U.S. patent application number 13/744702 was filed with the patent office on 2013-07-04 for separators for electrochemical cells.
This patent application is currently assigned to Sihl GmbH. The applicant listed for this patent is Optodot Corporation, Sihl GmbH. Invention is credited to Steven A. Carlson, Zhong Xu.
Application Number | 20130171500 13/744702 |
Document ID | / |
Family ID | 44681400 |
Filed Date | 2013-07-04 |
United States Patent
Application |
20130171500 |
Kind Code |
A1 |
Xu; Zhong ; et al. |
July 4, 2013 |
SEPARATORS FOR ELECTROCHEMICAL CELLS
Abstract
Provided are separators for use in batteries and capacitors
comprising (a) at least 50% by weight of an aluminum oxide and (b)
an organic polymer, wherein the aluminum oxide is surface modified
by treatment with an organic acid to form a modified aluminum
oxide, and wherein the treatment provides dispersibility of the
aluminum oxide in aprotic solvents such as N-methyl pyrrolidone.
Preferably, the organic acid is a sulfonic acid, such as
p-toluenesulfonic acid. Also preferably, the organic polymer is a
fluorinated polymer, such as polyvinylidene fluoride. Also provided
are electrochemical cells and capacitors comprising such
separators.
Inventors: |
Xu; Zhong; (Holden, MA)
; Carlson; Steven A.; (Cambridge, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Optodot Corporation;
Sihl GmbH; |
Watertown
Duren |
MA |
US
DE |
|
|
Assignee: |
Sihl GmbH
Duren
MA
Optodot Corporation
Watertown
|
Family ID: |
44681400 |
Appl. No.: |
13/744702 |
Filed: |
January 18, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/US2011/001274 |
Jul 18, 2011 |
|
|
|
13744702 |
|
|
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61399883 |
Jul 19, 2010 |
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Current U.S.
Class: |
429/145 ;
361/524; 428/315.5; 429/188; 429/251; 521/89 |
Current CPC
Class: |
Y10T 428/249978
20150401; H01G 9/02 20130101; Y02E 60/13 20130101; H01M 10/0525
20130101; H01M 2/166 20130101; Y02E 60/10 20130101; H01G 11/52
20130101 |
Class at
Publication: |
429/145 ;
429/251; 428/315.5; 429/188; 521/89; 361/524 |
International
Class: |
H01M 2/16 20060101
H01M002/16; H01G 9/02 20060101 H01G009/02 |
Claims
1. A separator for an electric current producing cell, wherein said
separator comprises a microporous layer comprising (a) at least 50%
by weight of an aluminum oxide and (b) an organic polymer, wherein
said aluminum oxide is surface modified by treatment with an
organic acid to form a modified aluminum oxide, and wherein said
treatment provides dispersibility of said aluminum oxide in aprotic
solvents, and said organic polymer comprises a first fluorinated
organic monomer and a second organic monomer or said organic
polymer comprises a polyvinylidene fluoride polymer.
2. The separator of claim 1, wherein said organic acid is a
sulfonic acid.
3. The separator of claim 2, wherein said sulfonic acid is an aryl
sulfonic acid, preferably a toluenesulfonic acid.
4. The separator of claim 1, wherein said organic acid is a
carboxylic acid.
5. The separator of claim 1, wherein said aluminum oxide comprises
a hydrated aluminum oxide of the formula Al.sub.2O.sub.3xH.sub.2O,
wherein x is in the range of 1.0 to 1.5, and wherein said hydrated
aluminum oxide is surface modified by treatment with an organic
acid to form a modified hydrated aluminum oxide.
6. The separator of claim 5, wherein said separator comprises 60 to
90% by weight of said modified aluminum oxide.
7. The separator of claim 5, wherein said microporous layer has an
average pore diameter from 2 nm to 70 nm.
8. The separator of claim 1, wherein said electric current
producing cell is a secondary lithium ion cell, a primary lithium
cell, or a capacitor.
9. The separator of claim 1, wherein said microporous layer is
coated on one or both sides of at least one microporous polyolefin
layer.
