JP7439316B1 - Hydrogen storage alloy powder for nickel metal hydride battery negative electrode - Google Patents

Abstract

An object of the present invention is to achieve both reduction in raw material cost by reducing the Co content and improvement in the life characteristics of the negative electrode with respect to a Co-containing CaCu5 type hydrogen storage alloy used as a negative electrode of a nickel-metal hydride battery. [Solution] General formula MmNiaMnbAlcCod (where Mm is misch metal, 4.30≦a≦4.75, 0.25≦b≦0.50, 0.25≦c≦0.45, 0≦ A hydrogen storage alloy having a CaCu5 type crystal structure represented by d≦0.12, 5.20≦a+b+c+d≦5.55), in which some of the constituent atoms represented by the above general formula are substituted with other elements. is substituted with This is a hydrogen storage alloy characterized by the following characteristics. [Selection diagram] Figure 2

Description

The present invention relates to a hydrogen storage alloy powder having a CaCu type ₅ crystal structure used as a negative electrode of a nickel-metal hydride battery. Furthermore, the present invention relates to a negative electrode using this hydrogen storage alloy powder and a battery using this negative electrode.

Nickel-metal hydride batteries using a hydrogen storage alloy for the negative electrode were commercialized in the early 1990s, and have since become widespread.

Nickel metal hydride batteries were used as a power source for mobile phones and notebook computers when they were first commercialized, but they have since been gradually replaced by smaller and lighter lithium-ion batteries, and today they are cheaper and safer. Because of its good balance between energy density and energy density per volume, it is used in toys, small equipment, and even hybrid cars.

The hydrogen storage alloy used in such a nickel-metal hydride battery is an alloy that reacts with hydrogen to form a metal hydride. This hydrogen storage alloy can reversibly store and release a large amount of hydrogen at around room temperature.

Hydrogen storage alloys include AB ₅ type alloy represented by LaNi ₅ , AB ₂ type alloy represented by ZrV _0.4 Ni _1.5 , and various other types such as AB type, A ₂ B type, and AB ₃ type. types of alloys are known. These alloys contain elemental groups (rare earth elements, Ca, Mg, Ti, Zr, V, Nb, Pt, Pd, etc.) that have a high affinity with hydrogen and play a role in increasing hydrogen storage capacity, and It is composed of a combination with an element group (Ni, Mn, Co, Al, etc.) that plays a role of promoting the hydrogenation reaction and lowering the reaction temperature, although the storage amount is relatively low and the amount of storage is small.

Among these, an AB ₅ -type hydrogen storage alloy having a CaCu ₅ -type crystal structure, for example, misch metal (hereinafter referred to as "Mm"), which is a rare earth mixture, is used at the A site, and Ni, Mn, An alloy using elements such as Co and Al can form a negative electrode with a relatively inexpensive material compared to alloys with other compositions.

AB ₅ type hydrogen storage alloy can be used to fill a negative electrode by adjusting the ratio of the atomic weight of the B site to the atomic weight of the A site (AB ratio) and the amount of replacing part of Ni with Co, Mn, Al, etc. Various characteristics such as discharge capacity, input/output characteristics, and cycle life can be adjusted. The _AB5 type hydrogen storage alloy, which has such characteristics, makes it possible to manufacture nickel-metal hydride storage batteries for various uses.

In order to popularize hybrid vehicles, it is necessary to keep the manufacturing cost of nickel-metal hydride batteries low and to further improve the life characteristics and input/output characteristics of the negative electrode. One such study is to focus on the "segregation phase" in the _AB5 type hydrogen storage alloy, which uses as little as possible the amount of expensive Co, with the aim of maintaining and improving its life characteristics.

For example, in Patent Document 1, a hydrogen storage alloy powder having a CaCu type ₅ crystal structure and consisting of MmMgNiCoMnAl is used as an active material of a hydrogen storage alloy electrode, the hydrogen storage alloy powder having an MgNiCoMnAl alloy phase at least inside the hydrogen storage alloy powder. A hydrogen-absorbing alloy powder containing dispersed fine segregated phases is applied. Preferably, it has been proposed to apply a hydrogen storage alloy powder having a surface layer made of an alloy of Ni and Co on the surface of the hydrogen storage alloy powder. It is said that by doing this, the miniaturization of the hydrogen storage alloy powder is suppressed.

In addition, in Patent Document 2, it contains misch metal with an amount of La of 60 to 90 wt%, Mg, Ni, Co, Mn, and Al, and has an amount of Mg of 2 to 6 at% based on the total amount of misch metal and Mg. A rare earth hydrogen storage alloy is proposed, which is characterized in that at least one segregated phase exists in the matrix, and the Mg concentration in the segregation phase is higher than the Mg concentration in the matrix. has been done. By doing this, the segregated phase in the alloy is selectively corroded during battery activation, the surface area increases from the vicinity of the segregated phase, and the alloy is activated early.

