A process for making boehmite with controlled primary and secondary particle sizes

Abstract

A method of forming a crystalline Boehmite product for use in a coating that includes preparing an aqueous slurry by mixing together water, pseudo-Boehmite, and aluminum material configured to dissolve in alkaline solution, followed by performing a hydrothermal process using the slurry to form the crystalline Boehmite. The mass ratio of pseudo-Boehmite to aluminum material in the slurry is ≤ 30% with the slurry having an overall solid content of ≤ 50%. The crystalline Boehmite, pseudo-Boehmite, and aluminum material each have an average (D50) primary particle size and an average (D50) secondary particle size. The average (D50) secondary particle size of aluminum material is < 77.4 micrometers (μm); the average (D50) secondary particle size of pseudo-Boehmite is < 8.0 μm, the average (D50) secondary particle size of crystalline Boehmite product is < 5.0 μm, and an average (D50) secondary particle size / an average (D50) primary particle size ratio < 3.0.

Description

A PROCESS FOR MAKING BOEHMITE WITH CONTROLLED PRIMARY AND SECONDARY PARTICLE SIZESFIELD

[0001] This invention generally relates to a process for making a Boehmite product for use in a coating formulation in which the primary and secondary sizes for the Boehmite are controlled. One possible application for a formulation comprising such a Boehmite product is a coating applied to a substrate, such as a separator incorporated in a lithium-ion cell.BACKGROUND

[0002] The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

[0003] The high thermal stability exhibited by Boehmite (i.e., aluminum oxide hydroxide or AIOOH) makes this mineral filler desirable for incorporation into material formulations used in many applications, such as in forming engineered plastic components, ceramic composites, abrasive materials, fire-retardants, and cosmetic products to name a few. One technologically important application for Boehmite is its incorporation into a coating formulation applied to a separator for use in a lithium-ion cell or battery. Boehmite is important in this respect mainly because of the high thermal stability and excellent wetting ability exhibited by the Boehmite, as well as for its relative softness and lower cost as compared to alumina.

[0004] Boehmite may be prepared from a hydrothermal process using aluminum trihydroxide or Gibbsite (i.e., AI2O3-3H2O) as a precursor. This type of synthesis is typically performed at high temperature (120°-250°C) and / or under steam pressure. The use of a hydrothermal process generally leads to the formation of crystalline Boehmite primary particles with various shapes and sizes. The size and morphology of the Boehmite primary particles can generally be adjusted by changing the reactions conditions. For example, when a small amount of pseudo-Boehmite (also written as Pseudo Boehmite or pseudoboehmite) is utilized as “seeds” in the hydrothermal process, the primary particle size of the formed Boehmite will generally decrease as the amount of the seeds that are utilized increases. However, the primary particle size of the crystalline Boehmite represents only one type of size parameter associated withthe Boehmite and does not consider the occurrence of agglomeration, aggregation, or flocculation of the primary particles resulting in the formation of larger secondary particle sizes as further described herein.

[0005] The coating thickness for a separator utilized in a lithium-ion cell is typically in the range of 2 micrometers (pm) to 6 micrometers (pm) when considering the coating applied to both sides of the separator or 1 pm to 3 pm for the coating applied to a single side of the separator. Since the size of the Boehmite particles needs to be at least smaller than the coating thickness in order to form a uniform coating, the secondary particle size of Boehmite must be typically <1.0 pm in order to be incorporated into a separator coating. The particle size being referred here is the secondary particle size since one secondary particle may be made-up of many primary particles, which exhibit much smaller individual sizes.

[0006] The size of the primary particles may also be critical for the coating quality since surface area is determined by the primary particle size. Typically, the smaller the primary particle size, the larger the surface area. However, a high surface also reduces the polymer binder / particle surface area ratio in a coating, which results in a decrease of the coating’s adhesion strength. In order to achieve a high coating quality, it is preferred that the particle size of the secondary particles be similar to the particle size of the primary particles. Therefore, it is necessary to control the particle size of both secondary particles and primary particles during the synthesis of the crystalline Boehmite. Thus, there is a need to develop processes that can simultaneously control the sizes of both the primary particles and secondary particles for the Boehmite that is formed.SUMMARY

[0007] This disclosure relates generally to a method of forming a crystalline Boehmite product. This method comprises the steps of preparing an aqueous slurry by mixing together water, a pseudo-Boehmite, and an aluminum material configured to dissolve in an alkaline solution; and performing a hydrothermal synthesis process using the aqueous slurry to form the crystalline Boehmite product. The mass ratio of pseudo-Boehmite to aluminum material in the aqueous slurry being < 20% with the aqueous slurry having an overall solid content of < 50%. The crystalline Boehmite product, the pseudo-Boehmite, and the aluminum material each have an average (D50)primary particle size and an average (D50) secondary particle size. The average (D50) secondary particle size of the aluminum material is < 77.4 micrometers (pm); the average (D50) secondary particle size of the pseudo-Boehmite is < 8.0 pm, the average (D50) secondary particle size of the crystalline Boehmite product is < 5.0 pm, and the average (D50) secondary particle size / the average (D50) primary particle size ratio of the crystalline Boehmite product is < 3.0.

