U.S. patent application number 16/501985 was filed with the patent office on 2019-12-05 for apparatus and method for processing battery electrodes.
The applicant listed for this patent is Iftikhar Ahmad, Peter H. Aurora, Andrew Cardin, Clayton DeCamillis, Michael Hampton, William Hicks, James E. Webb, JR., Pu Zhang. Invention is credited to Iftikhar Ahmad, Peter H. Aurora, Andrew Cardin, Clayton DeCamillis, Michael Hampton, William Hicks, James E. Webb, JR., Pu Zhang.
Application Number | 20190372089 16/501985 |
Document ID | / |
Family ID | 58239027 |
Filed Date | 2019-12-05 |
United States Patent
Application |
20190372089 |
Kind Code |
A1 |
Ahmad; Iftikhar ; et
al. |
December 5, 2019 |
Apparatus and Method for Processing Battery Electrodes
Abstract
An apparatus for processing battery electrodes includes: a
microwave applicator cavity with slots on opposite ends to allow a
continuous sheet to move through the cavity in a first direction; a
processing chamber constructed of microwave-transparent material,
disposed within the applicator cavity and surrounding the
continuous sheet, the processing chamber having slots to allow the
continuous sheet to pass through it; a microwave power supply to
deliver power to the applicator cavity; a source of heated gas
providing a controlled gas flow through the processing chamber in a
direction opposite the first direction; and, at least one
non-contacting temperature measuring device positioned to measure a
surface temperature at a selected location on the continuous sheet
as it passes through the processing chamber. The apparatus is
particularly suited for removing polar solvents from porous
electrode coatings. A related method is also disclosed.
Inventors: |
Ahmad; Iftikhar; (Raleigh,
NC) ; Cardin; Andrew; (Cary, NC) ; DeCamillis;
Clayton; (Raleigh, NC) ; Hampton; Michael;
(Raleigh, NC) ; Webb, JR.; James E.; (Garner,
NC) ; Zhang; Pu; (Ann Arbor, MI) ; Hicks;
William; (Saline, MI) ; Aurora; Peter H.; (Ann
Arbor, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ahmad; Iftikhar
Cardin; Andrew
DeCamillis; Clayton
Hampton; Michael
Webb, JR.; James E.
Zhang; Pu
Hicks; William
Aurora; Peter H. |
Raleigh
Cary
Raleigh
Raleigh
Garner
Ann Arbor
Saline
Ann Arbor |
NC
NC
NC
NC
NC
MI
MI
MI |
US
US
US
US
US
US
US
US |
|
|
Family ID: |
58239027 |
Appl. No.: |
16/501985 |
Filed: |
July 16, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15330272 |
Sep 1, 2016 |
10403880 |
|
|
16501985 |
|
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|
62283785 |
Sep 11, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F26B 3/04 20130101; F26B
3/343 20130101; H01M 4/0471 20130101; F26B 3/347 20130101; B05C
9/12 20130101; H01M 4/0404 20130101; F26B 13/00 20130101; B05C 9/14
20130101; H01M 10/052 20130101; H01M 4/1393 20130101; H01M 4/1391
20130101 |
International
Class: |
H01M 4/04 20060101
H01M004/04; H01M 4/1391 20060101 H01M004/1391 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with Government support under
Contract No. DE-EE0006869 awarded by the U.S. Department of Energy
to Lambda Technologies, Inc., and the Government has certain rights
in this invention.
Claims
1. A method for processing battery electrodes comprising:
depositing an electrode material as a wet slurry containing a
solvent onto a continuous substrate; passing said coated substrate
through a microwave-transparent processing chamber disposed within
a microwave applicator cavity; and, applying microwave power to
said applicator cavity to heat said solvent while simultaneously
introducing heated gas from a source external to said processing
chamber so that said gas passes through said processing chamber in
the direction opposite to the direction of movement of the coated
substrate in order that dry gas enters said processing chamber
adjacent to the dry end of said coated substrate and moist gas
exits said processing chamber adjacent to the wet end of said
coated substrate.
2. The method of claim 1 wherein said coating is deposited on said
substrate by a process selected from the group consisting of:
casting; doctor blading; spray coating, dip coating, screen
printing, and extrusion.
3. The method of claim 1 wherein said solvent is a polar solvent
selected from the group consisting of: water;
N-methyl-2-pyrrolidone, ethanol, methanol, isopropanol, acetone,
n-propanol, n-butanol, formic acid, propylene carbonate, ethyl
acetate, dimethyl sulfoxide, acetonitrile, dimethyl formamide,
tetrahydrofuran, and dichloromethane.
4. The method of claim 1 wherein said microwave power is supplied
in a sweeping fashion over a selected bandwidth.
5. The method of claim 3 wherein said heated gas is introduced at a
temperature less than 240.degree. C.
6. The method of claim 1 wherein said moist gas is extracted from
said processing chamber at a rate equal to the rate at which said
heated gas is introduced into said processing chamber so that
neutral pressure is maintained in said processing chamber.
7. The method of claim 1 further including the step of pre-drying
said gas to a selected relative humidity before heating and
delivering said gas to said processing chamber.
8. The method of claim 1 further including the step of calendaring
said coated substrate after it exits from said processing
chamber.
9. The method of claim 1 further including the step of passing said
coated substrate through a heated drying oven after it exits from
said processing chamber.
10. The method of claim 1 wherein said electrode material contains
a binder phase and said coating is dried at such a rate that the
concentration of said binder adjacent to a free surface is no more
than 1.3 times the concentration of said binder adjacent to said
substrate.
11. A method for processing battery electrodes comprising:
depositing an electrode material as a wet slurry containing a
selected solvent onto a continuous substrate; passing said coated
substrate through a microwave-transparent processing chamber
disposed within a microwave applicator cavity; applying microwave
power to said applicator cavity to heat said solvent while
simultaneously introducing heated gas from a source external to
said processing chamber so that said gas passes through said
processing chamber in the direction opposite to the direction of
movement of the coated substrate in order that dry gas enters said
processing chamber adjacent to the dry end of said coated substrate
and moist gas exits said processing chamber adjacent to the wet end
of said coated substrate; and, passing said coated substrate out of
said applicator cavity and through a second chamber for further
processing via conventional heating.
12. The method of claim 11 wherein said wet slurry is deposited on
said substrate by a process selected from the group consisting of:
casting; doctor blading; spray coating, dip coating, screen
printing, and extrusion.
