Method for manufacturing a separator for a lithium secondary battery, a separator for a lithium secondary battery manufactured by the method, and a lithium secondary battery including the same

Abstract

The present invention discloses a method for producing a nonwoven fabric separator for a lithium secondary battery, the method comprising the steps of: (S1) preparing a first solution in which β-chitin is dissolved in a first solvent; and (S2) preparing a second solution in which vinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE) is dissolved in a second solvent; (S2) producing a nonwoven fabric formed from fibers with a core-shell structure consisting of a β-chitin core and a vinylidene fluoride-trifluoroethylene copolymer shell by coaxially electrospinning the first solution as a core solution and the second solution as a shell solution; and (S3) melting and recrystallizing the vinylidene fluoride-trifluoroethylene copolymer by heat-treating the nonwoven fabric at a temperature lower than the thermal decomposition point of β-chitin and higher than the melting point of the vinylidene fluoride-trifluoroethylene copolymer.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a separator for a lithium secondary battery, a separator for a lithium secondary battery manufactured by the method, and a lithium secondary battery including the separator.

[0002] This application claims priority based on Korean Patent Application No. 10-2023-0038832 filed on March 24, 2023, and Korean Patent Application No. 10-2023-0182432 filed on December 14, 2023, and the contents disclosed in the specifications and drawings of said applications are incorporated herein in their entirety. [Background technology]

[0003] Interest in energy storage technology has been growing in recent years. As its applications expand to include mobile phones, camcorders, notebook PCs, and even electric vehicles, research and development of electrochemical devices such as secondary batteries is progressing vigorously. Electrochemical devices are the field that has attracted the most attention in this regard, and the development of rechargeable secondary batteries is a particular focus of attention.

[0004] The basic required characteristics of the separator that constitutes such a battery are to separate and electrically insulate the positive and negative electrodes, while also increasing the permeability of ions, e.g., lithium ions, and enhancing ionic conductivity through high porosity. The substrate of a commonly used separator is primarily a porous polyolefin substrate made of polyolefin-based materials such as polyethylene (PE) and polypropylene (PP), which are advantageous for pore formation and have excellent chemical resistance, mechanical properties, and thermal characteristics. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention provides a method for manufacturing a nonwoven fabric separator for a lithium secondary battery, which has good heat shrinkage properties and exhibits low resistance and improved cycle performance when applied to a lithium secondary battery.

[0006] Another object of the present invention is to provide a nonwoven fabric separator for a lithium secondary battery that exhibits the above-mentioned properties.

[0007] Another object of the present invention is to provide a composite separator for a lithium secondary battery, which includes a nonwoven fabric separator having the above-mentioned properties.

[0008] It is yet another object of the present invention to provide an electrode assembly for a lithium secondary battery including a nonwoven fabric separator having the above-mentioned properties, and a lithium secondary battery. [Means for solving the problem]

[0009] One aspect of the present invention provides a method for manufacturing a nonwoven fabric separator for a lithium secondary battery according to the following embodiment.

[0010] The first embodiment is (S1) preparing a first solution in which β-chitin is dissolved in a first solvent and a second solution in which vinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE) is dissolved in a second solvent; (S2) preparing a nonwoven fabric formed from fibers having a core-shell structure consisting of a core portion of β-chitin and a shell portion of vinylidene fluoride-trifluoroethylene copolymer by coaxially electrospinning the first solution as a core solution and the second solution as a shell solution; (S3) a step of heat-treating the nonwoven fabric at a temperature lower than the thermal decomposition point of β-chitin and higher than the melting point of the vinylidene fluoride-trifluoroethylene copolymer to melt and recrystallize the vinylidene fluoride-trifluoroethylene copolymer.

[0011] The second embodiment is the same as the first embodiment, The present invention relates to a method for producing a nonwoven fabric separator for a lithium secondary battery, wherein the weight ratio of the β-chitin core to the vinylidene fluoride-trifluoroethylene copolymer shell is about 50:50 to 99:1.

[0012] The third embodiment is the same as the first or second embodiment. The present invention relates to a method for producing a nonwoven fabric separator for a lithium secondary battery, wherein the first solvent is HFIP (1,1,1,3,3,3-hexafluoro-2-propanol) and the second solvent is MEK (methylethylketone).

[0013] The fourth embodiment is the same as the first embodiment, The nonwoven fabric is heat-treated at a temperature of about 150°C to 250°C in accordance with a method for producing a nonwoven fabric separator for a lithium secondary battery.

[0014] The fifth embodiment is the same as the first embodiment, The vinylidene fluoride-trifluoroethylene copolymer has crystals that grow along a crystal axis, and the crystals that grow along the crystal axis are aligned in a direction that is not parallel to the longitudinal direction of the fibers, in accordance with a method for manufacturing a nonwoven fabric separator for a lithium secondary battery.

[0015] In another aspect of the present invention, there is provided a nonwoven separator for a lithium secondary battery according to the following embodiment.

[0016] The sixth embodiment is A core of β-chitin and It is formed from fibers having a core-shell structure including a shell portion of vinylidene fluoride-trifluoroethylene copolymer, The vinylidene fluoride-trifluoroethylene copolymer has crystals that grow along a crystal axis, and the crystals that grow along the crystal axis are aligned in a direction that is not parallel to the longitudinal direction of the fibers.

[0017] The seventh embodiment is the same as the sixth embodiment, The crystals grown along the crystal axis are aligned along a direction substantially perpendicular to the longitudinal direction of the fibers, in a nonwoven fabric separator for a lithium secondary battery.

