Nano-sized polyhedral α-alumina particles and their manufacturing method

Abstract

The present invention relates to a polyhedral crystal structure having an average particle size (D 50 The present invention provides a coating agent including α-alumina particles having a particle diameter of 100 to 900 nm. The α-alumina particles are manufactured by mixing pseudoboehmite with a fluoride-based mineralizer and ultrapure water, grinding the mixture, and then sintering the resulting powder to grow into polyhedrons. The polyhedral alumina particles are coated on the surface of a porous polymer substrate while forming surface contact, and the vacant space induced by the interstitial volume between the particles is larger than that of spherical particles, thereby effectively suppressing the thermal shrinkage of the porous polymer substrate and realizing excellent breathability. In addition, the nano-level particle size allows for excellent dispersibility and the formation of a thin coating layer.

Description

[Technical Field]

[0001] The present invention relates to α-alumina particles having a polyhedral crystal structure and nano-size, which are useful as coating materials for components such as secondary battery separators, and a method for producing the same. [Background technology]

[0002] Alumina (Al2O3) has excellent mechanical strength, including abrasion resistance, chemical stability, thermal conductivity, and heat resistance, and is used in a wide range of applications, including abrasives, electronic materials, heat-dissipating fillers, optical materials, and biomaterials. Alumina can be α-, γ-, or η-crystalline alumina, or amorphous alumina, but is generally referred to as α-alumina, and its uses vary depending on particle size, shape, surface properties, and degree of aggregation.

[0003] Recently, alumina has been used as a surface coating to impart thermal stability to separators in secondary batteries used in various electric / electronic devices, including mobile devices and electric vehicles.

[0004] In secondary batteries, the separator separates the positive and negative electrodes to prevent electrical shorts and absorbs the electrolyte necessary for battery reactions to maintain high ionic conductivity. To this end, it is typically made of a porous polymer substrate (e.g., polyolefin). The porous polymer substrate tends to shrink when exposed to heat, which can cause contact between the positive and negative electrodes, potentially resulting in safety issues such as fire and explosion. To address this issue, inorganic particles such as alumina are coated on one or both sides of the porous polymer substrate together with a binder to protect the separator from breakage and prevent thermal shrinkage.

[0005] The alumina used to coat these separators is mostly spherical or amorphous particles. The spherical alumina is coated on the surface of the separator, creating interstitial volume between the particles, which maintains the separator's breathability and allows for smooth ion movement within the battery. However, because spherical alumina is coated on the separator surface by forming point contacts, it has the disadvantage of being less able to suppress shrinkage when the separator is deformed by heat (see Figure 1). Furthermore, amorphous particles have an inconsistent shape, which increases the likelihood of coating defects on the separator surface.

[0006] Meanwhile, Korean Patent Publication No. 10-2018-0010477 (applicant: CIS) produced micro-sized plate-like alumina by mixing aluminum hydroxide, ammonium chloride, and sodium polyphosphate in a solvent, followed by high-temperature heat treatment, filtration, washing, and dry grinding to provide alumina for coating separators in secondary batteries. This plate-like alumina is excellent for preventing thermal shrinkage by coming into surface contact with the separator surface, but because it is layered and coated in plate-like layers on the separator surface through surface contact, it can clog the separator pores, reducing ion migration and resulting in reduced battery performance (see Figure 2). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Republic of Korea Patent Publication No. 10-2018-0010477 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention overcomes the drawbacks of the prior art by providing a coating agent containing α-alumina particles that can form surface contact with the surface of a porous substrate such as a separator for a secondary battery, thereby improving heat shrinkage resistance and realizing excellent breathability, and a method for manufacturing the same. [Means for solving the problem]

[0009] One aspect of the present invention is a method for producing a polyhedral crystal structure having an average particle size (D 50 The present invention provides a coating agent containing α-alumina particles having a particle size of 100 to 900 nm.

