HYDROGEN STORAGE ALLOY

Abstract

Hydrogen storage alloy has a composition expressed by a general formula (LaaNdb“A”c“D”d)1-wMgwNixAly“T”z, where “A”, “D” and “T” represent at least one element selected from a group consisting of Sm and Gd, a group consisting of Pr, Eu, etc., and a group of consisting of V, Nb, etc., respectively; subscripts a, b, c and d satisfy relationship shown by a≧0, b≧0, c>0, 0.1>d≧0, and a+b+c+d=1; and subscripts w, x, y and z fall in a range shown by 0<w≦0.25, 0.05≦y≦0.35, 0≦z≦0.5, and 3.15≦x+y+z≦3.35.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to hydrogen storage alloy.

2. Description of the Related Art

Hydrogen storage alloy is attracting attention as energy-conversion and energy-storage material for its property of storing hydrogen securely and easily. Alkaline storage cells, or especially nickel-hydrogen secondary cells, with negative electrodes using the hydrogen storage alloy are characterized by high capacity and cleanness, and are therefore in high demand as consumer cells.

Rare-earth-Mg—Ni-based hydrogen storage alloy has been developed as hydrogen storage alloy for the negative electrodes of nickel-hydrogen secondary cells (see Unexamined Japanese Patent Publication No. 11-323469, for example). The rare-earth-Mg—Ni-based hydrogen storage alloy has a larger hydrogen storage amount than rare earth-Ni-based hydrogen storage alloy that has been conventionally used, and is therefore suitable to achieve the high capacity of the nickel-hydrogen secondary cells.

One of the methods for improving the characteristics of the nickel-hydrogen secondary cells is to prevent self-discharge. Conventional nickel-hydrogen secondary cells have high self-discharge rate, and are reduced in capacity while being left unused, so that they need to be charged immediately before use.

In contrast, the capacity of nickel-hydrogen secondary cells having low self-discharge rate does not decrease much even while being left unused. The nickel-hydrogen secondary cells are thus advantageous in that they are ready to use as long as being charged once in the user's spare time. It is considered that, if this advantage is fully utilized, the nickel-hydrogen secondary cells can be used as conveniently as dry cells.

In order to fully utilize the above-mentioned advantage, it is required to prevent a decrease in operating voltage while the nickel-hydrogen secondary cells are left unused. This is because, when the nickel-hydrogen secondary cells that have been left unused for a long time are used for devices, such as digital cameras and electric shavers, which require high operating voltage, the devices sometimes do not work due to the decreased operating voltage in spite that the cell capacity is still available.

SUMMARY OF THE INVENTION

It is an object of the invention to provide rare-earth-Mg—Ni-based hydrogen storage alloy that is capable of preventing a decrease in operating voltage and delivering high operating voltage when used in a nickel-hydrogen secondary cell, even after the cell is left unused for a long time.

The hydrogen storage alloy of the invention has a composition expressed by a general formula:

(La_aNd_b“A”_c“D”_d)_1-wMg_wNi_xAl_y“T”_z

where “A” represents at least one element selected from a group consisting of Sm and Gd; “D” represents at least one element selected from a group consisting of Pr, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Zr, Hf, Ca and Y; and “T” represents at least one element selected from a group consisting of V, Nb, Ta, Mo, Mn, Fe, Co, Zn, In, Cu, Si, P and B. Subscripts a, b, c and d satisfy relationship shown by a≧0, b≧0, c>0, 0.1>d≧0, and a+b+c+d=1. Subscripts w, x, y and z fall in a range shown by 0<w≦0.25, 0.05≦y≦0.35, 0≦z≦0.5, and 3.15≦x+y+z≦3.35.

Since the composition of the hydrogen storage alloy of the invention includes at least either one of Sm and Gd, when the hydrogen storage alloy is used in a nickel-hydrogen secondary cell, a post-operating voltage of the nickel-hydrogen secondary cell after letting the cell in unused condition is prevented from being decreased.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirits and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein:

FIGURE is a perspective view partially broken away, showing a nickel-hydrogen secondary cell according to one embodiment of the invention, and schematically showing a part of a negative electrode in enlarged scale within a circle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to achieve the above-mentioned object, the inventors conducted various studies for preventing a decrease in operating voltage of a nickel-hydrogen secondary cell. Through these studies, the inventors have found that, a nickel-hydrogen secondary cell is prevented from being decreased in the post-operating voltage when containing rare-earth-Mg—Ni-based hydrogen storage alloy having a given composition in which a ratio of the number of atoms situated in a B-site to the number of atoms situated in an A-site (B/A ratio) ranges from 3.15 to 3.35. The inventors and the like have thus arrived at the invention.

