Abstract:
The present invention relates to a current collector for an electrochemical cell. The current collector is a substrate having a grid pattern comprising open areas converging at an imaginary focal point on a connector tab of the substrate. The openings are grouped into distinct regions with the larger openings immediately adjacent to the connector tab and the smaller openings distant there from. This provides more conductive pathways at greater distances from the tab.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from provisional application Ser. No. 60/333,943 filed Nov. 28, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the conversion of chemical energy to electrical energy. More particularly, the present invention relates to a current collector useful in electrochemical cells of both aqueous and non-aqueous chemistries. 
     BACKGROUND OF THE INVENTION 
     Present electrochemical cell designs primarily utilize two construction methods. Either the internal electrodes are spirally wound or they are assembled in a multiple plate or multiplate 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 substrate, such as a foil or screen. 
     The current collector of the present invention comprises a substrate having a unique pattern of openings that facilitate improved discharge. The openings are larger adjacent to the current collector tab, becoming smaller as the distance from the tab increases. The present current collector is useful in both spirally wound and multiplate cell types for both primary and secondary chemistries. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a novel current collector design in which the open areas of the grid pattern converge at an imaginary focal point on a connector tab of the substrate. The openings are grouped into distinct regions with the larger openings immediately adjacent to the connector tab and the smaller openings distant there from. This provides more conductive pathways at greater distances from the tab so that electrode active material contacting the current collector at the smaller openings is more efficiently discharged. 
     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 
         FIG. 1  is a plan view of one embodiment of a current collector  10  according to the present invention. 
         FIG. 1A  is a plan view of another embodiment of a current collector  10 A according to the present invention. 
         FIG. 1B  is a plan view of another embodiment of a current collector  10 B according to the present invention. 
         FIG. 2  is a plan view of another embodiment of a current collector  12  according to the present invention. 
         FIG. 3  is a plan view of a double winged current collector  14  according to the present invention. 
         FIG. 4  is a side elevational view of the current collector  10  of  FIG. 1  incorporated into an electrochemical cell  100 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings,  FIGS. 1 ,  1 A,  1 B and  2  are views of various embodiments of “single wing” current collectors  10 ,  10 A,  10 B and  12 , respectively, according to the present invention while  FIG. 3  shows another embodiment of a current collector  12  having a double wing configuration.  FIG. 4  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 one of the single wing configurations  10 ,  10 A,  10 B and  12  or of the double wing type  14  is not necessarily important. 
     As shown in the enlarged view of  FIG. 1 , the current collector  10  is a relatively thin substrate comprised of wire or bar-shaped conductor strands in the shape of a frame  16  surrounding a grid  18  and supporting a tab  20 . The conductors and tab are of a conductive material such as nickel, aluminum, copper, stainless steel, tantalum, cobalt and titanium, and alloys thereof. The frame  16  has spaced apart upper and lower strands  22  and  24  extending to and meeting with left and right strands  26  and  28 . Upper frame strand  22  meets left frame strand  26  at curved corner  30 , left frame strand  26  meets lower frame strand  24  at curved corner  32 , lower frame strand  24  meets right frame strand  28  at curved corner  34 , and right frame strand  28  meets upper frame strand  22  at curved corner  36 . 
     Tab  20  is a generally solid planar member and extends outwardly from the upper frame strand  22  spaced substantially equidistant from the left and right frame strands  26 ,  28 . Tab  20  includes left and right, sides  38  and  40  extending to and meeting with an intermediate edge  42 . The tab sides  38  and  40  are parallel to each other and generally parallel to the left and right frame strands  26 ,  28 . The tab sides  38  and  40  meet the upper frame strand at curved corners  44  and  46 , respectively. If desired, however, the tab  20  can be spaced closer to either of the left or the right frame strand than the other. 
     The grid  18  is interior of and supported by the frame  16  and generally comprises a first region of openings  48 , a second region of openings  50  and a third region of openings  52 . Openings  48  are larger than openings  50 , which, in turn, are larger than openings  48 . A first transition zone (shown as dashed line  54 ) delineates the extent of the first openings  48 . The area between the first transition zone  54  and a second transition zone (shown as dashed lines  56 ) delineates the area of the second openings  50 . The region between the second transition zone  56  and a distal portion of the left and right frame strands  26  and  28  adjacent to the lower frame strand  24  delineates the area of the third openings  52 . 
