ELECTRODE ASSEMBLY

Abstract: An electrode assembly of an anode (11), a cathode (1) and insulating separator material (15) between the cathode (1) and the anode (11), the entire anode, the entire separator and the entire cathode being laminated together in an accordian fold. The assembly is useful in electrochemical cells.

Description: The invention relates to an electrode assembly, a method for making the electrode assembly and electrochemical cells in which the electrode assembly is used. Various configurations of electrode assemblies consisting of an anode and a cathode have been disclosed. The configuration and the arrangement of such electrode assemblies in a particular housing is important in that the assembly and configuration impacts upon the 1) amount of active electrode material that can be included in each cell, 2) electrical performance, and 3) the ease of manufacturing. A useful electrode assembly configuration is disclosed in U.S. Patent 3,663,721. Therein, an electrochemical cell is disclosed in which the lithium anode comprises a unitary and continuous length of zig-zag pleated lithium of a selected width. Individual cathode plates are positioned between pairs of the pleated anode. A separator is placed between the cathode and the anode to prevent electrical contact between the anode and the cathode. While this electrode assembly is useful, it is disadvantageous in that it requires 1) the manufacturing step of placing separate cathode plates between the zig-zag pleated anode, and 2) a series of electrical connections to be made between the various cathode plates and the external electrical contact with the cathode thereby increasing the opportunities for short circuits to occur. The present invention provides an electrode assembly comprising an anode, a cathode and insulating separator positioned between the cathode and the anode, the anode, the separator and the cathode being folded to form an accordian fold; characterized in that the anode and separator are approximately twice the length of the cathode and are folded in half over the cathode prior to forming the accordian fold. An accordian fold comprises two or more "V" folds. The electrode assembly of this invention is advantageous in that it avoids 1) the need for the manufacturing steps involved in making separate cathode plates and subsequently inserting such plates into the pleats of a zig-zag anode and 2) the need to make a series of electrical connections between individual cathode plates. Since the cathode is itself continuous, only one electrical contact need be made to the cathode. In one useful embodiment, the electrode assembly of this invention comprises a three piece laminate comprising, in the following order: a) a metal foil current collector; b) a layer of a malleable anode-active material; and c) the insulating separator. The present invention also provides an electrochemical cell comprising an electrolyte, and an electrode assembly of the present invention. In the above embodiment, the electrochemical cell constructed with the accordian folded electrode assembly of this invention has 1) decreased internal resistance and 2) better utilization of the anode-active material compared to electrochemical cells in which the anode is coated on a metal grid current collector. The present invention further provides a method of making an electrochemical cell comprising: forming an anode by covering a metal foil current collector with a layer of anode-active material; placing an insulating separator over the anode-active material to form a laminate; forming a cathode by coating at least one side of a metal foil grid current collector with a cathode-active material; folding the laminate approximately in half to sandwich the cathode therebetween; folding the assembly so formed into accordian folds; inserting the accordian folds into a housing; and filling the housing with an electrolyte. The present invention will now be described by way of example with reference to the accompanying drawings, in which:- Fig. 1 is a schematic drawing of the anode assembly. Fig. 2 is a schematic drawing of a typical cathode assembly. Fig. 3 shows the anode and cathode being laminated together prior to the accordian fold. Fig. 4 shows the laminate of Fig. 3 in an accordian fold. Fig. 5 shows an electrochemical cell com- prising an electrode assembly of the invention. Applicants will now proceed to describe the anode and cathode construction, the separator and the technique by which they are brought together in a laminate structure and subsequently accordian folded. In this description of the invention, lithium anodes and MnO₂ cathodes are used. It will be recognized that the invention will work with most electrode assemblies using solid fuel. The anode assembly in Fig. 1 is a three piece laminate comprised of an anode-active material 12 such as lithium coated on a .0254 mm (1 mil) metal foil current collector 13 such as stainless steel. A portion of the stainless steel foil 13 is left uncoated and trimmed to form the anode terminal 14. A separator 15 such as Celgard® 4510 (available from Celanese) and/or non-woven polypropylene is rolled over the entire lithium anode 11 formed of the anode-active material 12 and the current collector 13. The separator is a porous electrically-insulating material which prevents electrical contact between the anode and the cathode but allows movement of ions. Other useful anode-active materials include any malleable anode material such as alkali metals (Na and K), Li-Al alloys, Li-Si alloys, Li-B alloys and the metals of Groups Ia and IIa of the periodic table of elements. Malleable anode-active materials are coatable on metal foil. Also, the separator can be coated onto the surface of the active material with considerable adherence of the separator to the anode layer. Metal foils which can be used as the current collector include metals such as nickel, stainless steel, aluminum and titanium. While this embodiment of the invention is exemplified by having the insulating separator placed over the anode, it is clear that the invention works equally well when the separator is placed over the cathode. In any case, a small excess of separator is left relative to the sides of the electrode. The total length and thickness of the anode will be dictated by the fuel loading requirements of the electrochemical cell or battery specification under consideration. The cathode 1 is a laminate as shown in Fig. 2. In the embodiment shown it is made with MnO₂. The MnO₂ cathode 1 comprises a grid current collector 22 such as stainless steel coated on one or both sides with a mixture of MnO₂, carbon and Teflon® 23. A small portion of the stainless steel current collector 22 is left uncoated and shaped at one end to function as a cathode terminal 24. The wide variety of cathode-active materials which would also be useful in the electrode assemblies of this invention include the various forms of polyfluorocarbons, i.e.(CF x ) n x < 4 and n is some indeterminate large number, FeS₂, FeS, CuO and Bi₂O₃. A complete electrode assembly is shown in Fig. 3. It is made by positioning the cathode 1 on top of the separator 15 attached to the anode 11 so that the cathode terminal 24 and the anode terminal 14 are side by side but are not in electrical contact. The cathode, in this embodiment of the invention, is about one-half the length of the anode. The entire anode 11 is then folded over the entire cathode 1 to form a laminate structure in which the cathode 1 is sandwiched between the folds of the anode 11. Next, the complete electrode assembly of Fig. 3 is then accordian folded as shown in Fig. 4. The accordian folded electrode assembly of Fig. 4 is a series of connected "V" folds. The accordian folded electrode assemblies of this invention comprise two or more "V" folds. The length of each leg in the fold and the number of folds and legs will, of course, be determined by the dimensions of the container in which the electrode assembly is to be inserted. Obviously, electrode assemblies having more than two "V" folds or four legs are possible, depending upon the fuel loading requirements of the intended power application and the specific anode and cathode materials used. One method for folding the electrode assembly of this invention, referring to Fig. 3, is to make sequential folds beginning at the end away from the contacts 14 and 24. As each fold is made, the loose ends of the separate components are thus free to shift enough to reduce stresses and strains at the fold thereby preventing pinching, cracking, breaking, delamination, shorting, or the like. After checking for internal electrical shorts with an ohmmeter, the electrode assembly is made into an electrochemical cell or battery by first inserting the assembly into a container 25 such as shown in Fig. 5. An electrolyte comprising, for example, a solvent mixture of a propylene carbonate and dimethoxyethane or butyrolactone and dimethoxyethane, with a lithium salt such as lithium perchlorate or lithium tetrafluoroborate, is added to the container 25. The electrode assembly is inserted in the container so that electrode terminals 14 and 24 protrude upward forming a complete electrochemical cell or battery. The cell can be sealed with a cap if desired. Such caps and methods of sealing the cap to the container are well known in the battery art. In the accordian folded electrode assembly of Fig. 4, electrical contact between each leg of the anode is achieved when the legs of the assembly are pressed together and inserted into the container of Fig. 5. This electrical contact is achieved through the physical contact of the current collectors 13 of the anode assembly 11. This electrical contact between each leg of metal foiled back anode decreased the internal resistance of electrochemical cells because electrons have a shorter path to travel to reach the anode terminal compared to anodes and electrode assemblies having a different configuration. Moreover, the anode construction of Fig. 1 facilitates greater utilization of the anode-active material since substantially all of the anode-active material is in direct electrical contact with the metal foil current collector. This is in contrast to metal grid collectors in which much of the anode-active material is only indirectly in electrical contact with the grid current collector. 'Celgard' and 'Teflon', referred to above, are trade marks.

