Li-S battery separator

Abstract

A lithium-sulfur battery is provided, comprising: (i) a Li anode; (ii) a separator between the anode and cathode; (iii) a Li-containing electrolyte; and (iv) a sulfur-containing cathode, wherein the separator comprises a porous substrate supporting a metal-organic framework containing at least two different metal ions, one of which is an iron ion.

Description

[Technical Field]

[0001] The present invention relates to bimetallic metal-organic frameworks (MOFs) supported on porous substrates that can be used as separators in Li-S batteries. The invention details the manufacturing process for the necessary separators and encompasses Li-S batteries using the separators. These batteries have outstanding performance, particularly with regard to battery capacity retention after repeated charging. [Background technology]

[0002] The ever-increasing reliance on portable / rechargeable energy sources and the urgency of energy storage for renewable energy and green transitions are driving the rapid development of battery technologies with long life, high energy density, material sustainability, and safety. In the rechargeable battery sector, lithium-ion batteries (LiBs) dominate the portable consumer electronics and electric mobility markets and are also expanding into industrial and utility-scale energy storage. LiBs are positioned to play a central role in achieving the European Green Deal, which aims for net-zero greenhouse gas emissions by 2050, particularly in transportation, marine, and grid support applications. However, after more than 30 years of development, current LiB technology is approaching fundamental limitations in terms of energy density, safety, and cost. For example, for electric vehicle (EV) applications, further improvements in energy density are urgently needed to extend the driving range to at least 1000 km. Therefore, the development of battery technologies that can achieve high energy density is actively pursued.

[0003] Lithium-sulfur batteries (Li-S batteries) are considered a breakthrough technology because they combine a specific capacity (1675 mAh / g) five times the theoretical specific capacity of LiB with high specific energy (2600 Wh / kg). Lithium-sulfur batteries (Li-S batteries) are a type of rechargeable battery. Due to the low atomic weight of lithium and the moderate atomic weight of sulfur, Li-S batteries are relatively lightweight, an attractive attribute in environments where weight reduction is critical. The use of sulfur as the cathode, rather than metals such as cobalt that are common in lithium-ion batteries, also makes these batteries economically attractive. Chemical processes in Li-S cells include lithium dissolution from the anode surface (and incorporation into alkali metal polysulfide salts) during discharge and reverse lithium deposition on the anode during charge.

[0004] Compared to current LiBs, Li-S batteries offer many inherent advantages in addition to high energy density, including:

[0005] i) Improved safety through "conversion reactions" that form new substances during charging and discharging.

[0006] ii) Higher mass-energy density than LiBs due to weight reduction achieved by using sulfur and carbon instead of heavy metal oxides, which is a key advantage for applications such as wearable devices, vehicles, medical equipment, drones, and aircraft.

[0007] iii) Significant reduction in raw material costs: Sulfur costs $0.22 / kg, less than 1% of the cost of lithium cobalt oxide (the main material used in LiB cathodes).

[0008] iv) Higher charge capacity rating. The chemical design allows for faster charging.

[0009] v) Low risk of battery failure as the highly reactive Li anode is passivated by sulfide material during operation.

[0010] However, despite these advantages, Li-S battery technology has not yet been fully commercialized due to the following significant problems:

[0011] i) Due to the insulating properties of sulfur and lithium sulfide, the utilization rate of sulfur is low, making it difficult to reach the theoretical capacity.

[0012] ii) The polysulfide "shuttle effect," i.e., the migration of dissolved intermediate polysulfides (i.e., Li2S8, Li2S6, and Li2S4) through the separator and their reduction on the anode side, causing loss of capacity and active material.

[0013] iii) Short circuiting of Li-S batteries occurs due to the formation of lithium dendrites caused by non-uniform transport of Li+ through the separator during battery cycling.

[0014] iv) Cathode degradation due to loss of active sulfur during charging and discharging.

[0015] Significant efforts have been made to address these issues. For example, numerous conductive matrix materials have been designed to optimize the cathode for good electrical conductivity and confine sulfur to prevent cathode expansion during charging and discharging. Studies have also been reported on the application of different coating materials to separators based on the polar interface between polar lithium polysulfide (LiPS) and polar host materials, surface chemistry for polysulfide grafting and chain reactions, and metal-sulfur bonding. However, these conventional coating materials and sulfur hosts cannot prevent the migration of soluble polysulfides from the cathode to the anode, and cathode materials still suffer from low electronic conductivity and low sulfur loading. The most serious problem is that the cycle stability achieved so far is still far from satisfactory due to volume changes in the cathode and loss of active material during charging and discharging.

[0016] The inventors have discovered that the application of a superselective separator that blocks dissolved polysulfides while allowing Li+ ions to pass through is an effective solution to many of the above problems. Thus, a carefully designed separator can mitigate, among other issues, the polysulfide shuttle effect and lithium dendrite formation.

[0017] Lithium metal is used as the anode in Li-S batteries. At the anode surface, dissolution of metallic lithium and production of electrons and lithium ions occur during discharge, and electrodeposition occurs during charge. The half-reactions are expressed as follows:

[0018] [ka]

[0019] Similar to lithium batteries, this dissolution / electrodeposition reaction leads to unstable growth of the solid electrolyte interface (SEI), creating active sites for lithium nucleation and dendrite growth, which can lead to internal short circuits in lithium batteries and ultimately end their lifespan.

[0020] In a Li-S battery, energy is stored in the sulfur cathode. During discharge, lithium ions in the electrolyte migrate to the cathode and sulfur is reduced to lithium sulfide (LiS). During charge, sulfur is reoxidized to S. Thus, the half-reactions are:

[0021] [ka]

[0022] In reality, the reduction of sulfur to lithium sulfide is much more complex, resulting in lithium polysulfides (Li2S x , 2≦x≦8).

[0023] Upon discharge, the end product is not simply Li2S, but a mixture of Li2S2 and Li2S. This is due to the slower reduction reaction rate of Li2S. This contrasts with conventional lithium-ion cells, where lithium ions reside in the anode and cathode. Each sulfur atom can host two lithium ions. Typically, lithium-ion batteries accommodate only 0.5-0.7 lithium ions per host atom.

[0024] Another significant problem with Li-S cells is undesirable reactions with the electrolyte. While S and Li2S are relatively insoluble in most electrolytes, many intermediate polysulfides are soluble. Li2S n Dissolution of (n>2) in the electrolyte causes irreversible loss of active sulfur from the cathode, which also severely limits battery life.

[0025] This phenomenon is known as the polysulfide "shuttle." Historically, the "shuttle" effect has been the primary cause of degradation in Li-S batteries. Lithium polysulfides, Li2Sx (6 ≤ x ≤ 8), are highly soluble in the electrolytes commonly used in Li-S batteries. They form during battery discharge, migrate out of the cathode, and diffuse to the anode, where they are reduced to short-chain polysulfides and then diffuse back to the cathode to reform long-chain polysulfides. This process results in continuous loss of active material from the cathode, lithium corrosion, low coulombic efficiency, and reduced battery life. Furthermore, the "shuttle effect" leads to the anomalous self-discharge of Li-S batteries due to the slow dissolution of polysulfides (which occurs even under resting conditions). The electrolyte plays an important role in Li-S batteries, both by providing the "shuttle" effect through polysulfide dissolution and by stabilizing the SEI on the anode surface.

[0026] Conventionally, Li-S batteries use a liquid organic electrolyte enclosed within the pores of a polypropylene separator that separates the anode and cathode. However, these separators do nothing to address the polysulfide shuttle effect.

[0027] For this reason, many researchers have used various MOFs to modify separators in Li-S batteries. US2020 / 0220136 shows an example of UiO-66 MOF with Zr ions on a polymer support. However, no examples of the use of bimetallic materials have been described.

[0028] CN113410575 also describes MOFs attached to supports to form membranes for Li-S batteries, proposing FJU-88 or FJU-90 MOFs using Co metal ions.

[0029] However, the interception of polysulfides solely through the specific pore size of MOF materials is insufficient for long-term control of polysulfide efflux. In this regard, it is crucial to introduce a second active metal site as an electrocatalyst for the electrocatalytic conversion of the intercepted or adsorbed polysulfides into active materials.

