NANO-SIZED POLYHEDRAL a-ALUMINA PARTICLE AND METHOD FOR PRODUCING SAME

Abstract

The present invention provides a coating agent including -alumina particles having a polyhedral crystal structure and having an average particle size (D50) of 100-900 nm. The -alumina particles are produced in such a way that pseudo-boehmite is mixed with a fluoride-based mineralizer and ultrapure water and pulverized to obtain a powder which is then fired and grown into a polyhedral shape. The polyhedral alumina particles make surface contact and are coated on the surface of a porous polymer substrate, and empty space induced by the interstitial volume between particles is formed larger than that of spherical particles, thereby being capable of achieving excellent air permeability while effectively suppressing thermal contraction of the porous polymer substrate. In addition, due to a nano-level particle size, excellent dispersibility and the formation of a thin coating layer can be achieved.

Claims

1. A coating composition comprising -alumina particles having a polyhedral crystal structure and an average particle diameter (D.sub.50) of 100 to 900 nm.

2. The coating composition according to claim 1, wherein the -alumina particles have an average particle diameter (D.sub.50) of 200 to 600 nm.

3. The coating composition according to claim 1, wherein the polyhedral crystal structure of the -alumina particles includes a 14-hedral crystal structure.

4. The coating composition according to claim 1, wherein the ratio of planes in the polyhedral crystal structure of the -alumina particles is 10 to 20% of the total crystal plane area.

5. A method for producing -alumina particles contained in the coating composition of claim 1, the method comprising: (S1) mixing and reacting an aqueous solution comprising one or more aluminum salts with an aqueous solution containing a pH adjusting agent, and filtering and washing the product to obtain pseudo-boehmite of the following structural formula 1; (S2) mixing the pseudo-boehmite with a fluorine-based mineralizer and ultrapure water, and pulverizing the mixture, followed by filtering and drying; and (S3) filtering and drying the product of step (S2) and then calcining it to obtain a powder of -alumina particles having a polyhedral crystal structure and an average particle diameter (D.sub.50) of 100 to 900 nm. ##STR00003##

6. The method according to claim 5, wherein the aluminum salt used in step (S1) includes aluminum sulfate (Al.sub.2(SO.sub.4).sub.3.Math.418H.sub.2O), aluminum nitrate (Al(NO.sub.3).sub.3.Math.9H.sub.2O), aluminum acetate (Al(CHCOO).sub.3OH), or a mixture thereof.

7. The method according to claim 5, wherein the pH adjusting agent used in step (S1) includes sodium carbonate (Na.sub.2CO.sub.3), sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium carbonate (CaCO.sub.3), or a mixture thereof.

8. The method according to claim 5, wherein the ultrapure water in step (S2) is used at a ratio of 1 to 10 times the weight of pseudo-boehmite.

9. The method according to claim 5, wherein the pulverization in step (S2) is performed for 1 to 100 hours by milling using a plurality of balls having a diameter of 1 to 20 mm.

10. The method according to claim 5, wherein the pseudo-boehmite and the fluoride-based mineralizer in step (S2) are used in a weight ratio of 100:0.1 to 100:2.

11. The method according to claim 5, wherein the fluoride-based mineralizer includes LiF.sub.2, AlF.sub.3, NaF, NaPF.sub.6, K.sub.2TiF.sub.6, MnF.sub.2, or a mixture thereof.

12. The method according to claim 5, wherein the calcination in step (S3) is performed by raising the temperature at 3 to 15 C./min and maintaining the temperature of 800 C. to 1000 C. for 2 to 5 hours.

13. A component comprising a porous polymer substrate and a coating layer formed on one or both sides of the substrate, wherein the coating layer includes the coating composition of claim 1.

14. The component according to claim 13, wherein the component includes a separator for a secondary battery.

15. The component according to claim 13, wherein the component has 50% or more of a dimension retention rate as defined by Equation 1 below, in a thermal stability test using a circular specimen $\begin{array}{cc}\mathrm{Dimension}\mathrm{retention}\mathrm{rate}\left(\%\right)={\left(d_1/d_0\right)}^2& \left[\mathrm{Equation}1\right]\end{array}$ wherein d.sub.0 is the diameter of the circular specimen before heat treatment, and d.sub.1 is the diameter of the circular specimen after heat treatment at 150 C. for 30 minutes.

16. The component according to claim 13, wherein the component exhibits an air permeability of 200 to 211 sec/100 cc, in the air permeability test measuring the time taken for 100 cc of air to permeate the circular specimen with a diameter of 1 inch.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0016] FIGS. 1 to 3 each schematically illustrate the surface contact form when coating a substrate according to the shape of -alumina particles.

