U.S. patent application number 13/192968 was filed with the patent office on 2012-01-19 for waterborne fluoropolymer composition.
This patent application is currently assigned to Arkema Inc.. Invention is credited to Ramin Amin-Sanayei, Scott Gaboury.
Application Number | 20120015246 13/192968 |
Document ID | / |
Family ID | 45467243 |
Filed Date | 2012-01-19 |
United States Patent
Application |
20120015246 |
Kind Code |
A1 |
Amin-Sanayei; Ramin ; et
al. |
January 19, 2012 |
WATERBORNE FLUOROPOLYMER COMPOSITION
Abstract
This invention relates to a waterborne fluoropolymer composition
useful for the fabrication of Li-Ion-Battery (LIE) electrodes. The
fluoropolymer composition contains an organic carbonate compound,
which is more environmentally friendly than other fugitive adhesion
promoters currently used in waterborne fluoropolymer binders. An
especially useful organic carbonate compound is ethylene carbonate
(EC) and vinylene carbonate (VC), which are solids at room
temperature, and other carbonates which are liquid at room
temperature such as propylene carbonate, methyl carbonate and ethyl
carbonate. The composition of the invention is low cost,
environmentally friendly, safer, and has enhanced performance
compared to current compositions.
Inventors: |
Amin-Sanayei; Ramin;
(Malvern, PA) ; Gaboury; Scott; (Blue Bell,
PA) |
Assignee: |
Arkema Inc.
King of Prussia
PA
|
Family ID: |
45467243 |
Appl. No.: |
13/192968 |
Filed: |
July 28, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12788427 |
May 27, 2010 |
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13192968 |
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Current U.S.
Class: |
429/217 ;
252/500; 252/511; 252/519.33; 252/519.34; 361/305; 428/421 |
Current CPC
Class: |
Y02E 60/10 20130101;
F04C 15/0096 20130101; H01G 11/38 20130101; H01M 4/131 20130101;
C09D 127/16 20130101; H01G 11/30 20130101; H01G 11/26 20130101;
H01M 4/136 20130101; C08K 5/1565 20130101; H01G 11/52 20130101;
H01M 4/623 20130101; H01M 10/0525 20130101; H01M 4/133 20130101;
H01G 11/48 20130101; H01M 4/0404 20130101; H01M 50/449 20210101;
H01M 4/62 20130101; Y10T 428/3154 20150401; Y02E 60/13
20130101 |
Class at
Publication: |
429/217 ;
428/421; 252/500; 252/519.33; 252/519.34; 252/511; 361/305 |
International
Class: |
H01M 4/62 20060101
H01M004/62; H01G 4/008 20060101 H01G004/008; H01B 1/12 20060101
H01B001/12; H01B 1/24 20060101 H01B001/24; H01M 4/13 20100101
H01M004/13; B32B 27/06 20060101 B32B027/06 |
Claims
1. An aqueous composition comprising: a) from 0.5 to 150 parts of
fluoropolymer particles having a weight average particle size of
less than 500 nm; b) from 10 to 500 parts of one or more powdery
electrode-forming materials; c) from 1 to 150 parts of one or more
organic carbonates; d) 100 parts of water; all parts are by weight
and based on 100 parts by weight of water.
2. The aqueous composition of claim 1 comprising: a) from 1 to 100
parts of fluoropolymer particles having a weight average particle
size of less than 400 nm; b) from 20 to 400 parts of one or more
powdery electrode-forming materials; c) from 2 to 100 parts of one
or more organic carbonates; d) 100 parts of water; all parts are by
weight and based on 100 parts by weight of water.
3. The aqueous composition of claim 2 comprising: a) from 5 to 75
parts of fluoropolymer particles having a weight average particle
size of less than 300 nm; b) from 25 to 300 parts of one or more
powdery electrode-forming materials; c) from 10 to 75 parts of one
or more organic carbonates; d) 100 parts of water; all parts are by
weight and based on 100 parts by weight of water.
4. The aqueous composition of claim 1, wherein said organic
carbonate is selected from the group consisting of carbonates
having the general formula: (R.sub.2)CO.sub.3(R.sub.1) where
R.sub.1 and R.sub.2 represent a linear or branched C.sub.1-4 alkyl
group; R.sub.1 and R.sub.2 can be the same of different; ethylene
carbonate; propylene carbonate; butylene carbonate isomers; and
vinylene carbonate, and mixtures thereof.
5. The aqueous composition of claim 4, wherein said organic
carbonate is ethylene carbonate, propylene carbonate, or a mixture
thereof.
