NON-WOVEN MATERIAL WITH PARTICLE FILLING

Abstract

A ply includes a fibrous nonwoven web fabric forming a foundational structure, wherein the foundational structure includes fibers forming first pores and is partially filled with particles, wherein the particles at least partially fill the first pores so as to form regions filled with particles, wherein the particles in the filled regions form second pores such that an average diameter of the particles is greater than an average pore size of more than 50% of the second pores.

Description

This application is a U.S. National Phase Application under 35 U.S.C. §371 of International Application No. PCT/EP2008/004824, filed on Jun. 16, 2008, and claiming priority to German Application No. DE 10 2007 042 554.8, filed on Sep. 7, 2007. The International Application was published in German on Mar. 19, 2009 as WO 2009/033514 under PCT article 21 (2).

This invention relates to a ply having a foundational structure composed of a fibrous nonwoven web fabric, the foundational structure consisting of fibers and having first pores formed by the fibers, the foundational structure being at least partially filled with particles, which particles at least partially fill the first pores and form regions filled with particles.

BACKGROUND

Plies of the type mentioned are already known from the prior art. Such plies are used as separators in batteries and capacitors in energy storage duty. Charge storage in batteries and capacitors takes place chemically, physically or in a mixed form, for example by chemisorption.

To avoid an internal discharge within the battery or capacitor, oppositely charged electrodes are separated from each other mechanically by means of materials which do not conduct electrons and are known as separators or spacers. At the same time, by virtue of their porosity being conformed to the energy storage system and its use, the separators or spacers make it possible for ionic charge-carriers of an electrolyte to move between the electrodes.

The separators known from the prior art have small, interlinked openings in the micrometer range. These openings are said to be as large as possible in order that electrolyte conductivity in the drenched separator be as high as possible and the battery thus have a high power density. However, if the openings are too large, then metal dendrites can lead to a short circuit between the two electrodes which are actually to be electrically separated from each other. The metal dendrites consist either of lithium or of other metals which can be present in the battery as impurities.

Furthermore, particles of electrically conductive electrode materials can migrate through the openings. These processes can give rise to a short circuit between the electrodes and greatly speed the self-discharging of the battery or capacitor.

A short circuit can result in the local flow of very high currents, which releases heat. This heat can cause the separator to melt, which in turn can lead to a distinct decrease in the insulating/isolating effect of the separator. A very rapidly self-discharging battery consequently constitutes a high safety risk because of its high energy content and also the combustibility of the electrolyte and of other constituents.

A further disadvantage with separators known from the prior art is their lack of stability in the event of rising temperatures. The melting point is around 130° C. when polyethylene is used and around 150° C. when polypropylene is used.

Causes of short circuits include shrinkage of the separator due to excessive high temperature in the battery, metal dendrite growth due to reduction of metal ions (lithium, iron, manganese or other metallic impurities), debris from electrode particles, cutting debris or broken covering on electrodes, and direct contact between the two flat electrodes under pressure.

EP 0 892 448 A2 discloses the shutdown mechanism. The shutdown mechanism responds to local heating, for example due to a short circuit, by counteracting the aerial spreading of the short circuit by prohibiting ion migration in the vicinity of the initial short circuit. The heat loss due to the short circuit causes polyethylene to heat up to such an extent that it will melt and blind the pores of the separator. Polypropylene, which has a higher melting point, stays mechanically intact.

US 2002/0168569 A1 describes the construction of a separator consisting of polyvinyl difluoride which, in the manufacturing operation, is incipiently solubilized with a solvent, mixed with silica particles and applied as a thin film. Removing the solvent leaves a porous membrane.

WO 2006/068428 A1 describes the production of separators for lithium ion batteries by using a polyolefin separator which is additionally filled with gellike polymers and inorganic particles.

WO 2004/021475 A1 describes the use of ceramic particles which are combined with organosilicon adhesion promoters and inorganic binders from oxides of the elements silicon, aluminum and/or zirconium to form a thin sheet material.

To achieve adequate mechanical flexibility, the ceramic particles are incorporated into a supporting material, for example a fibrous nonwoven web fabric. This is disclosed by WO 2005/038959 A1.

