Abstract:
Separator for cylindrical cell of the outwardly guided type, wherein a sheet material is wound around a mandrel, and starting from the winding step until the insertion of a separator into the cell, an outward support is used that renders the binding of neighboring turns of the separator winding unnecessary, and the separator sheet has an extended portion, with the extension being at least equal to the radius of the separator cylinder, and with this extended portion being wetted with distilled or de-ionized water until the material softens and the winding and the mandrel are rotated and the bottom part is folded back and heat fused to close the separator cylinder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional Patent Application of U.S. application Ser. No. 10/578,149, filed May 3, 2006, which is the U.S. National Stage (371) application of PCT International Application No. PCT/IB2003/005023 filed Nov. 5, 2003, the complete disclosures of all of the aforesaid applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to an improved separator for cylindrical cells, preferably for alkaline cells that have a cylindrical body and a closed bottom portion. 
     BACKGROUND OF THE INVENTION 
     Cylindrical cells, mostly alkaline cells are widely used. These cells are comprised of an elongated cylinder constituted by a metal can with press fitted cathode pellet rings containing manganese dioxide as the active electrode material in the interior of the can to constitute the positive cell electrode. An anode gel composed of zinc powder active material, gelling agent and an alkaline electrolyte filling the cylindrical central cavity of the positive cell electrode so that a cylindrical separator made of a specific sheet material separates the two electrodes. The separator must be composed of a material that allows ions to pass from one electrode to the other, but prevent particles of the two electrode materials from passing through, and also be an electrical insulator to prevent electrons from passing directly though. The active area of the separator is the portion where it directly separates active cathode material from anode material. A critical portion of the separator is the central bottom part, because the anode gel expands significantly during discharge of the cell and the bottom part has to remain intact separating the anode from the interior of the can, which would cause an internal short circuit and cell failure. 
     A conventional way of providing a reliable insulation is described in U.S. Pat. No. 6,099,987 wherein an outer and an inner isolating cup are attached to the lower end of the separator, and the interior bottom part of the separator is sealed by the application of a thermoplastic sealant. This is a perfect solution as far as isolation of the two electrodes are concerned but the presence of the cups and of the sealant takes a substantial cell volume, which cannot be utilized for cell function and requires the handling of several separate material parts. 
     U.S. Pat. No. 6,541,152 shows a different design also utilizing an insulating cup at the bottom and it has the same problem of decreasing useful cell volume and requires the handling of two separate material parts. 
     U.S. Pat. No. 6,270,833 does not use any cup but the separator is made longer than the required useful length in the cell, the windings of the cylindrical separator body are bound together with a binder, and the extended portion is first pushed inwardly by a tool moving normal to the cylinder axis then folded back to close the initially open end. The folded and closed separator forms a self containing unit that should then be inserted into the cell. The smooth insertion requires a small clearance between the inner diameter of the cathode rings and the separator, which could increase cell resistance. The closing operation of the bottom part is complicated and requires movements in different directions, and problems can arise by the inevitable appearance of wrinkles. 
     U.S. Pat. No. 6,035,518 describes a different method of making the separator, in which the separator material is wound around a mandrel and the winding is kept on the mandrel by a vacuum, and the separator does not constitute a self-containing unit, it should be guided until insertion into the semi-finished cell, wherein the winding tries to open up and fill the whole available space. While the idea of guiding the separator until insertion into the cell is preferable, the key problem, i.e. the closing of the bottom is solved here by the application of a hot melt sealant to fill the cell bottom including the bottom region of the separator. The presence of a sealant at the active lower region of the separator also decreases the available useful cell volume. 
     A further problem characteristic to separators used for secondary cells lie in that often a laminated structure should be used, since in case of secondary cells a thin semi permeable membrane layer, such as a cellophane layer should be provided. Two or more layered laminates are expensive and adhesives used to affix the layers contribute to higher internal resistance. 
     There is a further issue concerning separators that concern the need of synchronization with the general cell manufacturing process. State of the art processes produce at high speeds of 600 to 1200 parts per minute, and this high speed favors or requires easy to use technologies that can fit into the manufacturing line, rather than preparation of off-line, pre-fabricated separators, which can cause problems from additional handling. 
