JP2012102343A - Hydrogen-storage alloy, hydrogen-storage alloy electrode, and nickel-hydrogen secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrogen-storage alloy whose cost can be reduced while keeping good storage and discharge characteristics of hydrogen.SOLUTION: The hydrogen-storage alloy has a composition represented by a general composition formula: REMgNiAl(wherein, RE is one or more elements selected from a group composed of La, Ce, etc.; and subscripts of x, y, z, are in the range of 0.05≤x≤0.2, 4.0≤y≤4.4 and 0.1≤z≤0.3, respectively). The storage alloy is characterized in that a phase having CaCutype crystal structure is in the range of 40-90 wt.% of the total composition, a phase having CeCotype crystal structure is in the range of 5-39 wt.% of the total composition, and a phase having PrCotype crystal structure is in the range of 3-20 wt.% of the total composition.

Description

The present invention relates to a hydrogen storage alloy, a hydrogen storage alloy electrode, and a nickel-hydrogen secondary battery, and more specifically, a hydrogen storage alloy having a composition represented by a general composition formula: RE _(1-x) Mg _x Ni _y Al _z The present invention relates to a hydrogen storage alloy electrode and a nickel hydride secondary battery including the same.

  Hydrogen storage alloys are attracting widespread attention as energy conversion materials and energy storage materials because they can store and release hydrogen. In addition, alkaline secondary batteries using hydrogen storage alloys as negative electrodes, especially nickel hydride secondary batteries, are the fields where hydrogen storage alloys are most practically used. Portable, electric vehicles, hybrid vehicles, industrial use, etc. Has been used extensively. A nickel metal hydride secondary battery is a storage battery implemented in place of a nickel-cadmium secondary battery, and has characteristics such as low pollution and high capacity as compared with a nickel-cadmium secondary battery.

At present, as a negative electrode for a mainstream nickel-metal hydride secondary battery, an MmNi ₅ (AB ₅ ) hydrogen storage alloy has already been put into practical use (Mm = mixed rare earth elements such as La, Ce, Pr, and Nd). The crystal structure of the AB ₅ -based hydrogen storage alloy has a CaCu ₅ type phase as the main crystal structure, and a part of Ni in MmNi ₅ is substituted with elements such as Al and Mn. As an example of the method for producing the hydrogen storage alloy, the alloy composition of the rare earth mixture Mm (La, Ce, Nd, Pr, and other rare earth elements), Al, Mn, Ni, Cu, etc. may be the target composition. It is known that each metal sample is weighed and prepared, and the mixture is obtained by heating and dissolving in a high-frequency melting furnace in an inert gas atmosphere.

Furthermore, hydrogen storage / release cycle life characteristics can be improved by adding Co to the MmNi _5- based hydrogen storage alloy. However, Co is expensive and causes the manufacturing cost to increase. Furthermore, the system alloy has inferior initial charge / discharge characteristics, and even a battery equipped with a negative electrode corresponding to more than twice the positive electrode capacity is regulated by negative electrode capacity, and a predetermined discharge capacity cannot be obtained. There was a problem. The problem is that the initial charge efficiency of the charge / discharge cycle of the alloy negative electrode is inferior, so even if the positive electrode completes charging and generates oxygen, the negative electrode may be charged to the same extent as the positive electrode. Responsible. After the generation of oxygen, the reaction in which oxygen and hydrogen generated from the positive electrode and the negative electrode change to water preferentially progresses over the negative electrode charging reaction, so that the capacity is smaller than that of the positive electrode, and the negative electrode is regulated by discharge. In such a state, the battery voltage is slightly lower and the discharge capacity is smaller than in the case of positive electrode regulation. Further, in some cases, when charging and discharging are repeated, elements such as Co and Mn contained in the alloy are eluted in the alkaline electrolyte and adhere to the separator and cause a short circuit. The discharge characteristics are degraded.

On the other hand, rare earth -Mg-Ni alloy a part of the rare earth elements of AB ₅ type alloy was replaced with Mg have been developed. The rare earth-Mg-Ni alloy can occlude a large amount of hydrogen gas at around room temperature, but has a problem of poor battery cycle life due to low alkali resistance. Therefore, Patent Document 1 and Patent Document 2 state that alkali resistance can be imparted to the hydrogen storage alloy by limiting the content of La and Ce in the rare earth component of the rare earth-Mg-Ni alloy. However, in order to strictly control the La and Ce contents, pure La and Ce are required as raw materials. Therefore, there is a problem that the manufacturing cost is high because the inexpensive Mm (Misch metal) used for producing the conventional AB ₅ series alloy cannot be used directly.

Therefore, in Patent Document 3, (La _a Pr _b Nd _{c Ad} ) _1-x Mg _x Ni _y Al _z (where A is a rare earth element other than La, Pr, Nd, and subscripts a, b, c, d satisfies the relationship represented by 0.4 ≦ a, 0 <b, 0 ≦ c, 0 ≦ d, c <b ≦ a, a + b + c + d = 1, and the subscripts x, y, and z are 0.10, respectively. ≦ x ≦ 0.25, 0.05 ≦ z ≦ 0.35, 3.0 ≦ y + z ≦ 4.0), the hydrogen storage alloy having a high capacity and good cycle life characteristics Proposed. Here, the number x of Mg atoms is 0.1 ≦ x ≦ 0.25, and the number z of Al atoms is 0.05 ≦ z ≦ 0.35, and the alloy capacity is set by setting the above range. It is said that the corrosion resistance of the alloy can be improved. In addition, as a reason why the sum of y and z representing the ratio of the A site and the B site is set in a range represented by 3.0 ≦ y + z ≦ 4.0, if y + z becomes too small, hydrogen occlusion Since the hydrogen storage stability in the alloy increases, the hydrogen releasing ability deteriorates. On the other hand, if y + z becomes too large, the hydrogen storage site in the hydrogen storage alloy decreases and the hydrogen storage ability starts to deteriorate. It is explained that.

