Hydrogen-absorbing alloy electrode for metal hydride alkaline batteries and process for producing the same
BACKGROUND OF THE INVENTION 
     This application claims the priority of Japanese Patent Applications Serial No. 6-315555 filed on Nov. 25, 1994, Serial No. 7-262096 filed on Sep. 13, 1995, Serial No. 7-262097 filed on Sep. 13, 1995 and Serial No. 7-279685 filed on Oct. 2, 1995. 
     FIELD OF THE INVENTION 
     The present invention relates to a hydrogen-absorbing alloy electrode for metal hydride alkaline batteries and a process for producing the same. More specifically, the present invention relates to an improvement of a hydrogen-absorbing material of hydrogen-absorbing alloy, in order to obtain metal hydride alkaline batteries being excellent both in high-rate discharge characteristics at an initial period of charge-discharge cycles and in charge-discharge cycle characteristics. 
     DESCRIPTION OF THE PRIOR ART 
     In recent years, metal hydride alkaline batteries using a metal compound such as nickel hydroxide as positive electrode and a hydrogen-absorbing alloy, which is a novel material, as negative electrode have been attracting much attention as next-generation batteries which will replace nickel-cadmium batteries, since they have high energy density per unit weight or unit volume and can be provided with high capacity. 
     As the hydrogen-absorbing alloys for such metal hydride alkaline batteries, those obtained by solidifying an alloy melt in a die by cooling with water and then pulverizing the obtained solid are generally used (hereinafter this type hydrogen-absorbing alloy is called &#34;conventional solidified alloy&#34;. 
     However, these conventional solidified alloys contain much segregation (irregular distribution of component elements) and hence the alloy particles crack on absorbing or discharging hydrogen during charge and discharge, thereby increasing the specific surface area. As a result, metal hydride alkaline batteries using a conventional solidified alloy as negative electrode material become indeed excellent in high-rate discharge characteristics at an initial period of charge-discharge cycles. On the other hand, they have the problem of having generally short life, since the segregation portions readily become starting points of oxidation degradation (corrosion). 
     In order to improve the cycle life, Japanese Patent Application Laid-open No. 89066/1985 proposed annealing (heating at a prescribed temperature for a prescribed time) a conventional solidified alloy and using the annealed alloy for negative electrode. 
     However, although an annealed conventional solidified alloy, having a low level of segregation, has a longer cycle life compared with one before annealing, its crystal grains become less crackable due to, in addition to the reduced segregation, the fact that the crystal grains have too large a size (sum of the thicknesses of two neighboring layers in a laminar structure in which the layers of the constituting rare earth element having a high concentration and those having a low concentration are present alternately). Consequently, the high-rate discharge characteristics at an initial period of charge-discharge cycles deteriorates markedly. 
     In view of the above problems, which are difficult to solve, associated with conventional solidified alloys, there has been in recent years proposed, as a negative electrode material for metal hydride batteries, a hydrogen-absorbing alloy obtained by what is known as &#34;single roll process&#34;, which comprises jetting an alloy melt onto the peripheral surface of a roller rotating at a high speed to quench and solidify the melt (see, for example, Japanese Patent Application Laid-open No. 187979/1994). 
     The hydrogen-absorbing alloy prepared by the single roll process, i.e. by quenching and solidifying the corresponding alloy melt, is hardly influenced by a gravitational field and hence contains less segregation compared with conventional solidified alloys. 
     However, the crystal grains of the hydrogen-absorbing alloy prepared by the single roll process have nonuniform sizes. The alloy will therefore contain, as the charge-discharge cycle proceeds, readily crackable parts (open-surface side containing larger crystal grains) and less crackable parts (roll-surface side containing smaller crystal grains). 
     Japanese Patent Application Laid-open No. 291363/1988 describes that it is desirable to use as an electrode material a hydrogen-absorbing alloy powder obtained by pulverizing flakes of a hydrogen-absorbing alloy having a thickness of not more than 40 μm with crystal grains oriented along a certain crystal plane (seemingly (hk0) plane). However, this hydrogen-absorbing alloy, with which the selectively oriented plane appears on the surface of the alloy particles, have the problem of poor electrode catalytic activity and poor high-rate discharge characteristics at an initial period of charge-discharge cycles. In addition, use of flakes having a thickness of not more than 40 μm results in too fine alloy particles after pulverization, thereby increasing the contact resistance between the alloy particles. As a result, the rate of utilization of the hydrogen-absorbing alloy decreases and further the cycle life of the resulting electrode becomes short. 
     FIG. 2 is an enlarged cross-sectional view of a hydrogen-absorbing alloy B prepared by single roll process, which schematically shows the condition of crystal grains appearing, when the alloy is cut in the length direction of an alloy strip and along a plane perpendicular to the strip surface (only a part is shown). In the FIG., blank parts 21 show layers of the rare earth elements having a high concentration, while black parts 22 with those having a low concentration; and the sum of the thicknesses of the two neighboring layers is the size of crystal grains. 25 is the thickness of the thin strip. As seen from FIG. 2, the crystal grains present in the open-surface side O have a large size 23, and those in the roll-surface side R have a small size 24. While the open-surface side O with a large crystal grain size 23 readily cracks and becomes activated, the roll-surface side R with a small crystal grain size 24 does not crack easily and is difficult to activate. Consequently, the charge-discharge depth of the readily activatable open-surface side O becomes large, so that the hydrogen-absorbing alloy B tends to be finely pulverized upon repeated charge and discharge. 
     As stated above, metal hydride alkaline batteries using an hydrogen-absorbing alloy prepared by single roll process, which is readily pulverized into fine pieces on repetition of charge and discharge, have the disadvantage of short cycle life. There has been a strong demand for improvement of this disadvantage. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a hydrogen-absorbing alloy electrode which can provide metal hydride alkaline batteries being excellent in both high-rate discharge characteristics at an initial period of charge-discharge cycles and charge-discharge cycle characteristics. 
     Another object of the present invention is to provide a process for producing the above electrode. 
     The present invention provides a hydrogen-absorbing alloy electrode for metal hydride alkaline batteries, which uses as hydrogen-absorbing material a powder of a rare earth element-nickel hydrogen-absorbing alloy obtained by pulverizing thin strips of said alloy prepared by single roll process and having an average thickness of 0.08 to 0.35 mm and a minimum size of crystal grains present in the roll-surface side of at least 0.2 μm and a maximum size of crystal grains in the open-surface side of not more than 20 μm, the size of crystal grains being as defined below: 
