Separator, secondary battery including the same, and method for manufacturing secondary battery

Abstract

The present invention relates to a separator, a secondary battery including the same, and a manufacturing method thereof. The present invention provides a separator having excellent ion conductivity and satisfying both the resistance and stability required for a secondary battery by including two types of binder polymers that dissolve in an electrolyte at different temperatures in different regions of a porous coating layer of the separator, thereby ensuring lithium ion pathways within the separator.

Description

[Technical Field]

[0001] The present invention relates to a separator, a secondary battery including the same, and a method for manufacturing the secondary battery. Specifically, the present invention relates to a separator that has excellent ionic conductivity and satisfies both the resistance and stability required for a secondary battery, a secondary battery including the same, and a method for manufacturing the secondary battery.

[0002] This application claims priority based on Korean Patent Application No. 10-2022-0148632, filed on November 9, 2022, and the entire contents disclosed in the specification and drawings of that application are incorporated herein by reference. [Background technology]

[0003] In recent years, ensuring the safety of electrochemical devices has been attracting much attention. In particular, secondary batteries such as lithium secondary batteries have an electrode assembly including a positive electrode, a negative electrode, and a separator, and such electrode assemblies can be fabricated in a structure in which a separator is interposed between the positive electrode and the negative electrode.

[0004] The separator used in the secondary battery has improved heat resistance and safety by forming a porous coating layer, which is a mixture of inorganic particles and a binder, on at least one surface of a porous substrate.

[0005] The binder used in the manufacture of a separator serves to bind and fix inorganic particles and adhere the electrode and separator. However, such a binder not only coats the surface of the porous substrate but also penetrates into the pores of the porous substrate, impairing the ion distribution function of the separator and resulting in an increase in the resistance of the secondary battery.

[0006] In addition, if the binder content is low or if it is easily dissolved in the electrolyte, the adhesion between the electrode and separator may be reduced, causing them to separate, or the binder in the porous coating layer may be unable to fix the inorganic particles, making it difficult to ensure stability. Also, if the binder content is increased to improve adhesion, there is the problem of reduced ionic conductivity. Summary of the Invention [Problem to be solved by the invention]

[0007] Therefore, an object of the present invention is to solve the above-mentioned problems and provide a separator that secures a path through which ions can move within the separator, thereby providing excellent ionic conductivity and satisfying both low resistance and stability, a secondary battery including the same, and a method for manufacturing the secondary battery. [Means for solving the problem]

[0008] The present inventors have discovered that the above problems can be solved through the following separator, secondary battery, and manufacturing method thereof.

[0009] According to a first aspect, there is provided a porous coating layer disposed on at least one surface of the porous substrate, the porous coating layer including inorganic particles and a binder polymer disposed on a part or the entire surface of the inorganic particles to connect and fix the inorganic particles to each other, the binder polymer including a first binder polymer and a second binder polymer, the first binder polymer being a polymer that is dissolved in an electrolytic solution when immersed in the electrolytic solution at a temperature of 65 to 75°C for 3 to 5 minutes, and the second binder polymer being a polymer that is dissolved in an electrolytic solution when immersed in the electrolytic solution at a temperature of 65 to 75°C for 3 to 5 minutes. the porous coating layer comprises a first porous coating layer region and a second porous coating layer region, at least a portion of the first porous coating layer region is exposed on a surface portion of the porous coating layer, the first porous coating layer region comprises the first binder polymer as the binder polymer, and the second porous coating layer region comprises the second binder polymer as the binder polymer.

[0010] A second aspect relates to the separator according to the first aspect, wherein the first porous coating layer region includes two or more regions spaced apart from each other.

[0011] A third aspect relates to the separator according to the first or second aspect, wherein at least a portion of the second porous coating layer region is partitioned by the first porous coating layer region.

[0012] A fourth aspect relates to the separator according to any one of the first to third aspects, wherein the first binder polymer comprises poly(vinylidenefluoride-hexafluoropropylene) (PVdF-HFP) and the second binder polymer comprises poly(vinylidenefluoride-tetrafluoroethylene) (PVdF-TFE).

[0013] According to a fifth aspect, there is provided the separator according to any one of the first to fourth aspects, wherein the weight average molecular weight of the first binder polymer is 300,000 to 600,000.

[0014] According to a sixth aspect, there is provided the separator according to any one of the first to fifth aspects, wherein the weight average molecular weight of the second binder polymer is 300,000 to 400,000.

[0015] A seventh aspect relates to the separator according to any one of the first to sixth aspects, characterized in that the first binder polymer is a PVdF-HFP copolymer having a hexafluoropropylene (HFP) substitution rate of 5 to 20 wt%, and the second binder polymer is a PVdF-TFE copolymer having a tetrafluoroethylene (TFE) substitution rate of 15 to 25 wt%.

[0016] According to an eighth aspect, there is provided a secondary battery comprising a positive electrode, a negative electrode, and the separator according to any one of the first to seventh aspects interposed between the positive electrode and the negative electrode.

[0017] According to a ninth aspect, there is provided the secondary battery according to the eighth aspect, wherein the first porous coating layer region has one or more grooves having a predetermined width and depth.

[0018] According to a tenth aspect, there is provided a secondary battery according to the ninth aspect, characterized in that the area of ​​the grooves is 20 to 30% of the total area of ​​the surface of the porous coating layer, based on the area occupied by the grooves on the surface of the porous coating layer.

[0019] According to an eleventh aspect, there is provided the secondary battery according to the ninth or tenth aspect, wherein the depth of the groove is 0.5 to 1 μm.

[0020] A twelfth aspect relates to the secondary battery according to any one of the ninth to eleventh aspects, wherein the grooves are arranged at regular or irregular intervals.

