JP2008258121A6 - Hydrogen storage alloy, negative electrode for nickel metal hydride secondary battery, nickel metal hydride secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To use a hydrogen storage alloy having good cycle characteristics, high battery operating voltage and suppressing self-discharge when used as a negative electrode active material of a nickel metal hydride secondary battery, and using this hydrogen storage alloy An object of the present invention is to provide a negative electrode for a nickel-metal hydride secondary battery and a nickel-metal hydride secondary battery manufactured using the negative electrode. SOLUTION: RNI _a Co _b , Al _c Mn _{d Me} (wherein R is at least one selected from rare earth elements including Y, M is at least one selected from elements other than R, Ni, Co, Al, and Mn, a is 3.30 ≦ a ≦ 5.00, b Is 0.10 ≦ b ≦ 0.90, c is 0.30 ≦ c ≦ 0.80, d is 0 ≦ d ≦ 0.15, e is 0 ≦ e ≦ 0.30, 4.90 ≦ a + b + c + d + e ≦ 5 Hydrogen having a composition represented by. It is a stored alloy, and there is no precipitated phase having an Al concentration higher than that of the parent phase confirmed by the Comp image and Al element mapping image of 500 times the EPMA of the cross-sectional structure of the alloy and having a grain size of 2.0 μm or more. A hydrogen storage alloy is used for a nickel metal hydride secondary battery.
[Selection] Figure 2

Description

  The present invention relates to a nickel metal hydride secondary battery, a hydrogen storage alloy used therefor, and a negative electrode.

JP 2004-55494 A JP 2001-40442 A JP 2004-119353 A

  A nickel metal hydride secondary battery using a negative electrode containing a hydrogen storage alloy has a higher energy density than a nickel cadmium secondary battery, and does not use harmful Cd. Nickel metal hydride batteries are also used in portable devices such as digital cameras and electric tools, as well as electric vehicles or hybrid electric vehicles, and various battery characteristics are required depending on the application. Among these, nickel metal hydride rechargeable batteries that suppress self-discharge are attracting attention because they are convenient for consumers to use immediately after purchase at the store. In electric vehicle or hybrid electric vehicle applications, energy consumption can be reduced by suppressing self-discharge. In the same application, since it is necessary to frequently charge and discharge, good cycle characteristics are required.

Conventionally, as a negative electrode material of a nickel metal hydride secondary battery, an AB ₅ type hydrogen storage alloy is mainly used. In general, Mn, Al, Co, or the like is added to this alloy in order to satisfy requirements such as hydrogen storage capacity, equilibrium pressure, and corrosion resistance.

  In literature 1, when a negative electrode containing a hydrogen storage alloy containing Al is used, in order to prevent Al eluting from the hydrogen storage alloy from precipitating on the positive electrode and causing self-discharge, etc., Al and a complex are added to the negative electrode. It is disclosed to add a complexing agent to form.

In Document 2, the content ratio of Mn and Al is extremely reduced, the hydrogen storage characteristics are excellent, the fine powder characteristics, the good initial characteristics and the output characteristics, and the durability and the storage stability are high. As a hydrogen storage alloy, a general formula MmNi _a Mn _b Al _c Co _d (where Mm is Misch metal, 3.7 ≦ a ≦ 4.2, 0 <b ≦ 0.3, 0 <c ≦ 0.4, A hydrogen storage alloy having a CaCu ₅ type crystal structure represented by 0.2 ≦ d ≦ 0.4, b + c <0.5, 5.00 ≦ a + b + c + d ≦ 5.20) is disclosed.

Reference 3 describes a nickel hydride secondary battery that suppresses an increase in internal resistance even when left for a long period of time, has excellent charge / discharge cycle characteristics, and has a large capacity. Is _a LaNi _5- type crystal structure represented by MmNi _a Co _b Al _c Mn _d , the Mn addition molar ratio is 0.2 or less (0 <d ≦ 0.2), and the equilibrium hydrogen pressure at 40 ° C. is 0 A nickel metal hydride secondary battery using a hydrogen storage alloy of less than 10 MPa as a negative electrode is disclosed.

