Method for manufacturing aramid nanofiber-based hybrid thin film, thin film manufactured thereby, and lithium secondary battery comprising same

Abstract

Provided are a method for manufacturing an aramid nanofiber-based hybrid thin film, a thin film manufactured thereby, and a lithium secondary battery comprising same. The aramid nanofiber-based hybrid thin film comprises: aramid nanofibers; an interpenetrating polymer network including a polymer infiltrated between the aramid nanofibers and having a porous network structure; and inorganic particles dispersed in the interpenetrating polymer network, and thus, when applied to a lithium secondary battery, the growth of lithium dendrites can be effectively inhibited.

Description

Method for manufacturing a hybrid thin film based on aramid nanofibers, thin film manufactured thereby, and lithium secondary battery including the same

[0001] The present invention relates to a separator for a lithium secondary battery, and more particularly, to a hybrid thin film based on aramid nanofibers.

[0002] Secondary batteries are rechargeable and reusable electrochemical cells. Secondary batteries are categorized by the type of charging material used. Currently, lithium-ion secondary batteries, which are small and commonly used in mobile phones, laptops, and PCs, are the most widely used. However, recent performance improvements in lithium-ion secondary batteries have slowed, and concerns about capacity limitations and the flammability of electrolytes have led to growing interest in next-generation secondary batteries. These next-generation batteries include lithium metal batteries, all-solid-state batteries, lithium-air batteries, and sodium-ion batteries.

[0003] In particular, lithium metal batteries (LMB) have attracted significant attention due to their outstanding performance with high energy density, including the development of anodes with a theoretical specific capacity more than 10 times that of conventional lithium-ion batteries. However, uncontrolled growth of dendritic crystals (lithium dendrites) on the lithium metal anode during long-term charge-discharge cycles leads to the formation of dead lithium through repeated cycles, which causes capacity reduction, and excessive accumulation of a solid-electrolyte interface (SEI) layer on the surface of the lithium dendrites, which reduces and depletes the liquid electrolyte.

[0004] The technical problem to be solved by the present invention is to provide a method for manufacturing an aramid nanofiber-based hybrid thin film capable of minimizing the growth of lithium dendrites, a thin film manufactured thereby, and a lithium secondary battery including the same.

[0005] The technical problems of the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art from the description below.

[0006] In order to achieve the above task, one aspect of the present invention provides a method for manufacturing an aramid nanofiber-based hybrid thin film, including the steps of dispersing inorganic particles in an aramid nanoseed suspension in which aramid fibers are dissolved in a strong base to form a dispersion, adding a polar protic solvent to the dispersion to form the aramid nanoseeds into aramid nanofibers, adding an organic solvent to the dispersion in which the aramid nanofibers are formed to form an aramid nanofiber-inorganic particle composite organic gel, adding a polymer solution to the composite organic gel to form a slurry, and applying the slurry onto a substrate and then drying it.

[0007] The above-mentioned inorganic particles can be dispersed on the surface and inside of the above-mentioned thin film.

[0008] The above inorganic particles may have at least a Lewis acid site on the surface.

[0009] The above inorganic particles may include sulfated zirconia particles.

[0010] The above inorganic particles may be inorganic solid electrolyte particles.

[0011] The above inorganic solid electrolyte particles may be lithium lanthanum tantalum zirconate (LLZTO) doped with tantalum (Ta).

[0012] The above organic solvent can dissolve the above polymer.

[0013] The organic solvent may include at least one selected from N-methyl-2-pyrrolidone (NMP), dimethylformamide (N,N-dimethylformamide (DMF), acetone, tetrahydrofuran (THF), methylene chloride, chloroform, and cyclohexane.

[0014] The polymer may include at least one fluorinated polymer selected from polytetrafluoroethylene (PTFE), poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP, and polyvinylidene fluoride (PVDF).

[0015] Hydrogen bonds and dipole interactions can be formed between the amide groups of the aramid nanofibers and the difluoromethylene groups of the fluoropolymer.

[0016] The above aramid nanofibers can be mixed in a ratio of 3 to 93 parts by weight of the above inorganic particles and 55 to 58 parts by weight of the above polymer relative to 100 parts by weight of the above aramid nanofibers.

[0017] Another aspect of the present invention provides an aramid nanofiber-based hybrid thin film comprising aramid nanofibers, an interpenetrating polymer network having a porous network structure, a polymer interpenetrating between the aramid nanofibers, and inorganic particles dispersed within the interpenetrating polymer network.

[0018] Another aspect of the present invention provides a lithium secondary battery comprising a cathode, a cathode, a separator disposed between the cathode and the anode, and an electrolyte containing a lithium salt and a solvent impregnated in the separator, wherein the separator comprises an aramid nanofiber-based hybrid thin film comprising an interpenetrating polymer network having a porous network structure, including aramid nanofibers and a polymer penetrated between the aramid nanofibers, and inorganic particles dispersed within the interpenetrating polymer network.

[0019] The above negative electrode may be lithium metal.

[0020] The above lithium salt may include at least one selected from lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI).

[0021] The above battery may be a lithium metal battery.

[0022] The method for manufacturing an aramid nanofiber-based hybrid thin film of the present invention, the thin film manufactured thereby, and a lithium secondary battery including the same can suppress the growth of lithium dendrites.

[0023] Accordingly, the cycling stability of secondary batteries can be improved.

[0024] In addition, the aramid nanofiber-based hybrid thin film of the present invention can be manufactured in a freestanding form and thus be thermally and mechanically robust.

[0025] In addition, the method for manufacturing a hybrid thin film based on aramid nanofibers of the present invention forms a three-dimensional interpenetrating polymer network, thereby uniformly dispersing inorganic particles on the surface and inside of the thin film, thereby easily manufacturing a thin thin film.

[0026] In addition, when zirconia sulfate is applied as an inorganic particle of the present invention, dissociation of lithium salt can be promoted, thereby forming a uniform distribution of lithium ions within the battery.

[0027] In addition, when LLZTO is applied as the inorganic particle of the present invention, high lithium ion conductivity can be achieved, so that the movement of lithium ions within the battery can become smoother.

[0028] The technical effects of the present invention are not limited to those mentioned above, and other technical effects not mentioned will be clearly understood by those skilled in the art from the description below.

[0029] Figure 1 is a flowchart showing a method for manufacturing an aramid nanofiber-based hybrid thin film according to one embodiment of the present invention.

[0030] Figure 2 is a schematic diagram showing a method for manufacturing an aramid nanofiber-based hybrid thin film according to one embodiment of the present invention.

[0031] FIG. 3 is a schematic diagram illustrating an aramid nanofiber-based hybrid thin film according to one embodiment of the present invention.

[0032] FIG. 4(a) to FIG. 4(c) are images and graphs analyzing the characteristics of zirconia sulfate particles of Manufacturing Example 2 of the present invention, and FIG. 4(d) is a graph showing the XRD spectra of the ANF / PVDF-HFP thin film of Comparative Example 2 of the present invention and the SZAP3 thin film of Manufacturing Example 3-3.

[0033] FIG. 5(a) to FIG. 5(h) are images of thin films of Manufacturing Examples 3-1 to 3-3 of the present invention and thin films of Comparative Examples 1 to 3 observed using a field emission scanning electron microscope (FE-SEM).

[0034] FIG. 6(a) and FIG. 6(b) are graphs showing thermogravimetric analysis (TGA) curves of zirconia (SZ) of Manufacturing Example 2 of the present invention, aramid nanofibers of Comparative Example 1, PVDF-HFP of Comparative Example 4, and SZAP thin films of Manufacturing Examples 3-1 to 3-3, and commercial PP film (celgard 2400).

[0035] FIG. 7(a) to FIG. 7(d) are graphs showing the Fourier transform infrared (FT-IR) spectra, major peak shifts of NH, C=O, and C-F2 of the zirconia sulfate (SZ) powder of Manufacturing Example 2 of the present invention, the aramid nanofiber (ANF) thin film of Comparative Example 1, the PVDF-HFP thin film of Comparative Example 4, the aramid nanofiber-based hybrid thin films (SZAP1, SZAP2, SZAP3) of Manufacturing Examples 3-1 to 3-3, and the commercial PP (celgard 2400) film.

[0036] FIG. 8(a) to FIG. 8(c) are graphs showing the results of uniaxial tensile tests of thin films of Comparative Examples 2 and 3 and Manufacturing Examples 3-1 to 3-3 of the present invention, as well as the electrochemical impedance spectroscopy (EIS) spectrum of a lithium / lithium symmetric cell and the initial part of the measured EIS.

