Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1998-12-17
2001-10-16
Chaney, Carol (Department: 1745)
Metal working
Method of mechanical manufacture
Electrical device making
C429S213000
Reexamination Certificate
active
06302928
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 methods of forming solid composite cathodes with cathode active layers which comprise an electroactive sulfur-containing material and an electrically conductive material, wherein the electroactive sulfur-containing material is heated to a temperature above its melting point and then the melted electroactive sulfur-containing material is resolidified to form a cathode active layer having redistributed sulfur-containing material of high volumetric density. The present invention also pertains to methods of forming electric current producing cells comprising such solid composite cathodes and having a high electrochemical utilization, and to solid composite cathodes and electric current producing cells formed using such methods.
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 density, and light weight 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 active layer. For example, the volume of the cathode active layer in an AA size battery is typically about 2 cm
3
. If the specific capacity of the electroactive material is a very high value, such as 1000 mAh/g, the amount or volumetric density of the electroactive material in the cathode active 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. If the volumetric density of electroactive material in the cathode active layer can be increased to higher levels, such as greater than 900 mg/cm
3
, the capacity of the battery may be proportionately increased to higher levels if the specific capacity of the electroactive material does not decrease significantly when the cathode active layer becomes denser and less porous.
There are a wide variety of electroactive materials that may be utilized in the cathode active 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, to Mukherjee et al. to the common assignee. These electroactive materials vary widely in their specific densities (g/cm
3
) and in their specific capacities (mAh/g) so the desired volumetric densities in mg/cm
3
of the electroactive material in the cathode active layer 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 may be present as the pure metal, in an alloy, or in an intercalated form, and the sulfur may be present as elemental sulfur or as a component in an organic or inorganic material with a high sulfur content, preferably above 75 weight percent sulfur. For example, in combination with a lithium anode, elemental sulfur has a specific capacity of 1680 mAh/g, and sulfur-containing polymers with trisulfide and longer polysulfide groups in the polymers have shown specific capacities of more than 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 “sulfur-containing polymers” pertains to polymers comprising sulfur-sulfur bonds forming trisulfide (—S—S—S—) and higher polysulfide linkages. These sulfur-containing polymers comprise, in their oxidized state, a polysulfide moiety, S
m
, selected from the group consisting of covalent —S
m
— moieties, ionic —S
m
−
moieties , and ionic S
m
2−
moieties, wherein m is an integer equal to or greater than 3. For example, sulfur-containing polymers comprising covalent —S
m
— moieties are described in U.S. Pat. Nos. 5,601,947; 5,690,702; 5,529,860; and copending U.S. Patent application Ser. No. 08/602,323, all to Skotheim et al. Sulfur-containing polymers comprising ionic —S
m
−
moieties are described in U.S. Pat. No. 4,664,991 to Perichaud et al. Also, for example, sulfur-containing polymers comprising ionic S
m
−
moieties are described in the aforementioned U.S. Pat. No. 4,664,991 and in European Patent No. 250,518 B 1 to Genies. Organo-sulfur materials with only disulfide (—S—S—) moieties typically show specific capacities only in the range of 300 to 700 mAh/g and are accordingly much less desirable for those applications requiring high specific capacities.
It is known to those skilled in the art of battery design and fabrication that practical battery cells comprising the electroactive cathode and anode materials also typically contain other non-electroactive materials such as a container, current collectors, separator, and electrolyte, in addition to polymeric binders, electrically conductive additives, and other additives in the electrodes. The electrolyte is typically an aqueous or nonaqueous liquid, gel, or solid material containing dissolved salts or ionic compounds with good ionic conductance, but with poor electronic conductivity. All of these additional non-electroactive components are typically utilized 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 active layer, it is desirable to maximize the weight percent for electroactive materials in the cathode active layer, for example, 65 to 85 weight percent of electroactive materials of a specific density of about 2 g/cm
3
, such as most high energy density sulfur-containing materials have, and to mai
Gernov Yordan M.
Skotheim Terje A.
Xu Zhe-Sheng
Chaney Carol
Moltech Corporation
Morrison & Foerster / LLP
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