KR102168816B1 - Aqueous coating composition and separator using the same - Google Patents

Abstract

The present invention comprises a multifunctional resin, a water-dispersible binder and inorganic particles, the multifunctional resin is a water-based coating composition containing polyvinyl alcohol (PVA) having a degree of polymerization of 1,500 or less and a saponification degree of 95% or less, and a separator coated therewith Provides.

Description

Aqueous coating composition and separator using the same {AQUEOUS COATING COMPOSITION AND SEPARATOR USING THE SAME}

The present invention relates to an aqueous coating composition for coating a separator, more particularly, to an aqueous coating composition having excellent heat resistance, coating properties and adhesive strength, and a separator coated therewith.

As technology development and demand for portable information and communication devices and electric vehicles increase in recent years, the demand for lithium secondary batteries is rapidly increasing as an energy source to maintain them. In particular, the demand is expanding from small electronic equipment such as cell phones and laptop computers that have been widely used in the past to fields requiring high capacity and high output such as complex electric vehicles, electric bicycles, and energy storage systems.

Polyolefin-based porous microporous membranes have excellent chemical stability and cell characteristics and are suitably used as separators for small batteries. However, the melting temperature is about 130℃~150℃, so there is no problem when applied to small batteries, but when applied to high capacity and high output batteries that require high energy density and discharge voltage, the separator is Since the possibility of ignition and explosion due to deformation is very high, thermal stability is required above all.

In order to prevent damage to the separator due to the high temperature as described above, a slurry in which heat-resistant inorganic particles and a binder are mixed is coated on a porous microporous substrate, and the deformation of the substrate is suppressed by the coating layer and thermal and electrochemical stability are improved. The obtained organic-inorganic composite porous separator was proposed.

The conventional separator coated with an organic slurry uses a large amount of organic solvent to dissolve a binder for providing bonding strength between inorganic particles and between the porous substrate and the coating layer. When the solvent is completely evaporated after coating a slurry in which the uniformly dispersed inorganic particles and a binder completely dissolved in an organic solvent are coated, sufficient interconnectivity is provided between the binder and the inorganic material.

However, a coating product using an organic solvent has the following problems. First, in order to control risk factors such as volatility, flammability, and toxicity of organic solvents, facilities such as explosion-proof facilities, deodorization facilities, and gas treatment facilities for inhalation of vapors are required, and economic feasibility decreases due to increased installation and maintenance costs of these facilities. do. Second, product cost increases due to the high cost of organic solvents. Third, the polyolefin constituting the microporous membrane and the binder dissolved in an organic solvent having good wettability penetrate into the pores, causing clogging of the pores and reduction in pore size. This causes problems such as a decrease in air permeability of the microporous membrane and a decrease in battery cycle characteristics due to a decrease in ionic conductivity. As described above, when the coating layer is formed using an organic solvent, environmental and economic problems, as well as safety and battery characteristics, may be deteriorated due to deterioration of physical properties of the separator even when a heat-resistant material is applied.

On the other hand, the aqueous slurry-coated separator does not have a problem caused by an organic solvent, but coating unevenness and battery performance deterioration occur due to the low wettability of the polyolefin-based porous membrane having a low surface energy.

In general, corona surface treatment technology, which is a hydrophilic treatment method, is used for uniform coating properties. In the case of corona treatment, it is possible to obtain improved wettability due to the effect of physical surface modification and chemical surface modification by generation of polar functional groups. However, the physical surface modification is a method of increasing the roughness of the surface with a strong electric field, which is not uniform and damages the surface of the substrate, thereby deteriorating the withstand voltage characteristic. On the other hand, in order to improve the wettability without corona treatment, a large amount of additives are required. The additives included in the coating layer are electrochemically unstable, causing degradation of battery performance due to addition reactions in the battery.

The present invention is to provide a water-based coating composition containing a minimum amount of additives excellent in heat resistance, coating properties and adhesive strength, and a separator coated therewith.

The present invention comprises a multifunctional resin, a water-dispersible binder and inorganic particles, the multifunctional resin is a water-based coating composition containing polyvinyl alcohol (PVA) having a degree of polymerization of 1,500 or less and a saponification degree of 95% or less, and a separator coated therewith Provides.

