JPH0945331A - Hydrogen storage alloy electrode and nickel-hydrogen secondary battery using the same - Google Patents

Abstract

(57) [Abstract] [Problem] A hydrogen storage alloy electrode which is useful as a negative electrode of a nickel-hydrogen secondary battery in which an increase in battery internal pressure during overcharge is suppressed and which has an excellent charge-discharge cycle life, and the same are incorporated. We provide high-capacity nickel-hydrogen secondary batteries. In this hydrogen storage alloy electrode, a mixture containing hydrogen storage alloy powder as a main component is supported on a current collector, and the mixture has water repellency, is flexible, and has a strong binding force. The fluororubber possessed is used as a binder.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen storage alloy electrode and a nickel-hydrogen secondary battery in which the hydrogen storage alloy electrode is incorporated as a negative electrode. More specifically, it is possible to suppress an increase in battery internal pressure during overcharge, The present invention also relates to a hydrogen storage alloy electrode capable of maintaining the initial discharge capacity of the battery for a long time and a high capacity nickel-hydrogen secondary battery incorporating the same as a negative electrode.

[0002]

2. Description of the Related Art Recently, with the progress of power saving in the field of electronic technology and the progress of mounting technology for semiconductor devices, it has become possible to make various electronic devices cordless and portable. A nickel-cadmium secondary battery has been widely used as a driving power source for these electronic devices.

However, the nickel-cadmium secondary battery is not so large in energy density per unit weight or unit volume and is likely to cause environmental pollution. Therefore, recently, the nickel-cadmium secondary battery has been used. Instead of batteries, clean, high-capacity nickel-hydrogen secondary batteries have been drawing attention and are being developed.

This nickel-hydrogen secondary battery operates using hydrogen as a negative electrode active material, and a negative electrode formed by carrying a hydrogen storage alloy capable of electrochemically storing and releasing hydrogen on a current collector ( (Hydrogen storage alloy electrode) and a positive electrode (nickel electrode) formed by similarly supporting nickel hydroxide, which operates as a positive electrode active material, on a current collector via an electrically insulating, liquid-retaining and air-permeable separator. The power generating elements are stacked on top of each other to form a power generating element, the power generating element is housed in a conductive bottomed can body, and a predetermined alkaline electrolyte is poured into the can body.

The hydrogen storage alloy electrode incorporated in a battery as the negative electrode is usually manufactured as follows.
First, a hydrogen storage alloy powder having a predetermined particle size, a conductive material powder having a predetermined particle size such as carbonyl nickel powder, cobalt powder, copper powder, carbon powder, and, if necessary, further polytetrafluoroethylene powder , A polyethylene powder, a polypropylene powder, and a polyvinylidene fluoride powder, and a binder powder having a predetermined particle diameter are mixed at a predetermined ratio to form a mixed powder.

Next, an aqueous solution of a thickener prepared by dissolving one or more thickeners such as methylcellulose, carboxymethylcellulose, polyethylene oxide and polyvinyl alcohol in ion-exchanged water or distilled water is added to the mixed powder. A predetermined amount is added and the whole is kneaded to prepare a mixture paste having a predetermined viscosity. Then, a predetermined amount of the above-mentioned mixture paste is applied on both surfaces of a current collector such as punching metal (perforated plate) or expanded metal (net) and then dried, and the whole is roll-rolled to a predetermined thickness. The target hydrogen storage alloy electrode is obtained by adjusting.

When the binder powder as described above is used in preparing the mixture paste, the rolling treatment is followed by, for example, firing in a nitrogen atmosphere at a temperature near the melting point of the binder powder. As a result, these binders are softened to bind each powder.

[0008]

By the way, this nickel-hydrogen secondary battery has the property that oxygen gas is generated at the nickel electrode (positive electrode) during overcharge and the gas pressure inside the battery rises. Therefore, the nickel-hydrogen secondary battery has a safety valve, and the internal pressure of the battery is a standard value (usually
If it exceeds (1.5 MPa), the safety valve operates to release the gas inside. However, in the case of a battery whose internal pressure is high during overcharging, each time the safety valve operates and the gas inside escapes, the alkaline electrolyte in the battery will also be carried away little by little, and the battery will gradually disappear. There is a problem that the amount of the alkaline electrolyte in the battery decreases and the battery life is exhausted in a short period.

