Sealed zinc secondary battery and zinc electrode therefor

Abstract: A zinc negative electrode comprising a zinc active material, one of a Ba(OH)₂ and Sr(OH)₂ material and a conductive matrix including a metallic oxide which is more electropositive than zinc. The zinc negative electrode is incorporated into a zinc secondary battery having an electrolyte whose electrolyte constituent is a low percentage of the electrolyte. The zinc negative electrode is split into electrode assemblies separated by a porous hydrophobic element.

Description: This invention relates to rechargeable or secondary batteries and, in particular, to rechargeable or secondary batteries using zinc negative electrodes. Various types of zinc secondary batteries are known in the art. Typical batteries are Ni-Zn, Ag-Zn, Zn-O 2 and MnO 2 -Zn. Zinc secondary batteries employ zinc negative electrodes which exhibit a decay or reduction in capacity with the repetitive charge-discharge cycling of the battery. As a zinc secondary battery and, therefore, each of its zinc negative electrodes, is cycled, the zinc active material of each electrode becomes redistributed over the surface of the electrode. This redistribution of active material or so-called "shape change" of each zinc electrode is a result of the solubility of the zinc active material in the alkaline electrolyte of the battery and causes a reduction in the capacity of the battery. The solubility of the zinc active material in the battery electrolyte also causes an increase in the battery gas pressure due to the decreased availability of the zinc active material to recombine with oxygen generated during charging. It similarly leads to an increase in the rate of hydrogen evolution by the battery. Accordingly, a rapid buildup of gas pressure occurs in the battery, often requiring that the battery be vented. Constant venting of the battery, however, causes loss of water through electrolysis. This, in turn, causes dryout of the battery electrodes, reducing battery life. Over the years, many different zinc negative electrodes have been proposed to reduce electrode shape change. One such zinc negative electrode is disclosed in U.S. Patent No. 3,516,862 issued to W. Van der Grinten. In the '862 Patent, Ca(OH) 2 is added to the zinc active material (ZnO) to reduce the solubility of the active material through the formation of CaZn 2 (OH) 6 (calcium zincate). In U.S. Patent 3,816,178, Maki, et al. disclose a zinc electrode containing Ca(OH) 2 along with the addition of oxides'of lead. In U.S. Patent 4,418,130, Soltis, et al. disclose the use of Ba(OH) 2 octahydrate. However, in these electrodes, the formed calcium or barium zincates experience thermodynamic instability due to dissociation in the battery electrolyte and finite solubility as evidenced by improved but still low cycle life. FR 2,180,940 discloses the addition of Ca(OH) 2 , Ba(OH) 2 and/or Sr(OH) 2 to a negative electrode comprising zinc in order to avoid problems due to electrode shape change. US 4,292,357 discloses a zinc or zinc oxide anode having at the center of the anode a body of an inert adsorptive material which establishes an electrolyte reservoir within the anode itself; the reservoir is sandwiched between a three-layer laminate comprising a current collector, a zinc/zinc oxide active layer and a separator. The disclosed anode is said to display low shape change. By inclusion of mercuric chloride or oxide into the active material gas evolution inside the anode is controlled. Other sealed zinc batteries have been proposed to improve gas recombination in the sealed battery. In these batteries, the shape change in the zinc electrode is still a concern. It is, therefore, an object of the present invention to provide an improved zinc negative electrode and zinc secondary battery. It is a further object of the present invention to provide a zinc negative electrode and zinc secondary battery with reduced shape change and solubility of the zinc electrode and increased cycle life for the battery. It is a yet further object of the present invention to provide a zinc negative electrode and zinc secondary battery meeting the above objectives and having improved gas recombination, permitting the battery to be sealed and maintenance free. Summary of the Invention In accordance with the principles of the present invention, the above and other objectives are realized in a zinc negative electrode comprised of a zinc active material, one of (Ca(OH) 2 , a Ba(OH) 2 and Sr(OH) 2 material and a conductive matrix containing a metal oxide material which is more electropositive than the zinc active material. In order order to promote gas recombination, the zinc negative electrode is further configured as a split electrode with adjacent like electrode