Separator membrane for secondary batteries

Abstract

The separator for a secondary battery according to the present invention includes a separator substrate having a porous structure and made of a polymer material, and a coating layer coated on at least one surface of the separator substrate. The coating layer includes a crystalline binder and an amorphous binder that are miscible with each other. The crystalline binder includes a first crystalline binder and a second crystalline binder. The content of the amorphous binder is more than 1 wt % to less than 5 wt % based on the total solid weight of the coating layer. This reduces resistance within the separator coating layer and improves ionic conductivity.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 2021-0132448 filed on October 6, 2021, and Korean Patent Application No. 2022-0127529 filed on October 6, 2022, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a separator for a secondary battery, and more particularly to a separator for a secondary battery having reduced resistance of a separator coating layer and improved adhesion to an electrode. [Background technology]

[0003] Among the components of a secondary battery, the separator is a porous polymer membrane located between the positive and negative electrodes, which serves to separate the positive and negative electrodes, prevent electrical shorts between the electrodes, and allow electrolyte and ions to pass through. Although the separator itself does not participate in the electrochemical reaction of the battery, its physical properties, such as wettability with the electrolyte, degree of porosity, and thermal shrinkage, affect the performance and safety of the battery.

[0004] Therefore, in order to enhance the physical properties of the separator, various methods have been attempted to add a coating layer to the separator substrate and to change the physical properties of the coating layer by adding various materials to the coating layer. For example, an inorganic substance may be added to the coating layer to improve the mechanical strength of the separator, or an inorganic substance or hydrate may be added to the coating layer to improve the flame retardancy and heat resistance of the separator substrate.

[0005] The separator can be bonded to the electrode by a lamination process. To ensure adhesion between the electrode and the separator, a binder can be added to the coating layer composition of the separator, and the binder can be introduced to the surface of the coating layer by a humid phase separation method, thereby forming an adhesive layer on the surface of the coating layer.

[0006] Generally, the behavior of the binder in the electrolyte varies depending on the crystallinity of the binder. For example, a binder with high crystallinity has low electrolyte impregnation rate, which reduces mobility and increases separator resistance. In addition, a battery using a separator with high resistance may have reduced lifespan.

[0007] Therefore, various studies have been conducted to reduce the resistance of the separator and extend the life of the battery.

[0008] The separator of Patent Document 1 has a coating layer formed on at least one surface of a porous polymer substrate, the coating layer including inorganic particles and a binder polymer, the binder polymer including an amorphous adhesive binder polymer and one or more fluorine-based copolymers, and the content of the amorphous adhesive binder polymer is 50 to 84 parts by weight based on 100 parts by weight of the total binder polymer content.

[0009] The separation membrane of Patent Document 2 includes a porous substrate and a porous coating layer formed on at least one surface of the porous substrate and including inorganic particles and a binder, wherein the weight of the binder is 5 wt % to 40 wt % of the total weight of the porous coating layer, and the binder includes a fluorine-based binder and a rubber-based binder.

[0010] The separators of Patent Documents 1 and 2 have low resistance and exhibit the effect of improving the adhesive strength between the separator and the electrode.

[0011] However, Patent Documents 1 and 2 do not disclose a technique for reducing the crystallinity of a crystalline binder contained in a separator coating layer.

[0012] Therefore, a technology capable of reducing the resistance of a separator is needed even in the case of a separator containing a crystalline binder in the coating layer. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Korean Patent Publication No. 2020-0034470 [Patent Document 2] Korean Patent Publication No. 2017-0138958 Summary of the Invention [Problem to be solved by the invention]

[0014] The present invention has been made to solve the above problems, and aims to provide a separator for a secondary battery that has reduced resistance and improved adhesion to an electrode by improving the mobility of a binder constituting a separator coating layer. [Means for solving the problem]

[0015] To achieve this object, the separator for a secondary battery according to the present invention includes a separator substrate having a porous structure and made of a polymer material, and a coating layer coated on at least one surface of the separator substrate. The coating layer includes a crystalline binder and an amorphous binder that are miscible with each other. The crystalline binder includes a first crystalline binder and a second crystalline binder. The content of the amorphous binder may be more than 1 wt % and less than 10 wt % based on the total solid weight of the coating layer.

[0016] The crystalline binder and the non-crystalline binder may be non-aqueous binders.

[0017] The crystalline binder may be a polyvinylidene fluoride (PVDF) copolymer.