     OBJECT OF THE INVENTION 
     The primary object of the invention is to enable maximum utilization of available cell volume. A further object is the combination of the unfolding nature of the guided separator as taught in the above referred U.S. Pat. No. 6,035,518 with the reliable establishment of a closed bottom that does not require the application of a sealant in the useful cell area, or overcoming the disadvantages of the methods described in the cited Japanese publications. Yet another object of the invention is to provide on-line adjustments of the sheet material length without the changes of any hardware components. A different object is to provide a separator that does not require the use of a laminate sheet if a multi-layered structure is required e.g. for secondary cells. A still further object is to provide a method that is simple, easy to make and which can provide synchronization with the cell manufacturing process. 
     SUMMARY OF THE INVENTION 
     According to the present invention a guided separator has been provided, wherein a sheet material is wound around a mandrel, and starting from the winding step till the insertion of the separator into the cell, an outward support is used that renders the binding of the neighboring turns of the separator winding unnecessary, and the separator sheet has an extended lower portion, wherein the extension is at least equal to the radius of the separator cylinder, and this extended portion is wetted with distilled or de-ionized water and wetted until the material softens and the winding and the mandrel are rotated and the bottom part is folded back to close the cylinder. The folding step is followed by a heat forming and fusing step, wherein a heated die is pressed against the bottom of the mandrel pressing and heating the folded portion therebetween, which causes the sheet material to fuse together. At this step the separator and the mandrel are not rotated any more. 
     The bottom sealing can be improved by the application of a predetermined small amount of thermoplastic sealant at the central bottom region that corresponds to an inactive part of the cell to ensure that the bottom seal is free of any potential pinholes. 
     The insertion of the separator can be facilitated if the mandrel consists of an outer sleeve and an inner pin, wherein the pin has a head portion corresponding to the required shape of the bottom portion of the separator. In that case the pin can hold the separator in place inside the semi-finished cell, while the sleeve is withdrawn first, and after removal of the sleeve the pin can be removed with the risk of pulling back the separator from the semi-finished call. 
     As a preferable embodiment multiple sheets can be wound together eliminating thereby the need of using an expensive laminate. 
     The present invention provides a maximum utilization of the available useful cell volume, since the full vertical surface of the separator is active, and there is no need of leaving an unnecessary clearances between the cathode interior and the separator. The non-use of any binder material improves the performance of the separator by decreasing its internal resistance. Last but not least, the wet folding followed by a heat forming step leaves essentially no wrinkles, providing an excellent separator bottom closing, which is easy to carry out. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in connection with preferable embodiments thereof, wherein reference will be made to the accompanying drawings. In the drawing: 
         FIG. 1  shows the schematic top view of the sheet feeding station; 
         FIG. 1   a  is the simplified elevation view of a part of  FIG. 1 ; 
         FIG. 2  is a similar view as  FIG. 1  adapted for feeding two sheets; 
         FIG. 3  shows the elevation view of the contact zone of the winding nest and the vacuum wheel, partially in section; 
         FIGS. 4 and 4   a  show the sectional elevation view of the two-part mandrel; 
         FIGS. 5 to 10  illustrate the folding of the separator bottom part; 
         FIGS. 11 and 12  show the forming and pressing step by a heated die; 
         FIGS. 13 and 14  are enlarged sectional views showing the application of a sealant; and 
         FIGS. 15 to 17  illustrate the insertion of the separator in the cell. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 and 2  show a similarity with FIG. 1 of U.S. Pat. No. 6,035,518 of Slivar, wherein a separator sheet material  10  is pulled down from a reel (not shown) by means of a pair of feed rollers  11 ,  12  rotating in opposite directions as shown by the arrows. The sheet material is a fibrous, porous, non-woven paper-like material in case of forming separators for alkaline cells. The sheet material can be a semi permeable membrane, such as a cellophane film, grafted micro-porous polyolefin membrane or the like, which can be laminated to a non-woven sheet material in case of secondary cells. 
