Ceramic-modified, acid-trapping polyolefin separator

Abstract

The present disclosure relates to the formation of free-standing, microporous polyolefin membranes that exhibit both high-temperature dimensional stability and acid scavenging capabilities. Such membranes can include hydrotalcite particles, which can contribute to high-temperature dimensional stability and acid scavenging capabilities. Such membranes can be used to improve the fabrication, performance (e.g., cycle life), and safety of energy storage devices, such as lithium-ion batteries.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 63 / 370,214, filed August 2, 2022, entitled "CERAMIC-MODIFIED, ACID-SCAVENGING POLYOLEFIN SEPARATORS," the entire contents of which are incorporated herein by reference.

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

[0003] The present disclosure relates to the formation of free-standing, microporous polyolefin membranes that have been modified within their bulk structure or on at least one surface with a mixture of inorganic particles that impart both high-temperature dimensional stability and acid-scavenging capabilities. These membranes (1) exhibit in-plane high-temperature dimensional stability (i.e., low shrinkage (e.g., less than 10%)) both above and below the melting point of the polyolefin membrane's polymer matrix, and (2) exhibit acid-scavenging capabilities via an anion-exchange mechanism. At high temperatures (above 135°C), pores within the bulk structure can begin to collapse or close, thereby altering the permeability through the membrane with little change in in-plane dimensions. Such membranes can be used to improve the fabrication, performance (e.g., cycle life), and safety of energy storage devices, such as lithium-ion batteries. [Background technology]

[0004] Separators are an integral component of lithium-ion and rechargeable Li-metal batteries, contributing to their performance, safety, and cost. Under normal operation, the separator's primary function is to prevent electronic conduction (i.e., short-circuiting or direct contact) between the anode and cathode while allowing ionic conduction through the electrolyte. For small commercial batteries, under transient conditions, such as an external short circuit or overcharge, the separator must shut down at a temperature well below where thermal runaway can occur. Shutdown occurs from the collapse of pores within the separator due to melting or viscous flow of the polymer, resulting in a slowdown or cessation of ionic flow between the electrodes. Nearly all Li-ion battery separators contain polyethylene, either as part of a single structure or multilayer structure, which initiates shutdown at approximately 130°C, near the melting point of polyethylene.

[0005] Separators for the lithium-ion market are currently manufactured using either "dry" or "wet" processes. Colgard LLC and others have demonstrated a dry process in which polypropylene (PP) or polyethylene (PE) is extruded into a thin sheet and subjected to rapid drawdown. The sheet is then annealed at temperatures 10–25°C below the polymer's melting point to control the crystallite size and orientation. The sheet is then rapidly stretched in the machine direction (MD) to form slit-like pores or voids. Trilayer PP / PE / PP separators have anisotropic mechanical properties and are difficult to fabricate with high yields at thicknesses below 14 μm, thereby limiting the volumetric energy density of lithium-ion batteries.

[0006] Wet-process separators composed of high-molecular-weight polyethylene are produced by extrusion of a plasticizer / polymer mixture at high temperatures, followed by phase separation, biaxial stretching, and extraction of the pore-forming agent (i.e., plasticizer). The resulting separators have ellipsoidal or spherical pores with good mechanical properties in both the longitudinal and transverse directions. PE-based separators produced in this manner have found wide application in Li-ion batteries by Toray, SEMCorp, Shenzhen Senior, W-Scope, Asahi-Kasei, SKIET, and ENTEK. Wet-process separators can be easily manufactured in thicknesses ranging from 5 to 12 μm and are the preferred choice for OEMs fabricating Li-ion batteries for electric vehicles.

[0007] For large Li-ion or rechargeable Li-metal batteries designed for electric vehicle applications, the benefits of separator shutdown have been openly questioned because of the difficulty of ensuring sufficient speed and uniformity of shutdown throughout the battery. As such, many companies have focused their efforts on modifying lithium-ion battery construction to include (1) heat-resistant separators or (2) heat-resistant layers coated on both electrodes of conventional polyolefin separators. These approaches are designed to prevent oxidation of the polyolefin surface in high-voltage cells and minimize in-plane separator shrinkage, preventing current collector edges from touching each other and thereby reducing the possibility of short circuits and thermal runaway.

