Color-coded, ceramic-coated battery separator

Abstract

Ceramic-coated, microporous polyolefin membranes having a contrast agent incorporated into the polyolefin membrane or the ceramic coating are disclosed herein. The contrast agent can include a dye, pigment, inorganic oxide, and / or other material that imparts color to the membrane or coating. The contrast agent enables one to easily determine which side of the membrane includes the ceramic coating. Such ceramic-coated, polyolefin membranes can be used as separators to improve the manufacturability, performance, and safety of energy storage devices such as lithium-ion batteries.

Description

COLOR-CODED, CERAMIC-COATED BATTERY SEPARATORRelated Applications

[0001] This application claims priority to United States Provisional Application No. 63 / 621 ,852, filed on January 17, 2024, and titled Color-Coded, Ceramic-Coated Battery Separator, which is incorporated herein by reference in its entirety.Copyright Notice

[0002] © 2025 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).Technical Field

[0003] The present invention relates to the formation of a ceramic-coated, microporous polyolefin membrane in which a contrast agent is incorporated into the polyolefin membrane or the ceramic coating to highlight which side is coated. The contrast agent can be a dye, pigment, inorganic oxide, or other material that imparts color to the membrane or coating Such ceramic-coated, polyolefin membranes can be used as separators to improve the manufacturability, performance, and safety of energy storage devices such as lithium-ion batteries.Background of the Invention

[0004] Separators are an integral part of the performance, safety, and cost of lithium-ion batteries. During normal operation, the principal functions of the separator are to prevent electronic conduction (i.e., shorts or direct contact) between the anode and cathode while permitting ionic conduction via the electrolyte Under abuse conditions, such as external short circuit or overcharge, the separator is required to shutdown at temperatures well below where thermal runaway can occur. Shutdown results from the collapse of pores in the separator due to melting and viscous flow of the polymer, thus slowing down or stopping ion flow between the electrodes. Nearly all Li-ion battery separators contain polyethylene as part of a single- or multi-layer construction so that shutdown begins at -130 °C, the melting point of polyethylene.

[0005] Separators for the lithium-ion market are presently manufactured via “dry” or “wet” processes In a dry process, polypropylene (PP) or polyethylene (PE) is extruded into a thin sheet and subjected to rapid drawdown The sheet is then annealed at 10-25 °C below the polymer melting point such that crystallite size and orientation are controlled. Next, the sheet is rapidly stretched in the machine direction (MD) to achieve slit-like pores or voids. Trilayer 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 an oil / polymer mixture at elevated temperature, followed by phase separation, biaxial stretching, and extraction of the process oil (i.e., plasticizer). The resultant separators have elliptical or spherical pores with good mechanical properties in both the machine and transverse directionsPE-based separators manufactured this way using cast film or blown film technologies have found wide use in Li-ion batteries.

[0007] In the case of large format Li-ion cells designed for hybrid, plug-in hybrid or electric vehicle applications (HEV, PHEV, EV), the benefits of separator shutdown have been openly questioned because it is difficult to guarantee a sufficient rate and uniformity of shutdown throughout the complete cell. The principal reason is that, after shutting down, residual stress and reduced mechanical properties above the polymer melting point can lead to shrinkage, tearing, or pinhole formation. The exposed electrodes can then touch and create an internal short circuit that leads to more heating, thermal runaway and explosion.

[0008] As such, battery manufacturers are focused on the use of separators with outstanding, high temperature in-plane stability. In United States Patent Application Publication No. 20190097196A1, a method for preparing an aromatic polyamide membrane (e.g., poly m-phenylene isophthalamide) for use as a Li-ion battery separator is described. Such polymers have glass transition temperatures above 300 °C and thermal decomposition temperatures that exceed 500 °C.

[0009] In an alternative approach, a microporous polyolefin membrane is coated with ceramic particles and a binder (e.g., a polymer binder) on one or both sides. At sufficient loading levels, the ceramic will impart high temperature dimensional stability as defined as < 5% areal shrinkage at temperatures that are above the melting point of the polyolefin. The ceramic layer also protects the polyolefin from oxidation when placed in contact with a high voltage cathode (e.g., NMC 622), and its tortuous pore structure mitigates dendrite growth.

