Negative electrode active material and method for producing same

Abstract

The invention relates to a negative electrode active material and a method for manufacturing the same. The manufacturing method of the negative electrode active material comprises the following steps: cooling a hydrogen storage alloy melt containing Ti, Zr, Cr, Mn and Ni to 500 DEG C at a speed of 1 * 102-1 * 104 DEG C / sec to obtain a sheet; and heat treating the flakes in vacuum or in an inert gas atmosphere at 500-900 DEG C for 1-10 hours. The negative electrode active material obtained by the production method contains a hydrogen storage alloy including a plurality of main phases and grain boundary phases present between the main phases adjacent to each other, the main phases include an AB2-type alloy phase, the grain boundary phases include an AB-type alloy phase, and the average distance between the main phases adjacent to each other is 1.0 [mu] m or less.

Description

Technical Field

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

[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 and A2B7-type alloys require rare earth elements as essential elements, resulting in high resource risks. Furthermore, AB5-type alloys sometimes require a high content of expensive cobalt.

[0003] Therefore, for example, Japanese Patent Application Laid-Open No. 10-36930 discloses a hydrogen storage alloy that does not contain rare earth elements and does not require a large amount of high-cost elements such as Co. 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, and the intermetallic compound is a cubic C15 type or C14 type Laves phase. Summary of the Invention

[0004] When the hydrogen storage alloy disclosed in Japanese Patent Application Laid-Open No. 10-36930 is used as a negative electrode active material, although rare earth elements are not used and the amount of expensive elements used is small, the discharge capacity is sometimes insufficient. Therefore, the present inventors have discovered the following problem: a negative electrode active material and a method for producing the same are desired that can improve the discharge capacity with low resource risk and low cost.

[0005] 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 and a method for producing the negative electrode active material that can improve discharge capacity with low resource risk and low cost.

[0006] To achieve the above-mentioned objectives, the present inventors have conducted intensive research and have finally completed the negative electrode active material and the method for producing the same disclosed herein.

[0007] <Scheme 1> A method for producing a negative electrode active material, comprising: preparing a melt of a hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni at a temperature of 1×10 2 ~1×10 4 The method comprises cooling the film at a rate of at least 100° C. / second to less than 500° C. to obtain a thin sheet; and heat-treating the thin sheet at 500-900° C. in a vacuum or inert gas atmosphere for 1-10 hours.

[0008] <Scheme 2> The method for producing a negative electrode active material according to Scheme 1, wherein a portion of Ti, Zr, Cr, Mn, and Ni is replaced with one or more elements selected from Fe, Co, V, Nb, Mo, B, Mg, and Al.

[0009] <Scheme 3> The method for producing a negative electrode active material according to Scheme 1 or 2, wherein the molten metal is cooled using a strip casting method.

[0010] <Scheme 4> A negative electrode active material, comprising a hydrogen storage alloy containing Ti, Zr, Cr, Mn and Ni, wherein the hydrogen storage alloy includes a plurality of main phases and grain boundary phases existing between the mutually adjacent main phases, the main phases comprising an AB2-type alloy phase, and the grain boundary phases comprising an AB-type alloy phase, in which A is one or more elements selected from Ti and Zr, and B is one or more elements selected from Cr, Mn and Ni, and in which A is one or more elements selected from Ti and Zr, and B is Ni, and the average distance between the mutually adjacent main phases is 1.0 μm or less.

[0011] <Scheme 5> The negative electrode active material according to Scheme 4, wherein part of Ti, Zr, Cr, Mn, and Ni is replaced by one or more elements selected from Fe, Co, V, Nb, Mo, B, Mg, and Al.

[0012] According to the present disclosure, a molten hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni is rapidly quenched to obtain a thin sheet comprising multiple main phases and grain boundary phases between adjacent main phases, wherein the main phases and grain boundary phases are miniaturized. Heat treating the thin sheet under specified conditions modifies the grain boundary phases, resulting in a hydrogen storage alloy in which the average distance between adjacent main phases falls within a specified range. Furthermore, using this hydrogen storage alloy, it is possible to provide a negative electrode active material and a method for producing the same, which improves discharge capacity with low resource risk and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] 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:

[0014] Figure 1A This is the SEM image of the sample of Example 1.

