Hydrogen storage alloy powder

Abstract

This invention provides a hydrogen storage alloy powder that reduces raw material costs by lowering the Co content, while exhibiting excellent lifespan characteristics when used in nickel-metal hydride batteries. [Solution] General formula: MmNi a Mn b Al c Co d This hydrogen storage alloy powder has a CaCu5-type crystalline structure represented by the formula (wherein Mm is mischmetal, 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), and is characterized in that the particle size passes through a sieve with a mesh size of 20 μm, and the magnetization after alkaline surface treatment by immersion in a 31 mass% KOH aqueous solution at a liquid temperature of 120°C for 4 hours is 1.50 to 2.50 emu / g.

Description

[Technical Field]

[0001] This invention relates to a hydrogen storage alloy powder having a CaCu5-type crystal structure, and more preferably to a hydrogen storage alloy powder used in the negative electrode of a nickel-metal hydride battery. [Background technology]

[0002] Nickel-metal hydride batteries, which use hydrogen storage alloys in the negative electrode, were commercialized in the early 1990s and have since become widely used.

[0003] Nickel-metal hydride batteries were initially used as power sources for mobile phones and laptops, but they were gradually replaced by smaller and lighter lithium-ion batteries. Currently, due to their low cost, high safety, and good balance of energy density per unit volume, nickel-metal hydride batteries are used in toys, small devices, and even hybrid cars.

[0004] The hydrogen storage alloys used in nickel-metal hydride batteries are alloys that react with hydrogen to form metal hydrides. These hydrogen storage alloys can reversibly absorb and release large amounts of hydrogen at or near room temperature.

[0005] Examples of hydrogen storage alloys include AB5 type alloys, such as LaNi5, and ZrV 0.4 Ni 1.5 In addition to the AB2 type alloy, which is representative of the above, various other types of alloys such as AB type, A2B type, and AB3 type are known. These alloys are generally composed of a combination of element groups that have a high affinity for hydrogen and play a role in increasing hydrogen storage capacity (rare earth elements, Ca, Mg, Ti, Zr, V, Nb, Pt, Pd, etc.) and element groups that have a relatively low affinity for hydrogen and have low storage capacity, but play a role in promoting the hydrogenation reaction and lowering the reaction temperature (Ni, Mn, Co, Al, etc.).

[0006] In particular, AB5 type hydrogen storage alloys having a CaCu5 type crystal structure, such as those using mischmetal (hereinafter referred to as "Mm"), a rare earth mixture, at the A site and elements such as Ni, Mn, Co, and Al at the B site, can form the negative electrode of a nickel-metal hydride battery using relatively inexpensive materials compared to alloys of other compositions.

[0007] In AB5 type hydrogen storage alloys, it is known that the characteristics of the negative electrode, such as charge / discharge capacity, input / output characteristics, and cycle life, can be adjusted by changing the ratio of the atomic weight of the B site to the atomic weight of the A site (AB ratio) or by changing the amount of substitutions such as Co, Mn, and Al that partially replace Ni. Therefore, AB5 type hydrogen storage alloys enable the creation of nickel-metal hydride batteries tailored to specific applications.

[0008] For example, to expand the adoption of hybrid vehicles, it is necessary to keep the manufacturing costs of nickel-metal hydride batteries low and further improve the lifespan and input / output characteristics of the negative electrode. To achieve this objective, research and development of AB5 type hydrogen storage alloys is being actively pursued. In particular, various studies are being conducted to maintain and improve the lifespan characteristics of AB5 type hydrogen storage alloys while minimizing the use of Co, an expensive rare metal.

[0009] As an example, Patent Document 1 describes a hydrogen storage alloy having a CaCu5-type crystal structure with reduced Co usage, in which the increase in specific surface area ΔCS of the hydrogen storage alloy particles before and after PCT (pressure-composition-temperature) measurement following activation treatment under predetermined conditions was 0.11 m². 2 / cc or more 0.22m 2 Items listed below / cc are included.

[0010] Furthermore, Patent Document 2 describes a hydrogen storage alloy having a CaCu5-type crystal structure with reduced Co usage, in which the ratio of hardness H(GPa) to composite elastic modulus E(GPa) measured by nanoindentation method, H / E, is 0.08 or more and 0.15 or less. According to these hydrogen storage alloys in Patent Documents 1 and 2, nickel-metal hydride batteries with a balanced lifespan and output characteristics can be obtained.

