JPS5951710B2 - Manufacturing method for battery electrodes - Google Patents

Description

Detailed Description of the Invention The present invention relates to a battery electrode formed by directly filling an active material inside a metal porous body having a three-dimensional network structure (hereinafter referred to as a spongy metal), particularly an alkaline electrolytic solution. This invention relates to improvements in battery electrodes used.

The object of the present invention is to provide an electrode with a three-layer structure in which a central layer filled with an active material and porous layers with fine pores that are hardly filled with an active material are formed on both sides. It is an object of the present invention to provide a battery electrode that exhibits good performance over a long period of time, with very little falling-off of active material inside the electrode, low electrical resistance of the electrode, and high mechanical strength.

Another object of the present invention is to provide a method for obtaining the above electrode using a simple manufacturing method. BACKGROUND ART Conventionally, nickel electrodes for alkaline storage batteries, metal hydride cathodes for metal hydride secondary batteries, and the like have been proposed as battery electrodes employing a spongy metal as an active material holder.

As a manufacturing method for these electrodes, for example, there is a method in which an active material mixed with a conductive material and a binder is directly filled inside a spongy metal, and then pressure is applied. Among these, nickel electrodes can be manufactured in a simpler manufacturing process than typical general-purpose sintered nickel electrodes, and are therefore expected to reduce costs. The porosity of the spongy metal is usually ~97%, which is higher than the 80% of the nickel porous material used in sintered nickel electrodes, which makes it possible to create electrodes with high energy density. It has the characteristics of However, on the other hand, the pore size of spongy metal is usually 50 to several hundred microns, and the pore size of the electrode body obtained by applying pressure after filling the active material is slightly smaller, but the pore size of the sintered nickel porous The active material is larger than the sintered nickel electrode because it is larger than the sintered nickel electrode due to gas generation during charging, resulting in a decrease in battery capacity and deterioration of battery performance due to short circuits inside the battery. become the cause.

In addition, because the proportion of the metal part of the holder in the electrode body is small, it has drawbacks such as being slightly inferior to sintered nickel electrodes in terms of performance during high rate discharge and mechanical strength of the electrode. On the other hand, the cathode of a metal hydride secondary battery is a spongy metal filled with an alloy powder that reversibly absorbs and releases hydrogen, as in the case of a nickel electrode. In the case of a metal hydride cathode, the alloy powder becomes finer due to repeated absorption and release of hydrogen during charging and discharging. When the alloy powder absorbs hydrogen, its volume increases by 10 to 20%, and the electrode's volume changes during charging and discharging. As a result, the metal hydride alloy powder that absorbs hydrogen, which serves as the active material, tends to fall off the electrode, creating the same problem as with nickel electrodes. The electrode obtained by the method of the present invention is characterized by having a three-layer structure in which a central layer filled with an active material and porous layers with fine pores that are hardly filled with an active material are formed on both sides. .

Therefore, the active material filled in the central layer inside the electrode is
Since it is contained in pores with fine pores, it hardly falls off. In other words, the pore diameter of a spongy metal filled with active material cannot be significantly reduced even when pressurized;
The part that is not filled with active material is crushed, and the metal part overlaps with the hole, so the hole diameter can be made considerably smaller, making it possible to make the hole diameter equivalent to that of a sintered nickel electrode. I found out something.

It was discovered that since the conductive metals are in close contact with each other in that part, the electrical resistance of the electrode plate can be reduced and the mechanical strength can also be improved. In the electrode of the present invention, there is a contradictory relationship between the ratio occupied by the outer layer containing almost no active material, the active material filling capacity density of the electrode, and the effect of preventing falling off.

Based on the previous experience of nickel poles using spongy metal,
When calculating the appropriate thickness of the outer layer from the viewpoint of packing density, the packing capacity density of the active material of the electrode is usually 300 to 6
450-550mAh with pressurization of 00kg/Cm2
/Cc, so in order to obtain an electrode close to this, it is thought that the outer layer should have a maximum thickness of around 15% of the electrode thickness. That is, in the case of a nickel electrode, approximately 10 of the cavities in the spongy metal
A paste-like active material having a water content of 20 to 30% can be filled into the container. When it is pressurized with a pressure of 500 kg/Cm2, the thickness of the electrode obtained is 6 times the thickness of the spongy metal.
At about 0%, the packing density is around 500mAh/Cc.
Further, when only the spongy metal without being filled with active material is pressurized, the thickness is about 10% of the original thickness and the porosity is about 40%. For example, for a cotton-like metal having a thickness of 2.6 mm (7), the relationship between the thickness of the outer layer and the active material filling density of the electrode is estimated as shown in Table 1. The most advantageous method for industrially obtaining the electrode of the present invention is to uniformly fill a paste-like active material inside a sheet-like spongy metal, and then leave the filled active material in a semi-dry state to activate the surface layers on both the upper and lower surfaces. The substance is removed to a predetermined depth with a brush made of wire or resin to form a layer that is hardly filled with the active material of both outer layers, and then pressurized to form a three-layer structure with a desired thickness. This is a method of using it as an electrode plate.

