High-performance lithium metal electrode based on piezoelectric nanocomposite protective layer, and manufacturing method therefor

Abstract

The present invention relates to a high-performance lithium metal electrode based on a piezoelectric nanocomposite protective layer, and a manufacturing method therefor. According to the present invention, when a piezoelectric nanocomposite is prepared, a high-temperature process at 1,000°C or higher as in a conventional technique, is not required, synthesis is possible at low cost under a room temperature condition, and nanofillers can be dispersed in the piezoelectric nanocomposite by a method such as spin coating. In addition, the lithium metal electrode having a protective layer including a piezoelectric nanocomposite, of the present invention, enhances lithium metal electrode-based lithium secondary battery electrochemical performance such as excellent lithium-ion mobility, low electrode resistance and lithium dendrite growth inhibition. Furthermore, the piezoelectric nanocomposite can be applied to various technical fields such as energy storage devices, sensors and medical devices.

Description

High-performance lithium metal electrode based on a piezoelectric nanocomposite protective layer and method for manufacturing the same

[0001] The present invention relates to a high-performance lithium metal electrode based on a piezoelectric nanocomposite protective layer and a method for manufacturing the same.

[0002] Energy harvesting technology, which converts external energy sources into electrical energy, has recently garnered attention as an eco-friendly technology. One such technology utilizes the piezoelectric properties of ferroelectric materials to convert mechanical energy from external vibrations into electrical energy, a task currently being pursued by various research groups. Meanwhile, adding ceramic nanofillers to a polymer matrix increases the electric double layer dipole moment, thereby enhancing energy storage performance. Reducing the distance between nanoparticles also increases interactions, which in turn increases the electric displacement field. Based on these principles, research is underway to introduce one-dimensional and two-dimensional structures into nanofillers to enhance connectivity and strengthen piezoelectric and dielectric properties.

[0003] Meanwhile, when synthesizing piezoelectric particles in wire, tube, or rod forms, composites are generally fabricated by aligning piezoelectric elements after compressing or molding piezoelectric materials, or by partially processing sintered piezoelectric materials and filling them with polymers. However, these methods are not economically viable due to high costs and low yields. On the other hand, while conventional bulk piezoelectric composites may exhibit slightly lower performance compared to composites with other connectivity types due to lower stress concentration, they are widely used in practical applications because of their manufacturing advantages.

[0004] Meanwhile, interest in electric vehicles (EVs), which can replace fossil fuel-based vehicles—one of the major causes of air pollution—is continuously increasing, and the development of lithium-ion batteries, which are primarily used as a power source for EVs due to their high discharge voltage and output stability, has recently been actively underway.

[0005] In this regard, lithium metal, which has a relatively high capacity (3,860 mAh / g) and a low redox potential (-3.04 V vs. SHE), has recently attracted attention as one of the promising anode materials, and research is also being conducted on lithium-ion batteries or lithium-metal batteries containing such lithium metal anodes (LMA).

[0006] However, when lithium metal is used as the anode, the high reactivity of lithium metal causes continuous side reactions with the electrolyte, forming an unstable film and resulting in poor lifespan characteristics. In particular, lithium dendrites, which are formed during battery charging and discharging, not only significantly reduce the battery's lifespan efficiency but can also cause sudden internal short circuits, potentially leading to fire and explosion; therefore, technology to effectively control them is required.

[0007] Accordingly, the inventors have developed a technology to enhance the electrochemical performance of lithium metal electrode-based lithium secondary batteries by utilizing nanofillers with zero to three-dimensional structures, combining engineering of polymer entanglement and polymer molecular weight to induce selective connectivity between nanofillers, and developing a cluster-structured percolation network piezoelectric nanocomposite having a percolation network structure in which nanofillers are clustered in a polymer matrix, which can be synthesized at room temperature at low cost, and applying it as a protective film for a lithium metal electrode.

[0008] The present invention, conceived under the above technical background, aims to provide a high-performance lithium metal electrode based on a piezoelectric nanocomposite protective layer and a method for manufacturing the same, which enhances the electrochemical performance of a lithium secondary battery based on a lithium metal electrode, such as high lithium ion mobility, low electrode resistance, and inhibition of lithium dendrite growth, by dispersing nanofillers with a zero- to three-dimensional structure within a polymer matrix while controlling polymer entanglement, polymer molecular weight, and the type and content of the nanofillers used to form a clustered (optionally connected) percolation network structure among the nanofillers and applying it to a lithium metal electrode.

[0009] The technical problems that the present invention aims to solve are not limited to those mentioned above, and other unmentioned technical problems will be clearly understood by those skilled in the art to which the present invention belongs from the description below.

[0010] The present specification provides a piezoelectric nanocomposite comprising: a polymer matrix; and piezoelectric ceramic nanofillers dispersed within the polymer matrix.

[0011] For example, the polymer matrix may include one type of piezoelectric resin selected from polyvinylidene fluoride (PVDF), polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polytrifluoroethylene (PTFE), polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene (P(VDF-TrFE-CTFE)), polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene (P(VDF-TrFE-CFE)), poly-L-lactic acid (PLLA), polyvinylidene fluoride-tetrafluoroethylene (P(VDF-TeFE)), triglycine sulfate (TGS), and combinations thereof.

[0012] For example, the polymer matrix comprises polyvinylidene fluoride (PVDF), and the polyvinylidene fluoride may contain a β-phase structure in the range of 0.01 to 95.0%.

[0013] For example, the weight-average molecular weight (Mw) of the polymer constituting the polymer matrix may range from 10,000 to 2,000,000.

[0014] For example, the piezoelectric ceramic nanofiller is a nanoparticle with a zero to three-dimensional structure and may have a diameter in the range of 1 to 900 nm.