     the size of crystal grains is the sum of the thicknesses of two neighboring layers in a laminar structure in which the layers of constituting rare earth elements having a high concentration and those having a low concentration are present alternately. 
     The present invention further provides a hydrogen-absorbing alloy electrode for metal hydride alkaline batteries, which uses as hydrogen-absorbing material a powder of an Mm.Ni.Co.Al.Mn alloy (Mm: misch metal; R: Ni, Co, Al and Mn; and x is 4.5 to 4.9) obtained by pulverizing thin strips of said alloy prepared by single roll process and having an average thickness of 0.08 to 0.35 mm and a minimum size of crystal grains present in the roll-surface side of at least 0.2 μm and a maximum size of crystal grains in the open-surface side of not more than 18 μm, the size of crystal grains being as defined above. 
     The present invention still further provides a process for producing hydrogen-absorbing alloy electrodes for metal hydride alkaline batteries, which comprises the steps of: preparing a thin strip-like rare earth element-nickel hydrogen-absorbing alloy by permitting a melt of the alloy to flow onto the peripheral surface of a single roller rotating at a surface speed of 50 to 1,000 cm/sec in an inert gas or under vacuum; annealing said thin strip-like rare earth element-nickel hydrogen-absorbing alloy in an inert gas or under vacuum at a temperature of 620° to 1,000° C. for a prescribed time; pulverizing the annealed thin strip-like rare earth element-nickel hydrogen-absorbing alloy into a powder of the alloy; and preparing an electrode by using the obtained alloy powder as hydrogen-absorbing material. 
     The present invention yet further provides a process for producing hydrogen-absorbing alloy electrodes for metal hydride alkaline batteries, which comprises the steps of: preparing a thin strip-like Mm.Ni.Co.Al.Mn alloy represented by the general formula MmRx (Mm: misch metal; R: Ni, Co, Al and Mn; and x is 4.4 to 5.2) by permitting a melt of the alloy to flow onto the peripheral surface of a single roll rotating at a surface speed of 50 to 1,000 cm/sec in an inert gas or under vacuum; annealing said thin strip-like Mm.Ni.Co.Al.Mn alloy in an inert gas or under vacuum at a temperature of 750° to 950° C. for a prescribed time; pulverizing the annealed alloy strips into a powder of the alloy; and preparing an electrode by using the obtained alloy powder as hydrogen-absorbing material. 
     The electrodes of the present invention can, when used as a negative electrode, provide metal hydride alkaline batteries which are excellent in both high-rate discharge characteristics at an initial period of charge-discharge cycles and charge discharge cycle characteristics. According to the processes of the present invention, there can be obtained the electrodes of the present invention having such excellent electrode characteristics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation and many of the attendant advantages thereof will be readily obtained as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
     FIG. 1 is a schematic view showing the condition of crystal grains present in and appearing on the cross-section of a hydrogen-absorbing alloy having been quenched and solidified by single roll process and then annealed; and 
     FIG. 2 is a schematic view showing the condition of crystal grains present in and appearing on the cross-section of a hydrogen-absorbing alloy having been quenched and solidified by single roll process and, thereafter, not annealed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The average thickness of the thin strip-like rare earth element-nickel hydrogen-absorbing alloy used in the present invention is restricted within the range of 0.08 to 0.35 mm for the following reasons. Use of thin strips having an average thickness exceeding 0.35 mm, in which crystal grains have nonuniform sizes in the thickness direction of the strips, shortens the cycle life. On the other hand, use of thin strips having an average thickness of less than 0.08 mm, in which the surface of alloy particles are oriented in (hk0) plane to deteriorate the catalytic activity of the electrode, decreases the high-rate discharge characteristics at an initial period of charge-discharge cycles and, besides, shortens the cycle life. The shortening of the cycle life is because: since the alloy particles constituting the electrode have a small particle size, the contact resistance between the alloy particles in the electrode increases, thereby decreasing the utilization rate of the hydrogen-absorbing alloy powder. 
     The minimum size of crystal grains of the rare earth element-nickel hydrogen-absorbing alloy used in the invention present in the roll-surface side and the maximum size thereof in the open-surface side are each restricted within the above range. Deviation from the range either decreases the high-rate discharge characteristics at an initial period of charge-discharge cycles or shortens the cycle life. That is, if a crystal grain having a size of less than 0.2 μm is present, together with larger particles, in the roll-surface side, the hydrogen-absorbing alloy will become hardly crackable, thereby decreasing the high-rate discharge characteristics at an initial period of charge-discharge cycles. On the other hand, if a crystal grain having a size exceeding 20 μm is present, together with smaller particles, in the open-surface side, the hydrogen-absorbing alloy will become readily finely pulverized finely and degradable by oxidation, thereby shortening the cycle life. 
     In order to obtain a hydrogen-absorbing alloy electrode capable of providing batteries having, besides excellent high-rate discharge characteristics at an initial period of charge-discharge cycles and charge-discharge characteristics, excellent internal-pressure characteristics, it is desirable to use a hydrogen-absorbing alloy having the following features. That is, the alloy powder preferably has an average particle diameter of 25 to 70 μm. More preferably, the alloy powder is one obtained by pulverizing a thin strip-like rare earth element-nickel hydrogen-absorbing alloy prepared by single roll process and then surface-treating the obtained powder by immersion in an acid solution and having an average diameter of 25 to 60 μm. Examples of the acid solution are aqueous solutions of hydrochloric acid, sulfuric acid and nitric acid. 
     With the average particle diameter exceeding 70 μm, the specific surface area becomes too small so that the oxygen generated on the positive electrode upon overcharge cannot be consumed through the hydrogen-absorbing alloy electrode (negative electrode). As a result, the internal pressure of the battery tends to increase. On the other hand, with a small average particle diameter of less than 25 μm, the alloy particles contact each other insufficiently, so that only part of the alloy particles can participate in the reaction. As a result, the heat of reaction increases and hence the battery internal pressure tends to increase. 
     The average particle diameter herein means a volume-weighted mean diameter MV as defined by the following formula (1). This volume-weighted mean diameter MV is introduced hereinto based on the consideration that, for any powder having a distribution of particle diameters, use of this value as representative diameter is more convenient as long as the effect of its particle diameter on a certain phenonmenon is the same as that produced by uniform particles having a diameter of MV. 
     
