Separator substrate, method for preparing the same, and separator

Abstract

The present invention relates to a separator substrate, a separator, an electrode assembly, and an electrochemical device, and the separator substrate according to one embodiment of the present invention has a surface roughness value (Sa) of a predetermined range. Accordingly, the separator using the separator base material maintains excellent adhesion strength, and the wear resistance of the separator is improved.

Description

Technical Field

[0001] The present disclosure relates to a separator substrate, a method for preparing the separator substrate, and a separator including the separator substrate. More particularly, the present disclosure relates to a separator having improved adhesion strength and wear resistance characteristics, and an electrode assembly and an electrochemical device including the separator.

[0002] This application claims the priority of Korean Patent Application No. 2023-0095292, filed in Korea on July 21, 2023, the disclosure of which is incorporated herein by reference. Background Art

[0003] A lithium secondary battery is manufactured through a process of inserting an electrode assembly including a positive electrode / separator / negative electrode into a battery case, injecting an electrolyte, and sealing the battery case. Separators for lithium secondary batteries typically include a polyolefin-based porous substrate, and in order to solve the problem of short circuits between the positive and negative electrodes caused by the thermal shrinkage behavior of the polyolefin-based porous substrate, separators having a coating of a mixture of inorganic particles and an adhesive polymer on the surface of the porous substrate have been developed to improve the strength and heat resistance of the separator. For example, such separators may include a safety reinforced separator (SRS) and a ceramic coated separator (CCS).

[0004] Since the inorganic particles in the coating act as spacers to maintain the physical shape of the separator, when exposed to high temperatures, the SRS or CCS can inhibit the shrinkage of the porous substrate, thereby preventing direct contact between the positive and negative electrodes. Therefore, an electrode assembly is manufactured by adhering the positive electrode and the negative electrode with a separator having porous coatings on both surfaces therebetween.

[0005] On the other hand, common types of unit cells of secondary batteries include cylindrical, square, and pouch types. In the case of a cylindrical secondary battery unit cell, the battery is manufactured by winding a positive electrode and a negative electrode with a separator interposed therebetween as an insulator to form a jelly roll type electrode assembly, and placing the electrode assembly in a battery can.

[0006] In this case, due to the volume expansion of the negative electrode during repeated charge and discharge of the battery, the porous coating of the separator is continuously scratched, and finally the separator is broken, resulting in an internal short circuit, that is, impingement. The internal short circuit may cause a fire in the cylindrical battery unit cell.

[0007] In addition, when the porous coating of the separator is scratched, the inorganic particles and / or the adhesive polymer of the porous coating may drift as impurities in the battery, resulting in a voltage drop behavior greater than the self-discharge rate of the battery.

[0008] There is a need to develop a separator having a porous coating to improve heat resistance and ensure frictional durability. Summary of the Invention

[0009] Technical Problem

[0010] The present disclosure aims to provide a separator, an electrode assembly including the separator, and an electrochemical device that can solve the above problems.

[0011] Specifically, the present disclosure aims to provide a separator having a porous coating stably bonded thereto and improved frictional durability; and an electrochemical device including the separator, such as a lithium secondary battery, in which voltage drop behavior is minimized.

[0012] In particular, the present disclosure aims to provide a separator substrate in which, when a porous coating is formed on the separator substrate, a high degree of impregnation of the coating-forming slurry is maintained to improve the adhesion strength between the separator substrate and the porous coating and surface characteristics such as wear resistance against friction; and a method for manufacturing the same.

[0013] Technical Solution

[0014] To solve the above problems,

[0015] According to one aspect of the present disclosure, a separator substrate according to the following embodiments is provided.

[0016] The separator substrate according to the first embodiment is

[0017] a porous polymer substrate having a surface roughness (Sa) of 80 nm to 160 nm on at least one surface.

[0018] According to the second embodiment, in the first embodiment,

[0019] the porous polymer substrate may have a surface roughness (Sa) of 85 nm to 150 nm on both surfaces.

[0020] According to another aspect of the present disclosure, a separator according to the following embodiments is provided.

[0021] The separator according to the third embodiment includes:

[0022] a separator substrate according to the first embodiment or the second embodiment; and

[0023] a porous coating present on at least one of the surfaces of the separator substrate having a surface roughness (Sa) of 80 nm to 160 nm, the porous coating including inorganic particles and a binder polymer.

[0024] According to the fourth embodiment, in the third embodiment,

[0025] The adhesion strength between the separator substrate and the porous coating can be 100 gf / 15 mm or more.

[0026] According to the fifth embodiment, in the third or fourth embodiment,

[0027] The adhesion strength between the separator substrate and the porous coating can be from 100 gf / 15 mm to 300 gf / 15 mm.

[0028] According to the sixth embodiment, in any one of the third to fifth embodiments,

[0029] The coefficient of friction of the separator can be 0.25 or more.

[0030] According to the seventh embodiment, in any one of the third to sixth embodiments,

[0031] The coefficient of friction of the separator can be from 0.5 to 1.0.

[0032] According to the eighth embodiment, in any one of the third to seventh embodiments,

[0033] The puncture strength of the separator can be 450 gf or more.

[0034] According to the ninth embodiment, in any one of the third to eighth embodiments,

[0035] The puncture strength of the separator can be from 500 gf to 600 gf.

[0036] According to another aspect of the present disclosure, an electrode assembly of the following embodiments is provided.

[0037] The electrode assembly according to the tenth embodiment may include:

[0038] A separator according to any one of the third to ninth embodiments; and

[0039] A positive electrode and a negative electrode, each electrode being present on respective ones of the two surfaces of the separator.

[0040] According to the eleventh embodiment, in the tenth embodiment,

[0041] The voltage drop (dOCV) of the electrode assembly within 48 hours can be 2.0 or less.

[0042] According to the twelfth embodiment, in the tenth or eleventh embodiment,

[0043] The voltage drop (dOCV) of the electrode assembly within 48 hours can be 1.5 or less.

[0044] According to another aspect of the present disclosure, there is provided an electrochemical device according to the following embodiments.

[0045] The electrochemical device according to the thirteenth embodiment includes:

[0046] The electrode assembly according to any one of the tenth to twelfth embodiments and a housing that houses the electrode assembly.

