Anode active material and method of manufacturing the same

Abstract

To provide an anode active material capable of enhancing discharge capacity with low resource risk and at low cost, and a method of manufacturing the same.SOLUTION: A method of manufacturing an anode active material includes: obtaining a foil by cooling a molten metal of a hydrogen storage alloy containing Ti, Zr, Cr, Mn and Ni at a speed of 1×103 to 1×106°C / sec and at least to lower than 500°C; and performing heat treatment on the foil in vacuum or in an inert gas atmosphere at 500 to 900°C and for 1 to 10 hours. The anode active material obtained by the manufacturing method contains the hydrogen storage alloy including a plurality of main phases and a grain boundary phase being present between the main phases adjacent to each other. The main phase includes AB2 type alloy phase, the grain boundary phase includes an AB type alloy phase, and an average distance between the main phases adjacent to each other is 1.0 μm or less.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode active material 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. Known hydrogen storage alloys include AB5-type alloys (e.g., La(NiCoMnAl)5 alloy) and A2B7-type alloys (e.g., (LaSmMg)2(NiAl)7 alloy). Both AB5-type alloys and A2B7-type alloys require rare earth elements, which poses a high resource risk. Furthermore, AB5-type alloys may require a large amount of expensive Co.

[0003] Therefore, a hydrogen storage alloy that does not contain rare earth elements and does not require the inclusion of large amounts of expensive elements such as Co is 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, and the intermetallic compound is a cubic C15 type or C14 type belonging to the Laves phase. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-36930 Summary of the Invention [Problem to be solved by the invention]

[0005] When the hydrogen storage alloy disclosed in Patent Document 1 is used as a negative electrode active material, no rare earth elements are used and the amount of expensive elements used is small, but the discharge capacity may be insufficient. Therefore, the present inventors have found that there is a need for a negative electrode active material that can improve the discharge capacity with low resource risk and low cost, and a method for producing the same.

[0006] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a negative electrode active material that can improve discharge capacity with low resource risk and low cost, and a method for producing the same. [Means for solving the problem]

[0007] The present inventors have conducted extensive research to achieve the above object and have completed the negative electrode active material and the method for producing the same according to the present disclosure. <Aspect 1> A molten metal of a hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni was added to a 1×10 2 ~1×10 4 Cooling at a rate of 100°C / sec to at least below 500°C to obtain a flake; and heat-treating the flakes at 500 to 900°C for 1 to 10 hours in vacuum or in an inert gas atmosphere; Including, A method for producing a negative electrode active material. Aspect 2: The method for producing the negative electrode active material of Aspect 1, wherein a portion of Ti, Zr, Cr, Mn, and Ni is substituted with one or more elements selected from the group consisting of Fe, Co, V, Nb, Mo, B, Mg, and Al. <Aspect 3> 3. The method for producing a negative electrode active material according to aspect 1 or 2, wherein the molten metal is cooled using a strip casting method. <Aspect 4> a hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni; the hydrogen storage alloy comprises a plurality of main phases and a grain boundary phase present between adjacent main phases, the main phase includes an AB2 type alloy phase, and the grain boundary phase includes 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 The average distance between adjacent main phases is 1.0 μm or less. Negative electrode active material. Aspect 5: The negative electrode active material of Aspect 4, wherein a portion of Ti, Zr, Cr, Mn, and Ni is substituted with one or more elements selected from the group consisting of Fe, Co, V, Nb, Mo, B, Mg, and Al. [Effects of the Invention]

[0008] According to the present disclosure, flakes are obtained by rapidly cooling a molten hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni, which has multiple main phases and grain boundary phases between adjacent main phases, and in which the main phases and grain boundary phases are refined. These flakes are then heat-treated under predetermined conditions to modify the grain boundary phases, resulting in a hydrogen storage alloy in which the average distance between adjacent main phases falls within a predetermined range. Furthermore, by using this hydrogen storage alloy, it is possible to provide a negative electrode active material with improved discharge capacity, low resource risk, and a method for producing the same. [Brief explanation of the drawings]

