Separator for secondary battery and method for manufacturing the same

Abstract

The separator for a secondary battery of the present invention includes a porous polymer substrate and a porous coating layer disposed on at least one side of the porous polymer substrate. The porous coating layer contains inorganic particles, a binder polymer, and a dispersant. The binder polymer contains at least three or more fluorine-based polymers. The dispersant contains a first dispersant and a second dispersant having a weight average molecular weight smaller than that of the first dispersant.

Description

Technical Field

[0001] The present invention relates to a separator for a secondary battery and a method for manufacturing the same. More specifically, the present invention relates to a separator in the form of a thin film containing inorganic particles and having improved adhesion to an electrode, and a method for manufacturing the same.

[0002] This application claims priority based on Korean Patent Application No. 10-2022-0147984 filed on November 8, 2022, and all of the contents disclosed in the specification and drawings of the application are incorporated into this application.

Background Art

[0003] The recent development of technologies related to mobile devices and the increasing demand are remarkable. Along with this, the demand for secondary batteries as an energy source is rapidly increasing. Among such secondary batteries, lithium secondary batteries, which exhibit high energy density, operating potential, long cycle life, and low self-discharge rate, have been commercialized and widely used.

[0004] In addition, recently, as interest in environmental issues has been increasing, efforts have been actively made in research on electric vehicles (EVs), hybrid electric vehicles (HEVs), etc., which can replace gasoline vehicles, diesel vehicles, etc., which use fossil fuels and are one of the main causes of air pollution. As power sources for such electric vehicles, hybrid electric vehicles, etc., research has mainly been conducted on and lithium secondary batteries having high energy density, high discharge voltage, and output stability are used.

[0005] Although a lithium secondary battery is a stable electrochemical element insulated by a separator, due to the possibility of short circuit between the positive electrode and the negative electrode caused by internal or external battery abnormal phenomena or impacts, resulting in heat generation and explosion, ensuring the thermal / chemical safety of the separator as an insulator is the most important matter to be considered.

[0006] In lithium secondary batteries, polyolefin-based separators that are widely used commercially are porous polymer substrates that function to provide pores for lithium ion passage while preventing electrical short circuits between the positive and negative electrodes, and mainly use materials such as polyethylene and polypropylene.

[0007] Generally, polyolefin-based porous separators manufactured using the process of film stretching fundamentally cannot avoid volume changes such as shrinkage or melting of the separator when the battery temperature rises due to internal or external stimuli above 100°C, and there are concerns that explosions may occur due to electrical short circuits between the positive and negative electrodes caused by this. Also, there is a problem that when the separator is ruptured due to dendrite growth inside the battery, it may cause an explosion of the battery due to an internal short circuit. In order to suppress such heat shrinkage at high temperatures and battery instability due to dendrites, by coating one or both sides of such a porous polymer substrate with inorganic particles together with a binder, the inorganic particles are given the function of suppressing the shrinkage rate of the substrate, and a coated separator is disclosed that provides a safer separator with a porous coating layer.

[0008] The porous coating layer contains inorganic particles and a binder polymer, and is incorporated to improve the durability (such as prevention of film breakage) and heat resistance stability (such as prevention of heat shrinkage) of the separator substrate. As such a binder polymer for the separator, a polyvinylidene fluoride (PVdF) - based resin is mainly used. However, since the adhesive force of the PVdF - based resin itself is not high, it is difficult to ensure a high level of binding force. It is conceivable to increase the content of the binder polymer or the coating amount to improve the adhesive force, but this may increase the layer thickness of the adhesive layer, resulting in a lower energy density, and moreover, there is a risk of increased resistance, which is not desirable. The adhesive layer of the separator must achieve both high adhesive force and high ionic conductivity even with a thin layer thickness. For the battery to be stably used over a long period of time, the separator must be a chemically and electrochemically stable substance. For these reasons, there is an urgent demand for the development of a separator for a secondary battery that satisfies the above - mentioned requirements.

Summary of the Invention

Problems to be Solved by the Invention

[0009] One problem to be solved by an aspect of the present invention is to provide a separator for a thin - film secondary battery coated with a coating layer containing inorganic particles and having improved adhesion to an electrode.

[0010] Another problem to be solved by another aspect of the present invention is to provide a method for manufacturing the separator for the secondary battery.

[0011] In addition to these, it will be easily understood that other objects and advantages of the present invention can be realized by the means and combinations thereof disclosed in the claims.

Means for Solving the Problems

[0012] In order to solve the problems of the present invention, a separator for a secondary battery according to the following aspects and a method for manufacturing the separator for a secondary battery are provided.

[0013] According to a first aspect, a separator for a secondary battery includes a porous polymer substrate and a porous coating layer disposed on at least one side of the porous polymer substrate. The porous coating layer contains inorganic particles, a binder polymer, and a dispersant. The binder polymer contains at least three or more fluorine-based polymers. The dispersant includes a first dispersant and a second dispersant having a weight average molecular weight smaller than that of the first dispersant. A separator for a secondary battery is provided.

[0014] According to a second aspect, in the first aspect, the weight average molecular weight of the first dispersant may be 100,000 to 500,000, and the weight average molecular weight of the second dispersant may be 1,000 to 20,000.

[0015] According to a third aspect, in any one of the first and second aspects, the weight ratio of the first dispersant to the total weight of the dispersant is 70 wt% to 90 wt%, and the weight ratio of the second dispersant to the total weight of the dispersant may be 10 wt% to 30 wt%.

[0016] According to a fourth aspect, in any one of the first to third aspects, the first dispersant may include a cyanoethyl group-containing copolymer.

[0017] According to a fifth aspect, in the fourth aspect, the first dispersant may include at least one selected from the group consisting of cyanoethyl polyvinyl alcohol, cyanoethyl pullulan, cyanoethyl sucrose, cyanoethyl cellulose, cyanoethyl hydroxyethyl cellulose, and cyanoethyl hydroxypropyl cellulose.

[0018] According to a sixth aspect, in any one of the first to fifth aspects, the second dispersant may include an unsaturated fatty acid-based polymer compound.

[0019] According to the seventh aspect, in the sixth aspect, the second dispersant may include at least one selected from the group consisting of an unsaturated carboxylic acid polymer, an unsaturated oleic acid polymer, an unsaturated eicosenoic acid polymer, an unsaturated nervonic acid polymer, and an unsaturated linoleic acid polymer.

[0020] According to the eighth aspect, in any one of the first to seventh aspects, the binder polymer includes PVdF-HFP as the first fluoropolymer, includes PVdF-CTFE as the second fluoropolymer, and may include at least one of PVdF-TFE and PVdF-HFP-AA as the third fluoropolymer.

[0021] According to the ninth aspect, in the eighth aspect, with respect to the mixture of the first to third fluoropolymers, the weight ratio of the first fluoropolymer is 40 wt% to 80 wt%, the weight ratio of the second fluoropolymer is 15 wt% to 40 wt%, and the weight ratio of the third fluoropolymer may be 1 wt% to 20 wt%.

