Cylindrical alkaline batteries

Abstract: The present invention provides a cylindrical alkaline battery including a reinforcing washer provided between the bottom plate being the negative electrode terminal and the gasket. In this battery, electric insulation is established between the bottom plate being the negative electrode terminal and the washer or between the gasket and the washer and the presence of the electric insulation cuts off the possible electric reaction route forming a local cell and prevents the resultant leakage of the electrolytic solution.

Description: The present invention relates to a cylindrical alkaline battery comprising an uncovered battery body and a heat-shrinkable cover member such as a shrink label, a shrink tube or the like. More particularly, the present invention relates to a cylindrical alkaline battery comprising an uncovered battery body and a heat-shrinkable cover member, wherein short circuits caused by the contact between the positive electrode case and a bottom plate being the negative electrode terminal via a third electroconductive material are prevented, and which further possesses improved leakage resistance. In alkaline batteries, in order that the cover can have a simplified structure and thereby allow the metallic battery case accommodating an electricity-generating element to have a larger diameter, the metallic battery case accommodating an electricity-generating element and functioning as the positive electrode terminal has been covered and insulated with a shrink label or tube having metal deposition on one side. This arrangement, when compared with a metallic battery case covered with a metallic can, enables the accommodation of a larger amount of an electricity-generating element inside the battery case, and provides a larger battery capacity. In the label-covered batteries, the shrink label is adhered to the outer surface of the battery case using an adhesive, and the projected portions of the label at the top and bottom ends of the case are subjected to shrinkage. However, in some cases, the label may peel at its ends due to deterioration of the adhesive or friction applied during transfer or storage of the battery. For the tube-covered batteries wherein a battery case is inserted into a cylindrical shrink tube and the tube is heat-shrunk, there occurs at times during high temperature storage of the battery further heat shrinkage of the upper and lower ends of the shrunk tube leading to an increase in the extent of exposure of the top and bottom ends of the battery case and reduced strength of the cover. In these label- or tube-covered batteries, besides the above-mentioned peeling or abnormal shrinkage of the label or tube in the vicinity of the opening of the battery case, there tends to occur damage (e.g. peeling, cutting) to the label or tube by contacting with terminals of an electric apparatus using such a battery, at the time of insertion or removal of the battery into or from the apparatus. Therefore, there may be a tendency for exterior short circuits to occur between the caulked portion of the positive electrode case and the bottom plate being the negative electrode terminal which are both metallic and are located adjacent each other, when the battery is inserted into an electric apparatus or is placed on a metal plate with the positive electrode terminal directed upward, whereby rupturing of the battery or leakage is induced. Generally, in alkaline batteries covered with a heat-shrinkable member, it is necessary to provide an exhaust port on the bottom plate being the negative electrode terminal to enable the discharge of gas generated inside the battery with a tiny explosion-proof mechanism having a thin nylon gasket breakable under increase in the internal pressure of the battery. In such alkaline batteries, when the sealing strength of the sealing gasket is weak, leakage of the electrolytic solution tends to occur along the sealing gasket during storage of the battery, because the caustic potash electrolytic solution used in alkaline batteries has a high creeping property. Hence, in order to obtain an alkaline battery of increased leakage resistance, there is generally adopted such a sealing structure as providing a metallic washer between the gasket and the bottom plate being the negative electrode terminal to increase sealing strength for the sealing gasket. This is the case with the construction of an alkaline battery described in Patent Abstracts of Japan, vol. 9, No. 150, (E-324) (1873) which also includes a sealing agent film coating the underside and peripheral edges of the bottom plate. The sealing film also forms a seal over the exhaust port on the bottom plate, although this is broken when the internal gas pressure of the battery increases to an abnormal level. In an alkaline battery having such a sealing structure, however, potassium ion (K⁺) permeates the nylon gasket according to the mechanism described later when the battery is stored under highly humid conditions (e.g. 90% relative humidity and 60°C) ; therefore, severe leakage occurs when the battery is stored under highly humid conditions over a long period of time such as one month or longer. The reason for this is that when the above alkaline battery is stored under highly humid conditions, moisture in the atmosphere penetrates into the battery through the exhaust port on the bottom plate being the negative electrode terminal, reaches the space between the metallic washer and the nylon gasket, and causes dew condensation and as the result, the interface between the metallic washer and the nylon gasket is moistened by water and the gasket turns to a hydrous nylon, thereby forming an electrochemical reaction route. The reaction mechanism is as follows. On the surface of the zinc present in the aqueous caustic potash solution, there occurs a zinc dissolution reaction represented by the following formula (1). Zn + 4K + + 4OH - → 2K + + Zn(OH) 4 -- + 2K + + 2e - On the water-moistened surface of the iron constituting the metallic washer which is electrically connected to the zinc via the bottom plate being the negative electrode terminal, there occurs a hydrogen formation reaction represented by the following formula (2). 