AIR CATHODES AND MATERIALS THEREFOR

Abstract: A sheetlike laminated air cathode (16) including first (34) and second (38) layers having opposed major surfaces (17, 18) respectively exposed for contact with a liquid electrolyte and with air, the layers also having facing major surfaces, and the second layer (38) being permeable to air but not to the liquid electrolyte; and a current collector (36) in contact with the first layer (34) and connectable to external electrical cir­ cuitry. The first layer (34) is a nonwoven fibrous web impreg­ nated with a mixture of carbon particles and a nonfibrous polymeric substance for holding the carbon particles in the web. The facing major surfaces of the first and second layers (34, 38) are bonded together by heat seal coating material (40) distributed on those major surfaces in such manner as to provide an array or network of areas free of coating material extending substantially uniformly thereover.

Description: Background of the invention This invention relates to air cathodes suitable for use in fuel cells or metal/air batteries and to methods for their manufacture. Metal/air batteries produce electricity by the electrochemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. As is well known in the art, an air cathode is a typically sheet-like member, having opposite surfaces respectively exposed to the atmosphere and to the aqueous electrolyte of the cell, in which atmospheric oxygen dissociates while metal of the anode oxidizes, providing a usable electric current flow through external circuitry connected between the anode and cathode. The air cathode must be permeable to air but substantially hydrophobic, and must incorporate an electrically conductive element to which the external circuitry can be connected; for instance, in present-day commercial practice, the air cathode is commonly constituted of active carbon containing a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element. A variety of anode metals have been used, with alloys of aluminum and alloys of magnesium being considered especially advantageous for particular application, owing to their low cost, light weight, and ability to function as anodes in metal/air batteries using neutral electrolytes such as sea water or other aqueous saline solutions. A typical metal/air battery is described in Hamlen et al, U.S. Patent No. 4,626,482, issued December 2, 1986. Notwithstanding the many uses and advantages of metal/air batteries, their application has been limited owing to the cost and difficulty of producing satisfactory air cathodes. For instance, it is conventional in present-day practice to produce air cathode sheet material by extruding a mixture of carbon and fluorinated polymer and pressing the mixture onto a metal mesh. The resulting material is relatively expensive; moreover, it is difficult to extrude material having the high carbon content required for air cathodes. Other problems have been encountered in achieving and maintaining satisfactory cohesion of laminated (multilayer) air cathodes. Summary of the invention The present invention in a first aspect broadly contemplates the provision of an air cathode comprising a sheetlike laminate that includes first and second layers having opposed major surfaces respectively exposed for contact with a liquid electrolyte and with air, these layers also having facing major surfaces, and the second layer being permeable to air but not to the liquid electrolyte; and current-collecting means in contact with the first layer and connectable to external electrical circuitry. The first layer of this cathode comprises a nonwoven fibrous web impregnated with a mixture of carbon particles and a nonfibrous polymeric substance for holding the carbon particles in the web. The facing major surfaces of the first and second layers are bonded together by heat seal coating material distributed on those facing major surfaces in such a manner as to provide an array or network of areas free of coating material extending substantially uniformly thereover. As a still further feature of the invention, in currently preferred embodiments, the fibrous web is a nonwoven web of conductive carbon fibers. The coating material may, for example, be distributed as a multiplicity of spaced-apart dots, or as a mesh having coating-material-free interstices. The provision of coating-free spaces maintains sufficient unclogged pores in the second layer (i.e. pores not sealed by the coating) to enable the air cathode to function as intended, yet with effective lamination of the layers to each other and/or to the current-collecting means. In illustrative embodiments of the air cathode of the invention, the current-collecting means comprises a layer of metal mesh. For example, the layer of metal mesh may be interposed between and may be substantially coextensive