A CONTAINER FOR A CLOSED, CYLINDRICAL, RECHARGEABLE BATTERY CELL

Abstract: In a closed, cylindrical, rechargeable battery cell, preferably a nickel/metal hydride battery, the electrode complex (8-12) and electrolyte of the cell are arranged directly in a cylindrical tube (15) of a thermoplastic material. This tube is provided with two end-closing, circular disks (16, 17) of an electrically conductive plate material. The disks are so melted into or pressed into the tube that a pressure-tight sealing is obtained.

Description: A CONTAINER FOR A CLOSED, CYLINDRICAL, RECHARGEABLE BATTERY CELL Technical Field The present invention relates to a container for a closed, cylindrical, rechargeable battery cell, primarily an alkaline cell, such as a nickel/metal hydride battery or the like. Background of the Invention A battery cell, typical for the state of the art, preferably a nickel/metal hydride battery, comprises a cy¬ lindrical, deep-drawn vessel, usually of nickel plated steel plate. In the vessel there is a complex, including a negative and a positive electrode, which together with a separator are wound to a cylinder shaped package. Together with the complex there is also an ion conducting fluid in the form of an electrolyte, usually a water solution of KOH or KOH+LiOH, which acts as an electron carrier between the electrodes. As a sealing for the vessel is used a cover, consisting of a circular metal disk with an annular pack¬ ing, which covers the circumference of the disk. On the cover there is a safety valve, usually consisting of a rubber block and a valve cover. The vessel has a pressed waist, which functions as a support for the cover. The cover is pushed down into the vessel to its waist, where¬ after the edges of the the vessel extending up over the cover are folded down. These edges are then pressed against the cover, so that the annular packing is squeezed between the waist and the folded edges. A problem with these batteries is to accomplish a fluid and gas tight sealing between the vessel and the cover. Certain manufacturers have succeeded in satis- factorily solving this problem but at the expense of a com¬ plicated and expensive manufacturing. For accomplishing this tight sealing very small tolerances on the upper part of the vessel are required and that the vessel material has to be free from stress. These requirements lead to a neces¬ sity for heat treatment of the vessel for increasing the ductility before the forming operation. However, it has appeared that even after the treatment the forming opera¬ tion leads to the exposure of the vessel wall to stresses and a weakening through thinning and crack formation of the vessel wall in the formed area thereof. This can lead to stress corrosion, which means that the vessel wall becomes perforated and leaks after exposure to the corrosive battery environment for a certain time. Another problem is the isolation between the positive and the negative pole. This problem is partly solved by the cover packing, which isolates the vessel (the negative pole) from the cover (the positive pole) . However, the positive contact lug is not isolated herewith, but instead a tape portion has to be applied around this lug in order to prevent short circuiting between the lug and the vessel. This tape application is a complicated step in the produc¬ tion, and a tape failure often leads to a short circuited battery. In this battery cell the whole vessel further con¬ stitutes the negative pole, isolated from the cover (the positive pole) only by the cover packing. In order to stall obvious risks for short circuiting the cylindrical part of the container is isolated, for example with a plastic shrink tube. Another type of problem appears when the safety valve has become faulty and does not open at an over-pressure, which can lead to a bursting of the battery. A bursting of a battery with metal sleeve can cause damages to the sur¬ roundings, when metal pieces are pushed away with high velocity. As further problems can be stated environmental problems with nickel plated vessels in very great numbers, high material costs for the vessel and comparatively large weight. At the recovery of batteries with metal vessels (steel vessels) the result is further undesired waste material and intermixing of other higher quality materials in the batteries. The Invention The object of the invention is to remove the above stated and other problems with a prior container of the kind described above. This is according to the invention accomplished in that the electrode complex and electrolyte of the battery cell are arranged directly in a cylindrical tube of a thermoplastic material, provided with two end-sealing, circular disks of an electrically conductive plate material, the disks being so melted into the tube that a pressure-tight sealing is accomplished. Preferred embodiments within this general idea are described below and stated in the appended subclaims. Short Description of the Drawings The invention shall be further described below under reference to the appended drawings, in which Fig. 1 is a partly sectioned side view of a typical prior art battery cell, Fig. 2 is a view corresponding to Fig. 1 of a battery cell according to the invention, Fig. 3 is a cross sec- tional view to a larger scale of a lower part of a tube for the battery cell according to Fig. 2, and Figs. 4-11 are sections which show different alternative embodiments of the sealings of the tube and parts therefore. Detailed Description of Preferred Embodiments Fig. 1 shows a