2H + + 2OH - + 2e - → H 2 + 2OH - Concurrently with the above reactions, the hydrous nylon acts as a diaphragm through which K⁺ migrates, and water present on the iron surface is gradually converted into an aqueous caustic potash solution (K⁺ + OH⁻), to finally turn the solution highly alkaline. The thus formed alkaline solution leaks out through the exhaust port on the bottom plate being the negative electrode terminal, thus causing severe leakage. As mentioned above, conventional alkaline batteries covered with a heat-shrinkable member have various problems to be solved with respect to the reliability of cover, safety of battery and leakage. It is an object of the present invention to provide means for cutting off the above-mentioned electrochemical reaction route resulting in the increased leakage resistance of cylindrical alkaline batteries. Means to achieve this object in the construction of a cylindrical alkaline battery which has a washer between the nylon gasket sealing the opening of the positive electrode case and the bottom plate being the negative electrode terminal, comprises (a) employing the bottom plate being the negative electrode terminal treated to be electrically insulating on at least one side which contacts with the metallic washer, or (b) employing the metallic washer treated to be electrically insulating on at least one side contacting with the bottom plate being the negative electrode terminal, or (c) employing the metallic washer treated to be electrically insulating on at least one side contacting with the nylon gasket, or (d) employing an electric insulating washer, or (e) employing the nylon washer treated to be electrically insulating on at least one side contacting with the metallic washer, or (f) installing an insulator between the bottom plate being the negative electrode terminal and the metallic washer, or (g) installing an insulator between the metallic washer and the nylon gasket. According to the present invention there is provided a cylindrical alkaline battery comprising a positive electrode case of a cylindrical form having an opening at one end and accommodating an electricity-generating element consisting of a positive electrode, a negative electrode, a separator and an alkaline electrolytic solution, a gasket made of an electrically insulating synthetic resin and installed within the opening of the positive electrode case, a bottom plate being the negative electrode terminal electrically connected to the negative electrode in the electricity-generating element and installed on the gasket, a reinforcing metallic washer provided between the bottom plate being the negative electrode terminal and the gasket, and a heat-shrinkable resin member covering the outer surface of the positive electrode case, wherein the opening of the positive electrode case is sealed by bending the open end inwardly and thereby tightening the bottom plate being the negative electrode terminal and the washer against the gasket, characterised in that electrical insulation is established between the bottom plate being the negative electrode terminal and the washer or, electrical insulation and ion-impermeability are established between the gasket and the washer thereby stopping leakage of electrolyte by preventing an electrochemical reaction which causes such leakage from occurring between the bottom plate and the washer or between the washer and the gasket. Fig. 1 is a view of an embodiment of a cylindrical alkaline battery (partial sectional view of the left half); Fig. 2 is an enlarged sectional view showing the sealed portion of the battery in Fig. 1 but including an embodiment according to the present invention; Fig. 3 is a partial sectional view of another embodiment of a cylindrical alkaline battery; Fig. 4 is a partial sectional view of a further embodiment of a cylindrical alkaline battery; Fig. 5 is a partial sectional view of a still further embodiment of a cylindrical alkaline battery; Fig. 6 is a drawing shown schematically an electrochemical reaction for the embodiment of the present invention; Fig. 7 is an enlarged sectional view showing the sealed portion of a cylindrical alkaline battery for another embodiment of the present invention; and Fig. 8 is a drawing showing schematically an electrochemical reaction for the embodiment of the present invention. The embodiments of the present invention are explained in detail referring to the accompanying drawings. In Fig. 1, 1 is a metallic battery case functioning as a positive electrode terminal, which contains a positive electrode mix 2 composed mainly of MnO₂ and moulded into a cylindrical shape, a bag-shaped separator 4 and a gel-like zinc negative electrode 3. Symbol 5 is a negative electrode collector. The bottom plate 6 acting as a negative electrode terminal is spot-welded to the end of the negative electrode collector 5 and has an exhaust port 7. This bottom plate 6 is held by bending the opening 11 of the battery case inwardly and tightening the bottom plate 6 together with a metallic washer 8 provided between the bottom plate 6 and the nylon gasket 10 against a nylon gasket 10. As shown in Fig. 2, at least the open end 11 and the open outer surface of the battery case 1 are covered with a ring-shaped electric insulating film 14. As shown in Fig. 3, the