Abstract:
The present invention relates to a current collector for an electrochemical cell. The current collector has a unique grid structure comprised of a frame supporting a plurality of radial strands as conductors radiating outwardly from a focal point on a connector tab. The frame and radial conductors are maintained in a fan-like orientation with respect to each other by two groups of concentric conductor strands, one located adjacent to the tab, the other spaced a substantial distance therefrom. The radiating conductors provide a more direct path to the connector tab for electron flow. This results in the current collector having reduced internal resistance in comparison to conventional current collector designs.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority from provisional application Serial No. 60/269,131, filed Feb. 15, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to the conversion of chemical energy to electrical energy. More particularly, the present invention relates to a current collector having lower internal resistance (Rdc) in comparison to current collectors of a conventional design. The present current collector is useful in electrochemical cells of both aqueous and non-aqueous chemistries.  
         BACKGROUND OF THE INVENTION  
         [0003]    Present electrochemical cell designs utilize two primary construction methods. Either the internal electrodes are spirally wound or, they are assembled in a multiple plate configuration. In either case, each of the positive and negative electrodes is comprised of a current collector and active chemical constituents contacted thereto. The current collector can either be the casing housing the cell or, a conductive foil or screen.  
           [0004]    The current collector of the present invention is useful in both cell types for both primary and secondary cell and has a unique grid structure comprised of a frame supporting a plurality of radial strands as conductors radiating outwardly from a focal point on a connector tab. The frame and radial conductors are maintained in a fan-like and generally planar orientation with respect to each other by two groups of concentric conductor strands, one located adjacent to the tab, the other spaced a substantial distance therefrom. While the spaced apart groups of concentric conductor strands maintain proper spacing and structural integrity for the current collector grid, the radiating conductors provide a more direct path to the connector tab for electron flow. This results in the current collector having reduced internal resistance in comparison to conventional current collector designs.  
         SUMMARY OF THE INVENTION  
         [0005]    A characteristic of any electrochemical cell is its internal resistance (Rdc). It is well known that the design, shape and configuration of a current collector and its grid pattern affect Rdc. Placing an external load on an electrochemical cell causes a chemical reaction that produces a flow of electrons through the current collector to the associated external battery terminal. The basis of the present invention is to reduce Rdc by reducing the flow path of electrons. In other words, the distance electrons must travel between all points on the current collector and the cell terminal is reduced.  
           [0006]    Accordingly, the present invention is a novel current collector design in which the open areas of the grid pattern are not symmetric and not constant in size and orientation. Additionally, the solid elements of the conductor grid are oriented such that they converge at a common focal point. In that manner, if the solid material within the conductor grid is modeled as a wire, then a well known equation for total resistance is, 
           
         R=pL/A 
       
           [0007]    Where, p is the resistance of the conductor material, L is the conductor length and A is the cross sectional area of the conductor. Reducing L reduces R.  
           [0008]    In that light, the internal resistance of the present current collector is reduced with respect to conventional designs by providing a more direct path to the connector tab. The improved grid structure has a fan-like configuration of radiating conductors so that every portion of the current collector is either along a straight, radiating path to the connector tab or, is relatively proximate a radiating conductor.  
           [0009]    These and other aspects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and the appended drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a plan view of a current collector  10  according to the present invention.  
         [0011]    [0011]FIG. 1A is an alternate embodiment of a current collector  10 A according to the present invention.  
         [0012]    [0012]FIG. 2 is a plan view of a double winged current collector  12  according to the present invention.  
         [0013]    [0013]FIG. 3 is an elevational view of the present current collector  10  incorporated into an electrochemical cell  100 .  
         [0014]    [0014]FIG. 4 is a plan view of a current collector  150  according to the prior art.  
         [0015]    [0015]FIG. 5 is a schematic of a resistance test performed on the present current collector  10  and one according to the prior art  150  shown in FIG. 4 FIG. 6 is a graph of the resistance results from the test performed according to FIG. 5. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    Referring now to the drawings, FIG. 1 shows an enlarged view of one embodiment of a current collector  10  according to the present invention while FIG. 2 shows another embodiment of the present current collector  12  having a double wing configuration. FIG. 3 is of an exemplary electrochemical cell  100  of a multi-plate configuration comprising one of the present current collectors. Whether the current collector of the cell  100  is of the single wing configuration  10  or of the double wing type  12  is not necessarily important.  
