Functional coatings for separators

Abstract

A coated separator comprising a microporous film and a coating on at least one side of the microporous film, wherein the coated separator shuts down at a temperature of 140°C or less. In some embodiments, the coating causes the separator to shut down at a temperature lower than the temperature at which the microporous film would shrink by more than 15%, more than 12%, more than 10%, or preferably more than 5% without any coating. The microporous film of the separator itself (uncoated) does not shut down or does not shut down at a temperature of 140°C or less. The microporous film can shut down at a temperature between 140°C and 350°C. The coating of the coated separator may contain polyethylene, a binder, and inorganic or heat-resistant particulates. The particulates may have a particle size D50 of 500 nm or less.

Description

[Technical field]

[0001] This application is directed to, among other things, new or improved battery separators or membranes having improved safety properties, one or more coatings, various functional coatings, and the like. [Background technology]

[0002] Increasing performance standards, safety standards, manufacturing demands, and / or environmental concerns have made the development of new and / or improved coating compositions for battery separators desirable.

[0003] One major safety issue with lithium-ion batteries is thermal runaway. Abuse conditions, such as overcharging, overdischarging, and internal short circuits, can cause battery temperatures to far exceed those intended by battery manufacturers for use. Tests to mimic abuse conditions can include, but are not limited to, nail penetration tests and hot box tests. For example, shutting down the battery between the anode and cathode in the event of thermal runaway, such as stopping ion flow across the separator, is a safety mechanism used to prevent thermal runaway. The separators in at least certain lithium-ion batteries must provide the ability to shut down at least slightly lower temperatures than the temperature at which thermal runaway occurs, while still retaining their mechanical properties. For example, faster shutdown at lower temperatures and longer durations is highly desirable so that the user or device has more time to shut down the system. In some embodiments, shutdown can occur due to the filling and / or closing of separator pores by molten polymer.

[0004] A nail penetration test is a type of battery safety test done to mimic an internal short circuit (e.g., due to lithium dendrite growth in lithium ion batteries). Typically, a sample battery is prepared containing an anode, a cathode, and a separator between the anode and the cathode. A nail is penetrated through the sample battery to mimic an internal short circuit and verify that the battery does not catch fire or explode. Various nail penetration rates are used in the industry. One way to prevent the sample battery from catching fire or exploding (a possible consequence of thermal runaway) is to use a battery separator that shuts down. Typically, most battery separators are capable of shutting down, but some shut down at higher temperatures than others. However, some battery separators that shut down may fail all or some nail penetration tests (e.g., tests using some nail penetration rates but not others). Therefore, a battery separator that passes all or many of the industry nail penetration tests is desirable or beneficial. Summary of the Invention [Problem to be solved by the invention]

[0005] The coated separators or membranes described herein can include a microporous film having a coating that provides shutdown at low temperatures regardless of the ability of the microporous film to shut down or shut down at low temperatures.

[0006] It is theorized by the present inventors that dimensional changes (e.g., shrinkage) of the separator at increasing temperatures may be one reason why the separator fails the nail penetration test. If the battery separator does not shut down before the shrinkage exceeds a threshold amount, this may lead to failure of the nail penetration test. Typically, batteries are designed such that the separator covers the electrodes as shown in FIG. 1. However, if the shrinkage exceeds a threshold amount, the electrodes may become exposed (see FIG. 2), leading to a thermal runaway situation that may lead to fire or explosion if it occurs before the separator can shut down.

[0007] To solve this problem, the present inventors propose a separator that shuts down before dimensional change (eg, shrinkage) exceeds a threshold amount. [Means for solving the problem]

[0008] In one aspect, the separator is a coated separator comprising a microporous film and a coating. The coated separator shuts down at a temperature less than 140° C. In some embodiments, the coated separator shuts down at a temperature less than 135° C., less than 130° C., less than 125° C., less than 120° C., less than 115° C., less than 110° C., less than 105° C., or less than 100° C.

[0009] In some preferred embodiments, the microporous film itself (uncoated) does not shut down at temperatures below 140° C. The microporous film, in some embodiments, does not shut down or shuts down at temperatures between 140° C. and 350° C. In some embodiments, the microporous film itself (uncoated) does not shut down at temperatures below 135° C. In some embodiments, it does not shut down or shuts down at temperatures between 135° C. and 350° C. In some embodiments, the microporous film does not shut down or shuts down at temperatures between 160° C. and 350° C. In some embodiments, it does not shut down or shuts down at temperatures between 135° C. and 160° C.

[0010] In some embodiments, the microporous film comprises, consists of, or consists essentially of a polyolefin. In some embodiments, the polyolefin is polypropylene or another polyolefin having a melting temperature of 160° C. or greater. In some embodiments, the microporous film is a monolayer film made of polypropylene or another polyolefin having a melting temperature of 160° C. or greater.

[0011] The microporous film may be a monolayer, bilayer, trilayer, or multilayer film. In some embodiments, the microporous film may be a monolayer film that comprises, consists of, or consists essentially of polypropylene. In some embodiments, the microporous film may be a film that has an average porosity of greater than 30%. In some embodiments, the microporous film may be a film that has pores with an average pore size of greater than 0.03 microns, greater than 0.04 microns, or greater than 0.045 microns.