[0047] According to the fourteenth embodiment, in the thirteenth embodiment,

[0048] The housing may be a cylindrical housing.

[0049] According to another aspect of the present disclosure, there is provided a method for manufacturing a separator substrate according to the following embodiments.

[0050] The method for manufacturing a separator substrate according to the fifteenth embodiment includes:

[0051] S1) Extruding a polymer slurry to obtain a polymer sheet; and

[0052] S2) Placing the obtained polymer sheet on a casting roll and cooling the polymer sheet,

[0053] wherein the method further includes, between the step S1 and the step S2, placing the polymer sheet in a temperature atmosphere between 28°C and 45°C.

[0054] Advantageous Effects

[0055] For the separator substrate according to the embodiment of the present disclosure, when a porous coating is formed on the separator substrate by improving the surface roughness Sa, the impregnation degree of the slurry for coating formation can be improved, thereby improving the adhesion strength between the separator substrate and the porous coating.

[0056] Therefore, due to the surface characteristics of the separator substrate, the wear resistance of the porous coating against friction can be improved, and the mechanical properties such as puncture strength can be improved.

[0057] Thus, an electrochemical device including a separator using the separator substrate can prevent and / or reduce the rupture of the separator caused by the volume expansion of an electrode, such as a negative electrode, during repeated charge and discharge; and the subsequent short circuit between the positive electrode and the negative electrode.

[0058] In addition, the separator having a porous coating can improve the heat resistance of the electrochemical device and the wear resistance of the separator, thereby preventing and / or reducing the formation of impurities in the device during charge and discharge of the electrochemical device, and minimizing the voltage drop of the electrochemical device. Description of the Drawings

[0059] Figure 1It is a diagram of an apparatus for manufacturing a separator substrate according to the prior art. Figure 1 The apparatus includes an extrusion unit 200 for extruding a polymer sheet and casting rollers 201, 202 for cooling the extruded polymer sheet.

[0060] Figure 2 It is a diagram of an apparatus for manufacturing a separator substrate according to an embodiment of the present disclosure. Figure 2 The apparatus includes a cooling unit 300 for cooling one surface of the separator substrate between the extrusion unit 200 and the casting rollers 201, 202.

[0061] Figure 3 It is a diagram of an apparatus for manufacturing a separator substrate according to an embodiment of the present disclosure. Figure 3 The apparatus includes a cooling unit 300 for cooling both surfaces of the separator substrate between the extrusion unit 200 and the casting rollers 201, 202. Detailed Embodiments

[0062] Hereinafter, the present disclosure will be described in detail.

[0063] When used in this specification, the terms "comprising" or "including" specify the presence of the recited elements, but do not preclude the presence or addition of one or more other elements, unless the context appears otherwise.

[0064] In the specification, "A and / or B" means A or B or both.

[0065] The specific terms used in the following description are for convenience of description only and are not intended to limit the present disclosure. Additionally, terms indicating directions such as up, down, left, right, front, back, inside, and outside indicate the directions in the accompanying drawings or the directions facing or away from the geometric center of the device, system, and elements.

[0066] According to one aspect of the present disclosure, there is provided a separator substrate which is a porous polymer substrate having a surface roughness Sa of 30 nm to 160 nm on at least one surface.

[0067] Separator Substrate and Method for Manufacturing the Same

[0068] According to one aspect of the present disclosure, there is provided a separator substrate which is a porous polymer substrate having a surface roughness Sa of 80 nm to 160 nm on at least one surface.

[0069] To describe this, a method for manufacturing a separator substrate according to an embodiment of the present disclosure will be described first.

[0070] A method for manufacturing a separator substrate according to one aspect of the present disclosure includes:

[0071] S1) Extrude the polymer slurry to obtain a polymer sheet; and

[0072] S2) Place the obtained polymer sheet on a casting roll and cool it.

[0073] In this case, according to one aspect of the present disclosure, the method further includes a step of placing the polymer sheet in a temperature atmosphere of 28°C to 45°C between step S1 and step S2.

[0074] Figure 1 is a diagram showing a part of an apparatus for manufacturing a separator substrate according to the prior art. Figure 1 The apparatus includes an extrusion unit 200 for extruding a polymer sheet and casting rolls 201, 202 for conveying the polymer sheet for cooling and subsequent processes. Generally, the polymer sheet extruded by the extrusion unit is cooled in the air while contacting the casting roll at a high temperature.

[0075] Figure 2 and Figure 3 is a diagram showing a part of an apparatus for manufacturing a separator substrate according to an embodiment of the present disclosure. Figure 2 and Figure 3 The apparatus includes a cooling unit 300 for cooling the extruded polymer sheet between the extrusion unit 200 and the casting rolls 201, 202. Figure 2 shows the cooling unit 300 on one side of the polymer sheet, and Figure 3 shows the cooling unit 300 on both sides of the polymer sheet.

[0076] First, in an embodiment of the present disclosure, step S1) of extruding the polymer slurry to obtain a polymer sheet may include melt-extruding a polymer resin as a raw material.

[0077] In an embodiment of the present disclosure, the polymer resin may include those used as raw materials for manufacturing a separator substrate, but is not limited thereto. The polymer resin may include any type of resin as a raw material for manufacturing a separator substrate, such as polyolefin, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene ether, polyphenylene sulfide, polyethylene naphthalate, or a mixture thereof. The polyolefin resin is formed by polymerization of olefins and refers to a polymer made from olefins that are usually used as monomers in a separator substrate. For example, the polyolefin resin may include polyethylene; polypropylene; polybutene; polyisopentene; polyhexene; polyoctene; homopolymers of monomers selected from the following: ethylene, propylene, butene, isopentene, 4-methylpentene, hexene, and octene; copolymers of two or more of them; or mixtures thereof, but is not limited thereto.

[0078] In an embodiment of the present disclosure, the porous polymer substrate may be a polyolefin substrate.

[0079] In an embodiment of the present disclosure, the porous polymer substrate may be a polyethylene substrate.

[0080] In an embodiment of the present disclosure, a common diluent can be used to extrude the polymer resin fed into the extruder. The diluent may include liquid or solid paraffin oil, wax, and soybean oil commonly used in the manufacture of diaphragms.

