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 an elastic material that serves to maintain the anode and cathode in a face-to-face 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 misalignment. However, the stack holder prevents such misalignment by maintaining a constrictive force on the electrode assembly throughout discharge.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from U.S. Provisional Application Ser. No. 60/974,496, filed Sep. 24, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    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 with each other, even as their dimensions change during cell discharge. 
         [0004]    2. Description of Related Art 
         [0005]    A typical electrochemical cell that is used to power implantable medical devices is comprised of a casing housing an anode and a cathode. The anode and cathode are separated from each other, typically by enclosing at least one of them within an envelope or bag of insulative separator material. The separator material 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 cathode are generally formed as one or more respective plates of anode and cathode active material. The plates are then aligned face-to-face with each other to form an electrode assembly or electrode 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 walls of the casing while occupying as much internal volume as possible. 
         [0006]    During cell discharge, the thicknesses of the plates of cathode active material and anode active material change. The thicknesses of the cathode plates increase while those of the anode decrease. In some cells, 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, the electrodes may become loosely held or confined within the casing. There may eventually be sufficient space inside the casing for the electrode assembly to move around within it. This condition is disadvantageous. As the electrodes move, they may no longer be directly opposite each other in their original face-to-face orientation. Misalignment may result in an increase in cell resistance between the cathode and anode, thereby causing lower pulse voltages, faster cell polarization, cell voltage fluctuations, and in general, more delivered capacity variation. 
         [0007]    What is needed is an electrochemical cell comprising an electrode assembly having an anode and a cathode that are tightly held together in their original face-to-face alignment throughout the entire discharge life of the cell. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention meets this need by providing an electrochemical cell comprising a conductive casing housing 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 the electrode assembly. The stack holder may be formed as a bag that envelopes the electrode assembly. Alternatively, the stack holder may be formed as a band disposed around a perimeter of the electrode assembly. In embodiments in which both plates of the anode and cathode active materials are enclosed by separators, the stack holder may be formed by joining the separators along their respective perimeters that contact each other. 
         [0009]    The stack holder is preferably made of an elastic material. In that manner, as the volume of the electrode assembly varies during cell discharge, the volume encircled within or surrounded by the stack holder varies a like amount. In particular, as the circumference of the electrode assembly decreases, the circumference encircled or surrounded within the stack holder also decreases, thus maintaining the desired face-to-face alignment between the anode and cathode plates. 
         [0010]    Either or both of the anode and 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 by the stack holder. In that respect, the stack holder is a component or part that is separate or in addition to that portion of the separator material disposed at an intermediate location between the opposite polarity electrodes. 
         [0011]    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 
         [0012]    The present invention will be described by reference to the following drawings, in which like numerals refer to like elements, and in which: 
           [0013]      FIG. 1  is a cross-sectional view of a first embodiment of an electrochemical cell of the present invention comprised of single anode and cathode plates forming the electrode assembly, and a stack holder provided as a bag enclosing the electrode assembly; 
           [0014]      FIG. 1A  is a cross-sectional view taken along line  1 A- 1 A of  FIG. 1 ; 
           [0015]      FIG. 2  is a cross-sectional view of a second embodiment of an electrochemical cell of the present invention comprised of a stack holder provided as a band disposed around the electrode assembly; 
           [0016]      FIGS. 2A and 2B  are cross-sectional views of the cell shown in  FIG. 2 , but illustrating alternate embodiments of band-type stack holders; 
           [0017]      FIG. 3  is a cross-sectional view of an alternative embodiment of an electrochemical cell of the present invention comprised of three electrode plates forming the electrode assembly; and 
           [0018]      FIG. 4  is a cross-sectional view of another embodiment of an electrochemical cell comprised of single anode and cathode plates forming the electrode assembly, wherein the electrode plates are held in a face-to-face alignment with each other by joining their respective separators together. 
       
    
    
       [0019]    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 
       [0020]    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  form an open ended container that receives an electrode assembly  20 , as will be described hereinafter. The open ended container housing the electrode assembly is then closed by a lid  42 . The casing and lid 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. The casing lid  42  is typically provided with a first opening to accommodate a glass-to-metal seal/terminal pin feedthrough and a second opening for electrolyte filling. 
         [0021]    The electrode assembly or electrode stack  20  comprises a cathode  22  and an anode  24  housed within the casing  12 . The cathode  22  is comprised of opposed plates  26  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. 
         [0022]    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 to the casing major face wall  14 . That&#39;s because only anode material directly facing the cathode 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. The anode current collector  30  includes a grounding tab  32  that is joined to the major face wall  14  of the casing  12 . 
         [0023]    Referring to  FIG. 1A , the cathode current collector  34  also includes a tab  36  that is joined to a terminal pin  38 . The positive terminal pin  38  is typically of molybdenum. An insulative seal  40  surrounds the terminal pin  38  where it passes through the first opening in the lid  42 , sealing the terminal pin  38  and isolating it from electrical contact with the casing  12 . 
         [0024]    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. 
         [0025]    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. 1 and 1A , the separator  48  envelopes the cathode plates  26 , thereby insulating them from direct physical contact with the anode plate  28  and the negative polarity casing  12 . For the sake of redundancy, the cell  10  may further include a second separator  50  enclosing the anode plate  28 . 
         [0026]    The separators  48 ,  50  are of an electrically insulative material that is chemically unreactive with the anode and cathode active materials and both chemically unreactive with 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 the electrochemical reaction of the cell. Illustrative separator materials include 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.). 
         [0027]    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 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 the electrochemical reactions of the cell. For both a primary and a 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 the anode ions to intercalate or react with the cathode active materials. Suitable lithium salts include 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. 
