Note: Descriptions are shown in the official language in which they were submitted.
123~9~1 4693-3536
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SECONDARY ELECTRICAL ENERGY STORAGE DEVICE
AND ELECTRODE THEREFOR
The invention resides in the use of a
carbonaceous material in conjunction with an electron
collector as an electrode for secondary electrical
energy storage devices. The carbonaceous ma-terial of
the electrode, is stable in the presence of an
electrolyte system containing anions such as perchlorates,
hexafluoroarsenates, and the like, under ambient or
normal operating temperatures of use of the electrode.
That i5 to say, the carbonaceous material does not
appreciably irreversibly swell or contract during deep
electrical charge and discharge cycles such as may be
performed in the operation of a secondary electrical
energy storage device.
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Numerous patents and technical literature
describe electrical energy storage devices utilizing a
carbonaceous material such as carbon or graphite as an
electrode material. Of course, one of the earliest of
these devices was the Laclanche' battery of 1866 wherein
carbon was used as an electron collector in a
20~ Zn/NH4Cl/MnO2 primary battery. Since then carbon has
been used extensively as a component of the electrode
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in primary batteries, Primary fuel cells, secondary fuel cells,
secondary batteries and capacitors. The function of the carbon or
qraphite in these aforementioned devices has been primarily that
of a current collector or as a reactive material to Eorm new com-
pounds with fluorine t~hich have different structures and prop-
erties than the ori~inal carhon/qraphite, and most recently, as
semiconductor materials which form salts with ions of the electro-
lyte. These prior art devices can be categorized as: primary
batteries such as is disclosed in Coleman et al. in U.S. Patent
No. 2,597,451, Panasonic Lithlum Battery literature, an~ IJ.S.
Patent Nos. ~,271,242, 3,700,502, and 4,224,389; fuel cells, such
as Japanese Publication ~o. 54-082043; and, secondary fuel cells,
wlth limited recharqeabi]ity, such as is described in Dey et al.
U.~. Patent No. 4,037,025, a rechargeable fuel cell employin~ an
activated (high surface area) qraphite; recharqeable secondary
batteries (accumulators) such as is disclosed in ~art U.S. Patent
No. 4,251,568 employing graphite as a current collector, and
Bennion U.S. Patent Nos. 3,844,837 and 4,009,323 and capacitors
such as in ~utherus et al. U.S. Patent No. 3,700,975 or German
Patent No. 3,231,243 using a high surface area carbon (~raphi~e~.
Some of these devices also utilize ionizable salts dissolved in a
nonconductive so]vent.
The carbonaceous materials described in the patents and
in the literature are materials graphitized or carhonized until
the materials become electrically conductive. These materials are
derived from polyacetylenes, polyphenylenes, polyacrylonitriles,
and petroleum pitch which have been heated to "carbonize and/or
graphitize" the precursor material to impart
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some degree of electrical conductivity. Some of -the
graphites used in the prior art literature are graphites
such as RPG (Reinforced Pyrolytic Graphite), R-1 nuclear
reactor grade graphite, PGCP tPyrolytic Graphite Carbon
Paper), and GRAFOIL (a Trademark of the Union Carbide
Corporation) comprising 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, pp. 29-33, April 19, 1982, in
an article entitled "Conducting Polymers R & D Continues
to Grow"; Journal Electrochem Society, Electrochemical
Science, 118, No. 12, pp. 1886-1890, December 1971; and
Chemical & Engineering News, 59, No. 41, pp. 34~35,
October 12, 1981, entitled "Polymer Cell Offers More
Power, Less Weight".
The problems attendant with these reported
cells are that they do not have a long life since the
electrode made from such carbonaceous material is
susceptible to degradation when subjected to repeated
electrical charge and discharge cycling.
For example, U.S. Patent No. 3,844,837 (Bennion
et al.) describe~ a battery employing a nuclear grade
graphite impregnated with chips of Li2o as the positive
electrode and copper as the negative electrode in a
LiCF3SO3-dimethyl sulfi-te (DMSU) electrolyte. The
graphite electrode was made from a grade R-l nuclear
graphite (sold by Great Lakes Carbon Company) and was
reported to b~ flaky after 9 cycles of electrical
charge and discharge. The patentees also tested a
graphite cloth and concluded it to be unsatisfactory.
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Several other gr~phites were used with equallv unsatisfactory
results with the best results o~tained from pyrolytic graphite
which failed after 33 cycles. Dey et al. employs a hiqh surface
area carbon or qraphitic material, ~ithin the pores of which a
chemical reaction occurs, but said material ls qenerally thought
to be of a low conductance throuqh lack of continuity of the carb-
on surface. Further, it is believed that such materials do not
maintain the flimensiona] stability and structural integrity neces-
sary for the reversible formation of carbon complexes required for
long rechargeable cycle Iife of secondary batteries.