In this case, it is extremely important to remove the active material on the surface layers of both the upper and lower surfaces in a semi-dry state in order to achieve the object of the present invention. The reason for this is that if the surface layer of the active material is removed after the paste-like active material is completely dry, the active material will firmly adhere to the spongy metal support, if the binding agent is sufficiently effective. The active material on the surface layer cannot be removed sufficiently. Conversely, if the bonding force is insufficient, even the active material filled in the middle layer will be removed. Conversely, even if the active material in the surface layer is removed when the active material has a high moisture content, the active material may move within the spongy metal or adhere to the brush, resulting in the activation of only the desired surface layer. It becomes impossible to remove the substance. Therefore, the appropriate amount of water is about 10% to about 20% in a semi-dry state in which the surface of the spongy metal filled with the active material is slightly damp. In addition, removal of the active material on the surface layer can be achieved by using a metal brush with a thin wire diameter or a hard synthetic resin brush, and the removal depth can be determined by using a rotating brush and a sponge filled with the active material. This can be done by setting the degree of contact with the shaped metal to a depth that removes the active material and performing it on both the upper and lower surfaces at the same time.

Note that since the removed active material can be reused, there is no loss even if it is removed in this way. Hereinafter, the present invention will be explained in more detail with reference to more specific examples.

Example 1 The material of the active material holder was nickel, and the structure shown in Figure 1 had a thickness of 2.6 mm, a porosity of 96%, and an average pore diameter of 150 μm.
using a spongy metal.

In the figure, 1 is the nickel core material, and 2 is the hole. An active material of nickel hydroxide mixed with nickel powder as a conductive material and carboxyl methyl cellulose as a binder is made into a paste with a water content of 30%, and is uniformly filled into the entire holder. After semi-drying, the upper and lower surfaces are coated. of active material was removed with a rotating wire brush made of stainless steel, and 500 kg/
An electrode plate was obtained by applying a pressure of Af.

In addition, after filling the spongy metal with the active material, we investigated the water content of the active material when removing the active material on the surface layer with a rotating brush.

That is, an attempt was made to remove the active material using a rotating brush from a sticky state with a moisture content of about 30% immediately after filling the active material to an almost completely dry state. As a result, it was found that it is best to remove the active material on the surface layer when the entire surface is semi-dry with a moisture content of about 10 to 20%, and the required areas can be removed uniformly and cleanly.
Next, electrodes were made by changing the contact depth of the brush with the spongy metal when removing the semi-dry active material, and the electrode thickness and active material packing density after pressurization were determined. Table 2 shows the results. show.

The table also shows the thickness of the portion of the electrode outer layer that is not filled with active material, calculated from the active material packing density. The contact dimension of the brush is 0.3 mm (the thickness of the spongy metal is 2.6 mm)
Therefore, the distance between the brushes placed on both sides is 2.3
mm), there was no effect of removing the active material and it was almost the same as the conventional electrode. Brush contact dimension is 0.5m
m or more. In this case, the active material removal effect was observed, and a densely compressed spongy metal layer containing no active material was observed on the electrode surface. Regarding the contact dimension of the brush, a range of 0.5 to 1.0 mm is appropriate, which does not significantly reduce the active material packing density of the obtained electrode compared to conventional electrodes. Furthermore, it has been found that it is better to set the degree of contact with the brush to be slightly larger than the thickness of the surface layer from which the active material is to be removed.

Schematic diagrams of the obtained electrodes are shown in FIGS. 2 and 3. FIG. 2 shows an electrode with a three-layer structure according to the present invention, and FIG. 3 shows a conventional electrode.
In FIGS. 2 and 3, numeral 3 represents the metal nickel of the active material holder, numeral 4 represents the cavity of the electrode body, and numeral 5 represents the active material. A secondary battery shown in FIG. 4 was constructed in which these electrodes were combined with an iron electrode as an anode, and a charge/discharge test was conducted.