[0015] Figure 1B For Figure 1A A distribution diagram showing the region of the AB2 type alloy phase (main phase) in black and the region of the AB type alloy phase (grain boundary phase) in white.

[0016] Figure 2AThis is the SEM image of the sample of Reference Example 1.

[0017] Figure 2B For Figure 2A A distribution diagram showing the region of the AB2 type alloy phase (main phase) in black and the region of the AB type alloy phase (grain boundary phase) in white.

[0018] Figure 3 This is a graph showing XRD analysis results for each sample of Examples 1 and 2, Comparative Example 1, and Reference Example 1.

[0019] Figure 4 This is a graph showing the relationship between current density and discharge capacity for each sample of Examples 1 and 2, Comparative Example 1, and Reference Example 1. DETAILED DESCRIPTION

[0020] The following describes in detail embodiments of the negative electrode active material and the method for producing the same according to the present disclosure. The embodiments described below do not limit the negative electrode active material and the method for producing the same according to the present disclosure.

[0021] Without being bound by theory, the following describes the findings obtained by the present inventors regarding why the negative electrode active material disclosed herein can improve discharge capacity with low resource risk and low cost.

[0022] As hydrogen storage alloys that do not contain rare earth elements and do not require a large amount of high-priced elements such as Co, alloys containing Ti, Zr, Cr, Mn and Ni can be cited. If a melt having the composition of this alloy is cooled at a normal rate and solidified, a two-phase hydrogen storage alloy is obtained, which includes an AB2-type alloy phase having a Laves structure and an AB-type alloy phase having a CsCl-type crystal structure. Moreover, since the melt is cooled at a normal rate, both the AB2-type alloy phase and the AB-type alloy phase are coarse. In addition, in this specification, unless otherwise specified, the so-called "cooling the melt at a normal rate" means, for example, arc melting the raw materials loaded into a crucible to obtain a melt, and cooling the melt directly in the crucible.

[0023] In a two-phase hydrogen storage alloy, the AB2 type alloy phase constitutes the main phase responsible for hydrogen storage, and the AB type alloy phase constitutes the grain boundary phase responsible for charge and discharge activity. A two-phase hydrogen storage alloy includes multiple main phases and grain boundary phases between adjacent main phases.

[0024] To ensure sufficient discharge capacity with a two-phase hydrogen storage alloy, the volume fraction of the AB2-type alloy phase constituting the main phase must be sufficiently maintained. Therefore, the volume fraction of the AB-type alloy phase constituting the grain boundary phase is preferably set to a maximum of approximately 30%. Furthermore, when a melt is cooled at a normal rate, the AB-type alloy phase constituting the grain boundary phase segregates.

[0025] In order to obtain sufficient charge and discharge activity even when the volume fraction of the AB type alloy phase constituting the grain boundary phase is small, that is, when the AB type alloy phase constituting the grain boundary phase is small, it is effective to quench the melt to refine the structure of the two-phase hydrogen storage alloy. As a result, the dispersion of the AB type alloy phase constituting the grain boundary phase is improved. Moreover, by heat treating the two-phase hydrogen storage alloy with a miniaturized structure under specified conditions, the AB type alloy phase constituting the grain boundary phase is modified to reduce its crystallinity, which is beneficial to the improvement of charge and discharge activity. It is believed that this is because the diffusion of hydrogen is promoted by reducing the crystallinity.

[0026] Furthermore, if the structure of the two-phase hydrogen storage alloy is refined, the distance between adjacent main phases is reduced. Furthermore, if 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, the reduction in the distance between adjacent main phases means that the grain boundary phase sandwiched between adjacent main phases becomes thinner. Furthermore, 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.

[0027] Hereinafter, a method for producing the negative electrode active material of the present disclosure based on the findings described above and essential structural features of the negative electrode active material of the present disclosure obtained thereby will be described.

