Hydrogen storage electrode

Abstract: A C15 type alloy hydrogen storage electrode is described which comprises a body of an alloy of the general formula ZrMo d Ni e , in which d = 0.1 to 1.20 and e = 1.1 to 2.5.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to secondary cells or batteries and more particularly, to C15 type alloy hydrogen storage electrodes using hydrogen storage alloys of the general formula ZrMo d Ni e , in which d = 0.1 to 1.20 and e = 1.1 to 2.5, or their hydrides capable of reversibly storing or absorbing and releasing hydrogen. The hydrogen storage electrodes can be utilized in 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. 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 term when the hydrogen-storing and releasing cycle is repeated. It is a still further object of the invention to provide a method for making a hydrogen storage electrode of the type mentioned above. Broadly, the hydrogen storage electrode according to the invention is made of a body which comprises an alloy of the following general formula, or its hydride, ZrMo d Ni e in which d = 0.1 to 1.20 and e = 1.1 to 2.5, said alloy having an alloy phase which is substantially a Laves phase of an intermetallic compound Zr, Mo and Ni and having a crystal structure of a cubically symmetric C15 type having a crystal lattice constant of from 0.692 to 0.77 nm. With the cubically symmetric MgCu₂ or 15 type structure, the crystal lattice constant, a, is from 0.692 to 0.770 nm (6.92 to 7.70 angstroms). With this range, a high discharge capacity is obtained. The hydrides of this alloy 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 amount 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 term. A method of fabricating the hydrogen storage alloy or hydride electrode as described above includes a non-sintering or sintering process. The non-sintering process comprises providing pieces or powder of the alloy or hydride, mixing the powder with a resin binder dissolved in a solvent and a conductive agent to obtain a dispersion or paste, applying the dispersion to a conductive core material, and drying the applied dispersion in a vacuum to obtain an electrode. Preferably, the conductive core material is a porous metal body, such as of Ni, having an average pore size of from 100 to 150 micrometers and a density of from 700 to 800 mg/cm³ and is applied with the dispersion at a packing rate of 4.5 to 5.0 g/cm³ in the dry basis. Alternatively, the sintering process may be used in which the powder of the alloy or hydride is mixed with a resin binder dissolved in a solvent and a sintering aid. The dispersion is applied to a conductive core material such a punching metal, dried in vacuum and press molded along with the conductive core material. The resultant molding is sintered in vacuum at a temperature of from 700 to 1200°C, during which the resin binder is thermally decomposed at approximately 400°C. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a X-ray refraction pattern of hydrogen storage alloys used in the invention; Figs. 2, 3 and 4 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. 5 is a schematic perspective view, in section, of a sealed cell using hydrogen storage electrodes of the invention; and DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION Broadly, the hydrogen storage electrode of the invention comprises a body of an alloy of the general formula, ZrMo d Ni e as defined in the claim. The alloy useful in the present invention has an alloy phase which is substantially a Laves phase of an intermetallic compound of Zr, Mo and Ni, and has a crystal structure of a face-centered cubically symmetric MgCu₂ or C15 type. It has been experimentally confirmed that the crystal lattice constant a of the C15 type structure should be in the range of from 0.692 to 0.770 nm (6.92 to 7.70 angstroms). Within this range of the constant, good discharge capacity and charged or discharged ampere-hour characteristics are obtained. The hydrogen storage alloys having the Laves phase having appropriate lattice constant 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 term. 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 is in the defined range. 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 C15 type structure in which the crystal lattice constant is within the certain range as set forth before are only useful as a hydrogen storage electrode. 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. For illustration, typical examples of binary alloys 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. For illustration, typical X-ray diffraction pattern of the C15-type Laves phase of ZrMn 0.6 Cr 0.2 Ni 1.2 is shown in Fig. 1. The numerals in the figure, 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. 1. 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. As stated above, the formation of the C15 type Laves phase depends mainly upon the combination of elements, and also upon the manner or conditions of preparation of alloy. It is not necessary that alloys of the invention be completely composed of the C15-type phase structure, but other phases may be contained only in small amounts, e.g. up to 10% of the total phases. 