in area with the first and second layers of the cathode. Alternatively, the layer of metal mesh may be bonded to the exposed surface of the first layer. In a further aspect, the invention contemplates the provision of a method of making a sheetlike laminated air cathode, including the steps of impregnating a nonwoven fibrous web with a suspension in a liquid vehicle, of material comprising carbon particles and a nonfibrous polymeric substance for holding the carbon particles in the web; and laminating the first and second layers together by juxtaposing them with a heat seal coating distributed between their facing major surfaces in such a manner as to provide an array or network of coating-free areas extending substantially uniformly thereover, and heating the juxtaposed layers to activate the coating. The air cathodes of the invention are characterized by highly satisfactory performance in metal/air batteries, as well as having other uses, e.g. in fuel cells. Particular advantages of the invention reside in the ease and low cost with which the laminated air cathode can be produced in large commercial-scale operations, as contrasted with air cathodes made by methods heretofore employed. Further features and advantages of the invention will be apparent from the detailed description hereinbelow set forth, together with the accompanying drawings. Brief description of the drawings Fig. 1 is a simplified schematic elevational sectional view of a metal-air battery incorporating an illustrative embodiment of the air cathode of the present invention; Fig. 2 is a fragmentary exploded perspective view of the air cathode of Fig. 1; Fig. 3 is a view similar to Fig. 2 of another embodiment of the air cathode of the invention; Fig. 4 is a fragmentary sectional elevational view of a further embodiment of the air cathode of the present invention; Fig. 5 is a view similar to Fig. 4 of yet another embodiment of the air cathode of the invention; and Fig. 6 is a view similar to Fig. 4 of a still further embodiment of the invention. Detailed description Referring first to Fig. 1, there is shown a metal/air battery 10 which includes a housing 11 defining a chamber 12 adapted to be substantially filled with a body of a liquid electrolyte 14, e.g. an aqueous solution of sodium chloride. A sheetlike air cathode 16 having opposed parallel major surfaces respectively designated 17 and 18 is mounted in one wall of the housing 11 so that the cathode major surface 17 is exposed to and in contact with the contained body of electrolyte 14, while the other cathode major surface 18 is exposed to the ambient air outside the chamber. The housing 11 defines a large vertical aperture across which the air cathode extends, with the periphery of the cathode sealed to the periphery of the housing aperture in a liquid-tight manner. A metal (e.g. aluminum) anode 20, shown as mounted in a lid 22 of the housing 11, and having the form of a plate with opposed parallel major surfaces, extends downwardly into the body of electrolyte 14 in the chamber 12. The anode 20 is disposed with one of its major surfaces in parallel, proximate but spaced relation to the major surface 17 of the air cathode 16 such that there is a small electrolyte-filled gap 24 between the anode and cathode. The general arrangement of this air battery may for example be substantially the same as that of one of the cells of the plural-cell battery described in U.S. Patent No. 4,626,482. External electrical contacts respectively designated 26 and 28 are provided for the cathode and anode of the battery which may thus be connected in an electrical circuit 29, e.g. including a switch 30 and a light bulb 32, either alone or in series with one or more other like cells. When the metal-air battery is assembled as shown, filled with electrolyte 14, and connected in the circuit 29 (with the switch 30 closed), the battery produces electricity for energizing and lighting the bulb 32, in known manner. As best seen in Fig. 2, the air cathode 16 of Fig. 1 is a laminate constituted of a carbon-containing layer 34, a layer of metal mesh 36, and a layer 38 constituted of a film or web which is permeable to air but impermeable to water of the electrolyte 14, so that the electrolyte cannot weep or bleed through the air cathode to the exterior of the battery. This layer 38 may, for example, be a water-repellent film or nonwoven web of polypropylene or polyethylene, e.g. polypropylene films available under the trade marks Celgard and Gelman, and a fibrous polyethylene nonwoven web commercially available from duPont under the trade mark Tyvek. The metal mesh layer 36, which is the current-collecting means of the air cathode of Figs. 1 and 2, may conveniently be a layer of nickel mesh (formed, for instance, by an expanded-metal technique) substantially coextensive with the layers 34 