JP5481803B2 - Nickel metal hydride storage battery - Google Patents

Description

  The present invention relates to a nickel metal hydride storage battery.

  Nickel metal hydride storage batteries have high energy density, so they are used as power sources for small electronic devices such as digital cameras and notebook computers, and because the operating voltage is equivalent to and compatible with primary batteries such as alkaline manganese batteries. As a substitute for the primary battery, the battery is widely used, and the demand for the battery is expanding dramatically.

  By the way, this type of nickel metal hydride storage battery includes a nickel electrode containing a positive electrode active material mainly composed of nickel hydroxide, a negative electrode mainly composed of a hydrogen storage alloy, a separator, and an alkaline electrolyte. Is. Among these battery constituent materials, the hydrogen storage alloy, which is the main material of the negative electrode, has a great influence on the performance of nickel-metal hydride storage batteries such as discharge capacity and cycle characteristics. Various hydrogen storage alloys have been studied in the past. Has been.

As the hydrogen storage alloy, a rare earth-Mg-Ni alloy such as a LaCaMgNi ₉ alloy having a PuNi ₃ type crystal structure (Patent Document 1) is known, and by using the alloy as a negative electrode of a nickel metal hydride storage battery, It has been reported that a discharge capacity exceeding that of an AB ₅ type rare earth-Ni alloy having a CaCu ₅ type crystal structure can be obtained.

JP-A-11-217643

  However, in a nickel-metal hydride storage battery using a conventional rare earth-Mg-Ni-based alloy as an active material for a negative electrode, although the initial discharge capacity shows a relatively good value, a long time has passed since the nickel-metal hydride storage battery was created. In such a case, the discharge capacity tended to decrease gradually.

  In view of the above-described problems of the prior art, the present invention is a nickel metal hydride storage battery using a rare earth-Mg—Ni based alloy as an active material for a negative electrode, and a long time has passed since the nickel hydride storage battery was created. Even in such a case, an object is to provide a nickel-metal hydride storage battery in which the discharge capacity is unlikely to decrease.

In order to solve the above-mentioned problems, the present inventors have conducted intensive research. As a result, different crystal structures such as a crystal phase having a Pr ₅ Co ₁₉ type crystal structure and a crystal phase having a Ce ₂ Ni ₇ type crystal structure have been obtained. A rare earth-Mg-Ni-based hydrogen storage alloy in which a plurality of crystal phases are stacked is used as a negative electrode, and nickel is used using an alkaline electrolyte in which sodium ion concentration and potassium ion concentration are adjusted to a predetermined concentration range It has been found that by configuring a hydrogen storage battery, a nickel hydride storage battery in which the discharge capacity is unlikely to decrease can be configured, and the present invention has been completed.

That is, in the present invention, two or more crystal phases having different crystal structures are laminated in the c-axis direction of the crystal structure, and the content of the crystal phase having a CaCu ₅ type crystal structure is 15% by mass or less. A negative electrode comprising a rare earth-Mg-Ni-based hydrogen storage alloy;
Alkaline electrolyte having a sodium ion concentration of 4.0 mol / liter or more, a potassium ion concentration and a lithium ion concentration of less than 2.0 mol / liter, and a total of the ion concentrations of 8.0 mol / liter or less And a nickel-metal hydride storage battery.

  Moreover, this invention provides the said nickel hydride storage battery whose content of the cobalt element of the said hydrogen storage alloy is 2.0 atomic% or less.

  The present invention also provides the nickel-metal hydride storage battery, wherein the alkaline electrolyte has a lithium ion concentration of less than 1.0 mol / liter.

According to the nickel metal hydride storage battery of the present invention, by using a hydrogen storage alloy in which two or more crystal phases having different crystal structures are stacked in the c-axis direction of the crystal structure as a negative electrode, charging and discharging can be performed. The distortion of the crystal phase caused by the storage and release of hydrogen at the time is relaxed, and the deterioration of the electrode is suppressed.
The rare earth-Mg-Ni-based hydrogen storage alloy has a CaCu ₅ type crystal structure content of 15% by mass or less, a sodium ion concentration of 4.0 mol / liter or more, and potassium By using an alkaline electrolyte having an ion concentration and a lithium ion concentration of 2.0 mol / liter or less, oxidative deterioration of the hydrogen storage alloy by the alkaline electrolyte is prevented.
As described above, according to the present invention, both the deterioration of the electrode due to the distortion of the alloy due to charging and discharging and the oxidative deterioration of the electrode due to the alkaline electrolyte are prevented. The reduction of the positive electrode due to the positive electrode reduction and the element eluted from the negative electrode can be suppressed, and the nickel-metal hydride storage battery having this configuration is unlikely to have a reduced discharge capacity after storage.