6. The aqueous composition of claim 1, further comprising an
effective amount of one or more additives selected from the group
consisting of surfactants, anti-settling agents, wetting agents,
thickeners, rheology modifiers, fillers, leveling agents,
anti-foaming agents, and pH buffers.
7. The aqueous composition of claim 1, consisting of said
fluoropolymer; water; and wherein said organic carbonate is
ethylene carbonate, propylene carbonate, or a mixture thereof.
8. The aqueous composition of claim 1, wherein said fluoropolymer
is a polyvinylidene fluoride (PVDF) homopolymer or copolymer
comprising at least 70 mole percent of vinylidene fluoride
units.
9. The aqueous composition of claim 1, wherein said PVDF has a melt
viscosity of greater than 1.0 kp, by ASTM D-3835 at 450.degree. F.
and 100.sup.-1 sec.
10. The aqueous composition of claim 1, wherein said powdery
electrode material comprises one or more materials selected from
the group consisting of lithium-salts of metal oxides, sulfides and
hydroxides; :LiCoO.sub.2, LiNi.sub.xCo.sub.1-xO.sub.2,
LiMn.sub.2O.sub.2, LiFePO4, LiNi.sub.xCo.sub.yMn.sub.zO.sub.m,
LiNi.sub.xMn.sub.yAl.sub.zO.sub.m where x+y+z=1 and m is an integer
representing the number of oxygen atom in the oxide to provide an
electron-balanced molecule; lithium cobalt oxide, lithium iron
phosphate, lithium manganese phosphate, lithium-nickel oxide, and
lithium-manganese oxide, carbonaceous materials, nano-titanates,
graphite, activated carbon, carbon black, phenolic resin, pitch,
tar, and carbon fibers.
11. An electrode comprising an electroconductive substrate coated
on at least one surface with the aqueous composition of claim 1 in
dried form, wherein said electrode exhibits interconnectivity.
12. The electrode of claim 11, wherein said powdery electrode
materials are not fully coated by PVDF.
13. A device comprising at least one electrode of claim 12,
selected from the group consisting of an non-aqueous-type battery,
a capacitor, and a membrane electrode assembly.
14. An electrode comprising an electroconductive substrate coated
on at least one surface with the aqueous composition of claim 2 in
dried form, wherein said electrode exhibits interconnectivity.
15. A device comprising at least one electrode of claim 14,
selected from the group consisting of an non-aqueous-type battery,
a capacitor, and a membrane electrode assembly.
Description
[0001] This application is a continuation-in-put of copending U.S.
application Ser. No. 12/788,427, filed May 27, 2010, from which
priority is claimed. This application also claims benefit, under
U.S.C..sctn.119(e) of U.S. Provisional Application No. 61/182364,
filed May 29, 2009. The cited references are incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] This invention relates to a waterborne fluoropolymer
composition useful for the fabrication of Li-Ion-Battery (LIB)
electrodes. The fluoropolymer composition contains an organic
carbonate compound, which is more environmentally friendly than
other fugitive adhesion promoters currently used in waterborne
fluoropolymer binders. An especially useful organic carbonate
compound is ethylene carbonate (EC) and vinylene carbonate (VC),
which are solids at room temperature, and other carbonates which
are liquid at room temperature such as propylene carbonate, methyl
carbonate and ethyl carbonate. The composition of the invention is
low cost, environmentally friendly, safer, and has enhanced
performance compared to current compositions.
BACKGROUND OF THE INVENTION
[0003] Fluoropolymers are a unique family of materials that are
most often used where exceptionally high performance,
maintenance-free, and long-lasting service-life is required. Among
fluoropolymers, polyvinylidene fluoride (PVDF) has a great balance
of properties and a long legacy as the only commercially accepted
binder for LIB cathodes and as a separator in polymer battery. This
usefulness comes from alternating CH.sub.2 and CF.sub.2 groups in
PVDF backbone that produce a high dipole moment, resulting in good
adhesion and compatibility with a vast array of materials.
[0004] U.S. Pat. No. 5,776,637 and U.S. Pat. No. 6,200,703 describe
a PVDF binder solution in organic solvents, particularly in NMP,
mixed with a powdery electrode material to form an electrode to be
used in a non-aqueous-type battery. The role of the organic solvent
is primarily to dissolve PVDF to provide good adhesion
(non-reversible adhesion) and interconnectivity between the powdery
electrode material particles upon evaporation of the organic
solvent. The bonded powdery electrode materials together should be
able to tolerate large volume expansion and contraction during
charge and discharge cycles without losing interconnectivity within
the electrodes. Interconnectivity of the active ingredients in an
electrode is extremely important in battery performance, especially
during charging and discharging cycles, as electrons must move
across the electrode to current collector, and lithium ions must
move within powdery particles of active materials as well as
between anode and cathode. In order to achieve desired performance,
PVDF binder is dissolved in a large volume of organic solvents,
such as NMP with 20 to 1 ratio, and subsequently is admixed with
powdery electrode forming material to produce slurry which upon
casting and drying will form electrode.
