Multilayer nanoporous separator

Abstract

To provide a lithium battery separator that is lighter, lower cost, and has a thinner inorganic oxide / polymer coating layer. [Solution] The lithium battery separator comprises a porous polymer layer having a first side surface and a second side surface, and a first layer adjacent to the first side surface of the porous polymer layer, the first layer comprising a plurality of pores and a solid material, wherein (a) the solid material comprises a plurality of inorganic oxide particles and at least one polymer, (b) the plurality of inorganic oxide particles in the first layer comprises crystallites having a size in the range of 5 nm to 90 nm, (c) the thickness of the first layer is 3 μm or less, the total coating weight of the first layer is less than 6.0 gsm, and (d) the ratio of the thickness of the first layer to the total thickness of the separator is 10% to 80%.

Description

Cross - reference to related applications

[0001] This application claims the benefit and priority of U.S. Provisional Patent Application No. 62 / 572,083, filed on October 13, 2017, the entire content of which is incorporated herein by reference.

Technical Field

[0002] The present invention generally relates to porous separators for batteries and other current - generating cells. In particular, the present invention relates to a multilayer porous separator having an inorganic oxide / polymer layer on a polymer layer and a lithium battery comprising such a separator.

Background Art

[0003] Lithium batteries are widely used in portable electronic devices such as smartphones and portable computers. New applications for lithium batteries include high - power batteries for hybrid vehicles, plug - in hybrid vehicles, and fully electric vehicles. Existing lithium batteries, including rechargeable and non - rechargeable lithium batteries, often utilize multilayer separators. Such separators may include porous films of extruded polyethylene or polypropylene coated with an inorganic oxide / polymer coating layer. As the demand for larger, higher - energy - density, and lower - cost lithium batteries increases, manufacturers have come to rely on battery active materials that are not as safe in order to achieve these goals. This has led to a need for battery separators having high dimensional stability at temperatures such as above 150°C.

[0004] Thus, it would be advantageous if an inorganic oxide / polymer coating layer provided even higher dimensional stability at high temperatures while allowing for lower manufacturing costs by requiring only lower coating weights and coating thicknesses.

Summary of the Invention

Problems to be Solved by the Invention

[0005] The object of the present invention is to provide a lighter, lower-cost, and thinner inorganic oxide / polymer coating layer for polymer-based separators for lithium batteries and other current-generating cells. [Means for solving the problem]

[0006] The present invention achieves these objectives by providing a separator for lithium batteries comprising (a) a porous polymer layer and (b) a nanoporous inorganic oxide / polymer composite layer coated on both sides of the polymer layer. In one embodiment, the nanoporous layer comprises an inorganic oxide (e.g., boehmite, SiO2) and one or more polymers, wherein the volume fraction of one or more polymers in the solid portion of the nanoporous layer may be about 10% to about 50%, preferably about 12% to about 45%, and the crystal size of the inorganic oxide particles is 5 nm to 90 nm. This combination of inorganic oxide particles with a very small crystal size (e.g., boehmite particles) and a large volume of organic polymer creates a porous thermally stable layer that provides high dimensional stability at high temperatures and high levels of porosity for excellent ionic conductivity of the electrolyte within the pores of the separator. This combination has also been found to significantly reduce the coating weight and coating thickness of the thermally stable layer.

[0007] In another embodiment, the nanoporous layer comprises (a) a blend of inorganic oxide particles (e.g., boehmite particles) and inorganic nitride particles (e.g., boron nitride or aluminum nitride particles), and (b) one or more polymers, wherein the volume fraction of one or more polymers in the solid portion of the nanoporous layer may be about 10% to about 50%, preferably about 12% to about 45%, and the crystal size of the inorganic particles is 5 nm to 90 nm. This combination of (1) inorganic oxide particles and inorganic nitride particles (e.g., boehmite particles and boron nitride or aluminum nitride particles) with very small crystal sizes and (2) a large volume of organic polymers creates a porous, thermally stable layer that provides high dimensional stability at high temperatures and high levels of porosity for excellent ionic conductivity of the electrolyte within the pores of the separator.

[0008] Preferably, the thermally stable inorganic oxide / polymer layer (or inorganic oxide and nitride / polymer layer) is coated on both sides of the porous polymer layer, such as a porous polyethylene layer or a porous polypropylene layer, in contrast to coating only one side of the porous polymer layer. This method has been shown to provide improved thermal dimensional stability in thin, low-cost separators. However, having the thermally stable layer on only one side of the polymer layer is also useful.

[0009] Another aspect of the lithium battery separator of the present invention relates to a separator comprising (a) a porous polymer layer and (b) a nanoporous inorganic oxide / polymer composite layer on one or both sides of the polymer layer, wherein the nanoporous layer comprises an inorganic oxide (e.g., boehmite, SiO2) and a polymer, and the crystal size of the inorganic oxide is 5 nm to 25 nm. It has been found that this smaller crystal size results in a coating layer with higher cohesive force and a thinner coating layer, and a higher adhesion to the polymer layer even at a lower coating weight than equivalent layers with a larger inorganic oxide crystal size, for example, about 80 nm.

[0010] In another embodiment, the present invention includes a lithium battery comprising an improved separator of the present invention.

[0011] A further aspect of the present invention is a method for producing a separator, the method comprising coating a solution comprising an inorganic oxide and / or inorganic nitride and one or more polymers onto one or both sides of a porous polymer layer to form the separator of the present invention. [Brief explanation of the drawing]

[0012] The features and advantages of this disclosure should be more fully understood by referring to the following detailed description in conjunction with the attached drawings. [Figure 1] Figure 1 is a graph of the coating thickness as a function of coating weight for a nanoporous separator according to one embodiment of the present invention, fabricated according to Example 1. [Figure 2]Figure 2 is a graph of shrinkage as a function of coating thickness of a nanoporous separator according to one embodiment of the present invention, fabricated according to Example 1. [Figure 3] Figure 3 is a graph of the coating thickness as a function of coating weight for a nanoporous separator according to one embodiment of the present invention, fabricated according to Example 2. [Figure 4] Figure 4 is a graph of shrinkage as a function of coating weight of a nanoporous separator according to one embodiment of the present invention, fabricated according to Example 2. [Figure 5] Figure 5 is a graph of the coating thickness as a function of coating weight for a nanoporous separator according to one embodiment of the present invention, fabricated according to Example 2. [Figure 6] Figure 6 is a graph of shrinkage as a function of coating weight of a nanoporous separator according to one embodiment of the present invention, fabricated according to Example 2. [Figure 7] Figure 7 is a graph of the coating thickness as a function of coating weight for a nanoporous separator according to one embodiment of the present invention, fabricated according to Example 3. [Figure 8] Figure 8 is a graph of shrinkage as a function of coating weight of a nanoporous separator according to one embodiment of the present invention, fabricated according to Example 3. [Figure 9] Figure 9 is a graph of the coating thickness as a function of coating weight for a nanoporous separator according to one embodiment of the present invention, fabricated according to Example 3. [Figure 10] Figure 10 is a graph of shrinkage as a function of coating weight of a nanoporous separator according to one embodiment of the present invention, fabricated according to Example 3. [Figure 11] Figure 11 is a graph of the water content as a function of the weight fraction of boron nitride in a nanoporous separator according to one embodiment of the present invention, which was prepared according to Example 4. [Modes for carrying out the invention]

[0013] The separator of the present invention provides excellent safety, lower cost, thinner thickness, and other important performance characteristics for use in lithium batteries and other current generating cells.

