ALKALINE CELL CONTAINER HAVING INTERIOR CONDUCTIVE COATING

Abstract: A hard conductive coating (14) for the interior of alkaline cell containers (12) is formed from a composition containing carbon particles carried in a binder and a volatile carrier. The method of making this coating includes evaporating the carrier. The resultant coating is substantially impervious to alkaline electrolyte.

Description: ALKALINE CELL CONTAIN-* HAVING INTERIOR CONDUCTIVE COATING FIELD OF THE INVENTION This invention relates to alkaline cells, and particularly to containers in which alkaline cells are assembled. The invention finds its principal use in cylindrical, alkaline cells which have a substantial axial length as compared to their diameter. BACKGROUND OF THE INVENTION One of the principal causes for the loss of apparent energy capacity of alkaline dry cells, once they have been manufactured and stored for extended periods of time, has been the increase of contact resistance between the material of the cathode within the cell and the container in which the cell has been assembled. This increase in contact resistance may be manifested by a reduced on-load terminal voltage, faster reduction to a cut-off voltage, or reduced photoflash capabilities. It occurs because of the fact that the material of the cell container or can, usually nickel plated steel, is subject to corrosion, particularly in the presence of alkaline electrolyte such as potassium hydroxide. One approach to overcoming the problem of internal corrosion, apart from nickel plating, is to provide an additional conductive coating on the interior surface of the can, which coating may then provide a low resistance interface between the cathode material and the can, while at the same time protecting the material of the can from corrosion. Relevant prior work done to provide an electrically conductive layer on the interior surface of the cathode can is evidenced by the following review of known prior art: WHITBY et al., U.S. Patent No. 2,806,078, issued September 10, 1957, discloses a cylindrical dry cell battery in which the inner can wall and the adjacent anode surface are coated by a layer of electrically conductive "grease" - which is a dimethyl βiloxane filled with silica. SUSOTΓΠJTΞ SHEET Thiβϊ "grease" provides a contact between the anode and the can. In order to uniformly distribute the coating on the surface of the can, the can is subjected to radio frequency heating. KHDDFF, U.S. Patent No.3,751,301, issued August 7, 1973, has provided an electrically conductive underlayer to a metal support body. This electrically conductive material is applied to the metal support body in admixture with a thermosetting resin. The thermosetting resin may be a water βnulβifiable epσxy resin, and is admixed with a conductive material such as carbon or graphite. After curing, a second coating which consists of t mixture of lead dioxide and a thermosetting binder is applied, and the structure is then used as a reserve battery electrode in an acid system. KUWAZAKI et al.. U.S. Patent No. 3,764,392, issued October 9, 1973, discloses a cell in which the metallic container is coated with a conductive mixture of a thermoplastic resin and graphite or acetylene black. This cell is said to have good performance characteristics, particularly when operating in a deep discharge mode to a high current load. None of the above United States patents have, however, provided a suitable coating for the interior surface of alkaline cells, particularly where the coating must provide high conductivity; i.e., a low resistance current path between the cathode material of the cell and the cell con¬ tainer; while, at the same time, also providing a coating which will withstand the rigors of manufacturing steps where the assembly of a cell is fully automated and is accomplished at very high speeds such any one assembly step may only take fractions of a second. Still further, it is not desirable to use a conductive coating which would occupy a significant volume, thereby reducing the volume within the can which is available for active electrode or electrolyte material. 'ΪTJ7S SHEET Japanese Patent Publication 48361-1983, published March 22, 1983 by SHINODA et al., discloses an alkaline dry cell battery where the interior surface of the cathode can is coated with a conductive coating which comprises polyvinyl isobutyl ether and carbon, which may be graphite or flake shaped graphite, and/or acetylene black. However, Shinoda et al. have provided a coating which is sticky to the touch, and which has rubber elasticity. This suggests that the electrically conductive layer on the interior surface of the can is easily scraped off when the cathode material is loaded into the can. Because of that problem, Shinoda et al. teach.that their cathode is packed into the can. However, that requires significant time during the manufacturing process, and cannot