Negative electrode active material for nickel-metal hydride battery and method for manufacturing the same

Abstract

To provide a negative electrode active material for a nickel-metal hydride battery and a method for manufacturing the same, which has excellent initial activation and can increase discharge capacity even with fewer charge-discharge cycles.SOLUTION: A negative electrode active material for a nickel-metal hydride battery includes a hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni, and further contains one or more elements selected from the group consisting of La and Ce, and the total content of the one or more elements selected from the group consisting of La and Ce is 8 atomic % or less relative to the entire hydrogen storage alloy, and there is also provided a method for manufacturing the same.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode active material for a nickel-metal hydride battery and a method for producing the same. [Background technology]

[0002] Hydrogen storage alloys are used as negative electrode active materials in nickel-metal hydride batteries. Hydrogen storage alloys are disclosed, for example, in Patent Document 1. The hydrogen storage alloy disclosed in Patent Document 1 contains predetermined amounts of Zr, Ti, Nb, V, Ni, Mn, Cr, Co, Fe, Si, Mo, and B, and has an intermetallic compound as an alloy phase, which is a cubic C15 or C14 type intermetallic compound belonging to the Laves phase. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-36930 Summary of the Invention [Problem to be solved by the invention]

[0004] The present inventors have found that when the hydrogen storage alloy disclosed in Patent Document 1 is used as a negative electrode active material, there is a problem that initial activation is difficult, and in order to increase the discharge capacity, the number of break-in charge and discharge cycles must be increased.

[0005] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a negative electrode active material for a nickel-metal hydride battery that has excellent initial activation and is capable of increasing discharge capacity even with a small number of charge-discharge cycles, and a method for producing the same. [Means for solving the problem]

[0006] The active material for a nickel-metal hydride battery and the method for producing the same according to the present disclosure may take the following forms. <Aspect 1> A hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni, and further containing one or more elements selected from the group consisting of La and Ce, and the total content of one or more elements selected from the group consisting of La and Ce is 8 atomic % or less relative to the entire hydrogen storage alloy; Negative electrode active material for nickel-metal hydride batteries. <Aspect 2> The hydrogen storage alloy has a main phase, a grain boundary phase, and a third phase, The main phase is an AB2 type alloy phase, and the grain boundary phase is an AB type alloy phase, In the AB2 type alloy phase, A is one or more elements selected from the group consisting of Ti and Zr, and B is one or more elements selected from the group consisting of Cr, Mn, and Ni; In the AB type alloy phase, A is one or more elements selected from the group consisting of Ti and Zr, B is Ni, and 2. The negative electrode active material for a nickel-metal hydride battery according to aspect 1, wherein the third phase is an alloy phase containing Ni and one or more elements selected from the group consisting of La and Ce. Aspect 3: The negative electrode active material for a nickel-metal hydride battery according to Aspect 1 or 2, wherein the Ni content in the main phase is 14 atomic % or more and 21 atomic % or less with respect to the entire main phase. <Aspect 4> Melting raw materials containing Ti, Zr, Cr, Mn, and Ni, and further containing one or more elements selected from the group consisting of La and Ce, to obtain a molten metal; and cooling the molten metal to solidify the molten metal and obtain an ingot of a hydrogen storage alloy; Including, The total content of one or more elements selected from the group consisting of La and Ce is 8 atomic % or less with respect to the entire molten metal. A method for producing a negative electrode active material for a nickel-metal hydride battery. Aspect 5: The method for producing a negative electrode active material for a nickel-metal hydride battery according to Aspect 4, further comprising exposing the ingot to an alkaline solution. [Effects of the Invention]

[0007] According to the present disclosure, by further adding a small amount of one or more elements selected from the group consisting of La and Ce to a hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni, it is possible to provide a negative electrode active material for a nickel-metal hydride battery that is excellent in initial activation and can increase discharge capacity even with a small number of running-in charge-discharge cycles, and a method for producing the same. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a graph showing the relationship between the number of break-in cycles and the discharge capacity. [Figure 2] FIG. 2 is an explanatory diagram showing the results of EDX area analysis of the sample of Example 1. [Figure 3] FIG. 3 is an explanatory diagram showing the results of EDX area analysis of the sample of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0009] The embodiments described below do not limit the negative electrode active material for a nickel-metal hydride battery of the present disclosure (hereinafter, sometimes simply referred to as the "negative electrode active material of the present disclosure") and its manufacturing method.