To prevent short circuits in the initial stages of metal dendrite formation, WO 2005/104269 A1 describes the use of comparatively low-melting waxes as an admixture to a ceramic paste.

WO 2007/028662 A1 describes the addition of polymer particles having a melting point of above 100° C. to ceramic fillers in order that the mechanical properties of the separator may be improved. The materials described are intended for use as a separator for lithium ion materials. Although these separators do provide a higher thermal stability than membranes, they have so far not been a commercial success. This may be due to their relatively high costs and to the excessive thickness of the material, which is above 25 μm.

WO 2000/024075 A1 describes the production of a membrane which can be used in fuel cells. This membrane consists of glass fiber materials in which fluorinated hydrocarbon polymers are fixed by means of a silicate binder.

Finally, JP 2005268096 A describes a separator for lithium ion batteries which is produced by melting together thermoplastic particles in a polyethylene/polypropylene fibrous supporting material by heating. This separator has a bubble-shaped porous structure having a pore diameter of 0.1-15 μm.

The prior art does not show an inexpensive separator which combines low thickness with high porosity and high thermal stability and can be safely used, over a wide temperature range, in batteries having high power and energy density.

SUMMARY OF THE INVENTION

An aspect of the present invention is to develop and refine a ply of the type mentioned at the beginning such that it combine low thickness with high porosity and high thermal stability following inexpensive fabrication.

According to that, the ply is characterized in that the particles in the filled regions form second pores, the average diameter of the particles being greater than the average pore size of the majority of the second pores.

The frequency distribution of the average pore sizes is set according to the present invention such that more than 50% of the second pores have average pore sizes which are below the average diameter of the particles. The inventors recognized that the pore structure of an inexpensive fibrous nonwoven web fabric can be modified through suitable arrangement and selection of particles. Specifically, the porosity of the ply of the present invention was recognized to be enhanceable compared to polyolefin membranes without reducing its stability. The arrangement of a multiplicity of particles whose average diameter is greater than the average pore size of the majority of the second pores in the filled region makes it possible to develop a high porosity and hence an enhanced imbibition of electrolyte by the fibrous nonwoven web fabric. At the same time, the pore structure created makes it virtually impossible for harmful metal dendrites to form therein. The present invention provides an arrangement for the particles which engenders a pore structure which is not bubblelike but is labyrinthine and includes elongate pores. In such a pore structure, it is virtually impossible for dendritic growths to form that extend all the way from one side of the ply to the other. This is efficacious in preventing short circuits in batteries or capacitors. The ply of the present invention is therefore very useful as a separator for batteries and capacitors having high power and energy density. The ply of the present invention is safe to use over a wide temperature range.

The particles could be spherical. This may advantageously produce an overwhelmingly closest packing of spheres in the first pores in the fibrous nonwoven web fabric. The average pore size of the majority of the second pores is essentially determined by geometric conditions in the packings of spheres. There are an infinite number of ways to produce a closest packing of spheres. Their common feature is that they consist of hexagonal layers of spheres. The two most important representatives are the hexagonally closest packing of spheres (layer sequence A, B, A, B, A, B) and the cubically closest packing of spheres (layer sequence A, B, C, A, B, C, A). The cubically closest packing of spheres is also known as the face-centered cubic packing of spheres. Each sphere in a closest packing of spheres has 12 neighbors, six in its own layer and three each above and below. They form a cuboctahedron in the cubic arrangement and an anticuboctahedron in the hexagonal arrangement. The packing density of a closest packing of spheres is 74%. However, the desire is to produce as high a porosity as possible. Therefore, not all particles in the first pores of the fibrous nonwoven web fabric will form a closest packing of spheres. Rather, there will also be zones where the particles are packed loosely, which promotes high porosity.

The particles could form a sheetlike homogeneous distribution in the foundational structure. This concrete form is a particularly effective way to prevent short circuits. Metal dendrites and detritus find it virtually impossible to migrate through a homogeneously covered sheet. Furthermore, such a sheet prevents direct contact between electrodes on application of pressure. It is specifically conceivable against this background that all the first pores in the fibrous nonwoven web fabric are homogeneously filled with the particles such that the ply predominantly exhibits average pore sizes which are smaller than the average diameters of the particles.

