KR20160109669A - High heat resistance separator for secondary batteries and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a separation membrane for satisfying required properties of a high capacity and large area secondary battery. The separation membrane includes: a porous base material layer; a heat resistant layer formed on one side or both sides of the porous base material layer; and an adhesion layer formed on the heat resistant layer. Inorganic particles are fixed and connected by binder polymer in the heat resistant layer, and the adhesion layer contains polymer particles. The separation membrane has a high strength property and a high heat resistant property. The separation membrane is evenly attached to a whole area of an anode and a cathode. As ions smoothly moves through an air hole evenly arranged on each layer, the separation membrane is very advantageous for improving performance of the high capacity and the large area secondary battery.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a separator for a secondary battery having excellent heat resistance,

The present invention relates to a separator for a lithium ion secondary battery excellent in heat resistance and a method for producing the same.

As lithium ion secondary batteries are used more and more, demands for large-sized and high-capacity lithium ion secondary batteries are becoming stronger. Accordingly, various methods for imparting functionality to a porous thin film made of polyethylene or polypropylene of a separator used in a lithium ion secondary battery have been attempted.

The separator is a microporous thin film made of polyethylene, which is made of microporous and homogeneous pores by increasing the strength and thinning through stretching, and by the phase separation with plasticizer, so that it is not torn well and has high mechanical safety. However, when the temperature of the battery is increased, the separator shrinks in the direction in which the separator is stretched. As a result, a short circuit occurs between the positive and negative electrodes inside the battery, which causes the battery to easily ignite and break. As the capacity per unit volume of the battery increases, the amount of heat generated when the internal short circuit of the battery is generated also increases.

In addition, the assembly method, which is widely used for producing a lithium ion secondary battery at low cost, is a method of manufacturing a positive electrode plate, a negative electrode plate and a separator by rolling. As the area of the electrode plate is getting wider, And there is a case where uneven charging / discharging phenomenon due to difference in distance between electrodes occurs. Such nonuniformity becomes more severe as the charge and discharge are repeated, shortening the lifetime of the battery. Therefore, attempts have been made to remove the excited portion by bonding the separator to the anode and the cathode plate. Therefore, the adhesion required for the separator in a large-area battery is further increased.

In Japanese Patent No. 4127989 (Patent Document 1), a porous adhesive layer made of an organic polymer which swells an electrolyte solution on both surfaces of a substrate of a polyolefin microporous membrane is arranged to have a thickness of 1 m or more and 20 m or less in total And the like. The above technology has advantages in ionic conductivity and adhesion, but has a problem that the heat resistant layer is not included and the heat resistance is insufficient and the thickness of the adhesive layer is thick, so that the separation membrane is required to be thinned.

In Korean Patent No. 0646508 (Patent Document 2), in order to solve the disadvantage that the adhesive polymer layer covers the entire area of the electrode, the performance of the battery is deteriorated by interfering with the movement of the lithium ion, To form an adhesive polymer membrane. This results in a significant difference in lithium ion conductivity between the portion having the adhesive polymer and the portion having no adhesive polymer, which causes a problem that non-uniform charging / discharging between the electrodes becomes serious.

Korean Patent No. 1156961 (Patent Document 3) discloses that a silane-based compound can be partially coated only on the upper and lower peripheral surfaces of an electrode and a separator to prevent the deterioration of ionic conductivity due to the adhesive layer while increasing the adhesive strength. However, in the large-area battery, there is a limit as it is impossible to prevent the middle portion of the electrode from being lifted when only the vicinity of the upper and lower outer circumferential surfaces of the separator is adhered.

