Secondary battery separator and method for manufacturing same

Abstract

The separator for a secondary battery according to the present disclosure includes a porous polymer substrate; and a porous coating layer disposed on at least one surface of the porous polymer substrate wherein the porous coating layer comprises inorganic particles, a binder polymer and a dispersant wherein the binder polymer comprises at least three types of fluorine-based polymers, and wherein the dispersing agent comprises a first dispersing agent and a second dispersing agent having a lower weight average molecular weight than the first dispersing agent.

Description

Technical Field

[0001] The present disclosure relates to a separator for a secondary battery and a method for manufacturing the same. More particularly, the present disclosure relates to a thin film separator including inorganic particles and having improved bonding strength with an electrode and a method for manufacturing the same.

[0002] This application claims priority to Korean Patent Application No. 10-2022-0147984 filed in Korea on November 8, 2022, the disclosure of which is incorporated herein by reference. Background Art

[0003] As the technology of mobile devices and the demand for mobile devices increase, the demand for secondary batteries as energy sources has also grown rapidly. Among secondary batteries, lithium secondary batteries have been brought to the market and widely used due to their high energy density and working potential, long cycle life and low self-discharge rate.

[0004] Recently, with the growing interest in environmental issues, many studies are being conducted on electric vehicles (EV) and hybrid electric vehicles (HEV) as alternatives to vehicles using fossil fuels such as gasoline vehicles and diesel vehicles (considered to be one of the main causes of air pollution). Lithium secondary batteries with high energy density, high discharge voltage and output stability are mainly studied and used as power sources for electric vehicles and hybrid electric vehicles.

[0005] Lithium secondary batteries are stable electrochemical devices because the separator isolates the positive electrode from the negative electrode, but in the event of a short circuit between the positive electrode and the negative electrode due to an internal or external malfunction or defect of the battery or an impact, heat generation and explosion may occur, so ensuring the thermal / chemical safety of the separator as an isolation member is the most important consideration.

[0006] Commercially available polyolefin-based separators widely used in lithium secondary batteries include polyethylene or polypropylene as a main material of a porous polymer substrate that prevents electrical short circuits of positive and negative electrodes and provides pores that serve as channels for lithium ions.

[0007] When the temperature of the battery rises to more than 100°C due to internal or external stimulation, the polyolefin-based porous separator, which is generally manufactured by a film stretching process, cannot avoid volume changes such as shrinkage or melting, and thus, may explode due to an electrical short circuit between the positive / negative electrodes. In addition, dendrite growth in the battery may penetrate the separator, causing an internal short circuit and a corresponding explosion. In order to suppress high temperature-induced thermal shrinkage and instability of the battery caused by dendrites, a coated separator is disclosed in which inorganic particles are coated on one or both surfaces of a porous polymer substrate together with a binder to allow the inorganic particles to suppress shrinkage of the substrate and improve safety due to the porous coating.

[0008] The porous coating layer comprises inorganic particles and a binder polymer and is introduced to improve the durability (preventing film rupture) or the stability of heat resistance (preventing thermal shrinkage) of the separator substrate. The binder polymer of the separator mainly includes a resin based on polyvinylidene fluoride (PVdF). However, the bonding strength of the resin itself based on PVdF is not high, so it is difficult to ensure a high level of bonding strength. In order to improve the bonding strength, the amount of binder polymer or the coating amount can be increased, but the thickness of the bonding layer may increase, resulting in a lower energy density and a higher resistance. Preferably, the bonding layer of the separator realizes high bonding strength and high ion conductivity at a small thickness, and in order to stably use the battery for a long time, the separator should be a chemically and electrochemically stable material. Therefore, there is a growing demand for developing a separator for a secondary battery that meets these requirements. Summary of the invention

[0009] Technical issues

[0010] According to one aspect, the present disclosure is directed to providing a separator for a thin film secondary battery having a coating layer including inorganic particles and having improved bonding strength with an electrode.

[0011] According to another aspect, the present disclosure is also directed to providing a method for manufacturing a separator for a secondary battery.

[0012] It will be easily understood that these and other objects and advantages of the present disclosure can be achieved by the means or methods set forth in the appended claims, and combinations thereof.

[0013] Technical Solution

[0014] In order to solve the above-mentioned problems of the present disclosure, a separator for a secondary battery and a method for manufacturing the separator for a secondary battery according to the following embodiments are provided.

[0015] According to a first embodiment, there is provided a separator for a secondary battery, which includes a porous polymer substrate; and a porous coating layer provided on at least one surface of the porous polymer substrate,

[0016] wherein the porous coating comprises inorganic particles, a binder polymer and a dispersant,

[0017] The binder polymer comprises at least three types of fluorine-based polymers, and

[0018] The dispersant includes a first dispersant and a second dispersant having a lower weight average molecular weight than the first dispersant.

[0019] According to the second embodiment, in the first embodiment, the weight average molecular weight of the first dispersant may be 100,000 to 500,000, and

[0020] The second dispersant may have a weight average molecular weight of 1,000 to 20,000.

[0021] According to a third embodiment, in the first or second embodiment, the ratio of the weight of the first dispersant to the total weight of the dispersants may be 70 wt % to 90 wt %, and

[0022] The ratio of the weight of the second dispersant to the total weight of the dispersant may be 10 wt % to 30 wt %.

[0023] According to a fourth embodiment, in any one of the first to third embodiments, the first dispersant may include a copolymer including a cyanoethyl group.

[0024] According to a fifth embodiment, in the fourth embodiment, 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.

[0025] According to a sixth embodiment, in any one of the first to fifth embodiments, the second dispersant may include an unsaturated fatty acid-based polymer compound.

[0026] According to a seventh embodiment, in the sixth embodiment, 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.

[0027] According to an eighth embodiment, in any one of the first to seventh embodiments, the binder polymer may include PVdF-HFP as the first fluorine-based polymer, PVdF-CTFE as the second fluorine-based polymer, and at least one of PVdF-TFE and PVdF-HFP-AA as the third fluorine-based polymer.

[0028] According to the ninth embodiment, in the eighth embodiment, based on the mixture of the first fluorine-based polymer to the third fluorine-based polymer, the weight ratio of the first fluorine-based polymer may be 40 wt % to 80 wt %,

[0029] A weight ratio of the second fluorine-based polymer may be 15 wt % to 40 wt % based on the mixture of the first to third fluorine-based polymers, and

[0030] The third fluorine-based polymer may be included in an amount of 1% to 20% by weight based on the mixture of the first to third fluorine-based polymers.