In addition, the present invention provides a lithium secondary battery including the above-described separator.

The water-based coating composition of the present invention is a low-cost, eco-friendly manufacturing method that satisfies the coating properties equivalent to or higher than that of the conventional coating composition containing a large amount of additives, and minimizes the moisture content to provide an organic-inorganic composite porous separator that satisfies excellent product stability. to provide.

Hereinafter, the present invention will be described in detail. However, it is not limited only by the following contents, and each component may be variously modified or selectively used as necessary. Therefore, it is to be understood as including all changes, equivalents, and substitutes included in the spirit and scope of the present invention.

<Water-based coating composition>

The aqueous coating composition according to the present invention comprises a multifunctional resin, a water-dispersible binder, and inorganic particles. Hereinafter, the composition of the coating composition will be described in detail.

bookbinder

The aqueous coating solution of the present invention uses two types of a multifunctional resin and a water-dispersible binder as a binder.

The multifunctional resin includes polyvinyl alcohol (PVA) or a derivative thereof. The degree of polymerization of the polyvinyl alcohol may be 1,500 or less, for example, in the range of 200-1,500. The degree of polymerization refers to the average of the number of monomers bound to one chain of a polymer. The higher the degree of polymerization, the longer the molecular chain, the wider the surface area of the molecule, and as a result, the attractive force between the molecules increases, resulting in an increase in viscosity and some aggregation. Therefore, the higher the degree of polymerization, the higher the peel strength, but there is a problem in that the coating property decreases, such as agglomeration occurs due to strong bonding between particles and defects are generated on the surface.

In addition, the saponification degree of the polyvinyl alcohol may be 95% or less, for example, 80 to 95%. The degree of saponification refers to the degree of hydrolysis when polyvinyl acetate (PVAc) is hydrolyzed to make polyvinyl alcohol, and can be measured through neutralization titration. If the degree of saponification exceeds 95%, the moisture content in the separator increases, which may cause problems in product stability.

When the degree of polymerization and the degree of saponification are within the range, polyvinyl alcohol may serve as an additive such as a dispersant and a wetting agent in addition to the role of a binder. As a result, not only the use of the organic solvent is unnecessary, but also the amount of the additive is reduced to reduce side reactions and internal resistance by the additive in the battery.

The multifunctional resin may be used in an amount of about 0.1 to 1.0% by weight based on the total weight of the solid content of the aqueous coating composition. When the multifunctional resin is used in the above content range, the separator coated with the water-based coating composition contains a minimum of moisture, and sufficient adhesion between the coating layer and the substrate and excellent battery cycle characteristics may be exhibited.

The water-dispersible binder includes a (meth)acrylic resin. Non-limiting examples of the (meth)acrylic resin include polyethyl (meth) acrylate, polypropyl (meth) acrylate, polyisopropyl (meth) acrylate, polybutyl (meth) acrylate, polyhexyl (meth) Acrylate, polyethylhexyl (meth)acrylate, polylauryl (meth)acrylate, and the like. The resin may be used alone or in combination of two or more.

The water-dispersible binder may be used in an amount of about 0.5 to 5% by weight based on the total weight of the solid content of the aqueous coating composition. When a binder is used in the above content range, suitable strength and adhesion may be imparted to the separator coated with the water-based coating composition.

In the present invention, the multifunctional resin and the water-dispersible binder may be mixed and used in a weight ratio of 1:2 to 1:30. By using a water-dispersible binder as a main component, it is possible to reduce the deterioration of the life and cycle characteristics of the battery due to the addition reaction of the additive, and secure sufficient heat resistance and adhesion. On the other hand, when the multifunctional resin is used in an amount less than the above range, a problem of reducing heat resistance may occur.

In the present invention, the content of the total binder including the multifunctional resin and the water-dispersible binder may be in the range of about 6% by weight or less, for example, 0.6 to 6% by weight, based on the total weight of the solid content of the aqueous coating composition. According to the composition of the present invention, even when 6% by weight or less of a binder is used, heat resistance and strength equal to or higher than that of a conventional coating composition containing a large amount of polyvinyl alcohol and additives having a high degree of polymerization can be secured.

