Method for preparing porous substrate of separator for lithium secondary battery, porous substrate prepared therefrom, separator for lithium secondary battery comprising same, and lithium secondary battery comprising same

Abstract

To provide a method for manufacturing a porous substrate of a separator for a lithium secondary battery.SOLUTION: Provided are a method of manufacturing a porous substrate of a separator for a lithium secondary battery, a porous substrate manufactured thereby, a separator for a lithium secondary battery including the same, and a lithium secondary battery including the same, wherein the method of manufacturing a porous substrate of a separator for a lithium secondary battery includes stretching an unstretched film including a resin, the resin includes a resin having a weight average molecular weight (MW) of 1,000,000 or less, the stretching includes multi-stage stretching, and a total stretching ratio is 150 times or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application claims priority to and the benefit of Korean Patent Application No. 10-2024-0104600, filed with the Korean Intellectual Property Office on August 6, 2024, the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates to a method for manufacturing a porous substrate for a separator for a lithium secondary battery, a porous substrate manufactured from the porous substrate, a separator for a lithium secondary battery including the porous substrate, and a lithium secondary battery including the porous substrate. [Background technology]

[0003] In recent years, the rapid spread of battery-powered electronic devices such as mobile phones, laptops, and electric vehicles has led to a rapid increase in demand for high-energy-density, high-capacity secondary batteries, which has led to active research and development into improving the performance of lithium secondary batteries.

[0004] A lithium secondary battery is a battery that includes a positive electrode and a negative electrode, each containing an active material capable of intercalating and deintercalating lithium ions, and an electrolyte, and generates electrical energy through oxidation and reduction reactions that occur when lithium ions are inserted into and deintercalated from the positive electrode and the negative electrode.

[0005] The demand for higher capacity and power output of lithium secondary batteries necessitates thinner separators, which are required to have high strength to prevent short circuits during the battery assembly process and to enhance battery safety, such as collision safety.

[0006] Conventionally, polyethylene resins with a weight-average molecular weight of over 1,000,000 have been used as the base material for porous substrates of separators. This is because polyethylene-based resins can provide high strength. However, these polyethylene-based resins are difficult to knead before extrusion and require long heat setting times, which can reduce processability and productivity. Furthermore, porous substrates made from these polyethylene-based resins exhibit trade-offs against improved strength due to increased shutdown temperature, increased melt shrinkage, and increased heat shrinkage caused by increased residual stress during stretching and heat setting. Summary of the Invention [Problem to be solved by the invention]

[0007] One embodiment provides a method for manufacturing a porous substrate of a separator for a lithium secondary battery, which has improved productivity and processability by using a resin having a weight-average molecular weight of 1,000,000 or less.

[0008] Another aspect of the present invention is to provide a method for manufacturing a porous substrate of a separator for a lithium secondary battery, which provides excellent mechanical strength and improves molten shrinkage, shutdown temperature, and heat shrinkage, which are in a trade-off relationship with the mechanical strength.

[0009] Yet another embodiment provides a porous substrate for a separator for a lithium secondary battery manufactured by the manufacturing method, a separator for a lithium secondary battery including the porous substrate, and a lithium secondary battery including the separator for a lithium secondary battery. [Means for solving the problem]

[0010] In one embodiment, there is provided a method for manufacturing a porous substrate of a separator for a lithium secondary battery, the method comprising: stretching an unstretched film containing a resin, the resin comprising a resin having a weight average molecular weight MW of 1,000,000 or less; and the stretching step comprising multi-stage stretching, wherein a total stretch ratio is 150 times or more.

[0011] Another embodiment provides a porous substrate manufactured by the above manufacturing method.

[0012] Yet another embodiment provides a separator for a lithium secondary battery, comprising the porous substrate.

[0013] Yet another embodiment provides a lithium secondary battery including a positive electrode, a negative electrode, and the separator for a lithium secondary battery disposed between the positive electrode and the negative electrode.

[0014] The method for manufacturing a porous substrate according to an embodiment provides a porous substrate having excellent productivity and processability, and having improved mechanical strength, melt shrinkage, shutdown temperature, and thermal shrinkage, which are in a trade-off relationship with each other. Therefore, the method for manufacturing a porous substrate according to an embodiment can suppress short circuits in a lithium secondary battery and improve the safety of the lithium secondary battery, such as impact characteristics and venting characteristics. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment. [Figure 3] FIG. 3 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment. [Figure 4] FIG. 4 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] DETAILED DESCRIPTION OF THE INVENTION The following detailed description of the present invention is provided by way of example only and is not intended to limit the scope of the present invention, which is defined solely by the scope of the claims set forth below.

[0017] Unless otherwise specified in this specification, when a layer, film, region, plate, or other part is said to be "on" another part, this does not only mean that it is "directly on" the other part, but also includes cases where there are other parts between them.