[0010] Another aspect of the present invention is a method for producing α-alumina particles contained in the coating agent, comprising: (Step S1) mixing and reacting an aqueous solution containing one or more aluminum salts with an aqueous solution containing a pH adjuster, filtering and washing the resulting product to obtain pseudoboehmite of the following structural formula 1; (Step S2) mixing the pseudoboehmite with a fluorine-based mineralizer and ultrapure water, pulverizing the mixture, filtering and drying the mixture; and (Step S3) filtering and drying the product of Step S2, and then calcining the mixture to obtain α-alumina particles having a polyhedral crystal structure and an average particle size (D 50 obtaining a powder of α-alumina particles having a particle diameter of 100 to 900 nm.

[0011] [ka] (Structural formula 1)

[0012] In yet another aspect of the present invention, there is provided a member including a porous polymer substrate and a coating layer formed on one or both surfaces of the substrate, wherein the coating layer includes a coating agent including the nano-sized polyhedral α-alumina particles. [Effects of the Invention]

[0013] The α-alumina particles contained in the coating agent of the present invention are produced by mixing pseudoboehmite with a fluoride-based mineralizer and ultrapure water, grinding the resulting powder, and then calcining it to grow it into a polyhedron. The polyhedral alumina particles are coated onto the surface of the porous polymer substrate while forming surface contact. The interstitial volume between the particles creates larger void spaces than spherical particles, effectively suppressing thermal shrinkage of the porous polymer substrate and achieving excellent breathability. In addition, the nano-level particle size allows for excellent dispersibility, enabling the formation of a thin coating layer. [Brief explanation of the drawings]

[0014] [Figure 1] 1A and 1B are schematic illustrations of the surface contact morphology of α-alumina particles when coating a substrate. [Figure 2] 1A and 1B are schematic illustrations of the surface contact morphology of α-alumina particles when coating a substrate. [Figure 3] 1A and 1B are schematic illustrations of the surface contact morphology of α-alumina particles when coating a substrate. [Figure 4] 2 shows scanning electron microscope (SEM) and transmission electron microscope (TEM) photographs of polyhedral α-alumina particles prepared in Examples 1 and 2 and Comparative Example 1. [Figure 5] 1 is an SEM photograph of plate-shaped α-alumina particles produced in Comparative Example 2. [Figure 6] Dimensional changes due to thermal shrinkage were observed for each size and type of alumina particles coated on circular specimens in Experimental Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0015] Since the present invention can be modified in various ways and has various embodiments, specific embodiments are illustrated in the drawings and described in detail in the detailed description. However, it should be understood that this is not intended to limit the present invention to only the specific embodiments, but also to include all modifications, equivalents, or alternatives within the spirit and technical scope of the present invention. In describing the present invention, if it is determined that detailed descriptions of related publicly known technologies may obscure the gist of the present invention, such detailed descriptions will be omitted.

[0016] The present invention will be described in more detail below.

[0017] One embodiment of the present invention relates to a coating agent comprising α-alumina particles having a polyhedral crystal structure and nano-size.

[0018] The polyhedral crystal structure means that the ratio (D / H) of the diameter (D) perpendicular to the crystallographic C-plane (0001) to the height (H) parallel to it is close to 1.

[0019] In particular, the α-alumina particles according to the present invention may have a tetradecahedral crystal structure in which the <0001> planes account for 10-20%, specifically 15-20%, of the total crystal face area. If the <0001> plane area is less than 10%, the particles will be rod-shaped, and if it exceeds 20%, the particles will be closer to a plate-like shape. Meanwhile, "amorphous" refers to an irregular state in which the outer shape is not uniform, and is distinguished from the polyhedral crystal structure of the present invention, which has distinct crystal faces.

[0020] When the alumina particles having a polyhedral crystal structure of the present invention are coated on the surface of a porous substrate, void spaces are formed due to the angles at which the polyhedral crystal faces meet as the particles are dispersed and in contact with each other. These void spaces are referred to as pores formed by interstitial volume between particles. Comparing Figures 1 and 3, the void spaces formed by polyhedral particles are larger than those formed by spherical particles.

[0021] Referring again to FIG. 3, the particles having the polyhedral crystal structure form surface contact when coated on the surface of a substrate, and are therefore more effective in preventing thermal shrinkage of the substrate than spherical particles (FIG. 1) which form point contact with the surface of the substrate.

