Materials for electrical devices

Melt-extruded polymeric electrolyte material for electrochemical power cells may be coextruded with other components of the cell, notably a lithium metal anode.

BACKGROUND OF THE INVENTION 
This invention relates to electrolyte and electrode materials for 
electrochemical current-generating cells, hereinafter referred to as 
electrochemical power cells. 
Some electrical components, for example some electrode materials, are 
sensitive insofar as they are difficult to handle during manufacture of 
electrical devices owing to physical weakness or high chemical reactivity, 
which may necessitate inconvenient handling procedures and/or special 
conditions such as dry room assembly. Examples of such sensitive materials 
include alkali metals and alkaline earth metals, notably lithium metal 
electrodes for lithium cells. In one kind of such cells, the electrodes 
are assembled with sheets of polymer compositions which are inherently 
ionically conductive, often in liquid-free form commonly known as 
"polymeric electrolytes". 
Lithium metal is difficult to roll into thin strips for use as an 
electrode, and U.S. Pat. No. 3,721,113 describes a method of alleviating 
this difficulty by rolling the lithium between smooth (surface asperities 
less than one micron) polymeric surfaces having sufficiently low critical 
surface tension to prevent adhesion to the lithium. The polymer may be a 
coating on the surface of rolls used to roll the lithium, or may be in the 
form of sheeting enclosing or facing the lithium, which does not adhere to 
the lithium and is peeled off the lithium strip after rolling. While this 
method facilitates the rolling operation, which produces the thin lithium 
strip, it does not improve the efficiency of assembling the delicate 
lithium strip into electrical devices. 
Numerous variations of the materials and structure of individual cell 
electrodes have previously been described, with the emphasis on the 
chemical and electrical performance of the materials and with little 
attention to the assembly process itself. For example, British Patent No. 
1533279 describes the use of an adherent thin coating of a vinyl polymer 
film on the surface of lithium strip electrodes for lithium/thionyl 
chloride cells to prevent electrode passivation, which tends to occur on 
storage of that particular kind of cell. The vinyl polymer film is 
insoluble in the thionyl chloride and must not be degraded or decomposed 
in the presence of the same. It must be sufficiently thin to permit ion 
transfer between the lithium and the thionyl chloride as required for 
current flow in operation of the cell. It is stated, though not 
demonstrated in the examples of the patent, that the vinyl polymer film 
may also serve as the sole electrode separator of the cell or may permit 
the use of a thinner separator than would normally be required. Somewhat 
thicker films of the vinyl polymer are recommended for that purpose, but 
it is made clear that the ion transfer needed for acceptable electrical 
performance of the cell will be adversely affected by thus increasing the 
film thickness. Electrode separators of polystyrene are described in U.S. 
Pat. No. 4,315,602 for alkaline cells, the separators again being 
necessarily thin enough to permit ion transfer. 
French Patent No. 7832978 (Publication No. 2442513) describes preparation 
of polymeric electrolyte films and their application to reactive metal 
electrodes by techniques such as solvent casting or pressure lamination of 
the polymeric film onto the metal or flowing the molten metal onto the 
polymer film and cooling.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides articles and processes whereby significant 
improvements in electrical power cell assembly can be achieved, as 
hereinafter described. 
In its broadest aspects, the invention provides melt extruded polymeric 
electrolyte materials and/or melt extruded cathode materials for 
electrochemical power cells. Either or both of these may be extruded onto 
other components of a cell, either sequentially or in some cases by 
coextrusion, for example to form a lithium cell in which a lithium anode 
is encapsulated in polymeric electrolyte with extruded cathode material 
and a current collector in contact with the electrolyte. Metallic anodes 
of suitable meltinq point, such as lithium, may advantageously be 
coextruded with the electrolyte and other components. 
The invention can thus provide an advantageous article comprising sensitive 
electrode material carrying an extruded, preferably flexible, layer of 
polymeric electrolyte material by which is meant material which is 
inherently capable of sufficient ionic conductivity to provide an 
electrolyte, and preferably also carrying an extruded cathode layer in 
contact with the electrolyte. Preferably, the article is in a form 
suitable for feeding to automatic equipment capable of assembling the said 
device. The ionically conductive electrolyte layer and the cathode layer 
are preferably applied to the sensitive material in a continuous process, 
which may preferably also include the aforementioned automatic assembly 
into electrical devices. 
