charging current is relatively large, the oxidation of the hydrogen storage alloy by oxygen gas and the oxygen absorption on the negative electrode surface or the ionization of oxygen are promoted in a well-balanced manner to suppress the increase in the internal pressure of the battery and to improve the high-rate discharge characteristics and charge / discharge. The purpose is to extend the discharge cycle life.

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 the surface of the negative electrode is expanded. The purpose is to provide an oxygen gas absorption layer composed of one or both of graphite and amorphous carbon and a binder. More preferably, the expanded graphite powder and the amorphous carbon powder have an average particle size smaller than that of the hydrogen storage alloy or hydride powder, and the particle size is mainly in the range of 0.01 to 50 μm. It is a thing.

As described above, one or both of expanded graphite and amorphous carbon, which have conductivity, catalytic action and oxidation resistance, on the surface of the negative electrode made of a hydrogen storage alloy or hydride and a binder such as a fluororesin. By forming an oxygen gas absorption layer composed of a mixture, the conductivity is mainly improved by the expanded graphite powder, and the oxygen gas absorption effect on the negative electrode surface by the amorphous carbon powder having a large specific surface area and the oxidation by both of these are highly corrosion resistant. Has a suppressive effect. Since this oxygen gas absorption layer is porous, has a large specific surface area, and is excellent in conductivity and oxidation resistance, it is safe by suppressing an increase in battery internal pressure, and has high rate discharge characteristics, charge / discharge cycle life characteristics. Can be improved.

The details will be described in the following examples.

Example A sample composed of commercially available Mm (Misch metal), La, Ni, and Co was weighed and mixed at a constant composition ratio, and heated and melted by an arc melting method. As an example, the alloy composition is Mm _0.5 La.
_0.5 Ni _3.5 Co _1.5 was used as the hydrogen storage alloy for the negative electrode. This alloy was crushed to 50 μm or less by a crusher, filled into a foam metal together with a binder, and then pressed and dried to obtain an electrode sample a. Next, a powder in which the surface of the alloy particles crushed by a crusher was coated with carbonaceous material was similarly filled in a foam metal together with a binder, and then pressed and dried to obtain an electrode sample b. In contrast to these conventional electrode samples, the electrode sample of the present invention was manufactured as follows. That is, as shown in FIGS. 1A and 1B, a paste-like fine powder made of expanded graphite containing a fluororesin as a binder is applied to the surface of an electrode plate of an electrode sample a of a certain size made of the hydrogen storage alloy 10. Then, the electrode sample on which the oxygen gas absorption layer 11 is formed by performing dry baking and heat treatment is c
And Further, an electrode sample using amorphous carbon instead of the expanded graphite was designated as d. Furthermore, hydrogen storage alloy 10
A paste-like fine powder made of a mixture of expanded graphite containing fluororesin as a binder and amorphous carbon was applied to the surface of the electrode plate of the electrode sample a of a certain size made of, and dried and heat-treated. The electrode sample on which the oxygen gas absorption layer 11 is formed is
And The expanded graphite used here has a particle size of about 1 to 50 μm, and the amorphous carbon has a particle size of about 0.01 to 10 μm.

Leads were attached to the various electrode samples a, b, c, d, and e to obtain the negative electrode plate 1. The amount of the hydrogen storage alloy used in the negative electrode plate 1 was 15 g so that it would be in excess of the positive electrode capacity. A known foamed nickel positive electrode and various negative electrodes are spirally wound through a separator to form a sealed alkaline storage battery of nominal 2000 mAh shown in Fig. 2, corresponding to various negative electrodes a, b, c, d, e. A battery to
It was designated as B, C, D, and E.

In FIG. 2, a negative electrode plate 1 made of a hydrogen storage alloy and a nickel positive electrode 2 are spirally wound via a separator 3 and arranged in a case 4, and insulating plates 5 and 6 are inserted above and below the safety valve to form a safety valve. It is hermetically sealed by a sealing plate 8 provided with 7. Reference numeral 9 is a positive electrode terminal connected to the positive electrode lead. 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 to control the positive electrode. 0 as an electron charge / discharge condition.
Charged at 2C (400mA) for 75 hours (charged 150%), 0.2C (400m
A) was discharged. The temperature of the charge / discharge cycle test was set to room temperature, and the internal pressure of each battery was measured at 150% charge. The battery internal pressure was compared at the 50th charge / discharge cycle. The measurement results are shown in Table 1 in comparison with the conventional battery. The voltage-current characteristics of the conventional battery and the battery of the present invention are shown in FIG.

However, the discharge capacity ratio in Table 1 is a percentage comparison of the capacity at 0.2 C after 200 cycles with respect to the initial capacity.