[0037] FIGS. 9(a) to 9(d) are images of the surface of a lithium metal negative electrode observed using a field emission scanning electron microscope (FE-SEM) after 200 cycles of LFP / lithium full cells in which the thin films of Manufacturing Example 5, Comparative Example 6, and Comparative Example 7 of the present invention were applied as separators, respectively.

[0038] Figure 10 is a schematic diagram of the electrochemical deposition behavior of the lithium metal negative electrode of the battery of Comparative Example 7 of the present invention (left) and the battery of Manufacturing Example 5 (right).

[0039] Figure 11 shows the 1 mA·cm of the battery (celgard 2400) of comparative example 7 of the present invention and the battery (SZAP3) of manufacturing example 5. -2 , 1 mAh·cm -2This graph shows the voltage profile of a lithium / lithium symmetric cell tested under conditions.

[0040] FIG. 12(a) and FIG. 12(b) are graphs showing typical charge / discharge curves and cycle performance and coulombic efficiency at 0.5C of the LFP / lithium full cells of Manufacturing Example 5 (SZAP3) of the present invention and Comparative Examples 6 to 8.

[0041] Figure 13 is a scanning electron microscope (SEM) image of the thin films of Manufacturing Examples 4-1 to 4-3 of the present invention.

[0042] Figure 14 is a graph showing the electrochemical impedance spectroscopy (EIS) spectra of the lithium / lithium symmetrical cells of Manufacturing Example 6-1 (LLZTO 25 wt%) and Manufacturing Example 6-2 (LLZTO 50 wt%) of the present invention.

[0043] Figure 15 is a graph showing the voltage profile of the lithium / lithium symmetric cell of Manufacturing Example 6-1 (LLZTO 25 wt%) and Manufacturing Example 6-2 (LLZTO 50 wt%) of the present invention.

[0044] Hereinafter, an embodiment of the present invention will be described in detail with reference to the attached drawings.

[0045] While the present invention is susceptible to numerous modifications and variations, specific embodiments thereof are illustrated in the drawings and will be described in detail below. However, the invention is not intended to be limited to the particular forms disclosed; rather, the invention includes all modifications, equivalents, and alternatives consistent with the spirit of the invention as defined by the claims.

[0046] When an element such as a layer, region or substrate is referred to as existing "on" another element, it will be understood that this may be directly on the other element, or that there may be intermediate elements in between.

[0047] Although the terms first, second, etc. may be used to describe various elements, components, regions, layers and / or regions, it will be understood that these elements, components, regions, layers and / or regions should not be limited by these terms.

[0048] Manufacturing method of hybrid thin film based on aramid nanofibers

[0049] FIG. 1 is a flowchart showing a method for manufacturing an aramid nanofiber-based hybrid thin film according to one embodiment of the present invention, and FIG. 2 is a schematic diagram showing a method for manufacturing an aramid nanofiber-based hybrid thin film according to one embodiment of the present invention.

[0050] Referring to Figures 1 and 2, first, inorganic particles can be dispersed in an aramid nanoseed suspension in which aramid fibers are dissolved in a strong base to form a dispersion (S100). That is, after adding the inorganic particles to the aramid nanoseed suspension, the suspension can be stirred to evenly disperse the inorganic particles within the suspension. Dispersion can be achieved using a known dispersing device such as ultrasonic treatment.

[0051] The aramid fiber may be an aromatic polyamide resin. The aromatic polyamide resin may include para-aramid and meta-aramid. Preferably, the aramid fiber may be polyparaphenylene terephthalamide. In the present embodiment, Kevlar, a commercially available product of para-aramid having a structure in which benzene rings are linearly connected via amide groups (CONH), was used as the aramid fiber, but is not limited thereto.

[0052] The aramid fiber may be an aramid microfiber having a diameter in the micrometer range. Specifically, in the aramid fiber, a plurality of polymer backbones may form nanofibers having a diameter in the nanometer range that are bonded by hydrogen bonds between amide groups, and these nanofibers may be further bonded by hydrogen bonds between amide groups to form aramid microfiber having a micrometer size. Specifically, for example, the average diameter of the aramid fiber may be 1 to 100 μm, specifically 5 to 20 μm, and more specifically 10 to 15 μm. In addition, the average length of the aramid fiber may be 0.1 to 100 μm, and the weight average molecular weight may be 1,000,000 to 5,000,000 g / mol. The above aramid fiber can be manufactured through a general manufacturing process of forming a para-polyamide polymer and then neutralizing it, but is not limited thereto.

[0053] The above aramid nanoseed suspension can be formed by adding aramid fibers to a solvent containing a strong base and stirring until the aramid fibers are completely dissolved and dissolved in the solvent. Due to this stirring, the aramid fibers can be separated (split) within the suspension and formed into the form of aramid nanoseeds (or aramid nanofiber precursors).

[0054] The amide groups of some polymer backbones included in the aramid fiber, i.e., the aramid microfiber, can be deprotonated by the strong base to generate amide anions (-CON-). As a result, the aramid microfiber can be split in the solvent due to the strong electrostatic repulsion between the amide anions (-CON-). In this process, the amide groups of all polymer backbones cannot be deprotonated, and the hydrogen bonds between the amide groups of some polymer backbones can be maintained. In this way, the aramid microfiber can be converted into aramid nanoseeds in which some polymer backbones are aggregated by the remaining hydrogen bonds while being split due to the electrostatic repulsion resulting from the deprotonation of some amide groups. The aramid nanoseeds can be suspended in the solvent and in a sol state, and when sufficiently suspended, the aramid nanoseeds may not be visible to the naked eye. Stirring may be used to facilitate the above suspension.

[0055] The solvent in the above suspension may include at least one substance selected from N-methyl-2-pyrrolidone (NMP), dimethylformamide (N,N-dimethylformamide (DMF), dimethyl sulfide (DMS), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and hexamethylphosphoramide (HMPA).

[0056] The strong base may be a typical alkali metal hydroxide substance. Specifically, for example, the strong base may include one or more strong bases of potassium hydroxide (KOH), sodium hydroxide (NaOH), and calcium hydroxide (Ca(OH)2).

[0057] The above inorganic particles may refer to inorganic materials having excellent ionic conductivity and capable of assisting the conduction of lithium ions. In one embodiment, the above inorganic particles may be prepared in powder form.

[0058] The above-mentioned inorganic particles can be dispersed on the surface and inside of the hybrid thin film manufactured in the present invention. To this end, as described above, the present invention may form a thin film by dissolving conventional aramid fibers in a strong base to prepare an aramid nanoseed suspension, and then dispersing the above-mentioned inorganic particles in the prepared aramid nanoseed suspension.

[0059] In one embodiment, the inorganic particle may have at least a Lewis acid site on the surface. In this case, the sulfonyl group in the lithium salt may act as a Lewis base site, so that the inorganic particle having at least a Lewis acid site on the surface may attract the Lewis base site in the lithium salt. Accordingly, lithium ions (Li) in the lithium salt + ) can make movement more smooth.

[0060] Specifically, for example, the inorganic particle having a Lewis acid moiety on the surface may be sulfated zirconia (SO4-ZrO2). The sulfated zirconia (SZ) can provide mechanical strength to the aramid nanofiber-based hybrid thin film of the present invention through its inherent high modulus. In addition, the sulfated zirconia (SZ) can act as a solid acid that promotes the dissociation of a lithium salt, specifically, lithium bis trifluoromethanesulfonimide (LiTFSI), in a lithium secondary battery. In addition, the surface of the aramid nanoseeds has a negative charge (-), and the surface of the sulfated zirconia (SZ) also has a negative charge, so that the aramid nanoseeds and the sulfated zirconia (SZ) can be stably distributed without clumping due to the repulsive force between the particles. Accordingly, the inorganic particles can be more uniformly dispersed on the surface and inside of the thin film.

[0061] The above sulfated zirconia (SZ) may be obtained by treating a zirconium oxide precursor with sulfuric acid. The zirconium oxide precursor may include at least one selected from zirconium propoxide, zirconium oxychloride, zirconium tetrachloride, zirconium (IV) nitrate, and zirconium (IV) sulfate.

[0062] The above sulfated zirconia (SZ) can be manufactured using a conventional method. For example, the above sulfated zirconia (SZ) can be manufactured by adding sulfuric acid and a solvent to a zirconium oxide precursor, followed by drying and calcination, but is not limited thereto.

