METHOD FOR PRODUCING AN ELECTRODE CONTAINING ELECTROLYTE-ABSORBED POLYMER PARTICLES

Abstract: An electrode for a galvanic cell is provided, comprising a particulate electrochemically active material, an electrolyte, and cross-linked electrolyte-absorbed polymer particles wherein the electrolyte-absorbed polymer particles are at least 1000 mu m in length, width or height and are distributed throughout the electrode. Also provided is a method for producing an electrode for a galvanic cell, comprising mixing a particulate electrochemically active material, an electrolyte, and dehydrated liquid absorbing cross-linked polymer particles which are sized to flow through a 20 Tyler mesh screen and be retained on a 200 Tyler mesh screen, such that, after absorbing the electrolyte, at least 50 % of the liquid absorbing cross-linked polymer particles are at least 1000 mu m in length, width or height and are substantially distributed throughout the electrode. The electrode is particularly suitable as a gelled anode for alkaline galvanic cells, especially alkaline zinc-manganese dioxide cell.

Description: METHOD FOR PRODUCING AN ELECTRODE CONTAINING ELECTROLYTE- ABSORBED POLYMER PARTICLES This invention relates to electrodes such as zinc powder-gel anodes for galvanic cells, to galvanic cells employing such electrodes, and to a method for producing electrodes containing electrolyte-absorbed polymer particles. A conventional type of alkaline cell employs a positive electrode (cathode) comprising predominantly an oxidising depolariser such as manganese dioxide, usually admixed with a binder, and a conductive material such as graphite. The negative electrode (anode) usually comprises a consumable anodic material such as powdered zinc, admixed with a gelling agent such as carboxymethyl cellulose, and a suitable alkaline electrolyte such as an aqueous potassium hydroxide solution. US-A-3, 884,721 discloses a composite negative electrode for use in an alkaline cell comprising, in combination, zinc particles, an alkaline electrolyte and a cross-linked polyacrylamide to form electrolyte nuggets. The zinc particles are said to be distributed throughout the composite negative electrode in a manner such that the zinc particles are in contacting relation with the electrolyte nuggets and with each other. US-A-5,376,480 discloses a gel form negative electrode of an alkaline battery that is produced without mercury and enables uniform dispersion of zinc or zinc alloy powder and an additive metal, which can be one or more of an oxide or hydroxide of indium, lead, gallium, or bismuth. The zinc or zinc alloy powder and the additive are dry-mixed before mixing with a gel form alkaline electrolyte. In order to obtain a satisfactorily high vibration strength and impact resistance, a fibre material can be added to the gel form negative electrode. The fibre material may be selected from rayon, vinylon, acryl, vinyon, polyamide, polypropylene, polyethylene, mercerised pulp, and linter pulp. US-A-4,963,447 discloses an alkaline cell having a gelled zinc negative electrode using a granular cross-linking type branched polyacrylic acid, polymethacrylic acid or salt thereof, as a gelling agent to hold zinc powder in the alkaline electrolyte. This gelling agent, holding alkaline electrolyte, swells and properly maintains the thickness of the electrolyte, so that the electrolyte can be sufficiently supplied to the reactive area of the cell. US-A-5,587,254 discloses a gel type negative electrode comprising a zinc alloy powder, an alkaline electrolyte, and three gelling agents in combination, namely a cross- linked polyacrylate-type water-absorbing polymer having a dispersion viscosity at 25 °C of at least 15,000 cps as a 0.5 wt % aqueous solution and having a particle size of mainly 100-900 μm, a cross-linked and branched-type polyacrylic acid or salt thereof having a dispersion viscosity at 25 °C of at least 15,000 cps as a 0.5 wt % aqueous solution and having a particle size of mainly 100 μm or smaller, and a granular cross-linked and branched-type polyacrylic acid or salt thereof having a dispersion viscosity at 25 °C of at least 15,000 cps as 0.5 wt % aqueous solution and having a particle size of mainly 100- 900 μm. If desired, mercury can be included to raise the hydrogen over-potential of the negative electrode zinc surface, thereby suppressing corrosion of the zinc and reducing the hydrogen gas generation that usually accompanies such corrosion. Since mercury is harmful to the environment, successful attempts have been made to eliminate it from batteries. However, with the elimination of mercury from the zinc anode, electrical continuity suffered due to decreased particle-to-particle contact between zinc particles and decreased contact between the zinc particles and the current collector. Also, the occurrence of strong shock to the cell can cause loss of stability within the anode. It would therefore be desirable to be able to provide an alkaline cell that exhibits good cell stability as well as good electrical continuity within an electrode, particularly the anode. We have now found that this may be achieved by forming an electrode, particularly the negative