Li-S battery cathode

Abstract

Provided is a process for preparing a cathode material for a Li-S battery, comprising: (i) nucleating metal ions on graphene oxide or reduced graphene oxide sheets such that the metal ions are chemically bonded to the basal plane of the graphene oxide or reduced graphene oxide sheets; (ii) then growing a metal-organic framework containing the chemically bonded metal ions by adding a multifunctional ligand to the product of step (i) and optionally heating the resulting mixture to a temperature of at least 20°C (e.g., 100-250°C) to form a metal-organic framework (MOF@rGO) bonded to the reduced graphene oxide sheets; and (iii) impregnating the metal-organic framework with elemental sulfur to form S-MOF@rGO, wherein the weight of the sulfur is 50%-90% of the weight of the S-MOF@rGO.

Description

[Technical Field]

[0001] The present invention relates to the use of sulfur-impregnated metal-organic frameworks (MOFs) bonded to reduced graphene oxide in the cathode of a lithium-sulfur battery. The invention details a process for fabricating the sulfur-impregnated metal-organic framework bonded to reduced graphene oxide, and encompasses cathodes comprising the sulfur-impregnated metal-organic framework bonded to reduced graphene oxide and batteries comprising the cathodes. Batteries using the cathodes of the invention have outstanding performance, particularly with respect to battery capacity retention after repeated charging. [Background technology]

[0002] 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 property in environments where weight reduction is important. The use of sulfur in 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.

[0003] 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:

[0004] [ka]

[0005] 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.

[0006] 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:

[0007] [ka]

[0008] In reality, the reduction of sulfur to lithium sulfide is much more complex, resulting in lithium polysulfides (Li2S x , 2≦x≦8).

[0009] Upon discharge, the end product is not simply Li2S, but a mixture of Li2S2 and Li2S. This is due to the slow reduction reaction rate of Li2S. This contrasts with conventional lithium-ion batteries, 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. Therefore, Li-S batteries offer many inherent advantages over current lithium-ion batteries, including:

[0010] i) Improved safety through "conversion reactions" that form new substances during charging and discharging.

[0011] ii) Higher energy density per unit mass than lithium-ion batteries due to the weight savings achieved by using sulfur and carbon instead of heavy metal oxides, which is a key advantage for applications such as wearable devices, electric vehicles, medical equipment, drones, and aircraft.

[0012] iii) Significant reduction in raw material costs: the cost of sulfur is less than 1% of the cost of lithium cobalt oxide (the material primarily used in lithium-ion battery cathodes).

[0013] iv) Higher charge capacity rating. The chemical design allows for faster charging.

[0014] v) Low risk of battery failure as the highly reactive Li anode is passivated by sulfide material during operation.

[0015] In this scenario, Li-S batteries are considered a breakthrough technology because they combine a specific capacity (1675 mAh / g) five times the theoretical capacity of lithium-ion batteries with high specific energy (2600 Wh / kg).

[0016] The main problem with Li-S batteries is that sulfur has poor electrical conductivity, which causes a significant volume change during discharge. Therefore, finding a suitable cathode material is difficult. Many solutions use a carbon / sulfur cathode and a lithium anode. Sulfur is very cheap, but has virtually no electrical conductivity, so a carbon coating compensates for the lack of electrical conductivity.

[0017] One problem with the Li-S cathode design is that the sulfur in the cathode absorbs lithium, resulting in Li x The volume expansion of the S composition is predicted to reach approximately 80% of the original sulfur volume. This causes significant mechanical stress on the cathode, which is the main cause of its rapid degradation. This expansion process reduces the contact area between carbon and sulfur, inhibiting the migration of lithium ions to the carbon surface.

[0018] 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 nDissolution of (n>2) in the electrolyte causes irreversible loss of active sulfur from the cathode, which also severely limits battery life.

[0019] 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 leaching of active material from the cathode, lithium corrosion, low coulombic efficiency, and reduced battery life. Furthermore, the "shuttle effect" contributes to the anomalous self-discharge of Li-S batteries due to the slow dissolution of polysulfides, which occurs even at rest.

[0020] Conventionally, Li-S batteries contain a liquid organic electrolyte enclosed within the pores of a polypropylene separator that separates the anode and cathode. The electrolyte plays a key role in Li-S batteries, both by providing a "shuttle effect" through polysulfide dissolution and by stabilizing the SEI on the anode surface.

