Abstract:
A method of manufacturing an electrochemical battery where the electrolyte is introduced into the cell stack with minimal or no loss of electrolyte. The method of fabricating the battery, comprises the steps of separately forming an electrode cell stack and a sealed electrolyte pouch, placing the cell stack and the electrolyte pouch together into a cell package, applying a vacuum to the cell package, sealing the cell package, and rupturing the pouch to release the electrolyte into the cell stack. The pouch is ruptured by squeezing the pouch until the electrolyte squirts out of the pouch. Also, the cell package is sealed by heat sealing. With this fabrication technique there is little or no electrolyte loss. In particular, since the electrolyte is injected into the electrode cell stack after the package has been sealed, substantially all of the electrolyte is suctioned into the electrode cell stack without any of the electrolyte escaping from the package. In addition, since the electrolyte is not poured into the cell stack during the manufacturing of the cell stack, the cell manufacturing machine does not have to provide a glove box environment. Also, contamination of the cell stack manufacturing machine is avoided. The cell package is made of a laminate of polyester, aluminum and polypropylene to allow the heat sealing of the polypropylene layers to each other even when the cell package is contaminated with electrolyte. Accordingly, it is not necessary to transfer the cell stack to an uncontaminated cell package.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method of fabricating an electrochemical battery and, more particularly, to a method of injecting the electrolyte into the cell stack of the battery by first placing the electrolyte in a sealed pouch that is placed together with the cell stack into a cell package. The invention also relates to a cell package which can be heat sealed even after it has been contaminated during the process of introducing the electrolyte into the cell stack.  
           [0003]    2. Background  
           [0004]    A important consideration in the manufacturing of electrochemical batteries is the manner in which the electrolyte is introduced into the cell stack. A current technique includes the steps of pouring the electrolyte into the cell stack during the manufacturing of the cell stack in a machine, placing the electrolyte impregnated cell stack into the cell package, evacuating the cell package and heat sealing the package.  
           [0005]    This technique suffers from many disadvantages. The first problem is the loss of electrolyte during the step of pouring the electrolyte into the cell stack and the subsequent step of evacuating the package. It is estimated that between 10 and 30% of the electrolyte is lost during these steps. The electrolyte is a relatively expensive component of the electrochemical cell. Thus, the loss of electrolyte increases the overall cost of manufacturing the battery. Further, since the amount of lost electrolyte cannot be gauged, the final volume of electrolyte that remains in the cell stack is unknown.  
           [0006]    A second problem is that the electrolyte that is naturally suctioned from the cell stack during the evacuating step contaminates the inside of the package. Such contamination of the package makes it difficult to securely seal the package. As such, subsequent leakage of the electrolyte from the sealed package may result. A further problem is that the pouring step must be performed in a glove box environment (i.e., dry and inert atmosphere) . Since this step is an intermediate step in the manufacturing of the cell stack, the machine which manufactures the cell stack must consequently have a glove box environment, thus driving up the cost of the machine. In addition, when the electrolyte is poured into the cell stack, the electrolyte contaminates the machine thus requiring that it be cleaned on a regular basis. In addition, the vacuum that is applied to the cell stack during the evacuating step may change the solvent ratio of the impregnated electrolyte.  
           [0007]    The current cell package is formed of a laminate of a polyester outer layer, an aluminum barrier layer and a polyethylene acrylic acid (EAA) inner layer. The polyester layer provides, strength, the aluminum layer prevents water from penetrating the cell package and the inner layer allows for the heat sealing of the cell package. Specifically, generally, the cell package includes two parts that are bonded together around their periphery by heating sealing the EAA inner layers to each other. The problem with this laminate is that once contaminated with electrolyte, the EAA inner layers cannot form a secure heat seal. This makes degassing and resealing of the cell package a problem. Also, the current material has a relatively high permeability to water necessitating the use of wider heat sealing areas in order to ensure a long shelf life. Finally, the current laminate will absorb electrolyte at elevated temperatures which can interact with the adhesive layer between the EAA inner layer and the aluminum barrier layer.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an object of the invention to provide a method of manufacturing an electrochemical battery which overcomes the above problems. In particular, an object of the invention is to provide a method of manufacturing a battery where the electrolyte is introduced into the cell stack with minimal or no loss of electrolyte. Another object of the invention is to provide a method in which the electrolyte filling step is performed after the cell stack is manufactured so that the cell stack manufacturing machine does not have to maintain a glove box environment and contamination of the machine is eliminated.  
