Note: Descriptions are shown in the official language in which they were submitted.
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PROC~SS FOR ~YCT.ING OF GA~S nURING T~F. ~PNUFACTURING OF
COMPONENTS FOR TITHIUM BATTERIES
FIELD OF THE INVENTION
This invention relates generally to the field of
battery manufacturing and the recycling of solvents during such
manufacturing. More particularly, the invention relates to the
recovery of solvents in lithium ion battery processes.
BACKGROUND OF T~ INV~NTION
Lithium metal batteries are made of various
constructions and by various methods. In one type of battery
lithium metal is used to make the anode of the battery and
lithium dioxy cobaltate is employed to make the cathode. In
this particular battery, a LiCF3So3 salt dissolved in N-methyl
pyrrolidone (NMP) acts as the liquid electrolyte for the
battery.
In other lithium metal batteries, the anode is made
from either lithium metal or a lithium aluminum alloy, and the
cathode is made from a lithium manganese oxide.
Lithium metal batteries are used as electrical
storage devises that are re-chargeable and re-useable. These
batteries are used in electronic devices such as video cameras,
cellular phones, computers, and cameras. Since the batteries
can be recharged they are effective for increasing the portable
nature of electronic devices. They also provide an economic
energy source.
There are however a few drawbacks associated with the
use of lithium metal batteries. One drawback is that the
presence of lithium metal in a battery poses a potential fire
hazard because lithium metal reacts violently when brought into
contact with water. Another drawback is that lithium metal
batteries require higher operating temperatures. A further
drawback is that discarded lithium metal batteries may leak and
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contaminate soil and water when they are disposed of in
landfills.
The above drawbacks, along with othe~ commercial
reasons, prompted the battery industry to develop batteries
having improved safety, environmental and performance features.
Accordingly, lithium ion secondary batteries were developed.
Lithium ion secondary batteries do not contain a lithium metal
electrode but instead contain electrodes made by coating thick
slurries of carbonaceous materials and polymeric binders onto
thin metal substrates. The slurries can also contain lithium
alloys.
Because the slurries formed from carbonaceous
materials and polymeric binders are highly viscous, and in some
cases can almost be gel-like in their consistency, solvents are
added to the slurries to decrease their viscosity. The
solvents generally used are aprotic solvents having low vapor
pressures and high flash points. These solvents may be mixed
with a lower boiling oxygenated solvent such as ether, ester,
alcohol or ketone. These lower boiling solvents are added to
change the drying properties and rates of the applied slurries.
For example, anodes are made from alloys that allow
lithium ions to be held in them (lithium - intercalatable). In
forming these anode~, oxides or carbon are mixed with
polytrifluoro ethylene binders and a carrier solve~lt such as
methyl ethyl ketone, ethyl acetate or NMP. The electrolyte
used is a non-aqueous solution containing LiCF3 S03 .
Lithium ion batteries are known that have a
carbonaceous material/polyvinylidene fluoride mixture applied
to a metal substrate. NMP is used as the carrier solvent for
application of the mixture to the metal substrate. The
electrolyte system in the battery construction is a solution of
LiC104 in propylene carbonate and di-methoxyethane.
Lithium ion batteries, known as rechargeable polymer
electrolyte batteries, have a carbon anode, a lithium nickel
oxide cathode and a polyacrylonitrile or polyvinyl chloride
matrix that contains an electrolyte made of lithium salts and
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small amounts of solvent within the polymer matrix.
Typically, when electrodes for use in lithium ion
batteries are manufactured, the metal substrate is coated with
a slurry and drawn through a drying oven using a roll coating
apparatus. Warm air moving over the surface of the coating
pulls the solvent from the slurry. The solvent laden heated
air is pulled by an exhaust fan and exits the drying oven
through an exhaust stack.
A rate limiting factor of the coating operation is
lo the speed at which the oven can cause the liquid to be removed
form the coating. An increase in the speed of drying means the
roll coating apparatus can be operated at a fast rate. Thus,
rapid removal of solvent from the slurry coatings is desired.
Up to 30 pounds per hour of solvent are evaporated during
production of the electrodes. Many production facilities
operate continuously so that substantial amounts of evaporated
solvents are generated. The solvents that exit through an
exhaust vent are handled in various ways. One method of
handling the solvent vapors generated from evaporating the
solvent is to simply allow the vapors to be emitted into the
atmosphere.
Another method of handling the solvent is to pass the
solvent vapors, after they have exited the drying oven, through
a thermal oxidizer which burns the vapors thereby,l emitting
carbon dioxide, carbon monoxide, and water into atmosphere.
Although this method drastically reduces the amount of solvent
emission, it is an expensive process since it destroys the
solvent.