[0030] CN107681091 describes a bimetallic separator based on BMZIF-5, which is calcined with zinc and cobalt ions for carbonization. The carbonized material is then mixed with PVDF to form a membrane coating. Thus, functionalized composite membranes for lithium-sulfur batteries are characterized by a membrane substrate, a nitrogen-cobalt-doped graphitized carbon material, and a binder. However, the fabrication of such materials is cumbersome and costly. It would be desirable to eliminate the need for carbonization of MOFs. From a sustainability perspective, calcination of MOFs requires high temperatures (approximately 1000°C) and the prolonged use of expensive gases (such as argon and hydrogen), which not only increases the cost of the materials but also poses environmental risks. Furthermore, high-temperature heating causes the MOF materials to lose their inherent porosity, degrading the MOF structure that actually aids in sorting polysulfides and lithium ions. [Prior art documents] [Patent documents]

[0031] [Patent Document 1] US Patent Application Publication No. 2020 / 0220136 [Patent Document 2] Chinese Patent No. 113410575 [Patent Document 3] Patent No. 107681091 Specification [Non-patent literature]

[0032] [Non-Patent Document 1] Xiaolong et al., J. Energy Chem, vol 82, 2023 [Non-patent document 2] Pingli et al., Chinese Chem. Letters vol 34, 2022 Summary of the Invention [Problem to be solved by the invention]

[0033] Xiaolong et al. (J. Energy Chem, vol 82, 2023) describe a modified battery separator of Ni-Co bimetallic MOF@PAN, fabricated by electrospinning, and Pingli et al. (Chinese Chem. Letters vol 34, 2022) describe a Ni-Co bimetallic MOF with CNTs as a battery separator. [Means for solving the problem]

[0034] We have established that cost-effective bimetallic MOF separators can be fabricated to selectively block and convert dissolved polysulfides in Li-S batteries while selectively passing Li+ ions. Our Fe-doped ZIF-8 and Fe-doped NHI-IO66 exhibited significantly higher catalytic activity for polysulfide conversion compared to their parent compounds, ZIF-8 and NHI-IO66. Meanwhile, the incorporation of Fe(II) ions into the ZIF structure dramatically improved the specific capacity and rate capability. Li-S batteries using the Fe-ZIF-8 / PP separator exemplified herein exhibited high cycle life of 1000 cycles and high initial capacities of 863 mAh / g at 0.5 C and 746 mAh / g at 3 C. Our Li-S batteries using bimetallic MOF-modified separators exhibited high capacity and long cycle life, even at high charge / discharge rates. Furthermore, the Fe-ZIF-8 / PP separator offers good sulfur electrochemical performance even under the relevant conditions of high sulfur loading and low-concentration electrolyte, and the Li||Li symmetric cell using this Fe-ZIF-8 / PP separator exhibits currents up to 10 mA / cm 2 The results showed excellent cycle performance at high current densities of 1000 sq. m.

[0035] The use of Fe as one of the constituent metals in bimetallic MOFs offers several additional advantages: iron is abundant and inexpensive, making iron-based bimetallic MOFs economically attractive, and iron is safe and poses limited environmental concerns.

[0036] Iron is non-toxic and does not pose a significant health risk when recycled, unlike many other metals that can be harmful to humans and the environment. [Brief explanation of the drawings]

[0037] [Figure 1]Figure 1a shows a coin cell fabricated with an S-carbon cathode, Celgard (polypropylene) or the MOF-modified separator prepared in Example 1, liquid electrolyte, Li anode, spacer, spring, and top lid. Figure 1b is a schematic diagram of the functional bimetallic 3D MOF-based separator of Example 1, specifically designed for Li-S batteries. [Figure 2] 2a and 2b show SEM images of a Celgard (PP) separator and the MOF-coated Celgard separator of Example 1. [Figure 3] Figures 3a-3d are digital images of the modified separator. [Figure 4] FIG. 4 shows the heat shrinkage test results of Celgard and the MOF-coated Celgard separator of Example 1 at room temperature and 150°C. [Figure 5] 5a and 5b show polysulfide permeation tests using a PP separator and the MOF-coated PP separator of Example 1. [Figure 6] Figure 6 shows the Li deposition / dissolution performance under different current densities at an areal capacity of 1 mAh / cm2 in symmetric cells using a PP separator and a MOF / PP separator. [Figure 7] FIG. 7 shows the ultra-long cycle performance of the symmetric cell using the MOF / PP separator of Example 1. [Figure 8] Figures 8a and 8b are SEM images of Li anodes with a PP separator (Figure 8a) and a MOF-protected PP separator (Figure 8b). [Figure 9] Figure 9 shows the cyclic voltammetry curves of Li-S batteries using PP separator and MOF / PP separator in the voltage window of 1.7–2.8 V at a scan rate of 0.1 mV / s. [Figure 10] Figure 10 shows the rate performance at different charge-discharge rates (C-rates) of Li-S batteries using a PP separator (lower curve) and an MOF / PP separator (upper curve) in the voltage window of 1.7-2.8 V. [Figure 11]FIG. 11 shows the cycling performance of Li-S batteries using a PP separator (lower curve) and a MOF / PP separator (upper curve), an S-carbon cathode, and a Li anode in a coin cell described herein. [Figure 12] FIG. 12 shows electrochemical polarization tests of the PP separator and MOF / PP separator to further confirm the improved conversion efficiency of polysulfides. DETAILED DESCRIPTION OF THE INVENTION

[0038] [Summary of the Invention] In one aspect, the present invention provides a method for producing a semiconductor device comprising: (i) a Li anode; (ii) a separator between the anode and the cathode; (iii) a Li-containing electrolyte; (iv) a sulfur-containing cathode; The separator provides a lithium-sulfur battery that includes a porous substrate supporting a metal-organic framework containing at least two different metal ions, one of which is an iron ion.

[0039] In another aspect, the present invention provides a method for producing a method of manufacturing a semiconductor device comprising: (i) a Li anode; (ii) a separator between the anode and the cathode; (iii) a Li-containing electrolyte; (iv) a sulfur-containing cathode; The separator provides a lithium-sulfur battery that includes a porous substrate supporting a non-carbonized metal-organic framework containing at least two different metal ions.

[0040] In another aspect, the present invention provides a method for producing a method of manufacturing a semiconductor device comprising: (i) a Li anode; (ii) a separator between the anode and the cathode; (iii) a Li-containing electrolyte; (iv) a sulfur-containing cathode; The separator provides a lithium-sulfur battery, wherein the separator includes a porous substrate supporting a metal-organic framework containing at least two different metal ions, the metal-organic framework being a zeolite-like imidazolate structure.

[0041] Viewed from another aspect, the present invention provides a separator suitable for use in batteries, such as lithium-sulfur batteries, comprising a substrate having deposited thereon a metal-organic framework containing at least two different metal ions, one of which is an iron ion.

[0042] In another aspect, the present invention provides a method for producing a method of manufacturing a semiconductor device comprising: 1) dissolving and mixing two metal salts in a solvent in the presence of an imidazole-based ligand or a tricarboxylic or dicarboxylic acid (e.g., 1,4-benzenedicarboxylic acid) to form a precipitate containing a metal-organic framework containing at least two different metal ions, one of which is an iron ion; 2) separating the precipitate, forming a slurry with the separated material, and coating the slurry on a porous substrate.

[0043] [Mode for Carrying Out the Invention] The present invention relates to a novel separator for use in batteries, particularly Li-S batteries. In Li-S batteries, a separator is used to separate electrodes, i.e., the separator separates the anode and the cathode. Preferably, the anode contacts one side of the separator, and the cathode contacts the other side of the separator. The separator typically has a layered structure in Li-S battery cells, comprising a substrate layer coated on one or both sides with a metal-organic framework containing at least two different metal ions, one of which is iron ion. The metal-organic framework containing at least two different metal ions can be further protected by an inert protective layer.

[0044] Traditionally, separators for Li-S batteries have simply been made from porous polymer materials such as polyolefins (e.g., polyethylene, more preferably polypropylene). Other polymers used as separators include polyesters, polyolefins, polyamides, polyacrylonitriles, polyimides, polyetherimides, polysulfones, polyamideimides, polyethers, polyphenylene sulfides, and aramids, as well as mixtures of two or more of these polymers. These conventional separators can be used as substrates in the present invention.

[0045] In the present invention, the separator used in the present invention comprises a substrate layer on which is deposited a metal-organic framework containing at least two different metal ions, one of which is an iron ion, referred to herein as a bimetallic MOF.

[0046] In particular, the separator used in the present invention includes a porous substrate layer functionalized to support a bimetallic MOF. The bimetallic MOF used in the separator of the present invention is preferably synthesized separately using conventional techniques and then coated onto the substrate. Thus, the bimetallic MOF can be formed on the substrate as a thin film having a thickness of, for example, 15 μm or less, e.g., 7 μm or less, e.g., 1.0 to 15 μm, preferably 1.0 to 5.0 μm. The bimetallic MOF can be deposited on one or both sides of the substrate. However, it is preferred that the bimetallic MOF layer be present between the cathode and the substrate layer.

[0047] The porous substrate can be made of any suitable material, but is typically a polymer substrate such as polyolefin (e.g., polyethylene and polypropylene), polyester, polyamide, polyacrylonitrile, polyimide, polyetherimide, polysulfone, polyamideimide, polyether, polyphenylene sulfide, and aramid. Polypropylene is preferably used as the substrate material. The substrate layer may have a thickness of 10 to 50 μm. The polypropylene can be woven or nonwoven.

