Abstract:
An electrochemical cell comprising a conductive casing housing an electrode assembly provided with a stack holder surrounding the electrode assembly is described. The stack holder is of a shape memory material that serves to maintain the anode and cathode in a face-to-face close physical proximity alignment throughout discharge. This is particularly important in later stages of cell life. As the cell discharges, anode active material is physically moved from the anode to intercalate with the cathode active material. As this mass transfer occurs, the cathode becomes physically larger and the anode smaller. This can lead to gaps forming between the anode and the cathode. However, the stack holder inhibits the formation of such gaps by maintaining a compressive force on the electrode assembly throughout cell discharge.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Application Ser. No. 61/098,875, filed Sep. 22, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an electrochemical cell. More particularly, the present invention relates to an electrochemical cell having a stack holder that keeps the electrodes in proper electrochemical alignment and close proximity with respect to each other, even as their dimensions change during cell discharge. 
     2. Description of Related Art 
     A typical electrochemical cell used to power an implantable medical device comprises a casing housing an anode and a cathode. The anode and the cathode are physically segregated from each other, typically by enclosing at least one of them within an envelope or bag of insulative separator material. The separator is typically provided as a thin porous sheet material that is saturated with electrolyte and allows the transport of ions in the electrolyte there through. The anode and the cathode are generally formed as one or more respective plates of anode active material and cathode active material. The plates are then aligned face-to-face and spaced apart from each other by the separator material, to form an electrode assembly or electrode stack (a.k.a., cell stack) within the cell casing. In order to maximize discharge efficiency and stabilize the location of the electrodes within the casing, it is preferable that the electrode assembly be tightly fitted within the casing&#39;s walls while occupying as much internal volume as possible. It is understood that electrochemical cells perform most efficiently when the anode and the cathode plates are positioned in close physical proximity to each other. Close physical proximity minimizes the path length that current carrying ions must travel. Ultimately, the close physical proximity minimizes the electrical impedance of the cell as measured at the cell terminals. 
     During discharge of lithium anode-type cells, the thicknesses of the cathode and anode plates change. In lithium anode systems, the thicknesses of the cathode plates increase while those of the anode decrease, but the total thickness of the electrode assembly decreases continuously throughout discharge. This occurs because the rate of cathode thickness increase due to lithium intercalation is smaller than the rate of lithium consumption at the anode. As the overall electrode assembly thickness decreases, gaps can form between the anode plates and the cathode plates. The gaps may eventually be sufficient to allow the electrode assembly to move within the casing. That outcome is undesirable. As the electrodes form gaps, they may no longer be in close physical proximity to each other. As the cell discharges, this may result in an increase in electrical impedance along with increased cell resistance between the cathode and the anode, thereby causing lower pulse voltages (cell terminal voltage under conditions after an intermittent high current pulse load), faster cell polarization, greater voltage fluctuations, and in general, more delivered capacity variation. 
     What is needed is an electrochemical cell comprising an electrode assembly having an anode and a cathode that maintain close physical proximity to each other throughout the entire discharge life of the cell. 
     SUMMARY OF THE INVENTION 
     The present invention meets this need by providing an electrochemical cell comprising a conductive casing housing and an electrode assembly. The casing comprises a side wall structure extending to an open end closed by a lid. The electrode assembly comprises a cathode of at least a first plate of cathode active material, an anode of at least a first plate of anode active material, and a separator disposed at an intermediate location between the plates of cathode active material and anode active material. The cell further includes a stack holder surrounding or partially surrounding the electrode assembly. The stack holder may be formed as a bag that receives and envelopes the electrode assembly. Alternatively, the stack holder may be formed as a band disposed around a perimeter of the electrode assembly. 
     The stack holder is preferably made of a shape memory material with protrusions. Each protrusion applies a force along the electrode assembly surface or case wall surface. Taken together, the protrusions provide the electrode assembly with a uniformly distributed compressive force, sufficient in magnitude and compliance to maintain close physical contact between the active components in the electrode assembly over the life of the cell. In that manner, as the volume of the electrode assembly varies during cell discharge, the protrusions extend further to apply a uniformly distributed compressive force upon the electrode assembly to maintain the desired close physical proximity of the anode and the cathode plates. 
