KR20220057374A - Method for manufacturing separator, separator manufactured by the method and method for manufacturing electrochemical device including the separator - Google Patents

Abstract

The present invention relates to a method for manufacturing a separator for an electrochemical device, including a step of impregnating a porous substrate with a pretreatment solution containing first and second solvents having different non-solvent exchange rates, and a separator for an electrochemical device manufactured by the method. According to the present invention, formation of dendrites is suppressed.

Description

Method for manufacturing a separation membrane, a separation membrane formed therefrom, and an electrochemical device having the same

The present invention relates to a method for manufacturing a separator for an electrochemical device such as a lithium secondary battery, a separator formed therefrom, and an electrochemical device having the same, and more particularly, to a porous organic/inorganic composite comprising a mixture of inorganic particles and a binder polymer It relates to a method for manufacturing a separator in which a porous layer is coated on at least one surface of a porous substrate, a separator formed therefrom, and an electrochemical device having the same.

Recently, interest in energy storage technology is increasing. Efforts for research and development of electrochemical devices are becoming more concrete as the field of application expands to cell phones, camcorders, notebook PCs, and even the energy of electric vehicles. Electrochemical devices are receiving the most attention in this aspect, and among them, the development of rechargeable batteries that can be charged and discharged is the focus of interest. and research and development of battery design.

Among the currently applied secondary batteries, lithium secondary batteries developed in the early 1990s have a higher operating voltage and significantly higher energy density than conventional batteries such as Ni-MH, Ni-Cd, and lead sulfate batteries using aqueous electrolyte solutions. is in the spotlight as However, these lithium ion batteries have safety problems such as ignition and explosion due to the use of an organic electrolyte, and are difficult to manufacture. Recent lithium ion polymer batteries have improved the weaknesses of lithium ion batteries and are considered one of the next-generation batteries. This is urgently required.

Although the above electrochemical devices are being produced by many companies, their safety characteristics show different aspects. It is very important to evaluate the safety and secure the safety of these electrochemical devices. The most important consideration is that when an electrochemical device malfunctions, it should not cause injury to users, and for this purpose, safety standards strictly regulate ignition and fuming within the electrochemical device. In the safety characteristics of the electrochemical device, there is a high risk of causing an explosion when the electrochemical device is overheated and thermal runaway occurs or the separator is penetrated. In particular, polyolefin-based porous membranes commonly used as separators for electrochemical devices exhibit extreme thermal contraction behavior at a temperature of 100 degrees or more due to material properties and characteristics of the manufacturing process including stretching, thereby preventing a short circuit between the anode and the cathode. There is a problem that causes

In order to solve the safety problem of the electrochemical device, a separation membrane in which a porous organic/inorganic composite porous layer is formed by coating a mixture of inorganic particles and a binder polymer on at least one surface of a porous substrate having a plurality of pores has been proposed. For example, in Korean Patent Application Laid-Open No. 2007-0019958, inorganic particles are dispersed on a porous substrate, and a slurry in which a binder polymer is dissolved in a solvent is coated on a porous substrate such as a polyolefin membrane and dried to form a porous organic/inorganic composite porous layer. A manufacturing method for a separation membrane prepared on a porous substrate is disclosed.

In the separator in which the organic/inorganic composite porous layer is formed, the inorganic particles present in the porous layer formed on the porous substrate act as a kind of spacer that can maintain the physical shape of the porous layer, so that the porous substrate is heated when the electrochemical device is overheated. It suppresses shrinkage or prevents short circuit of both electrodes during thermal runaway. In addition, voids exist between the inorganic particles to form micropores.

As such, the organic/inorganic composite porous layer contributes to the thermal stability of the separator, but when the organic/inorganic composite porous layer is formed, the binder polymer flows into the pores of the porous substrate and blocks some of the pores, thereby increasing the resistance of the separator There is this. In addition, in the case of a porous substrate, if the size of the pores is large, the inflow of the electrolyte is promoted, but mechanical strength may be lowered and heat resistance stability may be lowered. In order to solve this problem, a method of controlling the size of the pores to the nanometer level has been proposed, but there is a problem in that the inflow of the electrolyte into the micropores is lowered, so that the resistance characteristics and the ionic conductivity characteristics are lowered.

Therefore, the problem to be solved by the present invention is to solve the above problems, and a method for manufacturing a separation membrane capable of minimizing the clogging of pores of a porous substrate by a binder polymer when forming an organic/inorganic composite porous layer, and a separation membrane formed therefrom and to provide an electrochemical device having the same.

A first aspect of the present invention relates to a method for manufacturing a separator for an electrochemical device, the method comprising:

(S1) preparing a porous substrate;

(S2) impregnating the porous substrate with a pretreatment solution

(S3) applying a slurry for forming an organic/inorganic composite porous layer on at least one surface of the pretreated porous substrate;

(S4) step of immersing the resultant of (S3) in a coagulating solution containing a non-solvent to solidify the slurry;

Including the inorganic slurry particles and binder resin for forming the organic/inorganic composite porous layer,

The binder resin includes a polyvinylidene fluoride (PVDF)-based binder resin,

The pretreatment solution includes first and second solvents having different non-solvent exchange rates,

The first solvent has a low exchange rate for nonsolvent compared to NMP, and the second solvent is the same as or more than NMP.

