Hydrogen storage alloy, negative electrode for alkaline storage battery, and alkaline storage battery

Abstract

To provide a hydrogen storage alloy which enables easy reduction of raw material cost and is less likely to crack under repeated hydrogen absorption and desorption, and to provide a negative electrode for an alkaline storage battery and an alkaline storage battery employing the hydrogen storage alloy.SOLUTION: The hydrogen storage alloy has a chemical composition comprising Ti: 5 at% or more and 35 at% or less, Zr: 5 at% or more and 35 at% or less, Ni: 5 at% or more and 35 at% or less, Cr: 5 at% or more and 35 at% or less, and Mn: 5 at% or more and 35 at% or less, and one or two elements selected from the group consisting of Si: 0.50 at% or more and 10 at% or less and B: 0.50 at% or more and 10 at% or less, and a metallographic structure comprising a C14 phase having a C14-type crystal structure and a B2 phase having a B2-type crystal structure, where the proportion of the C14 phase is the highest. A mixing entropy ΔSmix of the hydrogen storage alloy is 1.5R or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a hydrogen storage alloy, a negative electrode for an alkaline storage battery, and an alkaline storage battery. [Background technology]

[0002] Conventionally, hydrogen storage alloys have been widely used as negative electrode active materials in alkaline storage batteries such as nickel-metal hydride batteries. In recent years, development of technology using hydrogen storage alloys to store hydrogen at hydrogen stations for supplying hydrogen to fuel cell vehicles and the like has been progressing.

[0003] A hydrogen storage alloy is an alloy composed of an element A having a high affinity for hydrogen and an element B having a low affinity for hydrogen, and known types include AB5 type alloys, AB2 type alloys, and AB type alloys. For example, an AB5 type hydrogen storage alloy containing a rare earth mixed metal has already been put to practical use as a negative electrode active material for nickel-metal hydride batteries (Patent Document 1).

[0004] However, the hydrogen storage capacity of the hydrogen storage alloy in Patent Document 1 is approximately 1.2 mass%, and from the perspective of improving the performance of alkaline storage batteries, such as the energy density, and increasing the amount of hydrogen stored in hydrogen stations, a hydrogen storage alloy with a higher hydrogen storage capacity than the hydrogen storage alloy in Patent Document 1 is desired. Furthermore, the rare earth mixed metal contained in the hydrogen storage alloy in Patent Document 1 is relatively expensive and is produced in uneven regions, so its price is prone to fluctuate depending on changes in social conditions, etc. Therefore, a hydrogen storage alloy that does not use expensive or rare metals, or that uses relatively small amounts of these metals, is desired.

[0005] Therefore, for example, Patent Document 2 describes a high-entropy hydrogen storage alloy having a chemical composition consisting of Ti: 5 atomic % to 35 atomic % inclusive, Zr: 5 atomic % to 35 atomic % inclusive, Ni: 5 atomic % to 35 atomic % inclusive, Cr: 5 atomic % to 35 atomic % inclusive, and Mn: 5 atomic % to 35 atomic % inclusive. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-167375 [Patent Document 2] International Publication No. 2022 / 250093 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the hydrogen storage alloy of Patent Document 2 has a problem in that when hydrogen absorption and desorption are repeatedly performed, cracks tend to occur in the hydrogen storage alloy as the number of repetitions increases.

[0008] The present invention has been made in view of the above background, and aims to provide a hydrogen storage alloy that can easily reduce raw material costs and is resistant to cracking when hydrogen is repeatedly absorbed and released, as well as a negative electrode for an alkaline storage battery and an alkaline storage battery that use this hydrogen storage alloy. [Means for solving the problem]

[0009] One aspect of the present invention is a composition comprising a chemical component consisting of one or two elements selected from the group consisting of Ti (titanium): 5 atomic % to 35 atomic % inclusive, Zr (zirconium): 5 atomic % to 35 atomic % inclusive, Ni (nickel): 5 atomic % to 35 atomic % inclusive, Cr (chromium): 5 atomic % to 35 atomic % inclusive, and Mn (manganese): 5 atomic % to 35 atomic % inclusive, and Si (silicon): 0.50 atomic % to 10 atomic % inclusive, and B (boron): 0.50 atomic % to 10 atomic % inclusive; a metal structure including a C14 phase having a C14 type crystal structure and a B2 phase having a B2 type crystal structure, the metal structure having the highest constituent ratio of the C14 phase; The mixing entropy ΔS is expressed by the following formula (1): mix The hydrogen storage alloy has a resistance of 1.5R or more.

[0010]

number

[0011] In the above formula (1), R is the gas constant, and x i is the mole fraction of each element contained in the hydrogen storage alloy. [Effects of the Invention]

[0012] The hydrogen storage alloy has a high mixing entropy ΔS of 1.5 times or more the gas constant by setting the contents of Ti, Zr, Ni, Cr, Mn, Si and B within the above-mentioned specific ranges. mix The hydrogen storage alloy can achieve the above-mentioned specific metal structure containing the C14 phase and the B2 phase. A hydrogen storage alloy having such chemical composition and metal structure has a high hydrogen storage capacity and can easily release the stored hydrogen. Furthermore, the hydrogen storage alloy contains at least one of Si and B in addition to Ti, Zr, Ni, Cr, and Mn, thereby suppressing the occurrence of cracks when hydrogen is repeatedly absorbed and released.

[0013] Furthermore, the elements contained in the hydrogen storage alloy are all relatively uniformly distributed in different regions. Furthermore, in the hydrogen storage alloy, the specific metal structure can be easily achieved as long as the content of each element is within the specific range. This makes it easy to reduce the amounts of Zr and Ni, which are relatively expensive among the elements mentioned above. Therefore, the hydrogen storage alloy makes it easy to reduce raw material costs.

