High pressure type hydride secondary battery

Abstract

A hydride secondary battery includes: a pressure vessel; a positive electrode disposed in the pressure vessel; a negative electrode disposed in the pressure vessel; and hydrogen gas with which the pressure vessel is filled. The negative electrode contains a hydrogen-absorbing alloy. In a pressure-composition-temperature diagram, a desorption curve at 25 C. of the hydrogen-absorbing alloy has a plateau pressure of 0.15 MPa or more and 10 MPa or less. The hydrogen gas has a pressure equal to or higher than the plateau pressure at 25 C. of the hydrogen-absorbing alloy.

Claims

1. A hydride secondary battery comprising: a pressure vessel; a positive electrode disposed in the pressure vessel; a negative electrode disposed in the pressure vessel; and hydrogen gas with which the pressure vessel is filled, wherein the negative electrode contains a hydrogen-absorbing alloy, and in a pressure-composition-temperature diagram, a desorption curve at 25 C. of the hydrogen-absorbing alloy has a plateau pressure of between 0.15 MPa or more and 10 MPa or less, wherein the hydrogen-absorbing alloy is an AB.sub.5 type alloy expressed by MmNi.sub.5-x8-y8Cr.sub.x8Mn.sub.y8, provided that Mm represents a mischmetal, x8 is between 0.25 or more and 0.45 or less, y8 is between 0.05 or more and 0.25 or less, and a sum of x8 and y8 is 0.5, and wherein the hydrogen gas has a pressure equal to or higher than the plateau pressure at 25 C. of the hydrogen-absorbing alloy, and wherein the mischmetal is an alloy comprising Ce and La.

2. The hydride secondary battery according to claim 1, wherein the desorption curve at 25 C. of the hydrogen-absorbing alloy has a plateau pressure of between 0.15 MPa or more and 5 MPa or less.

3. The hydride secondary battery according to claim 1, wherein the desorption curve at 25 C. of the hydrogen-absorbing alloy has a plateau pressure of between 0.3 MPa or more and 2.3 MPa or less.

4. The hydride secondary battery according to claim 1, wherein the hydrogen-absorbing alloy is at least one selected from the group consisting of MmNi.sub.4.5Cr.sub.0.45Mn.sub.0.05, and MmNi.sub.4.5Cr.sub.0.25Mn.sub.0.25, provided that Mm represents a mischmetal.

5. The hydride secondary battery according to claim 1, wherein the hydrogen-absorbing alloy has a hydrogen absorption amount of between 1.3 mass % or more and 1.5 mass % or less.

6. The hydride secondary battery according to claim 1, wherein the hydrogen gas has a pressure at 25 C. of between 10 MPa or more and 70 MPa or less.

7. The hydride secondary battery according to claim 1, wherein the positive electrode contains at least one of nickel oxyhydroxide and nickel hydroxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Features, advantages, and technical and industrial significance of exemplary embodiments will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

(2) FIG. 1 is an example of the PCT diagram of a hydrogen-absorbing alloy;

(3) FIG. 2 is the PCT diagram of the hydrogen-absorbing alloy in the related art; and

(4) FIG. 3 is a conceptual diagram illustrating a high pressure type hydride secondary battery according to an embodiment of this disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

(5) Hereinafter, an embodiment of this disclosure (hereinafter, referred to as this embodiment) will be described. However, the following description does not limit the scope of this disclosure.

(6) High Pressure Type Hydride Secondary battery

(7) A high pressure type hydride secondary battery of this embodiment has a high mass capacity density. Therefore, the high pressure type hydride secondary battery of this embodiment is suitable as a power source for, for example, HV and electric vehicles (EV). However, the application of the high pressure type hydride secondary battery of this embodiment is not limited to such applications for vehicles. The high pressure type hydride secondary battery of this embodiment can be applied to any applications.

(8) Hereinafter, the high pressure type hydride secondary battery is abbreviated as battery in some cases. FIG. 3 is a conceptual diagram illustrating the high pressure type hydride secondary battery according to this embodiment. A battery 100 includes a pressure vessel 50. In the pressure vessel 50, a positive electrode 10 and a negative electrode 20 are disposed. The positive electrode 10 and the negative electrode 20 are led out to the outside by lead wires. A separator 30 is disposed between the positive electrode 10 and the negative electrode 20. Although not illustrated, the positive electrode 10, the negative electrode 20, and the separator 30 are impregnated with an electrolyte. High pressure hydrogen gas is sealed in the space inside the pressure vessel 50.

