NO844833L - LEAD - Google Patents

Description

This invention relates to the use of a carbonaceous material in connection with an electron collector as an electrode for secondary storage devices for electrical energy. The carbonaceous material in the electrode is stable in the presence of an electrolyte system containing anions such as perchlorates, hexafluoroarsenates and the like, at ambient or normal operating temperatures for the electrode's use. This means that the carbonaceous material does not irreversibly swell or contract to a significant extent during cycles of deep electrical charge and discharge as may be done in the application of a secondary storage device for electrical energy.

Numerous patents and technical literature describe devices for storing electrical energy in which a carbonaceous material such as carbon or graphite is used as electrode material. One of the first devices was, as is well known, Leclanché's battery from 1866 in which carbon was used as

an electron collector in a primary Zn/NH^Cl/MnC^ battery. Since then, carbon has found extensive use as a constituent

part of the electrode in primary batteries, primary fuel cells, secondary fuel cells, secondary batteries and capacitors. The function of carbon or graphite in these above-mentioned devices has primarily been as a current collector or as a reactive material to form new compounds with fluorine which have different structures and properties than the original carbon/graphite, and more recently as semiconductor materials that form salts with the ions of the electrolyte. These previously known devices can be categorized as: primary batteries, which is described by Coleman et al. in the U.S. patent 2,597,451, Panasonic Lithium Battery literature, and U.S. patent no. 4,271,242, 3,700,502 and

4,224,389; fuel cells, such as Japanese Patent Publication No. 54-082043; and secondary fuel cells with limited recharging, as described by Dey et al. in the U.S. patent no. 4 0 37 025, a rechargeable fuel cell in which an activated graphite (with a large surface area) is used; rechargeable secondary batteries (accumulators) as described by Hart in the U.S. patent 4 251 568, where graphite is used as current collector, and Bennion in the U.S. patent nos. 3,844,837 and 4,009,323, as well as capacitors as described by Butherus et al. in the U.S. patent 3 700 975 or DE patent 3 231 243 using a carbon material (graphite) with a large surface area. In some of these devices, ionizable salts dissolved in a non-conductive solvent are also used.

The carbonaceous materials described in the patents and in the literature are materials that are graphitized or carbonized until the materials become electrically conductive. These materials are derived from polyacetylenes, polyphenylenes, polyacrylonitriles and petroleum pitch that have been heated so that the precursor material becomes "carbonized and/or graphitized", whereby the material becomes electrically conductive to some extent. Part of the graphites that have been used previously according to the literature are graphites such as reinforced pyrolytic graphite, RPG (Reinforced Pyrolytic Graphite),

R-1 nuclear reactor graphite grade, PGCP (Pyrolytic Graphite

Carbon Paper) and "GRAFOIL" (trademark, Union Carbide Corporation) consisting of an expanded and compressed graphite, and the like.

Doping of analogous carbonaceous materials has also been reported in Chemical and Engineering News, Volume 60, No. 16, pages 29-33, April 19, 1982, in an article entitled "Conducting Polymers R&D Continues to Grow", Journal Electrochem Society, Electrochemical Science, 118, No. 12, pages 1886-1890, December 1971:, and Chemical&Engineering News, 59, No. 41, pages 34-35, October 12, 1982, in an article entitled "Polymer Cell Offers More Power, Less Weight".

The problems with these reported cells is that they do not

has a long life, as the electrode made of such carbonaceous materials easily deteriorates when subjected to repeated electrical charge and discharge cycles.

For example, the U.S. describes patent 3,844,837 (Bennion

et al.) a battery in which a core-reactor grade graphite impregnated with grains of Li2° is used as the positive electrode and copper as the negative electrode in a LiCF^SO^-dimethylsulphite (DMSU) electrolyte. The graphite electrode was made from a nuclear reactor graphite, grade R-1 (from Great Lakes Carbon Company) and was reported to be flake-like

after electrical charge and discharge for 9 cycles. The patentees also tested a graphite cloth and found it unsatisfactory.

Several other graphites were used with equally unsatisfactory results; the best results were obtained with pyrolytic graphite which failed after 33 cycles. Dey et al. uses a carbon or graphite material with a high specific surface, and a chemical reaction takes place in the material's pores, but said material is generally assumed to have a low conductivity of

due to lack of continuity in the carbon surface. Furthermore, it is believed that such materials do not maintain the dimensional stability and material resistance that is necessary for the reversible formation of carbon complexes that is required for secondary batteries to have a long life during charge and discharge cycles.

Experiments conducted during the development of the present invention included the use of "GRAFOIL" (Trade Mark), which failed after the first electrical charge, and RPG ("Super Temp") graphite electrodes, which also failed. It was found that a proportion greater than 20% of the positive electrode made of RPG graphite was lost as flakes, chips and powder after only 27 electrical charge and discharge cycles.

