Abstract:
A process of co-extrusion of a thin electrode sheet with a thin electrolyte polymer sheet directly onto a current collector sheet for a lithium polymer battery. The process includes the steps of:
       (a) mixing a polymer with active electrode material, lithium salt and electronic conductive material in a first mixing chamber to form an electrode slurry;   (b) mixing a polymer with a lithium salt in a second mixing chamber to form an electrolyte slurry;   (c) feeding the electrode slurry through a first flow channel and the electrolyte slurry through a second flow channel;   (d) extruding the electrode slurry in the form of a thin electrode sheet through a first die opening connected to the first flow channel, the electrode slurry being extruded directly onto a current collector sheet; and   (e) concurrently extruding the electrolyte slurry in the form of a thin electrolyte sheet through a second die opening adjacent to the first die opening and connected to the second flow channel, the thin electrolyte sheet being extruded directly onto the thin electrode sheet.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority of U.S. provisional application Ser. No. 60/430,083, filed on Dec. 2, 2002. 
    
    
     FIELD OF INVENTION 
     The present invention relates generally to lithium polymer batteries and more specifically to a manufacturing process for extruding and assembling components of electrochemical cells for lithium polymer batteries. 
     BACKGROUND OF THE INVENTION 
     Rechargeable batteries manufactured from laminates of solid polymer electrolytes and sheet-like anodes and cathodes display many advantages over conventional liquid electrolyte batteries. These advantages include lower overall battery weight, high power density, high specific energy, longer service life, as well as being environmentally friendly since the danger of spilling toxic liquid into the environment is eliminated. 
     Solid lithium polymer battery components include positive electrodes, negative electrodes and an insulating material capable of permitting ionic conductivity, such as a solid electrolyte consisting of a polymer and a lithium salt sandwiched between the positive and negative electrodes. The anodes or negative electrodes are usually made of light-weight metals foils, such as alkali metals and alloys, typically lithium metal, lithium oxide, lithium-aluminum alloys and the like. The composite cathodes or positive electrodes are usually formed of a mixture of active material such as transitional metal oxide, an electrically conductive filler, usually carbon particles, and an ionically conductive polymer electrolyte material, the mixture being set on a current collector, which is usually a thin sheet of aluminum. Since solid polymer electrolytes are less conductive than liquid polymer electrolytes, solid or dry electrochemical cells must be prepared from very thin films (total thickness of approximately 50 to 250 microns) to compensate the lower conductivity with high film contact surfaces and to provide electrochemical cells with high power density. 
     Composite cathode thin films are usually obtained by solvent coating onto a current collector or by melt extrusion. Similarly, the polymer electrolyte separator layer is typically produced by solvent coating or by melt extrusion. 
     Solid lithium polymer electrochemical cells are typically manufactured by separately preparing the positive electrode, the electrolyte separator and the negative electrode. The positive electrode is initially coated onto a metallic foil (for example aluminum) or onto a metallized plastic film, which serves as a current collector. The polymer electrolyte is coated onto a plastic substrate, such as a film of polypropylene. The positive electrode is thereafter laminated onto one face of the electrolyte, then the plastic substrate is removed from the second face of the electrolyte and the lithium negative electrode is applied thereon. This manufacturing process which is reasonably efficient for research and development and small scale production of lithium polymer electrochemical cells is inadequate for large scale production. U.S. Pat. No. 5,536,278 to Armand et al. disclosed one such method of assembling the various components of a solid lithium polymer electrochemical cells. 
     U.S. Pat. No. 5,100,746 to Gauthier disclosed a method of laminating simultaneously a plurality of layers of components of an electrochemical cell that is adapted to speed up the manufacturing process, wherein double-layer solid polymer electrolyte/composite positive electrode sub-assemblies are subsequently associated with the other constituent layers of the electrochemical cell. However, the double-layer solid polymer electrolyte/composite positive electrode sub-assemblies are previously produced by successive lamination of positive electrodes and solid polymer electrolytes. 
     In order to improve the efficiency of the production process for large scale manufacturing of lithium polymer batteries, there is a need for a faster yet reliable method and apparatus for the production of multiple-layer solid polymer electrolyte/composite positive electrode sub-assemblies for thin film solid lithium polymer electrochemical cells. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved method of making and assembling components of a thin film solid lithium polymer electrochemical cell. 
     It is another object of the present invention to provide an apparatus for simultaneously making and assembling components of a thin film solid lithium polymer electrochemical cell. 