[0014] One aspect of the present invention is a separator for a lithium battery comprising (a) a porous polymer layer and (b) nanoporous inorganic oxide / polymer composite layers on both surfaces of the polymer layer, the nanoporous layer comprising boehmite and one or more polymers, the volume fraction of one or more polymers in the solid portion of the nanoporous layer being from about 10% to about 50%, preferably from about 12% to 45%, and the crystal size of the boehmite being from 5 nm to 90 nm. Alternatively, the nanoporous layer may further comprise particles of other inorganic oxides (e.g., SiO2) or nitrides (e.g., BN or AlN), or combinations thereof.

[0015] The term "lithium battery" may include, for example, a battery in which the electroactive anode material comprises lithium. This includes, but is not limited to, secondary or rechargeable lithium ion batteries, secondary lithium metal batteries, and primary or non-rechargeable lithium metal batteries.

[0016] The term "separator" may include, for example, a porous material within a battery that is inserted between the cathode and anode of the battery to provide electrical insulation against short circuits and to provide pores for filling the electrolyte of the battery.

[0017] The term "porous polymer layer" may include, for example, a layer of a polymer material that is porous such that it has permeability to the flow of air through the polymer layer. Typically, the battery industry measures the air permeability corresponding to this permeability using a porosimeter that provides a value of the time (seconds) for 100 cubic centimeters (cc) of air to pass through the porous layer. This value is generally referred to as the Gurley number. In the battery industry, it is preferred to make the Gurley number of the separator as low as possible without sacrificing mechanical integrity. The typical Gurley number for the air permeability of a porous polyethylene separator or a porous polypropylene separator used in the battery industry is 100 seconds / 100 cc to 300 seconds / 100 cc. In the present invention, the porous polymer layer does not include a non-woven polymer layer used as a separator in the battery industry, for example, a non-woven polymer layer comprising polyester fibers.

[0018] The term "nanoporous" may include a layer having pores with a diameter up to about 100 nm, preferably at least 90% of the pores having a diameter less than 100 nm. The pore size and relative number can be measured by mercury porosimetry or by the cross-section of the layer using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0019] As discussed herein, the nanoporous layer may be present on both sides of a substrate, support, layer, or coating. For example, the nanoporous layer may be present on both sides of a porous polymer layer such as a porous polyethylene layer. The general advantage of the nanoporous layer of the present invention is also found when the nanoporous layer is present on only one side of the porous polymer layer, but the overall performance is typically superior when the nanoporous layer is present on both sides of the porous polymer layer.

[0020] The superior performance characteristics of double-sided polymer coating include less shrinkage at high temperatures and improved battery safety compared to single-sided coating at the same total coating weight. Improved safety is typically the most important performance characteristic required for separators in lithium batteries, especially as batteries become larger and have higher energy densities, and as less safe battery active materials are being used to achieve higher energy densities and lower costs.

[0021] Another advantage of double-sided coating of a porous polymer layer with a nanoporous coating layer is that it provides the opportunity to have different compositions for the coatings on each surface. This allows the nanoporous layer to be optimized to achieve best performance when it faces the anode and when it faces the cathode. Generally, by adjusting the composition and coating thickness of the ceramic coating on either surface of the separator, it is possible to optimize (a) adhesion between the separator and a particular electrode, (b) interfacial impedance between the separator and a particular electrode, i.e., either the anode or the cathode, and (c) complex formation of migratory species (e.g., solubilized transition metal ions from the cathode).

[0022] For example, if the coating faces the cathode, the coating is particularly useful in that it is resistant to oxidation, especially at higher charging voltages, e.g., voltages exceeding 4.25 volts. This can be achieved, for example, by selecting an organic polymer for the nanoporous layer that is resistant to oxidation at a desired higher voltage, e.g., up to 5.0 volts. Furthermore, the nanoporous layer on one side can be designed with particularly small pore sizes, such as pores less than 10 nm in diameter, by using, for example, boehmite having a major crystal size of less than 10 nm in diameter. It should be understood that the present invention is not limited to the examples provided herein. For example, surface-modified alumina particles containing boehmite particles, as described in PCT / IB2015 / 000272 by Loscutova et al. of Sasol, may be used in the separator layer described herein. This patent application discloses the dispersion of these surface-modified alumina particles within nano-sized single-particle crystals. This reference also states that the average crystal size can be measured by X-ray diffraction on a 120 plane. As an example, the crystal size (or average crystal size) of inorganic particles in a separator comprising a nanoporous layer according to one embodiment of the present invention may be measured by (i) taking a cross-sectional sample of the nanoporous layer, (ii) measuring the diameter / width of 100 single primary (inorganic) particles randomly selected in the TEM or SEM image of this sample using a TEM or ultra-high power SEM at their widest dimension, (iii) excluding the 25 highest diameter values ​​and 25 lowest diameter values ​​from the 100 diameter / width values, and (iv) recording the crystal size as the average of the central 50 diameter / width values, i.e., as the average crystal size of the measured particles. Since the pore size diameter of ceramic particles in a nanoporous layer is usually close to the crystal size of ceramic particles such as boehmite particles used to fabricate the nanoporous layer, the pore size diameter may indicate the crystal size of ceramic particles relative to a nanoporous layer containing ceramic particles with a narrow crystal size.However, in the case of blends of ceramic particles of various crystal sizes in nanoporous coatings, the pore size diameter of the coating layer is typically intermediate between multiple crystal size diameters of the various ceramic particles, or a blend thereof, and individual peaks do not reflect different crystal sizes. The TEM or SEM methods described above for measuring crystal size analyze single particles rather than blends, so this limitation does not apply, and are therefore preferred methods for use in the present invention. These separator configurations using very small pore sizes are useful when placed against a lithium metal anode, in which case the very small pore size helps prevent the growth of lithium metal dendrites into and through the separator, which reduces the cell's cycle life, capacity, and safety. When very small boehmite particles are used, and some of these boehmite particles enter the pores of the polymer separator substrate, this is particularly useful in preventing the growth of lithium metal dendrites into and through the separator.

[0023] In another example, boehmite can be modified by reaction with fluorinated ethylene carbonate to improve cycle life when such a nanoporous layer is placed against an anode comprising silicon or lithium metal. This nanoporous layer, comprising boehmite covalently reacted with fluorinated ethylene carbonate, helps achieve excellent stability in high-voltage operation of cells, such as up to 5.0 volts, even when used against a cathode.

[0024] In a further example, a nanoporous layer having pores with a diameter of less than 10 nm can be placed relative to the cathode to suppress the diffusion of transition metals such as nickel ions from the cathode to the anode, which degrades the cell's cycle life and other performance characteristics. Optionally, to reduce the diffusion of transition metal ions from the cathode by placing a nanoporous layer relative to the cathode, very small boehmite crystal sizes of less than 10 nm can be combined with the reaction of this boehmite with fluorinated ethylene carbonate or other organic carbonates.

[0025] The term "boehmite" is understood in the art. It may include, for example, the hydrated aluminum oxide of the empirical chemical formula Al2O3H2O, where x is 1.0–2.0. Here, another term used interchangeably for boehmite is AlOOH. Boehmite may be characterized by its characteristic Fourier transform infrared ("FTIR") fingerprint. Boehmite is characterized by its distinctive X-ray diffraction pattern, and by its 3200 cm⁻¹ -1 It may also be characterized by the characteristic infrared spectrum of the double peaks of the hydroxyl groups in the vicinity. Boehmite may have an x ​​value greater than 1.5 for water molecules, and at most about 2. However, commercially available grade boehmite of all particle sizes typically has an x ​​value in the range of 1.0 to 1.5. The value of x can be measured by first drying the boehmite at about 120°C for 1 hour to remove water adsorbed on the surface, and then heating the boehmite sample at 1000°C for 1 hour to measure the water loss that occurs when it is dehydrated to anhydrous aluminum oxide Al2O3.