[0018] The eighth embodiment is the same as the sixth or seventh embodiment, The weight ratio of the core part to the shell part is about 50:50 to 99:1.

[0019] The ninth embodiment is any one of the sixth to eighth embodiments, The core-shell structured fibers have an average diameter of about 200 nm to 2,000 nm in the nonwoven fabric separator for lithium secondary batteries.

[0020] The tenth embodiment is any one of the sixth to ninth embodiments, The nonwoven fabric separator for a lithium secondary battery has a thickness of about 2 μm to 30 μm.

[0021] The eleventh embodiment is any one of the sixth to tenth embodiments, The basis weight of the nonwoven fabric separator is about 2 g / m 2 ~10g / m 2 The present invention relates to a nonwoven fabric separator for a lithium secondary battery.

[0022] The twelfth embodiment is any one of the sixth to eleventh embodiments, The nonwoven fabric separator for lithium secondary batteries has an air permeability of about 50s / 100cc or less.

[0023] The thirteenth embodiment is any one of the sixth to twelfth embodiments, The nonwoven fabric separator for a lithium secondary battery further includes a mixture of inorganic particles and a polymer coated on at least one surface of the nonwoven fabric.

[0024] In yet another aspect of the present invention, there is provided a composite separator for a lithium secondary battery according to the following embodiment.

[0025] The fourteenth embodiment is A nonwoven fabric separation membrane according to any one of the sixth to thirteenth embodiments; and a porous polymer film laminated on at least one surface of the nonwoven fabric separator.

[0026] In yet another aspect of the present invention, there is provided an electrode assembly for a lithium secondary battery according to the following embodiment.

[0027] The fifteenth embodiment is An electrode assembly for a lithium secondary battery, comprising: a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, The present invention relates to an electrode assembly for a lithium secondary battery, wherein the separator is the nonwoven fabric separator according to any one of the sixth to thirteenth embodiments.

[0028] The sixteenth embodiment is An electrode assembly for a lithium secondary battery, comprising: a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, The separator relates to an electrode assembly for a lithium secondary battery, wherein the separator is the composite separator according to the fourteenth embodiment.

[0029] In yet another aspect of the present invention, there is provided a lithium secondary battery according to the following embodiment.

[0030] The seventeenth embodiment is The present invention relates to a lithium secondary battery including the electrode assembly according to the fifteenth embodiment.

[0031] The eighteenth embodiment is The present invention relates to a lithium secondary battery including an electrode assembly according to a sixteenth embodiment. [Effects of the Invention]

[0032] According to the present invention, a nonwoven fabric separator for a lithium secondary battery is formed from fibers having a core-shell structure including a core of β-chitin and a shell of vinylidene fluoride-trifluoroethylene copolymer, the vinylidene fluoride-trifluoroethylene copolymer having crystals grown along a crystal axis, the crystals grown along the crystal axis being aligned in a direction not parallel to the longitudinal direction of the fibers.

[0033] The β-chitin that makes up the fiber core has excellent heat resistance, maintaining fiber properties even at high temperatures and exhibiting good thermal shrinkage characteristics. Furthermore, β-chitin is a ferroelectric polymer that exhibits ferroelectricity due to functional groups in the molecule, and therefore has a high affinity for lithium ions. Therefore, fibers with a β-chitin core induce uniform movement of lithium ions, reducing resistance.

[0034] The vinylidene fluoride-trifluoroethylene copolymer constituting the shell of the fiber is also a ferroelectric polymer with β-crystallinity. The vinylidene fluoride-trifluoroethylene copolymer shell formed on the surface of the β-chitin core by coaxial electrospinning is melted and recrystallized by heat treatment, resulting in crystals growing along a crystalline axis, which are aligned in a direction not parallel to the longitudinal direction of the fiber. The crystals growing along the crystalline axis of the vinylidene fluoride-trifluoroethylene copolymer maximize their ferroelectricity, particularly when aligned in a direction substantially perpendicular to the longitudinal direction of the fiber. This further enhances the affinity for lithium ions of the nonwoven separator formed from the core-shell structured fiber of the present invention, resulting in low resistance and improved cycle performance when applied to lithium secondary batteries. [Brief explanation of the drawings]

[0035] [Figure 1]1 is a SEM photograph of the surface of a nonwoven fabric separation membrane according to an embodiment of the present invention. [Figure 2] 1 is a SEM photograph of the surface of a nonwoven fabric separation membrane according to a comparative example of the present invention. [Figure 3] GIWAXS analysis results of a nonwoven fabric separator according to an embodiment of the present invention. In some of the accompanying drawings, corresponding elements are designated by the same reference numerals. Those skilled in the art should understand that the drawings illustrate elements simply and clearly and are not necessarily drawn to scale. For example, to facilitate understanding of various embodiments, the dimensions of some elements may be exaggerated relative to other elements. Furthermore, in commercially viable embodiments, useful or essential elements of the prior art may be omitted so as not to detract from the scope of various embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0036] Hereinafter, the terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that corresponds to the technical idea of ​​the present invention, in accordance with the principle that the inventor himself can appropriately define the concept of the term in order to best explain the invention.

[0037] Throughout this specification, the terms "comprises" or "includes" a certain element do not mean to exclude other elements, but to mean that the element may further include or comprise other elements, unless otherwise specified.