A second aspect of the present invention is that, in the first aspect, the pretreatment solution contains the first solvent in a ratio of 0.1 wt% to 50 wt% relative to 100 wt% of the pretreatment solution.

In a third aspect of the present invention, in the first or second aspect, the first solvent includes at least one of a linear carbonate-based compound, a cyclic carbonate-based compound, a linear ester-based compound, and a nitrile-based compound.

A fourth aspect of the present invention, in the third aspect, the linear carbonate-based compound is at least one selected from dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, and ethylpropyl carbonate will include

A fifth aspect of the present invention, in the third aspect, the linear ester compound is methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate and butyl propionate It is to include one or more selected from among.

A sixth aspect of the present invention, in the third aspect, the cyclic carbonate-based compound is ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate , 2,3-pentylene carbonate, vinylene carbonate, and one or more selected from fluoroethylene carbonate.

A seventh aspect of the present invention, in the third aspect, the nitrile-based compound is succinonitrile (succinonitrile), glutaronitrile (glutaronitrile), adiponitrile (adiponitrile), pimelonitrile (pimelonitrile) and water It will include at least one selected from among veronitrile (suberonitrile).

In an eighth aspect of the present invention, in any one of the first to seventh aspects, the second solvent is acetone, tetrahydrofuran, methylene chloride, chloroform, It will include one or more selected from dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, and water.

Claim 9 of the present invention relates to a separation membrane prepared by any one of the first to eighth aspects, wherein the separation membrane includes a porous substrate including a plurality of pores and organic/inorganic composite pores formed on both sides of the substrate. Including a layer, the difference in peel strength between the porous substrate and the organic / inorganic composite porous layer on both sides of the separator is less than 10%, and the difference in peel strength is calculated by Equation 1 below.

[Formula 1]

Difference in peel strength (%) = (|Peel strength of the first side-Peel strength of the second side| / (the larger of the peel strength of the first side and the peel strength of the second side)) x 100.

A tenth aspect of the present invention is the ninth aspect, wherein the organic/inorganic composite porous layer includes a binder resin and inorganic particles, has a plurality of micropores therein, and has a structure in which these micropores are connected, , which has a porous layer structure in which gas or liquid can pass from one surface to the other.

The separation membrane prepared according to the method of the present invention exhibits the following characteristics.

First, the porous organic/inorganic composite porous layer suppresses thermal shrinkage of the porous substrate during overheating of the electrochemical device, and prevents short circuit of both electrodes during thermal runaway.

Second, when the organic/inorganic composite porous layer is formed, the phenomenon that the binder polymer in the slurry flows into the pores of the porous substrate is improved, so that the clogging of the pores of the porous substrate is minimized. Accordingly, the increase in the resistance of the separator due to the formation of the porous organic/inorganic composite porous layer is reduced.

Third, the wettability after injection is improved by the wettability improving material impregnated in the porous substrate, thereby preventing capacity reduction and dendrite formation.

Fourth, it is possible to manufacture a separation membrane with a small difference in peel strength between the porous substrate and the organic/inorganic composite porous layer on both sides of the separation membrane.

Hereinafter, the present invention will be described in detail. Prior to this, the terms or words used in the present specification and claims should not be construed as being limited to conventional or dictionary meanings, and the inventor should properly understand the concept of the term in order to best describe his invention. Based on the principle that it can be defined, it should be interpreted as meaning and concept consistent with the technical idea of the present invention. Therefore, the configuration described in the embodiment described in this specification is only the most preferred embodiment of the present invention and does not represent all the technical spirit of the present invention, so at the time of the present application, various equivalents and It should be understood that there may be variations.

Throughout this specification, when a part "includes" a certain component, it means that other components may be further included, rather than excluding other components, unless otherwise stated.

In addition, the terms "about", "substantially", etc. used throughout this specification are used as meanings at or close to the numerical values when manufacturing and material tolerances inherent in the stated meaning are presented to help the understanding of the present application. It is used to prevent an unconscionable infringer from using the mentioned disclosure in an unreasonable way.

Throughout this specification, the description of “A and/or B” means “A or B or both”.

The present invention relates to a method for manufacturing a separator for an electrochemical device. In the present invention, the electrochemical device is a device that converts chemical energy into electrical energy by an electrochemical reaction, and is a concept including a primary battery and a secondary battery, and the secondary battery is capable of charging and discharging. , a concept encompassing a lithium ion battery, a nickel-cadmium battery, a nickel-hydrogen battery, and the like as specific examples thereof.

