Patent Publication Number: US-2009220861-A1

Title: Method for producing alkaline battery, and alkaline battery

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for producing an alkaline battery and also to an alkaline battery. 
     2. Description of the Related Art 
     Conventionally, in a coin-shaped, button-shaped, or other-shaped flat alkaline battery for use in a small electronic device such as a wrist watch, as shown in  FIG. 4 , the open end of a positive electrode can  51  is sealed with a negative electrode can  52  via a gasket G. The negative electrode can  52  has, at the open edge thereof, a folded portion  52   a  and a folded bottom  52   b  formed along the outer peripheral surface to have a U-shaped cross section. The negative electrode can  52  is compressed at the folded portion  52   a  by the inner peripheral surface of the open edge of the positive electrode can  51  via a gasket G, and thereby maintained in a hermetically sealed state. 
     The negative electrode can  52  is formed from a three-layer clad material pressed into a cup-like shape, comprising a nickel layer S 1  of nickel, a stainless steel layer S 2  of stainless steel, and a collector layer S 3  of copper. 
     The positive electrode can  51  accommodates a positive electrode mixture  53 . The negative electrode can  52  contains a negative electrode mixture  55  separated from the positive electrode mixture  53  by a separator  54  and containing mercury-free zinc or zinc alloy powder as a negative electrode active material. An alkaline electrolyte is injected thereinto. 
     In the negative electrode mixture  55 , amalgamated zinc comprising zinc or zinc alloy powder amalgamated with mercury is used to suppress the generation of hydrogen gas (H 2 ) from zinc or zinc alloy powder, and also suppress the generation of hydrogen gas (H 2 ) from the collector layer S 3  due to contact between zinc or zinc alloy powder and copper of the collector layer S 3  of the negative electrode can  52  via the alkaline electrolyte. 
     The reaction to generate hydrogen gas is caused by the dissolution of zinc or zinc alloy powder in an alkaline electrolyte, whereby zinc is oxidized to zinc oxide. However, as mentioned above, use of amalgamated zinc that has been amalgamated with mercury can suppress the generation of hydrogen. This provides advantages such as the prevention of reduction in capacity retention that accompanies hydrogen generation, reduction in leak resistance due to an increase in internal pressure, and expansion of the battery. 
     However, in recent years, from an environmental perspective, there is a tendency to avoid use of mercury in coin-shaped or button-shaped flat alkaline batteries, and a number of researches have been conducted on the avoidance of the use of mercury. 
     In order to effectively suppress the generation of hydrogen gas, a method has been proposed, which plates the surface of the collector layer S 3  with a coating layer of tin, a metal having a higher hydrogen overpotential than copper used in the collector layer S 3 . A coating layer (tin plating layer) is formed by depositing the above-mentioned tin by electroless plating, electrolytic plating, etc. 
     Further, another method has also been proposed, according to which tin is deposited all over the copper surface of the negative electrode can by plating, and then thermally treated at 120° C. to 180° C. for 2 minutes or more to give a copper-tin diffusion alloy layer occupying 30% or more the thickness of the tin plating. 
     Such an alkaline battery is incapable of completely preventing the generation of hydrogen gas. 
     Because the coating layer (tin plating layer) provided in the negative electrode can  52  is as thin as 5 μm or less and is formed by electroless plating, etc., defects such as pinholes, cracks, and the like are easily created in the coating layer (tin plating layer). If a pinhole, a crack, or the like exists in a coating layer (tin plating layer), hydrogen is generated from such a defective portion, which causes reduction in capacity retention, reduction in leak resistance, expansion of the battery can, etc. 
     Further, when a clad material is used for the negative electrode can  52 , rolling is employed for the manufacture, and it thus is highly possible that impurities adhere to the copper surface. Such adhesion of impurities may cause defects in the coating layer (tin plating layer), which will result in the generation of hydrogen gas mentioned above. 
     According to the method that thermally treats a coating layer (tin plating layer) to form a copper-tin diffusion alloy layer, although a diffusion alloy layer grows, because the temperature of the thermal treatment is 120° C. to 180° C., which is lower than the melting point of tin, when a pinhole, a crack, etc., which are main causes of hydrogen gas generation, exist in the tin plating layer (coating layer), such defects in the tin plating layer (coating layer) cannot be repaired. 