(9) That is, the battery 100 includes the pressure vessel 50, the positive electrode 10 disposed in the pressure vessel 50, the negative electrode 20 disposed in the pressure vessel 50, and the hydrogen gas with which the pressure vessel 50 is filled. The positive electrode 10 and the negative electrode 20 may be wound with the separator 30 interposed therebetween. The battery 100 may include a plurality of the positive electrodes 10 and a plurality of the negative electrodes 20. For example, the positive electrodes 10 and the negative electrodes 20 may be alternately laminated with the separator 30 interposed therebetween.

(10) Pressure Vessel

(11) The structure of the pressure vessel 50 is not particularly limited as long as the pressure vessel 50 can withstand the pressure of the hydrogen gas. For example, the wall of the pressure vessel 50 may have a multilayer structure. The multilayer structure may include a carbon fiber reinforced plastic (CFRP) layer. The CFRP layer is expected to have pressure resistance. The multilayer structure may include at least one of a metal liner and a resin liner. The metal liner and the resin liner are expected to have an action of sealing the hydrogen gas. It is desirable that the metal liner is made of a material that is less likely to be affected by hydrogen gas. The metal liner is made of, for example, an Al alloy (Alloy 6061 or the like), or stainless steel (SUS316L or the like). The resin liner is made of, for example, polyamide (PA).

(12) Negative Electrode

(13) The negative electrode 20 contains a hydrogen-absorbing alloy. The shape of the negative electrode 20 is not particularly limited. The negative electrode 20 may be, for example, a plate-like member. The planar shape of the negative electrode 20 may be a rectangular shape, a band shape, or the like. The negative electrode 20 may be, for example, a molded body of the hydrogen-absorbing alloy. The negative electrode 20 may be formed, for example, by coating a current collector with a negative electrode mixture material containing a powder-like hydrogen-absorbing alloy, a conductive material, and a binder. That is, the negative electrode 20 may contain a conductive material, a binder, a current collector, and the like as long as a hydrogen-absorbing alloy is contained therein.

(14) The conductive material may be metal powder such as copper (Cu) and Ni, carbon fiber, carbon powder such as carbon black, or the like. The binder may be, for example, polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), polyethylene (PE), styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), or a copolymer of tetrafluoroethylene and hexafluoropropylene (FEP). The current collector may be, for example, punched metal, expanded metal, or foamed metal. The material of the current collector may be, for example, Ni or Fe plated with Ni. The negative electrode mixture material may contain, for example, 70 mass % to 98 mass % of the hydrogen-absorbing alloy, 1 mass % to 20 mass % of the conductive material, and 1 mass % to 10 mass % of the binder.

(15) High Pressure Type Hydrogen-Absorbing Alloy

(16) In this embodiment, the negative electrode 20 contains a high pressure type hydrogen-absorbing alloy. That is, in a PCT diagram, a desorption curve at 25 C. of the hydrogen-absorbing alloy has a plateau pressure of 0.15 MPa or more and 10 MPa or less. The plateau pressure varies depending on the alloy composition of the hydrogen-absorbing alloy. As the plateau pressure increases, the improvement in the operating voltage of the battery can be expected. The plateau pressure may be, for example, 0.15 MPa or more and 5 MPa or less, 0.15 MPa or more and 2.3 MPa or less, 0.2 MPa or more and 2.3 MPa or less, or 0.3 MPa or more and 2.3 MPa or less, 0.57 MPa or more and 2.3 MPa or less, or 1.6 MPa or more and 2.3 MPa or less.

(17) The desorption curve at 25 C. in this specification is measured by the method based on JIS H 7201. In the measurement, a known Sieverts' instrument is used. During the measurement, when a thermometer disposed in a sample chamber (thermostat) indicates a temperature of 24 C. to 26 C., it is assumed that the desorption curve at 25 C. is measured.

(18) As illustrated in FIG. 1, in the PCT diagram, the vertical axis represents the dissociation pressure and the horizontal axis represents the hydrogen composition (H/M). The vertical axis has a common logarithmic scale. As described above, the hydrogen composition (H/M) represents the ratio (dimensionless number) of the number of hydrogen atoms to the number of metal atoms. The desorption curve at 25 C. may be created by connecting at least 10 measurement points, and preferably 20 measurement points.

