Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1997-12-19
2001-02-27
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S212000, C429S215000, C429S218100, C029S623500
Reexamination Certificate
active
06194099
ABSTRACT:
TECHNICAL FIELD
The present invention pertains generally to the field of cathodes and electric current producing cells. More particularly, the present invention pertains to solid composite cathodes which comprise (a) an electroactive sulfur-containing cathode material, which in its oxidized state, comprises a polysulfide moiety of the formula, -S
m
-, wherein m is an integer from 3 to 10; and (b) non-activated carbon nanofibers. The present invention also pertains to electric current producing cells comprising such composite cathodes, and to methods of making such composite cathodes and electric current producing cells.
BACKGROUND
Throughout this application, various publications, patents and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
An electroactive material that has been fabricated into a structure for use in a battery is referred to as an electrode. Of a pair of electrodes used in a battery, herein referred to as an electric current producing cell, the electrode on the electrochemically higher potential side is referred to as the positive electrode, or the cathode, while the electrode on the electrochemically lower potential side is referred to as the negative electrode, or the anode.
An electrochemically active material used in the cathode or positive electrode is referred to hereinafter as a cathode active material. An electrochemically active material used in the anode or negative electrode is hereinafter referred to as an anode active material. An electric current producing cell or battery comprising a cathode with the cathode active material in an oxidized state and an anode with the anode active material in a reduced state is referred to as being in a charged state. Accordingly, an electric current producing cell comprising a cathode with the cathode active material in a reduced state, and an anode with the anode active material in an oxidized state, is referred to as being in a discharged state.
As the evolution of batteries continues, and particularly as lithium batteries become more widely accepted for a variety of uses, the need for safe, long lasting, high energy batteries becomes more important. There has been considerable interest in recent years in developing high energy density cathode-active materials and alkali metals as anode active materials for high energy primary and secondary batteries.
To achieve high capacity in electric current producing cells or batteries, it is desirable to have a high quantity or loading of electroactive material in the cathode layer. For example, the volume of cathode coating layer in an AA size battery is typically about 2 cm
3
. If thc specific capacity of the electroactive material is 1000 mAh/g, the amount or volumetric density of the electroactive material in the cathode coating layer would need to be at least 500 mg/cm
3
in order to have the 1 gram of cathode active material in the AA size battery necessary to provide a capacity of 1000 mAh or 1 Ah. If the volumetric density of electroactive material in the cathode coating layer can be increased to higher levels, such as greater than 700 mg/cm
3
, the capacity of the battery can be proportionately increased to higher levels.
There are a wide variety of electroactive materials that are utilized in the cathode layers of electric current producing cells. For example, a number of these are described in copending U.S. patent application Ser. No. 08/859,996, titled “Novel Composite Cathodes, Electrochemical Cells Comprising Novel Composite Cathodes, and Processes for Fabricating Same” by common assignee. These electroactive materials vary widely in their specific densities and in their specific capacities so the desired volumetric densities in mg/cm
3
correspondingly vary over a wide range. Lithium and sulfur are highly desirable as the electrochemically active materials for the anode and cathode, respectively, of electric current producing cells because they provide nearly the highest energy density possible on a weight or volume basis of any of the known combinations of active materials. To obtain high energy densities, the lithium can be present as the pure metal, in an alloy, or in an intercalated form, and the sulfur can be present as elemental sulfur or as a component in an organic or inorganic material with a high sulfur content, preferably above 50 weight per cent sulfur. For example, in combination with a lithium anode, elemental sulfur has a theoretical specific capacity of 1680 mAh/g, and carbon-sulfur polymer materials with trisulfide and longer polysulfide groups in the polymer have shown specific capacities of 1200 mAh/g. These high specific capacities are particularly desirable for applications, such as portable electronic devices and electric vehicles, where low weight of the battery is important.
Herein, the term “carbon-sulfur polymer materials” means materials comprising carbon-sulfur polymers with carbon-sulfur single bonds and with sulfur-sulfur bonds forming trisulfide (-SSS-) and higher polysulfide linkages. These carbon-sulfur polymer materials comprise, in their oxidized state, a polysulfide moiety of the formula, -S
m
-, wherein m is an integer equal to or greater than 3. For example, these carbon-sulfur polymer materials are described in U.S. Pat. Nos. 5,601,947; 5,609,702; 5,529,860; and in copending U.S. patent application Ser. No. 08/602,323 to Skotheim et al., now abandoned. Organo-sulfur materials with only disulfide groups typically show specific capacities in the range of 300 to 700 mAh/g and are accordingly less desirable for those applications requiring high specific capacities.
It is well known to those skilled in the art of battery design and fabrication that practical battery cells comprising the electroactive cathode and anode materials also contain other non-electroactive materials such as a container, current collectors, electrode separators, polymeric binders, conductive additives and other additives in the electrodes, and an electrolyte (typically an aqueous or non-aqueous liquid, gel, or solid material containing dissolved salts or ionic compounds with good ionic conductance but poor electronic conductivity). All of these additional non-electroactive components are typically required to make the battery perform efficiently, but they also serve to reduce the gravimetric and volumetric energy density of the cell. It is, therefore, desirable to keep the quantities of these non-electroactive materials to a minimum so as to maximize the amount of electroactive material in the battery cell.
To achieve the highest possible volumetric density of the electroactive material in the cathode coating layer, it is desirable to maximize the weight per cent for electroactive materials in the coating layer, for example, 65 to 85 weight per cent for electroactive materials of a specific density of 2 g/cm
3
, and to maintain the porosity or air voids in the cathode coating layer as low as possible, for example, 40 to 60 volume percent. Particularly, the porosity of the cathode coating layer must be kept low because higher porosities, for example, 70 to 85 volume per cent, do not provide enough electroactive material to obtain very high cell capacities. With the electroactive transition metal oxides, this is often relatively easy to achieve because these oxides typically have electrically conductive properties and are typically microporous so that high levels of added conductive fillers and microporous additives are not required. With electroactive sulfur-based compounds, which have much higher specific capacities than the electroactive transition metal oxides, it is difficult to obtain efficient electrochemical utilization of the sulfur-based compounds at high volumetric densities because the sulfur-based compounds are highly electrically non-conducting or insulative and
Gernov Yordan M.
Xu Zhe-Sheng
Dove Tracy
Kalafut Stephen
Moltech Corporation
Morrison & Foerster / LLP
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