JP7740788B2 - Separation membrane for secondary batteries - Google Patents

Description

This application claims priority to Korean Patent Application No. 10-2021-0127062, filed on September 27, 2021.

This invention relates to a separator for a secondary battery. Specifically, it relates to a separator for a secondary battery that contains an aqueous emulsion-type binder in the separator coating layer, and that has improved adhesive strength regardless of the presence or absence of an electrolyte.

Among the components of a secondary battery, the separator is a porous polymer membrane located between the positive and negative electrodes. It separates the positive and negative electrodes, preventing electrical short circuits between the two electrodes and allowing electrolyte and ions to pass through. While the separator itself is not involved in the battery's electrochemical reactions, its physical properties, such as its wettability with the electrolyte, degree of porosity, and thermal shrinkage rate, affect the battery's performance and safety.

Therefore, in order to strengthen the physical properties of the separation membrane, various methods have been attempted to change the physical properties of the coating layer by adding a coating layer to the separation membrane substrate and adding various substances to the coating layer. For example, inorganic substances can be added to the coating layer to improve the mechanical strength of the separation membrane, or inorganic substances or hydrates can be added to the coating layer to improve the flame retardancy and heat resistance of the separation membrane substrate.

The separator can be attached to the electrode through a lamination process, and a binder can be added to the separator coating layer composition to ensure adhesion between the electrode and separator. As shown in Figure 2, the lamination process can attach the positive and negative electrodes to both sides of the separator.

Generally, the crystallinity of a binder can affect the binder's adhesive strength. For example, amorphous polymers have better adhesive strength with electrodes in an environment without electrolyte than in the presence of electrolyte. However, secondary batteries are manufactured by sealing an electrode assembly and electrolyte in a battery case, and charging and discharging are carried out by lithium ions moving through the electrolyte. Therefore, technology is needed to ensure adhesive strength with electrodes in the presence of electrolyte.

In this regard, Patent Document 1 discloses an organic/inorganic composite coated porous separator in which a coating liquid is coated on a porous substrate, the coating liquid being a dispersion of a polymeric compound binder, the polymeric compound binder including a first organic binder being an emulsion or suspension in which a water-insoluble polymeric compound is dispersed in water, and a second organic binder being a water-soluble polymeric compound.

The separation membrane in Patent Document 1 has improved wettability with the electrolyte, resulting in high ionic conductivity and excellent adhesion.

The separation membrane of Patent Document 2 has a coating layer formed on at least one surface of a porous polymer substrate. The coating layer includes inorganic particles and a binder polymer. The binder polymer includes an amorphous adhesive binder polymer and one or more fluorine-based copolymers. The content of the amorphous adhesive binder polymer is 50 to 84 parts by weight based on 100 parts by weight of the total binder polymer content.

The separation membrane of Patent Document 2 has low resistance and exhibits the effect of improving the adhesive strength between the separation membrane and the electrode.

However, Patent Documents 1 and 2 fail to present a technology that can improve the adhesive strength between the electrode and the separator not only when no electrolyte is present, but also when an electrolyte is present.

Korean Patent No. 1341196 Korean Patent Publication No. 2020-0034470

The present invention aims to solve the above problems by providing a separator for a secondary battery that includes a coating layer that provides dry adhesion between the electrodes and separator in the absence of electrolyte, enabling cell assembly, and wet adhesion between the electrodes and separator in the presence of electrolyte, as well as a method for manufacturing the same.

To this end, the objective is to design and provide a combination of an aqueous emulsion-type binder and an inorganic substance to form the coating layer, as well as a slurry containing the combination.

To achieve these objectives, the separator for a secondary battery according to the present invention includes a porous separator substrate containing a polymer and a coating layer on at least one side of the porous separator substrate, the coating layer including inorganic particles, a crystalline first binder, and an amorphous second binder, each of which is independently an aqueous emulsion binder, a concentration gradient of the inorganic particles and the amorphous second binder exists across the cross section of the coating layer, the inorganic particles are present in greater amounts on the surface of the coating layer facing the porous separator substrate than on the opposite side of the porous separator substrate, and when the separator is in a dry state, the amorphous second binder may be present in greater amounts on the surface of the coating layer facing the porous separator substrate than on the surface of the coating layer facing the porous separator substrate.

When the separator is immersed in an electrolyte solution, a concentration gradient of the crystalline first binder exists across the cross section of the coating layer, and the crystalline first binder may be present in greater amounts on the side of the coating layer opposite the porous separator substrate than on the surface facing the porous separator substrate.

The weight ratio of the crystalline first binder to the amorphous second binder may be 1:9 to 9:1.

The adhesive strength of the separator for the secondary battery when immersed in the electrolyte may be 1.0 gf/20 mm to 20.0 gf/20 mm.

The adhesive strength of the separator for the secondary battery in a dry state may be 10 gf/20 mm or more and less than 100 gf/20 mm.

The crystalline first binder may include a polyvinylidene fluoride-based binder, and the amorphous second binder may include an acrylate-based binder.

At least one of the crystalline first binder and the amorphous second binder may be a copolymer.

The first crystalline binder may have the same polymer skeleton as the binder used in the positive electrode, and the second amorphous binder may have the same polymer skeleton as the binder used in the negative electrode.

The crystalline first binder may be one or more selected from the group consisting of polyvinylidene fluoride, polyhexafluoropropylene, polytetrafluoroethylene, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-trichloroethylene (PVDF-TCE), and polyvinylidene fluoride-chlorotrifluoroethylene (PVDF-CTFE).

The amorphous second binder may be one or more selected from the group consisting of styrene-butadiene rubber, nitrile-butadiene rubber, acrylonitrile-butadiene rubber, acrylonitrile-butadiene-styrene rubber, and acrylate-containing polymers.

The crystalline first binder may be formed from particles having an average particle size (D50) of 200 nm to 350 nm.

The amorphous second binder may be formed from particles with an average particle size (D50) of 350 nm to 500 nm.

The density of the inorganic particles may be higher than the density of the crystalline first binder and the amorphous second binder.

The inorganic particles may have an aspect ratio of 1 to 2 and a BET specific surface area of 5 to 25 m ² /g.

The weight ratio of the first and second binders to the inorganic particles on the surface of the porous layer facing the porous separator substrate may be 45:55 to 0:100, and the weight ratio of the first and second binders to the inorganic particles on the surface of the porous layer opposite the porous separator substrate may be 50:50 to 100:0.