[0018] The amorphous binder may be at least one selected from the group consisting of an acrylate polymer or a copolymer thereof, a polyvinylpyrrolidone-polyvinyl acetate copolymer, and polyvinyl acetate.

[0019] The acrylate polymer or copolymer thereof may be represented by the following Chemical Formula 1, or may be at least one selected from the group consisting of methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, and ethyl methacrylate.

[0020] [ka]

[0021] The content of the amorphous binder may be more than 5 wt % and less than 50 wt % based on the total weight of the crystalline binder and the amorphous binder.

[0022] The weight average molecular weight of the amorphous binder may be less than 1 million.

[0023] When the separator is heat-treated at 80° C. for 3 hours, the resistance of the separator may be lower than 1.00 Ω.

[0024] When the separator for a secondary battery is immersed in an electrolyte and heat-treated, the inorganic material in the coating layer may be exposed to the surface.

[0025] The present invention provides a secondary battery including an electrode assembly in which the separator for a secondary battery is interposed between a positive electrode and a negative electrode and stacked.

[0026] The binder on the surface of the coating layer of the separator for the secondary battery can dissolve in the electrolyte and migrate and distribute inside the positive and negative electrodes.

[0027] The present invention also provides a battery module including the secondary battery.

[0028] The present invention can also be provided in the form of various combinations of the above technical solutions. [Effects of the Invention]

[0029] As described above, the separator for a secondary battery according to the present invention includes a crystalline binder and an amorphous binder in the coating layer, which improves binder solubility and binder mobility in an electrolyte compared to a separator including only a crystalline binder, thereby reducing the separator resistance and improving ionic conductivity.

[0030] In addition, when the separator for a secondary battery of the present invention is laminated and bonded to an electrode, the binder on the surface of the separator coating layer dissolves in the electrolyte and diffuses uniformly into the electrode, thereby improving the resistance of the battery cell. [Brief explanation of the drawings]

[0031] [Figure 1] 1 is a SEM photograph of the separator according to Example 1 before being immersed in an electrolyte solution. [Figure 2] 1 is a SEM photograph of the separator according to Example 1 after immersion in an electrolyte at 25° C. [Figure 3] 1 is a SEM photograph of the separator according to Example 1 after immersion in an electrolyte at 80° C. [Figure 4] 1 is a graph showing the DSC results of the separation membrane according to Example 2. [Figure 5] 1 is a graph showing the DSC results of the separation membrane according to Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0032] Hereinafter, with reference to the accompanying drawings, a detailed description will be given of an embodiment of the present invention that will enable a person skilled in the art to easily carry out the present invention. However, in describing the operation principle of the preferred embodiment of the present invention in detail, detailed description of related well-known functions or configurations will be omitted if it is determined that such detailed description may unnecessarily obscure the gist of the present invention.

[0033] Furthermore, the same reference numerals are used throughout the drawings for parts having similar functions and actions. Throughout the specification, when a part is said to be connected to another part, this includes not only a direct connection but also an indirect connection via another element therebetween. Furthermore, unless otherwise specified, "including a certain element" does not mean that other elements are excluded, but that other elements may also be included.

[0034] Furthermore, descriptions that limit or add specific elements are applicable to all inventions and are not limited to a particular invention unless otherwise specified.

[0035] Furthermore, throughout the description of the present invention and the claims, the singular includes the plural unless otherwise stated.

[0036] Furthermore, throughout the description of the present invention and the claims, unless otherwise specified, "or" includes "and." Therefore, "including A or B" means the three cases of including A, including B, or including both A and B.

[0037] Furthermore, throughout the description and claims of the present invention, the solid content of the coating layer refers to the remainder after removing the solvent from the coating layer, and may correspond to inorganic substances, binders, dispersants, etc. added to the solvent when preparing the coating layer slurry.

[0038] The present invention will now be described in more detail.

[0039] The separator for a secondary battery according to the present invention includes a separator substrate made of a porous polymer material, and a coating layer coated on at least one surface of the separator substrate, the coating layer including a crystalline binder and an amorphous binder that are miscible with each other, and the crystalline binder may include a first crystalline binder and a second crystalline binder.

