Composite film and its preparation method

Abstract

An embodiment of the present invention provides a composite membrane including a polyolefin-based porous carrier and a solid electrolyte impregnated inside the carrier, and a method for preparing the same.

Description

Technical Field

[0001] The present invention relates to a composite film and a method for preparing the same, and more particularly, to a composite film including a solid electrolyte that can be used as a battery separator and a method for preparing the same. Background Art

[0002] Compared with other batteries, lithium secondary batteries have excellent energy density and service life. Therefore, lithium secondary batteries have been widely used in fields ranging from small secondary batteries such as smartphones and laptop computers to fields that require large and medium-sized secondary batteries such as electric vehicles and energy storage systems (ESS).

[0003] In existing lithium secondary batteries, a liquid electrolyte is used. The liquid electrolyte has excellent ionic conductivity. However, on the contrary, there are risks such as fire and explosion caused by physical impact or high temperature, so the stability is poor. In particular, for use in electric vehicles and ESS that use a large number of lithium secondary batteries, further improvement in stability is required.

[0004] To solve this problem, research on next-generation lithium secondary batteries using solid electrolytes has been actively carried out mainly by universities. As a representative solid electrolyte, there is a NASICON-type inorganic electrolyte, which is characterized by having a composition of LiM2(PO4)3. However, compared with liquid electrolytes, these NASICON-type solid electrolytes have poor ionic conductivity, high manufacturing costs due to complex manufacturing methods, and an increase in interfacial resistance between the electrodes when manufacturing battery cells due to being hard and brittle, resulting in problems in manufacturing large-capacity batteries.

[0005] As another example of a solid electrolyte, there is a gel electrolyte formed by compounding a poly(ethylene oxide)-based polymer and a lithium salt. However, the disadvantages of the gel electrolyte are poor physical strength, poor heat resistance due to the low melting point of the polymer, and a decrease in ionic conductivity at room temperature and high temperature due to the crystallization tendency and ion migration resistance.

[0006] Prior Art Documents

[0007] (Non-Patent Document 1) Yun-Chae Jung et al., All Solid-State Lithium Batteries Assembled with Hybrid Solid Electrolytes, Journal of The Electrochemical Society, 162(4), A704-A710. (2015) Summary of the Invention

[0008] The present invention is developed to solve the above-mentioned prior art problems, and an object of the present invention is to provide a composite film that can be used in next-generation batteries and is formed by impregnating a solid electrolyte into a polyolefin-based porous carrier.

[0009] One aspect of the present invention provides a composite film including a polyolefin-based porous carrier and a solid electrolyte impregnated inside the carrier.

[0010] In one embodiment, the solid electrolyte may be impregnated in at least a part of the micropores inside the polyolefin-based porous carrier.

[0011] In one embodiment, the solid electrolyte may be coated on at least a part of the surface of the carrier.

[0012] In one embodiment, the carrier may include two polyethylenes with different weight average molecular weights, 30 wt% to 90 wt%, and 10 wt% to 70 wt% of an inorganic filler.

[0013] In one embodiment, the polyethylene may include: a first polyethylene having a weight average molecular weight of 1,000,000 to 3,000,000; and a second polyethylene having a weight average molecular weight of 200,000 to 500,000.

[0014] In one embodiment, the carrier may have a porosity of 50% to 90% and an average pore diameter of 20 nm to 100 nm.

[0015] In one embodiment, the solid electrolyte may include a lithium salt, an inorganic electrolyte, and a polymer.

[0016] In one embodiment, the inorganic electrolyte may be a compound represented by the following Chemical Formula 1 or 2:

[0017] [Chemical Formula 1]

[0018] Li 1+x Al xGe 2-x (PO4)3

[0019] [Chemical Formula 2]

[0020] Li 1+x Al x Ti 2-x (PO4)3

[0021] In the above Chemical Formula 1 or 2, 0 ≤ x ≤ 2.

[0022] In one embodiment, the above polymer may be one selected from the group consisting of an oxide-based polymer, a nitrile-based polymer, a fluorine-based polymer, an acrylic-based polymer, a sulfone-based polymer, and a copolymer of two or more thereof.

[0023] In one embodiment, the above composite film may have a thickness of 5 μm to 100 μm, a tensile strength of 500 kgf / cm 2 to 2500 kgf / cm 2 a tensile elongation of 50% to 300%, and a puncture strength of 100 gf to 600 gf.