With reference to FIGURE, a nickel-hydrogen secondary cell according to one embodiment of the invention will be described in detail.

FIGURE shows a cylindrical cell, for example, of AA size. The cell includes an exterior can 10 formed into a cylinder with a bottom. The exterior can 10 has an open upper end. A bottom wall of the exterior can 10 has conductivity and functions as a negative terminal. A disc-like lid plate 14 is set in the upper end of the exterior can 10 with a ring-like insulating packing 12 intervening between the lid plate 14 and the exterior can 10. The lid plate 14 is conductive. The lid plate 14 and the insulating packing 12 are fixed to the open end of the exterior can 10 by caulking a circumferential edge of the open end of the exterior can 10.

The lid plate 14 has a gas-vent hole 16 in the center thereof. A rubber valve element 18 is set on an outer surface of the lid plate 14. The valve element 18 blocks up the gas-vent hole 16. A cylindrical positive terminal 20 with a flange is also fixed onto the outer surface of the lid plate 14. The positive terminal 20 covers the valve element 18, and at the same time, presses the valve element 18 against the lid plate 14. In other words, the exterior can 10 is usually airtightly closed by the lid plate 14 through the insulating packing 12 and the valve element 18. When gas is produced inside the exterior can 10, and inner pressure is increased, the valve element 18 is compressed to open the gas-vent hole 16. In result, the gas within the exterior can 10 is emitted from the exterior can 10 through the gas-vent hole 16. In short, the lid plate 14, the valve element 18 and the positive terminal 20 form a safety valve for the cell.

The exterior can 10 contains an electrode assembly 22. The electrode assembly 22 includes a positive electrode 24, a negative electrode 26, and a separator 28, each formed into a strip. The positive electrode 24, the negative electrode 26, and the separator 28 are rolled in a spiral shape. The separator 28 is sandwiched between the positive electrode 24 and the negative electrode 26. To put it differently, the positive electrode 24 and the negative electrode 26 are fit together with the separator 28 sandwiched therebetween. An outermost circumference of the electrode assembly 22 is made of a part (outermost circumferential part) of the negative electrode 26. The outermost circumferential part of the negative electrode 26 is in contact with an inner circumferential wall of the exterior can 10. The negative electrode 26 and the exterior can 10 are thus electrically connected to each other. The positive electrode 24, the negative electrode 26, and the separator 28 will be described later in detail.

A positive lead 30 is accommodated in the exterior can 10. The positive lead 30 is placed between one end of the electrode assembly 22 and the lid plate 14. Both ends of the positive lead 30 are connected to the positive electrode 24 and the lid plate 14, respectively. Accordingly, the positive electrode 24 is electrically connected to the lid plate 14 through the positive lead 30. A circular insulating member 32 is disposed between the lid plate 14 and the electrode assembly 22. The insulating member 32 has a slit that allows the positive lead 30 to pass therethrough. The positive lead 30 thus extends through the slit. A circular insulating member 34 is disposed between the electrode assembly 22 and the bottom wall of the exterior can 10.

Moreover, the exterior can 10 is filled with a given amount of an alkaline electrolyte (not shown). A charge-and-discharge reaction caused between the positive electrode 24 and the negative electrode 26 progresses through the alkaline electrolyte contained in the separator 28. The alkaline electrolyte is not particularly restricted in terms of kind. For example, the alkaline electrolyte may be a sodium hydroxide solution, a lithium hydroxide solution, a potassium hydroxide solution, a solution obtained by mixing two or more of these solutions, or the like. It is preferable to use an aqueous solution containing sodium hydroxide as a main solute. The alkaline electrolyte is not particularly restricted also in terms of concentration. For example, alkaline electrolyte of 8N (normal) concentration may be used.

An applicable material for the separator 28 is, for example, polyamide fiber unwoven fabric or polyolefin fiber unwoven fabric, such as polyethylene and polypropylene, which is provided with a hydrophilic functional group. A preferable hydrophilic functional group is a sulfone group (SO₃H).

