Method for producing electrode for alkali batteries

[0001] [Technical field to which the invention belongs] The present invention relates to a method for manufacturing an electrode for an alkaline storage battery, which is formed by coating an active material slurry composed of an active material powder, a binder and a solvent of the binder on a conductive core body. [0002] [Conventional technology] In recent years, with the increase in small portable devices, the demand for rechargeable and dischargeable secondary batteries (storage batteries) has increased. In particular, with the miniaturization, thinning, and space efficiency of devices, large capacity can be obtained. Demand for alkaline storage batteries such as inexpensive nickel-hydrogen storage batteries has increased rapidly. The electrodes used in this type of alkaline storage battery serve as a solvent for the active material powder, the water-soluble binder, and the water-soluble binder on both sides of a conductive core made of punching metal or the like that holds the active material layer. It is formed by applying an active material slurry formed by kneading pure water or water, but usually, after applying the active material slurry on both sides of the conductive core body, at room temperature (about 20 ° C). It is produced through a process of natural drying. [0003] Here, when the electrode for the alkaline storage battery coated with the active material slurry is naturally dried, the drying speed is slow, and it usually takes a long time of about 5 to 6 hours for the electrode for the alkaline storage battery to dry. Therefore, there is a problem that the production efficiency of the electrode for the alkaline storage battery is poor. Therefore, in order to solve such a problem, a method has been proposed in which the active material slurry is coated on both sides of the conductive core body and then dried at a high temperature (about 60 ° C.). When the electrode for the alkaline storage battery coated with the active material slurry is dried at a high temperature in this way, the drying time becomes about 15 to 30 minutes, and the production efficiency of the electrode for the alkaline storage battery is improved. It will be. [0004] [Problems to be Solved by the Invention] By the way, in order to increase the drying speed of the electrode for the alkaline storage battery and improve the production efficiency, it is necessary to dry the electrode for the alkaline storage battery at a high temperature as described above, but when the electrode for the alkaline storage battery is dried at a high temperature, Since the evaporation rate of water becomes faster, the water contained in the active material layer rapidly moves from the inside of the electrode for the alkaline storage battery to the surface (dry surface side). However, when the water contained in the active material layer moves rapidly, the binder contained in the active material layer also moves with the movement of the water. As a result, the transferred binder is unevenly distributed and solidified on the surface of the electrode for the alkaline storage battery, so that the amount of the binder in the active material layer near the conductive core arranged in the center of the electrode for the alkaline storage battery Has occurred. [0005] Here, when the amount of the binder in the active material layer near the conductive core body decreases, the adhesive force between the active materials existing near the conductive core body or the adhesive force between the conductive core body and the active material decreases. Therefore, there is a problem that the active material layer is more likely to fall off than the conductive core body. Therefore, the present invention has been made to solve the above problems, and even if the drying temperature of the electrode for an alkaline storage battery coated with an active material is raised to improve the production efficiency, the electrode for an alkaline storage battery It is an object of the present invention to provide a manufacturing method capable of suppressing a decrease in the strength of the active material layer so that the active material layer can be prevented from falling off from the conductive core body and a high quality alkaline storage battery can be obtained. [0006] [Means for solving problems] In order to achieve the above object, the present invention is a method for manufacturing an electrode for an alkaline storage battery, which is formed by coating an active material slurry composed of an active material powder, a binder and a solvent of the binder on a conductive core body. Therefore, an active material slurry preparation step of mixing an active material powder, a binder, and a solvent of the binder to prepare an active material slurry, a slurry heating step of heating the active material slurry, and heating are performed. With the slurry coating process of applying the slurry to the conductive core body<u style="single">A drying step of heating and drying the electrode coated on the conductive core body in the slurry coating step and a pressurizing step of rolling the dried electrode are provided, and the slurry is added in the slurry heating step. The temperature is 30 ° C or higher, which is lower than the heating temperature of the electrode in the drying process, and is heated so that the temperature gradient between the ambient temperature in the drying process and the temperature of the slurry becomes small.