A coiled lithium battery

Abstract: In a coil type lithium battery comprising a coil electrode body formed by winding a strip like negative electrode plate of lithium or lithium alloy and strip like positive electrode through a separator, the entire outer cylindrical surface of the coil of the electrode body is covered with an insulating film to prevent internal short cirucit of the battery.

Description: The present invention relates to a lithium battery, and more particularly to a lithium primary battery having coiled electrode members for use in supplying high current. In a conventional coiled type lithium battery, an electrode body of a cylindrical shape of the battery is formed in such a manner that a negative electrode plate 1 made of a strip sheet of lithium or lithium alloy and a positive electrode 2 made of metal charge collecting mesh holding positive electrode agent are laminated through a separator 3 made of non woven cloth such as polypropylene fibers and are coiled in a roll. Moreover, in order to avoid collapse of the roll of the electrode members, the outer end part of the separator 3 wound around the outer cylindrical surface of the electrode body is adapted to contact with the inner cylindrical surface of a negative electrode can except for a portion of an adhesive tape wound around the central portion of the outer cylindrical surface of the roll of the electrode body. At the inner central portion of the roll of the electrode body, namely at the beginning of the winding of the electrode body, a rolled portion 3a of only the separator 3 is formed and the strip like positive electrode 2 is disposed outside of the rolled portion 3a and the strip like negative electrode 1 is disposed outside of positive electrode 2 through the separator 3. The inner end 1a of the strip like negative electrode plate 1 is ended beyond the inner end 2a of the positive electrode plate 2 toward the inner portion of the roll of the electrode body in order to prevent slip of the respective members of the electrode body and to facilitate to wind the electrode body when the electrode body is wound by a winding machine by clamping the inner end of the separator 3 by the rotation spindle of the machine. Accordingly, as shown by 3a. only the separator 3 is rolled along some length (normally 90° to 180°) from the inner end 1a of the strip like negative electrode plate 1. As the collecting mesh of the positive electrode, there is used an expand metal plate made of stainless which a metal plate is cut by a number of slits directed in one direction and the metal plate is expanded in the direction perpendicular to the slit to open the slits. Conventionally, the strip like positive electrode plate is formed in such a manner that the expand metal is sandwiched by positive electrode agent sheets made of positive active agent, electrically conductive powder and binder with the positive electrode agent sheets bonded to the expand metal by a rolling machine. However, according to the recent development of wide use field of the battery, there are increasing such cases of wrong use of the battery of this kind that the battery, despite that the battery is a primary battery, is erroneously subjected to charging or forced discharging by an external power source. It is an example of forced discharging, which occurs many times, that one or more old batteries which are already used and the discharging capacity thereof is finished or small, are connected with one or more new batteries of which discharging capacity is plenty. In this case, if the battery of which discharging capacity is finished is forced to be discharged, the battery is loaded with minus voltage by the current supplied from the other healthy battery. In the conventional coiled type lithium battery, if such error occurs, the battery is apt to generate heat due to an internal short circuit and there are many cases that the batteries fire. The holding of the positive electrode agent in the positive electrode is done mainly by bonding of the positive electrode mix carried on the expand metal on both sides of the slit. However, in case of expand metal, if the slit is opened too wide, the expand metal is apt to be cut off. Therefore, in general, the opening rate of the expand metal is small about less than 70 %, whereby the bonding areas are insufficient, the sufficient holding force can not be obtained and therefore there is a problem that the positive electrode mix can be easily separated from the expand metal at the time of formation of the coiled electrode body. In case the pressing force of the positive electrode agent sheet is increased for increasing the bonding force of the positive electrode agent and electric collecting ability, there is such a problem that rigid deformation of and cut off in the expand metal sheet