Abstract:
A current collector for an electrode of an electrochemical cell is described. The current collector has a peripheral edge between first and second major faces, the edge comprising at least a first edge contiguous with a second edge angled with respect to each other. A protrusion extends outwardly from the junction of the first and second edges. This protrusion helps to precisely position the current collector in a pressing fixture. That way, active material is contacted to each of the major faces of the current collector and is of a uniform thickness about its edges. Later, when the resulting electrode plate is assembled into an electrochemical cell, such as of a multi-plate construction, the protrusion also serves to maintain strict alignment of the plate inside the casing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of application Ser. No. 10/870,740, filed Jun. 17, 2004, now abandoned, which claims priority from provisional application Ser. No. 60/478,990, filed Jun. 17, 2003. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrochemical cells generating electrical energy by means of a chemical reaction. Electrolytic cells, for example of the lithium/silver vanadium oxide (Li/SVO) type, are typically constructed of one or more layers of anode, separator, and cathode. A screen or foil current collector is enclosed in the anode and cathode to transport electrons. An electrode assembly may be built by stacking multiple layers or plates on top of each other or by winding one or more long strips of the stacked layers around a mandrel. The electrode assembly is placed inside a case and immersed in an electrolyte, which transports ions. 
     One of the concerns in constructing an electrochemical cell is ensuring that the anode and cathode electrodes are properly aligned. This is not as great a problem is jellyroll electrode assemblies where the electrodes are of plates that are substantially longer than they are wide. The electrodes are then laid one on top of the other and spirally wound into the jellyroll configuration. 
     However, in an electrochemical cell having a multi-plate construction, electrode misalignment is a concern. Misalignment results in there being electrode plates that are not directly opposed by plates of an opposite polarity. In that respect, electrode plate misalignment detracts from the cell&#39;s discharge efficiency, as there will be active material that may not be fully reacted during electrochemical discharge. This is particularly likely to occur at the electrode edges. 
     The present invention prevents such misalignment by providing at least one of the electrode current collectors with projections emanating from its corners. These protrusions help to precisely position the current collector in a pressing fixture for contacting an active material to both sides thereof. That way, active material is contacted to each of the major faces of the current collector and is of a uniform thickness about its edges. Later, when the electrode plate is assembled into an electrochemical cell, such as of a multi-plate construction, the protrusions also serve to maintain strict alignment of the plate inside the casing. 
     Without protrusions according to the present invention, it is possible for the current collector to be positioned inside a pressing fixture with one portion of its edge too close to the fixture sidewall and another portion positioned too far away from the fixture. The result is that there is too much active material at the current collector edge spaced from the fixture sidewall and not enough at the other edge. This unbalanced active material contact can result in diminished discharge efficiency when the plate is incorporated into an electrochemical cell. 
     2. Prior Art 
     U.S. Pat. Nos. 627,134 to McDougall and 1,600,083 to Webster relate to current collectors having apertured projections. The projections do not contact the casing sidewall to ensure proper alignment. Instead, they receive locking rods for maintaining alignment inside a battery. Also, the prior art projections are not capable of centering the current collector in a pressing fixture. For example, with a generally rectangular shaped current collector, the centering projections must emanate from the corners at about a 45° angle, or essentially centered between the two contiguous sides. That way, with the current collector positioned in a fixture having the protrusion nested in a fixture corner, the immediately adjacent current collector sides are spaced from the pressing fixture sidewall by a like distance. The prior art current collectors do not provide for this type of centering as their protrusions emanate from a current collector edge adjacent to a corner. A corner emanating protrusion provides for proper spacing along the current collector edge having the protrusion, but not along the adjacent edge. 
     An example of this is shown with the current collector  10  illustrated in  FIG. 1 . The current collector  10  comprises first and second major faces  12  and  14  extending to a surrounding perimeter edge formed by opposed right and left edges  16  and  18  extending to upper and lower edges  20  and  22 . The right and left edges  16 ,  18  and the upper edge  20  are straight while the lower edge  22  is curved. The current collector  10  has an interior perforated region  24 . Spaced apart protrusions  26  and  28  emanate from the upper edge  20  adjacent to the respective right and left edges  16  and  18 . Similarly, spaced apart protrusions  30  and  32  emanate from the curved edge  22  adjacent to the respective right and left edges  16  and  18 . Having a protrusion only emanating from one edge of a current collector, instead of a corner between adjacent edges, means that there is no structure for regulating the spacing of the other current collector edge within a pressing fixture or a casing sidewall, as the case may be. In other words, protrusion  26  correctly spaced the upper edge  20  from a fixture sidewall (not shown), but is incapable of regulating the distance between the fixture and the right edge  16  of the current collector  10 . A similar problem exists with respect to protrusion  30  and edge  16  and protrusions  28  and  32  and edge  18 . 