[0048] In one embodiment, the substrate, such as a polypropylene substrate, can be a nonwoven or meltblown fabric.

[0049] It is particularly preferable to use a porous polypropylene film or a porous polypropylene nonwoven fabric or meltblown fabric.

[0050] Furthermore, to meet the general requirements of battery separators, such as high chemical stability to electrolyte and electrode materials, thinness, adequate porosity (about 40%), good wettability, low thermal shrinkage, pore size smaller than that of the active material and narrow pore size distribution, good mechanical stability, and strength, polypropylene can be modified with different polymer solutions, such as Nafion, polysulfone, and AF-2400, to enhance the porous structure of the substrate in Li-S battery applications.

[0051] The bimetallic MOF is preferably non-carbonized. Non-carbonized means that the formed MOF is not subjected to a calcination treatment aimed at carbonizing the contained organic matter. The calcination treatment is usually carried out at a temperature of at least 700°C. One advantage of this example is that the required bimetallic MOF can be prepared by a low-temperature process using only water as a solvent, as described below. In this invention, the MOF is synthesized and the pristine bimetallic MOF is prepared by adding Fe as a second metal to the parent MOF (e.g., ZIF-8, which contains Zn metal and has a unique 3D flower-like morphology). Furthermore, the bimetallic MOF is synthesized only in water at extremely low temperatures (e.g., 35°C).

[0052] The bimetallic MOFs of the present invention contain at least two metal ions, one of which is an iron ion, such as an Fe ion, and at least one of which is a first transition series metal ion. + It is preferred to use metal ions in their oxidized state.

[0053] In a preferred embodiment, only two metals, ideally two first transition series metals including Fe (e.g., Fe and one of Co, Zn, Zr, Mn, and Cr), are used to prepare the bimetallic MOF used in the separator. In particular, Fe ions are used in combination with a second first transition series metal (e.g., Zn ions and Fe ions), or Cr ions and Fe ions. Preferably, Ni ions are absent. Preferably, Co ions are absent.

[0054] One advantage of using Fe and Zn together, especially with ZIFs such as ZIF-8, is the cost, safety, and environmental impact. Such a combination avoids the use of more toxic metals such as Co and Ni, which are also much more difficult to source, resulting in increased costs and potential supply chain risks.

[0055] The molar ratio of metal ions may be in the range of 20:1 to 1:20, for example, 10:1 to 1:10, preferably 1:4 to 4:1. When Zn and Fe are used, it is preferable to use Zn metal ions in molar excess. When Cr and Fe are used, it is preferable to use Cr metal ions in molar excess. For example, Cr or Zn:Fe = 20:1 to 1:1 (e.g., 10:1 to 1:1 or 10:1 to 3:1).

[0056] The Fe sites act as efficient electrocatalysts for polysulfide adsorption and conversion. The use of Fe(II) is particularly preferred. Generally, a second metal ion is preferred as an electrocatalyst for polysulfide adsorption and conversion. Therefore, adding another metal to MOFs to form so-called bimetallic MOFs improves polysulfide conversion and leads to high specific discharge capacities.

[0057] The MOF preferably comprises at least 60 wt%, such as 75 wt% or more of the weight of the bimetallic MOF, and thus metal ions comprise no more than 40 wt%, such as no more than 25 wt%.

[0058] Applying the bimetallic MOF to the substrate may require the use of an adhesive. Any conventional adhesive can be used as long as it does not affect the battery's functionality and is inert.

[0059] The use of bimetallic MOFs as coatings on substrates offers the advantage that this type of separator prevents the permeation of lithium sulfide. The metal nodes within the MOF act as anchoring sites for lithium sulfide adsorption. The small pore diameter and pore window size allow it to act as a molecular sieve for Li and polysulfides.

[0060] Therefore, the present invention solves the polysulfide shuttle problem because the sulfur within the pores of the MOF is electrochemically attracted to the metal ions and cannot easily escape from the pores of the MOF. Furthermore, during battery operation, the separator of the present invention promotes the formation of LiS and LiS rather than soluble sulfides such as LiS (where x = 8, 6, 4, or 3).

[0061] Li-S batteries using the claimed bimetallic MOF separators exhibit high specific capacity even at high charge / discharge rates. Furthermore, the Li-S batteries of the present invention exhibit excellent capacity retention. This is achieved through an environmentally friendly and cost-effective synthesis that uses only water as a solvent and can be carried out at only 35°C.

[0062] [Metal-organic structure] Metal-organic frameworks (MOFs) are porous nanomaterials in which metal ion clusters are linked in a three-dimensional structure by organic ligands. Due to the variety of available building blocks, over 20,000 different MOFs have been reported to date. Between the organic and inorganic building blocks, cavities with distinct openings are formed, referred to herein as pores. These pores have a volume (i.e., pore diameter) and pore openings (i.e., pore windows), which determine the size of molecules that can enter and escape from the pores. Depending on the choice of building blocks, pore openings can be as large as 10 nm, resulting in internal pore surface areas of 1,000–10,000 m².2 / g range.

[0063] The growth mechanism of MOFs has been widely studied, and it is generally believed that the MOF formation process occurs via nucleation and diffusion, i.e., nuclei with adsorbed organic ligands on the surface assemble to form inorganic-organic crystals. MOF formation can be explained in three steps. First, the organic ligands are deprotonated, followed by complexation with metal ions. Second, these metal-ligand complexes or oligomers are formed in large quantities, which can then fuse to form MOF crystals. Further growth of these particles is driven by the diffusion of oligomers to the particle surface. Finally, growth is terminated when the system reaches equilibrium with the solvated species in solution or by the use of end-capping agents.

[0064] Of course, as the MOF grows, more and more metal ions are incorporated into the structure, so the MOF growth process typically requires the presence of the same metal salts used in the coordination step.

[0065] Ligands used in MOF growth are well known and are based on multifunctional organic ligands (e.g., containing carboxyl groups, amine groups, and optionally other functional groups). The MOFs used in the separators of the present invention are preferably prepared using nitrogen-containing ligands. In one embodiment, imidazole-based ligands, such as 2-methylimidazole salts, are used. In one embodiment, multifunctional organic ligands containing at least one carboxyl group, such as carboxylic acids, are used. Ligands containing at least two carboxyl groups are preferred. The ligands are generally small molecules with Mw up to 300 g / mol.

[0066] Therefore, one option is to use small tricarboxylic or dicarboxylic acids such as 1,4-benzenedicarboxylic acid or 1,3,5-benzenetricarboxylic acid. The ligands of interest often contain aromatic rings, such as phenyl rings. Some ligands, such as 2-aminoterephthalic acid, contain both carboxyl and amino groups.

[0067] NH2-UiO-66(Zr) or MIL101(Cr) are options for MOFs. By adding Fe ions to these, the bimetallic MOFs required for this invention can be formed. Therefore, the metal ion combination can be Zr / Fe or Cr / Fe.

[0068] In a preferred embodiment, the MOFs used are zeolite-like imidazolate frameworks (ZIFs). These are a type of metal-organic framework (MOF) that are topologically identical to zeolites. ZIF glasses can be synthesized by melt-quenching and contain tetrahedrally coordinated transition metal ions (e.g., Fe, Co, Cu, Zn) linked by imidazolate linkers. The metal-imidazole-metal angle is close to the 145° Si-O-Si angle of zeolites, so ZIFs have a zeolite-like topology. ZIFs such as ZIF-6, ZIF-7, ZIF-8, and ZIF-L are preferred.

[0069] ZIFs are a preferred choice herein because of the excellent chemical stability afforded by the presence of metal-nitrogen bonds in their structure. Ideally, ZIFs contain metal ions or clusters linked by imidazolate-based ligands. Compared to various organic ligands such as carboxylates, sulfonates, and phosphonates, imidazolate-based ligands offer unique advantages, including versatility, stability, tunable properties, redox activity, and structural diversity.

[0070] Furthermore, ZIFs tend to have smaller pore sizes and narrower pore size distributions than other MOFs, which may contribute to suppressing the polysulfide shuttle effect in Li-S batteries.

[0071] In one embodiment, the lithium-sulfur battery of the present invention employs a separator comprising a porous substrate supporting a metal-organic framework containing at least two different metal ions, the metal-organic framework being a zeolitic imidazolate framework. In this embodiment, one of the metal ions is preferably Fe, although any suitable combination of metal ions, such as a combination of first transition series metal ions, can be used in this embodiment. All embodiments described herein relating to the use of Fe ions and a second metal ion in any MOF are applicable to this embodiment of the present invention, in which the MOF is particularly a ZIF.

[0072] Hydrothermal and solvothermal methods are the most frequently reported synthetic techniques for MOF synthesis. In these solution-based methods, a solution containing metal ion precursors and ligand precursors is placed in a sealed reaction vessel and heated to near the boiling point of the solvent used. At this high temperature (and optionally under pressures of 1-200 bar), crystallization of the product occurs.