13. The method of claim 11 wherein said solvent is a polar solvent
selected from the group consisting of: water;
N-methyl-2-pyrrolidone, ethanol, methanol, isopropanol, acetone,
n-propanol, n-butanol, formic acid, propylene carbonate, ethyl
acetate, dimethyl sulfoxide, acetonitrile, dimethyl formamide,
tetrahydrofuran, and dichloromethane.
14. The method of claim 11 wherein said microwave power is supplied
in a sweeping fashion over a selected bandwidth.
15. The method of claim 13 wherein said heated gas is introduced at
a temperature less than 240.degree. C.
16. The method of claim 11 wherein said moist gas is extracted from
said processing chamber at a rate equal to the rate at which said
heated gas is introduced into said processing chamber so that
neutral pressure is maintained in said processing chamber.
17. The method of claim 11 further including the step of pre-drying
said gas to a selected relative humidity before heating and
delivering said gas to said processing chamber.
18. The method of claim 11 further including the step of
calendaring said coated substrate after it exits from said
processing chamber.
19. The method of claim 11 wherein said electrode material contains
a binder phase and said coating is dried at such a rate that the
concentration of said binder adjacent to a free surface is no more
than 1.3 times the concentration of said binder adjacent to said
substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional of U.S. patent application
Ser. No. 15/330,272 filed on Sep. 1, 2016 entitled, Apparatus and
method for drying battery electrodes, and further claims the
benefit of Provisional Patent Application No. 62/283,785, filed on
Sep. 11, 2015 by the present inventors, the entire disclosures of
which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
Field of the Invention
[0003] The invention pertains to methods and apparatus for drying
anode and cathode materials in the fabrication of lithium ion
batteries. More specifically, it pertains to methods involving the
simultaneous application of microwave heating and hot air
drying.
Description of Related Art
[0004] In high volume production of electrodes for lithium ion
batteries drying is the highest-cost unit operation. In
conventional processing, anode or cathode slurries are cast onto
metal foils and dried under highly controlled, conditions in very
long furnaces. The furnace length (which can be .about.40 m long)
is dictated by the limited rate at which water,
N-methyl-2-pyrrolidone (NMP), or other selected solvent can be
removed from the slurry. Higher temperature drying can increase the
drying speed but can cause binder migration, surface cracking,
particle segregation, orange peel defects or pore-blocking skin
formation; in general, controlling these issues becomes
increasingly difficult as the electrode thickness increases.
[0005] There is a need for improved drying processes to speed the
drying process without creating the aforementioned defects,
particularly when processing thicker electrodes.
Objects and Advantages
[0006] Objects of the invention include: providing an effective and
rapid processing method for anode and cathode slurries to fabricate
electrodes for lithium ion batteries; providing an improved drying
process for inorganic slurry coatings on metal foil; providing a
continuous drying process for slurry-coated metal foils; providing
a drying process that prevents defect formation in lithium battery
anode and cathode materials; providing a drying apparatus for
lithium battery electrodes that is relatively compact; providing a
more controllable and energy-efficient process for drying lithium
battery electrodes; and providing a rapid processing method that
enables the manufacture of thicker anodes and cathodes while
minimizing binder segregation.
SUMMARY OF THE INVENTION
[0007] According to one aspect of the invention, an apparatus for
processing battery electrodes comprises:
[0008] a microwave applicator cavity with slots on opposite ends to
allow a continuous sheet to move through the cavity in a first
direction;
[0009] a processing chamber constructed of microwave-transparent
material, disposed within the applicator cavity and surrounding the
continuous sheet, the processing chamber having slots to allow the
continuous sheet to pass through it;
[0010] a microwave power supply to deliver power to the applicator
cavity;
[0011] a source of heated gas providing a controlled gas flow
through the processing chamber in a direction opposite the first
direction; and,
[0012] at least one temperature measuring device positioned to
measure a surface temperature at a selected location on the
continuous sheet as it passes through the processing chamber.
[0013] According to another aspect of the invention, a method for
processing battery electrodes comprises:
[0014] depositing an electrode material as a wet slurry onto a
continuous metal foil;
[0015] passing the coated metal foil through a
microwave-transparent processing chamber disposed within a
microwave applicator cavity; and,
[0016] applying microwave power to the applicator cavity while
simultaneously passing heated gas through the processing chamber in
a direction opposite to the direction of movement of the coated
metal foil.
[0017] According to another aspect of the invention, an apparatus
for processing battery electrodes comprises:
[0018] a microwave applicator cavity with slots on opposite ends to
allow a continuous sheet to move through the cavity in a first
direction;
[0019] a processing chamber constructed of microwave-transparent
material, disposed within the applicator cavity and surrounding the
continuous sheet, the processing chamber having slots to allow the
continuous sheet to pass through it;
[0020] a microwave power supply to deliver power to the applicator
cavity;
[0021] a source of heated gas providing a controlled gas flow
through the processing chamber in a direction opposite the first
direction;
[0022] at least one temperature measuring device positioned to
measure a surface temperature at a selected location on the
continuous sheet as it passes through the processing chamber;
and,
[0023] a heated chamber, located downstream from the microwave
applicator cavity and having slots in opposite ends to allow the
continuous sheet to pass through it for further processing after
exiting the microwave applicator cavity.
[0024] According to another aspect of the invention, a method for
processing battery electrodes comprises:
[0025] depositing an electrode material as a wet slurry onto a
continuous metal foil;
[0026] passing the coated metal foil through a
microwave-transparent processing chamber disposed within a
microwave applicator cavity;
[0027] applying microwave power to the applicator cavity while
simultaneously passing heated gas through the processing chamber in
a direction opposite to the direction of movement of the coated
metal foil; and,
[0028] passing the coated foil through a second chamber for further
processing via conventional heating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The drawings accompanying and forming part of this
specification are included to depict certain aspects of the
invention. A clearer conception of the invention, and of the
components and operation of systems provided with the invention,
will become more readily apparent by referring to the exemplary,
and therefore non-limiting embodiments illustrated in the drawing
figures, wherein like numerals (if they occur in more than one
view) designate the same elements. The features in the drawings are
not necessarily drawn to scale.
[0030] FIGS. 1A-C illustrate schematically an apparatus in
accordance with some examples of the invention. FIG. 1A illustrates
a processing system; FIGS. 1B and 1C show the front cross sectional
view and side views of the processing system, respectively.
[0031] FIG. 2 presents capacity retention data for anode materials
prepared according to one aspect of the invention.
[0032] FIG. 3 presents discharge capacity retention for materials
prepared according to another aspect of the invention and prepared
by conventional methods.
[0033] FIG. 4 compares the binder segregation observed by SEM, for
materials prepared according to one aspect of the invention versus
material dried rapidly by conventional methods.