assemblies spaced by a porous hydrophobic element. Each electrode assembly includes an active element comprised of zinc active material, one of a Sr(OH) 2 , Ba(OH) 2 and Ca(OH) 2 material and a conductive matrix formed with a metallic oxide. One side of the active element abuts a metallic current collector element which, in turn, abuts a gas recombination catalytic element formed from a material more electropositive than zinc and having a chemical or electrochemical affinity for reacting with oxygen. The gas recombination catalytic elements of the electrode assemblies abut the hydrophobic element. The latter element and the electrode assemblies are enveloped by a separator material to complete the electrode. The zinc electrode is utilized with an electrolyte having a low concentration of electrolyte constituent and the electrode, electrolyte and a positive electrode are arranged in a container to form a zinc secondary battery. Preferably, the (Ca(OH) 2 , Ba(OH) 2 or Sr(OH) 2 material of the zinc negative electrode is in the range of 15-40 percent of the weight of the electrode and the metallic oxide material is in the range of 5-20 percent of the weight of the electrode. Also, preferably, the electrolyte contains an electrolyte constituent which is in the range of 5-20 percent of the electrolyte. With the above construction of the zinc negative electrode and the zinc secondary battery, the shape change of the zinc negative electrode is reduced due to the production of calcium, barium or strontium zincate which remains thermodynamically stable and substantially insoluble as a result of the low concentration of electrolyte. Moreover, the zinc negative electrode retains its conductivity in spite of the low concentration of electrolyte constituent due to its conductive matrix. Brief Description of the Drawings The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings in which: FIG. 1 shows a zinc secondary battery employing a zinc negative electrode in accordance with the principles of the present invention; FIG. 2 shows the zinc negative electrode of the battery of FIG. 1 in greater detail; and FIG. 3 is a graph which plots capacity versus cycles for a zinc secondary battery constructed in accordance with the principles of the present invention. Detailed Description FIG. 1 shows a sealed zinc secondary battery 1 in accordance with the principles of the present invention. As shown, the battery 1 comprises a plurality of positive electrodes 2 and a plurality of zinc negative electrodes 3 configured in accordance with the principles of the present invention. The positive electrodes 2 and negative electrodes 3 are interleaved with each other and the interleaved arrangement of electrodes is disposed in a sealed container 4 having positive and negative terminals 4A and 4B. The container 4 also holds an electrolyte 5 having an electrolyte constituent 6. In accordance with the principles of the present invention, each zinc negative electrode 3 comprises a zinc active material, a Ba(OH) 2 or Sr(OH) 2 material and a metallic matrix formed from a metallic oxide more electropositive than zinc. A useable zinc active material is ZnO. Useable metallic oxide materials are PbO, Bi 2 O 3 , CdO, Ga 2 O 3 and Tl 2 O 3 , with the first three being preferable. The aforesaid metallic oxides are more electropositive than zinc and are easily reduced to metal during charging without lowering the hydrogen overpotential of the respective electrode 3. Also, in accord with the invention, the concentration of electrolyte constituent 6 in the electrolyte 5 is maintained relatively low and within a range of 5-20 percent of the weight of the electrolyte. A useable electrolyte constituent is KOH of this weight concentration. Moreover, in yet further accord with the invention, the percentage of the Ba(OH) 2 or Sr(OH) 2 material in each negative electrode 3 is in the range of 15-40 percent of the weight of the electrode and the percentage of metallic oxide in each electrode 3 is in the range of 5-20 percent of the weight of the electrode. The latter range of the metallic oxide insures that a uniformly dispersed conductive matrix is present in the electrode. With this construction of the zinc negative electrodes 3 and the battery 1, it has been found that the production of zinc dendrites during charging and, therefore, the resultant shape change of the electrodes 3 is greatly reduced. At the same time, the conductivity and electrochemical utilization of the electrodes is maintained and the passivity of the electrodes is inhibited. More particularly, the reduced shape change of each electrode 3 is attributable to the low concentration of electrolyte constituent 6 in the electrolyte 5. This prevents the dissolution and enhances the thermodynamic