[0011] Furthermore, Patent Document 3 discloses a hydrogen storage alloy having a CaCu5-type crystal structure with reduced Co usage, wherein the ratio of the c-axis length to the a-axis length in the CaCu5-type crystal structure is 0.8092 or higher. It is claimed that such a hydrogen storage alloy can suppress the deterioration of the battery's lifespan characteristics when used as a negative electrode active material in a nickel-metal hydride battery.

[0012] Furthermore, Patent Document 4 discloses a hydrogen storage alloy in which the magnetization after a predetermined alkaline surface treatment is 1.60 emu / g or less. In this hydrogen storage alloy, the aim is to prevent a decrease in the battery's lifespan characteristics by setting the ratio of Al to Mn (Al / Mn) to 0.35 to 1.10 when the molar ratio of Mm constituting the A site is 1.00. In other words, the magnetization after the alkaline surface treatment is defined as an indicator, and if the magnetization after the alkaline surface treatment is high, it indicates that the alloy is easily affected by the surface treatment and is prone to corrosion by the alkaline electrolyte when used in a battery. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] Japanese Patent Publication No. 2023-71811 [Patent Document 2] Patent No. 7260722 [Patent Document 3] Patent No. 7014937 [Patent Document 4] Japanese Patent Publication No. 2022-60224

Summary of the Invention

Problems to be Solved by the Invention

[0014] In recent years, as the trading price of Co, a rare metal, has soared, in order to maintain or reduce the raw material cost of AB5 type hydrogen storage alloys containing Co, it is necessary to reduce the use of Co as much as possible.

[0015] However, when the Co content of AB5 type hydrogen storage alloys is reduced, pulverization of the alloy progresses due to repeated hydrogen absorption and desorption, and the life characteristics of the negative electrode tend to deteriorate. Although various studies have been made to maintain and improve the life characteristics of AB5 type hydrogen storage alloys, no effective solution has been found to achieve both reduction of Co content and improvement of the life characteristics of the negative electrode.

[0016] Therefore, as a result of intensive studies on the above problems, the inventors have found that by making a hydrogen storage alloy with reduced crystal anisotropy while using magnetization after a predetermined alkali surface treatment as an index, a nickel-hydrogen battery with excellent life characteristics can be obtained, and thus completed the present invention.

[0017] Therefore, an object of the present invention is to provide a hydrogen storage alloy powder that can suppress the raw material cost by reducing the Co content, prevent the elution of alloy components into an alkaline electrolyte when made into a battery as much as possible, and obtain a nickel-hydrogen battery with excellent life characteristics.

Means for Solving the Problems

[0018] That is, the gist of the present invention is as follows. [1] General formula: MmNi a Mn b Al c Co d(where Mm is mischmetal, 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 CaCu5-type crystal structure, After subjecting it to an alkali surface treatment such that it has a particle size that passes through a 20 μm sieve mesh and is immersed in a 31 mass% KOH aqueous solution at a liquid temperature of 120 °C for 4 hours, the magnetization is 1.50 to 2.50 emu / g. A hydrogen storage alloy powder characterized by this. A nickel-hydrogen battery negative electrode characterized by using the hydrogen storage alloy powder described in [2][1] as a negative electrode active material. A nickel-hydrogen battery characterized by using the nickel-hydrogen battery negative electrode described in [3][2].

Effect of the Invention

[0019] According to the hydrogen storage alloy powder of the present invention, a nickel-hydrogen battery having excellent life characteristics can be obtained while suppressing raw material costs by reducing the Co content.

Brief Description of the Drawings

[0020] [Figure 1] Figure 1 shows the particle size distribution of the sample for alkali surface treatment obtained in Example 1. [Figure 2] Figure 2 is an X-ray diffraction pattern of the hydrogen storage alloy powders obtained in Example 1 and Comparative Example 1.

Modes for Carrying Out the Invention

[0021] Hereinafter, the present invention will be described in detail. The hydrogen storage alloy powder in the present invention has the general formula: MmNi a Mn b Al c Co dA hydrogen storage alloy powder having a CaCu5-type crystalline structure represented by the formula (wherein Mm is mischmetal, 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), wherein the particle size is such that it passes through a sieve with a mesh size of 20 μm, and the magnetization after alkaline surface treatment by immersion in a 31 mass% KOH aqueous solution at a liquid temperature of 120°C for 4 hours is 1.50 to 2.50 emu / g.