This aspect of the invention accordingly provides a method of making an 
electrochemical power cell comprising melt-extruding polymeric electrolyte 
material onto another component to be incorporated in the cell. 
The protective encapsulation of sensitive material by the flexible extruded 
layer(s) permits the development of assembly processes which conveniently 
include the step of deforming the sensitive electrode materials while 
protected by the extruded material, for example to reduce the thickness of 
the electrode material, and/or the step of arranging it in a desired form 
such as a coiled electrode, as will be further described hereinafter. The 
invention includes such deformed articles whether or not in a feedable 
form. For this purpose, the electrode material may advantageously be a 
metal which is malleable under temperatures and pressures which do not 
unacceptably damage the extruded layer(s). 
The realisation that the previously unknown extrusion of the polymeric 
electrolyte and the cathode material can be used to form the active 
components of an electrochemical power cell leads to important processing 
advantages. Adherent extruded layers are preferred, for which purpose the 
or each layer may have suitable adhesion-promoting surface properties, 
such as surface asperities greater than one micron. The feedable article 
is preferably in the form of an elongate strip, preferably of sufficient 
length to make a plurality of the said electrical devices. 
The or each layer of extruded material is preferably "able to survive" 
mechanical deformation of the electrode material in the sense that it will 
retain its integrity and maintain a useful degree of protection both 
against mechanical damage and against contamination of the electrode 
material after a significant amount of deformation, for example for the 
aforementioned purposes. The precise amount of deformation which the 
protective material must survive will be a matter of commonsense for 
practical readers. Brittle layers which would crack so as to reduce the 
protection unacceptably are accordingly excluded, except for end uses 
where substantial flexibility is not required. Materials which would react 
unacceptably in other ways, either chemically or physically, for example 
very thin layers which would become unacceptably scuffed or torn, are of 
course to be avoided. 
The extruded electrolyte is capable of sufficient ionic conductivity to 
provide an electrolyte, preferably independent of the action of liquid. 
It is an advantage of the present invention that the extruded materials 
will provide protection against contamination of the electrode material. 
This is especially advantageous in connection with electrode materials 
which may react violently with certain contaminants, for example alkali 
metals with water, since the protective material will reduce the 
likelihood of violent reaction. 
The extruded material may thus provided a unitary pre-assembled electrical 
device such as a cell, thus eliminating some of the problems of handling 
and aligning electrodes, current collectors and other components during 
the assembly of the electrical devices, and facilitating automated 
processing. 
It will be understood that the sensitive electrode material may require 
protection for various reasons, for example materials which are subject to 
attack by atmospheric gases or moisture during storage; materials which 
may react prematurely with liquid with which they may come into contact 
during assembly materials which are subject to poisoning by contaminants 
during storage; and materials which lack physical strength or integrity 
and thus require protection from physical damage. The invention is 
especially useful for materials which require physical protection owing to 
physical weakness, e.g. lithium metal. 
It is a further advantage of the present invention that the processes can 
apply the extruded materials continuously and can therefore readily be 
integrated into a system for assembling successive portions of the article 
into a succession of the electrical devices as aforesaid, preferably 
automatically and continuously. The advantages of such an automated 
process over the piece-by-piece hand assembly methods hitherto used in the 
absence of the articles according to this invention, especially for alkali 
metal or alkaline earth metal electrode materials, will be appreciated. 
Pre-extruded films of electrolyte and/or cathode materials according to 
this invention can be assembled with the opposing electrode material and 
other components of the electrical device, but the preferred direct 
extrusion polymeric electrolyte material reduces the difficulties which 
are encountered in handling and aligning separate layers of materials, 
especially with reactive metals such as alkali metals or alkaline earth 
metals. 
