JP2002270166A - Nickel electrode and nickel.hydrogen secondary battery with usage of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nickel electrode having a nickel hydroxide particle as an active material, which is often used as the active material, and a nickel.hydrogen secondary battery exhibiting excellent cycle life property where in the nickel electrode is installed. SOLUTION: The nickel hydroxide particles are covered with a conductive film mainly consisting of cobalt oxyhydroxide formed by heat-treating metal cobalt or cobalt compound and alkaline aqueous solution under co-existence of oxygen. The nickel hydroxide particles covered with the conductive film are carried on a current collecting substrate to form the nickel electrode. When thermal decomposition temperatures of the nickel hydroxide particle before and after forming the conductive film are T0 deg.C and T1 deg.C respectively and the temperature at heat treatment is T, following relations are formed among T0 , T1 , and T: T deg.C<T0 <=270 deg.C, and 0.1<=(T0 -T1 )×100/T0 <=5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nickel electrode using nickel hydroxide particles as an active material, and a nickel-hydrogen secondary battery using the same. The present invention relates to a nickel-hydrogen secondary battery having excellent cycle life characteristics when incorporated as a positive electrode.

[0002]

2. Description of the Related Art A typical example of an alkaline secondary battery is a nickel-hydrogen secondary battery and a nickel-cadmium secondary battery. Both of these batteries mainly use nickel hydroxide particles as a positive electrode active material. A nickel electrode carried as a component is incorporated as a positive electrode. The nickel electrode has two types: a sintered type and a paste type.
The types are known.

[0003] Among these, the paste-type nickel electrode is generally manufactured as follows. That is, first, nickel hydroxide particles functioning as a positive electrode active material and a binder such as carboxymethylcellulose, methylcellulose, sodium polyacrylate, and polytetrafluoroethylene are kneaded with water to form a viscous positive electrode mixture. A paste is prepared. Then, the paste of the positive electrode mixture is applied to, for example, a foamed nickel substrate, a net-like sintered substrate of metal fibers, or a three-dimensional substrate in which the surface of a nonwoven fabric is nickel-plated,
Filled or coated on a current collecting substrate which is a two-dimensional substrate such as a nickel punched sheet or expanded nickel, further dried, and then subjected to pressure molding, and the dried positive electrode mixture is filled and carried on the current collecting substrate Is done.

[0004] Since the paste-type nickel electrode manufactured in this manner has a higher packing density of nickel hydroxide particles (active material) than that of a sintered nickel electrode, it is possible to provide a battery having a high discharge capacity. This is advantageous in that it can be performed. By using the above-mentioned nickel electrode as a positive electrode, a nickel-hydrogen secondary battery is assembled as follows.

First, a separator having electrical insulation and liquid retaining properties is interposed between the above-mentioned nickel electrode and a negative electrode having a negative electrode mixture mainly composed of a hydrogen storage alloy supported on a current collecting substrate. The power generation element is manufactured. As the separator, for example, a nonwoven fabric of a polyamide fiber or a nonwoven fabric of a polyolefin fiber such as a polyethylene fiber or a polypropylene fiber subjected to a hydrophilic treatment is generally used.

Next, the above-described power generating element is placed in a bottomed battery can (also serving as a negative electrode terminal) made of, for example, a nickel-plated steel sheet, and a predetermined amount of a predetermined alkaline electrolyte is further injected. As the alkaline electrolyte, an aqueous solution of sodium hydroxide, an aqueous solution of potassium hydroxide, an aqueous solution of lithium hydroxide, and an appropriate mixed aqueous solution thereof are usually used.

After the positive electrode terminal is provided in the opening of the battery can, the whole is sealed and the intended nickel-hydrogen secondary battery is assembled. By the way, in the case of the above-mentioned nickel electrode, there is a problem that the utilization rate of nickel hydroxide at the time of charge and discharge is low, and therefore, the cycle life characteristics tend to be short. The cause of the problem that the utilization rate of nickel hydroxide in the nickel electrode having the above-described structure is low can be roughly described as follows.