Experiments conducted in the course of the development
of the present invention included the use of GRAFOIL (r~rade Mark)
which failed on the first electrical charqe and RPG (~uper Temp)
graphite electrodes which also failed. It was found that an
amount greater than 20% of the positive electrode made from RPG
qraphite was lost as flakes, chips and powder after only 27 elec-
trical charge and discharge cycles.
It is to be noted that the prior art identifies the
disinte~ration and damaqe to the electrode as beinq a result of a
swelling and ~hrinking of the electrode body and that this
swelling and shrinking increases with each electrical charqe and
discharge cycle which distorts the graphite platelets which flake
off due to the stress of swelling and shrinking. In conducting
these experiments in the course of the development of the present
invention, it was confirmed that such flaking-off of the graphite
platelets occurs when the aforementioned graphite materials were
subjected to repeated electrical charge and discharge cyclesO
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According to a first aspect, -the present
invention provides an electrode for use in a secondary
electrical energy storage device comprising an electrode
body of electrically conductive carbonaceous material,
having a skeletal orientation, at least at or near the
surface, and a current collector electrically associated
therewith, wherein said carbonaceous material has a
Youngs modulus of greater than 1,000,000 psi (6.9 GPa)
and undergoes a physical dimensional change of less
than 5% during repeated electrical charge and discharge
cycling.
According to another aspect, the invention
provides a secondary electrical energy storage device
comprising a housing having an electrically non-conductive
interior surface and a moisture impervious exteriox
surface or laminar body and having at least one cell
positioned in said housing, each cell comprising at
least one pair of electro-conductive electrodes
electrically insulated from contact with each other,
said housing containing a substantially non-aqueous
electrolyte, wherein at least one of the electrodes of
each cell is an electrode of the invention.
According to a further aspect, the invention
resides in a secondary electrical energy storage device
comprising a housing having an electrically non-conduc-tive
interior surface and a moisture impervious outer surface
or laminated body and having at least one cell positioned
in said housing, each cell.comprising at least one pair
of electroconductive electrodes electrically insulated
from contact with each other, said housing containing a
substantially non-a~ueous electrolyte, wherein each of
the electrodes of each cell is an electrode
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and wherein each said electrode has the freedom of
choice of polarity on recharge and the ability to be
partially and/or fully reverse polarized without harm.
The electrodes can be separated from each other
by distance or by a non-electrically conductive ion-
permeable material.
Ph~sical Properties
Preferably, the electrically conductive
carbonaceous material of the electrode should have the
following physical property criteria:
(1) A Young's modulus of greater than 1,000,000
psi (6.9 GPa), preferably from 10,000,000 psi (69 GPa)
to 55,000,000 psi (380, GPa), more preferably from
20,000,000 to 45,000,000 psi (138 GPa to 311 GPa).
(2) An aspect ratio of greater than 100:1. The
aspect ratio is defined herein as the length to
diameter ~/d ratio of a fibrous or filament strand of
the carbonaceous material or as the length to depth
ratio when the carbonaceous material is formed as a
planar sheet.
(3) The structural and mechanical integrity of
the carbonaceous material in whatever fabricated form
it may be (woven, knit or non-woven from continuous
filament or staple fibers or a film) should be such
that it does not require the presence of a support such
as a pressure plate (face films or mesh) to maintain
the carbonaceous material in the desired sheet or plate
like shapes throughout at least 100 charge/discharge
cycles.
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(4) A surface area of at least 0.1 m2/g but
less than one associates with activated absorp-tive
carbon, suitably less than 50 m2/g, preferably less
than 10 m2/g, and more preferably less than a 5 m2/g.
(5) Sufficent integrity of the form of the
carbonaceous material to enable the carbonaceous material
to retain its plate or sheet like shape when of a size
greater than 1 in2 (6.45 cm2) to greater than 144 in2
(930 cm2) without support other than a metallic
current collector ~rame forming the edge portion of the
electrode.
(6) The secondary electrical energy storage
device in which -the electrode of this invention is
employed should be substantially free of water to the
extent o less than 100 ppm. Preferably, the water
content should be less than 20 ppm and most preferably
less than 10 ppm. The device of the invention is
capable of operating with a water content of up to 300
ppm but will have a somewhat reduced cycle life.
Further, it is to be understood that should the water
con-tent level become onerous, the device may be
disassembled, dried and reassembled in such dry state
without substantial damage to its continued
operability.
Performance Crlteria
(7) The carbonaceous material of an electrode
should be capable of sustaining more than 100 electrical
charge and discharge cycles without any appreciable
damage due to 1aking of the carbonaceous material.
Preferably, no app.reciable damage should occur after
more than 500 electrical charge and discharge cycles,
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at a discharge capacity of greater than 150 coulombs
per gram of carbonaceous material of an electrode.
