Coating method and resulting ceramic modified separator

Abstract

The present disclosure relates to a continuous process for coating a microporous polyolefin web with a ceramic composition or slurry, followed by drying at elevated temperatures while under lateral restraint. Such webs can be used to improve the manufacturability, performance, and safety of energy storage devices, such as lithium batteries.

Description

[Technical Field]

[0001] Related Applications This application claims priority to U.S. Provisional Patent Application No. 63 / 376,952, filed September 23, 2022, entitled COATING METHOD AND RESULTING CERAMIC-MODIFIED SEPARATOR, which is incorporated herein by reference in its entirety.

[0002] Copyright Notice (Copyright) 2023 Amtek Research International LLC. A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR §1.71(d).

[0003] The present invention relates to a continuous process in which a microporous polyolefin film is coated with a ceramic composition or slurry, followed by drying at elevated temperatures while laterally restrained, trimming off damaged edges, and winding onto a finished roll. The improvements include (1) the ability to overcome edge effects that result in defects and reduced yield when using conventional flotation furnaces, and (2) faster line speeds compared to stenters used in the textile industry as a result of offsetting the air knives. The resulting film exhibits good in-plane dimensional stability (i.e., low shrinkage) both above and below the melting point of the polyolefin matrix. At elevated temperatures (>135°C), pores within the film's bulk structure can begin to collapse or shut down, thereby modifying permeability through the film without appreciable change in in-plane dimensions. Such films can be used to improve the manufacturability, performance, and safety of energy storage devices, such as lithium batteries. Throughout this application, the term "lithium battery" refers to Li-ion and / or rechargeable Li-metal-based batteries. [Background technology]

[0004] The separator is an essential part of the performance, safety, and cost of lithium-ion batteries. During normal operation, the separator's primary function is to prevent electronic conduction (i.e., short circuit or direct contact) between the anode and cathode while allowing ionic conduction through the electrolyte. In small, commercially available cells under abusive conditions, such as an external short circuit or overcharging, the separator must shut down at a temperature well below the point at which thermal runaway can occur. Shutdown occurs from the collapse of the separator's pores due to polymer melting and viscous flow, thus slowing or stopping ionic flow between the electrodes. Nearly all Li-ion battery separators contain polyethylene as a single-layer or multi-layer component, so that shutdown begins at approximately 130°C, close to the melting point of polyethylene.

[0005] Separators for the lithium-ion battery market are currently manufactured via either "dry" or "wet" processes. Companies such as Celgard LLC describe a dry process in which polypropylene (PP) or polyethylene (PE) is extruded into a thin sheet and rapidly reduced in size. The sheet is then tempered at 10-25°C below the polymer's melting point to control the crystallite size and orientation. The sheet is then rapidly stretched in the machine direction (MD) to achieve slit-like holes or voids. Three-layer PP / PE / PP separators produced by the dry process are commonly used in lithium-ion rechargeable batteries.

[0006] Wet-process separators composed of high molecular weight polyethylene are produced by extrusion of a plasticizer / polymer mixture at elevated temperatures, followed by phase separation, biaxial stretching, and extraction of the pore-forming agent (i.e., plasticizer). The resulting separators have ellipsoidal or spherical pores with good mechanical properties in both the longitudinal and transverse directions. PE-based separators produced by this method by Toray, Asahi, SEM Corp, and ENTEK have found widespread use in Li-ion batteries.

[0007] For larger or higher capacity Li-ion cells designed for electric vehicles, the benefits of separator shutdown have been questioned because it is difficult to ensure a sufficient rate and uniformity of shutdown throughout the completed cell. Therefore, many companies have focused on modifying lithium-ion cell structures to include (1) high-temperature separators or (2) heat-resistant layers coated on either the electrodes or conventional polyolefin separators. Heat-resistant separators constructed from high-temperature polymers (e.g., polyimides) are produced on a limited basis from solution casting, electrospinning, or other process techniques. In these cases, the melting points of the high-temperature polymers prevent shutdown at temperatures below 200°C.

