Aqueous coating composition and separator for secondary battery using the same

Abstract

The present invention relates to a water-based coating composition that forms a coating layer with excellent flexibility, adhesive strength, heat resistance, and coating properties, and a separator for a secondary battery coated therewith.

Description

Technology Field

[0001] The present invention relates to a water-based coating composition that forms a coating layer with excellent flexibility, adhesive strength, heat resistance, and coating properties, and a separator for a secondary battery coated therewith. Background Technology

[0002] Lithium-ion rechargeable batteries are widely used as power sources for portable electronic devices such as mobile phones, laptops, and camcorders, as well as electric vehicles. Polyolefin-based microporous membranes, used as separators in lithium-ion rechargeable batteries, are closely related to battery performance; therefore, to achieve excellent battery performance, these membranes require superior mechanical properties, heat resistance, and stability.

[0003] Polyolefin-based porous micromembranes are suitable for use as separators in small batteries due to their excellent chemical stability and battery properties. However, while their relatively low melting point poses no problem when applied to small batteries, ensuring thermal stability is paramount when used in high-capacity, high-output batteries that require high energy density and discharge voltage; in such applications, deformation of the separator caused by rising battery temperatures poses a very high risk of ignition or explosion.

[0004] In order to prevent damage to the membrane caused by the high temperature mentioned above, an organic-inorganic composite porous membrane is proposed in which a slurry of heat-resistant inorganic particles and a binder is coated onto a porous microporous membrane substrate, thereby suppressing deformation of the substrate and ensuring thermal and electrochemical stability through the coating layer.

[0005] However, while conventional separator coating compositions exhibit excellent heat resistance, coating properties, and adhesive strength, there is a problem where the coating layer detaches at the cut sites during the process of repeated cutting in the battery manufacturing process. Therefore, there is a need to develop a coating composition with excellent adhesion to the separator. The problem to be solved

[0006] The present invention provides a water-based coating composition that forms a coating layer with excellent flexibility, adhesive strength, heat resistance, and coating properties, and a separator for a secondary battery coated therewith. means of solving the problem

[0007] The present invention provides a water-based coating composition comprising, based on the total weight of the coating composition, 30 to 60 weight% of ceramic particles, 0.1 to 10 weight% of a binder, and 0.1 to 10 weight% of a thickener, wherein the thickener comprises acrylamide, octylinone, and methylisothiazolinone, and a separator for a secondary battery coated therewith. Furthermore, the present invention provides a lithium-ion secondary battery comprising the aforementioned separator. Effects of the invention

[0008] The present invention provides a water-based coating composition that forms a coating layer with excellent flexibility, adhesive strength, heat resistance, and coating properties. The water-based coating composition according to the present invention forms a coating layer with excellent flexibility, thereby minimizing the detachment of the fine coating layer at the cutting site even when multiple repetitive cuttings are performed during a battery manufacturing process using a separator containing the same, while simultaneously improving adhesive strength to provide excellent heat resistance and coating properties. Specific details for implementing the invention

[0009] The present invention will be described in detail below. However, it is not limited to the following description, and each component may be modified in various ways or selectively combined as needed. Accordingly, it should be understood that the invention includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the invention.

[0011] Water-based coating composition

[0012] The water-based coating composition of the present invention comprises ceramic particles, a binder, and a thickener.

[0014] ceramic particles

[0015] The water-based coating composition of the present invention comprises ceramic particles. The ceramic particles used in the relevant technical field may be used without special limitations.

[0016] The ceramic particles mentioned above include Al2O3, BaTiO3, CaO, CeO2, NiO, MgO, SiO2, SnO2, SrTiO3, TiO2, Y2O3, ZnO, ZrO2, Pb(Zr,Ti)O3(PZT), and Pb 1-x La x Zr 1-y Ti y O3(PLZT), PB(Mg3Nb 2 / 3 O3-PbTiO3 (PMN-PT), HfO2, etc. can be used, but are not limited to these. The ceramic particles can be used alone or in a mixture of two or more types.

