GB1558725A

GB1558725A - Secondary batteris having a zinc negative electrode

Info

Publication number: GB1558725A
Application number: GB5553/77A
Authority: GB
Original assignee: Energy Research Corp
Current assignee: Fuelcell Energy Inc
Priority date: 1977-01-13
Filing date: 1977-02-10
Publication date: 1980-01-09
Also published as: JPS5388133A; JPS6148220B2; FR2377710A1; DE2707051B2; DE2707051C3; DE2707051A1; FR2377710B1

Description

(54) SECONDARY BATTERIES HAVING A ZINC NEGATIVE ELECTRODE (71) We, ENERGY RESEARCH CORPORATION, a corporation organized under the laws of the State of New York, United States of America, of 3 Great Pasture Road, Danbury, Connecticut 06810, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to secondary batteries having negative electrodes including zinc material.

Silver-zinc, nickel-zinc and oxgen(air)zinc alkaline batteries have not, in embodiments known prior to the invention in the above-identified patent application, satisfied the need of the battery industry to provide a secondary battery of these types suitable for use in powering electric vehicles and in end uses of like demand. These uses require deep (65%) discharge-recharge cycling capacity in the order of three hundred cycles or more, a figure of merit not attained in such previously known zinccontaining secondary batteries.

The inability of previous known zinc alkaline batteries to meet such cycling capability is believed to be based principally on limitations of their negative electrodes.

Thus, zinc active material electrodes known prior to the invention fail to provide repetitively uniform surface adherence for zinc electro-reduced from the solid zinc compound state or deposited thereon from the electrolyte. This observation is appreciated in some measure in Morrison United States reissue patent Re. 13,174 wherein it is noted, as introduction to the invention therein, that electrodes not originally including zinc active material fail to retain zinc deposited thereon from alkaline electrolytes containing zinc on repetitive discharge-recharge cycling. while some improvement in short term cycling is noted in this reissue patent on pre-plating cadmium or silver on the electrode and amalgamating it to form a surface suitable to receive the zinc, it is reported that electrolyte must be employed with such modified electrode in quantity exceeding that necessary to do the work involved. In its solution to the problem, this reissue patent departs from such use of cadmium plating, stated to be impractical, and discloses a mechanical electrode structure involving multiple wire screens to provide a retention bed for zinc active material. West German Offenlegungsschrift 2,601,571 discloses a secondary battery in which cycling capacity is said to be increased by preventing the dissolution in the electrolyte of the discharge product of the zinc electrode. This is done by incorporating calcium oxide and/or calcium hydroxide in a negative electrode comprising a fluorinecontaining synthetic resin, at least one of bismuth oxide, bismuth hydroxide, cadmium oxide and cadmium hydroxide, and zinc, zinc oxide or zinc hydroxide. The calcium oxide and/or hydroxide forms a precipitate of calcium zincate upon discharge. The use of a calcium compound has certain disadvantages. Thus the formation of a precipitate interferes with the ability of zinc to deposit upon the negative electrode on recharging. Also the size of the electrode has to be increased to accommodate a given quantity of zinc.

The present invention provides a secondary battery having a liquid electrolyte, a positive electrode comprising an electrochemically active material and a negative electrode comprising electrochemically active zinc material and particulate cadmium material dispersed therein, said negative electrode containing essentially no calcium compound capable of forming a precipitate with electrolytesoluble zinc material, said battery being constructed with said positive electrode active material and said zinc material in quantities such that said positive electrode active material is electrochemically completely depleted before electrochemical depletion of said zinc material in discharging said battery, any particulate cadmium material other than cadmium metal being converted to cadmium metal upon initial charging of the battery and remaining electrochemically inactive with respect to said positive electrode active material in such battery discharging.

Batteries of this type exhibit improved adherence for zinc deposited thereon from an electrolyte to meet present demands for extended deep discharge-recharge cycling capacity in secondary batteries.

The terminal voltage of batteries according to the invention is established in the course of discharge by the zinc material and positive electrode active material and does not exhibit a voltage step attributable to electrochemical involvement of the positive electrode active material and the cadmium metal formed upon initial charging. Secondary batteries in accordance with the invention exhibit deep discharge-recharge cycling capacity in excess of three hundred cycles. It is believed that the particulate cadmium greatly improves electrode surface morphology to retain zinc electroreduced from the solid zinc compound state or deposited thereon from solution in the electrolyte and provides the basis for the improved secondary battery performance.

