Patent Publication Number: US-2020303726-A1

Title: Negative electrode for alkali secondary battery, alkali secondary battery comprising the negative electrode, and method for producing the negative electrode

Description:
BACKGROUND 
     The present disclosure relates to a negative electrode for an alkali secondary battery, an alkali secondary battery comprising the negative electrode, and a method for producing the negative electrode. 
     Nickel hydrogen secondary batteries are known as one type of alkali secondary battery. The nickel hydrogen secondary batteries have higher capacity and are preferable for environmental safety in comparison to nickel cadmium secondary batteries, and have been used in various types of apparatuses such as various types of portable devices and hybrid electric cars, and have expanded their applications. With the expansion of these applications, nickel hydrogen secondary batteries with higher performance are particularly desirable. 
     Low-temperature discharge characteristics constitute an aspect of performance to be improved for the nickel hydrogen secondary batteries. A low-temperature discharge characteristic is an extent of how deeply discharge can be carried out in a low-temperature environment. The low-temperature discharge characteristic of a battery can be considered excellent if the battery is capable of discharging at a high capacity even in a low-temperature environment. 
     Much research has been done to improve the low-temperature discharge characteristics of nickel hydrogen secondary batteries. Japanese Patent Laid-Open No. 2000-030702, for example, discloses a nickel hydrogen secondary battery with low-temperature discharge characteristic improvements. In this nickel hydrogen secondary battery, an improvement in the low-temperature discharge characteristic is attained through a modification of the surface of a hydrogen absorbing alloy. 
     SUMMARY 
     As applications for the above-mentioned types of devices proliferate, usage condition become more severe. A further improvement in the low-temperature discharge characteristics of nickel hydrogen secondary batteries is consequently necessary. 
     In conventional nickel hydrogen secondary batteries as represented by Patent Document 1, although the low-temperature discharge characteristic is improved, nickel hydrogen secondary batteries are presently unable to fully adapt to current severe usage conditions. That is, the low-temperature discharge characteristics of the nickel hydrogen secondary batteries are not yet sufficient. 
     The present disclosure relates to the aforementioned challenge, and an object of the present disclosure is to provide a negative electrode for an alkali secondary battery which can attain a further improvement of the low-temperature discharge characteristic, an alkali secondary battery comprising the negative electrode, and a method for producing the negative electrode. 
     According to the present disclosure, there is provided a negative electrode for an alkali secondary battery, comprising: a negative electrode core body; and a negative electrode mixture held on the negative electrode core body, wherein the negative electrode mixture comprises a hydrogen absorbing alloy powder composed of particles of a hydrogen absorbing alloy, a powder of an electroconductive agent, and a powder of a negative electrode additive; the electroconductive agent is a hollow carbon black whose primary particle has a hollow-shell structure; and the negative electrode additive is a fluorine-containing anionic surfactant. 
     In the negative electrode for an alkali secondary battery, preferably, the powder of the electroconductive agent composed of the hollow carbon black is added in an amount of 0.25 parts by mass or more and 0.5 parts by mass or less based on 100 parts by mass of the hydrogen absorbing alloy powder. 
     In the negative electrode for an alkali secondary battery, preferably, the powder of the negative electrode additive composed of the fluorine-containing anionic surfactant is added in an amount of 0.01 parts by mass or more and 0.05 parts by mass or less based on 100 parts by mass of the hydrogen absorbing alloy powder. 
     In the negative electrode for an alkali secondary battery, preferably, the fluorine-containing anionic surfactant is a perfluorobutanesulfonate salt. 
     Further according to the present disclosure, there is provided an alkali secondary battery comprising: a container; and an electrode group accommodated together with an alkali electrolyte solution in the container, wherein the electrode group is composed of a positive electrode and a negative electrode stacked through a separator; and the negative electrode is a negative electrode for an alkali secondary battery having any one above-mentioned constitution. 