     The invention is aimed to solve the above problems. An object of the invention is to provide a method for producing a mercury-free alkaline battery that does not allow generation of hydrogen gas, and an alkaline battery. 
     SUMMARY OF THE INVENTION 
     The method for producing an alkaline battery of the invention is a method for producing an alkaline battery comprising a positive electrode can and a negative electrode can with an opening of the negative electrode can being fitted into an opening of the positive electrode can, the positive electrode can and the negative electrode can being hermetically sealed via a gasket to create an enclosed space. The enclosed space has disposed therein a separator, a positive electrode mixture on the positive electrode can side of the separator, and a negative electrode mixture containing zinc powder or zinc alloy powder on the negative electrode can side of the separator. The enclosed space is further filled with an alkaline electrolyte. The method includes a first step of chemically polishing the surface of a collector layer of copper included in the negative electrode can with an acid, a second step of surface-treating the chemically polished surface of the collector layer of the negative electrode can with a conductive polymer so as to place monovalent copper ions, a third step of forming a coating layer of a metal or an alloy having a higher hydrogen overpotential than copper on the surface-treated collector layer of the negative electrode can, and a fourth step of caulking, with the gasket in between, the positive electrode can and the negative electrode can containing the positive electrode mixture, the negative electrode mixture, the separator, and the alkaline electrolyte, thereby effecting sealing. 
     According to the method for producing an alkaline battery of the invention, before a coating layer of a metal or an alloy having a higher hydrogen overpotential than copper is formed on the collector copper layer of the negative electrode can, the collector copper layer of the negative electrode can is chemically polished with an acid; as a result, foreign substances adhering to a collector layer, small cracks, and the like can be eliminated. That is, foreign substances adhering to the surface of a collector layer, cracks, and the like created upon the production of the negative electrode can by pressing, which hinder the formation of a uniform and dense coating layer, can be sufficiently eliminated. Subsequently, the collector copper layer of the negative electrode can is surface-treated with a conductive polymer; as a result, only Cu+ (monovalent copper ions) exists on the surface of the collector copper layer. In other words, the use of a conductive polymer prevents divalent copper ions, which hinder the formation of a uniform and dense coating layer of a metal or an alloy having a higher hydrogen overpotential than copper metal, from being present at random with monovalent copper ions on the surface of the collector layer of copper of the negative electrode can. 
     Therefore, owing to the first step and the second step, a coating layer of a metal or an alloy having a higher hydrogen overpotential than copper can be formed in a negative electrode can as a tin coating layer having a uniform and precise thickness without pinholes or cracks. As a result, the collector layer is prevented from being exposed from the coating layer and generating hydrogen gas. 
     In the method for producing an alkaline battery, the acid used for the chemical polishing may be a mixed aqueous solution of sulfuric acid and hydrogen peroxide. 
     Accordingly, before a coating layer of a metal or an alloy having a higher hydrogen overpotential than copper is formed on the collector copper layer of the negative electrode can, the collector copper layer of the negative electrode can is chemically polished with a mixed aqueous solution of sulfuric acid and hydrogen peroxide, thereby eliminating foreign substances adhering to a collector layer, small cracks, etc. 
     In the method for producing an alkaline battery, the conductive polymer used for the surface treatment may be a polyaniline-based conductive polymer solution. 
     Accordingly, the collector copper layer of the negative electrode can is surface-treated with a polyaniline-based conductive polymer solution, so that only Cu+ (monovalent copper ions) exists on the surface of the collector copper layer. In other words, the use of a conductive polymer prevents divalent copper ions, which hinder the formation of a uniform coating layer of a metal or an alloy having a higher hydrogen overpotential than copper metal, from being present at random with monovalent copper ions on the collector layer of copper of the negative electrode can. 
     In the method for producing an alkaline battery, the coating layer of a metal or an alloy having a higher hydrogen overpotential than copper may be a tin coating layer formed of tin or tin alloy having a thickness of 0.03 to 0.1 μm using an electroless tin plating liquid. 
     Accordingly, a uniform and thin tin coating layer having no defects such as pinholes, cracks can be formed within a short period of time on the chemically polished and surface-treated collector layer. 
