Lithiophilic carbon nanotube assembly-based janus separator, method for manufacturing same, and lithium metal secondary battery comprising same

Abstract

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

Description

Lithium-affinity carbon nanotube assembly-based Janus separator, method for manufacturing the same, and lithium metal secondary battery including the same

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

[0002] As demand for portable electronic devices, large-capacity storage devices, and electric vehicles increases, the need for development of high-capacity secondary batteries is increasing.

[0003] Recently, lithium metal electrodes with a theoretical capacity of 3840 mAh / g and a low reduction potential are attracting attention as high-capacity secondary battery anode materials that can replace existing graphite (372 mAh / g), and as next-generation secondary battery anode materials along with lithium-sulfur secondary batteries and lithium-air secondary batteries.

[0004] However, lithium metal electrodes have a problem in that when the secondary battery is charged and discharged, lithium dendrites form sharply, repeatedly breaking and regenerating the SEI (Solid Electrolyte Interphase) layer, resulting in the loss of unnecessary lithium ions and charges. Furthermore, if the lithium dendrites continue to grow and penetrate the separator and come into contact with the opposite electrode, a short circuit may occur, potentially leading to an explosion.

[0005] Accordingly, active research is underway to coat separators with functional materials to improve the stability of lithium metal secondary batteries. However, strategies are needed to enhance the stability of lithium metal secondary batteries, including uniform lithium ion transport, enhanced lithium ion selectivity, and controlled lithium growth direction.

[0006] The present invention is intended to provide a Janus separator for a lithium metal secondary battery, which comprises a separator for suppressing lithium dendrite formation and controlling lithium growth direction, and a carbon nanotube assembly coated on one surface of the separator and including carbon nanotubes surface-modified with carboxyl groups and a linker containing amine groups.

[0007] However, the technical problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

[0008] 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 including carbon nanotubes surface-modified with an amine group-containing linker and a carboxyl group.

[0009] The above separator may be made of a polyolefin-based polymer material.

[0010] The surface of the above membrane can be modified with oxygen-containing functional groups by UV irradiation or plasma treatment.

[0011] The above amine group-containing linker may contain two or more amine groups.

[0012] The weight average molecular weight of the above amine group-containing linker is 30 g mol -1 5,000 g mol -1 It could be.

[0013] The above amine group-containing linker may include at least one selected from the group consisting of hydrazine, diethylenetriamine, tris(2-aminoethyl)amine, and polyethyleneimine.

[0014] The above carbon nanotube assembly may be in a binder-free form.

[0015] The above carbon nanotube assembly may be coated on one surface of the separator and may face toward the lithium metal electrode.

[0016] Within the above carbon nanotube assembly, carbon nanotubes surface-modified with an amine group-containing linker and a carboxyl group can be repeatedly stacked through hydrogen bonding.

[0017] The above carbon nanotube assembly may have a contact angle of 30° or less for an electrolyte containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and a mixed solvent of dimethyl ether (DME) and 1,4-dioxane (DIOX).

[0018] As one embodiment of the present invention, a method for manufacturing a Janus separator for a lithium metal secondary battery is provided, comprising the steps of: (a) surface-modifying a carbon nanotube with a carboxyl group; and (b) coating a carbon nanotube assembly comprising an amine group-containing linker and the carbon nanotube surface-modified with the carboxyl group on one surface of the separator.

[0019] In another embodiment of the present invention, a lithium metal secondary battery including the Janus separator is provided.

[0020] The Janus separator according to the present invention is characterized by including a separator; and a carbon nanotube assembly coated on one surface of the separator and including carbon nanotubes surface-modified with an amine group-containing linker and a carboxyl group. Accordingly, when a lithium metal secondary battery using the Janus separator according to the present invention is operated, not only can lithium ions be evenly dispersed to suppress lithium dendrite formation, but also the direction of lithium growth can be controlled, thereby ultimately securing high stability.

[0021] Therefore, the present invention can be applied to various high-capacity secondary batteries that require excellent driving stability and power density.

[0022] Figure 1 shows (A) a schematic description for the fabrication of n-MWCF separators and (B) a comparison of lithium growth behavior according to the separators in repeated constant current lithium plating.

[0023] Figure 2 illustrates the fabrication of n-MWCNTs: (A) Digital images of pristine (left) and acid-treated MWCNTs (right) dispersed in ethanol. (B) FE-TEM image of COOH-MWCNTs. (C) Diagram showing the LbL assembly of COOH-MWCNTs and NH2-TAA via specific interactions. (D) (NH2-TAA / COOH-MWCNT) as a function of the number of bilayers (n). n Changes in the FTIR spectra of multilayers. (E) (NH2-TAA / COOH-MWCNT) as a function of the number of bilayers (n). n Variation of UV-vis spectra of multilayers. The inset shows the UV-vis absorbance at 233 nm as a function of the number of bilayers (n). (F) (NH2-TAA / COOH-MWCNT) n Film thickness as a function of the number of bilayers (n). The inset shows a cross-sectional FE-SEM image. (G) (NH2-TAA / COOH-MWCNT) with different bilayer numbers (n). n Multilayer QCM results.

[0024] Figure 3 shows the characterization of n-MWCF: (A) FE-SEM images of n-MWCF membranes with different bilayer numbers (n) of 0 (bare membrane), 1, 3, and 5. The insets show the corresponding digital images of each n-MWCF membrane. (B) Sheet resistance (Ω sq) as a function of bilayer number (n) of n-MWCF membranes with different linkers, NH2-TAA (red circles) and NH2-PEI (blue circles). -1) change. (C) Electrical conductivity (σ / σ0) change of 3-MWCF membrane at 10,000 bending cycles with a bending radius of 2.5 mm (inset). (D) Wettability test of electrolyte (LiTFSI in DME / DIOX mixture) on n-MWCF membrane through contact angle change with different double layer number (n). (E) Electrolyte absorption capacity of n-MWCF membrane. (F) Electrostatic potential maps for bare PP membrane, pristine MWCNT, COOH-MWCNT, and MWCF, respectively. (G) Li + Trends in adsorption energies for ions.