[0017] Based on the total weight of the coating composition, the ceramic particles may comprise 30 to 60 weight percent, for example, 40 to 50 weight percent. If the content of the ceramic particles is less than the aforementioned range, the content of the binder becomes excessively large, which may lead to a decrease in pore size and porosity and an increase in ionic resistance due to a reduction in the empty space formed between the ceramic particles, thereby causing a degradation in battery performance. On the other hand, if it exceeds the aforementioned range, the adhesion to the ceramic particles and the adhesion between the separator and the electrode may decrease because the content of the binder is too low.

[0019] bookbinder

[0020] The water-based coating composition of the present invention comprises a binder. As the binder, any binder used in the relevant technical field may be used without special limitations.

[0021] Non-limiting examples of usable binders include polyvinylidene fluoride, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polyacrylonitrile, e.g., butyl acrylate-acrylonitrile copolymer, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate copolymer, polyethylene oxide, cellulose acetate, and cellulose acetate butyrate. Examples include cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, acrylonitrile-styrene-butadiene copolymer, and polyimide. The above binders may be used alone or in a mixture of two or more. The above binders may be used in a water-dispersed state.

[0022] Based on the total weight of the coating composition, the composition may contain 0.1 to 10 weight percent, for example, 0.5 to 3 weight percent of a binder. In the present invention, by increasing the binder content and decreasing the thickener content, the viscosity of the coating composition can be lowered while improving the adhesive strength. If the binder content is below the aforementioned range, the adhesive strength may decrease, and if it exceeds the aforementioned range, the battery performance may decrease as the pore size and porosity decrease and the ionic resistance increases due to the reduction of the empty space formed between the ceramic particles.

[0024] Thickener

[0025] The water-based coating composition of the present invention includes a thickener. The thickener includes a polyacrylamide-based thickener comprising acrylamide, octylinone, and methylisothiazolinone. In the present invention, by using a thickener of the above composition, heat resistance can be maintained even when the thickener content is reduced.

[0026] For example, the polyacrylamide-based thickener may comprise, based on the total weight of the polyacrylamide-based thickener, 0.01 to 6 weight% of acrylamide, for example, 0.5 to 1.8 weight%, 0.01 to 1 weight% of octylinone, for example, 0.1 to 0.6 weight%, and 0.01 to 3 weight% of methylisothiazolinone, for example, 0.4 to 1.2 weight%. If the content of acrylamide is below the aforementioned range, heat resistance may be reduced, and if it exceeds the aforementioned range, it may dissolve in an unstable state, resulting in an insufficient thickening effect. If the content of octylinone is below the aforementioned range, dispersion and mixing may be insufficient, and if it exceeds the aforementioned range, it may affect the polymer structure and interfere with intermolecular interactions, thereby reducing its function as a thickener. If the content of methylisothiazolinone is below the aforementioned range, the thickener may deteriorate or its performance may be reduced, and if it exceeds the aforementioned range, the polymer structure may become unstable, and the thickening performance, such as viscosity or cohesiveness, may be reduced.

[0027] The above-mentioned thickener may further include thickener components commonly used in the relevant technical field in addition to the aforementioned polyacrylamide-based thickener. For example, it may further include carboxymethylcellulose (CMC), methylcellulose (MC), polyacrylic acid (PAA), polyethylene oxide (PEO), polyacrylimide (PAM), etc.

[0028] Based on the total weight of the coating composition, it may contain 0.1 to 10 weight percent, for example, 0.5 to 3 weight percent, of a thickener. If the content of the thickener is less than the aforementioned range, heat resistance may be reduced, and if it exceeds the aforementioned range, flexibility may be reduced.

[0029] In the present invention, by increasing the binder content and decreasing the thickener content, the viscosity of the coating composition can be lowered while improving the adhesive strength. Additionally, by reducing the thickener content, flexibility is imparted to the coating layer. Consequently, even if multiple repetitive cutting operations are performed during the battery manufacturing process using a separator containing the coating layer, the detachment of the fine coating layer at the cutting site where the cutter is inserted can be minimized. Typically, reducing the thickener content leads to a decrease in heat resistance; however, in the present invention, by using a thickener of the aforementioned composition, heat resistance can be maintained even when the thickener content is reduced.