[0018] Unless otherwise specified herein, the singular may also include the plural. Furthermore, unless otherwise specified, "A or B" may mean "including A, including B, or including A and B."

[0019] As used herein, "combinations thereof" can mean mixtures, laminates, composites, copolymers, alloys, blends, reaction products, and the like of compositions.

[0020] Unless otherwise defined herein, particle size may refer to the average particle size. Furthermore, particle size refers to the average particle size D50, which refers to the diameter of particles with a cumulative volume of 50% by volume in the particle size distribution. The average particle size D50 may be measured by methods well known to those skilled in the art, such as using a particle size analyzer or a transmission electron microscope or scanning electron microscope. Alternatively, the average particle size D50 may be measured using a measuring device using dynamic light scattering, followed by data analysis to count the number of particles in each particle size range and then calculating the average particle size D50. Alternatively, the average particle size D50 may be measured using a laser diffraction method. More specifically, when measuring by the laser diffraction method, the particles to be measured are dispersed in a dispersion medium, then introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac's MT 3000), and irradiated with ultrasonic waves of approximately 28 kHz at an output of 60 W. The average particle size D50 based on 50% of the particle size distribution measured by the measuring device can then be calculated.

[0021] In this specification, the term "polyolefin-based porous substrate" refers to a porous substrate containing polyolefin as a main component, for example, a porous substrate containing 90% by weight or more of polyolefin relative to the total weight of the porous substrate.

[0022] Method for producing porous substrate According to one embodiment, a method for producing a porous substrate includes stretching an unstretched film containing a resin, the resin having a weight average molecular weight (MW) of 1,000,000 or less, the stretching step including multi-stage stretching, and a total stretch ratio of 150 times or more.

[0023] The manufacturing method aims to provide a porous substrate that contains a resin having a weight-average molecular weight of 1 million or less, such as polyethylene, has a thickness of 10 μm or less, a puncture strength to thickness ratio of 75 gf / μm or more, an air permeability of 120 sec / 100 cc or less, heat shrinkage rates in both MD and TD of 4.5% or less, melt shrinkage rates in both MD and TD of -5.0% or more, and a shutdown temperature of 143°C or less.

[0024] The manufacturing method aims to produce a polyolefin-based porous substrate.

[0025] The thickness of the porous substrate is 10 μm or less, for example, 1 to 10 μm, which can provide the effect of reducing the thickness of the separation membrane.

[0026] The ratio of the puncture strength to the thickness of the porous substrate is 75 gf / μm or more, for example, 75 to 100 gf / μm, and the porous substrate has high mechanical strength even when thin, making it easy to improve short circuit defects and impact properties.

[0027] The porous substrate may have an air permeability of 120 sec / 100 cc or less, for example, 50 to 120 sec / 100 cc, which allows easy movement of lithium ions.

[0028] The porous substrate may have a heat shrinkage rate of 4.5% or less, for example, 1 to 4.5%, in both the MD (machine direction) and TD (transverse direction), which can reduce the heat shrinkage rate of the separator and improve the reliability of the battery.

[0029] The porous substrate may have a melt shrinkage rate of −5.0% or more in both MD and TD, for example, −5.0 to −2.0%.

[0030] Here, the "melt shrinkage rate" refers to the force with which the porous substrate shrinks when it is shut down and melts due to an abnormal reaction at high temperature in a lithium secondary battery. Within this range, a separator with excellent heat resistance can be obtained. When a battery is manufactured using the separator, the separator can withstand a considerable temperature without melting even when the battery overheats during use, thereby preventing electrode short circuits and battery explosions.

[0031] The porous substrate may have a shutdown temperature of 143° C. or less, for example, 140 to 143° C. Within this range, the reliability of the battery during thermal runaway can be improved.

[0032] Therefore, the porous substrate can be used as a substrate for a separator for a lithium secondary battery, and can improve short circuit defects and collision characteristics of the battery, thereby enhancing the reliability of the battery.

[0033] In the production method, a resin having a weight-average molecular weight of 1 million or less is used. A resin having a weight-average molecular weight of 1 million or less is easy to knead before extrusion, requires a short heat setting time, and can improve the processability and productivity of the porous substrate production. Here, the weight-average molecular weight is a value determined by gel permeation chromatography in terms of polystyrene, and the unit is g / mol. In one embodiment, the resin may have a weight-average molecular weight of 600,000 to 1 million, for example, 500,000 to 800,000, for example, 500,000 to 700,000.

[0034] The resin may include one or more polymers selected from the group consisting of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyether ketone, polyaryl ether ketone, polyetherimide, polyamide imide, polybenzimidazole, polyether sulfone, polyphenylene oxide, cyclic olefin copolymer, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon (registered trademark), and polytetrafluoroethylene, or a copolymer of two or more of these. For example, the resin may be a polyolefin-based resin, such as a polyethylene-based resin.