           [0009]    These and other objects are achieved by a method of fabricating an electrochemical battery, comprising the steps of separately forming an electrode cell stack and a sealed electrolyte pouch, placing the cell stack and the electrolyte pouch together into a cell package, applying a vacuum to the cell package, sealing the cell package, and rupturing the pouch to release the electrolyte into the cell stack. The rupturing step includes the step of squeezing the pouch until the electrolyte squirts out of the pouch. Further, the sealing step includes the step of heat sealing the cell package.  
           [0010]    With this fabrication technique there is no electrolyte loss. In particular, since the electrolyte is injected into the electrode cell stack after the package has been sealed, substantially all of the electrolyte is suctioned into the electrode cell stack without any of the electrolyte escaping from the package. In addition, since the electrolyte is not poured into the cell stack during the manufacturing of the cell stack, the cell manufacturing machine does not have to provide a glove box environment. Also, contamination of the machine is avoided. Accordingly, all of the disadvantages discussed above with respect to the current technique are overcome.  
           [0011]    Another aspect of the invention is to form the cell package so that it includes two separate enclosures with a tube extending between the two enclosures such that they communicate with each other through the tube. The cell stack is placed in a first one of the enclosures which is then evacuated. The sealed electrolyte pouch is placed in the second enclosure which is also evacuated after the pouch is inserted. Thereafter, as with the previous embodiment, the pouch is ruptured to release the electrolyte from the pouch such that it flows through the tube and into the cell stack contained in the first enclosure. The tube is then sealed and the second enclosure is removed.  
           [0012]    Also, with this technique, after formation it is possible to degas the first enclosure. The formation is naturally done after the electrolyte has been released into the cell stack. The degassing step is performed by puncturing the second enclosure and applying a vacuum thereto so that the first enclosure is degassed via the tube. The tube is then sealed and the second enclosure is removed.  
           [0013]    It is a further object of the invention to provide a cell package made of a laminate which can be heat sealed even when the cell package is contaminated with electrolyte. This is achieved by a laminate which includes a polyester outer layer, an aluminum barrier layer and a polypropylene inner layer which are adhered together using a unique adhesive which does not break down when contaminated with electrolyte. It has been discovered that the polypropylene layers associated with the overlapped portions of the cell package can be securely heat sealed to each other even when contaminated with electrolyte. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The above and other objects, features and advantages of the present invention will be better understood from the following specification when read in conjunction with the accompanying drawings in which:  
         [0015]    [0015]FIG. 1 is a plan view showing the pouch filled with electrolyte according to a first embodiment of the invention;  
         [0016]    [0016]FIG. 2 is a sectional view taken along line  2 - 2  of FIG. 1 showing the electrolyte pouch;  
         [0017]    [0017]FIG. 3 is a plan view showing the partially sealed cell package with cell stack located therein, according to the embodiment of FIG. 1;  
         [0018]    [0018]FIG. 4 is a plan view showing the completely sealed cell package with both the cell stack and the electrolyte pouch contained therein, according to the embodiment of FIG. 1;  
         [0019]    [0019]FIG. 5 is a plan view showing the cell package after the electrolyte package has been ruptured and the electrolyte released inside the cell stack, according to the embodiment of FIG. 1;  
         [0020]    [0020]FIG. 6 is a plan view showing the cell package having a tube interconnecting the cell stack enclosure and the pouch enclosure, according to a second embodiment of the invention;  
         [0021]    [0021]FIG. 7 is a plan view showing cell package of FIG. 6 with the tube sealed after the introduction of the electrolyte into the cell stack enclosure;  
         [0022]    [0022]FIG. 8 is a plan view showing the cell package of FIG. 6 with the pouch enclosure portion having been removed;  
         [0023]    [0023]FIG. 9 is a side elevational view showing the tube;  
         [0024]    [0024]FIG. 10 is a plan view showing the cell package where the tube is connected to an external electrolyte source, according to another embodiment of the invention;  
         [0025]    [0025]FIG. 11 is a perspective view showing the preferred construction of the cell package; and  
         [0026]    [0026]FIG. 12 is an exploded sectional view showing the sealed portion of the cell package. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    Referring to FIGS.  1 - 4 , according to a first embodiment of the invention, an electrolyte  8  is first introduced into a pouch  10  and the pouch is evacuated and sealed. The sealed pouch is then placed along with a pre-manufactured cell stack  12  inside a cell package  14  which is then evacuated and sealed. Thereafter, the sealed pouch  10  is ruptured so that the electrolyte is released from the open end  16  of the pouch and impregnated inside the cell stack  12 .  
