Manufacturing method of sulfur-carbon composite positive electrode

Abstract

To provide a sulfur-carbon composite positive electrode having high capacity and high cycle characteristics.SOLUTION: A sulfur-carbon composite positive electrode includes a positive electrode current collector and a positive electrode mixture layer formed on one or both surfaces of the positive electrode current collector. The positive electrode mixture layer contains carbon particles having a plurality of pores, sulfur, and a binder. The carbon particles are bound by the binder, such that adjacent carbon particles are in contact with each other. The sulfur includes first sulfur present in the plurality of pores of the carbon particles located on the surface and inside of the positive electrode mixture layer, and second sulfur present on the surfaces of the carbon particles located on the surface and inside of the positive electrode mixture layer, excluding the contact areas between the carbon particles.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a positive electrode for a lithium-sulfur secondary battery, a lithium-sulfur secondary battery, and a method for manufacturing a positive electrode for a lithium-sulfur secondary battery. [Background technology]

[0002] In recent years, the applications of lithium-ion secondary batteries have expanded significantly to include industries such as portable devices, power tools, and automobiles. Accordingly, there is a demand for even higher energy densities in lithium-ion secondary batteries. However, this trend is reaching a plateau, necessitating the development of new battery materials and systems.

[0003] Lithium-sulfur secondary batteries, which use sulfur as the positive electrode active material, are attracting attention as a next-generation battery due to their expected high energy density. A typical lithium-sulfur secondary battery consists of a sulfur positive electrode, a lithium metal negative electrode, and a separator containing an organic electrolyte.

[0004] One of the challenges of lithium-sulfur secondary batteries is the low electrical conductivity of sulfur, the cathode active material. For this reason, cathodes have been developed that contain a composite material made by mixing sulfur with a conductive additive (e.g., carbon particles), and that have a cathode composite layer in which a conductive network is formed by the carbon particles.

[0005] Patent Document 1 discloses a method for manufacturing a sulfur-carbon composite positive electrode, which includes the steps of: preparing a sulfur-carbon composite positive electrode material by mechanically milling a sulfur active material, a conductive material (e.g., carbon particles), and a solid electrolyte; preparing a slurry by adding and mixing a binder and a solvent to the sulfur-carbon composite positive electrode material; coating the slurry obtained in the slurry-forming step onto a positive electrode current collector; and drying the coated slurry. The method also includes the mechanical milling step of intimately mixing the sulfur active material, conductive material, and solid electrolyte while pulverizing them by mechanical milling.

[0006] Non-Patent Document 1 discloses a method for producing a positive electrode by preparing a sulfur-carbon composite material by a melt impregnation method in which molten sulfur is absorbed into the pores of carbon particles by capillary force, and then casting a slurry of the resulting positive electrode material and a binder on the surface of a current collector to form a positive electrode composite layer. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2019-145206 [Non-patent literature]

[0008] [Non-Patent Document 1] Xiulei Ji et al. “Nature Materials”, volume 8, June2009, p500-p506 Summary of the Invention [Problem to be solved by the invention]

[0009] However, because the mechanical milling process disclosed in Patent Document 1 involves mixing sulfur particles, which are physically solid-solid particles, with a conductive additive (e.g., carbon particles), the sulfur particles do not fill (exist) in the pores of the carbon particles but merely adhere to the surfaces of the carbon particles. As a result, the contact area of ​​the sulfur particles with the surfaces of the carbon particles is small, so the conductive paths between the carbon particles and the sulfur particles that form the conductive network do not function effectively, reducing the utilization rate of the sulfur particles in the positive electrode mixture layer and making it difficult to obtain a high-capacity positive electrode.

[0010] Furthermore, in a lithium-sulfur secondary battery incorporating the positive electrode described in Patent Document 1, sulfur contained in the positive electrode mixture layer generates lithium polysulfide, a reaction intermediate, during charge and discharge. Lithium polysulfide has the property of easily dissolving in nonaqueous electrolytes. In the positive electrode described in Patent Document 1, the sulfur particles of the sulfur-carbon composite contained in the positive electrode mixture layer are simply attached to the surface of the carbon particles, as described above. Therefore, when the sulfur particles generate lithium polysulfide during charge and discharge, they are easily dissolved and detached upon contact with the nonaqueous electrolyte. As a result, the number of sulfur particles in the positive electrode mixture layer decreases with repeated charge and discharge, reducing the capacity retention rate of the lithium-sulfur secondary battery. In other words, this shortens the lifespan of the lithium-sulfur secondary battery.

