KR102368306B1 - Separator for secondary battery, preparing method thereof, and secondary battery including the same - Google Patents

Abstract

A first high-density polyethylene resin, a second high-density polyethylene resin having a greater density than the first high-density polyethylene resin, a secondary battery separator comprising a low molecular weight wax and inorganic particles, a manufacturing method thereof, and a secondary battery including the same are provided.

Description

Separator for secondary battery, manufacturing method thereof, and secondary battery including same {Separator for secondary battery, preparing method thereof, and secondary battery including the same}

It relates to a separator for a secondary battery, a manufacturing method thereof, and a secondary battery including the same.

A secondary battery represented by a lithium secondary battery has a high energy density and is widely used as a main power source for portable electronic devices such as mobile phones and notebook computers. Although higher energy density is required for such a secondary battery, securing safety becomes a technical problem.

In a secondary battery, when an electrolyte decomposition reaction and thermal runaway occur due to heat generated by an internal short circuit, overcharging, or overdischarging, the pressure inside the battery rapidly rises, which may cause an explosion of the battery.

The role of the separator is very important for the production of high-capacity batteries along with the recent trend of lighter and smaller batteries. In order to achieve high performance such as high output and high capacity as well as stability and reliability of the battery, a separator with a lower battery resistance is required. However, in general, when the electrical resistance is lowered, the withstand voltage and mechanical strength are lowered, which may cause a short circuit between the electrodes, so improvement is required.

One aspect is to provide a separator for a secondary battery having improved mechanical strength and productivity, and a method for manufacturing the same.

Another aspect is to provide a secondary battery with improved safety including the above-described separator.

According to one aspect,

There is provided a separator for a secondary battery comprising a first high-density polyethylene resin, a second high-density polyethylene resin having a greater density than that of the first high-density polyethylene resin, a low molecular weight wax, and inorganic particles.

Obtaining a precursor film by extrusion molding a resin composition comprising a first high-density polyethylene resin, a second high-density polyethylene resin, a low molecular weight wax and inorganic particles according to another aspect;

heat-treating the precursor film; and

There is provided a method of manufacturing a separator for a secondary battery comprising; stretching the heat-treated precursor film.

Anode according to another aspect; cathode; and the above-described separator interposed between the positive electrode and the negative electrode is provided.

By using the separator according to one embodiment, it is possible to manufacture a separator having excellent mechanical strength and low manufacturing cost while improving productivity by increasing moldability and extensibility. By using such a separator, it is possible to manufacture a secondary battery with improved safety without increasing cell resistance and lowering output.

1 is a schematic view showing the structure of a lithium secondary battery according to an embodiment.

Hereinafter, a separator according to an embodiment, a method for manufacturing the same, and a secondary battery including the same will be described in more detail.

There is provided a separator for a secondary battery comprising a first high-density polyethylene resin, a second high-density polyethylene resin having a greater density than that of the first high-density polyethylene resin, a low molecular weight wax, and inorganic particles.

The density of the first high-density polyethylene resin is 945 to 955 kg/m ³ , for example 950 kg/m ³ , and the flow index of the first high-density polyethylene resin is 0.05 g/10min to 0.2 g/10min, for example 0.08 g/m 10 min to 0.15 g/10 min, for example 0.1 g/10 min. And the density of the second high-density polyethylene resin is greater than 955 kg/m ³ and less than or equal to 965 kg/m ³ , for example, 960 kg/m ³ , and the flow index of the second high-density polyethylene resin is 0.25 to 0.5 g/10min, e.g. for example 0.25 to 0.45 g/10min, for example 0.27 to 0.35 g/10min, for example 0.32 g/10min.

The first high-density polyethylene resin and the second high-density polyethylene resin are high molecular weight polyethylene resins having different densities, and the first high-density polyethylene resin has a lower density than the second high-density polyethylene resin, improves mechanical strength, and is a polymer resin having elongation stabilization characteristics. The second high-density polyethylene resin is a polymer resin that is easy to cast and stretch and has excellent impact strength.

The first high-density polyethylene resin has a melting point of 120 to 145 °C, for example 125 to 140 °C, for example 134 °C. The first high-density polyethylene resin has an average molecular weight of 100,000 to 6,500,000 g/mol, for example 600,000 g/mol, and an average particle size of 50 to 200 μm, for example 100 to 150 μm, for example 110 μm. And the internal viscosity of the first high-density polyethylene resin is 500 cm ³ /g.

The second high-density polyethylene resin has a melting point of 120 to 145°C, for example, 125 to 140°C, for example, 134°C. And the density of the second high-density polyethylene resin is greater than 955 kg/m ³ and less than 965 kg/m ³ , for example, 960 kg/m ³ and environmental stress cracking resistance (ESCR) is 20 to 50 hours. , for example 24 to 45 hours, for example 40 hours. And the second high-density polyethylene resin has a tensile strength (tensile stress) of 20 to 35 MPa, for example, 21 to 33 MPa, for example, 31 MPa.

