CA1273677A

CA1273677A - Polymeric component

Info

Publication number: CA1273677A
Application number: CA000513482A
Authority: CA
Inventors: Robert Hamilton Mcloughlin
Original assignee: Scimat Ltd
Current assignee: Scimat Ltd
Priority date: 1985-07-11
Filing date: 1986-07-10
Publication date: 1990-09-04
Anticipated expiration: 2007-09-04
Also published as: JPS6215753A; ATE53710T1; DE3672034D1; US4849144A; GB8517571D0; EP0219190B1; EP0219190A1

Abstract

Abstract POLYMERIC COMPONENT FOR USE AS BATTERY SEPARATOR

A polymeric component, for use as a separator in an electrochemical device in which the anode comprises metallic lithium, comprises a thermoplastic polymer, a tertiary plasticiser and a filler that comprises a lithium compound which is substantially insoluble in both the polymer and the plasticiser. The component is rendered porous by deformation which increases its sur-face area. The porosity can be enhanced by extracting one or both of the plasticiser and the filler.

Description

PROCESS FOR MAKING AN

ELECTROCHEMICAL DEVICE SEPARATOR
-The present invention relates to a process for making separator for use in an electrochemical device, such as an electrochemical cell which employs a lithium anode.

To minimise the ris~ of formation of internal short cirucits in electrochemical devices such as electroche-mical cells, it is common practice to position a thin porous layer of material between the electrodes as a separator. The porosity of the separator should be carefully controlled since it affect~ directly the internal resistance of the cell. Several methods are available for the manufacture of separators. For example, separators may be constructed from woven, meshed or otherwise interlaced fibres ~uch a~ glass fibres, or they may be made from sheets of polymeric material (produced for example by solvent casting or by extrusion) which can be rendered porous, either physi-cally for example by stretching or perforation, or che~
mically by removal of a component from the material.
For example separators for u~e in storage bQtteries which employ acid or alkaline electrolytes are disclo~ed in US Patent No. 3351495. The ~eparator~
comprise a polymer composition which contains a polyolefin, a plasticiser and inert filler material that is insoluble in the plastici~er. After forming the composition into a sheet, at least some of the filler and/or the plasticiser is extracted.

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The invention provides a process for making a separator for an electrochemical device, comprising the steps of blending together a thermoplastic polymer, a filler which comprises a lithium compound which is substantially insolub~e in the polymer and a tertiary plasticiser which i9 substantially immiscib~e in the polymer in the absence of the filler, the blend comprising from 1 to 50 parts by weight of plasticiser and from 150 to 350 parts by weight of filler per hundred parts of polymer; forming the b~end into a film; and stretching the film to render it porous.

The selection of a lithium compound as the inert filler of the component renders the separator suitable for use in many types o electrochemical device.
Hitherto, the problam of ensuring electrochemical com-patibility between the metal of the anode and the cationic component of a filler incorporated in the separator has been overcome by careful choice of a filler to be compatible with the cell component.
Separators of this invention may however be usad in many types of cell, without risk of reaction b~tween the anode material and the cationic component of the filler, an* are particularly attxactive for use in 1 lithium cells b~cause of the high energy density and high power output which they can offer.

The filler is selected to be in~oluble in the ther-moplastic polymer so that the filler particles remain substantially intact in the polymer matrix and create pore sites therein. Preferably, the filler is selected also to be insoluble in li~uid with which the separator come~ into contact when incorporated into a device, such as the electrolyte. Such an insoluble filler will not affect the phy~ical characteristics of the electro-lyte such as its viscosity, or conductivity, as can happen in ~.~7~ 7 RK27~ EPC

the case of separators containing soluble fillers such as soluble salts and soluble polymers.

The plasticiser used in the separator is a tertiary plasticiser. The term "tertiary plasticiser" encom-passes plasticisers which are substantially immiscible with the thermoplastic polymer in the absence of the filler. In the composition of the present separator, without being limited to any particular theory, it is believed that there is an interaction between the plasticiser and the surfaces of the filler particles which results in the filler particles being encap-sulated in a layer of the plasticiser. The material of the plasticiser will preferably be subqtantially electrochemically inert towards the other cell com-ponents and will not affect the other components physi-cally. Beneficial interactions are not, however, excluded; for example, the plasticiser may be relied on to improve the processability of the composition of the separa~or~

Porosity in the film may be achieved in any of a number of ways~ The coating of the plasticiser around the filler'particles modifies the interactions b~tween the filler and the ~urrounding polymer matrix, and allows pores to be created in the pore sites defined by the Eiller particles by deformation of the film, par-ticularly by stretching. In contrast to components consisting essentially of polymer and filler particles which are made porous solely by stretching to cause fibrillation of the polymer matrix, the provision of plasticiser allows greater control to be placed over the pore forming process. This can provide the signi-ficant advantage of a more reproducible electrochemical performance.

