Manufacturing method for sulfur-carbon composite cathodes

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] This 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 into industries such as portable devices, power tools, and automobiles. Consequently, there is a growing demand for even higher energy density in lithium-ion secondary batteries. However, the rate at which the energy density of lithium-ion secondary batteries can be increased is plateauing, necessitating further research into battery materials and systems.

[0003] Lithium-sulfur secondary batteries, which use sulfur as the positive electrode active material, are attracting attention as one of the next-generation batteries due to their potential for 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, which is the positive electrode active material. For this reason, positive electrodes have been developed that include a composite material in which sulfur is mixed with a conductive additive (e.g., carbon particles), and have a positive electrode composite layer in which a conductive network of carbon particles is formed.

[0005] Patent Document 1 discloses a method for manufacturing a sulfur-carbon composite cathode, comprising the steps of: producing a sulfur-carbon composite cathode material by mechanical milling a sulfur-active material, a conductive material (e.g., carbon particles), and a solid electrolyte; preparing a slurry by adding a binder and a solvent to the sulfur-carbon composite cathode material and mixing them; coating the slurry obtained in the slurry formation step onto a cathode current collector; and drying the slurry coated in the coating step. A method for manufacturing a cathode is disclosed that includes a mechanical milling step in which the sulfur-active material, conductive additive, and solid electrolyte are densely mixed while being crushed by mechanical milling.

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

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

[0008] [Non-Patent Document 1] Xiulei Ji et al. “Nature Materials”, volume 8, June2009, p500-p506 [Overview of the project] [Problems that the invention aims to solve]

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

[0010] Also, in a lithium-sulfur secondary battery incorporating the positive electrode described in Patent Document 1, sulfur contained in the positive electrode composite layer generates lithium polysulfide, which is a reaction intermediate, during charge and discharge. Lithium polysulfide has a property of being easily dissolved in a non-aqueous electrolyte. In the positive electrode described in Patent Document 1, since the sulfur particles of the sulfur-carbon composite material contained in the positive electrode composite layer are simply attached to the surface of the carbon particles as described above, when the sulfur particles generate lithium polysulfide during charge and discharge, they are easily dissolved and detached by contact with the non-aqueous electrolyte. As a result, the sulfur particles in the positive electrode composite layer decrease with repeated charge and discharge, and the capacity retention rate of the lithium-sulfur secondary battery decreases. That is, the life of the lithium-sulfur secondary battery is reduced.

[0011] Further, the melt impregnation method described in Non-Patent Document 1 is a method of physically mixing molten sulfur and carbon particles. Since molten sulfur has high viscosity, only a small amount of sulfur is filled (present) in a plurality of pores of the carbon particles, and most of it exists 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, since most of the sulfur exists on the surface of the carbon particles, when sulfur generates lithium polysulfide, which is a reaction intermediate, during charge and discharge, it is easily dissolved and detached by contact with the non-aqueous electrolyte. As a result, the sulfur particles in the positive electrode composite layer decrease with repeated charge and discharge, and the capacity retention rate of the lithium-sulfur secondary battery decreases. That is, the life of the lithium-sulfur secondary battery is reduced.

[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 Problems

[0013] The present invention relates to a sulfur-carbon composite positive electrode comprising 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 together by the binder, and adjacent carbon particles are in contact with each other. The sulfur consists of first sulfur present in the plurality of pores of the carbon particles located on the surface and inside the positive electrode composite layer, and second sulfur present on the surface of the carbon particles, excluding the contact areas between carbon particles, located on the surface and inside the positive electrode composite layer. [Effects of the Invention]

[0014] According to the present invention, a sulfur-carbon composite cathode is provided that can achieve high capacity and high cycle characteristics when incorporated into a lithium-sulfur secondary battery. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 shows an example of an electrolytic cell used in the manufacture of a sulfur-carbon composite cathode according to the first embodiment. [Figure 2] Figure 2 shows an example of the configuration of a lithium-sulfur secondary battery according to the second embodiment. [Modes for carrying out the invention]

[0016] Embodiments of the present invention will be described below, but the present invention is not limited to the following description. Furthermore, various modifications or improvements can be made to the embodiments, and such modified or improved forms may also be included in the present invention.