[0035] In one embodiment, the unstretched film may contain 95% by weight or more, for example, 95 to 100% by weight, or 100% by weight, of a resin having a weight-average molecular weight of 1,000,000 or less. Within this range, the physical properties of the porous substrate described above can be easily achieved.

[0036] In one embodiment, the resin may contain 95% by weight or more, for example, 95 to 100% by weight, or 100% by weight, of a resin having a weight-average molecular weight of 1,000,000 or less. Within this range, the porous substrate can easily achieve the physical properties described above.

[0037] As a method for producing a porous substrate, for example, a polyolefin-based porous substrate, for example, a dry film-forming method and a wet film-forming method can be mentioned. As a method for producing a porous substrate of this embodiment, a wet film-forming method is preferred from the viewpoint of ease of control of the structure and physical properties of the substrate.

[0038] A resin solution is prepared by melt-kneading a resin and a membrane-forming solvent.

[0039] The resin may be the same as that described above. The resin solution may also contain various additives such as oil, for example, liquid paraffin, antioxidant, heat stabilizer, antistatic agent, ultraviolet absorber, antiblocking agent, filler, crystal nucleating agent, and crystallization retarder, as long as the effects of the present invention are not impaired.

[0040] The resin solution produced as described above is fed from an extruder to a die and extruded into a sheet, and the resulting extrudate is cooled to produce an unstretched film, i.e., a cast film. The unstretched film may also be produced by feeding multiple resin solutions of the same or different compositions from multiple extruders to a single die, laminating them in layers, and extruding them into a sheet.

[0041] In one embodiment, the thickness of the unstretched film, i.e., the cast film, may be 1500 to 5000 μm, for example, 2000 to 3500 μm. Within this range, the porous substrate can be easily produced by the stretching ratio described below.

[0042] The cast film is then stretched at least uniaxially.

[0043] The stretching includes multistage stretching, and the total stretch ratio is 150 times or more. Within this range of total stretch ratio, a porous substrate produced from a cast film containing a resin with a weight-average molecular weight of 1 million or less is likely to satisfy the above-mentioned physical properties (thickness, puncture strength, air permeability, puncture strength / thickness ratio, heat shrinkage, melt shrinkage, and shutdown temperature). For example, the total stretch ratio may be 150 times to 300 times, 163 times or more, or 163 times to 300 times.

[0044] As used herein, the term "total stretch ratio" refers to a value calculated based on the area ratio, and is the final area stretch ratio. The stretch ratio in this production method refers to the stretch ratio of the immediately preceding cast film provided in the next step, based on the immediately preceding cast film in this production method. The TD direction is the direction perpendicular to the MD direction when the film is viewed in plan. Here, "MD" may refer to the mechanical direction of the unstretched film, i.e., the cast film. When the unstretched film, i.e., the cast film, is a polyolefin-based resin film, the mechanical direction of the unstretched film may be the direction in which the unstretched film is produced when the polyolefin-based resin is produced by melt extrusion or solution casting.

[0045] This manufacturing method uses a resin with a weight-average molecular weight of 1 million or less, but when a cast film made from the resin is stretched from a single stretching stage to a total stretch ratio of 150 times or more, breakage may occur. Multi-stage stretching can prevent breakage of the film when the cast film is stretched at a total stretch ratio.

[0046] In one embodiment, the multi-stage drawing includes a first drawing and a second drawing, and the first drawing and the second drawing may be performed sequentially.

[0047] The first and second stretching may be either uniaxial or biaxial, although biaxial stretching may be preferred. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be used.

[0048] The primary stretching may be carried out at a ratio of at least 2 times the total stretching ratio, for example, 2 to 5. Within this range, the total stretching ratio can be easily controlled and the process handling during the secondary stretching can be easily carried out.

[0049] In one embodiment, the first stretching may be performed by simultaneous biaxial stretching or sequential biaxial stretching of MD uniaxial stretching and TD uniaxial stretching, or a combination thereof. In this case, the MD uniaxial stretching may have a stretch ratio of 1.4 times or more, for example, 1.5 times or more, for example, 1.5 to 3.0 times, and the TD uniaxial stretching may have a stretch ratio of 1.4 times or more, for example, 1.5 times or more, for example, 1.5 to 3.0 times.

[0050] The secondary stretching may be performed at a total stretch ratio of 64 times or more, for example, 64 to 75 times. Within this range, a substrate for a lithium secondary battery separator can be obtained that has improved productivity and processability, high strength, low shutdown, and low melt shrinkage, using a resin with a weight-average molecular weight of 1,000,000 or less.

[0051] In one embodiment, the secondary stretching may be performed by simultaneous biaxial stretching of MD uniaxial stretching and TD uniaxial stretching, or sequential biaxial stretching, or a combination thereof. In this case, the MD uniaxial stretching may have a stretch ratio of 8.0 times or more, for example, 8.0 times to 10.0 times, and the TD uniaxial stretching may have a stretch ratio of 8.0 times or more, for example, 8.0 times to 10.0 times.