[0040] The crystalline binder and the amorphous binder contained in the coating layer are miscible with each other. Therefore, when the crystalline binder and the amorphous binder are mixed, the crystallinity of the binder is reduced compared to when only the crystalline binder is used, and the solubility in the electrolyte is increased. Therefore, the resistance in the coating layer of the separator can be reduced, thereby enabling the manufacture of a separator with low resistance.

[0041] In addition, when the separator for the secondary battery and the electrode are laminated, the binder on the surface of the coating layer of the separator for the secondary battery dissolves in the electrolyte and becomes uniformly distributed within the electrode and separator, thereby improving the adhesive strength between the electrode and the separator and manufacturing a secondary battery with improved resistance.

[0042] The separator substrate contains pores that electrically insulate the negative and positive electrodes to prevent short circuits while allowing lithium ions to pass through. The separator substrate can be a porous membrane with high resistance to organic solvent electrolytes and small pore diameters. Any material that can be used as a separator material for a typical secondary battery can be used without any particular limitations. Examples of suitable resins include polyolefins, including polyethylene, polypropylene, and polybutene; polyvinyl chloride; polyethylene terephthalate; polycycloolefins; polyethersulfones; polyamides; polyimides; polyimideamides; polyaramids; nylons; polytetrafluoroethylenes; and mixtures or copolymers thereof. Among these, polyolefin resins have excellent applicability for coating layer slurries, and can reduce the thickness of the separator for a secondary battery, thereby increasing the ratio of electrode active material layers in the battery and increasing capacity per volume.

[0043] The thickness of the separation membrane substrate may be 1 μm to 100 μm, specifically 1 μm to 30 μm, and the diameter of the pores of the separation membrane substrate may generally be 0.01 μm to 10 μm.

[0044] The coating layer includes an inorganic substance for improving the mechanical properties and insulation of the separation membrane substrate, and a binder for maintaining the bond between the inorganic particles and improving the adhesion between the electrode and the separation membrane.

[0045] The inorganic substance is not particularly limited as long as it can uniformly form the thickness of the coating layer and does not undergo oxidation and / or reduction reactions within the operating voltage range of the applied secondary battery. In particular, when using inorganic particles having ion transfer ability, the ionic conductivity in the electrochemical device can be increased to improve the performance. Also, when using inorganic particles with a high dielectric constant as the inorganic substance, it can contribute to an increase in the dissociation degree of electrolyte salts, such as lithium salts, in the liquid electrolyte and improve the ionic conductivity of the electrolyte solution.

[0046] As an example of the inorganic substance, an inorganic substance having at least one of the characteristics of lithium ion transfer ability, piezoelectricity, and flame retardancy can be mentioned.

[0047] The inorganic substance with good lithium ion transfer ability contains lithium element, but does not store lithium and shows inorganic particles having the function of moving lithium ions. The inorganic particles having lithium ion transfer ability can transfer and move lithium ions due to a kind of defect existing inside the particle structure. Therefore, the lithium ion conductivity in the battery is improved, and thus the battery performance can be improved.

[0048] Examples of the inorganic particles having lithium ion transfer ability include lithium phosphate, lithium titanium phosphate, lithium aluminum titanium phosphate, (LiAlTiP) x O y -based glass, lithium lanthanum titanate, lithium germanium thiophosphate, lithium nitride such as Li3N (Li x N y , 0 < x < 4, 0 < y < 2), SiS2-based glass such as Li3PO4-Li2S-SiS2 (Li x Si y S z, (0 < x < 3, 0 < y < 2, 0 < z < 4), P2S5-based glass such as LiI-Li2S-P2S5 (Li x P y S z , 0 < x < 3, 0 < y < 3, 0 < z < 7) or one or more selected from the group consisting of these mixtures, but is not limited thereto.

[0049] The inorganic particles having piezoelectricity are insulators under normal pressure, but when a certain pressure is applied, it means a substance having the property of conducting electricity due to the change of the internal structure. It not only exhibits a high dielectric constant characteristic with a dielectric constant of 100 or more, but also when it is stretched or compressed by the application of a certain pressure, charges are generated, and one side is positively charged and the opposite side is negatively charged respectively, so that a potential difference is generated between both sides. It is a substance having such a function.

[0050] When using inorganic particles having the above characteristics, if an internal short circuit occurs between the positive electrode and the negative electrode due to an external impact such as Local crush or Nail, the inorganic particles coated on the separator not only prevent the positive electrode and the negative electrode from directly contacting, but also a potential difference occurs inside the particles due to the piezoelectricity of the inorganic particles. As a result, electron transfer between the positive electrode and the negative electrode, that is, a minute current flows, thereby reducing the loose battery voltage and improving the safety thereby.

