GB2098636A

GB2098636A - Separator for electrochemical energy-storage units and a process for its manufacture

Info

Publication number: GB2098636A
Application number: GB8212165A
Authority: GB
Original assignee: Carl Freudenberg KG
Current assignee: Carl Freudenberg KG
Priority date: 1981-04-28
Filing date: 1982-04-27
Publication date: 1982-11-24
Also published as: FR2504734A1; DE3116738A1; GB2098636B; DE3116738C2; FR2504734B1

Abstract

Separator material for electrochemical energy-storage units, e.g. nickel-cadmium cells and lead batteries, consists essentially of an electrolyte-resistant nonwoven fabric made from hydrophobic synthetic fibres bonded without binder and has at least the surface of the fibres rendered hydrophilic by a plasma treatment. The separator material may be manufactured by the action of an electric gas discharge in an evacuated chamber on the bonded nonwoven fabric until an O:C atomic ratio of 0.3:1 to 0.4:1 on the fibre surface is reached.

Description

SPECIFICATION Separator for electrochemical energy-storage units and a process for its manufacture The invention relates to a separator for electrochemical energy-storage units, comprising an electrolyte-resistant nonwoven fabric made from synthetic fibres. The invention also relates to a process for manufacturing such a separator.

Nonwoven fabric separators are in themselves known. They all have various advantages and disadvantages. Separators can be classified as macroporous (average pore radius > 1 0 ,um) or microporous (average pore radius < 10,am).

Macroporous separators are required, for example in the manufacture of a nickel-cadmium cell. In this case, not only the positive electrode but also the negative electrode consists of a thin, rollable strip of the active mass. The separator material is placed between the two electrodes and rolled up with them. After the strip consisting of the positive and negative electrode and also the separator has been rolled up, the three-layer structure is inserted into a beaker-shaped housing which is then filled with an electrolytic liquid, the electrolyte being absorbed rapidly and in a sufficient quantity, mainly by the separator material used, and stored in its pores. The separator must be resistant to the particular electrolytic liquid used and at the same time also rollable and flexible, in order to be able to fit closely against the positive and negative electrode.

Although nonwoven fabric separators made from synthetic fibres are in themselves highly suitable as separator material owing to their good resistance to the electrolytic liquid and, at the same time, high flexibility, there are serious disadvantages because hydrophobic fibres frequently do not have the required electrolyte absorption capacity and the required retention capacity for the electrolytic liquid. If the separator materials are given a hydrophilic finish, there is a danger of the electrolytic liquid becoming contaminated by the customarily used wetting agents and of the life of the energy-storage unit being shortened as a result.

The present invention therefore seeks to provide an electrolyte-resistant separator made from a nonwoven fabric, which separator has the known good behavioural characteristics, in particular superior behaviour in respect of flexibility and in respect of the filter effect due to its labyrinthine structure, whilst avoiding the risk of foreign substances, for example surfactants, passing from the separator into the electrolytic liquid during use. It would be particularly desirable to improve the wettability and the electrolyte absorption capacity and electrolyte retention capacity of the separator considerably and to give the electrochemical energy-storage units a considerably extended life.

According to the invention there is provided separator material for electrochemical energy-storage units, consisting essentially of an electrolyte-resistant nonwoven fabric made from synthetic fibres bonded without binder and having labyrinthically arranged pores of radius from 0.2 to 50 ym, at least the surface of the fibres of the inherently hydrophobic nonwoven fabric having been rendered hydrophilic by a plasma treatment.

The invention also provides a process for the manufacture of a separator according to the invention wherein an electrolyte-resistant nonwoven fabric made from synthetic fibres of hydrophobic polymeric material bonded without binder and having labyrinthically arranged pores of radius from 0.2 to 50 ,um is passed at a pressure of from 1 0-2 to 10 mbar through an evacuated chamber and exposed there to a glow discharge which is generated by two electrodes connected to a constant-voltage generator or an alternating-current voltage or a high-frequency generator, the output power of the generator being varied according to the geometric arrangement of the electrodes.

The separator thus consists essentially of an electrolyte-resistant nonwoven fabric which is bonded without binder. Nonwoven fabrics which are bonded thermally, for example by spot-welding, are particularly suitable for this purpose. Thermoplastic fibres are advantageously used and are then bonded by the application of heat and/or pressure and thus impart the required tensile and tear strength.

