GB2181594A - Electrochemical devices - Google Patents

Abstract

An electrochemical device comprises a separator 20 in the form of a generally tubular wall having an inwardly folded portion which at least partially closes one end thereof, and a fluid material 27 which is bonded to the inwardly folded portion so as to close and seal the one end. A method for forming such separator includes a step of reducing the mechanical rigidity of the one end before it is inwardly folded. The separator 20 may be formed by winding a separator material on a mandrel. The one end of the tubular wall may be folded by a spinning action between the wall and a crimping means acting on the wall. The method may include clamping the tubular wall against the mandrel but it may be such as to allow the separator 20 to rotate if the frictional drag on the separator due to spinning becomes excessive. The pressure within the tubular wall may be reduced so as to assist inwardly folding. An electrochemical device comprising the separator 20 may be formed by placing an outer cell portion 30 and other electrochemical parts 32 pre-placed in the outer cell portion over the separator before or after the settable fluid material 27 has cured. <IMAGE>

Description

SPECIFICATION Electrochemical devices This invention relates to electrochemical devices which incorporate separators and to the manufacture ofthe devices and the separators.

The invention is particularly applicable to separators and to the manufacture of generallytubuiar separators for use in cylindrical electrochemical devices.

Many types of battery have cylindrical or columnar positive and negative electrodes disposed coaxially, with an intervening separator. Such separators are commonly in theform of a cylindrical tube, open at one end (thetop of the device or cell) and usually closed at the other end (the device or cell bottom).

Batteries which have the structure described have previously been constructed in general by moulding separator material in order to form a cylindrical, closed end structure. Typical examples are the formation of this structure by the extrusion of paper strips through a die, or by means of ultrasonic welding, gluing, or heat-sealing. Heat sealing has recently been employed to seal the lower end of the separator after twisting (Japanese Patent No.41690/1979). Another method of sealing the lower end of a cylindrical separator comprises clamping the ends of a con tinuouslywelded separator by means of a pair of jigs and welding closed the exposed ends of the separator (see British Patent Application No. 2139408A).

Further examples of closure of cylindrical battery separators includethewell-known closure utilised in commercially available dry (or Leclanch6) batteries and in some alkaline-zinc batteries which include mercury oxides as depolarizing agents, wherein the tubular separator is closed by the inclusion of a porous or non-porous plug of material, usually accompanied by some folding over of the separator material in order to provide some additional sealing properties. The plug is a simple push fit, for example of cardboard, polyolefine or nylon.It has also been proposed (Japanese Patent Publication 1983 154177)to insert the separator in the device or cell can and then applyfusible material to the bottom of the cell can so that it spreads into contact with the bottom end of the separator.

Although the previously mentioned methods are perfectly adequate for use in most cylindrical batteries, especially those where few soluble species are yielded, orthose electrochemical systems that do not produce conductive by-products as a result of discharge, it is obvious that separators which are not inherently sealed againstthe production of such species will suffer from potential contact between the electrodes, thereby yielding greatly shortened discharge lives. The problem is particularly serious in cells based on mercuric oxide, in which mercury metal is produced in operation.Furthermore, the methods described above wherein an active sealing ofthe separator is undertaken are dependentonthe thermal characteristics of such separator materials and require several undesirable extra manufacturing stages, including the assessment of the integrity of such a seal.

An object of the present invention is to provide a novel separator, a novel method offorming a separator, and apparatus for forming a separator so that the separator is capable of providing a reliable seal particularly against soluble species and conductive by-products formed in use, in a simple and inexpensive manner.

It is well known to use prefabricated tubular separators in the manufacture of cells. A serious problem however is that the separator materials are easily damaged and contaminated, and therefore are dif ficuitto handle safely. The handling and transfer of prefabricated separatortubes before and during assembly into cells, leads to waste and handling difficulties caused by the presence of damaged separators. Furthermore conventional methods of assembling cells with prefabricated separators tend to allow variations in the placing ofthe separators and other cell constituents in the cell.