The nickel metal hydride storage battery according to the present invention has a crystal phase content of 15% by mass obtained by laminating two or more crystal phases having different crystal structures in the c-axis direction of the crystal structure and having a CaCu ₅ type crystal structure. The composition is R1 _a Mg _b Ni _c (where R1 is one or more elements selected from the group consisting of La, Pr, Nd and Y, and 18.2 <a <18. 7, 3.5 <b <4.7, 73.3 <c <75.0), cobalt element content is 0 atomic%, and hydrogen equilibrium pressure is 0.1 Mpa or less A negative electrode comprising a Mg—Ni-based hydrogen storage alloy, a sodium ion concentration of 5.0 mol / liter to 8.0 mol / liter, and a potassium ion concentration and a lithium ion concentration of 0 mol / liter, respectively. And an alkaline electrolyte.

As described above, the hydrogen storage alloy included in the negative electrode is a rare earth-Mg—Ni-based hydrogen storage alloy in which two or more crystal phases having different crystal structures are laminated in the c-axis direction of the crystal structure. It is.
Examples of the crystal phase include a crystal phase composed of a rhombohedral La ₅ MgNi ₂₄ type crystal structure (hereinafter also simply referred to as a La ₅ MgNi ₂₄ phase), and a crystal phase composed of a hexagonal Pr ₅ Co ₁₉ type crystal structure (hereinafter simply referred to as a simple phase). Pr ₅ Co ₁₉ phase), crystal phase composed of rhombohedral Ce ₅ Co ₁₉ type crystal structure (hereinafter also simply referred to as Ce ₅ Co ₁₉ phase), crystal phase composed of hexagonal Ce ₂ Ni ₇ type crystal structure (Hereinafter also simply referred to as Ce ₂ Ni ₇ phase), crystal phase composed of rhombohedral Gd ₂ Co ₇ type crystal structure (hereinafter also simply referred to as Gd ₂ Co ₇ phase), hexagonal CaCu ₅ type crystal structure Crystal phase (hereinafter also simply referred to as CaCu ₅ phase), crystal phase consisting of cubic AuBe ₅ type crystal structure (hereinafter also simply referred to as AuBe ₅ phase) crystal phase consisting of rhombohedral PuNi ₃ type crystal structure (hereinafter simply referred to as simply “AuBe ₅ phase”) PuNi _3-phase also called), and the like ani Rukoto can.
Among these, a hydrogen storage alloy formed by laminating two or more selected from the group consisting of La ₅ MgNi ₂₄ phase, Pr ₅ Co ₁₉ phase, Ce ₅ Co ₁₉ phase, and Ce ₂ Ni ₇ phase is preferably used. . The hydrogen storage alloy formed by laminating these crystal phases has excellent characteristics that the difference in expansion and contraction between the crystal phases is small, so that distortion is not easily generated, and deterioration is not easily caused by repeated storage and release of hydrogen. Have.

Here, the La ₅ MgNi ₂₄ type crystal structure is a crystal structure in which _four AB ₅ units are inserted between A ₂ B ₄ units, and the Pr ₅ Co ₁₉ type crystal structure is A ₂ B It is a crystal structure in which 3 units of AB ₅ units are inserted between ₄ units, and Ce ₅ Co ₁₉ type crystal structure is that 3 units of AB ₅ units are inserted between A ₂ B ₄ units. It is a crystal structure, and the Ce ₂ Ni ₇ type crystal structure is a crystal structure in which _two AB ₅ units are inserted between A ₂ B ₄ units. What is the Gd ₂ Co ₇ type crystal structure? , ₂ units of AB ₅ units are inserted between A ₂ B ₄ units, and the AuBe ₅ type crystal structure is a crystal structure composed of only A ₂ B ₄ units.

The A ₂ B ₄ unit is a structural unit having a hexagonal MgZn ₂ type crystal structure (C14 structure) or a hexagonal MgCu ₂ type crystal structure (C15 structure), and the AB ₅ unit is a hexagonal CaCu ₅ unit. It is a structural unit with a type crystal structure.
A represents any element selected from the group consisting of rare earth elements and Mg, and B represents any element selected from the group consisting of transition metal elements and Al.

  The order of lamination of the crystal phases is not particularly limited, and a combination of specific crystal phases may be such that the crystal phases are laminated with periodicity, and each crystal phase is randomly and periodically laminated. It may be.