The separator has labyrinthically arranged open pores of radius from 0.2 to 50 ,um, a pore radius of 10 ,um or more as a rule being sufficient to retain residues precipitated from the active electrode material. Particles which are precipitated in nickel-cadmium cells of conventional type are sufficiently large to become caught in these pores. The pore radius has to be matched to the particular electrode material and it is never more than 50 ym.

A microporous separator whose pores have a radius below 10 ,um is required for lead batteries of conventional type, so that dendritic lead crystal growth observed in charging and discharging processes is prevented. For this purpose very small labyrinthine pores are necessary.

Polyester fibres, polyolefin fibres, polycarbonate fibres and/or polysulphone fibres are usually employed as the synthetic fibres in the case of an acid electrolyte. For accumulators having alkaline electrolytes, separators made from polyamide, polyolefin and/or polysulphone fibres are usually employed.

Separators according to the invention thus admittedly consist of fibres of the type employed in known separators, but these fibres are bonded without binder in the separators according to the invention and they have markedly different wettability, electrolyte absorption capacity and electrolyte retention capacity as compared to the originally hydrophobic fibre material. Plasma-treated separators, which have thus become hydrophilic at least at the fibre surfaces, are excellently wettable by the electrolytic liquid.

U.S. Patent Specification 3,947,537 admittedly has also already disclosed a process in which the fibre surfaces of a nonwoven fabric are treated with a surface-active agent. According to German Offenlegungsschrift 2,543,149, hydrophilic substances for the fibres are added to the hydrophobic polymer. The two abovementioned nonwoven fabrics are however not suitable as separators or only partly suitable, because the introduction of additional chemicals disturbs the sensitive system of the electrochemical energy-storage units. Though the wettability is improved here, the known disturbance caused by the action of wetting agents and, if present, binders arises to a still greater extent. Separators made from nonwoven fabrics which are finished in this way are therefore unsuitable for practical use.

The nonwoven fabrics which are used in the separator material according to the invention, are rendered hydrophilic by a plasma treatment, based on the action of an electric gas discharge under reduced pressure, and do not contain any foreign components. The electric gas discharge is preferably carried out in air or in a gas whose molecules contain chemical elements having a high electronegativity. In addition to air and its constituents O2 and N2, carbon dioxide has proved particularly suitable for this purpose. It can be advantageous to increase the wettability by the electrolytic liquid still further by passing the nonwoven fabric, after the action of the electric gas discharge under reduced pressure, through a bath containing an unsaturated carboxylic acid, for example acrylic acid.

It has been found that the action of an electric gas discharge under reduced pressure in an atmosphere of air causes an increase in the oxygen content at the surface of the fibres of the nonwoven fabric. The electric gas discharge is preferably allowed to continue until the atomic ratio at the fibre surface of oxygen to carbon has reached about 0.3:1 to 0.4:1 (determined from the observed atom % of O2 divided by atom % of C).

In measuring this O:C atomic ratio, use is made of the ESCA principle (electron spectroscopy for chemical applications), which is described by D. T. Clark in Polymer Surfaces, Wiley, London 1978, page 310 et seq. and in the literature references cited there. This method is also suitable for very thin layers which are only a few ,um thick.

The treatment of the fibre surfaces in the nonwoven fabric is advantageously carried out by passing a continuous web of the material through an evacuated chamber, which contains air or a gas whole molecules consist of elements of high electronegativity, under greatly reduced pressure, the air or gas molecules being decomposed to their atomic constituents by an electron pulse or ion pulse. Atomic oxygen and nitrogen (in the case of air) or the other atomic constituents present react with the fibre surface, which becomes hydrophilic. The action of the electric gas discharge is preferably carried out until the O:C ratio indicated has been reached. The duration of the action is adapted to the particular end use.

The following Examples illustrate the invention.

EXAMPLE 1 A fibre web which has a weight of 80 g/m2 is produced, on a card, from polypropylene fibres which have a denier of 2 dtex and a staple length of 38 mm. This web is then spot-welded, the individual square welding spots having a side length of 0.48 mm and a square centimetre of the separator area containing 48 welding spots. The welded areas constitute about 11% of the total area of the separator.

