Abstract:
A cell including a cathode having a second cathode active material of a relatively high energy density but of a relatively low rate capability sandwiched between two current collectors and with a first cathode active material having a relatively low energy density but of a relatively high rate capability in contact with the opposite sides of the current collectors, is described. In this type of cell construction, it is important that the weight ratio of the first and second cathode active materials is within a strict tolerance. Further, it is important to be able to track and record this information, as well as other data, for each cell built in a production facility. Marking the current collectors and the cell casing with identifying I.D. matrixes that are read and recorded during cell manufacture does this.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority to U.S. provisional patent application Serial No. 60/413,076, filed on Sep. 24, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to the conversion of chemical energy to electrical energy. More particularly, the present invention is directed to the precise regulation of the gram amount of electrode active materials contacted to the opposite sides of a current collector. The precise weight of the current collector is also regulated within strict tolerance. Current collectors that are outside the weight criteria, whether before being contacted with the electrode active material or after, are rejected as being out of tolerance. The strict regulation of the weight of the electrode active material in a cell is particularly important when different active materials are contacted to opposite sides of the current collector. Such a configuration has, for example: silver vanadium oxide (SVO)/current collector/fluorinated carbon (CF x ), and it is important that the weight ratio of active materials is closely regulated for proper cell functioning.  
         SUMMARY OF THE INVENTION  
         [0003]    The present invention relates to a cell including a cathode having a second cathode active material of a relatively high energy density but a relatively low rate capability sandwiched between two current collectors and with a first cathode active material having a relatively low energy density but a relatively high rate capability in contact with the opposite sides of the current collectors. It is important for proper cell functioning that the weight ratio of the first and second cathode active materials is vitnin a strict tolerance. Further, it is important to be able to track and record this information, as well as other data, for each cell built in a production facility. Marking the current collectors with an identifying I.D. matrix that is read and recorded for each electrode and each cell does this.  
           [0004]    The present cell is useful for powering an implantable medical device, such as an automatic implantable cardioverter defibrillator, cardiac pacemaker, neurostimulator, drug puirp, bone growth stimulator, and hearing assist device.  
           [0005]    These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and to the appended drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a perspective view, partly broken away, of an electrochemical (word missing)  10  accordingly to the present invention.  
         [0007]    [0007]FIG. 2 is a plan view of a current collector  30  including an ID matrix identifier  62 .  
         [0008]    [0008]FIG. 3 is an enlarged view of the indicated area on FIG. 2.  
         [0009]    [0009]FIG. 4 is an exploded view of one embodiment of a sandwich cathode  32  of the present invention.  
         [0010]    [0010]FIG. 5 is a flow chart depicting the steps for building a cathode electrode according to the present invention.  
         [0011]    [0011]FIG. 6 is a flow chart depicting the steps for building an electrochemical cell including the cathode assembled according to FIG. 5. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]    [0012]FIG. 1 is a perspective view of an exemplary electrochemical cell  10 . The cell  10  includes a casing  12  housing an electrode assembly of an anode electrode comprising a plurality of anode plates  14  and a cathode electrode comprising a plurality of cathode plates  16  prevented from directly contacting each other by an intermediate separator  18 . The anode/cathode electrode assembly is in a prismatic configuration housed in the deep-drawn casing  12  closed by a lid  20 .  
         [0013]    The lid  20  includes an opening supporting a terminal lead  22  insulated from the lid by an insulating glass  24 . This structure is commonly referred to as a glass-to-metal seal. The terminal lead  22  is connected to one of the electrodes, typically the current collector (not shown in FIG. 1) for the cathode electrode, and serves as the positive terminal. The current collector for the anode electrode is connected to the casing  12  or lid  20 , or both, which serve as the negative terminal. This type of cell construction is referred to as a case-negative configuration. A case-positive configuration has the cathode connected to the case and the negative electrode connected to the terminal lead  22 . An activating electrolyte is filled into the other lid opening  26  and a closure member  28  hermetically sealed therein completes the cell  10 .  
         [0014]    While the exemplary cell  10  shown in FIG. 1 is of a prismatic design, the present invention is not intended to be so limited. In a broader sense, the present system is useful with many different types of cell designs including those of jellyroll or spirally-wound electrode assemblies, button-type cells, coin-cells, and the like. The present system is also useful with capacitors of either an electrochemical, electrolyte or hybrid design. This is what is meant by the term “electrical energy storage device” as used in this description.  