Inorganic particles

Inorganic particles are not deformed by external impact or high temperature, and are combined with a polymeric binder to fix the substrate and the coating layer, and the melting temperature is relatively low, preventing shrinkage or deformation of the polyolefin-based substrate requiring high temperature stability, thereby increasing battery stability. Do it.

The inorganic particles used in the aqueous coating composition of the present invention are not particularly limited as long as they are inorganic particles generally used in the art.

Non-limiting examples of inorganic particles include Alumina, Aluminum Hydroxide, Barium Titanium Oxide, Magnesium Oxide, Magnesium Hydroxide, Clay ), Titanium Oxide, Glass powder, Boehmite, and the like. These may be used alone or in combination of two or more.

The size or shape of the inorganic particles is not particularly limited, and for example, inorganic particles having an average particle diameter in the range of 0.1 to 2.0 µm may be used.

The inorganic particles may be used in an amount of about 94 to 99% by weight based on the total weight of the solid content of the aqueous coating composition. When the solid content weight of the inorganic particles is within the above range, it has the most stabilized aqueous slurry solution and excellent coating properties.

The inorganic particles may be mixed with a solvent and prepared in the form of an aqueous slurry through a stirring process using a beads mill, a ball mill, or the like.

menstruum

The aqueous coating composition of the present invention is prepared by mixing a slurry of inorganic particles dispersed in a solvent and a binder dissolved in a solvent. In the aqueous coating composition of the present invention, water is used as a solvent, and an organic solvent is not used. Therefore, the problem of the prior art does not occur due to the use of an organic solvent.

Polyvinyl alcohol of low polymerization degree used in the present invention is well adsorbed to the two-phase interface due to the balance of hydrophilicity and hydrophobicity, and serves to reduce the difference in high surface tension between the interfaces. Due to these properties, polyvinyl alcohol not only serves as a binder, but also serves as a dispersant or a wetting agent. As a result, not only does the use of an organic solvent unnecessary, but also reduces the amount of additives to reduce side reactions and internal resistance caused by additives in the battery.

The solvent may be used in an amount of about 50 to 69% by weight based on the total weight of the aqueous coating composition. When a solvent is used in the above content range, the phase separation of the mixture of the inorganic dispersion and the polymeric binder does not occur, and the coating property is excellent by having an appropriate viscosity.

The method of preparing the aqueous coating composition and the method of forming the coating layer on the porous substrate are not particularly limited, and general methods known in the art may be used. For example, a coating layer can be formed on the substrate using various methods such as dip coating, die coating, comma coating, gravure coating, bar coating, or a mixture method thereof. have.

The water content of the separator coated with the aqueous coating composition of the present invention is 600 to 750 ppm, the peel strength is 11 gf/15 mm or more, and the resistance is 0.80 Ω or less. By securing these physical properties, excellent coating properties, thermal stability, and adhesion can be secured.

Since the separator coated with the water-based coating composition according to the present invention contains minimal moisture and has excellent adhesion and thermal stability between the coating layer and the substrate, it prevents shrinkage and rupture of the separator when heat and deterioration occurs inside the battery cell. can do.

<Separator>

The present invention provides a separator coated with the above water-based coating composition. The separator according to the present invention includes a porous substrate and a coating layer formed on at least one surface of the porous substrate and formed of the aqueous coating composition of the present invention.

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

The content of the polyethylene resin may be, for example, 50 % by mass or more, and in another example, 60 % by mass or more with respect to the total mass of the polyolefin resin. The polyethylene resin may have a weight average molecular weight (Mw) in the range of, for example, 2.0×10 ⁴ to 8.0×10 ⁵ , and as another example, 3.0×10 ⁴ to 7.0×10 ⁵ It can be a range. As the polyethylene, high-density polyethylene (HDPE), medium-density polyethylene (MDPE), or low-density polyethylene (LDPE) may be used without limitation. Alternatively, two or more kinds of those having different weight average molecular weight (Mw) or density may be mixed.

