Electrochemical Cathode

This invention relates to an improved process for manufacturing electrodes and electrodes produced thereby, and more particularly to a process for manufacturing electrochemical cathodes of higher carbon content for use in electrochemical cells and electrochemical cathodes layers produced thereby.

Metal/air batteries produce electricity by electrochemically coupling in a cell a reactive metallic anode to an air cathode through a suitable electrolyte. As is well known in the art, an air cathode is a typically sheetlike member having opposite surfaces respectively exposed to the atmosphere and to an aqueous electrolye of the cell, in which (during cell operation) 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 (so that aqueous electrolyte will not seep or leak through it), and must incorporate an electrically conductive element for external circuitry.

In present-day commercial practice, the air cathode is commonly constituted of active carbon (with or without an added dissocation-promoting catalyst) containing a finely divided hydrophoboic polymeric material and incorporating a metal screen as the conductive element. A variety of anode metals are used including iron, zinc, magnesium, aluminum, alloys of aluminum, etc. Alloys of aluminum and magnesium are considered especially advantageous for particular applications owing to 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. Metal/air batteries have an essentially infinite shelf-storage life rendering them very suitable for standby or emergency uses in that the metal-air battery may be activated by immersing the electrode in an electrolyte.

In the aforementioned copending application U.S. Ser. No. 07/070,183, there is disclosed an air cathode comprised of a sheetlike laminate including first and second layers having opposed major surfaces, respectively, exposed for contact with a liquid electrolyte and with air and facing major surfaces. The second layer is permeable to air but not to a liquid electrolye. A current-collecting layer is in cotnact with the first layer and is connected to external electrical circuitry. The first layer of the cathode is comprised of a nonwoven fibrous web, preferably of conductive carbon fibers, impregnated with a mixture of carbon particles and 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 facing major surfaces in manner to provide an array or network of areas free of coating material being distributed as a multiplicity of spaced-apart dots, or as a mesh having coating-material-free interstices. Coating-free spaces maintains sufficient unclogged pores in the second layer 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, such as a layer of metal mesh interposed between and coextensive with the first and second layer.

In copending application U.S. Ser. No. 07/287,482, there is disclosed a process for producing an electrochemical cathode for an electrochemical cell comprised of a current collecting layer or substrate laminated between layers of a nonwoven conductive fibrous web, preferably of conductive carbon fibers, impregnated with a mixture of carbon particles and a nonfibrous polymeric substance, and optionally with a hydrophobic microporous film or layer disposed on one of the layers of the nonwoven conductive fibrous web as well as the product produced thereby.

While such an air cathode has been effective, attempts to increase carbon content of the active layers by successive coatings, etc. have resulted in thicker layers, however, the carbon tends to flake off the cathodes, are difficult to handle and resulted in layers of low density and thus reduced conductivity.

An object of the present invention is to provide an improved cathode of higher carbon content for electrochemical cells.

Another object of the present invention is to provide an improved electrochemical cathode of higher carbon content for electrochemical cells providing improved electrochemical characteristics, such as conductivity.

A further object of the present invention is to provide an improved electrochemical cathode of higher carbon content for electrochemical cells exhibiting improved durability.

Still another object of the present invention is to provide an improved process for producing an improved cathode of higher carbon content for electrochemical cells of greater structural integrity.

Yet another object of the present invention is to provide an improved process for producing an improved cathode of higher carbon content for electrochemical cells exhibiting improved uniformity and thus improved electrochemical characteristics, such as conductivity.

A still further object of the present invention is to provide an improved process for producing an improved cathode of higher carbon content for electrochemical cells at nominal temperature ranges.

These and other objects of the present invention are achieved by a process for producing an electrochemical cathode for an electrochemical cell comprised of a current collecting layer or substrate laminated between layers of a nonwoven conductive fibrous web, preferably of conductive carbon fibers, impregnated with a mixture of carbon particles, carbon fibers of a length of less than about 1/8" and a nonfibrous polymeric substance, and optionally with a hydrophobic micorporous film or layer disposed on one of the layers of the nonwoven conductive fibrous web as well as the product produced thereby.

Other features and advantages of the present invention will become more readily apparent from the following detailed description when taken with the accompanying drawings wherein:

FIG. 1 is a schematic elevational sectional view of an electrochemical metal/air cell incorporating an electrochemical cathode of one embodiment of the present invention;

FIG. 2 is a schematic exploded view of the embodiment of an electrochemical cathode of FIG. 1;

FIG. 3 is a schematic exploded view of another embodiment of an electrochemical cathode, such as for a lithium battery, of the present invention; and

FIG. 4 is a schematic view of another embodiment of the present invention in bipolar electrode configuration.