  The nickel metal hydride secondary battery of Document 1 can suppress self-discharge to some extent by capturing Al as a complex from the hydrogen storage alloy in the electrolyte solution. Precipitation causes reduction of the positive electrode active material and increase in internal resistance, resulting in self-discharge and a decrease in battery operating voltage. Further, in the hydrogen storage alloys of Reference 2 and Reference 3, even if the cast alloy is heat-treated, the segregation of Al or Mn cannot be sufficiently homogenized. Will occur. As a result, self-discharge occurs, and the cycle characteristics deteriorate.

It is possible to adjust the equilibrium pressure at the time of hydrogen storage and release without greatly reducing the amount of hydrogen stored by Mn added to the AB ₅ type hydrogen storage alloy, but because Mn is easily eluted into the electrolyte , Self-discharge and battery operating voltage drop. Further, when Mn is contained in a large amount, the alloy is pulverized with the absorption and release of hydrogen, which adversely affects the cycle characteristics of the battery. However, if the amount of Mn added is reduced in order to reduce the adverse effects of adding Mn, Al needs to be increased to adjust the equilibrium pressure. In that case, Al is uniformly dispersed in the alloy structure. It is difficult. If there is segregation of Al, the pulverization of the alloy is promoted, so that the cycle characteristics are deteriorated, and Al and Co eluted with the pulverization cause an increase in the internal resistance of the battery and a short circuit.

  Therefore, the present invention reduces the amount of Mn added and increases the amount of Al added even when the hydrogen storage alloy is used as a negative electrode active material for a nickel metal hydride secondary battery. Provided are a hydrogen storage alloy having a high operating voltage and capable of suppressing self-discharge, a negative electrode for a nickel metal hydride secondary battery manufactured using the hydrogen storage alloy, and a nickel metal hydride secondary battery manufactured using the negative electrode. For the purpose.

In order to solve the above-described problems, according to the present invention, as a hydrogen storage alloy having a low Mn content and a low Al segregation despite a high Al content, the formula RNi _a Co _b Al _{c is used.} Mnd _d M. (In the formula, R represents at least one selected from rare earth elements including Y, M represents at least one selected from elements other than R, Ni, Co, Al, and Mn. A represents 3.30 ≦ a ≦ 5. 00, b is 0.10 ≦ b ≦ 0.90, c is 0.30 ≦ c ≦ 0.80, d is 0 ≦ d ≦ 0.15, e is 0 ≦ e ≦ 0.30, 4.90 ≦ a + b + c + d + e ≦ 5.50), which is a hydrogen storage alloy having a composition represented by the following formula: 500 times as large as the Comp image and EP elemental mapping image by EPMA of the cross-sectional structure of the alloy. And a hydrogen storage alloy having no precipitated phase having a particle size of 2.0 μm or more is provided.

  Moreover, according to this invention, the negative electrode for nickel hydride secondary batteries containing the said hydrogen storage alloy is provided.

  Furthermore, according to this invention, the nickel hydride secondary battery using the said negative electrode for nickel hydride secondary batteries is provided.

The hydrogen storage alloy of the present invention has the following formula.
Formula _{RNi a Co b Al c Mn d} M e
In the formula, R represents at least one selected from rare earth elements including Y. R is selected depending on the required characteristics, but usually, in terms of cost, misch metal is mostly used for part or all of the raw material. R is mainly a light rare earth such as La, Ce, Pr, or Nd. Elements are used. The La content mainly affects the hydrogen storage amount and the equilibrium pressure. When misch metal is used as a raw material, when increasing the La content in the misch metal, a raw material with a high La content such as a single metal of La is used at the same time so that the desired La content is obtained. adjust.

  a represents the Ni content when the number of moles of R is 1. The Ni content mainly affects the micronization. a is 3.30 ≦ a ≦ 5.00. Preferably, 3.70 ≦ x ≦ 4.45. If a is less than 3.30, pulverization cannot be suppressed, and if a is more than 5.00, the hydrogen storage amount decreases, which is not preferable.

