METHOD OF FABRICATING IMPROVED SEPARATOR

Abstract: Method of fabricating a titanium and silver complex-impreg­ nated cellulosic separator for electrochemical cells involves impregnating a sheet of regenerated cellulose at elevated tem­ perature in an aqueous environment simultaneously with two different selected reagents, one containing silver and the other titanium. By the impregnation, silver and titanium are thus separately but simultaneously introduced into the cellulose to form a silver-titanium complex therewith, improving the oxidation resistance of the cellulose when the cellulose is used as a separator in an electrochemical cell. The sheet also exhibits improved resistance to the penetration of silver when incorpo­ rated into an electrochemical cell having silver as one of the electrodes, zinc as the other electrode and an alkali hydroxide as the electrolyte.

Description: Background of the Invention Field of the Invention The present invention generally relates to separators for alkaline batteries and more particularly to improved silverized and titanized cellulosic separators. Prior Art U.SO Patent No, 4,091,185, issued May 23, 1978, is directed to the preparation of separators for electrochemical batteries, such as silver-zinc batteries. In accordance with the disclosure therein, a silverized cellulosic separator is introduced into a silver-zinc ---battery; a titanium-containing compound is then added to the cell either by painting in onto the silverized cellulosic material or by adding it to the electrolyte or to the zinc electrode. It can also be added to a separate sheet placed between the separator and the electrode, which sheet functions as a negative inter separator. The titanium is then complexed with silver in the separator by subjecting the cell to a series of chargedischarge cycles. Although this procedure results in a satisfactory separator which is capable of inhibiting the migration of silver ions from the silver electrode through the electrolyte to the zinc electrode, thus inhibiting self-discharge and eventual cell circulating, the cell is not immediately fully protected against such migration since the titanium-silver complex is only generated as the cell is used. It would therefore be desirable to be able to provide an improved simplified method of silverizing and titanizing a cellulosic separator outside of a cell in order to form a titanium-silver complex therein before introducing the separator into the electrochemical cell. It would also be desirable if such method utilized readily available reagents to provide rapid and highly efficient results. Summary of the Invention The improved method of the present invention satisfies the foregoing needs. The invention is substantially as set forth in the Abstract above, It comprises a method wherein a sheet of cellulosic material to be used as a separator in an electrochemical cell is contacted in a treating zone outside of an electrochemical cell simultaneously with a plurality of reagents. One of the reagents contains a silverizing material and another reagent contains a titanizing material. The contact is maintained at a temperature and for a time sufficient to substantially introduce into the cellulosic material an effective concentration of each of silver and titanium to form Åan effective concentration of a titanium-silver complex in the sheet, Thereafter, the sheet is separated from contact with the reagents, then washed and dried. The finished separator can then be introduced into an alkaline electrolyte-containing battery having electrodes of silver and cadmin or nickel and cadmin or nickel and iron or nickel and zinc or silver and zinc or mercuric oxide and zinc or mecuric oxide and cadmin. In each instance, the separator effectively prevents migration of dissolved ions from one electrode through the electrolyte to the other electrode, therefore inhibiting selfdischarging and eventual short circuiting of the battery. In the case of silver and zinc electrodes, silver ions are inhibited from migrating to the zinc electrode. Moreover, rhe separator resists chemical degradation including oxidative attack. Since the method can be carried out rapidly and effectively in a separate bath outside the electrochemical cell, a fully operational, fully protective separator which includes the formed silver-titanium complex is introducible into the cell for maximum initial effectiveness, in contrast to silver-titanium complex-containing separators wherein the complex is generated in the electrochemical cell during operation of the cell. Further features of the present invention are set forth in the following description. Detailed Description In accordance with the method of the present invention, a cellulosic separator for use in electrochemical cells is selectively impregnated