The content of each crystal phase is not particularly limited, but the content of the crystal phase having the La ₅ MgNi ₂₄ type crystal structure is 0 to 50% by mass, and the Pr ₅ Co ₁₉ type crystal structure. The content of the crystal phase having _{3 to} 80% by mass, the content of the crystal phase having the Ce ₅ Co ₁₉ type crystal structure is 15 to 80% by mass, and the content of the crystal phase having the Ce ₂ Ni ₇ type crystal structure The rate is preferably 0 to 65% by mass.

  The crystal phase having each crystal structure can be identified by, for example, performing X-ray diffraction measurement on the pulverized alloy powder and analyzing the obtained X-ray diffraction pattern by the Rietveld method. .

A schematic diagram of an embodiment of the hydrogen storage alloy is shown in FIG. As shown in FIG. 1, the hydrogen-absorbing alloy as one embodiment, a CaCu ₅ phase, two and Pr ₅ Co ₁₉ phase adjacent the CaCu ₅ phase, two adjacent to the Pr ₅ Co ₁₉ phase The Ce ₂ Ni ₇ phase is laminated in the c-axis direction of the crystal structure.

It can be confirmed by observing the lattice image of the alloy using TEM that two or more crystal phases having different crystal structures are laminated in the c-axis direction of the crystal structure. An example of a lattice image of the first hydrogen storage alloy according to the present invention is shown in FIGS.
According to these figures, AB ₅ hydrogen storage alloy, a crystalline phase sequence AB ₅ unit is inserted 3 units worth between A ₂ B ₄ units are repeated, between A ₂ B ₄ units It is shown that the crystal phase in which the arrangement in which four units are inserted is repeated is laminated in the c-axis direction. The former crystal phase is a crystal phase composed of a Ce ₅ Co ₁₉ type crystal structure, and the latter is a crystal phase composed of a LaMgNi ₂₄ type crystal structure. Thus, by observing the lattice image by TEM, it is possible to confirm that the two or more crystal phases having different crystal structures are laminated in the c-axis direction, which is a constituent element of the present invention. it can.

As described above, the hydrogen storage alloy constituting the negative electrode in the present invention is formed by stacking two or more crystal phases having different crystal structures in the c-axis direction of the crystal structure. The distortion of the crystal phase at the time of occlusion is relaxed by the other adjacent crystal phase. Therefore, the negative electrode comprising the hydrogen storage alloy has an effect that the alloy is less likely to be pulverized even if hydrogen storage and release are repeated by charging and discharging, and deterioration is unlikely to proceed.
Further, since the hydrogen storage alloy is a rare earth-Mg-Ni based hydrogen storage alloy, there is an effect that the discharge capacity is large.

The hydrogen storage alloy has a composition of the general formula R1 _a R2 _b R3 _c (where R1 is one or more elements selected from the group consisting of rare earth elements including Y, R2 is Mg, Ca, Sr) And one or more elements selected from the group consisting of Ba and Ba, R3 is Ni, Co, Mn, Al, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr and 1 or 2 or more elements selected from the group consisting of Hf, preferably 10 ≦ a ≦ 30, 1 ≦ b ≦ 10, 65 ≦ c ≦ 90, satisfying a + b + c = 100) Can be used for
In particular, from the viewpoint of setting the CaCu ₅ phase content to 15% by mass or less, R2 is mainly composed of Mg, R3 is mainly composed of Ni, and components other than Ni and Co are 5 atomic% or less. Furthermore, it is preferable that 3.0 ≦ c / (a + b) ≦ 4.0.

Further, the content of the cobalt (Co) element is preferably 2.0 atomic% or less, more preferably 1.0 atomic% or less, and in particular, substantially free of Co element (that is, More preferably, the content is 0.1 atomic% or less.
Co element was conventionally added to enhance the corrosion resistance of the hydrogen storage alloy with respect to the alkaline electrolyte, but in the nickel metal hydride storage battery according to the present invention, the predetermined hydrogen storage alloy and the alkaline electrolyte were provided. Thus, even when the content of the Co element is small, the corrosion resistance of the hydrogen storage alloy with respect to the alkaline electrolyte can be improved.
In addition, by limiting the content of the Co element to the above range, there is an effect that the recoverability when the charge / discharge of the nickel-metal hydride storage battery is repeated or left for a long time is remarkably improved.

Further, the hydrogen storage alloy has a content of a crystal phase having a CaCu ₅ type crystal structure (CaCu ₅ phase) of 15% or less, preferably 10% or less.
Conventionally, when a hydrogen storage alloy containing a CaCu ₅ phase is used, the discharge capacity is reduced, but it is known that the nickel hydrogen storage battery has excellent cycle characteristics. In the present invention, the CaCu ₅ phase By making the content rate of the above range, the deterioration resistance performance of the negative electrode containing the hydrogen storage alloy to the alkaline electrolyte can be enhanced.