[0012] The coatings described herein may comprise, consist of, or consist essentially of polyethylene and a binder, in some embodiments, the coating may further comprise, consist of, or consist essentially of inorganic particulates in an amount of 10% or less, or 5% or less of the total solids in the coating.

[0013] In some embodiments, the inorganic particulate may comprise a metal oxide having a particle size D50 of about 500 nm or less, 250 nm or less, or 200 nm or less, In some embodiments, the metal oxide may comprise, consist of, or consist essentially of alumina.

[0014] In one embodiment, the separator is a coated separator that includes a microporous film, and a coating is described. The microporous film itself can be used as a battery separator, but by coating the microporous film to form a separator, the separator shuts down at a temperature lower than the temperature at which the microporous film would shrink by more than 15% without any coating. The coating may be applied to one or both sides of the microporous film.

[0015] In some embodiments, the coating causes the separator to shut down at a temperature lower than the temperature at which the microporous film would shrink by more than 12% without any coating. In some embodiments, the coating causes the separator to shut down at a temperature lower than the temperature at which the microporous film would shrink by more than 10% without any coating. In some embodiments, the coating causes the separator to shut down at a temperature lower than the temperature at which the microporous film would shrink by more than 5% without any coating.

[0016] In some embodiments, the coated separators described herein shut down at temperatures below 140° C., below 135° C., below 130° C., below 125° C., below 120° C., below 115° C., below 110° C., or below 100° C. In all cases, the shutdown temperature of the separator is lower than the shutdown temperature of the microporous film itself, i.e., without any coating.

[0017] In some preferred embodiments, the microporous film itself (uncoated) does not shut down at temperatures below 140° C. The microporous film, in some embodiments, does not shut down or shuts down at temperatures between 140° C. and 350° C. In some embodiments, the microporous film itself (uncoated) does not shut down at temperatures below 135° C. In some embodiments, it does not shut down or shuts down at temperatures between 135° C. and 350° C. In some embodiments, the microporous film does not shut down or shuts down at temperatures between 160° C. and 350° C. In some embodiments, it does not shut down or shuts down at temperatures between 135° C. and 160° C.

[0018] In some embodiments, the microporous film comprises, consists of, or consists essentially of a polyolefin. In some embodiments, the polyolefin is polypropylene or another polyolefin having a melting temperature of 160° C. or greater. In some embodiments, the microporous film is a monolayer film made of polypropylene or another polyolefin having a melting temperature of 160° C. or greater.

[0019] The microporous film may be a monolayer, bilayer, trilayer, or multilayer film. In some embodiments, the microporous film may be a monolayer film that comprises, consists of, or consists essentially of polypropylene. In some embodiments, the microporous film may be a film that has an average porosity of greater than 30%. In some embodiments, the microporous film may be a film that has pores with an average pore size of greater than 0.03 microns, greater than 0.04 microns, or greater than 0.045 microns.

[0020] In some embodiments, the coating may comprise, consist of, or consist essentially of polyethylene and a binder, hi some embodiments, the coating may further comprise, consist of, or consist essentially of inorganic particulates in an amount of 10% or less of total coating solids or in an amount of 5% or less of total coating solids.

[0021] In some embodiments, the inorganic particulate has a particle size D50 of 500 nm or less, 250 nm or less, or 200 nm or less. In some embodiments, the inorganic particulate comprises, consists of, or consists essentially of a metal oxide having a particle size of 250 nm or less, or 200 nm or less. In some embodiments, the metal oxide is alumina.

[0022] In another aspect, a secondary battery is described that includes a coated separator according to any of the embodiments described herein. The battery may include at least an electrode, a separator, and an electrolyte.

[0023] In another aspect, a capacitor is described that includes a battery separator according to any of the embodiments described herein. [Brief description of the drawings]

