Coating containing polytetrafluoroethylene particles for a battery separator and a battery separator coated therewith

Abstract

A coated battery separator is described herein, and the coating is provided on one or both sides of the battery separator. The coating includes polytetrafluoroethylene (PTFE) particles, and the coating is the outermost coating layer. In some cases, the coating may also include ceramic particles.

Description

Technical Field

[0001] This application relates to coatings containing polytetrafluoroethylene (PTFE) particles for battery separators and such coated separators. Specifically, this application relates to an outermost coating containing PTFE particles. These coatings and coated separators can be used in secondary batteries including lithium-ion batteries.

Background Art

[0002] Celgard's important U.S. Patent No. 6,432,586 (now RE47520) describes a ceramic coating for battery separators including battery separators used in lithium-ion batteries. The ceramic coating has become an industry standard and has significantly improved the safety of lithium-ion batteries.

[0003] One problem associated with certain ceramic-coated separators is that they can easily wear or damage the equipment used by battery manufacturers. In addition, certain ceramic coatings generally exhibit a high moisture content, for example, about 600 - 700 ppm. Therefore, it is required to address these problems.

[0004] High-temperature resistant polymers such as PTFE have been used to coat battery separators to provide thermal stability. The organic solvent-based coating of PTFE results in an increased JIS Garreau value, at least 1000 seconds higher than that of an uncoated separator. In many cases, these organic solvent-based coatings resulted in an infinite Garreau measurement value even when using a coating with a thickness of 4 microns or less. Typically, a high Garreau measurement value means that the flow of electrolyte across the separator is blocked or significantly restricted. Although thermal stability is provided, there is a need for a coating that does not increase the Garreau value so much.

[0005] Furthermore, particulate fluoropolymer coatings, such as those containing polyvinylidene fluoride (PVDF), have been used for coatings for battery separators. However, these fluoropolymers are known to make the surface "sticky" and facilitate adhesion between the separator and the electrode surface. Although they provide thermal stability, there is a need for coatings that do not make the surface "sticky" and do not facilitate adhesion between the separator and the electrode surface. Also, there is a need for non-sticky coatings that provide a low coefficient of friction.

Summary of the Invention

[0006] In at least one aspect, objective, or embodiment, coatings and coated battery separators are described herein that solve or address one or more of the above-described problems or needs. For example, the coating of the present invention for the coated separator exhibits low friction, and as a result, the potential for damage to battery manufacturing equipment that may be observed with the use of ceramic coatings is reduced. The coating may also have a reduced moisture content, for example, on the order of 300 to 400 ppm. Additionally, the coating material may have a melting temperature above 300 degrees Celsius, which can help absorb heat generated in the battery during use. This heat absorption will improve safety by avoiding or delaying thermal runaway. The coated battery separator of the present invention comprises 1) a battery separator and 2) an outermost coating on at least one side of the battery separator. The outermost coating contains PTFE particles in an amount of 10 wt% to 100 wt% based on the particles. For example, the PTFE particles are 10 wt% of the total particles (90 wt% are other types of particles). The outermost coating containing PTFE particles may be added, for example, at about 1 to 20 grams per square meter (gsm), 1 to 15 gsm, or in some cases preferably 1 to 10 gsm of the separator. In some embodiments, the outermost coating may further comprise ceramic particles selected from, but not limited to, SiO2, Al2O3, CaCO3, TiO2, SiS2, SiPO4, boehmite or γ-AlO(OH), and AlO(OH). The coating may further comprise a binder containing an acrylic binder. The battery separator may be a polyolefin battery separator and / or a shutdown separator. In at least one possible preferred aspect, objective, or embodiment, an aqueous coating containing polytetrafluoroethylene (PTFE) particles and a coated battery separator having the same are described herein.

[0007] In some embodiments, the outermost coating may have a coefficient of friction (COF) that is less than 0.4, less than 0.35, less than 0.3, or less than 0.25.

[0008] In some embodiments, the outermost coating may have a melting point that is greater than about 300 °C or greater than about 320 °C.

[0009] In some embodiments, the outermost coating has a moisture content of 300 ppm to 400 ppm or less.

[0010] In some embodiments, the JIS Gurley value (seconds) of the coated separator, when the applied coating has a thickness of 0.1 to 4 microns, is about 1 second to about 300 seconds or less, or less than 50 seconds higher than that of the battery separator itself, i.e., than when this coating is absent or when no coating is present. As the coating becomes thicker, typically there is a greater increase in the Gurley value. This additional Gurley value is the "add-on" Gurley value.

[0011] In one aspect, a lithium-ion battery is formed that includes the coated separator described above herein. In some embodiments, the coating of the present invention is the outermost coating and may face the cathode of the lithium-ion battery.