  However, while the rare earth minerals produced in nature contain more Nd than Pr, the hydrogen storage alloy represented by the composition disclosed in Patent Document 3 is required to be richer in Pr than Nd. Therefore, even if inexpensive La is used as the main component (0.4 ≦ a), it is unavoidable that pure Pr is eventually required, and there is a problem that it is difficult to reduce the material cost.

  In addition, when using a hydrogen storage alloy as the negative electrode of a nickel-metal hydride secondary battery, it is generally activated to obtain the original battery capacity because there is an oxide film on the particle surface of the hydrogen storage alloy that causes a decrease in capacity. It is necessary to remove the oxide film. The activation process (activation process) performed before the nickel hydride secondary battery is manufactured and shipped is required to reduce the time from the viewpoint of cost reduction. Usually, this activation treatment is performed by repeating several tens of cycles of charging / discharging or by repeating slow charging-similarly slow discharging like 0.1 to 0.2C rate (similarly 0.1 to 0.2C rate). Is the amount of current that can discharge or charge the entire capacity of the battery in 5 to 10 hours). However, since it takes more than one day, there is a demand for shortening the activation processing time. Therefore, Patent Document 4 proposes a surface modification treatment method in which a hydrogen storage alloy is acid-treated and an oxide film present on the surface of the alloy particles is removed in order to reduce the number of charge / discharge cycles as much as possible.

  However, in the surface modification treatment by acid treatment, a water washing step for removing the treatment liquid adhering to the hydrogen storage alloy must be provided as a subsequent step, which may increase the manufacturing cost.

JP 2005-290473 A JP 2006-040847 A JP 2008-208428 A JP-A-5-225975

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hydrogen storage alloy capable of reducing the cost while maintaining good hydrogen storage / release characteristics.

  In addition, the present invention uses such a hydrogen storage alloy as an electrode, so that a nickel-hydrogen secondary battery excellent in high rate discharge characteristics and self-discharge characteristics can be obtained at low cost while maintaining good discharge capacity and cycle life characteristics. The purpose is to provide in.

We general composition formula: focusing on the crystal structure of the rare earth _(1-x) Mg _{x Ni} hydrogen-absorbing alloy represented by y Al _z, subscripts x, y, by setting the value of z in the respective predetermined ranges, It has been clarified that excellent characteristics can be obtained by setting the content ratio of two or more crystal phases each having a phase having a CaCu ₅ type crystal structure as a main phase within a predetermined range.

  That is, the present invention provides the following hydrogen storage alloy, hydrogen storage alloy electrode, and nickel-hydrogen secondary battery.

Item 1:
General composition formula:
RE _(1-x) Mg _x Ni _y Al _z
(Wherein, RE is one or more selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu. And the subscripts x, y, and z are in the ranges indicated by 0.05 ≦ x ≦ 0.2, 4.0 ≦ y ≦ 4.4, and 0.1 ≦ z ≦ 0.3, respectively. Having a composition represented by
The phase having the CaCu ₅ type crystal structure is 40 to 90% by weight of the total composition, the phase having the Ce ₅ Co ₁₉ type crystal structure is _{5 to} 39% by weight of the total composition, and the phase having the Pr ₅ Co ₁₉ type crystal structure is all A hydrogen storage alloy characterized by being 3 to 20% by weight of the composition.
Item 2:
The difference between the a-axis length of the Ce ₅ Co ₁₉ type crystal structure and the a-axis length of the Pr ₅ Co ₁₉ type crystal structure with the a-axis length of the CaCu ₅ type crystal structure is ± 0.02 mm. Item 2. The hydrogen storage alloy according to Item 1, which is in a range.
Item 3:
The a-axis lengths of the crystal structures of all crystal phases are all in the range of 4.95 to 5.05 mm,
Item 3. The hydrogen storage alloy according to Item 1 or 2, wherein the c-axis length of the CaCu _5- type crystal structure is in the range of 3.98 to 4.02 mm.
Item 4:
Item 4. The hydrogen storage alloy according to any one of Items 1 to 3, which is subjected to an alkali heat treatment at a temperature of 60 to 100 ° C using a caustic aqueous solution having a concentration of 0.1 mol / L to 10 mol / L.
Item 5:
Item 5. The hydrogen storage alloy according to Item 4, wherein the residual magnetization value is in the range of 0.5 to 0.9 emu / g.
Item 6:
Item 6. A hydrogen storage alloy electrode comprising particles comprising the hydrogen storage alloy according to any one of Items 1 to 5 and a conductive core body that holds the particles.
Item 7:
A nickel hydride secondary battery comprising the hydrogen storage alloy electrode according to Item 6 as a negative electrode.

  According to the hydrogen storage alloy of the present invention, it is possible to reduce the cost while maintaining good hydrogen storage / release characteristics.

  In addition, by providing such a hydrogen storage alloy as a negative electrode for a nickel metal hydride secondary battery, the nickel metal hydride secondary having excellent high rate discharge characteristics and self-discharge characteristics while maintaining good discharge capacity and cycle life characteristics. A battery can be provided at low cost.

It is a figure which shows the PCT characteristic of an Example and a comparative example. It is a figure which shows the synchrotron radiation XRD pattern of an Example and a comparative example. It is a schematic block diagram of a test cell. It is a figure which shows the charging / discharging lifetime characteristic of an Example and a comparative example. It is a figure which shows the relationship between the discharge rate of an Example and a comparative example, and an average voltage. It is a figure which shows the relationship between the residual magnetization of an Example, and alkali treatment conditions.

The general composition formula of the hydrogen storage alloy according to the present invention is:
RE _(1-x) Mg _x Ni _y Al _z (1)
It is represented by

  Here, RE in the formula (1) is selected from the group consisting of rare earth elements La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Or one or more elements.

