Lithiophilic carbonnanotube assembly based janus separator and preparing method for manufacturing of the same and lithium metatl secondary battery comprising the same

Abstract

The present invention relates to a Janus separator for a lithium metal secondary battery comprising: a separator; and a carbon nanotube assembly coated on one surface of the separator and comprising carbon nanotubes surface-modified with an amine group-containing linker and a carboxyl group; a method for manufacturing the same; and a lithium metal secondary battery comprising the same.

Description

Technology Field

[0001] The present invention relates to a lithium-affinity carbon nanotube assembly-based Janus separator for suppressing lithium dendrite formation and controlling the lithium growth direction, a method for manufacturing the same, and a lithium metal secondary battery including the same. Background Technology

[0003] As demand for portable electronic devices, high-capacity storage devices, and electric vehicles increases, the need for the development of high-capacity secondary batteries is on the rise.

[0004] Recently, lithium metal electrodes with a theoretical capacity of 3840 mAh / g and a low reduction potential are gaining attention as next-generation secondary battery anode materials, along with lithium-sulfur secondary batteries and lithium-air secondary batteries, as high-capacity secondary battery anode materials that can replace conventional graphite (372 mAh / g).

[0005] However, lithium metal electrodes have a problem in that unnecessary lithium ions and charges are lost as lithium dendrites form sharply during the charging and discharging of the secondary battery, repeatedly breaking and regenerating the SEI (Solid Electrolyte Interphase) layer. In addition, as lithium dendrites continue to grow, they can penetrate the separator and come into contact with the opposite electrode, causing a short circuit and potentially leading to an explosion.

[0006] Accordingly, research on coating functional materials onto separators is actively underway to improve the stability of lithium metal secondary batteries. However, strategies to enhance the stability of lithium metal secondary batteries, such as uniform lithium ion transport, improved lithium ion selectivity, and control of lithium growth direction, are currently required. The problem to be solved

[0008] The present invention aims to provide a Janus separator for a lithium metal secondary battery, etc., for suppressing lithium dendrite formation and controlling the lithium growth direction, comprising: a separator; and a carbon nanotube assembly coated on one surface of the separator, comprising a carbon nanotube surface modified with a carboxyl group and an amine group-containing linker.

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

[0011] The present invention provides a Janus separator for a lithium metal secondary battery comprising: a separator; and a carbon nanotube assembly coated on one surface of the separator and comprising carbon nanotubes surface-modified with an amine group-containing linker and a carboxyl group.

[0012] The above separator may be made of a polyolefin-based polymer material.

[0013] The surface of the above separation membrane can be modified to oxygen-containing functional groups by UV irradiation or plasma treatment.

[0014] The above amine group-containing linker may contain two or more amine groups.

[0015] The weight-average molecular weight of the above amine group-containing linker is 30 g mol -1 to 5,000 g mol -1 It could be.

[0016] The above amine group-containing linker may include one or more selected from the group consisting of hydrazine, diethylenetriamine, tris(2-aminoethyl)amine, and polyethyleneimine.

[0017] The above carbon nanotube assembly may be in a binder-free form.

[0018] The carbon nanotube assembly above can be coated on one side of the separator and directed toward the lithium metal electrode.

[0019] In the above carbon nanotube assembly, carbon nanotubes surface-modified with amine group-containing linkers and carboxyl groups can be repeatedly stacked through hydrogen bonding.

[0020] The above carbon nanotube assembly may have a contact angle of 30° or less with respect to an electrolyte comprising lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), dimethyl ether (DME), and 1,4-dioxane (DIOX) mixed solvent.

[0021] In one embodiment of the present invention, a method for manufacturing a Janus separator for a lithium metal secondary battery is provided, comprising: (a) a step of surface-modifying carbon nanotubes with carboxyl groups; and (b) a step of coating a carbon nanotube assembly comprising an amine group-containing linker and carbon nanotubes surface-modified with carboxyl groups on one surface of the separator.

[0022] In another embodiment of the present invention, a lithium metal secondary battery comprising the Janus separator is provided. Effects of the invention

[0024] The Janus separator according to the present invention is characterized by comprising: a separator; and a carbon nanotube assembly coated on one surface of the separator and comprising carbon nanotubes surface-modified with an amine group-containing linker and a carboxyl group. Accordingly, when operating a lithium metal secondary battery with the Janus separator according to the present invention, it is possible to evenly disperse lithium ions to suppress the formation of lithium dendrites, and also control the direction of lithium growth, thereby having the advantage of ultimately securing high stability.

[0025] Therefore, the present invention can be utilized in various high-capacity secondary batteries requiring excellent driving stability and power density. Brief explanation of the drawing

[0027] Figure 1 shows (A) a schematic description of the fabrication of an n-MWCF separator and (B) a comparison of lithium growth behavior according to the separator in repetitive constant current lithium plating. Figure 2 illustrates the preparation of n-MWCF: (A) Digital images of pristine dispersed in ethanol (left) and acid-treated MWCNT (right). (B) FE-TEM image of COOH-MWCNT. (C) Diagram showing the LbL assembly of COOH-MWCNT and NH2-TAA through specific interactions. (D) (NH2-TAA / COOH-MWCNT) as a function of the number of bilayers (n). n Changes in the FTIR spectrum of the multilayer. (E) (NH2-TAA / COOH-MWCNT) as a function of the number of double layers (n) n Variation of the UV-vis spectrum of the multilayer. The inset shows the UV-vis absorbance at 233 nm as a function of the number of double layers (n). (F) (NH2-TAA / COOH-MWCNT) n Film thickness according to the number of double layers (n) in the multilayer. The inset shows a cross-sectional FE-SEM image. (G) (NH2-TAA / COOH-MWCNT) with different numbers of double layers (n) n Multilayer QCM results. Figure 3 shows the characterization of n-MWCF: (A) FE-SEM images of n-MWCF membranes with different double-layer numbers (n) of 0 (bare membrane), 1, 3, and 5. The inset shows the corresponding digital image of each n-MWCF membrane. (B) Sheet resistance (Ω sq) as a function of double-layer number (n) of n-MWCF membranes with different linkers, NH2-TAA (red circle) and NH2-PEI (blue circle). -1(C) Change in electrical conductivity (σ / σ0) of the 3-MWCF separator after 10,000 bending cycles with a bending radius of 2.5 mm (inset). (D) Wetness test of the electrolyte (LiTFSI in DME / DIOX mixture) on the n-MWCF separator through contact angle change with different double layer numbers (n). (E) Electrolyte absorption capacity of the n-MWCF separator. (F) Electrostatic potential maps for bare PP separator, pristine MWCNT, COOH-MWCNT, and MWCF, respectively. (G) Li on each configuration + Trends in adsorption energy for ions. Figure 4 shows the electrochemical characteristics of an n-MWCF separator-based half cell: (A) 0.5 mA cm⁻¹ -2 Voltage (V) versus time (sec) curves of an n-MWCF separator-based Li|Ni cell during Li nucleation. (B), (C) Nyquist plot of an n-MWCF separator-based Li|Ni cell and corresponding ion diffusion coefficient (D Li+ ). (D) 1 mA cm -2 and 1 mAh cm -2 Coulomb efficiency (CE) of an n-MWCF-based half-cell during repetitive Li plating / stripping cycles. (E) FE-SEM images of the Ni plate surface of a Li|Ni cell with a top separator (left) and a 3-MWCF separator (right) after cycling. (G) Effect of the n-MWCF intermediate layer on the Li plating mechanism in the LMB system. Figure 5 illustrates the symmetrical cell test: (A) Comparison of Li plating behavior in Li | Li symmetrical cells using a bare separator and (B) a 3-MWCF separator. FE-SEM images show the surface morphology of each separator and the Li anode. The inset shows a digital image of the decomposed Li anode. (C) Nyquist plots and representative equivalent circuits of n-MWCF separator-based Li symmetrical cells with different double layer numbers (n). (D) Ionic conductivity and Li+ transfer number of n-MWCF separator-based Li symmetrical cells. (E) 1 mA cm⁻¹ -2 (1 mAh cm -2 ) and (F) 3 mA cm -2 (1 mAh cm -2 (G) Constant current cycling of Li symmetric cells with each separator. (G) Comparison of cycle stability of Li symmetric cells based on 3-MWCF separators and functional separators from previous studies. Figure 6 shows the performance of an asymmetric complete cell with an n-MWCF separator: (A) Cycling test of a Li | NMC811 complete cell with a bare element and a 3-MWCF separator. (B) Rate-performance test of a Li | NMC811 complete cell with a bare element and a 3-MWCF separator. (C) 21.5 mg cm⁻¹ -2 Cycling test of a Li | NMC811 full cell with a bare battery and 3-MWCF separator under anode loading (N / P ratio ~ 1.5). The test was conducted at 0.4 mA cm⁻¹. -2 2 mA cm after precycling -2 Performed at. (D) 21.5 mg cm⁻¹ -2 Discharge profile of a Li | NMC811 complete cell with anode loading as a function of current density. The inset shows energy and power densities at various current densities. (E) Spider chart of a Li | NMC811 complete cell with different separators for comparison of various performance parameters. (F) Li | 10.5 mg cm⁻¹ -2An NMC811 pouch battery with a positive loading of 1 mA cm⁻¹ -2 Tested in. The inset shows a digital image of the assembled pouch-type battery. (G) Cycling test of a Li |LFP full cell with bare and 3-MWCF separator. Specific details for implementing the invention