A separation membrane prepared according to the method for manufacturing a separation membrane according to the present invention includes a porous substrate including a plurality of pores and an organic/inorganic composite porous layer formed on at least one surface or both surfaces of the porous substrate. The organic/inorganic composite porous layer includes a binder resin and inorganic particles, has a plurality of micropores therein, has a structure in which these micropores are connected, and gas or liquid flows from one side to the other. It can have the structure of the porous layer which became passable. The micropores may be derived from an interstitial volume filled with inorganic particles and formed between the inorganic particles. Meanwhile, in one embodiment of the present invention, a region (adhesive portion) having a relatively high content of the binder resin may be disposed on the surface of the organic/inorganic composite porous layer. In the manufacturing method described below, the adhesive part is integrally and inseparably bonded to the surface of the organic/inorganic composite porous layer as a result of the phase separation action of the binder resin during drying of the organic/inorganic composite porous layer. In addition, the bonding portion includes micropores formed as a result of the phase separation action.

A detailed description of the separation membrane manufacturing method of the present invention is as follows.

First, a porous substrate having pores is prepared (step S1).

As such a porous substrate, any porous substrate commonly used in electrochemical devices, such as a porous membrane formed of various polymers or a nonwoven fabric, may be used. For example, a polyolefin-based porous membrane used as a separator for an electrochemical device, in particular, a lithium secondary battery, a nonwoven fabric made of polyethylene terephthalate fiber, etc. can be used, and the material and shape can be variously selected according to the purpose. For example, a polyolefin-based porous membrane (membrane) is a polyolefin-based polymer such as polyethylene, polypropylene, polybutylene, polypentene, such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene, each alone or a mixture thereof. It can be formed of a polymer, and the nonwoven fabric can also be made of a fiber using a polyolefin-based polymer or a polymer having higher heat resistance than this. The thickness of the porous substrate is not particularly limited, but is preferably 1 to 100 μm, more preferably 5 to 50 μm, and the pore size and pores present in the porous substrate are also not particularly limited, but each of 0.001 μm to 50 μm and It is preferably 10 vol% to 95 vol%.

Then, the porous substrate is impregnated by immersing it in the pretreatment solution (step S2).

In the present invention, the pretreatment solution includes two types of first and second solvents having different exchange rates with respect to the nonsolvent. The first solvent has a lower exchange rate with respect to the nonsolvent than the second solvent.

The first solvent has a lower exchange rate with respect to the non-solvent than the second solvent, so even after drying the porous layer of the separator, the second solvent remains in the porous substrate, unlike the removal of the porous substrate, and the wettability improving additive of the porous substrate ( Wetting agent).

In the total lysis solution, the first solvent may be included in a ratio of 0.1 wt% to 50 wt% compared to 100 wt% of the pretreatment solution.

In a specific embodiment of the present invention, the first solvent may have a lower exchange rate with respect to nonsolvent than NMP, and the second solvent may have an exchange rate equal to or greater than NMP.

Non-limiting examples of the first solvent include a linear carbonate-based compound, a cyclic carbonate-based compound, a linear ester-based compound, and a nitrile-based compound, and may include at least one of them.

The linear ester-based compound may include at least one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate and butyl propionate.

The cyclic carbonate-based compound is ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, and fluoro Any one selected from the group consisting of ethylene carbonate or two or more of them may be included.

The linear carbonate-based compound may include any one selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, and ethylpropyl carbonate, or two or more of these.

The nitrile-based compound is selected from the group consisting of succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, suberonitrile, and combinations thereof. Any one or two or more of these may be included.

On the other hand, in a specific embodiment of the present invention, the second solvent is capable of dissolving the binder resin in the step (S3) to be described later, and preferably has a solubility index similar to that of the binder polymer to be used.

In the present invention, non-limiting examples of the second solvent that can be used include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N- and methyl-2-pyrrolidone (N-methyl-2-pyrrolidone, NMP), cyclohexane, water, and the like, and may include one or two or more of them.

In the present invention, the exchange rate can be measured in the following way.

First, a mixed solution in which the first solvent and the second solvent are mixed is prepared, the porous substrate is immersed therein, and then taken out (A1). Thereafter, an organic/inorganic composite porous layer is formed on the surface of the porous substrate, and the resultant result is prepared as an exchange rate measurement specimen (A2). Thereafter, the specimen is immersed in a non-solvent for a predetermined time, taken out (A3), and the result is analyzed with a photoelectron spectrometer (A4), and the content of the first solvent and the second solvent remaining in the specimen is measured (A5) .

In the step (A1), the first solvent may be included in an amount of 50 wt% compared to 100 wt% of the mixed solution. For example, the first solvent and the second solvent may be included in a ratio of 10:90 by weight. In addition, the immersion time in A1 may be performed for less than 1 minute, for example, about 30 seconds. On the other hand, after taking out the porous substrate, the surface of the porous substrate may be wiped with a scraper, and then subsequent steps may be performed. For the method of forming the organic/inorganic composite porous layer in step (A2), reference may be made to the contents described below. In one embodiment of the present invention, the organic / inorganic composite porous layer may be an actual manufacturing object. Meanwhile, as the non-solvent in step (A3), water or a non-solvent used in the production of the actual separation membrane may be applied. The immersion time in the nonsolvent in (A3) may be 1 minute or less. Meanwhile, a washing step may be performed after taking out in step (A3). In addition, when the content of the first solvent in the separation membrane is higher than the content of the second solvent as a result of the measurement in step (A5), the first and second solvents may be selected and applied to the actual production of the separation membrane.