RE preferably contains La and further contains one or more rare earth elements other than La. In this case, the general composition formula of the hydrogen storage alloy is
(La _a R1 _b ) _(1-x) Mg _x Ni _y Al _z (2)
Can be expressed as (However, R1 is one or more elements selected from the rare earth element group other than La. Also, a + b = 1)

  The subscript a in the formula (2) is preferably 0.45 ≦ a ≦ 0.95, more preferably 0.5 ≦ a ≦ 0.9, and still more preferably 0.6 ≦ a ≦ 0.8. . If the proportion of La contained in the rare earth is too small, the discharge capacity at high temperatures tends to be insufficient when a hydrogen storage alloy is used as the negative electrode. That is, by setting the value of a to 0.45 or more, the absolute amount of heat of hydrogenation (ΔH 2) of the entire rare earth can be reduced, and the average dissociation pressure of the entire hydrogen storage alloy can be reduced. It is possible to suppress a decrease in discharge capacity. Moreover, it is advantageous to raise the ratio of La also from the point of cost reduction.

  On the other hand, the amount of hydrogen occlusion increases as the proportion of La contained in the rare earth increases. However, if the value of a is too large, the crystal phase of the obtained hydrogen occlusion alloy is likely to be phase-separated. It becomes difficult to maintain well.

  In addition, the subscript b of the rare earth element R1 other than La in the formula (2) is, for example, b ≦ 0.5 in the case of Ce, b ≦ 0.15 in the case of Pr, and b ≦ 0.5 in the case of Nd. The range is preferably 0 <b ≦ 0.5 (in the case of Ce), 0.01 ≦ b ≦ 0.15 (in the case of Pr), and 0.05 ≦ b ≦ 0.5 (in the case of Nd). ) Is even better. Since all of these elements (Ce, Pr, Nd) have a heat of hydrogenation (ΔH) larger than that of La, if they are too large, the hydrogen storage pressure increases, and the hydrogen storage amount decreases accordingly. Moreover, since Nd is highly demanded and expensive as a raw material for Nd—Fe—B magnets, for example, it is industrially expensive to use such an element, which is not economically preferable. In addition, for example, Sm is expensive as a raw material for Sm—Co magnets, and Tb is expensive as a raw material for phosphors.

In the above formulas (1) and (2), the subscripts x, y, and z of Mg, Ni, and Al are 0.05 ≦ x ≦ 0.3, 4.0 ≦ y ≦ 4.4, 0.1, respectively. ≦ z ≦ 0.3. Here, when x is less than 0.05, the crystal structure of the obtained hydrogen storage alloy does not easily include a superlattice phase (Ce ₂ Ni ₇ type, Ce ₅ Co ₁₉ type or Pr ₅ Co ₁₉ type), Crystal structure collapses and becomes amorphous during hydrogen storage. Therefore, when used as a negative electrode of a battery, the cycle life characteristics are poor. On the other hand, when x is larger than 0.3, segregation of the MgNi ₂ Laves phase, which is difficult to store hydrogen, increases, resulting in a decrease in hydrogen storage amount and deterioration in cycle life characteristics. A more preferable range of x is 0.1 ≦ x ≦ 0.2.

  On the other hand, if y is less than 4.0, the hydrogen storage amount decreases and the discharge voltage decreases. When y exceeds 4.4, the effect of adding Mg or Al is weakened. If z is less than 0.1, the resulting hydrogen storage alloy has a crystal structure with a small lattice spacing, so that the hydrogen dissociation pressure increases. Therefore, when assembled as a battery, not only does the amount of hydrogen in the negative electrode decrease, but the cycle life characteristics become poor. Conversely, when z exceeds 0.3, the hydrogen dissociation pressure decreases, but the lattice spacing of the crystal structure excessively expands, the equilibrium dissociation pressure decreases, and the hydrogen storage capacity decreases.

  By setting y and z within the above ranges, the sum of y and z representing the ratio of the A site and the B site is in the range indicated by 4.1 to 4.7. If y + z is too small, the amount of rare earths required, particularly Nd, Ce, and Pr, increases, so that the hydrogen storage stability of the alloy increases and the hydrogen releasing ability deteriorates, and the material cost also increases. Conversely, if y + z becomes too large, the hydrogen storage sites of the alloy decrease and the hydrogen storage capacity deteriorates. Preferred y + z is 4.2 to 4.7, and more preferably 4.25 to 4.5.

  For the above reasons, by increasing La in the rare earth and substituting a suitable amount of Ni with Al, a hydrogen storage alloy having a large hydrogen storage capacity and a low equilibrium dissociation pressure can be obtained. Originally, elements such as Al and La having a lower equilibrium potential than the hydrogen oxidation-reduction potential are eluted in an alkaline aqueous solution as an electrolytic solution, so that the alloy deteriorates. Therefore, when Al is added and the amount of La is further increased, the resulting hydrogen storage alloy is likely to deteriorate. Therefore, by including Mg in the hydrogen storage alloy, deterioration of the electrode characteristics is suppressed.

  An example of an alloy manufacturing method is described as follows. For example, La, Pr, Nd, Mg, Ni, and Al metal raw materials are adjusted to have a desired composition, and heat treatment is performed in an inert atmosphere using a high-frequency heating device. Then, the melted predetermined raw material is cooled and solidified. And in order to produce | generate the target crystal phase in a desired ratio, it heat-processes in inert atmosphere, controls heat processing temperature (800-1200 degreeC) and heat processing time (5-20 hours), An ingot-like hydrogen storage alloy having two or more crystal phases having different structures can be obtained.

As described above, the general composition formula: RE _(1-x) Mg _x Ni _y Al _z (where RE is La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er , Tm, Yb, Lu selected from the group consisting of one or more elements), the composition ratio x, y, z is set to the above numerical ranges, respectively, to produce an alloy. The obtained hydrogen storage alloy includes, for example, a phase having a CaCu ₅ type crystal structure and a phase having a Ce ₅ Co ₁₉ type crystal structure, a phase having a CaCu ₅ type crystal structure, and a Pr ₅ Co ₁₉ type crystal. Including three phases including a phase having a structure, a phase having a CaCu ₅ type crystal structure, a phase having a Ce ₅ Co ₁₉ type crystal structure, and a phase having a Pr ₅ Co ₁₉ type crystal structure; Become.