[0011] Furthermore, the melt impregnation method described in Non-Patent Document 1 involves physically mixing molten sulfur with carbon particles. Because molten sulfur has high viscosity, only a small amount of sulfur fills (is present) within the pores of the carbon particles, with the majority of sulfur remaining on the surface of the carbon particles. As a result, in a lithium-sulfur secondary battery incorporating the positive electrode described in Non-Patent Document 1, most of the sulfur is present on the surface of the carbon particles. During charging and discharging, sulfur generates lithium polysulfides, a reaction intermediate. These sulfur polysulfides are easily dissolved and released upon contact with the non-aqueous electrolyte. As a result, the number of sulfur particles in the positive electrode mixture layer decreases with repeated charging and discharging, resulting in a decrease in the capacity retention rate of the lithium-sulfur secondary battery. This, in turn, shortens the lifespan of the lithium-sulfur secondary battery.

[0012] The present invention provides a sulfur-carbon composite positive electrode that can achieve high capacity and high cycle characteristics when incorporated into a lithium-sulfur secondary battery. [Means for solving the problem]

[0013] The present invention provides a sulfur-carbon composite positive electrode comprising a positive electrode current collector and a positive electrode mixture layer formed on one or both surfaces of the positive electrode current collector. The positive electrode mixture layer contains carbon particles having a plurality of pores, sulfur, and a binder. The carbon particles are bound by the binder, so that adjacent carbon particles are in contact with each other. The sulfur includes first sulfur present in the plurality of pores of the carbon particles located on the surface and in the interior of the positive electrode mixture layer, and second sulfur present on the surfaces of the carbon particles located on the surface and in the interior of the positive electrode mixture layer, excluding the contact areas between the carbon particles. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a sulfur-carbon composite positive electrode that can achieve high capacity and high cycle characteristics when incorporated into a lithium-sulfur secondary battery. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a diagram showing an example of an electrolytic deposition cell used in producing a sulfur-carbon composite positive electrode according to the first embodiment. [Figure 2] FIG. 2 is a diagram showing an example of the configuration of a lithium-sulfur secondary battery according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following description. Furthermore, various modifications and improvements can be made to the embodiments, and such modifications and improvements can also be included in the present invention.

[0017] First Embodiment The sulfur-carbon composite positive electrode according to the first embodiment includes a positive electrode current collector and a positive electrode mixture layer formed on one or both surfaces of the positive electrode current collector. The positive electrode mixture layer contains carbon particles having a plurality of pores, sulfur, and a binder.

[0018] The material constituting the positive electrode current collector is not particularly limited, and examples thereof include metals such as aluminum, nickel, copper, stainless steel, and carbon-processed metals processed into foils, meshes, expanded grids, punched metals, and the like.

[0019] The carbon particles in the positive electrode mixture layer are bound by a binder, so that adjacent carbon particles are in contact with each other, forming a conductive network of carbon particles in the positive electrode mixture layer that connects to the positive electrode current collector.

[0020] The carbon particles in the positive electrode composite layer preferably have an average pore diameter of 0.1 nm to 20 nm, more preferably 1 nm to 15 nm, even more preferably 3 nm to 10 nm, and most preferably 4 nm to 8 nm. If the average pore diameter is less than 0.1 nm, it may be difficult for sulfur to penetrate into the pores of the carbon particles. On the other hand, if the average pore diameter exceeds 20 nm, contact between the carbon particles and the first sulfur present in the multiple pores of the carbon particles described below may be insufficient, and a good conductive path between the carbon particles and sulfur may not function effectively, resulting in a decrease in sulfur utilization. In addition, the carbon particles in the positive electrode composite layer should have a specific surface area of ​​200 m or less. 2 ·g -1 More than 2500m 2 ·g -1 It is preferable that it is less than 1000m 2 ·g -1 More than 2200m 2 ·g -1 Furthermore, the carbon particles in the positive electrode mixture layer preferably have a pore volume of 0.5 cm or less. 3 ·g -1 The upper limit of the pore volume is not particularly limited, but in reality it is 5.0 cm 3 ·g -1 The specific surface area and pore volume of the carbon particles in the positive electrode mixture layer are within these ranges. A sulfur-carbon composite positive electrode exhibits excellent rate and cycle characteristics and exhibits reduced polarization. The average pore diameter and specific surface area of ​​the carbon particles can be measured by a nitrogen adsorption / desorption method, and the pore volume can be measured by a mercury porosimeter method.

[0021] Known carbon particles can be used, and specifically, one or a mixture of two or more selected from the group consisting of Ketjen black, carbon nanotubes, graphene, acetylene black, porous carbon (for example, CNovel (registered trademark) (Toyo Tanso Co., Ltd.)), etc. When a mixture is used, the combination and ratio thereof can be selected as desired depending on the purpose.

[0022] The sulfur contained in the positive electrode mixture layer is composed of first sulfur present in the pores of the carbon particles located on the surface and in the interior of the positive electrode mixture layer, and second sulfur present on the surfaces of the carbon particles located on the surface and in the interior of the positive electrode mixture layer, excluding the contact areas between the carbon particles. In other words, the first and second sulfurs are present in the positive electrode mixture layer without impairing the conductive network of the carbon particles.