[0031] According to the tenth embodiment, in the ninth embodiment, the third fluorine-based polymer may include PVdF-TFE and PVdF-HFP-AA,

[0032] A weight ratio of PVdF-TFE may be 3 wt % to 10 wt % based on the mixture of the first to third fluorine-based polymers, and

[0033] A weight ratio of PVdF-HFP-AA may be 3 wt % to 10 wt % based on the mixture of the first to third fluorine-based polymers.

[0034] According to the eleventh embodiment, in any one of the eighth to tenth embodiments, the PVdF-HFP may satisfy at least one of the following: a viscosity of 3 to 8 mPa·sec, a Tm of 130° C. to 160° C., a weight average molecular weight of 200,000 to 400,000, and a HFP substitution degree of 5 to 20 mol %.

[0035] According to a twelfth embodiment, in any one of the eighth to eleventh embodiments, the PVdF-CTFE may satisfy at least one of the following: a viscosity of 4 to 10 mPa·sec, 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 to 30 mol%.

[0036] According to the thirteenth embodiment, in any one of the eighth to twelfth embodiments, the PVdF-TFE may satisfy at least one of the following: a viscosity of 13 to 23 mPa·seconds, 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 to 40 mol%.

[0037] According to the fourteenth embodiment, in any one of the eighth to thirteenth embodiments, the PVdF-HFP-AA may satisfy at least one of the following: a viscosity of 15 to 25 mPa·sec, a Tm of 130° C. to 160° C., a weight average molecular weight of 800,000 to 1,200,000, and a HFP substitution degree of 1.5 to 5 mol%.

[0038] According to a fifteenth embodiment, there is provided a method for manufacturing a separator for a secondary battery, comprising:

[0039] preparing a first slurry comprising inorganic particles and a dispersant;

[0040] preparing a binder solution comprising a binder polymer and an organic solvent;

[0041] mixing the first slurry with a binder solution to prepare a second slurry; and

[0042] coating a second slurry on at least one surface of the porous polymer substrate,

[0043] wherein the binder polymer comprises at least three types of fluorine-based polymers, and

[0044] The dispersant includes a first dispersant and a second dispersant having a lower weight average molecular weight than the first dispersant.

[0045] According to the sixteenth embodiment, in the fifteenth embodiment, the inorganic particles may be present in an amount of 50 wt % to 99 wt % based on the total wt % of the binder polymer and the inorganic particles.

[0046] According to a seventeenth embodiment, in any one of the fifteenth or sixteenth embodiment, a weight ratio of solids in the second slurry may be 10 wt % to 40 wt %.

[0047] According to an eighteenth embodiment, in any one of the fifteenth to seventeenth embodiments, the viscosity of the second slurry may be 4 to 10 mPa·sec.

[0048] According to a nineteenth embodiment, in any one of the fifteenth to eighteenth embodiments, the manufacturing method may further include: inducing vapor-induced phase separation of the second slurry coated on the porous polymer substrate at a temperature of 40° C. to 60° C. and a relative humidity of 50% to 70%; and

[0049] The second slurry subjected to phase separation is dried at a temperature of 50 to 70° C. and a relative humidity of 0 to 5%.

[0050] Beneficial Effects

[0051] A separator for a secondary battery according to an embodiment of the present disclosure has a porous coating on at least one surface of a porous polymer substrate, the porous coating comprising inorganic particles, a binder polymer and a dispersant, wherein the binder polymer comprises at least three types of fluorine-based polymers, and the dispersant comprises a first dispersant and a second dispersant.

[0052] In one embodiment, the binder polymer may include a first fluorine-based polymer and a second fluorine-based polymer with a lower viscosity and a third fluorine-based polymer with a higher viscosity. The bonding strength between the separator and the electrode can be improved by a binder polymer with a lower viscosity. In addition, the dispersion of inorganic particles can be reduced by a low-viscosity binder polymer, but the reduction in dispersion can be compensated by a binder polymer with a higher viscosity.

[0053] In one embodiment, the first dispersant may be a high molecular weight dispersant having a relatively high weight average molecular weight, and the second dispersant may be a low molecular weight dispersant having a relatively low weight average molecular weight. A high bonding strength between the porous coating and the porous polymer substrate may be achieved by a first dispersant having a relatively high weight average molecular weight. In contrast, when the dispersant includes the first dispersant in a very large amount, due to the less dispersion of inorganic particles, there may be uncoated exposed areas that are not coated with the porous coating, but since the second dispersant has a relatively low weight average molecular weight, the dispersion of inorganic particles can be improved, and therefore, even if the porous coating is coated with a small thickness, an uncoated area may not be formed. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The accompanying drawings illustrate exemplary embodiments of the present disclosure and, together with the following detailed description, serve to provide a further understanding of the technical aspects of the present disclosure, and thus the present disclosure should not be construed as being limited to the accompanying drawings.

[0055] Figure 1 A table summarizing measurement results of the separator for secondary batteries according to an embodiment of the present disclosure is shown.

[0056] Figure 2 A table summarizing the measurement results of the separators for secondary batteries according to Comparative Examples of the present disclosure is shown.

[0057] Figures 3 to 8 According to Figure 1 Scanning electron microscope (SEM) images of the secondary battery separators of Examples 1 to 6.

[0058] Figures 9 to 15 According to Figure 2 SEM images of the secondary battery separators of Comparative Examples 1 to 7. DETAILED DESCRIPTION

[0059] Hereinafter, the present disclosure will be described in more detail.

[0060] It should be understood that the terms or words used in the present disclosure and the appended claims should not be interpreted as limited to the general meaning and dictionary meaning, but should be interpreted based on the meaning and concepts corresponding to the technical aspects of the present disclosure on the basis of the principle allowing the inventor to appropriately define the terms for the best description.

[0061] Unless the context clearly indicates otherwise, when used in the present disclosure, the terms “comprising”, “including” or “having” specify the presence of stated elements, but do not preclude the presence or addition of one or more other elements.

[0062] In addition, the terms "about" and "substantially" as used herein are used in the sense of being within, or close to, the stated tolerances, given the manufacturing and material tolerances inherent in the circumstances, and are used to prevent unscrupulous infringers from taking unfair advantage of the disclosure where precise or absolute values ​​are stated to aid in the understanding of the disclosure.