Further, in Patent Document 3, the negative electrode active material contains Mm (Mm represents a mixture of two or more rare earth elements containing 30% by weight or more of La) and at least Ni, Co, Mn, and Al as constituent elements. The hydrogen storage alloy is a hydrogen storage alloy, which has a segregated phase mainly composed of Ni, and the number of segregated phases exposed in any 15 μm square area of the cross section of the alloy (however, the number of segregated phases in the minimum diameter of the segregated phase is It has been proposed to use a hydrogen storage alloy in which the number of segregated phases whose circumscribed circles have a diameter of 0.05 μm or more is 1 to 40. By doing so, it is possible to achieve high capacity, high-rate discharge at low temperatures, and excellent high-temperature storage characteristics.

In Patent Document 4, in a hydrogen storage alloy having a matrix having a CaCu type ₅ crystal structure, the segregated phase is The Fe peak intensity ratio [{(Fe peak intensity of segregation)/(Fe peak intensity of parent phase)}×100(%)] is the ratio of the Fe peak intensity of the segregated phase to the Mn peak intensity of the parent phase. The Fe/Mn peak ratio [Fe/ A hydrogen storage alloy has been proposed in which the Mn ratio] is 0.12<[Fe/Mn ratio]<0.37. This is because it is known that containing Fe improves the pulverization characteristics (life characteristics), so by replacing a portion of Ni with Fe, excellent cycle characteristics can be obtained. Specifically, it is said that the segregated phase plays a role (a cushion role) in mitigating strain caused by expansion and contraction of the lattice accompanying hydrogen absorption and release in the parent phase.

Furthermore, in Patent Document 5, the general formula MmNix My (Mm is a misch metal or a mixture of rare earth elements, M is at least one member selected from the group consisting of Al, Mn, Co, Cu, Fe, Cr, Ti, and V) is disclosed. 5.0≦x+y≦5.5), and its alloy structure consists of a phase that has a CaCu type ₅ crystal structure and reversibly absorbs and releases hydrogen, and an element other than Mm as a main component. Hydrogen storage alloys have been proposed that are composed of one or more phases that do not store hydrogen, and in which the latter phase is dispersed in the form of islands in the former phase. By doing so, it is possible to achieve both long life and excellent high rate discharge characteristics without containing a large amount of Co. In other words, some of these have compositions in which a portion of Ni is replaced by Fe or Cr, and as a result, the phase that does not absorb hydrogen makes the phase that absorbs hydrogen itself a structure that is resistant to stress. .

Further, in Patent Document 6, _AB5 type hydrogen storage alloy in which Mm (misch metal) consisting of La and Ce occupies the A site and Ni, Co, Mn, and Al, or Ni, Mn, and Al occupies the B site. Then, the Co content was reduced to a Co/Mm molar ratio of 0.11 or less, the Al/Mn molar ratio was 0.35 to 1.10, and the c-axis length/a-axis length of the crystal was 0.8092 or more. Accordingly, it is possible to reduce the amount of Co and prevent deterioration in battery life characteristics when used as a negative electrode active material in a nickel-metal hydride battery. According to the above description, only La and Ce occupy the A site, but it is extremely difficult to form a CaCu type ₅ crystal structure in a composition where the molar ratio x of ABx is 5.0 or more. It is. This is because if the A site is only La and Ce and the ABx molar ratio is 5+α, the crystal structure is set to [A _5/(5+α) V ^A _1-5/(5+α) ]B ₅ Although it is necessary to form A-site vacancies by 1-5/(5+α), this is because some B atoms actually occupy a part of the A-site.

Japanese Patent Application Publication No. 2005-93297 Japanese Patent Application Publication No. 2004-269929 Japanese Patent Application Publication No. 2000-353542 Japanese Patent Application Publication No. 2009-30158 Japanese Patent Application Publication No. 7-286225 WO2021-220824

As mentioned above, studies are being conducted to maintain and improve life characteristics, but as the trading price of Co has soared in recent years, in order to maintain or reduce the raw material cost of AB ₅ type hydrogen storage alloy containing Co, It is desired to further reduce the content of or eliminate it.

However, when the Co content of the AB ₅ type hydrogen storage alloy is reduced, pulverization of the alloy due to repeated storage and release of hydrogen is promoted, which tends to reduce the life characteristics of the negative electrode. In the examples of Patent Documents 1 to 6 mentioned above, attention is paid to the segregated phase and attempts are made to maintain the life characteristics by replacing some constituent atoms with other elements, but in reality, a certain amount of Co is contained. Therefore, further study is required in order to achieve both reduction in Co content and improvement in the life characteristics of the negative electrode.

The present invention has been made in view of the above-mentioned problems, and is aimed at reducing the raw material cost by reducing the Co content in a hydrogen storage alloy having a CaCu type ₅ crystal structure, and by repeatedly absorbing and releasing hydrogen. It is an object of the present invention to provide a hydrogen storage alloy that can suppress pulverization, and a negative electrode and a battery using the hydrogen storage alloy.