     Either or both of the anode and the cathode may be comprised of a plurality of plates of their respective electrode active materials. The cell may be provided in either a case-positive or case-negative configuration. Each of the respective plates of electrode active material may be enveloped in its own separator, with the entire electrode assembly then being encircled, enveloped or partially enveloped by the stack holder. 
     The foregoing and additional objects, advantages, and characterizing features of the present invention will become increasingly more apparent upon a reading of the following detailed description together with the included drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described by reference to the following drawings, in which like numerals refer to like elements, and in which: 
         FIG. 1  is a perspective view of an electrochemical cell  10  according to the present invention. 
         FIG. 2  is a cross-sectional view of  FIG. 1  taken along the lines  2 - 2  and illustrates a first embodiment of an electrochemical cell of the present invention comprised of a first and second anode plates and cathode plates forming the electrode assembly, and a stack holder provided as a bag enclosing the electrode assembly. 
         FIG. 2A  is an enlarged section of  FIG. 2  showing the gap D 1  between the interior surface  150 B of the stack holder&#39;s wall contacting surface  150  and the electrode assembly&#39;s exterior surface  20 A at a first period of time. 
         FIG. 3  is  FIG. 2  over the discharge life in the case of a primary cell and, after numerous cell discharges in the case of a secondary cell. 
         FIG. 3A  is an enlarged section of  FIG. 3  showing the gap D 2  between the interior surface  150 B of the stack holder&#39;s wall contacting surface  150  and the electrode assembly&#39;s exterior surface  20 A at a second, later period of time. 
         FIG. 4  illustrates one embodiment of a stack holder; 
         FIGS. 5 to 7  illustrate alternative embodiments of a portion of the stack holder shown in  FIG. 4 . 
         FIGS. 8 and 8A  are cross-sectional views of the cell shown in  FIG. 2 , but illustrating alternate embodiments of band-type stack holders. 
         FIG. 9  is an alternative embodiment of  FIG. 2 . 
     
    
    
     The present invention will be described in connection with preferred embodiments, however, it will be understood that there is no intent to limit the invention to the embodiments described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning first to  FIG. 1 , an electrochemical cell  10  of either a primary or secondary, rechargeable chemistry is shown. The cell  10  is comprised of a conductive casing  12  having first and second opposed major face walls  14  and  16  joined to a surrounding side wall  18 . The face walls  14 ,  16  and surrounding side wall  18 , as illustrated at  FIG. 2 , form an open ended container that receives an electrode assembly  20 , as will be described hereinafter. The open ended container  12  housing the electrode assembly  20  is then closed by a lid  42 . The casing  12  and lid  42  may be comprised of materials such as stainless steel, mild steel, nickel-plated mild steel, titanium, tantalum or aluminum, but not limited thereto, so long as the metallic material is compatible for use with the other cell components and medically acceptable. The casing lid  42  is typically provided with a first opening  70  to accommodate a glass-to-metal seal  40 /terminal pin feedthrough  38  (see  FIGS. 1 ,  2 ,  3 ,  8 ,  8 A and  9 ) and a second opening  72  for electrolyte filling (see  FIG. 1 ). 