[0017] <First Embodiment> The sulfur-carbon composite cathode according to the first embodiment comprises 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.

[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 metal, which are processed into foils, meshes, expanded grids, punched metals, and the like.

[0019] The carbon particles in the positive electrode composite layer are bonded by a binder, and adjacent carbon particles are in contact with each other. With such a configuration, a conductive network of carbon particles connecting to the positive electrode current collector is formed in the positive electrode composite layer.

[0020] The carbon particles in the positive electrode composite layer preferably have an average pore diameter of 0.1 nm or more and 20 nm or less, more preferably 1 nm or more and 15 nm or less, still more preferably 3 nm or more and 10 nm or less, and most preferably 4 nm or more and 8 nm or less. When the average pore diameter is less than 0.1 nm, there is a risk that it may be difficult for sulfur to penetrate into the pores of the carbon particles. On the other hand, when the average pore diameter exceeds 20 nm, the contact between the carbon particles and the first sulfur present in a plurality of pores of the carbon particles described later becomes insufficient, and a good conductive path between the carbon particles and sulfur does not function effectively, and there is a risk that the utilization rate of sulfur may decrease. Further, the carbon particles in the positive electrode composite layer have a specific surface area of 200 m 2 ·g -1 or more and 2500 m 2 ·g -1 or less, preferably 1000 m 2 ·g -1 or more and 2200 m 2 ·g -1 or less. Furthermore, the carbon particles in the positive electrode composite layer preferably have a pore volume of 0.5 cm 3 ·g -1 or more. The upper limit value of the pore volume is not particularly limited, but in reality, it is 5.0 cm 3 ·g -1 or less. The sulfur-carbon composite positive electrode in which the specific surface area and pore volume of the carbon particles in the positive electrode composite layer are within this range has excellent rate characteristics and cycle characteristics, and further has a smaller polarization. The average pore diameter and specific surface area of the carbon particles can be measured by the nitrogen adsorption-desorption method, and the pore volume can be measured by the mercury porosimeter method.

[0021] The carbon particles can be any known type, specifically, one or more mixtures selected from the group including Ketjenblack, carbon nanotubes, graphene, acetylene black, and porous carbon (e.g., CNovel® (Toyo Tanso Co., Ltd.)). When using a mixture, the combination and ratio of these can be arbitrarily selected depending on the purpose.

[0022] The sulfur contained in the positive electrode composite layer consists of a first type of sulfur present in multiple pores of carbon particles located on the surface and within the positive electrode composite layer, and a second type of sulfur present on the surface of carbon particles, excluding the contact points between carbon particles, and located on the surface and within the positive electrode composite layer. In other words, the first and second types of sulfur are present in the positive electrode composite layer without impairing the conductive network formed by the carbon particles.

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

[0024] The first sulfur is present within multiple pores of the carbon particles, which act as a conductive additive, enabling a good conductive path to be established between the carbon particles and the sulfur. The second sulfur is present, for example, in a thin film on the surface of the carbon particles (excluding the contact areas between carbon particles). As a result, the contact area between the first and second sulfur and the carbon particles can be increased, thereby improving the contribution rate of the sulfur present in the cathode composite layer to the oxidation-reduction reaction. In other words, the utilization rate of sulfur in the cathode composite layer can be significantly increased compared to conventional sulfur-carbon composite cathodes, enabling higher capacity.

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

[0026] The binder included in the positive electrode composite layer is not particularly limited and known binders can be used. Specifically, one or more mixtures selected from the group including 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), carboxymethylcellulose (CMC), polyacrylonitrile (PAN), and polyimide (PI) can be used. When using mixtures, their combinations and ratios can be arbitrarily selected depending on the purpose.

