WO2016159658A1 - Cellulose-based multilayer separation membrane - Google Patents

Abstract

The present invention relates to a separation membrane for a secondary battery which is capable of improving a shut-down function of a cellulose-based multilayer separation membrane physically having high strength, and provides a separation membrane for a secondary battery comprising: a base layer formed of cellulose-based nanofibers and polyethylene nanoparticles; and a resin layer formed on one surface or both surfaces of the base layer, and formed of a polyolefin resin.

Description

Cellulose-based multilayer separator

The present invention relates to a secondary battery separator, and more particularly to a secondary battery separator with improved shutdown function.

This application claims priority based on Korean Application No. 10-2015-0044449, filed March 30, 2015 and Korean Application No. 10-2016-0037844, filed March 29, 2016. All contents disclosed in the specification are incorporated in this application.

Recently, interest in energy storage technology is increasing. As the field of application extends to the energy of mobile phones, camcorders, notebook PCs, and even electric vehicles, efforts for research and development of electrochemical devices are becoming more concrete. The electrochemical device is the most attracting field in this respect, and the development of a secondary battery capable of charging and discharging has been the focus of attention, and in recent years in the development of such a battery in order to improve the capacity density and specific energy The research and development of the design of the battery is progressing.

Among the secondary batteries currently applied, lithium secondary batteries developed in the early 1990s have a higher operating voltage and greater energy density than conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries that use an aqueous electrolyte solution. I am in the spotlight.

As the development and demand for mobile devices increases, the demand for secondary batteries as energy sources is increasing rapidly. Recently, the use of secondary batteries as a power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs) has been realized. It is becoming. Accordingly, many studies have been conducted on secondary batteries capable of meeting various needs, and in particular, there is a high demand for lithium secondary batteries having high energy density, high discharge voltage, and output stability.

The lithium secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. Among them, a required characteristic of the separator is to separate the positive electrode and the negative electrode and to electrically insulate the lithium ion based on its high porosity. To increase the ion conductivity. In general, the separator used may be a polyolefin-based material or cellulose. Polyolefin-based polymer substrates are advantageous for pore formation and excellent in chemical resistance, mechanical properties, and thermal properties, but have severe heat shrinkage at high temperatures and are physically weak. The cellulose substrate may be manufactured as a separator having a physically high strength, but has a problem in that it cannot act to cut off current in an abnormal operating environment.

The present invention has been made in view of the prior art as described above, and an object of the present invention is to provide a secondary battery separator and a method of manufacturing the same that can improve the shutdown function of the cellulose-based separator having a physically high strength.

The secondary battery separator according to the present invention for achieving the above technical problem is a base layer comprising cellulose-based nanofibers and polyethylene nanoparticles and

It is provided on one side or both sides of the said base material layer, and is provided with the resin layer which consists of polyolefin resin.

Preferably, the polyolefin resin may be at least one selected from the group consisting of polyethylene, polypropylene, polybutylene, and polypentene.

The polyethylene nanoparticles may have a melting point of 80 to 100 ° C., and the polyolefin resin may have a melting point of 100 to 140 ° C.

Preferably, the resin layer may further include inorganic particles and a polymer binder.

The resin layer may be laminated in a lamination method.

The resin layer may have a thickness of 3 to 20 μm, and the base layer may have a thickness of 5 to 20 μm.

Preferably, the cellulose-based nanofibers may be made of one selected from the group consisting of cellulose acetate, cellulose triacetate and cellulose butyrate.

Preferably, the diameter of the cellulose-based nanofibers is 10 nm to 500 nm, the length is 1 ㎛ to 10 mm, the polyethylene nanoparticles may have a particle size of 50 nm to 500 nm.

Preferably, the cellulose-based nanofibers and polyethylene nanoparticles may be included in a weight ratio of 7: 3 to 9: 1.

In addition, the method for producing a secondary battery separator according to the present invention, using a mixture of cellulose nanofibers and polyethylene nanoparticles dispersed in a solvent, preparing a sheet; Removing the solvent of the mixture to prepare a porous base layer; And laminating a resin layer made of a polyolefin resin on one or both surfaces of the substrate layer in a lamination method.

Preferably, the solvent may be the one paper phase selected from the group consisting of water, methyl alcohol, ethyl alcohol, propyl alcohol, acetone, ethyl acetate, methyl ethyl ketone and toluene to the pore-forming resin.