[0022] According to the tenth aspect, in the ninth aspect, the third fluoropolymer includes PVdF-TFE and PVdF-HFP-AA, and with respect to the mixture of the first to third fluoropolymers, the weight ratio of PVdF-TFE is 3 wt% to 10 wt%, and the weight ratio of PVdF-HFP-AA may be 3 wt% to 10 wt%.

[0023] According to the eleventh aspect, in any one of the eighth to tenth aspects, the PVdF-HFP may satisfy one or more of a viscosity of 3 mPa·s to 8 mPa·s, a Tm of 130 °C to 160 °C, a weight average molecular weight of 200,000 to 400,000, and an HFP substitution degree of 5 mol% to 20 mol%.

[0024] According to the 12th aspect, in any one of the 8th to 11th aspects, the PVdF-CTFE may satisfy one or more of a viscosity of 4 mPa·s to 10 mPa·s, a Tm of 150°C to 180°C, a weight average molecular weight of 200,000 to 400,000, and a CTFE substitution degree of 10 mol% to 30 mol%.

[0025] According to the 13th aspect, in any one of the 8th to 12th aspects, the PVdF-TFE may satisfy one or more of a viscosity of 13 mPa·s to 23 mPa·s, a Tm of 120°C to 150°C, a weight average molecular weight of 1,200,000 to 1,600,000, and a TFE substitution degree of 20 mol% to 40 mol%.

[0026] According to the 14th aspect, in any one of the 8th to 13th aspects, the PVdF-HFP-AA may satisfy one or more of a viscosity of 15 mPa·s to 25 mPa·s, a Tm of 130°C to 160°C, a weight average molecular weight of 800,000 to 1,200,000, and an HFP substitution degree of 1.5 mol% to 5 mol%.

[0027] According to the 15th aspect, there is provided a method for manufacturing a separator for a secondary battery, including the steps of manufacturing a first slurry containing inorganic particles and a dispersant, manufacturing a binder solution containing a binder polymer and an organic solvent, mixing the first slurry and the binder solution to manufacture a second slurry, and coating the second slurry on at least one side of a porous polymer substrate, wherein the binder polymer contains at least three or more fluorine-based polymers, and the dispersant contains a first dispersant and a second dispersant having a weight average molecular weight smaller than that of the first dispersant.

[0028] According to the 16th aspect, in the 15th aspect, the content of the inorganic particles may be 50 wt% to 99 wt% based on the total weight% of the binder polymer and the inorganic particles.

[0029] According to the 17th aspect, in any one of the 15th and 16th aspects, the weight ratio of the solid content in the second slurry can be 10 wt% to 40 wt%.

[0030] According to the 18th aspect, in any one of the 15th to 17th aspects, the viscosity of the second slurry can be 4 mPa·s to 10 mPa·s.

[0031] According to the 19th aspect, in any one of the 15th to 18th aspects, the manufacturing method may further include a step of inducing vapor-induced phase separation on the second slurry coated on the porous polymer substrate under the conditions of a temperature of 40°C to 60°C and a relative humidity of 50% to 70%, and a step of drying the phase-separated second slurry under the conditions of a temperature of 50°C to 70°C and a relative humidity of 0% to 5%.

Advantages of the Invention

[0032] The separator for a secondary battery according to an embodiment of the present invention includes a porous coating layer disposed on at least one side of a porous polymer substrate, and the porous coating layer includes inorganic particles, at least three or more fluorine-based polymers as a binder polymer, and a first dispersant and a second dispersant as dispersants.

[0033] In one embodiment, the binder polymer may include a first and a second fluorine-based polymer with relatively low viscosities, and a third fluorine-based polymer with relatively high viscosity. The binding force between the separator and the electrode is improved by the binder polymer with relatively low viscosity. Also, although the dispersing force of the inorganic particles may be weakened by the low-viscosity binder polymer, this can be complemented by the binder polymer with relatively high viscosity.

[0034] In one embodiment, the first dispersant can be a high molecular weight dispersant with a relatively large weight average molecular weight, and the second dispersant can be a low molecular weight dispersant with a relatively small weight average molecular weight. The first dispersant with a relatively large weight average molecular weight makes it possible to ensure a high adhesion force of the porous coating layer to the porous polymer substrate. On the other hand, when the first dispersant is contained in excess in the dispersant, there is a problem that the dispersing power of the inorganic particles decreases and an uncoated region where the porous coating layer is not coated and is exposed occurs. However, since the second dispersant has a relatively small weight average molecular weight, the dispersing power of the inorganic particles is improved. Therefore, even if the porous coating layer is thinly coated, no uncoated region occurs.

[0035] The drawings attached to this specification illustrate desirable embodiments of the present invention and serve to further understand the technical idea of the present invention together with the detailed description of the invention to be described later. Therefore, the present invention is not to be construed as being limited only to the matters described in the drawings.

Brief Description of the Drawings

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Mode for Carrying Out the Invention

[0037] Hereinafter, the present invention will be described in more detail.

[0038] The terms and words used in this specification and the claims are not to be construed as being limited to ordinary or dictionary meanings, but rather are to be construed in accordance with the meaning and concept corresponding to the technical idea of the present invention in accordance with the principle that the inventor himself can appropriately define the concept of the terms in order to explain the invention in the best way.

[0039] Throughout the specification, when a part is described as "including", "comprising", or "having" a certain component, this means that, unless otherwise specified, it does not exclude other components and may further include other components.

[0040] Also, terms and expressions such as "about" and "substantially" used throughout this specification are used to mean at or near the numerical value when manufacturing and material tolerances specific to the recited meaning are presented, and are used to prevent unscrupulous infringers from improperly using the disclosed content where exact or absolute numerical values are recited to assist in the understanding of the present invention.

[0041] The present invention relates to a separator, an electrochemical cell including the same, and a method for manufacturing the same. The electrochemical cell in the invention can cover any battery that undergoes an electrochemical reaction. Specific examples include capacitors such as all kinds of primary batteries, secondary batteries, fuel cells, solar cells, or supercapacitor elements. In particular, the electrochemical cell can be a secondary battery, and the secondary battery can be for a lithium-ion secondary battery. Examples of the lithium-ion secondary battery include lithium-metal batteries, lithium-sulfur batteries, all-solid-state batteries, lithium-polymer batteries, etc., and among them, a lithium-sulfur battery is preferably used.

[0042] According to one aspect of the present invention, there is provided a separator for a secondary battery, which includes a porous polymer substrate and a porous coating layer disposed on at least one side of the porous polymer substrate. The porous coating layer includes inorganic particles, a binder polymer, and a dispersant. The binder polymer includes at least three or more fluorine-based polymers, and the dispersant includes a first dispersant and a second dispersant having a weight average molecular weight smaller than that of the first dispersant.