     Thus, there is a need for a current collector design that enhances alignment in a pressing fixture so that a desired thickness of active material contacts both major current collector faces and the surrounding edge. Additionally, the current collector must provide for proper alignment with the opposite polarity electrode when it is incorporated into an electrode assembly housed inside a cell casing. The present current collector design provides both of these benefits. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a current collector design that ensures proper alignment of the current collector in both a pressing fixture for production of an electrode plate and later when the plate is incorporated into a electrode assembly. Providing the current collector with protrusions emanating from its corners does this, regardless whether the current collector is of a generally square shape having edges of substantially similar lengths or of a rectangular shape. In the latter case, the current collector can be significantly longer than it is wide as in a jellyroll electrode assembly, or not as in a prismatic cell design. In any event, the protrusions emanate from the corners centered between the edges. That way, they provide for spacing the current collector from the fixture sidewall a similar distance at the adjacent edges. This ensures a uniform thickness of active material contacted to the current collector at the edges. After the electrode plate has been built, the protrusions provide for properly aligning the electrode plate housed inside the casing. 
     The foregoing and additional advances and characterizing features of the present invention will become clearly apparent upon reading the ensuing description together with the included drawings wherein: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an exemplary current collector  10  according to the prior art. 
         FIG. 2  is a side elevational view of a current collector  40  according to the present invention. 
         FIGS. 2A to 2D  are enlarged views of the indicated area in  FIG. 2 . 
         FIG. 3  is a plan view showing the current collector  40  of  FIG. 2  is a pressing fixture  42  for forming an electrode plate. 
         FIG. 4  is a perspective view of a cell  100  comprising a casing  102  in a shadowed outline containing both anode and cathode plates with the anode plates connected to the case. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings,  FIGS. 2 and 3  illustrate an exemplary current collector  40  according to the present invention positioned within a pressing fixture  42 . The current collector  40  is a conductive member, typically selected from such materials as nickel, aluminum, stainless steel (for example according to U.S. Pat. No. 5,114,811 to Frysz et al.), mild steel, titanium, tantalum, platinum, gold, and cobalt-alloys (U.S. Pat. Nos. 6,541,158 and 6,110,622, both to Frysz et al.). These patents are assigned to the assignee of the present invention and incorporated herein by reference. 
     With respect to the orientation shown in the drawings, the current collector  40  comprises first and second major faces (only face  44  is shown) extending to a surrounding perimeter edge. Opposed right and left edges  46  and  48  extending to and meeting with upper and lower edges  50  and  52  form the perimeter edge. The right and left edges  46 ,  48  and the upper edge  50  are generally planar or straight while the lower edge  52  is curved. The current collector  40  has a solid frame  54  bordered by the edges and extending inwardly a relatively short distance to an interior perforated region  56 . The perforations are shown having the shape of diamonds, although virtually any opening shape is contemplated by the scope of the invention. This includes an expanded screen. Also, the current collector  40  need not be perforated at all, but instead, can be a solid member. 
     A first protrusion  58  emanates from the junction of the contiguous right and upper edges  46  and  50 . In  FIG. 2 , an imaginary projection of the right edge  46  is depicted by dashed line  60  and an imaginary projection of the upper edge  50  is depicted by dashed line  62 . The dashed lines  60  and  62  form a right angle. A first portion  58 A of the protrusion  58  resides between the dashed line  60  aligned with the right edge  46  and the protrusion perimeter. Similarly, a second portion  58 B of the protrusion resides between the dashed line  62  aligned with the upper edge  50  and the protrusion perimeter. The protrusion  58  is preferably centered at the corner of the right and upper edges  46 ,  50  with protrusion portions  58 A and  58 B being substantially equal. 
     As shown in the enlarged view of  FIG. 2A , a first distance  61  from the right edge  46  of the current collector to a distant most tangent  46 A of the perimeter of the first protrusion portion  58 A adjacent to the right edge  46  is substantially equal to a second distance  63  from the upper edge  50  to a distant most tangent  50 A of the perimeter of the second protrusion portion  58 B adjacent to the upper edge. The significance of this will be described in detail with respect to  FIG. 3 . 
