Abstract:
An electrochemical cell comprising an electrode assembly in which opposite polarity electrodes are wound together in a bi-directional fashion yielding a high energy density cell stack with low internal impedance is described. Each electrodes is constructed having a slot provided into its width at about a midportion thereof. The slots are brought into registry with each other to form a collapsible X-shaped electrode assembly, which is then bi-directionally folded to provide a wound electrode assembly.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation-in-part application of Ser. No. 09/262,245, filed Mar. 4, 1999, now abandoned. 
    
    
     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 that 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 amount of active electrode material that can be fit into the case and the ease of manufacturing the unit. 
     Some typical electrode assemblies include the “Z” folded electrode assembly that 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 zigzag 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 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 coiled up. Such an 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 assembly is generally recognized as preferred for high discharge and current pulse applications. However, in some applications, a cylindrically shaped electrode assembly is not desired because of other factors, such as the shape of the battery case. 
     U.S. Pat. No. 4,761,352 to Bakos et al. discloses yet another electrode assembly design comprising an accordion folded electrode assembly with unitary members for both the anode and cathode strips. The cathode strip is approximately half the length of the anode strip, and the anode strip is folded over the cathode strip to “sandwich” the cathode between two layers of the anode. The resulting form is then manually folded in an alternating series of “V” folds (best shown in FIG. 4 of the patent). However, that design provides some undesirable gaps which reduce the volumetric density of the electrochemically active materials. 
     What is needed is an improved multi-layer, folded electrode assembly design for high energy devices that includes many of the desirable features of the jelly roll design, such as unitary anode and cathode electrodes. 
     SUMMARY OF THE INVENTION 
     The present invention fills the above-described need by providing an electrochemical cell comprising an electrode assembly in which the electrodes are wound together in a bi-directional fashion, yielding a high energy density cell with low internal impedance. The anode and cathode electrodes are arranged in the cell in such a fashion that provides efficient utilization of the active components. The resultant wound assembly is configured such that it can be conveniently packaged in either a cylindrical or prismatic housing. 
     In one embodiment of the electrochemical cell, the electrodes are provided as two anode assemblies and one cathode assembly configured such that each anode is positioned on either side of the cathode assembly, and extending in opposing directions. At the center most portion of the assembly there is an overlap of anodes. This assembly is then wound about the overlapping region in a bi-directional fashion. The resultant assembly produces a wound cell stack configuration with a uniform contact of anode and cathode, such that the cell is balanced electrochemically and provides for optimum volume utilization within the battery enclosure. Each anode has one or more tabs that can be welded to the case. Alternately, two cathode assemblies can be paired with one anode assembly, with a resultant cathode tab welded to the case. In both of the above configurations, the opposite electrode may contain one or more tabs which are then electrically connected to the battery feedthrough pin. 
     An alternate embodiment of this invention provides for an anode electrode and a cathode electrode, wherein the electrodes are slotted. The electrodes are inserted, one into the other, essentially forming an “X”. Upon collapsing the electrodes, a variation of the above-described invention is obtained wherein the anode is approximately equally disposed on opposite sides of the cathode, radiating outwardly from the midportion thereof. This assembly is then wound from the center, resulting in a preferred cell stack assembly. This configuration provides the additional advantage of having the anode registered to the cathode, and mitigates the need for aligning two distinct anodes to the cathode. 
     Other features and advantages of the present invention will become apparent upon reading the following detailed description of embodiments of the invention, when taken in conjunction with the accompanying drawings and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of the cathode strip and separator of the present invention; 
     FIG. 2 is a side elevational view of the anode strip and separator of the present invention; 
     FIG. 3 is a bottom plan view of the cell stack assembly of the present invention; 
     FIG. 4 is a side elevational view of the cell stack assembly of the present invention; 
     FIG. 5 is a partial plan view of the wound electrode assembly of the present invention; 
     FIG. 6 is a perspective view of an alternate embodiment of the electrode strips of the present invention; 
     FIG. 7 is a partial plan view of the wound electrode assembly of the alternative embodiment; and 
     FIG. 8 is an exploded view of an electrochemical cell of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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 applications as capacitors and batteries, including aqueous and nonaqueous primary and secondary batteries. 