[0021] According to a thirteenth aspect, there is provided a method for manufacturing a secondary battery having a lithium ion path, the method including the steps of: preparing a first slurry containing inorganic particles, a first binder polymer, and a solvent; and preparing a second slurry containing inorganic particles, a second binder polymer, and a solvent; and coating the first slurry and the second slurry on at least one surface of a porous substrate having pores to prepare a separator having a porous coating layer, the porous coating layer comprising a first porous coating layer region containing the first binder polymer as the binder polymer and a second porous coating layer region containing the second binder polymer as the binder polymer. a step of preparing a spare secondary battery by interposing the separator between a positive electrode and a negative electrode, incorporating the electrode assembly into an electrode case, and injecting an electrolyte solution; and a step of performing a pressure and heat pretreatment on the spare secondary battery by placing the spare secondary battery between two jig plates and applying heat at a temperature of 65 to 75°C to the secondary battery using the jig plates for 3 to 5 minutes, wherein during the pressure and heat pretreatment, at least a portion of the first binder polymer contained in the first porous coating layer region is dissolved in the electrolyte solution, and one or more grooves having a predetermined width and depth are formed on the surface of the porous coating layer.

[0022] According to a fourteenth aspect, the method for producing a porous substrate according to the thirteenth aspect is characterized in that the step of coating the first slurry and the second slurry on at least one surface of the porous substrate includes: (1) a step of alternately coating the first slurry and the second slurry on the porous substrate; (2) a step of coating the second slurry on the porous substrate to a predetermined thickness, and then alternately coating the first slurry and the second slurry on the coated second slurry; or (3) a step of coating the second slurry on the porous substrate to a predetermined thickness, and then coating the first slurry in a predetermined shape on the coated second slurry with gaps.

[0023] A fifteenth aspect relates to the manufacturing method according to the thirteenth or fourteenth aspect, wherein the first binder polymer comprises poly(vinylidenefluoride-hexafluoropropylene) (PVdF-HFP), and the second binder polymer comprises poly(vinylidenefluoride-tetrafluoroethylene) (PVdF-TFE).

[0024] According to a 16th aspect, there is provided the method according to any one of the 13th to 15th aspects, wherein the weight average molecular weight of the first binder polymer is 300,000 to 600,000.

[0025] According to a seventeenth aspect, there is provided the method according to any one of the thirteenth to sixteenth aspects, wherein the weight average molecular weight of the second binder polymer is 300,000 to 400,000.

[0026] According to an 18th aspect, there is provided the method according to any one of the 13th to 17th aspects, wherein the first binder polymer is a PVdF-HFP copolymer having a hexafluoropropylene (HFP) substitution rate of 5 to 20 wt%, and the second binder polymer is a PVdF-TFE copolymer having a tetrafluoroethylene (TFE) substitution rate of 15 to 25 wt%.

[0027] According to a 19th aspect, the present invention relates to a manufacturing method according to any one of the 13th to 18th aspects, characterized in that the area of ​​the grooves is 20 to 30% of the total area of ​​the surface of the porous coating layer, based on the area occupied by the grooves on the surface of the porous coating layer.

[0028] According to a twentieth aspect, there is provided the method according to any one of the thirteenth to nineteenth aspects, wherein the depth of the groove is 0.5 to 1 μm. [Effects of the Invention]

[0029] According to the present invention, a separator having lithium ion pathways within the separator can be provided by providing two types of binder polymers having different dissolving temperatures in an electrolyte solution in different regions of a porous coating layer.

[0030] In addition, the present invention provides a secondary battery that is manufactured by subjecting a battery to a heat treatment under predetermined conditions, whereby one of two different binder polymers contained in the porous coating layer of the separator is dissolved in the electrolyte, thereby forming grooves in the porous coating layer of the separator. The grooves formed in the porous coating layer serve as paths through which ions can move within the separator, thereby providing a secondary battery and a method for manufacturing the same that exhibit excellent ionic conductivity, low resistance, and excellent air permeability.

[0031] The drawings attached to this specification illustrate preferred embodiments of the present invention and serve to further understand the technical concepts of the present invention as well as the content of the invention, and therefore the present invention should not be interpreted as being limited to only the matters depicted in the drawings. [Brief explanation of the drawings]

[0032] [Figure 1] 1 is a schematic illustration of a cross section of a porous coating layer according to one embodiment of the present invention; [Figure 2] 10 is a schematic cross-sectional view of a porous coating layer according to another embodiment of the present invention; [Figure 3] 1A-1C are top views of separators formed in various patterns according to various embodiments of the present invention. [Figure 4] FIG. 2 is a top view of the separator produced in Example 1. [Figure 5] FIG. 1 is a top view of a separator produced in Example 2. [Figure 6] FIG. 10 is a top view of the separator produced in Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0033] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, the terms and words used in this specification and claims should not be construed as being limited to their ordinary or dictionary meanings, but should be construed as meanings and concepts corresponding to the technical ideas of the present invention, in accordance with the principle that the inventor can appropriately define the concepts of terms himself / herself in order to best describe the invention. Therefore, it should be understood that the embodiment described in this specification and the configuration shown in the drawings are merely a preferred embodiment of the present invention and do not represent the entire technical ideas of the present invention, and therefore, various equivalent and modified embodiments may be available as of the time of filing this application.

[0034] Throughout this specification, when a part is described as "comprising" or "having" a certain component, this does not mean that other components are excluded, but that other components may also be included, unless otherwise specified.

[0035] Throughout this specification, the phrase "A and / or B" means "A or B or both."

