Abstract:
Heat generation is an important concern of electrochemical cell design. The invention is directed toward a cell design that efficiently and responsively dissipates heat by transfer from the cell to the casing through multiple parallel connections. This invention relates to battery designs having cell stacks in which both the anode and cathode are of a plate structure and the anode plates are independently connected to the cell casing or connected thereto via a bridge or bus. They may also consist of cell assemblies of wound electrode configurations or plate-serpentine configurations having multiple parallel connections to the cell casing. Surface area is an important criterion in battery design since it increases the current draw capability of the battery. The total surface area of this cell is potentially larger than any of the above mentioned types without sacrificing safety since the rate of heat conduction along the electrode pathway is approximately proportional to half an individual plate length, rather than the total anode length. Also, since the rate of heat leaving the cell is in excess of that found in traditional, galaxy or cathode plate and serpentine anode types of cells, the probability of cell venting is minimal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     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.  
         [0003]     The number of electrode layers in a cell is a trade-off between current and capacity requirements. More plates or winds of the electrodes give more surface area between the anode and cathode and subsequently, higher current capability to the cell. However, less plates or winds require less passive parts (separator and screen or foil) and allow for more active material (cathode and anode), which results in higher capacity.  
         [0004]     Safety is another important consideration in selecting design options involving plates, winds and the nature of the electrical connections. Particularly important is a design option that enhances transport of heat out of the cell. This is critical for batteries used to power implantable medical devices such as cardiac pacemakers and defibrillators. During an internal electrical short, electrical energy is converted to heat energy. To facilitate heat dissipation, it is desirable to conduct heat outside the cell as rapidly as possible so that internal temperature does not exceed the melting point of lithium. Such a temperature rise could cause hazardous venting of the cell.  
         [0005]     2. Prior Art  
         [0006]      FIG. 1  shows a traditionally wound Li/SVO cell  10  having a cathode connection  12  to the insulated terminal pin (not shown) and anode connections  14  to the cell casing (not shown). One end of the unitary wound anode is in the center and the other end is connected to the case wall. If there were an internal short at the center, bottom of the electrode assembly, one pathway would be for the heat to travel the full length and height of the anode to leave the cell.  
         [0007]      FIG. 2  shows another form of a traditionally wound cell  20 , often referred to as a galaxy-wound cell, having cathode connection tabs  22  and anode connection tabs  24 . One of each of the two anode ends is in the center and one of each of the other anode ends is connected to the case wall. If there were an internal short in such a galaxy-wound cell, a similar heat travel pathway from the center, bottom of the electrode assembly is approximately half that of the traditionally wound cell.  
         [0008]      FIG. 3  shows a traditional Li/SVO cell  30  having the cathode  32  made up of individual plates connected in parallel and the anode  34  wrapped around the cathode plates in a serpentine manner. Tabs  36  to a bridge-like connection structure  38  connect the cathode plates together and tabs  40  connect the anode to the cell casing (not shown). Heat transfer out of this traditional-plate cell design is similar to that of the galaxy-wound cell shown in  FIG. 2 .  
         [0009]     However, there is a need to develop an electrode arrangement that enhances heat transfer from the cell in the event of an internal short circuit. This extends the applicability of current electrochemical cells to new varieties of applications.  
       SUMMARY OF THE INVENTION  
       [0010]     The distance from the location of an internal short to the cell casing is critical. The present connection designs shorten this distance as compared to prior art arrangements to efficiently and responsively dissipate heat from an internal short to the casing through multiple parallel connections to the case wall. One form of this relates to battery designs having electrode assemblies in which both the anode and cathode are of a plate structure and the anode plates are independently connected to the case wall or connected via a bridge or bus to the case wall. The cell design may also consist of stacks of single or multiple anode elements having multiple parallel connections to the case wall. The potential for cell venting in such designs is minimized because the rate of heat leaving the cell is greater than that in traditional wound, galaxy wound, or cathode plate and serpentine anode type cells.  