The content ratio of each crystal phase can be adjusted by controlling the heat treatment conditions (particularly the heat treatment temperature) under an inert atmosphere. For example, in order to increase the proportion of a phase having a CaCu ₅ type crystal structure and reduce the proportion of a phase having a Ce ₅ Co ₁₉ type crystal structure or a Pr ₅ Co ₁₉ type crystal structure, the heat treatment temperature is relatively high. (1100 to 1200 ° C.). On the other hand, in order to reduce the proportion of the phase having the CaCu ₅ type crystal structure and increase the proportion of the phase having the Ce ₅ Co ₁₉ type crystal structure or the Pr ₅ Co ₁₉ type crystal structure, the heat treatment temperature is relatively low. (800-1000 ° C).

Each crystal phase has a phase having a CaCu ₅ type crystal structure as a main phase, a phase having a CaCu ₅ type crystal structure is 40 to 90 wt%, a phase having a Ce ₅ Co ₁₉ type crystal structure is ₅ to 39 wt%, and Pr ₅ The phase having the Co ₁₉ type crystal structure needs to be in the range of 3 to 20 wt%. In particular, it is preferable that CaCu ₅ type is present in the range of 45 to 85 wt%, Ce ₅ Co ₁₉ type is 6 to 35 wt%, and Pr ₅ Co ₁₉ type is 6 to 15 wt%. In the present specification, the main phase refers to the crystal phase constituting the alloy having the largest abundance ratio (weight ratio). A phase having another crystal structure such as a Ce ₂ Ni ₇ type crystal structure may be included.

It was clarified that by setting the weight ratio of each crystal phase within the above range, the size of the gap between metal atoms can be adjusted, and the equilibrium hydrogen pressure of the alloy represented by the above composition can be optimized. . Specifically, the a-axis length of the Ce ₅ Co ₁₉ type crystal structure and the a-axis length of the Pr ₅ Co ₁₉ type crystal structure with respect to the a-axis length of the CaCu ₅ type crystal structure of the hydrogen storage alloy It became possible to make the lengths approximately equal. Also, the lattice constant c-axis length of the CaCu ₅ type crystal structure, conventional MmNi ₅ system (AB5 type) so can be reduced as compared with the lattice constant c-axis length of the CaCu ₅ type crystal structure of the hydrogen storage alloy became. As described above, the hydrogen storage alloy with the optimized lattice constant can store a large amount of hydrogen electrochemically and release a large amount of hydrogen.

The range of the lattice constant c-axis length of the CaCu _5- type crystal structure which is the main phase is preferably in the range of 3.980 to 4.020 mm, and more preferably in the range of 3.982 to 4.01 mm. Also, the difference between the lattice constant a-axis length of the CaCu ₅ type crystal structure and the lattice constant a-axis length of the Ce ₅ Co ₁₉ type and Pr ₅ Co ₁₉ type crystal structures is within a range of ± 0.02Å. It is preferable that it is within a range of ± 0.01 mm. The a-axis length of each crystal structure is preferably 4.95 to 5.05 cm, and more preferably 4.97 to 5.03 mm.

  For the weight ratio, a-axis length, and c-axis length of each crystal phase, for example, an X-ray diffraction pattern is measured by a large synchrotron radiation facility “SPring-8” and structural analysis is performed by the Rietveld method. And you can ask for it.

When the presence of the CaCu ₅ type exceeds 90 wt%, the lattice constant c-axis length of the CaCu ₅ type is difficult to decrease. On the contrary, if it is less than 40 wt%, the CaCu ₅ type lattice constant c-axis length is difficult to increase. That is, by setting the lattice constant a-axis length and c-axis length in the above ranges, the size of the gap between metal atoms is adjusted, and the equilibrium hydrogen pressure of the alloy represented by the above composition is optimized. For example, when the values of the lattice constant a-axis and c-axis of the alloy are large, the unit cell volume increases, and the gap between metal atoms constituting the unit cell increases. When the gap is large, hydrogen easily enters the metal lattice, but it is difficult to hold hydrogen. Thus, the higher the unit cell volume of the alloy, the lower the equilibrium hydrogen dissociation pressure. Conversely, when the values of the lattice constant a-axis and c-axis of the alloy are small, the volume of the unit cell is small, and the gap between the metal atoms constituting the unit cell is small. If the gap is small, the metal atoms are densely packed, so that it is easy to hold hydrogen, but it is difficult for hydrogen to enter the metal lattice. Thus, the smaller the unit cell volume of the alloy, the higher the equilibrium hydrogen dissociation pressure.

  The ingot-like hydrogen storage alloy is coarsely pulverized by a crusher in an inert atmosphere, and then dry pulverized using a pin mill in an inert atmosphere, whereby powder particles of the hydrogen storage alloy can be obtained. These particles may contain MgO on the surface of the manufactured alloy due to the influence of oxygen contained in the atmosphere during processing and raw materials in the manufacturing process. Therefore, when the obtained particles are used as a hydrogen storage alloy electrode, MgO contained in the negative electrode active material is eluted into the electrolytic solution, so that the electrolytic solution is easily deteriorated, which affects the life of the battery.

  Therefore, it is preferable to subject the hydrogen storage alloy to alkali heat treatment to remove MgO present on the alloy surface and prevent deterioration of the electrolytic solution. The alkali heat treatment may be performed by immersing the hydrogen storage alloy in a granular (powdered) form or dipping it in a caustic aqueous solution after electrode formation. Examples of the caustic aqueous solution include NaOH, KOH, LiOH and the like, and the concentration is preferably 0.1 to 10 mol / L, and more preferably 2 to 8 mol / L. If the concentration is less than 0.1 mol / L, MgO may not be completely removed. On the other hand, if the concentration exceeds 10 mol, not only MgO but also the alloy itself may be dissolved. The treatment time is preferably 0.1 to 4 hours. If it is less than 0.1 hour, reproducibility is poor, and overtreatment exceeding 4 hours may dissolve not only MgO but also the alloy itself. As will be described later, the heat treatment temperature is preferably set in the range of 60 to 100 ° C, more preferably about 80 ° C. Further, as will be described later, the residual magnetization value of the alloy is preferably in the range of 0.5 to 0.9 emu / g, more preferably 0.6 to 0.8 emu / g. Here, the residual magnetization value is a value when an external magnetic field of 18 koe is applied using a vibrating sample magnetometer (VSM).

