Patent Publication Number: US-2019190083-A1

Title: Nickel hydrogen secondary battery

Description:
BACKGROUND 
     Technical Field 
     The present invention relates to a nickel hydrogen secondary battery. 
     Description of the Related Art 
     Secondary batteries are commonly used as power sources to drive electronic devices. In these and similar applications, the residual capacity of the secondary batteries often must be detected in order to determine whether the secondary batteries are capable of driving the electronic devices, and/or to determine a suitable timing for switching between discharge and charge of the secondary batteries. 
     The residual capacity of a battery can be indicated, for example, by a State of Charge (referred to hereinafter as “SOC”), which is a proportion of electric charge as a percentage of battery capacity. To this end, methods of detecting SOC and indirectly the residual capacity of the battery include a method of exploiting a discharge characteristic curve of the battery (see, for example, Japanese Patent Laid-Open No. 11-111348). 
     According to one such method, a discharge characteristic curve is determined from a relation between the voltage and the discharge time of a battery, or a relation between the voltage and the discharge capacity of the battery when the battery is discharged at a certain current value. By using this discharge characteristic curve, the relation between the voltage and SOC of the battery can be grasped. That is, with the discharge characteristic curve determined previously by some experiment, by detecting the voltage at a certain current value and exploiting the discharge characteristic curve at the current value, SOC and thereby the residual capacity of the battery can be detected. 
     In one specific example, voltage corresponding to an SOC of 25% (referred to hereinafter as a reference value “V 1 ”) and a voltage corresponding to an SOC of 75% (referred to hereinafter as a reference value “V 2 ”) are determined from a discharge characteristic curve. A measured value of the voltage of the battery is then compared with these reference values V 1  and V 2 . When the measured value of the voltage of the battery is nearly the reference value V 2 , it can be determined that sufficiently capacity still remains. When the measured value of the voltage of the battery is nearly the reference value e V 1 , it can be determined that little capacity is left. For example, where SOC=25% corresponds to the allowable lower limit of the residual capacity of the battery, it is possible to detect, by measuring the voltage of the battery, whether the residual capacity in the discharge last period of the battery has reached the allowable lower limit, that is, SOC=25%. Since discharge exceeding this becomes overdischarge, control of terminating discharge and switching to charge can be carried out. 
     Alkali secondary batteries are known as one type of secondary batteries. Nickel hydrogen secondary batteries have particularly high capacity among alkali secondary batteries, and have consequently come to be used in many applications such as various types of electronic devices and hybrid electric cars. There exists a need for a method capable of accurately determining residual capacity or SOC of a nickel hydrogen secondary battery. In the case of the above method, larger differences between the reference value V 1  and the reference value V 2  permit easier detection of the residual capacity of a battery. When being used for devices, however, conventional nickel hydrogen secondary batteries, exhibit a stable voltage until their residual capacity is exhausted. That is, the discharge characteristic curve exhibits nearly a constant voltage from an early period to a last period. Hence, the difference between the reference value V 1  and the reference value V 2  is relatively small. For this reason, nickel hydrogen secondary batteries pose a challenge with regard to detection of residual battery capacity, in comparison to other secondary batteries. 
     SUMMARY OF THE INVENTION 
     The present disclosure is directed to a nickel hydrogen secondary battery comprising a container and an electrode group accommodated together with an alkali electrolyte solution in the container. The electrode group has a positive electrode and a negative electrode stacked through a separator. The positive electrode comprises two or more positive electrode active substances having different compositions. The positive electrode active substances each comprise a nickel hydroxide as a main component and mutually differ in the amount of magnesium solid-dissolved. 
     The present summary is provided only by way of example, and not limitation. Other aspects of the present disclosure will be appreciated in view of the entirety of the present disclosure, including the entire text, claims, and accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrated by partially rupturing a nickel hydrogen secondary battery according to one embodiment of the present invention; and 
         FIG. 2  is a graph showing charge and discharge characteristics curves of each battery. 
     
    
    