[0009] [Figure 1A] FIG. 1A is an SEM image of the sample of Example 1. [Figure 1B] FIG. 1B is a distribution diagram of FIG. 1A, in which the AB2 type alloy phase (main phase) region is shown in black and the AB type alloy phase (grain boundary phase) region is shown in white. [Figure 2A] FIG. 2A is an SEM image of the sample of Reference Example 1. [Figure 2B] FIG. 2B is a distribution diagram of FIG. 2A, in which the AB2 type alloy phase (main phase) region is shown in black and the AB type alloy phase (grain boundary phase) region is shown in white. [Figure 3] FIG. 3 is a graph showing the results of XRD analysis of the samples of Examples 1 and 2, Comparative Example 1, and Reference Example 1. [Figure 4] FIG. 4 is a graph showing the relationship between current density and discharge capacity for each of the samples of Examples 1 and 2, Comparative Example 1, and Reference Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the negative electrode active material and the method for producing the same according to the present disclosure will be described in detail. Note that the following embodiments do not limit the negative electrode active material and the method for producing the same according to the present disclosure.

[0011] Without being bound by theory, the inventors will now explain their findings regarding why the negative electrode active material of the present disclosure can improve discharge capacity with low resource risk and low cost.

[0012] An example of a hydrogen storage alloy that does not contain rare earth elements and does not require the inclusion of large amounts of expensive elements such as Co is an alloy containing Ti, Zr, Cr, Mn, and Ni. When a molten metal having this alloy composition is cooled and solidified at a normal rate, a two-phase hydrogen storage alloy is obtained, comprising an AB2-type alloy phase with a Laves structure and an AB-type alloy phase with a CsCl-type crystal structure. Because the molten metal is cooled at a normal rate, both the AB2-type alloy phase and the AB-type alloy phase are coarse. In this specification, unless otherwise specified, "cooling the molten metal at a normal rate" refers to, for example, arc-melting raw materials charged in a crucible to obtain a molten metal, which is then cooled directly in the crucible.

[0013] In the dual-phase hydrogen storage alloy, the AB2 type alloy phase constitutes the main phase that is primarily responsible for hydrogen storage, and the AB type alloy phase constitutes the grain boundary phase that is primarily responsible for charge / discharge activity. The dual-phase hydrogen storage alloy comprises multiple main phases and grain boundary phases that exist between adjacent main phases.

[0014] To ensure sufficient discharge capacity in a dual-phase hydrogen storage alloy, it is necessary to ensure a sufficient volume fraction of the AB2 type alloy phase that constitutes the main phase. Therefore, it is preferable to set the volume fraction of the AB type alloy phase that constitutes the grain boundary phase to a maximum of approximately 30%. Furthermore, when the molten alloy is cooled at a normal rate, the AB type alloy phase that constitutes the grain boundary phase segregates.

[0015] Even when the volume fraction of the AB-type alloy phase constituting the grain boundary phase is small, i.e., when the amount of the AB-type alloy phase constituting the grain boundary phase is small, rapid cooling of the molten alloy to refine the structure of the two-phase hydrogen storage alloy is effective in obtaining sufficient charge-discharge activity. This improves the dispersion of the AB-type alloy phase constituting the grain boundary phase. Furthermore, heat-treating the two-phase hydrogen storage alloy having the refined structure under specified conditions to modify the AB-type alloy phase constituting the grain boundary phase and reduce its crystallinity is advantageous for improving charge-discharge activity. This is thought to be because the reduced crystallinity promotes hydrogen diffusion.

[0016] Furthermore, when the structure of a two-phase hydrogen storage alloy is refined, the distance between adjacent main phases is reduced, and when the grain boundary phase is modified, the distance between adjacent main phases is further reduced. Since the grain boundary phase exists between adjacent main phases, reducing the distance between adjacent main phases means that the grain boundary phase sandwiched between adjacent main phases becomes thinner. The inventors have discovered that using a two-phase hydrogen storage alloy having such a structure as a negative electrode active material can improve discharge capacity.

[0017] A method for producing the negative electrode active material of the present disclosure based on the findings described above and the constituent elements of the negative electrode active material of the present disclosure obtained thereby will be described below.

[0018] <<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 quenching of a melt and heat treatment. The quenching of a melt and heat treatment will be described below.