     The path of the sheet material  10  extends between a pair of counter-rotating cutting rollers  15 ,  16 . The roller  15  is a cylinder and acts as an anvil, while the roller  16  is spaced from the path of the sheet  10  and a pair of cutter prisms  17   a  and  17   b  with respective cutting edges is attached to the roller  16  at diametrically opposing positions. Twice in each revolution of the rollers  15 ,  16  one of the cutting edges presses against the anvil, and the sheet material gets cut along the edge. The cutting rollers are geared with the main assembly machine carrying the winding nest  22 . The length of the cut sheet section is determined by the speed of feed rollers  11 ,  12 , which are preferably servo-driven to enable on-line adjustments of the sheet length without the changes of any hardware components. The sheet material  10  proceeds in forward direction and reaches the periphery of a vacuum wheel  18  rotating in the direction as indicated by the arrow. These elements together constitute a first sheet feeder assembly  13 . 
     The vacuum wheel  18  has a stationary center  20  with a partially hollow cross section, and a vacuum is provided in the hollow inner space defined between the interior of the wheel  18  and the center  20 . A plurality of bores  21  is provided in the wall of the vacuum wheel  18 . When the sheet material  10  reaches the periphery of the vacuum wheel  18 , the vacuum through the first of the bores  21  will temporarily fasten the material to the wheel  18  by means of the sucking force, and the cut piece of sheet material will be transported till a zone of engagement with a winding nest  22 . The winding nest  22  is part of a cell assembly line (not shown) and it moves along a circular path having a diameter much larger than that of the vacuum wheel  18 . The cut sheet material  10  will continue in the interior of the winding nest  22 , because in the engagement zone of the winding nest  22  and the vacuum wheel  18  the stationary center  20  closes down the path of the vacuum allowing the front portion of the sheet material to be guided away from the vacuum wheel  18  into the winding nest  22 , while the remaining portion of the sheet material is still held in place by vacuum. As the separator is wound up in the winding nest  22 , a portion of the sheet material is always released from the vacuum wheel until the whole sheet is wound up. 
     The arrangement of  FIG. 2  is similar to that of  FIG. 1 , the difference lies in the existence of a second sheet feeder assembly  14  placed around the periphery of the vacuum wheel angularly offset from the first sheet feeder assembly  13  in a direction against the sense of rotation. The second feeder assembly  14  is used to provide a second sheet material  10   b  with predetermined length that will already be attached to the periphery of the vacuum wheel  18  when the first feeder assembly  13  feeds the sheet material. In this way two sections of sheet material  10  and  10   b  will travel one on top of another towards the winding nest  22 . The length of the sections can be equal but also different. In case of different length sheets, the position of the shorter sheet can be in any region of the longer sheet, which results in different positioning of the shorter sheet material inside the finished separator. The use of two or more sheet material sections eliminates the need of using a laminate as sheet material when separators for rechargeable batteries are made, wherein one of the sheet materials can be a semi permeable membrane, such as a cellophane layer. 
     In the elevation view of  FIG. 1   a  the sheet material  10  extends over the bottom edge of the vacuum wheel  18 . The height of the separator to be prepared corresponds substantially to the height of the vacuum wheel  18  and the extending portion will later form the separator bottom. A spray head  50  is used to spray a predetermined amount of distilled or de-ionized water to the extending portion of the sheet material  10  to slightly wet and soften the sheet material. Other means of water application for wetting such as nozzles, belts or others are acceptable as well. 