(8) The coulometric (coulombic) efflciency
of the carbonaceous material of the electrode should be
greater than 70 percent, preferably greater than 80
percent and most preferably greater than 90 percent.
(9) The carbonaceous material of the electrode
should be capable of sustaining deep electrical discharges
of greater than 70 percent of its electrical charge
capacity for at least 100 cycles of electrical chaxge
and discharge, and preferably greater than 80% for more
than 500 electrical charge and discharge cycles.
Accordingly, the carbonaceous material of an
electrode having the physical properties hereinbefore
described preferably should be capable of sustaining
electrical discharge and recharge of more than 100
cycles at a discharge capacity of greater than 150
coulombs per gram of carbonaceous material in an
electrode and at a coulometric efficiency of greater
than 70% without any substantial irreversible change in
dimensions (dimensional change of less than about 5%).
Usually, the carbonaceous material will be
obtained by heating a precursor material to a temperature
above 850C until electrically conductive. Carbonaceous
precursor starting materials capable of forming the
electrically conductive oriented carbonaceous material
; ~ portion of the electrode may be formed from pitch
(petroleum or coal tar), polyacetylene, polyacrylonitrile,
polyphenylene, SARAN (Trade Mark), and the like. The
carbonaceous precursor starting material should have
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some degree of skeletal orien-ta-tion, i.e., many of
these materials either have substantial concentrations
of oriented benzenoid structural moieties or moieties
which are capable of conversion, on heating, to
benzenoid or equivalent skeletal orientation at or near
the surface because of the skeletal orientation of the
starting material.
Exemplary of preferred carbonaceous precursor
materials which exhibit such skeletal orientation on
heating are assemblies of multi or monofilament strands
or fibers prepared from petroleum pitch or polyacrylo-
nitrile. Such multi or monofilament strands or fibers
are readily converted into threads or yarns which can
then be fabricated into a cloth-like product. One
technigue for producing suitable monofilament fibers is
disclosed in U.S. Patent No. 4,005,183 where the fibers
are made into a yarn which is then woven into a cloth.
The cloth is then subjected to a temperature, usually
above 1000C, sufficient to carbonize the cloth to make
~0 the carbonaceous material electrically conductive and
so as to provide the material with the physical property
characteristics hereinbefore described under paragraphs
(1) through (6). Such a cloth, in con~unction with an
electron collector, is particularly suitable for use as
an electrode in the secondary electrical energy storage
device of the present invention.
Advantageously, the carbonaceous precursor
material is in the form of a continuous filament fiber,
thread(s) constituted o continuous filament(s) or
non-continuous fiber tow (yarn) which can be made into
assemblies such as woven, non-woven, or knitted
assemblies, or the staple fibers per se can be layered
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to form a cloth, paper-like or felt-like planar member.
However, acceptable results are obtained when yarns
made from short fibers, having a length of from l to lO
cm, are woven into a cloth-like product (provided such
short fibers still have, when heat treated, the required
physical properties hereinbefore mentioned under (1)
through (6~). It is of course to be understood that
while it is advantageous to form the precursor material,
preferably in a stabilized state (such as is obtained
by oxidation3, into the desired form (knit, woven or
felt) prior to carbonization, such construction may be
done after carbonization if the modulus is below about
55,000,000 psi (380 GPa) and preferably below about
39,000,000 psi (269 GPa) for machine fabrication. It
is of course to be understood that the carbonaceous
material may be formed from a film precursor.
The degree of carbonization and/or graphiti-
zation does not appear to be a controlling factor in
the performance of the material as an electrode element
in an electrical storage device except tha-t it must be
enough to render the material sufficiently electrically
conductive and is also enough to provide the afore-
mentioned physical and mechanical properties under the
designated use conditions. Carbonaceous materials
; 25 having about 90 percent carbonization, are refexred to
in the literature as partially carbonized. Carbonaceous
materials having from 91 to 98 percent carbonization
are referred to in the literature as a carbonized
material, while materials having a carbonization of
greater than 98 percent are referred to as graphitized.
; It has surprisingly been found that carbonaceous materials
having a degree of carbonization, of from 90 to 99
percent, have failed as electrode materials unless
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the carbonaceous material has the required dimensional
stability during electrical charge and discharge cycling.
For example, ~PG graphite and GRAFOIL, while having the
requisite degree of carbonization, electrical conduc-tivity
and surface area, do not have the required physical
properties of Young's Modulus and aspect ratio and thus
have failed.
In accordance with the invention, a rechargable
and polarity reversible electrical storage device can
be prepared by aligning at least one pair of electrodes,
made from the aforedescribed carbonaceous material and
its associated electron collector (which are electrically
conductive) in a housing. The housing has a non-conductive
interior surface and is impervious to moisture. The
electrodes are immersed in a non-aqueous (water being
present in an amount of less than about 100 ppm) fluid
contained in said housing. The fluid itself must be
capable of forming, or contains dissolved therein, at
least one ionizable metal salt. Each electrode is
comprised of the carbonaceous heat-treated material, of
the present invention, associated with an electron
collector which is preferably insulated against contact
uith the electrolyte fluid.