This increase in the battery internal pressure generally occurs based on the following mechanism. At the time of overcharge, first, oxygen gas is generated on the surface of the nickel electrode based on the reaction represented by the following formula: OH ⁻ → 1/4 O ₂ +1/2 H ₂ O + e ⁻ (1), and also hydrogen storage On the surface of the alloy electrode, hydrogen is absorbed in the alloy based on the reaction represented by the following formula: M + H ₂ O + e ⁻ → MH−OH ⁻ (2) (where M represents a hydrogen storage alloy).

Then, the oxygen gas generated based on the equation (1) passes through the separator and reaches the hydrogen storage alloy electrode, and on the surface thereof, the following equation: M−H + 1/4 O ₂ → M + 1/2 A gas absorption reaction represented by H ₂ O (3) occurs, and oxygen gas is restored to water and consumed. In this case, if the above gas absorption reaction does not occur properly on the surface of the hydrogen storage alloy electrode,
The oxygen gas diffused from the nickel electrode is not restored to water, and at the same time, it oxidizes the surface of the hydrogen storage alloy electrode to inhibit the hydrogen storage capacity. Therefore, as the overcharge operation progresses, the gas is accumulated in the battery as a gas, and the internal pressure of the battery rises.

To solve the problem of increase in battery internal pressure during overcharge, a measure called positive electrode regulation which has been conventionally applied to nickel-cadmium secondary batteries has been taken. That is, it is a method of configuring the hydrogen storage alloy electrode as a negative electrode so that the theoretical capacity of the hydrogen storage alloy electrode becomes larger than the theoretical capacity of the active material (nickel hydroxide) supported by the nickel electrode (positive electrode). .

[0012] By doing so, theoretically, a large portion of the surface of the hydrogen storage alloy electrode that can contribute to the reaction represented by the equation (3) is increased, and the effect of restoring the diffused oxygen gas to water is effective. It is possible to suppress the increase in the internal pressure of the battery. However, as a practical problem, in the case of a nickel-hydrogen secondary battery, there is a problem that even if the above-mentioned positive electrode regulation is carried out, the rise in the battery internal pressure at the time of overcharge is not so suppressed. The reason is not always clear, but on the surface of the hydrogen storage alloy electrode during overcharge, the reaction represented by the following formula: H ₂ O + e ⁻ → 1/2 H ₂ + OH ⁻ (4) It is thought that this is because hydrogen has been generated due to the occurrence of.

By the way, in order for the reactions represented by the above equations (2) and (3) to properly proceed on the surface of the hydrogen storage alloy electrode, the hydrogen storage alloy electrode ( It is necessary that the three-phase interface consisting of solid), alkaline electrolyte (liquid), and oxygen gas or hydrogen gas (gas) is properly formed. If only the solid-liquid interface or the liquid-gas interface is formed on the surface of the hydrogen storage alloy electrode, the gas absorption reaction based on Eq. (3) does not occur, and the internal pressure of the battery changes over time. Because it rises to.

Here, forming a gas-solid-liquid three-phase interface in a proper state on the surface of the hydrogen storage alloy electrode means imparting proper hydrophilicity to the electrode surface and at the same time having proper hydrophobicity. It means to let. Based on this idea, conventionally, for example, polytetrafluoroethylene (PT) is used as a binder when manufacturing a hydrogen storage alloy electrode.
FE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (PFE)
P), tetrafluoroethylene-ethylene copolymer (P
ETFE), polychlorotrifluoroethylene (PCT
FE) or polyvinylidene fluoride (PVDF) is used to give water repellency to the electrode surface by using a slightly larger amount of fluororesin, and the hydrogen storage alloy electrode that has already been manufactured is provided with, for example, polytetrafluoro of a predetermined concentration. Efforts have been made to impart sufficient water repellency to the surface of an electrode by immersing it in an ethylene dispersion to sufficiently secure a gas-solid-liquid three-phase interface on the electrode surface.