[0024] [Figure 1] 1 and 2 include schematic diagrams illustrating the effect of separator dimensional changes (e.g., shrinkage) in a battery. The battery separator may cover the electrodes when the cell is assembled (FIG. 1), but may later shrink to expose the electrodes (FIG. 2). [Diagram 2] 1 and 2 include schematic diagrams illustrating the effect of separator dimensional changes (e.g., shrinkage) in a battery. The battery separator may cover the electrodes when the cell is assembled (FIG. 1), but may later shrink to expose the electrodes (FIG. 2). [Diagram 3] FIG. 3 shows a typical shutdown profile. [Figure 4] FIG. 4 includes schematic diagrams of one side and two side coated battery separators. [Diagram 5] FIG. 5 contains a diagram of a typical structure of a dry-processed porous membrane. [Figure 6] 6A and 6B are SEMs showing a typical structure of a dry-processed porous membrane. [Figure 7] FIG. 7 is a schematic diagram illustrating the concept of twisting. [Figure 8] FIG. 8 shows a schematic diagram of the coating described herein. [Figure 9] FIG. 9 includes a shutdown profile for the embodiments described herein. [Figure 10] FIG. 10 is a schematic diagram showing the effect of smaller and larger inorganic particles on packing. [Figure 11] FIG. 11 illustrates the curl of an embodiment described herein. [Figure 12] FIG. 12 shows a comparison of the properties of an uncoated tri-ply product and a tri-ply product with a 95° C. shutdown coating. [Figure 13] FIG. 13 shows the shutdown shift after coating for some embodiments described herein. [Figure 14] FIG. 14 is a graph showing that the shutdown coating described herein reduces pin removal force. [Figure 15] FIG. 15 is a schematic diagram showing good results for the pin removal test. [Figure 16] FIG. 16 is a graph showing MD shrinkage and Gurley at 115° C., 120° C., 125° C., and 130° C. [Figure 17] FIG. 17 includes a photograph of a film according to some embodiments described herein. [Figure 18] FIG. 18 is a graph showing the shutdown behavior for coatings with reference alumina versus nano alumina. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Preferred coated battery separators described herein are those that shut down at temperatures of 140° C. or less, 135° C. or less, 130° C. or less, 125° C. or less, 120° C. or less, 115° C. or less, 110° C. or less, 105° C. or less, or 100° C. or less. In some embodiments, the coated separators described herein shut down before experiencing a threshold amount of dimensional change (e.g., shrinkage). Dimensional change beyond a threshold amount can lead to a thermal runaway situation that can lead to fire or explosion if it occurs before the separator can shut down when the separator is used in a battery where the electrodes are exposed to one another (i.e., there is no separator between the electrodes as shown in FIG. 2).

[0026] A typical shutdown profile is shown in Figure 3. Shutdown is indicated in the profile at "shutdown" and not at the onset of "shutdown". When the temperature of shutdown is mentioned, it is the temperature indicated by "shutdown" and not at "onset of shutdown".

[0027] For purposes of this application, shutdown occurs when the resistance level across the separator reaches 1,000 ohms or greater and continues or remains above this value for at least 5° C. In some embodiments, shutdown can occur when the resistance across the separator can be 2,000 ohms or greater, 4,000 ohms or greater, 5,000 ohms or greater, 6,000 ohms or greater, 7,000 ohms or greater, 8,000 ohms or greater, 9,000 ohms or greater, or 10,000 ohms or greater and continues above this level for a period of at least 5° C. Sometimes the period can be a period of at least 10° C., at least 15° C., at least 20° C., at least 30° C., at least 40° C., or at least 50° C. In some embodiments, the period is from the start of the shutdown to the end of the shutdown window. Sometimes this is the shutdown window.

[0028] The battery separators described herein are not so limited and may be coated or uncoated. In a preferred embodiment, the battery separator is a coated battery separator that includes a coating on at least one side of the microporous film. In some embodiments, the coating may be applied to both sides of the microporous film. Exemplary one-side and two-side coated battery separators are shown in FIG. 4. In some embodiments, the coating described herein may be on one side of the microporous film in a two-side coated separator, and the other side of the microporous film may have a different coating. For example, it may have a ceramic coating. In some embodiments, the coating described herein may be on both sides of the microporous film.

[0029] In some embodiments, the coated separators described herein shut down at temperatures below 140° C., below 135° C., below 130° C., below 125° C., below 120° C., below 115° C., below 110° C., or below 100° C. In preferred cases, the shutdown temperature of the coated separator is lower than the shutdown temperature of the microporous film itself, i.e., without any coating.

[0030] coating The coatings described herein are not so limited, and any coating that is not inconsistent with the goals described herein (and that is not damaging to the battery) may be used. In some preferred embodiments, the coating causes the separator to shut down at a lower temperature than the microporous film itself would shut down. Sometimes, the coating causes the separator to shut down at temperatures below 140° C., below 130° C., below 120° C., below 110° C., or below 100° C., where the microporous film itself either does not shut down or shuts down at a higher temperature.

[0031] In some preferred embodiments, the coating causes the separator to shut down at a temperature lower than the temperature at which the microporous film would shrink by more than 15%, more than 12%, more than 10%, or more than 5% without any coating. In some embodiments, the coating causes the separator to shut down at a temperature lower than the temperature at which the microporous film would shrink by more than 20%, more than 15%, more than 14%, more than 13%, more than 11%, more than 10%, more than 9%, more than 8%, more than 7%, more than 6%, more than 5%, more than 4%, more than 3%, more than 2%, or more than 1% without any coating.

[0032] In some embodiments, the coating may comprise, consist of, or consist essentially of polyethylene and a binder. In some embodiments, the coating may further comprise, consist of, or consist essentially of inorganic particulates. The amount of inorganic particulates in the coating may not exceed 10% of the total solids in the coating. In some embodiments, they may not exceed 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1% of the total solids in the coating.

[0033] In some preferred embodiments, the coating may be an aqueous or water-based coating. "Water-based" means that the coating is formed from a slurry in which the solvent is water or water and a small amount, less than 5%, of another solvent, such as alcohol. The coating may also be a solvent-based coating, which is a coating formed from a slurry in which the solvent is an organic solvent. Solvent-based and water-based coatings are structurally different. In some embodiments, water-based coatings may be preferred due to the high uniformity of such coatings.