         [0011]     In that respect, the surface area of a plate structure cell according to the present invention can be larger than any of the above conventional designs without sacrificing safety since the rate of heat conduction out of the cell is augmented in contract to conventional cells. The present invention is also applicable to cells having wound electrode assemblies and plate-serpentine electrode arrangements.  
         [0012]     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  
       [0013]      FIG. 1  is a perspective view of a traditionally wound cell having one wound cathode element and one wound anode element with a single anode connection to the cell wall at each side.  
         [0014]      FIG. 2  is a perspective view of another traditionally wound cell having one wound cathode element and two wound anode elements with each anode element provided with a connector.  
         [0015]      FIG. 3  is a perspective view of a traditional cell with cathode plates interleaved between a serpentine anode element.  
         [0016]      FIG. 4  is a perspective view of a cell stack having a casing shadow outline containing both anode and cathode plates with the anode plates independently connected to the case wall.  
         [0017]      FIG. 5  is a perspective view of a cell stack with the casing shadow outline containing anode and cathode plates with the anode plates connected to the case via a bridge or bus.  
         [0018]      FIG. 6  is a perspective view of an exemplary casing according to the present invention.  
         [0019]      FIG. 7  is a sectional view taken along line  7 - 7  in  FIG. 6  depicting one embodiment of electrode connections including a terminal pin.  
         [0020]      FIG. 8  is a sectional view taken along line  8 - 8  in  FIG. 6  depicting a second embodiment of electrode connections including a terminal pin.  
         [0021]      FIG. 9  is a perspective view of a wound cell incorporating the invention having several anode elements with multiple parallel connections.  
         [0022]      FIG. 10  is a perspective view of a cell incorporating the invention having cathode plate elements interleaved between the folds of a serpentine anode element. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     As previously described, in electrochemical cells to which the present invention is applicable, the number of layers of electrodes is a trade-off between current and capacity requirements. More electrode plates or winds give more inter-electrode surface area between the anode and cathode and, subsequently, higher current capability. Fewer plates or winds require fewer passive parts (separator and screen or foil) and allow for more active material (cathode and anode), which results in higher capacity. Winding an electrode assembly may be the preferred technique for manufacturing a cell stack because it eliminates extra connections between “like” plates, which add labor cost to a cell construction. It may also be desirable from a design point of view because these connections do not contribute to increased capacity or electrode surface area.  
         [0024]     However, from a safety perspective, a wound electrode assembly may be less desirable than a stack of electrode plates with multiple connections between “like” plates because multiple connections enhance heat transport out of the cell. Electrical energy is converted to heat energy during an internal cell short. It can be shown that the greatest amount of power generated by a cell occurs when the resistance across a short equals the internal cell resistance. During maximum power the greatest amount of heat is generated. To facilitate heat dissipation during an internal short, it is desirable to conduct heat outside the cell as rapidly as possible so that the lithium temperature does not exceed its melting point (approximately 180° C.). Such a temperature rise may cause cell venting.  
         [0025]     In a case-negative cell design, one pathway is for heat to travel through the anode connections and the case wall via conduction. Heat then leaves the cell via convection. In this pathway, the rate of heat flow by conduction through the anode assembly is proportional to the anode area normal to the direction of the heat flow and inversely proportional to its length of travel along the anode. The rate of heat flow by convection is related to the surface area of the outside case wall.  
         [0026]     In a present invention cell with anode plates independently connected to the case wall, one of the more problematic areas is for an internal short to occur at the center, bottom of any plate. In this pathway, the maximum length that the heat must travel to the case wall is half the length and the height of an individual plate. However, the potential for cell venting in such designs is minimized because the rate of heat leaving the cell is greater than that leaving any of the conventional cell types illustrated in FIGS.  1  to  3 . For example, heat conduction through the anode pathway is approximately 16 times faster in a cell with eight anode plates as compared to a traditionally wound cell with the same total anode length. In other words, total inter-electrode surface area in the present cell designs is larger than any of the conventional cell types without jeopardizing safety because the rate of heat conduction through the anode pathway is approximately proportional to half an individual plate length, rather than the total anode length. Surface area is important in battery design because it increases the current draw capability of the battery.  
