Title of Invention

A PASTE-TYPE MANGANESE DRY BATTERY

Abstract

A manganese dry battery having excellent discharge characteristics and excellent storage characteristics is provided by making it possible to maintain the corrosion resistance of an anode can over an extended period of time from its initial stage. In the manganese dry battery, an electrolyte contained in a paste layer contains ammonium chloride in an amount not smaller than 10% by weight, and the paste layer contains bismuth in an amount equivalent to 0.0005 to 0.5% by weight of the electrolyte contained in the paste layer.

Full Text

DESCRIPTION MANGANESE DRY BATTERY TECHNICAL FIELD [0001] The present invention relates to a mercury-free manganese dry battery, and more particularly to improvements in discharge characteristics and storage characteristics of a paste type manganese dry battery. BACKGROUND ART [0002] Conventionally, in a paste type manganese dry battery, a mercury compound such as mercuric chloride is added in a paste layer in an amount of 0.01 to 0.1% by weight in order to suppress self discharge. By adding such a mercury compound, corrosion of zinc can be prevented since zinc surface is amalgamated and held in a high hydrogen overvoltage condition. [0003] However, with ever increasing awareness of global environmental pollution problems, there has also developed a need to eliminate the use of mercury in paste type manganese dry batteries. As a means for improving the corrosion resistance of an anode zinc can without using mercury, it has been proposed to add indium chloride into the paste layer (refer, for example, to Japanese Unexamined Patent Publication No. H06-163019). [0004] By the way, to improve the corrosion resistance of the anode can, a zinc alloy containing 0.4% by weight of lead is usually used for the anode can. Here, if the content of lead in the anode can is reduced to less then 0.4% by weight in consideration of environmental reasons, a larger amount of indium chloride has to be added in order to maintain the corrosion resistance of the anode can. As the result, the surface of the anode can is covered with an indium coating, the internal resistance increases, and the discharge characteristics degrade. Furthermore, the cost increases because of the increased amount of the indium chloride used. DISCLOSURE OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION [0005] In view of the above problem associated with the prior art, it is an object of the present invention to provide a manganese dry battery having excellent discharge characteristics and excellent storage characteristics by making it possible, without using mercury or lead, to maintain the corrosion resistance of the anode can over an extended period of time from its initial stage. MEANS FOR SOLVING THE PROBLEM [0006] A manganese dry battery according to the present invention comprises: a cathode mixture containing manganese dioxide; an anode can containing zinc; and a paste layer, containing starch, a water soluble paste material, and an electrolyte, for separating the cathode mixture and the anode can, wherein the electrolyte in the paste layer contains ammonium chloride in an amount not smaller than 10% by weight, and the paste layer contains bismuth in an amount equivalent to 0.0005 to 0.5% by weight of the electrolyte contained in the paste layer. [0007] Preferably, the bismuth is contained in the form of a bismuth compound. Preferably, the bismuth compound is at least one selected from the group consisting of a bismuth chloride and a bismuth oxide. Preferably, the paste layer further contains an indium chloride containing indium in an amount equivalent to 0.001 to 0.5% by weight of the electrolyte contained in the paste layer. Preferably, the anode can is made of zinc or a zinc alloy that does not contain lead. EFFECT OF THE INVENTION [0008] According to the present invention, a manganese dry battery having excellent discharge characteristics and excellent storage characteristics can be provided by making it possible, without using mercury or lead, to maintain the corrosion resistance of the anode can over an extended period of time from its initial stage. BRIEF DESCRIPTION OF THE DRAWINGS [0009] FIG. 1 is a front view, partly in cross section, of a manganese dry battery according to an example of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION [0010] The present invention is characterized in that the paste layer provided as a separator for separating the anode and cathode in a manganese dry battery contains bismuth. The paste layer comprises starch, a water soluble paste material, and an electrolyte. The electrolyte is an aqueous solution of zinc chloride and ammonium chloride. The bismuth is added in the paste layer in the form of a bismuth compound. For the water soluble paste material, use is made, for example, of polyethylene glycol-based alkyl phenol. It is considered that the bismuth compound reacts with ammonium ions in the electrolyte and exists in the form of an ammonium salt of bismuth in the paste layer. The ammonium salt is quickly precipitated as metal bismuth by electron exchange reaction with metal zinc on the zinc surface, and the hydrogen overvoltage on the zinc surface