[0012] The coating method is not limited and can include known coating methods such as spraying, printing, dipping, rolling, knife coating, extrusion, etc. The coating slurry can be cured or dried by known methods at ambient temperature or in an oven, depending on the type of slurry.

[0013] In another aspect, object, or embodiment, a coated polyolefin film of the present invention is described (such as a polyolefin film by a coated dry process or a dry stretching process). The coated polyolefin film may comprise a polyolefin film and a coating or a coating layer on at least one side of the polyolefin film, the coating contains polytetrafluoroethylene (PTFE) particles, and the coating is an outermost coating or a coating layer, and preferably, it can face the cathode of a secondary lithium ion battery or cell.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0015] The coated battery separator of at least some particular preferred embodiments herein comprises 1) a battery separator and 2) an outermost coating containing PTFE particles in an amount of 10 wt% to 100 wt% based on the particles.

[0016] Battery separator The battery separator is not particularly limited, and any membrane or film having both electrical insulation and ion conductivity and that can be used as a battery separator may be used.

[0017] The battery separator may have a thickness of 5 to 25 microns, 5 to 20 microns, 5 to 15 microns, or 5 to 10 microns.

[0018] In some preferred embodiments, the battery separator may be microporous, nanoporous, or mesoporous. In some preferred embodiments, the battery separator may have an average pore diameter in the range of 0.01 to 1.0 micron.

[0019] The battery separator may be made of any acceptable thermoplastic material. In some preferred embodiments, the battery separator may be made of a polyolefin. The polyolefin may be a blend, homopolymer, copolymer, or terpolymer of polyethylene or polypropylene.

[0020] The battery separator may be formed by a wet process that utilizes a solvent, oil, and / or plasticizer, or a dry process that does not utilize a solvent, oil, and / or plasticizer. The dry process may use a particulate pore former (e.g., beta-nucleated biaxial stretching process) or may not use one (e.g., Celgard® dry stretching process).

[0021] The battery separator may be a single-layer, two-layer, three-layer, or multi-layer separator. In some embodiments, the battery separator may be a shutdown separator having a structure of PP / PE, PP / PE / PP, PP / PE / PE / PP, or PP / PE / PP / PP, where PP is a polypropylene-containing layer and PE is a polyethylene-containing layer.

[0022] Outermost coating layer As can be understood, there is no layer above the outermost layer. The outermost layer may be provided directly on the surface of the battery separator, or the outermost layer may be formed on top of one, two, three, or more other layers. For example, in some embodiments, the outermost layer may be formed on top of a ceramic coating layer. The outermost layer may be a continuous layer or a discontinuous (e.g., patterned) layer.

[0023] The outermost layer described herein may contain PTFE particles in an amount of 10 wt% - 100 wt%, 15 wt% - 100 wt%, 20 wt% - 100 wt%, 25 wt% - 100 wt%, 30 wt% - 100 wt%, 35 wt% - 100 wt%, 40 wt% - 100 wt%, 45 wt% - 100 wt%, 50 wt% - 100 wt%, 55 wt% - 100 wt%, 60 wt% - 100 wt%, 65 wt% - 100 wt%, 70 wt% - 100 wt%, 75 wt% - 100 wt%, 80 wt% - 100 wt%, 85 wt% - 100 wt%, 90 wt% - 100 wt%, or 95 wt% - 100 wt% with respect to the particles. When forming the outermost layer using an aqueous coating slurry, PTFE particles are present. When a solvent-based coating slurry is used, PTFE dissolves and the coating does not contain PTFE particles. An SEM image of the coating is shown in Figure 1, where PTFE particles are visible. The outermost coating containing PTFE particles may be added, for example, at about 1 - 20 grams per square meter (gsm), 1 - 15 gsm, or optionally preferably 1 - 10 gsm of the separator.

[0024] In some embodiments, the outermost coating layer may further contain ceramic particles in addition to the PTFE particles. The ceramic particles may be one or more selected from, but not limited to, SiO2, Al2O3, CaCO3, TiO2, SiS2, SiPO4, boehmite or γ - AlO(OH), and AlO(OH).

[0025] In embodiments where ceramic is added, the ratio of PTFE particles to ceramic particles may be in the range of 1:100 to 100:1, 1:75 to 75:1, 1:50 to 50:1, 1:25 to 25:1, 1:10 to 10:1, 1:9 to 9:1, 1:8 to 8:1, 1:7 to 7:1, 1:6 to 6:1, 1:5 to 5:1, 1:4 to 4:1, 1:3 to 3:1, or 1:2 to 2:1, by weight of the particles. In some preferred embodiments, the ratio can be in the range of about 1:5 to 10:1.