It should be noted that the state of the art states that the degradation of and damage to the electrode is a result of swelling and shrinking of the electrode body, and that this swelling and shrinking increases with each electrical charge and discharge cycle, which deforms the small graphite plates, which flaking due to the stress of swelling and shrinking. In carrying out these experiments during the development of the present invention, it was confirmed that such flaking of the graphite sheets takes place when the above-mentioned graphite materials were subjected to repeated electrical charge and discharge cycles.

According to a first aspect, the present invention provides an electrode for use in a secondary device for storing electrical energy, comprising an electrode body of electrically conductive carbonaceous material, having a shell orientation, at least at or near the surface, and a -

with electrically connected current collectors, where the carbonaceous material has a Young's modulus greater than 6.9 GPa and undergoes a physical dimensional change of less than 5% during repeated electrical charge and discharge cycles.

According to another aspect, the invention provides a secondary device for storing electrical energy, comprising a housing having an electrically non-conductive inner surface and a moisture-impermeable outer surface or laminar body and with at least one cell arranged in said housing , where each cell comprises at least one pair of electrically conductive electrodes which are electrically isolated from each other, which housing contains a mainly non-aqueous electrolyte, where at least one of the electrodes in each cell is an electrode according to the invention.

According to a further aspect, the invention resides in a secondary device for storing electrical energy, comprising a housing having an electrically non-conductive inner surface and a moisture-impermeable outer surface or laminated body and with at least one cell arranged in said housing, each cell comprising at least one pair of electrically conductive electrodes which are electrically isolated from each other, which housing contains a substantially non-aqueous electrolyte, each of the electrodes in each cell being an electrode as set forth in claim 1, 2 or 3 , and where each of the mentioned electrodes offers freedom of choice with regard to polarity during recharging and has the ability to be partially and/or completely oppositely polarized without damage.

The electrodes can be separated from each other by a distance or by a non-electrically conductive, ion-permeable material.

PHYSICAL PROPERTIES

The electrically conductive carbonaceous material in the electrode should preferably meet the following criteria: (1) A Young's modulus greater than 6.9 GPa, preferably from 69 GPa to 380 GPa, more preferably from 138 GPa to 311 GPa. (2) An aspect ratio greater than 100:1. The aspect ratio is defined herein as the ratio between the length and the diameter, l/d, of a fibrous or filamentous cord of the carbonaceous material, or as the length/depth ratio when the carbonaceous material is shaped as a flat sheet.

(3) The structural and mechanical resistance of

the carbonaceous material, regardless of its manufacturing form

(woven, knitted or non-woven of continuous filaments or staple fibers or a film) should be such that it does not require the presence of a support such as a press plate (front films or cloth) to maintain the carbonaceous material in the desired sheet or plate-like form through at least 100 charge/discharge cycles. (4) A specific surface area of at least 0.1 m 2 /g, but less than that associated with activated absorbent carbon, suitably less than 50 m 2 /g, preferably less than 10 m 2 /g, and more preferably less than 5 m2/g. (5) Sufficient strength/durability of the shape of the carbonaceous material so that the material retains its plate- or sheet-like shape when it has a size from at least 6.5 cm 2 to at least 930 cm 2 , without support other than a metallic current collector- frame that forms the edge part of the electrode. (6) The secondary device for storing electrical energy in which the electrode according to the invention is used should be essentially water-free so that the water content is less than 100 ppm. Preferably, the water content should be below 20 ppm and most preferably below 10 ppm. The device according to the invention is able to work even if the water content is up to 300 ppm, but will have a somewhat reduced cycle life. Furthermore, it will be understood that should the water content become too high, the device can be disassembled, dried and reassembled in a dry state without significant damage to its continued operability.

PERFORMANCE CRITERIA.

(7) The carbonaceous material in an electrode should be able to withstand more than 100 electrical charge and discharge cycles without any significant damage due to flaking of the carbonaceous material. Preferably, no significant damage should have occurred after at least 500 electrical charge and discharge cycles at a discharge capacity of at least 150 coulombs per second. grams of carbonaceous material in an electrode. (8) The coulometric (coulomb) yield of the carbonaceous material in the electrode should be greater than 70%, preferably greater than 80% and most preferably greater than 90%. (9) The carbonaceous material in the electrode should be capable of withstanding deep electrical discharges of over 70% of the electrical charge capacity for at least 100 electrical charge and discharge cycles, preferably over 80% for at least 500 electrical charge and discharge cycles.

Accordingly, the carbonaceous material in an electrode exhibiting the physical properties described above should preferably be capable of withstanding electrical discharge and recharging over at least 100 cycles at a discharge capacity of at least 150 coulombs per second. gram of carbonaceous material in an electrode and at a coulometric yield of at least 70% without any significant irreversible change in dimensions (dimensional change less than approx. 5%).