[0063] The present disclosure relates to separators, electrochemical cells including the same, and methods for making the same. In the present disclosure, the electrochemical cell may include any battery in which an electrochemical reaction occurs. Specific examples of the electrochemical cell may include any type of primary battery, secondary battery, fuel cell, solar cell, or capacitor such as a supercapacitor. In particular, the electrochemical cell may be a secondary battery, and the secondary battery may be a lithium-ion secondary battery. The lithium-ion secondary battery may include, for example, a lithium-metal battery, a lithium-sulfur battery, an all-solid-state battery, a lithium polymer battery, etc., and is preferably a lithium-sulfur battery.

[0064] According to one aspect of the present disclosure, there is provided a separator for a secondary battery, comprising: a porous polymer substrate; and a porous coating layer disposed on at least one surface of the porous polymer substrate,

[0065] wherein the porous coating comprises inorganic particles, a binder polymer and a dispersant,

[0066] wherein the binder polymer comprises at least three types of fluorine-based polymers, and

[0067] The dispersant includes a first dispersant and a second dispersant having a lower weight average molecular weight than the first dispersant.

[0068] The porous polymer substrate can electrically isolate the negative electrode from the positive electrode to prevent short circuits and provide a path for the movement of lithium ions, is not limited to a specific type, and can include any porous polymer substrate commonly used as a separator for an electrochemical device. The porous polymer substrate can include, for example, a porous polymer film or a nonwoven fabric containing at least one of a polymer resin such as polyolefin, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalene.

[0069] In a specific embodiment of the present disclosure, the thickness of the porous polymer substrate may be selected from a thickness in the range of 5 μm to 50 μm. The thickness of the porous polymer substrate is not particularly limited to the above range, but when the thickness is much less than the above lower limit, the mechanical properties may be reduced and the separator may be easily damaged when the battery is used. On the other hand, the pore size and porosity present in the porous polymer substrate are not limited to a specific range, but may be 0.01 μm to 50 μm, 10% to 95%, respectively.

[0070] The porous coating can be arranged on at least one surface of the porous polymer substrate, and can include inorganic particles, a binder polymer and a dispersant. The inorganic particles in the porous coating can be maintained and stacked together by the binder polymer, and are discontinuously dispersed adjacent to or in contact with each other by the dispersant. Therefore, the porous coating can have a porous structure by the interstitial volume between the inorganic particles. Therefore, the porous coating can alleviate the high thermal shrinkage of the porous polymer substrate, and in addition, provide a mobile path for lithium ions due to ion permeability.

[0071] In a specific embodiment of the present disclosure, the thickness of the porous coating layer can be appropriately selected from a thickness in the range of 0.5 μm to 20 μm, or 1 μm to 10 μm, or 1 μm to 5 μm. The thickness of the porous coating layer is not particularly limited to the above range, but the present disclosure can provide a separator that prevents the formation of an uncoated area and improves adhesion strength and resistance even when the coating layer is formed with a small thickness.

[0072] The inorganic particles of the porous coating are not limited to a specific type and may include any electrochemically stable inorganic particles. Examples of the inorganic particles may include at least one of the following: 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), hafnium dioxide (HfO2), SrTiO3, SnO2, CeO2, MgO, Mg(OH)2, NiO, CaO, ZnO, ZrO2, SiO2, Y2O3, Al2O3, AlOOH, Al(OH)3, SiC and TiO2.

[0073] The binder polymer of porous coating comprises at least three types of fluorine-based polymers. The binder polymer can include a fluorine-based polymer as a binder component alone. In one embodiment, the binder polymer can include a first fluorine-based polymer, a second fluorine-based polymer and a third fluorine-based polymer. Alternatively, the binder polymer can include a first fluorine-based polymer, a second fluorine-based polymer and a third fluorine-based polymer as a binder component. The first fluorine-based polymer to the third fluorine-based polymer can include polyvinylidene fluoride (PVdF) polymer resin including vinylidene fluoride (VDF) as a monomer. The fluorine-based polymer resin may include a copolymer of VDF (as a monomer) and a comonomer of at least one of the following: hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), hexafluoroisobutylene, perfluorobutylethylene, perfluoropropyl vinyl ether (PPVE), perfluoroethyl vinyl ether (PEVE), perfluoromethyl vinyl ether (PMVE), perfluoro-2,2-dimethyl-1,3-dioxole (PDD) and perfluoro-2-methylene-4-methyl-1,3-dioxolane (PMD).

[0074] In one embodiment, the first fluorine-based polymer may be polyvinylidene fluoride-co-hexafluoropropylene (PVdF-HFP). For example, PVdF-HFP may satisfy at least one of the following: a viscosity of 3 to 8 mPa·sec, a Tm of 130°C to 160°C, a weight average molecular weight of 200,000 to 400,000, and a HFP substitution degree of 5 mol% to 20 mol%.

[0075] In one embodiment, the second fluorine-based polymer may be polyvinylidene fluoride-co-chlorotrifluoroethylene (PVdF-CTFE). For example, PVdF-CTFE may satisfy at least one of the following: a viscosity of 4 to 10 mPa·sec, 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 to 30 mol%.

[0076] In one embodiment, the third fluorine-based polymer may include at least one of polyvinylidene fluoride-tetrafluoroethylene (PVdF-TFE) and polyvinylidene fluoride-hexafluoropropylene acrylic acid (PVdF-HFP acrylic acid graft copolymer). For example, PVdF-TFE may satisfy at least one of the following: a viscosity of 13mPa·seconds to 23mPa·seconds, 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 20mol% to 40mol%. In addition, PVdF-HFP-AA may satisfy at least one of the following: a viscosity of 15mPa·seconds to 25mPa·seconds, a Tm of 130°C to 160°C, a weight average molecular weight of 800,000 to 1,200,000, a HFP substitution degree of 1.5mol% to 5mol%, and an AA substitution degree of 0.5mol% to 1.9mol%.

[0077] At least one of the fluorine-based polymers included in the binder polymer preferably has a viscosity of 10 to 15 mPa·sec, but the present disclosure is not limited thereto.

[0078] In one embodiment, the weight ratio of the first fluorine-based polymer may be 40 wt % to 80 wt % relative to the mixture of the first fluorine-based polymer to the third fluorine-based polymer, the weight ratio of the second fluorine-based polymer may be 15 wt % to 40 wt % relative to the mixture of the first fluorine-based polymer to the third fluorine-based polymer, and the weight ratio of the third fluorine-based polymer may be 1 wt % to 20 wt % relative to the mixture of the first fluorine-based polymer to the third fluorine-based polymer. Here, the third fluorine-based polymer may be any one of PVdF-TFE or PVdF-HFP-AA.

