Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF INVENTION
This invention relates in general to a high voltage
lithium rechargeable electrochemical cell and in particular to
such a cell with a thin conducting polymer cathode and electrolyte
ccntaining an alkyl-carbonate solvent.
BACKGROUND OF THE INVENTION
Rechargeable lithium batteries, especially those contain-
ing organic liquid based solvents, have generally suffered from
poor cycling efficiencies of lithium, solvent oxidation (degrada-
lo tion) on charge, diminishing cathode capacity with increased
cycling and hazardous situations resulting from cell abuse con-
ditions aR for example short circuit and overdischarge. Some sol-
vents allow,good lithium cycling efficiencies but are unstable to
the high anode potentials required during charging. Similarly,
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electrolytes that are stable to oxidation often allow poor lithiurn
cycling efficiency. Cell short circuiting that results from
dendrite formations or from overdischarge may cause explosions.
Overcharge may degrade the electrolyte or irreversibly diminish
performance of certain cathodes.
SUMMARY OF THE INVENTION
The general object of this inventlon is to provide a high
voltage lithium rechargeable electroc~lcal ~ell in which the
aforementioned difficultie~ are overcome. A more particular
object of the invention is to provide such a cell that is highly
efficient on discharge and charge. A more particular object of
the invention is to provide such a cell that is stable to oxida-
tion of the electrolyte on charge and also stable to modest over-
charge of the cathode. A still further object of the invention is
to provide such a cell that allows much more cathode area to be
packaged per unit volume than is possible for state-of-the-art
porous carbon cathodes. Another object of the invention is to
provide such a cell in which the ~afety hazards normally associ-
ated with lithium cells that are overdischarged or short circuited
are diminished because the cathode becomes electrically insulating
on discharge. A particular ob~ect of the invention is to provide
such a cell that is efficient during discharge and charge over
hundreds of cycles.
It has now been found that a high voltage, rechargeable,
lithium electrochemical cell can be provided that exhibits high
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cycling efficiency over many cycles, the cell includin~ a metallic
lithium anode, poly 3-methylthiophene (PMT) polymer cathode, and
electrolyte including lithium hexafluoroar~enate LiAsF salt
dissolved in dimethyl carbonate (DMC).
Cell features include a high voltage cell employing a
lithium anode which is able to be recharged. Also included is an
electrically conductive polymer film as the cathode that is also
rechargeable with excellent cycling efficiency. The electrolyte
is composed of LiAs~ in DMC, providing an electrolyte that is
neither oxidized nor reduced during cell operation. On discharge,
no harmful products or adverse chemical reactions occur other than
the release of Li cations and AsF anions into the electrolyte.
There is a built-in safety feature to render extreme conditions
such as short-circuit or overdischarge less hazardous; because the
polymer becomes electrically more insulating during undoping (which
occurs during cell discharge), as the polymer becomes less conduc-
tive and cell resistance increases, the polymer will act as an
internal fuse to terminate cell operation. On charge, Li is
replated at the lithium anode and AsF migrates to the oxidized
(positively charged) P~T cathode to electrically neutralize its
charge. Another attractive feature of the cell is the ability
to overcharge the cathode over many cycles without deleterious
effects. The cell reactions are:
DISCHARGE
o +
Anode: Li -~ xLi + x electrons
o
Cathode: [PMT AsF ] + x electrons ~ xPMT + xAsF
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CHARGE
Anode: xLi + x electrons _ ~ Li
o -- +
Cathode: xPMT + xAsF > [PMT AsF ] + x electrons
6 6 x
Since no deleterious reactions occur, hundreds or
thousands of cycles may be expected with negligible loss in
discharge capacity or charge ef~iciency.
The metallic lithium anode is desired to create a high
cell potential (> 3 v) when coupled with the cathode. To ~i~imi ze
the quantity of lithium for increased safety, one can also use a
lithium intercalating compound, such as graphite or one of the
metal oxide compounds. Intercalating compounds would be useful to
reduce the hazards associated with metallic lithium such as cell
shorting resulting from such conditions as dendrite formation with
cycling, abuse during discharge/charge and disposal.
The cathode is comprised of electrochemically formed,
electrically conducting poly 3-methylthiophene polymer film.