     The alkaline battery of the invention comprises a positive electrode can and a negative electrode can with an opening of the negative electrode can being fitted into an opening of the positive electrode can, the positive electrode can and the negative electrode can being hermetically sealed via a gasket to create an enclosed space. The enclosed space has disposed therein a separator, a positive electrode mixture on the positive electrode can side of the separator, and a negative electrode mixture containing zinc powder or zinc alloy powder on the negative electrode can side of the separator. The enclosed space is further filled with an alkaline electrolyte. The negative electrode can has a collector layer of copper whose surface is chemically polished and surface-treated, and coated with tin or tin alloy having a thickness of 0.03 to 0.1 μm without pinholes or cracks. 
     According to the alkaline battery of the invention, the collector layer is prevented from being exposed from the tin coating layer and generating hydrogen gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a sectional view of an alkaline battery according to the invention. 
         FIG. 2  is a sectional view for explaining the structure of a negative electrode can. 
         FIGS. 3A-3C  are explanatory view showing the method for forming a plate coating layer formed in a negative electrode can.  FIG. 3A  is an explanatory view for explaining chemical polishing of a collector layer,  FIG. 3B  is an explanatory view for explaining surface treatment of a collector layer, and  FIG. 3C  is an explanatory view for explaining electroless tin plating on a collector layer. 
         FIG. 4  is a sectional view of a conventional alkaline battery. 
         FIG. 5  is a table showing the incidence of leakage and the self-discharge of an alkaline battery according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     The alkaline battery of the invention is explained with reference to  FIG. 1  and  FIG. 2 .  FIG. 1  shows a sectional view of a button-shaped (flat) alkaline battery  10 . The open end of a positive electrode can  11  is sealed by a negative electrode can  12  via a gasket G having a J-shaped cross section. 
     The positive electrode can  11  comprises a stainless steel plate plated with nickel, and serves also as a positive terminal. The positive electrode can  11  accommodates a positive electrode mixture  13  in the form of a coin- or button-shaped pellet. 
     On the positive electrode mixture  13  in this positive electrode can  11 , a separator  14  is disposed. The separator  14  has a three-layer structure of a nonwoven fabric, cellophane, and a polyethylene graft polymerization film, for example. The separator  14  is impregnated with an alkaline electrolyte. The alkaline electrolyte may be an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, or a mixed aqueous solution of an aqueous sodium hydroxide solution and an aqueous potassium hydroxide solution, for example. 
     A ring-shaped gasket G is disposed on the inner peripheral surface of the open edge of the positive electrode can  11 , and a negative electrode mixture  15  is placed on the separator  14 . The negative electrode mixture  15  consists of mercury-free, i. e., non-mercury-containing, zinc or zinc alloy powder, an alkaline electrolyte, a thickener, etc., and is in a gel state. 
     The negative electrode can  12  is inserted into the opening of the positive electrode can  11  so as to accommodate the negative electrode mixture  15 . The negative electrode can  12  has, at the open edge thereof, a U-shaped folded portion  12   a  and a folded bottom  12   b  formed along the outer peripheral surface to have a U-shaped cross section. The negative electrode can  12  is compressed at the folded portion  12   a  by the inner peripheral surface of the open edge of the positive electrode can  11  via the gasket G, and thereby sealed (fourth step). 
     The negative electrode can  12  is formed from a three-layer clad material pressed into a cup-like shape, comprising a nickel layer S 1  of nickel, a stainless steel layer S 2  of stainless steel, and a collector layer S 3  of copper, with the collector layer S 3  being on the inner side. After the clad material is pressed into a cup-like shape, the negative electrode can  12  is processed as follows. The collecting inner surface  12   c  (surface of the collector layer S 3 ) of the negative electrode can  12  is chemically polished with a mixed aqueous solution of sulfuric acid and oxygenated water, etc., (first step) and then surface-treated with polyaniline or a like conductive polymer material (second step). Subsequently, a tin coating layer S 4  is formed in the inner surface area of the negative electrode can  12  by electroless tin plating or the like (third step). 