[0019] <Rapid cooling of molten metal> In the manufacturing method of the present disclosure, a molten metal of a hydrogen storage alloy containing Ti (titanium), Zr (zirconium), Cr (chromium), Mn (manganese), and Ni (nickel) is added to a molten metal of 10 3 ~10 5 Cool at a rate of 100°C / sec to at least below 500°C to obtain a flake.

[0020] The molten metal only needs to contain Ti, Zr, Cr, Mn, and Ni and have a composition that allows it to function as a hydrogen storage alloy after solidification. The content ratios of Ti, Zr, Cr, Mn, and Ni may be, for example, as follows:

[0021] The content of Ti, Zr, Cr, and Mn in the molten metal may be 10 atomic % or more, 12 atomic % or more, 15 atomic % or more, or 18 atomic % or more, or 50 atomic % or less, 45 atomic % or less, 40 atomic % or less, 35 atomic % or less, 30 atomic % or less, 28 atomic % or less, 26 atomic % or less, 24 atomic % or less, 22 atomic % or less, or 20 atomic % or less, based on the total molten metal. In order to make the volume fraction of the AB-type alloy phase constituting the grain boundary phase about 30% at most, the content of Ti, Zr, Cr, and Mn in the molten metal is preferably 12 to 24 atomic %.

[0022] The Ni content in the molten metal may be 15 atomic % or more, 20 atomic % or more, 22 atomic % or more, 24 atomic % or more, or 25 atomic % or more, or may be 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, 30 atomic % or less, 28 atomic % or less, or 26 atomic % or less. In order to make the volume fraction of the AB-type alloy phase that constitutes the grain boundary phase about 30% at most, the Ni content in the molten metal is preferably 20 to 30 atomic % with respect to the entire molten metal.

[0023] A portion of the Ti, Zr, Cr, Mn, and Ni in the molten metal 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 Fe, Co, V, Nb, Mo, B, Mg, and Al relative to the entire molten metal is preferably less than 50 atomic %, 45 atomic % or less, 40 atomic % or less, 35 atomic % or less, 30 atomic % or less, 25 atomic % or less, 20 atomic % or less, 15 atomic % or less, 10 atomic % or less, 5 atomic % or less, or 0 atomic %. As long as the total content of Fe, Co, V, Nb, Mo, B, Mg, and Al relative to the entire molten metal is within the above range, there is no adverse effect on the practical application of the manufacturing method of the present disclosure and the products produced therefrom. The total content of 0 atomic % means that a portion of Ti, Zr, Cr, Mn, and Ni in the molten metal is not substituted with one or more elements selected from the group consisting of Fe, Co, V, Nb, Mo, B, Mg, and Al.

[0024] The molten metal may contain elements other than Ti, Zr, Cr, Mn, Ni, Fe, Co, V, Nb, Mo, B, Mg, and Al, to the extent that such elements do not adversely affect the manufacturing method and the resulting product of the present disclosure in practical use. The content of such elements is preferably 8 atomic % or less, 6 atomic % or less, 4 atomic % or less, 2 atomic % or less, or 0 atomic % relative to the entire molten metal. When multiple types of such elements are present, the total content of these elements satisfies the above-mentioned requirements. A content of 0 atomic % means that the molten metal contains no elements other than Ti, Zr, Cr, Mn, Ni, Fe, Co, V, Nb, Mo, B, Mg, and Al. Elements other than Ti, Zr, Cr, Mn, Ni, Fe, Co, V, Nb, Mo, B, Mg, and Al are typically unavoidable impurities.

[0025] That is, the molten metal contains Ti, Zr, Cr, Mn, and Ni, 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. In this specification, unless otherwise specified, 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.

[0026] The above molten metal was mixed with 1×10 2 ~1×10 4 Cool at a rate of 100°C / sec to at least below 500°C.

[0027] The molten metal is 1×10 2 If the alloy is cooled at a rate of 5 × 10 °C / sec or faster, a hydrogen storage alloy with a fine structure can be obtained. 2 °C / sec or more, 7 x 10 2 °C / sec or more, 9 x 10 2 °C / sec or more, or 1 x 10 3 °C / sec or more. 4 If the cooling rate is less than 9 × 10 °C / sec, the structure will be refined and the cooling rate will not be unnecessarily high. 3 ℃ / sec or less, 7×10 3 ℃ / sec or less, 5×10 3 °C / sec or less, or 3 x 10 3 °C / sec or less.