In order to solve the above-mentioned problems, the present inventor conducted extensive research and developed a hydrogen storage alloy having a CaCu type ₅ crystal structure, which suppresses the raw material cost by reducing the Co content and replaces the pulverization suppressing effect of Co. As a result of considering methods to do this, we found that if we ensure sufficient intralattice porosity in the crystal lattice formed by filling the atoms that make up the hydrogen storage alloy so that the volume change is small even when hydrogen atoms enter, it is possible to We came up with the idea that it is possible to suppress the pulverization of the alloy due to repeated occlusion and release of . On top of that, some of the constituent atoms of the hydrogen storage alloy having a CaCu type ₅ crystal structure are replaced with a certain element The present invention was completed by discovering that it is possible to achieve both reduction and suppression of pulverization.

The gist of the present invention is as follows.

(1) General formula MmNi _a Mn _b Al _c Co _d (wherein, Mm is misch metal, 4.30≦a≦4.75, 0.25≦b≦0.50, 0.25≦c≦ 0.45, 0≦d≦0.12, 5.20≦a+b+c+d≦5.55) A hydrogen storage alloy powder having a CaCu type ₅ crystal structure and used for a negative electrode of a nickel-hydrogen battery. , some of the constituent atoms represented by the above general formula are substituted with other substituent elements X, and the intralattice porosity represented by the following formula is 23.20% or more and 23.70% or less. Hydrogen storage alloy powder for nickel-metal hydride battery negative electrodes .
Intralattice porosity = (crystal lattice volume - total volume of constituent atoms) / crystal lattice volume x 100
(2) Among the constituent atoms represented by the general formula, one or more target elements Y2 selected from the group consisting of Ni, Mn, and Al are substituted with the substituting element X, resulting in the composition formula MmNi _a-a1 Mn _{b-b1 Al c -} c1 Alloy powder .
(3) The hydrogen storage alloy powder for a negative electrode of a nickel-hydrogen battery according to (2), wherein the substitution element X has a larger metal bond radius than the atom to be replaced.
(4) The hydrogen storage alloy powder for a nickel-metal hydride battery negative electrode according to (1) or (2), wherein Mm contains a rare earth metal whose atomic radius is smaller than that of Ce.
(5) The hydrogen storage alloy powder for a nickel -hydrogen battery negative electrode according to (1) or (2), wherein the hydrogen storage alloy has a degree of difficulty in pulverization of 0.55 or more.
(6) A negative electrode for a nickel-metal hydride battery, characterized in that the hydrogen-absorbing alloy powder according to (1) or (2) is used as a negative electrode active material for a nickel-metal hydride battery .
(7) A nickel-metal hydride battery characterized by using the nickel- metal hydride battery negative electrode according to (6).

According to the present invention, it is possible to provide a hydrogen storage alloy that suppresses raw material costs by reducing Co content and suppresses pulverization of the alloy due to repeated absorption and release of hydrogen, and can be effectively used in negative electrodes and batteries. be able to. When the hydrogen storage alloy of the present invention is used as a negative electrode active material of a battery, the life characteristics can be maintained or improved, and the amount of Co used can be reduced.

Figure 1 is a schematic diagram showing the crystal structure of AB type ₅ alloy. (a) shows the case where the stoichiometric ratio of A and B is 5.0, and (b) shows the case where some of the A site atoms are missing. The case where the stoichiometric ratio exceeds 5.0 is shown. FIG. 2 is a schematic diagram showing how two B atoms are substituted in a dumbbell shape at the A site of the crystal structure shown in FIG.

The present invention will be explained in detail below.

The present inventors conducted a detailed study on an alternative method for pulverization due to hydrogen storage and release and its suppression in a hydrogen storage alloy having a CaCu type ₅ crystal structure, considering the pulverization suppressing effect of Co.
First, as an effect of suppressing pulverization by Co, when Co is added to a hydrogen storage alloy with a CaCu ₅ -type crystal structure (replaced with Co), the metallic bond radius of Co becomes the atomic radius of Ni in the LaNi _5- crystal lattice to be replaced. Since they are almost the same (translation supervised by Shigeo Fujiwara, “Sanderson Inorganic Chemistry Vol. 1”, Hirokawa Shoten, 1969, p. 87; see Non-Patent Document 1), even if Co atoms are substituted for Ni atoms, the crystal lattice size (crystal lattice volume) remains almost unchanged. However, in the case of a crystal of a hydrogen storage alloy to which Co is added, the change in volume during hydrogenation (hydrogen storage) becomes small, making it difficult to be pulverized.