[0036] A secondary battery according to one aspect of the present invention is a secondary battery including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, the separator including a porous substrate having pores, and a porous coating layer disposed on at least one surface of the porous substrate, the porous coating layer including a plurality of inorganic particles and a binder polymer disposed on a part or the entire surface of the inorganic particles to connect and fix the inorganic particles to each other, the binder polymer including a first binder polymer and a second binder polymer, the first binder polymer being capable of forming an electrolytic solution when immersed in an electrolyte at a temperature of 65 to 75°C for 3 to 5 minutes. the second binder polymer is a polymer that is dissolved in the electrolyte solution when immersed in the electrolyte solution at a temperature of 65 to 75°C for 3 to 5 minutes; the porous coating layer includes a first porous coating layer region and a second porous coating layer region, at least a portion of the first porous coating layer region is exposed to a surface portion of the porous coating layer; the first porous coating layer region includes the first binder polymer as a binder polymer; and the second porous coating layer region includes the second binder polymer as a binder polymer.

[0037] The porous substrate used in the separator of the present invention can be any planar porous substrate typically used in secondary batteries, such as porous membranes or nonwoven fabrics formed from a variety of polymers. For example, polyolefin-based porous membranes and nonwoven fabrics made of polyethylene terephthalate fibers, which are used as separators for secondary batteries, particularly lithium secondary batteries, can be used. Their materials and shapes can be selected from a wide variety of materials depending on the intended purpose. For example, polyolefin-based porous membranes can be formed from polymers containing one or more polyolefin-based polymers selected from the group consisting of polyethylene (e.g., high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene), polypropylene, polybutylene, and polypentene. Similarly, nonwoven fabrics can be made from fibers made of polyolefin-based polymers or polymers with higher heat resistance than these.

[0038] The porous substrate may have a thickness in the range of 1 μm to 30 μm. For example, the porous substrate may have a thickness in the range of 1 μm to 20 μm or 5 μm to 20 μm. When the thickness of the porous substrate satisfies this range, mechanical properties are easily maintained and an increase in battery resistance can be prevented.

[0039] The porosity of the porous substrate may be in the range of 30% to 75%. For example, the porosity of the porous substrate may be in the range of 35% to 65%. When the porosity is in this range, an increase in battery resistance can be prevented and the mechanical properties of the porous substrate can be maintained.

[0040] The pore size of the porous substrate may be in the range of 0.01 μm to 5.0 μm. For example, the pore size of the porous substrate may be in the range of 0.1 μm to 1.0 μm. When the pore size satisfies this range, an increase in battery resistance due to a blocked pore structure can be prevented, and the self-discharge characteristics of a typical secondary battery can be maintained.

[0041] The binder polymer includes a first binder polymer and a second binder polymer.

[0042] The first binder polymer and the second binder polymer are selected from those capable of providing binding strength between inorganic particles and between the inorganic particles and the porous substrate layer, and the first binder polymer and the second polymer binder have differentiated properties in that they dissolve in the electrolyte at different temperatures.

[0043] According to one aspect of the present invention, the first binder polymer is a polymer that dissolves in the electrolyte solution when immersed in the electrolyte solution at a temperature of 65 to 75°C for 3 to 5 minutes, and the second binder polymer is a polymer that does not dissolve in the electrolyte solution when immersed in the electrolyte solution at a temperature of 65 to 75°C for 3 to 5 minutes.

[0044] That is, the temperatures at which the first binder polymer and the second binder polymer dissolve in the electrolyte do not overlap. Therefore, when heat treatment is performed for 3 to 5 minutes at a temperature of 65 to 75°C, only a portion of the first binder polymer dissolves in the electrolyte, thereby providing wet adhesion and ensuring a path for lithium ion migration. Specifically, pores can be formed in the portion of the porous coating layer where the first binder polymer is dissolved. When the portion of the porous coating layer where the first binder polymer is dissolved is exposed to the surface of the porous coating layer, the pores form channels that can serve as paths for lithium ion migration. The second binder polymer remains undissolved, providing wet adhesion and maintaining the cell rigidity. Therefore, when such a separator is applied to a secondary battery, it can increase lithium ion conductivity, exhibit low resistance, and exhibit excellent air permeability.

[0045] According to one aspect of the present invention, the solubility of the first binder polymer and the second binder polymer in the electrolyte solution can be determined by comparing the proportion of the dissolved binder polymer after immersing each binder polymer in the electrolyte solution and storing it at 70°C for 5 minutes. If the proportion of the dissolved binder polymer is 10 wt% or more of the total weight of the binder polymer, it can be determined that the binder polymer is dissolved. If the proportion of the dissolved binder polymer is less than 10 wt% of the total weight of the binder polymer, it can be determined that the binder polymer is not dissolved. In this case, any conventional electrolyte solution known in the art containing an organic solvent, a lithium salt, and / or an additive can be used, and an electrolyte solution in which the lithium salt and additive content is 5 wt% or less relative to 100 wt% of the organic solvent can be used. For example, a solution can be used in which LiPF6 is dissolved to a concentration of 1.0 M in an organic solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed in a composition ratio of 3:7 (volume ratio), and an additive of 0.5 wt% vinylene carbonate (VC) is dissolved.

[0046] The electrolyte is A + B - A salt having the structure: + Li + , Na + , K. + or a combination thereof, such as B - is PF6 - , BF4 - , Cl - , Br - , I - , ClO4 - , AsF6 - , CH3CO2 - , CF3SO3 - , N(CF3SO2)2 - , C(CF2SO2)3 -Examples of suitable solvents include, but are not limited to, salts containing negative ions such as those listed above or combinations thereof dissolved or dissociated in organic solvents such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), γ-butyrolactone, and mixtures thereof. Examples of suitable additives include vinylene carbonate (VC), catechol carbonate (CC), biphenyl, cyclohexylbenzene, PS (propanesultone), and ESA (ethylene sulfate).

[0047] The salt can be used by appropriately changing it within a range that is normally usable, and the salt can be contained in the electrolyte solution at a concentration of 0.5 M to 2 M, specifically, 0.9 M to 1.5 M. For example, a lithium salt can be used as the salt.

[0048] In the electrolyte solution of the present invention, the content of the lithium salt and additives is preferably 5 wt % or less relative to 100 wt % of the organic solvent, from the viewpoint of the solubility of the first binder polymer and the second binder polymer.