[0081] In an embodiment of the present disclosure, for melting and extrusion, a common single-screw extruder or twin-screw extruder can be used, but not limited thereto. In an embodiment of the present disclosure, a mixture of the diluent and the polymer resin can be fed into the extruder, and then the polymer resin is melted and mixed at a high temperature to obtain a molten composition.

[0082] In this case, since the extrusion of the polymer sheet is usually carried out at a high temperature, for example, from 150 °C to 300 °C, the polymer sheet extruded through the extrusion unit can have a high surface temperature, for example, from 130 °C to 200 °C, immediately after extrusion, and the polymer sheet having the aforementioned surface temperature contacts the casting roll. When considering that the temperature of the casting roll is usually 20 °C to 45 °C, 25 °C to 45 °C, 35 °C to 45 °C, or 25 °C, the polymer sheet is rapidly cooled by contacting the casting roll and cannot form crystals on the surface.

[0083] According to one aspect of the present disclosure, before the polymer sheet extruded through the extrusion unit contacts the casting roll, it can be cooled once by the cooling unit 300 to cause surface crystallization of the polymer sheet, thereby increasing the surface roughness.

[0084] In an embodiment of the present disclosure, the cooling unit may also be referred to as a "cooler unit" for cooling the polymer sheet at a high temperature, and the cooling unit may include, for example, an "air heater" for contacting the polymer sheet at a temperature higher than room temperature. In an embodiment of the present disclosure, the cooling unit may include a device for applying hot air at 28 °C to 45 °C to at least one surface of the polymer sheet.

[0085] Conventionally, a porous polymer substrate as a diaphragm substrate is manufactured by extruding a polymer slurry to obtain a polymer sheet and placing the obtained polymer sheet on a casting roll to cool the polymer sheet. The present inventors found that placing the polymer sheet in an atmosphere with a specific temperature before cooling the polymer sheet would cause surface crystallization of the polymer sheet, resulting in different surface characteristics. Therefore, the manufacture of the diaphragm substrate also includes the step of placing the polymer sheet in a specific temperature range before cooling to obtain a diaphragm substrate having a surface roughness Sa of 80 nm to 160 nm through surface crystallization of the diaphragm substrate.

[0086] In an embodiment of the present disclosure, a step of placing the polymer sheet in a temperature atmosphere of 28°C to 45°C is performed between step S1 and step S2. Specifically, this step can be performed at a temperature of 30°C to 45°C or 30°C to 40°C. Additionally, within this temperature range, the polymer sheet can be placed in normal pressure air.

[0087] In an embodiment of the present disclosure, the additional step can be performed between step S1 and step S2 by a device for manufacturing the separator substrate that also includes a "cooling unit 300" in a manner continuous with step S1 and S2.

[0088] In an embodiment of the present disclosure, between step S1 and step S2, the speed at which the polymer sheet travels within a temperature range of 28°C to 45°C can be, for example, 5 m / minute to 20 m / minute, specifically 5 m / minute to 15 m / minute or 10 m / minute. However, without departing from the range of the surface roughness of the separator substrate, the speed can vary according to the speed of the entire process, and the present disclosure is not limited thereto.

[0089] When the polymer sheet is placed within the above temperature range before cooling in step S2, the surface roughness Sa can be 80 nm to 160 nm. Additionally, when the temperature is too high, the surface roughness of the separator substrate may decrease, resulting in a low adhesion strength between the separator substrate and the porous coating, and when the temperature is too low, the surface roughness of the separator substrate may increase, resulting in an adhesion strength between the separator substrate and the porous coating that is high and exceeds the optimal level, and the impact resistance on the surface of the separator is poor, but the present disclosure is not limited thereto.

[0090] In an embodiment of the present disclosure, the method for manufacturing the separator substrate may further include a step S3) of stretching and heat-setting the polymer sheet formed by the casting roll after cooling in step S2.

[0091] In an embodiment of the present disclosure, stretching may include stretching the polymer sheet in the same direction or different directions. For example, stretching may include stretching the polymer sheet independently in a direction parallel to each of the machine direction (MD) and / or the transverse direction (TD).

[0092] As used herein, the "machine direction" refers to the direction parallel to the traveling direction of [extrusion -> forming -> stretching...] in the process of manufacturing the separator substrate. The machine direction can be confirmed by the fiber orientation in the polymer of the separator substrate, and the direction parallel to the fiber orientation is the machine direction. Accordingly, the "transverse direction" refers to the direction perpendicular to the machine direction. The transverse direction can be confirmed as the direction perpendicular to the fiber orientation in the polymer of the separator substrate.

[0093] In an embodiment of the present disclosure, stretching can be carried out, for example, by roll type, stentering type, sequentially or simultaneously.

[0094] In an embodiment of the present disclosure, the stretching can each be carried out, for example, at a stretching ratio of more than 3 times, or from 5 times to 12 times or from 6 times to 7 times. When the stretching ratio satisfies the above numerical range, it can have an advantageous effect on the thickness uniformity of the prepared separator substrate and the performance balance between the machine direction and the transverse direction, but the present disclosure is not limited thereto.

[0095] In an embodiment of the present disclosure, the method can include extracting a diluent from the stretched sheet after stretching to form pores and heat setting.

[0096] In an embodiment of the present disclosure, an organic solvent can be used to carry out the extraction of the diluent. The organic solvent can contain solvents with high extraction efficiency and fast drying, and can suitably include, for example, methyl ethyl ketone, dichloromethane, hexane or a mixture thereof, but the present disclosure is not limited thereto.

[0097] In an embodiment of the present disclosure, the temperature for carrying out the extraction is not limited to a specific range and can include a temperature range in which the surface roughness of the porous substrate does not change.

[0098] In an embodiment of the present disclosure, after the stretching and the extraction of the diluent, heat setting can be carried out to forcibly hold the porous sheet and remove the residual stress in the sheet. However, the purpose of the heat setting is not limited thereto.

[0099] In an embodiment of the present disclosure, the heat setting temperature can vary according to the type of polymer resin used to manufacture the separator substrate. The heat setting temperature can be, for example, from 100 °C to 180 °C, specifically from 110 °C to 150 °C, more specifically from 120 °C to 140 °C, such as 130 °C, but the present disclosure is not limited thereto.