[0073] Another synthetic technique available for MOF synthesis is ultrasound-assisted synthesis (also known as sonochemical synthesis). In this solution-based method, a solution containing metal ion precursors and ligand precursors is placed in a sonication bath and exposed to high-energy ultrasound for a period of time. The high-energy waves interact with the liquid, forming alternating regions of high and low pressure, which generate cavities within the liquid. These cavities grow due to ultrasound-induced solute vapor diffusion into the cavities, which then become unstable and collapse. At this point, the ultrasonic energy stored within the cavities is rapidly released upon collapse, resulting in localized heating and cooling rates reaching 1000 K / s. This extreme condition leads to molecular excitation, molecular bond scission, and the generation of radicals capable of further reactions, leading to the nucleation and growth of MOF nanoparticles. Compared to other MOF synthesis techniques, sonochemical synthesis can be performed at room temperature or at relatively low synthesis temperatures. Furthermore, the dissolution and mixing of precursors results in a more homogeneous product.

[0074] In the present invention, the hydrothermal method is preferred, so after the coordination step, it is preferred to add a solution of the polyfunctional ligand to a metal salt solution and heat the mixture.

[0075] The molar ratio of the ligand to the metal ion can be varied in the range of 0.25:1 to 4:1, preferably 0.5:1 to 2:1.

[0076] MOF growth can be controlled by using modulators that adjust the nucleation and growth rates of MOF crystals. The role of many modulators, particularly monocarboxylic acids, is to trap MOF particles in the early stages of the nucleation and growth process, reducing the local metal ion concentration and effectively slowing down the growth rate. In this specification, monocarboxylic acids such as acetic acid and formic acid can be used as modulators.

[0077] When a modulator is used, its amount may be in the range of 0.001% to 50% of the solvent used. The modulator can be used to reduce the average particle size of the MOF. In some cases, it is possible to use equal amounts of the solvent and the modulator. Using a large amount of modulator tends to increase the particle size of the MOF in the structure.

[0078] The MOF preferably has a pore size of less than 30 Å, e.g., 2-25 Å or 2-20 Å, and ideally 15 Å or less, e.g., 8-12 Å. The pore size can be adjusted by controlling the nature of the metal ions and ligands, e.g., the length of the ligands. The pore window is preferably 2-10 Å, e.g., 3-5 Å. Such pore sizes are designed to allow the permeation of Li+ ions while preventing the escape of dissolved polysulfides through the pores. Furthermore, the pore windows are designed to completely prevent polysulfides from passing through the separator. These pore sizes and pore windows mitigate the problems of polysulfide shuttle effect and lithium dendrite formation.

[0079] To produce the required separator, the bimetallic MOF must be applied to the substrate surface. The surface coverage of the substrate can be high. Preferably, at least 50% (e.g., at least 80%) of the substrate surface is covered with the MOF. Preferably, the entire surface is covered. An adhesive may be required to help bond the bimetallic MOF to the support. An inert adhesive can be used. Thus, the bimetallic MOF can be applied to the substrate in combination with an inert adhesive. Application can be achieved by simply casting a solution of the bimetallic MOF onto the substrate.

[0080] In a preferred embodiment, bimetallic MOFs are prepared by dissolving metal salts of the required metals in a solvent (typically water). Suitable salts include nitrates, sulfates, and halide salts, although any soluble salt can be used. Sulfates are preferably used.

[0081] The solution also contains the ligands necessary for MOF formation, such as imidazole ligands such as 2-methylimidazole. The mixture can be stirred for extended periods, such as 12 hours or more, to allow for MOF growth. The process can be carried out at low temperatures, between 20 and 50 °C, and excessive heating is not required for MOF growth.

[0082] A notable advantage of the present invention is that bimetallic MOFs, particularly Fe-doped ZIFs (e.g., Fe-doped ZIF-8), can be prepared by a single-step solution-phase synthesis using water as the solvent. Such a process is environmentally friendly and cost-effective. Furthermore, the process can be carried out at low temperatures, such as 20-50°C (e.g., 35°C). In contrast, the process disclosed in (Journal of Energy Chemistry 82 (2023) 484-496) requires heating at 170°C for 6 hours and the use of ethanol as a wash solvent.

[0083] The desired ratio of metal ions in the final MOF can be adjusted by varying the amount of starting salt added.

[0084] The resulting precipitate can be separated and post-treated, for example by recovery by centrifugation, washing, and drying.

[0085] The bimetallic MOF can then be applied to a substrate. This can be easily achieved by simply casting a slurry of the bimetallic MOF onto the substrate. A slurry containing the bimetallic MOF can be easily prepared using SuperP (carbon black) and PVDF in an inert solvent (e.g., N-methylpyrrolidone). The resulting slurry is cast onto the substrate and dried. Carbon black facilitates electron transport and serves to trap polysulfides. Therefore, it is preferable to apply the bimetallic MOF to the substrate in the presence of carbon black in an inert solvent.

[0086] Finally, to protect the bimetallic MOF from damage, a porous protective layer can be applied over it. A separate substrate layer can also be used for this purpose.

[0087] [battery] The separator of the present invention can be used in a Li-S battery. The rest of the battery can be conventional. The anode of such a battery is conventional and can be composed of a Li alloy (e.g., an alloy with Al or Sn) or pure Li. The Li anode can be supported on a current collector such as a steel substrate. The Li anode can be combined with carbon to prevent problems associated with expansion during charge-discharge cycling.

[0088] The cathode of a Li-S battery is elemental sulfur or other electroactive sulfur-containing material, which may be mixed with a conductive material (e.g., carbon black) to improve electrical conductivity.

[0089] Known cathode manufacturing methods involve grinding carbon and sulfur to form a physical mixture, then mixing with a solvent and binder to form a slurry. This slurry is applied to a current collector and dried to remove the solvent. The resulting structure is calendared to form a composite electrode precursor, which is then cut to the desired shape to form the cathode.

[0090] A separator is placed on the cathode, and a lithium anode is placed on the opposite side of the separator. The coated separator can be placed anywhere between the cathode and the lithium anode. An electrolyte is introduced into the cell, wetting the cathode and separator.

[0091] The electrolyte used is typically a liquid organic electrolyte, which may be enclosed within the pores of the separator separating the electrodes. The electrolyte is a non-aqueous electrolyte. The electrolyte contains an organic solvent and a conductive salt. The organic solvent that can be used is one that is inert under the reaction conditions in the battery. The organic solvent is preferably selected from ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, dipropyl carbonate, cyclopentanone, sulfolane, dimethyl sulfoxide, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, 1,2-diethoxymethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate, nitromethane, 1,3-propanesulfone, and mixtures of two or more of these solvents.

[0092] Conductive salts include LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, and LiSO3C. x F 2x+1 , LiN(SO2C x F 2x-1 )2, or LiC(SO2C x F 2x+1 )3 (where 0≦x≦8), Li[(C2O4)2B], and mixtures of two or more of these salts.

[0093] The electrolyte plays an important role in Li-S batteries, both by providing a "shuttle effect" through polysulfide dissolution and by stabilizing the SEI on the anode surface. Traditionally, cyclic ethers (e.g., DOL), short-chain ethers (e.g., DME), and glycol ethers such as DEGDME and TEGDME have been used in Li-S batteries. A typical electrolyte is a 1:1 (volume ratio) DOL:DME solution containing LiTFSI and LiNO3 added as a lithium surface passivator.

[0094] Preferably, polysulfide anions are added to the electrolyte of a lithium-sulfur battery, for example, in the form of Li2S3, Li2S4, Li2S6, or Li2S8. In one embodiment, the amount of polysulfide added is such that the electrolyte is saturated with polysulfide, thereby compensating for sulfur loss in the negative electrode. The polysulfide is preferably added before the battery is put into service.

[0095] [Performance] The Li-S battery of the present invention has outstanding performance, particularly in terms of capacity degradation per cycle. All rechargeable batteries experience a decline in performance over time as they are repeatedly used and charged. The key to the value of a rechargeable battery is minimal degradation of battery performance per cycle (i.e., charge-discharge cycle). It has been demonstrated that the capacity degradation per cycle is less than 0.04% over 1,000 cycles under 0.5 C conditions. The battery of the present invention remains usable even after 4,000 charge-discharge cycles.

[0096] Experimental results confirm that the present invention minimizes the degradation rate per cycle and provides high cycling stability.

[0097] The Li-S battery of the present invention was cycled at 0.5C, 1C, 2C, and 3C, maintaining capacities of 903mAh / g, 830mAh / g, 785mAh / g, and 746mAh / g, respectively. Finally, at 0.5C, the capacity recovered to 882mAh / g, with a capacity degradation rate of only 0.06% per cycle.