[0028] The molten metal is cooled at the above-mentioned rate to at least below 500°C. This makes it possible to suppress the crystal growth of the AB2 type alloy phase constituting the main phase and the AB type alloy phase constituting the grain boundary phase after the molten metal starts to solidify, thereby suppressing the coarsening of these alloy phases. From this perspective, the molten metal may be cooled at the above-mentioned rate to at least 480°C or less, 460°C or less, 440°C or less, 420°C or less, or 400°C or less.

[0029] There are no particular limitations on the cooling method of the molten metal, as long as the molten metal can be cooled under the above-mentioned conditions to obtain flakes. Typically, the molten metal is cooled using a strip casting method.

[0030] The strip casting method is outlined below. Raw materials are melted in a melting furnace to prepare a molten metal having the above-mentioned composition. The molten metal is then fed into a tundish. The molten metal is then fed from the end of the tundish onto the surface of a cooling drum by its own weight. The molten metal is cooled on the outer periphery of a rotating cooling roll, where it solidifies and turns into flakes, which are then recovered.

[0031] The peripheral speed of the chill roll may be appropriately determined in consideration of the material of the chill roll, etc., so that the cooling rate of the molten metal falls within the desired range. The peripheral speed of the chill roll, in terms of the surface speed of the chill roll, may be 0.1 m / sec or more, 0.5 m / sec or more, 1.0 m / sec or more, 1.5 m / sec or more, or 2.0 m / sec or more, and may be 10.0 m / sec or less, 7.0 m / sec or less, 4.0 m / sec or less, 3.5 m / sec or less, or 3.0 m / sec or less.

[0032] In order to prevent oxidation of the molten metal and the flakes, it is preferable to cool the molten metal in an inert gas atmosphere, particularly an argon gas atmosphere, which also includes a nitrogen gas atmosphere.

[0033] <Heat treatment> The flakes obtained by quenching the melt are heat treated at 500 to 900°C for 1 to 10 hours in a vacuum or in an inert gas atmosphere.

[0034] If the heat treatment temperature is 500°C or higher, the AB type alloy phase constituting the grain boundary phase can be modified. From this perspective, the heat treatment temperature may be 550°C or higher, 600°C or higher, or 650°C or higher. If the heat treatment temperature is 900°C or lower, the AB2 type alloy phase constituting the main phase and the AB type alloy phase constituting the grain boundary phase do not coarsen. From this perspective, the heat treatment temperature may be 850°C or lower, 800°C or lower, 750°C or lower, or 700°C or lower.

[0035] The heat treatment time is determined taking into consideration the heat treatment temperature and other factors so as to modify the AB type alloy phase that constitutes the grain boundary phase. If the heat treatment time is 1 hour or longer, the modification of the AB type alloy phase that constitutes the grain boundary phase can be practically recognized. From this perspective, the heat treatment time may be 1.5 hours or longer, 2 hours or longer, or 2.5 hours or longer. If the heat treatment time is 10 hours or shorter, the AB2 type alloy phase that constitutes the main phase and the AB type alloy phase that constitutes the grain boundary phase do not coarsen. From this perspective, the heat treatment time may be 9 hours or shorter, 8 hours or shorter, 7 hours or shorter, 6 hours or shorter, 5 hours or shorter, 4 hours or shorter, or 3 hours or shorter.

[0036] To prevent oxidation of the flakes, the flakes are heat-treated in a vacuum or in an inert gas atmosphere. When heat-treating in a vacuum, the absolute pressure of the atmosphere is 1×10 -7 Pa or more, 1×10 -6 Pa or more, or 1×10 -5 Pa or more, 1 × 10 -2 Pa or less, 1×10 -3 Pa or less, or 1×10 -4 The pressure may be less than 1 Pa. The inert gas atmosphere is typically an argon gas atmosphere. For the inert gas atmosphere, an 18th group element such as argon is recommended.