[0027] The sulfur-carbon positive electrode according to the first embodiment described above comprises 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 together by the binder, and adjacent carbon particles are in contact with each other. The sulfur consists of first sulfur present in the plurality of pores of carbon particles located on the surface and inside the positive electrode composite layer, and second sulfur present on the surface of carbon particles, excluding the contact points between carbon particles, located on the surface and inside the positive electrode composite layer. Therefore, when incorporated into a lithium-sulfur secondary battery, it can achieve high capacity and high cycle characteristics.

[0028] In other words, in the positive electrode composite layer, adjacent carbon particles come into contact with each other to form a conductive network, and this conductive network of carbon particles comes into contact with the positive electrode current collector that holds the positive electrode composite layer. The first sulfur is present in multiple pores of carbon particles located on the surface and inside the positive electrode composite layer. That is, the first sulfur is trapped throughout the multiple pores of the carbon particles that form the conductive network, and as a result is in close contact with the carbon particles. The second sulfur is located on the surface and inside the positive electrode composite layer and is present on the surface of the carbon particles, excluding the contact points between carbon particles. That is, the second sulfur is also in close contact with the surface of the carbon particles that form the conductive network, for example, by covering it. As a result, both the first and second sulfur particles do not become isolated from the carbon particles forming the conductive network, allowing for good conductive paths to be obtained. Therefore, most of the sulfur present in the cathode composite layer contributes to the redox reaction, improving its utilization rate. Consequently, a high-capacity sulfur-carbon cathode can be realized.

[0029] Furthermore, in a lithium-sulfur secondary battery incorporating the sulfur-carbon cathode according to the first embodiment, the sulfur contained in the cathode composite layer generates lithium polysulfide, a reaction intermediate, during charging and discharging, as described above. Lithium polysulfide has the property of being easily soluble in non-aqueous electrolytes. The second sulfur contained in the cathode composite layer is present on the surface of the cathode layer and on the surface of the carbon particles located inside the cathode layer. Therefore, when it is converted to lithium polysulfide during charging and discharging, it may dissolve upon contact with the non-aqueous electrolyte. On the other hand, the first sulfur is present in multiple pores of the carbon particles located on the surface of the cathode composite layer and inside the cathode composite layer. In other words, it is sealed throughout the multiple pores of the carbon particles. Therefore, even if the first sulfur is converted to lithium polysulfide during charging and discharging, it remains in the multiple pores of the carbon particles, and desorption due to contact with the non-aqueous electrolyte can be suppressed. As a result, the decrease in sulfur in the cathode composite layer can be suppressed by repeated charging and discharging, thus suppressing the decrease in the capacity retention rate of the lithium-sulfur secondary battery. In other words, compared to the Lithium Sulfur secondary battery described in Patent Document 1 and Non-Patent Document 1 in the background technology section, the lifespan of the Lithium Sulfur secondary battery can be improved and high cycle characteristics can be achieved.

[0030] Next, a method for manufacturing a sulfur-carbon composite cathode according to the first embodiment will be described. (1) A carbon electrode is prepared by forming a carbon particle-containing layer, which includes carbon particles having multiple pores and a binder, on at least one surface of a positive electrode current collector. (2) Prepare an electrolytic solution containing a sulfur compound and a solvent. (3) The carbon-containing layer of the carbon electrode is impregnated with an electrolytic solution. (4) As shown in Figure 1, the carbon electrode 2, lithium ion conductor 4, and lithium metal electrode 3 are stacked in this order, and the carbon particle-containing layer 22 is facing the lithium ion conductor 4 to assemble the electrolytic cell 1. (5) A carbon electrode is used as the positive electrode and a lithium metal electrode as the negative electrode. A DC voltage is applied between these electrodes to electrolyze sulfur from the electrolytic solution present in the multiple pores of the carbon particles in the carbon particle-containing layer, as well as on the surface and in the voids of the carbon particle-containing layer. In this process, sulfur compounds (e.g., Li2S8) in the electrolytic 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, and sulfur is electrolyzed. The positive lithium ions pass through a lithium ion conductor and are attracted to the lithium metal electrode, which is the negative electrode, and lithium is electrolyzed. (6) The electrolytic emission cell is disassembled, the carbon electrode from which sulfur has been deposited is removed, washed, and then dried to produce a sulfur-carbon composite cathode.