[0015] For example, the piezoelectric ceramic nanofillers are barium titanate (BaTiO3, BTO), strontium titanate (SrTiO3), lead zirconate-titanate (Pb(Zr,Ti)O3, PZT), bismuth sodium titanate (Bi 0.5 Na 0.5 TiO3, BNT), bismuth ferrite (BiFeO3), calcium titanate (CaTiO3), lead magnesium niobate-lead titanate (Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3, PMN-PT), lithium lead niobate (PbNb2O6), berlinite (α-AlPO4), quartz (α-SiO2), lithium phosphate (LiTaO3), lithium niobium oxide (LiNbO3), strontium barium niobium oxide (Sr x Ba γ Nb2O8), lead germanate (Pb5Ge3O 11 ), terbium molybdenum (Tb2(MoO4)3), lithium borate (Li2B4O7), cadmium sulfide (CdS), zinc oxide (ZnO), bismuth oxide (Bi 12 SiO 20 ), bismuth-germanium oxide (Bi 12 GeO 20It may be one selected from lead titanate (PT-based, PbTiO3), PZT-complex perovskite, aluminum nitride (AlN), and combinations thereof.

[0016] For example, the piezoelectric ceramic nanofiller may be included in a range of 0.01 to 50.0 wt% based on the total weight of the piezoelectric nanocomposite.

[0017] In addition, the present specification provides a lithium metal electrode for a secondary battery comprising: a current collector; and a protective layer comprising the piezoelectric nanocomposite coated on the current collector.

[0018] For example, the above-mentioned current collector may be copper (Cu).

[0019] For example, the protective layer may have a thickness in the range of 0.01 to 100 μm.

[0020] For example, the lithium metal electrode may be a lithium metal negative electrode.

[0021] For example, the lithium metal negative electrode has a capacity of 3 mAh / cm² 2 Under current density conditions, dendrite formation is prevented, and a uniform and dense lithium electrodeposition with a thickness of 14.9 μm or less can be achieved between the current collector and the protective layer.

[0022] In addition, the present specification provides a method for manufacturing a lithium metal electrode for a secondary battery, comprising: a) dispersing piezoelectric ceramic nanofillers having a zero- to three-dimensional structure within a polymer matrix to form a piezoelectric nanocomposite having a clustered percolation network structure; and b) coating a composition comprising the piezoelectric nanocomposite formed in step a onto a current collector to form a protective layer.

[0023] According to the present invention, when manufacturing a piezoelectric nanocomposite, a high-temperature process of 1,000 ℃ or higher according to conventional technology is not required, synthesis can be performed at low cost under room temperature conditions, and nanofillers can be dispersed within the piezoelectric nanocomposite by methods such as spin coating.

[0024] In addition, the lithium metal electrode equipped with a protective layer containing the piezoelectric nanocomposite of the present invention enhances the electrochemical performance of lithium secondary batteries based on lithium metal electrodes, such as excellent lithium ion mobility, low electrode resistance, and inhibition of lithium dendrite growth. Furthermore, the piezoelectric nanocomposite can be applied to various technological fields, such as energy storage devices, sensors, and medical devices.

[0025] FIG. 1 schematically illustrates (a) the manufacturing process of a piezoelectric nanocomposite and (b) the optimization of the protective layer structure by controlling polymer entanglement and polymer molecular weight (Mw) according to an embodiment of the present invention.

[0026] FIG. 2 shows SEM images of (a) to (f) piezoelectric nanocomposites according to an embodiment and a comparative example of the present invention, (g) cross-sectional SEM image of BTO / PVDF(MM), and EDS mapping corresponding to each element.

[0027] FIG. 3 shows the XRD patterns of (a) to (b) piezoelectric nanocomposites and (c) FT-IR spectra according to an embodiment and a comparative example of the present invention.

[0028] Figure 4 shows peak current values ​​measured by cyclic voltammetry (CV) at various scan speeds using a piezoelectric nanocomposite according to an embodiment and a comparative example of the present invention.

[0029] Figure 5 is the result of performing PFM measurements for piezoelectricity measurement on BTO / PVDF(MM) according to an embodiment of the present invention.

[0030] Figure 6 shows Nyquist plots by EIS when the polymer molecular weight of the polymer matrix and the content of the piezoelectric ceramic nanofiller are different in an embodiment and a comparative example of the present invention.

[0031] Figure 7 shows the results of measuring the electrochemical performance of a lithium metal anode in which a piezoelectric nanocomposite of an embodiment of the invention and a comparative example is applied as a protective layer.

[0032] Figure 8 shows SEM images of a lithium metal anode with a piezoelectric nanocomposite applied as a protective layer according to an embodiment of the invention and a comparative example, a cross-sectional SEM image and a comparison of lithium thickness, a comparison of dendrite formation, and EDX mapping results of specific elements.

[0033] Figure 9 shows the results of measuring the electrochemical performance of an LFP cathode in a complete cell in which a lithium metal cathode with a piezoelectric nanocomposite applied as a protective layer is applied to an embodiment of the invention and a comparative example.

[0034] FIG. 10 schematically illustrates the operating mechanism of a protective layer for a lithium metal anode to which a piezoelectric nanocomposite of an embodiment and a comparative example of the invention is applied as a protective layer.

[0035] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that in assigning reference numerals to the components of each drawing, the same components are given the same reference numeral whenever possible, even if they are shown in different drawings. Furthermore, in describing the embodiments of the present invention, if it is determined that a detailed description of related known components or functions would hinder understanding of the embodiments of the present invention, such detailed description is omitted.

[0036]

[0037] The high-performance lithium metal electrode based on a piezoelectric nanocomposite protective layer with a clustered percolation network structure and the method for manufacturing the same will be described in more detail below.

[0038]

[0039] Method for manufacturing a lithium metal electrode for a secondary battery

[0040] A method for manufacturing a lithium metal electrode for a secondary battery according to one embodiment of the present invention comprises: a) a step of forming a piezoelectric nanocomposite having a clustered percolation network structure by dispersing piezoelectric ceramic nanofillers having a zero- to three-dimensional structure within a polymer matrix; and b) a step of forming a protective layer by coating a composition comprising the piezoelectric nanocomposite of step a onto a current collector.

[0041]

[0042] First, piezoelectric ceramic nanofillers with a zero to three-dimensional structure are dispersed within a polymer matrix to form a piezoelectric nanocomposite with a clustered percolation network structure (step a).