are easily occur. There is a further disadvantage that since the edge of the opened slit and the cut off edge are acute in the expand metal sheet, the separator is apt to be damaged. It has also been discovered that when the lithium batteries are subjected to a work using relatively large current, the batteries generate a lot of heat and the temperature of battery becomes higher at the central portion, and the insulation layer 3a is molten, whereby an internal short circuit occurs at the inner end 1a of the negative electrode plate 1 and the inner end 2a of the positive electrode 2 and a further large amount of current flows there with generation of further heat, and causing the lithium and lithium alloy to be molten resulting in firing or explosion of the battery. An essential object of the present invention is to provide a coiled type lithium battery in which the internal short circuit is hard to occur and the degree of safety is high. A further object of the present invention is to provide a coiled type lithium battery having a strip like positive electrode in which the positive electrode mix is not easily separated and has a good ability of collection of the electric charge. In order to accomplish the object of the present invention, the inventors studies the technical aspect of generation of the inner short circuit of the conventional lithium battery when the battery was erroneously used. As the result of the study by the inventors, it has been discovered that needle like or acicular crystal metal lithium is deposited in the process of the charge and discharge of the battery, in particular, lithium deposited at the interface between the negative can and coil electrode body breaks the outermost separator and the internal short circuit occurs, whereby a large current flows through the lithium at the short circuit portion and a large amount of heat is generated, resulting in melting of lithium, thereby causing an over reaction to be generated with a further large amount of heat. In order to eliminate the internal short circuit in the lithium battery, according to the present invention there is provided a coil type lithium battery as defined in claim 1. In the preferred embodiment, preferably, the insulation film is one of polyolefin film, polytetrafluoroethylene film and polyester film and the peripheral portion of the bottom of the electrode body is further covered by insulating film. Further embodiments can be found in the dependent claims. In the preferred embodiment, the plane weaving mesh is formed of the wires of 0.05 to 0.10 mm diameter with the wire distance of 0.25 to 1.2 mm and the wires are are bonded together at their cross points by heating process. Fig. 1 is a cross sectional view of an electrode body of a conventional coiled type lithium battery, Fig. 2 is a cross sectional view of an example of a coiled type lithium battery according to the present invention, Fig. 3 is a side elevational view of an example of a coiled type lithium battery according to the present invention with a part broken, Fig. 4 is an partial enlarged view showing an essential portion of the battery shown in Fig. 3, and Fig. 5 is a perspective view of the battery shown in Fig. 3.Fig. 6 shows the charge collecting mesh bonding the positive electrode mix. Referring to Figs. 2 to 5, a coil electrode body 10 is constructed in such a manner that a strip like negative electrode plate 11 of lithium or lithium alloy and a strip like positive electrode 12 having positive electrode mix held on the electric charge collection mesh of metal are laminated and coiled in a roll shape through a separator 13 therebetween so as to electrically insulate both electrodes 11 and 12. The coiled electrode body 10 has its outer cylindrical surface and the peripheral portion of the bottom surface covered with an insulation film 14. The separator 13 is formed in a bag shape and the positive electrode 12 is enclosed in the bag with a suitable length of a room left at the upper portion of the bag. As the insulation film 14 although various kind of material can be used, one of the material selected from polyolefin film, polytetrafluoroethylene film and polyester film may be suitable. As the insulation film 14, a simple film may be used, however, to use an adhesive film having its one surface formed as the adhesive layer using silicon adhesives makes the work of covering the coiled electrode body 10 easy. If the insulation film is too thin, the mechanical strength is insufficient and if it is too thick, the capacity of the battery is decreased, therefore, it is desired that the thickness of the insulation film is selected in a range of 50 to 100 µm. In the arrangement of the lithium battery mentioned above, even if the battery is subjected to a charging or a forcible discharging due to wrong