         [0027]     Turning now to the drawings,  FIG. 4  illustrates an electrochemical cell  50  according to one embodiment of the present invention. In this embodiment, the electrode assembly has both anode and cathode plate electrodes, and the anode plates are independently connected to the case wall. The anode electrical extension is shown in conjunction with a casing  52 , for example of mating first and second clamshell portions  54  and  56  ( FIG. 6 ). However, as those who are skilled in the art will realize, the anode electrode extension is useful with any casing design which allows access to the surface of the anode lead and the inside walls of the casing housing. The available designs include clamshell, prismatic, cylindrical, or button shapes. The casing  52  is of a conductive material, such as stainless steel. The mated clamshell portions  54  and  56  are sealed about their periphery to provide a hermetic enclosure for an electrode assembly.  
         [0028]     In particular, the first clamshell portion  54  comprises spaced apart sidewalls  60  and  62  extending to and meeting with spaced apart sidewalls  64  and  66 . The pairs of sidewalls  60 ,  62  and sidewalls  64 ,  66  meet each other at rounded corners and extend to an end wall  68 . Opposite the end wall  68  is a continuous edge  70  ( FIGS. 7 and 8 ) of the sidewalls  60 ,  62 ,  64 , and  66 .  
         [0029]     The second clamshell portion  56  comprises spaced apart sidewalls  72  and  74  extending to and connecting with spaced apart sidewalls  76  and  78 . The pairs of sidewalls  72  and  74  and sidewalls  76  and  78  meet at rounded corners and extend to an end wall  80 . Opposite the end wall  80  is a continuous edge  82  of the sidewalls  72 ,  74 ,  76 , and  78 .  
         [0030]     In one preferred embodiment shown in  FIGS. 6, 7 , and  8 , the first clamshell  54  is sized to fit inside the periphery of the second clamshell  56  in a closely spaced relationship. This means that sidewalls  60  and  62  are somewhat shorter in length than sidewalls  72  and  74 , sidewalls  64  and  66  are somewhat shorter in length than sidewalls  76  and  78 , and end wall  68  is somewhat smaller in area than end wall  80 .  
         [0031]     In the constructions of  FIGS. 7 and 8 , lapping continuous edge  82  of clamshell  56  over continuous edge  70  of clamshell  54  creates a peripheral seam. In this embodiment, clamshell  54  is partially disposed inside clamshell  56  and weld  86  provides a hermetic seal at the peripheral seam. Butting the clamshell portions  54  and  56  together also can form the cell case. A laser beam from a welding source (not shown) creates weld  86 .  
         [0032]     The embodiment depicted in  FIG. 6  shows an electrolyte fill opening  88  contained in sidewall  64  and a glass-to-metal seal  90  supporting a cathode terminal lead  92 . The electrolyte fill opening  88  is closed using a closure means (not shown) to hermetically seal the cell  50 .  
         [0033]     The casing described herein is readily 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  50  housed in the illustrated casing is of the liquid electrolyte type comprising a cathode electrode  100  having a body of solid cathode material in the form of plates  102  comprising sheets pressed together and bonded against a cathode current collector. The cathode active material is preferably comprised of a metal, a metal oxide, a mixed metal oxide or a metal sulfide, and the cathode current collector is fabricated from a thin sheet of metal. Suitable materials for the current collector include nickel, aluminum, stainless steel, mild steel and titanium, with titanium being preferred.  
         [0034]     Referring again to  FIG. 4 , cell  50  further includes an anode electrode, generally designated  104 . The anode electrode comprises anode active plates  106 , preferably lithium sheets pressed to the opposite sides of an anode current collector. The anode current collector is fabricated from a thin sheet of metal such as of nickel. The anode electrode  104  is in operative contact with the cathode electrode  100  through a thin sheet of separator material  108 . The separator divides the cathode and anode plates to prevent shorting of the cell  50  by direct physical contact between the electrode plates.  