[0010] In the case of cylindrical cells, the ceramic coating is often applied to only one side of the polyolefin membrane because it is easier to extract the “jellyroll” from the winding pin when the polyolefin surface is in contact with it. As such, it is important to know which side of the polyolefin membrane is coated with ceramic particles, but that is difficult to tell since both the uncoated and ceramic-coated polyolefin surfaces appear white upon visual inspection.

[0011] Heretofore, no consideration has been given to color-coding the polyolefin membrane or the ceramic coating of the separator so that each surface is easily distinguished. In this invention, a contrast agent is added to either the polyolefin or the ceramic coating to allow the coated and uncoated surfaces to be differentiated to achieve the above objective.Summary of the Invention

[0012] An object of the present disclosure is to achieve thin, freestanding, ceramic-coated microporous polyolefin membranes with good heat resistance above the melting point of the polyolefin and a coated surface that is easily distinguished from the uncoated surface of the membranes. The pore size range for microporous membranes is generally from about 10 nanometers to several microns, with an average pore size less than about 1 micrometer. Such membranes are generally opaque because the pore size and polymer matrix are of sufficient size to scatter visible light. The term “membrane” as used, is inclusive of other descriptions used in the scientific and patent literature such as “film”, “sheet”, and “web”. The microporous membranes can also exhibit free-standing properties, and have interconnected pores that extend throughout the membrane. “Free-standing”refers to a membrane having sufficient mechanical properties that permit manipulation such as winding and unwinding in sheet form for use in an energy storage device assembly.

[0013] In a first preferred embodiment, one or more contrast agents (e.g., a pigment ) is combined with inorganic particles (e.g., boehmite) in an aqueous solution (e.g., deionized water) and bead-milled to form an aqueous dispersion. A small amount of binder (e.g., carboxymethyl cellulose, polyvinyl pyrrolidone, acrylic) is added to the dispersion. The dispersion is then coated on one side of a microporous, polyolefin membrane using a Mayer rod and is then subsequently dried in an oven at 100 °C.

[0014] In a second preferred embodiment, one or more contrast agents (e.g., a water-soluble dye) is added to an aqueous dispersion of inorganic particles (e.g., fumed alumina and nanoboehmite) to which a small amount of binder was added. The aqueous dispersion is then coated onto one side of a microporous polyolefin membrane. The coated membrane is then dried in an oven at 100 °C.

[0015] In a third preferred embodiment, one or more contrast agents (e g., a blue pigment) is combined with polyethylene and a process oil in a twin-screw extruder to form an oil-filled sheet that is then biaxially-oriented. Next, the biaxially oriented sheet is passed through a solvent extraction bath to remove the oil and the solvent-laden sheet is then conveyed through a dryer at elevated temperature to remove the solvent and a create a microporous polyethylene membrane in which the pigment imparts a color other than white.

[0016] The colored, microporous polyethylene membrane is then coated on one side with an aqueous dispersion of inorganic particles (e.g., boehmite) and a binder (e.g., an acrylate binder). The coated side of the membrane is easily distinguished from the uncoated side.

[0017] The resultant color-coded, polyolefin membranes can be wound or stacked in a package to separate the electrodes in an energy storage device, for example, a battery, capacitor, supercapacitor, or fuel cell. Such membranes are beneficial to the manufacture of energy storage devices, particularly since they combine an easily distinguishable coated surface with excellent inplane dimensional stability at temperatures that exceed the melting point of the polymer matrix.

[0018] Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.Brief Description of the Drawings

[0019] The drawings illustrate several embodiments of the present disclosure.

[0020] Figure 1 is a schematic view of a two-layer, microporous polyolefin membrane with a ceramic coating layer and a polyethylene base film.