This invention is especially useful in relation to reactive metal 
electrodes such as alkali metal or alkaline earth metal electrodes, 
especially lithium electrodes for lithium cells. Production of thin sheet 
electrodes of these and other materials can be facilitated by deforming 
the electrode material, for example by rolling, while in contact with the 
extruded layer(s) so as to increase its surface area, e.g. to decrease the 
thickness of the electrode material or otherwise alter its form or surface 
configuration. In this way, thin sheets of lithium, for example of about 
0.075 millimeters thickness, which would otherwise be difficult and 
expensive to make and handle, can be produced from more readily available 
0.25 millimeter strip. The extruded material may be deformed, e.g. 
stretched, so as to enhance its function in the device, e.g. its ability 
to provide an electrode separator and/or electrolyte. This may be useful 
for extruded materials which require permeation by liquid in order to 
provide adequate ionic conductivity. However, substantially dry materials 
are preferred. 
The polymeric electrolyte of the invention may comprise other materials in 
addition to the salt-loaded polymer. These materials may advantageously be 
mixed into the polymer during the blending process. Melt blending 
techniques are particularly suitable for mixing the materials of the 
electrolyte because of the high homogeneity of the mixtures produced 
thereby. Materials may be selected that provide one or more functions, but 
it is, of course, important that the materials are both miscible with the 
electrolyte, and chemically compatible with the other components in the 
cell with which they will come into contact, for example a lithium anode. 
It is particularly advantageous to add to the polymer a plasticising agent 
to inhibit or substantially to prevent the change in morphology of the 
polymer from the amorphous phase in which it exists when molten, to a 
phase in which it is at least partially crystalline. This is advantageous 
since the ionic conductivity in the amorphous phase is generally higher 
than that in the crystalline phase. The following list contains examples 
of plasticising agents which may be mixed into a polymer for this purpose, 
the choice of agent depending on chemical compatibility and other factors: 
Propylene carbonate 
Ethylene carbonate 
Tetramethylene sulphone (Sulpholane*) 
.gamma.-Butyrolactone 
Dimethylformamide 
Dioctyl phthalate 
Dibutyl phthalate 
Thiokol TP-90B* plasticiser (Thiokol Corporation) 
Thiokol TP-759* plasticiser (Thiokol Corporation) 
Vulkanol OT* plasticiser (Bayer UK Ltd) *Trade mark 
The amount of plasticising agent or agents to be added will depend on many 
factors, particularly the nature of the polymer and other components of 
the electrolyte, and the temperature. However, it is generally appropriate 
to add betwee 5 and 60% by weight, especially between 15 and 50%, more 
especially between 25 and 40%. 
It is preferred to select a plasticising agent that has a relatively high 
dielectric constant to enhance dissociation of the ions of the salt, and 
thereby to improve ionic conductivity throughout the electrolyte. A 
dielectric constant of at least 15, preferably at least 20, especially at 
least 30, more especially at least 40 is preferred; for example, propylene 
carbonate (dielectric constant 64.4 at 25.degree. C.), ethylene carbonate 
(89.6 at 40.degree. C.) and tetramethylene sulphone (43.3 at 30.degree. 
C.). 
The invention includes electrodes for electrical devices carrying extruded 
polymeric electrolyte and electrical devices including such electrodes. 
Suitable ionically conductive materials include inorganic salts dispersed 
in extrudable organic polymer material, preferably capable of permitting 
sufficient transfer of dissociated ions of the salt to provide the 
required ionic conductivity in the substantial absence of any liquid. 
Examples include salt-loaded polymers having the repeating unit 
##STR1## 
wherein R is hydrogen or a group Ra, --CH.sub.2 OR.sub.a, --CH.sub.2 
OR.sub.e R.sub.a, --CH.sub.2 N(CH.sub.3).sub.2, in which R.sub.a is 
C.sub.1 -C.sub.16, preferably C.sub.1 -4 alkyl or cycloalkyl, and R.sub.e 
is an ether group of formula --(CH.sub.2 CH.sub.2 O--)p wherein p is a 
number from 1 to 100, preferably 1 or 2; or having the repeating unit 
##STR2## 
wherein R.sup.11 is R.sub.a, or ReRa, as defined above: or having the 
repeating unit 
##STR3## 
wherein Re and Ra are as defined above. The preferred salts are the strong 
acid salts of alkali metals or ammonium. 