         MV=Σ(ViDi)/ΣVi                                 (1) 
    
     where Di=representative diameter of the i-th particle diameter fraction, and 
     Vi=volume ratio of the i-th particle diameter fraction 
     Examples of the rare earth element-nickel alloy used the invention are Mm.Ni.Co.Al.Mn alloys represented by the general formula MmRx (Mm: Misch metal; R: Ni, Co, Al and Mn; x: 4.4 to 5.2). 
     The suitable Co content in the above Mm.Ni.Co.Al.Mn alloys is 0.4 to 0.9 mole per mole of Mm, and the suitable Ni content is 2.8 to 3.6 moles per mole of Mm. 
     In order to obtain a hydrogen-absorbing alloy electrode capable of providing batteries having, besides excellent high-rate discharge characteristics at an initial period of charge-discharge cycles and charge-discharge cycle characteristics, excellent resistance to overcharge, it is desirable to use a hydrogen-absorbing alloy powder obtained by pulverizing a thin strip-like Mm.Ni.Co.Al.Mn represented by the general formula MmRx (Mm: Misch metal; R: Ni, Co, Al and Mn; x: 4.4. to 4.9) having an average thickness of 0.08 to 0.35 mm and a minimum size of crystal grains present in the roll-surface side of at least 0.2 μm and a maximum size of crystal grains in the open-surface side of not more than 18 μm. In this case, the suitable Co content in the Mm.Ni.Co.Al.Mn alloys is 0.5 to 0.9 mole per mole of Mm, and the suitable Ni content is 2.9 to 3.3 moles per mole of Mm. 
     The electrode of the present invention can be obtained by, for example, the following process. That is, the process of the present invention comprises the steps of: preparing a thin strip-like rare earth element-nickel hydrogen-absorbing alloy by quenching and solidifying the alloy melt by single roll process in an inert gas or under vacuum at a roll surface speed of 50 to 1,000 cm/sec; annealing the thin strip-like rare earth element-nickel hydrogen-absorbing alloy in an inert gas or under vacuum at a temperature of 620° to 1,000° C.; pulverizing the annealed race earth element-nickel hydrogen-absorbing alloy into a powder of the alloy; mixing the alloy powder and a binder solution to obtain a slurry; and coating or impregnating a substrate (core) with the slurry. Specific examples of the core used in the invention are foamed porous metals, metal fibers, carbon fiber, metal meshes and perforated metals. Quenching and solidifying an alloy metal by single roll process at a roll surface speed of 50 to 1,000 cm/sec can permit the resulting thin strips to have a thickness of 0.08 to 0.35 mm. Annealing at a temperature of 620° to 1,000° C. after the quenching and solidification can permit the resulting crystal grains to have a minimum size of at least 0.2 μm and a maximum size of not more than 20 μm. In order to obtain crystal grains having a minimum size of at least 0.2 μm and a maximum size of not more than 18 μm, it is recommended to set the annealing temperature at 750° to 950° C. 
     The thickness of the above thin strip-like rare earth element-nickel hydrogen-absorbing alloy depends on the roll surface speed. The thickness decreases with increasing roll surface speed, i.e. increasing solidifying speed. The size of crystal grains depends on the annealing temperature. In general, elevating the annealing temperature results in an increased minimum crystal grain size but almost no change in the maximum size. In other words, elevating the annealing temperature decreases the dispersion of crystal grain sizes. If the annealing temperature exceeds 1,000° C. and gets close to the melting point of a hydrogen-absorbing alloy (about 1,200° C.), the alloy will partly re-melt at the grain boundary, thereby becoming very uncrackable and inactivated. The annealing time is about 1 to 10 hours. The annealing effect generally saturates after about 10 hours. 
     The thin strip-like rare earth element-nickel hydrogen-absorbing alloy used in the invention, having a small thickness of 0.08 to 0.35 mm, contains crystal grains having a uniform size throughout the thickness direction of the strips. The resulting alloy powder is therefore difficult to become finer during charge-discharge cycles. Since the lower limit of the strip thickness is set at at least 0.08 mm, the surface of the alloy particles is selectively oriented only to a small extent, so that the alloy has high electrode catalytic activity. 
     The rare earth element-nickel hydrogen-absorbing alloy used in the invention, containing no crystal grains which are too fine in the roll-surface side nor too coarse ones in the open-surface side, cracks to a moderate extent at an initial period of charge-discharge cycles and, nevertheless, does not become extremely fine upon repetition of charge-discharge cycles. 
     Accordingly, metal hydride alkaline batteries using the electrode of the present invention as the negative electrode are excellent in both high-rate discharge characteristics at an initial period of charge-discharge cycles and charge-discharge cycle characteristics. 
     In particular, use of a powder having an average particle diameter of 25 to 70 μm, preferably one surface-treated by immersion in an acid solution and having an average particle diameter after the treatment of 25 to 60 μm provides metal hydride alkaline batteries which are excellent not only in high-rate discharge characteristics at an initial period of charge-discharge cycles and charge-discharge cycle characteristics but in internal pressure characteristics. 
     EXAMPLES 
     Other features of the invention will become apparent in the course of the following detailed descriptions of exemplary embodiments which are given for illustration of the invention and are not intended to be limiting thereof. 
     EXAMPLES 1 THROUGH 13 
     Preparation of Hydrogen-Absorbing Alloy 
     Metals (all having a purity of at least: 99.9%) constituting the alloy to prepare were each weighed and mixed together and the mixture was melted under vacuum in a high-frequency melting furnace. The melt was cooled by a single roll process (roll diameter: 350 mm) at various cooling rates as shown in Table 1 (roll surface speed: 50, 100, 300, 500 or 1,000 cm/sec) to give thin strip-like rare earth element-nickel hydrogen-absorbing alloys represented by the composition formula MmNi 3 .4 Co 0 .8 Al 0 .3 Mn 0 .5 (strip length: about 30 to about 100 mm; strip width: about 20 to about 50 mm). Ten optional strips of each alloy were tested for thickness and the average was taken as the thickness of the alloy. 
     These rare earth element-nickel hydrogen-absorbing alloys were then annealed at various temperatures as shown in Table 1 (620°, 700°, 800°, 900°, 1,000° or 1,200° C.) in argon for 6 hours. 
     Each of the rare earth element-nickel hydrogen-absorbing alloys thus annealed was cut along the strip length and in a direction perpendicular to the strip surface. The reflected electron beam image of each cross-section was observed in a scanning electron microscope (JEOL866, made by JEOL Co.) to obtain the maximum size of crystal grains present in the open-surface side and the minimum size of those in the roll-surface side. The maximum size and minimum size of crystal grains were determined by measuring the distance between two neighboring blank lines, in a direction perpendicular thereto, observed on the reflected electron beam image (hereinafter the maximum and minimum sizes of crystal grains were obtained in the same manner). The maximum and minimum sizes of crystal grains as shown are average of measurements obtained with 10 samples for each rare earth element-nickel hydrogen-absorbing alloy. 
     FIG. 1 is a partial schematic view showing the condition of crystal grains appearing on the cross-section of the annealed thin strip-like hydrogen-absorbing alloy A. In the Figure, blank parts 1 show layers having a high concentration of rare earth elements and cobalt and a low concentration of manganese, and black parts 2 show layers having a low concentration of rare earth elements and cobalt and a high concentration of manganese. 3 shows the size of crystal grains present in the open-surface side O, while 4 that in the roll-surface side. 5 is the thickness of the thin strip. As seen from FIG. 1, the annealing had increased the size 4 of crystal grains in the roll-surface side, thereby reducing the difference between the size 4 and the size 3 of crystal grains in the open-surface side. As a result, the size distribution in the thickness direction the thin strip became more uniform than that of the hydrogen-absorbing alloy B before annealing (see FIG. 2). 
     Preparation of Hydrogen-Absorbing Alloy Electrodes 
     Each of the annealed rare earth element-nickel hydrogen-absorbing alloys was mechanically pulverized under an atmosphere of an inert gas (argon) and the obtained powders were sieved into powders having a maximum particle diameter of 100 mesh (in the Examples and Comparative Examples that follow, the maximum particle diameters were all adjusted at 100 mesh) and an average particle diameter (volume-weighted average diameter) of 70 μm. The average particle diameter was obtained by 2-point method [i=2 in the above equation (1)]. Further this average particle diameter was obtained by measuring particle size distribution by laser diffraction method with a laser light, which uses the Fraunhofer diffraction phenomenon. The measurement was conducted with a microtruck particle size analyzer (Type 7991 , made by Leeds &amp; Northrup Co.). Slurries were then prepared by mixing 90 parts by weight of each of the powders thus obtained and 10 parts by weight of 2.5% aqueous solution of polyethylene oxide. 
     These slurries were each applied on a perforated metal of nickel-plated iron and dried, to prepare hydrogen-absorbing alloy electrodes. 
     Preparation of Nickel Hydride Alkaline Batteries 
     AA-size nickel hydride alkaline batteries of positive electrode-dominating type (battery capacity: 1,200±10 mAh) A1 through A13 were prepared with the above hydrogen-absorbing alloy electrodes. A known sintered nickel electrode, a polyamide nonwoven fabric and a 30% aqueous potassium hydroxide solution were used as the positive electrode, separator and alkaline electrolyte, respectively. 
     Table 1 shows the alloy preparation conditions (roll surface speed and annealing temperature) used in Examples 1 through 13, the average particle diameter of the alloy powders prepared, the thickness of the thin strips and the maximum and minimum crystal grain sizes. 
     