[0008] In U.S. Patent No. 9,896,555 B2, ENTEK describes a free-standing, microporous, ultra-high molecular weight polyethylene (UHMWPE)-based film containing sufficient inorganic filler particles to provide low shrinkage while maintaining high porosity at temperatures above the melting point of the polymer matrix (>135° C.). Such free-standing, heat-resistant films have excellent hydration properties and ultra-low impedance, but do not exhibit shutdown properties due to the high inorganic filler loading level.

[0009] In U.S. Patent No. 7,638,230 B2, a porous heat-resistant layer was coated on the negative electrode. The heat-resistant layer consisted of an inorganic filler and a polymeric binder. The inorganic filler included magnesia, titania, zirconia, or silica. The polymeric binder included a modified rubber mixture containing polyvinylidene fluoride and acrylonitrile units. The heat-resistant layer contained 1 to 5 parts by weight of binder per 100 parts by weight of inorganic filler. A higher binder content adversely affected the high-rate discharge characteristics of the battery. Furthermore, the thickness of the porous heat-resistant layer had to be limited to 1 to 10 μm to achieve high discharge rates, and it could not be removed as a free-standing film. Free-standing refers to a sheet with sufficient mechanical properties to allow operations such as winding and unwinding of the film during assembly of the energy storage device.

[0010] U.S. Patent Application Publication Nos. 2008 / 0292968 A1 and 2009 / 0111025 A1 disclose organic / inorganic separators in which a porous substrate is coated with a mixture of inorganic particles and a polymeric binder to form an active layer on at least one surface of the porous substrate. The porous substrate can be either a nonwoven fabric, a membrane, or a polyolefin-based separator. The inorganic particles are selected from the group consisting of those exhibiting a dielectric constant greater than 5, piezoelectricity, and / or lithium ion conductivity. Various polymeric binders are described. The composite separators are claimed to exhibit superior thermal safety, dimensional stability, electrochemical safety, and lithium ion conductivity compared to uncoated polyolefin-based separators used in lithium batteries. Certain polymeric binders mixed with inorganic particles can provide a high degree of electrolyte swelling at the surface layer, but rapid wetting or swelling is not achieved in polyolefin substrates.

[0011] When a ceramic composition or slurry is applied to a microporous polymer matrix or polyolefin web and then dried to form a lithium-ion separator that is dimensionally stable at high temperatures and has improved oxidation resistance while in contact with a high-pressure cathode, there is a need to reduce costs through improved throughput and yield. Often, the rate-limiting step involves handling and drying the web through either a floatation furnace or a conventional hot air oven. Improved yield and throughput are made even more difficult when only one side of the polyolefin web is first coated, and then the other (second) surface of the web is coated in a secondary unwinding and coating step.

[0012] Until now, no consideration has been given to combining the benefits from a stenter, where typically the liquid saturated fabric is held under lateral tension with pins or clips as it traverses a conventional hot air oven equipped with high velocity air knives used in a flotation oven for drying. In the present invention, applicants have discovered that such a combination is a cost-effective method for drying polyolefin webs where a ceramic composition or slurry is applied to both surfaces of the web. Summary of the Invention [Problem to be solved by the invention]

[0013] An object of the present invention is to achieve a thin, free-standing, microporous polyolefin membrane with good oxidation resistance and high-temperature dimensional stability, as provided through a porous ceramic layer added to one or both surfaces of the web or polymer matrix. "Free-standing" refers to a web or membrane that has sufficient mechanical properties to be used for unwinding, coating, winding, scoring, and other web-manipulating operations. The terms "film," "sheet," "substrate," "web," and "membrane" can be used interchangeably, and the term membrane can be used to encompass webs, films, substrates, and sheets.