The secondary electrical energy storage
device of the invention may be constructed without the
polarity reversing capability by aligning the aforementioned
electrically conductive carbonaceous fiber assembly,
such as a cloth, and its electron collector as the
positive electrode alternating with a negative electrode
which may be constxucted of a metal, such as lithium,
or a metal alloy and immersing the electrodes in a
substantially non-aqueous fluid, which fluid itself is
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capable of forming or which contains at least one
ionizable soluble metal salt dissolved therein to
provide electrolyte ions~
In the construction of a preferred embodiment
of the secondary electrical energy storage device of
the present invention, conventional porous separators
of fiberglass, polymeric materials, or composites of
polymeric materials, may ~e and are preferably employed
to separate the positive and negative electrodes from
each other. Most preferably a nonwoven polypropylene
sheet is employed as the separator since it has the
desired degree of porosity and yet has a sufficient
tortuous path to prevent carbonaceous fibers from
- penetrating through it, thus preventing electrical
shorting. The porous separators also beneficially act
as stiffeners or supports for the electrodes.
Energy storage devices which are contained in
fluid-tight housings are generally known in the art.
Such housings may be suitably employed in the present
invention as long as the housing material is electrically
non-conductive and impervious to gases and/or moisture
(water or water vapor).
The materials found chemically compatible as
a housing material include polyvinylchloride, polyethylene,
poIypropylene, polytrifluoroethylene and relaked per-
fluorinated polymers, instant set polymer (ISP), a
rapidly solidifying reactive urethane mixture, the
aramids, a metal clad with a non-conductive polymeric
material such as an epoxy e.g. DER* 331 or with DERAKANE*,
ZETABON* and/or glass or a metal oxide, fluoride or the
*Trademark of The Dow Chemical Company
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like. Houslng materials found not to be suitable in
the preferred propylene carbonate system include acrylic,
polycarbonate and nylon. Acrylics craze and poly-
carbonates both crazes and becomes extremely brittle,
while nylon (except for aramides) is chemically reactive.
In addition to being compatible, a housing
material must also offer an absolute barrier of less
than 0.2 grams of H20/yr/ft2 (2.15 grams of H2O/yr/m2)
against the transmission of water vapor from the external
environment of the housing. No presently known thermo-
plastic materials alone offers this absolute barrier
against moisture at a thickness which would be useful
for a battery housing. At present only metals, for
example aluminum or mild s-teel, offer an absolute
barrier against moisture at oil thicknesses. Aluminum
foil having a thickness of greater than 0.0015 in.
(0.038 mm) has been shown to be essentially impervious
to water vapor transmission. It has also been shown
that when laminated to other materials, aluminum foil
as thin as 0.00035 in. (0.009 mm) can provide adequate
protection against water vapor transmission. Suitable
housings made of metal-plastic lamina-te, CED-epoxy-coated
metal (cathodic electro deposited~, or metal with an
internal liner of plastic or glass presen-tly satisfies
the requirements for both chemical compatability and
moisture barrier ability. Most of the cells and
batteries built to date have been tested in either a
dry box having a H20 level of <5 ppm, a glass cell or a
double walled housing with the space between the walls
filled with an activated molecular sieve e.g. 5A zeolite.
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The electrolyte fluld preferably consists ofa non-conductive, chemically stable, non-aqueous solvent
for ionizable salt or salts wherein the ionizable salt
is dissolved in the solvent. One can employ as the
solvent those compounds that are generally known in the
art such as, for example, compounds having oxygen,
sulfur, and/or nitrogen atoms bound to caxbon atoms in
an electrochemically non-reactive state. Preferably,
one can employ nitriles such as acetonitrile; amides
such as dimethyl formamide; ethers, such as tetrahydrofuran;
sulfur compounds, such as dimethyl sulfite; and other
compounds such as propylene carbonate. It is, of
course, to be understood that the solvent itself may be
ionizable under conditions of use sufficient to provide
the necessary ions in the solvent. Thus, the ionizable
salt must be at least partially soluble and ionizable
either when it is dissolved and goes into solution into
the solvent or upon liquification. While it is to be
understood that slightly soluble salts are operable, it
will be recognized that the rate of electrical charginy
and discharging may be adversely affected by the low
concentration of such salts in solution.
Ionizable salts which may be employed in the
practice of the invention are those taught in the prior
art and include salts of the more active metals, such
asj for example, the alkali metal salts, preferably
lithium, sodium or potassium, or mixtures thereof
containing stable anions such as perchlorate (ClO~=),
tetrafluoroborate (BF4=), hexafluoroarsenate (AsF6=),
hexafluoroant1monate (SbF6=) or hexafluorophosphate (PF6=).