Here, the occurrence of cracks in hydrogen storage alloys due to hydrogenation (hydrogen storage) occurs when hydrogen atoms H enter the alloy crystal in an interstitial manner and the crystal expands. It is thought that stress is generated between the crystal part and the unhydrogenated part at the interface between the part and the part which has not yet been hydrogenated. This is because stress is similarly generated in the reverse reaction of hydrogenation (hydrogen release). In other words, if the stress generated above exceeds the strength of the alloy crystal, it will crack, so if the amount of deformation (expansion) of the alloy crystal due to hydrogenation is small, the stress generated will also be small, making the alloy crystal difficult to crack. becomes difficult to change.

Next, consider that when a Co atom having substantially the same metal bond radius as a Ni atom is added, the change in volume of the hydrogen storage alloy becomes smaller even when the hydrogen storage alloy is hydrogenated. As mentioned above, since hydrogen storage alloys are hydrogenated in an interstitial manner in which hydrogen atoms H enter between the atoms constituting the alloy crystals, the alloy crystals usually expand significantly. However, they came up with the idea that the expansion of the alloy crystal is suppressed because the electron cloud of Co atoms easily forms covalent bonds that overlap with the electron cloud of hydrogen atoms. Comparing the d-electron configurations of Co and Ni, they are 3d ⁷ 4s ² and 3d ⁸ 4s ² , respectively, so the Co atom has a higher covalent bond order through the hydrogen atom and the d-electron, and the electron clouds overlap. big. That is, even if a hydrogen atom H enters the alloy crystal lattice, a small expansion occurs.

Based on the above, when considering an alternative to Co, it becomes an element that has a d electron configuration that allows for covalent bonding. However, although iron (Fe) and manganese (Mn) have a d-electron configuration that increases the covalent bond order, they have a large metallic bond radius (see Non-Patent Document 1), and covalent bonds with hydrogen H reduce the expansion of alloy crystals. It has little effect.

By the way, another idea is to expand the lattice of the alloy crystal in advance. The simple idea is that if the lattice of the alloy crystal is made larger in advance, the expansion will be smaller even when hydrogenated. To expand the alloy crystal lattice, replace the Ni site (B site) with an atom with a larger metallic bond radius than nickel (e.g., J.-M. Jouberta, et.al., Journal of Alloys and Compounds 330 -332 (2002) 208-214; see Non-Patent Document 2), and make the B/A molar ratio (x value of ABx) of A site and B site non-stoichiometric to 5.0 or more (for example, M. There is a method such as Latroche, et.al., Journal of Solid State Chemistry 146, 343-321 (1999); see Non-Patent Document 3).

When x of the latter ABx composition is made non-stoichiometric to 5.0 or more, the crystal structure becomes [AB _2/7×α ]B _5+(5/7)α , unlike the above-mentioned Patent Document 6. B atoms occupy part of the A site. When a B atom occupies the A site, two B atoms are substituted at the A site in a dumbbell shape. Figure 1 schematically shows the crystal structure of AB type ₅ alloy, where (a) shows the case where the stoichiometric ratio of A and B is 5.0, and (b) shows the part of the A site atoms. This is the case when the stoichiometric ratio exceeds 5.0. FIG. 2 shows a state in which two B atoms are substituted at the A site in a dumbbell shape. The present inventors have attempted various alloy designs in the belief that by expanding the lattice of the alloy crystal, particularly in the c-axis direction, cracking (pulverization) of the alloy crystal due to hydrogen absorption and release can be suppressed.

Further detailed studies by the present inventors revealed that alloys with expanded crystal lattices do not necessarily become difficult to pulverize, and in particular, when the Co content is low or zero, simply expanding the crystal lattice does not make it difficult to pulverize. It was found that it becomes difficult to suppress the

As a result of investigating the above-mentioned causes, the present inventors found that even if the crystal lattice of the hydrogen storage alloy is expanded, if space for hydrogen atoms H cannot be secured, even alloys with large crystal lattices will not be pulverized. It was also discovered that this could not be suppressed, leading to the present invention. That is, it is important to form larger spaces in the crystal lattice of the alloy so that even if hydrogen atoms H enter, there will be little change. Specifically, it has been found that pulverization can be suppressed by setting the intralattice porosity expressed by the following formula to 23.20% or more.
Intralattice porosity = (crystal lattice volume - total volume of constituent atoms) / crystal lattice volume x 100
It was also discovered that as the electron density (=number of electrons/atomic volume) of the atoms constituting the hydrogen storage alloy decreases as well as the intralattice porosity, the change when hydrogen atoms invade decreases. Note that the above-mentioned intralattice porosity was determined by excluding the volume of each constituent atom occupying the crystal lattice volume of the hydrogen storage alloy.

Here, in order to increase the lattice porosity, it is not enough to simply substitute atoms with a large metal bond radius or to widen the crystal lattice by making a dumbbell-shaped substitution at the A site as described above; It is necessary to select an atom (substitution element) to actually replace the target atom, and the present inventors have found that it is particularly advantageous to appropriately substitute an atom with a small metal bond radius to secure space.

