Separation Membrane for Lithium Secondary Battery and Method for Producing the Same

Abstract

A separator for a lithium secondary battery is provided, comprising a porous polymer substrate and a hydrophilic modified layer positioned within the porous polymer substrate or within and on at least one surface of the porous polymer substrate, the hydrophilic modified layer including a polymer compound represented by Chemical Formula 1 and a surfactant. According to one embodiment of the present invention, the use of a polymer compound with a unique structure and a surfactant can increase the migration rate of lithium ions within the separator. As a result, batteries using such a separator can have improved output and cycle characteristics.

Description

Technical Field

[0001] The present invention relates to a separator that can be used in electrochemical elements such as lithium secondary batteries, and a method for manufacturing the same.

[0002] This application claims priority based on Korean Patent Application No. 10-2020-0125066 filed on September 25, 2020, and all of the contents disclosed in the specification and drawings of the application are incorporated into this application.

Background Art

[0003] In recent years, interest in energy storage technology has been increasing. The application fields are expanding to include mobile phones, camcorders, notebook computers, and even the energy of electric vehicles, and efforts in the research and development of electrochemical elements are becoming more and more concrete. Electrochemical elements are the most spotlighted fields in this regard, and in particular, attention has been focused on the development of rechargeable secondary batteries. In recent years, in the development of such batteries, research and development related to new electrode and battery designs have been carried out to improve the capacity density and specific energy.

[0004] Among the currently applied secondary batteries, the lithium secondary battery developed in the early 1990s has attracted attention due to its advantages of high operating voltage and significantly high energy density compared to conventional batteries such as Ni-MH, Ni-Cd, and lead-sulfuric acid batteries that use aqueous electrolytes.

[0005] Although electrochemical elements such as lithium secondary batteries are produced by many manufacturers, their safety characteristics show different aspects. It is very important to evaluate and ensure the safety of such electrochemical elements. For example, the separator is an important factor that affects the safety and output characteristics of the battery because it prevents short circuits between the positive electrode and the negative electrode and at the same time provides a migration path for lithium ions.

[0006] Since the separator for lithium secondary batteries is made of stretched polyolefin as the main raw material, it has hydrophobicity. Therefore, the thicker the porous polymer substrate, the less easy it is for the electrolyte to be impregnated.

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0007] The problem to be solved by the present invention is to provide a separator in which the properties of the porous polymer substrate are modified from hydrophobic to hydrophilic.

[0008] Also, thereby, it is intended to improve the performance of a battery using such a separator.

MEANS FOR SOLVING THE PROBLEM

[0009] One aspect of the present invention provides a separator for a lithium secondary battery according to the following embodiments.

[0010] According to the first embodiment, a porous polymer substrate, and a hydrophilic modification layer located inside the porous polymer substrate or inside and on at least one surface of the porous polymer substrate, the hydrophilic modification layer containing a polymer compound represented by the following Chemical Formula 1 and a surfactant, wherein the separator for a lithium secondary battery is provided.

[0011]

CHEM.

[0012] In Chemical Formula 1, n is any integer from 6 to 8.

[0013] According to the second embodiment, in the first embodiment, the polymer compound may include α-cyclodextrin, β-cyclodextrin, γ-cyclodextrin, or two or more thereof.

[0014] According to the third embodiment, in the first or second embodiment, The content of the polymer compound may be 1 part by weight or less based on 100 parts by weight of the porous polymer substrate.

[0015] According to a fourth embodiment, in any one of the first to third embodiments, The surfactant may be a fluorine-based surfactant, a silicone-based surfactant, a nonionic surfactant, a cationic surfactant, an anionic surfactant, or a mixture of two or more thereof.

[0016] According to a fifth embodiment, in any one of the first to fourth embodiments, The surfactant has an electrolyte permeability of 30% or more even after high-temperature storage, and the electrolyte permeability is 1M LiFP in ethylene carbonate and ethyl methyl carbonate (3:7 volume ratio). 6 The steps of producing a composition by adding 0.5 part by weight of the surfactant to 100 parts by weight of an electrolyte containing the composition, and storing the produced composition in a sealed oven at 70 ° C for 6 hours, and the resulting product is measured using an ultraviolet-visible spectrophotometer (UV-Vis spectrophotometer) under normal temperature (25 ° C) conditions.

[0017] According to a sixth embodiment, in any one of the first to fifth embodiments, The content of the surfactant may be 0.5 part by weight or less based on 100 parts by weight of the porous polymer substrate.

[0018] According to a seventh embodiment, in any one of the first to sixth embodiments, The separation membrane may include a porous coating layer including inorganic particles on at least one surface thereof and a binder polymer for fixing and connecting the inorganic particles.

[0019] According to an eighth embodiment, in the seventh embodiment, The porous coating layer may further include a polymer compound represented by the following Chemical Formula 1.

[0020] [Chem.]

[0021] In Chemical Formula 1, n is any one integer from 6 to 8.

[0022] According to the ninth embodiment, in the eighth embodiment, The content of the polymer compound represented by the Chemical Formula 1 may be 10 parts by weight or less based on 100 parts by weight of the inorganic particles.

[0023] According to the tenth embodiment, in any one of the seventh to ninth embodiments, The binder polymer may be particulate or non-particulate.

[0024] According to the eleventh embodiment, in any one of the seventh to tenth embodiments, The inorganic particles may be inorganic particles having a dielectric constant of 5 or more, inorganic particles having lithium ion transport ability, or a mixture of two or more of these.