[0043] The piezoelectric nanocomposite of the present invention has a structure in which piezoelectric ceramic nanofillers are dispersed in a clustered percolation network shape within a polymer matrix.

[0044] The polymer matrix serves as a substrate for the piezoelectric nanocomposite and may include, for example, a piezoelectric resin such as a fluorine-based one that has excellent chemical resistance and good mechanical, thermal, and electrical properties. The above piezoelectric resin may be one selected from polyvinylidene fluoride (PVDF), polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polytrifluoroethylene (PTFE), polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene (P(VDF-TrFE-CTFE)), polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene (P(VDF-TrFE-CFE)), poly-L-lactic acid (PLLA), polyvinylidene fluoride-tetrafluoroethylene (P(VDF-TeFE)), triglycine sulfate (TGS), and combinations thereof, and specifically may be polyvinylidene fluoride (PVDF). When the above polymer matrix is ​​composed of polyvinylidene fluoride (PVDF), it is easy for carbon (C) and fluorine (F) atoms to form a β-phase structure having a Gaussian structure.

[0045] Meanwhile, when the polymer matrix according to the present invention comprises polyvinylidene fluoride (PVDF) and the piezoelectric ceramic nanofillers described below have a clustered (optional connected) structure, the β-phase structure of the polymer (polyvinylidene fluoride) is enhanced. In the polyvinylidene fluoride constituting the polymer matrix, the β-phase structure may be included in a range of 0.01 to 95.0%, specifically in a range of 10 to 95%, and more specifically in a range of 25 to 95%. Meanwhile, within the above range, the piezoelectric properties are improved due to the β-phase structure in which carbon (C) and fluorine (F) atoms have a Gaussian structure, thereby providing the advantage of securing excellent electrical performance.

[0046]

[0047] Meanwhile, the polymer constituting the polymer matrix according to one embodiment of the present invention may have a weight-average molecular weight (Mw) in the range of 10,000 to 2,000,000, more specifically in the range of 350,000 to 650,000, and more specifically in the range of 450,000 to 600,000. Specifically, when forming the piezoelectric nanocomposite in step a, the piezoelectric resin constituting the polymer matrix may have a weight-average molecular weight (Mw) and a polymer entanglement structure in different ranges depending on the type of polymer used. For example, when polyvinylidene fluoride (PVDF) is used as the piezoelectric resin, a polymer having a weight-average molecular weight in the range of 450,000 to 600,000 may be used. Meanwhile, if necessary, a non-piezoelectric resin may be further included, and the non-piezoelectric resin may be used, for example, polyacrylic acid (PAA) having a weight-average molecular weight in the range of 10,000 to 2,000,000, specifically in the range of 300,000 to 1,400,000.

[0048] Meanwhile, the structure of the piezoelectric nanocomposite can be controlled by the polymer entanglement structure and the polymer molecular weight. For example, if the polymer has a weight-average molecular weight below the above range, it has a low radius of gyration (Rg), low melt viscosity, and low entanglement density. Consequently, the piezoelectric ceramic nanofillers move freely within the polymer matrix, which makes it difficult to expect a dipole enhancement effect.

[0049] Conversely, if the weight-average molecular weight exceeds the above range, the movement of piezoelectric ceramic nanofiller particles is extremely restricted due to high entanglement density and high melt viscosity, and they form aggregates with one another, which can ultimately increase the movement path of lithium ions and cause a problem of increasing electrode resistance (see Fig. 1).

[0050] Therefore, when the weight-average molecular weight of the polymer matrix is ​​controlled within a specific range, it has an appropriate radius of gyration (Rg), melt viscosity, and entanglement density, so the piezoelectric ceramic nanofillers described below form a clustered (optionally connected) percolation network structure, which can enhance the dipole effect of the composite due to the strong electric field of the piezoelectric ceramic nanofillers. Meanwhile, the term "clustered (optionally connected) percolation network structure" in the claims and throughout the specification refers to a form in which nanofillers are physically in contact or in close proximity without clumping together and are selectively connected and arranged, and should be understood as a structure capable of forming a network shape through this.

[0051]

[0052] In step a above, piezoelectric ceramic nanofillers are dispersed within the polymer matrix. The piezoelectric ceramic nanofillers of the present invention are added to the polymer matrix to increase the electric double layer dipole moment and improve the energy storage performance of the composite; when the distance between nanofillers is reduced, the interaction increases, thereby achieving the effect of increasing the electric displacement field.

[0053] Specifically, the piezoelectric ceramic nanofiller according to the present invention may be a nanoparticle with a zero-dimensional to three-dimensional structure, specifically a nanoparticle with a zero-dimensional structure. When the piezoelectric ceramic nanofiller is a nanoparticle with a zero-dimensional structure, it may have a diameter in the range of 1 to 900 nm.

[0054] In addition, when the piezoelectric ceramic nanofiller has a 1 to 3-dimensional structure, it can have various shapes such as nanoparticle type, wire type, tube type, rod type, and microparticle type.

[0055] Meanwhile, when manufacturing a piezoelectric nanocomposite, controlling the content of the 0- to 3-dimensional piezoelectric ceramic nanofillers forms a clustered (selectively connected) percolation network structure within the polymer matrix. At this time, the structural integrity of the electrode is improved, and the clustered (selectively connected) effect improves the local electric field and strengthens the β-phase structure of the polymer matrix, thereby increasing the dipole moment. In addition, by effectively suppressing lithium dendrite growth through this, a uniform lithium electrodeposition process can be induced, thereby improving the overall stability and performance of the lithium metal electrode.

