Dimension-stable microporous web

Abstract

A multilayer structure comprising a microporous polymer web having two main surfaces and an inorganic material comprising nano-particles and micro-particles formed as a first porous layer on one or both of the main surfaces of the microporous polymer web is disclosed herein. The first porous layer provides high-temperature dimensional stability and a preserved multilayer structure above the melting point of the microporous polymer web, even when the fluid permeability of the single multilayer structure is reduced at elevated temperatures. The first porous layer has enhanced peel strength compared to an equivalent layer without nano-particles.

Description

Technology Field

[0001] Copyright Notice

[0002] ⓒ 2019 Amtek Research International LLC. The disclosed portion of this patent document contains copyrighted material. The copyright holder has no objection to the copying of the patent document or patent disclosure by anyone, as indicated in the patent file or record of the Patent Office, but retains all copyrights by other means. 37 CFR §1.71(d).

[0003] Technology field

[0004] The present invention relates to the fabrication of a freestanding microporous polymer web that (1) exhibits excellent in-plane dimensional stability (i.e., low shrinkage) and a preserved multilayer structure at temperatures both higher and lower than the melting point of the base polymer membrane, (2) maintains shutdown characteristics, and (3) has excellent adhesion between (i) a porous layer containing an inorganic material and (ii) the base polymer membrane. At high temperatures, pores within the bulk structure of the base polymer membrane may begin to collapse or shut down, thereby altering its permeability. Such a web can be used as a separator to improve the manufacturability, performance, and safety of energy storage devices such as lithium-ion batteries. Background Technology

[0005] Separators are an essential part of the performance, safety, and cost of lithium-ion batteries. During normal operation, the primary function of a separator is to prevent electrical conduction (i.e., short circuit or direct contact) between the anode and cathode while allowing ion conduction by the electrolyte. For small commercial cells under abuse conditions, such as external short circuits or overcharging, separators are required to shut down at temperatures significantly lower than the temperature at which thermal runaway could occur. These requirements are described by Doughty. D. Proceedings of the Advanced Automotive Battery Conference This is described in Honolulu, HI (June 2005). Shutdown occurs due to the collapse of pores within the separator caused by the melting and viscous flow of the polymer, which slows down or stops the flow of ions between the electrodes. Since almost all lithium-ion battery separators contain polyethylene as part of a single-layer or multi-layer structure, shutdown usually begins at about 130°C, which is the melting point of polyethylene.

[0006] Separators for the lithium-ion market are currently manufactured using either “dry” or “wet” processes. Celgard LLC et al. described a dry process in which polypropylene (PP) or polyethylene (PE) is extruded into a thin sheet and rapidly drawn down. The sheet is then annealed at a temperature 10–25°C below the polymer melting point to control crystal size and orientation. Next, the sheet is rapidly stretched in the machine direction (MD) to create slit-shaped pores or voids. Three-layer PP / PE / PP separators manufactured by the dry process are commonly used in lithium-ion batteries.

[0007] Wet process separators composed of polyethylene are manufactured by extruding a plasticizer / polymer mixture at an elevated temperature, followed by phase separation, biaxial stretching, and extraction of the pore-forming agent (i.e., plasticizer). The manufactured separators have elliptical or spherical pores that exhibit excellent mechanical properties in both mechanical and transverse directions. Toray Tonen Specialty Separator, Asahi Kasei Corp., SK Innovation Co., Ltd., and Entek ® PE-based separators manufactured in this manner by Membranes LLC are widely used in lithium-ion batteries.

[0008] More recently, battery failures occurring in commercial operation have demonstrated that shutdown does not guarantee safety. The main reason is that after shutdown, residual stress and reduced mechanical properties at temperatures above the polymer melting point can lead to shrinkage, tearing, or the formation of pinholes. Exposed electrodes can come into contact with other electrodes, forming an internal short circuit that can lead to further heating, thermal runaway, and explosion.