However, in the case of the hydrogen storage alloy electrode produced by these methods, the water repellency imparted is not sufficient, and the performance of suppressing the rise in the battery internal pressure during overcharge is not very good. On the other hand, in the case of the nickel-hydrogen secondary battery, as the charge / discharge cycle is repeated, in the hydrogen storage alloy electrode, the volume change of the hydrogen storage alloy powder carried on the current collector is about 10 to 20% ( Swelling and shrinking), resulting in miniaturization, and the situation where the current collector gradually peels off. This means
This is an inconvenient situation in that the discharge capacity of the battery is reduced.

To solve such a problem, Japanese Patent Application Laid-Open No. 2-135
According to Japanese Patent No. 665, a thermoplastic elastomer, such as a styrene-butadiene copolymer, which is rich in flexibility and has an excellent binding ability, a tetrafluoroethylene resin, and a tetrafluoroethylene-hexafluoropropylene copolymer. It has been proposed to use a material obtained by kneading such a fluororesin as a binder in the production of hydrogen storage alloy electrodes.

In the case of the hydrogen storage alloy electrode disclosed in this prior art, since the binder is flexible and has excellent binding ability as described above, the binder is hydrogen during the charge / discharge cycle. The hydrogen storage alloy powder can be appropriately deformed in accordance with the volume change of the storage alloy powder, and the hydrogen storage alloy powder is prevented from peeling off, so that the discharge capacity of the battery is maintained for a relatively long time.

However, this hydrogen storage alloy electrode is
It cannot be said to be sufficiently effective in terms of suppressing the rise in battery internal pressure during overcharging. INDUSTRIAL APPLICABILITY The present invention can solve the above-mentioned problems in the hydrogen storage alloy electrode of the nickel-hydrogen secondary battery and suppress the increase of the battery internal pressure at the time of overcharging of the nickel-hydrogen secondary battery incorporating the same as the negative electrode. At the same time, it is an object of the present invention to provide a hydrogen storage alloy electrode having a long charge / discharge cycle life, that is, capable of suppressing a decrease in the discharge capacity of the battery for a long period of time, and a high capacity nickel-hydrogen secondary battery.

[0019]

In order to achieve the above-mentioned object, in the present invention, in a hydrogen storage alloy electrode in which a mixture containing hydrogen storage alloy powder as a main component is supported on a current collector, The agent is provided with a hydrogen storage alloy electrode, characterized in that it contains fluororubber as a binder, and the hydrogen storage alloy electrode as a negative electrode, and,
There is provided a nickel-hydrogen secondary battery characterized in that a nickel electrode, in which a mixture containing nickel hydroxide as a main component is carried on a current collector, is incorporated as a positive electrode.

In the hydrogen storage alloy electrode of the present invention, the above-mentioned fluororubber functions as a binder for the alloy powder. In this case, since this fluororubber is flexible and has a strong binding force, it is possible to appropriately follow the volume change of the alloy powder that occurs during the charge / discharge cycle and prevent the alloy powder from peeling off. Suppress the decrease in capacity for a long time.

The fluororubber itself has water repellency. Therefore, since the water-repellent fluororubber is properly exposed on the surface of the hydrogen storage alloy electrode and the three-phase interface is properly formed, the increase in the battery internal pressure at the time of overcharge is suppressed. Furthermore, as mentioned above,
Fluorine rubber has all the characteristics such as water repellency, flexibility, and binding force required as a binder for hydrogen-absorbing alloy electrodes by itself, so that each of the above-mentioned properties can be exhibited when preparing a mixture paste. It is not necessary to add another component to the hydrogen storage alloy powder, and this fluororubber is sufficient. Therefore, the relative amount of the hydrogen storage alloy powder in the prepared mixture paste increases, and the capacity of the hydrogen storage alloy electrode supporting the powder increases.