[0038] As used herein, the property of "having pores" means that an object contains a plurality of pores, and the interconnected structure of the pores allows gaseous and / or liquid fluids to pass from one side of the object to the other.

[0039] In this specification, the separator has a porous property including a plurality of pores, and serves as a porous ion-conducting barrier that blocks the electrical connection between the negative electrode and the positive electrode in the lithium secondary battery and allows ions to pass through.

[0040] As used herein, the term "nonwoven fabric" refers to a fabric formed by intertwining fibers, rather than a woven fabric woven with warp and weft threads using a loom or a knitted fabric using a knitting machine.

[0041] In this specification, the term "substantially perpendicular direction" can be interpreted to include a direction in which the angle is exactly 90°, that is, perpendicular, as well as a direction in which the angle is 90°±15°, that is, substantially perpendicular.

[0042] In this specification, the term "non-parallel directions" can be interpreted as including directions that form an angle of 20° to 90°.

[0043] As used herein, the terms "about," "approximately," and "substantially" are used to mean a range of values ​​or degrees, or approximations thereof, taking into account inherent manufacturing and material tolerances, and are used to prevent infringers from unfairly exploiting the disclosure in which exact or absolute values ​​are recited, provided to aid in the understanding of the present invention.

[0044] The separator is one of the important elements of a lithium secondary battery, along with the negative electrode, positive electrode, and electrolyte, and serves to prevent physical contact between the positive electrode and the negative electrode. This is closely related to the efficiency and safety of the lithium secondary battery.

[0045] Polyolefin-based separators are most commonly used in lithium secondary batteries due to their excellent chemical safety and mechanical strength. However, separators using porous polyolefin films have the risk of internal short circuits due to thermal shrinkage at high temperatures, due to material characteristics as well as manufacturing process characteristics, particularly stretching, and the risk of fire due to melting of the polymer separator substrate during thermal runaway.

[0046] In light of this, a method has been proposed in which a nonwoven fabric made from a heat-resistant fiber such as polyethylene terephthalate is used as a separator substrate. The heat-resistant nonwoven fabric can be applied to the separator as is, or in the form of a nonwoven fabric coated with a mixture of inorganic particles and a polymer. Alternatively, the separator can be applied in the form of a composite separator by laminating the nonwoven fabric with a porous polymer film.

[0047] In the present invention, core-shell structured nanofibers of β-chitin and copolymer (PVDF-TrFE) were prepared using the coaxial electrospinning method, and a nonwoven separator with maximized crystallinity was prepared by aligning the PVDF-TrFE polarization axis (b-axis) through appropriate heat treatment. The separator was applied to a lithium secondary battery, and the resistance of the nonwoven separator and the cycle performance of the cell were confirmed to be improved.

[0048] One embodiment of the present invention relates to a method for manufacturing a nonwoven fabric separator for a lithium secondary battery, (S1) preparing a first solution in which β-chitin is dissolved in a first solvent, and a second solution in which vinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE) is dissolved in a second solvent; (S2) preparing a nonwoven fabric formed from fibers having a core-shell structure consisting of a core portion of β-chitin and a shell portion of vinylidene fluoride-trifluoroethylene copolymer by coaxially electrospinning the first solution as a core solution and the second solution as a shell solution; (S3) heat-treating the nonwoven fabric at a temperature lower than the thermal decomposition point of β-chitin and higher than the melting point of the vinylidene fluoride-trifluoroethylene copolymer to melt and recrystallize the vinylidene fluoride-trifluoroethylene copolymer.

[0049] First, a first solution in which β-chitin is dissolved in a first solvent and a second solution in which vinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE) is dissolved in a second solvent are prepared (S1).

[0050] The first solvent for dissolving β-chitin is not limited as long as it can dissolve β-chitin and allow electrospinning, for example, HFIP (1,1,1,3,3,3-hexafluoro-2-propanol).

[0051] The second solvent for dissolving vinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE) is not limited as long as it can dissolve vinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE) and perform electrospinning. For example, MEK (methylethylketone) can be used.

[0052] Next, the first solution is used as a core solution, and the second solution is used as a shell solution, and coaxial electrospinning is performed to produce a nonwoven fabric formed from fibers with a core-shell structure consisting of a core of β-chitin and a shell of vinylidene fluoride-trifluoroethylene copolymer (S2).

[0053] As is well known, by using a coaxial electrospinning apparatus, a first solution prepared by dissolving β-chitin in a first solvent is supplied as a core solution, and a second solution prepared by dissolving vinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE) in a second solvent is supplied as a shell solution, and electrospinning is performed using a single nozzle, thereby producing a nonwoven fabric formed from fibers with a core-shell structure consisting of a core of β-chitin and a shell of vinylidene fluoride-trifluoroethylene copolymer.

[0054] According to one embodiment, a nonwoven fabric is produced by electrospinning by introducing a first solution of β-chitin and a second solution of vinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE) into a core solution and a shell solution, respectively, into a coaxial electrospinning nozzle maintained at room temperature to about 80°C. Then, electrospinning is performed while an electric field is formed between the electrospinning nozzle and the substrate. For example, the electric field may be formed by applying a voltage of 1 kV to 30 kV. The first and second solutions are then ejected from the coaxial electrospinning nozzle to which high voltage is applied and coated onto the substrate in the form of fibers, thereby forming a nonwoven fabric. The formed nonwoven fabric can be peeled off from the substrate and used as a separator. When using another separator or electrode as the substrate, the nonwoven fabric can be used in the form of a coating on the substrate.

