ION TRANSPORT STRUCTURE USING TWO-DIMENSIONAL NANOMATERIAL HAVING DEFECTS, METHOD FOR PREPARING SAME AND LITHIUM EXTRACTION APPARATUS COMPRISING SAME

Description

TECHNICAL FIELD

The present invention relates to an ion transport structure using a two-dimensional nanomaterial having defects, a method for preparing the same, and a lithium extraction apparatus comprising the same, and more specifically, it pertains to an ion transport structure with improved ion transportability and selectivity through a structure in which nanomaterials in the form of nanoflakes grow on defects of a two-dimensional nanomaterial membrane, a method for preparing the same, and a lithium extraction apparatus comprising the same.

BACKGROUND ART

Lithium-ion batteries (LIBs) are widely used in portable electronic devices, electric vehicles, and large-scale energy storage systems, and their market size is growing rapidly. The high growth of the lithium-ion battery market has led to a surge in demand for lithium (Li), which is a raw material for lithium-ion batteries, resulting in active development of technologies for lithium extraction.

Typical sources of lithium include minerals such as petalite (LiAl(Si₂O₅)₂), from which lithium is extracted by converting them into chemically convertible compounds such as Li₂CO₃, LiCl, and Li(OH). For example, Korean patent publication No. 10-2021-0080058 describes a method for extracting lithium by chlorinating lithium-containing ore to obtain chlorides, dissolving the chlorides in an organic solvent with selective solubility for lithium chlorides to eliminate impurities and extract lithium. However, because processes such as flotation separation must be performed when using minerals, the process is complicated, and there is a limitation in that lithium minerals are concentrated in specific countries, making a smooth supply difficult.

In addition, technology has been developed that utilizes brine containing lithium-rich salt as a lithium source. As an example of such technology, Korean registered patent publication No. 10-1321070 describes a method of extracting high-purity lithium phosphate from brine, specifically by introducing hydroxide anions into the brine to remove impurities, and then introducing a lithium supply substance into the supernatant to precipitate lithium as lithium phosphate. However, this technology has a problem in that the resources cannot be utilized as they are, and in the case of brine, about 70% of the world's brine is concentrated in South America, making it difficult to obtain readily.

Accordingly, technologies are being developed to extract lithium from easily accessible lithium sources, among which the technology for extracting lithium from seawater has been receiving attention. Seawater contains 2,400 times more lithium than land and is not concentrated in specific areas, making it easier to obtain lithium sources, and it is expected that a system for selectively extracting lithium from seawater could yield a large amount of lithium.

In this way, a method for extracting lithium from seawater primarily involves introducing a recovery device containing an adsorbent into seawater to selectively adsorb lithium, and then treating it with acid to extract lithium. However, this method has the disadvantage of low extraction efficiency and poor economic viability. Additionally, a technology using a porous membrane for lithium extraction has been proposed, but the precise control of the pore size in porous membranes is difficult, making it challenging to selectively extract only lithium ions among various cations present in seawater.

While technologies for extracting lithium using various lithium sources have been developed, there are limitations such as difficulties in obtaining or utilizing resources, handling challenges due to the high reactivity of lithium, and the difficulty in obtaining high-purity lithium when using sources containing various cations other than lithium. Therefore, there is a need for the development of a technology that can efficiently extract high-purity lithium using easily accessible lithium sources.

DISCLOSURE

Technical Problem

The objective of the present invention to solve the problems is to provide an ion transport structure with excellent ion transportability and selectivity.

Another objective of the present invention is to provide a method for preparing the ion transport structure.

A further objective of the present invention is to provide a lithium extraction apparatus comprising the ion transport structure.

Technical Solution

To achieve the above objectives, the present invention provides an ion transport structure comprising a membrane made of a two-dimensional nanomaterial having defects; and nanoflakes which are located on one surface of the membrane and formed to cover the defects.

In the present invention, the ion transport structure may have a primary ion channel formed by the defects, and a secondary ion channel formed by the interlayer gap between the membrane and the nanoflake.

In the present invention, both the membrane and the nanoflake, each independently, may consist of a nanomaterial selected from the group consisting of graphene, transition metal dichalcogenides (TMD), hexagonal boron nitride (hBN), and halide perovskite.