         [0028]    In more detail, the electrolyte pouch  10  is illustrated in FIGS. 1 and 2. The pouch is made of a polyethylene film or the like having, for example, a thickness of 50μ, and is formed by folding the film in half and heat sealing the two longitudinal sides  18  to form heat sealed seams  19 . The bottom end  20  of the pouch is closed by virtue of the fold while the top end of the pouch  22  is initially open (FIG. 1 shows the pouch after the electrolyte has been inserted and the pouch completely sealed). There are, of course, alternative techniques for forming the pouch. For example, the pouch can be formed by folding the film along the length of the film and heat sealing the longitudinal side opposite the fold and the bottom. Also, the pouch can be formed so it is seamless with an open top. Alternatively, the pouch can be formed of two separate sheets which are heat sealed together.  
         [0029]    The next step is to fill the pouch  10  with a predetermined amount of electrolyte. This step is preferably conducted in a glove box having a dry and inert atmosphere and using a dropper. After the pouch  10  is filled, the top  22  of the pouch  10  is heat sealed in a condition that there are no air bubbles inside the pouch. One technique to accomplish this is to seal the pouch just below the top level of the electrolyte, although the invention is no so limited. This ensures that all of the air is removed from the pouch prior to sealing. After the top  22  of the pouch  10  is heat sealed, the excess portion of the pouch (i.e., above the seal) is removed by cutting the pouch just above the seal. Thereafter, the top edge is cleaned to remove the electrolyte on the outside of the pouch so as to avoid subsequent contamination. The electrolyte pouch  10  is now ready to be inserted into the cell package  14 .  
         [0030]    Referring to FIG. 3, the cell stack  12  is positioned inside the cell package  14  with its leads  24  extending outside the package  14 . Like the pouch, the package can be formed by folding the film in either the longitudinal or transverse direction and heat sealing two of the remaining three sides to leave one end open. Since the leads must be accessible from the exterior of the cell package, the cell package should be folded and heat sealed around the cell stack. In the illustrated embodiment, the cell package has been formed by folding the film along a longitudinal side  26  to cover the cell stack and thereafter heat sealing the opposite longitudinal side  28  and the top end  30  around the leads  24 . This leaves the bottom end  32  of the cell package open. Alternatively, the cell package can be formed from two separate sheets  31  which are heat sealed together, as shown in FIG. 11, and discussed in greater detail below.  
         [0031]    Referring to FIG. 4, after the cell package  14  has been formed, the pouch  10  is inserted into the cell package  14  through the open bottom end  32 . It is preferred that the pouch  10  be positioned so that a sealed end (e.g., the top end  22 ) of the pouch is closest to the cell stack, as shown. The cell package  14  is then evacuated and the open bottom end  32  of the package  14  is heat sealed, as designated by reference numeral  34 . This results in an evacuated cell package  14  with the cell stack  12  and the electrolyte pouch  10  located inside.  
         [0032]    Referring to FIG. 5, the pouch  10  is then ruptured to release the electrolyte from the pouch. Since the cell package  14  is evacuated, the released electrolyte is suctioned inside the cell package to activate the cell stack  12 . A preferred technique for rupturing the pouch is to apply a sufficient pressure to the pouch to burst the pouch. It is important that the technique used for rupturing the cell does not rupture the package. In the illustrated embodiment, the top, heat sealed, end  22  of the pouch, closest to the cell stack, is ruptured. This is achieved by rolling a roller (not shown) along the pouch starting from the bottom end  20  and proceeding toward the top end  22  or by applying a mechanical shock to the pouch.  