[0043] The porous polymer substrate can electrically insulate the negative electrode and the positive electrode to prevent short circuits while providing a migration path for lithium ions. Generally, any material that can be used as a separator material for an electrochemical device can be used without particular limitation. As such a porous polymer substrate, for example, a porous polymer film or non-woven fabric containing any one or more of polymer resins such as polyolefin, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalene can be used.

[0044] In a specific embodiment of the present invention, the thickness of the porous polymer substrate can be appropriately selected within the range of 5 to 50 μm. Although the thickness of the porous polymer substrate is not particularly limited to the above-described range, if the thickness is excessively thinner than the above-described lower limit, the mechanical physical properties may deteriorate and the separator may be easily damaged during battery use. On the other hand, although the pore diameter and porosity of the pores present in the porous polymer substrate are not particularly limited, they can be 0.01 to 50 μm and 10 to 95%, respectively.

[0045] The porous coating layer is located on at least one side of the porous polymer substrate and may contain inorganic particles, a binder polymer, and a dispersant. The inorganic particles in the porous coating layer are bound and filled with each other by the binder polymer, and may be dispersed discontinuously in a state of being adjacent to or in contact with each other by the dispersant. Thereby, the porous coating layer can have a porous structure derived from the interstitial volume between the inorganic particles. For this reason, the porous coating layer can complement the high thermal shrinkage of the porous polymer substrate and has ion permeability to provide a migration path for lithium ions.

[0046] In a specific embodiment of the present invention, the layer thickness of the porous coating layer can be appropriately selected within the range of 0.5 to 20 μm, 1 to 10 μm, or 1 to 5 μm. Although the layer thickness of the porous coating layer is not particularly limited to the above-mentioned range, in the present invention, even when a thin coating layer is formed, there is no uncoated region, and a separator with good adhesion and resistance can be provided.

[0047] The inorganic particles of the porous coating layer are not particularly limited as long as they are electrochemically stable. Examples of such inorganic particles include BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb 1-x La 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), hafnia (HfO2), SrTiO3, SnO2, CeO2, MgO, Mg(OH)2, NiO, CaO, ZnO, ZrO2, SiO2, Y2O3, Al2O3, AlOOH, Al(OH)3, SiC, and TiO2, etc., and any one or more of these can be included.

[0048] The binder polymer of the porous coating layer contains at least three or more fluorine-based polymers. The binder polymer may contain only a fluorine-based polymer as a binder component. In one embodiment, the binder polymer may contain a first fluorine-based polymer, a second fluorine-based polymer, and a third fluorine-based polymer. Alternatively, the binder polymer may contain a composition consisting of a first fluorine-based polymer, a second fluorine-based polymer, and a third fluorine-based polymer as a binder component. The first to third fluorine-based polymers may include a polyvinylidene fluoride (PVdF)-based polymer resin containing vinylidene fluoride (VDF) as a monomer. The fluorine-based polymer resin may include, as a monomer, copolymerized with VDF, hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), hexafluoro isobutylene, perfluorobutyl ethylene, perfluoropropyl vinyl ether (PPVE), perfluoroethyl vinyl ether (PEVE), perfluoromethyl vinyl ether (PMVE), perfluoro-2,2-dimethyl-1,3-dioxole (PDD), perfluoro-2-methylene-4-methyl-1,3-dioxolane (PMD), or a copolymer with two or more of these comonomers.

[0049] In one embodiment, the first fluorine-based polymer may be a polyvinylidene fluoride-co-hexafluoropropylene (PVdF-HFP). For example, the PVdF-HFP may satisfy one or more of a viscosity of 3 mPa·s to 8 mPa·s, a Tm of 130°C to 160°C, a weight average molecular weight of 200,000 to 400,000, and an HFP substitution degree of 5 mol% to 20 mol%.

[0050] In one embodiment, the second fluorine-based polymer may be a polyvinylidene fluoride-co-chlorotrifluoroethylene (PVdF-CTFE). For example, the PVdF-CTFE may satisfy one or more of a viscosity of 4 mPa·s to 10 mPa·s, a Tm of 150°C to 180°C, a weight average molecular weight of 200,000 to 400,000, and a CTFE substitution degree of 10 mol% to 30 mol%.

[0051] In one embodiment, the third fluorine-based polymer may include at least one of polyvinylidene fluoride-tetrafluoroethylene (PVdF-TFE) and a vinylidene fluoride-hexafluoropropylene acrylic acid graft copolymer (PVdF-HFP acrylic acid graft copolymer). For example, the PVdF-TFE may satisfy one or more of a viscosity of 13 mPa·s to 23 mPa·s, a Tm of 120°C to 150°C, a weight average molecular weight of 1,200,000 to 1,600,000, and a TFE substitution degree of 20 mol% to 40 mol%. Also, the PVdF-HFP-AA may satisfy one or more of a viscosity of 15 mPa·s to 25 mPa·s, a Tm of 130°C to 160°C, a weight average molecular weight of 800,000 to 1,200,000, an HFP substitution degree of 1.5 mol% to 5 mol%, and an AA substitution degree of 0.5 mol% to 1.9 mol%.

[0052] It is preferable that at least one of the fluorine-based polymers included in the binder polymer described above has a viscosity of 10 mPa·s to 15 mPa·s, but the present invention is not limited thereto.

[0053] In one embodiment, with respect to the mixture of the first to third fluoropolymers, the weight ratio of the first fluoropolymer may be 40 wt% to 80 wt%, the weight ratio of the second fluoropolymer may be 15 wt% to 40 wt%, and the weight ratio of the third fluoropolymer may be 1 wt% to 20 wt%. Here, the third fluoropolymer may be either PVdF-TFE or PVdF-HFP-AA.

[0054] In one embodiment, when the third fluoropolymer contains both PVdF-TFE and PVdF-HFP-AA, with respect to the mixture of the first to third fluoropolymers, the weight ratio of PVdF-TFE may be 3 wt% to 10 wt%, and the weight ratio of PVdF-HFP-AA may be 3 wt% to 10 wt%. Also, for example, the weight ratio of PVdF-TFE and the weight ratio of PVdF-HFP-AA may be the same as each other.

[0055] The dispersant of the porous coating layer includes a first dispersant and a second dispersant. The second dispersant has a smaller weight average molecular weight than the first dispersant. Therefore, the first dispersant may be a high molecular weight dispersant having a weight average molecular weight of 100,000 to 500,000, and the second dispersant may be a low molecular weight dispersant having a weight average molecular weight of 1,000 to 20,000. For example, the weight average molecular weight of the first dispersant may be in the range of 100,000 to 500,000, or 200,000 to 400,000. The weight average molecular weight of the second dispersant may be in the range of 1,000 to 20,000, 1,000 to 10,000, or 1,000 to 5,000.

