Abstract:
An electrochemical cell having a cell package made of a metallic material to allow the cell package to be sealed by welding, even when contaminated. The electrochemical cell further includes an electrode cell stack and a metallic cell package having a base portion and a lid portion which are welded to each other (peripheral seam) to define an enclosure. The cell package includes a first section for receiving the cell stack and a second section having an inlet port and a degassing port which communicate with the first section. The lid portion is welded to the base portion to form a weld seam located between the first and second sections to seal off the first section from the second section. The weld seam extends from the peripheral seam on a first side of the cell package to the peripheral seam on a second side of the cell package.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improved cell package which can be welded to hermetically seal the cell package after the cell stack and electrolyte have been placed in the package and a method of manufacturing the same. 
     2. Background 
     A important consideration in the manufacturing of electrochemical batteries is the manner in which the electrolyte is introduced into the cell stack. As discussed in a related patent application, one 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. 
     This technique has shortcomings. One potential concern is the loss of electrolyte during the step of pouring the electrolyte into the cell stack and the subsequent step of evacuating the package. The electrolyte is a relatively expensive component of the electrochemical cell. Thus, the loss of electrolyte increases the overall cost of manufacturing the cell. 
     A second potential concern is that the electrolyte that is suctioned from the cell stack during the evacuating step contaminates the inside of the package. Such contamination of the package may make it difficult to securely seal the package. As such, subsequent leakage of the electrolyte from the sealed package may result. A further concern 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. 
     The current cell package is formed of a laminate of a polyester outer layer, an aluminum barrier layer and a polyethylene or polypropylene 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 heat sealing the inner layers to each other. The problem with this laminate is that once contaminated with electrolyte, the inner layers may not form a secure heat seal. This makes degassing and resealing of the cell package a problem. Finally, when a polymer is used as an inner layer, the electrolyte may still be able to permeate through the polymer itself This is especially true if the battery is exposed to elevated temperatures. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method of manufacturing an electrochemical cell which overcomes the above problems. In particular, an object of the invention is to provide a method of manufacturing a cell 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. Another object of the invention is to provide a packaging, which minimizes the possibility of electrolyte permeation through the seals. 
     These and other objects are achieved by a method of fabricating an electrochemical cell, comprising the steps of forming an electrode cell stack and a metallic cell package having a base portion and a lid portion which are welded to each other to define an enclosure, the cell package including a first section for receiving the cell stack and a second section having an inlet port which communicates with the first section; placing the cell stack into the enclosure in the first section; sealing the lid portion to the base portion around a periphery of the cell package to form a peripheral seam; applying a vacuum to the enclosure through the inlet port in the cell package; introducing an electrolyte into the enclosure via the inlet port; and welding the lid portion to the base portion to form a first weld seam located between the first and second sections to seal off the first section from the second section. The first weld seam extends from the peripheral seam on a first side of the cell package to the peripheral seam a second side of the cell package. 
     The method further includes the steps of partially charging the cell stack resulting in generation of gases inside the enclosure; puncturing the cell package to form an evacuation port located in a third section of the cell; applying a vacuum to the evacuation port of the cell stack to withdraw the gases; and sealing the lid portion to the base portion across a second weld seam located between the first section and the third section. According to one preferred aspect of the invention the second weld seam extends from the first side to a third side opposite the first side. After the second weld seam has been formed, the excess portions of the cell package, corresponding to the second and third sections, are removed from the first section which holds the cell stack. 
     The electrochemical cell according to a preferred embodiment of the invention comprises: a casing; and an electrode cell stack contained within the casing along with an electrolyte. The casing includes a base and a lid that are made of a metallic material such that they can be welded to each other along a seam weld to form an enclosure for receiving the cell stack. This is different from conventional cell packages where the parts are heat sealed to each other. The metallic material of the casing is, for example, aluminum, copper, nickel or stainless steel. The weld seam extends around a periphery of the cell package. 
     The cell stack includes first and second tabs of opposite polarity. The electrochemical cell further comprises a pass-through terminal secured to the casing and electrically connected to the second tab while the first tab is electrically connected to the casing. According to one aspect of the invention, the pass-through terminal comprises an eyelet having a first through-hole, an insulator located in the first through-hole of the eyelet and having a second through-hole; and a terminal post located in the second through-hole of the insulator so as to be insulated from the eyelet, wherein the eyelet is welded to the metallic material of the casing and the terminal post is electrically connected to the second tab of the cell stack. If the casing is made of copper, the eyelet is nickel plated iron, the insulator is glass and the terminal post is molybdenum. Also, the terminal post and the second tab have a positive polarity and the first terminal and the copper casing have a negative polarity. 
     On the other hand, when the casing is made of aluminum, the eyelet is aluminum, the insulator is ceramic and the terminal post is copper. In this case the terminal post and the second tab have a negative polarity and the first terminal and the aluminum casing have a positive polarity. 
