U.S. patent application number 15/595276 was filed with the patent office on 2017-09-14 for methods for forming ionically conductive composite separators.
The applicant listed for this patent is Ford Global Technologies, LLC. Invention is credited to Venkataramani Anandan, Andrew Robert Drews, John Matthew Ginder.
Application Number | 20170263904 15/595276 |
Document ID | / |
Family ID | 55531272 |
Filed Date | 2017-09-14 |
United States Patent
Application |
20170263904 |
Kind Code |
A1 |
Anandan; Venkataramani ; et
al. |
September 14, 2017 |
METHODS FOR FORMING IONICALLY CONDUCTIVE COMPOSITE SEPARATORS
Abstract
A method of forming a conductive composite separator. The method
includes providing a plurality of particles within a bulk separator
material; and applying an AC electric field to the particles and
the bulk separator material while the bulk separator material is in
a liquid state to align the particles into at least one ionically
conductive aligned particle region within the bulk separator
material.
Inventors: |
Anandan; Venkataramani;
(Farmington Hills, MI) ; Drews; Andrew Robert;
(Ann Arbor, MI) ; Ginder; John Matthew; (Plymouth,
MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ford Global Technologies, LLC |
Dearborn |
MI |
US |
|
|
Family ID: |
55531272 |
Appl. No.: |
15/595276 |
Filed: |
May 15, 2017 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14504762 |
Oct 2, 2014 |
9666852 |
|
|
15595276 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 2300/0068 20130101;
H01M 10/052 20130101; H01M 2300/0088 20130101; H01M 2010/4271
20130101; H01M 2/145 20130101; H01M 2/18 20130101; H01M 2300/0082
20130101; H01M 10/056 20130101; H01M 2/20 20130101; Y02E 60/10
20130101; H01M 2/166 20130101; H01M 10/14 20130101 |
International
Class: |
H01M 2/14 20060101
H01M002/14; H01M 2/20 20060101 H01M002/20; H01M 2/18 20060101
H01M002/18; H01M 2/16 20060101 H01M002/16; H01M 10/14 20060101
H01M010/14 |
Claims
1. A method of forming a conductive composite separator,
comprising: providing a plurality of particles within a bulk
separator material; and applying an AC electric field to the
particles and the bulk separator material while the bulk separator
material is in a liquid state to align the particles into at least
one ionically conductive aligned particle region within the bulk
separator material.
2. The method of claim 1, wherein the AC electric field has a
strength of 100 to 2,000 V/mm and a frequency of 10 Hz to 10
kHz.
3. The method of claim 1, wherein the electric field is applied for
1 second to 1 hour.
4. The method of claim 1 further comprising heating the bulk
material such that it is in a liquid state prior to the applying
step.
5. The method of claim 1, wherein the at least one ionically
conductive aligned particle region extends in a straight line
parallel to a thickness of the bulk separator material.
6. The method of claim 1, wherein a volume fraction of particles in
the at least one ionically conductive aligned particle region is at
least 90%.
7. The method of claim 1, wherein the bulk separator material
includes polyethylene oxide (PEO), polyethylene-glycol (PEG),
polymethylmethacrylate (PMMA), or polyacrylonitrile (PAN).
8. A method of forming an ionically conductive composite separator,
comprising: providing a plurality of particles within an ionically
conductive polymer bulk separator material; and applying an AC
electric field to the particles and the bulk separator material
while the bulk separator material is in a liquid state to align the
particles into at least one ionically conductive aligned particle
region within the bulk separator material.
9. The method of claim 8, wherein the at least one ionically
conductive aligned particle region is an at least one ionically
conductive aligned solid particle region.
10. The method of claim 8, wherein the AC electric field has a
strength of 100 to 2,000 V/mm and a frequency of 10 Hz to 10
kHz.
11. The method of claim 8, wherein the electric field is applied
for 1 second to 1 hour.
12. The method of claim 8 further comprising heating the bulk
material such that it is in a liquid state prior to the applying
step.
13. The method of claim 8, wherein the at least one ionically
conductive aligned particle region extends in a straight line
parallel to a thickness of the bulk separator material.
14. The method of claim 8, wherein a volume fraction of particles
in the at least one ionically conductive aligned particle region is
at least 90%.