         [0028]    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. 
         [0029]    In order to maintain the electrode plates  26  and  28  in proper electrochemical face-to-face alignment with each other during cell discharge, a stack holder  52  according to the present invention surrounds the electrode assembly  20 . Referring to FIG.  1 , in one embodiment the stack holder  52  is formed as a bag that encloses or envelopes the electrode assembly  20  to maintain proper face-to-face electrochemical alignment between the anode and cathode plates. The stack holder  52  covers the outwardly facing side walls of the electrode assembly as well as the opposed ends adjacent to the casing bottom wall  18  and the lid  42 . 
         [0030]    Referring next to  FIG. 2 , 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 constrictive, face-to-face alignment as the cell is discharged, as indicated by arrows  56  and  58  shown in  FIGS. 1 ,  1 A,  2  to  2 B and  3 . As long as it provides constrictive forces to hold the electrode plates together, it is not necessary that the stack holder cover the entire electrode assembly  20  (as the shown stack holder  52  does). The function of the stack holder is to maintain proper face-to-face electrochemical alignment between the anode and cathode plates. 
         [0031]    In that respect,  FIG. 2  illustrates the electrode assembly  20  comprising the cathode  22  and anode  24  being aligned in a face-to-face 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 . 
         [0032]      FIG. 2A  illustrates another embodiment of cell  11  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. 2  as long as their cumulative heights H 3 +H 4  are at least 5% of the height H 1  of the electrode assembly. Stack holders  54 A and  54 B can have the same or different heights. 
         [0033]      FIG. 2B  illustrates still another embodiment of cell  11  where stack holders  54  is supplemented with additional stack holders  54 A and  54 B. As with the embodiment show in  FIG. 2A , the stack holder  54 A encircles the circumference of the electrode assembly adjacent to the bottom wall  18  of the casing while stack holder  54 B encircles the circumference of the electrode assembly adjacent to the lid  42 . The cumulative heights H 2 , H 3  and H 4  of the respective stack holders  54 ,  54 A and  54 B are preferably at least 5% of the height H 1  of the electrode assembly. 
         [0034]    It will also be apparent to those skilled in the art that while three stack holders are shown in  FIG. 2B , 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. 
         [0035]    The stack holders  52 ,  54 ,  54 A and  54 B may be made of the same materials used for the separators  48  and  50 . In one preferred embodiment, the holder material is an elastic 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 stack  20  during later stages of cell discharge. The term elastic is defined as a material that is capable of quickly recovering its original size and shape after a deformation force is removed. 
         [0036]    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  and include 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.). 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). 
         [0037]    In fabrication, the stack holder material may be wrapped around the electrode assembly and held under tension in a fixture to provide constrictive 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 . 
         [0038]    As long as they are elastic, 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. 
         [0039]    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. 3 . Cell  13  is built in a case-negative design having a cathode  22  comprised of a cathode plate  26  disposed at an intermediate location between an anode  24  comprised of a first anode plate  28 A contacting one side of an anode current collector  30 A and a second anode plate  28 B contacting one side of a second anode current collector  30 B. The anode plates  28 A,  28 B face the central cathode plates  26  because only anode active material directly opposite cathode active material participates in electrochemical discharge. The stack holder  62  applies constrictive forces indicated by arrows  56  and  58  against electrode plates  26 ,  28 A and  28 B, thereby maintaining proper face-to-face electrochemical alignment between the plates during cell discharge. It will be apparent that cell  13  may be comprised of additional plates of anode and cathode active material compressed or constricted into face-to-face alignment by the stack holder  62 . 
         [0040]    It is noted that the exemplary cells  10 ,  11  and  13  of respective  FIGS. 1 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. 
         [0041]      FIG. 4  is a cross-sectional view of a freshly built electrochemical cell  15  that has not yet been discharged. The cell is comprised of single anode and cathode plates forming the electrode assembly. The electrodes are held in proper face-to-face electrochemical alignment with each other by joining their respective elastic separators together. In that respect, cell  15  is similar in construction to the cell  10  of  FIGS. 1 and 1A , except the stack holder that encircles the electrode assembly  20  has been removed. Instead, constrictive forces between the face-to-face opposite polarity electrodes are provided along the perimeter of the separators  48  and  50  where they contact each other. Electrode plates  26  and  28  are held in close contact with each other by joining their respective separators  48  and  50  to each other. Since the separators  48 ,  50  are made of an elastic material, that portion of each separator lying against a major face wall of the anode and cathode tends to pull or constrict that electrode toward the other. This is possible because the separators are provided in a stretched state in comparison to a relaxed, non-deformed condition. Separators  48  and  50  may be joined intermittently along portions of their respective perimeters that contact each other, or along the entire perimeter of contact. 
         [0042]    In one preferred embodiment, separators  48  and  50  are joined to each other by a heat seal  60 . For the sake of clarity of illustration, heat seal  60  is depicted as being relatively thick compared to respective electrode plates  26  and  28 . It is to be understood that the respective separators  48  and  50  for electrodes  26  and  28  are in closer contact with each other than is shown in  FIG. 4 , and this contact relationship is maintained throughout the cell discharge. Regardless whether the stack holder is an envelope as shown in  FIGS. 1 and 1A , at least one band-type structure as shown in  FIGS. 2 to 2B  and  3 , or a heat seal between respective separators enveloping the anode and cathode, the opposite polarity electrodes must be close enough to each other to ensure that electrolyte wets the entire interface between them by capillary action. This must persist through the discharge life of the cell and is the primary purpose of the stack holder. 
         [0043]    It is, therefore, apparent that an electrochemical cell is provided with a stack holder that surrounds the electrode assembly or 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.