The carbonaceous material is usually prepared by heating a precursor material to a temperature above 850°C until it is electrically conductive. Carbonaceous precursor starting materials that can form the electrically conductive, oriented carbonaceous material in the electrode can be made from pitch (petroleum or coal tar), polyacetylene, polyacrylonitrile, polyphenylene, "SÅRAN" (trademark) and the like. The carbonaceous precursor starting material should have some degree of skeletal orientation; many of these materials either have significant concentrations of oriented groupings with a benzenoid structure or groupings which, when heated, can undergo conversion to benzenoid or equivalent skeletal orientation at or near the surface due to the skeletal orientation of the starting material.

Examples of preferred carbonaceous precursor materials which exhibit such skeletal orientation after heating are assemblies of multifilament or monofilament threads or fibers made from petroleum pitch or polyacrylonitrile. Such multifilament or monofilament threads or fibers can easily be converted into yarn, from which clothing-like products can then be produced. A technique for making suitable monofilament fibers is described in U.S. Pat. patent 4 005 183, where the fibers are converted into a yarn which is then woven into a cloth. This is then subjected to a temperature, usually above 1000°C, which is sufficient for the cloth to carbonise so that the carbon-containing material becomes electrically conductive, and whereby the material is given the physical properties described above under sections (1) to (6). Such a cloth is, in connection with an electron collector, particularly well suited for use as an electrode in the secondary device according to the present invention for storing electrical energy.

The carbonaceous precursor material is advantageously i

form of a continuous filament fiber, one or more threads consisting of one or more continuous filaments or non-continuous fiber rope (yarn), from which products such as woven, non-woven or knitted products can be made, or staple fibers as such can be layered to form of cloth, paper-like or foot-like flat products. However, acceptable results are obtained when yarns made from short fibers, with a length of from 1 to 10 cm, are woven into a garment-like product (provided that such short fibers, when heat-treated, still have the necessary physical properties mentioned above under ( 1)

to (6)). It should of course be understood that while it is advantageous to convert the precursor material, preferably in a stabilized state (such as obtained by oxidation) into the desired form (knitted, woven or felted) before the carbonization, then such construction can

tion is carried out after carbonisation if the modulus is below approx.

380 GPa and preferably below 269 GPa for machine fabrication.

It should of course be understood that the carbonaceous material can be produced from a film precursor.

The degree of carbonization and/or graphitization appears

not be a decisive factor for the material's performance as an electrode element in an electrical storage device, except that it must be sufficient to make the material sufficiently

electrically conductive and also sufficient to provide the above-mentioned physical and mechanical properties under the

ted conditions of use. Carbonaceous materials that have approx. 90% carbonation is referred to in the literature as partially carbonized. Carbonaceous materials with from 91 - 98% carbonization are referred to in the literature as a carbonized material, while materials with a degree of carbonization above 98% are referred to as graphitized. It has surprisingly been found that carbonaceous materials

with a degree of carbonization of from 90 to 99% have not been suitable as electrode materials unless the carbonaceous material has the required dimensional ionic stability during electrical charge and discharge cycling. For example, it is mentioned that RPG-graphite and "GRAFOIL", which have the required degree of carbonisation, electrical conductivity and specific surface, do not have the required physical properties with regard to Young's modulus and aspect ratio and have thus not proven to be satisfactory.

According to the invention, a rechargeable and polarity-reversible device for storing electrical energy can be produced by arranging at least one pair of electrodes made of the above-described carbon-based material and the associated electron collector (which is electrically conductive)

in a container/house. The container has a non-conductive inner surface and is impermeable to moisture. The electrodes are immersed in a non-aqueous (as water is present in an amount less than about 100 ppm) fluid in the container. The fluid itself must be able to form, or contain dissolved, at least one ionizable metal salt. Each electrode consists of the carbon-based heat-treated material, according to the present invention, connected to an electron collector which is preferably insulated against contact with the electrolyte fluid.

The secondary device according to the invention for storing electrical energy can be constructed without the capability of polarity reversal by placing the above-mentioned electrically conductive carbon-based fiber assembly, such as a cloth, and the electron collector as the positive electrode alternating with a negative electrode, which can be constructed of a metal, such as lithium, or a metal alloy, and the electrodes are placed in a substantially non-aqueous fluid, which fluid itself is capable of forming, or which contains, at least one ionizable soluble metal salt dissolved therein to provide of electrolyte ions.

In the construction of a preferred embodiment of the secondary electrical storage device according to the present invention, conventional porous separators made of fiberglass, polymeric materials or composites of polymeric materials or composites of polymeric materials can be used, and are preferably used, to separate the positive and negative electrodes from each other . Preferably, a non-woven polypropylene sheet is used as a separator, as it has the desired degree of porosity and yet has a sufficiently curved path/way to prevent carbon-based fibers from penetrating, thereby preventing electrical short-circuiting. The porous separators also have a beneficial effect in that they constitute a stiffening or support for the electrodes.