[0034] polyethylene The polyethylene used in the coating is not so limited. Any polyethylene consistent with the objectives described herein may be used. In some preferred embodiments, lower molecular weight (and therefore lower melting point) polyethylene may be used. In some embodiments, lower molecular weight polyolefins may be used. In some embodiments, the polyolefins, including polyethylene, may have a melting temperature between 90°C and 140°C, between 100°C and 140°C, between 110°C and 140°C, between 120°C and 140°C, or between 130°C and 140°C. In some embodiments, the particle size of the polyethylene or polyolefin may be between 0.5 and 5 microns, between 0.5 and 4 microns, between 0.5 and 3 microns, between 0.5 and 2 microns, or between 0.5 and 1 micron. Coatings containing polyethylene particles or beads may be preferred.

[0035] Binder The binder used in the coating is not significantly limited: any binder not inconsistent with the goals described herein may be used.

[0036] In some embodiments, the binder may be acrylic. In some embodiments, the binder may be a polymeric binder including, consisting of, or consisting essentially of a polymeric, oligomeric, or elastomeric material, but is not limited thereto. Any polymeric, oligomeric, or elastomeric material consistent with the present disclosure may be used. The binder may be ionically conductive, semiconductive, or nonconductive. Any gel-forming polymer recommended for use in lithium polymer batteries or solid electrolyte batteries may be used. For example, the polymeric binder may include at least one, or two, or three, etc. selected from polylactam polymers, polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyvinyl acetate (PVAc), carboxymethyl cellulose (CMC), isobutylene polymers, acrylic resins, latex, aramid, or any combination of these materials.

[0037] In some preferred embodiments, the polymeric binder comprises, consists of, or consists essentially of a polylactam polymer that is a homopolymer, copolymer, block polymer, or block copolymer derived from a lactam. In some embodiments, the polymeric material comprises a homopolymer, copolymer, block polymer, or block copolymer according to formula (1).

[0038] [ka]

[0039] In the formula, R 1 , R 2 , R 3 , and R 4 may be an alkyl or aromatic substituent, R 5may be an alkyl substituent, an aryl substituent, or a substituent containing a fused ring; preferred polylactams may be homopolymers or copolymers in which the copolymeric group X may be derived from vinyl, substituted or unsubstituted alkyl vinyl, vinyl alcohol, vinyl acetate, acrylic acid, alkyl acrylate, acrylonitrile, maleic anhydride, maleimide, styrene, polyvinylpyrrolidone (PVP), polyvinylvalerolactam, polyvinylcaprolactam (PVCap), polyamide, or polyimide; m may be an integer between 1 and 10, preferably between 2 and 4, such that the ratio of l to n is 0≦l:n≦10 or 0≦l:n≦1. In some preferred embodiments, the homopolymer, copolymer, block polymer, or block copolymer derived from lactams is at least one, at least two, or at least three selected from the group consisting of polyvinylpyrrolidone (PVP), polyvinylcaprolactam (PVCap), and polyvinyl-valerolactam.

[0040] In another preferred embodiment, the polymeric binder comprises, consists of, or consists essentially of polyvinyl alcohol (PVA). The use of PVA can result in a low curl coating layer, which helps the substrate to which the coating is applied remain stable and flat, for example, helping to prevent the substrate from curling. PVA may be added in combination with any other polymeric, oligomeric, or elastomeric material described herein, especially when low curl is desired.

[0041] In another preferred embodiment, the polymeric binder may comprise, consist of, or consist essentially of an acrylic resin. The type of acrylic resin is not particularly limited and may be any acrylic resin that is not contrary to the objectives described herein, for example, to provide a new and improved coating composition that can be used to make battery separators with improved safety. For example, the acrylic resin may be at least one, or two, or three, or four selected from the group consisting of polyacrylic acid (PAA), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), and polymethyl acrylate (PMA).

[0042] In other preferred embodiments, the polymeric binder may comprise, consist of, or consist essentially of carboxymethyl cellulose (CMC), isobutylene polymer, latex, or any combination thereof, which may be added alone or together with any other suitable oligomeric, polymeric, or elastomeric materials.

[0043] In some embodiments, the polymeric binder may include a solvent that is only water, an aqueous or water-based solvent, and / or a non-aqueous solvent. When the solvent is water, in some embodiments, no other solvent is present. The aqueous or water-based solvent may include a majority (greater than 50%) of water, more than 60% water, more than 70% water, more than 80% water, more than 90% water, more than 95% water, or more than 99% water, but less than 100% water. The aqueous or water-based solvent may include a polar or non-polar organic solvent in addition to water. The non-aqueous solvent may be any polar or non-polar organic solvent that is compatible with the goals set forth in this application, without limitation. In some embodiments, the polymeric binder includes only trace amounts of solvent, and in other embodiments includes 50% or more of solvent, sometimes 60% or more, sometimes 70% or more, sometimes 80% or more, etc.