15. A method of forming a conductive composite separator,
comprising: providing a plurality of particles within a bulk
separator material; and dielectrophoretic aligning the particles
into at least one ionically conductive aligned particle region
within the bulk separator material while the bulk separator
material is in a liquid state to align.
16. The method of claim 15 further comprising heating the bulk
material such that it is in a liquid state prior to the
dielectrophoretic aligning step.
17. The method of claim 15, wherein the dielectrophoretic aligning
step includes applying an electric field to the particles and the
bulk separator material.
18. The method of claim 17 further comprising controlling the
strength and/or frequency of the electric field.
19. The method of claim 17, wherein the electric field is an AC
electric field.
20. The method of claim 19, wherein the AC electric field has a
strength of 100 to 2,000 V/mm and a frequency of 10 Hz to 10 kHz.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 14/504,762 filed Oct. 2, 2014, now U.S. Patent No. ______
issued ______, the disclosure of which is hereby incorporated by
reference herein.
TECHNICAL FIELD
[0002] This disclosure relates to methods for forming ionically
conductive composite separators.
BACKGROUND
[0003] Rechargeable batteries (e.g., lithium-ion batteries)
generally include separators, which provide mechanical and
electronic separation of the electrodes while allowing ionic
conduction. One common separator is a porous polymer film that is
soaked in a liquid electrolyte. However, the common liquid
electrolytes for many batteries (e.g., Li-ion batteries) may be
flammable and could contribute to a fire hazard in severe
incidents. If an overcharge of a Li-ion battery occurs lithium
dendrites may grow at the negative electrode and penetrate the
porous membrane, leading to an internal short circuit. If the two
electrodes make electrical contact, the cell can begin to
self-discharge through the short, which may lead to a
thermal-runaway event. Thermal-runaway may, in turn, lead to a fire
hazard.
SUMMARY
[0004] In at least one embodiment, a method of forming a conductive
composite separator is disclosed. The method includes providing a
plurality of particles within a bulk separator material; and
applying an AC electric field to the particles and the bulk
separator material while the bulk separator material is in a liquid
state to align the particles into at least one ionically conductive
aligned particle region within the bulk separator material.
[0005] In another embodiment, a method of forming an ionically
conductive composite separator is disclosed. The method includes
providing a plurality of particles within an ionically conductive
polymer bulk separator material; and applying an AC electric field
to the particles and the bulk separator material while the bulk
separator material is in a liquid state to align the particles into
at least one ionically conductive aligned particle region within
the bulk separator material.
[0006] In yet another embodiment, a method of forming a conductive
composite separator is disclosed. The method includes providing a
plurality of particles within a bulk separator material; and
dielectrophoretic aligning the particles into at least one
ionically conductive aligned particle region within the bulk
separator material while the bulk separator material is in a liquid
state to align.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a schematic cross-section of a composite separator
having aligned particles, according to an embodiment;
[0008] FIG. 2 is a schematic of a particle alignment process,
according to an embodiment;
[0009] FIG. 3 is a schematic of a composite separator forming and
aligning process, according to an embodiment;
[0010] FIG. 4 is a schematic of a re-melt aligning process,
according to an embodiment;
[0011] FIG. 5A is an SEM image of a cross-section of an epoxy film
including BaTiO.sub.3 particles formed without an external electric
field;
[0012] FIG. 5B is an SEM image of a cross-section of an epoxy film
including BaTiO.sub.3 particles formed with an external electric
field;
[0013] FIG. 6A is an SEM image of a cross-section of an epoxy film
including LLZO particles formed without an external electric
field;
[0014] FIG. 6B is an SEM image of a cross-section of an epoxy film
including LLZO particles formed with an external electric field;
and
[0015] FIG. 7 is a graph showing calculated ionic conductivities
for a separator having homogeneously dispersed particles and a
disclosed separator having aligned particles.
DETAILED DESCRIPTION
[0016] As required, detailed embodiments of the present invention
are disclosed herein; however, it is to be understood that the
disclosed embodiments are merely exemplary of the invention that
may be embodied in various and alternative forms. The figures are
not necessarily to scale; some features may be exaggerated or
minimized to show details of particular components. Therefore,
specific structural and functional details disclosed herein are not
to be interpreted as limiting, but merely as a representative basis
for teaching one skilled in the art to variously employ the present
invention.