[0025] Figure 4 shows the electrochemical characteristics of the n-MWCF membrane-based half-cell: (A) 0.5 mA cm -2 Voltage (V) versus time (sec) curves of n-MWCF membrane-based Li|Ni cells during Li nucleation. (B), (C) Nyquist plots and corresponding ion diffusion coefficients (D) of n-MWCF membrane-based Li|Ni cells. Li+ ). (D) 1 mA cm -2 and 1 mAh cm -2 Coulombic efficiency (CE) of n-MWCF-based half-cells during repeated Li plating / stripping cycles. (E) FE-SEM images of the Ni plate surface of Li|Ni cells with bare separator (left) and 3-MWCF separator (right) after cycling. (G) Effect of the n-MWCF interlayer on the Li plating mechanism in the LMB system.

[0026] Figure 5 shows the symmetric cell tests: Comparison of Li plating behavior of Li|Li symmetric cells using (A) bare separators and (B) 3-MWCF separators. FE-SEM images show the surface morphology of each separator and Li anode. The inset shows a digital image of a disassembled Li anode. (C) Nyquist plots and representative equivalent circuits of Li symmetric cells based on n-MWCF separators with different double layer numbers (n). (D) Ionic conductivity and Li+ transfer number of Li symmetric cells based on n-MWCF separators. (E) 1 mA cm -2 (1 mAh cm -2 ) and (F) 3 mA cm -2 (1 mAh cm -2 ) Galvanostatic cycling of Li symmetric cells with each separator. (G) Cycling stability of Li symmetric cells based on 3-MWCF separator and comparison with previous studies on functional separators.

[0027] Figure 6 shows the performance of asymmetric full batteries with n-MWCF separators: (A) Cycling tests of Li|NMC811 full batteries with bare and 3-MWCF separators. (B) Rate-performance tests of Li|NMC811 full batteries with bare and 3-MWCF separators. (C) 21.5 mg cm -2 Cycling test of Li|NMC811 full cell with bare and 3-MWCF separator under anodic loading of (N / P ratio ~ 1.5). Test was performed at 0.4 mA cm -2 After pre-cycling at 2 mA cm -2 was performed at (D) 21.5 mg cm -2Current density-dependent discharge profiles of Li|NMC811 full cells with anode loadings of 10.5 mg cm. The insets show the energy and power densities at various current densities. (E) Spider charts of Li|NMC811 full cells with different separators to compare various performance parameters. (F) Li|10.5 mg cm -2 NMC811 pouch cell with positive loading of 1 mA cm -2 Tested in. The inset shows a digital image of the assembled pouch cell. (G) Cycling test of Li|LFP full cell with bare and 3-MWCF separator.

[0028] Lithium metal is a promising high-energy battery anode. However, the growth of lithium dendrites during charge / discharge cycling poses safety and durability issues. Here, we introduce a novel approach to control lithium dendrite growth using layer-by-layer (LbL)-assembled multi-walled carbon nanotube forests (MWCFs) composed solely of a lithium-affinity configuration (carboxylic acid-functionalized MWCNTs / amine-functionalized linkers) without an inert binder. The 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, alleviate local current density, and provide pore space for uniform lithium plating. Importantly, the MWCFs guide Li growth onto the Li metal anode and prevent dendrite penetration into the anode, resulting in a Li|Li symmetric battery with a long-life of 10,000 h (1 mA cm -2 ) can achieve excellent stability during cycling. Surprisingly, the NMC811-based asymmetric battery maintains ~81.9% capacity retention and 678 Wh kg after 600 cycles at 1 C. -1 It can achieve ultra-high energy density. In addition, LFP-based asymmetric batteries can exhibit excellent cycle stability.

[0029]

[0030] The present inventors designed a novel lithium-affinity molecular linker-mediated layer-by-layer (LbL) assembly method on a membrane, with a thickness of about 62 nm and a mass of 16.5 μg cm. -2 We propose a lithium-affinity multi-walled carbon nanotube forest (MWCF) without ultrathin electrochemically inactive components (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 exploiting the nanometer-scale controlled complementary interactions between the lithium-affinity COOH-functionalized MWCNTs (COOH-MWCNTs) and the lithium-affinity NH2-functionalized small molecule linker (tris(2-aminoethyl)amine, TAA), and between the NH2-TAA and the separator, this approach can effectively control lithium dendrite growth on the LMA surface, resulting in unprecedented operational stability. In particular, the lithiophilic NH2-functionalized molecular linker can directly connect adjacent lithiophilic COOH-MWCNTs with well-defined interactions, ensuring structural integrity that can effectively withstand mechanical (i.e., volume expansion and shrinkage during electrochemical cycling) and / or electrochemical stresses (i.e., SEI formation and lithium dendrite growth) during repeated lithium plating and stripping processes. Furthermore, the present approach can leverage the tailored chemical and physical properties of the nanometer-scale assembled functional interlayer films without an insulating polymer binder to influence the electrochemical kinetics of LMA via the interfacial interactions between the two different lithiophilic functional configurations (i.e., the COOH groups of MWCNTs and the NH2 groups of the molecular linker).

[0031] To achieve these goals, MWCFs were formed via lithiophilic linker-mediated LbL assembly of COOH-MWCNTs and NH2-TAA (i.e., NH2-TAA / COOH-MWCNT multilayers) onto commercial battery separators. Importantly, this LbL approach ensures a uniform distribution of abundant lithiophilic organic moieties (i.e., oxygen- and nitrogen-containing functional groups) within the conductive MWCF network, leading to rapid and homogeneous Li + It was found that the small molecule linker-induced LbL assembly, excluding the bulky / insulating polymer linker, can form a highly entangled ultrathin forest interlayer with a robust interfacial structure without lithiophilic inactive components, thereby preserving the porosity and electrical conductivity of the COOH-MWCNTs themselves. This MWCF configuration can form a uniform Li thin layer at the MWCF / separator interface, which can guide Li growth onto the LMA surface and prevent its penetration into the separator (Fig. 1B). The symmetric Li|Li battery generated using this MWCF interlayer-coated separator has a discharge current of 1 mA cm -2 Current density of 1 mAh cm -2 It showed an excellent lifespan exceeding 10,000 hours at a capacity of . In addition, LMA and LiNi 0.8 Mn 0.1 Co 0.1 A full-cell configuration consisting of O2(NMC811) cathode achieved an impressive cycling stability of 81.9% after 600 cycles at 1C and a current density of 0.1 mA cm -2 678 Wh kg -1delivered the highest energy density. Surprisingly, the full battery employing LiFePO4 (LFP) cathode exhibited an excellent cycling stability of 107% even after 1,500 cycles at 1C. This result significantly outperforms the conventional layer-based batteries reported to date, which are limited by unfavorable interfacial designs. Although 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 compositions and thus the physical and chemical properties of the interlayer, can be considered to provide new insights and a foundation for the development of future high-performance LMBs.