Afurther object of the present invention is to pro vide a method a nd and apparatus for processing separ- ators and assembling them into electrochemical cells, capable of substantially eliminating damage to the separator and unwanted variations in placement of cell components.

According to the present invention, there is provided a separator for an electrochemical device, wherein the separator comprises a generallytubular wall having an inwardly folded portion which at least partially closes one end thereof, and a set fluid material which is bonded to the inwardly folded portion so as to close and seal the one end.

Thegenerallytubularwall of the separator may be formed from a fibrous material and/orfilm and ends of the fibres contained within the fibrous material and/orfilm are preferably embedded in the setfluid material.

The setfluid material is preferably a thermoplastic resin, for example, a polyamide having a softening point in the range of 500 to 3500C. Alternatively, the thermoplastic resin may be a polyethylene, a polypropylene, a polyalkene, a polystyrene or a poly(isobutylene).

Afurtheralternativethermoplastic resin which could be used is a copolymer based on ethyl orvinylacetate, or ethylene-vinyl acetate copolymer, etc.

According to another aspect ofthe present invention, there is provided a method of forming a separatorforan electrochemical device, the method comprising: providing a generally tubularwall having one end thereof inwardly folded to at least partially close the said one end thereof; and depositing a desired quantity of settable fluid material on the inwardlyfolded end so asto close and seal the one end.

The method of forming a separator preferably includes a step of reducing the mechanical rigidity of the one end prior to the inwardly folding.

The generallytubularwall may be formed bywinding a separator material on a mandrel. Alternatively, the method of forming the separator may include prefabricating the generallytubularwall, placing it on to a mandrel, and inwardly folding the said one endofthetubularwallonthemandrel.

Whilethetubularwall is on the mandrel,the tubularwall is preferably inwardly folded by a spinn ing action between the tubularwall and a crimping means acting onthetubularwall.

The method preferably includes clamping the gen erallytubularwall againstthe mandrel,theclamping being such asto restrain the generally tubular wall from rotating on the mandrel during the spinning action.

The clamping may be such as to allow the separatorto rotate on the mandrel if frictional drag exerted on the separator due to the spinning becomes great enough to damage the separator.

The method offorming the separator may include the step of reducing pressure within the tubularwall so as to assist the inwardlyfolding.

The mandrel, by supporting and closely fitting in the separator, protects the separatorfrom mechanical damage and handling until the separator is exter naliy protected by the said outer part of the cell.

Furthermore, the mandrel can accurately locate the separator relative to the said outer part of the cell, thereby providing very accurate location ofthe cell components relative to one another and effectively reducing or elminating waste space within the cell, so that the effective capacity of the cell can be enhanced.

According to the present invention there isfurther provided an apparatus for forming a separatorforan electrochemical device, the apparatus comprising: means for inwardly folding one end of a generally tubularwall to at least partially close the one end; and means for depositing a desired quantity of settable fluid material on the inwardlyfolded end so as to close and seal the one end.

The apparatus may include means for reducing the mechanical rigidity of the one end priorto the inwardlyfolding.

Theapparatus mayinclude meansforwinding a separator material onto a mandrel so as to form the generally tubularwall. Alternatively, the apparatus may include means for placing a prefabricated separatoron to a mandrel.

A crimping means is preferably provided for acting onthegenerallytubularwall,whenthewall is on the mandrel, and spinning relative thereto inwardlyfolding the one end ofthe generallytubularwall.

The apparatus preferably includes a clamping means for clamping the tubularwall againstthe mandrel thereby to restrain the wall from rotating on the mandrel when the crimping means is acting on the one end during spinning. The clamping means may be operative for allowing the separator to rotate on the mandrel iffrictional drag on the separatordueto the spinning becomes great enough to damage the separator.

The apparatus may include means for reducing pressure within the tubularwall so as to assistthe inwardly folding.

