Separator for electrochemical device including inorganic coating layer and method for manufacturing same

Abstract

To provide a separation membrane which secures adhesion and has low resistance characteristics and a low electrolyte absorption ratio, and a battery including the same, and a method for manufacturing the separation membrane having the characteristics and a method for manufacturing the battery.SOLUTION: There is provided a separation membrane for an electrochemical element, wherein the separation membrane includes a porous base material, and an inorganic coating layer formed on at least one surface of the base material. The inorganic coating layer contains a binder resin and inorganic particles, and the binder resin contains a polyvinylidene fluoride-based polymer resin, and exhibits a low electrolyte absorption ratio. The separation membrane is excellent in adhesive force to an electrode, and exhibits low resistance characteristics.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a separator for an electrochemical device, the electrochemical device may be a primary battery or a secondary battery, and the secondary battery includes a lithium ion secondary battery.

[0002] This application claims priority based on Korean Patent Application No. 10-2018-0067486, filed on June 12, 2018, the entire contents of which are incorporated herein by reference in their entirety. [Background technology]

[0003] Non-aqueous secondary batteries, such as lithium-ion secondary batteries, are widely used as power sources for portable electronic devices such as notebook PCs, mobile phones, digital cameras, camcorders, etc. Recently, these batteries have been considered for use in automobiles and other applications due to their high energy density.

[0004] As portable electronic devices become smaller and lighter, the packaging of nonaqueous secondary batteries is becoming simpler. Initially, stainless steel battery cans were used as the packaging, but aluminum cans were developed, and now soft-pack packaging made of aluminum laminate packs has also been developed. In the case of soft-pack packaging made of aluminum laminate, the packaging is flexible, so gaps can form between the electrodes and the separator during charging and discharging, posing a technical issue of reduced cycle life. To solve this issue, technology for bonding the electrodes and the separator has become important, and many technical proposals have been made.

[0005] One proposed technique is to use a separator in which a porous layer made of a polyvinylidene fluoride resin (hereinafter also referred to as an adhesive porous layer) is molded onto a conventional polyolefin microporous membrane. The adhesive porous layer, when placed on an electrode and hot-pressed while still containing an electrolyte solution, can effectively bond the electrode and separator, functioning as an adhesive. This can improve the cycle life of soft-pack batteries.

[0006] In addition, when manufacturing a battery using a conventional metal can exterior, electrodes and a separator are stacked and wound together to form a battery element, and the element is then sealed in a metal can exterior together with an electrolyte to form a battery. On the other hand, when manufacturing a soft-pack battery using a conventional separator, a battery element is manufactured in the same manner as the metal can exterior battery, and the battery is then sealed in a soft-pack exterior together with an electrolyte, and finally a hot pressing process is performed to form the battery. Therefore, when using a separator having an adhesive porous layer as described above, a battery element can be manufactured in the same manner as the metal can exterior battery, which has the advantage of not requiring significant changes to the manufacturing process of conventional metal can exterior batteries.

[0007] In light of the above-mentioned background, various technologies have been proposed for separation membranes in which an adhesive porous layer is laminated on a polyolefin microporous membrane. For example, new technologies have been proposed that focus on the porous structure and thickness of the polyvinylidene fluoride resin layer from the viewpoint of ensuring both sufficient adhesiveness and ion permeability. Summary of the Invention [Problem to be solved by the invention]

[0008] SUMMARY OF THE INVENTION An object of the present invention is to provide a separator having low resistance and low electrolyte absorption while ensuring adhesion, and a battery including the separator.

[0009] Another object of the present invention is to provide a method for manufacturing a separator having the above properties and a method for manufacturing the battery.

[0010] The objects and advantages of the present invention can be realized by the means and combinations thereof set forth in the claims. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention provides a separator for an electrochemical device.

[0012] A first aspect of the present invention relates to such a separation membrane, the separation membrane comprising a porous substrate and an inorganic coating layer formed on at least one surface of the porous substrate, The inorganic coating layer includes a binder resin and inorganic particles, and the binder resin is a polyvinylidene fluoride polymer resin including at least one polymer unit selected from the group consisting of trifluoroethylene and tetrafluoroethylene.

[0013] A second aspect of the present invention is that, in the first aspect, the polyvinylidene fluoride polymer resin has a substitution rate of the polymer unit of 5 mol % to 30 mol %.

[0014] A third aspect of the present invention is that, in any one of the aforementioned aspects, the polyvinylidene fluoride polymer resin has a substitution rate of the polymer unit of 5 mol % to 15 mol %.

[0015] A fourth aspect of the present invention is any one of the aforementioned aspects, wherein the polyvinylidene fluoride polymer resin has a melting point (Tm) of 145° C. or lower.

[0016] A fifth aspect of the present invention is any one of the aforementioned aspects, wherein the polyvinylidene fluoride polymer resin has an electrolyte absorption rate of 30% or less according to the following mathematical formula 1:

[0017] [Number 1] Electrolyte absorption rate (%) = [(weight of polymer resin after immersion - initial weight of polymer resin) / initial weight of polymer resin] x 100

[0018] In a sixth aspect of the present invention, in any one of the aforementioned aspects, the electrolytic solution comprises an organic solvent and a lithium salt, and the organic solvent contains 30% by weight or more of an ester compound relative to 100% by weight of the organic solvent.

[0019] The seventh aspect of the present invention is any one of the aforementioned aspects, wherein the binder resin and inorganic particles in the inorganic coating layer are contained in a weight ratio of 15:85 to 50:50.

