Production of a Cell Electrode System

The present invention relates to a process for preparing an electrode active porous composition composed of an active material held by a fibrous polymeric material. More particularly it concerns a process for preparing a porous, conductive, flexible cell electrode.

The use of polymers as binders and wet proofing agents in cell electrodes containing finely divided electrochemically active material has been described and used for many years. In general, efforts have been made to improve the materials so that a minimum amount of binder can provide a maximum bonding effect for the active particles. Fibrous polymers have been proposed for this purpose. In U.S. Pat. Nos. 3,630,781 and 3,954,501, for example, electrodes are prepared from a fluorocarbon polymer very finely dispersed in a liquid, e.g., water. The liquid-type dispersion, which is in essence an emulsion, is mixed with a surfactant and finely divided active material, and the mixture is used in a paste or a liquid consistency to coat or impregnate a structural material. The fluorocarbon is minutely distributed within the liquid carrier, preferably on the colloidal level. At a stage in the process, the fluorocarbon is precipitated as a fibrous material, e.g., by freezing, increasing the pH, or by solvent extraction. Existing processes for converting polymers to fibrous form from such liquid-type dispersion have many problems. One of the most serious limitation of existing processes employing liquid dispersions is removal of the liquid in which the polymer is dispersed. This requirement introduces an energy intensive step which limits and complicates the processing of the material. Dispersing agents as usually formulated may contain a dispersing additive such as a surfactant, and such additives may have a detrimental effect on the performance of an electrode. It also may be difficult to control or obtain a desired uniform distribution of active material throughout the dispersion during processing. The liquid dispersion system also has constraints with respect to processing alternatives in fabricating the end product. For example, it is difficult to produce a membrane via a milling process using a liquid or paste. A liquid or paste must be applied to a support in order to be fabricated into a shaped product. Also, the polymeric binders prepared from liquid-type formulations limit the exposure of the active material, which limits the capacity of electrodes made from such materials.

It is an object of the present invention to overcome or alleviate the above-noted problems of the prior art. It is also an object of the present invention to provide a thin, flexible electrode material. An additional object is to provide a process for preparing a polymeric cell electrode which is amenable to standard polymer processing equipment.

Additional objects will become apparent from the disclosure which follows.

In accordance with the present invention, a method is provided for producing a porous, conductive, electrode system comprised of electrode active particles and a fibrillated polymeric material comprising subjecting a fibrillatable polymer, a particulate electrode active (or activatable) material, a major amount of removable pore-former to conditions for fibrillating the polymer by a dry processing technique and for forming a precursor electrode composition comprising discrete polymeric fibers throughout an intimate mixture of electrode active particles, and a major amount of a removable, polymeric pore-former. Conversion to the electrode system is achieved by removal of the pore-former. Advantageously, a conductive carbon is present.

The porous, conductive electrode system produced is comprised of electrode active particles held by a fibrillated polymeric material. The electrode system has structural integrity, without the necessity of sintering the material or using a separate supporting structure. It is composed predominantly of the electrode active material, i.e. over 50% by weight is active material. Preferably the electrode active material constitutes at least about 80%, and typically it constitutes about 70% to 99%, by weight, of the electrode system.

The electrode system is formed from a precursor electrode composition, which contains the removable, polymeric pore-former. After removal of the pore-former, the resultant electrode system is porous, fibrous and conductive, with the active material present as the predominant component. The pore-former, which is present in a major amount, also serves as a processing agent. The use of a major amount of a removable, polymeric pore-former, which also serves as a processing agent, in combination with a fibrillatable polymer and electrode active material, is a key feature of this invention.

In accordance with this invention, the polymer is converted to fibrous form using a dry processing technique thereby permitting greater freedom in the choice of materials which can be used in the electrode and in the processing steps. Also by using a dry processing technique for the polymer, the electrode system can be produced in standard polymer processing equipment, such as mills, extruders, and molding machines.

It is another aspect of the invention that the process allows many options regarding the ultimate structural form produced. The particular shape desired can be obtained simultaneously with the conversion to fibrous forms or immediately thereafter in conventional equipment. It is still another aspect of the invention that the electrode system can be produced in the form of a flexible sheet or film. This can be accomplished, e.g., on a mill or calendered directly as it comes from an extruder. To form the electrode the flexible film is then combined with a current collector, which may be for example, a metallic deposit on the film or a conductive film, wire screen, or expanded metal.

As indicated above, the electrode system is composed essentially of an electrode active material, fibrous polymeric material and advantageously conductive carbon, and it is characterized in that it is porous and conductive. It is further characterized in that the pores are formed in-situ by a method which maximizes the accessibility of the active material.