stability of the strontium or barium zincate compound produced in each negative electrode during battery discharging. Moreover, since the conductive matrix of each negative electrode is in intimate contact with the active material of the electrode, the conductivity and electrochemical utilization of the electrode is enhanced and passivation of the electrode is prevented, even though the concentration of electrolyte constituent is low. Accordingly, the battery 1 with the electrodes 3 provides improved cycle life, without significant degradation in performance. Specifically, batteries constructed in this manner and including 25% Sr(OH) 2 or Ba(OH) 2 have been cycled over 200 times, with less than a 15% capacity loss and virtually no shape change. In further accordance with the invention, the zinc electrodes 3 of the battery 1 are also constructed such that the battery exhibits improved gas recombination properties. In particular, the battery 1 exhibits a low pressure rise at the end of charge and rapid pressure decline during discharge. The battery 1 can thus be sealed without the need of substantial venting, thereby avoiding electrolyte loss and drying out of the electrodes. More particularly, each zinc electrode 3 is configured as a split electrode so as to provide easier access to the electrodes 3 for the oxygen evolved at the positive electrodes 2 during charge. This enhances oxygen recombination with the formation of barium or strontium zincate at the zinc electrodes. Specifically, each zinc electrode 3 comprises like electrode assemblies 3A and 3B separated by a porous hydrophobic element 3C. The like electrode assemblies 3A and 3B and the element 3C are shown in more detail in FIG. 2. As it can be seen, each assembly 3A and 3B comprises an active element 21 which includes a part (e.g., one-half by weight) of the zinc active material (ZnO in the illustrative case), the Ba(OH) 2 or Sr(OH) 2 material and the metallic oxide material (shown as PbO in the illustrative case) of the electrode. The active element 21 abuts a current collector element 22 comprised of a foil (shown as a copper foil in the illustrative case) plated with a metal ( shown as Ag in the illustrative case). The current collector element 22 is followed by a further gas recombination catalytic element 23 comprised of a material (shown as CdO in the illustrative case) more electropositive than zinc and having a chemical or electrochemical affinity for reacting with oxygen. The gas recombination catalytic element 23 of each electrode assembly 3A and 3B abuts the porous hydrophobic element 3C. A separator 3D of the electrode 3 envelops the electrode assemblies 3A and 3B and hydrophobic element 3C to complete each electrode 3. With this construction of each of the electrodes 3 of battery 1, the gas recombination catalytic elements 23 of each electrode act as catalysts to enhance the oxygen recombination occurring at the electrode. Oxygen evolved at the positive electrodes 2 will now initially react at the elements 23 of the electrodes 3, thereby keeping the gas pressure of the battery low. Additionally, rapid diffusion of oxygen through the split electrode assemblies is promoted by the porous hydrophobic layers 3C, which also keep the interior of the electrode assemblies from filling up with electrolyte. The zinc negative electrodes 3 and sealed zinc secondary battery 1 thus provide a battery which exhibits very little shape change in its negative electrodes, thereby enhancing the cycle life of the battery. Additionally, the battery exhibits enhanced gas recombination and is, thus, able to be sealed without venting to prevent electrolyte loss, thereby avoiding the need for maintenance. An overall improved sealed zinc secondary battery is thus realized. In further accordance with the aforesaid gas recombination, split electrode aspect of the invention, the split zinc electrode 3 shown in FIG. 2 can also be constructed by utilizing Ca(OH) 2 in place of the Ba(OH) 2 or Sr(OH) 2 of the electrode assemblies 3A and 3B. A battery 1 employing such a zinc electrode with Ca(OH) 2 exhibits enhanced gas recombination properties similar to that exhibited when Ba(OH) 2 or Sr(OH) 2 are used to construct the electrode. Example 1 A sealed rechargeable nickel-zinc battery with a rated capacity of 15 ampere-hours was constructed with zinc negative electrodes 3 and electrolyte 5 in accordance with the invention. Each zinc electrode 3 was formed from two like electrode assemblies. Each electrode assembly comprised a zinc active element formed of 64.5 percent ZnO, 25 percent Sr(OH) 2 , 8 percent PbO and 2.5 percent PTFE. The zinc active element was fabricated by a plastic roll-bonding process as described in U.S. Patent No. 4,976,904, assigned to the same