     Further according to the present disclosure, there is provided a method for producing the negative electrode for an alkali secondary battery, the method comprising: a preparation step of preparing a hydrogen absorbing alloy powder, a powder of an electroconductive agent, a powder of a negative electrode additive, and water in respective predetermined amounts; a paste-production step of producing a paste of a negative electrode mixture by kneading the prepared hydrogen absorbing alloy powder, powder of the electroconductive agent, powder of the negative electrode additive and water; a paste-coating step of coating a negative electrode core body with the paste of the negative electrode mixture obtained in the paste-production step; and a drying step of drying the paste of the negative electrode mixture with which the negative electrode core body is coated, wherein the electroconductive agent is a hollow carbon black whose primary particle has a hollow-shell structure, and the negative electrode additive is a fluorine-containing anionic surfactant. 
     In the negative electrode for an alkali secondary battery according to the present disclosure, the negative electrode mixture comprises a hydrogen absorbing alloy powder composed of particles of a hydrogen absorbing alloy, a powder of an electroconductive agent, and a powder of a negative electrode additive wherein the electroconductive agent is a hollow carbon black whose primary particle has a hollow-shell structure, and the negative electrode additive is a fluorine-containing anionic surfactant. This construction provides a larger electrode reaction area as the whole negative electrode than conventional ones, and thereby enables an electrochemical reaction to be made active even in a low-temperature environment. Hence, the alkali secondary battery comprising the negative electrode according to the present disclosure can attain a further improvement of the low-temperature discharge characteristic. 
     The aforementioned method for producing the negative electrode for an alkali secondary battery according to the present disclosure can further comprise a paste-production step of kneading a hydrogen absorbing alloy powder, a powder of an electroconductive agent composed of a hollow carbon black whose primary particle has a hollow-shell structure, a powder of a negative electrode additive composed of a fluorine-containing anionic surfactant and water to thereby produce a paste of a negative electrode mixture, so that the step homogeneously disperses the hollow carbon black and generates a large number of extremely fine bubbles to thereby contribute to an increase in the electrode reaction area of the negative electrode to be obtained. Hence, according to the method for producing the negative electrode for an alkali secondary battery according to the present disclosure, there can easily be produced the negative electrode for an alkali secondary battery excellent in the low-temperature discharge characteristic. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a perspective view of a nickel hydrogen secondary battery according to one embodiment of the present disclosure illustrated by partially breaking the battery. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, the nickel hydrogen secondary battery (hereinafter, referred to simply as a battery)  2  according to the present disclosure will be described by reference to the drawing. 
     The battery  2  to which the present disclosure is applied is not especially limited, but descriptions will be made by taking as an example the case of applying the present disclosure to an AA-size cylindrical battery  2  illustrated in  FIG. 1 . 
     As illustrated in  FIG. 1 , the battery  2  has an upper end-opened, bottomed, cylinder-shaped outer packaging can  10 . The outer packaging can  10  has electroconductivity, and its bottom wall functions as a negative electrode terminal. On the opening of the outer packaging can  10 , a sealing body  11  is fixed. The sealing body  11  comprises a lid plate  14  and a positive electrode terminal  20 , and seals the outer packaging can  10  and provides the positive electrode terminal  20 . The lid plate  14  is a disc-shaped member having electroconductivity. In the opening of the outer packaging can  10 , there are disposed the lid plate  14  and a ring-shaped insulating packing  12  encircling the lid plate  14 , and the insulating packing  12  is fixed on an opening edge  37  of the outer packaging can  10  by caulking the opening edge  37  of the outer packaging can  10 . That is, the lid plate  14  and the insulating packing  12  mutually cooperatively block hermetically the opening of the outer packaging can  10 . 
     Here, the lid plate  14  has a central throughhole  16  on its center, and on the outer surface of the lid plate  14 , a rubber-made valve disc  18  to close the central throughhole  16  is disposed. Further to the outer surface of the lid plate  14 , the metal-made positive electrode terminal  20  taking on a cylindrical shape having 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 . Then, on the positive electrode terminal  20 , there is opened a vent hole not illustrated in the FIGURE. 
     In the usual time, the central throughhole  16  is hermetically closed by the valve disc  18 . On the other hand, if gas is generated in the outer packaging can  10  and the internal pressure is raised, the valve disc  18  is compressed by the internal pressure and the central throughhole  16  is opened; consequently, the gas is released from in the outer packaging can  10  through the central throughhole  16  and the vent hole (not illustrated in FIGURE) of the positive electrode terminal  20  to the outside. That is, the central throughhole  16 , the valve disc  18  and the positive electrode terminal  20  form a safety valve for the battery. 
