CA1293018C - Microporous films - Google Patents

Abstract

ABSTRACT

The invention provides microporous polymer films which comprise a halopolymer in which the repeat units are -(Cn H2n)- and -(Cm X2m)-, where each X indepen_ dently represents fluorine or chlorine and the values of n and m are more than one and less than six. The films of the invention can be suitable for use in high tem-perature applications and can be substantially inert to certain chemically aggressive substances such as alkali and alkaline earth metals. The films find particular application in electrochemical cells comprising a lithium anode and thionyl chloride electrolyte.

Description

1'~93~i8 Microporous Films The present invention relates to microporous polymer films.
Microporous films are used in a wide range of applica-tions, generally to provide a selective barrier. For example they may be used as battery separators, ion-exchange membra-nes, electrolysis membranes as well as in breathable fabrics and medical and packaging applications. Commonly used poly-meric films comprise polyolefins such as polyethylene and polypropylene which can conveniently be made porous by extraction of a soluble component. Such films are chemically inert towards many acids and alkalis and towards many reac-tive metals. However, there exist a number of solvents with which a polyolefin film cannot be used because of chemical incompatibility. Furthermore, the maximum operating tem-perature of polyolefin films is about 120C, and their use in high temperature applications is therefore not possible.
The present invention provides a microporous polymeric film which comprises a halopolymer. Such films can be suitable for use in high temperature applications and can be substantially inert to certain chemically aggressive substan-ces, for example alkali and alkaline earth metals.
In one aspect, the invention provides a microporous polymeric film comprising a halopolymer in which the repeat units are ~tCn H2n)- and ~tCm X2m)-, where each X indepen-dently represents fluorine or chlorine and the values of n and m are greater than one and less than six, the film having a porosity of not less than 20~ by volume.
Preferably, the film comprises a copolymer, for example one which comprises ethylene and tetrafluoroethylene as the ~k ~ ~:93~18 monomer units, although chloroethylenes and fluorochloro-ethylenes can however also be used as the monomer units. In another embodiment, the film may comprise a copolymer that comprises longer chain monomer units such as propylene, buty-lene and halogenated analogues thereof. Paricularly pre-ferred halopolymers for use in the invention are those sold under the Trade Marks Tefzel (ethylene/tetrafluoroethylene) and Halar (ethylene/chlorotrifluoroethylene).
The term "film" is used to denote a non-fibrous self-supporting sheet. A microporous film is a porous film in which the details of pore configuration and/or arrangement are discernable only by microscopic examination. Preferably the pores or open cells in the films are smaller than those which can be seen using an optical microscope, when electron microscopy techniques may be used to resolve details of the pore structure. Generally, the maximum dimension of a substantial number of the pores will be less than 5 microme-ters, preferably less than 2 micrometers, measured by mercury intrusion porosimetry according to ASTM D 2873-70.
The porosity of the films may advantageously be not less than 30~, preferably not less than 40% by volume, for example from 40 to 50~ by volume, measured by mercury intrusion poro-simetry, again according to ASTM D2873-70.
Microporous films of the halopolymers defined above have chemical and physical properties which are advantageous for use in a variety of high performance applications, such as battery separators, ion-exchange membranes and electrolysis membranes, as well as for less demanding applications such as in breathable fabrics, and in packaging and medical applica-tions.
A significant advantage of the microporous film of the invention, is that it can be used in high temperature appli-cations. For example a film formed from Tefzel may be used 12~3(~18 _ 3 _ RK259 at temperatures up to at least about 175C without signifi-cant change in dimensions or porosity. The superior high temperature performance of the film of the invention allows them to be used in high temperature applicationS, for example, high temperature electrochemical cells where pre-viously used microporous films cannot function.
In accordance with the invention, films can be produced which are chemically inert towards reactive metals commonly used as anodes in electrochemical cells, such as for example metals of Groups I and II of the periodic table. This pro-perty of the films is surprising in view of the reactivity, towards lithium and sodium (at least), of the well-known halogenated polymers polyvinylidene fluoride (PVF2) and poly-tetrafluoroethylene (PTFE).
The films of the invention can also be chemically inert towards many aggressive liquids found for example in electrochemical cells, electrolysis cells and in other appli-cations. Thus, the preferred films of the invention are inert towards acids and alkalis, as well as towards reactive fluids such as oxyhalides of elements of Group VA and Group VIA of the periodic table (as published in The Condensed Chemical Dictionary, 9th Edition, Van Nostrand Reinhold, 1977), for example thionyl chloride, sulphuryl chloride and phosphoryl chloride. The films can therefore be used in many applications where the use of cumbersome non-woven glass fibre mats has previously been unavoidable, and a significant saving in size and weight may also thereby by obtained. An example of such an application is as a separator in a lithium/thionyl chloride cell. Accordingly, in another f aspect, the invention provides an electrochemica~ cel~/in which the separator comprises a microporous film which comprises a halopolymer in which the repeat units are -(Cn H2n)~ and ~(Cm X2m)-, where each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six.