[0026] Examples of polymers that may be used in the present invention include, but are not limited to, cellulose polymers such as hydroxyethylcellulose, vinylpyrrolidone polymers and their polymers, polysaccharides such as chitosan, polyethylene oxide, and polyvinyl alcohol. Preferably, the polymers, either alone or in combination, provide cohesive and adhesive forces to the nanoporous layer, are insoluble in the electrolyte of the lithium battery, and are electrochemically stable over the voltage range in which the lithium battery is used. One or more polymers may be crosslinked if necessary to achieve these properties.

[0027] The term "volume fraction of one or more polymers in the solid portion of a nanoporous layer" refers to the percentage of polymer content in the solid portion of an inorganic particle / polymer composite layer, expressed as a volume percentage. As explained below, nanoporous layers have a porosity of approximately 30% to 70%. In other words, much of the volume of a nanoporous layer is air, not solid material. The volume of air (e.g., pores) is not included in the solid portion for the purpose of determining the volume fraction of one or more polymers in the solid portion of a nanoporous layer. The volume fractions of polymer and inorganic particle contents in a nanoporous layer can be calculated based on the relative densities of the materials. For example, if the inorganic particles are only boehmite with a density of 3.03 g / cc, the density of the organic polymer is 1.3 g / cc, and the weight ratio of boehmite to organic polymer is 12:1, then the volume fraction of boehmite is 83.7%, and the volume fraction of organic polymer is 16.3%. When the weight ratio of boehmite to organic polymer is changed to 4:1, the volume fraction of boehmite becomes 63.2%, and the volume fraction of organic polymer becomes 36.8%. When the density of organic polymer is lower, such as 1.1 g / cc, the volume fraction at the same weight ratio of inorganic particles / polymer is higher than when the polymer density is higher, such as 1.3 g / cc. To determine the volume fraction of one or more organic polymers in the solid portion of the nanoporous layer, the nanoporous layer is removed from the separator, for example, by carefully scraping the layer or by not removing any polymer substrate. The weighed nanoporous layer sample in the crucible is then burned at a temperature sufficient to burn the organic polymer and leave a residue of ceramic particles. This yields the weight of the burned ceramic particles. If the particles are only boehmite, the weight of boehmite in the sample before burning is determined by dividing this weight of the burned particles by 0.85, assuming that one molecule of water is equivalent to one molecule of boehmite. It should be noted that while most boehmite particles experience a 15% weight loss during combustion, some boehmite particles may have different weight loss coefficients, such as 0.84 or 0.82. This same information can also be obtained by recording the weight of the nanoporous layer when it is heated to over 900°C using a thermogravimetric analyzer until a certain weight is achieved.As mentioned above, the density of boehmite is 3.03 g / cc. The density of most organic polymers is 1.1 to 1.4 g / cc, but the density of some polymers, such as polyvinylidene difluoride (PVdF), is approximately 1.8 g / cc. After determining the density of the organic polymer by analyzing its identity and weight ratio, the volume percentage of the organic polymer in the solid portion of a nanoporous layer of boehmite alone can be calculated. For example, if the weight percentage of boehmite is 88.9%, the weight ratio of pigment to binder is 8:1, the weight percentage of organic polymer is 11.1%, and its density is 1.3 g / cc, then by dividing the weight percentages of boehmite and organic polymer by their respective densities, the relative volume percentage of boehmite is given as 29.34%, and the relative volume percentage of organic polymer is given as 8.54%. By normalizing these volume percentages to 100% of the total volume percentage of the solid portion of the nanoporous layer, the volume percentage of the organic polymer is given as 22.54%. If the nanoporous layer is a blend of boehmite and other ceramic particles, or if the density of the organic polymer is unknown, further analysis may be performed to determine the volume percentage of the organic polymer, as is known in the art.

[0028] The term "crystal size" of a particle refers to the average size of the principal crystal or single crystal of an inorganic particle. This is typically measured using X-ray diffraction, transmission electron microscopy (TEM), or very high-resolution scanning electron microscopy (SEM). PCT / IB2015 / 000272 by Roskatova et al. of Sasol, cited above, provides an example of determining the average crystal size of inorganic particles such as alumina and boehmite using X-ray diffraction. As mentioned above, the crystal size may be determined using TEM or SEM. Many inorganic oxide particles, such as boehmite particles, are approximately cubic in shape, but can also be in other shapes such as platelets and needles. In a dry solid state in solution, boehmite particles typically aggregate into larger particles consisting of many principal boehmite particles, such as 10 to 20 principal particles. The inorganic particles in the nanoporous layer of the present invention result in characteristics such as pore size diameter, which are related to the crystal size of the particles.

[0029] In one embodiment of the separator of the present invention, the ratio of the total thickness of the nanoporous layers on both sides of the polymer layer to the total thickness of the separator is 10% to 80%. The thickness of these layers may be measured by SEM analysis of the cross-section of the separator. It has been found that by setting the lower limit of the proportional thickness of the nanoporous layer to about 10%, rather than 15% or more, more typically 25% or more, of the total thickness of the separator, due to the nanoporous pore size in the nanoporous layer and the high volume fraction of the organic polymer, a significant improvement in the dimensional stability of the separator at high temperatures, which is necessary for improved safety, is achieved. It has been found that the combination of the nanoporous pore size in the nanoporous layer and the very high volume fraction of the organic polymer (e.g., about 10% to about 50%) results in an acceptable porosity (e.g., about 30% to about 70%), along with good ionic conductivity of the electrolyte. It has also been found that the separator of the present invention functions well with a total thickness of the nanoporous layer equivalent to 80% of the total thickness of the separator.

[0030] In one embodiment, the thermal shrinkage of the separator at 150°C for 1 hour is less than 5%. This thermal shrinkage may be measured by the change in the area of ​​the separator after heating in an oven for 1 hour. This measurement of thermal shrinkage may be performed by marking the dimensions and area of ​​a square or rectangle, such as 10cm x 10cm, on a smaller portion of the separator sheet, measuring the measurements, and then suspending the sheet in an oven. After heating in the oven, the percentage change or percentage shrinkage of the dimensions and area of ​​the marked area is measured.

[0031] In one embodiment, the thermal shrinkage of the separator at 200°C for 1 hour is less than 5%. It has been found that the nanoporous separator of the present invention maintains its dimensional stability even when heated well above the melting point of the polymer layer, such as the porous polyethylene layer.

[0032] In one embodiment of the separator of the present invention, the total coating weight on both sides of the nanoporous layer is less than 4.5 grams / square meter (gsm), while the thermal shrinkage of the separator at 150°C for 1 minute is less than 5%. Furthermore, the use of boehmite with a lower density of approximately 3.03 g / cc helps to further reduce the coating weight of the heat-stabilized layer compared to higher density anhydrous aluminum oxide or alumina (density of approximately 4 g / cc). The lower the density of inorganic oxide particles, the more proportionally the coating weight required for a given thickness and porosity.

[0033] For example, a 4-micron thick alumina / polymer layer with 45% porosity for polyethylene-based separators typically weighs about 8 grams / square meter (gsm). However, switching to boehmite (or a blend of boehmite and other inorganic particles of similar or lower density) at the same thickness and % porosity reduces the coating weight to about 6 gsm or less. The high volume fraction of polymer in the nanoporous layer of the separator of the present invention, combined with the use of boehmite with a crystal size of 5 nm to 90 nm, enhances the dimensional stability of the separator layer at a specific thickness. While it is still possible to make the total thickness of the inorganic particle / polymer layer in the separator 4 microns or more, it is desirable to make the inorganic particle / polymer layer as thin as possible, such as a total thickness of about 3 microns, while achieving excellent dimensional stability at high temperatures, in order to reduce the cost of the separator and decrease the layer thickness. In one embodiment, the thermal shrinkage of the separator at 200°C for 1 hour is less than 5%.