[0034] FIG. 5 shows first formation cycle for Single Layer Pouch
(SLP) cells with electrodes (anode and cathode) dried using
conventional and ADP drying systems; using CC-CV (Constant
Current/Constant Voltage) charge and CC (Constant Current)
discharge protocols at C/10 from 3.0 to 4.2V.
[0035] FIG. 6 shows rate capability plots where all cells were
charged at 0.1 C, and discharged at 0.1, 0.2, 0.5, 1.0, 2.0 and 5.0
C.
DETAILED DESCRIPTION OF THE INVENTION
[0036] Microwave drying is well known and widely used in a number
of industries, owing to the fact that microwave energy can
penetrate for some distance into dielectric materials and is
readily absorbed by water molecules. However, drying wet slurries
on metal foils by conventional microwave drying using a fixed
frequency (typically 2.45 GHz) creates arcing on the metal foil and
is therefore not practical. The invention employs a variable
frequency microwave (VFM) source, rapidly sweeping through a
bandwidth of frequencies, to eliminate arcing and also to provide
uniformity within the processing cavity. In some examples, the
frequency range was typically 5.85 GHz to 6.65 GHz, but it will be
appreciated that other ranges may work equally well depending on
the cavity dimensions and other process variables. The basic VFM
approach is well-known and taught in several U.S. Patents [see,
e.g., U.S. Pat. No. 5,321,222 for a basic description of the
technique].
[0037] At the same time, it will be appreciated that rapid drying
of a slurry coating that has been deposited onto a metal foil can
lead to various defects. For example, rapidly drying the surface of
the slurry coating can form a relatively dense "skin", which would
be detrimental to the performance of a battery that relies on a
large amount of porosity and accessible surface area in the
electrodes for optimal electrical characteristics. The invention is
therefore directed to process improvements that can speed
processing and reduce the size of the process equipment while
avoiding detrimental effects on the materials being processed.
[0038] Referring to FIGS. 1A-C, an exemplary apparatus for
processing battery electrodes comprises a cold-wall applicator
cavity 100 of a suitable size and shape to accommodate foils of the
desired thickness for the battery component to be manufactured. A
port is provided on the upper surface of the cavity to admit
microwave power, which is typically introduced via a conventional
waveguide 200 and launcher. Microwave power is preferably provided
by a VFM power supply 300 operating over a selected bandwidth as
taught in the aforementioned patent literature. The applicator
cavity contains a slot 120 at each end, to accommodate the passage
of the coated metal foil, which is shown in FIG. 1B as running from
left to right. Broadband microwave chokes 110 are preferably
provided as shown, to minimize microwave leakage from the
slots.
[0039] A processing chamber 150 is disposed within the applicator
cavity 100, the purpose of which is to provide a controlled
processing environment, contain the flow of heated gas, and manage
the outflow of evaporating solvent from the film. This chamber is
constructed of microwave-transparent material such as polymer,
quartz, glass, or a ceramic material having low dielectric loss,
and further has a slot in each end for the continuous sheet to pass
through.
[0040] Heated gas (typically air, although any generally inert or
nonreactive gas, e.g., nitrogen, may be used) is supplied by a
blower or pump, not shown, entering as indicated in FIG. 1B on the
right at 160 and exiting on the left at 170, so that the gas flows
in the opposite direction from the movement of the passing metal
foil 130. Applicants have discovered that the counter-flowing
arrangement is important to achieving high efficiency (i.e., rapid
drying) while avoiding deleterious effects such as skin formation
on the electrode material. Part of the reason for this, as the
skilled artisan will appreciate, is that the gas is most dry in the
area where the film is also most dry (i.e., on the right in FIG.
1B), and the gas becomes progressively more moist as it moves
toward the area where the film is likewise most moist. This
produces a relatively even evaporation rate from the film and
avoids skin formation that can occur if the wet film encounters
very dry heated air at the "wet" end of the process.
[0041] At least one temperature monitor 140 is positioned to
measure the temperature of the passing film, and preferably two
such monitors are provided near the entry and exit points of the
film from the chamber as shown in FIG. 1B. The monitors may be any
type of non-contacting IR sensors, as are well known in the
art.
[0042] The following examples will further illustrate various
aspects of the inventive apparatus and describe exemplary
processing conditions and the results that were observed.
Example
[0043] A microwave applicator cavity 100 was configured in the form
of a rectangular prism 60.times.46.times.33 cm with 180.times.13 mm
slots 120 on opposite ends, sufficient to accommodate a continuous
sheet 130 typically 150-170 mm wide moving, as shown in FIG. 1B,
from left to right at 1 cm/sec. A microwave choke 110 was placed on
each end to suppress outward radiation of microwave energy. Each
choke contained a set of slotted dielectric plates spaced to absorb
microwave energy of the frequencies being used. VFM power was
delivered via waveguide 200 from power supply 300 [MicroCure
1600-2000, Lambda Technologies, Morrisville, N.C.) operating over
the range of 5.85 GHz to 6.65 GHz with a maximum power of 1600 W.
Frequency sweeping covered 4096 discrete frequencies across the
indicated bandwidth every 0.1 s, so that the dwell time at any one
frequency was about 25 .mu.s. Two IR sensors 140 [Williamson Silver
Model U2-1] were placed as shown in FIG. 1B.
Example
[0044] A microwave-transparent processing chamber 150 was
constructed as shown generally in FIG. 1B. The material used was 3
mm thick quartz and the inner cross-section dimensions were
210.times.58 mm extending over the entire 600 mm length of the
metallic microwave cavity 100. The processing chamber or subcavity
in this prototype system was constructed from quartz glass because
of its very low dielectric loss at microwave frequencies. However,
other materials such as low-loss ceramic materials or high
performance composites used for radomes and electromagnetic windows
(e.g. Saint-Gobain's RAYDEL.RTM. family of microwave transmissive
polytetrafluoroethylene PTFE or Teflon.RTM. composites) may be
suitable for larger production systems.
Example
[0045] A blower (not shown) supplies a controlled flow of heated
gas into the right-hand end of the process chamber at 160. The gas
exits from the left-hand end of the chamber at 170. The gas
(typically air) is initially supplied at a temperature somewhat
less than the boiling point of the solvent in the film (typically
85-90.degree. C. for aqueous systems). The process chamber has
slots 120 at the ends to allow the film to pass, and Applicants
have discovered that in many cases it is preferable to add a second
blower or pump to extract gas on the exit side 170 of the hot air
path and operate the two blowers at substantially identical flow
rates. It will be appreciated that this arrangement thereby creates
a condition of essentially neutral pressure, i.e., the gas pressure
inside the chamber is equal to that outside the chamber. As a
consequence, there is virtually no convection of gas outwardly
through the chamber slots and the temperature, humidity,
composition, and flow of the gas past the moving film may be
precisely controlled. This arrangement has the further advantage of
eliminating waste or loss of process heat as well as avoiding the
escape of annoying fumes into the workplace and the deposition or
condensation of moisture or other contaminants in the microwave
chokes.
