Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-08-15
2003-04-15
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C029S623100, C429S057000, C429S163000, C429S176000, C429S229000
Reexamination Certificate
active
06548201
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a metal/air cell having an anode comprising zinc and an air cathode. The invention relates to employing an anode comprising zinc particles plated with indium and heat treating a copper surface forming the inside surface of the cell's anode casing, such as by passing a heated gas in contact therewith.
BACKGROUND
Zinc/air cells are typically in the form of button cells which have particular utility as batteries for electronic hearing aids including programmable type hearing aids. Such miniature cells typically have a disk-like cylindrical shape of diameter between about 4 and 12 mm and a height between about 2 and 6 mm. Zinc air cells can also be produced in somewhat larger sizes having a cylindrical casing of size comparable to conventional AAAA, AAA, AA, C and D size Zn/MnO
2
alkaline cells and even larger sizes.
The miniature zinc/air button cell typically comprises an anode casing (anode cup), and a cathode casing (cathode cup). The anode casing and cathode casing each can have a closed end and an open end. An electrical insulating material can be placed around the outside surface of the anode casing. After the necessary materials are inserted into the anode and cathode casings, the open end of the anode casing is typically inserted into the open end of the cathode casing and the cell sealed by crimping. The anode casing can be filled with a mixture comprising particulate zinc. The zinc mixture contains mercury (typically about 3 percent by weight of the anode) and also a gelling agent and becomes gelled when electrolyte is added to the mixture. The electrolyte is usually an aqueous solution of potassium hydroxide, however, other aqueous alkaline electrolytes can be used. The cathode casing contains an air diffuser (air filter) which lines the inside surface of the cathode casing's closed end. The air diffuser can be selected from a variety of air permeable materials including paper and porous polymeric material. The air diffuser is placed adjacent to air holes in the surface of the closed end of the cathode casing. Catalytic material typically comprising a mixture of particulate manganese dioxide, carbon and hydrophobic binder can be inserted into the cathode casing over the air diffuser on the side of the air diffuser not contacting the air holes. An ion permeable separator is typically applied over the catalytic material so that it faces the open end of the cathode casing.
The cathode casing can typically be of nickel plated steel or nickel plated stainless steel, for example, with the nickel plate forming the cathode casing's outside surface and stainless steel forming the casing's inside surface. The anode casing can also be of nickel plated stainless steel, typically with the nickel plate forming the casing's outside surface. The anode casing can be of a triclad material composed of stainless steel having an outer layer of nickel and an inner layer of copper. In such embodiment the nickel layer typically forms the anode casing's outside surface and the copper layer forms the anode casing's inside surface. The copper inside layer is desirable in that it provides a highly conductive pathway between the zinc particles and the cell's negative terminal at the closed end of the anode casing. An insulator ring of a durable, polymeric material can be inserted over the outside surface of the anode casing. The insulator ring is typically of high density polyethylene, polypropylene or nylon which resists flow (cold flow) when squeezed.
After the anode casing is filled with the zinc mixture and after the air diffuser, catalyst, and ion permeable separator is placed into the cathode casing, the open end of the anode casing can be inserted into the open end of the cathode casing. The peripheral edge of the cathode casing can then be crimped over the peripheral edge of the anode casing to form a tightly sealed cell. The insulator ring around the anode casing prevents electrical contact between the anode and cathode cups. A removable tab is placed over the air holes on the surface of the cathode casing. Before use, the tab is removed to expose the air holes allowing air to ingress and activate the cell. A portion of the closed end of the anode casing can function as the cell's negative terminal and a portion of the closed end of the cathode casing can function as the cell's positive terminal.
Typically, mercury is added in amount of at least one percent by weight, for example, about 3 percent by weight of the zinc in the anode mix. The mercury is added to the anode mix to reduce the hydrogen gassing which can occur as a side reaction in the zinc/air cell during discharge and when the cell is placed in storage before or after discharge. The gassing, if excessive, can reduce the cell capacity and increase the chance of electrolyte leakage. Such leakage can damage or destroy the hearing aid or other electronic component being powered. The mercury also improves electrical conductivity between the zinc particles. Many regions around the world now greatly restrict the use of mercury in electrochemical cells because of environmental concerns.
Although mercury can now be eliminated from conventional zinc/MnO
2
alkaline cells by addition of various gassing inhibitors, to the anode mix, the elimination of mercury from zinc/air cells has proved to pose a far more difficult problem. This is because the zinc/air cells are provided with air holes at the end of the cathode casing, and zinc/air cells are typically much smaller cells. The air holes can provide a path for electrolyte to escape if there is even a moderate amount of gassing. Additionally, if conventional gassing inhibitors are added to the zinc/air cell anode mix instead of mercury, they either significantly reduce the anode conductivity or have to be added in quantity, thereby significantly reducing the cell's capacity (mAmp-hrs).
U.S. Pat. No. 3,897,265 discloses a representative zinc/air button cell construction with an anode casing inserted into the cathode casing. There is disclosed an insulator between the anode and cathode casings. The anode comprises zinc amalgamated with mercury. The cell includes an assembly comprising an air diffuser, cathode catalyst, and separator at the closed end of the cathode casing facing air holes in the surface of the cathode casing.
U.S. Pat. No. 5,279,905 discloses a miniature zinc/air cell wherein little or no mercury has been added to the anode mix. Instead, the inner layer of the anode casing has been coated with a layer of indium. The disclosed anode casing can be a triclad material composed of stainless steel plated on the outside surface with nickel and on the inside surface with copper. The copper layer is at least 1 microinch (25.4×10
−6
mm). The reference discloses coating the copper layer on the anode casing's inside surface with a layer of indium. The indium layer is disclosed as being between about 1 microinch and 5 microinches (25.4×10
−6
mm and 127×10
−6
mm).
It is thus desired to produce a zinc/air cell without added mercury.
It is desired to eliminate the need to add mercury to the zinc/air cell without increasing gassing within the cell, yet while obtaining good cell performance.
SUMMARY OF THE INVENTION
An aspect of the invention is directed to a zinc/air depolarized cell employing as anode active material zinc particles wherein the zinc particle surface has been plated with indium. The zinc average particle size is desirably between about 30 and 350 micron (30×10
−6
meter and 350×10
−6
meter). The zinc particles can be pure zinc or in the form of particulate zinc alloyed with a small amount, for example, between about 100 and 2000 ppm (based on pure zinc) of an alloy material. Suitable alloy materials for particulate zinc, for example, can be alloy materials of indium or indium and lead, or indium, lead and aluminum. Another desirable alloy material of indium and bismuth can also be used. These particulate z
Gibbons Daniel
Huq Rokeya
Moses Peter R.
Pappas David
Wei Guang
Chaney Carol
Douglas Paul I.
Dove Tracy
Josephs Barry D.
The Gillette Company
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