Japanese Laid-Open Patent Application No. 2013-020769 (Patent Document 4) discloses a porous substrate having a porous layer containing a heat-resistant resin on both sides of the porous substrate in order to satisfy all the various characteristics required for the separator, A separator having an adhesive porous layer containing a resin is disclosed. In Comparative Example 4, the case where a porous layer containing a heat-resistant resin is cross-sectioned and a cell is oriented so as to be oriented in a negative direction is considered to be inadequate. Although the above-mentioned technology tries to improve both the thermal stability and the adhesion of the separator by including both the heat resistant layer and the adhesive layer, since the heat resistant layer is limited to both sides and the main component of the adhesive layer is limited to the fluororesin, productivity and adhesive strength are limited , And when the adhesive layer is not on both sides, adhesion with the electrode can not be ensured.

In addition, it is common that the separation membrane is wrapped and rolled up in rolls like rolled toilet paper, and is transported and stored. However, since the surface of the separation membrane is smooth so that it is not loosened during transportation and storage, It is necessary to tightly wind it. However, when the ambient temperature is high during transportation, there is a case where the overlapping heat-sealable layers are adhered to each other, making it impossible to use them as a separation membrane. In order to solve such a problem, a method of inserting a PET film or the like between the adhesive layer of the separator and the adhesive layer to prevent the adhesive layers contacting the separator from sticking to each other is also used. In this case, The manufacturing cost is increased.

Japanese Patent No. 4127989 Korean Patent No. 0646508 Korean Patent No. 1156961 Japanese Laid-Open Patent Application No. 2013-020769

The present invention has various requirements such as high strength, porosity, chemical resistance, electrochemical resistance, heat resistance, uniformity of physical properties and low manufacturing cost, while being good in adhesion to an electrode while being a thin film, which is a requirement that conventional separator for secondary batteries can not satisfy The purpose is to provide.

In order to increase the capacity of the battery and lower the internal resistance of the battery, it is required to be a thin film. In order to prevent a short circuit between the anode and the cathode due to the impact of the battery assembly process and the use of the finished product, high strength, It is necessary to uniformly maintain fine pores having a size of several tens to several hundreds of nanometers in order to suppress the occurrence of internal short circuits due to irregularities and impurities on the electrode surface. In order to use the battery stably for a long period of time, the separator must be chemically and electrochemically stable. In order to suppress accidents such as ignition and rupture due to abnormal temperature rise inside the battery, heat resistance In order to mass-produce and distribute a uniform quality battery, the physical properties of the membrane should be uniform and the manufacturing cost should be reduced.

The particulate polymer material of the adhesive layer is not particularly limited as long as it can secure the adhesive force between the electrode and the separator. However, there is no problem in carrying and storing the separator without inserting the separator, It is preferable to use the substance to be expressed. The separation membrane is rolled up into rolls, rolled up in rolls, rolled and stored in the form of a rolled toilet paper, and is usually unwound during the manufacture of a battery. However, if the ambient temperature during transportation is high, the overlapped thermofusion layers are bonded together, And the like. It is also possible to use a method of interposing a separator between the heat-welding and interlayer adhesives by inserting a separator between the separators at the time of winding the separator, but if the separator is not required, the manufacturing cost can be reduced.

In order to achieve the above object,

A porous substrate layer;

A heat resistant layer formed on one or both surfaces of the porous substrate layer;

And an adhesive layer formed on the heat resistant layer,

Wherein the heat resistant layer is a composite separator for a secondary battery, wherein the inorganic particles are connected and fixed by a binder polymer, and the adhesive layer contains polymer particles.

The porous substrate layer of the present invention is a microporous stretched film made of polyolefin, and the polyolefin resin is one or two selected from polyethylene, polypropylene and copolymers thereof.

The polymer material constituting the adhesive layer and the heat resistant layer may be selected from the group consisting of polyacrylate, polybenzoate, polyvinyl pyrrolidone, polystyrene, polymethyl methacrylate, polybutyl acrylate, polyvinylidene fluoride and copolymers thereof And may include one or more selected polymeric materials.

The inorganic particles of the heat resistant layer are composed of one or more selected from among aluminum oxide such as alumina and boehmite, barium titanium oxide, titanium oxide, magnesium oxide, and clay glass powder. .