[0010] Without being bound by theory, the inventors will now explain their findings regarding why the negative electrode active material of the present disclosure has excellent initial activation and can increase the discharge capacity even with a small number of charge-discharge cycles.

[0011] A hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni is a two-phase alloy with an AB2 type alloy phase that is primarily responsible for hydrogen storage and an AB type alloy phase that is primarily responsible for charge-discharge activity. Both the AB2 type alloy phase and the AB type alloy phase contain Ni. Ni in the AB type alloy phase contributes to charge-discharge activity, but Ni in the AB2 type alloy phase contributes very little to charge-discharge activity.

[0012] When a small amount of one or more elements selected from the group consisting of La and Ce is added to the above-mentioned two-phase alloy, the added element and an element containing Ni form an alloy phase, which constitutes a third phase coexisting with the AB2 alloy phase and the AB alloy phase. The formation of the third phase reduces the Ni content in the AB2 alloy phase, and the third phase contributes to charge / discharge activity together with the AB alloy phase. The inventors have discovered that this results in excellent initial activation and enables increased discharge capacity even with a small number of break-in charge / discharge cycles.

[0013] The constituent elements of the negative electrode active material and the method for producing the same according to the present disclosure based on the above findings will now be described.

[0014] 《Negative electrode active material》 The negative electrode active material of the present disclosure includes a hydrogen storage alloy containing Ti (titanium), Zr (zirconium), Cr (chromium), Mn (manganese), and Ni (nickel), and further containing one or more elements selected from the group consisting of La (lanthanum) and Ce (cerium). The content ratios of these elements are not particularly limited as long as they are valid as a hydrogen storage alloy, but are preferably as follows. Note that the fact that the total upper limit of the content ratios of each element exceeds 100 atomic % means that the content ratios of all elements are not all upper limits.

[0015] <Ti and Zr> The Ti and Zr contents are preferably 10 atomic % or more, 12 atomic % or more, 14 atomic % or more, or 16 atomic % or more, and are preferably 50 atomic % or less, 40 atomic % or less, 30 atomic % or less, 25 atomic % or less, 24 atomic % or less, 23 atomic % or less, 22 atomic % or less, 21 atomic % or less, 20 atomic % or less, or 19 atomic % or less, based on the entire hydrogen storage alloy.

[0016] <Cr> The Cr content is preferably 10 atomic % or more, 12 atomic % or more, 14 atomic % or more, 16 atomic % or more, 18 atomic % or more, 20 atomic % or more, 22 atomic % or more, 24 atomic % or more, or 26 atomic % or more, and is preferably 50 atomic % or less, 40 atomic % or less, 35 atomic % or less, 30 atomic % or less, 29 atomic % or less, or 28 atomic % or less, based on the entire hydrogen storage alloy.

[0017] The Mn content is preferably 2 atomic % or more, 4 atomic % or more, 6 atomic % or more, or 8 atomic % or more, and is preferably 50 atomic % or less, 40 atomic % or less, 30 atomic % or less, 25 atomic % or less, 20 atomic % or less, 18 atomic % or less, 16 atomic % or less, 14 atomic % or less, 12 atomic % or less, or 10 atomic % or less, based on the entire hydrogen storage alloy. To improve the corrosion resistance of the hydrogen storage alloy, the Mn content is particularly preferably 14 atomic % or less, 12 atomic % or less, or 10 atomic % or less, based on the entire hydrogen storage alloy. Even if the Mn content is reduced in this way, the hydrogen storage alloy used in the negative electrode active material of the present disclosure is relatively easy to dissolve in alkali, contributing to improved charge / discharge activity. This is due to the presence of a third phase in the hydrogen storage alloy.