[0017] FIG. 4 shows a scanning electron microscope (SEM) photograph and transmission microscope (TEM) photograph of polyhedral -alumina particles prepared in Examples 1 and 2 and Comparative Example 1.

[0018] FIG. 5 shows an SEM photograph of plate-like -alumina particles prepared in Comparative Example 2.

[0019] FIG. 6 shows the observed dimensional change due to heat shrinkage according to the size and type of coated alumina particles of the circular specimen in Experimental Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

[0020] Since the present invention can be modified in various ways and can have various embodiments, specific embodiments will be illustrated in the drawings and described in detail in the detailed description. However, it is not intended to limit the present invention to specific embodiments, and should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention. In describing the present invention, if it is determined that a detailed description of a related known technology may obscure the gist of the present invention, the detailed description thereof will be omitted.

[0021] Hereinafter, the present invention will be described in more detail.

[0022] One embodiment of the present invention relates to a coating composition comprising nano-sized -alumina particles having a polyhedral crystal structure.

[0023] The polyhedral crystal structure crystallographically means that it has about 1 of a ratio (D/H) of the diameter (D) perpendicular to the [0001] plane that is the C plane, and the height (H) parallel thereto.

[0024] In particular, the -alumina particle according to the present invention may have a 14-hedral crystal structure in which the [0001] planes represent 10 to 20%, specifically to 20%, of the total crystal plane area in the polyhedral crystal structure. If the area of the [0001] planes is less than 10%, it has a rod shape, and if the area exceeds 20%, it has a shape close to a plate. Meanwhile, amorphous refers to an irregular state in which the appearance is not uniform and is distinguished from the polyhedral crystal structure of the present invention with clear crystal planes.

[0025] When the alumina particles having a polyhedral crystal structure of the present invention are coated on the surface of a porous substrate, the particles disperse and come into contact with each other, and an empty space is formed by a certain angle formed by meeting of polyhedron crystal planes. The empty space may be referred to as a pore formed by the interstitial volume between particles, and with comparing FIG. 1 and FIG. 3, the empty space formed by polyhedral particles is larger than the empty space formed by spherical particles.

[0026] Referring again to FIG. 3, since the particles of the polyhedral crystal structure form surface contact when coated on the surface of the substrate, the particles are more effective in preventing heat shrinkage of the substrate, compared to spherical particles (FIG. 1) that form point contact on the surface of the substrate.

[0027] Referring to FIG. 2, the plate-like particles form surface contact with the surface of the substrate, so they are excellent in preventing heat shrinkage of the substrate, but as the particles are stacked in a plate shape, the formation of empty spaces is small, which is disadvantageous in terms of air permeability.

[0028] Therefore, the -alumina particles having a polyhedral crystal structure of the present invention can be usefully used as a coating composition that does not impair battery performance since they not only promote thermal stability by effectively suppressing heat shrinkage when coated on a porous polymer substrate such as a secondary battery separator, but also enable smooth movement of lithium ions to the porous substrate.

[0029] In addition, the -alumina particles of the polyhedral crystal structure of the present invention are characterized by an average particle diameter (D.sub.50) in the range of 100 to 900 nm, in particular 200 to 600 nm.

[0030] The D.sub.50 represents the median value in the particle size distribution measured using a conventional method in the art, for example, a laser particle size analyzer. In the present invention, the -alumina particles have a nano level of D.sub.50, so they can improve the dispersion in the coating solution. It is advantageous in that it can reduce the weight and volume of the secondary battery to which the separator is applied, by forming a thin coating layer compared to micro-sized particles.

[0031] When the polyhedral -alumina particles are coated on the surface of a porous substrate such as a separator, it is preferred that the average particle diameter of the alumina particles is selected to be larger than the pore size of the porous substrate to avoid particles filling the pores of the porous substrate.

[0032] Another embodiment of the present invention relates to a method of manufacturing an abrasive comprising the -alumina particles of the polyhedral crystal structure. Hereinafter, the method will be described for each step.

[0033] First, an aqueous solution comprising one or more aluminum salts and an aqueous solution containing a pH adjusting agent are mixed and reacted (S1).

[0034] The aluminum salt may include aluminum sulfate (Al.sub.2(SO.sub.4).sub.3.Math.418H.sub.2O), aluminum nitrate (Al(NO.sub.3).sub.3.Math.9H.sub.2O), aluminum acetate (Al(CHCOO).sub.3OH), or a mixture thereof. For complete dissolution, it may be dissolved in warmed water (e.g., about 60 C.) at a concentration of 5% to 30% to prepare an aqueous solution.