[0020] In an eighth aspect of the present invention, in any one of the aforementioned aspects, the inorganic coating layer has an average pore size of 20 nm to 800 nm.

[0021] A ninth aspect of the present invention relates to an electrochemical device, the electrochemical device including an electrode assembly and an electrolyte, the electrode assembly including a positive electrode, a negative electrode, and a separator sandwiched between the positive electrode and the negative electrode, the separator being as described in any one of the preceding aspects.

[0022] In a tenth aspect of the present invention, in any one of the aforementioned aspects, the electrolytic solution contains an organic solvent and a lithium salt, and the organic solvent contains 30% by weight or more of an ester compound relative to 100% by weight of the organic solvent. [Effects of the Invention]

[0023] The separator and the electrochemical device including the separator according to the present invention have excellent adhesive strength between the separator and the electrode, low resistance characteristics, and the binder resin exhibits low electrolyte absorption.

[0024] The following drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the detailed description of the invention, serve to further understand the technical concept of the present invention, so the present invention should not be interpreted as being limited to the matters depicted in the drawings. Note that the shape, size, scale, or ratio of elements in the drawings attached to this specification may be exaggerated to emphasize a clearer description. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a SEM image of the surface of the inorganic coating layer of the separator prepared in Example 1. [Figure 2] 1 is a SEM image of the surface of the inorganic coating layer of the separator prepared in Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, the terms and words used in this specification and claims should not be construed as being limited to their ordinary or dictionary meanings, but should be construed as meanings and concepts corresponding to the technical ideas of the present invention, in accordance with the principle that the inventor himself can appropriately define the concepts of terms in order to best explain the invention. Therefore, it should be understood that the embodiments described in this specification and the configurations shown in the drawings are merely the most preferred embodiment of the present invention and do not represent the entire technical ideas of the present invention, and therefore, various equivalents and modifications that can be substituted therefor may exist at the time of filing this application.

[0027] Furthermore, throughout the specification, when a part is said to "include" a certain component, this does not mean that it excludes other components, but that it may further include other components, unless otherwise specified.

[0028] The terms "about," "substantially," and the like used throughout this specification are used to mean a numerical value or a approximation thereof when inherent manufacturing and material tolerances are present in the stated meaning, and are used to prevent unscrupulous infringers from unfairly using disclosures in which precise or absolute numerical values ​​are recited to aid in the understanding of this application.

[0029] Throughout this specification, the phrase "A and / or B" means "A or B, or all of them."

[0030] Certain terminology used in the following detailed description is for convenience only and is not intended to be limiting. The words "right," "left," "top," and "bottom" designate directions in the drawings to which reference is made. The words "inward" and "outward" designate directions toward or away from the geometric center of the designated device, system, and components thereof, respectively. "Front," "rear," "upper," "lower," and related words and phrases designate locations and orientations in the drawings to which reference is made and are not intended to be limiting. Such terms include the example words above, derivatives thereof, and words of similar import.

[0031] The present invention relates to a separator for an electrochemical device and an electrochemical device including the same. In the present invention, the electrochemical device is a device that converts chemical energy into electrical energy through an electrochemical reaction and is a concept that includes primary batteries and secondary batteries. The secondary batteries are rechargeable and include lithium ion batteries, nickel-cadmium batteries, nickel-metal hydride batteries, etc.

[0032] 1.Separation membrane (Structure of Separator) The separator according to the present invention comprises a porous substrate having a plurality of pores and an inorganic coating layer formed on at least one surface of the porous substrate.

[0033] In one embodiment of the present invention, the thickness of the separation membrane may be 5 μm to 20 μm, and may be appropriately adjusted within this range. For example, the thickness may be 18 μm or less, or 15 μm or less. The separation membrane also has an air permeability in the range of about 38% to 60%.

[0034] The term "air permeability" as used herein refers to the time it takes for 100 cc of air to pass through an object to be measured, such as a separation membrane or a porous substrate. The unit of measurement is seconds / 100 cc, and it can be used interchangeably with permeability, which is usually expressed as a Gurley value. In a specific embodiment of the present invention, the air permeability can be measured in accordance with JIS P8117. Furthermore, the air permeability P1 measured from an object having a thickness T1 can be converted to the permeability P2 when the object is 20 μm thick using the formula: P2 = (P1 × 20) / T1.

[0035] Meanwhile, in the present invention, the separator has an electrolyte absorption rate of more than 0% but not more than 30%. If the electrolyte absorption rate is high, the binder resin contained in the inorganic coating layer may expand, blocking pores and reducing the ionic conductivity of the separator. Therefore, the upper limit of the electrolyte absorption rate of the binder resin in the separator must be controlled to 30%. Within this range, the electrolyte absorption rate may be 10% or more, 15% or more, or 20% or more, or may be 27% or less, or 25% or less.

[0036] In the present invention, the electrolyte absorption rate (%) means the weight change rate (Equation 1) after the separator is immersed in the electrolyte for 72 hours. [Number 1] Electrolyte absorption rate (%) = [(weight of polymer resin after immersion - initial weight of polymer resin) / initial weight of polymer resin] x 100

[0037] In one embodiment of the present invention, when measuring the electrolyte absorption rate, the electrolyte solution includes an organic solvent, and the organic solvent includes 30 wt % or more of an ester compound relative to 100 wt % of the organic solvent. For example, the electrolyte solution may be a mixture of ethylene carbonate (EC) and propyl propionate (PP) in a volume ratio of EC:PP = 30:70. The electrolyte solution used when measuring the electrolyte absorption rate may not contain a lithium salt or may selectively contain a lithium salt. In one embodiment of the present invention, the electrolyte solution may be a mixture of an organic solvent in a volume ratio of EC:PP = 30:70, and may contain a lithium salt such as LiPF6 at a 1 mol concentration.