     The tin coating layer S 4  is preferably formed only in the inner surface area of the negative electrode can  12  to improve leak resistance. An inner surface area refers to an area on the inner side (the side in contact with an electrolyte) of the negative electrode can  12  and inward from the folded bottom  12   b.  The tin coating layer S 4  is not formed at the folded portion  12   a  and the folded bottom  12   b  that are in contact with the gasket G. This prevents the electrolyte from creeping up due to the creep phenomenon, and thus improves the leak resistance. More specifically, as compared with the collector layer S 3 , the tin coating layer S 4  allows an alkaline electrolyte to creep up more easily. 
     A mixed aqueous solution of sulfuric acid and oxygenated water, etc., polyaniline or a like conductive polymer solution, and an electroless tin plating liquid are applied dropwise onto the necessary part, i.e., the inner surface except for the folded portion  12   a  and the folded bottom  12   b  formed along the outer peripheral surface to have a U-shaped cross section, and then removed, followed by washing and dryness, repetitively. As a result, a uniform and dense tin coating layer S 4  can be formed. 
     The thickness of the tin coating layer S 4  is preferably 0.03 to 0.1 μm. This is because when the thickness is less than 0.03 μm, a uniform tin coating layer S 4  cannot be formed even after chemical polishing with a mixed acid and surface treatment with a conductive polymer, thus causing defects such as pinholes, cracks, etc. A thickness of over 0.1 μm accordingly requires more time for the formation of a tin coating layer S 4 , but such thickening of the tin coating layer S 4  provides no particular advantages. 
     A space D formed between the inner peripheral surface of the folded portion  12   a  of the negative electrode can  12  and a central-side protruding portion Ga of the gasket G is preferably no wider than the thickness W of the negative electrode can  12 . The length L 1  of the protruding portion Ga is preferably less than, but at least ½, the length L 2  of the folded portion  12   a.    
     The reasons for this are as follows. First, when the space D between the negative electrode can  12  and the gasket G is no wider than the thickness W of the negative electrode can  12 , and the length L 1  of the central-side protruding portion Ga of the gasket G is less than, but at least ½, the length L 2  of the folded portion  12   a  of the negative electrode can  12 , upon caulking and sealing the battery  10 , because the space D between the negative electrode can  12  and the gasket G is narrow, and the length L 1  of the central-side protruding portion Ga of the gasket G is as long as ½ or more the length L 2  of the folded portion  12   a  of the negative electrode can  12 , the negative electrode mixture  15  can be prevented from biting between the negative electrode can  12  and the gasket G. 
     Moreover, when the central-side protruding portion Ga of the gasket G is designed so that even its maximum length is shorter than the length L 2  of the folded portion  12   a  and never makes strong contact with the inner surface (surface of the inner surface area) of the negative electrode can  12 , the central-side protruding portion Ga of the gasket G does not serve as a prop for the negative electrode can  12 . Accordingly, when the battery  10  is caulked and sealed, the folded bottom  12   b  of the negative electrode can  12  strongly presses against the bottom flat part Gb of the gasket G, thereby preventing the negative electrode mixture  15  from biting between the negative electrode can  12  and the gasket G. 
     Examples of positive electrode active materials usable for the positive electrode mixture  13  used in the invention include, but are not limited to, silver oxide, manganese dioxide, a nickel-silver composite oxide, and nickel oxyhydroxide. 
     EXAMPLE 1  
     As Example 1, an SR626SW battery having the structure shown in  FIG. 1  was produced. A three-layer clad material having a thickness W of 0.2 mm and comprising a nickel layers S 1 , a stainless steel layer S 2  of SUS304, and a collector layer S 3  of copper was pressed to give a negative electrode can  12  having a folded portion  12   a  and a folded bottom  12   b.  Then, as shown in  FIG. 3A , a mixed aqueous solution  21  of sulfuric acid and hydrogen peroxide was applied dropwise onto a collecting inner surface  12   c  of the collector layer S 3  of the negative electrode can  12  to chemically polish only the collecting inner surface  12   c  area, followed by washing with water. 
     Subsequently, as shown in  FIG. 3B , a polyaniline-based conductive polymer solution  22  was applied dropwise to perform surface treatment. The conductive polymer solution  22  is then collected, followed by washing with water and drying. 