electrode, having a gel structure in accordance with the present invention. Accordingly, in a first aspect, the present invention provides an electrode for a galvanic cell comprising a particulate electrochemically active material, an electrolyte, and cross-linked electrolyte-absorbed polymer particles wherein the electrolyte-absorbed polymer particles are at least 1000 μm in length, width or height and are distributed throughout the electrode. In a second aspect, the present invention provides a method for producing an electrode for a galvanic cell, comprising: mixing a particulate electrochemically active material, an electrolyte, and dehydrated liquid absorbing cross-linked polymer particles which are sized to flow through a 20 Tyler mesh screen and be retained on a 200 Tyler mesh screen, such that, after absorbing the electrolyte, at least 50% of the liquid absorbing cross-linked polymer particles are at least 1000 μm in length, width or height and are substantially distributed throughout the electrode. In a third aspect, the present invention provides an electrochemical cell comprising an electrode as defined above. Advantageously, the gel electrode in accordance with the present invention, when used in a galvanic cell, improves the contact between the electrochemically active particles, such as zinc particles, and the contact between the particles and the current collector. Furthermore, the electrode can improve the shock resistance of the cell to provide better cell performance, especially during pulse discharge under high drains. These advantages may be achieved according to the present invention by providing a gel electrode, preferably the anode, using cross-linked polymer absorbed electrolyte particles that are at least 1,000 μm in length, width or height and are distributed throughout the gel electrode. In an embodiment of the first aspect of present invention, a negative electrode for use in an alkaline galvanic cell comprises an electrochemically active material, an electrolyte, and cross-linked electrolyte-absorbed polymer particles wherein the electrolyte-absorbed polymer particles are at least 1000 μm in length, width or height and are substantially distributed throughout the negative electrode. In a preferred embodiment of the first aspect, a negative electrode for an alkaline galvanic cell comprises zinc powder as an electrochemically active material, an alkaline electrolyte and cross-linked electrolyte-absorbed polymer particles wherein the particles are at least 1,000 μm in length, width or height and said particles are distributed substantially homogeneously throughout the negative electrode. Preferably, the negative electrode contains a gelling agent and the cross-linked electrolyte-absorbed polymer particles are present in the negative electrode in an amount between about 15% volume and about 50% volume of the total volume occupied by the gelling agent and the electrolyte-absorbed polymer particles. In an embodiment of the second aspect of present invention, a method for producing a negative electrode for a galvanic cell, comprises the steps of: (a) selecting dehydrated liquid absorbing cross-linked polymer particles which are sized to flow through a 20 Tyler mesh screen and be retained on a 200 Tyler mesh screen, and (b) mixing at least one electrochemically active material, an electrolyte solution, and the selected liquid absorbing cross-linked polymer particles of step (a), wherein after absorbing the electrolyte, at least 50% of the liquid absorbing cross-linked polymer particles are at least 1000 μm in length, width or height and are substantially distributed throughout the negative electrode. The present invention may be further understood from the following description: In the method according to the present invention, the cross-linked polymer particles can for example be contacted with some or all of the electrolyte prior to the particles being mixed with the electrochemically active material. Preferably, the liquid absorbing cross-linked polymer particles are gently mixed into the active material and electrolyte so that the particles can absorb the electrolyte, to produce the electrolyte- absorbed polymer particles that are at least 1,000 μm in length, width or height. Such particles may be irregularly shaped provided that the longest dimension is at least 1000 μm in size. The preferred method for preparing the polymer particles is to gently mix the polymer particles into water, preferably deionised water, making sure that the polymer particles do not form an agglomerate. The mixture, in a gel consistency, is then dehydrated, for example by dispensing on a surface and placing in an environment to permit the water to evaporate. Preferably, the water-absorbed polymer particles are placed, for example, in a heated vented oven, between 50 °C and 100 C, for a time period sufficient to evaporate the water and produce dehydrated polymer particles. The dehydrated polymer particles are, if necessary, first ground and then sorted. Preferably, the dehydrated polymer particles are sorted by selecting those particles that pass through a 20 Tyler mesh screen but are retained on a 200 Tyler mesh screen. More preferably, the