In the present invention, the defects may comprise at least one selected from vacancies and grain boundaries.

In the present invention, the defects may carry a negative charge.

In the present invention, the membrane may have a multilayer structure comprising 2 to 20 layers of two-dimensional nanomaterials.

In the present invention, the size of the primary ion channel may be 1 to 100 nm.

In the present invention, the size of the secondary ion channel may be 0.2 to 1 nm.

In the present invention, the ion transport structure may be a lithium ion transport structure.

The present invention also provides a method for preparing the ion transport structure.

The method for preparing the ion transport structure of the present invention may comprise steps of forming a membrane made of a two-dimensional nanomaterial having defects by depositing a first nanomaterial on a substrate; and depositing a second nanomaterial on the membrane to form nanoflakes over the defects.

In the present invention, the deposition of the first nanomaterial and the deposition of the second nanomaterial may be performed by chemical vapor deposition (CVD).

In the present invention, the deposition of the first nanomaterial may be carried out at a temperature of 800 to 1,200° C., and the deposition of the second nanomaterial may be carried out at a temperature of 500 to 900° C.

The present invention also provides a lithium extraction apparatus comprising the ion transport structure.

In the present invention, the lithium extraction apparatus may comprise an anodic cell comprising an anode and a lithium ion-containing solution; a cathodic cell comprising a cathode and an acidic solution; and the ion transport structure interposed between the anodic cell and the cathodic cell.

In the present invention, the solution level of the acidic solution in the cathodic cell is preferably lower than that of the lithium ion-containing solution in the anodic cell.

In the present invention, the lithium ion-containing solution may comprise seawater or brine.

In the present invention, the acidic solution may comprise at least one acid selected from the group consisting of hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), and phosphoric acid (H₂PO₄).

Advantageous Effects

According to the present invention, by utilizing a structure in which nanoflakes covering defects grow on a two-dimensional nanomaterial membrane having defects, unidirectional transport of cations is possible, and controlled-size ion channels can be formed. Therefore, the ion transport structure of the present invention is useful in various fields requiring selective ion transport due to its excellent ion transportability and selectivity. In particular, by manufacturing a lithium ion transport structure using the present invention and applying it to the separator of a lithium extraction apparatus, high-purity lithium can be efficiently extracted.

DESCRIPTION OF DRAWINGS

FIG. 1 schematically illustrates the method of manufacturing and the structure of the ion transport structure according to one embodiment of the present invention.

FIGS. 2a and 2b schematically illustrate the ion channel structure of the ion transport structure according to one embodiment of the present invention.

FIG. 3 schematically illustrates the structure of a lithium extraction apparatus fabricated in one embodiment of the present invention.

FIG. 4 schematically illustrates the structure of a separator fabricated using the lithium ion transport layer manufactured according to one embodiment of the present invention.

FIG. 5 shows a photograph of a lithium extraction apparatus fabricated in one embodiment of the present invention.

FIG. 6 shows the scanning electron microscope (SEM) images of the lithium ion transport layer fabricated in one embodiment of the present invention.

FIG. 7 shows the Raman spectrum analysis results of the lithium ion transport layer fabricated in one embodiment of the present invention.

FIG. 8 shows the transmission electron microscope (TEM) images of the lithium ion transport layer fabricated in one embodiment of the present invention.

FIG. 9 shows the selected area electron diffraction (SAED) results of the lithium ion transport layer fabricated in one embodiment of the present invention.

FIG. 10 shows the dark-field transmission electron microscope (DF-TEM) images of the lithium ion transport layer fabricated in one embodiment of the present invention.

FIG. 11 schematically shows the method of electrolyte penetration experiment for the lithium ion transport layer fabricated in one embodiment of the present invention.

FIG. 12 is a photograph showing the results of the electrolyte penetration experiment for the lithium ion transport layer fabricated in one embodiment of the present invention.

FIG. 13 schematically shows the system structure for testing the ion transport behavior of the lithium ion transport layer fabricated in one embodiment of the present invention.

FIG. 14 shows the chronoamperometry graph obtained through ion transport experiments on the lithium ion transport layer fabricated in one embodiment of the present invention.

FIG. 15 is a graph showing the ion transport ratio for each electrolyte from the ion transport experiment results of the lithium ion transport layer fabricated in one embodiment of the present invention.