with two different reagents simultaneously to separately introduce silver and titanium into the cellulose and to form the same into a complex in the cellulose, to improve the oxidation resistance of the cellulose and the operating characteristics of the electrochemical cell. The cellulosic sheet which is impregnated is a flexible sheet or plurality of sheets having cellulosic surface components. For example, the cellulosic material can include regenerated cellulose produced by the xanthate process, by the viscose process, by denitration of cellulose nitrate, by the cuprammonium process or by deacetylation of cellulose acetate. It is preferred that the cellulosic sheet material consist of semipermeable regenerated cellulose produced by the viscose process. Material such as cellophane 150-PD (available from DuPont de Nemours & Co.) is preferred or a similar product sold under the designation PUDO-193. However, unplasticized cellulose film, clear cellulose casing and fibrous cellulose casing material are also desirable. The impregnation is carried out in any suitable treating zone outside of an electrochemical cell at concentrations, times and temperatures sufficient to result in a total concentration of silver and titanium in the cellulose of about 0.05 to about 15 weight percent. However, the complex preferably is in a concentration of about 2-11 weight percent, The silver is introduced into the cellulose by any suitable technique. The impregnating solution containing the silver preferably is in the form of an aqueous solution of silver nitrate wherein the concentration of silver nitrate in the water is about t percent by weight and wherein the solution also contains about 1 percent by weight of sodium acetate or the like. The cellulose is immersed in a tank, beaker or the like containing the aqueous silver nitrate-sodium acetate-containing water solution any suitable length of time, but preferably about 1-15 minutes while the solution is maintained at an elevated temperature of, for example, about 70-800C. It will be understood that, for example, potassium acetate can be substituted for the sodium acetate and that any water soluble silver salt can be used to supply the silver to the cellulose. For example, soluble silver compounds such as the following can be us used: silver-acetate, silver propionate and silver fluoride. In most instances such soluble salts are utilized in an aqueous solution with either sodium acetate or potassium acetate or a mixture thereof and the solution temperature is between about 50 C. and about 10000. The impregnating solution in the treating zone also contains a titanium compound which comprises titanium-containing material selected from the group consisting of organic titanate, titanium dioxide, alkali metal titanate and alkaline earth metal titanate. Preferably the titanium-containing material comprises potassium titanate, sodium titanate, magnesium titanate or titanium dioxide. The treating bath also contains an aqueous solution of alkali metal hydroxide, for example, potassium hydroxide, lithium hydroxide or sodium hydroxide in a suitable concentration, that is, containing preferably 30-35 weight percent of the hydroxide in the water, The titanium-containing material is dissolved in the aqueous hydroxide, preferably in a saturated concentration. The weight ratio of the hydroxide plus titanium-containing material to the silver salt plus acetate is preferably about 5:1. After the cellulosic material has been impregnated in the treating bath to the desired extent, so that a titanium-silver complex is formed and is attached to the cellulosic material in a concentration of approximately 2-11 percent by weight, depending upon the residence, time and other factors, the cellulosic material is then withdrawn from the treating bath, washed with water, preferably deionized water, and then dried and is ready for use. The titanium complex attaches to the cellulosic chain to provide the cellulosic material with improved oxidation resistance so as to inhibit the degradation of the material when using it as a separator in an electrochemical cell such as a silver-zinc battery. The cellulosic separator material is also utilizable with other types of alkaline batteries incorporating silver and cadmium electrodes, nickel and cadmium electrodes, nickel and iron electrodes, mercuric oxide and zinc electrodes and mercuric oxide and cadmium electrodes. The purpose of the separator is to physically isolate the two electrodes of the battery or cell from each other in order to prevent their contacting or contaminating each other, Contamination arises when salts of a metal of one of the electrodes such as silver salts are formed because of the solubility of the electrode in the alkaline electrolyte, which is usually highly concentrated potassium