[0005] An organic-solvent-based slurry presents safety, health and
environmental dangers. Organic solvents are generally toxic and
flammable, volatile in nature, and involve special manufacturing
controls to mitigate and reduce environmental pollution and safety
risks. Moreover, the large carbon footprint associated with use of
organic solvents, is not environmentally desirable. Furthermore,
extra manufacturing steps are associated with capturing and
recycling large amount of NMP used in the preparation of slurry and
fabrication of electrodes. A suitable waterborne fluoropolymer
(particularly one that is PVDF-based), along with proper
formulation could eliminate the need for large volume of organic
solvents in the fabrication of electrodes for secondary
Li-ion-batteries and overcome environmental hazards associated with
use of such solvents.
[0006] There is an environmentally-driven, and safety-driven desire
to be able to produce excellent, interconnected PVDF-based
electrodes, without the massive use of organic solvents.
[0007] To effectively employ waterborne slurries in
electrode-forming processes, it is important to develop binder
systems that are compatible with current manufacturing practices
and provide desired properties of the intermediate and final
products. Some common criteria include: a) stability of the
waterborne fluoropolymer dispersion, having sufficient shelf-life,
b) stability of the slurry after admixing the powdery material, c)
appropriate viscosity of the slurry to facilitate good aqueous
casting, and d) sufficient interconnectivity within the electrode
which is non-reversible after drying. Additionally, from a
regulatory view, fluoropolymers made without fluorosurfactants are
preferred.
[0008] U.S. Pat. No. 7,282,528 entitled "electrode additive"
describes fluoropolymer dispersions for cathode electrodes, which
are made by using per-fluorinated surfactants. The patent fails to
teach or suggest the use of any fugitive adhesion promoters, and
specifically the use of organic carbonates in the latex to provide
and facilitate interconnectivity in the electrode that is
non-reversible. Polytetrafluoroethylene (PTFE) binders, or blends
of other fluoropolymers with 50% or more PTFE are preferred and
exemplified. The negative electrode of the examples uses a
conventional solvent-based PVDF solution.
[0009] U.S. Pat. No. 7,659,335 describes similar fluoropolymer
dispersions used as electrode binders, having a specific class of
non-ionic stabilizer used in post-polymerization. PTFE is either
preferred since melt-processing or dissolution is substantially
impossible. There is no mention of fugitive adhesion promoters or
adding organic carbonates to the latex to provide interconnectivity
within the electrode. There are large differences in the properties
and processing of the final electrodes formed from PTFE and PVDF
binders. PTFE polymers have very high melting points and exhibit
very strong resistance to dissolution in common solvents. As a
result, PTFE particles are not able to soften, flow, and adhere to
powdery particles to provide interconnectivity within an electrode.
Additionally, PTFE and its blends with other fluoropolymers do not
meet some of the criteria needed as a viable binder, including the
stability needed for waterborne fluoropolymer dispersion. Moreover,
PTFE binders do not provide sufficient interconnectivity in
electrodes which is non-reversible. Waterborne PVDF-based binders
with organic carbonates of the present invention exhibit sufficient
shelf stability, do not need concentrating steps, and provide
interconnectivity when properly formulated.
[0010] A waterborne binder is described in US20100304270 for making
an electrode using water as the media to prepare slurry instead of
using conventional NMP solution. The disclosed slurry formulation
requires anti-foaming agent, thickener, adhesion promoter, and a
relatively high binder loading. In general, any additive used in
the slurry formulation could have negative impact on the long-term
performance of a lithium ion battery, because the additives could
be oxidized in the cathode and generate off-gases. There is an
interest in reducing the amount of non-active ingredients in a
lithium ion battery in order to increase the energy density, so it
is desirable to reduce the non-active materials in the slurry.
[0011] Surprisingly, it has been found that a mixture of a
waterborne fluoropolymer with an organic carbonate as a fugitive
adhesion promoter can provide an effective, economically friendly
waterborne binder for a lithium ion battery. An especially useful
organic carbonate is ethylene carbonate (EC) which is solid at room
temperature, as the additive to binder media eliminates the need
for organic solvent, wetting agent, or anti-foaming agent for
slurry preparation. The waterborne fluoropolymer composition
provides interconnectivity between active ingredients and
sufficient adhesion to current collectors upon drying. Even though
EC is solid at room temperature, it has been surprisingly found
that the addition of EC to the waterborne fluoropolymer provides
good interconnectivity and adhesion for powdery materials without a
need for other additives.