can thus be increased. In this way, the bismuth compound, like mercury, has the effect of deterring the corrosion of the zinc anode can and suppressing self discharge. [0011] The electrolyte in the paste layer contains ammonium chloride in an amount not smaller than 10% by weight, and the paste layer contains bismuth in an amount equivalent to 0.0005 to 0.5% by weight of the electrolyte contained in the paste layer. In this case, excellent discharge performance can be obtained; in particular, since excellent discharge performance can be retained even after storage for an extended period of time, the storage characteristics improve. If the bismuth content exceeds 0.5% by weight, the internal resistance increases and the discharge performance drops. On the other hand, if the bismuth content is less than 0.0005% by weight, a sufficient effect to deter the corrosion of the anode can cannot be obtained. [0012] Preferably, the bismuth compound is at least one selected from the group consisting of a bismuth chloride and a bismuth oxide. BiCl3 can be given as an example of the bismuth chloride. Bi203 can be given as an example of the bismuth oxide. Further, if the concentration of ammonium chloride in the electrolyte is less than 10% by weight, since the bismuth becomes difficult to dissolve in the electrolyte, the amount of bismuth precipitation on the anode can decreases, reducing the effect of the bismuth added to improve the corrosion resistance of the anode can. [0013] Preferably, the paste layer further contains an indium chloride containing indium in an amount equivalent to 0.001 to 0.5% by weight of the electrolyte contained in the paste layer. When only the bismuth compound is added in the paste layer, the variation in voltage among products tends to more or less increase compared with the case where mercury is added, but when an indium chloride is added in addition to the bismuth compound, since the surface of the anode can is uniformly covered with a layer comprising indium and bismuth, the variation in voltage can be reduced to substantially the same level as when mercury is added. [0014] It is speculated that the addition of indium works to suppress the variation in voltage in the following mechanism. When an ammonium salt of bismuth exists in the electrolyte, metal bismuth is preferentially precipitated on the surface of the zinc anode can. Metal bismuth has poor ductility and low electron conductivity. However, it is believed that, when both bismuth and indium are present, metal indium having high ductility and high electron conductivity is precipitated together with the bismuth, and the surface of the zinc anode can is uniformly covered with these both metals to give stable corrosion resistance is thus obtained. [0015] Examples of indium chlorides include InCl, InCl3, In2Cl3, In4Cl7, and In5Cl9. With other halides, a similar effect can be obtained, examples of such halides including InBr3, InF3, and Inl3. Conventionally, a zinc alloy containing 0.4% by weight of lead is used for the anode can in order to improve the corrosion resistance of the anode can. However, even when the content of lead is reduced or lead is not added at all in consideration of environmental protection, an anode can having excellent corrosion resistance can be obtained by using the above-described paste layer, and thus a paste type manganese dry battery having excellent discharge characteristics and excellent storage characteristics can be achieved. That is, according to the present invention, a paste type manganese dry battery can be provided that does not use mercury or lead and is thus friendly to the environment. EXAMPLE [0016] Examples of the present invention are described in detail in the following. > The following method was employed to examine whether bismuth was dissolved in the electrolyte solution. The electrolyte was prepared by mixing zinc chloride, ammonium chloride, and water in proportions shown in Table 1. Bismuth in the form of BiCl3 or Bi203 was added in the electrolyte in an amount of 0.1% by weight relative to the weight of a zinc plate, followed by stirring. Then, the zinc plate was immersed into the electrolyte solution. After leaving it in the electrolyte for one hour, the amount of bismuth precipitated on the surface of the zinc plate was measured. The results are shown in Table 1. [0018] It was found that, in the case of electrolytes containing ammonium chloride in an amount not smaller than 10% by weight, the amount of bismuth precipitation was not smaller than 0,06% by weight when the bismuth was BiCl3, and not smaller than 0.04% by weight when the bismuth was Bi203. On the other hand, when the concentration of ammonium chloride was less than 10% by weight, the amount of bismuth precipitation was not larger than 0.01% by weight. It was shown that when the concentration of ammonium chloride was less than 10% by weight, the bismuth was difficult to dissolve in the electrolyte solution and was therefore difficult to precipitate on the zinc plate. [0019] The bismuth was nearly completely precipitated on the zinc plate after 24 hours in EXAMPLES 1 to 3 and after 96 hours in EXAMPLE 4. This led to the discovery of the fact that when an electrolyte