[0008] U.S. Patent No. 9,896,555 B2 (Pekala et al.) discloses a free-standing, microporous, ultra-high molecular weight polyethylene (UHMWPE)-based separator that contains sufficient inorganic filler particles dispersed throughout the polymer matrix (bulk structure) to maintain high porosity at temperatures above the melting point of the polymer matrix (above 135°C) while exhibiting low shrinkage. Although such free-standing, heat-resistant separators have excellent wettability and ultra-low impedance, they may not exhibit shutdown performance due to the high inorganic filler loading levels.

[0009] OEM battery manufacturers desire an ideal separator that (1) possesses high-temperature dimensional stability and (2) still exhibits shutdown characteristics. To meet this requirement, PE separators are coated on one or both sides with a heat-resistant inorganic (ceramic) layer. Boehmite and alumina are the two most commonly used ceramics in the coating process, and in some cases, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) or acrylates are blended with the ceramic to promote adhesion to the electrode. A major advantage of ceramic-coated PE separators is that they can exhibit through-thickness shutdown while maintaining excellent high-temperature dimensional stability (within 5% in-plane shrinkage) even at 180°C, approximately 50°C higher than the melting point of polyethylene, as a result of the sufficient ceramic coating.

[0010] In U.S. Patent No. 7,638,230 B2, a porous heat-resistant layer was coated on the negative electrode. The heat-resistant layer consisted of an inorganic filler and a polymer binder. The inorganic filler included magnesia, titania, zirconia, or silica. The polymer binder included a modified rubber mixture containing polyvinylidene fluoride and acrylonitrile units. The heat-resistant layer consisted of 1 to 5 parts binder per 100 parts inorganic filler by weight. Higher binder contents adversely affected the battery's high-rate discharge characteristics. Furthermore, to achieve high discharge rates, the thickness of the porous heat-resistant layer had to be limited to 1 to 10 μm, and it was impossible to remove it as a free-standing film. "Free-standing" refers to a sheet with sufficient mechanical properties to be rolled and unrolled on an unsupported web.

[0011] U.S. Patent Application Publication Nos. 2008 / 0292968 A1 and 2009 / 0111025 A1 disclose organic / inorganic separators in which a porous substrate is coated with a mixture of inorganic particles and a polymer binder to form an active layer on at least one surface of the porous substrate. The porous substrate may be a nonwoven fabric, a membrane, or a polyolefin-based separator. The inorganic particles are selected from the group consisting of those exhibiting a dielectric constant, piezoelectricity, and / or lithium-ion electrical conductivity greater than 5. Various polymer binders are described. The composite separators are claimed to exhibit superior thermal safety, dimensional stability, electrochemical safety, and lithium-ion electrical conductivity compared to uncoated polyolefin-based separators used in Li-ion batteries. In the case of certain polymer binders mixed with inorganic particles, the surface layer exhibits a high degree of electrolyte swelling, but does not achieve rapid wetting or swelling within the polyolefin substrate.

[0012] The latter approach involves the presence of an inorganic layer applied in a secondary coating operation on the surface of the electrode or porous substrate to provide heat resistance and prevent internal short circuits in the battery. The idea that inorganic particles could provide acid scavenging capabilities to improve the cycle life and performance of lithium-ion batteries has not previously existed. U.S. Patent Nos. 10,050,313 B2 and 10,741,812 B2 discuss organic chelating agents or nitrogen-containing polymers that can be coated onto at least one surface of a polyolefin separator or dispersed throughout its bulk structure for acid scavenging, but these approaches do not impart high-temperature dimensional stability to the separator. Summary of the Invention [Problem to be solved by the invention]