SUMMARY OF THE INVENTION
[0012] The invention relates to a composition comprising: [0013] a)
waterborne fluoropolymer having a solids of 0.5 to 150 parts
fluoropolymer particles having a weight average particle size of
less than 500 nm; [0014] b) from 10 to 500 parts of one or more
powdery electrode-forming materials; [0015] c) from 1 to 150 parts
of an organic carbonate; [0016] d) 100 parts water; All parts being
parts by weight based on 100 parts by weight of water. Preferably
the waterborne fluoropolymer is a polyvinylidene fluoride-based
polymer.
[0017] The invention further relates to a binder for an electrode
comprising an electroconductive substrate coated with slurry made
with the waterborne fluoropolymer of the invention, and a method
for producing the electrode from the composition(s) of the
invention.
[0018] The invention further relates to a non-aqueous-type
Li-ion-Battery having at least one electrode made with binder of
the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] All references listed in this application are incorporated
herein by reference. All percentages in a composition are weight
percent, unless otherwise indicated, and all molecular weights are
given as weight average monlecular weight, unless stated
otherwise.
[0020] By "irreversible" as used herein in relation to an electrode
formed from the polymer binder of the aqueous composition, is meant
that following the drying of the aqueous composition in Which the
polymer binder binds the powdery electrode-forming materials to
each together and to the electroconductive substrate, the polymer
binder is not soluble or redispersible in water. The
irreversibility is due to the fact that the polymer particles flow
and adhere to the powdery electrode-forming materials, providing
interconnectivity within the electrode, This is opposed to an
electrode formed from a PTFE dispersion or excessive water soluble
thickener (such as carboxylated methyl cellulose) which form a
binder without interconnectivity, and thus when the coating is
placed in water it redispeses.
[0021] By "interconnectivity" is meant that the powdery
electrode-forming materials are permanently bonded together by the
polymeric binder, providing low electrical resistance and high ion
mobility within the electrode.
[0022] The manner of practicing the invention will now be generally
described with respect to a specific embodiment thereof, namely
polyvinylidene fluoride-based polymer prepared by aqueous emulsion
polymerization using non-fluorinated emulsifier as the principle
emulsifier and used in preparation of an electrode. Although the
process of the invention has been generally illustrated with
respect to PVDF based polymers, one of skill in the art will
recognize that analogous polymerization techniques can be applied
to the preparation of homopolymers and copolymers of fluorinated
monomers and their formulation for the preparation of electrodes in
general, and more specifically to homopolymers or copolymers of
vinylidene fluoride (VDF), tetrafluoroethylene (TFE), and/or
chlorotrifluoroethylene (CTFE)--with co-reactive monomers
fluorinated or non-fluorinated such as hexafluoropropylene,
perfluorovinyl ether, and the like. While non-fluorinated
surfactants are preferred, the use of fluorosurfactants is also
anticipated by this invention.
PVDF
[0023] The term "vinylidene fluoride polymer" (PVDF) used herein
includes both normally high molecular weight homopolymers,
copolymers, and terpolymers within its meaning. Such copolymers
include those containing at least 50 mole percent, preferably at
least 75 mole %, more preferably at least 80 mole %, and even more
preferably at least 85 mole % of vinylidene fluoride copolymerized
with one or more comonomers selected from the group consisting of
tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene,
hexafluoropropene, vinyl fluoride, pentafluoropropene,
tetrafluoropropene, trifluoropropene, perfluoromethyl vinyl ether,
perfluoropropyl vinyl ether and any other monomer that would
readily copolymerize with vinylidene fluoride.
[0024] In one embodiment, up to 30%, preferably up to 25%, and more
preferably up to 15% by weight of hexafluoropropene (HFP) units and
70%, preferably 75%, more preferably 85% by weight or more of VDF
units are present in the vinylidene fluoride polymer. It is desired
that the HFP units be distributed as homogeneously as possible to
provide PVDF-HFP copolymer with excellent dimensional stability in
an end-use environment--such as in a battery.
[0025] The PVDF for use in the electrode composition preferably has
a high molecular weight. By high molecular weight, as used herein
is meant PVDF having a melt viscosity of greater than 1.0 kilopoise
according to ASTM method D-3835 measured at 450.degree. F. and 100
sec.sup.-1.
