1. Field of the Invention
The present invention relates to a hydrogen-absorbing alloy for a battery, a method of manufacturing the same and a nickel-metal hydride battery using the alloy, and more specifically, to a hydrogen-absorbing alloy for a battery capable of, when applied to a negative electrode of the battery, satisfying all of the three leading characteristics or a high electrode capacity (battery capacity), long life (long cycle life) durable for repeated use and excellent initial activity as well as a stabilized electric potential (evenness of a voltage), a method of manufacturing the same and a secondary nickel-metal hydride battery.
2. Description of the Related Art
Recently, the miniaturization and portability of electronic appliances, which cannot be conventionally expected, has been achieved by the progress of a power saving technology and mounting technology realized by the progress of electronics. Under such a circumstance, a secondary battery used as a power source of these electronic appliances is required to have a large capacity and long life. For example, in the field of office automation appliances, telephone devices, audio/visual appliances having been developed for personal use and portable use, the development of battery having a high performance is desired to operate these appliance for a longer time without using a power supply cable. Although a non-sintering type nickel-cadmium battery having the electrode substrate, which is composed of three-dimensional structure, of a conventional sintering type nickel-cadmium battery has been developed, the capacity of this battery has not been remarkably increased.
Thus, there is recently proposed and highlighted a secondary alkaline battery (secondary nickel-metal hydride battery) using the hydrogen-absorbing alloy powder fixed to a collector as a negative electrode. The electrode used to this nickel-metal hydride battery is made by the following procedure. That is, hydrogen-absorbing alloy is melted by a high frequency induction melting method, arc melting method or the like and then cooled and pulverized and the thus obtained pulverized powder is added with an electric conductive agent and binder to form a kneaded material, and this kneaded material is coated to or pressingly attached to a collector. The negative electrode using the hydrogen-absorbing alloy is characterized in that it can not only increase an effective energy density per a unit weight or capacity but also has a less amount of poisonous property and a less possibility of environmental pollution as compared with cadmium used as a material for the negative electrode of a conventional typical secondary alkaline battery.
The negative electrode containing the hydrogen-absorbing alloy, however, is immersed into a thick alkaline solution as a battery electrolyte when it is assembled to a secondary battery as well as exposed to oxygen evolved from a positive electrode when the battery is excessively charged, and thus the hydrogen-absorbing alloy is corroded and the electrode characteristics thereof are liable to be deteriorated. Further, when the battery is charged, hydrogen is absorbed into and released from the hydrogen-absorbing alloy to cause the volume of the alloy to be expanded and shrank, and thus cracks are produced to the hydrogen-absorbing alloy, by which the pulverization of the hydrogen-absorbing alloy is progressed. When the pulverization of the hydrogen-absorbing alloy is progressed, the increase of the specific surface area of the hydrogen-absorbing alloy is accelerated, and thus the ratio of the surface area of the hydrogen-absorbing alloy deteriorated by the alkaline battery electrolyte is increased.
Moreover, since the electric conductivity between the hydrogen-absorbing alloy and the collector is also deteriorated, a cycle life is shortened as well as the electrode characteristics are also deteriorated.
To solve the above problems, although there are proposed such methods as providing the hydrogen-absorbing alloy with a multi-element structure, preventing the direct contact of the hydrogen-absorbing alloy with the battery electrolyte by covering a copper thin film or nickel thin film onto the surface of the hydrogen-absorbing alloy powder or the surface of a negative electrode containing the hydrogen-absorbing alloy by a plating method, vapor deposition method or the like to improve the corrosion resistance of the hydrogen-absorbing alloy, preventing cracks by increasing the mechanical strength of the hydrogen-absorbing alloy, or suppressing the deterioration of the surface of the hydrogen-absorbing alloy by drying the same after it has been immersed into an alkaline solution, these methods cannot always achieve a sufficient improvement and sometimes lower an electrode capacity on the contrary.
Further, as described above, although the electrode characteristics of the conventional hydrogen-absorbing alloy are deteriorated by a kind of corrosive reaction caused by the alkaline battery electrolyte, the battery electrolyte is consumed by the reaction. Therefore, the battery electrolyte in the conventional battery has an amount larger than that necessary to smoothly cause a battery reaction so that the battery reaction is not prevented even if the amount of the battery electrolyte is reduced to some extent. When, however, the amount of the electrolyte is increased and the surface of the hydrogen-absorbing alloy electrode is covered with it, the reaction speed for consuming an oxygen gas evolved in an excessively charged region is lowered, and thus a problem arises in that a battery internal pressure is increased.