[0079] In one embodiment, when the third fluorine-based polymer includes PVdF-TFE and PVdF-HFP-AA, the weight ratio of PVdF-TFE may be 3 wt% to 10 wt% relative to the mixture of the first fluorine-based polymer to the third fluorine-based polymer, and the weight ratio of PVdF-HFP-AA may be 3 wt% to 10 wt% relative to the mixture of the first fluorine-based polymer to the third fluorine-based polymer. In addition, as an example, the weight ratio of PVdF-TFE may be equal to the weight ratio of PVdF-HFP-AA.

[0080] The dispersant of the porous coating layer includes a first dispersant and a second dispersant. The second dispersant has a lower weight average molecular weight than the first dispersant. Therefore, the first dispersant can be a high molecular weight dispersant with a weight average molecular weight of 100,000 to 500,000, and the second dispersant can be a low molecular weight dispersant with a weight average molecular weight of 1,000 to 20,000. For example, the weight average molecular weight of the first dispersant can 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 can be in the range of 1,000 to 20,000, 1,000 to 10,000 or 1,000 to 5,000.

[0081] When the weight average molecular weight of the first dispersant satisfies the above range, high bonding strength between the porous coating and the porous polymer substrate can be ensured, and the dispersion of inorganic particles can be improved. When the weight average molecular weight of the first dispersant is outside the above range, it may not be favorable for the thickness and air permeability of the separator, and the resistance increase may be significant. In addition, when the weight average molecular weight of the second dispersant satisfies the above range, the dispersion of inorganic particles in the porous coating can be improved, and the formation of uncoated areas can be prevented even when the porous coating is coated with a small thickness.

[0082] In particular, when the weight average molecular weight of the first dispersant is lower than 100,000, the porous coating layer may be peeled off due to low adhesive strength between the porous coating layer and the porous polymer substrate. In addition, when the weight average molecular weight of the first dispersant is higher than 500,000, the dispersion of inorganic particles may be significantly reduced.

[0083] Meanwhile, in the present disclosure, the weight average molecular weight may be measured by gel permeation chromatography (GPC). In the present disclosure, detailed GPC measurement conditions are as follows.

[0084] [GPC measurement conditions]

[0085] GPC device: ACQUITY APC(waters)

[0086] Column: PL gel Mixed-B+C (Agilent)

[0087] Column temperature: RT (20°C to 25°C)

[0088] Eluent: DMF / 0.05M LiBr

[0089] Flow rate: 1.0ml / min

[0090] Detector: Waters 2414RI detector

[0091] Sample concentration: ~1 mg / ml (0.45 μm syringe filter)

[0092] Injection volume: 10 μl

[0093] Curve Fitting: PMMA

[0094] Meanwhile, in the present disclosure, viscosity can be measured using a viscometer commonly used in the art. Specific examples of the viscometer may include Brookfile's DVELV or LV DV-II + Pro viscometer (cone plate). In a specific embodiment of the present disclosure, after each polymer to be measured is dissolved in acetone at a predetermined concentration, the viscosity is measured at 25° C. by Brookfield's LVDV-II + Pro viscometer (cone plate), and during the measurement, the RPM of the axis LV-1 (61) may be 100 rpm to 200 rpm, and the sample loading volume may be 100 mL to 250 mL.

[0095] In addition, in the present disclosure, in fluorine-based polymers, the degree of substitution can be measured by measuring the amount of functional groups (comonomers) in each polymer, for example, by one-dimensional normal nuclear magnetic resonance spectroscopy (1D normal NMR). The measurement method can be performed using a Bruker 500MHz NMR analyzer, and the sample to be measured can be dissolved in a DMSO-d6 solvent. For example, the measurement conditions can be set to 1H NMR (zg30), ns=32, d1=3 seconds.

[0096] Alternatively, NMR can be measured by fluorine atomic nuclear magnetic resonance spectroscopy. Fluorine atomic nuclear magnetic resonance spectroscopy will be described in more detail below.

[0097] For example, the measurement can be performed using CFCl3 as a reference material using a 19F NMR (Bruker DRX-300) analyzer by the following method. The measurement method starts with dissolving the sample to be measured in deuterated acetone at 8 wt%. The weight average molecular weight is determined by GPC (high temperature PL 220, Waters) equipped with a refractometer and two PLgel-10μm Mixed-B columns (Polymer Laboratory). The mobile phase solvent may include 0.1M LiBr and dimethylformamide, and the measurement conditions are: 80°C and a flow rate of 1.0mL / min. The reference sample may be, for example, a polystyrene having a weight average molecular weight in the range of 2,000g / mol to 2,000,000g / mol.

[0098] In another embodiment of the present disclosure, the method of fluorine atomic nuclear magnetic resonance spectroscopy analysis can use, for example, a 376.3 MHz Unity 400 spectrometer. For example, the spectrum can be obtained in 50° C. deuterated dimethylformamide by recirculating excitation pulses with a pulse width of 8.0 microseconds and a delay of 10 seconds, or in 50° C. deuterated dimethyl sulfoxide by recirculating excitation pulses with a pulse width of 6.0 microseconds and a delay of 5 seconds, or in 50° C. deuterated acetone by recirculating excitation pulses with a pulse width of 8.0 microseconds and a delay of 20 seconds.

[0099] In one embodiment, the first dispersant may include a copolymer including a cyanoethyl group. Examples of the first dispersant may include at least one of cyanoethyl polyvinyl alcohol, cyanoethyl pullulan, cyanoethyl sucrose, cyanoethyl cellulose, cyanoethyl hydroxyethyl cellulose, and cyanoethyl hydroxypropyl cellulose.

[0100] In one embodiment, the second dispersant may include an unsaturated fatty acid-based polymer compound. Examples of the second dispersant may include at least one of the following: 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.

[0101] Since the first dispersant has a high weight average molecular weight, a high bonding strength between the porous coating layer and the porous polymer substrate can be ensured. In contrast, when the dispersant contains the first dispersant in a very large amount, the dispersion of the inorganic particles is reduced, forming an uncoated exposed area that is not coated with the porous coating layer. On the contrary, since the second dispersant has a lower weight average molecular weight, the dispersion of the inorganic particles can be improved, and the formation of an uncoated area is prevented even when the thickness of the porous coating layer is reduced.

[0102] Therefore, the weight ratio of the first dispersant to the total weight of the dispersant may be 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 %.

[0103] When the weight ratio of the first dispersant and the weight ratio of the second dispersant satisfy the above range, it is possible to ensure high bonding strength of the porous coating layer to the porous polymer substrate, improve dispersion of inorganic particles, and prevent formation of uncoated areas.