[0009] For large lithium-ion batteries designed for hybrid or plug-in hybrid applications (HEV, PHEV), the benefits of separator shutdown are being publicly questioned because it is difficult to guarantee sufficient speed and uniformity of shutdown across the entire cell. This issue is addressed by Roth, EP, Proceedings of Lithium Mobile Power ConferenceThis is described in San Diego, CA (October 2007). Accordingly, many companies are focusing on modifying the structure of lithium-ion batteries to include (1) a heat-resistant separator or (2) an electrode coated with a heat-resistant layer or a conventional polyolefin separator. Heat-resistant separators composed of high-temperature polymers (e.g., polyimide, polyester, and polyphenylene sulfide) have been manufactured to a limited extent from solution casting, electrospinning, or other process technologies. In these cases, the high-temperature polymer melting point prevents shutdown at temperatures lower than 200°C.

[0010] U.S. Patent Publication US2012 / 0145468 describes a freestanding, microporous, ultra-high molecular weight polyethylene (UHMWPE)-based separator containing inorganic filler micros sufficient to provide low shrinkage while maintaining high porosity at temperatures (> 135°C) above the melting point of the polymer matrix. This freestanding, heat-resistant separator has excellent wettability and very low impedance, but does not exhibit shutdown characteristics due to the high loading level of the inorganic filler.

[0011] U.S. Patent No. 7,638,230 B2 describes a porous heat-resistant layer coated on the negative electrode of a lithium-ion secondary battery. The heat-resistant layer consists of an inorganic filler and a polymer binder. The inorganic filler includes magnesia, titania, zirconia, or silica. The polymer binder includes a modified rubber mixture containing polyvinylidene fluoride and acrylonitrile units. A higher binder content has a negative effect on the high-rate discharge characteristics of the battery.

[0012] U.S. Patent Publications US 2008 / 0292968 A1 and US 2009 / 0111025 A1, respectively, describe an inorganic / organic separator in which an active layer is formed on at least one surface of a porous substrate by coating the porous substrate with a mixture of inorganic particles and a polymer binder. The porous substrate may be a nonwoven fabric, a membrane, or a polyolefin-based separator. The inorganic particles are selected from the group comprising those exhibiting one or more of a dielectric constant greater than 5, piezoelectricity, electrochemical stability, and lithium ion conductivity. The selected polymer binder is described. The composite separator exhibits superior thermal stability, dimensional stability, electrochemical stability, and lithium ion conductivity compared to uncoated polyolefin-based separators used in lithium-ion batteries. In the case of certain polymer binders mixed with inorganic particles, a high degree of swelling with the electrolyte may result in a surface layer, but rapid wetting or expansion in the polyolefin substrate is not achieved.

[0013] In the latter two approaches among the above approaches, there is an inorganic charge layer applied during a secondary coating process on the surface of an electrode or porous substrate to provide heat resistance and prevent internal short circuits in the battery.

[0014] Several embodiments of freestanding microporous polymer webs rely on ultra-high molecular weight polyethylene (UHMWPE) as a polyolefin-based membrane component. The repeating unit of polyethylene is (-CH2CH2-) x x represents the average number of repeating units within individual polymer chains. For polyethylene, which is used in many film and molded part applications, x is approximately 10,000; for UHMWPE, x is approximately 150,000. This extreme difference in the number of repeating units is due to the higher degree of chain entanglement and unique characteristics associated with UHMWPE.

[0015] One of these characteristics is the ability of UHMWPE to resist material flow by its own weight when heated to temperatures above its melting point. This phenomenon is due to its extremely high buoyancy and the associated long relaxation time even at elevated temperatures. While UHMWPE is generally available, it is difficult to process into fibers, sheets, or membranes. Its high melt viscosity requires compatible plasticizers and twin-screw extruders to unwind the polymer chains, allowing the resulting gel to be processed into a useful form. This approach is commonly referred to as the “gel process.” In many cases, other polyolefins are blended with UHMWPE to lower the molecular weight distribution, which affects the properties following plasticizer extraction and results in a porous membrane.

[0016] In most of the described preferred embodiments, the microporous polyolefin membrane is prepared by mixing UHMWPE, high-density polyethylene (HDPE), and a plasticizer (e.g., mineral oil). The mixture of UHMWPE and HDPE is mixed with a sufficient amount of plasticizer and extruded to form a homogeneous, aggregated mass. The mass is processed into a blown film, a cast film, or by a calendering method to provide an oil-filled sheet of reasonable thickness (<250 μm). The oil-filled sheet may be further oriented in a biaxial direction to reduce its thickness and affect its mechanical properties. In an extraction operation, the oil is removed by a solvent that subsequently evaporates to form a microporous polyolefin membrane that is subsequently coated with an inorganic surface layer.