The coating layer can be formed by drying a slurry containing inorganic particles, a crystalline first binder, a non-crystalline second binder, and water, with a solids content of 15 to 30 wt % and a viscosity of 20 to 150 cP.

The coating layer can be formed by applying the slurry to the porous separator substrate and then drying it at 45 to 65°C.

The drying may be performed at the highest initial drying temperature.

The present invention provides a secondary battery including an electrode assembly including a stack of a positive electrode, a negative electrode, and a separator for the secondary battery disposed between the positive electrode and the negative electrode.

The secondary battery may have a bending strength of 9.0 MPa to 26.0 MPa.

The present invention also provides a battery module including the secondary battery.

The present invention can also provide various combinations of means for solving the above problems.

As described above, the separator for a secondary battery according to the present invention includes a crystalline binder and an amorphous binder in the coating layer. The amorphous binder ensures adhesive strength (dry adhesive strength) between the electrode and separator when no electrolyte is present, and the crystalline binder ensures adhesive strength (wet adhesive strength) between the electrode and separator when an electrolyte is present. This makes it possible to manufacture an electrode assembly by first providing dry adhesive strength between the electrode and separator, and then providing wet adhesive strength, thereby maintaining stable adhesion between the electrode and separator in the presence of an electrolyte.

As a result, a certain level of adhesive strength between the electrode and the separator can always be maintained, regardless of whether or not an electrolyte is present.

In addition, by using a binder in the separator coating layer that has similar characteristics to the binders used in the positive and negative electrodes (for example, a binder in the separator coating layer that has the same skeleton as the binder used in the electrodes), it is possible to ensure adhesion between the positive electrode and separator, and between the negative electrode and separator.

The separator has a coating layer formed on one surface of a porous separator substrate, and is an example of a separator in a dry state and in a state impregnated with an electrolyte. Each coating layer includes a crystalline first binder, an amorphous second binder, and inorganic particles. 1 illustrates an example of an electrode assembly formed by laminating a positive electrode and a negative electrode attached to both sides of a separator. 1 is a scanning electron microscope (SEM) image of a coating layer showing the difference between a crystalline first binder and an amorphous second binder. 1 is a graph showing the change in adhesive strength between an electrode and a separator depending on drying conditions.

Hereinafter, with reference to the accompanying drawings, a detailed description will be given of an embodiment of the present invention that will enable a person of ordinary skill in the art to easily implement the present invention. However, when describing the operating principles of a preferred embodiment of the present invention in detail, if a detailed description of related well-known functions or configurations is deemed to unnecessarily obscure the gist of the present invention, such a detailed description will be omitted.

Furthermore, the same reference numerals will be used throughout the drawings for parts that have similar functions and actions. Throughout the specification, when a part is said to be connected to another part, this includes not only direct connection but also indirect connection via an intervening element. Furthermore, unless otherwise specified, the inclusion of a certain element does not mean the exclusion of other elements, but means that other elements may also be included.

Furthermore, unless otherwise specified, descriptions that specify the elements by limiting or adding them can be applied to all inventions and are not limited to descriptions of specific inventions.

Furthermore, throughout the description of the invention and claims of this application, terms used in the singular also include the plural unless otherwise specified.

Furthermore, throughout the description of the invention and claims of this application, "or" includes "and" unless otherwise specified. Therefore, "including A or B" means all three cases: including A, including B, and including A and B.

The present invention is explained in more detail below.

The separator for a secondary battery according to the present invention has a porous structure and includes a separator substrate made of a polymer material and a coating layer coated on at least one surface of the separator substrate. The coating layer includes a crystalline first binder and an amorphous second binder, and the first binder and the second binder may each independently be an aqueous emulsion-type binder.

In the present invention, crystallinity can be expressed as a percentage of the melting enthalpy value measured by DSC (differential scanning calorimetry) relative to the melting enthalpy value of a theoretically perfect crystal (crystallinity of 100%). In the present invention, a non-crystalline binder means one with a crystallinity of 30% or less, and a crystalline binder means one with a crystallinity of more than 30%.

The present invention contains a coating layer that contains a crystalline first binder and a non-crystalline second binder. When an electrolyte is present, the crystalline first binder ensures adhesion to the electrode, and when an electrolyte is not present, the non-crystalline second binder ensures adhesion to the electrode.

In the present invention, it may be preferable for the adhesive strength of the amorphous second binder to be higher than that of the crystalline first binder, and the difference in adhesive strength may be caused by temperature and pressure. The temperature and pressure during lamination may be 20°C to 130°C and 100 kgf/ ^cm2 to 2000 kgf/ ^cm2 , respectively. Preferably, the pressure may be 100 kgf/ ^cm2 to 900 kgf/ ^cm2 , more preferably 100 kgf/ ^cm2 to 500 kgf/ ^cm2 . If the temperature exceeds 130°C, the shape of the binder may be distorted and the binder may not move properly, making it difficult to ensure sufficient adhesive strength. Therefore, in a coating layer containing a crystalline first binder and an amorphous second binder, both of the two binders may contribute to adhesive strength with the electrode, but in a dry state, adhesive strength with the electrode may be mainly formed by the amorphous second binder.

During cell assembly, dry adhesive force due to the second binder acts between the electrode and binder, and wet adhesive force due to the first binder acts when an electrolyte solution is injected. Specifically, when the coating layer is dried and no electrolyte solution is present, the first and second binders may be randomly distributed on the surface of the porous separator substrate. When bonding the electrode and separator, the second binder, which has a relatively low glass transition temperature and a large surface area, deforms due to temperature and pressure during bonding and may be present in greater amounts on the surface, forming dry adhesive force with the electrode. When an electrolyte solution is injected, the amorphous second binder, which has high solubility and fluidity in the electrolyte solution, may swell and migrate into the coating layer or the electrode. For example, when an organic solvent is used, a polymer binder with a high amorphous region may swell more than a binder with high crystallinity. The expansion of the polymer binder may cause fluidity, resulting in migration into the coating layer or the electrode. Therefore, since the crystalline first binder is primarily present on the surface of the coating layer, wet adhesion with the electrode can be formed by the crystalline first binder when an electrolyte is present.

The present invention uses a crystalline first binder with an average particle size (D50) of 200 nm or more, preferably 200 nm to 350 nm, and more preferably 200 nm to 300 nm. During the formation of the coating layer, a crystalline first binder that satisfies this range may migrate faster than other components and be distributed to the surface of the coating layer. The crystalline first binder may be more abundant on the surface of the coating layer located opposite the substrate.