[0021] We have identified a novel cathode material for Li-S batteries. This cathode contains a graphene derivative (e.g., partially reduced graphene oxide or reduced graphene oxide). Therefore, this material provides the cathode with electrical conductivity. The graphene derivative is functionalized by growing a metal-organic framework (MOF) on top of it, which can then be impregnated with sulfur. Importantly, the sulfur impregnated into the pores of the MOU is confined by the framework and cannot escape from the cathode during charge-discharge cycling. As previously mentioned, when sulfur reacts with Li during battery operation, lithium polysulfides, which are much larger than elemental sulfur, are produced. However, these compounds are too large to escape from the pores of the MOU, significantly suppressing cathode degradation and significantly reducing the polysulfide shuttle effect. Two-dimensional graphene-based nanosheets with spaces between their basal planes accommodate the volume expansion of the sulfur derivative during the charge-discharge process.

[0022] Surprisingly, we found that cathodes containing metal-organic frameworks (MOFs) grown on a two-dimensional graphene oxide derivative (referred to herein as GO), which can be partially or fully reduced during use (referred to herein as rGO), exhibit attractive properties for Li-S batteries. These MOF@rGO-containing cathodes have very high capacities and can be fabricated into flexible, foldable batteries with potentially advantageous size, safety, and efficiency.

[0023] MOFs have been investigated in Li-S batteries for some time. CN110492088 discloses a composite material, ZIF-8@reduced graphene oxide, loaded with sulfur, as a cathode material for Li-S batteries. ZIF-8 contains zinc ions and imidazole ligands. This cathode is fabricated by first reducing graphene oxide and then synthesizing ZIF-8 in situ on the surface of reduced graphene oxide in the presence of zinc salt and urea.

[0024] Chemical Engineering Journal, vol. 450, 4, 2022, S. Qiu et al., "Tunable MOFs derivatives for stable and fast sulfur electrodes in Li-S batteries," discusses tunable MOF derivatives for stable and fast sulfur electrodes in Li-S batteries. However, in this structure, the MOF is not anchored to graphene.

[0025] WO2022 / 020631 describes mixing sulfur-loaded MOFs with graphene flakes and polymer residues to form composites, however, the MOFs are not bonded to the graphene.

[0026] CN11241133 describes graphene / MOF structures obtained by simple mixing of materials. In Example 1, MOFs are pre-synthesized and combined with graphene.

[0027] CN109301191 also discloses graphene / MOF materials, but the MOFs are prepared separately and then combined with graphene oxide, thus making them a physical mixture of the components.

[0028] In CN111653729, graphene acts as a support for layered electrodes, and sulfur is coated onto the graphene before the MOF is added.

[0029] CN109950487 aims to provide a high-specific-capacity Li-S battery cathode material. This invention involves growing a metal-organic framework (MOR) ZIF-67 on graphene sheets by a simple hydrothermal method to form a composite material for Li-S battery cathode materials. [Prior art documents] [Patent documents]

[0030] [Patent Document 1] International Publication No. 2022 / 020631 Brochure [Patent Document 2] Chinese Patent No. 11241133 [Patent Document 3] Chinese Patent No. 109301191 [Patent Document 4] Chinese Patent No. 111653729 [Patent Document 5] Chinese Patent No. 109950487 [Non-patent literature]

[0031] [Non-Patent Document 1] S. Qiu et al., Chemical Engineering Journal, vol. 450, 4, 2022. Summary of the Invention [Problem to be solved by the invention]

[0032] Therefore, conventional techniques simply mix graphene oxide derivatives with MOFs or graphene derivatives with MOF synthetic precursors, and we have found that this MOF growth method does not provide optimal performance. [Means for solving the problem]

[0033] We designed a facile method to obtain dense, ordered, and uniformly sized MOF nanoparticles on rGO by efficiently utilizing the functional groups of GO.

[0034] In this example, careful pretreatment of GO ensures that the MOFs are ultimately attached to the basal plane of the graphene oxide by growing on metal nucleation sites. When metal-organic frameworks were attached to two-dimensional graphene derivatives (graphene oxide, partially reduced graphene oxide, and reduced graphene oxide), the cathode performance improved. This material was tested as a cathode in Li-S batteries and showed persistently enhanced cycling stability and low capacity fade rates after over 3,000 cycles.