[0052] In one embodiment, the stretch ratio of the first stretching may be smaller than the stretch ratio of the second stretching. When the cast film is first stretched, if the stretch ratio is too high, film breakage may occur.

[0053] In one embodiment, the ratio of the total stretch ratio of the second stretching to the total stretch ratio of the first stretching may be 16 times or more, for example, 16 to 40 times. Within this range, the physical properties of the porous substrate can be easily achieved.

[0054] The first stretching and the second stretching may each be performed by wet stretching.

[0055] In the first and second stretching, the stretching temperature is preferably within the range of the crystal dispersion temperature (Tcd) of the resin to Tcd + 30°C, more preferably within the range of the crystal dispersion temperature (Tcd) + 5°C to the crystal dispersion temperature (Tcd) + 28°C, and particularly preferably within the range of Tcd + 10°C to Tcd + 26°C. When the stretching temperature is within this range, film breakage due to resin stretching is suppressed, and high-magnification stretching is possible. Here, the crystal dispersion temperature (Tcd) refers to a value determined by measuring the temperature characteristics of dynamic viscoelasticity according to ASTM D4065. The stretching temperature may be, for example, from 90°C to 130°C.

[0056] Next, the membrane-forming solvent is removed from the stretched cast film. The solvent is removed by washing with a washing solvent. Because the resin, e.g., polyolefin phase, is phase-separated from the membrane-forming solvent phase, removing the membrane-forming solvent results in a porous membrane composed of fibrils that form a fine three-dimensional network structure and has three-dimensionally irregularly interconnected pores (voids). Washing solvents and methods for removing the membrane-forming solvent using them are well known, so a detailed description is omitted here.

[0057] After stretching, oil or the like contained in the resin solution may be removed.

[0058] Next, the microporous membrane from which the membrane-forming solvent has been removed is dried by heat drying or air drying. The drying temperature is preferably equal to or lower than the crystal dispersion temperature (Tcd) of the resin, and particularly preferably 5°C or more lower than Tcd. Drying is preferably carried out until the amount of remaining washing solvent is 5% by mass or less, more preferably 3% by mass or less, based on 100% by mass (dry weight) of the cast film. When the amount of remaining washing solvent is within this range, the porosity is maintained and deterioration of permeability is suppressed when the cast film is stretched and heat-treated.

[0059] Next, the dried porous substrate may be subjected to a heat treatment. As a heat treatment method, a heat setting treatment and / or a heat relaxation treatment may be used. The heat setting treatment is a heat treatment in which the membrane is heated while maintaining the dimension in the TD direction so as not to change. The heat relaxation treatment is a treatment in which the membrane is thermally shrunk in the MD direction and / or the TD direction during heating. The heat setting treatment is preferably performed by a tenter method or a roll method. The heat treatment temperature is preferably within the range of Tcd to Tm of the resin.

[0060] In one embodiment, heat setting may be performed while shrinking the film by 5-10% in the TD.

[0061] porous substrate Another embodiment provides a porous substrate produced by the above-described method.

[0062] The porous substrate is produced by the above-described production method.

[0063] According to one embodiment, the porous substrate comprises a resin having a weight-average molecular weight of 1 million or less, such as polyethylene, and the porous substrate may have a thickness of 10 μm or less, a puncture strength-to-thickness ratio of 75 gf / μm or more, an air permeability of 120 sec / 100 cc or less, heat shrinkage rates in both MD and TD of 4.5% or less, melt shrinkage rates in both MD and TD of -5.0% or more, and a shutdown temperature of 143°C or less.

[0064] The contents for the porous substrate may be substantially the same as those described above.

[0065] Therefore, the porous substrate can be used as a substrate for a separator for a lithium secondary battery to improve short circuit defects and collision characteristics of the battery and increase the reliability of the battery.

[0066] Separator for lithium secondary batteries Another embodiment provides a separator for a lithium secondary battery, comprising a porous substrate.

[0067] In one embodiment, the separator for a lithium secondary battery may be a porous substrate alone.

[0068] In another embodiment, the separator for a lithium secondary battery may include a porous substrate and a coating layer including an organic material, an inorganic material, or a combination thereof located on one or both sides of the porous substrate.

[0069] The organic material may include a polyvinylidene fluoride-based polymer or a (meth)acrylic-based polymer.

[0070] The inorganic material may include, but is not limited to, inorganic particles selected from Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, Mg(OH)2, boehmite, and combinations thereof.

[0071] The organic material and the inorganic material may be mixed in one coating layer, or may be present in a form in which a coating layer containing an organic material and a coating layer containing an inorganic material are laminated.