The foundational structure could have a coating of the particles. A coating likewise is an advantageous way of effecting the aforementioned prevention of short circuits. When a ply has a coating, the foundational structure will inevitably have a boundary region which is at least partly filled with particles.

The particles could be united with the fibrous nonwoven web fabric, or with each other, by a binder. This binder could consist of organic polymers. The use of a binder consisting of organic polymers makes it possible to produce a ply having sufficient mechanical flexibility. Polyvinylpyrrolidone surprisingly shows excellent binder properties.

It could be preferable to use thermoplastic and/or thermosetting binders. Examples which may be mentioned against this background are polyvinylpyrrolidone, polyacrylic acid, polyacrylates, polymethacrylic acid, polymethacrylates, polystyrene, polyvinyl alcohol, polyvinyl acetate, polyacrylamide and copolymers of the aforementioned, cellulose and its derivatives, polyethers, phenolic resins, melamine resins, polyurethanes, nitrile rubber (NBR), styrene-butadiene rubber (SBR) and also latex.

The melting point of the binder and/or of the particles could be below the melting points of the fibers of the fibrous nonwoven web fabric. By choosing such a binder/particles it is possible for the ply to realize a shutdown mechanism. In a shutdown mechanism, the melting particles and/or the binder blind the pores of the fibrous nonwoven web fabric, so that no dendritic growths through the pores and hence short circuits can occur.

It is conceivable against this background to use mixtures of particles having different melting points. This can be used to achieve stepwise or stagewise blinding of the pores with increasing temperature.

The particles could have an average diameter in the range from 0.01 to 10 μm. The selection of the average diameter from this range will be found particularly advantageous to avoid short circuits through formation of dendritic growths or debris.

The particles could be fabricated from organic polymers, in particular from polypropylene, polyvinylpyrrolidone, polyvinylidene fluoride, polyester, polytetrafluoroethylene, perfluoroethylene-propylene (FEP), polystyrene, styrene-butadiene copolymers, polyacrylates or nitrile-butadiene polymers and also copolymers of the aforementioned polymers. The use of organic polymers for the particles permits unproblematic melting of the particles to obtain a shutdown effect. It is further possible to fabricate a ply which is easy to cut to size without crumbling. Crumbling of the ply will usually occur when there is a relatively high proportion of inorganic particles in the ply. It is conceivable against this background to use mixtures of different particles or core-shell particles. This can be used to achieve stepwise or stagewise blinding of the pores with increasing temperature.

It is also possible to use inorganic particles or inorganic-organic hybrid particles. These particles do not melt below a temperature of 400° C. It is further possible to choose these particles with basic properties in order that the proton activity present in batteries may be at least partially reduced.

The fibers of the fibrous nonwoven web fabric could be fabricated from organic polymers, in particular from polybutyl terephthalate, polyethylene terephthalate, polyacrylonitrile, polyvinylidene fluoride, polyether ether ketones, polyethylene naphthalate, polysulfones, polyimide, polyester, polypropylene, polyoxymethylene, polyamide or polyvinylpyrrolidone. It is also conceivable to use bicomponent fibers which include the aforementioned polymers. The use of these organic polymers makes it possible to produce a ply having only minimal thermal shrinkage. Furthermore, these materials are substantially electrochemically stable to the electrolytes and gases used in batteries and capacitors.

The average length of the fibers of the fibrous nonwoven web fabric could exceed their average diameter by at least a factor of two or more, preferably by a multiple. This concrete development makes it possible to fabricate a particularly strong fibrous nonwoven web fabric, since the fibers can become intertwined with each other.

At least 90% of the fibers of the fibrous nonwoven web fabric could have an average diameter of not more than 12 μm. This concrete development makes it possible to construct a ply having relatively small pore sizes for the first pores. Still finer porosity is obtainable when at least 40% of the fibers of the fibrous nonwoven web fabric have an average diameter of not more than 8 μm.