[0037] 《Negative electrode active material》 The negative electrode active material of the present disclosure is a product of the manufacturing method of the present disclosure. The component composition and structure of the negative electrode active material of the present disclosure will be described below.

[0038] <Component composition> The negative electrode active material obtained by the manufacturing method of the present disclosure includes a hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni, and 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.

[0039] The composition range of the hydrogen storage alloy is basically the same as the composition range of the molten metal described in "<<Method for producing a negative electrode active material>>." However, if the composition varies during the production process due to depletion of specific elements, etc., the variation in composition may be taken into consideration in advance when melting the raw materials so that the composition range of the molten metal and the composition range of the hydrogen storage alloy are the same. In the description of "<Method for producing a negative electrode active material>", "the entire molten metal" is read as "the entire hydrogen storage alloy."

[0040] <Organization> The hydrogen storage alloy used in the negative electrode active material of the present disclosure comprises a plurality of main phases and a grain boundary phase present between adjacent main phases. The main phases primarily have a hydrogen storage function. The grain boundary phases primarily have a charge / discharge activation function. The presence of the grain boundary phases between adjacent main phases promotes the migration of hydrogen stored in the main phases, thereby activating the charge / discharge of the battery.

[0041] The main phase includes 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). The form of the alloy phase is typically an intermetallic compound, but is not limited thereto. For example, part or all of the alloy phase may be in the form of a solid solution of one element with the other.

[0042] The grain boundary phase includes 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). The form of the alloy phase is typically an intermetallic compound, but is not limited thereto. For example, for part or all of the alloy phase, one may be solid-dissolved in the other.

[0043] In both the AB2 type alloy phase and the AB type alloy phase, a portion of the constituent elements may be substituted with one or more elements selected from the group consisting of Fe, Co, V, Nb, Mo, B, Mg, and Al.

[0044] 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 also be written as, for example, the "ab2 type alloy phase" and the "ab type alloy phase", respectively.

[0045] The hydrogen storage alloy in the negative electrode active material of the present disclosure is a two-phase alloy consisting of a main phase and a grain boundary phase, with the grain boundary phase sandwiched between the main phases and existing between adjacent main phases. Rapid quenching of the melt reduces the grain size of the main phase and thins the grain boundary phase. Heat treatment modifies the grain boundary phase, reducing its crystallinity. This makes the grain boundary phase even thinner. Therefore, the structural change caused by rapid quenching and heat treatment can be evaluated by the thickness of the grain boundary phase.

[0046] The structure of an actual hydrogen storage alloy has a three-dimensional shape. When a grain boundary phase is sandwiched between two main phases, it is easy to define the "thickness of the grain boundary phase" in three-dimensional space. However, as described below, the thickness of the grain boundary phase is measured using two-dimensional SEM images. In two-dimensional space, it is not easy to define the "thickness of the grain boundary phase." The "thickness of the grain boundary phase" in three-dimensional space corresponds to the "distance between adjacent main phases" in two-dimensional space. Furthermore, since measurements are taken at multiple locations within the SEM image, the structural changes due to quenching of the molten metal and heat treatment are evaluated using the "average distance between adjacent main phases."

[0047] Next, a specific method for measuring the "average distance between adjacent main phases" will be described.

[0048] The hydrogen storage alloy is cut and polished to obtain an SEM image (backscattered electron image) of the polished surface. The SEM image is then subjected to EDX area analysis. If the hydrogen storage alloy is in powder form, for example, after grinding, the powder may be embedded in resin and polished to obtain an SEM image. The magnification of the SEM image is 1000x. Since the A and B elements are nearly identical within the AB-type alloy phase, the AB-type alloy phase region can be identified from the EDX area analysis results. Furthermore, a general-purpose object detection model is used to identify the AB2-type alloy phase region from the SEM image. Based on the EDX area analysis results and the identification results using the object detection model, the AB2-type alloy phase region is defined as black, and the AB-type alloy phase region is defined as white.