The extruded electrolyte preferably will not interact with the electrode 
materials, although beneficial interactions are not excluded from the 
invention. 
Preferred processing conditions for polyethylene oxide to achieve 
polymer/salt complication using Brabender, twin-srew internal cavity mixer 
at 30 RPM for a 10:1 complex, include blending temperatures of greater 
than 100.degree. C., ideally 140.degree.-180.degree. C., and not greater 
than 180.degree. C. (to avoid polymer degradation). Total blending time 
less than 20 minutes is preferred to reduce possibility of polymer 
degradation. 
For a Baker-Perkins MPC 30 compounding line, the optimum temperature on 
twin screw intensive mixing section is 120.degree. C.; optimum temperature 
during extrusion; 150.degree. C. 
The following Examples illustrate aspects of this invention. 
EXAMPLE 1 
10 g polyethylene oxide (Union Carbide WSR 205) was dissolved in 
acetonitrile (pre-distilled) with stirring to give a 3% solution. The 
appropriate amount of the salt LiCF.sub.3 SO.sub.3 (vacuum dried at 
130.degree. C. for 4 hours) to give a polymer oxygen lithium ion ratio 
(0:Li) of 10:1 was then added to the solution. The solution was then 
stirred at room temeprature for 4 hours. 
Polymer film of thickness 0.2-0.3 mm was then solvent cast from the 
solution by placing the solution in a flat glass petrie dish and allowing 
the solvent to evaporate slowly. The films were then vacuum dried at 
105.degree. C. for 8 hours, before being placed in a vacuum desiccator and 
transferred to a dry box. 
To ensure that the films remained totally anhydrous all subsequent handling 
operations on the materials were performed in the dry box. 
The DSC trace of the film was precorded on a Dupont 1090 Thermal analyser 
operating in the DSC mode. FIG. 1 depicts the DSC trace obtained. 
The conductivity of the film was measured on a 0.85 cm diameter sample 
using the Griffin conductivity bridge operating at 1 k Hz and the 
conductivity cell shown in FIG. 2. 
The conductivity at 100.degree. C. was 2.4.times.10.sup.-4 ohm.sup.-1 
cm.sup.-1. 
EXAMPLE 2 
The appropriate amounts of poly(ethylene oxide) and salt LiCF.sub.3 
SO.sub.3, to give a 10:1 (0:Li) complex were mixed as powders and then 
melt blended on a Brabender twin screw cavity mixer at 160.degree. C. for 
20 minutes (30 RPM). The material was then pressed at 120.degree. C. under 
15 tonnes pressure to give a film of thickness 0.2-0.3 mm. The film was 
then dried and handled as described in Example 1. 
The DSC trace of the material is shown in FIG. 3 and is essentially the 
same as that obtained from the material in the previous example. 
The conductivity of the film at 100.degree. C. was 4.2.times.10.sup.-4 
ohm.sup.-1 cm.sup.1. 
EXAMPLE 3 
The material described in Example 2 was melt blended on a Baker-Perkins 
compounding line. The material was then formed into tape using a single 
screw extruder (32 mm Baughn single screw L/D ratio 25/1). The tape was 
produced in thickness 0.3-0.4 mm. 
The tape was dried and handled as described in Example 1. 
The DSC trace obtained is shown in FIG. 4, and is essentially the same as 
those obtained in the two previous examples. 
The conductivity of the tape was measured using the method described in 
Example 1. 
The conductivity of the tape at 100.degree. C. was 2.1.times.10.sup.-4 
ohm.sup.-1 cm.sup.-1. 