typical prior art battery cell. The battery container thereof is comprised of a deep-pressed vessel 1, usually of nickel plated steel plate, and a circular cover 2 in the form of a metal disk, inserted in an annular packing 3. This packing 3 is arranged between an upper rim 1' and a "waist" 1" of the vessel 1. On the cover is arranged a safety valve 4, which consists of a rubber block 5 over a center hole 6 in the cover 2 and a valve cap 7, which is spot welded to the cover for allowing exit of gas under pressure. In the container is arranged a battery complex, con¬ sisting of a negative electrode 8 with a current conducting lug 9 and a positive electrode 10 with a current conducting lug 11. The electrodes 8 and 10 are together with a sepa¬ rator 12 wound to a cylindrical electrode package. In the container there is also an ion conducting fluid as an elec¬ trolyte, usually a water solution of KOH or KOH+LiOH, which acts as an electron carrier between the electrodes. A preferred embodiment of a battery cell according to the invention is shown in Fig. 2. This cell has a con- tainer, which in principle consists of a cylindrical tube 15 of a thermoplastic material, preferably polypropylene, with a cover 16 at its upper end and a bottom 17 at its lower end. The container contains a battery complex of prin- cipally the same kind as has been shown in Fig. 1, namely electrodes 8 and 10, lugs 9 and 11, a separator 12 and electrolyte. The electrodes and the separator are also here wound to a cylindrical electrode package. At the manufacture the bottom 17 is introduced from above in the cylindrical tube 15, whose lower part hereby can have the appearance shown in Fig. 3. The outer diameter of the bottom 17 corresponds to the inner diameter of a lower tube part 15' in the tube 15, whereas the tube part thereunder has a slightly smaller diameter than a down- wardly extending embossment 17' (Fig. 2) of the bottom 17, so that the bottom has to be pressed in place. Hereafter the inner surface of the tube material in the vicinity of the bottom 17 is warmed to such a temperature that the bottom is melted into the tube 15. Alternatively, the bottom can be placed in a tool at the manufacture of the cylindrical tube, so that an inte¬ grated construction is accomplished in one step. After the mounting of the electrode complex 8-12 described above the cover 16 is introduced into the tube 15, whereafter the upper edge 15" thereof is melted to the form shown in Fig. 2, for example by means of hot air di¬ rected thereto. The tube rim 15" will hereby become co¬ planar with the upper surface of the cover 16, which has the form shown in Fig. 2. The cover 16 has a center hole, in which a rubber block 18 is arranged. A valve cover 19 is arranged over the rubber block 18 and is spot welded to the cover 16, so that exit of gas under pressure is allowed. The rubber block 18 and the valve cover 19 thus form a safety valve. The valve cover 19 can be folded down at its circumference as appears from Fig. 2, so that the battery cell obtains a neat appearance, but measures have then to be taken to provide for gas exit. Figs. 4-11 illustrate different embodiments of the closure of the tube, especially at its bottom. For the sake of clarity these figures are not provided with reference numerals and only show the cylindrical tube, which in Fig. 2 has the numeral 15, and the bottom, which has the numeral 17. In the embodiment according to Fig. 4, where the appearance of the tube and the bottom, respectively, before mounting and sealing are shown in Figs. 5 and 6, the bottom is pushed in from below in the recess provided for this purpose in the lower part of the tube and having slightly smaller inner diameter than the outer diameter of the bottom. The circumference of the bottom can be embossed. The tube edges extending down past the bottom are heated with hot air and are folded down against the bottom, so that the completed sealing obtains the appearance shown in Fig. 4. For simplifying the introduction of the bottom in the tube the recess therein can be bevelled, as appears in Fig. 5. Also in the embodiment according to Fig. 7, where the appearance of the tube and the bottom, respectively, before the mounting and sealing are shown in Figs. 8 and 9, the bottom, whose circumference can be smooth, is introduced from below - in this case against an inner shoulder in the tube, serving as a support for the bottom. The edge of the tube extending down over the bottom can be heated and folded down. As an alternative this embodiment also enables the bottom to be molded into tube already in the molding tool. This method can also be used for the cover. Thus, if it is desired to mold both the cover and the bottom into the tube, the electrode complex with its lugs already on beforehand has to be joined to the cover and the bottom, whereupon the whole package with complex, cover and bottom is molded into the tube. In this case the electrolyte can be filled into the valve hole in the cover after the molding but before the mounting of the safety valve. A bottom in the form of a cup with punched out edge holes is shown in Fig. 11. The edge of the cup is intended to be pushed into a circumferential channel in the lower part of the tube and to be attached by melting. The punched out edge holes are thereby intended to reinforce the con¬ nection in the axial direction, as high pressures in the battery otherwise would be able to push the bottom out of the tube. The embodiments mentioned above can be combined in different ways, and the same