[0021] Figure 2 depicts an image of a ceramic-coated, microporous polyolefin membrane having a pigment and a dye incorporated into the ceramic coating to show clear contrast between the coated and uncoated surfaces

[0022] Figure 3 depicts an image of a ceramic-coated, microporous polyolefin membrane having a dye incorporated into the ceramic coating to show clear contrast between the coated and uncoated surfaces.

[0023] Figure 4 depicts an image of a ceramic-coated, microporous polyolefin membrane having a pigment incorporated into the polyethylene base film to show contrast with a side having the ceramic-coating.

[0024] Figure 5 depicts images of electrolyte compatibility testing at 60 °C of AOH 70 boehmite (Nabaltec), Blue 3J Pigment (Shepard Color), and Blue 30C59 Pigment (Shepard Color) in 1 M LiPFe in 1:1 EC:EMC (ethylene carbonate: ethyl methyl carbonate) electrolyte.Detailed Description

[0025] Wet process separators composed of high molecular weight polyethylene are produced by extrusion of an oil / polymer mixture at elevated temperature, followed by phase separation, biaxial stretching, and extraction of the process oil (i.e., plasticizer). The resultant separators have elliptical or spherical pores with good mechanical properties in both the machine and transverse directions. PE-based separators manufactured this way using cast film or blown film technologies have found wide use in Li-ion batteries.

[0026] The polyethylene used in the manufacture of such separators can various types of polyethylene. For instance, in some embodiments, the polyethylene includes ultrahigh molecular weight polyethylene (UHMWPE), very high molecular weight polyethylene (VHMWPE), high density polyethylene (HDPE), or mixtures thereof. In one embodiment, the polyethylene comprises UHMWPE generally corresponding to a molecular weight range of between about 3.1 to about 10 million grams / mol. In another embodiment, the polyethylene used comprises a molecular weight between 500,000 g / mol to 3.1 million grams per mol. In yet another embodiment, the polyethylene used comprises a molecular weight between 500,000 g / mol to 10 million g / mol. Representative polymers include 150 U and VH035 from KPIC (Korea), GUR 4120 and 4012 from Celanese (USA), and UH650 from Asahi-Kasei (Japan).

[0027] The plasticizer employed in the present invention is a nonevaporative solvent for the polymer, and is preferably a liquid at room temperature. The plasticizer has little or no solvating effect on the polymer at room temperature; it performs its solvating action at temperatures at or above the softening temperature of the polymer. For polyethylene homopolymers, the solvating temperature would be above about 180 °C, and preferably in the range of between about 200 °C and about 225 °C. It is preferred to use a processing oil, such as a paraffinic oil, naphthenic oil, aromatic oil, or a mixture of two or more such oils. Examples of suitable processing oils include: Risella 430X by Shell Oil Company; and Hydrocal™ 800 by Calumet Specialty Products; and Nytex 820 by Nynas Inc

[0028] The polymer / oil mixture is extruded through a sheet die or annular die, and then it is biaxially-oriented to form a thin, oil-filled sheet. Any solvent that is compatible with the oil can be used for the extraction step, provided it has a boiling point that makes it practical to separate the solvent from the plasticizer by distillation. Such solvents include 1,1 ,2 trichloroethylene, perchloroethylene,1.2-dichloroethane, 1 ,1 ,1-trichloroethane, 1 ,1,2-trichloroethane, methylene chloride, 1 , 1 ,2-trichloro-1.2.2-trifluoroethane, various trans-dichloroethylene azeotropes (e.g , Tergo MCF - MicroCare LLC), isoPar-G, hexane, heptane, decane, and toluene In some cases, it is desirable to select the processing oil such that any residual oil in the polymer sheet after extraction is electrochemically inactive The resultant membrane after extraction is microporous, having a porosity from about 35-65%. The pore size range is generally from about 10 nanometers to several microns, with an average pore size of less than about 1 micrometer. The thickness of the membrane can be in the about 3-25 pm range. Other thicknesses are also contemplated