[0056] Specifically, the piezoelectric ceramic nanofiller according to one embodiment of the present invention is barium titanate (BaTiO3, BTO), strontium titanate (SrTiO3), lead zirconate-titanate (Pb(Zr,Ti)O3, PZT), bismuth sodium titanate (Bi 0.5 Na 0.5 TiO3, BNT), bismuth ferrite (BiFeO3), calcium titanate (CaTiO3), lead magnesium niobate-lead titanate (Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3, PMN-PT), lithium lead niobate (PbNb2O6), berlinite (α-AlPO4), quartz (α-SiO2), lithium phosphate (LiTaO3), lithium niobium oxide (LiNbO3), strontium barium niobium oxide (Sr x Ba γ Nb2O8), lead germanate (Pb5Ge3O 11 ), terbium molybdenum (Tb2(MoO4)3), lithium borate (Li2B4O7), cadmium sulfide (CdS), zinc oxide (ZnO), bismuth oxide (Bi 12 SiO 20 ), bismuth-germanium oxide (Bi 12 GeO 20It may be one selected from lead titanate (PT-based, PbTiO3), PZT-complex perovskite, aluminum nitride (AlN), and combinations thereof. Meanwhile, when ABX3-based ceramics such as barium titanate are used as the piezoelectric ceramic nanofiller, the clustering effect (percolation effect) is enhanced, thereby improving piezoelectric and dielectric properties, so the effect of increasing the conductivity and electrochemical stability of lithium ions can be expected.

[0057] In the piezoelectric nanocomposite manufactured through the above steps, the piezoelectric ceramic nanofiller may be included in a range of 0.01 to 50.0 wt% based on the total weight of the piezoelectric nanocomposite, specifically in a range of 0.01 to 30.0 wt%. Meanwhile, the piezoelectric ceramic nanofiller may be included in different amounts depending on the type and form used, and for example, when zero-dimensional barium titanate (BaTiO3) is included alone, it may be included in a range of 15.0 to 30.0 wt% based on the total weight of the piezoelectric nanocomposite.

[0058] If the above-mentioned piezoelectric ceramic filler is included in an amount less than the above range, the effect of increasing lithium ion mobility is negligible due to the unconnected dispersed form of the piezoelectric ceramic nanofiller, making it difficult to sufficiently exhibit the effect of reducing electrode resistance. If it is included in an amount exceeding the above range, the excessive aggregation of the piezoelectric ceramic nanofiller causes non-uniform dispersion concentrated only in specific areas, which may actually increase the total resistance of the lithium metal electrode. Therefore, when the piezoelectric ceramic nanofiller is included within the above range, it is possible to reduce electrode resistance by simultaneously improving lithium ion mobility and charge mobility.

[0059]

[0060] Next, a composition containing the piezoelectric nanocomposite of step a is coated onto a current collector to form a protective layer (step b).

[0061] In the above step, the piezoelectric nanocomposite obtained in step a described above is added to dimethylformamide (DMF) to prepare a composition, and a protective layer is formed on the electrode surface by coating the composition onto a current collector. Specifically, the current collector may be made of copper (Cu), and the protective layer may have a thickness in the range of 0.01 to 100 μm, more specifically in the range of 1 to 10 μm, and more specifically in the range of 2 to 5 μm. Meanwhile, the protective layer has the advantage of improving piezoelectric properties within the thickness range described above, thereby ensuring excellent electrical performance of a lithium metal electrode-based lithium secondary battery.

[0062]

[0063] Piezoelectric nanocomposite and lithium metal electrode for secondary battery containing the same

[0064] A piezoelectric nanocomposite prepared according to one embodiment of the present invention may comprise a polymer matrix; and piezoelectric ceramic nanofillers dispersed within the polymer matrix. The polymer matrix and the piezoelectric ceramic nanofillers are as described above.

[0065]

[0066] A lithium metal electrode for a secondary battery according to another embodiment of the present invention may, for example, be a lithium metal negative electrode, and said lithium metal negative electrode may, for example, be 3 mAh / cm² 2 Under high current density conditions, the formation of dendrites with an irregular structure is prevented, while a uniform and dense lithium electrodeposition (~14.9 μm) can be achieved between the current collector and the protective layer.

[0067] According to the present invention described above, when manufacturing a piezoelectric nanocomposite, a high-temperature process of 1,000°C or higher according to conventional technology is not required, and synthesis can be performed at low cost under room temperature conditions, and nanofillers can be dispersed within the piezoelectric nanocomposite by methods such as spin coating. Furthermore, a lithium metal electrode equipped with a protective layer containing the piezoelectric nanocomposite of the present invention enhances the electrochemical performance of a lithium secondary battery based on a lithium metal electrode, such as excellent lithium ion mobility, low electrode resistance, and inhibition of lithium dendrite growth. Moreover, the piezoelectric nanocomposite can be applied to various technological fields, such as energy storage devices, sensors, and medical devices.

[0068]

[0069] Examples

[0070] The present invention is capable of various modifications and may take various forms, and specific embodiments are illustrated and described in detail below. However, this is not intended to limit the invention to the specific disclosed forms, and it should be understood that the invention includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the invention.

[0071]

[0072] Examples 1-1, 1-2, 1-3, 1-4

[0073] The process for manufacturing the piezoelectric nanocomposite of the present invention and the lithium metal electrode for a secondary battery containing the same is as follows. First, 50 mL of a solution containing 15 wt% polyvinylidene fluoride (PVDF) was prepared by stirring in NN dimethylformamide (DMF, anhydrous 99.8%) at 35 °C for 8 hours. Subsequently, barium titanate (BTO) was added to 5 mL of the solution relative to the total solution, and the mixture was mixed and stirred at 700 rpm for 3 hours. The stirred solution was vacuum-treated for 30 minutes to remove internal gases, including air bubbles. Meanwhile, the molecular weight of the PVDF used to control the structure of the composite, including the dispersion of the piezoelectric inorganic filler and the entanglement structure of the polymer, was set to 534,000 g / mol, classified as medium molecular weight (Medium Mw), and named PVDF (MM).

[0074] Subsequently, a composite thin film was fabricated by coating a BTO / PVDF precursor solution as a protective layer on a copper (Cu) substrate, while maintaining the same PVDF / BTO composition except for the molecular weight. For this purpose, copper foil pretreated with 1M hydrochloric acid (HCl) was used, and a two-step spin coating process was applied. The first step of all composites was performed at 300 rpm for 30 seconds, and in the second step, BTO / PVDF (MM) was coated at 2,000 rpm for 60 seconds.