     As embodied and broadly described herein, the invention provides a process of co-extrusion of a thin electrode sheet with a thin electrolyte polymer sheet directly onto a current collector sheet for a lithium polymer battery, the process comprising the steps of:
         (a) mixing a polymer with active electrode material, lithium salt and electronic conductive material in a first mixing chamber to form an electrode slurry;   (b) mixing a polymer with a lithium salt in a second mixing chamber to form an electrolyte slurry;   (c) feeding the electrode slurry through a first flow channel and the electrolyte slurry through a second flow channel;   (d) extruding the electrode slurry in the form of a thin electrode sheet through a first die opening connected to the first flow channel, the electrode slurry being extruded directly onto a current collector sheet; and   (e) extruding the electrolyte slurry in the form of a thin electrolyte sheet through a second die opening adjacent to the first die opening and connected to the second flow channel; the thin electrolyte sheet being extruded directly onto the thin electrode sheet.       

     As embodied and broadly described herein, the invention also provides an apparatus for co-extruding components of an electrochemical cell of a lithium polymer battery onto a current collector sheet, the apparatus comprising a plurality of passageways linking a plurality of extruders to at least one die; the at least one die having at least two flow channels connected to at least two die openings, the at least one die adapted to extrude distinct sheets of material onto a current collector sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and other advantages will appear by means of the following description and the following drawings in which: 
         FIG. 1  is a schematic frontal cross-sectional view of a co-extrusion apparatus according to a first embodiment of the invention; 
         FIG. 2  is a schematic frontal cross-sectional view of a co-extrusion apparatus according to a second embodiment of the invention; 
         FIG. 3  is an enlarged cross-sectional view of a multiple slot die shown in  FIG. 2  adapted for co-extrusion on each side of a current collector sheet; 
         FIG. 4  is a schematic view of a measuring apparatus for measuring the thickness of a bi-face co-extrusion assembly; 
         FIG. 5  is a schematic cross-sectional view of a pair of co-extrusion apparatus according to a third embodiment of the invention positioned one after the other along the traveling path of a current collector; 
         FIG. 6  is a schematic cross-sectional view of a pair of co-extrusion apparatus according to a fourth embodiment of the invention; 
         FIG. 7  is a schematic cross-sectional view of a co-extrusion apparatus according to a fifth embodiment of the invention; and 
         FIG. 8  is a schematic cross-sectional view of a co-extrusion apparatus according to a sixth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In general, the production of thin sheets of polymer electrolyte separator and thin sheets of composite cathode thin sheets is most efficient by melt extrusion through a slit die. The various constituents of the electrolyte separator or of the composite cathode are fed from one or more hoppers into an extruder, where they are melted, mixed and transported through an air tight cylinder via a mixing screw. The molten material is extruded toward the slit die and discharged through an elongated discharge port of the slit die at a constant rate onto a substrate sheet or film, where the slit die is adjusted to the desired thickness of film or sheet. Electrolyte separator and composite cathode materials are different from typical thermoplastic resins for instance and are difficult to extrude as some of their constituents remain in solid form through the melting step of the extrusion process. Furthermore, polymers of the polyether family, such as polyethylene oxide, have low melting points and become unstable under normal extrusion conditions such as high temperature and high shear conditions. As disclosed in co-pending U.S. application No. 60/362,079 which is hereby incorporated by reference, the applicants were able to solve the problems associated with the extrusion process of such material and, based on their ability to reliably extrude composite cathode thin sheets and electrolyte polymer separator thin sheets, have further improved the production process of thin film or sheet electrochemical cells as is described below. 
     To further improve the production process, multiple discharge slot dies were developed such that a composite cathode thin sheet and an electrolyte polymer separator thin sheet may be extruded concurrently onto a substrate such as a current collector. The current collector for the cathode material is typically a thin aluminum foil, nickel foil, iron or stainless steel foil or a polypropylene substrate with a thin layer of conductive metal particles thereon. The so-called co-extrusion production process may further comprise optical and/or ultra-sonic and/or Gamma gauges and/or Beta gauges measuring devices or any suitable measuring devices known to those skilled in the art adapted to measure the thickness of the various layers being extruded to ensure that the extruded layers remain within strict tolerances. 
     With reference to  FIG. 1 , there is shown an co-extrusion apparatus  10  according to a first embodiment of the invention adapted to produce mono-face current collector/cathode/electrolyte laminates. Co-extrusion apparatus  10  comprises a main structural body  12  to which is connected a first extruder  14 , a second extruder  16  and a double slot die  15 . First extruder  14  mixes and extrudes composite cathodic material and second extruder  16  mixes and extrudes polymer electrolyte material. 