[0063] In another embodiment, the inorganic particles may be inorganic solid electrolyte particles. Specifically, for example, the inorganic solid electrolyte particles may use a garnet-based oxide doped with tantalum (Ta). The garnet-based oxide may be Li 7+x-y A x B 3-x C 2-y D y O 12 It may be a material having the chemical formula of . At this time, A is a divalent cation, B is a trivalent cation, C is a tetravalent cation, and D is a pentavalent cation, and 0≤x<3 and 0≤y<2 may be satisfied. Preferably, 0≤x≤2 and 0≤y≤1 may be satisfied. The garnet oxide may have a cubic crystal system. That is, the present invention can implement high ionic conductivity characteristics by using a garnet oxide doped with tantalum (Ta) having a cubic crystal system as the inorganic particle, and can increase stability at room temperature.

[0064] The above inorganic particles may be lithium lanthanum tantalum zirconate (LLZTO) doped with tantalum (Ta). Specifically, the above inorganic particles may be Li 7-x La3Ta x Zr 2-x O 12 It may be a material having the chemical formula of . At this time, 0.1≤x≤1.0 may be satisfied, and preferably, 0.4≤x≤0.6 may be satisfied. More specifically, the inorganic particles may be Li 6.4 La3Ta 0.6 Zr 1.4 O 12 , Li 6.5 La3Ta 0.5 Zr 1.5 O 12 , and Li 6.6 La3Ta 0.4 Zr 1.6 O 12It may include one or more selected from among. Preferably, the inorganic particles are Li 6.4 La3Ta 0.6 Zr 1.4 O 12 It could be.

[0065] In the above S100, the inorganic particles may be mixed in a ratio of 3 to 93 parts by weight per 100 parts by weight of the aramid nanofibers. Specifically, the inorganic particles may be mixed in a ratio of 93 parts by weight per 100 parts by weight of the aramid nanofibers. If the mixing ratio of the aramid nanoseed suspension and the inorganic particles is outside the above-described range, the mechanical properties of the aramid nanofiber-based hybrid thin film of the present invention may deteriorate, or it may be difficult to easily minimize lithium dendrite growth.

[0066] Referring to FIGS. 1 and 2, a polar protic solvent can be added to the dispersion to form the aramid nanoseeds into nanofibers (S200). Specifically, when a polar protic solvent is added to the aramid nanoseed suspension and the dispersion in which the inorganic particles are dispersed, the solvent constituting the suspension can be replaced by the polar protic solvent. For example, the polar protic solvent may be distilled water or a primary alcohol, but is not limited thereto.

[0067] The amide groups (-CONH-) provided on the aramid nanoseeds can form hydrogen bonds by the above-described substituted polar protic solvent. By this hydrogen bond, the aramid nanoseeds can self-assemble and nanofiberize, thereby forming aramid nanofibers. Specifically, the polar protic solvent re-protonates the amide groups of the polymer backbones on the surface of the aramid nanoseeds in the suspension from amide anions (-CON-) to amide groups (-CONH-), and the original hydrogen bonds are restored between the re-protonated polymer backbones and the polymer backbones having the existing amide groups, thereby self-assembling to form a nanofiber network in which aramid nanofibers are irregularly networked. In addition, due to the difference in solubility parameters between aramid and the polar protic solvent, phase separation may be induced between the polar protic solvent and the aramid nanofiber network. Accordingly, the polar protic solvent may be impregnated into the aramid nanofiber network, so that the aramid nanofiber network may be in a gel state. In some embodiments, when the polar protic solvent is added to the dispersion, a solvent other than the polar protic solvent may be further included.

[0068] Referring to FIGS. 1 and 2, an aramid nanofiber-inorganic particle composite organic gel can be formed by adding an organic solvent to the dispersion in which the aramid nanofibers are formed (S300). That is, by adding the organic solvent to the dispersion in which the aramid nanofibers are formed in S200, the polar protic solvent constituting the dispersion undergoes solvent exchange with the organic solvent, and as the dispersion gels, an aramid nanofiber-inorganic particle composite organic gel can be formed. Thereafter, the aramid nanofiber-inorganic particle composite organic gel can be separated from the dispersion in which the aramid nanofiber-inorganic particle composite organic gel is formed using a conventional separation method such as centrifugation, but the separation method is not limited thereto.

[0069] In addition, the organic solvent added to the dispersion may be a substance capable of dissolving the polymer described below. Specifically, the organic solvent may include at least one selected from N-methyl-2-pyrrolidone (NMP), dimethylformamide (N,N-dimethylformamide (DMF), acetone, tetrahydrofuran (THF), methylene chloride, chloroform, and cyclohexane.

[0070] Referring to FIGS. 1 and 2, a polymer solution may be added to the composite organogel to form a slurry (S400). After adding the polymer to the aramid nanofiber-inorganic particle composite organogel, the polymer may be sufficiently stirred to form a slurry having a homogeneous viscosity. The polymer may be dissolved by the organic solvent added to the dispersion in S300 and mixed with the aramid nanofiber-inorganic particle composite organogel. That is, the polymer may penetrate between the aramid nanofibers constituting the composite organogel to form an interpenetrating polymer network (IPN). Specifically, the interpenetrating polymer network may be formed as hydrogen bonds and dipole interactions are formed between the amide groups of the aramid nanofibers and the difluoromethylene groups of the fluoropolymer.

[0071] The polymer may be an interpenetrating polymer capable of interpenetrating each other. Specifically, the polymer may be a fluoropolymer having a difluoromethylene group. The fluoropolymer may include at least one selected from a polyvinylidene fluoride (PVDF) homopolymer, a polytetrafluoroethylene (PTFE) homopolymer, and a polyvinylidene fluoride-fluorine comonomer copolymer. The above fluorinated comonomer may include at least one selected from hexafluoropropylene (HFP), chloro trifluoro ethylene (CTFE), trifluoro ethylene, hexafluoro isobutylene, perfluoro butyl ethylene, perfluoro propyl vinyl ether, perfluoro ethyl vinyl ether, and perfluoro methyl vinyl ether.

[0072] More specifically, the polymer may include at least one fluoropolymer selected from polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) and polyvinylidene fluoride (PVDF). In this example, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) was used as the polymer.

[0073] In the above S400, the polymer may be mixed in a ratio of 55 to 58 parts by weight per 100 parts by weight of the aramid nanofibers. If the mixing ratio of the aramid nanoseed suspension and the polymer is outside the above-described range, the desired interpenetrating network may not be properly formed.

[0074] Referring to FIGS. 1 and 2, the slurry may be applied onto a substrate and then dried (S500). This is a process for forming the slurry into a thin film, and since a conventional thin film manufacturing device using slurry can be used, there are no particular limitations.

[0075] The substrate may be any conventional thin film forming substrate. For example, the substrate may be a glass, metal, semiconductor, or polymer substrate, but is not limited thereto. Specifically, for example, the semiconductor substrate may be a silicon (Si) substrate, the metal substrate may be stainless steel, aluminum (Al), or copper (Cu) alloy, and the polymer substrate may be a polyethylene terephthalate (PET) substrate.

[0076] When applying the slurry to the substrate, any conventional slurry application method can be applied. For example, the application method may be blade coating, spin coating, bar coating, dip coating, gravure coating, spray coating, roll coating, or die coating, but is not limited thereto.

[0077] The slurry applied on the substrate can be dried at a temperature of 50 to 100°C for 16 to 20 hours to form an aramid nanofiber-based hybrid thin film. Specifically, for example, the drying temperature of the slurry applied on the substrate can be 60 to 80°C, and the drying time can be 18 to 19 hours.

[0078] After this, a process of separating the aramid nanofiber-based hybrid thin film from the substrate can be performed.

[0079] As described above, the method for manufacturing an aramid nanofiber-based hybrid thin film of the present invention can be formed in the form of a free-standing thin film having a thin thickness by uniformly dispersing the inorganic particles between the aramid nanofibers formed by nanofiberizing the aramid nanoseeds while the inorganic particles are dispersed in the aramid nanoseed suspension.

[0080] In addition, the method for manufacturing an aramid nanofiber-based hybrid thin film of the present invention can effectively allow the polymer to penetrate between the aramid nanofibers by adding a polymer and an organic solvent capable of dissolving the polymer to a composite organic gel including aramid nanofibers. Accordingly, the method for manufacturing an aramid nanofiber-based hybrid thin film of the present invention can easily manufacture a hybrid thin film including an interpenetrating polymer network having a porous network structure formed by the aramid nanofibers and the polymer penetrated between the aramid nanofibers, and inorganic particles dispersed therein.

[0081]

[0082] Aramid nanofiber-based hybrid thin film

[0083] FIG. 3 is a schematic diagram illustrating an aramid nanofiber-based hybrid thin film according to one embodiment of the present invention.