     A second protrusion  64  emanates from the junction of the contiguous right and lower edges  46  and  52 . The dashed line  60  aligned with the right edge  46  of the current collector passes through this projection, as does an imaginary projection of the curved lower edge  52  depicted by the dashed line  66 . The angle between the dashed lines  60  and  66  is obtuse. A first portion  64 A of the protrusion  64  resides between the dashed line  60  aligned with the right edge  46  and the protrusion perimeter. Similarly, a second portion  64 B of the protrusion resides between the dashed line  66  aligned with the curved bottom edge  52  and the protrusion perimeter. The protrusion  64  is preferably centered at the corner of the right and lower edges  46 ,  52  with protrusion portions  64 A and  64 B being substantially equal. 
     As shown in the enlarged view of  FIG. 2B , a first distance  65  from the right edge  46  of the current collector to the distant most tangent  46 A of the perimeter of the first protrusion portion  64 A adjacent to the right edge  46  is substantially equal to a second distance  67  from the lower edge  52  to a distant most tangent  52 A of the perimeter of the second protrusion portion  64 B adjacent to the lower edge. The distant most tangent  46 A also defines the first distance  61  from the right edge  46  of the first portion  58 A of protrusion  58 . The significance of this will be described in detail hereinafter with respect to  FIG. 3 . 
     The current collector  40  is also provided with a protrusion  68  at the junction of the contiguous left and upper edges  48 ,  50  and a protrusion  70  at the junction of the contiguous left and bottom edges  48 ,  52 . An imaginary projection of the left edge  48  is depicted by dashed line  72 . In that respect, protrusion  68  includes a first portion  68 A that resides between the dashed line  62  aligned with the upper edge  50  and the protrusion perimeter. A second portion  68 B of the protrusion  68  resides between the dashed line  72  aligned with the left edge  48  and the protrusion perimeter. The protrusion  68  is preferably centered at the corner of the left and upper edges  48 ,  50  with protrusion portions  68 A and  68 B being substantially equal. 
     As shown in the enlarged view of  FIG. 2C , a first distance  69  from the upper edge  50  of the current collector to the distant most tangent  50 A of the perimeter of the first protrusion portion  68 A adjacent to the upper edge  50  is substantially equal to a second distance  71  from the left edge  48  to the distant most tangent  48 A of the perimeter of the second protrusion portion  68 B adjacent to the left edge. The distant most tangent  50 A also defines the second distance  63  from the upper edge  50  of the second portion  58 B of protrusion  58 . 
     The other protrusion  70  has first and second portions  70 A and  70 B residing between the imaginary line projections  66  and  72  of the respective curved lower edge  52  and the left edge  48  and its perimeter. The protrusion  70  is preferably centered at the corner of the left and lower edges  48 ,  52  with protrusion portions  70 A and  70 B being substantially equal. 
     As shown in the enlarged view of  FIG. 2D , a first distance  73  from the left edge  48  of the current collector edge to the distant most tangent  48 A of the perimeter of the second protrusion portion  70 B adjacent to the left edge  48  is substantially equal to a second distance  75  from the curved lower edge  52  to a distant most tangent  52 B of the perimeter of the first protrusion portion  70 A adjacent to the lower edge. The distant most tangent  48 A also defines the second distance  71  from the left edge  48  of the second portion  68 B of protrusion  68 . Again, the significance of this will be described in detail hereinafter with respect to  FIG. 3 . 
     A tab  74  extending from the upper side  50  completes current collector  40 . 
     As shown in  FIG. 3 , the current collector  40  is received in the pressing fixture  42  comprising a bottom wall  76  supporting upstanding right and left sidewalls  78  and  80  and upstanding upper and lower sidewalls  82  and  84 . The sidewalls meet each other at curved corners and surround an opening leading into the fixture. A gap  86  is provided in the upper sidewall  82  to receive the current collector tab  74 . 
     The fixture  42  is used to build an electrode plate containing the current collector  40 . This is done by first loading an electrode active material (not shown) therein. The active material is preferably in a granular form or a blank cut from a freestanding sheet and has a substantially uniform thickness such that its upper surface is spaced below the upper edge of the fixture sidewalls  78 ,  80 ,  82  and  84 . The current collector  40  is then moved into the fixture  42  lying on top of the active material. In this position, protrusion  58  nests into contact with the curved corner between the right and upper fixture sidewalls  78 ,  82 . Similarly, protrusion  68  nests into contact with the curved corner between the upper and left fixture sidewalls  82 ,  80 , protrusion  70  nests into contact with the curved corner between the left and bottom fixture sidewalls  80 ,  84 , and protrusion  64  nests into contact with the curved corner between the bottom and left fixture sidewalls  84 ,  80 . 