First, the nickel hydroxide particles present as an active material have no conductivity. When a three-dimensional substrate is used as the current collecting substrate, the diameter of the holes existing in the skeleton (conductive path) of the three-dimensional structure is usually 100
Although it is about 500 μm, which is much larger than that of the nickel hydroxide particles, the pores are filled with a positive electrode mixture mainly composed of nickel hydroxide particles. Therefore, even if a charge is generated on the surface of the nickel hydroxide particles as an active material due to the battery reaction, the current collection efficiency of the current collection substrate is high because the resistance between the nickel hydroxide particles and the skeleton of the current collection substrate is large. And the polarization of the nickel electrode tends to increase.

That is, since the collection of the generated electric charge does not proceed smoothly, the battery reaction is hindered, and the utilization rate of the active material (nickel hydroxide) decreases. Also,
An increase in polarization causes a decrease in discharge voltage. Then, at the time of charging, γ-nickel oxyhydroxide, which is an irreversible charge product that does not discharge, is generated.
Nickel hydroxide having a layered crystal structure penetrates between layers of the crystal structure, and is released at the time of discharge. Therefore, as the charge / discharge proceeds, the amount of this γ-nickel oxyhydroxide in the crystal structure of nickel hydroxide increases,
The amount of nickel hydroxide that contributes to the battery reaction is reduced, resulting in a decrease in its utilization.

[0010] Further, γ-nickel oxyhydroxide is a higher-order oxide and has a lower density than β-nickel oxyhydroxide which contributes to charging and discharging of a battery. Therefore, the crystal strain increases due to the repetition of intrusion and release into the crystal structure during charge and discharge, the swelling of the active material supported on the nickel electrode accompanying the increase in the amount of γ-nickel oxyhydroxide, and the resulting electrolysis. A situation of liquid absorption occurs, causing a significant decrease in cycle life characteristics, and in the worst case, a situation in which the battery reaction stops in a short period of time.

From the above, it is possible to increase the conductivity between the active materials (nickel hydroxide particles) and between the active material and the current collecting substrate, and to improve the conductivity of γ-nickel oxyhydroxide during charging. Minimizing generation is an important technical issue for improving the utilization rate of the active material and improving the cycle life characteristics. First, with regard to the former problem, for example, when preparing a paste for a positive electrode mixture, particles of cobalt compound such as metallic cobalt, cobalt hydroxide, cobalt trioxide, cobalt tetroxide, cobalt monoxide, or a mixture thereof are used. There is a method in which a predetermined amount is added to nickel hydroxide particles as a conductive material to form a mixture, which is used as an active material.

When a nickel electrode carrying such a mixture is incorporated as a positive electrode of a nickel-hydrogen secondary battery, any of the metallic cobalt and the cobalt compound contained in the above-mentioned mixture is once added to the alkaline electrolyte. It dissolves as complex ions, which are distributed on the surface of the nickel hydroxide particles. Then, at the time of initial charging of the battery, these complex ions are oxidized prior to nickel hydroxide and converted into conductive cobalt oxyhydroxide, which is used as an active material between the nickel hydroxide particles. It precipitates between the material layer and the current collecting substrate, and a so-called conductive matrix is formed on the entire positive electrode mixture supported on the current collecting substrate. As a result, the conductivity between the active materials and between the active material and the current collecting substrate is improved, and the utilization rate of the active material is improved.

Japanese Patent Application Laid-Open No. 9-73900 discloses that
The following method is disclosed. In this method, a mixture of nickel hydroxide particles mixed with metal cobalt or a cobalt compound is charged into a stirring and mixing device, and while the mixture is being stirred, an aqueous alkali solution having a predetermined concentration is added thereto. Below, the whole is subjected to a heat treatment by irradiating radiation such as microwaves from a magnetron, for example.

Therefore, the treated particles obtained by this method are aggregates of particles in which the surface of the base nickel hydroxide particles is entirely or partially covered with a film mainly composed of cobalt oxyhydroxide. ing. And since the above-mentioned film of cobalt oxyhydroxide is a conductive film,
The surface of the treated particles and the aggregate thereof is conductive.