The second high-density polyethylene resin has excellent tensile strength, rigidity, and impact resistance, and also has excellent thermal stability during molding, excellent stress cracking resistance, long-term weatherability and creep resistance. (creep resistance) is excellent.

In the present specification, the flow index (Melt flow rate, MFR) is measured in accordance with ISO 1133 at a melt temperature of 190 ° C. and a melt load of 2.16 kg. And the density is measured according to ISO1183, and the internal viscosity is measured according to ISO 1628-3.

In addition, in the present specification, environmental stress cracking resistance (ESCR) is a measure of resistance characteristics such as cracks occurring in a harsh external environment, and is evaluated by D1693. And tensile strength can be measured according to ISO 527-1, 527-2.

As an example of the first high-density polyethylene resin, Celanese's Ticona GUR4018 having a flow index of 0.05 to 0.2 g/10 min, for example, 0.1 g/10 min at 190° C. and 2.16 kg according to ISO 1133 may be used. The density of Ticona GUR4018 in Celanese is about 950 kg/m ³ . And as an example of the second high-density polyethylene resin, the resin has a flow index of 0.25 g/10min to 0.5g/10min at 190°C and 2.16kg according to ISO 1133, for example, 5202BS of Prime polymer of 0.32g/10min. . The density of 5202BS of Prime Polymer is about 960 kg/m ³ .

A battery separator is required to have a thin film and mechanical strength as the capacity increases. In the case of a vehicle battery, the electrode area is expanded to increase the output of the battery, and accordingly, the ratio of the manufacturing cost of the separator to the total manufacturing cost of the battery increases.

Accordingly, the present inventors used a high-density, high-molecular-weight polyethylene resin having two different densities as a polymer resin when manufacturing a separator, and using a low-molecular wax together to improve the productivity and improve the moldability and stretchability of the unstretched film. The present invention for a secondary battery separator having low resistance and improved mechanical strength and a secondary battery using the same has been completed.

The low molecular weight wax may be one selected from low molecular weight polymers and oligomers. The low molecular weight wax should be selected to be miscible with the first high-density polyolefin resin and the second high-density polyolefin resin. For example, the low molecular weight wax may be a low molecular weight polyethylene wax.

The low molecular weight polyethylene wax has a molecular weight of 1000 to 8,000 g/mol, for example, 2,000 to 7,000 g/mol, for example, 3500 to 6500. The density is 910 to 990 kg/m ³ , for example 980 kg/m ³ , and the flow index is 2.0 to 7.0 g/10 min, for example 3.0 to 6.0 g/10 min, for example 5.0 g/10 min. And the crystallinity of the low molecular weight polyethylene wax is 60 to 88%, for example 82 to 87%, for example 85%. And the melting point is 115 to 128 ℃, for example, 126 ℃, and the softening point is 129 to 140 ℃, for example 136 ℃. Melt viscosity is 400 to 700 mPa ^. s, for example 600 mPa ^. s, and the hardness is less than 1X10 ^-1 mm.

Molecular weight and melt viscosity were measured using a viscometry. Density is measured according to JIS K7112, crystallinity is obtained through X-ray diffraction analysis, melting point is determined through DSC, and softening point and hardness are measured according to JIS K2207.

When the low molecular weight wax is used, the shutdown temperature of the separator is remarkably lowered, and when it is used, the releasability of the first high-density polyethylene resin and the first high-density polyethylene resin is improved, and the injection workability is improved.

Low molecular weight waxes include, for example, paraffin wax having a low molecular weight, microcrystalline wax, polyethylene wax, polypropylene wax, by-product polyethylene wax, Fischer-Tropsch wax, oxidized Fischer-Tropsch wax and functionalized wax. waxes may be mentioned. Functionalized waxes include, for example, hydroxy stearamide waxes and fatty amide waxes. As the low molecular weight wax, for example, high density low molecular weight polyethylene wax, by-product polyethylene wax, Fischer-Tropsch wax, etc. may be used.

The paraffin wax is, for example, PACEMAKER ® 30, 32, 35, 37, 40, 42, 45 & 53 available from Citgo Petroleum, Co.; ASTOR OKERIN® 236 available from Honeywell; R-7152 paraffin wax available from Moore &Munger; R-2540 available from Moore &Munzer; and other paraffin waxes such as those available from Sasol Wax under the trade names Sasolwax 5603, 6203 and 6805.

