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

Abstract

The present invention relates to a negative electrode active material for a nickel-metal hydride battery and a method for manufacturing the same. A negative electrode active material for nickel-metal hydride batteries, which contains a hydrogen storage alloy that contains Ti, Zr, Cr, Mn, and Ni, and also contains one or more elements selected from among La and Ce, and the total content ratio of the one or more elements selected from among La and Ce is 8 at% or less with respect to the entire hydrogen storage alloy.

Description

Technical Field

[0001] The present disclosure relates to a negative electrode active material for nickel-hydrogen batteries and a method for producing the same. Background Art

[0002] Hydrogen storage alloys are used as negative electrode active materials for nickel-hydrogen batteries. For example, Japanese Patent Application Laid-Open No. 10-36930 discloses a hydrogen storage alloy. The hydrogen storage alloy disclosed in Japanese Patent Application Laid-Open No. 10-36930 contains specified amounts of Zr, Ti, Nb, V, Ni, Mn, Cr, Co, Fe, Si, Mo, and B, and includes an intermetallic compound as an alloy phase. This intermetallic compound is a cubic C15-type or C14-type Laves phase. Summary of the Invention

[0003] When the hydrogen storage alloy disclosed in Japanese Patent Application Laid-Open No. 10-36930 is used as a negative electrode active material, the present inventors have discovered that initial activation is difficult and the number of break-in charge / discharge cycles required to increase discharge capacity is increased.

[0004] The present disclosure has been made to solve the above-mentioned problems, and an object of the present disclosure is to provide a negative electrode active material for nickel-metal hydride batteries that has excellent initial activation and can improve discharge capacity even with a small number of test charge and discharge cycles, and a method for producing the same.

[0005] The nickel-metal hydride battery active material and the method for producing the same disclosed herein can adopt the following embodiments.

[0006] <Scheme 1>

[0007] A negative electrode active material for a nickel-hydrogen battery, comprising a hydrogen storage alloy containing Ti, Zr, Cr, Mn and Ni, and further containing one or more elements selected from La and Ce, wherein the total content ratio of the one or more elements selected from La and Ce is less than 8 atomic % relative to the entire hydrogen storage alloy.

[0008] Option 2

[0009] According to the negative electrode active material for a nickel-metal hydride battery according to scheme 1, the hydrogen storage alloy includes 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 Ti and Zr, and B is one or more elements selected from Cr, Mn, and Ni, in the AB type alloy phase, A is one or more elements selected from Ti and Zr, and B is Ni, and the third phase is an alloy phase containing Ni and one or more elements selected from La and Ce.

[0010] <Scheme 3>

[0011] The negative electrode active material for a nickel-metal hydride battery according to claim 1 or 2, wherein the content ratio of Ni in the main phase is 14 atomic % or more and 21 atomic % or less relative to the entire main phase.

[0012] <Scheme 4>

[0013] A method for producing a negative electrode active material for a nickel-hydrogen battery comprises: melting raw materials to obtain a melt, wherein the raw materials contain Ti, Zr, Cr, Mn, and Ni, and further contain one or more elements selected from La and Ce; and cooling the melt to solidify the melt to obtain an ingot of a hydrogen storage alloy, wherein the total content ratio of the one or more elements selected from La and Ce relative to the entire melt is 8 atomic % or less.

[0014] <Scheme 5>

[0015] The method for manufacturing a negative electrode active material for a nickel-hydrogen battery according to scheme 4 further comprises: exposing the ingot to an alkaline solution.

[0016] According to the present disclosure, by further adding a small amount of one or more elements selected from La and Ce to a hydrogen storage alloy containing Ti, Zr, Cr, Mn and Ni, a negative electrode active material for nickel-hydrogen batteries can be provided, which has excellent initial activation and can increase the discharge capacity even with a small number of charge and discharge adjustments, and a method for manufacturing the same. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described below with reference to the accompanying drawings, wherein like reference numerals represent like elements, and wherein:

[0018] Figure 1 It is a graph showing the relationship between the number of debugging times and the discharge capacity.