[0028] Lithium metal is a promising anode for high-energy batteries. However, the growth of lithium dendrites during charge / discharge cycles poses issues regarding safety and durability. Accordingly, the inventors introduce a novel approach to control lithium dendrite growth using a layer-by-layer (LbL) assembled multi-walled carbon nanotube forest (MWCF) composed solely of a lithium-affinity composition (carboxylic acid-functionalized MWCNT / amine-functionalized linker) without an inert binder. Lithium-affinity MWCFs, directly deposited on one side of a separator via LbL assembly, can maintain excellent electrical conductivity and a gapless interface with the separator, mitigate local current density, and provide pore space for uniform lithium plating. Importantly, the MWCF induces Li growth toward the Li metal anode and prevents dendrites from penetrating the cathode, thereby enabling 10,000 hours (1 mA cm⁻¹) of operation in a Li|Li symmetrical battery. -2 The point is that excellent stability can be achieved during... Surprisingly, the NMC811-based asymmetric battery maintains a capacity retention rate of ~81.9% even after 600 cycles at 1 C and 678 Wh kg -1 It is possible to achieve ultra-high energy density. In addition, LFP-based asymmetric batteries can exhibit excellent cycle stability.

[0030] The inventors have designed a novel lithium-affinity molecular linker-mediated layer-by-layer (LbL) assembly method on a separator, having a thickness of about 62 nm and a mass of 16.5 µg cm⁻¹. -2We propose an ultrathin, electrochemically inactive lithium-affinity multi-walled carbon nanotube forest (MWCF) (Fig. 1A). This innovative yet simple methodology can enable the creation of a gapless and intimate interface between the lithium-affinity MWCF interlayer and the separator. By utilizing nanometer-scale controlled complementary interactions between the lithium-affinity COOH-functionalized MWCNT (COOH-MWCNT) and the lithium-affinity NH2-functionalized small molecule linker (tris(2-aminoethyl)amine, TAA), and between NH2-TAA and the separator, this approach can effectively control lithium dendrite growth on the LMA surface, providing unprecedented operational stability. In particular, lithium-affinity NH2-functionalized molecular linkers can directly link adjacent lithium-affinity COOH-MWCNTs through clearly defined interactions, thereby ensuring structural integrity capable of effectively withstanding mechanical (i.e., volume expansion and contraction during electrochemical cycles) and / or electrochemical stresses (i.e., SEI formation and lithium dendrite growth) during repetitive lithium plating and stripping processes. Furthermore, this approach can influence the electrochemical reaction rate of LMA through interfacial interactions between two different lithium-affinity functional configurations (i.e., COOH groups of MWCNTs and NH2 groups of molecular linkers) by utilizing the customized chemical and physical properties of functional interlayer films assembled at the nanometer scale without insulating polymer binders.

[0031] To achieve these goals, MWCFs were formed by assembling COOH-MWCNT and NH2-TAA (i.e., NH2-TAA / COOH-MWCNT multilayers) on commercial battery separators via lithium-affinity linker-mediated LbL assembly. Importantly, this LbL approach ensures a uniform distribution of abundant lithium-affinity organic components (i.e., oxygen- and nitrogen-containing functional groups) within the conductive MWCF network, thereby enabling rapid and homogeneous Li +It was found that it promotes ion flux and effectively suppresses irregular lithium dendrite growth. In particular, small molecule linker-induced LbL assembly, excluding bulky and insulating polymer linkers, forms a highly entangled, ultrathin forest interlayer with a robust interfacial structure free of lithium-affinity inactive components, thereby preserving the porosity and electrical conductivity of the COOH-MWCNT itself. This MWCF configuration forms a uniform thin layer of Li at the MWCF / separator interface, inducing Li growth toward the LMA surface and preventing penetration into the separator (Fig. 1B). A symmetric Li|Li cell produced using such an MWCF interlayer coated separator has a conductivity of 1 mA cm⁻¹. -2 current density and 1 mAh cm -2 It demonstrated an excellent lifespan exceeding 10,000 hours at the capacity. In addition, LMA and LiNi 0.8 Mn 0.1 Co 0.1 The complete cell configuration composed of an O2 (NMC811) cathode achieved remarkable cycle stability of 81.9% after 600 cycles at 1C, and 0.1 mA cm⁻¹ -2 at 678 Wh kg -1 It delivered the maximum energy density. Surprisingly, the complete cell employing the LiFePO4 (LFP) cathode exhibited excellent cycle stability of 107% even after 1,500 cycles at 1C. These results demonstrated significantly superior performance compared to conventional interlayer-based cells reported to date, which are limited by unfavorable interface designs. While MWCNTs have been widely used in conventional slurry-based interlayers, this novel structural and interfacial interaction design can provide unprecedented performance in terms of energy and stability. Therefore, this approach, which allows for precise and simple control of all configurations and the consequent physical and chemical properties of the interlayer, can be seen as providing new insights and a foundation for the development of future high-performance LMBs.

[0033] The present invention will be described in detail below.

[0035] In this specification, the term "Janus membrane" refers to a membrane in which one side and the other side have different forms, wherein (NH2-TAA / COOH-MWCNT) coated or deposited on one side of the membrane n It was named an n-Janus separator based on its multilayer structure.

[0036] In this specification, "lithium dendrite" refers to a needle or tree-branch structured precipitate formed when lithium crystals form on the surface of the negative electrode during the charging and discharging process of lithium metal, and these crystals become nuclei that gradually accumulate.

[0038] Janus separator

[0040] The present invention provides a Janus separator for a lithium metal secondary battery comprising: a separator; and a carbon nanotube assembly coated on one surface of the separator and comprising carbon nanotubes surface-modified with an amine group-containing linker and a carboxyl group.

[0042] First, the Janus separator for a lithium metal secondary battery according to the present invention comprises a separator.

[0043] The above-mentioned separator refers to a bare separator, which may be in a single-layer or multi-layer form and may be made of various known porous materials. Specifically, the separator may be a polyolefin-based polymer material, and in a specific embodiment of the present invention, a porous polypropylene polymer material was used.