  The hydrogen storage alloy of the present invention can be effectively used as a negative electrode for an alkaline battery, for example, a negative electrode active material for a nickel-hydrogen secondary battery, an air-hydrogen secondary battery, or the like. A negative electrode using this hydrogen storage alloy can have the same structure as a normal battery. For example, in a nickel-hydrogen secondary battery, the battery is accommodated in a battery can through a separator having electrical insulation between a negative electrode and a positive electrode using a hydrogen storage alloy as an active material, and filled with an alkaline electrolyte. can do.

  As the negative electrode, a paste type and a pressure type (non-paste type) described below can be adopted.

  The paste-type hydrogen storage alloy electrode is prepared by mixing a hydrogen storage alloy powder obtained by pulverizing the hydrogen storage alloy, a binder, and a conductive powder added as necessary. After applying, filling, and drying the current collector, it can be produced by rolling with a roller press or the like.

  The pressure-bonding type hydrogen storage alloy electrode is prepared by stirring the hydrogen storage alloy powder, the binder, and the conductive powder added as necessary, coating the current collector, and rolling with a roller press or the like. Can be manufactured.

  As a method for pulverizing the hydrogen storage alloy, for example, a mechanical pulverization method such as a ball mill, a pulverizer, a jet mill or the like, or a method in which high-pressure hydrogen gas is occluded / released and pulverized by volume expansion at that time can be adopted.

  Examples of the binder include polyacrylic acid soda, polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), styrene butadiene rubber (SBR), and polyvinylidene fluoride (PVdF). Can do. Such a binder is preferably blended at 0.1 to 5 wt% with respect to 100 wt% of the hydrogen storage alloy. However, in the case of producing a pressure bonding type hydrogen storage alloy electrode, polytetrafluoroethylene (which can be fixed into a network by fibrosis by stirring and fixing the hydrogen storage alloy powder and the conductive additive added as necessary, in a network form. It is preferable to use PTFE) as a binder.

  The conductive auxiliary agent may be any conductive powder, and examples thereof include carbon materials such as graphite powder and carbon black, and metal powders such as nickel and copper. Such a conductive additive is preferably blended in the range of 0.1 to 5 wt% with respect to 100 wt% of the hydrogen storage alloy.

  The current collector is, for example, a two-dimensional substrate such as a punching metal, an expanded metal, a wire mesh, a foil shape, a plate shape, or a foam metal substrate, a mesh-like sintered fiber substrate, a substrate obtained by plating a metal on a woven / nonwoven fabric, etc. A substrate etc. can be mentioned. However, in the case of producing a pressure bonding type hydrogen storage alloy electrode, it is preferable to use a two-dimensional substrate as a conductive substrate because a mixture containing hydrogen storage alloy powder is applied.

A general-purpose thing is used for the positive electrode combined with the hydrogen storage alloy electrode mentioned above. For example, as a known positive electrode, there is a non-sintered nickel positive electrode. The non-sintered nickel positive electrode is made of, for example, a mixture of nickel hydroxide and cobalt hydroxide (Co (OH) ₂ ), cobalt monoxide (CoO), metallic cobalt and the like which are added as necessary. Carboxymethyl cellulose (CMC) In addition, polyacrylates such as sodium polyacrylate are appropriately blended into a paste, and this paste is filled into a three-dimensional substrate such as a foamed metal substrate, a reticulated sintered fiber substrate, or a felt-plated substrate obtained by plating a metal onto a nonwoven fabric. And after drying, it can manufacture by rolling with a roller press etc.

  The separator only needs to have electrical insulation and allow hydroxide ions to pass therethrough. Examples thereof include single polymer fibers such as nylon, polypropylene (PP), and polyethylene (PE), those obtained by subjecting these polymer fibers to a hydrophilic treatment, and composite polymer fibers obtained by blending these fibers.

  As the alkaline electrolyte, for example, a potassium hydroxide solution having a concentration of 20 to 40 wt% or a mixture of lithium hydroxide, sodium hydroxide or the like in the potassium hydroxide solution is used.

According to one embodiment of the present invention, the main phase is CaCu ₅ type that does not contain Mn or expensive Co in the alloy and has a crystal structure with excellent hydrogen storage characteristics, and the lattice constant a-axis length is A hydrogen storage alloy having 4.95 to 5.05 mm and a c-axis length of 3.98 to 4.02 mm is obtained. By providing the hydrogen storage alloy as a negative electrode for a nickel metal hydride secondary battery, a battery having a high capacity maintenance rate with little self-discharge, a high discharge voltage, and a very fast initial activation can be obtained.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated more concretely, this invention is not limited to a following example, unless the summary is exceeded.

Each metal raw material of La, Pr, Nd, Mg, Ni, and Al is weighed to have the predetermined composition shown in Table 1, and until the predetermined metal raw material is dissolved in an argon gas atmosphere using a high-frequency heating device Heat treatment was performed. The melt | dissolved predetermined raw material was poured on the water-cooling type casting board, and cooling solidification was performed. This is an alloy having the target crystal phase by controlling the heat treatment temperature (1000 ° C.) and the heat treatment time (6 hours) in order to produce the target crystal phase at an appropriate ratio in an argon gas atmosphere. Got an ingot. The obtained alloy ingot was coarsely pulverized by a crusher under an argon gas atmosphere, and then dry pulverized using a pin mill under an argon gas atmosphere to obtain an alloy powder having an average particle diameter of about 40 μm (Examples 1 to 11). And Comparative Examples 1-4, 6). In Comparative Example 5, a conventional AB5-based alloy (MmNi _3.7 Co _0.7 Mn _0.3 Al _0.3 ) was pulverized in the same manner as described above to form a powder. Only the / A value is shown.

  Comparative Example 7 is the same as Example 4 except that the heat treatment temperature was set to 700 ° C.