According to the present invention there isfurther provided a method offorming an electrochemical device comprising forming a separator according to the method defined above, and placing an outer cell portion and other electrochemical parts pre-placed in the outer cell portion overthe separator before or afterthe settablefluid material has cured.

During the process of manufacture of the separator and assembly ofthe cell, the separator is preferably supported by the same mandrel throughoutthe manufacturing steps.

According to the present invention there isfurther provided an apparatus for forming an electrochemical device comprising apparatus for forming a separator as defined above, and means for placing an outer cell portion and otherelectrochemical parts pre-placed in the outer cell portion over the separator before or after the settablefluid material has cured.

In the case where the outer cell portion and other electrochemical parts are placed overthe separator before the settable fluid material has cured, intimate contact of the separator with the bottom of the cell can be ensured when the separator is pressed onto the bottom ofthe cell can, curing or setting ofthe fluid material following in situ.The fluid material may also bond itself and therefore the separator to the cell bottom or other cell components, thereby accurately locating the separator and maintaining its shape during subsequent operations such as insertion offurthercell components. Ifthefluid material is applied to the separator and allowed to set or cure before the sealed separator is inserted in the celI,the set plug or layer of sealing material maintains the shape of the separator during insertion into the cell and can thus provide accurate centering of the separator in the cell.

Byvirtue of the use of the invention, the separator, electrodes and other cell components can be maintained in a closely contacted accurately positioned condition, with minimal distance between the electrodes and concomitant improvement in cell char acteristicsand performance.

Suitable separator materials are well known to those skilled in the art. The separator/barrier materials must be substantially dimensionally stable in contact with the electrolyte utilised in the cell. Furthermore, it is necessary that the settable fluid or other material which is used to form the seal at the lower end of the finished separator is also stable (both chemically and dimensionally) to both electrodes and electrolyte which are used in the cell, as well as being stable with respect to intimate contact with the separator material or combination of materials.

Furthermore, the materials used to form the separator preferably have low electrical resistance.

The separator will in most cases be generally cylindrical, for example in the form of a relatively long tube in a conventional cylindrical cell, or a relatively short ring in the case of a button cell.

Examples of absorbent separator materials (i.e.

fibres from which absorbent materials can be made) include polyvinyl- alcohol, polypropylene, polyethylene, rayon, polyamide etc. Furthermore, barrier materials where considered necessary may include microporous or semipermeable materials such as polethylene film, polypropylene film, ion exchange orfilter membranes (e.g. resins embedded in woven matrix), polysulfonefibres etc. The separator material may be laminted or non-laminated. The adhesives which are used can generally be described as acrylic materials and copolymers of acrylic materials with other moieties, and polyvinyl alcohol alone or copolymers thereof. More specifically, some useful materials are styrene acrylic copolymers, acetate emulsions, polyvinyl acetate stabilised with polyvinyl alcohol, and sodium salt acrylate solutions.

One specific material is Texicryl 13/011, which is avai lable from Scott Bader Ltd. of Well ingborough, England. Otherusefultrademarked materials are Vinacryl45l2,Vinamul 8450,orVinamul3253.

The present invention will be further described with reference to the accompanying drawings in which: Figures 1 to 9 show successive stages of separator manufacture and cell assembly, Figure lOshows the general layout of a machine for separator manufacture and cell assembly, Figure 11 shows a detail ofthe machine, and Figure 12iilustrates a modification of the machine for making separators.

Figure 1 shows a separator support 1,to be called a stool. This comprises a pedestal 2 by which it is mounted in a movable carrier 3, for example an indexing turntable by which the stool is moved in steps from each work position to the next.

Above the pedestal 2 is a seat 5, from which extends upwards a cylindrical mandrel 4. Below the pedestal 2 a pin 6 extends through the carrier 3. A coaxial air passage 7 extendsthrough the stool from the bottom ofthe pin to the tip of the mandrel, open- ing at one or more apertures in the end face of the mandrel or in the the side surface of the mandrel just below the end face.