[0025] According to the twelfth embodiment, in any one of the seventh to eleventh embodiments, The inorganic particles may be hydrophilic.

[0026] According to the thirteenth embodiment, in any one of the seventh to twelfth embodiments, A silane group may be grafted onto the surface of the inorganic particles.

[0027] Another aspect of the present invention provides a lithium secondary battery according to the following embodiments.

[0028] According to the fourteenth embodiment, comprising a positive electrode, a negative electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode, There is provided a lithium secondary battery, wherein the separation membrane is a separation membrane according to any one of the first to thirteenth embodiments.

[0029] Still another aspect of the present invention provides a method for manufacturing a separation membrane for a lithium secondary battery according to the following embodiments.

[0030] According to the fifteenth embodiment, (S1) preparing a composition for modifying a substrate in which a polymer compound represented by the following Chemical Formula 1 and a surfactant are dispersed or dissolved in an aqueous solvent; (S2) applying and drying the composition for modifying the substrate on at least one surface of a porous polymer substrate having a large number of pores and being hydrophobic, a method for manufacturing a separation membrane for a lithium secondary battery is provided, the method including the steps.

[0031]

Chemical formula

[0032] In Chemical Formula 1, n is any integer from 6 to 8.

[0033] According to the sixteenth embodiment, in the fifteenth embodiment, the step (S2) may be a step of modifying the porous polymer substrate to be hydrophilic by allowing the surfactant and the polymer compound to penetrate into the porous polymer substrate.

[0034] According to the seventeenth embodiment, in the fifteenth or sixteenth embodiment, after the step (S2), it may further include a step (S3) of forming a porous coating layer by drying and applying a composition for forming a porous coating layer including a solvent, inorganic particles, and a binder polymer on at least one surface of the porous polymer substrate on which the composition for modifying the substrate has been applied and dried.

[0035] According to the eighteenth embodiment, in the seventeenth embodiment, The solvent may be an aqueous solvent or an organic solvent.

[0036] According to the 19th embodiment, in the 17th embodiment or the 18th embodiment, The composition for forming the porous coating layer may further include the polymer compound represented by Chemical Formula 1.

[0037] According to the 20th embodiment, in any one of the 17th to 19th embodiments, The step (S3) may be performed at a relative humidity of 30 to 80%.

Advantages of the Invention

[0038] According to one embodiment of the present invention, by using a polymer compound having a unique structure and a surfactant, the impregnation property of the electrolyte and the migration rate of lithium ions in the separation membrane can be increased.

[0039] Thereby, a battery using such a separation membrane can improve output characteristics and cycle characteristics.

[0040] In addition, a method for manufacturing the separation membrane and the battery can be provided.

[0041] The drawings attached to this specification illustrate desirable embodiments of the present invention and serve to further understand the technical idea of the present invention together with the content of the invention. Therefore, the present invention should not be construed as being limited only to the matters described in the drawings. On the other hand, the shape, size, scale, or ratio of elements in the drawings attached to this specification may be exaggerated in order to emphasize a clearer explanation.

Brief Description of the Drawings

[0042]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Best Mode for Carrying Out the Invention

[0043] Hereinafter, embodiments of the present invention will be described in detail. Prior to this, the terms and words used in this specification and claims should not be construed as being limited to their ordinary and dictionary meanings. The inventor himself must interpret them in accordance with the meaning and concept corresponding to the technical idea of the present invention in accordance with the principle that he can appropriately define the concept of the terms in order to explain the invention in the best way. Therefore, it must be understood that the embodiments described in this specification and the configurations shown in the drawings are merely the most desirable embodiment of the present invention and do not represent all of the technical ideas of the present invention. Therefore, there may be various equivalents and modifications that can replace them at the time of this application.

[0044] Throughout this specification, when a certain part "includes" or "comprises" other components, unless otherwise specified, it does not mean excluding other components, but rather may further include other components.

[0045] Also, terms such as "about" and "substantially" used throughout this specification are used to mean the numerical value or a value close to it when manufacturing and material tolerances inherent in the mentioned meaning are presented. They are used to prevent unscrupulous infringers from improperly using the disclosed content where exact or absolute numerical values are mentioned to assist in the understanding of this application.

[0046] One aspect of the present invention relates to a separator for a lithium secondary battery and a method for manufacturing the same.

[0047] The separator used in an electrochemical device is an important factor affecting the safety and output characteristics of the battery because it prevents short circuits between the positive electrode and the negative electrode and at the same time provides a migration path for lithium ions.

[0048] The separator for a lithium secondary battery is hydrophobic because its main raw material is a polymer such as stretched polyolefin. Therefore, the thicker the porous polymer substrate, the less easy it is to impregnate the electrolyte.

[0049] In order to facilitate the impregnation of the electrolyte, inorganic particles were introduced onto the porous polymer substrate, but in this case as well, the impregnation of the electrolyte was not significantly improved. Even when the surface of the porous polymer substrate was modified by corona surface treatment, there was a problem that the surface modification was not continuously maintained. There was no successful example in other methods of introducing a hydrophilic organic additive onto the porous polymer substrate. Finally, an attempt was made to modify the interfacial properties of the porous polymer substrate from hydrophobic to hydrophilic using a dispersant, but the effect was not sufficient.