[0028] 《Method for producing negative electrode active material》

[0029] The method for producing the negative electrode active material disclosed herein (hereinafter sometimes referred to as the "production method disclosed herein") includes quenching a melt and heat treatment. The quenching and heat treatment are described below.

[0030] 〈Quick cooling of melt〉

[0031] In the manufacturing method disclosed herein, a melt of a hydrogen storage alloy containing Ti (titanium), Zr (zirconium), Cr (chromium), Mn (manganese), and Ni (nickel) is prepared at a temperature of 1×10 2 ~1×10 4 The molten steel is cooled at a rate of 100°C / s to at least less than 500°C to obtain a thin sheet.

[0032] The melt only needs to contain Ti, Zr, Cr, Mn, and Ni and have a composition that forms a hydrogen storage alloy after solidification. The content ratios of Ti, Zr, Cr, Mn, and Ni can be, for example, as follows.

[0033] The content ratio of Ti, Zr, Cr, and Mn in the melt can 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, relative to the total melt. In order to maximize the volume fraction of the AB-type alloy phase constituting the grain boundary phase to approximately 30%, the content ratio of Ti, Zr, Cr, and Mn in the melt is preferably 12 to 24 atomic % relative to the total melt.

[0034] The Ni content in the melt can be 15 atomic % or more, 20 atomic % or more, 22 atomic % or more, 24 atomic % or more, or 25 atomic % or more, and can 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 maximize the volume fraction of the AB-type alloy phase constituting the grain boundary phase to approximately 30%, the Ni content in the melt is preferably 20 to 30 atomic % relative to the total melt.

[0035] A portion of the Ti, Zr, Cr, Mn, and Ni in the melt may be replaced with one or more elements selected from 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 melt is preferably less than 50 atomic %, less than 45 atomic %, less than 40 atomic %, less than 35 atomic %, less than 30 atomic %, less than 25 atomic %, less than 20 atomic %, less than 15 atomic %, less than 10 atomic %, less than 5 atomic %, or less than 0 atomic %. As long as the total content of Fe, Co, V, Nb, Mo, B, Mg, and Al relative to the entire melt is as described above, there is no adverse effect on the practical application of the production method and its product disclosed herein. The total content ratio of 0 atomic % means that part of Ti, Zr, Cr, Mn and Ni in the melt is not replaced by one or more elements selected from Fe, Co, V, Nb, Mo, B, Mg and Al.

[0036] The melt may contain elements other than Ti, Zr, Cr, Mn, Ni, Fe, Co, V, Nb, Mo, B, Mg, and Al, as long as this does not adversely affect the practical application of the manufacturing method and its products disclosed herein. The content of these 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 total melt. If multiple elements are present, the sum of their content ratios satisfies the aforementioned technical characteristics. A content ratio of 0 atomic % means that the melt 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.

[0037] That is, the melt may contain Ti, Zr, Cr, Mn, and Ni, with a portion of the Ti, Zr, Cr, Mn, and Ni optionally replaced with one or more elements selected from Fe, Co, V, Nb, Mo, B, Mg, and Al, with the remainder (the remaining portion) being unavoidable impurities. In this specification, unless otherwise specified, "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.

[0038] The above solution was mixed with 1×10 2 ~1×10 4 ℃ / second to at least less than 500℃.

[0039] If the melt is 1×10 2 ℃ / s or more, a hydrogen storage alloy with a refined structure can be obtained. From this point of view, the cooling rate of the melt can be 5×10 2 ℃ / second or more, 7×10 2 ℃ / second or more, 9×10 2 ℃ / second or more, or 1×10 3 ℃ / second or more. If the melt is heated to 1×10 4 ℃ / s or less, there will be no unnecessary high-speed cooling when the microstructure is already saturated. From this point of view, the cooling rate of the melt can be 9×10 3 ℃ / second or less, 7×10 3 ℃ / second or less, 5×10 3 ℃ / second or less, or 3×10 3 ℃ / second or less.

[0040] The melt is cooled at the above-mentioned rate to at least less than 500°C. This suppresses 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 melt solidifies, thereby suppressing the coarsening of these alloy phases. From this perspective, the melt can 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.