Energy storage devices which are placed in fluid-tight containers are known, generally speaking. Such containers can be suitably used in the present invention provided that the material in the container is electrically non-conductive and impermeable to gases and/or moisture (water or water vapour). The materials found to be chemically compatible as the container material include polyvinyl chloride, polyethylene, polypropylene, polytrifluoroethylene and related perfluorinated polymers "instant set polymer" (ISP)", a rapid setting reactive urethane compound, of amides, a metal coated with a non-conductive polymeric material, such as an epoxy material, for example "DER" 331 or with "DERAKANE", "ZETABON" (trademarks of The Dow Chemical Company) and/or glass or a metal oxide, fluoride or the like. Container materials which are not found suitable in the preferred propylene carbonate system include acrylic plastics, polycarbonate and nylon.Acrylic plastics crack, and polycarbonates both crack and become very brittle, while nylons (except aramids) are chemically reactive.

A container material must, in addition to being compatible, also form an absolute barrier corresponding to less than 2.15 g H20/yr r/m 2 against the penetration of water vapor from the container's surroundings. No hitherto known thermoplastic material alone forms this absolute barrier against moisture at a thickness usable for battery containers. Currently there are only metals, e.g. aluminum or mild steel, which forms an absolute barrier against moisture at foil thicknesses. Aluminum foil with a thickness greater than 0.038 mm has been shown to be substantially impermeable to water vapor. It has also been shown that an aluminum foil as thin as 0.009 mm when laminated to other materials can provide sufficient protection against the penetration of water vapour. Suitable containers made of metal/plastic laminate CED-epoxy-coated metal (cathodically electro-deposited), or metal with an inner lining of plastic or glass currently meet the requirements both with regard to chemical compatibility and barrier against moisture. Most of the cells and batteries that have been manufactured to date have been tested either in a dry box with an H^O content of less than 5 ppm, a glass cell or a double-walled container where the space between the walls is filled with an activated molecular sieve material, for example 5A zeolite.

The electrolyte fluid consists mainly of a non-conductive, chemically stable, non-aqueous solvent for the ionizable salt or salts, wherein the ionizable salt is dissolved in the solvent. The compounds that are generally known in the field can be used as solvents, such as, for example, compounds that have oxygen, sulfur and/or nitrogen atoms bound to carbon atoms in an electrochemically non-reactive state. Nitriles such as acetonitrile, amides such as dimethylformamide, ethers such as tetrahydrofuran, sulfur compounds such as dimethylsulphite, and other compounds such as propylene carbonate are preferably used. It will of course be understood that the solvent itself can be ionizable under the conditions of use, to a sufficient extent to provide the necessary ions in the solvent. The ionizable salt must thus be at least partially soluble and ionizable either when it dissolves and dissolves in the solvent or after liquefaction. While it will be understood that sparingly soluble salts can be used, it will be clear that the rate of electrical charging and discharging can be adversely affected by the low concentration of such salts in solution.

Ionizable salts that can be used in the execution

of the invention, are those indicated in the state of the art and include salts of the more active metals, such as for example alkali metal salts, preferably of lithium, sodium or potassium, or mixtures thereof containing stable anions such as perchlorate (C104=), tetrafluoroborate (BF =), hexafluoroarsenate (AsFb ,=), hexafluoroantimonate (SbF 6=) or hexafluorophosphate (PF6=).

The electrolyte (solvent and salt) must be essentially anhydrous, i.e. that it should contain less than 100 ppm water, preferably less than 20 ppm water and preferably less than 10 ppm water. Of course, the electrolyte can be prepared with more than the desired amount of water and dried, e.g. over activated molecular sieve materials of the type zeolite 5A. Such funds may also be included

in the final battery to ensure that the requirement for low water content is met. The electrolyte should also be such that it allows ions (anions and cations) of the ionizable salt to move freely through the solvent when the electrical charge-

and discharge potential moves the ions to and from their respective poles (electrodes).

When the electrode is constructed as a cloth or sheet, it includes an electron collector in conductive connection with at least one of the edges of the carbon-based fibers or sheet. The edge or edges are preferably further protected by a material to insulate the collector and substantially protect the electron collector from contact with the fluid and its electrolyte ions. The protective material must of course be unaffected by the fluid or the electrolyte ions.

The current collector is in intimate contact with the carbon-based material in the electrode at least along one edge and preferably at all four edges when the carbon-based material is in the form of an assembly such as a flat cloth, sheet or felt. The electrode can also be constructed with other shapes, such as in the form of a cylindrical or tubular bundle of fibres, threads or yarn, where the ends of the bundle are provided with a current collector.

It is also clear that an electrode in the form of a flat body of cloth, sheet or felt can be rolled up with a porous separator between the layers of the carbon-based material, and with the opposite edges of the rolled-up material connected to a current collector. Copper metal has been used as a current collector, but any electrically conductive metal or alloy may be used, such as silver, gold, platinum, cobalt, palladium and alloys thereof. Likewise, while electrolytic deposition has been used to bond a metal or metal alloy to the carbon-based material, other coating techniques can

(including fusion applications) or electroless deposition is used provided that the edges or ends of the electrode, including the majority of the fiber ends at the edges of the carbon-based material, are sufficiently wetted by the metal to provide an electrical contact and current path of substantially low resistance.