dehydrated polymer particles are sorted by selecting those particles that pass through a 20 Tyler mesh screen but are retained on a 60 Tyler mesh screen, and most preferably, by selecting those particles that pass through a 40 Tyler mesh screen but are retained on a 60 Tyler mesh screen. Preferably, the dehydrated polymer particles are gently folded into (gently added to) the active material and electrolyte to ensure that at least 75% of the dehydrated polymer particles absorb electrolyte and produce electrolyte-absorbed polymer particles that are at least about 1,000 μm in length, width or height, preferably between 1000 μm and 10,000 μm, more preferably between 1,000 μm to 8,000 μm, still more preferably between about 2,000 μm to 6000 μm, and most preferably about 5,000 μm. In a preferred embodiment, for producing a gelled anode for alkaline cells, the electrochemically active material such as zinc powder is mixed with an electrolyte and a gelling agent, and then the dehydrated polymer particles are mixed or folded into the zinc, electrolyte, and gelling agent composition to form electrolyte-absorbed polymer particles in an amount of between about 15% by volume and about 50% by volume, preferably between about 20% by volume and about 35% by volume and most preferably 6 about 30% by volume, of the total volume occupied by the gelling agent and electrolyte- absorbed polymer particles. The gelling agent for the zinc-electrolyte composite (pregel) can be the same polymer material as the cross-linked polymer used to produce the electrolyte-absorbed polymer particles, except that the polymer particles are then mixed with the zinc- electrolyte composite at a high blending rate so that the polymer particles do not effectively absorb the electrolyte, thereby functioning primarily as a gelling agent distinguishable from the polymer particles. Other conventional gelling agents used in the battery art can be used as the gelling agent for the pregel anode. For certain applications, it may be beneficial to include a mixture of gelling agents. The electrolyte-absorbed polymer particles substantially retain their swollen state in the anode. The electrolyte-swollen polymer particles in the anode push the electrochemically active particles, such as zinc particles, together so that the zinc particles are concentrated between the swollen polymer particles, thus forming a matrix- type structure that ensures good electrical contact between the zinc particles, and between the zinc particles and the anode current collector. The use of electrolyte swollen polymer particles greater than 1 ,000 μm in length, width or height provides benefits in that it reduces shock and vibration sensitivity by concentrating the zinc or other electrochemically active particles to maintain particle-to-particle contact, it serves as a reservoir for the electrolyte by holding the electrolyte in the anode without allowing the electrolyte to migrate to the cathode but releasing the electrolyte to the anode for reaction as needed during discharge, it imparts gel stiffness to prevent movement of the zinc or other particles, and can improve high rate discharge. In general, the cross-linked polymer employed according to the present invention preferably should: a. be capable of absorbing the electrolyte and after absorption assuming an expanded or swollen condition; b. be substantially insoluble in the electrolyte; c. be stable at the temperatures of use, i.e. it should not release absorbed electrolyte or change physical form at temperature of use; d. resist chemical degradation in a caustic environment; e. be capable of absorbing a minimum of about 5 times its weight of electrolyte; and f. when swollen with electrolyte, should not be extremely gummy or sticky to any significant degree, i.e. there should be no gluing or cementing effect. The water-insoluble particulate cross-linked polymer of the type herein contemplated is an absorbent material that maintains its particulate character as it absorbs electrolyte, and in doing so swells. As previously indicated, the absorbent, water- insoluble, particulate cross-linked polymer contemplated herein is preferably capable of absorbing about 5 times to 40 or more times its weight of electrolyte. Consequently, each individual absorbent particle swells or enlarges to several times its initial size without destruction of its initial integrity. Suitable cross-linked polymers for use in this invention are preferably selected from cross-linked carboxyvinyl polymers, cross-linked polyacrylate polymers and cross- linked polyacrylamide polymers. In a preferred embodiment, the cross-linked polymer is a polyacrylamide material. Prior to cross-linking, the polyacrylamide absorbent materials for use in the invention may suitably be compounds having the following structural formula: [{-CH 2 -CH(CONH 2 )-} m {-CH 2 -CH(COOY)-} n ] z , wherein Y is hydrogen, ammonium or an alkali metal; m is a number from 1 to 100; n is zero or a number up to 99 which may be regarded as an index of the degree of hydrolysis of m + n amide groups originally present; 8 m + n is equal to 100; and Z is a number from about 0.1 to 30, where Z times 100 is equal to the number of