The electrode active material, which is the predominant component of the electrode system--and may constitute even over 98% of the structure, is determined by the particular cell design. It can be any material which exhibits electrode activity, such as nickel, copper, mercury, cadmium, lead, manganese, silver, platinum, group metals, and compounds, e.g. oxides of mercury, silver, cadmium, lead or manganese, alloys, mixtures and other combinations thereof. The term "electrode active" material or "active" material will be understood to include elements, alloys, compounds, mixtures thereof which active or activatable, i.e. they can be converted even after incorporation in the electrode system into the active form. In general, in the present electrode systems, the active material the active material is in powder form, preferably it is very finely divided. Typically, for example, the manganese dioxide active particles pass a -100 mesh screen and 60-70% pass a -325 mesh screen. It is an advantage of the present invention that the active particles can be very fine and the system can accommodate changes in size and volume of the electrode active material. The fibers minimize losses due to attrition.

With respect to the fibrous component of the electrode system, generally it is important that the fibrillatable polymer is not detrimental to the electrochemical activity of the active material, and for use in many cells, it is important that it is highly resistant to corrosive environments. Fluorocarbon and propylene polymers have these suitable characteristics. To be useful for the process of the present invention, it is also important for the polymer to be fibrillatable in a dry-type process. Such polymers are known. For example, polytetrafluoroethylene (PTFE) can be fibrillated from the dry powder and it is commercially available as duPont's "Teflon" 6A and 7A. Fibrillatable polypropylene is available, for example, as strands, tape or film which can be used, e.g. as such or cut or chopped to appropriate size. The fibrous component is effective even when present in small amounts, e.g. about 0.5 to 5%, typically about 1-2%, by weight.

The conductive carbon powder is present in an amount of up to about 25% of the system, by weight and advantageously in an amount to provide at least some electrical conductivity to the electrode system. Examples of suitable carbon powders are acetylene black and graphite. Examples of commercially available conductive carbon powders are "Shawinigan" black (a product of Shawinigan Products Corp.), "Vulcan" XC-72 and "Regal" 660 ("Vulcan" and "Regal" carbon blacks are products of Cabot Corp.), Ashland "United" (a product of Ashland Chemical Co.) and "Ketjen" black (a product of Akzo Chemie NV).

To develop the porous electrode system after conversion of the polymer to fibers, the pore-forming agent is removed leaving the active particles encradle by fibers. Hence, the pore-forming agent must be removable. It must also serve as a processing agent. The pore-forming agents are further characterized in that they are polymers, they are non-reactive with the active material and the electrode fibers, and preferably they wet or can be made to wet the surfaces of active material. That is, the pore-forming agents will spread on or can be made to spread on the active material. It is believed that if the pore-former wets the surfaces of the active material, i.e. before removal, this will contribute to the accessibility of the active materials when the pore-former is removed. It is preferred that the pore-forming agents melt substantially below the temperature at which the active material will react. By removable is meant that the pore-former can be removed in any way that is not harmful to the remaining components and in a way that will leave the remaining material in a porous condition. In accordance with the invention, the pore-former serves as incipient sites for the pores and after removal of the pore-former, the pores can be interconnecting and distributed throughout the material. The pore-former may be, for example, leachable directly, convertible to leachable compositions or to gaseous products. Leachable pore-forming agents may be selected, for example, from water-soluble, base-soluble, acid-soluble or organic solvent-soluble resins. Water-soluble resins can be removed by leaching with water. It is noted in this regard that addition of water to the formulated fibrous material is not critical or disadvantageous since water added at this stage does not interfere with the advantageous flexibility in processing of the dry mix. Examples of pore-formers of the water-soluble type are polyolefin oxides such as polyethylene oxide and polypropylene oxide. Examples of other leachable pore-formers are polyethylenimine, which can be leached with dilute sulfuric acid; polyacrylic acid, which can be leached with dilute sodium hydroxide; and suitable paraffin waxes, which can be leached with halogenated solvents. Removal can also be effected by enzymatic breakdown. Generally, it is possible to use any of the known pore-forming techniques for polymers so long as it is compatible with the polymer system.