Microcrystalline waxes are, for example, those having at least 50% by weight of cyclo or branched alkanes having a length of from 30 to 100 carbons. It is generally less crystalline than paraffin and polyethylene waxes, and has a melting point in excess of about 70°C. Examples of microcrystalline waxes include: VICTORY® Amber wax, a 70°C melting point wax available from Baker Petrolite Corp.; BARECO® ES-796 amber wax, a 70°C melting point wax available from BARECO; BESQUARE ® 175 and 195 amber waxes, all 80° C. and 90° C. melting point microcrystalline waxes available from Baker Petroleum Corporation; Indramic® 91, a 90°C melting point wax available from Industrial Raw Materials; and PETROWAX® 9508 Light, a 90°C melting point wax available from Petrowax. Other examples of microcrystalline waxes are Sasol Wax 3971 available from Sasol Wax and Microwax K4001 available from Alfred Kochem GmBH.

Exemplary high-density, low-molecular-weight polyethylene waxes include POLYWAX ^™ from Baker Petrolit Corporation. 500, an ethylene homopolymer available as Polywax ^TM 1500 and Polywax ^TM 2000, a polyethylene wax, HI-WAX 400P from Mitsui chemicals with a flow index of 5.0 g/min at 2.16 kg at 190° C. according to ISO 1133 (Melting point: 126° C., density: 0.965 to 0.985 g/cm ³ , for example about 0.98 g/cm ³ (molecular weight: 3500 to 6500), etc. Polywax ^TM 2000 has a molecular weight of approximately 2000, Mw/Mn of approximately 1.0 , has a density of about 0.97 g/cm ³ at 16° C. and a melting point of about 126° C.

The mixing volume ratio of the first high-density polyethylene resin and the second high-density polyethylene resin is 1.1:1 to 3:1, for example, 1.5:1 to 3:1. When the mixing volume ratio of the first high-density polyethylene resin and the second high-density polyethylene resin is within the above range, a separator having excellent processability and improved strength can be obtained.

The content of the low molecular weight wax is 10 to 30 parts by volume, for example, 15 to 25 parts by volume, based on 100 parts by volume of the total volume of the separator. If the low molecular weight wax is not added, the processability of the separation membrane is greatly reduced, and the air permeability and tensile strength are lowered.

Hereinafter, the separation membrane manufacturing method of the present invention will be described. In the present invention, by stretching a precursor film containing inorganic particles as a filler, the polymer resin/inorganic particle interface is peeled off to form empty pores, so that a separator, which is a porous film, can be manufactured.

First, a precursor film is obtained by extrusion molding a resin composition including the first high-density polyethylene resin, the second high-density polyethylene resin, low molecular weight wax, and inorganic particles.

The total content of the first high-density polyethylene resin and the second high-density polyethylene resin in the resin composition is 5 to 80 parts by volume based on 100 parts by weight of the total volume of the resin composition. The total volume of the resin composition is the same as the total volume of the separator, and refers to the total volume of the first high-density polyethylene resin, the second high-density polyethylene resin, low molecular weight wax, and inorganic particles. When the total content of the first high-density polyethylene resin and the second high-density polyethylene resin is within the above range, physical properties such as mechanical strength and thermal stability of the separator are improved.

The resin composition may use other polymer resins in addition to the high-density polyolefin-based resin.

The content of the low molecular weight wax in the resin composition is 10 to 30 parts by volume, for example, 15 to 25 parts by volume, based on 100 parts by volume of the total volume of the resin composition.

During the extrusion molding, the first high-density polyethylene resin, the second high-density polyethylene resin, the low molecular weight wax and the inorganic particles are mixed and dispersed in advance and supplied to the extruder to undergo melt extrusion molding. Alternatively, it is also possible to separately supply the first high-density polyethylene resin, the second high-density polyethylene resin, the low-molecular-weight wax, and the inorganic particles, and melt-extrusion the same. Extrusion molding may use a single-screw or twin-screw extruder, or an air-cooled inflation molding method may be used.

For example, in the air-cooled inflation molding method, the resin and inorganic particles are kneaded into pellets, put into a hopper, which is an inflation molding device, and the extrusion temperature is set to 160~220℃, extruded in a tube form from a circular die, and then air is sealed and expanded. How to wind it up. An unstretched precursor film can be made by adjusting the extrusion amount so that the resin passing speed of the circular die and the draw ratio (DR) of the winding machine are 30-200dl. When the DR is within the above range, it is easy to adjust the film thickness during film formation, and uniform pores can be formed in the separator without a problem in which breakage (film breakage) occurs a lot. In the case of inflation molding, the blow-up ratio (BUR) is usually set to 1.0 to 2.0, and more preferably 1.2 to 1.8. In the case of extrusion molding under the above conditions, a desired precursor film can be made, and then, a separation membrane having a desired porosity can be obtained through a stretching process. The stretching step may be performed according to the process of stretching 0.3 times to 3 times at room temperature first, and 0.3 times to 3 times at second high temperature in the MD direction (machine direction). Here, the room temperature is 25°C and the high temperature is 100 to 150°C, for example, 120°C.