[0019] Figure 2 This is an explanatory diagram showing the results of EDX surface analysis of the sample of Example 1.

[0020] Figure 3 This is an explanatory diagram showing the results of EDX surface analysis of the sample of Comparative Example 1. DETAILED DESCRIPTION

[0021] The embodiments described below do not limit the negative electrode active material for nickel-metal hydride batteries of the present disclosure (hereinafter sometimes simply referred to as “the negative electrode active material of the present disclosure”) and its production method.

[0022] Without being bound by theory, the present inventors will now explain the reasons why the negative electrode active material disclosed herein is excellent in initial activation and can improve discharge capacity even with a small number of test charge and discharge cycles.

[0023] Hydrogen storage alloys containing Ti, Zr, Cr, Mn, and Ni are two-phase alloys consisting of an AB2-type alloy phase, which primarily performs hydrogen storage (hydrogen occlusion), and an AB2-type alloy phase, which primarily performs charge-discharge activity. Both the AB2-type alloy phase and the AB2-type alloy phase contain Ni. The Ni in the AB2-type alloy phase contributes to charge-discharge activity, while the Ni in the AB2-type alloy phase contributes little to charge-discharge activity.

[0024] When a small amount of one or more elements selected from La and Ce is added to the aforementioned two-phase alloy, an alloy phase consisting of the added element and Ni forms. This alloy phase constitutes a third phase that coexists with the AB2-type alloy phase and the AB-type alloy phase. The formation of this third phase reduces the Ni content in the AB2-type alloy phase, and together with the AB-type alloy phase, it contributes to charge-discharge activity. The inventors have discovered that this results in excellent initial activation, enabling increased discharge capacity even with a small number of charge-discharge cycles.

[0025] Based on the above findings, the constituent elements of the negative electrode active material and the method for producing the negative electrode active material of the present disclosure are described below.

[0026] Negative Electrode Active Material

[0027] The negative electrode active material disclosed herein comprises 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 La (lanthanum) and Ce (cerium). The proportions of each of these elements are not particularly limited as long as they form a hydrogen storage alloy, but are preferably as follows. It should be noted that the sum of the upper limits of the content of each element exceeds 100 atomic %, which means that the content of each element is not necessarily the upper limit.

[0028] Ti and Zr

[0029] Relative to the hydrogen storage alloy as a whole, the content ratio of Ti and Zr is preferably 10 atomic % or more, 12 atomic % or more, 14 atomic % or more, or 16 atomic % or more, and 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.

[0030] 〈Cr〉

[0031] Relative to the entire hydrogen storage alloy, the Cr content ratio 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 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.

[0032] 〈Mn〉

[0033] The Mn content relative to the entire hydrogen storage alloy is preferably 2 atomic % or more, 4 atomic % or more, 6 atomic % or more, or 8 atomic % or more, 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. In order to improve the corrosion resistance of the hydrogen storage alloy, the Mn content relative to the entire hydrogen storage alloy is particularly preferably 14 atomic % or less, 12 atomic % or less, or 10 atomic % or less. Even if the Mn content is reduced in this way, the alkali dissolution treatment of the hydrogen storage alloy used for the negative electrode active material of the present invention is relatively easy, which helps to improve the charge and discharge activity. This is because a third phase exists in the hydrogen storage alloy.

[0034] 〈Ni〉

[0035] Relative to the entire hydrogen storage alloy, the Ni content ratio is preferably greater than 15 atomic %, greater than 20 atomic %, greater than 22 atomic %, greater than 24 atomic %, or greater than 25 atomic %, and preferably less than 60 atomic %, less than 55 atomic %, less than 50 atomic %, less than 45 atomic %, less than 40 atomic %, less than 38 atomic %, less than 36 atomic %, less than 34 atomic %, less than 32 atomic %, or less than 30 atomic %.