[0050] One aspect of the present invention is to solve the above problems. That is, it is intended to epochally improve the performance (output and capacity retention rate) of the battery by enabling the electrolyte to smoothly penetrate into the pores in the separator.

[0051] The separator for a lithium secondary battery according to one aspect of the present invention comprises a porous polymer substrate and a hydrophilic modification layer located inside the porous polymer substrate or inside and on at least one surface of the porous polymer substrate, the hydrophilic modification layer containing a polymer compound represented by the following Chemical Formula 1 and a surfactant.

[0052]

Chemical formula

[0053] In Chemical Formula 1, n is any integer from 6 to 8.

[0054] As described above, the separation membrane according to one aspect of the present invention necessarily includes a porous polymer substrate containing a large number of fibrils, the polymer compound represented by Chemical Formula 1, and a surfactant. At this time, the polymer compound has a unique structure, and when it is included in the separation membrane together with the surfactant, it facilitates the penetration of the electrolyte into the interior of the separation membrane. This is because the surfactant allows the hydrophilic polymer compound to approach the surface of the fibrils in the hydrophobic porous polymer substrate. As a result, the electrolyte can be captured inside the hollow portion of the polymer compound represented by Chemical Formula 1. Consequently, the hydrophobic porous polymer substrate can be modified to be hydrophilic.

[0055] The separation membrane according to one aspect of the present invention contains a polymer compound represented by the following Chemical Formula 1 in a porous polymer substrate.

[0056]

Chemical Formula

[0057] In Chemical Formula 1, n is any integer from 6 to 8.

[0058] The polymer compound can be cyclodextrin in which 6 to 8 glucose units are arranged in a ring. At this time, when the number of glucose units is 6, it is α-cyclodextrin, when it is 7, it is β-cyclodextrin, and when it is 8, it is γ-cyclodextrin. Therefore, in the present invention, the polymer compound can be α-cyclodextrin, β-cyclodextrin, γ-cyclodextrin, or two or more of these.

[0059] In a specific embodiment of the present invention, the content of the polymer compound may be 1 part by weight or less, 0.8 part by weight or less, or 0.5 part by weight or less, and 0.01 part by weight or more, or 0.3 part by weight or more, based on 100 parts by weight of the porous polymer substrate. Within the above numerical range, the amount of the polymer compound is appropriate, and the hydrophilicity of the porous polymer substrate can be enhanced to achieve the effect of the present invention.

[0060] The separation membrane according to one aspect of the present invention contains a surfactant in the porous polymer substrate.

[0061] As described above, the surfactant is an auxiliary agent that helps the polymer compound approach the fibrils in the porous polymer substrate.

[0062] In a specific embodiment of the present invention, the surfactant may be a fluorosurfactant, a silicone surfactant, a nonionic surfactant, a cationic surfactant, an anionic surfactant, or a mixture of two or more thereof.

[0063] Among them, it is particularly desirable to use a fluorosurfactant. The fluorosurfactant has the advantages of better coating properties on the porous polymer substrate compared to nonionic surfactants, a higher oxidation potential, and a lower risk of side reactions in the battery.

[0064] In a specific embodiment of the present invention, the fluorosurfactant may contain 5 to 25 fluorines per molecule and may be one or more selected from the compounds represented by the following structural formulas.

[0065]

Chemical formula

[0066] In the above structural formula, n and m are each an integer from 1 to 10.

[0067] Examples of the fluorosurfactant may include 2-[methyl[(nonafluorobutyl)sulfonyl]amino]ethyl acrylate (FC4430, manufactured by 3M).

[0068] In a specific embodiment of the present invention, the surfactant may have an electrolyte permeability of 30% or more, or 40 - 90% even after high-temperature storage. At this time, the electrolyte permeability is measured by adding 0.5 part by weight of the surfactant to 100 parts by weight of an electrolyte containing ethylene carbonate and ethyl methyl carbonate (volume ratio 3:7) and 1M LiFP 6 The result obtained through the steps of manufacturing a composition by adding 0.5 part by weight of the surfactant to 100 parts by weight of the electrolyte and then storing the manufactured composition sealed in an oven at 70°C for 6 hours can be measured using an ultraviolet-visible spectroscopic analyzer under normal temperature (25°C) conditions.

[0069] When the electrolyte permeability of the surfactant satisfies such a range, it is advantageous in terms of controlling side reactions in the battery.

[0070] In a specific embodiment of the present invention, the content of the surfactant may be 1.0 part by weight or less, 0.7 part by weight or less, or 0.5 part by weight or less, and 0.05 part by weight or more, 0.1 part by weight or more, or 0.5 part by weight or more based on 100 parts by weight of the polymer compound. Within the above numerical range, the amount of the polymer compound is appropriate, which can enhance the hydrophilicity of the porous polymer substrate and achieve the effects of the present invention. Also, side effects on battery performance can be minimized within the above range.

[0071] On the other hand, the separation membrane according to one aspect of the present invention may further include a porous coating layer provided with inorganic particles and a binder polymer on at least one surface of the separation membrane.

[0072] At this time, the inorganic particles are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles that can be used in the present invention are within the operating voltage range of the applied electrochemical device (for example, Li / Li +There is no particular limitation as long as no oxidation and / or reduction reaction occurs at a reference of 0 to 5 V. In particular, when using inorganic particles with a high dielectric constant as the inorganic particles, it is possible to contribute to an increase in the dissociation degree of an electrolyte salt, such as a lithium salt, in the liquid electrolyte and improve the ionic conductivity of the electrolyte solution.