[0018] 〈Ni〉 The Ni content is preferably 15 atomic % or more, 20 atomic % or more, 22 atomic % or more, 24 atomic % or more, or 25 atomic % or more, and is preferably 60 atomic % or less, 55 atomic % or less, 50 atomic % or less, 45 atomic % or less, 40 atomic % or less, 38 atomic % or less, 36 atomic % or less, 34 atomic % or less, 32 atomic % or less, or 30 atomic % or less, based on the entire hydrogen storage alloy.

[0019] <La and Ce> The one or more elements selected from the group consisting of La and Ce may be contained in a small amount. The total content of the one or more elements selected from the group consisting of La and Ce is 8 atomic % or less, 7 atomic % or less, 6 atomic % or less, 5 atomic % or less, 4 atomic % or less, 3 atomic % or less, 2 atomic % or less, or 1 atomic % or less, based on the total hydrogen storage alloy. If the total content of the one or more elements selected from the group consisting of La and Ce exceeds 8 atomic %, the battery capacity will decrease.

[0020] When alkaline dissolution treatment is performed, a portion of one or more elements selected from the group consisting of La and Ce is released from the hydrogen storage alloy by the alkaline dissolution treatment. However, it is practically difficult to release all of the one or more elements selected from the group consisting of La and Ce from the hydrogen storage alloy by the alkaline dissolution treatment. Therefore, the total content of one or more elements selected from the group consisting of La and Ce may be 0.1 atomic % or more, 0.2 atomic % or more, 0.3 atomic % or more, 0.4 atomic % or more, 0.5 atomic % or more, 0.6 atomic % or more, 0.7 atomic % or more, 0.8 atomic % or more, or 0.9 atomic % or less.

[0021] When only La or Ce is contained among the group consisting of La and Ce, the content ratio of either of them may be the above-mentioned total content ratio. Note that B (boron) may be added to the group consisting of La and Ce, and the explanation so far may be changed to "one or more elements selected from the group consisting of La and Ce" as "one or more elements selected from the group consisting of La, Ce, and B." Furthermore, when only La, Ce, or B is contained among the group consisting of La, Ce, and B, the content ratio of either of them may be the above-mentioned total content ratio.

[0022] <Other elements> A portion of the Ti, Zr, Cr, Mn, and Ni in the hydrogen storage alloy may be substituted with one or more elements selected from the group consisting of Fe (iron), Co (cobalt), V (vanadium), Nb (niobium), Mo (molybdenum), B (boron), Mg (magnesium), and Al (aluminum). The total content of the substitution elements, such as Fe, relative to the entire hydrogen storage alloy is preferably 50 atomic % or less, 40 atomic % or less, 30 atomic % or less, 20 atomic % or less, 10 atomic % or less, 5 atomic % or less, or 0 atomic %. This does not adversely affect the practical application of the negative electrode active material and its manufacturing method of the present disclosure. A total content of 0 atomic % means that the above-mentioned substitution is not performed. Furthermore, when only one type of substitution element, such as Fe, is used, the content of that element may be the total content described above.

[0023] That is, the hydrogen storage alloy contains Ti, Zr, Cr, Mn, and Ni, and further essentially contains one or more elements selected from the group consisting of La and Ce, and optionally, a portion of the Ti, Zr, Cr, Mn, and Ni may be substituted with one or more elements selected from the group consisting of Fe, Co, V, Nb, Mo, B, Mg, and Al, with the remainder being unavoidable impurities.Unless otherwise specified, in this specification, unavoidable impurities refer to substances such as impurities contained in raw materials or impurities mixed in during the manufacturing process, whose inclusion cannot be avoided, or whose avoidance would result in a significant increase in manufacturing costs.

[0024] The negative electrode active material of the present disclosure may also contain substances other than the hydrogen storage alloy and unavoidable impurities to the extent that their function is not impaired. Examples of substances other than the hydrogen storage alloy include rust inhibitors. The total content of the substances other than the hydrogen storage alloy and the unavoidable impurities may be 5% by mass or less, 3% by mass or less, or 1% by mass or less, based on the total amount of the negative electrode active material. Taking the unavoidable impurities into consideration, the total content of the substances other than the hydrogen storage alloy and the unavoidable impurities may be 0.1% by mass or more, 0.3% by mass or more, or 0.5% by mass or more.

