Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2001-01-23
2003-09-16
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S232000
Reexamination Certificate
active
06620550
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for manufacturing an electrically conductive electrode for use in batteries. The invention also relates to the application of said electrode as the cathode in a primary alkaline battery.
BACKGROUND OF THE INVENTION
Primary alkaline electrochemical cells typically include a zinc anode active material, an alkaline electrolyte, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulosic and synthetic fibers. The anode active material can include for example, zinc particles admixed with conventional gelling agents, such as sodium carboxymethyl cellulose or the sodium salt of an acrylic acid copolymer, and an electrolyte. The gelling agent serves to suspend the zinc particles and to maintain them in contact with one another. Typically, a conductive metal nail inserted into the anode active material serves as the anode current collector. The electrolyte can be an aqueous solution of an alkali metal hydroxide for example, potassium hydroxide, sodium hydroxide or lithium hydroxide. The cathode typically includes manganese dioxide as the electrochemically active material admixed with an electrically conductive additive to enhance electrical conductivity, an optional polymeric binder, and other optional additives, such as titanium-containing compounds including anatase-type titanium dioxide and other alkaline earth metal titanates. Because manganese dioxide typically exhibits relatively low electrical conductivity, an electrically conductive additive is needed to improve the electrical conductivity between individual manganese dioxide particles and also between the manganese dioxide particles and the steel container that encloses the cell components and that also serves as cathode current collector. Suitable electrically conductive additives can include, for example, conductive carbon powders, such as carbon blacks, including acetylene blacks, natural graphites, synthetic graphites, including expanded or exfoliated graphites, and combinations thereof.
It is desirable for a primary alkaline battery to have a high discharge capacity (i.e., long service life). Since commercial cell sizes have been fixed, it is known that the performance and/or useful service life of a cell can be enhanced by increasing the interfacial surface area of the electrode active materials as well as by packing greater amounts of the electrode active materials into the cell. However, these approaches have practical limitations such as, for example, if the electrode active material is packed too densely in the cell, the rates of electrochemical reactions during cell discharge can be reduced, in turn reducing service life. Other deleterious effects such as cell polarization can occur as well. Polarization limits the mobility of ions within both the electrolyte and the electrodes, which in turn degrades cell performance and service life. Although the amount of active material included in the cathode typically can be increased by decreasing the amount of non-electrochemically active materials such as polymeric binder or conductive additive, a sufficient quantity of conductive additive must be maintained to ensure an adequate level of bulk conductivity in the cathode. Thus, the total active cathode material is effectively limited by the amount of conductive additive required to provide an adequate level of conductivity.
Further, it is highly desirable to enhance the performance of an alkaline cell at high rates of discharge. Typically, this is accomplished by increasing the volume fraction of conductive additive in the cathode in order to increase overall (viz., bulk) electrical conductivity of the cathode. The volume fraction of conductive additive within the cathode must be sufficient to form a suitable percolative network of conductive particles. Typically, when the conductive additive is a conductive carbon, about 5 to 15 weight percent of the total mixture is required. However, an increase in the amount of conductive carbon produces a corresponding decrease in the amount of active cathode material, giving lower service life. Conventional powdery conductive carbons such as acetylene black and flaky, crystalline natural or synthetic graphites have intrinsic drawbacks including low packing density, high electrolyte absorption, and high levels of impurities that can lead to excessive hydrogen gassing by the cell.
It is well known to use a specific type of graphite called expanded graphite in place of conventional powdery conductive carbons in battery cathodes. As used herein, expanded graphite comprises natural or synthetic graphite in which the crystal lattice has been uniaxially expanded or exfoliated. Various methods can be employed to form expanded graphite including, for example, the incorporation of a strong acid such as sulfuric, nitric, or chromic acid or mixtures thereof and a strong oxidant such as hydrogen peroxoide, perchloric acid, iodic or periodic acid, perchloric acid salts, permanganate salts, and the like followed by a rapid high temperature treatment as disclosed, for example, in U.S. Pat. Nos. 1,137,373; 1,191,383; 3,404,061; and Japanese Unexamined Patent Application (Kokai) No. 16406/1994. Following the heat-treatment, the expanded graphite typically is washed, compacted, and milled by attrition to produce the desired average particle sizes. After milling, expanded graphite particles typically exhibit reduced thickness in the direction of the graphite crystallographic c-axis. Since decreased particle thickness results in an increase in the number of conductive graphite particles per unit weight, a specific weight fraction of expanded graphite can impart a higher degree of conductivity in a cathode than the same amount of a non-expanded graphite. When admixed with manganese dioxide to form a cathode, less expanded graphite can be used resulting in increased service life. In addition, as disclosed in U.S. Pat. No. 5,482,798, expanded graphite has a flaky particle morphology, high compressibility, high lubricity, and good moldability thereby facilitating cathode fabrication.
The use of expanded graphite as a conductive additive in cathodes of conventional alkaline primary cells is known and disclosed for example, in U.S. Pat. No. 5,482,798; PCT publication no. WO 93/08123; European Application EP 0170,411; and also in Japanese Unexamined Patent Applications (Kokai) JP56-128579 and JP56-118267. A suitable expanded graphite having an average particle size ranging from 0.5 to 15 microns, preferably from 2 to 6 microns is disclosed in the '798 patent. The smaller average particle size of expanded graphite relative to conventional natural or synthetic crystalline graphite (e.g., 15 to 30 microns) was hypothesized to facilitate the formation of a conductive network typically at a lower volume fraction of graphite. An expanded graphite having an average particle size greater than about 30 microns was disclosed to provide no performance advantage in alkaline cells compared to a conventional non-expanded natural graphite having a comparable particle size. The '798 patent also disclosed that suitable amounts of expanded graphite can range from about 2 to 8 weight percent, and preferably from about 3 to 6 weight percent of the total cathode. Further, for expanded graphite contents of greater than about 10 weight percent no performance advantage is provided relative to equivalent amounts of unexpanded graphite particles in alkaline primary cells.
Various methods for preparing mixtures of manganese dioxide and graphite are known to provide suitable levels of electrical conductivity in cathodes of alkaline cells. Typically, graphite can be mixed dry with manganese dioxide using any of a variety of conventional blending, mixing or milling equipment. For example, U.S. Pat. No. 5,938,798 discloses the use of a twin cylinder mixer or a rotary tumbling mixer to dry mix graphite and manganese dioxide. In a subsequent step disclosed in the cited '798 patent, the formed mixture was wet-pulverized
Christian Paul A.
Mao Ou
Zeng Shuming
Chaney Carol
Douglas Paul I.
Josephs Barry D.
Krivulka Thomas G.
The Gillette Company
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