HYDROGEN STORAGE ELECTRODES

Abstract: A hydrogen storage electrode comprising a body of an alloy of the general formula, AB a , or a hydride thereof in which A represents at least one element selected from the group consisting of Zr, Ti, Hf, Ta, Y, Ca, Mg, La, Ce, Pr, Mm, Nb, Nd, Mo, Al and Si, B represents at least one element selected from Ni, V, Cr, Mn, Fe, Co, Cu, Zn, Al, Si, Nb, Mo, W, Mg, Ca, Y, Ta, Pd, Ag, Au, Cd, In, Sn, Bi, La, Ce, Mm where Mm is a mixture of rare earth elements, Pr, Nd, Th and Sm provided that A and B are different from each other, and ª is a value of from 1.0 to 2.5. The alloy has an alloy phase which is substantially a Laves phase of an intermetallic compound of A and B, and has a crystal structure of a hexagonally symmetric C14 type having crystal lattice constants, a and c, of from 4.8 to 5.2 angstroms and from 7.9 to 8.3 angstroms, respectively, and/or a cubically symmetric C15 type having a crystal lattice constant of from 6.92 to 7.70 angstroms. A method for making the electrode is also described using a sintering or non-sintering process.

Description: This invention relates to C15 type alloy hydrogen storage electrodes using a hydrogen storage alloy or its hydride capable of reversibly storing or absorbing and releasing hydrogen. The hydrogen storage electrodes can be utilized in secondary cells or alkaline batteries which are unlikely to produce public nuisance and have high energy densities. Description of the Prior Art Currently, lead acid storage batteries and alkaline storage batteries have been widely used as secondary batteries. Among the alkaline storage batteries, nickel-cadmium batteries are most widely employed. However, the recent development of portable appliances demands a secondary battery or cell of a higher energy density substantially free of any ecological problems. Attempts have been made to use zinc as a negative electrode in place of cadmium. However, the cell using the zinc electrode has not been in wide use because of the problem of life cycle. Now, attention has been drawn to alkaline storage batteries using a hydrogen storage alloy capable of reversibly storing and releasing hydrogen as a negative electrode and a nickel oxide as a positive electrode. This battery may be called a nickel-hydrogen secondary battery. The hydrogen storage battery has a substantially dischargeable energy density higher than existing cadmium batteries and is free of any formation of dendrite as with Zn. In addition, the hydrogen storage battery can be fabricated by known methods. Accordingly, the hydrogen storage battery has been expected as a secondary battery which has a high energy density, long life cycle and low public nuisance. Known hydrogen storage alloys useful as the hydrogen storage electrode include, for example, Ti₂Ni, TiNi, LaNi₅, MnNi₅, and CaNi₅ with or without other additive elements. These alloys are described, for example, in The Journal of Less-Common Metals 129 (1987), pp. 13-30 and 131(1987), pp.311-319. Moreover, United States Patent Nos. 4,551,400 and 4,716,088 describe improved alloys of the formula, Ti 2-x Zr x V 4-y Ni y , in which 0<x≦ 1.5 and 0.6≦ y≦ 3.5. Although Ti₂Ni, TiNi and substituted alloys thereof have a relatively high dischargeable capacity in electrochemical charge and discharge cycles, a Ti-rich stable phase is formed after repetition of the charge and discharge cycles. This leads to a cycle life problem. On the other hand, LaNi₅, MnNi₅ and substituted alloys thereof are low in hydrogen storage capacity, so that the discharge capacity is relatively small. Moreover, this type of alloy has further disadvantages in that a variation in performance is great relative to the variation in temperature, the alloy is expensive, and the purities of starting metals are not necessarily satisfactory. Secondary batteries using CaNi₅ and substituted alloys thereof exhibit a high discharge capacity at an initial stage of the charge and discharge cycle, but considerably lower in performance,similar to those using Ti₂N and TiNi, after repetition of the charge and discharge cycle. The alloys of Ti 2-x Zr x V 4-x Ni y gradually convert into more stable hydrides and, thus, an effective alloy phase reduces in amounts with a problem of the cycle life. From JP-A-54-45608 hydrogen storage materials of the C15 type are known. The storage materials must contain Ti and Zr. In Journal of the Less-Common Metals, Vol. 131, 15th March 1987, pp. 311-319, hydrogen storage alloys are described