ensure consistent characteristics from cell to cell. Moreover, Shinoda et al. require that a further manufacturing step be taken: heating the container to dissolve the material of the electrically conductive layer after the cathode blend has been incorporated into the cell. The present invention has none of the shortcomings of Shinoda et al. In particular, the present invention provides a coating composition which achieves essentially the same beneficial results; that is, significantly reduce corrosion of the steel can and increased short circuit performance, with much lower cathode/can interface voltage drop. Moreover, the present invention provides a cell can having a coating on its interior surface, where the coating is hard and not subject to scraping. Thus, the coating will not be displaced during manufacturing. Furthermore, the coating of the present invention is substantially impervious to alkaline electrolyte, while at the same time having good electrical conductivity. C -T . T' TE SHEET With the use of the conductive coating of this invention, the contact resistance between the can an . the cell depolarizer may be initially in the;-range of froβ zero to twenty milliOh β when measured at room temperature, and after storage (even under extreme conditions of temperature) the contact resistance between the can and the cell depolar¬ izer may increase only minimally; i.e., from zero to four times the initial resistance. Alternative methods of placing the cell depolarizer i -a; can, apart from pressing depolarizer pellets which are in inter¬ ference fit with the can, but which will ultimately result in the same characteristics as discussed immediately above, include placing loosely fitting pellets into the can and then recompacting them by forcing a rod into the central portion of the cathode pellets and then applying compacting pressure against the pellets so as to recompact them and spread the cathode material outwardly; or by extruding the cathode material around and past a rod placed in the center of the can, so as to ensure that, the length of the can which is intended to be filled with cathode or cell depolarizer material is substantially completely filled with that material. It is characteristic of the present invention that the hard coating, once formed on the interior surface of the cathode can, has substantially no tendency to swell in the presence of alkaline electrolyte. Therefore, efficient employment of the interior volume capacity of the can may be achieved. It has been found, unexpectedly, that certain carbon based lacquers such; as several commercially available materials said to be semi- conductive, and intended for uses entirely different from the present purpose, are suitable for the purposes of the present invention. They include a product marketed by W. . Grace & Co., in association with the trademark "ECCOCOAT 257", a nitrocellulose lacquer containing carbon __.„. .- ιj ε SHEET _7 particles, and another marketed by Acheson Industries, Inc. in association with th* trademark lE ECTRODAG 109", a PVC resin in methyl isobutyl acetone, also containing carbon particles. Generally, however, it can be said that coating compositions useful in the present invention includes carbon particles such as graphite particles, carbon black, acetylene black, and mixtures thereof carried in a binder with a volatile carrier, where the carrier is such that it will evaporate at room temperature, where the binder is such as to form a bard coating, over a surface onto which it has been applied after the carrier has evaporated, and where the hard coating is substantially impervious to alkaline electrolyte and is electrically conductive. The coating composition may also include, as a conductive component thereof, metal particles such as nickel or silver. The conductive particles should have a maximum size of about 10, preferably 5, microns. The binder may be a nitrocellulose lacquer, polyvinyl chloride (PVC), nitrile rubber, or other fortified organic polymer which will form a hard, adherent coating; and the coating .composition will be admixed with a diluent such as butyl acetate before it is applied, or it may include methyl ethyl ketone, methyl isobutyl ketone, or other similar diluent. BRIEF DESCRIPTION OF THE DRAWINGS: The above features and advantages of the present invention are more fully described hereafter, and a typical preferred embodiment is illustrated in the accompanying drawings, in which: Figure 1 is a vertical cross section of a typical alkaline cell according to the present invention at a stage during its manufacture when the cell depolarizer has been inserted into the container; and Figure 2 is a horizontal cross section of the cell of Figure 1. S* T:TUT SHEET DESCRIPTION OF THE PREFERRED EMBODIMENTS: As noted above, it is the principal purpose of this invention to provide a cathode container having a coating on at leaβt the major portion