[0013] As such, there is a need to develop ceramic-modified separators that can provide both high-temperature dimensional stability and acid scavenging capability to improve the safety and performance of lithium-ion batteries or rechargeable Li-metal batteries. In this disclosure, the inventors use synthetic hydrotalcite as the inorganic particles, which can simultaneously contribute to high-temperature dimensional stability and acid scavenging capability. [Means for solving the problem]

[0014] One objective of the present disclosure is to achieve a thin, free-standing, microporous polyolefin membrane with excellent high-temperature dimensional stability and acid scavenging capabilities due to inorganic particles dispersed throughout the polymer matrix of its bulk structure or incorporated as a surface coating. Synthetic hydrotalcite has been identified as an inorganic particle capable of imparting the above-mentioned properties. Such particles can be used alone or in combination with other inorganic or organic particles to produce the desired polyolefin membrane. Such polyolefin membranes can be used to form effective separators for energy storage devices, such as lithium-ion batteries.

[0015] As used herein, "freestanding" refers to a membrane that has sufficient mechanical properties to allow manipulation, such as rolling up and unrolling into a membrane shape for use in an energy storage device assembly. The terms "membrane," "film," and "sheet" can be used interchangeably throughout this specification to describe products made according to the disclosed embodiments, and the term "membrane" can be used to encompass webs, films, and sheets.

[0016] In a first embodiment of the present disclosure, the microporous polyolefin membrane is passed through an aqueous dispersion of hydrotalcite particles containing a small proportion of a polymer binder to deposit an inorganic layer of controlled thickness on one or both of its first and second major surfaces. Gravure printing, microgravure printing, slot die printing, dip coating, direct metering, or other approaches can be used to control the thickness of the surface coating. The wet membrane is then dried, for example, using a series of air knives in an oven in which heated air is used to evaporate the solvent. The coated membrane is then wound up into a master roll, which can later be used to form a battery separator.

[0017] In a second embodiment of the present disclosure, the microporous polyolefin membrane is passed through an aqueous dispersion containing hydrotalcite particles, at least one other type of inorganic particle (e.g., fumed alumina, boehmite, or a mixture thereof), and a small percentage of a polymer binder to deposit an inorganic layer of controlled thickness on one or both of the first and second major surfaces. By way of example, the inorganic particles in the aqueous dispersion may comprise about 0.1 to 12% hydrotalcite particles and about 88 to 99.9% at least one other type of inorganic particle. Gravure printing, microgravure printing, slot-die printing, dip-coating, direct metering, or other approaches can be used to control the thickness of the surface coating. The wet membrane is then dried, for example, using a series of air knives in an oven in which heated air is used to evaporate the solvent. In this case, the surface layer(s) contain a mixture of inorganic particles, all of which contribute to high temperature dimensional stability, but only the hydrotalcite has the ability to function as an acid scavenger via anion exchange.

[0018] In a third embodiment of the present disclosure, the microporous, self-supporting polyolefin membrane is made by combining polyethylene (e.g., VHMWPE = very high molecular weight polyethylene), hydrotalcite particles (or a mixture of hydrotalcite particles and at least one other type of inorganic particle), and a plasticizer (e.g., mineral oil). For example, a dry blend of polyethylene powder and hydrotalcite particles (or a mixture of hydrotalcite particles and at least one other type of inorganic particle) is combined with a plasticizer and extruded into a uniform, coherent mass containing the hydrotalcite particles (or a mixture of hydrotalcite particles and at least one other type of inorganic particle) dispersed throughout. The mass can be processed using blown film, cast film, or calendaring to produce an oil-filled sheet that can be further biaxially oriented after exiting the die to reduce its thickness and develop its mechanical properties. In an extraction operation, the oil is removed with a solvent (which is then evaporated) to produce a microporous, free-standing membrane containing hydrotalcite particles (or a mixture of hydrotalcite particles and at least one other type of inorganic particle) dispersed throughout its bulk matrix. By way of example, the microporous membrane can contain from about 0.1% to about 80% by weight of hydrotalcite particles (or a mixture of hydrotalcite particles and at least one other type of inorganic particle). Optionally, the hydrotalcite-containing membrane can then be coated with at least one other type of inorganic particle (e.g., fumed alumina, boehmite, or a mixture thereof) at a coating weight sufficient to impart good high-temperature dimensional stability.

