This invention relates to the field of electrochemical power cells, and in particular to zinc anodes in such cells.
In today""s world of portable electronics and electric power tools, batteries are more important to our daily lives than ever before. Along with the growth of the portable consumer electronics market, the demand for inexpensive, long-lasting, powerful batteries has increased dramatically. Battery manufacturers continue to look for new ways to pull more power, for a longer duration, and more efficiently from their products. In addition to the drive for more powerful and longer lasting batteries, manufacturers are aware of the need to be environmentally conscious and the need to eliminate or minimize the use of harmful additives, such as mercury (Hg), from their products.
Many of the batteries marketed today are alkaline cells. A typical alkaline cell includes a cathode, an anode, an alkaline electrolyte, and a container. Generally speaking, the cathode is usually composed of manganese dioxide (MnO2), and the anode is typically made of zinc or a mixture of zinc and other compounds. The electrolyte usually consists of mainly potassium hydroxide (KOH), but often contains other additives. These components are usually encapsulated in a container. There are currently several shapes of containers, but two of the most common are cylinder shapes in varying sizes such as in the well known xe2x80x9cAAxe2x80x9d, xe2x80x9cCxe2x80x9d, and xe2x80x9cDxe2x80x9d cells, and smaller flat button cell batteries that are used in such devices as cameras and hearing aids.
Like all batteries in general, alkaline cells produce power through chemical reactions known as oxidation-reduction (redox) reactions. In zinc alkaline batteries this reaction consumes the zinc anode material and converts it to zinc oxide/hydroxide. As the redox reaction proceeds the zinc oxide/hydroxide deposits and accumulates within the electrolyte domain between and around the zinc anode particles. As it accumulates, it blocks the reaction site pathways by forming a barrier between the electrolyte solution and the zinc anode. The accumulation of these deposits decreases the power capacity of the battery significantly, particularly on high rate drains.
A decrease of reactivity, known as passivation, of the zinc is caused by this build-up of zinc oxide on the surface of the zinc anode. The problem is increased when the surface area of the zinc anode is low. With a small surface area and high discharge rates, surface current densities are high. This causes the anode to become highly polarized and leads to passivation of the zinc until the current density is reduced. The problems of polarization and passivation are especially of concern at low temperatures because at lower temperatures the solubility of zinc oxide in the electrolyte decreases. Lower solubility of the zinc oxide leads to a faster accumulation of zinc oxide, quickly blocking reaction site pathways. However, this problem can be reduced by presenting a higher surface area of zinc. An increase in the surface area of zinc lowers the surface current density and helps to delay the onset of passivation. Thus, with all other factors being equal the higher the surface area of the zinc anode the better the overall performance of the alkaline cell.
The problem of passivation is of particular concern under high rate drain conditions. For example, in a cell utilizing an electrolyte of potassium hydroxide, high rate drains drive electrolyte concentrations to extremes creating low potassium hydroxide concentrations in the anode cavity and very high concentrations around and within the pores of the cathode. While zinc oxide solubility dramatically increases within the pores of the cathode and within the cathode/separator/anode cavity interface, zinc oxide solubility falls off dramatically in dominantly electrochemically active areas of the anode cavity. As the interdiffusion of electrolyte species occurs in the attempt to sustain equilibrium, electrolyte of high potassium hydroxide strength and high zinc oxide solubility diffuses toward electrolyte domains low in potassium hydroxide and zinc oxide content. The result is a localized precipitation of zinc oxide close to the separator but within the anode cavity. A zinc oxide compaction zone results which eventually inhibits electrolyte diffusion at the rate necessary to support the high rate drain.
The current art typically uses zinc powder in order to achieve a maximum surface area and to delay the onset of passivation. This zinc powder provides a very large surface area. However, the use of zinc powder has a number of drawbacks. First, if zinc powder is used alone there is a tendency for it to be mobile and sensitive to shock allowing particle to particle contact to be intermittently disrupted. Anodes of powdered zinc require intimate particle to particle contact as well as intimate current collector contact. To prevent unwanted movement of particles, a suspension agent or gelling agent is usually added to the electrolyte solution of the battery. The suspension agent inhibits the zinc powder mobility and helps maintain particle contact throughout the cell container. It would be more efficient if a suspension agent were not needed since it also interferes with ion transport. By decreasing the amount of suspension agent or gelling agent required, the battery design is simplified and costs are reduced. Another drawback to using zinc powder is its relative cost. The world is undergoing a battery-grade zinc powder shortage that drives the relative cost of zinc powder up. Thus, zinc powder is more expensive when compared with non-powdered zinc such as zinc mesh, zinc strip, or other solid zinc products.