electric insulating film 14 may cover the entire outer surface of the battery case 1 from the bent open portion of the bottom end of the cylinder. Thereon, a shrink label or shrink tube 15 is applied as a cover. In such a structure, since the opening of the case is covered with the electric insulating film 14, good electric insulation can be maintained between the positive electrode case and the metallic bottom plate being the negative electrode terminal even when a portion of the case undergoes peeling or damage due to deterioration during storage or friction, and this can prevent rupture of the positive electrode case or the leakage of electrolytic solution which is induced by an exterior short circuit due to contact of a third electroconductive material with both of the positive electrode case and the bottom plate. Incidentally, the electric insulating film 14 can be formed by, after bending the opening 11 of the battery case 1 and sealing the opening 11 with the gasket, applying a synthetic resin coating (e.g. phenolic resin coating, epoxy resin coating) to the outer surface of the bent portion of the battery case 1 and then drying the coating. Thus, the battery, wherein an electric insulating film 14 is formed on at least the open end and the open outer surface of the positive electrode case and then a heat-shrinkable member is applied as the cover, not only solves problems of conventional alkaline batteries having only a resin film cover but also can prevent an exterior short circuit and provide the increased reliability. Therefore, the battery has a very high value. It is possible to form an electric insulating film by applying and curing, in place of a synthetic resin coating requiring a long drying time, an ultraviolet-curable resin composed mainly of, for example, an epoxy acrylate or an aromatic monoacrylate curable in a short time by ultraviolet irradiation. That is, a synthetic resin coating requires about 30 minutes for drying and formation of film, however, an ultraviolet-curable resin can provide a cured film by ultraviolet irradiation of only about 3 seconds and can give substantially increased work efficiency. As a means to prevent an exterior short circuit, an insulating ring 16 may be fixed onto the outer surface of the cover film 15 at the bent portion of the positive electrode case 1 after the cover film 15 consisting of a shrink label or shrink tube has been applied on the entire outer surface of the sealed positive electrode case 1, as shown in Fig. 4. The presence of this insulating ring 16 can prevent peeling or damage of the cover label or cover tube at the battery bottom, can maintain good electric insulation between the positive electrode case 1 and the metallic bottom plate 6, and can sufficiently prevent exterior short circuit. Incidentally, the insulating ring 16 can be fixed, for example, by ultrasonic-bonding of an insulating ring 16 to the cover film 15 at the bent portion of the positive electrode case 1 or by adhering an insulating ring 16 to the cover film 15 at the bent portion of the positive electrode case 1 with an adhesive of solution type, aqueous dispersion type, reactive type or the like. Alternatively, the insulating ring 16 can be fixed according to a method as shown in Fig. 5 wherein a lobe 6' is provided at the outer surface of the bottom plate 6 being the negative electrode terminal and an insulating ring 16 is fixed below the lobe 6'. Thus, the battery, wherein an insulating ring is fixed onto the outer surface of the cover film at the bent opening portion of the positive electrode case, solves a problem of conventional alkaline batteries having only a resin film cover, prevents exterior short circuit and provides increased reliability. An embodiment of the present invention for preventing the leakage phenomenon caused by the above-mentioned electrochemical reaction is now described referring to the drawings. Fig. 2 is an enlarged sectional view of the sealed portion of a cylindrical alkaline battery of the present invention. In Fig. 2, 17 is an insulating film applied onto the upper surface of the metallic washer 8 and characterises the present embodiment. The constituting elements of this battery excluding the insulating film 17 are same as those of the battery of Fig. 1. This insulating film 17 insulates electrically the metallic washer 8 from the bottom plate 6 being the negative electrode terminal. In the present embodiment, since the bottom plate 6 and the metallic washer 8 are electrically insulated from each other by the insulating film 17 applied onto the metallic washer 8, no leakage occurs, for example, even if moisture reaches the space 13 between the nylon gasket 10 and the metallic washer 8, and the interface between them is wetted by water, when the battery is stored under highly humid condition. The mechanism for this electrochemical reaction is explained in Fig. 6 which illustrates a schematic case of the electrochemical reaction. In Fig. 6, 17' is an insulating film corresponding to the insulating film 17 of Fig. 2. In Fig. 6, since Zn and Fe are insulated electrically, any of the reactions of the formulae (1) and (2) do not proceed; there occurs no movement of K⁺ into the water which wets the iron washer; accordingly, the water does not become alkaline. Thus, according to this embodiment of the present invention, the application of the insulating film 17 onto the upper surface of the metallic washer 8 can prevent leakage even when the battery has been stored under highly humid conditions. Fig. 7 is an enlarged sectional view of the sealed open portion of a cylindrical alkaline battery (drawn in the form of an uncovered battery) embodiment as another embodiment of the present invention. In Fig. 7, 17 is an insulating film