[0044] In particular, the surface of the above-mentioned membrane may be modified by UV irradiation or plasma treatment to have oxygen-containing functional groups, particularly hydroxyl groups, and these hydroxyl groups may form hydrogen bonds with amine groups present in the amine group-containing linker described later.

[0046] Next, the Janus separator for a lithium metal secondary battery according to the present invention comprises a carbon nanotube assembly, wherein the carbon nanotube assembly is coated on one surface of the separator. Additionally, the carbon nanotube assembly comprises an amine group-containing linker and carbon nanotubes surface-modified with carboxyl groups.

[0047] The carbon nanotube surface-modified with the above carboxyl group has lithium affinity and hydrophilicity, and the carboxyl group present in the carbon nanotube can form hydrogen bonds with the amine group present in the amine group-containing linker described later.

[0048] The above amine group-containing linker also possesses lithium affinity and hydrophilicity. The amine group present in the amine group-containing linker that is coated first can form hydrogen bonds with oxygen-containing functional groups present in the separator, particularly hydroxyl groups, while simultaneously forming hydrogen bonds with carboxyl groups present in adjacent carbon nanotubes. Subsequently, the amine group-containing linker coated thereafter can form hydrogen bonds with carboxyl groups present in adjacent carbon nanotubes, thereby enabling LbL assembly.

[0049] Accordingly, the above amine group-containing linker preferably contains two or more amine groups to perform the role of a linker.

[0050] In addition, the weight-average molecular weight of the amine group-containing linker is 30 g mol so as to minimize contact resistance. -1 to 5,000 g mol -1 It can be, 30 g mol -1 up to 500 g mol -1 It is desirable, but not limited to this.

[0051] Specifically, the amine group-containing linker may include one or more selected from the group consisting of hydrazine, diethylenetriamine, tris(2-aminoethyl)amine, and polyethyleneimine, and preferably includes one or more selected from the group consisting of hydrazine, diethylenetriamine, and tris(2-aminoethyl)amine, but is not limited thereto.

[0052] Due to the presence of such amine group-containing linkers, the carbon nanotube assembly does not require a polymer binder, so it can be in a binder-free form.

[0053] Meanwhile, the carbon nanotube assembly can be assembled to form a uniform nanoporous structure and be coated on one surface of the separator so as to face the lithium metal electrode. This allows for the even dispersion of lithium ions to suppress the formation of lithium dendrites, as well as the control of the lithium growth direction.

[0054] The carbon nanotube assembly can be coated on the above-mentioned separator while its surface is modified with oxygen-containing functional groups, particularly hydroxyl groups, by UV irradiation or plasma treatment. The amine group-containing linker that is first coated within the carbon nanotube assembly can form hydrogen bonds with the separator and simultaneously form hydrogen bonds with adjacent carbon nanotubes. Subsequently, the amine group-containing linker that is coated can be repeatedly stacked through hydrogen bonding between the adjacent carbon nanotubes. The number of layers of carbon nanotubes surface-modified with carboxyl groups can be viewed as the number of repetitions, and the number of repetitions is intended to control the thickness of the coated carbon nanotube assembly; it may be 1 to 20 times, preferably 2 to 10 times, and more preferably 2 to 4 times based on a comprehensive assessment of lithium affinity, ion diffusion coefficient, cycle stability, etc., but is not limited thereto.

[0055] Depending on the number of such repetitions, the thickness of the carbon nanotube assembly may be 1 nm to 500 nm, preferably 40 nm to 200 nm, and more preferably 40 nm to 80 nm, but is not limited thereto.

[0056] The carbon nanotube assembly has lithium affinity and hydrophilicity, and in particular, the contact angle with respect to an electrolyte comprising a mixed solvent of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), dimethyl ether (DME), and 1,4-dioxane (DIOX) may be 30° or less, preferably 10° or less, and more preferably 0°, but is not limited thereto.

[0058] Method for manufacturing a Janus separator for a lithium metal secondary battery

[0060] The present invention provides a method for manufacturing a Janus separator for a lithium metal secondary battery, comprising the steps of: (a) surface-modifying carbon nanotubes with carboxyl groups; and (b) coating one surface of a separator with a carbon nanotube assembly comprising an amine group-containing linker and carbon nanotubes surface-modified with carboxyl groups.

[0062] First, the method for manufacturing a Janus separator for a lithium metal secondary battery according to the present invention comprises the step of surface-modifying carbon nanotubes with carboxyl groups [step (a)].

[0063] The carbon nanotubes are hydrophobic prior to surface modification and are characterized by surface modification with carboxyl groups to possess lithium affinity and hydrophilicity. At this time, the surface modification can be performed using an acidic solution, and is preferably performed using a mixed solution of sulfuric acid and nitric acid, but is not limited thereto.

[0065] Next, the method for manufacturing a Janus separator for a lithium metal secondary battery according to the present invention comprises the step of coating a carbon nanotube assembly comprising an amine group-containing linker and a carbon nanotube surface-modified with the carboxyl group on one surface of the separator [step (b)].

[0066] Since the above-mentioned separator, the above-mentioned amine group-containing linker, and the above-mentioned carbon nanotube surface-modified with carboxyl groups have been described above, a redundant explanation will be omitted.

[0067] The above coating is performed via a solution-process-based layer-by-layer (LbL) method, and the carbon nanotube assembly can be coated with the surface of the separator membrane modified with oxygen-containing functional groups, particularly hydroxyl groups, by UV irradiation or plasma treatment. The amine group-containing linker initially coated within the carbon nanotube assembly can form hydrogen bonds with the separator membrane while simultaneously forming hydrogen bonds with adjacent carbon nanotubes. Subsequently, the amine group-containing linkers can be repeatedly stacked through hydrogen bonding between the adjacent carbon nanotubes. The number of layers of carbon nanotubes surface-modified with the carboxyl group can be viewed as the number of repetitions, and the number of repetitions is for controlling the thickness of the coated carbon nanotube assembly, and may be 1 to 20 times, preferably 2 to 10 times, and more preferably 2 to 4 times based on a comprehensive assessment of lithium affinity, ion diffusion coefficient, cycle stability, etc., but is not limited thereto.

[0069] lithium metal secondary battery

[0071] The present invention provides a lithium metal secondary battery comprising the above Janus separator.

[0073] Specifically, the lithium metal secondary battery according to the present invention comprises the Janus separator, a positive electrode and a negative electrode disposed between the Janus separator, and an electrolyte in contact with the positive electrode and the negative electrode.

[0074] In particular, the carbon nanotube assembly coated on one side of the separator in the above Janus separator forms a uniform nanoporous structure and can be coated in the direction of the lithium metal electrode, which is the negative electrode. This allows for the even dispersion of lithium ions to suppress the formation of lithium dendrites, as well as the control of the lithium growth direction. At this time, the lithium metal electrode has a theoretical capacity of 3840 mAh / g and a low reduction potential, thereby significantly increasing the theoretical storage capacity.

[0076] As described above, the Janus separator according to the present invention is characterized by comprising: a separator; and a carbon nanotube assembly coated on one surface of the separator and comprising carbon nanotubes surface-modified with an amine group-containing linker and a carboxyl group. Accordingly, when operating a lithium metal secondary battery with the Janus separator according to the present invention, it is possible to evenly disperse lithium ions to suppress the formation of lithium dendrites, and also control the direction of lithium growth, thereby having the advantage of ultimately securing high stability.

[0077] Therefore, the present invention can be utilized in various high-capacity secondary batteries requiring excellent driving stability and power density.

[0079] Preferred embodiments are presented below to aid in understanding the present invention. However, the following embodiments are provided merely to facilitate a better understanding of the invention, and the scope of the invention is not limited by the following embodiments.