FIG. 16 shows the chronoamperometry analysis results for the lithium extraction apparatus fabricated in one embodiment of the present invention.

FIG. 17 shows the results of performing inductively coupled plasma optical emission spectroscopy (ICP-OES) on the solution in the supply reservoir after operating the lithium extraction apparatus fabricated in one embodiment of the present invention.

FIG. 18 is a graph showing the concentration ratio of Li⁺ to other cations in the collection reservoir after operating the lithium extraction apparatus fabricated in one embodiment of the present invention.

MODE FOR INVENTION

Hereinafter, the specific embodiments of the present invention will be described in more detail. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Generally, the nomenclature used herein is well known and commonly used in the art.

The present invention relates to an ion transport structure having ion channels capable of transporting cations.

In the present invention, by utilizing a structure where nanomaterials in the form of nanoflakes grow over defects on a two-dimensional nanomaterial membrane, unidirectional transport of cations is possible, and ion channels with controlled sizes can be formed. Therefore, the ion transport structure of the present invention can be usefully applied in various fields requiring selective ion transport, and in particular, when the lithium ion transport structure manufactured using the present invention is applied to the separator of a lithium extraction apparatus, it can efficiently extract high-purity lithium.

The ion transport structure according to the present invention may comprise a membrane made of a two-dimensional nanomaterial having defects; and nanoflakes positioned on one surface of the membrane and formed to cover the defects.

In the present invention, the membrane refers to a planar layer formed of a two-dimensional nanomaterial, and the nanoflake refers to a piece form formed of nanomaterial. In describing the present invention, the nanomaterials constituting the membrane and nanoflake may be referred to as the first nanomaterial and the second nanomaterial, respectively.

In the present invention, “nanomaterial” refers to a material having at least one dimension in the nanoscale, 100 nm or less in external size, and “two-dimensional nanomaterial” refers to a single-layer or multilayer material having nanoscale thickness.

In the present invention, the nanoflake is formed on one surface of the membrane to enable unidirectional transport of ions, and it is preferable to use two-dimensional nanomaterials as the first and second nanomaterials to consistently form the interlayer spacing.

In the present invention, the first nanomaterial and the second nanomaterial are independent and can be the same or different. The nanomaterials may include graphene, transition metal dichalcogenides (TMD), hexagonal boron nitride (hBN), and halide perovskite.

The transition metal dichalcogenides have a structural formula of MX₂, where M denotes a transition metal and X denotes a chalcogen element. The transition metal may be molybdenum (Mo), tungsten (W), or niobium (Nb), and the chalcogen clement may be oxygen (O), sulfur(S), selenium (Se), or tellurium (Te). For example, MoS₂ and WS₂ are commonly used as transition metal dichalcogenides.

The halide perovskites may be two-dimensional materials with a structural formula of A₂BX₄, ABX₄, or A_n−1B_nX_3n+1, where A is a monovalent alkali metal, B is a metal, and X is a halogen. For example, the alkali metal may be rubidium (Rb), cesium (Cs), or francium (Fr); the metal may be lead (Pb), copper (Cu), manganese (Mn), germanium (Ge), tin (Sn), nickel (Ni), cobalt (Co), iron (Fe), chromium (Cr), cadmium (Cd), or ytterbium (Yb); and the halogen may be fluorine (F), chlorine (Cl), bromine (Br), or iodine (I).

In the present invention, the membrane is characterized by having defects. The defects refer to areas where some atoms are lost due to the incompleteness of the nanomaterial, typically including vacancies and grain boundaries as examples. These defects form passages in the vertical direction of the membrane to provide channels for ion movement, and in the description of the present invention, the channel formed by these defects can be referred to as the primary ion channel.

In the present invention, the primary ion channel, being a passage formed by defects, has the characteristic of uneven sizes; for example, the size of the primary ion channel may be about 1 to 100 nm. In describing the present invention, the size of the ion channel can be defined as the maximum diameter of ions that can pass through the channel.

The primary ion channel formed by the defects has the property of carrying a negative charge. In this case, the movement of cations can be facilitated by the overlap of the Debye length due to the negative charge, thereby exhibiting excellent ion transportability. The size of the defects may vary depending on the material and manufacturing method, and generally has a size that allows for the passage of cation hydrates, forming a pathway for ion movement. For example, if the defect is a vacancy, the average diameter may range from 1 to 100 nm, but this is not limited thereto.