in which the concentration of ammonium chloride is not smaller than 10% by weight is used, the bismuth is fully dissolved in the electrolyte and, as a result, the bismuth can be nearly completely precipitated on the zinc plate and can thus be utilized effectively. [0020] > (1) Preparation of a paste for forming the paste layer An electrolyte was prepared by mixing zinc chloride, ammonium chloride, and water in a ratio of 10:20:70 by weight. Then, the electrolyte, starch powder, and a water soluble paste material were mixed in a ratio of 75:24:1 by weight, and a paste for forming the paste layer was prepared by adding compounds in the resulting mixture in the amounts shown in Table 2 relative to the amount of the electrolyte. In Table 2, the amounts of InCl3, BiCl3 and Bi203 added are each shown by converting the amount to the amount of indium or bismuth. (2) Fabrication of the paste type manganese dry battery The paste type manganese dry battery was fabricated in accordance with the following procedure- A front view of the paste type manganese dry battery of the present invention is shown partly in cross section in FIG. 1. A zinc alloy containing 0.4% by weight of lead was molded in shape of a cylinder having a closed bottom, to construct an anode can 3. After placing bottom paper 5 on the bottom inside surface of the anode can 3, a paste for forming the paste layer was filled into the anode can 3. Then, a cathode mixture 2 in which a carbon rod 1 formed by sintering carbon powder was inserted was placed inside the anode can 3, resulting in the structure in which the paste layer 4 was sandwiched between the cathode mixture 2 and the anode can 3. The cathode mixture 2 used here was prepared by mixing manganese dioxide as an active material, acetylene black as an electrically conductive material, and an aqueous solution of 30% by weight of zinc chloride as an electrolyte, in a ratio of 50:10:40 by weight. [0023] An aperture for inserting the carbon rod 1 therethrough was formed in the center of a seal member 9 formed from a polyolefin-based resin. The carbon rod 1 was inserted under pressure through the aperture of the seal member 9, thereby causing the periphery of the seal member 9 to fix into the edge of the open end of the anode can 3. Then, a cathode terminal 6 was fixed onto the top of the carbon rod 1, i.e., the cathode collector. A resin tube 11 made of a heat shrinkable resin film for providing insulation was placed around the outer circumference of the anode can 3, with the upper end portion of the film covering the upper surface of the outer circumferential portion of the seal member 9 and the lower end portion covering the lower surface of a seal ring 13. A seal plate 7 was placed to cover the seal member 9. [0024] The cathode terminal 6 made of a tin plate was formed into a shape having a cap-like center portion, which covers the upper end portion of the carbon rod 1, and a flat plate-like brim portion. An insulating ring 8 made of a resin was provided on the brim portion of the cathode terminal 6 to isolate the cathode terminal 6 from the seal plate 7. A bottom plate 10 which also serves as the anode terminal was provided on the bottom surface of the anode can 3, and the seal ring 13 was mounted around the outer surface of the flat plate-like outer peripheral portion of the bottom plate 10. An outer jacket 12 constructed from a cylindrically shaped tin plate was placed around the outer circumference of the resin tube 11. The lower end portion of the outer jacket 12 was bent inwardly while the upper end portion thereof was curled inwardly. Further, the tip of the upper end portion crimped onto the outer peripheral edge of the seal plate 7 to obtain an manganese dry battery. [0025] [Evaluation] (1) Evaluation of initial characteristics Immediately after fabrication, each battery was discharged under a load of 2.2 Ω, and the discharge time was measured. The terminal voltage was set to 0.8 V. Five samples were tested. and the average value was taken as the discharge time of the battery. If the discharge time was 125 minutes or longer, the battery was judged to have good discharge characteristics. [0026] (2) Evaluation of storage characteristics After measuring the battery voltage of the battery immediately after fabrication, the battery was stored at room temperature for 12 months. After 12 months of storage, the battery voltage of the battery was measured again, to check how much the battery voltage had dropped during storage from the initial voltage. One hundred samples were tested, and the average value was taken as the battery voltage drop for the battery. If the amount of battery voltage drop was 45 mV or less, the battery was judged to have good storage characteristics. Further, the variation in battery voltage among different samples was obtained for the battery after storage. One hundred samples were tested, and the difference between the highest battery voltage and the lowest battery voltage was obtained as the variation. The results are shown in Table 3. [0028] (A) Characteristics when the bismuth compound was added in the paste layer. In COMPARATIVE EXAMPLE 5 which improved the corrosion resistance of the anode can over COMPARATIVE