[0044] The amount of binder may be less than 20%, less than 15%, less than 10%, or less than 5% of the total solids in the coating in some preferred embodiments, hi some particularly preferred embodiments, the amount of binder may be no more than 10%, or no more than 5% of the total solids in the coating.

[0045] inorganic fine particles The inorganic particulate is not so limited. Any inorganic particulate that is not inconsistent with the objectives described herein may be used. The inorganic particulate may have a particle size D50 of less than 500nm, less than 450nm, less than 400nm, less than 350nm, less than 300nm, less than 250nm, less than 225nm, less than 200nm, less than 175nm, less than 150nm, less than 125nm, less than 100nm, less than 75nm, or less than 50nm. Without wishing to be bound by any particular theory, it is believed that the use of larger particles can suppress shutdown by blocking the flow of polymer, such as polyethylene, from the coating and into the pores of the separator. The use of large amounts of inorganic particles of any size can also block the flow of polymer into the pores of the separator to block ion flow.

[0046] In some embodiments, the inorganic particulate may comprise, consist of, or consist essentially of one or more metal oxides, hi some embodiments, the metal oxide (or one of the metal oxides) may be alumina.

[0047] In some embodiments, the inorganic particulates include: iron oxide, silicon dioxide (SiO 2 ), aluminum oxide (Al 2 O 3 ), boehmite (Al(O)OH), zirconium dioxide (ZrO 2 ), titanium dioxide (TiO 2 ), barium titanium oxide (BaTiO 3 ), tin dioxide (SnO 2), indium tin oxide, oxides of transition metals, graphite, carbon, metals, and any combination thereof.

[0048] In preferred embodiments, the ratio of the size of the inorganic particulate to the size of the polymeric particle is 0.5:1 or less. In some preferred embodiments, the ratio is 0.4:1 or less. 0.3:1 or less, 0.2:1 or less, 0.1:1 or less, or 0.05:1 or less. In some embodiments, the polymeric particle is 2, 3, 5, 10, 12, 15, or 20 times the size of the inorganic particulate.

[0049] Microporous Film The microporous film is not particularly limited and any microporous film not contrary to the goals described herein may be used. In some preferred embodiments, the microporous film may be one described in U.S. Patent No. 8,795,565 to Celgard®, entitled "Biaxially Oriented Microporous Membrane."

[0050] The microporous film may be a monolayer, bilayer, trilayer, or multilayer film. In some preferred embodiments, the microporous film may be a monolayer, bilayer, trilayer, or multilayer film made by a dry process, including the Celgard® dry-stretch process, or a wet process known in the art.

[0051] In some embodiments, the microporous film may comprise, consist of, or consist essentially of a polyolefin, hi some embodiments, the microporous film is a monolayer film comprising, consisting of, or consisting essentially of polypropylene or a polypropylene-polyethylene block copolymer having 1-10% polyethylene.

[0052] In a preferred embodiment, the microporous film may have an average pore size between 0.1 and 1.0 microns. In some embodiments, the microporous film may have a porosity of 20% or more, 30% or more, 40% or more, 50% or more, or 60% or more, up to 80% or 90%. Without wishing to be bound by any particular theory, it is believed that films with higher porosity and / or larger pores may have a more difficult time shutting down by themselves, i.e., without any coating. This is because when a microporous film made of a polymer melts, it may not be enough to completely block or close the pores. The blocking of the pores is believed to stop the flow of ions across the film.

[0053] The dry process, in some embodiments, is a process that does not use any pore formers / pore forming agents or beta-nucleating agents / beta-nucleating agents. In some embodiments, the dry process is a process that does not use any solvents, waxes, or oils. In some embodiments, the dry process is a process that does not use any pore formers / pore forming agents or beta-nucleating agents / beta-nucleating agents and does not use any solvents, waxes, or oils. In such embodiments, the dry process may be a dry stretch process. An exemplary dry stretch process known as the Celgard® dry stretch process is described in Chen et al., Structural Characterization of Celgard® Microporous Membrane Precursors: Melt-Extruded Polyethylene Films, J. of Applied Polymer Sci., vol. 53, 471-483 (1994), which is incorporated herein by reference in its entirety. Celgard® dry stretching process refers to a process in which pore formation results from stretching a non-porous oriented precursor at least in the machine direction. Kesting, Robert E., Synthetic Polymeric Membranes, A Structural Perspective, Second Edition, John Wiley & Sons, New York, NY, (1985), pages 290-297 also discloses a dry stretching process, which is incorporated herein by reference in its entirety. In the dry stretching process according to some preferred embodiments, the process may include a stretching step. The stretching step may include, consist of, or essentially consist of uniaxial stretching (e.g., stretching only in MD or only in TD), biaxial stretching (e.g., stretching in MD and TD), or multiaxial stretching (e.g., stretching along three or more different axes, e.g., MD, TD, and another axis).In some embodiments, the dry stretching process may include, consist of, or consist essentially of extrusion and stretching steps, in this order or not. In some embodiments, the dry stretching process may include, consist of, or consist essentially of extrusion, annealing, and stretching steps, in this order or not. The extrusion step may be a blown film extrusion process or a cast film extrusion process in some embodiments. In some embodiments, a non-porous precursor is extruded and stretched to form pores. In some embodiments, a non-porous precursor is extruded, annealed, and then stretched to form pores. In other embodiments, a porous or non-porous precursor may be formed by a method other than extrusion, for example, by sintering or printing, and stretching may be performed on the precursor to form pores or enlarge existing pores.