[0036] La and Ce

[0037] A small amount of one or more elements selected from La and Ce is sufficient. The total content of the one or more elements selected from La and Ce relative to the entire hydrogen storage alloy 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. If the total content of the one or more elements selected from La and Ce exceeds 8 atomic %, the battery capacity decreases.

[0038] When an alkali dissolution treatment is performed, a portion of one or more elements selected from La and Ce is removed from the hydrogen storage alloy by the alkali dissolution treatment. However, it is actually difficult to remove all of the one or more elements selected from La and Ce from the hydrogen storage alloy by the alkali dissolution treatment. Therefore, the total content of the one or more elements selected from La and Ce can 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.

[0039] When only one of La or Ce is included among La and Ce, the content ratio of that one element may be the above-mentioned total content ratio. Furthermore, B (boron) may be added to La and Ce. In the descriptions thus far, "one or more elements selected from La and Ce" may be replaced with "one or more elements selected from La, Ce, and B." Furthermore, when only one of La, Ce, or B is included among La, Ce, and B, the content ratio of that one element may be the above-mentioned total content ratio.

[0040] Other elements

[0041] Part of the Ti, Zr, Cr, Mn and Ni in the hydrogen storage alloy can be replaced by one or more elements selected from Fe (iron), Co (cobalt), V (vanadium), Nb (niobium), Mo (molybdenum), B (boron), Mg (magnesium), and Al (aluminum). Relative to the entire hydrogen storage alloy, the total content of the replacement elements such as Fe 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 %. As a result, in practice, there is no adverse effect on the negative electrode active material and its manufacturing method disclosed in the present invention. The total content of 0 atomic % means that the above-mentioned replacement is not performed. In addition, when the replacement element such as Fe is only one element, the content of the element can be the above-mentioned total content.

[0042] That is, the hydrogen storage alloy contains Ti, Zr, Cr, Mn, and Ni, and further necessarily contains one or more elements selected from La and Ce. Optionally, a portion of the Ti, Zr, Cr, Mn, and Ni is replaced with one or more elements selected from Fe, Co, V, Nb, Mo, B, Mg, and Al, and the remainder (balance) may be unavoidable impurities. In this specification, unless otherwise specified, the term "unavoidable impurities" refers to impurities contained in raw materials or impurities introduced during the manufacturing process, substances whose inclusion is unavoidable or whose avoidance would significantly increase manufacturing costs.

[0043] In addition, the negative electrode active material disclosed herein can contain substances other than hydrogen storage alloys and inevitable impurities within a range that does not impair its function. Examples of substances other than hydrogen storage alloys include rust inhibitors and the like. Relative to the entire negative electrode active material, the total content of substances other than hydrogen storage alloys and inevitable impurities may be 5% by mass or less, 3% by mass or less, or 1% by mass or less. Taking inevitable impurities into account, the total content of substances other than hydrogen storage alloys and inevitable impurities may be 0.1% by mass or more, 0.3% by mass or more, or 0.5% by mass or more.

[0044] 〈Organization (Metallographic Structure)〉

[0045] The hydrogen storage alloy used in the negative electrode active material disclosed herein includes an AB2-type alloy phase as the main phase, an AB2-type alloy phase as the grain boundary phase, and a tertiary phase. It should be noted that the terms "AB2-type alloy phase" and "AB-type alloy phase" follow conventional notation for alloy phases in hydrogen storage alloys. In cases where confusion with element B (boron) is possible, "AB2-type alloy phase" and "AB-type alloy phase" may be represented as, for example, "ab2-type alloy phase" and "ab-type alloy phase," respectively. The main phase, grain boundary phase, and tertiary phase are described below.

[0046] 〈Main Phase〉

[0047] The main phase is an AB2-type alloy phase. The term "AB2-type alloy phase" refers to an alloy phase consisting of one mole of an "A" element and two moles of a "B" element. A is one or more elements selected from Ti (titanium) and Zr (zirconium), and B is one or more elements selected from 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 function as hydrogen storage.

