Positive electrode carrier material for lithium sulfur secondary battery, positive electrode using the same, lithium sulfur secondary battery using the same, and method for manufacturing the same

Abstract

To provide a positive electrode carrier material for a lithium sulfur secondary battery capable of suppressing a shuttle effect, a positive electrode using the same, a lithium sulfur secondary battery using the same, and a method for manufacturing the same.SOLUTION: A positive electrode carrier material for a lithium sulfur secondary battery of contains a composite body formed of tantalum oxide nano-mesh and a pair of graphene nano sheets holding the tantalum oxide nano-mesh. In the positive electrode for the lithium sulfur secondary battery, sulfur is carried by the carrier material.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a positive electrode support material for a lithium-sulfur secondary battery, a positive electrode using the same, a secondary battery using the same, and methods for producing these. [Background technology]

[0002] Lithium-sulfur secondary batteries have attracted attention as a next-generation energy storage technology due to their high theoretical capacity, abundant reserves, and relatively low cost. The main problem preventing their practical use is the formation of lithium polysulfides (Li2S x This is called the shuttle effect, in which the oxides (x, 4≦x≦8) dissolve into the electrolyte and reach the anode, which reduces the capacity of the cathode and shortens the cycle life. In addition, there are problems such as deterioration due to cracks in the electrode material caused by large volume changes during charging and discharging.

[0003] A technology for suppressing the shuttle effect using nitrogen-doped graphene has been developed (see, for example, Patent Document 1). According to Patent Document 1, a nano-sulfur positive electrode composite material coated with nitrogen-doped graphene has been developed, which is characterized by an effective three-dimensional conductive network formed by stacking nitrogen-doped graphene together, and nano-sulfur particles uniformly coated with nitrogen-doped graphene sheet layers. However, in the case of the technology of Patent Document 1, not only lithium sulfide and lithium polysulfide are trapped in the positive electrode, but also the diffusion of lithium ions is inhibited, so there is a limit to the increase in capacity. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2017-521847 A Summary of the Invention [Problem to be solved by the invention]

[0005] In view of the above, an object of the present invention is to provide a positive electrode support material for a lithium-sulfur secondary battery that suppresses the shuttle effect, a positive electrode using the same, a lithium-sulfur secondary battery using the same, and methods for producing the same. [Means for solving the problem]

[0006] The positive electrode support material for a lithium-sulfur secondary battery according to the present invention contains a composite made of a tantalum oxide nanomesh and a pair of graphene nanosheets sandwiching the tantalum oxide nanomesh, thereby solving the above-mentioned problems. The ratio of the mass of the tantalum oxide nanomesh to the mass of the graphene nanosheets may be in the range of 3 or more and 8 or less. The ratio of the mass of the tantalum oxide nanomesh to the mass of the graphene nanosheets may be in the range of 4 or more and 6 or less. The tantalum oxide nanomesh may be a monolayer exfoliated from layered tantalum oxide. The tantalum oxide nanomesh may have openings with diameters in the range of 0.1 nm to 0.3 nm within a plane. The graphene nanosheet may be a reduced graphene oxide nanosheet. Either the tantalum oxide nanomesh or the graphene nanosheets may comprise a cationic polymer. The cationic polymer may be selected from the group consisting of polydiallyldimethylammonium (PDDA), polyallylamine (PAH), polyethyleneimine (PEI), polypyrrole (PPy), polyaniline (PANI) and poly(3,4-ethylenedioxythiophene) (PEDOT). It may be in the form of pellets or thin films. The sulfur-containing positive electrode for a lithium-sulfur secondary battery according to the present invention has the sulfur supported on the above-mentioned positive electrode carrier material, thereby solving the above-mentioned problems. The sulfur is 1 mg / cm 2 More than 25mg / cm 2 The following ranges may be supported: The method for producing the above-mentioned positive electrode support material for lithium-sulfur secondary batteries according to the present invention comprises mixing a dispersion liquid in which a tantalum oxide nanomesh is dispersed with a dispersion liquid in which a graphene nanosheet having a cationic polymer is dispersed, or mixing a dispersion liquid in which a tantalum oxide nanomesh having a cationic polymer is dispersed with a dispersion liquid in which a graphene nanosheet is dispersed, thereby solving the above-mentioned problems. The dispersion medium in the dispersion liquid may be selected from the group consisting of water, ethanol, N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP). The method may further comprise heating the product obtained by said mixing in an inert or reducing gas atmosphere. The heating may involve heating the product at a temperature in the range of 300° C. to 400° C. for a period of 30 minutes to 3 hours. The method may further comprise freeze-drying the product obtained by said mixing. The method for producing the above-mentioned positive electrode for a lithium-sulfur secondary battery according to the present invention comprises impregnating the above-mentioned positive electrode support material for a lithium-sulfur secondary battery with sulfur, thereby solving the above-mentioned problems. The method may further include processing the positive electrode support material into a pellet or film prior to the impregnation. The lithium-sulfur secondary battery according to the present invention comprises a positive electrode, a negative electrode, and an electrolyte located between the positive electrode and the negative electrode, the positive electrode being the above-mentioned positive electrode, and the negative electrode containing at least lithium, thereby solving the above-mentioned problems. Effect of the Invention

[0007] The positive electrode support material for lithium-sulfur secondary batteries of the present invention contains a composite consisting of a tantalum oxide nanomesh and a pair of graphene nanosheets sandwiching the tantalum oxide nanomesh. Sulfur can be supported between such composites, and the graphene nanosheets provide electrical conductivity, so the material functions as a positive electrode for lithium-sulfur secondary batteries. In addition, the tantalum oxide nanomesh has openings larger than lithium ions and smaller than lithium polysulfides, so that the lithium polysulfides are effectively trapped in the support material and the shuttle effect is suppressed. Lithium ions can move three-dimensionally inside and outside the support material, which can improve battery characteristics. By using a positive electrode in which sulfur is supported on the support material of the present invention, a lithium-sulfur secondary battery in which the shuttle effect is suppressed can be provided.

[0008] The method for producing a positive electrode support material for a lithium-sulfur secondary battery of the present invention is advantageous in practice since it is only necessary to mix a dispersion liquid in which a tantalum oxide nanomesh is dispersed with a dispersion liquid in which a graphene nanosheet having a cationic polymer is dispersed, or to mix a dispersion liquid in which a tantalum oxide nanomesh having a cationic polymer is dispersed with a dispersion liquid in which a graphene nanosheet is dispersed, and therefore does not require expensive equipment or skilled techniques. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram showing a positive electrode support material for a lithium-sulfur secondary battery according to the present invention. [Diagram 2] Schematic diagram of tantalum oxide nanomesh [Diagram 3] FIG. 1 is a diagram showing a procedure for producing a positive electrode support material for a lithium-sulfur secondary battery according to the present invention. [Figure 4] FIG. 1 is a schematic diagram showing a positive electrode for a lithium-sulfur secondary battery according to the present invention. [Diagram 5] FIG. 1 is a schematic diagram showing a lithium-sulfur secondary battery of the present invention. [Figure 6] Schematic diagram showing the mechanism for trapping lithium sulfide in the positive electrode [Figure 7]AFM image (a), in-plane X-ray diffraction pattern (b), HAADF-STEM image (c), simulated HAADF image (d), and FFT pattern (e) of tantalum oxide nanomesh. [Figure 8] Figure showing the results of a polysulfide penetration test of tantalum oxide nanomesh. [Figure 9] Figure 1 shows an AFM image of PDDA-rGO nanosheets. [Figure 10] Zeta potentials of TaO3 nanomesh, PDDA-rGO nanosheets, and GO nanosheets. [Figure 11] FIG. 1 shows the appearance of the aggregated powder obtained in Example 1. [Figure 12] FIG. 1 shows powder XRD patterns of the aggregated powders of Example 1 and Comparative Examples 1 to 3. [Figure 13] Figure 1 shows simulated XRD patterns of TaO3 nanomesh, rGO nanosheets, and their stacked units, and a TEM image of the aggregated powder of Example 1. [Figure 14] FIG. 1 shows an SEM image of the aggregated powder of Example 1. [Figure 15] FIG. 1 shows an SEM image and EDS mapping of the aggregated powder of Example 1. [Figure 16] FIG. 1 shows a TG-DTA curve of the agglomerated powder of Example 1. [Figure 17] FIG. 1 shows the appearance of non-S-impregnated pellets of Example 1. [Figure 18] FIG. 1 shows the CV characteristics of Li2S6 symmetric batteries using non-S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2. [Figure 19] FIG. 1 shows electrochemical impedance spectroscopy (EIS) curves and their equivalent circuits for Li2S6 symmetric batteries using non-S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2. [Figure 20] FIG. 1 shows the current-time characteristics (2.05 V) of Li2S8 coin batteries using non-S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2. [Figure 21] FIG. 21 shows Raman spectra of the S-non-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2 after the measurement of the current-time characteristics of FIG. 20. [Figure 22] FIG. 21 shows electrochemical impedance spectroscopy (EIS) curves and their equivalent circuits for Li2S8 coin batteries using non-S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2 after measuring the current-time characteristics of FIG. 20. [Figure 23] FIG. 1 shows the UV-visible absorption spectra of non-S-impregnated pellets of Example 1 and Comparative Example 1 after contact with Li2S6 electrolyte. [Figure 24] FIG. 1 shows the CV characteristics of lithium-sulfur batteries using the S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2 as the positive electrode. [Diagram 25] FIG. 1 shows the CV characteristics of a lithium-sulfur battery using the S-impregnated pellets of Example 1. [Figure 26] FIG. 1 shows the CV characteristics of a lithium-sulfur battery using the S-impregnated pellets of Comparative Example 1. [Figure 27] FIG. 13 shows the CV characteristics of a lithium-sulfur battery using the S-impregnated pellets of Comparative Example 2. [Figure 28] FIG. 1 shows the relationship between the peak current and the square root of the scan rate for a lithium-sulfur battery using the S-impregnated pellets of Example 1. [Figure 29] FIG. 1 is a graph showing the relationship between the peak current and the square root of the scan rate of a lithium-sulfur battery using the S-impregnated pellet of Comparative Example 1. [Diagram 30] FIG. 1 shows the relationship between the peak current and the square root of the scan rate for a lithium-sulfur battery using the S-impregnated pellets of Comparative Example 2. [Diagram 31] FIG. 1 shows electrochemical impedance spectroscopy (EIS) curves of lithium-sulfur batteries using the S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2. [Diagram 32] FIG. 1 shows the charge / discharge characteristics of lithium-sulfur batteries using the S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2. [Diagram 33] FIG. 1 shows cycle characteristics of lithium-sulfur batteries using the S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2. [Diagram 34] FIG. 1 shows cycle characteristics of lithium-sulfur batteries using non-S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2. [Diagram 35] FIG. 33 shows the charge / discharge potential difference of each lithium-sulfur battery obtained from the charge / discharge characteristics of FIG. 32. [Diagram 36] A diagram showing the capacities Q1 and Q2 of each lithium-sulfur battery obtained from the charge / discharge characteristics of FIG. 32. [Figure 37] FIG. 1 shows the cycle characteristics of a lithium-sulfur battery using another S-impregnated pellet of Example 1. [Figure 38] FIG. 1 shows the cycle characteristics of a lithium-sulfur battery using yet another S-impregnated pellet of Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that like elements are given like reference numerals and their description will be omitted.