[0008] The aluminum material may selected from the group consisting of Gibbsite, bayerite, doyleite, nordstrandite, amorphous aluminum hydroxide, and a combination thereof. The average (D50) secondary particle size of the aluminum material may alternatively be described as < 20.0 pm; alternatively, the average (D50) secondary particle size of the aluminum material is < 5.0 pm; alternatively, the average (D50) secondary particle size of the aluminum material is < 3.0 pm; alternatively, the average (D50) secondary particle size of the aluminum material is < 2.0 pm.

[0009] The average (D50) secondary particle size of the pseudo-Boehmite may alternatively, be described as < 3.0 pm; alternatively, the average (D50) secondary particle size of the pseudo-Boehmite is < 1.0 pm; alternatively, the average (D50) secondary particle size of the pseudo-Boehmite is < 0.5 pm.

[0010] The average (D50) secondary particle size of the crystalline Boehmite product may alternatively, be described as < 3 pm; alternatively, the average (D50) secondary particle size of the crystalline Boehmite product is < 1 pm; alternatively, the average (D50) secondary particle size of the crystalline Boehmite product is < 0.5 pm.

[0011] The average (D50) secondary particle size / average (D50) primary particle size ratio of the crystalline Boehmite product may be < 2.5; alternatively, the average (D50) secondary particle size / average (D50) primary particle size ratio of the crystalline Boehmite product is < 2.0.

[0012] The mass ratio of pseudo-Boehmite to aluminum material in the aqueous slurry may alternatively, be described as < 15.0%; alternatively, the mass ratio of pseudo-Boehmite to aluminum material in the aqueous slurry is < 1.0%; alternatively, the mass ratio of pseudo-Boehmite to aluminum material in the aqueous slurry is < 0.1 %. The overall solid content of the aqueous slurry may alternatively, be described as < 30%.

[0013] According to another aspect of the present disclosure, the hydrothermal synthesis process generally comprises the steps of: adjusting the pH of the aqueous slurry to be between about 8.5 to about 13.5; heating the aqueous slurry to a temperature between 120°C and 250°C for a duration of time that is from about 1 .0 hour up to about 72 hours; collecting the aqueous slurry to form a wet cake; and drying the wet cake to obtain the crystalline Boehmite product.

[0014] The pH of the aqueous slurry may be adjusted to be in the range of 1 1 .0 to 12.0. The slurry may be heated to a temperature between 160°C to 210°C for a duration of time that ranges from 3 hours to 24 hours.

[0015] According to yet another aspect of the present disclosure, a coating composition may be applied to a substrate, wherein the coating composition comprising the Boehmite as described above and as further defined herein. This substrate may be a separator used in an electrochemical cell. The coating as applied to the substrate may have a thickness of < 3 pm.

[0016] Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.DESCRIPTION OF THE DRAWINGS

[0017] In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings. The elements in each of the drawings may not necessarily be drawn to scale, but rather emphasis is placed upon illustrating the principles of the invention.

[0018] Figure 1 is a schematic representation of the particle size associated with primary particles and secondary particles.

[0019] Figure 2A is a flowchart depicting a method of forming a crystalline Boehmite product for use in a coating formulation according to the teachings of the present disclosure.

[0020] Figure 2B is another flowchart that further describes the steps associated with the hydrothermal synthesis process shown in Figure 2A.

[0021] Figure 3A to 3D are scanning electron micrographs (SEM) at a magnification (x) of a) x=5,000 and b) x=40,000 that describe the Boehmite formed in samples A1 to A12 according the methodology shown in Figures 2A and / or 2B.

[0022] Figure 4 is a flowchart depicting a method for applying a coating to a substrate in which the coating formulation includes the Boehmite product prepared by the methods shown in Figure 2A or 2B.