[0025] <Organization> The hydrogen storage alloy used in the negative electrode active material of the present disclosure comprises an AB2 type alloy phase as a main phase, an AB type alloy phase as a grain boundary phase, and a third phase. Note that both the "AB2 type alloy phase" and the "AB type alloy phase" conform to the notation used for alloy phases in conventional hydrogen storage alloys. However, in cases where there is a risk of confusion with element B (boron), the "AB2 type alloy phase" and the "AB type alloy phase" may be written as, for example, the "ab2 type alloy phase" and the "ab type alloy phase," respectively. The main phase, the grain boundary phase, and the third phase will be described below.

[0026] <Main phase> The main phase is an AB2 type alloy phase. "AB2 type alloy phase" means an alloy phase of 1 mole of "A" element and 2 moles of "B" element. A is one or more elements selected from the group consisting of Ti (titanium) and Zr (zirconium), and B is one or more elements selected from the group consisting of Cr (chromium), Mn (manganese), and Ni (nickel). AB2 type alloy phases containing these elements typically have a Laves structure. AB2 type alloy phases with such a structure primarily have a hydrogen storage function.

[0027] Typically, the main phase may contain 14 atomic % or more and 21 atomic % or less of Ni as the "B" element, based on the entire main phase. In conventional hydrogen storage alloys without a third phase, the main phase typically contains 21.3 atomic % or more of Ni, based on the entire main phase. As described above, the main phase is an AB2 type alloy phase having a Laves structure, and Ni present in such a structure hardly exhibits charge / discharge activity.

[0028] In the hydrogen storage alloy used in the negative electrode active material of the present disclosure, the main phase, grain boundary phase, and third phase coexist, and the Ni content of the main phase can be reduced to 14 atomic % or more and 21 atomic % or less, based on the entire main phase. From this perspective, the main phase may contain 14 atomic % or more, 15 atomic % or more, or 16 atomic % or more, and 21 atomic % or less, 20 atomic % or less, 19 atomic % or less, 18 atomic % or less, or 17 atomic % or less of Ni, based on the entire main phase.

[0029] Since the main phase exists as the main phase in the hydrogen storage alloy, the volume fraction of the main phase may be, for example, 50% or more, 60% or more, 70% or more, or 75% or more, and may be 95% or less, 90% or less, 85% or less, or 80% or less, relative to the entire hydrogen storage alloy.

[0030] A portion of the constituent elements of the AB2 type alloy phase may be substituted with one or more elements selected from the group consisting of Fe, Co, V, Nb, Mo, B, Mg, and Al, as with the AB type alloy phase and the third phase described below.

[0031] <Grain boundary phase> The grain boundary phase is an AB-type alloy phase. "AB-type alloy phase" means an alloy phase of 1 mole of "A" element and 1 mole of "B" element. A is one or more elements selected from the group consisting of Ti (titanium) and Zr (zirconium), and B is Ni (nickel). AB-type alloy phases containing these elements typically have a CsCl-type crystal structure. AB-type alloy phases with such a crystal structure have charge / discharge activity.

[0032] The grain boundary phase exists between adjacent main phases among the plurality of main phases. The third phase, which will be described later, also exists between adjacent main phases among the plurality of main phases. In the hydrogen storage alloy, the grain boundary phase and the third phase are the remainder of the main phase. Therefore, the total volume ratio of the grain boundary phase and the third phase may be, for example, 5% or more, 10% or more, 15% or more, or 20% or more, and may be 50% or less, 40% or less, 30% or less, or 25% or less, relative to the entire hydrogen storage alloy.

[0033] The hydrogen storage alloy may contain phases other than the main phase, grain boundary phase, and third phase. Phases other than the main phase, grain boundary phase, and third phase typically contain unavoidable impurities, and their volume fraction may be 5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0% relative to the entire hydrogen storage alloy. A volume fraction of 0% means that no phases other than the main phase, grain boundary phase, and third phase are present. When it is practically difficult to completely eliminate phases other than the main phase, grain boundary phase, and third phase, the volume fraction of phases other than the main phase, grain boundary phase, and third phase may be 0.1% or more, 0.3% or more, 0.5% or more, 0.7% or more, or 0.9% or more.