The heat resistant layer preferably contains 60 to 99% by weight of inorganic particles and 40 to 1% by weight of binder polymer based on 100% by weight of the total composition, and preferably has an inorganic particle size of 100 to 800 nm.

Wherein the adhesive layer comprises 70 to 97% by weight of polymer particles and 30 to 3% by weight of inorganic particles based on 100% by weight of the total composition, wherein the size of the polymer particles is 100 to 800 nm, .

The porous heat-resistant layer of the present invention is obtained by mixing a small amount of a heat-resistant binder with hard inorganic particles and bonding to the base layer to suppress shrinkage of the base layer generated at a high temperature and to improve the thermal stability of the battery. Pores having a proper size must be uniformly distributed over the entire area so that the ion movement of the cell is free.

The porous adhesive layer of the present invention functions to uniformly adhere the separator and the anode and cathode plates respectively disposed on both sides in order to closely adhere the gap in the entire area of the anode and the cathode plate and prevent lifting. If the polymer layer is formed of a particle type binder like the heat resistant layer made of inorganic particles, the ion mobility can be secured through the space between the polymer particles, and the size of the pores can be adjusted by adjusting the particle size. And the productivity can be increased by increasing the drying speed.

The separator of the present invention has a gas permeability of 500 seconds or less, a water content of 1000 ppm or less, and a heat shrinkage at 130 占 폚 of 10% or less.

When the membrane is laminated in two layers, the adhesive force between the overlapped porous adhesive layer is less than 0.3 gf / cm at a temperature of 60 ° C or less and a pressure of 1 MPa or less, and the adhesive force between the electrode and the separator at a temperature of 110 ° C or more and a pressure of 3 MPa or more Cm < 2 > / g or higher, and an adhesive layer was formed.

Since the separation membrane according to the present invention has high strength and high heat resistance and uniformly adheres to the entire area of the anode and the cathode and smoothly moves the ions through the uniformly distributed pores of each layer, It is advantageous for improving the performance of the battery.

Fig. 1 is an enlarged photograph of the surface of the adhesive layer at a magnification of 10,000 times of the separation membrane produced in Example 1 of the present invention.
2 is an enlarged photograph of a cross section of 5,000 times in cross section of the separation membrane produced in Example 1 of the present invention.
Fig. 3 is an enlarged photograph of the surface of the adhesive layer of 10,000 times the surface of the separation membrane produced in Example 3 of the present invention.

Hereinafter, the method for producing a separation membrane of the present invention and the separation membrane produced therefrom will be described in detail. The following embodiments and drawings are provided by way of example so that those skilled in the art can fully understand the spirit of the present invention. Further, it is to be understood that, unless otherwise defined, technical terms and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Descriptions of known functions and configurations that may be unnecessarily blurred are omitted.

The present invention relates to a separator for a secondary battery having high strength and high heat resistance characteristics and a method for producing the same, and more particularly, to a separator for a secondary battery comprising a porous substrate layer, a porous heat resistant layer and a porous adhesive layer formed on both outermost surfaces.

In order to develop such a separation membrane, a microporous stretched film made of polyolefin is used as the porous substrate layer. The polyolefin resin may be selected from polyethylene, polypropylene, and copolymers thereof, but is not limited thereto.

The porous substrate layer uses a thin film microporous stretched film having a thickness of 5 to 30 mu m. The tensile strength of the porous base layer is not less than 500 kgf / cm ^{2 in} all directions and is produced through biaxial stretching rather than uniaxial stretching in order to evenly increase the strength in all directions. The porous polymer film produced through stretching has an advantage of increasing the tensile strength in the stretched direction, but has a disadvantage that the stress to shrink in the stretched direction remains and shrinks when the temperature is increased.