[0034] In one embodiment, other inorganic oxides and / or inorganic nitrides with a density of about 3.0 g / cc or less, such as silicon dioxide and boron nitride, may be blended with the boehmite pigment. This results in other desirable properties, such as a lower total coating weight and lower water content of the nanoporous layer, as will be discussed further below.

[0035] In another embodiment of the separator of the present invention, the total coating weight on both sides of the nanoporous layer is less than 3.5 grams / square meter (gsm), while the thermal shrinkage of the separator at 150°C for 1 minute is less than 5%. In one embodiment, the thermal shrinkage of the separator at 200°C for 1 hour is less than 5%.

[0036] In one embodiment of the separator of the present invention, the crystal size of the inorganic particles may be about 30 nm to about 50 nm, preferably about 5 nm to about 25 nm, and more preferably about 5 nm to about 10 nm. In one embodiment, the inorganic particles comprise boehmite particles having a crystal size of about 30 nm to about 50 nm. In one embodiment, the crystal size of the boehmite may be about 5 nm to about 10 nm, preferably about 5 nm to about 8 nm.

[0037] In another embodiment of the present invention, the % porosity of the nanoporous layer is between 30% and 55%. The % porosity of the nanoporous layer may be calculated from the density of the material in g / cc in the dry coating, by assuming that the material in the dry coating is a 100% solid coating with 0% porosity and comparing it to the actual density of the coating. The actual density of the nanoporous layer may be calculated by dividing its coating weight in gsm by its coating thickness to provide the density of the nanoporous layer in g / cc. The % solid fraction by volume in the nanoporous layer is obtained by dividing the density of the nanoporous layer by the calculated density of the same material in the nonporous layer and multiplying by 100%. Subtracting this % solid fraction value from 100% gives the % pore volume or % porosity of the nanoporous layer. The % porosity can also be measured by mercury porosimetry analysis.

[0038] In one embodiment, the pore volume of the nanoporous layer is between 0.8 cc / g and 1.0 cc / g. The pore volume can be calculated by measuring the coating weight of the nanoporous layer in gsm and measuring the thickness of the nanoporous layer to determine the total cc per square meter. Then, the pore volume in cc / g is calculated by dividing the total cc per square meter by the number of grams measured in gsm. In one embodiment, the % porosity of the nanoporous layer is 55% to 70%. Above this level of % porosity, it becomes difficult to obtain the cohesive and adhesive strength of the nanoporous layer desirable for long-term and safer lithium battery separator applications.

[0039] In one embodiment of the separator of the present invention, the volume fraction of one or more polymers in the solid portion of the nanoporous layer is 15% to 45%. In one embodiment, the volume fraction of one or more polymers in the solid portion of the nanoporous layer is 20% to 45%. In one embodiment, the volume fraction of one or more polymers in the solid portion of the nanoporous layer is 25% to 45%.

[0040] In one embodiment of the separator of the present invention, the nanoporous layer comprises pores having an average pore diameter of 80 nm or less, with at least 70% of the pores having a diameter of less than 80 nm. In one embodiment of the separator of the present invention, the nanoporous layer comprises pores having an average pore diameter of 50 nm or less, with at least 70% of the pores having a diameter of less than 50 nm. In one embodiment of the separator of the present invention, the nanoporous layer comprises pores having an average pore diameter of 25 nm or less, with at least 70% of the pores having a diameter of less than 25 nm.

[0041] In one embodiment of the separator of the present invention, the ratio of the separator's air permeability to the polymer layer's air permeability in the absence of the nanoporous layer is 1.0 to 1.4. As described above, this air permeability is typically measured with an instrument that provides a Gurley number, which is the number of seconds it takes for 100cc of air to pass through the separator. A small increase in air permeability is an important indicator that the expected decrease in the ionic conductivity of the electrolyte within the separator by adding the inorganic oxide / polymer layer is low. A significant decrease in ionic conductivity can impair the lithium battery's cycle rate capability, particularly its power rate characteristics, as well as its cycling lifetime.

[0042] In one embodiment of the separator of the present invention, the polymer layer comprises a polyolefin. Suitable polyolefins include, but are not limited to, polyethylene and polypropylene. Other examples of polyolefin polymers include, but are not limited to, homopolymers of 1-butene, 1-pentene, 1-hexene, and 1-octene, as well as copolymers and ternary polymers thereof, which are used alone or in combination with one or more types of polymers. In one embodiment, the polymer layer comprises polyethylene. The polymer layer may be of a "wet process" type, providing porosity by extrusion followed by solvent extraction, or of a "dry process" type, providing porosity by extrusion followed by stretching. In the porous polymer layer, various molecular weights, densities, and stereochemical structures of polyethylene and other polymers may be utilized along with various melt flow and mechanical properties.

[0043] For the purpose of better uniform wetting of the coating fluid of the nanoporous layer and better adhesion of the nanoporous layer to the substrate, the porous polymer layer may have a coating layer on the surface to which the nanoporous layer is applied.

[0044] In one embodiment of a separator comprising polyethylene in a polymer layer, the thermal shrinkage of the separator at 120°C for 1 hour is less than 1.0%. Without the nanoporous layer of the separator of the present invention, the thermal shrinkage of a polyethylene separator of equivalent total thickness is typically greater than 1.0% at 105°C. In one embodiment, the ratio of the air permeability of the separator after thermal shrinkage at 120°C for 1 hour to the air permeability of the separator before thermal shrinkage at 120°C is 0.8 to 1.2. This is an important feature for enabling vacuum drying of the polyethylene-based separator of the present invention at a high temperature of 120°C without adversely affecting air permeability and ionic conductivity, and in some cases with a positive effect on ionic conductivity due to heat treatment. Vacuum drying is beneficial in reducing the moisture content of the separator to increase the cycling lifetime and capacity of lithium batteries and to improve the lithium battery cycling rate capability. Vacuum drying is typically performed at approximately 80°C for 4–24 hours for polyethylene separators, and at approximately 90°C for 4–24 hours for polyethylene separators with one or more inorganic oxide / polymer layers. For the polyethylene-based separators of the present invention, the possibility of using a higher temperature of 120°C and a shorter drying time of 1–3 hours is useful.

[0045] In another embodiment of the separator of the present invention, the nanoporous layer is a laminated layer on a polymer layer. The term "laminated layer" means that the nanoporous layer is not initially formed on the polymer layer by direct coating or the like, but rather laminated onto the polymer layer in a subsequent process. For example, the nanoporous layer is coated onto a release substrate, and then this nanoporous layer is transferred onto the polymer layer by lamination. This lamination may optionally involve the use of heat and / or some organic solvent to adhere the nanoporous layer to the polymer layer and to assist in the subsequent release of the release substrate without impairing the quality of the laminated multilayer separator.

[0046] Another embodiment of the lithium battery separator of the present invention relates to a separator comprising (a) a porous polymer layer and (b) a nanoporous inorganic particle / polymer composite layer on one or both sides of the polymer layer, wherein the nanoporous layer comprises an inorganic oxide (e.g., boehmite) or a blend of an inorganic oxide and an inorganic nitride (e.g., BN or AlN) and a polymer, and the crystal size of the inorganic particles is about 5 nm to about 25 nm, preferably about 5 nm to about 8 nm. In one embodiment, the nanoporous layer comprises pores having an average pore diameter of about 25 nm or less, and at least about 70% of the pores have a diameter of about 25 nm or less.