containing titanium, zirconium and nickel. SUMMARY OF THE INVENTION It is an object of the present invention to provide a hydrogen storage electrode which is adapted for use in secondary batteries whereby the battery has a high discharge capacity with a long life cycle and involves little ecological problem. It is another object of the invention to provide a hydrogen storage electrode which is made of an alloy or its hydride whose alloy phase is substantially a Laves phase of an intermetallic compound. It is a further object of the invention to provide a hydrogen storage electrode made of an alloy which is capable of storing or releasing a greater amount of hydrogen than known alloys and rarely forms a stable hydride or oxide phase, so that it can maintain a high energy density over a long period when the hydrogen-storing and releasing cycle is repeated. The objects are solved by a C15 type alloy hydrogen storage electrode which comprises a body of an alloy of the following general formula AB a or its hydride, in which A represents Zr or a mixture of at least 30 atomic percent of Zr and the balance of at least one element selected from the group consisting of Ti, Hf, Al and Si, B represents Ni and at least one element selected from the group consisting of V, Cr, Mn, Fe, Co, Cu, Zn, Al, Si, Nb, Mo, W, Mg, Ca, Y, Ta, Pd, Ag, Au, Cd, In, Sn, Bi, La, Ce, Pr, Nd, Th, Sm and Mm where Mm is a mixture of rare earth element and a is a value of from 1.0 to 2.5, said alloy having an alloy phase which is substantially a Laves phase of an intermetallic compound A and B and having a crystal structure of a cubically symmetric C15 type having a crystal lattice constant of from 0,692 to 0.77 nm. Mm defined as a mixture of rare earth elements may be a so-called Misch metal. In the MgCu₂ type structure, the crystal lattice constants, a and c, should be, respectively, in the ranges of from 0.48 to 0.52 nm and from 0.79 to 8.3 nm. Within these ranges, a high discharge capacity is obtained. More specific Zr-V-Ni or Zr-Mo-Ni alloys or hydrides, or Zr-V-Ni-M alloys in which M is an other metal, are also preferred in the practice of the invention because these alloys exhibit an improved discharge capacity or life cycle. The hydrides of these alloys are similarly used. The hydrogen storage alloys or hydrides used in the present invention permit theoretically a larger number of hydrogen atoms per unit lattice than known alloys and can thus store or release a larger amounts of hydrogen. This is advantageous in that when the storing and releasing cycle of hydrogen is repeated, more stable hydride and oxide phases are unlikely to form. Accordingly, a hydrogen storage electrode using the hydrogen storage alloy maintains a high energy density over a long period. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A and 1B are, respectively, X-ray diffraction patterns of hydrogen storage alloys used in the invention; Fig. 2 is a curve of a relative discharge capacity in relation to the variation in crystal lattice constant; Fig. 3 is a graphical representation of a discharge capacity in relation to the variation in charge and discharge cycle for different hydrogen storage electrodes using alloys according to one embodiment of the invention and for comparison; Figs. 4 to 7 are, respectively, a graphical representation of a discharge capacity in relation to the variation in charge and discharge cycle for different hydrogen storage electrodes obtained according to embodiments of the invention; Fig. 8 is a schematic perspective view, in section, of a sealed cell using hydrogen storage electrodes of the invention; and Figs. 9, 10 and 11 are, respectively, a graphical representation of a charge and discharge cycle life characteristic of hydrogen storage electrodes according to further embodiments of the invention. DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION In general, AB a alloys, in which A and B have the meanings as defined above, may have alloy phases other than the Laves phase of the C14 and/or C15 type, depending upon the combination of metal elements and the manner of preparation of the alloy. The alloys of the AB a type may take various structures of from C to C54 types defined according to the Structturberichte classification. For instance, MoSi₂ will be likely to have a structure of the C11b type, CaAl₂ will take a C54 type structure, and TiSi₂ will take a C54 type structure. These structures are mainly dependent upon the types of constituent elements