Collectors made of a non-precious metal, such as

copper, nickel, silver or alloys of such metals must be protected against the electrolyte and are therefore preferably coated with a synthetic resin material or an oxide, fluoride or the like which will not be attacked by the electrolyte or undergo any significant degradation under the operating conditions in a cell .

Electrodes according to the present invention made of the electrically conductive carbon-based material and its current collector can be used as the positive electrode in a secondary energy storage device. No significant damage to the electrode itself or the electrolyte, i.e. the solvent and the ionizable salt, is observed during repeated charging at a capacity above 150 coulombs per grams of active carbon-based material and deep discharges at a depth of more than 80% of the electrode's overall capacity during fast or slow charge/discharge.

Alternatively, electrodes according to the invention made of the electrically conductive carbon-based material and its current collector can also be used both as the positive and the negative electrode in an accumulator (secondary battery) with similar favorable operating characteristics as described above.

A surface area of at least 0.5 m<2>per grams and a low resistivity of less than 0.05 ohm/cm are desirable properties for the carbon-based material used for the electrode according to the invention. A battery built with the carbon-based material in electrodes according to the invention thus has a very low internal resistance and a very high corresponding coulometric efficiency, which is usually over 80%.

In the investigations carried out regarding the limits of the present invention, it was found that initial current densities after charging greater than 15.5 - 31 mA/cm<2> can result in damage to the carbon-based material in the electrode.

EXAMPLE 1

A pair of electrodes, each with an area of 71 cm 2 , was prepared from a sheet of "Panex" PWB-6 (Trade Mark), a sheet which had been heat-treated at a temperature higher than 1000°C by the factory edge and which made this sheet electrically conductive. The cloth was woven from a polyacrylonitrile (PAN) precursor, the yarn being made from non-continuous filaments (staple fibers)

which had an average length of approx. 5 cm and a diameter of 7 - 8 ym and an aspect ratio of approx. 700:1. The cloth was heat-treated by the manufacturer after weaving. The edges of the heat treated cloth were coated with copper by electroplating to provide a current collector. A metal wire was soldered to one end of the copper-plated edges. All four edges of each electrode (current collector and wire junction) were coated with an amine curable epoxy resin, "DER" (Trade Mark) 331, supplied by The Dow Chemical Company, thereby insulating the metal from the corrosive effects of the electrolyte under the conditions of use. The electrode pair was immersed in an electrolyte comprising a 15% solution of LiClO₂ in propylene carbonate placed in a polyvinyl chloride (PVC) container. The electrodes were arranged with a mutual distance of less than 0.6 cm. The assembly of the electrodes in the container was carried out in a dry box. The container was sealed while inside the dry case,

with the metal wires protruding from the container. The water content in the assembled container was less than 10 ppm. The fibers had a Young's modulus of approx. 230 GPa and an area-to-weight ratio

of o 0.6 - 1.0 m 2/g. The overall electrical capacity of the active carbon-based material in the electrode was determined to be approx. 250 coulombs/g.

The cell thus prepared was electrically charged

at a maximum voltage of 5.3 volts, below which the current density was not allowed to exceed 5.4 milliamperes per cm 2 electrode surface area. The cell was electrically charged and discharged for 1250 cycles over an 11 month period and exhibited a coulometric yield of over 90%, performed at a discharge capacity of over 85%. The cell was then disassembled, and the fibers from each of the cloth electrodes were examined under a microscope at 1000x

enlargement. According to the measurements that were possible, the fibers had the same diameter as the fibers from the same lot that had not been used in the cell. The cell was again assembled and the testing carried out in the same way as described above. The cell has so far been subjected to over 2800 charge and discharge cycles over a 23 month period with no reduction in coulometric yields, and the cell still exhibits a coulometric yield of over 90%.

EXAMPLE 2

Six electrodes similar to the electrodes in example 1 were produced and connected in a unit comprising 3 cells so that each of the three pairs of electrodes was sealed in separate polyethylene pockets (sacks). The electrodes were connected in series. The cell unit was used in the same way as in example 1, except that the voltage was approx. 16 volts. The starting voltage at open circuit was approx. 13.5 volts. After 228 electrical charge and discharge cycles, during which the discharge was a deep discharge corresponding to at least 78% of total capacity, the cells were disassembled, and the electrodes were removed from the pockets and the fibers examined for signs of deterioration, i.e. flaking and significant swelling and shrinkage of the fibers. The investigation showed no detectable change in the fiber diameter compared to fibers in the same batch of fabric that had not been used for the manufacture of the electrode in the present example. The measurements were carried out with a laser interferometer.

