JPH0388270A - Electrode for nickel-cadmium storage battery - Google Patents

Abstract

PURPOSE:To restrain the separation and coming off of an active material during formation and other processes by applying a specific active material mix to a conductive, two-dimensional substrate whose surface is made uneven by metal thermal-spray. CONSTITUTION:A conductive two-dimensional substrate whose surface is made uneven by metal thermal-spray is used as an active material holding body. An active material mix containing at least one of cadmium oxide, cadmium hydroxide, and metallic cadmium bonded with polytetrafluoroethylene is applied to this substrate. The amount of solvent in the paste is capable of reducing, and high density paste necessary for high density filling can be obtained. Mechanical and electrical contact of the active material with the two-dimensional substrate is increased. The separation and coming off of the active material from the substrate can be avoided.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to electrodes for nickel-cadmium storage batteries, particularly cadmium electrodes.

[Prior Art] Sealed nickel-cadmium storage batteries have high reliability and excellent charging and discharging characteristics, and as electrical equipment becomes more portable, demand for them is increasing significantly. In particular, as electronic devices become more compact, there is a strong demand for higher capacity batteries, which serve as power sources.

The common method for manufacturing electrodes for nickel-cadmium storage batteries is the so-called sintered electrode method, in which the active material is impregnated into the pores of a porous nickel material obtained by sintering carbonyl nickel powder. .

Although the sintered electrode has excellent characteristics such as being capable of rapid charging and discharging, the diameter of the sintered body formed is 10 μm.
There is a limit to the ability to uniformly and densely fill the active material into the pores of the same size, and there is a problem that it cannot sufficiently meet the demand for higher capacity, which is the desire to obtain more electricity from a battery of the same shape.

Therefore, there is a need for batteries with electrodes in which the active material accounts for a larger proportion, and paste-type electrodes have been proposed to meet this demand. This paste-type electrode has a structure in which the active material powder is directly filled into the current collector, and not only can it be filled with a large amount of active material since it does not use sintered nickel, but also
Since complicated operations such as sintering and impregnation can be omitted, the proportion of production is gradually increasing.

This paste type electrode is mainly applied to cadmium electrodes because the strength of the active material layer is weaker and the conductivity of the active material is poorer than that of the sintered type electrode. This paste-type electrode is manufactured using an active material mixture whose main component is a cadmium compound such as cadmium oxide, polyvinyl alcohol (hereinafter referred to as PvA), carboxymethyl cellulose (hereinafter referred to as CMC), etc. A method in which a solution of a binder dissolved in a solvent such as water or ethylene glycol is kneaded to form a slurry, and this is applied onto a two-dimensional conductive substrate such as a perforated plate or wire mesh and dried (conventional method) 1
...For example, JP-A-63-86259, JP-A No. 56-
No. 46668) or the active material mixture and polytetrafluoroethylene (hereinafter referred to as PTFE) are kneaded using water as a dispersion medium, and then applied onto a two-dimensional substrate, and the PTFE is made into fibers by hot pressing or the like. , a method of binding to form an electrode (
Conventional Example 2 (Japanese Patent Publication No. 54-1894) has been proposed.

[Problems to be Solved by the Invention] Paste-type electrodes generally have a problem in that they are inferior to sintered-type electrodes in terms of strength and conductivity.

In terms of strength, compared to sintered electrodes that use nickel sintered bodies as the electrode framework, paste electrodes have lower mechanical strength because the active material paste itself maintains the electrode shape. Pressing Step 9 In the electrode winding step for forming a cylindrical battery, there is a problem in that the active material is likely to peel off or fall off. For this reason, with paste-type electrodes, the yield is reduced or short circuits occur after filling a container.

Regarding conductivity, compared to a sintered electrode in which the active material is filled into the micropores formed by nickel particles, a paste electrode has a relatively larger proportion of the active material, so the resistance tends to be higher. Therefore, there is a problem that the conductivity decreases.

To solve this problem, in paste-type electrodes, a conductive material such as fine nickel powder is added to the active material. However, even with such paste-type electrodes, the interfacial contact between the active material layer and the electrode base is insufficient compared to sintered-type electrodes.
It is necessary to increase the microscopic contact area.

Conventional Example 1 using a solution-type binder such as PVA is a molding method in which a slurry containing the above-mentioned active material mixture is passed through a perforated plate, etc., which serves as a holder, and the slurry is applied to the holder. Generally, the desired amount of active material is obtained by molding the material using a slit or the like and drying it. Therefore, in order to obtain a uniform slurry, a certain degree of fluidity is required of the slurry, and therefore the amount of solvent in the slurry is usually large. This results in a relative decrease in the content of the active material mixture, which causes a decrease in the packing density of the slurry and thus of the electrode after drying.