[0035] The pH adjusting agent may include sodium carbonate (Na.sub.2CO.sub.3), sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium carbonate (CaCO.sub.3), or a mixture thereof. For complete dissolution, it may be dissolved in warmed water (e.g., about 40 C.) at a concentration of 5% to 30% to prepare an aqueous solution.

[0036] The aqueous solution of the aluminum salt and the aqueous solution of the pH adjusting agent may be mixed at a constant rate (e.g., 25 ml/min) in the range of room temperature to 95 C. to perform a sol-gel reaction. The pH of the reactant may range from 6 to 10.

[0037] Through the above reaction, pseudo-boehmite with a chemical composition represented by AlO(OH) is produced as a solid as shown in Structural formula 1 below:

##STR00002##

[0038] Pseudo-boehmite of the Structural formula 1 has water (H.sub.2O) bonded to the octahedral unit cell, so it has a high water content and thus has a small crystallite size. Therefore, it can be formed under lower pH conditions than aluminum hydroxide (Al(OH).sub.3), which was mainly used as a starting material in the production of conventional alumina. At a later stage, when it is transformed into -Al.sub.2O.sub.3 through a high-temperature calcinating process, particle agglomeration by seed and phase transition occur at a relatively low temperature, which is advantageous for obtaining a polyhedral crystal structure.

[0039] The pseudo-boehmite solids are filtered and washed, then mixed with a fluorine-based mineralizer and ultrapure water and pulverized (S2).

[0040] The fluoride-based mineralizer is an additive for growing crystals of -alumina particles, and it includes LiF.sub.2, AlF.sub.3, NaF, NaPF.sub.6, K.sub.2TiF.sub.6, MnF.sub.2, or a mixture thereof.

[0041] When used in excessive amounts, such fluoride-based mineralizer may remain in the final -alumina or form aggregates during the calcination process. In order to minimize such disadvantages, it is preferred to use the precursor powder and the fluoride-based mineralizer at a weight ratio of 100:0.1 to 100:2, specifically 100:0.5 to 100:1.5.

[0042] The ultrapure water is intended to increase pulverizing efficiency while promoting wet dispersion of pseudo-boehmite solids and fluoride-based mineralizers and may be used at a ratio of 1 to 10 times the weight of pseudo-boehmite. The wet dispersion promotes uniform dispersion of the fluoride-based mineralizer and minimizes agglomeration of precursor (pseudo-boehmite) particles, thereby affecting the polyhedral crystal structure of the final -alumina particles.

[0043] The pulverization may be performed for 1 to 100 hours by milling using a plurality of balls having a diameter of 1 to 20 mm.

[0044] After filtering and drying the pulverized product, the obtained powder is calcined to obtain a powder of -alumina particles having a nano-sized polyhedral crystal structure (S3).

[0045] The calcination which is a process of melt synthesis by heat treating dry powders at high temperature, may be performed in a crucible made of high purity alumina or zirconia.

[0046] Specifically, the calcination may be performed by raising the temperature at 3 to 15 C./min and maintaining the temperature of 800 C. to 1000 C. for 2 to 5 hours. The calcination condition can be appropriately changed considering the reaction and volatility due to differences in melting points of each material in the mixture and the amount of heat required for synthesis.

[0047] As described above, the nano-sized polyhedral -alumina particles prepared as described above are coated while forming surface contact on the surface of the porous substrate, and the empty space induced by the interstitial volume between particles is formed larger than that of spherical particles, thereby being capable of achieving excellent air permeability while effectively suppressing thermal contraction of the porous polymer substrate.

[0048] Therefore, the present invention further provides a component comprising a porous polymer substrate and a coating layer formed on one or both sides of the substrate, wherein the coating layer includes -alumina particles manufactured according to the present invention and having a polyhedral crystal structure and an average particle diameter (D.sub.50) of 100 to 900 nm.

[0049] In one embodiment of the present invention, the component may include a separator for a secondary battery. The thickness of the porous polymer substrate included in the separator may range from 1 to 100 m, the pore diameter present in the porous substrate may range from 10 to 100 nm, or 10 to 70 nm, or 10 to 50 nm, and the average particle diameter of the polyhedral alumina particles may be selected to be larger than the pore size of the porous substrate.