When the separator is in a dry state, the amorphous second binder is present in greater amounts on the side of the coating layer opposite the porous separator substrate than on the surface facing the porous separator substrate. When the separator is impregnated with an electrolyte, the crystalline first binder is present in greater amounts on the side of the coating layer opposite the porous separator substrate than on the surface facing the porous separator substrate.

In addition, several factors, such as the particle size, aspect ratio, and density of the inorganic material, the density of the binder, the slurry viscosity, and the drying conditions, can affect the migration of the binder into the coating layer, and these factors may act together to affect the distribution of the binder. In particular, the migration of the binder can be significantly affected by the drying conditions of the coating layer.

FIG. 1 is a conceptual diagram showing the separator immediately after coating, not after impregnation with the electrolyte. Migration of the inorganic material and binder can occur depending on the density and particle size of the raw materials, the solvent (H ₂ O), and the drying temperature. The density of the inorganic particles can be 2 to 4.5 g/cm ³ , the density of the first binder can be 1.1 to 1.5 g/cm ³ , and the density of the second binder can be 0.5 to 1.1 g/cm ^3. The average particle size (D50) of the inorganic particles can be 500 nm to 5000 nm, the average particle size (D50) of the first binder can be 200 nm or more, preferably 200 nm to 350 nm, and the average particle size (D50) of the second binder can be 300 nm or more, preferably 350 nm to 500 nm.

The separator substrate contains pores that electrically insulate the negative and positive electrodes to prevent short circuits while allowing lithium ions to pass through. The separator substrate can be a porous membrane with high resistance to organic solvent electrolytes and fine pore diameters. The separator substrate can be any material that can be used as a separator material for secondary batteries, without particular limitations. Examples of suitable resins include polyolefins (e.g., polyethylene, polypropylene, and polybutene), polyvinyl chloride, polyethylene terephthalate, polycycloolefins, polyethersulfone, polyamides, polyimides, polyimideamides, polyaramids, polycycloolefins, nylons, polytetrafluoroethylene, and mixtures or copolymers thereof. Among these, polyolefin resins have excellent coating properties for coating layer slurries, allowing for a thinner separator for secondary batteries and a higher proportion of the electrode active material layer in the battery, thereby increasing capacity per volume.

The thickness of the separation membrane substrate may be 1 μm to 100 μm, specifically 1 μm to 30 μm, and the pore diameter of the separation membrane substrate may generally be 0.01 μm to 10 μm.

A coating layer can be formed on the separator substrate by applying and drying the slurry described below. Prior to applying the slurry, surface treatments such as plasma treatment or corona discharge can be performed to improve impregnation with the electrolyte.

The coating layer includes an inorganic material for improving the mechanical properties and insulating properties of the separator substrate and a binder for improving adhesion between the electrode and the separator. The binder provides adhesion between the electrode and the separator while also binding adjacent inorganic particles to maintain the bond. The binder includes a crystalline first binder and an amorphous second binder, and the first binder and the second binder are aqueous emulsion-type binders that use water as a solvent.

The thickness of the coating layer formed on one side of the porous separator substrate is 0.1 μm to 5 μm. Within this range, the binder can migrate to the surface of the coating layer depending on the drying conditions described below.

Crystalline First Binder and Non-Crystalline Second Binder The first binder may be polyvinylidene fluoride-based, and the second binder may be acrylate-based.

Specific examples of the crystalline first binder include polyvinylidene fluoride, polyhexafluoropropylene, polytetrafluoroethylene, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-trichloroethylene (PVDF-TCE), and polyvinylidene fluoride-chlorotrifluoroethylene (PVDF-TCE). It may be one or more selected from the group consisting of polyvinylidene fluoride-chlorotrifluoroethylene, PVDF-CTFE), and more specifically, it may be a copolymer containing polyvinylidene fluoride.

The crystalline first binder may have an average particle size (D50) of 200 nm or more, preferably 200 nm to 350 nm.

The weight average molecular weight of the crystalline first binder may be 10,000 to 10,000,000. The density of the crystalline first binder may be 1.1 to 1.5 g/cm ³ .

The crystalline first binder may have a glass transition temperature of 80°C to 200°C, preferably 80°C to 150°C, and more preferably 110°C to 145°C, and may exhibit high oxidation resistance to the electrolyte.

The amorphous second binder may be a polyacrylate-based binder. For example, the amorphous second binder may be one or more selected from the group consisting of styrene-butadiene rubber, nitrile-butadiene rubber, acrylonitrile-butadiene rubber, acrylonitrile-butadiene-styrene rubber, and acrylate-based polymers, and more specifically, may be a copolymer containing acrylate.

The amorphous second binder may have an average particle size (D50) of 350 nm or more, preferably 350 nm to 500 nm, and more preferably 350 nm to 450 nm. If the average particle size of the amorphous second binder exceeds 500 nm, mobility to the interface between the electrode and the coating layer in the absence of an electrolyte solution decreases, weakening the adhesion between the electrode and the separator, making it difficult to manufacture an electrode assembly and achieving uniform coating quality when forming the coating layer. Furthermore, as the number of binder particles decreases for the same binder content, the number of contact points between adjacent inorganic particles decreases, increasing the likelihood of inorganic particles detaching, reducing interstitial volume and resulting in degraded electrical resistance characteristics.

The amorphous second binder has a larger average particle size (D50) than the crystalline first binder, so it has a larger contact area with the electrode than the crystalline first binder and can provide better dry adhesion.

The weight average molecular weight of the amorphous second binder may be 10,000 to 10,000,000. The density of the amorphous second binder may be 0.5 to 1.1 g/ ^cm .

The amorphous second binder has a glass transition temperature of 40°C or higher, preferably 45°C to 60°C, and more preferably 48°C to 60°C. Conventionally, acrylic binder particles with a relatively small particle size of 100nm to 150nm have been used. However, these acrylic binder particles generally contain diene-based butadiene rubber, which has a low glass transition temperature and the presence of double bonds, which can cause side reactions with the electrolyte and generate gas. Therefore, rather than directly using an acrylic binder with a low glass transition temperature of less than 0°C as the second binder, a copolymer with a high glass transition temperature obtained by copolymerizing acrylic with styrene can be used to prevent side reactions with the electrolyte. Furthermore, within this glass transition temperature range, the amorphous second binder maintains its shape even when the initial coating layer dries, allowing it to exhibit dry adhesion to the electrode.

In one specific example, the first binder may have characteristics similar to those of the binder used in the positive electrode, and the second binder may have characteristics similar to those of the binder used in the negative electrode.