[0054] In some embodiments, pore formers / pore forming agents or beta-nucleating agents / beta-nucleating agents may be used and the process still be considered a dry process. For example, a particle stretching process may be considered a dry process because oil or solvent is not extruded with the polymer and is not extracted from the extruded polymer to form pores. In a particle stretching process, particles, such as silica or calcium carbonate, are added to the polymer mixture and these particles help to form pores. In such a method, for example, a polymer mixture containing particles and polymer is extruded to form a stretched precursor and voids are created around the particles. In some embodiments, the particles can be removed after the voids are created. Although the particle stretching process may include a stretching step before or after the removal of the particles, the particle stretching process is not considered a dry stretching process because the principle pore formation mechanism is the use of particles that do not stretch.

[0055] In some preferred embodiments, the structure of the dry process porous membrane may have one or more distinguishing features. For example, the dry process membrane may contain more than 10% polypropylene. Wet processes, or other processes using solvents, are generally not compatible with polypropylene because the solvents degrade polypropylene. Therefore, wet process porous membranes typically contain no more than 10% polypropylene, most typically 5% or less. Another distinguishing feature of some dry process porous membranes, especially one used as a battery separator, is that they can have a shutdown function. The shutdown function can be imparted in some cases by a PP / PE / PP structure. This is unique to dry process membranes, because layers that contain mainly polypropylene (PP) generally cannot be formed in wet processes. The dry process is uniquely suited to form PP / PE / PP shutdown membrane structures.

[0056] In some embodiments, the identification of a dry-process porous membrane can be the presence of lamellae and fibrils. For example, the porous membrane can have a structure like that shown in FIG. 5 or FIG. 6A and FIG. 6B. FIG. 6A and FIG. 6B are FESM images showing slit-like micropores in a Celgard® microporous membrane with PE (A) and PP (B). In some embodiments, the pores or micropores of the dry-process porous membrane can be circular, elliptical, semicircular, trapezoidal, etc.

[0057] In some embodiments, a distinguishing feature of a dry-processed porous membrane is that it does not contain or is substantially free of pinholes. Pinholes are considered defects and generally are not an intentionally created feature of a dry-processed porous membrane. In some embodiments, a dry-processed microporous membrane may not contain or may substantially not contain pinholes larger than 10 nm. In some preferred embodiments, the pores of the dry-processed porous membrane are tortuous. In some embodiments, a distinguishing feature of a dry-processed porous membrane is tortuosity. In some embodiments, the tortuosity of the dry-processed porous membrane is greater than 1, greater than 1.2, greater than 1.3, greater than 1.4, greater than 1.5, greater than 1.6, greater than 1.7, greater than 1.8, greater than 1.9, or greater than 2.0. In some embodiments, the formula for roughly calculating tortuosity is formula (2): Torsion = x / t (2) where "x" is the length of the opening or pore in the porous membrane and "t" is the thickness of the membrane. A pinhole has a tortuosity of 1 because the length of the pinhole is the same as the thickness of the membrane. A tortuous pore has a tortuosity greater than 1 as shown in Figure 7 because the length of the pore is longer than the thickness of the membrane.

[0058] In some embodiments, the dry-stretched porous membrane is semi-crystalline. In some embodiments, the dry-stretched porous membrane is semi-crystalline and oriented in a single direction. For example, the membrane may be MD-oriented. Porous films formed by wet processes, such as films formed by beta-nucleation processes, may be randomly oriented.

[0059] In some embodiments, a coated separator includes a microporous film and a coating on at least one side of the microporous film, where the coated separator shuts down at a temperature of 140° C. or less. In some embodiments, the coating causes the separator to shut down at a temperature lower than the temperature at which the microporous film would shrink by more than 15%, more than 12%, more than 10%, or preferably more than 5% without any coating. The microporous film of the separator itself (uncoated) does not shut down or does not shut down at a temperature of 140° C. or less. The microporous film may shut down at a temperature between 140° C. and 350° C. The coating of the coated separator may contain polyethylene, a binder, and optional inorganic or heat-resistant particulates. EXAMPLES

[0060] Example 1 In Example 1, a coated separator was formed by coating a solution containing polyethylene, nano-sized alumina, a binder, and water or a water-based solvent onto one side of a polypropylene monolayer microporous film. Figure 8 shows a schematic diagram of the coating. The microporous film may be a biaxially oriented microporous membrane as disclosed in Celgard® Patent No. US8,795,565.