(19) The plateau pressure in this specification is calculated as follows. First, a straight line passing through three consecutive points in the desorption curve is drawn. The slope of the straight line is obtained. In a case where the three points do not lie on one straight line, the slope of the straight line is obtained by the least squares method. The combination of three points with the smallest slope is determined. The arithmetic average value of the dissociation pressures of the three points is regarded as the plateau pressure.

(20) Typically, in a case where the desorption curve is measured, an absorption curve is also measured. The absorption curve is a curve created by plotting the pressure (absorption pressure) when the hydrogen-absorbing alloy absorbs hydrogen against the hydrogen composition. In general, the absorption curve shifts to the higher pressure side than the desorption curve. Therefore, the fact that the desorption curve at 25 C. has a plateau pressure of 0.15 MPa or more means that both the desorption curve and the absorption curve at 25 C. have a plateau pressure of 0.15 MPa or more.

(21) Desorption curves and plateau pressures at other than 25 C. may also be measured. From the plateau pressure at 25 C. and the plateau pressures at the other temperatures, the standard enthalpy change and the standard entropy change when the hydrogen-absorbing alloy releases hydrogen are obtained. It is thought that the plateau pressure, the standard enthalpy change, and the standard entropy change satisfy the following relational equation.
ln PH.sub.2=H/RTS/R

(22) (in the equation, PH.sub.2 represents the plateau pressure, H represents the standard enthalpy change, S represents the standard entropy change, R represents the gas constant, and T represents the temperature at which the desorption curve is measured.) Therefore, since the natural logarithm of the plateau pressure (PH.sub.2) is plotted against the reciprocal of the temperature (1/T), H is obtained from the slope of the straight line, and S is obtained from the intercept of the straight line.

(23) Alloy Composition

(24) As the hydrogen-absorbing alloy, an AB type alloy (an alloy having a CsCl type crystal structure), an A.sub.2B type alloy (an alloy having a Mg.sub.2Ni type crystal structure), an AB.sub.5 type alloy (an alloy having a CaCu.sub.5 type crystal structure), and the like are known. Among them, the AB.sub.5 type alloy can become the high pressure type hydrogen-absorbing alloy. Furthermore, the AB.sub.5 type alloys expressed by Formulas (1) and (2) and LaNi.sub.5 are high pressure type hydrogen-absorbing alloys and tend to have a large hydrogen absorption amount.
MmNi.sub.5-x1B.sub.x,(1)

(25) (in the formula, Mm represents the mischmetal, B represents at least one selected from the group consisting of Fe, Cr, Mn, and Al, and x1 is 0 or more and 0.6 or less.)
MmNi.sub.5-x2Co.sub.x2(2)

(26) (in the formula, Mm represents the mischmetal, and x2 is more than 0 and 1.0 or less.)

(27) The negative electrode 20 may contain one kind of hydrogen-absorbing alloy singly or may contain two or more kinds of hydrogen-absorbing alloy. Therefore, the hydrogen-absorbing alloy is at least one selected from the group consisting of (i) the AB.sub.5 type alloy expressed by Formula (1), (ii) the AB.sub.5 type alloy represented by Formula (2), and (iii) LaNi.sub.5.

(28) The mischmetal (Mm) represents a mixture of rare earth elements primarily containing cerium (Ce) and La. Primarily containing Ce and La means that the sum of Ce and La occupies 50 mass % or more of the entire mixture. In addition to Ce and La, Mm may contain neodymium (Nd), praseodymium (Pr), samarium (Sm), magnesium (Mg), Al, Fe, and the like. Mm contains, for example, 40 mass % to 60 mass % of Ce, 10 mass % to 35 mass % of La, and Nd, Pr, Sm, and the like as the remainder. Specific examples include Mm containing 53.7 mass % of Ce, 24.1 mass % of La, 16.5 mass % of Nd, and 5.8 mass % Pr. Mm with a high content of La (so-called lanthanum-rich mischmetal) may also be used. The lanthanum-rich mischmetal contains, for example, 10 mass % to 30 mass % of Ce, 40 mass % to 70 mass % of La, and Nd, Pr, Sm, and the like as the remainder. Specific examples include Mm containing 25.8 mass % of Ce, 63.8 mass % of La, 7.9 mass % of Nd, and 2.4 mass % of Pr.