[0048] Typically, the main phase contains Ni as the "B" element at a concentration of 14 atomic % to 21 atomic % relative to the total main phase. Conventional hydrogen storage alloys that do not include a third phase typically contain Ni at a concentration of 21.3 atomic % or greater relative to the total main phase. As mentioned above, the main phase is an AB2-type alloy phase with a Laves structure. Ni present in such a structure exhibits little charge and discharge activity.

[0049] The hydrogen storage alloy used as the negative electrode active material of the present disclosure can reduce the Ni content of the main phase to 14 atomic % or more and 21 atomic % or less relative to the total main phase due to the coexistence of the main phase, the grain boundary phase, and the third phase. From this perspective, the main phase can 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 relative to the total main phase.

[0050] The main phase exists as the main phase in the hydrogen storage alloy, so the volume ratio of the main phase relative to the entire hydrogen storage alloy can be, for example, more than 50%, more than 60%, more than 70%, or more than 75%, or less than 95%, less than 90%, less than 85%, or less than 80%.

[0051] A portion of the constituent elements of the AB2 type alloy phase may be substituted with one or more elements selected from Fe, Co, V, Nb, Mo, B, Mg, and Al. The same applies to the AB type alloy phase and the third phase described below.

[0052] Grain boundary phase

[0053] The grain boundary phase is an AB-type alloy phase. The term "AB-type alloy phase" refers to an alloy phase consisting of one mole of element "A" and one mole of element "B." A is one or more elements selected from 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 this crystal structure possess charge and discharge active functions.

[0054] The grain boundary phase exists between adjacent main phases among the plurality of main phases. The tertiary phase, 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 tertiary phase constitute the remainder of the main phase. Thus, the combined volume ratio of the grain boundary phase and the tertiary phase relative to the entire hydrogen storage alloy may be, for example, 5% or more, 10% or more, 15% or more, or 20% or less, or 50% or less, 40% or less, 30% or less, or 25% or less.

[0055] In hydrogen storage alloys, phases other than the main phase, grain boundary phase, and tertiary phase may exist. Phases other than the main phase, grain boundary phase, and tertiary phase are typically phases containing unavoidable impurities, and their volume ratio relative to the entire hydrogen storage alloy may be less than 5%, less than 4%, less than 3%, less than 2%, less than 1%, or 0%. A volume ratio of 0% means that phases other than the main phase, grain boundary phase, and tertiary phase are completely absent. In cases where "the complete absence of phases other than the main phase, grain boundary phase, and tertiary phase" is practically difficult, the volume ratio of phases other than the main phase, grain boundary phase, and tertiary phase may be greater than 0.1%, greater than 0.3%, greater than 0.5%, greater than 0.7%, or greater than 0.9%.

[0056] The Third Phase

[0057] The third phase is an alloy phase containing Ni and one or more elements selected from La and Ce. The alloy phase is typically in the form of an intermetallic compound, but is not limited thereto. For example, one element may be dissolved in the other element in part or in whole.

[0058] In the absence of alkaline solution treatment, the content of Ni relative to the entire third phase may be 20 atomic % or more, 30 atomic % or more, or 40 atomic % or more, or 70 atomic % or less, 60 atomic % or less, or 50 atomic % or less. In the absence of alkaline solution treatment, the total content of one or more elements selected from La and Ce relative to the entire third phase may be 20 atomic % or more, 30 atomic % or more, or 40 atomic % or more, or 70 atomic % or less, 60 atomic % or less, or 50 atomic % or less. In addition, in the absence of alkaline solution treatment, the total content of Ti and Zr may be 1 atomic % or more, 2 atomic % or more, or 3 atomic % or more, or 6 atomic % or less, 5 atomic % or less, or 4 atomic % or less. The remainder is unavoidable impurities. Unavoidable impurities include Cr and Mn.