[0023] The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.DETAILED DESCRIPTION

[0024] The following description is merely exemplary in nature and is in no way intended to limit the present disclosure or its application or uses. For example, the Boehmite product incorporated into a coating formulation and used according to the teachings contained herein are described throughout the present disclosure in relation to a coating applied to a separator in a lithium-ion cell in order to more fully illustrate the structural elements and the use thereof. The incorporation and use of such a Boehmite product and / or coating formulation containing such Boehmite product in other applications, including without limitation, any application in which the secondary particle size and / or primary particle size of the Boehmite product is a key component or concern, is contemplated to be within the scope of the present disclosure. It should be understood that throughout the description and drawings, corresponding reference numerals indicate like or corresponding parts and features.

[0025] For the purpose of this disclosure, the terms "about" and "substantially" are used herein with respect to measurable values and ranges due to expected variations known to those skilled in the art (e.g., limitations and variability in measurements).

[0026] For the purpose of this disclosure, the terms "at least one" and "one or more of” an element are used interchangeably and may have the same meaning. These terms, which refer to the inclusion of a single element or a plurality of the elements, may also be represented by the suffix "(s)" at the end of the element. For example, "at least one particle", "one or more particles", and "particle(s)" may be used interchangeably and are intended to have the same meaning.

[0027] The present disclosure generally describes the synthesis of Boehmite with adjustable primary and secondary particle sizes in a hydrothermal process in whichthe secondary particle sizes of Gibbsite and pseudo-Boehmite are controlled. The pseudo-Boehmite is used as the seeds from which larger Boehmite particles are grown. The sizes of the secondary particles of both Gibbsite and pseudo-Boehmite used in the method are important for the formation of the primary and secondary particle sizes exhibited by the synthesized Boehmite particles. If the Gibbsite secondary particle size is too large (for example D50 ^77.4 pm), then the synthesized Boehmite will have large secondary particles even in the presence of pseudo- Boehmite with small particle sizes (samples A1-A4 in Table 1 ). If the pseudo-Boehmite has large secondary particle sizes (> 8.0 pm), then it could reduce the primary particle size of the Boehmite that forms, but not the secondary particle size of the Boehmite (samples A9 and A10 in Table 1 ). In order to be able to adjust the secondary particle size of the Boehmite, the secondary particle size of the pseudo-Boehmite must be < 8.0 micrometers (pm).

[0028] Referring to Figure 1 , primary particles 1 and secondary particles 5 are defined. A definition for the term “primary particle” has been proposed by the National Institute of Standards and Technology to be “the smallest identifiable subdivision in a particulate system.” In terms of particles formed by crystallization, primary particles represent the particles that are grown from a critical crystal nucleus or “seed”. Thus, the term “primary particle” is generally used to indicate the smallest unit of a particle that exhibits individuality prior to the occurrence of flocculation, aggregation, and / or agglomeration. Each individual particle may comprise the growth of one or more crystallites in the spatial form of a crystal lattice. Each primary particle 1 comprises a primary particle diameter 3.

[0029] A “secondary particle” 5 represents multiple primary particles 1 that are attracted to each other or are weakly associated through mechanisms, such as for example, van der Waal forces, electrostatic charges, or hydrogen bonding, to name a few. In other words a plurality of primary particles 1 may flocculate together to form agglomerates or aggregates, which are also called secondary particles 5. Each secondary particle 5 comprises a secondary particle diameter 7. Since the secondary particles 5 comprise a plurality of primary particles 1 , the secondary particle diameter 7 is generally greater than the primary particle diameter s.

[0030] Referring now to Figures 2A and 2B, the method 10A, 10B for forming a Boehmite product for use in a coating formulation, generally comprises preparing 15an aqueous slurry having an overall solids content of < 50% by mixing together an aluminum material configured to dissolve in an alkaline solution with water and a pseudo-Boehmite, followed by performing 20A, 20B a hydrothermal synthesis process using the aqueous slurry to form a crystalline Boehmite product that has an average (D50) secondary particle size of < 5.0 pm and an average (D50) secondary particle size / an average (D50) primary particle size ratio < 3.0. The mass ratio of pseudo- Boehmite to aluminum material in the aqueous slurry is < 30%. The aluminum material used in this method generally has an average (D50) secondary particle size of < 77.4 micrometers (pm), while the pseudo-Boehmite has an average (D50) secondary particle size of < 8.0 pm.

[0031] When desirable, the process for performing 20B a hydrothermal synthesis using the aqueous slurry to form a crystalline Boehmite product may further include the steps of: adjusting the pH of the aqueous slurry to be between about 8.5 to about 13.5; heating 30 the aqueous slurry to a temperature between 120°C and 250°C for a duration of time that is from about 1 .0 hour up to about 72 hours; collecting 35 the aqueous slurry to form a wet cake; and drying 40 wet cake to obtain the crystalline Boehmite product having an average (D50) secondary particle size of < 5.0 pm.