[0031] In step (1) above, the carbon-containing layer can be formed by dissolving and dispersing carbon particles and a binder in a solvent to prepare a slurry, coating the slurry onto at least one surface of the positive electrode current collector, and then drying it. It is preferable to minimize the amount of binder contained in the carbon-containing layer while maintaining the shape of the carbon-containing layer. For example, it is preferable to have 65% to 95% by weight of carbon particles and 5% to 35% by weight of binder; more preferably 70% to 90% by weight of carbon particles and 10% to 30% by weight of binder; and even more preferably 70% to 85% by weight of carbon particles and 15% to 30% by weight of binder. By setting the binder content to 5% to 35% by weight, it is possible to maintain the shape of the carbon-containing layer while reducing the blockage of the pores of the carbon particles by the binder, and to allow the electrolytic solution to penetrate into more of the pores of the carbon particles.

[0032] Examples of sulfur compounds used in step (2) above include one or more mixtures selected from the group consisting of Li2S8, Li2S6, Li2S4, Li2S2, and Li2S.

[0033] The solvent used in step (2) above is preferably a compound that does not decompose easily by electrolysis, and examples include sulfone compounds such as sulfolanes and nitrile compounds such as acetonitrile.

[0034] The electrolytic solution used in step (2) above preferably has a sulfur compound concentration of 0.1% by weight or more and 50% by weight or less, more preferably 1.0% by weight or more and 40% by weight or less, and even more preferably 2.0% by weight or more and 30% by weight or less. If the concentration of the electrolytic solution exceeds 50% by weight, the viscosity of the electrolytic solution will increase, which may make it difficult for the electrolytic solution to penetrate into the pores of the carbon particles. In addition, the particle size of the precipitated sulfur will increase, which may result in insufficient contact between the carbon particles and the sulfur, preventing the conductive path from working effectively and potentially reducing the utilization rate of sulfur. On the other hand, if the concentration of the electrolytic solution is less than 0.1% by weight, the electrodeposition efficiency of sulfur will decrease, which may make it impossible to secure an appropriate amount of sulfur electrolytically deposited.

[0035] The lithium metal electrode used in step (4) above has a structure in which a lithium metal foil is bonded to a copper current collector, for example. The lithium ion conductor used in step (4) above is not particularly limited; any material that conducts only lithium ions is acceptable. Specifically, Li7La3Zr2O 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 Examples include oxide-based solid electrolytes and cation exchange membranes.

[0036] In the electrolytic discharge of step (5) above, from the viewpoint of precipitating small-particle sulfur onto carbon particles, the current density of the DC current applied between the carbon electrode and the lithium metal electrode is 0.01 mA·cm². -2 1.0mA cm or more -2 The following is preferable: 0.01 mA·cm -2 More than 0.5mA cm -2 The following is more preferable. Furthermore, the cell voltage (target voltage) is preferably 2.6V to 3.5V under these current density conditions.

[0037] According to the manufacturing method of the first embodiment described above, an electrolytic extraction solution containing a sulfur compound and a solvent is impregnated into the carbon-containing layer, and sulfur can be electrolytically extracted to the carbon particles located on the surface and inside the carbon-containing layer by ionization of the sulfur compound in the electrolytic extraction solution impregnated into the carbon-containing layer. As a result, a sulfur-carbon cathode that functions as a cathode composite layer can be obtained from the carbon-containing layer of the carbon electrode removed from the electrolytic extraction cell, because sulfur has been electrolytically extracted from it. The carbon particles in the cathode composite layer are bound together by the binder because the morphology of the carbon-containing layer is maintained, and adjacent carbon particles can come into contact with each other to form a conductive network. Furthermore, the electrolytically extracted sulfur is first sulfur present in multiple pores of carbon particles located on the surface and inside the cathode composite layer, and second sulfur present on the surface of carbon particles located on the surface and inside the cathode composite layer, excluding the contact areas between carbon particles.