The hydrogen storage alloy preferably has a hydrogen equilibrium pressure of 0.07 MPa or less. Conventional hydrogen storage alloys have the property that when hydrogen equilibrium pressure is high, it is difficult to absorb hydrogen and it is easy to release the absorbed hydrogen. When the high rate characteristics of the hydrogen storage alloy are enhanced, hydrogen self-releases. It was easy to do.
However, it is a rare earth-Mg-Ni-based hydrogen storage alloy in which two or more crystal phases having different crystal structures are laminated, and particularly when the content of the crystal phase having a CaCu ₅ type crystal structure is 15% by mass or less. In some hydrogen storage alloys, good high rate characteristics can be obtained even when the hydrogen equilibrium pressure is set to a low value of 0.07 MPa or less. A nickel metal hydride battery using the hydrogen storage alloy as a negative electrode has excellent high rate characteristics and Hydrogen self-release (self-discharge in a battery) is unlikely to occur. This is considered to be because the diffusibility of hydrogen in the alloy was improved.

  The hydrogen equilibrium pressure means an equilibrium pressure (release side) of H / M = 0.5 in the 80 ° C. PCT curve (pressure-composition isotherm).

The hydrogen storage alloy which comprises a negative electrode can be obtained with the following manufacturing methods.
That is, the method for producing a hydrogen storage alloy as one embodiment includes a melting step of melting an alloy raw material blended to have a predetermined composition ratio, and rapid solidification of the molten alloy raw material at a cooling rate of 1000 K / second or more. And an annealing step of annealing the cooled alloy in a pressurized inert gas atmosphere in a temperature range of 860 to 1000 ° C.

More specifically, first, a predetermined amount of raw material ingot (alloy raw material) is weighed based on the chemical composition of the target hydrogen storage alloy.
In the melting step, the alloy raw material is put in a crucible and heated to 1200 to 1600 ° C., for example, in an inert gas atmosphere or in a vacuum to melt the alloy raw material.

  In the cooling step, the molten alloy material is cooled and solidified. The cooling rate is preferably 1000 K / second or more (also called rapid cooling). By rapidly cooling at 1000 K / second or more, there is an effect that the alloy composition is refined and homogenized.

  As the cooling method, specifically, a melt spinning method having a cooling rate of 100,000 K / second or more, a gas atomizing method having a cooling rate of about 10,000 K / second, or the like can be preferably used.

In the annealing step, heating is performed at 860 to 1000 ° C. using, for example, an electric furnace in a pressurized state under an inert gas atmosphere. As a pressurizing condition, 0.2 to 1.0 MPa (gauge pressure) is preferable. Moreover, it is preferable that the processing time in this annealing process shall be 3 to 50 hours.
The distortion of the crystal lattice is removed by such an annealing process, and the hydrogen storage alloy that has undergone the annealing process finally becomes a hydrogen storage alloy in which two or more crystal phases having different crystal structures are laminated.

The negative electrode constituting the nickel-metal hydride storage battery of the present invention includes the hydrogen storage alloy obtained as described above as a hydrogen storage medium. When the hydrogen storage alloy is used for an electrode as a hydrogen storage medium, the hydrogen storage alloy is preferably used after being pulverized.
The hydrogen storage alloy may be pulverized at the time of electrode fabrication either before or after annealing, but since the surface area is increased by pulverization, it is desirable to pulverize after annealing from the viewpoint of preventing surface oxidation of the alloy. The pulverization is preferably performed in an inert atmosphere to prevent oxidation of the alloy surface.
For the pulverization, for example, mechanical pulverization or hydrogenation pulverization is used.

Next, the alkaline electrolyte that constitutes the nickel-metal hydride storage battery according to the present invention will be described.
As the alkaline electrolyte, one having a sodium ion concentration of 4.0 mol / liter or more and a potassium ion concentration and a lithium ion concentration of less than 2.0 mol / liter, preferably a sodium ion concentration of 5. A material having a concentration of 0 mol / liter to 7.0 mol / liter, a potassium ion concentration of less than 2.0 mol / liter, and a lithium ion concentration of 0 to 1.0 mol / liter is used.
In particular, from the viewpoint of further reducing the corrosion of the alloy and suppressing self-discharge, the lithium ion concentration is preferably 0.1 mol / liter or less, and more preferably not containing lithium ions.

In the alkaline electrolyte, the total ion concentration of the sodium ion, the potassium ion, and the lithium ion is preferably 8.0 mol / liter or less, and is 5.0 to 7.0 mol / liter. Is more preferable.
Furthermore, a sodium ion single system may be used from a viewpoint of suppressing alloy corrosion.

