COATED SEPARATOR WITH FLUOROPOLYMERS FOR LITHIUM ION BATTERY

Abstract

The invention relates to a fluoropolymer-acrylic coating composition that can be used, for example, in coating electrodes and/or separators in electrochemical devices. A coated separator for a lithium ion battery contains the porous separator substrate, and coatings on at least one side of the separator. The coating consists of an inorganic coating on at least one side of the separator, and an adhesive organic coating on at least one side of the inorganic coating or the separator. The organic coating contains an improved fluoropolymer-acrylic composition or a mixture of fluoropolymer and acrylic. The present invention can improve the adhesion of the coated separator to electrodes.

Claims

1. A coated separator for a lithium ion battery comprising an adhesive layer (binder coating) on at least one side of a separator, wherein the adhesive layer comprises a fluoropolymer-acrylic composition, wherein said composition comprises a fluoropolymer-acrylic resin, the resin comprising from 5 to 50 wt % acrylic monomer units based upon the total weight of the fluoropolymer-acrylic resin, wherein the resin is cross linked, wherein the resin is a composition comprising an acrylic monomer polymerized in the presence of a fluoropolymer seed.

2. (canceled)

3. The coated separator of claim 1, wherein the fluoropolymer seed comprises a vinylidene fluoride polymer comprising at least 50 weight percent VDF.

4. The coated separator of claim 1, wherein the fluoropolymer seed comprises a polyvinylidene fluoride-hexafluoropropylene copolymer, wherein the total weight percent of hexafluoropropylene monomeric units in the fluoropolymer-acrylic resin is from 5 to 20 wt % based on the total weight percent of fluoropolymer-acrylic resin in the adhesive layer.

5. The coated separator of claim 1, wherein the fluoropolymer seed comprises from 3 to 30 wt % hexafluoropropylene.

6. (canceled)

7. The coated separator of claim 1, wherein the fluoropolymer-acrylic resin contains monomers that contain functional groups that can crosslink.

8. (canceled)

9. The coated separator of claim 1, wherein the fluoropolymer-acrylic resin contains monomer units selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, fumaric acid, N-methylolacrylamide, N-methylolmethacrylamide, diacetone acrylamide, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate and allyl glycidyl ether.

10. The coated separator of claim 1, wherein at least one or more of the acrylic monomers is selected from the group consisting of methyl methacrylate, methacrylic acid, methacrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl methacrylate, ethyl acrylate, butyl acrylate, propyl acrylate, acrylic acid, diacetone acrylamide, polymethoxydiethylene glycol (meth)acrylate and combinations thereof.

11. (canceled)

12. (canceled)

13. (canceled)

14. The coated separator of claim 1, wherein said adhesive layer further comprises 50 to 99 weight percent of inorganic particles, based on the combined weight of polymer and inorganic particles, wherein said inorganic particles being electrochemically stable inorganic particles.

15. (canceled)

16. The coated separator of claim 1, wherein said adhesive layer further comprises 50 to 99 weight percent of inorganic particles, based on the combined weight of polymer and inorganic particles, wherein said inorganic particles being electrochemically stable inorganic particles, and said inorganic particles are selected from the group consisting of MgO, bohemite (y-AlO(OH)), Al.sub.2O.sub.3, nano-clays, or mixtures thereof.

17. (canceled)

18. (canceled)

19. (canceled)

20. A component of an electrochemical device, wherein said component has directly coated on at least one side thereof a dried crosslinked fluoropolymer-acrylic composition, wherein the fluoropolymer-acrylic composition comprises a fluoropolymer-acrylic resin, the resin comprising from 5 to 50 wt % acrylic monomer units based upon the total weight of the fluoropolymer-acrylic resin, wherein the resin is a composition comprising an acrylic monomer polymerized in the presence of a fluoropolymer seed, wherein said coated component is a separator or electrode; wherein said dried, fluoropolymer-acrylic composition has a dry adhesive strength of greater than 10 N/m, preferably greater than 15 N/m, as measured by 180 degree peel strength measurement.

21. A method for forming a coated separator comprising a) the steps of dip-coating, spray coating, micro-gravure coating or slot coating at least one side of a separator with a crosslinkable fluoropolymer-acrylic composition, b) drying said coated separator at a temperature of from 25 to 85 C, to form a dried adhesive layer, on the separator, wherein the composition comprises a fluoropolymer-acrylic resin, the resin having and from 5 to 50 wt % acrylic monomer units based upon the total weight of the fluoropolymer-acrylic resin, wherein the resin is a composition comprising an acrylic monomer polymerized with a fluoropolymer seed.