[0017] To address the issues known to affect conventional porous
polymer separators, alternative separator materials and/or
configurations may be beneficial. Separators formed from dense
sheets of solid electrolyte (SE) could potentially address both the
flammability and dendrite formation issues. However, dense SE
sheets generally cannot be used in current manufacturing techniques
that require a flexible separator. A non-porous, ionically
conductive polymer membrane would be another potentially attractive
solution, but current polymers do not have sufficient conductivity
to be useful in some applications (e.g., automotive applications).
A composite of a polymer with randomly dispersed particles of SE
could potentially combine some of the advantages of both options.
However, at high solids loadings, the membrane is both highly
conductive and brittle, while at low loadings, it has poor
conductivity but is flexible. Accordingly, an alternative approach
that can use SE particles more efficiently to provide high-level
performance at moderate loadings would address the known issues of
the conventional separator and the short-comings of potential
alternatives.
[0018] With reference to FIG. 1, a schematic cross-section of a
rechargeable battery 10 (e.g., a lithium-ion battery) is shown
having a negative electrode (anode) 12, a positive electrode
(cathode) 14, and a composite separator 16. Within the bulk of the
separator 16 are a plurality of particles 18. The particles 18 may
be arranged into one or more aligned groups, such as strings or
chains 20 that extend across a thickness of the separator 16 (e.g.,
from the anode side to the cathode side). The strings or chains 20
may be unbroken, however, they may also include some gaps or
breaks. The chains 20 may be linear or substantially linear and may
form a straight line across the thickness of the separator 16. In
some embodiments, the chain(s) 20 may be non-linear, but still
extend across the thickness of the separator 16 to form a high
conduction path. In one embodiment, the chains 20 are oriented
parallel to the thickness direction (i.e., perpendicular to the
anode and cathode). The chains may have a thickness of a single
particle or several particles (e.g., up to 5 or up to 10
particles). As shown in FIG. 1, the chains 20 may be formed of
particles 18 that are in contact with one another or the particles
may be closely adjacent but not in physical contact. In one
embodiment, the chain(s) 20 may extend unbroken across the entire
thickness of the separator 16. In another embodiment, the chain(s)
20 may extend substantially unbroken across the entire thickness of
the separator 16, having small gaps in the chain 20. The chain(s)
20 may also extend across a majority of the thickness of the
separator 16 or over at least a certain percentage of the
thickness. For example, the chain(s) 18 may extend across at least
50, 60, 70, 80, 90, or 95 percent of the thickness of the separator
16.
[0019] While the particles 18 are shown in FIG. 1 as being circular
in cross-section, the particles 18 may have any shape. For example,
the particles 18 may be spherical, platelets, discs, needle-like,
cylindrical, irregular, flakes, cubical, angular, acicular, lath,
or other known particle shapes. The chains 20 are shown in FIG. 1
as having a single particle 18 width, however, the chains 20 may be
more than one particle wide, for example, up to 3, 5, or 10
particles wide. The particles 18 may have any suitable size or
diameter, for example, 3 nm to 100 .mu.m, or any sub-range therein,
such as 50 nm to 50 .mu.m or 500 nm to 30 .mu.m. The maximum
particle size may be limited by the thickness of the separator. In
one embodiment, the particles 18 may have a maximum particle
size/diameter of one half the thickness of the separator 16. The
size of the particles 18 may selected or prepared based on factors
such as interfacial resistance, mechanical strength, and ionic
conductivity. If the particles are not ionically conductive, then
the size of the particles may be based on factors including the
mechanical and ionic conductivity properties of the final composite
separator 16.
[0020] In one embodiment, the chains 20 may have a width of 5 nm to
300 .mu.m, or any sub-range therein. For example, the chains 20 may
have a width of 5 nm to 200 .mu.m, 5 nm to 100 .mu.m, 50 nm to 100
.mu.m, 100 nm to 100 .mu.m, 100 nm to 50 .mu.m, 100 nm to 30 .mu.m,
100 nm to 10 .mu.m, or 100 nm to 1.mu.m. The chains 20 may extend
across the entire thickness of the separator 16, which may be from
5 to 100 .mu.m, or any sub-range therein. For example, the
separator 16 may have a thickness of 5 to 30 .mu.m, 5 to 25 .mu.m,
5 to 20 .mu.m, 5 to 15 .mu.m, or 5 to 10 .mu.m.