     According to another aspect of the invention, when the casing is aluminum, the pass-through terminal comprises a copper rivet, at least one insulator circumscribing the rivet so as to insulate the rivet from the casing, and a nickel washer disposed on an outside of the casing and contacting the rivet with the insulator insulating the washer from the casing. In this case, the rivet and the second tab have a negative polarity and the first terminal and the aluminum casing have a positive polarity. 
     With the above electrochemical cell and related 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, contamination of the cell manufacturing machine with electrolyte is minimized. Accordingly, all of the concerns discussed above with respect to the current technique are overcome. 
     Further, since the casing is made of a metallic material such as copper or aluminum, the package can be sealed by welding, instead of by heating. It has been, discovered that electrolyte contamination does not interfere with a welded seal. The welding process can be, but is not limited to, ultrasonic welding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a plan view showing the cell package with the cell stack located therein; 
     FIG. 2 is a sectional view taken along line  2 — 2  of FIG. 1 showing the cell package; 
     FIG. 3 is a sectional view showing the pass-through terminal of the present invention for interconnecting one of the tabs of the cell stack to an external equipment, according to one aspect of the invention; 
     FIG. 4 is a sectional view showing the pass-through terminal of the present invention for interconnecting one of the tabs of the cell stack to an external equipment, according to another aspect of the invention; 
     FIG. 5 is a plan view showing the cell package after the electrolyte has been introduced into the cell package; 
     FIG. 6 is a sectional view taken along lines  6 — 6  of FIG. 5; 
     FIG. 7 is a plan view of the cell package after the degassing step; 
     FIG. 8 is a sectional view taken along line  8 — 8  of FIG. 7; 
     FIG. 9 is a plan view of the cell package after the removal of the excess material of the cell package; and 
     FIG. 10 is a sectional view taken along line  10 — 10  of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, the electrochemical cell  8  includes a cell package  10  having the shape of an envelope which is formed of a metallic sheet or sheets  12  so as to define an opening  14  therein for receiving a cell stack  16 . The package is preferably formed of two metallic sheets  12  that are welded along the edge to form weld seam  18 . The welding process can be, but is not limited to, ultrasonic welding. 
     One of the two sheets is a base  19  that has a cell stack cup  20  in which the cell stack  16  is placed. The other sheet is a lid  21 . Alternatively, the package  10  can be formed from a single metallic sheet that is folded in half leaving only three edges to be welded. According to the preferred embodiment, the metal sheet is made from aluminum or copper, although the invention is not to be limited to these materials. For example, other suitable materials include stainless steel and nickel. 
     As is conventional, the cell stack  16  includes a first tab  22  (of a first polarity) and a second tab  24  (of a second polarity). Since the cell package  10  is made of a metallic material which is conductive, according to the invention, the first tab  22  of the cell stack  16  is electrically connected directly to the cell package at weld  23 . On the other hand, the second tab  24  is connected to a pass-through terminal assembly  26  (shown schematically in FIGS. 1 and 2) which is provided in one of the sheets of the cell package to allow external connection to the second tab  24  of the cell stack. As discussed below, the polarity of the first and second tabs is dependent on the material of the cell package. 
     Referring to FIG. 1, the cell package also includes an electrolyte introducing portion  28  and degassing portion  30  which communicate with the inside of the package. Each of these portions includes a washer  29  which is welded to the inside surface of the cell package material, as shown in FIGS. 2 and 8. As discussed in greater detail below, the electrolyte introducing portion  28  has an electrolyte port  31  therein. The port  31  can be formed before or after the cell package is formed. On the other hand, as discussed below, the degassing portion  30  is punctured after the cell stack formation process to form degassing port  33  therein. The electrolyte port  31  is used to introduce the electrolyte into the cell package  10  to activate the cell stack  16  and the degassing port  33  is used to degas the cell package  10  after formation. The material of the washer  29  must be compatible with that of the cell package. If the cell package  10  is made of copper, it is preferable that the washer  29  be nickel plated iron; if the cell package  10  is made of aluminum, it is preferable that the washer be aluminum. 
     The following is a description of the design of the pass through terminal assembly  26 . There are two alternative designs respectively illustrated in detail in FIGS. 3 and 4. With reference to FIG. 3, according to a first of these designs, the pass-through terminal assembly  26  includes an eyelet  32 , an insulator  34  and a terminal post  36 . The eyelet  32  and insulator  34  are tubular members. The insulator  34  is located inside the eyelet  32  and the terminal post  36  is located inside the insulator  34 . The cell package  10  has a hole  38  therein through which the terminal post  36  protrudes. The eyelet  32  is located on the inside of the cell package  10  with the back surface  40  of the eyelet welded to the inside surface  42  of the cell package. A plastic washer  44  is adhered to the outside surface  46  of the cell package. 