[0051] Examples of the inorganic particles having piezoelectricity 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) or mixtures thereof, etc., but is not limited thereto.

[0052] The flame-retardant inorganic material can prevent overcharging of the secondary battery, impart flame-retardant properties to the separator, or prevent a sudden increase in the internal temperature of the battery. The flame-retardant inorganic material is at least one selected from the group consisting of antimony-containing compounds, metal oxides, metal hydroxides or hydrates, guanidine-based compounds, boron-containing compounds, and zinc stannate compounds.

[0053] Specifically, the antimony-containing compound is selected from antimony trioxide (Sb2O3), antimony tetroxide (Sb2O4), and antimony pentoxide (Sb2O5); the metal oxide, metal hydroxide, or metal hydrate is selected from alumina (Al2O3), magnesium hydroxide, aluminum hydroxide (Al(OH)3), aluminum oxyhydroxide (AlO(OH)), and CaO·Al2O3·6H2O; the guanidine-based compound is selected from the group consisting of guanidine nitrate, guanidine sulfamate, guanidine phosphate, and guanylurea phosphate; the boron-containing compound is H3BO3 or HBO2; and the zinc stannate compound is selected from Zn2SnO4, ZnSnO3, and ZnSn(OH)6.

[0054] The inorganic material may be included in an amount of 10 wt % to 90 wt % of the total weight of the solid content of the coating layer. If the inorganic material is included in an amount of less than 10 wt %, it is difficult to obtain the effects of adding the inorganic material, and if the inorganic material is included in an amount of more than 90 wt %, the binder content is too low, which may reduce the adhesive strength between the inorganic material, causing the inorganic material coating layer to detach from the separator substrate or leaving uncoated areas during coating, which is also undesirable.

[0055] The binder includes a crystalline binder and a non-crystalline binder, and the crystalline binder includes a first crystalline binder and a second crystalline binder, that is, the crystalline binder of the present invention is composed of two types of crystalline binders.

[0056] The crystalline binder and the amorphous binder may be non-aqueous binders.

[0057] The crystalline binder and the amorphous binder dissolve in a solvent and change their crystallinity, thereby reducing the resistance of the separator and improving cell lifespan, whereas the aqueous binder does not dissolve in water but remains dispersed, preventing changes in crystallinity and making it difficult to achieve a resistance reduction effect.

[0058] The first crystalline binder and the second crystalline binder are different from each other, and examples thereof include polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-trifluoroethylene (PVDF=TrFE), polyvinylidene fluoride-tetrafluoroethylene (PVDF-TFE), polyvinylidene fluoride-trichloroethylene (PVDF-TCE), and polyvinylidene fluoride-chlorotrifluoroethylene (PVDF-TCE). The polymer may be any one selected from the group consisting of polyvinylidene fluoride copolymers including PVDF-CTFE (PVDF-chlorotrifluoroethylene), polybutylacrylate, polyacrylonitrile (PAN), polyethylene-co-vinylacetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, and carboxyl methyl cellulose, or a mixture or copolymer of two or more of these.

[0059] The amorphous binder may be an acrylate polymer, polyvinylpyrrolidone-vinyl acetate copolymer (PVP-VAc), polyvinyl acetate (PVAc), cyanoethyl polyvinyl alcohol (PVA-CN), polybutadiene, styrene butadiene rubber (SBR), or polymethylmethacrylate (PMMA).

[0060] The content of the amorphous binder may be 2 wt% to 5 wt% based on the total weight of the solid content of the coating layer. If the content of the amorphous binder is less than 2 wt% based on the total weight of the solid content of the coating layer, the effect of reducing crystallinity by adding the amorphous binder is low, and the effect of reducing solubility in the electrolyte is low. Therefore, it is difficult to achieve a reduction in separator resistance. If the content of the amorphous binder is more than 5 wt% based on the total weight of the solid content of the coating layer, the content of the amorphous binder that dissolves in the electrolyte increases, which can increase the viscosity of the electrolyte, resulting in a problem of increased separator resistance.

[0061] The content of the amorphous binder may be more than 5 wt % and less than 50 wt % based on the total weight of the crystalline binder and the amorphous binder.