[0032]

[0033] Hereinafter, the present invention will be described in detail.

[0034]

[0035] The term "Janus membrane" in this specification means that one side and the other side of the membrane have different shapes, and (NH2-TAA / COOH-MWCNT) coated or deposited on one side of the membrane n It was named n-Janus separator according to its multilayer structure.

[0036] The term "lithium dendrite" in this specification refers to a needle or branch-shaped precipitate that is formed when lithium crystals are formed on the surface of the negative electrode during the charging and discharging process of lithium metal, and these become nuclei and gradually accumulate.

[0037]

[0038] Janus membrane

[0039]

[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 including carbon nanotubes surface-modified with an amine group-containing linker and a carboxyl group.

[0041]

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

[0043] The above-mentioned separator refers to a bare separator, may be single-layered or multi-layered, and may be made of various known porous materials. Specifically, the separator may be made of 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 separator may have a surface modified with oxygen-containing functional groups, particularly hydroxyl groups, by UV irradiation or plasma treatment, and these hydroxyl groups may form hydrogen bonds with amine groups present in an amine group-containing linker described below.

[0045]

[0046] Next, the Janus separator for a lithium metal secondary battery according to the present invention comprises a carbon nanotube assembly, characterized in that the carbon nanotube assembly is coated on one surface of the separator. In addition, the carbon nanotube assembly is characterized in that it comprises a carbon nanotube surface-modified with an amine group-containing linker and a carboxyl group.

[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 a hydrogen bond with an amine group present in an amine group-containing linker described below.

[0048] The above amine-containing linker also has lithium affinity and hydrophilicity, and the amine group present in the amine-containing linker that is first coated can form a hydrogen bond with an oxygen-containing functional group, particularly a hydroxyl group, present in the separator, and at the same time, form a hydrogen bond with a carboxyl group present in an adjacent carbon nanotube. The amine-containing linker that is subsequently coated can form a hydrogen bond with each carboxyl group present in an adjacent carbon nanotube, thereby enabling LbL assembly as it is interposed between adjacent carbon nanotubes.

[0049] Accordingly, the amine group-containing linker preferably contains two or more amine groups in order to function as a linker.

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

[0051] Specifically, the amine group-containing linker may include at least one selected from the group consisting of hydrazine, diethylenetriamine, tris(2-aminoethyl)amine, and polyethyleneimine, and preferably includes at least one 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 can be in a binder-free form, as no polymer binder is required.

[0053] Meanwhile, the carbon nanotube assembly can be assembled to form a uniform nanoporous structure, coated on one surface of the separator, and oriented toward the lithium metal electrode. This not only suppresses lithium dendrite formation by evenly dispersing lithium ions, but also controls the direction of lithium growth.

[0054] The carbon nanotube assembly can be coated on the separator while its surface is modified with oxygen-containing functional groups, particularly hydroxyl groups, through UV irradiation or plasma treatment. The amine-containing linker that is first coated within the carbon nanotube assembly can form hydrogen bonds with the separator and, at the same time, hydrogen bonds with adjacent carbon nanotubes. The amine-containing linkers that are subsequently coated can be repeatedly stacked through hydrogen bonds between the adjacent carbon nanotubes. The number of layers of carbon nanotubes surface-modified with carboxyl groups can be regarded as the number of repetitions, and the number of repetitions is for controlling the thickness of the coated carbon nanotube assembly, and can be 1 to 20 times, preferably 2 to 10 times, and in consideration of lithium affinity, ion diffusion coefficient, cycle stability, etc., 2 to 4 times is more preferable, but is not limited thereto.

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

[0056] The above carbon nanotube assembly has lithium affinity and hydrophilicity, and in particular, the contact angle for an electrolyte including lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and a mixed solvent of 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.

[0057]

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

[0059]

[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 a carbon nanotube with a carboxyl group; and (b) coating one surface of the separator with a carbon nanotube assembly comprising an amine group-containing linker and the carbon nanotube surface-modified with the carboxyl group.

[0061]

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

[0063] The above carbon nanotubes are hydrophobic before surface modification, and are characterized by surface modification with carboxyl groups to provide 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.

[0064]

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

[0066] Since the above separation membrane, the amine group-containing linker, and the carbon nanotube surface-modified with the carboxyl group have been described above, a duplicate description will be omitted.

[0067] The above coating is performed through a solution-based layer-by-layer self-assembly (LbL) process, and the carbon nanotube assembly can be coated while the surface of the separator is modified with oxygen-containing functional groups, particularly, hydroxyl groups, through UV irradiation or plasma treatment. The amine-containing linker that is first coated within the carbon nanotube assembly can form hydrogen bonds with the separator and, at the same time, hydrogen bonds with adjacent carbon nanotubes. The amine-containing linkers that are subsequently coated can be repeatedly stacked through hydrogen bonds between the respective carbon nanotubes. The number of layers of the carbon nanotubes surface-modified with the above carboxyl group can be seen as the number of repetitions, and the number of repetitions is for controlling the thickness of the coated carbon nanotube assembly, and can be 1 to 20 times, preferably 2 to 10 times, and comprehensively considering lithium affinity, ion diffusion coefficient, cycle stability, etc., 2 to 4 times is more preferable, but is not limited thereto.

[0068]

[0069] Lithium metal secondary battery

[0070]

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

[0072]

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

[0074] In particular, in the Janus separator, the carbon nanotube assembly coated on one side of the separator forms a uniform nanoporous structure and can be coated in the direction of the negative lithium metal electrode. This not only suppresses the formation of lithium dendrites by evenly dispersing lithium ions, but also controls the direction of lithium growth. At this time, since the lithium metal electrode has a theoretical capacity of 3840 mAh / g and a low reduction potential, the theoretical storage capacity can be significantly increased.