[0072] Lithium secondary battery Yet another embodiment provides a lithium secondary battery including a positive electrode, a negative electrode, and a separator for a lithium secondary battery.

[0073] The separator for the lithium secondary battery has been described above, so a detailed description will be omitted.

[0074] The positive electrode for a lithium secondary battery may include a current collector and a positive electrode active material layer formed on the current collector. The positive electrode active material layer includes a positive electrode active material, and may further include a binder and / or a conductive material.

[0075] In one example, the positive electrode may further include an additive that can act as a sacrificial positive electrode.

[0076] The content of the positive electrode active material may be 90% by weight to 99.5% by weight relative to 100% by weight of the positive electrode active material layer, and the contents of the binder and conductive material may be 0.5% by weight to 5% by weight each relative to 100% by weight of the positive electrode active material layer.

[0077] The positive electrode active material may be a compound capable of reversibly inserting and extracting lithium (lithiate intercalation compound). Specifically, one or more of composite oxides of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof may be used.

[0078] The composite oxide may be a lithium transition metal composite oxide, and specific examples include lithium nickel oxide, lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate compound, cobalt-free nickel-manganese oxide, or a combination thereof.

[0079] As an example, a compound represented by any of the following chemical formulas may be used: Li a A 1-b X b O 2-c D c (0.90≦a≦1.8,0≦b≦0.5,0≦c≦0.05);Li a Mn 2-b X b O 4-c D c (0.90≦a≦1.8,0≦b≦0.5,0≦c≦0.05);Li a Ni 1-b-c Co b X c O 2-α D α (0.90≦a≦1.8,0≦b≦0.5,0≦c≦0.5,0<α<2);Li a Ni 1-b-c Mn b X c O 2-α D α (0.90≦a≦1.8,0≦b≦0.5,0≦c≦0.5,0<α<2);Li a Ni b Co c L1 d G e O2(0.90≦a≦1.8,0≦b≦0.9,0≦c≦0.5,0≦d≦0.5,0≦e≦0.1);Li a NiG b O2(0.90≦a≦1.8,0.001≦b≦0.1);Li a CoG b O2(0.90≦a≦1.8,0.001≦b≦0.1);Li a Mn 1-b G b O2(0.90≦a≦1.8,0.001≦b≦0.1);Li a Mn2G b O4(0.90≦a≦1.8,0.001≦b≦0.1);Li a Mn 1-g G g PO4(0.90≦a≦1.8,0≦g≦0.5);Li (3-f) Fe2(PO4)3(0≦f≦2);Li a FePO4(0.90≦a≦1.8).

[0080] In the chemical formula, A is Ni, Co, Mn, or a combination thereof; X is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element, or a combination thereof; D is O, F, S, P, or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; and L is 1 is Mn, Al, or a combination thereof.

[0081] For example, the positive electrode active material may be a high-nickel positive electrode active material in which the nickel content is 80 mol% or more, 85 mol% or more, 90 mol% or more, 91 mol% or more, or 94 mol% or more, but not more than 99 mol%, relative to 100 mol% of metals excluding lithium in a lithium transition metal composite oxide. The high-nickel positive electrode active material can provide high capacity and may be applied to high-capacity, high-density lithium secondary batteries.

[0082] The binder serves to firmly adhere the positive electrode active material particles to each other and to the current collector. Representative examples of binders include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, (meth)acrylate styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, and nylon.

[0083] The conductive material is used to impart conductivity to the electrodes, and any material that is electron-conductive without undergoing chemical changes in the battery that is being constructed can be used. Examples of the conductive material include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials in the form of metal powder or metal fiber, including copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; and mixtures thereof.

[0084] The current collector may be made of Al, but is not limited to this.

[0085] The negative electrode for a lithium secondary battery includes a current collector and a negative electrode active material layer disposed on the current collector. The negative electrode active material layer includes a negative electrode active material and may further include a binder and / or a conductive material.

[0086] For example, the negative electrode active material layer may contain 90% to 99% by weight of the negative electrode active material, 0.5% to 5% by weight of the binder, and 0% to 5% by weight of the conductive material.

[0087] The negative electrode active material includes a material capable of reversibly inserting / desorbing lithium ions, lithium metal, an alloy of lithium metal, a material capable of doping and undoping lithium, or a transition metal oxide.

[0088] The material capable of reversibly inserting / desorbing lithium ions is a carbon-based negative electrode active material, and may include, for example, crystalline carbon, amorphous carbon, or a combination thereof. Examples of crystalline carbon include graphite such as amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, and examples of amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, fired coke, and the like.

[0089] As the alloy of lithium metal, an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn may be used.

[0090] As the material capable of doping and undoping lithium, a Si-based negative electrode active material or a Sn-based negative electrode active material may be used. The Si-based negative electrode active material may be silicon, a silicon-carbon composite, SiOx (0 < x < 2), a Si-Q alloy (Q is selected from an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element (excluding Si), a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and a combination thereof), or a combination thereof. The Sn-based negative electrode active material may be Sn, SnO2, a Sn-based alloy, or a combination thereof.