[0075] The BTO / PVDF composite thin film coated in this way was dried in a vacuum oven at 60°C for 12 hours, and then dried for an additional 2 hours at 100°C. Finally, to maximize piezoelectric properties, the coated film was subjected to a poling process at 100°C with a DC voltage of 150 kV / cm for 1 hour. Through the above process, a lithium metal electrode for a secondary battery equipped with a piezoelectric nanocomposite protective layer was prepared.

[0076] Meanwhile, Example 1-1 was defined as containing 15 wt% of barium titanate (BTO) based on the total weight of the piezoelectric nanocomposite constituting the protective layer, Example 1-2 as containing 20 wt%, Example 1-3 as containing 25 wt%, and Example 1-4 as containing 30 wt%.

[0077]

[0078] Comparative Examples 1-1, 1-2

[0079] The procedure was carried out in the same manner as in Examples 1-3, except that in order to control the entanglement structure of the polymer matrix when manufacturing the piezoelectric nanocomposite, the weight-average molecular weight of the polymer was set to 180,000 g / mol (low molecular weight (Low Mw): PVDF (LM), Comparative Example 1-1) and 1,000,000 g / mol (high molecular weight (High Mw): PVDF (HM), Comparative Example 1-2), respectively, and in order to maintain a constant thickness for each composite, in the second step, the only difference was that BTO / PVDF (LM) was coated at 500 rpm for 60 seconds and BTO / PVDF (HM) was coated at 2,500 rpm for 60 seconds.

[0080]

[0081] Comparative Examples 2-1 to 2-5

[0082] The same procedure as in Examples 1-3 was performed, except that barium titanate (BTO) was included in amounts of 0 wt% (Comparative Example 2-1), 5 wt% (Comparative Example 2-2), 10 wt% (Comparative Example 2-3), 35 wt% (Comparative Example 2-4), and 40 wt% (Comparative Example 2-5) based on the total weight of the piezoelectric nanocomposite.

[0083]

[0084] [Experiment 1: SEM Image Measurement and EDS Mapping]

[0085] SEM image measurement and EDS mapping were performed on the protective layer of the lithium metal electrode prepared according to Examples 1-3 and Comparative Examples 1-1 and 1-2 above.

[0086] Specifically, as can be seen in the SEM images (Top view) of Figures 2 (a) to (c), in Comparative Example 1-1, the BTO nanoparticles are simply uniformly dispersed within the PVDF matrix without inter-particle connectivity, whereas in Comparative Example 1-2, aggregation of the BTO nanoparticles is observed due to the high entanglement and melt viscosity of PVDF(HM) in the morphology. These BTO nanoparticles may further aggregate within the PVDF(HM) polymer with an increased amount of BTO, which can lead to an increase in the overall resistance of the electrochemical cell. In contrast, in Example 1-3, it can be seen that the BTO nanoparticles are dispersed within the PVDF matrix in a percolation network structure that is clustered (optionally connected) between particles.

[0087] Therefore, referring to the results in Figure 2, it can be confirmed that controlling the molecular weight of the polymer constituting the polymer matrix plays an important role in controlling the dispersion / arrangement structure of BTO particles and the polymer entanglement structure in the composite structure, and furthermore, the structure of the entire organic / inorganic composite composed of BTO particles and PVDF. Otherwise, it can be confirmed that the regularity of the composite structure decreases, and performance instability and irregular filling paths occur due to the heterogeneous distribution of nanoparticles.

[0088]

[0089] [Experiment 2: XRD Pattern Verification and FT-IR Spectrum Verification]

[0090] Figure 3 (a) shows the XRD pattern of a PVDF single film, and signature peaks are observed at 18.34° and 19.9° (2θ) in all patterns, indicating an α-phase crystal structure. Meanwhile, Figure 3 (b) shows the XRD pattern of the BTO / PVDF protective layer (composite film) according to Examples 1-3, Comparative Example 1-1, and Comparative Example 1-2 after spin stretching and annealing treatment. A diffraction peak appears at 20.74° (2θ) in all molecular weight ranges of polymers, indicating the formation of β-phase PVDF, which is a structure with maximum polarization, and the same peaks appear at 32°, 39°, and 45° (2θ), indicating the formation of BaTiO3 (BTO), which is an orthorhombic crystal structure with maximum dielectric constant. Meanwhile, referring to Fig. 3 (c), it can be confirmed that a β-phase composite film with piezoelectric properties is formed before and after the preparation of the composite film by analyzing changes in chemical functionality and bonding. In summary, through X-ray diffraction analysis and FT-IR spectrum analysis, excellent crystal structure and bonding structure possessing piezoelectric properties of the piezoelectric nanocomposite were confirmed regardless of molecular weight.

[0091]

[0092] [Experiment 3: CV Measurement at Various Scan Speeds]

[0093] In FIGS. 4 (a) to (d), cyclic voltammetry (CV) was performed on Bare Li and BTO / PVDF (LM, MM, HM)@Li of Examples 1-3 and Comparative Examples 1-1 and 1-2 at various scan rates (0.1, 0.2, 0.4, 0.8 mV / s -1 The peak current values ​​occurring at ) were measured, and these values ​​were used to evaluate the diffusion characteristics of lithium ions. Meanwhile, in Fig. 4 (h), the scan voltage (v 1 / 2 ) and peak current (I p By analyzing the relationship between ) and applying the Randles-Sevick equation to calculate the slope of this relationship, the respective slopes are as follows. Table 1 below shows the peak current (Ip ) and scan voltage (v 1 / 2 This is the slope value data of ).

[0094]

[0095] Electrode Name Slope Bare Li 0.094 ± 0.026 BTO / PVDF(LM) @ Li 0.80 ± 0.19 BTO / PVDF(MM) @ Li 1.24 ± 0.045 BTO / PVDF(HM) @ Li 0.0332 ± 0.027

[0096]

[0097] As can be seen in Table 1, BTO / PVDF(MM)@Li according to Examples 1-3 showed the largest slope (1.23974 ± 0.04513), suggesting that the diffusion of lithium ions was increased due to the clustering effect of selectively connected BTO particles. On the other hand, BTO / PVDF(HM)@Li showed the lowest slope (0.03272 ± 0.02655), which is judged to be due to the aggregation of BTO within the complex hindering the diffusion of lithium ions, resulting in a reduced slope.