[0059] When treated with an alkaline solution, the elements other than Ni in the third phase are largely removed by the alkaline solution treatment, forming a Ni-concentrated phase with one or more elements selected from La and Ce as its core. When treated with an alkaline solution, the total content of one or more elements selected from La and Ce in the third phase relative to the entire third phase can 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 can be 3 atomic % or less, 2 atomic % or less, or 1 atomic % or less. The remainder is Ni and unavoidable impurities.

[0060] The composition of the third phase varies greatly depending on whether or not the alkaline solution treatment is applied. However, the composition of the main phase hardly changes with the alkaline solution treatment, so the hydrogen storage alloy used for the negative electrode active material of the present disclosure can be specified by the Ni content in the main phase.

[0061] As described above, the combined volume ratio of the grain boundary phase and the tertiary phase can be 5% or more, 10% or more, 15% or more, 20% or more, 25% or more, or 30% or more, or can be 50% or less, 45% or less, or 40% or less. A volume ratio of the tertiary phase of 5% or more relative to the combined volume ratio of the grain boundary phase and the tertiary phase results in excellent initial activation, allowing for a practically discernible improvement in discharge capacity even with a small number of charge and discharge cycles. A volume ratio of the tertiary phase of 50% or less relative to the combined volume ratio of the grain boundary phase and the tertiary phase advantageously allows for the coexistence of the main phase, grain boundary phase, and tertiary phase.

[0062] <Organization determination method>

[0063] The hydrogen storage alloy is cut and ground to obtain an SEM image (backscattered electron image) of the ground surface. The SEM image is then subjected to EDX surface analysis. If the hydrogen storage alloy is powdered by pulverization, the powder can be filled with resin and ground to obtain an SEM image.

[0064] Because the main phase, grain boundary phase, and tertiary phase regions each have different compositional components, EDX surface analysis results allow for the identification of these regions. Using an image processing device, the area ratios of these regions are measured to determine the volume ratios of the main phase, grain boundary phase, and tertiary phase.

[0065] 《Method for producing negative electrode active material》

[0066] The method for producing a negative electrode active material of the present disclosure (hereinafter sometimes simply referred to as “the production method of the present disclosure”) includes melt preparation and melt cooling, and optionally includes an alkali dissolution treatment.

[0067] Melt Preparation

[0068] A raw material containing Ti, Zr, Cr, Mn, and Ni, and further containing one or more elements selected from La and Ce, is melted to obtain a melt (molten metal).

[0069] The composition range of the melt is essentially the same as that of the hydrogen storage alloy described in the "Negative Electrode Active Material" section. However, if the composition fluctuates during the manufacturing process due to loss of specific elements, the compositional fluctuations can be accounted for when melting the raw materials so that the melt's composition range is consistent with that of the hydrogen storage alloy. In the description of the "Negative Electrode Active Material Manufacturing Method," "the entire hydrogen storage alloy" can be replaced with "the entire melt."

[0070] The melting method is not particularly limited; examples include arc melting the raw materials placed in a crucible to produce a melt. To prevent oxidation of the raw materials and the melt, the raw materials are preferably melted in an inert gas atmosphere, particularly argon. Group 18 elements such as argon are recommended for this inert gas atmosphere.

[0071] Melt Cooling

[0072] The melt is cooled and solidified to produce an ingot of hydrogen storage alloy. The melt cooling method is not particularly limited; for example, arc melting the raw materials placed in a crucible and then cooling the melt directly within the crucible can be used. The cooling rate in such a method is generally 0.1°C / second or higher and less than 100°C / second.

[0073] To prevent oxidation of the raw materials and the melt, the melt is preferably cooled in an inert gas atmosphere, particularly an argon atmosphere. Inert gas atmospheres include nitrogen atmospheres. Furthermore, to achieve a uniform composition within the ingot, the above-described melting and melt cooling can be repeated multiple times.