The positive electrode 24 includes a conductive positive substrate having a porous structure and a positive mixture carried in pores of the positive substrate.

For example, a metal porous body made of nickel, or preferably a sintered substrate, may be used as the positive substrate.

The positive mixture includes positive active particles, various additive particles used to improve characteristics of the positive electrode 24 as needed, and a binder for binding the mixed particles of the positive active particles and the additive particles onto the positive substrate as needed.

If the cell is a nickel-hydrogen secondary cell, the positive active particles are nickel hydroxide particles. Instead of using the nickel hydroxide particles, it is possible to use nickel hydroxide particles contained cobalt, zinc, cadmium or the like in the form of a solid solution. Alternatively, it is also possible to use nickel hydroxide particles coated with cobalt compound with a surface subjected to alkaline thermal treatment.

Applicable as the additive particles are not only yttrium oxide but also cobalt compounds, such as cobalt oxide, metallic cobalt, and cobalt hydroxide, zinc compounds, such as metallic zinc, zinc oxide, and zinc hydroxide, rare-earth compounds such as erbium oxide.

Preferably, the positive mixture contains oxide or hydroxide of 0.5 to 5.0 percent by mass, which contains one or more elements selected from a group consisting of Y, Yb, Er, Ti, W and Nb.

As the binder, hydrophilic or hydrophobic polymer may be used.

The negative electrode 26 includes a conductive negative substrate (core body). A negative mixture is carried by the negative substrate. The negative substrate is made of a metal sheet. There are a large number of through holes distributed in the metal sheet. The negative substrate may be made, for example, of perforated metal sheet or a sintered sheet of metallic powder. The negative mixture includes a portion that is filled in the through holes of the negative substrate and a portion that is formed into a layer covering both entire surfaces of the negative substrate.

The negative mixture is schematically shown in enlarged size within a circle of FIGURE. The negative mixture contains hydrogen storage alloy particles 36 capable of storing and releasing hydrogen that functions as a negative active material, a conducting aid (not shown), such as carbon, which is used as needed, and a binder 38 for binding the hydrogen storage alloy and the conducting aid onto the negative substrate. As the binder 38, hydrophilic or hydrophobic polymer or the like may be used. The conducting aid may be carbon black or graphite. If the active material is hydrogen, a negative capacity is determined by the amount of the hydrogen storage alloy. According to the invention, therefore, the hydrogen storage alloy and the negative electrode 24 are referred to also as a negative active material and a hydrogen storage alloy electrode, respectively.

The hydrogen storage alloy forming the particles 36 is rare-earth-Mg—Ni-based hydrogen storage alloy. A main crystal structure of the hydrogen storage alloy is not a CaCu₅-type structure but a superlattice structure obtained by combining AB₅-type and AB₂-type structures. A composition thereof is expressed by a general formula:

(La_aNd_b“A”_c“D”_d)_1-wMg_wNi_xAl_y“T”_z  (1)

where “A” represents at least one element selected from a group consisting of Sm and Gd; “D” represents at least one element selected from a group consisting of Pr, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Zr, Hf, Ca and Y; and “T” represents at least one element selected from a group consisting of V, Nb, Ta, Mo, Mn, Fe, Co, Zn, In, Cu, Si, P and B. Subscripts a, b, c and d satisfy relationship shown by a≧0, b≧0, c>0, 0.1>d≧0, and a+b+c+d=1. Subscripts w, x, y and z fall in a range shown by 0<w≦0.25, 0.05≦y≦0.35, 0≦z≦0.5, and 3.15≦x+y+z≦3.35.

In the superlattice structure, La, Nd, the elements denoted by “A” and “D”, and Mg are situated in the A-site, whereas Ni, Al, and the element denoted by “T” are situated in the B-site. In this specification, among the elements situated in the A-site, La, Nd, and the elements denoted by “A” and “D” are referred to also as rare-earth components.

The hydrogen storage alloy particles 36 can be obtained, for example, as described below.

First, metal materials are measured and mixed together according to the above-mentioned composition. The obtained mixture is melted, for example, in a high-frequency melting furnace, and then formed into an ingot. The ingot is heated for 5 to 24 hours in an inert gas atmosphere at a temperature ranging from 900 to 1200 degrees centigrade. This thermal treatment makes a metal structure of the ingot into a superlattice structure including the AB₅-type and AB₂-type structures combined together. The ingot is subsequently pulverized into particles. The particles are sieved and classified by size, to thereby obtain the hydrogen storage alloy particles 36 of a desired particle size.