[0100] In an embodiment of the present disclosure, in the heat setting process, heat can be applied while performing uniaxial stretching in the MD or TD direction, or heat can be applied while performing biaxial stretching in the MD and TD directions, but it is not limited thereto.

[0101] According to an embodiment of the present disclosure, a separator substrate having a surface roughness Sa of 80 nm to 160 nm on two surfaces of a porous polymer substrate can be provided.

[0102] According to an embodiment of the present disclosure, the surface roughness Sa of at least one surface of the porous polymer substrate may be from 85 nm to 150 nm. Additionally, the surface roughness Sa of at least two surfaces of the porous polymer substrate may be from 85 nm to 150 nm.

[0103] According to an embodiment of the present disclosure, the surface roughness Sa on at least one surface of the porous polymer substrate may be from 90 nm to 140 nm.

[0104] According to an embodiment of the present disclosure, when the surface roughness Sa of at least one surface of the porous polymer substrate is within the above range, it may have beneficial effects on the adhesion strength between the separator substrate and the porous coating and the wear resistance and impact resistance of the porous coating during the formation of the porous coating. However, the effects of the present disclosure are not limited thereto.

[0105] On the other hand, the method for measuring the surface roughness of the separator substrate may include known methods for two-dimensional (2D) surface roughness measurement in addition to Sa measurement, such as Ra (center line average roughness), Rmax (maximum peak-to-valley roughness height), Rz (ten-point height), and Rg (root mean square (RMS)). However, Ra, Rmax, Rz, and Rg are measures of 2D (line direction) surface roughness and cannot confirm the surface roughness morphology. That is, two different surfaces may have the same value for at least one of Ra, Rmax, Rz, or Rg, but the 3D roughness morphology is different. Therefore, the impregnation amount of the slurry for forming the porous coating or the shape of the porous coating may change, and the adhesion strength between the separator substrate and the porous coating and the mechanical strength of the porous coating may change. Considering this, the present disclosure defines the surface roughness of the separator substrate by Sa.

[0106] In an embodiment of the present disclosure, the surface roughness Sa may be measured by surface analysis of the porous polymer substrate using an atomic force microscope (AFM). For example, a sample with a size of 30 μm × 30 μm is obtained from the porous polymer substrate whose surface roughness is to be measured, and the surface characteristics of each of the two surfaces are measured. In this case, the surface roughness Sa is measured by analyzing the average height difference over the entire surface for the average surface.

[0107] Hereinafter, the structure of the porous polymer substrate will be described by way of example. However, the porous substrate is not limited to the components described below.

[0108] In an embodiment of the present disclosure, the porous polymer substrate is a porous ion-conductive barrier that prevents electrical contact between the negative electrode and the positive electrode and allows ions to pass through, and refers to a substrate having pores therein. The pores are interconnected to allow gas or liquid to pass from one side of the substrate to the other side.

[0109] In an embodiment of the present disclosure, the porous polymer substrate may include a porous polymer membrane, and the porous polymer membrane includes a thermoplastic resin to provide a closed pore function. Here, the closed pore function refers to the following function: when the battery temperature is high, the thermoplastic resin melts to block the pores of the porous polymer substrate and prevent ion migration, thereby preventing thermal runaway in the battery.

[0110] In an embodiment of the present disclosure, the thickness of the porous polymer substrate is not limited to a specific range, but may include the above thickness range based on the total thickness of the separator, for example, from 5 μm to 300 μm, specifically from 5 μm to 100 μm, from 5 μm to 50 μm, from 5 μm to 20 μm, from 5 μm to 15 μm, or from 9 μm to 12 μm.

[0111] In an embodiment of the present disclosure, the "thickness" of the porous polymer substrate can be measured by a common method for measuring the thickness of each component of the separator. For example, the thickness of the porous polymer substrate can be measured using a common thickness measuring instrument, for example, using a commercially available thickness measuring instrument (VL-50S-B of Mitutoyo Corporation).

[0112] Separator

[0113] According to one aspect of the present disclosure, there is provided a separator including the above porous polymer substrate as a separator substrate and a porous coating formed on at least one surface of the separator substrate and including inorganic particles and a binder polymer. In this case, the porous coating may be formed on at least one of the surfaces of the separator substrate having a surface roughness Sa of 30 nm to 160 nm.

[0114] According to another aspect of the present disclosure, there is provided a separator including the above porous polymer substrate as a separator substrate and a porous coating formed on at least one surface of the separator substrate and including inorganic particles and a binder polymer. In this case, the porous coating is formed on at least one of the surfaces of the separator substrate having a surface roughness Sa of 80 nm to 160 nm.

[0115] The porous coating contains inorganic particles to improve the safety of the separator, and contains an adhesive polymer to hold the inorganic particles together. The inorganic particles can improve the heat resistance of the separator, and the adhesive polymer can impart adhesion strength to the surface of the separator. In this case, when the surface of the separator substrate on which the porous coating is formed is flat, the adhesion strength at the interface between the separator substrate and the porous coating may be low. However, when the surface of the separator substrate is too rough, the adhesion strength at the interface between the separator substrate and the porous coating is very high, and the amount of the adhesive at the interface between the separator substrate and the porous coating increases compared to the surface of the porous coating, resulting in poor impact resistance on the surface of the porous coating and low mechanical properties of the separator. Therefore, according to one aspect of the present disclosure, a separator having a porous coating on a separator substrate can achieve a predetermined level of surface roughness, resulting in good adhesion strength between the separator substrate and the porous coating and improved wear resistance and impact resistance of the separator.

[0116] In an embodiment of the present disclosure, the separator may be characterized in that the adhesion strength between the surface of a separator substrate having a surface roughness Sa of 80 nm to 160 nm, specifically 85 nm to 150 nm, and the porous coating is 30 gf / 15 mm or more. Specifically, the adhesion strength between the surface of a separator substrate having the above surface roughness Sa range and the porous coating may be 50 gf / 15 mm to 300 gf / 15 mm, 100 gf / 15 mm to 300 gf / 15 mm, 150 gf / 15 mm to 300 gf / 15 mm, or 195 gf / 15 mm to 284 gf / 15 mm. When the adhesion strength between the separator substrate and the porous coating of the separator is within the above range, this may have a beneficial effect on the process performance of manufacturing an electrode assembly using the separator and the stability of the separator.