The inorganic particles are boehmite, alumina, aluminum oxyhydroside (AlOOH), zirconia, yttria, ceria, magnesia, titania, silica, aluminum carbide, titanium carbide, tungsten carbide, boron nitride, aluminum nitride. Calcium carbonate, barium sulfate, aluminum hydroxide, magnesium hydroxide, or a combination thereof, and the size of the inorganic particles is 0.1 μm or more, for example 0.1 to 10 μm, for example 1 to 5 μm. And the porosity of the separator is 10 to 95%, for example, 35 to 50%, and the size of the pores is 0.01 to 50㎛. In the present specification, the term "size of inorganic particles" indicates an average diameter when the inorganic particles are spherical, and indicates a major axis length when the inorganic particles are non-spherical. The size of the inorganic particles can be measured using a particle size measuring device or using an electron scanning microscope or the like. And, in the present specification, the term "pore size" indicates a diameter when the pores are spherical, and a major axis length when the pores are non-spherical.

The inorganic particles may have a spherical shape, a plate shape, or the like.

The content of inorganic particles in the resin composition is 10 to 20 parts by volume based on 100 parts by volume of the resin composition. 100 parts by volume of the resin composition is equal to 100 parts by volume of the total volume of the separator. When the content of inorganic particles is within the above range, the separation membrane has excellent pore uniformity and excellent mechanical strength, air permeability, and thermal stability.

Then, the precursor film is heat-treated. The heat treatment may be annealing.

Annealing is a process that promotes the formation of micropores during stretching by improving the crystal structure and orientation structure by performing heat treatment. By proceeding with annealing, it is possible to alleviate the influence of variations in crystal orientation that occurs during film formation and to achieve uniformity of characteristics. The annealing is determined in consideration of the melting points of the first high-density polyethylene resin and the second high-density polyethylene resin, and is performed, for example, at 80 to 135°C.

Then, the heat-treated precursor film is stretched.

The stretching process consists of low-temperature stretching, high-temperature stretching, and heat treatment (relaxation and heat setting) processes. The stretching step may be one step or multiple steps of two or more steps. However, when the number of stages is two or more, the properties as a porous film are further stabilized.

The stretching temperature of the low-temperature stretching process is possible within the range of -10 to 50 ° C. However, in order to form micropores having adequate air permeability as a porous film, it is suitable to set the stretching temperature to 10 to 35 ° C. When the stretching temperature is within the above range, condensation appears on the roll when the stretching temperature is low, and the film slides off the roll, making it difficult to match the actual stretching ratio or the stretching ratio in the width direction of the film (TD) is non-uniform. Porosity can be achieved. In addition, when stretching at the above temperature, the elastic modulus of the resin composition, which is a matrix, is lowered, so that the degree of concentration of stress at the interface with the inorganic particles is reduced, thereby preventing a phenomenon in which porosity becomes difficult. The draw ratio is possible in the range of 0.25 to 3 times, and 0.5 to 2 times is more appropriate. When the draw ratio is within the above range, the ventilation time of the porous film is appropriate, and the phenomenon of fracture during low-temperature stretching is prevented in advance.

The stretching temperature of the high-temperature stretching process can be set within the range of 100 to 130°C. When the stretching temperature of the high-temperature stretching process is within the above range, the stretching temperature is low, the film thickness and width of the stretched film are increased, and the pore size is small, so that there is no problem in that the ventilation time of the finished porous film is long. Also, there is no occurrence of a phenomenon in which the film is melted due to a high stretching temperature and thus porosity becomes difficult. The draw ratio is possible within the range of 0.25 to 5 times, but 0.3 to 3 times is more appropriate. The ratio of the low-temperature stretch ratio and the high-temperature stretch ratio is different for each material, and the state of pore formation can be controlled depending on how the draw ratio of the low-temperature stretching and the high-temperature stretching is divided according to the overall stretch ratio.

Extending|stretching can be performed at the temperature of 100 degreeC - 130 degreeC, for example. If the stretching is carried out in the above-described range, it can have adequate air permeability and mechanical strength without blocking the pores in the separation membrane.

After the above-described stretching, the heat treatment may be further rough. The heat treatment is performed below the melting point and softening point of the first high-density polyethylene resin, the second polyethylene resin, and the low molecular weight wax to control so that the shape change of the separator does not substantially occur. The heat treatment temperature is carried out at a temperature higher than the stretching temperature, but in the range of 20° C. or less compared to the stretching temperature. If the heat treatment temperature is the same as the stretching temperature or lower than the stretching temperature, the distortion caused by the high temperature stretching is not removed. In addition, when the heat treatment temperature is within the above range, it is possible to prevent the product value from being reduced because the film shrinkage limit is reached and sagging occurs.