[0032] As used herein the average particle size (D50) represents the mean average particle size as measured for particles exhibiting multiple particle sizes in a particle size distribution. In other words the average (D50) particle size represents the measured value for which 50% of the particles exhibit a larger size and 50% of the particles exhibit a smaller size. Similarly, other measurements, such as Dw and D90, may be obtained directly from a cumulative particle size distribution. More specifically, D10, D50, and D90 represent percentile values, which are statistical parameters that indicate the size below which 10%, 50% or 90% of all particles in the measured sample are found.

[0033] Scanning electron microscopy (SEM) or other optical or digital imaging methodology known in the art may be used to determine the shape, morphology, and particle size of primary particles. The average primary and secondary particle sizes and particle size distributions may also be measured using any conventional technique, such as sieving, microscopy, Coulter counting, dynamic light scattering, or particle imaging analysis, to name a few. Alternatively, a laser particle analyzer is used for the determination of average secondary particle size and its corresponding particlesize distribution. Alternatively, scanning electron microscopy (SEM) may be used to check and confirm the average primary particle size as demonstrated by measurements taken at a magnification (x) of 40,000 as shown in Figures 3A to 3D.

[0034] The secondary particle sizes of the Gibbsite and pseudo-Boehmite utilized in the preparation method 10A, 10B was un-expectantly found to have a dramatic effect on the Boehmite product that is formed. The method 10A, 10B was performed multiple times utilizing Gibbsite and pseudo-Boehmite having different average (D50) secondary particle sizes as described in Runs A1 to A12 shown in Table 1 below. The shape, morphology, and particle size of primary particles of the Boehmite product formed in Runs A1 to A12 are shown in the SEM micrographs provided in Figures 3A to 3D at a magnification (x) of 5,000 and 40,000 with measurement of primary particle size being confirmed therein for the Boehmite that is formed.

[0035] Table 1. Effect of Pseudo-Boehmite and Gibbsite Sizes on theSecondary Particle Size of the Boehmite Product FormedPseudo Pseudo mite Second Primary Secondary Boeh Pseudo Gibbsite Boehmite ary size Gibbsit Particle Particle to Primary Sample size Boehmite, e, to(D50), g Size Size Particle (D50), ms Gibbsite gms |im pm mass (D50), (D50), pm (D50) ratio (%) |im Ratio

[0036] In general, the primary particle size of the Boehmite that is formed decreases upon increasing the amount of pseudo-Boehmite utilized in the preparation method 10A, 10B, which supports that pseudo-Boehmite acts as “seeds” that induce Boehmite crystal growth. However, surprisingly, when the Gibbsite that is used in the preparation method 10A, 10B has a large average (D50) secondary particle size, i.e.,77 A pm (see Runs A1 to A4, T able 1 ), there is no substantial size reduction in average secondary particles sizes for the Boehmite product that is formed regardless of the pseudo-Boehmite average (D50) secondary particle sizes that are utilized, e.g., average secondary particle size for the pseudo-Boehmite ranged from 0.22 pm to 8.0 pm. Consistent with the changes in the secondary particles and primary particles, the average (D50) secondary particle size / the average (D50) primary particle size increased significantly with the use of pseudo-Boehmite regardless of the secondary particle size of the pseudo-Boehmite, from 4.1 without any seed to > 11 .1 .

[0037] In addition, with a decrease in the Gibbsite average (D50) secondary particle size (compare Runs A1-A4 to A5-A8 to A9-A12), both the primary particle size and the secondary particle size of the synthesized Boehmite product are reduced. The reduction in Gibbsite average (D50) secondary particle size has a greater effect on the size of the secondary particles rather than the primary particles. Alternatively, this effect occurs when relatively small average (D50) secondary particle sizes of Gibbsite (e.g., D50 3.77 pm) are utilized in the preparation method.

[0038] When the secondary average (D50) particle size of the Gibbsite changes from 77.4 pm (Runs A1-A4) to 3.77 pm (Runs A5-A8), the secondary average (D50) particle size of the formed Boehmite decreases. For example, a decrease in secondary size of the Boehmite from 19.68 pm (Run A4) to 1.04 pm (Run A8) is observed when using 1 % pseudo-Boehmite seeds having an average (D50) secondary particle size at 0.22 pm. In these same runs (Run 4 vs Run 8), the average (D50) primary particle size was measured to decrease from about 1 pm to 0.2-0.3 pm (see Figures 3B & 3C). Upon changing the average (D50) secondary Gibbsite size from 3.77 pm (Run A8) to 1 .8 pm (Run A12), while still using 1 % pseudo-Boehmite seeds having an average (D50) particle size at 0.22 pm, the average (D50) secondary particle size of formed Boehmite decreased only from 1.04 pm (Run A8) to 0.46 pm (Run A12). The primary particles in these runs were measured to still have sizes in the range of 0.2- 0.3 pm (see Figures 3C & 3D). Thus, a small average (D50) secondary particle size is preferred for the pseudo-Boehmite.