[0073] For the reasons described above, the inorganic particles can be inorganic particles having a dielectric constant of 5 or more, inorganic particles having lithium ion transfer ability, or a mixture of two or more of these.

[0074] The inorganic particles having a dielectric constant of 5 or more are Al 2 O 3 , SiO 2 , ZrO 2 , AlO(OH), Al(OH) 3 , TiO 2 , BaTiO 3 , Pb(Zr x Ti 1-x )O 3 (PZT, where 0 < x < 1), Pb 1-x La x Zr 1-y Ti y O 3 (PLZT, where 0 < x < 1, 0 < y < 1), (1 - x)Pb(Mg 1 / 3 Nb 2 / 3 )O 3 -xPbTiO 3 (PMN - PT, where 0 < x < 1), hafnia (HfO 2 ), SrTiO 3 , SnO 2 , CeO 2 , MgO, NiO, CaO, ZnO, and SiC, and can be a mixture of one or more selected from the group consisting of them.

[0075] The inorganic particles having lithium ion transfer ability are lithium phosphate (Li 3 PO 4 ), lithium titanium phosphate (Li x Ti y (PO 4 ) 3, (0 < x < 2, 0 < y < 3), lithium aluminum titanium phosphate (Li x Al y Ti z (PO 4 ) 3 , (0 < x < 2, 0 < y < 1, 0 < z < 3), (LiAlTiP) x O y -based glass (0 < x < 4, 0 < y < 13), lithium lanthanum titanate (Li x La y TiO 3 , (0 < x < 2, 0 < y < 3), lithium germanium thiophosphate (Li x Ge y P z S w , (0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), lithium nitride (Li x N y , (0 < x < 4, 0 < y < 2), SiS 2 -based glass (Li x Si y S z , (0 < x < 3, 0 < y < 2, 0 < z < 4) and P 2 S 5 -based glass (Li x P y S z , (0 < x < 3, 0 < y < 3, 0 < z < 7) and may be one or more mixtures selected from the group consisting of.

[0076] The average particle size of the inorganic particles is not particularly limited, but for the formation of a porous coating layer with a uniform thickness and an appropriate porosity, it is preferably in the range of 0.001 to 10 μm, more preferably 100 nm to 2 μm, and even more preferably 150 nm to 1 μm.

[0077] In a specific embodiment of the present invention, the inorganic particles can be hydrophilic. By using hydrophilic inorganic particles, the electrolyte impregnation property can be further enhanced. For example, the inorganic particles can be Al(OH) 3 or Mg(OH) 2 .

[0078] In a specific embodiment of the present invention, the porous coating layer may include a binder polymer having a glass transition temperature (T g ) of -200 to 200 °C, because this can improve the mechanical properties such as flexibility and elasticity of the finally formed porous separation membrane. Such a binder polymer serves as a binder that connects and stably fixes between inorganic particles, contributing to preventing a decrease in the mechanical properties of the porous separation membrane.

[0079] In addition, the binder polymer does not necessarily have ionic conductivity, but when a polymer having ionic conductivity is used, the performance of the electrochemical device can be further improved. Therefore, the binder polymer can preferably be one having as high a dielectric constant as possible. In fact, since the degree of dissociation of salts in the electrolyte depends on the dielectric constant of the electrolyte solvent, the higher the dielectric constant of the binder polymer, the more the degree of salt dissociation in the electrolyte can be improved. The dielectric constant of such a binder polymer can be in the range of 1.0 to 100 (measurement frequency = 1 kHz), and can particularly be 10 or more.

[0080] In addition to the functions described above, the binder polymer may have a characteristic of showing a high degree of swelling of the electrolyte by gelling when impregnated with the liquid electrolyte. Therefore, the solubility index of the binder polymer, that is, the Hildebrand solubility parameter, is 15 to 45 MPa 1 / 2 , 15 to 25 MPa 1 / 2 or 30 to 45 MPa 1 / 2 in the range. Therefore, a hydrophilic polymer having more polar groups can be more preferably used than a hydrophobic polymer such as polyolefins. This is because when the solubility index is less than 15 MPa 1 / 2 or exceeds 45 MPa 1 / 2 , it may be difficult to be swollen by a normal liquid electrolyte for a battery.

[0081] In the porous separation membrane, inorganic particles are filled and in contact with each other, and are bound to each other by the binder polymer, whereby an interstitial volume is formed between the inorganic particles. The interstitial volume between the inorganic particles becomes free space and forms pores.

[0082] That is, the binder polymer adheres the inorganic particles to each other so as to maintain the state where the inorganic particles are bound to each other. For example, the binder polymer connects and fixes between the inorganic particles. Further, the pores of the porous separation membrane are pores formed when the interstitial volume between the inorganic particles becomes free space, and this is a space limited by the inorganic particles that are substantially in contact with each other in a closely packed or densely packed structure by the inorganic particles.

[0083] As such, the further included binder polymer can be applied without limitation as long as it is commonly used in the art. Examples include polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinyl pyrrolidone, polyvinyl acetate, ethylene vinyl acetate copolymer, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, and carboxymethyl cellulose, but are not limited thereto.

[0084] At this time, the binder polymer can be particulate or non-particulate. When the binder polymer is particulate, it is a case where the binder polymer is not dissolved in the solvent. When the binder polymer is non-particulate, it is a case where the binder polymer is dissolved in the solvent. At this time, the solvent can be an aqueous solvent or an organic solvent.