  b represents the Co content when the number of moles of R is 1. The Co content mainly affects pulverization. b is 0.10 ≦ b ≦ 0.90. Preferably, 0.40 ≦ b ≦ 0.70. If b is less than 0.10, the cast alloy cannot be sufficiently homogenized even if heat-treated, and there is a precipitated phase with a high Al concentration, and fine powdering cannot be suppressed. The hydrogen storage amount is small and the initial activity is not sufficient.

  c represents the Al content when the number of moles of R is 1. The Al content mainly affects the equilibrium pressure. c is 0.30 ≦ c ≦ 0.80. Preferably, 0.45 ≦ x ≦ 0.60. Since the hydrogen storage alloy of the present invention regulates the Mn content to be small as will be described later, it is necessary to adjust the equilibrium pressure by increasing the Al content. If c is less than 0.30, the equilibrium pressure is not as desired, and if it is greater than 0.80, the cast alloy cannot be sufficiently homogenized even by heat treatment, and the alloy structure has a higher Al concentration than the parent phase. There is a phase. When such a precipitated phase exists, cracks are generated starting from this precipitated phase, the alloy is pulverized, and the cycle characteristics deteriorate. Moreover, when Al elutes from such a deposited phase, the internal resistance of the battery increases and the operating voltage of the battery decreases.

  d represents the Mn content when the number of moles of R is 1. The Mn content mainly affects the equilibrium pressure. d is 0 ≦ d ≦ 0.15. As described above, in the present application, the Mn content is reduced as much as possible in order to solve the problem that the adverse effect of Mn, that is, self-discharge occurs and the operating voltage of the battery decreases. Preferably 0 ≦ d ≦ 0.10, more preferably d is 0. However, it is permissible to contain Mn at the impurity level. If d is greater than 0.15, self-discharge tends to occur, and the operating voltage of the battery decreases.

  e represents the content of M when the number of moles of R is 1. e is 0 ≦ e ≦ 0.30. M is not always necessary, and is included when fine adjustment of characteristics is required depending on the use of the battery. M represents at least one selected from elements other than R, Ni, Co, Al, and Mn. Specific examples of M include Mg, Ca, Ti, Zr, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Zn, B, Ga, Sn, and Sb. Preferably, M is It is at least one selected from Mg, Ti, Zr, Nb, Mo, W, Fe, Cu, B, and Sn. For example, when Mg is contained, the hydrogen storage amount increases. When Fe, Cu, Zr, Sn, Ti, Nb, Mo, W, and B are contained, pulverization is suppressed, or elution of Al, Mn, and Co into the electrolytic solution is suppressed. When e is larger than 0.30, various properties required for the hydrogen storage alloy of the present invention cannot be obtained.

  a + b + c + d + e represents the content of components other than R when the number of moles of R is 1. The content of all components other than R mainly affects the pulverization. a + b + c + d + e is 4.90 ≦ a + b + c + d + e ≦ 5.50. If a + b + c + d + e is less than 4.90, the pulverization cannot be suppressed, and the hydrogen storage amount is less than 5.50, which is not preferable. When a + b + c + d + e is large, segregation of Al or Mn is likely to occur. Therefore, in a hydrogen storage alloy having a large Al content and causing segregation of Al as in the hydrogen storage alloy of the present invention, 5.10 ≦ a + b + c + d + e ≦ 5 .30 is preferable.

  The hydrogen storage alloy of the present invention is an alloy having a small amount of Mn and a small amount of segregation of Al even though the content of Al is large. The cross-sectional structure of the hydrogen storage alloy was observed with EPMA by the method shown below, and the degree of segregation of Al was evaluated.

The thickness of the cross-section substantially parallel to the heat removal direction during casting of the hydrogen storage alloy is observed 500 times with EPMA. The hydrogen storage alloy of the present invention is a so-called AB ₅ system, and the matrix (matrix) of the EPMA observation image has a LaNi ₅ type crystal structure. The hydrogen storage alloy of the present invention can be confirmed by a Comp image and an element mapping image of Al, and there is no precipitated phase having an Al concentration higher than that of the parent phase and a particle size of 2.0 μm or more. First, the presence of the precipitated phase is confirmed by the Comp image, and then, it is confirmed by the element mapping image of Al whether the precipitated phase has a higher Al concentration than the parent phase. The particle size of the precipitated phase here means the length of the minor axis. Since there is no precipitated phase having an Al concentration higher than that of the parent phase and a particle size of 2.0 μm or more, pulverization of the alloy starting from the precipitated phase is suppressed, so that the cycle characteristics are improved. Moreover, since the elution of Al is suppressed, it is possible to suppress the battery internal voltage from increasing and the operating voltage of the battery from decreasing. Preferably, there is no precipitated phase having an Al concentration higher than that of the parent phase and a particle size of 1.0 μm or more.