[0049] The entire SEM image then appears as an irregular mesh pattern. The AB2 type alloy phase regions shown in black are granular, while the AB type alloy phase regions shown in white are shaped as a combination of multiple straight lines and / or curves. The length of the AB type alloy phase regions shown in white in the direction perpendicular to their longitudinal direction is measured at 20 points, and the average of these measurements is taken as the "average distance between adjacent main phases."

[0050] If the average distance between adjacent main phases is 1.0 μm or less, the structure is refined and the grain boundary phase is modified satisfactorily, resulting in improved discharge capacity. From this perspective, the average distance between adjacent main phases may be 0.9 μm or less, 0.8 μm or less, 0.7 μm or less, or 0.6 μm or less. If the structure is excessively refined, the discharge capacity will actually decrease, so the average distance between adjacent main phases is preferably 0.2 μm or more, 0.3 μm or more, or 0.4 μm or more.

[0051] Transformation The negative electrode active material and the method for producing the same of the present disclosure can be modified as appropriate within the scope of the claims. For example, the flakes after the heat treatment may be crushed and classified as necessary. [Example]

[0052] The negative electrode active material and the method for producing the same according to the present disclosure will be described in more detail below with reference to examples and comparative examples. Note that the negative electrode active material and the method for producing the same according to the present disclosure are not limited to the conditions used in the following examples.

[0053] <Sample Preparation> Each sample was prepared as follows.

[0054] Example 1 Raw materials containing 18.6 atomic percent Ti, 18.6 atomic percent Zr, 18.6 atomic percent Cr, 18.6 atomic percent Mn, and 25.6 atomic percent Ni were melted by high-frequency induction and strip-cast to obtain thin flakes. The cooling rate of the melt was 1×10 3 The chill roll was rotated at a surface speed of 2 m / s to achieve a temperature of 100 °C / s. High-frequency melting and strip casting were performed in an argon gas atmosphere. The purity of each of the Ti, Cr, Mn, and Ni raw materials was 99.8 mass% or higher, and the purity of the Zr raw material was 98%.

[0055] The thin pieces were heat-treated at 900°C for 2 hours. The heat treatment was carried out in a vacuum. The temperature was increased at a rate of 5°C / min up to 900°C. The 2 hours mentioned above was the holding time after the temperature reached 900°C. After holding, the pieces were cooled in the furnace.

[0056] The heat-treated flakes were 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 a 38 μm sieve and a 100 μm sieve.

[0057] Example 2 A sample of Example 2 was prepared in the same manner as in Example 1, except that the heat treatment temperature was 500°C and the heating rate up to 500°C was 5°C / min. The flakes subjected to the heat treatment were those that had been subjected to high-frequency melting and strip casting when preparing the sample of Example 1.

[0058] Comparative Example 1 A sample of Comparative Example 1 was prepared in the same manner as in Example 1, except that the flakes were not heat-treated. The flakes subjected to the heat treatment were those that had been high-frequency melted and strip-cast when preparing the sample of Example 1.

[0059] <Reference example 1> Raw materials containing 18.6 atomic percent Ti, 18.6 atomic percent Zr, 18.6 atomic percent Cr, 18.6 atomic percent Mn, and 25.6 atomic percent Ni 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 ingot's composition. The purity of each of the Ti, Cr, Mn, and Ni raw materials was 99.8 mass% or higher, and the purity of the Zr raw material was 98%.

[0060] 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.

[0061] "evaluation" Each sample was evaluated as follows.

[0062] <Component composition> The sample of Example 1 and the sample of Reference Example 1 were quantitatively analyzed by ICP. The sample of Example 1 had 18.51 atomic % Ti, 18.06 atomic % Zr, 19.45 atomic % Cr, 16.94 atomic % Mn, and 27.04 atomic % Ni. The sample of Reference Example 1 had 19.80 atomic % Ti, 17.70 atomic % Zr, 18.10 atomic % Cr, 17.00 atomic % Mn, and 27.40 atomic % Ni. It was confirmed that the variation in the composition between the raw material composition and the sample composition for both Example 1 and Reference Example 1 was within a range that does not pose any practical problems.