     The protrusions  58 ,  64 ,  68  and  70  are of substantially the same size, i.e., of a similar radius, to ensure that there is equal spacing between the current collector edges and the immediately adjacent fixture sidewalls. This means that the distance between the right current collector edge  46  and the fixture sidewall  78  (the distances  61  and  65  from the right current collector edge  46  to the distant most tangent  46 A of the respective protrusion portions  58 A and  64 A) is the same as the distance between the upper current collector edge  50  and upper fixture sidewall  82  (the distances  63  and  69  from the upper current collector edge  50  and the distant most tangent  50 A of the respective protrusion portions  58 B and  68 B), the left current collector edge  48  and left fixture sidewall  80  (the distances  71  and  73  from the left current collector edge  48  and the distant most tangent  48 A of the respective protrusion portions  68 B and  70 B), and between the lower current collector edge  52  and the lower fixture sidewall  84  (the distances  67  and  75  from the lower current collector edge  52  and the distant most tangents  52 A,  52 B of the respective protrusion portions  64 B,  70 A). If desired, however, the various protrusions can be off center in their respective corners and of unequal radii. 
     With the current collector  40  so positioned in the fixture  42 , another charge of active material is provided on top of the current collector. This active material, current collector, active material sandwich is then subjected to a pressing force sufficient to contact the active material to the major current collector faces and locked thereon through the perforations  54 . A suitable pressing force is about 10 to 20 tons/in 2  for about 30 to 60 seconds. In that manner, the protrusions ensure that there is a uniform amount of active material about the entire periphery of the current collector. A suitable process for forming blanks of active material is described in U.S. Pat. Nos. 5,435,874 and 5,571,640, both to Takeuchi et al. U.S. Pat. Nos. 4,830,960 and 4,964,877, both to Keister et al. describe a method for making an electrode component using a pressing fixture. All of these patents are assigned to the assignee of the present invention and incorporated herein by reference. 
     The thusly-manufactured electrode component can be either a cathode plate for a primary or secondary cell, or an anode plate for a secondary cell. 
       FIG. 4  illustrates an electrochemical cell  100  incorporating the current collector  40  of the present invention. The electrode assembly for the cell has both anode and cathode plates with the anode plates comprising the current collector  40  and electrically connected to the casing  102  serving as the negative terminal in a case-negative cell design. The casing  102  is of mating first and second clamshell portions  104  and  106  as described in U.S. Pat. No. 6,613,474 to Frustaci et al., which is assigned to the assignee of the present invention and incorporated herein by reference. However, as those who are skilled in the art will realize, the present invention current collector  40  is useful with any casing design including prismatic, cylindrical, or button shapes. The casing  102  is of a conductive material, such as of stainless steel or titanium. 
     The casing is adapted for housing various types of electrochemical chemistries such as alkali metal/solid cathode or alkali metal/oxyhalide electrochemical cells of both the solid cathode and liquid cathode types. The electrochemical cell  100  illustrated in  FIG. 4  is of the liquid electrolyte type comprising a cathode electrode having a body of solid cathode material in the form of plates  108 A and  108 B comprising cathode active material pressed together and bonded against a cathode current collectors  110  that are of a similar shape, but somewhat smaller in size than the current collector  40  described in  FIGS. 1 to 3  and being used for the anode electrode. This is because the anode current collectors  40  contact the inner surface of the casing  102  in the case-negative design. The cathode current collectors  110  for plates  108 A and  108 B are provided with a U-shaped tab  110 A connecting between them. This type of construction is referred to as a butterfly current collector, and is described in U.S. Pat. No. 5,250,373 to Muffoletto et al., the disclosure of which is hereby incorporated by reference. The U-shaped tab  110 A of the cathode is then connected to a terminal (not shown) insulated from the casing by a glass-to-metal seal (not shown), as is well known by those skilled in the art. Other cathode current collector designs can also be used. The cathode active material is preferably comprised of a metal, a metal oxide, a mixed metal oxide, a metal sulfide or a carbonaceous material. 
     The cell  100  further includes an anode electrode comprised of anode active plates  112 A,  112 B and  112 C, preferably of lithium sheets pressed to the opposite sides of the present invention current collector  40 . The outermost anode plates  112 A and  112 C are only provided with lithium on their inner surfaces facing cathode plates  108 A and  108 B, respectively. The anode current collector  40  is fabricated from a thin sheet of metal such as of nickel. The anode plates are in operative contact with the cathode plates through a thin sheet of separator material  114 . The separator divides the cathode and anode plates to prevent shorting by direct physical contact between the electrode plates while allowing ions to move between the plates. 