[0061] Example 2 In Example 2, a coated separator was formed by coating a solution containing polyethylene, nano-sized alumina, a binder, and water or a water-based solvent on both sides of a polypropylene monolayer microporous film. Figure 8 shows a schematic diagram of the coating. The microporous film may be a biaxially oriented microporous membrane as disclosed in Celgard® Patent No. US8,795,565.

[0062] FIG. 8 shows the absorption of moisture at the surface of the coating, which improves film curl. Nano-sized alumina absorbs moisture. Although the large surface area may attract a relatively large amount of moisture, the small particle size should not affect the packing of PE. The use of larger size inorganic particles may affect the packing of PE and therefore the shut down. In a preferred embodiment, the ratio of the size of the inorganic particles to the size of the PE particles is 0.5:1 or less. In some preferred embodiments, the ratio is 0.4:1 or less, 0.3:1 or less, 0.2:1 or less, 0.1:1 or less, or 0.05:1 or less. In some embodiments, the PE particles are as much as 12, 15, or 20 times the size of the inorganic particulates.

[0063] Figure 9 shows the difference in shutdown temperature for an uncoated microporous film (blue line) and a microporous film coated as a separator described herein (black line). The microporous film used has a shrinkage of 15% at temperatures between 120°C and 125°C. The shrinkage is 13% at approximately 120°C, 19% at approximately 130°C, and over 50% at 160°C.

[0064] The addition of alumina nanoparticles (inorganic nanoparticles) has been shown to improve curl as shown in Figure 11. The upper sample has no alumina added, while the bottom sample has alumina added. While not wishing to be bound by any particular theory, the use of alumina (inorganic particles) is said to improve curl through water adsorption. Alumina can attract a relatively large amount of water due to its small particle size and large surface area, but the small particle size should not affect the packing of the PE, so the impact on shutdown is not significant compared to using larger alumina particles as in the past. Hereinafter, in Figure 10, it is demonstrated why smaller inorganic particles are preferred herein. By increasing the surface area (smaller particles), a relatively large amount of charge-neutralizing water molecules can be attracted, while at the same time, the smaller particles do not interfere with the packing uniformity as larger particles do as shown in Figure 10.

[0065] Example 3 A water-based coating containing polyethylene, a binder, and nano-sized alumina was provided on one side (Example 3A) and two sides (Example 3B) of a microporous film made of a polymer having a melting point above 200 °C. The microporous film may not shut down or may shut down at a temperature above 200 °C.

[0066] Example 4 A water-based coating containing polyethylene, a binder, and nano-sized alumina was provided on one side (Example 4A) and two sides (Example 4B) of a microporous film made of a polymer having a melting point above 250 °C. The microporous film may not shut down or may shut down at a temperature above 250 °C.

[0067] Example 5 A water-based coating containing polyethylene, a binder, and nano-sized alumina was applied to one side (Example 5A) and two sides (Example 5B) of a microporous film made of a polymer with a melting point above 300° C. The microporous film may be non-shutdown or shut down at temperatures above 300° C.

[0068] Example 6 A water-based coating containing polyethylene, a binder, and nano-sized alumina was applied to one side (Example 6A) and two sides (Example 6B) of a microporous film made of a polymer with a melting point above 180° C. The microporous film may be non-shutdown or shut down at temperatures above 180° C.

[0069] Example 7 A tri-layer product (PP / PE / PP) was coated with a shutdown coating at 95°C. A comparison of the properties of an uncoated tri-layer product and a tri-layer product with a shutdown coating at 95°C is seen in Figure 12. A graph showing the MD shrinkage and Gurley at 115°C, 120°C, 125°C, and 130°C is in Figure 16. The base film w / o shutdown coating has high shrinkage but no shutdown at 125°C. The base film with shutdown coating has pore blocking but shrinkage remains low (<15%). Figure 17 shows the resulting film after baking at 115°C for 2 minutes. The coated film starts to become transparent, suggesting pore blocking at 115°C by the shutdown coating. The base film shows signs of shrinkage but no pore blocking (remains opaque).

[0070] Example 8 A tri-layer product (PP / PE / PP) was coated with a shutdown coating at 115°C. A comparison of the properties of an uncoated tri-layer product and a tri-layer product with a shutdown coating at 115°C is seen in Figure 12. A graph showing MD shrinkage and Gurley at 115°C, 120°C, 125°C, and 130°C is in Figure 16. The base film w / o shutdown coating has high shrinkage but no shutdown at 125°C. The base film with shutdown coating has pore blocking but shrinkage remains low (<15%). Figure 17 shows the resulting film after baking at 115°C for 2 minutes. The coated film starts to clear, suggesting pore blocking at 115°C by the shutdown coating. The base film shows signs of shrinkage but no pore blocking (remains opaque).

[0071] Example 9 A base film that has a shutdown at about 160° C. when uncoated is coated with a 120° C. shutdown coating, thereby shifting the shutdown to about 120° C. This is shown in Figure 13. This shows that a shutdown coating can be applied to a desired base film and that a desired shutdown shift can be achieved.

[0072] Example 10 A base film that has a shutdown of about 130° C. when uncoated is coated with a 95° C. shutdown coating. This shifts the shutdown of the base film to about 95° C. This is shown in Figure 13. This shows that a shutdown coating can be applied to a desired base film and that a desired shutdown shift can be achieved.