~Z93~18 _ 4 _ RK259 In a further aspect, the invention provides a method of making a polymeric film having a porosity of not less than 20~ which comprises providing a film of a polymer composition comprising:
(a) a halopolymer in which the repeat units are ~(Cn H2n)- and -(Cm X2m)-, where each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six;
(b) at least one extractable component which is substantially insoluble in the halopolymer;
and subsequently extracting at least some of the extractable component so as to render the film microporous.
The method enables microporous films of halopolymers to be made conveniently. By careful selection of the extrac-table components, a porosity of not less than 30~ by volume, for example not less than 40~ can be achieved. Preferably the polymer composition comprises an extractable salt and an extractable polymer. The extractable salt may be present in an amount of from 10 to 150 parts per 100 parts of the halo-polymer, preferably from 50 to 120 parts, especially from 95 to 105 parts. The extractable polymer may be present in an amount of not more than 60 parts per 100 parts of the halopo-lymer, preferably from 5 to 30 parts, especially from 20 to 25 parts. The proportions of the components of the com-position may be varied, depending on such factors as the desired porosity of the film, the desired porosity profile through the thickness of the film, the size of the pores, the nature and chemical compatibilities of the components.
A high porosity can be achieved using a combination of extractable polymer and salt, and although the reasons for this are not fully understood, it would appear that the extractable polymer makes easier the extraction of the salt ~s3als 5 _ RK259 from the halopolymer matrix, possibly by acting as a wetting agent. Furthermore, as shown by scanning electron micrographs of a film produced by the preferred method, the presence of the extractable polymer appears to increase the surface porosity of the film compared with the porosity of films prepared by extraction of a salt alone, in which latter films the surface porosity limits performance significantly.
Preferably the method includes the step of deforming the film so as to reduce its thickness prior to extraction of the extractable component. The film may be deformed by up to 25~, up to 50~, up to 85~ or by more, depending on for example the dimensions of the film, the desired nature of the pores, the nature of the halopolymer and the extractable com-ponents. The deformation is preferably carried out using rollers, for example nip rollers in line with an extrusion die, although other techniques including stretching of the film may also be used. Deformation of the film can increase the efficiency of the extraction step and can also affect the nature of the pores; for example passing the film though nip rollers can affect the tortuosity of the pores.
In another aspect, the invention provides a polymer com-position which comprises:
a) a halopolymer in which the repeat units are -(Cn H2n)-and ~(Cm X2m)-, where each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six;
b) an extractable salt;
c) an extractable polymer For some applications, it will be acceptable for some of the extractable polymer or the salt or both to remain in the halopolymer matrix after extraction. Generally however, it is preferred that substantially alI of the polymer and salt are extraeted from the matrix, since the porosity is thereby maximised.
It is particularly preferred that the extraetable polymer and the salt are selected to be soluble in one solvent.
This makes more convenient the extraction of polymer and salt since significantly fewer extractions need be performed. For convenience, the polymer and salt will be selected to be soluble in an aqueous solvent, such as water or an aqueous acid solution. Other solvents may, however, be selected. In some applications, the extracting solvent may be a liquid with which the film comes into contact when in use, such as for example the electrolyte of an electrochemical cell.
The extractable polymer is selected to have a solubility in the extracting solvent that is significantly higher than the solubility of the halopolymer. When water or another aqueous based solvent is selected as the solvent, the extrac-table polymer may be selected from the following list ~which is not exhaustive):
- alkylene oxide homo- and copolymers.
- vinyl alcohol homo- and copolymers.
- vinyl pyrrolidone homo- and copolymers.
- acrylic acid homo- and copolymers.
- methacrylic acid homo- and copolymers.
Certain naturally occurring polymers such as poly-saeeharides may also be used as the extraetable polymer com-ponent in certain applieations.
Particularly preferred materials are ethylene oxide polymers, sueh as that sold under the Trade Mark Polyox. The use of ethylene oxide polymers as the extractable polymer is advantageous since they are water soluble and melt-processable.
The extractable salt should be selected according to the end use of the porous film, since at least a small amount of lZ~3~18 7 _ RK259 the salt is likely to remain in the film after extraction, and any remaining salt must be chemically compatible with other materials with which the film comes into contact when in use. For example, if the film is to be used as a separa-tor in an electrochemical cell which has a reactive metal anode, the extractable salt should be electrochemically com-patible with other cell components. Thus the salt should be of a metal which is at least as electropositive as the metal of the anode; for example when the film is to be used as a separator in a lithium cell, the salt should be a lithium salt; preferred lithium salts include the carbonate, chloride, phosphate and aluminate, although other lithium salts may be used including the nitrate, sulphate, trifluoro-methylsulphonate and tetrafluoroborate.
It will be understood that in some circumstances, it will be appropriate to add other components to the polymer composition such as antioxidants, UV stabilisers, processing aids, dispersal aids, cross-linking agents, prorads and/or antirads and so on.
The components of the film may be blended using conven-tional polymer blending apparatus such as a twin-screw extruder or a two-roll mill. The film is preferably formed as a thin strip of sheet, and it may be made in this form by a melt processing technique, for example by extrusion, although blow and compression moulding techniques are example of alternative techniques which may be used. Melt processing techniques are preferred since they allow films to be made with consistent properties and permit the production of thin films. Furthermore, melt processing techniques allow a film to be made continuously. The film may be extruded onto, or coextruded with, another component with which it will be in contact when in use. Once formed, the film may be cut into pieces of suitable size, or it may be formed into a roll, for ease of transportation and storage.