     A pair of inner passageways  20  and  22  link the exit ports of extruders  14  and  16  to double slot die  15 . The exit port  18  of extruder  14  is aligned with inner passageway  20 , through which molten material fed from extruder  14  is directed toward double slot die  15 . The exit port  17  of extruder  16  is aligned with inner passageway  22 , through which molten material fed from extruder  16  is directed toward double slot die  15 . Passageway  20  is divided into two main sections  24  and  26 . Section  24  comprises an expansion chamber  28  adapted to regulate the flow of molten material; from expansion chamber  28 , the molten material is fed into a wider section  26  leading directly into double slot die  15 . Section  26  of passageway  20  comprises a tubular ram  30 , whose diameter is equal to that of section  26  and is adapted to move into the path of the molten material to partially block the exit port  29  of section  24 , thereby regulating the flow of molten material fed into double slot die  15 . The motion of tubular ram  30  is controlled by either a hydraulic motor or an electric motor, capable of exact positioning of the tubular ram  30  relative to exit port  29 . The electric or hydraulic motor is connected to a control system that regulates the debit of molten material in response to various parameters, which are described further down. Similarly, passageway  22  is divided into two main sections  34  and  38 . Section  24  comprises an expansion chamber  36  adapted to regulate the flow of molten material fed from extruder  16 ; from expansion chamber  36 , the molten material is fed at constant pressure into a wider section  38  leading directly into double slot die  15 . Section  38  of passageway  22  comprises a tubular ram  32  adapted to move into the path of the molten material and partially block the exit port  39  of section  34 , thereby regulating the flow of molten material fed into double slot die  15 . 
     Double slot die  15  is mounted onto the main structural body  12  of co-extrusion apparatus  10 . Double slot die  15  comprises a pair of flow channels  42  and  44  aligned with the exit ports  41  and  43  respectively. Flow channels  42  and  44  are shaped as fish tails or coat hangers to reconfigure the flow of each molten material into a thin film, which exit through adjacent slit openings  46  and  48  calibrated to the desired thickness of extruded films ranging from 10 to 100 Microns. The slit openings  46  and  48  may be calibrated during machining of the double slot die  15  or provided with adjustments for fine tuning, as is well known in the art of die making. Slit opening  46  is calibrated or adjusted to discharge a cathode thin sheet of about between 20 and 80 microns and slit opening  48  is calibrated or adjusted to discharge an polymer electrolyte separator of about between 10 and 50 microns, depending on the end use of the electrochemical cell to be produced. It is understood that double slot die  15  may be calibrated or adjusted to any thickness required; however, the efficiency of the electrochemical cells being produced is maximized with very thin sheets. When calibrated slit openings are worn such that the thickness of the extruded thin sheets exceeds a set tolerance by, for example, 5 microns, the entire double slot die  15  is replaced. Co-extrusion apparatus  10  may be equipped with a turret (not shown) comprising two or more double slot dies  15 , such that rotation of the turret will align a new double slot die  15  with exit ports  41  and  43  and co-extrusion may resume with minimal delay when the die must be replaced. The worn out die is removed from the turret and a new die installed in its place without undue interruption of production. 
     A continuous composite cathode thin sheet  50  of, for example, 60 μm±5 μm exits slit opening  46  and is deposited directly onto a moving current collector thin sheet  54 . Current collector  54  may be a thin metallic foil of, for example, 15-50 μm, or a thin metallized polymer film of similar thickness. Simultaneously, a continuous polymer electrolyte separator thin sheet  52  of for example 25 μm±5 μm exits slit opening  48  and is deposited onto the composite cathode thin sheet  50 . As shown in  FIG. 1 , in a preferred embodiment of the co-extrusion manufacturing process, the assembly of current collector/composite cathode/electrolyte separator  55  immediately passes between a pair of flat cylinder rollers  60  and  62  driven at constant speed, flat cylinder roller  60  driving current collector  54  at a set speed. Flat cylinder rollers  60  and  62  are mounted on pivotal support structure  64  and  66 , each having an hydraulic cylinder  68  adapted to adjust the exact position of the cylinder rollers  60  and  62  and also to adjust the pressure applied onto assembly  55  as it passes between the cylinder rollers  60  and  62  it is understood that other means and constructions for adjusting the position of the cylinder rollers  60  and  62  and the pressure applied on the assembly  55  by the cylinder rollers  60  and  62  are contemplated and within the reach of a person skilled in the art and as such are within the scope of the present invention. The pressure applied by cylinder rollers  60  and  62  helps to promote adhesion of the various layers of assembly  55  together. To prevent adhesion of the electrolyte separator thin sheet  52  to flat cylinder roller  62 , flat cylinder roller  62  may be maintained at temperatures below the ambient temperature and preferably at a temperature ranging from −40° C. to 10° C. To prevent adhesion, flat cylinder roller  62  may also be provided with an anti-adhesive liner. 
     As a variant of the co-extrusion manufacturing process, the extruded cathode sheet  50  and electrolyte sheet  52  may be stretched onto current collector  54  in order to marginally reduce the overall thickness of assembly  55 . The stretching is achieved by selecting the speed at which the current collector  54  travels at the contact point between the extruded cathode material  50  and the current collector  54 , such that the speed of current collector  54  exceeds the rate of discharge of double slot die  15 . The speed differential between the current collector  54  and the cathode material exiting slit die opening  46  marginally stretches the extruded cathode sheet  50 , thereby reducing its thickness and consequently the overall thickness of assembly  55 . Since the current collector may be a very thin metallic foil such as aluminum foil of 25 μm, stretching cathode sheet  50  with speed differential may cause ripping of the metallic foil, in which case it is no longer feasible. 