[0024] Another aspect of the present invention provides a method for preparing a composite film, characterized by including: step (a) of preparing an electrolyte solution by mixing a lithium salt, an inorganic electrolyte, and a polymer in a solvent; and step (b) of filling the above electrolyte solution into the pores of the above porous carrier.

[0025] In one embodiment, the weight ratio of the above inorganic electrolyte and polymer may be 6 to 8:2 to 4, respectively.

[0026] According to one aspect of the present invention, a composite film including a solid electrolyte can be provided.

[0027] According to another aspect of the present invention, a composite film having excellent physical strength and ion conductivity compared to an existing NASICON-type electrolyte separator membrane or gel electrolyte membrane can be provided.

[0028] According to still another aspect of the present invention, a composite film that can be easily thinned compared to an existing electrolyte separator membrane, and thus is easy to increase the capacity and area of a battery, can be provided.

[0029] The effects of the present invention are not limited to the above effects, and it should be understood that all effects deduced from the structure of the invention described in the detailed description or claims of the present invention are included. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a diagram illustrating a method for preparing a composite film including a solid electrolyte and a porous carrier according to an embodiment of the present invention.

[0031] Figure 2 It is a graph showing the result values of the ionic conductivity of the composite film prepared according to an embodiment of the present invention. Detailed Description of the Invention

[0032] The present invention will be described more fully hereinafter with reference to the accompanying drawings. However, the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. In the drawings, parts not relevant to the description are omitted for simplicity of illustration, and the same reference numerals always denote the same elements.

[0033] Throughout the specification, when it is described that a certain part is "connected" to another part, this includes not only the case of "direct connection" but also the case of "indirect connection" with other components interposed therebetween. And when it is described that a certain part "includes" a certain component, it means that other components may also be present without particularly contrary description, rather than excluding other components.

[0034] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0035] Generally, the term "electrolyte" refers to a material that has conductivity by being dissolved in a polar solvent and dissociating into ionic states. An electrolyte is an essential component in battery manufacturing, and a material with excellent ionic conductivity while restricting the transfer of electrons must be used. As the electrolyte, carbonate-based liquid electrolytes have mainly been used in the past, but they have safety problems such as being prone to evaporation, leakage, ignition, and explosion.

[0036] A "solid electrolyte" is a material in which ions can easily move in a solid state. Its ionic conductivity is poorer than that of a liquid electrolyte, but it is an essential component in the development of next-generation batteries because it does not have the above-mentioned safety problems.

[0037] The composite film according to one aspect of the present invention may include a polyolefin-based porous carrier and a solid electrolyte impregnated inside the carrier. Further, the solid electrolyte may be coated on at least a part of the surface of the carrier.

[0038] The solid electrolyte may be impregnated in at least some of the micropores inside the polyolefin-based porous carrier.

[0039] When manufacturing a battery using the composite film of the present invention, the solid electrolyte impregnated inside the above-mentioned carrier can come into contact with the electrode included on one side of the above-mentioned battery to move ions to the electrode on the other side. In addition, when the above-mentioned solid electrolyte is coated on at least a part of the surface of the above-mentioned carrier, the contact area with the electrode increases, thereby promoting the movement of the above-mentioned ions. Therefore, as the area coated with the above-mentioned solid electrolyte increases, the performance of the manufactured battery can be improved.

[0040] The above-mentioned carrier may include two polyethylenes with different weight average molecular weights of 30% to 90% by weight and 10% to 70% by weight of an inorganic filler. The above-mentioned polyethylene may include: a first polyethylene having a weight average molecular weight of 1,000,000 to 3,000,000 and a molecular weight distribution of 3 to 4; and a second polyethylene having a weight average molecular weight of 200,000 to 500,000 and a molecular weight distribution of 4 to 7. For example, the above-mentioned polyethylene may include 30% to 70% by weight of the above-mentioned first polyethylene and 30% to 70% by weight of the second polyethylene.