[0074] <Alkali dissolution treatment>

[0075] The ingot can be exposed to an alkaline solution to release elements other than Ni from the third phase, resulting in a Ni-rich phase. This further enhances initial activation and increases discharge capacity even with fewer charge and discharge cycles.

[0076] The type of alkaline solution is not particularly limited, but typically, a NaOH (sodium hydroxide) aqueous solution is used. The concentration of the alkaline solution can be appropriately determined. In the case of a NaOH aqueous solution, its concentration can be, for example, 10% by mass or more, 20% by mass or more, or 30% by mass or more, and can be 50% by mass or less, 45% by mass or less, or 40% by mass or less.

[0077] The treatment temperature and treatment time can be appropriately determined according to the type and concentration of the alkaline solution, the treatment amount, etc. The treatment temperature can be, for example, 70°C or higher, 80°C or higher, or 90°C or higher, and can be 130°C or lower, 120°C or lower, or 110°C or lower. The treatment time can be, for example, 1 hour or higher, 2 hours or higher, or 3 hours or higher, and can be 7 hours or lower, 6 hours or lower, or 5 hours or lower.

[0078] Preparation of Samples

[0079] The 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. Melting and cooling in the crucible were repeated three times to homogenize the composition of the ingot. The purity of the raw materials for Zr was 98% by mass, and the purity of the raw materials for the other elements was 99.9% by mass.

[0080]

Table 1

[0081] Unit: atomic %

[0082] Ti Zr Cr Mn Ni La Ce B Comparative Example 1 17.78 17.78 26.67 8.89 28.89 – – – Example 1 17.02 17.02 25.53 8.51 27.66 – 4.26 – Example 2 17.02 17.02 25.53 8.51 27.66 4.26 – – Example 3 17.47 17.47 26.20 8.73 28.38 – 1.75 – Example 4 17.62 17.62 26.43 8.81 28.63 – 0.88 – Comparative Example 2 16.33 16.33 24.49 8.16 26.53 – 8.16 – Reference Example 1 17.02 17.02 25.53 8.51 27.66 – – 4.26

[0083] The ingot was pulverized using a Tokyo Glass Equipment Co., Ltd. hand-operated stamp mill DA-30, and the obtained powder was classified into 38 to 100 μm using 38 μm and 100 μm sieves.

[0084] The classified powder was placed in a NaOH aqueous solution for alkaline dissolution. The concentration of the NaOH aqueous solution was 35% by mass. The amount of the NaOH aqueous solution was 80 ml. The amount of the powder was 2.0 g. The treatment temperature was 100°C and the treatment time was 4 hours.

[0085] The powder after alkali dissolution treatment was washed with water, filtered, and then vacuum dried.

[0086] "evaluate"

[0087] Each sample was evaluated as follows: Unless otherwise specified, each evaluation was performed after the alkali dissolution treatment.

[0088] 〈Ingredients〉

[0089] Before the alkali dissolution treatment, each sample was quantitatively analyzed by ICP. The results are shown in Table 2. It was confirmed that the variations in the composition between the raw material composition and the sample composition for all samples were within a range that did not pose a practical problem.

[0090]

Table 2

[0091] Unit: atomic %

[0092] Ti Zr Cr Mn Ni La Ce B Comparative Example 1 18.18 17.75 26.85 8.56 28.66 – – – Example 1 17.22 16.94 25.95 6.65 28.24 – 5.00 – Example 2 16.45 15.81 23.09 7.48 30.15 7.02 – Example 3 18.31 17.66 27.84 6.06 28.45 – 1.68 – Example 4 18.11 17.70 26.60 6.95 29.66 – 0.97 – Comparative Example 2 15.61 15.11 21.99 6.45 31.39 – 9.44 – Reference Example 1 17.41 17.26 25.57 7.92 28.27 – – 3.58

[0093] <Discharge Characteristics>

[0094] Evaluation battery cells were produced as follows.