The extent to which the film is deformed depends on may factors, including, for example, the nature of the polymer, plasticiser and filler, whether the plasticiser and/or the ~iller are to be extracted from the pores, the required pore size etc. Generally, a high degree of deformation will bè
preferred, to create a high degree of fibrilla~ion and therefore a relatively high porosity; in practice, the degree of deformation will be as high as possible without causing physical damage, such as rupture, to the component. Preferably, the film is stretched so that the dimension in the direction of stretching increases by at least 50~, more preferably at least 100%, especially at least 250%, ideally at least 450~.
By thi~ process, the thickness of the film can be reduced by a factor of five or more. Deformation to such an extent is possib~e with film which comprises substantial amounts of filler, for example from 150 to 300 parts by weight of filler per hundred parts of thermoplastic polymer, without physical damage to the film. This surprising effect is b~lieved to arise from the presence of relatively small amount~ o~ tertiary plasticiser in the composition of the film, and thus allows relatively thin porous film5to be made by con-venient processe~.

As an alternative to the step of deforming the polymer, or in addition thereto, the porosity of the separator may be increased by extraction of at least some of the plasticiser or the filler or both from the separator. The extraction may be carried out before assembly of the cell using an independent solvent, or after as~embly of the cell by mean3 of liquid with which the separator come~ into contact when incor-porated into the cell, such as the electrolyte, or in the case of a lithium/oxyhalide cell, the liquid oxy-~ ~73~t~

RK27~ EPC

halide. It is pre~erred to extract the plasticisPrfrom a deformed film which results in the filler par-ticles being left in pores which are substantially larger than the particles themselves. This is believed to reduce significantly collapse of the pores when the separator is in use. Furthermore, thorouqh wetting of the separator by che elec-trolyte is believed to b~
enhanced by residual filler. It is advantageous to stretch the film prior to extraction of the plasticiser or filler since extraction from the elongated pores can be more efficient than that from the unstretched sheet.

From the above t it will be clear that the size of the pores in the film will b~ influenced to a large extent by the size of the filler particles. This fac-tor will also influence the minimum thickness of a separator that can be made from a particular film For many applications, it is preferred that the size of the filler particles is generally less than 20 micro-metres, preferably less than 15 micrometre~, more pre-ferably leq~ than 8 micrometres, especially less than 5 micrometres. The film preferably has a laminate f~rm.
For example it may have the form of a strip or a sheet.
Preferably the thicknes~ of the lamina prior to defor-mation i~ le~s than 450 micrometres, more preferably less than 350 micrometres, especially le~s than 150 micrometres. Surprisingly, despite relatively high filler loading~, such as between 150 and 300 parts b~
weight per hundred parts of polymer, it has been possible to form such thin films by melt processing techniques such as extrusion with satisfactory unifor-mity and homogeneity. After deformation, the film pre-ferably has a thickness of less than 75 micrometres, especially from 25 to 50 micrometrex.

y ~ It'--1 The thermoplastic polymer of the component may be a polymer of a compound with one or more polymerisable double bonds, or a condensation polymer of a condensable compound.
Useful polymer of compounds with polymerisable double bonds may be selected from polymers of ethyleni-cally unsaturated hydrocarbons having 2 to 12 carbonsl such as ethylene, propylene, n-hexylene, n-dodecene or 4-tert butylstyrene and of vinyl ethers such as methyl or ethyl vinyl ether. Preferred among these compounds are polyethylene and polypropylene due to their low cost.
Copolymers of the above monomeric compounds may also be used.
Useful condensation polymers may be selected from self-condensates of omega-amino-fatty acids and their lactams, such as condensation polymers from caprolactam and from 11-amino-undecanoic acid.
The condensation polymers can be polyamides of diamines having 6 to 9 carbons and dicarboxylic acids having 6 to lO carbons. Typical useful diamines include hexamethylenediamine, nonamethylenediamine and aryldiamines such as m- and p phenylenediamine. Typical useful dicarboxylic acids include adipic acid, suberic acid, azelaic acid, terephthalic acid and isophthalic acid. The preferred polyamide is the condensate of hexamethylenediamine and adipic acid, for reasons of general availability.

The condensation polymers can also be selected from polyesters of aryldicarboxylic acids such as phtha-lic, terephthalic and isophthalic acids and glycols ,. ..

73~7~
7 _ RK27 having 2 to 6 carbons, such as ethylene, butylene- and hexylene-glycols.