[0039] The average (D50) secondary particle size of pseudo-Boehmite is important for determining the average (D50) secondary particle size of Boehmite product that is formed in the method 10A, 10B. Upon using 1 % of pseudo-Boehmite as seeds that have an average (D50) secondary size of 8.0 pm, the average (D50) secondary particlesize of the formed Boehmite changes from 15.39 m (Run A1 ) to 20.87 pm (Run A2). The Boehmite secondary particle size changed from 2.31 pm (Run A5) to 3.13 pm (Run A6) when Gibbsite having an average (Dso) secondary particle size of 3.77 pm (D50) Gibbsite; and from 1.87 pm (Run A9) to 1.99 pm (Run A10) when Gibbsite having an average (D50) secondary particle size of 1.8 pm. In all of these cases, the average (Dso) secondary particle size increased even if the primary particle size decreased. Thus, pseudo-Boehmite cannot be used to reduce the average (Dso) secondary particle size of the formed Boehmite if the average (Dso) particle size of the pseudo-Boehmite is 8.0 pm or more. Consistent with the changes in the sizes of the secondary particles and the primary particles, the average (Dso) secondary particle size / the average (Dso) primary particle size ratio increased by using pseudo-Boehmite with the average (Dso) secondary particle size of 8.0 pm, which is not desired for the coating application.

[0040] When pseudo-Boehmite with an average (Dso) secondary particle size of 2.58 pm (Runs A3, A7, A11 ) was used, the average (Dso) secondary particle size of the formed Boehmite was reduced significantly with the changes from 2.31 pm (Run A5) to 0.86 pm (Run A7) using Gibbsite having an average (Dso) secondary particle size of 3.77 pm, and from 1 .87 pm (Run A9) to 0.56 pm (Run 11 ) using Gibbsite having an average (Dso) secondary particle size of 1.8 pm. With the same size of pseudo- Boehmite of 2.58 pm, the average (Dso) secondary particle size / the average (Dso) primary particle size ratio of the formed Boehmite decreased from 26.8 (Run A3) to 2.6 (Run A7) to 1 .8 (Run A11 ) when the Gibbsite size decreased from 77.4 pm to 3.77 pm to 1 .8 pm. Therefore, it is desirable to use relatively small particles of both Gibbsite and pseudo-Boehmite for the synthesis of Boehmite with both the average (Dso) particle size for the primary and secondary particles being adjustable for use in a coating application, In general, the smaller amount of seeds will contribute to the formation of Boehmite having a larger average (Dso) primary particle size, while the smaller average (Dso) secondary particle sizes of the Gibbsite and pseudo-Boehmite particles will help reduce the average (Dso) secondary particle sizes of the Boehmite.

[0041] It is generally expected that both the average (Dso) primary particle sizes and average (Dso) secondary particle sizes of the Boehmite will be reduced with an increase in the amount of pseudo-Boehmite that is used as the seeds. Generally, Gibbsite particles dissolve and precipitate onto the seeding particles in order to growinto Boehmite particles. The more seeding particles that are present results in a smaller size for the Boehmite particles that are grown. Surprisingly, however, this expectation is true only when the pseudo-Boehmite has an average (Dso) secondary size that is small enough, but false when the average (D50) size of the pseudo- Boehmite is relatively large.

[0042] In Table 2, several experimental runs B1-B3 demonstrate that the average (D50) secondary particle size for the Boehmite formed stays at 0.56 to 0.57 pm, when the mass amount of the pseudo-Boehmite having an average (D50) secondary particle size of 2.58 pm is increased from 0.9 wt.% (Run B2) to 4.5 wt.% (Run B3). As a further comparison, in Table 3, several Runs C1-C6 demonstrate that the average (D50) secondary particle size of the Boehmite formed decreases from 0.47 pm (Run C4) to 0.34 pm (Run C5) when the mass amount increases from 0.9 wt.% to 5.3 wt.% for pseudo-Boehmite that exhibits a much smaller average (D50) secondary particle size of 0.22 pm. Thus, pseudo-Boehmite with a small average (D50) secondary particle size is preferred so that the average (D50) secondary particle size of the synthesized Boehmite will be more sensitive to the added amount of the seeds. With smaller secondary particles of Gibbsite and smaller secondary particles of pseudo-Boehmite, the average (D50) secondary particle size / the average (D50) primary particle size of the synthesized Boehmite was relatively constant with a value in the range of 1.4 to 2.0 (Table 3), which is desirable for the coating application.