10. An electrochemical cell comprising an anode, a cathode, an
organic electrolyte comprising a lithium salt, and a separator
interposed between said anode and said cathode, wherein said
separator comprises a microporous layer comprising (a) at least 50%
by weight of an aluminum oxide and (b) an organic polymer, wherein
said aluminum oxide is surface modified by treatment with an
organic acid to form a modified aluminum oxide, and wherein said
treatment provides dispersibility of said aluminum oxide in aprotic
solvents, and said organic polymer comprises a first fluorinated
organic monomer and a second organic monomer or said organic
polymer comprises a polyvinylidene fluoride polymer.
11. The cell of claim 10, wherein said organic acid is a sulfonic
acid.
12. The cell of claim 11, wherein said sulfonic acid is an aryl
sulfonic acid, preferably a toluenesulfonic acid.
13. The cell of claim 10, wherein said organic acid is a carboxylic
acid.
14. The cell of claim 10, wherein said aluminum oxide comprises a
hydrated aluminum oxide of the formula Al.sub.2O.sub.3xH.sub.2O,
wherein x is in the range of 1.0 to 1.5, and wherein said hydrated
aluminum oxide is surface modified by treatment with an organic
acid to form a modified hydrated aluminum oxide.
15. The cell of claim 10, wherein said microporous layer has an
average pore diameter from 2 nm to 70 nm.
16. The cell of claim 10, wherein said microporous layer is coated
on one or both sides of at least one microporous polyolefin
layer.
17. The cell of claim 10, wherein the anode active material of said
anode is lithium.
18. A capacitor comprising two electrodes, an organic electrolyte
comprising a tetraalkyl ammonium salt, and a separator interposed
between said two electrodes, wherein said separator comprises a
microporous layer comprising (a) at least 50% by weight of an
aluminum oxide and (b) an organic polymer, wherein said aluminum
oxide is surface modified by treatment with an organic acid to form
a modified aluminum oxide, and wherein said treatment provides
dispersibility of said aluminum oxide in aprotic solvents, and said
organic polymer comprises a first fluorinated organic monomer and a
second organic monomer or said organic polymer comprises a
polyvinylidene fluoride polymer.
19. The capacitor of claim 18, wherein said organic acid is a
sulfonic acid.
20. The capacitor of claim 18, wherein said inorganic oxide
comprises a hydrated aluminum oxide of the formula
Al.sub.2O.sub.3xH.sub.2O, wherein x is in the range of 1.0 to 1.5,
and wherein said aluminum oxide is surface modified by treatment
with an organic acid to form a modified aluminum oxide.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of International
Application Ser. No. PCT/US2011/001274, filed Jul. 18, 2011, which
claims the benefit of U.S. provisional patent application Ser. No.
61/399,883, filed Jul. 19, 2010, the entirety of each which is
hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to the field of
porous membranes and to the fields of electric current producing
cells and of separators for use in electric current producing
cells. More particularly, this invention pertains to a porous
separator membrane comprising an aluminum oxide and an organic
polymer where the aluminum oxide has been surface modified by
treatment with an organic acid to provide dispersibility in aprotic
organic solvents. Also, the present invention pertains to electric
current producing cells, such as lithium ion cells and capacitors,
comprising such porous separators.
BACKGROUND
[0003] An electroactive material that has been fabricated into a
structure for use in an electrochemical cell is referred to as an
electrode. Of a pair of electrodes used in an electrochemical cell,
the electrode on the electrochemically higher potential side is
referred to as the positive electrode or the cathode, while the
electrode on the electrochemically lower potential side is referred
to as the negative electrode, or the anode. A battery may contain
one or more electrochemical cells.
[0004] To prevent the undesirable flow of the electrons in a short
circuit internally from the anode to the cathode, an electrolyte
element is interposed between the cathode and the anode. This
electrolyte element must be electronically non-conductive to
prevent short circuits, but must permit the transport of ions
between the anode and the cathode. The electrolyte element should
also be stable electrochemically and chemically toward both the
anode and the cathode.