(29) The hydrogen-absorbing alloy may be an AB.sub.5 type alloy expressed by Formula (3).
MmNi.sub.5-x3Fe.sub.x3(3)

(30) (in the formula, Mm represents the mischmetal, and x3 is 0.2 or more and 0.4 or less.)

(31) As the AB.sub.5 type alloy expressed by Formula (3), for example, MmNi.sub.4.7Fe.sub.0.3 or the like can be exemplified.

(32) The hydrogen-absorbing alloy may be an AB.sub.5 type alloy expressed by Formula (4).
MmNi.sub.5-x4Cr.sub.x4(4)

(33) (in the formula, Mm represents the mischmetal, and x4 is 0.4 or more and 0.6 or less.) As the AB.sub.5 type alloy expressed by Formula (4), for example, MmNi.sub.4.5Cr.sub.0.5 or the like can be exemplified.

(34) The hydrogen-absorbing alloy may be an AB.sub.5 type alloy expressed by Formula (5).
MmNi.sub.5-x5Co.sub.x5(5)

(35) (in the formula, Mm represents the mischmetal, and x5 is 0.7 or more and 0.9 or less.)

(36) As the AB.sub.5 type alloy expressed by Formula (5), for example, MmNi.sub.4.2Co.sub.0.8 or the like can be exemplified.

(37) The hydrogen-absorbing alloy may be an AB.sub.5 type alloy expressed by Formula (6).
MmNi.sub.5-x6Mn.sub.x6(6)

(38) (in the formula, Mm represents the mischmetal, and x6 is 0.4 or more and 0.6 or less.)

(39) As the AB.sub.5 type alloy expressed by Formula (6), for example, MmNi.sub.4.5Mn.sub.0.5 or the like can be exemplified.

(40) The hydrogen-absorbing alloy may be an AB.sub.5 type alloy expressed by Formula (7).
MmNi.sub.5-x7Al.sub.x7(7)

(41) (in the formula, Mm represents the mischmetal, and x7 is 0.4 or more and 0.6 or less.)

(42) As the AB.sub.5 type alloy expressed by Formula (7), for example, MmNi.sub.4.5Al.sub.0.5 or the like can be exemplified.

(43) The hydrogen-absorbing alloy may be an AB.sub.5 type alloy expressed by Formula (8).
MmNi.sub.5-x8-y8Cr.sub.x8Mn.sub.y8(8)

(44) (In the formula, Mm represents the mischmetal, x8 is 0.25 or more and 0.45 or less, y8 is 0.05 or more and 0.25 or less, and the sum of x8 and y8 is 0.5.)

(45) Examples of the AB.sub.5 type alloy expressed by Formula (8) include MmNi.sub.4.5Cr.sub.0.45Mn.sub.0.05 and MmNi.sub.4.5Cr.sub.0.25Mn.sub.0.25.

(46) Therefore, the hydrogen-absorbing alloy may be at least one selected from the group consisting of (i) MmNi.sub.5, (ii) the AB.sub.5 type alloys expressed by Formulas (3) to (8), and (iii) LaNi.sub.5.

(47) Alternatively, the hydrogen-absorbing alloy may be at least one selected from the group consisting of MmNi.sub.5, MmNi.sub.4.7Fe.sub.0.3, MmNi.sub.4.5Cr.sub.0.5, MmNi.sub.4.2Co.sub.0.8, MmNi.sub.4.5Mn.sub.0.5, MmNi.sub.4.5Al.sub.0.5, MmNi.sub.4.5Cr.sub.0.45Mn.sub.0.05, MmNi.sub.4.5Cr.sub.0.25Mn.sub.0.25, and LaNi.sub.5.

(48) As the hydrogen absorption amount of the hydrogen-absorbing alloy increases, the improvement in the mass capacity density of the battery 100 can be expected. The hydrogen absorption amount represents the ratio of the mass of hydrogen to the mass of the alloy in a state where all the alloys are hydrogenated. The high pressure type hydrogen-absorbing alloy can have, for example, a hydrogen absorption amount of 1.3 mass % or more and 1.5 mass % or less. Furthermore, the high pressure type hydrogen-absorbing alloy may also have a hydrogen absorption amount of 1.4 mass % or more and 1.5 mass % or less.