[0022] On the other hand, referring to FIG. 2, the plate-shaped particles form surface contact with the surface of the substrate, which is effective in preventing thermal shrinkage of the substrate. However, since the particles are stacked in a plate-like shape, there is little free space, which is disadvantageous in terms of breathability.

[0023] Therefore, the α-alumina particles having a polyhedral crystal structure according to the present invention can be effectively used as a coating agent that not only effectively suppresses thermal shrinkage when coated on a porous polymer substrate such as a separator for a secondary battery, thereby improving thermal stability, but also enables smooth movement of lithium ions to the porous substrate, thereby preventing deterioration of battery performance.

[0024] In addition, the α-alumina particles of the present invention having a polyhedral crystal structure have an average particle size (D 50 ) is in the range of 100 to 900 nm, specifically 200 to 600 nm.

[0025] The above D 50 is a median value of the particle size distribution measured by a method commonly used in the art, for example, using a laser particle size analyzer. In the present invention, D 50 The nano-sized particles have the advantage of improving dispersibility in the coating solution and forming a thinner coating layer than micro-sized particles, thereby reducing the weight and volume of the secondary battery to which the separator is applied.

[0026] Meanwhile, when the polyhedral α-alumina particles are coated on the surface of a porous substrate such as a separator, the average particle size of the alumina particles is preferably selected to be larger than the pore size of the porous substrate to prevent the particles from filling the pores of the porous substrate.

[0027] Another embodiment of the present invention relates to a method for producing an abrasive containing the α-alumina particles having the polyhedral crystal structure. The method will be described below step by step.

[0028] First, an aqueous solution containing one or more aluminum salts and an aqueous solution containing a pH adjuster are mixed and reacted (step S1).

[0029] The aluminum salt may include aluminum sulfate (Al2(SO4)3·4~18H2O), aluminum nitrate (Al(NO3)3·9H2O), aluminum acetate (Al(CHCOO)3OH), or a mixture thereof, and is dissolved in heated water (e.g., about 60°C) at a concentration of 5 to 30% to prepare an aqueous solution.

[0030] The pH adjuster may include sodium carbonate (Na2CO3), sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium carbonate (CaCO3), or a mixture thereof, and is dissolved in heated water (e.g., about 40°C) at a concentration of 5 to 30% to prepare an aqueous solution.

[0031] The aluminum salt aqueous solution and the pH adjuster aqueous solution may be mixed at a constant rate (e.g., 25 ml / min) at room temperature to 95° C. The pH of the reactant may range from 6 to 10.

[0032] Through the reaction, pseudoboehmite, whose chemical composition is expressed as AlO(OH) as shown in the following structural formula 1, is produced as a solid.

[0033] [ka] (Structural formula 1)

[0034] Pseudoboehmite of structural formula 1 has a high water content due to water (H2O) being bound to the octahedral unit cell, resulting in a small crystallite size. Therefore, it is formed under lower pH conditions than aluminum hydroxide (Al(OH)3), which is the starting material commonly used in the production of conventional alumina. When it is subsequently transformed into α-Al2O3 through a high-temperature calcination process, particle aggregation and phase transition due to seeds occur at a relatively low temperature, which is advantageous for obtaining a polyhedral crystal structure.

[0035] The pseudoboehmite solid is filtered and washed, then mixed with a fluorine-based mineralizer and ultrapure water, and pulverized (step S2).

[0036] The fluoride-based mineralizer is an additive for growing the crystals of α-alumina particles, and LiF2, AlF3, NaF, NaPF6, K2TiF6, MnF2, or a mixture thereof is used.

[0037] When used in excess, such fluoride-based mineralizers may remain in the final α-alumina or form agglomerates during the calcination process. To minimize such drawbacks, it is advantageous to use the precursor powder and the fluoride-based mineralizer in a weight ratio of 100:0.1 to 100:2, more preferably 100:0.5 to 100:1.5.

[0038] The ultrapure water is used in a ratio of 1 to 10 times the weight of the pseudoboehmite to improve the efficiency of grinding while wet-dispersing the pseudoboehmite solids and the fluoride-based mineralizer. The wet dispersion uniformly disperses the fluoride-based mineralizer and minimizes the aggregation of the precursor (pseudoboehmite) particles, thereby affecting the polyhedral crystal structure of the resulting α-alumina particles.

