JP2005340003A - Electrode for nickel-hydrogen secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hydrogen storage alloy electrode (electrode for a nickel-hydrogen secondary battery) with high capacity, on which, the problem of extreme lowering of discharging capacity due to repeated charging and discharging cycles is solved. <P>SOLUTION: As the most important factor, the nickel-hydrogen secondary battery uses the hydrogen storage alloy electrode composed of Mg, Ca, and V, having a body-centered cubic lattice crystal structure as a main component. Especially, the hydrogen storage alloy is expressed by general formula: Mg<SB>1-x</SB>Ca<SB>x</SB>V<SB>y</SB>(0<x≤0.35, 1<y≤5). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrode for a nickel metal hydride secondary battery, and more particularly to a hydrogen storage alloy electrode for constituting a high capacity nickel metal hydride secondary battery.

  Nickel metal hydride rechargeable batteries using hydrogen storage alloys as negative electrodes are based on electrochemical hydrogen storage and release reactions, and are used in a wide range of applications from portable devices to large-sized applications as batteries that can be highly reliable and reduced in size and weight. I came. In recent years, research and development of electrode materials has been energetically deployed to meet the demand for higher capacity.

The current of the nickel hydrogen battery negative electrode have been used AB ₅ type alloy represented by MmNi _5, but boasts an overwhelming share, its capacity is already reached more than 85% of the theoretical capacity 372 mAh / g In order to further increase the capacity, it is necessary to develop a new alloy material. Recently, application of Mg-based and V-based alloys having a high hydrogen storage capacity has been studied.

For example, Mg ₂ Ni and MgNi have high theoretical capacities of about 1000 and 670 mAh / g, respectively, but these alloys are difficult to store and release hydrogen at room temperature, and the actual discharge capacities are about 750 and 500 mAh / g, respectively. is there. Moreover, in the strong alkaline aqueous solution used as the electrolytic solution, Mg (OH) ₂ is generated along with the charge / discharge cycle, and a rapid capacity reduction is observed (for example, see Non-Patent Documents 1 and 2). On the other hand, various V ₃ TiNi _0.56 alloys have been developed in which the capacity reduction accompanying the charge / discharge cycle is suppressed, but the discharge capacity is about 400 mAh / g, and a significant increase in capacity has not been achieved ( For example, refer nonpatent literature 3).

As described above, the development of an electrode for a nickel hydride secondary battery having a hydrogen storage alloy having a high capacity and excellent cycle characteristics has not been developed.
T.A. Kohno, M .; Kanda, J .; Electrochem. Soc. 143 (1996) L198. Y. Q. Lei, Y .; M.M. Wu, Q. M.M. Yang, J. et al. Wu, Q. D. Wang, Z .; Phys. Chem. 183 (1994) 379. M.M. Tsukahara, K .; Takahashi, T .; Misima, A .; Isomura, T .; Sakai, J. et al. Alloys Comp. 253 (1997) 583.

  The present invention has been made under the above-described state of the art, and an object of the present invention is to solve the problem that the discharge capacity is remarkably reduced with the charge / discharge cycle. It is to provide an electrode for a hydrogen secondary battery.

  The present invention is an electrode for a nickel hydride secondary battery characterized by using a hydrogen storage alloy mainly composed of an alloy having a body-centered cubic lattice crystal structure composed of Mg, Ca and V.

  The electrode for a nickel hydride secondary battery of the present invention is characterized by using a hydrogen storage alloy mainly composed of an alloy having a body-centered cubic lattice crystal structure composed of Mg, Ca and V. This hydrogen storage alloy is lightweight and contains Mg and Ca, which can store a large amount of hydrogen as constituent elements, and has a body-centered cubic lattice crystal structure similar to V which can store hydrogen at room temperature. Forms an unstable hydride having a distorted body-centered cubic lattice structure, so that a large amount of hydrogen is electrochemically occluded and released, and the corrosion resistance to the electrolyte is improved by alloying Mg, V and Ca. In order to improve, the electrode for high capacity | capacitance nickel metal hydride secondary batteries with few discharge capacity falls with the charging / discharging cycle which was a problem from the past is realizable.