  The hydrogen storage alloy of the present invention has an equilibrium pressure at 40 ° C. of 0.02 to 0.15 MPa. The equilibrium pressure of the hydrogen storage alloy of the present invention was measured at 40 ° C. according to JIS H7201 (1991) “Measurement Method of Pressure-Composition Isotherm (PCT Line) of Hydrogen Storage Alloy”, H / M = 0.5. The occlusion pressure at the time. When the equilibrium pressure is lower than 0.02 MPa, the discharge performance is lowered. When the equilibrium pressure is higher than 0.15 MPa, the internal pressure of the battery is increased.

  The method for producing the hydrogen storage alloy of the present invention is not particularly limited as long as the hydrogen storage alloy of the present invention is obtained. For example, the raw material which mix | blended the metal and mother alloy containing R, Ni, Co, Mn, Al, and M so that it may become the above-mentioned composition is prepared. Next, the blended raw materials are heated and melted under an inert gas atmosphere to obtain an alloy melt. The obtained alloy melt is cooled and solidified. In the hydrogen storage alloy of the present invention, Al segregation needs to be almost eliminated, and therefore, the cooling and solidification process is preferably performed by a strip casting method using a single roll or a twin roll that can obtain a large cooling rate. For example, in the case of a strip casting method using a single roll, the thickness of the resulting slab is preferably controlled in the range of 0.05 to 0.50 mm. In addition, the temperature of the alloy melt immediately before cooling and solidification is important because the mixed state of each component varies depending on the temperature even if the temperature is higher than the melting point. Is preferable, and more preferably 250 ° C. or higher. The alloy melt needs to be sufficiently mixed. In this respect, it is preferable to carry out in a high-frequency melting furnace in which the alloy melt largely convects.

  Next, the slab obtained by cooling and solidifying the alloy melt is heat-treated. The heat treatment is performed at 1,000 to 1,200 ° C. in an inert atmosphere. Although it depends on the degree of segregation of Al in the alloy slab before heat treatment, it is preferably performed at a temperature exceeding 1,100 ° C., more preferably at 1,150 ° C. or more.

  The negative electrode for a nickel metal hydride secondary battery of the present invention contains the hydrogen storage alloy of the present invention. The hydrogen storage alloy of the present invention is preferably contained as pulverized powder. It grind | pulverizes by a known grinding | pulverization means and it is set as the pulverized powder of MV (volume average diameter) 10-50 micrometers, More preferably, 20-40 micrometers. The pulverized powder can be subjected to a known treatment such as plating, surface coating with a polymer or the like, or a surface treatment with a solution of acid, alkali, or the like according to desired characteristics.

  Content of the hydrogen storage alloy of this invention shall be 80 mass% or more with respect to the total amount of materials which comprise negative electrodes other than collectors, such as an electrically conductive material and a binder. More preferably, it is 95 mass% or more. As the conductive material, known materials can be used, and examples thereof include carbonaceous materials such as graphite and carbon black (acetylene black, furnace black, etc.), copper, nickel, cobalt and the like. As the binder, known materials can be used, such as carboxymethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, polyethylene oxide, polytetrafluoroethylene (PTFE), 4-fluoroethylene-6-fluoropropylene copolymer. (FEP) and the like.

  As the current collector, for example, punching metal, foam metal, or the like can be used. Usually, since the negative electrode for nickel metal hydride secondary batteries is produced by what is called a paste type, punching metal is used. The paste type negative electrode is mixed with the hydrogen storage alloy (ground powder) of the present invention, the above-mentioned binder and, if necessary, a conductive material, an antioxidant, a surfactant, a thickener, etc. and mixed with water as a solvent. The paste is made into a paste, and this paste is applied to a current collector, filled, dried, and then subjected to a roller press or the like.