Therefore, when a positive electrode mixture is prepared using the treated particles and filled into a current collecting substrate to produce a nickel electrode, the positive electrode mixture is already in a state where conductivity has been imparted to the positive electrode mixture. Therefore, the nickel hydroxide particles as the active material show a high utilization rate. In this method, the nickel hydroxide particles, which are the raw material of the positive electrode mixture, already have surface conductivity. Since the prepared positive electrode mixture is carried, the obtained nickel electrode has an advantage that the current collection efficiency is already high at the time of production.

On the other hand, when attention is paid to nickel hydroxide itself, which is an active material, it is necessary to exhibit a high utilization factor and at the same time to improve the cycle life characteristics of the battery. Even during charging, it is necessary to have a property that γ-nickel oxyhydroxide, which is an irreversible charge product and causes swelling of the nickel electrode, is not generated as much as possible.

As one of the defining factors of this property, a thermal decomposition temperature measured by thermogravimetric analysis (TGA) is known (see JP-A-7-57730). In particular,
A nickel electrode with improved utilization by using nickel hydroxide particles having a thermal decomposition temperature of 270 ° C. or lower is disclosed.

[0018]

By the way, the present inventor has applied the processing method disclosed in JP-A-9-73900 to the nickel hydroxide particles disclosed in JP-A-7-57730. When the above-mentioned conductive film is formed on the surface of the nickel hydroxide particles by application and a nickel electrode is produced using the obtained treated particles, the nickel electrode is not necessarily treated with nickel hydroxide particles (conductive Of the active material is not so much improved as compared with the nickel electrode manufactured using the non-conductive film.

That is, all of the nickel hydroxide particles exhibiting the thermal decomposition temperature disclosed in JP-A-7-57730 are effective for the treatment method described in JP-A-9-73900. It is hard to say. In the present invention, when the nickel hydroxide particles are subjected to the treatment described in JP-A-9-73900 to impart high surface conductivity, the regulation on the thermal decomposition temperature of nickel hydroxide to be used is stricter and higher. An object of the present invention is to provide a nickel electrode exhibiting a utilization rate of an active material, and a nickel-hydrogen secondary battery having excellent cycle life characteristics because it is incorporated.

[0020]

The inventor of the present invention has found that the above-mentioned phenomenon in the nickel electrode manufactured by using the nickel hydroxide particles before the treatment and the nickel hydroxide particles after the treatment is as follows. It was considered to be expressed. That is, in the above-described treatment process for nickel hydroxide, although the surface conductivity of the nickel hydroxide particles is improved, more than that, the nickel hydroxide particles are altered under the influence of the treatment conditions at that time, and as a result, It is a consideration that the property may be exhibited because the property of easily generating γ-nickel oxyhydroxide is added to the nickel hydroxide particles after the treatment.

In this case, the factors that have the greatest effect on the alteration of the nickel hydroxide particles are considered to be, for example, the heat treatment temperature and the radiation intensity. When these conditions are severe (for example, when the irradiation energy of the radiation is too large or the heat treatment temperature is too high), the layered crystal structure of nickel hydroxide is damaged, that is, the crystallinity is deteriorated, It is also conceivable to lose the function as an active material.
The degree of damage to the crystal structure is determined by the magnitude of the binding energy in the crystal, but can be macroscopically evaluated by the level of the thermal decomposition temperature.

For example, low pyrolysis temperature means that
This is because the bonding energy in the crystal structure is small and the crystal structure is easily damaged, and the high thermal decomposition temperature means that the crystal structure is stable and hardly damaged. Therefore, it is considered that the rate of change of the thermal decomposition temperature before and after the above-mentioned treatment process can be positioned as an index indicating the degree of deterioration of the crystallinity of the nickel hydroxide particles to be treated.

Therefore, various processing particles were prepared by changing the processing conditions in the above-mentioned processing steps, and the thermal decomposition temperatures of these processing particles were measured by TGA. It was found that the thermal decomposition temperature before treatment was the value described below, and that the rate of change described above improved the active material utilization rate in the nickel electrode using the treated particles having a value within the range described below. Obtained. And based on the knowledge, they came to develop the nickel electrode of the present invention.