The content of the polypropylene resin may be, for example, 50 % by mass or less, for example, 40 % by mass or less, based on the total mass of the polyolefin resin. The polypropylene resin may have a weight average molecular weight (Mw), for example, in the range of 7.0×10 ⁴ to 3.2×10 ⁶ , and as another example, may be in the range of 9.5×10 ⁴ to 3.0×10 ⁶ . As the polypropylene, high-density polypropylene (HDPP), medium-density polypropylene (MDPP), or low-density polypropylene (LDPP) may be used without limitation. Alternatively, two or more kinds of those having different weight average molecular weight (Mw) or density may be mixed.

In order to improve characteristics as a battery separator use, the polyolefin resin may further contain a polyolefin that imparts a shutdown function. Examples of polyolefins that impart a shutdown function include LDPE and polyethylene wax. At this time, as LDPE, at least one selected from the group consisting of branched LDPE, linear LDPE (LLDPE), and ethylene/α-olefin copolymer prepared by a single site catalyst may be used, and the amount of it added is per It can be appropriately adjusted within the range known in the field.

If necessary, conventional additives known in the art, for example, antioxidants, pore formers, etc. may be included within a range that does not impair the effects of the present invention.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are only intended to aid understanding of the present invention, and the scope of the present invention is not limited to the examples in any sense.

[ Manufacturing example One]

5% by weight of PVA (Sigma aldrich, 360627) having a weight average molecular weight of 9,000 to 10,000 g/mol was added to water and stirred at 80° C. for 4 hours to prepare a multifunctional resin of Preparation Example 1.

[ Manufacturing example 2]

A multifunctional resin of Preparation Example 2 was prepared in the same manner as in Preparation Example 1, except that PVA (Sigma Aldrich, 363170) having a weight average molecular weight of 13,000 to 23,000 g/mol was used.

[ Manufacturing example 3]

The multifunctional resin of Preparation Example 3 was prepared in the same manner as in Preparation Example 1, except that PVA (Sigma Aldrich, P8136) having a weight average molecular weight of 30,000 to 70,000 g/mol was used.

[ Comparative Production Example One]

A multifunctional resin of Comparative Preparation Example 1 was prepared in the same manner as in Preparation Example 1, except that PVA (Sigma Aldrich, 348406) having a weight average molecular weight of 13,000 to 23,000 g/mol was used.

[ Comparative Production Example 2]

A multifunctional resin of Comparative Preparation Example 2 was prepared in the same manner as in Preparation Example 1, except that PVA (Sigma Aldrich, 363103) having a weight average molecular weight of 146,000 to 186,000 g/mol was used.

The degree of polymerization and saponification of the multifunctional resin prepared in Preparation Example 1-3 and Comparative Preparation Example 1-2 are shown in Table 1 below.

[ Example 1-3]

Each of the multifunctional resins prepared in Preparation Example 1-3 at room temperature, water-dispersible binder (acrylic resin, TOYOCHEM CO. CSB-130), inorganic particles (alumina, SUMITOMO CHEMICAL Co., HAKB41210KG3), and water 0.1: 0.5: 47.7 : A water-based coating composition was prepared by mixing at a weight ratio of 51.7. The aqueous coating composition was coated and dried to a thickness of 4 um on a polyethylene (PE) porous membrane by a bar coating method to form the coating layer of Example 1-3.

[ Comparative example 1-3]

A coating film of Comparative Example 1-2 was formed in the same manner as in Example, except that each of the multifunctional resins prepared in Comparative Preparation Example 1-2 was used instead of the multifunctional resin prepared in Preparation Example.

Except that a surfactant (3M, Fluorosurfactant FC-4430) was used instead of a multifunctional resin (PVA), and a surfactant, a water-dispersible binder, inorganic particles, and water were mixed in a weight ratio of 0.1: 1.4: 47.3: 51.2, The coating film of Comparative Example 3 was formed in the same manner as in Example.

[ Of the coating film Property evaluation]

The physical property tests of the coating films of Examples 1-3 and Comparative Examples 1-3 were performed as follows, and the results are shown in Table 2 below.