[0011] (Embodiment 1) In the first embodiment, a positive electrode support material for a lithium-sulfur secondary battery and a method for producing the same according to the present invention will be described. FIG. 1 is a schematic diagram showing a positive electrode support material for a lithium-sulfur secondary battery according to the present invention.

[0012] The positive electrode support material 100 for lithium-sulfur secondary batteries of the present invention (hereinafter, simply referred to as the support material for simplicity) contains a composite 130 consisting of a tantalum oxide nanomesh 110 and a pair of graphene nanosheets 120 sandwiching the tantalum oxide nanomesh 110. In detail, half of each of both sides of the tantalum oxide nanomesh 110 is covered with the graphene nanosheet 120, forming a sandwich structure of rGO / TaO3 / rGO. The gap between such composites 130 can support sulfur, which is a positive electrode active material, and the graphene nanosheet 120 provides electrical conductivity, so that the composite functions as a positive electrode of a lithium-sulfur secondary battery. In this specification, the composite 130 of the tantalum oxide nanomesh 110 sandwiched between a pair of graphene nanosheets 120 may be referred to as a sandwich structure from the viewpoint of ease of understanding.

[0013] The tantalum oxide nanomesh 110 has openings 210 (FIG. 2) in its surface that are derived from the crystallographic structure. The inventors of the present application have clarified that the diameter of such openings is larger than that of lithium ions and smaller than that of lithium polysulfides, and have found that lithium polysulfides (Li2S x It has been found that the lithium polysulfide (x: 4≦x≦8) can be effectively confined in the support material 100 (specifically, in the gaps between the composites 130). When the support material 100 of the present invention is used in the positive electrode of a lithium-sulfur secondary battery, the lithium polysulfide is confined in the support material 100, thereby preventing the lithium sulfide from dissolving in the electrolyte and suppressing the shuttle effect.

[0014] In addition, since lithium ions are smaller than the opening 210, they can move three-dimensionally inside and outside the support material, which can improve the battery characteristics of the lithium-sulfur secondary battery. For simplicity, in the following, lithium sulfide (Li2S) and lithium polysulfide (Li2S x :2≦x≦8) are sometimes collectively referred to simply as lithium sulfide. Next, each of the components of the carrier material 100 of the present invention will be described in detail.

[0015] FIG. 2 is a schematic diagram of a tantalum oxide nanomesh.

[0016] The tantalum oxide nanomesh 110 is exfoliated from the layered tantalum oxide, which is the parent crystal, as shown in JP 2007-284277 A. The layered tantalum oxide has a monoclinic crystal structure, in which metal-oxygen octahedra are linked by edge sharing or apex sharing to form a two-dimensional framework structure of TaO6 octahedra having regular holes, and is represented by the general formula ATaO3 (A is selected from the group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs)). As represented by the method described in JP 2007-284277 A, the layered tantalum oxide is exfoliated into a single layer by proton exchange of A ions between negatively charged TaO3 by a special treatment, so that the tantalum oxide nanomesh 110 has a negative charge.

[0017] The thickness of the tantalum oxide nanomesh 110 may be a crystallographic thickness, and is in the range of 0.9 nm to 2 nm. The lateral size of the tantalum oxide nanomesh 110 may be in the range of submicrons to tens of microns, preferably in the range of 0.05 μm to 10 μm, more preferably in the range of 0.1 μm to 5 μm. This range makes it easy to form the sandwich structure described above.

[0018] In addition, by having the above-mentioned thickness and lateral size of the tantalum oxide nanomesh 110, it is possible to expose more highly reactive sites for the conversion of tantalum oxide to lithium sulfide. In addition, when used as an electrode for a lithium-sulfur secondary battery, it is possible to shorten the diffusion distance of lithium ions, thereby improving the rate characteristics.

[0019] The tantalum oxide nanomesh 110 has an opening 210 in a plane derived from the crystallographic structure of the host layer. As shown in FIG. 2, the opening 210 may be rhombic, and the diameter thereof is preferably in the range of 0.1 nm to 0.3 nm. The diameter is more preferably in the range of 0.1 nm to 0.2 nm. In this range, lithium polysulfides can be trapped while allowing lithium ions to pass through. The diameter of the opening 210 is determined by creating a circle inscribed in the rhombic opening.

[0020] The graphene nanosheet 120 is an atomic layer containing sp2-bonded carbon atoms, and is a well-known material produced by the modified Hummers method or the like. The longitudinal size of the graphene nanosheet 120 is preferably in the range of 10 nm to 10 μm, more preferably in the range of 0.1 μm to 5 μm. If it is in this range, it is easy to form the above-mentioned sandwich structure.

[0021] The thickness of the graphene nanosheet 120 may range from a crystallographic thickness (monolayer) to a multilayer. From this viewpoint, the thickness of the graphene nanosheet 120 is preferably in the range of 0.3 nm to 10 nm, more preferably in the range of 0.3 nm to 2 nm. Within this range, the graphene nanosheet 120 can exhibit excellent electrical conductivity when used as an electrode.

[0022] Furthermore, since the graphene nanosheet 120 has the above-mentioned thickness and lateral size, it is endowed with electrical conductivity, and can therefore function as the positive electrode of a lithium-sulfur secondary battery.

[0023] The graphene nanosheet 120 may have a functional group (not shown) on the surface, such as a carboxyl group (COOH group), a hydroxyl group (OH group), a carbonyl group (CO group), or an aldehyde group (CHO group). Such a functional group may remain on the surface of the graphene nanosheet 120 during production, and even if the graphene nanosheet 120 has such a functional group, the penetration and movement of electrolyte ions and the electrical conductivity are not inferior. From this viewpoint, the graphene nanosheet 120 may be a reduced graphene oxide (rGO) nanosheet obtained by reducing graphene oxide. Graphene mechanically peeled off from graphite is electrically neutral, but the graphene nanosheet 120 obtained by reduction using the improved Hummers method has a negative charge.

[0024] As described above, since both the tantalum oxide nanomesh 110 and the graphene nanosheet 120 have a negative charge, either the tantalum oxide nanomesh 110 or the graphene nanosheet 120 may have the cationic polymer 140. This maintains a sandwich structure in which the pair of graphene nanosheets 120 sandwich the tantalum oxide nanomesh 110 by electrostatic interaction.

[0025] Here, the type of cationic polymer 140 is not particularly limited, but may be selected from the group consisting of polydiallyldimethylammonium (PDDA), polyallylamine (PAH), polyethyleneimine (PEI), polypyrrole (PPy), polyaniline (PANI) and poly(3,4-ethylenedioxythiophene) (PEDOT). These can be easily applied to either the tantalum oxide nanomesh 110 or the graphene nanosheet 120, and the amount of positive charge can be controlled.

[0026] In the composite 130, the mass ratio of the tantalum oxide nanomesh 110 to the mass of the graphene nanosheet 120 is preferably in the range of 3 to 8. Within this range, the tantalum oxide nanomesh 110 does not aggregate, and the formation of a sandwich structure is promoted. The mass ratio of the tantalum oxide nanomesh 110 to the mass of the graphene nanosheet 120 is more preferably in the range of 4 to 6. Within this range, the formation of a sandwich structure is further promoted by a method described below.

[0027] In the composite 130, the interlayer distance between the tantalum oxide nanomesh 110 and the graphene nanosheet 120 is preferably in the range of 0.5 nm to 5 nm. Within this range, electrical conductivity can be effectively imparted to the insulating tantalum oxide nanomesh 110. The interlayer distance is more preferably in the range of 1 nm to 2 nm. Within this range, high-speed movement of lithium ions is possible, and a lithium-sulfur secondary battery with excellent rate characteristics and cycle characteristics can be provided.

[0028] The support material 100 of the present invention is obtained in the form of a powder, but the powder may be processed into pellets or a thin film. Since the support material 100 of the present invention contains graphene nanosheets 120, it can be processed into a self-supporting pellet or thin film without using a binder, making a current collector unnecessary.

[0029] When the support material 100 of the present invention is in the form of pellets or a thin film, it does not have a binder, and therefore can support a sufficient amount of sulfur (S8), which is an active material, while exposing reaction sites, thereby improving the battery characteristics of a lithium-sulfur secondary battery. In addition, since the pellets can follow the volume change caused by the confinement of lithium sulfide, it is expected that the cycle characteristics will also be improved.

[0030] Next, a method for producing the positive electrode support material for a lithium-sulfur secondary battery of the present invention will be described. FIG. 3 is a diagram showing a procedure for producing a positive electrode support material for a lithium-sulfur secondary battery of the present invention.

[0031] As shown in Fig. 3, the carrier material 100 (Fig. 1) of the present invention is obtained by mixing a dispersion 310 in which a tantalum oxide nanomesh 110 is dispersed with a dispersion 320 in which a graphene nanosheet 120 having a cationic polymer 140 is dispersed. By simply mixing, a complex 130 is formed by electrostatic interaction between the negatively charged tantalum oxide nanomesh 110 and the positively charged graphene nanosheet 120, making it possible to eliminate the need for expensive equipment or skilled techniques.

[0032] The dispersion liquid 310 in which the tantalum oxide nanomesh 110 is dispersed is prepared by the method disclosed in Japanese Patent Application Laid-Open No. 2007-284277. The dispersion medium is not particularly limited as long as it is a protic polar solvent, but for example, a solvent selected from the group consisting of water, ethanol, N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP) is used. These solvents can maintain the tantalum oxide nanomesh 110 in a well-dispersed state without re-aggregation. The water may be distilled water, ion-exchanged water, Milli-Q water, ultrapure water, or the like.