[0038] Therefore, the obtained sulfur-carbon electrode can achieve high capacity and high cycle characteristics through 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 comprises the aforementioned sulfur-carbon composite positive electrode, a negative electrode, a separator interposed between the positive and negative electrodes, and a non-aqueous electrolyte.

[0040] The negative electrode comprises 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 from, for example, copper.

[0042] The negative electrode active material can be metallic lithium or a lithium alloy. Examples of lithium alloys include lithium aluminum alloy, lithium tin alloy, lithium lead alloy, and lithium silicon alloy. The negative electrode active material can also be one or more carbon materials selected from the group 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 using two or more carbon materials, their combination and ratio can be arbitrarily selected according to the purpose.

[0043] If the negative electrode active material is metallic lithium or a lithium alloy, the negative electrode can be manufactured by forming a negative electrode layer by attaching it in the form of a foil to one or both sides of the negative electrode current collector.

[0044] On the other hand, if 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 can be applied to one or both sides 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 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 any known salt can be used, specifically one or more selected from the group including lithium hexafluoride phosphate (LiPF6), lithium bromide (LiBr), lithium perchlorate (LiClO4), lithium bisoxalate borate (LiB(C2O4)), lithium borofluoride (LiBF4), lithium nitrate (LiNO3), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), etc. When a mixture is used, the combination and ratio can be arbitrarily selected depending on the purpose.

[0046] The non-aqueous solvent is not particularly limited and any known solvent can be used, specifically including 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. Sodium, 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- Examples include pyrpyrrolidinium bis(trifluoromethanesulfonyl)imide, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide, tributyldodecylphosphonium bis(trifluoromethanesulfonyl)imide, methyltributylammonium methyl sulfate, butyltrimethylammonium bis(trifluoromethanesulfonyl)imide, and trimethylhexylammonium bis(trifluoromethanesulfonyl)imide.The non-aqueous solvent can be one or more mixtures selected from the above group, and if a mixture is used, the combination and ratio thereof can be arbitrarily selected according to the purpose.

[0047] The separator can be an organic polymer-based separator or an inorganic separator that does not react with the positive electrode, negative electrode, and non-aqueous electrolyte. Examples of polymers constituting the organic polymer-based 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 including ceramic coating and structural control. When two or more treatments are performed, the combination and ratio thereof can be arbitrarily selected according to the purpose.

[0048] The positive electrode, negative electrode, separator, and non-aqueous electrolyte are housed within the outer casing. The outer casing is not particularly limited, but examples include a bag-shaped casing with a laminate film, a coin-shaped metal can, a cylindrical metal can, and a rectangular metal can.

[0049] The structure of a lithium-sulfur secondary battery according to this 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 comprises a sulfur-carbon composite positive electrode 110, a negative electrode 120, and a separator 130 positioned between the positive electrode 110 and the negative electrode 120. These positive electrode 110, negative electrode 120, and separator 130 are housed in an outer casing (not shown). The sulfur-carbon composite positive electrode 110 consists of a positive electrode current collector 111 and a positive electrode composite layer 112 containing carbon particles, sulfur, and a binder, provided on the surface of the positive electrode current collector 111 facing the separator 130. The negative electrode 120 consists of a negative electrode current collector 121 and a negative electrode layer 122 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 providing the sulfur-carbon composite cathode according to the first embodiment described above, a lithium sulfur secondary battery with high capacity and high cycle characteristics can be provided. [Examples]