[0035] The method of the present invention and the cathode of the present invention overcome a significant problem present in current Li-S batteries due to the high affinity of the cathode for lithium polysulfide adsorption and catalytic conversion in Li-S batteries.

[0036] Compared with previously reported cathode materials prepared by simply mixing MOFs and rGO, in this invention, MOFs are chemically coordinated to the basal plane of graphene. This key innovation significantly improves the capacity, performance, and stability of the battery. Such functional cathode materials can be considered the first on the market to improve battery performance with ultra-long cycle life and minimal capacity degradation. In particular, the Li-S battery of this invention using the S-MOF@rGO cathode exhibits high sulfur loading per area (0.1-9 mg / cm). 2 ) and electrolyte to sulfur ratio (E / S = 5-50 μL electrolyte / 1 mg sulfur), promising specific capacity was exhibited. [Brief explanation of the drawings]

[0037] [Figure 1] FIG. 1 shows the fabrication of a coin cell. [Figure 2] Figure 2 shows the coulombic efficiency (right axis) and discharge capacity (left axis) versus cycle number (x-axis) for S-MOF@rGO (MOF bound with rGO—upper line) and S-MOF+rGO (unbound MOF, i.e., physically mixed with rGO—lower line). [Figure 3] Figure 3 shows the rate performance of the Li-S battery with the S-MOF@rGO cathode at different current densities. [Figure 4] Figure 4 shows the cycling performance of the Li-S battery with the S-MOF@rGO cathode at 0.1 C. [Figure 5] Figure 5 shows the cycling performance of the Li-S battery with the S-MOF@rGO cathode at 0.2 C. [Figure 6] Figure 6 shows the cycling performance of the Li-S battery with the S-MOF@rGO cathode at 0.1 C. 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) nucleating metal ions on a graphene oxide or reduced graphene oxide sheet such that the metal ions are chemically bonded to the basal plane of the graphene oxide or reduced graphene oxide sheet; (ii) then growing a metal-organic framework containing said chemically bound metal ions by adding a multifunctional ligand to the product of step (i) and optionally heating the resulting mixture to a temperature of at least 20°C (e.g., 100-250°C) to form a metal-organic framework bound to reduced graphene oxide sheets (MOF@rGO); (iii) impregnating the metal-organic framework with elemental sulfur to form S-MOF@rGO, wherein the weight of the sulfur is 50%-90% of the weight of the S-MOF@rGO.

[0039] In another aspect, the present invention provides a method for producing a method of manufacturing a semiconductor device comprising: (i) nucleating metal ions on the graphene oxide sheets such that the metal ions are chemically bonded to the basal surfaces of the graphene oxide sheets; (ii) then growing a metal-organic framework containing the chemically bound metal ions by adding a multifunctional ligand to the product of step (i) and heating the resulting mixture to a temperature of at least 20°C (e.g., 100-250°C) to form a metal-organic framework bound to reduced graphene oxide sheets (MOF@rGO); (iii) impregnating the metal-organic framework with elemental sulfur to form S-MOF@rGO, wherein the weight of the sulfur is 50%-90% of the weight of the S-MOF@rGO.

[0040] In another aspect, the present invention provides a cathode for a Li-S battery, comprising reduced graphene oxide sheets 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 impregnated with sulfur to form an S-MOF@rGO structure, wherein the weight of the sulfur is 50% to 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.

[0041] 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 cathode as defined hereinbefore.

[0042] In a preferred embodiment, the separator comprises a bimetallic MOF.

[0043] [Definition] The abbreviation MOF stands for metal-organic framework.

[0044] The abbreviation GO stands for graphene oxide. The abbreviation rGO stands for reduced or partially reduced graphene oxide.

[0045] A single-layer graphene sheet has two distinct structural regions: (1) a basal plane composed of two-dimensional conjugated sp2 carbon atoms, and (2) edges composed of graphitic lines of carbon atoms containing one-atom-thick defects, dangling bonds, and various capping moieties (e.g., hydrogen, hydroxyl, carbonyl, and carboxyl groups). Thus, the basal plane is composed of two-dimensional conjugated sp2 carbon atoms. 2 It is made up of carbon atoms.

[0046] The abbreviation MOF@rGO is used herein to refer to a metal-organic framework attached to partially reduced graphene oxide sheets or reduced graphene oxide sheets and prepared according to the protocol of the present invention, meaning that the MOF is attached to the reduced graphene oxide via metal ions coordinated to oxygen atoms on the basal plane of the graphene oxide.