     An electrode group  22  is accommodated in the outer packaging can  10 . The electrode group  22  is composed of a strip positive electrode  24 , negative electrode  26  and separator  28 , and these are spirally wound 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 wall of the outer packaging can  10 . That is, the negative electrode  26  and the outer packaging can  10  are mutually electrically connected. 
     Then, in the outer packaging can  10 , a positive electrode lead  30  is disposed between one end of the electrode group  22  and the lid plate  14 . In detail, 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 , an upper circular insulating member  32  is disposed, and the positive electrode lead  30  extends by being passed through a slit  39  provided in the upper insulating member  32 . Further also between the electrode group  22  and the bottom part of the outer packaging can  10 , a lower circular insulating member  34  is disposed. 
     Further, in the outer packaging can  10 , a predetermined amount of an alkali electrolyte solution (not shown in FIGURE) is injected. The alkali electrolyte solution is impregnated in the electrode group  22 , and advances an electrochemical reaction (charge and discharge reaction) in charge and discharge between the positive electrode  24  and the negative electrode  26 . As the alkali electrolyte solution, it is preferable to use an alkali electrolyte solution comprising at least one of KOH, NaOH and LiOH as a solute. 
     As a material of the separator  28 , there can be used, for example, a polyamide fiber-made nonwoven fabric or a nonwoven fabric made of a polyolefin such as polyethylene or polypropylene, imparted with a hydrophilic functional group. 
     The positive electrode  24  is composed of an electroconductive positive electrode base material having a porous structure, and a positive electrode mixture held in pores of the positive electrode base material. 
     As such a positive electrode base material, for example, a sheet of a nickel foam can be used. 
     The positive electrode mixture comprises a positive electrode active substance particle and a binder. Then as required, positive electrode auxiliary additives may be added to the positive electrode mixture. The binder mutually binds the positive electrode active substance particles and also functions to cause the positive electrode active substance particle to bind with the positive electrode base material. Here, as the binder, there can be used, for example, a carboxymethylcellulose, a methylcellulose, a PTFE (polytetrafluoroethylene) dispersion, or an HPC (hydroxypropylcellulose) dispersion. 
     As the positive electrode active substance particle, there is used a nickel hydroxide particle usually used for nickel hydrogen secondary batteries. It is preferable to adopt, as the nickel hydroxide particle, a higher-order nickel hydroxide particle. 
     Further it is preferable to use, as the nickel hydroxide particle, one comprising Co as a solid solution component. The Co as a solid solution component contributes to the improvement of the electroconductivity between the positive electrode active substance particles, and improves the charge acceptability. Here, when the content of Co comprised as a solid solution component in the nickel hydroxide particle is low, the effect of improving the charge acceptability is small; and conversely when being too high, the particle growth of the nickel hydroxide particle becomes inhibited. Hence, it is preferable to use, as the nickel hydroxide particle, one comprising 0.5% by mass or more and 5.0% by mass or less of Co as a solid solution component. 
     Then, it is preferable to make the nickel hydroxide particle to further comprise Zn as a solid solution component. Here, the Zn suppresses the expansion of the positive electrode accompanied by the progress of the charge and discharge cycles, and contributes to the improvement of the cycle lifetime characteristics of the battery. 
     It is preferable to make the content of Zn comprised as a solid solution component in the nickel hydroxide particle to be 2.0% by mass or higher and 5.0% by mass or lower with respect to the nickel hydroxide. 
     Further it is preferable to cover the surface of the nickel hydroxide particle with a surface layer composed of a cobalt compound. It is preferable to adopt, as the surface layer, a higher-order cobalt compound layer composed of a cobalt compound order-heightened to tri- or more valent. 
     The higher-order cobalt compound layer is excellent in electroconductivity, and forms an electroconductive network. It is preferable to adopt, as the higher-order cobalt compound layer, a layer composed of a cobalt compound, such as cobalt oxyhydroxide (CoOOH), order-heightened to tri- or more valent. 
     As described above, the positive electrode active substance particle is produced by a production method usually used for nickel hydrogen secondary batteries. 
     Then, the positive electrode  24  can be produced, for example, as follows. 
     First, a positive electrode mixture slurry comprising the positive electrode active substance particle, water and the binder is prepared. The prepared positive electrode mixture slurry is filled, for example, in a foamed nickel sheet, and dried. After the drying, the foamed nickel sheet filled with the nickel hydroxide particle and the like is rolled and then cut to thereby fabricate the positive electrode  24 . 