  Comparative Example 8 is the same as Example 4 except that the heat treatment temperature was 1300 ° C.

  In Table 1, the A site indicates the atomic ratio of La, Pr, and Nd after fixing the atomic weight ratio of Mg in the entire alloy to 10%. The atomic ratio of Pr to Nd was set to 1: 4, and Nd was set to be larger than Pr as in the case of rare earth minerals produced in nature. On the other hand, the B site is represented by the atomic ratio of Ni and Al to the A site, and corresponds to y and z in the above formulas (1) and (2), respectively. “B / A” in Table 1 corresponds to y + z.

<Confirmation of hydrogen storage capacity>
The pressure-composition isothermal (PCT) curves in Examples 1 to 6 and Comparative Example 1 were measured. For the measurement, a PCT automatic characteristic measuring device (manufactured by Rigaku) was used. The maximum pressure in the hydrogen atmosphere was 1 MPa, the measurement temperatures were 20 ° C., 40 ° C. and 80 ° C., and the relationship between the hydrogen pressure and the hydrogen absorption amount (H / M) of the hydrogen storage alloy at each temperature is shown in the figure. The results are shown in FIG.

  In each of Examples 1 to 6, a large plateau can be confirmed in a specific hydrogen pressure region as compared with Comparative Example 1, indicating that hydrogen storage / release is possible. Further, as the B / A value increased, the plateau became more prominent. In particular, from the comparison between Examples 4 and 5, even with the same B / A value, a plateau could be confirmed under a lower hydrogen pressure as the Al content increased.

<Structural analysis>
It was measured at a large synchrotron radiation facility SPring-8 (beam line BL19B2, wavelength λ = 0.4012Å). As an example, the synchrotron radiation X-ray diffraction patterns of Examples 1 to 5 and Comparative Example 1 are shown in FIG.

When structural analysis was performed by the Rietveld method, Examples 1 to 11 were a CaCu ₅ type phase (P6 / mmm), a Ce ₂ Ni ₇ type phase (P63 / mmc), a Ce ₅ Co ₁₉ type phase (R-3m) and By assuming four types of models of the Pr ₅ Co ₁₉ type phase (P63 / mmc), it was possible to accurately reproduce the peak position. Table 2 shows the existence ratio of each phase derived from the Rietveld analysis results of Examples 1 to 11 and Comparative Examples 1 to 8 in terms of weight fraction.

Examples 1 to 11 have a CaCu ₅ type phase as a main phase, and include three phases of a Ce ₅ Co ₁₉ type phase and a Pr ₅ Co ₁₉ type phase, whereas Comparative Examples 1 to 2 and 6 to 7 are Ce ₂ Ni _7- type phase, Ce ₅ Co _19- type phase and Pr ₅ Co _19- type phase were mostly contained, and Ce ₅ Co _19- type phase was the main phase. In addition, Comparative Examples 3 to 5 and 8 have a CaCu ₅ type phase as a main phase, but included an excess of 90 wt% or more.

<Evaluation of discharge capacity based on JIS H7205>
The evaluation of electrochemical hydrogen storage is JIS
Based on H7205, the prescribed alloy powders listed in Table 1 and -325 mesh Cu powder (made by high purity chemical) are mixed at 20:80 (weight ratio), press-molded at a total pressure of 5 t, and φ12 mm pellets are formed. Produced. The pellet was sandwiched between 60 mesh nickel meshes, the periphery was spot welded to fix the pellets, and then a nickel plate was welded to the nickel mesh to produce a test electrode (working electrode). The counter electrode was a foamed nickel electrode, and the reference electrode was a mercury oxide (Hg / HgO) electrode. These working electrode, counter electrode, and reference electrode were immersed in a 6 mol / L KOH aqueous solution to produce an H-shaped test cell shown in FIG. The test temperature is 25 ° C. unless otherwise specified.

  Table 3 shows the discharge capacities measured based on JIS H7205 in Examples 1 to 9 and Comparative Examples 1 to 5.

  In Examples 1 to 11 having a B / A value of 4.7 or less, a large discharge capacity of 330 mAh / g or more was obtained, while in comparison examples 3 to 3 having a B / A value of 4.8 or more. No. 5 had a discharge capacity as small as about 300 mAh / g. Further, Comparative Example 6 was a case where the composition ratio of aluminum was small (z = 0.02) as shown in Table 1, and the discharge capacity in this case was also lower than that of the Example. In Comparative Examples 7 and 8, the discharge capacity was lower than that in Examples. Although the composition ratio of each metal raw material of Comparative Examples 7 and 8 is the same as that of Example 4, the proportion of each crystal phase is different from Example 4 by changing the heat treatment temperature. Thus, it was confirmed that not only the composition ratio of each metal raw material was set to a predetermined range but also a high discharge capacity was obtained by controlling the content ratio of each crystal phase to be within a predetermined range. .

<Evaluation as a sealed battery>
The alloy powders of Examples 1 to 9 and Comparative Examples 1 to 5 were mixed with carboxymethyl cellulose (CMC) and polytetrafluoroethylene (PTFE), and then an appropriate amount of ion-exchanged water was added to prepare a slurry. The slurry was filled in foamed nickel (Celmet # 8; manufactured by Sumitomo Electric Industries), dried at 80 ° C. for 1 hour or longer, and then rolled with a roller press to be used for the negative electrode. The electrode thickness of the negative electrode is an average of 400 μm.

  After mixing nickel hydroxide powder coated with cobalt oxyhydroxide, carboxymethyl cellulose (CMC) and polytetrafluoroethylene (PTFE), an appropriate amount of ion-exchanged water was added to prepare a slurry. The slurry was filled in foamed nickel (Celmet # 8; manufactured by Sumitomo Electric Industries), dried at 80 ° C. for 1 hour or longer, and then rolled with a roller press to be used for the positive electrode. The average thickness of the positive electrode is 450 μm.

