TITLE: SOLID POLYMER ELECTROLYTES
European Patent EP0599924
 B1

ABSTRACT:
Abstract not available for EP0599924
Abstract of corresponding document: US5581394
PCT No. PCT/GB92/01500 Sec. 371 Date Feb. 15, 1994 Sec. 102(e) Date
Feb. 15, 1994 PCT Filed Aug. 13, 1992 PCT Pub. No. WO93/04507 PCT Pub.
Date Mar. 4, 1993Solid polymer electrolytes for electrochromic devices
are produced by dissolving lithium perchlorate and
polymethylmethacrylate in propylene carbonate, all dried to below 10
ppm water, and screen printing or casting a film of the resulting
solution followed by removal of solvent to leave a final polymer
concentration of about 40% by weight based on the combined weight of
polymer and solvent. Screen printability is conferred by a resin
modifier such as MODAFLOW (TM).

INVENTORS:
Green, Mino (55 Gerard Road, London, SW13 9QH, GB)
Kang, Karam Singh (822 Pembridge Crescent, Kingston, Ontario, K7M 6A4,
CA)

APPLICATION NUMBER: EP19920917574
PUBLICATION DATE: 11/18/1998
FILING DATE: 08/13/1992

ASSIGNEE: BTG INTERNATIONAL LIMITED (10 Fleet Place, London, EC4M 7SB, GB)
INTERNATIONAL CLASSES: C08K3/22; C08K3/16; C08L101/00; C09D11/00; C09D11/02; G02F1/15;
H01B1/12; H01M6/18; (IPC1-7): H01M6/18; C09D11/02; G02F1/15; H01B1/12
EUROPEAN CLASSES: C08K3/16+L33/12; G02F1/15W2; H01B1/12F; H01M6/18B

DOMESTIC PATENT REFERENCES:
EP0331342 Solid electrolyte devices.
EP0424827 Cross-linked polymeric electrolyte.

FOREIGN REFERENCES:
GB2246872A

OTHER REFERENCES:
DATABASE WPIL Derwent Publications Ltd., London, GB; AN 91-041083
DATABASE WPIL Derwent Publications Ltd., London, GB; AN 91-320228
DATABASE WPIL Derwent Publications Ltd., London, GB; AN 92-214346
Attorney, Agent or Firm:
Neville, Peter Warwick (Patents Department British Technology Group Ltd
10 Fleet Place, London, EC4M 7SB, GB)

CLAIMS:
1. A process for making a solid polymer electrolyte comprising forming
a solution by dissolving a conductive salt and a polymer in an alkylene
carbonate, forming a thin layer of said solution and removing
sufficient of said alkylene carbonate to solidify the electrolyte.
2. A process as claimed in Claim 1, wherein said solution is screen
printed to form said thin layer.
3. A process as claimed in Claim 1, wherein said solution is cast on to
a support to form said thin layer.
4. A process as claimed in any one of Claims 1 to 3, wherein said
solution contains a resin modifier conferring on said solution the
properties of bubble release, surface levelling and clean snap-back so
as to render the solution screen printable.
5. A process as claimed in any one of Claims 1 to 4, wherein the solid
electrolyte produced is a substantially water free solid polymer
electrolyte for an electrochromic device comprising an alkylene
carbonate, a polymer in a quantity sufficient to render the electrolyte
solid and a conductive lithium salt, said electrolyte containing no
more than 500 ppm water.
6. A process as claimed in any preceding claim, wherein the polymer is
a polyalkylmethacrylate.
7. A process as claimed in Claim 6, wherein the polymer is a
polymethylmethacrylate.
8. A process as claimed in any preceding claim, wherein the alkylene
carbonate is propylene carbonate.
9. A process as claimed in any preceding claim, wherein said salt is a
lithium salt.
10. A process claimed in any preceding claim, wherein the content of
said polymer in the solid electrolyte is from 35 to 45 percent by
weight based upon the combined weight of said polymer and said alkylene
carbonate.
11. A process as claimed in Claim 10, wherein said percentage is about
40 percent.
12. A precursor for a solid polymer electrolyte, comprising an alkylene
carbonate solvent, a polyalkylmethacrylate and a conductive salt, which
precursor is solidifiable by removal of solvent and contains no more
than 500 ppm of water.