The foregoing and other features of the invention will be evident from the drawings and from detailed description of preferred embodiments of batteries and electrode structures set forth below.

In the drawings: Fig. 1 shows typical discharge curves for a silver-zinc alkaline secondary battery constructed in accordance with the invention.

Fig. 2 shows curves indicative of amperehour capacities of a battery constructed in accordance with the invention and a comparison battery constructed otherwise.

In a particularly preferred embodiment of a battery in accordance with the invention, the negative electrode comprises a mixture of zinc oxide, particulate cadmium oxide and a binder therefor, such as polytetrafluoroethylene (PTFE). The mixture is kneaded to a doughlike consistency with the assistance of a nonaqueous lubricant. The kneaded mixture is then rolled to form continuous sheets in the range of thickness of from 10 to 100 mils, and preferably from 30 to 50 mils. The lubricant is next removed from the sheet, providing a flexible porous sheet of PTFE containing zinc oxide cadmium oxide.

Cadmium oxide and PTFE are now separately mixed and this mixture is kneaded to a dough-like consistency with the assistance of a non-aqueous lubricant.

The kneaded mixture is then rolled to form continuous sheets in thxe range of thickness of from 2.0 to 20 mils, and preferably from 2.0 to 5.0 mils.

A sandwich negative electrode structure is now fabricated, first by applying the cadmium oxide-PTFE sheet, made as above-discussed, to each of the opposed sides of a metallic foil current collector, for example, a solid copper foil of 2.0 mils thickness and then by applying the zinc oxide-cadmium oxide-PTFE sheet, made as above-discussed, to the exposed surface of each of the cadmium oxide-PTFE sheets on the current collector.

In the foregoing practice, the cadmium oxide may be a commercially available powder known as ASARCO cadmium oxide. The zinc oxide may be the commercially available powder known as New Jersey zinc oxide, designated U.S.P.

12. The PTFE may be a powder commercially available as Dupont "Teflon" powder No. 6C ("Teflon" is a Registered Trade Mark). A suitable solvent is commercially available as Shell Sol V, a petroleum solvent having a gravity of 70.8 APT (aniline point temperature), an aniline point of 129"F, and a composition by volume of 65.5 per cent paraffins, 32 per cent naphthene and 2.5 per cent aromatics.

Such solvent is used in amount 50 to 90 percent by weight of the total weight of the solids in the mixtures.

Cadmium oxide is preferably employed in the electrode structure in an amount by weight of from 1.0 to 10 percent of the weight of the zinc active material. In the particular practice under discussion, the cadmium oxide is 5.0 per cent by weight of the total weight of the solids in the zinc oxide-cadmium oxide-PTFE sheets and the PTFE weight is 2.5 per cent of the total weight of the solids in these sheets. An equal amount by weight of cadmium oxide is employed in the cadmium oxide-PTFE sheet, with the PTFE weight thereof being 2.0 per cent of the cadmium oxide weight.

Depending on selected thickness of the latter sheets, the weight of cadmium oxide therein may be as low as 15 per cent by weight of the cadmium oxide in the zinc oxide-cadmium oxide-PTFE sheets. As mentioned above, the total weight of the cadmium addition to the entire electrode structure is preferably from 1.0 to 10 per cent of the weight of the zinc active material.

To attain preferred surface morphology for the negative electrode structure, the particle size and surface area of the cadmium oxide is desirably selected such that the metallic cadmium particles situate in the electrode structure upon conversion of the cadmium oxide exhibit particle size not greater than 10 microns and have surface area not less than 1.0 square meters per gram. Metallic cadmium powder may be employed as the starting material in place of cadmium oxide, in which case starting material particle size and surface area selection are made directly within the above-stated limits. Where either cadmium oxide powder or cadmium hydroxide powder, both less dense than metallic cadmium powder, is employed as the starting material, particle size and surface area parameters for the starting material are increased by from 20 to 30 per cent above from the stated limits to ensure that the metallic cadmium particles in the electrode structure upon conversion of the starting material cadmium compound exhibit such preferred particle size and surface area.