[0023] Generally, charging and discharging of lithium-sulfur secondary batteries is carried out through the oxidation-reduction reaction between sulfur and lithium. In order for sulfur, which has low electrical conductivity, to contribute to the oxidation-reduction reaction, it is desirable for sulfur to be in contact with and dispersed in carbon particles.

[0024] The first sulfur exists within the pores of the carbon particles, which acts as a conductive additive, enabling a good conductive path to be established between the carbon particles. The second sulfur exists, for example, in the form of a thin film, on the surface of the carbon particles (excluding the contact areas between the carbon particles). This increases the contact area between the first and second sulfur and the carbon particles, thereby improving the contribution of the sulfur present in the positive electrode mixture layer to the redox reaction. This significantly increases the utilization rate of sulfur in the positive electrode mixture layer compared to conventional sulfur-carbon composite positive electrodes, achieving higher capacity.

[0025] The volume ratio of the first sulfur to the second sulfur is preferably 60:40 to 95:5, and more preferably 80:20 to 90:10.

[0026] The binder contained in the positive electrode mixture layer is not particularly limited, and known binders can be used, specifically, one or a mixture of two or more selected from the group consisting of polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyacrylic acid (PAA), lithium polyacrylate (PAALi), styrene butadiene rubber (SBR), polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyethylene glycol (PEG), carboxymethyl cellulose (CMC), polyacrylonitrile (PAN), polyimide (PI), etc. When a mixture is used, the combination and ratio thereof can be selected as desired depending on the purpose.

[0027] The sulfur-carbon positive electrode according to the first embodiment described above includes a positive electrode current collector and a positive electrode composite layer formed on one or both surfaces of the positive electrode current collector. The positive electrode composite layer contains carbon particles having a plurality of pores, sulfur, and a binder. The carbon particles are bound by the binder so that adjacent carbon particles are in contact with each other. The sulfur comprises first sulfur present in the plurality of pores of the carbon particles located on the surface and in the interior of the positive electrode composite layer, and second sulfur present on the surfaces of the carbon particles located on the surface and in the interior of the positive electrode composite layer, excluding the contact portions between the carbon particles. Therefore, when incorporated into a lithium-sulfur secondary battery, high capacity and excellent cycle characteristics can be achieved.

[0028] That is, adjacent carbon particles in the positive electrode mixture layer contact each other to form a conductive network, and the conductive network of the carbon particles contacts the positive electrode current collector that holds the positive electrode mixture layer. The first sulfur is present in the pores of the carbon particles located on the surface and inside of the positive electrode mixture layer. That is, the first sulfur is enclosed throughout the pores of the carbon particles that form the conductive network, resulting in intimate contact with the carbon particles. The second sulfur is located on the surface and inside of the positive electrode mixture layer, and is present on the surfaces of the carbon particles except for the contact areas between the carbon particles. That is, the second sulfur is also in intimate contact with the surfaces of the carbon particles that form the conductive network, for example, covering them. As a result, both the first and second sulfur atoms are not isolated from the carbon particles that form the conductive network, and a good conductive path can be obtained, so that most of the sulfur in the positive electrode mixture layer contributes to the redox reaction, improving the utilization rate and enabling the realization of a high-capacity sulfur-carbon positive electrode.

[0029] Furthermore, in a lithium-sulfur secondary battery incorporating the sulfur-carbon positive electrode according to the first embodiment, sulfur contained in the positive electrode mixture layer generates lithium polysulfide, a reaction intermediate, during charge and discharge, as described above. Lithium polysulfide is easily soluble in nonaqueous electrolytes. The second sulfur contained in the positive electrode mixture layer is present on the surface of carbon particles located on the surface and in the interior of the positive electrode layer. Therefore, if it is converted to lithium polysulfide during charge and discharge, it may dissolve upon contact with the nonaqueous electrolyte. On the other hand, the first sulfur is present within the pores of the carbon particles located on the surface and in the interior of the positive electrode mixture layer. That is, it is encapsulated throughout the pores of the carbon particles. Therefore, even if the first sulfur is converted to lithium polysulfide during charge and discharge, it remains within the pores of the carbon particles, preventing its desorption upon contact with the nonaqueous electrolyte. As a result, the reduction of sulfur in the positive electrode mixture layer due to repeated charge and discharge cycles is suppressed, thereby preventing a decrease in the capacity retention rate of the lithium-sulfur secondary battery. That is, compared with Patent Document 1 and Non-Patent Document 1 described in the background art, the life of the lithium-sulfur secondary battery can be improved, and high cycle characteristics can be achieved.