     Referring to FIG. 1, a first electrode  10  is preferably a continuous structure comprising an active material  11  contacted to a current collector  12  (shown in dashed lines). The active material for a cathode electrode is preferably comprised of a metal, a metal oxide, a metal sulfide, a mixed metal oxide, a carbonaceous material, or the like and is combined with the current collector of a conductive material such as a conductive screen. For an anode electrode, the preferred active material is an alkali metal selected from Group  1 A of the Periodic Table of Elements and contacted to an anode current collector. A preferred anode electrode comprises lithium contacted to a nickel current collector. In a preferred form of the present invention, the electrode strip  10  is a cathode electrode having a set of cathode tabs  15  provided for making an electrical connection to a positive terminal. 
     Turning to FIGS. 2 and 3, a second electrode  16  includes a pair of second electrode strips of a second electrode active material  17  contacted to a current collector  18  (shown in dashed lines) disposed on opposite sides of the first electrode  10 . The second electrode strips  16  overlap along a midportion  19  of the first electrode  10  (FIG.  3 ). Preferably, the second electrode strips  16  are part of the anode electrode. The anode electrode strips  16  have anode tabs  22  that provide for electrical connection to a negative terminal. 
     As shown in FIGS. 1,  2  and  4 , a separator material  13  is disposed behind each electrode to prevent contact between overlayed layers of electrodes. Alternatively, the separator  13  is disposed in front of each electrode strip. In a preferred embodiment, which is not shown in the drawings, a separator  13  in the form of an envelope encapsulates each of the first and second electrodes  10 ,  16 . In that respect, whether the separator  13  is disposed between immediately adjacent electrode strips or, the separator serves as an envelope encapsulating at least one of the electrodes, the separator must prevent direct physical contact between the electrodes  10 ,  16 . 
     Turning to FIG. 4, an electrode assembly according to the present invention comprises a cathode electrode  10  and two anode electrodes  16 A,  16 B, which are each preferably elongate, flat, and rectangular. The anode electrodes  16 A,  16 B are disposed on opposite sides of the cathode  10  and aligned such that they overlap across the midportion  19  thereof. The anode electrodes  16 A,  16 B are a little more than half the length of the cathode electrode  10 , and extend a short distance across the midportion  19  in order to overlap. Alternately, two cathode electrode assemblies are paired with one anode electrode in a similar overlapping configuration. 
     From the alignment shown in FIGS. 3 and 4, the electrode strips  10  and  16  are then folded about the overlapping region in a bi-directional fashion to provide the electrode assembly  25 . As shown in FIG. 5, those portions of anode strips  16 A and  16 B on the outside of the assembly  25  have the outside of the current collector devoid of anode active material. This is because there is no opposing cathode active material, and such anode active material would provide very little, if any, additional volumetric efficiency. Also, the ends of the anode strips  16 A and  16 B extend somewhat beyond the end of the cathode electrode  10  to fully utilize the discharge efficiency of the cathode electrode. 
     The term bi-directional refers to the fact that one side is folded downwardly and the opposite side is folded upwardly, either in succession or simultaneously, to generate the electrode assembly  25  shown in FIG.  5 . The electrode assembly  25  produces a wound cell stack configuration with uniform contact of anode and cathode electrodes such that the cell is balanced electrochemically and provides for optimum volume utilization within the battery enclose. 
     An alternate embodiment of the present invention is shown in FIGS. 6 and 7. In this embodiment, a cathode electrode strip  50  comprising a cathode active material  52  contacted to a cathode current collector  54  has a downwardly facing slot  53  disposed in a midportion  56  thereof. The slot  53  extends from a lower edge  58 A toward an upper edge  58 B, but spaced therefrom. The lower and upper edges  58 A and  58 B define the length of the strip  50 . An anode electrode strip  60  comprises an anode active material  62  contacted to an anode current collector  64  and includes an upwardly facing slot  63  disposed in a midportion  66 . The slot  63  extends from an upper edge  68 A toward a lower edge  68 B, but spaced therefrom. The upper and lower edges  68 A and  68 B define the length of the strip  60 . 