[0098] The Li-S battery of the present invention has a sulfur loading per area of ​​0.1 to 9 mg / cm 2 , preferably 0.5 to 5.0 mg / cm 2 may be.

[0099] The Li-S battery of the present invention has a sulfur loading per area of ​​0.1 to 9 mg / cm 2 and can be used with different volumes of electrolyte, such as 5 to 50 μL.

[0100] The amount of sulfur supported per area is calculated by dividing the total area of ​​the electrode (y cm 2 ) means the amount of sulfur (x mg) contained in the Li-S battery. The higher the sulfur loading, the higher the energy density of the Li-S battery. If the sulfur loading is low and a large amount of electrolyte is added, the energy density of the battery decreases. Therefore, the reported sulfur loading per area allows for a reduction in the volume of the electrolyte.

[0101] In the Li-S battery of the present invention, the ratio E:S of electrolyte to sulfur can be 5 to 50 μL of electrolyte: 1 mg of sulfur.

[0102] The bimetallic MOF-coated separator of the present invention has a current density of 10 mA / cm 2 Even under very high currents of 1000 mAh, they exhibit no polarization effect and promote uniform lithium dissolution and deposition in Li||Li symmetric cells, a highly desirable property for the long-term cycling stability of the battery.

[0103] The thermal shrinkage of the separator is an important factor in battery safety. The separator of the present invention is extremely thermally stable and does not shrink, even when heated to, for example, 150°C. Since the substrate shrinks at this temperature, the combination of the MOF and the substrate significantly enhances thermal stability. This excellent heat resistance would prevent internal electrical short circuits at high temperatures during the charge-discharge cycle of the cell. Therefore, from another perspective, the separator of the present invention does not undergo thermal shrinkage even when heated to 150°C.

[0104] The battery of the present invention is capable of rapid charging and is easy to manufacture on a large scale.

[0105] [use] The lithium-sulfur battery of the present invention can be used to supply energy to mobile information devices, tools, electric vehicles, hybrid-powered vehicles, and the like, among others.

[0106] Although the present invention has been described with respect to the use of the separator of the present invention in Li-S batteries, it is believed that the claimed separator is also useful as a separator in other batteries, such as metal-sulfur batteries and lithium-ion batteries.

[0107] Alternative Cathode Embodiments While the separator of the present invention can be used with conventional sulfur-containing cathodes, in another embodiment, the separator of the present invention can be used in combination with certain cathodes, as described below.

[0108] Thus, in one embodiment, the cathode used comprises reduced graphene oxide sheets chemically bonded from the basal plane of the reduced graphene oxide to a metal-organic framework via an oxygen-metal linker, and the metal-organic framework is impregnated with sulfur to form an S-MOF@rGO structure, with the weight of sulfur being 50%-90% of the weight of the S-MOF@GO. Preferably, at least 50% by weight of the MOFs present are bonded to the reduced graphene oxide sheets.

[0109] [Graphene oxide] Graphene oxide (GO) is a derivative of graphene and is characterized as a two-dimensional nanomaterial. Graphene nanosheets are composed of aromatic sp 2 While GO is composed of only hybridized carbon atoms, it has a single-layer graphitic structure of carbon atoms with randomly distributed aromatic and oxygenated aliphatic regions (sp 3GO can be described as having oxygen atoms in functional groups such as hydroxyl, epoxy, carbonyl, and carboxyl. The hydroxyl and epoxy groups are primarily located on the basal planes of GO, while the carbonyl and carboxyl functional groups are located at the edges of the GO sheets. Herein, coordination of metal ions via the hydroxyl and epoxy groups is preferred.

[0110] GO can be prepared by exfoliating it into single-layer GO with a thickness of approximately 1 nm by ultrasonic treatment, which destroys the interactions between adjacent layers. These functional groups also enable GO to form relatively stable dispersions in polar solvents such as water and DMF through hydrogen bonding between the polar groups of the solvent and the epoxy groups on the basal surface of GO.

[0111] The disappearance or reduction of oxygen-containing groups in reduced graphene oxide (which includes partially reduced graphene oxide in this specification) reduces the dispersibility of the flakes in water. The presence of reduced graphene oxide can be confirmed by a color change that occurs upon reduction. Aqueous solutions of reduced graphene oxide are pale yellow. Aqueous solutions of reduced graphene oxide are black.

[0112] rGO can be obtained by subjecting GO to various chemical, thermal, and electrochemical treatments, such as treating GO with hydrazine hydrate, exposing GO to hydrogen plasma for a few seconds, exposing GO to intense pulsed light generated by a xenon flash tube, or heating GO in distilled water at various temperatures and for different durations.

[0113] The cathode fabrication process begins with graphene oxide or reduced graphene oxide sheets containing oxygen atoms capable of coordinating with metal ions. These oxygen atoms are located on the basal plane, ideally from epoxy or hydroxyl groups, and also contain carboxyl groups capable of coordinating with metal ions.

[0114] Ideally, the starting graphene oxide is unreduced, as this maximizes the number of oxygen atoms available for coordination with metal ions. However, in the final cathode, reduced graphene oxide is preferred because of the significantly better electrical conductivity of rGO.

[0115] GO or rGO sheets can be exfoliated using ultrasound, which maximizes the number of nucleation sites on the GO or rGO surface. At this process step, the GO or rGO is typically present in an inert solvent such as DMF, water, or methanol.

[0116] In the first step of the process, metal ions coordinate to graphene oxide or rGO via heterogeneous nucleation sites provided by oxygen atoms on the graphene oxide or rGO surface. Suitable metal ions are transition metal ions, such as those from the first transition series. Zr, Co, Zn, Cr, or Cu are preferred, with Zr or Cr being particularly preferred. The metal ions are anchored to the graphene oxide or rGO sheets via oxygen atoms on the sheets, thereby chemically bonding the metal-organic framework to the graphene oxide or rGO surface as it grows. Any metal-organic framework that is not chemically bonded to the surface must be removed later in the process. This strong chemical bond between the graphene oxide sheets and the metal ions is crucial for cathode performance.

[0117] Coordination of metal ions to the graphene oxide or rGO surface can occur on one or both sides, preferably both sides.

[0118] Metal ions are typically applied to the surface of graphene oxide or rGO in an inert solvent using a salt of the target metal that dissolves in the solvent. If the GO or rGO is already in a solvent, simply add the appropriate amount of the required metal salt to the solvent. Preferred metal salts are nitrates, sulfates, acetonates, or halides such as chlorides.

[0119] For example, a 0.1–1.0 M aqueous solution of metal salt can be added to the GO or rGO dispersion solvent. The amount of metal ion added can vary, but typically 0.1–1.0 mmol of metal salt is appropriate for 20 mg of GO or rGO.

[0120] The graphene oxide or reduced graphene oxide sheets can be contacted with the metal ions for at least 10 minutes (e.g., up to 60 minutes). Sonication can be used to encourage the metal ions to coordinate to oxygen atoms on the basal plane of the graphene oxide or reduced graphene oxide sheets.

[0121] pH can be used to control metal ion binding. Generally, a more basic pH (especially above 7, e.g., 8–12) is preferred. Increasing the pH promotes nucleation and improves oxygen deprotonation. Increasing the pH of the starting GO dispersion can deprotonate oxygen functional groups, which facilitates metal cation interaction with the GO or rGO surface. Increasing the pH above 9.8 results in ionization of hydroxyl groups on the GO surface, maximizing the coordination of metal ions to the basal plane. Increasing the pH also increases the packing density of the MOF.

[0122] Once the initial coordination to the graphene oxide or reduced graphene oxide surface is complete, the unbound metal ions can be separated from those bound to the graphene oxide or reduced graphene oxide by centrifugation and decantation. This allows the level of metal binding to be measured by the difference between the amount of metal ions supplied and the amount of unbound metal ions in the supernatant. However, it is desirable to omit this step, as residual metal ions may be incorporated into the MOFs during MOF growth.

[0123] After the initial metal ions are coordinated to the graphene oxide or reduced graphene oxide surface, metal-organic framework growth can begin. Therefore, it is important that the metal ions are coordinated to the basal plane of the GO or rGO before the MOF ligands are added.

[0124] This growth method has several advantages, as it allows for the physical mixing of the components as well as ensuring grafting between the MOFs and GO or rGO, while simultaneously yielding dense and fine MOFs that offer large specific surface areas.

[0125] [Metal-organic structure] The MOFs used in the cathode in this embodiment may be the same or different from those present in the separator, and the principles of MOF growth remain the same.

[0126] The ligands used in this embodiment for MOF growth are well known and are based on polyfunctional organic ligands (e.g., those containing carboxyl groups, amine groups, and optionally other functional groups). In one embodiment, imidazole-based ligands, such as 2-methylimidazole salts, are used. In one embodiment, polyfunctional organic ligands containing at least one carboxyl group, such as a carboxylic acid, are used. Ligands containing at least two carboxyl groups are preferred. The ligands are generally small molecules with Mw up to 300 g / mol.