         [0017]    As shown in the enlarged view of FIG. 1, the current collector  10  generally comprises wire or bar-shaped conductor strands in the shape of a frame  14  surrounding a grid  16  and supporting a tab  18 . The conductors and tab are of a conductive material such as nickel, aluminum, copper, stainless steel, tantalum, cobalt and titanium, and alloys thereof. As shown in this figure, the frame  14  has spaced apart upper and lower strands  20  and  22  extending to and meeting with left and right strands  24  and  26 . Upper frame strand  20  meets left frame strand  24  at curved corner  28 , left frame strand  24  meets lower frame strand  22  at curved corner  30  and lower frame strand  22  meets right frame strand  26  at curved corner  32 .  
         [0018]    Tab  18  is a generally solid planar member and extends outwardly from the junction of the upper frame strand  20  and the right frame strand  26 . In that respect, tab  18  includes upper and lower sides  34  and  36  extending to and meeting with an intermediate edge  38 . The tab sides  34  and  36  are parallel to each other and are generally parallel to the upper and lower frame strands  20 ,  22 . However, the upper tab side  34  is spaced somewhat below the upper frame strand  20  to provide a curved junction  40  where the upper strand  20  transitions into the upper tab side  34 . The lower tab side  36  meets the right frame strand  26  at a perpendicular angle.  
         [0019]    The grid  16  is interior of and supported by the frame  14  and comprises a fan-shaped configuration of radial strands as conductors in the form of wires or elongate bars. The radial conductor strands are of two general types. Conductors  42 ,  44  and  46  begin at the tab  18  and radiate outwardly to terminate at either the left frame strand  24  (conductor  42 ) or the lower frame strand  22  (conductors  44  and  46 ). The other type of radial conductor begins at a position spaced from the tab  18  and radiates outwardly to terminate at either the left frame strand  24  or the lower frame strand  22 . Regardless, both types of radial conductors have their terminus aimed at an imaginary focal point  48 , indicated by the area outlined by the dashed lines, and residing on the tab  18 .  
         [0020]    The grid structure further comprises a first and a second group of curved or concentric strands as conductors in the form of wires or elongated bars. The first group includes conductors  50 ,  52 ,  54 ,  56  and  58 , which are at progressively greater distances from the focal point  48  and concentric therewith. Each of the first group of concentric conductors extends from the upper frame strand  20  to the right frame strand  26  and intersects with each of the radial conductors  42 ,  44  and  46  emanating from the focal point  48  of tab  18 . Other of the radial conductors do not begin at the tab  18 , but radiate outwardly from various ones of the first group of concentric conductors. In that respect, radial conductors  60  and  62  both begin at concentric conductor  58  while radial conductor  64 , intermediate conductors  60  and  62 , begins at concentric conductor  50 . Conductors  60 ,  62  and  64  terminate at the left frame strand  24 . Radial conductor  66  begins at concentric conductor  54  and terminates in the vicinity of the curved corner  30  between the left frame strand  24  and the bottom frame strand  22 .  
         [0021]    The remaining radial conductors  68 ,  70 ,  72 ,  74 ,  76 ,  78 ,  80 ,  82  and  84  begin at various ones of the first group of concentric conductors, but terminate at the lower frame strand  22 . In that respect, radial conductor  68  begins at concentric conductor  58 . Moving downwardly and towards the right frame strand  26  in FIG. 1, conductor  70 , which is adjacent to focal point radial conductor  44 , begins at concentric conductor  58 . Radial conductors  72  and  74  begin at concentric conductors  54  and  58 , respectively. Radial conductor  76  begins at concentric conductor  50 , and radial conductor  78  begins at concentric conductor  54 . Finally, conductors  80 ,  82  and  84  begin at concentric conductor  54  and terminate at the lower frame strand  22 . Focal point radial conductor  46  is directly adjacent to radial conductors  78  and  80 .  
         [0022]    The grid  16  is completed by the second group of curved strands comprising conductors  86 ,  88  and  90 . As is the case with the first group of concentric conductors, these, too, are concentric with the focal point  48 . However, they are spaced a substantial distance from the first group of concentric conductors  50 ,  52 ,  54 ,  56  and  58 . The first group of concentric conductors  86 ,  88  and  90  extend from the upper frame strand  20  to the lower frame strand  22 .  
         [0023]    As shown in FIG. 1, the substantial distance from the first group of concentric conductors to the second group is defined as a factor of “x”. The distance x is defined as being from the focal point  48  to the outer most radial conductor  58  of the first group. Then, the distance from radial conductor  58  to the first one of the second group of radial conductors  86  is n(x) with n ranging from about 1 to about 10, and fractions thereof.  