Mg, V and Ca alone absorb about 7.6 wt%, 3.8 wt% and 4.8 wt% hydrogen, respectively, but Mg and Ca hydrides are very stable and electrochemical hydrogen release That is, it is difficult to discharge. On the other hand, V having a body-centered cubic lattice crystal structure absorbs hydrogen up to VH ₂ at room temperature, and the theoretical capacity reaches 1018 mAh / g. In the hydrogen storage process, there are two types of hydrides in which the crystal structure of metal atoms is a body-centered tetragonal lattice structure and a face-centered cubic lattice structure. Since the former is stable, only a discharge capacity of about half the theoretical capacity can be obtained in practice.

  The hydrogen storage alloy constituting the electrode for the nickel metal hydride secondary battery of the present invention has a body-centered cubic lattice crystal structure similar to V, is lighter than V, and has a potentially high hydrogen storage amount of Mg and Ca. Since it is contained as a constituent element, more hydrogen can be occluded electrochemically at room temperature than V. Further, since a part of V is substituted with Mg and Ca having an atomic radius larger than V, the V crystal lattice is greatly distorted, and the metal atom does not form a stable hydride having a body-centered tetragonal lattice structure, An unstable hydride having a distorted body-centered cubic lattice structure is formed. Therefore, almost all of the stored hydrogen can be electrochemically released.

  Furthermore, since Mg, V, and Ca are alloyed, the corrosion resistance with respect to the alkaline electrolyte is improved, and the capacity reduction due to the charge / discharge cycle is suppressed. Note that a hydrogen storage alloy composed mainly of an alloy having a body-centered cubic lattice crystal structure composed of Mg, Ca, and V is milled using a ball mill device, and mechanically mixed and mixed. It can be manufactured by mechanical alloying (MA) for alloying.

In the present invention, the alloy having a body-centered cubic lattice crystal structure composed of Mg, Ca and V is preferably represented by the general formula Mg _1-x Ca _x V _y (0 <x ≦ 0.35, 1 ≦ y ≦ 5). ). Since the atomic radius of Ca is considerably larger than V, when the Ca content exceeds 0.35, all Ca cannot be replaced with V, and there are many by-products other than the body-centered cubic lattice structure. The substantial discharge capacity per weight is reduced. Further, even when the V amount is smaller than 1, it is difficult to completely replace Mg and Ca. On the other hand, when the amount of V exceeds 5, the ratio of lightweight Mg and Ca decreases, so the substantial discharge capacity per unit weight becomes small.

  In addition, when a hydrogen storage alloy mainly composed of an alloy having a body-centered cubic lattice crystal structure composed of Mg, Ca and V is milled together with a carbon material by a ball mill apparatus, an oxide film on the alloy surface is removed. Carbon atoms partially dissolve on the alloy surface, and a large number of phase interfaces are formed. Therefore, the hydrogen diffusion path is increased, the reaction resistance in the hydrogen storage alloy electrode (nickel metal hydride secondary battery electrode) is reduced, and the charge / discharge characteristics are remarkably improved. The carbon material is preferably at least one selected from graphite, amorphous carbon, activated carbon, activated carbon fiber, and carbon nanotube.

  The carbon material is preferably added at a weight ratio of alloy powder: carbon material = 9: 1 or less. Since the carbon material does not absorb hydrogen, if the amount of the carbon material increases, the hydrogen storage amount per unit weight may be substantially reduced.

  In addition, when a hydrogen dissociation catalytic material is present on at least the surface of an alloy composed of Mg, Ca and V and having a body-centered cubic lattice crystal structure, the electrochemical hydrogen storage / release reaction proceeds more easily due to the catalytic effect. Therefore, the improvement of the charge / discharge characteristic in the electrode for nickel metal hydride secondary batteries can be achieved.

  The hydrogen dissociation catalyst material is preferably at least one selected from Ti, Cr, Mn, Fe, Co, Ni, Cu, Rh, Pd, Ru and Pt, which have a particularly high catalytic effect. The abundance ratio of the hydrogen dissociation catalyst material is preferably such that the total number of atoms constituting the hydrogen dissociation catalyst material is 1% or more and 10% or less of the total number of atoms constituting the hydrogen storage alloy. This is because if it is less than 1%, the catalytic effect is not sufficiently exhibited, and if it exceeds 10%, the substantial discharge capacity per unit weight becomes small. The method of existing on the surface is not particularly limited, and may be appropriately selected from mechanical mixing by milling, plating, thermal spraying, vacuum deposition, sputtering, ion plating, and the like.

  Examples of the present invention will be specifically described below, but the present invention is not limited to these examples.

