JPH027144B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a flat plate-shaped framed electrode used in a metal-bromine secondary battery. FIG. 1 is a block diagram showing an example of a metal-bromine battery to which the electrode of the present invention is applied. Here, the state during charging is shown. The electrolytic cell 1 has an ion exchange membrane 4 partitioning the electrolytic cell 1 at its center, and a positive electrode 5 is placed on one side and a negative electrode 6 is placed on the other side. Around the positive electrode 5, a positive electrode electrolyte (MBr ₂ +
Br ₂ ) 2 is supplied, and a negative electrode electrolyte (MBr ₂ ) 3 is provided around the negative electrode 6 from a negative electrode storage tank 10 by a pump 12 . Conventionally, the electrodes of metal-bromine batteries with such a configuration have mainly been made of metal materials with high corrosion resistance due to the corrosive nature of bromine as the positive electrode material.
The main electrodes were Pt electrodes and _RuO2 electrodes. However, the high cost of these metal materials makes them unsuitable for electrodes in the present battery system, which is intended to provide an inexpensive means of energy storage. On the other hand, inexpensive carbon electrodes have been proposed as an alternative to these electrode materials, but they are not suitable as electrodes for long-life batteries because they have drawbacks in mechanical strength and are subject to wear. In order to improve these problems and to be lightweight and suitable for mass production, carbon plastic electrodes are used, which are made by kneading and kneading conductive substances such as carbon black into polyolefin plastics. By the way, a metal-bromine secondary battery is put into practical use as a stacked battery in which electrodes and separators are stacked alternately and integrated by tightening with bolts or the like. In this case as well, a carbon plastic electrode is used as the electrode, and the electrode is formed by forming an insulating frame part around the electrode part by heat pressing, injection molding, or the like. but,
There have been problems in that the electrode portion thermally shrinks unevenly, causing warpage in the entire electrode and cracking at the joint between the electrode portion and the frame portion. If a stacked battery is constructed using such warped electrodes, stress for tightening will be applied to the warped portions, which may cause cracks or cause insufficient tightening force to be applied. There has been a problem in that the electrolyte leaks to the outside when the solution is circulated. The present invention has been made in view of the above, and an object of the present invention is to provide a flat framed carbon plastic electrode that is free from warping and bending and has properties suitable for practical use in terms of electrical resistance and mechanical strength. That is. In the present invention, 35 to 77wt of polyolefin plastic made of one of polyethylene, polypropylene, and ethylene-propylene copolymer
%, carbon black 20-45wt% and silicon oxide,
The flat electrode part is formed by kneading and molding 3 to 20 wt% of at least one metal oxide powder selected from alumina under heat and pressure, and the polyolefin plastic formed around the entire circumference of the electrode part is the same. The above object is achieved by an electrode having a frame part made of plastic. Here, the carbon black used in the present invention is preferably a carbon black with low electrical resistance such as acetylene black, and specifically,
Conductexsc (trade name: manufactured by Columbia Carbon Black Co., Ltd.), Vulcan XC-72 (trade name: manufactured by Kabot Corporation), Denka Black (trade name: manufactured by Denki Kagaku Kogyo Co., Ltd.), etc. are suitable. Furthermore, the metal oxide used in the present invention is selected so that its addition does not adversely affect properties such as mechanical strength and electrical resistance that are originally required for an electrode, and also has excellent corrosion resistance against electrolytes. . By adding this metal oxide, the coefficient of thermal expansion of the electrode portion is reduced, and deformation of the electrode during the heating step when forming a frame portion around the electrode portion can be prevented. Specifically, metal oxides such as SiO ₂ and Al ₂ O ₃ can be mentioned. Next, a method of manufacturing a sheet as an electrode using the above material will be explained. First, predetermined amounts of base plastic powder, carbon black, and metal oxide powder are weighed and heated and kneaded in a kneader. In this case, an ordinary pressure kneader is most suitable, but it is preferable to thoroughly mix the ingredients with a mixer before kneading, in order to keep the heating and kneading time by the pressure kneader short and to ensure uniform kneading. Here, shortening the kneading time is an effective means to prevent deterioration of the base plastic. The resistance value of the electrode portion in the present invention basically depends on the amount of carbon black added to the polyolefin plastic. As shown in FIG. 4, the electrical resistance decreases as the amount added increases, and at 45% (by weight) it reaches a low value of 0.1 Ω·cm.
However, even if more than 45% (by weight) is added, the rate of decrease becomes slow, and the resistance value does not decrease even though the amount added is increased.In fact, the mechanical strength decreases, and the original characteristics of the plastic are lost. It disappears. Furthermore, since the kneading time is long and it is difficult to produce an electrode with a uniform composition, there is a risk that the electric liquid will penetrate into the electrode even through the interface between the carbon black particles and the plastic. On the other hand, if it is less than 20% (by weight), the electrical resistance will increase rapidly as shown in FIG. From the above viewpoint, in the present invention, the amount of carbon black added is 20 to 45% of the total kneaded material constituting the electrode part.
(weight)% (details will be described later). In addition, the amount of metal oxide added in the present invention is determined by the mechanical strength and electrical resistance required for the electrode (basically determined by the amount of carbon black added, but even if the amount of carbon black is approximately the same, the amount of metal oxide The amount is set at 3 to 20% (by weight) of the entire kneaded material constituting the electrode portion, both to ensure that the resistance value increases with addition (the resistance value increases with addition) and to prevent thermal deformation (details will be described later). Hereinafter, the present invention will be explained in more detail with reference to Examples. 74.1% by weight of high-density polyethylene, 22.2% by weight of carbon black (30g per 100g of high-density polyethylene), 3.7% by weight of SiO ₂ (high-density polyethylene
5g for 100g), mixed for about 10 minutes in a mixer, and then kneaded for about 5 minutes in a pressure kneader preheated to 180-200℃ to form carbon black and SiO ₂ in high-density polyethylene. A homogeneously kneaded lump was obtained, and then a sheet-like electrode portion of an arbitrary thickness was manufactured from the above lump by adjusting the roll gap of a heat roller. Furthermore, a sheet-like electrode portion having a metal net layer between the layers may be produced by overlapping this sheet-like electrode and a metal net and passing it through a heat roller. In the case of the above-mentioned kneaded product, the appropriate temperature of the heat roller at this time was in the range of 170 to 180°C. Table 1 shows the blending amounts of carbon black and metal oxides in electrode parts manufactured with different compositions by the same method as in the above examples.
The table shows the characteristics of the produced sheet-like electrode portion. The amounts in Table 1 are weights (g) per 100 g of plastic, and the thicknesses are all 1 mm.