[0127] Therefore, it is preferable to use low molecular weight tricarboxylic or dicarboxylic acids such as 1,4-benzenedicarboxylic acid or 1,3,5-benzenetricarboxylic acid. The ligands of interest often contain aromatic rings such as phenyl rings. Therefore, the most preferred ligands are based on polycarboxylic acids containing aromatic rings.

[0128] Some ligands, such as 2-aminoterephthalic acid, contain both carboxyl and amino groups.

[0129] Hydrothermal and solvothermal methods are the most frequently reported synthetic techniques for the synthesis of MOFs according to this embodiment. In these solution-based methods, a solution containing metal ion precursors and ligand precursors is placed in a sealed reaction vessel and heated to near the boiling point of the solvent used. At this elevated temperature (and optionally under pressures of 1-200 bar), crystallization of the product occurs.

[0130] Another synthetic technique that can be used to synthesize MOFs in this embodiment is ultrasound-assisted synthesis (also known as sonochemical synthesis). In this solution-based method, a solution containing metal ion precursors and ligand precursors is placed in a sonication bath and exposed to high-energy ultrasound for a period of time. The high-energy waves interact with the liquid, forming alternating regions of high and low pressure, which generate cavities within the liquid. These cavities grow due to ultrasound-induced solute vapor diffusion into the cavities, which then become unstable and collapse. At this point, the ultrasonic energy stored within the cavities is rapidly released upon collapse, resulting in localized heating and cooling rates reaching 1000 K / s. This extreme condition leads to molecular excitation, molecular bond scission, and the generation of further reactive radicals, which trigger the nucleation and growth of MOF nanoparticles. Compared to other MOF synthesis techniques, sonochemical synthesis can be performed at room temperature or at relatively low synthesis temperatures. Furthermore, the dissolution and mixing of precursors results in a more homogeneous product.

[0131] In this embodiment, a hydrothermal method is preferred, and therefore, after the coordination step, it is preferred to add a solution of the polyfunctional ligand to a metal salt solution and heat the mixture to a heating temperature of 20 to 250°C, preferably 50 to 250°C, for example 100 to 200°C.

[0132] The molar ratio of the ligand to the metal ion can be varied in the range of 0.25:1 to 4:1, preferably 0.5:1 to 2:1.

[0133] MOF growth can be controlled by using modulators that adjust the nucleation and growth rates of MOF crystals. The role of many modulators, particularly monocarboxylic acids, is to trap MOF particles in the early stages of the nucleation and growth process, reducing the local metal ion concentration and effectively slowing down the growth rate. In this specification, monocarboxylic acids such as acetic acid and formic acid can be used as modulators.

[0134] When a modulator is used, its amount may be in the range of 0.001% to 50% of the solvent used. The modulator can be used to reduce the average particle size of the MOF. In some cases, it is possible to use equal amounts of the solvent and the modulator. Using a large amount of modulator tends to increase the particle size of the MOF in the structure.

[0135] Preferably, NH2-UiO-66(Zr) or MIL101(Cr) are used as MOFs, which can be grown according to the techniques described for the separator MOFs.

[0136] At the temperatures used in MOF synthesis, graphene oxide is typically reduced to reduced graphene oxide, as evidenced by the color change of the GO solution after heating from pale yellow to black (reduced form of GO).

[0137] After the structure is grown, the material can be purified to remove unbound MOFs. While many unreacted (unbound) MOFs may have grown during the fabrication process, it is clear that some MOFs are bound to the reduced graphene oxide surface via metal ions initially coordinated to the basal plane of the graphene oxide. Because the cathode in this example contains MOFs physically bound to the basal plane of the reduced GO, it is desirable to remove unbound MOFs that could adversely affect performance.

[0138] The bound MOFs remain chemically bound to the rGO surface even after sonication of the material. Therefore, the reaction mixture can be sonicated and / or centrifuged, so that unreacted MOFs remain in the supernatant and the MOF@rGO precipitates to the bottom of the tube. Multiple centrifugations are possible to ensure purity. Speeds between 2500 and 6000 rpm are appropriate. Preferably, at least 90% by weight of the MOFs are physically bound to the rGO. After centrifugation, the material can be dried, e.g., to remove water.

[0139] In the MOF@rGO structure, the MOF preferably accounts for the majority of the structure weight, preferably 60-98 wt % of the structure, e.g., 70-97.5 wt %, while the rGO accounts for 2-40 wt %, e.g., 2.5-30 wt % of the structure.

[0140] MOFs preferably have pore diameters less than 30 Å, e.g., 2-25 Å, ideally 15 Å or less. Pore diameters can be tuned by controlling the nature of the metal ions and ligands, e.g., ligand length. The pore window is preferably 4-11 Å. Such pore diameters are ideal for allowing Li+ ions to pass through while blocking the escape of dissolved polysulfides from the pores. Such pore diameters mitigate the problems of polysulfide shuttle effects and lithium dendrite formation.

[0141] The surface coverage of the rGO sheets can be high and can be adjusted by changing the pH or the nucleation time of the metal coordination. Preferably, at least 50 wt% of the MOFs present are attached to the reduced graphene oxide sheets. It is preferred that the MOFs are uniformly distributed throughout the reduced graphene oxide sheets.

[0142] As mentioned above, the temperature used in MOF synthesis can also have a reduction effect on GO. Thus, in one embodiment, the process of the present invention uses GO as a starting material, and after metal ion coordination to the GO basal plane, MOF growth at elevated temperatures simultaneously results in MOF growth and GO reduction. Alternatively, one skilled in the art can begin the process of the present invention with reduced graphene oxide sheets. The presence of reduced graphene oxide in the final cathode is important.

[0143] [Sulfur addition] After the MOFs are present on the rGO, sulfur must be introduced to form the actual cathode. Sulfur is preferably loaded onto the MOF@rGO by a melt-diffusion method. This method allows elemental sulfur to be melted and impregnated into the MOFs, as the elemental sulfur can pass through the pore windows of the MOFs and be retained there.

[0144] Therefore, sulfur loading can be achieved by a melt-diffusion method, in which sulfur is simply melted and impregnated into the pores of the MOF. Lithium sulfide formed during battery operation is trapped within the pores. The sulfur loading is preferably at least 50 wt% of the S-MOF@rGO material, e.g., 60-90 wt% of the S-MOF@rGO material. The sulfur loading can be measured by thermogravimetric analysis. For example, a thermogravimetric analysis system (TGA) can be used under an inert atmosphere by measuring the weight loss of sulfur over time and / or with increasing temperature.

[0145] Alternatively, the loading can be determined based on the weight of MOF+rGO used and the amount of S added during the manufacturing process (accounting for any S recovered unbound during the process).

[0146] In one embodiment, sulfur loading onto the MOF@rGO can be achieved by mixing sulfur with the MOF@rGO and grinding it into a fine powder. The mixture is then heated, for example in an autoclave, to a temperature above the melting point of sulfur (e.g., above 120°C, such as 130-200°C, e.g., 155°C). The sulfur-loaded MOF@rGO can be recovered after cooling. The product can be further ground if necessary. This is labeled herein as S-MOF@rGO.

[0147] The resulting material is suitable for use as a cathode in Li-S batteries, although adjustments to the material morphology may be necessary for use in actual cells.

[0148] The cathode may be supported on a current collector such as aluminum foil or carbon-coated aluminum foil, and thus may be formed as a thin layer on the current collector.

[0149] To fabricate a battery of the present invention, the cathode material can be pressed onto an aluminum foil current collector. To fabricate an anode, a film containing lithium or a lithium alloy can be pressed onto a suitable support. A separator can be impregnated with electrolyte, and the electrodes can be laminated onto the saturated separator. This results in a charged battery.

[0150] The invention will now be described with reference to the following non-limiting examples and figures.

[0151] BRIEF DESCRIPTION OF THE DRAWINGS A Zn / Fe-ZIF-8 metal-organic framework was fabricated by a one-step solution-phase synthesis method using a Zn and Fe binuclear paddle structure and a methylimidazole linker in water at 35 °C.

[0152] {Synthesis of bimetallic ZIF-8 and Fe-doped ZIF-8} ZIF-8 was synthesized according to the following steps.

[0153] 1) Dissolve ZnSO4·7H2O (575 mg) in 30 mL of water and dissolve 1.314 g of 2-methylimidazole in 30 mL of water in a separate beaker.

[0154] 2) After stirring each solution at room temperature for several minutes, the metal solution is added to the 2-methylimidazole solution and stirred continuously at 35°C for 24 hours.

[0155] 3) The white precipitate in the beaker is collected by centrifugation, washed with deionized water, and then dried at 70°C for 12 hours.