The nickel-hydrogen secondary cell has a high capacity because the hydrogen storage alloy particles 36 contain rare-earth-Mg—Ni-based hydrogen storage alloy as a main component.

The rare-earth-Mg—Ni-based hydrogen storage alloy used in the nickel-hydrogen secondary cell contains at least either one of Sm and Gd, and has a given composition in which a ratio of the number of elements in the B-site to the number of elements in the A-site (B/A ratio) ranges from 3.15 to 3.35. This prevents a decrease in the post-operating voltage of the nickel-hydrogen secondary cell in which the negative electrode 26 is a hydrogen storage alloy electrode made of the hydrogen storage alloy particles 36.

The invention thus provides the nickel-hydrogen secondary cell that is prevented from being decreased in operating voltage, and the cell has a very high industrial value.

EMBODIMENTS

1. Assembly of Cells

Embodiment 1

(1) Fabrication of Negative Electrodes

Raw materials of rare-earth components were prepared. The rare-earth components contained 22 percent of La, 30 percent of Nd, 20 percent of Sm, 20 percent of Gd, and 8 percent of Y in the ratio of the number of atoms. The raw materials of the rare-earth components were mixed with metal raw materials including Mg, Ni, Al and Co. The obtained mixture was melted in an induction melting furnace to produce an ingot of hydrogen storage alloy. The hydrogen storage alloy ingot contained the raw materials of the rare-earth components, Mg, Ni, Al and Co at the proportion of 0.90:0.10:2.9:0.2:0.1 in terms of the ratio of the number of atoms. The hydrogen storage alloy ingot was subsequently subjected to thermal treatment in an argon atmosphere at a temperature of 1000 degrees centigrade for 10 hours. The thermal treatment provided an ingot of rare-earth-Mg—Ni-based hydrogen storage alloy having a superlattice structure whose composition is shown by (La_0.22Nd_0.30Sm_0.20Gd_0.20Y_0.08)_0.90Mg_0.10Ni_2.90Al_0.20Co_0.10.

The obtained ingot was mechanically pulverized in an inert gas atmosphere. Particles obtained by the pulverization were sieved, and alloy particles of a particle size ranging from 400 to 200 meshes were sorted out. Subsequently, a particle size distribution of the alloy particles was measured by using a laser diffraction/scattering-method particle size distribution analyzer to find that the alloy particles of 50 percent in convolution integral had an average particle size of 30 μm, and a largest particle size of 45 μm.

A mixture was produced by adding 100 parts by mass of the alloy particles with 0.4 parts by mass of sodium polyacrylate, 0.1 parts by mass of carboxymethyl cellulose, and 2.5 parts by mass of a polytetrafluoro-ethylene dispersion liquid (dispersion medium: water, 60 parts by mass of solid content). The mixture was then kneaded into a negative mixture slurry.

The slurry was evenly coated in given thickness onto both entire surfaces of a Ni-coated iron perforated metal sheet with a thickness of 60 μm. After the slurry was dried, the perforated metal sheet was pressed and cut into negative electrodes for an AA-size nickel-hydrogen secondary cell, which had 9.0 grams of hydrogen storage alloy per sheet.

(2) Fabrication of Positive Electrodes

A mixed solution of nickel sulfate, zinc sulfate, and cobalt sulfate was prepared at a ratio of 3 percent by mass of Zn and 1 percent by mass of Co to metallic Ni. A sodium hydroxide solution was gradually added into the mixed solution while being kept stirred, to thereby react the sodium hydroxide solution with the mixed solution. In this process, reacting pH was maintained within a range from 13 to 14, to thereby separate out nickel hydroxide particles. The nickel hydroxide particles were rinsed three times in pure water of 10 times as much amount as the nickel hydroxide particles. The nickel hydroxide particles were then dehydrated and dried.

The obtained nickel hydroxide particles were mixed with 40 percent by mass of an HPC dispersion liquid, to thereby prepare a positive mixture slurry. The slurry was subjected to drying treatment after being filled into a nickel substrate having a porous structure. The substrate was then flat-rolled and cut into positive electrodes for an AA-size nickel-hydrogen secondary cell.

