Abstract:
A current interrupt device for rechargeable, electrochemical cells having a safety valve, insulating spacer and a thin metal plate for coupling to an electrode assembly is provided. The thin metal plate electrically couples to the safety valve by way of a conductive polymer, preferably having a positive temperature coefficient characteristic. The positive temperature coefficient characteristic is such that the impedance of the polymer increases with increasing temperature. In normal operation, the polymer conducts current. In high current situations, where internal components heat due to parasitic resistances, the polymer becomes an insulator and disconnects the cell from the exterior can. When gasses build within the cell, the safety valve deforms, thereby causing the safety valve to separate from the thin metal plate. Once the safety valve and thin metal plate have separated, the electrode assembly is electrically isolated from the exterior can.

Description:
BACKGROUND 
     1. Technical Field 
     This invention relates generally to protection devices for rechargeable cells, including electrochemical cells and super capacitors, and more particularly to devices that interrupt current when pressure builds within a rechargeable cell. 
     2. Background Art 
     Rechargeable batteries have become integral components of everyday life. Portable electronic devices like cellular telephones, two way radios and laptop computers rely upon rechargeable batteries for their portability. The rechargeable battery offers a way to slip the surly bonds of wall mounted power supplies and touch the face of the wireless world. 
     Battery packs generally include a plastic housing, electronic circuitry and at least one rechargeable cell. The cell within the battery pack is the device that stores and releases electrochemical energy. Many of these cells are sealed within cylindrical, aluminum (or steel) cans. Within these cans exist the cell&#39;s electrode assembly: electrode materials, a separator to keep the electrodes apart, and electrolyte. One of the most popular cans in use today is known as the “18-650” can. It is so named because it is 18 mm in diameter and 65 mm long. 
     Some cell chemistry types, like lithium-ion for example, produce gas when they are improperly charged, shorted or exposed to high temperatures. This gas can be combustible and may compromise the reliability of the cell. As such, protection circuitry is placed within the battery pack and about the cell to ensure that the cell is not over charged. The protection circuitry generally consists of integrated circuits and other components. Like any physical system, there is a small possibility that one of the components in the protection circuit may fail in the field. For this reason, the cells themselves often include back-up, or redundant, safety components to ensure that the cell is not overcharged. 
     A popular way of providing secondary protection for a cell is by way of a current interrupt device. One of the more popular current interrupt devices in use today is recited in U.S. Pat. No. 5,418,082, entitled Sealed Battery with Current Cut Off Means, issued May 23, 1995, to Taki et al., incorporated herein by reference. Such a device is illustrated in FIG.  1 . 
       FIG. 1  is a cross-sectional view of the current interrupt device. A sealed cell has a safety valve  5  made of a metal plate that may be deformed by an increase of pressure within the cell. The current interrupt device  6  is actuated by the deformation of the safety valve  5 . An insulating disk  23  is fixed between the safety valve  5  and the cell electrode  1 . This disk  23  has a central aperture  21  through which the projection  9  of the safety valve  5  is inserted. The disk further includes gas apertures  22  through which a gas, if and when it is generated by the cell, is passed. 
     A thin metal plate  24 , which is electrically connected to one ribbon lead  7  of the cell electrode  1  is attached to the electrode side of the disk  23  in such a manner as to close the central aperture  21 . The safety valve projection  9  is welded through the central aperture  21  to the thin metal plate  24 . 
     Referring now to  FIG. 2 , illustrated therein is the current interrupt device of  FIG. 1  after being actuated by gas within the cell. The safety valve  5  has been pushed up by gas generated within the cell that passed through aperture  22  and placed pressure upon the safety valve  5 . The safety valve  5  then deformed by swelling toward the cap  3  of the cell. In so doing, the weld between the safety valve  5  and the thin metal plate  24  is broken, thereby interrupting current flow. 
     The problem with this invention is the weld (between the projection  9  and the metal plate  24 ). To begin, an expensive, precision welder is required to make the tiny weld through the aperture  21  of the disk  23 . Next, if this weld is slightly too strong, the current interrupt device will not open quickly enough. If the weld is slightly too weak, there will be nuisance opening of the current interrupt device. If an operator manufacturing the current interrupt device errs ever so slightly during the welding process, reliability of the device will be compromised. In short, if the welding process is not an extremely precise, six-sigma or better manufacturing process, the current interrupt device will not function as designed. 