Further, the aforesaid deterioration of the hydrogen-absorbing alloy is also a problem when battery is designed.
That is, a secondary alkaline battery is designed to sealed in such a manner that when the battery is discharged, a portion of the hydrogen-absorbing alloy electrode usually remains in a charged state and when the battery is charged, a portion of the hydrogen-absorbing alloy electrode partially remains in an uncharged state. Since, however, the hydrogen-absorbing alloy is deteriorated with the progress of charge and discharge, a large amount of the hydrogen-absorbing alloy must be contained so that the above relationship can be maintained even if the alloy is deteriorated in order to obtain a sufficient cycle life as a battery. Consequently, since the volume of a nickel electrode as a capacity-limiting electrode is reduced and the volume of the hydrogen-absorbing alloy electrode is increased, the increase of the battery capacity is prevented as well as a cost of the battery is increased because the hydrogen-absorbing alloy is expensive.
Incidentally, the aforesaid hydrogen-absorbing alloy is composed of, for example, AB.sub.2 type or A.sub.2 B type hydrogen-absorbing alloy represented by Zr-Ti-Mn-Fe-Ag-V-Al-W, Ti.sub.15 Zr.sub.21 V.sub.15 Ni.sub.29 Cr.sub.5 Co.sub.5 Fe.sub.1 Mn.sub.8 and the like. These hydrogen-absorbing alloys are made by a usual method of pulverizing an alloy made by being melted and cast. When this series of alloys are used to a negative electrode, they exhibit a high electrode capacity and provide a good capacity of about 300 mAh/g, 400 mAh/g, respectively as well as almost all the metal materials constituting the alloys are less expensive.
These alloys, however, have a drawback in that they are generally difficult to be made to have a composition distributed uniformly. Further, a battery using this series of alloys as an electrode material has a drawback in that the battery has a delayed rising-up of a capacity, and thus a high electrode capacity can be obtained for the first time after an activating operation (charge/discharge operation) of several tens of cycles is repeated after the battery has been assembled. Moreover, this series of alloys also have a drawback in that discharging characteristics are bad in a large current and further a voltage greatly drops at a low temperature. That is, this series of the alloys cannot achieve the high capacity in the three leading characteristics or a high electrode capacity, long life and excellent initial activity, whereas they cannot satisfy the technical requirements in the aspect of the initial activity (rising-up property).
On the other hand, there is an AB.sub.5 type alloy represented by LaNi.sub.5 as another hydrogen-absorbing alloy used to secondary alkaline battery. A negative electrode using this series of an alloy having a hexagonal-crystal structure has the feature that it can increase the effective energy density per a unit weight or unit volume of a battery and a battery capacity as well as has a less possibility to cause the environmental pollution due to cadmium and the like and good battery characteristics when compared with the case in which cadmium as a conventional typical negative electrode material for secondary alkaline battery is used. Further, the battery using the AB.sub.5 type alloy has an advantage that it can discharge a large current. In this connection, an AB.sub.5 type hydrogen-absorbing alloy composed of Mm-Ni-Co-Al alloy (Mm is referred to as misch metal) has a low electrode capacity of less than 200 mAh/g and a cycle life determined by charge/discharge is about 400 cycles, which does not reach the level for satisfying the electrode capacity and cycle life needed by the recent technical requirements.
Thus, a technology of relatively increasing the content ratio of the A site is also employed to increase the electrode capacity of the battery using the AB.sub.5 type hydrogen-absorbing alloy. According to this technology, although the electrode capacity can be increased by about 30%, a drawback arises in that the charge/discharge cycle life is shortened.
Further, there is also employed a technology for increasing the amount of La content in misch metal (Mm: a mixture of rare earth elements containing 10-50 wt % of La, 5-60 wt % of Ce, 2-10 wt % of Pr, 10-45 wt % of Nd and the like) constituting the A component. That is, it is possible to increase the electrode capacity by about 30% by using misch metal containing an reduced amount of a Ce element and a relatively increased amount of La. In this case, however, it is also difficult to increase the cycle life.
As described above, a hydrogen-absorbing alloy suitable for secondary nickel-metal hydride battery for satisfying the electrode capacity, cycle life, initial rising-up characteristics and stability of electric potential required by the recent technical level is not yet realized for practical use.