[0043] Table 2. Effect of the Amount of pseudo-Boehmite (2.58 pm) on the Average (D50) Particle Size of the formed Boehmite.Pseudo Pseudo Secondary Primary SecondarySample Boehmite, g Gibbsite, g Boehmite to Particle Particle (D50) ( (D50 = 1.8 Gibbsite to D50 = 2.58 size (D50), size (D50), p Mass Ratio Primary (D50) gm) m)(%) gm gm Particle Ratio

[0044] Table 3. Effect of Amount of Pseudo Boehmite (D50: 0.22 m) on ParticleSizesPseudo PseudoGibb Secondary Primary SecondarySample Boehmite, g site, g Boehmite to (D50 = 0.22 (D50 = 1.8 Gibbsite Particle Particle (D50) to size (D50), size (D50), Primary (D50 m) gm) Mass Ratio ) g gm pm Particle Ratio (%)

[0045] The preparation of Boehmite from a hydrothermal process as described above was described via the use of Gibbsite and pseudo-Boehmite as the raw materials in aqueous solution. In principle, the Gibbsite may be replaced with another aluminum material generally comprising AI-OH present within the structure. Thus, the aluminum material utilized in the method 10A, 10b of the present disclosure may be Gibbsite, bayerite, doyleite, nordstrandite, amorphous aluminum hydroxide or a combination thereof. Alternatively, the aluminum material is Gibbsite. The aluminum material will dissolve in an alkaline solution at high hydrothermal temperature. Once the aluminum material is dissolved, it precipitates as Boehmite (AIOOH) since Boehmite is insoluble under the synthesis conditions. Therefore, alternatively, any aluminum material that can dissolve under the claimed synthesis conditions may be used as the precursor or raw material for the synthesis of the Boehmite.

[0046] The average (D50) secondary particle size of the aluminum material or Gibbsite should be < 77.4 pm. In general, a smaller particle size is preferable. Alternatively, the average (D50) secondary particle size may be < 20 pm; alternatively, < 10 pm; alternatively, < 5 pm; or alternatively, < 2 pm.

[0047] Pseudo-Boehmite has a crystal structure similar to Boehmite, but with a much smaller crystalline size and with the inclusion of more water molecules. Once treated under hydrothermal conditions, the small crystalline particles may be exposed and act as seeds for crystal growth into larger Boehmite particles. However, in order to adjust the average (D50) secondary particle size of the formed Boehmite, the pseudo-Boehmite must have a relatively small average (D50) secondary particle size.The average (D50) secondary particle size of the pseudo-Boehmite may be < 8.0 micrometers (pm); alternatively, < 5 pm; alternatively, < 3 pm; alternatively, < 1 pm; or alternatively, < 0.5 pm.

[0048] The amount of the pseudo-Boehmite that is utilized as the seeds in the method of the present disclosure will affect the average (D50) primary and secondary particle sizes of the Boehmite that is formed. The use of a higher amount of seeds generally results in the formation of smaller primary Boehmite particles. However, the average (D50) secondary particle size of the Boehmite also is affected by the average (D50) secondary particle size of the pseudo-Boehmite. In general, the use of a higher amount of seeds will also result in smaller secondary particles provided the average (D50) secondary particle size of the pseudo-Boehmite is small enough. A mass ratio of pseudo-Boehmite to aluminum material that ranges from 99% to 0.001 % can be used to make Boehmite. However, for a coating application, a relatively large primary particle size is preferred, such that a lower mass ratio should be used. Thus, the mass ratio of the pseudo-Boehmite to Gibbsite may be < 20%; alternatively, < 10%; alternatively, < 1 %; and alternatively, < 0.1 %. It is even possible that < 0.01 % could be used as the mass ratio considering the small size of Boehmite crystallites present in the pseudo-Boehmite (e.g. 4.9 nanometers).

[0049] According to one aspect of the present disclosure, the aqueous slurry comprises aluminum material having an average (D50) secondary particle size that < 5 pm and pseudo-Boehmite having an average (D50) secondary particle size that is < 3 pm in order to form a crystalline Boehmite product having an average (D50) secondary particle size that is < 3 pm and an average (D50) secondary particle size / average (D50) primary particle size that is < 3.0. Alternatively, the average (D50) secondary particle size of the aluminum material is < 3 pm, the average (D50) secondary particle size of the pseudo-Boehmite is < 1 pm, the average (D50) secondary particle size of the formed crystalline Boehmite product is < 1 pm; and the mass ratio of the pseudo-Boehmite to aluminum material present in the aqueous slurry is < 20%.