[0095] First, a negative electrode was prepared. More specifically, 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 aid, and 2 parts by mass of carboxymethyl cellulose (CMC) as a binder were mixed to prepare a paste composition. The paste composition was filled into the negative electrode collector, then vacuum-dried at 80°C, and then rolled with a pressure of about 8kN (gap 300μm) to obtain a negative electrode. In addition, as the negative electrode collector, porous nickel (Celmet#7 manufactured by Sumitomo Electric Industries, Ltd., thickness 1.6mm) with a Ni pole piece welded thereto was used. In addition, the capacity was adjusted with a target of about 240mAh.

[0096] Next, the positive electrode was prepared. More specifically, 88 parts by mass of nickel hydroxide (Ni(OH)2) as the positive electrode active material, 10 parts by mass of cobalt oxide (CoO) as the conductive additive, and 1 part by mass each of two binders (carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA)) were mixed to produce a paste composition. This paste composition was filled into the porous nickel described above, then vacuum-dried at 80°C, and then roller-pressed with a pressure of approximately 8kN to obtain the positive electrode. Furthermore, the capacity ratio of the negative electrode to the positive electrode was adjusted to negative electrode:positive electrode = 1:4.5.

[0097] Next, an electrolyte solution was prepared. Pure water was added to KOH to adjust the KOH concentration to 6 mol / L, resulting in 90 ml of electrolyte solution. The electrolyte solution and a separator (PE / PP nonwoven fabric, 150 μm thick) were then placed in a container. Furthermore, the negative electrode (working electrode), positive electrode (counter electrode), and Hg / HgO electrode (reference electrode) were placed to obtain an evaluation battery cell.

[0098] Using the obtained evaluation battery cell, a charge and discharge test (25° C.) was performed. First, a conditioning charge and discharge was performed until the negative electrode capacity was saturated. More specifically, the following charge and discharge steps (1) to (4) were performed.

[0099] (1) Charge at 0.1C for 14 hours, then discharge at 0.1C until the negative electrode potential reaches -0.6V.

[0100] (2) Charge at 0.5C for 2.2 hours, then discharge at 0.5C until the negative electrode potential reaches -0.6V.

[0101] (3) Next, charge at 0.1C for 14 hours, and then discharge at 0.1C until the negative electrode potential reaches -0.6V.

[0102] (4) Repeat the charge and discharge steps (1) to (3) above until the capacity of the 0.1 C discharge is saturated, and the resulting electrode capacity is set as the initial capacity.

[0103] The test charge and discharge were performed 1 to 4 times (1 to 5 times in Comparative Example 1). The evaluation battery cell that completed each test charge and discharge was charged at 0.2C for 7.5 hours and then discharged at 0.1C until the negative electrode potential reached -0.5V, and the capacity was confirmed.

[0104] The results are shown in Figure 1 and Table 3. Figure 1 Each sample has two results because two tests were conducted under the same conditions. In addition, the results in Table 3 are the average values of multiple tests.

[0105]

Table 3

[0106] Unit: mAh / g

[0107] first Second time The third time Fourth time Fifth Comparative Example 1 211.6 292.0 305.5 317.6 317 Example 1 303.5 314.9 315.0 313.2 – Example 2 234.9 284.0 291.9 292.5 – Example 3 326.2 342.6 343.8 342.2 – Example 4 238.4 313.1 323.7 328.2 – Comparative Example 2 263.9 268.3 264.4 257.5 – Reference Example 1 269.9 299.9 304.4 304.4 –

[0108] Depend on Figure 1 As can be understood from Table 3, the samples of Examples 1 and 2 exhibited higher discharge capacities than the sample of Comparative Example 1 (a sample not containing one or more elements selected from La and Ce), even after a single commissioning charge and discharge cycle. Furthermore, it was confirmed that the sample of Comparative Example 2 exhibited higher discharge capacity than the sample of Comparative Example 1, even after a single commissioning charge and discharge cycle, but the excessive Ce content resulted in a decrease in battery capacity.