[0056] By satisfying the weight average molecular weight of the first dispersant within the above-described range, it becomes possible to ensure a high adhesive strength of the porous coating layer to the porous polymer substrate, and the dispersing power of the inorganic particles is improved. When the weight average molecular weight of the first dispersant is out of the above-described range, it is disadvantageous in terms of the thickness and air permeability of the separator, and there is a risk of a problem that the resistance increase width becomes large. Further, by satisfying the weight average molecular weight of the second dispersant within the above-described range, the dispersing power of the inorganic particles in the porous coating layer is improved, and even when the porous coating layer is thinly coated, an uncoated region does not occur.

[0057] In particular, when the weight average molecular weight of the first dispersant is less than 100,000, there is a concern that the adhesive strength between the porous coating layer and the porous polymer substrate may become weak and a peel-off phenomenon may occur. Further, when the weight average molecular weight of the first dispersant is greater than 500,000, there is a risk that the dispersing power of the inorganic particles may be significantly reduced.

[0058] On the other hand, in the present specification, the weight average molecular weight can be measured using Gel Permeation Chromatography (GPC). In the present invention, refer to the following for detailed GPC measurement conditions.

[0059] [GPC Measurement Conditions] GPC measuring device: ACQUITY APC (manufactured by Waters Corporation) Column: PL gel Mixed-B + C (manufactured by Agilent Technologies, Inc.) Column temperature: RT (20 to 25°C) Eluent: DMF / 0.05M LiBr Flow rate: 1.0 ml / min Detector: Waters 2414 RI detector Sample concentration: ~1 mg / ml (0.45 μm syringe filter) Injection volume: 10 μl Calibration curve: PMMA

[0060] On the one hand, in this specification, the viscosity can be measured using an ordinary viscometer. For specific examples, as the viscometer, devices such as DVELV and LVDV-II+Pro Viscometer (cone plate type) manufactured by Brookfield can be used. In a specific embodiment of the present invention, after dissolving the polymer to be measured in acetone at a predetermined concentration, the viscosity is measured at 25°C using an LVDV-II+Pro Viscometer (cone plate type) manufactured by Brookfield. When measuring, the spindle can be LV-1 (No. 61) and 100 to 200 rpm, and the loading amount of the sample can be 100 to 250 mL.

[0061] Also, in the fluorine-based polymer in this specification, the substitution degree can be measured by measuring the content of functional groups (copolymerized monomers) in each polymer. For example, it can be measured by the method of one-dimensional normal nuclear magnetic resonance (1D Normal NMR). As a measurement method, a method using a Bruker 500 MHz NMR analyzer can be mentioned, and the sample to be measured can be dissolved in a DMSO-d6 solvent for use. The measurement conditions can be set, for example, 1 H NMR (zg30), ns = 32, d1 = 3 sec.

[0062] Alternatively, the NMR can be measured by fluorine atomic nuclear magnetic resonance spectroscopy. A more detailed description of the method of fluorine atomic nuclear magnetic resonance is as follows.

[0063] For example, it can be a method of measurement according to the following method using a 19F NMR (DRX-300 manufactured by Bruker) analyzer and using CFCl3 as a reference substance. In this method, first, a sample to be measured is dissolved at 8 wt% in acetone substituted with deuterium. The weight average molecular weight is confirmed using a refractive index detector and gel permeation chromatography (GPC, High Temperature PL 220, manufactured by Waters Corporation) with two PL gel-10μm Mixed-B columns (manufactured by Polymer Laboratories). As the mobile phase solvent, dimethylformamide containing 0.1 M LiBr is used, and the measurement conditions can be a flow rate of 1.0 mL / min at 80°C. As a standard sample, for example, polystyrene having a range of average molecular weights from 2,000 g / mol to 2,000,000 g / mol can be used.

[0064] In another aspect of the present invention, as a method using the fluorine nuclear magnetic resonance spectroscopy, for example, a method using a Unity 400 spectrophotometer at 376.3 MHz can be used. For example, the spectrum can be obtained with an excitation pulse width of 8.0 microseconds in deuterated dimethylformamide at 50°C, a recycle delay of 10 seconds, or an excitation pulse width of 6.0 microseconds in deuterated dimethyl sulfoxide at 50°C, a recycle delay of 5 seconds, or an excitation pulse width of 8.0 microseconds in deuterated acetone at 50°C, and a recycle delay of 20 seconds.

[0065] In one embodiment, the first dispersant may include a cyanoethyl group-containing copolymer. Examples of such the first dispersant include cyanoethyl polyvinyl alcohol, cyanoethyl pullulan, cyanoethyl sucrose, cyanoethyl cellulose, cyanoethyl hydroxyethyl cellulose, cyanoethyl hydroxypropyl cellulose, etc., and one or more of these can be used.

[0066] In one embodiment, the second dispersant may include an unsaturated fatty acid-based polymer compound. Examples of such a second dispersant include unsaturated carboxylic acid polymers, unsaturated oleic acid polymers, unsaturated eicosenoic acid polymers, unsaturated nervonic acid polymers, unsaturated linoleic acid polymers, etc., and one or more of these can be used.

[0067] Since the first dispersant has a relatively large weight average molecular weight, it becomes possible to ensure a high adhesion force of the porous coating layer to the porous polymer substrate. On the other hand, when the first dispersant is contained in excess in the dispersant, there is a problem that the dispersing power of the inorganic particles decreases and an uncoated region where the porous coating layer is not coated and is exposed occurs. On the contrary, since the second dispersant has a relatively small weight average molecular weight, the dispersing power of the inorganic particles is improved and no uncoated region occurs even when the layer thickness of the porous coating layer is reduced.

[0068] Therefore, the weight ratio of the first dispersant to the total weight of the dispersant can be 70 wt% to 90 wt%, and the weight ratio of the second dispersant can be 10 wt% to 30 wt%.

[0069] When the weight ratio of the first dispersant and the second dispersant satisfies the above-described range, it becomes possible to ensure a high adhesion force of the porous coating layer to the porous polymer substrate, the dispersing power of the inorganic particles is improved, and no uncoated region occurs.

[0070] On the other hand, when the weight ratio of the first dispersant is less than 70 wt%, there is a concern that the adhesion force between the porous coating layer and the porous polymer substrate becomes weak and a peel-off phenomenon occurs. Also, when the weight ratio of the first dispersant is greater than 90 wt%, there is a concern that an uncoated region will occur.

[0071] When the weight ratio of the second dispersant is less than 10 wt%, there is a concern that the uncoated region may occur due to a decrease in the dispersing power of the inorganic particles. Further, when the weight ratio of the second dispersant is greater than 30 wt%, there is a concern that the adhesive force between the porous coating layer and the porous polymer substrate may become weak and a peeling-off phenomenon may occur.

[0072] The separator for a secondary battery of the present invention includes a porous coating layer disposed on at least one side of a porous polymer substrate, and the porous coating layer includes inorganic particles, at least three or more fluorine-based polymers as a binder polymer, and a first dispersant and a second dispersant as dispersants.