[0091] The silicon-carbon composite may be a composite of silicon and amorphous carbon. According to one embodiment, the silicon-carbon composite may be in the form of silicon particles and the surfaces of the silicon particles coated with amorphous carbon. For example, the silicon-carbon composite may include secondary particles (cores) formed by assembling primary silicon particles and an amorphous carbon coating layer (shell) located on the surfaces of the secondary particles. Amorphous carbon may also be located between the primary silicon particles, for example, the primary silicon particles may be coated with amorphous carbon. The secondary particles may be dispersed in an amorphous carbon matrix.

[0092] The silicon-carbon composite may further comprise crystalline carbon. For example, the silicon-carbon composite may comprise a core comprising crystalline carbon and silicon particles and an amorphous carbon coating layer disposed on the core.

[0093] The Si-based negative electrode active material or the Sn-based negative electrode active material may be used in combination with a carbon-based negative electrode active material.

[0094] The binder serves to firmly adhere the negative electrode active material particles to each other and to the current collector. The binder may be a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof.

[0095] Examples of non-aqueous binders include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or combinations thereof.

[0096] The aqueous binder may be selected from styrene-butadiene rubber, (meth)acrylate styrene-butadiene rubber, (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, butyl rubber, fluororubber, polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, poly(meth)acrylonitrile, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0097] When an aqueous binder is used as the negative electrode binder, it may further contain a cellulose-based compound that can impart viscosity. The cellulose-based compound may be a mixture of one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metal may be sodium, potassium, or lithium.

[0098] The dry binder is a fiberizable polymeric material, which may be, for example, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or a combination thereof.

[0099] The conductive material is used to impart conductivity to the electrodes, and any material that is electron-conductive without undergoing chemical changes in the battery that is being constructed can be used. Specific examples include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials in the form of metal powder or metal fiber, including copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; and mixtures thereof.

[0100] The negative electrode current collector may be selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0101] The lithium secondary battery may further include an electrolyte.

[0102] The electrolyte for the lithium secondary battery includes a non-aqueous organic solvent and a lithium salt.

[0103] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reactions of the battery can migrate.

[0104] The non-aqueous organic solvent may be a carbonate, ester, ether, ketone, or alcohol solvent, an aprotic solvent, or a combination thereof.

[0105] Examples of carbonate solvents that may be used include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC).

[0106] As the ester-based solvent, methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolide, mevalonolactone, valerolactone, caprolactone, and the like may be used.

[0107] Examples of ether solvents that may be used include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and tetrahydrofuran. Examples of ketone solvents that may be used include cyclohexanone. Examples of alcohol solvents that may be used include ethyl alcohol and isopropyl alcohol. Examples of aprotic solvents that may be used include nitriles such as R-CN (where R is a hydrocarbon group having a linear, branched, or cyclic structure and having 2 to 20 carbon atoms, and may contain a double bond, an aromatic ring, or an ether group), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane and 1,4-dioxolane, and sulfolanes.

[0108] The non-aqueous organic solvents may be used alone or in combination of two or more.

[0109] When a carbonate-based solvent is used, a cyclic carbonate and a chain carbonate may be mixed and used, and the cyclic carbonate and the chain carbonate may be mixed in a volume ratio of 1:1 to 1:9.

[0110] Lithium salts are substances that dissolve in organic solvents and act as a source of lithium ions within the battery, enabling basic lithium secondary battery operation and facilitating the movement of lithium ions between the positive and negative electrodes. Representative examples of lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide (LiFSI), LiC4F9SO3, and LiN(C x F 2x+1 SO2)(C y F 2y+1SO2) (x and y are integers of 1 to 20), lithium trifluoromethanesulfonate, lithium tetrafluoroethersulfonate, lithium difluorobis(oxalate)phosphate (LiDFOB), and lithium bis(oxalate)borate (LiBOB).

[0111] Lithium secondary batteries may be classified into cylindrical, prismatic, pouch-shaped, coin-shaped, and other shapes depending on their shape. FIGS. 1 to 4 are schematic diagrams showing lithium secondary batteries according to an embodiment, with FIG. 1 showing a cylindrical battery, FIG. 2 showing a prismatic battery, and FIGS. 3 and 4 showing pouch-shaped batteries. Referring to FIGS. 1 to 4, a lithium secondary battery 100 may include an electrode assembly 40 having a separator 30 interposed between a positive electrode 10 and a negative electrode 20, and a case 50 housing the electrode assembly 40. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with an electrolyte (not shown). The lithium secondary battery 100 may include a sealing member 60 that seals the case 50, as shown in FIG. 1. Also, in FIG. 2, the lithium secondary battery 100 may include a positive electrode lead tab 11, a positive electrode terminal 12, a negative electrode lead tab 21, and a negative electrode terminal 22. As shown in FIGS. 3 and 4, the lithium secondary battery 100 may include electrode tabs 70, i.e., a positive electrode tab 71 and a negative electrode tab 72, which function as electrical paths for conducting current formed in the electrode assembly 40 to the outside.