The porous heat-resistant layer introduced to improve the heat resistance is composed of inorganic particles and a binder on one side or both sides of the base layer, and the content of the inorganic particles in the heat-resistant layer is 60 to 99 per 100% by weight of the mixture containing the inorganic particles and the binder polymer Weight%. If the amount is less than 60% by weight, the content of the polymer may be excessively increased, resulting in a decrease in pore size and porosity due to a decrease in void space formed between the inorganic particles, resulting in deterioration of final battery performance. When the amount of the organic solvent is more than 99% by weight, the mechanical properties of the final organic / inorganic composite porous separator are deteriorated due to the weak adhesion between the inorganic materials because the polymer content is too small.

The porous heat-resistant layer is prepared by mixing a small amount of a heat-resistant binder with a hard inorganic particle having a size of 100 to 800 nm and coating the surface of the base layer with a thickness of 1 to 10 μm on both sides or both sides.

The porous adhesive layer is a particulate binder such that the polymer particles included in the porous adhesive layer constitute a spherical polymer layer having a diameter of 100 to 800 nm as in the case of the heat resistant layer made of inorganic particles. In order to stably maintain the pores when the pressure is applied , The adhesive polymer material and the inorganic particles may be mixed together. The amount of the inorganic particles is preferably 1 to 30% by volume. When the amount of the inorganic particles exceeds 30% by weight, the adhesive strength is lowered.

The inorganic particles or polymeric materials used as raw materials for the separator should be chemically and electrochemically stable in the secondary battery. Examples of the inorganic particles include aluminum oxide such as alumina and boehmite, barium titanium oxide, titanium oxide, magnesium oxide, and clay glass powder. Among them, high purity Of aluminum oxide can be used. As the polymer substance, polyolefin, polyacrylate series, polybenzoate series, polyvinyl pyrrolidone, polystyrene, polymethyl methacrylate, polybutyl acrylate, polyvinylidene fluoride and the like can be used. The manufacturing cost is minimized by using the multilayer coating method simultaneously with the heat resistant layer and the adhesive layer.

Various methods can be used for the method of bonding the separator and the electrode, and a method advantageous for realizing productivity and battery performance can be used. The first method is to cut the electrode, place it on the separator, apply heat and pressure to the electrode, and roll the separator with the electrode to make the cell. In the second method, a jelly roll is formed by winding an electrode and a separator, and then the electrode and separator are adhered to each other by applying heat and pressure. Then, the integrated jelly roll is put into a battery case of a metal can or an aluminum pouch type, To make a battery. The third method is to insert the jelly roll into the battery outer casing, inject the electrolyte, and apply heat and pressure to bond the electrode and the separator. The first and second methods can be classified into a dry bonding method in the absence of an electrolyte solution and a wet bonding method in a state in which an electrolyte solution is contained in a third method. According to the kind of the adhesive polymer material, . Since the adhesive strength depends on the presence or absence of the electrolyte and the constituent materials of the positive electrode and the negative electrode, it is preferable that the constituent materials of the adhesive layer are different depending on the battery design.

Hereinafter, the present invention will be described in more detail with reference to the following examples. However, the present invention is not limited to the following examples.

[Example 1]

One. Manufacture of anode

The mixture was added to NMP (N-methyl-2-pyrrolidone) as a solvent so that LiCoO2 was used as a cathode active material, 2.5 wt% of polyvinylidene fluoride as an adhesive and 3.5 wt% of Super- To prepare a positive electrode slurry. The slurry was coated on an aluminum foil having a thickness of 30 탆, dried and pressed to prepare a positive electrode plate having a thickness of 150 탆.

2. Manufacture of cathode

95% by weight of artificial graphite as an anode active material, 3% by weight of acrylic latex having a Tg of -52 캜 as an adhesive, and 2% by weight of CMC (Carboxymethyl cellulose) as a thickening agent were added to water as a solvent, A negative electrode slurry was prepared. The slurry was coated on a copper foil having a thickness of 20 탆, dried and pressed to prepare a negative electrode plate having a thickness of 150 탆.

3. Preparation of Membrane

3.1. Preparation of heat resistant layer slurry

94% by weight of boehmite particles having an average particle diameter of 600 nm, 2% by weight of polyvinyl alcohol having a melting point of 220 占 폚 and a saponification degree of 99%, 4% by weight of Acrylic latex having a Tg of -52 占 폚 The resulting mixture was added to water as a solvent and stirred to prepare a uniform heat resistant layer slurry.