[0084] Referring to FIG. 3, the aramid nanofiber-based hybrid thin film (1) may include aramid nanofibers (100), an interpenetrating polymer network (200) having a porous network structure and including a polymer that has penetrated between the aramid nanofibers (100), and inorganic particles (300) dispersed within the interpenetrating polymer network (200).

[0085] The aramid nanofibers (100) may have a structure in which a plurality of aramid nanofibers are irregularly entangled. The aramid nanofiber-based hybrid thin film (1) of the present invention may include micropores and nanopores irregularly mixed by the aramid nanofibers (100). When the aramid nanofiber-based hybrid thin film (1) of the present invention is applied as a separator of a lithium secondary battery, the movement of lithium ions may become smooth due to the pores formed by the aramid nanofibers (100), and the mechanical properties may be improved due to the inclusion of inorganic substances, thereby minimizing the growth of lithium dendrites.

[0086] The above interpenetrating polymer network (200) may include a polymer that is provided in a form that is entangled between the aramid nanofibers (100) by penetrating between the aramid nanofibers (100). That is, the interpenetrating polymer network (200) may be a hybrid of a network structure in which the aramid nanofibers (100) are irregularly entangled and an interpenetrating polymer network in which the polymer is penetrating between the aramid nanofibers (100). Through this, the aramid nanofiber-based hybrid thin film (1) of the present invention can maintain its own mechanical properties without a separate substrate or support.

[0087] The above-described inorganic particles (300) may be dispersed within the interpenetrating polymer network (200). That is, the above-described inorganic particles (300) may be dispersed between the aramid nanofibers (100) and the interpenetrating polymers (200). As the above-described inorganic particles (300) are evenly dispersed, the aramid nanofiber-based hybrid thin film (1) of the present invention may be formed with a uniform and thin thickness. In addition, the above-described inorganic particles (300) have high ionic conductivity for lithium ions and may assist in the dissociation or movement of lithium ions depending on the type of the inorganic particles, so that the aramid nanofiber-based hybrid thin film (1) of the present invention may be actively utilized as a separator for a lithium secondary battery. Specifically, the description of the above-described aramid nanofibers (100), the above-described polymer, and the above-described inorganic particles (300) may refer to the above-described method for manufacturing a hybrid thin film to avoid redundant description.

[0088] The aramid nanofiber-based hybrid thin film (1) may be in the form of a freestanding thin film having a thickness of 12 to 16 μm. A freestanding thin film may be defined as a thin film having a portion that is not attached to a support such as a substrate. That is, the aramid nanofiber-based hybrid thin film (1) of the present invention may be formed to have a thickness of 12 to 16 μm, and thus may be utilized as a thinner and more uniform separator than a separator of a conventional lithium secondary battery. In addition, the aramid nanofiber-based hybrid thin film (1) of the present invention is formed in the form of a freestanding thin film, and thus is mechanically robust despite its thinness and uniformity, thereby further enhancing the stability of the separator.

[0089] The elastic modulus of the aramid nanofiber-based hybrid thin film (1) may be 0.5 to 1.8 GPa. In addition, the tensile strength of the hybrid thin film may be 21 to 58 MPa. Specifically, this can be referred to the following manufacturing examples and experimental examples.

[0090]

[0091] Lithium secondary batteries containing hybrid thin films based on aramid nanofibers

[0092] Another aspect of the present invention can provide a lithium secondary battery including the aramid nanofiber-based hybrid thin film described above. The lithium secondary battery can include a cathode, an anode, a separator disposed between the cathode and the anode, and an electrolyte containing a lithium salt and a solvent impregnated in the separator. In this case, the separator can include an aramid nanofiber-based hybrid thin film including an interpenetrating polymer network having a porous network structure, a polymer permeating between the aramid nanofibers, and inorganic particles dispersed within the interpenetrating polymer network.

[0093] The lithium secondary battery may be any one selected from a lithium metal battery, a lithium ion battery, and a lithium air battery. Specifically, the lithium secondary battery may be a lithium metal battery.

[0094] The lithium secondary battery of the present invention can be manufactured using a conventional and known method. For example, it can be formed by including the aforementioned aramid nanofiber-based hybrid thin film as a separator between the positive and negative electrodes and injecting an electrolyte therein.

[0095] The above-mentioned negative electrode can be manufactured according to a known conventional method. For example, the negative electrode may include at least one negative electrode active material layer selected from lithium metal or a lithium alloy. The lithium alloy may be an alloy composed of lithium and at least one metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al, and Sn, but is not limited thereto. The negative electrode may further include a conventional negative electrode current collector under the negative electrode active material layer. Specifically, for example, the negative electrode current collector may be made of, but is not limited to, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, or silver, an aluminum-cadmium alloy, or the like. The negative electrode may further include a solvent, a binder, a conductive agent, or a dispersant, as needed. More specifically, the negative electrode may be lithium metal.

[0096] The positive electrode can be manufactured using a known, conventional method. For example, the positive electrode may include a positive electrode active material, a solvent, a binder, and a conductive agent. The positive electrode active material can be applied in various ways depending on the type of lithium secondary battery, and is not particularly limited.

[0097] The above electrolyte may contain a lithium salt and a solvent impregnated in the separator. The above electrolyte may be applied in various ways depending on the type of lithium secondary battery. The above electrolyte may be manufactured in various forms, such as a coin cell in which an electrode current collector is housed in a pouch outer material and then the electrolyte is injected or the electrolyte is applied. Specifically, the lithium salt may include at least one selected from lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI).

[0098] The separator may serve to insulate the electrodes between the anode and cathode. The aramid nanofiber-based hybrid thin film of the present invention may be used as the separator. For a description of the aramid nanofiber-based hybrid thin film, reference may be made to the above-described content.

[0099] The aramid nanofiber-based hybrid thin film of the present invention, as a separator for the lithium secondary battery, can suppress the formation of lithium dendrites through the porous network structure, thereby improving the cycling stability of the battery. In addition, the aramid nanofiber-based hybrid thin film of the present invention can uniformly distribute lithium ions through inorganic particles, facilitate the movement of lithium ions, and physically block the growth of dendrites with its high mechanical properties, thereby improving the performance and lifespan stability of the battery.

[0100] Below, preferred experimental examples are presented to aid understanding of the present invention. However, the following experimental examples are provided solely to aid understanding of the present invention, and the present invention is not limited to the following experimental examples.

[0101]

[0102] Manufacturing Example 1: Aramid nanoseed suspension

[0103] 1.25 g of Kevlar 69 (Thread Exchange) fibers cut into 5 to 7 cm lengths and 1 g of potassium hydroxide (KOH, 90% purity, Sigma-Aldrich) were stirred in 100 mL of dimethyl sulfoxide (DMSO) (99.5% purity, Daejung Chemical) at 25°C for 2 weeks to obtain a viscous, dark red suspension of aramid nanoseeds.

[0104]

[0105] Manufacturing Example 2: Zirconia sulfate powder

[0106] A Zr precursor solution was formed by mixing 20 mL of zirconium n-propoxide (70 wt% in 1-propanol, Sigma-Aldrich) and 62.2 mL of anhydrous 1-propanol (99.5% purity, Sigma-Aldrich). Then, 1.10 mL of sulfuric acid (95% purity, Daejung Chemical) and 14.81 mL of deionized water were sequentially added to the Zr precursor solution at 50°C. The solution was then magnetically stirred at the same temperature for 1 hour. Next, 100 mL of ethanol (95% purity or higher, Daejung Chemical) was added, and the solution was centrifuged at 6000 rpm for 10 minutes to remove moisture to suppress particle growth during drying and calcination. The centrifugation washing was repeated three times. The obtained powder was dried at 80°C for 12 hours to remove residual ethanol, then calcined in a muffle furnace at 600°C for 3 hours and ball milled to synthesize zirconium sulfate oxide (SO4-ZrO2, hereinafter referred to as SZ) particles of about 300 to 400 nm.

[0107]

[0108] Manufacturing Examples 3-1 to 3-3: Aramid nanofiber-based hybrid thin film

[0109] The aramid nanoseed suspension of the above manufacturing example 1 was diluted to a concentration of 0.125 wt% by adding dimethyl sulfoxide (DMSO), a diluent.

