Patent Publication Number: US-2006014075-A1

Title: Process for manufacturing plate electrode stackings

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to a process for manufacturing plate electrode stackings for use in electrochemical cells, such as batteries, and a separator material for use in such a process.  
      Lead acid batteries are widely used and include adjacent positive and negative electrodes immersed in an electrolyte and spaced by separators.  
      The electrodes used in the manufacture of such batteries are generally made of lead dioxide for the positive electrode and lead for the negative electrode. The electrodes are separated from each other by a battery separator that prevents the adjacent electrodes from coming into physical contact and that provides space for an electrolyte.  
      Two different lead acid battery designs are used commercially: the flooded cell and the recombinant cell.  
      In a flooded cell, only a small portion of the electrolyte is absorbed into the separator. Thus, the battery separator typically has ribs extending from one or both planar surfaces to provide open space for “free” electrolyte. The separator typically used in flooded cells is an extruded microporous ribbed polymer sheet.  
      In a recombinant cell, wherein the oxygen gas generated at the positive electrode during charge is recombined at the negative electrode to form water, the electrolyte is gelified.  
      Furthermore, electrodes for battery assembly often comprise an additional layer of retention mat to reduce the loss of active material.  
      Numerous assembly steps are necessary to manufacture an electrode stacking for use in battery assembly. Known processes often require manual steps or very complex automation equipment to manufacture protected plate electrodes or electrode stackings for use in battery assembly.  
      U.S. Pat. No. 4,418,464 describes an apparatus for wrapping and enveloping a battery plate with three layers of different materials.  
      Typically, a layer of retention tape, such as Hydramatic, is first vertically wrapped around a positive electrode and is then clamped in place using a cap or boot. In a preassembling step, a glass mat and a Koroseal layer are unrolled and heat sealed together, then cut to size. This cut laminate is then positioned below the Hydramatic and horizontally wrapped around the electrode and its ends are sealed together. In an additional step, the wrapped positive electrode, a polyethylene separator cut to size and a negative electrode are alternatively positioned and stacked to form an electrode stacking for use in a battery.  
      Prior art methods are either labor intensive and time consuming, implying partially manual procedures, or require complex and expensive equipment to automate the numerous cutting, folding, wrapping, assembling and sealing steps.  
     SUMMARY OF THE INVENTION  
      The object of the present invention is therefore to provide a process for manufacturing a plate electrode stacking for use in battery assembly, which allows a simplified, efficient and more economical procedure, while ensuring safe and high quality assembly. A further object of the invention is to provide a separator material for use in such a process.  
      In order to overcome the abovementioned problems, the present invention proposes a process for manufacturing of a plate electrode stacking for use in batteries, comprising the steps of 
          a) supplying a separator material comprising a polymer sheet layer and a glass mat layer, dimensions of said glass mat layer being chosen in order to leave an overlapping portion of said polymer sheet layer uncovered,     b) placing a positive plate electrode onto said glass mat layer of the separator material,     c) folding and wrapping said separator material around said positive plate electrode,     d) attaching said overlapping portion to form a sleeve with an upper and a lower open end, and     e) placing a negative plate electrode onto said sleeve containing the positive plate electrode.        

      The invention allows the manufacturing of electrode stackings with alternate positive and negative plate electrodes ready for insertion in a battery case, while necessitating a reduced number of discrete operating steps. The enveloping of the electrode with the separator material can actually be achieved in a single basic folding and wrapping operation, which essentially consist in bending both sides of the separator material around the electrode. The fact that one single separator is needed to replace the current several separately folded layers of Hydramatic, glass mat, Koroseal and PE separator significantly reduces the number of required steps and hence increases the productivity of the operation. It also greatly facilitates the implementation of the process in a fully automated continuous system and, as a consequence, the reliability of the manufacturing process. Furthermore, the combined polymer sheet and glass mat separator improves the packing and the performance of the battery.  
      Hence, the present process is particularly advantageous for the battery manufacturer as high quality batteries may be obtained without complicated and expensive machinery.  
      The separator material used in step a) comprises a polymer sheet layer and a glass mat layer, the dimensions of the latter being chosen in order to leave an overlapping portion of said polymer sheet uncovered. In fact, the size of the glass mat is chosen to allow a complete wrap around the plate electrode, both sides of the wrapped glass mat forming a flush joint. The polymer sheet layer is larger than the glass mat layer to form an overlapping portion upon wrapping in step c).  
      The polymer sheet may be made of any suitable porous, hydrophilic and chemically stable polymer or copolymer, or mixtures of such (co)polymers, providing sufficient flexibility, appropriate porosity, low electrical resistance and high puncture resistance. A particularly preferred polymer sheet comprises porous or microporous polyvinyl chloride, containing silica, the silica being preferably physically bonded to the polymer. The thickness of the polymer sheet layer largely depends on the constituents used and the technical requirements of the separator inside the electrochemical cell. Generally, its thickness will range from 0.15 to 5.00 mm, preferably from 0.50 to 4.00 mm.  