Electrode structure so fabricated, with zinc oxide active material in the amount of 130 grams, is employed as the negative electrode in an alkaline battery having as its positive electrode a silver plate containing 140 grams of silver active material. An aqueous electrolyte having 40 per cent potassium hydroxide is employed and the negative and positive electrode are separated by three layers of commercially available Union Carbide fibrous sausage casing cellulosic separator material. The battery (Cell 1) is subjected to dischargerecharge cycling wherein it is discharged at 8.0 amperes for 5.0 hours to remove 40 ampere-hours of the cell capacity and is then charged at 2.5 amperes to 2.0 volts.

In initial charging, the cadmium oxide is converted to cadmium metal since it is electrochemically more positive than zinc.

The cadmium metal is thereafter inert since it does not participate in electrochemical oxidation or reduction in the potential region for oxidation and reduction of the zinc active material during battery discharging.

The electrochemical equations applicable during discharge are: 2AgO+H2O+2eAg2O+2OH (1) Ag2O+H2O+2e2Ag+2OH (2) Zn+2OH- < Zn(OH)2+2e (3) As is seen in Fig. 1, which shows typical discharge curves for a battery having a negative electrode constructed in accordance with the invention, equations (1) and (2) apply initially in the course of battery terminal voltage drop from 1.8 volts to 1.5 volts with equation (3) applicable thereafter. The discharge cycle in Fig. 1 is terminated when 40 ampere-hours are removed. The two discharge cycles illustrated, i.e. at five cycles and at two hundred cycles, are substantially identical.

Equations (l)-(3) are reversible, the reversed version thereof being applicable during charging.

Referring to Fig. 2, the solid line curve therein shows the ampere-hours capacity of the above-discussed battery in the course of cycling thereof in excess of three hundred cycles. The battery yields its rated 40 ampere-hours throughout such cycling.

Initially and after each of the first, second and third hundred cycles, the battery is maintained on discharge until its terminal voltage drops below 1.5 volts to 1.3 volts, i.e.

by continuing to draw 8.0 amperes therefrom for about 0.5 hour beyond the normal 5.0 hour discharge time. On initial discharge, the battery yields 44.5 ampere-hours. After the one-hundredth cycle, 43.5 ampere-hours are drawn from the battery on such discharge to 1.3 volts. After the twohundredth and three-hundredth cycle, 41.5 and 42 ampere-hours, respectively, are removed on such discharge to 1.3 volts, with the battery continuing to provide its rated capacity in full during subsequent typical discharge cycles.

To provide a basis for evaluating the performance of this cadmium-additive battery, Fig. 2 further shows a broken line curve illustrating the discharge cycling capacity of a comparison battery differing from the cadmium-additive battery solely in respect to its negative electrode. The comparison battery negative electrode is prepared from a mixture containing 2.0 percent by weight of mercuric oxide, 2.5 percent by weight of PTFE and the remainder zinc oxide. With addition of a lubricant, as above-discussed in preparation of the negative electrode structure of the cadmium-additive battery, this mixture is kneaded and rolled to a thickness of 30 to 50 mils and pressed onto a solid foil copper collector.

With an initial ampere-hour capacity as shown in Fig. 2, the comparison cell is subjected to the same discharge-recharge cycling as in the cadmium-additive battery, except for the practice of discharge of the latter to 1.30 volts initially and then after each one hundred cycles. As shown in Fig.

2, the comparison battery ampere-hour yield deteriorates continuously throughout such cycling, the battery yielding but twenty ampere-hours after somewhat less than one hundred and fifty cycles.