[0019] In a fourth embodiment, a microporous free-standing membrane can be produced using one of the fabrication processes described above, which includes at least one type of inorganic particle (e.g., fumed alumina, boehmite, or a mixture thereof) in its bulk structure. Optionally, the microporous membrane is then coated with a hydrotalcite-containing surface layer at a coating weight sufficient to impart good high-temperature stability. The hydrotalcite-containing surface layer can include only hydrotalcite particles as inorganic particles, or a mixture of hydrotalcite particles and at least one other type of inorganic particle.

[0020] The microporous, freestanding polyolefin membranes described above can be rolled or stacked into packages to separate electrodes in energy storage devices, such as batteries, capacitors, supercapacitors, or fuel cells. The pores of the membrane can be filled with electrolyte, both in the surface layers and throughout the inorganic-modified bulk structure. Such membranes are advantageous for fabricating energy storage devices because they offer a good combination of heat resistance, in-plane dimensional stability, acid scavenging capabilities, and shutdown properties, among other things.

[0021] Further objects and advantages of the present disclosure will become apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a titration curve showing the titration of a dispersion of hydrotalcite with concentrated HCl. [Figures 2A-2D] FIG. 1 is a diagram of a vial showing the effect of hydrotalcite on electrolyte degradation (compared to a control vial). DETAILED DESCRIPTION OF THE INVENTION

[0023] The membranes used in the present disclosure are composed of a polyolefin matrix. The type of polyolefin and its molecular weight generally depend on the fabrication process (e.g., blown film vs. cast film). The most suitable polyolefin for use in blown film processes is ultra-high molecular weight polyethylene (UHMWPE), having an intrinsic viscosity of at least 10 dL / g, preferably in the range of 18-22 dL / g. The molecular weight of UHMWPE generally corresponds to a range between about 3.1 million g / mol and about 10 million g / mol. It may also be desirable to blend UHMWPE with other polyolefins, such as HDPE or linear low-density polyethylene (LLDPE), to affect the membrane's shutdown performance. In the case of cast film processes, the most suitable polyolefin is very high molecular weight, high-density polyethylene (VHMW-HDPE) with a molecular weight of about 600,000 g / mol to about 1.5 million g / mol.

[0024] The plasticizer employed in this disclosure is a non-evaporative solvent for the polyolefin polymer, preferably a liquid at room temperature. The plasticizer has little or no solvating effect with the polymer at room temperature, but exerts its solvating effect at temperatures above the softening temperature of the polymer. For UHMWPE, the solvation temperature will be greater than about 160°C, preferably between about 160°C and about 220°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 oils sold by Shell Oil Company, such as Risella® 430X; and oils sold by Calumet Lubricants, such as Hydrocal™ 800; and oils sold by Nynas Inc., such as Nypar® 330.

[0025] The polymer / oil mixture is extruded through a sheet or annular die and then biaxially oriented to form a thin, oil-filled sheet. The extraction step can use a solvent that is miscible with the oil, provided that its boiling point makes it practical to separate the solvent from the plasticizer by distillation to dryness. Examples of 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, and toluene. In some cases, it is preferable to select a processing oil that is electrochemically inert, if any, remaining in the polymer sheet after extraction. The resulting polyolefin membrane after extraction is microporous, having a porosity of about 35-65%. The pore size range is generally about 10 nanometers to several microns, with an average pore size of less than about 1 micrometer. The thickness of the polyolefin membrane can range from about 3 to 25 μm.