Processing finishes on the fibre are removed by subsequent washing.

After drying, the sheet-like structure is subjected to an electric gas discharge under reduced pressure. The sample is passed through a vacuum chamber for this purpose. The vacuum chamber contains two electrodes, in between which the nonwoven fabric passes through, and it works at a pressure of 10-2 to 10 mbar. The pressure is set at 10-' mbar and can be varied by means of a control valve, through which just as much gas flows as is removed by the pump. The glow discharge is excited by means of a constant voltage generator or an alternating-current voltage or high-frequency generator.

The frequency in this example is 30 kHz. The output power of the generator can be varied with the geometric arrangement of the electrodes, and the treatment period can be varied between a few seconds and minutes depending on the treatment intensity and the degree of hydrophilisation desired.

Here, the treatment is carried out for 20 seconds at an output power of the generator of 200 W.

Distance between the electrodes: 45 mm, area of the electrodes: 100 cm2.

The sample (a round disc having a radius of 1 cm) is placed on the surface of a 30% strength aqueous KOH solution. A wetting time of 5 seconds is observed, whilst a counter sample, which has not been treated in the manner described, has a wetting time of more than 300 seconds.

This sample as subjected to ESCA investigations. ESCA (electron spectroscopy for chemical applications) is the method best suited to the investigation of surfaces of materials even in the case of very thin layers of only a few ,z thickness, as are obtained by the plasma treatment. Irradiation of the sample with monochromatic, soft X-rays knocks electrons out of the inner shells of atoms taking part in a chemical bond (photo-ionisation). It is possible to base inferences about chemical elements taking part in a bond and the nature of their chemical bond (for example double bond or single bond) on the energy of these electrons.

Spectra, forming Figures 1 and 2 of the accompanying drawings, of the untreated and of the plasma-treated non-woven fabric show the following: The plasma treatment increases the percentage content (atom %) of oxygen from 4.82% to 26.58%, which corresponds to an increase in the O:C ratio from 0.051:1 to 0.36:1. Essentially, oxygen has been stored. The exact investigation of the Cls signals permits inferences about the chemical structure of the groupings formed (see Figures 3 and 4 of the accompanying drawings). Whilst the untreated nonwoven fabric shows virtually only C-C and C-H bonds, the plasma treatment 5 additionally produces

groupings and there is a suggestion of EXAMPLE 2

groupings.

The procedure indicated in Example 1 is followed to produce a web having a weight per unit area of 85 g/m2 from polyolefin fibres, the fibre substance of which consists of 60% of polypropylene and 40% of polyethylene (% by weight) and which have a denier of 3.3 dtex and a staple length of 64 mm.

The web is spot-welded, the individual square welding spots having a side length of 0.30 mm and one square centimetre of the separator surface containing 64 welding spots. The welded areas constitute about 6% of the total area of the separator. Finishes on the fibre are removed by subsequent washing.

After drying, the nonwoven fabric is subjected to an electric gas discharge, in a manner which is described in Example 1, the vacuum chamber being filled with air under an operating pressure of 0.1 mbar. The nonwoven fabric is treated for 1 5 seconds at an output power of the generator of 1 00 W. The geometric arrangement of the electrodes is as in Example 1.

A test for the wettability corresponding to Example 1 gave a wetting time of 2 seconds in comparison to 250 seconds for an untreated sample.

An ESCA investigation has the result shown below in tabular form:

O:C ratio Asignment of Cls Untreated sample 0.08:1 C-C, C-H Plasma-treated sample 0.31:1 C-C, C-H O=C-O-, C=O I EXAMPLE 3 A web made from a fibre mixture of 50% of polypropylene fibres (2 dtex/38 mm) and 50% of a fibre which has a core of polypropylene and a sheath of polyethylene (3.3 dtex/64 mm), and having a weight per unit area of 55 g/m2 is bonded in an oven at a temperature of 1 45 C. Finishes on the fibre are removed by subsequent washing.

After drying, the nonwoven fabric is treated with an electric gas discharge in a C02-filled chamber under an operating pressure of 0.2 mbar.