[0038] 1) Porous base material The porous substrate is a substrate having a plurality of pores formed therein, which act as a porous ion-conducting barrier that allows ions to pass through while blocking electrical contact between the anode and cathode. The pores have an interconnected structure, allowing gas or liquid to pass from one side of the substrate to the other.

[0039] The material constituting such a porous substrate can be either an organic material or an inorganic material having electrical insulation properties. In particular, from the viewpoint of imparting a shutdown function to the substrate, it is desirable to use a thermoplastic resin as the constituent material of the substrate. Here, the shutdown function refers to the function of blocking the movement of ions and preventing thermal runaway of the battery when the battery temperature becomes high by dissolving the thermoplastic resin and blocking the holes in the porous substrate. A thermoplastic resin with a melting point of less than 200°C is suitable as the thermoplastic resin, and polyolefin is particularly desirable.

[0040] In addition, the porous substrate may further include at least one polymer resin such as polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalate. The porous substrate may be, but is not limited to, a nonwoven fabric, a porous polymer film, or a laminate of two or more thereof.

[0041] Specifically, the porous polymer substrate is any one of the following a) to e). a) Porous film formed by melting / extruding polymer resin b) A multilayer film in which two or more layers of the porous film of a) are laminated. c) A nonwoven web produced by accumulating filaments obtained by melting / spinning a polymer resin d) A multilayer film in which two or more layers of the nonwoven fabric web of b) are laminated. e) A porous composite membrane with a multilayer structure comprising two or more of the above a) to d).

[0042] In the present invention, the porous substrate preferably has a thickness of 3 μm to 12 μm or 5 μm to 12 μm. If the thickness is less than this range, the conductive barrier function may be insufficient, while if the thickness exceeds this range (i.e., is too thick), the resistance of the separator may increase excessively.

[0043] In one embodiment of the present invention, the weight-average molecular weight of the polyolefin is preferably 100,000 to 5,000,000. If the weight-average molecular weight is less than 100,000, it may be difficult to ensure sufficient mechanical properties. On the other hand, if it is greater than 5,000,000, the shutdown characteristics may be poor and molding may be difficult. Furthermore, the puncture strength of the porous substrate may be 300 gf or more, from the viewpoint of improving production yield. The puncture strength of the porous substrate refers to the maximum puncture load (gf) measured by a puncture test using a Kato Tech KES-G5 handy compression tester under the conditions of a needle tip curvature radius of 0.5 mm and a puncture speed of 2 mm / sec.

[0044] In one specific embodiment of the present invention, the porous polymer substrate may be any planar porous polymer substrate used in electrochemical devices. For example, an insulating thin film having high ion permeability and mechanical strength, a pore diameter of typically 10 nm to 100 nm, and a thickness of typically 5 μm to 12 μm may be used.

[0045] 2) Inorganic coating layer A. Structure of inorganic coating layer In the present invention, the separator includes an inorganic coating layer formed on one surface of the porous substrate. The inorganic coating layer includes a mixture containing a binder resin and inorganic particles, and the binder resin includes a polyvinylidene fluoride resin (PVdF-based polymer resin). The inorganic coating layer is densely packed with inorganic particles and has a plurality of micropores resulting from interstitial volumes formed between the inorganic particles. These micropores are interconnected, providing a porous structure that allows gas or liquid to pass from one surface to the other. In one embodiment of the present invention, the inorganic particles are coated on all or at least a portion of their surfaces with a binder resin, and are surface-bonded and / or point-bonded via the binder resin. In one embodiment of the present invention, the binder resin and inorganic particles in the inorganic coating layer are present in a weight ratio of 15:85 to 50:50. In other words, the binder resin content in the inorganic coating layer may be in the range of 15% to 50% by weight, with the total weight of the binder resin and the inorganic particles being 100% by weight. Within this range, the binder resin content may be 20% by weight or more, 30% by weight or more, or 40% by weight or less, 30% by weight or less, or 20% by weight or less. For example, the binder resin content may be 15% to 25% by weight, with the total weight of the binder resin and the inorganic particles being 100% by weight.

[0046] In one embodiment of the present invention, the inorganic coating layer has an average pore size of 20 nm to 1,000 nm. Within this range, the average pore size of the inorganic coating layer may be 800 nm or less or 500 nm or less, and independently or in addition, may be 20 nm or more, 50 nm or more, or 100 nm or more. For example, the average pore size of the inorganic coating layer is 20 nm to 800 nm. The pore size can be calculated by shape analysis using SEM images. If the pore size is smaller than this range, the pores are likely to be blocked due to expansion of the binder resin in the coating layer. If the pore size is outside this range, the coating layer will not function as an insulating film and the self-discharge characteristics of the secondary battery after manufacture may be deteriorated.

[0047] In one embodiment of the present invention, the porosity of the inorganic coating layer is preferably 30% to 80%. A porosity of 30% or more is advantageous in terms of lithium ion permeability, while a porosity of 80% or less is preferable in terms of ensuring adhesion between the separator and the electrode without increasing the surface opening ratio.

[0048] Meanwhile, in the present invention, the porosity and pore size can be measured using an adsorption gas such as nitrogen using a BELSORP (BET equipment) manufactured by BEL JAPAN, or by methods such as mercury intrusion porosimetry and capillary flow porosimetry. Alternatively, in one embodiment of the present invention, the thickness and weight of the obtained coating layer can be measured, and the porosity can be calculated from the theoretical density of the coating layer.