  Mg powder (average particle size 800 μm) and Ca powder (average particle size 2 mm) in a molar ratio of 2: 1 and a chromium steel planetary ball mill device container (capacity 45 ml) together with chrome steel balls (7 mmφ × 22) Then, mechanical alloying (MA) was performed for 80 hours at room temperature under an Ar gas atmosphere using a planetary ball mill.

The weight ratio of the ball to the sample was 30: 1, and the rotational speed of the planetary ball mill device was 600 rpm. As a result of analyzing the powder in the container by X-ray diffraction (XRD), the production of CaMg ₂ was confirmed.

The CaMg ₂ and V powder (average particle size 50 μm) are put in a chrome steel planetary ball mill container (capacity 45 ml) together with chrome steel balls (7 mmφ × 22) in a molar ratio of 1: 3. Using a ball mill apparatus, MA was performed for 1 to 40 hours at room temperature in an Ar gas atmosphere. The weight ratio of the ball to the sample was 30: 1, and the rotational speed of the planetary ball mill device was 600 rpm. The result of analyzing the obtained alloy powder by XRD is shown in FIG. In FIG. 1, 1 is an XRD pattern in a sample before MA, 2 is an XRD pattern in a sample that has been subjected to MA for 1 hour, 3 is an XRD pattern in a sample that has been subjected to MA for 2 hours, and 4 is a sample that has been subjected to MA for 5 hours An XRD pattern 5 indicates an XRD pattern in a sample subjected to MA for 10 hours.

The peaks of the (011) plane, the (002) plane, and the (112) plane corresponding to the body-centered cubic lattice structure of V shift to a low angle side with an increase in MA time, and peaks attributed to CaMg ₂ Disappeared, and it was found that a part of V was gradually replaced with Mg and Ca having larger atomic radii while maintaining the body-centered cubic lattice structure. FIG. 2 shows the change in the lattice constant estimated from the X-ray diffraction peak. The lattice constant increased up to 10 hours with the MA time, and after that, showed a substantially constant value. Therefore, it was found that 10 hours of MA was sufficient for the preparation of the alloy in this system.

Further, when the element distribution in the alloy powder was examined by energy dispersive X-ray spectroscopy (EDS), each element of Mg, Ca, and V was uniformly distributed, not just segregation of CaMg ₂ and V, It was confirmed that the three elements Mg, Ca, and V were indeed alloyed. This alloy powder produced by MA for 10 hours is referred to as Alloy A.

  Further, alloy A and graphite powder were mixed at a weight ratio of 9: 1 and sealed together with a chromium steel ball (7 mmφ × 22) in a chromium steel planetary ball mill device container (capacity 45 ml). The weight ratio of the balls to the mixed powder was 30: 1. Using a planetary ball mill apparatus, mechanical milling was performed for 30 minutes at room temperature in an Ar gas atmosphere. The rotation speed of the planetary ball mill device was 400 rpm. The sample thus obtained is named Alloy B.

  Further, Pd powder as a hydrogen dissociation catalyst material is added to alloy B so that the number of atoms in the alloy B is 2 atomic%, and a chromium steel planet with chrome steel balls (7 mmφ × 22). It was enclosed in a container for a mold ball mill (capacity 45 ml). The weight ratio of the balls to the mixed powder was 30: 1. Using a planetary ball mill apparatus, a mechanical milling treatment was carried out at a rotation speed of 400 rpm for 1 hour in an Ar gas atmosphere at room temperature, and the catalyst material was supported on the surface of the alloy B. The obtained sample is referred to as Alloy C. As a result of analyzing the surface of the alloy C with an electron probe X-ray microanalyzer (EPMA), it was found that fine particles of Pd were uniformly dispersed on the surface.

  Further, 1 g of the alloy A was weighed, and Ni corresponding to 5 atomic% of the total number of atoms in the alloy A was vacuum deposited. Vapor deposition was performed in five steps, and the alloy powder was stirred between the vapor depositions in order to deposit as uniformly as possible. As a result of analyzing the surface of the obtained sample by EPMA, it was found that the alloy surface was uniformly coated with Ni. The obtained sample is referred to as Alloy D.