[0047] A further embodiment of the lithium battery separator of the present invention relates to a separator comprising (a) a porous polyethylene layer and (b) a nanoporous inorganic particle / polymer composite layer on both sides of the polyethylene layer, wherein the nanoporous layer comprises boehmite or a blend of boehmite and boron nitride and one or more polymers, the volume fraction of one or more polymers in the solid portion of the nanoporous layer is about 10% to about 50%, the total coating weight of the nanoporous layer on both sides is 5.5 grams or less per square meter, and the thermal shrinkage of the separator at 150°C for 1 hour is less than 5%. In one embodiment, the total coating weight of the nanoporous layer on both sides is 3.5 grams or less per square meter, and the thickness of the polyethylene layer is between 5 microns and 16 microns.

[0048] In another embodiment, the present invention is a lithium battery comprising the separator of the present invention.

[0049] Another aspect of the present invention is a method for producing a separator of the present invention, the method comprising coating one or both sides of a porous polymer layer with a solution comprising inorganic particles and one or more polymers. In one embodiment, the inorganic particles comprise boehmite particles having a crystal size of 5 nm to 90 nm. In one embodiment, the boehmite particles have a crystal size of about 5 nm to about 25 nm, preferably about 5 nm to about 8 nm. In one embodiment, the volume fraction of one or more polymers in the solid portion of the coating after drying is about 10% to about 50%, preferably about 12% to about 45%, more preferably about 15% to about 45%. [Examples]

[0050] Some embodiments of the present invention will be described in the following examples, which are provided as examples only and not as limitations.

[0051] Example 1 A boehmite slurry was prepared by adding 12 parts of boehmite (DISPAL 10F4 supplied by Sasol Inc., Houston, Texas) and 1 part of chitosan (deacetylated chitin poly(D-glucosamine), medium molecular weight, sourced from Sigma-Aldrich, St. Louis, MO) to 73 parts of water:isopropyl alcohol in a 9:1 ratio, and stirring the slurry. The main particle size of the boehmite was approximately 40 nm. The boehmite slurry was coated onto both sides of a 12-micron thick porous polyethylene separator (EP12 sourced from ENTek, Lebanon, Oregon) using wound rods of different wire thicknesses, obtaining coating weights ranging from 1.2 grams / m² (gsm) to 5.2 gsm for both sides. First, a boehmite coating was applied to one side of the polyethylene separator and dried in a convection oven at 70°C for 2 minutes. Next, the boehmite coating was applied to the uncoated opposite side of the polyethylene separator and dried under the same oven heating conditions as the first coating.

[0052] A 12:1 weight ratio of boehmite pigment to chitosan binder is calculated to result in a porous boehmite coating with 15.3 volume% chitosan binder in the solid portion. This calculation is based on the specific gravity of boehmite (3.03 g / cc) and chitosan (1.4 g / cc).

[0053] The coating thickness was measured using a Dorsey gauge on a polyethylene separator before and after applying the boehmite coating to both surfaces. The difference in thickness between before and after coating is in the microns. An alternative method for measuring the coating thickness is to use a Dorsey gauge on a coating separator before and after removing the boehmite coating from both surfaces by either peeling it off with adhesive tape or rubbing it off with an alcohol-soaked cloth.

[0054] The coating weight was measured by weighing a 10cm x 10cm sample of polyethylene separator before and after applying the boehmite coating to both sides. Multiplying the difference in weight before and after coating by 1,000 gives the coating weight in grams per square meter (gsm). An alternative method for measuring the coating weight was to weigh a 10cm x 10cm sample of coated separator before and after removing the boehmite coating from both sides by either peeling it off with adhesive tape or rubbing it off with an alcohol-soaked cloth.

[0055] Figure 1 shows a plot of the average boehmite coating thickness and coating weight per side, ranging from approximately 0.6 gsm to approximately 2.6 gsm. For double-sided coating, doubling these values ​​gives a total coating weight range of approximately 1.2 gsm to approximately 5.2 gsm. The total boehmite coating thickness on both sides varies from approximately 1.2 microns to approximately 4.4 microns. If more accurate measurement of coating thickness is desired, scanning electron microscopy (SEM) analysis of the cross-section of the boehmite-coated plastic separator can be used.

[0056] SEM analysis of the total coating thickness revealed a density of approximately 1.06 gsm per micron of coating thickness. From this, the % porosity is estimated to be 62% for this 12:1 ratio of pigment to binder.

[0057] One objective in minimizing the cost of boehmite coating is to apply the total coating weight required to reduce thermal shrinkage to a target value, such as less than 5% shrinkage at 150°C. Figure 2 shows that this level of thermal shrinkage can be achieved on a 12-micron polyethylene substrate in this embodiment with an average coating thickness of approximately 1.9 microns or more per side. For double-sided coating, doubling this value gives a total coating thickness of approximately 3.8 microns or more. Based on the density of the boehmite coating determined by SEM analysis, a total coating thickness of approximately 3.8 microns on both sides corresponds to a coating weight of approximately 4.0 gsm.

[0058] Corona treatment of the polyethylene substrate is preferred for better wetting and coating adhesion of the boehmite slurry. In addition to the wound coating rod, various other coating methods may be used, but are not limited to gravure coating, screen printing coating, reverse roll coating, blade coating, and slot die coating.

[0059] Example 2 A boehmite slurry was prepared by adding 12 parts of boehmite (DISPERAL D60 supplied by Sasol Inc., Hamburg, Germany) and 1 part of vinylpyrrolidone copolymer (Soteras CCS grade supplied by Ashland Inc., Wilmington, DE) to 73 parts of water:isopropyl alcohol in a 9:1 ratio, and stirring the slurry. The main particle size of the boehmite was approximately 75 nm. The boehmite slurry was coated onto both sides of a 12-micron thick porous polyethylene separator (EP12 supplied by ENTEK, Lebanon, Oregon) using wound rods of different wire thicknesses, obtaining coating weights ranging from approximately 2.4 grams / square meter (gsm) to approximately 9.2 gsm for both sides. First, a boehmite coating was applied to one side of the polyethylene separator and dried in a convection oven at 70°C for 2 minutes. Next, the boehmite coating was applied to the uncoated opposite side of the polyethylene separator and dried under the same oven heating conditions as the first coating.

[0060] A 12:1 weight ratio of boehmite pigment to vinylpyrrolidone copolymer binder is calculated to result in a porous boehmite coating with 18.7 volume% polymer binder in the solid portion. This calculation is based on the specific gravity of boehmite (3.03 g / cc) and vinylpyrrolidone copolymer (1.1 g / cc).

[0061] Figure 3 shows plots of the average boehmite coating thickness and coating weight per side, ranging from approximately 1.2 gsm to approximately 4.6 gsm. For double-sided coating, doubling these values ​​gives a total coating weight range of approximately 2.4 gsm to approximately 9.2 gsm. The total boehmite coating thickness on both sides varies from approximately 2.8 microns to approximately 9.0 microns.

[0062] As already discussed, one objective of minimizing the cost of boehmite coating is to apply the total weight of coating required to reduce thermal shrinkage to a target value, such as less than 5% shrinkage at 150°C for one hour. Figure 4 shows that this level of thermal shrinkage can be achieved on the 12-micron polyethylene substrate of this embodiment with an average coating thickness of approximately 2.0 microns or more per side. For double-sided coating, doubling this value gives a total coating thickness of approximately 4.0 microns or more. Based on Figure 3, a total coating thickness of approximately 4.0 microns on both sides corresponds to a coating weight of approximately 3.9 gsm.

