MULTILAYER SEPARATOR FOR A BATTERY

Abstract

A multilayer (200) for a lithium-ion battery having a structure including at least a polyolefin based substrate layer (204) forming the inner layer of the multilayer separator (200); a resin layer (203) stacked on both surface of the polyolefin substrate layer (204), the resin layer (203) being formed from a polyolefin; a cellulose fibers based outer layer (202) stacked on the surface of each resin layer (203).

Claims

1. A multilayer separator (200) for a lithium-ion battery having a structure comprising at least: a polyolefin based substrate layer (204) forming the inner layer of the multilayer separator (200); a resin layer (203) stacked on both surface of the polyolefin substrate layer (204), the resin layer (203) being formed from a polyolefin; a cellulose fibers based outer layer (202) stacked on the surface of each resin layer (203).

2. The multilayer separator (200) for a lithium-ion battery according to claim 1, wherein said polyolefin based substrate layer (204) is made of at least one selected group consisting of polyethylene, polypropylene, or a blend comprising substantially polypropylene or polyethylene.

3. The multilayer separator (200) for a lithium-ion battery according to claim 1, wherein said polyolefin based substrate layer (204) is made of low-density polyethylene (LPDE), linear low-density polyethylene (LLDPE), or a blend comprising substantially LPDE, LLDPE or a mixture thereof.

4. The multilayer separator (200) for a lithium-ion battery according to claim 1, wherein said polyolefin based substrate layer (204) is made of a random copolymer of ethylene and alpha-olefins selected from the group consisting of coalpha-olefins having a polymerized alpha olefin content about 20% by weight, and preferably 16% by weight.

5. The multilayer separator (200) for a lithium-ion battery according to claim 1, wherein said polyolefin based resin layer (203) is made of at least one selected group consisting of polyethylene, polypropylene, or a blend comprising substantially polypropylene or polyethylene.

6. The multilayer separator (200) for a lithium-ion battery according to claim 1, wherein said cellulose fibers based outer layer (202) is made of at least one selected group consisting of consisting of ethyl cellulose, methyl cellulose, hydroxypropyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, carboxymethyl cellulose, cellulose acetate, cellulose triacetate, cellulose acetate phthalate, nitrocellulose, cellulose acetate butylate, cellulose acetate propionate, ammonium thereof, and salt thereof.

7. The multilayer separator (200) for a lithium-ion battery according to claim 1, wherein said cellulose fibers of the cellulose fibers based outer layer (202) are refined cellulose fibers, cellulose microfibrils, cellulose nanofibrils, lignin and derivatives thereof.

8. The multilayer separator (200) for a lithium-ion battery according to claim 1, wherein said cellulose fibers based outer layer (202) has size of fibers comprised between 2 mm and 100 nm in length.

9. The multilayer separator (200) for a lithium-ion battery according to claim 1, wherein said cellulose fibers of the cellulose fibers based outer layer (202) are refined cellulose fibers having a size of 200 nm in length.

10. The multilayer separator (200) for a lithium-ion battery according to claim 1, wherein the polyolefin based substrate layer (204) has a thickness of 5 to 15 μm, the resin layer (203) has a thickness of 12 to 75 μm, preferably 25 μm, and the cellulose fibers based outer layer (202) has a thickness of 1 to 5 μm, preferably 2 μm.

11. The multilayer separator (200) for a lithium-ion battery according to claim 1, wherein the polyolefin based substrate layer (204) has a melting point of 80 to 124° C. and the resin layer (203) has a melting point of 130 to 160° C.

12. The multilayer separator (200) for a lithium-ion battery according to claim 1, wherein the resin layer (203) is stacked in lamination method, a hydraulic pressure method or a filtration method.

13. The multilayer separator (200) for a lithium-ion battery according to claim 1, wherein the cellulose fibers based outer layer (202) is stacked in lamination method, a hydraulic pressure method, a filtration method or slot dye coating method.

14. A lithium-ion battery wherein said a lithium-ion battery comprises a multilayer separator (200) for a lithium-ion battery having a structure comprising at least: a polyolefin based substrate layer (204) forming the inner layer of the multilayer separator (200); a resin layer (203) stacked on both surface of the polyolefin substrate layer (204), the resin layer (203) being formed from a polyolefin; a cellulose fibers based outer layer (202) stacked on the surface of each resin layer (203).