The treatment is carried out for 20 seconds at an output power of the generator of 1 20 W. The geometric arrangement of the elctrodes is as in Example 1. A wetting time of 1 second results, in comparison to 200 seconds for an untreated sample.

An ESCA investigation has the result shown below in tabular form:

O:C ratio Assignment of Cls Untreated sample 0.12:1 C-C, C-H Plasma-treated sample 0.34:1 C-C, C-H C=O, O=C -0-, (-C-OH) EXAMPLE 4 A web which has a weight per unit area of 1 10 g/m2 and consists of 70% of polyamide (6, 6) fibres (1.7 dtex/40 mm) and 30% of polyamide fibres (3.3 dtex/40 mm), which contain 40% of polyamide 6,6 as a core and 40% of polyamide 6 as a sheath, is bonded in an oven at 2230 C. Processing finishes on the fibre are removed by washing.

After drying, the sheet-like structure is exposed to an electric gas discharge in an oxygen-filled chamber under an operating pressure of 0.15 mbar. The output power of the generator is 180 W and the treatment period is 10 seconds. The geometric arrangement of the electrodes is as in Example 1.

The wettability is 1 second, in comparison to 250 seconds for an untreated sample.

An ESCA investigation has the result shown below in tabular form:

O:C ratio Assignment of Cls Untreated sample 0.16:1 C-C, C-H, C=O Plasma-treated sample 0.30:1 C-C, C-H, C=O O=C=O, -C-OH EXAMPLE 5 A web which has a weight of 60 g/m2 and consists of continuous polypropylene filaments having a widely varying fibre diameter of about 10 ym is bonded by the action of heat and pressure. A treatment with an electric gas discharge is carried out for 1 5 seconds in an air-filled chamber under an operating pressure of 0.2 mbar at an output power of the generator of 80 W. The geometric arrangement of the electrodes is as in Example 1.

The wetting time in an alkaline electrolyte is 2 seconds, in comparison to over 300 seconds for an untreated sample. In 30% strength aqueous sulphuric acid, treatment times of 3 seconds are observed in comparison to more than 300 seconds for an untreated sample.

An ESCA investigation has the result shown below in tabular form:

O:C ratio Assignment of Cls Untreated sample 0.02:1 C-C, C-H Plasma-treated sample 0.38:1 C-C, C-H C=O, O=C-0 I -C-OH EXAMPLE 6 A web made from a fibre mixture which consists of 50% of polyester fibres (1.7 dtex/40 mm) and 50% of undrawn polyester fibres (1 ,7 dtex/40 mm), and having a weight per unit area of 100 g/m2 is welded over the whole surface between two calender rollers at 2050C. Finishes on the fibre are removed by subsequent washing.

After drying, a treatment with an electric gas discharge in an oxygen-filled chamber under an operating pressure of 0.1 5 mbar is carried out. The output power of the generator is 120 W and the duration of the treatment is 10 seconds. The geometric arrangement of the electrodes is as in Example 1.

On testing the wettability of 30% strength aqueous sulphuric acid, a wetting time of 1 second results, in comparison to more than 300 seconds for an untreated sample.

An ESCA investigation has the result shown below in tabular form:

O:C ratio Assignment of Cls Untreated sample 0.33:1 C-C, C-H, 7C-O, O=C-O I Plasma-treated sample 0.40:1 C-C, C-H, -C-O, O=C-O (enhanced) C=O EXAMPLE 7 Micro-fibres are produced from a 12% strength solution of a polycarbonate in dichloromethane by the electrodynamic spray process (German Auslegeschrift 2,620,399) and laid onto a nonwoven fabric conveyor belt which continuously passes close to the spray electrode. The spray field is 4 kV/cm at a temperature of 250C and a relative humidity in the spray space of 30%.The goods web thus produced is compacted in a separate process step until a pore radius distribution of 7 to 16 ,um has been reached.

The weight of the web is 1 60 g/m2. The web is exposed to an electric gas discharge in an air-filled chamber under an operating pressure of 0.1 mbar, the output of the generator being 200 W. The treatment lasts 20 seconds. The geometric arrangement of the electrodes is as in Example 1.

On testing the wettability by a 30% strength aqueous sulphuric acid solution, a wetting time of 60 seconds results, compared with more than 300 seconds for an untreated sample.