[0021] The separator 16 may include any amount of particles 18 that
provides sufficient ionic conductivity for a battery's
requirements. The alignment of the particles 18 reduces the number
of particles 18 needed to attain a certain level of ionic
conductivity, compared to randomly dispersed particles.
Accordingly, the volume fraction of particles 18 in the composite
separator 16 may be reduced in order to maintain good flexibility
of the separator 16 while also increasing the ionic conductivity.
In one embodiment, the separator 16 includes from 0.1 to 40% by
volume of particles 18, or any sub-range therein. In another
embodiment, the separator 16 includes from 0.1 to 20% by volume of
particles 18. In another embodiment, the separator 16 includes from
0.1 to 15% by volume of particles 18. In another embodiment, the
separator 16 includes from 0.1 to 10% by volume of particles 18. In
another embodiment, the separator 16 includes from 0.1 to 5% by
volume of particles 18. The volume percent of the particles 18 may
be adjusted based on factors such as the shape of the particle,
size of the particle, ionic conductivity of the particle, or other
properties of the particles and/or the composite separator.
[0022] In at least one embodiment, the particles 18 include solid
electrolyte (SE) particles (also known as fast ion conductors).
These particles may be electrically insulating but ionically
conductive. Non-limiting examples of solid electrolyte materials
include lithium lanthanum zirconium oxide (LLZO, e.g.,
Li.sub.7La.sub.3Zr.sub.2O.sub.12), lithium phosphorus oxynitride
(LiPON, e.g., Li.sub.3.3PO.sub.3.8N.sub.0.24 to
Li.sub.3.6PO.sub.3.3N.sub.0.69), LISICON or Thio-LISICON (Lithium
Superlonic CONductor, e.g.,
Li.sub.3.25Ge.sub.0.25P.sub.0.75S.sub.4),Li.sub.2S--P.sub.2S.sub.5,
Li--Al--Ge--PO.sub.4, Li--Ti--Al--PO.sub.4, Li--V--Si--O, LiBSiO,
LiBON, lithium lanthanum titanate, and NASICON.
[0023] The particles 18 may also include dielectric or ceramic
particles that have little or no ionic conductivity. Non-limiting
examples of such particles may include barium titanate (e.g.,
BaTiO.sub.3), alumina (Al.sub.2O.sub.3) Silica (SiO2), Ceria
(CeO.sub.2), and Titanium oxide (TiO2). It has been found that high
surface area ceramic particles may enhance the ionic conductivity
of a surrounding polymer, even if the ceramic particles themselves
do not have ionic conductivity. Without being held to any
particular theory, it is believed that this effect is due to a
surface interaction wherein the polymer chains near the surface of
the particles have a higher free volume and thus allow for a higher
ionic diffusivity. Aligning these particles into chains 20 may
further enhance the ionic conductivity of the composite separator
16 by connecting or aligning these regions of higher free volume
into chains that span the separator 16.
[0024] The particles 18 may also include particles having
anisotropic dielectric properties and/or shapes. Particles 18
having anisotropic shapes may include needle-shaped particles or
plates. The long axes of the particles 18 may be aligned with each
other to form the chains 20. Alternatively, in some embodiments,
the long axes may be aligned perpendicular to the direction of the
chain 20 such that the particles are side-by-side rather than
tip-to-tip.
[0025] The composite separator 16 includes a bulk or matrix
material 22 that forms a bulk of the separator 16 and surrounds or
encapsulates the chains 20 of particles 18. The bulk material 22
may be a polymer, which may or may not be an ionic conductor.
Non-limiting examples of suitable ionically conductive polymers
include polyethylene oxide (PEO), polyethylene-glycol (PEG),
polymethylmethacrylate (PMMA), and polyacrylonitrile (PAN).
Non-limiting examples of suitable non-ionically conductive polymers
include epoxies, polypropylene (PP), and polyethylene (PE).
[0026] In at least one embodiment, the particles 18 may be aligned
within one or more ionically conductive aligned particle regions
24. The aligned particle regions may be three-dimensional regions
extending across a thickness of the matrix material 22 of the
composite separator 16 from the anode side to the cathode side. The
regions 24 may encapsulate or surround the particle chains 20
described above. For example, a region 24 may be a defined shape
such as a cylinder or rectangular prism, or it may be irregular.