  By using an alkaline electrolyte having such an ion concentration, the alkaline electrolyte can prevent adverse effects such as oxidation on the rare earth-Mg-Ni-based hydrogen storage alloy and is immersed in the alkaline electrolyte in the battery. There is an effect that the oxidative deterioration of the hydrogen storage alloy that is brought into the state of being formed can be suppressed.

  In addition, various additives can be added to the electrolytic solution in order to improve the corrosion resistance to the alloy, improve the overvoltage at the positive electrode, improve the corrosion resistance of the negative electrode, and improve self-discharge. As this additive, oxides, hydroxides, etc., such as yttrium, ytterbium, erbium, calcium, and zinc, can be used singly or in combination.

On the other hand, the positive electrode of the nickel metal hydride storage battery is not particularly limited, but generally a nickel hydroxide composite oxide mainly composed of nickel hydroxide and mixed with zinc hydroxide or cobalt hydroxide. A positive electrode including a positive electrode active material can be preferably used, and a positive electrode including the nickel hydroxide composite oxide uniformly dispersed by a coprecipitation method can be more preferably used.
As additives other than the nickel hydroxide composite oxide, cobalt hydroxide, cobalt oxide and the like as a conductive modifier can be used, and the nickel hydroxide composite oxide is coated with cobalt hydroxide. A part of these nickel hydroxide composite oxides oxidized using oxygen or oxygen-containing gas, or a chemical such as K ₂ S ₂ O ₈ or hypochlorous acid can be used.
In addition, as the additive, as a substance for improving the oxygen overvoltage, a rare earth element compound such as Y or Yb or a Ca compound can be used. Since some of rare earth elements such as Y and Yb are dissolved and disposed on the negative electrode surface, an effect of suppressing corrosion of the negative electrode active material can be expected.

The positive electrode and the negative electrode may contain a conductive agent, a binder, a thickener, and the like as other components in addition to the main components as described above.
The conductive agent is not limited as long as it is an electron conductive material that does not adversely affect battery performance. Usually, natural graphite (such as scale-like graphite, scale-like graphite, earth-like graphite), artificial graphite, carbon black, acetylene black, A conductive material such as ketjen black, carbon whisker, carbon fiber, vapor-grown carbon, metal (nickel, gold, etc.) powder, metal fiber, or the like can be included as one or a mixture of two or more.
Among these, as the conductive agent, acetylene black is preferable from the viewpoints of electron conductivity and coatability. The addition amount of the conductive agent is preferably 0.1% by mass to 10% by mass with respect to the total weight of the positive electrode or the negative electrode. In particular, it is desirable to use acetylene black after being pulverized into ultrafine particles of 0.1 to 0.5 μm because the necessary carbon amount can be reduced.
As a method of mixing them, a method that can be as uniform as possible is preferable. For example, a powder mixer such as a V-type mixer, an S-type mixer, a grinder, a ball mill, a planetary ball mill, or the like may be dry or wet. The method used in the above can be adopted.
As the binder, usually, thermoplastic resins such as polytetrafluoroethylene (PTFE), polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber A polymer having rubber elasticity such as can be used as one kind or a mixture of two or more kinds. The addition amount of the binder is preferably 0.1 to 3% by mass with respect to the total amount of the positive electrode or the negative electrode.
As said thickener, polysaccharides, such as carboxymethylcellulose, methylcellulose, xanthan gum, etc. can be normally used as 1 type, or 2 or more types of mixtures. The addition amount of the thickener is preferably 0.1 to 0.3% by mass with respect to the total amount of the positive electrode or the negative electrode.

  The positive electrode and the negative electrode are prepared by mixing the active material, the conductive agent, and the binder in an organic solvent such as water, alcohol, and toluene, and then applying the obtained mixed liquid onto a current collector and drying it. Produced suitably. As the application method, for example, a method of applying an arbitrary thickness and an arbitrary shape using means such as roller coating such as an applicator roll, screen coating, blade coating, spin coating, and per coating is preferable. It is not limited to.

As the current collector, an electron conductor that does not adversely affect the exchange of electrons with the active material in the battery that is configured can be used without particular limitation. As the current collector, for example, from the viewpoint of reduction resistance and oxidation resistance, nickel or nickel-plated steel plate can be suitably used as a material, and as a shape, a foam, a molded body of a fiber group, A three-dimensional base material subjected to uneven processing or a two-dimensional base material such as a punching plate can be suitably used. Moreover, there is no limitation in particular also about the thickness of this electrical power collector, The thing of 5-700 micrometers is used suitably.
Among these current collectors, for the positive electrode, it is necessary to use a material having a porous structure, which is made of nickel having excellent corrosion resistance and oxidation resistance to alkali, and having a structure excellent in current collection. preferable. In addition, as a negative electrode, it is preferable to use a punching plate obtained by applying nickel plating for improving reduction resistance to an iron foil which is inexpensive and excellent in conductivity.
The punching diameter is preferably 2.0 mm or less, and the opening ratio is preferably 40% or more. This makes it possible to improve the adhesion between the negative electrode active material and the current collector even with a small amount of binder.