use, since the interface between the outer peripheral surface of the coiled electrode body 10 and the inner cylindrical surface of the negative electrode can 15 is insulated by the insulation film 14, crystallization of the needle like crystal of lithium is effectively suppressed and even if the crystallization is generated, breakage of the outermost separator 13 by the needle like crystal can be prevented because of the presence of the insulation film 14, whereby it is possible to avoid the danger of the internal short circuit in the battery. In addition, in the battery shown in the embodiment, the lower edge portion of the electrode body 10 where the electric current concentration tends to occur can be protected by the insulation film 14, so that it is assured to suppress the internal short circuit The specific structure of the coiled electrode body 10 is characterized at the beginning of the winding. Namely, as shown in Fig. 2, the innermost portion of the electrode body 10 is constructed by only the coil 13a of the separator 13, the innermost coiled portion of the strip like positive electrode 12 is wound on the outer side of the coil 13a, moreover, the negative electrode plate 11 is wound outside of the coiled part of the positive electrode 12. The innermost end of the negative electrode plate 11 is ended behind the inner end 12a of the strip like positive electrode plate 12. Accordingly, the entire coil portion of the negative electrode plate 11 is so arranged that the positive electrode plate 12 is situated inside of the coil portion of the negative electrode plate 11 beginning from the coil end 11a. In the lithium battery mentioned above, when the battery is used for supplying a large current, even if the coiled portion 13a of the separator 13 which faces to the central hole 10a of the coiled electrode body 10 is molten by the heat at the central portion of the battery, the separator 13 which is situated out side than the innermost peripheral portion of the positive electrode 12 can be protected by the strip like positive electrode 12 and is never molten. Therefore, the inner end 11a of the strip like negative electrode plate 11 can be always separated from both of the strip like positive electrode plates 12, whereby the inner short circuit never occurs and firing of the battery and explosion of the battery due to melt of the strip like negative electrode plate 11 can be prevented. Preferably, the the inner end 11a of the negative electrode 11 is positioned in an angular range from 45° to 360° behind the inner end 12a of the positive electrode plate 12 partly because it is not desired to position the inner end 11a of the negative electrode plate 11 near the inner end 12a of the positive electrode plate 12 since the bad effect of the melt of the separator 13 reaches such region near the inner end 12a and partly because if the inner end 11a is positioned more than 360° behind the inner end 12a, the battery capacity is lowered due to the shortage of the amount of the material of the negative electrode plate 12. The positive electrode plate 12 is constructed in such a manner that, as shown in Fig. 4, the positive electrode mix 16 consisting of positive electrode active agent such as manganese dioxide, conductive powder such as carbon black and binder such as polytetrafluoroethylene is held, as shown in Fig. 6, to a charge collecting mesh 30 made of plane weave metal mesh formed by weaving metal wires 30a such as stainless steel wires. In the coiled lithium battery as mentioned above, since the charge collecting mesh 30 of the positive electrode plate 12 is made of the plane weave metal mesh, the aperture factor can be made large, whereby the bonding areas of the positive electrode mix to the charge collecting mesh 30 through the apertures of the mesh can be made large and therefore, sufficient holding force for holding the positive electrode mix to the mash can be obtained. In addition, since the charge collecting mesh 30 is formed by laminating the metal wires 30a and the positive electrode mix 16 enters in the spaces of the crossing parts of the metal wires 30a, the bonding force between the charge collecting mesh 30 and the positive electrode mix 16 can be increased. By the effects mentioned above, in the process of forming the coil electrode body 10 by winding, it is hard to occur the separation of the positive electrode mix 16 from the mesh, thereby resulting in improving the battery efficiency. In addition, when the positive electrode mix in the form of a sheet is bonded to the charge collecting mesh 30 using rolling work, even if the rolling pressure is increased, the wires forming the mesh 30 are hard to be cut because the wires are easily extensible, accordingly, it is possible to make the bonding of the collecting mesh 30 and the positive