[0047] [Mode for Carrying Out the Invention] The present invention relates to a material based on reduced graphene oxide chemically bonded to a MOF, suitable for use in the cathode of a Li-S battery.

[0048] The MOF@rGO structure can be impregnated with sulfur to form a material suitable for use as a cathode in Li-S batteries, along with conventional anodes and electrolytes. The key to this invention is that the MOF binds to the basal plane of reduced graphene oxide (rGO), providing significantly improved properties compared to solutions in which such specific binding does not occur.

[0049] [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 3 GO 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] The process begins with graphene oxide or reduced graphene oxide sheets containing oxygen atoms capable of coordinating with metal ions. These oxygen atoms are typically located on the basal plane, ideally from epoxy or hydroxyl groups, and also contain carboxyl groups capable of coordinating with metal ions.

[0054] 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.

[0055] GO or rGO sheets can be exfoliated using ultrasound, which maximizes the number of nucleation sites on the GO or rGO surface. At this stage of the process, GO or rGO is typically present in an inert solvent, such as DMF, water, or methanol. It is preferable not to use sulfuric acid during the exfoliation process (e.g., to make the graphene porous), as sulfuric acid can cause dissolution of polysulfides and potentially destroy the two-dimensional structure of GO. Therefore, graphene oxide is preferably exfoliated ultrasonically in the presence of only a suitable organic solvent (e.g., DMF). This process introduces metal sites onto the open two-dimensional structure of graphene oxide.

[0056] It is preferable to perform the ultrasonic treatment without heating the graphene oxide. It is not necessary to subject the graphene oxide to a solvothermal reaction. Solvothermal treatment, for example at 100 °C, reduces GO, but this step is conveniently performed during MOF growth because it reduces the active sites for metal binding. Ideally, the exfoliation step is therefore performed at a relatively low temperature, such as below 60 °C (e.g., room temperature). Therefore, it is particularly preferable to perform the reduction of graphene oxide to form rGO simultaneously with MOF formation. This maximizes the efficiency of the process, since only a single thermal treatment step is required.

[0057] Therefore, in another aspect, the present invention comprises: (i) exfoliating graphene oxide dispersed in an organic solvent by ultrasonic treatment, preferably in the absence of sulfuric acid, to obtain exfoliated graphene oxide sheets dispersed in an organic solvent; (ii) nucleating metal ions on the exfoliated graphene oxide sheets such that the metal ions are chemically bonded to the basal plane of the graphene oxide sheets; (iii) then growing a metal-organic framework containing said chemically bound metal ions by adding a multifunctional ligand to the product of step (i) and optionally heating the resulting mixture to a temperature of at least 20°C (e.g., 100-250°C) to form a metal-organic framework bound to reduced graphene oxide sheets (MOF@rGO); (iv) impregnating a metal-organic framework with elemental sulfur to form S-MOF@rGO, where the weight of sulfur is 50%-90% of the weight of S-MOF@rGO.

[0058] Therefore, in another aspect, the present invention comprises: (i) exfoliating graphene oxide dispersed in an organic solvent by ultrasonic treatment at a temperature below 60°C (e.g., room temperature) to obtain exfoliated graphene oxide sheets dispersed in an organic solvent; (ii) nucleating metal ions on the exfoliated graphene oxide sheets such that the metal ions are chemically bonded to the basal plane of the graphene oxide sheets; (iii) then growing a metal-organic framework containing said chemically bound metal ions by adding a multifunctional ligand to the product of step (i) and optionally heating the resulting mixture to a temperature of at least 20°C (e.g., 100-250°C) to form a metal-organic framework bound to reduced graphene oxide sheets (MOF@rGO); (iv) impregnating a metal-organic framework with elemental sulfur to form S-MOF@rGO, where the weight of sulfur is 50%-90% of the weight of S-MOF@rGO.

[0059] 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.

[0060] Coordination of metal ions to the graphene oxide or rGO surface can occur on one or both sides, preferably both sides.

[0061] 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.

[0062] 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.

[0063] The process can be carried out in the absence of an amino compound.The process can be carried out in the absence of urea.