[0049] The thickness of the inorganic coating layer is preferably 1.5 μm to 5.0 μm on one side of the porous substrate. The thickness is preferably 1.5 μm or more, and within this range, the adhesive strength with the electrode is excellent, resulting in increased cell strength of the battery. On the other hand, if the thickness is 5.0 μm or less, it is advantageous in terms of the cycle characteristics and resistance characteristics of the battery.

[0050] B. Inorganic coating layer material B1. Binder resin In one embodiment of the present invention, the binder resin includes a polyvinylidene fluoride-based resin (PVdF-based polymer resin), and preferably, the PVdF-based polymer resin accounts for 80% by weight or more or 90% by weight or more of the binder resin (100% by weight). The PVdF-based polymer resin has a weight-average molecular weight of 600,000 or less, preferably 400,000 or less. A weight-average molecular weight of 600,000 or less is advantageous for increasing flexibility and improving adhesive strength. Here, the weight-average molecular weight of the PVdF-based polymer resin can be determined by gel permeation chromatography (GPC). The molecular weight is expressed in g / mol.

[0051] In one embodiment of the present invention, the PVdF-based polymer resin may include at least one of a homopolymer of vinylidene fluoride (i.e., polyvinylidene fluoride), a copolymer of vinylidene fluoride and a copolymerizable monomer, and a mixture thereof. In one embodiment of the present invention, the monomer may be, for example, a fluorinated monomer and / or a chlorinated monomer. Non-limiting examples of the fluorinated monomer include vinyl fluoride; trifluoroethylene (TrFE); chlorofluoroethylene (CTFE); 1,2-difluoroethylene; tetrafluoroethylene (TFE); hexafluoropropylene (HFP); perfluoro(alkyl vinyl) ethers such as perfluoro(methyl vinyl) ether (PMVE), perfluoro(ethyl vinyl) ether (PEVE), and perfluoro(propyl vinyl) ether (PPVE); perfluoro(1,3-dioxole); and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD), among others.

[0052] The PVdF polymer resin satisfies at least one of the following: a melting point (Tm) of 145°C or less, preferably 140°C or less; a crystallinity of 30% to 45%; and a crystallization temperature of 120°C or less.

[0053] In one embodiment of the present invention, the PVDF-based polymer resin may have a Tm of 145°C or less, preferably 140°C or less, in terms of adhesive strength during heat bonding. A crystallinity of 30% or more is advantageous in terms of improving resistance characteristics, but if the crystallinity is too high, adhesive strength decreases. Therefore, it is preferable to control the crystallinity to 45% or less. Furthermore, since a high crystallization temperature (Tc) tends to increase the crystallinity and lead to α-crystallization, it is preferable that the crystallization temperature be 120°C or less.

[0054] In the present invention, the crystallization temperature can be measured by differential scanning calorimetry (DSC) and is defined as the temperature at which the highest enthalpy (peak temperature) is observed during crystallization. The crystallinity is expressed as a percentage by dividing the melting enthalpy (ΔH) measured by DSC by the melting enthalpy (ΔH) of a theoretical perfect crystal (100% crystallinity). The theoretical melting enthalpy of a perfect crystal can be found in the Polymer Handbook for known polymers. For unknown or newly synthesized materials, it can be calculated by extrapolation from two or more crystallinity points.

[0055] In one embodiment of the present invention, the PVDF-based polymer resin has a melting temperature of 145° C. or less, a crystallinity of 30% to 45%, and a crystallization temperature of 120° C. or less. When these ranges are satisfied, the resin exhibits a sufficient degree of crystallinity so as not to reduce the resistance characteristics, but also exhibits a weak crystalline form (mainly including β crystals), thereby improving the flexibility of the inorganic coating layer and, as a result, ensuring low resistance characteristics along with the adhesive strength between the electrode and the separator.

[0056] In one embodiment of the present invention, the PVdF-based polymer resin preferably contains a copolymer of vinylidene fluoride units and another copolymerizable monomer, and the PVdF-based polymer resin preferably contains 80 wt% or more, 90 wt% or more, or 99 wt% or more of such a copolymer based on 100 wt% of the PVdF-based polymer resin. In one embodiment of the present invention, the copolymer contains 70 mol% or more of vinylidene fluoride as copolymerized units, and the substitution rate with the monomer is 5 mol% to 30 mol%. In one embodiment, the substitution rate may range from 8 mol% to 10 mol%, 15 mol% to 18 mol%, and may be 25 mol% to 20 mol%, 17 mol% to 15 mol%. For example, the substitution rate is 10 mol% to 25 mol%.

[0057] The PVdF polymer resin having such a relatively low molecular weight can be obtained preferably by emulsion polymerization or suspension polymerization, and particularly preferably by suspension polymerization.

[0058] In one embodiment of the present invention, the PVdF-based polymer resin contains 80% by weight or more, 90% by weight or more, or 99% by weight or more of a copolymer of vinylidene fluoride units and other copolymerizable monomers, including both trifluoroethylene (TrFE) and tetrafluoroethylene (TFE), and the substitution rate with the monomers is 5 mol% to 30 mol%. In one embodiment, the substitution rate can range from 8 mol% to 10 mol%, 15 mol% to 18 mol%, and can be 25 mol% to 20 mol%, 17 mol% to 15 mol%. For example, the substitution rate is 10 mol% to 25 mol%.

