JP3402785B2 - Manufacturing method of hydrogen storage alloy electrode - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a hydrogen storage alloy electrode using a hydrogen storage alloy which reversibly stores and releases hydrogen as a negative electrode material. [0002] Conventional storage batteries include nickel-cadmium storage batteries and lead storage batteries.
However, in recent years, metal hydride storage batteries using a hydrogen storage alloy for the negative electrode have attracted particular attention because they are lighter, have higher capacities, have higher energy densities than these batteries, and are clean storage batteries. As a method for producing a hydrogen storage alloy electrode, Japanese Patent Application Laid-Open No. 61-66366 and Japanese Patent Application Laid-Open No. 61-6637
No. 2, adding polytetrafluoroethylene (hereinafter referred to as PTFE) as a binder to the hydrogen storage alloy powder,
A method is disclosed in which a uniformly kneaded mixture is rolled, then placed on both sides of a current collector, pressed and dried to obtain a predetermined electrode. A negative electrode made of the above-mentioned hydrogen storage alloy is combined with a known nickel positive electrode, wound between a positive electrode and a negative electrode with a separator interposed therebetween, stored in a battery outer can, injected with an electrolytic solution, and sealed. Thus, a metal hydride storage battery is manufactured. In this metal hydride storage battery, the capacity of the negative electrode is designed to be larger than the capacity of the positive electrode in order to seal the battery. This is because oxygen gas generated from the positive electrode during charging is reduced to water at the negative electrode to suppress an increase in battery internal pressure. The reaction at this time is as follows. [0006] As the battery is charged, the positive electrode having a small capacity is first fully charged, and when the battery is further charged, the battery is in an overcharged state, and oxygen gas is generated from the positive electrode as shown in equation (1). 4OH ⁻ → 2H ₂ O + O ₂ + 4e ⁻ (1) Oxygen gas generated from the positive electrode is consumed at the negative electrode by a reaction represented by the following formulas (2) and (3). [0008] _{O 2 + 2H 2 O + 4e} - → 4OH - (2) 4MH + O 2 → 4M + 2H 2 O (3) (3) water generated by the equation, the negative electrode surface is a hydrophilic, The negative electrode surface becomes wet, impeding diffusion of oxygen gas to the negative electrode surface, and the consumption rate of oxygen gas at the negative electrode is reduced.
For this reason, the internal pressure of the battery increases, the safety valve operates, and the gas in the battery is released, and at the same time, the electrolyte leaks out of the battery. As a result, the electrolyte in the separator is depleted, so-called dry-out of the separator occurs, and there is a problem that the cycle characteristics of the battery deteriorate. As a method for solving such a problem, it has been proposed to treat the surface of the hydrogen storage alloy with water repellency. It is known that this water-repellent treatment forms a so-called gas-liquid-solid three-phase interface on the surface of the oxygen gas, the electrolytic solution and the hydrogen storage alloy, and the oxygen gas consumption reaction becomes smooth. For example, a water-repellent layer is provided on a part of a hydrogen-absorbing alloy electrode as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 61-118963. It has been proposed that the hydrogen-absorbing alloy electrode be subjected to a water-repellent treatment by immersion in a fluororesin dispersion, and that the hydrogen-absorbing alloy electrode be subjected to a water-repellent treatment with an ester as disclosed in JP-A-5-159799. Among these, the water-repellent treatment with a fluororesin is used as a water-repellent treatment agent because the fluororesin powder and the fluororesin dispersion are easy to handle and the fluororesin itself has excellent alkali resistance and is hardly deteriorated. Promising. As a method of performing water repellency treatment on the surface of a hydrogen storage alloy electrode using a solution containing a fluororesin, a method of dipping the hydrogen storage alloy electrode in a fluororesin dispersion or a method of applying a fluororesin dispersion is used. Are known. However, in such a water-repellent treatment such as immersion or coating, the surface treatment solution enters pores of the electrode, which are gaps between the alloy particles. Since the pores are as small as several μm to several tens of μm, the processing liquid enters the inside of the electrode by capillary action. When such a water-repellent electrode is dried, the solvent gradually evaporates, but the fluorine resin remains in the pores while being condensed. Therefore, the pores inside the water-repellent treated electrode plate after drying are covered or closed by the fluororesin remaining in the pores inside the electrode. When a battery is manufactured using such an electrode and charged and discharged, the alloy present in the portion closed with the fluororesin does not participate in charging and discharging,