[0033] The concentration of the tantalum oxide nanomesh 110 in the dispersion liquid 310 is not particularly limited as long as it is dispersed. 3 More than 1.5g / dm 3 More preferably, it is 0.3 g / dm3 More than 0.7g / dm 3 The range is as follows: Within this range, the formation of the above-mentioned sandwich structure is promoted.

[0034] The graphene nanosheet 120 having the cationic polymer 140 is prepared, for example, by applying the cationic polymer 140 to a graphene oxide nanosheet (GO nanosheet) exfoliated from graphite oxide, followed by reduction. Alternatively, the graphene oxide nanosheet (GO nanosheet) may be reduced, and then the cationic polymer 140 may be applied thereto.

[0035] Graphite oxide is obtained from natural graphite pieces or flake-like graphite by the well-known modified Hummer process. The graphite oxide thus obtained is dispersed in a dispersion medium and ultrasonically treated, whereby the graphite oxide is exfoliated into a single layer to become a GO nanosheet. A reducing agent such as hydrazine or urea may be added to the dispersion liquid containing the GO nanosheet, and the mixture may be heated. As a result, the GO nanosheet is reduced to become a graphene (rGO) nanosheet. Then, the above-mentioned cationic polymer may be added to obtain a graphene nanosheet 120 having a cationic polymer. The heating may be performed, for example, at a temperature range of 70° C. to 100° C. for 1 hour to 24 hours. The amount of the cationic polymer added may be 2 times to 10 times the mass of the GO nanosheet (or rGO nanosheet). The rGO nanosheet to which the cationic polymer is added thus obtained may be collected by centrifugation and dispersed in the above-mentioned dispersion medium.

[0036] Since the dispersion liquid 310 and the dispersion liquid 320 are mixed, it is preferable that the dispersion medium used in the dispersion liquid 310 is the same as the dispersion medium used in the dispersion liquid 320. After dispersing the graphene nanosheet 120 having the cationic polymer 140 in the dispersion medium, it is preferable to perform high-speed centrifugation (for example, 5000 rpm to 30000 rpm) and use the supernatant as the dispersion liquid 320.

[0037] The concentration of the graphene nanosheets 120 in the dispersion liquid 320 is not particularly limited as long as they are dispersed. For example, 3 More than 1.0g / dm 3 More preferably, it is 0.05 g / dm 3 More than 0.15g / dm 3 The range is as follows: Within this range, the formation of the above-mentioned sandwich structure is promoted.

[0038] The mixing is preferably performed so that the mass ratio of the tantalum oxide nanomesh 110 in the dispersion 310 to the graphene nanosheets 120 in the dispersion 320 is 3 or more and 8 or less. This suppresses the tantalum oxide nanomesh 110 from self-re-aggregating, and promotes the formation of a sandwich structure between the tantalum oxide nanomesh 110 and the graphene nanosheets 120. The mixing is more preferably performed so that the mass ratio is in the range of 4 or more and 6 or less.

[0039] Mixing may be performed using a stirrer such as a magnetic stirrer, and is performed, for example, at a rotation speed of 100 rpm to 500 rpm for 10 to 30 minutes.

[0040] In this way, the carrier material 100 having the complex 130 of the present invention is obtained as a product (precipitate). The product may be collected by repeatedly washing with alcohol such as ethanol and centrifuging. The collected product may be freeze-dried. This allows the dispersion medium remaining in the product to be completely removed.

[0041] Furthermore, the product may be heat-treated in an inert or reducing gas atmosphere. This further promotes the reduction of the graphene nanosheet 120 to graphene, and the oxygen-containing functional groups remaining on the surface can be removed, so that the electrical conductivity can be improved. In addition, the cationic polymer 140 is burned and removed by the heat treatment, so that the active sites of the tantalum oxide nanomesh 110 can be increased. Examples of the inert or reducing gas atmosphere include argon gas, xenon gas, helium gas, neon gas, nitrogen gas, hydrogen gas, a mixture of hydrogen and nitrogen, and ammonia decomposition gas. The heat treatment conditions may be, for example, a temperature range of 300°C to 400°C for a time of 30 minutes to 3 hours.

[0042] 3, the graphene nanosheet 120 has the cationic polymer 140, but the tantalum oxide nanomesh 110 may have the cationic polymer 140. That is, a dispersion liquid in which the tantalum oxide nanomesh 110 (positive charge) having the positively charged cationic polymer 140 is dispersed may be mixed with a dispersion liquid in which the graphene nanosheet 120 having the negative charge is dispersed. The tantalum oxide nanomesh 110 having the cationic polymer 140 may be obtained by adding the cationic polymer 140 to the above-mentioned dispersion liquid 310 and stirring the mixture.

[0043] The obtained product may be processed into pellets or a thin film. For processing into pellets, known methods such as die pressing and cold isostatic pressing (CIP) can be used. For processing into a thin film, known methods such as dropping, dip coating, flow coating, flow coating, curtain coating, spin coating, spray coating, airless spray coating, bar coating, roll coating, brush coating, can be used.

[0044] In this manner, a positive electrode support material 100 for a lithium-sulfur secondary battery is produced, which contains a composite 130 having a sandwich structure consisting of the tantalum oxide nanomesh 110 of the present invention and a pair of graphene nanosheets 120 that sandwich it.

[0045] (Embodiment 2) In the second embodiment, a positive electrode for a lithium-sulfur secondary battery of the present invention, a lithium-sulfur secondary battery of the present invention using the same, and methods for producing the same will be described.

[0046] FIG. 4 is a diagram illustrating a schematic diagram of a positive electrode for a lithium-sulfur secondary battery according to the present invention.

[0047] The positive electrode 400 for a lithium-sulfur secondary battery of the present invention includes a support material 100 supporting sulfur 410. Here, the support material is the support material 100 described in the first embodiment, so the description will be omitted. The sulfur 410 is a sulfur particle, and may be located on the surface of the tantalum oxide nanomesh 110 and the graphene nanosheet 120 in the composite 130 of the support material 100, and in the gaps between the composites 130. FIG. 4 shows the composites 130 stacked, but the composites 130 may be randomly aggregated without being limited to stacking. In this case, the sulfur 410 is also located in the gaps between the composites 130.

[0048] The particle size of the sulfur 410 is preferably in the range of 1 nm to 10 nm, inclusive, in which case the sulfur 410 can be sufficiently positioned in the gaps between the composites 130, thereby improving the battery characteristics.

[0049] The amount of sulfur 410 supported is not particularly limited, but is illustratively 1 mg / cm 2 If the amount of sulfur 410 supported is equal to or greater than 1 mg / cm, the positive electrode can function advantageously. 2 More than 25mg / cm 2 More preferably, it is in the range of 5 mg / cm 2 More than 20mg / cm 2 The range is preferably 10 mg / cm or less. 2 More than 20mg / cm 2 By increasing the supported amount, the sulfur utilization rate is improved, and therefore a lithium-sulfur secondary battery having a high energy density can be provided.

[0050] Next, a method for producing the positive electrode 400 for a lithium-sulfur secondary battery of the present invention will be described. The positive electrode 400 for a lithium-sulfur secondary battery of the present invention is produced by impregnating the support material 100 of the first embodiment with sulfur 410. The impregnation of sulfur 410 may be performed, for example, by immersing the support material 100 in a solution containing sulfur 410, or by dropping, applying, or spraying the solution containing sulfur 410 onto the support material 100. From this viewpoint, it is preferable to process the support material 100 into a pellet or a thin film prior to the impregnation.

[0051] The solvent of the solution containing sulfur 410 is not particularly limited as long as the sulfur 410 dissolves therein, and examples thereof include carbon disulfide, toluene, and benzene. The concentration of sulfur 410 in the solution may be controlled by the amount of sulfur supported, and is preferably 10 mg / cm. 3 More than 50mg / cm 3 The following ranges can be used:

[0052] In addition, the carrier material 100 may be immersed in, dropped onto, coated on, or sprayed onto a solution containing sulfur 410, and then dried to remove the solvent. Drying may be air drying, heat drying, freeze drying, or the like.

[0053] FIG. 5 is a diagram illustrating a lithium-sulfur secondary battery of the present invention.

[0054] The lithium-sulfur secondary battery 500 of the present invention includes a positive electrode 510, a negative electrode 520, and an electrolyte 530 located between the positive electrode 510 and the negative electrode 520. The positive electrode 510 is the above-mentioned positive electrode 400, and the negative electrode 520 contains at least lithium (Li).

[0055] FIG. 6 is a diagram illustrating the mechanism by which lithium sulfide is trapped in the positive electrode.

[0056] As described above, in the lithium-sulfur secondary battery 500 of the present invention, the above-mentioned support material 100 supporting sulfur is used in the positive electrode 510. In the positive electrode 510, sulfur (S8) and lithium react in multiple stages, and the charge / discharge reaction proceeds by repeating the process of finally reacting to Li2S and the process of returning from Li2S to S8. Since the lithium sulfide generated in the charge / discharge reaction cannot pass through the opening 210 of the tantalum oxide nanomesh 110 in the composite 130 in the positive electrode 510, the lithium sulfide is effectively confined in the positive electrode 510 (specifically, between the composite 130 and another adjacent composite 130). As a result, the lithium polysulfide is less likely to diffuse to the negative electrode 520, so the shuttle effect is suppressed and the amount of sulfur in the positive electrode 510 is not reduced, and therefore the decrease in the charge / discharge capacity can be suppressed. On the other hand, the lithium ions can move three-dimensionally inside and outside the positive electrode 510 (specifically, the composite 130), which enables high-speed movement and high-speed charge / discharge. Each component will be described.

[0057] The positive electrode 510 is the above-mentioned positive electrode 400, and therefore description thereof will be omitted, but the thickness of the positive electrode 510 is illustratively in the range of 5 μm to 500 μm. In consideration of practical application, the thickness of the positive electrode 510 may be 10 μm to 400 μm. The positive electrode 510 may further include a positive electrode current collector (not shown). This makes it easy to extract power to the outside. The positive electrode current collector may be a metal material such as aluminum (Al), nickel (Ni), stainless steel, or gold (Au). The positive electrode current collector may be in the form of a sheet.

[0058] The negative electrode 520 may be a known electrode containing lithium, and may be, for example, a simple lithium metal, an alloy containing lithium, or an inorganic material doped with lithium. Examples of the alloy containing lithium include an alloy of lithium and aluminum (Al), and an alloy of lithium and indium (In). Examples of the inorganic material doped with lithium include Li-doped Si, Li-doped SiO, Li-doped Sn, and Li-doped SnO2.