Depending on the level of doping, electrical conductivity of the
polymer can be in the range of 1~ to 2 x 10 S cm . Because
it i5 formed electrochemically, a very thin film can be produced
on a suitable substrate which can then serve as the current
collector in the cell. PMT films on the order of one micrometer
thick can be formed, allowing more electrode area to be packaged
per unit volume (compared to carbon electrodes common to lithium
cells). Although there are many methods that one skilled in the
art might use to prepare the polymer, a suitable procedure used
for polymerizing PMT on a substrate is as follows:
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Preparation of PMT is in a 125 ml Europeall flask (Ace
Glass) using a 1 cm platinum flag counter electrode, a satur-
ated sodi~n calomel reference electrode, ~nd a platinum rod work-
ing electrode. The platin~n rod is polished to a mirror finish
with 0.1 micron alumina/water paste and sheathed in heat shrink-
able Teflon so as to expose only the 0.071 cm cross sectional
area at the polished end of tlle rod. The cell is also fitted with
a glass tube for bubbling gas and a gas outlet. The cell is
flooded with electrolyte contailling hig~ purity 3-methylthiopllene
monomer and litlliwn hexafluoroarsenate at 0.1 molar concentratiol~s
in redistilled acetonitrile as the solvent. Ultra higll purity dry
argon is bubbled througl~ the electrolyte to remove oxygen.
PMT polymerizes when the potential (working vs reference)
is 1.5 V and above. ~n adherent film 1.4 microns thick is pro-
duced by pulse depositioJI. This is carried out at a constant cur-
rent of 10 r~A cm by passing ~.25 coulombs per Clll 011 five
successive cycles with five minute rest periods (at open circuit
between cycles to restore equilibrium conditiolls. Films of poor
quality formed if the rest periods were omitted. The PM'r-coa~ed
2~ platinum sur~ace is ~hell rinsed in acetonitrile and dried under
vacuum at 50 C. In the oxidized (~sF -doped, electrically
conductive) state PMT is blue in color, while reduced (undoped,
electrically insulating) PMT is red. During cell cycling, t~le
polymer becomes oxidized and reduced, being electrically neutra-
lized by the insertion and loss of AsF anions. Once the
polymer becomes doped to its maximum level, an overcharge
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condition will exist where no more anion~ will be in~erted, and
additional charge will ~e wasted. If mode~tly overcharged, no
harmful cell reactions will occur.
A conductive electrolyte that i8 ~table during cell charg-
ing has been a concern in lithium systems because of the high oxi-
dation potential~ required. It is difficult to find ~olvents that
are ~table (will not become oxidized) during cell charging and
will permit good lithium cycling efficiencieR. One suitable sol-
vent is dimethyl carbonate. DMC is stable to oxidation potentials
up to 4.4 V. ~ stable, conductive electrolyte is formed with the
addition of dry, high purity LiAsF salt in redistilled DMC. In
a 1.5 M LiAsF -DMC electrolyte, conductivity i~ approximately
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0.01 S cm
Constant current recharge of the system described herein
is most efficient up to a cutoff potential of 3.8 V. Charging to
a potential of 4.0 V provides additional capacity on discharge but
exhibits a loss in efficiency on charge. These potentials are
well within the limits of electrolyte stability.
DESCRIPTION OF THE P~EFERRED EM~ODIMENT
A cell is constructed with a lithium metal anode, a 1.4
micrometer thick poly 3-methylthiophene polymer cathode doped with
AsF and supported on a platinum substrate, and a lithium
reference electrode. The cell is flooded with 10 ml of
electrolyte composed of 1.46 ~ LiAsF in dimethyl carbonate.
The cell is di~charged at o.1 mA cm constant current
until cell voltage falls to 2.7 V. After a one minute rest period
at open circuit, the cell i8 charged at 0.05 mA cm con~tant
current until cell potential reaches 3.8 V, allowing a one minute
rest period prior to the next discharge. Under these conditions,
cell discharge is reproducible over many cycles; likewise, cell
recharge is reproducible over many cycles, replacing exactly the
same number of coulombs as are rem~ved on discharge.
DESCRIPTION OF THE D~AWING
Figure 1 shows a Li/1.46 M LiAsF -DMC~1.4~4m thick PMT
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cell di~charge at 0.1 mA cm constant current to a 2.7 V
cutoff. Discharge curves are shown for di~charge numbers 20, 30,
40, S0 and 63. Recharge is by constant current at 0.~5 mA cm
to a 3.8 V cutoff.