[0014] There are a wide variety of drying and curing ovens commonly used for coating plastic films, papers, or foils and roll-to-roll processing. Such ovens can include roll support or belt / conveyor support for single-sided coating of such substrates. For double-sided coating, the web cannot touch the support roller before it is completely dry. This is where flotation ovens are best utilized for ceramic slurry coating of microporous polyolefin webs, especially on both sides, for applications as separators in lithium batteries. [Means for solving the problem]

[0015] A side view of an open levitation furnace is shown in FIG. 1 (depicting an exemplary levitation furnace with offset air knives 10). During use of such a furnace, a double-sided coated plastic film, paper, or foil is expected to follow a sinusoidal pattern established by the air knives as it traverses the furnace. Simultaneously, the liquid carrier (e.g., water or organic solvent) from the coating solution evaporates and is removed from the surface of the plastic film, paper, or foil. An advantage of a levitation furnace is that the air velocity and air knife geometry can be adjusted to achieve rapid drying. In the case of porous materials such as paper, there are also applied capillary forces within the pores that result in stress and shrinkage during the drying process.

[0016] Special care must be taken with microporous polyolefin webs due to their thickness (<20-25 um) and porosity, which results in lower modulus (i.e., stiffness) compared to conventional PET films (approximately 100 um) that are commonly coated and dried in such ovens. Additionally, the distance between the top of the air knife and the microporous polyolefin web advancing through the oven, and the associated air velocity, must be rebuilt for each different thickness, not only of the web but also of the thickness to which the coating solution is applied when it is wet ("wet" thickness).

[0017] An additional challenge with flotation furnaces is the edge effect that results from "baggy edges" in the master roll of microporous polyolefin web as it unwinds and passes through a coating station before entering the flotation furnace. Even without the application of a coating, the "baggy edges" make it difficult to establish a uniform sinusoidal pattern from the air knife. This effect is exacerbated once a coating solution is applied, and defects occur when the coated web touches the air knife, leaving ceramic deposits and uncoated areas on the web. The coated edge can also curl up due to uneven deposition of coating solution at the edge.

[0018] Edge effects and other potential defects become more prevalent as thin microporous polyolefin webs widen because of the increased tendency for longitudinal wrinkles to form. As a result, most flotation furnaces used to produce ceramic-coated polyolefin separators do not exceed 1.2 meters in width.

[0019] To overcome the above challenges, ENTEK combined its floating furnace design with the stenter technique used in the textile industry, where wet fabric is held laterally by pins or clips while the material passes through a hot air oven where convection is used to dry it. A schematic diagram of an exemplary stenter is shown in Figure 2 (depicting the fabric (with chemicals applied) being held laterally via pin or needle chains as the fabric passes through the oven or dryer).

[0020] The present invention is a microporous, free-standing, heat-resistant polyolefin membrane obtained by applying a ceramic composition or slurry to one or both major surfaces and then drying the membrane by impinging it with an air knife while simultaneously being laterally restrained (as shown in Figure 3). The lateral restraint serves to minimize capillary forces that typically lead to shrinkage or pore collapse during solvent evaporation. [Brief explanation of the drawings]

[0021] [Figure 1] 1 depicts a schematic diagram of an exemplary levitation furnace. [Figure 2] 1 depicts a schematic diagram of an exemplary stenter. [Figure 3] 1 depicts a schematic diagram of an exemplary drying oven according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0022] The microporous membranes used in the present invention are composed of a polyolefin polymer matrix or bulk structure. The polyolefin most preferably used for the polyolefin matrix is ​​very high molecular weight polyethylene (VHMWPE) with a Mw greater than about 300,000 g / mol (e.g., from about 300,000 to about 3.1 million g / mol). In some cases, it may be desirable to blend VHMWPE with other polyolefins, such as ultra-high molecular weight polyethylene (UHMWPE, Mw greater than about 3.1 to about 10 million g / mol), high density polyethylene (HDPE), or linear low density polyethylene (LLDPE), to affect properties such as the membrane's shutdown or melting characteristics. Microporous membranes generally contain about 35-65% porosity. Pore sizes generally range from about 10 nanometers to several microns, with an average pore size of less than about 1 micrometer. The membrane thickness (excluding the coating) is generally about 3-25 μm, or about 20 μm or less.