mer units between cross-links. Cross-linked polyacrylamide material useful in this invention may be prepared by known techniques, for example by cross-linking a linear polyacrylamide or, preferably, by copolymerising an acrylamide monomer with a non-conjugated divinyl compound. Acrylic acid, methacrylic acid, or salts thereof may be employed with or in place of the acrylamide. The polymerisation may be carried out by any of the standard methods, such as using peroxide catalysts or by photo-polymerisation with a riboflavin activator. It will be appreciated that the amount of cross-linking compound required to give the desired end product will depend on the reactants employed and the conditions of reaction. Examples of non-conjugated, divinyl cross-linking compounds are 1,4-di vinyl benzene; N,N-diallylacrylamide; diallylamine; diallylmethacrylamide;, 2,5-dimethyl-l,7-octadiene; p,p'-diisopropenylbenzene; 2,8-dimethyl-l,8-nonadiene and diethylene glycol divinyl ether, divinyl sulphone, and methylene-bisacrylamide. Suitable carboxyvinyl polymer materials for use in this invention can be formed in the presence of a mixture of polyvalent allyl cross-linking agent with a polyvalent vinyl cross-linking agent, and further granulated. This granular branched carboxyvinyl polymer may be formed by incorporating the above cross-linking agents in a suitable mixing ratio, adding solvents such as water or an alcohol during deposition polymerisation and forming granular particles from the resultant deposited small particles, or forming particles in the presence of solvents such as water or alcohol, after the deposition polymerisation and granulating the particles. Cross-linked polyacrylate polymers suitable for use in this invention include polyacrylic acid and salts thereof. The particulate electrochemically active material employed according to the present invention is preferably zinc. The zinc may be of the type commonly employed in the art, for example in the form of zinc particles or powder. The particles preferably 9 should have a size within the Tyler standard screen range of through 60 mesh but substantially retained on 325 mesh. They can for example be present in the anode in an amount of 30 to 85% by weight preferably 40 to 70% by weight, based on the total weight of the ingredients in the anode. The electrolyte material is preferably an aqueous alkaline metal hydroxide such as sodium hydroxide or potassium hydroxide, or mixtures thereof. Potassium hydroxide is preferred. The electrolyte material is preferably present in the anode structure in an amount of 10 to 65% by weight, more preferably 25 to 55% by weight, based on the total weight of the ingredients in the anode. Suitable gelling agents for use in the anode of this invention include for example sodium carboxymethyl cellulose (CMC), methyl cellulose (MC), poly-N-vinyl pyrrolidone (PNVP), and polymethacrylic acid (PMA). The electrode of this invention is ideally suited for use as an anode in alkaline galvanic cells, preferably alkaline-manganese dioxide-zinc system cells. EXAMPLE A gelled zinc negative electrode is made of an alkaline electrolyte comprising potassium hydroxide, zinc oxide and water, a gelling agent and a non-amalgamated zinc powder. The zinc negative electrode is prepared as follows: A glass beaker was filled with 400 millilitres of deionised water, with a mixing action at 500-800 rpm. A total of 93.5 grams of Carbopol 940 (trademark of B. F. Goodrich) was added to the water in small amounts while mixing with the water to hydrate. The saturated Carbopol 940 solution had the consistency of jelly. The saturated solution was poured into a stainless steel pan, and placed into a vented 100 °C oven for 24 hours. After the water was fully evaporated, the pan was spotted with dehydrated particles. The dehydrated particles were approximately 0.6 cm (1/4 inch) in diameter and hard. The pan was removed from the oven and cooled. The dehydrated particles were scraped off the pan with a stainless steel 10 putty knife. The dehydrated particles were then placed into a grinder for 10 seconds on high speed. The ground dehydrated particles were then poured through a 40 Tyler mesh size sieve. Any particles that could not pass through the 40 Tyler mesh screen were re- ground until all the particles passed through the 40 Tyler mesh screen and only the particles retained on a 60 Tyler mesh screen were used. The anode and dehydrated particles were made as follows: A dry blend was used to manufacture anodes that contain pregel anodes and/or dehydrated particles. For an anode with pregel, the dry ingredients of zinc and indium hydroxide are mixed for about five minutes. While the zinc and the indium hydroxide were mixing, a 0.1 N potassium hydroxide solution was fed into the mix. The pregel was thoroughly mixed. The pregel was again mixed for 15 to 20 seconds while sprinkling the 60 Tyler mesh dehydrated particles on top. The anode was aged for one day before use to allow the dehydrated particles to fully hydrate. The resulting mixture was a gelled anode having electrolyte-absorbed polymer particles greater than 1000 μm in length, width or height, as determined by scanning electron microscopy.