(3) Assembly of Nickel-Hydrogen Secondary Cells

The negative and positive electrodes obtained in the above-descried manner were rolled in a spiral shape with separators, which were made of polypropylene or nylon unwoven fabric, sandwiched between the respective negative and positive electrodes. In result, an electrode assembly was formed. The electrode assembly was put into an exterior can. The exterior can was then filled with a sodium hydroxide solution as an alkaline electrolyte. The solution contained lithium and potassium, and had a concentration of 30 percent by mass. The exterior can was tightly sealed with a lid plate or the like. In this manner, the AA-size nickel-hydrogen secondary cell shown in FIGURE was assembled. This cell had a nominal capacity of 2500 mAh.

Embodiment 2

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.22Nd_0.30Gd_0.40Y_0.08)_0.90Mg_0.10Ni_2.90Al_0.20Cu_0.10.

Embodiment 3

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.22Nd_0.30Sm_0.40Y_0.08)_0.90Mg_0.10Ni_2.90Al_0.20Co_0.10.

Embodiment 4

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.12Nd_0.20Sm_0.30Gd_0.30Y_0.08)_0.90Mg_0.10Ni_2.90Al_0.20Co_0.10.

Embodiment 5

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.16Nd_0.16Sm_0.30Gd_0.30Y_0.08)_0.90Mg_0.10Ni_2.90Al_0.20Co_0.10.

Embodiment 6

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.20Nd_0.12Sm_0.30Gd_0.30Y_0.08)_0.90Mg_0.10Ni_2.90Al_0.20Co_0.10.

Embodiment 7

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.20Nd_0.12Sm_0.33Gd_0.33Y_0.02)_0.90Mg_0.10Ni_2.90Al_0.20Co_0.10.

Embodiment 8

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.21Nd_0.12Sm_0.33Gd_0.33Y_0.01)_0.90Mg_0.10Ni_2.90Al_0.20Co_0.10.

Embodiment 9

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.21Nd_0.12Sm_0.33Gd_0.33Y_0.01)_0.90Mg_0.10Ni_3.00Al_0.20.

Embodiment 10

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.21Nd_0.12Sm_0.33Gd_0.33Y_0.01)_0.90Mg_0.10Ni_2.85Al_0.35.

Embodiment 11

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.21Nd_0.12Sm_0.33Gd_0.33Y_0.01)_0.90Mg_0.10Ni_3.15Al_0.05.

Embodiment 12

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.21Nd_0.12Sm_0.33Gd_0.33Y_0.01)_0.90Mg_0.10Ni_2.95Al_0.20.

Embodiment 13

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.21Nd_0.12Sm_0.33Gd_0.33Y_0.01)_0.90Mg_0.10Ni_3.15Al_0.20.

Embodiment 14

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.21Nd_0.12Sm_0.33Gd_0.33Y_0.01)_0.75Mg_0.25Ni_3.00Al_0.20.

Embodiment 15

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.21Nd_0.12Sm_0.33Gd_0.33Y_0.01)_0.90Mg_0.10Ni_2.90Al_0.20Cu_0.10.

Comparative Example 1

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.22Nd_0.70Y_0.08)_0.90Mg_0.10Ni_3.15Al_0.20Co_0.10.

Comparative Example 2

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.22Nd_0.70Y_0.08)_0.90Mg_0.10Ni_2.90Al_0.20Co_0.10.

Comparative Example 3

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.22Ce_0.40Nd_0.30Y_0.08)_0.90Mg_0.10Ni_2.90Al_0.20Co_0.10.

Comparative Example 4

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.12Nd_0.12Sm_0.33Gd_0.33Y_0.10)_0.90Mg_0.10Ni_2.90Al_0.20Co_0.10.

Comparative Example 5

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.21Nd_0.12Sm_0.33Gd_0.33Y_0.10)_0.90Mg_0.10Ni_2.82Al_0.38.

Comparative Example 6

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.21Nd_0.12Sm_0.33Gd_0.33Y_0.10)_0.90Mg_0.10Ni_3.18Al_0.02.

Comparative Example 7

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.21Nd_0.12Sm_0.33Gd_0.33Y_0.10)_0.90Mg_0.10Ni_2.92Al_0.02.

Comparative Example 8

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.21Nd_0.12Sm_0.33Gd_0.33Y_0.10)_0.90Mg_0.10Ni_3.17Al_0.20.