and also on the manner or conditions of preparation of the alloys. In the practice of the invention, the alloys having structures other than the C14 and/or C15 type structure will not be effective as a hydrogen storage electrode. Moreover, it has been experimentally confirmed that the crystal lattice constants a and b of the C14 type structure should, respectively, values of from 0.48 to 0.52 nm and from 0.79 to 0.83 nm. With the C15 type structure, the crystal lattice constant, a, should be in the range of from 0.692 to 0.77 nm. Within this or these ranges of the constants, good discharge capacity and charged or discharged ampere-hour characteristics are obtained. The hydrogen storage alloys having the Laves phase having appropriate lattice constant or constants have theoretically greater capability of occupying hydrogen atoms per unit lattice than known alloys. This allows a greater amount of either storing or releasing hydrogen. In addition, the alloys rarely form stable, not reversible, hydride or oxide phases when a storing and releasing cycle of hydrogen is repeated. This is why the electrode using the alloy of the invention can maintain a high energy density over a long period. Through a number of experiments of alloys as defined in the formula indicated above, it has been found that when the alloy has the C15 type Laves phase, such an alloy is effective as a hydrogen storage electrode provided that the crystal lattice constant or constants are in the defined range or ranges. The cubically symmetric C15 type Laves phase alloy has a MgCu₂ type crystal structure. The performance of the C15-type alloy as a hydrogen storage material is greatly influenced by the crystal lattice constant as stated above. However, when an alloy having good characteristics in a hydrogen gas phase is used as a hydrogen storage electrode, good results cannot always be obtained. This is because the electrochemical stability of the alloy in an alkaline electrolytic solution is important. It is general that alloys having a satisfactorily high charged and discharged ampere-hour characteristic are small in discharged ampere-hour. We have found that specific alloy compositions of the C14 type structure and the C15 type structure in which the crystal lattice constant or constants are within a certain range or ranges as set forth before are only useful as a hydrogen storage electrode. All the alloys defined by the formula, AB a , in which A, B and a have, respectively, the same meanings as defined before, are not suitable as hydrogen storage electrode. These alloys should have the C14 structure having a crystal lattice constants a and c of from 0.48 to 0.52 nm and from 0.79 to 0.83 nm, respectively, and/or the C15 structure having a crystal lattice constant, a, from 0.692 to 0.77 nm. The crystal structure is dependent greatly on the combination of elements used although the manner and conditions of preparation including a cooling speed, an annealing temperature and the like will, more or less, influence the formation of the structure. Other alloy phases such as of the types C11b, C12, C16 and C 23 may develop from combinations of A and B in the formula. These alloys of the different alloy phases are outside the scope of the invention. Typical examples of binary alloys having the C14 type structure are ZrMn₂, TiMn₂, ZnCr₂ and the like, and those having the C15 type structure are TiCr₂, ZrCo₂, ZrMo₂ and the like. These type structures will develop when an alloy melt is cooled relatively slowly. If the melt is quenched, crystallization may not occur, or other phases may appear predominantly. The crystal structure of the alloys is readily confirmed, for example, through X-ray diffraction analysis. Typical X-ray diffraction patterns of the C15-type Laves phase of ZrMn 0.6 Cr 0.2 Ni 1.2 and the C14-type Laves phase of ZrMn 0.6 Cr 0.2 Al 0.2 Ni 1.0 are shown in Figs. 1A and 1B, respectively. The numerals in the figures, e.g. (311), indicate Miller Indices. The formation of the C15-type Laves phase will be seen from the existence of peaks corresponding to the Miller indices (222), (311) and (220) in Fig. 1A. Likewise, the peaks corresponding to the Miller indices (004), (201), (112), (103) and (110) ensure the formation of the C14-type Laves phase. In this manner, the crystal structure of prepared alloys can be readily confirmed through the X-ray diffraction analysis. The existence of other peaks will show the development of other phases. The formation of the C14 type and/or