An ESCA investigation has the result shown below in tabular form:

O:C ratio Assignment of Cls Untreated sample 0.28:1 C-C, -C-H C=O, O=C-O I Plasma-treated sample 0.34:1 C-C, C-H C=O, O=C-O (enhanced) EXAMPLE 8 Micro-fibres are produced in accordance with Example 7 from a 12% strength solution of a polysulphone in dichloromethane by the electrodynamic spray process and laid on a web conveyor belt which continuously passes close to the spray electrode. The goods web is compacted in a separate process step until a pore radius distribution of 10 to 30 um has been reached. The weight of the microfibre web is 20 g/m2.The sheet-like structure is exposed to an electric gas discharge in an air-filled chamber under an operating pressure of 0.1 mbar, the output power of the generator being 120 W and the treatment period 10 seconds. The geometric arrangement of the elctrodes is the same as in Example 1.

On testing the wettability by 30% strength aqueous sulphuric acid, a wetting time of 12 seconds results, compared to more than 300 seconds for an untreated sample. The corresponding wetting times for 30% strength aqueous KOH solution are 10 seconds compared with more than 300 seconds for an untreated nonwoven fabric.

An ESCA investigation has the result shown below in tabular form:

Assignment Assignment O:C ratio Of Cls of S2p Untreated sample 0.16:1 C-C, C-H S(Vl) -C-O Plasma-treated sample 0.33:1 C-C, C-H (S(Vl), S-C --CC-O. C=O, S-H 1/ O=C~O

Claims

1. Separator material for electrochemical energy-storage units, consisting essentially of an electrolyte-resistant nonwoven fabric made from synthetic fibres bonded without binder and having labyrinthically arranged pores of radius from 0.2 to 50 ym, at least the surface of the fibres of the inherently hydrophobic nonwoven fabric having been rendered hydrophilic by a plasma treatment.

2. Separator material according to claim 1, in the form of a sheet-like structure which is made from electrolyte-resistant fibres of hydrophobic polymeric material but has hydrophilic properties without the addition of any kind of surface-active substances and the surfaces of whose fibres have an oxygen:carbon atomic ratio of 0.3:1 to 0.4:1.

3. Separator material according to claim 1 or 2, in which the fabric is bonded by spot-welding or other thermally-induced bonding.

4. Separator material according to any of claims 1 to 3, wherein the nonwoven fabric is made from polyolefin, polycarbonate, polyamide, polysulphone and/or polyester fibres.

5. A process for the manufacture of separator material according to claim 1 wherein an electrolyte-resistant non-woven fabric made from synthetic fibres of hydrophobic polymeric material bonded without binder and having labyrinthically arranged pores of radius from 0.2 to 50 ,um is passed at a pressure of from 10-2 to 10 mbar through an evacuated chamber and exposed there to a glow discharge which is generated by two electrodes connected to a constant-voltage generator or an alternating-current voltage or a high-frequency generator, the output power of the generator being varied according to the geometric arrangement of the electrodes.

6. A process according to claim 5, wherein the nonwoven fabric is after-treated with acrylic acid or other unsaturated carboxylic acid after the action of the glow discharge.

7. A process according to claim 5 or 6, wherein the nonwoven fabric is exposed for at least 20 seconds to a plasma treatment, using a generator of which the output power is 200 W and electrodes having a separation of 4.5 cm and an area of 100 cm2.

8. A process according to any of claims 5 to 7, wherein the treatment chamber contains air or a gas whose molecules have chemical elements of high electronegativity.

9. A process according to any of claims 5 to 8, wherein the glow discharge is caused to act on the nonwoven fabric until its fibres have an oxygen:carbon atomic ratio of 0.3:1 to 0.4:1.

10. A process for the manufacture of separator material according to claim 1 carried out substantially as hereinbefore described or exemplified.

11. Separator material for electrochemical energy-storage units when manufactured by a process according to any of claims 5 to 10.

12. A nickel-cadmium cell containing a separator material according to any of claims 1 to 4 or 11 but with pores of radius at least 10 ,um as separator between its electrodes.

13. A lead battery containing a separator material according to any of claims 1 to 4 or 1 1 but with pores of radius below 10 ,um as separator between its electrodes.