[0064] In another aspect, the present invention provides a method for producing a method of manufacturing a semiconductor device comprising: (i) nucleating metal ions on graphene oxide or reduced graphene oxide sheets in the absence of urea so that the metal ions are chemically bonded to the basal plane of the graphene oxide or reduced graphene oxide sheets; (ii) then growing a metal-organic framework containing said chemically bound metal ions by adding a multifunctional ligand to the product of step (i) and optionally heating the resulting mixture to a temperature of at least 20°C (e.g., 100-250°C) to form a metal-organic framework bound to reduced graphene oxide sheets (MOF@rGO); (iii) impregnating the metal-organic framework with elemental sulfur to form S-MOF@rGO, where the weight of sulfur is 50%-90% of the weight of the S-MOF@rGO.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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 a large specific surface area.

[0070] [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.

[0071] 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.

[0072] 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.

[0073] Ligands used in 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. Ligands are generally small molecules with Mw up to 300 g / mol.

[0074] 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.

[0075] Some ligands, such as 2-aminoterephthalic acid, contain both carboxyl and amino groups.

[0076] 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.

[0077] 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.

[0078] In the present invention, 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 at a heating temperature of 20 to 250°C, preferably 50 to 250°C, for example 100 to 200°C.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] As the MOF, it is preferable to use NH2-UiO-66(Zr) or MIL101(Cr).

[0083] 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).

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] [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. Using this method, elemental sulfur can be melted and impregnated into the MOFs, as the elemental sulfur passes through the pore windows of the MOFs and is retained there. This process can be carried out in an inert atmosphere (e.g., in the absence of air). In one embodiment, sulfur and MOF@rGO are placed in a sealed vial and heated to a temperature above the melting point of sulfur, allowing impregnation to occur.

[0091] 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.

[0092] 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 unbound S recovered during the process).

[0093] 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., 100°C or higher, e.g., 120°C or higher, e.g., 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.

[0094] In one embodiment, the sulfur-loaded MOF@rGO can be heated for 8 to 30 hours (e.g., 10 to 20 hours).

[0095] Generally, all steps of the present invention can be carried out in the absence of strong acids, such as mineral acids. Generally, the process of the present invention can be carried out in the absence of sulfuric acid.

[0096] Generally, the process of the present invention can be carried out in the absence of an inert atmosphere (e.g., using an argon atmosphere). Processes requiring an inert atmosphere have not received much attention in industry.

[0097] In another aspect, the present invention provides a method for producing a method of manufacturing a semiconductor device comprising: (i) nucleating metal ions on the graphene oxide or reduced graphene oxide sheets such that the metal ions are chemically bonded to the basal plane of the graphene oxide or reduced graphene oxide sheets; (ii) then growing a metal-organic framework containing said chemically bound metal ions by adding a multifunctional ligand to the product of step (i) and optionally heating the resulting mixture to a temperature of at least 20°C (e.g., 100-250°C) to form a metal-organic framework bound to reduced graphene oxide sheets (MOF@rGO); (iii) impregnating a metal-organic framework with elemental sulfur in an air atmosphere to form S-MOF@rGO, where the weight of sulfur is 50%-90% of the weight of the S-MOF@rGO.

[0098] The resulting material is suitable for use as a cathode in a Li-S battery, although the material morphology may need to be adjusted for use in an actual cell.

[0099] 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.

[0100] [battery] The sulfur-impregnated MOF@rGOs of the present invention can be used as cathodes in Li-S batteries. The cathode can be fabricated by mixing S-MOF@rGO with the necessary additives and grinding the resulting mixture. Typical additives include polyvinyl fluoride and polycarboxylic acid dispersants. The cathode preferably contains at least 60 wt% S-MOF@rGO.

[0101] The S-MOF@rGO powder can be dispersed in a liquid carrier (along with additives), and the dispersion can be cast onto a metal foil, such as aluminum foil, and dried. The cathode can then be cut to size.

[0102] The remainder of the battery can be conventional. The anode of such a battery is conventional and can be composed of a Li alloy (e.g., alloyed 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.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] [Separator] A separator can be used between the electrodes, and is often a porous polymer separator such as polypropylene. Other useful polymers for the separator 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.

[0108] In one embodiment, a separator containing a MOF can be used. Thus, in one embodiment, the separator between the anode and cathode is functionalized to support a MOF. Unlike the MOF used in the cathode, this MOF can be synthesized separately and coated onto a separator, such as a polymer separator. Thus, the MOF can form a thin film on the separator, 1 to 5 μm thick.