[0156] Fe-doped ZIF-8 was also synthesized by the same method, except that FeSO4 7H2O (46.3 mg) was added to the ZnSO4 7H2O aqueous solution in step 1. The subsequent steps were the same as those for ZIF-8. The obtained samples were labeled ZIF-8 and Fe-ZIF-8.

[0157] {Production of modified separator} ZIF-8 and Fe-ZIF-8 modified separators were fabricated by coating a slurry containing ZIF-8, Fe-ZIF-8, SuperP, and PVDF onto Celgard 2400. Briefly, Fe-ZIF-8, SuperP, and PVDF were mixed in a weight ratio of 75:15:10 in NMP and ball-milled in a sealed Teflon jar for approximately 40 minutes to prepare a slurry. The resulting slurry was cast onto a Celgard 2400 separator and dried at 60 °C for 12 hours. After drying, the modified separators were punched into 19 mm diameter circles.

[0158] {Manufacturing sulfur electrodes} Sulfur was loaded onto CNT / GO (1:1) by a conventional melt-diffusion method. Briefly, sulfur powder and CNT / GO were thoroughly mixed by grinding and then sealed in a glass vial. The glass vial was transferred to an autoclave and heated at 155 °C for 12 h. The cathode slurry was prepared by mixing S-CNT / GO powder (90 wt%) and PVDF binder (10 wt%) in NMP as a solvent. The resulting slurry was applied to a carbon-coated Al foil with a doctor blade and dried at 60 °C for 12 h to fabricate an electrode. Finally, the electrode was cut into a disk with a diameter of 12 mm. The sulfur loading per area of ​​the resulting cathode was 1–3.6 mg / cm. 2 The range is.

[0159] The thickness of the bimetallic Fe-ZIF-8 separator was 1 to 15 μm. The thickness of the Celgard layer was 25 μm.

[0160] The bimetallic Fe-ZIF-8 separator prepared in Example 1 was tested as illustrated in Figures 1-13. Figure 1a shows a coin cell prepared using an S-carbon cathode, Celgard (polypropylene) or the MOF-modified separator prepared in Example 1, liquid electrolyte, Li anode, spacer, spring, and top cover. The liquid electrolyte used was 1 M LiTFSI and 0.1 M LiNO3 dissolved in 1,2-dimethoxyethane (DME) and 1,3-dioxacyclopentane (DOL) (volume ratio 1:1).

[0161] The electrochemical cell contains a Li anode, a cathode containing sulfur and carbon black, a Celgard 2400 separator, and a liquid electrolyte in an Ar-filled glove box. This is shown in Figure 1a. The sulfur loading of the cathode is at least 50 wt% of the weight of the S-carbon material, e.g., 60-90 wt% of the S-carbon material. The sulfur loading can be measured by thermogravimetric analysis. For example, a thermogravimetric analysis system (TGA) can be used under an inert atmosphere by measuring the weight loss of sulfur over time and / or with increasing temperature.

[0162] Figure 1b is a schematic diagram of the functional bimetallic 3D MOF-based separator of Example 1, specifically designed for Li-S batteries. It is characterized by highly ordered micropores with pore diameters of approximately 10 Å, significantly smaller than the diameter of medium-chain-length lithium polysulfides, allowing for the efficient transport of lithium. + It selectively blocks and converts polysulfides while allowing uniform transport of ions.

[0163] Figures 2a and 2b show SEM images of a Celgard (PP) separator and the MOF-coated Celgard separator of Example 1. The SEM image of untreated PP shows a porous structure with extensive pores measuring several hundred nanometers, allowing polysulfides dissolved in the electrolyte to pass through this structure (Figure 2a). On the other hand, the SEM image of the MOF-coated separator clearly shows that the separator is completely coated with MOFs (Figure 2b). This coating provides the pore structure and the ability to block polysulfides.

[0164] Figures 3a–d show digital images of the modified separator, showing that the PP separator is completely coated with MOF (Figure 3a). The MOF-coated separator retains its original shape even after being folded twice, suggesting that the MOF is tightly bonded to the separator and has high mechanical stability with sufficient flexibility (Figures 3c–d).

[0165] Figure 4 shows the thermal shrinkage tests of Celgard and the MOF-coated Celgard separator of Example 1 at room temperature and 150°C. To prevent internal short circuits in batteries at high temperatures, the separator must undergo minimal thermal shrinkage. As shown in Figure 4, the commercially available Celgard separator completely shrinks at 150°C. This tendency of Celgard to shrink easily when heated could potentially increase safety risks in the event of thermal runaway. However, the MOF-coated separator exhibited superior heat resistance, maintaining its original shape even at 150°C.

[0166] Figures 5a and 5b show polysulfide permeation tests using a PP separator and the MOF-coated PP separator of Example 1. High permeation resistance to soluble polysulfides is important for modified separators in Li-S batteries. In this regard, permeation experiments were conducted to investigate polysulfide permeation through the separator using an H-shaped cell (Figures 5a and 5b). Polysulfide solution (Li2S6) was added to the left side of the cell (black), and blank electrolyte was introduced to the right side (colorless). Polysulfide permeation was investigated for PP and the MOF / PP modified separator of Example 1 under similar conditions over different time periods.

[0167] In the PP separator, polysulfides rapidly diffused to the right side of the H-shaped cell after 10 minutes, suggesting that the highly porous PP separator had poor ability to prevent polysulfide diffusion (Figure 5a). In contrast, the MOF / PP separator of Example 1 exhibited significantly improved polysulfide blocking performance. Even after 12 hours, polysulfide migration was still negligible, demonstrating the excellent polysulfide blocking ability of the MOF / PP separator (Figure 5b). Polysulfide permeability tests confirmed that the MOF / PP separator had the ability to mitigate polysulfide shuttle migration.

[0168] Figure 6 shows the areal capacity of 1 mAh / cm for symmetric cells using a PP separator and a MOF / PP separator. 2 The Li deposition / dissolution performance under different current densities at 1000 kJ / cm2 is shown. The polarization effect of the PP separator and MOF / PP separator was evaluated using a Li||Li symmetric cell. The Li electrode with the PP separator was measured at a current density of 0.5 mA / cm2. 2 and area capacity of 1mAh / cm 2 However, the MOF / PP Li||Li symmetric cell achieves minimal polarization (36 mV). Similarly, the MOF / PP separator symmetric cell achieves an areal capacity of 1 mAh / cm. 2 At a current density of 1mA / cm 2 to 10mA / cm 2The voltage hysteresis of the symmetric cell with a PP separator began to increase after about 700 h, likely due to the growth of Li dendrites and the consumption of electrolyte.

[0169] Figure 7 shows the ultra-long cycle performance of a symmetric cell using the MOF / PP separator of Example 1. Current density: 0.5 mA / cm 2 and area capacity of 1mAh / cm 2 It operated stably for over 4000 hours while maintaining low voltage hysteresis.

[0170] Figures 8a and 8b show SEM images of Li anodes using a PP separator (Figure 8a) and a MOF-protected PP separator (Figure 8b). To clarify the role of Fe-ZIF-8 / PP in the Li deposition / dissolution process, the surface morphology of the Li plating layer after 1000 h of cycling was observed by SEM. On the Li metal surface using the conventional PP separator, Li dendrites formed in a chaotic state (Figure 8a). On the other hand, the MOF / PP separator still maintained a smooth surface (Figure 8b). This indicates that the MOF / PP separator works more effectively in suppressing the growth of lithium dendrites.

[0171] Figure 9 shows the cyclic voltammetry curves of Li-S batteries using PP and MOF / PP separators over a voltage window of 1.7–2.8 V at a scan rate of 0.1 mV / s. Two distinct reduction peaks, I and II, are attributed to the conversion of S molecules to higher soluble polysulfides, which then convert to Li2S2 and Li2S. The oxidation peaks (III and IV) correspond to the conversion of Li2S2 and Li2S to sulfur molecules. However, the CV curve of the MOF / PP separator exhibits two sharp redox peaks: the oxidation peak shifted significantly in the negative direction and the reduction peak shifted significantly in the positive direction, suggesting reduced polarization and significantly improved electrocatalysis. The well-matched peaks also indicate the occurrence of reversible electrochemical reactions within the electrode materials.

[0172] Figure 10 shows the rate performance of Li-S batteries with a PP separator (lower curve) and an MOF / PP separator (upper curve) at different charge-discharge rates (C-rates) over the voltage window of 1.7–2.8 V. The battery with the PP separator exhibited significant capacity degradation at various C-rates. In contrast, the Li-S battery with the MOF-coated separator demonstrated much better performance (1036 mAh / g at 0.3 C). The capacities remained stable at 903 mAh / g, 830 mAh / g, 785 mAh / g, and 746 mAh / g during cycling tests at 0.5 C, 1 C, 2 C, and 3 C, respectively. Finally, the capacity recovered to 882 mAh / g at 0.5 C, with a capacity degradation rate of only 0.06% per cycle.