[0039] The pulverization is carried out by a milling method using a plurality of balls having a diameter of 1 to 20 mm for 1 to 100 hours.

[0040] The pulverized product is filtered and dried, and then the obtained powder is calcined to obtain a powder of α-alumina particles having a nano-sized polyhedral crystal structure (step S3).

[0041] The firing is a process of melting and synthesizing the dried powder by heat treatment at high temperature, and is carried out in a crucible made of high-purity alumina or zirconia.

[0042] Specifically, the calcination is carried out by increasing the temperature at a rate of 3 to 15°C / min and then maintaining the temperature at 800 to 1000°C for 2 to 5 hours. Meanwhile, the calcination conditions can be appropriately changed taking into consideration the reactivity and volatility due to the difference in melting point between the materials in the mixture and the amount of heat required for synthesis.

[0043] The nano-sized polyhedral α-alumina particles prepared as described above are coated on the surface of the porous substrate while forming surface contact, and the void space induced by the interstitial volume between the particles is larger than that of spherical particles, thereby effectively suppressing the thermal shrinkage of the porous substrate and realizing excellent breathability.

[0044] Therefore, the present invention further provides a member comprising a porous polymer substrate and a coating layer formed on one or both sides of the substrate, wherein the coating layer is manufactured according to the present invention, has a polyhedral crystal structure, and has an average particle size (D 50 The present invention provides a member containing α-alumina particles having a particle size of 100 to 900 nm.

[0045] 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 be in the range of 1 to 100 μm, the pore diameter of the porous substrate may be in the range of 10 to 100 nm, 10 to 70 nm, or 10 to 50 nm, and the average particle size of the polyhedral alumina particles may be selected to be larger than the pore size of the porous substrate.

[0046] The coating layer may contain a binder to provide adhesion of the nano-sized polyhedral α-alumina to the surface of the substrate, and the binder may be selected from adhesive polyethylene, polypropylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polystyrene, polyacrylic, and mixtures thereof. The thickness of the coating layer is not particularly limited, but is in the range of 0.5 to 50 μm or 1 to 10 μm, taking into account the intended performance of the porous substrate.

[0047] A member having a coating layer containing the nano-sized polyhedral α-alumina particles has a dimensional retention rate of 50% or more, as defined by the following equation 1, in a thermal stability test using a circular specimen.

[0048] (Number 1) Dimensional retention rate (%) = (d1 / d0) 2

[0049] In the above formula, d0 is the diameter of the circular specimen before heat treatment, and d1 is the diameter of the circular specimen after heat treatment at 150°C for 30 minutes.

[0050] Furthermore, the member can satisfy an air permeability of 215 sec / 100 cc or less, for example, 200 to 211 sec / 100 cc, in an air permeability test that measures the time it takes for 100 cc of air to permeate a circular test piece with a diameter of 1 inch.

[0051] While the present invention may be embodied in many different forms, it is to be understood that the invention is not limited to the specific embodiments set forth herein. [Example]

[0052] Example 1:

[0053] (Stage 1) Aqueous solution (a) was prepared by completely dissolving 199.8 g of Al2(SO4)3·14~18H2O in 982.8 g of pure water heated to 60°C, and aqueous solution (b) was prepared by completely dissolving 95.4 g of Na2CO3 in 528 g of pure water heated to 40°C. Aqueous solution (b) was added to aqueous solution (a) at a rate of 25 mL / min and stirred for 10 minutes to allow the reaction to proceed. The reaction product (pH 7.3~7.8) was filtered and washed to obtain pseudoboehmite solids.

[0054] (Stage 2) 40 g of the pseudoboehmite solid and 30.2 g of AlF were mixed with 120 g of ultrapure water, and the mixture was milled using a ball with a diameter of 5 mm for 48 hours, followed by filtration and drying.

[0055] (Stage 3) The obtained product was filtered and dried, and then calcined by heat treatment at 900°C for 5 hours with a temperature increase rate of 10°C / min. After the heat treatment, α-alumina particle powder was finally obtained.

