Expanded zinc mesh anode

Details Expanded zinc mesh anode Expanded zinc mesh anode

2002-02-15

2004-01-06

Gulakowski, Randy (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S229000, C429S243000

Reexamination Certificate

active

06673494

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of electrochemical power cells, and in particular to zinc anodes in such cells.
BACKGROUND OF INVENTION
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 (MnO
2
), 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 “AA”, “C”, and “D” 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 “autopsied”, 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, ther

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