[0085] The separation membrane according to one aspect of the present invention may further contain other additives as components of the porous coating layer in addition to the inorganic particles and the binder polymer described above.

[0086] On the other hand, in a specific embodiment of the present invention, the porous coating layer may contain the polymer compound represented by the above Chemical Formula 1.

[0087] Regarding the above polymer compound, the above-described content can be incorporated by reference.

[0088] When the polymer compound is further contained in the porous coating layer in this way, the impregnation property with the electrolyte can be further improved.

[0089] Another aspect of the present invention provides a method for manufacturing a separation membrane for a lithium secondary battery.

[0090] Specifically, it includes: (S1) a step of preparing a composition for modifying a substrate in which the polymer compound represented by the above Chemical Formula 1 and a surfactant are dispersed or dissolved in an aqueous solvent; and (S2) a step of applying and drying the composition for modifying a substrate on at least one surface of a porous polymer substrate having a large number of pores and being hydrophobic. This is the feature of the invention.

[0091] Hereinafter, it will be specifically described.

[0092] In step (S1), a composition for modifying a substrate in which the polymer compound represented by the above Chemical Formula 1 and a surfactant are dispersed or dissolved in an aqueous solvent is prepared.

[0093] At this time, the aqueous solvent may be water and may further contain a predetermined amount of alcohol. At this time, the polymer compound and the surfactant may be dispersed in the aqueous solvent. Regarding the polymer compound and the surfactant, the above-described content can be incorporated by reference.

[0094] At this time, as a method for dispersing the polymer compound and the surfactant in an aqueous solvent, an ordinary method well-known in the art is used. For example, an ultrasonic disperser, a ball mill, a bead mill, a disperser, a mixer, etc. can be used, and a ball mill or a bead mill is particularly desirable. At this time, the treatment time can vary depending on the volume, but 1 to 20 hours is appropriate, and the particle sizes of the crushed polymer compound and the surfactant can be controlled by the size of the beads used in the ball mill or bead mill and the ball mill (or bead mill) time.

[0095] Next, the composition for modifying the substrate is applied and dried on at least one surface of the porous polymer substrate having a large number of pores and being hydrophobic (S2).

[0096] In the present invention, the porous polymer substrate is a porous membrane, which can electrically insulate the positive electrode and the negative electrode to prevent short circuit and provide a migration path for lithium ions, and can be used without particular limitation as long as it can be usually used as a separator material for electrochemical elements.

[0097] Specifically, the porous polymer substrate can be a porous polymer film substrate or a porous polymer nonwoven fabric substrate.

[0098] The porous polymer film substrate can be a porous polymer film made of polyolefin such as polyethylene and polypropylene. Such a polyolefin porous polymer film substrate exhibits a shut-down function at a temperature of, for example, 80 to 150°C.

[0099] At this time, the polyolefin porous polymer film can be formed of polyethylene such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, ultra-high molecular weight polyethylene, polypropylene, polybutylene, polypentene, etc. alone or by mixing two or more of these.

[0100] In addition, the porous polymer film substrate may be manufactured by forming it into a film using various polymers such as polyester in addition to polyolefin. Further, the porous polymer film substrate may be formed in a structure in which two or more film layers are laminated, and each film layer may be formed from a polymer such as the above-described polyolefin, polyester, etc. alone or a polymer in which two or more of these are mixed.

[0101] In addition to the polyolefin-based materials as described above, the porous polymer film substrate and the porous nonwoven fabric substrate may be formed from polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalate, etc., either alone or as a mixture of these.

[0102] The thickness of the porous polymer substrate is not particularly limited, but specifically it is 1 to 100 μm, more specifically 5 to 50 μm. Since the high output / high capacity of batteries has been progressing recently, it is advantageous to use a thin film for the porous polymer substrate. The diameter of the pores present in the porous polymer substrate is 10 nm to 100 nm, 10 nm to 70 nm, 10 nm to 50 nm, or 10 nm to 35 nm, and the porosity can be 5% to 90%, preferably 20% to 80%. However, in the present invention, such numerical ranges can be easily deformed according to specific embodiments or as required.

[0103] The pores of the porous polymer substrate have various types of pore structures, and as long as any one of the average sizes of the pores measured using a porosimeter or observed on FE-SEM satisfies the above-described conditions, it is included in the present invention.

[0104] Here, in the case of a generally known uniaxially stretched dry separation membrane, the central pore size in the vertical direction (TD, transverse direction) rather than the pore size in the machine direction (MD, longitudinal direction) is used as a reference on FE-SEM. For other porous polymer substrates having a network structure (e.g., wet PE separation membranes), the pore size measured by a porosimeter can be used as a reference.

[0105] The method of applying the composition for substrate modification to the porous polymer substrate is not particularly limited, but it is desirable to use a slot coating or dip coating method. Slot coating is a method in which the composition supplied through a slot die is applied to the entire surface of the substrate, and the thickness of the coating layer can be adjusted by the flow rate supplied from a metering pump. Dip coating is a method in which the substrate is immersed in a tank filled with the composition for coating, and the thickness of the coating layer can be adjusted by the concentration of the composition and the speed at which the substrate is pulled up from the composition tank. In order to control the coating thickness more accurately, after immersion, post-metering may be performed using a Mayer bar or the like.