</u>It is characterized by that. [0007] In this way, after heating the active material slurry, if the heated active material slurry is applied to the conductive core body, the active material slurry is later heated and dried in a drying furnace. Even if it is charged, the temperature gradient between the ambient temperature in the drying furnace and the temperature of the active material slurry becomes small. Therefore, the movement of the binder in the active material slurry is suppressed as described above, so that the amount of the binder in the active material layer in the vicinity of the conductive core can be prevented from decreasing. become. As a result, it becomes possible to maintain the adhesive force between the active material powders in the vicinity of the conductive core body or the adhesive force between the conductive core body and the active material powder, and the adhesive strength of this type of electrode for an alkaline storage battery is improved. , It becomes possible to prevent the active material layer from falling off from the conductive core body. [0008] Then, in order to exert the effect of heating the active material slurry as in the present invention, a drying step of heating and drying the electrode coated on the conductive core body with the heated active material slurry is required. It is particularly effective when applied to a method for producing an electrode for an alkaline storage battery, which comprises a pressurizing step of rolling a dried electrode. The heating temperature of the slurry in the active material slurry heating step needs to be lower than the drying temperature. [0009] In this case, if the temperature of the heated active material slurry is less than 30 ° C, the temperature gradient between the ambient temperature in the drying furnace and the temperature of the active material slurry in the subsequent drying step does not become small. In the drying furnace, the binder in the active material slurry moves to the electrode surface, and the amount of the binder in the active material layer near the conductive core body decreases, so that the conductive core body and the active material The adhesive force with the powder will be reduced. Therefore, it is necessary to specify the heating temperature of the slurry in the active material slurry heating step to be 30 ° C. or higher. If the temperature is higher than the temperature at which the binder is decomposed, the function of the binder cannot be exerted. Therefore, the upper limit of the temperature at which the active material slurry is heated is a temperature at which the binder is not decomposed. Need to be. [0010] BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an embodiment in the case where the present invention is applied to a hydrogen storage alloy electrode used for a negative electrode of a nickel-hydrogen storage battery will be described with reference to FIG. FIG. 1 is a cross-sectional view schematically showing a series of manufacturing processes in which an active material slurry is coated on a conductive core body, the applied active material slurry is dried, and then rolled. [0011] 1. Hydrogen storage alloy electrode manufacturing equipment As shown in FIG. 1, an example of the apparatus for producing a hydrogen storage alloy electrode of the present invention is a roll of a conductive core material 11 made of a nickel-plated perforated steel plate (for example, a punched metal having a thickness of 0.06 mm). The unwinding roll 21 that winds up in a roll, the winding roll 22 that winds up the obtained hydrogen storage alloy electrode 10 in a roll shape, and the transport rollers 23,24,25,26 that convey the conductive core material 11. A slurry tank 20 for forming a coating film by coating the conductive core material 11 with the active material slurry 12a, a slit 27 for adjusting the coated active material slurry 12a to a predetermined thickness, and a coated active material. It is composed of a drying furnace 28 for drying the slurry 12a and a pair of pressure rolls 29 for rolling the dried electrodes to a predetermined thickness. [0012] In the present embodiment, the unwinding roll 21 in which the conductive core material 11 made of a nickel-plated perforated steel plate is