In a specific embodiment of the present invention, the measurement of the exchange rate may be utilized to select a pretreatment solution to be used in the actual manufacture of the separation membrane. That is, except for the first solvent and the second solvent, after establishing the process conditions for the preparation of the separation membrane, the established process conditions can be applied to the measurement of the exchange rate, and when measuring the exchange rate, various first solvents and agents 2 Appropriate ones can be selected and applied from the candidate group of solvents. In one embodiment of the present invention, as described above, when NMP is used as the second solvent in the exchange rate measurement, a component having a lower exchange rate for a non-solvent than NMP, that is, more remaining in the specimen than NMP. The component to be used can be used as the first solvent.

As such, as the porous substrate is impregnated with the pretreatment solution, at least some or all of the pores of the porous substrate are filled with the pretreatment solution. In particular, since the pores located on the surface layer of the porous substrate are preferentially filled by the pretreatment solution, even if the slurry for forming a porous layer is applied to the surface of the porous substrate in a step to be described later, the slurry does not flow into the pores of the porous substrate, and the pretreatment solution Since the second solvent in the coagulation tank is exchanged with a nonsolvent or removed through a drying process, the pores of the porous substrate may be maintained as empty spaces in the finally obtained separation membrane or may be filled with the first solvent in the pretreatment solution. In addition, the pores may be filled with an electrolyte after the electrolyte is injected to provide a path for ion transfer during charging and discharging of the battery.

Next, a slurry for forming an organic/inorganic composite porous layer is applied to the surface of the resultant of (S2) (step S3). The slurry may be applied and dried on at least one or both surfaces of the porous substrate to be formed on both surfaces of the substrate.

In the slurry for forming the porous layer, inorganic particles are dispersed and the binder resin is dissolved in the third organic solvent.

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 not particularly limited as long as oxidation and/or reduction reactions do not occur in the operating voltage range of the applied electrochemical device (eg, 0 to 5V based on Li/Li ⁺ ). In particular, when inorganic particles having a high dielectric constant are used as the inorganic particles, the ionic conductivity of the electrolyte can be improved by contributing to an increase in the degree of dissociation of an electrolyte salt, such as a lithium salt, in a liquid electrolyte.

For the above reasons, the inorganic particles preferably 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 BaTiO ₃ , Pb(Zr,Ti)O ₃ (PZT), Pb _1-x La _x Zr _1-y Ti _y O ₃ (PLZT, where 0 < x < 1, 0 < y < 1), Pb(Mg _1/3 Nb _2/3 )O ₃ -PbTiO ₃ (PMN-PT), Hafnia (HfO ₂ ), SrTiO ₃ , SnO ₂ , CeO ₂ , MgO , NiO, CaO, ZnO, ZrO ₂ , Y ₂ O ₃ , Al ₂ O ₃ , TiO _{2 ,} SiC or mixtures thereof.

In addition, as the inorganic particles, inorganic particles having lithium ion transport ability, that is, inorganic particles containing lithium element but not storing lithium and having a function of transporting lithium ions may be used. Non-limiting examples of inorganic particles having lithium ion transport ability include 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), 14Li ₂ O-9Al ₂ O ₃ -38TiO ₂ -39P ₂ (LiAlTiP) _x O _y series glass such as O ₅ (0 < x < 4, 0 < y < 13), lithium lanthanum titanate (Li _x La _y TiO ₃ , 0 < x < 2, 0 < y < 3) Lithium germanium thiophosphate such as , Li _3.25 Ge _0.25 P _0.75 S ₄ , etc. (Li _x Ge _y P _z S _w , 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5 ), Li ₃ N, etc. Lithium nitride (Li _x N _y , 0 < x < 4, 0 < y < 2), Li ₃ PO ₄ -Li ₂ S-SiS ₂ SiS ₂ based glass (Li _x Si P ₂ S ₅ series glass, such as _y S _z , 0 < x < 3, 0 < y < 2, 0 < z < 4), LiI-Li ₂ SP ₂ S ₅ , etc. (Li _x P _y S _z , 0 < x < 3, 0 < y < 3, 0 < z < 7) or mixtures thereof.

In addition, the average particle diameter of the inorganic particles is not particularly limited, but is preferably in the range of 0.001 to 10 μm for the formation of a coating layer having a uniform thickness and an appropriate porosity. If it is less than 0.001 μm, dispersibility may be reduced, and if it exceeds 10 μm, the thickness of the coating layer formed may increase.