[0073] In one embodiment, each of the first fluorine-based polymer and the second fluorine-based polymer can be PVdF-HFP and PVdF-CTFE having relatively low viscosities, and the third fluorine-based polymer can be PVdF-TFE and / or PVDF-HFP-AA having relatively high viscosities. The binding force between the separator and the electrode is improved by the binder polymer having a relatively low viscosity. Further, although there is a possibility that the dispersing power of the inorganic particles may be weakened by the low-viscosity binder polymer, this can be complemented by the binder polymer having a relatively high viscosity.

[0074] In one embodiment, the first dispersant can be a high molecular weight dispersant such as a cyanoethyl group-containing copolymer having a relatively large weight average molecular weight, and the second dispersant can be an unsaturated fatty acid-based high molecular compound having a relatively small weight average molecular weight. The first dispersant having a relatively large weight average molecular weight makes it possible to ensure a high adhesive force of the porous coating layer to the porous polymer substrate. On the other hand, when the first dispersant is contained in excess in the dispersant, there is a problem that the dispersing power of the inorganic particles decreases and an uncoated region where the porous coating layer is exposed without being coated occurs. This is because the second dispersant having a relatively small weight average molecular weight improves the dispersing power of the inorganic particles, and therefore, even if the porous coating layer is thinly coated, the uncoated region does not occur.

[0075] Hereinafter, the manufacturing method of the separator for the secondary battery described above will be described.

[0076] According to one aspect of the present invention, there is provided a method for manufacturing a separator for a secondary battery, including the steps of manufacturing a first slurry containing inorganic particles and a dispersant, manufacturing a binder solution containing a binder polymer and an organic solvent, mixing the first slurry and the binder solution to manufacture a second slurry, and coating the second slurry on at least one side of a porous polymer substrate, wherein the binder polymer contains at least three or more fluorine-based polymers, and the dispersant includes a first dispersant and a second dispersant having a weight average molecular weight smaller than that of the first dispersant.

[0077] Further, in one embodiment, the manufacturing method may further include the steps of inducing phase separation in the second slurry coated on the porous polymer substrate under conditions of a temperature of 40°C to 60°C and a relative humidity of 50% to 70%, and drying the phase-separated second slurry under conditions of a temperature of 50°C to 70°C and a relative humidity of 0% to 5%.

[0078] The first slurry can be produced by mixing and dispersing an organic solvent, inorganic particles, and a dispersant.

[0079] Examples of the organic solvent include cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane; aromatic hydrocarbons such as toluene, xylene, and ethylbenzene; ketones such as acetone, ethyl methyl ketone, diisopropyl ketone, cyclohexanone, methylcyclohexane, and ethylcyclohexane; chlorine-based aliphatic hydrocarbons such as methylene chloride, chloroform, and carbon tetrachloride; esters such as ethyl acetate, butyl acetate, γ-butyrolactone, and ε-caprolactone; acetonitriles such as acetonitrile and propionitrile; ethers such as tetrahydrofuran and ethylene glycol diethyl ether; alcohols such as methanol, ethanol, isopropanol, ethylene glycol, and ethylene glycol monomethyl ether; and amides such as N-methylpyrrolidone and N,N-dimethylformamide. According to a specific embodiment of the present invention, the organic solvent may contain acetone in consideration of the advantages in the drying process.

[0080] These solvents may be used alone or as a mixed solvent of two or more of them. Among these, in particular, it is preferable to use a solvent having a low boiling point and high volatility because it can be removed in a short time and at a low temperature. Specifically, acetone, toluene, cyclohexanone, cyclopentane, tetrahydrofuran, cyclohexane, xylene, or N-methylpyrrolidone, or a mixed solvent thereof is preferable.

[0081] The above-described substances can be used as the inorganic particles and the dispersant.

[0082] The binder solution can be produced by mixing a binder polymer and an organic solvent. As the binder polymer, the above-described first to third fluorine-based polymers can be used. As the organic solvent, the above-described solvents can be used, and preferably, it may contain acetone.

[0083] The second slurry can be produced by introducing the binder solution into the first slurry and crushing and dispersing it while controlling the inorganic particles to have a predetermined average particle size using a ball milling method or the like.

[0084] At this time, the inorganic particles can be 50 wt% to 99 wt% based on the total weight % of the binder polymer and the inorganic particles, and the weight ratio of the solid content in the second slurry can be 10 wt% to 40 wt%. Also, the viscosity of the second slurry can be 4 mPa·s to 10 mPa·s.

[0085] The produced second slurry can be applied to one or both sides of a porous polymer substrate to form a porous coating layer. There is no limitation on the method of coating the second slurry, and examples of the method include a dip coat method, a die coat method, a roll coat method, a comma coat method, a doctor blade coat method, a reverse roll coat method, a direct roll coat method, and the like.

[0086] The coating process for forming the porous coating layer is preferably carried out under a certain range of humidity. After applying the composition for forming the porous coating layer to one or both sides of the porous polymer substrate, while undergoing a drying process, the binder dissolved in the coating layer (composition) will have phase transition characteristics due to the vapor-induced phase separation phenomenon known in the art. The phase separation can be carried out by humidification phase separation or immersion phase separation.

[0087] The humidification phase separation is as described below.

[0088] First, a non-solvent can be incorporated in a gaseous state for phase separation. The non-solvent is not particularly limited as long as it is partially miscible with the solvent without dissolving the binder, and can be, for example, one or more selected from the group consisting of water, methanol, ethanol, isopropanol, and butanol.

[0089] When adding by incorporating a non-solvent in a gaseous state, it is possible to perform phase separation using a small amount of the non-solvent, and there is an advantage that drying of the inorganic composition becomes even easier.

[0090] At this time, the temperature at which the non-solvent in the gaseous state is added can be in the range of 15°C to 70°C. If it is less than 15°C, it is difficult for the non-solvent to maintain the gaseous state, the drying rate of the inorganic composition is slow, and productivity is low. If it exceeds 70°C, the drying rates of the solvent and the non-solvent are too high, and it is difficult for phase separation to be sufficiently performed.

[0091] Also, in the phase separation process, the non-solvent can be added so that the vapor pressure of the non-solvent becomes 15% to 80% or 30% to 50% with respect to the saturated vapor pressure, and the phase separation process can be sequentially performed. If the vapor pressure of the non-solvent is less than 15% with respect to the saturated water vapor, the amount of the non-solvent is excessively small, and it is difficult for phase separation to be sufficiently performed. If it exceeds 80%, phase separation is excessively vigorous, and it is difficult to perform a uniform coating.

[0092] In order for phase separation to be performed by adding a non-solvent in a gaseous state, it is advantageous that the boiling point of the solvent is low and evaporation easily occurs. That is, when the temperature is lowered while the solvent is evaporated, it becomes easier for the non-solvent in the gas phase to exchange with the solvent while being condensed. In one specific example, when adding a non-solvent in a gaseous state, the solvent can have a boiling point in the range of 30°C to 80°C. Also, the solvent of the inorganic composition to which the non-solvent in the gaseous state is added can be, for example, one or more selected from the group consisting of acetone and methyl ethyl ketone.