As is clear from Table 1 and FIG. 3, the internal pressure of the conventional type battery A reached 10 kg / cm ² , and some leakage phenomenon was observed, and the discharge capacity of the internal battery increased due to the decrease of the electrolyte. The decrease is also large. The discharge capacity after 200 cycles is 1.2 Ah, which is about 40% of the initial capacity. 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. In the conventional battery B, the increase in the internal pressure of the battery is reduced and the absorption of oxygen gas on the surface of the negative electrode proceeds relatively smoothly, but the discharge capacity after 200 cycles shows 1.5 Ah, which is about 25% of the initial capacity. is doing. It is considered that this decrease in battery capacity is caused by the internal resistance of the electrode itself increasing with charge / discharge cycles. That is, the bonding force between the carbonaceous coating alloy particles in the electrode is weakened, the contact resistance is increased, and the discharge capacity is reduced. As shown in FIG. 3, the current-voltage characteristics of the battery A in the initial stage are excellent, whereas the characteristics of the battery B are poor. Especially, the higher the current, the larger the drop in terminal voltage. This also shows that the battery B has a high internal resistance even in the initial stage. In this way, the conventional battery A, especially in terms of battery internal pressure and charge / discharge cycle life,
Battery B also has a problem in high rate discharge characteristics.

On the other hand, the batteries C, D, E of the present invention also have a battery internal pressure of 2.5 to 4.0 k.
It shows a value of g / cm ² , discharge capacity is maintained at 92 to 95%, and the current-voltage characteristics are excellent. In the battery C of the present invention, since the surface of the negative electrode is surrounded by the fine powder of expanded graphite having high conductivity, the oxygen gas absorption function is slightly low, but the high rate discharge performance is excellent. In Battery D, since the surface of the negative electrode is surrounded by fine powder of amorphous carbon having a large specific surface area, the high-rate discharge performance is slightly inferior, but the oxygen gas absorption function is excellent, and the increase in battery internal pressure is suppressed. Further, since the battery E surrounds the surface of the negative electrode with a mixture of fine powder of expanded graphite having high conductivity and fine powder of amorphous carbon having a large specific surface area, the residual amount of both expanded graphite and amorphous carbon is retained. Depending on the effect, it has excellent properties such as oxygen gas absorption capacity on the negative electrode surface, oxidation resistance by oxygen gas, electrode resistance, etc., so that the pressure rise in the battery during overcharge is small and the charge / discharge cycle life is long. Moreover, it exhibits excellent characteristics such as high rate discharge characteristics.

Preferably, the average particle size of the expanded graphite powder and the amorphous carbon powder forming the oxygen gas absorption layer on the surface of the negative electrode is smaller than the average particle size of the hydrogen storage alloy or hydride powder.
The finer the particles that make up the oxygen gas absorption layer, the better the adhesion on the electrode surface, and the less likely it is to drop out during charging and discharging.
Since the average particle size of the hydrogen storage alloy or hydride powder is excellent at about 50 μm or less in terms of characteristics, at least the particles forming the oxygen gas absorption layer are optimally in the range of 0.01 to 50 μm.
On the other hand, if it is too fine, the cost will increase due to manufacturing problems, which is not preferable from the economical viewpoint.

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. The hydrogenated powder was dehydrogenated as needed to form a negative electrode, and a sealed alkaline storage battery was constructed. The characteristics were measured, and similar values were obtained. Further, when a catalyst metal is supported on the expanded graphite powder or the amorphous carbon powder, the oxygen gas absorption capacity is improved, but this poses a problem in terms of cost, so it is desirable to add a small amount by a simple method.

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 high rate discharge characteristics, and long charge / discharge cycle life. To be

[Brief description of drawings]

1A and 1B are a side view and a front view showing the structure of a negative electrode plate in the present invention, FIG. 2 is a view showing the constitution of a sealed alkaline storage battery in an embodiment of the present invention, and FIG. 3 is a battery of the present invention. FIG. 6 is a diagram showing a comparison of current-voltage characteristics in a conventional battery and a conventional battery. 1 ... Anode plate, 10 ... Hydrogen storage alloy, 11 ... Oxygen gas absorption layer.

Claims (2)

[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 the surface of the negative electrode is bonded to either or both of expanded graphite and amorphous carbon. A sealed alkaline storage battery provided with an oxygen gas absorption layer composed of an agent. 2. The average particle size of the expanded graphite powder and the amorphous carbon powder forming the oxygen gas absorption layer on the surface of the negative electrode is smaller than the average particle size of the hydrogen storage alloy or hydride powder, and the particle size is mainly The sealed alkaline storage battery according to claim 1, which is within a range of 0.01 to 50 μm.