[0030] Next, a method for producing the sulfur carbon composite positive electrode according to the first embodiment will be described. (1) A carbon particle-containing layer containing carbon particles having a plurality of pores and a binder is formed on at least one surface of a positive electrode current collector to prepare a carbon electrode. (2) An electrodeposition solution containing a sulfur compound and a solvent is prepared. (3) The carbon-containing layer of the carbon electrode is impregnated with an electrolytic deposition solution. (4) As shown in FIG. 1, the carbon electrode 2, the lithium ion conductor 4, and the lithium metal electrode 3 are stacked in this order so that the carbon particle-containing layer 22 faces the lithium ion conductor 4 to assemble the electrodeposition cell 1. (5) Using the carbon electrode as the positive electrode and the lithium metal electrode as the negative electrode, a DC voltage is applied between these electrodes to electrolytically deposit sulfur from the electrodeposition solution present in the pores of the carbon particles in the carbon particle-containing layer and in the surface and voids of the carbon particle-containing layer. In this process, sulfur compounds (e.g., Li2S8) in the electrodeposition solution are ionized into negative sulfide ions and positive lithium ions. The negative sulfide ions are attracted to the carbon electrode, which is the positive electrode, to electrolytically deposit sulfur, and the positive lithium ions pass through the lithium ion conductor and are attracted to the lithium metal electrode, which is the negative electrode, to electrolytically deposit lithium. (6) The electrolytic deposition cell is disassembled, and the carbon electrode on which sulfur is deposited is taken out, washed, and then dried to produce a sulfur-carbon composite positive electrode.

[0031] In the step (1), the carbon-containing layer can be formed by dissolving and dispersing carbon particles and a binder in a solvent to prepare a slurry, applying the slurry to at least one surface of the positive electrode current collector, and then drying the slurry. The amount of binder contained in the carbon-containing layer is preferably kept to a minimum while still maintaining the shape of the carbon-containing layer. For example, the carbon particles are preferably 65% ​​to 95% by weight and the binder is 5% to 35% by weight, more preferably 70% to 90% by weight and the binder is 10% to 30% by weight, and even more preferably 70% to 85% by weight and the binder is 15% to 30% by weight. By keeping the binder content between 5% and 35% by weight, the shape of the carbon-containing layer is maintained while reducing clogging of the pores of the carbon particles with the binder, allowing the electrolytic deposition solution to penetrate more of the pores of the carbon particles.

[0032] The sulfur compound used in the step (2) can be, for example, one or a mixture of two or more selected from the group consisting of Li2S8, Li2S6, Li2S4, Li2S2 and Li2S.

[0033] The solvent used in the step (2) is preferably a compound that is difficult to decompose during electrodeposition, and examples thereof include sulfone compounds such as sulfolane, and nitrile compounds such as acetonitrile.

[0034] The electrolytic deposition solution used in step (2) preferably has a sulfur compound concentration of 0.1 wt% to 50 wt%, more preferably 1.0 wt% to 40 wt%, and even more preferably 2.0 wt% to 30 wt%. If the concentration of the electrolytic deposition solution exceeds 50 wt%, the viscosity of the electrolytic deposition solution increases, potentially making it difficult for the electrolytic deposition solution to penetrate into the pores of the carbon particles. Furthermore, the particle size of the precipitated sulfur increases, resulting in insufficient contact between the carbon particles and sulfur, which may result in an ineffective conductive path and reduced sulfur utilization. On the other hand, if the concentration of the electrolytic deposition solution is less than 0.1 wt%, the efficiency of sulfur electrodeposition decreases, potentially making it impossible to ensure an appropriate amount of sulfur electrolytically deposited.

[0035] The lithium metal electrode used in the step (4) has a structure in which, for example, a lithium metal foil is attached to a copper current collector. The lithium ion conductor used in the step (4) is not particularly limited, and may be any material that conducts only lithium ions. 12 , Li 0.35 La 0.55 TiO3, Li 1+x+y Al x (Ti,Ge) 2-x Si y P 3-y O 12 (0≦x≦2,0≦y≦3), Li 2.9 PO 3.3 N 0.46 and cation exchange membranes.

[0036] During the electrolytic deposition in step (5), in order to deposit sulfur with a small particle size on the carbon particles, the current density of the direct current applied between the carbon electrode and the lithium metal electrode is set to 0.01 mA cm -2 1.0mA cm or more -2 Preferably less than 0.01 mA cm -2 More than 0.5mA cm -2 The following is more preferable: Furthermore, the cell voltage (ultimate voltage) is preferably 2.6 V or more and 3.5 V or less under this current density condition.