[0104] In contrast, when the weight ratio of the first dispersant is less than 70 wt %, the porous coating layer may be peeled off due to the low bonding strength between the porous coating layer and the porous polymer substrate. In addition, when the weight ratio of the first dispersant is greater than 90 wt %, an uncoated area may be formed.

[0105] When the weight ratio of the second dispersant is less than 10 wt %, an uncoated area may be formed due to less dispersion of inorganic particles. In addition, when the weight ratio of the second dispersant is greater than 30 wt %, the porous coating layer may be peeled off due to low bonding strength between the porous coating layer and the porous polymer substrate.

[0106] The separator for a secondary battery of the present disclosure has a porous coating on at least one surface of a porous polymer substrate, the porous coating comprising inorganic particles, a binder polymer and a dispersant, wherein the binder polymer comprises at least three types of fluorine-based polymers, and the dispersant comprises a first dispersant and a second dispersant.

[0107] In one embodiment, each of the first fluorine-based polymer and the second fluorine-based polymer can be PVdF-HFP and PVdF-CTFE with lower viscosity, respectively, and the third fluorine-based polymer can be PVdF-TFE and / or PVdF-HFP-AA with higher viscosity. The bonding strength between the separator and the electrode can be improved by a binder polymer with lower viscosity. In addition, the dispersion of inorganic particles can be reduced by a low-viscosity binder polymer, but the reduction in dispersion can be compensated by a binder polymer with higher viscosity.

[0108] In one embodiment, the first dispersant can be a high molecular weight dispersant with a higher weight average molecular weight, such as a copolymer containing cyanoethyl, and the second dispersant can be a polymer compound based on unsaturated fatty acids with a lower weight average molecular weight. The high bonding strength between the porous coating and the porous polymer substrate can be ensured by a first dispersant with a higher weight average molecular weight. In contrast, when the dispersant contains the first dispersant in a very large amount, the dispersion of the inorganic particles is reduced, forming an uncoated exposed area that is not coated with the porous coating. Since the second dispersant has a lower weight average molecular weight, the dispersion of the inorganic particles can be improved, thereby preventing the formation of uncoated areas even when the porous coating is formed with a small thickness.

[0109] Hereinafter, a method for manufacturing a separator for a secondary battery will be described.

[0110] According to one aspect of the present disclosure, there is provided a method for manufacturing a separator for a secondary battery, comprising:

[0111] preparing a first slurry comprising inorganic particles and a dispersant;

[0112] preparing a binder solution comprising a binder polymer and an organic solvent;

[0113] mixing the first slurry with a binder solution to prepare a second slurry; and

[0114] coating a second slurry on at least one surface of the porous polymer substrate,

[0115] wherein the binder polymer comprises at least three types of fluorine-based polymers, and

[0116] The dispersant includes a first dispersant and a second dispersant having a lower weight average molecular weight than the first dispersant.

[0117] Furthermore, in one embodiment, the manufacturing method may further include inducing phase separation of the second slurry coated on the porous polymer substrate at a temperature of 40° C. to 60° C. and a relative humidity of 50% to 70%; and

[0118] The second slurry subjected to phase separation is dried at a temperature of 50 to 70° C. and a relative humidity of 0 to 5%.

[0119] The first slurry may be prepared by mixing and dispersing an organic solvent, inorganic particles, and a dispersant.

[0120] The organic solvent may include cyclic aliphatic hydrocarbons, such as cyclopentane, cyclohexane; aromatic hydrocarbons, such as toluene, xylene, ethylbenzene; ketones, such as acetone, ethyl methyl ketone, diisopropyl ketone, cyclohexanone, methylcyclohexane, ethylcyclohexane; chlorinated aliphatic hydrocarbons, such as dichloromethane, chloroform, carbon tetrachloride; esters, such as ethyl acetate, butyl acetate, γ-butyrolactone, ε-caprolactone; nitriles, such as acetonitrile, propionitrile; ethers, such as tetrahydrofuran, ethylene glycol diethyl ether; alcohols, such as methanol, ethanol, isopropanol, ethylene glycol, ethylene glycol monomethyl ether; amides, such as N-methylpyrrolidone, N,N-dimethylformamide. According to a specific embodiment of the present disclosure, in view of the advantages of the drying process, the organic solvent may include acetone.

[0121] These solvents can be used alone or in combination. Especially, some solvents with low boiling point and high volatility are preferred because they can be removed in a short time at low temperatures. Specifically, the solvent can preferably include acetone, toluene, cyclohexanone, cyclopentane, tetrahydrofuran, cyclohexane, dimethylbenzene, N-methylpyrrolidone or its mixture.

[0122] The inorganic particles and the dispersant may contain the materials described above.

[0123] The binder solution may be prepared by mixing a binder polymer, which may include the first to third fluorine-based polymers, with an organic solvent, and the organic solvent may include the above-mentioned solvent, preferably, acetone.

[0124] The second slurry may be prepared by adding the binder solution to the first slurry, and grinding and dispersing the inorganic particles while controlling the average particle size to a predetermined level using a ball milling method.

[0125] In this case, the inorganic particles may be present in an amount of 50 to 99 wt % based on the total weight % of the binder polymer and the inorganic particles, and the weight ratio of the solid in the second slurry may be 10 to 40 wt %. In addition, the viscosity of the second slurry may be 4 to 10 mPa·sec.

[0126] The prepared second slurry is coated on one or both surfaces of the porous polymer substrate to form a porous coating. The method for coating the second slurry is not limited to a specific method, and may include dip coating, die coating, roll coating, comma coating, knife coating, reverse roll coating, direct roll coating, etc.

[0127] The coating process for forming the porous coating is preferably carried out within a predetermined humidity range. After the composition for forming the porous coating is applied on one or both surfaces of the porous polymer substrate, the binder dissolved in the coating (the composition) has phase change characteristics through phase separation (steam induced phase separation) known in the art through a drying process. Phase separation can be carried out by wet phase separation or impregnation phase separation.

[0128] The wet phase separation will be described below.

[0129] First, for phase separation, a non-solvent can be introduced in a gaseous state. The non-solvent is not limited to a specific type and can include any non-solvent that does not dissolve the binder and is partially compatible with the solvent. The non-solvent can be, for example, at least one selected from the following: water, methanol, ethanol, isopropanol and butanol.

[0130] When the non-solvent is introduced and added in a gaseous state, phase separation can be performed using a small amount of the non-solvent, and the inorganic composition can be dried more easily.