[0049] According to one embodiment of the present invention, the first binder polymer may include poly(vinylidenefluoride-hexafluoropropylene) (PVdF-HFP).

[0050] The second binder polymer may include poly(vinylidenefluoride-tetrafluoroethylene) (PVdF-TFE).

[0051] According to one embodiment of the present invention, the first binder polymer can be poly(vinylidenefluoride-hexafluoropropylene) (PVdF-HFP), and the second binder polymer can be poly(vinylidenefluoride-tetrafluoroethylene) (PVdF-TFE).

[0052] According to one aspect of the present invention, the first binder polymer may be a PVdF-HFP copolymer having a hexafluoropropylene (HFP) substitution rate of 5 to 20 wt %, or 10 to 15 wt %. When the hexafluoropropylene (HFP) content is within the above range, excellent ionic conductivity and air permeability can be exhibited when a separator containing the PVdF-HFP is used in a battery.

[0053] According to one embodiment of the present invention, the second binder polymer may be a PVdF-TFE copolymer having a tetrafluoroethylene (TFE) substitution rate of 15 to 25 wt% or 18 to 23 wt%. When the tetrafluoroethylene (TFE) content is within the above range, when a separator containing this is included in a battery, the binding force between inorganic particles in the separator can be maintained, and sufficient heat resistance and stability can be achieved.

[0054] Here, the substitution rate means the percentage (%) of the number of specific repeating units relative to the total number of repeating units in the polymer.

[0055] The first binder polymer has a weight average molecular weight (M w Preferably, the first binder polymer has a weight average molecular weight (M) in the range of 350,000 to 500,000. wWhen the weight-average molecular weight of the first binder polymer satisfies this range, it can be dissolved in an electrolyte solution at a temperature of 65 to 75°C, and the viscosity of a slurry containing the first binder polymer can be controlled to ensure coatability, thereby enabling a uniform coating layer to be obtained.

[0056] The second binder polymer has a weight average molecular weight (M w When the weight-average molecular weight of the second binder polymer satisfies this range, the viscosity of the slurry containing the second binder polymer is controlled, ensuring coatability and providing a uniform coating layer.

[0057] The inorganic particles according to one embodiment of the present invention are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles usable in the present invention are those that can be used in the range of the operating voltage of the applied battery (e.g., Li / Li + There are no particular limitations on the inorganic particles, as long as they do not undergo oxidation and / or reduction reactions at a voltage (0 to 5 V relative to the reference voltage). As a non-limiting example, the inorganic particles may have an average particle size in the range of 0.001 to 3 μm or 0.001 to 2 μm. When the average particle size of the inorganic particles falls within this range, dispersibility is improved and excessive buildup of a coating layer can be prevented. Here, the average particle size of the inorganic particles refers to the particle size (D50) at 50% of the integrated value from the smallest particle size side, calculated based on the particle size distribution of the particles after classification using a conventional particle size distribution analyzer.

[0058] Non-limiting examples of the inorganic particles include Al2O3, AlOOH, Al(OH)3, AlN, BN, MgO, Mg(OH)2, SiO2, ZnO, TiO2, BaTiO3, or a mixture of two or more of these.

[0059] The content of the inorganic particles in the porous coating layer may be in the range of 50% by weight to 90% by weight, or 50% by weight to 80% by weight, based on 100% by weight of the porous coating layer.

[0060] According to one aspect of the present invention, the porous coating layer includes inorganic particles and a binder polymer that binds the inorganic particles together (i.e., the binder connects and fixes the inorganic particles together) so that the inorganic particles can be maintained in a bound state, and the binder can maintain the inorganic particles and the porous substrate in a bound state.

[0061] According to one embodiment of the present invention, the average pore size of the porous coating layer may be 10 nm to 900 nm, or 20 nm to 100 nm. The pore size can be calculated by analyzing the shape of the pores using an SEM image. The pore size can be calculated by taking the closed curve formed by the intersection of binder threads as the pore shape. The pore size of the porous coating layer according to one embodiment of the present invention can be measured using capillary flow porometry. Capillary flow porometry measures the diameter of the smallest pore in the thickness direction. Therefore, to measure the pore size of the porous coating layer alone using capillary flow porometry, the porous coating layer must be separated from the porous substrate and then wrapped in a supportive nonwoven fabric. In this case, the pore size of the nonwoven fabric must be significantly larger than that of the coating layer. The porosity of the porous coating layer according to one embodiment of the present invention is preferably 50% to 85%. A porosity of 85% or less ensures mechanical properties that can withstand the pressing process for bonding to the electrode, and is advantageous for ensuring adhesive strength because the surface opening ratio does not become excessively high. On the other hand, a porosity of 50% or more is higher than the porosity of most porous substrates and is advantageous from the viewpoint of ion permeability.

[0062] Meanwhile, the porosity according to one embodiment of the present invention can be measured using an adsorption gas such as nitrogen using a BELSORP (registered trademark) (BET equipment) manufactured by BELJAPAN, or can be measured by methods such as mercury intrusion porosimetry, capillary flow porosimetry, etc. Alternatively, the true density of the electrode active material layer obtained according to one embodiment of the present invention can be calculated from the density (apparent density) of the electrode (electrode active material layer) obtained according to one embodiment of the present invention, the composition ratio of materials contained in the electrode (electrode active material layer), and the density of each component, and the porosity of the electrode active material layer can be calculated from the difference between the apparent density and the true density.

[0063] The thickness of the porous coating layer is preferably 0.5 μm to 5 μm on one side of the porous substrate. The thickness may be 0.5 μm or more, preferably 1 μm or more. Within this range, the adhesive strength with the electrode is excellent, resulting in increased cell strength of the battery. On the other hand, a thickness of 5 μm or less is advantageous in terms of the cycle characteristics and resistance characteristics of the battery. From this perspective, the thickness is preferably 4 μm or less, more preferably 3 μm or less.