Specifically, the positive electrode can be made of a highly crystalline polyvinylidene fluoride homopolymer with a crystallinity of 95% or more. Therefore, for example, homo PVDF, which can be used as a crystalline first binder, can be included in the positive electrode mixture. The negative electrode can be made of a water-based emulsion-type amorphous acrylic binder.

In this way, because the separator coating layer uses a binder with similar characteristics to the binders used in the positive and negative electrodes, the adhesion between the positive electrode and separator, and between the negative electrode and separator, can be improved. For example, the same binder can be used in the coating layer and the corresponding positive and negative electrodes. Alternatively, the first crystalline binder can have the same polymer backbone as the binder used in the positive electrode, and the second amorphous binder can have the same polymer backbone as the binder used in the negative electrode. For example, the binder that can be used in the positive or negative electrode can be a PVdF homopolymer, and the binder used in the separator can be a PVdF-HFP binder with an HFP substitution rate of 10%.

The weight ratio of the first binder to the second binder may be 1:9 to 9:1, and more specifically, 3:7 to 7:3.

When the content of the first binder is less than 10% by weight based on the total weight of the first binder and the second binder, the dry adhesion strength, which is the adhesion strength in the absence of electrolyte, is excellent, but the wet adhesion strength, which is the adhesion strength in the presence of electrolyte, is too weak. When the content of the second binder is less than 10% by weight, the wet adhesion strength is excellent, but the dry adhesion strength is too weak.

Figure 3 is a top-down SEM image of the coating layer. The image shows the distribution of the crystalline first binder and the amorphous second binder contained in the coating layer.

The inorganic particles may be any inorganic material that can uniformly form a coating layer and does not undergo oxidation and/or reduction reactions within the operating voltage range of the secondary battery to be used. In one embodiment, the thickness uniformity may have a tolerance of ±2 μm. In particular, inorganic particles with ion transport ability can increase ionic conductivity within an electrochemical device, thereby improving performance. Furthermore, inorganic particles with a high dielectric constant can increase the degree of dissociation of electrolyte salts, such as lithium salts, in a liquid electrolyte, thereby improving the ionic conductivity of the electrolyte solution.

An example of such an inorganic material is an inorganic material that has at least one of the following properties: lithium ion transfer ability, piezoelectricity, and flame retardancy.

The inorganic material with good lithium ion transport ability refers to inorganic particles that contain lithium but have the function of transporting lithium ions without storing lithium. Inorganic particles with lithium ion transport ability can transport and move lithium ions through a type of defect present within the particle structure. This improves the lithium ion conductivity within the battery, thereby improving battery performance.

Examples of the inorganic particles having lithium ion transfer ability include lithium phosphate, lithium titanium phosphate, lithium aluminum titanium phosphate, (LiAlTiP) _x O _y -based glass, lithium lanthanum titanate, lithium germanium thiophosphate, lithium nitrides such as Li ₃ N (Li _x N _y , 0<x<4, 0<y<2), SiS ₂ -based glasses such as Li ₃ PO ₄ -Li ₂ S-SiS ₂ (Li _x Si _y S _z , 0<x<3, 0<y<2, 0<z<4), and P ₂ S ₅ -based glasses such as LiI-Li ₂ S-P ₂ S ₅ (Li _x P _y S _z , 0<x<3, 0<y<3, 0<z<7), LLZO-based, or mixtures thereof, but is not limited thereto.

These inorganic particles with piezoelectricity are non-conductors at normal pressure, but when a certain amount of pressure is applied, their internal structure changes and they become electrically conductive. They not only exhibit high dielectric constant characteristics with a dielectric constant of 100 or more, but also generate electric charges when stretched or compressed by the application of a certain amount of pressure, causing one side to become positively charged and the other side to become negatively charged, creating a potential difference between the two surfaces.

When inorganic particles with the above characteristics are used, if an internal short circuit occurs between the positive and negative electrodes due to an external impact such as a local crush or a nail, the inorganic particles coated on the separator not only prevent the positive and negative electrodes from coming into direct contact, but also create a potential difference within the particles due to the piezoelectricity of the inorganic particles. This allows electrons to move between the positive and negative electrodes, i.e., a minute current to flow, resulting in a gradual decrease in battery voltage and improved safety.

Examples of the inorganic particles having piezoelectricity include, but are not limited to, BaTiO ₃ , Pb(Zr,Ti)O ₃ (PZT), Pb _1-x La _x Zr _1-y Ti _y O ₃ (PLZT) (0<x<1, 0<y<1), Pb(Mg _1/3 Nb _2/3 )O ₃ -PbTiO ₃ (PMN-PT), hafnia (HfO ₂ ), or mixtures thereof.

The flame-retardant inorganic material not only prevents overcharging of the secondary battery, but also imparts flame-retardant properties to the separator and prevents a sudden rise in temperature inside the battery. The flame-retardant inorganic material is at least one selected from the group consisting of antimony-containing compounds, metal hydroxides or metal hydrates, guanidine-based compounds, boron-containing compounds, and zinc stannate compounds.

Specifically, the antimony-containing compound is selected from antimony trioxide ( _Sb2O3 ), _{antimony tetroxide} ( _Sb2O4 ), and antimony pentoxide ( _Sb2O5 ); the metal hydroxide or metal hydrate is selected from _{alumina (Al2O3), magnesium hydroxide, aluminum hydroxide (Al(OH)3 ) , aluminum oxyhydroxide} (AlO(OH)), and _CaOAl2O36H2O ; the guanidine-based compound is selected from the group consisting of guanidine nitride, guanidine sulfamate, guanidine phosphate, _and guanylurea phosphate; the boron-containing compound is _H3BO3 or _HBO2 ; _and the zinc stannate compound is selected from _Zn2SnO4 , _ZnSnO3 , and ZnSn(OH) _{6 .}

The average particle size (D50) of the inorganic particles can be 50 nm to 5,000 nm, preferably 200 nm to 1,000 nm, and more preferably 300 nm to 700 nm. If the average particle size of the inorganic particles is less than 50 nm, the specific surface area increases, requiring an additional binder to bond the inorganic particles together, which is disadvantageous from the perspective of electrical resistance. Furthermore, the greater the amount of binder present, the lower the peel strength and binder peeling may be. If the average particle size of the inorganic particles exceeds 5,000 nm, the uniformity of the coating layer surface decreases. Furthermore, if the size of the protruding particles after coating is large, the separator and electrodes may be damaged during lamination, potentially causing a short circuit.