     There is thus a need for an improved secondary protection device for rechargeable electrochemical cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrated a prior art current interrupt device. 
         FIG. 2  illustrates a prior art current interrupt device after actuation. 
         FIG. 3  illustrates a current interrupt device in accordance with the invention. 
         FIG. 4  illustrates a positive temperature coefficient polymer at an ambient temperature. 
         FIG. 5  illustrates a positive temperature coefficient polymer at an elevated temperature. 
         FIG. 6  illustrates a current interrupt device that has been actuated in accordance with the invention. 
         FIG. 7  illustrates one cross section of a protrusion in accordance with the invention. 
         FIG. 8  illustrates a preferred cross section of a protrusion in accordance with the invention. 
         FIG. 9  illustrates a way to reduce the contact impedance between the thin metal plate and the positive temperature coefficient impedance in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” 
     Referring now to  FIG. 3 , illustrated therein is a current interrupt device (CID)  106  in accordance with the invention. This CID  106  offers advantages over the prior art in that it not only provides cell protection against internal pressure, but also provides overcurrent and thermal protection as well. Additionally, this CID  106  eliminates the need for costly welding equipment and precision welds. 
     The CID  106  is implemented in a battery can  102 , which is preferably steel, aluminum or an equivalent thereof. The can  102  houses the electrode assembly  101  of the cell, which may include cathode material, anode material, separator, electrolyte and other electrochemical storage components. (Note that with respect to batteries, the cathode is the positive electrode and the anode is the negative electrode.) The can  102  is closed with a lid  103  that is preferably hermetically sealed by crimping, gluing or welding to the can  103 . An optional gasket  104  may be included to provide a gas or liquid tight seal. The can  102  traditionally serves as the external anode of the cell, while the lid  103  traditionally serves as the external cathode of the cell. 
     The CID  106  itself comprises a safety valve  105  with a protrusion  109 . The safety valve  105  is preferably manufactured from a soft, deformable metal. The protrusion  109  is preferably disposed towards the center of the safety valve  105 . An insulating spacer  123  is disposed between the safety valve  105  and the electrode assembly  101 . The insulating spacer  123  includes a plurality of apertures, including a protrusion aperture  121  and at least one gas aperture  122 . The protrusion aperture  121  is preferably disposed towards the center of the insulating spacer  123 . The insulating spacer  123  is preferably held in place by a spacer holder  131 . The alignment of the insulating spacer  123  relative to the safety valve  105  is such that the protrusion  109  fits within the protrusion aperture  121 . 
     A thin metal plate  124  is disposed against the insulating spacer  123  opposite the safety valve  105  (i.e. on the same side of the insulating spacer  123  as the electrode assembly  101 ). The thin metal plate  124  is aligned relative to the insulating spacer  123  so as to cover the protrusion aperture  121 . An electrically conductive ribbon lead  107  couples the electrode assembly  101  to the thin metal plate  124 . 
     The protrusion  109  electrically couples to the thin metal plate by way of an electrically conductive polymer  100 . The polymer is preferably a positive temperature coefficient polymer (PTCP). As illustrated in  FIGS. 4 and 5 , a PTCP comprises composite of semi-crystalline polymer and conductive particles. The PTCP may either be a pure or composite polymer. At normal temperature, as shown in  FIG. 4 , the conductive particles are disposed closely together to form low-impedance networks capable of conducting current through the polymer. Once the polymer temperature rises beyond a predetermined threshold (determined by the chemical composition of the polymer), however, the crystallites in the polymer melt and become amorphous. The resulting increase in volume due to crystalline phase melting causes separation of the conductive particles, as shown in FIG.  5 . This results in a large, non-linear increase in the impedance of the PTCP. The increase in impedance is exponential in nature and rapidly stops any flow of current through the polymer. Temperature rise occurs when abnormally high currents flow through the polymer and cause resistive heating of the surrounding components. 