[0046] Having described the key components and features of the
apparatus, various aspects of the inventive process will be
described to provide a fuller understanding of the process and how
the apparatus provides a wide flexibility for process control and
optimization.
[0047] The hot gas is typically introduced at a temperature T.sub.a
preferably 10-20.degree. C. lower than the intended process
temperature T.sub.p; thus for an aqueous slurry T.sub.a is
preferably .about.85-90.degree. C. (This will warm up the inner
process chamber. The purpose of the hot gas flow is two-fold.
First, it carries away the solvent volatilized by the efficient
penetration of VFM energy through the entire coating thickness; if
the evaporated solvent molecules are not carried away they will
have the tendency to condense back on the dried porous slurry or
onto the walls of chamber 150. Second, the heated gas preheats the
metal foil, which is not heated by the microwave energy and would
otherwise act as a heat sink for the heat generated via dielectric
loss in the slurry. The flow of hot gas is such that it carries the
solvent molecules to the upstream side, where the freshly cast wet
slurry coated foil enters the chamber, i.e. from the drier side to
the wetter side. The lower hot gas temperature at the wet end
(<T.sub.a) will insure that there are no "orange peel" defects
or pore-blocking skin formation, when the slurry cast electrode
material enters the process chamber.)
[0048] VFM power is introduced into the applicator cavity 100 and
passes into the microwave transparent chamber 150, penetrates the
volume of the slurry film, interacts with the polar solvent
molecules and drives them out to the surface of the coated slurry.
(As the solvent evaporates the cooling effect associated with it
will be observed on the IR sensor 140 monitoring the entrance
slurry temperature T.sub.e on the left. As the web travels to the
right and evaporation diminishes, the temperature will start to
rise, reaching the process temperature T.sub.p, by the time it
arrives under the right-hand IR monitor 140. When the slurry coated
foil arrives at the chamber exit on the right, the penetrating VFM
has driven much of the moisture out of the slurry thickness and as
the coated foil approaches the hot gas entrance side where the hot
gas temperature is the highest, the hot gas primarily carries out a
surface drying function. Thus, the invention exploits a synergy of
the microwave-driven internal heating and drying and the hot gas
driven surface drying. As a result, the electrode slurry can be
dried very effectively and at temperature less than those required
by conventional IR or hot air drying.)
[0049] It will be appreciated that optimizing the processing
parameters will include adjusting the VFM power delivered into the
chamber: higher microwave power will produce higher moisture or
solvent removal and higher the process temperature T.sub.p. It will
be further understood that, in general, one will avoid process
temperatures at or above the boiling point of the solvent
(100.degree. C. for aqueous slurries); otherwise voids can be
created. To control the final exit temperature, the hot gas
temperature T.sub.a can be decreased, so that the relatively cooler
gas will dry and cool the electrode surface temperature, yet the
higher microwave power will drive the rapid removal of solvent from
the interior.
[0050] Another controllable process variable includes
simultaneously reducing the flow rate for hot air and exhaust,
which allows the surface temperatures to increase; this might be
necessary at temperatures where higher air flow may cool the foil
electrode more than desired.
[0051] Another process variable is the speed of the web, which can
be adjusted for various loadings, and as the web speed is varied
the above parameters can be optimized through routine
experimentation to provide drying of a thick slurry without orange
peel defects or pore-blocking skin formation.
[0052] The adjustability of variables described above allows the
user to adapt the process for different slurry compositions and
thicknesses, and for slurries containing other solvents. Solvents
may include, but are not limited to, water and NMP. In each case,
rapid solvent removal will be possible at relatively lower
temperature, without creating problems (orange peel defects or
pore-blocking skin formation and binder migration) normally
associated with otherwise high temperatures or rapid drying.
[0053] Although in principle the invention may be usefully employed
in the drying of virtually any wet slurry on a metal foil, it is
particularly applicable to anode or cathode slurry mixtures formed
on metal foil by any suitable process. Such slurries typically
comprise a mixture of various inorganic particulates, a binder
phase, and a volatile solvent. In the following examples, electrode
slurries were cast onto metal foils by conventional methods. Some
drying experiments were conducted in a static environment, in which
a sample of coated foil was placed in a microwave cavity and was
not moving as a continuous sheet or web; in other cases, the slurry
was deposited continuously onto a moving metal foil, which passed
through the process as generally shown in FIGS. 1A-B at a
predetermined speed. These will be referred to as "static" and
"dynamic" tests, respectively.
Example
[0054] An anode was prepared as follows: 97 wt. % natural graphite
powder and 3 wt. % aqueous binder [styrene-butadiene rubber (SBR)]
were mixed with water to form an anode slurry at 50% solids
content, using a Flacktek SpeedMixer DAC150. The slurry was cast
onto a 10 .mu.m Cu foil using a doctor blade casting applicator.
The coated anode was dried under static conditions in the VFM
chamber with flowing hot air at 80.degree. C. With VFM power on,
the slurry temperature reached 95.degree. C. and coating was dried
in 1 minute to form an anode with a loading of 11 mg/cm.sup.2.
[0055] A similarly prepared anode was conventionally dried with
flowing hot air at 95.degree. C. for comparison. The coating was
dried in 5 minutes to form an anode with a loading of 11
mg/cm.sup.2. The inventive process, in this example, provides a
five-fold improvement in process speed.
Example
[0056] A cathode structure was prepared as follows: 93 wt. % NCM
(Ni--Co--Mn oxide) powder, 3 wt. % conductive carbon, and 4%
polyvinylidine difluoride (PVDF) binder were mixed with NMP to form
a cathode slurry at 60% solids content by using a Flacktek
SpeedMixer DAC150. The slurry was cast onto a 20 .mu.m Al foil
using a doctor blade casting applicator. The coated cathode was
then dried under static conditions by the inventive method with
flowing hot air at 100.degree. C. When VFM power was on, the
temperature reached 110.degree. C. and the coating was dried in 3.5
minutes to form a cathode with a loading of 18 mg/cm.sup.2.
[0057] When the coated cathode was dried conventionally with
flowing hot air at 110.degree. C., the coating dried in 7 minutes
to form a cathode with a loading of 18 mg/cm.sup.2. The inventive
process, in this example, provides a two-fold improvement in
process speed.