[0014] As a result, according to the above-described embodiment, it is possible to provide a hydrogen storage alloy that is easy to reduce raw material costs and that is less likely to crack when hydrogen is repeatedly absorbed and released. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is an explanatory diagram showing the X-ray diffraction patterns of alloys A1, A4, A7, and B1 in Example 1. [Figure 2]FIG. 2 is an explanatory diagram showing the particle size distribution of alloy A4 in Example 1. [Figure 3] FIG. 3 is an explanatory diagram showing the particle size distribution of alloy B1 in Example 1. [Figure 4] FIG. 4 is a partial cross-sectional view showing a main part of the negative electrode for an alkaline storage battery in Example 2. As shown in FIG. [Figure 5] FIG. 5 is an explanatory diagram showing the results of the charge-discharge cycle test of specimens C4 and D1 in Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0016] (hydrogen storage alloy) The hydrogen storage alloy has a chemical composition containing Ti and Zr as elements A and Ni, Cr, and Mn as elements B as main constituent elements, and further contains at least one of Si and B. The entropy of mixing ΔS of the hydrogen storage alloy is expressed by the following formula (1): mix is 1.5R or more.

[0017]

number

[0018] In the above formula (1), R is the gas constant, and x i is the mole fraction of each element contained in the hydrogen storage alloy.

[0019] Alloys with a high mixing entropy of 1.5R or more, such as the hydrogen storage alloy, have properties different from those of ordinary alloys. For example, the mixing entropy ΔS of the hydrogen storage alloy is mix The height of the nucleus results in disordered atomic arrangement within the crystal structure. A hydrogen storage alloy with this characteristic atomic arrangement not only has a high hydrogen storage capacity but also can easily release the stored hydrogen even in a room temperature environment.

[0020] The hydrogen storage alloy has a metal structure containing a C14 phase having a C14 type crystal structure and a B2 phase having a B2 type crystal structure, with the C14 phase having the highest composition ratio. Such a metal structure is determined by the element contents and the mixing entropy ΔS mix This can be easily achieved by adjusting the chemical components to fall within the specific ranges. The "C14 type crystal structure" is identical to the hexagonal MgZn2 type structure. The "B2 type crystal structure" is identical to the cubic CsCl type structure.

[0021] The composition ratio of the crystalline phases contained in the hydrogen storage alloy can be determined based on the X-ray diffraction chart of the hydrogen storage alloy. More specifically, the mass ratio of each crystalline phase present in the hydrogen storage alloy can be estimated by performing Rietveld analysis on the X-ray diffraction chart of the hydrogen storage alloy.

[0022] The proportion of the C14 phase in the hydrogen storage alloy is preferably 75% by mass or more, and more preferably 80% by mass or more. By increasing the proportion of the C14 phase in this manner, a higher hydrogen storage capacity than that of a typical AB5-type rare earth nickel-based hydrogen storage alloy can be easily achieved. Furthermore, when such a hydrogen storage alloy is used as an active material for a negative electrode of an alkaline storage battery, it can easily achieve a high discharge capacity.

[0023] The composition ratio of the B2 phase in the hydrogen storage alloy is preferably more than 0 mass% and not more than 20 mass%. By setting the composition ratio of the B2 phase within the specific range, the effect of increasing the hydrogen storage capacity can be more reliably obtained. In this case, the hydrogen absorbed in the hydrogen storage alloy is more likely to be released to the outside of the hydrogen storage alloy. Therefore, such a hydrogen storage alloy is suitable, for example, as a hydrogen storage material in hydrogen stations or as an active material for negative electrodes of alkaline storage batteries.

[0024] From the viewpoint of more reliably obtaining the above-mentioned effects, the composition ratio of the B2 phase in the hydrogen storage alloy is more preferably 0.5% by mass or more, even more preferably 1.0% by mass or more, particularly preferably 1.5% by mass or more, and most preferably 2.0% by mass or more. On the other hand, from the viewpoint of further increasing the hydrogen storage capacity of the hydrogen storage alloy, the composition ratio of the B2 phase in the hydrogen storage alloy is more preferably 19% by mass or less, even more preferably 18% by mass or less, even more preferably 16% by mass or less, particularly preferably less than 10% by mass, and most preferably 5.0% by mass or less.

[0025] When determining a preferred range of the B2 phase content, the above-described upper and lower limits of the B2 phase content can be combined arbitrarily. For example, the preferred range of the B2 phase content may be 0.5% by mass to 20% by mass, 1.0% by mass to 20% by mass, 1.5% by mass to 20% by mass, 2.0% by mass to 20% by mass, or 2.0% by mass to 18% by mass.

[0026] The proportion of the B2 phase in the hydrogen storage alloy may be 10% by mass or more and 20% by mass or less. When used as a negative electrode active material for an alkaline storage battery, such a hydrogen storage alloy can further increase the discharge capacity. Furthermore, in this case, the decrease in discharge capacity during repeated charge-discharge cycles is suppressed, and a high discharge capacity can be maintained for a long period of time. Therefore, a hydrogen storage alloy having a proportion of the B2 phase within the specific range is suitable, for example, as a negative electrode active material for alkaline storage batteries. From the viewpoint of further increasing the discharge capacity of an alkaline storage battery while suppressing the decrease in discharge capacity during repeated charge-discharge cycles, the proportion of the B2 phase in the hydrogen storage alloy is preferably 10% by mass or more and 19% by mass or less, more preferably 10% by mass or more and 18% by mass or less, and even more preferably 10% by mass or more and 16% by mass or less.

