Ceramic membrane having support materials comprising polyaramid fibers and method for producing said membranes

Abstract

The present invention relates to a membrane comprising a flat, flexible substrate having a plurality of openings and having a porous inorganic coating situated on and in said substrate, the material of the substrate being selected from woven or non-woven, electrically non-conductive fibers, characterized in that the substrate comprises polyaramide fibers that are pure or connected to fibers of the further polymer or at least of one of these further polymers, wherein the fibers of at least one of said further polymers comprise a melting point that is lower than the decomposition point of the polyaramide fibers.

Claims

1. A membrane, comprising: a sheetlike, flexible, woven or nonwoven substrate comprising electrically nonconductive fibers, which is provided with a multiplicity of openings; and a porous inorganic coating on and inside the substrate, wherein the electrically nonconductive fibers comprise (i) polyaramid fibers melted to fibers of a further polymer having a melting point lower than the decomposition point of the polyaramid fibers, or (ii) polyaramid fibers melted to one another with a polymeric binder.

2. The membrane of claim 1, wherein the further polymer fibers are at least one selected from the group consisting of polyethylene terephthalate, a polyester, a polycarbonate, polyacrylonitrile, a polyimide, a polyamide, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, polyethylene, polypropylene, and polyolefin, and the polymeric binder is at least one selected from the group consisting of polyurethane, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride-PTFE copolymer, butadiene rubber, (meth)acrylate latex, polyvinylpyrrolidone, and polyvinyl alcohol.

3. The membrane of claim 1, wherein the polyaramid fibers are melted to further polymer polymer fibers and a mixing ratio of the polyaramid fibers to the further polymer fibers is in a range of 10:1 to 1:10.

4. The membrane of claim 1, wherein the substrate has a porosity of 50 to 97%.

5. The membrane of claim 1, wherein the substrate has a melting point of more than 200° C.

6. The membrane of claim 1, having a tensile strength of more than 5 N/cm.

7. A method for producing the membrane of claim 1, the method comprising: (I) applying a suspension comprising an inorganic component comprising a compound comprising a metal, a semimetal, or a mixed metal and a main group element selected from the group consisting of a group 3 to group 7 element, and a sol, to a sheetlike, flexible substrate provided with a multiplicity of openings and comprising (i) polyaramid fibers which are joined to further polymer fibers having a melting point lower than the decomposition point of the polyaramid fibers or (ii) polyaramid fibers joined to one another with a polymeric binder; and (II) heating the substrate to a temperature of between 200 to 350° C., to obtain a solidified suspension on the substrate, inside the substrate, or both.

8. The method of claim 7, wherein the further polymer fibers are at least one selected from the group consisting of polyethylene terephthalate, polyacrylonitrile, polyester, polyimide, polyamide, polytetrafluoroethylene, and polyolefin, the polymeric binder is at least one selected from the group consisting of polyurethane, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride-PTFE copolymer, butadiene rubber, (meth)acrylate latex, polyvinylpyrrolidone, polyvinyl alcohol, and the suspension comprises at least one selected from the group consisting of a sol, polyaramid particles, and at least one oxide of a metal selected from the group consisting of Al, Zr, Si, Ti, and Y.

9. The method of claim 7, wherein the suspension comprises a sol, polyaramid particles, and at least one oxide of a metal selected from the group consisting of Al, Zr, Si, Ti, and Y.

10. The method of claim 7, wherein the applying (I) is carried out by roll coating, printing, pressing, injecting, rolling, knifecoating, spreading, dipping, squirting, or pouring.

11. The membrane of claim 1, wherein the membrane is suitable for use as a separator in a battery, a wall lining, or a protective and support device casing.

12. A lithium ion battery, comprising: the membrane of claim 1, wherein the membrane is a separator.

13. The membrane of claim 1, wherein the polyaramid fibers have a diameter in a range of 0.5 to 50 μm.

14. The membrane of claim 1, wherein the polyaramid fibers have a diameter in a range of 0.5 to 20 μm.

15. The membrane of claim 3, wherein the further fibers are polyethylene terephthalate fibers.

16. The membrane of claim 15, wherein a mixing ratio of the polyaramid fibers and the polyethylene terephthalate fibers is 1:1.

17. The membrane of claim 4, wherein the substrate has a porosity of 75 to 90%.

18. The membrane of claim 1, wherein the electrically nonconductive fibers comprise (i) polyaramid fibers melted to fibers of a further polymer having a melting point lower than the decomposition point of the polyaramid fibers.

19. The membrane of claim 1, wherein the electrically nonconductive fibers comprise (ii) polyaramid fibers melted to one another with a polymeric binder.