In Fig. 4, 6 is a nickel pole, 7 is a lead plate of the nickel pole, 8 is an iron pole, 9 is a lead plate, 10 is a separator,
11 is an electrolytic solution mainly containing caustic potash, and 12 is a battery container.
Note that the test electrode has a brush contact dimension of 0.5 mm to 1.5 mm.
5mm is the three-layer structure electrode of the present invention, 0.3011n
was a conventional electrode. The charge/discharge tests included a life test using a current of 0.2C and a high rate discharge up to 2C discharge. Charging was performed at 150% of the discharge capacity in both cases. The results are shown in FIGS. 5 and 6. FIG. 5 shows the results of the life test, where the vertical axis shows the discharge utilization rate of the active material and the horizontal axis shows the number of charging and discharging cycles. 6th
The figure shows the results of a high-rate discharge test, where the vertical axis is the discharge utilization rate of the active material, and the horizontal axis is the discharge current, which is indicated by the discharge rate C. From these results, it can be seen that the three-layer structure electrode of the present invention has smaller polarization during high rate discharge than the conventional structure electrode, and maintains a high discharge utilization rate over a long period of time.

In addition, when observing the life test battery after 500 cycles, it was observed that the active material that had fallen to the bottom of the battery case had accumulated about 2 mm in the battery using the conventional electrode structure. In the electrode of the present invention, almost no active material was observed to fall off, except for a small amount observed in battery A, where the outer layer was thin. Therefore, it can be seen that the three-layer electrode of the present invention can provide a considerable effect even when the outer layer is very thin. Example 2 Titanium and nickel were mixed at a weight ratio of 62:38, and the synthesized alloy was pulverized in an arc melting furnace.

To this alloy powder, 5% by weight of acetylene black and 10% by weight of a solid suspension of tetrafluoroethylene resin as a binder were added, and the mixture was made into a paste and made into a sponge of the same thickness as used in Example 1. After filling the inside of the shaped metal and removing the alloy powder on the surface layer with a toothbrush-like wire brush in a semi-dry state, approximately 1 ton/CIn! An electrode body was obtained by applying pressure at a pressure of . A battery as shown in FIG. 4 was constructed using the three-layered electrode (a) of the present invention and a conventional electrode (b) in which the alloy powder on the surface layer was not removed. In the case of this example, the metal hydride cathode a or b described above was used as the electrode 8 in FIG. 4, and only a caustic potassium aqueous solution was used as the electrolyte 11. The charge/discharge test was conducted using 1g of alloy powder.
Repeated tests were performed with a current of 50 mA per sample.

The results are shown in FIG. As is clear from the figure, it was found that the three-layer structure electrode of the present invention exhibited stable performance over a long period of time, and the effectiveness of the electrode structure of the present invention was also demonstrated for metal hydride cathodes. . As described above, the electrode of the present invention forms dense layers on both sides of the middle layer filled with active material, and includes the middle layer, so that charging and discharging is possible. There is almost no shedding of the active material due to repetition of this process, and stable performance can be obtained over a long period of time.

[Brief explanation of the drawing]

FIG. 1 is an enlarged view of a metal porous body (spongy metal) used in an example of the present invention, and FIG. 2 is a schematic vertical cross-sectional view of an electrode of the present invention.
FIG. 3 is a schematic vertical cross-sectional view of a conventional electrode, FIG. 4 is a vertical cross-sectional view of a battery used in an example of the present invention, and FIG. 5 is a diagram showing the cycle characteristics of a nickel electrode in terms of utilization rate. FIG. 6 is a diagram showing the high rate discharge characteristics of the nickel electrode in terms of utilization rate.
FIG. 7 is a diagram showing the cycle characteristics of a metal hydride cathode. 3... Porous body, 4... Hole portion, 5
...Active material.

Claims (1)

[Claims] 1 After filling a paste-like active material inside a sheet-like porous metal body having a three-dimensional network structure, the active material in the pores of both the upper and lower surface layers of the porous metal body is removed with a brush in a semi-dry state. 1. A method for producing a battery electrode, which comprises forming a layer that is hardly filled with an active material, and then pressurizing a porous metal body in its thickness direction.