[0027] The Ti content in the hydrogen storage alloy is preferably 10 atomic % to 33 atomic % and more preferably 13 atomic % to 28 atomic %. In this case, the C14 phase is more likely to form in the hydrogen storage alloy. From the same viewpoint, the Zr content in the hydrogen storage alloy is preferably 10 atomic % to 30 atomic % and more preferably 13 atomic % to 25 atomic %. The Ni content in the hydrogen storage alloy is preferably 10 atomic % to 33 atomic % and more preferably 15 atomic % to 30 atomic %. The Cr content in the hydrogen storage alloy is preferably 10 atomic % to 30 atomic % and more preferably 15 atomic % to 25 atomic %. The Mn content in the hydrogen storage alloy is preferably 10 atomic % to 30 atomic % and more preferably 13 atomic % to 25 atomic %.

[0028] The hydrogen storage alloy contains, in addition to the five main elements mentioned above, one or two elements selected from the group consisting of Si: 0.50 atomic % to 10 atomic % and B: 0.50 atomic % to 10 atomic %. By adding Si and / or B to the hydrogen storage alloy at the specific content, cracking of the hydrogen storage alloy during repeated hydrogen absorption and desorption can be suppressed. Furthermore, by making the hydrogen storage alloy less likely to crack, the charge-discharge cycle characteristics can be improved when the hydrogen storage alloy is used as the active material of an alkaline storage battery.

[0029] To more reliably obtain the above-mentioned effects, the Si content in the hydrogen storage alloy is preferably 0.60 atomic % or more, more preferably 0.70 atomic % or more, and even more preferably 0.80 atomic % or more. From the same viewpoint, the B content in the hydrogen storage alloy is preferably 0.60 atomic % or more, more preferably 0.70 atomic % or more, and even more preferably 0.80 atomic % or more. If the Si content and B content in the hydrogen storage alloy are both less than 0.50 atomic %, it may be difficult to suppress cracking of the hydrogen storage alloy.

[0030] On the other hand, if the Si content in the hydrogen storage alloy is too high, it may result in a decrease in the hydrogen storage capacity. By setting the Si content in the hydrogen storage alloy to 10 atomic % or less, preferably 7.0 atomic % or less, more preferably 6.0 atomic % or less, even more preferably 4.0 atomic % or less, particularly preferably 2.0 atomic % or less, and most preferably 1.5 atomic % or less, this problem can be easily avoided and a hydrogen storage alloy with a high hydrogen storage capacity can be easily obtained.

[0031] Similarly, by setting the B content in the hydrogen storage alloy to 10 atomic % or less, preferably 7.0 atomic % or less, more preferably 6.0 atomic % or less, even more preferably 4.0 atomic % or less, particularly preferably 2.0 atomic % or less, and most preferably 1.5 atomic % or less, a hydrogen storage alloy exhibiting a high hydrogen storage capacity can be easily obtained.

[0032] In determining a preferred range of the Si content in the hydrogen storage alloy, the above-mentioned upper and lower limits of the Si content can be combined arbitrarily. For example, the preferred range of the Si content may be 0.50 atomic % to 8.0 atomic %, 0.50 atomic % to 6.0 atomic %, 0.50 atomic % to 4.0 atomic %, 0.60 atomic % to 4.0 atomic %, 0.70 atomic % to 4.0 atomic %, 0.70 atomic % to 2.0 atomic %, 0.70 atomic % to 1.5 atomic %, or 0.80 atomic % to 1.5 atomic %.

[0033] Similarly, when determining a preferred range of the B content in the hydrogen storage alloy, any combination of the above-mentioned upper and lower limits of the B content can be used. For example, the preferred range of the B content may be 0.50 atomic % to 8.0 atomic %, 0.50 atomic % to 6.0 atomic %, 0.50 atomic % to 4.0 atomic %, 0.60 atomic % to 4.0 atomic %, 0.70 atomic % to 4.0 atomic %, 0.70 atomic % to 2.0 atomic %, 0.70 atomic % to 1.5 atomic %, or 0.80 atomic % to 1.5 atomic %.

[0034] The reason why Si and B suppress cracking of the hydrogen storage alloy is thought to be, for example, as follows: A hydrogen storage alloy expands when it absorbs hydrogen and contracts when it releases the absorbed hydrogen. Therefore, when hydrogen is repeatedly absorbed into and released from a hydrogen storage alloy, expansion and contraction are repeated. As a result, it is thought that cracks occur in the hydrogen storage alloy.

[0035] In contrast, Si in the hydrogen storage alloy tends to be unevenly distributed in the C14 phase. It is also believed that Si dissolves in the C14 phase by substituting for Ni. The C14 phase with dissolved Si is solid-solution strengthened, which is believed to improve its strength and toughness. Therefore, it is believed that the hydrogen storage alloy containing Si becomes less susceptible to cracking due to the improved strength and toughness of the C14 phase.

[0036] Furthermore, B in the hydrogen storage alloy, together with Zr, easily forms a C32 phase having a C32-type crystal structure. The C32 phase in the hydrogen storage alloy is precipitation strengthened. Therefore, it is believed that the hydrogen storage alloy containing B becomes less susceptible to cracking due to precipitation strengthening.

[0037] When the hydrogen storage alloy contains B, it is preferable that a C32 phase having a C32-type crystal structure is formed in the hydrogen storage alloy. In this case, the effect of suppressing cracking of the hydrogen storage alloy can be more reliably obtained. Note that the "C32-type crystal structure" is the same as the hexagonal AlB2-type structure.

[0038] The hydrogen storage alloy may further contain Al (aluminum): 0.50 atomic % to 10 atomic %. In this case, the effect of suppressing cracking of the hydrogen storage alloy can be further enhanced. From the viewpoint of further enhancing this effect, the Al content in the hydrogen storage alloy is preferably 0.50 atomic % to 8.0 atomic %, more preferably 0.50 atomic % to 6.0 atomic %, even more preferably 0.50 atomic % to 4.0 atomic %, particularly preferably 0.50 atomic % to 2.0 atomic %, and most preferably 0.70 atomic % to 2.0 atomic %.