Therefore, the present invention is based on the general formula _{MmNiaMnbAlcCod} (wherein, Mm is misch _{metal ,} 4.30≦a≦4.75, 0.25 _≦ b≦0.50, 0.25 ≦c≦0.45, 0≦d≦0.12, 5.20≦a+b+c+d≦5.55), is a hydrogen storage alloy having a CaCu ₅ type crystal structure, and has a configuration represented by the above general formula. Part of an atom is replaced by an atom with a smaller metal bond radius than the constituent atom, or an atom with a larger metal bond radius than the constituent atom, or an atom with a smaller metal bond radius than the constituent atom and the constituent atom. It is a hydrogen storage alloy in which the lattice porosity is 23.20% or more and 23.70% or less expressed by the following formula.
Intralattice porosity = (crystal lattice volume - total volume of constituent atoms) / crystal lattice volume x 100

In the hydrogen storage alloy having the CaCu type ₅ crystal structure of the present invention, that is, the AB _type hydrogen storage alloy, first, the metal (atom) constituting the A site will be explained. In the present invention, misch metal (Mm) in which La or a part or all of La is a rare earth metal mixture is used as the metal constituting the A site. In Mm, it is preferable that La and Ce occupy a proportion within the range of 80% by mass or more and 100% by mass or less based on the total mass of Mm, in particular, La is 70 to 99% by mass and Ce is 1 to 30% by mass. Preferably, La is in the range of 74 to 97% by mass, and Ce is in the range of 3 to 26% by mass.

Next, the metal (atom) constituting the B site will be explained. In the present invention, Ni, Mn, Al, and Co are used as metals constituting the B site. The molar ratio of these metals in the general formula satisfies the following conditions.
Ni molar ratio (a) 4.30≦a≦4.75
Mn molar ratio (b) 0.25≦b≦0.50
Al molar ratio (c) 0.25≦c≦0.45
Co molar ratio (d) 0≦d≦0.12
AB ratio 5.20≦(a+b+c+d)≦5.55
Preferred conditions are as follows.
Ni molar ratio (a) 4.40≦a≦4.70
Mn molar ratio (b) 0.35≦b≦0.43
Al molar ratio (c) 0.38≦c≦0.42
Co molar ratio (d) 0≦d≦0.05
AB ratio 5.25≦(a+b+c+d)≦5.46

The molar ratio d of Co is preferably as small as possible in order to reduce raw material costs, and is set to 0≦d≦0.12. By setting d to be greater than 0 and less than or equal to 0.12, that is, by replacing with Co, it becomes difficult to pulverize even if hydrogen absorption and desorption is repeated, and this tendency becomes more pronounced as the amount of substitution increases. However, even if d=0, that is, without Co substitution, by using the AB ₅ type hydrogen storage alloy of the present invention, it is possible to make it difficult to pulverize even if hydrogen storage and release are repeated. Therefore, the Co molar ratio (d) is 0≦d≦0.12, preferably 0≦d≦0.05. Note that if d exceeds 0.12, it will not lead to reduction in raw material cost.

The reason why the ratio of La and Ce in Mm and the molar ratio of Ni, Mn, and Al were set as above is that the hydrogen storage capacity (H/M) of AB ₅ type hydrogen storage alloy is 0.85 to 1. 00 to ensure charge/discharge capacity, setting the equilibrium pressure to 0.04 to 0.07 MPa to facilitate initial activation, and widening the plateau region in the PCT curve as much as possible.
In Mm, by setting La and Ce within the range of 80% by mass or more and 100% by mass or less based on the total mass of Mm, the hydrogen storage capacity (H/M) can be set to 0.85 to 1.00. This is because discharge capacity can be secured. As mentioned above, the ratio (a) of Ni is within the range of 4.30 or more and 4.75 or less, but when a negative electrode is produced using the hydrogen storage alloy powder as an active material, it is easy to maintain its output characteristics, and There is no particular deterioration in its life characteristics. As mentioned above, the ratio of Mn (b) is within the range of 0.25 or more and 0.50 or less, but within this range, it is possible to easily maintain the difficulty of pulverizing the hydrogen storage alloy powder. . As mentioned above, the Al ratio (c) is within the range of 0.25 or more and 0.45 or less, but within this range, the hysteresis in the PCT characteristics is small and the charge/discharge efficiency of the hydrogen storage alloy powder is deteriorated. can be suppressed, and a decrease in the hydrogen storage amount of the hydrogen storage alloy powder can be suppressed.

The above is the general formula _MmNia Mn _b Al _c Co _d (where Mm is misch metal, 4.30≦a≦4.75, 0.25≦b≦0.50, 0.25≦c≦ 0.45, 0≦d≦0.12, 5.20≦a+b+c+d≦5.55) This is a basic explanation of an alloy having a CaCu type ₅ crystal structure.