Referring now to FIG. 1, there is illustrated a metal/air battery configuration (one cell illustrated), generally indicated as 10 comprised of a housing 12 defining a chamber 14 for receiving a liquid electrolyte 16, such as an aqueous solution of sodium chloride; and air cathode, generally indicated as 18, and a metal anode 20. The anode 20 is connected by line 22 to a light bulb 24 connected by line 26 including a switch 28 to the air cathode 18. The housing 12 defines a large vertical aperture 30 for receiving the air cathode 18 in liquid-tight fashion with the periphery of the air cathode 18 sealed to the periphery of the aperture 30 of the housing 12. Closing of the switch 28 closes an electrical circuit formed between the air cathode 18 and the anode 20 via the electrolyte 16 thereby illuminating the light bulb 24.

The air cathode 18 is a laminate structure comprised of a metal mesh substrate 32, reactive layers 34 laminated to the mesh substrate 32 and a hydrophobic microporous layer 36 mounted to the outer reactive layer 34. The metal mesh 32, is the current collector of the metal/air battery 10, and is formed of a suitable metallic material, such as nickel, stainless steel and the like, formed for example by expanded metal techniques and includes an upwardly extending tab portion 38 for connection to line 26.

The anode 20, may be formed of a suitable anodic material, such as aluminum, magnesium or the like, and is illustrated as being disposed in a top 40 of the housing 12 in the form of a plate member extending vertically into the chamber 14 including the electrolyte 16. The anode 20 is longitudinally disposed with reference to the housing and is spaced apart in parallelled relationship to the air cathode 18 a distance sufficient to form a gap therebetween for the electrolyte 16. The general arrangement of the metal/air battery 10 may be substantially the same as that of one of the cells of the plural-cell battery, such as described in U.S. Pat. No. 4,626,482 hereby incorporated by reference.

In FIG. 3, there is illustrated an electrochemical cathode, generally indicated as 42, constituting another embodiment of the present invention and a laminate structure comprised of the metal mesh substrate 32 laminated between an inner reactive layer 34 and an outer reactive layer 34 without any hydrophobic microporous layer mounted to an outer reactive layer 34. Such embodiment of the present invention finds application in the lithium battery art and would also include a metal mesh substrate 32 having only one side laminated to a reactive layer 34 thereby forming a two layered cathode.

In FIG. 4, there is illustrated a bipolar electrode configuration, generally indicated as 50, comprised of an anode layer 52 spaced-apart by an electrically conductive or dielectric substrate layer 54 from an electropositve layer 56. The anode layer 52 is formed of a suitable anodic material, such as aluminum, magnesium and the like as hereinabove described with reference to anode layer 20 of the metal/air battery 10 of FIG. 1. The electropositive layer 56 is similarly formed as the reactive layer 34 hereinabove discussed. The materials of the electric conductive or dielectric layer 54 is a function of cell-cell configuration of intended battery assembly. Should the anodes and electropositive layers be externally connected, the layer 54 is formed of a dielectric material whereas for internal connection, the layer 54 is an electrically conductive layer formed for example of a plastic material, such as a butyl or acrylic film including a dispersion of electrically conductive particles (not shown) in an amount sufficient to provide a volume resistivity of less than about 0.5 ohm cm.

The reactive layer 34 is comprised of a conductive nonwoven fibrous web impregnated with a mixture of carbon, carbon fiber particles and a nonfibrous polymeric substance for holding the carbon particles in the web. Other substances e.g. catalyst, may also be included in the impregnating mixture. The nonwoven fibrous web of the present invention is preferably formed of electrically conductive carbon fibers having a length of from about 0.5 to 1.25 inches and a diameter of from about 5.0 to 15.0 .mu.. The carbon content of such carbon fibers is preferably at least about 90 weight percent exhibiting a resistance of less than about 20 ohms/sq.

The conductive nonwoven fibrous web is formed into a thickness of from 5 to 20 mils, exhibiting a pore volume of at least about 90 percent, to provide a high pick-up of the impregnation mixture whether in suspension or in a coating format. The impregnation mixture should increase the basic weight of the conductive nonwoven fibrous web by about 20 to 120 g./m/.sup.2 as a function of solids content of the impregnation suspension or coating and pore volume of the conductive nonwoven fibrous web, and provide a front-to-back (F/B) electrical conductivity of less than about 1 ohm.

The impregnation mixture, in the form of a suspension or coating, is comprised of carbon particles, generally in the form of carbon black, carbon fibers and a nonfibrous adhesive polymeric material to bind the carbon particles to the web and a dispersion or mixing agents. The carbon particles are preferably of small size, expressed as surface area (m..sup.2 /g.), generally in excess of 1000, and preferably greater than about 1250, such as Black Pearls 2000, available from Cabot Corporation having a mean particle diameter of 0.015 .mu.. The carbon fibers are of a length less than about 1/8" and of a diameter of from about 5 to 15 .mu.. The carbon fibers are added to the impregnated mixture in an amount to constitute 1 to 5, preferably about 2 percent by weight of the solids content of the impregnated mixture.

The nonfibrous adhesive polymeric materials include the polytetrafluoroethylenes, such as Teflon.RTM. T-30, a registered trademark of duPont. The dispersion or mixing agents include the sodium salt of polymeric naphthalene sulfonic acid and those included with the polytetrafluoroethyene (PTFE).