[0117] The adhesion strength of the separator can be measured, for example, by the following method. A separator for which the adhesion strength is to be measured is sampled to a width of 15 mm, and the target surface for adhesion strength measurement is attached to a glass slide using an 18-mm-wide double-sided tape (3M Company) so that they are in contact with each other. Subsequently, the peel strength between the separator substrate and the porous coating is measured using a UTM machine (Instron Company) under the conditions of 180° and 300 mm / min.

[0118] In addition, in an embodiment of the present disclosure, the separator may be characterized in that the coefficient of friction is 0.25 or more. In this specification, the "coefficient of friction" refers to the coefficient of friction when the porous coating is detached due to breakage or damage caused by the frictional force applied to the separator, and as a result, the separator substrate is exposed on the surface. When the surface roughness Sa of the separator substrate is within the above range, the impregnation degree of the slurry for forming the porous coating on the separator substrate can be increased, the adhesion strength between the separator substrate and the porous coating can be increased, and the force required to detach the separator substrate and the porous coating by friction can be increased, and the coefficient of friction can be increased.

[0119] According to an embodiment of the present disclosure, the coefficient of friction of the separator may be 0.25 or more. Specifically, the coefficient of friction of the separator may be 0.5 to 1.0, 0.50 to 0.85, 0.60 to 0.80, or 0.64 to 0.78.

[0120] The coefficient of friction of the separator can be measured, for example, using a commonly used friction and wear tester. Specifically, the coefficient of friction of the separator can be measured, for example, by measuring the coefficient of friction when the porous coating is detached when friction is repeatedly applied with a 5 g / Dia tip using a Heidon friction and wear tester.

[0121] In addition, in an embodiment of the present disclosure, the separator may be characterized in that the puncture strength is 450 gf or more. In this specification, the "puncture strength" is a measure of the resistance of the separator to the applied normal force. When the surface roughness Sa of the separator substrate is within the above range, the impregnation degree of the slurry for forming the porous coating on the separator substrate can be increased, the adhesion strength between the separator substrate and the porous coating can be increased, and the mechanical strength of the separator substrate can be improved. On the contrary, when the surface roughness of the separator substrate is too high, defects may form inside the separator substrate, resulting in low mechanical strength and poor puncture strength, but the present disclosure is not limited thereto.

[0122] According to an embodiment of the present disclosure, the puncture strength of the separator may be 450 gf or more. Specifically, the puncture strength of the separator may be 450 gf to 650 gf, 500 gf to 600 gf, 530 gf to 590 gf, or 566 gf to 589 gf.

[0123] The puncture strength of the separator can be measured, for example, by a commonly used method for measuring puncture strength. Specifically, the puncture strength of the separator can be measured, for example, by measuring the force when penetrating the separator when a force is applied to the separator using an Instron UTM machine with a 1 mm tip under the conditions of 120 mm / min.

[0124] Hereinafter, the structure of the porous coating will be described by way of example. However, the structure of the porous coating is not limited thereto.

[0125] In an embodiment of the present disclosure, the porous coating may include inorganic particles and a binder polymer, and all or at least a part of the surface of the inorganic particles may be coated with the binder polymer. In this case, the inorganic particles are held together face-to-face and / or point-to-point through the binder polymer medium.

[0126] For example, the weight ratio of the inorganic particles to the binder resin contained in the porous coating may be from 95:5 to 50:50. Specifically, the weight ratio of the inorganic particles to the binder resin contained in the porous coating may be from 99:1 to 50:50, from 95:5 to 50:50, from 95:5 to 70:30, from 95:5 to 80:20, or from 95:5 to 90:10, but the present disclosure is not limited thereto. The porous coating has a plurality of micropores connected to each other and has a structural feature of a porous layer that allows gas or liquid to pass from one side to the other side.

[0127] In an embodiment of the present disclosure, the porous coating may have a pore structure formed by pores or voids (interstitial volume) between the inorganic particles. The pore size or porosity (ratio of pore volume) can be adjusted according to the particle size and particle size distribution. Through this structure, the resistance to metal impurities present in the electrode can be increased and the shrinkage of the porous polymer substrate can be suppressed, thereby enhancing the safety of the electrochemical device.

[0128] In an embodiment of the present disclosure, the porous coating may include: a plurality of nodes including the inorganic particles and the binder polymer covering at least a part of the surface of the inorganic particles; and at least one linear filament formed of the binder polymer of the nodes, wherein the filament has a node connection portion extending from the node and connecting the node to another node, and the node connection portion has a three-dimensional network structure formed by the filaments derived from the binder polymer being interconnected.

[0129] In an embodiment of the present disclosure, as described above, the porous coating may be formed by a safety enhanced separator (SRS) manufacturing method, a ceramic coated separator (CCS) manufacturing method, or any other known manufacturing method, but is not limited thereto.

[0130] In an embodiment of the present disclosure, the inorganic particles are not limited to a specific type and may include any type of electrochemically stable inorganic particles. That is, the inorganic particles that can be used in the present disclosure are not limited to a specific type and may include any type of inorganic particles that do not undergo oxidation and / or reduction reactions within the operating voltage range of the applied electrochemical device (for example, 0 to 5V vs Li / Li + ). Non-limiting examples of the inorganic particles may include BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb 1-xLa x Zr 1-y Ti y O3 (PLZT, 0 < x < 1, 0 < y < 1), Pb(Mg 1 / 3 Nb 2 / 3 )O3 - PbTiO3 (PMN - PT), hafnium dioxide (HfO2), SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, SiO2, Y2O3, Al2O3, SiC, or at least one of TiO2.

[0131] In an embodiment of the present disclosure, the average particle size (D 50 ) of the inorganic particles can be, for example, 100 nm or more. Specifically, the average particle size (D 50 ) of the inorganic particles can be from 100 nm to 1 μm or from 100 nm to 500 nm. When the average particle size of the inorganic particles is within the above range, this can have a beneficial effect on suppressing the increase in the resistance of the separator, but the present disclosure is not limited thereto.

[0132] The particle size of the inorganic particles can be measured by common methods for measuring particle size and can be measured, for example, using a Malvern particle size analyzer (PSA). Additionally, the average particle size (D 50 ) refers to the particle size at 50% of the cumulative particle size distribution and can be measured by the laser diffraction method commonly used in the art. In this case, the laser diffraction particle size measuring instrument can include, for example, a Microtrac S3500.