[0031] menstruum

[0032] The water-based coating composition of the present invention may include a solvent. For example, the water-based coating composition of the present invention may be prepared by mixing a ceramic particle slurry dispersed in a solvent with a binder dissolved in a solvent. In the water-based coating composition of the present invention, water is used as the solvent, and organic solvents are not used.

[0033] The above solvent can be used in an amount satisfying 100% by weight of the total weight of the water-based coating composition, and, for example, in an amount of 35 to 65% by weight based on the total weight of the coating composition. When the solvent is used within the above content range, phase separation of the ceramic particle slurry and the binder mixture does not occur, and excellent coating properties are achieved by having an appropriate viscosity.

[0035] The viscosity (25°C) of the water-based coating composition according to the present invention may be 70 cP or less, for example, 5 to 70 cP, or for other examples, 10 to 20 cP. When the viscosity satisfies the aforementioned range, a coating layer with excellent flexibility, adhesive strength, heat resistance, and coating properties can be formed. When the viscosity exceeds the aforementioned range, the coating properties may be reduced when manufacturing a separation membrane including a coating layer formed with the coating composition.

[0036] The method of forming a coating layer on a porous substrate using the above-mentioned water-based coating composition is not particularly limited, and conventional methods known in the art can be used. For example, a coating layer can be formed on a substrate using various methods such as dip coating, die coating, comma coating, gravure coating, bar coating, or a combination thereof.

[0038] Separator

[0039] The present invention provides a separation membrane coated with the aforementioned water-based coating composition. The separation membrane according to the present invention comprises a porous substrate and a coating layer formed on at least one surface of the porous substrate and formed with the water-based coating composition of the present invention.

[0040] The porous substrate is formed from a polyolefin resin, and any conventional polyolefin resin known in the art can be used without limitation. For example, it may include polyethylene resin, polypropylene resin, or both.

[0041] To improve the properties as a separator for batteries, the polyolefin resin may further include a polyolefin that provides a shutdown function. Low-density polyethylene (LDPE) or polyethylene wax may be used as the polyolefin that provides the shutdown function. As for the LDPE, one or more selected from the group consisting of branched LDPE, linear LDPE (LLDPE), and ethylene / α-olefin copolymers produced by a single-site catalyst may be used.

[0042] The above porous substrate may, if necessary, include conventional additives known in the art, such as antioxidants, pore-forming agents, etc., to a extent that does not impair the effects of the present invention.

[0044] Lithium-ion secondary battery

[0045] The present invention provides a secondary battery comprising the aforementioned separator, for example, a lithium-ion secondary battery.

[0046] The lithium-ion secondary battery of the present invention can be manufactured according to conventional methods known in the art, except for using a separator coated with a water-based coating composition according to the present invention. For example, it can be manufactured by interposing a separator between a positive electrode and a negative electrode and introducing a non-aqueous electrolyte.

[0048] The present invention will be explained in more detail below through examples. However, the following examples are intended only to aid in understanding the present invention and do not imply that the scope of the present invention is limited to these examples in any way.

[0050] [Preparation Example 1-2]

[0051] Water-based coating compositions of each manufacturing example were prepared according to Table 1 below.

[0053] [Comparative Manufacturing Examples 1-5]

[0054] Water-based coating compositions of each comparative example were prepared according to Table 1 below.

[0056]

[0058] Ceramic particles: BaSO4

[0059] Binder: Water-dispersed BA-AN (Buty acrylate-acrylonitrile) copolymer

[0060] Thickener 1: Polyacrylamide-based thickener (containing 1.6 wt% acrylamide, 0.4 wt% octylinone, and 1.0 wt% methylisothiazolinone)

[0061] Thickener 2: Polyacrylamide-based thickener (containing 1.4 wt% acrylamide, 0.6 wt% octyllinone, 1.7 wt% disodium disulfite, and 1.0 wt% acrylonitrile)

[0063] [Physical Property Evaluation]

[0064] The physical properties of the water-based coating compositions of each manufacturing example and comparative manufacturing example were measured by the following method, and the results are shown in Table 2 below.