     The electrode assembly or electrode stack  20  comprises a cathode  22  and an anode  24  as shown in  FIG. 9 , or a first anode  24 A, the cathode  22 , and a second anode  24 B as shown in  FIG. 2 . The anode  24 , the first anode  24 A and the second anode  24 B are collectively referred to as anode  24 . The electrode assembly  20  is housed within the casing  12 . The cathode  22  is comprised of opposed plates  26 A,  26 B of cathode active material sandwiching a cathode current collector  34 . Suitable cathode active materials include fluorinated carbon, silver vanadium oxide, copper silver vanadium oxide, Ag 2 O, Ag 2 O 2 , CuF 2 , Ag 2 CrO 4 , MnO 2 , V 2 O 5 , MnO 2 , TiS 2 , Cu 2 S, FeS, FeS 2 , copper oxide, copper vanadium oxide, and mixtures thereof. Suitable cathode current collector materials are selected from the group consisting of stainless steel, titanium, tantalum, platinum, gold, aluminum, cobalt nickel alloys, nickel-containing alloys, highly alloyed ferritic stainless steel containing molybdenum and chromium, and nickel-, chromium- and molybdenum-containing alloys. 
     The anode  24  is comprised of a plate  28  of anode active material contacting one side of an anode current collector  30 . The other, bare side of the anode current collector  30  resides adjacent, minus the separator and/or stack holder, to the casing major face wall  14  and/or  16 . That is because only anode material directly facing the cathode material participates in cell discharge. For a primary cell, lithium and its alloys and intermetallic compounds, for example, Li—Si, Li—Al, Li—B and Li—Si—B alloys, are preferred for the anode active material. For a secondary cell, the anode is of a carbonaceous material, for example graphite, that is capable of intercalating and de-intercalating lithium ions. Preferably, the anode is a thin metal sheet or foil of lithium metal or graphite, pressed or rolled on a metallic anode current collector selected from titanium, titanium alloy, nickel, copper, tungsten or tantalum. Each anode current collector  30  includes a grounding tab  32  that is joined to the major face wall  14  and/or  16  of the casing  12 . 
     Referring to  FIGS. 2 ,  3 ,  8 ,  8 A and  9 , the cathode current collector  34  also includes a tab  36  that is joined to the terminal pin  38 . The positive terminal pin  38  is typically of molybdenum. The insulative seal  40  surrounds the terminal pin  38  where it passes through the first opening  70  in the lid  42 , sealing the terminal pin  38  and isolating it from electrical contact with the casing  12 . 
     Seal  40  is preferably a glass-to-metal seal comprised of a ferrule  44  joined to the lid  42 , and a bead  46  of fused glass bonded within the annulus between the ferrule  44  and the terminal pin  38 . The ferrule  44  can be made of titanium although molybdenum, aluminum, nickel alloy and stainless steel are also suitable. The glass is of a corrosion resistant type having up to about 50% by weight silicon such as CABAL 12, TA 23, FUSITE 425 or FUSITE 435. Although the cell  10  shown in  FIG. 1  is of a case-negative design, it is to be understood that the present invention is also applicable to cells of a case-positive design. 
     Cell  10  is further comprised of a first separator enveloping at least one of the cathode  22  and the anode  24 . In the case-negative cell design shown in  FIGS. 2 ,  3 ,  8 ,  8 A and  9 , the separator  48  envelopes the cathode plates  26 , thereby insulating them from direct physical contact with the anode plate(s)  28  and the negative polarity casing  12 . For the sake of redundancy, the cell  10  may further include second separators  50  that enclose each anode plate  28  as illustrated at  FIGS. 2 ,  3 ,  8  and  8 A. Furthermore, any one of the separators enclosing or enveloping the anode and the cathode may be of a single or double layer construction. That is in addition to the stack holder of the present invention, which will be described in detail hereinafter. 
     Each separator  48 ,  50  is an electrically insulative material. The electrically insulative material is chemically unreactive with the anode active materials and the cathode active materials and insoluble in the electrolyte. In addition, the separator material has a degree of porosity sufficient to allow flow there through of the electrolyte during electrochemical reactions of the cell. Illustrative separator materials include and are not limited to fabrics woven from fluoropolymeric fibers including polyvinylidine fluoride, polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethylene used either alone or laminated with a fluoropolymeric microporous film, non-woven glass, polypropylene, polyethylene, glass fiber materials, ceramics, polytetrafluoroethylene membrane commercially available under the designation ZITEX (Chemplast Inc.), polypropylene membrane commercially available under the designation CELGARD (Celanese Plastic Company, Inc.) and a membrane commercially available under the designation DEXIGLAS (C. H. Dexter, Div., Dexter Corp.). 