[0023] The ceramic coating formulation used in the present invention comprises inorganic particles dispersed in an aqueous solvent mixture, which may contain a small amount of alcohol to improve wetting of the surface of the polyolefin membrane. Exemplary inorganic particles that can be used include at least one of inorganic oxides, carbonates, or hydroxides, such as alumina, silica, zirconia, titania, mica, boehmite, magnesium hydroxide, calcium carbonate, another suitable inorganic acid, or a mixture thereof. One or more hydrotalcites can also be used, alone or in combination with other types of inorganic particles. The inorganic particles are typically charge-stabilized to remain suspended within the aqueous mixture. Low molecular weight water-soluble polymers or polymer dispersions are typically used as binders for the inorganic particles. To minimize binder concentration and yet achieve a strong, microporous surface film that does not easily shed the inorganic particles, it is desirable to select a polymer with many hydrogen-bonding sites. Acrylic acid, polyvinylpyrrolidone, polyvinyl alcohol, carboxymethyl cellulose, and their copolymers or derivatives represent preferred polymeric binders, such that 10 parts by weight or less of the polymeric binder can be used with 90 parts by weight or more of the inorganic particles.

[0024] In another embodiment, an adhesive layer comprising an acrylic acid or polyvinylidene fluoride (PVDF) copolymer can be deposited onto the ceramic layer to form a four- or five-layer structure. As will become apparent, a four-layer structure can include a polyolefin layer coated on one major surface with a ceramic layer, followed by an adhesive layer coating each major surface of the polyolefin / ceramic structure. A five-layer structure can include a polyolefin layer coated on both major surfaces with a ceramic layer, followed by an adhesive layer coating each major surface of the ceramic layer. In another embodiment, the acrylic acid or polyvinylidene fluoride (PVDF) copolymer can be combined with inorganic particles so that the double-coated separator can then be bonded to the battery electrodes using heat and pressure.

[0025] In addition to controlling the amount of polymeric binder and inorganic particles in the ceramic coating formulation, applicants have discovered that it is important to control the particle size distribution of the inorganic particles. Furthermore, the coating formulation must be carefully applied to the polyolefin web to control the thickness of the resulting surface layer. Coating techniques include dip coating, microgravure, gravure, spray, slot die, and other processes that can achieve the correct "wet" thickness before drying. A preferred drying method, as shown in FIG. 3, utilizes an air knife 20 in combination with a pin or clip system 30 that holds and laterally guides the web 40 as it moves through an oven and then is wound into a roll. The air knife 20 can be aligned (as shown in FIG. 3) or offset. In such drying systems, the width of the polyolefin web 40 passing through the dryer or oven can exceed about 1.2 m (e.g., from about 1.2 m to about 2.4 m or more). The temperature of the dryer or oven can be from about 60°C to about 135°C, from about 80°C to about 130°C, or about 125°C or higher. When dry, the coating weight on the membrane is approximately 0.3 g / m 2 ~Approx. 12g / m 2 , about 1g / m 2 ~about 8g / m 2 , or about 4 g / m 2 ~about 6g / m 2 The coated membrane also exhibits a Gurley number of about 30 to about 250 seconds / 100 cc of air, or about 100 to about 200 seconds / 100 cc of air. [Example]

[0026] Example 1 A microporous polyolefin membrane (ENTEK12EPH, 12.4 μm, porosity 49.6%, Gurley air permeability = 104 s / 100 cc of air) was unwound under controlled tension and passed over a carbon composite roll into a horizontal position, at which point a #10 Mayer rod was used to coat the top membrane surface with a 20 wt.% solids aqueous ceramic slurry (97 / 3 fumed alumina / PVOH) containing 4 wt.% isopropanol to improve wetting. The solution coated the entire width of the sheet, except for approximately the outer 40 mm of each side, which were then held in clips on a chain system. The chains holding each side of the web were parallel to the entire length of the oven, which had offset top and bottom air knives approximately 100 mm from each surface of the polyethylene membrane. The wetting membrane advanced through the oven at 20 m / min while the oven temperature was maintained at 125 °C within the plenum that provided the air knives.