[0017] “Freestanding” refers to a web having sufficient mechanical properties to allow operations such as winding or unwinding in the form of a film for use in energy storage device assemblies.

[0018] In a first preferred embodiment, a polyolefin-based membrane is passed through an aqueous dispersion, such as an alcohol / water dispersion of an inorganic material. The inorganic material may include, for example, inorganic oxides, carbonates, or hydroxides such as alumina, silica, zirconia, titania, mica, boehmite, magnesium hydroxide, calcium carbonate, or mixtures thereof. As the membrane is pulled through the aqueous dispersion, a surface coating of controlled thickness may be formed using a wire-wound rod (e.g., a Mayer rod). The infiltrated membrane is then dried in an oven that evaporates the liquid phase using a series of air knives and hot air to form a first porous layer on one or both of the main surfaces of the microporous polymer web.

[0019] The first porous layer comprises inorganic material nanoparticles sufficient to provide excellent adhesion to a microporous polymer web. For example, the inorganic material of the first porous layer may comprise about 10 to about 60 weight percent of nanoparticles or about 20 to about 50 weight percent of nanoparticles. The remainder of the inorganic material of the first porous layer is microparticles such as boehmite particles or other alumina microparticles. Microparticles having a platelet-like structure, such as boehmite, can help improve adhesion. Preferably, the first porous layer has an average peel strength improved by at least 20% compared to an equivalent layer without nanoparticles. Preferably, the first porous layer comprises nanoparticles sufficient to impart an average peel strength of at least 31 N / m to the first porous layer, such as an average peel strength of, for example, 31 N / m to 200 N / m, 31 N / m to 100 N / m, 37 N / m to 94 N / m, 42 N / m to 89 N / m, or 47 N / m to 84 N / m.

[0020] At high temperatures, pores within the bulk structure of the base polyolefin membrane may begin to collapse or shut down, which can alter permeability and reduce ion conductivity. This ultimately leads to the shutdown of the battery cell. In the first preferred embodiment, the inorganic material surface coating has a weight-based threshold coating ratio of inorganic material to polyolefin sufficient to maintain in-plane dimensional stability (in a plane defined by the mechanical and transverse directions) and a preserved multilayer structure at a temperature at least higher than the melting point of the polyolefin membrane (such as a temperature about 45°C higher than the melting point of the polyolefin membrane). This prevents contact between the electrodes while the battery cell shuts down due to a loss of ion conductivity.

[0021] "Single, multilayer structure" refers to a microporous polymer web comprising an inorganic material formed on at least one of the main surfaces of the web. Both main surfaces of the web may have a porous layer formed thereon. The porous layer comprising the inorganic material may have additional layers formed thereon, such as a second porous layer composed of a gel-forming polymer material. The multilayer structure forms a single structure.

[0022] In a preferred embodiment, dimensional stability, including the multilayer structure, is maintained during shutdown (i.e., as the fluid permeability of the single multilayer structure decreases above the melting point of the microporous polymer web). In short, micro-buckling is prevented, and the boundaries of the layer interfaces are maintained. Fig. 9A shows an SEM image of a PE-based separator exposed to a temperature of 180°C with high shrinkage (> 20% shrinkage). As can be seen in Fig. 9A, as the separator shrinks, the inorganic material with insufficient adhesion separates from the PE. In contrast, as shown in Fig. 9B, an SEM image of an inorganic-coated PE-based separator exposed to a temperature of 180°C with a low shrinkage rate shows that the multilayer structure was maintained during shutdown, and the inorganic material maintained adhesion to the PE. Additionally, the boundaries of the interfaces were maintained. When the multilayer structure is maintained during shutdown, a clear distinction between the layer containing the inorganic material and the polymer layer can be seen under SEM.

[0023] Preferably, dimensional stability is sufficiently maintained during shutdown to avoid shrinkage of more than 10% in the machine direction or transverse direction.