[0081] [Example]

[0082] ingredient

[0083] Pristine multiwalled carbon nanotubes (MWCNT) and tris(2-aminoethyl)amine (TAA) were purchased from MERCK. The carboxylic acid (COOH) functionalization of the pristine MWCNTs was performed by oxidation with an H2SO4 / HNO3 mixture at 70°C for 2 hours. Subsequently, the resulting suspension was carefully washed using dialysis tubing to remove byproducts and residues. Organic solvents (ethanol, acetone) were purchased from Daejeong Hwakum (South Korea). Other chemical reagents were purchased from Sigma-Aldrich and used without further purification.

[0085] Fabrication of a Multiwalled Carbon Nanotube Forest (MWCF)

[0086] 1 mg mL each of the prepared COOH-MWCNT and NH2-TAA molecules -1It was dispersed in ethanol at the concentration of (NH2-TAA / COOH-MWCNT). n To construct a multilayer (MWCF), one side of a substrate (Celgard 2400 separator, quartz, or silicon wafer) was treated with UV-ozone for 5 minutes to introduce oxygen-containing functional groups onto the surface. Then, the substrate was first immersed in an NH2-TAA solution for 10 minutes to form an NH2-TAA layer through hydrogen bonding interactions, and then the weakly adsorbed TAA molecules were washed off with pure ethanol. Next, the NH2-TAA-coated substrate was immersed in a COOH-MWCNT solution for 10 minutes and then washed off with pure ethanol. This procedure was repeated to obtain an n-MWCF with a desired number of double layers (n).

[0088] LiFePO 4 (LFP) and LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) Anode Manufacturing

[0089] The anode fabrication involved mixing LFP or NMC811 powder with Super P carbon and PVDF binder in a weight ratio of 8:1:1. This mixture was dispersed in NMP to form a uniform slurry. Then, the slurry was coated onto carbon-coated aluminum foil (thickness 18 μm) using a doctor blade and dried overnight at 90°C. After the drying process, the anode sheet was pressurized at 20 MPa for 1 minute.

[0091] Battery Assembly and Electrochemical Measurement

[0092] Electrochemical measurements were performed using CR2032 type coin cells (MTI Corporation) and a WBCS3000 multichannel workstation. Cells were assembled in an argon-filled glove box (MBraun, O2 < 0.1 ppm, H2O < 0.1 ppm), using Li foil (thickness 35 μm) as the cathode, a prepared anode, and a Celgard 2400 separator or n-MWCF separator. For Li|Ni half-cells and Li|Li symmetric cells, the electrolyte used was 1 M LiTFSI in 1,2-dimethoxyethane (DME) / 1,3-dioxolane (DOL) (1:1 vol%) with 2 wt% LiNO3 added. For Li|NMC811 and Li|LFP cells, the electrolyte was 1 M LiPF6 in ethylene carbonate (EC) / diethyl carbonate (DEC) (1:1 vol%). For all coin cells, the electrolyte volume was fixed at 100 μL. Constant current charge-discharge tests for Li | NMC811 cells were performed at 0.1 C for the first two cycles in a potential range of 2.7–4.3 V, followed by subsequent cycles at various rates. Li | LFP cells were tested under similar conditions in the range of 3.5–4.0 V. The theoretical capacities of NMC811 and LFP used for C-rate calculations were 200 and 170 mAh g⁻¹, respectively. -1 It was. The Li|Ni battery has a cutoff voltage of 1.0 V and a voltage of 1 mA cm⁻¹. -2 Cycles were performed at a current density. Electrochemical impedance spectroscopy (EIS) measurements were performed in a frequency range of 100 kHz to 0.1 Hz with a perturbation amplitude of 0.01 mV. All electrochemical tests were performed at room temperature (25°C).

[0094] Assembly of pouch-type batteries

[0095] The pouch-type battery was assembled in an argon-filled glove box (MBraun, O2 < 0.1 ppm, H2O < 0.1 ppm). First, the Al and Ni tabs of the positive and negative electrodes were welded using an ultrasonic welder (GN-800, Gelon). After welding, a 20 μm thick Li-coated Cu foil (11 μm thickness), a 3-MWCF separator, and the prepared positive electrode were packaged into a laminate bag and filled with electrolyte. Then, the laminate bag was finally vacuum-sealed.

[0097] Specialization

[0098] High-resolution transmission electron microscopy (HR-TEM) was performed using a Technai 20 instrument (FEI). Field emission scanning electron microscopy (FE-SEM) was performed using a Hitachi S-4800 instrument. The adsorption behavior and surface functions of the LbL assembled multilayer were investigated using Fourier transform infrared (FTIR) spectroscopy. These analyses were performed at room temperature under ambient conditions at 4 cm -1 The analysis was performed using a CARY 600 spectrometer (Agilent Technologies) with the resolution [unclear]. Data obtained from 200 scans were processed using OMNIC software (Nicolet). The UV-vis spectra of the LbL assembled multilayer were measured using a Lambda 35 instrument (Perkin Elmer). X-ray photoelectron spectroscopy (XPS) was performed using an X-TOOL system (ULVAC-PHI). Quartz crystal microgravimetric analysis (QCM) was performed using a QCM200 (SRS) instrument. (NH2-TAA / COOH-MWCNT) n The mass of each layer of the multilayer (MWCF) was calculated from the frequency change during the LbL deposition process using the simplified Sauerbrey equation:

[0099] ΔF(Hz) = -56.6 × Δm

[0100] Here, ΔF and Δm are (NH2-TAA / COOH-MWCNT) n The frequency and mass changes for each layer of the multilayer are shown, respectively.

[0102] Calculation details

[0103] All calculations were performed using Q-Chem code version 5.4.0, and the B3LYP correlation function (a Becke 3-parameter hybrid function coupled with Lee-Yang-Parr) with a 6-311++G(d, p) base set was used. Celgard and H-terminated graphene molecules with or without carboxyl groups (-COOH) were modeled, and the convergence threshold was 10 -7 Structural optimization was performed using the Direct Inversion in Iterative Subspace (DIIS) algorithm. Using the optimized structure, various Li adsorption configurations and representative adsorption configurations were investigated.

[0105] Example 1: Preparation of Multilayer MWCF

[0106] To prepare lithium-affinity MWCFs, pristine hydrophobic MWCNTs were first converted into hydrophilic COOH-MWCNTs using the H2SO4 / HNO3 oxidation method to form COOH sites on the outer surface of the MWCNTs. The successful surface modification of the resulting COOH-MWCNTs was verified through high dispersion stability in ethanol and Fourier transform infrared (FTIR) spectroscopy (Fig. 2A). In this case, the COOH-MWCNTs maintained their original tubular properties without significant fragmentation after the given oxidation conditions (Fig. 2B).

[0107] Based on these results, COOH-MWCNT is NH2-functionalized TAA (abbreviated as NH2-TAA, molecular weight ~146 g mol) through hydrogen bonding interactions between the COOH portion of MWCNT and the NH2 portion of TAA in ethanol. -1) was sequentially assembled with the linker and LbL (Fig. 2C). The formed (NH2-TAA / COOH-MWCNT) n The FTIR spectra of the multilayer layers were each at 1704 cm⁻¹ -1 The carbonyl (C=O) stretching vibration of the COOH group and 1573 cm⁻¹ -1 A clear absorption peak originating from the NH bending vibration of the NH2 portion was observed, and the peak intensity of this characteristic vibration gradually increased as the number of bilayers (n) increased from 1 to 3 (Fig. 2D). Further verification of hydrogen bonding was performed by observing the formation of amide bonds after heat treatment, which was supported by density functional theory (DFT) calculations.