electrode mix tight and it is possible to improve the ability of charge collection of the charge collecting mesh. In addition, in the plane weaving, since the peripheral portions of the aperture are smooth, there is no fear of damaging the separator at the charge collecting mesh, and even if the ends of the wires are damaged, for example by cutting the wire or wires, the degree of damage is smaller very much than the degree of damage in the conventional expand metal. It is suitable to select the aperture factor of the charge collector mesh as 70 to 85 % so as to satisfy the holding force of the positive electrode mix 16 and the charge collecting ability. In a specific example, it is desired that the diameter  of the respective longitudinal and lateral wires 30a for the mesh shown in Fig. 6 is 0.05 to 1.2 mm and the distance t between two wires is 0.25 to 1.2 mm. If the diameter  is too small and distance t is too large, the charge collecting ability is insufficient and the mechanical strength of the positive electrode 12 is deteriorated. To the contrary, if the distance t between the two adjacent wires is too small, the holding force for holding the positive electrode mix becomes insufficient. If the diameter  of the wire is too large, the amount of the positive electrode mix held in the charge collecting mesh is decreased, thereby causing the battery ability to be deteriorated. As the plane weave metal mesh mentioned above, there may be used such a mesh that the longitudinal and lateral wires 30a are simply woven, however, it is desired to use such a mesh that the respective cross points of the longitudinal and lateral wires are heated with a high temperature so that both wires are welded and electrically connected because such mesh is superior to the conventional mesh in the electron transmission property and charge collecting property. In order to prepare the positive electrode plate 12, there may be used such a method that the positive electrode mix sheets 12 in the form of sheet are rolled on both sides of the charge collecting mesh 30, however, there may be used other method, for example, to paint the positive electrode mix of viscous liquid on the charge collecting mesh 30. Reference numeral 15 denotes a negative can of a cylindrical shape with a bottom, in which the coiled electrode body 10 is installed. The negative can 15 is made of iron plate with nickel coated, having its upper portion narrowed as indicated by 15a. 17 is a sealing plate made of SUS 430 stainless steel plate in a round disc shape with its central portion opened by a hole 17a. The sealing plate 17 is fitted within the upper portion of the negative can 15 by engaging the upwardly folded peripheral edge portion of the sealing plate 17 in the inside of the bent portion 15b of the upper end portion of the negative can 15 through an insulation packing 18. 19 denotes a positive electrode terminal plate made of a nickel plated iron plate and shaped in a hat shape by press forming. The positive electrode terminal plate 19 has its central portion formed a cut blade 19a projected slantingly in a downward direction with the flanged peripheral edge portion of the positive electrode terminal plate 19 clamped between the folded edge portion of the peripheral portion of the sealing plate 17 through a packing 20 of a ring shape. 21 denotes an explosion proof film made of titanium with about 15 µm thick. The film 17 is laid on the top surface of the sealing plate 17 with the outer peripheral edge clamped between the sealing plate 17 and the positive electrode terminal plate 19, whereby when the inner pressure of the battery rises, the film 17 is expanded upwardly and the film 17 is broken by the engagement with the blade 19a so as to exhaust the inner gas in the battery outward, thereby resulting in preventing explosion of the battery. 23 denotes a thermal deformable member made of a poly ethylene ring and mounted in a space between the explosion proof film 21 and the positive electrode terminal plate 19. The thermal deformable member 23 has an ability of preventing an erroneous operation of the explosion proof film 21. 24 denotes an insulation ring disposed on the coiled electrode body 10, 22 denotes an insulation plate disposed between the lower surface of the coiled electrode body 10 and the inner surface of the negative can 15. The insulation packing 18, the ring shaped packing 20, the insulation ring 24 and the insulation plate 22 are respectively made of molded members of polypropylene or the like. 25 denotes a positive lead member made of a strip of stainless steel with its upper end connected with the sealing plate 17 and its lower end connected with the central portion of the positive electrode plate 12 by spot welding, and the intermediate portion of the positive lead member 25 is covered with adhesive tape (not shown) made of teflon processed glass fiber cloth. 