       FIG. 3  shows the contact zone of the vacuum wheel  18  and of the winding nest  22 . A two-part mandrel  23  is inserted through a central opening of the winding nest  22  and rotated as illustrated by the arrow. The winding nest  22  is not rotated. The peripheral speed of the mandrel  23  is substantially the same or somewhat higher as that of the vacuum wheel  18  and of the speed of a pair of belts  24 ,  25  made of a resilient material e.g. rubber. The belts  24 ,  25  are arranged and led through the zone of contact between the vacuum wheel  18  and the winding nest  22  and they are contacting and pressing the sheet material  10  wound around the mandrel  23 . The belts  24 ,  25  contact the sheet material  10  through respective cut sections made in the winding nest  22 . The mandrel  23  shown in  FIGS. 4 and 4   a  comprise a sleeve  26  and a pin  27  extending through the central hollow opening of the sleeve  26 . The pin  27  has a wider head portion  28  having the shape as shown in  FIG. 4A  fitted in a nest on the end portion of the sleeve  26 . The pin  27  fits loosely in the sleeve  26 . Both the pin  27  and the sleeve  26  are rotated and moved together until a predetermined station of the assembly line is reached. 
     When  FIGS. 1 and 3  are considered together, it can bee seen that the sheet material  10  will move in the hollow interior of the winding nest  22 , and under the pressing effect of the belts  24 ,  25  it will be wound around the rotating mandrel  23 . The extending wet portion  19  will extend out over the head  28 . 
     The winding nest  22  together with the mandrel  23  and the wound sheet material  10  will then move out from the contact zone with the vacuum wheel, and the belts  24 ,  25  are led along this path of movement pressing continuously the wound and rotating sheet material  10  to the mandrel  23 . 
     Reference is made to  FIGS. 5 to 10  showing the mandrel  23  with the wound sheet  10  around it at three subsequent positions during the path of movement of the assembly in the manufacturing line. The winding nest  22  around the sheet  10  has not been shown, however, it travels together until full insertion of the separator in the battery console. A stationary rail  29  is arranged along the travel path of the assembly. The rail  29  defines a forming groove  30  in which the wet extending portion of the rotating sheet  10  is inserted.  FIGS. 5 and 6  show the forming groove  30  close to the initial position of engagement,  FIGS. 7 and 8  correspond to a medium position and  FIGS. 9 and 10  show the final position just before the assembly moves out of the zone of engagement with the rail  29 . The profile of the groove  30  corresponds to that of the head portion of the mandrel  23 , whereby the extending wet portion  19  of the sheet material  10  will be gradually formed. The soft sheet material will easily follow the profile of the groove because in wet state it is very flexible and the forming process will occur smoothly leaving essentially no wrinkles. The rotation facilitates a smooth forming operation. The extending portion  19  has a sufficient length that by the end of the forming process the central opening of the wound sheet material  10  gets completely closed, and thereby a separator  31  is produced. Shortly after the assembly moves out of the groove  30  end leaves the end of the rail  29 , the assembly leaves the path of the belts  24 ,  25  and the rotation of the mandrel  23  is stopped. 
     At this time the vertical position of the pin  27  is held in a fixed position, and a heated die  32  with an upwardly facing recess  33  is moved upwards ( FIG. 11 ) and pressed against the lower formed end of the separator  31 . The profile of the recess  33  corresponds to that of the head portion  28  of the pin  27 , and the pressure arising between the recess  33  and the head portion  28  fuses the separator  31  to take the shape of the head portion  28 . The heating is provided by a cartridge heater  34  installed in the interior of the die  32  as illustrated schematically on  FIGS. 11 and 12 , and the elevated temperature and pressure is sufficient to form and fuse the bottom portion of the sheet material to its final shape. Following this bottom forming and fusing step the so obtained structure will be essentially wrinkle-free and sufficiently strong to keep the lower part of the separator cylinder together. 
     The separator  31  should finally be inserted in the hollow interior of a cylindrical cell console (can/cathode) assembly  35 , which is a semi-final intermediate product and has the shape as illustrated in  FIGS. 15 to 17 . The console  35  comprises a cylindrical metal can  36  defining a cylindrical inner space, in which a plurality of hollow cathode rings  37  are inserted. The closed end portion of the can  36  has a short cylindrical tip  38 . The central region of the formed and closed bottom part of the separator  31  is just above the hollow interior space of the tip  38 , and this space is sufficient to allow application of a thermoplastic sealant referred also to as “hot melt” at the central bottom part of the separator  31 . The presence of this sealant is optional and provides additional short circuit prevention for the closed bottom part of the separator  31 . 