[0131] In this case, the temperature for adding the gaseous non-solvent can be in the range of 15°C to 70°C. When the temperature is lower than 15°C, the non-solvent does not remain in a gaseous state and the inorganic composition dries slowly, resulting in low productivity. When the temperature is higher than 70°C, the solvent and the non-solvent dry quickly, resulting in insufficient phase separation.

[0132] In addition, during the phase separation process, the non-solvent may be added sequentially so that the vapor pressure of the non-solvent is 15% to 80% or 30% to 50% of the saturated vapor pressure, and the phase separation is performed. When the vapor pressure of the non-solvent is lower than 15% of the saturated vapor pressure, sufficient phase separation cannot be performed due to the very small amount of the non-solvent, and when the vapor pressure of the non-solvent is higher than 80% of the saturated vapor pressure, the phase separation is excessively performed, making it difficult to obtain a uniform coating.

[0133] In order to induce phase separation when adding a non-solvent in a gaseous state, it is preferred to use a solvent with a low boiling point because it evaporates easily. That is, when the solvent evaporates, the temperature decreases and the non-solvent in a gaseous state becomes condensed, which makes it easy to exchange with the solvent. In a specific example, when a non-solvent in a gaseous state is added, the boiling point of the solvent may be in the range of 30° C. to 80° C. In addition, the solvent of the inorganic composition containing the non-solvent in a gaseous state may be, for example, at least one selected from acetone and methyl ethyl ketone.

[0134] Next, the impregnation phase separation will be described below.

[0135] The composition for forming a porous coating is coated on at least one surface of a porous polymer substrate and then immersed in a coagulation solution containing a suitable non-solvent for a predetermined time. This induces phase separation of the composition for forming a porous coating and coagulation of the binder resin applied. In this process, pores are formed in the coating containing the binder resin and inorganic particles. Subsequently, washing is performed to remove the coagulation solution, followed by drying to completely form a porous coating on the porous polymer substrate.

[0136] The solvent for dissolving the binder may include any solvent that dissolves the binder resin at a concentration of 5 wt % or more, preferably 15 wt % or more, more preferably 25 wt % or more at 25° C. Non-limiting examples of the solvent may include polar amide solvents such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide, acetone, cyclopentanone, methyl acetate, γ-butyrolactone, trimethyl phosphate, triethyl phosphate, and dimethylethoxymethane. When the solubility of the binder resin in the solvent is less than the above range, phase separation may be excessively performed.

[0137] The nonsolvent may include any nonsolvent having a binder resin solubility of less than 5 wt % at 25° C. The nonsolvent may include at least one selected from the group consisting of water, methanol, ethanol, propanol, butanol, butanediol, ethylene glycol, propylene glycol, and tripropylene glycol.

[0138] The coagulation solution may contain a nonsolvent alone or in combination with the above solvents. When a mixed solvent of the nonsolvent and the solvent is used, the amount of the nonsolvent is 95 wt % or more based on 100 wt % of the coagulation solution to form a good porous structure and improve productivity.

[0139] After phase separation, the separator for secondary batteries of the present disclosure can be dried. Specifically, the second slurry subjected to phase separation can be dried at a temperature of 50° C. to 70° C. and a relative humidity of 0% to 5%, or a temperature of 50° C. to 70° C. and a relative humidity of 0%. Drying can be carried out using methods known in the art, and can be carried out by an intermittent or continuous process using an oven or a heating chamber within a temperature range considering the vapor pressure of the solvent used. Drying is carried out to remove the solvent present in the composition, and it is preferably dried as soon as possible in view of productivity.

[0140] Hereinafter, embodiments will be described to help understand the present disclosure. However, embodiments according to the present disclosure may be modified into many different forms, and the scope of the present disclosure should not be construed as being limited to the following embodiments. The embodiments of the present disclosure are provided to fully and comprehensively illustrate the present disclosure to those of ordinary skill in the art to which the present disclosure belongs.

[0141] Example 1

[0142] First slurry

[0143] 400 g of acetone as an organic solvent, 130 g of Al2O3 (Korea Ceramics, ALK-S1) as inorganic particles, 3 g of cyanoethyl polyvinyl alcohol (Echo 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 (BYK, P-105, weight average molecular weight 1,500) as a second dispersant were mixed and dispersed to prepare a first slurry.

[0144] Binder solution

[0145] 400 g of acetone as an organic solvent, 36 g of PVdF-HFP (Solvay, Solef 20808) as a first fluorine-based polymer, 15 g of PVdF-CTFE (Solvay, Solef 32008) as a second fluorine-based polymer, and 3 g of PVdF-TFE (Daikin) as a third fluorine-based polymer were mixed to prepare a binder solution.

[0146] PVdF-HFP has a viscosity of 3.42 mPa·s, a Tm of 151°C, a weight average molecular weight of 300,000 and a HFP substitution degree of 8 mol%, PVdF-CTFE has a viscosity of 5.34 mPa·s, a Tm of 166°C, a weight average molecular weight of 300,000 and a CTFE substitution degree of 20 mol%, and PVdF-TFE has a viscosity of 18.69 mPa·s, a Tm of 135°C, a weight average molecular weight of 140,000 and a TFE substitution degree of 31 mol%.

[0147] Second slurry

[0148] The binder solution was added to the first slurry, and the inorganic particles were ground and dispersed while the average particle size was controlled to a predetermined level using a ball milling method to prepare a second slurry. The second slurry had a weight ratio of organic solvent to solid of 4:1 and a solid content of 20 wt%. The weight ratio of the binder polymer to the inorganic particles in the solid was 1:4, and the weight of the inorganic particles was 80 wt% relative to the total weight % of the binder polymer and the inorganic particles. The viscosity of the second slurry was 7.32 mPa·sec.

[0149] Separators for secondary batteries

[0150] The second slurry was coated on a polyolefin-based substrate (Toray, thickness 9 μm, air permeation time 70 seconds / 100 cubic centimeters, resistance 0.50 ohms, porosity 45 volume %) as a porous polymer substrate by a microgravure method. Subsequently, a phase transition was performed by inducing vapor-induced phase separation of the second slurry coated on the porous polymer substrate at a temperature of 60° C. and a relative humidity of 60% for 15 seconds, and dried at a temperature of 60° C. and a relative humidity of 0% to prepare a separator for a secondary battery.

[0151] Example 2

[0152] The separator was prepared by the same method as in Example 1, except that 400 g of acetone as an organic solvent, 34 g of PVdF-HFP (Solvay, Solef 20808) as a first fluorine-based polymer, 14 g of PVdF-CTFE (Solvay, Solef 32008) as a second fluorine-based polymer, and 6 g of PVdF-TFE (Daikin) as a third fluorine-based polymer were mixed to prepare a binder solution having a viscosity of 8.58 mPa·s.