[0108] (NH2-TAA / COOH-MWCNT) n The adsorption behavior of the multilayer was also investigated using UV-vis spectroscopy (Fig. 2E). As the number of double layers (n) was increased from 1 to 10, the intensity of the absorption spectrum increased almost linearly. In this case, (NH2-TAA / COOH-MWCNT) n The total film thickness of the multilayer increased to approximately 198 nm (n = 10), and the thickness per bilayer was predicted to be approximately 19.8 nm (Fig. 2F). The average mass change per bilayer (Δm) was calculated from the frequency change (ΔF) using quartz crystal microbalance (QCM) measurements and was approximately 5.5 µg cm⁻¹. -2 It was predicted to be (ΔF ~ 312Hz) (Fig. 2G). In this case, the proportion of TAA within the multilayer is approximately 12.8% (i.e., ~0.7 µg cm⁻¹ per bilayer). -2It was merely ) which had the advantage of fully utilizing the properties of COOH-MWCNT, including its porous structure and electrical characteristics. Consequently, these observations clearly meant that the adsorption amount per double layer was nearly regular and could be precisely controlled by the number of double layers (n). Therefore, each component (i.e., COOH-MWCNT and NH2-TAA) could be evenly distributed throughout the entire area of ​​the resulting LbL assembled multilayer.

[0109] In particular, the oxygen atoms in the COOH groups of MWCNT and the nitrogen atoms in the NH2 groups of TAA are Li + Because it has a strong affinity for ions, (NH2-TAA / COOH-MWCNT) has a uniform structure n The multilayer (i.e., n-MWCF) served as a uniform active site for Li deposition during electrochemical operation. Since the LbL assembled multilayer consists solely of COOH-MWCNT and NH2-TAA molecular linkers without electrochemical or lithium-affinity inert components or bulky polymer linkers, this MWCF was expected to exhibit strong lithium-affinity, a highly uniform / nanoporous structure, and excellent electrical conductivity. These characteristics are highly advantageous for Li + Ions penetrated rapidly and uniformly into the MWCF and effectively inhibited the growth of irregular Li resin.

[0111] Example 2: Characterization of MWCF on the separator

[0112] To verify these possibilities, MWCF was deposited on one side of a UV-irradiated polypropylene (PP) separator (Celgard 2400) using hydrogen bond interaction-mediated LbL assembly. As shown in Fig. 3A, the surface coverage of MWCF on the separator gradually increased as the number of multilayer bilayers (n) increased from 0 to 5. In particular, 3-MWCF with a thickness of approximately 60 nm exhibited a surface coverage of over 90% on the separator and maintained a highly nanoporous structure well. Furthermore, the formed MWCF displayed a very smooth surface morphology with nanometer-scale roughness, indicating uniform deposition of the composition through the LbL assembly process. Thanks to these characteristics, the LbL-assembled MWCF could serve as a high-quality functional intermediate layer with a uniform structure between the separator and the LMA. In contrast, bulky polymer binder-based approaches faced difficulties in effectively utilizing the physical advantages of MWCNT due to pore clogging and non-uniformity, which degraded ion transport kinetics. Specifically, a key advantage of this approach is (NH2-TAA / COOH-MWCNT) n The multilayer structure had strong interfacial interactions (i.e., hydrogen bonding interactions) formed not only between the multilayer and the separator but also between adjacent COOH-MWCNT layers, resulting in a gapless interface with the separator. This phenomenon was particularly important because it effectively eliminated the possibility of needle-shaped lithium dendrites penetrating the potential gap between the separator and the MWCF.

[0113] Furthermore, it was also very important to consider the electrical conductivity of the MWCF coated on the separator in the LMB system. Specifically, when the lithium-affinity MWCF-coated separator was in close contact with the Li metal anode, the conductive MWCF acted as the top electrode of the LMA, enabling the effective and uniform distribution of current density from the separator to the Li metal anode. This implied that nanoporous MWCF with higher electrical conductivity is significantly more advantageous for fabricating high-performance LMBs with low overpotential and long-term stability. It is worth noting that low molecular weight (M w ~ 146) It was found that conductive MWCF assembled with NH2-TAA has distinct advantages over MWCF with insulating NH2-functionalized polymer linkers (e.g., NH2-poly(ethyleneimine), NH2-PEI). Fig. 3B shows (NH2-TAA / COOH-MWCNT) n - Coated separator and (NH2-PEI / COOH-MWCNT) n - The change in sheet resistance of the coated separator was shown as a function of the number of double layers (n). In this case, the sheet resistance value of the NH2-TAA-based separator was 1.6 × 10⁻⁶ for the NH2-PEI-based separator (1.6 × 10⁻⁶ for n = 3). 6 Ω sq -1 It was two steps lower than ) (for n = 3, 1.3 × 10 4 Ω sq -1These results clearly demonstrated that the small molecule linker used in this approach minimizes contact resistance between adjacent COOH-MWCNTs. Furthermore, the (NH2-TAA / COOH-MWCNT)3-coated separator (abbreviated as 3-MWCF separator) exhibited excellent electrical stability even under repeated bending cycles (σ / σ0 = 100% after 10,000 bending cycles) due to strong interfacial interactions between all components (i.e., NH2-TAA, COOH-MWCNT, and separator) (Fig. 3C). Since the role of the MWCF on the separator is to ensure a uniform charge distribution in the space between the LMA and the separator, a gapless interface between the conductive MWCF and the LMA, and between adjacent MWCNTs, was very beneficial for improving the performance of the LMA.

[0114] Furthermore, the well-distributed functional groups within the MWCF were able to ensure good wettability of electrolytes composed of polar solvents (typically containing oxygen molecules) common in LIB applications. This was confirmed by monitoring the contact angle of the electrolyte solution (1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a 1:1 volume ratio dimethyl ether (DME) / 1,4-dioxane (DIOX) mixture) on the n-MWCF separator (Fig. 3D). As the number of double layers (n) of the n-MWCF separator increased from 1 to 5, the electrolyte contact angle decreased significantly, ultimately reaching complete wettability in the 3-MWCF separator. In addition to this good electrolyte wettability, it should be noted that the MWCF with a 3D structure containing numerous nanopores can serve as an efficient electrolyte reservoir. When the electrolyte absorption weight of the n-MWCF membrane was calculated using the following equation (1), it increased from 150% for the bare membrane to 311% for the 5-MWCF membrane (Fig. 3E).

[0115] (1)

[0116] Here, W o and Wt represents the weight of the n-MWCF separator before and after immersion in the electrolyte, respectively. In this case, the electrolyte absorption capacity of the MWCF continuously increases with the number of double layers, indicating that the specific surface area is precisely controlled through the LbL assembly process. These results suggest that the n-MWCF separator has excellent electrolyte compatibility, which is very advantageous for reducing internal ionic resistance and achieving long-term cycle maintenance.

[0117] In addition, density functional theory (DFT) calculations were used to gain insight into the influence of introduced functional groups on lithium dendrite formation. Based on experimental evidence regarding the impact of the surface of each configuration on electrolyte compatibility, the characteristics of five representative structures—bare PP separator, NH2-TAA, pristine MWCNT, COOH-MWCNT, and MWCF (NH2-TAA / COOH-MWCNT multilayer)—were compared (Fig. 3F). First, an electrostatic potential (ESP) map was constructed to visualize the charge density distribution (Fig. 3F). The bare separator exhibited a nearly neutral charge, whereas the pristine MWCNT, lacking carboxyl groups, showed a slight negative charge close to neutral. In contrast, the COOH-MWCNT exhibited a significant negative charge, which was localized particularly around the oxygen atoms of the carboxyl groups. Notably, increasing the oxidation time of COOH-MWCNT could generate more lithium-affinity carboxyl groups, but it reduced electrical conductivity and increased the internal resistance of the battery, thereby lowering cycle stability. Additionally, negative charges overlapped due to the complementary interaction between the amine group of NH2-TAA and the oxygen of the carboxyl group. This visualization highlighted potential adsorption sites for lithium atoms.