The aspect ratio of the inorganic particles is 1 to 2, preferably 1.2 to 2, and the BET specific surface area of the inorganic particles may be 5 m ² /g to 25 m ² /g. Binder migration through the voids formed between the inorganic particles within this range is easy, and particularly in the case of monodispersion, the smaller the aspect ratio and the smaller the BET specific surface area, the better the binder migration within the coating layer may be.

The density of the inorganic particles may be 2 g/cm ³ to 4.5 g/cm ³ , preferably 2 g/cm ³ to 4 g/cm ³ . The density difference between the inorganic particles and the first and second binders may be 0.5 g/cm ³ to 4 g/cm ³ . When the density difference between the inorganic particles and the binders is within this range, there is a possibility that the binder will not easily move due to the density difference when forming a coating layer.

Dispersant : The coating layer may further include a dispersant to further improve the dispersibility of the inorganic material. The dispersant functions to maintain the inorganic material in a uniformly dispersed state within the binder during preparation of the coating layer slurry. Examples of materials that can be used as the dispersant include at least one selected from the group consisting of oil-soluble polyamines, oil-soluble amine compounds, fatty acids, fatty alcohols, sorbitan fatty acid esters, tannic acid, and pyrogallol. When a dispersant is included, the coating layer may contain the dispersant in an amount of 5 wt% or less.

The coating layer may include a water-based emulsion formed by mixing the crystalline first binder, the amorphous second binder, and inorganic particles in water as a solvent. The mixture may contain water in an amount of 2000 ppm or less, preferably 500 ppm or less.

The slurry to be applied to the porous separator substrate to form a coating layer includes the above-described crystalline first binder, amorphous second binder, inorganic particles, and solvent, and may further include additives such as a dispersant.

The slurry may contain the inorganic particles and the binder, including the first and second binders, in a weight ratio of 90:10 to 10:90, preferably 80:20 to 20:80. The amount of solids contained in the slurry may be 15 to 30 wt %, and the viscosity of the slurry may be 25 to 150 cP. Outside this range, the movement of the binder in the coating layer may be hindered, making it difficult to ensure sufficient adhesive strength.

Drying Conditions and Concentration Gradient The slurry in which the first crystalline binder, the second amorphous binder, and the inorganic particles are mixed with an aqueous solvent may be applied to the porous polymer substrate and dried to form a coating layer. The coating layer may have a concentration gradient of the first crystalline binder, the second amorphous binder, and the inorganic particles in a thickness direction.

After the slurry is applied to at least one surface of a porous separator substrate, the substrate can be moved through a heating zone and dried to form a coating layer. The porous separator substrate with the applied slurry can be dried while moving through a heating zone heated to a predetermined temperature at a predetermined speed to form a separator with a coating layer. The heating temperature of the heating zone can be 45°C to 65°C. The porous separator substrate can be moved through the heating zone at a speed of 25 m/min to 150 m/min, preferably 40 m/min to 60 m/min. The drying conditions for the coating layer can be determined depending on the morphological differences between the binder and inorganic particles, and the drying conditions can determine the concentration gradient of each binder and inorganic particle distributed along the thickness of the coating layer.

More specifically, by adjusting the molecular weight, glass transition temperature, density, and average particle size of the binder; the aspect ratio, BET specific surface area, density, and average particle size of the inorganic particles; and the viscosity of the slurry within the aforementioned ranges, and by adjusting the drying temperature in the heating zone and the moving speed of the porous separator substrate moving within the heating zone, the distribution of the binder and inorganic material, and the corresponding concentration gradient, can be controlled in the coating layer formed.

For example, when inorganic particles have an aspect ratio of 1 to 2, a BET specific surface area of 5 to ^{25 m} /g, a density of ² g/cm to 4.5 g/cm, a density difference between the inorganic particles and ^{the first} and second binders of 0.5 g/cm to 4 g/cm, and a viscosity of a slurry containing these is 25 to 150 cP, the slurry can be applied and then dried for 2 to 10 minutes while passing through heating zones at a speed of 40 m/min, each heated at 45°C to 65°C. Specifically, the heating zones may be five to seven, each set at a temperature of 65°C to 45°C.

When drying in a heating zone, the initial drying temperature is the highest, and subsequent drying can be performed at temperatures equal to or lower than the initial drying temperature. Conventionally, the drying temperature was gradually increased or a peak (T-zone) where the drying temperature was particularly high was created to prevent binder migration within the coating layer. In the present invention, the initial drying temperature is set to the highest temperature, allowing binder migration during the coating layer formation process, ensuring both dry and wet adhesion.

For example, the drying temperature in the first heating zone may be 65°C, and the drying temperature in the final heating zone may be 45°C. Even if the first heating zone is 65°C, the second binder contained in the actual coating layer may not deform because it is heated below its glass transition temperature of 60°C due to the specific heat of the solvent. Because the binder has excellent mobility within the coating layer under these drying conditions, the binder is more widely distributed on the surface opposite the surface facing the porous separator substrate in the formed coating layer, and the inorganic particles are more widely distributed on the surface facing the porous separator substrate.

In the coating layer, the content of inorganic particles increases from the surface opposite the surface facing the porous separation membrane substrate toward the facing surface, and the content of binder increases from the surface facing the porous separation membrane substrate toward the opposite surface.

In one embodiment, the weight ratio of the first and second binders to the inorganic particles present on the surface of the coating layer facing the porous separator substrate may be 45:55 to 0:100, and the weight ratio of the first and second binders to the inorganic particles present on the surface opposite the surface may be 50:50 to 100:0.

Adhesion Strength of Separator In the separator for a secondary battery according to the present invention, the wet adhesion strength, which is the adhesion strength of the separator when immersed in an electrolyte, may be 1.0 gf/20 mm or more, specifically 10 gf/20 mm or more, and more specifically 7 gf/20 mm to 20 gf/20 mm. If the wet adhesion strength is less than 1 gf/20 mm, the stiffness of the cell may be reduced, leading to assembly problems and separator folding problems during electrode assembly manufacture. If the wet adhesion strength exceeds 20 gf/20 mm, impregnation of the electrolyte into the separator may be hindered, potentially resulting in the precipitation of lithium dendrites.

In addition, the dry adhesive strength of the separator for secondary batteries, which is the adhesive strength of the separator in a dry state, may be 10 gf/20 mm or more, specifically 10 gf/20 mm or more but less than 100 gf/20 mm, and more specifically 30 gf/20 mm to less than 100 gf/20 mm. If the dry adhesive strength is 100 gf/20 mm or more, impregnation of the electrolyte into the separator may be hindered, which may result in the precipitation of lithium dendrites. Referring to Figure 4, if the drying temperature is too low, the binder does not move sufficiently within the coating layer, resulting in a decrease in the adhesive strength of the binder. As the drying temperature increases, the binder content on the surface of the coating layer increases, thereby increasing the electrode adhesive strength. However, above a certain temperature, the binder soaks, reducing the adhesive strength.