[0041] The method for cooling the melt is not particularly limited as long as the melt can be cooled under the above-mentioned conditions to obtain thin sheets. Typically, the melt is cooled using a strip casting method.

[0042] The strip casting method is briefly described. The raw materials are melted in a melting furnace to prepare a molten metal with the aforementioned composition. The molten metal is then fed to a tundish. The molten metal is fed by gravity from the end of the tundish to the surface of a cooling drum. The molten metal cools on the outer circumference of the rotating cooling rollers, solidifying into thin flakes that are then recovered.

[0043] The peripheral speed of the cooling roll can be appropriately determined so that the cooling rate of the melt falls within the desired range, taking into account factors such as the material of the cooling roll. The peripheral speed (circumferential velocity) of the cooling roll, expressed as the surface speed of the cooling roll, can be 0.1 m / s or higher, 0.5 m / s or higher, 1.0 m / s or higher, 1.5 m / s or higher, or 2.0 m / s or higher, and can be 10.0 m / s or lower, 7.0 m / s or lower, 4.0 m / s or lower, 3.5 m / s or lower, or 3.0 m / s or lower.

[0044] From the viewpoint of preventing oxidation of the melt and the sheet, the melt is preferably cooled in an inert gas atmosphere, particularly an argon atmosphere. The inert gas atmosphere also includes a nitrogen atmosphere.

[0045] Heat treatment

[0046] The thin sheet obtained by rapid cooling of the melt is heat treated at 500 to 900° C. for 1 to 10 hours in vacuum or in an inert gas atmosphere.

[0047] As long as 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 can be 550°C or higher, 600°C or higher, or 650°C or higher. As long as 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 will not coarsen. From this perspective, the heat treatment temperature can be 850°C or lower, 800°C or lower, 750°C or lower, or 700°C or lower.

[0048] About heat treatment time, consider heat treatment temperature etc., with the mode of the AB type alloy phase modification that will constitute grain boundary phase, determine.As long as heat treatment time is more than 1 hour, then can confirm the modification of the AB type alloy phase that constitutes grain boundary phase practically.From this viewpoint, heat treatment time can be more than 1.5 hours, more than 2 hours or more than 2.5 hours.As long as heat treatment time is below 10 hours, then the AB2 type alloy phase that constitutes main phase and the AB type alloy phase that constitutes grain boundary phase can not coarsening.From this viewpoint, heat treatment time can be below 9 hours, below 8 hours, below 7 hours, below 6 hours, below 5 hours, below 4 hours or below 3 hours.

[0049] In order to suppress oxidation of the thin film, the thin film is heat treated in a vacuum or in an inert gas atmosphere. When heat treated in a vacuum, the absolute pressure of the atmosphere can be 1×10 -7 Pa or above, 1×10 -6 Pa or above, or 1×10 -5 Pa or above, can be 1×10 -2 Pa or less, 1×10 -3 Pa or less, or 1×10 -4 Pa or less. The inert gas atmosphere is typically an argon atmosphere. Group 18 elements such as argon are recommended for the inert gas atmosphere.

[0050] Negative Electrode Active Material

[0051] The negative electrode active material disclosed herein is a product of the manufacturing method disclosed herein. The component composition and structure of the negative electrode active material disclosed herein are described below.

[0052] 〈Ingredients〉

[0053] The negative electrode active material obtained by the manufacturing method of the present disclosure comprises a hydrogen storage alloy containing Ti, Zr, Cr, Mn, and Ni. Furthermore, a portion of the Ti, Zr, Cr, Mn, and Ni may be substituted with one or more elements selected from Fe, Co, V, Nb, Mo, B, Mg, and Al.

[0054] The composition range of the hydrogen storage alloy is essentially the same as the composition range of the melt described in the "Method for Producing Negative Electrode Active Materials." However, if the composition fluctuates during the manufacturing process due to consumption of specific elements, the composition fluctuations can be accounted for when melting the raw materials so that the composition range of the melt is consistent with that of the hydrogen storage alloy. In the description of the "Method for Producing Negative Electrode Active Materials," "the entire melt" can be replaced with "the entire hydrogen storage alloy."