     In a variant, a polypropylene thin sheet  56  shown in dotted lines is added on the side of the polymer electrolyte separator sheet  52  to protect the electrolyte separator. The polypropylene thin film  56  is removed prior to lamination of an anode film on the electrolyte separator layer, as described in U.S. Pat. No. 5,100,746 which is hereby incorporated by reference in its entirety. Although optional, when assembly  55  is brought to a further processing station where an anode film is laminated over assembly  55 , the polypropylene thin sheet  56  is important for storing assembly  55  into rolls for future use. 
     Assembly  55  winds through a series of rollers  70  to maintain a set tension on the continuous sheets and is brought to a measuring station  75  comprising a series of mechanical, optical, ultra sonic, Gamma or Beta measuring devices to control the thickness of the various layers of assembly  55 . In this particular embodiment, there are two layers to be measured; cathode sheet  50  and electrolyte separator sheet  52 . Therefore, measuring station  75  comprises three measuring devices  76 ,  77  and  78 . The first measuring device  76  measures the overall thickness of the entire assembly  55 , including current collector  54 , cathode sheet  50 , electrolyte separator sheet  52  and, optionally, polypropylene sheet  56 . Note that the current collector  54  and, when used, polypropylene sheet  56  are known quantities. The second measuring device  77  is for example an optical device calibrated to measure the thickness of electrolyte separator sheet  52 , which is a material allowing light transmission. A Light wave is sent though electrolyte sheet  52  at an angle. A portion of the light wave is reflected off the first surface of electrolyte sheet  52  and a portion of the light wave is transmitted through electrolyte sheet  52  and is reflected by the second surface of electrolyte sheet  52 . The two light reflections are received by optical measuring device  77 , which calculates the perpendicular thickness of electrolyte sheet  52 . If a polypropylene sheet  56  is used, the light reflected off its surfaces may be discarded. The third measuring device  78  is an ultra sonic, Gamma or Beta device calibrated to measure the thickness of cathode sheet  50 . Because the cathodic material layer is opaque, these types of waves are better suited to measure its thickness. 
     Measuring devices  76 ,  77  and  78  are linked to an electronic control unit such as a computer which is continuously fed data representative of the thickness of electrolyte sheet  52  and of cathode sheet  50 . This data is monitored by comparing it to pre-set thickness tolerances. When a thickness measurement falls outside the pre-set tolerances, an alarm signal is sent and the double slot die  15  may be adjusted manually by an operator receiving the alarm signal, or replaced altogether as previously described with a rotation of the die turret to change the double slot die  15 . The electronic control unit also monitors trends in the thickness measurement data received. The electronic control unit is linked to the system controlling the debit of molten material through passageways  20  and  22  via the motors controlling the position of both tubular rams  30  and  32 , and to the system controlling the speed and pressure of cylindrical roller  60  and  62 . The debit of cathode and electrolyte material may also be adjusted directly at the extruder&#39;s level by marginally increasing or decreasing the flow rate adjusting the throughput of the pumping device. The variables of debit, speed and pressure of rollers may be adjusted according to signals received from the electronic control unit, with the effect of providing minor adjustments to the thickness of electrolyte sheet  52  and of cathode sheet  50 . Other means of controlling the debits of cathode material and electrolyte material other than the illustrated tubular rams are possible and contemplated and within the competence of the skilled technician. Examples of such means are numerous and include valves and adjustable restrictions of the passageways or exit ports or even at the die exit. 
     For example, a detected increase in the thickness of cathode sheet  50  may be compensated by a decrease in the debit of molten cathodic material, which is effected by moving tubular ram  30  thereby partially blocking the flow of molten material through the exit port  29  of section  24  of passageway  20 , and simultaneously decreasing the flow rate of extruder  14  gear pump. Furthermore, the speed of current collector  54  may also be marginally increased by increasing the speed of rollers  60  and  62  to increase the stretching of cathode sheet  50 . Various responses to deviating thickness of electrolyte sheet  52  and cathode sheet  50  are pre-programmed, stored into memory, retrieved and initiated when corresponding thickness measurement data are received. Although limited, the ability of the system to effect minute adjustments of the thickness of electrolyte sheet  52  and cathode sheet  50  improves the quality of the final product. 
     Obviously, other means of measuring the thickness of the co-extruded layers are contemplated and well within the scope of the disclosed invention. The measurement is used to provide quality control of the co-extruded sheets and also to provide references for minute adjustments of the co-extrusion process. 