[0026] The outermost coating layer may further include a binder and / or other additives, materials, or components. For example, the binder may include one or more selected from polyvinyl alcohol (PVA), poly-N-vinylacetamide (PNVA), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyvinyl acetate (PVAc), or combinations thereof.

[0027] According to at least certain aspects, objectives, or embodiments, the coating of the present invention comprising PTFE particles is made from an aqueous coating slurry or formulation or an aqueous acrylic coating slurry having at least one binder, PTFE particles, and optionally further additives, materials, or components. Further, at least selected aspects, objectives, or embodiments include a novel or improved coating for a porous or microporous membrane or substrate including a battery separator or separator membrane or base film, a novel and / or improved coated porous or microporous membrane or substrate including a battery separator, a novel or improved coating for a porous or microporous membrane or substrate including a battery separator, a novel or improved coating, coating slurry, or coating formulation comprising at least (i) a polymer binder, (ii) PTFE particles, and (iii) at least one component selected from the group consisting of a crosslinking agent, a low-temperature shutdown agent, a diffusing agent, and a thickening agent, and / or a novel or improved coated porous or microporous polymer membrane or substrate, wherein the coating comprises at least PTFE particles.

[0028] In some embodiments, the outermost coating layer can have a thickness of 1 to 20 microns, 1 to 19 microns, 1 to 18 microns, 1 to 17 microns, 1 to 16 microns, 1 to 15 microns, 1 to 14 microns, 1 to 13 microns, 1 to 12 microns, 1 to 11 microns, 1 to 10 microns, 1 to 9 microns, 1 to 8 microns, 1 to 7 microns, 1 to 6 microns, 1 to 5 microns, 1 to 4 microns, 1 to 3 microns, or 1 to 2 microns.

[0029] In some embodiments, the outermost coating can have a coefficient of friction (COF) that is less than 0.4, less than 0.35, less than 0.3, or less than 0.25.

[0030] In some embodiments, the outermost coating can have a melting point that is greater than about 300 °C or greater than about 320 °C.

[0031] In some embodiments, the outermost coating containing PTFE particles can be added, for example, at about 1 to 20 grams per square meter (gsm), 1 to 15 gsm, or optionally preferably 1 to 10 gsm of the separator.

[0032] In some embodiments, the JIS Gurley value (seconds) of the coated separator is about 1 second to about 300 seconds, 1 second to 200 seconds, 1 second to about 100 seconds, 1 second to about 50 seconds, or 1 second to about 30 seconds higher than that of the battery separator itself, i.e., than in the case where this coating is absent or no coating at all. This is the case when the thickness of the added coating is 0.1 to 4 microns. As the coating gets thicker, typically there is a greater increase in the Gurley value.

[0033] Described herein are coated battery separators adapted or suitable for use in secondary batteries such as, but not limited to, secondary lithium ion batteries. The coating is provided on one or both sides of the battery separator. The coating contains polytetrafluoroethylene (PTFE) particles and the coating is the outermost coating layer. In some cases, the coating may also contain ceramic particles.

Examples

[0034] One comparative example and six examples were fabricated and tested. Figures 1 and 2 show SEMs of coated three-layer films according to some embodiments described herein. In all examples, the three-layer base film has a Gurley value of 178 seconds. Further details of the examples are as follows.

[0035] Comparative Example 1: A film of about 5 microns was provided on a three-layer polyolefin battery separator with a thickness of about 16 microns. The film contained only Al2O3 particles and no PTFE particles. Example 1: Example 1 is the same as Comparative Example 1 except that the coating contains only PTFE particles and does not contain ceramic particles (e.g., Al2O3). Example 2: Example 2 is the same as Example 1 except that it has more binder than Example 1, and as shown in the data, the coefficient of friction (COF) slightly increased. Example 3 (c-1): Example 3 is the same as the other examples except that the coating in Example 3 contained PTFE particles and Al2O3 particles. The ratio of PTFE to Al2O3 is 0.26:1. Example 4 (c-2): Example 4 is the same as Example 3 except that the ratio is 1.58:1. Example 5 (c-3): Example 5 is the same as Example 3 except that the ratio is 3.95:1. Example 6 (c-4): Example 6 is the same as Example 3 except that the ratio is 9.47:1.

[0036] The coefficient of friction (COF) of the coating of each example was tested, and the results are reported in Figure 3. Samples for the COF test were prepared, placed on an EZ Sled for the COF test, and the samples were made to be in a taut state without wrinkles or creases. Next, ASTM D1894 (Standard Test Method for Static and Kinetic Coefficient of Friction of Plastic Films and Sheeting, British Inch / Gram) was carried out. Parameters of force (N), distance (cm), and thread (200 g) were used.