The binder resin includes a polyvinylidene fluoride (PVdF)-based binder resin, and in 100 wt% of the total binder resin, the PVdF-based binder resin is 80 wt% or more, 90 wt% or more, or 99 wt% or more, and in one embodiment In this case, the total amount of the binder resin may be PVdF-based binder resin. In one embodiment of the present invention, the PVdF-based binder resin has a weight average molecular weight of 600,000 or less, 400,000 or less, or 300,000 or less. When the weight average molecular weight is 600,000 or less, flexibility is increased, which is advantageous in improving adhesion. In addition, when a low molecular weight PVdF-based binder resin is used, solubility in various solvents is increased, thereby increasing the degree of freedom in solvent selection, and pores are more uniformly formed during phase separation, which is advantageous for low resistance. Therefore, when this point is considered, it is preferable that a weight average molecular weight is 400,000 or less. However, when the weight average molecular weight is less than 50,000, the PVdF-based binder resin may have a weight average molecular weight of 50,000 or more because it is dissolved in the electrolyte to increase the viscosity of the electrolyte and thereby decrease ionic conductivity. Here, the weight average molecular weight of the PVdF-based binder resin can be determined by gel permeation chromatography (GPC method).

In the present invention, the PVdF-based binder resin may be a homopolymer of vinylidene fluoride (ie, polyvinylidene fluoride), a copolymer of vinylidene fluoride and other copolymerizable monomers, or a mixture thereof. Examples of the monomer copolymerizable with vinylidene fluoride include tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, chlorofluoroethylene, 1,2 difluoroethylene, perfluoro(methylvinyl)ether, purple Luoro (ethylvinyl) ether, perfluoro (propylvinyl) ether, more fluoro (1,3 dioxole), perfluoro (2,2-dimethyl-1,3-dioxole), trichloroethylene and One type or two or more types selected from vinyl fluoride and the like may be included. In one embodiment of the present invention, the PVdF-based binder resin has a Tm of 150° C. or less, preferably a Tm of 140° C. or less, in terms of adhesive strength during heat bonding. To this end, the binder resin may include a copolymer containing 70 mol% or more of vinylidene fluoride as a polymerization unit, and in this case, the copolymer has a degree of substitution by other copolymerizable monomers as described above of 5 mol% or more or 8 mol% or more. The PVdF-based binder resin having a relatively low molecular weight as described above can be preferably obtained by emulsion polymerization or suspension polymerization, particularly preferably suspension polymerization.

On the other hand, in one specific embodiment of the present invention, the binder resin, if necessary, polymethylmethacrylate (polymethylmethacrylate), polybutylacrylate (polybutylacrylate), polyacrylonitrile (polyacrylonitrile), polyvinylpyrrolidone ( polyvinylpyrrolidone), polyvinylacetate, ethylene vinyl acetate copolymer (polyethylene-co-vinyl acetate), polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate ( cellulose acetate butyrate), cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose , Pullulan (pullulan), carboxyl methyl cellulose (carboxyl methyl cellulose) at least one component may be further included in addition.

The weight ratio of the inorganic particles to the binder resin is, for example, preferably in the range of 50:50 to 99:1, more preferably 70:30 to 95:5. When the content ratio of the inorganic particles to the binder resin is less than 50:50, the content of the polymer increases, so that the pore size and porosity of the coating layer formed may be reduced. When the content of the inorganic particles exceeds 99 parts by weight, the peeling resistance of the formed coating layer may be weakened because the binder polymer content is small.

In the present invention, the third organic solvent means a solvent capable of dissolving the binder polymer. As a solvent for the binder polymer, it is preferable that the solubility index is similar to that of the binder polymer to be used, and the solvent has a low boiling point. This is to facilitate uniform mixing and subsequent solvent removal. Non-limiting examples of the third solvent that can be used include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrole money (N-methyl-2-pyrrolidone, NMP), cyclohexane, water, or a mixture thereof. The third solvent may be the same as or different from the second solvent described above.

In the separator manufacturing method of the present invention, in consideration of ease of coating and prevention of gelation of the binder polymer, the difference in solubility indices between the binder polymer, the second solvent, and the third solvent is preferably 5.0 Mpa ^1/2 or less. In this aspect, it is more preferable to use the same kind of solvent as the second solvent and the third solvent.

The slurry in which the inorganic particles are dispersed and the binder polymer is dissolved in the third solvent may be prepared by dissolving the binder polymer in the third solvent, then adding the inorganic particles and dispersing the same, but is not limited thereto. The inorganic particles may be added in a crushed state to an appropriate size, but it is preferable to disperse the inorganic particles while crushing them using a ball mill method after adding the inorganic particles to the solution of the binder polymer.

The coating thickness of the slurry to be coated on the porous substrate is preferably adjusted so that the thickness of the porous organic/inorganic composite porous layer finally formed after drying is 0.1 μm to 20 μm, in consideration of the over-resistance of improving the stability of the battery. .

The application of the slurry according to step (S3) can be performed continuously or discontinuously by applying one or more suitable methods among various known methods such as doctor blade, impregnation (dip coating), slot die coating, slide coating, and curtain coating. can

Next, the resultant of the step (S3) is immersed in a coagulating solution containing a non-solvent to solidify the slurry (S4). Thereby, a porous organic/inorganic composite porous layer is formed on the porous substrate. The slurry is solidified by exchanging the second and third solvents with the non-solvent in the coagulating solution.