Preferably, the pore-forming resin may be at least one selected from the group consisting of polyethylene glycol, polyvinyl talcol, and polyvinyl propylene.

Separation membrane according to the present invention has a high physical strength and excellent safety, it is possible to ensure a high safety by improving the shutdown function.

The following drawings, which are attached to this specification, illustrate preferred embodiments of the present invention, and together with the contents of the present invention serve to further understand the technical spirit of the present invention, the present invention is limited to the matters described in such drawings. It should not be construed as limited.

1 is a view schematically showing a cross-sectional structure of a separator according to an aspect of the present invention.

Hereinafter, the present invention will be described in detail with reference to the drawings. The terms or words used in this specification and claims are not to be construed as limiting in their usual or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best explain their invention in the best way possible. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

Conventional membranes made of cellulose have a high physical strength, but have a problem in that they cannot act to cut off current in abnormal operating environments.

The present invention can provide a shutdown function by providing a resin layer made of a polyolefin resin on at least one surface of the base material layer containing cellulose nanofibers, and has a relatively low melting point in the base material layer containing cellulose nanofibers. By including polyethylene nanoparticles, the shutdown characteristic can be further improved.

1 is a view schematically showing a cross-sectional structure of a separator according to an aspect of the present invention. Referring to FIG. 1, the separator 100 for a secondary battery according to an embodiment of the present invention includes a base layer 110 including cellulose-based nanofibers and polyethylene nanoparticles; And a resin layer 120 made of polyolefin resin formed on both surfaces of the substrate layer.

The polyethylene nanoparticles included in the base layer may have a melting point of 80 to 100 ° C. and a particle size of 50 nm to 500 nm. Unlike conventional polyethylene, the polyethylene nanoparticles may have a relatively low melting point, thereby improving shutdown characteristics of a separator including a cellulose substrate. In addition, the polyethylene nanoparticles may have an average molecular weight of 500 g / mole to 900 g / mole.

The polyethylene nanoparticles will be described in detail below, for example, but not limited thereto.

1. Ni-methyl complex (Me (OCH ₂ CH ₂ ) _n NH ₂ ) was added to 100 ml of distilled water at room temperature, and the mixture was stirred for 2 minutes to prepare a uniform solution.

2. The solution was placed in a pressurized reactor at a pressure of 40 bar and under stirring, a constant amount of ethylene was continuously injected.

3. After the titration reaction time, the reactor was opened to obtain a dispersion through the glass wool plug.

4. The obtained dispersion was washed three times with methanone and dried under vacuum at 50 ° C. to prepare polyethylene nanoparticles.

The polyethylene nanoparticles may be prepared in the form of nanoparticles using a Ni catalyst, prepared in the form of PE magnetic composite particles with iron oxide particles, and then prepared by a method of removing metal components through acid treatment. Can be.

The cellulose-based nanofibers and polyethylene nanoparticles may be included in a weight ratio of 7: 3 to 9: 1. When the weight ratio is satisfied, it may be easily prepared as a separator without interfering with the binding between the cellulose nanofibers, and may provide an appropriate level of shutdown characteristics.

The resin layer may be formed in a film form and formed on one or both sides of the base layer in a lamination method. In the general coating method, a method in which a polymer resin is dissolved and applied to a solvent is used, and a process is long and productivity is low. According to the present invention, the resin layer is manufactured in the form of a film and laminated in a lamination manner, so that it is not necessary to use a solvent, thereby simplifying the process. Moreover, since a resin layer is manufactured separately, characteristics, such as thickness, porosity, and composition of a resin layer, can be adjusted easily.

The resin layer may have a thickness of 3 to 20 μm, and the base layer may have a thickness of 5 to 20 μm.

The resin layer is to provide a shutdown function, the polyolefin resin may be at least one selected from the group consisting of polyethylene, polypropylene, polybutylene and polypentene. Preferably, the polyethylene may be low density polyethylene (LDPE). Also preferably, the polyolefin resin may be one having a melting point of 100 to 140 ° C.

In addition, the resin layer may further include inorganic particles and a polymer binder.

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 the oxidation and / or reduction reactions do not occur in the operating voltage range (for example, 0 to about 5 V on the basis of Li / Li ⁺ ) of the applied electrochemical device. In particular, in the case of using the inorganic particles having the ion transport ability, it is possible to improve the performance by increasing the ion conductivity in the electrochemical device.