[0064] According to one embodiment of the present invention, the porous coating layer includes a first porous coating layer region and a second porous coating layer region, wherein the first porous coating layer region includes only a first binder polymer as a binder polymer, and the second porous coating layer region includes only a second binder polymer as a binder polymer.

[0065] The porous coating layer of the present invention may include a region other than the first porous coating layer region and the second porous coating layer region, and may consist of the first porous coating layer region and the second porous coating layer region. However, the present invention may further include a plurality of regions as long as the porous coating layer is partitioned into at least two regions and each region contains a different binder polymer, and the porous coating layer is not limited to the content of this specification.

[0066] For example, referring to Figures 1 and 2, the first porous coating layer region (region B in Figure 1) may have two or more regions that are separated from each other, and at least a portion of the second porous coating layer region may be separated by the first porous coating layer region (region A in Figure 1).

[0067] According to one embodiment of the present invention, at least a portion of the first porous coating layer region is exposed to a surface portion of the porous coating layer. The surface portion of the porous coating layer in this invention refers to at least one surface of the porous coating layer that is not in contact with the porous substrate. By exposing at least a portion of the first porous coating layer region to the surface portion of the porous coating layer, one or more predetermined grooves can be provided in the first porous coating layer region.

[0068] The porous coating layer according to one embodiment of the present invention has one or more grooves on the surface, each having a predetermined width and depth.

[0069] The grooves are formed in a pressurized and heated pretreatment step of the secondary battery manufacturing method described below. Specifically, in the pressurized and heated pretreatment step, at least a portion of the first binder polymer contained in the first porous coating layer region is dissolved in the electrolyte solution to form at least one groove. That is, the shape of the grooves may correspond to the shape of the first porous coating layer region.

[0070] In this case, the width of the groove may be measured as the length of one cross section based on the surface of the porous coating layer, and the depth of the groove may be measured as the vertical distance to the bottom of the groove based on the surface of the porous coating layer.

[0071] According to one aspect of the present invention, the area of ​​the grooves on the surface of the porous coating layer may be 20 to 30% of the total area of ​​the surface of the porous coating layer, based on the area occupied on the surface of the porous coating layer.

[0072] The groove depth may be 0.5 to 1 μm, and the groove width may be 10 to 30 mm. In this case, the groove depth may refer to the average depth of the grooves present on the surface of the porous coating layer. For example, the groove depth may be expressed based on the average value of the depths of at least 10 grooves.

[0073] Furthermore, the width of the groove may be measured as the length of one cross section based on the surface of the porous coating layer. Specifically, for example, if the grooves are striped, the width of the groove may refer to the stripe width. If the grooves are dotted, the width may represent the average value of the diameters of at least 10 grooves. By ensuring that the area, depth, and width of the grooves fall within the above ranges, the heat resistance of the porous coating layer can be ensured, wrinkles due to variations in the thickness of the porous coating layer can be prevented, and the electrolyte can be sufficiently absorbed.

[0074] The grooves according to one aspect of the present invention may be formed at regular or irregular intervals. The shape of the grooves in the present invention is not particularly limited, but may be formed in the shape of stripes or dots that are formed regularly or irregularly.

[0075] The secondary battery according to the present invention includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. The electrode used in the present invention is not particularly limited, and may be prepared by a conventional method well known in the art, in which an electrode active material is bound to an electrode current collector.

[0076] Non-limiting examples of the positive electrode active material among the electrode active materials include conventional positive electrode active materials that can be used for the positive electrode of a lithium secondary battery. In particular, it is preferable to use lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a lithium composite oxide that is a combination of these.

[0077] Non-limiting examples of the negative electrode active material include conventional negative electrode active materials that have been used in the negative electrodes of lithium secondary batteries. In particular, lithium metal or lithium alloys, and lithium adsorbent materials such as carbon, petroleum coke, activated carbon, graphite, or other carbons can be suitably used.

[0078] Non-limiting examples of positive electrode current collectors include foils made of aluminum, nickel, or a combination thereof, and non-limiting examples of negative electrode current collectors include foils made of copper, gold, nickel, or a copper alloy, or a combination thereof.

[0079] A method for manufacturing a secondary battery having a lithium ion path according to one embodiment of the present invention includes the steps of preparing a first slurry containing inorganic particles, a first binder polymer, and a solvent, and a second slurry containing inorganic particles, a second binder polymer, and a solvent, and coating the first slurry and the second slurry on at least one surface of a porous substrate having pores to prepare a separator having a porous coating layer, the porous coating layer having a first porous coating layer region containing the first binder polymer as a binder polymer and a second porous coating region containing the second binder polymer as a binder polymer. a step of preparing a spare secondary battery by interposing the separator between a positive electrode and a negative electrode, incorporating the electrode assembly into an electrode case, and injecting an electrolyte solution; and a step of performing a pressure and heat pretreatment on the spare secondary battery by placing the spare secondary battery between two jig plates and applying heat at a temperature of 65 to 75°C to the secondary battery using the jig plates for 3 to 5 minutes, wherein during the pressure and heat pretreatment, at least a portion of the first binder polymer contained in the first porous coating layer region is dissolved in the electrolyte solution, and one or more grooves having a predetermined width and depth are formed on the surface of the porous coating layer.

[0080] First, a slurry for forming a porous coating layer is prepared by mixing a solvent, inorganic particles, and a binder polymer. At this time, by changing the type of binder, a first slurry containing a first binder polymer and a second slurry containing a second binder polymer are prepared. Please refer to the above description for details of the inorganic particles, the first binder polymer, and the second binder polymer.

[0081] The solvent preferably has a solubility index approximately equal to that of the first and second binder polymers to be used and a low boiling point. This facilitates uniform mixing and subsequent solvent removal. Non-limiting examples of solvents that can be used include one or a mixture of two or more selected from the group consisting of acetone, methyl ethyl ketone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), and cyclohexane.