In the manufacturing method of the present invention, a porous film is formed using an interfacial peeling method of inorganic particles. In this method, since dry porosity is possible without using a solvent, the environmental load is small, and the winding speed of the porous film is increased when manufactured with a stretching device, and thus productivity can be improved.

The separator of the present invention may prepare a multilayer precursor film of two or more layers by laminating two or more precursor films prepared according to the above process. In another example, a multi-layered precursor film having two or more layers may be manufactured by using co-extrusion.

The porosity of the separation membrane is 10 to 95%, for example, 30 to 80%, for example, 35 to 50%, and the size of the pores is 0.01 to 50㎛. The total thickness of the separation membrane is, for example, 10 to 20 μm, for example, 14 to 20 μm. The thickness of the separator is 14 to 20 μm. Within the above range, the pores formed in the separator are sufficiently opened, so that the ion conductivity is excellent and the battery output and battery performance can be improved.

The air permeability of the separation membrane can be measured according to JIS P 8117. For example, 10 specimens cut at 10 different points are prepared, and then, a separator having a circular area of 1 inch in diameter in each of the specimens using an air permeability measuring device (Asahi Seiko Co., Ltd.) transmits 100 cc of air. Measure the average time taken five times each, and then calculate the average value to measure the air permeability.

The separation membrane according to one embodiment has a tensile strength of 1,200 kgf/cm ² or more, a puncture strength per membrane thickness of 15 gf/μm or more, and a permeability of 10 to 12.5 sec/100cc per membrane thickness.

By employing the separator of the present invention, it is possible to manufacture a secondary battery with improved safety while securing thermal and mechanical properties.

According to another aspect, there is provided a secondary battery including a positive electrode, a negative electrode, and the above-described separator interposed between the positive electrode and the negative electrode.

The secondary battery may be, for example, a lithium secondary battery such as a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

The secondary battery may be manufactured using a method commonly used in the technical field of the present invention.

The positive electrode and the negative electrode are prepared by coating and drying a composition for forming a positive electrode active material layer and a composition for forming a negative electrode active material layer on a current collector, respectively.

The composition for forming the cathode active material is prepared by mixing a cathode active material, a conductive agent, a binder, and a solvent, and the cathode active material according to an embodiment is used as the cathode active material.

The binder is a component that assists in bonding between the active material and the conductive agent and bonding to the current collector, and includes, but is not limited to, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, and hydrogen. Roxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, Various copolymers, etc. are mentioned.

The conductive agent is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, graphite such as natural graphite or artificial graphite; carbon-based substances such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; conductive fibers such as carbon fibers and metal fibers; carbon fluoride; metal powders such as aluminum and nickel powder; conductive whiskeys such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used.

As a non-limiting example of the solvent, N-methyl 2-pyrrolidone and the like are used.

The content of the binder, the conductive agent and the solvent is at a typical level.

The positive electrode current collector has a thickness of 3 to 500 μm, and is not particularly limited as long as it has high conductivity without causing chemical change in the battery, for example, stainless steel, aluminum, nickel, titanium, heat-treated carbon, Alternatively, a surface treated with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel may be used. The current collector may increase the adhesion of the positive electrode active material by forming fine irregularities on the surface thereof, and various forms such as a film, sheet, foil, net, porous body, foam body, and non-woven body are possible.

Separately, a composition for forming the anode active material layer is prepared by mixing the anode active material, a binder, a conductive agent, and a solvent.

The negative active material is a non-limiting example, and graphite, a carbon-based material such as carbon, lithium metal, an alloy thereof, a silicon oxide-based material, and the like may be used.

The binder, the conductive agent, and the solvent may be the same type of material as that used for manufacturing the positive electrode.

As the negative electrode current collector, it is generally made to have a thickness of 3 to 500 μm. Such a negative current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, heat-treated carbon, or a surface of copper or stainless steel. Carbon, nickel, titanium, silver, etc. surface-treated, aluminum-cadmium alloy, etc. may be used. In addition, like the positive electrode current collector, the bonding strength of the negative electrode active material may be strengthened by forming fine irregularities on the surface, and may be used in various forms such as a film, sheet, foil, net, porous body, foam, non-woven body, and the like.

A secondary battery may be manufactured by interposing the separator according to an embodiment between the positive electrode and the negative electrode manufactured according to the above process.