hydroxide. In the case of silver cells, silver salts are formed. Such salts can migrate through the electrolyte to the other electrode, causing the cell or battery to self-discharge and eventually short circuit. The separator retards this migration through the electrolyte through the separator toboth electrodes. Oxidation and chemical degradation of the cellulosic separator normally occurs because the effects of the concentrated alkaline electrolyteíand the oxidixing effect of the dissolved metal salt, silver salts in the case of silver electrode-containing cells. In silver electrode-containing cells, metallic silver and silver oxide are disposed in the separator. There is considerable evidence to substantiate the conclusion that the degradation of the separator occurs in the form of a redox reaction involving an oxidative attack on the cellulosic separator by the soluble silver salt whilethe silver deposition results in a complementary reduction of the silver salt to its lower valance states. The following specific example further illustrates certain features of the present invention. Example In a first series- (series A), cellophane (PUDO193 - available from E0I. du Pont de Nemours & CoO) was soaked in 100 F. water for approximately one minute and then transferred wet to a treating bath outside of an electrochemical cell. The bath contained an aqueous solution having a 30 percent by weight concentration of potassium hydroxide saturated with dissolved potassium titanate. This aqueous solution had previously been combined with a 1 percent solution of silver nitrate saturated with sodium acetate, the silver nitrate solution being in a weight ratio to the potassium hydroxide solution of approximately 1:5. The bath was maintained at about 70 C. and the cellophane was immersed in the bath for a period of 5 minutes. Thereafter, the cellophane was withdracnfrom the bath, washed free of treating solution by deionized water and dried. Parallel runs in series A were made utilizing in one case a residence time of one minute and in a second case a residence time of 15 minutes, A second series (series B) was run in the manner described above, utilizing residence times of 1,5 and 15 minutes and incorporating the procedure described above except that the concentration of potassium hydroxide in aqueous solution was 5 weight percent. In a third series (series C), utilizing residence times of 1, 5 and 15 minutes, the bath contained an. aqueous solution of potassium hydroxide wherein the hydroxide was in a concentration of 15 weight percent. Each silver-titanium complex impregnated cellophane sample prepared in accordance with the above was then tested by incorporating it into a test cell which consisted of four positive (silver) electrodes, each 1.875 in, wide x3.0O in. high and 0.027 in. thick and five negative (zinc) electrodes, each measuring 1.875 in. wide x 3.00 in. high and 0.042 in. thick, The cellophane was disposed between the electrodes and the resulting cell pack was placed in a plastic cell case which was then sealed with a cover terminal assembly after the cell case was filled with a 45 weight percent solution of aqueous potassium hydroxide. The cell was allowed to soak for 72 hours and then subjected to two formation cycles consisting of two consecutive charge-discharge cycles (charge at 0o6 amperes to 2.05 volts cut-off and discharge at 4 amperes to 1.00 volt cut-offO The capacity (AH) obtained from the cell on the second cycle discharge was noted, The cell was then placed on a continuous cycling regime (charge at 0.6 amperes to 2005 volts and discharge at 4 amperes to 1.0 volts). The electrical resistance and the physical characteristics including dry and wet thickness and appearance were also noted for each impregnated cellophane and are set forth in the following table. TABLE TREATMENT Ti-Ag ELECTRICAL THICKNESS SERIES RESIDENCE TIME KOH COMPLEX RESISTANCE Dry Wet CAPACITY CYLES TO SHORT (MINUTES) (Wt-%) (Wt-%) (M#/inê (Mils) (Mils) MAXIMUM MINIMUM A 1 30 3.6 20 1.2 1.2 5 30 4.4 20 1.2 2.4 15 30 10.6 21 1.2 2.4 B 1 5 2.0 18 1.2 2.8 5 5 3.0 17 1.2 2.8 15 5 4.3 19 1.2 2.8 C 1 15 2.2 22 1.1 2.7 5 15 4.8 21 1.1 2.8 15 15 8.8 20 1.1 2.8 From the information set forth in the Table above, it is apparent that the electrical resistance of the cellophane impregnated with the titanium-silver complex varies between about 17 and 22 milliohms per square inch. The wet thickness of each cellophane sheet was about 2.4 - 2.8 mils and the dry thickness was about 1.1 - 1.2 mils. All of the impregnated cellophane sheets performed satisfactorily as separators in the test cell. However, it was preferred to utilize a residence time of about 5 minutes along with a concentration of about 30 weight percent of potassium hydroxide in the treating bath. Under those conditions the