[0039] The reason why Al suppresses cracking of hydrogen storage alloys is thought to be, for example, as follows. That is, Al in the hydrogen storage alloy tends to be unevenly distributed in the B2 phase. It is also thought that Al is solid-dissolved in the B2 phase by substituting for Mn. It is thought that the strength and toughness of the B2 phase in which Al is solid-dissolved are improved by solid-solution strengthening. Therefore, it is thought that the hydrogen storage alloy containing Al becomes less likely to crack due to the improved strength and toughness of the B2 phase.

[0040] From the viewpoint of further enhancing the effect of suppressing cracking of the hydrogen storage alloy, it is preferable that the hydrogen storage alloy contains both Si: 0.50 atomic % to 10 atomic % and Al: 0.50 atomic % to 10 atomic %, more preferable that it contains both Si: 0.50 atomic % to 6.0 atomic % and Al: 0.50 atomic % to 6.0 atomic %, even more preferable that it contains both Si: 0.50 atomic % to 4.0 atomic % and Al: 0.50 atomic % to 4.0 atomic %, and particularly preferable that it contains both Si: 0.70 atomic % to 1.5 atomic % and Al: 0.70 atomic % to 2.0 atomic %.

[0041] In determining the chemical composition of the hydrogen storage alloy, the preferred ranges for each element described above can be combined arbitrarily. For example, the hydrogen storage alloy may have a chemical composition consisting of one or two elements selected from the group consisting of Ti: 10 atomic % to 33 atomic %; Zr: 10 atomic % to 30 atomic %; Ni: 10 atomic % to 33 atomic %; Cr: 10 atomic % to 30 atomic %; and Mn: 10 atomic % to 30 atomic %; and Si: 0.50 atomic % to 10 atomic %; and B: 0.50 atomic % to 10 atomic %. The hydrogen storage alloy may have a chemical composition consisting of one or two elements selected from the group consisting of Ti: 13 atomic % to 28 atomic %; Zr: 15 atomic % to 25 atomic %; Ni: 15 atomic % to 30 atomic %; Cr: 10 atomic % to 30 atomic %; and Mn: 13 atomic % to 25 atomic %; and Si: 0.50 atomic % to 6.0 atomic %; and B: 0.50 atomic % to 6.0 atomic %.

[0042] The hydrogen storage alloy may also have a chemical composition consisting of Ti: 13 atomic % to 28 atomic %; Zr: 15 atomic % to 25 atomic %; Ni: 15 atomic % to 30 atomic %; Cr: 10 atomic % to 30 atomic %; Mn: 13 atomic % to 25 atomic %; Si: 0.50 atomic % to 6.0 atomic %; and Al: 0.50 atomic % to 6.0 atomic %.

[0043] From the viewpoint of improving the charge / discharge cycle characteristics of an alkaline storage battery and further improving the discharge capacity, the hydrogen storage alloy preferably has a chemical composition consisting of Ti: 13 atomic % to 28 atomic %; Zr: 15 atomic % to 25 atomic %; Ni: 15 atomic % to 30 atomic %; Cr: 10 atomic % to 30 atomic %; Mn: 13 atomic % to 25 atomic %; Si: 0.70 atomic % to 1.5 atomic %; and Al: 0.70 atomic % to 2.0 atomic %.

[0044] In addition to the elements described above, the hydrogen storage alloy may contain impurities that are inevitably mixed in during the manufacturing process. The content of these impurities should be 0.2 atomic % or less for each element, and 2.0 atomic % or less in total.

[0045] The hydrogen storage alloy preferably has static hydrogen storage characteristics in which the plateau pressure during the hydrogen absorption process and the plateau pressure during the hydrogen desorption process are 0.005 MPa or more and 0.50 MPa or less, respectively, in a pressure-composition isotherm obtained in an environment at a temperature of 30°C. A hydrogen storage alloy with such characteristics allows for easy control of the hydrogen pressure during the hydrogen absorption process and the hydrogen desorption process when used as a hydrogen storage material in a hydrogen station or as an active material for a negative electrode of an alkaline storage battery. Therefore, such a hydrogen storage alloy is more practical.

[0046] Furthermore, the hydrogen storage alloy preferably has static hydrogen storage characteristics such that the ratio of the plateau pressure during the hydrogen absorption process to the plateau pressure during the hydrogen desorption process is 0.5 to 3.0 times in a pressure-composition isotherm obtained in an environment at a temperature of 30°C. By using such a hydrogen storage alloy with small hysteresis as a hydrogen storage material in a hydrogen station, the amount of hydrogen supplied to the hydrogen storage alloy during the hydrogen absorption process and the amount of hydrogen released from the hydrogen storage alloy during the hydrogen desorption process can be controlled in the same way. Therefore, such a hydrogen storage alloy is more practical.

[0047] The hydrogen storage alloy has chemical components that are substantially free of Fe (iron). That is, the content of Fe in the hydrogen storage alloy is equal to or less than the content of unavoidable impurities. Fe may form Fe(OH)3 when it comes into contact with an alkaline aqueous solution such as the electrolyte of an alkaline storage battery. Because Fe(OH)3 is an insulator, when a hydrogen storage alloy containing Fe is used as an active material for a negative electrode of an alkaline storage battery, the electronic conductivity of the active material may decrease, leading to a deterioration in discharge rate characteristics. In contrast, because the hydrogen storage alloy does not contain Fe, Fe(OH)3 is not formed even when the hydrogen storage alloy comes into contact with an alkaline aqueous solution.

