Abstract:
The use of a polymer mesh made of material that melts under thermal runaway helps improve the safety of an electrochemical device. The mesh material can increase the impedance of the battery during the thermal runaway and absorb some of the heat produced.

Description:
This Application is a continuation of 08/883,644 filed Jun. 26, 1997 now U.S. Pat. No. 6,168,880. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to electrochemcial devices. Electrochemical devices are typically made of thin electrode and separator layers. 
     An example of such an electrochemical device is described in Kejha et al U.S. Pat. No. 5,521,023. Kejha et al. describes using a sheet of perforated plastic material along with a liquid ion-conductive polymer. The ion-conductive polymer is cured to form a solid- or semi-solid-state electrolyte composite. The perforated plastic material helps maintain the separation between the electrode layers. The liquid ion-conductive polymer by itself may not sufficiently maintain the separation between the electrode layers. 
     Another manner of maintaining the separation between the electrode layers is to use a separator using a co-polymer/plasticizer separation layer. The co-polymer/plasticizer layer can be laminated to electrode layers. The co-polymers of the separator will mix with co-ploymers of the electrodes. Later, the plasticizer is removed and an electrolyte solution added. The electrolyte solution provides and ion-conductive path between the electrodes. The co-polymer of the separator provides a relatively constant separation distance for the separator. The co-polymer remains not significantly soluble in the electrolyte solution, so that separation of the electrodes is maintained. 
     It is desired to have an improved electrochemical device. 
     SUMMARY OF THE INVENTION 
     A problem with electrochemical devices is that, under some conditions, a battery runaway can occur. A runaway is a short between the electrode layers which causes the battery to heat up. The head of the thermal runaway can cause additional damage to the battery, further reducing the impedance of the battery. Avoiding such runaway problems is especially important for lithium batteries. 
     In the present invention, a polymer mesh material made of a material that will melt during battery runaway is used. This material can be placed in a separator layer, electrode layer, or between the battery layers. The mesh is sized so that a solid battery material can be positioned in the mesh holes. 
     When the mesh material melts, the internal impedance is increased and thermal energy is absorbed to cause the melting. In effect, the holes of the polymer mesh are fused shut. This is useful during thermal runaway, abuse or other elevated temperature conditions. In a preferred embodiment, the melting temperature of the mesh is less than 150° C. but more than 100° C. to allow the battery layers to be laminated under normal lamination pressures and temperatures. Possible plastic materials for the polymer mesh include polyethylene, polypropylene, polyethylene terephthalate, and various co-polymers. 
     In a preferred embodiment, the mesh is made of ppolypropylene, polyethylene or a polyethylene/polypropylene co-polymer. 
     When a co-polymer is used in the separator that is substantially non-soluble in an electrolyte material, the separation and uniformity of the electrode layers can be effectively maintained under normal conditions without a polymer mesh. Adding the polymer mesh with the desired melting point has the advantage that thermal runaway can be avoided. 
     Additionally, placing the polymer mesh in an electrode layer can also help prevent the thermal runaway of an electrochemical device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings. 
     FIG. 1 is a diagram illustrating a die slot producing a mesh material within an active material layer for a battery. 
     FIGS. 2A and 2B are diagrams illustrating a solvent casting of a battery layer with a mesh material that is embedded therein. 
     FIG. 3 is a diagram illustrating the forming of a cell from individual die layers by lamination. 
     FIG. 4 is a diagram illustrating the forming of a bicell from individual battery layers. 
     FIG. 5 is a diagram illustrating the different battery layers including the non-conductive mesh of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a diagram illustrating the die slot production of a battery layer  10  including a battery material  12  with embedded electrically non-conductive mesh  14 . In this embodiment, the battery layer  10  is extruded from the die slot  16 . 
     In a preferred embodiment, the mesh material  14  is a plastic material. The mesh material  14  forms holes which allow the solid material  12  of the batter layer to be positioned therein. The mesh  14  is preferably expanded mesh formed by perforating and stretching a plastic layer. The mesh can also be produced by screen casting or by molding. 
     The mesh material is preferably made of material that melts at a temperature below 150° C. to allow for the mesh material to melt during thermal runaway of a battery. The mesh material preferably has a melting temperature above about 100° C. to allow the mesh to be used within battery layers that are laminated together. Plastic materials having a melting point in the desired range are believed to be polypropylene, polyethylene and a polyethylene/polypropylene co-polymer. In a preferred embodiment, the melting point of the mesh material is about 120-140° C. 
     The battery material  12  can be the separator film material used to form the separator layer, or alternatively be anode or cathode material. 
     Optionally, the mesh  14  can include an organic or inorganic filler material. Filler material can be used to improve the dielectric strength, change the dielectric constant and/or improve adhesion. Possible particle shape of the solid or hollow filler(s) used in the polymer mesh material include spherical, cubical, block, plate, flake or fiber. Possible filler raw materials include calcium carbonate, silica, glass, mica, alumina trihydrate, calcium metasilicate, aluminum silicate, antimony oxide, carbon or graphite, talc, barium sulfate or kaolin. 
     FIGS. 2A and 2B illustrate the solvent casting of a mesh  22  within a battery material  20 . In FIG. 2A, the battery material  20  can be a separator layer formed with a co-polymer and an intercellular compound, such as a plasticizer. An example of a co-polymer that is not soluble in the electrolyte is polyvinylidene fluoride/hexafluoropropylene (PVDF-HFT). The plasticizer can be removed by chemical treatment, and after the application of heat or temperature in FIG. 2B, the battery material  20  reduces to a film  24 . FIG. 2A and 2B illustrate the solvent casting of a separator layer, but the solvent casting could be used to produce an anode or cathode layer as well. 
     FIG. 3 illustrates the lamination of a cell  30  using a separator layer  32 , inner layer  34 , and cathode layer  36 . These layers are pressed together between rollers  38  and  40  to produce the laminated cell  30 . The mesh material can be part of the separator layer  32 , inner layer  34 , or cathode layer  36 . Alternately, the separator layer could be placed between any of these battery layers in the lamination process. 
     Another way to connect the battery layers to the polymer mesh is to melt or laminate the battery material to the polymer mesh. The co-polymers in the battery material will fuse with the polymer mesh. 
     FIG. 4 illustrates the production of a bicell  42  by lamination. In the production of a bicell, two cathode layers  44  and  46  are separated by separator layers  48  and  50  from a single anode layer  52 . As discussed above, the mesh material can be placed within any of these layers or between any of these layers. FIG. 5 is a diagram illustrating a battery cell  54 . The battery cell  54  includes an anode layer  56 , separator layer  58 , and cathode layer  60 . The separator layer  58  includes the mesh material  62 . The anode layer  56  is preferably made of a graphite-based carbon material. The anode layer  56  includes a current collector  64  and active material  66 . The cathode layer  60  in a preferred embodiment is lithiated manganese oxide or lithiated cobalt oxide. The cathode  60  includes a current collector  68  and active material  70 . 
     In a preferred embodiment, a liquid electrode material is added to the battery. The co-polymer of the separator is substantially insoluble in the electrolyte so that the co-polymer can maintain the separation between the electrodes. The polymer mesh material with the desired melting point has the advantage that the mesh will melt during thermal runaway. 
     Various details of the implementation and method are merely illustrative of the invention. It will be understood that various changes in such details may be within the scope of the invention, which is to be limited only by the appended claims.