22. (canceled)

23. The method of claim 21, wherein the fluoropolymer seed comprises a vinylidene fluoride polymer comprising at least 50 weight percent VDF.

24. The method of claim 21, wherein the seed comprises a polyvinylidene fluoride-hexafluoropropylene copolymer, wherein the total weight percent of hexafluoropropylene monomeric units in the fluoropolymer-acrylic resin is from 5 to 20 wt % based on the total weight percent of fluoropolymer-acrylic resin in the adhesive layer.

25. The method of claim 21, wherein the fluoropolymer seed comprises from 3 to 30 wt % hexafluoropropylene.

26. The method of claim 21, wherein the acrylic polymer contains monomers that contain functional groups that can crosslink.

27. (canceled)

28. The method of claim 21, wherein the acrylic polymer contains monomer selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, fumaric acid, N-methylolacrylamide, N-methylolmethacrylamide, diacetone acrylamide, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate and allyl glycidyl ether.

29. The method of claim 21, wherein at least one or more of the acrylic monomers is selected from the group consisting of methyl methacrylate, methacrylic acid, methacrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl methacrylate, ethyl acrylate, butyl acrylate, propyl acrylate, acrylic acid, diacetone acrylamide, polymethoxydiethylene glycol (meth)acrylate, and combination thereof.

30. (canceled)

31. The method of claim 21, wherein the fluoropolymer-acrylic resin is self cross linking.

32. The method of claim 21, wherein the fluoropolymer-acrylic composition comprises a cross-linking agent.

33. The method of claim 21, wherein said adhesive layer further comprises 50 to 99 weight percent of inorganic particles, based on the combined weight of polymer and inorganic particles, wherein said inorganic particles being electrochemically stable inorganic particles.

34. The method of claim 21, wherein said fluoropolymer-acrylic resin is dissolved in solvent prior to the coating step.

Description

EXAMPLES

Adhesive Strength to Positive Electrode:

[0125] Preparation of positive electrode: 27.16 g of Nickel Manganese Cobalt 622 powder as the positive active material, 0.42 g of carbon black powder as the conductive agent, and 0.42 g of polyvinylidene fluoride as binder were mixed in 4.83 g of N-methyl-pyrrolidone. The resultant solution were mixed under high speed, e.g. 2000 rpm. The positive electrode slurry was coated onto aluminum foil, dried in the oven and calendared by press to achieve a positive electrode.

[0126] Sample preparation for peel test: The coated separator and positive electrode were cut into shape of 2.5 cm by 5 cm. The adhesive organic layer coated side of the separator was laminated into contact with the positive electrode side. Lamination was carried at 85° C. and 0.62 MPa for 2 min to adhere coated separator to positive electrode. After lamination, attach single sided tape as the backing support layer to the coated separator. Then cut the composite of single sided tape, coated separator, and positive electrode to 1.5 cm by width and 5 cm by length.

[0127] Adhesive strength test: Apply a double sided tape onto a thick block (e.g. thickness around 1 cm) of steel plate, attach the uncoated side of aluminum foil in the composite of electrode and coated separator to the double sided tape, and run the 180 degree peel test by peeling off the single sided tape and coated separator. The peel test was run under tension mode, with a load cell of 10 N and peeling speed of 2 mm/min. The trend that the higher the tested adhesion force, the more transferred electrode material to the coated separator would be observed.

[0128] Swelling test in electrolyte: Electrolyte consists of ethylene carbonate, dimethyl carbonate, and diethyl carbonate with ratio of 1:1:1 by volume was used. Samples were prepared either by drying from solution with organic solvent or by drying from solution with water. Swelling test was carried at 60° C. with dried samples submerged completely in the electrolyte for 72 hours. Weight of the sample was measured before swelling test (m1) as well as after the swelling test (m2). Then the swelling ratio was characterized as (m2−m1)/m1*100%.

Example 1

[0129] A polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) copolymer latex was used as seed to synthesize a latex containing fluoropolymer-acrylic composition using emulsion polymerization process. Solids content of this latex is about 50 wt %. The mass percent of the HFP part in the PVDF-HFP copolymer is around 20 to 22 wt % and the acrylic part is about 50 wt % in total polymer. The acrylic part contains crosslinkable functional groups. The acrylic part has a glass transition temperature of −25° C.

[0130] The fluoropolymer-acrylic composition was directly used as latex (in water).