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The electrolyte (solvent and salt) must be
substantially water-free, -that is, it should contain
less than 100 ppm of water, preferably less than 20 ppm
of water and most preferably less than lO ppm of water.
Of course, the electrolyte can be made up having more
than the desired amount of water and dryed, for example,
over activated zeolite 5A molecular sieves. Such
agents may also be combined in-to the finished battery
to ensure that the low level water requirement ls
maintained. The electrolyte should also be such as to
permit ions (anions and cations) of the ionizable salt
to move freely through the solvent as the electrical
potential of charge and discharge move the ions to and
from their respective poles (electrodes).
The electrode, when constructed as a cloth or
sheet, includes an electron collector conductively
associated with at least one of the edges of the
carbonaceous fibers or sheet. The edge(s) is preferably
further protected by a material to insulate the collector
and to 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 intimately contacts the
carbonaceous material of the electrode at least along
one edge and preferably on all four edges thereof when
the carbonaceous material is in the form of an assembly
such as a planar cloth, sheet or felt. It is also
~nvisioned that the electrode may be constructed in
other shapes such as in the form of a cylindrical or
tubular bundle of fibers, threads or yarns in which the
ends of the bundle are provided with a current collector.
It is also apparent that an electrode in the form of a
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planar body of cloth, sheet or felt can be rolled up
with a porous separator between the layers of the
carbonaceous material, and with the opposed edges of
the rolled up material, connected to a current collector.
While copper metal has been used as a current collector,
any electro-conductive metal or alloy may be employed,
such as, for example, silver, gold, platinum, cobalt,
palladium, and alloys thereof. Likewise, while
electrodeposition has been used in bonding a metal or
metal alloy to the carbonaceous material, other coating
techniques (including melt applications) or electroless
deposition methods may be employed as long as the edges
or ends of the electrode, including a majority of the
fiber ends at the edges of the carbonaceous material
are wetted by the metal to an extent sufficient to
provide a substantially low-resistant electrical contact
and current path.
Collectors made from a non-noble metal, such
as copper, nickel, silver or alloys of such metals,
must be protected from the electrolyte and therefore
are preferably coated with a synthetic resinous material
or an oxide, fluoride or the like which will not be
attacked by the electrolyte or undergo any significant
degradation at the operating conditions of a cell.
Electrodes of the present invention made from
the electrically conductive carbonaceous material and
its current collector can be employed as the positive
electrode in a secondary energy storage device. No
substantial damage to the electrode itself or the
electrolyte, i.e., solvent and ioni~able salt, is
observed when undergoing repeated charges at a capacity
of greater than 150 coulombs per gram of ac-tive
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earbonaceous matexial, and deep diseharges at a depth of greater
than 80 percent of the total capacity of the electrode at fast
or slow rates of charge/discharge.
Alternatively, electrodes of the invention made from
the electrically conductive carbonaceous material and its current
collector can also be employed as both the positive and negative
eleetrodes in an accumulator (secondary battery) wi-th similar
benefieial operating eharaeteristics as hereinbefore deseribed.
A surfaee area of at least 0.5 square meters per gram
and a low resistivity of less than 0.05 ohm~em of the earbonaeeous
material employed for the eleetrode of the invention are desirable
properties. Thus, a battery eonstrueted with the earbonaeeous
material electrodes of the invention has an extremely low internal
resistanee and a very high eorresponding eoulometrie effieieney
whieh usually is greater than 80 pereent.
During the investigative period for the limits of the
present invention it was found tha-t initial eurrent densities on
eharge greater than 100 to 200 mA/in2 (15.5 to 31 mA/em ) can
result in damage to the carbonaeeous material of th~e eleetrode.
The present invention will now be more fully explained
in the following examples, with referenee to the aeeompanying
drawings, in which:
Figure 1 is a graphical representation of the relation-
ship between terminal vol-tage and discharge for a 0.9 ohm eell
which is fully described in Example 4 hereinbelow;
Figure 2 is a graphical representa-tion of the relation-
ship between terminal voltage and diseharge for a 0.7 ohm eell
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which is fully described in Example 4 hereinbelow;
Figure 3 graphically illustrates the differences between
the 0.9 ohm and 0.7 ohm cells;
Figure 4 graphically illustrates the maximum power
density against the state of charge for a battery cell containing
active carbonaceous ma-terial in accordance with the present
invention; and
Figure 5 graphically illustrates the high rate power
discharge of the cell referred to in Figure 4 and in Example 4
hereinbelow.