[0059] That is, in one embodiment of the present invention, the PVDF-based polymer resin may contain PVDF-TrFE, PVDF-TFE, or both, and the range of substitution rate of TrFE and / or TFE may refer to the above content.

[0060] In particular, trifluoroethylene (TrFE) and tetrafluoroethylene (TFE) have lower steric hindrance than chlorofluoroethylene (CTFE) and hexafluoropropylene (HFP). Even when substituted with PVdF, the PVdF chains are effectively spaced closely together, preventing free volume between the polymer chains. This reduces the electrolyte absorption rate and keeps it within the aforementioned range. In other words, the polymer resin is less likely to expand due to the reduced free volume between the polymer chains, preventing excessive expansion of the polymer resin and preventing the pores in the porous coating layer from being blocked by the expansion of the polymer resin. This effectively maintains the porosity within the separator at an appropriate level. Even when a porous coating layer is formed on the surface of the separator, the porosity is maintained at an appropriate level, improving the low resistance characteristics of the separator.

[0061] In the present invention, the PVdF-based binder resin substituted with the above monomer has a Tm of 145°C or less, which is advantageous for improving adhesive strength. Furthermore, since such a PVDF-based binder resin has a low crystallization temperature and a β-crystalline structure, it is advantageous for achieving desired adhesive strength and exhibits a degree of crystallinity that is advantageous for realizing low resistance characteristics of the separator. Furthermore, when a PVdF-based binder resin substituted with trifluoroethylene (TrFE) and / or tetrafluoroethylene (TFE) is used, it is preferable because it can control the electrolyte absorption rate (%) to 30% or less and appropriately maintain the porosity within the separator.

[0062] According to a specific embodiment of the present invention, the inorganic coating layer may further include a (meth)acrylic polymer resin as a binder resin. The (meth)acrylic polymer includes a (meth)acrylic acid ester as a monomer, and non-limiting examples thereof include (meth)acrylic polymers including butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, ethyl (meth)acrylate, methyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, n-oxyl (meth)acrylate, isooctyl (meth)acrylate, isononyl (meth)acrylate, lauryl (meth)acrylate, and tetradecyl (meth)acrylate as monomers. The (meth)acrylic polymer may be included in an amount of 20% by weight or less, based on 100% by weight of the total binder resin.

[0063] In one embodiment of the present invention, the inorganic coating layer may further include an additive, such as a dispersant and / or a thickener, in an amount of 1 to 3 wt % based on 100 wt % of the inorganic coating layer. In one embodiment of the present invention, the additive may be one or more selected from the group consisting of polyvinylpyrolidone (PVP), polyvinylalcohol (PVA), hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), ethylhydroxyethyl cellulose (EHEC), methyl cellulose (MC), carboxymethyl cellulose (CMC), hydroxyalkyl methyl cellulose, and cyanoethylene polyvinyl alcohol.

[0064] B2.Inorganic particles In one specific embodiment of the present invention, the inorganic particles are not particularly limited as long as they are electrochemically stable. That is, inorganic particles that can be used in the present invention are not particularly limited as long as they do not undergo oxidation and / or reduction reactions within the operating voltage range of the applied electrochemical device (e.g., 0 to 5 V based on Li / Li+). In particular, when inorganic particles with a high dielectric constant are used as the inorganic particles, they can contribute to increasing the degree of dissociation of electrolyte salts, such as lithium salts, in the liquid electrolyte, thereby improving the ionic conductivity of the electrolyte solution.

[0065] For the above reasons, it is desirable that the inorganic particles include high-dielectric-constant inorganic particles having a dielectric constant of 5 or more, preferably 10 or more. Non-limiting examples of inorganic particles having a dielectric constant of 5 or more include BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb 1-x La x Zr 1-y Ti y O3(PLZT,0 <x<1,0<y<1)、Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3 (PMN-PT), hafnia (HfO2), SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, SiO2, Y2O3, Al2O3, SiC and TiO2 or mixtures thereof.

[0066] In addition, inorganic particles having lithium ion transfer ability, i.e., inorganic particles containing lithium element but having the function of transferring lithium ions without storing lithium, can be used. Non-limiting examples of inorganic particles having lithium ion transfer ability include lithium phosphate (LiPO), lithium titanium phosphate (Li x Ti y (PO4)3,0 <x<2、0<y<3)、リチウムアルミニウムチタンホスフェート(Li x Al y Ti z (PO4)3,0 <x<2、0<y<1、0<z<3)、14Li2O-9Al2O3-38TiO2-39P2O5などのような(LiAlTiP) x Oy System glass (0 < x < 4, 0 < y < 13), lithium lanthanum titanate (Li x La y TiO3, 0 < x < 2, 0 < y < 3), Li 3.25 Ge 0.25 P 0.75 Lithium germanium thiophosphate such as S4 (Li x Ge y P z S w , 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), 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 mixtures thereof, etc. can be mentioned.

[0067] Also, the average particle size of the inorganic particles is not particularly limited, but for the formation of a coating layer with a uniform thickness and an appropriate porosity, it is desirable to be in the range of 0.1 μm to 1.5 μm. If it is less than 0.1 μm, the dispersibility may decrease, and if it exceeds 1.5 μm, the thickness of the formed coating layer may increase.

[0068] 2. Method for manufacturing the separation membrane The inorganic coating layer forming slurry containing the binder resin and inorganic particles according to the present invention is applied onto a porous substrate, and the binder resin is solidified, whereby the inorganic coating layer can be integrally formed on the porous substrate by a method.