The membrane may comprise one or more layers, preferably two or more layers of two-dimensional nanomaterials to provide a pathway for ion movement, and for example, may have a multilayer structure ranging from 2 to 20 layers. As an example, multi-layer graphene may be suitably used.

In the present invention, the second nanomaterial is grown on the membrane to form nanoflakes, thereby creating additional ion channels. At this time, due to the defects present in the membrane, the adsorption of atoms during the growth of the second nanomaterial is facilitated, and the defects advantageously act in nucleation and material growth. As a result, a defect healing phenomenon may occur, where the second nanomaterial is formed in a manner that covers the defects of the membrane.

FIG. 1 schematically illustrates the method of manufacturing and the structure of the ion transport structure according to one embodiment of the present invention. Referring to FIG. 1, a nanomaterial is grown on a membrane having defects to form nanoflakes that cover the defects. According to the structure, the nanoflakes have a larger area compared to the defects, and a uniform and appropriately sized van der Waals interlayer gap is formed in the overlapping region between the membrane and the nanoflakes.

The interlayer gap can be defined as the distance between the membrane and the nanoflake, through which ions passing through the primary ion channel formed by the defects can be transported. In the description of the present invention, the channel formed by the interlayer gap can be referred to as the secondary ion channel.

In the present invention, the size of the secondary ion channel, i.e., the interlayer gap, can be adjusted according to the type of cation being transported, and may range from 0.2 to 1 nm, preferably from 0.2 to 0.5 nm.

FIG. 2 schematically illustrates the channel structure of the ion transport structure according to one embodiment of the present invention. FIG. 2a illustrates the movement of ions through defects in a multilayer graphene membrane, enabling cation transport due to the negative charge characteristics of the ion channel formed by the defects. However, the ion channel through the defects may not exhibit precise ion selectivity due to the size and irregularity of the defects. In contrast, when nanoflakes of MoS₂ are grown on the defects of the multilayer graphene as shown in FIG. 2b, only those cations that have a size capable of passing through the interlayer gap can move, enabling selective cation transport.

The ion transport structure of the present invention can have varying interlayer gaps depending on the materials of the nanomaterials constituting the membrane and the nanoflakes. Therefore, by selecting suitable materials according to the size of the cations being transported and controlling the interlayer gap, it is possible to adjust ion selectivity.

In one embodiment of the present invention, the ion transport structure may be a structure for lithium ion transport. In this case, the interlayer gap can be utilized to ensure a size of at least 0.3 nm, for example, at least 0.32 nm, to allow lithium ions with a single hydration layer to pass through, and to prevent the passage of other cations to secure lithium selectivity, the interlayer gap is preferably 0.5 nm or less, more preferably 0.4 nm or less, and for example, 0.36 nm or less. In relation to this, in an embodiment of the present invention, when multilayer graphene is used as the nanomaterial for the membrane and MoS₂, a transition metal dichalcogenide, is used as the nanoflake, an interlayer gap of about 0.34 nm is formed, confirming the implementation of an optimal ion channel for selective transport of lithium ions.

The ion transport structure according to the present invention can be manufactured through a step of depositing a first nanomaterial on a substrate to form a membrane made of a two-dimensional nanomaterial having defects; and a step of depositing a second nanomaterial on the membrane to form nanoflakes over the defects.

In the present invention, the deposition of the first and second nanomaterials can be performed using chemical vapor deposition (CVD). Chemical vapor deposition is a process in which materials are deposited due to the reaction of precursor gases under high-temperature conditions, and as a substrate used in the membrane formation step, metal foils, for example, catalytic metal foils such as nickel foil or copper foil can be utilized.

After forming the membrane, a protective layer can be formed on one surface of the membrane and then the substrate can be removed. The protective layer may comprise polymer layers such as polymethyl methacrylate (PMMA), polyethylene oxide (PEO), or polyvinylidene fluoride (PVDF), and the substrate can be removed using an etchant. Furthermore, before forming the nanoflakes, the membrane with the protective layer can be transferred to another substrate, and the protective layer removed before proceeding with the process.