Next, an electrode for a nickel metal hydride secondary battery was prepared and a charge / discharge test was performed. First, production of an electrode will be described. The alloy powder and Cu powder were mixed with a mortar at a weight ratio of 1: 3, 1 g was weighed, and a load of 10 t / cm ² was applied by a press to obtain a pellet having a diameter of 13 mm. The pellet was sandwiched between Ni nets and subjected to spot welding to obtain an electrode. The electrodes for nickel metal hydride secondary batteries produced using alloys A to D are referred to as electrodes A to D, respectively.

  These electrodes are used as a negative electrode, a Ni plate having an excessive electric capacity at the counter electrode, a Hg / HgO electrode as a reference electrode, a 6M potassium hydroxide aqueous solution as an electrolyte, and an electrode for a nickel hydrogen secondary battery The charge / discharge test was conducted at 25 ° C. in an open triode cell with a capacity restriction. Charging was performed at a current density of 100 mA / g for 13 hours per weight of the hydrogen storage alloy, and at a current density of 50 mA / g, the final voltage was -0.6 Vvs. Discharge to Hg / HgO. The pause time between each step was 30 minutes. The above charge / discharge cycle was repeated a predetermined number of times.

  FIGS. 3 and 4 show the initial discharge curve of each electrode and the change in discharge capacity accompanying the charge / discharge cycle. In the figure, 6 is an initial discharge curve of the electrode A, 7 is an initial discharge curve of the electrode B, 8 is an initial discharge curve of the electrode C, and 9 is an initial discharge curve of the electrode D. Further, 10 indicates a change in the discharge capacity of the electrode A, 11 indicates a change in the discharge capacity of the electrode B, 12 indicates a change in the discharge capacity of the electrode C, and 13 indicates a change in the discharge capacity of the electrode D.

  In any of the electrodes, the discharge curve had one flat portion, and the decrease in discharge capacity with an increase in the number of cycles was small. The initial capacity of the electrode A was 950 mAh / g, and in the electrodes B to D, the discharge capacity increased to 1010 to 1050 mAh / g due to the increase of the hydrogen diffusion path or the catalytic effect of the hydrogen dissociation catalyst material. In the electrode C, the decrease in discharge capacity accompanying the increase in the number of cycles was the smallest, and the initial capacity was maintained even after 20 charge / discharge cycles.

  Further, in the above nickel-hydrogen secondary battery electrode, when mechanical milling treatment is performed with at least one selected from graphite, amorphous carbon, activated carbon, activated carbon fiber and carbon nanotube other than graphite, or other than Pd In the case of using at least one material selected from Ti, Cr, Mn, Fe, Co, Ni, Cu, Rh, Pd, Ru and Pt as a hydrogen dissociation catalyst material, or Ti as a deposition material other than Ni Similar results were obtained when at least one substance selected from Cr, Mn, Fe, Co, Ni, Cu, Rh, Pd, Ru and Pt was used.

As described above, the electrode for a nickel hydride secondary battery using a hydrogen storage alloy mainly composed of an alloy having a body-centered cubic lattice structure composed of Mg, Ca and V has a high capacity. And it became clear that the reduction of the discharge capacity accompanying a charge / discharge cycle was small.

  Mg powder (average particle size 800 μm) Ca powder (average particle size 2 mm) and V powder (average particle size 50 μm) in various molar ratios, together with chromium steel balls (7 mmφ × 22), planetary ball mill device made of chromium steel The sample was placed in a container (capacity 45 ml), and MA was performed for 40 hours at room temperature under an Ar gas atmosphere using a planetary ball mill apparatus. The weight ratio of the ball to the sample was 30: 1, and the rotational speed of the planetary ball mill device was 600 rpm.

When the structure of the obtained alloy powder was analyzed by XRD, a nickel-hydrogen secondary battery electrode was prepared using the obtained alloy powder in the same procedure as in Example 1, and the charge / discharge test was performed under the same conditions. went. The results are shown in Table 1. Mg _0.8 Ca _0.2 V ₁ , Mg _0.65 Ca _0.35 V ₁ , Mg _0.65 Ca _0.35 V ₂ and Mg _0.65 Ca _0.35 V ₅ have a body-centered cubic lattice structure. The formation of an alloy was observed, and the initial discharge capacity was 833 to 1100 mAh / g.

On the other hand, in Mg _0.6 Ca _0.4 V ₁ , Ca remained unreacted, formation of by-products other than the body-centered cubic lattice structure was observed by XRD, and the initial discharge capacity was small. Moreover, since Mg _0.65 Ca _0.35 V _0.5 lacks V, CaMg ₂ appeared as a by-product, and a significant decrease in the initial discharge capacity was observed. On the other hand, Mg _0.65 Ca _0.35 V ₆ forms an alloy having a body-centered cubic lattice structure. However, since the substitution amount of Mg and Ca is small, the discharge capacity is not substantially increased.