[0026] The PVDF used in the invention is generally prepared by
means known in the art, using aqueous free-radical emulsion
polymerization--although suspension, solution and supercritical
CO.sub.2 polymerization processes may also be used. In a general
emulsion polymerization process, a reactor is charged with
&ionized water, water-soluble surfactant capable of emulsifying
the reactant mass during polymerization and optional paraffin wax
antifoulant. The mixture is stirred and deoxygenated. A
predetermined amount of chain transfer agent. CTA, is then
introduced into the reactor, the reactor temperature raised to the
desired level and vinylidene fluoride (and possibly one or more
comonomers) are fed into the reactor. Once the initial charge of
vinylidene fluoride is introduced and the pressure in the reactor
has reached the desired level, an initiator emulsion or solution is
introduced to start the polymerization reaction. The temperature of
the reaction can vary depending on the characteristics of the
initiator used and one of skill in the art will know how to do so.
Typically the temperature will be from about 30.degree. to
150.degree. C., preferably from about 60.degree. to 120.degree. C.
Once the desired amount of polymer has been reached in the reactor,
the monomer feed will be stopped, but initiator feed is optionally
continued to consume residual monomer. Residual gases (containing
unreacted monomers) are vented and the latex recovered from the
reactor.
[0027] The surfactant used in the polymerization can be any
surfactant known in the art to be useful in PVDF emulsion
polymerization, including perfluorinated, partially fluorinated,
and non-fluorinated surfactants. Preferably the PVDF emulsion of
the invention is fluorosurfactant free, with no flurosurfactants
being used in any part of the polymerization. Non-fluorinated
surfactants useful in the PVDF polymerization could be both ionic
and non-ionic in nature including, but are not limited to,
3-allyloxy-2-hydroxy-1-propane sulfonic acid salt,
polyvinylphosphonic acid, polyacrylic acids, polyvinyl sulfonic
acid, and salts thereof, polyethylene glycol and/or polypropylene
glycol and the block copolymers thereof, alkyl phosphonates and
siloxane-based surfactants.
[0028] The PVDF polymerization results in a latex generally having
a solids level of 10 to 60 percent by weight, preferably 1.0 to 50
percent, and having a weight average particle size of less than 500
nm, preferably less than 400 nm, and more preferably less than 300
nm. The weight average particle size is generally at least 20 nm
and preferably at least 50 nm. Additional adhesion promoters may
also be added to improve the binding characteristics and provide
connectivity that is non-reversible. A minor amount of one or more
other water-miscible solvents, such as ethylene glycol, may be
mixed into the PVDF latex to improve freeze-thaw stability.
[0029] In the present invention, PVDF polymer binder is generally
used in the aqueous electrode-forming composition, however a blend
of several different polymer binders, preferably all fluoropolymer
binders, and most preferably all PVDF binders may also be used. In
one embodiment, only thermoplastic fluoropolymers that can be
softened by fugitive adhesion promoters, specifically organic
carbonates, and more specifically EC, are used as the polymeric
binder. The fluoropolymer of the invention is present in the
waterborne fluoropolymer composition at from 0.5 to 150 parts,
preferably 1 to 100 parts, and more preferably 5 to 75 parts of
fluoropolymer to 100 parts of water,
Organic Carbonates
[0030] The waterborne slurry of the invention contains at least one
organic carbonate, in addition to the fluoropolymer and water. The
organic carbonate acts as a fugitive adhesion promoter to produce
the interconnectivity needed in electrodes formed from the
composition of the invention. By "fugitive adhesion promoter" as
used herein is meant an agent that increases the interconnectivity
of the composition after coating on a substrate. The fugitive
adhesion promoter is then capable of being removed from the formed
electrode generally by evaporation (for a chemical) or by
dissipation (for added energy).
[0031] Organic carbonates of the invention include, but are not
limited to: [0032] a) carbonates having the general formula:
(R.sub.2)CO.sub.3(R.sub.1) where R.sub.1 and R.sub.2 represent a
linear or branched C.sub.1-4 alkyl group; R.sub.1 and R.sub.2 can
be the same of different. Examples include, for example, methyl
carbonate, ethyl carbonate, n-propyl carbonate, sec-propyl
carbonate, n-butyl carbonate, t-butyl carbonate, methyl-ethyl
carbonate, methyl propyl carbonate, ethyl-propyl carbonate,
methyl-butyl carbonate, and ethyl-butyl carbonate. [0033] b)
ethylene carbonate CAS# 96-49-1 having a melting point of
35-38.degree. C.: [0034] c) propylene carbonate CAS# 108-32-7
having boiling point of 240.degree. C.; [0035] d) butylene
carbonate isomers; and [0036] e) vinylene carbonate. Especially
preferred organic carbonates are ethylene carbonate, propylene
carbonate, and vinylene carbonate. Ethylene carbonate is of special
interest, since it is a solid at room temperature, yet easily
dissolves in water at any portion.