of its.interior surface* whereby the operating characteristics of the cell experience no significant deterioration following storage, either at the time when, the cell goes into the hands of the consumer who has purchased it, or later. The-present invention also provides a method for making an alkaline cell-,, at.least to the stage where at least the cell depolarizer is inserted, into the cathode can of the cell. Referring to Figures 1 and 2, a typical but exemplary configuration is shown of a portion of a cell 10, which comprises a container or can 12 which.may be formed of such material as steel, and may be stamped or drawn from:that material. The material ' of the can 12 may be plated with nickel or nickel alloy, at leaβt on the interior surface thereof. In accordance with the present invention a coating 14 covers the in erior surface of can 12. Also within the cell 10 is a cell depolarizer 16 which, has an opening 18 in its center for the insertion of the other electrode material, a current collector, and so on. The precise details of the- assembly of the cell are not relevant to the present invention. The can 12 may be formed with a plurality of ridges 20, each of which extends vertically for substantially the entire height of the can, the ridges being spaced circumferentially around the can. In a "D n size cell, the ridges might advantageously have an inwardly extending dimension of about 0.032 inches. SUBSTITUTE SHEBT As noted, the coating composition of the present invention has a carrier,which will evaporate at room temperature. When the volatile carrier has evaporated, a hard coating remains, and that coating has the general characteristic of a matrix which remains and is firmly bonded to the material of the can. The conductive component of the coating composition is securely retained in place in the interstices of the matrix; so that a contiguous, conductive, hard coating is formed, which coating is substantially impervious to the alkaline electrolyte - which may be potassium hydroxide and may have zinc oxide admixed thereto - and which has no tendency to swell in the presence of the alkaline electrolyte. However, because the hard coating has no tensile strength, it must be put in place after the container or can is formed. That may be accomplished, for example, by any of the following steps: (a) the can may be dipped into a bath of coating composition and withdrawn therefrom, so as to leave a residue of coating composition within the can; (b) the can may be filled with coating composition and then spilled, so as to leave a residue of the coating composition within the can; or (c) the interior of the can may be sprayed with the coating composition, and any residue may be permitted to run out from the can. (d) in a less preferred embodiment, the interior of the can may be brushed with the coating composition. Thereafter, the volatile solvent of the coating composition is evaporated, such as at room temperature for at least three hours, or at an elevated temperature of 55 to 90°C for at least 0.2 to 2 hours. SUBSTITUTE SHEET Alternative methods of applying the coating composition to the interior of the can include preheating the coating composition to 25 to 45°C. and spraying it into the can. In yet another coating method, the cana themselves may be preheated to between 50 and 150°C, with the coating composition being between 15 and 45°C. The coating composition is then sprayed,into the cans, which are then air dried at room temperature for at least 15 seconds. During that period of time, the volatile solvent is driven.off, and the can cools down at least to some extent. The:initial contact resistance between the can and the cell depolarizer 16. may be measured and may be found to be initially in the range. 1 of zero* (that is, below the measurement sensitivity of the instrument being used) up to about 20 milliOhms - usually in the range less.-than 4 milliOhms. Then, following storage under various conditions, such as from two weeks to fifty-two weeks at room temperature, two weeks at 55T°C, or one week at 71°C, tests have shown that the contact resistance between the can and the cell depolarizer will have increased not more than fourrtimes its initial resistance. Thus, even after storage under adverse conditions, the contact resistance between the can and the cell depolarizer may be in the range of from substantially zero up to 80 milliOhms at the worst. Similar uncoated cans - but having a nickel plating on their interior surface - have been tested under similar conditions using identical testing equipment, after they were stored in exactly the same conditions, and have demonstrated increases in contact resistance on storage up to 200 milliOhms or more. The thickness of the hard coating, once it has been placed and cured, may be 0.0002 to 0.003 inches, typically 0.0004 to 0.001 inches. Such thin coatings have no significant effect on the internal