As a separator of a nickel metal hydride storage battery, it is preferable that a porous film or a nonwoven fabric exhibiting excellent rate characteristics be used alone or in combination of two or more. Examples of the material constituting the separator include polyolefin resins such as polyethylene and polypropylene, and nylon.
The basis weight of the separator is preferably 40 g / m ² to 100 g / m ² . If it is less than 40 g / m ² , the short circuit and the self-discharge performance may be deteriorated, and if it exceeds 100 g / m ² , the proportion of the separator per unit volume increases, so that the battery capacity tends to decrease. The air permeability of the separator is preferably 1 cm / sec to 50 cm / sec. If it is less than 1 cm / sec, the internal pressure of the battery may increase, and if it exceeds 50 cm / sec, the short circuit and the self-discharge performance may be deteriorated. The average fiber diameter of the separator is preferably 1 μm to 20 μm. When the thickness is less than 1 μm, the strength of the separator decreases, and the defect rate in the battery assembly process may increase. When the thickness exceeds 20 μm, the short circuit and the self-discharge performance may decrease.
The separator is preferably subjected to a hydrophilic treatment. For example, the surface of polyolefin resin fibers such as polypropylene may be subjected to sulfonation treatment, corona treatment, fluorine gas treatment, plasma treatment, or a mixture of those already subjected to these treatments. In particular, separators that have been sulfonated have a high ability to adsorb impurities such as NO ₃ ⁻ , NO ₂ ⁻ , and NH ₃ ⁻ that cause the shuttle phenomenon and elements eluted from the negative electrode. ,preferable.

  The sealed nickel-metal hydride storage battery as one embodiment of the present invention is preferably produced by injecting the electrolyte solution before or after laminating the positive electrode, the separator, and the negative electrode, and sealing with an exterior material. Is done. In a sealed nickel-metal hydride storage battery in which a power generation element in which a positive electrode and a negative electrode are stacked via the separator is wound, the electrolyte is injected into the power generation element before or after winding. preferable. As an injection method, it is possible to inject at normal pressure, but a vacuum impregnation method, a pressure impregnation method, and a centrifugal impregnation method can also be used. Examples of the material of the outer package of the sealed nickel-metal hydride storage battery include nickel-plated iron, stainless steel, polyolefin resin, and the like.

  The configuration of the sealed nickel-metal hydride storage battery is not particularly limited, and batteries including a positive electrode, a negative electrode, and a single-layer or multi-layer separator, such as a coin battery, a button battery, a square battery, a flat battery, Alternatively, a cylindrical battery having a roll-shaped positive electrode, a negative electrode, and a separator can be given.

  Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

(Preparation of positive electrode)
An ammonium complex and an aqueous sodium hydroxide solution were added to an aqueous solution in which nickel sulfate, zinc sulfate and cobalt sulfate were dissolved at a predetermined ratio to form an ammine complex. While vigorously stirring the reaction system, caustic soda is further added dropwise, the pH of the reaction system is controlled to 10 to 13, and the spherical high density nickel hydroxide particles serving as the core layer base material are converted into nickel hydroxide: zinc hydroxide: cobalt hydroxide. = 93: 5: 2 The weight ratio was synthesized.
The high-density nickel hydroxide particles were put into an alkaline aqueous solution adjusted to pH 10 to 13 with caustic soda, and an aqueous solution containing cobalt sulfate and ammonia at predetermined concentrations was added dropwise while stirring the solution. During this time, an aqueous caustic soda solution was appropriately added dropwise to maintain the pH of the reaction bath in the range of 10-13. The pH was maintained in the range of 10 to 13 for about 1 hour, and a surface layer made of a mixed hydroxide containing Co was formed on the surface of the nickel hydroxide particles. The ratio of the surface layer of the mixed hydroxide was 4% by mass with respect to hydroxide core layer base material particles (hereinafter simply referred to as core layer).
Furthermore, nickel hydroxide particles having a surface layer made of the mixed hydroxide were put into a 30% by mass (10 mol / liter) aqueous caustic soda solution at a temperature of 110 ° C. and sufficiently stirred. Subsequently, excess K ₂ S ₂ O ₈ was added to the equivalent of the cobalt hydroxide contained in the surface layer, and it was confirmed that oxygen gas was generated from the particle surface. The active material particles were filtered, washed with water and dried. To the obtained active material particles, 0.2% by mass of a carboxymethyl cellulose (CMC) aqueous solution, 0.3% by mass of polytetrafluoroethylene (PTFE), and 2% by mass of Yb ₂ O ₃ are added to obtain the active material particles. : CMC solute: PTFE: Yb ₂ O ₃ = 97.5: 0.2: 0.3: 2.0% by mass (solid content ratio) paste was filled in a nickel porous body of 350 g / m. . Then, after drying at 80 degreeC, it pressed to predetermined thickness and it was set as the 2000 mAh nickel positive electrode plate.