[0098]

[0099] [Experiment 4: PFM Signal Measurement]

[0100] In Figures 5 (a) and (b), PFM measurements were performed to determine the piezoelectricity of the BTO / PVDF(MM) composite, and the piezoelectricity in the protective layer was confirmed through the high-contrast PFM phase and amplitude in the images. Meanwhile, in Figure 5 (c), it can be seen that the phase of the BTO / PVDF nanocomposite is in the range of -100° to 100°, showing a phase difference of 180°, and exhibits strong polarization intensity due to the inherently strong dielectric properties of BTO. Additionally, local spectroscopic analysis confirmed that BTO nanoparticles were integrated into highly active piezoelectric regions within the piezoelectric polymer matrix. As a result, it can be confirmed that the BTO nanoparticles were effectively integrated into the polymer matrix, significantly enhancing the piezoelectric region.

[0101] KPFM measurements were performed on the PVDF and BTO / PVDF(MM) electrodes, and the surface potential of each electrode was measured as shown in Figures 5 (e) and (g). When the surface potential of the corresponding electrodes was plotted as a Gaussian statistical distribution histogram as shown in Figures 5 (f) and (h), PVDF showed a mixed distribution of negative and positive charges, whereas BTO / PVDF(MM) showed potential values ​​in only one direction of negative charge.

[0102] When comparing the absolute values ​​of surface potential, BTO / PVDF (MM) exhibited a higher potential. The surface potential range of the PVDF film was -400 to 200 mV with a center value of -99.3 mV, whereas the BTO / PVDF composite film ranged from -400 to -50 mV with a center value of -264.1 mV, indicating that the BTO / PVDF composite material has a higher surface potential compared to PVDF. Furthermore, the directional alignment of the dipoles and the high surface potential acted as factors that increased piezoelectricity.

[0103] Since the piezoelectric effect of composite materials depends significantly on the initial dipole alignment state, piezoelectric composites require unidirectional dipole alignment in the initial electric field. Randomly aligned dipoles within the composite material do not exhibit a strong piezoelectric effect in subsequent electric fields. In particular, identical dipole alignment of BTO / PVDF composite films in the initial state can effectively enhance both the piezoelectric effect and structural stability in subsequent electric fields.

[0104]

[0105] [Experiment 5: Measurement of Electrochemical Performance Changes of Lithium Metal Anode / Protective Film Composites According to Composite Protective Film Structure through Control of Polymer Molecular Weight and Entanglement Structure]

[0106] As can be seen in Figures 6a, 6d and 6b, 6e, the total resistance of the lithium metal anode / protective film composite layer decreased as the BTO content increased from 0 wt% to 25 wt% (for BTO / PVDF (LM), the resistance decreased from 160.309 Ω to 88.266 Ω, and for BTO / PVDF (MM), it decreased from 307.684 Ω to 210.126 Ω). In particular, the resistance reduction effect was greater in Examples 1-1 to 1-4, which is due to the increased mobility of lithium ions caused by the high dielectric properties of BTO. Meanwhile, Figures 6c and 6f show that when the weight-average molecular weight of the polymer is 1,000,000 g / mol as in Comparative Example 1-2, the total resistance of the BTO / PVDF (HM) treated lithium anode increases when the BTO content exceeds 5 wt%. This is because BTO nanoparticles aggregate within a PVDF(HM) matrix with high entanglement and melt viscosity, resulting in non-uniform dispersion. This suggests that while an appropriate BTO content improves lithium ion mobility and charge mobility to reduce resistance, excessive BTO actually increases resistance through aggregation.

[0107] Meanwhile, Figures 6g to 6i illustrate the effect of polymer molecular weight on the composite structure and electrochemical performance. Low molecular weight polymers possess a low radius of gyration (Rg), low melt viscosity, and low entanglement density, which allows nanoparticles to move freely within the polymer matrix, forming a structure in which BaTiO3 (BTO) nanoparticles are not strongly connected. This strengthens the β-phase PVDF crystal structure and may partially increase the piezoelectric effect of the composite material. BTO nanoparticles within the medium molecular weight polymer matrix form a clustered (selectively connected) percolation network structure, causing the nanoparticles to accumulate in adjacent areas rather than being completely dispersed. This can amplify the dipole effect of the composite material due to the strong electric field of the BTO groups.

[0108] On the other hand, in high molecular weight polymers (PVDF(HM)), the movement of BTO nanoparticles is restricted due to high entanglement density and melt viscosity, forming aggregates. This alters the lithium ion migration pathway, increasing electrode resistance. Consequently, the total resistance of a BTO / PVDF-treated lithium anode can be controlled by adjusting the amount of BTO and the network structure of the composite material, and resistance may be higher in high molecular weight polymers due to nanoparticle aggregation.

[0109]

[0110] [Experiment 6: Measurement of Changes in Electrochemical Performance According to Polymer Molecular Weight]

[0111] According to FIGS. 7a and 7b, the lithium anode with a BTO / PVDF (MM) protective layer formed according to Example 1-3 maintained a high Coulomb efficiency of 97.1% up to 270 cycles, whereas the efficiency of the treated anodes with a protective layer formed by Bare Li, Comparative Example 1-1 (BTO / PVDF (LM)), and Comparative Example 1-2 (BTO / PVDF (HM)) dropped to below 90% after 63, 85, and 214 cycles, respectively. The decrease in efficiency of the Bare Li anode is due to irreversible lithium loss and the formation of SEI (Solid Electrolyte Interphase).