     The cell  10  is thereafter filled with the electrolyte solution and hermetically sealed such as by close-welding a stainless steel ball over the second opening  72  in the lid  42  serving as a fill-hole. The electrolyte serves as a medium for migration of ions between the anode  24  and the cathode  22  during electrochemical reactions of the cell. For both a primary and secondary cell chemistry, electrochemical reaction at the electrodes involves conversion of ions in atomic or molecular forms which migrate from the anode  24  to the cathode  22 . A suitable electrolyte has an inorganic, ionically conductive salt dissolved in a nonaqueous solvent, and more preferably, the electrolyte includes an ionizable lithium salt dissolved in a mixture of aprotic organic solvents comprising a low viscosity solvent and a high permittivity solvent. The inorganic, ionically conductive salt serves as the vehicle for migration of anode ions to intercalate or react with the cathode active materials. Suitable lithium salts include and are not limited to LiPF 6 , LiBF 4 , LiAsF 6 , LiSbF 6 , LiClO 4 , LiO 2 , LiAlCl 4 , LiGaCl 4 , LiC(SO 2 CF 3 ) 3 , LiN(SO 2 CF 3 ) 2 , LiSCN, LiO 3 SCF 3 , LiC 6 F 5 SO 3 , LiO 2 CCF 3 , LiSO 6 F, LiB(C 6 H 5 ) 4 , LiCF 3 SO 3 , and mixtures thereof. 
     Low viscosity solvents useful with the present invention include esters, linear and cyclic ethers and dialkyl carbonates such as tetrahydrofuran (THF), methyl acetate (MA), diglyme, trigylme, tetragylme, dimethyl carbonate (DMC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), 1-ethoxy, 2-methoxyethane (EME), ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, and mixtures thereof, and high permittivity solvents include cyclic carbonates, cyclic esters and cyclic amides such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate, acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, γ-valerolactone, γ-butyrolactone (GBL), N-methyl-pyrrolidinone (NMP), and mixtures thereof. 
     In order to maintain the electrode plates  26  and  28  in proper electrochemical close physical proximity alignment with each other during cell discharge, a stack holder  52  according to the present invention surrounds the electrode assembly  20 . Referring to  FIG. 2 , in one embodiment the stack holder  52  is formed as a bag that receives and envelopes the electrode assembly  20  on all surfaces excluding the surface adjacent the lid  42 . That is to maintain close physical proximity electrochemical alignment between the anode and cathode plates. In other embodiments, the stack holder  52  encases and envelopes the electric assembly except where the grounding tab and/or terminal pin are located. The stack holder  52  has a portion that contacts the interior surface of the casing&#39;s side walls  14 ,  16  and other portions contact the electrode assembly  20 . 
     Referring next to  FIG. 8 , the stack holder may alternatively be formed as a band  54  disposed in an encircling relationship with a portion of the electrode assembly  20 . As used herein with respect to a stack holder, the term “encircling” is meant to indicate that the stack holder is disposed around a portion of the perimeter of the electrode assembly  20  in an orientation such that it holds the two or more electrode plates in a compressive, face-to-face, close physical proximity alignment as the cell is discharged, as indicated by arrows  56  and  58  shown in  FIGS. 2 ,  3 ,  8 ,  8 A and  9 . As long as it provides compressive forces to hold the electrode plates together in a close physical proximity, it is not necessary that the stack holder cover more of electrode assembly  20  (as the shown stack holder  52  does). The function of the stack holder  52 ,  54  is to maintain proper face-to-face electrochemical, close physical proximity alignment between the anode and cathode plates. 