[0027] The coating weight for the ceramic coating on one side is approximately 1.1 g / m 2 The thickness was determined to be about 14.4 μm, and the air permeability was determined to be 96 sec / 100 cc of air, which was unexpectedly lower than that of the uncoated polyethylene membrane.

[0028] Example 2 A microporous polyolefin membrane (ENTEK12EPH, 12.4 μm, porosity 49.6%, Gurley air permeability = 104 s / 100 cc of air) was unwound under controlled tension and passed over a carbon composite roll into a horizontal position, at which point a #10 Mayer rod was used to coat the top membrane surface with a 20 wt.% solids aqueous ceramic slurry (97 / 3 fumed alumina / PVOH) containing 4 wt.% isopropanol to improve wetting. The solution coated the entire width of the sheet, except for approximately the outer 40 mm of each side, which were then held in clips on a chain system. The chains holding each side of the web were parallel to the entire length of the oven, which had offset top and bottom air knives approximately 100 mm from each surface of the polyethylene membrane. The wetting membrane advanced through the oven at 20 m / min while the oven temperature was maintained at 135 °C within the plenum that provided the air knives.

[0029] Even at this higher dryer temperature, which is close to the peak melting point of polyethylene, single-sided ceramic coated membranes with a coating weight of 1.25 g / m 2 and the Gurley value was 96 seconds / 100cc of air.

[0030] It will be understood that references throughout this specification to "an embodiment" or "the embodiment" mean that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment. Thus, the appearances of such phrases, or derivatives thereof, as recited throughout this specification do not necessarily all refer to the same embodiment.

[0031] Similarly, in the above description of the embodiments, it should be recognized that various features are sometimes grouped together in a single embodiment, drawing, or description to streamline the disclosure. However, this method of disclosure should not be interpreted as reflecting an intention that any claim require more features than are expressly recited in that claim. Rather, as the following claims reflect, inventive aspects lie in combinations of fewer than all features of a single foregoing disclosed embodiment. Accordingly, the claims following the Detailed Description are expressly incorporated into this Detailed Description, with each claim standing on its own as an embodiment. The present disclosure includes all permutations of independent claims with their dependent claims. Furthermore, additional embodiments that can be derived from the independent claims and the following dependent claims are also expressly incorporated into this written description.

[0032] The recitation in a claim of the term "first" with respect to a feature or element does not necessarily imply the presence of a second or additional such feature or element.

[0033] References to approximations, such as by use of the term "about," are made throughout this specification. For each such reference, it is understood that in some embodiments, the value, feature, or characteristic may be stated without approximation. When a modifier such as "about" is used, the term includes within its scope the modified word without the modifier. For example, when the term "about" is recited in connection with a feature, it is understood that in further embodiments, the feature may have the exact configuration. Unless otherwise stated, all ranges include both endpoints and all numbers between the endpoints.

[0034] Without further elaboration, it is believed that one skilled in the art can use the foregoing to utilize the present invention to its fullest extent. The claims and the embodiments disclosed herein should be construed as merely illustrative and exemplary and in no way limiting the scope of the present disclosure. It will be apparent to those skilled in the art that, using the present disclosure, changes may be made to the details of the above-described embodiments without departing from the underlying principles of the present disclosure herein. In other words, various modifications and improvements to the embodiments disclosed in detail in the above description are within the scope of the appended claims. Moreover, the order of steps or acts of methods disclosed herein may be changed by those skilled in the art without departing from the scope of the present disclosure. In other words, unless a specific order of steps or acts is necessary for the proper operation of an embodiment, the order or use of specific steps or acts may be modified. Accordingly, the scope of the present invention is defined by the following claims and their equivalents.