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 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. The C15 type alloy hydrogen storage electrode of the present invention comprises a body of an alloy of the general formula, ZrMo d Ni e , in which d = 0.1 to 1.20 and e = 1.1 to 2.5. Mo takes part in a charged ampere-hour characteristic. In this alloy, preferred ranges of d and e are, respectively, from 0.3 to 0.7 and from 1.4 to 1.9. This alloy is also economically advantageous. In these classes of the alloys, they should have C15-type structure, in which the crystal lattice constant is in the range defined before. For the preparation of the hydrogen storage alloys used in accordance with the present invention, commercially available metal pieces of a granular or plate form are provided and weighed in predetermined amounts, and these pieces are melted by argon arc melting or high frequency induction heating in vacuum or an inert gas such as argon. In order to improve the homogeneity and crystallinity of the alloy, once prepared alloys are preferably thermally treated at temperatures lower than a melting point of the alloy in vacuum, an inert gas or air. The treating temperatures below the melting point should preferably be higher than 900°C, although varying depending upon the type of alloy, for ensuring the effect of the thermal treatment. The alloy indicated with reference to illustrative Fig. 1 is thermally treated at 1080°C. The thermal treatment is generally effected for a time of at least one hour. For the fabrication of a hydrogen storage electrode using the hydrogen storage alloys, it is usual to first divide the alloy into pieces having, for example, a size of from 150 to 500 U.S. mesh. The alloy can be divided into pieces by known techniques using the Ellis motor, a jaw crusher, an automatic mortar by a wet process. In general, binary Zr-Ni alloys are relatively difficult to pulverize, but ternary or quaternary alloys are more likely to pulverize. In a sintering process, the pieces are subsequently mixed with from 2 to 10 wt% of a sintering aid based on the weight of the alloy pieces. The sintering aid is a powder of nickel or carbonyl nickel. The mixture is dispersed in a solution of a binder resin to obtain a paste or dispersion. The dispersion is applied to a conductive core material, dried in vacuum and press molded. The press molding is finally sintered in vacuum at a temperature of from 700 to 1200°C for a time sufficient for the sintering, say, 1 hour or over. During the sintering, the resin binder is thermally decomposed at approximately 400°C. The resin binder may be hydrophilic or hydrophobic in nature and is, for example, polyethylene, polyvinyl alcohol, CMC, fluorine resins, synthetic rubbers such as styrene-butadiene rubber, thermoplastic elastomers such as tetrafluoroethylenehexafluoropropylene copolymers, and the like. On the other hand, the electrode may be fabricated by a non-sintering process in which the alloy pieces are mixed with a solution of a resin binder as used in the sintering process along with a conductive agent, to obtain a paste or dispersion. The paste is applied to a conductive core material of a porous structure having a pore size of from 100 to 150 micrometers and a density of from 700 to 800 mg/cm³ at an appropriate packing rate, e.g. from 4.5 to 5.0 g/cm³ on the dry basis. The packed paste is dried in vacuum to obtain an electrode. The drying temperature may be not lower than 80°C, preferably about 120°C. The conductive agent used in the non-sintering process may be metal pieces such as Cu, Ni and the like, carbonyl nickel powder, carbon, and the like. The agent is used in an amount of from 0.1 to 60 wt% of the alloy pieces. In both sintering and non-sintering processes, the resin binders are dissolved in suitable solvents including water, alcohols such as ethyl alcohol, glycols such as ethylene glycol, and other organic solvents. Preferably, aqueous solutions of CMC, polyvinyl alcohol and the like having several weight percent are used. The conductive core materials may be a metal mesh, a punching metal sheet, foamed metals, expanded metals, metallic fibers or felts, and the like as is ordinarily used for this purpose. For the sintering process, a punching metal or a metal mesh is preferably used. On the other hand, foamed or expanded metals, and metallic fibers or felts are preferred for the non-sintering process. The fabrication of the electrode from alloy pieces has been described above. The alloy pieces may be absorbed with hydrogen to form an electrochemically reversible hydride. The hydride is advantageous in preventing oxidation of the alloy. The electrochemically stable hydride is different from a non-reversible, stable hydride phase which might be present in the alloys of the invention in small amounts as a kind of solid solution. The use of a hydride as a starting material instead of a hydrogen storage alloy is also within the scope of the invention. It should be noted that when hydrogen storage alloys of the invention are absorbed with hydrogen, a stable hydride phase is invariably formed in the alloy in an amount of about 3 to 7 wt% of the alloy, but the alloys of the invention do not form a higher amount of the stable hydride during the charge-discharge cycle. 