     As more particularly shown in  FIG. 1 , the first openings  48  are of a rectangular shape oriented with an apex pointed at each of the left and right frame strands  26 ,  28  and the upper and lower frame strands  22 ,  24 . The first openings  48  propagate or extend from an imaginary focal point  58  on the tab  20  and are uniformly spaced throughout the area bordered by the upper frame strand  22  and the first transition zone  54 . Triangular shaped openings  60  are provided at spaced intervals between the first openings  48  and the upper frame strand  22 . 
     The second openings  50  are of a rectangular shape positioned in a similar orientation as the first openings  48 . As with the first openings, the second openings are uniformly spaced throughout the region bordered by the first transition zone intersecting the upper frame strand  22  and the second-transition zone  56  intersecting the left and right frame strands  26 ,  28 . Triangular shaped openings  62  are provided at spaced intervals between the second openings  50  and the frame strands  22 ,  26  and  28 . 
     The third openings  52  are also of a rectangular shape positioned in a similar orientation as the first and second openings  48 ,  50 . The third openings are uniformly spaced throughout the region bordered by the second transition zone  56  and its intersection with the left and right frame strands  26 ,  28  and the lower frame strand  24 . As before, triangular shaped openings  64  are provided at spaced intervals between the second rectangular openings  52  and the lower, left and right frame shaped strands  24 ,  26  and  28 . 
     An important aspect of the present invention is the relationship between the regional extent of the first, large openings  48  to the intermediate sized second openings  50  to that of the smaller, third openings  52 . If the distance from the focal point  58  to the first transition zone  54  is “x” , then the distance from the first transition zone to the second transition zone  56  ranges from about 0.2x to about 10x. Also, the distance from the second transition zone  56  to the terminus of the third openings  52  ranges from about 0.2x to about 10x. 
     An important application of the present invention is use of the current collector  10  in a cathode electrode. During electrochemical cell discharge, electrons from the anode electrode travel through the load and are distributed to the cathode electrode to react with anode ions that have traveled directly through the separator to a reaction site on the cathode active material. It is important that these reactions occur uniformly throughout the cathode electrode, especially when the cathode active material has a higher resistivity than the current collector, such as silver vanadium oxide in a lithium cell (Li/SVO). Although current flow across the current collector is important, current flow across the cathode active material itself is critical because it has a greater impact on the even and uniform discharge of the anode and cathode electrodes. In other words, the transport of electrons to the cathode active material through the cathode current collector must be uniform for a cell to discharge at a constant rate, especially as end-of-life (EOL) discharge approaches. This is particularly the case when the current collector is provided with openings. 
     In a prior art current collector having openings of a fairly consistent size throughout, it is often seen that the anode material directly opposite or facing that portion of the cathode electrode proximate the tab reacts first. As discharge continues in a conventional cell design, anode material facing those portions of the cathode active material further and further from the cathode tab are reacted. Finally, anode material at the very outer reaches of the anode electrode and facing cathode active material most remote from the cathode tab is reacted. This results in non-uniform discharge, especially as EOL approaches when the cell is subjected to pulse discharge conditions in the Ampere range. An example is when the cell is used to power a cardiac defibrillator during device activation and the discharge is on the order of about 1 to about 4 amps. Non-uniform discharge is not so pronounced when the defibrillator is in a monitoring mode and current is on the order of about 1 microampere to about 100 microampere. 
     The unique structural configuration of the openings  48 ,  50  and  52  of the present current collector  10  prevents such non-uniform discharge. In those areas immediately proximate the current collector tab  20 , where the prior art current collector first experiences the majority of its discharge reactions, the distance from the edge of the current collector pathways bordering an opening to the cathode active material at the opening&#39;s center is greater, for example opening  48 , than in an opening of a smaller size, for example openings  50  and  62 . Therefore, while the cathode active material contacting a conductive portion of the current collector and immediately adjacent thereto is readily reacted, the cathode active material further removed from the conductive current collector portions or pathways and closer to the center of any one opening is not so readily reacted. In the present invention, this means the greater distance the electron must travel to react with the cathode active material at the center of a larger opening  48  acts to counterbalance the rapid discharge of the cathode active material proximate the tab. 