EXAMPLE 1
A pair of electrodes each having an area of 11 in (71
cm2) were prepared from a Panex (Trade Mark) PWB-6 cloth (a cloth
which had been heat treated at a temperature greater than 1000C
by the manufacturer which rendered this cloth electroconductive)
purchased from Stackpole Fibers Industry Company. The cloth was
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woven from a polyacrylonitrile (PAN) precursor in which
the yarn was manufactured from non-continuous filaments
(staple fibers) having an average length of about 2
inches (5 cm) and a diameter of 7 to 8 miGrometers and
an aspect ratio of about 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 wire
was soldered to one end of the copper coated edges.
All four edges of each electrode (current collector and
wire connector) were coated with an amine curable epoxy
resin, DER (Trade Mark) 331, manufactured by The Dow
Chemical Company, to insulate the metal from the corrosive
effects of the electrolyte under the conditions of use.
The pair of electrodes were immersed in an electrolyte
comprising a lS percent solution of LiCl04 in propylene
carbonate contained in a polyvinylchloride (PVC) housing.
The electrodes were spaced less than 0~25 inch (0.6 cm)
apart. The assembly of the electrodes into the housing
was carried out in a dry bo~. The housing was sealed
while in the dry box with the wires extending from the
housing. The water content in the assembled housing
was less than 10 ppm. The fibers had a Young's modulus
of about 33,000,000 psi (230 GPa) and an area to weight
ratio of 0.6 to 1.0 m2/g. The total electrical capacity
of the active carbonaceous material of the electrode
was determined to be about 250 coulombs/g.
The cell so-prepared was electrically charged
at a ma~imum voltage of 5.3 volts with the current
llmited from exceeding 35 milliamps per square inch
(5.4 milliamps/cm2) electrode face area. The cell was
electrically charged and discharged 1250 cycles over an
11 month period and exhibited a coulometric efficiency
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of greater than 90 percent, conducted at a discharge
capacity of greater -than 85 percent. The cell was then
dismantled and the fibers from each of the cloth
electrodes were examined under a microscope with lO00
power magnification. Insofar as measurable, the fibers
had the same diameter as the fibers from the same lot
which had not been used in the cell. The cell was
reassembled and testing continued in the same manner as
hereinbefore described. The cell has completed, thus
10 far, over 2,800 charge and discharge cycles over an 23
month period without a reduction in coulometric
efficiency, it still has a coulometric efficiency of
greater than 90 percent.
E~AMPLE 2
Six electrodes similar to the electrodes of
Example 1 were prepared and connected in a three cell
unit such that each of the three pairs of the electrodes
were sealed in separate polyethylene pockets (bags).
The electrodes were connected in series. The three
cell unit was operated in the same manner as in Example
1 except that the voltage was about 16 volts. The
initial open circuit voltage was about 13.5 volts.
After 228 electrical charge and discharge cycles,
during which the dischaxge was cond~cted at a deep
discharge of greater than 78 percent of total capacity,
the cells were dismantled and the electrodes were
removed from their pockets and the fibers examined for
signs of deterioration, i.e., flaking and excessive
swelling and shrinking of the fibers. The examination
showed no detectable change in fiber diameter from
fibers measured in the same lot of cloth that had not
been~used to prepare the electrode o this Example.
Measurements were conducted with a laser interEerometer.
,
. 30,532-F -19-
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EXAMPLE 3
Several planar sheets were cut from a cloth
woven from yarn made ~rom 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 (Trade
Mark). The precursor fiber tow yarn with an aspect
ratio of abou-t 800:1 had been woven into a cloth and
then heat treated at a temperature of greater than
2000C. The planar sheets each had a dimension of
about 1 ft2 (930 cm2) in area. The fiber had a Young's
modulus of 45,000,000 psi (315 GPa) and a surface area
of about 1 m2/gm after heat treatment. The sheets were
plated with copper metal along their four edges so that
all fibers were electrically connected to form an
electron collector frame. An insulated copper wire was
attached to one edge of the collector near a corner by
solder and the solder joint and copper collector was
coated with DERAKANE* brand of a curable vinyl ester
resin. Each pair of sheets were aligned parallel to
each other with the soldered wires at opposite ends of
the matching edges and separated by a foraminous,
non-woven, fibrous, polypropylene composite sheet
having a thickness of 5 mils (0.1 mm). A polyethylene
pocket (bag~ of a size of about 1 ft2 (930 cm2) was
employed as a cell container. Three cells were assembled
in a dry bo~ by placing a pair of the carbon ~iber
sheets and their separator into each of three pockets
and filling each pocket with about 500 grams of an
electrolyte of 15 weight percent solution of LiClO~ in
propylene carbonate. The electrolyte level in the pocket
*Trademark of The Dow Chemical Company
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was determined to provide 21 grams of active fiber per
electrode (the area of the electrode exposed to the
electrolyte). The remainder of the carbon fibers of
each electrode extended out of the solution or was
covered by the Derakane (Trade Mark) resin/copper metal
frame.