Bend Strength
After injecting an electrolyte solution to fabricate a cell, it is difficult to disassemble the cell to measure the adhesive strength between the separator and the electrode. When a peeling experiment is performed to measure the adhesive strength between the electrode and the separator after laminating the separator and the electrode, peeling between the porous separator substrate and the coating layer may occur instead of peeling between the electrode and the separator. Instead of performing the peeling experiment, the flexural strength can be measured indirectly by laminating the separator and one electrode together.

Bending strength can be measured as the maximum bending stress experienced by the sample at the maximum displacement, and the maximum bending stress can increase in proportion to the wet adhesive strength. For example, the maximum bending stress can be the maximum value of the stress applied while applying force to three points on the separator and electrode laminate to reach a maximum displacement of 20 mm.

For example, if the maximum bending stress is 1.3 MPa, the wet adhesive strength may be approximately 1 gf/20 mm, and if the maximum bending stress is 26 MPa, the wet adhesive strength may be approximately 20 gf/20 mm. If the maximum bending stress is less than 1.3 MPa, the adhesive strength between the electrode and separator is not secured, making cell assembly difficult and potentially resulting in the separator folding. If the maximum bending stress exceeds 26 MPa, the wet adhesive strength becomes too high, making it difficult for the separator to be impregnated with the electrolyte, potentially resulting in the precipitation of lithium dendrites in that area.

The present invention also provides a secondary battery including an electrode assembly in which the separator for a secondary battery is interposed between a positive electrode and a negative electrode, and the electrode assembly is housed in a battery case and then immersed in a non-aqueous electrolyte solution containing a lithium salt, which is poured into the battery case.

The positive electrode can be manufactured by, for example, applying a positive electrode mixture, which is a mixture of a positive electrode active material composed of positive electrode active material particles, a conductive material, and a binder, to a positive electrode current collector, and a filler can be further added to the positive electrode mixture as needed.

The positive electrode current collector is generally manufactured to a thickness of 3 μm to 500 μm. It is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, it can be made of stainless steel, aluminum, nickel, titanium, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver, and more specifically, aluminum can be used. The current collector can have fine irregularities on its surface to increase the adhesion of the positive electrode active material, and can be in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

The positive electrode active material may include, in addition to the positive electrode active material particles, layered compounds such as lithium nickel oxide (LiNiO ₂ ) and compounds substituted with one or more transition metals; lithium manganese oxides such as those having the chemical formula Li _1+x Mn _2-x O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , and LiMnO ₂ ; lithium copper oxide (Li ₂ CuO ₂ ); vanadium oxides such as LiV ₃ O ₈ , LiV ₃ O ₄ , V ₂ O ₅ , and Cu ₂ V ₂ O ₇ ; and those having the chemical formula LiNi _1-x M _x O ₂ The lithium manganese composite oxides may be composed of, but are not limited to, Ni-site lithium nickel oxides represented by the chemical formula LiMn _2-x M _x O ₂ (wherein M is Co, Ni, Fe, Cr, Zn, or Ta, and x is 0.01 to 0.1) or Li ₂ Mn ₃ MO ₈ (wherein M is Fe, Co, Ni, Cu, or Zn); LiMn ₂ O ₄ in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion; disulfide compounds; and Fe ₂ (MoO ₄ ) ₃ .

The conductive material is typically added in an amount of 0.1% to 30% by weight based on the total weight of the mixture including the positive electrode active material. There are no particular limitations on the conductive material, as long as it is conductive and does not induce chemical changes in the battery. Examples of such conductive materials include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

The binder contained in the positive electrode is a component that aids in bonding the active material and conductive material, etc., and in bonding to the current collector. It is typically added in an amount of 0.1 to 30% by weight based on the total weight of the mixture including the positive electrode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer, styrene-butadiene rubber, fluororubber, and various copolymers.

In one embodiment, the binder contained in the positive electrode has similar properties to the crystalline first binder contained in the coating layer. Specifically, a binder having the same polymer backbone as the crystalline first binder can be used as the positive electrode binder. For example, the crystalline first binder used in the coating layer can be a PVDF copolymer, and the binder used in the positive electrode can be a PVDF homopolymer. These binders have similar backbone structures, which can contribute to improving the adhesion between the electrode and the separator.

Negative Electrode The negative electrode is prepared by applying a negative electrode active material onto a negative electrode current collector and drying the applied material. If necessary, the negative electrode may further contain the components contained in the positive electrode.

The negative electrode current collector is generally made to a thickness of 3 μm to 500 μm. There are no particular limitations on the material of this negative electrode current collector, as long as it is conductive and does not induce chemical changes in the battery. Examples of materials that can be used include copper, stainless steel, aluminum, nickel, titanium, baked carbon, copper or stainless steel surfaces treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys. As with the positive electrode current collector, the surface can be made finely uneven to strengthen the binding strength of the negative electrode active material. The negative electrode current collector can be used in a variety of forms, including films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

Examples of the negative electrode active material include carbon such as non-graphitizable carbon and graphite-based carbon; metal composite _oxides such as _LixFe2O3 (0≦x≦1), _LixWO2 (0≦x≦1), _{and SnxMe1 -xMe'yOz (} Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, elements of Groups 1, 2, and 3 of the periodic table, halogens; 0<x≦1;1≦y≦3; 1≦z≦8) _; lithium metal; lithium alloys; silicon-based alloys _; tin-based alloys; _SnO , _SnO2 , PbO, _PbO2 , _Pb2O3 , _Pb3O4 , _Sb2O3 , _Sb2O4 , _Sb2O5 , _GeO , _GeO2 , _and Bi. Metal oxides such as _{Bi 2} O ₃ , Bi ₂ O ₄ and Bi ₂ O ₅ ; conductive polymers such as polyacetylene; Li—Co—Ni based materials, etc. can be used.

The negative electrode may further include a binder. The binder contained in the negative electrode is a component that aids in binding the active material and conductive material, etc., and in binding to the current collector. It is typically added in an amount of 0.1 to 30% by weight based on the total weight of the mixture including the negative electrode active material. Examples of such binders include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, polyacrylic acid and its hydrogen-substituted materials with Li, Na, Ca, etc., and various copolymers.