(49) Hydrogen Gas

(50) The pressure vessel 50 is filled with the hydrogen gas. Hydrogen gas has a pressure equal to or higher than the plateau pressure at 25 C. of the high pressure type hydrogen-absorbing alloy. Accordingly, the high pressure type hydrogen-absorbing alloy can function as a high-capacity negative electrode active material. The pressure of the hydrogen gas is measured with a pressure gauge. It is natural that the pressure gauge should be selected as appropriate depending on the pressure of the hydrogen gas, the form of the pressure vessel 50, and the like. For example, a pressure gauge for high pressure hydrogen gas at 70 MPa is also commercially available. The pressure gauge may be permanently installed in the pressure vessel 50.

(51) The hydrogen gas of this embodiment functions as the negative electrode active material, thereby contributing to the improvement in the mass capacity density of the battery 100. The hydrogen gas preferably has a pressure of 10 MPa or more and 70 MPa or less at 25 C. As the hydrogen gas (the negative electrode active material) is compressed, the improvement in the volumetric energy density of the battery 100 is expected. At 25 C., the hydrogen gas has a pressure of more preferably 20 MPa or more and 70 MPa or less, even more preferably 40 MPa or more and 70 MPa or less, and most preferably 60 MPa or more and 70 MPa or less.

(52) Positive Electrode

(53) The positive electrode 10 contains a positive electrode active material. The shape of the positive electrode 10 is also not particularly limited. The positive electrode 10 may also be, for example, a plate-like member. The planar shape of the positive electrode 10 may also be a rectangular shape, a band shape, or the like. The positive electrode active material may be, for example, at least one of NiOOH and Ni(OH).sub.2. That is, the positive electrode 10 may be a nickel positive electrode. The nickel positive electrode may be a sintered nickel positive electrode well known in the related art, or may be a paste type nickel positive electrode. For example, the positive electrode 10 may be formed by filling a base material such as foamed Ni with a positive electrode mixture material containing the positive electrode active material, a conductive material, and a binder. The conductive material may be, for example, cobalt oxide (CoO), or cobalt hydroxide (Co(OH).sub.2). The binder may be, for example, PVA, CMC, SBR, PTFE, or FEP. The positive electrode mixture material contains, for example, 80 mass % to 98 mass % of the positive electrode active material, 1 mass % to 10 mass % of the conductive material, and 1 mass % to 10 mass % of the binder.

(54) Separator

(55) The separator 30 is disposed between the positive electrode 10 and the negative electrode 20. The separator 30 may be, for example, a nonwoven fabric made of polypropylene (PP), or a nonwoven fabric made of polyamide (PA). The nonwoven fabric may be subjected to a treatment for imparting hydrophilicity to the surface of the fiber. Examples of the treatment for imparting hydrophilicity include a sulfonation treatment and a plasma treatment. The nonwoven fabric has, for example, a thickness of about 50 m to 500 m. The nonwoven fabric has, for example, a basis weight of about 50 g/m.sup.2 to 100 g/m.sup.2.

(56) Electrolyte

(57) Voids in the positive electrode 10, the negative electrode 20, and the separator 30 are impregnated with the electrolyte. The electrolyte may be, for example, a potassium hydroxide (KOH) aqueous solution. The electrolyte may contain, in addition to KOH, sodium hydroxide (NaOH), lithium hydroxide (LiOH), and the like. The concentration of hydroxide ions (OH.sup.) in the electrolyte may be, for example, about 1 mol/l to 20 mol/l.

(58) In a nickel-metal hydride secondary battery in the related art, water contained in the electrolyte may be electrolyzed during charging, and hydrogen gas may be generated. In the high pressure type hydride secondary battery of this embodiment, the inside of the pressure vessel 50 is filled with the high pressure hydrogen gas. Therefore, it is expected that electrolysis of water will hardly proceed due to Le Chatelier's principle.

(59) Hereinafter, examples will be described. However, the following examples do not limit the scope of this disclosure.

(60) Preparation of Hydrogen-Absorbing Alloy

(61) Various hydrogen-absorbing alloys were prepared in the following manner.

(62) Sample No. 1

(63) A Mm powder and a Ni powder were prepared. The Mm powder and the Ni powder were mixed in a molar ratio of Mm:Ni=1:5. Accordingly, a mixed powder was prepared. The mixed powder was dissolved in an arc melting furnace under an argon atmosphere. Accordingly, a molten alloy was prepared. The molten alloy was cooled by a strip casting method. Accordingly, a cast piece of a hydrogen-absorbing alloy (MmNi.sub.5) was obtained. The case piece was crushed by a ball mill. Accordingly, a powder of the hydrogen-absorbing alloy (AB.sub.5 type alloy) was prepared. The powder had a particle size of 20 m to 100 m.