[0082] Thereafter, the first slurry and the second slurry are coated on at least one surface of a planar porous substrate having a large number of pores to form a porous coating layer.

[0083] In this case, the porous coating layer has a first porous coating layer region containing only the first binder polymer as the binder polymer and a second porous coating layer region containing only the second binder polymer as the binder polymer.

[0084] According to one aspect of the present invention, when a first slurry and a second slurry are coated on at least one surface of the porous substrate to form a porous coating layer, (1) the first slurry and the second slurry are alternately coated on the porous substrate, or (2) the second slurry is coated on the porous substrate to a predetermined thickness, and the first slurry and the second slurry are alternately coated on the coated second slurry, or (3) the second slurry is coated on the porous substrate to a predetermined thickness, and the first slurry is coated in a predetermined shape on the coated second slurry with gaps.

[0085] As described above, by coating the first slurry and the second slurry on one side of the porous substrate, it is possible to create a first porous coating layer region containing only the first binder polymer as the binder polymer, and a second porous coating layer region containing only the second binder polymer as the binder polymer.

[0086] 1 to 3 illustrate examples of a porous coating layer having a first porous coating layer region and a second porous coating layer region. Region A in Figures 1 and 2 is the area where the second slurry (containing a second binder polymer and a polymer that is not dissolved in the electrolyte) is applied, and region B (containing the first binder polymer and a polymer that is dissolved in the electrolyte) is the area where the first slurry is applied.

[0087] The method for coating the first and second slurries is not particularly limited, and a conventional coating method can be used. For example, the first and / or second slurries are applied and then dried. A conventional coating method in the technical field of the present invention, such as a slot die coater, a Mayer bar, a reverse roll coater, or a gravure coater, can be used for coating, and a coating method using a slot die coater is preferably used.

[0088] Next, the separator is interposed between the positive electrode and the negative electrode to prepare an electrode assembly, and the electrode assembly is placed in an electrode case and an electrolyte is injected to prepare a spare secondary battery. For details of the positive electrode, negative electrode, and electrolyte, please refer to the above description.

[0089] Next, the spare secondary battery is placed between two jig plates, and a pressurized and heated pretreatment is performed by applying heat at a temperature of 65 to 75° C. to the spare secondary battery using the jig plates for 3 to 5 minutes.

[0090] In the pressure and heat pretreatment step, at least a portion of the first binder polymer contained in the first porous coating layer region is dissolved in the electrolyte solution, thereby forming one or more grooves having a predetermined width and depth on the surface of the porous coating layer.

[0091] At this time, the magnitude of the pressure applied to the spare secondary battery by the fixture plate is 2 to 5 kgf / cm 2 , or 3 to 4 kgf / cm 2 It could be.

[0092] As described above, the first binder polymer is a polymer that dissolves in the electrolyte when immersed in the electrolyte at a temperature of 65 to 75°C for 3 to 5 minutes, and the second binder polymer is a polymer that does not dissolve in the electrolyte when immersed in the electrolyte at a temperature of 65 to 75°C for 3 to 5 minutes. Therefore, in the pressurized and heated pretreatment step, only the first binder polymer contained in the first porous coating layer region is dissolved in the electrolyte.

[0093] The regions of the first porous coating layer where the first binder polymer is dissolved may become channels that serve as paths for lithium ions to move through. For details about the channels, please refer to the above description.

[0094] That is, the method for manufacturing a secondary battery according to the present invention provides a path through which lithium ions can move on the surface of the porous coating layer, thereby increasing the conductivity of lithium ions. When a separator having such a porous coating layer is used in a secondary battery, it can exhibit low resistance and excellent air permeability.

[0095] The present invention will be described in detail below with reference to examples. However, the examples according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the examples detailed below. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0096] Example Example 1 Separator manufacturing A first binder polymer (PVdF-HFP, HFP substitution rate: 15%, weight-average molecular weight: approximately 400,000) and inorganic particles (alumina Al2O3, manufactured by Nippon Light Metal Co., Ltd., LS235, with an average particle size (D50) of 500 nm) were added to N-methylpyrrolidone (NMP) as a solvent to prepare a first slurry. The weight ratio of the binder polymer to the inorganic particles was 20:80.

[0097] A second binder polymer, PVdF-TFE (polyvinylidene fluoride-tetrafluoroethylene, TFE substitution rate 20%, weight average molecular weight approximately 300,000), and inorganic particles, alumina Al2O3 (LS235, manufactured by Nippon Light Metal Co., Ltd.) with an average particle size (D50) of 500 nm, were added to N-methylpyrrolidone (NMP) as a solvent to prepare a second slurry. The weight ratio of the binder polymer to the inorganic particles was 20:80.

[0098] Then, the first slurry and the second slurry were coated alternately on a 12 μm thick polyethylene porous substrate (porosity 45%) to a thickness of about 1.5 μm and in a rectangular stripe pattern to finally produce a separator.

[0099] Manufacture of positive and negative electrodes 97 wt% LiCoO2, 1.5 wt% carbon black powder as a conductive material, and 1.5 wt% polyvinylidene fluoride (PVdF, manufactured by Kureha Corporation) were mixed and added to N-methyl-2-pyrrolidone solvent and stirred for 30 minutes using a mechanical stirrer to prepare a cathode active material slurry. The slurry was applied to a 20 μm thick aluminum current collector using a doctor blade to a thickness of approximately 60 μm, dried for 0.5 hours in a hot air dryer at 100°C, and then dried again for 4 hours under vacuum conditions at 120°C. The cathode was then rolled to prepare a cathode.