(Preparation of negative electrode)
A predetermined amount of a raw material ingot (alloy raw material) of each chemical composition shown in Tables 1 to 4 is weighed into a crucible and heated to 1200 ° C. to 1600 ° C. using a high-frequency melting furnace in a reduced pressure argon gas atmosphere. Melted. After melting, a melt spinning method was applied and quenched at a cooling rate of 1000 K / second or more to solidify the alloy.
Next, the obtained alloy ingot was heated in an electric furnace at 930 ° C. for 5 hours in an argon gas atmosphere pressurized to 0.2 MPa (gauge pressure, the same applies hereinafter).
About each obtained hydrogen storage alloy, the equilibrium in H / M = 0.5 of the PCT curve (pressure-composition isotherm) in 80 degreeC was carried out using the Siebelz type PCT measuring apparatus (the Suzuki Shokan Co., Ltd. P73-07). The pressure was determined.
Furthermore, each hydrogen storage alloy was pulverized to an average particle diameter D50 = 50 μm, and the obtained powder was subjected to conditions of 40 kV, 100 mA (Cu tube) using an X-ray diffractometer (manufactured by BrukerAXS, product number M06XCE). An X-ray diffraction test was performed. Furthermore, analysis by the Rietveld method (analysis software: Rietan 2000) was performed, and the generation ratio of the crystal phase was calculated. The results are shown in Tables 1-4.
Next, the alloy powder, a styrene-butadiene copolymer (SBR) aqueous solution, and a methyl cellulose (MC) aqueous solution were mixed at a solid content mass ratio of 99.0: 0.8: 0.2 to form a paste, and a blade Using a coater, it was applied to a punched steel plate in which iron was nickel-plated, dried at 80 ° C., and then pressed to a predetermined thickness to obtain a negative electrode. The amount of active material (alloy amount) contained in the negative electrode was 8.5 g.

(Production of evaluation battery)
The negative electrode, a 100 μm-thick polypropylene non-woven separator subjected to a sulfonation treatment, and the positive electrode are combined and wound into a roll, and 1.97 cc of an alkaline electrolyte having the composition shown in Table 1 is injected and opened. An AA-type sealed nickel-metal hydride storage battery (capacity 2000 mAh) having a valve with a valve pressure of 3.0 MPa was produced.
(Battery conversion treatment)

  This battery was charged with 0.1 ItA (200 mA) for 12 hours, and then the operation of discharging to 0.2 V with 1 ItA was repeated twice at 20 ° C., then stored at 40 ° C. for 48 hours, and formed (activated). .

(Measurement of capacity maintenance rate and recovery rate)
Charging / discharging was repeated 3 cycles in a constant temperature bath of 20 ° C. under a charging condition of 160% at 0.1 ItA and a discharging condition of 1 V at 0.2 ItA. Further, after 160% charging at 0.1 ItA, it was stored in a constant temperature bath at 60 ° C. for 30 days. Then, after storing for 3 hours in a constant temperature bath at 20 ° C., the battery was discharged at 0.2 ItA under the condition that the final voltage was 1 V, and the remaining capacity was measured. And the capacity | capacitance maintenance factor was computed based on the following formula.
Capacity maintenance rate (%) = remaining capacity / final discharge capacity before storage × 100
Subsequently, charging and discharging were performed in a constant temperature bath of 20 ° C. under a charging condition of 160% at 0.1 ItA and a discharging condition where the final voltage was 1 V at 0.2 ItA, and the recovery discharge capacity was obtained. And the recovery rate was computed based on the following formula.
Recovery rate (%) = recovery discharge capacity / final discharge capacity before storage × 100
The results are shown in Tables 1-4.

(Alloy immersion test in alkaline solution)
5 g of alloy powder (D50 = 50 μm) used in each test example and 50 cc of alkaline electrolyte were placed in an Erlenmeyer flask and immersed at 80 ° C. for 3 days. Thereafter, the alloy powder was washed with water and vacuum-dried.
The obtained powder was subjected to an X-ray diffraction test under the conditions of 40 kV, 100 mA (Cu tube) using an X-ray diffractometer (manufactured by Bruker AXS, product number M06XCE). Furthermore, analysis by the Rietveld method (analysis software: Rietan 2000) was performed to calculate the generation ratio of rare earth hydroxide. The results are shown in Tables 1-4.