[0005] Typically, the electrolyte element contains a porous
material, referred to as a separator (since it separates or
insulates the anode and the cathode from each other), and an
aqueous or non-aqueous electrolyte, that usually comprises an ionic
electrolyte salt and ionically conductive material, in the pores of
the separator. A variety of materials have been used for the porous
layer or separator of the electrolyte element in electrochemical
cells. These porous separator materials include polyolefins such as
polyethylenes and polypropylenes, glass fiber filter papers, and
ceramic materials. Usually these separator materials are supplied
as porous free-standing membranes that are interleaved with the
anodes and the cathodes in the fabrication of electrochemical
cells.
[0006] A liquid organic electrolyte containing organic solvents and
lithium salts is typically used as the electrolyte in the pores of
the separator in the electrolyte element for rechargeable or
secondary lithium ion and non-rechargeable or primary lithium
electrochemical cells. Alternatively, a gel or solid polymer
electrolyte containing an ionically conductive polymer and lithium
salts, and optionally organic solvents, might be utilized instead
of the liquid organic electrolyte.
[0007] In addition to being porous and chemically stable to the
other materials of the electric current producing cell, the
separator should be flexible, thin, economical in cost, and have
good mechanical strength and safety properties.
[0008] High porosity in the separator is important for obtaining
the high ionic conductivity needed for effective performance in
most batteries, except, for example, those batteries operating at
relatively low charge and discharge rates, and for efficiency in
capacitors, such as supercapacitors. It is desirable for the
separator to have a porosity of at least 30 percent, and preferably
40 percent or higher, in lithium ion batteries.
[0009] Another highly desirable feature of the separator in the
electrolyte element is that it is readily wetted by the electrolyte
materials that provide the ionic conductivity. When the separator
material is a polyolefin material that has non-polar surface
properties, the electrolyte materials (which typically have highly
polar properties) often poorly wet the separator material. This
results in longer times to fill the battery with electrolyte and
potentially in low capacities in the battery due to a non-uniform
distribution of electrolyte materials in the electrolyte
element.
[0010] The separators used for lithium ion batteries are typically
polyolefin separators, which melt at below 200.degree. C. and are
very flammable. The lithium ion batteries, as well as lithium
primary batteries and some capacitors, utilize highly flammable
organic solvents in their electrolytes. A non-melting and flame
retardant separator would help prevent the spread of any burning of
the organic electrolyte, caused by an internal short circuit,
thermal runaway, or other unsafe condition, that might spread into
a larger area of the battery or capacitor and cause a major
explosion. As lithium ion batteries are increasingly utilized for
high power applications, such as for electric vehicles, the need
for improved safety is greatly increased because of the very large
size and high power rates of these vehicle batteries.
[0011] A separator that is applicable for lithium ion and other
electric current producing cells and that has flame retardant and
non-melting properties that provide safety against internal short
shorts and thermal runaway, while maintaining the chemical
stability of the electrolyte and of the separator, would be of
great value to the battery and capacitor industry.
SUMMARY OF THE INVENTION
[0012] To achieve increased safety in separators for use in
electric current producing cells such as batteries and capacitors,
the present invention utilizes non-flammable inorganic oxides, such
as aluminum oxides, and preferably non-flammable organic polymers
having fluorinated groups in the separators. This invention
utilizes various inorganic oxide particle pretreatment, mixing,
coating, drying, and delaminating methods for preparing such
separators.