     With reference to  FIG. 2 , there is shown a co-extrusion apparatus  100  according to a second embodiment of the invention, adapted to produce bi-face current collector/cathode/electrolyte separator laminates. Co-extrusion apparatus  100  comprises a main structural body  102 , to which are connected four extruders  104 ,  106 ,  108  and  110  and a multiple slots die  105  mounted at the discharged end of co-extrusion apparatus  100 . Extruders  106  and  110  mix and extrude composite cathodic material. Extruders  104  and  108  mix and extrude polymer electrolyte material. Co-extrusion apparatus  100  comprises a central passageway  112  adapted to guide a current collector thin sheet  154  directly into multiple slots die  105 . Central passageway  112  extends the Length of co-extrusion apparatus  100 , from a first end  114  which receives current collector sheet  154  to a second end  116  which guides current collector sheet  154  into multiple slots die  105 . 
     Co-extrusion apparatus  100  comprises a first pair of inner passageways  120  and  122  linking the exit ports of extruders  106  and  110  to multiple slot die  105 . The path of passageways  120  and  122  leads the extruded cathode material toward the central portion of multiple slot die  105  on each side of current collector  154 , such that a sheet of extruded cathode material will be laid directly onto each side of current collector  154 . Passageways  120  and  122  are divided into two main sections  124  and  126 . Sections  124  comprises expansion chambers adapted to regulate the flow of the molten cathode material; from expansion chamber, the molten cathode material is fed into the wider sections  126  leading directly into multiple slot die  105 . Each section  126  comprises a tubular ram  130  whose diameter is equal to that of section  126  and is adapted to move into the path of the molten cathode material to partially block the exit ports of sections  124 , thereby regulating the flow of molten cathode material fed into multiple slot die  105 . The motion of tubular rams  130  is control by either a hydraulic motor or an electric motor (not shown) capable of exact positioning of the tubular rams  130  relative to exit ports of sections  124 . The electric or hydraulic motor is connected to a control system that regulates the debit of molten cathode material discharged by multiple slot die  105 . 
     Co-extrusion apparatus  100  comprises a second pair of inner passageways  140  and  142  linking the exit ports of extruders  104  and  108  to multiple slot die  105 . The path of passageways  140  and  142  leads the polymer electrolyte separator material toward the outer portions of multiple slot die  105  on each side of current collector  154 , such that a sheet of polymer electrolyte material will be laid onto the previously laid cathode sheets on each side of current collector  154 . Inner passageways  140  and  142  each comprise two distinct sections identical to inner passageways  120  and  122  and tubular rams  144  adapted to regulate the debit of molten polymer electrolyte material discharged by multiple slot die  105 . 
     As shown in  FIG. 3 , which is a cross-sectional view of multiple slot die  105 , multiple slot die  15  comprises a central channel  160  that guides current collector sheet  154  toward the discharge end of multiple slot die  105 . Multiple slot die  15  comprises four flow channels  162 ,  164 ,  166  and  168 , each shaped as fish tails, coat hangers or any other flow channel designs known to those skilled in the art of die making to reconfigure the flow of extruded materials into a thin films. Flow channels  162  and  164  aligned with passageways  120  and  122  reshape and discharge the molten cathode material as thin film onto each side of current collector  154 . Flow channels  166  and  168  aligned with passageways  140  and  142  reshape and discharge molten polymer electrolyte material as thin film onto the previously laid cathode material thin films. 
     Each flow channel  162 ,  164  comprises a discharge opening  170  calibrated to discharge a cathode thin sheet of about 20 to 80 μm (depending on end use) directly onto the moving current collector  154 . Each flow channel  166 ,  168  comprises a discharge opening  172  positioned downstream from discharge openings  170  and calibrated to discharge an electrolyte separator thin sheet of about 10 to 50 μm (depending on end use) onto the previously laid cathode sheets. The discharge openings  170  and  172  may be calibrated during machining of die  105  or manually adjustable. Adjustments of discharge openings  170  and  172  may be incorporated into the design of multiple slot die  105  as is well know in the art of die making. 
     As shown in  FIG. 2 , a bi-face assembly  155  electrolyte/cathode/current collector/cathode/electrolyte emerges from discharge nozzle  175  and immediately passes between a pair of flat cylinder rollers  180  and  182  driven at constant speed, moving bi-face assembly  155  at a set speed. As previously described and illustrated in  FIG. 1 , flat cylinder rollers  180  and  182  are mounted on pivotal support structure  184  and  186 , each having a hydraulic cylinder  188  adapted to adjust the exact position of the cylinder rollers  180  and  182  and the pressure applied onto bi-face assembly  155  as it passes between the cylinder rollers  180  and  182 . It is understood that other means and constructions for adjusting the position of the cylinder rollers  180  and  182  and the pressure applied on the bi-face assembly  155  by the cylinder rollers  180  and  182  are contemplated and within the reach of a person skilled in the art and as such are within the scope of the present invention. The pressure applied by cylinder rollers  180  and  182  helps promote adhesion of the various layers of bi-face assembly  155  together. To prevent adhesion of the electrolyte separator layer of bi-face assembly  155  to flat cylinder rollers  180  and  182 , each cylinder roller may be maintained at temperatures below the ambient temperature and preferably at a temperature ranging from −40° C. to 10° C. Alternatively, each cylinder roller is provided with an anti-adhesive liner. 