As described above, it has a crystal structure of a body-centered cubic lattice composed of Mg, Ca, and V, and has the general formula Mg _1-x Ca _x V _y (0 <x ≦ 0.35, 1 ≦ y ≦ 5) It has been clarified that the electrode for nickel-metal hydride secondary battery characterized by using the hydrogen storage alloy shown in FIG.

  The hydrogen storage alloy for nickel-metal hydride secondary battery electrodes according to the present invention contains Mg and Ca that are lightweight and can store a large amount of hydrogen as constituent elements, and is a body-centered cube similar to V that can store hydrogen at room temperature. It forms an unstable hydride having a lattice crystal structure and a body-centered cubic lattice structure in which metal atoms are distorted, so that a large amount of hydrogen is electrochemically occluded and released, and Mg, V and Since the corrosion resistance to the electrolytic solution is improved by the alloying of Ca, an electrode for a high-capacity nickel metal hydride secondary battery, which has been a conventional problem and has a small decrease in the discharge capacity accompanying the charge / discharge cycle, can be realized.

The figure which shows the time-dependent change of the XRD pattern at the time of Mg-Ca-V alloy production of Example 1. FIG. The figure which shows a time-dependent change of the lattice constant of the alloy crystal lattice at the time of Mg-Ca-V alloy preparation of Example 1. FIG. The figure which shows the initial stage discharge curve of each electrode for nickel-hydrogen secondary batteries in Example 1. FIG. The figure which shows the discharge capacity change of each electrode for nickel-hydrogen secondary batteries in Example 1. FIG.

Explanation of symbols

1 XRD pattern in sample before MA
2 XRD pattern in a sample that was subjected to MA for 1 hour
3 XRD pattern of sample after 2 hours of MA
4 XRD pattern in sample subjected to MA for 5 hours
5 XRD pattern of sample subjected to MA for 10 hours
6 Initial discharge curve of electrode A
7 Initial discharge curve of electrode B
8 Initial discharge curve of electrode C
9 Initial discharge curve of electrode D
10 Change in discharge capacity of electrode A
11 Discharge capacity change of electrode B
12 Change in discharge capacity of electrode C
13 Discharge capacity change of electrode D

Claims (9)

An electrode for a nickel hydride secondary battery using a hydrogen storage alloy composed mainly of an alloy having a body-centered cubic lattice crystal structure composed of Mg, Ca and V. According to claim 1, wherein the alloy having a crystal structure of the body-centered cubic lattice is represented by the general formula _{Mg 1-x Ca x V y} (0 <x ≦ 0.35,1 ≦ y ≦ 5) Electrode for nickel metal hydride secondary battery. 3. The electrode for a nickel-metal hydride secondary battery according to claim 1, wherein the hydrogen storage alloy is milled together with a carbon material. The electrode for a nickel metal hydride secondary battery according to claim 3, wherein the carbon material is added at a weight ratio of alloy: carbon material = 9: 1 or less. The nickel-hydrogen secondary battery electrode according to claim 3 or 4, wherein the carbon material is at least one selected from graphite, amorphous carbon, activated carbon, activated carbon fiber, and carbon nanotube. 6. The electrode for a nickel hydride secondary battery according to claim 1, wherein a hydrogen dissociation catalyst substance is present on at least the surface of the hydrogen storage alloy having a crystal structure of the body-centered cubic lattice. 7. The nickel according to claim 6, wherein the hydrogen dissociating butterfly substance is present in such a manner that the total number of atoms constituting the hydrogen dissociation catalyst substance is 1% or more and 10% or less of the total number of atoms constituting the hydrogen storage alloy. Electrode for hydrogen secondary battery. The nickel hydride according to claim 6 or 7, wherein the hydrogen dissociation catalyst material is at least one selected from Ti, Cr, Mn, Fe, Co, Ni, Cu, Rh, Pd, Ru and Pt. Secondary battery electrode. The electrode for a nickel hydride secondary battery according to any one of claims 6 to 8, wherein the hydrogen dissociation catalyst material is provided on a surface of the hydrogen storage alloy by vacuum deposition.

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