Useful polymers include:
Ethylene/tetrafluoroethylene copolymer (Tefzel Trade Mark) Ethylene/chlorotrifluoroethylene copolymer Poly(2-methylpropene) Polypropylene Polyethylene Poly(4-tert-butylstyrene) Poly(vinyl methyl ether) Poly(6-aminocaproic acid) Poly(11-aminoundecanoic acid) Poly(ethyleneterephthalate) Poly(decamethylene sebacamide) Poly(heptamethylene pimelamide) Poly(octamethylene suberamide) Poly(nonamethylene azelaamide) Poly(hexamethylene adipamide) Optionally, the polymeric composition may be cross-linked, for example, by means of a chemical cross-linking agent or physically by irradiation with high energy electrons.
According to the invention, it has been found that lithium compounds are most appropriate for use as the filler of the component, since they confer a versatility on the component, allowing it to be used as a separator in a range of electrochemical devices without risk of chemical degradati.on. Lithium salts are preferred, in particular, lithium salts which are insoluble in the cell electrolyte. Particularly preferred lithium salts include the carbonate, phosphate and aluminate, although ~ '; ,,,,,.,. - ; ., 3~i7`7 other lithium compounds may be used, such as the nitrate, sulphate, trifluoromethylsulphonate, tetrafluoroborate and other salts.
The selection of the plasticiser will generally be influenced by the materials chosen for the thermoplastic polymer and the filler, since the interactions between the plasticiser and the polymer and filler are of impor-tance. The choice of plasticiser may also be influenced by the cell electrolyte and the desired solu-bility of the plasticiser in the electrolyte.
Examples of tertiary plasticisers which may be used in the composition of the invention include ethylene carbonate, propylene carbonate (PC), ethylene glycol dimethylether ~DME), tetrahydrofuran t dimethylformamide and dimethylsulphoxide. Other plasticisers which may be used include triglyme, tetraglyme, and selected polyethylene oxides and polyethylene glycols. Solvents which are suitable for extracting the plasticiser, when that is desired, will be apparent to those skilled in the art. As an example, ethylene glycol dimethyl ether may be used as a solvent to extract propylene carbonate when used as a plasticiser.
It will be understood that in some circumstances, it will be appropriate to add certain other ingredients to the composition of the component, such as antioxi-dants, UV stabilisers, processing aids, dispersal aids, cross-linking agents and so on. It is particularly pre-ferred to add to the composition a dispersal aid, pre-ferably a salt of a fatty acid. The salt will generally be selected to be electrochemically compatible with other components which the separator will encounter when incorporated into a device. In the case of a device which comprises a lithium anode, the lithium salt of the ~ 3~ ~ RK27~

fatty acid will b~ preferred. ~tearates are the pre-ferred fatty acid salts. The stearate may for example b~ added directly to the polymer composition or pro-duced by reaction between the filler and s-tearic acid.
The quantity of the dispersal aid added to the com-position will be determined by the quantity of plasti-ciser. Preferably the ratio by weight of plasticiser to dispersal aid is from 1:1 to 15:1, more preferably from 3:1 to 12:1, especially about 10:1.

It is convenient to express the proportions of the major constituents of the composition in terms of parts b~ weight per hundred parts of thermoplastic polymer prior to extraction of one or more of the constituents, it is preferred that the composition comprises from one to 50, preferably from 5 to 15 parts by weight of plasticiser per hundred parts of polymer, and from 150 to 350 parts by weight of filler per hundred parts of polymer.

The composition may be b~ended by conventional polymer blending apparatus such as a twin screw extruder or a two-roll mill. As stated above, the com-ponent preferab~y takes the form of a thin strip or sheet; more preferably, it is made in this form by extrusion, although blow and comp-ression moulding tech-niques are alternatives. The optional step of sub sequently deforming the resulting strip may be relied on to obtain a component of desired thickness. Having b~en formed into a lamina of suitab~e thickness, the component may be cut into pieces of suitable size, or may b~ formed into a roll for ease of transportation and storage.

The component may be used to protect a lithium or other metal anode prior to assemb~y of an electrochemi-cal device, for example by encapsulation of the anode.

. . .

~ ~3~jt~ 7 ~K274 The u~e of a separator to protect reactive metal anode~
is di3clo~ed in EP-A-1435S2~ EP-A-143566 and EP-A-146246. Thu~, for example, the lithium and the component may be co-extruded and the lithium stretched to form a thin web while protected by, and in contact with, the component. Alternatively, the component may be extruded as a loose coating on to a lithium or other metal foil by pa~sing the foil through a crosqhead die on the extruder; the component may then be deformed to render it porous, either with or wlthout deformation of the metal.
Example~ of polymeric components, and details of their manufacture, will now be de~cribed. The conduc-tivity of each of the components was evaluated as follows, referring to the accompanying drawing which shows schematically, a conductivity cell:
A sample of a lamina of polymer 1 with connec-tion~ to the lithium 2 made by piece~ of nickel mesh 3 was sealed in the conductivity cell by 0-ring3 4. The specified electrolyte solution was added and the conduc-tance of the polymer wa measured on both side~ of the llthium usin~ electrodes 6 and conductance bridge~ 7,8.