[0093] Regarding the immersion phase separation among the above phase separations, it is as follows.

[0094] After coating a composition for forming a porous coating layer on at least one side of a porous polymer substrate, it is immersed in a coagulation liquid containing a suitable non-solvent for a predetermined time. Thereby, while a phase separation phenomenon is caused in the coated composition for forming a porous coating layer, the binder resin is solidified. In this step, the coating layer containing the binder resin and inorganic particles is made porous. Then, by washing with water, the coagulation liquid is removed, and by drying, a porous coating layer can be integrally formed on the porous polymer substrate.

[0095] As the solvent used for dissolving the binder, those capable of dissolving 5% by weight or more, preferably 15% by weight or more, more preferably 25% by weight or more of the binder resin under the temperature condition of 25°C can be used. Non-limiting examples of such solvents include polar amide solvents such as N-methylpyrrolidone, dimethylacetamide, and dimethylformamide, propanone, cyclopentanone, methylacetate, gamma-butyrolactone, trimethylphosphate, triethylphosphate, and dimethylethoxymethane. When the solubility of the binder resin in the solvent used does not reach the above-mentioned range, there is a problem that phase separation will occur excessively.

[0096] As the non-solvent, those having a solubility of less than 5% by weight of the binder resin under the temperature condition of 25°C can be used. As such non-solvents, any one or more selected from water, methanol, ethanol, propyl alcohol, butyl alcohol, butanediol, ethylene glycol, propylene glycol, or tripropylene glycol can be used.

[0097] As the coagulating liquid, only a non-solvent may be used, or a mixed solvent of a non-solvent and a solvent as described above may be used. When using a mixed solvent of a non-solvent and a solvent, from the viewpoint of forming a good porous structure and improving productivity, the content of the non-solvent with respect to 100% by weight of the coagulating liquid is 95% by weight or more.

[0098] The separator for a secondary battery of the present invention can be dried after phase separation. Specifically, the phase-separated second slurry can be dried under conditions of a temperature of 50°C to 70°C and a relative humidity of 0% to 5%, or under conditions of a temperature of 50°C to 70°C and a relative humidity of 0%. As the drying method, a known method in the industry can be used, and it can be carried out batchwise or continuously using an oven or a heating chamber within a temperature range considering the vapor pressure of the solvent used. The drying is to remove almost all of the solvent present in the composition, and this is preferably as fast as possible in consideration of productivity and the like.

[0099] Hereinafter, in order to deepen the understanding of the present invention, the present invention will be described in more detail with reference to examples. However, the examples according to the present invention can be deformed into various other forms, and it should not be construed that the scope of the present invention is limited to the following examples. The examples of the present invention are provided to more fully explain the present invention to those having average knowledge in the industry.

[0100] Example 1 First slurry 400 g of acetone as an organic solvent, 130 g of Al2O3 (manufactured by Korea's Daehan Ceramics Co., Ltd., ALK-S1) as inorganic particles, 3 g of cyanoethyl polyvinyl alcohol (manufactured by Korea's Eco Chemical Co., Ltd., CEPV-157, weight average molecular weight 263,000) as a first dispersant, and 0.6 g of an unsaturated carboxylic acid polymer (manufactured by BYK, P-105, weight average molecular weight 1,500) as a second dispersant were mixed and dispersed to produce a first slurry.

[0101] Binder solution 400 g of acetone as an organic solvent, 36 g of PVdF-HFP (manufactured by Solvay, Solef® 20808) as the first fluorine-based polymer, 15 g of PVdF-CTFE (manufactured by Solvay, Solef® 32008) as the second fluorine-based polymer, and 3 g of PVdF-TFE (manufactured by Daikin Industries, Ltd.) as the third fluorine-based polymer were mixed to produce a binder solution.

[0102] The viscosity of the PVdF-HFP is 3.42 mPa·s, the Tm is 151 °C, the weight average molecular weight is 300,000, and the HFP substitution degree is 8 mol%. The viscosity of the PVdF-CTFE is 5.34 mPa·s, the Tm is 166 °C, the weight average molecular weight is 300,000, and the CTFE substitution degree is 20 mol%. The viscosity of the PVdF-TFE is 18.69 mPa·s, the Tm is 135 °C, the weight average molecular weight is 140,000, and the TFE substitution degree is 31 mol%.

[0103] Second slurry The binder solution was put into the first slurry and crushed and dispersed while controlling the inorganic particles to have a predetermined average particle size using the ball mill method to produce a second slurry. The second slurry had a weight ratio of organic solvent to solid content of 4:1, a solid content weight ratio of 20 wt%, a weight ratio of binder polymer to inorganic particles in the solid content of 1:4, and a weight ratio of inorganic particles to the total weight% of binder polymer and inorganic particles of 80 wt%. The viscosity of the second slurry is 7.32 mPa·s.

[0104] Separator for secondary battery The second slurry was applied and coated on a polyolefin-based substrate as a porous polymer substrate (manufactured by Toray Industries, Inc., thickness: 9 μm, air permeability time: 70 s / 100 cc, resistance: 0.50 ohm, porosity: 45 vol%) by the microgravure method. Thereafter, vapor-induced phase separation was induced for 15 seconds under the conditions of a temperature of 60° C. and a relative humidity of 60% with respect to the second slurry coated on the porous polymer substrate to cause a phase transition, and drying was performed under the conditions of a temperature of 60° C. and a relative humidity of 0% to produce a separator for a secondary battery.

[0105] Example 2 A binder solution having a viscosity of 8.58 mPa·s was produced by mixing 400 g of acetone as an organic solvent, 34 g of PVdF-HFP (manufactured by Solvay, Solef (registered trademark) 20808) as a first fluorine-based polymer, 14 g of PVdF-CTFE (manufactured by Solvay, Solef (registered trademark) 32008) as a second fluorine-based polymer, and 6 g of PVdF-TFE (manufactured by Daikin Industries, Ltd.) as a third fluorine-based polymer, and the separator was produced in the same manner as in Example 1 except for this.

[0106] Example 3 A binder solution having a viscosity of 7.41 mPa·s was produced by mixing 400 g of acetone as an organic solvent, 36 g of PVdF-HFP (manufactured by Solvay, Solef (registered trademark) 20808) as a first fluorine-based polymer, 15 g of PVdF-CTFE (manufactured by Solvay, Solef (registered trademark) 32008) as a second fluorine-based polymer, and 3 g of PVdF-HFP-AA (manufactured by Solvay, Solef (registered trademark) 75130) as a third fluorine-based polymer, and the separator was produced in the same manner as in Example 1 except for this. The viscosity of the PVdF-HFP-AA is 19.22 mPa·s, Tm is 145° C., the weight average molecular weight is 1,000,000, and the HFP substitution degree is 2.3 mol%.