  The negative electrode for a nickel metal hydride secondary battery of the present invention can be formed with a water repellent layer, a conductive layer, or the like on the surface as necessary. These are performed by a known method. For example, the former is performed by applying and drying a fluororesin dispersion and the latter is performed by plating or the like.

  The nickel metal hydride secondary battery of the present invention uses the negative electrode for a nickel metal hydride secondary battery of the present invention. A publicly known thing can be used for the other composition. The nickel-hydrogen secondary battery of the present invention can have various shapes such as a cylindrical shape, a stacked shape, and a coin shape. Regardless of the shape, the nickel-metal hydride secondary battery stores an electrode group in which a negative electrode, a separator, and a positive electrode are stacked in a can made of stainless steel or the like. In the case of a cylindrical shape, the negative electrode and the negative electrode terminal are usually connected by winding the electrode group spirally with the negative electrode on the outside and inserting it into the can body so that the negative electrode terminal is the can body. The positive electrode is usually connected to the positive terminal by a lead.

  As the separator, a polymer fiber nonwoven fabric made of nylon, polypropylene, polyethylene or the like, a porous polymer film such as polyethylene, polypropylene, or the like can be used.

  The positive electrode contains nickel oxide, and for example, a non-sintered nickel electrode is used. A non-sintered nickel electrode is made by mixing nickel hydroxide and optionally added cobalt hydroxide, cobalt monoxide, metallic cobalt, etc., with a binder as a solvent to form a paste. It is manufactured by filling a current collector such as foam metal and drying, and then applying a roller press or the like.

  A 6-8 N potassium hydroxide solution is injected as an alkaline electrolyte into a container in which the electrode group is housed. An alkaline electrolyte to which lithium hydroxide, sodium hydroxide or the like is added can also be used. In the container, a gasket for sealing the battery, a safety valve that operates when the pressure in the battery rises abnormally, and the like are provided.

Effect of the invention

  A negative electrode for a nickel metal hydride secondary battery manufactured using the hydrogen storage alloy of the present invention, a nickel metal hydride secondary battery manufactured using this negative electrode has good cycle characteristics, a high operating voltage of the battery, and self-discharge. Can be suppressed.

Next, the present invention will be described in detail by way of examples.

Example 1
(La _0.85 Ce _0.11 Pr _0.01 Nd _0.03 ) Ni _4.15 Co _0.55 Al _0.50 The raw material Mm (Misch metal), La, Ni, Mn , Co and Al were weighed and melted in an argon atmosphere in a high-frequency melting furnace to obtain an alloy melt. Subsequently, the temperature of this melt was set to 1,550 ° C., and it was rapidly cooled and solidified by a strip casting method using a single roll casting apparatus of a copper water-cooled roll to obtain a flaky alloy having a thickness of approximately 0.2 mm. . The obtained alloy was heat-treated at 1,140 ° C. for 6 hours in an argon atmosphere.

  The obtained hydrogen storage alloy was observed with an EPMA (manufactured by JEOL Ltd., trade name JXA8800) at a magnification of 500 times. The obtained Comp image and Al element mapping image are shown in FIGS. 1 and 2, respectively. As a result, no precipitated phase having an Al concentration higher than that of the mother phase and having a particle size of 2.0 μm or more was confirmed.

(Evaluation of pulverization characteristics)
The obtained hydrogen storage alloy was pulverized by a ball mill to obtain an alloy powder having a D50 of 35 μm. The obtained alloy powder was evacuated at 80 ° C. for 1 hour using a PCT apparatus, and 2.3 MPa of hydrogen gas was introduced at 40 ° C. to absorb hydrogen until the hydrogen pressure reached equilibrium, and then at 80 ° C. Hydrogen was released by evacuation for 1 hour. The ratio of D50 after hydrogen storage / release to D50 before hydrogen storage / release was calculated and used as the pulverization residual rate. The residual rate of pulverization of this hydrogen storage alloy was 92%.