According to the results shown in Table 1, in the CaCu ₅ single phase which is a conventional AB ₅ type, the type of the alkaline solution from KOH (Comparative Example 5), when instead of NaOH (Comparative Example 4, 6), The amount of rare earth oxide produced by the immersion test is reduced by 39% and 28%, respectively. In the case of Ce ₂ Ni ₇ , which is a conventional A ₂ B ₇ type, the immersion test is performed when the type of alkaline solution is changed from KOH (Comparative Example 8 ) to NaOH (Comparative Examples 7 and 9 ). The production amounts of rare earth oxides due to are reduced by 38% and 24%, respectively.
In contrast, as in the present invention, two or more crystal phases having different crystal structures are laminated in the c-axis direction of the crystal structure, and the content of the crystal phase having a CaCu ₅ type crystal structure is 15 mass. % In the case of a rare earth-Mg—Ni-based hydrogen storage alloy, when the type of the alkaline solution is changed from KOH (Comparative Examples 1 and 3 ) to NaOH (Example 1 and Comparative Example 2), The amount of rare earth oxide produced by the immersion test is reduced by 55% and 50%, respectively, and it is recognized that an exceptional effect that cannot be expected from the conventional configuration is exhibited.

Further, according to the results shown in Table 2, in Comparative Example 11 in which the alkali ion concentration exceeded 8 mol / liter, the amount of rare earth oxide produced by the immersion test increased, and deterioration was likely to proceed due to the alkali solution. Is recognized. On the other hand, in the Example whose alkali ion concentration is 8 mol / liter or less, the production amount of the rare earth oxide by the immersion test is a relatively small value, and it is recognized that the deterioration resistance against the alkaline solution is excellent.

Further, according to the results shown in Table 3, in Comparative Example 19 in which the content of the crystal phase having the CaCu ₅ type crystal structure exceeds 15% by mass, the capacity retention rate is reduced, and the formation of rare earth oxides by the immersion test It can be seen that the amount is increasing and degradation is likely to proceed with the alkaline solution. On the other hand, when the rare earth-Mg-Ni-based hydrogen storage alloy of the present invention in which the content of the crystal phase having a CaCu ₅ type crystal structure is 15% by mass or less is used, the capacity retention rate is improved, In addition, it is recognized that the amount of rare earth oxide produced by the immersion test is suppressed to a low level.
Further, in Comparative Examples 16 and 17 in which the content of Co element is 2.0 atomic% or less, it is recognized that a remarkable effect that the recovery rate exceeds 99% is exhibited.

Furthermore, Table 4 describes the results of Examples and Comparative Examples when the production phase is changed with the alkali concentration of 7 mol / liter. As in Table 3, in the comparative example in which the content of the crystal phase having a CaCu ₅ type crystal structure exceeds 15% by mass, the amount of rare earth oxide produced by the immersion test increases and the deterioration tends to proceed due to the alkaline solution. Is recognized. On the other hand, when the rare earth-Mg-Ni-based hydrogen storage alloy of the present invention in which the content of the crystal phase having a CaCu ₅ type crystal structure is 15% by mass or less is used, the capacity retention rate is improved, In addition, it is recognized that the amount of rare earth oxide produced by the immersion test is suppressed to a low level.
Further, in Example 5 in which the content of Co element is 2.0 atomic% or less, it is recognized that a remarkable effect that the recovery rate exceeds 99% is exhibited.

The schematic diagram which showed one Embodiment of the hydrogen storage alloy. The TEM image which showed an example of the hydrogen storage alloy. The figure which expanded and showed a part of FIG.

Claims (1)

Two or more crystal phases having different crystal structures are laminated in the c-axis direction of the crystal structure, the content of the crystal phase having a CaCu ₅ type crystal structure is 15% by mass or less, and the composition is R1 _a Mg _b Ni _c (where R1 is one or more elements selected from the group consisting of La, Pr, Nd and Y, and 18.2 <a <18.7, 3.5 <b <4 , 73.3 <c <75.0) , a rare earth-Mg—Ni-based hydrogen storage alloy having a cobalt element content of 0 atomic% and a hydrogen equilibrium pressure of 0.1 Mpa or less And an alkaline electrolyte having a sodium ion concentration of 5.0 mol / liter to 8.0 mol / liter and a potassium ion concentration and a lithium ion concentration of 0 mol / liter, respectively. Nickel metal hydride storage battery.