and 38 but having an exposed tab portion projecting upwardly above the latter layers to serve as the cathode contact 26. In accordance with the invention and as a particular feature thereof, in the embodiment illustrated in Figs. 1 and 2, the layer 34 comprises a nonwoven fibrous web impregnated with a mixture of carbon particles and a nonfibrous polymeric substance for holding the carbon particles in the web. Other substances may optionally be included in the impregnating material as well. The fibers of the web may, for example, be polyester fibers, such as polyethylene terephthalate fibers. Satisfactory polyester nonwovens include grades commercially available under the trade marks Reemay 20024, Reemay 2415, Confil and Holytex 3234. Alternatively, cellulosic fibers, polyvinyl alcohol fibers, or mixtures of two or more of the foregoing types of fibers may be used. As still another alternative, the fibers of the nonwoven web may be electrically conductive carbon fibers. Such conductive fibers may, for example, be in the form of a carbon fiber nonwoven web made of carbon fibers coated with nickel, produced by International Paper Company as Product 8000251 (hereinafter termed IPN). A preferred nonfibrous polymeric substance for use as the adhesive polymer is the grade of polytetrafluoroethylene commercially available under the trade mark Teflon T-30, a product of duPont. Additional important features of the invention reside in the manner in which the layers of the air cathode 16 are bonded or laminated together. In the embodiment of Figs. 1 and 2, the metal mesh layer 36 is sandwiched between facing major surfaces of the layers 34 and 38, and in this embodiment, the layers are bonded together by means of a discontinuous heat-seal coating initially applied to that surface of the layer 38 which faces the layers 34 and 36. More particularly, the heat seal coating is applied to the last-mentioned major surface of the layer 38 in a pattern of discontinuous dots 40, such that there is a continuous network of coating-free spaces between adjacent dots. It is found that when the layers 34 and 36 and the layer 38 bearing the dots 40 are juxtaposed and subjected to bonding heat and pressure, the dot coating adheres the layer 48 both to the metal mesh 36 and to the layer 34 (through apertures of the mesh). At the same time, owing to the discontinuous pattern of the heat-seal coating, the passage of oxygen through and beyond the layer 38 is not prevented by the resultant bond. That is to say, the layer 38 retains its porosity because the heat seal coating in the illustrated dot pattern leaves a large proportion of the pores of layer 38 unclogged. An alternative embodiment of the cathode is illustrated in Fig. 3. Again, the cathode is made up of layer 34, 36 and 38 all as described above. In Fig. 3, however, the heat-seal coating used to laminate the layers is not applied in a dot pattern on a surface of the layer 38, but is instead interposed, as a net or mesh 42 of heat seal resin, between the layers 38 and 36. When the layers are assembled and subjected to bonding heat and pressure, the resin of the net 42 bonds layers 38 and 34 through the apertures of metal mesh 36, and the interstices of the net 42 provide an array of uncoated areas (i.e. areas of unclogged pores) extending entirely over the layer 38. In some instances, with a structure as generally illustrated in Fig. 3, it may be desirable to employ a second layer (not shown) of the heat-seal resin net 42, the second layer being interposed between the metal mesh 36 and the carbon-containing layer 34. Again, the interstices of the two net layers leave a sufficient area of unclogged (sealant-free) pores to enable effective functioning of the cathode. Where the nonwoven fibers of the web of layer 34 are electrically conductive, such as carbon fibers e.g. having a cladding of nickel, it is possible to dispense with at least most of the current-collecting metal mesh layer 36. Thus, as shown in Fig. 4, in such case the layer 38 may be directly bonded to a carbon-impregnated layer 34a incorporating the conductive-fiber web (once more employing, for example, a dot pattern or net of heat seal resin to effect the bonding) with a small piece 36a of nickel or other conductive metal mesh interposed between, and bonded to, a small area of the facing surfaces of layers 34a and 38. A portion of the mesh piece 36a projects as a tab beyond the layers 34 and 38a to serve as a contact 26a for the cathode. Fig. 5 illustrates an embodiment of the cathode of the invention wherein the facing major surfaces of the layers 34 and 38 are bonded directly together (by a dot pattern or net of heat-seal coating) throughout their extent, and the metal mesh current collector 36b is bonded to the exposed surface of layer 34 (i.e. at the cathode surface exposed to the