[0022] Here, when a hydrogen storage alloy is used as the negative electrode of a nickel-metal hydride battery, the decrease in the battery's capacity is thought to proceed as follows. In other words, hydrogen storage alloys undergo pulverization through repeated hydrogen absorption and release. The newly formed surfaces created by the increased specific surface area of ​​the pulverized alloy are corroded by the alkaline electrolyte, reducing its lifespan. During this process, rare earth components, Mn, Al, and other elements besides Ni dissolve into the alkaline electrolyte. Therefore, in order to prevent a decrease in the lifespan of nickel-metal hydride batteries, it is important not only to suppress the pulverization of the hydrogen storage alloy but also to minimize the dissolution of alloy components as much as possible.

[0023] Therefore, the present invention aims to suppress intracrystalline strain and grain size anisotropy as much as possible, improve the crystalline orientation and the directional dependence of amorphous components, and thereby prevent the hydrogen storage alloy from dissolving into the alkaline electrolyte when it is pulverized, so that the magnetization after alkaline surface treatment falls within a predetermined range.

[0024] More specifically, as described above, the magnetization after alkaline surface treatment, in which the particles are made to pass through a sieve with a mesh size of 20 μm and immersed in a 31 mass% KOH aqueous solution at a liquid temperature of 120°C for 4 hours, is 1.50 to 2.50 emu / g, preferably 1.70 to 2.45 emu / g.

[0025] Of these, the alkaline surface treatment, which involves immersion in a 31% by mass KOH solution at 120°C for 4 hours, serves as a substitute reaction for the corrosion reaction caused by the electrolyte when using nickel-metal hydride batteries. This surface treatment forms a Ni-rich layer on the surface of the hydrogen storage alloy powder, increasing its magnetization. Therefore, if the magnetization is high after this surface treatment, it indicates that the powder is more susceptible to the effects of the surface treatment and more prone to corrosion by the electrolyte. If the magnetization exceeds 2.5 emu / g, the lifespan characteristics deteriorate. However, if the magnetization is less than 1.5 emu / g, the amount of Ni, which has high electrical conductivity, becomes too small, which increases the internal resistance and may reduce the output characteristics. The treatment to make the particle size passable through a sieve with a mesh size of 20 μm is as shown in the examples described later.

[0026] Furthermore, the hydrogen storage alloy powder in the present invention preferably has a full width at half maximum (FWHM) of the diffraction peak of the (001) plane in the X-ray diffraction pattern using Cu-Kα rays as the X-ray source in the range of 0.095° / 2θ to 0.105° / 2θ, and more preferably in the range of 0.095° / 2θ to 0.102° / 2θ. Also, the value obtained by dividing the FWHM of the diffraction peak of the (100) plane by the FWHM of the diffraction peak of the (001) plane preferably has a value in the range of 0.90 to 1.10, and more preferably in the range of 0.94 to 1.01.

[0027] Here, the (001) plane peak appears at approximately 21.7° in 2θ values ​​obtained by X-ray diffraction using a Cu-Kα tube. The (100) plane peak also appears at approximately 20.3° in 2θ values ​​obtained by X-ray diffraction. In this invention, the full width at half maximum (h) of the (001) plane peak in X-ray diffraction is... 001 If the full width at half maximum (FWHM) of the (001) plane peak in X-ray diffraction is less than 0.095° / 2θ, the crystallinity is too high, making it brittle and prone to pulverization due to hydrogen absorption and release, which may reduce its lifetime characteristics. Conversely, if the full width at half maximum (FWHM) of the (001) plane peak in X-ray diffraction exceeds 0.105° / 2θ, the crystallinity decreases, increasing reactivity with the alkaline electrolyte and potentially reducing its lifetime characteristics. Also, if the full width at half maximum (FWHM) of the (100) plane peak in X-ray diffraction is less than 0.095° / 2θ, 100The range is preferably 0.095° / 2θ to 0.098° / 2θ, and more preferably 0.095° / 2θ to 0.096° / 2θ.

[0028] On the other hand, the full width at half maximum (h) of the diffraction peak of the (100) plane 100 ) is the full width at half maximum (h) of the diffraction peak of the (001) plane. 001 The value obtained by dividing by (h 100 / h 001 Regarding this, a value less than 0.9 or greater than 1.1 indicates increased crystallinity anisotropy. This can lead to increased leaching from one side or the other, potentially reducing the battery's lifespan.