connection way need not be used for the cover as for the bottom, but the method is chosen after the type of battery and after the machining conditions. The cover and the bottom are circular disks, which may be designed in different ways for suiting different sealing ways. When a press-fit is being utilized, it is of course necessary that the cover and the bottom have an outer diameter which is larger than the inner diameter of the tube. A simplification of the sealing process is attained if the tube has an internal bevel for easier in¬ sertion of the disk. The cover and the bottom are made a material with good conductivity. The material, which also shall be completely resistant in a strong alkaline electro¬ lyte, such as a concentrated solution of potassium hydroxide, can be stainless steel, pure nickel, nickel plated steel or nickel plated copper. The tube is a cylindrical tube with a generally con¬ stant inner diameter except in the ends, where the tube can be internally bevelled for accomplishing a simplified in- sertion of the cover or the bottom and possibly be provided with an internal shoulder assisting in getting the cover or the bottom in its exact position. The material in the tube is a polymer, which is resistant to a strong alkaline elec¬ trolyte and has good electrical isolation properties as well as low permeability for gases. For environmental reason the material shall also be free from halogen, such as polypropylene, polyethylene or polyamide. By dimensioning the sealing in a suitable way it may open at a desired pressure, and in this way the safety valve provided on present battery cells may be obviated. In order to see to it that the sealing opens at the desired place and at the desired pressure, the tube can be provided with a fracture indication in a suitable place, and the opening pressure may be controlled by the size of this in- dication. Experiments In order to investigate the tightness of the con¬ tainer empty containers without complex and electrolyte were manufactured and pressure tested in water. The empty containers were manufactured by a tube shaped plastic sleeve from polypropylene (NESTE SC13 10GN) with an inner diameter of 13.1 mm and a wall thickness of 0.6 mm. The plastic sleeve was closed in the bottom with a circular disk in accordance with Fig. 4. The disk was made of stainless steel (SS 2333) with a thickness of 0.3 mm and an outer diameter of 13.3 mm. The bottom was pushed into the sleeve about 2 mm, whereupon the sleeve edge was melted down over the bottom. A circular disk according to Fig. 2 was used as a cover. This cover was provided with an air nipple, which was threaded into its valve hole, whereupon a 6 mm air tube was connected. The air tube was connected over a manometer to an air tube with an over-pressure of 200 kg/cm 2 . By exposing the empty containers to an over-pressure of 5 kg/cm and submerging them into water, they were tested with regard to leakage. The first test was made in 20 °C water, where the containers were held under pressure in 5 minutes. The result of this test was that no air bubble leaked out from any container. Further tests were made with an over-pressure of 5 kg/cm but in a water tem¬ perature of 60 °C and 80 °C, respectively. The containers were first submerged for 10 minutes in the water for ob¬ taining the same temperature, whereafter the containers were tested. No leakage from the sealings could be seen at these tests. For investigating the pressure limits of the sealing the containers were exposed to an over-pressure of 15- 20 kg/cm 2 and were submerged for 5 minutes in 20 °C water. These tests did not show any leakage from the sealings.

Claims: CLAIMS 1. A container for a closed, cylindrical, recharge¬ able battery cell, c h a r a c t e r i z e d in that the electrode complex (8-12) and electrolyte of the battery cell are directly arranged in a cylindrical tube (15) of a thermoplastic material, provided with two end-closing, circular disks (16, 17) of an electrically conducting plate material, the disks being so melted into or pushed into the tube that a pressure-tight sealing is accomplished. 2. A container according to claim 1, c h a r a c t e r i z e d in that the tube (15) is made of polypropylene, polyethylene or polyamide. 3. A container according to claim 1, c h a r a c t e r i z e d in that the end closing disks (16, 17) are made of stainless steel, nickel, nickel plated steel or nickel plated copper. 4. A container according to claim 1, c h a r a c t e r i z e d in that the bottom (17) of the container is inserted from the upper end of the container until contact with a bottom shoulder. 5. A container according to claim 1, c h a r a c t e r i z e d in that the end-closing disks (16, 17) have a slightly larger outer diameter than the inner diameter of the tube (15) and are inserted from each end in the tube before the melting of the plastic. 6. A container according to claim 1, c h a r a c t e r i z e d in that a fracture indication is made on the tube (15) for functioning as a safety valve. 7. A container according to claim 1, c h a r a c t e r i z e d in that one of the end-closing disks (16) has a hole, in or over which is arranged a rubber block (18) and a valve cover (19) for forming a safety valve.

Inventors: BAERRING NILS-ERIK, OHLSSON PER-AAKE, BAERRING, NILS-ERIK, OHLSSON, PER-AAKE
Applicants: BAERRING NILS ERIK, NI ME HYDRID AB, OHLSSON PER AAKE, BAERRING, NILS-ERIK, NI-ME HYDRID AB, OHLSSON, PER-AAKE
Application year: 1995
Main classification: H01M2
IPC codes: H01M2, H01M10