[0112] As shown in Fig. 7c, the lithium anode treated with a BTO / PVDF(MM) composite protective film exhibited excellent cycle stability, maintaining a stable low overpotential (overpotential, <25 mV) for 1,060 cycles. In contrast, the untreated lithium anode and the anodes treated with BTO / PVDF(LM) and BTO / PVDF(HM) as comparison groups showed an increase in overpotential after 36, 46, and 117 cycles, respectively. At specific time intervals, BTO / PVDF(MM) initially showed a peak pattern and continued to show a peak pattern over time. On the other hand, the untreated lithium anode and the anodes treated with BTO / PVDF(LM) and BTO / PVDF(HM) showed an arc-like pattern in the initial overpotential.

[0113] The peak pattern is observed during the initial charging and discharging phases and is an important indicator of the initial performance of the lithium secondary battery. This phenomenon primarily occurs during the initial charging phase and reflects the active charging and discharging behavior of lithium ions. However, as the battery undergoes repeated cycles, a dead lithium layer forms within the lithium secondary battery. This dead lithium layer hinders the transport of lithium ions and negatively alters the chemical activity between the electrolyte and the lithium electrode. Due to these internal chemical changes, the initial peak pattern gradually changes into an arc pattern over time. Furthermore, the reason why the initial overvoltage pattern takes the form of an arc in the Bare Li case of Fig. 7c can be explained in detail.

[0114]

[0115] [Experiment 7: SEM image of lithium electrode based on Cu current collector coated with protective layer]

[0116] SEM images of bare Li and lithium electrodeposited on Cu current collectors coated with the BTO / PVDF(MM) protective layer of Examples 1-3 (Figs. 8 (a) to (d)): Figs. 8 (a) and (c) are each 1 mAh / cm² 2 and 3 mAh / cm 2 In the SEM image of a lithium metal electrode plated (electrodeposited) on bare Cu, irregular dendrite formation and non-uniform electrodeposition are observed, indicating unstable electrode surface electrodeposition after long-term cycling, whereas Figures 8 (b) and (d) show lithium uniformly plated on a copper (Cu) current collector with a BTO / PVDF(MM) protective layer formed under the same conditions, showing a very smooth and uniform surface, indicating that the BTO / PVDF(MM) protective layer coating is effective in promoting uniform lithium electrodeposition.

[0117] Comparison of cross-sectional SEM images and lithium thickness (Figs. 8 (e) to (f)): In Fig. 8 (e), electrodeposited lithium with a thickness of 40.5 μm is observed, exhibiting high porosity. This suggests that lithium ions may have been plated unevenly in specific regions, implying that porosity may increase in the spread-out lithium structure resulting from irregular electrodeposition. On the other hand, Fig. 8 (f) shows the cross-sectional structure of a BTO / PVDF(MM) coated lithium electrode, revealing a three-layer structure consisting of an upper layer of the BTO / PVDF(MM) composite, a middle layer of plated lithium, and a lower layer of the Cu substrate. The lithium plated beneath the BTO / PVDF(MM) protective layer exhibits a dense electrodeposition structure with a thickness of 5.2 μm, which is significantly thinner compared to the spread-out lithium (40.5 μm). This corresponds to 1 mAh / cm² 2 It matches the theoretically calculated thickness (~5 μm) under the current density.

[0118] Meanwhile, FIGS. 8 (g) and (h) show a high current density (3 mAh / cm²). 2 The formation of dendrites on bare Cu and BTO / PVDF(MM) coated Cu was compared. Irregular dendrite formation was observed on bare Cu, but uniform and high-density lithium electrodeposition (~14.9 μm) was observed on the electrode with the BTO / PVDF(MM) protective layer.

[0119] This suggests that the BTO / PVDF(MM) composite used as a protective layer plays a role in controlling the electrodeposition pattern of lithium. Compared to bare Li electrodeposited under various current density conditions, the BTO / PVDF(MM) coating exhibits low porosity and uniform electrodeposition. While bare Li shows a thickness higher than the theoretical capacity thickness, the BTO / PVDF(MM) protective layer coating maintains an electrodeposition thickness corresponding to the theoretical capacity thickness. This contributes to improving the stability and performance of the lithium metal cathode in the battery.

[0120]

[0121] [Experiment 8: Measurement of Electrochemical Performance of LFP Cathode in Complete Cell]

[0122] As shown in Fig. 9 (a), the cycle stability of the LiFePO4 (LFP) cathode at 0.5 C was evaluated. In terms of initial capacity, the Li-LFP and the BTO / PVDF(MM) coated Li-LFP according to Examples 1-3 showed 166.8 mAh / g and 164.6 mAh / g, respectively. After 30 cycles, the Li-LFP decreased to 120 mAh / g with a capacity retention rate of approximately 71.9%. On the other hand, the BTO / PVDF(MM) / Li-LFP demonstrated stable performance, maintaining an excellent capacity retention rate of 150.87 mAh / g even after 350 cycles. Additionally, as shown in Fig. 9 (b), the cycle stability was evaluated at 2.0 C. On the other hand, in comparative experiments on the Li-LFP cycle stability of BTO / PVDF(LM) and BTO / PVDF(HM), the initial capacity was 163.35 mAh / g for BTO / PVDF(LM) and 161.32 mAh / g for BTO / PVDF(HM). After 75 cycles, capacity retention rates of 65.4% and 48.0%, respectively, were recorded, confirming the superior cycle stability and capacity retention rate of BTO / PVDF(MM).

[0123] In Figure 9 (c), the current capability of BTO / PVDF(MM) / Li-LFP at various current densities was evaluated, and the BTO / PVDF(MM) coated Li-LFP exhibited excellent current capability at various current densities from 0.1 C to 15.0 C, and in particular, maintained a capacity of 72.0 mAh / g even at 15.0 C, showing a capacity retention rate of 45.7%. In Figures 9 (d) and 9 (e), the voltage profiles of BTO / PVDF(MM) / Li-LFP and Li-LFP during long-term cycles (>100 cycles) were compared, and the BTO / PVDF(MM) coated Li-LFP maintained a stable voltage profile without polarization even after 180 cycles at 0.5 C, retaining a capacity of 131.42 mAh / g.