     Co-extrusion apparatus  100  may be equipped with a turret (not shown) comprising two or more multiple slot dies  105 , such that rotation of the turret wilt align a new multiple slot die  105  with the exit ports of passageways  120 ,  122 ,  140 ,  142 . In this embodiment, the extrusion process and the current collector are stopped for a few seconds so that the rotation of the turret cuts the current collector sheet  154  at the exit end  116  of co-extrusion apparatus  100 . The cut end of current collector sheet  154  is fed though central channel  160  and reinserted between cylindrical rollers  180  and  182  such that co-extrusion may resume with minimal delay. The discarded die is removed from the turret and a newly calibrated or adjusted die installed in its stead without undue interruption of production. 
     As previously described for the co-extrusion of a monoface assembly illustrated in  FIG. 1 , a polypropylene thin film  156  shown in dotted lines may be added on each side of the bi-face assembly  155  to protect the electrolyte separator layers. The polypropylene thin films  156  are removed prior to lamination of anode films on each side of the bi-face assembly  155  as described in U.S. Pat. No. 5,100,746, which is hereby incorporated by reference in its entirety. Although not necessary, when bi-face assembly  155  is brought directly to a further processing station where an anode film is laminated on each side of bi-face assembly  155 , the polypropylene thin films  156  are important for storing bi-face assembly  155  into rolls for future use. 
     As illustrated in  FIG. 4 , bi-face assembly  155  winds through a series of rollers  190  to maintain a set tension on the continuous sheets and is brought to a measuring station  192  comprising a series of mechanical, optical, ultra sonic, Gamma or Beta measuring devices to control the thickness of the various layers of bi-face assembly  155 . In this particular embodiment, there are four layers to be measured; the cathode sheets on both sides of current collector  154  and the electrolyte separator sheets laid over each cathode sheets. Therefore, measuring station  192  comprises five measuring devices  194 ,  195 ,  196 ,  197  and  198 . The first measuring device  194  measures the overall thickness of the entire bi-face assembly  155 , including current collector  154 , the two cathode sheets, the two electrolyte separator sheets and, optionally, the two polypropylene films  156 . Note that current collector  154  and, when used, polypropylene sheets  156  are known quantities. 
     The second measuring device  195  is for example an optical device calibrated to measure the thickness of electrolyte separator sheet on a first side of bi-face assembly  155 . A light wave is sent though the electrolyte layer at an angle; a portion of the light wave is reflected off the first surface of electrolyte layer and a portion of the Light wave is transmitted through electrolyte layer and is reflected by the second surface of electrolyte layer. The two light reflections are received by optical measuring device  195 , which calculates the perpendicular thickness of electrolyte layer. If a polypropylene sheet  156  is used, the light reflected off its surfaces may be discarded. The third measuring device  196  is an ultra sonic, Gamma or Beta device calibrated to measure the thickness of cathode layer on the first side of bi-face assembly  155 . Because the cathode material is opaque, ultra sonic Gamma or Beta waves are better suited to measure its thickness. 
     The fourth measuring device  197  is a device calibrated to measure the thickness of electrolyte separator sheet on the second side of bi-face assembly  155  and is identical to measuring device  195 . The fifth and last measuring device  198  is a device calibrated to measure the thickness of cathode layer on the second side of bi-face assembly  155  and is identical to measuring device  196 . 
     Measuring devices  194 ,  195 ,  196 ,  197 , and  198  are individually linked to an electronic control unit, such as a computer, which is continuously fed data representative of the thickness of each cathode layers and each electrolyte layers. This data is monitored by comparing it to pre-set thickness tolerances. When a thickness measurement fall outside the pre-set tolerances, an alarm signal is sent and the multiple slot die  105  is either adjusted manually by a machine operator or replaced. The electronic control unit also monitors trends in the thickness measurements data received. The electronic control unit is linked to the system controlling the debit of molten material through the various passageways  120 ,  122 ,  140 , and  142  via the motors controlling the position of both tubular rams  130  and  144 , and to the system controlling the speed and pressure of cylindrical roller  180  and  182 . The debit of cathode and electrolyte material may also be adjusted directly at the extruder&#39;s level by marginally increasing or decreasing the flow rate by adjusting the throughput of the extruder(s) pumping device(s). The variables of debit, speed and pressure of rollers may be adjusted according to signals received from the electronic control unit with the effect of providing minor adjustments to the thickness of the electrolyte layers and of the cathode layers of assembly  155 . As previously mentioned, other means of controlling the debits of cathode material and electrolyte material other than the illustrated tubular rams are possible and contemplated and within the competence of the skilled technician. Examples of such means are numerous and include gear pumps adjustments, valves and adjustable restrictions of the passageways or exit ports  116  or even at the die exit. 