The region(s) 24 may each surround a chain 20. Each region 24 may
extend across the entire thickness of the separator (e.g., from the
anode side to the cathode side). The region 24 may have a width or
diameter that is the same as the width of the chain 20 at its
widest point or the region 24 may be defined as having the same
width as the chain 20 along its length (e.g., the region 24 width
corresponds to the chain width along the length of the chain). The
aligned particle regions may have a high volume fraction of
particles 18 compared to the matrix material 22. In one embodiment,
the volume fraction of particles 18 in the regions 24 is at least
75%. In another embodiment, the volume fraction of particles 18 in
the regions 24 is at least 80%. In another embodiment, the volume
fraction of particles 18 in the regions 24 is at least 85%. In
another embodiment, the volume fraction of particles 18 in the
regions 24 is at least 90%. In another embodiment, the volume
fraction of particles 18 in the regions 24 is at least 95%.
[0027] The particles 18 may be aligned into chains 20 in the matrix
material 22 using any suitable method. In at least one embodiment,
the particles 18 are aligned into chains 20 by dielectrophoretic
alignment, as illustrated in FIG. 2. Dielectrophoretic alignment
may include applying an electric field 26 normal to the surface of
the composite separator 16 using a voltage source 28 while the
matrix material 22 (e.g., polymer) is in a liquid state. In one
embodiment, the electric field 26 is an alternating electric field
(AC electric field). However, it may be possible to use a DC field.
An AC electric field 26 may cause the particles 18 to develop
oscillating dipole moments and electrical forces and torques that
generally encourage alignment of the dipoles along the field
direction and drift in the plane of the separator 16 (e.g., plane
parallel to the anode and cathode surfaces) to regions where the
local field is enhanced by an adjacent particle. By this action,
the particles 18 may self-assemble into chains 20 that span at
least a portion of the thickness of the separator 16, as shown in
FIGS. 1 and 2. Once the particles 18 have been given sufficient
time to form the chains, the matrix material 22 may be allowed to
solidify, thereby locking the chains 20 into position.
[0028] Dielectrophoretic alignment may be used to align any type of
particle 18 in a matrix material 22, provided that there is a
difference in dielectric constants for the materials of the
particles 18 and the matrix material 22. The force applied to the
particles 18 by the AC electric field may vary depending on the
properties of the particles 18 and the matrix material 22. For
example, the strength of the force may depend on the electrical
properties of the particles 18 and matrix material 22, on the
particles' shape and size, and on the strength and/or frequency of
the electric field. The alignment of the particles 18 into chains
20 in the matrix material 22, as described above, may be affected
by properties such as the polarizabilities of the particles 18 and
the matrix material 22, the size and/or shape of the particles 18
and the viscosity of the matrix material 22 when in a liquid
phase.
[0029] The electric field 26 may be an AC electric field having any
suitable frequency to apply a force to the particles 18 and
facilitate their alignment. In at least one embodiment, the
frequency of the AC electric field is from 10 Hz to 10,000 Hz
(10kHz), or any sub-range therein. For example, the AC electric
field 26 may have a frequency of 100 to 8,000, 500 to 7,000, 1,000
to 6,000, or 3,000 to 5,000 Hz. In some particle-matrix material
systems, the frequency used may affect the orientation of the
particles 18 within the chains 20. For example, over certain
frequency ranges, particles 18 having a long axis (e.g.,
needle-like or plate-shaped particles) may form chains 20 with
their long axes perpendicular to the field direction, rather than
parallel to it. Accordingly, the frequency of the AC electric field
26 may be controlled to adjust the orientation of the particles 18
in the chains 20. The frequency of the AC electric field 26 may be
constant throughout the dielectrophoretic alignment or it may be
adjusted during the alignment (e.g., dynamically adjusted) to
control the alignment and/or orientation of the particles 18.
[0030] The electric field 26 may have any suitable strength to
apply a force to the particles 18 and facilitate their alignment.