[0037] According to the manufacturing method of the first embodiment described above, a carbon-containing layer is impregnated with an electrolytic deposition solution containing a sulfur compound and a solvent. The sulfur compound in the electrolytic deposition solution impregnated into the carbon-containing layer is ionized, thereby electrolytically depositing sulfur onto carbon particles located on and within the carbon-containing layer. As a result, sulfur is electrolytically deposited in the carbon-containing layer of the carbon electrode removed from the electrolytic deposition cell, resulting in a sulfur-carbon positive electrode that functions as a positive electrode composite layer. Because the carbon particles in the positive electrode composite layer maintain their morphology, they are bound by the binder, allowing adjacent carbon particles to contact each other and form a conductive network. The electrolytically deposited sulfur includes first sulfur present in the pores of the carbon particles located on and within the positive electrode composite layer, and second sulfur present on the surfaces of the carbon particles located on and within the positive electrode composite layer, excluding contact areas between the carbon particles.

[0038] Therefore, the obtained sulfur carbon electrode can achieve high capacity and high cycle characteristics due to the same behavior as described above.

[0039] <Second embodiment> A lithium-sulfur secondary battery according to a second embodiment will be described. The lithium-sulfur secondary battery according to the second embodiment includes the above-described sulfur-carbon composite positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte solution.

[0040] The negative electrode includes a negative electrode current collector and a negative electrode layer containing a negative electrode active material formed on one or both surfaces of the negative electrode current collector.

[0041] The negative electrode current collector is made of, for example, copper.

[0042] The negative electrode active material can be metallic lithium or a lithium alloy. Examples of lithium alloys include lithium-aluminum alloys, lithium-tin alloys, lithium-lead alloys, and lithium-silicon alloys. The negative electrode active material can be one or more materials selected from the group consisting of carbon materials such as natural graphite, artificial graphite, carbon black, acetylene black, graphite, activated carbon, carbon fiber, coke, soft carbon, and hard carbon. When two or more carbon materials are used, the combination and ratio of the materials can be selected as desired depending on the purpose.

[0043] When the negative electrode active material is metallic lithium or a lithium alloy, the negative electrode can be produced by attaching the negative electrode active material in the form of a foil to one or both surfaces of a negative electrode current collector to form a negative electrode layer.

[0044] On the other hand, when the negative electrode active material is a carbon material, the negative electrode active material is dispersed in a solvent together with a binder and, if necessary, a conductive additive to prepare a negative electrode slurry. The negative electrode slurry is applied to one or both surfaces of a negative electrode current collector, dried, and, if necessary, pressed with a roller or the like to form a negative electrode layer containing the negative electrode active material and the binder, thereby producing a negative electrode.

[0045] The non-aqueous electrolyte contains a non-aqueous solvent in which a lithium salt is dissolved. The lithium salt is not particularly limited and can be any known salt, specifically, one or a mixture of two or more selected from the group consisting of lithium hexafluorophosphate (LiPF), lithium bromide (LiBr), lithium perchlorate (LiClO), lithium bis(oxalatoborate) (LiB(CO)), lithium fluoroborate (LiBF), lithium nitrate (LiNO), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). When a mixture is used, the combination and ratio thereof can be selected as desired depending on the purpose.

[0046] The non-aqueous solvent is not particularly limited, and known solvents can be used. Specific examples include ethylene carbonate, ethyl methyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, sulfolane, oxolane, tetraglyme, triglyme, fluoroethylene carbonate, and ionic liquids.Examples of ionic liquids include 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-methyl-3-propylimidazolium bis(trifluoromethanesulfonyl)imide, 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, and 1-ethyl-3-methylimidazolium methanesulfonyl. nate, 1-butyl-3-methylimidazolium methanesulfonate, 1,2,3-trimethylimidazolium methylsulfate, methylimidazolium chloride, methylimidazolium hydrogen sulfate, 1-ethyl-3-methylimidazolium hydrogen sulfate, 1-butyl-3-methylimidazolium hydrogen sulfate, 1-ethyl-3-methylimidazolium tetrachloroaluminate, 1-butyl-3-methylimidazolium tetrachloroaluminate, 1-ethyl-3-methylimidazolium acetate 1-Butyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium ethyl sulfate, 1-butyl-3-methylimidazolium methyl sulfate, 1-ethyl-3-methylimidazolium thiocyanate, 1-butyl-3-methylimidazolium thiocyanate, 1-ethyl-2,3-dimethylimidazolium ethyl sulfate, 1-butylpyridinium bis(trifluoromethanesulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluoromethanesulfonyl)imide, 1-methyl-1-propanol

[0043] Examples of suitable methyl ammonium bis(trifluoromethanesulfonyl)imide include propylpyrrolidinium bis(trifluoromethanesulfonyl)imide, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide, tributyldodecylphosphonium bis(trifluoromethanesulfonyl)imide, methyltributylammonium methylsulfate, butyltrimethylammonium bis(trifluoromethanesulfonyl)imide, and trimethylhexylammonium bis(trifluoromethanesulfonyl)imide.The non-aqueous solvent may be one or a mixture of two or more solvents selected from the above group, and when a mixture is used, the combination and ratio thereof may be selected arbitrarily depending on the purpose.