That is, the surface of the nickel electrode of the present invention is coated with a conductive film mainly composed of cobalt oxyhydroxide formed by heat-treating cobalt metal or a cobalt compound and an aqueous alkali solution in the presence of oxygen. A nickel electrode formed by supporting nickel hydroxide particles on a current collecting substrate, wherein the thermal decomposition temperatures of the nickel hydroxide particles before and after the formation of the conductive film are T ₀ (° C.) and T ₁ , respectively.
(° C.), and when the temperature at the time of the heat treatment is T,
Between T ₀ , T ₁ , and T, the following expressions: T ° <T ₀ ≦ 270 ° C. (1) and 0.1 ≦ (T ₀ −T ₁ ) × 100 / T ₀ ≦ 5 ( 2) is characterized by the following relationship:

Further, the present invention provides a nickel-hydrogen secondary battery, wherein the nickel electrode is used as a positive electrode and the hydrogen storage alloy electrode is used as a negative electrode.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Nickel hydroxide particles as an active material in a nickel electrode of the present invention are disclosed in JP-A-9-73900.
Patent No. 5,037,045, which is a nickel hydroxide particle that has been subjected to the treatment disclosed in Japanese Patent Application Laid-Open Publication No. H10-209,878. Therefore, a conductive film is formed on the surface. Specifically, the treatment on the nickel hydroxide particles is performed as follows.

First, nickel hydroxide particles (referred to as R ₀ ) as a raw material and a predetermined amount of cobalt metal or the above-mentioned cobalt compound are mixed, and at the same time, an aqueous alkali solution such as an aqueous sodium hydroxide solution is added thereto. Add and stir and mix the whole. The stirring and mixing are performed in an oxygen-containing atmosphere (for example, the atmosphere), and at the same time, proceed under a heating environment formed by irradiating the entire system with radiation such as microwaves having a predetermined intensity. Therefore, a reaction system composed of a heating environment in which an alkaline aqueous solution and oxygen coexist is formed around the mixture.

In this reaction system, first, metal cobalt or a cobalt compound constituting a part of the mixture is dissolved in an alkaline aqueous solution to form complex ions, and the surface of nickel hydroxide particles (R ₀ ), which is a main component of the mixture, is mixed. Is coated. The mixture, mainly the nickel hydroxide particles (R ₀ ), is heated by the irradiated radiation, and at the same time, the above-mentioned cobalt complex ions are oxidized by the coexisting oxygen to be converted into conductive cobalt oxyhydroxide. I do. That is, the obtained nickel hydroxide particles (referred to as R) are in a state where the surface is covered with a conductive film mainly composed of cobalt oxyhydroxide.

Using the nickel hydroxide particles (R) thus produced, a positive electrode mixture is prepared in the same manner as in the prior art, and the positive electrode mixture is supported on a current collecting substrate to form the nickel electrode of the present invention. Manufactured. The nickel hydroxide particles (R ₀ ) used as a raw material for producing the nickel hydroxide particles (R) have a thermal decomposition temperature T represented by the formula (1).
₀ (° C) is used.

When nickel hydroxide particles (R ₀ ) having a temperature T (° C.) or lower during the heat treatment performed by T ₀ are used, the nickel hydroxide particles (R ₀ ) are thermally decomposed during the formation of the conductive film. As a result, nickel hydroxide particles (R) intended for use as an active material cannot be produced, and nickel hydroxide particles (R ₀ ) having a T ₀ higher than 270 ° C. have deteriorated crystallinity. This is because it is considered to be too much, and the function as the active material is remarkably deteriorated.

The term "thermal decomposition temperature" as used herein refers to a temperature measured by thermogravimetry analysis. The sample is heated at a constant heating rate, and the weight of the sample in the process is measured over time. It is the temperature at which the sample weight starts to decrease sharply. Such T ₀ value is, as described above, macroscopically the degree of crystallinity of the nickel hydroxide particles (R ₀ ), which is the nickel hydroxide particles (R ₀ ).
₀ ) can be realized by appropriately selecting the conditions for manufacturing. Generally, nickel hydroxide particles are produced by a neutralization reaction between an aqueous solution of a nickel salt and an aqueous alkaline solution. At that time, the degree of crystallinity can be changed by adjusting the pH value and temperature of the reaction field. Because.