[0013] One aspect of the present invention pertains to a separator
for an electric current producing cell, wherein the separator
comprises a microporous layer comprising (a) at least 50% by weight
of an aluminum oxide and (b) an organic polymer, wherein the
aluminum oxide is surface modified by treatment with an organic
acid to form a modified aluminum oxide. In one embodiment, the
organic acid is a sulfonic acid, preferably an aryl sulfonic acid,
and more preferably a toluenesulfonic acid. In one embodiment, the
organic acid is a carboxylic acid. In one embodiment, the aluminum
oxide comprises a hydrated aluminum oxide of the formula
Al.sub.2O.sub.3xH.sub.2O, wherein x is in the range of 1.0 to 1.5,
and wherein the hydrated aluminum oxide is surface modified by
treatment with an organic acid to form a modified hydrated aluminum
oxide. In one embodiment, the modified aluminum oxide has an
Al.sub.2O.sub.3 content in the range of 50 to 85% by weight. In one
embodiment, the modified aluminum oxide has an Al.sub.2O.sub.3
content in the range of 65 to 80% by weight. In one embodiment, the
separator comprises 60 to 90% by weight of the modified aluminum
oxide. In one embodiment, the separator comprises 70 to 85% by
weight of the modified aluminum oxide. In one embodiment, the
microporous layer is a xerogel layer. In one embodiment, the
organic polymer comprises a polyvinylidene fluoride polymer. In one
embodiment, the separator comprises a copolymer of a first
fluorinated organic monomer and a second organic monomer. In one
embodiment, the second organic monomer is a second fluorinated
organic monomer.
[0014] In one embodiment of the separators of this invention, the
electric current producing cell is a secondary lithium ion cell. In
one embodiment, the electric current producing cell is a primary
lithium cell. In one embodiment, the electric current producing
cell is a capacitor. In one embodiment, the separator does not melt
at temperatures lower than 300.degree. C. In one embodiment, the
separator is a flame retardant separator.
[0015] Another aspect of the present invention pertains to an
electrochemical cell comprising an anode, a cathode, an organic
electrolyte comprising a lithium salt, and a separator interposed
between the anode and the cathode, wherein the separator comprises
a microporous layer comprising (a) at least 50% by weight of an
aluminum oxide and (b) an organic polymer, wherein the aluminum
oxide is surface modified by treatment with an organic acid. In one
embodiment, the organic acid is a sulfonic acid, preferably an aryl
sulfonic acid, and more preferably a toluenesulfonic acid. In one
embodiment, the organic acid is a carboxylic acid. In one
embodiment, the aluminum oxide comprises a hydrated aluminum oxide
of the formula Al.sub.2O.sub.3xH.sub.2O, wherein x is in the range
of 1.0 to 1.5, and wherein the hydrated aluminum oxide is surface
modified by treatment with an organic acid to form a modified
hydrated aluminum oxide.
[0016] In one embodiment of the electrochemical cells of this
invention, the microporous layer is a xerogel layer. In one
embodiment, the anode active material of the anode is lithium. In
one embodiment, the modified aluminum oxide has an Al.sub.2O.sub.3
content in the range of 50 to 85% by weight. In one embodiment, the
modified aluminum oxide has an Al.sub.2O.sub.3 content in the range
of 65 to 80% by weight. In one embodiment, the organic polymer
comprises a polyvinylidene fluoride polymer. In one embodiment, the
organic polymer comprises a copolymer of a first fluorinated
organic monomer and a second organic monomer. In one embodiment,
the second organic monomer is a second fluorinated organic monomer.
In one embodiment, the lithium salt is lithium
hexafluorophosphate.
[0017] Another aspect of the present invention relates to a
capacitor comprising two electrodes, an organic electrolyte
comprising a tetraalkyl ammonium salt, and a separator interposed
between the two electrodes, wherein the separator comprises a
microporous layer comprising (a) at least 50% by weight of an
aluminum oxide and (b) an organic polymer, wherein the aluminum
oxide is surface modified by treatment with an organic acid to form
a modified aluminum oxide. In one embodiment, the inorganic oxide
comprises a hydrated aluminum oxide of the formula
Al.sub.2O.sub.3xH.sub.2O wherein x is in the range of 1.0 to 1.5,
wherein the aluminum oxide is surface modified by treatment with an
organic acid to form a modified aluminum oxide. In one embodiment,
the organic acid is a sulfonic acid. In one embodiment, the
microporous layer is a xerogel layer. In one embodiment, the
organic polymer comprises a polyvinylidene fluoride polymer. In one
embodiment, the organic polymer comprises a copolymer of a first
fluorinated organic monomer and a second organic monomer.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The separators of the present invention provide superior
safety and other key performance properties for use in electric
current producing cells, including, but not limited to, lithium
batteries and capacitors. Methods of preparing microporous xerogel
separators for electrochemical cells are described in U.S. Pat.