[0063] For a boehmite slurry with a 15% solids content in 9:1 water:isopropyl alcohol, and a weight ratio of Disperal D60 to vinylpyrrolidone copolymer of 8:1, this example applied a 12:1 boehmite D60:polymer coating to a 12-micron polyethylene substrate using a winding rod and dried. The volume percentage of the polymer binder was calculated to be 25.6%. Figure 5 shows plots of the average boehmite coating weight and coating thickness per side, ranging from approximately 0.8 gsm to approximately 4.2 gsm. For double-sided coating of this boehmite coating with a weight ratio of 8:1 boehmite pigment:vinylpyrrolidone polymer binder, doubling these values ​​gives a range of approximately 1.6 gsm to approximately 8.4 gsm. The corresponding total boehmite coating thickness on both sides varies from approximately 1.6 microns to approximately 8.2 microns. Figure 6 shows that a thermal shrinkage of less than 5% at 150°C for 1 hour can be achieved with an average coating thickness of approximately 2.2 microns or more per side. For double-sided coating, doubling this value gives a total coating thickness of approximately 4.4 microns or more. Based on Figure 5, a total coating thickness of approximately 4.4 microns corresponds to a coating weight of approximately 4.8 gsm.

[0064] Example 3 Using the procedure of Example 2, a boehmite slurry of Dispal 25F4 boehmite and vinylpyrrolidone copolymer with a weight ratio of 12:1 and a solid content of 15% was prepared in 9:1 water:isopropyl alcohol. Dispal 25F4 was sourced from Sasol, Lake Charles, LA, and its main crystalline grain size is 8 nm. This boehmite slurry was coated onto a 12-micron polyethylene substrate of EP12 sourced from Entec, and dried, similar to the process used in Examples 1 and 2. The volume percentage of the polymer binder was calculated to be 18.7%. Figure 7 shows plots of the average boehmite coating thickness and coating weight per side, ranging from approximately 0.8 gsm to approximately 3.0 gsm. For double-sided coating of this boehmite coating with a boehmite pigment:vinylpyrrolidone polymer binder in a weight ratio of 12:1, doubling these values ​​gives a range of approximately 1.6 gsm to approximately 6.0 gsm. Figure 8 shows that a thermal shrinkage of less than 5% at 150°C for 1 hour is obtained with an average coating thickness of approximately 1.8 microns or more per side. For double-sided coating, doubling this value gives a total coating thickness of approximately 3.6 microns or more. Based on Figure 7, a total coating thickness of approximately 3.6 microns corresponds to a coating weight of approximately 3.6 gsm.

[0065] A boehmite slurry with a weight ratio of Dispal 25F4 to vinylpyrrolidone copolymer of 8:1 in a 9:1 water:isopropyl alcohol mixture was also coated onto a 12-micron polyethylene substrate and dried, similar to the process used in Examples 1 and 2. The volume percentage of the polymer binder was calculated to be 25.6%. Figure 9 shows plots of average boehmite coating thickness and coating weight per side, ranging from approximately 0.8 gsm to approximately 4.7 gsm. For double-sided coating of this boehmite coating with a weight ratio of 8:1 boehmite pigment:vinylpyrrolidone polymer binder, doubling these values ​​gives a total coating weight range of approximately 1.6 gsm to approximately 9.4 gsm. Figure 10 shows that a thermal shrinkage of less than 5% at 150°C for 1 hour is obtained with an average coating thickness of approximately 0.9 microns or more per side. For double-sided coating, doubling this value gives a total coating thickness of approximately 1.8 microns or more. Based on Figure 9, a total coating thickness of approximately 1.8 microns corresponds to a coating weight of approximately 2.9 gsm. For a boehmite coating with a thickness of approximately 1.8 microns, the coating weight should have been approximately 2.0 gsm, based on the density of the boehmite coating of approximately 1.1 g / cc shown in Example 1, considering that the boehmite weight percentage of the 8:1 boehmite pigment:vinylpyrrolidone copolymer is lower, and the weight percentage of the vinylpyrrolidone copolymer is higher and the density is lower compared to chitosan. This indicates that approximately 0.9 gsm of boehmite coating penetrated and dried into the pores of the polyethylene substrate, creating a boehmite / polyethylene substrate with a boehmite coating on the surface. With an 8:1 pigment:binder (P:B) ratio in this coating, approximately 0.9 gsm of boehmite particles in the pores correspond to approximately 0.8 gsm of boehmite particles in the pores of the boehmite / polyethylene substrate or layer. Since this boehmite coating was applied to both sides of the polyethylene substrate, it appears that about half of the boehmite, or about 0.4 gsm, penetrated into the pores on each side of the polyethylene substrate and dried, forming a boehmite / polyethylene substrate or layer that resulted in a boehmite-coated boehmite / polyethylene substrate.

[0066] Similar 8:1 boehmite coatings with Dispal 2SF4 were prepared using the procedure of this example, with chitosan instead of vinylpyrrolidone copolymer and a 12-micron thick polyethylene substrate sourced from Shenzhen Senior, China, instead of Entec's polyethylene substrate, resulting in a coating weight of 1.8 gsm. The thermal shrinkage of this coating was less than 5% when heated at 150°C for 1 hour. When using the procedure to remove the boehmite coating with adhesive tape, it was not possible to remove the boehmite coating from a 10 × 10 cm sample without peeling off any portion of the polyethylene substrate, even with careful pulling of the tape. This was in contrast to the tape pulling results of the easy and complete removal of the boehmite coating from the polyethylene substrate in Examples 1 and 2 and Comparative Examples 1 and 2. These results are consistent with the fact that when a portion of the boehmite coating penetrates the pores of the polyethylene layer and dries, adhesive and cohesive forces are exerted on the boehmite coating such that the point of failure during tape pulling occurs below the surface of the polyethylene layer in most areas, rather than at the interface between the polyethylene surface and the boehmite coating on the polyethylene surface. After tape pulling, several areas of the polyethylene layer that remained undisturbed were found. From these areas, the additional thickness of the boehmite layer was determined to be approximately 1.8 microns.