     While the above-identified figures set forth one or more embodiments of the present disclosure, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents the invention by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the invention. The figures may not be drawn to scale, and applications and embodiments of the present invention may include features and components not specifically shown in the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a nickel hydrogen secondary battery  2  according to the present invention (hereinafter, referred to as battery) will be described by reference to the drawing. 
     The battery  2  to which the present invention is applied is not especially limited, but the present invention is applied to, for example, an AA-size cylindrical battery  2  illustrated in  FIG. 1 . 
     As illustrated in  FIG. 1 , the battery  2  is equipped with an outer can  10  having an opened upper end and a bottomed cylindrical shape. The outer can  10  has conductivity and a bottom wall  35  thereof functions as a negative electrode terminal. A sealing body  11  is fixed to the opening of the outer can  10 . The sealing body  11  contains a lid plate  14  and a positive electrode terminal  20 , and seals the outer can  10  and provides the positive electrode terminal  20 . The lid plate  14  is a disc-shape member having conductivity. In the opening of the outer can  10 , the lid plate  14  and a ring-shape insulating packing  12  surrounding the lid plate  14  are disposed, and the insulating packing  12  is fixed to an opening edge  37  of the outer can  10  by caulking the opening edge  37  of the outer can  10 . That is, the lid plate  14  and the insulating packing  12  hermetically block the opening of the outer can  10  in cooperation with each other. 
     Here, the lid plate  14  has a center through-hole  16 , and on the outer surface of the lid plate  14 , a rubber-made valve disc  18  plugging up the center through-hole  16  is disposed. Further on the outer surface of the lid plate  14 , the metal-made positive electrode terminal  20  which has a cylindrical shape with a flange so as to cover the valve disc  18  is electrically connected. The positive electrode terminal  20  presses the valve disc  18  toward the lid plate  14 . Here, the positive electrode terminal  20  has a vent hole opened therein, which is not illustrated. 
     Usually, the center through-hole  16  is hermetically closed with the valve disc  18 . On the other hand, when a gas is evolved in the outer can  10  and the internal pressure thereof rises, the valve disc  18  is compressed by the internal pressure to open the center through-hole  16 , and consequently, the gas is released from in the outer can  10  to the outside through the center through-hole  16  and the vent hole (not illustrated) of the positive electrode terminal  20 . That is, the center through-hole  16 , the valve disc  18  and the positive electrode terminal  20  form a safety valve for the battery. 
     In the outer can  10 , an electrode group  22  is accommodated. The electrode group  22  comprises a strip-form positive electrode  24 , a strip-form negative electrode  26  and a strip-form separator  28 , and the electrode group  22  is wound in a spiral form in the state that the separator is interposed between the positive electrode  24  and the negative electrode  26 . That is, the positive electrode  24  and the negative electrode  26  are mutually stacked through the separator  28 . The outermost periphery of the electrode group  22  is formed of a part (outermost peripheral part) of the negative electrode  26 , and contacts with the inner peripheral wall of the outer can  10 . That is, the negative electrode  26  and the outer can  10  are mutually electrically connected. 
     In the outer can  10 , a positive electrode lead  30  is disposed between one end of the electrode group  22  and the lid plate  14 . Specifically, in some embodiments, one end of the positive electrode lead  30  is connected to the positive electrode  24 , and the other end thereof is connected to the lid plate  14 . Therefore, the positive electrode terminal  20  and the positive electrode  24  are mutually electrically connected through the positive electrode lead  30  and the lid plate  14 . Here, between the lid plate  14  and the electrode group  22 , a circular upper insulating member  32  is disposed, and the positive electrode lead  30  extends through a slit  39  installed in the upper insulating member  32 . Further, a circular lower insulating member  34  is also disposed between the electrode group  22  and the bottom part of the outer can  10 . 
     Further, a predetermined amount of an alkali electrolyte solution (not illustrated) is injected in the outer can  10 . The alkali electrolyte solution is impregnated in the electrode group  22  and advances charge and discharge reactions between the positive electrode  24  and the negative electrode  26 . As the alkali electrolyte solution, an alkali aqueous solution containing KOH, NaOH and LiOH as solutes can be used. 