[0047] The separator can be an organic polymer separator or an inorganic separator that does not react with the positive electrode, negative electrode, non-aqueous electrolyte, etc. Examples of polymers that make up the organic polymer separator include polypropylene, polyolefin, nitrocellulose, and polyimide. Examples of inorganic separators include silica glass nonwoven fabric. The separator may be subjected to one or more treatments selected from the group consisting of ceramic coating and structure control. When two or more treatments are performed, the combination and ratio of the treatments can be selected as desired depending on the purpose.

[0048] The positive electrode, negative electrode, separator, and non-aqueous electrolyte are housed in an exterior body. The exterior body is not particularly limited, but examples thereof include a bag-shaped exterior body having a laminate film, a coin-shaped metal can, a cylindrical metal can, and a rectangular metal can.

[0049] The structure of the lithium-sulfur secondary battery according to the embodiment will be described below with reference to the drawings: Figure 2 is a cross-sectional view showing an example of a lithium-sulfur secondary battery.

[0050] The lithium-sulfur secondary battery 100 includes a sulfur-carbon composite positive electrode 110, a negative electrode 120, and a separator 130 disposed between the positive electrode 110 and the negative electrode 120. The positive electrode 110, the negative electrode 120, and the separator 130 are housed in an exterior body (not shown). The sulfur-carbon composite positive electrode 110 is composed of a positive electrode current collector 111 and a positive electrode mixture layer 112 containing carbon particles, sulfur, and a binder, which is provided on the surface of the positive electrode current collector 111 facing the separator 130. The negative electrode 120 is composed of a negative electrode current collector 121 and a negative electrode layer 122, which is provided on the surface of the negative electrode current collector 121 facing the separator 130. The separator 130 is impregnated with a non-aqueous electrolyte.

[0051] As described above, according to the second embodiment, by including the sulfur-carbon composite positive electrode according to the first embodiment, it is possible to provide a lithium-sulfur secondary battery that achieves high capacity and high cycle characteristics. [Example]

[0052] The present invention will be described in more detail below by way of examples, but is not limited to these examples.

[0053] Example 1 Ketjen black (hereinafter abbreviated as KB), a carbon particle material, and carboxymethyl cellulose (hereinafter abbreviated as CMC), a binder, were mixed in ultrapure water at a weight ratio (KB:CMC) of 70:30 to prepare a slurry. The average pore diameter and specific surface area of ​​Ketjen black were measured by the nitrogen adsorption / desorption method, and the pore volume was measured by the mercury porosimeter method. The average pore diameter was 4.730 nm, and the specific surface area was 1323 m. 2 ·g -1 and the pore volume is 1.565 cm 3 ·g -1 The obtained slurry was applied to the surface of aluminum foil, which served as a positive electrode current collector, to a thickness of 50 μm, and then vacuum dried overnight at 80°C to form a carbon-containing layer on the positive electrode current collector, producing a carbon electrode. Sulfur and lithium sulfide (Li2S) were dissolved in sulfolane to a molar ratio (sulfur:Li2S) of 7:8, and the solution was stirred overnight at 60°C to prepare an electrodeposition solution containing Li2S8 with a Li2S8 concentration of 0.1 wt%. The prepared electrodeposition solution was impregnated into a carbon electrode.

[0054] Next, the lithium metal electrode and the lithium ion conductor Li7La3Zr2O 12 The electrodeposition cell was assembled by stacking the carbon particle-containing layer and the lithium metal electrode impregnated with the electrodeposition solution in this order, with the carbon particle-containing layer facing the lithium ion conductor. The carbon electrode was used as the positive electrode and the lithium metal electrode as the negative electrode. A current density of 0.1 mAcm was applied to these electrodes. -2A DC voltage of 1000kJ / cm2 was applied until the cell voltage reached 2.8 V, causing sulfur to be electrolytically deposited on the carbon-containing layer of the carbon electrode. The cell was then disassembled, and the carbon electrode with the deposited sulfur was removed. It was thoroughly washed with dimethyl ether (DME) and dried to produce a sulfur-carbon composite positive electrode with a carbon-containing layer (positive electrode composite layer) on which sulfur had been electrolytically deposited. The sulfur content per unit area of ​​the positive electrode was 1.5 mg cm2. -2 It was decided.

[0055] Example 2 A sulfur-carbon composite positive electrode was prepared in the same manner as in Example 1, except that the concentration of the sulfur compound in the electrodeposition solution was set to 5 wt %.

[0056] Example 3 A sulfur-carbon composite positive electrode was prepared in the same manner as in Example 1, except that the concentration of the sulfur compound in the electrodeposition solution was 10% by weight.

[0057] Example 4 A sulfur-carbon composite positive electrode was prepared in the same manner as in Example 1, except that the concentration of the sulfur compound in the electrodeposition solution was set to 50% by weight.