[0107] Example 4 400 g of acetone as an organic solvent, 34 g of PVdF-HFP (manufactured by Solvay, Solef (registered trademark) 20808) as the first fluorine-based polymer, 14 g of PVdF-CTFE (manufactured by Solvay, Solef (registered trademark) 32008) as the second fluorine-based polymer, and 6 g of PVdF-HFP-AA (manufactured by Solvay, Solef (registered trademark) 75130) as the third fluorine-based polymer were mixed to produce a binder solution with a viscosity of 9.01 mPa·s, except for this, it was produced in the same manner as in Example 1. The viscosity of the PVdF-HFP-AA is 19.22 mPa·s, the Tm is 145 °C, the weight average molecular weight is 1,000,000, and the HFP substitution degree is 2.3 mol%.

[0108] Example 5 400 g of acetone as an organic solvent, 34 g of PVdF-HFP (manufactured by Solvay, Solef (registered trademark) 20808) as the first fluorine-based polymer, 14 g of PVdF-CTFE (manufactured by Solvay, Solef (registered trademark) 32008) as the second fluorine-based polymer, 3 g of PVdF-TFE (manufactured by Daikin Industries, Ltd.) as the third fluorine-based polymer, and 3 g of PVdF-HFP-AA (manufactured by Solvay, Solef (registered trademark) 75130) were mixed to produce a binder solution with a viscosity of 8.741 mPa·s, except for this, it was produced in the same manner as in Example 1. The viscosity of the PVdF-HFP-AA is 19.22 mPa·s, the Tm is 145 °C, the weight average molecular weight is 1,000,000, and the HFP substitution degree is 2.3 mol%.

[0109] Example 6 400 g of acetone as an organic solvent, 130 g of Al2O3 (manufactured by Korea's Daehan Ceramics Co., Ltd., ALK-S1) as inorganic particles, 3 g of cyanoethyl polyvinyl alcohol (manufactured by Korea's Eco Chemical Co., Ltd., CEPV-208, weight average molecular weight 364,000) as a first dispersant, and 0.6 g of an unsaturated carboxylic acid polymer (BYK Co., P-105, weight average molecular weight 1,500) as a second dispersant were mixed and dispersed to produce a first slurry, except for this, it was produced in the same manner as in Example 1.

[0110] Comparative Example 1 A binder solution was produced by mixing 400 g of acetone as an organic solvent, 38 g of PVdF-HFP (manufactured by Solvay, Solef® 20808) as a fluorine-based polymer, and 16 g of PVdF-CTFE (manufactured by Solvay, Solef® 32008) as a fluorine-based polymer, except for this, it was produced in the same manner as in Example 1.

[0111] Comparative Example 2 A binder solution was produced by mixing 400 g of acetone as an organic solvent, 27 g of PVdF-HFP (manufactured by Solvay, Solef® 20808) as a fluorine-based polymer, and 27 g of PVdF-CTFE (manufactured by Solvay, Solef® 32008) as a fluorine-based polymer, except for this, it was produced in the same manner as in Example 1.

[0112] Comparative Example 3 A binder solution was produced by mixing 400 g of acetone as an organic solvent, 16 g of PVdF-HFP (manufactured by Solvay, Solef® 20808) as a fluorine-based polymer, and 38 g of PVdF-CTFE (manufactured by Solvay, Solef® 32008) as a fluorine-based polymer, except for this, it was produced in the same manner as in Example 1.

[0113] Comparative Example 4 A binder solution was produced in the same manner as in Example 1, except that 400 g of acetone as an organic solvent and 54 g of PVdF-TFE (manufactured by Daikin Industries, Ltd.) as a fluorine-based polymer were used.

[0114] Comparative Example 5 A binder solution was produced in the same manner as in Example 1, except that 400 g of acetone as an organic solvent and 54 g of PVdF-HFP-AA (manufactured by Solvay, Solef (registered trademark) 75130) as a fluorine-based polymer were used.

[0115] Comparative Example 6 A first slurry was produced in the same manner as in Example 1, except that 400 g of acetone as an organic solvent, 130 g of Al2O3 (manufactured by Korea's Daehan Ceramics Co., Ltd., ALK-S1) as inorganic particles, and 3 g of cyanoethyl polyvinyl alcohol (manufactured by Korea's Eco Chemical Co., Ltd., CEPV-157) as a dispersant were mixed and dispersed.

[0116] Comparative Example 7 A first slurry was produced in the same manner as in Example 1, except that 400 g of acetone as an organic solvent, 130 g of Al2O3 (manufactured by Korea's Daehan Ceramics Co., Ltd., ALK-S1) as inorganic particles, and 2 g of an unsaturated carboxylic acid polymer (BYK, P-105) as a dispersant were mixed and dispersed.

[0117] Figures 1 and 2 show the measurement results for Examples 1 to 6 and Comparative Examples 1 to 7, respectively.

[0118] The air permeability of the separator was measured using a Gurley air permeability meter in accordance with JIS P-8117. At this time, for the separator, the time taken for 100 cc of air to pass through a diameter of 28.6 mm and an area of 645 mm 2 was measured.

[0119] The adhesive strength of the electrode was measured by cutting the manufactured separator into a size of 25 mm × 70 mm, overlapping the separator and the electrode, sandwiching them between two sheets of A4 paper, and then bonding them using a flat press. At this time, the conditions of the flat press were heating and pressurization at a pressure of 1000 kgf for 1 second at a temperature of 60°C. After that, using a UTM (universal testing machine) (LLOYD instruments LF plus), the force required to peel the electrode and the porous coating layer opposing the electrode was measured by applying a force at 180° at a measurement speed of 300 mm / min.

[0120] The resistance is the resistance value when the separator is cut and stacked on a 2032 coin cell manufactured by Hokusen Co., Ltd. and impregnated with an electrolyte solution. It was measured by the alternating current method at 25°C using an electrochemical impedance spectroscopy (EIS) with a 1M LiPF6-ethylene carbonate / ethyl methyl carbonate (weight ratio 3:7) electrolyte solution.

[0121] As shown in FIGS. 1 and 2, the separators of Examples 1 to 6 were measured to have a coating layer thickness of 2.5 μm or less, making it possible to realize a separator with a relatively thin film compared to the comparative examples. Also, when the packing density, air permeability, electrode adhesive strength, peel strength, and resistance of the separators of Examples 1 to 6 were collectively compared, they were measured to be better than those of the comparative examples.

[0122] FIGS. 3 to 8 show the SEM images of the separators for secondary batteries according to Examples 1 to 6, and FIGS. 9 to 15 show the SEM images of the separators for secondary batteries according to Comparative Examples 1 to 7 in order.

[0123] When observing the SEM images of FIGS. 3 to 8, it was observed that there was substantially no uncoated region compared to the comparative examples. In particular, in FIG. 3, the portion composed of the thin binder layer appears black, confirming that it is all coated.

[0124] In contrast, in Comparative Examples 1, 2, 4, 5, and 6, the coating layer was too thick to achieve a thin separator, and in Comparative Examples 3 and 7, the coating layer was good in thickness, but Comparative Example 3 was measured to have excessively low electrode adhesion strength, and Comparative Example 7 was measured to have excessively low peel strength. Furthermore, in Figures 9 to 14, the white alumina on the surface clumped together, revealing the separator surface in the empty spaces, confirming the presence of uncoated areas.