A further method of handling the solvent vapor is to
direct the vapors into a condensing chamber where the vapors
are condensed onto metal surfaces that are kept at a
temperature cooler than the exhaust air. The temperature of
the metal surfaces is maintained by circulating a cooling
liquid that is provided by a chiller system. These cooling
units consume large amounts of energy and are expensive to
maintain.
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As will become apparent from the discussion below, it
is an object of the present invention to provide a cost
efficient means to recycle and reclaim solve~ts used to
manufacture electrodes of lithium ion batteries. It is another
object of the present invention to provide a closed loop
continuous feed condensation process. It is yet a further
object of the present invention to reduce the amount of
solvents destroyed and to prevent the release of such solvents
into the atmosphere. Other objects and advantages of the
present invention will be readily apparent from the description
of the invention as set out in the specification and claims
below.
SUMMARY OF THE INVENTION
Provided herein is a method of recovering a solvent
in a process for manufacturing lithium ion batteries. In
practicing the invention, a carrier solvent is released from a
slurry coated onto a metal substrate. The slurry contains both
the carrier solv~nt and the material used to fabricate
electrodes for use in lithium ion batteries. In fabricating
the electrodes, the solvent is mixed with a gas stream
comprising a gas and water after which the solvent is
condensed. The invention as described herein may also comprise
a closed looped and continuous process for the capture and re-
use of the solvent.
The invention provides a substantial improvement in
processes that require the removal of carrier solvent during
the forced air drying of paste-like films or slurries used to
manufacture electrodes for lithium ion batteries. The
improvements comprise an inexpensive means of recovering the
carrier solvent.
DF~TAIT~n D~.~CRIPTION OF T~ INV~NTION
In one aspect of the invention, anode construction
begins with the mixing together of: 1) a slurry of a
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carbonaceous material, usually graphite; 2) a polymeric binder,
usually polyvinylidene fluoride, polyethylene, or polytrifluoro
ethylene; and 3) a carrier. The preferred carrier comprises an
aprotic solvent having a low vapor pressure and a high flash
point. The carrier may be a combinatioh of an aprotic solvent
and oxygenated solvents having low boiling temperatures such as
an ether, ester, alcohol, or ketone. These solvents are added
to change the drying rates of the solvents from the metal
substrates. The preferred aprotic solvent is an alkyl
substituted pyrrolidone, most preferably N-methyl pyrrolidone.
The preferred combination of solvents used to make up the
carrier solvent is an akyl substituted pyrrolidone and methyl
ethyl ketone.
In manufacturing an anode according to an embodiment
of the present invention, a reverse roll coating apparatus has
strung through it a metal foil substrate, usually thin copper
(0.0005 - 0.002 inches thick). The copper foil is usually
several feet wide and is strung through the coating apparatus
from rolls of the copper foil that are usually several hundred
yards in length. The slurry is placed into a slurry
containment area of the reverse roll coating apparatus. The
coating apparatus is started up and the slurry is applied to
one face of the metal foil. The metal foil is usually moving
at a rate of several yards per minute, usually 1 to 3~1yards per
minute. Next, the slurry coated metal foil enters a drying
oven. The slurry at this point is still a highly viscous
liquid.
The drying oven is typically about 10 - 30 yards in
length. Heated air is provided in the oven and is pulled over
the surface of the slurry in order to dry the solvent out of
slurry. Typically the temperature of the air passing over the
surface of the slurry is about 200 ~F to 300 ~F. The air drawn
into the oven is preferably at room temperature with a relative
humidity at about 30%. The air drawn into the oven is pulled
preferably at a rate of about 5000 to about 8000 scfm. The
solvent is evaporated into the gas stream to create a solvent
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laden hot air gas stream. The solvent laden hot air gas is
pulled out of the oven and into a condenser where the solvent
is condensed. In another embodiment of the invention, the
water content of the air stream is increased ranging from about
greater than 30% to about 50%, by introducing the water into
the air stream after it exits the drying oven and before the
aprotic solvent is condensed.
In a preferred method a "precooling" heat exchanger
is used to precool the solvent laden air stream prior to its
entering a condensing exchanger. Using this method the air
cooled in the condensing exchanger is recycled back into the
"precooling" heat exchanger where it is used to cool the air
stream exiting the drying oven.
Once the aprotic solvent is condensed it can be
distilled from the water and recycled in a closed system
wherein the solvent is reintroduced into the slurry. The water
is separated from the NMP in the distillation unit.
In another embodiment of the invention a water
scrubber is introduced into the system in order to absorb NMP
vapors that remain in the air stream after it leaves the
condenser. Preferably, the scrubber is a packed column that
removes residual solvent vapors out of the exhaust air with
water mist. The water stream generated by the mist can be
recycled into a distillation unit to remove the residual NMP.
