Chemistry: electrical current producing apparatus – product – and – Means externally releasing internal gas pressure from closed... – Blowout type
Reexamination Certificate
2000-12-18
2003-02-04
Kim, John (Department: 1723)
Chemistry: electrical current producing apparatus, product, and
Means externally releasing internal gas pressure from closed...
Blowout type
C429S129000, C429S172000, C429S249000
Reexamination Certificate
active
06514637
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to separators for electrochemical cells, particularly separators for alkaline cells, which cover an electrode surface, wherein the separator is formed by treating a film applied to the electrode surface. The invention also relates to a protector disk contacting the top surface of the cathode to protect the cathode from shorting.
BACKGROUND
Primary (non-rechargeable) alkaline cells typically contain an anode comprising active material of zinc, alkaline electrolyte, a cathode comprising active material of manganese dioxide, and an ion permeable separator sheet between the anode and cathode. The alkaline electrolyte is typically an aqueous solution of potassium hydroxide, but other alkali solutions of sodium or lithium hydroxide may also be employed. The cell contents are typically housed in a cylindrical steel casing (housing). The anode material comprises zinc particles admixed with zinc oxide and conventional gelling agents, such as carboxymethylcellulose or acrylic acid copolymers, and electrolyte solution. The gelling agent holds the zinc particles in place and in contact with each other. The cathode material comprises manganese dioxide and small amount of electrolyte and may also include small amounts of carbon or graphite to increase conductivity. The cathode material is conventionally a solid material compressed against the inside surface of the cell casing (housing) forming a hard compacted annular mass.
The separator is conventionally premanufactured outside of the cell. The separator is typically cut into two or more strips and inserted in the cell casing so that the strip edges overlap. The separator material covers the inside surface of the cathode and lies adjacent the anode. Additional electrolyte may optionally be added after the separator is inserted. In such case the separator absorbs the additional electrolyte and a portion of the electrolyte is reabsorbed into the cathode. Anode material may then be inserted into the core of the casing. The separator keeps the anode and cathode from physical contact.
The separator may be of woven or nonwoven ion permeable material. The separator is ion permeable, but yet prevents passage of active anode or cathode material therethrough. Conventional ion permeable separators for alkaline cells may be formed of a single sheet of fibrous woven or nonwoven material. Such single sheet is prefabricated outside of the cell and inserted into the cell, typically in overlapping strips, after the cathode has been inserted into the casing. The fibrous woven or nonwoven sheet may comprise a combination of fibrous material, typically comprising polyvinylalcohol fibers and cellulosic fibers. Conventional separators may also be made of multiple layers of such fibrous nonwoven material laminated to each other.
Conventional separator sheets for alkaline cells may comprise a dual layer of an ion permeable film membrane laminated to a fibrous nonwoven material. The film membrane may be of a cellulosic material typically cellophane. The cellulosic film membrane may be laminated to a fibrous nonwoven material comprising polyvinylalcohol fibers and cellulosic fibers. The cellulosic film membrane prevents zinc dendrites from penetrating into the cathode where they may cause shorting of the cell. Zinc dendrites can form in the anode during prolonged discharge. The cellulosic film membrane, however, can noticeably increase the internal resistance of the cell, particularly, in high power application. The fibrous nonwoven material provides support for the cellulosic membrane and also functions to absorb electrolyte which can be reabsorbed by the electrodes. The polyvinylalcohol fiber component in the nonwoven material withstands attack by alkaline electrolyte and lends structural support to the nonwoven layer. The cellulosic fibers gives the nonwoven much of its absorbency characteristic.
Such conventional separators which are prefabricated outside of the cell, whether of single or multiple layer, have a significant thickness, typically between about 5 mil and 10 mil (0.127 mm and 0.254 mm), more typically about 8 mil (0.203 mm). As such, they may consume a significant percentage of the internal volume of a small size cell. For example, when inserted into AA alkaline cells, such conventional separators may typically consume between about 3 and 20 percent of the useable internal cell volume by a combination of separator thickness and non-uniform conformation. Such conventional separators can only be made to conform to and cover either flat surfaces or surfaces that are only gradually and uniformly contoured, that is, surfaces which do not have steep angle of curvature or surfaces which do not have reverse curvature.
It is desirable that the separator be applied to cover cathode surfaces of essentially any shape and surface contour.
It is desirable to protect the top surface of the cathode in a manner which reduces the chance of shorting in the event that the separator develops surface irregularites along the cathode surface.
It is desirable that the separator, when applied to alkaline cells, be of minimal thickness yet durable, ion permeable, and resistant to attack by alkaline electrolyte.
It is desirable that the separator's ionic resistance be low enough that it does not significantly impede the cell's performance especially at high rates of discharge.
It is desirable that the separator resist passage of zinc dendrites from anode to cathode.
SUMMARY OF THE INVENTION
An aspect of the invention is directed to a method of forming a separator in an electrochemical cell having casing, anode, cathode and electrolyte, comprising the steps of coating one of the anode or cathode surfaces with a material to form a film on said surface and treating said film to form a separator contacting said electrode surface. The method of the invention is particularly suitable for forming a separator on a surface of the cathode in the cell casing of an alkaline cell. The cathode surface on which the separator is formed may be of any shape or contour. The alkaline cell may have single or multiple anode cavities. The separator may be in the form of an ion permeable film contacting and conforming to the surface of said electrode.
The method of the invention has particular application generally to forming separator film insitu on the surface of an electrode in the cell casing of bobbin type electrochemical cells, particularly bobbin type alkaline cells. Such alkaline cells, for example, may conventionally have an anode comprising zinc and cathode comprising manganese dioxide and alkaline electrolyte, or they may be in the form of conventional zinc-air cells having alkaline electrolyte, and an anode comprising zinc which is depolarized with air. Such bobbin cells are characterized by having at least one of the electrodes in the form of a discrete lump of solid or semisolid mass which is separately inserted into the cell casing. Conventional bobbin cells typically have a cylindrical casing with an open end and a closed end. After the cell contents are inserted into the casing, an end cap assembly comprising an insulating sealing disk (plug) with current collector therethrough and an end cap is inserted into the open end and the casing. The casing is crimped over the peripheral edge of the insulating sealing disk to seal the cell. The bobbin cell to which the method of the invention is applicable is distinguishable from cells wherein the electrodes are in wrapped jelly roll configuration. In the latter cells the electrodes are coated as thin layers onto conductive substrates to form a laminate with separator therebetween. The laminate is rolled into a jelly roll configuration and the rolled laminate inserted into the cell casing. The process of the invention, therefore, may be applied to forming electrode/separator laminates useful in lithium/manganese dioxide primary cells or in forming electrode/separator laminates in secondary (rechargeable cells).
An aspect of the invention is directed to providing electrical insulating pro
Licata John
Sargeant Sean
Treger Jack
Douglas Paul I.
Josephs Barry D.
Kim John
Krivulka Thomas G.
Sorkin David
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