[0055] Most of the solvent can be dried immediately after electrospinning, but it is also possible to completely remove the solvent using a separate drying device.

[0056] In this case, the substrate facing the ejection nozzle and serving as a target for coaxial electrospinning may be disposed in a direction parallel to the bottom surface or in a direction perpendicular to the bottom surface.

[0057] Next, the produced nonwoven fabric is heat-treated at a temperature lower than the thermal decomposition point of β-chitin and higher than the melting point of the vinylidene fluoride-trifluoroethylene copolymer to melt and recrystallize the vinylidene fluoride-trifluoroethylene copolymer (S3). Since the melting point of the vinylidene fluoride-trifluoroethylene copolymer is about 146°C, the heat treatment temperature can be, for example, about 150°C to 250°C, but is not limited thereto.

[0058] The nonwoven fabric separator thus prepared is formed from fibers with a core-shell structure including a core of β-chitin and a shell of vinylidene fluoride-trifluoroethylene copolymer, and the vinylidene fluoride-trifluoroethylene copolymer has crystals that grow along a crystal axis, and the crystals that grow along the crystal axis are aligned in a direction that is not parallel to the longitudinal direction of the fiber.

[0059] β-chitin, which forms the core of the fiber, has excellent heat resistance and maintains its fiber properties even at high temperatures, demonstrating excellent heat shrinkage characteristics. Furthermore, β-chitin is a ferroelectric polymer that exhibits ferroelectricity due to functional groups in the molecule, and therefore has a high affinity for lithium ions. Therefore, fibers with a β-chitin core induce uniform movement of lithium ions, reducing resistance.

[0060] The vinylidene fluoride-trifluoroethylene copolymer (VFC) that constitutes the shell of the fiber is also a ferroelectric polymer with β-crystallinity. The VFC shell formed on the surface of the β-chitin core by coaxial electrospinning is melted and recrystallized by heat treatment, resulting in crystals that grow along the crystal axis (epitaxial growth). Epitaxial growth of VFC is also observed on the surface of polytetrafluoroethylene or graphene, but because the b-axis (polarization axis) is aligned parallel to the polytetrafluoroethylene or graphene crystals, enhancing the ferroelectricity of VFC is limited. The ferroelectricity of VFC is maximized when the crystals grow along the crystal axis and are aligned perpendicular to the longitudinal direction of the fiber. As a result, the nonwoven separator formed from the core-shell structured fiber of the present invention has a higher affinity for lithium ions, resulting in lower resistance and improved cycle performance when applied to lithium secondary batteries, and also suppressing problems such as the precipitation of lithium dendrites.

[0061] In this case, the weight ratio of the core portion of β-chitin to the shell portion of vinylidene fluoride-trifluoroethylene copolymer constituting the fibers of the nonwoven fabric may be about 50:50 to 99:1, for example, about 50:50 to 80:20, but is not limited thereto.

[0062] The average diameter of the core-shell structured fibers can be adjusted by the concentrations of the core solution and the shell solution and the diameter of the spray nozzle. For example, the average diameter of the core-shell structured fibers may be 200 nm to 2,000 nm, or 300 nm to 1,000 nm. A nonwoven fabric formed from such core-shell structured fibers is entangled to have a three-dimensional (3D) shape, and the pore size of the nonwoven fabric can be adjusted by the average diameter of the core-shell structured fibers and the fiber loading amount. The basis weight of the nonwoven fabric is 2 g / m 2 ~10g / m 2 The air permeability of the nonwoven fabric may be, but is not limited to, about 50s / 100cc or less.

[0063] The thickness of the nonwoven fabric may be, but is not limited to, 2 μm to 30 μm, and the nonwoven fabric formed by electrospinning can be easily made thin.

[0064] The manufactured nonwoven fabric separator may further include a mixture of inorganic particles and a polymer coated on at least one surface of the nonwoven fabric.

[0065] The inorganic particles are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles are suitable for use in a lithium secondary battery having an operating voltage range (e.g., Li / Li + There are no particular limitations on the dielectric constant as long as oxidation and / or reduction reactions do not occur at a potential of 0 V to 5 V relative to the reference potential. In particular, when inorganic particles with a high dielectric constant are used as the inorganic particles, they can contribute to an increase in the degree of dissociation of the electrolyte salt, for example, lithium salt, in the liquid electrolyte, thereby improving the ionic conductivity of the electrolyte solution.

[0066] For the reasons mentioned above, the inorganic particles may include inorganic particles with a high dielectric constant, that is, a dielectric constant of 5 or more, or 10 or more. Non-limiting examples of inorganic particles with a dielectric constant of 5 or more include BaTiO3, Pb(Zr x ,Ti 1-x )O3(PZT, where 0 <x<1である。)、Pb 1-x La x Zr1-y Ti y O3 (PLZT, where 0 < x < 1, 0 < y < 1.) (1 - x)Pb(Mg 1 / 3 Nb 2 / 3 )O 3-x PbTiO3 (PMN - PT, where 0 < x < 1.), hafnium (HfO2), SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, Y2O3, Al2O3, boehmite, AlOOH, SiC, and any one inorganic particle selected from the group consisting of TiO2 or a mixture of two or more of these.