Nos. 6,153,337 and 6,306,545, and in U.S. Pat. Application
20020092155, all to Carlson et al. The liquid mixture described in
these references for coating xerogel separators comprises an
inorganic oxide, an organic binder, and typically water as the
volatile liquid in the mixture. Optionally, the liquid mixture
comprises organic solvents, preferably protic organic solvents.
Examples of protic organic solvents are alcohols and glycols.
[0019] The drying process to form a xerogel layer involves the
removal of the liquid in the liquid mixture. As is known in the art
of inorganic oxide xerogel coatings, as the liquid is removed, the
colloidal particles of inorganic oxide sol form a gel that, upon
further loss of liquid, forms a 3-dimensional microporous network
of inorganic oxide. By the terms "xerogel layer" and "xerogel
structure," as used herein, is meant, respectively, a layer of a
coating or the structure of a coating layer in which the layer and
structure were formed by drying a liquid sol or sol-gel mixture to
form a solid gel matrix as, for example, described in Chem. Mater.,
Vol. 9, pages 1296 to 1298 (1997) by Ichinose et al. for coating
layers of inorganic oxide based xerogels. Thus, if the liquid of
the gel formed in the liquid sol-gel mixture is removed
substantially, for example, through the formation of a liquid-vapor
boundary phase, the resulting gel layer or film is termed, as used
herein, a xerogel layer. Thus, the microporous xerogel layers of
this invention comprise a dried microporous three-dimensional solid
network with pores which are interconnected in a substantially
continuous fashion from one outermost surface of the layer through
to the other outermost surface of the layer. A continuous xerogel
coating layer has the materials of the xerogel in a continuous
structure in the coating layer, i.e., the materials, such as
inorganic oxide particles, are in contact and do not have
discontinuities in the structure, such as a discontinuous layer of
solid pigment particles that are separated from each other.
[0020] In contrast, xerogel pigment particles may be formed by a
xerogel process involving drying a liquid solution of a suitable
precursor to the pigment to form a dried mass of xerogel pigment
particles, which is typically then ground to a fine powder to
provide xerogel pigment particles. The microporous inorganic oxide
layers of this invention may be, but are not limited to, xerogel
layers. The inorganic oxide layers of the present invention may
also be discontinuous layers of solid pigment particles that are
not a xerogel coating layer and have discontinuities of solid
pigment particles that are separated from each other in the
structure of the discontinuous layer. This separation typically
involves organic polymer interposed between the pigment particles.
The terms "xerogel coating" and "xerogel coating layer," as used
herein, are synonymous with the term "xerogel layer."
[0021] As used herein, the term "microporous" describes the
material of a layer or coating, in which the material possesses
pores of a diameter of about 1 micron or less. As used herein, the
term "nanoporous" describes the material of a layer or coating, in
which the material possesses pores of a diameter of about 100
nanometers or less.
[0022] Preferably for battery and capacitor separator applications,
these pores are connected in a substantially continuous fashion
from one outermost surface of the microporous layer through to the
other outermost surface of the layer. This substantially continuous
3-dimensional microporous inorganic oxide network is efficient in
allowing the diffusion of ions, such as lithium ions, through the
separator during the charging and discharging of the electric
current producing cell.
[0023] The amount of the pores in the separator may be
characterized by the percent porosity or percent pore volume, which
is the cubic centimeters of pores per cubic centimeters of the
separator. The porosity may be measured by filling the pores with a
relatively non-volatile liquid having a known density and then
calculated by the increase in weight of the separator with the
liquid present divided by the known density of the liquid and then
dividing this quotient by the volume of the separator, as
calculated from the area and average thickness of the
separator.
[0024] In one embodiment of the separators of this invention, the
average pore diameter of the microporous inorganic oxide layer is
from 2 nm to 70 nm. Typically, the average pore diameter of the
microporous inorganic oxide layer is from 30 to 50 nm. These
extremely small pores, that are about 5 to 10 times smaller than
the average pore dimensions of polyolefin separators, present no
limitation to high conductivity with lithium salt electrolytes.