[0056] Example 2: The same process as in Example 1 was carried out, except that in step 2, the grinding by ball milling was carried out for 24 hours.

[0057] Comparative Example 1: The same process as in Example 1 was carried out, except that ball milling was not performed in step 2.

[0058] Comparative Example 2: Aqueous solution (a) was prepared by completely dissolving 199.8 g of Al2(SO4)3·14~18H2O in 982.8 g of pure water heated to 60°C, and aqueous solution (b) was prepared by completely dissolving 72 g of NaOH in 528 g of pure water heated to 40°C. Aqueous solution (b) was added to aqueous solution (a) at a rate of 25 mL / min and stirred for 10 minutes to allow the reaction to proceed. The reaction product (pH 7.3~7.8) was filtered, washed, dried, and then crushed to obtain pseudoboehmite powder.

[0059] 40 g of the pseudoboehmite powder and 30.8 g of AlF were dry-mixed. The mixed powder was heat-treated at 900°C for 5 hours with a temperature increase rate of 10°C / min. After the heat treatment, α-alumina particle powder was finally obtained.

[0060] The physical properties of the α-alumina particles prepared in the examples and comparative examples were measured and are shown in Table 1 below.

[0061] [Table 1]

[0062] As can be seen from Table 1, the α-alumina particles prepared by crushing pseudoboehmite, wet mixing it with a fluoride-based mineralizer, and then calcining it according to the examples have an average nano-sized diameter (D 50 ) while D 50 The polyhedral crystal structure was observed with a thickness ratio close to 1.

[0063] Meanwhile, SEM and TEM photographs of the α-alumina particles prepared in Examples 1 and 2 and Comparative Example 1 are shown in FIG.

[0064] From FIG. 4, it can be seen that the α-alumina particles of Examples 1 and 2 have a nano-sized tetradecahedral crystal structure, while the α-alumina particles of Comparative Example 1 have a micro-sized tetradecahedral crystal structure.

[0065] FIG. 5 is an SEM photograph of the α-alumina particles produced in Comparative Example 2, in which the plate-like structure can be seen.

[0066] Experimental example 1: Air permeability test when coating secondary battery separator The α-alumina particles prepared in the examples and comparative examples were used to form a coating layer on one side of a secondary battery separator. Specifically, the α-alumina particles and an acrylic polymer binder were dispersed in water at a ratio of 95:5 to form a slurry, which was then coated on one side of a polyethylene (PE) porous substrate (thickness: 11 μm) and dried to form a coating layer with a thickness of 2 μm.

[0067] The porous substrates with the α-alumina particle coating layer formed thereon were cut into circular specimens with a diameter of 1 inch, and the time required for 100 cc of air to permeate each specimen was measured using an air permeability tester (ASAHI SEIKO) (measurement conditions: 25°C). The results are shown in Table 2 below.

[0068] [Table 2]

[0069] From Table 2, it can be seen that the PE substrates coated with nano-sized polyhedral α-alumina particles prepared in Examples 1 and 2 have excellent breathability because the time it takes for air to permeate is shorter than that of the substrate coated with plate-like alumina particles in Comparative Example 2. Meanwhile, the alumina particles in Comparative Example 1 have the best breathability due to their micro-size, but are inferior in thermal stability as shown in Experimental Example 2 below.

[0070] Experimental Example 2: Thermal stability test when coating secondary battery separator The α-alumina particles having a polyhedral crystal structure, which were confirmed to have excellent air permeability in Experimental Example 1, were subjected to a thermal stability test according to size.

[0071] First, four types of polyhedral α-alumina particles with different particle sizes (100 nm, 250 nm, 500 nm, and 2 μm) and common spherical α-alumina particles (average particle size: 500-700 nm) were prepared. Each particle was used to form a 2 μm-thick coating layer on a PE porous substrate using the same process as in Experimental Example 1, and then circular specimens with a diameter of 18 mm were manufactured. Three circular specimens were manufactured for each coated alumina particle.

[0072] Each of the circular specimens was heat-treated at 150° C. for 30 minutes, and then the dimensional changes were observed. The results are shown in FIG.

[0073] After the heat treatment of each specimen, the dimensional retention rate defined by the following equation 1 was measured, and the results are shown in Table 3 below.