     As a material of the separator  28 , there can be used, for example, a polyamide fiber-made nonwoven fabric imparted with hydrophilic functional groups, and a polyolefin, such as polyethylene or polypropylene, fiber-made nonwoven fabric imparted with hydrophilic functional groups. Specifically, a nonwoven fabric containing mainly polyolefin fibers having been subjected to a sulfonation treatment and being imparted with sulfone groups is preferably used. Here, the sulfone groups are imparted by using an acid containing sulfate groups such as sulfuric acid or fuming sulfuric acid and treating the nonwoven fabric. A battery using a separator containing such fibers having sulfone groups exhibits an excellent self-discharge characteristic. 
     The positive electrode  24  comprises a conductive positive electrode substrate having a porous structure, and a positive electrode mixture held in pores of the positive electrode substrate. 
     As such a positive electrode substrate, for example, a net-like, spongy or fibrous nickel-plated metallic body, or a nickel foam can be used. 
     The positive electrode mixture comprises a positive electrode active substance, a conductive material, a positive electrode additive and a binder. The binder functions to mutually bind the positive electrode active substance, the conductive material and the positive electrode additive and to bind the positive electrode mixture to the positive electrode substrate. Here, as the binder, for example, a carboxymethylcellulose, a methylcellulose, a PTFE (polytetrafluoroethylene) dispersion, or an HPC (hydroxypropylcellulose) dispersion can be used. 
     The positive electrode active substance comprises a first positive electrode active substance and a second positive electrode active substance. These first positive electrode active substance and second positive electrode active substance contain nickel hydroxide as a main component. Then, the first positive electrode active substance and the second positive electrode active substance differ in the amount of magnesium solid-dissolved. 
     It is preferable because the conductivity is improved that the nickel hydroxide constituting the positive electrode active substance be an order-heightened nickel hydroxide. Here, “containing nickel hydroxide as a main component” refers to that the content of nickel hydroxide in the first positive electrode active substance exceeds 50% by mass, and similarly refers to that the content of nickel hydroxide in the second positive electrode active substance exceeds 50% by mass. 
     The first positive electrode active substance has at least magnesium solid-dissolved. On the other hand, the second positive electrode active substance does not necessarily need to have magnesium solid-dissolved, but in the case of having magnesium solid-dissolved therein, it is preferable that the amount of magnesium solid-dissolved therein be smaller than the amount of magnesium solid-dissolved in the first positive electrode active substance. Then in the present invention, forms in which the amounts of magnesium solid-dissolved are different also include a form in which the first positive electrode active substance has magnesium solid-dissolved and the second positive electrode active substance has magnesium not solid-dissolved at all. 
     By solid-dissolving magnesium in nickel hydroxide, the equilibrium potential of the nickel hydroxide rises. Since the amount of magnesium solid-dissolved and the equilibrium potential correlate with each other, by mixing active substances differing in the amount of magnesium solid-dissolved, variations in the discharge curve occur. In particular, since active substances having higher equilibrium potentials discharge earlier, by making the amounts of magnesium different, variations in the discharge curve can be made. 
     In the first positive electrode active substance and the second positive electrode active substance, it is preferable to solid-dissolve at least one of zinc and cobalt, in addition to magnesium, as elements to be solid-dissolved in nickel hydroxide. 
     Here, zinc suppresses swelling of the positive electrode accompanying charge and discharge of the positive electrode, and prevents exhaustion of the electrolyte solution. Further cobalt has the effect of securing the conductivity of the nickel hydroxide. 
     As the conductive material, for example, one or two or more selected from cobalt compounds and cobalt (Co) can be used. The cobalt compounds include cobalt oxide (CoO) and cobalt hydroxide (Co(OH) 2 ). The conductive material, as required, is added to the positive electrode mixture, and as forms of the conductive material to be added, in addition to a powdery form, the conductive material may be contained in a form of a coating covering the surface of the positive electrode active substance in the positive electrode mixture. Preferably, a form is adopted in which a layer of cobalt hydroxide is formed on the surface of particles of the positive electrode active substance and the layer of cobalt hydroxide is oxidized to thereby provide a conductive layer of cobalt oxyhydroxide having a high conductivity. Further it is more preferable to provide a conductive layer of cobalt oxyhydroxide containing sodium. When sodium is contained in the cobalt oxyhydroxide, the conductivity is largely improved. Hence, the incorporation of sodium attains the effect of largely improving the utilization factor of the nickel hydroxide. 
     The positive electrode additive is added for improving characteristics of the positive electrode, and, for example, yttrium oxide or zinc oxide can be used. 