         [0015]    [0015]FIG. 2 shows a current collector  30  of a structure useful with the electrode  32  shown in FIG. 4. The illustrated electrode  32  is a cathode, although the present invention is equally applicable to an anode electrode. The cathode comprises a first current collector  30 A and a second current collector  30 B. The current collectors  30 A and  30 B are essentially identical and their structure will be described in detail with respect to the illustrated current collector  30  of FIGS. 2 and 3.  
         [0016]    The current collector  30  comprises opposed wing sections  32  and  34  connected together by an intermediate tab portion  36 . The tab  36  supports spaced apart projections  38  and  40 . The latter projection  40  has an aperture  42  while an aperture  44  is spaced a short distance inboard from the former one (FIG. 3). The projections  38 ,  40  and apertures  42 ,  44  serve as indexing structures for accurately and repeatably positioning the current collector in a fixture for building the electrode, as will be explained in detail hereinafter. The current collector wing sections  32 ,  34  each comprise an open grid structure  46 ,  48 , respectively, providing them in the form of a screen, and the like. One preferred method for providing the open grid current collectors is described in U.S. Pat. Nos. 6,110,622 and 6,461,771, both to Frysz et al. These patents are assigned to the assignee of the present invention and incorporated herein by reference.  
         [0017]    As shown in FIG. 4, an electrode, for example a cathode electrode, is built by positioning in an appropriately shaped fixture (not shown) a pair of blanks  50  and  52  of a first electrode active material, for example SVO, followed by the first current collector  30 A having its respective wings positioned on top of the blanks. Blanks  54  and  56  of a second electrode active material, for example CF x  are positioned on top of the opposite sides of the wings cf current collector  30 A.  
         [0018]    The second current collector  30 B is then positioned on top of the second electrode active material blanks  54 ,  56  opposite the first current collector  30 A. Finally, two blanks  58  and  60  of a third electrode active material, for example SVO, are positioned on the wings of the current collector  30 B opposite the second electrode active material.  
         [0019]    This assembly is then subjected to sufficient pressure to intimately contact the active materials to the opposite ides of the respective current collectors  30 A,  30 B. Direct bonding contact with the current collector sides is important to prevent delamination. However, it is also important that the SVO and CF, materials are segregated to their respective current collector sides so that the active material/current collector interfaces are not “contaminated ” by the opposite active material. In other words, it is important that one active material does not migrate through the screen grid to the other side of the current collector to interfere with direct bonding of the other active material to the current collector surface.  
         [0020]    The thusly assembly electrode assembly is referred to as a “sandwich electrode”. A preferred form is a cathode electrode with the first and third active materials of a greater rate capability, but a lesser energy density than the intermediate second active material. The second active material has a greater energy density, but a lesser rate capability than the first and third active materials. Silver vanadium oxide is preferred for the first and third active materials while CF x  is preferred for the intermediate second active material.  
         [0021]    In a broader sense, it is contemplated by the scope of the present invention that the first and third active materials of the present sandwich cathode design are any materials that have a relatively lower energy density but a relatively higher rate capability than the second active material. In addition to silver vanadium oxide, copper silver vanadium oxide, V 2 O 5 , MnO 2 , LiCoO 2 , LiNiO 2 , LiMn 2 O 4 , TiS 2 , Cu 2 S, FeS, FeS 2 , copper oxide, copper vanadium oxide, and mixtures thereof are useful as the first and third active materials, and in addition to fluorinated carbon, Ag 2 O, Ag 2 O 2 , CuF 2 , Ag 2 CrO 4 , MnO 2  are useful as the second active material. Even SVO is useful as the second active material when copper silver vanadium oxide is the first and third active material. For a more detail description of a “sandwich” electrode design, reference is made to U.S. Pat. No. 6,551,747 to Gan, which is assigned to the assignee of the present invention and incorporated herein by reference.  
         [0022]    In order to regulate the manufacturing process for the sandwich electrode, each of the current collectors  30 A,  30 B is provided with a unique identification code or ID matrix  62 . The ID matrix  62  is preferably etched, such as by a laser, onto the connecting tab  36 . This provides the matrix with a smaller footprint than a typical bar code, thus minimizing warping of the current collector due to excessive heat. Etching is also preferred because it is permanent and will not contaminate the cell as an ink jet marking system might.  