[0110] The sulfated zirconia (SZ) powder of Preparation Example 2 was added to the diluted aramid nanoseed suspension as shown in Table 1, and then dispersed by ultrasonic treatment for 30 minutes to form a dispersion. To form nanofibers based on reprotonation, a polar protic solvent was added to the dispersion at a weight ratio of 9:1, and the first solvent exchange was performed. The dispersion with the added polar protic solvent was centrifuged twice at 10,000 rpm for 15 minutes, and then the solvent was exchanged with an organic solvent, N-methyl-2-pyrrolidone (NMP, ≥99.5% purity, Daejung Chemical), and redispersed. After removing the supernatant from the solvent-exchanged dispersion, an aramid nanofiber-sulfated zirconia (SZ / ANF) gel was obtained. 8.7 wt% poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP, pellets, Mw = ~ 400,000, average Mn = ~ 130,000, Sigma-Aldrich) in N-methyl-2-pyrrolidone (NMP) solution was added to the SZ / ANF gel at a weight ratio of 1:3. The slurry was formed by magnetic stirring for 1 day to obtain a homogeneous viscous solution. The homogenized slurry was blade-coated on a glass substrate using a doctor blade at 1 mm intervals, and the formed cast film was dried in a vacuum oven at 60 °C for 18 h to obtain an aramid nanofiber-based hybrid thin film (SZ / ANF / PVDF-HFP).

[0111] Classification Aramid nanoseed suspension Zirconia sulfate (SZ) powder Manufacturing example 3-1 (SZAP1) 40g 0.013g Manufacturing example 3-2 (SZAP2) 40g 0.04g Manufacturing example 3-3 (SZAP3) 40g 0.12g

[0112] Manufacturing Examples 4-1 to 4-3: Aramid nanofiber-based hybrid thin film

[0113] An aramid nanofiber-based hybrid thin film (LLZTO / ANF / PVDF-HFP) was obtained by performing the same procedure as in Manufacturing Examples 3-1 to 3-3, except that LLZTO powder instead of zirconia sulfate powder was used.

[0114] Commercially available LLZTO powder was used. Here, in the case of the above Manufacturing Example 4-1 (LLZTO 25 wt%) and Manufacturing Example 4-2 (LLZTO 50 wt%), blade coating with a width of 1 mm was performed, and in the case of Manufacturing Example 4-3 (LLZTO 75 wt%), blade coating with a width of 500 μm was performed.

[0115] The amount of LLZTO added to the diluted aramid nanoseed suspension is as shown in Table 2 below.

[0116] Classification Aramid nanoseed suspension LLZTO powder Manufacturing example 4-1 (LLZTO 25 wt%) 40 g 0.013 g Manufacturing example 4-2 (LLZTO 50 wt%) 40 g 0.04 g Manufacturing example 4-3 (LLZTO 75 wt%) 40 g 0.12 g

[0117] Comparative Example 1: Aramid nanofiber thin film (ANF)

[0118] An ANF thin film composed of aramid nanofibers was obtained by performing the same process as in Manufacturing Examples 3-1 to 3-3 above, except that zirconia sulfate powder and PVDF-HFP were added.

[0119] Comparative Example 2: Aramid nanofiber / PVDF-HFP thin film (ANF / PVDF-HFP)

[0120] An ANF / PVDF-HFP thin film composed of aramid nanofibers and PVDF-HFP was obtained by performing the same procedure as in Manufacturing Examples 3-1 to 3-3 except that zirconia sulfate powder was added.

[0121] Comparative Example 3: Sulfated zirconia / PVDF-HFP thin film (SZ / PVDF-HFP)

[0122] Except for the process of adding the aramid nanoseed suspension in Manufacturing Examples 3-1 to 3-3, the process was performed in the same manner to obtain an SZ / PVDF-HFP thin film composed of zirconia sulfate (SZ) and PVDF-HFP.

[0123] Comparative Example 4: PVDF-HFP thin film

[0124] A thin film was obtained using PVDF-HFP by performing the same process as in Manufacturing Examples 3-1 to 3-3 above, except that the aramid nanoseed suspension and zirconia sulfate powder were added.

[0125] Comparative Example 5: Commercial polypropylene (PP) thin film

[0126] For comparison, a commercially available polypropylene (PP) thin film, Celgard 2400, was prepared.

[0127]

[0128] Experimental Example 1: Characterization of a hybrid thin film based on aramid nanofibers.

[0129] FIG. 4(a) to FIG. 4(c) are images and graphs analyzing the characteristics of zirconia sulfate (SZ) particles of Manufacturing Example 2 of the present invention, and FIG. 4(d) is a graph showing the XRD spectra of the ANF / PVDF-HFP thin film of Comparative Example 2 of the present invention and the SZAP3 thin film of Manufacturing Example 3-3.

[0130] Fig. 4(a) is an image of zirconia sulfate (SZ) particles of Manufacturing Example 2 of the present invention observed using a field emission scanning electron microscope (FE-SEM, Apreo S Hivac, FEI). Through this, the microstructure of the zirconia sulfate (SZ) particles can be analyzed. Looking at the FE-SEM image of Fig. 4(a), it can be confirmed that the size of the zirconia sulfate (SZ) particles is 364±64 nm.

[0131] Fig. 4(b) is a graph showing the Fourier transform infrared (FT-IR) spectrum of zirconia sulfate (SZ) particles of Manufacturing Example 2 of the present invention. Through this, the chemical structure of zirconia sulfate (SZ) particles can be analyzed. The Fourier transform infrared (FT-IR) spectrum was obtained by Nicolet iS50, Thermo to identify characteristic peaks of the functional part. In Fig. 4(b), the FT-IR spectrum of SZ particles is 1213, 1120, 1060, and 992 cm -1 It showed a characteristic peak in the two-digit array. This is Zr 4+ It is believed to be due to the sulfate ion coordinated to .

[0132] Figure 4(c) is a graph showing the X-ray diffraction analysis (XRD) spectrum of zirconia (SZ) particles of Manufacturing Example 2 of the present invention. The XRD pattern is 0.8°s in the diffraction angle range 2θ = 5 to 75°. -1 The results were recorded with an X-ray diffractometer (D8 ADVANCE, BRUKER) equipped with Cu-Kα radiation at a scan rate of 40 kV and 20 mA. In Fig. 4(c), the main peaks for the tetragonal system were observed at 2θ = 30°, 50°, and 60° for the (111), (220), and (311) planes belonging to JCPDS card No. 17-0923, respectively, and it was confirmed that a small amount of monoclinic phase was included.

[0133] Referring to Fig. 4(d), in the XRD results of the SZAP3 sample of Manufacturing Example 3-3, monoclinic peaks of zirconia sulfate (SZ) particles appeared instead at 28° and 32° for the (-111) and (111) planes, respectively, belonging to JCPDS card number 37-1484. In addition, relatively broad peaks appeared at the aramid crystallinity of 18° and 20° for the (110) and (200) planes, respectively. In contrast, the ANF / PVDF-HFP thin film of Comparative Example 2 showed only aramid nanofiber (ANF) crystal peaks.

[0134] This indicates that the phase difference between the zirconia sulfate (SZ) particles before and after manufacturing the aramid nanofiber-based hybrid thin film of the present invention is due to the transformation of the metastable tetragonal phase into the relatively more stable monoclinic phase. This can be attributed to the removal of oxygen ion vacancies by the addition of excessive water during the manufacturing process.

[0135]

[0136] Figures 5(a) to 5(h) are images of thin films of Manufacturing Examples 3-1 to 3-3 of the present invention and thin films of Comparative Examples 1 to 3 observed using a field emission scanning electron microscope (FE-SEM). At this time, Figures 5(d) and 5(h) are plan views, and the rest are cross-sectional images.

[0137] Looking at the shape of the pure ANF thin film of Comparative Example 1 in Fig. 5(a), it can be confirmed that aramid nanofibers are laminated, and the thickness of the thin film was found to be 1 μm.

[0138] In Fig. 5(b), the thickness of the ANF / PVDF-HFP thin film of Comparative Example 2 is 13 μm, and it can be seen that it is similar to the SZAP thin films of Figs. 5(e) to 5(h) except that zirconia sulfate (SZ) particles are not embedded.

[0139] In the case of the SZ / PVDF-HFP thin film of Comparative Example 3 in Fig. 5(c) and Fig. 5(d), the cross-sectional and planar images show that the zirconia (SZ) particles are agglomerated to form an uneven film, resulting in a very rough thin film surface. The thickness of the thin film was approximately 18 μm.

[0140] In contrast, referring to FIGS. 5(e) to 5(h), the SZAP thin films containing zirconia (SZ) particles had a flat surface and a thickness of about 12 to 16 μm. The SZAP thin films of Manufacturing Examples 3-1 to 3-3 were made by interconnecting aramid nanofibers (ANF) and had a three-dimensional nanonetwork structure rich in nanoporosity, and it can be seen that zirconia (SZ) was well distributed between them. Specifically, this can be seen as a result of the formation of a three-dimensional nanonetwork structure in which the aramid nanofibers (ANF) played an important role in preparing a thin and homogeneous free-standing film, thereby providing distribution and bonding of the zirconia (SZ) particles.