[0109] In a preferred embodiment, two metals, ideally two first-row transition metals, particularly Fe, and especially Zn and Fe, are used to prepare MOFs for separators. Therefore, the MOF is bimetallic. When Zn and Fe are used, it is preferable to use a molar excess of Zn metal ions (Zn:Fe = 10:1 to 3:1).

[0110] The use of bimetallic MOFs as coatings on separator supports offers the advantage that this type of separator prevents lithium sulfide permeation. 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.

[0111] The ligands used in the preparation of the MOF can be the same as those used in the preparation of the cathode as defined earlier in this specification.

[0112] 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 cathode 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).

[0113] 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.

[0114] [Performance] The Li-S battery of the present invention has outstanding performance, particularly in terms of capacity degradation per cycle. All rechargeable batteries lose performance over time as they are used and charged repeatedly. The key to the value of a rechargeable battery is minimal degradation of battery performance per cycle (i.e., charge-discharge cycle). We have demonstrated a capacity degradation per cycle of less than 0.05% over 1000 cycles.

[0115] Experimental results confirmed that the present invention minimizes the degradation rate per cycle and provides high cycling stability. The S-MOF@rGO with 75 wt% sulfur loading exhibited a capacity degradation per cycle of 0.02% after 1000 cycles, which is several times better than many previously reported materials synthesized as cathode materials for Li-S batteries.

[0116] The cathodes of the present invention are thermally stable and exhibit weight loss and thermal decomposition behavior similar to materials in which the MOFs are not bound to the rGO.

[0117] The cathodes of the present invention have very high initial capacities, e.g., at least 1000 mAh / g, and in some cases at least 1300 mAh / g. Values ​​up to 2000 mAh / g are envisioned. Reversible capacities after 20 cycles can be at least 1200 mAh / g.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] The battery of the present invention is capable of rapid charging and is easy to manufacture on a large scale.

[0123] [use] The lithium-sulfur battery of the present invention can be used to supply energy to mobile information devices, tools, electric vehicles, hybrid vehicles, and the like.

[0124] [Brief description of the drawing] The fabrication of a coin cell is shown in Figure 1. The coin cell for Li-S battery includes a bottom lid, an S-MOF@rGO cathode, a Celgard separator, a liquid electrolyte (not shown), a Li anode, a spacer, a spring, and a top lid.

[0125] Figure 2 shows the coulombic efficiency (right axis) and discharge capacity (left axis) versus cycle number (x-axis) for S-MOF@rGO (MOF bound with rGO - upper line) and S-MOF+rGO (unbound MOF, i.e., physically mixed with rGO - lower line). The cycling performance was measured at a current density of 0.5 C. The charge / discharge voltage range was 1.6–2.8 V.

[0126] Figure 3 shows the rate performance of a Li-S battery using an S-MOF@rGO cathode at different current densities. The cathode was fabricated as described in Example 3. The coin-type cell (shown in Figure 1) assembled for rate performance evaluation consisted of an S-MOF@rGO cathode, a Li anode, a Celgard separator, and a liquid electrolyte (1 M lithium bis(trifluoromethanesulfonyl)imide in 1,3-dioxolane / 1,2-dimethoxyethane (volume ratio 1:1) with lithium nitrate additive). The charge / discharge voltage range was 1.6–2.8 V. At 0.1 C, the initial discharge capacity could reach up to 1246 mAh / g. When cycled at various C-rates (0.3C, 0.5C, 1C, 2C, 4C, and 8C), the capacities remained at 870mAh / g, 764mAh / g, 675mAh / g, 594mAh / g, 455mAh / g, and 384mAh / g, respectively. When the current density was returned to 0.5, the capacity finally recovered to 691mAh / g, which means that the best reversible capacity was obtained under very high currents.

[0127] Figure 4 shows the cycling performance of a Li-S battery using an S-MOF@rGO cathode at 0.1 C. The cathode was fabricated as described in Example 3. The coin cells assembled for rate performance evaluation were similar to those described in Figure 3. The charge / discharge voltage range was 1.6 to 2.8 V. The Li-S coin cell using the S-MOF / rGO cathode exhibited an initial discharge capacity of 802 mAh / g. From the initial cycle to the 100th cycle, the discharge capacity was 394 mAh / g, with a degradation rate per cycle of 0.03%. However, from the 100th to 1831st cycles, the discharge capacity was 264 mAh / g, with a degradation rate per cycle of only 0.01%.