EXAMPLE 3

Some flat sheets were cut out from a woven cloth

yarn of an essentially continuous monofilament precursor fiber made from petroleum pitch. The fibers were manufactured by the Union Carbide Company and sold under the trade name "Thornel". The precursor fiber yarn, which had an aspect ratio

of approx. 800:1, had been woven into a cloth and then heat treated at a temperature of at least 2000°C. Each of the flat sheets had an area size of approx. 930 cm 2. The fiber had a Young<1>'s modulus of 315 GPa and a surface area of approx. 1 m 2/g after the heat treatment. The sheets were plated with copper metal along their length

four edges so that all fibers were electrically connected forming an electron-collector frame. An insulated copper wire was connected to one of the edges of the collector near a corner by soldering, and the solder joint and the copper collector were coated with a hardenable vinyl ester resin "DERAKANE" (trademark). The sheets in each pair were arranged parallel to each other with the soldered wires at opposite ends of the mating edges and separated by means of a perforated non-woven fibrous polypropylene composite sheet having a thickness of approx. 0.1 mm. A polyethylene

pocket (sack) with a size of 930 cm was used as a cell container. Three cells were assembled in a dry case by placing a pair of the carbon fiber sheets and their separator in each of the three pockets, and each pocket was filled with approx. 500 g of an electrolyte of a 15% by weight solution of LiClO₂ in propylene carbonate. The electrolyte level in the pocket was determined so that it gave 21 g of active fiber per electrode (the area of the electrode that was exposed to electrolyte). The rest of the carbon fibers in each electrode extended out of the solution or were covered by the "DERAKANE" resin/copper metal frame.

Mounting the cells in a dry case maintained the water content of less than 20 ppm of the electrolyte solution. Each pocket was sealed while in the dry case in such a way that the soldered wire ends extended through the seal at opposite ends of the sealed edge. The three cells thus produced were placed in a box of clear plastic and the wires were connected in series. Activated molecular sieve material of the zeolite 5A type (for absorption of moisture) was added over the top of the cells, and the whole was taken out of the dry case. The end wires of the two end plates in the 3-cell array were connected by connection contacts that extended through a cover or cover for the box, and the cover was quickly and tightly attached to the box.

The device was charged at a voltage of 15 - 16 volts and a current of 1.8-2 amperes for 45 minutes. The device was then discharged through a 12 volt car headlight which drew an average amperage of 2.0 - 2.5 amps. The device was discharged to 90% of its capacity within 30 minutes. The electrical charge and discharge cycles were performed over 850 times. The cell was then disassembled and the fibers were examined under a microscope at 1000x magnification and showed no detectable signs of swelling or deterioration due to flaking. The device received an electrical charge and deep discharge at 90% of capacity for each cycle.

EXAMPLE 4

A PAN-based (precursor fiber) cloth was purchased from R.K. Textile, Ltd., Heaton Moor, U.K. The cloth was sold under the trade name "Panox" and consisted of a non-conductive carbon fiber with an aspect ratio in excess of 250:1, formed into yarn and woven cloth and allegedly not heated to a temperature above 400°C . The cloth was heat treated at a temperature of approx. 1000°C for a sufficiently long time for the cloth to become electrically conductive. The heat-treated cloth had a Young's modulus of 160 GPa and a surface area of approx. 1 m2/g. Two samples of the cloth, each with a width of approx. 5 cm along one side and an area of approx. 26 cm 2 was cut from the heat-treated cloth, and the four edges of each cloth were plated with copper metal to form a current collector for the electrode. A wire was soldered to one of the corners of the current collector in each electrode. This current collector was coated with a coating mixture based on vinyl ester resin of the "DERAKANE" type. A non-woven polypropylene composite sheet, "Celgard" 5511, was placed between the two electrodes, and the electrodes were inserted into a plastic pocket (sheath). This device was placed in a dry box in which the water content was kept below 20 ppm of the electrolyte solution. A 10% by weight solution of LiClO₂ in a propylene carbonate solution was used to fill the casing until the two electrodes were immersed in the electrolyte solution. The wires from each electrode were connected with a double-pole two-way end, where one pole was connected to an electrical voltage source of 5.3 volts. The other pole was connected to an electrical resistance of 10 ohms. The cell was deeply discharged to at least 80% of its dual charge and subjected to over 800 electrical charge and discharge cycles with a coulometric yield of over 80%. The capacity of this cell was approx. 70% of the capacity in the PAN example (example 1) based on the total weight of the electrode.

Cells constructed in accordance with the present invention have been found to have an internal resistance averaging less than 0.038 ohms/ft 2 of electrode surface area in a 6 electrode cell. This value, which was originally measured as less than 1 ohm, included the wiring of the charging system, which had a length of approx. 6 m. By measuring the resistance in the wires and re-measuring the overall resistance in the system from the charge, the resistance in the accumulator 2 2 itself (the secondary battery) was calculated to be 0.038 ohm/ft (0.41 ohm/m ).