     Subsequently, as shown in  FIG. 3C , an electroless tin plating liquid  23  was applied dropwise. The electroless tin plating liquids  23  was then collected, followed by washing with water and drying to thereby form, in the collecting inner surface  12   c  area of the negative electrode can  12 , a dense tin coating layer S 4  having a thickness of 0.07 μm and having a large crystal structure. A negative electrode can  12  was thus obtained. 
     Meanwhile, an alkaline electrolyte comprising an aqueous sodium hydroxide solution and an aqueous potassium hydroxide solution was injected thereinto. Next, a disc-shaped pellet of a positive electrode mixture  13  was placed into the positive electrode can  11 , so as to impregnate the positive electrode mixture  13  with the alkaline electrolyte. 
     On the pellet of the positive electrode mixture  13  was placed a three-layer separator  14  comprising a nonwoven fabric, cellophane, and a polyethylene graft polymerization film punched into a circular shape. An alkaline electrolyte comprising an aqueous sodium hydroxide solution and an aqueous potassium hydroxide solution was applied dropwise onto the separator  14  for impregnation. 
     On the separator  14  is placed a gel negative electrode mixture  15  comprising a mercury-free, aluminum-, indium-, and bismuth-containing zinc alloy powder, zinc oxide, a thickener, an aqueous sodium hydroxide solution, and an aqueous potassium hydroxide solution. A ring-shaped gasket G made of nylon formed from 66 nylon coated with an asphalt+epoxy sealant was inserted into the open edge of the positive electrode can  11  to caulk the open edge of the positive electrode can  11 , thereby hermetically sealing the negative electrode can  12  over the negative electrode mixture  15 . An alkaline battery was thus obtained. 
     In this case, the width of a space D between the inner peripheral surface of the folded portion  12   a  of the negative electrode can  12  and the central-side protruding portion Ga of the gasket G was set at 0.05 mm, which is ¼ the thickness W of the negative electrode can  12 , and the length L 1  of the protruding portion Ga of the gasket G was set at ⅔ the length L 2  of the folded portion  12   a  of the negative electrode can  12 . 
     EXAMPLE 2  
     The structure in Example 2 is the same as in Example 1, except that the width of the space D between the inner peripheral surface of the folded portion  12   a  of the negative electrode can  12  and the central-side protruding portion Ga of the gasket G was set at 0.10 mm, and the length L 1  of the protruding portion Ga was set at ⅔ the length L 2  of the folded portion  12   a  of the negative electrode can  12 . 
     EXAMPLE 3  
     The structure in Example 3 is the same as in Example 1, except that the width of the space D between the inner peripheral surface of the folded portion  12   a  of the negative electrode can  12  and the central-side protruding portion Ga of the gasket G was set at 0.05 mm, and the length L 1  of the protruding portion Ga was set at ½ the length L 2  of the folded portion  12   a  of the negative electrode can  12 . 
     EXAMPLE 4  
     The structure in Example 4 is the same as in Example 1, except that the thickness of the tin coating layer S 4  was set at  0 .03 μm. 
     EXAMPLE 5  
     The structure in Example 5 is the same as in Example 1, except that the thickness of the tin coating layer S 4  was set at 0.1 μm. 
     COMPARATIVE EXAMPLE 1  
     The structure in Comparative Example 1 is the same as in Example 1, except that the thickness of the tin coating layer S 4  was set at 0.01 μm. 
     COMPARATIVE EXAMPLE 2  
     The structure in Comparative Example 2 is the same as in Example 1, except that the step of chemical polishing with a mixed aqueous solution of sulfuric acid and hydrogen peroxide was omitted. 
     COMPARATIVE EXAMPLE 3  
     The structure in Comparative Example 3 is the same as in Example 1, except that the step of chemical polishing with a mixed aqueous solution of sulfuric acid and hydrogen peroxide and the step of treatment with a polyaniline-based conductive polymer solution were omitted. 
     COMPARATIVE EXAMPLE 4  
     The structure in Comparative Example 4 is the same as in Example 1, except that the width of the space D between the inner peripheral surface of the folded portion  12   a  of the negative electrode can  12  and the central-side protruding portion Ga of the gasket G was set at 0.25 mm, and the length L 1  of the protruding portion Ga was set at ⅔ the length L 2  of the folded portion  12   a  of the negative electrode can  12 . 