Comparative Example 9

A nickel-hydrogen secondary cell was assembled in the same manner as in Embodiment 1, except that the composition of the hydrogen storage alloy was (La_0.21Nd_0.12Sm_0.33Gd_0.33Y_0.10)_0.72Mg_0.28Ni_3.00Al_0.20.

2. Method of Evaluating the Cells

(1) Activation Treatment

As activation treatment, the cells of Embodiments 1 to 15 and Comparative Examples 1 to 9 were charged at a current of 0.1 C for 16 hours and then discharged at a current of 0.2 C to a final voltage of 0.5 V. This charge-and-discharge treatment was repeated twice.

(2) Evaluation of Initial Operating Voltage

The cells of Embodiments 1 to 15 and Comparative Examples 1 to 9, which had been subjected to the activation treatment, were charged at a current of 1.0 C for one hour and then discharged at a current of 1.0 C. A discharge duration from the start of discharge to the time point when the voltage reached the final voltage (0.8 V) was measured. In results of the measurement, the voltage at a middle point of the discharge duration was obtained as initial operating voltage. These results are shown in TABLE 1 as difference (unit: mV) from a result of Comparative Example 1.

TABLE 1 includes not only the elements contained in the hydrogen storage alloy and the subscripts of the respective elements in the general formula (I) but also a ratio of the number of elements situated in the B-site to the number of elements situated in the A-site (B/A ratio).

(3) Evaluation of Post-Operating Voltage

The cells discharged in the evaluation of initial operating voltage were charged at a current of 1.0 C for one hour, and then left unused for one month in an atmosphere at a temperature of 60 degrees centigrade. After being thus left unused, the cells are charged at a current of 1.0 C for one hour and discharged at a current of 1.0 C, to thereby measure a discharge duration from the start of discharge to the time point when voltage reached a final voltage of 0.8 V. In results of the measurement, the voltage at a middle point of the discharge duration was obtained as post-operating voltage. TABLE 1 shows the amount of change (unit: mV) from the initial operating voltage to the post-operating voltage with respect to each cell.

(4) Evaluation of Cycle Life

The cells of Embodiments 1 to 15 and Comparative Examples 1 to 9, which had been subjected to the activation treatment, were repeatedly applied to the charge-and-discharge treatment in which the cells were charged at a current of 1.0 C for one hour and then discharged at a current of 1.0 C to a final voltage of 0.8 V. The number of cycles before the cells became unable to be discharged (cycle life) was counted. These results are shown in TABLE 1 in relative values in which the cycle life of Comparative Example 1 is indicated as 100.