     Referring again to  FIG. 3 , the CID  106  interrupts current in multiple ways. The first way is dependent upon pressure. When gasses build within the cell, the gasses pass through the gas aperture(s)  122  and place pressure on the safety valve  105 . As the safety valve  105  is preferably manufactured from a soft, deformable metal, this net pressure causes the safety valve  105  to deform away from the electrode assembly  101 . This deformation causes the protrusion  109  to pull free from the PTCP  100 , not unlike separating one cookie of an Oreo™ from the other cookie and the ever so tasty cream filling. This separation stops any current flow between the electrode assembly  101  and the world outside the can  102 . A deformed safety valve  105 , analogous to the split Oreo™, is illustrated in FIG.  6 . 
     The second way of interrupting current with the CID  106  depends upon current. When excessively high currents pass through the PTCP  100 , to parasitic, resistive losses cause both the polymer and the surrounding components (the thin metal plate  124 , the safety valve  105  and the ribbon lead  107 ) to heat. This heating causes the PTCP  100  to go from a low-impedance state to a high impedance state. The transition essentially stops the flow of current. 
     The third way of interrupting current depends upon temperature. It is well known in the art that cell performance is degraded with electrochemical cells are charged or discharged at high temperatures. When the cell can, and thus the cell contents, heat due to exothermic conditions, the internal components heat, thereby causing the PTCP  100  to go from a low-impedance state to a high impedance state. The transition stops the flow of current. 
     In  FIG. 3 , the protrusion  109  is illustrated as a semicircle, which is convenient for manufacture of the safety valve  105 . Referring now to  FIG. 7 , illustrated therein is a preferred geometry for the protrusion  705 . The protrusion  705  is preferably a semi-rectangle. A semi-rectangle offers greater surface area for interconnection to the PTCP  100 . This increased surface area reduces contact impedance between the thin metal plate  124  and the safety valve  705 , thereby reducing the overall internal impedance of the cell. 
     To this point, the CID has been analogous to an Oreo cookie, with PTCP sandwiched between two metal plates. It is the mechanical rigidity of safety valve material that keeps the PTCP sandwiched between the safety valve and the thin metal plate. Such a mechanical “pressure contact” works well in benign to normal environments. One issue that a designer must keep in mind when designing electronic devices, however, is the notorious “drop test”. Drop testing is a grueling design evaluation test where a finished product is dropped anywhere from three to five feet to a surface of wood, tile or even concrete. Most product design specifications require that the product withstand such a drop with no degradation in performance. One issue with pressure contacts subjected to these tests is that they may momentarily open and then “bounce back”. In the case of battery products, such a bounce may disrupt power to the host device. 
     Referring now to  FIG. 8 , illustrated therein is a preferred embodiment of a “bounce resistant” CID in accordance with the invention. In this particular embodiment, the protrusion  809  includes at least one aperture. PTCP  100  is then disposed both above and below the protrusion  809  so as to form columns of PTCP that pass through the protrusion  809  of the safety valve  805 . This structure requires a significant deformation of the safety valve  805  to completely decouple the safety valve from the thin metal plate  124 . This significant deformation requirement prevents nuisance disconnects resulting from bounce issues. 
     One other design issue that may arise involves spreading a PTCP across a smooth metal surface. If the PTCP does not adhere completely, tiny pockets of air may form between the PTCP and the metal. These pockets of air increase the contact impedance of the CID. Referring now to  FIG. 9 , illustrated therein is a CID with reduced contact impedance. The CID of  FIG. 9  includes two methods of reducing the contact impedance. These methods may be used together or in combination. 
     The first method involves roughening the surface of the thin metal plate  924 . The roughened portion is illustrated as portion  902 . By roughening the surface, small depressions and raised portions are formed. When the PTCP  100  is deposited upon the roughened portion  902 , the roughened surface causes the PTCP  100  to fill in the depressions, thereby reducing contact impedance. 
     The second method involves anchors  901  that are added to the small metal plate  924 . These anchors  901  serve as small barbs that penetrate the PTCP  100 , thereby increasing the surface area of the thin metal plate  924  that is in contact with the PTCP  100 . This increased surface area also reduces the contact impedance. 
     The present invention offers several advantages over the prior art. First, the PTCP requires no welding. As such, neither expensive welding equipment nor precision welding processes are required. Second, the invention offers added value in that a single CID provides not only protection from excess pressure, but protection from thermal and overcurrent conditions as well. 
     While the preferred embodiments of the invention have been illustrated and described, it is clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions, and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the following claims.