The inability of the comparison battery to provide its rated ampere-hour output on repetitive discharge and recharge is attributable to the inability of its zinc electrode to provide suitable adherence for zinc electroreduced from the solid zinc compound state or deposited thereon from the electrolyte. In the cadmium-additive battery constructed in accordance with the invention, cadmium metal is believed to be situate in the negative electrode in manner and form providing conductive nucleation sites for uniform zinc electroreduction or deposition from solution as evidenced by the results attained in Figs. I and 2. These results are believed to be due to the particulate cadmium metal which is present in the electrode structure in dispersed particulate configuration as contrasted with electroplating or electrodeposition thereof on the electrode structure as alluded to in the Morrison reissue patent abovediscussed. In the multiple layer electrode structure of the invention considered to this juncture, the addition of cadmium to the current collector by laminating the collector with the PTFE-cadmium oxide sheets provides electrically conductive nucleation sites for uniform zinc deposition on the collector. The addition of cadmium to the zinc active material by dispersion of particulate cadmium therein I provides electrochemically inert elEctrically conductive nucleation sites for individual zinc particles.

The preferred surface morphology of the electrode structure in accordance with the invention may be provided by practices other than that providing the' foregoing PTFE-bound multiple layer cofiguration.

Thus, cadmium/cadmium compound powder having the above-discussed surface area and size characteristics maybe applied by doctor blade or otherwise evenly mechanically spread onto the current collector and the zinc oxide powder pressed over the cadmium/cadmium con3pound. On initial charging, the cadmium metal is then sufficiently attached to both the current collector and the zinc oxide totprovide an electrode structure having . improved adherence for zinc electroreduced from the solid zinc compound state or deposited from solution.

While particulate cadmium metal, cadmium oxide, cadmium sulfate and cadmium hydroxide and mixtures thereof are among cadmium materials:employable in practicing the invention, cadmium oxide powder is preferred since it yields the highest surface area for the cadmium metal additive and is more economical than the other referenced cadmium materials.

As noted above, the cadmium material of the negative electrode is electrochemically inactive with respect to the active material of the positive electrode during battery discharge. This aspect of the invention is achieved by preselecting the quantities of zinc material and positive electrode active material such that the latter is electrochemically depleted in discharge prior to electrochemical depletion of the zinc material. This practice will be further understood from the additional examples now set forth.

A silver-zinc cell (Cell 2) is fabricated with a plurality of positive and negative plates such that the theoretical positive capacity is 77 Ah and the theoretical negative capacity is 122 Ah. The negative plates contain the cadmium additive at a theoretical capacity of 6.7 Ah. The cell is filled with 45% KOH and initially charged at 4 amperes to an end voltage of 2.02 volts with a resultant input of 62 Ah. The cell is then discharged at 8 amperes to a final voltage of 1.30 volts with a resultant output of 53 Ah. A zinc wire reference electrode inserted in the cell shows that the positive plate capacity limits the discharge at 1.30 volts and is depleted.

These results show that there is a reserve capacity of 9 Ah of charged zinc and 6.7 Ah of charged cadmium remaining at the end of the cycle. On subsequent cycles the cell is discharged at 8 amperes for 5 hours which removes 40 Ah per cycle, increasing further the charge reserve of zinc to 22 Ah while the 6.7 Ah of charged cadmium remains constant.

A nickel-zinc cell (Cell 3) is fabricated with a plurality of positive and negative plates such that the theoretical positive capacity is 38 Ah and the theoretical negative capacity is 80 Ah. The negative plates contain the cadmium additive at a theoretical capacity of 4.4 Ah. The cell is filled with 35% KOH containing 1% LiOH and initially charged at 2.5 amperes for 17 hours with a resultant input of 42.5 Ah. The cell is then discharged at 6 amperes to a final voltage of 1.35 volts with a resultant output of 32 Ah. A zinc wire reference electrode inserted in the cell shows that the positive plate capacity limits the discharge at 1.35 volts and is depleted.

These results show that there is a reserve capacity of 10.5 Ah of charged zinc and 4.4 Ah of charged cadmium remaining at the end of the cycle. On subsequent cycles the cell is discharged at 6 amperes for 4.17 hours which removes 25. Ah per cycle, increasing further the charge reserve of zinc to 17 Ah while the 4.4 Ah of charged cadmium remains constant.

An air-zinc cell (Cell 4) is fabricated with 2 oxygen (air) electrodes and one zinc electrode containing 25 Ah of theoretical zinc capacity. The zinc electrode contains the cadmium additive at a theoretical capacity of 1.4 Ah. The cell is filled with 35% KOH and initially charged at 1.25 amperes for 16 hours with a resultant input of 20 Ah. The cell is then discharged at 4 amperes to 1.00 volts with a resultant output of 16 Ah.