[0069] Specifically, a binder resin is first dissolved in a solvent to prepare a polymer solution, and then inorganic particles are added to and mixed with the polymer solution to prepare a slurry for forming the inorganic coating layer. This is then applied to a porous substrate and left for a predetermined time under conditions of approximately 40% to 80% relative humidity to solidify (dry) the binder resin. During this process, phase separation of the binder resin is induced. During the phase separation process, the solvent migrates to the surface of the inorganic coating layer, and as the binder resin migrates to the surface of the inorganic coating layer along with the solvent migration, the binder resin content in the surface of the inorganic coating layer increases. Pores are formed below the surface of the inorganic coating layer due to interstitial volume between the inorganic particles, resulting in the inorganic coating layer possessing porous properties. In one embodiment of the present invention, when a PVdF-based copolymer containing a predetermined proportion of TrFE and / or TFE as polymerization units is used as the binder resin, this binder resin exhibits a slower phase separation rate than a PVdF homopolymer or a PVdF-based copolymer containing HFP or CTFE as polymerization units. Therefore, excessive phase separation, such as the formation of a binder resin layer with a predetermined thickness at the top of the inorganic coating layer due to excessive migration of the binder resin to the surface of the inorganic coating layer during the drying process, is prevented. In one embodiment of the present invention, the inorganic coating layer may have an increasing binder resin content toward the surface, but exhibits a morphology in which the inorganic particles and the binder resin are mixed throughout the thickness of the inorganic coating layer, maintaining a pore structure resulting from interstitial volume between the inorganic particles throughout the inorganic coating layer. This allows the inorganic coating layer to maintain a low resistance. Furthermore, the method for manufacturing the inorganic coating layer allows for easy process control due to the slow phase separation rate of the binder resin.

[0070] The inventors of the present invention discovered that the higher the substitution ratio of a PVdF copolymer compared to a PVdF homopolymer, the slower the phase separation behavior during humid drying. They realized that this characteristic could be used to control the binder distribution through the thickness of an inorganic coating layer. Even in the case of copolymers, the rate of phase separation during drying can be more easily controlled by using polymerization units such as trifluoroethylene (TrFE) and tetrafluoroethylene (TFE), which maintain a higher density after substitution than hexafluoroethylene, and controlling the substitution ratio within a specific range. That is, the slower phase separation rate prevents the binder resin used in the inorganic coating layer from migrating to the surface of the inorganic coating layer during humid drying and becoming concentrated there, which would increase interfacial resistance. Furthermore, the binder resin remains uniformly distributed within the inorganic coating layer, thereby enhancing adhesion between the porous substrate and the inorganic coating layer even with a low binder content.

[0071] Thereafter, the obtained separation membrane is dried, whereby the inorganic coating layer can be integrally formed on the porous substrate.

[0072] The slurry may be prepared using a solvent capable of dissolving the PVdF resin, such as a polar amide solvent such as acetone, methyl ethyl ketone, N-methylpyrrolidone, dimethylacetamide, dimethylformamide, or the like.

[0073] The slurry can be applied using a conventional coating method such as a Mayer bar coater, a die coater, a reverse roll coater, a gravure coater, etc. When the inorganic coating layer is formed on both sides of the porous substrate, the coating liquid can be applied to each side, followed by humidification phase separation and drying. However, from the viewpoint of productivity, it is more preferable to apply the coating liquid to both sides of the porous substrate simultaneously, followed by humidification phase separation and drying.

[0074] The separation membrane of the present invention can also be produced by separately preparing the inorganic coating layer and the porous substrate, superposing these sheets, and combining them using thermocompression bonding or an adhesive. A method for obtaining the inorganic coating layer as an independent sheet includes applying the slurry onto a release sheet, forming the inorganic coating layer using the above-mentioned method, and then peeling off only the inorganic coating layer.

[0075] 3. An electrode assembly including the separator Meanwhile, the present invention provides a secondary battery including the separator, which includes an anode, a cathode, and a separator interposed between the anode and the cathode, and the separator is a low-resistance separator having the above-mentioned characteristics.

[0076] In the present invention, the positive electrode comprises a positive electrode current collector and a positive electrode active material layer on at least one surface of the current collector, the positive electrode active material layer including a positive electrode active material, a conductive material, and a binder resin. The positive electrode active material is a layered compound such as lithium manganese composite oxide (LiMn2O4, LiMnO2, etc.), lithium cobalt oxide (LiCoO2), 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 = 0 to 0.33), LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiFe3O4, V2O5, and Cu2V2O7; and the chemical formula LiNi 1-x M x Ni-site lithium nickel oxide represented by the chemical formula LiMnO2 (where M=Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x=0.01 to 0.3). 2-x M xO2 (where M is Co, Ni, Fe, Cr, Zn or Ta, and x = 0.01 - 0.1), or a lithium manganese composite oxide represented by Li2Mn3MO8 (where M is Fe, Co, Ni, Cu or Zn); LiMn2O4 in which a part of Li in the chemical formula is replaced by an alkaline earth metal ion; a disulfide compound; It may contain one or a mixture of two or more of Fe2(MoO4)3.

[0077] In the present invention, the negative electrode includes a negative electrode current collector and a negative electrode active material layer containing a negative electrode active material, a conductive material, and a binder resin on at least one surface of the current collector. The negative electrode uses, as the negative electrode active material, carbon such as lithium metal oxide, graphitized carbon, 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, Group 3 elements of the periodic table, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8), etc. metal composite oxides; lithium metal; lithium alloy; silicon-based alloy; tin-based alloy; 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; It may contain one or a mixture of two or more selected from titanium oxides.