A portion of the air exiting the water wash packed column
scrubber can be re-used as the air entering the drying oven.
As the solvent is driven from the oven a construction
is formed from the carbon/polymer composite bonded to the metal
foil. The construction is wound into rolls and the rolls are
put through the same coating/drying process. The final
composite that is made into the anode has the metal foil coated
on both sides with the carbon/polymer composition.
The cathode is constructed using the same process
used to make the anode with some minor differences. One
difference is in the composition of the slurry. In
constructing the cathode, the slurry comprises a large amount
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of a lithium oxide, such as LiCoO2 or LiNio2. Another difference
is that aluminum foil is used as the metal substrate. The
aluminum foil cathode is coated in much the same way as is the
copper foil that is used for the anode.
The following examples are illustrative only and are
not meant to limit the invention in any manner.
Example 1
(30% Relative humidity air stream at 75~F)
This example illustrates the use of a condenser for
condensing solvent vapor from an air stream exiting a drying
oven and the subsequent recovery of the solvent in a
distillation column. It also illustrates the use of a heat
exchanger to "precool" a solvent laden air stream before it
enters a primary condenser.
Room temperature air at approximately 75~F and with
a relative humidity of about 30% is drawn into the oven through
a reverse coating apparatus at a rate of about 5000 scfm using
exhaust fans. The temperature of the air rises from about 24~C
to about 150~C by the time it exits the oven. As the coated
metal foil passes through the drying oven, a total of about 100
lb/hr of NMP is evaporated from its surface into the air-
stream. The concentration of NMP vapor in the a~ir stream
leaving the drying oven is about 12% (by volume).
The solvent-laden air stream is pulled from the
drying oven through a series of two finned-typed heat
exchangers. The bulk of the NMP is condensed in the second
heat-exchanger that uses a chilled water-and glycol solution as
its cooling medium. When the oven is initially started-up,
the second exchanger cools the air stream to preferably the dew
point of the NMP. The air exiting the second exchanger is then
recycled to the first "precooling" exchanger where it is used
to cool air exiting the drying oven. When the system reaches
steady state, the precooling exchanger reduces the cooling load
in the second exchanger. The air stream exiting the final
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condenser contains only 15 ppmv NMP and has a temperature of
about -6~C (the dew point of the NMP and water vapor mixture).
The air can be recycled or treated further.
The NMP and water condensed in the second heat
exchanger are pumped from a reservoir to a distillation column
to recover the NMP from the water. The column feed has a
composition of about 70 weight percent NMP and about 30 weight
percent water. The column is best operated under a vacuum.
The temperature of the reboiler at the bottom of the is between
about 100 to 180~C. The temperature in the overhead condenser
is between about 25 to 55OC. The NMP is recovered as the
bottoms product from the column at a concentration in the
bottoms of 99.99 % (by weight). When the system reaches steady
state, the bottoms product is used to preheat the feed to the
distillation column.
Example 2
(50% Relative Humidity Air, at 75~F)
This example illustrates the rise in the dew point
caused by the addition of water vapor to the NMP-containing air
stream of Example 1. The equipment and conditions described in
this example are the same as those of Example 1 with the
following exceptions: Room temperature air at 75~F a,,nd with a
relative humidity of about 50% is drawn into the oven through
a reverse roll coating apparatus at a rate of about 5000 scfm
by the drying oven's exhaust fans. The air temperature rises
through the oven to about 150~C and a total of about 100 lb/hr
of NMP is evaporated into the air stream. The concentration of
NMP in the air is 12% (vol.).
Because of the additional water vapor present in the
air stream, the temperature required for condensing the NMP
from vapor is about 11~C. This is 5~C higher than the
condensation temperature (dew point) of Example 1. Despite the
higher dew point, the air stream exiting the final condenser
still contains only 14 ppmv NMP. The air can be recycled,
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treated or discharged. The condensed NMP is recovered from the
water in a distillation column as in Example 1.
Example 3
(30% Relative Humidity Air at 75~F)
This example illustrates the use of a water scrubber
downstream of the primary condenser for use in absorbing NMP
vapor remaining in the air stream exiting the condenser.
Air containing residual NMP, after it passes through
the condenser, is scrubbed by a water scrubber, such as a
packed column exhaust air scrubbing unit. The water stream,
containing about 2 weight percent NMP and about 9 weight
percent water, can be discharged from the system or sent to a
distillation column. Preferably, the whole process takes place
in a closed loop system. The air discharged from the scrubber
and containing about 1 ppm NMP can also be discharged from the
system or recycled back into the air stream prior to
condensation.
The invention has been described with reference to
various specific embodiments. However, many variations and
modifications may be made while remaining within the scope and
spirit of the invention.
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