     COMPARATIVE EXAMPLE 5  
     The structure in Comparative Example 5 is the same as in Example 1, except that the width of the space D between the inner peripheral surface of the folded portion  12   a  of the negative electrode can  12  and the central-side protruding portion Ga of the gasket G was set at 0.05 mm, and the length L 1  of the protruding portion Ga was set at ⅓ the length L 2  of the folded portion  12   a  of the negative electrode can  12 . 
     Two hundred batteries were produced for each of the above Examples 1 to 5 and Comparative Examples 1 to 5. One hundred of the batteries were stored in a severe environment where the temperature was 45° C. and the relative humidity was 93%, and the incidence of leakage 100 days later was evaluated. The results of evaluation are shown in the table of  FIG. 5 . One hundred of the batteries were stored in an environment where the temperature was 60° C. and the relative humidity was 0% for 100 days. The batteries were discharged at a constant resistance of 30 kΩ to a final voltage of 1.2 V. The thus-obtained discharge capacity [mAh] is shown in the table of  FIG. 5 . The initial discharge capacity of these batteries was around 28 mAh. 
     (1) First, Examples 4 and 5 and Comparative Example 1 are compared from the table of  FIG. 5 . It is found that when the thickness of the tin coating layer S 4  is 0.03 to 0.1 μm, the capacity retention can be improved. 
     This is because a tin coating layer S 4  thickness of 0.03 μm or more leads to, as combined with the effects of chemical polishing with a mixed acid or the like and surface treatment with a conductive polymer, the formation of a uniform tin coating layer S 4 , thus preventing defects such as pinholes, cracks, etc. The reason that the thickness of the tin coating layer was set at no more than 0.1 μm is that although it accordingly requires more time for the formation of the tin coating layer S 4 , thickening of the tin coating layer S 4  provides no particular advantages. 
     (2) Next, Example 1 and Comparative Examples 2 and 3 are compared from the table. It is found that chemical polishing with a mixed aqueous solution of sulfuric acid and hydrogen peroxide and treatment with a polyaniline-based conductive polymer solution improve the capacity retention. 
     This is possibly because that the chemical polishing with a mixed acid prior to the electroless tin plating and the surface treatment of the plating surface with polyaniline or a like conductive polymer material resulted in the formation of a dense tin coating layer S 4  without cracks or pinholes. 
     (3) Next, Examples 1 and 2 and Comparative Example 4 are compared from the table. It is found that when a space D between the inner peripheral surface of the folded portion  12   a  of the negative electrode can  12  and the central-side protruding portion Ga of the gasket G was no wider than the thickness W of the negative electrode can  12 , the leak-proof properties can be improved. This is possibly because when the battery  10  is caulked and sealed, because the space D between the negative electrode can  12  and the gasket G is narrow, the negative electrode mixture  15  can be prevented from biting between the negative electrode can  12  and the gasket G. 
     (4) Finally, Example 3 and Comparative Example 5 are compared from the table. It is found that when the length L 1  of the central-side protruding portion Ga of the gasket G is at least ½ the length L 2  of the folded portion  12   a  of the negative electrode can  12 , the leak-proof properties can be improved. This is also possibly because that when the battery  10  is caulked and sealed, because the length L 1  of the central-side protruding portion Ga of the gasket G is as long as ½ or more the length L 2  of the folded portion  12   a  of the negative electrode can  12 , the negative electrode mixture  15  can be prevented from biting between the negative electrode can  12  and the gasket G. 
     Moreover, when the central-side protruding portion Ga of the gasket G is designed so that even its maximum length never makes strong contact with the inner surface of the negative electrode can  12 , the central-side protruding portion Ga of the gasket G does not serve as a prop for the negative electrode can  12 . Therefore, when the battery  10  is caulked and sealed, the folded bottom  12   b  of the negative electrode can  12  strongly presses against the bottom flat part Gb of the gasket G, thereby possibly preventing the negative electrode mixture  15  from biting between the negative electrode can  12  and the gasket G. 
     Next, the advantages of the above-constructed embodiment are explained. 