【table】

【table】

[Table] When the compounding amounts shown in Table 1 are converted to weight percent, the compositions of electrodes A to K are as follows. Electrode A Plastic 75.2% by weight Carbon black 24.8% by weight Electrodes B, D, F Plastic 72.5% by weight Carbon black 23.9% by weight Metal oxide powder 3.6% by weight Electrodes C, E, G Plastic 69.9% by weight Carbon black 23.1% by weight Metal oxide Material powder 7.0% by weight Electrode H Plastic 78.7% by weight Carbon black 17.3% by weight Metal oxide powder 4.0% by weight Electrode I Plastic 75.8% by weight Carbon black 16.7% by weight Metal oxide powder 7.5% by weight Electrode J Plastic 48.8% by weight Carbon black 48.8 Weight% Metal oxide powder 2.4% by weight Electrode K Plastic 47.6% by weight Carbon black 47.6% by weight Metal oxide powder 4.8% by weight Figure 6 is a graph of Tables 1 and 2. The graph shows the relationship between the amount added (wt%) and the electrical resistance (Ω·cm) and tensile strength (Kg/cm ² ) of the electrode. In the figure, , are the tensile strength and electrical resistance when no metal oxide is added, , are the tensile strength and electrical resistance when SiO ₂ is added as a metal oxide, respectively, and , are the tensile strength and electrical resistance when SiO 2 is added as a metal oxide, respectively. The tensile strength and electrical resistance when Al ₂ O ₃ is added are the tensile strength and electrical resistance when carbon black is less than 20 wt% and SiO ₂ is added, , is the tensile strength and electrical resistance when SiO ₂ is added when carbon black is 45 wt% or more. This shows the tensile strength and electrical resistance when . Further, FIG. 5 is a graph showing the relationship between the amount of metal oxide added and the thermal expansion reduction rate % (rate of reduction in thermal expansion coefficient based on the case where no metal oxide is added). As shown in Figure 6, when the amount of carbon black added is less than 20wt%, the tensile strength is the same or slightly higher than that of 20wt% or more (compared with), but the electrical resistance value is lower than that of metal oxidation. Even if the amount of added substance is small, it will be considerably higher than that of 20wt% or more. (compare and). On the other hand, when more than 45wt% of carbon black is added, although the electrical resistance value becomes lower (compare and), the tensile strength is significantly lower than when it is less than 45wt% even if almost no metal oxide is added. (compare). From these facts, it is clear that in order to obtain an electrode suitable for practical use in terms of both electrical resistance and mechanical strength, the amount of carbon black added should be 20 to 45 wt%. Regarding the amount of metal oxide added, as is clear from the graphs in Figures 5 and 6, if the amount added is less than 3% by weight, the tensile strength is high and the electrical resistance is low (carbon black is 20 to 45wt%). %in the case of),
The decrease in the coefficient of thermal expansion of the electrode portion is slight. That is,
If less than 3% by weight is added, thermal deformation during the formation of the frame portion cannot be effectively prevented. On the other hand, if more than 20% by weight of metal oxide is added, the coefficient of thermal expansion of the electrode portion will be greatly reduced, but the tensile strength will also decrease and the electrical resistance will increase. If the tensile strength decreases, the electrodes will be damaged when bolting the stacked electrodes and separators together to form a stacked battery, and the electrodes will no longer function as electrodes. From the above, in the present invention, the amount of metal oxide added is set at 3 to 20% by weight. In other words, the amount of carbon black added is 20 to 45wt.
%, and by adjusting the amount of metal oxide added from 3 to 20% by weight, the coefficient of thermal expansion can be increased from 5 to 5%, as shown in Figure 5, while ensuring mechanical strength and electrical resistance suitable for practical use as an electrode. Can be reduced by 20%. Next, the above-mentioned sheet-like electrode part A (comparative example: plastic 75.2% by weight, carbon black
24.8% by weight, no metal oxide added) and B (Example: Plastic 72.5% by weight, carbon black)
23.9 wt%, metal oxide powder 3.6 wt%) each 10
Create one sheet at a time and form a frame around each.
A framed electrode was placed on a surface plate and the height h of the warp was measured. The results are shown in Table 3. In addition, when configuring a stacked battery, if the distance between the electrode and the separator is, for example, 1.6 mm, the allowable range for electrode warpage is less than 0.6 mm (approximately 40 mm of the distance to the separator).
(less than %).