Claims: 11 CLAIMS: 1. An electrode for a galvanic cell comprising a particulate electrochemically active material, an electrolyte, and cross-linked electrolyte-absorbed polymer particles wherein the electrolyte-absorbed polymer particles are at least 1000 μm in length, width or height and are distributed throughout the electrode. 2. An electrode according to claim 1 wherein the particulate electrochemically active material is concentrated at the interface between the polymer particles. 3. An electrode according to claim 1 or claim 2 further comprising a gelling agent, the gelling agent being distinguishable from the cross-linked electrolyte-absorbed polymer particles. 4. An electrode according to claim 3 wherein the cross-linked electrolyte-absorbed polymer particles are present in an amount between about 15% by volume and about 50% by volume of the total volume occupied by the gelling agent and electrolyte- absorbed polymer particles. 5. An electrode according to claim 4 wherein the electrochemically active material is zinc and the cross-linked electrolyte-absorbed polymer particles occupy between about 20% by volume and about 35% by volume of the total volume occupied by the gelling agent and electrolyte-absorbed polymer particles. 6. An electrochemical cell comprising an electrode according to any of claims 1 to 5. 7. A method for producing an electrode for a galvanic cell, comprising mixing a particulate electrochemically active material, an electrolyte, and dehydrated liquid 12 absorbing cross-linked polymer particles which are sized to flow through a 20 Tyler mesh screen and be retained on a 200 Tyler mesh screen, such that, after absorbing the electrolyte, at least 50% of the liquid absorbing cross-linked polymer particles are at least 1000 μm in length, width or height and are substantially distributed throughout the electrode. 8. A method according to claim 7, wherein the dehydrated liquid absorbing cross- linked polymer particles are prepared by mixing cross-linked polymer particles with water and then dehydrating the cross-linked polymer particles. 9. A method according claim 8, wherein the water is deionised water. 10. A method according to any of claims 7 to 9, wherein the dehydrated liquid absorbing cross-linked polymer particles are allowed to absorb electrolyte to produce electrolyte-absorbed polymer particles, prior to mixing with the particulate electrochemically active material. 11. A method according to any of claims 7 to 10, wherein the dehydrated liquid absorbing cross-linked polymer particles flow through a 40 Tyler mesh screen and are retained on a 200 Tyler mesh screen. 12. A method according to claim 11, wherein the dehydrated liquid absorbing cross- linked polymer particles flow through a 40 Tyler mesh screen and are retained on a 60 Tyler mesh screen. 13. A method according to any of claims 7 to 12, wherein at least 75% of the cross- linked electrolyte-absorbed polymer particles are at least 1000 μm in length, width or height. 13 14. A method according to claim 13 wherein at least 75% of the cross-linked electrolyte-absorbed polymer particles are between 1000 μm and 10,000 μm in length, width or height. 15. A method according to claim 14, wherein at least 80% of the cross-linked electrolyte-absorbed polymer particles are between 2000 μm and 6000 μm in length, width or height. 16. A method according to any of claims 7 to 15, wherein the cross-linked polymer particles are selected from cross-linked carboxyvinyl polymer particles, cross-linked polyacrylate polymer particles, and cross-linked polyacrylamide polymer particles.

Inventors: URRY LEWIS F, URRY, LEWIS, F.
Applicants: EVEREADY BATTERY INC, EVEREADY BATTERY COMPANY, INC.
Application year: 1999
Main classification: H01M4
IPC codes: H01M4, H01M6