[0100] A mixture of 96.5 wt% artificial graphite particles (LC1, manufactured by Ningbo Shanshan Co., Ltd., China) with an average particle size of 16 μm, 2.3 wt% styrene-butadiene rubber (SBR) binder (manufactured by Zeon Corporation), and 1.2 wt% carboxymethyl cellulose (CMC, manufactured by Daicel Corporation, Japan) was added to distilled water and stirred for 60 minutes using a mechanical stirrer to prepare anode active material slurry. The slurry was then coated onto an 8 μm-thick copper current collector using a doctor blade to a thickness of approximately 60 μm, dried in a hot air dryer at 100°C for 0.5 hours, and then dried again at 120°C in a vacuum atmosphere for 4 hours. The resulting mixture was then rolled to prepare anodes.

[0101] Manufacture of spare secondary batteries An electrode assembly was manufactured by interposing a separator between the positive electrode and negative electrode manufactured as described above and then housed in an electrode case. Then, an organic electrolyte solution was injected into the electrode assembly, which was prepared by dissolving LiPF6 to a concentration of 1.0 M in an organic solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a composition ratio of 3:7 (volume ratio) and dissolving 0.5 wt% of vinylene carbonate (VC) as an additive, to manufacture a spare secondary battery.

[0102] Manufacturing secondary batteries with secured lithium-ion pathways The prepared spare secondary battery was placed between two jig plates and subjected to a pressure of 3 kgf / cm 2Then, a pressure of 1000 kJ / cm2 was applied and heat at a temperature of 70°C was applied for 4 minutes to perform a pressure heating pretreatment.

[0103] As a result of the pressurized and heated pretreatment, a portion of the first binder polymer contained in the first porous coating layer region of the separator of the preliminary secondary battery was dissolved in the electrolyte, providing grooves on the surface of the porous coating layer and ensuring lithium ion pathways. The area of ​​the grooves was approximately 20% of the total area of ​​the porous coating layer, the length of the grooves (representing the width in the transverse direction (TD) of the stripes) was 10 to 30 mm, and the maximum depth of the grooves was within the range of 0.5 to 1 μm.

[0104] Examples 2 and 3 A separator was manufactured in the same manner as in Example 1, except that the second slurry was applied onto a porous substrate using a slot die coater, and the first slurry was coated onto the coated second slurry at a distance to form a circular shape. In this case, the area of ​​the grooves was about 25 to 30% of the total area of ​​the porous coating layer, the length of the grooves (referring to the diameter of the circular shape) was 20 to 30 mm, and the depth of the grooves was within the range of 0.5 to 1 μm.

[0105] Comparative Example 1 A separator was produced in the same manner as in Example 1, except that only the first slurry was applied onto the porous substrate.

[0106] Comparative Example 2 A separator was produced in the same manner as in Example 1, except that only the second slurry was applied onto the porous substrate.

[0107] Comparative Example 3 A separator was fabricated by coating a porous substrate with a slurry prepared by mixing PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene, HFP substitution rate 15%, weight average molecular weight approximately 400,000), PVdF-TFE (polyvinylidene fluoride-tetrafluoroethylene, TFE substitution rate 20%, weight average molecular weight approximately 300,000), and inorganic particles in N-methylpyrrolidone (NMP) as a solvent. The weight ratio of PVDF-HFP:PVDF-TFE:inorganic particles was 10:10:80.

[0108] Evaluation example Determining the solubility of the binder polymer in the electrolyte The solubility of the binder polymer in the electrolyte was judged by immersing each binder polymer in the electrolyte and storing it at 70°C for 5 minutes, and then comparing the proportion of the binder polymer dissolved. If 10% or more of the binder polymer was dissolved, it was judged to be dissolved.

[0109] The electrolyte used in this study was an organic solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a 3:7 (volume ratio) mixture, in which LiPF6 was dissolved to a concentration of 1.0 M and an additive of 0.5 wt% vinylene carbonate (VC) was dissolved.

[0110] The solubility of PVdF-HFP used as the first binder polymer in Example 1 was evaluated, and the dissolved ratio of the first binder polymer was approximately 10 wt % of the total weight of the first binder polymer, indicating that the first binder polymer was dissolved in the electrolyte. On the other hand, the solubility of PVdF-TFE used as the second binder polymer was evaluated, and the dissolved ratio of the second binder polymer was less than 10 wt % of the total weight of the second binder polymer, indicating that the second binder polymer was not dissolved in the electrolyte.

[0111] Cross-sectional observation of separator (analysis of groove specifications on the surface of the porous coating layer) The cross sections of the separators produced in the examples were observed using a field emission scanning electron microscope (FE-SEM) (Hitachi S-4800 Scanning Electron Microscope). Figures 4 to 6 are SEM images of one side (top surface) of the separators produced in Examples 1 to 3.

[0112] Wet adhesion (wet adhesion) measurement The wet adhesive strength of each of PVdF-HFP as the first binder polymer and PVdF-TFE as the second binder polymer used in Example 1 was measured.

[0113] Separators (Production Examples 1 to 4) manufactured using only the first slurry prepared in Example 1 and separators (Production Examples 5 and 6) manufactured using only the second slurry prepared in Example 1 were stacked with electrodes, inserted into pouches, and then injected with an electrolyte solution. They were then stored at room temperature for 24 hours. After this, they were subjected to a pressure of 3 kgf / cm at each measurement temperature in Table 1 below. 2 The electrode and separator were pressed together for 5 minutes using a universal testing machine (UTM), and the wet adhesive strength at the interface was measured while peeling them off.

[0114] In Preparation Examples 1 to 4, it was confirmed that the wet adhesive strength decreased as the temperature increased because the first binder polymer was dissolved in the electrolyte solution. In contrast, in Preparation Examples 5 and 6, it was confirmed that the wet adhesive strength was maintained because the second binder polymer was not dissolved in the electrolyte solution even when the temperature increased.