       FIG. 5  illustrates another variant of the invention, where two extrusion stations  201  and  203  are positioned adjacent one another along the path of a current collector  205 . Extrusion station  201  is adapted to lay directly onto current collector  205  a first layer of extruded cathode material  210  on both sides of current collector  205 . Extrusion station  201  comprises two extruders  212  and  213  mixing and extruding thin films of cathodic material as illustrated, but could easily comprise only one extruder with two feeding ports. Extruders  212  and  213  feed extruded cathodic material through an extrusion die  215  comprising a pair of flow channels  216  and  217  shaped as fish tails or coat hangers or any other shape that reconfigures the flow of extruded materials into a thin films. Flow channels  216  and  217  reshape the flow and discharge extruded cathode material as thin film onto each side of current collector  205 . The flow channels are provided with thickness adjustment means  219  and  220  adapted to adjust the thickness of the cathode sheets being laid onto current collector  205 . Adjustment means  219  and  220  are illustrated as mechanical but may also be hydraulically or electrically controlled. A primary assembly  218  comprising current collector  215  and two cathode sheets  210  exits die  215  and is compressed by a first pair of rollers  222  before entering second extrusion station  203  through an aperture  224  adapted to receive the marginally thicker primary assembly  218 . 
     Extrusion station  203  is adapted to lay directly onto primary assembly  218  a second layer of extruded polymer electrolyte material  226  on both sides of primary assembly  218 . Extrusion station  203  also comprises two extruders  230  and  232  mixing and extruding thin films of polymer electrolyte material as illustrated, but could easily comprise only one extruder with two feeding ports. Extruders  230  and  232  feed extruded polymer electrolyte material through an extrusion die  235  similar to extrusion die  215 , although adjusted for primary assembly  218 . Extrusion die  235  comprises a pair of flow channels  236  and  237  shaped as fish tails or coat hangers, which reconfigure the flow of extruded materials into a thin film. Flow channels  236  and  237  reshape the flow and discharge extruded polymer electrolyte material as thin film onto each side of primary assembly  218 . The flow channels are provided with thickness adjustment means  219  and  220  adapted to adjust the thickness of the extruded electrolyte sheets being laid onto primary assembly  218 . A multi-layer assembly  240  comprising current collector  215 , two cathode sheets  210  and two polymer electrolyte sheets exits extrusion die  235  and is compressed by a second pair of rollers  242  to complete the bi-face current collector/cathode/electrolyte separator laminates. 
     In this particular embodiment, the co-extrusion process may be carried out while the current collector is traveling upwardly. Advantageously when the various layers are deposited onto a sheet of current collector traveling vertically in the upward direction, the extruded cathode and electrolyte materials are spread more evenly due to the equal action of gravity on each layer pulling down on the extruded material. 
     As in previously described embodiments, the co-extrusion apparatus illustrated in  FIG. 5  may be complemented with mechanical, optical, ultra sonic, Gamma or beta measuring devices adapted to measure the thickness of the various layers. In this specific embodiment, two such measuring stations would be provided immediately after each co-extrusion apparatus  201  and  203 , so that the initial measurement of the extruded cathode layers  210  is taken without the interference of the electrolyte layers  226 . One or two electronic units such as computers receive the measurement data and adjust the extruders&#39; flow rates, the thickness of the extruded sheets via adjustment means  219  and  220  and the pressure exerted by cylindrical rollers  222  and  242  in order to provide minute adjustments of the thickness of the various layers  210  and  226 . 
       FIG. 6  illustrates another variant of a co-extrusion process and apparatus, in which two co-extrusion apparatuses  301  and  302  similar to co-extrusion apparatus  10  illustrated in  FIG. 1  are positioned on each sides of a moving current collector  305 . Each co-extrusion apparatus  301  and  302  comprises a double slot die  315  having a pair of flow channels  316  and  317 . Flow channels  317  extrude thin sheets of cathode material directly onto each side of current collector  305 , whereas flow channels  316  extrude a thin sheet of polymer electrolyte material over the previously laid cathode thin sheets. The discharge section of each double slot die  315  is angled relative to current collector  305  such that the extruded cathode sheets are properly laid first and then the electrolytes sheets are laid over the cathode sheets. Two cylindrical rollers  320  positioned directly after co-extrusion apparatus  301  and  302  apply a small pressure directly onto the surfaces of the electrolyte layers. As previously mentioned, the co-extrusion may be carried out with current collector  305  traveling vertically upward. 
     As in previously described embodiments, the co-extrusion apparatus illustrated in  FIG. 6  may be complemented with precise measuring devices adapted to measure the thickness of the various layers. In this specific embodiment, a single measuring stations would be provided immediately after each co-extrusion apparatus  301  and  302 , that measures the thickness of each extruded cathode layers and each electrolyte layers. One electronic unit such as computers receives the measurement data and adjust the extruders&#39; speeds, the thickness of the extruded sheets via internal debit adjustment means (not shown) and the pressure exerted by cylindrical rollers  320  in order to provide minute adjustments of the thickness of the various extruded layers. 
