Fabrication of electrodes and devices containing electrodes

Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode

Utility Patent

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C029S623100

Utility Patent

active

06168885

ABSTRACT:

FIELD OF THE INVENTION
The field of the invention is fabrication of electrodes, including fabrication of batteries, supercapacitors, and other devices utilizing electrodes.
BACKGROUND OF THE INVENTION
Electrolytic cells containing an anode, a cathode, and a solid electrolyte are known in the art, and are commonly referred to as “solid” or “polymer” batteries. In such batteries discharge is characterized by lithium or other ions from the anode passing through the electrolyte to the electrochemically active material of the cathode, where the ions are taken up with the simultaneous release of electrical energy. During charging, the flow of ions is reversed so that lithium or other ions pass from the electrochemically active cathode material through the electrolyte, and are reintroduced back onto or into the anode. Because solid electrolytes typically have very poor conductivity, the electrolyte typically also contains a solvent or “plasticizer”.
The negative electrode is the anode during discharge. Numerous anode active materials are known in the art, including lithium, alloys of lithium with aluminum, mercury, manganese, iron, or zinc, and intercalation anodes using various forms of carbon such as graphite, coke, mesocarbon microbeads, and tungsten, tin, or other oxides. Intercalation anodes typically also include a polymeric binder, i.e., a film-forming agent, suitable for forming a bound porous composite. The polymeric binder generally exhibits a molecular weight of from about 1,000 to about 5,000,000. Examples of suitable polymeric binders include ethylene propylene diene monomer (EPDM); polyvinylidene fluoride (PVDF), PVDF copolymers, ethylene acrylic acid copolymer (EAA), ethylene vinyl acetate copolymer (EVA), styrene-butadiene rubber (SBR), carboxymethylcellulose, polyacrylonitrile (PAN), and the like.
The positive electrode is the cathode during discharge. Numerous cathode active materials are also known in the art, including transition metal oxides, sulfides, and selenides. Representative materials include oxides of cobalt, nickel, manganese, molybdenum, and vanadium, sulfides of titanium, molybdenum, and niobium, chromium oxides, copper oxides, and lithiated oxides of cobalt, manganese and nickel, and the like. Intercalation cathodes are also known, and may utilize the same binders employed in the manufacture of anodes.
The active materials for anodes and cathodes are typically produced by suspending particulate material and the binder in a solvent to form an electrode paste. In the case of an anode, the particulate material would be an anode active material. In the case of a cathode, the particulate material would be cathode active material. The electrode paste is then layered onto a current collector, and the solvent is removed by volatilization (drying) or other methods. The porous electrode structure which remains includes particulate electrode material held adjacent to a current collector.
Anode current collectors typically include foils or grids comprising nickel, iron, stainless steel, or copper. Cathode current collectors typically include foils or grids comprising aluminum, nickel, iron, or stainless steel. An adhesion promoter can also be used to facilitate bonding between the anode or cathode material and its corresponding current collector.
Numerous solvents are known for the production of electrode pastes, depending on the desired process parameters. However, since the electrode paste solvent is generally removed by drying, the solvent is usually volatile. Commonly used solvents include acetone, xylene, alcohols, cyclohexanone, dichloromethane, dimethylacetamide (DMA), dimethylformamide (DMF), hexamethylphosphoramide (HMP), dimethylsulfoxide (DMSO), 1-methyl-2-pyrrolidone or N-methylpyrrolidone (NMP), etc., and mixtures thereof.
It is also known to remove the electrode paste solvent with an extracting solvent to produce a “dry” battery precursor, and then imbibe an electrolyte solvent and electrolyte salt into the dry precursor. Typical extracting solvents are diethyl ether and hexane.
In the manufacture of a solid polymer electrolytic cell, a viscous electrolyte precursor is typically deposited onto the anode or the cathode. The electrolyte precursor includes monomers or other polymerizable compounds, which are then cured to form the solid electrolyte. Curing is generally accomplished by application of heat, UV light, or other energy source. The final electrolyte typically comprises from about 5 to about 25 weight percent of the inorganic ion salt based on the total weight of the electrolyte. The percentage of salt depends on the type of salt and electrolytic solvent employed.
Solid polymer electrolytes are generally divided into the “dry” polymer electrolytes based on PEO, and gel polymers such as polyvinylidene fluoride (PVDF) based systems. Polymer systems that do not contain a plasticizer are known, but known plasticizer free systems must generally be operated at elevated temperatures. In secondary batteries, temperatures of the order of 80° C. are usually needed to obtain acceptable conductivity levels. This makes the PEO system impractical for use in many applications, in particular, the portable electronics markets. The gel based or plasticized polymer electrolyte systems offer a significant improvement in lithium ion conductivity at room temperature.
For systems that do contain plasticizers (electrolyte solvents), numerous such solvents are known. Examples include cyclic and non-cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), dipropyl carbonate (DPC), dibutyl carbonate (DBC), as well as acetates, diesters such as oxalate, succinate, adipate, suberate and sebacate, which may or may not be substituted, low molecular weight polymers such as polycarbonates, polyacrylates, polyesters, and various other substances including polysiloxanes, gamma-butyrolactone, triglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane, and so on. When using propylene carbonate based electrolytes in an electrolytic cell with graphite anodes, a sequestering agent, such as a crown ether, is also often added in the electrolyte.
A pre-wetting agent may be employed to improve the interface between electrode and electrolyte (see U.S. Pat. No. 5,700,300 to Jensen et al, December 1997). Such agents are known to include a plasticizing solvent and a matrix forming polymer. Suitable solvents are well known in the art and include, for example, organic solvents such as ethylene carbonate, propylene carbonate, as well as mixtures of these compounds. Higher boiling point plasticizer compounds, such as dibutyl phthalate, dimethyl phthalate, diethyl phthalate, and tris butyoxyethyl phosphate are also suitable as long as the viscosity of the pre-wet material can be maintained at a suitably low level. Suitable solid polymeric matrix precursors are well known in the art, and include inorganic polymers, organic polymers, or a mixture of polymers with inorganic non-polymeric materials. One typical polymeric matrix precursor is urethane acrylate.
A secondary battery typically comprises several solid electrolytic cells in which the current from each of the cells is accumulated by a current collector. The total current generated by the battery is roughly the sum of the current generated from each of the individual electrolytic cells employed in the battery. Such an arrangement enhances the overall current produced by the battery to levels that render such batteries commercially viable.
Often, the various cells are spiral wound before being provided with a protective coating, with a porous separator inserted between anode and cathode to prevent shorting. It is also contemplated that an inert filler can be added to the electrolyte formulation to act as a separator. This filler can be an inert oxide such as alumina, a polymer powder such as polypropylene, or specialized separator pillars or particles designed to improve the laminate stability. Alternatively, the filler could be a woven o

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