  A negative electrode and a positive electrode are wound in a spiral shape using a sulfonated polyolefin nonwoven fabric as a separator to form an electrode group, which is inserted into a battery case, and 6 mol / L KOH aqueous solution (LiOH-containing 30 g / L) is injected as an electrolyte. Thus, a Sub-C type nickel-hydrogen battery was constructed. N / P is 1.5 and the nominal capacity is 2.8 Ah.

  Connect the assembled battery to a charge / discharge tester (Charge / Discharge Test Equipment manufactured by Instrument Center) and charge and discharge 1C rate (103% charge, 0.8V discharge cut-off) in a constant temperature bath set at 25 ° C. Ten cycles were performed for activation. The 1C rate is an amount of current that can discharge or charge the entire capacity of the battery in one hour.

Cycle life characteristics test results (initial characteristics)
The life characteristics of the batteries using Examples 1, 2 and Comparative Example 5 as the negative electrode are shown in FIG. In the figure, the utilization rate is a value obtained by converting the discharge capacity with the positive electrode capacity (289 mAh / g) at the time of one-electron reaction as 100% utilization rate. Hereinafter, unless otherwise specified, the utilization rate has the same meaning. In Examples 1 and 2, a utilization rate exceeding 90% was obtained from the initial stage. On the other hand, the conventional MmNi _5- based alloy (Comparative Example 5) required about 50 cycles to stabilize the capacity.

Test results of cycle life characteristics Table 4 shows the test results of the cycle life characteristics of batteries using Examples 1 to 9 and Comparative Examples 1 to 5 as negative electrodes. Table 4 shows that the 1C rate charge / discharge (105% charge = 63 minute charge, 0.8V discharge cut-off) was repeatedly performed without the alkali treatment of the negative electrode, and the capacity retention rate after 100 cycles, after 300 cycles and after 500 cycles. (Ratio of discharge capacity to rated battery capacity (%)).

  In all of Examples 1 to 9 and Comparative Examples 1 to 5, no significant decrease in capacity was observed up to 100 cycles, but at 500 cycles, Comparative Example 3 having a B / A value of 4.8 or more. Compared to 5, Examples 1 to 9 showed higher capacity retention rates.

Test Results of Output Characteristics FIG. 5 shows the results of a high rate discharge test in batteries using Examples 3 to 5 and Comparative Examples 2, 3, and 5 as the negative electrode. FIG. 5 shows an average discharge voltage when discharging to 1 V after 103% charging at a 1 C rate using the discharge rate as a parameter. Examples 3 to 5 show a higher discharge voltage than Comparative Examples 2, 3, and 5. In particular, in Example 5, the average voltage exceeds 1.05 V even in discharge at a rate of 10 C (the amount of current that can discharge the rated battery capacity in 6 minutes), and it can be said that it has a high discharge voltage. About Examples 1-9 and Comparative Examples 1-5, the utilization factor in said high rate discharge conditions is shown in Table 5.

  The batteries having the negative electrodes of Examples 1 to 9 have a high utilization rate at a high discharge rate as compared with the batteries having the negative electrodes of Comparative Examples 1 to 5, and the utilization is 40% or more even at 10C rate discharge. Showed the rate. In particular, the batteries having the negative electrodes of Examples 4 and 5 exhibited a high utilization rate exceeding 50% in 10C rate discharge.

Test results of self-discharge characteristics As for the self-discharge characteristics, the batteries of Examples 1 to 9 and Comparative Examples 1 to 5 after the 300-cycle life test were charged at 105% at a 1C rate and left in a high temperature bath at 45 ° C. for 1 month. Thereafter, the discharge capacity when discharged to 0.8 V and the capacity before being left to stand are compared, and the result of obtaining the capacity storage rate is shown in Table 6.

  Compared with Comparative Examples 1-5, Examples 1-9 showed a high capacity | capacitance preservation | save rate. Among them, Examples 2 to 7 obtained particularly excellent self-discharge characteristics.

High temperature characteristics The degree of battery capacity recovery after standing at high temperatures required for practical use was investigated. 100 cells of each of Examples 1 to 9 and Comparative Examples 1 to 5 were produced, discharged at a 1C rate to 0.8 V, and left in a high temperature bath at 65 ° C. for 1.5 months. Thereafter, the circuit voltage of any battery was 0V. When each battery was charged at a constant current of 1.4 A, all of the batteries of the present application recovered to the discharge capacity before being left, but in the batteries of Comparative Examples 1 to 5, 5 of the 100 cells were short-circuited, and thus the batteries were charged. It became impossible.

<Lattice constant of each crystal phase>
Tables 7 and 8 show the lattice constants of the respective phases derived from the Rietveld analysis results of Examples 1 to 9 and Comparative Examples 1 to 5, respectively. It can be seen that the lattice constant c-axis length of the crystal structure of the CaCu ₅ type phase of Examples 1 to 9 is smaller than the lattice constant c axis length of the CaCu ₅ type phase of Comparative Example 5.

In Examples 1 to 9, the lattice constant a-axis length of the CaCu ₅ type phase and the lattice constant a axis length of the crystal structure of the Ce ₅ Co ₁₉ type phase, the Pr ₅ Co ₁₉ type phase, and the Ce ₂ Ni ₇ type phase are used. It can be seen that the difference between these values is within the range of ± 0.01 mm. On the other hand, in Comparative Examples 1 to 5, the CaCu ₅ type phase has a lattice constant a-axis length selected from a Ce ₅ Co ₁₉ type phase, a Pr ₅ Co ₁₉ type phase, and a Ce ₂ Ni ₇ type phase. The difference in the lattice constant a-axis length of the crystal structure of one or more phases exceeded 0.02 超 え る.

As described above, in Examples 1 to 9, the c-axis lattice constant was smaller than that of the general-purpose AB ₅ alloy (Comparative Example 5), and the a-axis lengths of the respective crystal phases were substantially the same. As is clear from the above experimental results, the batteries using Examples 1 to 9 are excellent in high rate discharge characteristics and self-discharge characteristics while maintaining good discharge capacity and cycle life characteristics. This is probably because volume expansion in the c-axis direction of the alloy during charging and discharging was suppressed.