[0029] The hydrogen storage alloy powder in this invention has the general formula: MmNi a Mn b Al c Co d This represents a CaCu5-type crystal structure, i.e., an AB5-type hydrogen storage alloy. In this invention, Mm (mischmetal) is used as the metal (atom) constituting the A site. This Mm is preferably La, or a mixture of rare earth metals in part or all of the La. Preferably, La and Ce are present in proportions of 80% to 100% by mass relative to the total mass of Mm, more preferably 70-99% by mass of La and 1-30% by mass of Ce, and even more preferably 74-97% by mass of La and 3-26% by mass of Ce.

[0030] Furthermore, in this invention, Ni, Mn, Al, and Co are used as the metals (atoms) constituting the B site. The molar ratios of these metals in the general formula shown above satisfy 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

[0031] Furthermore, the 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

[0032] Of the above conditions, the molar ratio of Co (d) is preferably as low as possible to reduce raw material costs, and is set to 0 ≤ d ≤ 0.12. By setting d to be greater than 0 and 0.12 or less, that is, by Co substitution, pulverization becomes less likely even when hydrogen absorption and release are repeated, and this tendency becomes more pronounced as the amount of substitution increases. However, even without Co substitution, if d = 0, that is, by using the AB5 type hydrogen storage alloy of the present invention, pulverization can be suppressed even when hydrogen absorption and release are repeated. Therefore, the molar ratio of Co (d) is 0 ≤ d ≤ 0.12, and preferably 0 ≤ d ≤ 0.05. Note that if d exceeds 0.12, it will not lead to a reduction in raw material costs.

[0033] The reasons for setting the above-mentioned ratios of Mm, La, and Ce, as well as the molar ratios of Ni, Mn, and Al, are to ensure sufficient charge / discharge capacity by setting the hydrogen storage capacity (H / M) of the AB5 type hydrogen storage alloy to 0.85-1.00, to facilitate initial activation by setting the equilibrium pressure to 0.04-0.07 MPa, and to make the plateau region in the PCT curve as wide as possible.

[0034] In addition, regarding Mm, preferably, by setting La and Ce within the range of 80% to 100% by mass relative to the total mass of Mm, it becomes easier to secure a hydrogen storage capacity (H / M) of 0.85 to 1.00 and thus a greater charge / discharge capacity.

[0035] Furthermore, the molar ratio of Ni (a) is within the range of 4.30 to 4.75, as described above. Within this range, when a negative electrode is fabricated using hydrogen storage alloy powder as the active material, its output characteristics are easily maintained, and the lifetime characteristics are not significantly worsened. Moreover, by setting the molar ratio of Mn (b) within the range of 0.25 to 0.50, it is possible to suppress the pulverization of the hydrogen storage alloy powder. Furthermore, by setting the molar ratio of Al (c) within the range of 0.25 to 0.45, hysteresis in the PCT characteristics is small, the deterioration of the charge-discharge efficiency of the hydrogen storage alloy powder is suppressed, and the decrease in the amount of hydrogen absorbed by the hydrogen storage alloy powder is suppressed.

[0036] The hydrogen storage alloy powder used in this invention only needs to satisfy the requirements described above, and there are no particular restrictions on the method of obtaining it. However, a preferred manufacturing method is described below.

[0037] First, a preferred method for producing hydrogen storage alloy powder according to the present invention is a method that involves a weighing step, a mixing step, a casting step, a grinding step, and a heat treatment step. In the weighing step, each raw material of the hydrogen storage alloy is weighed to obtain a desired alloy composition. In the mixing step, the weighed multiple types of raw materials are mixed.

[0038] Next, in the casting process, the mixed raw materials are placed in a high-frequency heating melting furnace and melted to form molten metal. This molten metal is then poured, for example, into a mold and cast at a temperature in the range of 1150°C to 1550°C (casting temperature = molten metal temperature in the crucible at the start of casting). When casting in a mold, the resulting alloy solidifies from the bottom in contact with the mold towards the top of the mold, making it prone to a unidirectional solidification structure. In other words, there is a risk of increased anisotropy in the resulting crystals. Therefore, the thickness of the alloy cast in the mold should be 50 mm or less, preferably 30 mm or less. By controlling the thickness of the alloy ingot in this way, variations in the crystallinity of the final hydrogen storage alloy powder can be suppressed.

[0039] The alloy after casting is crushed in a crushing process. There are no particular restrictions on the crushing process, but generally, in the case of a Brown horizontal crusher or a planetary ball mill, force is applied evenly to the entire material to be crushed, so stress may accumulate inside during the crushing process and fine distortion may remain. However, in the case of a cutting mill, where localized force is applied and fracture occurs, distortion is less likely to occur in the material to be crushed, making it easier to handle. For this reason, it is preferable to use a cutting mill. Furthermore, it is preferable to crush the material to the required particle size by coarse crushing and fine crushing in a low oxygen concentration. For example, the alloy ingot cast in a mold is crushed by adjusting the oxygen concentration until it is small enough to pass through a 500 μm sieve.