wound in a roll shape is arranged at a predetermined position, and then the unwinding roll is placed. The tip of the conductive core material 11 wound around 21 is passed through a transfer roller 23, a slurry tank 20, a slit 27, a drying furnace 28, transfer rollers 24, 25, a pressure roll 29, and a transfer roller 26. It is assumed that it is wrapped around. A heater 20a for heating the active material slurry 12a contained in the slurry tank 20 to a predetermined temperature is arranged inside the wall of the slurry tank 20, and a current is supplied to the heater 20a outside the heater 20a. A power supply 20b is provided for this purpose. [0013] 2. Preparation of hydrogen storage alloy powder MmNi_3.4Co_0.8Al_0.2Mn_0.6(Mm is a misch metal), each commercially available metal element Mm, Ni, Co, Al, Mn was weighed and mixed at a predetermined ratio. This product is put into a high-frequency melting furnace to melt it, then poured into a mold, cooled, and MmNi._3.4Co_0.8Al_0.2Mn_0.6A lump (ingot) of a hydrogen storage alloy composed of the above was prepared. After the lump of the hydrogen storage alloy was roughly pulverized, it was mechanically pulverized in an inert gas atmosphere until the average particle size became about 50 μm to prepare a hydrogen storage alloy powder. The average particle size of the obtained hydrogen storage alloy powder is a value measured by a laser diffraction method. [0014] 3. Fabrication of hydrogen storage alloy electrode Then, to 99% by mass of the hydrogen storage alloy powder prepared as described above, 1% by mass of polyethylene oxide (PEO) powder as a water-soluble binder with respect to the mass of the hydrogen storage alloy powder, an appropriate amount of water (or pure water). Water) was added and kneaded to prepare a hydrogen storage alloy slurry (active material slurry) 12a, and then the hydrogen storage alloy slurry 12a was housed in the slurry tank 20. Then, after connecting the power supply 20b to the heater 20a of the slurry tank 20, a heater current was supplied from the power supply 20b to the heater 20a to heat the inside of the slurry tank 20 to a predetermined temperature. After that, by winding the winding roll 22 at a predetermined speed, the conductive core material 11 wound in a roll shape on the winding roll 21 is wound from the winding roll 21. [0015] As a result, the hydrogen storage alloy slurry 12a adheres to both surfaces of the conductive core material 11 in the process of passing through the hydrogen storage alloy slurry 12a in the slurry tank 20, and the hydrogen storage alloy layer 12a is formed. Then, when the conductive core material 11 passes through the slit 27, the excess hydrogen storage alloy slurry 12a is scraped off, and the coating thickness of the hydrogen storage alloy layer 12a is adjusted. The conductive core material 11 in which the hydrogen storage alloy layer 12a is adjusted to a predetermined thickness enters the drying furnace 28 whose temperature is maintained at about 60 ° C., and the hydrogen storage alloy layer is formed after a predetermined time elapses. 12a is dried. At this time, the rotation speed of the take-up roll 22 is adjusted so that the time remaining in the drying furnace 28 is 20 minutes. [0016] Next, the conductive core material 11 in which the hydrogen storage alloy layer 12a is dried passes between the pair of pressure rolls 29 and is rolled so that the hydrogen storage alloy layer 12a has a predetermined thickness (0.6 mm) to be conductive. A hydrogen storage alloy electrode 10 having a hydrogen storage alloy layer 12 is formed on both sides of the sex core material 11, and then the electrode 10 is wound on a take-up roll 22. The hydrogen storage alloy electrode 10 produced by heating the inside of the slurry tank 20 and setting the temperature of the hydrogen storage alloy slurry 12a to 30 ° C was used as the electrode A of Example 1. Similarly, the hydrogen storage alloy electrode 10 produced by setting the temperature of the hydrogen storage alloy slurry 12a to 35 ° C is used as the electrode B of Example 2, and the hydrogen storage alloy produced by setting the temperature of the hydrogen storage alloy slurry 12a to 40 ° C. The electrode 10 was used as the electrode C of Example 3. Further, the hydrogen storage alloy electrode 10 produced by keeping the temperature of the hydrogen storage alloy slurry 12a at room temperature (20 ° C) without heating the inside of the slurry tank 20 was used as the electrode X of the comparative example. [0017] 4. Measurement of adhesion strength of hydrogen storage alloy electrode Then, after cutting each hydrogen storage alloy electrode 10 (A, B, C, X) produced as described above to a predetermined size, the hydrogen storage alloy layer 12 on one side is cut, and the cut surface is lightly rubbed with a waste cloth. After removing the cutting chips on the surface to