[0066] Separator manufacturing

[0067] Viscosity-average molecular weight (Mv) is 3 x 10 4 20 wt% of polyethylene resin with g / mol and viscosity-average molecular weight (Mv) of 3 × 10 6 Separator membranes of each example and comparative example were prepared by gravure coating a water-based coating composition of each example and comparative example to a thickness of 2.7 μm on one side of a porous polyolefin substrate prepared using 80 wt% of polyethylene resin with g / mol.

[0068] In addition, for the heat shrinkage test, the separation membranes of each example and comparative example (20% down) were prepared using the same method as above, except that the amount of water-based coating composition used was reduced by 20% by weight.

[0070] thermal shrinkage

[0071] After leaving the separator and separator (20% down) of each example and comparative example in a hot air drying oven at 150°C for 60 minutes, the shrinkage rate (%) in the longitudinal and transverse directions was checked.

[0073] Peel strength

[0074] An 18 mm double-sided tape (3M) was attached to a steel plate, and the separator membranes of each example and comparative example were adhered to the tape. Using a tensile strength tester (Instron), the strength was measured when the adhesive tape and the coating layer peeled off by pulling 180° at a speed of 100 mm / min.

[0076] Coating layer peels off after cutting

[0077] The separator membranes of each example and comparative example were cut with a cutter (saw type), and the amount of coating layer detachment was measured using SEM (1,000x magnification) and evaluated according to the following criteria.

[0078] Lv3: Eliminated area is 50% or more

[0079] Lv2: Dropped area is 5% or more and less than 50%

[0080] Lv1: Dropped area less than 5%

[0082] Coating properties

[0083] The surface coating condition of the separator membranes of each example and comparative example was visually observed to check for defects such as non-coating, clumping of coating liquid, or cracking, and then the coating performance was evaluated according to the following criteria.

[0084] ◎: No defects (uniform application)

[0085] △: Defect area less than 10%

[0086] X: Defect area is 10% or more

[0088] viscosity

[0089] After leaving the water-based coating compositions of each manufacturing example and comparative manufacturing example at 25°C for 1 minute, measurements were taken using a viscometer (BROOKFIELD DVNXLVCJG, Threed spindle 20 rpm, 3 minutes).

[0091]

[0093] As shown in Table 2 above, the water-based coating compositions of Preparation Examples 1 and 2 according to the present invention and the separation membranes of Examples 1 and 2 coated therewith exhibited excellent physical properties across all measured items. On the other hand, Comparative Preparation Examples 1 and 2 using a thickener (thickener 2) outside the scope of the present invention, and Comparative Examples 3-5 water-based coating compositions in which the content of the components constituting the coating composition is outside the scope of the present invention, and the separation membranes of Comparative Examples 1-5 coated therewith exhibited generally inferior physical properties compared to Examples 1 and 2.

Claims

Claim 1 A water-based coating composition comprising, based on the total weight of the coating composition, 30 to 60 weight% of ceramic particles, 0.1 to 10 weight% of a binder, and 0.1 to 10 weight% of a thickener, wherein the thickener comprises a polyacrylamide-based thickener including acrylamide, octylinone, and methylisothiazolinone. Claim 2 A water-based coating composition according to claim 1, wherein the polyacrylamide-based thickener comprises 0.01 to 6 weight% of acrylamide, 0.01 to 1 weight% of octylinone, and 0.01 to 3 weight% of methylisothiazolinone, based on the total weight of the polyacrylamide-based thickener. Claim 3 A water-based coating composition according to claim 1, wherein the viscosity (25 ℃) is 70 cP or less. Claim 4 A separator for a secondary battery coated with a water-based coating composition of any one of claims 1 to 3. Claim 5 A lithium-ion secondary battery comprising a separator for a secondary battery according to claim 4.

Images