[0062] If the content of the amorphous binder is less than 5 wt% based on the total weight of the binder, the effect of reducing crystallinity by adding the amorphous binder is small, and the effect of reducing solubility in the electrolyte is small. Therefore, it is difficult to reduce the separator resistance. If the content of the amorphous binder is more than 50 wt% based on the total weight of the binder, the content of the amorphous binder that dissolves in the electrolyte increases, which can increase the viscosity of the electrolyte, resulting in a problem of increased separator resistance.

[0063] The weight-average molecular weight of the amorphous binder may be less than 1,000,000. If the weight-average molecular weight of the amorphous binder is more than 1,000,000, the weight-average molecular weight of the binder is too large, which reduces the solubility in the electrolyte and reduces the effect of reducing resistance. In addition, some of the high molecular weight binder dissolved in the electrolyte may increase the viscosity of the electrolyte, thereby increasing the resistance of the separator, which is undesirable.

[0064] The coating layer may further include a dispersant to further improve the dispersibility of the inorganic material. The dispersant serves to maintain uniform dispersion of the inorganic material within the binder during preparation of the coating layer slurry. Examples of materials that can be used as the dispersant include at least one selected from the group consisting of oil-soluble polyamines, oil-soluble amine compounds, fatty acids, fatty alcohols, sorbitan fatty acid esters, tannic acid, and pyrogallic acid.

[0065] The secondary battery according to the present invention may include an electrode assembly in which the separator for a secondary battery is interposed between a positive electrode and a negative electrode, and the electrode assembly may be impregnated with a non-aqueous electrolyte solution containing a lithium salt.

[0066] The separator for a secondary battery includes an amorphous binder with excellent electrolyte solubility. When a coating layer slurry containing a mixture of a crystalline binder and an amorphous binder is coated on a separator substrate and then treated at high temperature, the binder on the surface of the coating layer of the separator for a secondary battery dissolves in the electrolyte. The binder dissolved in the electrolyte migrates and distributes inside the positive and negative electrodes. As a result, the mixed binder on the surface of the coating layer of the separator for a secondary battery is uniformly distributed within the separator and the positive and negative electrodes, reducing resistance within the coating layer and improving adhesion between the electrodes and the separator.

[0067] The positive electrode can be manufactured, for example, by applying a positive electrode active material composed of positive electrode active material particles and a positive electrode mixture in which a conductive material and a binder are mixed onto a positive electrode current collector, and a filler can be further added to the positive electrode mixture as needed.

[0068] The positive electrode current collector is generally manufactured to have a thickness of 3 μm to 500 μm, and is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity, and may be made of, for example, stainless steel, aluminum, nickel, titanium, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver, and more particularly, aluminum. The current collector may have microscopic irregularities on its surface to increase the adhesive strength of the positive electrode active material, and may be in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0069] The positive electrode active material may include, in addition to the positive electrode active material particles, a layered compound such as lithium nickel oxide (LiNiO2) or a compound substituted with one or more transition metals; 1+x Mn 2-x Lithium manganese oxides such as LiMnO4 (where x is 0 to 0.33), LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5, and Cu2V2O7; and the chemical formula LiNi 1-x M x O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x = 0.01 to 0.3); 2-x M xThe lithium manganese composite oxides may be represented by the formula LiMnO2 (where M=Co, Ni, Fe, Cr, Zn, or Ta, and x=0.01 to 0.1) or Li2Mn3MO8 (where M=Fe, Co, Ni, Cu, or Zn); LiMn2O4, in which part of the Li in the chemical formula is replaced with an alkaline earth metal ion; disulfide compounds; Fe2(MoO4)3, etc., but are not limited to these.

[0070] The conductive material is usually added in an amount of 0.1 wt % to 30 wt % based on the total weight of the mixture including the positive electrode active material. Such a conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity, and examples thereof include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0071] The binder contained in the positive electrode is a component that helps bind the active material and conductive material and the current collector, and is typically added in an amount of 0.1 to 30 wt % based on the total weight of the mixture including the positive electrode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer, styrene-butadiene rubber, fluororubber, and various copolymers.

[0072] The negative electrode is fabricated by coating a negative electrode active material on a negative electrode current collector and drying the coated negative electrode active material. If necessary, the negative electrode may further include the components contained in the positive electrode.