Therefore, the nickel-hydrogen secondary battery in which the hydrogen storage alloy electrode is incorporated as the negative electrode suppresses an increase in the battery internal pressure during overcharging, prolongs the charge / discharge cycle life, and becomes a high-capacity battery.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION In the hydrogen storage alloy electrode of the present invention, the binder is made of fluororubber which is flexible, has a strong binding force, and has water repellency. As the fluororubber, the following can be cited as a suitable example. That is, 1. The following formula:

[0024]

[Equation 1]

1. A copolymer of vinylidene fluoride and hexafluoropropylene represented by (m and n are integers of 1 or more); The following formula:

[0026]

[Equation 2]

2. A copolymer of tetrafluoroethylene and propylene represented by (m and n are integers of 1 or more); The following formula:

[0028]

(Equation 3)

(M and n are integers of 1 or more, R ₁ is carbon number 1
3. an alkyl group of 4 to 4) and a copolymer of tetrafluoroethylene and perfluorovinyl ether; The following formula:

[0030]

(Equation 4)

4. A copolymer of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene represented by (m, n and r are integers of 1 or more); The following formula:

[0032]

(Equation 5)

5. A copolymer of vinylidene fluoride and pentafluoropropylene represented by (m and n are integers of 1 or more); The following formula:

[0034]

(Equation 6)

6. A copolymer of vinylidene fluoride, pentafluoropropylene and tetrafluoroethylene represented by (m and n.r are integers of 1 or more); The following formula:

[0036]

(Equation 7) 7. A copolymer of vinylidene fluoride and chlorotrifluoroethylene represented by (m and n are integers of 1 or more); The following formula:

[0037]

(Equation 8)

1 to 7 described above and a hard segment which is a fluororesin such as PTFE or PFEP.
For example, a copolymer with a soft segment which is a fluororubber such as a copolymer of. Further, a compound having at least a part of each of the above structural blocks in the molecular structure can also be used. The hydrogen-absorbing alloy electrode of the present invention is a mixture of hydrogen-absorbing alloy powder, a conductive material, and the above-mentioned fluororubber, and a mixture paste by adding a predetermined amount of a thickener aqueous solution such as a carboxymethylcellulose aqueous solution to the mixture. It can be manufactured by preparing and filling the current collector with the mixture paste, drying and rolling, and supporting the current on the current collector.

In this case, the fluororubber can be used in any of solid, liquid and latex forms. When the amount of this fluororubber is adjusted to be within a range of 0.05 to 10 parts by weight with respect to 100 parts by weight of the hydrogen storage alloy powder, the hydrogen storage alloy electrode produced is overcharged at the time of overcharging. This is preferable because the rise in battery internal pressure can be effectively suppressed and the decrease in discharge capacity can be prevented over a long period of time. Particularly, when it is 0.3 to 3 parts by weight, excellent effects are exhibited.

[0040]

【Example】

Examples 1 to 15 and Comparative Examples 1 and 2 Composition: MmNi _3.23 Co _0.98 Mn _0.29 Al _0.39 (where Mm represents misch metal, La: 26% by weight,
Ce: 50 wt%, Nd: 15 wt%, Pr: 9 wt%
The hydrogen storage alloy of (1) is crushed with a hammer mill, further crushed with a cross beater mill, and then classified to obtain a particle size of 2
Powders of 0-60 μm were prepared.

After 100 parts by weight of this alloy powder, 10 parts by weight of nickel powder having an average particle size of 0.3 μm and the above-mentioned fluororubber in the amount (parts by weight) shown in Table 1 as a solid content were uniformly mixed. Then, 24 parts by weight of 1% carboxymethyl cellulose aqueous solution was added thereto to prepare various mixture pastes. A steel punching sheet with a thickness of 60 μm (aperture ratio 38
%) On both surfaces, and then dried at a temperature of 80 ° C. and further rolled with a roller press to adjust the thickness to obtain a hydrogen storage alloy electrode having a thickness of about 0.4 mm.

The hydrogen storage alloy electrode and a nickel electrode having an active material mixture containing nickel hydroxide as a main component supported on a foamed nickel plate are superposed with a nylon nonwoven fabric separator therebetween and spirally wound. Then, it is housed in a battery case that also serves as a negative electrode terminal, and a predetermined amount of alkaline electrolyte is injected into the battery case.
A 00 mAh nickel-hydrogen battery was assembled.

At this time, the theoretical capacity of the nickel electrode is set to 1 / 1.6 of the theoretical capacity of the hydrogen storage alloy electrode. Also, the safety valve attached to the lid plate is about 3.
The valve was adjusted to operate at an internal pressure of 0 MPa. Furthermore, a small hole having a diameter of 1 mm was formed in the bottom of the battery case, and a pressure sensor for measuring the internal pressure was attached to the small hole.