Thus, the pore sizes of the separators of this invention may
provide ion transport and conductivity with lithium ion battery
electrolytes that is at least equal to that of polyolefin
separators.
[0025] In one embodiment of the separators of this invention, the
inorganic oxide is an aluminum oxide. Other inorganic oxides, such
as zirconium oxides and silicas, as known in the art of electrolyte
elements and separators for electrochemical cells, may be utilized
alone or in combination with other inorganic oxides including
aluminum oxides. Preferred aluminum oxides are aluminum boehmites.
The term "pseudo-boehmite," as used herein, pertains to hydrated
aluminum oxides having the chemical formula,
Al.sub.2O.sub.3xH.sub.2O, wherein x is in the range of 1.0 to 1.5.
Terms used herein, which are synonymous with "pseudo-boehmite,"
include "aluminum boehmite," "boehmite," "AlOOH," and "hydrated
alumina." The materials referred to herein as "pseudo-boehmite" are
distinct from anhydrous aluminum oxides or aluminas
(Al.sub.2O.sub.3 such as alpha-alumina or gamma-alumina) and
hydrated aluminum oxides of the formula Al.sub.2O.sub.3xH.sub.2O
wherein x is less than 1.0 or greater than 1.5. In one embodiment
of the separators of the present invention, the weight percent of
the aluminum oxide in the separator is greater than 50%. This
loading of the aluminum oxide helps to provide the porosity of the
separator that is needed for conductivity and for rapid wetting by
the electrolyte when manufacturing the electric current producing
cell.
[0026] One aspect of the present invention pertains to a separator
for an electric current producing cell, wherein the separator
comprises a microporous layer comprising (a) at least 50% by weight
of an aluminum oxide and (b) an organic polymer, wherein the
aluminum oxide is surface modified by treatment with an organic
acid to form a modified aluminum oxide. The separator may contain
only the microporous layer of this invention or may contain
additional microporous layers, such as porous polyolefin layers as
typically used in lithium ion batteries. For example, the
microporous layer of the present invention may be coated on one or
both sides of a microporous polyolefin layer, such as Celgard 2500,
the trade name for a polyolefin separator membrane available from
Polypore, Inc., of Charlotte, N.C.. Whereas a thickness of 5 to 20
microns is typical for the separators of this invention that
contain only the microporous layers of this invention, the
thickness of a coating of the microporous layer of this invention
onto a polyolefin microporous separator is typically in, but not
limited to, the range of 1 to 4 microns.
[0027] In one embodiment of the separators of the present
invention, the organic acid is a sulfonic acid, preferably an aryl
sulfonic acid, and more preferably a toluenesulfonic acid. In one
embodiment, the organic acid is a carboxylic acid. One purpose of
the surface modification of the aluminum oxide is to make the
aluminum oxide particles dispersible in organic solvents,
especially in aprotic organic solvents. This broader scope of
dispersibility is advantageous in enabling a wider range of organic
polymers that are soluble in aprotic organic solvents, but not in
water and alcohols, to be used. Other types of surface modification
of inorganic oxides, as known in the art of surface modification of
inorganic oxides for excellent dispersibility in aprotic organic
solvents may be utilized in the present invention.
[0028] In one embodiment of the separators of this invention, the
aluminum oxide comprises a hydrated aluminum oxide of the formula
Al.sub.2O.sub.3xH.sub.2O, wherein x is in the range of 1.0 to 1.5,
and wherein the hydrated aluminum oxide is surface modified by
treatment with an organic acid to form a modified hydrated aluminum
oxide. In one embodiment, the modified aluminum oxide has an
Al.sub.2O.sub.3 content in the range of 50 to 85% by weight. In one
embodiment, the modified aluminum oxide has an Al.sub.2O.sub.3
content in the range of 65 to 80% by weight. In one embodiment, the
separator comprises 60 to 90% by weight of the modified aluminum
oxide. In one embodiment, the separator comprises 70 to 85% by
weight of the modified aluminum oxide. In one embodiment, the
microporous layer is a xerogel layer. In one embodiment, the
organic polymer comprises a polyvinylidene fluoride (PVDF) polymer,
such as KYNAR HSV 900, the trade name for a PVDF polymer for
lithium battery and other applications available from Arkema, Inc.