Thus, the two-dimensional nanomaterial in the form of membranes formed by chemical vapor deposition may comprise inherent defects. These defects carry a negative charge, which facilitates the adsorption of atoms during additional nanomaterial formation and shows favorable effects for nucleation and material growth. Therefore, when growing the nanomaterial on the membrane, it can grow in the form of nanoflakes covering the defects of the membrane.

In an exemplary embodiment of the present invention, when using graphene as the nanomaterial, graphene can be deposited by injecting methane (CH₄) gas, a carbon source, at a temperature range of 800 to 1,200° C. In this case, the pressure condition may be maintained at a vacuum condition of less than 3×10⁻³ torr. Additionally, when using transition metal chalcogenides such as MoS₂ as the nanomaterial, a CVD process can be conducted by injecting transition metal and chalcogen precursors at a temperature range of 500 to 900° C., maintaining the reaction pressure conditions between 5 to 20 torr, for example, between 8 to 15 torr.

The ion transport structure of the present invention is excellent in cation transportability, enabling selective transport of cations based on the size of the passageway formed by the interlayer gap between the membrane and the nanoflakes. Therefore, the present invention can be widely applied to the development of various ion transport layers for lithium ions, sodium ions, magnesium ions, calcium ions, sodium ions, potassium ions, and other ions, and is particularly useful for unidirectional transport of small cations such as lithium ions.

Accordingly, the present invention may also provide a lithium extraction apparatus comprising the ion transport structure.

The lithium extraction apparatus comprises an anodic cell and a cathodic cell, and the ion transport structure of the present invention serves as a separator for lithium ion extraction positioned between the anodic cell and the cathodic cell.

In the lithium extraction apparatus, the anodic cell may comprise an anode and a lithium ion-containing solution, while the cathodic cell may comprise a cathode and an acidic solution and lithium can be extracted by applying voltage to the lithium extraction apparatus to move lithium ions from the anodic cell to the cathodic cell.

The lithium ion-containing solution applied to the anodic cell may be a solution containing lithium ions that can serve as a source of lithium, for example, natural resources such as seawater or brine. By using the ion transport structure of the present invention, high-purity lithium can be extracted from natural resources containing a large number of other cations in addition to lithium ions.

The acidic solution applied to the cathodic cell may be a solution containing at least one acid selected from the group consisting of hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), and phosphoric acid (H₂PO₄). By applying an acidic solution to the cathodic cell, when an electrical imbalance occurs between the cells due to the movement of lithium ions, hydrogen ions can be consumed to maintain electrical neutrality. As a result, lithium ions can continuously move from the anodic cell to the cathodic cell, and hydrogen gas can be generated in the cathodic cell, thereby simultaneously securing additional energy resources.

In the lithium extraction apparatus of the present invention, the structure is not limited as long as a separator for lithium extraction is interposed on the surface where the anodic cell and the cathodic cell contact, configured to allow lithium ions to move from the lithium ion-containing solution of the anodic cell to the acidic solution of the cathodic cell. For example, the anodic cell and cathodic cell may be arranged horizontally, or the cathodic cell may be positioned within the anodic cell.

In the present invention, it is preferable that the solution level of the acidic solution in the cathodic cell is lower than that of the lithium ion-containing solution in the anodic cell. For example, the difference in solution levels may range from 0.1 to 10 cm, preferably from 0.3 to 1 cm.

Preferably, the solution level of the acidic solution may be lower by 2 to 20%, for example, by 3 to 10%, compared to the solution level of the lithium ion-containing solution. When the solution level of the acidic solution is lower, the transmembrane hydraulic pressure (TMHP) due to the difference in solution levels acts, making it possible to operate the apparatus at a lower voltage compared to the case with no difference in levels, thus allowing lithium to be effectively extracted with a small amount of electrical energy. For example, lithium extraction from the apparatus is possible even when applying a voltage of 1.5V or less, preferably 1.4V or less, due to the difference in solution levels.

In the present invention, the transmembrane hydraulic pressure generated by the difference in levels may range from, for example, 5 to 500 Pa, specifically from 20 to 100 Pa.

In the lithium extraction apparatus of the present invention, when the anodic cell and the cathodic cell are arranged in a horizontal direction, it is preferable to partition the area near the surface of the solution to prevent the movement of the solution, thereby maintaining different solution levels.