[0041] Generally, the wider the molecular weight distribution (M w / M n ), the lower the shear stress, resulting in a decrease in viscosity, so the processability is improved, but the physical properties are reduced; while the narrower the molecular weight distribution, the worse the processability, but the physical properties are improved. As described above, even if two or more polymer materials are kneaded and used in the form of a composition, if the respective molecular weight distributions are similar, the physical properties and processability cannot be coordinated. Therefore, as described above, by mixing the first polyethylene and the second polyethylene with different weight average molecular weights and molecular weight distributions, the physical properties and processability of the porous membrane can be more coordinated.

[0042] The above-mentioned inorganic filler may be one selected from the group consisting of silica (SiO2), TiO2, Al2O3, zeolite, AlOOH, BaTiO2, talc (Talk), Al(OH)3, CaCO3, and a mixture of two or more thereof. Preferably, it may be spherical nanoparticles having an average particle diameter of 10 nm to 1,000 nm. Preferably, the nanoparticles are surface-hydrophobized or hydrophilized. The content of the above-mentioned inorganic filler in the above-mentioned porous membrane may be 10% to 70% by weight, preferably 10% to 60% by weight. When the content of the above-mentioned inorganic filler is less than 10% by weight, the mechanical strength, acid resistance, chemical resistance, and flame retardancy of the above-mentioned porous membrane may be reduced, while when the content of the above-mentioned inorganic filler is greater than 70% by weight, the flexibility and processability of the above-mentioned porous membrane may be reduced. By mixing the inorganic filler in the above-mentioned carrier, a carrier with excellent mechanical strength can be prepared even at a high porosity.

[0043] For example, silicon dioxide (SiO2) may have a hydrocarbon layer formed on its surface and composed of hydrophobic linear hydrocarbon molecules. Since silicon dioxide itself is hydrophilic, linear hydrocarbon molecules, for example, spherical silicon dioxide nanoparticles coated with (polyethylene) ethylene are suitable for improving the compatibility with hydrophobic polyethylene.

[0044] The above-mentioned carrier may have a porosity of 50% to 90%, preferably 70% to 80%, and may have an average pore diameter of 20 nm to 100 nm.

[0045] When the porosity of the above-mentioned carrier is less than 50%, the impregnation amount of the solid electrolyte decreases, resulting in a decrease in ionic conductivity. When the porosity of the above-mentioned carrier is greater than 90%, the mechanical strength of the composite film will decrease. In particular, compared with the existing liquid electrolyte, the ionic conductivity of the solid electrolyte is poor, so a carrier with a porosity of more than 60% needs to be used.

[0046] When the average pore diameter of the above-mentioned carrier is less than 20 nm, it is difficult to impregnate the solid electrolyte. When the average pore diameter of the above-mentioned carrier is greater than 100 nm, the mechanical strength of the prepared composite film will be poor.

[0047] The above-mentioned solid electrolyte may include a lithium salt, an inorganic electrolyte, and a polymer.

[0048] The above-mentioned inorganic electrolyte may be a compound represented by the following Chemical Formula 1 or 2.

[0049] [Chemical Formula 1]

[0050] Li 1+x Al x Ge 2-x (PO4)3

[0051] [Chemical Formula 2]

[0052] Li 1+x Al x Ti 2-x (PO4)3

[0053] In the above Chemical Formula 1 or 2, 0 ≤ x ≤ 2.

[0054] "Sodium superionic conductor (NASICON: Natrium super ionic conductor)" is an inorganic compound represented by the chemical formula of Na 1+x Zr2Si x P 3- x O 12 (0 < x < 3), which has an ionic conductivity similar to that of a liquid electrolyte at about 10 -3a high ionic conductivity of S / cm, which is considered to be because Na + ions can jump between the unique crystal lattices of NASICON.

[0055] NASICON-type inorganic electrolytes are inorganic compounds having a lattice structure similar to that of NASICON. Among them, for example, in the case of LAGP, due to the easy movement of Li + ions, it can have a high ionic conductivity.

[0056] The above inorganic electrolyte can be LAGP or LATP as a NASICON-type inorganic electrolyte.

[0057] "Gel electrolyte" is a kind of solid electrolyte in which a liquid is impregnated inside a flexible lattice framework, and thus has properties similar to those of a liquid electrolyte. For example, a liquid containing a lithium salt such as lithium perchlorate is impregnated inside a lattice composed of polymers such as polyethyleneoxide, polypropylene oxide, polyacrylonitrile (PAN), polyvinylidene fluoride (PVdF), polymethylmethacrylate, etc.