     The first positive electrode active substance can be produced, for example, as follows. First, nickel sulfate and magnesium sulfate weighed so as to provide a predetermined composition are placed in a predetermined amount of water to thereby prepare a first mixed aqueous solution of nickel sulfate and magnesium sulfate. Then, while the first mixed aqueous solution is being stirred, a sodium hydroxide aqueous solution is slowly added to the first mixed aqueous solution and allowed to react to thereby cause particles containing nickel hydroxide as a main component and having magnesium solid-dissolved to deposit. The first positive electrode active substance can thus be obtained. 
     When the amount of magnesium solid-dissolved in the first positive electrode active substance is 2.0% by mass or larger, the discharge characteristic curve of an obtained battery becomes a shape in which the gradient varies in the discharge last period, and the effect of detecting SOC and eventually the residual capacity with good accuracy from the voltage of the battery is attained. With the amount of magnesium solid-dissolved being smaller than 2.0% by mass, however, it becomes difficult to satisfactorily attain such an effect. Hence, the amount of magnesium solid-dissolved is adjusted to preferably 2.0% by mass or larger, and more preferably 2.48% by mass or larger. 
     On the other hand, with the amount of magnesium solid-dissolved being larger than 5.0% by mass, accompanying by that, the purity of nickel in the nickel hydroxide decreases and the capacity decreases. Further the particle diameter of the nickel hydroxide is liable to become small and the cycle operating life characteristic lowers. Additionally, since the equilibrium potential rises, and consequently, the difference from the oxygen evolution potential becomes small, the charge acceptance at high temperatures lowers. Hence, it is preferable to adjust the amount of magnesium solid-dissolved to 5.0% by mass or smaller. It is more preferable to adjust the amount of magnesium solid-dissolved to 4.05% by mass or smaller. 
     Then in the case of solid-dissolving zinc and cobalt in the first positive electrode active substance, zinc sulfate and cobalt sulfate weighed so as to provide a predetermined composition are further added to thereby prepare a first mixed aqueous solution; and a sodium hydroxide aqueous solution is slowly added to the first mixed aqueous solution and allowed to react to thereby cause particles of a first positive electrode active substance containing nickel hydroxide as a main component and having magnesium, zinc and cobalt solid-dissolved to deposit. 
     The second positive electrode active substance can be produced, for example, as follows. First, an aqueous solution of nickel sulfate is prepared. A sodium hydroxide aqueous solution was slowly added to the nickel sulfate aqueous solution and allowed to react to thereby cause particles of nickel hydroxide to deposit. Particles of a second positive electrode active substance can thus be obtained. Here in the case of solid-dissolving zinc, magnesium and cobalt in particles of a second positive electrode active substance, nickel sulfate, magnesium sulfate, zinc sulfate and cobalt sulfate are weighed so as to provide a predetermined composition to thereby prepare a second mixed aqueous solution. While the second mixed aqueous solution is being stirred, a sodium hydroxide aqueous solution is slowly added to the second mixed aqueous solution and allowed to react to thereby cause particles of a second positive electrode active substance containing nickel hydroxide as a main component and having magnesium, zinc and cobalt solid-dissolved to deposit. Here, in the case of solid-dissolving magnesium in the second positive electrode active substance, it is preferable to set the amount of magnesium solid-dissolved in the second positive electrode active substance at a smaller amount than that in the first positive electrode active substance. That is, it is preferable that a relation of A&gt;B≥0 be satisfied, where A represents the amount of magnesium solid-dissolved in the first positive electrode active substance, and B represents the amount of magnesium solid-dissolved in the second positive electrode active substance. Here, the second positive electrode active substance is allowed to have also a form of having an amount of magnesium solid-dissolved of 0 and containing no magnesium at all. 
     An aggregate of the particles of the first positive electrode active substance and an aggregate of the particles of the second positive electrode active substance obtained as described above are mixed together to thereby obtain a positive electrode active substance powder being an aggregate of positive electrode active substance particles. 
     Here, in the positive electrode active substance powder, it is preferable to provide a form in which the amount of the particles of the first positive electrode active substance is larger than the amount of the particles of the second positive electrode active substance. This is because when the amount of the first positive electrode active substance, which contains more of magnesium than the second positive electrode active substance, is smaller than the amount of the second positive electrode active substance, it becomes difficult for the discharge characteristic curve of an obtained battery to assume a shape thereof in which the gradient in the discharge last period varies. 