[0075]

[0076] As described above, the Janus separator according to the present invention is characterized by including a separator; and a carbon nanotube assembly coated on one surface of the separator and including carbon nanotubes surface-modified with an amine group-containing linker and a carboxyl group. Accordingly, when a lithium metal secondary battery using the Janus separator according to the present invention is operated, not only can lithium ions be evenly dispersed to suppress lithium dendrite formation, but also the direction of lithium growth can be controlled, so that ultimately high stability can be secured, which is an advantage.

[0077] Therefore, the present invention can be applied to various high-capacity secondary batteries that require excellent driving stability and power density.

[0078]

[0079] Hereinafter, preferred examples are presented to aid in understanding the present invention. However, the following examples are provided solely to facilitate a better understanding of the present invention, and the scope of the present invention is not limited by the following examples.

[0080]

[0081] [Example]

[0082] ingredient

[0083] Pristine multi-walled carbon nanotubes (MWCNTs) and tris(2-aminoethyl)amine (TAA) were purchased from MERCK. Carboxylic acid (COOH) functionalization of pristine MWCNTs was performed by oxidation with a H2SO4 / HNO3 mixture at 70°C for 2 h. The resulting suspension was then carefully washed using dialysis tubing to remove byproducts and residues. Organic solvents (ethanol, acetone) were purchased from Daejung Chemicals (Korea). Other chemical reagents were purchased from Sigma-Aldrich and used without further purification.

[0084]

[0085] Fabrication of multi-walled carbon nanotube forests (MWCFs)

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

[0087]

[0088] Fabrication of LiFePO4 (LFP) and LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes

[0089] The cathode fabrication involved mixing LFP or NMC811 powders 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. The slurry was then coated onto carbon-coated aluminum foil (18 μm thick) using a doctor blade and dried overnight at 90°C. After the drying process, the cathode sheet was pressed at 20 MPa for 1 minute.

[0090]

[0091] Battery assembly and electrochemical measurements

[0092] Electrochemical measurements were performed using CR2032 type coin cells (MTI Corporation) and a WBCS3000 multichannel workstation. The cells were assembled in an argon-filled glovebox (MBraun, O2<0.1 ppm, H2O<0.1 ppm) using Li foil (35 μm thick) as the cathode, as-prepared cathodes, and Celgard 2400 separators or n-MWCF separators. 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%) supplemented with 2 wt% LiNO3. 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-type cells, the electrolyte volume was fixed at 100 μL. Galvanostatic charge-discharge tests of Li|NMC811 cells were performed in the potential range of 2.7–4.3 V at 0.1 C for the first two cycles, followed by subsequent cycles at various rates. Li|LFP cells were tested in the potential range of 3.5–4.0 V under similar conditions. The theoretical capacities of NMC811 and LFP used in the C-rate calculations were 200 and 170 mAh g, respectively. -1was. Li | Ni battery was 1 mA cm at a cutoff voltage of 1.0 V. -2 Cycles were performed at a current density of . Electrochemical impedance spectroscopy (EIS) measurements were performed in the 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).

[0093]

[0094] Assembly of pouch-type batteries

[0095] Pouch-type batteries were 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 thick), a 3-MWCF separator, and the prepared positive electrode were packaged in a laminate bag and filled with electrolyte. The laminate bag was then vacuum-sealed.

[0096]

[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 functionality of the LbL assembled multilayers were investigated using Fourier transform infrared (FTIR) spectroscopy. These analyses were performed at room temperature under ambient conditions at a wavelength of 4 cm -1The measurements were performed using a CARY 600 spectrometer (Agilent Technologies) at a resolution of 1000 nm. The data obtained from 200 scans were processed using OMNIC software (Nicolet). The UV-vis spectra of the LbL assembled multilayers 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 carbon fiber (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 It shows the frequency and mass changes for each layer of the multilayer, respectively.

[0101]

[0102] Calculation Details

[0103] All calculations were performed using the Q-Chem code version 5.4.0, employing the B3LYP correlation functional (a three-parameter Becke hybrid functional combined with Lee-Yang-Parr) with the 6-311++G(d, p) basis set. Celgard, H-terminated molecular graphene with and without carboxyl groups (-COOH) was modeled, and a convergence threshold of 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.

[0104]

[0105] Example 1: Preparation of multilayer MWCF

[0106] To fabricate lithium-compatible MWCs, pristine hydrophobic MWCNTs were first converted into hydrophilic COOH-MWCNTs using a H2SO4 / HNO3 oxidation method, thereby forming COOH sites on the external surface of the MWCNTs. The successful surface modification of the resulting COOH-MWCNTs was verified by their high dispersion stability in ethanol and Fourier transform infrared (FTIR) spectroscopy (Fig. 2A). In this case, the COOH-MWCNTs retained their original tubular characteristics without significant fragmentation after the given oxidation conditions (Fig. 2B).

[0107] Based on these results, COOH-MWCNTs were synthesized by NH2-functionalized TAA (abbreviated as NH2-TAA, molecular weight ~146 g mol) through hydrogen bonding interactions between the COOH moiety of MWCNTs and the NH2 moiety of TAA in ethanol. -1 ) were sequentially assembled with LbL linkers (Fig. 2C). The formed (NH2-TAA / COOH-MWCNT) n The FTIR spectra of the multilayers are 1704 cm -1 Carbonyl (C=O) stretching vibration of COOH moiety at 1573 cm -1 A distinct absorption peak arising from the NH bending vibration of the NH2 moiety 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 achieved 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 multilayers was also investigated using UV-vis spectroscopy (Fig. 2E). As the number of bilayers (n) increased from 1 to 10, the intensity of the absorption spectrum increased almost linearly. In this case, (NH2-TAA / COOH-MWCNT) nThe total film thickness of the multilayer increased to approximately 198 nm (n = 10), resulting in a predicted thickness per bilayer of approximately 19.8 nm (Fig. 2F). The average mass change per bilayer (ㅿm) was calculated from the frequency change (ㅿF) using a quartz crystal microbalance (QCM) measurement and was approximately 5.5 μg cm -2 (ㅿF ~ 312 Hz) was predicted (Fig. 2G). In this case, the proportion of TAA within the multilayer was approximately 12.8% (i.e., ~0.7 μg cm per bilayer). -2 ) had the advantage of being able to fully utilize the properties of COOH-MWCNTs, including their porous structure and electrical properties. Consequently, these observations clearly implied that the adsorption amount per bilayer was almost regular and could be precisely controlled by the number of bilayers (n). Therefore, each component (i.e., COOH-MWCNT and NH2-TAA) could be evenly distributed within the entire area of ​​the resulting LbL assembled multilayer.