         [0024]    In the current collector  10  shown, the substantial distance between the two groups of radial conductors is measured along upper frame stand  20 . However, it is contemplated by the scope of the present invention that the current collector need not necessarily have the generally rectangular frame shape shown. Instead, the frame can be squared, circular, or some other irregular shaped dictated by the design requirements of a specific cell construction. No matter what the specific shape of the current collector, according to the present invention, it has a series of radial conductors fanning out from a focal point and supported by spaced apart first and second groups of concentric conductors with the distance between the two groups of concentric conductors being at least “x”, as measured is some direction from the focal point. A single concentric conductor is sufficient to constitute a group for the purpose of this invention.  
         [0025]    Also, a concentric conductor need not necessarily connect to the spaced apart portions of the frame. It is within the scope of the present invention that any one of the first group of concentric conductors  50 ,  52 ,  54 ,  56  and  58  and of the second group of concentric conductors  86 ,  88  and  90  can terminate at one of the radiating conductors  42 ,  44 ,  46 ,  60 ,  62 ,  64 ,  66 ,  68 ,  70 ,  72 ,  74 ,  76 ,  78 ,  80 ,  82  and  84  and, in that respect, not terminate at the frame. For example, in FIG. 1A the opposed ends of concentric conductor  58  terminate at radial conductors  42  and  84  instead of upper frame strand  20  and right frame strand  26 , respectively. Similarly, the opposed ends of centric conductor  84  terminate at radial conductors  42  and  68  instead of upper frame strand  20  and lower frame strand  22 , respectively.  
         [0026]    The double wing current collector  12  of FIG. 2 is essentially comprised of two current collector portions  12 A and  12 B, each similar to current collector  10  of FIG. 1 as mirror images of each other. The mirror image current collectors  12 A,  12 B are positioned side-by-side connected together at a tab  18 A.  
         [0027]    [0027]FIG. 3 shows the exemplary electrochemical cell  100  useful with either one of the current collectors  10 ,  12 . For sake of clarity, the single wing collector  10  is shown. The cell includes a casing  102  having spaced apart front and back side walls (not shown) joined by side walls  104  and  106  and a planar bottom wall  108 . The junctions between the various side walls and bottom wall are curved. The open top of the casing  102  is closed by a lid  110 . Lid  110  has an opening  112  that serves as a port for filling an electrolyte (not shown) into the casing after the cell internal components have been assembled therein and lid  110  has been sealed to the side walls. In the final and fully assembled condition, a sealing plug, such as a ball  114 , is hermetically sealed in the electrolyte fill opening  112  to close the cell in a gas tight manner. The casing  102 , lid  110  and sealing ball  114  are preferably of a conductive material. Suitable materials include nickel, aluminum, stainless steel, mild steel, nickel plated mild steel and titanium. Preferably, the casing, lid and sealing ball are of the same material.  
         [0028]    A terminal lead  116  for one of the anode electrode and the cathode electrode is electrically insulated from the lid  110  and the casing  102  by a glass-to-metal seal  118 . In a case-negative cell configuration, the lead  116  serves as the cathode terminal and the lid  110  and casing  102  serve as the negative or anode terminal, as is well known to those skilled in the art. A case-positive cell configuration has the positive electrode or cathode contacted to the casing  102  with the anode supported on the current collector  10  connected to the lead  116 .  
         [0029]    In either case, the exemplary cell  100  shown in FIG. 3 includes a central electrode  120  comprising the current collector  10  of the present invention supporting at least one of the opposite polarity active materials. For the sake of clarity, the active materials are not shown supported on the current collector  10 . However, in a case-negative cell configuration, current collector  10  supports opposed layers of cathode active material contacting the opposite major sides thereof locked together through its many open areas. The tab  18  is then connected to the terminal lead  116  such as by welding. In a case-positive cell configuration, anode active material is locked together supported on the opposite major sides of the current collector.  
         [0030]    The central electrode  120  of cell  100  is sealed in a separator envelope  122  to prevent direct contact with the opposite polarity electrode. While not shown in FIG. 3, in a case-negative design the opposite polarity electrode is the anode comprised of anode active material contacted to the inner major sides of the current collector  12  shown in FIG. 2. The wing portions  12 A and  12 B of collector  12 , joined by the intermediate tab  18 A, are folded downwardly toward each other with respect to tab  18 A and into electrical association with the opposed major sides of the intermediate cathode. In a case-positive cell configuration, the opposed cathode plates are carried by the wing portions  12 A,  12 B and folded down toward each other and with respect to tab  18 A into electrical association with the opposed major sides of the central anode.  