In addition, when inorganic particles having a high dielectric constant are used as the inorganic particles, the ionic conductivity of the electrolyte may be improved by contributing to an increase in the dissociation degree of the electrolyte salt such as lithium salt in the liquid electrolyte.

For the foregoing reasons, the inorganic particles may include high dielectric constant inorganic particles having a dielectric constant of about 5 or more, such as about 10 or more, inorganic particles having a lithium ion transfer ability, or a mixture thereof. Non-limiting examples of inorganic particles having a dielectric constant of about 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), 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, boehmite or mixtures thereof.

In particular, BaTiO ₃ , Pb (Zr, Ti) O ₃ (PZT), Pb _{1 - x} La _x Zr _{1 - y} Ti _y O ₃ (PLZT), PB (Mg _1/3 Nb _2/3 ) O _{3 described above} Inorganic particles such as -PbTiO ₃ (PMN-PT) and hafnia (HfO ₂ ) not only exhibit high dielectric constants with dielectric constants of about 100 or more, but also generate charge when stretched or compressed under constant pressure By having piezoelectricity at which a potential difference occurs, it is possible to prevent the occurrence of an internal short circuit of both electrodes due to an external impact, thereby improving the safety of the electrochemical device. In addition, synergistic effects of the high dielectric constant inorganic particles and the inorganic particles having lithium ion transfer ability may be doubled.

The inorganic particles having a lithium ion transfer capacity refers to inorganic particles containing lithium elements but having a function of transferring lithium ions without storing lithium, and the inorganic particles having lithium ion transfer ability are present in the particle structure. Since the lithium ions can be transferred and moved due to a kind of defect, the lithium ion conductivity in the battery is improved, thereby improving battery performance. Non-limiting examples of the inorganic particles having a lithium ion transfer capacity is 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 (LixAlyTiz (PO ₄ ) ₃ , 0 <x <2, 0 <y <1, 0 <z <3), 14Li ₂ O-9Al ₂ O ₃ -38TiO ₂ -39P ₂ O ₅ , etc. (LiAlTiP) _x O _y series glass (0 <x <4, 0 <y <13), lithium lanthanum titanate (Li _x La _y TiO ₃ , 0 <x <2, 0 <y <3), Li _{3 . 25 Ge 0 .25 P 0. 75 S} 4 Mani lithium germanium thiophosphate Titanium _{(Li x Ge y P z S} w, 0 <x <4, 0 <y <1, 0 <z <1, 0 <w such as <5), lithium nitrides such as Li ₃ N (Li _x N _y , 0 <x <4, 0 <y <2), and SiS ₂ based glass such as Li ₃ PO ₄ -Li ₂ S-SiS ₂ (Li _x Si _y S _z , 0 <x <3, 0 <y <2, 0 <z <4), P ₂ S ₅ series glass such as 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.

The polymer binder may be polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, ethylene-co-vinyl acetate, Polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinyl alcohol (cyanoethylpolyvinylalcohol), cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, acrylonitrile-styrene-butadiene copolymer (acrylonitrile-styrene-butadiene copolymer, polyimide, etc. Or combinations thereof, but can be used by mixing two or more kinds, without being limited thereto.

The cellulose-based nanofibers are ethyl cellulose (Ethylcellulose), methyl cellulose (Methylcellulose), hydroxypropyl cellulose (Hydroxypropyl cellulose), hydroxyethyl cellulose (Hydroxyethyl cellulose), hydroxypropyl methyl cellulose (Hydroxypropyl methyl cellulose), hydroxyethyl Hydroxyethyl methyl cellulose, Carboxymethyl cellulose, Cellulose acetate, Cellulose triacetate, Cellulose acetate phthalate, Nitrocellulose, Cellulose acetate , Cellulose acetate propionate and their ammonium or salts.

The diameter of the cellulose-based nanofibers may be 10 to 500 nm, the length may be 1 ㎛ to 10 mm.

In addition, according to another aspect of the present invention, the method for producing a secondary battery separator, using a mixture of cellulose nanofibers and polyethylene nanoparticles dispersed in a solvent, preparing a sheet; Removing the solvent of the sheet to prepare a porous base layer; And laminating a resin layer made of polyolefin resin on one or both surfaces of the substrate layer in a lamination method. The method of manufacturing a separator for a secondary battery is provided.