[0125] The weight average molecular weight is measured using gel permeation chromatography (GPC), and the detailed GPC measurement conditions are as follows.

[0126] [GPC measurement conditions] GPC measurement device: ACQUITY APC (Waters Corporation) Column: PL gel Mixed-B+C (Agilent Technologies) Column temperature: RT (20-25°C) Eluent:DMF / 0.05M LiBr Flow rate: 1.0ml / min Detector: Waters 2414 RI detector Sample concentration: ~1mg / ml (0.45μm syringe filter) Injection volume: 10μL Correction curve: PMMA

[0127] The viscosity was measured using a Brookfield LVDV-II+Pro Viscometer (cone-plate type). First, each polymer to be measured was dissolved in acetone at a predetermined concentration, and then the viscosity was measured at 25°C using the device. The spindle was LV-1 (No. 61), the speed was 100 to 200 rpm, and the sample loading amount was 100 to 250 mL.

[0128] The degree of substitution was measured by a method of one-dimensional normal nuclear magnetic resonance spectroscopy (1D Normal NMR). As a detailed measurement method, a method using a Bruker 500 MHz NMR analyzer can be mentioned, and it was used after dissolving about 10 mg of the sample in about 0.5 ml of DMSO-d6 at 50°C. The measurement was carried out, for example, 1 under the conditions of 1H NMR (zg30), ns = 32, and d1 = 3 sec.

Claims

1. A separator for a secondary battery, comprising a porous polymer substrate and a porous coating layer disposed on at least one side of the porous polymer substrate, wherein the porous coating layer contains inorganic particles, a binder polymer, and a dispersant, the binder polymer contains at least three or more fluorine-based polymers, and the dispersant contains a first dispersant and a second dispersant having a weight average molecular weight smaller than that of the first dispersant.

2. The weight average molecular weight of the first dispersant is 100,000 to 500,000, and the weight average molecular weight of the second dispersant is 1,000 to 20,000. The separator for a secondary battery according to Claim 1.

3. The weight ratio of the first dispersant to the total weight of the dispersant is 70 wt% to 90 wt%, and the weight ratio of the second dispersant to the total weight of the dispersant is 10 wt% to 30 wt%. The separator for a secondary battery according to Claim 1.

4. The first dispersant contains a cyanoethyl group-containing copolymer. The separator for a secondary battery according to Claim 1.

5. The first dispersant contains at least one selected from the group consisting of cyanoethyl polyvinyl alcohol, cyanoethyl pullulan, cyanoethyl sucrose, cyanoethyl cellulose, cyanoethyl hydroxyethyl cellulose, and cyanoethyl hydroxypropyl cellulose. The separator for a secondary battery according to Claim 4.

6. The second dispersant contains an unsaturated fatty acid-based polymer compound. The separator for a secondary battery according to Claim 1.

7. The second dispersant contains at least one selected from the group consisting of an unsaturated carboxylic acid polymer, an unsaturated oleic acid polymer, an unsaturated eicosenoic acid polymer, an unsaturated nervonic acid polymer, and an unsaturated linoleic acid polymer. The separator for a secondary battery according to Claim 6.

8. The binder polymer, contains PVdF-HFP as a first fluorine-based polymer, contains PVdF-CTFE as a second fluorine-based polymer, and contains at least one of PVdF-TFE and PVdF-HFP-AA as a third fluorine-based polymer. The separator for a secondary battery according to Claim 1.

9. With respect to the mixture of the first to third fluorine-based polymers, the weight ratio of the first fluorine-based polymer is 40 wt% to 80 wt%, The weight ratio of the second fluorine-based polymer is 15 wt% to 40 wt%, The weight ratio of the third fluorine-based polymer is 1 wt% to 20 wt%. The separator for secondary battery according to claim 8.

10. The third fluorine-based polymer includes PVdF-TFE and PVdF-HFP-AA, With respect to the mixture of the first to third fluorine-based polymers, The weight ratio of the PVdF-TFE is 3 wt% to 10 wt%, The weight ratio of the PVdF-HFP-AA is 3 wt% to 10 wt%. The separator for secondary battery according to claim 9.

11. The PVdF-HFP satisfies one or more of a viscosity of 3 mPa·s to 8 mPa·s, a Tm of 130°C to 160°C, a weight average molecular weight of 200,000 to 400,000, and an HFP substitution degree of 5 mol% to 20 mol%. The separator for secondary battery according to claim 8.

12. The PVdF-CTFE satisfies one or more of a viscosity of 4 mPa·s to 10 mPa·s, a Tm of 150°C to 180°C, a weight average molecular weight of 200,000 to 400,000, and a CTFE substitution degree of 10 mol% to 30 mol%. The separator for secondary battery according to claim 8.

13. The PVdF-TFE satisfies one or more of a viscosity of 13 mPa·s to 23 mPa·s, a Tm of 120°C to 150°C, a weight average molecular weight of 1,200,000 to 1,600,000, and a TFE substitution degree of 20 mol% to 40 mol%. The separator for secondary battery according to claim 8.

14. The PVdF-HFP-AA satisfies one or more of a viscosity of 15 mPa·s to 25 mPa·s, a Tm of 130°C to 160°C, a weight average molecular weight of 800,000 to 1,200,000, and an HFP substitution degree of 1.5 mol% to 5 mol%. The separator for secondary battery according to claim 8.

15. The step of manufacturing a first slurry containing inorganic particles and a dispersant, The step of manufacturing a binder solution containing a binder polymer and an organic solvent, The step of mixing the first slurry and the binder solution to manufacture a second slurry, The step of coating at least one side of the porous polymer substrate with the second slurry, Including, The binder polymer contains at least three or more kinds of fluorine-based polymers, The method for manufacturing a separator for a secondary battery, wherein the dispersant includes a first dispersant and a second dispersant having a weight average molecular weight smaller than that of the first dispersant.

16. The method for manufacturing a separator for a secondary battery according to claim 15, wherein the content of the inorganic particles is 50 wt% to 99 wt% based on the total weight% of the binder polymer and the inorganic particles.

17. The method for manufacturing a separator for a secondary battery according to claim 15, wherein the weight ratio of the solid content in the second slurry is 10 wt% to 40 wt%.

18. The method for manufacturing a separator for a secondary battery according to claim 15, wherein the viscosity of the second slurry is 4 mPa·s to 10 mPa·s.

19. Inducing vapor-induced phase separation on the second slurry coated on the porous polymer substrate under conditions of a temperature of 40°C to 60°C and a relative humidity of 50% to 70%; Drying the phase-separated second slurry under conditions of a temperature of 50°C to 70°C and a relative humidity of 0% to 5%; The method for manufacturing a separator for a secondary battery according to claim 15, further comprising the above steps.

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