Since the electrode reaction becomes non-uniform, there is a problem that the charge / discharge cycle characteristics are not good. The present invention has been made in view of the above problems, and an object of the present invention is to provide a metal hydride storage battery having a good oxygen gas consuming capacity at a negative electrode and excellent charge / discharge cycle characteristics. And In a method for manufacturing a hydrogen storage alloy electrode using a hydrogen storage alloy as a negative electrode material, an aqueous dispersion of a fluorine resin in which a fluorine resin is dispersed in water with a surfactant and an organic solvent are used. A step of immersing the surface of the negative electrode in a mixed solution with a solvent or applying the mixed solution to the surface of the negative electrode. In order for oxygen gas generated from the positive electrode to react smoothly on the negative electrode surface, water repellency is imparted to the negative electrode surface to form a three-phase interface between the oxygen gas, the electrolytic solution and the gas-liquid solid of the electrode. It is important that In the present invention, in order to impart water repellency to the negative electrode surface, the negative electrode surface is immersed in a mixed solution of an aqueous dispersion of a fluororesin dispersed in water with a surfactant and an organic solvent or an organic solvent. The solution has been applied. Since the fluororesin is hydrophobic, it does not disperse and immediately settles out in an aqueous solution unless an external force such as stirring is applied. If left for a long time, the precipitate adheres to the container, which is not preferable. Therefore, as a method of dispersing the fluororesin, a method of forming and dispersing polymer micelles by the use of a surfactant, in particular, is generally known. Examples of such a material include an aqueous dispersion of a fluororesin. This is because the fluororesin is uniformly dispersed in the aqueous solution by the action of the surfactant, so that the water repellency treatment can be easily performed on the electrode surface. However, when the water-repellent treatment is performed using an aqueous dispersion of a fluororesin, the solution permeates into the inside of the electrode because the fluororesins do not aggregate with each other and has low viscosity. Therefore, the pores of the electrode are blocked by the fluorine resin during drying, which is inconvenient for the electrode reaction. In the present invention, when an organic solvent is mixed with an aqueous dispersion of a fluororesin dispersed in water with a surfactant, the uniformly dispersed fluororesins can be aggregated. Therefore, it becomes difficult for the fluororesin to enter the gaps between the alloy particles constituting the electrode, and adhere to the alloy particles on the electrode surface. In addition, the gelation of the solution increases the viscosity of the solution, making it difficult for the solution to penetrate inside the electrode. Accordingly, water repellency can be surely imparted to the electrode surface, and the pores of the electrode are not blocked by the fluororesin, whereby a uniform charge / discharge reaction proceeds. In the present invention, a mixed solvent of water and an organic solvent is used as the solvent. Desirable are water-soluble organic solvents, such as alcohols. It mixes with water with a balance of hydrophilicity and hydrophobicity. Water-soluble solvents such as alcohols do not form polymeric micelles. Therefore, when such an organic solvent is added to an aqueous solution in which a fluororesin is dispersed, for example, a solution in which polymer micelles are formed by a surfactant, alcohols are mixed with water, but do not form micelles in alcohol. The fluororesin contained in the portion cannot be dispersed, and the fluororesins attract each other and aggregate. Furthermore, the solution gels and the viscosity of the solution increases. When such a surface treatment solution is used, since the viscosity is large, the solution does not enter the pores of the electrode, and the fluororesin adheres to the electrode surface. On the other hand, when a hydrophobic organic solvent such as cyclohexane or heptane is used by mixing with water, the water and the hydrophobic solvent are separated even when the mixture is stirred. However, when the dispersion containing the fluororesin and the hydrophobic solvent are stirred and mixed, water and the hydrophobic solvent can be mixed together by the action of the surfactant. Also in this case, although the mechanism is not clear, the fluororesin cannot form polymer micelles, and the fluororesins are aggregated. Further, like the hydrophilic solvent, the solvent gels and the viscosity of the solution increases.