         [0023]    [0023]FIGS. 5 and 6 are flow charts illustrating an industrial production line for precisely and accurately controlling the processes that constitute the manufacture of the sandwich electrode and, more generally, the associated electrochemical cell  10 . The processes begin with a bulk CF x  powder input  64 , a bulk SVO sheet coupon input  66  and a current collector input  68 . First moving along the CF x  flow path, the bulk powder is moved to a sifter  70  that separates out or sieves out any particles greater than a specified size. The sifted out particles are moved to a pulverizer (not shown) that commutes them to the desired size before they are re-introduced into the sifter. The CF x  powder leaving the sifter is filled into a fixture having the precise shape of the product cathode electrode. A specified weight amount of CF x  powder in the fixture is leveled smooth  72  and then pressed with sufficient force to form a blank  74 . The blank  74  is weighed on a tare scale  76 , and if it is within tolerance, moved to a holding bin. If not, the blank is rejected as being out of specification  78 . In order to pass tolerance, a CF x  blank must be within at least about ±0.1 grams of a specified weight and, more preferably, within about ±0.005 grams of a specified weight.  
         [0024]    Formation of an SVO blank takes place in a somewhat different manner. Silver vanadium oxide blank formation is carried out according to the process described in U.S. Pat. Nos. 5,435,874 and 5,571,604, both to Takeuchi et al. These patents are assigned to the assignee of the present invention and incorporated herein by reference. As described in the Takeuchi et al. patents, a freestanding active sheet or coupon is made from SVO of a specified granular size, carbon black or graphite as a conductive additive and a powder fluoro-resin binder such as PTFE powder. These ingredients are mixed in a solvent of either water or an inert organic medium such as mineral spirits. The resulting paste is either run through a series of compacting roll mills to form a thin sheet having a tape form, or it is turned into briquettes that are then calendered into the freestanding sheet as a continuous tape. In any event, the tape is subjected to a drying step that removes any residual solvent or water and then moved to a machine that punches coupons  66  from the tape. The coupons  66  are transferred to a blanking station where a hydraulic press having platens or fixtures presses them into blanks  80  of the precise shape of the product cathode electrode. Each blank  80  is weighed on a tare scale  82 , and if it is within tolerance, moved to a holding bin. If not, the blank is rejected as being out of specification  84 . In order to pass tolerance, a SVO blank must be within at least about ±0.1 grams of a specified weight and, more preferably, within about ±0.005 grams of a specified weight.  
         [0025]    The current collector input  68  begins with a bin holding a plurality of the current collectors  30  (FIG. 2). A chemical machining process, such as described in U.S. Pat. Nos. 6,110,622 and 6,461,771, both to Frysz et al., preferably produces the current collectors. These patents are assigned to the assignee of the present invention and incorporated herein by reference. The current collectors  30  are moved to an etching station  86  where the ID matrix  62  is applied to the connecting tab  36 . The etched current collector is moved to a reader  88  that electronically confirms the ID matrix marking  62 . After ID matrix confirmation, the current collector is weighed on a tare scale  90 , and if it is within tolerance, moved to a holding bin for the etched and weighed current collector screens  92 . If not, the current collector is rejected as being out of specification  94 . In order to pass tolerance, a current collector must be within at least about ±0.1 grams of a specified weight and, more preferably, within about ±0.006 grams of a specified weight.  
         [0026]    The thusly-manufactured CF x  blank  74 , SVO blank  80 , and current collectors  92  are then fed to a linear slide equipped with a Cartesian robot  96 . This machine is programmable to assemble the three input components into any one of a number of different electrode configurations.  
         [0027]    One is of a sandwich cathode as shown in FIG. 4 having the dual wing current collectors  30 A,  30 B each of a configuration: SVO/current collector/CF x /current collector/SVO. Another preferred embodiment is of the same configuration but without the current collectors being of a dual wing construction. Another preferred embodiment is of the configuration: SVO/current collector/SVO/CF x SVO/current collector/SVO. Still another preferred embodiment is of the configuration: SVO/current collector/CF x  with the SVO side facing the lithium anode. This latter cathode configuration provides a cell referred to as a “medium-rate design”. The others are referred to as being of “high-rate designs”.  