[0128] Figure 5 shows the cycling performance of a Li-S battery using an S-MOF@rGO cathode at 0.2 C. The cathode was fabricated as described in Example 3. The coin cells assembled for rate performance evaluation were similar to those described in Figure 3. The charge / discharge voltage range was 1.6 to 2.8 V. The Li-S coin cell using the S-MOF / rGO cathode exhibited a high discharge capacity of 413 mAh / g. However, after 3,817 cycles, the discharge capacity was 159 mAh / g, with a degradation rate per cycle of only 0.01%.

[0129] Figure 6 shows the cycling performance of a Li-S battery using an S-MOF@rGO cathode at 0.1 C. In this study, the area-specific sulfur loading was high and the amount of electrolyte was minimized. Addressing the challenge of high sulfur loading is crucial for the commercialization of Li-S batteries. To address this challenge, a thick-film electrode was fabricated, achieving an area-specific sulfur loading of 2.4 mg / cm. 2 to 8 mg / cm 2 The electrolyte to sulfur ratio was 6.6 μL electrolyte per mg sulfur. The cathode was fabricated by the method described in Example 3, except that a circular cathode with a diameter of 14 mm was used. The coin cell assembled for rate performance evaluation was the same as that described in Figure 3. The charge / discharge voltage range was 1.6 to 2.8 V. The Li-S coin cell using the S-MOF / rGO cathode had an area loading of 8 mg / cm. 2 , 6.3 mg / cm 2 , 4.6 mg / cm 2 , 4 mg / cm 2 , 3.3 mg / cm 2 , 3 mg / cm 2 , and 2.4 mg / cm 2 The discharge capacities were high at 138mAh / g, 215mAh / g, 254mAh / g, 283mAh / g, 321mAh / g, 575mAh / g, and 603mAh / g, respectively.

[0130] Example 1 - 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.

[0131] 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.

[0132] The freshly prepared solution was transferred to a 125 mL Teflon-lined steel autoclave and treated in a Termaks TS8024 Lab Drying Convection Oven at 120 °C for 12 hours. This process reduces GO to rGO. To remove unreacted (unbound) MOF particles 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.

[0133] {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.

[0134] Specifically, sulfur loading onto MOF@rGO was carried out in a closed system. Sulfur powder and MOF@rGO were thoroughly mixed by grinding and then sealed in a glass vial. The glass vial was then transferred to an autoclave and heated at 155 °C for 12 h using a vacuum oven.

[0135] [Example 2] 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.

[0136] [Comparative example 1: S-MOF+rGO mixture] For the S-MOF+rGO cathode, MOF particles were first synthesized using the method described above without adding rGO. After MOF particle synthesis, rGO and MOF particles were physically mixed using a piston and mortar, which was labeled MOF+rGO. Sulfur was then loaded onto the MOF+rGO by the melt-diffusion method, which was labeled S-MOF+rGO.

[0137] [Example 3] An electrochemical cell for a Li-S battery was fabricated using S-MOF@rGO (loaded with 75% sulfur by weight) by reacting 20 mL of GO dispersed in DMF at a concentration of 1 mg / mL with 0.257 mmol of ZrCl4 and 0.386 mmol of 2-aminoterephthalic acid as a ligand.

[0138] The electrochemical cell has a Li anode, a cathode containing S-MOF@rGO, a Celgard separator, and a liquid electrolyte.

[0139] S-MOF@rGO, SuperP, and polyvinylidene fluoride binder were mixed in a weight ratio of 75:15:10 in N-methyl-2-pyrrolidone solvent and ball-milled for approximately 60 minutes in a sealed Teflon jar to prepare a cathode slurry. The resulting slurry was cast onto aluminum foil and dried overnight at 60 °C. After drying, the cathode was punched into a circle with a diameter of 12 mm. The sulfur loading per area was 0.5–5 mg / cm. 2 It was.

[0140] CR-2032 coin cells were assembled in an Ar-filled glovebox using a lithium metal anode, a Celgard 2400 separator, an S-MOF@rGO cathode, and an electrolyte. This is shown in Figure 1. The electrolyte consisted of 1 M lithium bis(trifluoromethanesulfonyl)imide in 1,3-dioxolane / 1,2-dimethoxyethane (volume ratio: 1:1) with a lithium nitrate additive. The electrolyte to sulfur ratio was 5-50 μL electrolyte:1 mg sulfur. The charge / discharge voltage range was 1.6-2.8 V. Rate performance was tested at room temperature using a Landt test system, varying the current density from 0.1 C to 8 C (1 C = 1675 mA / g). Cycling performance was measured using a Biologic at different scan rates.