1;~93(~18 It is particularly surprising that the defined halopoly-mers can be blended with the preferred extractable polymers mentioned above using melt processing techniques, in view of the wide difference between the melting points of the two classes of materials. In some cases, care may be necessary in selecting quantities and mixing conditions to avoid degra-dation of the extractable polymer and halopolymer.
For certain applications, i~ may be advantageous to cross-link the polymer of the film. This may be effected by irradiation, for example by electrons, gamma or radiation, or by use of a chemical cross-linking agent.
The chosen final thickness of the film is dependent on the end use, and factors such as the desired strength, flexi-bility, barrier properties and so on will generally have to be considered. The materials of the film may be produced to a thickness of less than 75 micrometres, preferably by less than 50 micrometres. Most preferred films for use as a bat-tery separator have a thickness of less than 35 micrometres, for example between 20 and 30 micrometres.
In yet another aspect, the invention provides a method of making an electrode separator suitable for use in an electrochemical cell which comprises a reacti~e metal anode, the method comprising extruding onto, or coextruding with, another component of the cell a film of a polymer composition which comprises a halopolymer in which the repeat units are ~(Cn H2n)~ and ~(Cm X2m)-, where each X independently repre-sents fluorine or chlorine, and the values of n and m are greater than one and less than six. Preferably the extruded film will contain an extractable component which can be extracted.
The microporous film of the invention may be used to protect sensitive material for greater convenience during handling. For example it may be used to protect sensitive 3(~18 electrode materlal such as reactlve metal anodes, for example llthlum anodes, as dlsclosed In EP-A-143562.

The Inventlon wlll now be descrlbed wlth reference to examp I es, and to the accompanylng drawlngs in wh I ch:-Figure 1 Is a graph showlng how the ultlmate elongatlon of certaln polymerlc materlals Is affected by exposure to a solu-tlon of LIAIC14 In SOC12; and Flgure 2 shows a test cell as used to measure the elec-trlcal reslstance of a cell separator.

Example 1 Non-porous samples of ethylene/tetrafluoroethylene copolymer (TEFZEL 210), ethylene/chlorotrlfluoroethylene copolymer (HALAR 300), polyvlnylldene ~luorlde (KYNAR 460 - Trade Mark) and polyethylene (GULF 9606 Trade Mark) were malntalned In a 1.9 M
solutlon of LIAIC14 I n SOC/2 under reflux, and the effect on the ultImate elongatlon of the samples was monltored. The results are shown In Flgure 1, expressed In terms of the percentage of the Inltlal ultImate elongatlon whlch was retalned after the exposures. The measurements were made at room temperature.

Samples of TEFZEL210 and HAlAR 300 were also malntalned In a 1.9 M solutlon of LIAIC14 In thlonyl chlorlde (SOC12) at room temperature. The effect of the treatment on the mechanlcal prop-ertles of the samples Is shown In Table 1, the results belng expressed as In Flgure 1.
Table 1:
(a) 8 weeks at room temperature:

~ _ g _ 1;~93~i8 % Retalned % Retalned Tenslle Strength Ult. Elongatlon _ - 9a -'~

~293C~18 (b) 32 hours under reflux at 850C:

~Retained % Retained Tensile Strength Ult. Elongation ..
Tefzel 210 9 77 Halar 300 61 57 Example 2 The stability of polytetrafluoroethylene (Zitex G110 -Trade Mark), Kynar 460, Halar 300 and Tefzel 210 towards lithium metal was evaluated at elevated temperature.
Samples of the four polymeric materials were dried by placing them in a vacuum oven at 150C for 8 hours. The samples were then maintained in contact under pressure with lithium in a dry environment at 850C for 16 days. The following results were obtained on inspection:
Tefzel 210: ~o change in the appearance of lithium or of Tefzel.
Halar 300: Lithium blackened over approximately 80~ of the surface which contacted the polymer sample.
Kynar 460: Lithium blackened over approximately 95% of the surface which contacted the polymer sample.
Zitex G110: Lithium blackened over the entire surface which contacted the polymer sample, and bonded weakly to the sample.