These results show that there is a reserve capacity of 4 Ah of charged zinc and 1.4 Ah of charged cadmium remaining at the end of the cycle. On subsequent cycles the cell is discharged at 4 amperes for 3 hours which removed 12 Ah per cycle, increasing further the charge reserve of zinc to 8 Ah while the 1.4 Ah of charged cadmium remains constant.

WHAT WE CLAIM IS: 1. A secondary battery having a liquid electrolyte, a positive electrode comprising an electrochemically active material and a negative electrode comprising electrochemically active zinc material and particulate cadmium material dispersed therein said negative electrode containing essentially no calcium compound capable of forming a precipitate with electrolytesoluble zinc material, said battery being constructed with said positive electrode active material and said zinc material in quantities such that said positive electrode active material is electrochemically completely depleted before electrochemical depletion of said zinc material in discharging said battery, any particulate cadmium material other than cadmium metal being converted to cadmium metal upon initial charging of the battery and remaining electrochemically inactive with respect to said positive electrode active material in such battery discharging.

2. A battery according to claim 1, wherein said positive electrode active material is a silver material.

3. A battery according to claim 1, wherein said positive electrode active material is a nickel material.

4. A battery according to claim 1, wherein said positive electrode is an oxygen electrode.

5. A battery according to claim 1, 2 or 3, wherein said particulate cadmium material is selected from (a) cadmium metal having particle size of not greater than 10 microns and surface area not less than 1.0 square meters per gram, (b) a cadmium compound electrochemically convertible to cadmium metal having particle size of not greater than 10 microns and surface area not less than 1.0 square meters per gram and (c) mixtures of said cadmium metal and said cadmium compound.

6. A battery according to claim 5, wherein said particulate cadmium material is cadmium oxide.

7. A battery according to claim 6, wherein said zinc material is zinc oxide.

8. A battery according to claim 7, wherein the negative electrode also comprises a binder for said cadmium oxide and said zinc oxide: 9. A battery according to claim 8, wherein said binder comprises polytetrafluoroethylene.

10. A battery according to any one of claims I ,to 5, wherein said particulate cadmium material is cadmium hydroxide.

11. A battery according to any one of claims 1 to 5, wherein said particulate cadmium material is cadmium sulfate.

12. A battery according to any one of the preceding. claims, wherein said negative electrode comprises an electrially conductive support, a first layer overlying said conductive support and comprising said particulate' cadmium material and a first binder therefor, and a second layer overlying said first layer and comprising said zinc material, said particulate cadmium material aird a second binder therefor.

13. A battery according to claim 12, wherein said first and second binders comprise pblytetrafluoroethylene.

14. A battery according to any one of the preceding claims, wherein the weight of said particulate cadmium material is from 1.0 to 10 per cedt of the weight of said zinc material.

15. A battery according to claim 1 substantially as described with reference to any one of yells 1 to 4.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. of charged cadmium remaining at the end of the cycle. On subsequent cycles the cell is discharged at 4 amperes for 3 hours which removed 12 Ah per cycle, increasing further the charge reserve of zinc to 8 Ah while the 1.4 Ah of charged cadmium remains constant. WHAT WE CLAIM IS:

1. A secondary battery having a liquid electrolyte, a positive electrode comprising an electrochemically active material and a negative electrode comprising electrochemically active zinc material and particulate cadmium material dispersed therein said negative electrode containing essentially no calcium compound capable of forming a precipitate with electrolytesoluble zinc material, said battery being constructed with said positive electrode active material and said zinc material in quantities such that said positive electrode active material is electrochemically completely depleted before electrochemical depletion of said zinc material in discharging said battery, any particulate cadmium material other than cadmium metal being converted to cadmium metal upon initial charging of the battery and remaining electrochemically inactive with respect to said positive electrode active material in such battery discharging.

7. A battery according to claim 6, wherein said zinc material is zinc oxide.

8. A battery according to claim 7, wherein the negative electrode also comprises a binder for said cadmium oxide and said zinc oxide:

9. A battery according to claim 8, wherein said binder comprises polytetrafluoroethylene.