[0034] <Third phase> The third phase is an alloy phase containing Ni and one or more elements selected from the group consisting of La and Ce. The form of this alloy phase is typically, but not limited to, an intermetallic compound. For example, part or all of this alloy phase may be in the form of a solid solution of one element in the other.

[0035] When alkaline solution treatment is not performed, the Ni content may be 20 atomic % or more, 30 atomic % or more, or 40 atomic % or more, and 70 atomic % or less, 60 atomic % or less, or 50 atomic % or less, based on the entire third phase. When alkaline solution treatment is not performed, the total content of one or more elements selected from the group consisting of La and Ce may be 20 atomic % or more, 30 atomic % or more, or 40 atomic % or more, and 70 atomic % or less, 60 atomic % or less, or 50 atomic % or less, based on the entire third phase. Furthermore, when alkaline solution treatment is not performed, optionally, the total content of Ti and Zr may be 1 atomic % or more, 2 atomic % or more, or 3 atomic % or more, and 6 atomic % or less, 5 atomic % or less, or 4 atomic % or less. The remainder is unavoidable impurities. The unavoidable impurities include Cr and Mn.

[0036] When the alkaline solution treatment is performed, most of the elements other than Ni in the third phase are removed by the alkaline solution treatment, and a Ni-enriched phase is formed with one or more elements selected from the group consisting of La and Ce as nuclei. When the alkaline solution treatment is performed, the total content of one or more elements selected from the group consisting of La and Ce in the third phase may be, for example, 0.1 atomic % or more, 0.3 atomic % or more, 0.5 atomic % or more, 0.7 atomic % or more, or 0.9 atomic % or more, or 3 atomic % or less, 2 atomic % or less, or 1 atomic % or less, relative to the entire third phase. The remainder is Ni and unavoidable impurities.

[0037] The composition of the third phase differs significantly depending on whether or not the alkaline solution treatment is performed, but the composition of the main phase remains almost unchanged depending on whether or not the alkaline solution treatment is performed. Therefore, the hydrogen storage alloy used in the negative electrode active material of the present disclosure can be specified by the Ni content in the main phase as described above.

[0038] The total volume fraction of the grain boundary phase and the third phase is as described above, and the volume fraction of the third phase relative to the total volume fraction of the grain boundary phase and the third phase may be 5% or more, 10% or more, 15% or more, 20% or more, 25% or more, or 30% or more, and may be 50% or less, 45% or less, or 40% or less. When the volume fraction of the third phase relative to the total volume fraction of the grain boundary phase and the third phase is 5% or more, it can be practically recognized that the initial activation is excellent and the discharge capacity is increased even with a small number of break-in charge / discharge cycles. When the volume fraction of the third phase relative to the total volume fraction of the grain boundary phase and the third phase is 50% or less, the coexistence of the main phase, grain boundary phase, and third phase can be advantageously realized.

[0039] <Organization identification method> The hydrogen storage alloy is cut and polished, and an SEM image (backscattered electron image) of the polished surface is obtained. The SEM image is then subjected to EDX area analysis. If the hydrogen storage alloy is in powder form after pulverization or the like, the powder may be embedded in resin and polished to obtain an SEM image.

[0040] Since the main phase, grain boundary phase, and third phase regions each have a different elemental composition, the main phase, grain boundary phase, and third phase regions can be distinguished and recognized from the EDX area analysis results. Then, by measuring the area ratio of each region using an image processing device, the volume fractions of the main phase, grain boundary phase, and third phase can be determined.

[0041] <<Method for producing negative electrode active material>> The method for producing a negative electrode active material according to the present disclosure (hereinafter, sometimes simply referred to as the "production method according to the present disclosure") includes preparing a molten metal and cooling the molten metal, and optionally includes an alkali dissolution treatment.