[0067] In addition, as the inorganic particles, inorganic particles having lithium ion transfer ability, that is, inorganic particles containing lithium element but having a function of moving lithium ions without storing lithium can be used. Non - limiting examples of inorganic particles having lithium ion transfer ability include lithium phosphate (Li3PO4), lithium titanium phosphate (Li x Ti y (PO4)3, 0 < x < 2, 0 < y < 3), lithium aluminum titanium phosphate (Li x Al y Ti z (PO4)3, 0 < x < 2, 0 < y < 1, 0 < z < 3), (LiAlTiP) x O y - based glass (0 < x < 4, 0 < y < 13), lithium lanthanum titanate (Li x La y TiO3, 0 < x < 2, 0 < y < 3), Li 3.25 Ge 0.25 P 0.75 S4 and the like such as lithium germanium thiophosphate (Li x Ge y P z S w , 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), lithium nitride such as Li3N (Li x N y, (0 < x < 4, 0 < y < 2), SiS2-based glasses such as Li3PO4-Li2S-SiS2 (Li x Si y S z , (0 < x < 3, 0 < y < 2, 0 < z < 4), P2S5-based glasses such as LiI-Li2S-P2S5 (Li x P y S z , (0 < x < 3, 0 < y < 3, 0 < z < 7) or mixtures of two or more of these, etc.

[0068] The inorganic particles may be mixed with a polymer and coated on at least one side of the nonwoven fabric. Non-limiting examples of polymers include polyvinylidene fluoride, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloro ethylene, polyvinylidene fluoride-co-chlorotrifluoro ethylene, polymethyl methacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate. Examples of suitable polymers include acrylonitrile-styrene-butadiene copolymer, cyanoethyl propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxyl methyl cellulose, acrylonitrile-styrene-butadiene copolymer, polyimide, and mixtures of two or more of these. Both ion-conductive and ion-non-conductive polymers can be used.

[0069] Coating with a mixture of inorganic particles and a polymer can be performed by dispersing inorganic particles in a slurry prepared by dissolving or dispersing a polymer in a solvent, coating the slurry on at least one surface of a nonwoven fabric, and then drying the mixture. Depending on the size of the pores of the nonwoven fabric, the mixture of inorganic particles and a polymer can be distributed only on the surface of the nonwoven fabric, or distributed both on the surface and in the internal pores of the nonwoven fabric, or distributed primarily inside the porous polymer nonwoven fabric.

[0070] The inorganic particles may be disposed in a state where they are substantially in contact with each other, except for a state where fibers are interposed between them. In this case, when the mixed polymer is formed as a coating layer on some or all of the surfaces of the inorganic particles, the polymer may connect and fix the inorganic particles to each other so that the inorganic particles remain bound to each other.

[0071] The nonwoven fabric separator of the above-described type may be laminated on at least one surface of a porous polymer film to be used in the form of a composite separator.

[0072] The porous polymer film may be simply laminated with the nonwoven fabric separator, or may be attached to the nonwoven fabric separator by a process such as lamination.

[0073] The porous polymer film may be, for example, a polyolefin porous polymer film. Any polyolefin porous polymer film commonly used as a separation membrane can be used. For example, a polyolefin-based polymer such as polyethylene (e.g., high-density polyethylene, linear low-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene), polypropylene, polybutylene, or polypentene may be used alone or in combination to form a polymer. When such a polyolefin porous polymer film is laminated with a nonwoven fabric separation membrane to form a composite separation membrane, the polyolefin porous polymer film exhibits a shutdown function at temperatures of, for example, 80°C to 150°C. In this respect, it is preferable that the polyolefin porous polymer film be made of polyethylene.

[0074] The nonwoven fabric separator or composite nonwoven fabric separator may be interposed between a positive electrode and a negative electrode to form an electrode assembly for a lithium secondary battery.

[0075] The positive and negative electrodes that constitute the electrode assembly will be described below by way of example.

[0076] The positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector.

[0077] The positive electrode current collector in the positive electrode is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector typically has a thickness of about 3 μm to 500 μm, and the surface of the positive electrode current collector may be provided with fine irregularities to enhance the adhesive strength of the positive electrode active material. For example, the positive electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0078] The positive electrode active material layer may include a known positive electrode active material, a conductive material, and a binder.

[0079] Positive electrode active materials include layered compounds such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and compounds substituted with one or more transition metals; 1+x Mn 2-x Lithium manganese oxides such as LiMnO4 (where x is 0 to 0.33), LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, and Cu2V2O7; and the chemical formula LiNi 1-x M x Ni-site lithium nickel oxide represented by the chemical formula LiMnO2 (where M=Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x=0.01 to 0.3). 2-x M x Examples of suitable lithium manganese oxides include, but are not limited to, lithium manganese composite oxides represented by Li2Mn3MO8 (where M is Co, Ni, Fe, Cr, Zn, or Ta, and x is 0.01 to 0.1) or Li2Mn3MO8 (where M is Fe, Co, Ni, Cu, or Zn); LiMn2O4, in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion; disulfide compounds; and Fe2(MoO4)3.

[0080] The conductive material is used to impart conductivity to the electrode. Any conductive material can be used without particular limitations, as long as it does not undergo chemical changes in the resulting battery and has electronic conductivity. Examples include black black (e.g., natural black or artificial black); carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or fibers (e.g., copper, nickel, aluminum, and silver); conductive whiskers (e.g., zinc oxide and potassium titanate); conductive metal oxides (e.g., titanium oxide); and conductive polymers (e.g., polyphenylene derivatives). These materials can be used alone or in combination. The conductive material is typically present in an amount of about 1% to 30% by weight based on the total weight of the positive electrode active material layer.