[0141] In addition, looking at FIGS. 5(e) to 5(h), it can be seen that as the content of zirconia (SZ) in the SZAP thin film increases, more inorganic particles are uniformly included throughout the network structure. As described above, the aramid nanofiber-based hybrid thin film of the present invention, when applied as a separator in a secondary battery, is much thinner than a commercially available PP separator (thin film thickness: approximately 25 μm), so it can achieve a high energy density, and it is expected to be advantageous in reducing the overall internal resistance of the battery.

[0142]

[0143] Figures 6(a) and 6(b) are graphs showing thermogravimetric analysis (TGA) curves of zirconia sulfate (SZ) particles of Manufacturing Example 2 of the present invention, aramid nanofiber (ANF) thin film of Comparative Example 1, PVDF-HFP thin film of Comparative Example 4, and SZAP thin films of Manufacturing Examples 3-1 to 3-3, and a commercial PP film (celgard 2400). The thermogravimetric analysis (TGA) was measured by STA 449 F3, Netzsch, in a nitrogen (N2) atmosphere from 25°C to 900°C at a heating rate of 10°C / min.

[0144] In Fig. 6(a), the thermogravimetric analysis (TGA) results of the sulfated zirconia (SZ) sample of Manufacturing Example 2 showed a retention rate of approximately 85 wt% at 900°C, and the main decomposition occurred at approximately 650°C due to sulfur (S) decomposition. This indicates that the content of sulfate anions was approximately 10 wt%.

[0145] Referring to FIG. 6(b), it can be seen that the SZAP thin films of Manufacturing Examples 3-1 to 3-3 of the present invention contain thermally stable zirconia sulfate (SZ) particles, and thus have improved thermal stability in the high temperature range compared to the commercial separator of Celgard 2400, which is completely decomposed at 490°C. In addition, looking at FIG. 6(b), at the same temperature of 490°C, the SZAP1 thin films of Manufacturing Example 3-1, the SZAP2 thin films of Manufacturing Example 3-2, and the SZAP3 thin films of Manufacturing Example 3-3 showed retention rates of 45, 51, and 64 wt%, respectively. In addition, they showed weight retention rates of 34, 40, and 52 wt%, respectively, even at a higher temperature of 900°C.

[0146] Referring back to Fig. 6(a), the inclusion of aramid nanofibers (ANF) increased thermal stability, showing a retention rate of approximately 38 wt% at 900°C. Based on the weight retention rate, the content ratios of zirconia sulfate (SZ), aramid nanofibers (ANF), and PVDF-HFP components in the SZAP thin films of Manufacturing Examples 3-1 to 3-3 were calculated.

[0147] Table 3 shows the content ratios of SZ, ANF, and PVDF-HFP of the SZAP thin films calculated from the TGA results of Figs. 6(a) and 6(b). The volume ratio was calculated by converting the weight ratio to the density of the pure components of SZ, ANF, and PVDF-HFP.

[0148] Referring to Table 3, the contents of zirconia sulfate (SZ) in the thin films of SZAP1 of Manufacturing Example 3-1, SZAP2 of Manufacturing Example 3-2, and SZAP3 of Manufacturing Example 3-3 were 2, 14, and 37 wt%, respectively.

[0149] Sample sulfated zirconia (SZ) aramid nanofiber (ANF) interpenetrating polymer (PVDF-HFP) SZAP 12 wt%, 0.6 vol% 63 wt%, 68.6 vol% 35 wt%, 30.8 vol% SZAP 2 14 wt%, 4.2 vol% 55 wt%, 65.8 vol% 31 wt%, 30 vol% SZAP 3 37 wt%, 13.8 vol% 40 wt%, 58.9 vol% 23 wt%, 27.4 vol%

[0150] This graph shows the Fourier transform infrared (FT-IR) spectra of the thin films of Comparative Examples 1 to 4 and the aramid nanofiber-based hybrid thin films of Preparation Examples 3-1 to 3-3, and the major peak shifts of NH, C=O, and C-F2. Through this, the interactions of molecules within the thin films can be investigated. Referring to Fig. 7(a), the aramid nanofiber (ANF) thin film of Comparative Example 1 exhibited peak shifts at 3323, 1644, 1540, and 1504 cm2, which are attributed to NH, C=O, CN, and C-C groups, respectively. -1 It showed a characteristic peak in .

[0151] In Fig. 7(a), for the PVDF-HFP thin film of Comparative Example 4, a strong and broad asymmetric stretching vibration peak of CF2 is observed at 1178 cm -1 . The α-phase peaks were 1072, 976, and 614 cm -1 appeared in , and the β-phase peak was at 841 cm -1 In addition, the shaking and bending vibration peaks of CF2 were observed at 485 and 510 cm -1 appeared in .

[0152] In Fig. 7(a), in the case of the aramid nanofiber-based hybrid thin films (SZAP1, SZAP2, SZAP3) of Manufacturing Examples 3-1 to 3-3, the main characteristic peaks corresponding to ANF and PVDF-HFP were mostly observed, indicating that the hybrid thin films were successfully composited. On the other hand, in the FT-IR spectra of the SZAP thin films of Manufacturing Examples 3-1 to 3-3, it was difficult to observe the characteristic peak of zirconia sulfate (SZ) because the intensity of the characteristic peak of zirconia sulfate (SZ) was relatively low.

[0153] As shown in Figs. 7(b) to 7(d), the SZAP films showed major peak shifts in the NH and C=O stretching bands of the aramid nanofiber (ANF) component and the C-F2 stretching peak of the PVDF-HFP component. Referring to Figs. 7(b) and 7(c), after introducing PVDF-HFP to the ANF / PVDF-HFP and SZAP films, the NH and C=O stretching bands of the aramid nanofiber (ANF) were blue-shifted. In addition, after adding zirconia sulfate (SZ) particles to the SZAP films, the degree of blue-shift was slightly reduced.

[0154] In Fig. 7(d), the C-F2 stretching peak of the PVDF-HFP thin film of Comparative Example 4 red-shifted after adding aramid nanofibers (ANF) to PVDF-HFP, but the magnitude of the peak shift decreased again as the zirconia (SZ) content increased in the order of SZAP1, SZAP2, and SZAP3. This can be seen as an indication of the possibility of hydrogen bonding and dipole interactions between the amide groups of the aramid nanofibers (ANF) and the difluoromethylene groups of PVDF-HFP. These interactions can be partially disrupted by the addition of zirconia (SZ) particles rich in highly polar sulfate groups.

[0155]

[0156] Manufacturing Example 5: Secondary battery including aramid nanofiber-based hybrid thin film (SZ / ANF / PVDF-HFP)

[0157] The secondary battery was assembled using a CR2032 coin cell as a lithium secondary battery inside a glove box (MBRAUN) in an argon (Ar) atmosphere with both moisture and oxygen contents of less than 0.5 ppm.

[0158] In the symmetric cell test, lithium metal foils with a thickness of 300 μm were used as the negative and positive electrodes.

[0159] In the full cell test, the negative electrode used a lithium metal foil with a thickness of 300 μm, and the positive electrode was prepared using a slurry composed of LiFePO4 (LFP), Super P carbon, and polyvinylidene fluoride (PVDF) in a weight ratio of 8:1:1 with NMP. The slurry was then cast onto aluminum (Al) foil using a doctor blade and dried overnight in a vacuum oven at 120°C. The loading per area of ​​the active material was 5.1 to 6.9 mg cm -2 It was.

[0160] The liquid electrolyte used was 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a volume of DOL:DME=1:1 containing 1 wt% LiNO3.

[0161] As a separator, an aramid nanofiber-based hybrid thin film (SZAP3) of Manufacturing Example 3-3 was used.

[0162]

[0163] Manufacturing Examples 6-1 to 6-3: Secondary battery including aramid nanofiber-based hybrid thin film (LLZTO / ANF / PVDF-HFP)

[0164] The same procedure was followed as in Manufacturing Example 5 above, except that the LLZTO / ANF / PVDF-HFP thin films of Manufacturing Examples 4-1 to 4-3 were used as the separation membranes.

[0165] Comparative Example 6: Secondary battery containing aramid nanofiber / PVDF-HFP

[0166] The same method as Manufacturing Example 5 was used except that the aramid nanofiber / PVDF-HFP thin film of Comparative Example 2 was used as a separator in Manufacturing Example 5.

[0167] Comparative Example 7: Secondary battery containing commercial polypropylene (PP)

[0168] The same method as Manufacturing Example 5 was used except that celgard 2400, a commercial polypropylene (PP) film of Comparative Example 5, was used as a separator in Manufacturing Example 5.