[0109] In particular, the oxygen atoms in the COOH group of MWCNT and the nitrogen atoms in the NH2 group of TAA are Li + Because it has a strong affinity for ions, (NH2-TAA / COOH-MWCNT) with 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 only of COOH-MWCNTs and NH2-TAA molecular linkers without any electrochemical and lithiophilic inactive components or bulky polymer linkers, the present MWCFs were expected to exhibit strong lithiophilic properties, highly uniform / nanoporous structure, and excellent electrical conductivity. These properties are very advantageous for Li + The ions penetrated rapidly and uniformly into the MWCF, effectively suppressing the growth of irregular Li resins.

[0110]

[0111] Example 2: Characterization of MWCF on a membrane

[0112] To verify this possibility, MWCFs were deposited on one side of a UV-irradiated polypropylene (PP) membrane (Celgard 2400) using hydrogen-bonding interaction-mediated LbL assembly. As shown in Figure 3A, the surface coverage of MWCFs on the membrane gradually increased as the number of bilayers (n) in the multilayer increased from 0 to 5. In particular, 3-MWCFs with a thickness of approximately 60 nm exhibited a surface coverage of over 90% on the membrane and well maintained a highly nanoporous structure. Furthermore, the formed MWCFs exhibited a very smooth surface morphology with nanometer-scale roughness, indicating the uniform deposition of the composition through the LbL assembly process. These characteristics enabled the LbL-assembled MWCFs to function as high-quality functional interlayers with a uniform structure between the membrane and the LMA. On the other hand, approaches based on bulky polymer binders have had difficulties in effectively utilizing the physical benefits of MWCNTs due to pore blockage and non-uniformity, which deteriorate ion transfer kinetics. Specifically, the key advantage of the present approach is (NH2-TAA / COOH-MWCNT) n The multilayer exhibited a gap-free interface with the separator, with strong interfacial interactions (i.e., hydrogen bonding) not only between the multilayer and the separator, but also between adjacent COOH-MWCNT layers. This phenomenon was particularly significant, as it effectively eliminated the possibility of needle-like lithium dendrites penetrating the potential gap between the separator and the MWCF.

[0113] In addition, it was 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 could act as the upper electrode of the LMB, effectively and uniformly distributing the current density from the separator to the Li metal anode. This meant that the nanoporous MWCF with higher electrical conductivity was much more advantageous in fabricating high-performance LMB with low overpotential and long-term stability. It is noteworthy that the nanoporous MWCF with low molecular weight (M w ~ 146) The conductive MWCF assembled with NH2-TAA had a distinct advantage over the MWCF with an insulating NH2-functionalized polymer linker (e.g., NH2-poly(ethylene imine), NH2-PEI). Figure 3B shows (NH2-TAA / COOH-MWCNT) n -Coated separator and (NH2-PEI / COOH-MWCNT) n -The sheet resistance change 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 when n = 3, which is higher than that of the NH2-PEI-based separator (1.6 × 10 when n = 3). 6 Ω sq -1 ) was two steps lower than (1.3 × 10 for n = 3) 4 Ω sq -1) These results clearly indicated that the small molecule linker used in this approach minimized the contact resistance between adjacent COOH-MWCNTs. In addition, the (NH2-TAA / COOH-MWCNT)3-coated separator (abbreviated as 3-MWCF separator) exhibited excellent electrical stability even after repeated bending cycles (σ / σ0 = 100% after 10,000 bending cycles) due to the strong interfacial interactions among all components (i.e., NH2-TAA, COOH-MWCNT, and separator) (Fig. 3C). Since the role of the MWCF on the separator is to ensure uniform charge distribution in the space between the LMA and the separator, the gapless interface between the conductive MWCF and LMA, as well as between the adjacent MWCNTs, was very beneficial for improving the performance of the LMA.

[0114] Furthermore, the well-distributed functional groups within the MWCFs could 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 ( Figure 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 3D-structured MWCF with 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 membrane before and after immersion in 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 membrane has excellent electrolyte compatibility, which is very advantageous for reducing internal ionic resistance and achieving long-term cycling.

[0117] Additionally, density functional theory (DFT) calculations were used to gain insight into the effect of the introduced functional groups on lithium dendrite formation. Based on experimental evidence on the surface compatibility of each configuration, the properties of five representative structures: bare PP separator, NH2-TAA, pristine MWCNT, COOH-MWCNT, and MWCF (NH2-TAA / COOH-MWCNT multilayer) were compared ( Figure 3F ). First, an electrostatic potential (ESP) map was constructed to visualize the charge density distribution ( Figure 3F ). The bare PP separator exhibited a nearly neutral charge, whereas the pristine MWCNT without carboxyl groups exhibited a slightly negative charge that was close to neutral. In contrast, the COOH-MWCNT exhibited a significant negative charge, which was particularly localized around the oxygen atoms of the carboxyl groups. It is noteworthy that increasing the oxidation time of COOH-MWCNTs resulted in the generation of more lithium-affinity carboxyl groups, but this decreased electrical conductivity and increased the internal resistance of the battery, thereby lowering cycling stability. Furthermore, the complementary interaction between the amine group and the oxygen in the carboxyl group of NH2-TAA resulted in an overlap of negative charges. This visualization highlighted potential lithium atom adsorption sites.