[0058] The following examples present some of the analytical
comparisons performed on samples dried by the standard method as
well those dried with the inventive Advanced Drying Process
(ADP).
Example
[0059] Wet Adhesion Test
[0060] Electrodes prepared with standard and ADP method were soaked
in electrolyte for 2 h at 80.degree. C. then cooled to room
temperature. The adhesion of the electrode material to the metal
foil was tested using the visual cross hatch comparison method,
which is well known in the paints and coatings industry. In this
method, the coating is scored in two orthogonal directions using a
razor blade to form a cross-hatched pattern. Poor adhesion will
cause some coating material to separate from the substrate and
break away near the cut lines. In this test, both samples
(conventional and microwave processed) showed similar adhesion.
Example
[0061] Moisture Analysis
[0062] Moisture analysis was also performed on the samples prepared
by standard convection drying as well as by the inventive ADP
method. The measured solvent content of the conventionally dried
film was 1968.97 ppm, whereas that of the film processed by
continuous ADP drying was 1974.02 ppm. In both cases, therefore,
the solvent content (as dried) was .about.2000 ppm (the target
value is <5000 ppm) for both samples, even though the sample was
dried using the inventive process in a fraction of the time
required by the conventional method.
Example
[0063] Anode Formation Half Cell Testing
[0064] The anode formation half-cell testing data comparison is
shown in FIG. 2, which shows capacity retention vs. discharge rates
expressed in terms of C for anode electrodes dried under standard
static and continuous ADP conditions. [C=1 means that the battery
is discharged at a rate that would deplete its nominal capacity in
1 hour; C=10 means that the battery is discharged in 1/10th of an
hour; C=0.1 means that the battery is discharged in 10 hours]. Data
are summarized below in Table 1.
TABLE-US-00001 TABLE 1 Half-cell formation parameters.sup.a Drying
Loading Reversible conditions (mg/cm.sup.2) (mAh/cm.sup.2) Capacity
(mAh/g) ICL (%) Standard 10.1 3.7 343.2 7 Continuous ADP 10.2 3.8
345.1 7 .sup.aThe electrode is an anode with a water-based
binder
[0065] FIG. 2 and Table 1 show results of the electrochemical
characterization performed using half coin cells. Specific
capacities and initial capacity loss (ICL) are similar for both
sets of cells. Cells produced by static or continuous drying had
almost identical cell voltage, and, as shown in FIG. 2, they have
virtually identical capacity retention.
Example
[0066] Life Cycle Testing
[0067] Additionally, life cycle data for single layer pouch (SLP)
cell [3.0 mAh/cm.sup.2, at C/2,100% Depth of Discharge (DoD)] made
with electrodes dried under both conditions is shown in FIG. 3. The
plots show comparable performance between the baseline and the cell
fabricated with the inventive ADP dried electrodes.
Example
[0068] Binder Migration
[0069] All of the analytic work and data presented above shows that
the inventive ADP produces virtually identical electrical
properties to conventional drying while significantly reducing the
overall drying time. More surprisingly, Applicants have discovered
that the inventive process has a significant impact on the
migration of binder within the film; ADP actually produces better
properties compared to rapid drying attempts with conventional
methods. FIG. 4 shows SEM photographs under backscattered mode for
rapid ADP and rapid conventional drying. Each sample is shown in
cross section, and the binder distribution is evident as bright
spots relative to the darker gray graphite particles. There is
clearly an advantage to using the inventive process (left), because
binder segregation near the surface of the electrode (seen in the
conventionally dried sample on right and indicated by the area
bounded by dashed lines) will have various deleterious effects on
mechanical and electrical performance.
[0070] Because the binder is uniformly dispersed within the
material to begin with, the ratio of binder concentration near the
electrode surface to that near the metal foil substrate therefore
reflects the degree of binder migration during the drying process.
A smaller number indicates a lower degree of binder migration,
where a ratio of 1 implies zero migration. Table 2 below shows the
ratios computed by image processing software for the two methods.
ADP has minimal binder migration (1.05) whereas rapid conventional
drying causes significantly higher binder migration (1.77).
TABLE-US-00002 TABLE 2 Binder migration Drying method Binder
concentration ratio.sup.a This invention 1.05 Conventional 1.77
.sup.aBinder at electrode surface/binder at Cu foil electrode
interface
[0071] The above analytical data demonstrate that rapid ADP can
meet all the battery specifications without any negative impact,
whereas conventional attempts to rapidly dry the electrode slurries
cause performance to deteriorate. A deficit in binder near the
current collector can diminish adhesion and life of the cell.
Excess binder at the electrode surface can lead to skin formation
that impedes electrolyte access to the porous electrode
structure.
[0072] Electrode manufacturing often includes a calendering step,
in which the electrodes are compressed (typically between
cylindrical rollers at pressure from 300-2000 kg/cm.sup.2), which
reduces the electrode thickness by a controlled amount and improves
adhesion and density. Calendering may also influence the wetting of
the electrode by the electrolyte.
Example
[0073] Some preferred ranges of process variables were determined
for a graphite anode composition comprising 97 wt. % natural
graphite powder and 3 wt. % aqueous styrene-butadiene rubber (SBR)
binder:
[0074] VFM processing temperature: 70-95.degree. C.;
[0075] Hot air temperature at entrance (with VFM): ambient to
90.degree. C.;
[0076] Hot air temperature at entrance (no VFM): 80-150.degree.
C.;
[0077] Hot air flow rate: 150-350 L/min;
[0078] Foil speed through chamber: 100-500 mm/min.
Example
[0079] Some preferred ranges of process variables were determined
for a cathode composition comprising 93 wt. % NCM (Ni--Co--Mn
oxide) powder, 3 wt. % conductive carbon, and 4 wt. %
polyvinylidine difluoride (PVDF) binder, using NMP as the
solvent:
[0080] VFM processing temperature: 80-140.degree. C.;
[0081] Hot air temperature at entrance (with VFM): 80 to
120.degree. C.;
[0082] Hot air temperature at entrance (no VFM): 80-150.degree.
C.;
[0083] Hot air flow rate: 150-650 L/min;
[0084] Foil speed through chamber: 100-500 mm/min.
Example
[0085] A comparison was made using the graphite anode composition
at a loading of 10.4 to 10.6 mg/cm.sup.2. The coated substrate was
moving at 500 mm/min in each case. The inventive process, using VFM
and hot air, was accomplished using a chamber 0.5 m long. The
conventional process, using a 2-zone IR heated system, used a
chamber 2.5 m long. The invention, therefore, provided a five-fold
reduction in the required length of the processing line.