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

[0053] (Example 1) A slurry was prepared by kneading Ketjenblack (hereinafter abbreviated as KB), a carbon particle material, and carboxymethylcellulose (hereinafter abbreviated as CMC), a binder, in ultrapure water in a weight ratio (KB:CMC) of 70:30. The average pore diameter and specific surface area of ​​Ketjenblack were measured by nitrogen adsorption / desorption, and the pore volume was measured by mercury porosimeter. The average pore diameter was found to be 4.730 nm, and the specific surface area was 1323 m². 2 ·g -1 The pore volume is 1.565 cm³. 3 ·g -1 The obtained slurry was applied to the surface of the aluminum foil, which was the 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, thereby fabricating a carbon electrode. Sulfur and lithium sulfide (Li2S) were dissolved in sulfolane in a molar ratio (sulfur:Li2S) of 7:8, and stirred overnight at 60°C to prepare an electrolytic solution containing Li2S8 with a Li2S8 concentration of 0.1 wt%. The prepared electrolytic solution was impregnated into the carbon electrode.

[0054] Next, the lithium metal electrode and the lithium ion conductor Li7La3Zr2O 12 An electrolytic cell was assembled by stacking a carbon electrode impregnated with electrolytic 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, and a current density of 0.1 mAcm² was applied to these electrodes. -2A DC voltage was applied until the cell voltage reached 2.8V, causing sulfur to be electrolytically deposited in the carbon-containing layer of the carbon electrode. The cell was then disassembled, the sulfur-deposited carbon electrode was removed, thoroughly washed with dimethyl ether (DME), and dried to produce a sulfur-carbon composite cathode containing the electrolytically deposited sulfur-containing carbon layer (cathode composite layer). The sulfur content per unit area of ​​the cathode was 1.5 mg·cm². -2 That's what I decided.

[0055] (Example 2) A sulfur-carbon composite cathode was prepared in the same manner as in Example 1, except that the concentration of the sulfur compound in the electrolytic solution was set to 5% by weight.

[0056] (Example 3) A sulfur-carbon composite cathode was prepared in the same manner as in Example 1, except that the concentration of the sulfur compound in the electrolytic efflux solution was set to 10% by weight.

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

[0058] (Example 5) A sulfur-carbon composite cathode was prepared in the same manner as in Example 1, except that acetonitrile was used as the solvent for the electrolytic deposition solution.

[0059] (Example 6) A sulfur-carbon composite cathode was prepared in the same manner as in Example 1, except that acetonitrile was used as the solvent for the electrolytic deposition solution and the concentration of the sulfur compound was 5% by weight.

[0060] (Example 7) A sulfur-carbon composite cathode was prepared in the same manner as in Example 1, except that acetonitrile was used as the solvent for the electrolytic deposition solution and the concentration of the sulfur compound was set to 10% by weight.

[0061] (Example 8) A sulfur-carbon composite cathode was prepared in the same manner as in Example 1, except that acetonitrile was used as the solvent for the electrolytic deposition solution and the concentration of the sulfur compound was set to 50% by weight.

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

[0063] (Example 10) A sulfur-carbon composite cathode was prepared in the same manner as in Example 1, except that a mixture of Li2S6 and Li2S8 with a weight ratio of 50:50 (Li2S6:Li2S8) was used as the sulfur compound in the electrolytic solution, and the concentration of the sulfur compound was set to 50% by weight.

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

[0065] (Comparative Example 1) A sulfur-carbon composite was obtained by a melt impregnation method, in which sulfur and KB were dry-mixed using a mortar and pestle in a weight ratio (sulfur:KB) of 75:25, and this mixture was heated in an inert gas atmosphere at 155°C for 12 hours to allow the molten sulfur to be absorbed by the KB. Next, the sulfur-carbon composite, KB, and CMC were kneaded in ultrapure water in a weight ratio (sulfur-carbon composite:KB:CMC) of 80:10:10 to prepare a slurry. The obtained slurry was applied to the surface of the aluminum foil, which is the positive electrode current collector, to a thickness of 50 μm, and vacuum-dried overnight at 80°C to form a positive electrode composite layer, thereby producing a sulfur-carbon composite positive electrode. The sulfur content per unit area of ​​the positive electrode was 1.5 mg·cm². -2 That's what I decided.