[0058] The above lithium salt can be one selected from the group consisting of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium hexafluoroantimonate, lithium hexafluoroacetate, lithium trifluoromethanesulfonate, and combinations of two or more thereof, but the present invention is not limited thereto.

[0059] The above polymer can be one selected from the group consisting of oxide-based polymers, nitrile-based polymers, fluorine-based polymers, acrylic-based polymers, sulfone-based polymers, and copolymers including two or more of them. When the above lithium salt and polymer are compounded, a gel electrolyte can be obtained.

[0060] The above oxide-based polymer can be polyethyleneoxide, polypropylene oxide, or a polymer having an oxidizing group in the side chain. The above nitrile-based polymer can be polyacrylonitrile or a polymer having a nitrile group in the side chain. The above fluorine-based polymer can be polyvinylidene fluoride, polytetrafluoroethylene, or a polymer having fluorine in the side chain. The above acrylic-based polymer can be polymethylmethacrylate or a polymer having an acrylic group in the side chain. The above sulfone-based polymer can be a polymer having a sulfone group in the main chain or side chain, but the present invention is not limited thereto.

[0061] As described above, the present invention can make up for the disadvantages of each electrolyte by mixing and using an inorganic electrolyte and a gel electrolyte. For example, by mixing the gel electrolyte, it is possible to suppress the interfacial resistance of the electrode, which is a disadvantage of the inorganic electrolyte, and the increase in manufacturing cost, and thus it is beneficial to manufacture a large-capacity battery.

[0062] The above solid electrolyte may include 0.01% by weight to 0.1% by weight of an additive capable of increasing ionic conductivity, such as boron trioxide (B2O3), relative to 100% by weight of the solid electrolyte. The above additive can increase the ionic conductivity by lowering the crystallization temperature of LAGP, and can even improve the stability inside the solution.

[0063] In the prior art, a separator in the form of a film is prepared by forming a solid electrolyte, but this type of composite film has the following limitations compared to the separators used in the past, that is, it lacks mechanical strength and cannot reduce the thickness of the thin film to below a certain level, so there is a disadvantage that it cannot be substantially used. However, the present invention can prepare a composite film with excellent mechanical strength by impregnating a solid electrolyte obtained by mixing an inorganic electrolyte and a gel electrolyte into a porous carrier. The above composite film may have a thickness of 5 μm to 100 μm, a tensile strength of 500 kgf / cm 2 to 2,500 kgf / cm 2 elongation at break of 50% to 300% and a puncture strength of 100 gf to 600 gf.

[0064] Referring to Figure 1 , a method for preparing a composite film according to another aspect of the present invention may include: step (a) of preparing an electrolyte solution by mixing a lithium salt, an inorganic electrolyte, and a polymer in a solvent; and step (b) of filling the pores of the above porous carrier with the above electrolyte solution.

[0065] In the above step (a), the above inorganic electrolyte can be prepared in powder form and then added to and dispersed in a solution containing the above lithium salt and polymer to prepare an electrolyte solution. The above dispersion can be achieved by using a beads mill or a mixer, etc., which adopt a direct grinding method. A beads mill is a general term for equipment that uses beads as grinding media, and it can be classified into a ball mill, an attrition mill, a vertical mill (disk type, pin type), a horizontal mill (disk type, pin type, high energy mill), a paint shaker, etc. Among them, a representative ball mill is a rotating mill that uses spherical grinding media, and an attrition mill is a device that crushes by using the frictional force of rollers. On the other hand, examples of mixers include a three-roll mill, a planetary mixer, and a paste mixer, etc. As such a direct grinding method, the wet method is widely used, but the present invention is not limited thereto.

[0066] The above inorganic electrolyte can be prepared by a method including the following steps: step (i), preparing a solid solution by mixing a lithium salt, an aluminum salt, and a phosphate compound with a germanium salt or a titanium salt; step (ii), adding an additive to the above solid solution and dispersing it in a solvent; and step (iii), calcining the product of the above step (ii).

[0067] As the solvent in the above step (a), for example, anhydrous acetonitrile can be used. As long as the lithium salt, the above inorganic electrolyte, and the polymer are sufficiently dissolved, the amount of the above solvent is sufficient, but when an excessive amount of solvent is used, the drying process that can be performed after the above step (b) may require excessive time.