In the present invention, some of the constituent atoms represented by the above general formula are substituted with other substituent elements X other than these. In this case, the substitution may be made with an atom having a smaller metal bond radius than the constituent atoms. For example, among the constituent atoms represented by the above general formula, when Mm is the target element to be replaced (target element Y1), more specifically, when La or Ce, etc. are the target element Y1, the metal The A site may be substituted with an atom (substituting element X) having a small bond radius. Further, the B site may be substituted with an atom having a smaller metal bond atomic radius than one or more target elements Y2 selected from the group consisting of Ni, Mn, Al, and Co. More specifically, examples of atoms having a smaller metal bond radius than La or Ce include Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Y, and Sc. On the other hand, examples of atoms having a smaller metal bond atomic radius than Ni, Mn, Al, or Co include Be, B, C, and N.

Further, an atom having a larger metal bond radius than the constituent atoms may be substituted. For example, when replacing the target element Y1 among the constituent atoms represented by the above general formula, the A site may be replaced with an atom with a larger metal bond radius than these atoms, and the target element Y2 may also be replaced with an atom having a larger metal bond radius than these atoms. In the case of substitution, the B site may be substituted with an atom having a larger metal bond radius than these atoms. To give more specific examples of these, examples of atoms having a larger metal bond radius than La or Ce include Eu, Yb, Ba, Ca, and the like. On the other hand, examples of atoms having a larger metal bond atomic radius than Ni, Mn, Al, and Co include Fe, Cu, Zn, Ga, Ge, Cr, V, Ti, Zr, Nb, Mo, and Mg.

Alternatively, an atom with a smaller metal bond radius and an atom with a larger metal bond radius than the constituent atoms may be combined to replace the A site, the B site, or both. Among these, as mentioned above, by expanding the crystal lattice by substitution with atoms with a large metal bond radius or dumbbell-shaped substitution at the A site, and replacing appropriately with atoms with a small metal bond radius, the intralattice porosity can be increased to 23 .20% or more, thereby suppressing pulverization. This is particularly effective when the Co content is low or zero.

In particular, the target to be substituted is the target element Y2 _, and the substitutional element X substitutes one or more of Ni, _{Mn , or} Al _. It is more preferable to use a hydrogen storage alloy that satisfies the following formula: e=a1+b1+c1, 5.20≦a+b+c+d+e≦5.55).

The crystal lattice volume of the present invention is determined as follows. In other words, the crystal lattice volume can be determined from the lattice constants (a-axis, c-axis) calculated from the X-ray diffraction pattern measured by the X-ray diffraction method (P. H. L. Notten, et.al., Journal of The Electrochemical Society 146). 9), 3181-3189 (1999); same as Non-Patent Document 4).

On the other hand, the total volume of atoms occupying the crystal lattice volume is calculated using a spherical model from the metal bond radius of each atom constituting the crystal lattice. Then, the intralattice porosity of the present invention is calculated from the following formula.
Intralattice porosity = (crystal lattice volume - total volume of constituent atoms) / crystal lattice volume x 100
For the metal bond radius of the present invention, the value described in "Chemical Handbook Basic Edition" edited by the Chemical Society of Japan, published in 1984 is used. Further, in the case of a typical element, one-half of the bond distance between like atoms described in the representative value of atomic spacing is used.

If the intralattice porosity is set to 23.20% or more as described above, pulverization due to hydrogen absorption can be suppressed, and the larger the porosity, the greater the pulverization suppressing effect. Therefore, although there is no upper limit for the intralattice porosity, it was set to 23.70% or less from the viewpoint of stability of the CaCu type ₅ crystal structure.

In the hydrogen storage alloy of the present invention, since the degree of difficulty in pulverization is 0.55 to 0.70, when the hydrogen storage alloy powder is used as a negative electrode, it becomes a negative electrode with long life characteristics and easy initial activation. .
Here, the degree of difficulty in pulverization is defined as the "particle size of the hydrogen storage alloy powder after 10 hydrogen storage/release cycles in an environment with a holding temperature of 45°C and hydrogen pressure adjustment of 1.82 MPa" and "initial particle size of the hydrogen storage alloy powder". ” is the value divided by . Moreover, the plateau pressure is the equilibrium hydrogen pressure (MPa) at H/M=0.5 on the hydrogen release side at a measurement temperature of 45°C. In detail, the method for measuring the degree of difficulty in pulverization in the present invention will be described below. Furthermore, in the Examples described below, the degree of difficulty in pulverizing a hydrogen storage alloy obtained according to the method described below was measured.