[0055] <organize>

[0056] The hydrogen storage alloy used as the negative electrode active material of the present disclosure comprises multiple main phases and grain boundary phases present between adjacent main phases. The main phases primarily function as hydrogen storage. The grain boundary phases primarily function as charge and discharge activation. The presence of the grain boundary phases between adjacent main phases promotes the movement of hydrogen stored in the main phases, activating the battery's charge and discharge.

[0057] The main phase comprises an AB2-type alloy phase. "AB2-type alloy phase" refers to an alloy phase consisting of one mole of element "A" and two moles of element "B." 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). The alloy phase typically takes the form of an intermetallic compound, but is not limited thereto. For example, one of A and B may be dissolved in the other of A and B in a portion or all of the alloy phase.

[0058] The grain boundary phase includes an AB-type alloy phase. "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). The alloy phase typically takes the form of an intermetallic compound, but is not limited to this. For example, one of A and B may be dissolved in the other of A and B in part or all of the alloy phase.

[0059] Part of the constituent elements of the AB2 type alloy phase and the AB type alloy phase may be replaced by one or more elements selected from Fe, Co, V, Nb, Mo, B, Mg and Al.

[0060] It should be noted that "AB2 type alloy phase" and "AB type alloy phase" are both recorded according to the previous method of notating alloy phases in hydrogen storage alloys. In cases where there is a possibility of confusion with element B (boron), "AB2 type alloy phase" and "AB type alloy phase" can be expressed as, for example, "ab2 type alloy phase" and "ab type alloy phase", respectively.

[0061] The hydrogen storage alloy in the negative electrode active material disclosed herein is a two-phase alloy of a main phase and a grain boundary phase, with a grain boundary phase sandwiched between the main phases, and the grain boundary phase exists between adjacent main phases. By quenching the melt, the particle size of the main phase becomes smaller and the grain boundary phase becomes thinner. By heat treatment, the grain boundary phase is modified and the crystallinity is reduced. As a result, the grain boundary phase becomes thinner. Therefore, the structural changes caused by quenching the melt and heat treatment can be evaluated by the thickness of the grain boundary phase.

[0062] The structure of an actual hydrogen storage alloy has a three-dimensional shape. When a grain boundary phase is sandwiched between the main phases, it is easy to define the "thickness of the grain boundary phase" in three-dimensional space. However, as described later, the thickness of the grain boundary phase is measured using a two-dimensional SEM image. It is not easy to define the "thickness of the grain boundary phase" in two-dimensional space. The "thickness of the grain boundary phase" in three-dimensional space is equivalent to the "distance between adjacent main phases" in two-dimensional space. In addition, the measurement is performed at multiple locations in the SEM image, so the structural changes caused by rapid cooling of the melt and heat treatment are evaluated by the "average distance between adjacent main phases."

[0063] Next, a specific method for measuring the “average distance between mutually adjacent main phases” will be described.

[0064] The hydrogen storage alloy is cut and ground to obtain an SEM image (reflected electron image) of its ground surface. Then, EDX surface analysis is performed on its SEM image. When the hydrogen storage alloy is powdered by crushing, the powder can be resin-embedded and ground to obtain an SEM image. The magnification of the SEM image is 1000 times. In the AB type alloy phase, since the A element and the B element are roughly the same, the area of the AB type alloy phase can be distinguished by the EDX surface analysis results. In addition, using a general object detection model, the area of the AB2 type alloy phase is distinguished by the SEM image. Then, based on the EDX surface analysis results and the discrimination results using the object detection model, the area of the AB2 type alloy phase is defined in black, and the area of the AB type alloy phase is defined in white.

[0065] Thus, the SEM image as a whole shows an irregular mesh pattern. The region of the AB2 type alloy phase represented by black is granular, and the region of the AB type alloy phase represented by white is a shape composed of a combination of multiple straight lines and / or curves. The length of the region of the AB type alloy phase represented by white in the direction perpendicular to the longitudinal direction was measured at 20 points, and the average of these measured values was defined as the "average distance between adjacent main phases."