[0109] For the samples of Example 1 and Comparative Example 1, EDX surface analysis was performed before the alkali dissolution treatment. Figure 2 This is an explanatory diagram showing the results of EDX surface analysis of the sample of Example 1. Figure 3 This is an explanatory diagram showing the results of EDX surface analysis of the sample of Comparative Example 1.

[0110] Depend on Figure 2 and Figure 3 It can be understood that in the sample of Example 1, in addition to the main phase and the grain boundary phase, a third phase exists, whereas in the sample of Comparative Example 1, no third phase exists.

[0111] For the sample of Example 1, composition analysis was carried out at 5 points in the main phase area. The results showed that, as an average value, Ti was 16.20 atomic%, Zr was 17.40 atomic%, Cr was 43.98 atomic%, Mn was 6.75 atomic%, Ni was 15.68 atomic%, and Ce was below the measurement limit.

[0112] Composition analysis of the sample of Example 1 at five points in the grain boundary phase region revealed an average of 28.38 atomic percent Ti, 18.24 atomic percent Zr, 1.95 atomic percent Cr, 2.34 atomic percent Mn, and 49.09 atomic percent Ni. Ce was below the detection limit. Cr and Mn are believed to be influenced by the presence of the main phase below the grain boundary phase (toward the back of the drawing).

[0113] Composition analysis was performed on the sample of Example 1 at five points in the third phase region. The results showed that, on average, Ti was 2.76 atomic%, Zr was 3.46 atomic%, Cr was below the detection limit, Mn was below the detection limit, Ni was 46.12 atomic%, and Ce was 48.91 atomic%. It is believed that Cr and Mn may also be affected by the main phase existing below the grain boundary phase (toward the back of the paper).

[0114] For the sample of Comparative Example 1, component analysis was performed at 5 points in the main phase area. The results showed that, as an average value, Ti was 15.99 atomic %, Zr was 17.93 atomic %, Cr was 35.84 atomic %, Mn was 8.91 atomic %, and Ni was 21.33 atomic %.

[0115] For the sample of Comparative Example 1, component analysis was performed at five points in the grain boundary phase region. The results showed that, as an average value, Ti was 27.63 atomic %, Zr was 18.03 atomic %, Cr was 2.70 atomic %, Mn was 3.39 atomic %, and Ni was 48.26 atomic %.

[0116] The results of the above-mentioned point analysis (composition analysis) show that a third phase with a high Ni content exists in the sample of Example 1. Furthermore, it can be seen that the Ni content in the main phase of the sample of Example 1 is reduced from 21.33 atomic % to 15.68 atomic % compared to the sample of Comparative Example 1.

Claims

1. A negative electrode active material for a nickel-hydrogen battery, comprising a hydrogen storage alloy containing Ti, Zr, Cr, Mn and Ni, and further containing one or more elements selected from La and Ce, wherein the total content ratio of the one or more elements selected from La and Ce relative to the entire hydrogen storage alloy is 8 atomic % or less.

2. The negative electrode active material for nickel-metal hydride batteries according to claim 1, wherein The hydrogen storage alloy includes a main phase, a grain boundary phase, and a third phase, The main phase is AB2 type alloy phase, and the grain boundary phase is AB type alloy phase, In the AB2 type alloy phase, A is one or more elements selected from Ti and Zr, and B is one or more elements selected from Cr, Mn, and Ni, In the AB type alloy phase, A is one or more elements selected from Ti and Zr, and B is Ni, and The third phase is an alloy phase containing Ni and one or more elements selected from La and Ce.

3. The negative electrode active material for nickel-metal hydride batteries according to claim 1 or 2, wherein The content ratio of Ni in the main phase is 14 atomic % or more and 21 atomic % or less relative to the entire main phase.

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

5. The method for producing a negative electrode active material for a nickel-hydrogen battery according to claim 4, further comprising: The ingot is exposed to an alkaline solution.

Images