applied onto the lower surface and both ends of the metallic washer 8. The constituting elements of this battery excluding the insulating film 17 are same as those of the battery of Fig. 1. This film 17 cuts off the ionic conduction between the metallic washer 8 and the hydrous nylon gasket 10. In the present embodiment, since the ionic conduction between the metallic washer 8 and the hydrous nylon gasket 10 is cut off by the insulation film 17 applied onto the lower surface and both ends of the metallic washer 8, no leakage occurs for example, even when the battery is stored under highly humid conditions and moisture reaches the space 13 between the nylon gasket 10 and the metallic washer 8. The mechanism for this electrochemical reaction is explained in Fig. 8 which illustrates a schematic case of the electrochemical reaction. In Fig. 8, 17' is an insulating film corresponding to the insulating film 17 of Fig. 7. In Fig. 8, since any portion of Fe is not in direct contact with water, the H₂ formation reaction represented by the formula (2) does not take place on the Fe surface, and accordingly, the reaction of the formula (1) does not take place, either; there occurs no movement of K⁺ into the water which wets the washer; consequently, the water does not become alkaline. Thus, according to this embodiment of the present invention as illustrated in Fig. 7 the application of the insulation film 17 onto the lower surface and two ends of the metallic washer 8 can prevent leakage even when the battery has been stored under highly humid conditions. As is understood from the above embodiments of the present invention, in a cylindrical alkaline battery wherein a washer 8 is provided between the nylon gasket 10 and the bottom plate 6 being the negative electrode terminal each acting as a sealant for the opening of the positive electrode case 1, leakage occurring when the battery has been stored under highly humid conditions can be prevented by insulating the bottom plate 6 and the metallic washer 8 by (1) applying an insulating film onto the metallic washer 8, (2) placing a ring-shaped insulating washer between them, or (3) using an electric insulant as the metallic washer 8, or by electrically or ion-conductively insulating the metallic washer 8 and the nylon gasket 10 by placing a chlorosulfonated polyethylene film between them. As a result, excellent cylindrical alkaline batteries can be provided. Table 1 shows the results of leakage resistance when alkaline batteries meeting the IEC standard, LR 6 were stored under conditions of 60°C and 90% relative humidity. As is clear from Table 1, the alkaline batteries of the embodiments of the invention as compared with a conventional alkaline battery of same size, show remarkably improved leakage resistance. Run No. Storage period, days 7 14 21 28 56 1 (first embodiment) 0 0 0 0 0 2 (second embodiment) 0 0 0 0 0 3 (conventional battery) 0 5 50 90 100

Claims: A cylindrical alkaline battery comprising a positive electrode case (1) of a cylindrical form having an opening (11) at one end and accommodating an electricity-generating element consisting of a positive electrode (2), a negative electrode (3), a separator (4) and an alkaline electrolytic solution, a gasket (10) made of an electrically insulating synthetic resin and installed within the opening (11) of the positive electrode case (1), a bottom plate (6) being the negative electrode terminal electrically connected to the negative electrode in the electricity-generating element and installed on the gasket, a reinforcing metallic washer (8) provided between the bottom plate (6) being the negative electrode terminal and the gasket (10), and a heat-shrinkable resin member (15) covering the outer surface of the positive electrode case, wherein the opening of the positive electrode case is sealed by bending the open end inwardly and thereby tightening the bottom plate (6) being the negative electrode terminal and the washer (8) against the gasket (10), characterised in that electrical insulation is established between the bottom plate (6) being the negative electrode terminal and the washer (8) or, electrical insulation and ion-impermeability are established between the gasket (10) and the washer (8) thereby stopping leakage of electrolyte by preventing an electrochemical reaction which causes such leakage from occurring between the bottom plate (6) and the washer (8) or between the washer (8) and the gasket (10). A cylindrical alkaline battery according to claim 1, characterised in that electrical insulation between the bottom plate (6) and the washer (8) is established by a layer of electrical insulation on at least the upper surface of the washer (8). A cylindrical alkaline battery according to claim 1, characterised in that electrical insulation and ion-impermeability between the washer (8) and the gasket (10) is established by a layer (17) of insulation on the lower surface and side ends of the washer (8). A cylindrical alkaline battery according to claim 1, characterised in that electrical insulation between the bottom plate (6) and the metallic washer (8) is provided by a ring-shaped insulator. A cylindrical alkaline battery according to claim 1, characterised in that electric insulation between the gasket (10) and the metallic washer (8) is provided by a film of chlorosulfonated polyethylene. A cylindrical alkaline battery according to claim 1, characterised in that the washer (8) is an electrical insulator.

Inventors: ASAOKA JUNICHI, IWAKI HIROFUMI, KAJIKAWA YOJI, KANEKO TOSHIKAZU, KUBO ISAO, MIYOSHI JUN, ASAOKA, JUNICHI, IWAKI, HIROFUMI, KAJIKAWA, YOJI, KANEKO, TOSHIKAZU, KUBO, ISAO, MIYOSHI, JUN
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: H01M2
IPC codes: H01M2, B29C61, B29C63, F16J15