Description

EXAMPLE 1: PRODUCTION OF A MEMBRANE OF THE INVENTION

(1) To 160 g of ethanol were initially added 15 g of a 5% strength by weight aqueous HCl solution, 10 g of tetraethoxysilane, 2.5 g of methyltriethoxysilane, and 7.5 g of Dynasilan GLYMO (manufacturer of all Dynasilanes: Evonik AG). This sol, which was initially stirred for a number of hours, was then used to suspend 125 g each of Martoxid MZS-1 and Martoxid MZS-3 aluminum oxides (manufacturer of both aluminum oxides: Martinswerke). This slip was homogenized for at least a further 24 hours with a magnetic stirrer, during which the stirred vessel had to be covered so that there was no loss of solvent.

(2) A nonwoven composed of a polyaramid/PET fiber blend, with a ratio of 50% polyaramid fiber (Kevlar, manufacturers include DuPont) and 50% PET (manufacturer Advansa, Dacron type), with a thickness of about 30 μm and a basis weight of about 20 g/m.sup.2, was coated with the above slip in a continuous roll application process (belt speed about 8 m/h, T=200° C.). In this roll application process, the slip was applied to the nonwoven using a roller which moved in the opposite direction to the belt direction (direction of movement of the nonwoven). The nonwoven subsequently passed through an oven which had a temperature of 200° C. In the following example, the same technique and arrangement were used. The end result obtained was a membrane having an average pore size of 450 nm.

EXAMPLE 2: PRODUCTION OF A MEMBRANE OF THE INVENTION

(3) To 160 g of ethanol were added first of all 15 g of a 5% strength by weight aqueous HCl solution, 10 g of tetraethoxysilane, 2.5 g of methyltriethoxysilane, and 7.5 g of Dynasilan GLYMO. This sol, which was initially stirred for a number of hours, was then used to suspend 280 g of Alcoa CT 1200 SG aluminum oxide.

(4) This slip was homogenized for at least a further 24 hours with a magnetic stirrer, during which it was necessary to cover the stirred vessel so that there was no loss of solvent.

(5) A polyaramid fiber/PET nonwoven (ratio 80% polyaramid fiber (manufacturer, for example, DuPont, Kevlar type) and 20% PET (manufacturer Advansa, Dacron type) having a thickness of about 100 μm and a basis weight of 22 g/m.sup.2 was coated with the above slip in a continuous roll application process (belt speed about 8 m/h, T=250° C.). The end result obtained was a membrane having an average pore size of 240 nm.

EXAMPLE 3: LI ION BATTERY WITH THE MEMBRANE OF THE INVENTION AS SEPARATOR

(6) A membrane produced as in example 1 was installed in an Li ion cell consisting of a positive mass of LiCoO.sub.2, a negative mass consisting of graphite, and an electrolyte composed of LiPF.sub.6 in ethylene carbonate/dimethyl carbonate [LiCoO2 (36.3 mg), active mass 86%/S-450-PET_2, EC/DMC 1:1, 1M LiPF6/graphite (17.0 mg), active mass 90%].

(7) Four specimen cells were constructed, and the charging capacity achieved was recorded as a function of the charge and discharge cycles. The charge and discharge capacities against the cycle run are shown as a graph in FIG. 1 for the various specimen cells. In FIG. 1, Ch. Cap. denotes charging capacity and Dis. Cap. denotes discharging capacity.

(8) For all four cells, no drop in capacity was found over the measured time period, and the charging and discharging capacities were virtually identical. Even after 26 cycles, no drop in battery performance was found.

EXAMPLE 4: INVESTIGATION OF TEMPERATURE STABILITY

(9) To investigate the contraction behavior, cut-to-size sections of the ceramic composite membranes and of the polyaramid fiber nonwovens were exposed to elevated temperatures. In the course of this exposure it was found that contraction was observed only for the comparative material (PET nonwoven). For the remaining materials, the contraction was below 1%, as shown in Table 1.

(10) TABLE-US-00002 TABLE 1 Contraction Contraction at 100° C. at 200° C. PET nonwoven  3% 10% Polyaramid fiber nonwoven <1% <1% Polyaramid fiber/PET nonwoven <1% <1% Ceramic composite membrane with PET  3%  5% nonwoven, described in WO 2008/038960 Membrane of the invention with <1% <1% polyaramid fiber/PET nonwoven

(11) Moreover, no change in the color of the material was found to accompany increase in temperature. Even when the membrane of the invention was exposed to flames, it retained its integrity in the form of the test component.