electrolyte), with an upwardly projecting tab exposed to serve as the cathode contact 26b. In this instance, the layer 34 need not incorporate a conductive-fiber web, the current collector mesh being substantially coextensive with the layers 34 and 38. The embodiment of Fig. 5 affords superior strength of lamination, in that the layers 34 and 38 are adhered directly to each other rather than (as in Figs. 1-3) through a metal mesh layer. The metal mesh may be bonded to the layer 34 with latex, the latex being precoated on the metal mesh. Fig. 6 illustrates a currently particularly preferred embodiment of the cathode of the invention, in which the metal mesh 36c is sandwiched between (and substantially coextensive with) two layers 34a and 34a′ each constituted of a conductive carbon fiber nonwoven web impregnated with carbon and an adhesive polymer such as Teflon R , and the air-side (water-impermeable) layer 38 is laminated to the layer 34a. Bonding of layer 38 to layer 34a is effected in the same manner as described above for the embodiment of Fig. 5, and the mesh 36c is adhered to both layers 34a and 34a′ with latex precoated on the mesh, again as described above with reference to Fig. 6. As before, a protruding tab portion of the mesh serves as the electrical contact 26c for the cathode. The method of fabricating the described air cathodes of the invention may now be readily explained. To produce the carbon-impregnated web material for layer 34 or 34a, a suitable nonwoven web is impregnated an aqueous suspension of carbon particles, the aforementioned nonfibrous polymeric substances, and other ingredients as may be desired, for example a catalyst as is commonly employed in the carbon-containing layer of an air cathode. When the web has been thus impregnated and dried, it and the material for layers 36 and 38 are juxtaposed in the arrangement shown in Fig. 2, 3, 4 or 5, with a heat seal coating either applied as a dot pattern on the inner major surface of layer 38 or interposed as one or more layers of heat-seal net 42 at the appropriate location or locations in the "sandwich" of layers, and the sandwich is subjected to bonding heat and pressure sufficient to activate the coating to form the bond. As further discussed below, the materials of the cathode are so selected that the temperature required to activate the heat-seal coating to bond the layers does not damage the other layers or their constituent substances. Once the layers are bonded, the laminate is cut to size to provide individual cathodes. In this manner, the invention provides highly satisfactory air cathodes at substantially reduced cost (as compared to present-day commercially available air cathodes), not only because the materials used are relatively inexpensive, but also because the manufacturing operations involved (impregnation and lamination with heat) are simple, straightforward and readily capable of performance on a large production scale. There follows a further more detailed description of particular embodiments of the product and method of the invention, specifying materials and process conditions therefor. I. PREPARATION OF AIR-CATHODE ACTIVE LAYERS BY IMPREGNATING NONWOVENS Herein are described the provision of active (carbon-containing) layers in accordance with the invention, viz. layers 34 or 34a in the illustrated embodiments, for air cathodes suitable for use in fuel cells or as cathodes in metal/air batteries. In a broader sense, the products and techniques herein described may be employed to supply thin layers of chemicals for other uses in batteries as well, including electrode applications where air is not involved. As explained above, there are typically three components in an air cathode embodying the invention: a carbon-containing layer which consists of a nonwoven web or fabric material impregnated with a carbon-polymer mixture; a current-collector layer containing strands of metal or metal-containing conductive material; and a water-repellent, air-permeable membrane material. Factors to be considered in preparing the carbon containing layer include selection of the nonwoven web material, formulation of the carbon-polymer impregnating mixture, and determination of process steps and conditions for producing the impregnated product. In selecting the nonwoven material, the primary variables are chemical composition, thickness (or basis weight) and porosity (or void fraction). Pertinent to the choice of these variables are such conditions as the temperatures employed in process steps (e.g. curing and laminating) to which the carbon-containing layer is subjected, and the impregnation of the web. As further discussed below, the impregnation step involves dipping the nonwoven web into an aqueous suspension of carbon-polymer mixture, followed by scraping or wiping off the excess suspension and drying