[0074] (Number 1) Dimensional retention rate (%) = (d1 / d0) 2

[0075] In the above formula, d0 is the diameter of the circular specimen before heat treatment, and d1 is the diameter of the circular specimen after heat treatment at 150°C for 30 minutes.

[0076] [Table 3]

[0077] Referring to Table 3 and FIG. 6, it can be seen that the polyhedral particles have superior thermal stability after being coated on the PE porous substrate compared to the spherical particles.

[0078] Although the present invention has been described in detail above, it is obvious to those skilled in the art that the specific details are merely preferred embodiments and are not intended to limit the scope of the present invention. Therefore, the true scope of the present invention is defined by the claims and their equivalents.

Claims

1. It has a polyhedral crystal structure and an average particle size (D ５０ ) is 100 to 900 nm, The coating agent, wherein the ratio of [0001] faces in the polyhedral crystal structure of the α-alumina particles is 10 to 20% of the total crystal face area.

2. The average particle size (D ５０ 2. The coating agent according to claim 1, wherein the average particle diameter of the nanoparticles is 200 to 600 nm.

3. 2. The coating agent according to claim 1, wherein the polyhedral crystal structure of the α-alumina particles comprises a tetradecahedral crystal structure.

4. A method for producing α-alumina particles contained in the coating agent of claim 1, comprising the steps of: (Step S1) mixing and reacting an aqueous solution containing one or more aluminum salts with an aqueous solution containing a pH adjuster, and filtering and washing the product to obtain pseudoboehmite of the following structural formula 1: (Step S2) Mixing the pseudoboehmite with a fluorine-based mineralizer and ultrapure water, pulverizing the mixture, filtering, and drying; (Step S3) The product of step S2 is filtered and dried, and then calcined to obtain a polyhedral crystal structure having an average particle size (D ５０ and obtaining a powder of α-alumina particles having a particle size of 100 to 900 nm. 【Chemical 1】

5. The aluminum salt used in step S1 is aluminum sulfate (Al ２ (SO ４ ) ３ ・4 to 18 hours ２ O), aluminum nitrate (Al(NO ３ ) ３ ・9H ２ O), aluminum acetate (Al(CHCOO) ３ 5. The method of claim 4, comprising:

6. The pH adjuster used in step S1 is sodium carbonate (Na ２ CO ３ ), sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium carbonate (CaCO ３ 5. The method of claim 4, comprising:

7. The method of claim 4, wherein the ultrapure water is used in step S2 in a ratio of 1 to 10 times the weight of the pseudoboehmite.

8. The method according to claim 4, wherein the pulverization in step S2 is carried out for 1 to 100 hours by a milling method using a plurality of balls having a diameter of 1 to 20 mm.

9. 5. The method of claim 4, wherein in step S2, the pseudoboehmite and the fluorine-based mineralizer are used in a weight ratio of 100:0.1 to 100:

2.

10. The fluorine-based mineralizer is LiF ２ , AlF ３ , NaF, NaPF ６ , K. ２ TiF ６ , MnF ２ or a mixture thereof.

11. 5. The method according to claim 4, wherein the firing in step S3 is performed by increasing the temperature at a rate of 3 to 15° C. / min and then maintaining the temperature at 800 to 1000° C. for 2 to 5 hours.

12. A member comprising a porous polymer substrate and a coating layer formed on one or both surfaces of the substrate, A member, wherein the coating layer comprises the coating agent according to claim 1 .

13. The component of claim 12 , wherein the component comprises a separator for a secondary battery.

14. The member according to claim 12, wherein the member has a dimensional retention rate of 50% or more, as defined by the following Equation 1, in a thermal stability test using a circular specimen: (Equation 1) Dimensional retention rate (%) = (d １ / d ０ ) ２ In the above formula, d ０ is the diameter of the circular specimen before heat treatment, and d １ is the diameter of the circular specimen after heat treatment at 150°C for 30 minutes.

15. 13. The member according to claim 12, wherein the member exhibits an air permeability of 200 to 211 seconds / 100 cc in a test measuring the time it takes for 100 cc of air to permeate a circular specimen having a diameter of 1 inch.

Images