In the present invention, the non-solvent may have a solubility of less than 5 wt% of the PVdF-based resin at 25°C. As the non-solvent, one or more selected from water, methanol, ethanol, propyl alcohol, butyl alcohol, butanediol, ethylene glycol, propylene glycol, and tripropylene glycol may be used.

In one embodiment of the present invention, by the phase separation of the above step, the binder resin is moved to the surface layer portion of the organic/inorganic composite porous layer of the separator, so that an adhesive portion having a high binder resin content may be formed in the surface layer portion. The adhesive portion is formed by the phase separation and is formed in an integral and indivisible form on the surface layer of the organic/inorganic composite porous layer, and is not physically distinguishable from the organic/inorganic composite porous layer.

On the other hand, the first solvent included in the pretreatment solution in step (S4) has a lower exchange rate with the nonsolvent than the second solvent and remains in the pores. In one embodiment of the present invention, the remaining first solvent may, for example, be attached to the pore walls or fill all or part of the pores. The first solvent is a component having high affinity with the organic solvent of the electrolyte, as will be described later, and serves to assist the electrolyte from flowing into the porous substrate. Accordingly, the separation membrane manufactured by the manufacturing method of the present invention has an effect of improving the wettability by the electrolyte.

In addition, during the solidification process of the slurry, the inorganic particles exist in a substantially close contact state, and as they are connected and fixed to each other by the formed binder polymer coating layer, empty spaces are formed between the inorganic particles to form pores.

As such, diffusion of the binder polymer in the slurry into the pores of the porous substrate is minimized due to the pretreatment solution. Due to this, since the clogging of the pores of the porous substrate by the binder polymer in the slurry is minimized, the problem of increasing the resistance of the separator due to the formation of the porous organic/inorganic composite porous layer is improved.

In a specific embodiment of the present invention, after the polymer binder resin is solidified, the process of additionally washing the separation membrane with water or the like may be performed once or twice or more multiple times. In addition, a drying process may be performed to remove the liquid used for washing after the washing process.

In a specific embodiment of the present invention, in the method for manufacturing the separation membrane, the separation membrane manufacturing process may be performed as a continuous process by a conveying means such as a conveyor belt or roll-to-roll.

In addition, the present invention relates to a separator for an electrochemical device manufactured according to the above manufacturing method. The separation membrane, as described above, includes a porous substrate including a plurality of pores and an organic/inorganic composite porous layer formed on at least one surface or both surfaces of the porous substrate. The organic/inorganic composite porous layer includes a binder resin and inorganic particles, has a plurality of micropores therein, has a structure in which these micropores are connected, and gas or liquid flows from one side to the other. It can have the structure of the porous layer which became passable. The micropores may be derived from an interstitial volume filled with inorganic particles and formed between the inorganic particles. In addition, in one embodiment of the present invention, the organic / inorganic composite porous layer may include an adhesive part, and the description of the adhesive part refers to the foregoing.

On the other hand, in the separator according to the present invention, the organic/inorganic composite porous layer includes a binder resin and and inorganic particles on both sides of the porous substrate, and the difference in peel strength between the porous substrate and the organic/inorganic composite porous layer on both sides is small. will be. In one embodiment of the present invention, the difference in peel strength is less than 10%, preferably less than 7%. The peel strength can be measured by the following [Equation 1].

[Formula 1]

Difference in peel strength (%) = (|Peel strength of the first side-Peel strength of the second side| / (the larger of the peel strength of the first side and the peel strength of the second side)) x 100

In the separator of the present invention, the difference in the peel strength of both sides of the separator is small by impregnating the porous substrate with the pretreatment solution so that the surface state of both surfaces of the porous substrate before application of the slurry is even, resulting in a porous layer coated on both sides of the porous substrate and The peel strength of , shows similar values.

Meanwhile, the present invention provides an electrochemical device including a separator manufactured according to the above-described method. The electrochemical device includes all devices that undergo an electrochemical reaction, and specific examples thereof include all kinds of primary, secondary batteries, fuel cells, solar cells, or capacitors such as supercapacitor devices. In particular, 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 is preferable.

The electrochemical device may be, for example, a lithium ion secondary battery. The lithium ion secondary battery may be manufactured by, for example, preparing a positive electrode and a negative electrode, and winding or laminating with a separator obtained by the above-described method interposed therebetween.

The positive electrode and the negative electrode to be applied together with the separator of the present invention are not particularly limited, and the electrode active material may be prepared in a form bound to the electrode current collector according to a conventional method known in the art. Among the electrode active materials, non-limiting examples of the positive electrode active material include conventional positive electrode active materials that can be used in positive electrodes of conventional electrochemical devices, and in particular, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a combination thereof. It is preferable to use one lithium composite oxide. As a non-limiting example of the negative electrode active material, a conventional negative electrode active material that can be used for the negative electrode of a conventional electrochemical device can be used, and in particular, lithium metal or lithium alloy, carbon, petroleum coke, activated carbon, A lithium adsorbent material such as graphite or other carbons is preferable. Non-limiting examples of the positive current collector include a foil made of aluminum, nickel, or a combination thereof, and non-limiting examples of the negative current collector include copper, gold, nickel, or a copper alloy or a combination thereof. There are manufactured foils and the like.