[0024] In a preferred embodiment, the organic hydrogen bonding component may be present in the aqueous dispersion at about 5% or less. The preferred organic hydrogen bonding component comprises both polymers and small molecules having a plurality of hydrogen bonding sites. Preferred polymers include polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), polyacrylic compounds, polyethylene oxide, polyvinyl alcohol, and mixtures thereof. Preferred small molecules include catechol, sucrose, tannic acid, maltitol, dimethylol dihydroxyethylene urea (DMDHEU), and pentaerythritol.

[0025] Preferably, the first porous layer comprises nanoparticles sufficient to impart an average peel strength of at least 31 N / m, e.g., 31 N / m to 200 N / m, having an organic hydrogen-containing component of about 10 weight% or less, e.g., about 1 to about 10%, about 1% to about 8%, or 1% to about 6% in the first porous layer. For example, by incorporating sufficient nanoparticles, a desirable average peel strength can be achieved with an organic hydrogen-bonding component consisting of about 10% or less of a PVP-based polymer, a mixture of polymers mainly comprising polyacrylic, or a mixture thereof.

[0026] Preferably, the first porous layer has a median pore size of about 15 nm to about 100 nm. The porosity of the porous layer containing the inorganic material can be controlled by adjusting the ratio of micro-particles to nano-particles. If the ratio of micro-particles to nano-particles in the porous layer is low, the median pore size becomes small. If the ratio of micro-particles to nano-particles is high, a large median pore size becomes large. For example, a ratio of about 2:1 can achieve a median pore size of about 12-40 nm. In another example, a ratio of about 4:1 can achieve a median pore size of about 40-60 nm. In yet another example, a ratio of about 8:1 can achieve a median pore size of about 80-100 nm.

[0027] Additionally, the inorganic material preferably has a sufficient ratio of nano-particles to micro-particles at a limiting coating ratio that minimizes the thickness of the first porous layer.

[0028] In addition, the first porous layer may be further coated with a second porous layer comprising a gel-forming polymer material to increase the laminability of the separator to the electrode.

[0029] Finally, for each of the above embodiments, corona treatment of the polyolefin-based membrane can improve the overall average peel strength of the coated separator. The applicant believes that oxygen-containing species (e.g., hydroxyl groups) derived from the corona treatment of the polyolefin membrane surface hydrogen bond with inorganic particles to improve adhesion at the interface between the inorganic surface layer and the polyolefin membrane.

[0030] A microporous, freestanding polyolefin separator produced as described in a preferred embodiment can be wound or laminated within a package to separate electrodes in an energy storage device, e.g., a battery, capacitor, supercapacitor, or fuel cell. A gel-forming polymer material can be gelled, and an electrolyte can be added to fill the pores in both the bulk structure of the base polymer membrane and the interior of the inorganic material. Such a separator is advantageous for the manufacture of energy storage devices because it combines excellent heat resistance, in-plane dimensional stability, in-layer adhesion, lamination capability, and shutdown characteristics.

[0031] Accordingly, to the advantage of the present specification, those skilled in the art can achieve (1) sufficient adhesion to the microporous polymer web and a multilayer structure preserved above the melting point of the microporous polymer web to achieve high-temperature dimensional stability by adjusting the ratio of micro-particles to nano-particles in the porous layer; (2) minimized moisture content of the porous layer; and (3) sufficient porosity of the porous layer including inorganic materials.

[0032] Additional aspects and advantages will become apparent from the detailed description of a preferred embodiment with reference to the attached drawings. Brief explanation of the drawing