assignee hereof. The total weight of each zinc active element mass was about 7.85 g. Each zinc active element was laminated to one face of a current collector element formed from a silver plated perforated copper foil which was 0.0762 mm (.003") thick. A gas recombination catalytic element was then laminated to the opposite face of the current collector element to complete each electrode assembly. The gas recombination element comprised a sheet of plastic bonded CdO containing 5 percent PTFE and was approximately 0.127 mm (.005") thick. A porous PTFE sheet (~65 percent porosity manufactured by Norton) which was 0.127 mm (.005") thick was then sandwiched between two zinc electrode assemblies and the assemblies were placed in two layers of nonwoven polyamide absorber Pellon FS2519 (manufactured by Freudenberg). The resultant electrode 3 was about 1,651 mm (.065") thick. Seven such zinc negative electrodes were constructed. Each positive electrode of the battery comprised 66 percent Ni(OH) 2 , 30 percent graphite and 4 percent PTFE. The graphite component was coated with 5 percent Co 3 O 4 (cobalt oxide spinel) as described in U.S. Patent 4,546,058, also assigned to the same assignee hereof. An electrode sheet was fabricated by the same plastic roll-bonding process of the '904 patent which was used for the zinc active elements of the negative electrodes. The resultant positive electrode had a mass which weighed about 14.16 g. Six positive electrodes were constructed. The six positive electrodes were each wrapped in three layers of Celgard 3400 microporous polypropylene separator 0.0254 mm (.001") thick (manufactured by Hoerscht Chemical) and then assembled in a container in interleaved fashion with the seven negative electrodes. The container was then filled with 20 percent KOH electrolyte containing 1 percent LiOH and subjected to three formation charge-discharge cycles. The container was then sealed and a pressure gauge was installed to monitor gas pressure changes with cycling. The battery was then placed on a cyclic charge-discharge regimen consisting of charge at 3.83 amperes for 3.6 hours and discharge at 5 amperes for 2.4 hours (depth of discharge 80 percent of rated capacity). The battery was given a deep discharge at 5 amperes to 1.20 volts every 25 cycles to determine its capacity retention; the results are shown in FIG. 3. As can be seen from FIG. 3, during cycling the capacity of the battery remained at 100 percent of rated for over 200 cycles. This compares quite favorably to a conventionally constructed zinc secondary battery which typically exhibits loss of more than 40 percent of its original capacity after less than 200 cycles. Observation of the pressure variation with cycling of the constructed battery indicated a maximum pressure of about 137.9 kPa excess pressure (20 psig) after charge and a pressure of less than 34.5 kPa excess pressure (5 psig) after discharge, owing to a rapid oxygen recombination during discharge. This again compares favorably with the conventional zinc secondary battery which exhibits a pressure rise to 206.85 kPa excess pressure (30 psig) at the end of charge and only falls to about 103.43 kPa excess pressure (15 psig) after discharge. No evidence of zinc shape change was seen at the edges of the zinc negative electrode in the constructed battery. Again, this was favorable as compared to the conventional battery in which considerable deposits of irregular and spongy zinc masses are accumulated at the edges of the zinc negative electrodes. The above example demonstrates that, in the battery of the invention, the solubility of the zinc active material in the zinc electrodes is decreased leading to improved cycle life. It also demonstrates that the split electrode assemblies of the battery of the invention provide additional access to gases generated in the battery, thereby resulting in improved gas recombination. Example 2 A similar battery was constructed as in the Example 1 with the only change being that the Sr(OH) 2 in the zinc negative electrode was replaced by Ba(OH) 2 . Example 3 A similar battery was constructed as in the Example 1 with the only change being that the Sr(OH) 2 in the zinc negative electrode was replaced by Ca(OH) 2 . In all cases it is understood that the above-described arrangements are merely illustrative of the many possible specific embodiments which represent applications of the present invention. Numerous and varied other arrangements, can be readily devised in accordance with the principles of the present invention without departing from the scope of the invention as defined in the claims. Thus, for example, the positive electrodes 2 of the battery 1 could have also contained a MnO 2 active material, instead of a Ni(OH) 2 active material.