     The enlarged view on  FIG. 13  illustrates the application of a predetermined amount of hot melt on the central bottom part  39  of the separator  31 . A rotating wheel  40  is provided with shallow spherical or conical recesses  41 , and the surface of the wheel  40  just contacts or is arranged in the close vicinity of the central bottom part  39  of the separator  31 . A predetermined amount of hot melt is measured in the recesses  41 , e.g. by immersing the rotating wheel  40  into a heated tank of hot melt along the path of rotation (not shown), and after removal of excess hot melt from the recessed surface of the wheel  40  this latter will move into the position shown in  FIG. 13 , where the central bottom part  39  of the separator  31  contacts the recess  41 . Here the absorbent separator material will take up the volume of hot melt in the recess  41 . The wheel  40  moves then away from the contact zone, and the hot melt soaked in the separator material cools down, solidifies and provides an efficient sealing  42  at the central bottom part  39  as shown in  FIG. 14 . The application of a predetermined amount of hot melt in a uniform area on the separator bottom, which is a neutral area from the point of view of the operation of the cell as there is no active cathode material on the opposing side, and therefore will not cause any interference with cell performance and also not contribute to any added internal resistance. The application of the hot melt is in no way indispensable and represents an optional feature, whereby an increased protection against cell shorts is provided. 
     Reference will be made again to  FIGS. 15 to 17  wherein the insertion of the separator  31  into the hollow cylindrical interior of the console (can/cathode) assembly  35  is illustrated. The manufacturing line is in constant movement, and the separator  31  wrapped around the sleeve  26  and kept in the winding nest  22  ( FIGS. 11 ,  12 ) leaves the position where the hot melt sealing  42  is provided and moves to a station where the console assembly  35  is moved under the separator  31  so that their vertical axes fall in the same line. The console assembly  35  comprises the cell can  36  with the cathode rings  37  positioned therein. Now the sleeve  26  and the pin  27  are moved down together and the separator  31  is inserted in the hollow interior of the console assembly  35 . The wide head portion  28  of the pin  27  pushes the closed bottom part of the separator  31  during the whole path and ensures a perfect abutment with the bottom of the interior of the can  36 . The separator  31  slips out of the winding nest  22  when being already inserted partially into the console assembly  35 , thus the windings of the separator  31  cannot loosen or get unwrapped. After insertion the separator&#39;s winding can stretch out a bit to fill the whole inner space defined by the hollow inner surface of the cathode rings  37 . 
     The next step is the withdrawal of the sleeve  26  in upward direction as shown in  FIG. 16 . The pin  27  presses the bottom of the separator  31  during this withdrawal step and keeps the separator  31  in position. Finally, the pin  27  is withdrawn, and the semi-finished cell contains the console assembly  35  with the separator  31  duly positioned, and being ready for the next operation on the assembly line, i.e. electrolyte dispensing. The advantage of having a two-part mandrel  23 , where the outer sleeve  26  can be retracted first, is that there will be no chance of separator pull-back out of the semi-finished cell, thereby increasing efficiency and yield of the manufacturing process. 
     This design and assembly of the separator  31  is preferable because the full active length of the cell can be used for cell function. The separator can fill out the interior of the cathode rings, i.e. no gap will be formed, as it is the case in pre-formed separators. Comparative cell measurements have demonstrated that the average performance of cells provided with the separator according to the present invention was by about 10 to 20% better than in case of identical cells with conventional separator. The improvement was measurable in cell capacity and in decrease of the internal cell resistance resulting in higher short circuit currents and better high drain performance. 
     There will be no need for using any adhesive between adjacent layers of the separator as it is the case in most known designs, and the separator function will not be decreased by the presence of adhesives. A further advantage lies in the elimination for the need of using laminated sheets, which are more expensive than using bare sheets and have slightly worse performance due to the application of an adhesive between the layers. 
     The manufacturing process is also favorable because it is performed with the speed and machinery of the cell manufacturing line, i.e. the operation of the preparation of the separator can be synchronized with the manufacturing line.