Claims: An electrode assembly comprising an anode (11), a cathode (1) and insulating separator (15) positioned between the cathode (1) and the anode (11), the anode (11), the separator (15) and the cathode (1) being folded to form an accordian fold; characterized in that the anode (11) and separator (15) are approximately twice the length of the cathode (1) and are folded in half over the cathode (1) prior to forming the accordian fold. An assembly according to claim 1, wherein the anode (11) comprises a three-piece laminate including, in the following order: a) a metal foil current collector (13); b) a layer (12) of a malleable anode-active material; and c) the insulating separator (15). An assembly according to claim 2, wherein the anode-active material of the anode is a malleable metal. An assembly according to claim 3, wherein the cathode (1) comprises a layer (23) of cathode-active material coated onto a metal grid current collector (22). An assembly according to claim 4, wherein the cathode-active material is a polyfluorocarbon, FeS₂, FeS, CuO, MnO₂ or Bi₂O₃. An assembly according to claim 5, wherein the anode-active material is lithium and the cathode-active material is MnO₂. An assembly according to claim 4, wherein the metal grid current collector (23) is made of stainless steel and is coated on both sides with the cathode-active material. An electrochemical cell comprising an electrolyte, and an electrode assembly according to any one of the previous claims. An electrochemical cell according to claim 8, wherein the electrolyte comprises a solvent mixture of propylene carbonate and dimethoxyethane and a lithium salt. An electrochemical cell according to claim 8, wherein the electrolyte comprises a solvent mixture of dimethoxyethane and butyrolactone and a lithium salt. A method of making an electrochemical cell comprising: forming an anode (11) by covering a metal foil current collector (13) with a layer (12) of anode-active material; placing an insulating separator (15) over the anode-active material to form a laminate; forming a cathode (1) by coating at least one side of a metal foil grid current collector (22) with a cathode-active material; folding the laminate approximately in half to sandwich the cathode (1) therebetween; folding the assembly so formed into accordian folds; inserting the accordian folds into a housing; and filling the housing with an electrolyte.

Inventors: BAKOS, VINCENT WILLIAM, MARTIN, THOMAS WILLIAM
Applicants: EASTMAN KODAK COMPANY (A NEW JERSEY CORPORATION)
Application year: 1986
Main classification: H01M6
IPC codes: H01M6