Claims: Apparatus comprising: at least one zinc negative electrode, said zinc negative electrode including: a zinc-active material; one of a Ba(OH) 2 , Ba(OH) 2 and Sr(OH) 2 material; and a conductive matrix including a metallic oxide which is more electropositive than zinc; said zinc negative electrode further including a porous hydrophobic element and being formed by first and second electrode assemblies with said porous hydrophobic element disposed therebetween, each of said first and second electrode assemblies comprising: an active element containing a part of said zinc active material, said one of said Ca(OH) 2 , Ba(OH) 2 and Sr(OH) 2 material and said metallic oxide material; a current collector element having a first face abutting said active element; and a gas recombination catalytic element containing a metallic oxide which is more electro-positive than zinc and has a chemical or electrochemical affinity for oxygen, said gas recombination catalytic element abutting a second face of said current collector element and said porous hydrophobic element. Apparatus in accordance with claim 1 wherein: said one zinc negative electrode further comprises a separator element enveloping said first and second electrode assemblies and said porous hydrophobic element. Apparatus in accordance with claim 2 wherein: said current collector element comprises a copper foil plated with silver; said metallic oxide of said gas recombination catalytic element comprises CdO; and said active element and said gas recombination catalytic element each further comprise PTFE. Apparatus in accordance with claim 1, wherein: each of said electrode assemblies contains one-half of said zinc active material, said one of said Ca(OH) 2 , Ba(OH) 2 and Sr(OH) 2 material and said metallic oxide material in weight percent of said zinc negative electrode. Apparatus in accordance with claim 1 wherein: said metallic oxide material of said active element of each electrode assembly is one of PbO, Bi 2 O 3 , CdO, Ga 2 O 3 and Tl 2 O 3 . Apparatus in accordance with claim 1 wherein: said one of said Ca(OH) 2 , Ba(OH) 2 and Sr(OH) 2 material is in the range of 15-40 percent of the weight of said zinc negative electrode; and said metallic oxide material of said active elements of said electrode assemblies is in the range of 5-20 percent of the weight of said zinc negative electrode. Apparatus in accordance with claim 6 wherein: said metallic oxide material of said active element of each electrode assembly is one of PbO, Bi 2 O 3 , CdO, Ga 2 O 3 and Tl 2 O 3 . Apparatus in accordance with claim 7 further comprising: an electrolyte containing an electrolyte constituent which is 5-20 percent of the weight of said electrolyte. Apparatus in accordance with claim 8 further comprising: at least one positive electrode; and a container in which is disposed each said zinc negative electrode, each said positive electrode and said electrolyte. Apparatus in accordance with 9 wherein: said electrolyte is KOH. Apparatus in accordance with claim 1 further comprising: at least one positive electrode; and a container in which is disposed each said zinc negative electrode, each said positive electrode and an electrolyte. Apparatus in accordance with claim 11 further comprising: an electrolyte disposed in said container. Apparatus in accordance with claim 11 wherein: said container is sealed. Apparatus in accordance with claim 11 wherein: said positive and negative electrodes are disposed in an interleaved manner in said container.

Inventors: CHARKEY ALLEN, CHARKEY, ALLEN
Applicants: EVERCEL INC, EVERCEL, INC.
Application year: 1995
Main classification: H01M4
IPC codes: H01M10, H01M4