                                           TABLE 1                                 
__________________________________________________________________________
       Roll Thickness           Average                                   
                                     High-rate                            
       surface                                                            
            of thin                                                       
                  Annealing                                               
                        Size of crystal                                   
                                particle                                  
                                     discharge                            
                                          Cycle                           
       speed                                                              
            strips                                                        
                  tempera-                                                
                        grains (μm)                                    
                                diameter                                  
                                     capacity                             
                                          life                            
       (cm/sec)                                                           
            (mm)  ture (°C.)                                       
                        Min.                                              
                            Max.                                          
                                (μm)                                   
                                     (mAh)                                
                                          (times)                         
__________________________________________________________________________
Battery A1                                                                
       50   0.35  700   0.2 20  70   970  650                             
Battery A2                                                                
       50   0.35  900   0.35                                              
                            15  70   930  660                             
Battery A3                                                                
       100  0.27  700   0.2 15  70   980  650                             
Battery A4                                                                
       100  0.27  800   0.25                                              
                            12  70   970  630                             
Battery A5                                                                
       100  0.27  900   0.35                                              
                            12  70   940  640                             
Battery A6                                                                
       300  0.23  620   0.2 15  70   990  660                             
Battery A7                                                                
       300  0.23  700   0.2 13  70   920  630                             
Battery A8                                                                
       300  0.23  900   0.2 15  70   960  620                             
Battery A9                                                                
       300  0.23  1,000 0.2 13  70   930  640                             
Battery A10                                                               
       500  0.15  700   0.2 10  70   920  670                             
Battery A11                                                               
       500  0.15  900   0.25                                              
                            9   70   920  630                             
Battery A12                                                               
       1,000                                                              
            0.08  700   0.2 8   70   940  620                             
Battery A13                                                               
       1,000                                                              
            0.08  900   0.3 9   70   930  600                             
__________________________________________________________________________
 
    
     Comparative Example 1 
     The same procedure as in Examples 1 through 13 was followed except that conventional solidification process was used instead of single roll process, to prepare a block-like rare earth element-nickel hydrogen-absorbing alloy having the same composition. 
     The alloy thus obtained was then annealed at 900° C. for 6 hours (in the Examples and Comparative Examples that follow, the same annealing time of 6 hours was used) and, thereafter, mechanically pulverized and sieved, to give a powder having an average particle diameter of 70 μm and a maximum and minimum crystal grain size of 35 μm and 10 μm respectively. 
     Battery B1 was prepared in the same manner as in Examples 1 through 13 except for using, as hydrogen-absorbing material, the powder thus obtained. 
     Comparative Example 2 
     The same procedure as in Examples 1 through 13 was followed except that the roll surface speed was changed to 10 cm/sec and that the annealing temperature was set at 900° C., to prepare a powder having an average particle diameter of 70 μm, an average thickness of 0.57 mm, a maximum crystal grain size in the open-surface side of 40 μm and a minimum crystal grain size in the roll-surface side of 0.2 μm. 
     Battery B2 was prepared in the same manner as in Examples 1 through 13 except for using, as hydrogen-absorbing material, the powder thus obtained. 
     Comparative Example 3 
     The same procedure as in Examples 1 through 13 was followed except that the roll surface speed was changed to 300 cm/sec and that the annealing temperature was set at 500° C., to prepare a powder having an average particle diameter of 70 μm, an average thickness of 0.23 mm, a maximum crystal grain size in the open-surface side of 15 μm and a minimum crystal grain size in the roll-surface side of not more than 0.01 μm. 
     Battery B3 was prepared in the same manner as in Examples 1 through 13 except for using, as hydrogen-absorbing material, the powder thus obtained. 
     Comparative Example 4 
     The same procedure as in Examples 1 through 13 was followed except that the roll surface speed was changed to 300 cm/sec and that the annealing temperature was set at 1200° C., to prepare a powder having an average particle diameter of 70 μm, an average thickness of 0.23 mm, a maximum crystal grain size in the open-surface side of 25 μm and a minimum crystal grain size in the roll-surface side of 0.4 μm. 
     Battery B4 was prepared in the same manner as in Examples 1 through 13 except for using, as hydrogen-absorbing material, the powder thus obtained. 
     Comparative Example 5 
     The same procedure as in Examples 1 through 13 was followed except that conventional solidification process (water-cooling solidification) was used instead of single roll process and that annealing was not conducted, to prepare a powder having an average particle diameter of 70 μm and a maximum and minimum crystal grain sizes of 25 μm and 7 μm, respectively. 
     Battery B5 was prepared in the same manner as in Examples 1 through 13 except for using, as hydrogen-absorbing material, the powder thus obtained. 
     Comparative Example 6 
     The same procedure as in. Examples 1 through 13 was followed except that the roll surface speed was changed to 300 cm/sec and that annealing was not conducted, to prepare a powder having an average particle diameter of 70 μm, an average thickness of 0.23 mm, a maximum crystal grain size in the open-surface side of 10 μm and a minimum crystal grain size in the roll-surface side of not more than 0.1 μm. 
     Battery B6 was prepared in the same manner as in Examples 1 through 13 except for using, as hydrogen-absorbing material, the powder thus obtained. 
     Comparative Example 7 
     The same alloy melt (MmNi 3 .4 Co 0 .8 Al 0 .3 Mn 0 .5) as used in Examples 1 through 13 was solidified by gas atomization process and then annealed at 900° C. for 6 hours, to give a hydrogen-absorbing alloy powder. The average particle diameter and the maximum and minimum crystal grain sizes were 70 μm, 8 μm and 0.1 μm, respectively. 
     Battery B7 was prepared in the same manner as in Examples 1 through 13 except for using as hydrogen-absorbing material, the powder thus obtained. 
     Comparative Example 8 
     The same procedure as in Examples 1 through 13 was followed except that the roll surface speed was changed to 1,500 cm/sec and that the annealing temperature was set at 900° C., to prepare a powder having an average particle diameter of 70 μm, an average thickness of 0.06 mm, a maximum crystal grain size in the open-surface side of 5 μm and a minimum crystal grain size in the roll-surface side of 0.2 μm. 
     Battery B8 was prepared in the same manner as in Examples 1 through 13 except for using, as hydrogen-absorbing material, the powder thus obtained. 
     Comparative Example 9 
     The same procedure as in Examples 1 through 13 was followed except that the roll surface speed was changed to 3,000 cm/sec (roll diameter: 150 mm) and that the annealing temperature was set at 900° C., to prepare a powder having an average particle diameter of 55 μm, an average thickness of 0.04 mm, a maximum crystal grain size in the open-surface side of 2 μm and a minimum crystal grain size in the roll-surface side of 0.2 μm. The average particle diameter could not reach 70 μm, because of the high roll surface speed of 3,000 cm/sec. 
     Battery B9 was prepared in the same manner as in Examples 1 through 13 except for using, as hydrogen-absorbing material, the powder thus obtained. 
     Comparative Example 10 
     The same procedure as in Examples 1 through 13 was followed except that the roll surface speed was changed to 5,000 cm/sec (roll diameter: 150 mm) and that the annealing temperature was set at 900° C., to prepare a powder having an average particle diameter of 48 μm, an average thickness of 0.03 mm, a maximum crystal grain size in the open-surface side of 2 μm and a minimum crystal grain size in the roll-surface side of 0.15 μm. The average particle diameter could not reach 70 μm, because of the high roll surface speed of 5,000 cm/sec. 
     Battery B10 was prepared in the same manner as in Examples 1 through 13 except for using, as hydrogen-absorbing material, the powder thus obtained. 
     Table 2 shows the alloy preparation conditions (roll surface speed and annealing temperature) used in Comparative Examples 1 through 10, the average particle diameter of the alloy powders prepared, the thickness of the thin strips and the maximum and minimum crystal grain sizes. 
     