In one embodiment, the separator comprises a copolymer of a first
fluorinated organic monomer and a second organic monomer. In one
embodiment, the second organic monomer is a second fluorinated
organic monomer.
[0029] In one embodiment of the separators of this invention, the
electric current producing cell is a secondary lithium ion cell. In
one embodiment, the electric current producing cell is a primary
lithium cell. In one embodiment, the electric current producing
cell is a capacitor. In one embodiment, the separator does not melt
at temperatures lower than 300.degree. C. The aluminum oxide or
other inorganic oxide material is primarily responsible for
providing this non-melting and dimensionally stable property at
high temperatures. In one embodiment, the separator is a flame
retardant separator. The aluminum oxide or other inorganic oxide is
a flame retardant material and, in combination with a highly
fluorinated organic polymer which is also flame retardant, provides
a flame retardant separator.
[0030] Another aspect of the present invention pertains to an
electrochemical cell comprising an anode, a cathode, an organic
electrolyte comprising a lithium salt, and a separator interposed
between the anode and the cathode, wherein the separator comprises
a microporous layer comprising (a) at least 50% by weight of an
aluminum oxide and (b) an organic polymer, wherein the aluminum
oxide is surface modified by treatment with an organic acid. In one
embodiment, the organic acid is a sulfonic acid, preferably an aryl
sulfonic acid, and more preferably a toluenesulfonic acid. In one
embodiment, the organic acid is a carboxylic acid. In one
embodiment, the aluminum oxide comprises a hydrated aluminum oxide
of the formula Al.sub.2O.sub.3xH.sub.2O, wherein x is in the range
of 1.0 to 1.5, and wherein the hydrated aluminum oxide is surface
modified by treatment with an organic acid to form a modified
hydrated aluminum oxide.
[0031] In one embodiment of the electrochemical cells of this
invention, the microporous layer is a xerogel layer. In one
embodiment, the anode active material of the anode is lithium. In
one embodiment, the modified aluminum oxide has an Al.sub.2O.sub.3
content in the range of 50 to 85% by weight. In one embodiment, the
modified aluminum oxide has an Al.sub.2O.sub.3 content in the range
of 65 to 80% by weight. In one embodiment, the organic polymer
comprises a polyvinylidene fluoride polymer. In one embodiment, the
organic polymer comprises a copolymer of a first fluorinated
organic monomer and a second organic monomer. In one embodiment,
the second organic monomer is a second fluorinated organic monomer.
In one embodiment, the lithium salt is lithium
hexafluorophosphate.
[0032] Another aspect of the present invention relates to a
capacitor comprising two electrodes, an organic electrolyte
comprising a tetraalkyl ammonium salt, and a separator interposed
between the two electrodes, wherein the separator comprises a
microporous layer comprising (a) at least 50% by weight of an
aluminum oxide and (b) an organic polymer, wherein the aluminum
oxide is surface modified by treatment with an organic acid to form
a modified aluminum oxide. In one embodiment, the inorganic oxide
comprises a hydrated aluminum oxide of the formula
Al.sub.2O.sub.3xH.sub.2O wherein x is in the range of 1.0 to 1.5,
wherein the aluminum oxide is surface modified by treatment with an
organic acid to form a modified aluminum oxide. In one embodiment,
the organic acid is a sulfonic acid. In one embodiment, the
microporous layer is a xerogel layer. In one embodiment, the
organic polymer comprises a polyvinylidene fluoride polymer. In one
embodiment, the organic polymer comprises a copolymer of a first
fluorinated organic monomer and a second organic monomer.
EXAMPLES
[0033] Several embodiments of the present invention are described
in the following examples, which are offered by way of illustration
and not by way of limitation.
Example 1
[0034] A 20% by weight dispersion of DISPAL 10SR, the trade name
for a surface-modified aluminum oxide available from SASOL North
America, Houston, Tex., in methyl ethyl ketone was prepared.