[0169] Comparative Example 8: Secondary battery containing PVDF-HFP

[0170] Manufacturing Example 5 was performed in the same manner as Manufacturing Example 5, except that the PVDF-HFP thin film of Comparative Example 4 was used as a separator.

[0171]

[0172] Experimental Example 2: Analysis of Mechanical and Electrochemical Properties of Hybrid Thin Films Based on Aramid Nanofibers

[0173] FIG. 8(a) to FIG. 8(c) are graphs showing the results of uniaxial tensile tests of thin films of Comparative Examples 2 and 3 and Manufacturing Examples 3-1 to 3-3 of the present invention, as well as the electrochemical impedance spectroscopy (EIS) spectrum of a lithium / lithium symmetric cell and the initial part of the measured EIS.

[0174] As shown in Fig. 8 and Table 4 below, the ionic conduction behavior of the sample was investigated using electrochemical impedance spectroscopy (EIS). The sample was tested in a cell configuration of lithium / separator / lithium symmetric cell at a constant temperature of 30°C with an ether-based electrolyte (1.0 M LiTFSI dissolved in a 1:1 volume ratio of DOL:DME containing 1 wt% LiNO3) as in Preparation Example 5.

[0175] Specifically, the uniaxial tensile test was performed using a DMA 850 from TA Instruments. The Nyquist plot shows the bulk resistance (R) of the membrane. b ) and interface resistance (R int ) was fitted by the equivalent circuit model shown in the inset of Fig. 8b to obtain R b To see in detail, an enlarged EIS spectrum in the high frequency region is shown in Fig. 8(c).

[0176] Table 4 below shows the mechanical and electrochemical properties of the thin films of Comparative Examples 2 and 3 and Manufacturing Examples 3-1 to 3-3, as well as the thicknesses of the thin films.

[0177] Sample elastic modulus [GPa] Tensile strength [MPa] R b [Ω]R int [Ω]σ[mS cm -1 ]Thickness [㎛]Comparative Example 20.69±0.0634.5±1.511.60±1.08159.0±23.50.11±0.0113Manufacturing Example 3-10.50±0.0221.5±1.28.82±0.52213.5±13.40.15±0.0112Manufacturing Example 3-20.657±0.00326.5±0.46.36±0.30202.8±13.60.20±0.0112Manufacturing Example 3-31.75±0.0757.2±2.26.62±0.56130.3±17.30.25±0.0216Comparative Example 30.26±0.025.7±0.7___18

[0178] Referring to Fig. 8(a) and Table 4, the SZ / PVDF-HFP thin film of Comparative Example 3 exhibited an elastic modulus of 0.26±0.02 GPa and a tensile strength of 5.7±0.7 MPa. These values ​​were much lower than expected, which may be caused by inefficient stress transfer from one phase to another due to weak interfacial interactions. As the content of zirconia sulfate (SZ) particles increased in the SZAP thin films of Preparation Examples 3-1 to 3-3, the elastic modulus of the thin films gradually increased to 0.50±0.02 GPa, 0.657±0.003 GPa, and 1.75±0.07 GPa, and the mechanical strength also increased with a similar trend.

[0179] In comparison, the ANF / PVDF-HFP thin film of Comparative Example 2 exhibited a moderately high elastic modulus value of 0.69±0.06 GPa due to the inclusion of mechanically robust aramid nanofibers (ANF), which is still lower than the value of SZAP3 of Manufacturing Example 3-3, which had the highest zirconia sulfate (SZ) content.

[0180] Referring to Fig. 8(c), as the SZ content of the SZAP membrane increases, the bulk resistance (R b ) and interface resistance (R int ) all gradually decreased. This is because the SZ particles can act as a solid acid that strongly binds to the TFSI anion and promotes the dissociation of the lithium salt.

[0181] As shown in Table 4, Manufacturing Example 3-3, which shows the best ionic conductivity, is R b 6.62±0.56Ω, R int The ionic conductivity is 130.3±17.3Ω and 0.25±0.02 mScm. -1 It was found that. In comparison, Comparative Example 2 is R b 11.60±1.08 Ω, R int The conductivity is 159.0±23.5Ω and the ionic conductivity is 0.11±0.01 mScm. -1, which may be due to the fact that it was filled with a relatively dense ANF network structure without a specific acid group, which is a worse result than Manufacturing Example 3-3.

[0182]

[0183] FIGS. 9(a) to 9(d) are field emission scanning electron microscope (FE-SEM) images of the surface of the lithium metal anode of LFP / lithium full cells, each of which applied the thin films of Manufacturing Example 5, Comparative Example 6, and Comparative Example 7 of the present invention as separators, after 200 cycles. The LFP / lithium full cell tests were performed using a battery cycler (WBCS3000, WonAtech) within a voltage range of 2.5 to 4.0 V at a C-rate of 0.5 C. All tests were performed at a constant temperature of 30°C, and for SEM imaging, each cell was disassembled and the lithium metal anode was washed three times with a DOL / DME (1:1) solvent. All tests were performed at a constant temperature of 30°C.

[0184] Referring to Figs. 9(a) and 9(b), each cell including the Celgard 2400 thin film of Comparative Example 7 and the ANF / PVDF-HFP thin film of Comparative Example 6 exhibited severe dendrite growth. In contrast, looking at Figs. 9(c) and 9(d), it can be seen that the lithium metal anode surface of the cell including the SZAP3 thin film of Preparation Example 5 was electrodeposited with lithium in a micro-sized, relatively round shape without dendrites. It is believed that lithium electrodeposits with this specific shape can improve cycling performance by reducing SEI formation, liquid electrolyte consumption, and dead lithium formation because they have large grain boundaries and a relatively small surface area.

[0185] Figure 10 is a schematic diagram of the electrochemical deposition behavior of the lithium metal negative electrode of the battery of Comparative Example 7 of the present invention (left) and the battery of Manufacturing Example 5 (right).

[0186] Referring to Fig. 10, the SZAP separator of the battery of Manufacturing Example 5 can suppress lithium dendrites due to synergistic effects such as physically blocking lithium dendrite growth due to high mechanical properties, enhanced ion transport by introducing SZ, and uniform ion distribution through the ANF three-dimensional nano-network structure. On the other hand, in the PP separator of the battery of Comparative Example 7, the lithium ions can be seen to be localized in the pores because the large ion insulating framework and free anions released by the lithium salt gather in the channels and hinder lithium ion transport. Therefore, it can be seen that sharp lithium dendrites are formed due to the uneven distribution of lithium ions.

[0187] Figure 11 shows the 1 mA·cm of the battery (celgard 2400) of comparative example 7 of the present invention and the battery (SZAP3) of manufacturing example 5. -2 , 1 mAh·cm -2 This graph shows the voltage profile of a lithium / lithium symmetric cell tested under conditions. The voltage versus cycle plot is 1 mA·cm -2 (1mAh·cm -2 ) was obtained under the conditions of

[0188] Referring to Fig. 11, the SZAP3 separator of the battery of Manufacturing Example 5 operated stably, exhibiting an overvoltage of less than 20 mV for 800 cycles, except for the early part of the cycle. In contrast, the Celgard 2400 separator of the battery of Comparative Example 7 exhibited a much higher overvoltage than SZAP3, showed a sharp increase, and a short circuit occurred before 300 cycles. This is due to the large consumption of electrolyte and the accumulation of lithium dendrites and dead lithium, which is a general result when using existing separators. As described above, it can be seen that the aramid nanofiber-based hybrid thin film of the present invention exhibits superior properties compared to existing separators when applied as a separator of a secondary battery.

[0189] Figures 12(a) and 12(b) are graphs showing typical charge / discharge curves and cycle performance and coulombic efficiency at 0.5C of the LFP / lithium full cells of Manufacturing Example 5 (SZAP3) of the present invention and Comparative Examples 6 to 8. The full cell cycle performance was measured at 0.5C in a LiFePO4 (LFP) / lithium cell (1.0 M LiTFSI electrolyte dissolved in a volume of DOL:DME=1:1 containing 1 wt% LiNO3, 5.1 to 6.9 mg cm -2 (negative mass loading) was tested.

[0190] Referring to Fig. 12(a), the charge / discharge curves of the first cycle, 50 cycles, 150 cycles, and 200 cycles of the battery of Preparation Example 5 (SZAP3) showed almost no change with a slight decrease in capacity. In particular, as shown in Fig. 12(b), the SZAP3 sample showed a relatively high discharge capacity retention rate of 96.8% and a coulombic efficiency of 99.8% after 300 cycles at 0.5 C. This can be seen as indicating the stable operation of the SZAP sample without a significant decrease in cell performance. As described above, the aramid nanofiber-based hybrid thin film of the present invention is expected to be promising in the development of a highly stable hybrid separator for lithium metal batteries (LMB) due to its enhanced safety.