In one embodiment, the binder contained in the negative electrode has properties similar to those of the second amorphous binder contained in the coating layer. Specifically, a binder having the same polymer skeleton as the second amorphous binder can be used as the negative electrode binder.

Electrolyte The non-aqueous electrolyte solution containing a lithium salt comprises an electrolyte solution and a lithium salt, and as the electrolyte solution, a non-aqueous organic solvent, an organic solid electrolyte, an inorganic solid electrolyte, or the like is used.

Examples of the non-aqueous organic solvent that can be used include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran (franc), 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate.

Examples of the organic solid electrolyte that can be used include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyagitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and polymerizing agents containing ionic dissociating groups.

Examples of the inorganic solid electrolyte that can be used include Li nitrides, _halides , and sulfates such as Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ NLiI-LiOH, LiSiO ₄ , LiSiO _{4 -LiI} -LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Li 4 SiO 4 -LiI-LiOH, and Li ₃ PO ₄ -Li ₂ S-SiS ₂ .

The lithium salt is a substance that is easily soluble in the non-aqueous electrolyte _{, and examples thereof include LiCl, LiBr, LiI, LiClO4, LiBF4, LiB10Cl10, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6 , LiSbF6 , LiAlCl4 , CH3SO3Li , ( CF3SO2 ) 2NLi , lithium chloroborane , lithium} lower aliphatic carboxylate, lithium tetraphenylborate, and imide.

The present invention also provides a battery module and a battery pack that include the secondary battery as a unit cell, and a device that includes the battery module or battery pack.

Specific examples of such devices include, but are not limited to, small devices such as computers, mobile phones, and power tools; power tools powered by battery-powered motors; electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters; electric golf carts; and medium- to large-sized devices such as power storage systems.

The structures of the battery modules, battery packs, and devices are well known in the art, so detailed descriptions thereof will be omitted in this specification.

The following description will be given with reference to examples of the present invention, but these are intended to facilitate understanding of the present invention and are not intended to limit the scope of the present invention.

Example 1
The inorganic substance used was alumina (Sumitomo, AES11) with an average particle size (D50) of 500 nm, a density of 4 g/cm ³ , an aspect ratio of 1.3, and a BET specific surface area of 8 m ² /g. The crystalline polymer used was a fluorine-based water-dispersed emulsion (Arkema, Aquatec 9530, solids content 30 wt%) with an average particle size (D50) of 200 nm. The amorphous polymer used was an acrylic-based water-dispersed emulsion (LGC, SA22, solids content 30 wt%) with an average particle size (D50) of 350 nm and a density of 1 g/cm ^3. Carboxymethylcellulose (GLCem, SG-L02). These were mixed in the following ratios, in order: 79:18:2:1, to produce a slurry. The viscosity of the slurry was 50 cP (solids content 30 wt%). A 9 μm thick polyethylene SV-9 (SEM corp) was used as the porous separator substrate, and the separation membrane coating slurry was coated on both sides of the polyolefin-based separator substrate to a thickness of 3 μm using a bar coater.

The porous separator substrate coated with the separation membrane coating slurry was placed in seven heating zones at temperatures of 65°C, 65°C, 60°C, 55°C, 50°C, 45°C, and 45°C, respectively, and dried for a total of 2-3 minutes while moving at a speed of 40 m/min, producing a separation membrane with an overall thickness of 15 μm.

Example 2
A separator was prepared using the same method as in Example 1, except that the weight ratio of the first crystalline binder to the second amorphous binder was changed from 9:1 to 7:3.

Example 3
A separator was prepared using the same method as in Example 1, except that the weight ratio of the first crystalline binder to the second amorphous binder was changed from 9:1 to 5:5.

Example 4
A separator was manufactured using the same method as in Example 1, except that the weight ratio of the first crystalline binder to the second amorphous binder was changed from 9:1 to 3:7.

Example 5
A separator was manufactured using the same method as in Example 1, except that the weight ratio of the first crystalline binder to the second amorphous binder was changed from 9:1 to 1:9.

(Comparative Example 1)
A separator was manufactured using the same method as in Example 1, except that the weight ratio of the first crystalline binder to the second amorphous binder was changed from 9:1 to 10:0.

(Comparative Example 2)
A separator was prepared using the same method as in Example 1, except that the weight ratio of the first crystalline binder to the second amorphous binder was changed from 9:1 to 0:10.

(Comparative Example 3)
A separator was manufactured using the same method as in Example 1, except that the average particle size (D50) of the amorphous second binder was changed to 800 nm.

(Comparative Example 4)
A separator was prepared using the same method as in Example 1, except that the amorphous second binder was changed to an acrylate homopolymer having an average particle size (D50) of 350 nm and a glass transition temperature of less than 0°C.

(Comparative Example 5)
A separator was prepared using the same method as in Example 1, except that the inorganic material was changed to aluminum hydroxide having a density of 2 g/cm ³ , an aspect ratio of 2.3, and a BET specific surface area of 40 m ² /g.

(Comparative Example 6)
A separator was manufactured using the same method as in Example 1, except that the viscosity of the slurry for coating the separator was changed to 10 cP.

(Comparative Example 7)
A separator was manufactured in the same manner as in Example 1, except that the heating zone was changed to 80° C. for one drying step during the formation of the coating layer.

(Experimental Example 1) Measurement of air permeability (Gurley)
The air permeability measuring device used was the EG01-55-1MR model manufactured by Asahi Seiko Co., Ltd.

The separation membranes manufactured in Examples 1 to 5 and Comparative Examples 1 to 7 were fixed to the hollow upper and lower tips of an air permeability measuring device, and the time it took for 100 cc of air to pass through the separation membrane substrate was measured using a set differential pressure applied to the upper and lower tips to determine the air permeability, and the results are shown in Table 1 below.

(Experimental Example 2) Measurement of dry adhesive strength
The negative electrode, in which a carbon-based negative electrode mixture is coated on a copper foil, and the separators prepared in Examples 1 to 5 and Comparative Examples 1 to 7 were each cut to a width of 25 mm and arranged to overlap each other.

This was then pressed using a hot press at 60°C, 6.5 MPa, and for 1 second to prepare a sample for measuring electrode adhesion strength.

To measure the adhesive strength between the negative electrode and the separator, a 180° peel test was performed at 300 mm/min using an Instron UTM device.

The dry adhesive strength measured in this manner is shown in Table 1 below.