     In that respect,  FIG. 8  illustrates the electrode assembly  20  comprising the cathode  22  and anode  24  being aligned in a face-to-face, close physical proximity relationship suitable for acceptable electrochemical discharge. The electrode assembly  20  has a total height H 1  determined by measuring the cathode  22  and anode  24  from adjacent to the bottom wall  18  of the casing to adjacent the lid  42 . The stack holder  54  encircles the circumference of the electrode assembly  20  and has a height H 2  that is at least 5% of H 1  to a maximum of 100% of H 1 . 
       FIG. 8A  illustrates another embodiment of cell  10  where stack holder  54  has been replaced by stack holders  54 A and  54 B. Stack holder  54 A has a height H 3  and encircles the circumference of the electrode assembly  20  adjacent to the casing bottom wall  18  while stack holder  54 B has a height H 4  and encircles the circumference of the electrode assembly adjacent to the lid  42 . The respective heights H 3  and H 4  of the stack holders  54 A and  54 B can be less than the height of H 2  of stack holder  54  shown in  FIG. 8  as long as their cumulative heights H 3 +H 4  are at least 5% of the height H 1  of the electrode assembly to a maximum of 100% of H 1 . Stack holders  54 A and  54 B can have the same or different heights. 
     It will also be apparent to those skilled in the art that while two stack holders  54 A,  54 B are shown in  FIG. 8B , that should not be taken as limiting. Any number of band-type stack holders can be provided in a surrounding, encircling relationship with the electrode assembly  20 , just as long as their cumulative heights are at least 5% of the total height of the electrode assembly. 
     The stack holders  52 ,  54 ,  54 A and  54 B (collectively referred to as stack holders  52 ) may be made of the same materials used for the separators  48  and  50 . In one preferred embodiment, the stack holder material has a shape memory material capable of accommodating an initial expansion of the cathode that may occur at the early stage of cell discharge, and subsequent shrinkage of the electrode assembly  20  during later stages of cell discharge. The term shape memory is defined as a material that is capable of reverting to its original size and shape after a deformation force is removed. 
     An example of the shape memory is illustrated by comparing  FIG. 2  to  FIG. 3 .  FIG. 2  illustrates the electrode assembly  20  at a first period of time and  FIG. 3  is  FIG. 2  at a later period in its discharge life and, in the case of a secondary cell, after numerous cell discharge cycles. In  FIG. 2 , the anode plates and the cathode plates are surrounded by the stack holder  52 . The stack holder  52  is firmly positioned in the casing  12  and has a portion that contacts the face walls  14 ,  16  and a portion that contacts the electrode assembly  20 . 
     The stack holder  52  (along with  54 ,  54 A and  54 B) has a wall contacting surface  150  and a plurality of protrusions  152 . Each protrusion  152  has a force contacting surface  154 , and an extension area  156  ( FIGS. 4 to 7 ) positioned between the wall contacting surface  150  and the force contacting surface  154 . Each protrusion  152  can be a dimple as illustrated at  FIGS. 2 ,  3 ,  4 ,  5 ,  8 ,  8 A, and  9  or variations thereof which include conical, cylindrical, cubic, polygonic, or rounded shapes that are randomly spaced; an extended embodiment of the dimple, conical, cylindrical, cubic, polygonic, or rounded shapes which are adjacent to another protrusion as illustrated at  FIG. 6 ; an extended embodiment of the dimple, conical, cylindrical, cubic, polygonic, or rounded shapes which are spaced a predetermined distance from another protrusion as illustrated at  FIG. 7 ; or combinations or mixtures thereof. 
     A characteristic of each protrusion  152  is that it has a maximum extension E 1  as shown in  FIG. 4 . The maximum extension E 1  is designed to exceed the maximum gap space between the interior surface  150 B of the stack holder&#39;s wall contacting surface  150  and the electrode assembly&#39;s exterior surface  20 A over the cell&#39;s  10  discharge life. The protrusion  152 , which is made from a shape memory material, should not reach maximum extension E 1  when it is in the casing  12 . If the protrusion  152  reaches maximum extension E 1  and the gap between the wall contacting surface&#39;s  150  interior surface  150 B and the electrode assembly&#39;s exterior surface  20 A exceeds the maximum extension E 1 , then the protrusions  152 , in that rare instance, may not provide the desired compressive force against the electrode assembly  20 . Without the desired compressive force being applied, the electrode assembly may not retain the appropriate close physical proximity between the anode and the cathode plates for maximum cell discharge efficiency. Accordingly, the protrusion&#39;s maximum extension E 1  is designed to exceed the maximum gap space between the wall contacting surface&#39;s  150  interior surface  150 B and the electrode assembly&#39;s exterior surface  20 A over the cell&#39;s  10  discharge life. 