[0033] Figure 1 shows the weight of the inorganic material coating of Example 1 corresponding to 180°C shrinkage of more than 10% machine direction (MD) and less than 10% MD as a function of nanoparticle concentration. Figure 2 illustrates the thermal shrinkage result morphology of Example 1 as a function of nanoparticle concentration for a polyolefin separator coated with inorganic particles using binder A. Figure 3 illustrates the thermal shrinkage result morphology of Example 1 as a function of nanoparticle concentration for a polyolefin separator coated with inorganic particles using binder B. Figure 4 shows the adhesion strength as a function of nanoparticle concentration for the coated separator of Example 1. Figure 5 shows the thermogravimetric weight loss (corresponding to moisture content) as a function of nanoparticle concentration for the coated separator of Example 1. Figure 6 shows an SEM image of a coated separator of Example 1 containing about 33 wt% of nanoparticles and additionally coated with a PVDF-HFP coating. Figure 7 illustrates an experiment showing the effect of the micro-nano-particle ratio in an inorganic coating on the pore size distribution in a porous layer containing inorganic materials. Figure 8 shows the pore size distribution for the micro-:nano-particle ratio tested in Example 1. Figure 9A shows an SEM image of an inorganic-coated PE-based separator exposed to a temperature of 180°C with high shrinkage. Figure 9B shows an SEM image of an inorganic-coated PE-based separator exposed to a temperature of 180 °C with low shrinkage, where the multilayer structure was preserved and the porous layer of the inorganic material maintained adhesion to PE during shutdown. Specific details for implementing the invention

[0034] The base membrane uses a polyolefin matrix. The polyolefin most preferably used is ultra-high molecular weight polyethylene (UHMWPE) having an intrinsic viscosity of at least 10 deciliters / gram, preferably in the range of 18 to 22 deciliters / gram. It is desirable to mix UHMWPE with other polyolefins, such as HDPE or linear low-density polyethylene (LLDPE), to affect the shutdown characteristics of the membrane. The membrane may also be prepared from other polyolefins, such as ethylene-propylene copolymer, polypropylene, and polymethylpentene, or mixtures thereof.

[0035] The plasticizer used is non-volatile with respect to the polymer and is preferably liquid at room temperature. The plasticizer has little to no solvation effect on the polymer at room temperature; the plasticizer performs solvation at the polymer's softening temperature or at a higher temperature. For UHMWPE, the solvation temperature may be higher than about 160°C, and preferably in the range between about 180°C and about 240°C. It is preferable to use processing oils such as paraffinic oils, naphthenic oils, aroma oils, or mixtures of two or more. Examples of suitable processing oils include: oils sold by Shell Oil Company, such as Gravex™ 942; oils sold by Calumet Lubricants, such as Hydrocal™ 800; and HR Tufflo. ® Includes oils sold by Nynas Inc. as shown above.

[0036] The polymer / oil mixture is extruded through a sheet die or annular die and then biaxially oriented to form a thin, oil-filled sheet. Any solvent available for use with oil may be used in the extraction step and is provided to have a boiling point that makes it practical to separate the plasticizer from the solvent by distillation. Such solvents include 1,1,2-trichloroethylene; perchloroethylene; 1,2-dichloroethane; 1,1,1-trichloroethane; 1,1,2-trichloroethane; methylene chloride; chloroform; 1,1,2-trichloro-1,2,2-trifluoroethane; isopropyl alcohol; diethyl ether; acetone; hexane; heptane; and toluene. In some cases, it is desirable to select a processing oil such that the residual oil in the polyolefin membrane becomes electrochemically inert after extraction.

[0037] The coating formulation used in the first aqueous dispersion of the two preferred embodiments consists of inorganic particles in which more than 50% of the water is counted as the liquid phase. The inorganic particles are generally charge-stabilized and remain suspended in an alcohol / water mixture. Inorganic hydrogen bonding components, such as low molecular weight, water-soluble polymers, are also present. It is desirable to select a polymer having a number of hydrogen bonding sites to obtain a robust microporous inorganic surface that does not easily shed the inorganic particles while minimizing their concentration.

[0038] In addition to controlling the amount of organic hydrogen bonding components and inorganic particles within the coating formulation, the applicant considers it important to control the particle size distribution of the inorganic particles. Furthermore, the coating formulation controls the thickness of the inorganic surface layer created by carefully applying it to a polyolefin-based membrane.

[0039] A small amount of nanoparticles can substantially reduce the coating weight required to achieve high-temperature dimensional stability (180°C). However, particles with a higher surface area retain more moisture than particles with a lower surface area (i.e., larger particles). One approach to addressing moisture retention is to use a mixture of high and low surface area particles. Low surface area particles (“micro-particles”) did not retain as much moisture as high surface area particles (“nano-particles”).