[0068] The weight ratios of the above inorganic electrolyte and polymer can be 6 - 8:2 - 4 respectively, and the weight ratio of the above polymer and lithium salt can be 15 - 20:1.

[0069] In the above step (b), various structures can be added so that the electrolyte solution is fully filled in the pores of the porous carrier. The above filling can be performed by immersing the above electrolyte solution on at least one surface of the above porous carrier by using methods such as dip coating, roll coating, bar coating, and spray coating. In addition, a viscosity reducer can be added to the above electrolyte solution, or the content of the solvent can be increased. Or, if the impregnation is carried out under high-temperature conditions, the above electrolyte solution can be more easily impregnated.

[0070] As another method, during the execution of step (b) above, the impregnation amount of the above electrolyte solution can be increased by applying a pressure gradient to one side and the other side of the porous carrier. In particular, when performing the above filling by the roll coating method, by positioning a suction device or a roll equipped with a suction device on the surface opposite to the surface coated with the above electrolyte solution, the above electrolyte solution coated on one side of the above porous carrier can be smoothly and quickly filled into the pores of the above porous carrier. As needed, a part of the above electrolyte solution coated on one side of the above porous carrier does not fill into the pores of the above porous carrier, but can completely pass through the pores of the above porous carrier and be recovered by the suction device. In this case, the recovered above electrolyte solution can be reused, thereby improving productivity.

[0071] As another method, when a polar material is coated inside and / or on the surface of the pores of the above porous carrier, the wettability of the above electrolyte solution is improved, so that the electrolyte solution can be more easily filled. The above methods can be used alone or in combination.

[0072] Hereinafter, embodiments of the present invention will be described in more detail. However, the following experimental results only record the representative experimental results in the above embodiments, and the embodiments etc. cannot be interpreted as reducing or limiting the scope and content of the present invention. And the effects of various embodiments of the present invention not clearly presented hereinafter can be specifically found in the corresponding parts.

[0073] Examples 1 to 5

[0074] Mix 20 parts by weight of nano-silica particles with an average particle size of 600 nm coated with ethylene on the surface, 60 to 100 parts by weight of liquid paraffin oil with a kinematic viscosity of 70 cSt (at 40 °C), 20 parts by weight of a first polyethylene with a weight average molecular weight of 1,500,000, 20 parts by weight of a second polyethylene with a weight average molecular weight of 350,000, and 1 part by weight of phosphite as an antioxidant, and disperse the nano-silica particles using a high-speed mixer.

[0075] After that, microbubbles generated during the mixing process are removed by a vacuum defoaming process. A twin-screw extruder equipped with a 350 mm wide T-die is used to melt and knead and discharge at a temperature of 190 °C to 230 °C. At this time, the addition amount is controlled so that the nano-silica content in the porous carrier is 51.4% by weight. The melt-kneaded product extruded through the T-die is cooled by casting at 60 °C and solidified to room temperature, and the thickness of the sheet is adjusted to 1 mm to 2 mm.

[0076] Then, a biaxial stretcher heated to 120 °C is used to stretch the extruded porous carrier by 600% to 1,000% in the horizontal direction and 600% to 1,000% in the vertical direction to prepare a film. The stretched film is immersed in dichloromethane at 40 °C for 1 hour to remove liquid paraffin oil, and then dried at room temperature to remove the residual solvent. Thereafter, it is stretched by 10% in the horizontal direction by a biaxial stretcher, shrunk by 5% after stretching, stretched by 10% in the vertical direction and shrunk by 5% thereafter, and held for 30 seconds for heat setting, thereby preparing a porous carrier with a porosity of 50% to 90% and an average pore diameter of 20 nm to 100 nm.

[0077] LAGP (Li 1.5 Al 0.5 Ge 1.5 (PO4)3) solid solution is prepared by proportionally mixing lithium carbonate (Li2CO3), aluminium oxide (Al2O3), germanium oxide (GeO2), and ammonium dihydrogen phosphate (NH4H2PO4). Based on the total weight of the above solid solution, 0.05 wt% of B2O3 is mixed, and ball-milled for 24 hours to disperse it in isopropyl alcohol. After drying at 25 °C for 24 hours to evaporate the volatile solvent, it is heated to 700 °C at a rate of 5 °C / min to further remove volatile substances for 2 hours, and then heated to 850 °C at a rate of 5 °C / min and calcined in an argon atmosphere for 12 hours to prepare LAGP powder.