[0048] Furthermore, when a hydrogen storage alloy containing Mn is used as the active material for a negative electrode of an alkaline storage battery, Mn may dissolve into the electrolyte, causing the composition of the hydrogen storage alloy to change from the intended composition. In particular, repeated charging and discharging can cause cracks in the hydrogen storage alloy, increasing the contact area with the electrolyte and facilitating the dissolution of Mn into the electrolyte. In contrast, the hydrogen storage alloy is less likely to crack even when repeatedly storing and releasing hydrogen. Therefore, when such a hydrogen storage alloy is used as the active material for a negative electrode of an alkaline storage battery, Mn is less likely to dissolve into the electrolyte upon contact with the electrolyte, even during repeated charging and discharging. Therefore, using the hydrogen storage alloy as the active material for a negative electrode of an alkaline storage battery can improve charge-discharge cycle characteristics and suppress a decrease in discharge capacity even during repeated charging and discharging.

[0049] As described above, the hydrogen storage alloy has excellent charge-discharge cycle characteristics and discharge rate characteristics, and is therefore suitable as an active material for the negative electrode of an alkaline storage battery.

[0050] (Method of manufacturing hydrogen storage alloy) The hydrogen storage alloy can be easily produced, for example, by simply casting an alloy having the specific chemical composition. The method for melting the hydrogen storage alloy is not particularly limited, and various melting furnaces, such as a vacuum high-frequency melting furnace, can be used. Furthermore, various methods, such as a strip casting method, can be used to cast the hydrogen storage alloy.

[0051] Immediately after casting, hydrogen storage alloys may contain defects such as vacancies and lattice distortions that arise during the solidification process, as well as dislocations. Defects and dislocations in hydrogen storage alloys cause a decrease in the hydrogen storage capacity. Therefore, reducing defects and dislocations in hydrogen storage alloys can increase the hydrogen storage capacity.

[0052] In order to remove defects and dislocations in the hydrogen storage alloy, it is preferable to heat the cast hydrogen storage alloy to 1000°C or less in an inert gas atmosphere. By heating under the specific conditions, it is possible to remove defects and dislocations present inside the hydrogen storage alloy while maintaining the specific crystal structure. As a result, it is possible to increase the hydrogen storage capacity of the hydrogen storage alloy.

[0053] (negative electrode for alkaline storage batteries) The negative electrode for an alkaline storage battery made using the hydrogen storage alloy may have, for example, the following configuration: That is, the negative electrode for an alkaline storage battery includes a current collector made of a conductor, a binder, and a powdered active material held on the current collector via the binder, The active material is a core portion made of the hydrogen storage alloy; and a surface layer that contains Ni hydroxide and is present on the surface of the core portion.

[0054] In the negative electrode, various types of conductors such as metal foil, punched metal, expanded metal, and metal mesh can be used as the current collector.

[0055] The binder acts to hold the active material to the current collector by being interposed between the current collector and the active material. For example, polyvinyl alcohol can be used as the binder. Furthermore, the binder may contain known additives such as a thickener, if necessary.

[0056] The negative electrode may contain, if necessary, a known conductive agent or conductive aid such as Cu (copper) powder or Ni powder, which, like the active material, is held on the current collector via a binder.

[0057] The core portion of the active material is composed of the hydrogen storage alloy. As described above, the hydrogen storage alloy is capable of absorbing and releasing hydrogen, and therefore is applicable to the active material of a negative electrode for an alkaline storage battery. That is, the negative electrode can absorb a large amount of hydrogen in the core portion of the active material during charging. Furthermore, the negative electrode can easily release the hydrogen absorbed in the core portion from the core portion to the outside during discharging.

[0058] A surface layer containing Ni hydroxide is present on the surface of the core portion. In addition to Ni hydroxide, the surface layer may contain Ni oxide and insulating compounds such as ZrO2 and TiO2 remaining after the activation treatment described below. The surface layer may also be composed of fine particles containing Ni hydroxide. By forming a surface layer on the surface of the core portion, the discharge rate characteristics of the negative electrode can be further improved, and the difference between the discharge capacity of the alkaline storage battery when discharged at a high discharge rate and the discharge capacity of the alkaline storage battery when discharged at a low discharge rate can be further reduced.

[0059] (Method of manufacturing negative electrodes for alkaline storage batteries) To prepare the negative electrode for an alkaline storage battery, first, a hydrogen storage alloy is cast by the method described above. This hydrogen storage alloy is then pulverized to prepare a powdered active material. On the surface of the active material thus obtained, insulating compounds such as ZrO2 and TiO2 are present due to contact with the atmosphere. Next, the active material is mixed with a binder to prepare a negative electrode mixture. This negative electrode mixture is then applied to a current collector and dried, allowing the active material to be retained on the current collector.

[0060] The active material held on the current collector is then activated by boiling in a strong alkaline aqueous solution. The strong alkaline aqueous solution used in the activation process can be, for example, an aqueous solution at a temperature of 105°C or higher and a pH of 14 or higher. By activating the active material, insulating compounds present on the surface of the active material can be removed and Ni atoms present on the surface of the active material can be converted into hydroxides. As a result, the surface layer is formed on the surface of the active material, and the negative electrode can be obtained.