[0037] The composition of the invention contains 1 to 150 parts,
preferably from 2 to 100 parts and more preferably from 10 to 50
parts by weight, of one or more organic carbonates per 100 parts by
weight of water. The useful organic carbonates that are liquids are
soluble or miscible in water. This organic carbonate acts as a
plasticizer for PVDF particles, making them tacky and capable of
acting as discrete adhesion points during the drying step. The PVDF
polymer particles are able to soften, flow and adhere to powdery
materials during manufacture, resulting in electrodes with high
connectivity that are non-reversible. In one embodiment the organic
carbonate is a latent solvent, which is a solvent that does not
dissolve or substantially swell PVDF resin at room temperature, but
will solvate the PVDF resin at elevated temperatures.
Powdery Electrode-Forming Material
[0038] The composition of the invention contains 10 to 500 parts,
preferably 20 to 400 parts, more preferably 25 to 300 parts of one
or more powdery electrode-forming materials per 100 parts of water.
The nature of the powdery electrode-forming material depends on
whether the composition will be used to form a positive or a
negative electrode. In the case of a positive electrode, the active
electrode material may be an oxide, sulfide or hydroxide of lithium
and/or a transition metal (including but not limited to cobalt,
manganese, aluminum, titanium, or nickel, and iron phosphates,
manganese phosphate). Double, and triple salts of lithium are also
contemplated. Preferred positive electrode materials include, but
are not limited to, LiCoO.sub.2, LiNi.sub.xCo.sub.1-xO.sub.2,
LiMn.sub.2O.sub.2, LiNiO.sub.2, LiFePO4,
LiNi.sub.xCo.sub.yMn.sub.zO.sub.m,
LiNi.sub.xMn.sub.yAl.sub.zO.sub.m where x+y+z=1 and m is an integer
representing the number of oxygen atom in the oxide to provide an
electron-balanced molecule; as well as lithium-metal oxides such as
lithium cobalt oxide, lithium iron phosphate, lithium manganese
phosphate, lithium-nickel oxide, and lithium-manganese oxide.
[0039] In the case of a negative electrode, the active material is
generally a carbonaceous material, nano-titanate, or other matrix
capable of being doped with lithium ions. Useful carbonaceous
materials include, but are not limited to graphite, manmade
graphite, carbon, carbon black, acetylene black, phenolic resin,
pitch, tar, etc. In the present invention carbon fibers can also be
used.
[0040] The ratio of PVDF solids to powdery electrode-forming
material is from 0.5-25, parts by weight of PVDF solids to 75 to
99.5 parts by weight powdery electrode material, preferably from
0.5-15, parts by weight of PVDF solids to 85 to 99.5 parts by
weight powdery electrode material, more preferably from 1-10 parts
by weight of PVDF solids to 90 to 99 parts by weight powdery
electrode material, and in one embodiment from 0.5-8, parts by
weight of PVDF solids to 92 to 99.5 parts by weight powdery
electrode material. If less PVDF is used, complete
interconnectivity may not be achieved, and if more PVDF is used,
there is a reduction in conductivity, and also the composition
takes up volume and adds weight--and one use of the composition is
for very small and light batteries.
Other Additives
[0041] The composition of the invention optionally contains other
additives in effective amounts, such as surfactants or
anti-settling agents, wetting agents, thickeners and rheology
modifiers, fillers, leveling agents, anti-foaming agents, pH
buffers, and other adjuvants typically used in waterborne
formulation while meeting desired electrode requirements.
[0042] The composition of the invention contains 0 to 10 parts,
preferably from 0.1 to 10 parts, and more preferably 0.5 to 5 parts
of one or more anti-settling agents and/or surfactants per 100
parts of water. These anti-settling agents or surfactants are added
to the PVDF dispersion post-polymerization, generally to improve
the shelf stability, and provide additional stabilization during
slurry preparation. Useful anti-settling agents include, but are
not limited to, ionic substances, such as salts of alkyl sulfates,
sulfonates, phosphates, phophonates (such as sodium lauryl sulfate
and ammonium lauryl sulfate) and salts of partially fluorinated
alkyl sulfates, carboxylates, phosphates, phosphonates (such as
those sold under the CAPSTONE brandname by DuPont), and non-ionic
surfactants such as the TRITON X series (from Dow) and PLURONIC
series (from BASF). In one embodiment, only anionic surfactants or
in combination with non-ionic surfactants are used. It is preferred
that no fluorinated surfactants are present in the composition,
either residual surfactant from the polymerization process, or
added post-polymerization in forming or concentrating an aqueous
dispersion.