[0026] The coating formulation used in this disclosure consists of inorganic particles dispersed in an aqueous mixture, which may contain a small amount of alcohol to improve wetting on the surface of the polyolefin membrane. The inorganic particles are electrically stabilized and suspended in the aqueous mixture. Typically, a polymer dispersion or a water-soluble polymer is used as a binder for the inorganic particles. To minimize binder concentration and achieve a robust, microporous surface layer that does not easily peel off the inorganic particles, it is desirable to select a polymer with hydrogen bonding sites. Representative examples of preferred polymer binders include acrylates, polyvinylpyrrolidone, polyvinyl alcohol, carboxymethyl cellulose, and their copolymers or derivatives, allowing the use of 10 parts or less of the polymer binder with 90 parts or more of the inorganic particles.

[0027] In the case of Li-ion or rechargeable Li-metal batteries, the electrolyte typically consists of 1–1.2 M LiPF6 (lithium hexafluorophosphate) dissolved in a mixture of organic carbonates (e.g., dimethyl carbonate (DMC) and ethyl-methyl carbonate (EMC)). This lithium salt is highly reactive with water and can readily generate hydrofluoric acid (HF) within the electrochemical cell. The presence of hydrofluoric acid accelerates other concomitant reactions (e.g., dissolution of transition metals from the cathode active material), negatively impacting cycle life and Li-ion battery performance. Fumed alumina, boehmite, and other inorganic oxides can often adsorb hydrofluoric acid onto their surfaces, preventing the ion-exchange mechanism from immobilizing halogen anions in the interior region of the hydrotalcite intermediate membrane.

[0028] Hydrotalcites have a layered structure composed of magnesium oxide and aluminum oxide. They are considered an unusual type of layered material, a layered double hydroxide, with a positively charged hydroxide layer and a charge-balancing mobile anion located in the intermediate membrane region. These hydrotalcites can function as acid scavengers.

[0029] Hydrotalcite is an anionic clay that can be found in nature, but is also available in high-purity synthetic form from companies such as Kisuma America, Inc. (Houston, TX). As will be explained in the Examples below, these high-purity, synthetic hydrotalcites may be of greatest interest in Li-ion batteries.

[0030] As mentioned above, in some embodiments, in addition to hydrotalcite, at least one other type of inorganic particle can be used, including, but not limited to, inorganic oxides, carbonates, or hydroxides such as alumina (e.g., fumed alumina), silica, zirconia, titania, mica, boehmite, magnesium hydroxide, calcium carbonate, and mixtures thereof.

[0031] As previously noted, the polymeric membranes disclosed herein having inorganic particles dispersed within a polyolefin matrix and / or coated on one or both of their first and second major surfaces exhibit in-plane high temperature dimensional stability. In some embodiments, the polymeric membrane exhibits less than 10% shrinkage in each of the machine and transverse directions when exposed to 180°C for at least 10 minutes. In other embodiments, the polymeric membrane exhibits less than 10% shrinkage in each of the machine and transverse directions when exposed to a temperature at least 50°C above the melting point of the polyolefin matrix (or polyolefin within the polyolefin matrix) for at least 10 minutes. In some such embodiments, the coating weight is 6 g / m or less. 2 is less than. [Example]

[0032] Example 1 A 9 μm thick microporous ultra-high molecular weight polyethylene-containing separator, ENTEK EPX (ENTEK Membranes LLC, Oregon), was coated with an aqueous-based dispersion containing: 52.9 g Selvol 09-325 (aqueous-based polyvinyl alcohol solution; 98% hydrolysis; 8.5% solids by weight; Sekisui) 281.6g deionized water 20g isopropanol 2g BYK 154 dispersant 145.5g Hydrotalcite (DHT-4C, Kisuma)

[0033] The coating dispersion contained 30% solids by weight and had a 97 / 3 hydrotalcite / polyvinyl alcohol (PVOH) mass ratio. Separators were dip-coated in a bath containing the aqueous-based dispersion, and a #7 Mayer rod was used to control the thickness of the wet layer on each side. The wet separators were then dried using a series of air knives and passed through a vertical oven at 80°C, wound onto a core, and tested. Table I shows the physical properties of the coated separators prepared in Example 1.