     Then, the negative electrode  26  will be described. 
     The negative electrode  26  has a strip electroconductive negative electrode core body, and a negative electrode mixture is held on the negative electrode core body. 
     The negative electrode core body is composed of a metal material sheet having distributed throughholes, and for example, a punching metal sheet can be used. The negative electrode mixture is not only filled in the throughholes of the negative electrode core body but also held as layers on both surfaces of the electrolyte core body. 
     The negative electrode mixture comprises a hydrogen absorbing alloy particle capable of absorbing and desorbing hydrogen as a negative electrode active substance, an electroconductive agent, a binder and a negative electrode additive. Further as required, negative electrode auxiliary additives may be added to the negative electrode mixture. 
     The binder mutually binds the hydrogen absorbing alloy particles, the electroconductive agent and the like, and simultaneously functions to bind the hydrogen absorbing alloy particles, the electroconductive agent and the like to the negative electrode core body. Here, the binder is not especially limited, and there can be used, for example, a binder usually used for nickel hydrogen secondary batteries, such as a hydrophilic or hydrophobic polymer. 
     A hydrogen absorbing alloy in the hydrogen absorbing alloy particle is not especially limited, and there can be used a hydrogen absorbing alloy usually used for nickel hydrogen secondary batteries. Here, as a preferable hydrogen absorbing alloy, there can be used a rare earth element-Mg—Ni-based hydrogen absorbing alloy comprising a rare earth element, Mg and Ni. 
     As the electroconductive agent, there is used a hollow carbon black whose primary particle has a hollow-shell structure. Here, the carbon black refers to a microparticle of carbon having a diameter of about 3 to 500 nm. Usual carbon black has a compact interior of its primary particle. The hollow carbon black to be used in the present disclosure, however, has the interior of its primary particle having a void. That is, a portion of the carbon excluding the void assumes a shape like a shell. Therefore, in the present disclosure, the structure in which the carbon assumes a shape like a shell and has a void in its interior is determined to be called a hollow-shell structure. 
     As the hollow carbon black, it is preferable to use one having a voidage of 55% or higher, and it is more preferable to use one having a voidage of 60% or higher and 80% or lower. Further as the hollow carbon black, it is preferable to use one having a specific surface area determined by a BET method of 700 m 2 /g or larger and 3,000 m 2 /g or smaller, and it is more preferable to use one having the specific surface area of 800 m 2 /g or larger and 1,500 m 2 /g or smaller. 
     When the hollow carbon black having such a hollow-shell structure is used as the electroconductive agent, the specific surface area as the whole negative electrode can be increased and along therewith, the electrode reaction area can be increased. Hence, use of such an electroconductive agent contributes to the improvement of the low-temperature discharge characteristic of the nickel hydrogen secondary battery. 
     It is preferable to add 0.25 parts by mass or more and 0.50 parts by mass or less of the powder of the electroconductive agent composed of the hollow carbon black with respect to 100 parts by mass of the hydrogen absorbing alloy powder composed of the hydrogen absorbing alloy particle. When the addition amount of the powder of the electroconductive agent composed of the hollow carbon black is smaller than 0.25 parts by mass, the electroconductivity becomes insufficient, and the low-temperature discharge characteristic lowers. On the other hand, when the addition amount thereof exceeds 0.50 parts by mass, the effect of the improvement of the low-temperature discharge characteristic saturates and furthermore due to the lowness of the bulk density of the hollow carbon black, the thickness of an obtained negative electrode plate increases and it becomes difficult to accommodate the negative electrode plate in the outer packaging can. 
     Then, as the negative electrode additive, a fluorine-containing anionic surfactant is used. The fluorine-containing anionic surfactant homogeneously disperses the powder of the electroconductive agent in a negative electrode mixture paste in a process of production of the negative electrode, and forms extremely fine bubbles in the m order. Such extremely fine bubbles remain as fine voids also after the negative electrode mixture is dried, and contribute to the increase of the electrode reaction area in the negative electrode. 
     The fluorine-containing anionic surfactant is superior in the alkali resistance to surfactants of hydrocarbon chains, and is suitable because not decomposing in the battery and not adversely affecting the battery reaction. 