[0042] <Molten metal preparation> A molten metal is obtained by melting raw materials containing Ti, Zr, Cr, Mn, and Ni, and further containing one or more elements selected from the group consisting of La and Ce.

[0043] The range of the component composition of the molten metal is basically the same as the range of the component composition of the hydrogen storage alloy explained in "<Negative Electrode Active Material>". However, if the component composition changes during the manufacturing process due to depletion of specific elements, etc., the change in component composition may be taken into consideration in advance when melting the raw materials so that the component composition range of the molten metal and the component composition range of the hydrogen storage alloy are the same. In the explanation of "<Method for Manufacturing a Negative Electrode Active Material>", "the entire hydrogen storage alloy" should be read as "the entire molten metal".

[0044] The melting method is not particularly limited, but for example, raw materials placed in a crucible are arc-melted to obtain a molten metal. To prevent oxidation of the raw materials and the molten metal, it is preferable to melt the raw materials in an inert gas atmosphere, particularly an argon gas atmosphere. For the inert gas atmosphere, a Group 18 element such as argon is recommended.

[0045] <Cooling of molten metal> The molten metal is cooled and solidified to obtain an ingot of the hydrogen storage alloy. The molten metal cooling method is not particularly limited, but examples include arc-melting raw materials placed in a crucible and then cooling the molten metal in the crucible. The cooling rate in such a method is generally 0.1°C / sec or more and less than 100°C / sec.

[0046] To prevent oxidation of the raw materials and the molten metal, it is preferable to cool the molten metal in an inert gas atmosphere, particularly an argon gas atmosphere. Examples of the inert gas atmosphere include a nitrogen gas atmosphere. Furthermore, to homogenize the composition of the ingot, the above-described melting and molten metal cooling may be repeated multiple times.

[0047] <Alkaline dissolution treatment> The ingot may be exposed to an alkaline solution to remove elements other than Ni from the third phase, thereby obtaining a Ni-enriched phase, which further enhances initial activation and further increases discharge capacity even with fewer charge-discharge cycles.

[0048] The type of alkaline solution is not particularly limited, but typically, an aqueous solution of NaOH (sodium hydroxide) is used. The concentration of the alkaline solution may be determined appropriately. In the case of an aqueous NaOH solution, the concentration may be, for example, 10% by mass or more, 20% by mass or more, or 30% by mass or more, and 50% by mass or less, 45% by mass or less, or 40% by mass or less.

[0049] The treatment temperature and treatment time may be appropriately determined depending on the type and concentration of the alkaline solution, the treatment amount, etc. The treatment temperature may be, for example, 70° C. or higher, 80° C. or higher, or 90° C. or higher, and may be 130° C. or lower, 120° C. or lower, or 110° C. or lower. The treatment time may be, for example, 1 hour or higher, 2 hours or higher, or 3 hours or higher, and may be 7 hours or lower, 6 hours or lower, or 5 hours or lower. [Example]

[0050] <Sample Preparation> Raw materials having the composition shown in Table 1 were placed in a crucible and arc-melted to obtain a molten metal. The molten metal was then cooled in the crucible and solidified to obtain an ingot. The melting and cooling process was repeated three times in the crucible to homogenize the composition of the ingot. The purity of the Zr raw material was 98% by mass, and the purity of the raw materials for the other elements was 99.9% by mass.

[0051] [Table 1]

[0052] The ingot was pulverized using a manual stamp mill DA-30 manufactured by Tokyo Glass Instruments Co., Ltd., and the resulting powder was classified into particles of 38 to 100 μm using sieves of 38 μm and 100 μm.

[0053] The classified powder was placed in an aqueous NaOH solution and subjected to an alkali dissolution treatment. The concentration of the aqueous NaOH solution was 35% by mass. The amount of the aqueous NaOH solution was 80 ml. The amount of powder was 2.0 g. The treatment temperature was 100°C. The treatment time was 4 hours.

[0054] The powder after the alkaline dissolution treatment was washed with water, filtered, and dried in vacuum.

[0055] "evaluation" Each sample was evaluated as follows. Unless otherwise specified, each evaluation was carried out after the alkaline dissolution treatment.