Figure 2 shows a Li/1.46 M LiAsF -DMC/1.4 ~m thick
PMT cell cycled after recharge at 0.05 mA cm to 3.8 V follow-
ing short-circuiting of the cell and allowing it to sit for two
days. Curves for 0.1 mA cm constant current discharge to a
2.7 V are ~hown for cycle numbers 63, 90 and 116.
Figure 3 shows a Li/1.46 M LiAsF -DMC/1.4 ~m thick PMT
cell discharge at 0.1 mA cm constant current to a 2.7 V cutoff.
Recharge is by constant current at 0.05 mA cm to a 4.0 V cut-
off over fourteen cycles. Cycle numbers 117, 125 and 130 are shown.
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Figure 1 illustrates some of the cell discharges during
the first 63 cyclefi. Cell capacity is extremely reproducible.
Each recharge cycle replaces exactly 1~0~ of the charge previou~ly
remo~ed. Cell operating po~ential exceeds 3 V for nearly the
entire di~charge.
After cycle 63, the cell is intentionally ~hort-circuited
and remains sitting for two days. The cell is again recharged (to
the 3.8 V cutoff) and cycling continues. Approximately 12% of
cell discharge capacity i8 irreversibly lost, but no further loss
is observed over the next 53 cycles to cycle 116 (Figure 2).
The next 14 cycles (Figure 3, cycles 117 through 130) as
performed are with a recharge voltage cutoff o~ 4.0 v. Capacity
increases over the first couple of cycles and then stabilizes for
the remaining cycles. The increase in discharge capacity is
presumed a result of doping the polymer to a higher level with
AsF ani~ns . Recharge of the cell to 4.0 V results in an
overcharge condition. Approximately 108~ of the coulombs removed
on discharge are passed during charging. After the initial
increase in discharge capacity, overcharge remains at about 108%,
2~ and discharge capacity remains constant. This i8 important
because it shows that in addition to the electrolyte being stable
at a potential as high as 4.~ V, the polymer cathode i8 also
stable to this potential. Further, the polymer cathode is stable
to overcharge conditions, capable of continuing to provide a
- reproducible discharge.
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There i~ ~ome charging voltage cutoff, not yet deter-
mined, that better balances cell cycling; precluding current being
wasted on cell overcharging while allowing maximum discharge
capacity.
In the invention, in lieu o~ l~thium as the anode, one
might u~e lithium intercalating materials such as gr~phite, or any
of ~everal metal oxide6 or metal sulfides. The anode material
might also be a metal such a~ calcium, sodium, magnesium, barium,
potassium, titanium or strontium. The anode could also be com-
prised of alloy~ of lithium, sodium, aluminum, magnesium, calcium,
barium, pota~sium, titanium or 6trontium. Then too, the anode
might be metallic cation intercalating materials such as graphite
or any of several metal oxides or metal sulfides.
As for the cathode, one might use poly 3-methylthiophene
prepared by other methods to alter physical, chemical or electro-
; nic characteristic~ of the polymer. Al~o, one might prepare PMT
on other ~ubstrates such as nickel or aluminum foil. One might
also use other electrically conductive polymers with electro-
chemical characteristics ~imilar to PMT.
A~ for the electrolyte, one might use a mixed solvent
including DMC with methylformate, methylacetate, or some other
solvent that provide~ higher electrolyte conductivity and lithium
cycling ef~iciency. One might also use diethylcarbonate which is
resistant to oxidation or diethylcarbonate mixed with methylfor-
nate, methylactate or some other solvent. One might also use
other stable ~alt~ and/or ~olvents, organic or inorganic. One
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might also use mixtures of these salts or solvents or mixtures of
both salts and solvents.
The electrochemical cell of the invention can be use for
high voltage electrical power in the form of a rechargeable
battery. The cell can also be used as a power source where there
is a requirement for a high degree of 6afety and a,large number of
cycles. The cell might also find use as a high pulse power device
when configured in a bipolar arrangement, since one is able to
stack many cells in a small volume due to the very thin cathode.
Then too, the cell might find use as a backup power in circuit
board applications or as a reserve cell, especially in cases where
it is desired to maintain constant trickle charge to ensure
battery readiness.
We wish it to be understood that we do not desire to be
limited to the exact details of construction shown and described
for obvious modifications will occur to a person skilled in the
art.
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