The ply could be characterized by a thickness of not more than 100 μm. A ply of this thickness can still be rolled up without problems and permits very safe battery operation. The thickness could preferably be not more than 60 μm. This thickness permits improved rollability and yet a safe battery operation. The thickness could more preferably be not more than 25 μm. Plies having such a thickness can be used to build very compact batteries and capacitors.

The ply could have a porosity of at least 25%. A ply of this porosity is by virtue of its density of material particularly effective in suppressing the formation of short circuits. The ply could preferably have a porosity of at least 35%. A ply of this porosity can be used to produce a battery of high power density. The ply described herein combines very high porosity with nonetheless very small second pores, so that no dendritic growths extending from one side to the other side of the ply can form. It is conceivable against this background that the second pores form a labyrinthine microstructure in which no dendritic growths from one side to the other side of the ply can form.

The ply could have pore sizes of not more than 3 μm. The choice of this pore size will be found particularly advantageous in avoiding short circuits. The pore sizes could more preferably be not more than 1 μm. Such a ply is particularly advantageous in avoiding short circuits due to metal dendrite growth, due to debris from electrode particles and due to direct contact between the electrodes on pressure application.

The ply could have an ultimate tensile strength force in the longitudinal direction of at least 15 newtons/5 cm. A ply of this strength is particularly easy to roll up on the electrodes of a battery without rupturing.

The ply could be mechanically consolidated by calendering. Calendering is effective in reducing surface roughness. The particles used at the surface of the fibrous nonwoven web fabric exhibit flattening after calendering.

The ply described herein can be used as a separator in batteries and capacitors in particular, since it is particularly efficacious in preventing short circuits.

The ply described herein can also be used as a gas diffusion layer or membrane in fuel cells, since it exhibits good wetting properties and can transport liquids.

There are, then, various ways of advantageously developing and refining the teaching of the present invention. Reference must be made, on the one hand, to the subordinate claims and, on the other, to the following elucidation of a preferred illustrative embodiment of the present invention with reference to the drawing.

The elucidation of the preferred illustrative embodiment of the present invention with reference to the drawing will also serve to elucidate generally preferred developments and refinements of the teaching.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing

FIG. 1 shows a scanning electron micrograph of a ply in which the particles are present in first pores in a fibrous nonwoven web fabric and form a porous region filled with particles,

FIG. 2 shows a scanning electron micrograph of the particles of a filled region configured as a coating, and

FIG. 3 shows a greatly magnified scanning electron micrograph of the particles of a filled region.

DETAILED DESCRIPTION

FIG. 1 shows a ply having a foundational structure composed of a fibrous nonwoven web fabric, the foundational structure consisting of fibers 1 and having first pores 2 formed by the fibers 1, the foundational structure being at least partially filled with particles 3, which particles 3 at least partially fill the first pores 2 and form regions 4 filled with particles 3.

FIG. 3 shows a filled region 4 in a magnified view. With reference to FIG. 3, the particles 3 form second pores 5 in the filled regions 4, the average diameter of the particles 3 being greater than the average pore size of the majority of the second pores 5. The particles 3 are spherical and tend to form a closest packing of spheres in regions.

FIG. 2 shows a coating of the particles 3 which has been applied to the fibrous nonwoven web fabric.

FIGS. 1 to 3 show scanning electron micrographs of a ply comprising a fibrous nonwoven web fabric, the fibers 1 of which are fabricated from polyester. The particles 3 are spherical in configuration and form in regions agglomerates which fill the first pores 2 in the fibrous nonwoven web fabric. The fibers 1 have an average diameter of less than 12 μm. The ply has a thickness of 25 μm. It exhibits a shrinkage in the transverse direction of less than 1% at a temperature of 170° C.

The average diameter of the particles 3 is 200 nm The particles 3 consist of polyvinylidene fluoride and were secured to the fibers 1 by a polyvinylpyrrolidone binder.