     As such a fluorine-containing anionic surfactant, it is preferable to use a perfluorobutanesulfonate salt. Then, as the perfluorobutanesulfonate salt, it is preferable to use potassium perfluorobutanesulfonate, sodium perfluorobutanesulfonate, lithium perfluorobutanesulfonate, cesium perfluorobutanesulfonate, rubidium perfluorobutanesulfonate or the like. 
     It is preferable to add 0.01 parts by mass or more and 0.05 parts by mass or less of the powder of the negative electrode additive composed of the fluorine-containing anionic surfactant with respect to 100 parts by mass of the hydrogen absorbing alloy powder composed of the hydrogen absorbing alloy particle. When the addition amount of the powder of the negative electrode additive composed of the fluorine-containing anionic surfactant is smaller than 0.01 parts by mass, the ability to disperse the powder of the electroconductive agent, that is, the dispersibility becomes insufficient. On the other hand, when the addition amount of the powder of the negative electrode additive composed of the fluorine-containing anionic surfactant exceeds 0.05 parts by mass, relatively large bubbles in the mm order are formed in a negative electrode mixture paste in the process of production of the negative electrode. Such relatively large bubbles remain also after the negative electrode mixture is dried, and end in forming large irregularities on the negative electrode surface. Such irregularities have risk of causing the internal short circuit in the battery. Further the fluorine-containing anionic surfactant has no electroconductivity and when the addition amount thereof is increased, inhibits the discharge reaction of the negative electrode. Therefore, it is preferable to make the addition amount of the powder of the negative electrode additive composed of the fluorine-containing anionic surfactant to be 0.05 parts by mass or smaller. 
     The negative electrode  26  can be produced, for example, as follows. 
     First, the hydrogen absorbing alloy powder composed of the hydrogen absorbing alloy particle, the powder of the electroconductive agent composed of the hollow carbon black whose primary particle has a hollow-shell structure, the powder of the negative electrode additive composed of the fluorine-containing anionic surfactant and water are prepared in respective predetermined amounts (preparation step). 
     Then, the prepared hydrogen absorbing alloy powder, powder of the electroconductive agent powder, binder and water are kneaded to thereby produce a negative electrode mixture paste (paste production step). This kneading operation in the paste production step homogeneously disperses the powder of the electroconductive agent and generates extremely fine bubbles in the negative electrode mixture paste by the action of the negative electrode additive. 
     The negative electrode core body is coated with the obtained negative electrode mixture paste (paste coating step). Thereafter, the resultant is dried (drying step). 
     After the negative electrode mixture paste is dried, the negative electrode core body on which the negative electrode mixture comprising the hydrogen absorbing alloy particle and the like are adhered is subjected to rolling and cutting to thereby fabricate the negative electrode  26 . 
     The positive electrode  24  and the negative electrode  26  fabricated as in the above are spirally wound in the state that the separator  28  is interposed to thereby form the electrode group  22 . 
     The electrode group  22  thus obtained is accommodated in the outer packaging can  10 . Successively, a predetermined amount of an alkali electrolyte solution is injected in the outer packaging can  10 . Thereafter, the outer packaging can  10  accommodating the electrode group  22  and the alkali electrolyte solution is sealed with the sealing body  11  equipped with the positive electrode terminal  20  to thereby obtain the battery  2  according to the present disclosure. The obtained battery  2  is subjected to an initial activation treatment to be thereby put in a usable state. 
     EXAMPLES 
     1. Production of Batteries 
     Example 1 
     (1) Fabrication of a Positive Electrode 
     Nickel sulfate, zinc sulfate and cobalt sulfate were weighed so that with respect to Ni, Zn became 2.5% by mass and Co became 1.0% by mass, and added to a 1N sodium hydroxide aqueous solution comprising ammonium ions to thereby prepare a mixed aqueous solution. While the obtained mixed aqueous solution was stirred, a 10N sodium hydroxide aqueous solution was little by little added to the mixed aqueous solution to be allowed to react to thereby cause base particles composed of nickel hydroxide particles comprising nickel hydroxide as a main component and comprising Zn and Co as solid solution components to be produced by stabilizing the pH during the reaction at 13 to 14. 
     The obtained base particles were three times washed with pure water of a volume 10 times that of the base particles, and were subjected to a dehydration and drying treatment. For the obtained base particles, the particle diameter was measured by using a laser diffraction scattering-type particle size distribution analyzer, and as a result, the volume-average particle diameter (MV) of the base particles was 8 μm. 