[0078] In one embodiment of the present invention, the conductive material may be, for example, any one selected from the group consisting of graphite, carbon black, carbon or metal fiber, metal powder, conductive whisker, conductive metal oxide, activated carbon, and polyphenylene derivatives, or a mixture of two or more thereof. More specifically, the conductive material may be any one selected from the group consisting of natural graphite, artificial graphite, Super-P, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, Denka black, aluminum powder, nickel powder, zinc oxide, potassium titanate, and titanium oxide, or a mixture of two or more thereof.

[0079] The current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity, and examples thereof include stainless steel, copper, aluminum, nickel, titanium, baked carbon, and aluminum or stainless steel whose surfaces are surface-treated with carbon, nickel, titanium, silver, or the like.

[0080] The binder resin may be a polymer commonly used in electrodes in the art. Non-limiting examples of such binder resins include vinylidene fluoride-co-hexafluoropropylene (PVdF-HFP), vinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polyethylhexyl acrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate. propionate, cyanoethyl pullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, and the like.

[0081] The electrode assembly thus prepared can be placed in a suitable case and an electrolyte injected to manufacture a battery.

[0082] In the present invention, the electrolyte solution is+ B - A salt having the structure: + Li + , Na + , K. + or a combination thereof, such as B - PF6 - , BF4 - , Cl - , Br - , I - , ClO4 - , AsF6 - , CH3CO2 - , CF3SO3 - , N(CF3SO2)2 - , C(CF2SO2)3 - The salts containing anions such as those listed above or ions consisting of a combination thereof may be dissolved or dissociated in organic solvents including, but not limited to, 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, ester compounds, and mixtures of one or more selected from the above.

[0083] Meanwhile, in a specific embodiment of the present invention, the organic solvent contains an ester-based compound, and preferably, the ester-based compound is present in an amount of 30 wt % or more, 50 wt % or more, 60 wt % or more, or 65 wt % or more relative to 100 wt % of the organic solvent.

[0084] In one embodiment of the present invention, the ester-based compound includes at least one selected from the group consisting of isobutyl propionate, isoamyl propionate, isobutyl butyrate, isopropyl propionate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl butyrate, ethyl butyrate, propyl butyrate, and butyl butyrate.

[0085] The use of such ester-based compounds exhibits high ionic conductivity, making them highly advantageous for use in secondary batteries for high-power vehicles. However, such ester-based compounds can reduce the porosity of the inorganic coating layer due to their high electrolyte absorption rate for PVdF-based copolymer binder resins, such as PVdF-HFP. However, as described above, the separator of the present invention includes PVdF-TrFE and / or PVdF-TFE as the binder resin for the inorganic coating layer, and PVdF-TrFE and PVdF-TFE can maintain an absorption rate for ester-based compounds of 30% or less. Therefore, when PVdF-TrFE and / or PVdF-TFE are used as the binder resin for the inorganic coating layer and an ester-based compound is used in the electrolyte, ionic conductivity can be maximized, making them highly advantageous for manufacturing batteries for high-power devices such as electric vehicles.

[0086] The present invention also provides a battery module including a battery having the electrode assembly as a unit cell, a battery pack including the battery module, and a device including the battery pack as a power source. Specific examples of the device include, but are not limited to, power tools powered by electric 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 power storage systems.

[0087] The present invention will be described below with reference to 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 completely explain the present invention to those skilled in the art.

[0088] Example 1 A polymer solution was prepared by dissolving 18 g of PVdF-TFE (Daikin, VT-475, Tm: 138°C, absorbency: 24%, TFE 14 mol%) and 2 g of cyanoethylene polyvinyl alcohol as a dispersant in 400 g of acetone. The crystallization temperature of the PVdF-TFE was 117°C, and the crystallinity was 37.7%. 80 g of Al2O3 (Nippon Light Metal, LS235) was added to the solution and dispersed using a ball mill to prepare a slurry for the inorganic coating layer. The slurry was applied to a porous substrate (Toray, B12PA1) by dip coating, and humidified phase separation was induced at a relative humidity (RH) of 60%. An inorganic coating layer was formed on both sides of the substrate to a total thickness of 8 μm. Figure 1 is an electron microscope image of the surface of the inorganic coating layer of the separator obtained in Example 1. As shown in Figure 1, the separator of Example 1 had an appropriate level of phase separation, and it was confirmed that the binder resin was distributed within the inorganic coating layer and did not migrate excessively to the surface of the inorganic coating layer. Furthermore, analysis of electron microscope images confirmed that the average pore size of the inorganic coating layer was in the range of 20 nm to 800 nm.

[0089] The electrical resistance of the separation membrane obtained in this example was measured to be about 0.9 Ω, and the peel force was good at 65 gf / 25 mm.

[0090] Example 2 A separator was prepared in the same manner as in Example 1, except that PVdF-TrFE (Sigma-Aldrich, Solvene 200, Tm: 131°C, electrolyte absorption: 27%, TrFE 20 mol%) was used. The crystallization temperature of the PVdF-TrFE was 119°C, and the crystallinity was 32.4%. The resistance of the obtained separator was 0.92 Ω, and the peel strength was 60 gf / 25 mm.

[0091] Example 3 A separator was prepared in the same manner as in Example 1, except that 9 g of PVdF-TFE (Daikin, VT-475) used in Example 1 was mixed with 9 g of PVdF-TrFE (Sigma-Aldrich, Solvene 200) used in Example 2. The resistance of the obtained separator was 0.91 Ω, and the peel strength was 63 gf / 25 mm.