3.2. Preparation of adhesive layer slurry

The polymer particles containing polymethylmethacrylate and polybutylcrylate as main components and dispersed in water and having a spherical shape with an average particle size of 500 nm were diluted to a ratio of 20% by weight with respect to water and used as an adhesive layer slurry.

3.3 Preparation of membranes by coating and drying

As a coating base material, a high strength oriented polyolefin microporous film product (ENPASS) having a thickness of 9 탆 and manufactured by SK Innovation was used. A multilayer slot coating die was used to simultaneously coat the heat resistant layer slurry and the adhesive layer slurry on one side of the substrate Thereafter, only the adhesive layer slurry was coated on the other side of the substrate using a single layer slot coating die. The solvent was evaporated through a drier and then rolled into a roll. The thickness of the endothermic layer was 3 μm, and the thickness of the double-sided adhesive layer was 0.8 μm.

[Example 2]

A separator was prepared in the same manner as in Example 1 except that the heat resistant layer was coated on both sides of the substrate layer, not on the end face thereof, to a thickness of 1.5 탆.

[Example 3]

Except that alumina particles having an average particle diameter of 600 nm were added in the production of the adhesive layer slurry so that the volume ratio of the polymer particles to the alumina particles was 75:25. Respectively.

[Example 4]

A separation membrane was prepared in the same manner as in Example 1 except that the average particle diameter of the polymer particles used in the production of the adhesive layer slurry was 200 nm.

[Comparative Example 1]

A separator was prepared in the same manner as in Example 1 except that there was no heat resistant layer.

[Comparative Example 2]

A separation membrane was prepared in the same manner as in Example 1 except that the average particle size of the polymer particles used in the production of the adhesive layer slurry was 50 nm.

[Comparative Example 3]

Except that alumina particles having an average particle diameter of 600 nm were added during the production of the adhesive layer slurry so that the volume ratio of the polymer particles to the alumina particles was 65:35. Respectively.

[Experimental Examples] 1. Tensile strength measurement

The method of measuring the tensile strength of the base layer was conducted according to ASTM D882 standard, and was measured for each of the longitudinal direction and the transverse direction, and then the value was taken to be low.

2. Measurement of heat shrinkage at 130 ℃

The method of measuring the heat shrinkage of the membrane at 130 ° C is as follows. The membrane is cut into a rectangular shape with a side length of 10 cm to prepare a sample. The area of the sample before the experiment is measured and recorded using a camera. Place five sheets of paper on each of the top and bottom of the sample so that the sample is in the center, and then fasten the four sides of the paper with a clip. The sample wrapped in paper was left in a hot air drying oven at 130 占 폚 for 1 hour. After leaving the sample, the sample was taken out and the area of the separation membrane was measured with a camera to calculate the area contraction ratio.

3. Measurement of gas permeability

The gas permeability of the separator was measured according to JIS P8117 standard, and the time required for 100 cc of air to pass through the area of ¹ inch ² was recorded in seconds.

4. Adhesion strength measurement

The adhesion between the separators and the adhesion between the separator and the electrodes were measured using a hot press and a 180 ° peel strength meter. The sample for the measurement of the adhesion between the separating films was prepared by stacking two separators on a hot press machine and then applying heat and pressure of 1 MPa at 60 ° C for 1 minute. A sample for measuring the adhesion between the separator and the electrode was prepared by placing a separator between the anode and the cathode plate, placing the separator on a hot press, and applying heat and pressure of 110 ° C and 3 MPa for 1 minute. The samples thus prepared were each measured for 180 ° peel strength.

5. Moisture Content Measurement

In order to measure the content of water contained in the membrane, Karl Fischer moisture determination method was used. The measurement equipment used was a 831 KFC Coulometer from Metrohm and 885 Compact Oven. The measurement conditions were a sample weight of 0.3 g, an oven temperature of 150 ° C, and a measurement time of 600 seconds.