[0118] Furthermore, comparative analysis of the adsorption energies of Li atoms showed 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 exhibited stronger adsorption energies of -2.73 eV for COOH-MWCNT and -3.11 eV for MWCF, respectively. These results demonstrated that NH2-TAA not only acts as a linker but also provides additional favorable adsorption sites, effectively controlling the charge distribution for uniform Li deposition. In addition, the atomic charge of Li tends to increase with the adsorption energy of the functional sites, indicating that the adsorption of Li atoms is induced by negatively charged functional sites and positively charged Li atoms. + It was shown that the strong electrostatic interaction between ions could be driven. Therefore, DFT calculations suggested that MWCFs with strong and uniform co-adsorption sites for Li atoms could significantly alleviate the local current density and effectively prevent significant dendrite formation. Based on these results, the present approach suggests the possibility of precisely designing and controlling the function of n-MWCF separators without incorporating a lithiophilic inactive component through hydrogen bond-mediated LbL assembly between lithiophilic COOH-MWCNTs and a lithiophilic small molecule linker (NH2-TAA).

[0119]

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

[0121] To systematically evaluate the effect of lithiating MWCFs on lithium dendrite growth behavior during Li plating and stripping processes, Li|Ni half-cells were assembled using n-MWCF separators with various bilayer numbers (n). As previously confirmed in Figure 3, the electrolyte wettability and absorption capacity of the MWCFs clearly varied with the number of bilayers (or configurations), each of which induced a thickness change of approximately 20 nm. This also implied that the electrochemical performance of n-MWCF separator-based cells could be effectively controlled and optimized through the LbL assembly approach. Furthermore, the abundant lithiating sites in the LbL-assembled MWCFs significantly reduced the energy barrier in the initial Li nucleation step, promoting uniform Li deposition. As shown in Figure 4A, the half-cells with n-MWCF separators exhibited relatively lower nucleation overpotentials compared to cells based on bare separators. Specifically, when the number of double layers (n) is increased from 0 (for a bare membrane) to 5 (for a 5-MWCF membrane), the overvoltage value increases with a current density of 0.5 mA cm -2 The voltage was significantly reduced from 97 mV to 44 mV. 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 were performed to determine the internal resistance and ion (Li) interactions at the interface of the n-MWCF membrane-based system. + ) was further performed at room temperature to analyze the charge transfer dynamics (Fig. 4B). In this case, the charge transfer resistance (R ct ) values ​​range from 244 (for bare membrane) to 126 Ω cm 2 (for 5-MWCF membranes with a thickness of 101 nm) decreased sharply, and the equivalent series resistance (R s ) values ​​also range from 7.8 to 3.8 Ω cm 2 was reduced to . In general, R ct The value is Li in the electrolyte by receiving electrons from the electrode. +The process of desolvating and depositing ions was reflected. R of n-MWCF membrane-based battery ct The lower the value, the more Li + This indicates that ion transport is easier, which suggests that less activation energy is required to overcome the barrier and undergo the deposition process.

[0123] These results were consistent with those presented in Figure 4A. Interestingly, the Warburg slope of the 5-MWCF membrane-based battery was slightly reduced compared to the 3-MWCF membrane-based battery, indicating an increased resistance to ion diffusion. This phenomenon was further elucidated by calculating the diffusion coefficient derived from the measured Warburg impedance (Figure 4C). The calculated diffusion coefficient (D Li+ ) 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 is well known that various electrode factors including electrical conductivity, thickness, porosity, and surface area are closely related to the ion diffusion rate of electrochemical energy storage systems, which affect the growth of lithium dendrites and the overall performance. In the present invention, the n-MWCF assembled LbL on the separator was found to increase in thickness and surface area as the number of double layers (n) increased, while the pore size gradually decreased, which was confirmed by the Brunauer-Emmett-Teller (BET) analyzer.

[0124] To more clearly evaluate the influence of n-MWCF membrane on the electrochemical performance of LMB, the current density at 1 mA cm in Li|Ni half-cell configuration was measured according to the number of double layers (n). -2 and 1 mAh cm -2The Coulombic efficiency (CE) was monitored during Li plating / stripping cycles (Fig. 4D). Initially, the CE of the bare separator-based cell decreased rapidly after 70 cycles, indicating that lithium dendrites grew uncontrollably and rapidly, resulting in short circuits. However, as the bilayer number (n) of the MWCF separator increased to 3, the cycling stability was significantly improved, showing a high CE of ~97.2% after 190 cycles. Noteworthy, this improved stability decreased again as the bilayer number further increased from 3 to 5, consistent with the observed trend in ion diffusion kinetics with bilayer number. Indeed, the Li plating behavior on the electrode surface was strongly influenced by the ion diffusion conditions at the electrode / electrolyte interface, which was also directly related to dendrite growth. Therefore, these observations suggested that the 3-MWCF separator could provide an optimal structure to effectively suppress the indiscriminate lithium dendrite growth during repeated Li plating / stripping cycles. As a result, the bare separator-based Li | The Ni half-cell exhibited typical needle-like lithium dendrites on the Ni plates after cycling, which led to severe short-circuiting (Figure 4E). In contrast, the 3-MWCF separator-based cell exhibited mossy-like deposition of Li, as shown in Figure 4F, indicating well-controlled current flow through the electrode surface (i.e., Ni plates). Notably, the enhanced ion diffusion properties of the n-MWCF separator-based cell could effectively maintain an appropriate ion concentration for forming a mossy Li layer during long-term cycling by alleviating the diffusion limitation at the electrode / electrolyte interface. Therefore, the small molecule linker (i.e., TAA)-mediated LbL assembly could precisely tune the functionality of the MWCF interlayer at the nanometer scale, and effectively maintain the highly porous structure and electrical conductivity of the MWCNT itself to facilitate charge transfer.In this context, the LbL-assembled conductive MWCFs represented a promising alternative to insulating polymer binder-based composite interlayers. Specifically, conventional simple mechanical mixing (i.e., blending) approaches, lacking sufficient interaction between the individual components, could result in reduced charge conductivity due to partial blockage of the nanopores. In addition to providing a highly porous network for efficient ion diffusion channels, the conductive MWCF interlayer with its rich lithium affinity served as a robust and effective host material for stable lithium plating. Specifically, the high surface coverage of the MWCFs on the separator promoted the sequential deposition of Li at the MWCF / separator interface, along with the formation of a gapless interface with favorable complementary interactions. This induced Li growth onto the LMA surface in the MWCF separator, effectively preventing lithium dendrite penetration through the separator (Figure 4G).