[0059] The thickness of the negative electrode 520 is not particularly limited, but is illustratively in the range of 5 μm to 500 μm. In consideration of practical application, the thickness of the negative electrode 520 may be 50 μm to 300 μm. The negative electrode 520 may further include a negative electrode current collector (not shown). This makes it easy to extract power to the outside. The negative electrode current collector may be a metal material such as copper (Cu), nickel (Ni), stainless steel, or gold (Au). The negative electrode current collector may be in the form of a sheet.

[0060] The electrolyte 530 may be a known electrolyte used in lithium-sulfur secondary batteries, and may be, for example, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), LiPF6, LiBF4, or the like. These electrolytes are used by dissolving them in tetrahydrofuran, glyme, diglyme, triglyme, tetraglyme, diethoxyethane (DEE), or dimethoxyethane (DME). The electrolyte 530 may further contain lithium nitrate. This forms a coating on the surface of the negative electrode 520, making it possible to suppress the passage of lithium sulfide.

[0061] Alternatively, the electrolyte 530 may be impregnated into a separator (not shown) to hold the electrolyte 530. The separator is a porous film made of a resin such as polyethylene or polypropylene. The thickness of the electrolyte 530 is not particularly limited, but may be 1 μm or more and 50 μm or less.

[0062] The lithium-sulfur secondary battery 500 of the present invention may be housed in a case (not shown). The case is made of aluminum (Al), stainless steel, nickel-plated steel, or the like. The shape of the lithium-sulfur secondary battery 500 may be a coin type, a button type, a sheet type, a cylindrical type, a square type, or the like. In FIG. 5, the lithium-sulfur secondary battery 500 of the present invention shows a set of a positive electrode 510, a negative electrode 520, and an electrolyte 530, but a plurality of these may be integrated. Such modifications are within the scope of the present invention and will be understood by those skilled in the art.

[0063] The lithium-sulfur secondary battery 500 of the present invention is charged by connecting it to an external power source and applying a positive potential to the positive electrode 510 and a negative potential to the negative electrode 520. The lithium-sulfur secondary battery 500 is discharged by connecting a discharge circuit to the positive electrode 510 and the negative electrode 520 of the lithium-sulfur secondary battery 500 and passing current through the discharge circuit of an electronic device, an electric vehicle, or the like.

[0064] Next, a method for producing the lithium-sulfur secondary battery 500 of the present invention will be described. The lithium-sulfur secondary battery 500 may be produced, for example, by separately producing the positive electrode 510, the negative electrode 520, and the electrolyte 530 and laminating them together. Alternatively, when the negative electrode 520 is a lithium metal foil, the negative electrode 520 may be formed as a film by physical vapor deposition or chemical vapor deposition after laminating the positive electrode 510 and the electrolyte 530.

[0065] The present invention will now be described in detail with reference to specific examples, but it should be noted that the present invention is not limited to these examples. EXAMPLES

[0066] [reagent] The following reagents were used: Rubidium carbonate (Rb2CO3, 99.9%, Rare Metallic Co., Ltd.), tantalum oxide (Ta2O5, 99.99%, Rare Metallic Co., Ltd.), hydrochloric acid (concentrated HCl, Kishida Chemical Co., Ltd.), tetrabutylammonium hydroxide solution (TBAOH, 10wt%, Wako special grade, Fujifilm Wako Pure Chemical Industries, Ltd.), polydiallyldimethylammonium chloride (PDDA, 20wt% aqueous solution, Aldrich), sulfur (S, 99.998%, Aldrich), The following fluorocarbons were used: lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 99.95%, Aldrich), lithium nitrate (LiNO3, SAJ Grade 1, Sigma-Aldrich), 1,2-dimethoxyethane (DME, 99.5%, Sigma-Aldrich), 1,3-dioxolane (DOL, 99.8%, Sigma-Aldrich), carbon disulfide (CS2, 99%, Fujifilm Wako Pure Chemical Industries, Ltd.), lithium foil (Li, 99.8%, Alfa Aesar), lithium sulfide (Li2S, 99.98%, Aldrich), tetraglyme (99.9%, Sigma-Aldrich), and graphite (Fujifilm Wako Pure Chemical Industries, Ltd.).

[0067] [Preparation of tantalum oxide nanomesh] Tantalum oxide nanomesh was prepared according to the manufacturing method described in JP 2007-284277 A. In detail, rubidium carbonate and tantalum oxide were mixed in a molar ratio of 1.02:1 and fired at 900°C for 20 hours to generate layered rubidium tantalum oxide (RbTaO3) as a parent crystal. Rubidium tantalum oxide (10 g) was dissolved in a hydrochloric acid solution (1 mol / dm 3 , 1dm 3 ) and stirred for three days to exchange the interlayer rubidium for hydrogen ions. The solution was decanted and replaced every 24 hours. In this way, layered tantalum oxide (HTaO3·1.3H2O), a hydrogen ion exchanger, was obtained.

[0068] The obtained layered tantalum oxide (4 g) was dissolved in tetrabutylammonium hydroxide (TBAOH) solution (1 dm 3 ), and the mixture was shaken to separate into single layers.+ and H + The molar ratio of TaO3 to TaO3 was 1:1. The dispersion was shaken at 180 rpm for 10 days and centrifuged at 2000 rpm for 10 minutes to remove unexfoliated layered tantalum oxide. The resulting suspension was centrifuged again at 20000 rpm for 30 minutes to obtain a single layer of exfoliated tantalum oxide (TaO3) nanomesh. The supernatant was washed with ultrapure water until it became transparent, and the tantalum oxide nanomesh was dispersed in water to obtain a dispersion.

[0069] This dispersion was dropped onto a silicon substrate and dried, and the tantalum oxide nanomesh was observed with an atomic force microscope (AFM, Probe Station AFM5000II, Hitachi High-Technologies Corporation) and a scanning transmission electron microscope (STEM, JEM-2000, JEOL Ltd.). The tantalum oxide nanomesh was structurally analyzed by in-plane X-ray diffraction (wavelength of synchrotron radiation = 0.11991(2) nm, BL-6C, High Energy Accelerator Research Organization), and the image taken with a 300 kV field emission electron microscope (TEM, JEM-3000F, JEOL Ltd.) was subjected to fast Fourier transform (FFT) to obtain the FFT pattern. These results are shown in Figure 7.

[0070] FIG. 7 shows an AFM image (a) of the tantalum oxide nanomesh, an in-plane X-ray diffraction pattern (b), an HAADF-STEM image (c), a simulated HAADF image (d), and an FFT pattern (e).

[0071] According to FIG. 7(a), the tantalum oxide nanomesh was confirmed to be a two-dimensional sheet with a crystallographic thickness of 1 nm and a lateral dimension of 0.1 μm to 5 μm. According to FIG. 7(b), it was confirmed that the tantalum oxide nanomesh exhibited a sharp diffraction peak and had high crystallinity. The diffraction peak of the tantalum oxide nanomesh was indexed to a face-centered rectangular lattice with unit cell lengths a=0.9607 nm and b=0.8479 nm, and these values ​​were in good agreement with the in-plane lattice constants of RbTaO3. This confirmed that the tantalum oxide nanomesh maintained the crystal structure of the RbTaO3 host layer before the monolayer exfoliation.

[0072] According to Figures 7(c)-(e), it was confirmed that the tantalum oxide nanomesh had openings as shown in the inset of Figure 7(b). Furthermore, the distance between two orthogonal oxygen atoms in the diamond-shaped openings A and B shown in the inset of Figure 7(b) was estimated to be 0.39 nm and 0.46 nm, respectively, from the crystallographic data of the original RbTaO3. Considering the radius of the oxygen ion (approximately 0.14 nm), the diameters of the openings A and B were calculated to be 0.11 nm and 0.18 nm, respectively, which is approximately 0.03 nm for the lithium ion (Li + ) and other metal ions for batteries.

[0073] Next, a lithium polysulfide permeation test was conducted on the obtained tantalum oxide nanomesh. Two L-shaped chambers were separated by a separator (Celgard 2325, Celgard Corporation) and an H-shaped container was assembled. One of the separators was filled with 0.1 mg / cm2 lithium polysulfide by vacuum filtration. 2 One separator was supported with a tantalum oxide nanomesh, and the other separator was not supported with a tantalum oxide nanomesh. The left chamber of the H-shaped container was filled with a 0.005 mol / L Li2S6 solution (solvent was a mixed solvent of DOL and DME mixed at a volume ratio of 1:1, yellow solution), and the right chamber was filled with a mixed solvent of pure DOL and DME (volume ratio 1:1, colorless and transparent solution), and the sieving ability of the tantalum oxide nanomesh for lithium polysulfides was examined. The results are shown in Figure 8.

[0074] FIG. 8 shows the results of a polysulfide penetration test of tantalum oxide nanomesh.

[0075] The upper part of FIG. 8 shows the change over time of only the separator not carrying a tantalum oxide nanomesh, and the lower part of FIG. 8 shows the change over time of the separator carrying a tantalum oxide nanomesh.

[0076] As shown in the upper part of Figure 8, the right chamber changed from colorless and transparent to yellow over time due to the penetration of lithium polysulfides from the left chamber. On the other hand, as shown in the lower part of Figure 8, even after 2 hours (Figure 8(h)), the right chamber remained colorless and transparent, and no penetration of lithium polysulfides from the left chamber was observed. This suggests that the tantalum oxide nanomesh is effective in suppressing the shuttle effect because it does not allow lithium polysulfides to penetrate.

[0077] [Preparation of graphene (rGO) nanosheets] First, graphene oxide nanosheets (hereafter referred to as GO nanosheets) were prepared from graphite by the well-known modified Hummers method.

[0078] The resulting GO nanosheets were dispersed in water at 200 cm 3 (Concentration: 0.2g / dm 3 ) in 1.5 cm of an aqueous solution containing polydiallyldimethylammonium (PDDA) as a cationic polymer. 3 (concentration: 20 wt%) to modify the GO nanosheets with PDDA. Then, 15 mm of hydrazine monohydrate was added. 3 (concentration: 98 wt%) was added and heated at 90° C. for 3 hours with stirring. As a result, the GO nanosheets were reduced to graphene nanosheets (hereafter referred to as rGO nanosheets).