When such a surface treatment solution is used, since the viscosity is large, the solution does not enter the pores of the electrode, and the fluororesin adheres to the electrode surface. (Embodiment 1) FIG. 1 is a sectional view of a cylindrical nickel-hydride alkaline storage battery showing an example of the present invention. As shown in FIG. 1, an electrode group 4 including a positive electrode 1 provided with a nickel active material, a negative electrode 2 containing a hydrogen storage alloy powder, and a separator 3 interposed between the positive and negative electrodes 1 and 2 has a spiral shape. It is wound around. The electrode group 4 includes an outer can 6 also serving as a negative electrode terminal.
The outer can 6 and the negative electrode 2 are electrically connected to the bottom surface of the outer can 6 by a negative electrode current collector 5. On the other hand, a gasket 1
1, a sealing plate 12 having a through hole in the center is installed, and a positive electrode terminal 13 is mounted on the sealing plate 12. A valve plate 8 and a holding plate 9 are placed on a sealing plate 12 having a through hole in the center, and the holding plate 9 is configured to be pressed by a spring 10. The positive electrode terminal 13 and the positive electrode plate 1 are connected via the positive electrode current collector 7 and the sealing plate 12. When the internal pressure of the battery rises, the valve plate 8, the holding plate 9, and the coil spring 10 are pushed in the direction of arrow A, and a gap is formed in the valve plate to release the gas inside to the atmosphere. It is configured to be able to. Here, the cylindrical nickel-hydride alkaline storage battery was manufactured as follows. First, commercially available Mm (mixture of misch metal: rare earth element), nickel, cobalt, aluminum, and manganese in an element ratio of 1.0: 3.2: 1.0:
After weighing so as to have a ratio of 0.2: 0.6, the mixture was melted in a high-frequency induction furnace in an argon gas atmosphere, and the molten metal was cooled to obtain Mm _1.0 Ni _3.2 Co _1.0 Al _0.2
A hydrogen storage alloy ingot represented by Mn _0.6 was produced. Next, the hydrogen storage alloy ingot was mechanically pulverized to produce a hydrogen storage alloy powder having an average particle size of 50 μm. To the alloy powder, 1 wt% of polyethylene oxide was added as a binder, and an appropriate amount of water as a dispersion medium was added and kneaded to prepare a slurry. The slurry was then subjected to a conductive support made of punching metal. It was applied to both sides of the body, dried and rolled to produce a base electrode. Next, a 15% by weight PTFE dispersion (trade name: Teflon 30-) was used as a surface treatment solution.
J, manufactured by Du Pont-Mitsui Fluorochemicals Co., Ltd.) and ethanol at a volume ratio of 1: 1 to prepare a mixture. The base electrode was immersed in this surface treatment solution, dried, and ethanol was removed to remove the electrode of the present invention. a was produced. Then, the negative electrode 2 made of the surface-treated hydrogen-absorbing alloy and the known sintered nickel positive electrode 1 were
The electrode group 4 is produced by spirally winding it through a nonwoven fabric separator 3, inserted into an outer can 6, and a 30 wt% aqueous potassium hydroxide solution is injected into the outer can 6. The battery A of the present invention having a nominal capacity of 1000 mAh was produced by closing the container. Example 2 An electrode b of the present invention and a battery B of the present invention were produced in the same manner as in Example 1 except that cyclohexane was used instead of ethanol as the surface treatment solution. (Comparative Example 1)
Comparative Battery C was prepared in the same manner as in Example 1 except that the battery was immersed in a 5% by weight PTFE dispersion (trade name: Teflon 30-J, manufactured by Du Pont-Mitsui Fluorochemicals). Comparative Example 2 A comparative battery D was prepared in the same manner as in Example 1 except that the base electrode of Example 1 having no surface treatment was used as a negative electrode. (Experiment 1) The internal pressures of the batteries of each of the batteries A and B of the present invention and the comparative batteries C and D were measured, and the average value is shown in Table 1. In this experiment, each battery A, B, C, D
After charging at 0.1 C (100 mA), 0.2 C (2
A charge / discharge cycle of discharging at 00 mA) was repeated 10 times to activate. Thereafter, a pressure sensor was attached to the bottom of the outer can, and the internal pressure of the battery when charged at 1 C (1000 mA) for 90 minutes was measured. [Table 1] From Table 1, it was confirmed that the batteries A and B of the present invention had lower battery internal pressure during charging than the comparative batteries C and D. This is because the batteries A and B of the present invention have a PTF
Since the E-resin is attached, the water repellency of the surface of the hydrogen storage alloy electrode is high, and a three-phase interface between the alloy surface and the electrolyte and oxygen gas is formed near the electrode surface. The generated oxygen gas consumption reaction proceeds smoothly. Therefore, the comparative battery D which was not surface-treated at all with the PTFE resin
The internal pressure of the battery is significantly reduced. Further, in the comparative battery C, the surface treatment with the PTFE resin is performed, so that the oxygen gas consumption ability is good. However, since the fluororesin is not aggregated as in the present invention, the fluororesin enters the pores of the electrode, and the fluororesin remaining in the pores blocks the pores of the electrode.