In other words, using a PCT (hydrogen pressure-composition-isotherm diagram) characteristic evaluation device, the hydrogen storage alloy powder was evaluated after 10 hydrogen storage and desorption cycles under an environment of a holding temperature of 45°C and a hydrogen pressure adjustment of 1.82 MPa. The value obtained by dividing the "particle size" by the "initial particle size of the hydrogen storage alloy powder" was indexed as the degree of difficulty in pulverization. That is, the closer the pulverization difficulty level is to 1, the harder it is to pulverize the hydrogen storage alloy powder, and the closer it is to 0, the easier it is to pulverize the hydrogen storage alloy powder. In determining the degree of difficulty in pulverization, the "initial particle size of the hydrogen storage alloy powder" refers to the average particle size D50 measured using a particle size distribution analyzer 7997SRA manufactured by Lees & Northrup. "Particle size of hydrogen storage alloy powder after 10 hydrogen storage and desorption cycles in an environment with a holding temperature of 45°C and hydrogen pressure adjustment of 1.82 MPa" refers to the fully automatic PCT measuring device (1/2 After 10 cycles of hydrogen storage and desorption using an inch straight tube sample cell (sample amount: 3 g) at a holding temperature of 45°C and hydrogen pressure adjustment of 1.82 MPa, particle size distribution measurements were performed using a Leeds and Northrup Co., Ltd. It refers to the average particle diameter D50 measured using the device 7997SRA. The activation treatment of the hydrogen storage alloy powder in the fully automatic PCT measuring device is performed under an environment with an activation temperature of 80°C and a hydrogen pressure of 1.82 MPa, and the hydrogen storage and release cycle of the hydrogen storage alloy powder in the device is The test was conducted under an environment of a temperature of 45° C., a hydrogen storage pressure of 1.82 MPa, and a hydrogen release pressure of 0 MPa.

The reason why the pulverization difficulty level is set to 0.55 to 0.70 is that if it is too high, initial activation will be difficult and the input/output characteristics of the battery will deteriorate.On the other hand, if it is too low, the life characteristics of the battery will be ensured. This is so that it will not happen.

The hydrogen storage alloy of the present invention may be one that satisfies the requirements of the present invention described above, and there are no particular limitations on the method for obtaining it. Below, one example that can be adopted as a method for manufacturing the hydrogen storage alloy of the present invention will be shown.

The hydrogen storage alloy can be manufactured through a weighing process, a mixing process, a casting process, and a heat treatment process. For use in batteries, the hydrogen storage alloy produced in this way is pulverized and used (used after a pulverization process). In the weighing step, each raw material of the hydrogen storage alloy is weighed so as to have a desired alloy composition. At that time, each raw material including the substituent element X is weighed so that the hydrogen storage alloy according to the present invention can be obtained. In the mixing step, a plurality of weighed raw materials are mixed. In the casting process, a mixed raw material is put into a high-frequency heating melting furnace, the mixed raw material is melted to form a molten metal, and this molten metal is poured into a mold, for example, at a temperature in the range of 1150°C to 1550°C (casting temperature = the temperature at the start of casting). The temperature of the molten metal in the crucible) is then cast.

The alloy after casting is heat treated in a heat treatment step at a temperature of 950° C. to 1250° C. in a non-oxidizing atmosphere. Further, the heat treatment time is, for example, 5 hours or more and 18 hours or less, although it depends on the size of the ingot (hydrogen storage alloy piece) after casting. The time may be set so that the temperature reaches a predetermined temperature to the center of the ingot and grains grow to a desired crystal size.
For use in batteries, the hydrogen storage alloy thus produced is pulverized. In this pulverization process, the hydrogen storage alloy powder is made into a hydrogen storage alloy powder having the required particle size by coarse pulverization and fine pulverization. For example, an ingot is ground to a size that passes through a sieve of 500 μm to obtain a hydrogen storage alloy powder.

The hydrogen storage alloy powder thus obtained can be used to prepare a battery negative electrode by a known method. That is, a hydrogen storage alloy negative electrode can be constructed by mixing and molding a binder, a conductive aid, etc. using a known method. Then, it is preferably used as a negative electrode constituting a nickel-metal hydride battery.

Hereinafter, the present invention will be explained based on examples. Note that the present invention is not limited to the examples.

[Examples 1 to 8, Comparative Examples 1 to 7]
Each metal raw material of Ni, Mn, Al, Co, La and Ce as Mm, and the substitution element X was weighed so as to have the alloy composition shown in Tables 1 and 2. In these tables, La and Ce as Mm are shown as elements at the A site, and other raw materials are shown as elements at the B site, expressed in molar ratio. These raw materials were placed in a crucible in a melting furnace and evacuated, followed by an argon gas atmosphere. Next, the molten metal was heated and melted using a high-frequency heating device, and the molten metal heated to 1540°C was held for 10 minutes, then lowered to 1250°C, and cast into a mold at a casting temperature of 1230°C. The cast alloy ingot was heat treated at 1080° C. for 12 hours in an argon atmosphere to obtain an alloy ingot. The obtained alloy ingot was coarsely crushed by a crusher under an inert atmosphere, and then crushed using a cutting mill (manufactured by Frütsch) under an inert atmosphere, and then the particle size passing through a sieve of 500 μm ( 500 μm or less) was made into a hydrogen storage alloy powder.

<Evaluation method>
Various evaluations were performed on the hydrogen storage alloy powders (samples) obtained in Examples and Comparative Examples as follows.