[0078] Polyethylene oxide (PEO) with a weight-average molecular weight of 150,000 and lithium perchlorate (LiClO4) are vacuum-dried at 100 °C for 24 hours, then dissolved in anhydrous acetonitrile and stirred at 80 °C for 12 hours. After adding the above LAGP powder, an electrolyte solution is prepared by ball-milling for 24 hours. The weight ratios of LAGP and PEO in the above electrolyte solution are 6 - 8:2 - 4 respectively, and the molar ratios of ethylene oxide groups and lithium ions are 15 - 20:1 respectively.

[0079] The above porous carrier is immersed in the above electrolyte solution to fully impregnate the electrolyte into the pores of the carrier. Thereafter, the above porous carrier is washed and dried at room temperature to obtain a composite membrane.

[0080] Examples 1 to 3 were prepared by impregnating an electrolyte solution with a weight ratio of LAGP to PEO of 7:3 into porous carriers with different porosities. Examples 4 and 5 were prepared by setting the porosity of the porous carrier to 75% and changing the weight ratio of LAGP to PEO to prepare the composite film.

[0081] Comparative Example 1

[0082] After coating the electrolyte solution with a weight ratio of LAGP to PEO of 7:3 in the above examples on a flat plate by the doctor blade method, the solvent was dried and removed under vacuum conditions at 40 °C to obtain the composite film.

[0083] Comparative Examples 2 and 3

[0084] Except for setting the porosity of the porous carrier to 75% and changing the weight ratio of LAGP to PEO, the composite film was prepared in the same manner as in the examples.

[0085] Comparative Example 4

[0086] 60 to 100 parts by weight of liquid paraffin oil with a kinematic viscosity of 70 cSt (at 40 °C), 20 parts by weight of a first polyethylene with a weight average molecular weight of 1,500,000, 20 parts by weight of a second polyethylene with a weight average molecular weight of 350,000, and 1 part by weight of phosphite as an antioxidant were mixed.

[0087] After that, microbubbles generated during the mixing process were removed by a vacuum defoaming process. A twin-screw extruder equipped with a 350 mm wide T-die was used to melt and knead and discharge at a temperature of 190 °C to 230 °C. The melt-kneaded material extruded through the T-die was cooled and solidified to room temperature by casting at 60 °C, and the thickness of the sheet was adjusted to 1 mm to 2 mm.

[0088] Then, a biaxial stretching machine heated to 120 °C was used to stretch the extruded porous carrier 600% to 1,000% in the horizontal direction and 600% to 1,000% in the vertical direction to prepare a thin film. The stretched thin film was immersed in dichloromethane at 40 °C for 1 hour to remove the liquid paraffin oil, and then dried at room temperature to remove the residual solvent. After that, it was stretched 10% in the horizontal direction by a biaxial stretching machine, shrank 5% after stretching, stretched 10% in the vertical direction and then shrank 5%, and held for 30 seconds for heat setting to prepare the carrier, but the porosity of the prepared carrier was less than 60%, or the average pore diameter was less than 20 nm due to shrinkage, or it could not be used due to breakage during the stretching process.

[0089] Preparation Example

[0090] Insert the composite film of Example 2 above between two copper foil electrodes, process it into a jelly roll shape, then connect nickel tabs to the electrode tabs and seal it with an aluminum pouch to prepare a battery cell.

[0091] Comparative Preparation Example 1

[0092] A battery cell was prepared in the same manner as the above Preparation Example, except that the composite film of Comparative Example 1 was used instead of the composite film of Example 2.

[0093] Comparative Preparation Example 2

[0094] Insert a porous carrier with a porosity of 75% prepared in the same manner as in the Example between two copper foil electrodes, process it into a jelly roll shape, then connect nickel tabs to the electrode tabs, inject the electrolyte, and seal it with an aluminum pouch to prepare a battery cell. As the above electrolyte, 1.15 M LiPF6 in ethyl carbonate (EC): ethyl methyl carbonate (EMC) = 3:7 (volume ratio) 0.85 g was used.