volume of the can; and since the coating shows no tendency to swell in the presence of alkaline electrolyte, there is no necessity for permitting additional SUBSTITUTESHEE volume,within the container to accommodate such swelling. This permits the*addition of more active material to the cell, thereby giving it longer life as well as better storage characteristics. The coating composition may be admixed with butyl acetate over a range of ratios of composition to butyl acetate of from 1:8 to 2:1. The choice offthe mixing ratio depends on such characteristics as the initial characteristics of the coating composition as it has been manufactured or purchased, the speed of the manufacturing line and -the method by which the coating composition will be applied to the interior surfaces of the cans, the temperature and rate at which the coating composition will be cured, and the size of the cell container (large or small). EXAMPLE Alkaline cells are made in accordance with standard practice, but in which the cell containers are made in accordance with this invention. The inside of each container is coated by spraying a composition of butyl acetate, nitrocellulose and carbon black particles thereon and permitting the butyl acetate to evaporate. The composition contains 80 percent by weight of butyl acetate and 20 percent solids. The solids are constituted by carbon black and nitrocellulose in a ratio of 2 to 1. The particle size of the carbon black is substantially all less than 5 microns. The resulting cell cotainers have substantially continuous hard, conductive coatings thereon of a thickness on the order of one-half mil. Representative test results have demonstrated the following: In one series of tests, control (i.e., uncoated) cells stored for six weeks at room temperature have shown an average increase in internal cathode/can contact resistance of 40 milliOhms, which would result in a loss of 16 millivolts of terminal voltage at a cell current into a ; ιι i w ; » • .-_ • C_-_-i_-Ξ-T relatively heavy load of 400 milliAmps. Coated cells, made in accordance with: he the Example, and stored under the same conditions as the control cell's, shewed an average increase of internal resistance of zero, and therefore no measurable loss in terminal voltage of the cell even into a 40D:milliAmp load. Likewise, control cells stored for two weeks at 55°C showed an average increase of internal resistance of 85 milliOhms, for a loss of terminal voltage of 34 millivolts into a 400 milliAmp load; whereas cells made in accordance with the Example, and stored under the same conditions, showed anr.average increase in internal resistance of 4 milliOhms for a loss of terminal voltage of 1.6 millivolta into a 400 milliAmp load. Other cells in sizes ranging from "AA" to "D", following storage for two. weeks at 55°C, showed improvements in operating characteristics of cufcoff voltage into various loads of up to 22X. Moreover, "AA" cells into a photoflash load showed improvements of 20% in terms of the number of flashes pemitetd, and 50Z recovery time after the fifth flash, as compared to control cells. The short circuit current of various cells were tested following differing storage conditions, against control cells. For example, test "D" cells showed no significant change of average short circuit current for cells according to this invention, after various storage conditions; so that cells stored at 55°C for two weeks, and an average short circuit current of 19.3 milliAmps, and cells stored at 71°C for one week had an average short circuit current of 19.4 milliAmps. Control "D" cells showed a decrease of average short circuit current to 14.0 milliAmps for cells stored at 55°C for two weeks, and to 11.0 milliAmps for cells stored at 71°C for one week. "C" cells showed an average short circuit current of 14.0 milliAmps for cells stored at 55°C for two weeks; whereas the control cells dropped from 9.9 milliAmps initially to 8.3 milliAmps following storage. Substantially similar results were obtained with "AA" cells. ' ' The benefits of a hard coating which is impervious to alkaline electrolyte, and which improves the internal contact resistance of alkaline cells, have been fully discussed and clearly demonstrated by the above. The fact that the coating is a hard coating precludes the possibility that the coating will be scraped in any substantial amounts into the bottom of the cell container when the cell depolarizer is inserted into it; and it also provides for much easier can storage where the cans may be stored in bulk containers without having to worry about the possibility of the coating on the inside of the cans running during storage. Various specific examples of coating composition have been provided, .but it is shown that in all events the coating composition includes at least carbon particles and may include additional conductive particles, carried in a binder with a