(64) Sample Nos. 2 to 13

(65) As shown in Table 1 below, powders of hydrogen-absorbing alloys according to Sample Nos. 2 to 13 were prepared in the same procedure as Sample No. 1 except that the alloy composition was changed.

(66) TABLE-US-00001 TABLE 1 Sample list Hydrogen-absorbing alloy Hydrogen Test cell Plateau pressure absorption Mass capacity Sample (25 C.) amount H S density Voltage No. Alloy composition [MPa] [mass %] [kJ/mol H.sub.2] [J/K .Math. mol H.sub.2] [mAh/g] [V] 1 MmNi.sub.5 2.3 1.5 21.1 97 400 1.26 2 MmNi.sub.4.7Fe.sub.0.3 1.6 1.4 22 97 370 1.25 3 MmNi.sub.4.5Cr.sub.0.5 0.57 1.3 25.5 100 340 1.24 4 MmNi.sub.4.2Co.sub.0.8 2.1 1.5 23.8 105 400 1.26 5 MmNi.sub.4.5Mn.sub.0.5 0.33 1.5 28 104 400 1.23 6 MmNi.sub.4.5Al.sub.0.5 0.38 1.3 28 105 340 1.24 7 MmNi.sub.4.5Cr.sub.0.45Mn.sub.0.05 0.30 1.5 30 104 400 1.23 8 MmNi.sub.4.5Cr.sub.0.25Mn.sub.0.25 0.20 1.5 30 105 390 1.22 9 LaNi.sub.5 0.15 1.4 31 108 370 1.22 10 MmNi.sub.4.2Cr.sub.0.2Mn.sub.0.5Al.sub.0.3 0.02 1.1 37 109 300 1.20 11 MmNi.sub.4.0Fe.sub.1.0 0.10 1.0 29 99 270 1.22 12 MmNi.sub.4.2Mn.sub.0.8 0.10 1.0 28 95 270 1.22 13 MmNi.sub.4.1Al.sub.0.9 0.10 1.0 33 105 260 1.22

(67) Production of Test Cell

(68) A test cell (hydride secondary battery) including a test electrode containing a powder of a hydrogen-absorbing alloy and a test electrode was produced according to the procedure based on JIS H 7205. Here, as the container for the test cell, a pressure vessel capable of withstanding the same pressure as the plateau pressure of the hydrogen-absorbing alloy was used, and the pressure vessel was filled with hydrogen gas so that the internal pressure of the pressure vessel was equal to or higher than the plateau pressure. Other configurations of the test cell are as follows.

(69) Test Cell

(70) Counter electrode: nickel hydroxide

(71) Electrolyte: KOH aqueous solution (6 mol/l)

(72) Separator: nonwoven fabric made of PP

(73) Evaluation

(74) By the method described above, desorption curves at 25 C. were measured. Furthermore, a PCT diagram was created. For the measurement, a Sieverts' instrument manufactured by Suzuki Shokan Co., Ltd. was used. Plateau pressures, hydrogen absorption amounts, H, and S were calculated. The results are shown in Table 1 above.

(75) As shown in Table 1 above, Sample Nos. 1 to 9 were the high pressure type hydrogen-absorbing alloys having a plateau pressure of 0.15 MPa or more and 10 MPa or less. Sample Nos. 10 to 13 were low pressure type hydrogen-absorbing alloys having a plateau pressure of less than 0.15 MPa.

(76) The discharge characteristics and voltages of the test cells were measured by the procedure based on JIS H 7205. Accordingly, the mass capacity density of the hydrogen-absorbing alloy was obtained. The results are shown in Table 1 above.

(77) As shown in Table 1 above, the test cells (that is, the high pressure type hydride secondary batteries) including the high pressure type hydrogen-absorbing alloys (Sample Nos. 1 to 9) had a higher mass capacity density than the test cells including the low pressure type hydrogen-absorbing alloys (Sample Nos. 10 to 13).

(78) The embodiments and examples disclosed herein are examples in all respects and should not be considered restrictive. The scope of the disclosure of this disclosure should be indicated not by the embodiments and examples described above but by the claims. The scope of this disclosure is intended to include meanings equivalent to claims and all changes within the scope.