      The role of the glass mat layer is to prevent loss of the active material. Although the thickness of such a glass mat may vary as a function of technical and economical considerations, it will generally be chosen in a range of 0.15 to 5.00 mm, preferably 0.40 to 2.00 mm.  
      The polymer sheet and the glass mat layer which make up the separator material are preferably attached together by any suitable means, e.g. using glue, hot-melt adhesive, etc.  
      In a further embodiment of the present invention, the polymer sheet is provided on at least part of its surface with corrugations, i.e. the polymer sheet is altered to display a wrinkled cross-section usually forming a parallel straight or sinusoidal surface pattern. This configuration is advantageous as corrugated polymer sheets exhibit improved physical properties compared to a standard ribbed separator. Depending on the polymer material used, the corrugation process may advantageously increase the volume porosity of the polymer sheet, thus reducing acid displacement and electrical resistance.  
      The corrugation of flat polymer sheets may be achieved by optionally heating the sheet and passing it between two corrugating rolls with generally complementary embossing patterns. A particularly preferred design of corrugation rolls will be described below.  
      As mentioned above, the overlapping portion of the polymer sheet is attached after folding and wrapping in step c) on the underlying polymer sheet by any appropriate means, such as gluing, hot-melt gluing, etc.  
      In the overlapping portion, the attachment of two layers of polymer sheet will generally result in an increased thickness of the resulting sleeve. This might be considered as a disadvantage during the electrode stacking and during its placement in the electrochemical cell. One option is to leave these parts uncorrugated, thereby reducing the thickness of the seam. Another, preferred option is to modify the shape and/or the extent of the corrugations in such a way that in the overlapping portion both polymer sheet layers closely fit together without getting such an extra thickness.  
      Therefore, in a further embodiment, the corrugations may be adapted on one or more parts of said polymer sheet, e.g. the overlapping portion and/or the overlapped portion, to avoid an excessive thickness in the overlapping part of the resulting sleeve. This can be achieve for example by using corrugation rolls showing a embossing pattern as described more in detail below.  
      In a further embodiment of the invention, the polymer sheet is not provided with corrugations on portions forming lateral edges of the sleeve containing the positive plate electrode. The lack of corrugations along the lateral sides of the sleeve is of particular advantage not only to facilitate the folding and wrapping in step c), but also to tightly control the width of the resulting sleeved electrode, thereby ensuring an optimal fit of the stackings inside the electrochemical cell.  
      To further enhance the retention of electrode material inside the sleeve and to further reduce the incidence of a short-circuit during charging and discharging operations, the process according to the invention, in a still further embodiment, preferably comprises the additional step of closing one of said open ends of the sleeve, in particular the lower open end, using any appropriate means, such as welding, gluing or, preferably, using a clip.  
      The clip used to close the lower end of the sleeve may be a preformed clip, preferably with a U-shaped cross section or, in a preferred embodiment, the clip is a U-shaped clip formed by heating and subsequently shaping a flat stock of polymer ribbon during the process, e.g. using a forming shoe or the like.  
      As explained above, the present process is particularly aimed at the battery manufacturer and therefore the separator material is preferably provided in a ready to use form, preferably cut to size according to the manufacturer&#39;s requirements. Hence, in a further embodiment, the separator material in step a) is supplied in leaf form and its size and corrugation pattern being adapted for the manufacture of a sleeved electrode.  
      Although the manufacturing steps described above appear as discrete and separate manufacturing steps, they may of course be combined in space or in time to further optimize the overall process or to reduce the time needed per electrode stacking. As an example, steps c) and d) may combined by directly attaching said overlapping portion during folding and wrapping, e.g. by first applying glue onto the overlapping portion and by subsequently folding and wrapping the separator material.  
      A further aspect of the invention provides for a separator material for use in a process according to the invention, comprising a polymer sheet layer, and, attached on one surface thereto, a glass mat sheet layer, wherein said polymer sheet is provided with corrugations on at least part of its surface and the dimension of said glass mat sheet layer being chosen to leave an overlapping portion of said polymer sheet uncovered.  
      As already explained above, in a preferred embodiment, the corrugations of the polymer sheet may be adapted on one or more parts of said polymer sheet to reduce the overall thickness in the overlapping portion of the resulting sleeved electrode.  
      Furthermore, the polymer sheet is preferably not provided with corrugations on one or more portions, in particular along the lateral sides of a sleeved electrode manufactured according to a process of the invention, to facilitate folding of said separator material and/or to control its shape upon folding. As stated above, the lack of corrugations along the lateral sides of the sleeve not only facilitates the folding and wrapping step, but also allows to tightly control the shape and especially the width of the resulting sleeved electrode, thereby ensuring a optimal fit of the stackings inside the electrochemical cell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic drawing of a conventional process showing the individual steps needed for the manufacture of a plate electrode stacking.  
       FIG. 2  is a schematic drawing of a process according to a preferred embodiment of the present invention showing the individual steps needed for the manufacture of a plate electrode stacking, wherein  FIG. 2A  is a side view and  FIG. 2B  a top view of a preferred embodiment.  