[0079] The obtained suspension was centrifuged at high speed (rotation speed: 20,000 rpm), and the collected material was further dispersed in ultrapure water. Centrifugation was performed again (rotation speed: 6,000 rpm), and the supernatant was used as a dispersion containing rGO nanosheets modified with PDDA (PDDA-rGO nanosheets). This dispersion was applied to a silicon substrate and observed with an AFM. For comparison, a dispersion containing rGO nanosheets reduced without using PDDA (rGO nanosheets) and a dispersion containing GO nanosheets before reduction were also prepared. The zeta potentials of these dispersions were measured (zeta potential / particle size measurement system, ELSZ-2, manufactured by Otsuka Electronics Co., Ltd.). These results are shown in Figures 9 and 10.

[0080] FIG. 9 shows an AFM image of PDDA-rGO nanosheets.

[0081] According to FIG. 9, it was confirmed that the PDDA-rGO nanosheet is a highly anisotropic two-dimensional crystal with a lateral size of 0.5 μm to 5 μm and a thickness of 1.5 nm. Although not shown, the GO nanosheet has a thickness of 0.8 nm, and the rGO nanosheet has a thickness of 0.6 nm. From this, it was found that the oxygen-containing functional groups present on the surface of the GO nanosheet were removed by heat treatment using hydrazine monohydrate, and the GO nanosheet was reduced to become a rGO nanosheet. In addition, the thickness of the PDDA-rGO nanosheet (1.5 nm) was thicker than that of the rGO nanosheet (0.6 nm), and it was found that PDDA was modified.

[0082] FIG. 10 shows the zeta potentials of TaO3 nanomesh, PDDA-rGO nanosheets, and GO nanosheets.

[0083] Figure 10 shows the zeta potentials of the TaO3 nanomesh and GO nanosheets in addition to the PDDA-rGO nanosheets. According to Figure 10, both the TaO3 nanomesh and the GO nanosheets had negative charges, but the PDDA-rGO nanosheets had positive charges. Although not shown, it was confirmed that the rGO nanosheets not modified with PDDA also had negative charges. This indicates that the charge was controlled by the addition of PDDA.

[0084] In the following Examples and Comparative Examples, a dispersion containing the TaO3 nanomesh prepared here, a dispersion containing the PDDA-rGO nanosheets, and a dispersion containing the rGO nanosheets were used.

[0085] [Example 1] In Example 1, according to the procedure in Figure 3, a dispersion liquid in which a tantalum oxide (TaO3) nanomesh was dispersed was mixed with a dispersion liquid in which a graphene nanosheet having PDDA as a cationic polymer (PDDA-rGO nanosheet) was dispersed, to produce a composite (S-TaO3 / rGO) consisting of a TaO3 nanomesh and a pair of PDDA-rGO nanosheets sandwiching it.

[0086] In detail, TaO3 nanomesh dispersion (TaO3 concentration 0.48 g / dm 3 , 0.2dm 3 ), and PDDA-rGO nanosheet dispersion (PDDA-rGO concentration 0.1 g / dm 3 (However, calculated based on the mass of graphene alone without considering PDDA), 0.2 dm 3 ) were mixed and stirred.

[0087] Here, the mass ratio of TaO3 nanomesh to rGO nanosheets was mixed to satisfy 4.8 so that the interface area between the TaO3 nanomesh and the PDDA-rGO nanosheets would be maximized. The interface area between the TaO3 nanomesh and the PDDA-rGO nanosheets was evaluated for the ideal graphene structure using the unit cell of the hexagonal rGO nanosheets (a = 0.25 nm) and the unit cell of the face-centered rectangular lattice of the TaO3 nanomesh (a = 0.98 nm, b = 0.87 nm). The two-dimensional weight density (W rGO ) and the two-dimensional weight density (W TaO3 ) is expressed by the following formula: W rGO =2M C / (a×a×sin120°×N A ) W TaO3 =8M TaO3 / (a×b×N A ) Here, N A is the Avogadro constant, and M C and M. TaO3 are the molecular weights of carbon and TaO3. The mass ratio of TaO3 to rGO is W TaO3 / W rGOThis resulted in a calculated value of 4.8.

[0088] The mixture of TaO3 nanomesh dispersion and PDDA-rGO nanosheet dispersion was centrifuged at a rotation speed of 5000 rpm for 10 minutes and washed. The washed aggregated powder was collected and freeze-dried. The aggregated powder was observed. The results are shown in Figure 11.

[0089] The agglomerated powder was subjected to powder X-ray diffraction and observed by TEM and scanning electron microscope (SEM, JSM-6700F, manufactured by JEOL Ltd.). In addition, elemental analysis was performed by energy dispersive X-ray spectroscopy (EDS) attached to the SEM. Thermogravimetric differential thermal analysis (TG-DTA, TG-DTA8122, manufactured by Rigaku Corporation) was performed on the agglomerated powder. These results are shown in Figures 12 to 16.

[0090] The agglomerated powder was then pelletized by a pelletizing method. Specifically, the agglomerated powder was filled into a circular mold with a diameter of 10 mm and compressed at a pressure of 2 MPa for 10 seconds. The appearance of the pellets was observed. The results are shown in Figure 17.

[0091] The adsorption capacity of the obtained pellets (15 mg) was evaluated. In detail, the pellets were immersed in a solution (yellow, 0.005 mol / dm 3 , 4cm 3 ) and evaluated based on the color change of the solution and the UV-visible spectrum. The results are shown in Figure 23.

[0092] The pellets obtained were then mixed with sulfur (20 mg) and carbon disulfide (CS2, 1 cm 3 The pellets were immersed in a sulfur solution containing sulfur dissolved in sulphur dioxide (S), and carbon disulfide was volatilized to form a positive electrode. The amount of sulfur supported was controlled by changing the amount of sulfur solution immersed and / or the thickness of the pellets. Electrochemical tests were performed using the positive electrodes prepared in this way. For ease of understanding, the pellets impregnated with sulfur will be referred to as S-impregnated pellets, and the pellets not impregnated with sulfur will be referred to as S-non-impregnated pellets.

[0093] A CR2032 coin battery (lithium-sulfur battery) was fabricated in an argon-filled glove box. The positive electrode, separator, negative electrode, and electrolyte were the above-mentioned S-impregnated pellets, Celgard 2325, lithium foil, and a DOL / DME (volume ratio 1:1) solution (1 mol / dm 3 The sulfur content in the S-impregnated pellets was controlled to 50 wt%. The electrolyte to sulfur ratio (E / S) was adjusted to a sulfur loading of 5 mg / cm. 2 , 10mg / cm 2 , 15mg / cm 2 The concentrations were 15.2 μL / mg, 7.6 μL / mg, and 5.1 μL / mg for the lithium-sulfur batteries, respectively. Electrochemical tests of the lithium-sulfur batteries were carried out using an electrochemical workstation (Solartron, 1280B). The charge / discharge performance of the batteries was evaluated in the potential range of 1.7 V to 2.8 V using a battery tester (Hokuto, HJ1001SD8). The results are shown in Figures 24, 25, 28, 31 to 33, and 35 to 38.

[0094] S and Li2S (molar ratio 5:1), LiTFSI and LiNO3 were added to DOL / DME (volume ratio 1:1), and the mixture was stirred at 60°C for 48 hours to form a DOL / DME electrolyte (0.2 mol / dm 3 Li2S6+1mol / dm 3 A Li2S6 symmetric battery was then fabricated using S-non-impregnated pellets as the working and counter electrodes, Celgard 2325 as the separator, and DOL / DME electrolyte as the electrolyte. The CV and EIS characteristics of the Li2S6 symmetric battery were measured using an electrochemical workstation. The results are shown in Figures 18 and 19.

[0095] S and Li2S (molar ratio 7:1) and LiTFSI were added to tetraglyme, and the mixture was stirred at 60 °C for 48 hours to obtain a tetraglyme electrolyte (0.3 mol / dm 3 Li2S8+1mol / dm 3A Li2S8 coin battery was then fabricated using a non-S-impregnated pellet as the positive electrode, lithium foil as the negative electrode, Celgard 2325 as the separator, tetraglyme electrolyte as the positive electrode side electrolyte, and tetraglyme (without additives) as the negative electrode side electrolyte. The Li2S8 coin battery was discharged to 2.06 V at a constant current of 112 μA, and the current-time characteristics and EIS characteristics were measured using an electrochemical workstation (Solartron, 1280B) while the battery was held at 2.05 V. The results are shown in Figures 20 to 22.

[0096] [Comparative Example 1] In Comparative Example 1, a dispersion containing tantalum oxide (TaO3) nanomesh and a dispersion containing graphene nanosheets (rGO nanosheets) were mixed to produce a random composite (R-TaO3 / rGO) in which TaO3 nanomesh and rGO nanosheets were randomly combined.

[0097] In detail, TaO3 nanomesh dispersion (TaO3 concentration 0.48 g / dm 3 , 0.2dm 3 ), and rGO nanosheet dispersion (rGO concentration 0.1 g / dm 3 , 0.2dm 3 ) was mixed and this was dissolved in a PDDA aqueous solution (PDDA concentration 10 g / dm 3 , 0.2dm 3 ) and stirred. Then, it was centrifuged at 5000 rpm for 30 minutes, washed, and freeze-dried. The physical properties and electrochemical properties of R-TaO3 / rGO were evaluated in the same manner as in Example 1. The results are shown in Figures 12, 18 to 24, 26, 29, and 31 to 36.

[0098] [Comparative Example 2] In Comparative Example 2, a rGO nanosheet dispersion (rGO concentration 0.1 g / dm 3 , 0.2dm 3 ) in a PDDA aqueous solution (PDDA concentration 5 g / dm 3 , 0.2dm 3) and stirred. The mixture was then centrifuged at 5000 rpm for 30 minutes, washed, and freeze-dried to obtain a restacked product of PDDA-rGO (rGO). The physical properties and electrochemical properties of the rGO were evaluated in the same manner as in Example 1. The results are shown in Figures 12, 13, 18 to 22, 24, 27, 30, and 31 to 36.

[0099] [Comparative Example 3] In Comparative Example 3, a TaO3 nanomesh dispersion (TaO3 concentration 0.48 g / dm 3 , 0.2dm 3 ) in a PDDA aqueous solution (PDDA concentration 5 g / dm 3 , 0.2dm 3 ) and stirred. Then, it was centrifuged at 5000 rpm for 30 minutes, washed, and freeze-dried to obtain a restacked body of TaO3 nanomesh (TaO3). The physical properties of TaO3 were evaluated in the same manner as in Example 1. The results are shown in Figures 12 and 13. Note that the electrochemical properties of the TaO3 nanomesh were not evaluated because it had high resistance and did not function as an electrode.

[0100] The results of Example 1 and Comparative Examples 1 to 3 will be explained together. FIG. 11 is a diagram showing the appearance of the aggregated powder obtained in Example 1.