This battery was subjected to a battery internal pressure test and a charge / discharge cycle test with the following specifications. Battery internal pressure test: Charge a charging current of 1 CmA at a temperature of 20 ° C. to 450% of the theoretical capacity of the nickel electrode and measure the battery internal pressure at that time. Charge / Discharge Cycle Test: A cycle in which at a temperature of 20 ° C., a charge current of 1 CmA overcharges the battery to 150% of the theoretical capacity of the nickel electrode, and a discharge current of 1 CmA continuously discharges the battery voltage to 1.0 V. After repeating 500 times, the discharge capacity was measured, and the percentage of the value with respect to the initial discharge capacity was calculated and evaluated as the discharge capacity maintenance rate.

The larger this value is, the more the battery is suppressed in the decrease of the discharge capacity for a long period of time. The initial discharge capacities of all the batteries used in the test were 1170 to 12
It was within the range of 00 mAh. Table 1 shows the above results. For comparison, in place of the fluororubber used in the examples, PTFE dispersion (D-1 manufactured by Daikin Industries, Ltd.) was blended in an amount of 1.5 parts by weight as a solid content, and with the examples. Similarly, a hydrogen storage alloy electrode (Comparative Example 1) was manufactured.

Instead of fluororubber, styrene / ethylene / butadiene / styrene latex (SEBS latex) manufactured by Japan Synthetic Rubber Co., Ltd. is used as a solid content of 1.5.
A hydrogen storage alloy electrode (Comparative Example 2) was produced in the same manner as in Example except that 1.0 part by weight of the above PTFE dispersion was mixed as a solid content.

A nickel-hydrogen battery incorporating these hydrogen storage alloy electrodes was also subjected to a battery internal pressure test and a charge / discharge cycle test, and the results are shown in Table 1.

[0048]

[Table 1]

As is clear from Table 1, in all the batteries incorporating the hydrogen storage alloy electrodes of the examples, the battery internal pressure was a low value of 0.3 to 0.6 MPa, and after 500 cycles of charge and discharge. The discharge capacity retention rate of each of the batteries was 92% or more, the internal pressure of the battery was 0.3 to 0.6 MPa lower than that of the comparative example, and the discharge capacity maintenance rate after 500 cycles of charge and discharge was 1%.
0% or more is excellent. This is 120mAh in capacity
The above also means that the battery of the present invention has a higher capacity than the battery of the comparative example.

Further, the batteries incorporating the electrodes of Examples 3 to 5 had extremely low battery internal pressure of less than 0.4 MPa and a discharge capacity maintenance ratio of more than 95%, and thus exhibited excellent characteristics. . This means that the amount of fluororubber used as the binder is 0.3 with respect to 100 parts by weight of the hydrogen storage alloy powder.
It has been shown that it is particularly suitable to use ˜3.0 parts by weight.

[0051]

As is apparent from the above description, the hydrogen storage alloy electrode of the present invention suppresses an increase in battery internal pressure during overcharge of a nickel-hydrogen secondary battery incorporating the same.
Moreover, even if the charge / discharge cycle is repeated over a long period of time, the decrease in battery capacity is reduced. This is an effect brought about by using, as the binder, a fluororubber having water repellency and being highly flexible and having a strong binding force at the same time.

The nickel-hydrogen secondary battery incorporating the hydrogen storage alloy electrode has a higher capacity than conventional batteries.

Claims (3)

[Claims] 1. A hydrogen storage alloy electrode in which a mixture containing hydrogen storage alloy powder as a main component is supported on a current collector, wherein the mixture contains fluororubber as a binder. And hydrogen storage alloy electrode. 2. The content of the fluororubber in the mixture is 0.05 to 10 with respect to 100 parts by weight of hydrogen storage alloy powder.
The hydrogen storage alloy electrode according to claim 1, which is parts by weight. 3. The hydrogen storage alloy electrode according to claim 1 is incorporated as a negative electrode, and the nickel electrode in which a mixture containing nickel hydroxide as a main component is carried on a current collector is incorporated as a positive electrode. Nickel-hydrogen secondary battery.