[0131] The slurry was applied to the porous separator, and dried at 60° C. (for latex slurry). The dried thickness of the adhesive layer is in the range of 1 to 2 μm. The adhesive strength of separator coated with fluoropolymer-acrylic composition in example 1 to cathode was 55.1 N/m for latex slurry. The average swelling ratio of the fluoropolymer-acrylic composition in electrolyte was 500%.

Example 2: Crosslinked AMF Polymer

[0132] A polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) copolymer latex was used as seed to synthesize a latex containing fluoropolymer-acrylic composition using emulsion polymerization process. Solids content of this latex is around 44 wt %. The mass percent of the HFP part in the PVDF-HFP copolymer is around 20 to 22 wt % and the acrylic part is around 30 wt % in total polymer and contained cross linkable groups. The acrylic part has a glass transition temperature of 46° C.

[0133] The fluoropolymer-acrylic composition was dissolved in solvent of cyclopentanone and the solution concentration was 10 wt % by mass.

[0134] The slurry was applied to the porous separator, and dried at 60° C. oven. The dried thickness of the adhesive layer is in the range of 1 to 2 μm. The adhesive strength of separator coated with the fluoropolymer-acrylic composition in example 2 to cathode was averaged as 31.8 N/m and the average swelling ratio of the fluoropolymer-acrylic composition in electrolyte was 900 wt %.

Example 3: Blend of Crosslinkable AMF Polymer with VDF/HFP Copolymer

[0135] Except that fluoropolymer-acrylic composition in Example 2 was mixed with polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP, with 10% HFP) copolymer to yield a composition containing 25 wt % of acrylic, the same procedure was carried out in Example 2 to coat separators.

[0136] The adhesive strength of separator coated with material in Example 3 to cathode was averaged as 17.1 N/m and the swelling ratio of the material in electrolyte was 650 wt %.

Comparative Example 1: PVDF Copolymer—No Acrylic

[0137] A polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) copolymer was dissolved in cyclopentanone and the solution concentration was 10 wt % by mass. The mass percent of the HFP part in the PVDF-HFP copolymer is around 4 to 6 wt %. The copolymer was dissolved in solvent of cyclopentanone and the solution concentration was 10 wt % by mass.

[0138] The slurry was applied to the porous separator, and dried at 60° C. oven. The dried thickness of the adhesive layer is in the range of 1 to 2 μm. The adhesive strength of separator coated with material in Comparative Example 1 to cathode was below 3 N/m and the swelling ratio of the material in electrolyte was averaged as 160 wt %.

Comparative Example 2: Not Crosslinked

[0139] A polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) copolymer latex was used as seed to synthesize a latex containing fluoropolymer-acrylic composition using emulsion polymerization process. Solids content of this latex is around 44 wt %. The mass percent of the HFP part in the PVDF-HFP copolymer is around 20 to 22 wt % and the acrylic part is around 30 wt % in total polymer. The acrylic part has a glass transition temperature of 55° C.

[0140] The fluoropolymer-acrylic composition was dissolved in solvent of cyclopentanone and the solution concentration was 10 wt % by mass.

[0141] The slurry was applied to the porous separator, and dried at 60° C. oven. The dried thickness of the adhesive layer is in the range of 1 to 2 μm. The adhesive strength of separator coated with fluoropolymer-acrylic composition in Comparative Example 2 to cathode was averaged as 13.7 N/m and the material in Example 2 dissolved in electrolyte.

Comparative Example 3: (Physical Blend) not Crosslinked

[0142] Polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) copolymer was mixed with acrylic type resin to yield acrylic mass ratio of 30 wt % and HFP content in the PVDF-HFP copolymer is around 8 to 10 wt %. The acrylic type resin has a glass transition temperature of about 70° C.

[0143] The material was dissolved in solvent of cyclopentanone and the solution concentration was 10 wt % by mass. The adhesive strength of separator coated with material in Comparative Example 3 to cathode was below 3 N/m and part of the material of dissolved in electrolyte as indicated by a mass loss.

TABLE-US-00001 TABLE 2 Tg of Adhesive Solids HFP Acrylic Acrylic 1 to 2 strength Swelling Example content wt % wt % ° C. micron N/m Wt % 1 50 21 30 55.1 500 2 44 21 30 46 C. 31.8 900 3 20 9 25 46 17.1 650 Comparative 1 5 0 3 160 Comparative 2 44 21 30 55 13.7 dissolved Comparative 3 30 9 70 3 Partially BLEND dissolved