Incidentally, in the above-described processing steps,
The nickel hydroxide particles (R ₀ ), which are a raw material, are subjected to a heat treatment or are subjected to external energy such as radiation, so that the obtained nickel hydroxide particles (R) have defects in the crystal structure. Often it has occurred. Therefore, the thermal decomposition temperature (T ₁ ) of the obtained nickel hydroxide particles (R) is generally considered to be lower than the thermal decomposition temperature (T ₀ ) of the nickel hydroxide particles (R ₀ ). .

In the present invention, the thermal decomposition temperature (T ₁ ) of the nickel hydroxide particles (R) is considered in consideration of the problem of the deterioration of the crystallinity, that is, the decrease of the thermal decomposition temperature. The degree of deterioration of the crystallinity of the nickel hydroxide particles before and after is represented by the rate of change (%) of the thermal decomposition temperature (T ₀ ) of the nickel hydroxide particles (R ₀ ), and the rate of change is expressed by the equation (2). It is defined as shown in.

If the rate of change is greater than 5%, the crystal structure of the nickel hydroxide particles (R ₀ ) is being destroyed during the above treatment, and the resulting nickel hydroxide particles (R)
If the rate of change is smaller than 0.1%, the deterioration of the nickel hydroxide particles (R ₀ ) is not so advanced at first, but this is because Since the processing conditions for forming the conductive film are slow, the surface conductivity is inferior, and there is a difficulty in improving the utilization rate.

This rate of change is limited by the processing conditions in the above-mentioned processing steps, in particular, the heat treatment temperature and the radiation intensity which affect the crystallinity of the nickel hydroxide particles (R ₀ ). In other words, when the processing conditions are severe, for example, when the heat treatment temperature is too high or the radiation intensity is too high, the thermal decomposition temperature (T ₁ ) of the nickel hydroxide particles (R) is greatly reduced, and in the first place. If the performance as an active material may be lost, and if the processing conditions are too slow, the thermal decomposition temperature (T ₁ ) of the obtained nickel hydroxide particles (R) will be T _{0 of the} nickel hydroxide particles (R ₀ ). Since the value does not change much as compared with the value, although the formation of γ-nickel oxyhydroxide is suppressed, the formation of a conductive film is hard to occur, and the effect of improving the utilization is impaired.

Using the nickel hydroxide particles (R) thus produced, a positive electrode mixture is prepared in the same manner as in the prior art.
After applying and filling the positive electrode mixture to the current collecting substrate, a drying process and pressure molding are performed to produce the nickel electrode of the present invention.
The nickel-hydrogen secondary battery of the present invention uses the above-mentioned nickel electrode as a positive electrode and the hydrogen storage alloy electrode as a negative electrode. In this case, if the capacity of the hydrogen storage alloy electrode (negative electrode) is set to be 1.2 to 2.0 times the capacity of the nickel electrode (positive electrode), the manufactured nickel-hydrogen secondary battery will be overcharged and overcharged. Electrode deterioration due to gas generation during discharge is suppressed,
This is preferable because the life of the battery can be extended.

[0037]

EXAMPLES (1) Production of Active Material Nickel hydroxide particles (R ₀ ) having the T ₀ values shown in Table 1 were prepared. In each case, the average particle size is 10 μm. To 100 parts by weight of each nickel hydroxide particle (R ₀ ), 5 parts by weight of cobalt hydroxide particles having an average particle diameter of 1 μm were mixed, and the mixture was charged into a fluidized-bed granulator and stirred. At this time, an 8N aqueous sodium hydroxide solution was added in such an amount that the mixture could infiltrate, and the whole was mixed.