[0073] The negative electrode current collector is generally made to have a thickness of 3 μm to 500 μm. Such a negative electrode current collector is not particularly limited as long as it has conductivity without causing a chemical change to the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, or those with a surface treatment of carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel, aluminum-cadmium alloy, etc. can be used. Also, similar to the positive electrode current collector, minute irregularities can be formed on the surface to strengthen the binding force of the negative electrode active material, and various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabrics, etc. can be used.

[0074] Examples of the negative electrode active material include carbon such as graphitizable carbon and graphite-based carbon; Li x Fe2O3 (0 ≦ x ≦ 1), Li x WO2 (0 ≦ x ≦ 1), Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1, Group 2, and Group 3 elements of the periodic table, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8) and other metal composite oxides; lithium metal; lithium alloys; silicon-based alloys; tin-based alloys; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, and Bi2O5; conductive polymers such as polyacetylene; Li-Co-Ni-based materials, etc. can be used.

[0075] The lithium salt-containing non-aqueous electrolyte consists of an electrolyte and a lithium salt. As the electrolyte, non-aqueous organic solvents, organic solid electrolytes, inorganic solid electrolytes, etc. are used.

[0076] Examples of the non-aqueous organic solvent that can be used include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethylene carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran (franc), 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0077] Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyagitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and polymers containing ionic dissociation groups.

[0078] Examples of the inorganic solid electrolyte that can be used include nitrides, halides, and sulfates of Li, such as LiN, LiI, LiNI, LiN-LiI-LiOH, LiSiO, LiSiO-LiI-LiOH, LiSiS, LiSiO, LiSiO-LiI-LiOH, and LiPO-LiS-SiS.

[0079] The lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte, and examples thereof include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10 Cl 10, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, lithium chloroborane, lithium lower aliphatic carboxylic acids, lithium 4-phenylborate, imides, etc. can be used.

[0080] The present invention also provides a battery module and a battery pack including the secondary battery as a unit cell, and a device including the battery module or the battery pack.

[0081] Specific examples of the device include, but are not limited to, small devices such as computers, mobile phones, and power tools; power tools that are powered by battery-powered motors; electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters; electric golf carts; and medium- to large-sized devices such as power storage systems.

[0082] The structures of the battery module, battery pack, and device are well known in the art, and therefore will not be described in detail in this specification.

[0083] The present invention will be described below with reference to examples, but these are for easier understanding of the present invention and are not intended to limit the scope of the present invention.

[0084] Example 1 To prepare the coating layer slurry for the separator for secondary batteries, 14 wt% of polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) with a hexafluoropropylene content of 15% and a weight-average molecular weight of 400,000, 4 wt% of polyvinylidene fluoride-chlorotrifluoroethylene (PVDF-CTFE) with a chlorotrifluoroethylene content of 20% and a weight-average molecular weight of 450,000, 2 wt% of an acrylate polymer, 79 wt% of alumina (Al2O3) as an inorganic substance, and 1 wt% of tannic acid with a molecular weight of 1,700 as a dispersant are mixed in acetone.

[0085] The acrylate polymer has a glass transition temperature of 26° C. and a weight average molecular weight of 200,000.

[0086] The coating layer slurry was applied to a polyolefin-based separator substrate by dip coating under a relative humidity of 40% to produce a separator. The coating layer thickness was 8 μm, and the loading amount of the coating layer slurry was 13.5 g / m. 2 is.

[0087] <Example 2> A separator was prepared in the same manner as in Example 1, except that polyvinylpyrrolidone-vinyl acetate copolymer was used instead of the acrylate polymer.

[0088] Example 3 A separator was prepared in the same manner as in Example 1, except that polyvinyl acetate was used instead of the acrylate polymer.

[0089] <Comparative Example 1> A separator was prepared in the same manner as in Example 1, except that the acrylate polymer was omitted and the mixture contained 15 wt% polyvinylidene fluoride-hexafluoropropylene and 5 wt% polyvinylidene fluoride-trichloroethylene.

[0090] <Comparative Example 2> A separator was prepared in the same manner as in Example 1, except that the mixture contained 15 wt % of polyvinylidene fluoride-hexafluoropropylene and 1 wt % of an acrylate polymer.

[0091] <Comparative Example 3> A separator was prepared in the same manner as in Example 1, except that the mixture contained 8 wt% polyvinylidene fluoride-hexafluoropropylene, 2 wt% polyvinylidene fluoride-trichloroethylene, and 10 wt% acrylate polymer.