[0125]

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

[0127] To accurately analyze the effect of the MWCF separator on the Li deposition behavior during galvanostatic cycling, Li | Li symmetric cell tests were performed. In these tests, the MWCF middle layer of the separator was kept facing the working electrode (cathode). 3 mA cm -2After 400 Li plating / stripping cycles, the cell assembled with the bare separator exhibited typical rough dendrite growth on the Li metal anode surface (Fig. 5A). In contrast, the Li symmetric cell based on the 3-MWCF separator exhibited very smooth and uniform Li deposition with a mossy-like morphology on the Li metal surface, which was consistent with the observation in the half-cell configuration (Fig. 5B) and ensured strong adhesion to the separator. Importantly, the 3-MWCF covering the separator with a gapless interface allowed dense Li deposition and guided Li metal growth to the anode surface, effectively suppressing its penetration into the bare separator. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis revealed that the SEI layer on the 3-MWCF separator remained stable after 400 cycles, confirming that the lithium plating was uniform and well-controlled without severe morphological cracking.

[0128] As mentioned earlier, the ionic conduction behavior at the interface plays a crucial role in achieving stable Li plating, which in turn determines the ionic conductivity of the battery and the Li + The ion transport capacity was reflected in the number of ions. 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, the R of each cell ct The values ​​range from 391.2 to 141.8 Ω cm as the number of double layers (n) increases from 0 to 5. 2 was significantly reduced (Fig. 5C). This trend was consistent with the improvement in electrical conductivity of the MWCF interface as the number of double layers increased. However, in the symmetric battery based on the 5-MWCF separator, R ct Despite the decrease in the values, the ionic conductivity and Li +It was noteworthy that the transfer number was lower than that of the 3-MWCF separator-based cell (Fig. 5D). This observation was consistent with the trends observed in the ion diffusion coefficients and CE values ​​of the half-cell tests. Specifically, increasing the number of double layers (n) consistently improved the electrical conductivity and lithiation affinity of the MWCF interlayer, 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 symmetric cell exhibited significantly higher ion conductivity (0.46 mS cm) than the other tested cells. -1 ) and Li + The transmission rate (0.76) was shown.

[0129] The excellent charge diffusion properties of the 3-MWCF membrane ensured a uniform ion distribution at the interface over a wide range of current densities, promoting electrochemical reactions while effectively suppressing dendritic Li deposition. This feature can be explained by the "sand capacity," which refers to the maximum capacity that mossy Li can form without deforming into a dendritic structure. This critical capacity is calculated by multiplying the sand time by the current density. In particular, the sand time is the period from the initial deposition of mossy Li to the cation (i.e., Li) at the electrode interface. + ions) is defined as the period until the concentration decreases to 0. This relationship can be expressed by the following equation (2):

[0130] (2)

[0131] z here c is the charge number of the cation (Li + In 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 relevant anion, D app is the apparent diffusion coefficient of the electrolyte.

[0132] According to these calculations, a Li|Li symmetric battery using a 3-MWCF separator has a C Sand = 35.2 / J, which represents the largest predicted area for mossy Li formation. In addition, the interfacial stability and cycling reversibility of the n-MWCF separator-based Li symmetric battery were investigated. For this purpose, 1 mAh cm -2 The voltage profiles of the devices were monitored during long-term discharge / charge cycles at various current densities while maintaining a constant capacity (Figures 5E and 5F). The voltage curves of all Li|Li symmetric cells using n-MWCF separators were 1 mA cm 2 (1 mAh cm -2 less than 3 mA cm -2 (1 mAh cm -2 All of them showed fluctuations during the initial 200-300 h of cycling (below 1 mA cm). This behavior could be due to the formation of an unstable interface between the MWCF-coated separator and the electrolyte in the early stage of cycling. In all cases, the Li-symmetric cells using bare separators showed limited cycle life 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 cycle life of the symmetric cells, leading to an increase in overpotential. In particular, the 3-MWCF separator-based cells showed a cycle life of less than 1 mA cm -2 5,000 cycles at 3 mA cm -2 It exhibited excellent cycling stability of 10,500 cycles, and a smooth and stable voltage plateau with low voltage hysteresis of less than ~28.3 mV. In addition, this stable cycling behavior was accompanied by a current density and capacity of up to 10 mA cm -2 and 10 mAh cm -2 Even when increased to , the excellent interfacial stability and Li of the 3-MWCF intermediate layer persist. +The ion transport capability was clearly demonstrated. This unprecedented high operational stability of the 3-MWCF membrane-based Li symmetric battery was significantly superior to that of previously reported functional membrane-based symmetric Li batteries (Figure 5G). Importantly, the small molecule linker (TAA)-mediated LbL design of the lithium-affinity MWCNT interlayer according to the present invention effectively optimized the interfacial structure, enabling excellent charge conduction at various current densities, outperforming the conventional slurry- or polymer linker-based approaches.

[0133]

[0134] Example 5: Characterization of an asymmetric full battery based on an n-MWCF separator.

[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 full battery was fabricated using O2 (NMC811) as a cathode material and its electrochemical performance was characterized. 1.1 mg cm at 1 C. -2 The cycling performance of a 3-MWCF membrane-based full battery (i.e., Li | 3-MWCF membrane | NMC811) with a loading of 100 μm was investigated. Surprisingly, it exhibited a capacity retention of 81.9% and a CE of 99.9% even after 600 cycles, which outperformed the bare membrane-based battery, which rapidly degraded 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 full battery was investigated, which delivered a higher capacity over a wide range of current rates compared to a bare separator-based full battery (Fig. 6B). In particular, the 3-MWCF separator-based full battery exhibited a high capacity of 191.7 mAh g at 0.1 C. -1 , 145.9 mAh g at 2 C -1The highest specific capacity was 118% of that of the bare membrane-based cell despite the same amount of active material loading. This improved rate performance indicated that the LbL assembled MWCF interlayer effectively overcomes the kinetic barrier and mass transfer limitations at the interface during high-speed operation, resulting in a significantly reduced overpotential (0.06 V) compared to the bare membrane-based cell (~0.3 V). This promising performance of the 3-MWCF separator-based full cell was further verified by EIS measurements, which showed that the bare membrane-based cell had a specific capacity of 163.4 Ω cm. 2 59.6 Ω cm, which is significantly lower than 2 R of ct The values ​​were shown. These results indicate that the 3-MWCF intermediate layer has a uniform Li + Promotes ion deposition and Li + It was clearly shown that the interfacial resistance within the battery was effectively reduced by improving ion mobility.