In at least one embodiment, the strength of the electric field 26
is from 100 V/mm to 2,000 V/mm, or any sub-range therein. For
example, the electric field 26 may have a strength of 200 to 1,500,
250 to 1,200, 300 to 1,000, 350 to 750, or 400 to 600 V/mm. The
strength of the electric field 26 may be constant throughout the
dielectrophoretic alignment or it may be adjusted during the
alignment (e.g., dynamically adjusted) to control the alignment of
the chains 20. The electric field 26 may be applied to the
particles 18 and liquid-phase matrix material 22 for any suitable
length of time to facilitate alignment of the particles 18 into
chains 20. In at least one embodiment, the electric field 26 may be
applied for 1 second to 3 hours, or any sub-range therein. For
example, the electric field may be applied for 1 second to 1 hour,
1 second to 30 minutes, 1 second to 15 minutes, 1 second to 5
minutes, 1 second to 1 minute, 1 to 45 seconds, 5 to 45 seconds, 5
to 30 seconds, or 10 to 30 seconds. In general, the particles 18
will be more aligned the longer that the electric field 26 is
applied. Accordingly, independent of the field strength or
frequency, increasing the time of the dielectrophoretic alignment
may allow for increased alignment of the particles 18 into chains
20 (e.g., the particles have less lateral deviation from the long
axis). However, longer alignment times may also increase the
possibility of particle agglomeration.
[0031] The alignment of the particles 18 in the matrix material 22
may be performed while the matrix material 22 is in a liquid state.
Non-limiting examples of the liquid state of the matrix material 22
include a polymer melt, a solution with a removable solvent, and a
reactive mixture of liquids (e.g., an epoxy). In one embodiment,
the particles 18 of the separator 16 may be aligned when the
separator is formed, as shown in FIG. 3. The particles 18 may be
incorporated into or added to the matrix material 22, while the
matrix material is in a liquid state. The particles 18 may
initially be randomly dispersed or dispersed without any
predetermined alignment. While the matrix material is still in a
liquid state, an alignment process may be performed, such as
dielectrophoretic alignment, to form the chains 20 and the
ionically conductive aligned particle regions 24.
[0032] With reference to FIG. 3, an embodiment of an alignment
system 100 is shown for aligning particles 18 during an initial
forming of the separator 16. A vessel 102 may hold a polymer melt
104 having particles 18 and a matrix material 22 included therein.
The vessel 102 may cast the melt 104 onto a moving conveyor 106,
which may include a spreader for a separator 16 (not shown). A
voltage source 108 may be connected to the conveyor 106 and an
electrode 110 to produce an electric field between the conveyor 106
and the electrode 110. The electrode 110 is shown located above the
conveyor 106 in FIG. 3, however, any suitable configuration may be
used to provide an electric field to the melt 104. As the melt 104
passes through the electric field, the particles 18 are able to
align into chains 20 via dielectrophoretic alignment, as described
above.
[0033] A heater 112 may be included in the system 100 to ensure
that the melt 104 stays in a liquid state and does not solidify
prior to the completion of the alignment of particles 18. The
heater 112 may be any suitable device for increasing the
temperature of the matrix material 22. Non-limiting examples may
include a hot-air heater, infrared energy, microwave energy, or
others. To assist in cooling the melt 104 down to its melting
temperature, a cooling plate 114 may be positioned and configured
to cool the melt 104 in order to solidify it and lock the particles
18 into their aligned configuration. Other methods of cooling other
than a cooling plate may also be used, such as the use of cooled
air, a cooled drum, or others. The electrode 110 may extend into
the cooling region such that the electric field is maintained as
the matrix material solidifies. Extending the electrode into the
cooling region may prevent the particles 18 from becoming unaligned
as the matrix material 22 solidifies. After the matrix material 22
has solidified, the newly formed and aligned separator 116 may be
removed from the conveyor 106. The aligned separator 116 may be
wound onto a pick-up roll 118 for later use or may be immediately
further processed (e.g., cut to size).
[0034] The degree of alignment of the particles 18 may be
controlled by the strength and/or frequency of the electric field
(described above) and/or by the amount of time exposed to the
electric field while the matrix material 22 is in the liquid state.