     The stack holder  52  is positioned over the electrode assembly  20 . As previously identified, the electrode assembly  20  can include and is not limited to: (a) the anode plate  24 , the cathode plate  26  and a separator positioned between the plates (see  FIGS. 2 ,  3 ,  8 ,  8 A and  9 ); (b) the anode plate  24  and the separator  50 , and the cathode plate  26 ; (c) the cathode plate and the separator  48 , and the anode plate  24  (see  FIG. 9 ); and (d) the anode plate  24  and the separator  50 , and the cathode plate and the separator  48  (see  FIGS. 2 ,  3 ,  8  and  8 A). Additional anode plates and cathode plates can be used so long as they comply with the format of an anode plate being separated from direct physical contact with a cathode plate. 
     After the stack holder  52  and the electrode assembly  20  are inserted into the case  12 , the wall contacting surface&#39;s  150  exterior surface  150 A contacts the case wall  14 ,  16  and at least portions of side wall  18  and, in some embodiments, the lid  42 . Where there is a protrusion  152  near the wall contacting surface, the wall contacting surface&#39;s  150  interior surface  150 B does not normally contact the electrode assembly&#39;s exterior surface  20 A. Nonetheless, after at least partial cell discharge, a first gap D 1  may form between the wall contacting surface&#39;s  150  interior surface  150 B and the electrode assembly&#39;s exterior surface  20 A. That gap D 1  is illustrated in  FIG. 2 . At a greater depth of discharge, the first gap D 1  expands to a second gap D 2  as illustrated in  FIG. 3 . 
     The first gap D 1  between the interior surface  150 B of the stack holder&#39;s wall contacting surface  150  and the electrode assembly&#39;s exterior surface  20 A is smaller than the second gap D 2 . The second gap D 2  is larger because as a primary cell is discharged through its useful life and, in the case of a secondary cell, after numerous discharge cycles, the cathode&#39;s width C 1  (see  FIG. 2 ) expands to width C 2  (see  FIG. 3 ), i.e. C 1 &gt;C 2 , at a slower rate than the thinning of the anode&#39;s width A 1  (see  FIG. 2 ) to width A 2  (see  FIG. 3 ), i.e. A 1 &lt;A 2 . That discrepancy in the cathode&#39;s expansion rate and the anode&#39;s thinning rate results in the gap increasing in size over time as illustrated by comparing the first gap D 1  ( FIG. 2 ) to the second gap D 2  ( FIG. 3 ), i.e. D 2 &gt;D 1 . 
     Despite the first gap D 1  ( FIG. 2 ) being less than the second gap D 2  ( FIG. 3 ), the stack holder  52  still contacts the electrode assembly&#39;s exterior surface  20 A through the protrusion  152 , as illustrated at  FIGS. 2 and 3 . Each protrusion  152  illustrated at  FIGS. 2 and 3 , is not at its maximum extension E 1 . In that manner, each protrusion  152  applies a compressive force upon the electrode assembly&#39;s exterior surface  20 A through the force contacting surface  154 . The collective compressive force of each protrusion results in the electrode assembly  20  being maintained in the proper and desired close physical proximity position. 
     Obviously the reverse structure can be made as well. In the reverse embodiment, the protrusions  152  apply a force upon the casing walls and the wall contacting surface  150  contacts the electrode assembly&#39;s walls. An example of a stack holder  52  illustrating those capabilities is illustrated in  FIG. 5 . 