[0092] <Comparative Example 4> A separator was prepared in the same manner as in Example 1, except that the glass transition temperature of the acrylate polymer was 30° C. and the weight average molecular weight was 1.2 million.

[0093] <Experimental Example 1> Crystallinity measurement The crystallinity of the separators prepared in Examples 1 to 3 and Comparative Examples 1 to 4 was measured using a differential scanning calorimeter (DSC) Discovery DSC250 manufactured by TA Instruments.

[0094] The peak temperature and heat quantity were analyzed from the temperature distribution measured by heating-cooling-heating at a rate of 10°C / min within the range of -60°C to 250°C. The crystallinity was calculated using the following formula. The calculated crystallinity is shown in Table 1 below, and the DSC results for Example 2 and Comparative Example 1 are shown in Figures 4 and 5, respectively.

[0095]

number

[0096] In the above equation, ΔHm is the heat of fusion of the binder.

[0097] <Experimental Example 2> Solubility in electrolyte (solvation, %) The separation membranes prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were prepared to have a width*length dimension of 5 cm*5 cm, and the initial weight (A) of the separation membrane was measured.

[0098] An electrolyte solution containing 1M LiPF6 and a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 3:7 was prepared. The separator was immersed in the electrolyte solution under the following conditions and separated by the electrolyte. -24 hours at 25℃ -24 hours at 80℃

[0099] The separation membrane was washed with ethanol, filtered, and then dried at 60° C., and the weight (B) of the dried separation membrane was measured.

[0100] The solubility of the separator binder was calculated using the following formula, and the results are shown in Table 1 below.

[0101]

number

[0102] In addition, SEM photographs of the separator of Example 1 taken before (A) immersion in the electrolyte, after (B) immersion at room temperature (25°C) for 24 hours, and after (C) treatment at 80°C for 3 hours are shown in Figures 1 to 3, respectively.

[0103] FIG. 1 is an SEM photograph of the separator according to Example 1 before it was immersed in an electrolyte solution, FIG. 2 is an SEM photograph of the separator according to Example 1 after it was immersed in an electrolyte solution at 25°C, and FIG. 3 is an SEM photograph of the separator according to Example 1 after it was immersed in an electrolyte solution at 80°C.

[0104] Referring to Figures 1 to 3, comparing Figure 1 with Figure 2, separator (B) shown in Figure 2 shows a state in which the electrolyte has entered between the binder chains, increasing the volume of the binder, and the PVDF-HFP on the surface has swelled in the electrolyte, resulting in a flattened surface.

[0105] The separator (C) heat-treated at a high temperature (80°C) shown in Figure 3 shows that the binder on the surface of the coating layer has dissolved, exposing the inorganic materials within the coating layer. Therefore, when this separator is laminated with an electrode, the binder dissolved in the electrolyte solution diffuses evenly into the separator coating layer and the electrode, reducing not only the resistance of the separator but also the resistance of the battery cell.

[0106] <Experimental Example 3> Separator resistance (Ω) To measure the resistance of the separator, the separator and the electrolyte were placed in a case and sealed to prepare a coin cell.

[0107] The electrolyte solution was composed of 1M LiPF6, ethylene carbonate (EC), and ethyl methyl carbonate (EMC) in a volume ratio of 3:7.

[0108] The resistance of the coin cell was measured using a Solartron analytical EIS under the conditions of a frequency of 100,000 to 10,000 Hz and an AC amplitude of 10 mA, and the results are shown in Table 1 below.

[0109] [Table 1]

[0110] Since the separator according to the present invention includes a non-crystalline binder as a coating layer composition, the crystallinity of the entire binder is reduced, and the solubility in the electrolyte is increased, thereby reducing the resistance in the separator coating layer.

[0111] Referring to Table 1, it can be seen that the separators prepared in the Examples have lower resistance than the separators prepared in the Comparative Examples. In particular, it can be seen that the resistance reduction effect of the separators prepared in the Examples is greater at 80°C.

[0112] Meanwhile, the binder solubility in the electrolyte solution was not significantly improved at 25°C, whereas the binder solubility of the separator prepared in the Examples was significantly improved at 80°C compared to the separator prepared in the Comparative Examples.

[0113] Specifically, Comparative Example 1, which does not contain a non-crystalline binder, and Comparative Example 2, which contains a small amount of non-crystalline binder, show a significant decrease in binder solubility at 80°C. This indicates that it is preferable to contain a certain amount of non-crystalline binder.