The pedestal 2 is securely fastened to the carrier 3.

The stool can be a single integral body of metal or other suitable material, or can be fabricated from separate components. Preferably, the stool or at least the mandrel has a surface colour or finish different from that of a separator, so that the presence or absence of a separator on the mandrel can easily be detected.

The stool material should have adequate thermal capacity, corrosion resistance to the operating environment including the chemical constituents of the cells to be produced, and mechanical strength and durability. Preferred materials are steel, in particular stainless steel, and ceramic-based materials; plastics can also be used.

Figure 10 shows the general layout of a machine for making separators and partly assembling cells. A suitable chassis or framework carries the turntable 3 and any convenient form of indexing drive for this.

Around the turntable are the following successive work stations: separator supply and placement station 10, separator closing station 11, sealant application station 12, can feed and placement station 13, ejection station 14.

The machine operates as follows.

At the separator placement station 10, preformed and cut cylindrical separator tubes 20 are placed, automatically or by hand, on the mandrels 4 ofthe stools 1 (Figure 2). For automatic placement of the separators, one can use a bowl feed and suitable feed chutes.

Stepwise rotation of the turntable 3 carries the stools and separators through a gate 21 dimensioned to detect or block incorrectly placed separators, to the separator closing station. This comprises two stages. In the first stage, each separator is presented to a pre-forming tool 22 which is advanced into contact with the tip ofthe separatorandforms in one side of it, an initial fold 23. Forthis purpose, it will be understood that the mandrel is somewhat shorter than the separator so that the latter is unsupported at its upper end. The initial fold can be only a small deformation, its purpose to destroy the mechanical rigidity provided by the cylindrical shape of the separator and thereby facilitate subsequent complete folding of the separator end.

The carrier is then indexed furtherto present the separatortoa U-shapedclampfinger24which is advanced to press the lower end of the separator againstthe mandrel. While the separator is thus held bythe clamp finger against rotating on the mandrel, a spin crimping head 25 is lowered onto the prefolded upper end of the separator and spins, thereby folding the material of the separator inwards, to sub stantiallyclosethe upper end of the separator.Preferably, this folding does not completely close the separator end, but leaves a small opening at the centre ofthe folding end, to allow efficient penetration ofthe subsequently applied sealant fluid (which may be applied to the folded end by injection or may be moulded thereon) and enablethelattertobond thoroughly with the ends of the fibres of the separator material to form a perfect seal. The spinning crimp head 25 will exert a frictional pull on the separ ator, tending to make the separatorturn on the mandrel. The clamping force exerted by the finger 24 is limited in orderto allow the separator to rotate between the clamp finger and the mandrel, if thefrictional drag exerted by the spinning crimping head 25 is great enough to damage the separator if the latter were prevented from rotating.

In the illustrated machine, two separators are present at each operating station simultaneously, therefore there are two adjacent clamp mechanisms, two adjacent crimping and folding mechanisms, and so on.

From the separator closing station 11, the separator is indexed by theturntableto the sealing station 12,atwhich it is placed belowasealant nozzlefrom which a carefully metered small amount of hot-melt adhesive sealant is ejected downwards onto the folded upper end of the separator, forming a sealing bead 27. Because the folded separator end has a small remaining central opening, the fluid sealant can penetratethroughthis opening, making very intimate contact with the edge regions of the folded separator material, thoroughly filling folds and any interstices and perfectly embedding the fibres of the separator material inside and outside the end of the separator, thereby forming a perfect seal at the end of the separator.

Stools approaching the sealing station are monitored to ensure that they are carrying separators. If a stool is detected that is not carrying a separator, the corresponding sealent ejector nozzle 26 is prevented from ejecting sealant onto that mandrel. In one convenient arrangement, the presence or absence of separators is detected by photoelectric sensing means, the mandrel 4 having a colour sufficiently differentfrom that of the separator material to enable the sensing means reliablyto distinguish between a separator on a mandrel, and a bare mandrel. Preferably, the mandrels are black, separators commonly being white.