Example
[0086] A similar comparison was made using a cathode composition of
NCM523 and NMP-based binder. The loading was 18.2 mg/cm.sup.2
(conventional process) and 18.9 mg/cm.sup.2 (inventive process). In
this case, the coated substrate was moving at 350 mm/min
(conventional) and 225 mm/min (inventive). The lengths of the two
chambers were the same as in the previous example. After allowing
for the greater linear speed of the conventional drier, the
invention still provided a three-fold improvement in overall
efficiency.
Example
[0087] Adhesion and Binder Migration: Standard adhesion tests
(performed using industry standards) were carried out on sample
electrodes (anode and cathode) dried using conventional and
advanced drying processes. Binder distribution measurements were
done using elemental mapping on 4 to 6 sections of the film (anode
or cathode) cross-section (from top to bottom). Binder distribution
ratio, from electrode surface and near the metal foil substrate,
should be under 1.3 in order to obtain acceptable electrode
properties. The results may be summarized as follows: For the
aqueous anode composition and a loading of 10 mg/cm.sup.2, both
conventionally processed and VFM processed samples passed the
adhesion test, and the VFM processed samples had superior binder
uniformity (1.07 vs. 1.19 for the conventional process). For the
cathode composition using NMP as the solvent and a loading of 18
mg/cm.sup.2, both conventionally processed and VFM processed
samples passed the adhesion test, and the VFM processed samples had
superior binder uniformity (1.03 vs. 1.09 for the conventional
process).
Example
[0088] Cathode Binder distribution: Cathode electrodes dried
conventionally and using the inventive process were analyzed in
cross section by SEM to study the binder distribution, using
fluorine concentration as a proxy for the PVDF binder material
across the thickness of the electrode. Fluorine elemental maps
indicated that the ratio of binder content (weight %) between
surface and foil interfaces is 1.10 for the conventionally dried
electrode. Conversely, the electrode dried under ADP has a ratio of
1.03. The latter confirms earlier observations, indicating less (or
no) binder migration to the electrode surface when ADP is used to
dry the electrodes. Examination of the structures by SEM also
showed a finer, denser and more uniform microstructure of the
electrode material produced by the inventive process compared to
conventional drying.
Example
[0089] Anode Binder Distribution: Anode electrodes with a loading
of .about.10 mg/cm.sup.2 dried conventionally and with the
inventive ADP were analyzed using SEM (cross-sectional view) to
study the binder distribution. Samples were stained with osmium
tetroxide prior to SEM studies. Backscattered electron (BSE)
microscopy was used to map osmium through the anode cross sections;
the metal is visible as white bright spots on cross section
micrographs. Using this method, the ratio of binder content between
surface and foil interfaces, were 1.2 and 1.1 for conventional and
ADP dried electrodes, Table 3. Again, Applicants observed a finer,
denser, and more uniform microstructure of the electrode material
for ADP as compared to conventional drying.
TABLE-US-00003 TABLE 3 Ratio of binder content in a zone vs.
content near foil Zone Conventional Drying This Invention 1 (near
foil) 1.0 1.0 2 1.04 1.09 3 1.15 1.07 4 1.18 1.13 5 (near surface)
1.19 1.07
Example
[0090] Electrochemical Performance: First formation cycle for
single layer pouch cells with electrodes (anode and cathode) dried
using conventional and ADP drying systems, was evaluated using
Constant Current-Constant Voltage (CC-CV) charge and Constant
Current (CC) discharge protocols at C/10 from 3.0 to 4.2 V, FIG. 5.
In general, no difference has been observed during formation among
ADP cells. The ADP cells show a slightly higher reversible capacity
of 170.5 vs 166.4 mAh/g for standard cells, with an initial
capacity loss (ICL) of 12% for ADP cells vs 13% for standard cells.
Since the active materials are the same, the slight improvement may
be attributed to the more uniform microstructure of ADP electrodes
as discussed in earlier examples.
Example
[0091] Cycle life (100% SOC): Life cycle experiments for single
layer pouch cells (both conventional and ADP dried electrodes) with
3.0 mAh/cm.sup.2 loadings were performed at 0.5 C current rate
(100% SOC). A cell with conventional anode and cathode made by
standard processes was compared to a cell made using the inventive
process (in each case the loading was equivalent to 3.1 mAh/cm and
the initial capacity was about 142 mAh/g). Plots of discharge
capacity retention versus cycle number were virtually identical
between the baseline and the cell fabricated with ADP dried
electrodes. After 500 cycles, the inventive cell retained 79% of
initial capacity, compared to 78% for the conventional cell.
[0092] The preceding examples were for the continuous-cast single
layer pouch cell. The following examples are for the 2.0 Ah High
Energy (HEC) Prismatic Cells.
Example
[0093] Formation Cycles: Formation cycles were taken in all cells
at C/10 from 3.0 to 4.2 V. Applicants observed that the voltage vs.
specific capacity plots for cells with electrodes dried with
conventional and ADP drying systems were virtually identical. It is
worth emphasizing that no difference has been observed during
formation of all cells, and ADP cells perform as well as the
standard cells.
Example
[0094] Rate Capability: After formation, three standard and three
ADP dried cells were used for rate capability experiments. Cells
were charged at 0.1 C and then discharged at different C rates
(0.1, 0.2. 0.5, 1.0, 2.0 and 5.0 C) from 2.7 to 4.2 V. Average
discharge capacity retention is shown in FIG. 6. The rate
performance shows that ADP dried electrodes are as good as the
standard electrodes. At rates >2 C, ADP electrodes show slightly
higher capacity retention than conventional cells, which Applicants
speculate could be due to their more uniform microstructure.
Example
[0095] Electrochemical performance: Several standard and ADP cells
(2.2 Ah) were fabricated. Formation cycles were taken in all cells
at C/10 from 3.0 to 4.2 V, showing average reversible capacity of
2.0 Ah and 16% ICL for all cells. The cathode electrochemical
loadings were set to 3.0 mAh/cm.sup.2.
[0096] Life cycle testing was carried out for the prismatic cells
(both conventional and ADP dried electrodes) at C/3 current rate
from 3.5 to 4.2 V. At this voltage window the cell discharges to
80% (or 80% depth of discharge, DOD). After 500 cycles have been
completed, Applicants observed capacity retention of 92% for cells
made with both types of dried electrodes.
[0097] The foregoing comparisons demonstrate that ADP fabricated
cells were identical in performance to the conventional process of
record. However, in conducting these evaluations the internal VFM
heating method has been demonstrated to minimize the binder
migration to the surface. In addition, there is some evidence that
the microstructure, porosity, pore size distribution, and
tortuosity can be better controlled with rapid internal VFM heating
and that should positively influence the electrical conductivity
and performance of the battery electrodes.