According to the Material Safety Data Sheet (MSDS) by SASOL for
DISPAL 10SR, the aluminum oxide is an aluminum boehmite, and the
surface modification comprises p-toluenesulfonic acid (PTSA).
Separately, a 10% by weight solution of KYNAR HSV 900 in N-methyl
pyrrolidone (NMP) was prepared. The aluminum oxide dispersion was
added to the stirred fluoropolymer solution to prepare a dispersion
containing the aluminum oxide and fluoropolymer in a dry weight
ratio of 5:1. The % solids of this dispersion was about 17%.
[0035] This dispersion was coated onto a 3 mil thick silicone
treated polyester (PET) film on the silicone release side to give a
dry coating thickness of about 20 microns and then delaminated from
the release substrate to provide a free standing aluminum oxide
microporous separator with a porosity of about 43%. Evaluation of
this aluminum oxide microporous separator in a typical lithium ion
button cell with a graphite-containing anode, a lithium
hexfluorophosphate-containing electrolyte in organic carbonate
solvents, and a cobalt oxide-containing cathode showed equal or
better chemical stability at 55.degree. C., cycling, and ionic
conductivity at 1C and 5C charge, in comparison to a control button
cell with an Ube polyolefin separator of the same thickness
substituted for the aluminum oxide separator.
[0036] The aluminum oxide separator did not melt at temperatures
below 300.degree. C. and was flame retardant, as shown by not
burning when exposed to an open flame.
Example 2
[0037] The aluminum oxide and fluoropolymer dispersion of Example 1
was coated onto a 20 micron thick polyolefin separator from Ube and
dried at 90.degree. C. to avoid shrinkage and melting of the
polyolefin separator. The thickness of the coating was varied from
1 to 4 microns dry and coated on one or both sides of the
polyolefin separator. Button cells as described in Example 1 were
prepared with a 2 micron thick aluminum oxide microporous coating
on one or both sides of the polyolefin separator and gave
comparable stability at 55.degree. C., cycling, and conductivity to
control button cells with the polyolefin separator only.
Comparative Example 1
[0038] A 20% by weight dispersion of DISPAL 10F4, the trade name
for a surface-modified aluminum boehmite available from SASOL North
America, Houston, Tex., in methyl ethyl ketone was mixed at 2200
rpm stirring for 40 minutes. No satisfactory dispersion was
obtained, and nearly all of the pigment settled to the bottom of
the mix container. According to the Material Safety Data Sheet
(MSDS) by SASOL for DISPAL 10F4, the surface modification comprises
formic acid. Separately, a 10% by weight solution of KYNAR HSV 900
in NMP was prepared. The non-dispersed DISPAL 10F4 mix in methyl
ethyl ketone was added to the stirred fluoropolymer solution at a
dry weight ratio of 3:1 of the aluminum boehmite and fluoropolymer
with continued stirring at 2200 rpm for 40 minutes. No satisfactory
dispersion was obtained, and nearly all of the pigment settled to
the bottom of the mix container. The mix was not suitable for
coating a separator layer on a release substrate.
Comparative Example 2
[0039] A 7.5% by weight solution of KYNAR HSV 900 in NMP was
prepared. To this fluoropolymer solution with stirring at 2200 rpm,
DISPAL 10F4 in a dry weight ratio of 5:1 of the aluminum boehmite
and fluoropolymer was added slowly with continued stirring at 2200
rpm for 40 minutes. No satisfactory dispersion was obtained, and
nearly all of the pigment settled to the bottom of the mix
container. The mix was not suitable for coating a separator layer
on a release substrate.
[0040] Comparative Examples 1 and 2 show that surface modification
of an aluminum oxide with formic acid does not provide
dispersibility in aprotic solvents as represented by NMP and methyl
ethyl ketone and consequently is not suitable for coating
separators with polyvinylidene fluoride (PVdF) and other
fluoropolymers that require aprotic solvents such as NMP for
solubility and for use in coatings, such as a separator or battery
electrode coating.
[0041] While the invention has been described in detail and with
reference to specific and general embodiments thereof, it will be
apparent to one skilled in the art that various changes and
modifications can be made therein without departing from the spirit
and scope thereof
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