[0066] As long as the average distance between adjacent main phases is 1.0 μm or less, both microstructure refinement and grain boundary phase modification are favorable, resulting in improved discharge capacity. From this perspective, the average distance between adjacent main phases can be 0.9 μm or less, 0.8 μm or less, 0.7 μm or less, or 0.6 μm or less. Excessive microstructure refinement can lead to a decrease in discharge capacity, 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.

[0067] Transformation

[0068] The negative electrode active material and its manufacturing method disclosed herein can be modified appropriately within the scope of the claims. For example, the heat-treated flakes can be crushed and classified as needed.

[0069] The negative electrode active material and its production method disclosed herein are described in more detail below using examples and comparative examples. Note that the negative electrode active material and its production method disclosed herein are not limited to the conditions used in the following examples.

[0070] Preparation of Samples

[0071] Each sample was prepared according to the following procedures.

[0072] <Example 1>

[0073] The raw material containing 18.6 atomic % of Ti, 18.6 atomic % of Zr, 18.6 atomic % of Cr, 18.6 atomic % of Mn, and 25.6 atomic % of Ni was melted by high frequency and thin sheets were obtained by strip casting. 3 The cooling rolls were rotated at a surface speed of 2 m / s at a rate of 100°C / s. High-frequency melting and strip casting were performed in an argon atmosphere. The purity of the raw materials for Ti, Cr, Mn, and Ni was 99.8% by mass or higher, and the purity of the raw material for Zr was 98%.

[0074] The sheet was heat treated at 900°C for 2 hours. The heat treatment was performed in a vacuum. The heating rate to 900°C was 5°C / minute. The 2 hours mentioned above is the holding time after reaching 900°C. Furthermore, the sheet was furnace-cooled after the holding time.

[0075] 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 38 to 100 μm using a 38 μm sieve and a 100 μm sieve.

[0076] <Example 2>

[0077] The 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 to 500°C was 5°C / min. Furthermore, the sheet subjected to the high-frequency melting and strip casting process used in preparing the sample of Example 1 was used for the heat treatment.

[0078] <Comparative Example 1>

[0079] 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 heat treatment were those subjected to high-frequency melting and strip casting in preparing the sample of Example 1.

[0080] Reference Example 1

[0081] 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 melt. This melt was then cooled and solidified within the crucible to produce an ingot. This melting and cooling cycle was repeated three times within the crucible to homogenize the ingot's composition. The purity of each of the raw materials for Ti, Cr, Mn, and Ni was 99.8% by mass or higher, and the purity of the raw material for Zr was 98%.

[0082] The ingot was pulverized using a manual Stamp Mill DA-30 manufactured by Tokyo Glass Instruments Co., Ltd., and the obtained powder was classified into 38 to 100 μm using a 38 μm sieve and a 100 μm sieve.

[0083] "evaluate"

[0084] Each sample was evaluated as follows.

[0085] 〈Ingredients〉

[0086] Quantitative analysis was performed using ICP on the samples of Example 1 and Reference Example 1. In the sample of Example 1, Ti was 18.51 atomic%, Zr was 18.06 atomic%, Cr was 19.45 atomic%, Mn was 16.94 atomic%, and Ni was 27.04 atomic%. In the sample of Reference Example 1, Ti was 19.80 atomic%, Zr was 17.70 atomic%, Cr was 18.10 atomic%, Mn was 17.00 atomic%, and Ni was 27.40 atomic%. It was confirmed that the variations in the composition between the raw material composition and the sample composition in both the samples of Example 1 and Reference Example 1 were within a range that does not pose a practical problem.

[0087] <Organization (average distance between adjacent main phases)>

[0088] SEM observation and EDX surface analysis were performed on the sample of Example 1 and the sample of Reference Example 1. Then, the average distance between adjacent main phases was determined using the above-mentioned method. Figure 1A This is the SEM image of the sample of Example 1. Figure 1B For Figure 1A A distribution diagram showing the region of the AB2 type alloy phase (main phase) in black and the region of the AB type alloy phase (grain boundary phase) in white. Figure 2A This is the SEM image of the sample of Reference Example 1. Figure 2B For Figure 2AA distribution diagram showing the region of the AB2 type alloy phase (main phase) in black and the region of the AB type alloy phase (grain boundary phase) in white.