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

[0138] To expand the applicability of MWCF membranes to industrial demands, 2 mAh cm -2A pouch-type battery with a capacity of 6 mAh was fabricated using an NMC811 cathode with an area capacity of (N / P ratio of ~2), and its electrochemical performance was investigated. Notably, the pouch-type battery based on the 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 membrane, it was paired with an LFP cathode to form a Li | 3-MWCF membrane | LFP full cell. As shown in Fig. 6G, a 3.3 mg cm -2 The MWCF membrane-based LFP cell with a loading of 20.6 mg cm maintained an impressive capacity retention of 107% after 1500 cycles at 1C. The gradual increase in capacitance observed during the first 100 cycles compared to the initial value was attributed to the enlarged interfacial area between the MWCF-coated separator, LFP, and electrolyte, and the enhanced system activation through repeated cycling. In contrast, the LFP cell using a bare separator with the same loading experienced a gradual capacity decay after 200 charge / discharge cycles. In addition, the capacity retention was 20.6 mg cm -2 MWCF separator-based LFP batteries with higher cathode loading showed a similar trend, reaching 2 mA cm -2The LbL assembled MWCF interlayer effectively promoted mass transport within the cell and prevented lithium dendrite formation. This was further supported by XPS analysis, which showed that the lithium metal anode paired with the 3-MWCF separator exhibited reduced strengths for LiF and LixPOyFz compared to the bare separator. This reduction suggested that the uniform Li ion flux enabled by the 3-MWCF separator resulted in a thinner and more uniform SEI layer. Based on this promising performance of the MWCF intermediate layer, 10.5 mg cm -2 Li|3-MWCF separator|LFP pouch cell with loading amount of 2 mA cm and total capacity of 5 mAh (N / P ratio ~2.3) -2 It showed an excellent capacity retention rate of 99.8% per cycle.

[0140]

[0141] By utilizing an ultrathin MWCF interlayer assembled at the interface with the separator, we developed a high-performance LMB, effectively suppressing and directing the growth of needle-like lithium dendrites during repeated lithium plating and stripping on the LMA, thereby achieving excellent operational stability. The MWCF interlayer was LbL assembled solely with lithiophilic COOH-MWCNTs and an NH2-functionalized molecular linker (TAA), exploiting well-defined complementary interfacial interactions without incorporating electrochemically inactive components. This unique approach significantly enhanced charge-conducting properties at the interface, facilitated uniform current distribution on the LMA surface, and provided sufficient pore space for stable lithium plating while maintaining robust structural integrity. Notably, the small molecular linker-mediated LbL assembly enabled 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, the optimized 3-MWCF separator-based symmetric battery (i.e., Li | 3-MWCF separator | Li symmetric battery) exhibited a current density of 3 mA cm -2 The NMC811 anode exhibited an unprecedentedly high cycle stability of 10,500 cycles at a current density of 10,000 mV, and the overvoltage was significantly reduced to 28.3 mV, which surpassed the stability performance of symmetrical batteries manufactured by the conventional slurry casting method. In addition, the asymmetrical full battery employing the NMC811 anode achieved a maximum energy density of 678 Wh kg. -1 And it achieved an excellent capacity retention rate of 81.9% even after 600 cycles. In particular, the 6 mAh pouch-type battery maintained ~99.85% (~2 mAh cm) per cycle. -2) were stably maintained. Considering that this approach can precisely control lithium dendritic growth through multilayer functionality and structural design, it can provide a basis for the development and design of high-performance LMBs that enable very long-term operating stability.

[0143]

[0144] The foregoing description of the present invention is for illustrative purposes only. Those skilled in the art will readily appreciate that modifications to other specific embodiments can be made without altering the technical spirit or essential characteristics of the present invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims

1. Separator; and A Janus separator for a lithium metal secondary battery comprising a carbon nanotube assembly coated on one surface of the separator and including carbon nanotubes surface-modified with an amine group-containing linker and a carboxyl group.

2. In paragraph 1, A Janus separator for a lithium metal secondary battery, characterized in that the separator is made of a polyolefin-based polymer material.

3. In paragraph 1, A Janus separator for a lithium metal secondary battery, characterized in that the surface of the separator is modified with oxygen-containing functional groups by UV irradiation or plasma treatment.

4. In paragraph 1, A Janus separator for a lithium metal secondary battery, characterized in that the above amine group-containing linker contains two or more amine groups.

5. In paragraph 1, The weight average molecular weight of the above amine group-containing linker is 30 g mol -1 5,000 g mol -1 A Janus separator for a lithium metal secondary battery, characterized by:

6. In paragraph 1, A Janus separator for a lithium metal secondary battery, characterized in that the amine group-containing linker comprises at least one selected from the group consisting of hydrazine, diethylenetriamine, tris(2-aminoethyl)amine, and polyethyleneimine.

7. In paragraph 1, A Janus separator for a lithium metal secondary battery, characterized in that the above carbon nanotube assembly is in a binder-free form.

8. In paragraph 1, A Janus separator for a lithium metal secondary battery, characterized in that the carbon nanotube assembly is coated on one surface of the separator and faces toward the lithium metal electrode.

9. In paragraph 1, A Janus separator for a lithium metal secondary battery, characterized in that carbon nanotubes surface-modified with an amine group-containing linker and a carboxyl group within the above carbon nanotube assembly are repeatedly stacked through hydrogen bonds.

10. In paragraph 1, A Janus separator for a lithium metal secondary battery, characterized in that the carbon nanotube assembly has a contact angle of 30° or less for an electrolyte containing a mixed solvent of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), dimethyl ether (DME), and 1,4-dioxane (DIOX). 11.(a) a step of surface-modifying a carbon nanotube with a carboxyl group; and (b) A method for manufacturing a Janus separator for a lithium metal secondary battery, comprising a step of coating a carbon nanotube assembly including an amine group-containing linker and a carbon nanotube surface-modified with the carboxyl group on one surface of the separator.

12. A lithium metal secondary battery comprising a Janus separator according to paragraph 1.

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