[0106] By drying the porous polymer substrate coated with the composition for substrate modification using a dryer such as an oven, the fibrils in the porous polymer substrate can be modified from hydrophobic to hydrophilic. This is because the surfactant and the polymer compound have penetrated into the porous polymer substrate.

[0107] After the step (S2), a step (S3) of forming a porous coating layer by drying and applying a composition for forming a porous coating layer containing the solvent, inorganic particles, and binder polymer to at least one surface of the porous polymer substrate coated and dried with the composition for substrate modification may be further included.

[0108] At this time, the solvent used can be an aqueous solvent or an organic solvent.

[0109] At this time, it is desirable that the solvent has a solubility index similar to that of the binder polymer to be used and a low boiling point. This is to facilitate uniform mixing and subsequent solvent removal. Non-limiting examples of solvents that can be used include one compound selected from water, acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone, and cyclohexane, or a mixture of two or more thereof.

[0110] As a method for dispersing inorganic particles, ordinary methods well-known in the art are used. For example, an ultrasonic disperser, a ball mill, a bead mill, a disperser, a mixer, etc. can be used, and a ball mill or a bead mill is particularly desirable. At this time, the treatment time can vary depending on the volume, but 1 to 20 hours is appropriate, and the particle size of the crushed inorganic particles can be controlled by the size of the beads used in the ball mill or bead mill and the ball mill (or bead mill) time.

[0111] The method for applying the composition for forming the porous coating layer to the porous polymer substrate is not particularly limited, but it is desirable to use a slot coating or a dip coating method. Slot coating is a method in which the composition supplied through a slot die is applied to the entire surface of the substrate, and the thickness of the coating layer can be adjusted by the flow rate supplied from a metering pump. Dip coating is a method in which the substrate is immersed in a tank filled with the composition for coating, and the thickness of the coating layer can be adjusted by the concentration of the composition and the speed at which the substrate is pulled up from the composition tank. In order to control the coating thickness more accurately, after immersion, a Meyer bar or the like may be used for post-metering.

[0112] By drying the porous polymer substrate coated with the composition for forming the porous coating layer using a dryer such as an oven, a porous coating layer is formed on the porous polymer substrate.

[0113] In the porous coating layer, the inorganic particles and the binder polymer are filled and in contact with each other, and are bound to each other by the binder polymer. As a result, an interstitial volume is formed between the inorganic particles, and the interstitial volume between the inorganic particles can become a free space to form pores.

[0114] That is, the binder polymer can adhere to these inorganic particles to connect and fix between the inorganic particles so as to maintain the state in which the inorganic particles are bound to each other. Further, the pores of the porous coating layer are pores formed as free spaces of the interstitial volume between the inorganic particles, and this can be a space limited by the inorganic particles substantially in contact with each other in a filling structure (closely packed or densely packed) by the inorganic particles.

[0115] The drying can be performed in a drying chamber, and at this time, the conditions of the drying chamber are not particularly limited for the application of the non-solvent.

[0116] However, in the case of the present invention, since drying is performed under humid conditions, the binder polymer can be mainly distributed on the surface of the porous coating layer. The drying step can be performed at a relative humidity of 30% or more, 35% or more, or 40% or more, and 80% or less, 75% or less, or 70% or less. For example, it can be performed in the range of 40% to 80%. Further, the drying step can be performed for 0.1 minute to 2 minutes in a temperature range of 20 to 70°C.

[0117] The thickness of the porous coating layer is not particularly limited, but specifically, it is 1 to 10 μm, more specifically, 1.5 to 8 μm. The porosity of the porous coating layer is also not particularly limited, but it is preferably 35 to 65%.

[0118] On the other hand, the composition for forming the porous coating layer may contain a polymer compound containing a repeating unit represented by the chemical formula 1.

[0119] Regarding the above-mentioned polymer compound, the above-described content can be incorporated by reference.

[0120] When the polymer compound is further included in the porous coating layer in this way, the impregnation property with the electrolyte can be further improved.

[0121] The electrochemical element according to one aspect of the present invention includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and the separator is the separator according to one embodiment of the present invention described above.

[0122] Such an electrochemical element includes any element that performs an electrochemical reaction, and specifically includes all types of primary batteries, secondary batteries, fuel cells, solar cells, or capacitors such as supercapacitor elements. In particular, it can be a lithium secondary battery including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery among the secondary batteries.

[0123] Both electrodes of the positive electrode and the negative electrode applied together with the separator of the present invention are not particularly limited, and can be manufactured in a form in which an electrode active material is bound to an electrode current collector by a conventional method well known in the art. As non-limiting examples of the electrode active material, ordinary positive electrode active materials used for the positive electrode of conventional electrochemical elements can be used, and in particular, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a lithium composite oxide combining these is preferably used. As non-limiting examples of the negative electrode active material, ordinary negative electrode active materials used for the negative electrode of conventional electrochemical elements can be used, and in particular, lithium metal or a lithium alloy, carbon, petroleum coke, activated carbon, graphite, or other carbonaceous materials such as lithium adsorbing substances are desirable. Non-limiting examples of the positive electrode current collector include foils made of aluminum, nickel, or combinations thereof, and non-limiting examples of the negative electrode current collector include foils made of copper, gold, nickel, or a copper alloy, or combinations thereof.