A confirmation of the data in the above examples was made by a co-worker using a 2-electrode cell made of "Thornel" cloth, (VCB-45) having a Young's modulus of 315 GPa, a surface area of 1 m 2 /g and an aspect ratio of over 10,000:1, in which each canvas had a size of 15.2 cm x 15.2 cm. Copper edges were plated around all four edges of the cloth to form the current collector. The current collector was then coated with "DERAKANE" 470-36. The current collector's edges were approx. 2.6 cm wide, leaving active areas of carbon-based material of approx. 10 cm x 10 cm. This area on

100 cm 2 for each electrode contained approx. 6 g of carbon fiber.

The electrodes were separated by placing an electrode

in a heat-sealed bag of a microporous polypropylene film ("Celgard" 5511).

The device consisting of electrodes and separator was placed in a polyethylene bag, the bag was filled with a dry electrolyte of 15 wt in LiClO^ in propylene carbonate (about 100 ml), and the device was pressed between two plastic crimp plates, which support the sides of the bag containing the electrolyte . The thickness of the "DERAKANE" coated copper current collector kept the fibrous part of the two electrodes from being pressed to the minimum mutual separation distance. In later experiments, a 10 cm x 10 cm spacer plate was inserted between the crimp plates to press the electrode-separator combination closer together. This reduced the cell resistance from approx. 0.9 ohm to approx. 0.7 ohms.

Discharge data at different discharge rates were recorded for two of the above described configurations of the cell. In one case (the 0.9 ohm cell) the electrode separation was limited by the epoxy coating on the current collector to approx. 4 mm. In the second case (0.7 ohm cells), the electrodes were pressed together at the center with only the porous polypropylene spacer between them (less than 1 mm).

Curve I in fig. 1 shows the terminal voltage as a function

of the discharge, coulomb per grams of fiber, for the 0.9 ohm cell at several discharge rates ranging from 6 hours to 3/4 hour. These discharges correspond to a so-called first plateau (2 volt reduction). If one assumes that the total capacity for the first plateau is 180 coulomb per gram down to 2 volt voltage drop, the values along the abscissa can be replaced by "% discharged", with "180 coulomb/g" equivalent to "100% discharged".

The total energy taken at a 3 hour speed at constant load is almost the same as at the speed corresponding to 6 hours. At the fast 3/4 hour discharge rate, inefficiencies arise due to the cell's resistance and electrode depolarization. The electrode current densities corresponding to these discharge rates are:

"Coulomb per gram of fiber" is based on the weight of the active carbon-based material in just one electrode.

Curve II shows data for the 0.7 ohm cell. Obviously is

more energy available to the cell with the lower resistance. Curve III shows the comparison between the two cells at the higher discharge rate (3/4 hour).

A lithium metal reference electrode was inserted into the cell to determine which electrode was polarized. The voltage drops between each electrode and the reference electrode were determined during charge and discharge and after opening the circuit.

After opening the circuit, the voltages between the negative electrode and the reference electrode were generally less than 100 mV and changed only significantly with time. When the voltage was measured between the positive electrode and the reference electrode, it changed with time, decreasing after each charge and increasing after each discharge.

A battery cell's maximum output at different stages of charge was determined by pulse discharge of the cell at loads that produced terminal voltages of half the open circuit voltage. The "pulses" were 10 seconds long, and the power was calculated as the average power over the 10 seconds.

The cell was first charged to 344 coulombs per gram of active carbon-based material in an electrode. This was taken as a 100% state of charge. The highest current drawn from the 10 cm x 10 cm electrode cell at 100% state of charge was 2.5 - 3.0 amps. Later power determinations were carried out at levels of 247 coulomb per grams (72% charge) and 224 coulombs per grams (65% charge). Curve V shows the results.

The maximum power from this cell was approx. 0.48 watts

per grams of fiber at 100% state of charge and dropped to approx. 0.31 watts per grams of fiber at 72% state of charge. The power performance drops rapidly after this, as the voltage drops and polarization begins. A pulse discharge that lasts longer than 10 seconds will not necessarily reduce the final effect to any great extent. Curve VI shows the voltage progression for a 40 second discharge rate at maximum power. After the first ten seconds, the voltage drop is small.

EXAMPLE 5

A battery comprising 3 cells was constructed from 12 plates, 4 per cell, of "Thornel" fiber described in Example 3. Each sheet had a size of approx. 930 cm 2 and had been copper plated on each edge. The copper plating along the edges was coated with curable vinyl ester resin, a quality of "DERAKANE". The plates had an active area of approx. 852 cm 2. The 4 plates in each cell were assembled with a perforated separator made of polypropylene ("scrim") between each plate. Pairs of plates in each cell were connected in parallel so that after charge/discharge the plates were alternately +, -, +, -. The four plates and their separators were arranged in a polypropylene sack which had a size of 33 cm x 33 cm, and which contained approx. 600 ml of an electrolyte solution of 15% by weight LiClO^ in propylene carbonate • This electrolyte content in each bag was sufficient to give approx. 37 g of active fibers per electrode plate.