The secondary battery includes a non-aqueous electrolyte. The non-aqueous electrolyte includes a lithium salt, and at least one selected from a non-aqueous electrolyte, an organic solid electrolyte, and an inorganic solid electrolyte.

Examples of the non-aqueous electrolyte include, but are not limited to, N-methyl 2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2- Dimethoxyethane, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, N,N-formamide, N,N-dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate , methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, pyro An aprotic organic solvent such as methyl pionate or ethyl propionate may be used.

Examples of the organic solid electrolyte include, but are not limited to, polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphoric acid ester polymers, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and the like.

The inorganic solid electrolyte may include, but is not limited to, Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ and the like may be used.

The lithium salt is a material readily soluble in the non-aqueous electrolyte, and includes, but is not limited to, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, (FSO ₂ ) ₂ NLi, lithium chloroborate, a compound represented by the following formula, a mixture thereof, or a combination thereof may be used.

1 is a cross-sectional view schematically illustrating a typical structure of a lithium secondary battery, which is one of secondary batteries according to an embodiment.

Referring to FIG. 1 , the lithium secondary battery 100 includes a positive electrode 40 , a negative electrode 50 , and a separator 10 . The electrode assembly 60 obtained by winding or folding the above-described positive electrode 40 , the negative electrode 50 , and the separator 10 is accommodated in the battery case 70 . Next, an electrolyte solution (not shown) is injected into the battery case 25 to complete a lithium secondary battery. The battery case 70 has a rectangular shape in FIG. 1 . The lithium secondary battery may be a lithium ion battery. The electrode assembly 60 may have a jelly roll shape. A plurality of the electrode assemblies are stacked to form a battery pack, and the battery pack can be used in any device requiring high capacity and high output. For example, it can be used in a laptop computer, a smart phone, an electric vehicle, and the like.

In addition, since the lithium secondary battery has excellent storage stability, lifespan characteristics, and high rate characteristics at high temperatures, it can be used in electric vehicles (EVs). For example, it may be used in a hybrid vehicle such as a plug-in hybrid electric vehicle (PHEV).

The present invention will be described in more detail through the following examples and comparative examples. However, the examples are provided to illustrate the present invention, and the scope of the present invention is not limited thereto.

Example One

After mixing the polymer resins A-1, A-2, A-3, and inorganic particles B-2 to be described later in the composition shown in Table 1 below, extruding at about 195° C. using an inflation film forming device, and blow-up ratio A precursor film having a thickness of 30 μm was prepared by setting (BUR) to 1.6.

The precursor film was annealed at 110° C. for at least 1 hr, 100% first stretched once in the MD direction at 25° C., and then 100% second stretched at 120° C. in the MD direction to form a porous film as a secondary battery separator having a thickness of 16 μm. prepared.

<Polymer resin>

A-1: High-density polyethylene resin, Ticona GUR4018 from Celanese with a flow index of 0.1 g/10 min at 190°C and 2.16 kg according to ISO 1133 (density: about 950 kg/m ³ )

A-2: High-density polyethylene resin, 5202BS of Prime Polymer with a flow index of 0.32g/10min at 190℃ and 2.16kg according to ISO 1133 (Density: about 960 kg/m ³ )

A-3: Low molecular weight polyethylene wax (wax) according to ISO 1133 at 190°C and 2.16kg, with a flow index of 5.0g/10min at Mitsui Chemicals' HI-WAX 400P (Melting point: 126°C, density: 0.98g/cm) ³ )

<Weapon Particles>

B-1: RA (CaCO3) octachloride from Dongho Calcium with a diameter of 60 nm as a spherical filler

B-2: HBT-006-M (Al2O3) from Sinocera with a diameter of 60 nm as a spherical filler

B-3: Sinocera's (AlOOH) with a plate shape and a diameter of 60 nm

B-4: Spherical filler with a diameter of 60 nm from Sakai (BaSO4)

<Examples 2-3 and Comparative Examples 1 to 7>

A porous film, which is a separator for a secondary battery, was prepared in the same manner as in Example 1, except that a polymer resin and inorganic particles were mixed with the composition shown in Tables 1 and 2 below.

resin composition Example 1 Example 2 Example 3 mixing ratio
(volume) profit A-1 45 45 40 A-2 22.5 30 25 A-3 22.5 15 25 inorganic particles B-1 - - - B-2 10 10 10 B-3 - - - B-4 - - -

resin composition comparative example
One Comparative Example 2 Comparative Example 3 comparative example
4 comparative example
5 comparative example
6 comparative example
7 mixing ratio
(volume) profit A-1 - - - - 70 45 - A-2 90 90 90 90 - 45 72 A-3 - - - - 20 - 18 inorganic particles B-1 10 - - - - - - B-2 - 10 - - 10 10 10 B-3 - 10 - - - - B-4 - - 10 - - -

Example 4: lithium secondary battery Produce

A lithium secondary battery was manufactured by using the separator of Example 1 according to the method to be described later.