[0061] In the method for producing the negative electrode, the activation treatment can reduce the adverse effects of insulating compounds such as ZrO2 and TiO2 present on the surface of the active material, which is believed to improve the discharge capacity and discharge rate characteristics of the negative electrode. [Example]

[0062] Example 1 An example of the hydrogen storage alloy will be described with reference to Figures 1 to 3. The hydrogen storage alloy of this example has a chemical composition consisting of Ti: 5 atomic % to 35 atomic %; Zr: 5 atomic % to 35 atomic %; Ni: 5 atomic % to 35 atomic %; Cr: 5 atomic % to 35 atomic %; and Mn: 5 atomic % to 35 atomic %; and one or two elements selected from the group consisting of Si: 0.50 atomic % to 10 atomic %; and B: 0.50 atomic % to 10 atomic %. The hydrogen storage alloy of this example also has a metallographic structure containing a C14 phase having a C14-type crystal structure and a B2 phase having a B2-type crystal structure, with the C14 phase having the highest proportion of the C14 phase. The entropy of mixing ΔS of the hydrogen storage alloy, expressed by the following formula (1), is: mix is 1.5R or more.

[0063]

number

[0064] In the above formula (1), R is the gas constant, and x i is the molar fraction of each element contained in the hydrogen storage alloy. The method for producing the hydrogen storage alloy of this example will be described in detail below.

[0065] <Preparation of hydrogen storage alloy> First, Ti (manufactured by Kojundo Chemical Laboratory Co., Ltd., powder, purity 99.9%), Zr (manufactured by Kojundo Chemical Laboratory Co., Ltd., sponge, purity 98.0%), Ni (manufactured by Kojundo Chemical Laboratory Co., Ltd., powder, purity 99.9%), Cr (manufactured by Kojundo Chemical Laboratory Co., Ltd., powder, purity 99.0%), Mn (manufactured by Kojundo Chemical Laboratory Co., Ltd., flake, purity 99.9%), Si (crushed pieces, purity 99.99%), B (manufactured by Kojundo Chemical Laboratory Co., Ltd., powder, purity 99.9%), and Al (manufactured by Kojundo Chemical Laboratory Co., Ltd., powder, purity 99.0%) were mixed in a desired ratio using an arc melting furnace and then melted to produce an ingot of hydrogen storage alloy. Note that a vacuum high-frequency melting furnace may be used to melt Ti, etc. Alternatively, a strip casting method may be used to cast the hydrogen storage alloy. Furthermore, the cast ingot may be subjected to heat treatment such as homogenization treatment, if necessary.

[0066] Next, the obtained specimen was divided into two equal parts using a wet cutting machine, and one of the ingots was used to measure the specific gravity and observe the structure. The other ingot was coarsely crushed in a tungsten carbide mortar to obtain powder. This powder was hydrogenated and crushed, and then the powder was refined by releasing hydrogen from the powder. The refined powder was then sieved to obtain powder with a diameter of 20 to 40 μm.

[0067] As described above, hydrogen storage alloys (alloys A1 to A8) having the chemical compositions shown in Table 1 were prepared. Alloy B1 in Table 1 is a hydrogen storage alloy for comparison with alloys A1 to A8. The method for preparing alloy B1 was the same as that for alloys A1 to A8, except that the chemical compositions were changed as shown in Table 1. The content of each element shown in Table 1 is a value measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). A high-frequency plasma atomic emission spectrometer (Shimadzu Corporation, model "ICPV-1017") was used to analyze the chemical compositions.

[0068] <Evaluation of hydrogen storage alloy properties> Next, the alloys A1 to A8 obtained above were subjected to crystal structure analysis and static hydrogen storage characteristics evaluation.

[0069] [Crystal structure analysis] Powder X-ray diffraction was performed using an X-ray diffractometer ("SmartLab (registered trademark)" manufactured by Rigaku Corporation) to obtain the X-ray diffraction pattern of each alloy. The crystalline phase contained in each alloy was identified based on the obtained X-ray diffraction pattern, and Rietveld analysis was performed on the X-ray diffraction pattern to estimate the composition ratio of the crystalline phase in each alloy. Powder X-ray diffraction was performed using CuKα radiation, and the X-ray tube output was set to 40 kV and 40 mA. Crystal structure analysis and Rietvelt analysis were performed using powder X-ray analysis software ("PDXL" manufactured by Rigaku Corporation).

[0070] As examples of X-ray diffraction patterns, the X-ray diffraction patterns of alloys A1, A4, A7, and B1 are shown in Figure 1. In Figure 1, the vertical axis represents the diffraction intensity (relative intensity), and the horizontal axis represents the diffraction angle 2θ (°).

[0071] The diffraction peaks appearing in the X-ray diffraction patterns were compared with a database, and it was found that alloys A1 to A8 and alloy B1 all had a C14 phase having a C14 type crystal structure, a B2 phase having a B2 type crystal structure, and a B33 phase having a B33 type crystal structure. The C14 phase in these alloys is presumed to have a crystal structure of ZrMn2 or ZrMn2 in which Zr atoms and / or Mn atoms are substituted with other atoms. The B2 phase is also presumed to have a crystal structure of Ti 0.64 Zr 0.36 Ni and Mn 0.15 Ti 0.85 The B33 phase is estimated to be ZrNi.

[0072] Table 2 shows the mass ratio of the crystalline phase in each alloy estimated by Rietvelt analysis. 0.5 Ti 0.5 Mn2, B2 phase Ti 0.6 Zr 0.4 To improve the validity of the analysis, the X-ray diffraction pattern was fitted assuming that the phase was Ni, ignoring the presence of the B33 phase. The lattice constants and volumes of the C14 and B2 phases were calculated using the Whole Powder Pattern Fitting (WPPF) method, and the results are shown in Table 2.