	TABLE 1
	Compositions of hydrogen storage alloy (subscripts)	Operating voltage
	La		Nd	Sm	Gd	Y	Mg	Ni	Al	Co	Cu	B/A	Initial	Post-Initial	Cycle
	(a)	Ce	(b)	(c)	(c)	(d)	(w)	(x)	(y)	(z)	(z)	ratio	(mV)	(mV)	life
Embodiment 1	0.22	0	0.30	0.20	0.20	0.08	0.10	2.90	0.20	0.10	0	3.20	±0	−41	98
Embodiment 2	0.22	0	0.30	0	0.40	0.08	0.10	2.90	0.20	0.10	0	3.20	+3	−40	98
Embodiment 3	0.22	0	0.30	0.40	0	0.08	0.10	2.90	0.20	0.10	0	3.20	−3	−42	98
Embodiment 4	0.12	0	0.20	0.30	0.30	0.08	0.10	2.90	0.20	0.10	0	3.20	+4	−39	104
Embodiment 5	0.16	0	0.16	0.30	0.30	0.08	0.10	2.90	0.20	0.10	0	3.20	+3	−35	110
Embodiment 6	0.20	0	0.12	0.30	0.30	0.08	0.10	2.90	0.20	0.10	0	3.20	+1	−34	110
Embodiment 7	0.20	0	0.12	0.33	0.33	0.02	0.10	2.90	0.20	0.10	0	3.20	+2	−32	120
Embodiment 8	0.21	0	0.12	0.33	0.33	0.01	0.10	2.90	0.20	0.10	0	3.20	+2	−32	122
Embodiment 9	0.21	0	0.12	0.33	0.33	0.01	0.10	3.00	0.20	0	0	3.20	+2	−28	120
Embodiment 10	0.21	0	0.12	0.33	0.33	0.01	0.10	2.85	0.35	0	0	3.20	−4	−36	150
Embodiment 11	0.21	0	0.12	0.33	0.33	0.01	0.10	3.15	0.05	0	0	3.20	+5	−26	95
Embodiment 12	0.21	0	0.12	0.33	0.33	0.01	0.10	2.95	0.20	0	0	3.15	±0	−28	92
Embodiment 13	0.21	0	0.12	0.33	0.33	0.01	0.10	3.15	0.20	0	0	3.35	+6	−29	112
Embodiment 14	0.21	0	0.12	0.33	0.33	0.01	0.25	3.00	0.20	0	0	3.20	+6	−28	94
Embodiment 15	0.21	0	0.12	0.33	0.33	0.01	0.10	2.90	0.20	0	0.10	3.20	+2	−32	110
Comparative Example 1	0.22	0	0.7	0	0	0.08	0.10	3.15	0.20	0.10	0	3.45	±0	−85	100
Comparative Example 2	0.22	0	0.7	0	0	0.08	0.10	2.90	0.20	0.10	0	3.20	−21	−52	85
Comparative Example 3	0.22	0.4	0.3	0	0	0.08	0.10	2.90	0.20	0.10	0	3.20	−3	−88	28
Comparative Example 4	0.12	0	0.12	0.33	0.33	0.10	0.10	2.90	0.20	0.10	0	3.20	+4	−37	64
Comparative Example 5	0.21	0	0.12	0.33	0.33	0.01	0.10	2.82	0.38	0	0	3.20	−9	−40	79
Comparative Example 6	0.21	0	0.12	0.33	0.33	0.01	0.10	3.18	0.02	0	0	3.20	−18	−25	32
Comparative Example 7	0.21	0	0.12	0.33	0.33	0.01	0.10	2.92	0.20	0	0	3.12	−1	−28	44
Comparative Example 8	0.21	0	0.12	0.33	0.33	0.01	0.10	3.17	0.20	0	0	3.37	+6	−51	99
Comparative Example 9	0.21	0	0.12	0.33	0.33	0.01	0.28	3.0	0.20	0	0	3.20	+7	−28	72

3. Results of Cell Evaluation

TABLE 1 evidently shows the following matters.

(1) As compared to Comparative Example 1 in which the B/A ratio of the hydrogen storage alloy is 3.45, Comparative Example 2 in which the B/A ratio is 3.20 significantly prevents a decrease in the post-operating voltage. On the other hand, in Comparative Example 2, the initial operating voltage is drastically decreased to a problematic level from a practical point.

According to Comparative Example 3 in which the rare-earth components are changed in order to prevent the decrease of the operating voltage, the initial operating voltage is relatively high, but the cycle life is drastically decreased. This is because the rare-earth-Mg—Ni-based hydrogen storage alloy containing Ce has an extremely low corrosion resistance.