[0081] The binder functions to improve adhesion between positive electrode active material particles and between the positive electrode current collector. Examples of binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. One or more of these may be used alone or in combination. The binder may be present in an amount of about 1 wt% to 30 wt% based on the total weight of the positive electrode active material layer.

[0082] The positive electrode may be fabricated by a conventional method for fabricating a positive electrode. For example, the positive electrode may be fabricated by applying a composition for forming a positive electrode active material layer, including a positive electrode active material and, optionally, a binder and a conductive material, to a positive electrode current collector, followed by drying and rolling. In this case, the types and amounts of the positive electrode active material, binder, and conductive material are as described above.

[0083] The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and may be used alone or in combination. The amount of the solvent used may be such that the viscosity is sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, and to provide a suitable thickness uniformity during application to produce a positive electrode, taking into consideration the coating thickness of the slurry and the production yield.

[0084] In another method, the positive electrode can be produced by casting the composition for forming the positive electrode active material layer onto a separate support, peeling the composition from the support, and laminating the resulting film onto a positive electrode current collector.

[0085] The negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0086] The negative electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy may be used. The negative electrode current collector typically has a thickness of about 3 μm to 500 μm. As with the positive electrode current collector, the surface of the current collector may be formed with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0087] The negative electrode active material layer includes a negative electrode active material and, optionally, a binder and a conductive material. For example, the negative electrode active material layer can be fabricated by applying a negative electrode-forming composition including the negative electrode active material and, optionally, the binder and the conductive material to a negative electrode current collector and drying the composition, or by casting the negative electrode-forming composition on a separate support, peeling it off from the support, and laminating the resulting film on the negative electrode current collector.

[0088] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium, such as a carbonaceous material, such as artificial graphite, natural graphite, graphitized carbon fiber, or amorphous carbon; a metallic compound capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, a Si alloy, a Sn alloy, or an Al alloy; or SiO β Examples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide (0<β<2); or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites. These may be used alone or in combination. A thin film of metallic lithium may also be used as the negative electrode active material. Both low-crystalline carbon and high-crystalline carbon may also be used as the carbon material. Typical low-crystalline carbons include soft carbon and hard carbon, and typical high-crystalline carbons include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-fired carbons such as petroleum or coal tar pitch-derived cokes.

[0089] The binder and conductive material may be the same as those described above for the positive electrode.

[0090] Meanwhile, a lithium secondary battery according to one aspect of the present invention includes the above-described electrode structure.

[0091] The lithium secondary battery includes the electrode structure and electrolyte described above. Examples of the electrolyte used in the present invention include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten polymer electrolytes that can be used in manufacturing lithium secondary batteries.

[0092] For example, the electrolyte may include an organic solvent and a lithium salt.

[0093] The organic solvent may be any solvent that functions as a medium through which ions involved in the electrochemical reaction of the battery can move. For example, the organic solvent may be an ester solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; an ether solvent such as dibutyl ether or tetrahydrofuran; a ketone solvent such as cyclohexanone; an aromatic hydrocarbon solvent such as benzene or fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylene carbonate (EC), or propylene carbonate. Examples of suitable solvents include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a C2-C20 linear, branched, or cyclic hydrocarbon group that may contain a double-bonded aromatic ring or ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with low viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) that have high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of batteries, are more preferred. In this case, mixing the cyclic carbonate and the chain carbonate in a volume ratio of about 1:1 to about 1:9 can result in excellent electrolyte performance.

[0094] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries. For example, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The concentration of the lithium salt is preferably within the range of about 0.1M to 2.0M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, resulting in good electrolyte performance and efficient lithium ion migration.

[0095] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethyl alcohol amine, cyclic ethers, ethylenediamine, m-glyme (n-glyme), hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethyl alcohol, or aluminum trichloride, to improve battery life characteristics, suppress battery capacity loss, and improve battery discharge capacity. In this case, the additives may be contained in an amount of about 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0096] The lithium secondary battery according to the present invention is useful in portable devices such as mobile phones, notebook computers, and digital cameras, and in electric vehicles such as hybrid electric vehicles (HEVs).

[0097] While the present invention may be embodied in many different forms, it is to be understood that the invention is not limited to the specific embodiments set forth herein.

[0098] Example 1: A first solution was prepared by dissolving β-chitin (Glycosyn, β-chitin) in HFIP (1,1,1,3,3,3-hexafluoro-2-propanol) at a concentration of approximately 0.6 wt%, and a second solution was prepared by dissolving vinylidene fluoride-trifluoroethylene copolymer (Solvay, VDF / TrFE = 75:25 mol%) in MEK (methylethylketone) at a concentration of approximately 10 wt%.

[0099] The prepared first solution was used as a core solution, and the second solution was used as a shell solution. These solutions were then introduced into a coaxial electrospinning nozzle controlled at a temperature of 25°C. A voltage of 1.5 kV was applied between the electrospinning nozzle and a support made of SUS foil to perform random electrospinning, and the resulting mixture was dried to produce a nonwoven fabric formed from fibers with a core-shell structure consisting of a β-chitin core and a vinylidene fluoride-trifluoroethylene copolymer shell.

[0100] The nonwoven fabric was then heat-treated at approximately 200°C for 30 minutes and then left at room temperature to obtain a nonwoven fabric separation membrane. In the nonwoven fabric separation membrane, the weight ratio of the β-chitin core to the vinylidene fluoride-trifluoroethylene copolymer shell was approximately 70:30.