[0040] Additionally, the ratio of nanoparticles to microparticles can be optimized to ensure maximum adhesion of the coated layer (i.e., the first porous layer) to the base membrane. An inorganic material comprising a combination of 20 to 50 wt% nanoparticles and a balanced mix of microparticles is believed to provide optimized adhesion of the first porous layer to the base membrane. In some cases, the nanoparticle fraction of the total inorganic material content may be as low as 10% or as high as 60%, and optimal adhesion may still be achieved. Without being bound by theory, a balance of inorganic material content consisting of 20 to 50 wt% nanoparticles and microparticles may provide optimal adhesion because the mixed particle system hinders crack propagation (e.g., see Fig. 6).

[0041] As used herein, “nano-particle” refers to individual particles or multi-particle aggregates having an average size of 100 nanometers or less. The term “micro-particle” refers to individual particles, multi-particle aggregates, or multi-aggregate clusters having an average size greater than 100 nanometers and up to about 2 microns. As used herein, nanoparticles are not small enough to penetrate into the bulk structure of the polyolefin membrane. Similarly, “nanoporous” indicates that pores exist with an average size of about 100 nm or less, and “microporous” indicates that pores exist with an average size greater than about 100 nm and up to about 1 micron.

[0042] As the percentage of nanoparticles increases, the thickness of the first porous layer can be reduced while maintaining dimensional stability. In other words, as the percentage of nanoparticles increases, the limiting coating ratio of inorganic particles to the base membrane (i.e., the minimum ratio required to maintain dimensional stability) decreases. It should be understood that the limiting coating ratio and the limiting coating thickness (i.e., the minimum coating thickness required to maintain dimensional stability for a given base membrane thickness) imply similar concepts. It should be understood that the limiting coating thickness can be achieved by coating one side of the membrane to its full thickness or by coating two sides of the membrane to half the limiting thickness.

[0043] It is possible to select a concentration of nanoparticles that achieves desired dimensional stability with an inorganic material-to-base membrane ratio and an acceptable moisture level while optimizing the adhesion of the inorganic material to the base membrane.

[0044] As the weight of the base web increases (due to increased thickness or decreased porosity), the required weight of the inorganic particles (and corresponding thickness) increases to achieve a selected limiting coating ratio. Nanoparticles with a higher surface area require less weight (and less corresponding thickness) to achieve the same dimensional stability as inorganic microparticles with a lower surface area. The limiting coating ratio of the inorganic particles is thought to be determined by the surface area and the weight of the inorganic particles relative to the weight of the microporous polymer web. Thus, the inorganic particle-coated microporous polymer web can be further coated in a second aqueous dispersion, for example, with a gel-forming polymer material, and can maintain dimensional stability.

[0045] Examples 1.

[0046] The effect of nanoparticle concentration on the critical coating weight required to achieve high-temperature dimensional stability was evaluated. Shrinkage tests were performed at 180°C for 30 minutes. Entec, a 12 μm thick microporous ultra-high molecular weight polyethylene-containing separator ® EPH (Entek Membranes LLC, Oregon) was coated with different aqueous dispersions. Two different binder systems, "Binder A" and "Binder B," at 6 wt% were evaluated. Binder A is a PVP-based polymer. Binder B is a mixture of polymers primarily comprising polyacrylic. Different aqueous dispersions were tested containing mixed grades of nanoparticle-type alumina (PG003, Cabot, aqueous dispersion with a major particle size of about 20 nm) and microparticle-type boehmite (average particle size of about 1.4 microns), with nanoparticle concentrations ranging from 0 wt% to 100 wt% of the total inorganic material concentration. The boehmite microparticles have a platelet-like structure (see Fig. 6).

[0047] The weight of the ceramic coating corresponding to 180° shrinkage in the machine direction (MD) of more than 10% and less than 10% was configured as a function of nanoparticle concentration (see Fig. 1).

[0048] The thermal shrinkage of a polyolefin separator coated with inorganic particles using binder A as a function of nanoparticle concentration is shown in Fig. 2. The thermal shrinkage of a polyolefin separator coated with inorganic particles using binder B as a function of nanoparticle concentration is shown in Fig. 3.