(2) According to Embodiment 1 in which the rare-earth components consist of La, Nd, Sm, Gd and Y with the B/A ratio maintained at 3.2, the post-operating voltage is considerably improved while both the initial operating voltage and the cycle life are retained at the same level as in Comparative Example 1.
(3) Comparing Embodiment 1 to Embodiments 2 and 3, the post-operating voltages of Embodiments 1 to 3 are substantially equal, regardless of the ratio between Sm and Gd. According to Embodiment 3 using relatively more Sm than Embodiment 1, the initial operating voltage is slightly lower than in Embodiment 1. However, the initial operating voltage of Embodiment 2, which uses relatively more Gd than Embodiment 1, is higher than that of Embodiment 1.
(4) In Embodiment 4, Sm and Gd quantities are not changed in ratio but increased higher than in Embodiment 1. In result, Embodiment 4 has high initial and the post-operating voltages and a long cycle life, as compared to Embodiment 1.
(5) As to Embodiments 1, 5 and 6, La and Nd quantities are examined. These results show that the post-operating voltage and the cycle life are enhanced by making the La quantity equal to or higher than the Nd quantity in the ratio of the number of atoms. In a range where the La quantity is equal to or higher than the Nd quantity, however, even if the La quantity is further increased, the effects are saturated.
(6) As to Embodiments 6, 7 and 8, and Comparative Example 4, an examination is made as to how much rare-earth components, other than La, Nd, Sm and Gd, may be contained. Embodiment 4 in which the subscript of Y is 0.1 has a shorter cycle life than Embodiment 6 in which the subscript of Y is 0.08. To the contrary, Embodiment 7 in which the subscript of Y is 0.02 has a longer cycle life than Embodiment 6. Embodiment 8 in which the subscript of Y is 0.01 is substantially the same as Embodiment 7 in characteristics. It is therefore desirable that subscript d in the general formula, which corresponds to the quantity of the rare-earth component other than La, Nd, Sm and Gd, be set less than 0.10 and fall in a range from 0.01 to 0.02.
(7) As to Embodiments 8, 9 and 15, an examination is made as to whether the components other than Ni and Al within the B-site can be cut in quantity. Embodiment 9 that includes no component in the B-site other than Ni and Al has higher the post-operating voltage than Embodiment 8. This result proves that it is effective to reduce the other components than Ni and Al, especially to reduce Co and Cu.
(8) As to Embodiments 9, 10 and 11, and Comparative Examples 5 and 6, upper and lower limits of Al quantity are examined. On the basis of the result, the subscript y of Al is set within a range from 0.05 to 0.35, and preferably from 0.10 to 0.20.
(9) As to Embodiments 9, 12 and 13, and Comparative Examples 7 and 8, upper and lower limits of the B/A ratio are examined. On the basis of the result, the B/A ratio is set within a range from 3.15 to 3.35, or preferably from 3.20 to 3.30. In other words, on the lower limit side, cycle characteristic is drastically degraded if the B/A ratio is reduced slightly less than 3.15. On the upper limit side, the post-operating voltage is decreased if the B/A ratio is increased slightly more than 3.35.
(10) As to Embodiments 9 and 14, and Comparative Example 9, an upper limit of Mg quantity is examined. On the basis of the result, the subscript w indicative of Mg quantity is set at 0.25 or lower, and preferably within a range from 0.10 to 0.20.

The invention is not restricted to the one embodiment and the exemplifying embodiments, and may be modified in various ways. For example, the nickel-hydrogen secondary cell may be a square cell, and a mechanical structure is not particularly restricted.

Concerning the nickel-hydrogen secondary cell of the one embodiment, the post-operating voltage of the cell after letting unused for long time is prevented from being decreased by using the rare-earth-Mg—Ni-based hydrogen storage alloy with the composition indicated by the general formula (1). However, the decrease of the operating voltage may be further prevented by applying other means at the same time.

More specifically, it is possible to simultaneously employ such means as the use of a separator including fibers with a sulfone group, the use of electrolyte containing sodium hydroxide as a main solute, the use of a positive active material whose surface is provided with a coating layer containing cobalt, thermal treatment that is applied to the cobalt-coating layer under the condition that alkali and oxygen coexist, to cause at least one or more kinds of oxides or hydroxides selected among Y, Yb, Er, Ti, W and Nb to exist on the surface of the positive active material, to set the existing quantity within a range from 0.5 to 5.0 percent by mass of the positive mixture, and to fill an active material into a nickel-made porous sintered substrate in the positive substrate.

The hydrogen storage alloy of the invention is applicable to other articles than a nickel-hydrogen secondary cell.

The invention thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

1. Hydrogen storage alloy comprising:

a composition expressed by a general formula: (LaaNdb“A”c“D”d)1-wMgwNixAly“T”z

where “A” represents at least one element selected from a group consisting of Sm and Gd;

“D” represents at least one element selected from a group consisting of Pr, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Zr, Hf, Ca and Y;

“T” represents at least one element selected from a group consisting of V, Nb, Ta, Mo, Mn, Fe, Co, Zn, In, Cu, Si, P and B;

subscripts a, b, c and d satisfy relationship expressed by a≧0, b≧0, c>0, 0.1>d≧0, and a+b+c+d=1; and

subscripts w, x, y and z fall in a range shown by 0<w≦0.25, 0.05≦y≦0.35, 0≦z≦0.5, and 3.15≦x+y+z≧3.35.

2. The hydrogen storage alloy according to claim 1, wherein

the subscripts a, b and c satisfy relationship expressed by c>a+b.

3. The hydrogen storage alloy according to claim 2, wherein

the subscripts a and b satisfy relationship expressed by a≧b.

4. The hydrogen storage alloy according to claim 3, wherein the subscript d is 0.02 or less.

5. The hydrogen storage alloy according to claim 4, wherein

the “T” contains at least one element other than Co and Cu.