[0114] On the other hand, Comparative Example 3, which contained a large amount of amorphous binder, exhibited higher binder solubility at 80° C. than the Examples, but exhibited higher separator resistance at both 25° C. and 80° C., which may lead to a decrease in ion conductivity due to an increase in separator resistance, resulting in a shortened battery cell lifespan. Therefore, it is difficult to evaluate the suitability of the material for use as a separator for secondary batteries solely based on the high content of amorphous binder.

[0115] In the case of Comparative Example 4, in which the weight-average molecular weight of the amorphous binder was 1.2 million, the binder solubility significantly decreased at 80°C and the separator resistance was also measured to be slightly higher, indicating that the weight-average molecular weight of the amorphous binder also affects the binder solubility at high temperatures.

[0116] Therefore, because the separator according to the present invention contains an amorphous binder within a specific content range, it not only has superior separator resistance compared to conventional separators that do not contain an amorphous binder, but also has the effect of significantly improving binder solubility at high temperatures (80°C), thereby increasing ionic conductivity and improving the lifespan of battery cells. Thus, when the separator according to the embodiment is heat-treated at 80°C for 3 hours, the resistance of the separator is lower than 1.00Ω, providing a separator with significantly reduced resistance.

[0117] FIG. 4 is a graph showing the DSC results of the separation membrane according to Example 2, and FIG. 5 is a graph showing the DSC results of the separation membrane according to Comparative Example 1.

[0118] Referring to Figures 4 and 5, in the separator of Comparative Example 1, which contained only a crystalline binder, the crystalline peaks of the first crystalline binder and the second crystalline binder were observed, whereas in Example 2, in which a crystalline binder and a non-crystalline binder were mixed, the crystalline peaks of the first crystalline binder and the second crystalline binder were observed to be smaller, and the overall crystallinity of the binder was reduced, resulting in a lower crystalline peak temperature than in Comparative Example 1.

[0119] As described above, since the separator coating layer according to the present invention includes a crystalline binder and an amorphous binder, the resistance of the separator is reduced, and a secondary battery with improved cell life performance can be provided.

[0120] Those skilled in the art will be able to make various applications and modifications within the scope of the present invention based on the above content.

Claims

1. a separation membrane substrate having a porous structure and made of a polymer material; a coating layer coated on at least one surface of the separation membrane substrate; Including, the coating layer comprises a crystalline binder and an amorphous binder that are miscible with each other; the crystalline binder includes a first crystalline binder and a second crystalline binder; The content of the amorphous binder is more than 1 wt % to 5 wt % based on the total weight of the solid content of the coating layer; The weight average molecular weight of the amorphous binder is less than 1 million, the amorphous binder is at least one selected from the group consisting of an acrylate polymer or a copolymer thereof, a polyvinylpyrrolidone-polyvinyl acetate copolymer, and polyvinyl acetate; The acrylate polymer or copolymer thereof does not contain a styrene monomer unit, The separator for a secondary battery, wherein the total crystallinity of the crystalline binder and the amorphous binder is 10.5% to 12.6%.

2. The separator for a secondary battery according to claim 1 , wherein the crystalline binder and the amorphous binder are non-aqueous binders.

3. The separator for a secondary battery according to claim 1 , wherein the crystalline binder is a polyvinylidene fluoride (PVDF) copolymer.

4. 2. The separator for a secondary battery of claim 1, wherein the content of the amorphous binder is more than 5 wt % and less than 50 wt % based on the total weight of the crystalline binder and the amorphous binder.

5. 10. The separator for a secondary battery according to claim 1, wherein the separator has a resistance of less than 1.00 ohms when heat-treated at 80[deg.] C. for 3 hours.

6. 2. The separator for a secondary battery according to claim 1, wherein the inorganic material in the coating layer is exposed to the surface when the separator for a secondary battery is immersed in an electrolyte and heat-treated.

7. A secondary battery comprising an electrode assembly in which the separator for a secondary battery according to claim 1 is interposed between a positive electrode and a negative electrode and stacked.

8. The secondary battery of claim 7 , wherein the binder on the surface of the coating layer of the separator for the secondary battery dissolves in the electrolyte and moves and distributes inside the positive and negative electrodes.

9. A battery module comprising the secondary battery according to claim 7 .

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