[0063] <Structure (average distance between adjacent main phases)> The sample of Example 1 and the sample of Reference Example 1 were subjected to SEM observation and EDX area analysis. Then, the average distance between adjacent main phases was determined using the method described above. FIG. 1A is an SEM image of the sample of Example 1. FIG. 1B is a distribution diagram of FIG. 1A, in which the AB2 type alloy phase (main phase) region is colored black and the AB type alloy phase (grain boundary phase) region is colored white. FIG. 2A is an SEM image of the sample of Reference Example 1. FIG. 2B is a distribution diagram of FIG. 2A, in which the AB2 type alloy phase (main phase) region is colored black and the AB type alloy phase (grain boundary phase) region is colored white.

[0064] In Figure 1A, the black dots are the AB2 type alloy phase (main phase), and it can be seen that the structure is extremely fine. Because the AB type alloy phase (grain boundary phase) is smaller than the AB2 type alloy phase (main phase), it is thought that not all of the AB type alloy phase (grain boundary phase) regions are shown in white in Figure 1B. In other words, it is thought that the white regions in Figure 1B represent relatively large AB type alloy phases (grain boundary phases) among the AB type alloy phases (grain boundary phases). Because the upper limit of the average distance between adjacent main phases is important, it is practically acceptable to use Figure 1B to determine the average distance between adjacent main phases.

[0065] Taking this into consideration, the average distance between adjacent main phases was determined by the above-mentioned method, and was found to be 0.6 μm for the sample of Example 1 and 2.2 μm for the sample of Reference Example 1. Furthermore, for the sample of Example 2, it was not easy to identify the region of the AB-type alloy phase (grain boundary phase), but it was confirmed that the average distance between adjacent main phases was equal to or less than that of the sample of Example 1.

[0066] <Crystal structure> XRD analysis was performed on each of the samples of Examples 1 and 2, Comparative Example 2, and Reference Example 1. The results are shown in Figure 3. As shown in Figure 3, in the samples of Examples 1 and 2 (quenched and heat-treated), some peaks of the AB type alloy phase (grain boundary phase) were not observed (see the dashed lines in Figure 3). On the other hand, in Comparative Example 1 (quenched only, no heat treatment), all peaks of the AB type alloy phase (grain boundary phase) were observed. From this, it is thought that the crystallinity of the AB type alloy phase (grain boundary phase) was reduced by the heat treatment.

[0067] <Discharge characteristics> An evaluation cell was prepared as follows.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] After the charge-discharge test, the evaluation cell was charged at 0.2 C for 7.5 hours, and then discharged at 2 C or 5 C until the negative electrode potential reached −0.5 V, and the capacity was confirmed.

[0073] The results are shown in Figure 4. There are two results for each sample (except Reference Example 1) because the test was performed twice under the same conditions. From Figure 4, it can be seen that the samples of Examples 1 and 2 have discharge capacities equal to or greater than that of the sample of Reference Example 1 (a molten alloy containing rare earth elements that was cooled at a normal rate). On the other hand, it can be seen that the sample of Comparative Example 1 (only rapid cooling, no heat treatment) has a significantly reduced discharge capacity.

Claims

1. A molten metal of a hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni was poured into a 1×10 ２ ~1 x 10 ４ Cooling at a rate of ° C. / sec to at least below 500° C. to obtain a flake; and heat treating the flakes at 500 to 900°C for 1 to 10 hours in vacuum or in an inert gas atmosphere; Including, A method for producing a negative electrode active material.

2. 2. The method for producing a negative electrode active material according to claim 1, wherein a portion of Ti, Zr, Cr, Mn, and Ni is substituted with one or more elements selected from the group consisting of Fe, Co, V, Nb, Mo, B, Mg, and Al.

3. The method for producing a negative electrode active material according to claim 1 or 2, wherein the molten metal is cooled using a strip casting method.

4. a hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni; the hydrogen storage alloy comprises a plurality of main phases and a grain boundary phase present between adjacent main phases, The main phase is AB ２ the grain boundary phase comprises 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 The average distance between adjacent main phases is 1.0 μm or less. Negative electrode active material.

5. 5. The negative electrode active material according to claim 4, wherein a portion of Ti, Zr, Cr, Mn, and Ni is substituted with one or more elements selected from the group consisting of Fe, Co, V, Nb, Mo, B, Mg, and Al.

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