The amount of time spent exposed to the electric field may be
controlled by adjusting the speed of the conveyor 106 and/or the
length of the conveyor 106 or electrode 110. If a longer exposure
time is desired, the conveyor speed may be reduced and/or the size
of the length of the conveyor 106 or the electrode 212 may be
increased. If a shorter exposure time is desired, then the opposite
adjustments may be made (e.g., faster conveyor speed, smaller
conveyor/electrode). As described above, the alignment of the
particles may be performed while the separator is in a liquid
state. In addition to a polymer melt, as shown and described with
respect to FIG. 3, other non-limiting examples of the liquid state
may also include a solution with a removable solvent or a reactive
mixture. One of ordinary skill in the art will appreciate that the
system 100 of FIG. 3 may be modified in order to align particles 18
within matrix materials in other liquid states.
[0035] In another embodiment, an example of which is shown in FIG.
4, the separator 16 may be formed and solidified prior to the
alignment process and may be returned to a liquid state for
alignment. The particles 18 may initially be randomly dispersed in
the matrix material 22, or otherwise dispersed without any
predetermined alignment. A portion or all of the matrix material 22
may then be returned to a liquid state, for example, by melting. An
electrode may then be configured to produce an electric field in
the region of the liquid matrix material 22 to align the particles
18 into chains 20 via dielectrophoretic alignment, as described
above. Once a certain amount of alignment has occurred, the matrix
material 22 may allowed to solidify, thereby locking the particles
18 into their aligned configuration. The matrix material 22 may be
actively cooled, for example, using a cooling plate, cooled air, or
other methods, or it may be allowed to passively cool to its
melting temperature at ambient conditions.
[0036] With reference to FIG. 4, an embodiment of a re-melt or
re-processing system 200 is shown. A feed roll 202 may be loaded
with an unaligned separator 204 having particles 18 dispersed
within a matrix material 22. The particles 18 may be randomly
dispersed or otherwise not aligned in a predetermined manner. The
unaligned separator 204 may be unrolled onto a rotating drum 206. A
heater 208, such as a heater plate, may be positioned and
configured to heat the separator 204 such that at least a portion
of the matrix material 22 of the separator 204 melts. A voltage
source 210 may be connected to the drum 206 and an electrode 212 to
produce an electric field between the drum 206 and the electrode
212. In one embodiment, the electrode may have a contour that
follows an outer contour of the drum 206, such that the electric
field is perpendicular to the surface of the separator (e.g.,
parallel to the thickness direction). As the melted portion of the
matrix material 22 passes through the electric field, the particles
18 are able to align into chains 20 via dielectrophoretic
alignment, as described above.
[0037] The degree of alignment may be controlled by the strength
and/or frequency of the electric field (described above) and/or by
the amount of time exposed to the electric field while the matrix
material 22 is in the liquid state. The amount of time spent
exposed to the electric field may be controlled by adjusting the
speed of rotation of the drum 206 and/or the size/diameter of the
drum 206 or electrode 212. If a longer exposure time is desired,
the drum speed may be reduced and/or the size of the drum 206 or
the size of the electrode 212 may be increased. If a shorter
exposure time is desired, then the opposite adjustments may be made
(e.g., faster drum speed, smaller drum/electrode). To assist in
cooling the liquid matrix material down to its melting temperature,
a cooling plate 214 may be positioned and configured to cool the
separator 204 in order to solidify it and lock the particles 18
into their aligned configuration. Other methods of cooling other
than a cooling plate may also be used, such as the use of cooled
air, a cooled drum, or others. The electrode 212 may extend into
the cooling region such that the electric field is maintained as
the matrix material solidifies. Extending the electrode into the
cooling region may prevent the particles 18 from becoming unaligned
as the matrix material 22 solidifies. After the matrix material 22
has solidified, the aligned separator 216 may be removed from the
drum 206, for example, by peeling. The aligned separator may be
wound onto a pick-up roll 218 for later use or may be immediately
further processed (e.g., cut to size).
[0038] While FIG. 4 shows the system 200 as using a cylindrical
drum 206, other configurations may be utilized. For example, the
unaligned separator 204 may be unrolled or placed on a flat
transport system (e.g., a conveyor belt) and moved passed a heater
208 and an electric field. The heater 208 may be any suitable
device for increasing the temperature of the matrix material 22.
Non-limiting examples may include a hot-air heater, infrared
energy, microwave energy, or others. In addition, the matrix
material 22 may be solidified using methods in addition to, or
instead of, cooling. For example, a polymer melt may be cured by
exposure to UV light or by the addition of a curing agent (e.g., a
catalyst). The electric field may be maintained during these
alternative solidifying processes, similar to above with respect to
the cooling plate.