[0191] Figure 13 is a scanning electron microscope (SEM) image of the thin films of Manufacturing Examples 4-1 to 4-3 of the present invention.

[0192] Referring to Figure 13, although some areas where inorganic particles are clumped together are observed, it can be confirmed that the inorganic material is uniformly distributed throughout. Furthermore, as the content of the inorganic particle LLZTO increases, the three-dimensional aramid nanofiber network becomes densely filled with a mixture of inorganic material and polymer, leaving no empty spaces between the particles.

[0193] Figure 14 is a graph showing the electrochemical impedance spectroscopy (EIS) spectra of the lithium / lithium symmetric cells of Manufacturing Example 6-1 (LLZTO 25 wt%) and Manufacturing Example 6-2 (LLZTO 50 wt%) of the present invention. Table 5 below is a chart showing the electrochemical characteristics and thickness of Manufacturing Example 6-1 (LLZTO 25 wt%) and Manufacturing Example 6-2 (LLZTO 50 wt%) of the present invention.

[0194] Sample R b [Ω]R int [Ω]σ[mS cm -1 ]Thickness [㎛] Manufacturing Example 6-1 (LLZTO 25 wt%) 9.97 ± 0.80 18 1.28 ± 12.2 6 0.12 ± 0.01 11 Manufacturing Example 6-2 (LLZTO 50 wt%) 8.85 ± 1.05 16 2.75 ± 3.8 10.16 ± 0.02 13

[0195] Referring to FIG. 14 and Table 5, when comparing LLZTO 25 wt% and 50 wt%, as the content of the inorganic material increases, both the bulk resistance and the interface resistance decrease, and the ionic conductivity shows a tendency to slightly increase. FIG. 15 is a graph showing the voltage profile of the lithium / lithium symmetric cell of Manufacturing Example 6-1 (LLZTO 25 wt%) and Manufacturing Example 6-2 (LLZTO 50 wt%) of the present invention.

[0196] Referring to Fig. 15, both LLZTO 25 wt% and LLZTO 50 wt% in a lithium / lithium symmetric cell appear to operate stably for more than 500 cycles. Among them, the LLZTO 50 wt% sample with a high LLZTO content was confirmed to operate relatively more stably with a low overvoltage and no popping.

[0197] As described above, the aramid fiber-based hybrid thin film of the present invention has a thin and uniform free-standing shape and is mechanically robust, with the tensile modulus and strength increasing to a maximum of 1.8 GPa and 59 MPa, respectively, for SZAP3. In addition, the ionic conductivity of the SZAP1 to SZAP3 thin films is 0.14 to 0.25 mS cm. -1The range was suitable for batteries, and the lithium / lithium symmetric cell using SZAP3 showed excellent cycling stability for 800 cycles.

[0198] Furthermore, at 0.5C, the LFP / lithium full cell operated for over 300 cycles with excellent capacity retention (96.8%) and Coulombic efficiency (99.8%). Furthermore, it was confirmed that even when equipped with inorganic particles such as LLZTO, it exhibited ionic conductivity and symmetric cell cycling behavior similar to that of zirconia.

[0199] Accordingly, it is expected that the aramid nanofiber-based hybrid thin film of the present invention can be actively utilized as a separator for lithium metal batteries (LMB) without lithium dendrites.

[0200] Above, the present invention has been described in detail with reference to preferred embodiments, but the present invention is not limited to the above embodiments, and various modifications and changes can be made by those skilled in the art within the technical spirit and scope of the present invention.

Claims

1. A step of forming a dispersion by dispersing inorganic particles in an aramid nanoseed suspension in which aramid fibers are dissolved in a strong base; A step of forming the aramid nanoseeds into aramid nanofibers by adding a polar proton solvent to the above dispersion; A step of forming an aramid nanofiber-inorganic particle composite organic gel by adding an organic solvent to a dispersion in which the above aramid nanofibers are formed; A step of forming a slurry by adding a polymer solution to the above composite organic gel; and A method for manufacturing an aramid nanofiber-based hybrid thin film, comprising the step of applying the above slurry onto a substrate and then drying it.

2. In paragraph 1, A method for manufacturing an aramid nanofiber-based hybrid thin film, wherein the above-mentioned inorganic particles are dispersed on the surface and inside of the thin film.

3. In paragraph 1, A method for manufacturing a hybrid thin film based on aramid nanofibers, wherein the above-mentioned inorganic particles have at least a Lewis acid site on the surface.

4. In paragraph 3, A method for manufacturing a hybrid thin film based on aramid nanofibers, wherein the above inorganic particles include sulfated zirconia particles.

5. In paragraph 1, A method for manufacturing a hybrid thin film based on aramid nanofibers, wherein the above inorganic particles are inorganic solid electrolyte particles.

6. In paragraph 5, A method for manufacturing a hybrid thin film based on aramid nanofibers, wherein the above-mentioned inorganic solid electrolyte particles are lithium lanthanum tantalum zirconate (LLZTO) doped with tantalum (Ta).

7. In paragraph 1, A method for manufacturing a hybrid thin film based on aramid nanofibers, wherein the organic solvent dissolves the polymer.

8. In paragraph 7, A method for manufacturing an aramid nanofiber-based hybrid thin film, wherein the organic solvent comprises at least one selected from N-methyl-2-pyrrolidone (NMP), dimethylformamide (N,N-dimethylformamide (DMF), acetone, tetrahydrofuran (THF), methylene chloride, chloroform, and cyclohexane.

9. In paragraph 1, A method for manufacturing an aramid nanofiber-based hybrid thin film, wherein the polymer comprises at least one fluorinated polymer selected from polytetrafluoroethylene (PTFE), poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP, and polyvinylidene fluoride (PVDF).

10. In paragraph 9, A method for manufacturing an aramid nanofiber-based hybrid thin film, wherein hydrogen bonds and dipole interactions are formed between the amide groups of the aramid nanofibers and the difluoromethylene groups of the fluoropolymer.

11. In paragraph 1, A method for producing an aramid nanofiber-based hybrid thin film, wherein 3 to 93 parts by weight of the inorganic particles and 55 to 58 parts by weight of the polymer are mixed with respect to 100 parts by weight of the aramid nanofibers.

12. A thin film manufactured by the method for manufacturing a hybrid thin film based on aramid nanofibers of paragraph 1.

13. Aramid nanofibers; An interpenetrating polymer network having a porous network structure and including a polymer penetrated between the aramid nanofibers; and Aramid nanofiber-based hybrid thin film comprising inorganic particles dispersed within the above interpenetrating polymer network.

14. In paragraph 13, A hybrid thin film based on aramid nanofibers, wherein the above-mentioned inorganic particles are dispersed on the surface and inside of the thin film.

15. In paragraph 13, The above-mentioned inorganic particles are a hybrid thin film based on aramid nanofibers having Lewis acid sites on the surface.

16. In paragraph 15, The above inorganic particles are a hybrid thin film based on aramid nanofibers, comprising sulfated zirconia particles.

17. In paragraph 13, The above inorganic particles are inorganic solid electrolyte particles, and are a hybrid thin film based on aramid nanofibers.

18. In paragraph 17, A method for manufacturing a hybrid thin film based on aramid nanofibers, wherein the above-mentioned inorganic solid electrolyte particles are lithium lanthanum tantalum zirconate (LLZTO) doped with tantalum (Ta).

19. In paragraph 13, An aramid nanofiber-based hybrid thin film comprising at least one fluorinated polymer selected from the group consisting of polytetrafluoroethylene (PTFE), poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP, and polyvinylidene fluoride (PVDF).

20. Bipolar; cathode; A separator disposed between the anode and cathode; and An electrolyte solution containing a lithium salt and a solvent impregnated in the above separator; A lithium secondary battery comprising an aramid nanofiber-based hybrid thin film comprising aramid nanofibers as the separator, an interpenetrating polymer network having a porous network structure, a polymer penetrating between the aramid nanofibers, and inorganic particles dispersed within the interpenetrating polymer network.

21. In paragraph 20, A lithium secondary battery, wherein the above negative electrode is lithium metal.

22. In paragraph 20, A lithium secondary battery, wherein the lithium salt comprises at least one selected from lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethane)sulfonimide (LiN(SO2CF3)2).

23. In paragraph 20, The above battery is a lithium secondary battery, a lithium metal battery.

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