Accordingly, in the lithium extraction apparatus, it is preferable that the length of the separator is shorter than the height of the anodic cell and the cathodic cell, and the areas not interposed by an ion channel between the anodic cell and the cathodic cell are partitioned to prevent the movement of lithium ions.

Specifically, in the structure where the anodic cell and the cathodic cell are partitioned, the separator for lithium extraction itself can be made shorter, or the edges of the separator can be molded with an impermeable material to ensure that the length of the ion-permeable arca is shorter than the height of the anodic cell and the cathodic cell. The upper part of the cells where the solution level is located can be partitioned to prevent the movement of lithium ions. For example, a structure can be used where a lower passage is formed between separate anodic and cathodic cells, and the separator is interposed only in this passage.

In one embodiment of the present invention, the lithium extraction apparatus may further include a feeding counterpart (FC) connected to the anodic cell and a collecting counterpart (CC) connected to the cathodic cell. The feeding counterpart can accommodate a lithium ion-containing solution and be connected to allow movement of the lithium ion solution with the anodic cell. The collecting counterpart can accommodate an acidic solution and be connected to allow movement of the acidic solution with the cathodic cell.

In this structure, the solution is circulated between the anodic cell and the feeding counterpart, allowing lithium ions to be continuously supplied to the anodic cell. Similarly, the circulation of solution between the cathodic cell and the collecting counterpart enables the extracted lithium ions from the cathodic cell to move to the collecting counterpart, where hydrogen ions are continuously supplied by the acidic solution. For solution circulation, the anodic cell and feeding counterpart, as well as the cathodic cell and collecting counterpart, can be connected by fluid transfer pumps, and the apparatus may further include a drive unit to operate these pumps.

Since the ion transport structure of the present invention excels in ion transportability and selectivity, applying it as a separator in a lithium extraction apparatus allows for the efficient extraction of high-purity lithium from solutions containing various ions such as seawater and brine.

Examples

The following examples are provided to describe the present invention in more detail. However, these examples are presented to illustrate certain experimental methods and compositions, and the scope of the present invention is not limited to these examples.

Preparation Example 1: Fabrication of Lithium Ion Transport Layer

To fabricate the lithium ion transport layer, MoS₂ nanoflakes were selectively grown on the defect sites of a multi-layer graphene membrane (MGM).

High-purity Ni foil (99.99%, Sigma-Aldrich) was loaded into a CVD reactor, maintaining a vacuum of less than 3×10⁻³ torr, and the reactor was heated to above 1,000° C. After heat-treating for 30 minutes, a mixture of Ar/H₂ (10:1) and CH₄ gas were injected into the reactor. After 2 minutes, the reactor was cooled to room temperature to form a multi-layer graphene membrane (MGM) on the Ni foil.

Next, polymethyl methacrylate (PMMA) was coated on one side of the MGM, and the underlying nickel was etched away using a Ni etchant (UN2796 sulfuric acid, Transene Company, Inc.). The MGM supported by PMMA was transferred to a SiO₂/Si wafer, and the PMMA was removed using acetone. The sample was then loaded into an MoS₂ MOCVD reactor, and molybdenum and sulfur precursors were injected with Ar gas at a pressure and temperature of 10 torr and 650° C., respectively, to grow MoS₂ in the form of nanoflakes on the defect areas of the MGM, thus creating the ion transport structure (MFs-on-MGM).

Preparation Example 2: Fabrication of a Lithium Extraction Apparatus Using the Lithium Ion Transport Layer

Using the lithium ion transport layer from Preparation Example 1, a lithium extraction apparatus with the structure shown in FIG. 3 was fabricated.

The lithium ion transport layer prepared in Preparation Example 1 was transferred onto an anodic aluminum oxide (AAO) support to ensure stability, and polydimethylsiloxane (PDMS) was molded around the edge, excluding a 0.5 cm diameter Li⁺ transport window, as shown in FIG. 4, to fabricate the separator.

The separator was placed between the anodic cell and cathodic cell of an electrochemical cell. Artificial seawater with a lithium concentration of 20 ppm was introduced into the anodic cell, and a 5 mM HCl solution was introduced into the cathodic cell. Electrodes were positioned in each cell, and the two electrodes were connected to an external voltage bias. Platinum (Pt) wire was used as both the anodic and cathodic electrodes.