volatile carrier. In general, given that the contents of an alkaline cell; that is, the amount of cathode material, anode material, electrolyte, the separators, and the cell construction including the can material, the seal, and the method of the cell construction, are constant between cells according to this invention and control cells or cells that are presently available, with the only difference being the addition of the coating composition and the presence of the hard coating on the interior surface of the cathode container in keeping with this invention, it follows that for the most part the total capacity in milliAαp-hours of cells according to this invention and ordinary cells is essentially the same. However, cells according to this invention have shown a higher initial current, higher terminal voltage on load, with a higher short circuit current. The cells provide a higher average current into a constant resistance, although perhaps for a slightly shorter period of time due to the maximum milliAmp hour capacity of the cell; but they provide better service hours for cells working into a constant current load, and a much shorter recharge time for cells operating with a photoflash load. Several examples of cell testing have been described and discussed, and a typical construction which is exemplary and not intended to limit the present invention, has been indicated. The scene of the present invention is defined by the appended claims. , a ύ ϊ < tl Λm w_ --- ET

Claims: WHAT IS CLAIMED IS: 1. A cathode container having a coating on at least the major portion of the interior surface thereof, said coating being a hard coating including conductive particles carried in a binder, said coating being electrically conductive and substantially impervious to alkaline electrolyte and having substantially no tendency to swell in the presence of alkaline electrolyte, wherein the contact resistance between said can and a cathode depolarizer tightly fitted therein will initially be not greater than 20 milliOhms when measured at room temperature. 2. The container of claim 1, where said binder is a nitrocellulose lacquer or a polyvinyl chloride resin. 3. The container of claim 2, wherein said coating is formed from a composition having the binder and conductive particles suspended in a volatile carrier from which the volatile carrier has been removed. 4. The container of claim 3, wherein said composition is a commercial composition sold as ELECTRODAG * 109 or ECCOCOAT * 257. 5. The container of claim 3 wherein said volatile carrier is butylscetate, methyl isobutyl or methyl ethylketone. 6. The container of claim 1, wherein said conductive particles are selected from the group consisting of graphite particles, carbon black, acetylene black, and mixtures thereof with each other or with metal particles. -T ' .Ts . i3-#- SHEET 77.. An electrochemical cell having a cathode container as defined in claim.1,, a cathode depolarizer tightly fitted therein, a separator, an anodej,and an alkaline electrolyte. 8. A method of preparing an alkaline cell, wherein at least a portion of- the cell depolarizer is inserted into a formed sheet steel can, characterized by the steps of: coating at least a major portion of the interior surface of the formed can with a coating composition which includes carbon particles carried,in a binder with a volatile carrier; evaporating the volatile carrier to leave a hard, conductive coating s on. the inside surface of said can, which coating is substantially impervious to alkaline electrolyte; and placing a depolarizer into said can in tight fit with said can, inserting: a separator, adding an anode and an anode current collector, and sealing the container. 9.. The method of claim 8, where the coating composition is applied to at least: the inside surface of the can by one of the following steps: (a) dipping the can into a bath of coating composition and withdrawing it from the bath so as to leave a residue within the can; and withdrawing it from the bath so as to leave a residue within the can; (b) filling the can with coaing composition and then spilling coating composition from the can so as to leave a residue within the can; (c) spraying the interior of the can with coating composition at room temperature; or (d) spraying the interior of the can with coating composition which has been re-heated to 25 to 45°C. SUBSTITUTESHEET 10. The method of claim 9, where the coating composition is applied to the inside surface of the can after the can has been pre-heated to a temperature between 50 and 150°C and wherein the coating is sprayed into the c'an at a temperature between 15 and 45°C; and the can is then allowed to air dry at room temperature for at least 15 seconds. S ' J23-πTU7_: SHEET

Inventors: MOHANTA SAMARESH, MOHANTA, SAMARESH
Applicants: DURACELL INT, DURACELL INTERNATIONAL INC.
Application year: 1986
Main classification: H01M2
IPC codes: H01M2