[0029] A ceramic coating layer can thereafter be applied to the membrane and dried. It will be appreciated that the ceramic coating can include various types of inorganic particles, including inorganic oxides, carbonates, or hydroxides, such as at least one of alumina, silica, zirconia, titania, mica, boehmite, magnesium hydroxide, calcium carbonate, or mixtures thereof One or more hydrotalcites can also be used either alone or in combination with another type of inorganic particle. The coating formulation can further comprise inorganic particles dispersed in aqueous mixtures that include a binder. A polymer dispersion or water-soluble polymer is typically used as the binder. Exemplary binders can include acrylates, polyvinyl pyrrolidone, polyvinyl alcohol, carboxy methyl cellulose, and their copolymers or derivatives. The thickness of the coating layer can be in the about 0.5-6 pm range. Other thicknesses are also contemplated. In another embodiment, the coating layer can comprise a coating weight of from about 0.3 g / m2to about 12 g / m2, such as from about 3 g / m2to about 6 g / m2Other coat weights are also contemplated.

[0030] As previously discussed, the contrast agent can be added into either the polyolefin base film or the ceramic coating. Various types of contrast agents can be used, including pigments, dyes, inorganic oxides, and combinations thereof. Exemplary pigments can one or more of metal oxides, carbon, carbides, or mixtures thereof. Exemplary dyes include water soluble dyes Other types of contrast agents can also be used.

[0031] The following examples are illustrative in nature and not intended to be limited in any way.Example 1

[0032] A 9 pm thick, microporous ultrahigh molecular weight polyethylene-containing membrane, ENTEK® EPH (ENTEK Membranes LLC, Oregon) was coated with an aqueous-based dispersion that contained the following:337 5 g - DI Water124 g - Boehmite8.2 g - Acrylic Emulsion Binder2.5 g - Rheology Modifier0.5 g - Dispersant3 g - Pigment: Shepard Color Blue 3J3 g - Dye: Blue McCormick Assorted Food Color

[0033] The coating dispersion was made by mixing boehmite, dispersant, acrylic emulsion binder, a rheology modifier, and Shepard Color Blue 3J in water using agitation. The resulting mixture was milled using a ball mill for three hours. The Blue McCormick Assorted Food Color was added to the post-milled solution under low shear mixing conditions The coating dispersion contained 26 wt% solids. The separator was coated on one side via a dip coating line. The wetted separator was then dried using a Heraeus IR heater and forced air through a horizontal oven and wound on a core, priorto testing (Figure 2). Table 1 shows physical properties of the prepared coated separator in Example 1.Table 1Example 2

[0034] A 9 pm thick, microporous ultrahigh molecular weight polyethylene-containing membrane, ENTEK® EPX (ENTEK Membranes LLC, Oregon) was coated with an aqueous-based dispersion that contained the following:337 5 g - DI Water124 g - Boehmite8.2 g - Acrylic Emulsion Binder2.5 g - Rheology Modifier0.5 g - Dispersant3 g - Dye: Blue McCormick Assorted Food Color

[0035] The coating dispersion was made by mixing boehmite, dispersant, acrylic emulsion binder, a rheology modifier, and Blue McCormick Assorted Food Color in water using agitation. The coating dispersion contained 26 wt% solids. The separator was coated on one side via a dip coating line The wetted separator was then dried using a Heraeus IR heater and forced air through a horizontal oven and wound on a core, prior to testing (Figure 3). Table 2 shows physical properties of the prepared coated separator in Example 2.Table 2Example 3

[0036] A 9 pm thick, microporous ultrahigh molecular weight polyethylene-containing membrane containing blue pigment, was coated with an aqueous-based dispersion that contained the following:337 5 g - DI Water124 g - Boehmite8.2 g - Acrylic Emulsion Binder2.5 g - Rheology Modifier0.5 g - Dispersant

[0037] The coating dispersion was made by mixing boehmite, dispersant, acrylic emulsion binder, and a rheology modifier in water using agitation. The coating dispersion contained 26 wt% solids. The separator was coated on one side via an automatic draw down coater using a doctorblade The wetted separator was then dried in an oven at 130°C for five minutes, prior to testing (Figure 4) Table 3 shows physical properties of the prepared coated separator in Example 3.Table 3Example 4