In order to increase the capacity, it was necessary to increase the packing density of the dried electrode through subsequent processing. As a solution to increase the packing density, it is common to densify semi-dried or dried electrodes by roll pressing, etc., but deformation due to shearing force during pressing causes bonds between active material particles. There were drawbacks such as a decrease in adhesion, a tendency to short-circuit, and a decrease in current collection performance and active material utilization rate due to the occurrence of cracks between the paste and the current collector.

On the other hand, in Conventional Example 2, the active material particles are bound by a network made of fibrous PTFE, which is different from the glue-like solution binder, so the solvent required to make it into a paste is Since the amount can be halved, the packing density of the paste and dried electrode can be improved. Furthermore, even in the event that it is necessary to increase the density by roll pressing, etc., the network-like bonds are more flexible against deformation due to pressing than solution-type binders, so the bonds between active material particles It has the advantage that it is less likely to be damaged. However, P.T.F.
Due to the nature of E, the paste bound by this has poor binding properties to two-dimensional substrates, especially perforated plates, and is not only significantly inferior in adhesiveness compared to conventional method 1, but also difficult to form in caustic alkali. When applied, regardless of whether or not densification is achieved by roll pressing or the like, the active material peels off and falls off from the two-dimensional substrate significantly, making it difficult to form an electrode.

[Means for Solving the Problems] The present invention has been made to solve the problems as described above. It is used as a holder and is coated with a cathode active material mixture bound with PTFE.

As a method of forming irregularities on the surface of an active material holder, which is a current collector, a method of press-molding a perforated plate is proposed in JP-A-50-78840. Although this method is effective in improving adhesion, there are limits to the number and shape of unevenness when forming unevenness through press working, and the binding between the active material mixture and the active material mixture is not necessarily sufficient. I couldn't say it.

The metal spraying used in the present invention as a method of creating unevenness is a method of melting and spraying metal such as wire or powder.
Generally, this is often carried out for the purpose of surface protection and uniformity. However, it has been found that by selecting conditions suitable for surface roughening, it is possible to create irregularities of approximately 50 to 200 μm, although this varies depending on the material. Furthermore, since the shape of the protrusion can be made more complex than in press processing, the contact surface area with the active material mixture can be dramatically increased.

Conductive two-dimensional substrates such as perforated plates, wire meshes, and expanded metals are suitable for these thermal sprays.

In particular, perforated plates are the most suitable in terms of material yield and effectiveness during thermal spraying.

On the other hand, compared to conventional sintered electrodes, sintered short metal fibers, or foamed metal substrates that have a three-dimensional structure, two-dimensional substrates are made of high-viscosity paste with a small amount of solvent to achieve high density. Suitable for painting. In particular, P with strong viscoelasticity
A paste bound with TFE is more effective in terms of manufacturing and performance than a three-dimensional substrate, as it is easy to fill with active material and it is easy to obtain electrodes with high packing density.

Furthermore, for the reasons mentioned above, even when using a thermally sprayed substrate,
If PVA, which is a solution type binder, is used, it is necessary to increase the density considerably by roll pressing, etc., so as with conventional method 1, the binding force between active materials will decrease, and there will be no countermeasure against short circuits. I don't get it. However, in electrodes using PTFE, for the reasons mentioned above, not only can the binding force of the active material be maintained, but also the active material particles near the substrate are difficult to move due to the protrusions, so it is difficult to move the active material particles near the surface by roll pressing etc. "Abrasion stress" that promotes fiberization of PTFE acts even more between the active material particles of
Since the binding by the TFE resin becomes even stronger, it is possible to significantly improve the electrode strength.

[Function] The present invention makes it possible to reduce the amount of solvent in the paste, and to create a high-density paste necessary for high-density filling, while also improving mechanical and electrical contact. Good contact between the substrate and the active material could be maintained. This means that the irregularities formed on the surface of the two-dimensional substrate by the projections by thermal spraying and the flexibility of the active material bound by PTFE maintain good contact between the active material layer and the two-dimensional substrate. This is due to a number of reasons.

[Example 1] A nickel-plated perforated steel plate (thickness 88 μm 1 porosity 50%), a 40-mesh nickel wire mesh (wire diameter 150 μm), a thickness 10 G, um were prepared by arc spraying a nickel wire rod.
Protrusions were formed on the expanded metal obtained by processing a nickel plate. An arc spraying machine manufactured by Metco was used for arc spraying, and the spraying amount was 20 mg/car from both the front and back surfaces. The protrusions obtained in this way are 3
It was 0 to 100 μm.

