Abstract:
An electrode assembly constructed of continuous anode and cathode electrodes that are overlaid in overlapping fashion and wound into a cell stack suitable for prismatic or other non-cylindrically-shaped cases. The cathode electrode strip has some regions where the electrode material is pressed to a high density and has some regions where the active material is pressed to a lower density, such that the lower density regions correspond to the bend regions in the wound cell stack.

Description:
FIELD OF INVENTION 
     The present invention generally relates to the art of electrochemical energy, and more particularly, to an electrode assembly, electrochemical cells in which the electrode assembly is used and a method for making the electrode assembly. 
     BACKGROUND OF THE INVENTION 
     Batteries or electrochemical cells are typically volumetrically constrained systems which cannot exceed the available volume of the battery case. The size and resulting volume of the battery case are dictated by the space requirements available for the particular application. The components that make up a battery namely, the cathode electrode, the anode electrode, the separator, the current collectors, and the electrolyte all have to fit into the limited space defined by the battery case. Therefore, the arrangement of the components impacts on the volume of electrode active material that can fit into the case and the ease of manufacturing the unit. 
     Some typical electrode assemblies that attempt to maximize volumetric efficiency include the “Z” folded electrode assembly which is disclosed in U.S. Pat. No. 3,663,721 to Blondel et al. In the “Z” folded electrode, a unitary and continuous lithium anode is folded back and forth in a zig-zag fashion. The length of the individual folds determines the width of the electrode assembly. Individual cathode plates are positioned between pairs of the pleated anode electrode and electrically connected to one another. The design has some drawbacks including the requirement that separate cathode plates be inserted between each pair of adjacent layers of anode electrode, and the requirement that electrical connections be made between all of the inserted cathode plates. This arrangement increases the time and costs associated with manufacturing. 
     Another typical volumetrically efficient electrode assembly configuration is the “jelly roll” design in which the anode electrode, the cathode electrode, and the separator are overlaid with respect to each other and wound into a coil. Such a wound electrode configuration is desirable because the continuous anode and cathode electrodes require a minimal number of mechanical connections to their respective terminal leads, and the jelly roll type assembly is generally recognized as preferred for high discharge and current pulse applications. Use of the winding method, however, often limits the density of the electrodes because as an electrode is pressed more densely to its current collector it becomes more brittle and has a greater tendency to crack and flake off the screen especially when wound about a small radius bend. Also, the electrode material may delaminate along the length of the electrode causing the material to lose contact with the current collector screen. 
     Because stacked or flat folded cathode plates as described above do not create the stresses in the bend regions that are associated with winding, the plate method has been able to accommodate higher density electrodes and therefore has traditionally provided a cell stack of higher total electrode weight and capacity than is possible using the wound method. 
     What is needed is an improved wound cell stack with a relatively high density electrode for use in a prismatic (cuboid-shaped) or other non-cylindrical case. 
     SUMMARY OF THE INVENTION 
     The present invention meets the above-described need by providing a wound electrode having some regions where the electrode material is pressed to a high density and having some regions where the active material is pressed to a lower density. 
     The wound electrode has some regions that lie in the “flat” and some regions that are curved. Moving from the inside of the wound stack to the outside of the stack, the bend regions have increasing radii. The flat regions and the bend regions with a longer radius curve are pressed to a high density similar to a cell stack formed from the plate method. In bend regions making relatively “tight” or short radius turns, the active material is pressed to a lower density to accommodate the stresses associated with the bending during the formation of the cell stack. 
     In a preferred embodiment, a wound electrode cell stack has electrode material pressed to a lower density in the regions corresponding to the shorter radius turns. The tool that presses the active material to the current collector is shaped such that alternating high density and low density regions are located on both sides of the current collector screen. 
     The present invention also includes a method of manufacturing a wound cell stack as described above. The method includes the steps of simultaneously pressing the electrode active material onto the current collector screen to a high density in some regions and to a low density in other regions. The location of the low density regions is predetermined such that when the anode and cathode electrodes are wound into a cell stack the low density regions correspond to the regions where the shortest radius curves are formed in the wound cell stack. 
     To assemble the cell stack, the cathode electrode is placed in alignment with the anode electrodes and the combination is then wound as described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a cathode electrode strip of the present invention; 
     FIG. 2 is a side elevation view of the cathode electrode strip of FIG. 1; 
     FIG. 3 is a plan view of a first anode electrode; 
     FIG. 4 is a plan view of a second anode electrode; 
     FIG. 5 is a side elevational view of the combined cathode and anode electrode strips prior to winding; 
     FIG. 6 is a plan view of the combined cathode and anode electrode strips prior to winding; and, 
     FIG. 7 is a side elevational view of the wound cell stack with high density and low density regions in the cathode electrode. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is designed for high energy devices such as batteries and capacitors and is adaptable in a wide variety of electrode configurations and shapes for such applications as capacitors and batteries including aqueous and nonaqueous primary and secondary batteries. 