     The anode current collector tabs can be an individual piece attached to the case wall or, alternatively, they can be in the form of a U-shaped member connecting between two anode current collectors  40 . In cell  100 , anode plate  112 A has its current collector  40  provided with a tab having a portion  116 A planar therewith and a bent portion  116 B that is contacted to the casing  102 , such as by welding. Anode plates  112 B and  112 C are provide with a U-shaped tab  118  connecting between them. The mid-point or apex of the U-shaped tab  118  is joined to the tab portion  116 B, preferably by welding. The anode tabs are made of the same material as the current collector, preferably nickel, however, other materials also may be satisfactory. 
     By way of example, in an illustrative primary cell, the active material of the cathode body is a silver vanadium oxide cathode material as described in U.S. Pat. Nos. 4,310,609 and 4,391,729 or copper silver vanadium oxide as described in U.S. Pat. Nos. 5,472,810 and 5,516,340, all assigned to the assignee of the present invention, the disclosures of which are hereby incorporated by reference. The cathode current collectors  110  can be titanium, the cathode terminal lead can be molybdenum, and the separators  114  can be of polypropylene. The activating electrolyte can be a 1.0M to 1.4M solution of LiAsF 6  or LiPF 6  in a 50:50 mixture of, by volume, 1,2-dimethoxyethene and propylene carbonate. The glass seal can be of TA-23 Hermetic sealing glass, while the casing can be of stainless steel. 
     This electrochemical system is of a primary cell type. However, those skilled in the art will readily recognize that the casing of the present invention is readily adopted to house both primary electrochemical systems of either a solid cathode or liquid catholyte type, or a secondary cell such as a lithium ion cell having a carbonaceous negative electrode and lithium cobalt oxide positive electrode. 
     In the secondary electrochemical cell, the anode or negative electrode comprises an anode material capable of intercalating and de-intercalating the anode active material, such as the preferred alkali metal lithium. A carbonaceous negative electrode comprising any of the various forms of carbon (e.g., coke, graphite, acetylene black, carbon black, glass carbon, “hairy carbon” etc.), which are capable of reversibly retaining the lithium species, is preferred for the anode material. A “hairy carbon” material is particularly preferred due to its relatively high lithium-retention capacity. “Hairy carbon” is a material described in U.S. Pat. No. 5,443,928 to Takeuchi et al., which is assigned to the assignee of the present invention and incorporated herein by reference. Graphite is another preferred material. Regardless of the form of the carbon, fibers of the carbonaceous material are particularly advantageous because they have excellent mechanical properties that permit them to be fabricated into rigid electrodes capable of withstanding degradation during repeated charge/discharge cycling. Moreover, the high surface area of carbon fibers allows for rapid charge/discharge rates. 
     Also in secondary systems, the positive electrode preferably comprises a lithiated material that is stable in air and readily handled. Examples of such air-stable lithiated cathode active materials include oxides, sulfides, selenides, and tellurides of such metals as vanadium, titanium, chromium, copper, molybdenum, niobium, iron, nickel, cobalt and manganese. The more preferred oxides include LiNiO 2 , LiMn 2 O 4 , LiCoO 2 , LiCo 0.92 Sn 0.08 O 2  and LiCo 1-x Ni x O 2 . 
     An electrolyte is also required to activate the anode/cathode combination in the secondary system. The composition of the electrolyte depends on the materials of construction of the anode and the cathode as well as the product application for the cell. A preferred electrolyte for a lithium ion secondary cell has a lithium salt dissolved in a solvent system of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and propylene carbonate. 
     The current collector of the present invention can also be employed in a cell having a case-positive electrical configuration. In particular, replacing lithium anode elements with cathode plates provides a case-positive electrical configuration. Accordingly, cathode plates would be replaced by lithium anode plates, sandwiched together and against the current collector of the present invention serving as an anode current collector that, in turn, is connected to the terminal lead and insulated from the casing by the glass-to-metal seal. In all other respects, the anode current collector in the case-positive configuration is similar to that previously described with respect to cell  100  having the case-negative configuration. 
     The present invention may also be used with acid or alkaline-based batteries. 
     Now, it is therefore apparent that the present invention accomplishes its intended objects. While embodiments of the present invention have been described in detail, that is for the purpose of illustration, not limitation.