When the pores of the electrode are closed as described above, a part of the hydrogen storage alloy does not participate in the charge / discharge reaction, and as the battery is charged, it reaches a full charge faster than the battery A of the present invention, and is further charged. And overcharge state, and hydrogen gas is generated from the negative electrode,
The internal pressure is higher than that of the batteries A and B of the present invention. (Experiment 2) The charge / discharge cycle characteristics of the batteries A and B of the present invention and the comparative batteries C and D were examined, and the results are shown in FIG. The experimental conditions were 0.5C (50
0 mAh) for 2.5 hours and then 0.5 C (500
(mAh), the cycle of terminating the discharge when the battery voltage reached 1.0 V was repeated, and the cycle number when the discharge capacity of the battery reached 60% of the initial capacity was defined as the cycle life. As is apparent from FIG. 2, it is recognized that the batteries A and B of the present invention have improved charge / discharge cycle characteristics as compared with the comparative batteries C and D. The reason for this is that when the batteries A and B of the present invention are compared with the comparative battery D, the surface of the hydrogen storage alloy electrode is subjected to a water-repellent treatment, and the oxygen gas consumption is smooth as shown in the above Experiment 1. This is considered to be because the internal pressure of the battery at the time of charging is reduced and the electrolyte does not leak out of the battery due to the operation of the safety valve. Next, when the batteries A and B of the present invention are compared with the comparative battery C, since the fluororesin adheres to the surface of the alloy and does not block the pores of the electrode, the charge / discharge reaction proceeds in the plane direction and the depth direction. It is considered that the cycle life was improved because the both proceeded uniformly. In this embodiment, ethanol is used as the hydrophilic organic solvent. However, similar effects can be obtained by using other alcohols such as methanol and butanol. In this embodiment, cyclohexane is used as the hydrophobic organic solvent. However, the present invention is not limited to this, and the same effect can be obtained with heptane or the like. According to the present invention, a gas-liquid-solid three-phase interface in which oxygen gas, an electrolytic solution, and a hydrogen storage alloy electrode coexist is formed near the surface of the hydrogen storage alloy, and oxygen gas consumption at the negative electrode And the internal pressure of the battery can be reduced. Accordingly, since the operation of the safety valve caused by the increase in the internal pressure of the battery can be prevented, deterioration of the battery performance due to leakage of the electrolyte can be suppressed. Further, since the fluororesin does not block the pores of the electrode, a part of the alloy does not participate in the charge / discharge reaction, the charge / discharge reaction becomes uniform, and the charge / discharge cycle characteristics of the battery can be improved. it can.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a nickel-hydride alkaline storage battery according to one embodiment of the present invention. FIG. 2 is a graph showing cycle characteristics of batteries A and B of the present invention and comparative batteries C and D. [Explanation of Codes] Positive electrode2. Negative electrode3. Separator 4. Electrode group5. Negative electrode current collector 6. Outer can 7. Positive electrode current collector8. Valve plate 9. Holding plate 10. Spring 11. Gasket12. Sealing plate 13. Positive terminal

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-1-206562 (JP, A) JP-A-62-139255 (JP, A) JP-A-1-206563 (JP, A) JP-A 55-205 3121 (JP, A) JP-A-55-93675 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/26

Claims (1)

(1) An aqueous dispersion of a fluororesin in which a fluororesin is dispersed in water by a surfactant in a method for producing a hydrogen storage alloy electrode using the hydrogen storage alloy as a negative electrode material. A method of dipping the surface of the negative electrode in a mixed solution of water and an organic solvent, or applying the mixed solution to the surface of the negative electrode.