[0067] Since the boehmite coating could not be cleanly removed from the polyethylene layer by tape pulling, the coating was washed and rubbed off using a soft cloth soaked in methanol. The weight of the 10 × 10 cm sample after washing was 0.0757 grams (g). This weight of the washed and rubbed sample did not change even after being rubbed twice more with methanol. The polyethylene substrate used in this example did not contain inorganic pigments such as alumina or boehmite. When the methanol-washed sample was incinerated in a crucible at a temperature of 1000°C for 70 minutes, 0.0045 g of anhydrous aluminum oxide was shown after dehydration of the boehmite pigment, with a weight loss of 15% of the boehmite pigment weight. Using this conversion coefficient from boehmite to aluminum oxide by dividing the weight of aluminum oxide by 0.85, a boehmite content of approximately 0.0053 was obtained, meaning that at least approximately 0.53 gsm of boehmite pigment was provided in the pores of the polyethylene layer. Wet friction to remove the boehmite coating from the surface would have removed a small amount of the coating from the upper pores of the polyethylene layer, so the weight of boehmite pigment in the pores may have been slightly greater than 0.53 gsm. Since there were two coatings on the polyethylene layer, the amount of boehmite in the pores from one coating was approximately half of this total, i.e., approximately 0.27 gsm. As further evidence of boehmite coating penetration into the pores of the polyethylene substrate, the dimensional stability of this "washed" polyethylene with approximately 0.53 gsm of boehmite in the pores after heating at 150°C for 1 hour was 54% and 36.5% in two directions (mechanical direction (MD) and transverse direction (CD)). In contrast, the uncoated polyethylene substrate showed thermal shrinkage of 70% and 75.5%. This improvement in thermal stability due to the boehmite pigment within the pores of the polyethylene substrate is one explanation for the relatively small coating weight and thickness applied to the surface of the polyethylene layer, resulting in high dimensional stability at 150°C.To analyze the presence and quantity of boehmite particles in the pores of the polymer layer, combustion and / or thermogravimetric analysis may be used if, as described above, the surface coating of the nanoporous layer is removed and the subsequent combustion of all organic polymer is performed, and only boehmite particles are found in the nanoporous layer, and the polymer layer did not contain inorganic particles or boehmite particles before the application of the boehmite coating. Another method is to use TEM or SEM, which are used to measure the average crystal size, to create an image of the presence of boehmite particles in the pores of the polymer layer and simultaneously determine their crystal size. By combining this method with the combustion and / or thermogravimetric analysis described above, the weight of boehmite particles in the separator can be obtained in gsm units by quantifying the weight of boehmite or other ceramic particles, if any, in the pores. If necessary, FTIR, X-ray diffraction, thermogravimetric analysis (TGA), high-resolution photoelectron spectroscopy (ESCA), and... 27The presence and quantity of boehmite particles in the pores of the polymer layer can be further confirmed by utilizing known analytical techniques for boehmite particles, such as Al nuclear magnetic resonance (NMR). An alternative method for collecting ceramic particles, such as boehmite particles, from the pores of the polymer layer after removing the surface coating from the polymer layer may be Soxhlet extraction of the polymer layer using a high-temperature solvent that dissolves the polymer and separates insoluble inorganic particles. In addition to providing the weight of the inorganic particles, this can be combined with the above analytical methods, combustion and / or thermogravimetric analysis, or TEM or SEM methods to confirm the presence and quantity of boehmite particles. One benefit of this incorporation of nano-sized boehmite particles may be the ability to manufacture and market boehmite / polyethylene substrates for various applications, which may include applications for manufacturing CCS, in which case the boehmite / polyethylene substrate may have several advantages over polyethylene substrates, which may include, but are not limited to, higher dimensional stability at high temperatures, an improved small pore size distribution, and other benefits such as ease of wetting by subsequent coatings. One example of a particularly useful application of boehmite / polymer substrates may be filtration applications, in which case the very small nano-sized boehmite particles in the polymer pores, with or without various surface treatments, may be able to provide desired nanofiltration and ultrafiltration properties that cannot be obtained from polymer substrates without boehmite particles in the pores, and may be suitable for desalination of seawater. This advantage may also be applicable to other boehmite / polymer substrates, such as boehmite / polypropylene and boehmite / other polyolefin substrates, in addition to polyethylene.

[0068] Example 4 Using the procedure of Example 1, a nanoporous separator containing boehmite and boron nitride pigment in a weight ratio of 50:50 was prepared. Here, instead of the Entec polyethylene separator, a 12-micron thick porous polyethylene separator procured from Toray Industries, Inc. in Tokyo, Japan was used, and vinylpyrrolidone polymer, Soteras CCS, was used instead of chitosan. The boron nitride pigment was NX1 procured from Momentive Performance Materials. The total coating weight on both sides was 2.5 gsm. Figure 11 shows the reduction in water content of this separator compared to a control containing only boehmite pigment and no boron nitride. Specifically, as shown in Figure 11, with a weight ratio of boehmite to boron nitride of 50:50, the water content at 20% RH was reduced to one-third of the value for boehmite pigment alone.

[0069] Comparative Example 1 An alumina mixture was prepared in water by adding 20 parts of alumina pigment (HJA-0719, sourced from Anhui Estone Materials Technology Co., Ltd., Anhui Province, China) and 1 part of hydroxypropyl cellulose polymer (Klucel H grade supplied by Ashland mc., Wilmington, DE) to 63 parts of water and stirring the slurry. This 20:1 weight ratio of pigment to binder resulted in similar Guarley permeability and electrolyte ionic conductivity as in Example 2, where the pigment to binder ratio was 8:1. As the 20:1 weight ratio decreased, the adverse effects gradually increased, accompanied by higher Guarley permeability and lower ionic conductivity. The main particle size of the alumina was larger than 400 nm. Using wound rods of different wire thicknesses, an alumina mixture was coated onto both sides of a 12-micron thick porous polyethylene separator (EP12 sourced from Entec, Lebanon, Oregon), resulting in total coating weights ranging from approximately 0.8 grams / square meter (gsm) to approximately 4.4 gsm for both sides. First, the alumina coating was applied to one side of the polyethylene separator and dried in a convection oven at 70°C for 2 minutes. Then, the alumina coating was applied to the uncoated opposite side of the polyethylene separator and dried under the same oven heating conditions as the first coating.

[0070] A 20:1 weight ratio of alumina pigment to hydroxypropyl cellulose polymer binder is calculated to result in 13.6 volume% of polymer binder in the solid portion of the porous alumina coating. This calculation is based on the specific gravity of alumina (4.0 g / cc) and hydroxypropyl cellulose polymer (1.27 g / cc). At a total coating weight of approximately 4.4 gsm, the shrinkage of this alumina coating was approximately 60% at 150°C for 1 hour.

[0071] Comparative Example 2 A boehmite mixture was prepared in water by adding 20 parts of boehmite pigment (BG-613, sourced from AnHui Estone Materials Technology Co., Ltd., Anhui Province, China) and 1 part of hydroxypropyl cellulose polymer (Klucel H grade supplied by Ashland Inc., Wilmington, DE) to 63 parts of water and stirring the slurry. This 20:1 weight ratio of pigment to binder resulted in similar Guarley permeability and electrolyte ionic conductivity as in Example 2, where the pigment to binder ratio was 8:1. As the 20:1 weight ratio decreased, the adverse effects gradually increased, accompanied by higher Guarley permeability and lower ionic conductivity. The main particle size of the boehmite was larger than 400 nm. Using wound rods of different wire thicknesses, a boehmite mixture was coated onto both sides of a 12-micron thick porous polyethylene separator (EP12 sourced from Entec, Lebanon, Oregon), resulting in total coating weights ranging from approximately 0.8 grams / square meter (gsm) to approximately 7.0 gsm for both sides. First, the boehmite coating was applied to one side of the polyethylene separator and dried in a convection oven at 70°C for 2 minutes. Then, the boehmite coating was applied to the uncoated opposite side of the polyethylene separator and dried under the same oven heating conditions as the first coating.

[0072] A 20:1 weight ratio of boehmite pigment to hydroxypropyl cellulose polymer binder is calculated to result in 10.7 volume% of polymer binder in the solid portion of the porous boehmite coating. This calculation is based on the specific gravity of boehmite (3.03 g / cc) and hydroxypropyl cellulose polymer (1.27 g / cc). At a total coating weight of approximately 7.0 gsm, the shrinkage of this boehmite coating was approximately 60% at 150°C for 1 hour.

[0073] Table 1 summarizes the data from Examples 1 to 3.

[0074] [Table 1]

[0075] One challenge in using nano-sized ceramic particles, such as nano-sized boehmite particles, is their greater tendency to absorb water due to their much larger surface area compared to micro-sized ceramic particles, such as micro-sized alumina and micro-sized boehmite particles. For lithium-ion cells, lithium metal cells, and other electrochemical cells sensitive to water content, the separator needs to have a low water content, such as less than 600 ppm, preferably less than 300 ppm, and more preferably less than 100 ppm, as is typically seen in separators made solely of porous polymers without ceramic material. Vacuum drying of the separator at up to 80°C helps reduce this water content in separators with nano-sized ceramic particles, and the separator of the present invention has enhanced thermal stability, allowing even higher vacuum drying temperatures of up to 90°C to about 110°C; however, some users of the separator may prefer not to perform this additional step of vacuum drying. It is preferable if the higher thermal stability of the separator of the present invention, the lower cost due to the use of less ceramic coating material, and the thinner ceramic coating thickness, including those due to the inclusion of ceramic coatings and ceramic particles within the porous polymer separator layer, include a lower moisture content in the separator due to the use of more hydrophobic ceramic particles and organic polymers. Having fewer ceramic particles due to the lower coating weight of the present invention is an important advantage for reducing the moisture content of the separator, but it is even more useful if the ceramic particles are more hydrophobic and less hygroscopic.