                                           TABLE 2                                 
__________________________________________________________________________
       Roll Thickness           Average                                   
                                     High-rate                            
       surface                                                            
            of thin                                                       
                  Annealing                                               
                        Size of crystal                                   
                                particle                                  
                                     discharge                            
                                          Cycle                           
       speed                                                              
            strips                                                        
                  tempera-                                                
                        grains (μm)                                    
                                diameter                                  
                                     capacity                             
                                          life                            
       (cm/sec)                                                           
            (mm)  ture (°C.)                                       
                        Min.                                              
                            Max.                                          
                                (μm)                                   
                                     (mAh)                                
                                          (times)                         
__________________________________________________________________________
Battery B1                                                                
       c.s.*                                                              
            --    900   10  35  70   340  660                             
Battery B2                                                                
       10   0.57  900   0.2 40  70   1,000                                
                                          310                             
Battery B3                                                                
       300  0.23  500   0.1 or                                            
                            15  70   1,000                                
                                          280                             
                        below                                             
Battery B4                                                                
       300  0.23  1,200 0.4 25  70   320  360                             
Battery B5                                                                
       c.s.*                                                              
            --    --    7   25  70   990  180                             
Battery B6                                                                
       300  0.23  --    0.1 or                                            
                            10  70   820  200                             
                        below                                             
Battery B7                                                                
       g.a.**                                                             
            --    900   0.1 8   70   750  280                             
Battery B8                                                                
       1,500                                                              
            0.06  900   0.2 5   70   780  530                             
Battery B9                                                                
       3,000                                                              
            0.04  900   0.2 2   55   520  380                             
Battery B10                                                               
       5,000                                                              
            0.03  900   0.15                                              
                            2   48   480  240                             
__________________________________________________________________________
 Only Batteries B9 and B10 used a 150 mmdia. roll.                        
 *conventional solidification process                                     
 **gas atomization process                                                
 
    
     High-Rate Discharge Capacity at an Initial Period of Charge-Discharge Cycles 
     Batteries A1 through A13 and B1 through B10 were activated by charging at 120 mA at a room temperature (about 25° C.) for 16 hours and then discharging at 120 mA to a voltage of 0.95 V. 
     Thereafter, each of the batteries was charged at 1,200 mA for 1.1 hours and discharged at 4.8 A to 0.95 V, to obtain high-rate discharge capacity. The high-rate discharge capacity was determined on 3 pieces for each battery and the average was taken as the high-rate discharge capacity of the battery. The results are shown in Tables 1 and 2 above. 
     Cycle Life 
     Batteries A1 through A13 and B1 through B10 were, after being activated under the same conditions as above, subjected to a charge-discharge cycle test, in which each cycle consists of charging at 1,200 mA at room temperature for 1.1 hours, resting for 1 hour and then discharging at 1,200 mA to 1.0 V, in order to test for their cycle lives. This test was conducted on 10 pieces of each battery and, with the shortest life and the longest life being omitted, the average of the residual 8 results was taken as its cycle life. The cycle life was obtained in terms of the number of cycles (times) until the battery capacity decreased to 960 mAh or below. The results are shown in Tables 1 and 2 above. 
     While Batteries Al through A13 shown in Table 1 have not only excellent high-rate discharge characteristics at an initial period of charge-discharge cycles but long cycle lives, Batteries B1 through B10 are inferior either in high-rate discharge characteristics at an initial period of charge-discharge cycles or in cycle life, or in both. 
     The excellent high-rate discharge characteristics at an initial period of charge-discharge cycles and long cycle lives of Batteries A1 through A13 are attributable to the fact that the hydrogen-absorbing alloy used for negative electrode contains less segregation and contains no too small or too large crystal grains. 
     Battery B1, though a long cycle life, has a small high-rate discharge capacity at an initial period of charge-discharge cycles. This is attributable to the fact that segregation had been diminished because of uniform Mn distribution and that the crystal grains had become too large, so that the hydrogen-absorbing alloy became less crackable. 
     Battery B2, having a large high-rate discharge capacity at an initial period of charge-discharge cycles though, has a short cycle life. This is attributable to the following fact. While the open-surface side, which is readily crackable, is easily activated, the roll-surface side is less crackable and hence less active. As a result, the charge-discharge depth of the open-surface side became large and the hydrogen-absorbing alloy broke into fine pieces. 
     Battery B3, though having a large high-rate discharge capacity at an initial period of charge-discharge cycles, has a short cycle life. This is attributable to the fact that the too low annealing temperature permitted insufficiently activated parts to remain, so that the charge-discharge depth of activated part became large and hence the hydrogen-absorbing alloy broke into fine pieces. 
     Battery B4 has a small high-rate discharge capacity at an initial period of charge-discharge cycles and a short cycle life. The small high-rate discharge capacity at an initial period of charge-discharge cycles is attributable to the fact that: because of the annealing temperature having been close to the melting point of the hydrogen-absorbing alloy, the alloy partially re-melted at grain boundaries, thereby becoming very uncrackable and less active. The short cycle life is attributable to the utilization rate of the hydrogen-absorbing alloy having decreased as the alloy became less active. 
     Battery B5, though having a large high-rate discharge capacity at an initial period of charge-discharge cycles, has a short cycle life. The short cycle life is attributable to the fact that: although possessing large crystal grains, the hydrogen-absorbing alloy contained segregation of Mn because it had not been annealed. 
     Battery B6, though having a large high-rate discharge capacity at an initial period of charge-discharge cycles, has a short cycle life. This is attributable to the following fact. While the open-surface side, which was readily crackable, was easily activated, the roll-surface side is less crackable and hence less active. As a result, the charge-discharge depth of the open-surface side became large and the hydrogen-absorbing alloy broke into fine pieces. 
     Battery B7 has a small high-rate discharge capacity at an initial period of charge-discharge cycles and a short cycle life. The short cycle life is attributable to the following fact. The alloy, having been solidified by gas atomization, had a large dispersion of grain size distribution. Upon annealing, such crystal grains having different sizes, in particular small grains having a size of 30 μm had been poorly activated, so that the utilization rate of the entire alloy became worse. 
     Battery B8 has a small high-rate discharge capacity at an initial period of charge-discharge cycles and a short cycle life. The small high-rate discharge capacity at an initial period or charge-discharge cycles is attributable to the fact that: since the roll-surface side of the alloy had a strong selective orientation in the (hk0) plane, a sufficient degree of activation could not be obtained, even if the minimum size of crystal grains in the roll-surface side had become as large as 0.2 by annealing. The short cycle life is attributable to the following fact. The alloy, although using an alloy powder having an average particle diameter of 70 μm, had a low strip thickness of 0.06 mm (60 μm), thereby containing many flat-shaped particles. As a result, the contact resistance between alloy particles became large, so that the utilization rate of the entire alloy decreased. 
     Batteries B9 and B10, both using alloy powders obtained by pulverizing still thinner strips than those used for Battery B8, have a smaller high-rate discharge capacity at an initial period of charge-discharge cycles and a shorter cycle life than those of Battery B8. 
     Relationship Between the Co Content in Mm.Ni.Co.Al.Mn Alloy and the High-Rate Discharge Capacity at an Initial Period of the Charge-Discharge Cycles and Cycle Life 
     The procedure of Example 8 (roll surface speed: 300 cm/sec; annealing temperature: 900° C.) was followed except for changing the Co content in the alloy to various levels, to prepare 7 types of hydrogen-absorbing alloy powders (average particle diameter: 70 μm) represented by composition formula MmNi 4 .2-y Co y  Al 0 .3 Mn 0 .5 (y=0.3, 0.4, 0.5, 0.6, 0.7, 0.9 or 1.0). 
     Table 3 shows the conditions of preparing the alloy powders (roll surface speed and annealing temperature), the average particle diameter of the alloy powders, the thickness of thin strips and the maximum and minimum sizes of crystal grains. 
     