~3~1 8 ~ RK259 Example 3 Ethylene/tetrafluoroethylene copolymer (Tefzel 210), lithium carbonate and polyethylene oxide (Polyox WSR 301 - Trade Mark) were compounded using a Baker Perkins twin screw extruder to give a blend containing 100 parts Tefzel, 100 parts lithium carbonate and 22 parts Polyox. The compound was further extruded using a Leistritz single screw extruder to produce a film of thickness ~.2 mm which was rolled using rollers at a temperature in the range 140-220 C to produce film having a thickness of approximately 27 micrometres.
This thinned film was then treated with concentrated HCl at room temperature (c. 23C) to remove the lithium carbonate and Polyox leaving a microporous web of Tefzel. The excess acid and reaction products were removed by washing with distilled water prior to drying of the ~ilm. The porosity of the resulting film, determined according to ASTM D2873-70, was found to be 45~.
Example 4:
The electrical resistance of a porous Tefzel film, pre-pared by the method described in Example 3, was measured in three different electrolytes using a test cell as shown in Figure 2 of the accompanying drawings. As shown in Figure 2, the cell comprises a container 1 incorporating electrodes 2 of stainless steel connected to a C conductance bridge 3. A
sample 4 to be tested is sealed into the test chamber 5 by sealing gaskets 6, and the necessary conductive liquid is introduced via feed ports 7.
The results are given in Table 3:

1~93~18 Table 3:
. _ Electrolyte Resistance at 30C
(ohm cm2) 1.9 Molar LiAlC14 in SOCl2 4 1.0 Molar LiCl04 in 50/50 7 Propylene carbonate 1,2-dimethyoxyethane 30~ ww KOH in water 0.45 Example 5:
Tefzel 210 and lithium chloride in the ratio 40:60 were blended using a Leistritz twin screw extruder. The resulting blend was then re-extruded to produce a web having a width 40 cm and thickness 0.5 mm. The thickness of the web was then reduced to 0.15 mm by passing it through the nips of a calen-dar roller stack. The web was soaked in water to leach out the lithium chloride, and then dried thoroughly. The poro-sity of the film was found to be 48% by volume.
The film was used as a separator in a test cell as described above, in which the electrolyte was 1.9 M LiAlCl4 in SOCl2. The resistance of the film was measured using a conductivity bridge, and found to be 161 ohm cm2 at room tem-perature.

Claims (10)

1. An electrochemical cell having a separator, wherein the separator comprises a porous film comprising a halopolymer in which the repeating units are -(CnH2n)- and -(CmX2m)- wherein each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, said cell further comprising a reactive metal anode.

2. An electrochemical cell according to Claim 1, wherein the anode comprises lithium.

3. An electrochemical cell according to Claim 1, wherein the electrolyte comprises an oxyhalide of an element of group VA
or VIA.

4. An electrochemical cell according to Claim 3, wherein the electrolyte comprises thionyl chloride, sulfuryl chloride or phosphoryl chloride.

5. An electrochemical cell as claimed in Claim 1, 2 or 3, wherein the halopolymer is an ethylene/chlorotrifluoroethylene copolymer.

6. An electrochemical cell as claimed in Claim 1, 2, or 3, wherein the halopolymer is an ethylene/tetrafluoroethylene copolymer.

7. An electrochemical cell as claimed in Claim 1, 2 or 3, wherein the separator consists essentially of said halopolymer film which is microporous and has a porosity of not less than 20 % by volume.

8. An electrochemical cell as claimed in Claim 1, comprising a container having two electrically isolated terminals; the container having therein said anode connected to one terminal, a cathode connected to the other terminal, a fluid electrolyte, an ionizable solute dissolved in said electrolyte, and said separator positioned between and in contact with the anode and the cathode, the porous film of the separator being a substantially continuous microporous film.

9. An electrochemical cell as claimed in Claim 1, 2 or 3, wherein the anode is in physical contact with the separator.

10. An electrochemical cell as claimed in Claim 1, 2 or 3, having an elevated operating temperature of up to about 175°C.