[0101] Example 2: Nonwoven fabric separators were prepared in the same manner as in Example 1, except that the average fiber diameter was changed under the conditions shown in Table 1.

[0102] Comparative Example 1: Instead of coaxial electrospinning, electrospinning was performed using only β-chitin under the conditions listed in Table 1 .

[0103] Comparative Example 2: Instead of coaxial electrospinning, electrospinning was performed using only vinylidene fluoride-trifluoroethylene copolymer under the conditions listed in Table 1 .

[0104] Comparative Example 3: The nonwoven fabric prepared in Comparative Example 1 was immersed in a solution in which vinylidene fluoride-trifluoroethylene copolymer was dissolved in MEK (methylethylketone) at a concentration of 10 wt %, and then dried to prepare a nonwoven fabric separator.

[0105] Comparative Example 4: A nonwoven fabric separator was prepared in the same manner as in Example 1, except that the nonwoven fabric prepared by coaxial electrospinning was not heat-treated.

[0106] Comparative Example 5: A nonwoven fabric separator was prepared in the same manner as in Example 1, except that the nonwoven fabric prepared by coaxial electrospinning was heat-treated at about 120° C. for 1 hour and then left at room temperature.

[0107] <Average fiber diameter> In this specification, the average fiber diameter is defined as being measured as follows.

[0108] A nonwoven fabric separator specimen was fixed to a mount for SEM measurement using carbon tape. The surface was then coated with platinum using ion sputtering for approximately 60 seconds. The platinum-coated sample was measured at 10 kV using FE-SEM (Hitachi S-4800). The diameter of a single fiber was measured at 10 points or more in the 500x image observed with the FE-SEM using software, and the average was used to calculate the fiber diameter.

[0109] <Basis weight of nonwoven fabric> In this specification, the basis weight of a nonwoven fabric is defined as being measured as follows.

[0110] The basis weight of nonwoven fabric is 1m 2 The weight per piece was measured.

[0111] <Air permeability of nonwoven fabric separation membrane> In this specification, the air permeability of a nonwoven fabric separator is defined as being measured as follows.

[0112] The air permeability of the separator was measured in accordance with JIS P8117.

[0113] <Measurement of Thermal Shrinkage Rate> The manufactured non-woven separator membrane was left in an oven maintained at about 150 °C for 30 minutes and then taken out. The lengths in the MD direction (Machine Direction) and TD direction (Transverse Direction) before and after putting it into the oven were measured to evaluate the thermal shrinkage rate.

[0114] <Measurement of ER and Cycle Performance> Each non-woven separator membrane of the examples and comparative examples was cut and stacked in a 2016 coin cell of Hoshen. After impregnating with an electrolyte, the resistance value of the battery was measured by the AC method at about 25 °C by EIS (Electrochemical Impedance Spectroscopy) using an electrolyte of 1M LiPF6-ethylene carbonate / ethyl methyl carbonate (weight ratio 3:7).

[0115] Also, charge and discharge were repeated in the range of about 2.5 V to 4.25 V at a rate of 1C at 25 °C, and the cycle performance (capacity retention rate) was measured by calculating the ratio of the discharge capacity after 200 cycles to the initial discharge capacity.

[0116]

Table 1

[0117] On the other hand, the nonwoven fabric separation membrane of Comparative Example 1 made from β-chitin fiber had a resistance characteristic of 0.64Ω, which was higher than that of the nonwoven fabric separation membrane of Example 1-2, and a cycle performance of 80%, which was lower than that of the nonwoven fabric separation membrane of Example 1-2. The nonwoven fabric separation membrane of Comparative Example 2 made from vinylidene fluoride-trifluoroethylene copolymer had a heat shrinkage rate of 28 / 27%, which was poor in heat resistance and high-temperature heat shrinkage characteristics.

[0118] The nonwoven fabric separation membrane of Comparative Example 3, in which vinylidene fluoride-trifluoroethylene copolymer was applied to a β-chitin nonwoven fabric by the immersion method, had a nonwoven fabric resistance of 0.71 Ω, which was poor in resistance characteristics. The nonwoven fabric separation membrane of Comparative Example 4, which was not subjected to a post-heat treatment process, had a resistance of 0.55 Ω, which was higher than the nonwoven fabric separation membrane of Example 1-2, but had a cycle performance of 84%, which was lower than the nonwoven fabric separation membrane of Example 1-2.

[0119] The nonwoven fabric separator of Comparative Example 5, which was heat-treated at a temperature lower than the melting point of vinylidene fluoride-trifluoroethylene copolymer, had a resistance characteristic of 0.41 Ω, which was higher than that of the nonwoven fabric separator of Example 1-2, and a cycle performance of 95%, which was lower than that of the nonwoven fabric separator of Example 1-2.

[0120] On the other hand, in the case of Example 2 in which the average fiber diameter was about 500 nm, the electrical resistance was about 0.32 Ω, which was lower than the electrical resistance of about 0.37 Ω in Example 1 in which the average fiber diameter was about 700 nm.

[0121] FIG. 1 is a SEM photograph of the surface of a nonwoven fabric separator according to Example 1 of the present invention, FIG. 2 is a SEM photograph of the surface of a nonwoven fabric separator according to Comparative Example 5 of the present invention, and FIG. 3 is the GIWAXS analysis result of the nonwoven fabric separator according to Example 1 of the present invention.

[0122] 1 and 2, the nonwoven fabric separator according to Example 1 in Fig. 1 has a smoother surface than the nonwoven fabric separator according to Comparative Example 5 in Fig. 2. That is, it can be seen that the surface of the nonwoven fabric separator according to the present invention is formed smoother.