[0133] In an embodiment of the present disclosure, the adhesive resin may include, for example, a polyvinylidene fluoride-based resin (PVdF-based resin) and / or an acrylic-based adhesive. In an embodiment of the present disclosure, the PVdF-based resin may include at least one of a vinylidene fluoride homopolymer (i.e., polyvinylidene fluoride), a copolymer of vinylidene fluoride and a copolymerizable monomer, or a mixture thereof. In an embodiment of the present disclosure, the monomer may include, for example, a fluorinated monomer and / or a chlorine-based monomer. Non-limiting examples of the fluorinated monomer may include at least one of the following: vinyl fluoride; trifluoroethylene (TrFE); chlorotrifluoroethylene (CTFE); 1,2-difluoroethylene; tetrafluoroethylene (TFE); hexafluoropropylene (HFP); perfluoro(alkyl vinyl ether), such as perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(propyl vinyl ether) (PPVE); perfluoro(1,3-dioxolene); or perfluoro(2,2-dimethyl-1,3-dioxolene) (PDD). The acrylic-based adhesive may include, for example, polyacrylic acid (PA), polyacrylonitrile (PAN), polyacrylamide (PAA), or a (meth)acrylic-based polymer or a mixture thereof, but the present disclosure is not limited thereto. The (meth)acrylic-based polymer refers to a polymer containing (meth)acrylate as a monomer. The monomer may include, for example, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, ethyl (meth)acrylate, methyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, tert-butyl (meth)acrylate, pentyl (meth)acrylate, n-hexyl (meth)acrylate, isooctyl (meth)acrylate, isononyl (meth)acrylate, lauryl (meth)acrylate, tetradecyl (meth)acrylate, or a mixture thereof, but the present disclosure is not limited thereto.

[0134] In an embodiment of the present disclosure, the porous coating is not limited to a specific thickness, but as long as the above ratio range with respect to the total thickness of the separator is satisfied, the thickness of the porous coating may be, for example, 0.5 μm to 50 μm, specifically 0.5 μm to 10 μm, 0.5 μm to 5 μm, or 1.5 μm to 3 μm.

[0135] Electrode Assembly

[0136] According to another aspect of the present disclosure, there is provided an electrode assembly including the above-described separator and a positive electrode and a negative electrode respectively formed on each of the two surfaces of the separator.

[0137] As described above, the separator according to one aspect of the present disclosure may have improved heat resistance due to the inclusion of a porous coating, high adhesion strength between the separator substrate and the porous coating, and improved impact resistance caused by the improved abrasion resistance of the porous coating and the good mechanical properties of the separator substrate. An electrode assembly using the same can prevent or delay voltage drop, thereby effectively solving the low voltage problem caused by the breakage of the separator.

[0138] In an embodiment of the present disclosure, the electrode assembly has little or no voltage drop during 200 repeated charge and discharge cycles.

[0139] For example, the electrode assembly may exhibit a voltage drop (dOCV) of 2.0 or less within 48 hours. For example, the electrode assembly may exhibit a voltage drop of 2.0 or less within 48 hours after 200 charge and discharge cycles. Additionally, the electrode assembly may exhibit a voltage drop of 1.5 or less within 48 hours, specifically 1.5 or less within 48 hours after 200 charge and discharge cycles. More specifically, the electrode assembly may exhibit a voltage drop of 1.0 or less within 48 hours, specifically 1.0 or less within 48 hours after 200 charge and discharge cycles. For example, within 48 hours, the electrode assembly may exhibit a voltage drop of 0.1 to 1.0, 0.1 to 0.8, 0.3 to 0.8, 0.3 to 0.7, or 0.4 to 0.6.

[0140] In an embodiment of the present disclosure, the voltage drop can be measured by calculating the difference between the OCV measured at a reference time (t0) and the OCV measured after 48 hours (t 48 ) Specifically, after charging to SOC60% set as the end point, the OCV of the electrode assembly is measured to measure the voltage drop over 48 hours.

[0141] In an embodiment of the present disclosure, when using a separator with a separator substrate, the improved abrasion resistance of the separator can have a beneficial effect on providing an electrochemical device with good battery safety and life characteristics.

[0142] Hereinafter, the configuration of the electrode will be described by way of example. However, the present disclosure is not limited thereto.

[0143] In an embodiment of the present disclosure, the positive electrode and the negative electrode may each include a current collector and an electrode active material coating on the current collector, and are not limited to a specific size or shape.

[0144] In an embodiment of the present disclosure, the positive electrode active material may include, for example, a lithium transition metal oxide; a lithium metal iron phosphate; a lithium nickel manganese cobalt oxide; an oxide in which a part of the lithium nickel manganese cobalt oxide is replaced by other transition metals; or two or more of them, but not limited thereto. Specifically, the positive electrode active material may include, for example, a layered compound, such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound replaced by one or more transition metals; a lithium manganese oxide having the formula Li 1+x Mn 2-x O4 (where x is from 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides, such as LiV3O8, LiV3O4, V2O5, Cu2V2O7; a Ni-site lithium nickel oxide represented by the formula LiNi 1-x M x O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, x = 0.01 to 0.3); a lithium manganese composite oxide represented by the formula LiMn 2-x M x O2 (where M = Co, Ni, Fe, Cr, Zn or Ta, x = 0.01 to 0.1) or Li2Mn3MO8 (where M = Fe, Co, Ni, Cu or Zn); a lithium metal phosphate LiMPO4 (where M = Fe, Co, Ni or Mn); a lithium nickel manganese cobalt oxide Li 1+x (Ni a Co b Mn c ) 1-x O2 (x = 0 to 0.03, a = 0.3 to 0.95, b = 0.01 to 0.35, c = 0.01 to 0.5, a + b + c = 1); an oxide in which a part of the lithium nickel manganese cobalt oxide is replaced by aluminum, Li a [Ni b Co c Mn d Al e 1-f M1 f O2 (M1 is at least one selected from the group consisting of Zr, B, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P and S, 0.8 ≤ a ≤ 1.2, 0.5 ≤ b ≤ 0.99, 0 < c < 0.5, 0 < d < 0.5, 0.01 ≤ e ≤ 0.1, 0 ≤ f ≤ 0.1); an oxide in which a part of the lithium nickel manganese cobalt oxide is replaced by other transition metals, Li 1+x (Ni a Co b Mn c M d ) 1-x ​O2 (where x = 0 to 0.03, a = 0.3 to 0.95, b = 0.01 to 0.35, c = 0.01 to 0.5, d = 0.001 to 0.03, a + b + c + d = 1, and M is any one selected from the group consisting of Fe, V, Cr, Ti, W, Ta, Mg, and Mo), a disulfide compound; Fe2(MoO4)3, but not limited thereto.