         [0033]    After the cell stack is impregnated with the electrolyte, it is desirable to keep the cell stack in the cell package, instead of transferring the cell stack to another uncontaminated cell package, although the invention is not limited in this respect. This can be done by heat sealing the cell package along seam line  36  extend transversely between the cell stack  12  and the electrolyte pouch  10 . This involves heat sealing the cell package when it is contaminated with electrolyte. The preferred cell package material is discussed below, although it is understood that the invention is not limited to the particular type of cell package material.  
         [0034]    According to another embodiment of the invention, a method is used which does not require the sealing of the cell package after it becomes contaminated. This method will be described in detail with reference to FIGS.  6 - 9 . According to this aspect of the invention, the cell package  14  is formed so that it includes two enclosure that are separated from each other. A first enclosure  50  has a cell stack cup  52  which is sized to receive the cell stack  12  and the second enclosure  54  has a pouch cup  56  which is sized to receive the electrolyte pouch  10 , discussed above. The cell package  14  is vacuum heat sealed along seams  58 , as shown in FIGS. 6 and 7 and discussed further below, so as to form the two enclosures. A tube  60  is provided inside the cell package  14  so that one end communicates with the cell stack cup  52  and the other end communicates with the pouch cup  56 . As a result, the two cups are in fluid communication with each other. A window  62  is provided in the cell package  14  between the two enclosures such that a portion  64  of the tube is exposed to the outside of the cell package to allow the tube to be ultra-sonically welded without interfering with the package.  
         [0035]    In the illustrated embodiment, the cell package  14  is formed from two sheets of packaging material that are heat sealed together along the seams  58  in a conventional manner. Prior to sealing the transverse seams  66 , the tube is positioned between the sheets with respective ends in communication with the cell stack cup  52  and the pouch cup  56 . Thereafter, the package is heat sealed to form the transverse seams. It is noted that the tube  60  is made of an annealed metal, preferably nickel, copper, aluminum or any other material that is electrochemically compatible and weldable. Further, it is preferable that the tube be no thicker than the leads. The tube can be circular, oval or the like. Since the tube is made of metal, as discussed below, the heat sealing of the cell package does not cause the tube to be sealed.  
         [0036]    As shown in FIG. 6, the cell stack  12  is provided in the cell stack cup  52  such that its leads extend to the exterior of the cell package  14 . Thus, the cell package  14  is sealed around the leads  24  and evacuated, as discussed above. Similarly, the electrolyte pouch  10  is provided in the pouch cup  56  in the second enclosure  54  and the second enclosure is evacuated. As with the previous embodiment, the cell package  14  is completely sealed without the presence of free electrolyte. Therefore, the cell package is not contaminated with the electrolyte when it is sealed.  
         [0037]    After the cell package  14  has been sealed and the two enclosures  50 ,  54  have been evacuated, the electrolyte pouch  10  is then ruptured in the manner discussed above. Thus, the electrolyte is released from the pouch  10 , flows through the tube  60 , and becomes impregnated in the cell stack  12  to thereby activate the cell.  
         [0038]    The cell stack  14  is then subjected to formation which causes gases to develop inside the first and second enclosures  50 ,  54  and the cell stack cup  52  and pouch cup  56 , which communicate with each other. To degas the enclosures, the second enclosure  54  containing the burst electrolyte pouch is punctured to form hole  68  and evacuated by drawing a vacuum as designated by arrow V in FIG. 7. Thus, the electrolyte gasses are evacuated from the enclosures. After degassing, the tube is sealed at seam  70  in the window  62 . Preferably, the tube  60  is ultrasonically welded so that contaminants are removed from the weld area. Of course, the invention is not to be limited in this respect, it being understood that any convention welding technique would suffice.  
         [0039]    Referring to FIG. 8, after degassing, the cell package is trimmed to remove the second enclosure portion. The tube  60  is then folded back over the side of the cell package  14  so that it does not protrude from the package.  