Claims

1. 1. A free-standing microporous membrane, comprising: a polymeric matrix extending from one surface to an opposite surface, said polymeric matrix comprising a polyolefin having a molecular weight of 300,000 g / mol or greater; a porous ceramic coating deposited on at least one side of the polymeric matrix, the microporous membrane being formed in a continuous process in which the ceramic coating is dried while the polymeric matrix is ​​laterally constrained at an elevated temperature.

2. 10. The microporous membrane of claim 1, wherein the polymer matrix comprises a blend of very high molecular weight polyethylene (VHMWPE) and at least one of ultra high molecular weight polyethylene (UHMWPE), high density polyethylene (HDPE), or linear low density polyethylene (LLDPE).

3. 3. The microporous membrane of claim 1 or 2, wherein the polymer matrix comprises a thickness of about 20 μm or less.

4. The microporous membrane of any one of claims 1 to 3, wherein the polymer matrix comprises a width of about 1.2 m or greater.

5. The microporous membrane of any one of claims 1 to 4, wherein the porous ceramic coating is deposited on two opposite sides of the polymer matrix.

6. The film has a thickness of about 0.3 to about 12 g / m ２ , about 1 to about 8 g / m ２ , or about 4 to about 6 g / m ２ The microporous membrane of any one of claims 1 to 5, having a coating weight of

7. 7. The microporous membrane of any one of claims 1 to 6, wherein the membrane exhibits a Gurley number of about 30 to about 250 seconds / 100 cc of air, or about 100 to about 200 seconds / 100 cc of air.

8. The microporous membrane of any one of claims 1 to 7 for use as a ceramic-modified separator in a lithium-ion or rechargeable Li-metal battery.

9. 1. A method of making a free-standing microporous membrane, comprising: constraining a polymer matrix laterally, the polymer matrix comprising a polyolefin having a molecular weight of 300,000 g / mol or greater; coating at least a portion of at least one side of the polymeric matrix with a ceramic coating; and exposing the polymeric matrix and the ceramic coating to an elevated temperature.

10. The method of claim 9 , wherein constraining the polymer matrix comprises constraining the polymer matrix with pins or clips.

11. 11. The method of claim 9 or 10, wherein exposing the polymeric matrix and the ceramic coating to an elevated temperature comprises exposing the polymeric matrix and the ceramic coating to a temperature of about 125°C or greater.

12. The method of any one of claims 9 to 11, wherein exposing the polymer matrix and the ceramic coating to an elevated temperature comprises arranging the polymer matrix and the ceramic coating in a drying oven.

13. The method of any one of claims 9 to 12, wherein the polymer matrix comprises a thickness of about 20 μm or less.

14. The method of any one of claims 9 to 13, wherein the polymer matrix comprises a width of about 1.2 m or more.

15. 15. The method of any one of claims 9 to 14, wherein the polymer matrix comprises a blend of very high molecular weight polyethylene (VHMWPE) and at least one of ultra high molecular weight polyethylene (UHMWPE), high density polyethylene (HDPE), or linear low density polyethylene (LLDPE).

16. The method of any one of claims 9 to 15, wherein the ceramic coating is on two opposite sides of the polymer matrix.

17. The film has a thickness of about 0.3 to about 12 g / m ２ , about 1 to about 8 g / m ２ , or about 4 to about 6 g / m ２ The method of any one of claims 9 to 16, wherein the coating has a coating weight of

18. The method of any one of claims 9 to 17, wherein the membrane exhibits a Gurley number of about 30 to about 250 seconds / 100 cc of air, or about 100 to about 200 seconds / 100 cc of air.

19. A free-standing microporous membrane formed according to the method of any one of claims 9 to 18.

20. A drying oven, The entrance and an outlet downstream from the inlet; a passageway extending between the inlet and the outlet; a plurality of air knives positioned within the passageway; a plurality of restraining elements configured to laterally restrain the polyolefin web as it moves from the inlet to the outlet.

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