26 denotes a negative lead member of a L character shape with its vertical member of the L character shape adhered to the central portion of the negative electrode plate 11 by an adhesive tape (not shown) and its horizontal member is spot welded to the inner surface of the bottom of the negative can 15 passing the central hole 10a and the central hole 22a of the insulation plate 22. The negative can 15 contains electrolyte in which lithium salt is molten in the non aqueous solution. It is noted that the battery according to the present invention, as the separator 13, a simple sheet separator may be used. In addition various detailed portions of the battery may be modified. According to the present invention, since the entire outer surface of the electrode body is covered with the insulation film, even if the battery is erroneously charged or forcibly discharged, the inner short circuit is hard to occur and therefore the fire proof and safe coiled lithium batteries can be provided. Moreover, by using one of the film selected from poly olefin film, polytetrafluoroethlene film as the insulation film, the insulation and mechanical strength of the insulation film are improved, whereby it is assured more to prevent the inner short circuit of the lithium battery. By covering the lower peripheral portion of the electrode body in addition to cover the outer surface of the coiled electrode body, the effect of preventing the inner short circuit can be much improved. Moreover, the inner end of the negative electrode plate made of lithium or lithium alloy is positioned behind the end of the positive electrode plate, and the innermost coil portion of the positive electrode plate is situated inside of the coil of the negative electrode plate, therefore, even if heat reservation occurs in the central portion of the battery, the inner short circuit is hard to occur, whereby according to the present invention, it is possible to provide coiled type lithium batteries which are safe and without fear of firing. Moreover, since the plane weaving mesh is used as the charge collecting mesh of the positive electrode, it is possible to increase the holding force for holding the positive electrode mix because of increment of the aperture rate of the mesh, whereby separation of the positive electrode mix is hard. Also it is possible to increase the boding force of the positive electrode mix by rolling the sheet of the positive electrode mix at the time of the production of the positive electrode plate, whereby it is possible to enhance the charge collecting ability and to decrease the damage of the separator by the charge collecting mesh, so that it is possible to provide the coil type lithium batteries of high ability. In addition, in case of using such plane weave metal mesh that the diameter of the wire is 0.05 to 0.10 mm and the wire distance is 0.25 to 1.2 mm, there can be obtained good properties of the holding force of the positive electrode mix and the mechanical strength with a suitable aperture rate. It is also possible to enhance the charge collecting property by using the plane weave metal mesh in which the longitudinal wire and lateral wire are welded and joined at the respective crossing points. EXAMPLES Example 1 (An embodiment of the invention of claim 1) The positive lead member made of a stainless steel plate was bonded to the central portion in the longitudinal direction of the strip like positive electrode plate (190 mm x 30 mm x 0.4 mm) made of the charge collecting mesh made of stainless steel 316 on which positive electrode mix comprising manganese dioxide of 10 parts by weight, carbon black of 1 part by weight and polytetrafluoroethylene of 0.2 parts by weight was held. The positive electrode plate was enclosed in the separator of a bag shape made of non woven cloth of polypropylene fibers and the opening edge of the bag was sealed by thermal bonding with the separator elongated 15 mm than the positive electrode so that the inner end of the separator was situated at a position higher than the inner end of the positive electrode plate. The negative lead member made of nickel was attached to the central portion in the longitudinal direction of the strip like negative electrode plate (190 mm x 28 mm x 0.17 mm)made of lithium by the insulation adhesive tape the base of which is made of teflon processed glass fiber cloth. The intermediate portion of the positive lead member was also covered by the adhesive tape as mentioned above. Subsequently, the inner end portion of the separator enclosing the strip like positive electrode plate was fixed to the spindle of a winding machine, locating the strip like negative electrode plate its outside, then the spindle was rotated so that the coil