LiNi _{0 . 33} Co _{0 . 33} Mn _{0 . 33} O ₂ 97.45 wt%, artificial graphite (SFG6, Timcal) powder 0.5 wt% as a conductive material, carbon black (Ketjenblack, ECP) 0.7 wt%, modified acrylonitrile rubber (BM-720H, Zeon Corporation) 0.25 wt%, Polyvinylidene fluoride (PVdF, S6020, Solvay) 0.9% by weight and polyvinylidene fluoride (PVdF, S5130, Solvay) 0.2% by weight were mixed and added to N-methyl-2-pyrrolidone solvent, followed by a mechanical stirrer was stirred for 30 minutes to prepare a cathode active material slurry. The slurry was applied to a thickness of about 60 μm on an aluminum current collector of a thickness of 20 μm using a doctor blade, dried in a hot air dryer at 100° C. for 0.5 hours, and then dried again for 4 hours under vacuum and 120° C. conditions, and rolled (roll press) to prepare a positive electrode plate.

The slurry prepared according to the above process was coated on an aluminum foil using a doctor blade to form a thin electrode plate, dried at 135° C. for more than 3 hours, and then rolled and vacuum dried to prepare a positive electrode.

98% by weight of artificial graphite (BSG-L, Tianjin BTR New Energy Technology Co., Ltd.), 1.0% by weight of a styrene-butadiene rubber (SBR) binder (ZEON), and 1.0% by weight of carboxymethyl cellulose (CMC, NIPPON A&L) After mixing, it was added to distilled water and stirred for 60 minutes using a mechanical stirrer to prepare a negative electrode active material slurry. The slurry was applied to a thickness of about 60 μm on a copper current collector having a thickness of 10 μm using a doctor blade, dried in a hot air dryer at 100° C. for 0.5 hours, and then dried again under vacuum and 120° C. conditions for 4 hours, A negative electrode was prepared by rolling (roll press).

As the electrolyte, a solution containing 1.5M LiPF ₆ dissolved in a solvent obtained by mixing ethylene carbonate (EC), ethylmethyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 2:2:6 was used. .

A separator was interposed between the positive electrode and the negative electrode, and electrolyte was injected to prepare a prismatic lithium secondary battery.

Example 5-6

A lithium secondary battery was manufactured in the same manner as in Example 4, except that the separator of Example 2 was used instead of the separator of Example 1.

evaluation example 1: Breathability

According to Examples 1 to 4 and Comparative Examples 1 to 7, the unit thickness air permeability of each polyethylene separator was measured according to JIS P 8117 using an air permeability measuring instrument (Asahi Seiko EGO-1).

After making 10 specimens cut at 10 different points to a size that can fit a circle with a diameter of 1 inch (inch), ASAHI SEIKO OKEN TYPE Air Permeation Tester EG01-55-1MR (Asahi Seiko g) was used to measure the passage time of 100 cc of air in each of the specimens. After measuring the time five times, the average value was calculated to measure the air permeability.

[Conditions for setting air permeability measuring device]

Measuring pressure: 0.5 kg/cm2, cylinder pressure: 2.5 kg/cm2, set time: 10 seconds

The average of the data was recorded by measuring 10 or more times at intervals of 10 cm for a 1 m specimen.

evaluation example 2: puncture strength

In order to measure the puncture strength of the separators prepared in Examples 1 to 3 and Comparative Examples 1 to 7, the following experiment was performed.

10 specimens were prepared by cutting each of the separators prepared in Examples and Comparative Examples at 10 different points with a width (MD) of 50 mm Х length (TD) of 50 mm, and then using the GATO Tech G5 equipment After placing the specimen on the 10 cm hole, the force of piercing was measured while pressing it with a 1 mm probe. The puncture strength of each specimen was measured three times, and then the average value was calculated. The evaluation results of the puncture strength are as shown in Table 3 below.

evaluation example 3: Tensile strength

In order to measure the tensile strength of the separators prepared in Examples 1 to 3 and Comparative Examples 1 to 7, the following experiment was performed.