       FIGS. 3A and 3B  are isometric views of steps b) and c) respectively, according to a preferred embodiment of the present invention.  
       FIG. 4A  illustrates a preferred roll design for the corrugation of a separator material for use in the present invention and  FIG. 4B  and  FIG. 4C  are detailed views of  FIG. 4A . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The abovementioned and further objects of the present invention will be more apparent from the following description of a not limiting embodiment with reference to the attached drawings.  
      In  FIG. 1 , which represents a prior art process for the manufacture of electrode plate stackings, the positive plates  10  are placed onto an assembly belt.  
      In step  1 , the Hydramatic  11  is unrolled vertically and taken by the advancing plates  10  in the middle. The Hydramatic  11  is cut to size and U-wrapped around the plate  10  in step  2 . In step  3 , the Hydramatic  11  is fixed with a clip to the bottom of the plate  10  by forming and mounting a cap or boot  12 . In step  4 , the glass mat  14  and the Koroseal  13  are unrolled beneath the chain conveyor, heat sealed together on an inline hot roller, then cut on a rotary knife against a back-up roll. This cut laminate is then positioned below the Hydramatic  11  and guided with “tooth jigs” on the main conveyor. The folding around the edges is accomplished with a shoe-like fixed anvil. An inline heat tape is used to seal the Koroseal into a vertical seam and onto the bottom cap  12 . In step  5 , the protected positive plates  15 , comprising a positive plate  10 , a Hydramatic layer  11 , a boot or cap  12 , a layer of glass mat  13  and a layer of Koroseal  14 , are then piled by vacuum or fork lifters onto stacks.  
      The stacks are transported to the assembly station (for example a Tekmax stacker) in step  6 , where a protected positive plate  15  is placed on a conveyor belt. In step  7 , a first separator  16  is lifted with vacuum heads onto the protected positive plate  15 . In step  8 , a negative plate  17  is laid on this first separator  16 . The positioning of a second separator  16  on the negative plate  17  is accomplished in step  9 . Finally, in step  10 , the units from step  9  comprising a protected positive plate  15 , a first separator  16 , a negative plate  17  and a second separator  16 , are piled up to form the electrode stackings for use in the battery assembly.  
      In  FIG. 2   a , resp.  2   b , the separator material  20  according to the invention, comprising a corrugated polymer sheet layer  21  and a glass mat layer  22  is positioned on a conveyor belt in step  1 . In step  2 , the positive plate electrode  10  is then placed on the separator material  20  with the glass mat layer  22  toward the positive plate  10 .  
      In step  3 , the folding and wrapping of the separator material  20  around the plate may be accomplished manually or preferably with a continuous folding in an automated step. In order to close the sleeve permanently, a strip of glue is applied on one side of the overlapping part and pressed together with a shoe, thereby forming a protected or sleeved positive plate electrode  25 , comprising a positive plate electrode  10  wrapped in a separator material  20  according to the invention.  
      The bottom of the sleeve may or may not be closed with a clip using a flat stock of PVC ribbon (step not represented). This ribbon is heated by a forming shoe to shape it and in the same time flattening the space of the separator were it is applied on.  
      In step  4 , a negative plate electrode  15  is positioned on the sleeved electrode and the resulting units, comprising a sleeved electrode  25  and a negative plate  15 , are piled up in step  5  to form the electrode stackings for use in the battery assembly.  
       FIGS. 3A and 3B  show the main steps of a process according to the invention. In  FIG. 3A , a positive plate electrode  10  is placed on a separator material  20  comprising a polymer sheet layer  21  and attached thereto a glass mat layer  22 . The width of the latter is chosen to form a flush joint after the folding and wrapping, whereas the polymer layer is wider so as to define an overlapping portion which is not covered by the glass mat layer. The portions C of the separator material which later form the lateral sides of the protected envelope are not corrugated in this preferred embodiment.  
       FIG. 3B  depicts the situation at the end of the folding and wrapping step wherein the separator material is going to be attached, e.g. glued in the overlapping portion to form a protected or sleeved plate electrode  25 , comprising the separator material with layers  21  and  22  and the positive plate electrode  10 . Layer  21 , which is the polymer sheet, has a general corrugation profile A over most of its surface, whereas the portions C forming the sides of the sleeved plate  25  are uncorrugated. In at least one of the portions B in the overlapping part (or in both overlapping and overlapped portions) of the sleeve, these corrugations are preferably adapted to prevent oversized seams.  
       FIG. 4A  illustrates a typical roll which may be used to corrugate the polymer sheet layer. In fact, the corrugating roll shown allows the corrugation of a double width (see line X) of polymer sheet layer which may be cut after corrugation or preferably after the attachment of the glass mat layer. A detailed view of the general corrugation profile A is shown in  FIG. 4C . As mentioned above, to reduce the overall thickness of the sleeve in the overlapping part, this general profile is adapted, e.g. as shown in reference B, or in the enlarged view in  FIG. 4B , by reducing the height of the embossing ribs, thereby allowing a better fit in the overlapping part. Portions referenced C display the uncorrugated lateral parts of the resulting sleeved plate electrode.