After application of the sealant, the turntable carries the separators to the can placement station 13, at which the cell outer assemblies 30, comprising a cell can together with the outer electrochemical constituents 32 ofthe cell pre-placed in the can, is placed over the separator held on the mandrel, as shown in Figure 8. The turntable is then indexed to carrythe cell outer assembly and separator to the ejection station 14. At this, an air ejector head 28 is raised into sealing engagement with the lower end ofthe pin 6 of the stool, and an air pulse is applied from the ejector head 28 through the air passage 7 to blowthe separator and cell assembly offthe stool, to a suitable ejection shute or conveyor. The empty stool then returns to the separator placement station 10 to repeat the production cycle.

Optionally, a test station may be provided between the sealant application station and the can placement station. The test station illustrated in Figure 7 may comprise a vacuum test head 29 similar to the ejector head 28. The vacuum test head 29 is raised into en gagementwith the lower end ofthe pen 6 and suction is applied to the interior ofthe separator underthe seal 27. Suitable sensing means determine whether or not there is any leakage through the separator seal. To permit such testing, the air passage 6 must open through one or more fixed apertures in the end face of the mandrel.

As shown in Figure 1 1,theturntable3 is supported on a ring of rollers 19, on a suitable bed plate 18 which also carry the various described operational stations. The turntable can be driven through one or more rollers 19, or separately.

Itwill readily be understood thatthe described method and machine supportthe separator, and therefore protect itfrom damage, throughout the operations involved in closing and sealing the separator and placing it in the cell.The separator is handled only to the extent necessary to feed itto the stool.Theriskofdamageduringprocessing is minimised by the design ofthe equipment in particular the design and operation of the clamp fingers 24 which on the one hand ensure accurate positioning ofthe separator relative to the crimping head, elim inatetheeffectsofinteractions between the crimping head and the separator material, and on the other hand eliminate distortion of the separator by allo wing ittospin in response to excess frictional drag from the crimping head 25.

The machine can be driven in any convenient manner, for example electrically, hydraulically or pneumatically, or by any desired combination of these drive means. Pneumatic operation is preferred forthe operation of machine components such as the preforming tools, clamping fingers and crimping head.

Because the cell outer assembly is placed on the separatorwhile the latter is on the mandrel, the relative positions of the separator and the cell outer assembly can be accurately controlled by proper choice of the dimensions of the mandrel relative to the cell outer assembly, and particularly in relation to the shape and dimensions ofthe cell can. In a preferred arrangement, the stool has, in the angle between the mandrel 4 and the seat 5, an annular bead orsim ilar configuration (not shown) designed to provide accurate positive location of the open end of the can relative to the mandrel and therefore relative to the separator.This provides particularly accurate control of the position of the separator in the cell and in particular, depth of insertion of the separator in the cell, thereby eliminating waste space within the cell and guaranteeing uniformity of placement and shape of separator, and thus the maximum internal volume of the assembled components and maximum cell capacity. That is, the stool acts as a positive stop forthe ceil buterassembly, by engaging eitherthe end ofthe can or the pre-placed cell outer material 32.

The described machine and method have a number of other advantages. For example,the initial fold 23 formed in the separator end tends to reduce the size ofthe hole left in the separator end by the spinning-crimping operation, and ensures that the hole is central.

The use of spinning forfolding the separator end provides simple operation together with reliablefolding producing a substantially smooth surface on the separator end. If the separator has been made by winding, the direction of spinning is preferably that in which the separator was wound (means being pro vided for orienting the separatortubes with a con sistentdirection of winding).

Preferably, the crimping head 25 executes a number of small bounces during spinning. This re duces any tendency to drag the separator round, or to tearthe separator material, and provides a part icularly smooth and stable fold. To assist folding, optionally a reduced pressure may be applied within the separatorthrough the hole 6. A hydrocheck unit can be provided to control the movement of the crimping head.