[0095] Experimental Example 1

[0096] The test methods for each physical property measured in the present invention are as follows. Unless otherwise mentioned about the temperature, the measurement is carried out at room temperature (25 °C).

[0097] - Thickness (μm): Measure the thickness of the carrier or composite film sample using a precision thickness gauge.

[0098] - Porosity (%): Measure the porosity of the carrier sample with a radius of 25 mm using a capillary porometer according to ASTM F316 - 03.

[0099] - Tensile strength (kgf / cm 2 ): Use a tensile strength measuring device to apply stress to a composite film sample with dimensions of 20×200 mm in the TD direction and measure the stress applied until the sample breaks.

[0100] - Tensile elongation (%): Use a tensile strength measuring device to apply stress to a composite film sample with dimensions of 20×200 mm in the TD direction, measure the maximum length stretched until the sample breaks, and calculate the tensile elongation using the following formula.

[0101] Tensile elongation (%) = {(l1 - l2) / (l1)} * 100

[0102] - Puncture strength (gf): Using a puncture strength measuring instrument, a force was applied to a composite film sample with dimensions of 100×50 mm at a speed of 0.05 cm / sec using a rod with a diameter of 0.5 mm, and the force applied when piercing the above sample was measured.

[0103] - Heat shrinkage rate (%): After inserting a composite film sample with dimensions of 200×200 mm between A4 papers and placing it in an oven at 105°C for 1 hour, it was cooled at room temperature. The lengths of the sample shrunk in the horizontal and vertical directions were measured, and the heat shrinkage rate was calculated using the following formula. The higher value was recorded for the heat shrinkage rates in the horizontal and vertical directions.

[0104] Heat shrinkage rate (%) = {(l3 - l4) / (l3)} * 100

[0105] (In the above formula, l3 is the length of the sample in the horizontal or vertical direction before shrinkage, and l4 is the length of the sample in the horizontal or vertical direction after shrinkage.)

[0106] - Ionic conductivity (mS / cm): Using electrochemical impedance spectroscopy (EIS: Electrochemical Impedance Spectroscopy), the ionic conductivity of the battery cells prepared in the above examples or comparative examples was measured 5 times under the conditions of a frequency range of 104 Hz to 106 Hz, a current of 10.0 mV, a voltage range of ±10 V, and a temperature of 50°C, and the average value was obtained.

[0107] The physical properties of the composite films prepared according to the above examples and comparative examples were measured, and the results are shown in Table 1, Table 2 below and Figure 2 in.

[0108]

Table 1

[0109]

[0110]

[0111]

Table 2

[0112] Distinguish Comparative Example 2 Example 4 Example 2 Example 5 Comparative Example 3 LAGP:PEO 5:5 6:4 7:3 8:2 9:1 Ionic conductivity 0.01 0.11 0.13 0.10 0.03

[0113] Referring to Table 1 above, compared with the composite films of Comparative Example 1 prepared by forming a thin film using a solid electrolyte, the composite films of Examples 1 to 3 obtained by impregnating a solid electrolyte in a porous carrier have significantly excellent mechanical strength and have a similar level of ionic conductivity.

[0114] Referring to Table 2 above and Figure 2, it can be confirmed that within the range where the weight ratio of LAGP to PEO is 6 - 8:2 - 4, the ionic conductivity of the composite film is excellent.

[0115] The ionic conductivity was further measured under high temperature conditions above 60°C. However, the ionic conductivity of Comparative Example 3 was less than 0.1 mS / cm, and the ionic conductivities of other Comparative Example 2, Example 2, 4, and 5 were 0.1 mS / cm to 1 mS / cm.

[0116] Experimental Example 2

[0117] After connecting the LED device to the battery cells prepared according to the above Preparation Examples and Comparative Preparation Examples 1 and 2, a penetration test was conducted by piercing the above battery cells with a nail to confirm the stability of the composite film.

[0118] Even after the penetration test of the battery cells in the Preparation Example and Comparative Preparation Example 1, the LED element emitted light, but the battery cell of Comparative Preparation Example 2 exploded after ignition.

[0119] That is, it can be confirmed that the battery cell using the existing liquid electrolyte has a safety problem of being easily penetrated, but the battery cell using the solid electrolyte will not leak, catch fire, or explode even if it is damaged, thus solving the safety problem.