[0173] Figure 11 shows the cycling performance of Li-S batteries using a PP separator (lower curve) and a MOF / PP separator (upper curve), an S-carbon cathode, and a Li anode in a coin cell described herein. The MOF / PP Li-S battery exhibited a discharge capacity of 865 mAh / g, which dropped to 409 mAh / g after 1000 cycles, with a Coulombic efficiency of approximately 100%. In contrast, the PP battery had a relatively low initial capacity (466 mAh / g) that deteriorated to 167 mAh / g after 1000 cycles, but still maintained a Coulombic efficiency greater than 100%.

[0174] Figure 12 shows electrochemical polarization tests of the PP separator and the MOF / PP separator to further confirm the improved conversion efficiency of polysulfides. The discharge plateau of Li-S with MOF / PP is flatter, and the discharge and charge capacities are higher. Furthermore, Fe-ZIF-8 / PP exhibits a smaller voltage hysteresis (ΔE = 0.16 V) compared to PP (ΔE = 0.30 V). In the galvanostatic discharge curves, Q H corresponds to the high discharge plateau, and Q L corresponds to the low discharge plateau, which indicates the conversion reaction of polysulfides. The MOF / PP Li-S battery has a higher Q than the PP cell. H and Q L The highest specific capacitance was observed in Fe-ZIF-8 / PP:Q H :378, Q L :658, PP:Q H :344, Q L :334mAh / g). Q H and Q L The high discharge capacity values ​​confirm that the electrocatalytic conversion of polysulfides improved the utilization of the active materials.

[0175] Example 2 - Synthesis of MOF@rGO The sulfur host MOF@rGO was synthesized as follows: GO dry powder (20 mg) was dispersed in 20 mL of DMF and sonicated for 3 h to exfoliate the GO nanosheets and obtain a stable dispersion. ZrCl4 was then added to the GO dispersion.

[0176] 0.343 mmol of ZrCl4 was added to 20 mL of GO dispersion. In this process, metal ions coordinate to graphene oxide via heterogeneous nucleation sites on the graphene oxide surface provided by oxygen atoms. This solution was treated in an ultrasonic cleaning bath (VWR Ultrasonic Cleaner) for 30 minutes. Next, x-fold amounts of 0.343 mmol of 2-aminoterephthalic acid (x = 1, 1.5, and 2) and y μL of deionized H2O (y = 20, 100, and 200) were added to the solution while stirring. Water concentration and molar ratio (ligand:metal) were tested as modifiers to control the MOF particle size.

[0177] The freshly prepared solution was transferred to a 125 mL Teflon-lined steel autoclave and treated in an oven (Termaks TS8024 Lab Drying Convection Oven) at 120 °C for 12 hours. This process reduces GO to rGO. To remove unreacted MOF particles (not bound to rGO) and purify the MOF@rGO, the sample was collected by centrifugation. The powder was redispersed in deionized water in an ultrasonic bath for 30 minutes and then transferred to a centrifuge tube. Between each treatment, the supernatant was removed eight times for 10 minutes and replaced with fresh deionized water. The purified sample was then dried in a vacuum oven at 60 °C for 24 hours.

[0178] [Sulfur-loaded (S-NH2-UiO66 / rGO)] After drying, sulfur was loaded onto the MOF@rGO by a melt-diffusion method. This method allows elemental sulfur to be melted and impregnated into the MOFs, since the elemental sulfur can pass through the pore windows of the MOFs and be retained there. Typically, the as-synthesized MOF@rGO and sulfur were ground into fine powders in different amounts. The mixture was then transferred to an autoclave and heated in an oven at 155 °C for 12 hours. The sulfur loaded onto the MOF@rGO was collected at room temperature and further ground into fine powder, labeled S-MOF@rGO.

[0179] [Example 3] The same procedure as above was followed, except that Cr was used instead of Zr. For Cr metal experiments, 0.5 mmol of Cr(III)(NO3)3·H2O was used with 0.5 mmol of benzene-1,4-dicarboxylic acid and x mL (x = 0.290, 0.435, and 0.625) of glacial acetic acid.

Claims

1. (i) a Li anode; (ii) a separator between the anode and the cathode; (iii) a Li-containing electrolyte; and (iv) a sulfur-containing cathode; The lithium-sulfur battery, wherein the separator comprises a porous substrate supporting a metal-organic framework containing at least two different metal ions, one of which is an iron ion.

2. 10. The lithium-sulfur battery of claim 1, wherein the metal-organic framework is non-carbonized.

3. 10. The lithium-sulfur battery of any one of the preceding claims, wherein the porous substrate of the separator is polypropylene or polyethylene, preferably polypropylene.

4. 10. The lithium-sulfur battery of claim 1, wherein the porous substrate of the separator is a porous polypropylene film or a porous polypropylene nonwoven fabric or a meltblown fabric.

5. 10. The lithium-sulfur battery of any one of the preceding claims, wherein the substrate of the separator has a thickness of 10 to 50 μm.

6. 10. The lithium-sulfur battery of any one of the preceding claims, wherein the metal-organic framework is a zeolitic imidazolate framework, such as ZIF-1-20, preferably ZIF-8.

7. 10. The lithium-sulfur battery of any one of the preceding claims, wherein the metal-organic framework contains at least two first transition series metal ions.

8. 10. The lithium-sulfur battery of any one of the preceding claims, wherein the metal-organic framework contains Fe ions and one of Co ions, Zn ions, Zr ions, Mn ions, Fe ions, and Cr ions.

9. 10. The lithium-sulfur battery of any one of the preceding claims, wherein the metal-organic framework contains Zn ions and Fe ions, such as Zn ions and Fe(II) ions.

10. 10. A lithium-sulfur battery according to any one of the preceding claims, wherein the metal ions have a molar ratio of 20:1 to 1:1, in particular Zn:Fe of 20:1 to 1:

1.

11. 10. The lithium-sulfur battery of any one of the preceding claims, wherein the metal-organic framework contains Cr ions and Fe ions, such as Cr ions and Fe(II) ions.

12. 10. A lithium-sulfur battery according to any one of the preceding claims, wherein the metal ions have a molar ratio of 20:1 to 1:1, in particular Cr:Fe of 20:1 to 1:

1.

13. 10. The lithium-sulfur battery of any one of the preceding claims, wherein the bimetallic MOF forms a layer on the substrate having a thickness of 1.0 to 15.0 μm (e.g., 1.0 to 5.0 μm).

14. The cathode comprises reduced graphene oxide sheets, the reduced graphene oxide sheets being chemically bonded to a metal-organic framework via an oxygen-metal linker from the basal plane of the reduced graphene oxide, and the metal-organic framework is impregnated with sulfur to form an S-MOF@rGO structure, the weight of the sulfur being 50%-90% of the weight of the S-MOF@GO, and the MOF is, for example, NH 2 - UiO-66(Zr) or MIL101(Cr).

15. A separator suitable for use in batteries such as Li—S batteries, comprising a porous substrate supporting a metal-organic framework containing at least two different metal ions, one of which is an iron ion.

16. The separator according to claim 15, wherein the MOF has a pore size in the range of 2 to 20 Å.

17. 17. The separator of claim 15 or claim 16, wherein the MOF is a zeolite-like imidazolate structure, such as ZIF-1-20, preferably ZIF-8.

18. The separator of any one of claims 15 to 17, wherein one of the metal ions acts as an electrocatalyst for the adsorption and conversion of polysulfides.

19. 19. The separator of claim 15 or claim 18, wherein the zeolite-like imidazolate structure contains Zn ions and Fe ions or Cr ions and Fe ions, such as Zn / Cr ions and Fe(II) ions.

20. The separator according to any one of claims 15 to 19, which does not undergo thermal shrinkage even when heated up to 150°C.

21. 21. The separator according to any one of claims 15 to 20, wherein the metal ions have a molar ratio of 20:1 to 1:1, in particular Cr or Zn:Fe of 20:1 to 1:

1.

22. 1) dissolving two metal salts (one of which is an iron salt) in a solvent in the presence of an imidazole-based ligand or a tricarboxylic or dicarboxylic acid (e.g., 1,4-benzenedicarboxylic acid) and mixing them to form a precipitate containing a metal-organic framework containing at least two different metal ions (one of which is an iron ion); 2) separating the precipitate, forming a slurry using the separated material, and coating the slurry on a porous substrate.

23. 23. The process of claim 22, wherein step 1) is carried out at a temperature of 50°C or less.

24. (v) a Li anode; and (vi) a separator between the anode and cathode; (vii) a Li-containing electrolyte; and (viii) a sulfur-containing cathode; the separator comprises a porous substrate supporting a metal-organic framework containing at least two different metal ions, the metal-organic framework being a zeolite-like imidazolate structure.

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