[0050] In addition, the coating layer may contain a binder to provide binding force of nano-sized polyhedral -alumina to the surface of the substrate, and the binder may be selected from adhesive polyethylene, polypropylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polystyrene, polyacrylic acid, and mixtures thereof. The thickness of the coating layer is not particularly limited, but may range from 0.5 to 50 m or from 1 to 10 m, considering the intended performance of the porous substrate.

[0051] The component having a coating layer comprising nano-sized polyhedral a-alumina particles may have 50% or more of a dimension retention rate as defined by Equation 1 below, in a thermal stability test using a circular specimen.

[00001]$\begin{array}{cc}\mathrm{Dimension}\mathrm{retention}\mathrm{rate}\left(\%\right)={\left(d_1/d_0\right)}^2& \left[\mathrm{Equation}1\right]\end{array}$

[0052] wherein do is the diameter of the circular specimen before heat treatment, and d.sub.1 is the diameter of the circular specimen after heat treatment at 150 C. for 30 minutes.

[0053] In addition, the component may satisfy an air permeability of 215 sec/100 cc or less, for example, 200 to 211 sec/100 cc, in the air permeability test measuring the time taken for 100 cc of air to permeate the circular specimen with a diameter of 1 inch.

MODE FOR CARRYING OUT THE INVENTION

[0054] Hereinafter, the present invention will be described in detail through specific embodiments so that those skilled in the art can easily implement the present invention. However, the present invention may be implemented in many different forms and is not limited to the embodiments described herein.

Example 1

(Step 1)

[0055] An aqueous solution (a) in which 199.8 g of Al.sub.2(SO.sub.4).sub.3.Math.1418H.sub.2O was completely dissolved in 982.8 g of pure water heated to 60 C., and an aqueous solution (b) in which 95.4 g of Na.sub.2CO.sub.3 was completely dissolved in 528 g of pure water heated to 40 C. were prepared. The aqueous solution (b) was added to the aqueous solution (a) at a rate of 25 ml/min and stirred for 10 minutes to react. The reaction product (pH 7.37.8) was filtered and washed to obtain pseudo-boehmite solids.

(Step 2)

[0056] 40 g of the pseudo-boehmite solids and 0.2 g of AlF.sub.3 were mixed with 120 g of ultrapure water and pulverized by milling for 48 hours using balls with a diameter of 5 mm. Then, filtration and drying were performed.

(Step 3)

[0057] The obtained product was filtered and dried, then heat-treated and calcined at 900 C. for 5 hours at a temperature increase of 10 C./min. After heat treatment, powder of -alumina particles was finally obtained.

Example 2

[0058] The same process as Example 1 was performed except that in step 2, pulverization by ball milling was performed for 24 hours.

Comparative Example 1

[0059] The same process as Example 1 was performed except that ball milling was not performed in Step 2.

Comparative Example 2

[0060] An aqueous solution (a) in which 199.8 g of Al.sub.2(SO.sub.4).sub.3.Math.1418H.sub.2O was completely dissolved in 982.8 g of pure water heated to 60 C., and an aqueous solution (b) in which 72 g of NaOH was completely dissolved in 528 g of pure water heated to 40 C. were prepared. The aqueous solution (b) was added to the aqueous solution (a) at a rate of 25 ml/min and stirred for 10 minutes to react. The reaction product (pH 7.37.8) was filtered, washed, dried, and then pulverized to obtain pseudo-boehmite powders.

[0061] g of the pseudo-boehmite powders and 0.8 g of AlF.sub.3 were mixed in a dry manner. The mixed powders were heat-treated and calcined at 900 C. for 5 hours at a temperature increase of 10 C./min. After heat treatment, powder of -alumina particles was finally obtained.

[0062] The physical properties of the -alumina particles prepared in Examples and Comparative Examples were measured and shown in Table 1 below.

TABLE-US-00001 TABLE 1 Production condition Pseudo- Result Ball Pore boehmite:AlF.sub.3 Morphology Raw milling volume.sup.1) Mixing (weight (crystallite D.sub.50.sup.2) material time (ml/g) mode ratio) structure) (m) Ex. 1 Pseudo- 48 0.266 Wet 100:0.5 Polyhedral 0.25 2 boehmite 24 0.291 100:0.5 Polyhedral 0.52 Comp. 1 Pseudo- 0 0.332 Wet 100:0.5 Polyhedral 1.9 Ex. boehmite 2 Aluminum 0 0.340 Dry 100:2.0 Plate-like 10.6 hydroxide .sup.1)Pore volume was measured using the gas adsorption method at the Korea Polymer Testing Laboratory. .sup.2)D.sub.50 was measured using an in-house particle distribution meter (CILAS 1090).