While continuing stirring and mixing in the atmosphere, a magnetron provided to the apparatus was operated to irradiate microwaves to perform a heat treatment for 20 minutes. At this time, five types of treated particles were produced by adjusting the irradiation intensity of the microwave and changing the overall heat treatment temperature. The T ₁ value of the obtained treated particles (R) was measured. The results are shown in Table 1.

[0039]

[Table 1]

(2) Preparation of Nickel Electrode First, 0.25 parts by weight of carboxymethyl cellulose, 0.25 parts by weight of sodium polyacrylate, and PTFE dispersion (specific gravity: 1) were added to 100 parts by weight of each nickel hydroxide particle (R). (3.0, 60% by weight of solid content) 3.0 parts by weight of water and 35 parts by weight of water were mixed and kneaded to prepare a positive electrode mixture paste.

This paste was prepared using a porosity of 95% and a thickness of 1.
A 7 mm nickel-plated porous sheet was filled, dried, and roll-rolled to form a nickel electrode. (3) Assembly of Nickel-Hydrogen Secondary Battery The above-mentioned nickel electrode and MmNi _4.0 Co _0.4 Mn _0.3 A
l _0.3 (Mm is a misch metal) as a main component, a hydrophilic polyolefin-based nonwoven fabric is disposed as a separator between the electrode and a hydrogen-absorbing alloy electrode. And then sealed with an AA-size nickel-hydrogen secondary battery (1300 mAh).

(4) Cycle Life Characteristics and Active Material Utilization Utilization rate After each battery is charged at 0.1 C, the battery is discharged at 0.2 C until the final voltage reaches 1.0 V, the discharge capacity at that time is obtained, and the value is divided by the initial capacity to be activated. The utilization rate (%) of the substance was determined. The results are shown in Table 2.

[0043]

[Table 2]

As is evident from Table 2, the utilization factor of the nickel electrode supporting the nickel hydroxide particles (R) satisfying the formulas (1) and (2) defined in the present invention as an active material at the same time is as follows. 100% or more. 2. Cycle life characteristics For each battery, charge and discharge at 1C for 1.5 hours, and discharge at 1C until the final voltage reaches 1.0 V as one cycle are repeated to determine the discharge capacity. The ratio (%) was determined. The result is shown in FIG.

As is apparent from FIG. 1, the battery incorporating the nickel electrode carrying the nickel hydroxide particles of the present invention has a capacity of 60% or more of the initial discharge capacity even when the charge / discharge cycle is 300 or more. The discharge capacity has been confirmed and the cycle life characteristics are very excellent.

[0046]

As is clear from the above description, the nickel electrode using the nickel hydroxide particles defined in the present invention as an active material has a very high active material utilization rate. Therefore, a nickel-hydrogen secondary battery in which this nickel electrode is incorporated as a positive electrode has excellent cycle life characteristics.

[Brief description of the drawings]

FIG. 1 is a graph showing cycle life characteristics of an example battery and a comparative example battery.

Claims (4)

[Claims] 1. A method for collecting nickel hydroxide particles whose surface is coated with a conductive film mainly composed of cobalt oxyhydroxide formed by heat-treating metal cobalt or a cobalt compound and an alkaline aqueous solution in the presence of oxygen. A nickel electrode supported on a substrate, wherein the thermal decomposition temperatures of the nickel hydroxide particles before and after the formation of the conductive film are T ₀ (° C.) and T ₁ (° C.), respectively.
When the temperature at the time of the heat treatment is T, T ₀ ,
The following relationship is established between T ₁ and T: T ° C. <T ₀ ≦ 270 ° C. and 0.1 ≦ (T ₀ −T ₁ ) × 100 / T ₀ ≦ 5. Characteristic nickel electrode. 2. The nickel electrode according to claim 1, wherein the cobalt compound is at least one selected from the group consisting of cobalt hydroxide, cobalt trioxide, cobalt tetroxide, and cobalt monoxide. 3. The nickel electrode according to claim 1, wherein said T is 35 ° C. or higher. 4. A nickel-hydrogen secondary battery, wherein the nickel electrode according to claim 1 is used as a positive electrode and the hydrogen storage alloy electrode is used as a negative electrode.