Still another problem with the use of zinc powder as the anode in alkaline cells is the increased cost and loss of performance due to waste. When an alkaline battery having a zinc powder anode is completely discharged at high discharge rates and is xe2x80x9cautopsiedxe2x80x9d, it is observed that only approximately 50% of the zinc powder has been consumed. The reaction of the balance of the zinc powder has been inhibited by disruption of interparticle electrical contact by the deposits of zinc oxide/hydroxide. This leaves a significant amount of zinc powder in the battery which was not utilized to produce power. This decreases the amount of time before the onset of cell failure due to a decrease in the available surface. This wasted zinc drives up the costs of alkaline batteries in two ways. First, larger amounts of zinc powder are required to achieve the same amount of battery output. Second, the volume wasted by this excess zinc could be used for extra electrolyte. Increasing the amount of electrolyte would allow the manufacturers to increase the battery""s capacity.
Zinc powder negative electrodes are much less efficient at high discharge rates than at low discharge rates. Currently, to improve efficiency some zinc anodes are amalgamated with mercury. Mercury dramatically improves interparticle contact inhibiting the onset of low interparticle contact. Since mercury is environmentally dangerous, use of mercury is highly undesirable. However, due to its ability to improve interparticle contact, the removal of mercury from cell designs without causing cell quality problems has been difficult. Thus, an alternative to zinc powder which is more efficient at high discharge rates without the use of mercury is needed. This alternative needs to retain the high surface area of powdered zinc but avoid the problems associated with powdered zinc.
One alternative that has been explored by the industry to the use of zinc powder while still yielding more surface area than solid zinc is the use of perforated zinc as an anode. This material is readily available in spite of the world shortage of battery-grade zinc powder. However, the use of perforated zinc has its own problems. First, perforated zinc is manufactured by punching holes in solid sheets of zinc. This creates two problems. First, the material punched from the zinc is wasted. Second, there is manufacturing difficulty in making precise perforations. While these problems are significant, the largest problem that makes the use of perforated zinc undesirable is that the perforation does not substantially increase the surface area of the electrode. While some new surface area is generated, there is also a loss of surface area associated with the material punched out of the zinc. Thus, when compared to powdered zinc, perforated zinc has a relatively low surface area.
Another alternative to zinc powder is the use of expanded zinc mesh. The use of expanded zinc mesh is desirable because it provides a large surface area and is currently more available and cheaper than zinc powder. However, previously disclosed methods of using zinc mesh have had their own problems. For example, a method of using expanded zinc mesh as an anode was disclosed in U.S. Pat. No. 4,175,168. The expanded zinc mesh was folded to create an anode. Zinc spacers were used to separate the folded layers of the zinc mesh anode in the cell. U.S. Pat. No. 4,226,920 discloses another expanded zinc mesh anode. This anode was formed by winding the zinc mesh to create a coil. Spacers were again used to provide a substantially uniform space between adjacent layers of the coil. The space between the electrode layers in these two anode designs was necessary to allow adequate electrolyte flow between the layers and to maintain adequate battery performance. However, this use of spacers to separate the layers of the anode is undesirable because it requires the use of additional materials and adds additional manufacturing costs. A zinc anode design that does not require the use of spacers to maintain battery performance is needed.
Alkaline batteries are used in hearing aids, flashlights, power hand tools, and electronic toys and games. Many new electronics require batteries that are efficient at high discharge rates. Therefore, there is a need for a battery that delivers more power capacity, is more efficient at high discharge rates, avoids the manufacturing expense of using suspension gels or spacers, minimizes the use of environmentally damaging compounds such as mercury, and uses a more available and inexpensive material than battery-grade zinc powder. In short, it is desirable to develop an alkaline battery anode comprised of expanded zinc mesh which reduces or eliminates the shortcomings of prior art anodes.
The present invention presents a solution to the above problems by providing zinc anodes having improved performance without the use of zinc powder. More specifically, the present invention provides for the use of expanded zinc mesh to increase battery performance at high discharge rates and to improve the efficiency of the anode design to decrease the amount of unreacted zinc in the oxidation-reduction reaction. The present invention also provides for electrochemical cells incorporating the improved zinc anode designs.
The present invention improves battery performance through the use of expanded zinc mesh to construct an anode of increased surface area with one of three basic designs. A xe2x80x9ccoiledxe2x80x9d zinc anode of the present invention comprises a single sheet of zinc mesh wrapped into a coil. A xe2x80x9clayeredxe2x80x9d zinc anode of the present invention comprises two or more sheets of zinc mesh layered together so that each layer is in electrical contact with adjacent layers. A xe2x80x9cfoldedxe2x80x9d zinc anode of the present invention comprises a single sheet of zinc mesh folded upon itself to form two or more layers that are in electrical contact with their adjacent layers.
In all three instances, the use of spacers between the layers of the mesh as taught by the prior art is avoided to lower material and manufacturing costs. Instead of maintaining a uniform space between the layers of the zinc mesh, the present invention utilizes anode configurations in which the layers of expanded zinc mesh are in direct physical contact. The direct contact of these layers creates electrical contact points between the zinc mesh layers of the anode. These electrical contacts work with the mesh to form a continuous lattice network that allows for better connectivity of the zinc to increase performance at high discharge rates.
The present invention also encompasses electrochemical cells employing each of the three disclosed types of zinc mesh anodes. These electrochemical cells are generally comprised of a casing, one of the described anodes, a cathode, an insulating separator between the anode and cathode, and electrolyte solution. These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and claims.