The sealant 37 may be allowed or caused to set hard before placement of the cell outer assembly. To accelerate setting, it may be placed in a cold air jet after leaving the sealing station. Alternatively, the sealant may be allowed to remain fluid or at least plastic, until after application of the cell outer assembly. This has the advantage that the cell can bottom will force the sealant into more intimate contact with the folded separator end, ensuring that sealant penetrates to the interior of the separator; furthermore the sealant can in this case also secure the separatorto the can bottom.

The sealing station 12 may be enclosed in a housing, to retainfumesorvapoursemitted by the sealant material. This housing may contain an inert atmosphere blanket, preferably nitrogen, in order to increase protection of personnel and to reduce degradation ofthe sealant material due to oxidation by the ambient atmosphere.

In one convenient arrangement, automatic placing of the cathode/can is by means means of a low resonance bowl feeder using a "C" chute configuration.

The cathode/cans are linearly fed to a stop position and selectively orientated by means of a rotary actuator: the cathode/cans may be transferred in correct alignment to the separatortubes by conventional hydraulic or pneumatic methods. Photoelectric devices, similarto those previously mentioned, control this assembly process, and no assembly will take place in the absence of a separatortube.

For particularly good retention of cell constituents, the separator material is preferably a three-layer laminate comprising inner and outer absorbent layers and an intervening barrier, and as described above, the separator is folded leaving a small central aperture which is sealed by the hot melt or other set table fluid material, which by virtue of the central hole makes intimate contact with the intermediate barrier layer of the separator material.

For less arduous requirements, other separator materials can be used; the central aperture can be omitted with sealant being placed on the folded end of the separator.

In the case ofthe machine described, the amount of settable fluid which may be dispensed can be varied depending on the part size but is controlled by nozzle size, pressure and time factors. The nozzle size can be 0.15 - 0.4 mm (preferably 0.15 mm),time of application is 1 Oo0msec.(preferably 5 to 25msec) and pressure ranges may be 5to 75 psi (preferably 18 to 25 psi). Most settable fluids can be dispensed by this method, e.g. those described herein above wherethe preferred temperature rangeof50-350C softening point is referred to, depending upon the substrates under consideration.

Byway of example only, the cylindrical tubular separator isformed from a triple laminate of polyvinyl alcohol(as absorbent)/polypropylene(as barrier)/-polyvinyl alcohol fibres, by means of a spin/ gluing technique.

In the case of an alkaline-zinc battery, by way of example, a positive material containing manganese dioxide/graphite, or mercury oxide/graphite, or mercury oxide/manganese oxide/graphite is moulded to make a cylindrical electrode, to be placed in the can, e.g. a nickel-plated mild steel can. A negative electrode of zinc/mercury amalgam/expander mix, together with an electrolyte, is added after insertion ofthe separator, either as a preformed gel orseparately, in order to form the active cell. The battery cell maythen be sealed in a conventional manner.

Numerous other variations are possible within the scope of the invention.

For example, the carrier, illustrated as a turntable, may alternatively provide for transport along a re ctilinear or other-shaped path, and may be integrated with other cell manufacturing processes including separator manufacture, manufacture of the outer cell assembly, insertion of inner cell components, and cell sealing.

The described machine uses separate pre-cut separatortubes. Alternatively, the machine may accept continuous feed of separator tubing, the separator tubing being cut to length before or after being fitted on the mandrels, within the operating cycle of the machine.

In a further alternative, the separator is made in the machine by wrapping on the mandrels, for example as shown in Figure 12.