[0049] Examples 2

[0050] A peel test was performed to test the adhesion of the inorganic coating to the polyolefin-based membrane. The coated separator was placed horizontally on a steel plate, and a magnetic strip was positioned at the edge of the separator to secure it, and an average peel strength test was performed. An adhesive tape (pressure-sensitive tape, 3M Scotch® Magic™ Tape 810, 3 / 4 inch (1.9 cm) wide) was applied to the coated separator. The free end of the tape was secured to a fixing clip, and the tape was peeled 180° from the original tape direction at a speed of 8.5 mm / sec and a distance of 100 mm (i.e., 180° peel test configuration). 10 ± 0.005 lbs. The force required to remove the coating layer from the base polyolefin membrane was measured using a force gauge (Chatillon, DFGS-R-10) with a load cell capacity of 4 kg ± 2.7 g, and the average load was recorded. All tests were performed at room temperature. Figure 4 shows the adhesion strength as a function of nanoparticle concentration for the coated separator. The data can be converted to N / m by dividing the values ​​by the width of the adhesive tape, 0.019 m. Optimal adhesion was observed when the nanoparticle concentration was approximately 20 wt% to approximately 50 wt%.

[0051] Examples 3

[0052] Thermogravimetric analysis (TGA) was performed on the coated separator of Example 1. Figure 5 shows the TGA weight loss (corresponding to moisture content) of the coated separator as a function of nanoparticle concentration. As the nanoparticle loading level increased, the moisture content of the separator increased. Table 1 lists the coating weight of the coating layer along with the data shown in Figure 5.

[0053] [Table 1]

[0054]

[0055] Examples 4

[0056] The coated separator of Example 1 having 33% nanoparticles was further coated with an aqueous dispersion comprising the following:

[0057] 233 g XPH 884 (25 wt% poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP); Solvay)

[0058] 216 g distilled water

[0059] 30 g isopropanol (ACS grade)

[0060] 21 g Selvol 09-325 aqueous polyvinyl alcohol solution (8.5 wt% solids; 98% hydrolyzed; Sekisui)

[0061] The coating dispersion contained 12 wt% solids with a 97 / 3 PVDF-HFP / PVOH mass ratio. The separator was dip-coated through a bath containing the aqueous dispersion, and the thickness of the wet layer on each side was controlled with a #4 Mayer rod. The infiltrated separator was dried with a series of air knives, transported through a vertical oven set to 80°C, and wound onto a core before testing. Figure 6 shows an SEM image of the coated separator of Example 1, which contains approximately 33 wt% nanoparticles and is additionally coated with a PVDF-HFP coating.

[0062] Examples 5

[0063] Mercury porosimetry differential intrusion was performed on various inorganic-coated separators. The experiment demonstrates the effect of the micro-particle:nano-particle ratio on the pore size distribution of the coatings. All inorganic coatings were applied to ENTEK EPH-based separators. The micro-particles consisted of CEH-1 (Saint Gobain, average particle size 0.5 microns). The nano-particles consisted of PG008 (Cabot, major particle size ~20 nm). The median pore size of the inorganic porous layer ranged from ~15 nm (100% nanoparticles for the inorganic portion) to ~100 nm (8:1 micro-particle:nano-particle ratio). The pore size distribution of the base separators and the different ratios tested are shown in Figure 7.

[0064] Examples 6

[0065] Mercury porosity differential intrusion of various inorganic coated separators was performed using the method disclosed in Example 1. The experiment demonstrates the effect of the micro-particle:nano-particle ratio on the pore size distribution of the coating. The pore size distributions for different ratios tested are shown in Figure 8.

[0066] It will be obvious to those skilled in the art that many variations may be made to the details of the embodiments described above without departing from the basic principles of the invention. For example, an inorganic surface layer may be applied as a coating on part or the entire surface of a polyolefin membrane.