[0073] [Static hydrogen absorption characteristics] A pressure-composition-temperature (PCT) characteristic measuring device (manufactured by Suzuki Shokan Co., Ltd.) was used to obtain pressure-composition isotherms at 30°C for each alloy listed in Table 1. The hydrogen storage capacity at a pressure of 0.9 MPa was read from the pressure-composition isotherm during the hydrogen absorption process for each alloy, and this value was designated as the maximum hydrogen storage capacity. Furthermore, the plateau pressure was determined from the pressure-composition isotherm during the hydrogen absorption or desorption process for each alloy using the following method. Specifically, a linear region with a relatively small slope and nearly parallel to the horizontal axis was identified on the pressure-composition isotherm, and this region was designated as the plateau region. The equilibrium hydrogen pressures in the plateau region were then arithmetically averaged, and this value was designated as the plateau pressure. The maximum hydrogen storage capacity, plateau pressure during the hydrogen absorption process, and plateau pressure during the hydrogen desorption process for each alloy were as shown in Table 3.

[0074] [Median diameter of hydrogen storage alloy before hydrogen absorption] The median diameter of each alloy before hydrogen absorption was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT3000II, manufactured by Microtrac Bell Corporation). Specifically, the particle size distribution based on the volume of each alloy dispersed in air was obtained using this analyzer. As an example, Figures 2 and 3 show the particle size distribution curves of alloy A4 and alloy B1 before hydrogen absorption. Note that the vertical axis of Figures 2 and 3 represents the frequency (unit: %) based on the volume, and the horizontal axis represents the particle size (unit: μm).

[0075] Table 4 also shows the median diameter (ie, d50) of each alloy before hydrogen absorption, calculated based on the particle size distribution.

[0076] [Median diameter of hydrogen storage alloy after hydrogen release] Using the same method as in the static hydrogen storage property evaluation described above, each alloy was allowed to absorb hydrogen and then release hydrogen from the alloy. This procedure was repeated five times. The median diameters of the alloys thus obtained were measured using the same method as for measuring the median diameter of the hydrogen storage alloy before hydrogen absorption. As examples, Figures 2 and 3 show particle size distribution curves for Alloy A4 and Alloy B1 after hydrogen release. Table 4 also shows the median diameters of each alloy after hydrogen release, calculated based on the particle size distribution, and the reduction rate of the median diameter after hydrogen release. The reduction rate of the median diameter is the amount of reduction in the median diameter after hydrogen release relative to the median diameter before hydrogen absorption, i.e., the ratio of the difference between the median diameter before hydrogen absorption and the median diameter after hydrogen release, expressed as a percentage.

[0077] [Table 1]

[0078] [Table 2]

[0079] [Table 3]

[0080] [Table 4]

[0081] As shown in Tables 1 and 2, alloys A1 to A8 have the specific chemical compositions and metal structures. mix is 1.5R or more. Therefore, alloys A1 to A8 can reversibly store and release hydrogen, as shown in Table 3. Furthermore, as shown in Table 4, alloys A1 to A8 had a smaller reduction rate of median diameter after hydrogen release than alloy B1, which does not contain Si or B, and were less likely to crack even when hydrogen storage and release were repeated.

[0082] Example 2 In this example, an example of a negative electrode for an alkaline storage battery using a hydrogen storage alloy as the active material will be described. As shown in FIG. 4, the negative electrode 1 for an alkaline storage battery of this example has a current collector 2 made of a conductor, a binder 3, and a powdered active material 4 held on the current collector 2 by the binder 3. Each particle 41 constituting the active material 4 is made of a hydrogen storage alloy having the specific chemical composition and a C14 type crystal structure of the main phase. A method for producing the negative electrode 1 of this example will be described in detail below.

[0083] [Method of manufacturing alkaline storage battery negative electrode 1] First, a hydrogen-absorbing alloy selected from powdered alloys A1 to A8 and alloy B1 obtained in Example 1, Ni powder, and PVA (polyvinyl alcohol) and CMC (carboxymethyl cellulose) as binder 3 were mixed in a mass ratio of active material 4:Ni powder:PVA:CMC=74:24:0.5:1.5 to prepare a paste-like negative electrode mixture. This negative electrode mixture was filled into a Ni mesh serving as a separately prepared current collector 2. Thereafter, the Ni mesh was rolled using a roll press to adhere the negative electrode mixture to the Ni mesh, thereby producing a negative electrode 1.

[0084] Next, the negative electrode 1 was subjected to activation treatment by boiling in a 6 mol / L potassium hydroxide aqueous solution for 2 to 4 hours. The temperature of the potassium hydroxide aqueous solution was set to 105°C. As a result, test specimens C1 to C8 and test specimen D1 shown in Table 5 were obtained. Although not shown in the figure, a surface layer (not shown) containing Ni hydroxide was formed on the surface of each particle 41 constituting the active material 4 in the test specimen after activation treatment.

[0085] Next, one of specimens C1 to C8 and specimen D1 was combined with a commercially available Ni(OH)2 / NiOOH positive electrode and Hg / HgO reference electrode, and a three-electrode battery cell was constructed using an electrolyte containing 6 mol / L potassium hydroxide and 1 mol / L lithium hydroxide.

[0086] [Charge / discharge cycle characteristics] The charge-discharge cycle characteristics of the battery cell were evaluated using the following method. Specifically, the battery cell was first charged for 5 hours at a temperature of 30°C and a current density of 25 mA / g, and then rested for 10 minutes to stabilize the potential. Next, the battery was discharged at a current density of 25 mA / g to a potential of -0.5 V relative to the Hg / HgO potential. This charge-discharge cycle was repeated 30 times, and the discharge capacity of the battery cell was measured from the start to the end of discharge in each cycle.

[0087] Table 5 shows the discharge capacity in the first charge / discharge cycle, the discharge capacity after 30 cycles, and the capacity retention rate. The capacity retention rate (unit: %) in Table 5 is the ratio of the discharge capacity after 30 cycles to the discharge capacity in the first charge / discharge cycle, expressed as a percentage. Figure 5 also shows the capacity retention rates for each cycle of specimens C4 and D1 as an example. The vertical axis of Figure 5 represents the capacity retention rate (unit: %), and the horizontal axis represents the number of charge / discharge cycles (unit: times).