(1) Air permeation (air permeation)

It was measured according to the JISP8117 measurement method using an Oken-type air permeation resistance meter (Asahi Seiko Co. Ltd., EGO-1T). Air permeability refers to the time (Sec) required for 100cc of air to pass through the coating film of a certain area under a certain pressure.

(2) Peel strength

An 18 mm double-sided tape (3M) was attached to the iron plate, and a test piece (18 mm X 100 mm) to measure the peel strength was adhered to the tape. Thereafter, the tensile strength measurement device (Instron) was used to measure the strength when the adhesive tape and the coating layer were peeled by pulling at 180° at a speed of 300 mm/min. Usually, the peel strength at 180° between the porous membrane and the coating layer is preferably 7 gf/15 mm or more, and 20 gf/15 mm or more is ideal.

(3) transparent point

The number of transparent dots with a diameter of 3 mm or more existing on the surface of the sample coated with an area of 10 m 2 was measured and expressed according to the following criteria.

◎: 0~2, ○: 3~4, △: 5~6, X: 7 or more

(4) moisture content

Leave the coated microporous membrane in a dry box at a dew point of -55°C or less for 30 minutes, put 0.2 g of a measurement sample in a heating tube, heat it at 150°C for 10 minutes, and then Karl Fischer Moisture Titrator, MKC-610 -NT was used to measure the moisture content.

(5) resistance

A coin cell including a coated microporous membrane, an electrode, and an electrolyte was manufactured, mounted on a resistance measuring device (Solar tron, 1255B), set to Voltage 10 (V), Current 4 (A), and measured resistance.

As shown in Table 2, the separator of Example 1-3 using a multifunctional resin (PVA) according to the present invention has battery characteristics (permeability, peel strength, coating properties, cycle characteristics) equal to or higher than that of a separator using a conventional coating composition. ), while minimizing the moisture content, it can be confirmed that excellent product stability is satisfied.

The separator of Comparative Example 1 uses a multifunctional resin (PVA) having a high saponification degree, and has a problem in that the moisture content is very high compared to the Example.

The separator of Comparative Example 2 was made of a multifunctional resin (PVA) having a high polymerization degree, and exhibited excellent physical properties in terms of air permeability, peel strength, and moisture content, but there is a problem in that many defects are generated on the surface and resistance is increased.

The separator of Comparative Example 3 was coated with a coating composition in which a surfactant was added instead of PVA, and a binder content was increased to increase the proportion of additives in the composition. As a result of measuring the physical properties, the peel strength improved with the increase of the binder content, but the air permeability decreased and the resistance increased, causing side reactions in the battery and deterioration of the battery cycle performance.

Claims (9)

Including a multifunctional resin, a water-dispersible binder and inorganic particles,
An aqueous coating composition wherein the multifunctional resin contains polyvinyl alcohol (PVA) or a derivative thereof having a polymerization degree of 1,500 or less and a saponification degree of 95% or less,
The multifunctional resin is 0.1 to 1.0 wt%, the water-dispersible binder is 0.5 to 5 wt%, based on the total weight of the solid content of the aqueous coating composition,
The multifunctional resin and the water-dispersible binder are mixed and used in a weight ratio of 1:2 to 1:30,
An aqueous coating composition containing no organic solvent. The aqueous coating composition according to claim 1, wherein the degree of polymerization of the polyvinyl alcohol is in the range of 200-1,500. The aqueous coating composition according to claim 1, wherein the degree of saponification of the polyvinyl alcohol is in the range of 80 to 95%. delete The aqueous coating composition according to claim 1, wherein the water-dispersible binder comprises a (meth)acrylic resin. delete The aqueous coating composition according to claim 1, wherein the total binder including the multifunctional resin and the water-dispersible binder is 0.6 to 6% by weight based on the total weight of the solid content of the aqueous coating composition. A separator coated with the water-based coating composition of any one of claims 1 to 3, 5 or 7. The separator according to claim 8, wherein the water content is 600 to 750 ppm, the peel strength is 11 gf/15 mm or more, and the resistance is 0.80 Ω or less.