     The positive electrode  24  can be produced, for example, as follows. The conductive material, the positive electrode additive, water and the binder are added to the positive electrode active substance powder being an aggregate of the positive electrode active substance particles obtained as described above, and kneaded to thereby prepare a positive electrode mixture slurry. The obtained positive electrode mixture slurry is filled, for example, in a nickel foam, and dried. After the drying, the nickel foam filled with nickel hydroxide particles and the like is rolled and then cut to thereby obtain the positive electrode  24  carrying the positive electrode mixture. 
     The negative electrode  26  has a strip-form conductive negative electrode substrate (core), and a negative electrode mixture is held on the negative electrode substrate. The negative electrode substrate is a sheet-form metal material having through-holes distributed thereon, and for example, a punching metal sheet or a sintered substrate obtained by molding and sintering a metal powder can be used. The negative electrode mixture is not only filled in the through-holes of the negative electrode substrate, but also held in a layer form on both surfaces of the negative electrode substrate. 
     The negative electrode mixture contains a hydrogen absorbing alloy particle capable of absorbing and releasing hydrogen as a negative electrode active substance, a conductive material, and a binder. The binder functions to mutually bind particles of the hydrogen absorbing alloy, a negative electrode additive, and the conductive material, and simultaneously to bind the negative electrode mixture to the negative electrode substrate. Here, as the binder, a hydrophilic or hydrophobic polymer or the like can be used. As the conductive material, carbon black or graphite can be used. As required, a negative electrode additive may be added. 
     The hydrogen absorbing alloy in the hydrogen absorbing alloy particles is not especially limited, and one used for common nickel hydrogen secondary batteries is used. 
     The above-mentioned hydrogen absorbing alloy particle is obtained, for example, as follows. First, metal raw materials are weighed so as to provide a predetermined composition, and mixed; and the mixture is melted, for example, by an induction melting furnace, and thereafter cooled to thereby make an ingot. The obtained ingot is subjected to a heat treatment in an inert gas atmosphere at 900 to 1,200° C. for 5 to 24 hours. Thereafter, the ingot cooled to room temperature is crushed and sieved to thereby obtain particles of the hydrogen absorbing alloy having a desired particle diameter. 
     The negative electrode  26  can be produced, for example, as follows. A hydrogen absorbing alloy powder being an aggregate of the hydrogen absorbing alloy particles, the conductive material, the binder and water are kneaded to thereby prepare a negative electrode mixture paste. The obtained negative electrode mixture paste is coated on the negative electrode substrate, and dried. After the drying, the negative electrode substrate having the hydrogen absorbing alloy particles and the like adhered thereon is rolled and cut. The negative electrode  26  is thus fabricated. 
     The positive electrode  24  and the negative electrode  26  fabricated as described above are wound in a spiral form in the state that the separator  28  is interposed therebetween to thereby form the electrode group  22 . 
     The electrode group  22  thus obtained is accommodated in the outer can  10 . Then, the alkali electrolyte solution is injected in a predetermined amount in the outer can  10 . Thereafter, the outer can  10  accommodating the electrode group  22  and the alkali electrolyte solution is sealed with the lid plate  14  equipped with the positive electrode terminal  20  to thereby obtain the battery  2  according to the present invention. The obtained battery  2  is subjected to an initial activation treatment and thereby brought into a usable state. 
     EXAMPLES 
     1. Production of batteries 
     Example 1 
     (1) Fabrication of a Hydrogen Absorbing Alloy and a Negative Electrode 
     A rare earth component containing 20% by mass of La and 80% by mass of Sm was prepared. The obtained rare earth component, Mg, Ni and Al were weighed to thereby prepare a mixture having these components having proportions thereof in molar ratio of 0.99:0.01:3.25:0.25. The obtained mixture was melted in an induction melting furnace; and the molten metal was poured in a mold and thereafter cooled to room temperature to thereby make an ingot of a hydrogen absorbing alloy. For a sample extracted from the ingot, a compositional analysis using a high-frequency inductively coupled plasma spectroscopy (ICP) was carried out. As a result, the composition of the hydrogen absorbing alloy was (La 0.20 Sm 0.80 ) 0.99 Mg 0.01 Ni 3.25 Al 0.25 . 