[0034] [Table 1]

[0035] Example 2 Naphthenic process oil (140 kg) was fed into a Ross mixer where it was agitated and degassed. The following were then added and mixed with the oil: 64kg UHMWPE (molecular weight: approx. 5 million g / mol) 32kg VHMWPE (molecular weight: approx. 1.2 million g / mol) 32kg HMW-HDPE (molecular weight: approx. 0.6 million g / mol) 1.2kg Li stearate 1.2kg antioxidant

[0036] The mixture was blended at approximately 40°C to form a homogeneous 47 wt% polymer slurry. The polymer slurry was then pumped into a 103 mm diameter co-rotating twin-screw extruder while maintaining a melt temperature of approximately 215°C. Simultaneously, hydrotalcite DHT-4V powder (Kisuma America, Inc.; Houston, TX) was added at a rate of 5 kg / h using a second loss-in-weight feeder. The extrudate was fed through a melt pump into a 257 mm diameter annular die with a 2.75 mm gap. The throughput through the die was 235 kg / h. Air was used to expand the extrudate to produce a biaxially oriented, oil-filled film approximately 2250 mm in diameter. The expanded extrudate was then passed through an upper nip at a speed of 20 m / min to collapse the air bubbles and form a bilayer, which was then side-slit into two individual layers.

[0037] The oil-filled layer was then extracted in-line with trichloroethylene (TCE), followed by TCE evaporation in a hot air oven equipped with a series of carbon beds for vapor collection. The extracted sheet was further stretched in the machine direction (50°C) and cross direction (128°C) to yield a 9 um thick separator that was wound into a master roll.

[0038] The hydrotalcite loading level in the separator was determined by thermogravimetric analysis and found to be about 4.4 wt %.

[0039] Example 3 The following powders were dry blended: 511g HMW-HDPE (molecular weight: approx. 0.6 million g / mol) 256g fumed alumina (Spectral 81) 33g Hydrotalcite DHT-4A-2 (Kisuma America Inc)

[0040] The mixed powder and 100 g of process oil (Hydrocal 800) were then combined and fed into a 27 mm twin-screw extruder. Additional oil was added at the extruder throat. The mixture was extruded at elevated temperature (approximately 160°C) through a sheet die into a horizontal calender roll stack. The thickness of the extruded sheet was approximately 560 μm. The oil-filled sheet was then stretched 9 × 9 at 115°C in a Karo-5 laboratory unit (Bruckner, Germany). The stretched sheet was then mounted on a frame and extracted with Tergo MCF solvent. The solvent-laden sheet, still in the frame, was placed in an oven at 80°C to dry the solvent. The resulting microporous membrane had a thickness of 11.9 μm, a breaking strength of 294 grams force, and a Gurley value (sec / 100 cc air) of 89.

[0041] Example 4 The following example describes a method for titrating hydrotalcite with concentrated HCl. 20 g of hydrotalcite (DHT-4C, Kisuma) was dispersed in 180 g of deionized water. The dispersion was stirred using a magnetic stir bar throughout the titration. Concentrated HCl (37% HCl, ACS Grade, VWR Analytical BHD) was added to a 25 mL burette and used as the titrant. A Milwaukee MW102 pH meter was used to measure the pH during the titration. Initially, concentrated HCl was added in 0.5 mL increments using the 25 mL burette. Measurements were recorded after the Milwaukee meter indicated the pH had stabilized. The HCl titration curve is shown in Figure 1. These results demonstrate that hydrotalcite has excellent performance as an acid scavenger.