[0118] Furthermore, a comparative analysis of the adsorption energies of Li atoms revealed distinct differences among the materials. Specifically, the Bare membrane, NH2-TAA, and MWCNT exhibited weak adsorption energies of -0.01 eV, -0.68 eV, and -0.82 eV, respectively (Fig. 3G). In contrast, materials containing oxygen functional groups with significant negative charges showed stronger adsorption energies, with COOH-MWCNT exhibiting -2.73 eV and MWCF exhibiting -3.11 eV, respectively. These results demonstrated that NH2-TAA not only acts as a linker but also provides additional advantageous adsorption sites, effectively controlling the charge distribution for uniform Li deposition. Additionally, since the atomic charge of Li tends to increase with the adsorption energy of the functional site, the adsorption of Li atoms involves negatively charged functional sites and positively charged Li. + It was indicated that it can be driven by strong electrostatic interactions between ions. Therefore, DFT calculations suggested that MWCFs with strong and uniform co-adsorption sites for Li atoms can significantly mitigate local current density and effectively prevent significant dendrite formation. Based on these results, this approach suggests the possibility of accurately designing and controlling the function of n-MWCF separators without incorporating a lithium-affinity inert configuration through hydrogen bond-mediated LbL assembly between lithium-affinity COOH-MWCNTs and lithium-affinity small molecule linkers (NH2-TAA).

[0120] Example 3: Electrochemical properties of n-MWCF separator in half-cell configuration

[0121] To systematically evaluate the effect of lithium-affinity MWCFs on lithium dendrite growth behavior during the Li plating and stripping processes, Li|Ni half-cells were assembled using n-MWCF separators with various double-layer numbers (n). As previously confirmed in Fig. 3, the electrolyte wettability and absorption capacity of the MWCFs clearly differed depending on the number of double layers (or configurations), each inducing a thickness change of approximately 20 nm. Furthermore, this implied that the electrochemical performance of n-MWCF separator-based batteries could be effectively controlled and optimized through the LbL assembly method. Additionally, the lithium-affinity sites in the LbL-assembled MWCFs significantly reduced the energy barrier during the initial Li nucleation phase, promoting uniform Li deposition. As shown in Fig. 4A, half-cells with n-MWCF separators exhibited relatively lower nucleation overpotential values ​​compared to bare separator-based batteries. Specifically, when the number of double layers (n) is increased from 0 (bare separator) to 5 (5-MWCF separator), the overvoltage value is at a current density of 0.5 mA cm⁻¹ -2 It decreased significantly from 97mV to 44mV. This showed that there is a strong correlation between the number of double layers (n) of MWCF and its lithium affinity.

[0122] Electrochemical impedance spectroscopy (EIS) tests on the internal resistance and ions (Li) at the interface of n-MWCF membrane-based systems + ) Further analysis of transport kinetics was performed at room temperature (Fig. 4B). In this case, the charge transfer resistance (R ct The value is 126 Ω cm at 244 (for the bare membrane). 2 (In the case of a 101 nm thick 5-MWCF separator) it decreased sharply, and the equivalent series resistance (R s The value is also 7.8 to 3.8 Ω cm 2 It decreased to . Generally, R ct The value is Li in the electrolyte that receives electrons from the electrode. +The process of deposition by desolvating ions was incorporated. R of the n-MWCF separator-based battery ct The lower the value, the more Li + This indicates that ion transport is easier, suggesting that less activation energy is required to overcome barriers and undergo the deposition process.

[0123] These results were consistent with those presented in Fig. 4A. Interestingly, the Warburg slope of the 5-MWCF separator-based battery decreased slightly compared to the 3-MWCF separator-based battery, indicating an increase in resistance to ion diffusion. This phenomenon became more evident by calculating the diffusion coefficient derived from the measured Warburg impedance (Fig. 4C). Calculated diffusion coefficient (D Li+ The value showed an upward trend as the number of double layers (n) increased from 0 to 3, but gradually decreased as the number of double layers (n) further increased from 4 to 7. It was well known that various electrode factors, including electrical conductivity, thickness, porosity, and surface area, are closely linked to the ion diffusion rate of electrochemical energy storage systems, causing lithium dendrite growth and affecting overall performance. In the present invention, the n-MWCF assembled with LbL on the separator showed that the thickness and surface area increased as the number of double layers (n) increased, while the pore size gradually decreased, which was confirmed by a Brunauer-Emmett-Teller (BET) analyzer.

[0124] To more clearly evaluate the effect of the n-MWCF separator on the electrochemical performance of the LMB, 1 mA cm⁻¹ for Li|Ni half-cell configurations according to the number of double layers (n) -2 and 1 mAh cm -2Coulomb efficiency (CE) was monitored during Li plating / stripping cycles (Fig. 4D). Initially, the CE of the bare separator-based battery decreased sharply after 70 cycles, indicating that lithium dendrites grew uncontrollably and rapidly, leading to a short circuit. However, as the number of double layers (n) of the MWCF separator increased to 3, cycle stability improved significantly, exhibiting a high CE of ~97.2% after 190 cycles. Notably, as the number of double layers further increased from 3 to 5, this improved stability decreased again, consistent with the trend observed in ion diffusion kinetics according to the number of double layers. Indeed, the behavior of Li plating on the electrode surface is significantly influenced by ion diffusion conditions at the electrode / electrolyte interface, which was also directly related to dendrite growth. Therefore, these observations suggest that the 3-MWCF separator can provide an optimal structure for effectively suppressing indiscriminate lithium dendrite growth during repeated Li plating / stripping cycles. Consequently, bare separator-based Li | The Ni half-cell exhibited typical needle-shaped lithium dendrites on the Ni plate after cycling, causing severe short circuits (Fig. 4E). In contrast, the 3-MWCF separator-based cell showed a mossy-like deposition of Li, as shown in Fig. 4F, which indicated well-controlled current flow through the electrode surface (i.e., the Ni plate). In particular, the enhanced ion diffusion characteristics of the n-MWCF separator-based cell alleviated the diffusion limit at the electrode / electrolyte interface, effectively maintaining an appropriate ion concentration to form a mossy Li layer during long-term cycling. Thus, the small molecule linker (i.e., TAA)-mediated LbL assembly could precisely tune the function of the MWCF interlayer at the nanometer scale and facilitate charge transfer by effectively maintaining the highly porous structure and electrical conductivity of the MWCNT itself.In this context, LbL assembled conductive MWCFs presented a promising alternative to insulating polymer binder-based composite interlayers. In particular, conventional simple mechanical mixing (i.e., blending) approaches, which lacked sufficient interaction between the individual components, could result in reduced charge conductivity due to partial blockage of nanopores. In addition to providing a highly porous network for efficient ion diffusion channels, the conductive MWCF interlayer, possessing rich lithium affinity, served as a robust and effective host material for stable lithium plating. Specifically, the high surface coating of MWCF on the separator facilitated the sequential deposition of Li at the MWCF / separator interface, accompanied by the formation of a gapless interface with favorable complementary interactions. This induced Li growth from the MWCF separator to the LMA surface, effectively preventing the penetration of lithium dendrites through the separator (Fig. 4G).

[0126] Example 4: n-MWCF separator-based symmetrical battery

[0127] To accurately analyze the effect of the MWCF separator on Li deposition behavior during constant current cycling, Li | Li symmetrical cell tests were performed. In these tests, the MWCF intermediate layer of the separator was maintained facing the working electrode (cathode). 3 mA cm⁻¹ -2After 400 Li plating / stripping cycles, the battery assembled with a bare separator exhibited typical rough dendrite growth on the surface of the Li metal anode (Fig. 5A). In contrast, the Li-symmetric battery based on a 3-MWCF separator showed very smooth and uniform Li deposition with a mossy-like morphology on the Li metal surface, which was consistent with what was observed in the half-cell configuration (Fig. 5B) and ensured strong adhesion to the separator. Importantly, the 3-MWCF, which covers the separator with a gapless interface, allows for dense Li deposition and induces Li metal growth toward the anode surface, effectively suppressing penetration into the bare separator. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis confirmed that the SEI layer on the 3-MWCF separator remained stable even after 400 cycles, indicating that the lithium plating was uniform and well controlled without severe morphological cracking.