obtain the sample hydrogen storage alloy electrode 10a, the adhesion strength of the sample hydrogen storage alloy electrode 10a was measured. In measuring the adhesion strength, as shown in FIG. 2, a cutter (not shown) was used at an angle of about 30 degrees with respect to the surface of the hydrogen storage alloy layer 12 of each of these sample hydrogen storage alloy electrodes 10a. After holding, the cutting edges x and y were drawn so as to cut the hydrogen storage alloy layer 12 so that a load of about 250 g was applied to the cutting edge of the cutter. The intervals between the grooves x and y were set to 1 mm, and 10 grooves x and y were drawn so as to intersect each other at right angles. [0018] Then, 10 grids x and y were drawn so as to intersect each other at right angles to form 100 grids. Next, using 10 samples of each sample hydrogen storage alloy electrode 10a in which 100 squares are formed in a grid pattern, the sample hydrogen storage alloy electrode 10a is set to be vertical, and the height is about 100 mm. After lifting up to, each sample hydrogen storage alloy electrode 10a was freely dropped. After repeating this drop test three times, the number of squares formed on each sample hydrogen storage alloy electrode 10a was counted, and the average value was calculated as shown in Table 1 below. [0019] [table 1]<img file="JP4390397B2_D0001.tif" />[0020] As is clear from the results in Table 1 above, the average number of dropouts of the sample hydrogen storage alloy electrode X prepared by setting the temperature of the hydrogen storage alloy slurry 12a to 20 ° C (normal temperature) is 30, whereas hydrogen Sample hydrogen storage alloy electrode A prepared with the temperature of the storage alloy slurry 12a set to 30 ° C The average number of drops was 10, and sample hydrogen storage alloy electrode B prepared with the temperature of the hydrogen storage alloy slurry 12a set to 35 ° C. The average number of dropouts is 8, and the average number of dropouts of the sample hydrogen storage alloy electrode C prepared by setting the temperature of the hydrogen storage alloy slurry 12a to 40 ° C is 7, which is a hydrogen storage alloy more than the sample hydrogen storage alloy electrode X. It can be seen that as the temperature of the slurry 12a is increased, the average number of drops is extremely reduced. [0021] [0021] This is because when the heated hydrogen storage alloy slurry 12a is coated on the conductive core body 11, even if the coated hydrogen storage alloy slurry 12a is put into the drying furnace 28, the inside of the drying furnace 28 Since the temperature gradient between the atmospheric temperature (about 60 ° C) and the temperature of the hydrogen storage alloy slurry 12a (30 to 40 ° C) becomes small, the binder in the hydrogen storage alloy slurry 12a is the hydrogen storage alloy electrode. Movement to the surface of 10 will be suppressed. Therefore, it is possible to prevent the amount of the binder in the active material layer in the vicinity of the conductive core 11 from decreasing, and the binding force or conductivity between the hydrogen storage alloy powders in the vicinity of the conductive core 11 can be prevented. It is considered that the binding force between the sex core 11 and the hydrogen storage alloy powder can be maintained. As a result, the adhesive strength of the hydrogen storage alloy electrode 10 is improved, and it is possible to prevent the active material layer 12 from falling off from the conductive core body 11. [0022] In this case, if the temperature of the heated hydrogen storage alloy slurry 12a is less than 30 ° C, the temperature gradient between the ambient temperature in the drying furnace 28 and the temperature of the hydrogen storage alloy slurry 12a becomes large. Therefore, when the hydrogen storage alloy slurry 12a is dried, the binder in the hydrogen storage alloy slurry 12a moves to the surface of the hydrogen storage alloy electrode 10, and the hydrogen storage alloy layer in the vicinity of the conductive core body 11 is formed. The amount of the binder in 12a is reduced, and the binding force between the hydrogen storage alloy powders in the vicinity of the conductive core body 11 or the binding force between the conductive core body 11 and the hydrogen storage alloy powder is reduced. Therefore, it is necessary to specify the temperature for heating the hydrogen storage alloy slurry 12a to 30 ° C or higher. In this case, if the hydrogen storage alloy slurry 12a is heated above the temperature at which the binder is decomposed, the function of the binder cannot be exhibited. Therefore, the upper limit of the temperature for heating the hydrogen storage alloy slurry 12a is It is necessary to set the temperature so that the binder is not decomposed. [0023] As described above, in the present invention, since the heated hydrogen storage