In this embodiment, panels 35 of laminate material are prepared in the form of a tape roll 36. The separator panels are stripped from the carrier tape at a stripper head 37 at the edge of the turntable 3, resting at the end of a rubbing block 38, from which position they are picked up by the mandrels as the latter are carried past on rotation oftheturntable. Friction with the inner surface of the rubbing block 38 wraps the separator panels on the mandrels. The latter may be provided with longitudinal vacuum slots in the mandrel surfaces, to which a vacuum can be applied from asuitablevacuum manifold, to hold the separator panels 35 properly in place on the mandrel during wrapping. The separator tubes thus formed on the mandrels can be secured in any convenient way for example by means of adhesives.The mandrels carrying the wrapped separators are then indexed to the pre-folding and subsequent operating stations as already described. The direction of spin at the spincrimping head should be the same as the direction of winding ofthe separators on the mandrels.

To effect rotation of the mandrelsforwrapping the separator panels, pinions 39 connected to the mandrels, placed belowthe turntable, mesh with a stationary arcuate rack 40.

The invention is applicable to a wide variety of cell types, including Leclanché cells, lithium nonaqueous cells, organic electrolyte cells, alkaline/zinc cells, zinc/mercury oxide cells.

Hitherto, there has been a great deal of difficulty in reliably manufacturing cylindrical separators, and known separators either fail to provide adequate sealing at the closed end, or require relatively complex and expensive sealing procedures. The present in vention provides a reliable procedure for manufacturing separators with sealed ends, cheaply and at high production rates.

The preferred form of separator described herein comprises a cylindrical tube, folded inwards at its end leaving a small central hole, and sealed by afluid sealant placed in and around the hole. This form of separator has advantages in addition to ease of manufacture and efficient sealing. In particular, because the sealant extends over only a relatively small proportion of the area ofthe closed end (typically about one half of the diameter or less), the absorbent constituent or components of the separator are exposed to the electrolyte over the outer part of the folded end of the separator as well as atthe sides ofthe separator. This provides an efficientwicking action which assists the take up of the electrolyte.

Claims (31)

1. A separatorfor an electrochemical device, wherein the separator comprises a generally tubular wall having an inwardly folded portion which at least partially closes one end thereof, and a set fluid material which is bonded to the inwardly folded portion so as to close and seal the one end.

2. A separator according to Claim 1, wherein the generally tubularwall is formed from a fibrous material and/orfilm and ends offibres contained within the fibrous material and/or film are embedded in the setfluid material.

3. A separator according to Claim 1 or Claim 2, wherein the setfluid material is a thermoplastic resin.

4. A separator according to Claim 3, wherein the thermoplastic resin is a polyamide and has a softening point in the range 50"-350"C.

5. A separator according to Claim 3, wherein the thermoplastic resin is a polyethylene, a polypropylene, a polyalkene, a polystyrene or a polyisobutylene.

6. A separator according to Claim 3, wherein the thermoplastic resin is a copolymer based on ethyl or vinyl-acetate, or an ethylene-vinyl acetate copolymer.

7. A method offorming a separator for an electrochemical device, the method comprising: providing a generally tubular wall having one end thereof inwardly folded to at least partially close the said one end thereof; and depositing a desired quantity of settable fluid material on the inwardly folded end so as to close and seal the one end.

8. A method of forming a separator according to Claim 7, comprising a step of reducing the mechanical rigidity ofthe one end priorto the inwardlyfolding.

9. A method offorming a separator according to Claim7orClaim 8, comprising forming the generally tubularwall by winding a separator material on a mandrel.

10. A method offorming a separator according to Claim 7 or Claim 8, comprising prefabricating the generallytubularwall, placing iton to a mandrel, and inwardlyfolding the said one end ofthe tubularwall on the mandrel.

11. A method offorming a separator according to Claim 9 or Claim 10, wherein the one end of the gene rallytubularwall is inwardlyfolded,whilethetubular wall is on the mandrel, by a spinning action between thetubularwall andacrimping means acting onthe tubularwall.

12. A method of forming a separator according to Claim 11, comprising clamping the generally tubular wall againstthe mandrel, wherein the clamping is such as to restrain the generallytubularwall from rotating on the mandrel during the spinning.

