Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2001-06-13
2003-07-22
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S224000, C429S229000, C429S206000
Reexamination Certificate
active
06596438
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an improved cathode mixture comprising manganese dioxide and carbon fibers, particularly graphitized mesophase pitch-based carbon fibers.
BACKGROUND OF THE INVENTION
Conventional alkaline electrochemical cells are formed of a cylindrical casing. The casing is initially formed with an enlarged open end and opposing closed 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 casing edge over an edge of the insulating plug and radially compressing the casing around the insulating plug to provide a tight seal. A portion of the cell casing at the closed end forms the positive terminal.
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 cellulose or cellulosic and polyvinylalcohol 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, which is electrically connected to the negative terminal end cap. 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 particulate manganese dioxide as the electrochemically active material admixed with an electrically conductive additive, typically graphite material, to enhance electrical conductivity. Optionally, polymeric binders, and other additives, such as titanium-containing compounds can be added to the cathode.
Since manganese dioxide typically exhibits relatively low electric conductivity, an electrically conductive additive is needed to improve the electric conductivity between individual manganese dioxide particles. Such electrically conductive additive also improves electric conductivity between the manganese dioxide particles and the cell housing, which also serves as cathode current collector. Suitable electrically conductive additives can include, for example, conductive carbon powders, such as carbon blacks, including acetylene blacks, flaky crystalline natural graphite, flaky crystalline synthetic graphite, including expanded or exfoliated graphite.
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 useful service life of a cell can be enhanced by packing greater amounts of the electrode active materials into the cell. However, such approach has 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 fraction of conductive additive in the cathode in order to increase overall (bulk) electric conductivity of the cathode. The fraction of conductive additive within the cathode must be sufficient to form a suitable network of conductive particles. Typically, when the conductive additive is a conductive carbon, about 3 to 15, desirably between 4 to 10 weight percent of the total mixture are 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 have large volume in a certain weight but lower electric conductivity than the flaky, crystalline natural or synthetic graphite, which possess a three-dimensional crystal structure as described below. The less conductive nature of carbon blacks leads to high carbon content in cathode electrode and less electrochemically active material, in turn, to the shorter service life of electrochemical cells.
To increase the electric conductivity of carbonaceous materials, a thermal process known as graphitization is applied to convert carbons into graphite material or graphite product. Such a graphitic product is characterized by a distinctive three-dimensional graphitic crystal structure. The crystalline structure is composed of individual unit cells which are repeatable in the “a” and “c” directions. The graphite crystalline structure has a unit cell which is generally of a three dimensional hexagonal (six sided) shape. The base of the hexaganol unit is defined by a hexaganol plane of sides “a” of equal size and separated from each other by 120 degrees. The hexaganol plane defines the “a” direction of the crystalline structure. The thickness of the unit cell is defined by the height of the unit cell defined by the axis “c” which is perpendicular to said hexagonal plane lying in the “a” direction. A reference to such hexaganol unit cells using the same standard nomenclature can be found, for example, in F. Daniels and R. Alberty,
Physical Chemistry,
2
nd
Edition, John Wiley & Sons (1961), pp. 622-623. The unit cells are repeatable in the “a” direction and “c” direction up to a point where they abruptly change orientation. This defines the bounds of the crystalline structure in the “a” and “c” directions. Different graphites have different number of repeatable unit cell in the “a” and “c” direction. (The size of each repeatable unit cell for graphites will be the same.) The size of the crystalline structure (crystalline size) for a specific graphite can be defined by the distance La of the crystalline structure in the “a” direction which spans the total number of repeat units in the “a” direction, and the distance Lc in the “c” direction which spans the total number of repeat units in the “c” direction. The number of unit cells in the “a” direction can be determined by dividing the distance La by the size of the unit cell in the “a” direction. Conversely, the number of unit cells in the “c” direction can be determined by dividing the distance Lc by the size of the unit cell in the “c” direction.
In most graphitic products, for example, natural graphite, the Lc and La distances defining the crystalline size as measured by x-ray diffraction are typically in the range of 1000 to 3000 Angstrom. Expanded graphite, a typical exfoliated graphite, can have a large La of about 500 to 1000 but typically a smaller Lc of about 300 to 1000, typically between about 400 to 600 Angstrom due to chemical exfoliation in this direction. Due to the anisotropy of graphite in the “a” and “c” directions of the crystal unit cell, the La normally contributes to electric and thermal conductivity of a graphite more than Lc does. In order to provide good electric conductivity, it is desirable to use graphite including natural and synthetic graphite that have a La greater than 100, more typically between 100 to 300 Angstrom in the cathode of an alkaline cell. Conventional powdery conductive carbons such as acetylene black have small La
Douglas Paul I.
Josephs Barry D.
Krivulka Thomas G.
The Gillette Company
Weiner Laura
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