[0050] In order to prepare an aqueous slurry for use in a hydrothermal process, the aluminum material, e.g., Gibbsite, and the pseudo-Boehmite are mixed together in water. The solid content of this aqueous slurry will not only affect the overall cost ofthe product, but also the average (D50) secondary particle size of the Boehmite product. An aqueous slurry having a higher solid content, which will reduce the process cost, is typically preferred. However, in order to make the product homogenous, it is necessary to ensure that the aqueous slurry is stirred or mixed well. In this regard, if the aqueous slurry comprises a solid content that is too high, the aqueous slurry may be too viscous to be adequately mixed. Therefore, the solid content of the aqueous slurry utilized in the process is generally in the range of 1 % to about 50%. Alternatively, the solid content may be < 40%; alternatively, < 30%; alternatively, < 20%; and alternatively, < 15%.

[0051] The aluminum material, e.g., Gibbsite AI(OH)3 will tend to dissolve in alkaline solution at high temperatures. If the pH is too high, the aluminum material will completely dissolve and become a sodium aluminate solution. Thus, the pH has to be in a specific range for Gibbsite to be dissolved and precipitated as Boehmite (AIOOH). The pH of the aqueous slurry may be in the range of 8.5 to 13.5; alternatively, in the range of 10.0 to 13.0; and alternatively, in the range of 11.0 to 12.0.

[0052] With a higher reaction temperature, the dissolution of the aluminum material, e.g., Gibbsite, will be easier and the crystallization of the Boehmite will become quicker. The temperature, however, is usually limited by the capability of the autoclave. In general, an autoclave can only handle a temperature < 200°C. An autoclave may be designed to operate at a temperature as high as 250-260°C, but with much higher cost associated therewith. A lower temperature, such as 100°C, could also be used, but with an extended treatment time such as more than 1 week. In order to be cost effective, the reaction temperature may range from 120°C to 250°C; alternatively, between 160°C and 210°C; alternatively, in the range of 170°C to 200°C.

[0053] In general, the reaction time may be shorter when using a higher temperature. To be cost effective, the reaction should be completed in less than one week. The time should not be too short since it requires time to heat the autoclave up and to cool it down as well. The reaction time may range from 1 hour to 72 hours; alternatively, in the range from 2 hours and 48 hours; and alternatively, in the range from 3 hours to 24 hours.

[0054] According to another aspect of the present disclosure, a method for applying a coating to a substrate in which the coating formulation includes the Boehmite product prepared by the methods described above and as further defined herein is provided.Referring now to Figure 4, this method 100 generally comprises the steps of forming 105 a crystalline Boehmite product according to the process described above and as further defined herein (e.g., see Figures 2A & 2B); dispersing the crystalline Boehmite product into a fluid to form a coating formulation; and applying the coating formulation to at least one side of the substrate. When desirable this substrate may be a separator used in an electrochemical cell with the coating applied to either one side or to both sides of the separator.

[0055] The specific examples provided in this disclosure are given to illustrate various embodiments of the invention and should not be construed to limit the scope of the disclosure. The embodiments have been described in a way which enables a clear and concise specification to be written, but it is intended and will be appreciated that embodiments may be variously combined or separated without departing from the invention. For example, it will be appreciated that all preferred features described herein are applicable to all aspects of the invention described herein.

[0056] EXAMPLES - Preparation of Boehmite in Runs A1-A12, B1 -B3, and C1 -C6

[0057] All examples or experimental runs described herein incorporate 5.5 grams of Gibbsite with a predetermined amount of Pseudo-Boehmite (see Tables 1 -3) in water to make ~30 ml of an aqueous slurry with adequate mixing to ensure homogeneity. The pH of the aqueous slurry is adjusted using 4N NaOH to be in the range of about 1 1-12. The aqueous slurry is then transferred into a 50 ml autoclave and heated at 180°C for 8 hours. The Boehmite formed is collected after filtering the heated slurry, washed with copious amount of water, and dried in an oven at about 130°C.

[0058] Those ski I led-i n-the-art, in light of the present disclosure, will appreciate that many changes can be made in the specific embodiments which are disclosed herein and still obtain alike or similar result without departing from or exceeding the spirit or scope of the disclosure. One skilled in the art will further understand that any properties reported herein represent properties that are routinely measured and can be obtained by multiple different methods. The methods described herein represent one such method and other methods may be utilized without exceeding the scope of the present disclosure.

[0059] The foregoing description of various forms of the invention has been presented for purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise forms disclosed. Numerous modifications or variations are possible in light of the above teachings. The forms discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various forms and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.