     Referring also to FIG. 1, the second tab  24  is electrically connected to the terminal post  36  of the terminal assembly  26  via a lead  48  which is welded at one end to the second tab  24  and at the other end to the terminal post  36 . Thus, with this arrangement, the second tab  24  is electrically connected to the terminal post  36 , which protrudes to the exterior of the cell package  10 , while being insulated from the metallic cell package by the insulator  34 . Hence, when connecting the cell to the external equipment, one terminal (not shown) of the equipment is simply placed in contact with the metallic cell package  10  to which the first tab  22  of the cell stack is connected, and the other terminal (not shown) of the external equipment is electrically connected to the terminal post  36 , to which the second tab  24  of the cell stack is electrically connected via lead  48 . 
     The lead  48  and the accessible parts of the stack  16 , with the opposite polarity of the metallic packaging  10 , should be insulated using internal insulators  80  and  81 , as shown in FIGS. 5 and 6. 
     As noted above, according to the preferred embodiment of the invention, the cell package can be made of either aluminum or copper. While nickel is also an option, it is relatively expensive and, hence, not preferred. When the cell package is made of copper, the first tab  22  of the cell stack  16  has a negative polarity and the second tab  24  of the cell stack  16  has a positive polarity. Therefore, in this case the cell package  10 , to which the first tab  22  is directly connected, has a negative polarity and the terminal post  36  of the terminal assembly  26  has a positive polarity. Also, it has been discovered that for best results, for a cell package made of a copper material, the eyelet  32  should be made of nickel plated iron, the insulator  34  should be made of a glass (e.g., Sandia TR 23™) and the terminal post should be made of molybdenum. 
     On the other hand, when the cell package  10  is made of aluminum, the first tab  22  of the cell stack  16  has a positive polarity and the second tab  24  of the cell stack  16  has a negative polarity. Therefore, in this case the cell package  10 , to which the first tab  22  is directly connected, has a positive polarity and the terminal post  36  of the terminal assembly  26  has a negative polarity. It is preferable that the eyelet  32  be made of aluminum, the insulator  34  be made of a ceramic (e.g., Al 2 O 3 ) and the terminal post be made of copper. 
     An alternative design of the pass-through terminal assembly  26  is illustrated in FIG.  4 . This design is preferred when using a cell package made of aluminum. According to this embodiment, the terminal assembly  26  includes a rivet  50 , an internal washer  52 , insulators  54  and an external washer  56 . According to a preferred embodiment, the rivet  50  is made of copper, the internal washer  52  is made of aluminum, the insulators  54  are made of polyethermide (e.g., Ultem™ made by General Electric) and the external washer  56  is made of nickel. 
     As shown in FIG. 4, the rivet  50  extends through the holes provided in the cell package  10 , the insulators  54 , and the internal and external washers  52  and  56  so that it protrudes from the cell package. With the head  58  of the rivet located on the inside of the cell package  10 , the opposite end  59  of the rivet  50  is flared outwardly until it contacts the nickel washer  56 . The insulators  54  prevent the rivet  50  and the external nickel washer  56  from contacting the aluminum cell package  10  to prevent shorting of the cell stack  16 . The outside face  60  of the internal washer  52  is welded to the inside surface  42  of the aluminum cell package such that the terminal assembly  26  is securely retained to the cell package. The purpose of the external nickel washer  56  is to make electrical contact with the negative terminal of the external equipment. As with the embodiment of FIG. 2, the second tab  24  of the cell stack  16  is electrically connected to the rivet via the lead  48 . Since the cell package in this embodiment is aluminum, the polarity of the second tab  44  is negative so that the polarity of the rivet  50  is likewise negative. Naturally, the lead  40  must be insulated to some degree so that it will not contact the cell package which has the opposite polarity. 
     A description of the method of activating the cell stack will now be provided with reference to the figures. As noted above, FIGS. 1 and 2 show the cell stack  16  located inside the cup  20  of the cell package  10 . After the cell stack has been placed in the cup  20 , the lid  21  is welded to the base  19  around the periphery of the cell package  10  as indicated by the weld seam  18 . After the cell package has been welded, a vacuum is applied to the electrolyte port  31  after which electrolyte is introduced through the electrolyte port into the cell package  10 . After the filling of the electrolyte, the cell package is welded along weld seam  62 , as shown in FIGS. 5 and 6. The electrochemical cell is then partially charged (i.e., formation) generating gases inside the cell package  10 . As shown in FIGS. 7 and 8, after formation, the degassing portion  30  is then punctured to form the degassing port  33  in the cell packaging material and a vacuum is then applied to withdraw the formation gases from the inside of the cell package  10 . The cell package is then welded along weld seam  64  shown in FIG.  7 . Referring also to FIGS. 9 and 10, the excess material  66  of the cell package is then trimmed leaving only the lower portion  68  where the cell stack  16  is located resulting in the electrochemical cell  8 . 
     Having described the invention with particular reference to the preferred embodiments, it will be obvious to those skilled in the art to which the invention pertains after understanding the invention, that various modifications and changes may be made therein without departing from the spirit and scope of the invention as defined by the claims appended hereto.