the impregnated web. To produce the impregnated web, the carbon/polymer dispersion is usually applied to both sides of the nonwoven. The porosity of the nonwoven should be sufficient to permit intermingling of the two impregnating applications of carbon/polymer, i.e. those respectively applied to the opposite sides of the web. The impregnated web may receive additional heat-curing and/or mechanical smoothing or compacting treatments, such as heat-calendering. Especially useful, for many applications, are "polyester" nonwoven webs. These webs are made from fibers of polyethylene terephthalate (PET), which are quite inert in the use environment. PET has a melting point in the range of 245° to 265°C, and can therefore withstand the curing and heat-lamination steps employed in making the present cathode, these steps being performed at lower temperatures. Satisfactory polyester nonwovens include the grades commercially available under the trade names Reemay 2024, Reemay 2415, Confil, and Hollytex 3234. Unsatisfactory results have been obtained with the grades of polyester nonwovens commercially available under the trade names Hollytex 3329 and Hollytex 3396, which are thicker and less porous than the grades found satisfactory. There are two tests which are helpful in selecting the nonwoven for impregnation. One is an air permeability test and the other is the front-to-back (F/B) electrical conductivity of the sheet after it is impregnated with the carbon/polymer and after it is dried. The nonwoven must have a high void volume, to permit a high pickup of the impregnating suspension. The impregnating suspension must penetrate into the nonwoven from each surface so as to achieve good intermingling between the top and bottom coatings, and to give a low front-to-back (F/B) electrical resistance. A properly porous nonwoven typically gives a front-to-back electrical resistance (after impregnation and drying) of 3 to 6 ohms. If there is inadequate penetration of the carbon suspension, the F/B resistance can be 8 to 50 ohms or higher. The impregnation treatment increases the basis weight of the sheet by 15 to 60 grams per square meter (GSM), depending on the solids content of the impregnating suspension and on the pore volume of the nonwoven. Table 1 below summarizes information on various nonwovens suitable for impregnation. All of these materials are made from fibers which have melting points significantly higher than the subsequent lamination temperatures. The carbon suspension is the liquid material used to impregnate the nonwoven. The primary ingredients in the carbon suspension are (1) carbon black, (2) a nonfibrous adhesive polymer to hold the carbon black in the web, (3) dispersing agents to permit preparation of the suspension, and (4) flow control agents which impart viscosity stability and fluidity. One example of a satisfactory and currently preferred carbon black is that available under the trade name Black Pearls-2000, a product of Cabot Corporation of Boston, MA. A currently preferred nonfibrous polymeric substance for use as an adhesive polymer is the grade of polytetrafluoroethylene commercially available under the trade name Teflon T-30, a product of duPont. It is purchased as a 60% solids suspension in water, containing its own dispersing agents. Cathodes of moderately good performance have also been made with a copolymer of polyvinyl chloride (PVC) used as the adhesive polymer. The PVC has advantages over the Teflon polymer (better adhesive stength which then requires less adhesive thereby raising the carbon percentage in the carbon-containing layer), but the cathode performance to date has been inferior. The dispersing agents are in the carbon dispersion and in the Teflon dispersion. Tests reported herein have used custom-dispersed carbon; the dispersing agent is calcium naphthenate. The dispersed carbon used is designated "Foamblak 991." The Teflon dispersion is understood to be dispersed with a nonionic dispersant available from Rohm & Haas under the trade name Triton X-100. Flow modifiers, discussed below, may also have dispersing agent properties. Flow modifiers have seldom been used in the tests and samples herein described, but have potential importance in overcoming a problem encountered in mixing the Teflon polymer and the carbon, viz. the tendency of such mixture to gradually gel upon standing, resulting in a material which has poor properties for impregnation. It is found that a few percent of sodium carboxy-methyl cellulose (CMC) added to the carbon will prevent much of the gelling which occurs upon the addition of the Teflon polymer to the carbon. Platinum-catalyzed carbons have also been used with beneficial effects. These were prepared by precipitating platinum in samples of the aforementioned Foamblak 991 dispersion, using a proportion of platinum equal to 2% of