[0112] The lithium secondary battery according to an embodiment of the present invention may be applied to automobiles, mobile phones, and / or various types of electronic devices, but the present invention is not limited thereto.

[0113] Examples and comparative examples of the present invention will be described below. However, the following examples are merely examples of the present invention, and the present invention is not limited to the following examples.

[0114] Example 1 A polyethylene resin-containing solution was produced by melt-kneading 28 parts by weight of a polyethylene resin (weight average molecular weight 600,000) and 72 parts by weight of liquid paraffin using a twin-screw extruder. The polyethylene resin-containing solution was fed from the twin-screw extruder to a T-die and extruded, and then the extruded molded body was cooled while being taken up by a cooling roll to produce a cast film.

[0115] The cast film was wet stretched at 110-120°C using a stretching machine, uniaxially stretched in the MD of the cast film at a stretch ratio of MD01, and then uniaxially stretched in the TD at a stretch ratio of TD01 (first stretching). The total stretch ratio in the first stretching was 1.5 x 1.5 = 2.25.

[0116] Next, the film was wet stretched at 110-120°C using a stretching machine, uniaxially stretched in the MD of the cast film at a stretch ratio of MD02, and then uniaxially stretched in the TD at a stretch ratio of TD02 (secondary stretching). The total stretch ratio in the secondary stretching was 8.5 x 8.5 = 72.25.

[0117] The total draw ratio is 2.25 x 72.25 = approximately 163.

[0118] The liquid paraffin was extracted from the wet-stretched film using methylene chloride and then dried at room temperature to produce a microporous membrane.

[0119] The microporous membrane was heat-set using a tenter method at 125°C to 130°C while shrinking by 8% in TD, to produce a polyethylene-based porous substrate.

[0120] Examples 2 to 5 In Example 1, a polyethylene-based resin having a weight-average molecular weight shown in Table 1 below was used to prepare a casting film having a thickness shown in Table 1 below, and a polyethylene-based porous substrate was prepared in the same manner as in Example 1, except that the film was stretched at stretch ratios MD01, TD01, MD02, and TD02 shown in Table 1 below in the first stretching and the second stretching, respectively.

[0121] Comparative Examples 1 to 11 In Example 1, a polyethylene-based resin having a weight-average molecular weight shown in Table 2 below was used to prepare a casting film having a thickness shown in Table 2 below, and a polyethylene-based porous substrate was prepared in the same manner as in Example 1, except that the film was stretched at stretch ratios MD01, TD01, MD02, and TD02 shown in Table 2 below in the first stretching and the second stretching, respectively.

[0122] The properties of the prepared porous substrates were evaluated in the following Tables 1 and 2, and the results are shown in Tables 1 and 2.

[0123] (1) Piercing strength (unit: gf) The porous substrates prepared in the Examples and Comparative Examples were cut at 10 different points along a 50mm x 50mm axis to prepare 10 specimens. Using a GATO Tech G5 instrument, the specimens were placed over a 10cm hole and the penetration force was measured by pressing a 1mm probe. The puncture strength of each specimen was evaluated three times, and the average puncture strength was calculated.

[0124] (2) Air permeability (unit: sec / 100cc) The air permeability of the porous substrates prepared in the examples and comparative examples was measured using a measuring device (EG01-55-1MR, Asahi Seiko) by measuring the time (unit: seconds) required for 100 cc of air to pass through the separation membrane. The air permeability was measured twice and calculated as the average value.

[0125] [Setting conditions for air permeability measurement device] Measurement pressure: 0.5 kg / cm 2 Cylinder pressure: 2.5kg / cm 2 Set time: 10 seconds

[0126] (3) Ratio of puncture strength to thickness (unit: gf / μm) The ratio of the thickness of the porous substrate to the measured puncture strength was calculated.

[0127] (4) Heat shrinkage rate (unit: %) The porous substrates of the Examples and Comparative Examples were cut into 8 cm x 8 cm pieces to prepare samples. A 5 cm x 5 cm square was drawn on the surface of the sample, which was then sandwiched between paper or alumina powder and placed in an oven at 150°C for 1 hour. The sample was then removed and the dimensions of the sides of the square were measured to calculate the shrinkage in both the MD and TD directions. The shrinkage was calculated using the following equation:

[0128] [Number 1] Shrinkage rate = (L0-L1) / L0 x 100 (L0 is the initial length of the porous substrate, and L1 is the length of the separation membrane after being left at 150°C for 1 hour)

[0129] 5) Melt shrinkage rate (unit: %) The porous substrates of the examples and comparative examples were cut into 1 cm x 8 cm samples. Each specimen was mounted in a TMA device and heated from room temperature (approximately 20°C) to approximately 200°C at a heating rate of 10°C / min under a load of 0.005 N. The change in length of each specimen was measured by measuring the melt-shrunk length relative to the initial length and calculating the average value.