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 of Zr, Mo and 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 and discharge capacity. 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 400 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. It was 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. The content of Ni is concerned with a charge and discharge cycle characteristic or durability, i.e. a larger content of Ni leads to a longer life with a tendency toward a smaller charged ampere-hour characteristic. This example illustrates an alloy of the formula, ZrMo d Ni e , in which d = 0.1 to 1.20 and e = 1.1 to 2.5. The alloy Nos. 1 to 6 are alloys of this invention and the alloy Nos. 7 to 10 consist of Zr-Mo-Ni but the atomic ratios are outside the scope of the invention or the type of alloy phase is not in conformity with the scope of the invention. The alloy No. 7 has a larger atomic ratio of Mo, the alloy Nos. 8 and 9 are not within the scope of the invention with respect to the alloy phase and have, respectively, a smaller atomic ratio of Mo and a smaller atomic ratio of Ni. The alloy No. 10 has a larger atomic ratio of Ni. These alloys were subjected to a performance test in which they were used as a negative electrode of alkaline battery as set out above. The discharged ampere-hour of an open system at the 10th charge and discharge cycle is indicated in Table 1. Alloy No. Alloy Composition Discharged Ampere-Hour After The 10th Cycle (mAh/g) 1 ZrMo 1.2 Ni 2.5 258 2 ZrMo 0.65 Ni 1.8 343 3 ZrMo 0.6 Ni 1.9 367 4 ZrMo 0.5 Ni 1.4 359 5 ZrMo 0.3 Ni 1.7 376 6 ZrMo 0.1 Ni 2.5 258 7 ZrMo 1.3 Ni 2.4 210 8 ZrMo 0.05 Ni 1.4 239 9 ZrMo 0.4 Ni 1.0 234 10 ZrMo 0.4 Ni 2.6 155 Fig. 2 shows a charge and discharge cycle life of the electrodes using the alloy Nos. 2, 3, 5 and 6. From the figure, the hydrogen storage alloy electrodes of the invention have a great discharge capacity and a good durability with a good initial discharge rise-up characteristic. This alloy system is better than other types of alloys with respect to an initial activation characteristic and rapid charge and discharge characteristics. When the atomic ratio of Mo is smaller than 0.1 or larger than 1.2, the discharge capacity is small. On the other hand, when the atomic ratio of Ni is smaller than 1.1 or larger than 2.5, the discharge capacity is small. This is because the content of Mo takes part in the charged ampere-hour, i.e. a larger content of Mo results in a larger charged ampere-hour but because of the formation of a stable hydride, the discharge efficiency is so small that the discharged ampere-hour reduces. The content of Ni has such effects as discussed above. Example 2 This example illustrates an electrode of an alloy No 17 of the formula, ZrMo 0.5 Ni 1.5 and, for comparision, electrodes of comparative alloys Nos. 11 to 16 shown in table 2. The alloy Nos. 11 to 14 do not form the Laves phase. The alloy Nos. 15 and 16 form the Laves phase but are so small in crystal lattice constant that hydrogen is rarely absorbed. The alloy No. 17 was found to consist of the C15 type Laves phase. With the C15 type crystal structure, the crystal lattice constant was in the range of 6.92 to 7.70 angstroms. 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. Alloy No. Alloy Composition 11 Zr₂Ni 12 ZrNi 13 ZrNi₂ 14 ZrNi₃ 15 ZrMn 0.5 Cr 1.5 16 ZrMn 1.0 Fe 1.0 17 ZrMo 0.5 Ni 1.5 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 Figs. 3 and 4 in which the numerals in the figures correspond to the alloy numbers, respectively. From the results of the figure 5, although the alloy Nos. 11 and 12 have a great amount of stored hydrogen, but an electrochemical release amount is very small. Since the alloy No. 14 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. 15 and 16, the Laves phase is formed but the crystal lattice constant is so small that the discharge capacity is very small. In contrast, the alloy No. 17 of this embodiment keep a high discharge capacity of above 250 mAh/g after repetition of the charge and discharge cycle and are thus excellent as a hydrogen storage electrode. Fig. 5 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. A C15 type alloy hydrogen storage electrode is described which comprises a body of an alloy of the general formula ZrMo d Ni e , in which d = 0.1 to 1.20 and e = 1.1 to 2.5.

Claims: A C15 type alloy hydrogen storage electrode which comprises a body of an alloy of the general formula ZrMo d Ni e , in which d = 0.1 to 1.20 and e = 1.1 to 2.5, said alloy having an alloy phase which is substantially a Laves phase of an intermetallic compound Zr, Mo and Ni and having a crystal structure of- a cubically symmetric C15 type having a crystal lattice constant of from 0.692 to 0.77 nm.

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