[0098] It will be appreciated that the foregoing experiments were
done using slurries that had been formulated and optimized for the
conventional drying process. It is possible that improved slurry
formulations may be developed that provide even greater benefits by
optimizing solvent, solids loading, binder composition, etc., in
view of the performance characteristics of the inventive
microwave-based process.
[0099] Applicants recognize that various anode and cathode
compositions are known to be of potential interest in the field of
lithium ion batteries. Anode materials include lithium, graphite,
lithium alloying materials, intermetallics, and silicon. Cathode
materials include lithium-metal oxides such as LiCoO.sub.2,
LiMn.sub.2O.sub.4, and Li(Ni.sub.xMn.sub.yCo.sub.z)O.sub.2,
vanadium oxides, and LiFePO.sub.4. All of these materials require
some kind of thermal treatment, such as drying, curing, or
annealing, and the inventive apparatus may be adapted for such
materials and processes through routine experimentation. The
following example describes the use of the invention to process
some alternative materials.
Example
[0100] To validate the robustness of the inventive process for
various electrode materials, powders were mixed in appropriate
ratios representing: a nanostructured Si--C composite anode; a
LiCoO.sub.2 (LCO) cathode; and a LiNiCoAlO.sub.2 (NCA) cathode.
Each slurry was cast individually on copper or aluminum foil and
processed under optimized processing parameters in the inventive
system. The results are presented in Table 4.
TABLE-US-00004 TABLE 4 Performance of the inventive drying system
applied to different active materials Electrochemical Process
Electrode Binder Loading, mAh/cm.sup.2 Benefit.sup.a Si composite
anode LiPAA/water 3.0 3.0 X LCO cathode PVDF/NMP 3.2 3.0 X NCA
cathode PVDF/NMP 3.0 2.5 X
[0101] It will be appreciated that the inventive process may not
only be used instead of a conventional drying line, but it may also
be used in combination with a significantly shortened conventional
dryer. Operated as a "booster module" the invention makes use of
the penetrating power of microwaves to drive out solvent from the
bulk of the film, after which the film passes into the conventional
dryer to perform rapid surface drying. .sup.aBenefit is defined as
the ratio of the length of the conventional drying oven divided by
the length of the inventive process chamber, corrected for changes
in web speed
Example
[0102] A cathode slurry using NCM523 powder at 55% solids content
was applied to a film at an electrochemical loading of 3.0
mAh/cm.sup.2. Using the ADP system along with a conventional drying
oven having four independently-controlled heating zones, cathode
films were coated and dried at a web speed of 800 mm/min, which in
this case was limited by the speed of the casting stage. No defects
were observed in the dried films.
[0103] In further tests, films with mass loading equivalent to
capacities as high as 4.0 mAh/cm.sup.2 were also processed. Without
the inventive system, surface cracks were observed in these
coatings. However, when the VFM system was engaged, the films were
completely dried without forming surface defects. Use of VFM
heating allowed the power and temperature of the convection oven to
be reduced by .about.20%, Table 5.
[0104] The cathode slurry was coated and dried at .about.1000
mm/min. Without using the ADP system there were surface cracks and
residual wet spots. With a slight increase in power and
temperature, the combination of the inventive ADP and convection
drying made it possible to completely dry the films without any
defects.
TABLE-US-00005 TABLE 5 Process results for conventional drying with
and without a VFM stage Loading Drying Air Zone Run mg/cm.sup.2
Method T.sub.a Opening Power, % 1 17.8 hot air 90 12.5% 65/70/75/80
2 25.1 hot air 90 12.5% 65/70/75/80 3 24.4 95% VFM 110 25.0%
60/60/65/70 4 25.5 95% VFM 110 25.0% 60/60/65/65 5 24.6 95% VFM 130
25.0% 60/65/65/70 Speed Binder migration.sup.a Run mm/min Surface
Zone 2 Zone 3 Zone 4 Foil 1 800 1.14 1.06 0.89 0.96 1.00 2 800 1.87
1.59 1.24 1.23 1.00 3 800 1.10 1.12 1.22 1.04 1.00 4 800 1.46 1.27
1.09 1.05 1.00 5 1000 1.48 1.29 1.18 1.10 1.00 .sup.aDefined as the
ratio of binder content in a particular section of the coating
divided by the binder in the section closest to the metal foil
substrate
[0105] The above examples demonstrate the robustness of the
invention to process a variety of electrode materials, increase the
mass loading and the drying speed, without any compromise of
properties. In fact, the rapid internal heating with VFM actually
improves (reduces) the binder migration observed with even slower
convection drying methods.
[0106] The above examples describe the case of drying single sided
wet cast slurries on metal foils. It will be appreciated that there
are methods to coat both sides of the metal foils and the skilled
artisan will see that the inventive apparatus and method may easily
be modified to dry both sides by allowing VFM exposure and hot air
flow on both the top and bottom sides of the metal foil.
Furthermore, the invention is not only applicable to electrode
materials applied to a metal substrate by casting or doctor
blading; spray coating, dip coating, screen printing, extrusion, or
any other suitable means may be used to deposit the electrode
material onto a substrate. Furthermore, the substrate may be
metallic or nonmetallic (e.g., a polymer film) and, alternatively,
the substrate may be a metal mesh or screen embedded in the porous
electrode material. Li-ion batteries can be fabricated by
sequentially spraying the component slurries onto desired surfaces.
The slurry might be applied to selected areas using masks, jet
printers, or other means to create a particular device geometry.
For some multilayer energy storage devices, an activation step
involves heating the layers to temperatures that range from
50.degree. C. to about 150.degree. C. This activation step can also
easily be performed with the inventive apparatus. The multilayer
energy storage material can also be deposited, screen printed or
electro-coated on to the substrate, before it enters the apparatus
shown in for the desired thermal treatment.
[0107] A separator is a critical component in liquid electrolyte
batteries; it is placed between the positive electrode and negative
electrode to prevent physical contact of the electrodes and also
enabling free ionic transport and isolating electronic flow. The
separator, anode and cathode can be individually formed into sheets
or films, which are subsequently stacked or rolled to form the
battery. Alternatively, the electrode material can be applied onto
the separator (referred to as integrated electrode separators),
which can be used in lithium ion batteries as replacements for free
standing separators. Thus, the separator alone and/or electrode
material or any other web configuration can be heat treated by
passing through the inventive apparatus.