[0088] [Table 5]

[0089] As shown in Table 5, the cores of the active materials of specimens C1 to C8 were composed of hydrogen-storage alloys having the specific chemical composition and metal structure and having an entropy of mixing within the specific range. Therefore, these specimens exhibited superior charge-discharge cycle characteristics compared to specimen D1, which contained an active material that did not contain Si or B, and were able to suppress the decrease in discharge capacity even when repeatedly charged and discharged. This is thought to be because cracking of the active material during repeated charge-discharge cycles was suppressed, making it less likely that Mn would be leached from the active material due to contact with the electrolyte.

[0090] Furthermore, among specimens C1 to C8, the active material of specimen C4 was composed of a hydrogen storage alloy having a chemical composition consisting of Ti: 13 atomic % to 28 atomic %; Zr: 15 atomic % to 25 atomic %; Ni: 15 atomic % to 30 atomic %; Cr: 10 atomic % to 30 atomic %; Mn: 13 atomic % to 25 atomic %; Si: 0.70 atomic % to 1.5 atomic %; and Al: 0.70 atomic % to 2.0 atomic %. Therefore, in addition to a high capacity retention rate, specimen C4 also had an excellent discharge capacity.

[0091] Specific embodiments of the hydrogen storage alloy, negative electrode for alkaline storage battery, and alkaline storage battery according to the present invention are not limited to those described in Examples 1 and 2, and can be modified as appropriate within the scope of the present invention.

[0092] For example, the hydrogen storage alloy may take the following forms (1) to (5).

[0093] [1] A chemical composition consisting of one or two elements selected from the group consisting of Ti: 5 atomic % or more and 35 atomic % or less, Zr: 5 atomic % or more and 35 atomic % or less, Ni: 5 atomic % or more and 35 atomic % or less, Cr: 5 atomic % or more and 35 atomic % or less, and Mn: 5 atomic % or more and 35 atomic % or less, and Si: 0.50 atomic % or more and 10 atomic % or less, and B: 0.50 atomic % or more and 10 atomic % or less; A metal structure including a C14 phase having a C14 type crystal structure and a B2 phase having a B2 type crystal structure, wherein the C14 phase has the highest composition ratio; The mixing entropy ΔS is expressed by the following formula (1): mix A hydrogen storage alloy having a resistance of 1.5R or more.

[0094]

number

[0095] In the above formula (1), R is the gas constant, and x i is the mole fraction of each element contained in the hydrogen storage alloy.

[0096] [2] The hydrogen storage alloy according to [1], further comprising Al: 0.50 atomic % or more and 10 atomic % or less. [3] The hydrogen storage alloy according to [1] or [2], wherein the composition ratio of the B2 phase is more than 0 mass % and not more than 20 mass %. [4] The hydrogen storage alloy according to [1] or [2], wherein the composition ratio of the B2 phase is 10% by mass or more and 18% by mass or less. [5] The hydrogen storage alloy according to any one of [1] to [4], which essentially contains B: 0.50 atomic % or more and 10 atomic % or less, and further contains a C32 phase having a C32 type crystal structure.

[0097] The negative electrode for an alkaline storage battery may have the following configuration [6]. [6] A negative electrode for an alkaline storage battery, comprising a current collector made of a conductor, a binder, and a powdered active material held on the current collector via the binder, The active material is A core portion made of the hydrogen storage alloy according to any one of [1] to [5]; a surface layer containing a hydroxide of Ni and present on the surface of the core portion;

[0098] The alkaline storage battery may also have the following configuration [7]. [7] An alkaline storage battery having the negative electrode for alkaline storage batteries according to [6]. [Explanation of symbols]

[0099] 1. Negative electrodes for alkaline storage batteries 2 Current collector 3. Binder 4 Active material 41 particles

Claims

1. a chemical composition consisting of one or two elements selected from the group consisting of Ti: 5 atomic % or more and 35 atomic % or less, Zr: 5 atomic % or more and 35 atomic % or less, Ni: 5 atomic % or more and 35 atomic % or less, Cr: 5 atomic % or more and 35 atomic % or less, and Mn: 5 atomic % or more and 35 atomic % or less, and Si: 0.50 atomic % or more and 10 atomic % or less, and B: 0.50 atomic % or more and 10 atomic % or less; A metal structure including a C14 phase having a C14 type crystal structure and a B2 phase having a B2 type crystal structure, wherein the C14 phase has the highest constituent ratio; Mixing entropy ΔS represented by the following formula (1) mix A hydrogen storage alloy having a resistance of 1.5R or more. [Equation 1] (where R in the formula (1) is the gas constant, and x i is the mole fraction of each element contained in the hydrogen storage alloy.

2. 2. The hydrogen storage alloy according to claim 1, further comprising Al: 0.50 atomic % or more and 10 atomic % or less.

3. 2. The hydrogen storage alloy according to claim 1, wherein the composition ratio of the B2 phase is more than 0 mass % and not more than 20 mass %.

4. 2. The hydrogen storage alloy according to claim 1, wherein the composition ratio of the B2 phase is 10 mass % or more and 18 mass % or less.

5. 2. The hydrogen storage alloy according to claim 1, which essentially contains 0.50 atomic % to 10 atomic % of B, and further contains a C32 phase having a C32 type crystal structure.

6. A negative electrode for an alkaline storage battery, comprising: a current collector made of a conductor; a binder; and a powdered active material held on the current collector via the binder, The active material is a core portion made of the hydrogen storage alloy according to any one of claims 1 to 5; a surface layer containing a hydroxide of Ni and present on the surface of the core portion.

7. An alkaline storage battery comprising the negative electrode for alkaline storage batteries according to claim 6.

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