Claims

CLAIMSWhat is claimed is:1 . A method of forming a crystalline Boehmite, the method comprising: preparing an aqueous slurry by mixing together water, a pseudo-Boehmite, and an aluminum material configured to dissolve in an alkaline solution; the mass ratio of pseudo-Boehmite to aluminum material in the aqueous slurry being < 20% with the aqueous slurry having an overall solid content of < 50%; and performing a hydrothermal synthesis process using the aqueous slurry to form the crystalline Boehmite product; wherein the crystalline Boehmite product, the pseudo-Boehmite, and the aluminum material each have an average (Dso) primary particle size and an average (Dso) secondary particle size; wherein the average (D50) secondary particle size of the aluminum material is < 77.4 micrometers (pm); the average (Dso) secondary particle size of the pseudo- Boehmite is < 8.0 pm, the average (D50) secondary particle size of the crystalline Boehmite product is < 5.0 pm, and the average (Dso) secondary particle size / the average (Dso) primary particle size ratio of the crystalline Boehmite product is < 3.0.

2. The method according to claim 1 , wherein the aluminum material is selected from the group consisting of Gibbsite, bayerite, doyleite, nordstrandite, amorphous aluminum hydroxide, and a combination thereof.

3. The method according to either claim 1 or 2, wherein the average (D50) secondary particle size of the aluminum material is < 20.0 pm.

4. The method according to any of claims 1 to 3, wherein the average (Dso) secondary particle size of the aluminum material is < 5.0 pm.

5. The method according to any of claims 1 to 4, wherein the average (Dso) secondary particle size of the aluminum material is < 3.0 pm.

6. The method according to any of claims 1 to 5, wherein the average (D50) secondary particle size of the aluminum material is < 2.0 pm.

7. The method according to any of claims 1 to 6, wherein the average (D50) secondary particle size of the pseudo-Boehmite is < 3.0 pm.

8. The method according to any of claims 1 to 7, wherein the average (D50) secondary particle size of the pseudo-Boehmite is < 1.0 pm.

9. The method according to any of claims 1 to 8, wherein the average (D50) secondary particle size of the pseudo-Boehmite is < 0.5 pm.

10. The method according to any of claims 1-9, wherein the average (D50) secondary particle size of the crystalline Boehmite product is < 3 pm.11 . The method according to any of claims 1 -10, wherein the average (D50) secondary particle size of the crystalline Boehmite product is < 1 pm.

12. The method according to any of claims 1 -1 1 , wherein the average (D50) secondary particle size of the crystalline Boehmite product is < 0.5 pm.

13. The method according to any of claims 1 -12, wherein the average (D50) secondary particle size / average (D50) primary particle size ratio of the crystalline Boehmite product is < 2.5.

14. The method according to any of claims 1 -13, wherein the average (D50) secondary particle size / average (D50) primary particle size ratio of the crystalline Boehmite product is < 2.0.

15. The method according to any of claims 1 to 14, wherein the mass ratio of pseudo-Boehmite to aluminum material in the aqueous slurry is < 15.0%.

16. The method according to any of claims 1 to 15, wherein the mass ratio of pseudo-Boehmite to aluminum material in the aqueous slurry is < 1.0%.

17. The method according to any of claims 1 to 16, wherein the mass ratio of pseudo-Boehmite to aluminum material in the aqueous slurry is < 0.1 %.

18. The method according to any of claims 1 to 17, wherein the overall solid content of the aqueous slurry is < 30%.

19. The method according to any of claims 1 to 18, wherein the hydrothermal synthesis process comprises the steps of: adjusting the pH of the aqueous slurry to be between about 8.5 to about 13.5; heating the aqueous slurry to a temperature between 120°C and 250°C for a duration of time that is from about 1 .0 hour up to about 72 hours; collecting the aqueous slurry to form a wet cake; and drying the wet cake to obtain the crystalline Boehmite product.

20. The method according claim 19, wherein the pH of the aqueous slurry is adjusted to be in the range of 11 .0 to 12.0.21 . The method according to either claim 19 or 20, wherein the slurry is heated to a temperature between 160°C to 210°C.

22. The method according to any of claims 19 to 21 , wherein the duration of time for which the slurry is heated ranges from 3 hours to 24 hours.

23. A coating composition applied to a substrate, the coating composition comprising the Boehmite according to any of claims 1-22.

24. The coating composition according to claim 23, wherein the substrate is a separator used in an electrochemical cell.

25. The coating composition according to any claims 23 to 24, wherein the coating as applied to the substrate has a thickness of < 3 pm.

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