Additionally, a feeding counterpart (FC) for seawater circulation was connected to the anodic cell, and a collecting counterpart (CC) for acidic solution circulation was connected to the cathodic cell. Fluidic pumps were introduced for solution circulation. Approximately 1 L each of seawater and HCl solution were placed in the feeding and collecting counterparts, respectively, to complete the lithium extraction system. A photograph of the lithium extraction system fabricated using an H-type electrochemical cell is shown in FIG. 5.

Experimental Example 1: Structural Analysis of the Lithium Ion Transport Layer Using Scanning Electron Microscopy

A field emission scanning electron microscope (FE-SEM) analysis was performed on the lithium ion transport layer from Preparation Example 1 using the JSM-7600 (JEOL Ltd.).

The left image in FIG. 6 shows an SEM image of the multi-layer graphene membrane (MGM), it can be confirmed that a linear network structure was observed on the surface in the MGM grown on Ni foil via CVD. When the Ni etchant was applied to the MGM, the etchant penetrated below the linear network, indicating that this network structure represents grain boundary defects in the MGM.

Meanwhile, the right image in FIG. 6 shows an SEM image after the growth of MoS₂ nanoflakes on the MGM. It shows that the growth of MoS₂ nanoflakes primarily followed the MGM network and was tightly connected in linear formations. Furthermore, each nanoflake was found to typically have a triangular shape with one side measuring between 80 and 120 nm.

FIG. 7 presents the Raman spectrum analysis results of the lithium ion transport layer, indicating that the MoS₂ nanoflakes exhibit crystallinity, primarily having a monolayer or bilayer structure, as evidenced by the 22.5 cm−1 in intensity ratio of 2D and G (difference between the I_2D/I₁₂g mode (383.6 cm⁻¹) and the A_1g mode (406.1 cm⁻¹)).

Experimental Example 2: Structural Analysis of the Lithium Ion Transport Layer Using Transmission Electron Microscopy

A transmission electron microscopy (TEM) image was obtained of the lithium ion transport layer from Preparation Example 1 using a JEM-2100F (JEOL Ltd.), and the crystal structure was analyzed through selected area electron diffraction (SAED).

FIG. 8 shows the TEM image of the lithium ion transport layer, confirming that the MoS2 nanoflakes are connected along the dotted lines.

FIG. 9 presents the SAED results for the region highlighted in red in FIG. 8, showing multiple pairs of hexagonal patterns. Referring to this crystallographic analysis, the 6-fold symmetric dot patterns indicated by the blue and red lines correspond to the hexagonal honeycomb lattice of graphene, while the other hexagonal patterns indicated by the yellow line represent MoS₂. This confirms the correlation to the grain boundaries of the MGM.

Additionally, referring to the dark-field transmission electron microscopy (dark-field TEM, DF-TEM) image of FIG. 10 obtained from MoS₂ diffraction patterns, it can be confirmed that the grain boundaries of the MGM are mostly covered by MoS₂ nanoflakes.

From these selective growth results, it can be seen that the defects in graphene favor the adsorption of Mo and S adsorbate atoms, as well as nucleation and the growth of nanoflakes, and some separated nanoflakes cover defects such as vacancies in graphene.

Experimental Example 3: Analysis of Material Transport Characteristics of the Lithium Ion Transport Layer

A liquid infiltration experiment was conducted on the lithium ion transport layer from Preparation Example 1 to verify its material transport characteristics.

To measure the amount of material movement, as shown in FIG. 11, the multi-layer graphene membrane (MGM) and ion transport layers (MFs-on-MGM) prepared in Preparation Example 1 were floated in the electrolyte, and the permeation time of 0.1 M LiCl droplets of 10 μL was recorded. To account for loss due to droplet evaporation, a SiO₂/Si wafer was used as a comparative sample, and the experimental photos are shown in FIG. 12.

The top portion of FIG. 12 shows the experimental image for the multi-layer graphene membrane without nanoflakes, where the droplet deposited on the multi-layer graphene gradually permeated through the membrane, completely disappearing within 18 minutes. In contrast, the droplet on the lithium ion transport layer according to the present invention, as seen in the bottom of FIG. 12, showed minimal permeation and maintained its original size, similar to the droplet on a SiO₂/Si substrate.