         [0028]    Regardless of the specific type of cell being built, the finished cathode leaving the Cartesian robot  96  moves to a tare scale  98  where a final weight is recorded. This weight must be within ±5% of the cumulative weights of the respective CF x  blanks, SVO blanks and current collectors, or the cathode is rejected  100  as being out of specification. After final weighing, the cathode electrode weight is checked and the ID matrix  62  etched onto the current collectors are scanned  102 . The ID matrix associated with the readings of the final weights of the various component blanks and current collectors  104  is recorded  106  in the memory of a central processor unit, or it is recorded in some other tangible medium such as on a disk, print out, and the like.  
         [0029]    [0029]FIG. 6 is a schematic representation of a cell constructed having one or more of the cathode configurations described with respect to FIG. 5. While not shown in the drawing, the cell has an anode as a continuous elongated element or structure of an alkali metal, preferably lithium or a lithium alloy, enclosed within a separator and folded into a serpentine shape. A plurality of cathode electrode assemblies with an associated ID matrix  108  produced according to the component flow chart of FIG. 5 are then interposed between the anode folds. In the case of the cathode shown in FIG. 4, the spaced apart plates are folded relative to the connecting tab  36  so that there is a portion of the anode disposed along opposite major sides or each cathode plate. The cell illustrated in FIG. 6 has two dual wing cathode electrode structures and a fifth cathode plate not of a dual wing construction.  
         [0030]    The cathode plates interleaved between the folds of the serpentine anode are fitted inside a suitably sized casing  12  that itself has been provided with a laser etched ID matrix. The case ID matrix is scanned  110  and this data is also recorded for later retrieval. That way, there is a permanent record of each cell detailing the specific electrode configurations with the exact weights of the various active blanks and current collectors housed in a specific casing. The cell is activated with an electrolyte such as of LiPF 6  of LiAsF 6  dissolved in a 50:50, by volume, mixture of propylene carbonate and 1,2-dimethoxyethane. For a case-negative cell design, the current collector of the serpentine anode is connected to the case or lid, or both, and the current collector connecting portions  36  are connected to the terminal lead  22 . If a case-positive design is desired, the reverse is true.  
         [0031]    One exemplary form of the ID matrix  62  includes a cell model number and a unique serial number. An example is the twenty-character sequence 20770000000000000001. The first four numbers designate the cell as a model 2077 cell, and the following  16  characters are the cell&#39;s unique serial number.  
         [0032]    In a sandwich electrode design, it is important that the weight ratios of the high rate active material, for example SVO, to that of the high-energy active material, for example CF x , be within a strict tolerance. In a lithium electrochemical cell, a sandwich cathode having the configuration of: SVO/current collector/CF x /current collector/SVO, provides for the high volumetric capacity CF x  active material being quantitatively converted into or used as the high power energy of the SVO material. In that respect, it is believed that during high energy pulsing, the SVO material provides all the discharge energy. Above the discharge voltage of the CF x  electrode material, only SVO electrode material is discharged, providing all of the energy for pulsing as well as for any background load discharging. Under these discharge conditions, the CF x  active material is polarized with respect to the SVO material discharge voltages. Then, when the lithium cell is discharged to the working voltage of the CF x  material, both the SVO and CF x  materials provide the energy for background load discharging. However, only the SVO material provides energy for high rate pulse discharging. After the SVO active material is pulse discharged, the potential of the SVO material tends to drop due to the loss of capacity. When the SVO background voltage drops below the working voltage of the CF x  material, the SVO material is charged by the CF x  material to bring the discharge voltage of the sandwich cathode materials to an equal value. Therefore, it is believed that the SVO material acts as a rechargeable electrode while at the same time the CF x  material acts as a charger or energy reservoir. As a result, both active materials reach end of service life at the same time.  
         [0033]    Thus, it is important for the proper functioning of a lithium cell containing a sandwich cathode of, for example the configuration of: SVO/current collector/CF x /current collector/SVO, to have the weights of the respective active materials properly regulated within strict tolerances. This is accomplished by the use of the ID matrix etched onto the current collectors and the casing of the present cells. As previously discussed, other sandwich cathode configurations include: SVO/current collector/SVO/CF x /SVO/current collector/SVO and SVO/current collector/CF x  with the SVO facing the lithium anode. In these alternate embodiments it is also important to strictly regulate the weight ratios of the active materials. The ID matrix can also be etched onto the anode current collector for tracking that component as well.  
         [0034]    It is appreciated that various modifications to the inventive concepts described herein may be apparent to those of ordinary skill in the art without departing from the spirit and scope of the present invention as defined by the appended claims.