     Then, the obtained base particles were charged in a cobalt sulfate aqueous solution, and while the cobalt sulfate aqueous solution was stirred, a 1 mol/l sodium hydroxide aqueous solution was little by little dropped to be allowed to react to thereby produce a precipitate while the pH during the reaction was kept at 11. Then, the produced precipitate was filtered off, and washed with pure water, and thereafter vacuum dried. Thereby, there were obtained intermediate product particles each having the base particle  38  as their core and having a layer of a 5-mass % cobalt hydroxide on the surface of the core. Then, the thickness of the layer of cobalt hydroxide was about 0.1 μm. 
     Then, the intermediate product particles were charged in a 25-mass % sodium hydroxide aqueous solution. Here, when the mass of a powder composed of the intermediate product particles was taken to be P, and the mass of the sodium hydroxide aqueous solution was taken to be Q, the mass ratio of these masses was set so that P:Q became 1:10. Then, the sodium hydroxide aqueous solution containing the powder of the intermediate product particles added therein was subjected to a heat treatment at a temperature of 85° C. held for 8 hours under stirring. 
     The powder composed of the intermediate product particles having been subjected to the heat treatment was washed with pure water, and dried by being exposed to warm air of 65° C. Thereby, there was obtained a positive electrode active substance powder composed of positive electrode active substance particles having a surface layer composed of a higher-order cobalt oxide on the surface of the base particles composed of the nickel hydroxide particles comprising Zn and Co as solid solution components. 
     Then, to 95 parts by mass of the positive electrode active substance powder composed of the nickel hydroxide particle fabricated as described above, 3.0 parts by mass of a powder of zinc oxide, 2.0 parts by mass of a powder of cobalt hydroxide, and 50.0 parts by mass of water comprising 0.2 part by mass of a powder of a hydroxypropylcellulose as a binder were added, and kneaded to thereby prepare a positive electrode mixture slurry. 
     Then, the positive electrode mixture slurry was filled in a sheet nickel foam as a positive electrode base material. Here, the nickel foam used was one having a surface density (basis weight) of about 600 g/m 2 , a porosity of 95% and a thickness of about 2 mm. 
     After the nickel foam filled with the positive electrode mixture slurry was dried, the nickel foam filled with the positive electrode mixture was so regulated and rolled that the filling density of the positive electrode active substance calculated by the following expression (1) became 2.9 g/cm 3 , and thereafter, cut into a predetermined size to thereby obtain a positive electrode  24  for an AA size composed of a non-sintered nickel electrode. 
     Filling density of the positive electrode active substance [g/cm 3 ]=a mass of the positive electrode mixture [g]/(a height of the electrode [cm]×a length of the electrode [cm]×a thickness of the electrode [cm]−a mass of the nickel foam [g]/a specific gravity of nickel [g/cm 3 ]). 
     (2) Fabrication of a Negative Electrode 
     Each metal material of La, Sm, Mg, Ni and Al was mixed so as to become a predetermined molar ratio, and thereafter charged and dissolved in an induction melting furnace, and cooled to thereby fabricate an ingot. 
     Then, the ingot was subjected to a heat treatment in an argon gas atmosphere at 1,000° C. for 10 hours to be homogenized, and thereafter mechanically crushed in an argon gas atmosphere to thereby obtain a rare earth element-Mg—Ni-based hydrogen absorbing alloy powder. For the obtained rare earth element-Mg—Ni-based hydrogen absorbing alloy powder, the particle size distribution was measured by a laser diffraction scattering-type particle size analyzer (apparatus name: SRA-150, manufactured by MicrotracBEL Corp.). As a result, the average particle diameter corresponding to 50% of cumulation in terms of mass was 65 m. 
     The composition of the hydrogen absorbing alloy powder was analyzed by an inductively coupled plasma spectroscopy (ICP), and was La 0.30 Sm 0.70 Mg 0.10 Ni 3.33 Al 0.17 . Further the X-ray diffractometry (XRD) was carried out on the hydrogen absorbing alloy powder, and the crystal structure was a Ce 2 Ni 7  type being a so-called superlattice structure. 
     To 100 parts by mass of the obtained hydrogen absorbing alloy powder, 0.50 parts by mass of a powder of an electroconductive agent composed of a hollow carbon black whose primary particle has a hollow-shell structure, 0.01 parts by mass of a powder of potassium perfluorobutanesulfonate, 1.0 part by mass of a powder of a styrene butadiene rubber, 0.25 parts by mass of a powder of sodium polyacrylate, 0.05 parts by mass of a powder of a carboxymethylcellulose, and 20 parts by mass of water were added and kneaded in an environment of 25° C. to thereby prepare a negative electrode mixture paste. 
     Here, the above hollow carbon black used was Ketjen Black®, manufactured by Lion Specialty Chemicals Co., Ltd. With respect to the physical properties of the hollow carbon black, the specific surface area by a BET method was 1,270 m 2 /g; the voidage was 80%; and the primary particle diameter was 34.0 nm. 