EXAMPLE 4, the amount of battery voltage drop during storage decreased, and the variation in battery voltage after storage also decreased. In EXAMPLES 5 to 13 in which the bismuth compound was added in the paste layer, the corrosion resistance of the anode can improved over COMPARATIVE EXAMPLE 5, and the amount of battery voltage drop during storage further decreased. Further, in EXAMPLES 5 to 13, excellent initial characteristics were obtained compared with COMPARATIVE EXAMPLE 5. In COMPARATIVE EXAMPLE 7, since the bismuth content was high, the internal resistance increased and the discharge characteristics degraded. In COMPARATIVE EXAMPLE 6, since the bismuth content was low, the corrosion resistance of the anode can was insufficient, and the amount of battery voltage drop during storage therefore increased. [0029] (B) Characteristics when the indium chloride was added in the paste layer containing the bismuth compound. In EXAMPLES 14 to 23 in which the indium chloride was added in the paste layer containing the bismuth compound, the corrosion resistance of the anode can improved, and the amount of battery voltage drop during storage decreased. In EXAMPLE 14, since the amount of the indium chloride added was small, the effect of suppressing the variation in battery voltage after storage was small. In EXAMPLE 21, the variation in battery voltage after storage was suppressed but, since the amount of the indium chloride added was large, the internal resistance increased, resulting in a degradation of the discharge characteristics. In EXAMPLES 15 to 20, 22 and 23, good initial discharge characteristics and good storage characteristics were achieved, and at the same time, the variation in battery voltage after storage was suppressed. [0030] > Paste type manganese dry batteries were fabricated using the same method as EXAMPLE 5, except that the content of lead in the anode can and the amounts of the bismuth chloride and the indium chloride added in the paste layer were varied as shown in Table 4, and their initial characteristics and storage characteristics were evaluated. In Table 4, the amounts of InCl3 and BiCl3 added are each shown by converting the amount to the amount of indium or bismuth. The results are shown in Table 5. not sufficient and, as a result, the battery voltage greatly dropped during storage. In COMPARATIVE EXAMPLE 9, since the bismuth content was high, the internal resistance increased and the initial discharge characteristics therefore degraded. On the other hand, in EXAMPLES 22 to 24 in which the content of lead in the anode can was reduced, and in EXAMPLES 27 to 32 in which lead was not contained, good initial discharge characteristics and good storage characteristics were obtained by adding a small amount of bismuth. [0034] (D) Characteristics when the content of lead in the anode can was reduced and the indium chloride was added in the paste layer containing the bismuth compound. In EXAMPLES 34 to 38 in which lead was not contained, good initial discharge characteristics and good storage characteristics were obtained by adding a small amount of bismuth, and at the same time, the variation in battery voltage after storage was suppressed because of the addition of the indium chloride. [0035] In EXAMPLE 33, since the amount of the indium chloride added was small, the effect of suppressing the variation in battery voltage after storage was small. In EXAMPLE 39, the variation in battery voltage after storage was suppressed but, since the amount of the indium chloride added was large, the internal resistance increased, resulting in a degradation of the discharge characteristics. INDUSTRIAL APPLICABILITY [0036] As described above, the paste type manganese dry battery of the present invention has excellent discharge characteristics and excellent storage characteristics and therefore can be applied as a power supply for a high performance small electronic apparatus, a portable apparatus, or the like. CLAIMS 1. A manganese dry battery comprising: a cathode mixture including manganese dioxide; an anode can including zinc; and a paste layer including starch, a water soluble paste material, and an electrolyte for separating said cathode mixture and said anode can, wherein said electrolyte in said paste layer contains ammonium chloride in an amount not smaller than 10% by weight, and said paste layer contains bismuth in an amount equivalent to 0,0005 to 0.5% by weight of said electrolyte contained in said paste layer. 2. The manganese dry battery in accordance with claim 1, wherein said bismuth is contained in the form of a bismuth compound. 3. The manganese dry battery in accordance with claim 2, wherein said bismuth compound is at least one selected from the group consisting of a bismuth chloride and a bismuth oxide. 4. The manganese dry battery in accordance with claim 1, wherein said paste layer further contains an indium chloride comprising indium in an amount equivalent to 0.001 to 0.5% by weight of said electrolyte contained in said paste layer. 5. The manganese dry battery in accordance with claim 1, wherein said anode can is formed from zinc or a zinc alloy that does not contain lead.

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