On these substrates, 100 g of cadmium oxide, 1 g of furnace black, reinforcing material (short fiber) Ig,
A paste consisting of 4 g of PTFE and 15 g of ethylene glycol was applied, and after drying, an electrode plate with a filling amount of 150 mg/car was obtained. An electrode plate with a thickness of 0.42 mm (packing density 3.8 g/CC) was obtained by roll pressing.

These electrode plates and electrode plates using unsprayed substrates prepared in the same manner were chemically formed in a 20% KOH aqueous solution.

Table 1 shows the active material utilization rate at that time. In both cases, thermal spraying improves the active material utilization rate. As for the cause of this, we investigated the electrodes after chemical formation and found that the lower the utilization rate, the more
Since there was a tendency for the active material to peel off and fall off from the substrate, it was found that the holding power of the active material, which is related to the surface area of the substrate, has a large influence.

Further, an electrode was produced in the same manner using a thermal sprayed substrate and using a paste with the same composition except that 40 g of PVA and ethylene glycol were used instead of PTFE. When the short-circuit rate of a battery made using these electrodes was examined, it was found to be PTFEo, 27%.

PVAI is 3%, which clearly shows that the short rate is improved when PTFE is used.

[Example 2] Arc spraying was performed in the same manner as in Example 1 except that a nickel wire and a zinc wire were used as a pair. The amount of spraying is 50mg
Thermal spraying was carried out from both surfaces at a density of /car.

By immersing the thus obtained substrate in a 10% cadmium sulfate solution for 3 br, the thermally sprayed metal zinc is used as a reducing agent, and the cadmium ions in the solution are used as a substitute for zinc by utilizing the difference in ionization tendency. was deposited as metallic cadmium on the substrate. A paste having the same composition as in Example 1 was applied onto these substrates and dried to form electrodes. For comparison, cadmium oxide 1 was added to the electrode of Example 1 and the substrate of Example 1.
00g.

Metal cadmium 25g, furnace black 1.25g
.

An electrode (Example 1') was produced in the same manner except that a paste consisting of 1.25 g of reinforcing material, 5 g of PTFE, and 2 Gg of ethylene glycol was used.

These electrodes were cut into pieces of 85 mm x 4 Gmm, and then wound with a sintered anode plate of 70 mm x 40 mm through a separator made of nylon nonwoven fabric without chemical conversion to form a battery. After charging at 70 mA for 15 hours, a test of discharging at 140 mA was repeated several times, and then discharging at 35 A.

Figure 1 shows the charging curve during the first charging.

Compared to Example N1.1', in Example 2, the charging voltage was kept low from the initial stage of charging, and good charging performance was exhibited.

The curve of 3.5A discharge is shown in FIG. Compared to Conventional Example 1, Example 1 shows better voltage characteristics. Example 1' has metal cadmium added to the paste as a discharge reserve, so it shows better voltage characteristics than Example 1, but Example 2, in which metal cadmium is precipitated by replacing zinc, is even better.

In addition, the above-mentioned nickel is used as the sprayed metal.

In addition to a mixture of nickel and other metals, an alloy containing nickel as a main component may also be used.

[Effects of the Invention] As explained above, in the electrode for a nickel-cadmium storage battery of the present invention, an active material mixture using PTFE as a binder is applied to a conductive two-dimensional substrate whose surface has been roughened by metal spraying. Because it is coated, it is not only possible to suppress the peeling and falling off of the active material during chemical formation, but also to improve the paste density when drying the coat and the electrode strength after increasing the density by roll pressing, etc.
This can be dramatically improved compared to the conventional method using VA.

This means that the packing density of the active material, which conventionally had a limit of about 3.2 g/cc, can be reduced to, for example, 3.6 to 4.1 g/cc.
It has great industrial value, as it not only improves the yield in the manufacturing process, but also improves the active material utilization rate, battery life, and rapid discharge performance.

[Brief explanation of drawings]

Figure 1 shows the charging curve of a battery using the electrode of this example chemically formed in a 20% KOH aqueous solution, and Figure 2 shows the charging curve of a battery using a 3.5 A (5 cm
A) It is a discharge curve diagram of a battery using electrodes of the present example and a conventional example during discharge.

Claims (1)

[Scope of Claims] A conductive two-dimensional substrate whose surface has been roughened by metal spraying, containing at least one of cadmium oxide, cadmium hydroxide, and metal cadmium bonded with polytetrafluoroethylene. Electrodes for nickel-cadmium storage batteries coated with an active material mixture. Electrodes for storage batteries.