       FIG. 7  illustrates yet another variant of a co-extrusion process and apparatus, in which four extrusion apparatus  401 ,  402 ,  403  and  404  are positioned in pairs on each side of a moving current collector  406 . The first pair of extrusion apparatuses  401  and  402  extrude a thin sheet of cathode material  410  directly onto each surface of current collector  406 . These first layers  410  passes through a first pair of cylindrical rollers  412 , which apply an even pressure onto cathode layers  410  to adjust their thickness. The first assembly  414  consisting of cathode/current collector/cathode is then fed through the second pair of extrusion apparatus  403  and  404  extrude directly onto each surfaces of cathode layers  410  a thin sheet of electrolyte material  416 . The final assembly  418  consisting of electrolyte/cathode/current collector/cathode/electrolyte is then fed through a second pair of cylindrical rollers  420 , which apply an even pressure onto final assembly  418  to adjust the final thickness of the extruded assembly. As mentioned, the co-extrusion may be carried out with current collector  406  traveling vertically upward. 
     As in the previously described embodiment of  FIG. 5 , the co-extrusion apparatus illustrated in  FIG. 7  may be complemented with measuring devices adapted to measure the thickness of the various layers. In this specific embodiment, two such measuring stations would be provided immediately after each pair of extrusion apparatus, so that the initial measurement of the extruded cathode layers  410  is taken without the interference of the electrolyte layers  416 . One or two electronic units such as computers receive the measurement data and adjust the extruder&#39;s speeds, the thickness of the extruded sheets via internal adjustment means of each extrusion apparatus  401 ,  402 ,  403 , and  404 , and the pressure exerted by cylindrical rollers  412  and  420  in order to provide minute adjustments of the thickness of the various layers  410  and  416 . 
       FIG. 8  illustrates yet another variant of a co-extrusion process and apparatus in which two co-extrusion apparatus  501  and  502  similar to co-extrusion apparatus  10  illustrated in  FIG. 1  are positioned on opposite sides of a moving current collector  505 . Current collector  505  winds its way through a series of rollers that effectively turn the current collector upside down such that co-extrusion apparatus  501  coats one side of the current collector  505  and co-extrusion apparatus  502  coats the other side of the current collector  505 . Each co-extrusion apparatus  501  and  502  comprises a double slot die  515  having a pair of flow channels  516  and  517 . Flow channels  517  extrude thin sheets of cathode material directly onto each side of current collector  505 , whereas flow channels  516  extrude a thin sheet of polymer electrolyte material over the previously laid thin sheets of cathode material. In operation, current collector  505  is initially re-directed by cylindrical roller  510  toward cylindrical roller  512  and co-extruder  501 . Co-extruder  501  discharges a thin layer of cathode material  520  directly onto the current collector  505  and a thin layer of a polymer electrolyte material  521  directly onto the layer of cathode material  520  through the flow channels  516  and  517  of its double slot die  515  as the current collector  505  is supported by roller  512 . The assembly of current collector  505 , cathode layer  520  and polymer electrolyte layer  521  remains in contact with roller  512  for approximately ½ turn or 180°, and is directed through cylindrical rollers  511  and  513  and toward the nip of cylindrical roller  518  and co-extruder  502  with the current collector  505  facing the double slot die  515  of co-extruder  502 . Co-extruder  502  discharges a thin layer of cathode material  522  directly onto the current collector  505  and a thin layer of a polymer electrolyte material  523  directly onto the layer cathode material  522  as the assembly is supported by roller  518 . The bi-face half cell assembly of electrolyte  523 /cathode  522 /current collector  505 /cathode  520 /electrolyte  521  is then completed and either appropriately stored for future processing or directed to a subsequent manufacturing station for further processing. Cylindrical rollers  513  and  518  may be cooled and kept at a low temperature to prevent the polymer electrolyte layer  521  from undesirably adhering thereto. As previously described for co-extrusion apparatus  301  and  302 , the discharge section of each double slot die  515  of co-extruder  501  and  502  may be angled relative to current collector  505  and its trajectory such that the extruded cathode layers  520  and  522  are appropriately laid first and then the polymer electrolyte layers  521  and  523  are suitably laid over the cathode sheets or layers  520  and  522 . Nip rollers may also be positioned directly after co-extruders  501  and  502  to apply small pressure directly onto the surfaces of previously laid cathode and electrolyte layers to promote adhesion and surface leveling. 
     As described for the previous embodiments, the co-extrusion apparatus illustrated in  FIG. 7  may be complemented with measuring devices adapted to measure the thickness of the various layers of the assembly. 
     Although the present invention has been described in relation to particular variations thereof, other variation and modifications are contemplated and are within the scope of the present invention. Therefore the present invention is not to be limited by the above description but is defined by the appended claims.