         [0035]     As shown in  FIG. 4 , the anode current collector extension can be an individual piece attached to the case wall or, alternatively, it can be in the form of a header extension secured to the wall as shown in  FIG. 5 . Depending on the battery application, either embodiment may be preferred. The extension and the header are made of metal, preferably nickel, however, other materials also may be satisfactory. The extension can be flat such as a strap or of another geometric shape such as hexagonal, or triangular, as long as it transfers heat from the cell and can be attached to the casing wall. However, in general, the greater surface area, the greater the heats transfer.  
         [0036]     In the embodiment shown in  FIG. 4 , two sets of anode current collector extensions are provided for connection to one of the casing halves shown in  FIG. 7 . In the first set, the current collector extension for the outermost anode plate comprises a first portion  114  extending from the anode current collector and being coplanar therewith and a second portion  116  extending substantially perpendicular to the first portion  114  and in a direction inwardly of the electrode assembly. Each of the remaining current collector extensions in the first set comprises a first portion  120  extending from the corresponding anode current collector and being coplanar therewith, a second portion  122  extending substantially perpendicular to the first portion and in a direction outwardly relative to the center of the electrode assembly, a third portion  124  extending substantially perpendicular to the second portion  122  and in a direction parallel to the first portion  120  and a fourth portion  126  extending substantially perpendicular to the third portion  124  and in a direction inwardly of the electrode assembly. Second portion  116  of the outermost extension and the fourth portions  126  of the remaining extensions are connected such as by welding to the same casing half, for example to casing half  54  shown in  FIG. 7 .  
         [0037]     In the second set of anode current collector extensions, the extension for the outermost anode plate comprises a single section  130  extending from the anode current collector and being coplanar therewith. Each of the remaining current collector extensions in the second set comprises a first portion  132  extending from the corresponding anode current collector and being coplanar therewith, a second portion  134  extending substantially perpendicular to the first portion and in a direction outwardly relative to the center of the stack and a third portion  136  extending substantially perpendicular to the second portion and in a direction parallel to the first portion. Section  130  of the outermost current collector extension and portions  136  of the remaining current collection extensions are connected such as by welding to the same casing half as the various extensions from the first set described above, for example to casing half  54  shown in  FIG. 7 . In the illustrative cell  50  shown in  FIG. 4 , individual cathode current connector tabs  140  extending from corresponding cathode current collectors are joined by a bridge connector  142  which, in turn, is connected to the cathode terminal lead, for example lead  94  shown in  FIG. 6 .  
         [0038]     In an alternative arrangement illustrated in  FIG. 8 , anode current collector extensions  124 ,  136  are connected to the respective casing halves  56  and  54 .  
         [0039]     In the embodiment shown in  FIG. 5 , the anode current collector extensions are joined by a bridge  150  which, in turn, is attached to the inside surface of the casing by a tab  152 . The anode current collector extensions comprise individual tabs extending from each of the anode plates that are joined together, such as by welding to bridge  150 . In this embodiment, however, bridge  150  is part of the extension for the outermost anode plate. In particular, the current collector extension for the outermost anode plate comprises a first portion  153  extending from the anode current collector and being coplanar therewith, and bridge  150  comprising a second portion extending substantially perpendicular to the first portion and in a direction across substantially the entire width or thickness of the cell stack. Each of the remaining current collector extensions in the cell stack is connected to the bridge  150 . Preferably, adjacent anode plates share a common U-shaped extension, these being designated  154  in  FIG. 5 . This is the result of providing a pair of anode current collectors joined by a central strip in a wing-like formation. For a more detailed description of such a current collector formation, reference is made to U.S. Pat. No. 5,250,373 to Muffoletto et al., the disclosure of which is hereby incorporated by reference. The mid-point or apex of the U-shaped extension is joined to the bridge  150 , preferably by welding.  