the weight of carbon present. In tests herein reported, impregnation has been performed by hand, painting the suspension onto both sides of the nonwoven web with a paint brush. However, for commercial-scale production the impregnation step can be performed on a continuous web machine. The carbon-containing layer described above has value in an air cathode when it is attached to a conductive current collector (layer 36 or 36b or 36c in Figs. 1-3, 5 and 6). The current collector is nickel mesh or any other metal mesh which will not be corroded in the use environment. Air cathodes also need an air-permeable, water-repellent layer in the laminate. The air-permeable layer will be desribed separately below. Alternative useful nonwoven web materials for the carbon-containing layer 34 of the air cathode of the invention include easily wettable, hydrophilic, cellulosic (or other absorptive) nonwovens impregnated with carbon/ adhesive (usually carbon/Teflon). In contrast, the polyester webs discussed above and other (e.g., polyolefin and polyamide) nonwoven substrates are not absorptive, although they could be made very wettable with treatment with wetting agents; it is not known at present if such wetting agents would interfere with the cathode's functional performance. Examples of absorptive nonwoven web materials that have been tested (being impregnated in the manner described above for PET nonwovens, and evaluated in use in or as the nonwoven web of layer 34 in an air cathode arranged in accordance with the invention) are the following products of the Chicopee Division of Johnson and Johnson: (a) Code 5710. The fibers of the web are 70% Rayon (regenerated cellulose) and 30% polyester (polyethylene terephthalate); the web also contains 2% extra binder (acrylic resin) to hold the web together. As tested, the basis weight was 78 grams per square meter before impregnation and 147 grams per square meter after impregnation, giving an add-on of 69 grams per square meter. Electrical performance (sample 184A in Table II below) was good. (b) Code 5524. The fibers are 60% wood pulp and 40% polyester (polyethylene terephthalate); the web is not acrylic-bonded. Basis weight was 70 grams per square meter before impregnation and 114 grams per square meter after impregnation, giving an add-on of 34 grams per square meter. Electrical performance (sample 184B in Table II) was fairly good for the first 24 hours of testing. (c) Code 9676-7519. The fibers are 30% Rayon (regenerated cellulose) and 70% polyvinyl alcohol. Basis weight was 42 grams per square meter before impregnation and 65 grams per square meter after impregnation, giving an add-on of 23 grams per square meter. Electrical performance (sample 184C in Table II) was good, but inferior to that of the Code 5710 and Code 5524 materials. (d) Code 9657-7814. The fibers are 100% polyvinyl alcohol. Basis weight was 67 grams per square meter before impregnation and 83 grams per square meter after impregnation, giving an add-on of 15 grams per square meter. The absorptive nonwovens just mentioned are well suited to processing on a production machine for making a carbon-impregnated air cathode layer, and in this regard they are much superior to the non-absorptive nonwovens. The polyvinyl alcohol fibrous nonwoven can be used as an impregnating base. This type of nonwoven is used in the alkaline battery industry. A primary attribute of polyvinyl alcohol is its resistance to alkali, an attribute that may be advantageous in specialized applications. In testing the absorptive materials, as reported in Table II below, a controlled-current half-cell (CCHC) test was used to indicate the performance of the cathode independent from the anode. The best results are those closest to zero. The cube cell voltage in Table II shows the output of the cell under continuous load for 20 hours. It is a type of life test, measuring performance of both anode and cathode. Cube cell tests are erratic after about 8 hours because of scale buildup in the cell. The best results are the highest voltages. The second CCHC test shows cathode performance after the cathode has been run for 24 hours in a cube cell test. Data show that samples 184A and 184B were superior to 184C and 185D. For many application the current density will be less than 5mA/cm². Most testing is done at higher current densities to accelerate the test and to magnify differences. Data show that the materials of samples 184A, 184B and 184C are acceptable. The higher carbon add-on with 184A is an indication that it may be superior, among those tested in this group. Conductive-fiber nonwoven webs (also separately discussed below) may alternatively be employed. These are materials made of carbon fibers with or without a coating (on the fibers) of metallic nickel. Such webs, with nickel-coated fibers, may be capable of use