[0124] The electrolytic solution used in the electrochemical device of the present invention is A + B - a salt having a structure such as, A + is Li + , Na + , K + ions composed of alkali metal cations such as these or combinations thereof, and B - is PF 6 - , BF 4 - , Cl - , Br - , I - , ClO 4 - , AsF 6 - , CH 3 CO 2 - , CF 3 SO 3 - , N(CF 3 SO 2 ) 2 - , C(CF 2 SO 2 ) 3 - a salt containing anions composed of these or combinations thereof, dissolved or dissociated in an organic solvent composed of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), γ-butyrolactone or a mixture thereof, but is not limited thereto.

[0125] The injection of the electrolytic solution may be performed at an appropriate stage in the battery manufacturing process according to the manufacturing process of the final product and the required physical properties. That is, it may be injected before battery assembly or at the final stage of battery assembly.

[0126] Hereinafter, the present invention will be described in detail with specific examples. However, the examples according to the present invention can be modified into many other forms, and the scope of the present invention should not be construed as being limited to the examples described below. The examples of the present invention are provided to more fully explain the present invention to those with average knowledge in the industry.

[0127] <Example 1> β-Cyclodextrin (manufactured by TCI) and 2-[methyl[(nonafluorobutyl)sulfonyl]amino]ethyl acrylate (FC4430, manufactured by 3M) as a surfactant were added to water as a solvent at a weight ratio of 99.5:0.5 to prepare a composition for substrate modification. At this time, the surfactant had an electrolyte permeability of 48% even after high-temperature storage. At this time, the electrolyte permeability was measured using an ultraviolet-visible spectrophotometer under normal temperature (25 °C) conditions for the product obtained through the steps of adding 0.5 part by weight of the surfactant to 100 parts by weight of an electrolyte containing 1M LiFP 6 in a composition manufacturing step, and then storing the manufactured composition sealed in an oven at 70 °C for 6 hours.

[0128] Thereafter, a polyethylene porous film with a thickness of 9 μm (porosity: 45%) as a porous polymer substrate was immersed in an impregnation tank containing the composition for substrate modification and then dried to remove the solvent to produce a separation membrane. At this time, based on 100 parts by weight of the polyethylene porous film, the sum of the content of β-cyclodextrin and the content of the surfactant was 0.5 part by weight. Here, the weight ratio of β-cyclodextrin to the surfactant in the 0.5 part by weight, which is the sum of the content of β-cyclodextrin and the content of the surfactant, maintained the ratio of 99.5:0.5, which is the weight ratio of β-cyclodextrin to the surfactant contained in the original composition for the substrate material. The test results for this are shown in Tables 1 and 2.

[0129] <Comparative Example 1> A polyethylene porous film with a thickness of 9 μm (porosity: 45%) was prepared as Comparative Example 1.

[0130] The test results for this are shown in Tables 1 and 2.

[0131] <Comparative Example 2> 0.5 g of a surfactant (FC4430, manufactured by 3M) was added to 99.5 ml of water as a solvent to prepare a composition for modifying a substrate.

[0132] Thereafter, a polyethylene porous film having a thickness of 9 μm (porosity: 45%) was immersed in an impregnation bath containing the composition for modifying a substrate and then dried to remove the solvent to produce a separation membrane. At this time, the content of the surfactant based on 100 parts by weight of the polyethylene porous film was 0.5 part by weight.

[0133] The test results for this are shown in Table 1.

[0134] <Comparative Example 3> 0.5 g of β-cyclodextrin was added to 99.5 ml of water as a solvent to prepare a composition for modifying a substrate.

[0135] Thereafter, a polyethylene porous film having a thickness of 9 μm (porosity: 45%) was immersed in an impregnation bath containing the composition for modifying a substrate and then dried to remove the solvent to produce a separation membrane. At this time, the content of the β-cyclodextrin based on 100 parts by weight of the polyethylene porous film had a large variation for each position and it was difficult to present a representative value. The test results for this are shown in Table 1.

[0136]

Table 1

[0137] At this time, for the tensile strength of the separation membrane, the separation membrane was cut into 15 mm × 100 mm to prepare a sample, and then, using the prepared sample, it was pulled in the MD direction and the TD direction at a speed of 500 mm / min in accordance with ASTM-D882, and the strength at the time when the specimen broke was measured.

[0138] The wettability aspect by the electrolyte droplet addition (drop) test was as follows: After injecting 2 μl of an electrolyte solution in which 1 M lithium hexafluorophosphate (LiPF 6 ) was dissolved into a 20:40:40 wt% solvent of propylene carbonate / dimethyl carbonate / ethyl methyl carbonate (PC / DMC / EMC) on the surface of the separation membrane, the elapsed time after 5 minutes was observed. The test results are shown in FIGS. 1 to 3.

[0139] On the other hand, the water droplet contact angle was measured using a contact angle measuring device (manufactured by KRUSS, model: DSA100). Specifically, after dropping a single drop of a certain amount of solution (water) on the surface of the separation membrane, an image was measured and the angle between the substrate and the water droplet was measured to measure the contact angle.

[0140]

Table 2

[0141] At this time, the thermal shrinkage rate was calculated as (initial length - length after heat shrinkage treatment at 120°C for 1 hour) / (initial length)×100.

[0142] When no surface treatment is performed at all as in Comparative Example 1, the contact angle is about 28% larger than that in Example 1, and as shown in FIG. 2, it can be confirmed that the wettability with the electrolyte is not improved.