10 specimens were prepared by cutting each of the separators prepared in Examples and Comparative Examples at 10 different points in a rectangular shape with a width of 10 mm Х and a length of 50 mm. It was mounted on a UTM (tensile tester) and bitten so as to have a measurement length of 20 mm, and then the specimen was pulled to measure average tensile strength in the MD and TD directions. The evaluation results of tensile strength are shown in Table 3 below.

division Example 1 Example 2 Example 3 comparative example
One comparative example
2 comparative example
3 comparative example
4 comparative example
5 comparative example
6 comparative example
7 per film thickness
breathability
(sec/100cc)/μm) 11.1 12.3 10.2 13.0 10.9 13.7 14.0 18.2 21.1 12.5 Puncture strength per film thickness
(gf/μm) 18.0 17.7 15.8 16.1 15.4 12.6 16.4 16.7 16.9 10.0 Tensile strength (kgf/cm ² ) 1508 1400 1270 1270 1147 1174 1348 1460 1659 851

As shown in Table 3, the separators of Examples 1 to 3 have excellent workability, puncture strength per film thickness of 15 gf/μm or more, and tensile strength of 1,200 kgf/cm ² or more. showed excellent properties by satisfying the range of 10 to 12.5 sec/100cc. The processability of the separator can be evaluated as excellent when the polymer resin constituting the separator for manufacturing the porous film using the interfacial peeling method is well mixed in an appropriate ratio and stretched to satisfy the desired level of air permeability characteristics.

In contrast, the separator of Comparative Example 1 had excellent puncture strength and tensile strength, but had insufficient air permeability characteristics. could not be reached And the separator of Comparative Example 3 had excellent puncture strength, but did not reach satisfactory levels of air permeability and tensile strength, and the separators of Comparative Examples 4 to 6 had excellent puncture strength and tensile strength, but did not reach satisfactory levels of air permeability characteristics. showed

Although the above has been described with reference to the preferred manufacturing examples, it will be understood by those skilled in the art that various modifications and changes can be made without departing from the spirit and scope set forth in the following claims.

10: separator 40: positive electrode
50: negative electrode 60: electrode assembly
70: battery case 100: lithium secondary battery

Claims (14)

It includes a first high-density polyethylene resin, a second high-density polyethylene resin having a greater density than the first high-density polyethylene resin, a low molecular weight wax, and inorganic particles,
The first high-density polyethylene resin has a density of 945 to 955 kg/m ³ ,
The density of the second high-density polyethylene resin is greater than 955 kg/m ³ and less than or equal to 965 kg/m ³ ,
The content of the low molecular weight wax is 10 to 30 parts by volume based on 100 parts by volume of the total volume of the separator,
The low molecular weight wax is a low molecular weight polyethylene wax,
The low molecular weight polyethylene wax has a molecular weight of 300 to 10,000 g/mol, a density of 910 to 990 kg/m ³ , a flow index of 2 to 7 g/10min, and a melting point of 115 to 128° C., a separator for secondary batteries. delete According to claim 1,
A secondary battery separator having a mixing volume ratio of the first high-density polyethylene resin and the second high-density polyethylene resin of 1.1:1 to 3:1. delete delete According to claim 1,
The flow index of the first high-density polyethylene resin is 0.05 to 0.2 g/10min, the average molecular weight is 100,000 to 6,500,000 g/mol,
The second high-density polyethylene resin has a flow index of 0.25 to 0.5 g/10 min, an environmental stress cracking resistance (ESCR) of 20 to 50 hours, and a tensile strength of 20 to 35 MPa. . According to claim 1,
The content of the inorganic particles is 10 to 15 parts by volume based on 100 parts by volume of the total volume of the separator,
The average particle diameter of the inorganic particles is 0.1㎛ or less secondary battery separator. According to claim 1,
The inorganic particles have an average particle diameter of 40 to 70 nm. According to claim 1,
The inorganic particles are boehmite, alumina, aluminum oxyhydroside (AlOOH), zirconia, yttria, ceria, magnesia, titania, silica, aluminum carbide, titanium carbide, tungsten carbide, boron nitride, aluminum nitride. A separator for secondary batteries comprising calcium carbonate, barium sulfate, aluminum hydroxide, magnesium hydroxide, or a combination thereof. According to claim 1,
The separator for a secondary battery has a tensile strength of 1,200 kgf/cm ² or more, a puncture strength per film thickness of 15 gf/μm or more, and a permeability per film thickness of 10 to 12.5 sec/100cc. Extruding a resin composition including the first high-density polyethylene resin, the second high-density polyethylene resin, and a low molecular weight wax and inorganic particles to obtain a precursor film;
heat-treating the precursor film; and
Including; stretching the heat-treated precursor film;
The blow-up ratio (BUR) at the time of extrusion is 1.0 to 2.0,
The stretching step is carried out according to the process of stretching 0.3 times to 3 times at the first room temperature in the MD direction (machine direction), and 0.3 times to 3 times at the second high temperature, so as to claim 1, 3, 6 to A method for producing a separator for a secondary battery to obtain the separator of any one of claims 10 to 11. delete delete anode; cathode; and a secondary battery comprising the separator of any one of claims 1, 3, and 6 to 10 interposed between the positive electrode and the negative electrode.

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