[0128] As mentioned earlier, ion conduction behavior at the interface plays a crucial role in achieving stable Li plating, which affects the ion conductivity of the battery and Li + It was reflected in the ion transfer rate. To evaluate these factors, EIS and DC polarization analyses were performed at room temperature on Li-symmetric cells using various separators (Fig. 5C). In this case, R of each cell ct The value ranges from 391.2 to 141.8 Ω cm as the number of double layers (n) increases from 0 to 5. 2 It decreased significantly (Fig. 5C). This trend was consistent with the improvement in electrical conductivity at the MWCF interface as the number of double layers increased. However, in 5-MWCF separator-based symmetric cells, R ct Despite the decrease in value, ionic conductivity and Li +It was notable that the number of transfers was lower compared to the 3-MWCF separator-based battery (Fig. 5D). These observations were consistent with the trends observed in the ion diffusion coefficient and CE values ​​of the half-cell test. Specifically, increasing the number of double layers (n) consistently improved the electrical conductivity and lithium affinity of the MWCF intermediate layer, but the ion diffusion kinetics decreased due to changes in pore size and thickness, as observed in the 5-MWCF separator-based half-cell (see Fig. 4C). As a result, the 3-MWCF separator-based symmetrical battery exhibited significantly higher ion conductivity (0.46 mS cm⁻¹) compared to other test cells. -1 ) and Li + The number of transmissions (0.76) was shown.

[0129] The excellent charge diffusion characteristics of the 3-MWCF separator ensured a uniform ion distribution at the interface across a wide range of current densities, thereby promoting electrochemical reactions while effectively suppressing dendrite Li deposition. This feature can be described by "sand capacity," which refers to the maximum capacity at which mossy Li can form without deforming into a dendrite structure. This critical capacity is calculated by multiplying the sand time by the current density. In particular, the sand time refers to the period from the initial deposition of mossy Li to the electrode interface where cations (i.e., Li + It is defined as the period until the concentration of ions decreases to zero. This relationship can be expressed by the following equation (2):

[0130] (2)

[0131] Here, z c is the charge number of the cation (Li + In the case of z c = 1), c0 is the bulk salt concentration, F is the Faraday constant, J is the current density, t a =1 - t Li is the transition number of the related anion, D app is the apparent diffusion coefficient of the electrolyte.

[0132] According to these calculations, the Li | Li symmetric cell using a 3-MWCF separator is C Sand Based on the relationship = 35.2 / J, the largest predicted region for mossy Li formation was presented. In addition, the interfacial stability and cycling reversibility of the n-MWCF separator-based Li-symmetric battery were investigated. To this end, 1 mAh cm⁻¹ -2 The voltage profile of the device was monitored during long-term discharge / charge cycles at various current densities while maintaining a constant capacity (Figs. 5E and 5F). The voltage curves of all Li | Li symmetric cells using n-MWCF separators are 1 mA cm⁻¹ 2 (1 mAh cm -2 Less than) and 3 mA cm -2 (1 mAh cm -2 All (less than) exhibited fluctuations during the initial 200–300 hours of cycling. This behavior may have been due to the formation of an unstable interface between the MWCF-coated separator and the electrolyte during the early stages of cycling. In all cases, Li-symmetric cells using bare separators showed a limited lifespan of less than 200 cycles and were accompanied by irregular voltage hysteresis. On the other hand, the use of n-MWCF separators significantly reduced the lifespan of symmetric cells and increased overvoltage. In particular, 3-MWCF separator-based cells exhibited 1 mA cm⁻¹ -2 At 5,000 cycles, 3 mA cm -2 It demonstrated excellent cycle stability for 10,500 cycles, and exhibited smooth and stable voltage flatness with low voltage hysteresis of less than ~28.3 mV. Furthermore, this stable cycling behavior supported current density and capacity up to 10 mA cm⁻¹. -2 and 10 mAh cm -2 Even if it increases up to, it persists, and the excellent interfacial stability of the 3-MWCF interlayer and Li +The ion transport capability was clearly demonstrated. The unprecedented high operational stability shown by the 3-MWCF separator-based Li symmetrical battery was significantly superior to that of previously reported functional separator-based symmetrical Li batteries (Fig. 5G). Importantly, the small molecule linker (TAA)-mediated LbL design according to the present invention for the lithium-affinity MWCNT interlayer effectively optimized the interfacial structure, enabling excellent charge conduction at various current densities, thereby outperforming conventional slurry- or polymer linker-based approaches.

[0134] Example 5: Characterization of an n-MWCF separator-based asymmetric complete cell

[0135] To further demonstrate the potential of MWCF membranes for practical LMB systems, commercially available LiNi 0.8 Mn 0.1 Co 0.1 An asymmetric complete cell was fabricated using O2 (NMC811) as the cathode material, and its electrochemical performance was characterized. 1.1 mg cm⁻¹ at 1 C -2 The cycle performance of a 3-MWCF separator-based complete cell (i.e., Li | 3-MWCF separator | NMC811) was investigated with a loading amount. Surprisingly, it showed a capacity retention rate of 81.9% and a CE of 99.9% even after 600 cycles, which was superior to the performance of a bare separator-based cell that decomposed rapidly after 320 cycles (Fig. 6A).

[0136] To further evaluate the effect of the MWCF interlayer on the separator, the rate performance of a 3-MWCF separator-based complete cell, which delivers higher capacity over a wide current rate range compared to a bare separator-based complete cell, was investigated (Fig. 6B). In particular, the 3-MWCF separator-based complete cell showed 191.7 mAh g at 0.1 C. -1 , 145.9 mAh g at 2 C -1It exhibited the highest specific capacity, which was 118% of that of the bare separator-based cell despite loading the same amount of active material. This enhanced rate performance implied that the LbL assembled MWCF interlayer effectively overcame kinetic barriers and mass transfer limitations at the interface during high-speed operation, resulting in a significant reduction in overpotential (0.06 V) compared to the bare separator-based cell (~0.3 V). This promising performance of the 3-MWCF separator-based complete cell was further validated by EIS measurements, which compared to 163.4 Ω cm⁻¹ of the bare separator-based cell. 2 59.6 Ω cm, significantly lower compared to 2 of R ct It showed values. These results indicate that the 3-MWCF interlayer is uniform Li + Promotes ion deposition and Li + It was clearly demonstrated that improving ion mobility effectively reduces interfacial resistance within the battery.

[0137] The important point is that realizing high-energy LMBs in practical applications requires considering several challenging factors, including a low cathode-to-anode capacity ratio (N / P ratio ≤ 2), high anode loading, and reduced electrolyte content. To clarify these critical issues, 21.5 mg cm⁻¹ -2 Long-term cycling tests were additionally performed on full cells with high anode (NMC811) loading and a low N / P ratio of ~1.5 (Fig. 6C). In all cases, 0.4 mA cm⁻¹ was used for a stable SEI layer. -2 After performing formation cycling at (0.1 C), 2 mA cm -2Cycling was performed at (0.5 C). As shown in Fig. 6C, the 3-MWCF separator-based complete cell exhibited an excellent capacity retention rate of 84.5% at 150 cycles, maintaining nearly 100%. Subsequently, the capacity dropped significantly after the 200th cycle and was accompanied by unstable CE, which may have been due to severe lithium loss. In contrast, the cell with a bare separator experienced significant capacity degradation after only 20 cycles, exhibited poor functionality, and the CE value dropped sharply at the 77th cycle. Furthermore, the 3-MWCF separator-based complete cell demonstrated significantly better rate performance compared to the bare separator-based cell, suggesting that the enhanced charge conductivity characteristics of the 3-MWCF interlayer could be particularly beneficial in mass-loaded applications (Fig. 6D). Consequently, the 3-MWCF separator-based complete cell had a capacity of 678 Wh / kg based on the total weight of the active material. -1 and 347 W / kg -1 It delivered maximum energy and power densities, demonstrating significantly superior performance compared to previously reported functional separator-based batteries and bare separator-based batteries (inset in Fig. 6D). In particular, the superior performance of the 3-MWCF separator-based battery remained excellent even when calculating energy and power densities including the weight of the inactive components (i.e., electrolyte, Al current collector, separator, and MWCF interlayer) and active materials. Fig. 6E summarizes the representative characteristics of the 3-MWCF separator-based full battery for comparison with bare separator-based batteries. These results clearly demonstrated the stable performance of the MWCF interlayer in controlling uniform current flux under rigorous real-world conditions.