[0058] Example 5 A sulfur-carbon composite positive electrode was prepared in the same manner as in Example 1, except that acetonitrile was used as the solvent for the electrodeposition solution.

[0059] Example 6 A sulfur-carbon composite positive electrode was prepared in the same manner as in Example 1, except that acetonitrile was used as the solvent for the electrodeposition solution and the concentration of the sulfur compound was 5 wt %.

[0060] Example 7 A sulfur-carbon composite positive electrode was prepared in the same manner as in Example 1, except that acetonitrile was used as the solvent for the electrodeposition solution and the concentration of the sulfur compound was 10 wt %.

[0061] Example 8 A sulfur-carbon composite positive electrode was prepared in the same manner as in Example 1, except that acetonitrile was used as the solvent for the electrodeposition solution and the concentration of the sulfur compound was set to 50 wt %.

[0062] Example 9 A sulfur-carbon composite positive electrode was prepared in the same manner as in Example 1, except that a mixture of Li2S6 and Li2S8 in a weight ratio (Li2S6:Li2S8) of 80:20 was used as the sulfur compound in the electrodeposition solution, and the concentration of the sulfur compound was 50 wt%.

[0063] Example 10 A sulfur-carbon composite positive electrode was prepared in the same manner as in Example 1, except that a mixture of Li2S6 and Li2S8 in a weight ratio (Li2S6:Li2S8) of 50:50 was used as the sulfur compound in the electrodeposition solution, and the concentration of the sulfur compound was 50 wt%.

[0064] Example 11 A sulfur-carbon composite positive electrode was prepared in the same manner as in Example 1, except that a mixture of Li2S6 and Li2S8 in a weight ratio (Li2S6:Li2S8) of 20:80 was used as the sulfur compound in the electrodeposition solution, and the concentration of the sulfur compound was 50 wt%.

[0065] (Comparative Example 1) Sulfur and KB were dry mixed using a mortar and pestle in a weight ratio (sulfur:KB) of 75:25, and the mixture was heated in an inert gas atmosphere at 155°C for 12 hours to absorb the molten sulfur into the KB. This resulted in a sulfur-carbon composite obtained by the melt impregnation method. Next, the sulfur-carbon composite, KB, and CMC were mixed in ultrapure water at a weight ratio (sulfur-carbon composite:KB:CMC) of 80:10:10 to prepare a slurry. The resulting slurry was applied to the surface of aluminum foil, which served as a positive electrode current collector, to a thickness of 50 μm and then vacuum-dried overnight at 80 °C to form a positive electrode composite layer, producing a sulfur-carbon composite positive electrode. The sulfur content per unit area of ​​the positive electrode was 1.5 mg cm. -2 It was decided.

[0066] (Comparative Example 2) A sulfur-carbon composite was prepared by mechanical milling, in which sulfur and KB were dry-mixed using a mortar and pestle in a weight ratio (sulfur:KB) of 75:25. The resulting sulfur-carbon composite, KB, and CMC were then kneaded in ultrapure water in a weight ratio (sulfur-carbon composite:KB:CMC) of 80:10:10 to prepare a slurry. This slurry was applied to the surface of aluminum foil, which served as a positive electrode current collector, to a thickness of 50 μm and dried overnight in vacuum at 80 °C to form a positive electrode composite layer, producing a sulfur-carbon composite positive electrode. The sulfur content per unit area of ​​the positive electrode was 1.5 mg cm. -2 It was decided.

[0067] The electrochemical properties were evaluated using an evaluation cell having the structure described below. The evaluation cells were fabricated using the sulfur-carbon composite positive electrodes of Examples 1 to 11 and Comparative Examples 1 and 2. Specifically, a polyimide separator was interposed between the sulfur-carbon composite positive electrode and the lithium metal electrode, and the separator was filled with a non-aqueous electrolyte to assemble a bipolar cell. The non-aqueous electrolyte contained 1 mol dm lithium bis(trifluoromethane)sulfonimide (LiTFSI), a lithium salt. -3 The solution was prepared by dissolving the solution in sulfolane so that the concentration became

[0068] <Discharge capacity measurement> Discharge tests were carried out on the evaluation cells of Examples 1 to 11 and Comparative Examples 1 and 2. The test temperature was 80°C, and the current density was 1.0 mAcm -2 The battery was discharged at 1000 kJ / s until the voltage reached 1.5 V, and the discharge capacity was measured, normalized by the amount of sulfur contained in the positive electrode mixture layer of the sulfur-carbon composite positive electrode. The results are shown in Table 1 below.