<Effect of alkali treatment (powder)>
Alkali treatment by charging 1 kg of the alloy powders of Examples 1 to 9 and Comparative Examples 1 to 5 into 2.5 kg of 6 mol / L potassium hydroxide aqueous solution heated to 50 to 110 ° C. and heating with stirring for 30 to 120 minutes. Went. For the heating, a throw-in heater was used, and the liquid temperature during the alkali treatment was appropriately confirmed by visually observing a thermometer different from the controller thermometer. The temperature change during the treatment was ± 2 ° C., and the temperature drop upon charging the alloy recovered to the predetermined temperature within 5 minutes.

  In the washing operation after the alkali treatment, first, fine powdery floating substances were removed together with the alkali solution by decantation. Subsequently, the alloy residue was stirred and washed with distilled water, and the suspended matter was removed again by decantation. After repeating this operation three times, water was removed by suction filtration and dried under reduced pressure at 40 ° C.

  It is known that the rare earth-based hydrogen storage alloy elutes the rare earth component of the surface layer by alkali treatment and reprecipitates as a needle-like hydroxide. At 70 ° C for 30 minutes, the acicular crystals are slightly precipitated on the alloy surface. However, the stricter the treatment conditions (the higher the temperature or the longer the time), the greater the amount of acicular crystals deposited. It was confirmed that the etching of the alloy surface progressed. Under the condition of 50 ° C. for 2 hours, the acicular crystal precipitates could not be confirmed on the alloy surface.

  FIG. 6 shows the result of measuring the residual magnetization of the alloy after the alkali treatment using a VSM (vibrating sample magnetometer; manufactured by Tamagawa Seisakusho). The value of remanent magnetization increased in proportion to the processing temperature and time, and it was confirmed from the magnetic measurement that the degree of etching of the alloy surface layer depends on both the processing temperature and time. On the other hand, the value of remanent magnetization became constant at 80 ° C. for 1 hour. From the above results, it was suggested that the optimum alkali treatment was at 80 ° C. for 1 hour.

  From the results shown in Table 9, it was confirmed that a long treatment time was required when the alkali treatment temperature was low, and that the effect was exhibited in a short time when the treatment temperature was high. Further, it was found that the discharge capacity is improved early when the treatment is strict. This means that the removal of the inert phase on the alloy surface is accelerated by tightening the processing conditions. On the other hand, it has been confirmed that excessive treatment for a long time or at a high temperature causes a reduction in discharge capacity. This is presumably because dissolution progressed to the inside of the alloy particles due to excessive treatment and the hydrogen storage phase was lost.

  That is, as the alkali treatment conditions for the hydrogen storage alloy of the present invention, the heating temperature is preferably set in the range of 60 to 100 ° C, more preferably about 80 ° C. The concentration of the caustic aqueous solution is not limited to that of the present embodiment, and may be practically set in the range of 0.1 mol / L to 10 mol / L.

  In order to examine how the effect of the alkali treatment affects the cycle life, Table 10 shows the capacity retention rate at the 500th cycle. In the charge / discharge cycle life test, 1C rate charge / discharge (105% charge = 63 minute charge, 0.8 V discharge cut-off) was repeatedly performed.

  It has been found that the cycle life of the alloy of the present invention can be further improved by performing an optimum alkali treatment.

<Effect of alkali treatment (electrode)>
The alloy electrodes (2 kg) of Examples 1 to 9 and Comparative Examples 1 to 5 were charged into 3 kg of a 6 mol / L potassium hydroxide aqueous solution heated to 70 to 90 ° C., and heated with stirring for 30 to 120 minutes for alkali treatment. Went. The heating method and the like are the same as in the alkali treatment of the powder, and the electrode production method is the same as the negative electrode production method used in the evaluation as a sealed battery.

  Table 11 shows the capacity retention rate at the 500th cycle. In the charge / discharge cycle life test, 1C rate charge / discharge (105% charge = 63 minute charge, 0.8 V discharge cut-off) was repeatedly performed.

When the alkali treatment was performed after the electrode formation, the conditions at a higher temperature were better than the powder alkali treatment. This is probably because the electrode contains a binder, so that a slightly strict alkali treatment is required.

Claims (7)

General composition formula:
RE _(1-x) Mg _x Ni _y Al _z
(Wherein, RE is one or more selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu. And the subscripts x, y, and z are in the ranges indicated by 0.05 ≦ x ≦ 0.2, 4.0 ≦ y ≦ 4.4, and 0.1 ≦ z ≦ 0.3, respectively. Having a composition represented by
The phase having the CaCu ₅ type crystal structure is 40 to 90% by weight of the total composition, the phase having the Ce ₅ Co ₁₉ type crystal structure is _{5 to} 39% by weight of the total composition, and the phase having the Pr ₅ Co ₁₉ type crystal structure is all A hydrogen storage alloy characterized by being 3 to 20% by weight of the composition. The difference between the a-axis length of the Ce ₅ Co ₁₉ type crystal structure and the a-axis length of the Pr ₅ Co ₁₉ type crystal structure with the a-axis length of the CaCu ₅ type crystal structure is ± 0.02 mm. The hydrogen storage alloy according to claim 1, wherein the hydrogen storage alloy is in a range. The a-axis lengths of the crystal structures of all crystal phases are all in the range of 4.95 to 5.05 mm,
3. The hydrogen storage alloy according to claim 1, wherein the c-axis length of the CaCu _5- type crystal structure is in the range of 3.98 to 4.02 mm.   The hydrogen storage alloy according to any one of claims 1 to 3, wherein an alkali heat treatment is performed at a temperature of 60 to 100 ° C using a caustic aqueous solution having a concentration of 0.1 mol / L to 10 mol / L.   The hydrogen storage alloy according to claim 4, wherein the remanent magnetization value is in the range of 0.5 to 0.9 emu / g.   6. A hydrogen storage alloy electrode comprising particles comprising the hydrogen storage alloy according to any one of claims 1 to 5 and a conductive core body that holds the particles.   A nickel-hydrogen secondary battery comprising the hydrogen storage alloy electrode according to claim 6 as a negative electrode.

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