[0038] Electrolyte aging tests were prepared by adding 0.5g of AOH 70 boehmite (Nabaltec), Blue 3J (Shepard Color), and Blue 30C59 (Shepard Color) to individual 20ml scintillation vials and drying at 120°C for 1 hour After drying, scintillation vials were transferred to a glove box, and 5ml of 1 M LIPF6 in 1 :1 EC:EMC (ethylene carbonate: ethyl methyl carbonate) electrolyte (Aldrich) was added to each of the scintillation vials. The scintillation vials were then sealed and placed in an oven at 60°C for 7 days. Figure 5 shows comparative images after electrolyte aging at 60°C; electrolyte containing AOH 70 boehmite, Blue 3J, and Blue 30C59 showed similar discoloration after 60°C exposure for 168 hours. Thus the contrast agents did not appear to adversely impact the electrolyte.

[0039] It will be apparent to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the disclosure. The scope of the present disclosure should, therefore, be determined only by the following claims.

Claims

Claims1. A free-standing, microporous polyolefin membrane comprising: a microporous polyolefin base membrane having a coated side and an uncoated side; and a ceramic coating layer disposed on a first major surface of the microporous polyolefin base membrane, the ceramic coating layer comprising inorganic particles and forming the coated side of the microporous polyolefin base membrane; wherein one of the microporous polyolefin base membrane or the ceramic coating layer comprises a contrast agent that allows the coated and uncoated sides of the microporous polyolefin base membrane to be visually distinguishable.

2. The free-standing, microporous polyolefin membrane of claim 1, wherein the ceramic coating layer comprises a sufficient coat weight to impart high temperature dimensional stability as defined by < 5% areal shrinkage at a temperature above the melting point of the polyolefin.

3. The free-standing, microporous polyolefin membrane of claim 1 or 2, wherein the contrast agent comprises at least one of a pigment or a dye.

4. The free-standing, microporous polyolefin membrane of claim 3, wherein the contrast agent comprises a water-soluble dye.

5. The free-standing, microporous polyolefin membrane of claim 3, wherein the contrast agent comprises a pigment including at least one of a metal oxide, carbon, carbide, or mixtures thereof.

6. The free-standing, microporous polyolefin membrane of any one of claims 1 to 5, wherein the microporous polyolefin base membrane comprises the contrast agent.

7. The free-standing, microporous polyolefin membrane of any one of claims 1 to 6, wherein the ceramic coating layer comprises the contrast agent.

8. An energy storage device comprising the free-standing, microporous polyolefin membrane of any one of claims 1 to 7.

9. A method of forming a free-standing, microporous polyolefin membrane comprising: obtaining a microporous polyolefin base membrane having first and second major surfaces; and applying a ceramic coating layer to the first major surface of the microporous polyolefin base membrane, the ceramic coating layer comprising inorganic particles and forming a coated side of the microporous polyolefin base membrane; wherein one of the microporous polyolefin base membrane or the ceramic coating layer comprises a contrast agent that allows the coated side of the microporous polyolefin base membrane to be visually distinguishable from an uncoated side of the microporous polyolefin base membrane10. The method of claim 9, wherein the ceramic coating layer comprises a sufficient coat weight to impart high temperature dimensional stability as defined by < 5% areal shrinkage at a temperature above the melting point of the polyolefin.11 The method of claim 9 or 10, wherein the contrast agent comprises at least one of a pigment or a dye.

12. The method of claim 11, wherein the contrast agent comprises a water-soluble dye.

13. The method of claim 11 , wherein the contrast agent comprises a pigment including at least one of a metal oxide, carbon, carbide, or mixtures thereof.

14. The method of any one of claims 9 to 13, wherein the microporous polyolefin base membrane comprises the contrast agent.

15. The method of any one of claims 9 to 13, wherein the ceramic coating layer comprises the contrast agent.

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