Metal-air cells typically include a metal anode, an air cathode, and a separator all disposed and supported in some sort of container. The metal anode usually comprises a fine-grained metal powder, such as zinc, aluminum, or magnesium, which is blended together with an aqueous electrolyte, such as potassium hydroxide, and a gelling agent into a paste. The separator is a porous material that allows the passage of electrolyte between the cathode and anode, but prevents direct electrical contact therebetween and short circuiting of the cell.
The air cathode is a catalytic structure designed to facilitate the reduction of oxygen. Typically, it is composed of active carbon, a binder, and a catalyst which, together with a metal current collector, are formed into a thin sheet. The air cathode also commonly incorporates a hydrophobic polymer, such as polytetrafluoroethylene or polystyrene, directly into the cathode sheet and sometimes also as a coextensive film. The hydrophobic polymer prevents electrolyte from flooding the cathode or passing through it and leaking from the cell. The container includes oxygen access openings, diffusion chambers and the like which are designed to allow sufficient oxygen to reach all parts of the air cathode.
Metal-air cells have high specific energies. In fact, zinc-air cells have the highest specific energy, up to 450 Wh/kg, of all aqueous primary systems, and high energy per unit volume as well. The components of zinc-air cells also are relatively benign.
Because of their high energy density, button cells incorporating zinc-air chemistry currently are the most popular energy source for hearing aids. The much larger majority of electronic devices, however, has higher energy requirements requiring the use of larger (i.e., greater than one ampere hour capacity) cells or batteries. Despite the electrochemical advantages of metal-air and especially zinc-air systems, carbon-zinc and alkaline manganese dioxide systems continue to dominate the much larger world market for larger primary batteries.
Many portable electronic devices, such as portable computers, also place severe constraints on battery weight and volume. In such applications, prismatic cells would be preferable over button or cylindrical cells, which latter type of cells, in general, require more space to be allocated in the device than the cells themselves actually occupy. Prismatic zinc-air cells also can be much thinner than alkaline cells of equivalent capacity.
Attempts to scale up and reconfigure zinc-air button cells to a larger, prismatic configuration, however, have generally failed. Zinc-air batteries currently are not a competitive option for use in the full spectrum of consumer and electrical products, and they represent a small portion of all primary batteries sold today.
A major problem has been in achieving an inexpensive, light-weight and easily constructed zinc-air cell configuration which is leak-proof, but which provides for efficient electrochemical discharge of the cell. Those problems are only exacerbated by the expansion of the zinc anode during the life of the battery.
That is, as a zinc-air cell is discharged, zinc is oxidized to zinc oxide, and zinc oxide has a lower density than zinc. The zinc anode therefore will expand during discharge of the cell. The amount of this expansion is reported to be from 17% up to 60%. See U.S. Pat. No. 4,687,714 to J. Oltman et al. (17%) and U.S. Pat. No. 3,855,000 to J. Jammet (60%).
This anodic expansion can cause various problems to develop. For example, Jammet '000 and Oltman '714 report that anodic expansion can cause the metal anode to directly contact the air cathode, thereby short-circuiting the cell.
U.S. Pat. No. 4,894,295 to M. Cheiky reports that the pressure buildup caused by anodic expansion can cause the container to expand, thereby causing air pockets to form between the air cathode and the electrolyte. This in turn diminishes electrolytic communication between the air cathode and the anode, reducing the battery output capacity.
Oltman '714 also reports that the internal pressure created by anodic expansion may force the air cathode so tightly against the container that complete exposure of the air cathode to oxygen may be diminished. To the extent that oxygen cannot efficiently reach all portions of the air cathode, the discharge rate of the battery is likewise diminished.
Further, as discussed in Oltman '714, the internal pressure buildup caused by anodic expansion can cause a cell to leak. While various strategies to prevent leakage have been devised, an increase in internal pressure necessarily increases the risk of leakage in any configuration. Likewise, although the degree to which the problems discussed are present will vary from design to design, all zinc-air cells must be designed to tolerate the increase in volume by the metal anode during discharge.
One such strategy, discussed in Oltman '714, is to provide free space inside the cell into which the anode may expand. When the individual anode particles expand freely into a void, however, they may not remain in physical contact with one another, and that may cause incomplete cell discharge. Oltman '714 addresses that problem by reducing the size of the expansion space and by allowing a portion of the battery container to expand as the cell is discharged.
While this approach may have some merit in button cells, such as those disclosed in Oltman '714, it has drawbacks in larger, prismatic cells. Button cells are relatively small and cylindrically shaped, comprising disc-shaped, layered electrochemical components. Typically, button cells are about 1/4 to 3/8 inches in diameter and about 1/4 inches or less in height. The metal anode is set initially into intimate contact with the separator during manufacture. The gelled anode, in small cells, usually is sufficiently strong to stay in position, despite the presence of a vacant expansion space and despite mechanical shocks that the cell might receive during subsequent handling.
In larger, prismatic cells, however, the strength of the gel alone may not be sufficient to hold the anode in place. The metal anode may shift into vacant expansion space, becoming separated from the desired electrochemical contact with the separator and air cathode as the cell is handled in the manufacturing and distribution process. If so, the desired cell discharge performance will be adversely impaired.
Cheiky '295 discloses a generally prismatic metal-air cell. There is no expansion space provided, and so the problem of anode separation should be minimized. The battery disclosed therein accommodates anodic expansion by incorporating a flexible bottom which can expand as the anode grows.