         [0040]    [0040]FIG. 9 illustrates one technique for forming the tube  60 . As noted above, the tube  60  is formed by a thin metallic sheet. The sheet is folded such that the inside surfaces of the sheet contact each other and are welded together at weld area  72 . Although a description has been provided of one technique for forming the tube, it is understood that the invention is not limited to this arrangement. For example, the tube could be formed by injection molding or the like so that it is seamless, much like a hollow needle.  
         [0041]    The following is a description of yet another embodiment of the invention illustrated in FIG. 10. Instead of providing two enclosures in the cell package, it is possible to provide just a single enclosure  50  for receiving the cell stack  14  and supply the electrolyte from an external source through the tube  60  which communicates with the single enclosure  50 . In more detail, according to this aspect of the invention, one end of the tube extends into the cell stack cup  52  in the first enclosure  50  and the other end of the tube  60  extends outside the cell package so as to be exposed. The exposed end of the tube  60  may then be connected to an electrolyte reservoir  80  via a three-way pump  82  which allows the enclosure to first be evacuated and then filled with the electrolyte. After the enclosure is filled, the tube is sealed and the cell is then subjected to formation. After formation, the tube is cut to allow for degassing by drawing a vacuum through the tube. After degassing, the tube is US welded a second time. The tube is then folded over the cell package.  
         [0042]    [0042]FIGS. 11 and 12 show the construction of the preferred cell package  14 , although the above methods are not limited to the use of this particular package. One of the sheets  31  has a cavity corresponding to the cell stack cup  52 . The material of the sheets  31  includes a polyester layer  37 , an Al (Aluminum) foil layer  38  and a polypropylene heat seal layer  40 . The polyester layer  37  is adhered to the Al foil layer  38  by adhesive  42  and the heat seal layer  40  is adhered to the Al foil layer using thermoset adhesive  44 . The polyester layer  37  provides mechanical strength, the Al foil layer  38  acts as a moisture barrier to prevent moisture from entering into the cell package and the heat seal layer  40  seals the cell package.  
         [0043]    The thickness of the heat seal layer is between 25μ and 100μ, and preferably between 50μ and 75μ. The primary purpose of the heat seal layer is to seal the cell package after the electrolyte has been impregnated into the cell stack  12 . Polypropylene has a good compatibility with the electrolyte so that it can be heat sealed even when the cell package is contaminated with the electrolyte.  
         [0044]    Referring to FIG. 12 showing the heat sealed edge  46  of the cell package, the edge does not have an Al foil layer  38  to act as a barrier. Thus, it is important that the heat seal layer  40  have low permeability for both water and the electrolyte to ensure a long shelf life. Because of the non-polar nature of polypropylene it has a lower moisture permeability than the polyethylene, polyethylene acrylic acid (EAA) and Surlyn™, which, as discussed above, are current packaging materials for lithium polymer batteries. Also, polypropylene is more resistant to organic solvents than these materials.  
         [0045]    Potential disadvantages associated with polypropylene is that is has a higher sealing temperature than the conventional materials (approximately 140° C. v. 100-120° C.) and cannot be sealed directly to the metal leads of the battery. The former problem can be addressed by using packaging equipment that is capable of heat sealing at this higher temperature. The latter problem can be overcome by coating the leads  24  with a suitable hotmelt EAA glue or EAA film in advance.  
         [0046]    Any convention adhesive can be used as the adhesive  42 , i.e., aqueous or solvent based adhesive. On the other hand, the adhesive  44  is preferably a thermoset polyurethane adhesive or an organosol type modified polypropylene dispersion adhesive (Morprime®10B), both available from Morton International, Inc. These adhesives are advantageous in that they will not break down in the event the electrolyte penetrates through the polypropylene heat seal layer  40  during extended use or storage of the battery. A conventional water or solvent based adhesive can be attacked by the electrolyte.  
         [0047]    The following test results demonstrate the improved strength associated with these adhesives  44 . Specifically, a test was conducted to compare the seal strength of the cell package of the present invention (in both the uncontaminated state and the contaminated state) with the current cell package.  