(Evaluation of self-discharge characteristics)
Similarly, a binder is added to the produced alloy powder to form a paste, which is applied to a current collector, dried and pressed to produce a negative electrode for a nickel metal hydride secondary battery. This negative electrode and a known nickel positive electrode, separator, A 2000 mAh AA size sealed nickel-metal hydride battery was fabricated using the electrolytic solution. This battery was charged at 120 ° C. at 120 ° C. for 120%, and the capacity obtained by discharging to 0.2 V at 1.0 C was taken as the standard capacity. Moreover, after charging 150% at 20 ° C. and 0.1 C, it was left at 40 ° C. for 28 days, and then discharged at 20 ° C. and 0.2 C to 1.0 V. The ratio of the capacity after 28 days storage to the standard capacity was calculated and used as the capacity retention rate after storage. The capacity maintenance rate of this alloy powder was 91%.

(Example 2)
(La _0.85 Ce _0.11 Pr _0.01 Nd _0.03 ) Ni _4.25 Co _0.45 Al _0.50 The raw material was adjusted so as to have an alloy composition, and quenched and solidified heat treatment was performed. The same procedure as in Example 1 was performed except that the procedure was performed at 1,165 ° C. for 6 hours. The Comp image and Al element mapping image observed by EPMA of the obtained slab are shown in FIGS. 3 and 4, respectively. No precipitated phase having an Al concentration higher than that of the mother phase and having a particle size of 2.0 μm or more was confirmed. Table 1 shows the pulverization residual rate and the capacity maintenance rate.

(Example 3)
(La _0.85 Ce _0.11 Pr _0.01 Nd _0.03 ) Ni _4.25 Co _0.45 Al _0.40 Mn _0.10 The raw material was adjusted so as to have an alloy composition, quenched and solidified. Example 1 was performed except that the heat treatment of the piece was performed at 1,120 ° C. for 6 hours. When the obtained slab was observed by EPMA, no precipitated phase having an Al concentration higher than that of the matrix and a particle size of 2.0 μm or more was confirmed. Table 1 shows the pulverization residual rate and the capacity maintenance rate.

Example 4
(La _0.85 Ce _0.11 Pr _0.01 Nd _0.03 ) Ni _4.40 Co _0.30 Al _0.50 The same procedure as in Example 1 was performed except that the procedure was performed at 1,165 ° C. for 6 hours. When the obtained slab was observed by EPMA, no precipitated phase having an Al concentration higher than that of the matrix and a particle size of 2.0 μm or more was confirmed. Table 1 shows the pulverization residual rate and the capacity maintenance rate.

(Comparative Example 1)
The same procedure as in Example 1 was performed except that the quenched and solidified slab was heat treated at 1,000 ° C. for 6 hours. A Comp image and an element mapping image of Al observed by EPMA of the obtained slab are shown in FIGS. 5 and 6, respectively. A precipitated phase having an Al concentration higher than that of the mother phase and having a particle size of 2.0 μm or more was confirmed. The maximum precipitated phase was 3.5 μm. Table 1 shows the pulverization residual rate and the capacity maintenance rate.

(Comparative Example 2)
The quenched and solidified slab was processed in the same manner as in Example 2 except that the heat treatment was performed at 1,140 ° C. for 6 hours. A Comp image and an element mapping image of Al observed by EPMA of the obtained slab are shown in FIGS. 7 and 8, respectively. A precipitated phase having an Al concentration higher than that of the mother phase and having a particle size of 2.0 μm or more was confirmed. The maximum precipitated phase was 2.5 μm. Table 1 shows the pulverization residual rate and the capacity maintenance rate.

(Comparative Example 3)
The same procedure as in Example 2 was performed except that a water-cooled copper mold was used to form an alloy ingot having a thickness of 3 cm. The Comp image and Al element mapping image observed by EPMA of the obtained slab are shown in FIGS. A precipitated phase having an Al concentration higher than that of the mother phase and having a particle size of 2.0 μm or more was confirmed. The maximum precipitated phase was 3.5 μm. Table 1 shows the pulverization residual rate and the capacity maintenance rate.