[0040] After pulverization, the alloy is heat-treated. Preferably, the heat treatment is performed in a non-oxidizing atmosphere. Furthermore, from the viewpoint of sufficiently increasing the crystallinity of the final hydrogen storage alloy powder, it is preferable to heat-treat at a temperature of 1070°C or higher. On the other hand, if the heat treatment temperature is too high, the alloy may begin to dissolve and its crystallinity may be disrupted, so it is preferable to heat-treat at a temperature of 1150°C or lower. More preferably, the heat treatment is performed at a temperature of 1090°C to 1130°C. The heat treatment time depends on the size of the resulting alloy powder, but several hours to more than ten hours is preferable, and the time should be set so that the center of the alloy powder reaches the predetermined temperature.

[0041] By performing heat treatment after the grinding process, lattice defects and lattice strains introduced by the grinding process are reduced by the heat treatment. As a result, as described above, the full width at half maximum and its ratio in the X-ray diffraction pattern can be controlled.

[0042] Next, a second example of a preferred method for producing hydrogen storage alloy powder according to the present invention comprises a weighing step, a mixing step, a casting step, and a cooling step, after which the hydrogen storage alloy powder may be obtained in the order of a grinding step and a heat treatment step, or the hydrogen storage alloy powder may be obtained in the order of a heat treatment step and a grinding step. The weighing step and mixing step in this second example are the same as in the first example described above.

[0043] Next, in the second example, in the casting process, the mixed raw materials are put into a high-frequency heating melting furnace and melted to form molten metal, and in the cooling process, this molten metal is poured directly into a cooling roll to cool it. In the first example, the casting process employs mold casting, whereas in this second example, the mold process employs strip casting, which includes a cooling process.

[0044] Specifically, after the mixed raw materials are placed in a high-frequency heating melting furnace and melted, preferably the resulting molten metal is poured directly into a cooling roll and cast at a temperature in the range of 1150°C to 1550°C, while simultaneously being rapidly cooled by removing heat with cooling water circulated within the cooling roll, thereby obtaining a flattened alloy piece.

[0045] In this second example, strip casting allows for rapid cooling compared to the cooling method of pouring molten metal into a mold when using mold casting. In this case, a single-roll or double-roll cooling roll may be used. The cooling roll may be made of cast iron or cast steel, but preferably, the outer circumference of the roll should be made of copper to improve heat transfer, and furthermore, a cavity should be provided inside the roll for constant water cooling. In this case, the full width at half maximum and its ratio of the predetermined diffraction peaks in the X-ray diffraction pattern described above can be controlled by the rate at which the molten metal is poured into the cooling roll (molten metal amount per hour) and the rotation speed of the cooling roll (roll peripheral speed). Therefore, these conditions should be adjusted as appropriate.

[0046] Furthermore, as mentioned above, the thickness of the flattened alloy pieces obtained varies depending on the peripheral speed of the cooling rolls, etc. However, if the alloy pieces are too thick, anisotropy tends to increase, and the energy required in the grinding process to grind them to a predetermined particle size increases, which may lead to the introduction of many lattice defects and lattice distortions. For this reason, the thickness of the alloy pieces is preferably 0.5 mm or less, and more preferably 0.3 mm or less. On the other hand, making the alloy pieces too thin increases the rotation speed of the cooling rolls, creating equipment constraints and reducing productivity by slowing down the rate at which the molten metal is poured onto the cooling rolls. Therefore, the practical lower limit for the thickness of the alloy pieces can be said to be 0.1 mm.

[0047] The flattened alloy pieces obtained as described above are rapidly cooled using cooling rolls, resulting in relatively high crystallinity and low anisotropy compared to the case of mold casting. Therefore, hydrogen storage alloy powder may be obtained in the same order as in the first example, by performing the crushing step followed by the heat treatment step, or by performing the heat treatment step followed by the crushing step. The conditions for the crushing step and the heat treatment step are the same as those described in the first example.