                                           TABLE 3                                 
__________________________________________________________________________
&#34;y&#34; of                                                                    
      Roll Thickness           Average                                    
                                    High-rate                             
MmNi.sub.4.2-y                                                            
      surface                                                             
           of thin                                                        
                 Annealing                                                
                       Size of crystal                                    
                               particle                                   
                                    discharge                             
                                         Cycle                            
Co.sub.y Al.sub.0.3                                                       
      speed                                                               
           strips                                                         
                 tempera-                                                 
                       grains (μm)                                     
                               diameter                                   
                                    capacity                              
                                         life                             
Mn.sub.0.5                                                                
      (cm/sec)                                                            
           (mm)  ture (°C.)                                        
                       Min.                                               
                           Max.                                           
                               (μm)                                    
                                    (mAh)                                 
                                         (times)                          
__________________________________________________________________________
0.35  300  0.23  900   0.2 17  70   1,000                                 
                                         490                              
0.4   300  0.23  900   0.2 14  70   1,000                                 
                                         500                              
0.5   300  0.22  900   0.2 15  70   1,000                                 
                                         530                              
0.6   300  0.23  900   0.2 13  70   980  550                              
0.7   300  0.24  900   0.2 18  70   970  590                              
0.8   300  0.23  900   0.2 15  70   960  620                              
0.9   300  0.23  900   0.2 16  70   930  750                              
0.95  300  0.24  900   0.2 19  70   810  800                              
__________________________________________________________________________
 
    
     Then, hydrogen-absorbing alloy electrodes and nickel hydride alkaline batteries were prepared in the same manner as in Example 8 except for using these hydrogen-absorbing alloy powders as hydrogen-absorbing material. 
     The nickel hydride alkaline batteries thus prepared were subjected to the high-rate discharge test and charge-discharge cycle test under the same conditions as above, in order to study their high-rate discharge characteristics at an initial period or charge-discharge cycles and cycle life. The results are shown in Table 3, which also includes the results obtained with Battery A8. 
     It is understood from Table 3 that, with Mm.Ni.Co.Al.Mn alloys, in order to obtain a nickel hydride alkaline batteries being excellent in both of high-rate discharge characteristics at an initial period or charge-discharge cycles and charge-discharge cycle characteristics, Cobalt is desirably contained in an amount or 0.4 to 0.9 mole per mole of Mm. 
     Relationship Between the Ni Content of Mm.Ni.Co.Al.Mn Alloy and the High-Rate Discharge Capacity at an Initial Period or the Charge-Discharge Cycles and Cycle Life 
     The procedure of Example 8 (roll surface speed: 300 cm/sec; annealing temperature: 900° C.) was followed except for changing the Ni content in the alloy to various levels, to prepare 5 types of hydrogen-absorbing alloy powders (average particle diameter: 70 μm) represented by composition formula MmNi z  Co 0 .8 Al 0 .3 Mn 0 .5 (z=2.6, 2.8, 3.2, 3.6 or 3.8) 
     Table 4 shows the conditions of preparing the alloy powders (roll surface speed and annealing temperature), the average particle diameter of the alloy powders, the thickness of thin strips and the maximum and minimum sizes of crystal grains. 
     
                                           TABLE 4                                 
__________________________________________________________________________
       Roll Thickness           Average                                   
                                     High-rate                            
&#34;z&#34; of surface                                                            
            of thin                                                       
                  Annealing                                               
                        Size of crystal                                   
                                particle                                  
                                     discharge                            
                                          Cycle                           
MmNi.sub.z Co.sub.0.8                                                     
       speed                                                              
            strips                                                        
                  tempera-                                                
                        grains (μm)                                    
                                diameter                                  
                                     capacity                             
                                          life                            
Al.sub.0.3 Mn.sub.0.5                                                     
       (cm/sec)                                                           
            (mm)  ture (°C.)                                       
                        Min.                                              
                            Max.                                          
                                (μm)                                   
                                     (mAh)                                
                                          (times)                         
__________________________________________________________________________
2.6    300  0.24  900   0.25                                              
                            16  70   1,000                                
                                          520                             
2.8    300  0.23  900   0.25                                              
                            15  70   1,000                                
                                          580                             
3.2    300  0.25  900   0.25                                              
                            17  70   980  630                             
3.4    300  0.23  900   0.2 15  70   960  620                             
3.6    300  0.26  900   0.2 18  70   980  570                             
3.8    300  0.25  900   0.2 18  70   1,000                                
                                          510                             
__________________________________________________________________________
 
    
     Then, hydrogen-absorbing alloy electrodes and nickel hydride alkaline batteries were prepared in the same manner as in Example 8 except for using these hydrogen-absorbing alloy powders as hydrogen-absorbing material. 
     The nickel hydride alkaline batteries thus prepared were subjected to the high-rate discharge test and charge-discharge cycle test under the same conditions as above, to study their high-rate discharge characteristics at an initial period of charge-discharge cycles and cycle life. The results are shown in Table 4, which also includes the results obtained with Battery A8. 
     It is understood from Table 4 that, with Mn.Ni.Co.Al.Mn alloys, in order to obtain a nickel hydride alkaline batteries being excellent in both of high-rate discharge characteristics at an initial period of charge-discharge cycles and charge-discharge cycle characteristics, nickel is desirably contained in an amount of 2.8 to 3.6 moles per mole of Mm. 
     Relationship Between the Average Particle Diameter of Alloy Powder and the Internal Pressure Characteristics of Battery 
     Metals (all having a purity of at least 99.9%) constituting the alloy to prepare were each weighed and mixed together and the mixture was melted under vacuum in a high-frequency melting furnace. The melt was cooled by single roll process (roll diameter: 350 mm) at a roll surface speed of 300 cm/sec, to give a thin strip-like rare earth element-nickel hydrogen-absorbing alloy represented by composition formula MmNi 3 .1 Co 0 .8 Al 0 .3 Mn 0 .5. 
     The rare earth element-nickel hydrogen-absorbing alloy thus obtained was then annealed at 900° C. for 6 hours and then mechanically pulverized under an atmosphere of an inert gas (argon) and sieved, to give hydrogen-absorbing alloy powders having various average particle diameters and having a maximum size of crystal grains in the open-surface side of 13 μm and a minimum size of those in the roll-surface side of 0.25 μm and an average thickness of thin strips of 0.23 mm. 
     AA-size nickel hydride alkaline batteries of positive electrode-dominating type (battery capacity: 1,200±10 mAh) A14 through A29 were prepared in the same manner as in Examples 1 through 13 except for using these hydrogen-absorbing powders as hydrogen-absorbing material. Among these batteries, A20 through A25 and A28 and A29 were prepared with the rare earth element-nickel hydrogen absorbing alloy powders having been further treated as follows. That is, the powders were each immersed in an aqueous hydrochloric acid solution having a pH of 0.5 and, after the mixture had been stirred to a pH of 7, taken out, washed with pure water and dried. 
     Separately, a block-like rare earth element-nickel hydrogen-absorbing alloy having the same composition was prepared by conventional solidification process, instead of single roll process. The alloy thus obtained was then annealed at 900° C. for 6 hours, mechanically pulverized and sieved, to give powders having various average particle diameters, Batteries B11 through B15 were prepared using these powders in the same manner as above. 
     Internal Pressure Characteristics of Battery 
     Batteries A14 through A29 and B11 through B15 were activated by charging at 120 mA at room temperature (about 25° C.) for 16 hours and then discharging at 60° C. and 120 mA to 0.95 V. 
     Each of the batteries was fitted with a pressure gauge on the can bottom and then charged at room temperature and at 1200 mA (1C), The charging time (minutes) required for the battery internal pressure to elevate to 10 kg/cm 2  was measured. For each battery, 4 pieces were tested for the charging time and the average of the 4 measurements was taken as an index of battery internal pressure characteristics. Table 5 shows the average particle diameter (μm) of the alloy powder used for each battery and the average charging time (minutes). The average particle diameters of surface-treated powders with acid are those after the treatment. 
     