The electrolyte solution that can be used in the electrochemical device of the present invention has an ionized form/dissociated form by including a salt having the same structure as A ⁺ B ⁻ in the fourth solvent. In the above salt, A ⁺ includes an ion consisting of an alkali metal cation such as Li ⁺ , Na ⁺ , K ⁺ or a combination thereof, and B ^- is PF ₆ ^- , BF ₄ ^- , Cl ^- , Br ^- , I ^- , ClO Including ions consisting of anions such as ₄ ^- , AsF ₆ ^- , CH ₃ CO ₂ ^- , CF ₃ SO ₃ ^- , N(CF ₃ SO ₂ ) ₂ ^- , C(CF ₂ SO ₂ ) ₃ ^- or combinations thereof salts are included.

The fourth solvent may refer to the description of the first solvent. Non-limiting examples of the fourth solvent include propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethyl ethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate (EMC), gamma butyrolactone (γ-butyrolactone), and the like. It may include one type or two or more types. However, the present invention is not limited thereto.

In a specific embodiment of the present invention, the fourth solvent is not particularly limited as long as it can be used as an organic solvent for an electrolyte of a secondary battery. For example, it is the same component as the first solvent or the same as the first solvent. ingredients may be included. Alternatively, the first solvent may be configured to include a fourth solvent to be used as an electrolyte component for a battery to which the separator is to be applied.

The electrolyte injection may be performed at an appropriate stage during the battery manufacturing process according to the manufacturing process and required physical properties of the final product. That is, it may be applied before assembling the battery or in the final stage of assembling the battery.

Hereinafter, examples will be given to describe the present invention in detail. However, the embodiments according to the present invention may be modified in various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided in order to more completely explain the present invention to those of ordinary skill in the art.

Example 1

A 9 μm thick polyethylene porous membrane (porosity 45%) was prepared. This was immersed in the pretreatment solution and taken out after 30 seconds. The pretreatment solution was prepared by mixing propylene carbonate and NMP, and the concentration of propylene carbonate as an additive was 10 wt%.

Next, PVdF-CTFE (polyvinylidene fluoride-chlorotrifluoroethylene copolymer) and Cyanoethylpullulan (cyanoethylpullulan) were added to acetone in a weight ratio of 10:2, respectively, and dissolved at 50° C. for about 12 hours or more. A polymer solution was prepared. Inorganic particles obtained by mixing Al ₂ O ₃ powder and BaTiO ₃ powder in a weight ratio of 9:1 were added to the prepared polymer solution so as to have a polymer/inorganic particle = 10/90 weight ratio, and then using a ball mill method for more than 12 hours. A slurry was prepared by crushing and dispersing inorganic particles. The average particle diameter of the inorganic particles of the slurry thus prepared was 600 nm.

The slurry was coated on one surface of the polyethylene porous membrane treated with the pretreatment solution. The coating amount of the slurry was such that the thickness of the finally formed porous organic/inorganic composite porous layer was 4 μm.

Then, the coated substrate was immersed in water as a solidified solution to induce phase separation and solidification of the binder resin. The temperature of the solidified liquid was 25°C. Thereafter, it was washed with water three times and dried in an oven at 80°C.

Example 2

A separation membrane was completed in the same manner as in Example 1, except that NMP containing 10 wt% of ethylene carbonate was used as a pretreatment solution. The coating amount of the slurry was adjusted so that the thickness of the finally formed porous organic/inorganic composite porous layer was 4 μm.

Comparative Example 1

A separation membrane was completed in the same manner as in Example 1, except that only the slurry was coated through a slot die without treatment with the pretreatment solution. The coating amount of the slurry was adjusted so that the thickness of the finally formed porous organic/inorganic composite porous layer was 4 μm.

How to measure resistance

For the separators of each Example and Comparative Example, resistance was measured in the following manner. An electrolyte solution was prepared by dissolving LiPF6 at a 1 molar concentration in a solvent in which ethylene carbonate, propylene carbonate, and propyl propionate were mixed at a ratio (volume ratio) of 25:10:65. After each separator was impregnated with the electrolyte, the electrical resistance was measured using a multi-probe analyzer (Hioki).

Peel strength (adhesive strength) evaluation method

Artificial graphite, conductive material (carbon black), CMC, and an acrylic copolymer as a binder are mixed in a weight ratio of 96:1:1:2dml and dispersed in water to prepare a negative electrode slurry, coated on a copper current collector, and dried and rolled Thus, an anode was prepared. A specimen was prepared by laminating the separator obtained in each Example or Comparative Example and the negative electrode at 60° C. under the condition of 1000 kgf. Each specimen obtained as described above was fixed to an adhesive strength measuring instrument (LLOYD Instrument, LF plus), peeled at an angle of 180° at 25° C. at a rate of 25 mm/min, and the strength at this time was measured.

air permeability measurement

The time (sec) through which 100 ml of air permeated was measured for each separation membrane. In a specific embodiment of the present invention, the ventilation time may be measured in accordance with JIS P8117.