[0042] Example 5 The effect of hydrotalcite on the aging of lithium-ion electrolytes was evaluated as follows. Hydrotalcite (0.54 g, DHT-4C, Kisuma) was added to 20 mL scintillation vials (borosilicate glass, melamine lids, PTFE seals). The scintillation vials containing hydrotalcite and control 20 mL scintillation vials (no hydrotalcite) were dried at 120 °C for 48 hours. After drying, the scintillation vials were transferred to a glovebox, and 5 mL of 1 M LiPF6 in 1:1 ethylene carbonate:ethyl methyl carbonate (EC:EMC) electrolyte (Aldrich) was added to each vial. The scintillation vials were then sealed and placed in an oven at 60 °C for 144 hours. Figures 2A-2D show the results of aging at 60°C, where Figure 2A shows the control vial at 0 hours; Figure 2B shows the hydrotalcite vial at 0 hours; Figure C2 shows the control vial at 144 hours; and Figure 2D shows the hydrotalcite vial at 144 hours. As can be seen, the control vial containing only electrolyte showed substantial darkening of the electrolyte (Figure 2C). In contrast, the electrolyte containing hydrotalcite showed minimal discoloration (Figure 2D). These results indicate that hydrotalcite effectively minimizes electrolyte degradation.

[0043] Example 6 A 9 μm thick microporous ultra-high molecular weight polyethylene-containing separator, ENTEK EPX (ENTEK Membranes LLC, Oregon), was coated with an aqueous-based dispersion containing: 362g deionized water 20g isopropanol 22.5 g Soteras V (polyvinylpyrrolidone-based aqueous solution; 20% solids by weight; Ashland) 0.5g Soteras B (crosslinker, Ashland) 45g boehmite (AOH 70, Nabaltec) 50g Hydrotalcite (DHT-4C, Kisuma)

[0044] The coating dispersion consisted of 20% solids by weight (45 / 50 / 5 mass ratio of boehmite / hydrotalcite / binder). Separators were dip-coated in a bath containing the aqueous-based dispersion, and a #10 Mayer rod was used to control the thickness of the wet layer on each side. The wet separators were then dried using a series of air knives and passed through a vertical oven at 80°C, wound onto cores, and tested. Table 2 shows the physical properties of the coated separators prepared in Example 6.

[0045] [Table 2]

[0046] Example 7 A 9 μm thick microporous ultra-high molecular weight polyethylene-containing separator, ENTEK EPX (ENTEK Membranes LLC, Oregon), was coated with an aqueous-based dispersion containing: 362g deionized water 20g isopropanol 22.5 g Soteras V (polyvinylpyrrolidone-based aqueous solution; 20% solids by weight; Ashland) 0.5g Soteras B (crosslinker, Ashland) 85g boehmite (AOH 70, Nabaltec) 10g Hydrotalcite (DHT-4C, Kisuma)

[0047] The coating dispersion consisted of 20% solids by weight (85 / 10 / 5 mass ratio of boehmite / hydrotalcite / binder). Separators were dip-coated in a bath containing the aqueous-based dispersion, and a #11 Mayer rod was used to control the thickness of the wet layer on each side. The wet separators were then dried using a series of air knives and passed through a vertical oven at 80°C, wound onto cores, and tested. Table 3 shows the physical properties of the coated separators prepared in Example 7.

[0048] [Table 3]

[0049] As will be apparent to those skilled in the art, many changes can be made to the details of the above-described embodiments without departing from the underlying principles of the disclosure, and the scope of the disclosure should therefore be determined solely by the claims which follow.

Claims

1. 1. A free-standing, microporous polymer membrane for use in an energy storage device, the free-standing, microporous polymer membrane comprising: a polyolefin matrix having first and second major surfaces, and inorganic particles dispersed within the polyolefin matrix and / or coated on one or both of the first and second major surfaces; the inorganic particles contain an acid scavenger; Freestanding, microporous polymer membrane.

2. 10. The free-standing, microporous polymer membrane of claim 1, wherein the free-standing, microporous polymer membrane exhibits in-plane, high temperature dimensional stability.

3. 3. The free-standing, microporous polymer membrane of claim 1 or 2, wherein the inorganic particles comprise hydrotalcite.

4. The free-standing, microporous polymer membrane of claim 3 , wherein the inorganic particles comprise synthetic hydrotalcite.