[0066] (Comparative Example 2) A sulfur-carbon composite was prepared by mechanical milling, dry-mixing sulfur and KB (carbon dioxide) using a mortar and pestle in a weight ratio (sulfur:KB) of 75:25. The obtained sulfur-carbon composite, KB, and CMC were 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 the aluminum foil, which served as the positive electrode current collector, to a thickness of 50 μm, and vacuum-dried overnight at 80°C to form a positive electrode composite layer, thereby producing a sulfur-carbon composite positive electrode. The sulfur content per unit area of ​​the positive electrode was 1.5 mg·cm². -2 That's what I decided.

[0067] The electrochemical properties were evaluated using evaluation cells having the structure described below. The evaluation cells were prepared using the sulfur-carbon composite cathodes of Examples 1-11 and Comparative Examples 1 and 2, respectively. Specifically, a polyimide separator was interposed between the sulfur-carbon composite cathode and the lithium metal electrode, and a non-aqueous electrolyte was filled into the separator to assemble a bielectrode cell. The non-aqueous electrolyte consisted of 1 mol·dm³ of lithium bis(trifluoromethane)sulfonimide (LiTFSI), a lithium salt. -3 It was prepared by dissolving it in sulfolane to achieve the desired result.

[0068] <Measurement of discharge capacity> Discharge tests were conducted on the evaluation cells of Examples 1-11 and Comparative Examples 1 and 2. The test temperature was 80°C, and the current density was 1.0 mAcm². -2 The device was discharged until the voltage reached 1.5V, and the discharge capacity, normalized by the amount of sulfur contained in the positive electrode composite layer of the sulfur-carbon composite cathode, was measured. The results are shown in Table 1 below.

[0069] <Evaluation test of charge / discharge cycle characteristics> The evaluation cells for Examples 1-11 and Comparative Examples 1 and 2 underwent evaluation tests of their charge-discharge cycle characteristics. The test temperature was set to 80°C, and the current density was 1.0 mA·cm². -2The battery was discharged until the voltage reached 1.5V, and then charged to 2.8V with a current density equivalent to the discharge current. This charge-discharge cycle was considered one cycle, and the charge-discharge cycle was repeated 100 times to measure the discharge capacity of the evaluation cell. Based on the measured discharge capacity of the evaluation cell in the first cycle and the discharge capacity in the 100th cycle, the discharge capacity retention rate was calculated using the following formula. Discharge capacity retention rate (%) = (Discharge capacity at 100 cycles / Discharge capacity at 1 cycle) × 100

[0070] [Table 1]

[0071] As is clear from Table 1, the evaluation cells (Examples 1-11) using sulfur-carbon composite cathodes fabricated by electrolytic emission all had a discharge capacity of 1300 mAh·g normalized for the amount of sulfur. -1 In addition to the above, it demonstrated a large discharge capacity and a high discharge capacity retention rate of over 61%. This is because, in the sulfur-carbon composite cathode fabricated by electrolysis, adjacent carbon particles in the cathode composite layer come into contact with each other to form a conductive network, and the sulfur consists of a first sulfur present in multiple pores of carbon particles located on the surface and inside the cathode composite layer, and a second sulfur present on the surface of carbon particles located on the surface and inside the cathode composite layer.