[0123] Figure 3 is a graph showing the results of GIWAXS (Grazing-Incidence Wide-Angle X-Ray Scattering) analysis of the nonwoven fabric separator according to Example 1 of the present invention. Referring to Figure 3, it can be seen that when the vinylidene fluoride-trifluoroethylene copolymer is heat-treated at a temperature higher than the melting point of the vinylidene fluoride-trifluoroethylene copolymer, it undergoes a melt-recrystallization process, resulting in a (200) peak at the equator and a (110) peak at a point 30° away from the meridian. That is, when the (200) peak is at the equator, the a-axis is aligned parallel to the substrate surface. When the (110) peak is at a point 30° away from the meridian, the b-axis is aligned perpendicular to the substrate, resulting in the formation of a crystalline phase in which the b-axis is aligned perpendicular to the substrate.

[0124] As a result, it was confirmed that the b-axis of PVDF-TrFE was aligned substantially perpendicular to the longitudinal direction of the fiber, thereby maximizing the ferroelectricity of the nonwoven fabric separator of the present invention.

[0125] Although the present invention has been described above with reference to preferred embodiments, it will be understood by those skilled in the art or those with ordinary skill in the art to which the invention pertains that various modifications and variations of the present invention are possible within the scope of the spirit and technical scope of the present invention as set forth in the claims below. Therefore, the technical scope of the present invention should not be limited to the content of the detailed description of the invention, but should be determined by the claims.

Claims

1. (S1) preparing a first solution in which β-chitin is dissolved in a first solvent and a second solution in which vinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE) is dissolved in a second solvent; (S2) preparing a nonwoven fabric formed from fibers having a core-shell structure consisting of a core portion of β-chitin and a shell portion of vinylidene fluoride-trifluoroethylene copolymer by coaxially electrospinning the first solution as a core solution and the second solution as a shell solution; (S3) heat-treating the nonwoven fabric at a temperature lower than the thermal decomposition point of β-chitin and higher than the melting point of the vinylidene fluoride-trifluoroethylene copolymer to melt and recrystallize the vinylidene fluoride-trifluoroethylene copolymer.

2. 2. The method for producing a nonwoven fabric separator for a lithium secondary battery according to claim 1, wherein the weight ratio of the β-chitin core to the vinylidene fluoride-trifluoroethylene copolymer shell is about 50:50 to 99:

1.

3. 2. The method for manufacturing a nonwoven fabric separator for a lithium secondary battery according to claim 1, wherein the first solvent is HFIP (1,1,1,3,3,3-hexafluoro-2-propanol) and the second solvent is MEK (methylethylketone).

4. 2. The method for producing a nonwoven fabric separator for a lithium secondary battery according to claim 1, wherein the nonwoven fabric is heat-treated at a temperature of about 150°C to 250°C.

5. 2. The method for manufacturing a nonwoven fabric separator for a lithium secondary battery according to claim 1, wherein the vinylidene fluoride-trifluoroethylene copolymer has crystals that grow along a crystal axis, and the crystals that grow along the crystal axis are aligned in a direction that is not parallel to the longitudinal direction of the fibers.

6. a β-chitin core; It is formed from fibers having a core-shell structure including a shell portion of vinylidene fluoride-trifluoroethylene copolymer, The vinylidene fluoride-trifluoroethylene copolymer has crystals that grow along a crystal axis, and the crystals that grow along the crystal axis are aligned in a direction that is not parallel to the longitudinal direction of the fibers.

7. 7. The nonwoven fabric separator for a lithium secondary battery according to claim 6, wherein the crystals grown along the crystal axis are aligned along a direction substantially perpendicular to the longitudinal direction of the fibers.

8. 7. The nonwoven fabric separator for a lithium secondary battery according to claim 6, wherein the weight ratio of the core to the shell is about 50:50 to 99:

1.

9. 7. The nonwoven fabric separator for a lithium secondary battery according to claim 6, wherein the average diameter of the core-shell structured fibers is about 200 nm to 2,000 nm.

10. 7. The nonwoven fabric separator for a lithium secondary battery according to claim 6, wherein the thickness of the nonwoven fabric separator is about 2 μm to 30 μm.

11. The nonwoven fabric separation membrane has a basis weight of about 2 g / m ２ ~10g / m ２ The nonwoven fabric separator for a lithium secondary battery according to claim 6,

12. 7. The nonwoven fabric separator for a lithium secondary battery according to claim 6, wherein the nonwoven fabric separator has an air permeability of about 50 s / 100 cc or less.

13. 7. The nonwoven fabric separator for a lithium secondary battery according to claim 6, further comprising a mixture of inorganic particles and a polymer coated on at least one surface of the nonwoven fabric.

14. The nonwoven fabric separation membrane according to any one of claims 6 to 13; a porous polymer film laminated on at least one surface of the nonwoven fabric separator.

15. An electrode assembly for a lithium secondary battery, comprising: a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, 14. An electrode assembly for a lithium secondary battery, wherein the separator is the nonwoven fabric separator according to claim 6.

16. An electrode assembly for a lithium secondary battery, comprising: a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, 15. An electrode assembly for a lithium secondary battery, wherein the separator is the composite separator according to claim 14.

17. A lithium secondary battery comprising the electrode assembly according to claim 15.

18. A lithium secondary battery comprising the electrode assembly according to claim 16.

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