[0141] The S-MOF@rGO Li-S battery showed a discharge capacity of 700 mAh / g, which reached 615 mAh / g after 200 cycles, with a capacity degradation rate of only 0.06% per cycle and a coulombic efficiency of nearly 100%.

[0142] On the other hand, the S-MOF+rGO cathode had a low initial capacity of 583 mAh / g and ultimately decreased to 343 mAh / g, with a capacity degradation rate of 0.20% per cycle at 0.5C.

Claims

1. (i) nucleating metal ions on graphene oxide or reduced graphene oxide sheets such that the metal ions are chemically bonded to the basal plane of the graphene oxide or reduced graphene oxide sheets; (ii) thereafter, growing a metal-organic framework containing said chemically bound metal ions by adding a multifunctional ligand to the product of step (i) and optionally heating the resulting mixture to a temperature of at least 20°C (e.g., 100-250°C) to form a metal-organic framework bound to reduced graphene oxide sheets (MOF@rGO); (iii) impregnating the metal-organic framework with elemental sulfur to form S-MOF@rGO, wherein the weight of the sulfur is 50%-90% of the weight of the S-MOF@rGO.

2. 10. The process of any one of the preceding claims, wherein the MOF is formed using polyfunctional organic ligands, such as polycarboxylic acid ligands.

3. 10. The process of any one of the preceding claims, wherein the MOF is prepared using a tricarboxylic or dicarboxylic acid (e.g., 4-benzenedicarboxylic acid, 1,3,5-benzenetricarboxylic acid, or 2-aminoterephthalic acid, or a salt thereof).

4. 10. The process of any one of the preceding claims, wherein the metal ion is a first row transition metal.

5. 10. A process according to any one of the preceding claims, wherein the metal ion is Zr, Co, Zn, Cr or Cu, in particular Zr or Cr.

6. 10. The process of any one of the preceding claims, wherein the graphene oxide is reduced in step (ii), for example by heating.

7. 10. The process of any one of the preceding claims, wherein the amount of sulfur present in the S-MOF@rGO material is 60-90 wt%.

8. 10. The process of any one of the preceding claims, wherein the amount of the MOF in the MOF@rGO is 60-98 wt%.

9. 10. The process of any one of the preceding claims, wherein step (i) is carried out in the absence of urea.

10. 10. The process according to any one of the preceding claims, wherein the graphene oxide in step (i) is obtained by exfoliating graphene oxide dispersed in an organic solvent by ultrasonic treatment in the absence of sulfuric acid.

11. 10. The process according to any one of the preceding claims, wherein the graphene oxide in step (i) is obtained by exfoliating graphene oxide dispersed in an organic solvent by ultrasonic treatment at a temperature below 60°C (e.g., room temperature).

12. 10. The process of any one of the preceding claims, wherein step (i) uses graphene oxide and step (ii) reduces the graphene oxide.

13. A cathode for a Li—S battery, comprising: reduced graphene oxide sheets, the reduced graphene oxide sheets being 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 being impregnated with sulfur to form an S-MOF@rGO structure, wherein the weight of the sulfur is 50% to 90% of the weight of the S-MOF@GO.

14. The MOF is NH 2 14. The cathode of claim 13, wherein the cathode is UiO-66(Zr) or MIL101(Cr).

15. (i) a Li anode; (ii) a separator between the anode and the cathode; (iii) a Li-containing electrolyte; and (iv) a cathode according to claim 13 or 14, and a lithium-sulfur battery comprising the cathode.

16. 16. The lithium-sulfur battery of claim 15, wherein the separator comprises a bimetallic MOF.

17. Sulfur loading per area: 0.1 to 9 mg / cm 2 The Li-S battery according to claim 15 or claim 16,

18. Sulfur loadings per area ranging from 0.1 to 9 mg / cm were used in different volumes of electrolyte ranging from 5 to 50 μL. 2 The Li-S battery according to any one of claims 15 to 17, wherein the Li-S battery has a high solubility in water.

19. The Li-S battery according to any one of claims 15 to 18, wherein different ratios of electrolyte to sulfur (E:S=5-50 μL:1 mg) are applied.

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