[0056] <Component composition> Before the alkaline dissolution treatment, each sample was quantitatively analyzed by ICP. The results are shown in Table 2. It was confirmed that the variation in the composition of each sample between the composition of the raw material and the composition of the sample was within a range that would not cause any practical problems.

[0057] [Table 2]

[0058] <Discharge characteristics> An evaluation cell was prepared as follows.

[0059] First, a negative electrode was prepared. More specifically, a paste-like composition was prepared by kneading 49 parts by mass of the above-mentioned hydrogen storage alloy as the negative electrode active material, 49 parts by mass of Ni powder as a conductive additive, and 2 parts by mass of carboxymethyl cellulose (CMC) as a binder. This paste-like composition was filled into a negative electrode current collector, subsequently dried in a vacuum at 80°C, and then roll-pressed (clearance 300 μm) at approximately 8 kN to obtain a negative electrode. The negative electrode current collector used was porous nickel (Celmet #7, manufactured by Sumitomo Electrochemical Co., Ltd., thickness 1.6 mm) with a Ni tab welded thereto. The capacity was adjusted to a target of approximately 240 mAh.

[0060] Next, a positive electrode was prepared. More specifically, a paste-like composition was prepared by kneading 88 parts by mass of nickel hydroxide (Ni(OH)2) as a positive electrode active material, 10 parts by mass of cobalt oxide (CoO) as a conductive additive, and 1 part by mass each of two binders (carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA)). This paste-like composition was filled into the porous nickel, followed by vacuum drying at 80°C and then roll pressing at approximately 8 kN to obtain a positive electrode. The capacity ratio of the negative electrode to the positive electrode was adjusted to negative electrode:positive electrode = 1:4.5.

[0061] Next, an electrolyte solution was prepared. Pure water was added to KOH to adjust the KOH concentration to 6 mol / L, obtaining 90 ml of electrolyte solution. The electrolyte solution and a separator (PE / PP nonwoven fabric, 150 μm thick) were then placed in a container, followed by a negative electrode (working electrode), a positive electrode (counter electrode), and a Hg / HgO electrode (reference electrode), to obtain an evaluation cell.

[0062] A charge-discharge test (25°C) was carried out using the obtained evaluation cell. First, a run-in charge-discharge test was carried out until the negative electrode capacity was saturated. More specifically, the following charge-discharge cycles (1) to (4) were carried out. (1) The battery was charged at 0.1 C for 14 hours and then discharged at 0.1 C until the negative electrode potential reached −0.6 V. (2) The battery was charged at 0.5 C for 2.2 hours and then discharged at 0.5 C until the negative electrode potential reached −0.6 V. (3) The battery was then charged at 0.1 C for 14 hours and then discharged at 0.1 C until the negative electrode potential reached −0.6 V. (4) The above charge / discharge cycles (1) to (3) were repeated until the 0.1 C discharge capacity was saturated, and the obtained electrode capacity was taken as the initial capacity.

[0063] The test cell was subjected to 1 to 4 charge-discharge cycles (1 to 5 cycles in Comparative Example 1), and after each charge-discharge cycle, it was charged at 0.2 C for 7.5 hours and then discharged at 0.1 C until the negative electrode potential reached −0.5 V, and the capacity was confirmed.

[0064] The results are shown in Figure 1 and Table 3. In Figure 1, there are two results for each sample because the test was performed twice under the same conditions. The results in Table 3 are the average values ​​of multiple tests.

[0065] [Table 3]

[0066] 1 and Table 3, it can be seen that the samples of Examples 1 and 2 have a higher discharge capacity than the sample of Comparative Example 1 (a sample that does not contain one or more elements selected from the group consisting of La and Ce) even after a single charge-discharge cycle. Also, it can be confirmed that the sample of Comparative Example 2 has a higher discharge capacity than the sample of Comparative Example 1 even after a single charge-discharge cycle, but the battery capacity was reduced due to the excessive Ce content.