By adjusting the mixing ratio of the cellulose nanofibers and the solvent included in the mixture, the porosity and thickness of the substrate layer can be easily adjusted. Preferably, the cellulose nanofibers and the solvent may be included in a weight ratio of 6: 4 to 8: 2.

The solvent is a pore-forming resin to bind the cellulose nanofibers, and includes a pore-forming resin to form a porous structure of the substrate layer, water, methyl alcohol, ethyl alcohol, propyl alcohol, acetone, ethyl acetate, methyl ethyl ketone And toluene may further comprise one paper selected from the group consisting of. The pore-forming resin may be used one or more selected from the group consisting of polyethylene glycol, polyvinyl talcol and polyvinyl propylene.

Specifically, in the process of producing a sheet, a mixture obtained by dispersing cellulose nanofibers and polyethylene nanoparticles in a solvent may be passed through a homogenizer to prepare a suspension, which may be prepared in a sheet form by reducing the pressure. The sheets produced exhibit high tensile strength through strong hydrogen bonding between cellulose fibers. The number of passes through the homogenizer may be 5 to 20 cycles. After decompression, the cellulose fibers are uniformly arranged to form a sheet of porous structure.

In order to remove the residual pore-forming resin, the porous sheet is formed by using at least one selected from the group consisting of water, methyl alcohol, ethyl alcohol, propyl alcohol, acetone, ethyl acetate, methyl ethyl ketone, and toluene. The washing step can be performed. After the resin for pore forming is removed, the sheet is dried, and the solvent of the sheet is removed to prepare a substrate layer having a porous structure.

The drying process of the sheet may be performed at 40 ° C. to 80 ° C., in an air or inert gas or vacuum environment for 1 to 30 hours. For example, the process of drying the sheet may be performed in air at a temperature of 50 to 70 ° C. for 20 to 30 hours. The solvent is removed through the drying process, and the portion from which the solvent is removed forms voids. The drying of the sheet may include a process of dehydration drying using a vacuum filter, but is not limited thereto.

Moreover, when laminating | stacking the said resin layer on both surfaces of the said base material layer, different resin layers can be used, respectively. Therefore, by using the resin layer with improved shutdown function and the resin layer with improved air permeability, a separation membrane having different characteristics on both sides can be manufactured.

According to another aspect of the present invention, an electrochemical device including an anode, a cathode, and the aforementioned separator interposed between the anode and the cathode may be manufactured. The electrochemical device of the present invention includes all devices that undergo an electrochemical reaction, and specific examples include capacitors such as all kinds of primary cells, secondary batteries, fuel cells, solar cells, or 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 is preferable among the secondary batteries.

Anodes, cathodes and the like can be readily prepared by processes and / or methods known in the art.

Specifically, the positive electrode is prepared by, for example, applying a mixture of a positive electrode active material, a conductive agent, and a binder onto a positive electrode current collector, followed by drying, and optionally, a filler is further added to the mixture.

The positive electrode is manufactured in a form in which a positive electrode active material is bound to a positive electrode current collector according to a conventional method known in the art. In this case, as the cathode active material, a conventional cathode active material that may be used for the cathode of the conventional electrochemical device may be used, and non-limiting examples include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni _a Co _b Mn _c ) O ₂ (0 <a <1, 0 <b <1, a + b + c = 1), LiNi _{1 - Y} Co _Y O ₂ , LiCo _1-Y Mn _Y O ₂ , LiNi _{1 - Y} Mn _Y O ₂ (where 0 ≦ Y <1), Li (Ni _a Co _b Mn _c ) O ₄ (0 <a <2, 0 <b <2, a + b + c = 2), LiMn _{2-Z Ni Z O 4, LiMn 2 -} Z Co Z O 4 ( where, 0 <Z <2) has, LiCoPO _4, LiFePO _4, and mixtures thereof and the like. As the anode current collector, a foil made of aluminum, nickel, or a combination thereof may be used.