[0037] The increase in Gurley value (add-on Gurley value) due to providing a coating on the battery separator was also tested, and the results are reported in FIG. 4. The data shows that the coated separator has a lower coefficient of friction (COF) compared to a coated separator of a comparative example having only ceramic (e.g., Al2O3) during coating. This alleviates the problem that the ceramic coating deteriorates the battery manufacturing apparatus. The coating described herein also has a low add-on Gurley value, and thus the ion flow across the membrane is not affected as in the case of using other types of coatings (e.g., PTFE coating using an organic solvent-based coating slurry, which results in a coated product having a high Gurley value or even an infinite Gurley value). The add-on Gurley value (JIS Gurley value measured in seconds) is the difference in Gurley value between the coated battery separator and the uncoated battery separator. This indicates the amount of Gurley value that is "added on" by the addition of the coating to the battery separator.

Claims

1. A battery separator, and A coating layer on at least one side of the battery separator, wherein the coating contains polytetrafluoroethylene (PTFE) particles, and the coating is the outermost coating layer, the coating layer; A coated battery separator comprising.

2. The coated battery separator according to claim 1, wherein the coating contains 10% to 100% PTFE particles based on the weight of the particles.

3. The coated battery separator according to claim 1, wherein the coating also contains ceramic particles.

4. The ceramic particles are SiO ２ , Al ２ O ３ , CaCO ３ , TiO ２ , SiS ２ , SiPO ４ , boehmite (γ - AlO(OH)), and one or more selected from AlO(OH), the coated battery separator according to claim 1.

5. The coated battery separator according to claim 3, wherein the coating is a ceramic coating.

6. The coated battery separator according to claim 1, wherein the coating does not contain fluoropolymers other than PTFE.

7. The coated battery separator according to claim 1, wherein the coating consists essentially of PTFE particles and optionally a binder.

8. The coated battery separator according to claim 7, wherein the binder is an acrylic binder.

9. The coated battery separator according to claim 1 or claim 3, wherein the coating has a coefficient of friction (COF) of less than 0.

4.

10. The coated battery separator according to claim 9, wherein the COF is less than 0.

35.

11. The coated battery separator according to claim 9, wherein the COF is less than 0.

30.

12. The coated battery separator according to claim 9, wherein the COF is less than 0.

25.

13. The coated battery separator according to claim 1, wherein the coating has a melting point above 300 °C or above 320 °C.

14. The coated battery separator according to claim 3, wherein the coating has a melting point above 300 °C or above 320 °C.

15. The coated battery separator according to claim 7, wherein the coating has a melting point above 300 °C or above 320 °C.

16. The coated battery separator according to claim 1, wherein the coating is formed using an aqueous coating slurry.

17. The battery separator is a shutdown separator, and the coating extends the shutdown. The coated battery separator according to claim 1.

18. The battery separator is a polyolefin battery separator. The coated battery separator according to claim 1.

19. The battery separator comprises another coating, the same as or different from the coating containing PTFE particles. The coated battery separator according to claim 1.

20. The coated battery separator has a coating thickness of 0.1 to 4 microns and has a JIS Garreau value (seconds) that is about 1 second to about 300 seconds greater than the JIS Garreau value (seconds) of the battery separator without coating. The coated battery separator according to claim 1.

21. The coated battery separator has a coating thickness of 0.1 to 4 microns and has a JIS Garreau value (seconds) that is less than 50 seconds greater than the JIS Garreau value (seconds) of the battery separator without coating. The coated battery separator according to claim 20.

22. The coating contains 10% PTFE particles by weight of the particles. The coated battery separator according to claim 1.

23. The coating contains 100% PTFE particles. The coated battery separator according to claim 1.

24. The coating layer has a moisture content of 300 to 400 ppm or less. The coated battery separator according to claim 1.

25. A lithium-ion battery comprising the coated battery separator according to any one of claims 1 to 24.

26. A polyolefin film, A coating layer on at least one side of the polyolefin film, wherein the coating contains polytetrafluoroethylene (PTFE) particles and the coating is the outermost coating layer. A coated polyolefin film comprising:

27. The outermost coating containing PTFE particles is added at about 1 to 20 grams per square meter (gsm), 1 to 15 gsm, or 1 to 10 gsm of the separator. The coated battery separator according to claim 1.

28. The coated polyolefin film according to claim 26, wherein the outermost coating containing PTFE particles is added at about 1 to 20 grams per square meter (gsm), 1 to 15 gsm, or 1 to 10 gsm of the separator.

Images