DESCRIPTION:
The present invention relates to processes for producing solid polymer
electrolytes and to precursors for solid polymer electrolytes. Such
polymer electrolytes may be incorporated into electrical devices such
as electrochromic devices.
The formulation of polymer electrolytes for use in electrochromic
devices is a long-standing problem. Attention has principally been
concentrated on the use of polyethylene oxide containing lithium
perchlorate as a conductive salt. Propylene carbonate has been used as
a solvent in combination with polyethylene oxide.
Japanese Patent Specification No. 60037530 discloses a polymer
electrolyte comprising polymethylmethacrylate, propylene carbonate and
lithium perchlorate. The content of polymethylmethacrylate is such that
the electrolyte is gelatinous, having a consistency such that it is
capable of being injected into an electrochromic cell.
Japanese Patent Specification No. 55-86070 discloses a battery
electrolyte comprising polymethylmethacrylate, propylene carbonate and
lithium fluoborate in the form of a gel possessing elasticity produced
by cooling a sol formed from the above materials. Battery electrolytes
are not generally suitable for use in electrochromic devices and such
suitability is not indicated in this specification. No conductivity
figures are disclosed, nor are any fabrication methods which would lend
themselves to the production of electrochromic devices.
EP-A-0424827 discloses cross-linked electroylyte polymers in which
propylene carbonate may be present as an optional plasticiser.
We have sought to produce an electrolyte which is solid, stable on
storage, electrically stable in use in electrochromic devices and
preferably capable of being fabricated using a process of screen
printing or of casting to form a film or sheet.
The present invention provides a process for making a solid polymer
electrolyte comprising forming a solution by dissolving a conductive
salt and a polymer in an alkylene carbonate, forming a thin layer of
said solution and removing sufficient of said alkylene carbonate to
solidify the electrolyte.
The polymer is preferably an acrylic polymer such as a polyacrylic acid
ester. Suitably the polymer is polyalkyl methacrylate, for instance
polymethylmethacrylate.
The alkylene carbonate is preferably propylene carbonate.
The lithium salt is preferably lithium perchlorate.
The content of the polymer in the solid polymer electrolyte produced is
sufficient to render the electrolyte solid. Preferably the polymer
content is from 35 to 60 percent by weight based upon the combined
weight of said polymer and said alkylene carbonate, e.g. 35 to 45
percent. More preferably, said percentage is about 40 percent, e.g.
from 38 to 42 percent.
The electrolyte may further comprise an opacifying and reflecting
material or pigment such as titanium dioxide. The object of this
material is optically to isolate the front from the rear electrode in a
display electrochromic cell and to reflect ambient light which has
passed through the front electrode material. Suitably, the electrolyte
contains from 10 to 30 percent of the opacifying material by weight
based upon the weight of the electrolyte, e.g. from 15 to 25 percent.
The electrolyte preferably also comprises a resin modifier such as
ethylacrylate, 2-ethylhexylacrylate copolymer. The purpose of the resin
modifier will be described below in connection with the fabrication of
the electrolyte.
The alkylene carbonate can be removed to solidify the electrolyte by
heating the solution.
The solution may be formed into a thin layer or thick film prior to
said solidification by casting or by screen printing on to a substrate
or support. Multiple layers of the polymer electrolyte may be built-up
by sequential cycles of laying down a layer of solution and removing
solvent to solidify the layer.
The invention includes a precursor for a solid polymer electrolyte,
comprising an alkylene carbonate solvent, a polyalkylmethacrylate and a
conductive salt, which precursor is solidifiable by removal of solvent
and contains no more than 500 ppm of water.
In preparing an electrical device using the electrolyte forming process
according to the invention, first and second substrates may be printed
with precursor formulations according to the invention and solvent may
be removed therefrom to produce solid electrolytes on the substrates. A
further print of precursor may be applied to one or both of the
substrates and partially solidified by solvent removal. The substrates
may then be mated together to form a substrate-solid
electrolyte-substrate sandwich. The partially dried print of precursor
serves to provide a tacky and readily matable surface over the
previously prepared solid electrolyte.
The precursor solution typically will be viscous and preferably has all
of the properties necessary for screen printability. The solution may
have suspended in it undissolved solids and these may comprise excess
of the salt and/or an opacifying material or pigment as described
above.
The solution preferably contains from 10 to 30 percent by weight of