in place of nickel mesh as current collectors, although samples thus far tested appear insufficiently effective for such use. With or without the nickel coating, however, they provide startlingly good results when used as the nonwoven which is impregnated with the carbon-Teflon polymer suspension to provide the carbon-containing layer (34a, Figs. 4 and 6) of an air cathode. In some instances, with the conductive nonwovens it is possible to combine the active (carbon-containing) layer function and the current collector function into one thin layer; consequently, it is to be understood that the foregoing discussion of air cathodes utilizing a metal mesh layer as current-collecting means is subject to exceptions which relate to the products made with active layers incorporating conductive nonwoven webs. II. AIR-SIDE LAYER OF AIR CATHODES The air-side layer 38 of the air cathode of the invention requires a combination of air-permeability and water-repellency. If the air-side layer imbibes liquid in its pores, the passage of air may be restricted, and undesirable leakage or weeping of electrolyte liquid through the air-side layer may occur. The air-side layer can be omitted from some types of air cathodes if the liquid-side (carbon-containing layer) has appropriate properties, although initial attempts to produce such cathodes have exhibited leakage of electrolyte through to the air side. Some prior proposals for provision of air-side layers have involved use of Teflon-carbon mixtures in which the Teflon content is lower than in the liquid side; but applicant's attempts to make such structures have produced erratic results. Illustrative examples of products found suitable for use as the air-side layer in cathodes of the present invention include two polypropylene films (respectively available under the trade names Celgard and Gelman), and a fibrous polyethylene nonwoven web commercially available from duPont under the trade name Tyvek. Polyester nonwovens tested as air-side materials have given invariably bad results. At least for a variety of applications, a currently preferred material for the air-side layer 38 is a Tyvek web, a product made from the partial compaction of a nonwoven fibrous material. There are many different grades of Tyvek, and most of them are surface-treated (usually oxidized) to make them more receptive to inks and less water-repellent. The Tyvek grade currently preferred for air cathodes of the present invention is grade 1073B, which is used in packaging of gas-sterilizable hospital supplies. It has no anti-static treatment and no corona treatment. It withstands a static head of 152 cm of water and the resistance to air flow is 21 Gurley seconds. A similar but less preferred Tyvek grade (though also believed satisfactory) is 1059B which has like properties but lower basis weight than 1073B. In order to use the Tyvek material, which has a melting point of about 130°C and a still lower softening point, it must be adhered to the nickel mesh 36 and/or layer 34 at temperatures which do not compact and ruin the porosity of the Tyvek. Also, any heat-seal adhesives should not plug the porosity of the Tyvek. III. ELECTRICALLY CONDUCTIVE NONWOVENS IN BATTERY ELECTRODES As stated, an electrically conductive carbon fiber nonwoven web impregnated with carbon and a nonfibrous polymeric substance (e.g. Teflon polymer) as an adhesive may be employed as a layer 34 or 34a in the air cathode of the invention. Such a layer may also have utility in battery applications other than air-cathode applications. It is known to apply active carbon material onto a wire conductor. There are commercial air-cathodes which apply a carbon-Teflon mixture directly to wire mesh. Five different types of impregnated nonwoven webs were made and compared in "Type 24" cathodes (described below, and having the arrangement shown in Fig. 5, discussed above): Sample 196A: International Paper Company's carbon fiber nonwoven web without nickel coating (Product 8000030), impregnated with 10% solids carbon/Teflon "dope", with an add-on of 70 grams per square meter; Sample 196B: The same uncoated carbon fiber web material impregnated with the carbon/Teflon dope diluted to about 7% solids, giving an add-on of 46 grams per square meter; Sample 196C: The same 7% dope, used to impregnated the IPN nickel-coated carbon fiber nonwoven web, with an add-on of 16 grams per square meter; Sample 196D: The 7% dope, used to impregnate the absorptive cellulosic web identified above as Code 5710, with an add-on of about 20 grams per square meter; Sample 196E: A carbon-containing layer produced with 43 grams per square meter add-on by carbon/Teflon impregnation onto the IPN nickel-coated carbon fiber nonwoven web. Test results with these five cathodes are summarized in Table III: The all-carbon nonwoven web (with no nickel