[0069] <Charge / discharge cycle characteristic evaluation test> A test to evaluate the charge-discharge cycle characteristics was carried out on the evaluation cells of Examples 1 to 11 and Comparative Examples 1 and 2. The test temperature was 80°C, and the current density was 1.0 mA cm -2The cell was discharged at 1000 kJ / s until the voltage reached 1.5 V, and then charged to 2.8 V at a current density equivalent to the discharge current. This charge / discharge cycle constitutes one cycle, and the charge / discharge cycle was repeated 100 times to measure the discharge capacity of the evaluation cell. The discharge capacity retention rate was calculated using the following formula based on the measured discharge capacities at the first cycle and the 100th cycle of the evaluation cell. Discharge capacity retention rate (%) = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) x 100

[0070] [Table 1]

[0071] As is clear from Table 1, the evaluation cells (Examples 1 to 11) using sulfur-carbon composite positive electrodes prepared by electrodeposition all had a discharge capacity normalized to the amount of sulfur of 1300 mAh g -1 The results show a large discharge capacity and a high discharge capacity retention rate of over 61%. This is because, in the sulfur-carbon composite positive electrode fabricated by electrodeposition, adjacent carbon particles in the positive electrode mixture layer come into contact with each other to form a conductive network, and the sulfur is composed of first sulfur present in the pores of the carbon particles located on the surface and in the interior of the positive electrode mixture layer, and second sulfur present on the surfaces of the carbon particles located on the surface and in the interior of the positive electrode mixture layer.

[0072] The evaluation cells equipped with the sulfur-carbon composite positive electrodes of Comparative Example 1, which was prepared by the melt-infiltration method, and Comparative Example 2, which was prepared by the mechanical milling method, both had a discharge capacity normalized to the amount of sulfur of 1100 mAh g -1 The discharge capacity was small, at 0.25 or less, and the discharge capacity retention rate was low, at 35% or less, and it was confirmed that the discharge capacity normalized to the amount of sulfur and the cycle characteristics were poor. [Explanation of symbols]

[0073] 1...electrolytic deposition cell, 2...positive electrode, 21...positive electrode current collector, 22...carbon particle-containing layer, 3...negative electrode, 31...negative electrode current collector, 32...lithium metal foil, 4...lithium ion conductor, 100...lithium-sulfur secondary battery, 110...sulfur-carbon composite positive electrode, 111...positive electrode current collector, 112...positive electrode composite layer, 120...negative electrode, 121...negative electrode current collector, 122...negative electrode layer, 130...separator

Claims

1. A sulfur-carbon composite positive electrode comprising a positive electrode current collector and a positive electrode mixture layer formed on one or both surfaces of the positive electrode current collector, the positive electrode mixture layer includes carbon particles having a plurality of pores, sulfur, and a binder; the carbon particles are bound by the binder, and adjacent carbon particles are in contact with each other; The sulfur is a first sulfur present in a plurality of pores of the carbon particles located on the surface and in the interior of the positive electrode mixture layer, and a second sulfur present on the surfaces of the carbon particles located on the surface and in the interior of the positive electrode mixture layer, excluding contact portions between the carbon particles.

2. 2. The sulfur-carbon composite positive electrode according to claim 1, wherein the carbon particles have pores with an average pore size of 0.1 nm or more and 20 nm or less.

3. 3. The sulfur-carbon composite positive electrode according to claim 1, wherein the carbon particles are Ketjen black.

4. A lithium-sulfur secondary battery comprising the sulfur-carbon composite positive electrode according to any one of claims 1 to 3, a negative electrode, a separator, and a non-aqueous electrolyte.

5. forming a carbon particle-containing layer containing carbon particles having a plurality of pores and a binder on at least one surface of a positive electrode current collector to prepare a carbon electrode; providing an electrodeposition solution comprising a sulfur compound and a solvent; impregnating the carbon particle-containing layer of the carbon electrode with the electrodeposition solution; assembling an electrolytic deposition cell by stacking a lithium metal electrode, a lithium ion conductor, and the carbon electrode in this order, with the carbon particle-containing layer facing the lithium ion conductor; a carbon electrode serving as a positive electrode and a lithium metal electrode serving as a negative electrode, and applying a DC voltage to these electrodes to electrolytically deposit sulfur from the electrodeposition solution present in the pores of the carbon particles in the carbon particle-containing layer and in the surfaces and voids of the carbon particle-containing layer.

6. The sulfur compound is Li ２ S ８ , Li ２ S ６ , Li ２ S ４ , Li ２ S ２ and Li ２ 6. The method for producing a sulfur-carbon composite positive electrode according to claim 5, wherein the sulfur-carbon composite positive electrode is at least one selected from the group consisting of S.

7. 7. The method for producing a sulfur-carbon composite positive electrode according to claim 5, wherein the solvent is a nitrile-based or sulfone-based organic solvent.

8. The method for producing a sulfur-carbon composite positive electrode according to any one of claims 5 to 7, wherein the concentration of the sulfur compound contained in the electrodeposition solution is 50 wt% or less.

Images