[0101] According to Fig. 11, the obtained agglomerated powder was a black powder. Although not shown, the agglomerated powders obtained in Comparative Examples 1 and 2 were also black and had a similar appearance. The agglomerated powder obtained in Comparative Example 3 was white.

[0102] FIG. 12 is a diagram showing powder XRD patterns of the aggregated powders of Example 1 and Comparative Examples 1 to 3. As shown in FIG.

[0103] The agglomerated powder of Comparative Example 3 showed strong peaks at 4.7° and 9.9°. These are due to a series of basal diffractions from the restacked structure of the TaO3 nanomesh, and are due to the TBA +This suggests that the diffraction peaks at 14.3°, 28.4°, 33.3°, 37.7°, and 43.0° were identified as diffraction peaks from the 110, 220, 130, 400, and 040 crystal planes using the unit cell (a=0.96 nm, b=0.85 nm) of the face-centered rectangular lattice of the TaO3 nanomesh. This confirmed that the aggregated powder of Comparative Example 3 was a restack of freeze-dried TaO3 nanomesh.

[0104] The agglomerated powder of Comparative Example 2 did not show clear diffraction peaks, due to the small scattering coefficient of carbon, indicating that the agglomerated powder of Comparative Example 2 was a restack of freeze-dried PDDA-rGO nanosheets.

[0105] The XRD pattern of the agglomerated powder of Example 1 had a series of in-plane reflections from the TaO3 nanomesh described above, and no peaks from the rGO nanosheets were detected. In addition, the background increased toward the low angle side, and had humps at approximately 7° and 12° as shown by the arrows. This is due to the layered structure of the TaO3 nanomesh and rGO nanosheets. This indicates that the agglomerated powder of Example 1 (S-TaO3 / rGO) is a superlattice-like layer of TaO3 nanomesh and PDDA-rGO nanosheets.

[0106] FIG. 13 shows the simulated XRD patterns of TaO nanomesh, rGO nanosheets, and their stacked units, and a TEM image of the aggregated powder of Example 1.

[0107] To understand the layered structure of the agglomerated powder, we simulated the XRD pattern by summing the scattering amplitudes of all atom pairs in the structure based on the following Debye equation:

[0108]

number

[0109] where N is the number of atoms, f i , f j is the scattering atomic coefficient for the i-th and j-th atoms, r ij is the distance between atoms, and Q is the scattering vector (=4πsinθ / λ). The calculation is done on a 100×100 nm 2 The calculations were performed on a structural model consisting of a horizontally elongated TaO3 nanomesh and a two-dimensional sheet of rGO nanosheets. The atomic positions were calculated based on the structures of RbTaO3 and graphene.

[0110] Figure 13(a) shows the simulated XRD patterns of a single TaO3 nanomesh layer and a single rGO nanosheet layer. Figure 13(b) shows the simulated XRD patterns of a single rGO / TaO3 unit and a double rGO / TaO3 unit. Figure 13(c) shows the simulated XRD pattern of a sandwich structure of rGO / TaO3 / rGO. Figure 13(d) shows the TEM image of the aggregated powder of Example 1.

[0111] As shown in Fig. 13(a), the intensity increased steeply and monotonically with decreasing 2θ angle in both cases. According to the XRD pattern of the rGO / TaO3 single unit in Fig. 13(b), when the two sheets are paired with a separation of 1.5 nm, the background gradually increases at low angles, which is in good agreement with the XRD pattern of the aggregated powder in Example 1 shown in Fig. 12.

[0112] According to Fig. 13(b), when a single unit of rGO / TaO3 is repeated twice, a small wave associated with the Laue interference function appears along with the diffraction peak. From these results, it is reasonable to consider that the agglomerated powder of Example 1 is a pair of a single layer of TaO3 nanomesh and a single layer of PDDA-rGO nanosheet, but from the viewpoint of chemical structure, it is a sandwich-like structure of rGO / TaO3 / rGO, in which half of each side of TaO3 is covered with rGO nanosheet. In fact, according to Fig. 13(c), the XRD pattern of the sandwich-like structure of rGO / TaO3 / rGO is in good agreement with the XRD pattern of the agglomerated powder of Example 1 shown in Fig. 12.

[0113] The inset in Figure 13(d) shows a cross-sectional TEM image of the agglomerated powder of Example 1, which shows three parallel fringes, indicating a sandwich-like structure of rGO / TaO3 / rGO with an interlayer distance of 1.5 nm. The TEM image shows a thin sheet-like morphology, indicating that the agglomerated powder of Example 1 has an ultrathin film structure.

[0114] On the other hand, when comparing the XRD patterns of the agglomerated powder of Example 1 and that of Comparative Example 1 in Fig. 12, the XRD pattern of Comparative Example 1 was similar to that of Example 1, except for the strong basal diffraction peaks at 4.5° and 9.8°. This suggests that the agglomerated powder of Comparative Example 1 is a mixture of those having rGO / TaO3 pairs, restacked TaO3 nanomesh, and restacked rGO nanosheets, and that the TaO3 nanomesh and rGO nanosheets are randomly composited.

[0115] FIG. 14 is a diagram showing an SEM image of the aggregated powder of Example 1.

[0116] 14, the agglomerated powder of Example 1 has a loosely bound microstructure and has spaces capable of supporting sulfur, suggesting that it can function as a positive electrode carrier material for lithium-sulfur secondary batteries. In addition, such a structure has an electronically conductive framework and a permeation path for the electrolyte, and low ohmic resistance and diffusion resistance can be expected by using the agglomerated powder of Example 1 in a positive electrode.

[0117] FIG. 15 is a diagram showing an SEM image and EDS mapping of the aggregated powder of Example 1.

[0118] FIG. 15(a) is an SEM image of the agglomerated powder of Example 1, and FIG. 15(b)-(d) show EDS mapping of carbon (C), oxygen (O) and tantalum (Ta), respectively. FIG. 15(b)-(d) are shown in grayscale, and each element is present in the brightly-illuminated areas. According to FIG. 15, C, O and Ta are uniformly distributed, and it was found that the agglomerated powder of Example 1 is homogeneous.

[0119] FIG. 16 is a diagram showing a TG-DTA curve of the agglomerated powder of Example 1.

[0120] According to Fig. 16, the agglomerated powder of Example 1 was dehydrated by 200°C. The weight loss from 200°C to 450°C is due to the phase transition from TaO3 to Ta2O5 and the combustion of rGO. When calculated ignoring the mass of PDDA, the mass ratio of TaO3 to rGO was 4.3. This value was in good agreement with the mass ratio (4.8) at the time of preparation, taking into account the mass of PDDA.

[0121] From the above, it was shown that the aggregated powder of Example 1 contains a composite (rGO / TaO3 / rGO) consisting of a tantalum oxide nanomesh and a pair of graphene nanosheets sandwiching the tantalum oxide nanomesh, has a space capable of supporting sulfur, and functions as a positive electrode carrier material for lithium-sulfur secondary batteries. It was also confirmed that the ratio of the mass of the tantalum oxide nanomesh to the mass of the graphene nanosheets satisfies the range of 3 to 8, and furthermore, the range of 4 to 6.

[0122] FIG. 17 is a diagram showing the appearance of the non-S-impregnated pellets of Example 1.

[0123] According to FIG. 17, the S-non-impregnated pellet of Example 1 could be held with tweezers and was self-supporting. Although not shown, the S-non-impregnated pellets of Comparative Examples 1 and 2 were similar. This shows that the composite of the present invention can provide a self-supporting pellet without requiring a binder or a current collector, and can increase the amount of sulfur carried. In addition, by forming the composite of the present invention (rGO / TaO3 / rGO) into a pellet, a positive electrode that can withstand large volume changes during charging and discharging can be provided.

[0124] Next, the results of investigating the catalytic activity for the conversion of lithium polysulfides in the non-S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2 will be summarized and described.

[0125] FIG. 18 is a diagram showing the CV characteristics of Li2S6 symmetric batteries using the S-non-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2.

[0126] 18 shows the CV characteristics at a scan rate of 1 mV / s. The CV characteristics show the catalytic activity of each agglomerated powder for the conversion of lithium polysulfides in the non-S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2. The CV curve of the non-S-impregnated pellets of Comparative Example 2 showed oxidation peaks at 0.14 V and 0.71 V, and reduction peaks at -0.18 V and -0.64 V, indicating a two-step reaction.

[0127] In contrast, the CV curve of the S-non-impregnated pellet of Comparative Example 1 showed a much smaller peak interval between the redox peaks than that of Comparative Example 2, indicating high reversibility. This is because the surface of the TaO3 nanomesh exhibits Lewis acidity, which contributes to high catalytic activity for the conversion of lithium polysulfide, which is a Lewis base. However, the S-non-impregnated pellet of Comparative Example 1 is composed of an aggregated powder in which insulating TaO3 nanomesh and rGO nanosheets are randomly stacked, so the current response was smaller than that of the pellet (rGO) of Comparative Example 3.

[0128] Meanwhile, surprisingly, the S-non-impregnated pellets (S-TaO3 / rGO) of Example 1 showed the highest catalytic activity according to the CV characteristics, which is attributed to the rGO / TaO3 / rGO sandwich structure in the S-non-impregnated pellets of Example 1, which exposes more reactive sites.

[0129] FIG. 19 is a diagram showing electrochemical impedance spectroscopy (EIS) curves and their equivalent circuits for Li2S6 symmetric batteries using the S-non-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2.

[0130] According to Fig. 19, the impedance data of the EIS curves of both non-S-impregnated pellets were fitted with two semicircles and one straight line. The two semicircles correspond to the two-step reaction in the conversion process from long-chain lithium polysulfides to short-chain lithium polysulfides. The response from the non-S-impregnated pellets (rGO) of Comparative Example 2, which has the highest charge transfer resistance (Rct1 = 61.0 Ω, Rct2 = 15.4 Ω), indicates that the reaction rate of lithium polysulfide conversion is the slowest. The S-non-impregnated pellet (R-TaO3 / rGO) of Comparative Example 1 exhibited a smaller charge transfer resistance (Rct1=34.0Ω, Rct2=26.5Ω) than the S-non-impregnated pellet (rGO) of Comparative Example 2. However, the ohmic resistance (Rs=12.5Ω) of the S-non-impregnated pellet (R-TaO3 / rGO) of Comparative Example 1 was higher than the ohmic resistance (Rs=5.7Ω) of the S-non-impregnated pellet (rGO) of Comparative Example 2.