[0076] One method to provide lower hygroscopicity is to treat the surface of ceramic particles with a hydrophobic material, such as by using hydrophobic-treated boehmite particles, in particular when the hydrophobic material chemically reacts with the boehmite particles and thus permanently bonds to them. Suitable hydrophobic materials for chemical modification of boehmite pigments include, but are not limited to, organic sulfonic acids, organic carbonates, and polymers having hydrophobic groups, as well as polymers having reactive groups such as carboxylic acid groups and hydroxyl groups that can react with the hydroxyl groups of boehmite or other ceramic particles.

[0077] An example of organic sulfonic acid-treated boehmite particles is Dispal® 25SR, available from Sasol in Lake Charles, Louisiana, which includes a boehmite pigment chemically reacted with p-toluenesulfonic acid via sulfonic acid groups. The reaction leaves hydrophobic p-toluene groups on the outer surface of the boehmite particles, making them less absorbent of water. An even more hydrophobic example available from Sasol is boehmite particles in which p-dodecylbenzenesulfonic acid surface-reacts via sulfonic acid groups, resulting in boehmite particles with extremely hydrophobic dodecyl groups on the outer surface of the boehmite pigment. This makes the boehmite particles hydrophobic so that the treated boehmite can be dispersed in a hydrocarbon solvent. The use of organic sulfonic acid-surface-treated boehmite particles in boehmite coatings for separators is disclosed in U.S. Patent Application Publication 2013 / 0171500 by Xu et al.

[0078] Examples of organic carbonate-treated boehmite particles include, but are not limited to, those comprising ethylene carbonate and fluoroethylene carbonate as the organic carbonate. In addition to the option of selecting an organic carbonate that is highly hydrophobic in the molecular portion facing away from the boehmite surface, there may also be an option of selecting an organic carbonate that provides increased ionic conductivity to the separator. The use of ethylene carbonate-surfaced boehmite particles to provide an enhanced conductive separator is described in U.S. Patent No. 8,883,354 to Carlson et al.

[0079] The use of boehmite particles surface-treated with an organic polymer to provide a separator is described in U.S. Patent No. 9,871,239 to Carlson et al. In boehmite separators, the major crystal size and the resulting pore size are reduced, and this smaller pore size results in lower ionic conductivity of the separator. Therefore, it is useful to perform surface treatments such as the three types described above, which can be used to increase the ionic conductivity of the separator and reduce its water content.

[0080] The preparation of ceramic mixtures or slurries containing surface-treated hydrophobic materials can be conveniently carried out by adding a precursor such as an organic carbonate to a slurry containing untreated boehmite particles, and then heating the slurry at a temperature such as 80°C for a sufficient amount of time, for example, 1 hour, to react the surface-treated material such as the organic carbonate with the boehmite particles to the desired level of surface treatment. While it is not necessary to separate the surface-treated boehmite particles by a drying process, it is rather common to dilute the slurry with water and / or a solvent to the desired % solids content level for coating porous polymer substrates, and then use the slurry as is. If it is desirable to further reduce the particle size of the ceramic particles such as boehmite particles, and / or homogenize the slurry for greater uniformity, and possibly for a lower viscosity at the desired % solids content level for coating, the slurry can be further mixed in an impingement mill or similar milling device before and / or after any surface treatment. Examples of the use of impingement mills to reduce and homogenize pigment-containing coating mixtures are described in U.S. Patents 5,210,114 and 5,292,588 to Katsen.

[0081] The present invention has been described in detail with reference to specific and general embodiments, but it will be apparent to those skilled in the art that various changes and modifications can be made without departing from its spirit and scope.

Claims

1. A separator for lithium batteries, A porous polymer layer having a first side and a second side, A first layer adjacent to the first side surface of the porous polymer layer, comprising a plurality of pores and a solid material, (a) The solid material comprises a plurality of inorganic oxide particles and at least one polymer, (b) The plurality of inorganic oxide particles in the first layer include crystallites having a size in the range of 5 nm to 90 nm. (c) The thickness of the first layer is 3 μm or less, and the total coating weight of the first layer is less than 6.0 gsm. (d) The ratio of the thickness of the first layer to the total thickness of the separator is 10% to 80%. Separator.

2. The diameter of the plurality of pores in the first layer is 100 nm or less. A separator according to claim 1.

3. The size of the crystallites is 5 nm to 80 nm. A separator according to claim 1.

4. The plurality of inorganic oxide particles in the first layer are hydrated Al ２ O ３ including, A separator according to claim 1.

5. The plurality of inorganic oxide particles in the first layer are boehmite, SiO ２ Boron nitride pigment, ALN, hydrated aluminum oxide, AlOOH, and Al ２ O ３ ・xH ２ Includes one or more of O, A separator according to claim 1.

6. When the plurality of inorganic oxide particles of the first layer contain Al ２ O ３ ·xH ２ O, x is 1.0 to 2.0, The separator according to claim 5.

7. The at least one polymer in the first layer comprises one or more of the following: cellulose polymer, hydroxyethylcellulose, vinylpyrrolidone polymer and its polymers, polysaccharides, chitosan, polyethylene oxide, polyvinyl alcohol, and polyolefin. A separator according to claim 1.

8. The volume fraction of at least one polymer in the solid material is 10% to 50%. A separator according to claim 1.

9. The porosity of the first layer is 30% to 70%. A separator according to claim 1.

10. A second layer adjacent to the second side surface of the porous polymer layer, comprising a plurality of inorganic oxide particles and one or more polymers, A separator according to claim 1.

11. The total coating weight of the first and second layers is less than 9.4 gsm. The separator according to claim 10.

12. The total coating weight of the first layer is 0.8 gsm to 3.0 gsm. A separator according to claim 1.

13. The first layer has a different composition from the second layer. The separator according to claim 10.

14. The weight ratio of inorganic oxide to polymer in the first layer is 12:

1. A separator according to claim 1.

15. The combined thickness of the first and second layers is less than 3.8 microns. The separator according to claim 10.

16. The pore volume of the first layer is between 0.8 cc / g and 1.0 cc / g. A separator according to claim 1.

17. The porous polymer layer contains a polyolefin, A separator according to claim 1.

18. The polyolefin is selected from the group consisting of polyethylene, polypropylene, 1-butene, 1-pentene, 1-hexene, and 1-octene homopolymers, as well as copolymers and ternary polymers thereof. The separator according to claim 17.

19. The porosity of the first layer is 30% to 55%. The separator according to claim 9.

20. The size of the crystallites is 5 nm to 25 nm. A separator according to claim 1.

21. The size of the crystallites is 30 nm to 50 nm. A separator according to claim 1.

22. The size of the crystallites is between 5 nm and 75 nm. A separator according to claim 1.

23. The aforementioned volume fraction is between 12% and 45%. The separator according to claim 8.

24. The aforementioned volume fraction is between 25% and 45%. The separator according to claim 8.

25. The aforementioned volume fraction is between 15.3% and 25.6%. The separator according to claim 8.

26. The total coating weight of the first and second layers is 8.4 gsm or less. The separator according to claim 10.

27. The total coating weight of the first layer is 0.8 gsm to 4.7 gsm. A separator according to claim 1.

28. The total coating weight of the first layer is 3.6 gsm or less. A separator according to claim 1.

29. The weight ratio of inorganic oxide to polymer in the first layer is 8:

1. A separator according to claim 1.

30. The weight ratio of inorganic oxide to polymer in the first layer is 4:

1. A separator according to claim 1.

Images