     Then, the ingot was subjected to a heat treatment in an argon gas atmosphere at a temperature of 1,000° C. for 10 hours. Then after the heat treatment, the ingot of the hydrogen absorbing alloy cooled to room temperature was mechanically crushed in an argon gas atmosphere to thereby obtain a hydrogen absorbing alloy powder being an aggregate of hydrogen absorbing alloy particles. Here, for the obtained hydrogen absorbing alloy powder, the particle size of the particles was measured by a laser diffraction scattering-type particle size distribution analyzer; as a result, the volume-average particle diameter (MV) of the hydrogen absorbing alloy particles was 60 μm. 
     To 100 parts by mass of the obtained powder of the hydrogen absorbing alloy, 0.4 parts by mass of a sodium polyacrylate, 0.1 parts by mass of a carboxymethylcellulose, 1.0 part by mass of a dispersion of a styrene butadiene rubber (SBR), 1.0 part by mass of a carbon black and 30 parts by mass of water were added, and the mixture was kneaded to thereby prepare a paste of a negative electrode mixture. 
     The paste of the negative electrode mixture was coated uniformly so the thickness as to become constant on both surfaces of an iron-made perforated plate as a negative electrode substrate. The perforated plate had a thickness of 60 μm, and was plated with nickel on its surfaces. 
     After the paste was dried, the perforated plate on which the hydrogen absorbing alloy powder adhered was rolled to raise the amount of the alloy per volume, and thereafter, cut to thereby obtain a negative electrode  26  for an AA-size. 
     (2) Fabrication of a Positive Electrode Active Substance (First Positive Electrode Active Substance Particle, Second Positive Electrode Active Substance Particle) and a Positive Electrode. 
     (i) Fabrication of First Positive Electrode Active Substance Particles 
     Nickel sulfate, zinc sulfate, magnesium sulfate and cobalt sulfate were weighed so as to become, based on nickel, 4.0% by mass of zinc, 2.48% by mass of magnesium and 1.0% by mass of cobalt; and these were added to a 1N sodium hydroxide aqueous solution containing ammonium ions to thereby prepare a first mixed aqueous solution. While the obtained first mixed aqueous solution was being stirred, a 10N sodium hydroxide aqueous solution was slowly added and allowed to react in the first mixed aqueous solution while the pH was stabilized at 13 to 14 during the reaction to thereby produce base particles containing nickel hydroxide as a main component and having zinc, magnesium and cobalt solid-dissolved. 
     Then, the obtained base particles were placed in an ammonia aqueous solution; a cobalt sulfate aqueous solution was added to the ammonia aqueous solution and allowed to react while the pH was maintained at 9 to 10 during the reaction. Thereby, cobalt hydroxide deposited on the surface of the base particles to thereby obtain intermediate particles having a cobalt hydroxide layer of about 0.1 μm in thickness. Then, the intermediate particles were placed in a reaction vessel, and convected in a high-temperature (80° C.) air containing oxygen in the reaction vessel. Then, a 12N sodium hydroxide aqueous solution was sprayed in the reaction vessel, and the resultant was subjected to a heat treatment for 45 min. Thereby, the cobalt hydroxide on the surface of the intermediate particles became a highly conductive cobalt oxyhydroxide, and sodium was incorporated in the cobalt oxyhydroxide layer, to thus form a conductive layer containing cobalt oxyhydroxide containing sodium. Thereafter, the intermediate particles having such a cobalt oxyhydroxide layer were filtered, washed with pure water in an amount 10 times the amount of the intermediate particles, thereafter dehydrated, and dried in a 60° C. environment. Thereby, there were obtained first positive electrode active substance particles having a conductive layer containing cobalt oxyhydroxide on the surface of the base particles. Here, as a result of measurement of the particle diameter of a first positive electrode active substance powder being an aggregate of the obtained first positive electrode active substance particles by using a laser diffraction scattering-type particle size distribution analyzer, the volume-average particle diameter (MV) of the first positive electrode active substance particles was 10 μm. 
     (ii) Fabrication of Second Positive Electrode Active Substance Particles 
     Nickel sulfate, zinc sulfate and cobalt sulfate were weighed so as to become, based on nickel, 4.0% by mass of zinc and 1.0% by mass of cobalt; and these were added to a 1N sodium hydroxide aqueous solution containing ammonium ions to thereby prepare a second mixed aqueous solution. While the obtained second mixed aqueous solution was being stirred, a 10N sodium hydroxide aqueous solution was slowly added and allowed to react in the second mixed aqueous solution while the pH was stabilized at 13 to 14 during the reaction to thereby produce base particles containing nickel hydroxide as a main component and having zinc and cobalt solid-dissolved. 
