Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Cell enclosure structure – e.g. – housing – casing – container,...
Reexamination Certificate
2000-06-13
2002-11-19
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Cell enclosure structure, e.g., housing, casing, container,...
C429S165000, C429S211000, C429S238000
Reexamination Certificate
active
06482543
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to electrochemical cells, particularly alkaline cells, having metal current collectors.
BACKGROUND
Conventional alkaline electrochemical cells are formed of a cylindrical housing (casing). The housing is initially formed with an enlarged open end. After the cell contents are supplied, an end cap with insulating plug is inserted into the open end. The cell is closed by crimping the housing edge over an edge of the insulating plug and radially compressing the housing around the insulating plug to provide a tight seal. A portion of the cell housing forms the positive terminal.
The cell contents of a primary alkaline cell (Zn/MnO
2
cells) typically contain an anode comprising zinc anode active material, alkaline electrolyte, a cathode comprising manganese dioxide cathode active material, and an electrolyte ion permeable separator, typically comprising a nonwoven material containing cellulosic fibers and polyvinylalcohol fibers. The anode active material comprises zinc particles admixed with zinc oxide and conventional gelling agents, such as carboxymethylcellulose or acrylic acid copolymers, and electrolyte. The gelling agent holds the zinc particles in place and in contact with each other. A single conductive metal nail, known as the anode current collector, is typically inserted into the anode material in contact with the end cap which forms the cell's negative terminal. The nail is conventionally inserted into the anode and located along the cell's central longitudinal axis. The current collector can be welded to the end cap. The alkaline electrolyte is typically an aqueous solution of potassium hydroxide, but other alkali solutions of sodium or lithium hydroxide may also be employed. Preferably there is no added mercury to the anode, that is, the anode is essentially mercury free, thus containing less than 50 parts mercury per million parts total cell weight. The cathode material is typically of manganese dioxide and may include small amounts of carbon or graphite to increase conductivity. Conventional alkaline cells have solid cathodes comprising battery grade particulate manganese dioxide. Battery grade manganese dioxide as used herein refers to manganese dioxide generally having a purity of at least about 91 percent by weight. Electrolytic MnO
2
(EMD) is the preferred form of manganese dioxide for alkaline cells because of its high density and since it is conveniently obtained at high purity by electrolytic methods. EMD is typically manufactured from direct electrolysis of a bath of manganese sulfate and sulfuric acid.
In the cathodes of conventional Zn/MnO
2
alkaline cells the manganese dioxide composition is typically between about 70 and 87 percent by weight. Particulate graphite and aqueous KOH solution can be added to the manganese dioxide to form a cathode mixture. Such mixtures form a moist solid mix which can be fully compacted into the cell housing using plungers or other such compacting devices forming a compacted solid cathode mass in contact with the cell casing. The cathode material can be preformed into the shape of disks forming annular rings inserted into the cell in stacked arrangement so that their outer surface abuts the inside surface of the cell housing. In such embodiment the cathode inside surface faces the cell's interior. The cell housing typically functions as the cathode current collector. The cathode inside surface is normally smooth and uniform resulting in a cathode having a uniform and constant wall (annular) thickness along the cell's length. The cathode disks can then be recompacted while in the casing. The separator can be placed so that it lines the inside surface of the cathode. The spearator can be inserted as a sheet over the cathode exposed surface or it can be sprayed or coated in liquid form onto the cathode and subsequently dried to form a film as described in commonly assigned copending patent application Ser. No. 09/280,367, filed Mar. 29, 1999. The anode mixture can be inserted into the central void space available within the cathode disks—the separator material being between the anode and cathode surfaces.
There are increasing commercial demands to make primary alkaline cells better suitable for high power application. Modern electronic devices such as cellular phones, digital cameras and toys, flash units, remote control toys, camcorders and high intensity lamps are examples of such high power applications. Such devices require high current drain rates, typically pulsed drain, of between about 0.5 and 2 Amp, more usually between about 0.5 and 1.5 Amp. Correspondingly, they require operation at power demands between about 0.5 and 2 Watt. Modern electronic devices such as cellular phones, digital cameras and toys, flash units, remote control toys, camcorders and high intensity lamps are examples of such high power applications. Thus, it is desirable to provide a way of reliably increasing the useful service life of conventional primary alkaline cells particularly for cells to be used in high power applications.
However, use of the cell in high power application tends to increase polarization effects. Polarization limits the mobility of ion transport within the electrode active material and within the electrolyte. Such polarization can result in a significant portion of the anode and cathode material remaining undischarged, thereby reducing the cell's actual capacity (mAmp-hours) or service life (hours). It is thus desirable to find ways of making the cell more amenable to high power application without significantly increasing polarization effects or otherwise adversely affecting cell performance.
Recent art describes an approach wherein the alkaline cell cathode has passages or tunnels therein for insertion of the anode material, as shown in U.S. Pat. No. 5,869,205. The tunnels are completely surrounded by cathode material which is lined with ion permeable separator material. In another approach the cathode disks appear to have spaced apart cutout or indented regions which run circumferentially along the cathode's inside surface (surface facing the cell's interior) as shown in International publication WO 00/01022. This results in a cathode wall thickness which varies along the cell's length. Alkaline cell cathode disks having cutout or indented regions which appear to run longitudinally along the cathode's inside surfaces are shown in U.S. patent application Ser. No. 09/358,578, filed Jul. 21, 1999, issued Jan. 29, 2002 as U.S. Pat. No. 6,342,317, commonly assigned with the present application. In such design the cathode wall thickness varies circumferentially. Alkaline cells having such indented surfaces provide greater interfacial surface area at the interface between anode and cathode than conventional cells. This reportedly reduces the average current density (mAmp/cm
2
) at the anode/cathode interface resulting in improved actual capacity (mAmp-hrs), particularly under high power application.
When conventional nail anode current collectors are used with such alkaline cells, however, the average distance between the nail and the cathode surface is typically less than it would e in conventional cells having cathodes of constant wall thickness. This has a tendency to increase the cell's internal resistance during discharge and therefore can result in less than optimum performance.
SUMMARY OF THE INVENTION
Applicant has determined that the performance of alkaline cells having an annular cathode with a lobed, namely an indented (curvilinear) surface can be improved by extending a portion of the anode current collector into the indented area. This reduces the cell's internal resistance, and improves performance extending further the cell's service life, particularly under high power application.
It has also been determined that the performance of alkaline cells having annular cathodes can be improved by employing cathode current collectors which are inserted into the cathode. Such cathode current collectors can be in the
Adamson David V.
Lee Tae-Won
Roberts Michael F.
Shelekhin Alexander B.
Sylvestre Matthew R.
Douglas Paul I.
Josephs Barry D.
Kalafut Stephen
Krivulka Thomas G.
The Gillette Company
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