[0072] The evaluation cells equipped with sulfur-carbon composite cathodes, Comparative Example 1 (fabricated by melt impregnation) and Comparative Example 2 (fabricated by mechanical milling), both had a discharge capacity normalized for the amount of sulfur of 1100 mAh·g. -1 The following characteristics were observed: a small discharge capacity and a low discharge capacity retention rate of 35% or less, indicating inferior discharge capacity and cycle characteristics when sulfur content is normalized. The invention described in the original claims of this application is listed below. [1] A sulfur-carbon composite positive electrode comprising a positive electrode current collector and a positive electrode composite material layer formed on one or both surfaces of the positive electrode current collector, The positive electrode composite layer comprises carbon particles having multiple pores, sulfur, and a binder. The carbon particles are bound together by the binder, and adjacent carbon particles are in contact with each other. The sulfur-carbon composite cathode is characterized in that the sulfur is a first sulfur present in a plurality of pores of the carbon particles located on the surface and inside the cathode composite layer, and a second sulfur present on the surface of the carbon particles, excluding the contact areas between carbon particles, located on the surface and inside the cathode composite layer. [2] The sulfur-carbon composite cathode of the carbon particles is characterized in that the average pore diameter of the pores is 0.1 nm or more and 20 nm or less [1]. [3] The sulfur-carbon composite cathode according to [1] or [2], characterized in that the carbon particles are Ketjenblack. [4] A lithium-sulfur secondary battery characterized by comprising one of the sulfur-carbon composite positive electrode, negative electrode, separator, and non-aqueous electrolyte according to [1] to [3]. [5] A carbon electrode is fabricated by forming a carbon particle-containing layer containing carbon particles having multiple pores and a binder on at least one surface of the positive electrode current collector. Prepare an electrolytic solution containing sulfur compounds and a solvent. The carbon particle-containing layer of the carbon electrode is impregnated with the electrolytic solution. Assembling an electrolytic cell by stacking a lithium metal electrode, a lithium ion conductor, and the carbon electrode in this order, and such that the carbon particle-containing layer faces the lithium ion conductor. A method for producing a sulfur-carbon composite positive electrode, characterized by using the carbon electrode as the positive electrode and the lithium metal electrode as the negative electrode, applying a DC voltage to these electrodes, and electrolytically extracting sulfur from the electrolytic solution present in the plurality of pores of the carbon particles in the carbon particle-containing layer and on the surface and voids of the carbon particle-containing layer. [6] The aforementioned sulfur compound is Li 2 S 8 Li 2 S 6 Li 2 S 4 Li 2 S 2 and Li 2 A method for producing a sulfur-carbon composite cathode, characterized in that it is at least one selected from the group consisting of S.[5] [7] A method for producing a sulfur-carbon composite cathode according to [5] or [6], characterized in that the solvent is a nitrile-based or sulfone-based organic solvent. [8] A method for producing a sulfur-carbon composite cathode according to any of [5] to [7], characterized in that the concentration of the sulfur compound contained in the electrolytic solution is 50% by weight or less. [Explanation of symbols]

[0073] 1...Electrode output 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 carbon electrode is manufactured by 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. Prepare an electrolytic solution containing sulfur compounds and a solvent. The carbon particle-containing layer of the carbon electrode is impregnated with the electrolytic solution. Assembling an electrolytic cell by stacking a lithium metal electrode, a lithium ion conductor, and the carbon electrode in this order, and such that the carbon particle-containing layer faces the lithium ion conductor. A method for producing a sulfur-carbon composite positive electrode, characterized by using the carbon electrode as the positive electrode and the lithium metal electrode as the negative electrode, applying a DC voltage to these electrodes, and electrolytically extracting sulfur from the electrolytic solution present in the plurality of pores of the carbon particles in the carbon particle-containing layer and on the surface and voids of the carbon particle-containing layer.

2. The method for producing a sulfur-carbon composite cathode according to Claim 1, characterized in that the sulfur compound is at least one selected from the group consisting of Li₂S₄, Li₂S₆, Li₂S₄, Li₂S₂, and Li₂S.

3. The method for producing a sulfur-carbon composite cathode according to claim 1 or 2, characterized in that the solvent is a nitrile-based or sulfone-based organic solvent.

4. The method for producing a sulfur-carbon composite cathode according to Claim 1, characterized in that the concentration of the sulfur compound contained in the electrolytic solution is 50% by weight or less.

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