     Then, the obtained base particles were placed in an ammonia aqueous solution; a cobalt sulfate aqueous solution was added to the ammonia aqueous solution and allowed to react while the pH was maintained at 9 to 10 during the reaction. Thereby, cobalt hydroxide deposited on the surface of the base particles to thereby obtain intermediate particles having a cobalt hydroxide layer of about 0.1 μm in thickness. Then, the intermediate particles were placed in a reaction vessel, and convected in a high-temperature (80° C.) air containing oxygen in the reaction vessel. Then, a 12N sodium hydroxide aqueous solution was sprayed in the reaction vessel, and the resultant was subjected to a heat treatment for 45 min. Thereby, the cobalt hydroxide on the surface of the intermediate particles became a highly conductive cobalt oxyhydroxide, and sodium was incorporated in the cobalt oxyhydroxide layer, to thus form a conductive layer containing cobalt oxyhydroxide containing sodium. Thereafter, the intermediate particles having such a cobalt oxyhydroxide layer were filtered, washed with pure water in an amount 10 times the amount of the intermediate particles, thereafter dehydrated, and dried in a 60° C. environment. Thereby, there were obtained second positive electrode active substance particles having a conductive layer containing cobalt oxyhydroxide on the surface of the base particles. Here, as a result of measurement of the particle diameter of a second positive electrode active substance powder being an aggregate of the obtained second positive electrode active substance particles by using a laser diffraction scattering-type particle size distribution analyzer, the volume-average particle diameter (MV) of the second positive electrode active substance particles was 10 μm. 
     (iii) Fabrication of a Positive Electrode 
     Then, 95 parts by mass of the first positive electrode active substance powder and 5 parts by mass of the second positive electrode active substance powder were mixed to thereby obtain a positive electrode active substance powder. The obtained positive electrode active substance powder was mixed with 0.3 parts by mass of yttrium oxide, 0.6 parts by mass of niobium oxide, 0.2 parts by mass of an HPC (hydroxypropylcellulose), 0.2 parts by mass of a PTFE dispersion liquid and 30 parts by mass of water to thereby prepare a positive electrode mixture slurry; and the positive electrode mixture slurry was filled in a sheet-form nickel foam as a positive electrode substrate. The nickel foam filled with the positive electrode mixture slurry was dried, and thereafter rolled and cut into a predetermined shape to thereby obtain a positive electrode  24  for an AA-size. 
     (3) Assembly of a Nickel Hydrogen Secondary Battery 
     The positive electrode  24  and the negative electrode  26  obtained were wound in a spiral form in the state of a separator  28  being interposed therebetween to thereby fabricate an electrode group  22 . The separator  28  used for the fabrication of the electrode group  22  was a polypropylene fiber-made nonwoven fabric having been subjected to a sulfonation treatment, and had a thickness of 0.1 mm (basis weight: 53 g/m 2 ). 
     Then, an alkali electrolyte solution, an aqueous solution containing KOH, NaOH and LiOH was provided. The alkali electrolyte solution contained KOH, NaOH and LiOH in a ratio of KOH:NaOH:LiOH=0.8:7.0:0.02. 
     Then, the electrode group  22  was accommodated in a bottomed cylindrical outer can  10 , and the provided alkali electrolyte solution in a predetermined amount was injected. Thereafter, an opening of the outer can  10  was closed with a sealing body  11  to thereby assemble a rated capacity-2,000 mAh AA-size nickel hydrogen secondary battery  2 . Here, the rated capacity was defined by a discharge capacity of a battery when the battery was charged in an environment of a temperature of 25° C. at 0.2 C for 16 hours, and thereafter discharged at 0.4 C until the battery voltage became 1.0 V. 
     (4) Initial Activation Treatment 
     The battery  2  was subjected to an initial activation treatment by repeating five times such a charge and discharge operation that the battery  2  was charged in an environment of a temperature of 25° C. at 0.2 C for 16 hours, and thereafter discharged at 0.4 C until the battery voltage became 1.0 V. The battery  2  was thus brought into a usable state. 
     Example 2 
     A nickel hydrogen secondary battery was assembled as in Example 1, except for adjusting the amount of magnesium contained in first positive electrode active substance particles to 3.62% by mass based on nickel contained therein. 
     Example 3 
     A nickel hydrogen secondary battery was assembled as in Example 1, except for adjusting the amount of magnesium contained in first positive electrode active substance particles to 4.05% by mass based on nickel contained therein. 
     Comparative Example 1 
     A nickel hydrogen secondary battery was assembled as in Example 1, except for adjusting the amount of magnesium contained in first positive electrode active substance particles to 0% by mass based on nickel contained therein (that is, except that magnesium was not contained in the first positive electrode active substance particles). 
     Here, Table 1 shows the whole amount of the first positive electrode active substance, the amount of magnesium contained in the first positive electrode active substance, the whole amount of the second positive electrode active substance, the amount of magnesium contained in the second positive electrode active substance, and the capacity per unit mass of the positive electrode, in Examples 1 to 3 and Comparative Example 1. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 First Positive Electrode 
                 Second Positive Electrode 
                 Capacity per Unit 
               