                       TABLE 5                                                     
______________________________________                                    
        Average Battery internal pressure                                 
        particle                                                          
                characteristics (minutes)                                 
        diameter                                                          
                Not acid-                                                 
        (μm) treated     Acid-treated                                  
______________________________________                                    
Single roll                                                               
          70        A14     110   A20    110                              
process   60        A15     115   A21    120                              
          55        A16     120   A22    125                              
          45        A17     120   A23    130                              
          35        A18     115   A24    130                              
          25        A19     110   A25    115                              
          80        A26     80    A28    70                               
          20        A27     70    A29    60                               
Conventional                                                              
          90        B11     60                                            
solidification                                                            
          85        B12     85                                            
          80        B13     85                                            
          70        B14     65                                            
          65        B15     60                                            
______________________________________                                    
 
    
     As seen from Table 5, Batteries A14 through A25 have longer charging time until the internal pressure reaches 10 kg/cm 2 , i.e. better battery internal pressure characteristics compared with Batteries A26 through A29 and Batteries B11 through B15. To summarize this result and the previous results, it is understood that use, as hydrogen-absorbing material, of an alloy powder having an average particle diameter of 25 to 70 μm obtained by pulverizing a thin strip-like rare earth element-nickel hydrogen-absorbing alloy prepared by single roll process and having an average thickness of 0.08 to 0.35 mm and a minimum crystal grain size of the roll-surface side of at least 0.2 μm and a maximum crystal grain size of the open-surface side of not more than 20 μm can yield a hydrogen-absorbing alloy electrode which will provide metal hydride alkaline batteries being excellent not only in high-rate discharge characteristics and charge-discharge cycle characteristics but in battery internal pressure characteristics. 
     The poor battery internal pressure characteristics of Battery A29, which used an alloy powder having an average particle diameter of 20 μm, is considered to be due to the following fact. The surface treatment had removed oxides, thereby causing the surface of alloy particles to become smooth, which worsened the contact between the alloy particles. As a result, only part of the alloy powder was able to participate in the reaction, so that the reaction heat elevated, which caused hydrogen to dissociate readily. 
     The poor battery internal pressure characteristics of Battery A28, which used an alloy powder having an average particle diameter of 80 μm, is considered to be due to the following fact. The surface treatment had removed oxides to smooth out the projections and recesses on the surface of the alloy particles to a certain degree, thereby further decreasing the specific surface area of the alloy powder. As a result, the reaction was effected only on a small, limited surface, so that the reaction heat elevated, which caused hydrogen to dissociate readily. 
     Among Batteries A14 through A25, those A21 through A25 are found to be particularly improved in the battery internal pressure characteristics. This fact shows that it is more desirable, in order to improve battery internal pressure characteristics, to use as hydrogen-absorbing material an alloy powder having an average particle diameter of 25 to 60 μm and obtained by pulverizing a thin strip-like alloy and further surface-treating the resulting powder by immersion in an acidified liquid. Batteries B11 through B15, using conventional solidified alloys, have poor battery internal pressure characteristics. This is considered to be due to the fact that the conventional solidified alloys had more marked segregation of Mn and the like as compared with quench-solidified alloys, thereby becoming readily crackable and oxidizable, so that consumption of oxygen could not proceed rapidly. 
     Although the above descriptions of the Examples have been made by reference to Mm.Ni.Co.Al.Mn alloys, the present invention is widely applicable to hydrogen-absorbing alloy electrodes using any rare earth element-nickel hydrogen-absorbing alloy as hydrogen-absorbing material, since rare earth element-nickel hydrogen-absorbing alloys have the tendency of, when solidified, having a similar distribution of constituting rare earth elements. 
     Relationship Between the x in MmRx (The Stoichiometric Ratio Between R and Mm) and Anti-Overcharge Characteristics 
     Metals (all having a purity of at least 99.9%) constituting the alloys to prepare were each weighed and mixed together and the obtained mixtures were melted under vacuum in a high-frequency melting furnace. The melts were cooled by single roll process (roll diameter: 350 mm) at a roll surface speed of 300 cm/sec, to give 8 types of thin strip-like rare earth element-nickel hydrogen-absorbing alloys represented by the following composition formulas: 
     MmNi 3 .6 Co 0 .8 Al 0 .3 Mn 0 .5 (x=5.2 in MmRx), MmNi 3 .3 Co 0 .8 Al 0 .3 Mn 0 .5 (x=4.9 in MmRx), MmNi 3 .2 Co 0 .8 Al 0 .3 Mn 0 .5 (x=4.8 in MmRx), MmNi 3 .1 Co 0 .8 Al 0 .3 Mn 0 .5 (x=4.7 in MmRx), MmNi 3 .0 Co 0 .8 Al 0 .3 Mn 0 .5 (x=4.6 in MmRx), MmNi 2 .9 Co 0 .8 Al 0 .3 Mn 0 .5 (x=4.5 in MmRx), MmNi 2 .8 Co 0 .8 Al 0 .3 Mn 0 .5 (x=4.4 in MmRx) and MmNi 2 .6 Co 0 .8 Al 0 .3 Mn 0 .5 (x=4.2 in MmRx). 
     The thin strip-like Mm.Ni.Co.Al.Mn alloys thus obtained were annealed at 900° C. in argon for 6 hours and then mechanically pulverized under an atmosphere of an inert gas (argon), to give alloy powders having an average particle diameter of 70 μm. Table 6 shows the stoichiometric ratio x (x in MmRx), the molar ratio of Ni to Mm, the thickness of thin strips and the maximum and minimum crystal grain sizes, of the alloys thus prepared. 
     
                       TABLE 6                                                     
______________________________________                                    
                    Thick-           Average of                           
Stoichio-   Molar   ness of  Size of charge-                              
metric      ratio   thin     crystal discharge                            
ratio of    of Ni   strips   grain (μm)                                
                                     cycles                               
Battery                                                                   
      alloy, x  to Mm   (mm)   min. max. (times)                          
______________________________________                                    
A30   5.2       3.6     0.26   0.3  18   11                               
B8    5.0       3.4     0.23   0.2  15   12                               
A31   4.9       3.3     0.25   0.3  18   20                               
A32   4.8       3.2     0.26   0.25 18   22                               
A33   4.7       3.1     0.24   0.2  15   22                               
A34   4.6       3.0     0.24   0.2  16   21                               
A35   4.5       2.9     0.25   0.2  15   20                               
A36   4.4       2.8     0.25   0.2  14   10                               
A37   4.2       2.6     0.24   0.2  14   7                                
______________________________________                                    
 