said polymer based upon the combined weight of polymer and the alkylene
carbonate, for instance about 25 percent by weight.
The solution preferably also contains a resin modifier as described
above conferring on said solution the properties of bubble release,
surface levelling and clean snap-back so as to render the solution
screen printable.
The polymer electrolyte precursor formations may additionally include a
plasticiser such as di-iso-octylphthalate in an amount similar to the
content of resin modifier described above.
For a successful screen printing operation, it is necessary that the
solution will not drip through a screen printing mesh (e.g. a stainless
steel 325 openings/inch (128 openings/cm), 50 µm opening, 60 µm thick
mesh) within the time needed to carry out the screen printing
operation, e.g. within 5 or more preferably 10 minutes. It is necessary
that any bubbles in the solution will be released after screen printing
within a reasonable resting time, for instance within 30 minutes or
more preferably 15 minutes. It is necessary that the surface of the
screen printed layer of solution levels itself after bubbles have been
released and have burst at the surface. Lastly, it is necessary that
when the screen used in screen printing snaps back off the surface, it
snaps back cleanly from the printed solution.
These properties can be obtained by a suitable choice of resin
modifier. A preferred ethylacrylate,2-ethylhexylacrylate copolymer
resin modifier is available from Monsanto under the name "Modaflow"
(CAS No. 26376-86-3). This is clear yellowish viscous liquid having a
flash point of 93°C and a boiling point of 205°C at 10 mm Hg pressure
(1.3 kPa). It has a specific gravity of 1.01 and is insoluble in water
but soluble in most organic salts.
Very little of the Modaflow resin need be included in a screen
printable formulation in accordance with the invention, for instance
from 0.1 to 5 percent, preferably from 0.5 to 1 percent by weight of
the weight of the polymer solution, including any suspended material.
Alternative materials to Modaflow resin modifiers include silicon oils
and stearates.
For practising the process according to the invention there may
therefore be provided a screen printable precursor for the solid
polymer electrolyte.
Generally, such a precursor comprises a solvent, a polymer and a
conductive salt. The nature of the solvent, polymer and conductive salt
is as described above.
The polymer electrolyte precursor preferably also comprises a resin
modifier as described above conferring on the precursor the required
properties for screen printing.
Precursor solutions of the kind described can be stored in dry
conditions for a period of months without deterioration.
The invention can produce a substantially water free solid polymer
electrolyte for an electrochromic device comprising an alkylene
carbonate, a polymer in a quantity sufficient to render the electrolyte
solid and a conductive lithium salt, said electrolyte containing no
more than 10 ppm water.
The invention may be used in producing an electrical device comprising
an electrode and a counter-electrode separated by a solid polymer
electrolyte and has particular relevance to electrochromic devices.
Electrochromic devices using the polymer electrolytes may have the
structures and characteristics described in for instance GB-A-2081922,
GB-A-2164170 and GB-A-2197527.
The lithium content of the solid polymer electrolyte may be chosen to
achieve a suitable conductivity for the purpose of the electrochromic
device or other device in which the electrolyte is to be used. For
electrochromic displays, a relatively high conductivity is desired. For
electrochromic windows and mirrors and other large electrochromic
devices where speed of writing and erasing is less important than
uniformity of coloration, a lower conductivity electrolyte may be
desired.
The lithium concentration in the electrolyte may however be from 0.1
molal (based on the quantity of solvent such as propylene carbonate
within the electrolyte) to 3 molal, e.g. from 0.1 molal to 0.4 molal
for low conductivity applications and from 0.6 molal to 1.5 molal for
higher conductivity uses.
To provide a satisfactory long term performance in an electrochromic
device, it is necessary to exclude significant amounts of water from
the electrolyte. We have found that water present in the electrolyte in
an electrochromic device will react with the electrochromic layer, e.g.
with lithium in a tungsten oxide layer, in such a device causing loss
of performance. Accordingly, the electrolytes according to the
invention contain no more than 500 ppm water. This may be achieved by
rigorous drying of the separate components of the electrolyte prior to
manufacture of the electrolyte by the method exemplified below.
Preferably said water content is not more than 200 ppm, e.g. no more
than 100 ppm.
Because it is difficult to prevent some take up of water during the
processing of the electrolyte to place it in situ in an electrochomic
device, it will generally be necessary to reduce the water content of
each of the materials used in the electrolyte substantially below these