               
                   
                 Active Substance 
                 Active Substance 
                 Mass of Positive 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Whole amount 
                 Amount of Mg 
                 Whole amount 
                 Amount of Mg 
                 Electrode 
               
               
                   
                 [parts by mass] 
                 [% by mass] 
                 [parts by mass] 
                 [% by mass] 
                 [mAh/g] 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 95 
                 2.48 
                 5 
                 0 
                 234 
               
               
                 Example 2 
                 95 
                 3.62 
                 5 
                 0 
                 222 
               
               
                 Example 3 
                 95 
                 4.05 
                 5 
                 0 
                 222 
               
               
                 Comparative 
                 95 
                 0 
                 5 
                 0 
                 245 
               
               
                 Example 1 
               
               
                   
               
            
           
         
       
     
     2. Evaluation of the Nickel Hydrogen Secondary Batteries 
     (1) Charge and Discharge Characteristics 
     The batteries having been subjected to the initial activation treatment in Examples 1 to 3 and Comparative Example 1 were each charged in an environment of 25° C. at 0.1 C for 720 min, and thereafter allowed to stand for 60 min. 
     The batteries were each discharged in the same environment at 0.2 C until the battery voltage became 1.0 V. 
     In the course of the above-mentioned charge, the relation between the voltage and the charge time of the batteries each was determined and the charge characteristic curve of the each battery was determined. 
     In the course of the above-mentioned discharge, the relation between the voltage and the discharge time of the batteries each was determined and the discharge characteristic curve of the each battery was determined. 
     From the obtained results, the charge and discharge characteristics curves of Examples 1 to 3 and Comparative Example 1 are shown in  FIG. 2 . In  FIG. 2 , curves rising in the voltage value with the passage of time indicate charge characteristic curves; and curves lowering in the voltage value with the passage of time indicate discharge characteristic curves. 
     (2) Consideration 
     The battery of Comparative Example 1, in which magnesium was not solid-dissolved in the positive electrode active substance, corresponds to conventional nickel hydrogen secondary batteries, and assumes a shape of its discharge characteristic curve being nearly flat between the discharge early period, for example, 50 min of discharge time and the discharge last period, for example, 250 min of discharge time. Hence, the difference in voltage value between the discharge early period and the discharge last period is small, and it is then difficult to detect SOC from the voltage of the battery. 
     By contrast, the discharge characteristic curves of the batteries of Examples 1 to 3, in which magnesium was solid-dissolved in their positive electrode active substance, have not so flat portions as the discharge characteristic curve of Comparative Example 1, and have shapes different from the discharge characteristic curve of Comparative Example 1. Hence, it can be said that the detection of SOC for the batteries of Examples 1 to 3 can be carried out easier than that for the battery of Comparative Example 1. 
     It can be confirmed that the gradients of the discharge characteristic curves of Examples 1 to 3 vary in their last period. Hence, the difference in voltage value between the discharge early period and the discharge last period becomes relatively large, and the detection of SOC from the voltage of the batteries can easily be carried out. In particular, since the battery of Example 3, in which 4.05% by mass of magnesium was solid-dissolved in the first positive electrode active substance, exhibits a higher operating voltage than the battery of Comparative Example 1, and moreover, the shape of the discharge characteristic curve has a step in the discharge last period, it is easier to detect SOC. Further, the capacity per unit mass of the positive electrode of Example 1 is 234 mAh/g and those of the positive electrodes of Examples 2 and 3 are 222 mAh/g; so, these positive electrodes secure a capacity of 91% to 96% based on the capacity per unit mass (245 mAh/g) of Comparative Example 1, and cause no large capacity reduction. Therefore, it can be said that the batteries according to Examples 1 to 3 can be used satisfactorily. 
     From the above, it can be said that it is preferable that the amount of magnesium solid-dissolved in the first positive electrode active substance be adjusted to 2.48% by mass or larger, because the variation in the shape of the discharge characteristic curve becomes remarkable. Then since a larger amount of magnesium solid-dissolved brings about a larger variation in the shape of the discharge characteristic curve, a larger amount of magnesium solid-dissolved is preferable. When the amount of magnesium in the positive electrode active substance is increased, however, the amount of nickel hydroxide is reduced along therewith. As a result, the positive electrode capacity decreases. Additionally, since the equilibrium potential rises, the difference from the oxygen evolution potential becomes small and the charge acceptance at high temperatures decreases. Such disadvantages, when the amount of magnesium solid-dissolved exceeds 5.0% by mass, become remarkable. Hence, in order to suppress the degrees of the above-mentioned disadvantages low and simultaneously make the variation in the shape of the discharge characteristic curve to be satisfactorily large, it is preferable that the amount of magnesium solid-dissolved in the first positive electrode active substance be adjusted to 4.05% by mass or smaller. 
     From the above, it can be said that according to the present invention, a nickel hydrogen secondary battery for which the detection of SOC of the battery and eventually the residual capacity of the battery can easily be carried out can be obtained. 
     In the above-mentioned embodiments and Examples, a form containing two positive electrode active substances of a first positive electrode active substance and a second positive electrode active substance differing in the amount of magnesium solid-dissolved has been described, but the present invention is not limited to this form, and it is allowed to adopt a form containing two or more positive electrode active substances differing the amount of magnesium solid-dissolved. Further the nickel hydrogen secondary battery according to the present invention may be a rectangular battery, and the shape thereof is not any more limited. 
     &lt;Aspects of the Present Invention&gt; 
     A first aspect of the present invention is a nickel hydrogen secondary battery, the nickel hydrogen secondary battery comprising a container and an electrode group accommodated together with an alkali electrolyte solution in the container, wherein the electrode group has a positive electrode and a negative electrode stacked through a separator; the positive electrode comprises two or more positive electrode active substances having different compositions; and the positive electrode active substances each comprise a nickel hydroxide as a main component and mutually differ in the amount of magnesium solid-dissolved. 
     A second aspect of the present invention is the nickel hydrogen secondary battery according to the first aspect of the present invention, wherein the positive electrode active substances comprise a first positive electrode active substance and a second positive electrode active substance; and a relation of A&gt;B≥0 is satisfied, where A represents the amount of magnesium solid-dissolved in the first positive electrode active substance, and B represents the amount of magnesium solid-dissolved in the second positive electrode active substance. 
     A third aspect of the present invention is the nickel hydrogen secondary battery according to the second aspect of the present invention, wherein the A is 2.0% by mass or larger and 5.0% by mass or smaller. 
     A fourth aspect of the present invention is the nickel hydrogen secondary battery according to the second aspect of the present invention, wherein the A is 2.48% by mass or larger and 4.05% by mass or smaller. 
     A fifth aspect of the present invention is the nickel hydrogen secondary battery according to the third aspect or fourth aspect of the present invention, wherein the B is 0% by mass. 
     A sixth aspect of the present invention is the nickel hydrogen secondary battery according to any one of the second to fifth aspects of the present invention, wherein the first positive electrode active substance is contained in a larger amount than the second positive electrode active substance. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.