[0092] Comparative Example 1 A separator was manufactured in the same manner as in Example 1, except that PVdF-HFP (Solvay Solef 21510, Tm: 132°C, electrolyte absorption: 110%, HFP 13 mol) was used as the binder. The crystallization temperature of the PVdF-HFP was 134°C, and the crystallinity was 25.4%. Figure 2 is an electron microscope image of the surface of the inorganic coating layer of the separator obtained in Comparative Example 1. It was confirmed that a large amount of the binder resin component had migrated excessively to the surface of the inorganic coating layer in the separator of Comparative Example 1, forming a layer with a high concentration of binder resin on the surface.

[0093] The electrical resistance of the obtained separation membrane was measured and found to be approximately 1.1 Ω. The peel force was 71 gf / 25 mm.

[0094] Comparative Example 2 A separator was manufactured in the same manner as in Example 1, except that PVdF-HFP (Arkema, Kynar 2500, Tm: 125, electrolyte absorption: 145%, HFP 16 mol%) was used as the binder. The crystallization temperature of the PVdF-HFP was 132°C, and the crystallinity was 23.7%. The electrical resistance of the obtained separator was measured and found to be approximately 1.2 Ω. The peel strength was a good 76 gf / 25 mm.

[0095] Comparative Example 3 A separator was manufactured in the same manner as in Example 1, except that PVdF-HFP (Kureha, 8200, Tm: 155°C, electrolyte absorption: 29%, HFP 2 mol%) was used as the binder. The crystallization temperature of the PVdF-HFP was 139°C, and the crystallinity was 28.1%. The electrical resistance of the obtained separator was measured to be about 0.94 Ω, and the peel strength was 45 gf / 25 mm.

[0096] Electrolyte absorption rate The electrolyte absorption rate (%) was determined by immersing each separator in the electrolyte for 72 hours and then measuring the weight change rate according to Equation 1. [Number 1] Electrolyte absorption rate (%) = [(weight of polymer resin after immersion - initial weight of polymer resin) / initial weight of polymer resin] x 100

[0097] When measuring the electrolyte absorption rate, the electrolyte used was an organic solvent in which ethylene carbonate (EC) and propyl propionate (PP) were mixed in a ratio (volume ratio) of 30:70.

[0098] How to measure resistance The resistance of the separators of each example and comparative example was measured as follows. An electrolyte was prepared by dissolving LiPF6 at 1 molar (M) in a solvent containing a 25:10:65 volume ratio of ethylene carbonate, propylene carbonate, and propyl propionate. After immersing each separator in the electrolyte, the electrical resistance was measured using a multi-probe analyzer (Hioki Corporation).

[0099] Peel strength (adhesion strength) evaluation method The separator samples obtained in each Example or Comparative Example (e.g., Example 1) were cut into pieces measuring 100 mm (length) x 25 mm (width) to prepare two test pieces for each. Two test pieces were stacked together and then hot-pressed at 100°C for 10 seconds. The resulting laminate was then fixed to an adhesive strength tester (LLOYD Instrument, LF plus). The upper separator test piece was peeled off at a 180° angle at 25°C and a rate of 25 mm / min, and the strength at this time was measured.

[0100] [Table 1]

[0101] The results of the electrical resistance and peel strength tests for the separators obtained in the examples and comparative examples are shown in Table 1. The separators according to the examples of the present invention exhibited low electrical resistance and appropriate peel strength. Meanwhile, the separators according to comparative examples 1 and 2 exhibited good peel strength but high electrical resistance, and the separator according to comparative example 3 exhibited electrical resistance values ​​similar to those of the separators according to the examples of the present invention but exhibited reduced peel strength.

Claims

1. A battery comprising an electrode assembly and an electrolyte, The electrode assembly is an electrochemical device including a positive electrode, a negative electrode, and a separator sandwiched between the positive electrode and the negative electrode, The separation membrane is The present invention includes a porous substrate and an inorganic coating layer formed on at least one surface of the porous substrate, the inorganic coating layer includes a binder resin and inorganic particles, the binder resin is a polyvinylidene fluoride resin containing at least one polymerization unit selected from the group consisting of trifluoroethylene and tetrafluoroethylene, and the polyvinylidene fluoride resin has a substitution rate with the polymerization unit of 14 mol % to 30 mol %; The polyvinylidene fluoride resin has a melting point of 145°C or less, a crystallization temperature of 117°C or more and 120°C or less, and a crystallinity of 30% to 45%; The inorganic coating layer has a binder resin content that increases toward a surface portion thereof, and exhibits a mixed form of inorganic particles and binder resin in a thickness direction of the inorganic coating layer, and the lower portion of the surface portion has a porous property due to pores caused by interstitial volumes between the inorganic particles, and is a separator for an electrochemical device, The electrochemical element, wherein the electrolyte solution contains an organic solvent and a lithium salt, and the organic solvent contains 30% by weight or more of an ester-based compound relative to 100% by weight of the organic solvent.

2. 2. The electrochemical device according to claim 1, wherein the polyvinylidene fluoride resin has an electrolyte absorption rate of 30% or less according to the following mathematical formula 1: [Equation 1] Electrolyte absorption rate (%) = [(weight of polymer resin after immersion - initial weight of polymer resin) / (initial weight of polymer resin)] x 100

3. 2. The electrochemical element according to claim 1, wherein the binder resin and the inorganic particles in the inorganic coating layer are contained in a weight ratio of 15:85 to 50:

50.

4. 2. The electrochemical device according to claim 1, wherein the inorganic coating layer has an average pore size of 20 nm to 800 nm.

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