(Measurement of difficulty in pulverization)
The degree of difficulty in pulverization was measured by the method described above using a particle size distribution measuring device and a PCT characteristic evaluation device.

(Intra-lattice porosity)
As mentioned above, the definition of intralattice porosity is as follows: Intralattice porosity = (crystal lattice volume - total volume of constituent atoms) / crystal lattice volume x 100
The crystal lattice volume was measured using an X-ray diffraction device (SmartLab manufactured by Rigaku Co., Ltd.), and calculated from the a-axis length and c-axis length obtained by Rietveld analysis. Specifically, a hydrogen storage alloy pulverized to 20 μm or less was analyzed using a powder X-ray diffraction device (Rigaku Corporation, SmartLab) with a goniometer radius of 300 mm, an X-ray source of CuKα rays, a tube voltage of 45 kV, and a tube current of 200 mA. It was measured. At that time, the diffraction angle was in the range of 2θ=15.0 to 85.0°, the scan speed was 4.000°/min, and the scan step was 0.020°. Based on the obtained X-ray diffraction results, the crystal structure was analyzed by the Rietveld method (analysis software SmartLab Studio II, PowderXRD plug-in).

On the other hand, as mentioned above, the total volume of the constituent atoms was calculated using a spherical model from the metal bond radius of each atom constituting the crystal lattice. At that time, for the metal bond radius, the value described in "Chemical Handbook Basic Edition" edited by the Chemical Society of Japan, published in 1984 was used. Further, in the case of typical elements, one-half of the bond distance between the same kind of atoms as the representative value of the atomic spacing is used. Furthermore, when the composition of ABx representing the crystal structure becomes ABx > 5 (that is, when the AB ratio > 5), the elements at the B site are substituted into the A site in a dumbbell shape as described above, so ABx = 5 + α In this case, the total volume is the sum of the molar ratio of each element shown in Tables 1 and 2 divided by (1+1/7α) multiplied by the atomic volume of each element determined from the metal bond radius. The total volume of In addition, in the "substitution element" column in Tables 1 and 2, the substitution element and the site of the target element to be substituted by the substitution element are shown. Furthermore, the "radius relationship" represents the relationship in the metal bond radius between the element to be replaced and the replacement element.

In addition to the above examples, based on the test results conducted by the present inventors so far, it was found that the difficulty of pulverization of AB ₅ type hydrogen storage alloy of Mm-Ni-Mn-Al-Co alloy is 0.55 or more. It has been found that in order to achieve this, it is preferable to substitute some of the constituent atoms with a predetermined substituting element so that the intralattice porosity is 23.30% or more and 23.70% or less. In Comparative Examples 1 to 7, although the compositions were almost the same as those of Examples, the intralattice porosity was 23.20% or less, so it was not possible to obtain a level of difficulty in pulverization comparable to Examples 1 to 8. . From this, it was confirmed that according to the present invention, an alloy with a low Co content, low raw material cost, excellent life characteristics, and suitable for a negative electrode for nickel-hydrogen batteries can be obtained.

Claims (7)

General formula _MmNia Mn _b Al _c Co _d (wherein, Mm is misch metal, 4.30≦a≦4.75, 0.25≦b≦0.50, 0.25≦c≦0.45 , 0≦d≦0.12, 5.20≦a+b+c+d≦5.55) A hydrogen storage alloy powder having a CaCu type ₅ crystal structure represented by A part of the constituent atoms represented by the formula is substituted with another substituent element Hydrogen storage alloy powder for negative electrodes of nickel-metal hydride batteries .
Intralattice porosity = (crystal lattice volume - total volume of constituent atoms) / crystal lattice volume x 100 Among the constituent atoms represented by the general formula, one or more target elements Y2 selected from the group consisting of Ni, Mn, and Al are substituted with the substituting element X, resulting in a compositional formula MmNi _a-a1 Mn _b- The hydrogen storage alloy powder for a negative electrode of a nickel-hydrogen battery according to claim 1, which satisfies _b1 Al _c−c1 X _e Co _d (wherein, e=a1+b1+c1, 5.20≦a+b+c+d+e≦ 5.55 ). 3. The hydrogen storage alloy powder for a negative electrode of a nickel-hydrogen battery according to claim 2, wherein the substitutional element X has a larger metal bond radius than the atom to be substituted. The hydrogen storage alloy powder for a nickel-hydrogen battery negative electrode according to claim 1 or 2, wherein Mm contains a rare earth metal whose atomic radius is smaller than that of Ce. The hydrogen storage alloy powder for a negative electrode of a nickel-hydrogen battery according to claim 1 or 2, wherein the hydrogen storage alloy has a degree of difficulty in pulverization of 0.55 or more. A nickel-metal hydride battery negative electrode , characterized in that the hydrogen storage alloy powder according to claim 1 or 2 is used as a negative electrode active material for a nickel-metal hydride battery . A nickel-metal hydride battery characterized by using the nickel -metal hydride battery negative electrode according to claim 6.

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