alloy slurry 12a is coated on the conductive core body 11, it dries even if it is rapidly dried at a high temperature in the drying furnace 28. The temperature gradient between the ambient temperature in the furnace 28 and the temperature of the hydrogen storage alloy slurry 12a becomes small. Therefore, even if the productivity of the hydrogen storage alloy electrode is improved by rapid drying at a high temperature, it is possible to prevent the amount of the binder in the active material layer in the vicinity of the conductive core 11 from decreasing. As a result, the binding force between the hydrogen storage alloy powders in the vicinity of the conductive core body 11 or the binding force between the conductive core body 11 and the hydrogen storage alloy powder can be maintained, and the adhesion strength of the hydrogen storage alloy electrode 10 can be maintained. It becomes possible to prevent the active material layer 12 from falling off from the conductive core body 11. [0024] In the above-described embodiment, the slurry tank 20 is provided with a heater 20a, a power source 20b is connected to the heater 20a to supply a heater current, and a hydrogen storage alloy slurry 12a housed in the slurry tank 20 is specified. The example of heating to the temperature of the above has been described, but the means for heating the hydrogen storage alloy slurry 12a to a predetermined temperature is not limited to this, and the slurry tank 20 may be heated using various heat sources. Alternatively, the hydrogen storage alloy slurry 12a may be heated outside the slurry tank 20 and then injected into the slurry tank 20. [0025] Further, in the above-described embodiment, an example in which the atmospheric temperature (heating temperature) in the drying furnace 28 is maintained at about 60 ° C. to dry the hydrogen storage alloy layer 12 has been described, but the drying furnace 28 has been described. The ambient temperature (heating temperature) inside may be any number of times as long as it is within the range of 50 to 100 ° C, and is appropriately selected in consideration of the active material constituting the active material slurry, the binder, the thickness of the obtained electrode, and the like. You just have to do it. When changing the drying temperature of the electrode coated with the active material, it is necessary to change the temperature of the active material slurry correspondingly so that the temperature gradient with the active material slurry does not become large. [0026] Further, in the above-described embodiment, MmNi_3.4Co_0.8Al_0.2Mn_0.6An example of using a hydrogen storage alloy represented by is described above, but Mm_aNi_{b b}Co_cMn_dAl_eHydrogen storage alloy represented by, LaNi₅AB in which a part of Ni is replaced with Co and Al, W, etc. in the system₅A type rare earth-based hydrogen storage alloy or the like may be used. Further, in the above-described embodiment, an example in which a mechanically pulverized hydrogen storage alloy is used has been described, but a hydrogen storage alloy produced by an atomizing method or a mixed powder obtained by mixing the pulverized alloy with the hydrogen storage alloy may be used. Good. [0027] Further, in the above-described embodiment, an example in which the production method of the present invention is applied to a hydrogen storage alloy electrode has been described, but the present invention is not limited to the hydrogen storage alloy electrode, but a conductive core such as a cadmium electrode or a nickel electrode. It can be applied to various electrodes for alkaline storage batteries formed by directly coating the active material slurry on the body. Further, in the above-described embodiment, an example in which polyethylene oxide (PEO) is used as the binder has been described, but the binder is not limited to PEO, and various binders such as PTFE and SBR are used. Can be done. In this case, it is necessary to select the solvent according to the binder to be used. [Simple explanation of drawings] FIG. 1 is a cross-sectional view schematically showing a series of manufacturing processes in which an active material slurry is coated on a conductive core body, the applied active material slurry is dried, and then rolled. [Figure 2] It is a perspective view which shows typically the state which made the grid-like cut groove in the active material layer in order to perform the dropout test of an active material. [Explanation of symbols] 10 ... Hydrogen storage alloy electrode plate, 11 ... Conductive core (punching metal), 12 ... Hydrogen storage alloy layer (active material layer), 12a ... Hydrogen storage alloy slurry (active material slurry) , 20 ... Slurry tank, 20a ... Heater, 20b ... Power supply, 21 ... Unwinding roll, 22 ... Winding roll, 23,24,25,26 ... Conveying roller, 27 ... slit, 28 ... drying furnace, 29 ... pressurized roll