[0043] The composition of the invention optionally contains 0 to 5
parts, preferably from 0 to 3 parts of one or more wetting agents
per 100 parts of water. Surfactants can serve as wetting agents,
but wetting agents may also include non-surfactants. In some
embodiments, the wetting agent can be an organic solvent. It has
been found that the presence of optional wetting agents permits
uniform dispersion of powdery electrode material(s) into aqueous
dispersion of vinylidene fluoride polymer. Some electrode
materials, such as carbonaceous materials will not disperse in an
aqueous dispersion without the use of wetting agent. Useful wetting
agents include, but are not limited to, ionic and non-ionic
surfactants such as the TRITON series (from Dow) and the PLURONIC
series (from BASF), and organic liquids that are compatible with
the aqueous dispersion, including but not limited to NMP, DMSO, and
acetone.
[0044] The composition of the invention may contain 0 to 10 parts,
preferably from 0 to 5 parts of one or more thickeners or rheology
modifiers per 100 parts of water. Addition of water-soluble
thickener or rheology modifier to the above dispersion prevents or
slows down the settling of powdery electrode materials while
providing appropriate slurry viscosity for a casting process Useful
thickeners include, but are not limited to the ACRYSOL series (from
Dow Chemical); partially neutralized poly (acrylic acid) or poly
(methacrylic acid) such as CARBOPOL from Lubrizol; and carboxylated
alkyl cellulose, such as carboxylated methyl cellulose (CMC).
Adjustment of the formulation pH can improve the effectiveness of
some of the thickeners. In addition to organic rheology modifiers,
inorganic rheology modifiers can also be used alone or in
combination. Useful inorganic rheology modifiers include, but are
not limited to, inorganic rheology modifiers including but not
limited to natural clays such as montmorillonite and bentonite,
manmade clays such as laponite, and others such as silica, and
talc.
[0045] The thickeners of the invention are used in the aqueous
composition containing the PVDF and powdery electrode material, and
are not used in pure form as a second. coating composition as has
been described in the JP 2000357505 reference.
Aqueous Dispersion Formulation
[0046] The aqueous electrode-forming composition of the invention
can be obtained in many different ways.
[0047] In one embodiment, a PVDF dispersion is formed (preferably
without any fluorosurfactant) and a predetermined amount of any
anti-settling agent(s) or surfactant(s), is diluted in water and
post-added to the PVDF dispersion latex with stirring, in order to
provide adequate storage stability for the latex. To this PVDF one
or more optional additives are added, with stirring. The pH can be
adjusted, if needed, for the thickener to be effective. The
electrode-forming powdery material(s) and other ingredients are
then added to the mixture. It may be advantageous to disperse the
electrode-forming powdery material(s) in the organic carbonate, the
latent solvent or wetting agent to provide wetting of the powdery
materials prior to admixing with the aqueous PVDF binder
formulation. The final composition is then subjected to a high
shear mixing to ensure uniform distribution of the powdery material
in the composition. The final aqueous composition of the invention
should have a viscosity useful for casting or coating onto a
substrate. The useful viscosity is in the range of from 2,000 to
20,000 cps at 20 rpm, and 25.degree. C. depending on application
methods.
[0048] The aqueous dispersion composition is very critical to
manufacturing a high quality and low cost electrode. A good and
well-balanced slurry formulation will help to achieve good
dispersion, which will not only lead to a uniform and high quality
electrode but also decrease the manufacturing cost by reducing the
scrap rate. Furthermore, addition of suitable dispersion agents
will decrease the mixing time, which in turn will increase
productivity. Secondly, the slurry formulation will affect the
slurry stability, the slurry settle-down time, and viscosity. The
unstable slurry will not only increase the production cost due to a
high scrap rate but also will lead to low quality product due high
variation in electrode thickness.
[0049] The aqueous formulation could affect the electrode
performance, for example additives such as anti-foaming agents,
coalescent agents, wetting agents, in slurry formulation have
tendency to be oxidized during charging cycle and generate off-gas
with is extremely undesired for LIB. Surprisingly, when the aqueous
fluoropolymer binder is used in conjunction with ethylene
carbonate, propylene carbonate and vinylene carbonate, the need for
all of these additives can be eliminated.