     The negative electrode mixture paste was applied on both surfaces of a punching metal sheet as a negative electrode substrate so as to be uniform and have a constant thickness. Further also in throughholes, the negative electrode mixture paste was filled. Here, the punching metal sheet was an iron-made strip body in which a large number of the throughholes were perforated, and had a thickness of 60 μm, and had a nickel plating on its surface. 
     After the negative electrode mixture paste was dried, the punching metal sheet holding the negative electrode mixture was so regulated and rolled that the filling density of the hydrogen absorbing alloy (hereinafter, referred to as hydrogen absorbing alloy filling density) calculated by the following expression (2) became 5.5 g/cm 3 , and thereafter cut into a predetermined size to thereby obtain a negative electrode  26  for an AA size. 
     Filling density of the hydrogen absorbing alloy [g/cm 3 ]=a mass of the hydrogen absorbing alloy [g]/(a height of the electrode [cm]×a length of the electrode [cm]×a thickness of the electrode [cm]−a mass of the punching metal sheet [g]/a specific gravity of iron [g/cm 3 ]). 
     (3) Assembling of a Nickel Hydrogen Secondary Battery 
     The obtained positive electrode  24  and negative electrode  26  were spirally wound in the state that a separator  28  was interposed therebetween to thereby fabricate an electrode group  22 . The separator  28  used here for fabrication of the electrode group  22  was one being composed of a polypropylene fiber-made nonwoven fabric having been subjected to a sulfonation treatment and having a thickness of 0.1 mm (basis weight: 40 g/m 2 ). 
     Meanwhile, there was prepared an alkali electrolyte solution composed of an aqueous solution comprising KOH, NaOH and LiOH as solutes. The alkali electrolyte solution had a mixing mass ratio of KOH:NaOH:LiOH=15:2:1, and had a specific gravity of 1.30. 
     Then, the above electrode group  22  was accommodated in a bottomed cylinder-shaped outer packaging can  10 , and 2.4 g of the prepared alkali electrolyte solution was injected. Thereafter, an opening of the outer packaging can  10  was closed by a sealing body  11  to thereby assemble an AA-size nickel hydrogen secondary battery  2  having a nominal capacity of 1,500 mAh. 
     (4) Initial Activation Treatment 
     There was repeated three times a charge and discharge cycle in which a charge and discharge operation of causing the obtained battery  2  to be charged at 1.0 It for 16 hours in an environment of a temperature of 25° C. and thereafter to be discharged at 1.0 It until the battery voltage became 1.0 V was taken as one cycle. The battery  2  was thus subjected to the initial activation treatment, being put in the state of being ready to be used. 
     Example 2 
     A nickel hydrogen secondary battery was fabricated as in Example 1, except for altering the addition amount of the powder of potassium perfluorobutanesulfonate to 0.02 parts by mass. 
     Example 3 
     A nickel hydrogen secondary battery was fabricated as in Example 1, except for altering the addition amount of the powder of potassium perfluorobutanesulfonate to 0.05 parts by mass. 
     Comparative Example 1 
     A nickel hydrogen secondary battery was fabricated as in Example 1, except for no addition of the powder of the electroconductive agent composed of the hollow carbon black and the powder of potassium perfluorobutanesulfonate. 
     Comparative Example 2 
     A nickel hydrogen secondary battery was fabricated as in Example 1, except for no addition of the powder of potassium perfluorobutanesulfonate. 
     Comparative Example 3 
     A nickel hydrogen secondary battery was fabricated as in Example 1, except for addition of 0.25 parts by mass of the powder of the electroconductive agent composed of the hollow carbon black and no addition of the powder of potassium perfluorobutanesulfonate. 
     Comparative Example 4 
     A nickel hydrogen secondary battery was fabricated as in Example 1, except for addition of 1.00 part by mass of the powder of the electroconductive agent composed of the hollow carbon black and no addition of the powder of potassium perfluorobutanesulfonate. 
     2. Evaluation of the Nickel Hydrogen Secondary Batteries 
     (1) Test for the Low-Temperature Discharge Characteristic 
     The fabricated batteries of Examples 1 to 3 and Comparative Examples 1 to 4 were each charged at 1.0 It in an environment of 25° C. until the battery voltage, after reaching a maximum value, decreased by 10 mV, and thereafter allowed to stand for 3 hours in an environment of −10° C. 
     Then, the battery after the being left for 3 hours was discharged at 1.0 It until the battery voltage became 1.0 V in an environment of −10° C.; and the discharge capacity at this time was determined. 
     Here, with the value of the discharge capacity of Comparative Example 1 being taken to be 100, the ratio of the value of the discharge capacity of the each battery to the 100 was determined; and the result is shown as a ratio of the low-temperature discharge characteristic in Table 1. 
     Here, it is indicated that the larger the value of the ratio of the low-temperature discharge characteristic, the better the low-temperature discharge characteristic. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Addition Amount 
                   