[0048] The hydrogen storage alloy powder obtained in this way can be used to prepare a negative electrode for a battery by known methods. Specifically, the hydrogen storage alloy powder according to the present invention can be used as the negative electrode active material for a nickel-metal hydride battery, and can be mixed with a binder, a conductive additive, etc., and molded to form the negative electrode of a nickel-metal hydride battery. Furthermore, the nickel-metal hydride battery negative electrode obtained in this way can be used together with a known positive electrode to constitute a nickel-metal hydride battery. [Examples]

[0049] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to these. The hydrogen storage alloy powders obtained in the examples were measured and evaluated as follows.

[0050] [Method for measuring the magnetization of hydrogen storage alloy powder] 20 g of hydrogen storage alloy powder was ground for 3 minutes using a cyclomill (Yoshida Seisakusho, model: 1033-300), and then passed through a sieve with a mesh size of 20 μm to obtain a sample for alkaline surface treatment with a particle size (D50 = 15 μm ± 0.5 μm). D50 was measured using a particle size distribution analyzer (Microtrac, Nikkiso Co., Ltd., model: MT3300EXII) under the following conditions, and determined from the resulting volume-based particle size distribution chart. Figure 1 shows the particle size distribution of the alkaline surface treatment sample in Example 1, which will be described later. (Set Zero time):30sec (Measurement time):30sec (Number of measurements): 1 (Solvent and refractive index): Water, 1.33 (particle condition permeability):reflection (Flow rate):75%

[0051] Six g of the obtained alkaline surface treatment sample was placed in 60 ml of a 31% by mass KOH aqueous solution at 120°C, and the sample was shaken using a shaker for 4 hours to perform the alkaline surface treatment. Subsequently, the sample was washed with water, filtered, and dried to obtain a sample for magnetization measurement.

[0052] Next, the hysteresis loop of the magnetization measurement sample obtained above was measured using a VSM (Vibration Sample Magnetometer, manufactured by Toei Kogyo Co., Ltd., model: VSM-C7-10A) to determine the magnetization. Before measuring the magnetization, a calibration procedure using a standard sample was performed to calibrate the instrument and confirm that it was functioning correctly. After that, the magnetization measurement described above was carried out. The measurement conditions are as follows. (Measurement conditions for vibrating sample type magnetometer) Maximum applied magnetic field (Oe): 10000 M Range: 0.5emu Sweep speed: 1 min / FS Electromagnet angle: 0.00 Measurement mode: 1 / 2 loop Furthermore, the magnetization was determined from the hysteresis loop obtained above as follows. Magnetization (emu / g)=M(10) / Measurement alloy weight In the equation, M(10) is the magnetization at 10[kOe].

[0053] [Measurement of the full width at half maximum of diffraction peaks in X-ray diffraction patterns] For the hydrogen storage alloy powders obtained in the examples and comparative examples, the full width at half maximum (H) of the diffraction peak of the (001) plane in the X-ray diffraction pattern was determined. 001 ), and similarly, the full width at half maximum (h) of the diffraction peak of the (100) plane. 100 ), and the ratio of these half-widths (h 100 / h 001 The following was measured: A SmartLab powder X-ray diffractometer manufactured by Rigaku Corporation was used. Hydrogen storage alloy powder, adjusted to a particle size (D50 = 15 μm ± 0.5 μm), was measured using the above powder X-ray diffractometer 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. The diffraction angle was set to 2θ = 15.0 to 30.0°, the scan speed to 4.000° / min, and the scan step to 0.020°.

[0054] Based on the obtained X-ray diffraction results, the full width at half maximum (FWHM) of the (100) plane peak appearing around 2θ = 20.3° and the FWHM of the (001) plane peak appearing around 2θ = 21.7° were calculated from the powder X-ray diffraction pattern using the analysis software SmartLab StudioII and the PowderXRD plugin. The FWHM ratio of (100) / (001) was calculated by dividing the obtained FWHM of the (100) plane diffraction peak by the FWHM of the (001) plane diffraction peak.

[0055] [Manufacturing of nickel-metal hydride batteries] A battery was prepared using the hydrogen storage alloy powder obtained in the examples and comparative examples as the negative electrode of a nickel-metal hydride battery by the following procedure. First, 0.5 g of hydrogen storage alloy powder, which had been ground to a particle size of 20 μm or less (D50 = 15 μm ± 0.5 μm), 0.1 g of polytetrafluoroethylene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 0.05 g of acetylene black (manufactured by Denka Co., Ltd., HS-100), and 0.6 g of distilled water were kneaded together to form an active material paste.