An important characteristic of the pore-former of this invention is that it is a polymer. As such it is compatible with the fibrillatable polymer and can serve as a system without the need for using very large quantities of additives which must be removed from the fibrillatable polymer, e.g. by filtration, before dry processing. That is, the polymeric pore-former serves as the medium which permits the combination of other components of the system to be uniformly distributed and conveniently worked. The amount used is predetermined primarily by the amount of porosity desired and not by the processing needs of the fibrillatable polymer. And in accordance with the present invention, processing can be carried out in standard polymer processing equipment. Since, the pore-former is by definition removable, no unwanted ingredients are added to the system. A further advantage of using a polymeric pore-former is that it can be used at normal working temperatures for polytetrafluoroethylene or polypropylene, i.e. about 75.degree.-100.degree. C. without reaching a hazardous flash point. For example, Stoddard solvent, which is sometimes considered a medium for working polytetrafluoroethylene, is unsuitable as a pore-former for the present process because it has a flash point of 100.degree. to 110.degree. F. Vehicles also not suitable as pore-formers for the present process are those, such as low molecular weight paraffins, naphthenes, or aromatics such as naphthalene, benzene, or industrial solvents that are mixtures of various organic compounds, e.g. Shell-Sol B or mineral salts, which may volatilize during working on the processing equipment. A critical problem with vehicles which volatilize during processing in that the pore-former is removed before it can serve as the incipient sites for the pores.

Pore-formers such as certain polyolefin oxides, e.g. polyethylene oxide and polypropylene oxide, in addition to being conveniently water soluble and having suitable melting temperatures (e.g. polyethylene oxide melts below 75.degree. C.), have the advantage that they serve as both pore-forming and processing agents for fibrillatable polymers such as polytetrafluoroethylene and polypropylene. In general, such pore-formers have molecular weights in the neighborhood of 100,000 to 1,000,000.

The amount of pore-former to be incorporated in the precursor electrode composition is dependent on the amount of porosity desired in the ultimate product. In general, the pore-former is present in the precursor electrode material in a major amount, i.e. at least about 10% by weight, typically about 15% to about 50%, and preferably over 25 or 30 weight %. In the electrode system, it is desirable to have maximum exposure of the electrode active material and to have free movement of electrolyte throughout the electrode mass. To achieve this the electrode system preferably has about 70 to 80 volume % porosity, and the pores should be interconnecting. For such porosity, an equivalent volume % of leachable pore-former is needed in the precursor electrode composition, i.e. about 70 to 80 volume %. This is equivalent to about 15 to 50 weight % of polyethylene oxide. To obtain porosity of this extent and type using fugitive gaseous pore-former considerably more pore-former would be necessary, and this could be estimated based on the volume of gas produced and conditions.

After removal of the pore-former, in general, the electrode system is predominantly electrode active material and is composed, by weight, of about 0.5 to about 5% fibrillated polymer, up to about 25% (e.g. 0.5 to about 25%) conductive carbon, and the remainder essentially electrode active material.

As indicated above, to prepare the porous electrically conductive electrode system the initial components, (i.e. the electrode active material, the fibrillatable polymer, and the pore-former) are first converted to the precursor electrode composition by a method comprising subjecting such components to conditions to convert the polymer by a dry processing technique, e.g. as a powder or tape under shear stresses, to fibrous form and to transform the components to a material composed of discrete fibers throughout an intimate mixture consisting of active material and pore-former. The components can all be blended together initially, e.g. in a blender such as a Banbury mixer or a ball mill, and then processed, e.g., in an extruder. Alternatively, the components can be mixed with each other and processed in various sequences depending on the desired ultimate configuration and the equipment used. By removal of the pore-former, the precursor electrode composition is converted to the electrode system.

The electrode system or precursor electrode composition can be formed as pellets and the pellets can be converted to the desired shape by the usual polymer processing techniques, e.g. extrusion, molding, blowing or combinations thereof. As indicated above, with appropriate choice of processing technique, the components can be processed from a dry powder mixture directly to a sheet or bar. The thickness of the sheet or bar can be varied and the electrode material can be produced directly as a flexible sheet or film. Alternatively, for example, it can be made into pellets and the pellets extruded and blown to a film or the pellets can be injection molded to a film. It is noteworthy that the electrode system in which PTFE is the major or only remaining polymer component can be made directly, e.g. by milling or any number of other standard polymer process techniques, into a material which has structural integrity--without sintering and has porosity. It is particularly an advantage of the present process that PTFE, which is uniquely difficult to process into shaped structures by methods other than sintering, can be readily shaped without sintering into a porous structure using standard polymer processing techniques when using the process of the present invention.