     In FIG. 1, a cathode electrode  10  is shown. The cathode electrode  10  is preferably formed from a continuous strip of active material  11  contacting a conductive member serving as a cathode current collector  12 . The active material  11  is pressed onto the current collector  12  through a process that is known to those of ordinary skill in the art. The active material  11  is preferably a metal, a metal oxide, a metal sulfide, a mixed metal oxide or a carbonaceous material. The cathode current collector  12  is preferably comprised of conductive material such as of a conductive screen and the like. The cathode electrode  10  has a plurality of flat regions  13  that correspond to the flat regions  14  shown in the final wound cell stack  20  (FIG.  7 ). The electrode  10  also has a plurality of bend regions  16  that correspond to the bend regions  17  in the final wound cell stack  20 . Starting at the left end of the cathode electrode strip the flat regions  13  are disposed such that there is a bend region  16  between each successive flat region  13 . The bend regions  16  become progressively shorter toward the center of the electrode strip  10 . The most centrally located bend regions  16  correspond to the shortest radius curves in the final wound cell stack  20 . The shortest radius curves in the wound cell stack  20  generate the greatest stresses. When these sections are curved during the winding process, the material  11  on the inside of the strip  10  is compressed and the material  11  on the outside of the strip  10  is placed under tension. Accordingly, the sharpest or tightest (smallest radius) sections are most likely to cause failures manifested by flaking off or separation from the current collector screen (delamination). 
     Accordingly, referring to FIG. 2, the bend regions  16  are the regions that are most in need of relief from the stresses associated with winding. As a result the bend regions  16  have cathode active material  11  pressed at a lower density. The material  11  is pressed to a high density in the regions  13  that correspond to the flat sections  14  of the wound stack  20 . The regions  16  where the material  11  has been pressed to a lower density are capable of bending without flaking or delaminating due to the curvature. These regions  16  are created by a modified press (not shown) that is capable of pressing different regions of the cathode electrode  10  at different densities according to the pattern best shown in FIG.  2 . 
     Once the cathode electrode  10  is provided with the high and low density regions  13 ,  16  respectively, a separator (not shown) and a pair of anode electrode strips  21  and  24  (FIGS. 3-4) are placed in alignment with the cathode electrode strip  10 . The anode strips  21 ,  24  have regions  22  and  25  that correspond to the flat regions  14  of the final wound stack  20  and have regions  23 ,  26  that correspond to the bend regions  17  of the final cell stack  20 . While it is not a requirement, the anode electrodes  21 ,  24  may also be pressed to a lower density in bend regions  23 ,  26 . The anode electrodes  21 ,  24  have connectors  27  for connecting to the case in a case negative design as known to those of ordinary skill in the art. The combined anode strip, electrode strip, and separator are then overlayed as shown in FIGS. 5-6. Next, the electrodes are wound around a mandrel disposed in the center  45  of the combined strips as shown and described in copending patent application Ser. No. 09/262,245 entitled Wound Stack for Enhanced Battery Performance, which is assigned to the assignee of the present invention and which is incorporated herein by reference. 
     Returning to FIG. 1, the electrode assembly  10  also has electrical connectors  33  and  36  for connecting to the case and/or the terminal pin. The connector  36  has an elongate section  37  for connecting to the terminal pin as shown and described in U.S. Pat. No. 5,750,286 to Muffoletto et al., which is assigned to the assignee of the present invention and which is incorporated herein by reference. 
     It will be readily apparent to those of ordinary skill in the art to which the invention pertains that a “jelly roll” configuration could also be used. For the jelly roll configuration, a unitary anode strip that is approximately the same length as the cathode electrode would be overlayed with the cathode electrode with a separator between. The combined electrodes would then be wound about a mandrel from one end to the other end as known to those of ordinary skill in the art. The low density regions would be disposed in the areas corresponding to the tightest curves in the final wound cell produced according to the jelly roll method. 
     Turning to FIG. 7, the wound cell stack  20  of the present invention includes continuous anode and cathode electrodes ( 10 ,  21 ,  24 ) wound such that they are disposed in flat regions  14  and bend regions  17 . The flat regions  14  correspond to regions  13  of the cathode electrode where the electrode active material  11  is pressed to a high density comparable to the densities used with flat folded electrode assemblies. The bend regions  17  correspond to the regions  16  of the cathode assembly  10  where electrode active material  11  has been pressed to a low density on the current collector screen  12  to facilitate the winding of the cell stack  20 . These low density regions  16  facilitate the winding of the cell stack  20  by preventing delamination from occurring along the longitudinal axis of the cathode electrode  10 . If low density regions  16  are not provided, the electrode active material  11  may start to peel away from the cathode current collector  12  in the bend regions  17  and spread down the longitudinal axis into the straight regions  14 . The discontinuity provided by the regions  16 , where the electrode active material is pressed to a lower density, prevents the delamination from developing. 
     The present invention provides several advantages. By utilizing continuous anode and cathode electrode strips, the device provides for elimination of the extra connections for “like” plates associated with some of the plate designs. These extra connections do not contribute to the capacity or surface area of the active materials. The present invention also provides for a cell with a higher capacity than a cell stack constructed of all high density plates or one constructed of uniformly low density wound elements. Also, the present invention provides for bending of the electrode without cracking of the active material in the tight bend regions. The present invention increases the capacity density of the cell by about ten percent or more depending on the case aspect ratio. 
     While the invention has been described in connection with certain preferred embodiments, it is not intended to limit the scope of the invention to the particular forms set forth, but, on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.