Diameter change rate measurement

LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , PVdF and carbon black were mixed in a weight ratio of 97.0:1.5:1.5 and dispersed in 2-methyl-2-pyrrolidone to prepare a positive electrode slurry, which was then coated on an aluminum current collector. A positive electrode was prepared by drying and rolling. A specimen was prepared by laminating the separator obtained in each Example or Comparative Example and the positive electrode at 60° C. under the condition of 1000 kgf. 2 μl of the electrolyte solution was dropped on the surface of the specimen in a circular fashion, and the diameter change was observed after 5 minutes. Meanwhile, the diameter of the electrolyte solution on the surface immediately after dropping (A) and the diameter after 5 minutes after dropping (B) were measured, and the diameter change rate was calculated, and the results are shown in [Table 1] below. The rate of change in diameter is the percentage of the difference between A and B with respect to A.

breathability
(sec/100mL) Resistance (Ω) Peel strength (gf/25mm) diameter change rate
(%) obverse The back Difference in Peel Strength (%) Example 1 80 0.5 82 80 2.5% 165 Example 2 85 0.55 85 89 5% 158 Comparative Example 1 85 0.57 63 97 35% 78

As can be seen in Table 1, the separation membrane prepared according to the manufacturing method of the present invention has similar air permeability and resistance values as compared to the separation membrane of Comparative Example. However, the rate of change in diameter of the separator according to the embodiment of the present invention is significantly superior to that of the separator of the comparative example. The diameter change rate indicates the degree of electrolyte impregnation of the separator, and it means that the electrolyte impregnation rate of the separator according to the present invention has a higher effect than that of the separator according to the comparative example. According to these results, in the case of the battery to which the separator according to the present invention is applied, the electrolyte impregnation rate can be improved, and the electrochemical properties such as cycle characteristics and output characteristics can be improved. In addition, it was confirmed that the separation membrane of Examples did not have a large difference in peel strength between the porous substrate and the organic/inorganic composite porous layer on both sides of the separation membrane when compared with the separation membrane of Comparative Example.

Claims (10)

(S1) preparing a porous substrate;
(S2) impregnating the porous substrate with a pretreatment solution
(S3) applying a slurry for forming an organic/inorganic composite porous layer on at least one surface of the pretreated porous substrate;
(S4) step of immersing the resultant of (S3) in a coagulating solution containing a non-solvent to solidify the slurry;
Including the inorganic slurry particles and binder resin for forming the organic/inorganic composite porous layer,
The binder resin includes a polyvinylidene fluoride (PVDF)-based binder resin,
The pretreatment solution includes first and second solvents having different non-solvent exchange rates,
The first solvent has a low exchange rate for non-solvent compared to NMP, and the second solvent is the same as or exceeding NMP.
According to claim 1,
The pretreatment solution is a method of manufacturing a separator for an electrochemical device in which the first solvent is included in a ratio of 0.1 wt% to 50 wt% compared to 100 wt% of the pretreatment solution.
According to claim 1,
The first solvent is a method of manufacturing a separator for an electrochemical device comprising at least one of a linear carbonate-based compound, a cyclic carbonate-based compound, a linear ester-based compound, and a nitrile-based compound.
4. The method of claim 3,
The linear carbonate-based compound is a method of manufacturing an electrochemical device separator comprising at least one selected from dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, and ethylpropyl carbonate.
4. The method of claim 3,
The linear ester-based compound is for an electrochemical device comprising at least one selected from among methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate and butyl propionate A method for manufacturing a separation membrane.
4. The method of claim 3,
The cyclic carbonate-based compound is ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, and fluoro A method of manufacturing a separator for an electrochemical device comprising at least one selected from among ethylene carbonate.
4. The method of claim 3,
The nitrile-based compound is succinonitrile (succinonitrile), glutaronitrile (glutaronitrile), adiponitrile (adiponitrile), pimelonitrile (pimelonitrile) and suberonitrile (suberonitrile) to include at least one selected from the group consisting of A method for manufacturing a separator for an electrochemical device.
According to claim 1,
The second solvent is acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (N-methyl- 2-pyrrolidone, NMP), cyclohexane (cyclohexane), and a method of manufacturing a separator for an electrochemical device comprising at least one selected from water.
It is manufactured by the manufacturing method of claim 1,
A porous substrate comprising a plurality of pores and an organic/inorganic composite porous layer formed on both sides of the substrate, wherein the difference in peel strength between the porous substrate and the organic/inorganic composite porous layer on both sides of the separator is less than 10%, and The difference in peel strength is a separator for an electrochemical device that is calculated by Equation 1 below:
[Formula 1]
Difference in peel strength (%) = (|Peel strength of the first side-Peel strength of the second side| / (the larger of the peel strength of the first side and the peel strength of the second side)) x 100.
10. The method of claim 9,
The organic/inorganic composite porous layer includes a binder resin and inorganic particles, has a plurality of micropores therein, has a structure in which these micropores are connected, and gas or liquid flows from one side to the other. A separator for an electrochemical device that has a structure of a porous layer that can pass through.