[Table] As is clear from the measurement results shown in Table 3, the electrode according to the present invention has significantly improved deformation during the heating process for forming the frame. Next, ZnBr ₂ used in zinc-bromine secondary batteries
Figures 2 and 3 show the results of measuring the polarization characteristics of each electrode shown in Table 1 using a solution in which 4 mol/l of KCl was added. FIG. 2 is a negative polarization curve of the electrode, and FIG. 3 is a positive polarization curve of the electrode. From the results shown in FIGS. 2 and 3, it can be seen that the electrode of the present invention has a flat voltage up to a high current density region, which is a sufficiently advantageous characteristic in terms of Zn overvoltage and bromine overvoltage in this battery system. As described above, in the present invention, by adding metal oxide powder to the electrode part to make the composition specific, the thermal expansion coefficient of the electrode part is ensured while ensuring electrical resistance and mechanical strength suitable for practical use as an electrode. is decreasing. Therefore, it is possible to prevent the entire electrode from warping or bending during the heating process when forming the frame around the electrode, and from cracking at the joint between the electrode and the frame. . Therefore, if the electrode according to the present invention is used, when the electrodes are stacked to form an electrolytic cell with the frame part and the electrode part, the electrolytic cell can be kept airtight and leakage of the electrolyte can be prevented. can. In addition, when forming a laminated battery by stacking electrodes and separators alternately and tightening them with bolts etc., the electrodes do not warp or otherwise deform, so the clamping pressure may cause cracks in the electrodes or the stacking The battery will not be deformed.

[Brief explanation of drawings]

Figure 1 is a block diagram of a metal-bromine secondary battery.
FIGS. 2 and 3 are graphs showing the polarization characteristics of electrodes according to examples of the present invention, with FIG. 2 showing a negative polarization curve and FIG. 3 showing a positive polarization curve. Also, Figure 4 is a graph showing the relationship between the amount of carbon black added and electrical resistance, Figure 5 is a graph showing the relationship between the amount of metal oxide added and the coefficient of thermal expansion reduction, and Figure 6 is the graph showing the relationship between the amount of metal oxide added and the reduction in thermal expansion. It is a graph showing the relationship between the amount, electrical resistance, and tensile strength.

Claims (1)

[Claims] 1. An electrode for a metal-bromine battery in which an electrode portion and an insulating frame portion are integrally formed, comprising: polyethylene, polypropylene, and ethylene.
35 to 77 wt% of polyolefin plastic made of one type of propylene copolymer, 20 to 45 wt% of carbon black, and 3 to 30% of at least one metal oxide powder selected from silicon oxide and alumina.
20 wt% is kneaded and molded under heat and pressure to form a flat plate-shaped electrode part, and a frame part made of the same polyolefin plastic as the polyolefin plastic integrally formed around the entire circumference of the electrode part. Features of metal-bromine battery electrodes.