[0073] Example 11 A base film with high pin removal was coated with a shutdown coating. Figure 14 shows that the shutdown coating reduced the pin removal force. Figure 15 demonstrates the good results of the pin removal test, i.e., the film does not stretch when the pin is removed. The use of a coating to reduce pin removal eliminates the need to add additives to the base film that may affect processability.

[0074] Example 12 A base film with low pin removal force was coated with a shutdown coating. Figure 14 shows that the shutdown coating reduced the pin removal force. Figure 15 demonstrates the good results of the pin removal test, i.e., the film does not stretch when the pin is removed. The use of a coating to reduce pin removal eliminates the need to add additives to the base film that may affect processability.

[0075] Example 13 Two identical base films were coated with two different water-based shutdown coatings. One coating included polyethylene, binder, and reference alumina with a size of about 0.7 microns (700 nm). The other coating included polyethylene, binder, and nano alumina with a size of 250 nm. As shown in FIG. 18 herein, the shutdown coating with nano alumina shuts down at a much lower temperature (about 100° C. compared to about 125° C.) and the shutdown window was extended to about 190° C. Therefore, the shutdown separator with nano alumina is considered to be much safer than the shutdown separator with reference alumina.

Claims

1. A coated separator comprising a microporous film and a coating on at least one side of the microporous film, the coated separator shuts down at a temperature below 140° C., or below 135° C., the coating being a water-based or solvent-based coating; the coating comprises polyethylene and a binder; the coating further comprises inorganic particulates; The separator, wherein the inorganic particulate comprises alumina and the microporous film has an average porosity of greater than 30%.

2. 10. The coated separator of claim 1, wherein the coated separator shuts down at a temperature below 130°C, below 125°C, below 120°C, or below 115°C.

3. 10. The coated separator of claim 1, wherein the coated separator shuts down at a temperature below 110°C or below 100°C.

4. 10. The coated separator of claim 1, wherein the microporous film itself (uncoated) does not shut down at temperatures below 140°C, does not shut down or shuts down at temperatures between 140°C and 350°C, does not shut down or shuts down at temperatures below 135°C, or does not shut down or shuts down at temperatures between 140°C and 350°C.

5. 10. The coated separator of claim 1, wherein the microporous film comprises a polyolefin, the polyolefin is polypropylene or another polyolefin having a melting temperature of 160°C or greater, or the microporous film is a monolayer film made of polypropylene or another polyolefin having a melting temperature of 160°C or greater.

6. 6. The coated separator of claim 5, wherein the microporous film does not shut down or shuts down at a temperature between 160°C and 350°C, or the microporous film does not shut down or shuts down at a temperature between 135°C and 160°C.

7. 2. The coated separator of claim 1, wherein the inorganic particulate comprises a metal oxide having a particle size D50 of 500 nm or less, 250 nm or less, or 200 nm or less.

8. 1. A coated separator, wherein the microporous film is a single layer microporous film, the monolayer microporous film comprises polypropylene; the microporous film has an average porosity of greater than 30%, or The coated separator of any one of claims 1 to 6, wherein the microporous film comprises polypropylene.

9. The coated separator of any one of claims 1 to 6, wherein the microporous film has an average pore size greater than 0.03 microns, greater than 0.04 microns, or greater than 0.045 microns.

10. The coated separator of any one of claims 1 to 6, 8, wherein the microporous film is a bi-layer, tri-layer, or multi-layer microporous film.

11. A coated membrane for a coated separator comprising a microporous film and a coating on at least one side of the microporous film, the coating of the coated membrane shuts down at a temperature below 140° C.; the coating comprises polyethylene and a binder; the coating further comprises inorganic particulates; The membrane, wherein the inorganic particulate comprises alumina and the microporous film has an average porosity of greater than 30%.

12. A coated membrane comprising a microporous polyolefin film and a porous coating on at least one side of said microporous film, said coating having or comprising a material that melts or flows to block pores of said porous coating at a temperature below 140° C.; the porous coating comprises polyethylene and a binder; the porous coating further comprises inorganic particulates; The membrane, wherein the inorganic particulate comprises alumina and the microporous film has an average porosity of greater than 30%.

13. A coated membrane comprising a microporous polyolefin film and a microporous coating on at least one side of said microporous film, said coating having or comprising a polymeric material that melts or flows to block pores of said coating at a temperature below 140°C; the porous coating comprises polyethylene and a binder; the porous coating further comprises inorganic particulates; The membrane, wherein the inorganic particulate comprises alumina and the microporous film has an average porosity of greater than 30%.

14. A separator having a shutdown coating, the coating being a water-based or solvent-based coating; the coating comprises polyethylene and a binder; the coating further comprises inorganic particulates; The inorganic fine particles include alumina, and the separator has an average porosity of more than 30%. The separator.

15. 15. The separator of claim 14, wherein the coating comprises the inorganic particulates having a particle size D50 of 500 nm or less, 250 nm or less, or 200 nm or less.

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