[0131] On the other hand, the charge transfer resistance (Rct1=3.8Ω, Rct2=8.1Ω) and ohmic resistance (Rs=2.0Ω) of the S-non-impregnated pellet (S-TaO3 / rGO) in Example 1 were the smallest compared to those in Comparative Examples 1 and 2, and showed extremely high electrical conductivity and fast reaction rate. This is because the TaO3 nanomesh and the rGO nanosheet were in surface contact at the molecular level, exposing a wealth of active sites, which is consistent with the results in Figure 18.

[0132] FIG. 20 is a diagram showing the current-time characteristics (2.05 V) of Li2S8 coin batteries using the S-non-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2.

[0133] The curves for the batteries using the S-non-impregnated pellets of Comparative Example 1 (R-TaO3 / rGO) and the S-non-impregnated pellets of Example 1 (S-TaO3 / rGO) had three regions, while the curve for the battery using the S-non-impregnated pellets of Comparative Example 2 (rGO) had two regions.

[0134] The first region where the current rapidly decreases with time corresponds to the liquid-liquid conversion process of soluble long-chain lithium polysulfides to short-chain lithium polysulfides. The second region, which shows particularly prominent peaks in R-TaO3 / rGO and S-TaO3 / rGO, corresponds to the liquid-solid nucleation process to produce solid Li2S2 or Li2S. The current rise in S-TaO3 / rGO was faster than that in R-TaO3 / rGO, suggesting that the precipitation of Li2S is faster in S-TaO3 / rGO. Since the peak area corresponds to the amount of precipitated Li2S, the amount of precipitated in S-TaO3 / rGO is larger than that in R-TaO3 / rGO. The third region where the current decreases and plateaus corresponds to the reduction of soluble lithium polysulfides in the diffusion-limited region.

[0135] FIG. 21 is a diagram showing Raman spectra of the S-non-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2 after the measurement of the current-time characteristics of FIG.

[0136] The Raman spectrum of Li2S is also shown in Fig. 21. The peak at 373 cm was observed in the S-non-impregnated pellet (R-TaO3 / rGO) of Comparative Example 1 and the S-non-impregnated pellet (S-TaO3 / rGO) of Example 1. -1 The bands of TaO3 nanomesh were consistent with those of Li2S. This confirmed that Li2S nucleated in these materials. On the other hand, no such bands were observed in the S-unimpregnated pellets (rGO) of Comparative Example 2. This indicates that the TaO3 nanomesh can efficiently promote the electrochemical precipitation of Li2S and reduce the overpotential for its nucleation.

[0137] FIG. 22 is a diagram showing electrochemical impedance spectroscopy (EIS) curves and their equivalent circuits of Li2S8 coin batteries using the S-non-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2 after the measurement of the current-time characteristics of FIG. 20.

[0138] 22, the impedance data of the EIS curves of the S-non-impregnated pellets (R-TaO3 / rGO) of Comparative Example 1 and the S-non-impregnated pellets (S-TaO3 / rGO) of Example 1 were fitted with one semicircle, while that of the S-non-impregnated pellets (rGO) of Comparative Example 2 was fitted with two semicircles. This difference is believed to be due to the difference in the overpotential for the nucleation of Li2S described above.

[0139] In the non-S-impregnated pellets (R-TaO3 / rGO) of Comparative Example 1 and the non-S-impregnated pellets (S-TaO3 / rGO) of Example 1 in the tetraglyme electrolyte containing Li2S8, the Li2S nucleation reaction is the main process. In the non-S-impregnated pellets (rGO) of Comparative Example 2, only the conversion reaction between soluble lithium polysulfides occurs due to the high overpotential for Li2S nucleation. This was confirmed by the similar EIS curves of the non-S-impregnated pellets (rGO) of Comparative Example 2 in the DOL / DME electrolyte containing Li2S6 and the tetraglyme electrolyte containing Li2S8.

[0140] The S-free pellet (S-TaO3 / rGO) of Example 1 maintained a small ohmic resistance (Rs=5.7Ω) even after the maximum amount of Li2S was deposited, which was smaller than the ohmic resistance Rs=7.9Ω of the S-free pellet (R-TaO3 / rGO) of Comparative Example 1 and the ohmic resistance Rs=9.5Ω of the S-free pellet (rGO) of Comparative Example 2. The superior performance of the S-free pellet of Example 1 is believed to be due to the uniform distribution and close contact of Li2S in the sandwich structure of rGO / TaO3 / rGO.

[0141] FIG. 23 shows the UV-visible absorption spectra of the non-S-impregnated pellets of Example 1 and Comparative Example 1 after contacting them with a Li2S6 electrolyte.

[0142] FIG. 23 also shows the Li2S6 electrolyte (a), the appearance of the S-non-impregnated pellets (R-TaO3 / rGO) of Comparative Example 1 after contact with the Li2S6 electrolyte (a) (b), and the appearance of the S-non-impregnated pellets (S-TaO3 / rGO) of Example 1 after contact with the Li2S6 electrolyte (a) (c). According to FIG. 23, the Li2S6 electrolyte before contact was pale yellow, but when it was contacted with the S-non-impregnated pellets (S-TaO3 / rGO) of Example 1, it became colorless and transparent and did not have an absorption peak of 550 nm or less. The S-non-impregnated pellets (R-TaO3 / rGO) of Comparative Example 1 also adsorbed some Li2S6, but not enough. This also showed that the S-non-impregnated pellets (S-TaO3 / rGO) of Example 1 had excellent affinity and adsorption. This is thought to be because more reaction sites are exposed due to the rGO / TaO3 / rGO sandwich structure in the pellets of Example 1.

[0143] These results suggest that the agglomerated powder of the present invention functions as a sulfur support material, and in particular, the rGO / TaO3 / rGO sandwich structure composite in the agglomerated powder can confine lithium polysulfides and reduce the movement of lithium polysulfides, thereby effectively suppressing the shuttle effect. In addition, as described above, the support material of the present invention has high catalytic activity of the TaO3 nanomesh and high nucleation of Li2S, and therefore, by impregnating sulfur, it can exhibit excellent electrochemical properties as a positive electrode of a lithium-sulfur secondary battery.

[0144] Next, the results of investigating the electrochemical characteristics of lithium-sulfur secondary batteries using the S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2 as the positive electrode will be summarized.

[0145] FIG. 24 is a diagram showing CV characteristics of lithium-sulfur batteries using the S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2 as the positive electrode.

[0146] Figure 24 shows the results of the study on the sulfur loading of 5 mg / cm 2The CV characteristics of the lithium-sulfur battery using the S-impregnated pellets of Comparative Example 2 at a scan rate of 0.1 mV / s are shown. When the S-impregnated pellets of Comparative Example 2 (rGO) were used, two reduction peaks were shown at 2.17 V and 1.81 V, and one broad oxidation peak was shown at 2.57 V. When the S-impregnated pellets of Comparative Example 1 (R-TaO3 / rGO) were used, two reduction peaks and two oxidation peaks at 2.39 V and 2.56 V were shown, similar to Comparative Example 2. These two oxidation peaks showed higher currents than those of Comparative Example 2, suggesting that the S-impregnated pellets of Comparative Example 1 (R-TaO3 / rGO) showed higher electrochemical activity than the S-impregnated pellets of Comparative Example 2 (rGO). This is due to the high catalytic activity of the TaO3 nanomesh.

[0147] On the other hand, when the S-impregnated pellets (S-TaO3 / rGO) of Example 1 were used, two sets of redox peaks were observed. The reduction peaks at 2.26 V and 1.87 V were the process in which sulfur (S8) was converted to long-chain lithium polysulfides, then to short-chain lithium polysulfides, and finally to Li2S. When the potential was swept in the reverse direction, the reverse reaction occurred. Comparing the results of Example 1 and Comparative Example 1, the positive electrode of the S-impregnated pellets (S-TaO3 / rGO) of Example 1 showed a higher peak current and a smaller peak potential separation than the positive electrode of the S-impregnated pellets (R-TaO3 / rGO) of Comparative Example 1. This indicates that the S-impregnated pellets of Example 1 have superior activity and reversibility compared to those of Comparative Example 1.

[0148] FIG. 25 is a diagram showing the CV characteristics of a lithium-sulfur battery using the S-impregnated pellet of Example 1. FIG. 26 is a diagram showing the CV characteristics of a lithium-sulfur battery using the S-impregnated pellet of Comparative Example 1. FIG. 27 is a diagram showing the CV characteristics of a lithium-sulfur battery using the S-impregnated pellet of Comparative Example 2. FIG. 28 is a graph showing the relationship between the peak current and the square root of the scan rate for a lithium-sulfur battery using the S-impregnated pellet of Example 1. FIG. 29 is a graph showing the relationship between the peak current and the square root of the scan rate for a lithium-sulfur battery using the S-impregnated pellets of Comparative Example 1. FIG. 30 is a graph showing the relationship between the peak current and the square root of the scan rate for a lithium-sulfur battery using the S-impregnated pellets of Comparative Example 2.

[0149] 25 to 27 show that the sulfur loading is 5 mg / cm 2 The CV characteristics of the lithium-sulfur battery using the S-impregnated pellets at various scan rates are shown, and the lithium ion (Li + ) was evaluated. As shown in Figures 28 to 30, the peak current (Ip) and the square root of the scan rate (v 1 / 2 ) was found to be a diffusion-controlled process, i.e., the diffusion coefficient of lithium ions was found to be approximately linear with Ip and v 1 / 2 The capacitance is proportional to the slope of the line, which indicates the mobility of lithium ions.

[0150] The diffusion coefficient of the S-impregnated pellet (S-TaO3 / rGO) of Example 1 was larger than those of the S-impregnated pellets of Comparative Example 1 and Comparative Example 2, and it was found to have the highest lithium ion mobility. This is because the openings in the TaO3 nanomesh surface not only allow the lithium ions to migrate in three dimensions, but also reduce the diffusion distance of the lithium ions. For this reason, if the S-impregnated pellet of Example 1 is used for the positive electrode of a lithium-sulfur secondary battery, high-speed charging and discharging can be achieved.

[0151] FIG. 31 is a diagram showing electrochemical impedance spectroscopy (EIS) curves of lithium-sulfur batteries using the S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2.

[0152] FIG. 31 also shows the equivalent circuit. All EIS curves had a semicircle in the high frequency region corresponding to the charge transfer resistance and a linear component in the low frequency region. The charge transfer resistance (Rct=119.5Ω) of the S-impregnated pellet (S-TaO3 / rGO) of Example 1 was significantly smaller than that (Rct=159.7Ω) of the S-impregnated pellet (R-TaO3 / rGO) of Comparative Example 1 and that (Rct=188.3Ω) of the S-impregnated pellet (rGO) of Comparative Example 2, indicating excellent catalytic activity.