[0056] Next, a foamed nickel sheet (Sumitomo Electric Industries' Cellmet #7, porosity = 96%) measuring 30 mm in length, 40 mm in width, and 1.6 mm in thickness was used as the current collector. The active material paste obtained above was uniformly filled into this sheet, and after being held in a 100°C constant temperature bath for 30 minutes for drying, it was pressure-molded at 100 MPa to form the negative electrode (capacity = approximately 180 mAh). Nickel hydroxide (capacity = approximately 500 mAh) was used as the positive electrode. A separator made of nonwoven fabric was placed between these two electrodes, and then PVC plates were used to compress both sides to form a battery with a limited negative electrode capacity.

[0057] [Charge-discharge cycle test] Using the nickel-metal hydride batteries prepared as described above, charge-discharge cycle tests were conducted using the methods shown in (1) and (2) below. (1) As explained above in [Fabrication of Nickel-Metal Hydride Battery], a nickel-metal hydride battery cell was assembled using a negative electrode made from hydrogen storage alloy powder adjusted to a particle size (D50 = 15 μm ± 0.5 μm), nickel hydroxide for the positive electrode, a 31% potassium hydroxide aqueous solution as the electrolyte, and a nonwoven fabric as the separator. (2) The nickel-metal hydride battery cell was charged at 45°C to 0.6C-100% and discharged at 0.6C-0.7V cut-off for 100 cycles, and the capacity retention rate after 100 cycles was calculated using the following formula. Capacity retention rate (%) = (Battery capacity after 100 cycles / Initial battery capacity) × 100

[0058] (Examples 1-5) Metal raw materials, La and Ce as the elements of site A, and Ni, Mn, Al, and Co as the elements of site B, were weighed to achieve the alloy composition (molar ratio) shown in Table 1. These metal raw materials were then mixed and placed in a crucible in a high-frequency heating melting furnace, which was then evacuated and subjected to an argon gas atmosphere. The mixed raw materials were then heated and melted in a high-frequency heating device, poured onto a cooling roll for strip casting, and rapidly cooled simultaneously with casting to obtain a flattened alloy piece with a thickness of 0.2 mm.

[0059] The obtained alloy pieces were heat-treated at 1100°C for 12 hours under an inert atmosphere, then coarsely crushed in a crusher under a low-oxygen atmosphere, and subsequently finely ground using a cutting mill (Retsch, model: SM300) to a particle size (500 μm or less) that could pass through a 500 μm sieve, to obtain the hydrogen storage alloy powders of Examples 1 to 5.

[0060] [Table 1]

[0061] (Examples 6-8) Metal raw materials, La and Ce as elements for site A, and Ni, Mn, Al, and Co as elements for site B, were weighed to achieve the alloy composition (molar ratio) shown in Table 1. These metal raw materials were then mixed and placed in a crucible in a high-frequency heating melting furnace. After vacuum evacuation, the atmosphere was changed to an argon gas atmosphere. The mixed raw materials were then heated and melted in a high-frequency heating device, poured into a mold, and cast to obtain an alloy ingot with a thickness of 30 mm.

[0062] The obtained alloy ingot was coarsely crushed in a low-oxygen atmosphere using a crusher, and then finely ground to a particle size (500 μm or less) that could pass through a 500 μm sieve using a cutting mill (Retsch, model: SM300). Next, it was heat-treated at 1100°C for 12 hours in an inert atmosphere to obtain the hydrogen storage alloy powders according to Examples 6 to 8.

[0063] (Comparative Examples 1-4) Metal raw materials, La and Ce as the elements of site A, and Ni, Mn, Al, and Co as the elements of site B, were weighed to achieve the alloy composition (molar ratio) shown in Table 1. These metal raw materials were then mixed and placed in a crucible in a high-frequency heating melting furnace, which was then evacuated and subjected to an argon gas atmosphere. The mixed raw materials were then heated and melted in a high-frequency heating device, poured into a mold, and cast to obtain alloy ingots of a predetermined thickness (comparative examples 1-3 were 30 mm thick, and comparative example 4 was 60 mm thick).

[0064] Next, the alloy ingots obtained in Comparative Examples 1 to 3 were heat-treated at 1100°C for 12 hours under an inert atmosphere, then coarsely crushed in a crusher under a low-oxygen atmosphere, and subsequently finely ground to a particle size (500 μm or less) that could pass through a 500 μm sieve using a cutting mill (Retsch, model: SM300) to obtain hydrogen storage alloy powder.