These results confirm that the ion transport layer fabricated according to the present invention, with nanoflakes grown on the defects of multi-layer graphene, successfully prevents indiscriminate material transport.

Experimental Example 4: Analysis of Ion Transport Characteristics of the Lithium Ion Transport Layer Using Chronoamperometry

Ion transport behavior of the ion transport layer from Preparation Example 1 was observed through chronoamperometry (CA) measurements.

In electrolytes containing LiCl, NaCl, KCl, MgCl₂, CaCl₂, NiCl₂, and CoCl₂, an inactive graphite electrode was positioned as shown in FIG. 13, with the ion transport layer from Preparation Example 1 applied as a separator. A bias voltage was applied between the electrodes to obtain a chronoamperometry graph.

Referring to the chronoamperometry graph in FIG. 14, it can be observed that the current instantaneously increases at each bias step and gradually saturates to a specific value. This current spike corresponds to the capacitive current (Ic) caused by the accumulation and release of ions in the electric double layer (EDL) of the graphite electrode. In contrast, the phenomenon of saturation of the current to a specific value signifies continuous ion transport through the separator. Comparing the curves for each electrolyte, it was confirmed that the ion transport characteristics of the LiCl electrolyte were significantly superior to those of the other electrolytes.

FIG. 15 illustrates the relative ratios of LiCl and other electrolytes (i.e., lithium ion selectivity) based on the I_M plot and bias voltage. It confirms that the ion transport layer from Preparation Example 1 demonstrates excellent lithium ion selectivity. Moreover, when a negative voltage is applied to sequentially pass cations through the primary and secondary ion channels, even better ion selectivity results were observed.

Experimental Example 5: Confirmation of Lithium Extraction Using Chronoamperometry

For the lithium extraction system from Preparation Example 2, chronoamperometry was performed under a cell voltage of −1.5V, and the results are shown in FIG. 16.

Referring to FIG. 16, it can be confirmed that lithium ions (Li⁺) are continuously extracted from the collecting compartment (CC) of the lithium extraction system. Additionally, to secure a high-purity Li⁺ solution, the solution in the feeding counterpart (FC) was changed to the solution from the collecting compartment (CC) obtained from the previous extraction process. Over a period of 20 hours, Li⁺ extraction was repeated 2, 3, and 4 times, and as a result, although the current gradually decreased over time and repetitions due to the decrease in molarity in the feeding counterpart (FC), there were no significant changes in the numerical values.

From the results, it was confirmed that the application of the ion transport layer of the present invention to the lithium extraction system allows stable ion transport and water decomposition reactions, and the system can operate stably to enable high-purity lithium extraction, even with continuous repeated extractions.

Experimental Example 6: Confirmation of Lithium Extraction Using Inductively Coupled Plasma Optical Emission Spectroscopy

The lithium extraction system from Preparation Example 2 was operated to extract lithium ions (Li⁺), and during this process, changes in the composition of the solutions in the feeding counterpart (FC) and collecting counterpart (CC) were monitored using inductively coupled plasma optical emission spectroscopy (ICP-OES).

FIG. 17 presents the ICP-OES results, showing that the concentration of lithium ions (Li⁺) in the artificial seawater of the feeding counterpart (FC) decreased from 23.4 ppm to 1.6 ppm, approximately a 95% reduction, confirming that Li⁺ was extracted into the collecting compartment (CC). Meanwhile, other cations also migrated toward the collecting compartment (CC) in small amounts, but their changes were found to be very minimal.

Additionally, FIG. 18 shows a graph depicting the concentration ratio of Lit to other cations in the collecting compartment (CC), indicating that all elements decreased by more than 10 times. In the fourth extraction, the concentration ratio of Li⁺ to Na⁺ decreased from 1,672 to 0.11, and no other cations were detected.

From these experimental results, it was confirmed that the lithium extraction system of the present invention can efficiently extract high-purity lithium.

In conclusion, specific aspects of the present invention have been described in detail, and it is evident to those skilled in the art that such specific descriptions are merely preferred embodiments and do not limit the scope of the present invention. Therefore, the practical scope of the present invention shall be defined by the appended claims and their equivalents.