Expansion of the container, however, is not entirely desirable. Such expansion or distortion of the container can make it more difficult to fit the battery in the battery compartment of an electrical device. While the batteries in Oltman '714 and Cheiky '295 to a certain extent purport to minimize such problems by limiting container expansion to certain portions of the container, such an approach needlessly complicates design of the container. Moreover, the Cheiky '295 design, with its unique surface geometry, may pose further constraints on the design of battery compartments and attempts to standardize external cell or battery configurations.
Further approaches to accommodating anodic expansion are disclosed in Jammet '000 and U.S. Pat. No. 4,054,726 to H. Sauer. Jammet '000 discloses cells having a cylindrical container in which a compressible structure, such as a spring-biased plate, a frictionally slidable piston, or a compressible open-cell polyethylene foam layer or equivalent material, is situated in an expansion space located within the container. The expansion space and compressible structure are adjacent to the metal anode.
The spring-biased plate and frictionally slidable piston disclosed in Jammet '000, however, involve relatively complicated designs which drive up the cost and efficiency of manufacture. They also add significant weight to the cell and occupy a significant amount of space which consequently can neither be filled with anode paste or reserved for anodic growth. All Jammet '000 embodiments, it is stated, operate to maintain the negative electrode in the desired state of compression during use. For example, the negative electrode may expand upwardly against the opposition provided by the foam such that the electrode is maintained in the desired state of compression against the electrolyte.
In Sauer '726, metal-air button cells are disclosed in which a compressible expansion body is positioned within the zinc electrode. The placing of the expansion body within the zinc electrode has the particular advantage, it is stated, that the entire inner surface of the metallic cover remains in electrical contact with the negative electrode. Sauer also states that it is necessary that the expansion body have closed compressible pores and preferably be hydrophobic.
To ensure trouble-free operation, Sauer '726 further states that it is necessary to coordinate the compressibility of the individual structural elements of the cell. Thus, the specific compression of the expansion body must be lower than the specific compression pressure of the layer performing the air distribution and support function as well of the air electrode layer and its adjoining layer.
While the Jammet '000 and Sauer '726 structures may be satisfactory to deal with anodic expansion in cylindrical and button cells, such structures would be unsuitable for use in prismatic zinc-air cells. Thus, what was not appreciated was that the resulting increase in the internal pressure within the cell, while probably less than that encountered in the absence of an expansion space, would be significant and would create severe problems in a prismatic design.
Button and cylindrical cells, such as those disclosed in Sauer '726 and Jammet '000, are inherently more rigid structures than are prismatic cells. The containers for button and cylindrical cells also are frequently made from relatively thick steel. Consequently, button and cylindrical cells, especially those with steel casings, can withstand higher pressures without significant change in their external diameters.
The weight of the battery, however, is frequently an important consideration in cell design, especially those cells which are intended for use in portable electrical devices, such as portable computers. Fabricating a casing from steel, however, tends to add considerable weight to a larger, prismatic cell.
On the other hand, prismatic containers which are fabricated from lighter-weight materials, such as structural plastics, tend to expand and swell as internal cell pressure increases during discharge. With this expansion of the container come various problems, as noted above.
Higher internal pressures causes problems in prismatic cells in other ways as well. That is, plastic container components in theory can be sealed relatively easily by, for example, ultrasonic welding. The materials also are hydrophobic, and thus, the seals have less of a problem of electrolyte creepage, which is common with metal containers and aqueous potassium hydroxide electrolytes.
It is not always possible, however, to achieve a defect-free seal between plastic container components or between the air cathode and the container during mass production of cells. Electrolyte or anode paste which spills on the sealing surfaces, as well as imperfections in the plastic parts themselves, can lead to minor defects in a seal. Even small defects can allow electrolyte to leak as the pressure within a cell increases. Moreover, larger cells have more extensive areas which must be sealed and, therefore, a somewhat higher likelihood that a given cell will have a defect in its seal. Cell designs which have minimal internal pressure buildup in service are required so as to lessen, if not eliminate, the probability of electrolyte leakage.
It seems apparent that, for considerable time, there has existed a substantial need for larger prismatic metal-air cells, especially for thin prismatic zinc-air cells, which can satisfy the energy requirements for a wide variety of applications. It seems further apparent that a considerable amount of effort has been directed to providing suitable cells capable of providing satisfactory performance. Yet, despite recognition of the need and the considerable efforts made to date, there still exists the need for prismatic, and especially for thin prismatic metal-air cells such as zinc-air, which can provide satisfactory and reliable performance for a wide variety of commercial applications.
An object of this invention, therefore, is to provide a prismatic metal-air cell which provides more reliable electrochemical performance in service. A related and more specific object is to provide a thin prismatic zinc-air cell having satisfactorily reliable electrochemical performance in service.
It also is an object to provide a prismatic zinc-air cell which is more leak resistant, and especially, which is more leak resistant notwithstanding the presence of minor defects which can occur in the seal during mass production of cells.
It is a further object of this invention to provide a prismatic zinc-air cell which can accommodate anodic expansion during discharge without significant expansion of the cell container, even when the container is a relatively light-weight, thin-walled plastic container. A related object is to provide a zinc-air cell which can accommodate anodic expansion during discharge without significant buildup of pressure within the container.
It also is an object to provide a prismatic zinc-air cell in which the metal anode is more reliably supported against the separator to prevent separation therefrom and to maintain electrolytic communication therebetween.
It is a further object of this invention to provide a prismatic zinc-air cell which is simple in design, easy and economical to assemble and does not place excessive constraints on the types of materials from which the cell may be constructed.
Yet another object of the subject invention is to provide a prismatic zinc-air cell wherein all of the above-mentioned advantages are realized.
These and other objects and advantages of the invention will be apparent to those skilled in the art upon reading the following detailed description and upon reference to the drawings.