[0124] In conclusion, it was confirmed that using a BTO / PVDF(MM) composite as a protective layer for a lithium metal anode enables uniform lithium electrodeposition and improves the electrochemical stability of a lithium metal electrode-based lithium secondary battery through piezoelectric properties, thereby controlling the electric field distribution and suppressing dendrite growth during charge-discharge cycles, resulting in high electrochemical stability even in full-scale batteries.

[0125]

[0126] [Experiment 9: Operating Mechanism of Piezoelectric Protective Layer on Lithium Metal Anode]

[0127] In Fig. 10, when Li ions are electrodeposited on the Cu current collector due to the non-uniform current distribution in Bare Cu, non-uniform nucleation and lithium electrodeposition occur, which can easily lead to the formation of a non-uniform lithium anode after long cycles (Fig. 10 (a)). On the other hand, BTO / PVDF(MM) / Cu (Fig. 10 (b)) exhibits a more uniform lithium electrodeposition process. BTO nanoparticles within the PVDF matrix are selectively linked to improve the structural integrity of the electrode, and the clustering effect improves the local electric field and strengthens the β-phase PVDF crystal structure, thereby increasing the dipole moment.

[0128] In addition, selectively linked structures between BTO nanoparticles are formed under specific ranges of Rg, melt viscosity, and entanglement density conditions of the polymer within the composite, which enhances the local electric field generated by the BTO groups, thereby strengthening the dipole effect and effectively suppressing lithium dendrite growth. Therefore, BTO / PVDF(MM) / Cu according to Examples 1-3 can induce a more controlled and uniform lithium electrodeposition process compared to Bare Cu, thereby improving the overall stability and performance of the lithium metal.

[0129]

[0130] The foregoing description is merely an illustrative explanation of the technical concept of the present invention, and those skilled in the art to which the present invention pertains will be able to make various modifications and variations within the scope of the essential characteristics of the present invention. Accordingly, the embodiments disclosed in the present invention are intended to explain, not limit, the technical concept of the present invention, and the scope of the technical concept of the present invention is not limited by these embodiments. The scope of protection of the present invention shall be interpreted by the claims below, and all technical concepts within an equivalent scope shall be interpreted as being included within the scope of rights of the present invention.

Claims

1. Polymer matrix; and A piezoelectric nanocomposite comprising piezoelectric ceramic nanofillers dispersed within the polymer matrix.

2. In Paragraph 1, The polymer matrix comprises a piezoelectric nanocomposite comprising one piezoelectric resin selected from polyvinylidene fluoride (PVDF), polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polytrifluoroethylene (PTFE), polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene (P(VDF-TrFE-CTFE)), polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene (P(VDF-TrFE-CFE)), poly-L-lactic acid (PLLA), polyvinylidene fluoride-tetrafluoroethylene (P(VDF-TeFE)), triglycine sulfate (TGS), and combinations thereof.

3. In Paragraph 2, The above polymer matrix comprises polyvinylidene fluoride (PVDF), and The above polyvinylidene fluoride is a piezoelectric nanocomposite comprising a β-phase structure in the range of 0.01 to 95.0%.

4. In Paragraph 1, A piezoelectric nanocomposite having a weight-average molecular weight (Mw) of the polymer constituting the polymer matrix in the range of 10,000 to 2,000,000.

5. In Paragraph 1, The above-mentioned piezoelectric ceramic nanofiller is a nanoparticle with a zero to three-dimensional structure, and A piezoelectric nanocomposite having a diameter in the range of 1 to 900 nm.

6. In Paragraph 1, The above piezoelectric ceramic nanofillers are barium titanate (BaTiO3, BTO), strontium titanate (SrTiO3), lead zirconate-titanate (Pb(Zr,Ti)O3, PZT), bismuth sodium titanate (Bi 0.5 Na 0.5 TiO3, BNT), bismuth ferrite (BiFeO3), calcium titanate (CaTiO3), lead magnesium niobate-lead titanate (Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3, PMN-PT), lithium lead niobate (PbNb2O6), berlinite (α-AlPO4), quartz (α-SiO2), lithium phosphate (LiTaO3), lithium niobium oxide (LiNbO3), strontium barium niobium oxide (Sr x Ba γ Nb2O8), lead germanate (Pb5Ge3O 11 ), terbium molybdenate (Tb2(MoO4)3), lithium borate (Li2B4O7), cadmium sulfide (CdS), zinc oxide (ZnO), bismuth oxide (Bi 12 SiO 20 ), bismuth-germanium oxide (Bi 12 GeO 20 A piezoelectric nanocomposite selected from lead titanate (PT-based, PbTiO3), PZT-complex perovskite, aluminum nitride (AlN), and combinations thereof.

7. In Paragraph 1, The above piezoelectric ceramic nanofiller is included in a range of 0.01 to 50.0 wt% based on the total weight of the piezoelectric nanocomposite, in a piezoelectric nanocomposite.

8. The entire house; and A lithium metal electrode for a secondary battery comprising: a protective layer comprising a piezoelectric nanocomposite according to claim 1 coated on the above-mentioned current collector.

9. In Paragraph 8, The above current collector is a lithium metal electrode for a secondary battery, which is copper (Cu).

10. In Paragraph 8, A lithium metal electrode for a secondary battery, wherein the protective layer has a thickness in the range of 0.01 to 100 μm.

11. In Paragraph 8, The above lithium metal electrode is a lithium metal negative electrode for a secondary battery.

12. In Paragraph 11, The above lithium metal cathode has 3 mAh / cm 2 A lithium metal electrode for a secondary battery, wherein dendrite formation is prevented under current density conditions and a uniform and dense lithium electrodeposition with a thickness of 14.9 μm or less is formed between the current collector and the protective layer. 13.a) A step of forming a piezoelectric nanocomposite having a clustered percolation network structure by dispersing piezoelectric ceramic nanofillers with a zero to three-dimensional structure within a polymer matrix; b) a step of forming a protective layer by coating a composition containing the piezoelectric nanocomposite of step a onto a current collector; a method for manufacturing a lithium metal electrode for a secondary battery.

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