type electrode body was produced. The entire outer cylindrical surface of the coil type electrode body and the periphery of the bottom of the electrode body were covered by the adhesive insulation film made of polypropylene film of 50 µm thick with its one side coated with adhesive layer of silicone adhesives of 30 µm thick. The coil type electrode body was installed in the iron negative can plated by nickel with the insulation plate and insulation ring each made of polypropylene and the horizontal member of the negative lead member of L shape was was spot welded to the bottom of the negative can. Subsequently, the top portion of the negative can was formed as the narrowed portion then the electrolyte of 1 : 1 mixed solution of propylene carbonate and 1·2 dimethoxyethane with lithium perchlorate solved with the concentration of 0.5 mol/ℓ was injected. The iron positive terminal plate coated by nickel, the titanium explosion proof film of 15 µm thick and the sealing plate of SUS 430 are laminated through the round shape thermal deformable member made of polyethylene and the ring shape packing of polypropylene, with the peripheral edge thereof fitted by the insulation packing made of polypropylene to form the sealing body in which the above members are integrally combined in one body. Subsequently, the positive lead member is spot welded to the bottom face of the sealing plate, then the sealing body is mounted in the negative can above the narrowed portion, thereafter the opening edge of the negative can was fastened, forming the curved portion, whereby the negative can was sealed and prepared the coil type lithium battery as shown in Figs. 2 to 5. Comparative Example As a comparative example, a coil type lithium batter was prepared in a similar manner as the example 1 except that the coil electrode body was not covered by the insulation film. 50 batteries of the example 1 and 30 batteries of the comparative example were subjected to the test in such a manner that the respective batteries were put in the forced discharging condition by causing the polarities of the batteries to be reversed to - 2.5 volts using stabilized D>C power source, and they were put for one day and the number of the batteries which fired was counted. The result is shown in the the table 1. N F / N T Example 1 0 / 50 comparative example 2 / 30 In the table 1, N F is the number of fired battery and N T is the number of the tested batteries.

Claims: A coil type lithium battery comprising a coil electrode body with a negative electrode current collecting can, wherein the outermost layer of the coil electrode body is the outermost separator layer and the outermost electrode of the coil electrode body is the outermost positive electrode layer formed by winding a strip like negative electrode plate of lithium or lithium alloy and strip like positive electrode through a separator characterized in that at least the entire outer cylindrical surface of the coil of the electrode body is covered with an insulating film. The battery according to claim 1, wherein the insulation film is one of polyolefin film, polytetrafluoroethylene film and polyester film. The battery according to claim 1, wherein the peripheral portion of the bottom of the electrode body is further covered by insulating film. A coil type lithium battery according to claim 1, characterized in that the inner end of the strip like negative plate is positioned behind the inner end of the strip like positive electrode plate with the coil electrode body negative electrode plate being arranged such that the positive electrode plate is situated inside the coil portion of the negative electrode plate, when viewed beginning from the inner coil end. A coil type lithium battery according to claim 1, characterized in that said strip like positive electrode plate comprises charge collecting mesh formed by plane weaving mesh woven by a plurality of longitudinal wires and a plurality of lateral wires and positive electrode mix containing electrode active material and held on the charge collecting mesh. The battery according to claim 5 wherein the plane weaving mesh is formed of the wires of 0.05 to 0.10 mm diameter with the wire distance of 0.25 to 1.2 mm. The battery according to claim 5, wherein the longitudinal wires and lateral wires are bonded together at their cross points by a heating process.

Inventors: EDAMOTO TOSHIYUKI, HISATOMI KAORU, KAJII OSAMU, KITAMURA TOMIO, SOMATOMO YOSHIKI, WATANABE OSAMU, YOSHIDA HIROSHI, EDAMOTO, TOSHIYUKI, HISATOMI, KAORU, KAJII, OSAMU, KITAMURA, TOMIO, SOMATOMO, YOSHIKI, WATANABE, OSAMU, YOSHIDA, HIROSHI, EDAMOTO, TOSHIYUKI HITACHI MAXELL LTD., YOSHIDA, HIROSHI HITACHI MAXELL, LTD.
Applicants: HITACHI MAXELL, HITACHI MAXELL LTD.
Application year: 1989
Main classification: H01M6
IPC codes: H01M4, H01M6