     As shown in FIG. 6, the anode strip  60  is provided with a separation  13  to prevent direct physical contact with the cathode strip  50 . Preferably, the separator  13  envelopes the anode strip  60 , and more preferably, each of the cathode strip  50  and the anode step  60  are housed in their own separate envelopes. 
     To construct the electrode assembly, the strips  50  and  60  are moved together with the slots  53 ,  63  registering with each other to form a collapsible X-shaped assembly. In this embodiment, the opposed ends  68 C and  68 D of the anode strip  60  extends outwardly a small distance past the opposed ends  58 C and  58 D of the cathode strip  50  and in a configuration such that each electrode  50 ,  60  radiates outwardly from the midportion  56 ,  66  of the other electrode. The electrode strips  50 ,  60  are then folded in a bi-directional fashion from the center or midportions  56 ,  66  to produce the wound electrode assembly  75  shown in FIG.  7 . The bi-directional folding is similar to that described with respect to the electrode assembly  25  shown in FIGS. 1 to  5 . 
     The completed electrode assembly  75  shown in FIG. 7 is similar to the electrode assembly  25  in the respect that those portions of anode strip  60  on the outside of the assembly have the outside of the current collector devoid of anode active material. As previously explained, this is because there is no opposing cathode active material there, and such anode active material would provide very little, if any, additional volumetric efficiency. Also, the ends of the anode strip  60  extend somewhat beyond the respective ends of the cathode strip  50  to fully utilize the discharge efficiency of the cathode electrode. This alternate embodiment provides the additional advantage of having the anode registered to the cathode and mitigates the need for aligning two distinct anodes to the cathode. 
     The present electrode assemblies  25 ,  75  provide several advantages to cell design, including high energy density with low internal impedance. Additionally, the anode and cathode electrodes  10 ,  16  for assembly  25  and the electrodes  50 ,  60  for assembly  75  are arranged in the cell in a way that provides efficient utilization of the active components. The resultant wound cell stacks are configured such that they can be conveniently packaged in either a cylindrical or prismatic shaped casing. These casing shapes are well known to those of ordinary skill in the art. The electrode assemblies  25 ,  75  also provide a cell stack construction in which the anode and cathode are uniformly utilized during cell discharge. Finally, the assemblies  25 ,  75  provide a cell having a relatively high inter electrode surface area which results in a high current rate capability. This is advantageous for use in applications such as powering an implantable defibrillator. 
     A preferred primary electrode chemistry for the electrode assemblies  25 ,  75  according to the present invention has the first electrode  10 ,  50  of a mixed metal oxide such as silver vanadium oxide (SVO), copper silver vanadium oxide (CSVO) or a fluorinated carbonaceous material (CF x ), and the second electrode  16 ,  60  comprising lithium. A Li/SVO or Li/CSVO electrochemical couple is activated with an electrolyte of 0.25M to 1.5M LiAsF 6  or LiPF 6  in a 50:50, by volume, mixture of propylene carbonate and 1,2-dimethoxyethane. For a Li/CF x  cell, the preferred electrolyte is 1.0M to 1.4M LiBF 4  in γ-butyrolactone. A preferred secondary chemistry has a carbonaceous negative electrode and a lithiated counter electrode. A preferred lithiated material is lithium cobalt oxide. This couple is activated with an electrolyte of 1M LiPF 6  or 1M LiAsF 6  in ethylene carbonate/1,2-dimethoxyethane (3:7). 
     Referring to FIGS. 1,  2  and  8 , the anode tabs  22  can be welded to the case  80  (negative). Alternately, two cathode assemblies can be paired with one anode assembly with the resultant cathode tabs (not shown) welded to the case  80  (positive). In both of the above configurations, the opposite electrode may contain one or more tabs (cathode tabs  15 ) that are electrically connected to the battery feedthrough or terminal pin  82 . The terminal pin  82  is electrically insulated from the lid  84  of the casing  80  by a glass-to-metal seal  86 . Similar electrical connections for the cathode strip  50  and the anode strip  60  are made for the electrode assembly  75  shown in FIGS. 6 and 7. 
     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.