[0063] As can be seen in Table 1, the -alumina particles prepared according to Examples by pulverizing pseudo-boehmite, wet mixing with a fluoride-based mineralizer, and then calcining had a nano-sized average diameter (D.sub.50) and exhibited a polyhedral crystal structure with a ratio of D.sub.50 and thickness close to 1.

[0064] Meanwhile, SEM and TEM observation photos of the -alumina particles prepared in Examples 1 and 2 and Comparative Example 1 are shown in FIG. 4.

[0065] From FIG. 4, it was found that the -alumina particles of Examples 1 and 2 have a nano-sized 14-hedral crystal structure, and the -alumina particles of Comparative Example 1 have a micro-sized 14-hedral crystal structure.

[0066] FIG. 5 is an SEM photograph of the -alumina particles prepared in Comparative Example 2, showing the plate-like structure.

Experimental Example 1: Air Permeability Test for Coating Secondary Battery Separator

[0067] A coating layer was formed on one side of the secondary battery separator using each of the -alumina particles prepared in Examples and Comparative Examples. Specifically, the slurry obtained by dispersing -alumina particles and acrylic polymer binder in water at a ratio of 95:5 was coated on one side of a polyethylene (PE) porous substrate (11 m thick) and then dried to form a 2 m thick coating layer.

[0068] The porous substrate on which the coating layer of -alumina particles was formed was manufactured into a circular specimen with a diameter of 1 inch, and then the time taken for 100 cc of air to permeate through each specimen was measured using an air permeability measuring device (Asahi Seiko) (measurement condition: 25 C.). The results are shown in Table 2 below.

TABLE-US-00002 TABLE 2 Air permeability (sec/100 cc) PE porous substrate 150 Example 1 211 Example 2 203 Comparative Example 1 185 Comparative Example 2 323

[0069] From Table 2, it can be seen that the PE substrate coated with the nano-sized polyhedral -alumina particles prepared in Examples 1 and 2 has excellent air permeability since the substrate has a shorter time for air to permeate compared to the substrate coated with plate-like alumina particles of Comparative Example 2.

[0070] On the other hand, the substrate coated with the alumina particles of Comparative Example 1 has excellent air permeability due to the micro particle size, but is disadvantageous in terms of thermal stability as shown in Experimental Example 2 below.

Experimental Example 2: Thermal Stability Test for Coating Secondary Battery Separator

[0071] A thermal stability test according to particle size was performed on -alumina particles with a polyhedral crystal structure that were confirmed to have excellent breathability in Experimental Example 1.

[0072] First, four types of polyhedral -alumina particles with different particle sizes (100 nm, 250 nm, 500 nm, and 2 m) and general spherical -alumina particles (average particle diameter: 500700 nm) were prepared, and each type of particles was used to form a 2 m thick coating layer on the PE porous substrate through the same process as in Experimental Example 1 to prepare a circular specimen with a diameter of 18 mm. The circular specimen was manufactured in triplicate for each of the coated alumina particles.

[0073] Each of the circular specimens was subjected to heat treatment at 150 C. for 30 minutes, and then the dimensional change was observed, and the results are shown in FIG. 6.

[0074] In addition, after heat treatment for each specimen, the dimension retention rate as defined by Equation 1 below was measured, and the results are shown in Table 3 below.

[00002]$\begin{array}{cc}\mathrm{Dimension}\mathrm{retention}\mathrm{rate}\left(\%\right)={\left(d_1/d_0\right)}^2& \left[\mathrm{Equation}1\right]\end{array}$

[0075] wherein d.sub.0 is the diameter of the circular specimen before heat treatment, and d.sub.1 is the diameter of the circular specimen after heat treatment at 150 C. for 30 minutes.

TABLE-US-00003 TABLE 3 Dimension retention rate (%) Room After heat treatment temperature at 150 C. PE 100 0 Polyhedral particle (100 nm) 100 52.1 coating Polyhedral particle (250 nm) 100 66.6 coating Polyhedral particle (500 nm) 100 60.5 coating Polyhedral particle (2 m) coating 100 57.9 Spherical particle (500 nm) coating 100 35.3

[0076] Referring to Table 3 and FIG. 6, it can be seen that polyhedral particles have superior thermal stability after being coated on a PE porous substrate compared to spherical particles.

[0077] Although embodiments according to the present invention have been described above, they are merely illustrative, and those skilled in the art will understand that various modifications and equivalent scope of embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention should be determined by the following patent claims.