         [0048]    The following is a description of the test.  
                                                                     Conventional packaging material                Polyester   23 μm (outer layer)           Adhesive layer   aqueous or solvent based           Aluminum foil layer   20 μm           Adhesive layer   aqueous or solvent based           Low density PE/Surlyn ®   70 μm (Heat seal layer)            Packaging material according to invention:                Polyester layer   12 μm (outer layer)           Adhesive layer   aqueous or solvent based               adhesive, or thermoset           Aluminum foil layer   13 μm           Adhesive layer   thermoset polyurethane           Polypropylene   100 μm (heat-seal layer)                      
 
         [0049]    Strips were cut of each packaging material and they were heat sealed together to form pouches. A first group of pouches were filled with organic electrolyte composed of a mixture of dimethylcarbonate, diethylcarbonate, ethylenecarbonate, and LiPF 6 . They were subsequent stored for 4 days at 60° C. 20 mm strips were then cut from the pouches so the heat seal area was in one end. The two pieces of packaging material, which previously made up the sides of the pouch were then pulled apart at a 180° angle, and the force and failure mechanism were determined. A second group of pouches were subjected to the tension test without being contaminated.  
         [0050]    In all cases the failure mode was delamination of the heat seal layer from the Aluminum foil, so the test showed the quality of the adhesive bonding those two layers together. The test results are shown in the following table.  
                                                                                             Seal strength of 20 mm wide strips                No Electrolyte   After 4 days @ 60° C. with           Contamination   Electrolyte                Conventional   Package of   Conventional   Package of           Packaging   Invention   Packaging   Invention                        Tensile   3.9 kg   9.1   kg   0.0 kg   4.7 kg       strength   3.2 kg   10.2   kg   0.0 kg   5.2 kg           4.3 kg   10.9   kg   1.1 kg               9.5   kg   2.1 kg       Average   3.8 kg   9.9   kg   0.8 kg   5.0 kg                                  
 
         [0051]    As is apparent, the cell package material of the present invention provides superior strength as compared to the conventional cell package, both when contaminated with electrolyte and when uncontaminated.  
         [0052]    The Al foil layer should be thick enough to avoid pinholes, preferably in the range of 25μ to 75μ. In the event that the cell package must be preformed, a formable grade Al foil should be used.  
         [0053]    As noted above, the outer polyester layer  37  is designed to provide mechanical strength to the cell package  14 . It should be understood that the invention is not limited to the use of polyester for this layer. For example, a laminate of PET and PE could be used as an alternative.  
         [0054]    The methods of the present invention provides the following important advantages. First, since the cell stack is sealed at the time that the pouch is ruptured, there is minimal loss of electrolyte. Also, there is no contamination of the equipment. Further, since the cell package is evacuated prior to the rupturing of the electrolyte pouch, electrolyte loss is further eliminate as it will not be suctioned from the cell stack. Further, the only step that must be performed in a glove box (i.e., a dry and inert atmosphere) is the filling of the pouch with electrolyte and the subsequent sealing of the pouch. The remaining assembly process steps can be performed in a dry room. Therefore, the equipment used for manufacturing the cell stack is less costly. Further, since the vacuum is applied to the package cell under the condition that the pouch is sealed inside the package, a higher vacuum pressure can be applied to ensure deep impregnation of the electrolyte into the cell stack without risking loss of electrolyte.  
         [0055]    In addition, the cell package construction with the polypropylene heat sealing layer and the special adhesive enables the cell package to be heat sealed after the impregnation of the electrolyte, using the first method described above, and retains its integrity even when contaminated by electrolyte. Therefore, additional steps associated with transferring the electrode stack to another, uncontaminated cell package can be eliminated.  
         [0056]    Further, using the tube technique for filling the cell stack with the electrolyte eliminates altogether the need for heat sealing a contaminated cell package.  
         [0057]    The foregoing detailed description is illustrative of the invention, and it is to be understood that additional embodiments thereof will be obvious to those skilled in the art. The embodiments disclosed herein together with those additional embodiments are considered to be within the scope of the invention as described in the claims appended hereto.