     Accordingly, an electron reacting at a cathode active material site proximate the center of one of the relatively smaller openings  50  and  52  does not travel as far from the conductive pathways as in one of the larger openings  48 . In this manner, the present current collector  10  promotes even and complete discharge of the cathode active material throughout the entire area of the cathode current collector, including those regions distal with respect to the tab  20 . 
       FIG. 1  shows the transition zones between the various opening regions having a generally elliptical shape. This is not necessary.  FIG. 1A  shows a current collector  10 A similar in construction to current collector  10  but having the rectangular shaped openings propagating or extending from focal point  58 A on tab  20 A to transition zones  54 A and  56 A of a partial circular shape. In other words, the transition zones  54 A and  56 A are of a generally fixed radius from the focal point  58 A. In all other respects, current collector  10 A is generally similar to current collector  10  of  FIG. 1 . For that reason, the parts of current collector  10 A corresponding to those of current collector  10  have been given the same numerical designation, but with the “A” suffix. 
     In a broader sense, however, the transition zone need not have an elliptical or a circular shape. It can also have an irregular shape. Furthermore, current collectors  10  and  10 A are shown having three distinct regions of openings propagating from the focal point  58 . However, according to the present invention there are at least two regions of openings, but there can be more than three regions. In any event, as the regions of openings propagate from the focal point, the openings are of a progressively smaller size. 
     Another embodiment of the present current collector  10 B has the openings having a gradual decrease in size as the distance from the tab increases. This is shown in  FIG. 1B  where the parts that are the same as those of current collector  10  are given the same numerical designation, but with the “B” suffix. The openings are designated  49 A to  49 T. 
     In a similar manner as the current collector  10 A of  FIG. 1A , current collector  12  of  FIG. 2  is generally similar to the current collector  10  of  FIG. 1  except the openings are circular instead of rectangular shaped. Also, the circular shaped openings propagate or extend from the focal point  58 C on tab  20 C to transition zones  54 C and  56 C having partial circular shapes. For that reason, the parts of current collector  12  corresponding to those of current collector  10  have been given the same numerical designations but with the “C” suffix. 
     It is also contemplated by the scope of the present invention that the openings need not necessarily be circular or rectangular. Instead, they can be of irregular shapes. They can also be of different shapes in the same current collector. What is important is that the size of the majority of the openings in a first zone or region closest to the current collector tab are larger than the majority of the openings in a second region further from the tab than the first region. A majority is greater than 50%. 
     The double wing current collector  14  of  FIG. 3  is essentially comprised of two current collector portions  14 A and  14 B, each similar to current collector  10 A of  FIG. 1A  as mirror images of each other. The mirror image current collectors  14 A,  14 B are positioned side-by-side, connected together at the third rectangular-shaped opening  52 A. 
       FIG. 4  shows the exemplary electrochemical cell  100  useful with any one of the current collectors  10 ,  10 A,  12  and  14 . 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 sidewalls  104  and  106  and a planar bottom wall  108 . The junctions between the various side walls and bottom wall are curved. A lid  110  closes the open top of the casing  102 . Lid  110  has an opening  112  that serves as a port for filling an electrolyte (not shown) into the casing after the cell&#39;s internal components have been assembled therein and lid  110  has been sealed to the side walls. In the final and fully assembled condition, a 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. 
     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 . 
     In either case, the exemplary cell  100  shown in  FIG. 4  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 openings. The tab  20  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. 
     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. 4 , 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  14  shown in  FIG. 3 . The wing portions  14 A and  14 B of collector  12  are then folded toward each other at about the mid-point of the third diamond-shaped opening  52 A so that the tabs  20 A line up with each other. In a case-positive cell configuration, the opposed cathode plates are carried by the wing portions  14 A,  14 B and folded toward each other and into electrical association with the opposed major sides of the central anode. 
     A more thorough and complete discussion of a cell construction having a current collector comprising wing-like portions that are folded into electrical association 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. 
     The cell  100  can be of either a primary or a secondary chemistry. A preferred primary electrochemical cell is of an alkali metal anode, such as of lithium, and a solid cathode active material. Exemplary cathode materials include silver vanadium oxide, copper silver vanadium oxide, manganese dioxide and fluorinated carbon (CF x ). An exemplary secondary cell has a carbonaceous anode and a lithiated cathode active material such as lithium cobalt oxide. In either type of cell chemistry, the activating electrolyte is of a nonaqueous nature. 
     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.