[0065] On the other hand, the alloy ingot obtained in Comparative Example 4 was coarsely crushed using a crusher in a low-oxygen atmosphere, then finely crushed using a cutting mill (Retsch, model: SM300) to a particle size that could pass through a 500 μm sieve (500 μm or less), and finally heat-treated at 1100°C for 12 hours in an inert atmosphere to obtain hydrogen storage alloy powder.

[0066] (Comparative Examples 5-7) Metal raw materials, La and Ce as the elements of site A, and Ni, Mn, Al, and Co as the elements of site B, were weighed to achieve the alloy composition (molar ratio) shown in Table 1. These metal raw materials were then mixed and placed in a crucible in a high-frequency heating melting furnace, which was then evacuated and subjected to an argon gas atmosphere. The mixed raw materials were then heated and melted in a high-frequency heating device, poured onto a cooling roll for strip casting, and rapidly cooled simultaneously with casting to obtain a flattened alloy piece with a thickness of 0.2 mm.

[0067] Of the obtained alloy pieces, Comparative Examples 5 and 7 were coarsely crushed using a crusher in a low-oxygen atmosphere, then finely ground to a particle size (500 μm or less) that could pass through a 500 μm sieve using a planetary ball mill (Nagao Systems, model: Planet M2-3F), and finally heat-treated at 1100°C for 12 hours in an inert atmosphere to obtain hydrogen storage alloy powder.

[0068] On the other hand, the alloy piece of Comparative Example 6 was coarsely crushed in a low-oxygen atmosphere using a crusher, then finely crushed using a cutting mill (Retsch, model: SM300) to a particle size that could pass through a 500 μm sieve (500 μm or less), and then heat-treated at 1050°C for 12 hours in an inert atmosphere to obtain hydrogen storage alloy powder.

[0069] Each hydrogen storage alloy powder obtained in Examples 1-8 and Comparative Examples 1-7 was pulverized using a cyclomill to adjust the particle size to pass through a sieve with a mesh size of 20 μm, as described above, and then subjected to a predetermined alkaline surface treatment to measure its magnetization. The results are shown in Table 2.

[0070] Furthermore, X-ray diffraction was performed on each hydrogen storage alloy powder obtained in Examples 1 to 8 and Comparative Examples 1 to 7 using the method described above, and the full width at half maximum (FWHM) of a predetermined diffraction peak was determined. At that time, the hydrogen storage alloy powder was pulverized for 3 minutes using a cyclomill (Yoshida Seisakusho Co., Ltd., model: 1033-300), sieved through a 20 μm sieve, and adjusted to a particle size (D50 = 15 μm ± 0.5 μm). The full width at half maximum (h) of the diffraction peak of the (001) plane in the X-ray diffraction pattern was determined. 001 ), (100) width at half maximum of diffraction peaks (h 100 ), and the ratio of these half-widths (h 100 / h 001 The results are shown in Table 2. Figure 2 also shows the X-ray diffraction patterns of the hydrogen storage alloy powders obtained in Example 1 and Comparative Example 1.

[0071] Furthermore, the hydrogen storage alloy powder obtained above was used to fabricate a negative electrode using the method described earlier, and a nickel-metal hydride battery cell was obtained, followed by a charge-discharge cycle test. The capacity retention rate (%), calculated from the ratio of the battery capacity after 100 cycles to the initial battery capacity, is shown in Table 2.

[0072] [Table 2]

[0073] As can be seen from Table 2, in Comparative Examples 1 to 7, where the magnetization after alkaline surface treatment fell outside the scope of the present invention, the battery capacity retention rate was at most 83%. In contrast, Examples 1 to 8 all showed higher battery capacity retention rates compared to the comparative examples.

[0074] Therefore, the hydrogen storage alloy powder of the present invention makes it possible to obtain a nickel-metal hydride battery with excellent lifespan characteristics while suppressing raw material costs by reducing the Co content.

Claims

1. General formula: MmNi ａ Mn ｂ Al ｃ Co ｄ CaCu is represented by (wherein Mm is mischmetal, and 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) ５ A hydrogen storage alloy powder having a type crystal structure, A hydrogen storage alloy powder characterized by having a particle size that can pass through a sieve with a mesh size of 20 μm, and a magnetization of 1.50 to 2.50 emu / g after alkaline surface treatment by immersion in a 31 mass% KOH aqueous solution at a liquid temperature of 120°C for 4 hours.

2. A nickel-metal hydride battery anode, characterized in that the hydrogen storage alloy powder described in claim 1 is used as the anode active material for the nickel-metal hydride battery.

3. A nickel-metal hydride battery characterized by using the nickel-metal hydride battery negative electrode described in claim 2.

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