[0067] EDX area analysis was performed on the samples of Example 1 and Comparative Example 1 before alkaline dissolution treatment. Fig. 2 is an explanatory diagram showing the results of EDX area analysis on the sample of Example 1. Fig. 3 is an explanatory diagram showing the results of EDX area analysis on the sample of Comparative Example 1.

[0068] 2 and 3, it can be seen that in the sample of Example 1, in addition to the main phase and the grain boundary phase, a third phase is present, whereas in the sample of Comparative Example 1, no third phase is present.

[0069] The sample of Example 1 was subjected to a component analysis at five points in the main phase region, and the average values ​​were 16.20 atomic % Ti, 17.40 atomic % Zr, 43.98 atomic % Cr, 6.75 atomic % Mn, 15.68 atomic % Ni, and Ce was below the measurement limit.

[0070] The sample of Example 1 was subjected to a component analysis at five points in the grain boundary phase region, and the average values ​​were 28.38 atomic % for Ti, 18.24 atomic % for Zr, 1.95 atomic % for Cr, 2.34 atomic % for Mn, 49.09 atomic % for Ni, and Ce was below the measurement limit. It is believed that Cr and Mn are influenced by the main phase present below the grain boundary phase (toward the back of the page).

[0071] The sample of Example 1 was subjected to a component analysis at five points in the third phase region, and the average values ​​were 2.76 atomic % for Ti, 3.46 atomic % for Zr, Cr below the measurement limit, Mn below the measurement limit, Ni 46.12 atomic %, and Ce 48.91 atomic %. It is possible that Cr and Mn are influenced by the main phase present below the grain boundary phase (toward the back of the page).

[0072] The sample of Comparative Example 1 was subjected to a component analysis at five points in the main phase region, and the average values ​​were 15.99 atomic % Ti, 17.93 atomic % Zr, 35.84 atomic % Cr, 8.91 atomic % Mn, and 21.33 atomic % Ni.

[0073] The sample of Comparative Example 1 was subjected to a component analysis at five points in the grain boundary phase region, and the average values ​​were 27.63 atomic % Ti, 18.03 atomic % Zr, 2.70 atomic % Cr, 3.39 atomic % Mn, and 48.26 atomic % Ni.

[0074] From the results of the point analysis (component analysis) described above, it can be seen that a third phase with a high Ni content exists in the sample of Example 1. It can also be seen that, compared to the sample of Comparative Example 1, the Ni content in the main phase of the sample of Example 1 is reduced from 21.33 atomic % to 15.68 atomic %.

Claims

1. A hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni, and further containing one or more elements selected from the group consisting of La and Ce, the total content of one or more elements selected from the group consisting of La and Ce is 8 atomic % or less relative to the entire hydrogen storage alloy; Negative electrode active material for nickel-metal hydride batteries.

2. the hydrogen storage alloy comprises a main phase, a grain boundary phase, and a third phase; The main phase is AB ２ the grain boundary phase is an AB type alloy phase, The A and B ２ For the alloy phase, A is one or more elements selected from the group consisting of Ti and Zr, and B is one or more elements selected from the group consisting of Cr, Mn, and Ni; In the AB type alloy phase, A is one or more elements selected from the group consisting of Ti and Zr, and B is Ni, and 2. The negative electrode active material for a nickel-metal hydride battery according to claim 1, wherein the third phase is an alloy phase containing Ni and at least one element selected from the group consisting of La and Ce.

3. 3. The negative electrode active material for a nickel-metal hydride battery according to claim 1, wherein the content of Ni in the main phase is 14 atomic % or more and 21 atomic % or less with respect to the entire main phase.

4. Melting raw materials containing Ti, Zr, Cr, Mn, and Ni and further containing one or more elements selected from the group consisting of La and Ce to obtain a molten metal; cooling the molten metal to solidify the molten metal and obtain an ingot of a hydrogen storage alloy; Including, The total content of one or more elements selected from the group consisting of La and Ce is 8 atomic % or less relative to the entire molten metal. A method for producing a negative electrode active material for a nickel-metal hydride battery.

5. The method for producing a negative electrode active material for a nickel-metal hydride battery according to claim 4 , further comprising exposing the ingot to an alkaline solution.

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