    
     AA-size nickel hydride alkaline batteries of positive electrode-dominating type (battery capacity: 1,200±10 mAh) A30 through A37 were prepared in the same manner as in Examples 1 through 13 except for using these hydrogen-absorbing powders as hydrogen-absorbing material. 
     Anti-Overcharge Characteristics 
     Batteries A30 through A37 were activated by charging at 1 20 mA at room temperature (about 25° C.) for 16 hours and then discharging at 60° C. and 120 mA to 0.95 V. 
     The batteries were then each subjected to a charge-discharge cycle test which repeals cycles, one cycle consisting of charging at 40° C. and at 240 mA (0.2C) for 10 days, resting for 1 hour to permit the circumferential temperature to return to room temperature and discharging at 1,200 mA to 1.0 V. In the test, the number of charge-discharge cycles required for the discharge capacity to decrease to 80% of that at the first cycle, or below, was recorded. For each battery, this required number of charge-discharge cycles was obtained on 10 pieces and, after exclusion of the largest and smallest numbers of cycles, the average of the remaining 8 numbers was taken as an index of the anti-overcharge characteristics. The results are also shown in the above Table 6, which includes the results obtained on Battery A8. 
     As seen from Table 6, Batteries A31 through A35, using alloys having a stoichiometric ratio x of 4.5 to 4.9, have far better anti-overcharge characteristics than those of other batteries using alloys having an x deviating from this range. The poor anti-overcharge characteristics of Batteries A30 and A8 are attributable to the fact that the alloys cracked and hence the oxygen consumption activity decreased, so that the electrolyte leaked out. The poor anti-overcharge characteristics of Batteries A36 and A37 are attributable to the fact that there was formed, different from mother phase, a second phase (Mm 2  R 7  phase) in which oxygen consumption reaction did not proceed smoothly on overcharge, so that oxidation of the alloys was promoted. To summarize this result and the previous results, it is understood that use, as hydrogen-absorbing material, of an alloy powder obtained by pulverizing a thin strip-like Mm.Ni.Co.Al.Mn alloy having an average thickness of 0.08 to 0.35 mm prepared by single roll process and represented by general formula MmRx (Mm: misch metal; R: Ni, Co, Al and Mn; x is 4.5 to 4.9) and having a minimum crystal grain size of the roll-surface side of at least 0.2 μm and a maximum crystal grain size of the open-surface side of not more than 18 μm can yield a hydrogen-absorbing alloy electrode which will provide metal hydride alkaline batteries being excellent not only in high-rate discharge characteristics at an initial period of charge-discharge cycles and charge-discharge cycle characteristics but in anti-overcharge characteristics. 
     Relationship Between the Molar Ratio Between Co and Mm and Anti-Overcharge Characteristics 
     Metals (all having a purity of at least 99.9%) constituting the alloys to prepare were each weighed and mixed together and the obtained mixtures were melted under vacuum in a high-frequency melting furnace. The melts were cooled by single roll process (roll diameter: 350 mm) at a roll surface speed of 300 cm/sec, to give 8 types of thin strip-like Mm.Ni.Co.Al.Mn alloys represented by the following composition formulas: MmNi 2 .8 Co 1 .1 Al 0 .3 Mn 0 .5 (molar ratio of Co to Mm: 1.1), MmNi 2 .95 Co 0 .95 Al 0 .3 Mn 0 .5 (molar ratio of Co to Mm: 0.95), MmNi 3 .0 Co 0 .9 Al 0 .3 Mn 0 .5 (molar ratio of Co to Mm: 0.9), MmNi3.2Co 0 .7 Al 0 .3 Mn0.5 (molar ratio of Co to Mm: 0.7), MmNi 3 .3 Co 0 .6 Al 0 .3 Mn 0 .5 (molar ratio of Co to Mm: 0.6), MmNi 3 .4 Co 0 .5 Al 0 .3 Mn 0 .5 (molar ratio of Co to Mm: 0.5), MmNi 3 .5 Co 0 .4 Al 0 .3 Mn 0 .5 (molar ratio of Co to Mm: 0.4) and MmNi 3 .55 Co 0 .35 Al 0 .3 Mn 0 .5 (molar ratio of Co to Mm: 0.35). 
     The thin strip-like Mm.Ni.Co.Al.Mn alloys thus obtained were annealed at 900° C. in argon for 6 hours and then mechanically pulverized under an atmosphere of all inert gas (argon), to give alloy powders having an average particle diameter of 70 μm. Table 7 shows the molar ratio of Co to Mm, the thickness of thin strips and the maximum and minimum crystal grain sizes, of the alloys thus prepared. 
     
                       TABLE 7                                                     
______________________________________                                    
           Thick-            Average of                                   
Molar      ness of Size of   charge-  High-rate                           
ratio      thin    crystal   discharge                                    
                                      discharge                           
of Co      strips  grain (μm)                                          
                             cycles   capacity                            
Battery                                                                   
      to Mm    (mm)    min. max. (times)  (mAh)                           
______________________________________                                    
A38   1.1      0.25    0.2  17   24       770                             
A39   0.95     0.24    0.2  16   24       810                             
A40   0.9      0.25    0.2  16   23       930                             
A33   0.8      0.24    0.2  15   22       960                             
A41   0.7      0.24    0.2  15   20       970                             
A42   0.6      0.23    0.2  14   20       980                             
A43   0.5      0.24    0.2  16   19       1000                            
A44   0.4      0.23    0.2  15   11       1000                            
A45   0.35     0.24    0.2  14   9        1000                            
______________________________________                                    
 
    
     AA-size nickel hydride alkaline batteries of positive electrode-dominating type (battery capacity: 1,200|10 mAh) A38 through A45 were prepared in the same manner as in Examples 1 through 13 except for using these hydrogen-absorbing powders as hydrogen-absorbing material. 
     The batteries were then each subjected to the high-rate discharge test and an anti-overcharge characteristics test under the same conditions as above, to be tested for the high-rate discharge characteristics at an initial period of charge-discharge cycles and the anti-overcharge characteristics. The results are also shown in the above Table 7, which includes the results obtained on Battery A33 as copied from Table 6. 
     It is understood from Table 7 that: with use of Mm.Ni.Co.Al.Mn alloy, it is desirable, for obtaining a hydrogen-absorbing alloy electrode capable of providing nickel hydride alkaline batteries which are excellent not only in high-rate discharge characteristics at an initial period of charge-discharge cycles but in anti-overcharge characteristics, to use an Mm.Ni.Co.Al.Mn alloy containing Co in an amount of 0.5 to 0.9 mole per mole of Mm. Batteries A38 and A39, using alloys having a molar ratio of Co to Mm exceeding 0.9, have too small a specific surface area of alloy powders, thereby having a small high-rate discharge capacity. Batteries A44 and A45, using alloys having a molar ratio of Co to Mm of less than 0.5, are readily crackable into fine pieces and hence have poor anti-overcharge characteristics, although they have a large high-rate discharge capacity thanks to the large specific area of the powders. 
     Relationship Between the Annealing Temperature and the Anti-Overcharge Characteristics 
     Metals (all having a purity of at least 99.9%) constituting the alloy to prepare were each weighed and mixed together and the obtained mixture were melted under vacuum in a high-frequency melting furnace. The melt was cooled by single roll process (roll diameter: 350 mm) at a roll surface speed of 300 cm/sec, to give a thin strip-like Mm.Ni.Co.Al.Mn alloy represented by the composition formula MmNi 3 .1 Co 0 .8 Al 0 .3 Mn 0 .5 (molar ratio of Co to Mm: 0.8). 
     The thin strip-like Mm.Ni.Co.Al.Mn alloy thus obtained was annealed at various temperatures (700°, 750°, 800°, 950° or 1,000° C.) in argon for 6 hours and then mechanically pulverized under an atmosphere of an inert gas (argon), to give alloy powders having an average particle diameter of 70 μm. Table 8 shows the annealing temperature, the thickness of thin strips and the maximum and minimum crystal grain sizes, of the alloys thus prepared. 
     
                       TABLE 8                                                     
______________________________________                                    
            Thickness             Average of                              
Annealing   of thin   Size of crystal                                     
                                  charge-dis-                             
tempera-    strips    grain (μm)                                       
                                  charge cycles                           
Battery                                                                   
      ture (°C.)                                                   
                (mm)      min.  max.  (times)                             
______________________________________                                    
A46   700       0.24      0.2   15    14                                  
A47   750       0.24      0.2   16    20                                  
A48   800       0.24      0.2   15    22                                  
A33   900       0.24      0.2   15    22                                  
A49   950       0.24      0.25  15    21                                  
A50   1,000     0.24      0.3   15    15                                  
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     AA-size nickel hydride alkaline batteries of positive electrode-dominating type (battery capacity: 1,200±10 mAh) A46 through A50 were prepared in the same manner as in Examples 1 through 13 except for using these hydrogen-absorbing powders as hydrogen-absorbing material. 
     The batteries were then each subjected to the anti-overcharge characteristics test under the same conditions as above, in order to study the anti-overcharge characteristics. The results are also shown in the above Table 8, which includes the results obtained on Battery A33 as copied from Table 6. 
     It is understood from Table 8 that: in order to obtain a hydrogen-absorbing alloy electrode capable of providing nickel hydride alkaline batteries being excellent not only in high-rate discharge characteristics at an initial period of charge-discharge cycles and charge-discharge cycle characteristics but in anti-overcharge characteristics, it is necessary to use an Mm.Ni.Co.Al.Mn alloy annealed at a temperature of 750° to 950° C. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.