[0153] FIG. 32 is a graph showing the charge / discharge characteristics of lithium-sulfur batteries using the S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2. FIG. 33 is a graph showing cycle characteristics of lithium-sulfur batteries using the S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2. FIG. 34 is a graph showing cycle characteristics of lithium-sulfur batteries using the S-non-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2. FIG. 35 is a diagram showing the charge / discharge potential difference of each lithium-sulfur battery obtained from the charge / discharge characteristics of FIG. FIG. 36 is a graph showing the capacities Q1 and Q2 of each lithium-sulfur battery obtained from the charge / discharge characteristics of FIG.

[0154] 32 and 33 show the results for the sulphur loading of 5 mg / cm 2 The charge / discharge current was 2mA / cm using S-impregnated pellets. 2 32 shows the charge / discharge characteristics and cycle characteristics of the lithium-sulfur battery in Example 1, Comparative Example 1, and Comparative Example 2. According to FIG. 32, the discharge capacities of the lithium-sulfur batteries using the S-impregnated pellets of Example 1, Comparative Example 1, and Comparative Example 2 were 1139 mAh / g, 871 mAh / g, and 584 mAh / g, respectively. On the other hand, according to FIG. 34, when pellets not impregnated with sulfur (S8) were used, all of the lithium-sulfur batteries showed low discharge capacities. This confirmed that sulfur is an active material in lithium-sulfur batteries.

[0155] In FIG. 33, the initial discharge capacity of the lithium-sulfur battery using the S-impregnated pellets of Example 1 was higher than those of Comparative Example 1 and Comparative Example 2. This means that the utilization rate of sulfur in the S-impregnated pellets of Example 1 is higher than those of Comparative Example 1 and Comparative Example 2. The discharge capacity of the lithium-sulfur battery of Example 1 gradually decreased from 1139 mAh / g to 559 mAh / g in the first 10 cycles, and the capacity was well maintained over the next 100 cycles. Meanwhile, the discharge capacities of the lithium-sulfur batteries of Comparative Example 1 and Comparative Example 2 further decreased to 484 mAh / g and 326 mAh / g, respectively, at 100 cycles.

[0156] FIG. 35 shows a list of the potential difference ΔE between the charge plateau and the discharge plateau in the charge / discharge characteristics of FIG. 32 for each lithium-sulfur battery. The potential difference ΔE indicates the polarization of the battery, and as the polarization of the battery increases, the utilization rate of sulfur decreases. According to FIG. 35, the polarization potential was significantly decreased when the S-impregnated pellets of Example 1 were used. This shows that the S-impregnated pellets of Example 1 increase the capacity of the positive electrode of the lithium-sulfur battery.

[0157] Referring again to Figure 32, two sets of charge-discharge plateaus were observed for all positive electrodes. The plateau at higher potential corresponds to the conversion of sulfur and long-chain lithium polysulfides, while the plateau at lower potential corresponds to the conversion of long-chain lithium polysulfides to short-chain lithium polysulfides and Li2S. The capacities corresponding to these two processes are designated Q1 and Q2 and plotted as shown in Figure 36.

[0158] According to Fig. 36, Q2>Q1, which indicates that the capacity contribution of the conversion process from long-chain lithium polysulfides to short-chain lithium polysulfides and Li2S is large. In particular, Q2 of the lithium-sulfur battery using the S-impregnated pellets of Example 1 was significantly larger than those using the S-impregnated pellets of Comparative Example 1 and Comparative Example 2. From this, it was found that the S-impregnated pellets of Example 1 can efficiently convert long-chain lithium polysulfides to short-chain lithium polysulfides and Li2S, and are effective as the positive electrode of lithium-sulfur secondary batteries.

[0159] FIG. 37 is a graph showing the cycle characteristics of a lithium-sulfur battery using another S-impregnated pellet of Example 1.

[0160] FIG. 37 shows the results of the experiment using a 10 mg / cm2 sulfur loading. 2 The S-impregnated pellets of Example 1 were used, and the charge / discharge current was 1 mA / cm 2 37 shows the cycle characteristics of a lithium-sulfur battery at 100 cycles. According to Fig. 37, the initial discharge capacity was 1190mAh / g, decreased to 741mAh / g after 100 cycles, and then stabilized.

[0161] FIG. 38 is a graph showing the cycle characteristics of a lithium-sulfur battery using still another S-impregnated pellet of Example 1.

[0162] FIG. 38 shows the results of a 15 mg / cm2 sulfur loading. 2 The S-impregnated pellets of Example 1 were used, and the charge / discharge current was 2 mA / cm 2 Figure 38 shows the cycling characteristics of a lithium-sulfur battery at 12.2 mAh / cm. As shown in Figure 38, the capacity decreased in the first few cycles, but the activation process increased the capacity to 12.2 mAh / cm. 2 Achieved a high areal capacity of 10.5mAh / cm even after 40 cycles. 2 The area capacity was 674 mAh / g. This area capacity was equivalent to a charge / discharge capacity of 674 mAh / g. As far as we know, this area capacity and sulfur loading are significantly improved among those reported so far, suggesting that the use of the positive electrode of the present invention can significantly improve the sulfur utilization rate and provide a lithium-sulfur secondary battery with high energy density.

[0163] From the above, it was shown that by supporting sulfur on the aggregate powder containing the composite of the present invention (rGO / TaO3 / rGO sandwich structure), it functions as a positive electrode for lithium-sulfur secondary batteries. The positive electrode of the present invention can confine lithium polysulfides between the composites of the rGO / TaO3 / rGO sandwich structure, reducing the movement of lithium polysulfides, thereby effectively suppressing the shuttle effect. In particular, since it can be pelletized without using a binder, etc., it can not only support a large amount of sulfur, but also withstand volume changes, and is extremely useful. [Industrial Applicability]

[0164] The positive electrode support material for lithium-sulfur secondary batteries of the present invention is a composite of tantalum oxide nanomesh and graphene nanosheets laminated together, and therefore can hold sulfur in the gaps between the composites and function as a positive electrode. In particular, the tantalum oxide nanomesh has openings larger than lithium ions and smaller than lithium polysulfides, so that it does not impede the diffusion of lithium ions, while the lithium polysulfides are effectively trapped in the support material, suppressing the shuttle effect. In addition, the support material of the present invention can be easily pelletized by the graphene nanosheets without using a binder or a current collector, and can support a larger amount of sulfur. By using such a positive electrode, a lithium-sulfur secondary battery with excellent battery characteristics can be provided. [Explanation of symbols]

[0165] 100 Positive electrode support material for lithium-sulfur secondary batteries 110 Tantalum Oxide Nanomesh 120 Graphene nanosheets 130 Complex 140 Cationic Polymers 210 Aperture 310, 320 dispersion 400 Positive electrode for lithium-sulfur secondary battery 410 Sulfur 500 Lithium-sulfur secondary battery 510 Positive electrode 520 negative electrode 530 Electrolytes

Claims

1. A positive electrode carrier material for a lithium-sulfur secondary battery, containing a composite comprising a tantalum oxide nanomesh and a pair of graphene nanosheets sandwiching the tantalum oxide nanomesh.

2. The positive electrode carrier material according to Claim 1, wherein the ratio of the mass of the tantalum oxide nanomesh to the mass of the graphene nanosheet is in the range of 3 or more and 8 or less.

3. The positive electrode carrier material according to Claim 2, wherein the ratio of the mass of the tantalum oxide nanomesh to the mass of the graphene nanosheet is in the range of 4 or more and 6 or less.

4. The positive electrode carrier material according to Claim 1, wherein the tantalum oxide nanomesh is monolayer exfoliated from a layered tantalum oxide.

5. The positive electrode carrier material according to Claim 1, wherein the tantalum oxide nanomesh has openings in the plane in the range of 0.1 nm or more and 0.3 nm or less in diameter.

6. The positive electrode carrier material according to Claim 1, wherein the graphene nanosheet is a reduced graphene oxide nanosheet.

7. The positive electrode carrier material according to Claim 1, wherein either the tantalum oxide nanomesh or the graphene nanosheet has a cationic polymer.

8. The positive electrode carrier material according to Claim 7, wherein the cationic polymer is selected from the group consisting of poly(diallyldimethylammonium) (PDDA), polyallylamine (PAH), polyethyleneimine (PEI), polypyrrole (PPy), polyaniline (PANI), and poly(3,4-ethylenedioxythiophene) (PEDOT).

9. The positive electrode carrier material according to Claim 1, which is in the form of a pellet or a thin film.

10. A positive electrode for a lithium-sulfur secondary battery containing sulfur, wherein the sulfur is supported on the positive electrode carrier material according to any one of Claims 1 to 9.

11.

12. A method for manufacturing the positive electrode carrier material for a lithium-sulfur secondary battery according to any one of Claims 1 to 9, comprising mixing a dispersion in which a tantalum oxide nanomesh is dispersed and a dispersion in which a graphene nanosheet having a cationic polymer is dispersed, or mixing a dispersion in which a tantalum oxide nanomesh having a cationic polymer is dispersed and a dispersion in which a graphene nanosheet is dispersed.

13. The sulfur is supported in the range of 1 mg / cm ２ or more and 25 mg / cm ２ or less, and the positive electrode according to claim 10. ​ ​ ​ ​ ​ The method according to claim 12, wherein the dispersion medium in the dispersion liquid is selected from the group consisting of water, ethanol, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and N-methyl-2-pyrrolidone (NMP).

14. The method according to claim 12, further comprising heating the product obtained by the mixing in an inert or reducing gas atmosphere.

15. The method according to claim 14, wherein the heating is performed by heating the product in a temperature range of 300°C or higher and 400°C or lower for 30 minutes or longer and 3 hours or shorter.

16. The method according to claim 13, further comprising lyophilizing the product obtained by the mixing.

17. A method for manufacturing a positive electrode for a lithium-sulfur secondary battery according to claim 10, comprising: impregnating sulfur into the positive electrode carrier material for a lithium-sulfur secondary battery according to any one of claims 1 to 9. A method.

18. The method according to claim 17, further comprising processing the positive electrode carrier material into pellets or a thin film prior to the impregnating.

19. A lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, and an electrolyte positioned between the positive electrode and the negative electrode, wherein: the positive electrode is the positive electrode according to claim 10; the negative electrode contains at least lithium. A secondary battery.

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