[0108] The apparatus depicted in FIG. 1 is shown with the web
passing through in a substantially horizontal plane. It will be
appreciated, however, that the cavity and slots may equally well be
oriented so that the web passes in a substantially vertical plane.
Such a configuration may be particularly useful for processing a
metal sheet having coatings on both sides, because it eliminates
the need for rollers or other supporting structures that might
otherwise interfere with processing a web that has a wet coating on
both sides. The vertical configuration would also be useful when
processing a web in which the substrate forms an internal mesh
completely contained within a slurry, rather than a solid sheet
with a slurry on one side.
[0109] As used herein, the terms "wet" and "dry" imply greater or
lesser amounts, respectively, of a volatile solvent, which may be
water or may be an organic solvent, such as NMP, that has at least
a partially polar nature so that it will absorb microwave energy
efficiently. Thus, while the relatively "wet" end of a coated film
enters the chamber and the relatively "dry" end exits the chamber,
it will be understood that the actual solvent content of the
exiting film will not, in general, be zero; it will simply be
significantly less than that of the incoming film.
[0110] Similarly, the incoming "dry" heated gas, introduced
adjacent to the dry end of the film, will have a lower
concentration of solvent than will the "moist" gas exiting the
chamber adjacent to the wet end of the film. In the case of
non-aqueous solvents such as NMP, the concentration in the incoming
gas may be very low, whereas in the case of aqueous systems, there
will likely be some non-zero moisture content in the incoming air,
but it will be significantly less than the moisture content or
relative humidity of the air as it exits the wet end of the
process.
[0111] It will be understood that the relative humidity of ambient
air will by definition decrease when that air is heated. Such air
will in many cases be suitable as is. However, if even lower
incoming relative humidity is desired, the air may be dried or
dehumidified by any suitable means prior to heating and injection
into the chamber.
[0112] As noted earlier, Applicants have obtained excellent results
using a VFM system, which, in one example, provided microwave power
in a sweeping fashion over a bandwidth of .+-.400 MHz about a
center frequency of 6.25 GHz. This represents a bandwidth of
.+-.6.4% of the center frequency. Frequency sweeping has two clear
advantages in this context: First, it eliminates arcing that might
occur at the edges of a metal foil. Second, by creating a large
number of independent modes within the cavity, more uniform heating
is obtained. The skilled artisan will appreciate that uniformity in
a cavity is a function of bandwidth, center frequency, and cavity
dimensions relative to the microwave wavelength. A smaller cavity
(relative to wavelength) will in general require a wider bandwidth
of sweeping to achieve a given level of uniformity compared to a
larger cavity. A user may therefore take these factors into account
when engineering a system for a particular purpose. It will be
further understood that various microwave power devices can produce
power over a suitable bandwidth; these include traveling wave tube
(TWT) amplifiers, solid state power amplifiers, and others.
[0113] The invention may be used with any suitable solvent. Table 6
lists some non-polar solvents, which have very small dipole moments
as well as dielectric constants. The non-polar solvent molecules
will not themselves respond well to microwaves, but it will be
appreciated that some electrode materials are lossy to some degree
at microwave frequencies and this will allow volumetric heating of
the film, which is one benefit of the inventive process.
TABLE-US-00006 TABLE 6 Properties of some non-polar solvents
Non-polar Chemical Dielectric Dipole Boiling Solvents Formula
Constant Moment Point, .degree. C. Pentane
CH.sub.3--CH.sub.2--CH.sub.2--CH.sub.2--CH.sub.3 1.84 0.00 D 36
Cyclopentane C.sub.5H.sub.10 1.97 0.00 D 40 Hexane
CH.sub.3--CH.sub.2--CH.sub.2--CH.sub.2--CH.sub.2--CH.sub.3 1.88
0.00 D 69 Cyclohexane C.sub.6H.sub.12 2.02 0.00 D 81 Benzene
C.sub.6H.sub.6 2.3 0.00 D 80 Toluene C.sub.6H.sub.5--CH.sub.3 2.38
0.36 D 111 1,4-Dioxane
/--CH.sub.2--CH.sub.2--O--CH.sub.2--CH.sub.2--O--\ 2.3 0.45 D 101
Chloroform CHCl.sub.3 4.81 1.04 D 61 Diethyl ether
CH.sub.3CH.sub.2--O--CH.sub.2--CH.sub.3 4.3 1.15 D 35
[0114] In contrast, the polar solvents listed in Table 7 have
significantly higher dielectric constants and dipole moments. Like
water molecules, in the presence of microwave energy these polar
molecules will be set into rotational movement. Anywhere these
solvents are present, even deep into the pores of the porous
dielectric film, microwave energy has the capability to agitate
these molecules and help drive evaporation. It will be preferred to
stay below the boiling point of the solvent or reagent to allow
some additional rotational movement within the pores without
boiling, which can lead to void formation.
TABLE-US-00007 TABLE 7 Properties of some polar solvents Polar
Chemical Dielectric Dipole Boiling Solvent Formula Constant Moment
Point, .degree. C. Water H--O--H 80 1.85 D 100 Ethanol
CH.sub.3--CH.sub.2--OH 24.5 1.69 D 79 Methanol CH.sub.3--OH 33 1.70
D 65 Isopropanol (IPA) CH.sub.3--CH(--OH)--CH.sub.3 18 1.66 D 82
Acetic acid CH.sub.3--C(.dbd.O)OH 6.2 1.74 D 118 Acetone
CH.sub.3--C(.dbd.O)--CH.sub.3 21 2.88 D 56 n-Propanol
CH.sub.3--CH.sub.2--CH.sub.2--OH 20 1.68 D 97 n-Butanol
CH.sub.3--CH.sub.2--CH.sub.2--CH.sub.2--OH 18 1.63 D 118 Formic
acid H--C(.dbd.O)OH 58 1.41 D 101 Propylene C.sub.4H.sub.6O.sub.3
64.0 4.9 D 240 carbonate Ethyl acetate
CH.sub.3--C(.dbd.O)--O--CH.sub.2--CH.sub.3 6.02 1.78 D 77 Dimethyl
sulfoxide CH.sub.3--S(.dbd.O)--CH.sub.3 46.7 3.96 D 189
Acetonitrile CH.sub.3--C.ident.N 37.5 3.92 D 82 (MeCN)
Dimethylformamide H--C(.dbd.O)N(CH.sub.3).sub.2 38 3.82 D 153
Tetrahydrofuran /--CH.sub.2--CH.sub.2--O--CH.sub.2--CH.sub.2--\ 7.5
1.75 D 66 Dichloromethane CH.sub.2Cl.sub.2 9.1 1.60 D 40
* * * * *