coating) is a currently preferred conductive nonwoven material for the invention. It can be made with much better formation (uniformity of fiber distribution) than the nickel-plated material, which tends to contain stalky fiber bundles. Also, the all-carbon material is 20% cheaper than the web of nickel-coated fibers. The cathodes referred to as "Type 24" have the arrangement WA/C20/OLA, in which WA refers to the nickel mesh which has been precoated with an adhesive and C20 refers to the conductive nonwoven impregnated with a carbon/Teflon mixture. In these cathodes, as Fig. 5 illustrates, the nickel mesh is on the exposed surface of layer 34 (C20) rather than being sandwiched between layers 34 and 38. In cathodes with conductive carbon fiber nonwoven webs in the layer 34 and with the nickel mesh on the outside, the conductive nonwoven web contributes significantly in insuring good results. Also, placing the mesh on the outside affords several advantages. First, it is easy to make outside connections to the nickel mesh. Second, it may give longer-lived cathodes because it now appears that many past failures could have been the result of delamination between the carbon layer and the porous film, and there may be reduced delamination problems if the OLA is bonded directly to the carbon-containing layer 34 rather than to a center layer of nickel mesh. A third consideration relates to avoiding leakage through the air-side layer 38: the carbon layer between the nickel mesh and the porous OLA film 38 should prevent the sharpness of the mesh from punching holes in the film. It is to be understood that the invention is not limited to the features and embodiments hereinabove specifically set forth but may be carried out in other ways without departure from its spirit.

Claims: An air cathode comprising a sheetlike laminate (16) including (a) first (34) and second (38) layers having opposed major surfaces (17,18) respectively exposed for contact with a liquid electrolyte and with air, said layers also having facing major surfaces, and said second layer (38) being permeable to air but not to said liquid electrolyte; and current-collecting means (36) in contact with said first layer (34) and connectable to external electrical circuitry; characterized in that said first layer (34) comprises a nonwoven fibrous web impregnated with a mixture of carbon particles and a nonfibrous polymeric substance for holding the carbon particles in the web; and said facing major surfaces of said first and second layers (34, 38) are bonded together by heat seal coating material (40) distributed on said facing major surfaces in such manner as to provide an array or network of areas free of coating material extending substantially uniformly thereover. An air cathode as claimed in claim 1, characterized in that said current-collecting means (36) comprises a layer of metal mesh. An air cathode as claimed in claim 2, characterized in that said layer of metal mesh (36) is interposed between and is substantially coextensive in area with said first and second layers. An air cathode as claimed in claim 2, characterized in that said layer of metal mesh (36) is bonded to the exposed surface of said first layer (34). An air cathode as claimed in any one of claims 1-4, characterized in that said fibrous web (34) is a nonwoven web of conductive carbon fibers. An air cathode as claimed in any one of claims 1-5, wherein said coating material (40) is distributed as a multiplicity of spaced-apart dots. An air cathode as claimed in any one of claims 1-5, wherein said coating material (40) is distributed as a mesh having coating-material-free interstices. A method of making an air cathode that comprises a sheetlike laminate including first and second layers (34, 38) and current-collecting means (36) in contact with said first layer (34) and connectable to external electrical circuitry, wherein said layers have opposed major surfaces respectively exposed for contact with a liquid electrolyte and with air, said layers also having facing major surfaces, and said second layer (38) being permeable to air but not to said liquid electrolyte; said method being characterized by impregnating a nonwoven fibrous web (34) with a suspension, in a liquid vehicle, of material comprising carbon particles and a nonfibrous polymeric substance for holding the carbon particles in the web, and laminating said first and second layers (34, 38) together by juxtaposing them with a heat seal coating (40) distributed between their facing major surfaces in such manner as to provide an array or network of coating-free areas extending substantially uniformly thereover, and heating the juxtaposed layers to activate the coating.

Inventors: HOGE, WILLIAM H.
Applicants: ALCAN INTERNATIONAL LIMITED
Application year: 1988
Main classification: H01M4
IPC codes: H01M4, H01M12