Assembly of the cells in a dry box maintained
the water content at less than 20 ppm of electrolyte
solution. Each pocket was sealed while in the dry box,
in a manner to allow the soldered wire ends -to extend
through the seal at opposite ends of the sealed edge.
The three cells so-prepared were placed in a clear
plastic box and the wires connected in series. A
quantity of activitated zeolite 5A molecular sieves (to
absorb moisture) was added over the top of the cells
and the assembly was removed from the dry box. The end
wires of the two end plates of the three cell series
were connected to terminals extending through a cover
or lid for the box and the cover ~uickly sealed to -the
box.
The assembly was charged at a potential of 15
to 16 volts, and at a current of 1.8 to 2 amps, for 45
minutes. Thereafter the device was discharged through
a 12 volt automobile headlight drawing an average
current of from 2.0 t~ 2.5 amps. The device was
discharged to 90 percent of its capacity in 30 minutes.
The electrical charge and discharge cycles were conducted
over 850 times. The cell was then disassemhled and the
fibers examined under a microscope at 1000 times magni-
fication and showed no detectable signs of swelling or
30,532-F -21-
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deterioration due to flaking. The device was accepting
an electrical charge and deep discharge at 90 percent
of capacity for each cycle.
EXAMPLE 4
A PAN base (precursor fiber) cloth was obtained
from R. K. Textile, Ltd., Heaton Moor, U.K. The cloth
was sold under the trade name Panox (Trade Mark) and
was a non-conductive carbon fiber with an aspect ratio
of greater than 250:1 made into yarn and woven cloth
and, reportedly, had not been heated to a temperature
above 400C. The cloth was heat treated at a temperature
of about 1000C for a time sufficient to make the cloth
electroconductive. The heat treated cloth had a Youngs
modulus of 23,000,000 psi (160 GPa~ and a surace area
of about 1 m2/gm. Two samples of cloth each having a
width of 2 inches (5 cm) on a side and an area of 4 in2
(26 cm2~ were 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 corner o~ the current collector of
each electrode. The solder and copper current collector
were coated with a Derakane (Trade Mark) brand vinyl
ester resin coating composition. A non-woven poly-
propylene composite sheet, Celgard (Trade Mark) 5511,
was positioned between the two electrodes and the
electrodes were inserted into a plastic pocket
(envelope). This assembly was placed in a dry box
wherein the water content was maintained at less than
20 ppm of electrolyte solution. A 10 weight percent
30~ solution of LiCl04 in a propylene carbonate solution
was used to fill the envelope until the two electrodes
were submersed in the electrolyte solution. The wires
from each electrode were connected to a double pole,
. 30,532-F -22-
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double throw switch, one terminal of which was connected
-to an electrical voltage source of 5.3 volts. The
other terminal was connected to an electrical resistance
load of 10 ohms. The cell was deep discharged to
greater than 80 percent of its total charge and operated
in excess of 800 electrical charge and discharge cycles
with a coulometric efficiency of greater than 80 percent.
The capacity of this cell was about 70 percent of that
of the PAN example (Example 1) on a total electrode
weight basis.
Cells constructed in accordance with the
present invention have been found to have an internal
resistance which is, on the average, less than 0.038 ohm/ft2
(0.41 ohm/m2) of electrode face area in a six electrode
cell. This value, originally measured as less than
1 ohm, included the lead wires to the charging system
having a length of about 6 meters. On measuring the
resistance of the leads and then remeasuring the total
resistance of the system from the charge, the resistance
of the accumulator (secondary battery) proper was
calculated to be 0.038 ohm/ft2 (0.41 ohm/m2).
A confirmation of the data of the above
examples was carried out by a co-worker in a 2 electrode
cell made from "~hornel" cloth, VCB-45 having a Youngs
modulus of 45,000,000 psi (315 GPa), a surface area of
1 m2/g and an aspect ratio of greater than 10,000:1, in
which each cloth had a dimension of 15.2 cm x 15.2 cm.
~opper edges were plated around all four edges of the
cloths to form the current collector. The current
30 ; collector was then coated with DERAKANE (Trade Mark)
470-36. The current collector edges were about 2.6 cm
wide, leaving active carbonaceous material areas of
~ .
3a ~ 532-F -23- -
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about 10 cm x 10 cm. The 100 cm2 area of each
electrode contained about 6 grams of carbon fiber.
The electrodes were separated by placing one
electrode in a heat sealed bag of "Celgard" (Trade
Mark) 511 microporous polypropylene film.