         [0031]    A more thorough and complete discussion of a cell construction having a current collector comprising wing-like portions which are folded into electrical assistance with a central electrode of an opposite polarity is shown in U.S. Pat. No. 5,312,458 to Muffoletto et al. This patent is assigned to the assignee of the present invention and incorporated herein by reference.  
         [0032]    The following example describes the manner and process of a current collector according to the present invention, and it sets forth the best mode contemplated by the inventors of carrying out the invention, but it is not to be construed as limiting.  
         [0033]    Voltage measurements related to Rdc for a current collector  10  according to the present invention (FIG. 1) and for current collector  150  (FIG. 4) according to the prior art were recorded using a multimeter and constant current power source. The prior art current collector  150  was made of a similar conductive material as the present invention collector  10  and had a similar area, as shown in the plan views of FIGS. 1 and 4.  
         [0034]    Current collector  150  is comprised wire or bar-shaped conductor strands in the shape of a frame  152  surrounding a grid  154  and supporting a tab  156 . The frame  152  has spaced apart upper and lower strands  158  and  160  extending to and meeting with left and right strands  162  and  164 . Upper frame strand  158  meets left frame strand  162  at curved corner  166 , left frame strand  162  meets lower frame strand  160  at curved corner  168  and lower frame strand  160  meets right frame strand  164  at curved corner  170 .  
         [0035]    The grid  154  is comprised of a plurality of intersecting wire or bar-shaped conductor strands  172  and  174 . Conductors  172  are parallel to each other as are conductors  174 . Conductors  172  extend from the upper frame strand  158  to either the lower frame strand  160  or the left frame strand  162  while conductors  174  extend from the upper frame strand  158  to either the lower frame strand  160  or the right frame strand  164 . This provides the grid  154  having a plurality of diamond shaped openings  176 , and portions thereof, formed between the intersecting conductors  172 ,  174 .  
         [0036]    To perform the comparative Rdc test, a power source  178  having positive and negative leads  180  and  182  (FIG. 5) was set at a current of 0.182 Amperes, which is equal to the current for one electrode plate in a typical battery powering an implantable medical device. Test nodes A, B, C, D, E, F, G, H, I, J, K and L, as indicated in FIGS. 1 and 4, were set up on the respective current collectors  10 ,  150 . The positive lead  180  was placed on node C which corresponds to the focal point  48  of tab  18  of collector  10  and on the tab  156  of current collector  150 . The negative lead  182  was then placed on the other nodes A, B and D to L. The voltage drop between these node points was measured using a multimeter  184  having positive and negative leads  186  and  188  by placing the positive multimeter lead on node C and the negative multimeter lead  188  on the node on which the negative power supply lead  182  was contacted.  
         [0037]    [0037]FIG. 6 is a graph of Rdc as a function of measured nodal point location for two of each of the current collectors  10  , 150 . Since a constant current power source was used, the resistance can be computed by, 
         
       R=V/I 
     
         [0038]    Where, V is the voltage drop and I is the current. Since R is a factor of the total Rdc of the cell, the Rdc of one current collector plate is equal to R as computed above. The Rdc measurements for two current collectors of each of the present invention and the prior art are set forth below in Table 1 and in the graph of FIG. 6. This data clearly illustrate that the novel current collectors  10 ,  12  of the present invention have significantly lower Rdc than the prior art current collector  150  described in FIG. 4. This, in turn, lowers total battery Rdc to improve overall battery and device performance.  
                                                     TABLE 1                           Present   Present                   Invention   Invention   Prior Art   Prior Art       Meas Pt   #1   #2   #1   #2                                A   15.1   15.3   26.5   24.9       B   12.4   15.0   15.0   15.4       C   5.2   5.1   8.8   8.4       D   18.5   18.7   42.0   42.8       E   19.5   19.8   43.7   42.9       F   19.9   19.6   37.6   39.2       G   36.2   35.8   78.1   78.1       H   41.5   40.8   76.5   76.3       I   38.0   37.4   74.5   77.9       J   75.8   80.9   130.8   130.7       K   68.0   66.4   123.6   125.3       L   61.5   59.0   112.9   114.1                  
 
         [0039]    Since the theory of this invention has been proven using the mathematical relations described herein, it can be applied to any battery construction technique, namely, the multiple plate and wound cell stack described above.  
         [0040]    It is appreciated that various modifications to the present 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 herein appended claims.