[0115] [Table 1] Battery resistance measurement The separators produced in Example 1 and Comparative Example 3 were punched into 18π circular shapes and immersed in an electrolyte solution at 70° C. for about 4 minutes to produce coin cells. The electrolyte solution used here was the same as the electrolyte solution used in producing the secondary battery of Example 1.

[0116] Thereafter, the resistance was measured using an electrochemical impedance spectroscopy (EIS) device.

[0117] It was confirmed that Example 1, in which grooves were provided on the surface of the porous coating layer to ensure lithium ion pathways, exhibited lower resistance than Comparative Example 3, in which the porous coating layer was prepared by simply mixing two types of binders.

[0118] [Table 2]

Claims

1. a porous substrate having pores; a porous coating layer disposed on at least one surface of the porous substrate, the porous coating layer comprising inorganic particles and a binder polymer disposed on a part or the entire surface of the inorganic particles to connect and fix the inorganic particles to each other; and the binder polymer includes a first binder polymer and a second binder polymer; the first binder polymer is a polymer that dissolves in the electrolyte solution when immersed in the electrolyte solution at a temperature of 65 to 75°C for 3 to 5 minutes; the second binder polymer is a polymer that is not dissolved in the electrolyte solution when immersed in the electrolyte solution at a temperature of 65 to 75°C for 3 to 5 minutes; the porous coating layer includes a first porous coating layer region and a second porous coating layer region; At least a portion of the first porous coating layer region is exposed to a surface portion of the porous coating layer; A separator, wherein the first porous coating layer region contains the first binder polymer as the binder polymer, and the second porous coating layer region contains the second binder polymer as the binder polymer.

2. The separator of claim 1 , wherein the first porous coating layer region comprises two or more regions spaced apart from one another.

3. The separator of claim 1 , wherein at least a portion of the second porous coating layer region is bounded by the first porous coating layer region.

4. the first binder polymer comprises poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP), 2. The separator of claim 1, wherein the second binder polymer comprises poly(vinylidenefluoride-tetrafluoroethylene) (PVdF-TFE).

5. 2. The separator according to claim 1, wherein the weight average molecular weight of the first binder polymer is 300,000 to 600,000.

6. 2. The separator according to claim 1, wherein the weight average molecular weight of the second binder polymer is 300,000 to 400,000.

7. the first binder polymer is a PVdF-HFP copolymer having a hexafluoropropylene (HFP) substitution rate of 5 to 20 wt %, 2. The separator according to claim 1, wherein the second binder polymer is a PVdF-TFE copolymer having a tetrafluoroethylene (TFE) substitution rate of 15 to 25 wt %.

8. A secondary battery, A secondary battery comprising a positive electrode, a negative electrode, and the separator according to claim 1 interposed between the positive electrode and the negative electrode.

9. The secondary battery according to claim 8 , wherein the first porous coating layer region has one or more grooves having a predetermined width and depth.

10. 10. The secondary battery of claim 9, wherein the area of ​​the grooves is 20 to 30% of the total area of ​​the surface of the porous coating layer, based on the area of ​​the surface of the porous coating layer occupied by the grooves.

11. 10. The secondary battery according to claim 9, wherein the depth of the groove is 0.5 to 1 μm.

12. 10. The secondary battery according to claim 9, wherein the grooves are arranged at regular or irregular intervals.

13. A method for manufacturing a secondary battery having a lithium ion pathway, comprising: preparing a first slurry containing inorganic particles, a first binder polymer, and a solvent, and a second slurry containing inorganic particles, a second binder polymer, and a solvent; a step of preparing a separator having a porous coating layer by coating a first slurry and a second slurry on at least one surface of a porous substrate having pores, the porous coating layer including a first porous coating layer region containing the first binder polymer as the binder polymer and a second porous coating layer region containing the second binder polymer as the binder polymer; preparing an electrode assembly by interposing the separator between a positive electrode and a negative electrode, and then incorporating the electrode assembly into an electrode case and injecting an electrolyte solution into the electrode case to prepare a spare secondary battery; a step of performing a pressure and heat preliminary treatment in which the spare secondary battery is placed between two jig plates and heat is applied to the spare secondary battery at a temperature of 65 to 75°C for 3 to 5 minutes using the jig plates; Including, During the pressure and heat pretreatment, at least a portion of the first binder polymer contained in the first porous coating layer region is dissolved in the electrolyte solution, thereby forming one or more grooves having a predetermined width and depth on the surface of the porous coating layer.

14. The step of coating the first slurry and the second slurry on at least one surface of the porous substrate includes: (1) coating the first slurry and the second slurry alternately onto the porous substrate; (2) coating the second slurry onto the porous substrate to a predetermined thickness, and then coating the first slurry and the second slurry alternately onto the coated second slurry; or (3) coating the second slurry onto the porous substrate to a predetermined thickness, and coating the first slurry onto the coated second slurry with a gap in a predetermined shape; The method of claim 13, comprising:

15. the first binder polymer comprises poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP), The method of claim 13, wherein the second binder polymer comprises poly(vinylidenefluoride-tetrafluoroethylene) (PVdF-TFE).

16. The method according to claim 13, wherein the weight average molecular weight of the first binder polymer is 300,000 to 600,000.

17. The method according to claim 13, wherein the weight average molecular weight of the second binder polymer is 300,000 to 400,000.

18. the first binder polymer is a PVdF-HFP copolymer having a hexafluoropropylene (HFP) substitution rate of 5 to 20 wt %, The manufacturing method according to claim 13, wherein the second binder polymer is a PVdF-TFE copolymer having a tetrafluoroethylene (TFE) substitution rate of 15 to 25 wt %.

19. The method of claim 13, wherein the area of ​​the grooves is 20 to 30% of the total area of ​​the surface of the porous coating layer, based on the area of ​​the surface of the porous coating layer occupied by the grooves.

20. The manufacturing method according to claim 13, wherein the depth of the groove is 0.5 to 1 μm.

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