It is known to convert PTFE to fibrous form in a dry process. For example, U.S. Pat. No. 3,838,092 describes a process for producing dustless powders and U.S. Pat. No. 4,071,946 describes a process for producing lead powders for tube electrodes. Both patents refer to fibrillating PTFE by a dry process. Neither patent discloses the porous material of the present invention or the method of obtaining it. In both patents powders are produced, and they are mainly concerned with the handling of fine powders, per se and the prevention of dusting. The latter patent, which involves a tube electrode, specifically seeks to reduce the dusting problem during filling of the electrode.

It is another aspect of the invention that the electrode system can be produced directly as a flexible, coherent sheet. The sheet can be produced in any desired thicknss and can be formed as a thin, flexible film, e.g. of the order of about 1 to 10 mils, which has structural integrity. Typically, suitable electrode system films can be made of about 1.5 to 5 mils, e.g. 2 to 3 mils. In one embodiment, the initial components of the precursor composition are mixed dry, heated to an elevated temperature, and subjected to shear stresses for a time sufficient to convert the polymer into fibrous form. As noted above, this can be effected in standard polymer equipment, and the sheet or film can be formed in one or more steps and one or more types of equipment. For example, the initial components can be milled, e.g. in a roll-type mill. During such milling the active material and pore-former can be homogeneously mixed and the fibers can be distributed throughout the mixture. The product from the mill is peeled from the rollers in sheet form. Porosity is obtained by removing the pore-former from the milled sheet, and the sheet can be produced in this way with good mechanical strength, and it is in the manageable form. Porosity and conductivity of the electrode system can be controlled, e.g., with appropriate use of pore-formers and carbon. The polymeric material as it comes from the mill can be processed immediately or stored in a dry state. The electrode can be made as a flexible film in any desired thickness, and it can be easily combined with a current collector such as a conductive film, wire screen or expanded metal.

This example illustrates the process of the invention carried out on a mill. The components used in the process are listed in order of loading preference on the mill.

A two-roll rubber mill is used, with mill processing temperature set at 75.degree. C. In a typical test mix: An 80 gram charge of pore-former (WSRN-80) is added to a running mill. When the resin has formed a band, a 2 gram charge of fibrillatable polymer (Teflon 6A) is added slowly and the fibrillatable polymer is permitted to join the resin band. The material is scraped from the mill and refolded to make a homogeneous mixture. Then 15 grams of carbon black (Shawinigan black) is added by sifting it slowly into the band and allowing it to be taken up into the resin. The carbon-containing material is scraped and folded until a smooth plastic band is observed. Thereafter, 105 grams of MnO.sub.2 dry powder is added. Again the powder is sifted into the mixture on the mill allowing time for the active material and resin to mix and band. Temperature control is important since at this point in the processing the mixture is heat sensitive. Using the components of the present tests, keeping the rolls at a 0.010 inch nip and at 75.degree. C. gives a safe working mix. Higher temperatures may lead to an uncontrollable exotherm and mix ignition. The mix will behave in a safe manner for mixing and sheeting, but should not be allowed to remain on the mill for extended periods of time after it is mixed.

The material coming off the mill is smooth and peels from the mill surface. Such material is soft and flexible, and strong enough to allow trimming to size without cracking at the edges. Storage of the material at this point is practical. For use it can be, for example, heat pressed on to wire screen grids.

To form the porous electrode system, the pore-former is leached, e.g. in hot tap water. After all the pore-former is leached out, the material can be dried (air drying of the material leaves approximately 1% moisture in the mix).

In this form, the dry, porous electrode absorbs both alkaline and acid electrolyte rapidly, and wets out completely.

Electrodes in accordance with the present invention can be fabricated using PbO.sub.2, Ni(OH).sub.2, CdO, or CdS as the active material instead of MnO.sub.2. Compositions can be formed having processing and handling characteristics similar to the material with MnO.sub.2 as the active material.

An electrode system, formed essentially as described in Example 1, with MnO.sub.2 as the active material, is hot pressed at 7000 psi on to gold screens and the resultant structures used as electrodes.

These electrodes are then used in the fabrication of a three-plate cell. The cell consists of two pieces of zinc foil, the MnO.sub.2 -containing electrode system on a screen, and a reference electrode. A non-woven fabric is used as a separator. The cell is flooded with 45% KOH, and after wetting out is discharged at a constant current rate of 4.25 mA/cm.sup.2. This rate gave 50% of the theoretical capacity. Monitoring of the cathode vs. the reference electrode shows that the cell was cathode limited.

The test shows that the MnO.sub.2 of the film prepared in accordance with the present invention functions as an active cathode material in a conventional type battery assembly. The ability to produce an electrode in this manner affords the opportunity for greater freedom in cell design, particularly with cell designs which require thin and/or flexible electrode materials.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.