[0138] To expand the applicability of MWCF separators to industrial demand, 2 mAh cm -2A pouch-type battery of 6 mAh was fabricated using an NMC811 cathode with an area capacity of (N / P ratio ~2) and its electrochemical performance was investigated. Notably, the pouch-type battery based on a 3-MWCF separator achieved an excellent capacity retention rate of 99.85% per cycle over 100 cycles (Fig. 6F).

[0139] To further demonstrate the versatility of the MWCF separator, it was paired with an LFP anode to form a Li | 3-MWCF separator | LFP complete cell. As shown in Fig. 6G, 3.3 mg cm⁻¹ -2 LFP cells based on MWCF separators with a loading of [value] maintained an impressive capacity retention rate of 107% even after 1,500 cycles at 1C. The gradual increase in capacitance observed during the first 100 cycles compared to the initial value appeared to be attributed to the expanded interface area between the MWCF-coated separator, LFP, and electrolyte, as well as improved system activation through repeated cycles. In contrast, LFP cells using a bare separator with the same loading experienced a gradual decrease in capacity after 200 charge / discharge cycles. Additionally, 20.6 mg cm⁻¹ -2 MWCF separator-based LFP cells with a higher anode loading showed a similar trend, and 2 mA cm -2It exhibited a capacity retention rate of 96.9% after 120 cycles (N / P ratio ~2). Furthermore, the overvoltage for the MWCF separator-based LFP cell at the 3rd and 80th cycles was 0.16 V and 0.23 V, respectively, which was significantly lower than that of the bare separator-based LFP cell (0.43 V at the 3rd cycle and 0.6 V at the 80th cycle). These results indicated that the LbL-assembled MWCF intermediate layer effectively promotes mass transfer within the cell and prevents the formation of lithium dendrites. This is further supported by XPS analysis, which showed that the lithium metal anode paired with a 3-MWCF separator exhibited reduced intensity for LiF and LixPOyFz compared to the bare separator. This reduction suggested that a thinner and more uniform SEI layer was formed, attributed to the uniform Li ion flux enabled by the 3-MWCF separator. Based on these promising performance results of the MWCF interlayer, 10.5 mg cm -2 A Li | 3-MWCF separator | LFP pouch-type battery with a loading amount and a total capacity of 5 mAh (N / P ratio ~2.3) has a load of 2 mA cm⁻¹. -2 It showed an excellent capacity retention rate of 99.8% per cycle.

[0141] The inventors developed a high-performance LMB utilizing an ultrathin MWCF interlayer assembled at the interface with the separator, thereby effectively suppressing and preventing the growth of needle-shaped lithium dendrites during repetitive lithium plating and stripping on the LMA, and achieving excellent operational stability. The MWCF interlayer was LbL assembled solely with lithium-affinity COOH-MWCNT and NH2-functionalized molecular linkers (TAAs), utilizing well-defined complementary interfacial interactions without incorporating electrochemically inert components. This unique approach significantly improved charge conductivity characteristics at the interface while maintaining robust structural integrity, facilitated uniform current distribution on the LMA surface, and provided sufficient pore space for stable lithium plating. In particular, LbL assembly mediated by small molecular linkers allowed for precise control of the chemical and physical functions of the MWCF interlayer at the nanometer scale, resulting in low internal resistance and high energy efficiency even during high-speed operation.

[0142] Based on this approach, an optimized 3-MWCF separator-based symmetric cell (i.e., Li | 3-MWCF separator | Li symmetric cell) yields 3 mA cm⁻¹ -2 It demonstrated unprecedentedly high cycle stability of 10,500 cycles at a current density, and the overvoltage was significantly reduced to 28.3 mV, surpassing the stability performance of symmetrical batteries manufactured by the conventional slurry casting method. Furthermore, the asymmetrical complete battery employing the NMC811 cathode exhibited a maximum energy density of 678 Wh kg⁻¹. -1 And it achieved an excellent capacity retention rate of 81.9% even after 600 cycles. In particular, the 6 mAh pouch-type battery achieved ~99.85% per cycle (~2 mAh cm⁻¹). -2The area capacity of the [unclear] was maintained stably. Considering that this approach can precisely control lithium dendrite growth through multilayer functionality and structural design, this can provide a basis for the development and design of high-performance LMBs that enable very long operational stability.

[0144] The foregoing description of the present invention is for illustrative purposes only, and those skilled in the art will understand that other specific forms can be easily modified without altering the technical spirit or essential features of the present invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims

Claim 1 A Janus separator for a lithium metal secondary battery, characterized by comprising: a separator; and a binder-free carbon nanotube assembly coated on one surface of the separator via layer-by-layer (LbL) method and comprising carbon nanotubes surface-modified with two or more amine group-containing linkers and carboxyl groups, wherein the amine group-containing linkers and the carbon nanotubes surface-modified with carboxyl groups are repeatedly stacked within the carbon nanotube assembly through hydrogen bonding. Claim 2 Janus separator for a lithium metal secondary battery, characterized in that, in claim 1, the separator is made of a polyolefin-based polymer material. Claim 3 A Janus separator for a lithium metal secondary battery according to claim 1, characterized in that the surface of the separator is modified to an oxygen-containing functional group by UV irradiation or plasma treatment. Claim 4 delete Claim 5 In claim 1, the weight-average molecular weight of the amine group-containing linker is 30 g mol -1 to 5,000 g mol -1 Janus separator for lithium metal secondary batteries, characterized by being Claim 6 A Janus separator for a lithium metal secondary battery according to claim 1, characterized in that the amine group-containing linker comprises one or more selected from the group consisting of hydrazine, diethylenetriamine, tris(2-aminoethyl)amine, and polyethyleneimine. Claim 7 delete Claim 8 A Janus separator for a lithium metal secondary battery according to claim 1, wherein the carbon nanotube assembly is coated on one surface of the separator and faces toward the lithium metal electrode. Claim 9 delete Claim 10 A Janus separator for a lithium metal secondary battery according to claim 1, characterized in that the carbon nanotube assembly has a contact angle of 30° or less with respect to an electrolyte comprising a mixed solvent of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), dimethyl ether (DME), and 1,4-dioxane (DIOX). Claim 11 A method for manufacturing a Janus separator for a lithium metal secondary battery, comprising: (a) a step of surface-modifying carbon nanotubes with carboxyl groups; and (b) a step of coating a binder-free carbon nanotube assembly comprising two or more amine group-containing linkers and carbon nanotubes surface-modified with carboxyl groups on one surface of a separator via layer-by-layer (LbL) assembly, wherein in step (b), the amine group-containing linkers and carbon nanotubes surface-modified with carboxyl groups are repeatedly stacked within the carbon nanotube assembly through hydrogen bonding. Claim 12 A lithium metal secondary battery comprising a Janus separator according to claim 1.

Images