. _ A porou~ polymeric component wa~ formed by melt blending, on a 2 roll mill, 100 parts, by weight, of a linear medium density polyethylene (Sclair 8405~U~rom Dupont~ with 200 parts, by weight, of lithium carbonate with had previouYly been ground and air cla~ified to a maximum particle ~ize o~ 5 micrometre~ After removal from the m~ll the blend was allowed to cool to room tem-perature prior to bein6 granulated and tumble blended, for 1 hour, with 30 parts, by weight, of propylene car-bonate.

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~ RK274 This material was fed into the feed section of a 32 mm (25:1 LD ratio) single screw extruder, operating at a temperature of 150C and melt extruded by a crosshead as a thin wall tube having a diameter of approximately 9mm.
After cooling, to below the crystalline melting tem-perature of the polyethelene, the tube was stretched, in the machine direction, by 450~.
A length of lithium foil, 5.84 mm wide by 0.51 mm thick was inserted inside the resulting thin wall, 35 micrometres thick, porous tube and its resistance measured using the conductivity cell as described above.
The specific resistance of the porous material was found to be 9.5 Qcm2 when measured in a 1 molar solution of LiCl04 in 50/50 PC/DME.

A compound mix manufactured and tested as pre-viously described but with 225 parts of Li2C03 and 32 parts of Triglyme per 100 parts of polyethylene had a specific resistance of 7~5Jlcm2.

A compound mix manufactured and tested as pre-viously described but with 250 parts of Li2C3, 35 parts of Triglyme and 3.5 parts of lithium stearate per lO0 parts of polyethylene had a specific resistance of 2.5 Q
cm2 when made with a wall thickness of 50 micrometres.

A compound mix, as described in example 3, was extruded as a loose coating on to a lithium foil by drawing the foil through the extruder crosshead. The coating was then made porous by stretching it in the 3~

machine direction without deforming the lithium, to a wall thickness of 35 micrometres. Its specific resistance, measured as previously described, was found to be 2.4fLcm2.

A compound mix manufactured and tested as described in example 3 but with 18 parts of propylene carbonate per 100 parts of polyethylene had a specific resistance of 6~Lcm2 when made with a wall thickness of 100 micrometres.

A compound mix, manufactured and tested as described in example 1 but with 350 pats of Li2C03 and 45 parts of Triglyme per 100 parts of polyethelene, was found to have a specific resistance of 2.5Q cm2 when made with a wall thickness of 200 micrometres.

Claims

The embodiments of the invention in which an exclusive property or priviledge is claimed are defined as follows:

1. A process for making a separator for an electrochemical device, comprising the steps of blending together a thermoplastic polymer, a filler which comprises a lithium compound which is substantially insoluble in the polymer and a tertiary plasticiser which is substan-tially immiscible in the polymer in the absence of the filler, the blend comprising from 1 to 50 parts by weight of plasticiser and from 150 to 350 parts by weight of filler per hundred parts of polymer; forming the blend into a film; and stretching the film to render it porous.

2. A process as claimed in claim 1, wherein the film is stretched by at least 100%.

3. A process as claimed in claim 1, wherein the film is stretched by at least 250%.

4. A process as claimed in claim 1, wherein the blend is formed into the film by extrusion.

5. A process as claimed in claim 1, further comprising the step of extracting at least some of the plasticiser from the film.

6. A process as claimed in claim 5, wherein the extraction step is performed after the stretching step.

7. A process as claimed in any one of claims 1, 2 or 4, further comprising the step of extracting at least some of the filler from the film.

8. A process as claimed in any one of claims 1, 2 or 4, wherein the thermoplastic polymer is a polyolefin.

9. A process as claimed in any one of claims 1, 2 or 4, wherein the thermoplastic polymer is polypropylene or polyethylene.

10. A process as claimed in any one of claims 1, 2 or 4, wherein the particle size of the filler is less than 8 micrometres.

11. A process as claimed in any one of claims 1, 2 or 4, wherein the filler comprises lithium carbonate, lithium phosphate, or lithium aluminate.

12. A process as claimed in claim 1, wherein the film is formed so as to protect a reactive metal anode.

13. A process as claimed in claim 12, wherein the film is formed so as to encapsulate a reactive metal anode.

14. A process as claimed in claim 13, wherein the reactive metal anode comprises lithium metal.

15. A process as claimed in claim 1, in which the film is applied to a body of reactive metal so as to protect it.

16. A process as claimed in claim 15, in which the film is applied to the body of reactive metal so as to encap-sulate it.