(Comparative Example 4)
(La _0.85 Ce _0.11 Pr _0.01 Nd _0.03 ) Ni _4.25 Co _0.45 Al _0.20 Mn _0.30 The raw material was adjusted to have an alloy composition, quenched and solidified. Example 1 was performed except that the heat treatment of the piece was performed at 1000 ° C. for 6 hours. When the obtained slab was observed by EPMA, no precipitated phase having an Al concentration higher than that of the matrix and a particle size of 2.0 μm or more was confirmed. Table 1 shows the pulverization residual rate and the capacity maintenance rate.

(Comparative Example 5)
_{(La 0.85 Ce 0.11 Pr 0.01 Nd} 0.03) except for adjusting the raw material so that the alloy composition of Ni _4.55 Co _{0.15 Al 0.50} was carried out in the same manner as in Example 2 . A Comp image and an element mapping image of Al observed by EPMA of the obtained slab are shown in FIGS. 11 and 12, respectively. A precipitated phase having an Al concentration higher than that of the mother phase and having a particle size of 2.0 μm or more was confirmed. The maximum precipitated phase was 3.5 μm. Table 1 shows the pulverization residual rate and the capacity maintenance rate.

2 is a Comp image of a cross-sectional structure of the hydrogen storage alloy of Example 1. FIG. 2 is an Al mapping image of a cross-sectional structure of the hydrogen storage alloy of Example 1. FIG. 4 is a Comp image of a cross-sectional structure of the hydrogen storage alloy of Example 2. 3 is an Al mapping image of a cross-sectional structure of the hydrogen storage alloy of Example 2. FIG. 3 is a Comp image of a cross-sectional structure of the hydrogen storage alloy of Comparative Example 1. 3 is an Al mapping image of a cross-sectional structure of the hydrogen storage alloy of Comparative Example 1. 6 is a Comp image of a cross-sectional structure of the hydrogen storage alloy of Comparative Example 2. 4 is an Al mapping image of a cross-sectional structure of the hydrogen storage alloy of Comparative Example 2. 6 is a Comp image of a cross-sectional structure of the hydrogen storage alloy of Comparative Example 3. 10 is an Al mapping image of a cross-sectional structure of the hydrogen storage alloy of Comparative Example 3. 10 is a Comp image of a cross-sectional structure of the hydrogen storage alloy of Comparative Example 5. 6 is an Al mapping image of a cross-sectional structure of the hydrogen storage alloy of Comparative Example 5.

Claims (9)

Formula _{RNi a Co b Al c Mn d} M e ( at least one wherein R is selected from rare earth elements including Y, M represents R, Ni, Co, Al, at least one selected from the elements other than Mn A is 3.30 ≦ a ≦ 5.00, b is 0.10 ≦ b ≦ 0.90, c is 0.30 ≦ c ≦ 0.80, d is 0 ≦ d ≦ 0.15, and e is 0 ≦ e ≦ 0.30, 4.90 ≦ a + b + c + d + e ≦ 5.50), and a COMP image and Al element of 500 times the EPMA of the cross-sectional structure of the alloy by EPMA A hydrogen storage alloy characterized by having a higher Al concentration than a parent phase confirmed by a mapping image and no precipitated phase having a particle size of 2.0 μm or more.   2. The hydrogen storage alloy according to claim 1, wherein M is at least one selected from Mg, Ti, Zr, Nb, Mo, W, Fe, Cu, B, and Sn.   The hydrogen storage alloy according to claim 1 or 2, wherein an equilibrium pressure at 40 ° C is 0.02 to 0.15 MPa.   d is 0, The hydrogen storage alloy of Claims 1-3 characterized by the above-mentioned.   5. The hydrogen storage alloy according to claim 1, wherein c is 0.45 or more and 0.60 or less.   b is 0.40 or more and 0.70 or less, The hydrogen storage alloy of Claims 1-5 characterized by the above-mentioned.   The hydrogen storage alloy according to claim 1, comprising a slab having a thickness of 0.05 to 0.50 mm.   A negative electrode for a nickel metal hydride secondary battery containing the hydrogen storage alloy according to claim 1.   A nickel metal hydride secondary battery using the negative electrode for a nickel metal hydride secondary battery according to claim 8.

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