C15 type Laves phase depends mainly upon the combination of elements, and also upon the manner or conditions of preparation of alloy. In some cases, the C14 and C15 structures appear in one alloy, and such an alloy is usable in the practice of the invention. It is not necessary that alloys of the invention be completely composed of the C14 and/or C15-type phase structure, but other phases may be contained only in small amounts, e.g. up to 10% of the total phases. In the foregoing general formula, A is Zr or a mixture of at least 30 atomic percent of Zr and the balance of at least one element selected from Ti, Hf, Al and Si. This is because Zr contributes to increase an amount of storage of hydrogen, and tends to form the desired phase structure of the C15 types. Moreover, B is preferably a mixture of at least 40 atomic percent of Ni and the balance of at least one element selected from V, Cr, Mn, Fe, Co, Cu, Zn, Al, Si, Nb, Mo, W, Mg, Ca, Y, Ta, Pd, Ag, Au, Cd, In, Sn, Bi, La, Ce, Mm, Pr, Nd, Th and Sm. In view of economy and characteristic properties, Ni, V, Zr, Cr, Mn and/or Mo is preferably contained in the alloy provided that the phase structure should be as defined before. Although the alloys are described above, the electrochemically reversible hydrides of these alloys which have the C15-type structure may also be used in the practice of the invention. These hydrides are advantageous in preventing oxidation of alloy powder before fabrication of an electrode and exhibit a higher initial charge and discharge speed. The specific Zr alloys of the general formula, AB a , in which A, B and a have, respectively, the same meanings as defined before, have advantages in view of the fact that Zr is likely to form homogeneous alloys which are longer in cycle life and higher in capacity when applied as a negative electrode for hydrogen storage battery. A preferred alloy has the general formula A'B' j Ni k , in which A' represents Zr or a mixture of at least 30 atomic percent of Zr and the balance of at least one element selected from Ti, Hf, Al and Si, B' represents at least one element selected from V, Cr, Mn, Fe, Co, Cu, Zn, Al, Si, Nb, Mo, W, Mg, Ca, Y, Ta, Pd, Ag, Au, Cd, In, Sn, Bi, La, Ce, Mm, Pr, Nd, Th and Sm in which Mm has the same meaning as defined before, j = 0.5 to 1.0 and k = 1.0 to 1.5 provided that j + k = 1.0 to 2.5 . The alloys of this class have a discharge capacity of from 200 to 350 mAh/g when used as a hydrogen storage electrode over a long cycle life and has thus good durability. In this sense, the alloys are preferred. If A' is a mixture of Zr and other elements, it is preferred that Al and/or Si is used as the other element particularly in view of economy. Al or Si is used as a substitute for part of Zr, the strong chemical activities of Zr-based alloys such as ignitable properties can be mitigated. Accordingly, easy handling is allowed. Preferably, B' represents two or more elements selected from Cr, Mn, Fe and Co because better discharge cycle properties are obtained with good affinity for Ni and good homogeneity without causing segregation. In view of the ease in formation of the C15-type Laves phase, preferable Zr-based alloys are those alloys of the general formula, ZrMn n Cr o Ni p , in which n = 0.4 to 0.8, o = 0.1 to 0.4, and p = 0.5 to 1.5 provided that n+o+p = 1.0 to 2.5 . In these classes of the alloys, they should have a C15-type structure, in which the crystal lattice constant or constants should be in the range or ranges defined before. The electrode may take any form such as a disk, an elongated strip, a sheet, a plate or the like. The present invention is more particularly described by way of examples. Example 1 Commercially available metal elements including Zr, Ti, Hf, Ta, Y, Ca, Mg, La, Ce, Mm (Misch metal), Nb, Nd, Sm, Mo, Al, Si, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Si, W and Cd were provided as starting elements and were weighed to give alloy compositions indicated in Table 1. The respective mixtures were molten in an argon arc melting furnace or in a high frequency induction heating furnace in vacuum or in an inert gas such as argon to obtain alloys indicated in Table 1. Part of the respective alloy samples were used for analyses of an alloy composition, crystal structure, crystal lattice constant and homogeneity. The other samples were, respectively, used for the measurement of an amount of stored hydrogen in a hydrogen gas (mainly by a P (pressure)-C (composition)-T (temperature) procedure) and also for evaluation of electrode performance by a single electrode test and a sealed cell test. The P-C-T measurement procedure was effected by placing a sample alloy in a stainless steel reaction container and maintained at a given temperature. The air in the container was evacuated, into which an H₂ gas was introduced to a predetermined level inherent to the sample alloy. The hydrogen gas gradually absorbed as H into the alloy and the absorbing reaction stopped upon arrival at a given equilibrium pressure. The PCT curve at the temperature could be obtained by plotting a logarithm of the variation in pressure as y-axis and the amount of H as x-axis. Part of the tested alloy samples is shown in Table 1 with respect to the alloy composition, crystal lattice constant and discharge capacity. Sample No. Alloy Composition Crystal Lattice Constant (nm) Discharge Capacity (mAh/g) 1 TiCo 0.6,700 5 2 NbCo₂ 0.6,755 11 3 TaCo₂ 0.6,757 9 4 TaFeNi 0.6,855 17 5 CaAl₂ 0.8,038 89 6 LaAl₂ 0.8,170 29 7 CeAl₂ 0.8,055 73 8 ZrV 0.5 Ni 1.5 0.7,340 365 9 Ti 0.7 Zr 0.3 Cr 1.5 Ni 0.5 0.7,422 367 The X-ray diffraction analysis of these alloys reveals that all the alloys indicated in Table 1 have the Laves phase of the C15 type. More particularly, the alloy Nos. 1 to 4 have the C15 type structure but the crystal lattice constant is smaller than 0.692 nm. The alloy Nos. 5 to 7 has a crystal lattice constant larger than 0.77 nm. In view of the above results, the crystal lattice constant should be in the range of from 0.692 to 0.77 nm. The alloy Nos. 8 and 9 were found to be better with respect to the amount of stored hydrogen when determined by the P-C-T measurement. The alloy No. 9 is a typical alloy containing Ni. These alloy samples were subjected to a cell test to evaluate the performance as a negative electrode for alkaline storage battery. The test was effected as follows. Each alloy obtained by melting was pulverized to obtain particles having a size of 300 µm (200 mesh) or below (U.S. mesh). About 5 g of the alloy powder was sufficiently mixed with 0.5 g of a polyethylene powder as a binder and 2 g of a carbonyl nickel powder used as a conducting agent. The mixture was applied to a nickel mesh conductive core material having a wire diameter of 0.2 mm and a size of 16 mesh and pressed to obtain a plate. The plate was allowed to stand at 120°C for about 1 hour in vacuum to melt the polyethylene, followed by attachment of lead wires to obtain a hydrogen storage electrode. For evaluation of the electrode as a negative electrode for secondary cell, a sintered nickel electrode or plate which is used in a commercial nickel-cadmium battery was provided as a positive or counter electrode in such a way that the quantity of the positive electrode was in excess in electric capacity over that of the hydrogen storage alloy electrode. A polyamide non-woven fabric was used as a separator and a solution obtained by adding 20 g/liter of lithium hydroxide to a caustic soda aqueous solution having a specific gravity of 1.30 was used as an electrolyte. Subsequently, a charging and discharging cycle at a constant current was repeated at 20°C. The charged ampere-hour was 500 mA x 5 hours and the discharge was effected at 300 mA while cutting 0.8 volts or below. The discharge capacity is shown in Table 1. Fig. 2 shows the relationship between a relative discharge capacity and the crystal lattice constant, in which the relative discharge capacity is a value by percent relative to the maximum discharge capacity. In Fig. 3, there is shown a discharge capacity in relation to the variation in cycle number for the electrodes using the alloy sample No. 9 and the electrodes for comparison using LaNi₅ and Ti₂Ni. As will be apparent from the results of the Table 1 and Fig. 2, the alloys of the invention having the elements defined in the present invention and the C15 type Laves phase are excellent as a negative electrode for alkaline storage battery. The alloy sample Nos. 1 to 4 which have the C15 type Laves phase but have a crystal lattice constant not larger than 0.692 nm are small in discharge capacity. On the other hand, the alloys having a crystal lattice constant larger than 0.77 nm is also small in discharge capacity. This is because the former alloys have the capability of storing only a small amount of hydrogen during a charging process or cycle. The latter alloys have the capability of storing a large amount of hydrogen, but form a stable hydride during the charge