         [0040]     Thus, the anode current collector extensions connect each of the anode plates to the conductive casing to enhance heat transfer out of the cell in the event of an internal short circuit. In the illustrated anode-cathode electrode assembly, the anode plates may be viewed as successive portions of the entire anode extending along a path cross-wise of the assembly cross-section, and the extensions connect each of these successive portions to the cell casing.  
         [0041]     As shown in  FIG. 9 , the use of anode electrode current collector extensions is not limited to a plate structure, but may also be used in wound electrode configurations, such as the jellyroll and galaxy wound arrangements described hereinabove and known to those skilled in the art. Anode electrode current collector extensions are provided for the portions of the length of the anode electrode  158  located in the cross-section of the wind. In particular, anode electrode  158  is in the form of a continuous strip having successive portions along its length, and when the strip is in a wound configuration those successive portions extend along a path cross-wise of the anode-cathode electrode assembly cross-section. The current collector extensions are connected to each of these successive portions to enhance heat transfer out of the cell in the event of an internal short circuit.  
         [0042]     In particular, the current collector extensions  160  and  162  for the outermost portions of the continuous anode  158  comprise single sections extending from the anode current collector and being generally coplanar therewith. Associated with extensions  160  and  162  are extensions for two other portions of the anode length. These extensions comprise first portions  164  and  166  extending from the anode current collector and being generally coplanar therewith, second portions  168  and  170  extending substantially perpendicular to the first portions  164  and  166 , respectively, and in a direction along the cross-section of the wind, and third portions  172  and  174  extending substantially perpendicular to the second portions  168  and  170 , respectively, and contacting current collector extension  160 ,  162  for the outermost portions of the continuous anode. The remaining portions of the length of the anode electrode located in the cross-section of the wind are provided with extensions comprising first portions  180  extending from the anode current collector and being coplanar therewith, second portions  182  extending generally perpendicular to the first portions and in a direction across the cross-section of the wind and third portions  184  extending generally perpendicular to the second portions and away from the wind and preferably joined together. The current collector extensions  160  and  162  contacted by respective portions  172  and  174  and the plurality of contacted third portions  184  are connected to the cell casing, preferably by welding. The cathode electrode is connected to an external cathode terminal pin in a known manner.  
         [0043]      FIG. 10  illustrates an electrode assembly having one of the electrodes comprising a plurality of plates and the other electrode in the form of a strip arranged in a serpentine path around the plates. In the illustrated cell  190 , the cathode comprises a plurality of plates  192  and the anode  194  is the continuous strip wound along a serpentine path around the cathode plates. The anode strip has successive portions along its length, and when the strip is in a serpentine configuration those successive portions extend along a path cross-wise of the cross-section of the anode-cathode assembly. The anode current collector extensions are connected to each of those successive portions and to the cell casing to enhance heat transfer out of the cell in the event of an internal short circuit.  
         [0044]     The cathode electrode  196  is similar to those of the cells of  FIGS. 4 and 5 . Two sets of anode current collector extensions  200  and  202  are provided in a manner similar as those in the cell of  FIG. 4 . Alternatively, an arrangement of anode current collector extensions like that of the cell of  FIG. 5  can be provided.  
         [0045]     As shown in  FIGS. 7 and 8 , the terminal lead  92  connected to the cathode current collector via bridge  142  and tabs  140  extends through a header assembly comprising the glass-to-metal seal  90  fitted in the wall of the casing. Lead  92  is the positive electrical terminal, being connected to the cathode electrode. As stated previously, the anode electrode is in operative contact with the conductive casing through direct physical contact of the anode current collector extensions with the casing, in particular with the inner surface of the casing.  
         [0046]     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 collector can be titanium, terminal lead  92  can be molybdenum, and the separators 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. Glass seal  90  can be of TA-23 Hermetic sealing glass, while the casing can be of stainless steel.  
         [0047]     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.  
         [0048]     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.  
         [0049]     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 .  
         [0050]     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.  
         [0051]     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  14  having the case-negative configuration.  
         [0052]     The present invention may also be used with acid or alkaline-based batteries.  
         [0053]     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.