The negative electrode is manufactured in a form in which the negative electrode active material is bound to the negative electrode current collector according to conventional methods known in the art. In this case, the negative electrode active material may be, for example, carbon such as hardly graphitized carbon or graphite carbon; Li _x Fe ₂ O ₃ (0 ≦ _x ≦ 1), Li _x WO ₂ (0 ≦ _x ≦ 1), Sn _x Me _{1 - x} Me ' _y O _z (Me: Mn, Fe, Pb, Ge; Me' Al, B, P, Si, a group 1, 2, 3 element of the periodic table, a halogen, a metal complex oxide of 0 <x≤1;1≤y≤3;1≤z≤8; Lithium metal; Lithium alloys; Silicon-based alloys; Tin-based alloys; SnO, SnO ₂ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , GeO, GeO ₂ , Bi ₂ O ₃ , Bi ₂ O ₄ , Oxides such as Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used. Meanwhile, as the cathode current collector, stainless steel, nickel, copper, titanium, or an alloy thereof may be used.

In addition, the electrolyte that may be inserted between the electrode and the separator A ⁺ B ^- A salt of the structure, such as, A ⁺ comprises Li ^+, Na ^+, an alkali metal cation or an ion composed of a combination thereof, such as K ⁺ and B ^- is _{^{PF 6 -, BF 4 -,}} Cl -, Br -, I -, ClO 4 -, AsF 6 -, CH 3 CO 2 -, CF 3 SO 3 -, N (CF 3 SO 2) 2 - , Salts containing ions consisting of anions such as C (CF ₂ SO ₂ ) ₃ ^- or combinations thereof include 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), ethylmethyl carbonate (EMC), gamma butyrolactone dissolved or dissociated in an organic solvent consisting of (γ-butyrolactone) or mixtures thereof, but not limited thereto. All.

The injection of the electrolyte may be performed at an appropriate step in the battery manufacturing process, depending on the manufacturing process and the required physical properties of the final product.

As a process of applying the separator of the present invention to a battery, a lamination (stack) and folding process of the separator and the electrode may be performed in addition to the general winding process.

On the other hand, the embodiments of the present invention disclosed herein are not intended to limit the scope of the present invention only presented a specific example to facilitate understanding. It will be apparent to those skilled in the art that other modifications based on the technical idea of the present invention can be carried out in addition to the embodiments disclosed herein.

Claims (12)

A base layer comprising cellulose-based nanofibers and polyethylene nanoparticles and Separation membrane for a secondary battery formed on one side or both sides of the base layer, and having a resin layer made of a polyolefin resin. The method of claim 1, The polyolefin resin is at least one member selected from the group consisting of polyethylene, polypropylene, polybutylene and polypentene, the secondary battery separator. The method of claim 1, The polyethylene nanoparticles have a melting point of 80 to 100 ℃, the polyolefin resin, characterized in that the melting point of 100 to 140 ℃, the separator for secondary batteries. The method of claim 1, The resin layer is a secondary battery separator, characterized in that it further comprises inorganic particles and a polymer binder. The method of claim 1, The resin layer is laminated in a lamination method, the secondary battery separator. The method of claim 1, The thickness of the resin layer is 3 to 20 ㎛, the thickness of the base layer is 5 to 20 ㎛, separator for secondary batteries. The method of claim 1, The cellulose-based nanofibers, characterized in that made of one selected from the group consisting of cellulose acetate, cellulose triacetate and cellulose butyrate, the secondary battery separator. The method of claim 1, The cellulose-based nanofibers have a diameter of 10 nm to 500 nm, a length of 1 μm to 10 mm, and the polyethylene nanoparticles have a particle size of 50 nm to 500 nm. The method of claim 1, The cellulose-based nanofibers and polyethylene nanoparticles are characterized in that the weight ratio of 7: 3 to 9: 1, characterized in that the secondary battery separator. Preparing a sheet using a mixture of cellulose nanofibers and polyethylene nanoparticles dispersed in a solvent; Removing the solvent of the mixture to prepare a porous base layer; And Laminating a resin layer made of a polyolefin resin on one side or both sides of the base layer in a lamination method; comprising, a secondary battery separator. The method of claim 10, The solvent is a pore-forming resin, water, methyl alcohol, ethyl alcohol, propyl alcohol, acetone, ethyl acetate, methyl ethyl ketone and toluene, characterized in that the addition of one paper phase selected from the group consisting of to prepare a secondary battery separator Way. The method of claim 11, The pore-forming resin is at least one member selected from the group consisting of polyethylene glycol, polyvinyl talcol and polyvinyl propylene, the method of manufacturing a secondary battery separator.

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