[0120] The above description of the present invention is for illustrative purposes. Those with ordinary knowledge in the technical field to which the present invention pertains should understand that it can be easily deformed into other specific forms without changing the technical idea or essential features of the present invention. Therefore, the above-described embodiments should be understood as being exemplary in all aspects and not restrictive. For example, the individual components described in a single form can also be implemented in a dispersed manner, and similarly, the components described in a dispersed manner can be implemented in a combined form.

[0121] The scope of the present invention should be represented by the claims, and all changes and deformations derived from the meaning, scope, and equivalent concepts of the claims should be interpreted as being fully included within the scope of the present invention.

Claims

1. A composite film, characterized in that, Comprising: a polyolefin-based porous carrier; and a solid electrolyte impregnated inside the above carrier, the above carrier comprises 30 wt% to 90 wt% of polyethylene and 10 wt% to 70 wt% of an inorganic filler, the above solid electrolyte comprises a lithium salt, an inorganic electrolyte and a polymer, the weight ratios of the above inorganic electrolyte and polymer are respectively 6 - 7:3 - 4, the above carrier has a porosity of 50% to 90% and an average pore diameter of 20 nm to 100 nm, the above polyethylene comprises 30 - 70 wt% of a first polyethylene having a weight-average molecular weight of 1,000,000 - 3,000,000 and a molecular weight distribution of 3 to 4 and 30 - 70 wt% of a second polyethylene having a weight-average molecular weight of 200,000 - 500,000 and a molecular weight distribution of 4 to 7, The above composite film has a tensile strength of 500 kgf / cm 2 to 2,500 kgf / cm 2 .

2. The composite film according to claim 1, wherein the above solid electrolyte is impregnated in at least a part of the micropores inside the above polyolefin-based porous carrier.

3. The composite film according to claim 1, wherein the above solid electrolyte is coated on at least a part of the surface of the above carrier.

4. The composite film according to claim 1, wherein the above solid electrolyte further comprises boron trioxide, the above boron trioxide comprises 0.01 wt% to 0.1 wt% relative to 100 wt% of the solid electrolyte.

5. The composite film according to claim 1, wherein the above inorganic electrolyte is a compound represented by the following Chemical Formula 1 or 2: [Chemical Formula 1] Li 1+x Al x Ge 2-x (PO4)3 [Chemical Formula 2] Li 1+x Al x Ti 2-x (PO4)3 In the above Chemical Formula 1 or 2, 0 ≤ x ≤ 2.

6. The composite film according to claim 1, wherein the above polymer is one selected from the group consisting of oxide-based polymers, nitrile-based polymers, fluorine-based polymers, acrylic-based polymers, sulfone-based polymers and copolymers of two or more thereof.

7. The composite film according to claim 1, wherein the above composite film has a thickness of 5 μm to 100 μm, a tensile elongation rate of 50% to 300% and a puncture strength of 100 gf to 600 gf.

8. A method for preparing a composite film, characterized in that, Comprising: step (a), preparing an electrolyte solution by mixing a lithium salt, an inorganic electrolyte and a polymer in a solvent; and step (b), filling the above electrolyte solution into the pores of a porous carrier, the above carrier comprises two kinds of polyethylene with different weight-average molecular weights of 30 wt% to 90 wt% and 10 wt% to 70 wt% of an inorganic filler, the weight ratios of the above inorganic electrolyte and polymer are respectively 6 - 7:3 - 4, the above carrier has a porosity of 50% to 90% and an average pore diameter of 20 nm to 100 nm, the above polyethylene comprises 30 - 70 wt% of a first polyethylene having a weight-average molecular weight of 1,000,000 - 3,000,000 and a molecular weight distribution of 3 to 4 and 30 - 70 wt% of a second polyethylene having a weight-average molecular weight of 200,000 - 500,000 and a molecular weight distribution of 4 to 7, The above composite film has a tensile strength of 500 kgf / cm 2 to 2,500 kgf / cm 2 .

9. The method for manufacturing the composite film according to claim 8, wherein The steps for preparing the electrolyte solution include the step of further mixing boron trioxide into the above-mentioned solvent, The above-mentioned boron trioxide is included in an amount of 0.01 wt% to 0.1 wt% based on 100 wt% of the solid electrolyte.

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