     (1) According to this embodiment, first, before the coating layer S 4  of tin having a higher hydrogen overpotential than copper was formed in the collector layer S 3  of copper of the negative electrode can  12 , the collector layer S 3  of the negative electrode can  12  was chemically polished with a mixed aqueous solution  21  of sulfuric acid and hydrogen peroxide, thereby eliminating foreign substances adhering to the collector layer S 3 , small cracks, etc. 
     Accordingly, foreign substances adhering to the surface of the collector layer S 3 , cracks, and the like created upon the production of the negative electrode can  12  by pressing, which hinder the formation of a uniform and dense tin coating layer S 4 , can be sufficiently eliminated. 
     (2) According to this embodiment, before the tin coating layer S 4  was formed and after the collector layer S 3  of the negative electrode can  12  was chemically polished, surface treatment was performed with a polyaniline-based conductive polymer so that only monovalent copper ions exist on the surface of the collector layer S 3  of the negative electrode can  12 . 
     It accordingly is possible to prevent divalent copper ions, which hinder the formation of a uniform and dense tin coating layer S 4 , from being present at random with monovalent copper ions on the surface of the collector layer S 3  of copper of the negative electrode can  12 . 
     (3) According to this embodiment, the tin coating layer S 4  was formed after the collector layer S 3  of copper of the negative electrode can  12  was chemically polished and surface-treated. Accordingly, the tin coating layer S 4  can be formed in the negative electrode can  12  from an electroless tin plating liquid  23  as a tin coating layer S 4  having a uniform and precise thickness without pinholes, cracks, or like defects. As a result, the collector layer is prevented from being exposed from the coating layer and generating hydrogen gas. 
     Further, because the tin coating layer S 4  was formed to have a thickness of 0.03 to 0.1 μm, it is possible to form a tin coating layer S 4  having a uniform and precise thickness without pinholes within a short period of time. 
     (4) According to this embodiment, the width of the space D between the inner peripheral surface of the folded portion  12   a  of the negative electrode can  12  and the central-side protruding portion Ga of the gasket G was set at no more than the thickness W of the negative electrode can  12 . 
     Therefore, when the battery  10  is caulked and sealed, because the space D between the negative electrode can  12  and the gasket G is narrow, the negative electrode mixture  15  can be prevented from biting between the negative electrode can  12  and the gasket G, and the leak-proof properties can be improved. 
     (5) According to this embodiment, the length L 1  of the central-side protruding portion Ga of the gasket G was set at less than, but at least ½, the length L 2  of the folded portion  12   a  of the negative electrode can  12 . 
     Therefore, because the length L 1  of the central-side protruding portion Ga of the gasket G is as long as ½ or more the length L 2  of the folded portion  12   a  of the negative electrode can  12 , when the battery  10  is caulked and sealed, the negative electrode mixture  15  can be prevented from biting between the negative electrode can  12  and the gasket G. 
     Moreover, because the central-side protruding portion Ga of the gasket G is designed so that even its maximum length is shorter than the length L 2  of the folded portion  12   a  and never makes strong contact with the inner surface of the negative electrode can  12 , the central-side protruding portion Ga of the gasket G does not serve as a prop for the negative electrode can  12 . As a result, when a battery  10  is caulked and sealed, the folded bottom  12   b  of the negative electrode can  12  strongly presses against the bottom flat part Gb of the gasket G, thereby preventing the negative electrode mixture  15  from biting between the negative electrode can  12  and the gasket G. 
     The above embodiment may be modified as follows. 
     The coating layer of the negative electrode can  12  may be made of not only tin but also of, as a metal or alloy having a higher hydrogen overpotential than copper, one or more metals such as indium (melting point: 156.6° C.) and bismuth (melting point: 271.4° C.) or alloys. 
     This also enables the formation of a coating layer in the negative electrode can  12 , which does not have defects due to pinholes, crack, impurities, and the like. Accordingly, the generation of hydrogen gas (H 2 ) due to contact between zinc, which is a negative electrode active material, and the collector layer S 3  of the negative electrode can  12  can be suppressed, thereby suppressing the corrosion of the zinc. At the same time, resistance to leakage due to the creep phenomenon of an alkaline electrolyte can be improved. According to the invention, an excellent alkaline battery can be obtained without using mercury. 
     The invention is not limited to the above examples, and naturally, various other structures are also possible without deviating from the scope of the invention.