Claims

Claim 1 A battery separator comprising a free-standing unitary multilayer structure having first and second main surfaces, wherein the structure comprises a microporous polymer web having two main surfaces, and one or both of the main surfaces of the microporous polymer web have an inorganic material comprising nanoparticles and microparticles formed as a first porous layer, wherein the nanoparticles have an average size of 100 nm or less, and the microparticles have an average size greater than 100 nm, and the nanoparticles form 20% to 50% by weight of the inorganic material, and the first porous layer has an average peel strength of at least 31 N / m, and the first porous layer provides high-temperature dimensional stability and a preserved multilayer structure above the melting point of the microporous polymer web even when the fluid permeability of the unitary multilayer structure is reduced at an elevated temperature. Claim 2 A battery separator according to claim 1, wherein the inorganic material comprises an inorganic oxide, carbonate, hydroxide, or a mixture thereof. Claim 3 A battery separator according to paragraph 2, wherein the inorganic material comprises alumina, silica, zirconia, titania, mica, boehmite, magnesium hydroxide, calcium carbonate, or a mixture thereof. Claim 4 A battery separator according to claim 1, wherein the first porous layer further comprises an organic hydrogen bonding component. Claim 5 A battery separator according to claim 1, wherein the structure further comprises a second porous layer comprising a gel-forming polymer material having a passage. Claim 6 A battery separator according to claim 5, wherein the gel-forming polymer material comprises poly(vinylidene fluoride-hexafluoropropylene). Claim 7 A battery separator according to claim 1, wherein the microporous polymer web comprises a polyolefin. Claim 8 A battery separator according to claim 7, wherein the polyolefin comprises polyethylene, polypropylene, or a mixture thereof. Claim 9 A battery separator according to claim 8, wherein the polyolefin comprises ultra-high molecular weight polyethylene (UHMWPE). Claim 10 A battery separator according to claim 1, wherein the first porous layer has an average peel strength improved by at least 20% compared to an equivalent layer without nanoparticles. Claim 11 A battery separator according to claim 1, wherein the first porous layer has a median pore size of 15 nm to 100 nm. Claim 12 A battery separator comprising a freestanding single multilayer structure having first and second main surfaces, wherein the structure comprises a microporous polymer web having two main surfaces, and one or both of the main surfaces of the microporous polymer web have an inorganic material comprising nanoparticles and microparticles formed as a first porous layer, wherein the nanoparticles have an average size of 100 nm or less and the microparticles have an average size greater than 100 nm, and the nanoparticles form 20% to 50% by weight of the inorganic material, and the first porous layer has an average peel strength of at least 31 N / m and has a median pore size of 12 nm to 60 nm, and the first porous layer provides less than 10% shrinkage in the mechanical or transverse direction above the melting point of the microporous polymer web. Claim 13 A battery separator according to claim 12, wherein the micro-particles of the first porous layer comprise platelet-like particles. Claim 14 A battery separator according to paragraph 12, wherein the inorganic material comprises an inorganic oxide, carbonate, hydroxide, or a mixture thereof. Claim 15 A battery separator according to claim 12, wherein the first porous layer comprises less than 10 weight percent of an organic hydrogen bonding component. Claim 16 A battery separator according to claim 15, wherein the first porous layer comprises less than 10 weight percent of polymer. Claim 17 A battery separator according to claim 12, wherein the structure further comprises a second porous layer comprising a gel-forming polymer material having a passage. Claim 18 A method for forming a free-standing unitary multilayer structure, the method comprising: providing a microporous polymer web having two main surfaces; selecting a weight ratio of micro-particles to nano-particles in an inorganic material in an aqueous solution such that when an inorganic material in an aqueous solution forms a first porous layer on the microporous polymer web, the median pore size is 12 nm to 100 nm, wherein the nano-particles have an average size of 100 nm or less, the micro-particles have an average size greater than 100 nm, and the nano-particles form 20% to 50% by weight of the inorganic material; and passing the microporous polymer web through an aqueous solution to form a first porous layer on one or both of the main surfaces of the microporous polymer web, wherein the first porous layer comprises an inorganic material. Claim 19 In claim 18, the method wherein a weight ratio of 2:1 of micro-particles to nano-particles achieves a median pore size of 12 to 40 nm. Claim 20 In claim 18, the method wherein a weight ratio of 4:1 of micro-particles to nano-particles achieves a median pore size of 40 to 60 nm. Claim 21 An energy storage device comprising a battery separator according to any one of claims 1 to 17. Claim 22 delete Claim 23 delete Claim 24 delete Claim 25 delete Claim 26 delete Claim 27 delete

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