15. A method of preparing a multilayer separator (200) for lithium-ion battery according to claim 1, said method comprising: a step of forming a polyolefin based substrate layer (204); a step of stacking a resin layer (203) formed from a polyolefin on both surface of the polyolefin substrate layer (204) formed in the precedent step, in a lamination method, a hydraulic pressure, or filtration method; a step of stacking a cellulose fibers based outer layer (202) on the surface of each resin layer (203) in a lamination method, a hydraulic pressure, filtration method or slot dye coating method.

16. The method of preparing a multilayer separator (200) for lithium-ion battery according to claim 15, wherein the step of forming a polyolefin based substrate layer (204) comprises: a sub-step of forming a mixture of a linear low-density polyethylene compounded with calcium carbonate particles; a sub-step of extruding said mixture into precursor film; a sub-step of cooling and of stressing said precursor film in order to obtain said polyolefin based substrate layer (204).

17. The method of preparing a multilayer separator (200) for lithium-ion battery according to claim 16, wherein the mixture comprises, expressed in weight, between 40% and 45% of linear low-density polyethylene and 50% of calcium carbonate.

18. The method of preparing a multilayer separator (200) for lithium-ion battery according to claim 15, wherein said step of stacking a cellulose fibers based outer layer (202) is a step of stacking a nano-fibrillated cellulose layer on the surface of each resin layer (203).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The present invention will be described subsequently in more detail with reference to the attached drawing, given by way of examples, but in no way limited thereto, in which:

[0048] FIG. 1 is a cross-sectional view of a multilayer separator for a lithium-ion battery in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0049] FIG. 1 is a cross-sectional view of a multilayer separator 10 for a lithium-ion battery in accordance with one embodiment of the present invention.

[0050] The multilayer separator 200 is an ionic conductive separator able to be inserted between the two electrodes of the lithium-ion battery in order to ensure an electrical insulating and an ionic conduction.

[0051] The multilayer separator 200 of the invention presents a structure comprising: [0052] a polyolefin based substrate layer 204 forming the inner layer of the multilayer structure, [0053] a resin layer 203 stacked on both surface of the polyolefin substrate layer 204, the resin layer 203 being formed from a polyolefin, [0054] a cellulose fibers based outer layer 202 stacked on the surface of each resin layer 203.

[0055] Thus, the multilayer separator 200 of the invention presents a structure comprising two cellulose fibers based outer layers 202 and three polyolefin based inner layers 203, 204, 203 sandwiched between the two cellulose fibers based outer layers 202.

[0056] The polyolefin based substrate layer 204 provides shutdown effect and may be at least one selected group consisting of polyethylene, polypropylene, or a blend comprising substantially polypropylene or polyethylene.

[0057] Preferably, to achieve lower shutdown temperatures, the polyolefin based substrate layer 204 may be low-density polyethylene (LPDE), linear low-density polyethylene (LLDPE), or a blend comprising substantially LPDE, LLDPE or a mixture thereof.

[0058] Preferably, the polyolefin based substrate layer 204 may be at least one selected group consisting of a random copolymer of ethylene and alpha-olefins selected from the group consisting of coalpha-olefins having a polymerized alpha olefin content about 20% by weight, and preferably 16% by weight.

[0059] Preferably, the polyolefin based substrate layer 204 may be include copolymers of ethylene-butene and copolymers of ethylene-hexene.

[0060] Preferably, the polyolefin based substrate layer 204 has a melting point of 80 to 124° C.

[0061] The polyolefin based resin layer 203 provides higher mechanical properties (higher puncture strength) and the may be at least one selected group consisting of polyethylene, polypropylene, or a blend comprising substantially polypropylene or polyethylene.

[0062] Preferably, the polyethylene may be low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), or a blend comprising substantially LPDE, LLDPE or a mixture thereof.

[0063] Preferably, the polyolefin based resin layer 203 has a melting point of 130 to 160° C.

[0064] The thickness of the resin layer 203 may be 12 to 75 μm and the thickness of the polyolefin substrate layer 204 may be 5 to 15 μm.

[0065] Preferably, the polyolefin based substrate layer 204 is polyethylene and the polyolefin based resin layer 203 is polypropylene.

[0066] The resin layer 203 may be prepared in the form of film, and may be stacked on both surfaces of the polyolefin based substrate layer 204 in a lamination method. In the case of a general coating method, a polymeric resin is melted in a solvent and then applied, but this method has problems of long processing time and low productivity. Since the resin layer according to the invention is prepared in the form of film and stacked in a lamination method, there is no need of use a solvent, thus simplifying the preparation process.