The conductive nonwoven web of carbon fibers is impregnated by immersion, coating extrusion or the like, with the aqueous suspension of the impregnation mixture of the carbon particles. carbon fibers, nonfibrous polymeric substance and other desired nonfibrous imgredients, e.g. a catalyst. A catalyst, such as potassium permangante, may be added to the impregnation mixture to improve (or reduce) polarization voltage of the resulting cathode. Generally, potassium permangante is added in an amount of from 8 to 20%, preferably above 4% based upon dried weight of carbon dispersion solids and dispersion agent. Silver oxide has also been found to be an effective catalyst in the system of the present invention generally added in the form of silver peroxide to the impregnation mixture.

The impregnated conductive nonwoven fibrous web is dried at a temperature of from about 150 to 400.degree. F., a temperature below the sintering temperature of the binder material, i.e. the PTFE, and a temperature high enough to ensure substantially complete moisture removal.

The microporous or hydrophobic layer 36 is a film or web permeable to air but impermeable to the solvent of the electrolyte system to prevent or minimize penetration of the electrolyte through the air cathode to the exterior of the metal/air battery 10 with concomitant disappearance of active interface sites. The hydrophoboic layer 36 is laminated on the reactive layer 34 for intended use particularly in conjunction with liquid electrolytes. It has been found that a film of polymeric hydrophobic material, such as films of polytetrafluoroethylene of a thickness of from 2 to 20 mils embossed with mesh of a netted configuration of 1/30" to 1/8"diameter provides particularly desirable hydrophobic properties while conmitantly providing for effective diffusion of a gaseous fuel.

As disclosed in the aforementioned copending application, the layers of the air cathode laminate 18 are bonded by a discontinuous heat seal coating applied to the reactive layers as a discontinuous dot matrix system or an interposed layer of meshed coating material to be subsequently heated and subjected to a pressure step during passage of the air cathode laminate through a pressure roller system (not shown). Bonding of a reactive layer to the metal mesh substrate is particularly effectively performed by passing the metal mesh substrate 38 through a liquid solution or dispersion of a sealing material prior to lamination with a reactive layer 34. In this aspect of the present invention, the sealing material is a Teflon.RTM.dispersion in water.

The reactive layers 34 with seal coating either applied as a dot pattern on inner facing major surfaces or heat seal netting material (not shown) interposed are juxtaposed to the current collecting substrate 32 in a "sandwich" of layers, and the sandwich subjected to bonding heat and pressure sufficient to activate the heat seal coating to bond the reactive layers 34 to the metal substrate 32. The materials of the electrochemical cathode are selected such that activation temperatures of the heat-seal coating to bond the layers does not damage any layers or component substance, generally a temperature less than 250.degree. F. The hydrophyllic layer 36 may be simultaneously laminated to one of the reactive layers 34 as part of the laminate sandwich, or in a separate but like bonding step. Once the layers are bonded, the resulting laminate is cut to size to provide the individual electrochemical cathodes.

The process of the present invention is set forth in the following specific examples which are intended to be merely illustrative and the present invention is intended not to be limited thereto.

A conductive nonwoven web of carbon fibers (International Paper Company), is introduced into a continuous web machine to effect coating and drying operations. The impregnated mixture (carbon/Teflon.RTM.) is comprised of 800 gms. of an aqueous dispersion of Black Pearls (carbon) 2000 (15% solids), 1.6 gms of carbon fibers (FORTAFIL 3C*) cut to a length of 1/8" and 96 gms. Teflon.RTM. T-30 (60% solids) applied to the nonwoven web. Impregnation is effected to provide an add-on of 100 GSM. The resulting impregnated layers and fine nickel mesh (Delker) precoated with an adhesive are passed through an oven maintained at a temperature of from 200 to 300.degree. F. and thence web and passed through laminating rolls at nip pressure of 600 pounds per lineal inch at a temperature of 250.degree. F. to form an electrochemical cathode.

The procedures set forth in EXAMPLE I similarly followed except that a hydrophobic polymeric (Teflon.RTM.) film is included in the sandwich to be laminated, i.e. included against a side of one of the impregnated webs to form a four layered electrochemical cathode suitable for use in the laminate of each Example is preheated at 180.degree. F. with the microporous polymeric film and passed through laminating rolls at a temperature of 200.degree. F.

The procedure as set forth in EXAMPLE III is similarly followed except an embossed roll is used and the laminate sandwich is presented so that the hydrophobic film comes into contact with the polymeric film

The process of the present invention permits heavy loading of aqueous slurries of carbon-teflon e.g. up to 5000 GSM, as well as other materials to the web substrate eliminating concomitant problems of mud cracking and the like.

While the invention has been described in connection with an exemplary embodiment thereof, it will be understood that many modifications will be apparent to those of ordinary skill in the art; and that this application is intended to cover any adaptations of variations thereof. Therefore, it is manifestly intended that this invention be only limited by the claims and the equivalents thereof.