13. A method offorming a separator according to Claim 12, wherein the clamping is such as to allow the separator to rotate on the mandrel if frictional drag exerted on the separator due to the spinning be comes great enough to damage the separator.

14. A method offorming a separator according to any one of Claims 7 to 13, comprising reducing pres surewithinthetubularwall so asto assist the in wardlyfolding.

15. Apparatusforforming a separator for an elec- trochemical device, the apparatus comprising: means for inwardly folding one end of a generally tubularwall to at least partially close the one end; and means for depositing a desired quantity of settable fluid material onthe inwardlyfolded end so as to close and seal the one end.

16. Apparatusforforming a separator according to Claim 15, comprising means for reducing the mechanical rigidity of the one end priorto the inwardlyfolding.

17. Apparatus forforming a separator according to Claim 15 or Claim 16, comprising meansforwinding a separator material on to a mandrel so as to form the generallytubularwall.

18. Apparatusforforming a separator according to Claim 15 or Claim 16, comprising meansforplacing a prefabricated separator on to a mandrel.

19. Apparatusforforming a separator according to Claim 17 or Claim 18, comprising crimping means foracting on the generallytubularwall, when onthe mandrel, and spinning relative thereto thereby inwardlyfolding the one end ofthe generally tubular wall.

20. Apparatusforforming a separator according to Claim 19, comprising clamping meansforclam pingthegenerallytubularwall against the mandrel thereby to restrain the generallytubularwall from rotating on the mandrel when the crimping means is acting on the one end during spinning.

21. Apparatus for forming a separator according to Claim 20, wherein the clamping means is operat ive for allowing the separatorto rotate on the mandrel if frictional drag exerted on the separator due to the spinning becomes great enough to damage the separator.

22. Apparatusforforming a separator according to any one of Claims 15 to 21, comprising means for reducing pressurewithin thetubularwall forassist- ing the inwardly folding.

23. A method of forming an electrochemical device comprising forming a separator according to the method defined in any one of Claims 7 to 14, and placing an outer cell portion and other electrochemical parts pre-placed in the outer cell portion overthe separator before or after the settable fluid material has cured.

24. A method offorming an electrical device according to claim 23, wherein the separator is on the same mandrel throughout the method steps defined in Claims 9 and 11 to 13 or Claims 10 to 13.

25. Apparatusforforming an electrochemical device comprising apparatus for forming a separator according to any one of Claims 1 to 22, and means for placing an outer cell portion and other electrochemical parts pre-placed in the outer cell portion over the separator before or after the settable fluid material has cured.

26. A separator substantially as described herein with reference to figures 1 to 9 of the accompanying drawings.

27. A method offorming a separator according to Claim 7, the method being substantially as described herein.

28. Apparatusforforming a separatorforanelectrochemical device substantially as described herein with reference to figure 10 and figure 11 or figures 10, 11 and 12 of the accompanying drawings.

29. A method offorming an electrochemical device according to Claim 23, the method being sub stantiallyas described herein.

30. A method of manufacturing an electrochemical device of generally cylindrical configuration, comprising forming atubularseparatoron a mandrel, placing in a cell container cell constituents forming the outer portion ofthe cell, inserting the separatorwithin the said cell constituents, placing within the separatorfurther cell constituents, and sealing the cell container, and in which the separator remains positioned on the mandrel during at least the later stages offorming the separator and during the insertion ofthe separator.

31. An apparatus for manufacturing an electrochemical device of generally cylindrical configuration, comprising means for forming a tubularsepar- atoron a mandrel, meansforplacing in a cell container cell constituents forming the outer portion of the cell, means for inserting the separator within the said cell constituents, means for placing within the separatorfurthercell constituents, and means for sealing the cell container, wherein when the apparatus is in use, the separator remains positioned on the mandrel during at leastthe later stages of for ming the separator and during the insertion ofthe separator.