[0153] Example 3

[0154] The separator was prepared by the same method as in Example 1, except that 400 g of acetone as an organic solvent, 36 g of PVdF-HFP (Solvay, Solef 20808) as a first fluorine-based polymer, 15 g of PVdF-CTFE (Solvay, Solef 32008) as a second fluorine-based polymer, and 3 g of PVdF-HFP-AA (Solvay, Solef 75130) as a third fluorine-based polymer were mixed to prepare a binder solution having a viscosity of 7.41 mPa·sec. PVdF-HFP-AA had a viscosity of 19.22 mPa·sec, a Tm of 145°C, a weight average molecular weight of 1,000,000, and a HFP substitution degree of 2.3 mol%.

[0155] Example 4

[0156] The separator was prepared by the same method as in Example 1, except that 400 g of acetone as an organic solvent, 34 g of PVdF-HFP (Solvay, Solef 20808) as a first fluorine-based polymer, 14 g of PVdF-CTFE (Solvay, Solef 32008) as a second fluorine-based polymer, and 6 g of PVdF-HFP-AA (Solvay, Solef 75130) as a third fluorine-based polymer were mixed to prepare a binder solution having a viscosity of 9.01 mPa·sec. PVdF-HFP-AA had a viscosity of 19.22 mPa·sec, a Tm of 145°C, a weight average molecular weight of 1,000,000, and a HFP substitution degree of 2.3 mol%.

[0157] Example 5

[0158] The separator was prepared by the same method as in Example 1, except that 400 g of acetone as an organic solvent, 34 g of PVdF-HFP (Solvay, Solef 20808) as a first fluorine-based polymer, 14 g of PVdF-CTFE (Solvay, Solef 32008) as a second fluorine-based polymer, and 3 g of PVdF-TFE (Daikin) and 3 g of PVdF-HFP-AA (Solvay, Solef 75130) as a third fluorine-based polymer were mixed to prepare a binder solution having a viscosity of 8.741 mPa·sec. PVdF-HFP-AA had a viscosity of 19.22 mPa·sec, a Tm of 145°C, a weight average molecular weight of 1,000,000, and a HFP substitution degree of 2.3 mol%.

[0159] Example 6

[0160] The separator was prepared by the same method as in Example 1, except that 400 g of acetone as an organic solvent, 130 g of Al2O3 (Korea Ceramics, ALK-S1) as inorganic particles, 3 g of cyanoethyl polyvinyl alcohol (Echo 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, P-105, weight average molecular weight 1,500) as a second dispersant were mixed and dispersed to prepare a first slurry.

[0161] Comparative Example 1

[0162] A separator was prepared by the same method as Example 1, except that 400 g of acetone as an organic solvent and 38 g of PVdF-HFP (Solvay, Solef 20808) and 16 g of PVdF-CTFE (Solvay, Solef 32008) as a fluorine-based polymer were mixed to prepare a binder solution.

[0163] Comparative Example 2

[0164] A separator was prepared by the same method as Example 1, except that 400 g of acetone as an organic solvent and 27 g of PVdF-HFP (Solvay, Solef 20808) and 27 g of PVdF-CTFE (Solvay, Solef 32008) as fluorine-based polymers were mixed to prepare a binder solution.

[0165] Comparative Example 3

[0166] A separator was prepared by the same method as Example 1, except that 400 g of acetone as an organic solvent and 16 g of PVdF-HFP (Solvay, Solef 20808) and 38 g of PVdF-CTFE (Solvay, Solef 32008) as fluorine-based polymers were mixed to prepare a binder solution.

[0167] Comparative Example 4

[0168] A separator was prepared by the same method as in Example 1, except that the binder solution was prepared using 400 g of acetone as an organic solvent and 54 g of PVdF-TFE (Daikin) as a fluorine-based polymer.

[0169] Comparative Example 5

[0170] A separator was prepared by the same method as in Example 1, except that 400 g of acetone as an organic solvent and 54 g of PVdF-HFP-AA (Solvay, Solef 75130) as a fluorine-based polymer were used to prepare the binder solution.

[0171] Comparative Example 6

[0172] The separator was prepared by the same method as Example 1, except that 400 g of acetone as an organic solvent, 130 g of Al2O3 (Korea Ceramics, ALK-S1) as inorganic particles and 3 g of cyanoethyl polyvinyl alcohol (Echo Chemical Co., Ltd., CEPV-157) as a dispersant were mixed and dispersed to prepare a first slurry.

[0173] Comparative Example 7

[0174] The separator was prepared by the same method as in Example 1, except that 400 g of acetone as an organic solvent, 130 g of Al2O3 (Korea Ceramic, ALK-S1) as inorganic particles, and 2 g of an unsaturated carboxylic acid polymer (BYK, P-105) as a dispersant were mixed and dispersed to prepare a first slurry.

[0175] Figure 1 and Figure 2 The measurement results of Examples 1 to 6 and Comparative Examples 1 to 7 are shown respectively.

[0176] The air permeability of the separator was measured using a Gurley air permeability tester according to JIS P-8117. In this case, 100 cubic centimeters of air was measured through a diameter of 28.6 mm and an area of ​​645 mm 2 The time required for the separators.

[0177] The separator-electrode adhesion strength was measured by cutting the separator into a size of 25 mm × 70 mm, stacking it with the electrode, inserting it between 2 A4 papers and adhering it using a flat press. In this case, hot pressing was performed under the conditions of a temperature of 60°C, a pressure of 1000 kgf and a flat press for 1 second. Subsequently, when a 180° force was applied using a UTM device (LLOYDinstrument LF plus) at a measuring speed of 300 mm / min, the force required to peel the porous coating opposite to the electrode from the electrode was measured.

[0178] The resistance is the resistance value when the separator is cut, put into a Hoshen 2032 button cell and immersed in an electrolyte solution, and is measured by an alternating current method using EIS (Electrochemical Impedance Spectroscopy) at 25°C using a 1M LiPF6-ethylene carbonate / ethyl methyl carbonate (weight ratio 3:7) electrolyte solution.

[0179] like Figure 1 and Figure 2 As shown, the separators of Examples 1 to 6 had a coating layer with a thickness of 2.5 μm or less when measured, and it was determined that a thinner film separator was achieved compared to the comparative example. In addition, when the bulk density, air permeability, separator-electrode bonding strength, peel strength and resistance of the separators of Examples 1 to 6 and the comparative example were compared, the separator of the embodiment was superior to the comparative example.