[0143] When, as in Comparative Example 2, it contains no cyclodextrin and only a surfactant, it can also be confirmed that the wettability of the electrolyte is not improved, similar to Comparative Example 1.

[0144] On the other hand, as shown in Example 1 of Table 1, when a surfactant and cyclodextrin are used simultaneously, it can be confirmed that the wettability with respect to the electrolyte is improved and the contact angle becomes smaller.

Claims

1. A porous polymer substrate and a hydrophilic modification layer located inside the porous polymer substrate or inside and on at least one surface of the porous polymer substrate, the hydrophilic modification layer containing a polymer compound represented by the following Chemical Formula 1 and a surfactant. 【Chemical 1】 In Chemical Formula 1, n is any integer from 6 to 8. A separator for a lithium secondary battery, wherein the content of the surfactant is 1.0 part by weight or less based on 100 parts by weight of the polymer compound.

2. The separator for a lithium secondary battery according to Claim 1, wherein the polymer compound contains α-cyclodextrin, β-cyclodextrin, γ-cyclodextrin, or two or more of these.

3. The separator for a lithium secondary battery according to Claim 1 or 2, wherein the content of the polymer compound is 1 part by weight or less based on 100 parts by weight of the porous polymer substrate.

4. The separator for a lithium secondary battery according to any one of Claims 1 to 3, wherein the surfactant is a fluorine-based surfactant, a silicone-based surfactant, a nonionic surfactant, a cationic surfactant, an anionic surfactant, or a mixture of two or more of these.

5. As a separator for a lithium secondary battery A porous polymer substrate and a hydrophilic modification layer located inside the porous polymer substrate or inside and on at least one surface of the porous polymer substrate, the hydrophilic modification layer containing a polymer compound represented by the following Chemical Formula 1 and a surfactant. The separator further includes a porous coating layer provided with inorganic particles on at least one surface of the separator and a binder polymer for fixing and connecting the inorganic particles. A separator for a lithium secondary battery, wherein the content of the surfactant is 1.0 part by weight or less based on 100 parts by weight of the polymer compound. [Chemical 2] In Chemical Formula 1, n is any integer from 6 to 8.

6. The porous coating layer of the separator for a lithium secondary battery according to Claim 5 further contains a polymer compound represented by the following Chemical Formula 1. 【Chemical Formula 3】 In Chemical Formula 1, n is any integer from 6 to 8. The separator for a lithium secondary battery according to Claim 5.

7. The separator for a lithium secondary battery according to Claim 6, wherein the content of the polymer compound represented by Chemical Formula 1 is 10 parts by weight or less based on 100 parts by weight of the inorganic particles.

8. The separator for a lithium secondary battery according to any one of Claims 5 to 7, wherein the binder polymer is in a particulate or non-particulate form.

9. The inorganic particles are inorganic particles having a relative dielectric constant of 5 or more, inorganic particles having lithium ion transport ability, or a mixture of two or more of these, and the separator for a lithium secondary battery according to any one of claims 5 to 8.

10. The inorganic particles are hydrophilic, and the separator for a lithium secondary battery according to claim 9.

11. The inorganic particles have a silane group grafted on the surface of the inorganic particles, and the separator for a lithium secondary battery according to any one of claims 5 to 10.

12. It includes a positive electrode, a negative electrode, and a separator and an electrolytic solution interposed between the positive electrode and the negative electrode, The separator is the separator according to any one of claims 1 to 11, and the lithium secondary battery.

13. (S1) preparing a composition for modifying a substrate in which a polymer compound represented by the following chemical formula 1 and a surfactant are dispersed or dissolved in an aqueous solvent; (S2) applying and drying the composition for modifying a substrate on at least one surface of a porous polymer substrate having a large number of pores and being hydrophobic, 【Chemical 4】 In Chemical Formula 1, n is any integer from 6 to 8, The content of the surfactant is 1.0 part by weight or less based on 100 parts by weight of the polymer compound, and a method for manufacturing a separator for a lithium secondary battery.

14. The step (S2) is a step of modifying the porous polymer substrate to be hydrophilic by allowing the surfactant and the polymer compound to penetrate into the porous polymer substrate, and the method for manufacturing a separator for a lithium secondary battery according to claim 13.

15. (S1) preparing a composition for modifying a substrate in which a polymer compound represented by the following chemical formula 1 and a surfactant are dispersed or dissolved in an aqueous solvent; (S2) applying and drying the composition for modifying a substrate on at least one surface of a porous polymer substrate having a large number of pores and being hydrophobic, After the step (S2), forming a porous coating layer by drying and applying a composition for forming a porous coating layer containing a solvent, inorganic particles, and a binder polymer on at least one surface of the porous polymer substrate on which the composition for modifying a substrate has been applied and dried (step (S3)), The content of the surfactant is 1.0 part by weight or less based on 100 parts by weight of the polymer compound, and a method for manufacturing a separator for a lithium secondary battery. [Chemical Formula 5] In Chemical Formula 1, n is any integer from 6 to 8.

16. The manufacturing method of the separator for a lithium secondary battery according to claim 15, wherein the solvent is an aqueous solvent or an organic solvent.

17. The manufacturing method of the separator for a lithium secondary battery according to claim 15 or 16, wherein the composition for forming the porous coating layer further contains the polymer compound represented by Chemical Formula 1.

18. The manufacturing method of the separator for a lithium secondary battery according to any one of claims 15 to 17, wherein the step (S3) is performed at a relative humidity of 30 to 80%.

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