The assembly of electrodes and separator was
placed in a polyethylene bag, the bag filled with a dry
electrolyte of 15 percent by weight LiCl04 in propylene
carbonate (about 100 cc) and the assembly squeezed
between two plastic edge pressure plates which support
the sides of the bag holding the electrolyte. The
thickness of the DERAKANE-coated copper current
collector kept the fibrous portion of the two
electrodes from being pressed into minimal separation
distance with each other. In later runs, a 10 cm x 10
cm spacer plate was inserted between the edge pressure
plates to press the electrode-separator combination
more tightly together. This lowered the cell resistance
from about 0.9 ohm down to about 0.7 ohm.
Discharge data at various discharge rates were
taken for two of the above described configurations of
the cell. In one case (0.9 ohm cell) the electrode
separation was limited by the epoxy coating on the
current collector to approximately 4 mm. In the other
case (0.7 ohm cells), the electrodes were forced
together at the center with only the porous poly-
propylene separator between them (less than 1 mm).
In the graph of Figure 1, the curves show the
terminal voltage vs. the discharge, coulombs per gram
of fiber, for the 0.9 ohm cell at several discharge
:
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rates ranging from 6 hours to 3/4 hour. These
discharges correspond to a so-called first plateau (2
volt cutoff). If one assumes that the total capacity
of the 1st plateau is 180 coulombs per gram to the 2
volt cutoff voltage, the values on the abscissa can be
replaced with "% discharged"; with "180 coulombs/gm"
equivalent to '1100% discharged".
The total energy recovered at a 3 hour rate at
constant load is almost the same as at the 6 hour rate.
At the fast 3/4 hour rate discharge, inefficiencies are
generated and occur due to cell resistance and
electrode polarization. The electrode current
densities corresponding to these discharge rates are:
Ave. Current density2at
Rate (hours) constant load (ma/cm )
6 0.5
3 1.0
1.5 2.0
0.75 4.0
The "coulombs per gram of fiber" is based on
the weight of the active carbonaceous material of one
electrode only.
In Figure 2, the curves show the data for the
3 0.7 ohm cell. Obviously, more energy is available for
the cell with the lower resistance. In Figure 3, the
curves show a comparison of the two cells at the higher
discharge rate (3/4 hr rate)~
A lithium metal reference electrode was
inserted into the cell to determine which electrode was
30,532-F -25-
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polarizing. The voltage drops between each electrode
and the reference electrode were determined during
charge and discharge and on opening the circuit.
On opening the circuit, the voltages between
the negative electrode and the reference electrode were
generally less than 100 mv and changed only slightly
with time. The voltage, when measured between the
positive electrode and the reference electrode, changed
with time, decreasing after each charge and increasing
after each discharge.
The maximum power capabilities of a battery
cell at different stages of charge were determined by
pulse discharging the cell at loads that gave terminal
voltages of one-half of 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 carbonaceous material in one electrode.
This was taken as a 100% state of charge. Maximum
current drawn from the 10 cm x 10 cm electrode cell at
100% state of charge was 2.5 to 3.0 amperes. Subsequent
power determinations were made at levels of 247 coulombs
per gram (72% charge) and 224 coulombs per gram (65%
charge). ~ shows the results.
Maximum power from this cell was about 0.48
watts per gram of fiber at 100% state of charge, dropping
to about 0.31 ~atts per gram of fiber at 72% state of
charge. The power capabilities drop rapidly after that
since the voltage drops and polarization sets in. A
pulse discharge of lonqer than 10 seconds does not
30,532-F -26-
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ne~essarily reduce the final power a great deal. ~v~
shows the voltage trace of a 40 second maximum power
rate discharge. After the lst 10 seconds, the voltage
drop is small.
EXAMPLE 5
A three cell battery was constructed from
twelve plates, four per cell, of Thornel brand fiber
described in Example 3. Each plate had a dimension of
approximately 12 in x 12 in (144 in2 or 930 cm2), and
had been copper plated on each edge. The copper
plating along the edges was coated with Derakane (Trade
Mark~ brand of curable vinyl ester resin. The plates
had an active area of about 132 in2 ~852 cm2). The
four plates of each cell were assembled with a foraminous
polypropylene scrim separator between each plate.
Pairs of plates in each cell were connected in parallel
so that on charge/discharge the plates were alternately
+, -, +, -. The four plates and their separators were
contained in a polypropylene bag having a dimension of
20 from 13 in x 13 in (33 cm x 33 cm) which contained
about 600 cc of an electrolyte solution of 15 percent
by weight of LiCl04 in propylene carbonate. This
electrolyte level in each bag was sufficient to provide
about 37 grams of active fibers per electrode plate.
The battery was initially charged over a
period of 1000 minutes to a capacity of 7.9 amp hours
at a potential of 14-16 volts. The cell was then
discharged over a 200 minute period through a 12 volt
automobile headlight putting out an average capacity of
6.2 amp hours representing greater than 80 percent
depth of discharge. Recharge was carried out over an
800 minute period. An average of coulometric efficiency
30,532-F -27-
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of approximately 90% on charge discharge cycling was
observed.
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