[0180] Figures 3 to 8 Scanning electron microscope (SEM) images of the separators for secondary batteries according to Examples 1 to 6 are shown respectively, Figures 9 to 15 SEM images of the separators for secondary batteries according to Comparative Examples 1 to 7 are respectively shown.

[0181] As an observation Figures 3 to 8 As a result of the SEM images, almost no or no uncoated area was observed compared to the comparative example. Figure 3 In the figure, the color of the thin adhesive layer is black, indicating that it is fully coated.

[0182] In contrast, in the case of Comparative Examples 1, 2, 4, 5, and 6, the coating was too thick and a thin film separator could not be achieved, and in the case of Comparative Examples 3 and 7, the coating thickness was appropriate, but the separator-electrode adhesion strength measured in Comparative Example 3 was too low, and the peel strength measured in Comparative Example 7 was too low. Figures 9 to 14 In the figure, a surface of the separator having void spaces was observed due to the aggregation of white aluminum oxide on the surface, confirming the presence of uncoated areas.

[0183] The weight average molecular weight is measured by GPC, and the GPC measurement conditions are as follows.

[0184] [GPC measurement conditions]

[0185] GPC device: ACQUITY APC(waters)

[0186] Column: PL gel Mixed-B+C (Agilent)

[0187] Column temperature: RT (20°C to 25°C)

[0188] Eluent: DMF / 0.05M LiBr

[0189] Flow rate: 1.0ml / min

[0190] Detector: Waters 2414RI detector

[0191] Sample concentration: ~1 mg / ml (0.45 μm syringe filter)

[0192] Injection volume: 10 μl

[0193] Curve Fitting: PMMA

[0194] The viscosity was measured using a Brookfile LV DV-II+Pro viscometer (cone and plate). After each polymer to be measured was dissolved in acetone at a predetermined concentration, the device was used to measure the viscosity at 25°C, with the RPM of the spindle LV-1 (61) being 100 to 200 rpm and the sample loading volume being 100 to 250 mL.

[0195] The degree of substitution is measured by 1D normal NMR. The measurement is performed using a Bruker 500MHz NMR analyzer, and about 10 mg of a sample is dissolved in about 0.5 ml of DMSO-d6 at 50°C. The measurement conditions are 1H NMR (zg30), ns=32, d1=3 seconds.

Claims

1. A separator for a secondary battery, comprising: a porous polymer substrate; and a porous coating disposed on at least one surface of the porous polymer substrate, wherein the porous coating layer comprises inorganic particles, a binder polymer and a dispersant, wherein the binder polymer comprises at least three types of fluorine-based polymers, and The dispersant comprises a first dispersant and a second dispersant having a lower weight average molecular weight than the first dispersant.

2. The separator for a secondary battery according to claim 1, wherein 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.

3. The separator for a secondary battery according to claim 1, wherein a ratio of the weight of the first dispersant to the total weight of the dispersant is 70 wt % to 90 wt %, and The ratio of the weight of the second dispersant to the total weight of the dispersant is 10 wt % to 30 wt %. 4 . The separator for a secondary battery according to claim 1 , wherein the first dispersant comprises a copolymer including a cyanoethyl group. 5 . The separator for a secondary battery according to claim 4 , wherein the first dispersant comprises 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. 6 . The separator for a secondary battery according to claim 1 , wherein the second dispersant comprises an unsaturated fatty acid-based polymer compound. 7 . The separator for a secondary battery according to claim 6 , wherein the second dispersant comprises 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. 8 . The separator for a secondary battery according to claim 1 , wherein the binder polymer comprises PVdF-HFP as a first fluorine-based polymer, PVdF-CTFE as a second fluorine-based polymer, and at least one of PVdF-TFE and PVdF-HFP-AA as a third fluorine-based polymer.

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

10. The separator for a secondary battery according to claim 9, wherein the third fluorine-based polymer comprises PVdF-TFE and PVdF-HFP-AA, wherein a weight ratio of the PVdF-TFE is 3 wt % to 10 wt % based on the mixture of the first fluorine-based polymer to the third fluorine-based polymer, and Wherein a weight ratio of the PVdF-HFP-AA is 3 wt % to 10 wt % based on the mixture of the first to third fluorine-based polymers. 11 . The separator for a secondary battery according to claim 8 , wherein the PVdF-HFP satisfies at least one of the following: a viscosity of 3 to 8 mPa·sec, a Tm of 130 to 160° C., a weight average molecular weight of 200,000 to 400,000, and a HFP substitution degree of 5 to 20 mol %. 12 . The separator for a secondary battery according to claim 8 , wherein the PVdF-CTFE satisfies at least one of the following: a viscosity of 4 to 10 mPa·sec, a Tm of 150 to 180° C., a weight average molecular weight of 200,000 to 400,000, and a CTFE substitution degree of 10 to 30 mol %. 13 . The separator for a secondary battery according to claim 8 , wherein the PVdF-TFE satisfies at least one of the following: a viscosity of 13 to 23 mPa·sec, 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 to 40 mol %. 14 . The separator for a secondary battery according to claim 8 , wherein the PVdF-HFP-AA satisfies at least one of the following: a viscosity of 15 to 25 mPa·sec, a Tm of 130° C. to 160° C., a weight average molecular weight of 800,000 to 1,200,000, and a HFP substitution degree of 1.5 to 5 mol %.

15. A method for manufacturing a separator for a secondary battery, comprising: preparing a first slurry comprising inorganic particles and a dispersant; preparing a binder solution comprising a binder polymer and an organic solvent; mixing the first slurry with the binder solution to prepare a second slurry; and coating the second slurry on at least one surface of the porous polymer substrate, wherein the binder polymer comprises at least three types of fluorine-based polymers, and The dispersant comprises a first dispersant and a second dispersant having a lower weight average molecular weight than the first dispersant. 16 . The method for manufacturing a separator for a secondary battery according to claim 15 , wherein the inorganic particles are present in an amount of 50 wt % to 99 wt % based on the total wt % of the binder polymer and the inorganic particles. 17 . The method for manufacturing a separator for a secondary battery according to claim 15 , wherein a weight ratio of solids 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 to 10 mPa·sec.

19. The method for manufacturing a separator for a secondary battery according to claim 15, further comprising: inducing vapor-induced phase separation of the second slurry coated on the porous polymer substrate at a temperature of 40° C. to 60° C. and a relative humidity of 50% to 70%; as well as The second slurry subjected to phase separation is dried at a temperature of 50 to 70° C. and a relative humidity of 0 to 5%.

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