and discharge cycle, so that hydrogen cannot be released readily. Thus, the discharge efficiency is small. From the practical point of view, it may be preferred that the discharge capacity of a cell is not less than approximately 250 mAh/g. This value is 75% or higher of the maximum discharge capacity of the electrodes using the alloys of the invention. As will becomes apparent from the cycle life characteristic of Fig. 3, the known electrodes using Ti₂Ni and LaNi₅ deteriorate considerably, whereas the hydrogen storage electrodes of the invention are better. Particularly, those alloys containing Ni, Mo or V are preferred. It was also found that the electrodes of the invention had highly rapid charging and discharging characteristics. This is considered due to the fact that the alloys used in the present invention have good electrochemical catalytic activity and a good oxidation resistance. In this example, the alloys of the AB a , in which a = 2, were shown. It should be noted that so far as intermetallic compounds of AB₂ are formed, the value of a is not necessarily at 2 but within a range of from 1.0 to 2.5. Example 2 This example illustrates electrodes of alloys of the general formula, A'B' j Ni k , in which A' represents Zr, or a mixture of at least 30 atomic percent of Zr and the balance of at least one element selected from the group consiting of Ti, Hf, Al and Si, B' represents at least one element selected from V, Cr, Mn, Fe, Co, Cu, Zn, Al, Si, Nb, Mo, W, Mg, Ca, Y, Ta, Pd, Ag, Au, Cd, In, Sn, Bi, La, Ce, Mm, Pr, Nd, Th and Sm where Mm is a mixture of rare earth elements, and j = 0.5 to 1.0 and k = 1.0 to 1.5 provided that j + k = 1.5 to 2.5 . The alloys prepared are shown in Table 2 in which the alloy Nos. 10 to 15 are for comparison. The alloy Nos. 10 to 13 do not form the Laves phase. The alloy Nos. 14 and 15 form the Saves phase but are so small in crystal lattice constant that hydrogen is rarely absorbed. The alloy Nos. 16 to 39 are alloys of the above general formula and were found to consist of the C14 or C15 type Laves phase. With the C15 type crystal structure, the crystal lattice constant was in the range of 0.692 to 0.770 nm. The C14 type structure was found to have crystal lattice constants, a and c, of 0.48 to 0.52 nm and 0.79 to 0.82 nm respectively. These alloys were subjected to measurement of the P-C-T characteristic using hydrogen gas, with the result that an amount of stored hydrogen was relatively high. These alloys were used to make a cell test in an alkaline electrolytic solution in the same manner as in the foregoing examples. The results are shown in Fig. 4 through 7 in which the numerals in the figures correspond to the alloy numbers, respectively. From the results of the figures, although the alloy Nos. 10 and 11 have a great amount of stored hydrogen, but an electrochemical release amount is very small. Since the alloy No. 13 has an excessive amount of Ni, the amount of stored hydrogen lowers and thus, a satisfactory discharge capacity cannot be obtained. With the alloy Nos. 14 and 15, the Laves phase is formed but the crystal lattice constant is so small that the discharge capacity is very small. In contrast, the alloy Nos. 16 to 39 of this embodiment keep a high discharge capacity of 250 to 350 mAh/g after repetition of the charge and discharge cycle and are thus excellent as a hydrogen storage electrode. Subsequently, the hydrogen storage alloy electrodes were used to make a cylindrical sealed nickel-hydrogen secondary cell of the UM 2 type for evaluation. The alloys used were those alloys of Nos. 30, 32, 35 and 37. These alloys were each reduced into pieces having a size not larger than 225 µm (150 mesh) (U.S. standard), to which a polyvinyl alcohol binder was added along with water to make a paste. The paste was applied to a nickel-plated punching metal sheet and dried, followed by cutting it to an elongated piece having a width of 3.9 cm and a length of 26 cm. Lead sheets were attached to the piece in position by spot welding to obtain a hydrogen storage alloy electrode. A counter electrode used was a known foamed nickel electrode having a width of 3.9 cm and a length of 22 cm. A separator was a polyamide non-woven fabric, and an electrolytic solution used was a solution of 20 g/liter of lithium hydroxide in a potassium hydroxide aqueous solution having a specific gravity of 1.20. A nominal capacity was 3.0 Ah. Fig. 8 shows a schematic view of the secondary cell of the cylindrical form which