[0067] Further, since the resin layer 203 is prepared separately, characteristics such as the thickness, porosity, composition and the like of the resin layer can be adjusted easily.

[0068] In a another embodiment, the resin layer 203 may be prepared in the form of film, and may be stacked on both surfaces of the polyolefin based substrate layer 204 in a hydraulic pressure method or a filtration method.

[0069] The outer layers 202 consist of cellulose fibers made from group consisting of ethyl cellulose, methyl cellulose, hydroxypropyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, carboxymethyl cellulose, cellulose acetate, cellulose triacetate, cellulose acetate phthalate, nitrocellulose, cellulose acetate butylate, cellulose acetate propionate, ammonium thereof, and salt thereof.

[0070] Cellulose fibers are preferably refined cellulose fibers, cellulose microfibrils, cellulose nanofibrils, lignin and derivatives thereof.

[0071] The Cellulose Fibers

[0072] The cellulose fibers of the of the outer layer 203 have size inferior or equal to 2 mm, in length.

[0073] Preferably, the cellulose fibers have size comprised between about 2 mm in length and about 100 nm in order to avoid the dendrite migration in the structure of the separator.

[0074] Preferably, cellulose fibers of the outer layer 203 are refined cellulose fibers, or cellulose nanofibrils, having a size of about 200 nm in length.

[0075] The cellulose fibers based outer layer 202 may be prepared separately, and may be stacked on the outer surface of each resin layer 203 in a lamination method, a hydraulic pressure, filtration method or slot die coating method.

[0076] According to one embodiment of the invention, the refined cellulose fibers (FBr) of the cellulose fibers based outer layer 202 can be obtained by a refining method comprising the following steps: [0077] a) a step of dispersion, in an aqueous medium, of previously dried cellulose fibers, to obtain a cellulose fiber paste in which the cellulose fiber content varies from 1 to 15% by weight relative to the total weight of said cellulose fiber paste; [0078] b) a step of shearing of said cellulose fiber paste, so as to obtain refined cellulose fibers, that is to say cellulose fibers exhibiting a Schopper-Riegler degree varying from 30 to 95° SR approximately.

[0079] As in the field of the paper industry, the refining method according to the present invention is a mechanical treatment of the cellulose fibers in order to obtain their hydration (step a) their fibrillation and their shortening as well as the creation of thin elements (step b).

[0080] The cellulose fibers based outer layer 202 according to the invention ensure greater electrolyte retention and thermal stability, while the polyolefin based resin layers 203 ensure higher puncture strength and the narrow pore size distribution fundamental to limit dendrite penetration in the separator (especially inside the polyolefin based substrate layer 204), and to avoid short circuit of the battery.

[0081] The higher puncture strength of resin layer 203 reduces risks of separator damaging in case of dendrite growing. The using of the polyolefin based resin layers 203 on each side of the polyolefin based substrate layer 204 allows considerably reducing the risk of failure.

[0082] Thanks to the multilayer separator 200 of the invention, the advantages of cellulose and polyolefin based materials are combined together to provide a multilayer separator, comprising the following material structure: cellulose material/polyolefin material/cellulose material, displaying high thermal stability, reduced shrinkage, high electrolyte retention and shutdown effect.

[0083] The multilayer separator 200 of the invention can be prepared according to manufacturing example shown hereinafter, but this example is no limitation thereto.

[0084] A linear low-density polyethylene (LLDPE) with a melt flow index of 2.0 was compounded with calcium carbonate particles that are surface-treated with calcium stearate. The calcium carbonate has an average particle size of 1 μm. Polymer compositions having LLPDE filled with 40%, 45%, and 50% by weight of CaCO respectively were then cast extruded into precursor films, each having a thickness of approximately 25 μm. Each resulting precursor film was cooled and subjected to tenter stress in the transverse direction with a stretch ratio of 2.5 to 1.

[0085] One layer of the resulting white porous film, forming the polyolefin based substrate layer 204 was then sandwiched between two stretched polypropylene (PP) microporous membranes, forming resin layer 203, and bonded together to form a trilayer structure PP/PE/PP forming the polyolefin based inner layers 203, 204, 203. Finally, the trilayer structure was coated by a slot dye technique with a nano-fibrillated cellulose (NFC) layer exhibiting a thickness of 2 μm from both side of the trilayer structure and dried over night at 80° C. Once dried the multilayer separator according to the invention has the following structure:

NFC/PP/PE/PP/NFC.

[0086] In the drawings and the description, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.