[0089] exist Figure 1A In the figure, the black dotted part is the AB2 type alloy phase (main phase), and it can be understood that the structure is very fine. Compared with the AB2 type alloy phase (main phase), the AB type alloy phase (grain boundary phase) is smaller, so it is believed that Figure 1B In FIG, not all the regions of the AB type alloy phase (grain boundary phase) are shown in white. Figure 1B In the figure, the white area shows the relatively large AB type alloy phase (grain boundary phase) in the AB type alloy phase (grain boundary phase). Since the upper limit of the average distance between the adjacent main phases is important, the upper limit of the average distance between the adjacent main phases is used. Figure 1B There is no practical problem in obtaining the average distance between mutually adjacent main phases.

[0090] Taking this into consideration, the average distance between adjacent main phases was determined using the above-mentioned method. The result was 0.6 μm for the sample of Example 1 and 2.2 μm for the sample of Reference Example 1. Furthermore, although the region of the AB-type alloy phase (grain boundary phase) was more difficult to identify in the sample of Example 2, the average distance between adjacent main phases was confirmed to be equal to or less than that of the sample of Example 1.

[0091] Crystal structure

[0092] XRD analysis was performed on the samples of Examples 1 and 2, Comparative Example 2, and Reference Example 1. The results are shown in Figure 3 .like Figure 3 As shown, for the samples of Examples 1 and 2 (quenching and heat treatment), the peaks of a part of the AB type alloy phase (grain boundary phase) were not confirmed (refer to Figure 3 On the other hand, in Comparative Example 1 (only rapid cooling without heat treatment), all peaks of the AB type alloy phase (grain boundary phase) were confirmed. Therefore, it is believed that the crystallinity of the AB type alloy phase (grain boundary phase) is reduced by heat treatment.

[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. Furthermore, 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 to 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 rolled at 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, a negative electrode (working electrode), a positive electrode (counter electrode), and a 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 conducted. First, break-in charge and discharge were 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 at 0.1 C discharge is saturated, and the resulting electrode capacity is taken as the initial capacity.

[0103] After the trial charge and discharge, the evaluation battery cells were charged at 0.2C for 7.5 hours and then discharged at 2C or 5C until the negative electrode potential reached -0.5V, and the capacity was checked.

[0104] The results are shown in Figure 4 Each sample (excluding Reference Example 1) has two results because two tests were conducted under the same conditions. Figure 4 It can be seen that the samples of Examples 1 and 2 achieved discharge capacities equivalent to or greater than those of the sample of Reference Example 1 (a product obtained by cooling a melt containing rare earth elements at a normal rate). On the other hand, it can be seen that the discharge capacity of the sample of Comparative Example 1 (which was subjected to only rapid cooling without heat treatment) was significantly reduced.

Claims

1. A method for producing a negative electrode active material, comprising: The melt of hydrogen storage alloy containing Ti, Zr, Cr, Mn and Ni was heated to 1×10 2 ~1×10 4 ° C / sec at least to less than 500 ° C, to obtain a thin sheet; and The sheet is heat-treated at 500 to 900° C. for 1 to 10 hours in vacuum or in an inert gas atmosphere.

2. The method for producing a negative electrode active material according to claim 1, wherein: Part of Ti, Zr, Cr, Mn, and Ni is replaced 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 melt is cooled using a strip casting method.

4. A negative electrode active material comprising a hydrogen storage alloy containing Ti, Zr, Cr, Mn and Ni, The hydrogen storage alloy includes a plurality of main phases and grain boundary phases existing between the main phases adjacent to each other. The main phase comprises an AB2 type alloy phase, and the grain boundary phase comprises 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 average distance between the adjacent main phases is 1.0 μm or less.

5. The negative electrode active material according to claim 4, wherein Part of Ti, Zr, Cr, Mn, and Ni is replaced by one or more elements selected from Fe, Co, V, Nb, Mo, B, Mg, and Al.

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