The average diameter of the particles 3 is determined from the number of particles 3 in the filled region 4. The particles 3 preferably exhibit a narrow distribution curve; that is, an average diameter having a low standard deviation. The average pore sizes of most, viz. the majority, of the second pores 5 is less than 200 nm. By average pore size of a second pore 5 is meant the diameter of an imaginative sphere 6 which has the same volume as the pore 5. The imaginative sphere resides between the particles 3 such that it touches the surfaces of the neighboring particles 3. Imaginative spheres 6 which characterize the dimension of the pores are depicted in FIG. 3 as black-bordered hollow circles.

A distribution curve where the x-axis indicates the average pore sizes of the second pores 5 and the y-axis indicates the number or frequency of the average pore sizes would show that more than 50% of the second pores 5 have average pore sizes which are below 200 nm.

With regard to further advantageous developments and refinements of the teaching of the present invention reference is made to the general part of the description and to the accompanying claims.

It may finally be emphasized most particularly that the previously purely arbitrarily selected illustrative embodiment merely serves to discuss the teaching of the present invention, but does not limit that teaching to this illustrative embodiment.

Claims

1-18. (canceled)

19. A ply comprising:

a fibrous nonwoven web fabric forming a foundational structure, wherein the foundational structure includes fibers forming first pores and is partially filled with particles, wherein the particles at least partially fill the first pores so as to form regions filled with particles, wherein the particles in the filled regions form second pores such that an average diameter of the particles is greater than an average pore size of more than 50% of the second pores.

20. The ply as recited in claim 19, wherein the particles are spherical.

21. The ply as recited in claim 19, wherein the particles form a sheet-like homogeneous distribution in the foundational structure.

22. The ply as recited in claim 19, wherein at least a portion of the filled regions forms a coating of the foundational structure.

23. The ply as recited in claim 19, wherein the particles are united with the fibrous nonwoven web fabric via a binder composed of organic polymers selected from the group consisting of polyvinylpyrrolidone, polyacrylic acid, polyacrylate, polymethacrylic acid, polymethacrylate, polystyrene, polyvinyl alcohol, polyvinyl acetate, polyacrylamide and copolymers thereof, cellulose and its derivatives, polyethers, phenolic resin, melamine resin, polyurethane, nitrile rubber (NBR), styrene-butadiene rubber (SBR) and latex.

24. The ply as recited in claim 23, wherein a melting point of the binder is below a melting point of at least one of the particles and the fibers.

25. The ply as recited in claim 19, wherein the particles have an average diameter between 0.01 and 10 μm.

26. The ply as recited in claim 19, wherein the particles are fabricated from organic polymers selected from the group consisting of polypropylene, polyvinylpyrrolidone, polyvinylidene fluoride, polyester, polytetrafluoroethylene, perfluoro-ethylene-propylene (FEP), polystyrene, styrene-butadiene copolymers, polyacrylate and nitrile-butadiene polymers and copolymers thereof.

27. The ply as recited in claim 19, wherein the fibers of the fibrous nonwoven web fabric are fabricated from organic polymers selected from the group consisting of polybutylene terephthalate, polyethylene terephthalate, polyacrylonitrile, polyvinylidene fluoride, polyether ether ketone, polyethylene naphthalate, polysulfone, polyimide, polyester, polypropylene, polyoxymethylene, polyamide and polyvinylpyrrolidone.

28. The ply as recited in claim 19, wherein an average length of the fibers exceeds an average diameter of the fibers by at least a factor of two.

29. The ply as recited in claim 19, wherein at least 90% of the fibers have an average diameter of not more than 12 μm.

30. The ply as recited in claim 19, wherein at least 40% of the fibers have an average diameter of not more than 8 μm.

31. The ply as recited in claim 19, wherein a thickness of the ply is not more than 100 μm.

32. The ply as recited in claim 19, wherein a porosity of the ply is at least 25%.

33. The ply as recited in claim 19, wherein the first and second pores form a labyrinthine microstructure.

34. The ply as recited in claim 19, wherein the first and second pores have a pore size of not more than 3 μm.

35. The ply as recited in claim 19, wherein an ultimate tensile strength force of the ply is at least 15 N/5 cm in a longitudinal direction.

36. The ply as recited in claim 19, wherein the foundational structure is calendered.