The results of the above Examples and Comparative Examples are shown in Tables 1 and 2.

Sample name Heat resistant layer Adhesive layer Total thickness (탆) Floor composition Inorganic Particle Diameter (nm) Thickness (㎛) Polymer particle diameter (nm) Inorganic particle
Volume ratio (%) Example 1 3 section 600 0.8 500 0 Example 2 3 both sides 600 0.8 500 0 Example 3 3 section 600 0.8 500 25 Example 4 3 section 600 0.8 200 0 Comparative Example 1 0 - - 0.8 500 0 Comparative Example 2 3 both sides 600 0.8 50 0 Comparative Example 3 3 section 600 0.8 500 35

Sample name 130 ℃
Heat shrinkage [%] Gas permeability (sec) Adhesive strength (gf / cm) Moisture Content (ppm) Separator interlayer Between membrane and electrode Example 1 3.0 400 0.1 2.0 700 Example 2 3.0 410 0.1 2.0 720 Example 3 2.5 350 0 1.3 760 Example 4 3.0 480 0.2 2.5 780 Comparative Example 1 35.0 350 0.2 2.0 300 Comparative Example 2 3.0 2000 0.4 3.0 1200 Comparative Example 3 2.3 330 0 0.8 850

Claims (15)

A porous substrate layer;
A heat resistant layer formed on one or both surfaces of the porous substrate layer;
And an adhesive layer formed on the heat resistant layer,
Wherein the heat resistant layer is formed by bonding and fixing inorganic particles with a binder polymer, and the adhesive layer is formed by containing polymer particles. The method according to claim 1,
Wherein the porous substrate layer is a microporous stretched film made of polyolefin, and the polyolefin resin is one or two selected from polyethylene, polypropylene, and copolymers thereof. The method according to claim 1,
The polymer material constituting the adhesive layer and the heat resistant layer may be selected from the group consisting of polyacrylate, polybenzoate, polyvinyl pyrrolidone, polystyrene, polymethyl methacrylate, polybutyl acrylate, polyvinylidene fluoride and copolymers thereof A composite separator for a secondary cell comprising one or more selected polymeric materials. The method according to claim 1,
The inorganic particles of the heat resistant layer are composed of one or more selected from among aluminum oxide such as alumina and boehmite, barium titanium oxide, titanium oxide, magnesium oxide, and clay glass powder. Composite separator for secondary battery. The method according to claim 1,
Wherein the heat resistant layer comprises 60 to 99% by weight of inorganic particles and 40 to 1% by weight of a binder polymer based on 100% by weight of the total composition. The method according to claim 1,
Wherein the heat resistant layer has an inorganic particle size of 100 to 800 nm. The method according to claim 1,
Wherein the adhesive layer comprises 70 to 97% by weight of polymer particles and 30 to 3% by weight of inorganic particles based on 100% by weight of the total composition. The method according to claim 1,
Wherein the adhesive layer has a polymer particle size of 100 to 800 nm. The method according to claim 1,
Wherein the thickness of one layer of the adhesive layer is 1 占 퐉 or less. The method according to claim 1,
A composite membrane for a secondary battery having a gas permeability of 500 seconds or less. The method according to claim 1,
A composite separator for a secondary battery having a water content of 1000 ppm or less. The method according to claim 1,
And a heat shrinkage ratio at 130 占 폚 of 10% or less. The method according to claim 1,
Wherein the adhesive strength between the overlapped porous adhesive layers is 0.3 gf / cm or less at a temperature of 60 DEG C or less and a pressure of 1 MPa or less when the separator is laminated in two layers. The method according to claim 1,
Wherein the adhesive strength between the separator and the electrode is 1.0 gf / cm or more at a temperature of 110 캜 or more and a pressure of 3 MPa or more. A secondary battery produced by any one of claims 1 to 14.

Images