titanium-silver complex was in a concentration of about 4.4 weight percent in the cellophane and exhibited an electrical resistance of about 20 milliohms. It had a capacity of and lasted approximately cycles until it shorted out. The performance of each of the silvertitanium complex-impregnated cellophane sheets as a separator was substantially improved over conventional unimpregnated and silver impregnated cellophane separators. Additional tests were run following the procedure set forth above but substituting magnesium titanate, titanium dioxide and various organic titanates for the potassium titanate. Further tests were run substituting sodium hydroxide for the potassium hydroxide and tests were also run substituting various fibrous regenerated cellulosic films and other cellulosic films for the cellophane. Finally, tests were run utilizing silver acetate and/or silver fluoride in place of silver nitrate and potassium acetate in place of sodium acetate in the treating solution. All the tests provided results comparable to those set forth above. The above Examples clearly illustrates that a suitable silver-titanium-containing complex can be formed in a single step in a cellulosic sheet outside of an electrochemical cell to cause the sheet to have improved electrical, electrochemical and physical properties as a separator when subsequently used in an electrochemical cell. Thus, the separator has improved resistance to physical deterioration and shorting out, among other properties. The method of formation involves the use of a single treating solution for a suitable residence time and temperature to achieve the desired result of in situ formation of the silver-titanium complex in the cellulose. Further features of the invention are as set forth in the foregoing. Various modifications, changes, alterations and additions can be made in the method of the present invention, its steps and parameters. All such modifications, changes, alterations and additions as are within the scope of the appended claims form part of the present invention.

Claims: CLAIMS: 1. A method of making a titanium and silver compleximpregnated cellulosic e.g. cellophane separator for electrochemical cells, wherein the method comprises impregnating a sheet of regenerated cellulose in an aqueous environment in a treating zone outside of an electrochemical cell simultaneously with separate selected reagents for a time and temperature sufficient to introduce silver and titanium separately into the cellulose, and wherein the silver and titanium form a complex in the cellulose to improve the oxidation resistance of the cellulose. 2. A method according to claim 1 wherein impregnation is carried out at a temperature between about 70 C and 800C. 3. A method according to claim 1 or 2, wherein the treating zone includes an aqueous solution of alkali metal hydroxide and a titanium-containing material selected from organic titanate, titanium dioxide, alkali metal titanate and alkaline earth metal titanate. 4. A method according to claim 3 wherein the titaniumcontaining material comprises potassium titanate and wherein the hydroxide comprises potassium hydroxide saturated with the potassium titanate. 5. A method according to claim 4 wherein the regenerated cellulose is first soaked in warm water and then, while wet, is transferred to the treating zone and wherein the potassium hydroxide is present in a concentration of about 30 percent by weight of the aqueous solution. 6. A method according to any of the preceding claims wherein one of the reagents is an aqueous solution of a water soluble silver salt. 7. A method according to claim 6 wherein the silver salt is silver nitrate. 8. A method according to claim 7 wherein the silver nitrate is present in the aqueous solution in a concentration of about 1 percent by weight, wherein the solution also contains an alkali metal acetate in a saturated concentration, and wherein the combined weight ratio of the alkali metal hydroxide and the titaniumcontaining material to the silver nitrate and alkali metal acetate is about 5:1. 9. A method according to any of the preceding claims 3 to 8 wherein the titanium-containing material comprises potassium titanate, wherein the hydroxide contains potassium hydroxide in a 30 percent by weight concentration in aqueous solution and wherein the potassium titanate is in a saturated concentration. 10. A method according to any of the preceding claims wherein the treating zone is maintained st a temperature of about 70 C. and the cellulose is trested in the treating zone with the reagents for about 5 minutes and thereafter removed from the treating zone and washed free of reagents.

Inventors: CHIREAU, ROLAND F.
Applicants: YARDNEY ELECTRIC CORPORATION
Application year: 1983
Main classification: H01M2
IPC codes: H01M2