               
               
                   
                 Addition Amount 
                 of Potassium 
                 Ratio of Low- 
               
               
                   
                 of Hollow 
                 Perfluoro- 
                 Temperature 
               
               
                   
                 Carbon Black 
                 butanesulfonate 
                 Discharge 
               
               
                   
                 [parts by mass] 
                 [parts by mass] 
                 Characteristic 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Example 1 
                 0.50 
                 0.01 
                 114 
               
               
                 Example 2 
                 0.50 
                 0.02 
                 122 
               
               
                 Example 3 
                 0.50 
                 0.05 
                 113 
               
               
                 Comparative 
                 none 
                 none 
                 100 
               
               
                 Example 1 
               
               
                 Comparative 
                 0.50 
                 none 
                 107 
               
               
                 Example 2 
               
               
                 Comparative 
                 0.25 
                 none 
                 107 
               
               
                 Example 3 
               
               
                 Comparative 
                 1.00 
                 none 
                 108 
               
               
                 Example 4 
               
               
                   
               
            
           
         
       
     
     (2) Discussion 
     (i) The battery of Comparative Example 1, using a negative electrode comprising neither of the hollow carbon black and potassium perfluorobutanesulfonate, had a ratio of the low-temperature discharge characteristic of 100. By contrast, the batteries of Comparative Examples 2 to 4, comprising the hollow carbon black and no potassium perfluorobutanesulfonate, had a ratio of the low-temperature discharge characteristic of 107 to 108, revealing that the low-temperature discharge characteristic was improved more than in the battery of Comparative Example 1. Hence, it can be said that the addition of the hollow carbon black has an effect on the improvement of the low-temperature discharge characteristic. 
     (ii) Meanwhile, comparing Comparative Examples, in Comparative Example 3, the addition amount of the hollow carbon black was 0.25 parts by mass, and the ratio of the low-temperature discharge characteristic was 107. By contrast, in Comparative Example 2, although the addition amount of the hollow carbon black was 0.50 parts by mass, which was an amount twice that in Comparative Example 3, the ratio of the low-temperature discharge characteristic was 107, which was the same as that of Comparative Example 3. Further in Comparative Example 4, although the addition amount of the hollow carbon black was 1.00 part by mass, which was an amount four times that in Comparative Example 3, the ratio of the low-temperature discharge characteristic was 108, which was only in a slightly improved degree. Hence, although the addition of the hollow carbon black improves the ratio of the low-temperature discharge characteristic, it can be said that even when 0.50 parts by mass or more thereof is added, the effect of improving the ratio of the low-temperature discharge characteristic ends in saturating. Further the ratio of the low-temperature discharge characteristic has a limit of 108, which cannot sufficiently meet recent year&#39;s usage condition. 
     (iii) It is found that the batteries of Examples 1 to 3, using a negative electrode comprising both of the hollow carbon black and potassium perfluorobutanesulfonate, were improved more in the low-temperature discharge characteristic than Comparative Examples 2 to 4, using a negative electrode comprising no potassium perfluorobutanesulfonate. 
     This is conceivably because that since by the potassium perfluorobutanesulfonate, the hollow carbon black being an electroconductive agent was more homogeneously dispersed, and extremely fine bubbles were formed and became voids in the negative electrode, the electrode reaction area in the whole negative electrode increased and the battery reaction was thereby made to be active even in a low temperature, more improving the ratio of the low-temperature discharge characteristic. 
     Here, with respect to the addition amount of potassium perfluorobutanesulfonate, in the case of 0.02 parts by mass in Example 2, the ratio of the low-temperature discharge characteristic was 122; and it can be said that Example 2 was the best aspect. In the case of the aspect of Example 2, it is conceivable that the dispersibility of the hollow carbon black was sufficiently high and the effect of increasing the electrode reaction area was sufficiently exhibited. 
     In the case of Example 1, in which the addition amount of potassium perfluorobutanesulfonate was 0.01 parts by mass, the ratio of the low-temperature discharge characteristic was 114, which was better than in Comparative Examples 1 to 4, but lower than in Example 2. This is conceivably because the addition amount of potassium perfluorobutanesulfonate was not sufficient and the dispersibility of the hollow carbon black and the effect of increasing the electrode reaction area were not exhibited so much as in Example 2. 
     In the case of Example 3, in which the addition amount of potassium perfluorobutanesulfonate was 0.05 parts by mass, the ratio of the low-temperature discharge characteristic was 113, which was better than in Comparative Examples 1 to 4, but lower than in Example 2. This is conceivably because the addition amount of potassium perfluorobutanesulfonate, which had no electroconductivity, was larger than in Example 2, and the discharge reaction was thereby inhibited. 
     From the above, it can be said that when the hollow carbon black is combined with potassium perfluorobutanesulfonate, the synergetic effect thereof improves the ratio of the low-temperature discharge characteristic more largely than conventionally, and the improvement of the low-temperature discharge characteristic of the battery can be attained. 
     Here, in the present embodiment, description of use only of potassium perfluorobutanesulfonate have been made, but there can be attained the similar effect also when there is used another perfluorobutanesulfonate salt, that is, sodium perfluorobutanesulfonate, lithium perfluorobutanesulfonate, cesium perfluorobutanesulfonate, rubidium perfluorobutanesulfonate, or the like. 
     Here, the present invention is not limited to the above embodiment and Examples, and various changes and modifications may be made. The battery to which the present disclosure is applied suffices as long as being an alkali secondary battery, and includes, in addition to the nickel hydrogen secondary battery, for example, nickel zinc secondary batteries. Further the structure of the battery is not especially limited, and may also be, in addition to a circular one, a rectangular one. 
     EXPLANATION OF REFERENCE SIGNS 
     
         
         
           
               2  NICKEL HYDROGEN SECONDARY BATTERY 
               22  ELECTRODE GROUP 
               24  POSITIVE ELECTRODE 
               26  NEGATIVE ELECTRODE 
               28  SEPARATOR