The battery was initially charged over a period of 1000 minutes to a capacity of 7.9 ampere-hours at a voltage of 14-16 volts. The cell was then discharged over a 200 minute period through a 12 volt car headlight drawing an average capacity of 6.2 amp hours, representing greater than 80% depth of discharge. Recharging was carried out over an 800 minute period. An average coulometric yield of approx. 90% by charge/discharge cyclization was observed.

Claims (22)

1. Electrode for use in a secondary device for storing electrical energy, comprising an electrode body of an electrically conductive carbon-based material, having a skeletal orientation, at least at or near the surface, and a current collector electrically connected thereto, wherein the carbon-based material has a Young's modulus greater than 6.9 GPa and undergoes a physical dimensional change of less than 5% during repeated electrical charge and discharge cycling. 2. Electrode according to claim 1, characterized in that the device comprising the carbon-based material has a Young's modulus of from 6.9 GPa to 380 GPa, and that the carbon-based material is derived from a precursor material selected from a polyacrylonitrile and pitch. 3. Electrode according to claim 2, characterized in that the carbon-based material has a Young<1>'s modulus of from 69 GPa to 380 GPa. 4. Electrode according to claim 2, characterized by. that the carbon-based material of a Young's modulus of from 138 GPa to 311 GPa. 5. Electrode according to one or more of the preceding claims, characterized in that the carbon-based material has a surface area of from 0.1 to 50 m 2 /g. 6. Electrode according to claim 5, characterized in that the carbon-based material has a surface area of from 0.1 to 10 m 2 /g. 7. Electrode according to claim 6, characterized in that the carbon-based material has a surface area of from 0.1 to 5 m 2 /g. 8. Electrode according to one or more of the preceding claims, characterized in that the carbon-based material has an aspect ratio (l/d) or equivalent ratio greater than 100:1. 9. Electrode according to one or more of the preceding claims, characterized in that the carbon-based material itself has a structural resistance in sizes from 2 2 6.45 cm to over 930 cm. 10. Electrode according to one or more of the preceding claims, characterized in that the carbon-based material is in the form of a cloth, film, paper, paper-like or foot-like flat sheet, or other combinations. 11. Electrode according to one or more of claims 1-9, characterized in that the carbon-based material is in the form of a cloth or a felt-like cloth or sheet comprising an assembly of at least one warp yarn of continuous filaments or staple fibers that have a length of 1 - 10 cm. 12. Electrode according to one or more of claims 1-9, characterized in that the carbon-based material is a woven or knitted fabric made from an assembly of warp yarns of continuous filaments or staple fibers that have a length of 1-10 cm. 13. Electrode according to one or more of claims 1-9, characterized in that the carbon-based material is a cloth or a felt-like cloth or sheet made from a string of continuous filaments or staple fibres. 14. Electrode according to one or more of the preceding claims, characterized in that the electron collector is an electrically conductive metal applied to at least one edge of the carbon-based material, and that the plated edge is coated with a non-conductive, non-reactive protective material . 15. Electrode according to one or more of the preceding claims, characterized in that the carbon-based material is able to withstand more than 100 electrical discharge cycles with at least 70% discharge depth at at least 70% coulometric yield without any significant damage to the material's structural stability. 16. Secondary device for storing electrical energy, comprising a container having an electrically non-conductive inner surface and a moisture-impermeable outer surface or laminar body and has at least one cell located in the container, each cell comprising a pair of electrically conductive electrodes that are electrically insulated from contact with each other, and the container contains a substantially non-aqueous electrolyte, wherein at least one of the electrodes in each cell is an electrode as specified in one or more of the preceding claims. 17. Device according to claim 16, characterized in that the electrolyte comprises a non-conductive, chemically stable non-aqueous solvent and an ionizable salt dissolved therein. 18. Device according to claim 16 or 17, characterized in that the electrolyte solvent is selected from compounds that have oxygen, sulfur and/or nitrogen atoms bound to carbon atoms in an electrochemically non-reactive state, and that the salt is an alkali metal salt. 19. Device according to claim 18, characterized in that the electrolyte solvent is propylene carbonate and the alkali metal salt is lithium perchlorate. 20. Secondary device for storing electrical energy, comprising a container having an electrically non-conductive inner surface and a moisture-impermeable outer surface or laminated body and having at least one cell arranged in the container, each cell comprising at least a pair of electrically conductive electrodes electrically isolated from contact with each other, and the container contains a mainly non-aqueous electrolyte, where each of the electrodes in each cell is an electrode as specified in one or more of claims 1 - 15. 21. Device according to claim 20, characterized in that each electrode in a cell has freedom of choice with regard to polarity when recharging and the ability to be partially and/or completely reverse-polarized without damage. 22. Device according to claim 20 or 21, which has a maximum power density of more than 0.31 watts per gram active positive carbon electrode for a 40 second pulse period at full charge.