[0130] (6) Shutdown temperature (unit: °C) While measuring the air permeability of the porous substrates of the Examples and Comparative Examples, the separation membranes were exposed to elevated temperatures (starting from 30°C at a temperature increase rate of 5°C / min). The temperature at which the air permeability (Gurley value) of the separation membrane exceeded 100,000 sec / 100 cc was measured. The air permeability was measured using an air permeability meter (Asahi Seiko, EGO-IT) in accordance with JIS P8117.

[0131] [Table 1]

[0132] [Table 2]

[0133] [Table 3]

[0134] As shown in Table 1, the porous substrates of the examples provided excellent mechanical strength and improved the melt shrinkage rate, shutdown temperature, and heat shrinkage rate, which are in a trade-off relationship with mechanical strength.

[0135] On the other hand, as shown in Tables 2 and 3, the porous substrates of the comparative examples did not satisfy all of the physical properties of the porous substrates of the examples.

[0136] Although the preferred embodiments of the present invention have been described above, the present invention is not limited thereto and can be embodied in various modifications within the scope of the claims, the detailed description of the invention, and the accompanying drawings, and it is to be understood that these modifications also fall within the scope of the present invention. [Explanation of symbols]

[0137] 100 Lithium secondary battery 10 positive electrode 11 Positive electrode lead tab 12 Positive terminal 20 negative electrode 21 Negative electrode lead tab 22 Negative terminal 30 Separator 40 Electrode assembly 50 cases 60 Sealing member 70 Electrode tab 71 Positive electrode tab 72 Negative electrode tab

Claims

1. The method includes a step of stretching an unstretched film containing a resin, wherein the resin includes a resin having a weight average molecular weight (MW) of 1,000,000 or less; The stretching step includes multi-stage stretching, and the total stretching ratio is 150 times or more.

2. The method according to claim 1 , wherein the multi-stage drawing includes a first drawing and a second drawing, and the first drawing and the second drawing are performed sequentially.

3. The primary stretching has a total stretching ratio of 2 times or more, The method according to claim 2 , wherein the secondary stretching has a total stretch ratio of 64 times or more.

4. The method according to claim 2, wherein the primary stretching is simultaneous biaxial stretching, sequential biaxial stretching, or a combination thereof, and the MD uniaxial stretching has a stretch ratio of 1.5 times or more, and the TD uniaxial stretching has a stretch ratio of 1.5 times or more.

5. The method according to claim 2, wherein the secondary stretching is simultaneous biaxial stretching, contraction-order biaxial stretching, or a combination thereof, and the MD uniaxial stretching has a stretch ratio of 8.0 times or more, and the TD uniaxial stretching has a stretch ratio of 8.0 times or more.

6. The method according to claim 2 , wherein the stretch ratio of the first stretching is smaller than the stretch ratio of the second stretching.

7. The method according to claim 6, wherein the ratio of the stretching ratio of the second stretching to the stretching ratio of the first stretching is 16 times or more.

8. The method according to claim 1 , wherein the unstretched film contains 95% by weight or more of the resin having a weight average molecular weight of 1,000,000 or less.

9. The method according to claim 1 , wherein the resin contains 95% by weight or more of a resin having a weight average molecular weight MW of 1,000,000 or less.

10. The method according to claim 1, wherein the resin having a weight average molecular weight of 1,000,000 or less is a polyolefin resin.

11. 2. The manufacturing method according to claim 1, wherein the porous substrate has a thickness of 10 μm or less, a puncture strength to thickness ratio of 75 gf / μm or more, an air permeability of 120 sec / 100 cc or less, a heat shrinkage rate in each of MD and TD of 4.5% or less, a melt shrinkage rate in each of MD and TD of −5.0% or more, and a shutdown temperature of 143° C. or less.

12. A porous substrate for a separator for a lithium secondary battery, manufactured by the manufacturing method according to any one of claims 1 to 11.

13. The porous substrate according to claim 12, wherein the porous substrate has a thickness of 10 μm or less, a puncture strength to thickness ratio of 75 gf / μm or more, an air permeability of 120 sec / 100 cc or less, a heat shrinkage rate in each of MD and TD of 4.5% or less, a melt shrinkage rate in each of MD and TD of −5.0% or more, and a shutdown temperature of 143° C. or less.

14. A separator for a lithium secondary battery, comprising the porous substrate according to claim 12 .

15. A lithium secondary battery comprising the separator for lithium secondary batteries according to claim 14.

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