5. 10. The free-standing, microporous polymeric membrane of claim 1, wherein the inorganic particles comprise a mixture of hydrotalcite and at least one other type of inorganic particles.

6. 6. The free-standing, microporous polymeric membrane of claim 5, wherein the inorganic particles comprise a mixture of about 0.1-12% hydrotalcite and about 88-99.9% of at least one other type of inorganic particles.

7. 7. The free-standing, microporous polymeric membrane of claim 5 or 6, wherein the at least one other type of inorganic particles comprises an inorganic oxide, carbonate, or hydroxide.

8. The free-standing, microporous polymer membrane of any one of claims 1 to 7, wherein the inorganic particles are dispersed throughout the polyolefin matrix.

9. 9. The free-standing, microporous polymeric membrane of claim 1, wherein the inorganic particles are coated on one or both of the first and second major surfaces.

10. 10. The free-standing, microporous polymer membrane of any one of claims 1 to 9, wherein the free-standing, microporous polymer membrane exhibits less than 10% shrinkage in each of the machine and cross directions when exposed to 180°C for at least 10 minutes.

11. The inorganic particles are coated on one or both of the first and second major surfaces, and the coating weight is 6 g / m ２ The free-standing, microporous polymer membrane of any one of claims 1 to 10, wherein the thickness is less than 1 / 2 mm.

12. 12. The free-standing, microporous polymer membrane of any one of claims 1 to 11, wherein the free-standing, microporous polymer membrane exhibits less than 10% shrinkage in each of the machine and cross directions when exposed to a temperature at least 50°C higher than the melting point of the polyolefin in the polyolefin matrix for at least 10 minutes.

13. 13. The free-standing, microporous polymeric membrane of claim 1, wherein inorganic particles comprising hydrotalcite are dispersed throughout the polyolefin matrix, and at least one other type of inorganic particle is coated on one or both of the first and second major surfaces.

14. 14. The free-standing, microporous polymeric membrane of claim 13, wherein the at least one other type of inorganic particles comprises an inorganic oxide, carbonate, or hydroxide.

15. An energy storage device comprising the free-standing, microporous polymer membrane of any one of claims 1 to 14.

16. 1. A free-standing, microporous polymer membrane for use in an energy storage device, the free-standing, microporous polymer membrane comprising: a polyolefin matrix having first and second major surfaces; a coating disposed on one or both of the first and second major surfaces, the coating including inorganic particles; and an acid scavenger dispersed in at least one of the polyolefin matrix or the coating; Including, The free-standing, microporous polymer membrane exhibits in-plane high temperature dimensional stability, and the coating weight is 6 g / m ２ is less than Freestanding, microporous polymer membrane.

17. 17. The free-standing, microporous polymeric membrane of claim 16, wherein the acid scavenger comprises hydrotalcite.

18. 18. The free-standing, microporous polymeric membrane of claim 17, wherein the acid scavenger comprises synthetic hydrotalcite.

19. The free-standing, microporous polymer membrane of any one of claims 16 to 18, wherein the inorganic particles comprise inorganic oxides, carbonates, or hydroxides.

20. 20. The free-standing, microporous polymer membrane of any one of claims 16 to 19, wherein the free-standing, microporous polymer membrane exhibits less than 10% shrinkage in each of the machine and cross directions when exposed to 180°C for at least 10 minutes.

21. 21. The free-standing, microporous polymer membrane of any one of claims 16 to 20, wherein the free-standing, microporous polymer membrane exhibits less than 10% shrinkage in each of the machine and cross directions when exposed to a temperature at least 50°C higher than the melting point of the polyolefin in the polyolefin matrix for at least 10 minutes.

22. The free-standing, microporous polymer membrane of any one of claims 16 to 21, wherein the acid scavenger is dispersed in the polyolefin matrix.

23. The free-standing, microporous polymer membrane of any one of claims 16 to 22, wherein the acid scavenger is dispersed in the coating.

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