includes a hydrogen storage alloy negative electrode 1, a nickel positive electrode 2 and a separator 3 provided between the electrodes 1 and 2. Indicated by 4 is a case serving also as a terminal for the negative electrode, by 5 and 6 are insulator plates, and by 7 is a safety valve. Indicated by 8 is a terminal for the positive electrode, by 9 is a positive lead, by 10 is a sealing plate, and by 11 is an insulating gasket. The cells were each subjected to a charge and discharge cycle test at 20°C. The charging was effected at 0.1 C or 10 hour rates to a level of 130% and the discharging was effected at 0.2 C or 5 hour rates to a final voltage of 1.0 volt. This charge and discharge cycle was repeated. As a result, it was found that the cells using the alloys of the invention kept a discharge capacity of approximately 3.0 Ah after repetition of 200 cycles, without any lowering of the performance. These results demonstrate that the alloys of the invention are appropriately applicable to sealed cells or batteries as shown in Fig. 8. Example 3 Similar to Example 2, there were prepared alloys of the general formula, AB l Ni m , in which A represents a mixture of at least 30 atomic percent of Zr and the balance of Al or Si, B₁ represents at least one element selected from V, Cr, Mn, Fe, Co, Cu, Zn, Al, Si, Nb, Mo, W, Mg, Ca, Y, Ta, Pd, Ag, Au, Cd, In, Sn, Bi, La, Ce, Mm, Pr, Nd, Th and Sm, l = 0.5 to 1.0 and m = 1.0 to 1.5 provided that l + m = 1.0 to 2.5 . The alloy compositions are indicated in Table 3. The alloy Nos. 40 to 61 were of the above general formula and were found to have a C15 type Laves phase. With the 15 type Laves phase alloy, the crystal lattice constant was in the range of from 0.692 to 0.77 nm for all the alloys. The alloys were subsequently subjected to a cell test in an alkaline electrolytic solution in the same manner as in the foregoing examples. The results are shown in Figs. 9 to 11 in which the numerals correspond to the alloy numbers, respectively These results demonstrate that the alloys of this example exhibit a discharge capacity of from 250 to 300 mAh/g after the charge and discharge cycle has been repeated and are thus excellent as a hydrogen storage electrode. Moreover, when the alloys were used to make a cylindrical sealed nickel-hydrogen secondary cell having such a construction as shown in Fig. 8 and evaluated, it was found that the cells using the hydrogen storage alloy electrodes of the invention kept a discharge capacity of approximately 3.0 Ah after repetition of 200 cycles or over. The alloys of this example are more advantageous than those of the foregoing examples in that since Al or Si is used by substitution for part of Zr, strong chemical activities such as ignitable properties of Zr-based alloys can be mitigated. Thus, these Al or Si-substituted alloys are more convenient in handling.

Claims: A C15 type alloy hydrogen storage electrode which comprises a body of an alloy of the general formula, AB a , or a hydride thereof, in which A represents Zr or a mixture of at least 30 atomic percent of Zr and the balance of at least one element selected from the group consisting of Ti, Hf, Al and Si, B represents Ni and at least one element selected from the group consisting of V, Cr, Mn, Fe, Co, Cu, Zn, Al, Si, Nb, Mo, W, Mg, Ca, Y, Ta, Pd, Ag, Au, Cd, In, Sn, Bi, La, Ce, Pr, Nd, Th, Sm and Mm where Mm is a mixture of rare earth element and a is a value of from 1.0 to 2.5, said alloy having an alloy phase which is substantially a Laves phase of an intermetallic compound A and B and having a crystal structure of a cubically symmetric C15 type having a crystal lattice constant of from 0.692 to 0.770 nm. The C15 type alloy hydrogen storage electrode according to Claim 1, wherein B represents a mixture of at least 40 atomic percent of Ni and the balance represents at least one element as mentioned above. The C15 type alloy hydrogen storage electrode according to Claim 1, wherein A represents Zr or a mixture of at least 30 atomic percent of Zr and Al or Si, and B represents two or more elements selected from the group consisting of Cr, Mn, Fe and Co. The C15 type alloy hydrogen storage electrode according to Claim 1, wherein said alloy is of the formula, ZrMn n Cr o Ni p , in which n = 0.4 to 0.8, o = 0.1 to 0.4 and p = 0.5 to 1.5 provided that n + o + p = 1.0 to 2.5 .

Inventors: GAMO, TAKAHARU, IWAKI, TSUTOMU, MORIWAKI, YOSHIO
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: H01M4
IPC codes: H01M4, C01B3