EXAMPLES
[0039] With reference to FIGS. 5A and 5B, sample composite
separators were prepared without and with dielectrophoretic
alignment, respectively. Both separators were prepared with barium
titanate (BaTiO.sub.3) particles dispersed in an uncured film of
epoxy. The separator in FIG. 5A was cured by successively raising
the temperature from room temperature to 80.degree. C., without an
external electric field applied. The separator in FIG. 5B was cured
in the same manner, but with an AC electric field of 500 V/mm
applied at a frequency of 4,000 Hz until the curing completed. The
samples were then fractured to produce cross-sections for imaging
using scanning electron microscopes. As shown in FIG. 5A, the
sample cured with no external electric field applied has a random
dispersion of BaTiO.sub.3 particles, with no alignment of the
particles. In contrast, the sample cured under an electric field,
shown in FIG. 5B, has a plurality of chains of BaTiO.sub.3
particles traversing the cross-section of the separator. The
particles are aligned into chains of about one particle width and
extend across the entire thickness of the separator.
[0040] With reference to FIGS. 6A and 6B, sample composite
separators were prepared without and with dielectrophoretic
alignment, respectively. Both separators were prepared with lithium
lanthanum zirconium oxide (LLZO) solid electrolyte particles
dispersed in an uncured film of epoxy. The separator in FIG. 6A was
cured by successively raising the temperature from room temperature
to 80.degree. C., without an external electric field applied. The
separator in FIG. 6B was cured in the same manner, but with an AC
electric field of 500 V/mm applied at a frequency of 4,000 Hz until
the curing completed. The samples were then fractured to produce
cross-sections for imaging using scanning electron microscopes. As
shown in FIG. 6A, the LLZO particles in the sample cured with no
external electric field applied have settled to one side of the
separator and do not form any aligned chains. In contrast, the
sample cured under an electric field, shown in FIG. 6B, has a
plurality of chains of LLZO particles traversing the cross-section
of the separator. The particles are aligned into chains of about
one particle width and extend across the entire thickness of the
separator.
[0041] With reference to FIG. 7, a graph is shown of calculated
ionic conductivity values for a separator with homogeneously
dispersed particles compared to a composite separator with aligned
particles. The ionic conductivity values for a homogeneous
dispersion were calculated using the following Maxwell
equation:
.sigma. .sigma. m = 1 + 3 ( .sigma. d - .sigma. m .sigma. d + 2
.sigma. m ) .phi. ##EQU00001##
[0042] Where .sigma., .sigma..sub.m, and .sigma..sub.d are the
ionic electrical conductivities of the composite separator, the
matrix material, and the particles, respectively, and .phi. is the
volume fraction of the particles.
[0043] For the separator with aligned particles, the aligned
particle chains were approximated as solid pillars, rather than as
individual particles. The ionic conductivity values were calculated
using a weighted parallel conductivity equation:
.sigma.=.sigma..sup.d*f+.sigma..sub.m*(1-f)
[0044] Where .sigma., .sigma..sub.m, and .sigma..sub.d are the same
as above and f is the volume fraction of the particles.
[0045] The calculations used to produce the graph of FIG. 7 were
performed using LLZO as the particle material and PEO as the bulk
matrix material. LLZO has an ionic conductivity of 4*10.sup.-4 S/cm
and PEO has an ionic conductivity of 1*10.sup.-8 S/cm. As shown in
FIG. 7, the ionic conductivity of the composite separator with
aligned particles has an ionic conductivity that is several orders
of magnitude higher at a given particle volume fraction than a
separator with homogeneously dispersed particles. This greatly
increased ionic conductivity per particle volume allows for the
amount of particles to be reduced in the separator, thereby
allowing the separator to maintain desirable mechanical properties,
such as flexibility.
[0046] While exemplary embodiments are described above, it is not
intended that these embodiments describe all possible forms of the
invention. Rather, the words used in the specification are words of
description rather than limitation, and it is understood that
various changes may be made without departing from the spirit and
scope of the invention. Additionally, the features of various
implementing embodiments may be combined to form further
embodiments of the invention.
* * * * *