(Experimental Example 3) Measurement of Wet Adhesion Strength
The positive electrode in which the positive electrode mixture was coated on the aluminum foil and the separator prepared in Examples 1 to 5 and Comparative Examples 1 to 7 were each cut to a width of 20 mm and placed in a pouch, and a carbonate-based electrolyte solution was poured into the pouch.

The pouch was pressurized at 5 kgf, 70°C, and 4 minutes to prepare a sample for measuring electrode adhesion strength.

To measure the adhesive strength between the positive electrode and the separator, a 90° peel test was performed at 200 mm/min using an Instron UTM device.

The wet adhesive strength measured in this manner is shown in Table 1 below.

(Experimental Example 4) Measurement of separation membrane resistance (Ω)
The separators prepared in Examples 1 to 5 and Comparative Examples 1 to 7 were interposed between SUS sheets to prepare coin cells.

An electrolyte containing 1M LiPF ₆ and a mixture of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:2 was poured into the coin cell.

To measure the resistance of the coin cell, electrochemical impedance spectroscopy was performed using a BioLogic Science Instruments VMP3 at 25°C with an amplitude of 10 mV and a scan range of 0.1 Hz to 1 MHz, and the results are shown in Table 1 below. The coin cell resistance is preferably 0.2 Ω to 1.8 Ω.

(Experimental Example 5) Measurement of bending strength
The separator and one electrode prepared in Example 1 were each prepared to a size of 3 cm x 3 cm, stacked, and laminated at 65°C under a pressure of 3 kgf/ ^cm² for 8 minutes, and then 3 g of the same electrolyte solution used in Experimental Example 4 was injected. The laminate was subjected to a 3-point bending test using a Universal Testing Machine (UTM) at a compression rate of 50 mm/min to measure the maximum bending stress for a maximum displacement of 20 mm. The results are shown in Table 1 below.

Referring to Table 1 above, the separators manufactured in the examples have a dry adhesion strength of 10 gf/20 mm or more and a wet adhesion strength of 1.0 gf/20 mm or more compared to the separators manufactured in the comparative examples. If the dry adhesion strength is too strong, as in comparative example 2, electrolyte impregnation becomes difficult, and lithium dendrites precipitate in those areas.

Furthermore, when the crystalline first binder and the amorphous second binder are contained in equal proportions, as in Example 3, the dry adhesive strength is 40.7 gf/20 mm and the wet adhesive strength is 17.0 gf/20 mm.

In other words, the separator for a secondary battery according to the present invention can ensure adhesive strength not only in the absence of electrolyte but also in the presence of electrolyte by configuring the weight ratio of the crystalline first binder to the amorphous second binder so that at least one of a dry adhesive strength of 30 gf/20 mm or more and a wet adhesive strength of 10 gf/20 mm or more is satisfied.

Anyone with ordinary skill in the art to which the present invention pertains will be able to make various applications and modifications within the scope of the present invention based on the above content.

Claims (17)

A separator for a secondary battery, comprising: a porous separator substrate containing a polymer; and a coating layer on at least one surface of the porous separator substrate,
the coating layer includes inorganic particles, a crystalline first binder, and a non-crystalline second binder;
the crystalline first binder and the non-crystalline second binder are each independently an aqueous emulsion binder;
a concentration gradient of the inorganic particles and the amorphous second binder exists across the cross section of the coating layer, the inorganic particles are present in greater amounts on the surface of the coating layer facing the porous separation membrane substrate than on the opposite side of the porous separation membrane substrate, and when the separation membrane is in a dry state, the amorphous second binder is present in greater amounts on the surface of the coating layer opposite the porous separation membrane substrate than on the surface facing the porous separation membrane substrate;
the crystalline first binder includes a polyvinylidene fluoride-based binder, and the non-crystalline second binder includes an acrylate-based binder;
When the separator is immersed in an electrolyte, a concentration gradient of the crystalline first binder exists across a cross section of the coating layer, and the crystalline first binder is present in a greater amount on the surface of the coating layer opposite the porous separator substrate than on the surface facing the porous separator substrate,
The separator for a secondary battery, wherein the second amorphous binder has a larger average particle size (D50) than the first crystalline binder. The separator for a secondary battery according to claim 1, wherein the weight ratio of the first crystalline binder to the second amorphous binder is 1:9 to 9:1. The separator for a secondary battery according to claim 1, wherein the adhesive strength of the separator when immersed in the electrolyte is 1.0 gf/20 mm to 20.0 gf/20 mm. The separator for a secondary battery according to claim 1, wherein the adhesive strength of the separator in a dry state is 10 gf/20 mm or more and less than 100 gf/20 mm. The separator for a secondary battery according to claim 1, wherein at least one of the first crystalline binder and the second amorphous binder is a copolymer. 2. The separator for a secondary battery of claim 1, wherein the first crystalline binder is at least one selected from the group consisting of polyvinylidene fluoride , polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-trichloroethylene (PVDF-TCE), and polyvinylidene fluoride-chlorotrifluoroethylene (PVDF-CTFE). The separator for a secondary battery according to claim 1 , wherein the second amorphous binder is an acrylate-containing polymer . The separator for a secondary battery according to claim 1, wherein the crystalline first binder is formed from particles having an average particle size (D50) of 200 nm to 350 nm. The separator for a secondary battery according to claim 1, wherein the amorphous second binder is formed from particles having an average particle size (D50) of 350 nm to 500 nm. The separator for a secondary battery according to claim 1, wherein the density of the inorganic particles is higher than the density of the first crystalline binder and the density of the second amorphous binder. 2. The separator for a secondary battery according to claim 1, wherein the inorganic particles have an aspect ratio of 1 to 2 and a BET specific surface area of 5 to 25 m ² /g. The separator for a secondary battery according to claim 1, wherein the coating layer is formed by drying a slurry containing inorganic particles, a crystalline first binder, a non-crystalline second binder, and water, with a solids content of 15 to 30 wt % and a viscosity of 25 to 150 cP. The separator for a secondary battery according to claim 12, wherein the coating layer is formed by applying the slurry to the porous separator substrate and then drying at 45 to 65°C. The separator for a secondary battery described in claim 13, wherein the drying is performed at the highest initial drying temperature. A secondary battery including an electrode assembly,
The electrode assembly comprises a stack of a positive electrode, a negative electrode, and the separator for a secondary battery according to claim 1 , disposed between the positive electrode and the negative electrode. The secondary battery of claim 15, wherein the bending strength of the secondary battery is 9.0 MPa to 26.0 MPa. A battery module including the secondary battery described in claim 15.