     Suitable materials that are also useful for the stack holders  52 ,  54 ,  54 A and  54 B are the same materials that are used for separators  48 ,  50 , preferably fluoropolymeric materials including polyvinylidine fluoride, polyethylenetetrafluoroethylene, ethylenetetrafluoroethylene (ETFE) and polyethylenechlorotrifluoroethylene used either alone or laminated with a fluoropolymeric microporous film, non-woven glass, polypropylene, polyethylene, ceramics, polytetrafluoroethylene membrane commercially available under the designation ZITEX (Chemplast Inc.), polypropylene membrane commercially available under the designation CELGARD (Celanese Plastic Company, Inc.) and a membrane commercially available under the designation DEXIGLAS (C. H. Dexter, Div., Dexter Corp.). These materials can be provided in a bi-layer or tri-layer construction. An example is a tri-layer polymeric material of polypropylene/polyethylene/polyethylene (PP/PE/PE). 
     While woven fabrics of the above materials are generally preferred for the cell separators, that type of construction is not necessarily favored for the stack holder of the present invention. Instead of a loose fabric, which is desired for permitting ion flow there through, the stack holder preferably has a uniformly closed and coherent texture. That&#39;s because such solid-type materials have better shape memory characteristics. A most preferred material for the stack holder is solid, non-woven ETFE having a thickness of about 0.005 inches. 
     In fabrication of a stack holder according to the present invention, the stack holder material may be wrapped around the electrode assembly  20  and held under tension in a fixture to allow the protrusions  152  to provide compressive forces against the electrode plates  26  and  28 . The stack holder material may be heat sealed in a manner similar to that used to fabricate individual electrode plate separators  48  and  50 . Alternatively, the electrode assembly  20  can be inserted into a cavity  200  of the stack holder  52 . 
     The stack holder  52  can obtain the protrusions through numerous possible methods. One exemplary method is to use a conventional insulator bag having an opening and the interior of the insulator bag defines the cavity  200 . The insulator bag is positioned around a tongue depressor instrument. The tongue depressor instrument directs the insulator bag toward a heated plate. The heated plate is at a temperature just below the melting temperature of the insulator bag and has mirror image protrusion forms. Upon contact with the heated plate, the insulator bag forms into the stack holder  52  with the protrusions  152  extending in the desired direction and having the desired shape and spacing. 
     As long as they are shape memory material, the stack holders may also be made from non-porous materials that are not typically used to construct cell separators. Examples are polyimide tape and polypropylene tape. The difference between these tapes and the previously mentioned separator materials is that the former are non-porous and contain adhesives. As used herein, the term “porous” refers to a material that has sufficient permeability to permit an acceptable degree of ion flow there through to support electrochemical discharge. On the other hand, a non-porous material may have some permeability, but not to a degree sufficient to permit ion flow to sustain an electrochemical discharge. 
     In other embodiments, either or both of the anode and cathode may be comprised of a plurality of plates of their respective electrode active materials. Each of the respective plates of electrode active material may be enveloped in its own separator, with the entire electrode assembly being further encircled by an elastic stack holder. One exemplary cell comprised of multiple electrode plates is shown in  FIG. 2 . 
     It is noted that the exemplary cell  10  of respective  FIGS. 2 to 3  are comprised of individual electrode plates that are typically fabricated separately. However, the present invention is not to be construed as limited to such an electrode configuration. Other cells having serpentine or jellyroll electrode configurations may be provided with a stack holder in accordance with the present invention. Therefore, the term “electrode plate” used herein is meant to indicate any structure of electrode active material that is alignable in a substantially face-to-face orientation or alignment with one or more adjacent portions of an opposite polarity electrode active material in a close physical proximity. 
     It is, therefore, apparent that an electrochemical cell is provided with a stack holder that surrounds the electrode assembly or cell stack thereof. The stack holder maintains the desired face-to-face electrical alignment between the opposite polarity electrode plates as the cell is discharged. While this invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variations that fall within the broad scope of the appended claims.