levels, e.g. to no more than 10 ppm water, e.g. no more than 5 ppm.
The invention will be further described and illustrated with reference
to the following specific examples.
Example 1
A polymer electrolyte precursor according to the invention is
formulated as follows:-
Polymethylmethacrylate powder (molecular weight = 80,000) is dried
under vacuum for 16 hours at 50°C. Higher temperatures are avoided to
prevent the fine powder coalescing into a solid mass which is more
difficult to dissolve.
Propylene carbonate is dried to a water content below 10 ppmw over 4 Å
molecular sieve for 12 hours and is distilled at 85°C at 1 mm of Hg
pressure (1.3 kPa). Lithium perchlorate fine powder is heated to 180°C
in air or dry nitrogen for 24 hours.
The dried propylene carbonate is added to the dried lithium perchlorate
powder and stirred for 2 hours at 50°C, or at a greater temperature,
e.g. 100°C. The polymethylmethacrylate powder is added and stirred at
50°C until all the powder is dissolved. A higher temperature such as
130°C can be used. The quantities employed are as follows:-
  Weight (gms)
pmma 25
lithium perchlorate 5.32
propylene carbonate 75
About 15g of the propylene carbonate is lost in this preparation stage.
Titanium dioxide powder is dried at 180°C for 24 hours and is sieved
before being added to the solution prepared previously. Further
propylene carbonate is added with the titanium dioxide. Modaflow resin
modifier is added also and is mixed at high speed with the rotor blade
of a grinder to produce a precursor solution according to the invention
for a polymer electrolyte. The composition of the precursor is as
follows:-
  Weight (gms)
ppma 25
Lithium perchlorate 5.32
Propylene carbonate 75
Titanium dioxide 10
Modaflow2
Example 2
A solid polymer electrolyte is produced from the precursor of Example 1
as follows. The precursor is printed through a stainless steel screen
patterned with a 23 µm emulsion thickness. The snap-off distance is set
at 40/1000" (1 mm) . The printed film is dried at 75°C in dry nitrogen
to solidify it. A film thickness of 100 µm of solid printed electrolyte
is produced by successive print and solvent removal cycles.
The composition of the solid polymer electrolyte after solvent removal
is found to be as follows:
  Weight (percent)
ppma 28.3
Lithium perchlorate 6.0
Propylene carbonate 40.8
Titanium dioxide 22.6
Modaflow2.3
The percentage of polymer based upon the combined weight of polymer and
solvent is therefore approximately 41 percent.
The conductivity of the solid polymer electrolyte is found to be 7.4 x
10^-5 Ω^-1 cm^-1 and molal concentration of lithium perchlorate is
1.66.
Example 3
Two further solid polymer electrolytes prepared generally by a method
as described above have the following compositions:
  Composition A weight percent Composition B weight percent
ppma 35.5 36.2
Lithium perchlorate 7.6  3.8
Propylene carbonate 39.9 34.3
Titanium dioxide 14.2 21.7
Modaflow2.8  4.0
The conductivity of composition A is 2.3 x 10 Ω^-1 cm^-1 and that of
composition B is 8.7 x 10 Ω^-1 cm^-1 whilst the molal concentration of
lithium perchlorate in composition A is 2.15 and in composition B is
1.25.
Example 4
A precursor for a solid polymer electrolyte according to the invention
made generally as described in Example 1 has the following
composition:-
  Parts by weight
Polymethylmethacrylate 100
Propylene carbonate 384
Lithium perchlorate 17
Modaflow20
Titanium dioxide 32
Di-iso-octylphthalate  16
Silica  4
The di-iso-octylphthalate is present as a plasticiser and the silica is
hydrophobic silica.
Example 5
A further precursor formulation is as follows:-
  Parts by weight
Polymethylmethacrylate 100
Propylene carbonate 384
Lithium perchlorate 17
Modaflow20
Titanium dioxide 32
Di-iso-octylphthalate  40
Silica  4
Example 6
An electrochromic device is constructed using the precursor
formulations of Examples 4 and 5 as follows. The composition of Example
4 is screen printed through a polyester screen of 110 counts per inch
(43 counts per cm) with a mesh orientation of 25° and a 30 µm emulsion
thickness at a snap-off distance of 0.1 mm. The composition is printed
on to previously prepared pairs of substrates bearing electrodes and
counter-electrodes of a known type for an electrochromic device. The
printed substrates are heated in an oven at 100°C for 20 minutes and
the process of printing and solvent removal is repeated 4 times to
build-up a satisfactory thickness of solid polymer electrolyte.
A fifth print is made using the composition of Example 5 on each
substrate and the resulting prints are dried for 2 minutes at 100°C
before being mated together in a tacky state under an even pressure of
approximately 5 lbs per sq inch (34 kPa). The completed device is
sealed around its edges in a water excluding manner to produce a
finished electrochromic device.
Example 7
A formulation for a transparent polymer electrolyte precursor which
upon 50 percent solvent removal yields a solid electrolyte suitable for
use in windows and mirrors is as follows:-
  weight percent
ppma 170g
Propylene carbonate 533g (444cc)
LiClO[4]21.2g
Modaflow1.5g