[0145] In an embodiment of the present disclosure, the negative electrode active material may include, for example, lithium metal or a lithium alloy, soft carbon, hard carbon, natural graphite, floating graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microspheres, mesophase pitch, coke derived from petroleum or coal tar pitch, a silicon (Si)-based compound (M - SiOx (M = Li, Mg, Ca, Al, or Ti, 0 ≤ x < 2)), or a mixture thereof, but not limited thereto.

[0146] In an embodiment of the present disclosure, the electrode assembly may be a jelly roll type electrode assembly having a structure in which a first electrode plate and a second electrode plate having a sheet shape are wound in one direction with a separator interposed therebetween. In this case, the separator may be the above-mentioned separator, and the first electrode plate and the second electrode plate may be a positive electrode plate and a negative electrode plate, respectively.

[0147] Electrochemical Device

[0148] According to another aspect of the present disclosure, an electrochemical device may be provided, which includes the above-mentioned electrode assembly and a housing that houses the electrode assembly.

[0149] In an embodiment of the present disclosure, the electrochemical device may include, for example, a primary battery, a secondary battery, a supercapacitor, and an electric double layer capacitor. More specifically, the secondary battery may be a lithium ion secondary battery.

[0150] In an embodiment of the present disclosure, the housing may include any common battery housing and is not limited to a specific shape depending on the use of the battery. For example, the housing may have a cylindrical shape, a square shape, a pouch shape, or a coin shape using a can.

[0151] In an embodiment of the present disclosure, the housing may be a cylindrical housing, and the electrochemical device may be a cylindrical battery.

[0152] In an embodiment of the present disclosure, the cylindrical battery includes: a jelly roll type electrode assembly that includes a positive electrode and a negative electrode wound in one direction with a separator interposed between the positive electrode plate and the negative electrode plate; a battery can that houses the electrode assembly; and a sealing body that seals an open end portion of the battery can, where the separator is the above-mentioned separator.

[0153] In an embodiment of the present disclosure, the cylindrical battery may be a large cylindrical battery cell having a form factor ratio of 0.4 or more (defined as a value obtained by dividing the diameter of the cylindrical battery by its height, i.e., the ratio of the diameter Φ to the height H).

[0154] In an embodiment of the present disclosure, the cylindrical battery may be, for example, a 46110 cell (diameter 46 mm, height 110 mm, form factor ratio 0.418), a 48750 cell (diameter 48 mm, height 75 mm, form factor ratio 0.640), a 48110 cell (diameter 48 mm, height 110 mm, form factor ratio 0.418), a 48800 cell (diameter 48 mm, height 80 mm, form factor ratio 0.600), or a 46800 cell (diameter 46 mm, height 80 mm, form factor ratio 0.575). In the numbers indicating the form factor, the first two numbers indicate the diameter of the cell, the next two numbers indicate the height of the cell, and the last number 0 indicates that the cross-section of the cell is circular.

[0155] When the electrode assembly is completed as described above, the electrode assembly may be housed in a case according to a conventional method and the case may be sealed, thereby manufacturing an electrochemical device, and in this case, the electrochemical device may be, for example, a lithium secondary battery.

[0156] Hereinafter, the present disclosure will be described in more detail by way of examples, but the following examples are provided for illustrative purposes only and the scope of the present disclosure is not limited thereto.

[0157] [Manufacture of separator substrate]

[0158] A porous polymer substrate is manufactured by the following method.

[0159] A polyethylene polymer having a molecular weight of 600,000 (g / mol) and an antioxidant are mixed in an extruder, then melted at a temperature of 200 °C and extruded through a T-die. The obtained polymer sheet is placed to pass through the temperature range described in Table 1 below, and then cooled by a stainless steel casting roll having a surface temperature of 25 °C, thereby being cooled. In this case, the traveling rate through this temperature range is 10 m / minute. Subsequently, the polymer sheet passing through the casting roll is stretched in the MD direction (stretching ratio 7 times, stretching temperature 115 °C) and the TD direction (stretching ratio 6 times, stretching temperature 125 °C) using a tenter-type sequential stretching machine behind the casting roll, the diluent is extracted using dichloromethane, and then the sheet is heat-set in the MD and TD directions at a temperature of 130 °C, thereby obtaining a porous polymer substrate. The obtained porous polymer substrate has a thickness of 9 μm and a porosity of 45 vol%.

[0160]

[0161] [Manufacture of separator]

[0162] Using the prepared porous polymer substrates of Comparative Examples 1 and 2 and Examples 1 and 2 respectively as separator substrates, a separator was manufactured by forming a porous coating on both surfaces of the separator substrate by the following method.

[0163] Preparation of Porous Coating

[0164] PAA (MW 350,000 g / mol) binder and inorganic particles (Al2O3) were mixed in an aqueous solvent at a weight ratio of 5:95 to prepare a coating slurry. The prepared inorganic coating slurry was applied to the entire surface of the porous substrate by a bar coating method and dried to form a porous coating with a thickness of 1.5 μm on each of the upper and lower surfaces of the porous substrate.

[0165] Thus, a separator with a total thickness of 12 μm was manufactured.

[0166] [Manufacture of electrode assembly]

[0167] Each of the prepared separators was used as a separator, and the negative electrode and the positive electrode were prepared as follows, and each was attached to one surface of the separator to manufacture an electrode assembly.

[0168] Manufacture of Negative Electrode

[0169] Active material (graphite), binder polymer (SBR), and conductive material (Super P) were mixed in distilled water at a weight ratio of 95:0.5:4.5 to prepare a negative electrode composition.