[0050] The aqueous electrode composition is applied onto at least
one surface, and preferably both face surfaces, of an
electroconductive substrate by means known in the art, such as by
brush, roller, ink jet, squeegee, foam applicator, curtain coating,
vacuum coating, or spraying. The electroconductive substrate is
generally thin, and usually consists of a foil, mesh or net of a
metal, such as aluminum, copper, lithium, iron, stainless steel,
nickel, titanium, or silver. The coated electroconductive substrate
is then dried to form a coherent composite electrode layer, that
may then be calendered, providing an interconnected composite
electrode usable in a non-aqueous-type battery. The aqueous
electrode composition can be optionally baked at elevated
temperature to achieve high adhesion strength. The dried electrode
can be optionally subjected to calendering at high pressure and
high temperature to further improve electrode adhesion.
[0051] The aqueous electrode composition of the present invention
has an advantage in processing, in that water has a boiling point
lower than the commonly used solvents for PVDF, and thus can be
dried faster, or at a lower temperature than solvent-based PVDF
compositions, and lower than compositions containing PTFE. Process
temperatures of 150.degree. C. or less, 120.degree. C. or less,
100.degree. C. or less and even 90.degree. C. or less may be used
and result in a useful electrode.
[0052] Another advantage of using the aqueous coating of the
present invention over solvent coatings, is that an aqueous PVDF
dispersion serves as a binder with polymer particles binding
together the powdery electrode materials only at specific discrete
points to produce interconnectivity, while a solution coating forms
a continuous coating on the powdery electrode materials. The
continuous polymer coating formed from solution coatings, while
very thin, still serves as an insulator, reducing the electrical
conductivity.
[0053] The electrodes of the invention can be used to form an
electrochemical device, such as a battery, capacitor, electric
double layer capacitor, membrane electrode assembly (MEA) or fuel
cell, by means known in the art. A non-aqueous-type battery can be
formed by placing a negative electrode and positive electrode on
either side of a separator. The separator is generally a polymeric
porous film impregnated with an electrolytic solution.
EXAMPLES
Synthesis of Waterborne Fluoropolymer:
[0054] Into an 80-gallon stainless steel reactor was charged, 345
lbs of deionized water and 250 grams of Pluronic 31R1 non-ionic
surfactant (from BASF). Following evacuation, agitation was begun
at 23 rpm and the reactor was heated to 100.degree. C. After
reactor temperature reached the desired set point, 0.6 lbs propane
was charged into the reactor. Reactor pressure was then raised to
650 psi by charging about 35 lbs vinylidene fluoride (VDF) into the
reactor. After the reactor pressure was stabilized, 5.2 lbs of an
aqueous initiator solution containing 1 wt % potassium persulfate
and 1 wt % sodium acetate was added to the reactor to jumpstart
polymerization. The rate of further addition of initiator solution
was so adjusted to obtain and maintain a VDF polymerization rate of
roughly 70 pounds per hour. The VDF homopolymerization was
continued until 100 lbs of VDF was fed to reactor, at this point a
1 wt % aqueous solution of sodium lauryl sulfate (SLS) was
introduced into the reactor at ratio to monomer of 1.5%. After
adding total of 150 lbs of monomer to the reactor, and 18.3 lbs of
initiator solution, all feeds were stopped. After 20 minutes, the
agitation was stopped. and the reactor was vented and the latex
recovered. Latex had 27% solids with particle size of 155 nm, To
the final latex, solution of SLS at ratio to solids of 0.5% was
added. Polymer resin was isolated by coagulating the latex, washing
the latex with deionized water, and drying. The resin had a melt
viscosity of 24 kilopoise measured at 232.degree. C., a DSC melt
point of 163-168.degree. C.
[0055] Ethylene carbonate (EC) was added to the resultant latex and
(designated SPS-2) where the ratio of EC to latex was 27/100
wt/wt.
Cell Cycle Life at 60.degree. C. Temperature
[0056] Two sets of 18650 cells with nominal capacity of 2.0 Ah
using LiCoO.sub.2 cathodes, one made with SPS-2 and the other one
with SPS-2 plus NMP, and they were cycled at 60.degree. C. The
results at 60.degree. C. are significant as the decay rate is
generally faster at elevated temperatures as compared to room
temperature. The discharge capacity vs. the cycle number at 60 C
was established for a pair of cells made with 1.5% SPS-2 binder,
and another pair made with 1.5% WF plus 5 wt % NMP. After 100
cycles, the cells lost, about 7% of its initial capacity,
indicating SPS-2 is surprisingly is a very good water based binder
for lithium ion cathode. The all cells were discharged to 2.8V at 1
A and charged to 4.2V at 1.5 A.
* * * * *