EXAMPLE 4 
A length of lithium foil supplied by Foote Mineral Co., 35 mm wide 0.25 mm 
thick, was encapsulated with a blend of Polyox WSR 205 and LiCF.sub.3 
SO.sub.3 made as described in Example 3. The encapsulation was done by 
passing the lithium through a crosshead die mounted on a 32 mm single scew 
Baughn extruder and drawing a tube of the Polyox/LiCF.sub.3 SO.sub.3 blend 
down on to it. The encapsulation was completed by drawing the composition 
between nip rollers immediately following extrusion. The resulting 
laminate had an overall thickness of 0.65 mm being composed of a lithium 
layer, 0.25 mm thick, coated on each side with a 0.2 mm thick layer of the 
Polyox, LiCF.sub.3 SO.sub.3 blend. 
The coating was removed from the lithium foil and its conductivity measured 
using the method described in example 1. 
The conductivity of the coating at 25.degree. C. was 2.5.times.10.sup.-8 
ohm.sup.-1 cm.sup.-1, this compares with a value of 3.8.times.10.sup.-8 
ohm.sup.-1 cm.sup.-1 obtained for the material described in Example 1. 
EXAMPLE 5 
23.8 grams of polyethylene oxide) was melted on a Brabender twin screw 
cavity mixer at 160.degree. C. for 5 minutes (30 RPM). To the melt was 
then added 23.8 grams electrolyte manganese dioxide (Mitsubushi 
Corporation GS grade) previously dried for 4 hours at 375.degree. C., and 
2.4 grams acetylene black (Cairn Chemical Limited). The materials were 
then melt blended for a further 5 minutes to give a homogeneous mix. 
A thin plaque of the composite cathode was fabricated from the mix by 
pressing at 120.degree. C. under 15 tonnes pressure. The final plaque 
thickness was 0.15 mm. 
EXAMPLE 6 
The composite cathode described in Example 5 was used to fabricate an 
eletrochemical cell in the following way. 
A 0.85 cm diameter disc was cut from the composite cathode plaque, dried at 
110.degree. C. under vacuum for 2 hours and then transferred to the dry 
box where all subsequent operations were performed. 
A 0.1 cm diameter disc of the ionically conducting material described in 
Example 3 was sandwiched between the composite cathode and a 0.85 cm 
diameter disc of lithium metal, thickness 0.25 mm. This assembly was then 
pressed between spring loaded stainless steel electrodes to ensure good 
interfacial contact between the solid components of the electrochemical 
cell. The stainless steel electrodes also acted as external electrical 
connection for the electrochemical cell. 
The electrochemical cell so described gave an open circuit voltage at 
70.degree. C. of 3.30 volt. When connected to a 100 K ohm external load 
the cell was capable of delivering 28 uA at 2.8 volt. Example 7 
The appropriate amounts of poly(ethylene oxide) and salt LiCF.sub.3 
SO.sub.3, to give a 10:1 (0:Li) complex were mixed as powders and then 
melt blended on a Brabender turn screw cavity mixer at 160.degree. C. for 
15 minutes. 30% w/w propylene carbonate (Aldrich Chemical Co. Ltd.) was 
then added as the melt cooled. The resulting material was pressed at 
80.degree. C. under 15 tonnes pressure to give a film of thickness 0.5 mm. 
The conductivity of the film at 23.degree. C. was 1.84.times.10.sup.-4 ohm 
cm.sup.-1. 
The above process was repeated to produce electrolytes containing ethylene 
carbonate and sulpholane. Conductivities of 1.45.times.10.sup.-4 and 
7.54.times.10.sup.-5 ohm.sup.-1 cm.sup.-1 respectively were measured at 
23.degree. C. 
EXAMPLE 8 
The appropriate amounts of poly(ethylene oxide) and salt LiCF.sub.3 
SO.sub.3, to give a 30:1 (0:Li) complex were mixed as powders and then 
melt blended on a continuous basis, in a Leistritz twin-screw extruder. 
Pre-distilled propylene carbonate was added to the pre-formed polymeric 
electrolyte during a second extrusion stage. A precise and reproducible 
level of addition (34.5% w/w) was achieved by means of a calibrated 
reciprocating pump. 
The conductivity of the polymeric electrolyte was 1.90.times.10.sup.-4 
ohm.sup.-1 cm.sup.-1 . 
Lithium metal was encapsulated as in example 4, prior to fabrication of 
electrochemical cells.