[0170] The negative electrode composition was applied to one surface of a copper current collector and dried to prepare a negative electrode. The loading amount of the negative electrode was 5.3 mAh / cm 2 .

[0171] Manufacture of Positive Electrode

[0172] Active material (NCMA), binder polymer (PVDF), and conductive material (CNT) were mixed in a solvent (NMP) at a weight ratio of 97:1:2 to prepare a positive electrode composition.

[0173] The positive electrode composition was applied to one surface of an aluminum current collector and dried to prepare a positive electrode. The loading amount of the positive electrode was 4.949 mAh / cm 2 .

[0174] Assembly of Electrode Assembly

[0175] Stack the prepared positive electrode / separator / negative electrode / separator in order and wind them around a core to fabricate a jelly roll type electrode assembly.

[0176] [Performance Evaluation]

[0177] Evaluate the characteristics of the separator substrate, separator, and electrode assembly respectively in the manufacturing order by the following methods, and the results are shown in Table 2 below.

[0178] In Table 2 below, the adhesion strength of the separator refers to the adhesion strength between the porous coating and the surface where the surface roughness is measured.

[0179] Measurement of Surface Roughness Sa of Separator Substrate

[0180] Perform surface analysis of the porous polymer substrate using an atomic force microscope (AFM) to obtain a height map of the surface, and measure the surface roughness Sa by analyzing the average height difference over the entire surface for the average surface.

[0181] Measure the surface roughness of both surfaces of the separator substrate.

[0182] Measurement of Adhesion Strength of Separator

[0183] Sample the separator to a width of 15 mm, and attach the target surface for adhesion strength measurement to a glass slide using an 18 mm wide double-sided tape (3M Company) so that they are in contact with each other. Subsequently, use a UTM machine (Instron Company) to measure the peel strength between the separator substrate and the porous coating under the conditions of 180° and 300 mm / min.

[0184] Measure the adhesion strength on both surfaces of the separator.

[0185] Measurement of Coefficient of Friction of Separator

[0186] The coefficient of friction is measured when the porous coating detaches by repeatedly applying friction to the separator using a Heidon friction and wear tester with a 5 g / Dia tip.

[0187] Measurement of Puncture Strength of Separator

[0188] The force when piercing the separator by applying a force to the separator using a UTM machine of Instron Company under the conditions of a 1 mm tip and 120 mm / min.

[0189] Measurement of Self-Discharge

[0190] Evaluate the self-discharge characteristics of the separator by the following method.

[0191] The prepared jelly roll type electrode assembly was housed in a cylindrical case, and electrolyte (DME / DOL 1:1 volume / volume, 1M LiFSI) was injected to fabricate a cylindrical battery.

[0192] Subsequently, it was set to SOC60% by charging, and the voltage drop was measured for 48 hours.

[0193]

[0194] As can be seen from Table 1 and Table 2 above, during the manufacturing process of the separator substrate, before cooling, the separator substrate was placed within a temperature range between 25°C and 45°C and then cooled. The surface roughness Sa of the obtained porous polymer substrate was 80 nm to 160 nm. Thus, it was confirmed that the porous polymer substrate had an optimal adhesion strength level and high wear resistance. In addition, it was confirmed that the voltage drop (low voltage) caused by self-discharge of the electrode assembly using this porous polymer substrate could be minimized.

[0195] In contrast, it was confirmed that the porous polymer substrate as the separator substrate obtained by being placed within a temperature range outside the above range before cooling and then cooled had a surface roughness Sa outside the range of 80 nm to 160 nm. In particular, in the case of Comparative Example 1 where the surface roughness Sa was less than 80 nm, it was confirmed that the adhesion strength between the separator substrate and the porous coating was low, and the mechanical strength such as wear resistance was also poor, and a voltage drop of the electrode assembly using this separator substrate was observed. In the case of Comparative Example 2 where the surface roughness Sa was greater than 160 nm, the adhesion strength was very high, and the impact resistance of the separator was found to be poor.

[0196] [Symbol Explanation]

[0197] 200 - Extrusion unit

[0198] 201, 202 - Casting rollers

[0199] 300 - Cooling unit

Claims

1. A separator substrate, which is a porous polymer substrate having a surface roughness (Sa) of 80 nm to 160 nm on at least one surface.

2. The diaphragm substrate according to claim 1, wherein, The porous polymer substrate has a surface roughness (Sa) of 85 nm to 150 nm on both surfaces.

3. A separator, comprising: the separator substrate according to claim 1; and a porous coating, which is present on at least one of the surfaces of the separator substrate having a surface roughness (Sa) of 80 nm to 160 nm, and the porous coating contains inorganic particles and a binder polymer.

4. The separator according to claim 3, wherein, The adhesion strength between the separator substrate and the porous coating is 100 gf / 15 mm or more.

5. The diaphragm according to claim 4, wherein, The adhesion strength between the separator substrate and the porous coating is 100 gf / 15 mm to 300 gf / 15 mm.

6. The diaphragm according to claim 3, wherein The coefficient of friction of the separator is 0.25 or more.

7. The separator according to claim 6, wherein The coefficient of friction of the separator is 0.5 to 1.

0.

8. The diaphragm according to claim 3, wherein, The puncture strength of the separator is 450 gf or more.

9. The diaphragm according to claim 8, wherein, The puncture strength of the separator is 500 gf to 600 gf.

10. An electrode assembly, comprising: the separator according to any one of claims 3 to 9; and a positive electrode and a negative electrode, and each electrode is present on each of the two surfaces of the separator.

11. The electrode assembly according to claim 10, wherein, The voltage drop (dOCV) of the electrode assembly within 48 hours is 2.0 or less.

12. The electrode assembly according to claim 11, wherein, The voltage drop (dOCV) of the electrode assembly within 48 hours is 1.5 or less.

13. An electrochemical device, comprising: the electrode assembly according to claim 10; and a housing for accommodating the electrode assembly.

14. The electrochemical device according to claim 13, wherein, The housing is a cylindrical housing.

15. A method for preparing a separator substrate, comprising: S1) extruding a polymer slurry to obtain a polymer sheet; and S2) placing the obtained polymer sheet on a casting roll and cooling the polymer sheet, wherein the method further comprises: between the step S1 and the step S2, placing the polymer sheet in a temperature atmosphere between 28 °C and 45 °C.

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