Carbon fiber sheet materials and methods of making and using...

Details Carbon fiber sheet materials and methods of making and using... Carbon fiber sheet materials and methods of making and using...

2000-12-05

2003-01-07

Juska, Cheryl A. (Department: 1771)

Fabric (woven, knitted, or nonwoven textile or cloth, etc.)

Nonwoven fabric

Including parallel strand or fiber material within the...

C442S343000, C442S354000, C442S355000, C442S381000, C442S367000, C442S368000, C442S400000, C442S035000, C442S268000, C442S036000

Reexamination Certificate

active

06503856

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to carbon fiber sheet materials and methods of making and using the same. More particularly, the invention relates to carbon fiber structures formed of substantially parallel carbon fibers and having an adhesive polymeric material adhered directly onto one or more surfaces thereof, which structures are useful as electrodes in electrical energy storage devices.
BACKGROUND OF THE INVENTION
Carbonaceous materials, such as those derived from pitch or polyacrylonitrile (PAN), can be used as an electrode material in electrical energy storage devices. For example, carbon can be used as a component of the electrode in primary batteries, primary fuel cells, secondary fuel cells, secondary batteries and capacitors. The carbon material functions as a current collector and/or as a reactive material to form new compounds which have different structures and properties than the original carbon material, and most recently, as semiconductor materials which form salts with ions of the electrolyte. Examples of carbon fiber based electrodes and batteries incorporating the same as a component are described, for example, in U.S. Pat. Nos. 4,865,931; 5,518,836; 4,830,938; 4,952,466; and 4,929,521.
In general, the batteries include electrodes formed of electrically conductive carbon fibers. The carbon fiber electrodes are placed in a suitable housing in contact with an electrolyte, typically an ionizable salt in a nonaqueous fluid. The electrodes are separated from one another in the housing to prevent short circuiting between the electrodes while allowing ions to travel between the electrodes. Typically the electrodes are separated from one another by materials that are separate and discrete from the individual electrodes. Examples of separators described in the art include sheets of fiberglass, nonwoven polymeric sheets or webs, coated metallic screens, porous films, and spacers (such as polymeric beads) extending between the electrodes.
Such batteries can offer an attractive alternative to traditional lead-acid reduction-oxidation batteries. For example, batteries including a carbon fiber based electrode can generate high power, and are generally efficient, compact, and non-toxic. However, there are difficulties associated with the production of carbon fiber based electrodes. It can be difficult to handle the carbon fiber assemblies, particularly on a commercial production scale. Carbon fiber assemblies, particularly those prepared from carbon fiber tows, have limited dimensional stability and are difficult to handle without touching and compromising the carbon fiber surface. This in turn can slow fabrication times in manufacturing the batteries and limit the reasonably expected quality and performance of batteries and components. Yet traditional techniques for stabilizing fibrous assemblies in the textile art are not readily translated into the production of carbon fiber assemblies for battery applications.
For example, carbon fiber tows have been impregnated with resin to form prepreg tapes and rovings. However, the impregnating resin would be expected to interfere with the electrical performance of the electrode. Also, impregnating a carbon fiber tow with resin would reduce the available surface area of the carbon fibers, which can also comprise performance in battery applications.
Woven and/or knit carbon fiber assemblies are typically more dimensionally stable than a fiber tow. However, it can be difficult to weave or knit carbon fibers without a size on the fiber surfaces because of the inherent stiffness and brittle nature of these fibers. The size, however, can be difficult to remove from the woven and/or knit assembly. If left on the fibers, the sizing can interfere with the electrical performance of the carbon fiber electrode. Still further, woven and knit carbon fiber assemblies can have significantly reduced percent surface area available for interaction with the electrolyte solution. Also, the surface geometry of a knit or woven fabric is irregular, which can cause electrical inefficiencies because ions will transfer preferentially to the high peaks of the fabric.
SUMMARY OF THE INVENTION
The present invention provides carbon fiber sheet materials, which are useful for a variety of applications, including use as electrodes in electrical energy storage devices. The carbon fiber sheets of the invention include a network formed of a plurality of carbon fibers or filaments, arranged substantially parallel relative to one another. To provide improved dimensional stability and ease of handling, the carbon fiber networks include an adhesive polymeric material adhered directly onto one, and preferably both, surfaces of the carbon fiber network as an integral part of the carbon fiber network. The adhesive polymeric material is in the form of a porous or permeable layer so as to allow the passage of ions therethrough and into contact with the carbon fiber network. Advantageously the adhesive layer can be releasably adhered to the surface of the carbon fiber network.
In one advantageous embodiment of the invention, the adhesive polymeric material is a melt blown web which is melt blown and adhered directly onto one or more surfaces of the carbon fiber network. However, the adhesive material can also be present in other forms, such as but not limited to, other types of fibrous webs (such as spunbonded webs), microporous films, a discontinuous pattern of adhesive, and the like.
Despite the presence of the adhesive on a surface of the carbon fibers, the electrical performance of the carbon fibers as an electrode is not significantly compromised. In this regard, the adhesive material is applied so that only a small percentage of the carbon fiber surface is used or contacted by the adhesive. This in turn allows ions to be less restricted in their movement.
The resultant stabilized carbon fiber assemblies can be more readily handled, particularly when fabricating batteries, thus reducing production times and costs. Further, the carbon fiber networks of the invention can provide cost benefits by providing an integral separator/carbon fiber electrode assembly, in contrast to conventional separators, which are discrete and separate from the electrode.
The carbon fiber network can be a woven, knit or nonwoven substrate. In one advantageous embodiment of the invention, the carbon fiber network is formed from one or more tows or bundles of carbon fibers. Preferably the fibers of the tow(s) are spread out relative to one another to form a substantially planar sheet prior to applying the adhesive layer web to a surface thereof. The added flexibility in production provided by stabilizing the tow with the adhesive layer is particularly advantageous because spreading the fibers of the tow to form a planar sheet increases the surface area available for interaction with the electrolyte solution.
This also has the advantage of improving control of the uniformity of the tow thickness across the width and length dimensions thereof. As a result, carbon fiber sheet materials can be manufactured that have a substantially uniform thickness. This in turn can be particularly advantageous for battery performance. In this regard, battery performance is related at least in part to providing a substantially uniform or homogeneous ratio of electrolyte mass to carbon fiber mass. Thus battery performance can be improved by minimizing inconsistencies in the distances between fibers, and thus the distances required for ions to travel between carbon fibers.
In another embodiment of the invention, the carbon fiber structure is a woven or nonwoven web formed of intersecting transverse (weft) yarns and longitudinal (warp) yarns. The warp yarns are formed of carbon fibers or filaments and can be mono-filament or multifilament yarns. Preferably the weft yarns are thermoplastic, polymer coated fiberglass yarns, and more preferably polyolefin coated fiberglass yarns, such as polypropylene coated fiberglass yarns. Alternatively, the weft yarns can be formed of a thermoplastic material, such as a

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