Carbon material for lithium-sulfur battery and method for producing same

Abstract

Provided is a method for producing a carbon material for a lithium-sulfur battery, with which it is possible to form a battery having improved charge / discharge capacity. The method for manufacturing the carbon material for the lithium-sulfur battery comprises the following steps: performing wet grinding treatment on a mixture containing a first carbon material, a dispersing agent and a liquid medium to obtain slurry; removing at least a portion of the liquid medium from the slurry to obtain a second carbon material; heat-treating the second carbon material to obtain a third carbon material; and activating the third carbon material to obtain a fourth carbon material.

Description

Technical Field

[0001] The present invention relates to a carbon material for lithium-sulfur batteries and a method for manufacturing the same. Background Art

[0002] Lithium-sulfur batteries are secondary batteries with high energy density per unit mass, for example, using mesoporous sulfur-carbon composites in which sulfur is arranged in the mesopores. Lithium-sulfur batteries are lighter than conventional lithium-ion batteries and are expected to be used in large power machines such as electric vehicles and in aviation applications such as drones. In these applications, it is hoped that the cycle capacity retention rate and the charge and discharge capacity will be improved.

[0003] Regarding the above, for example, Japanese Patent Application Laid-Open No. 2015-220225 proposes a positive electrode material in which lithium sulfide and a carbon material are composited. Summary of the invention

[0004] Problems to be solved by the invention

[0005] An object of one embodiment of the present disclosure is to provide a carbon material for a lithium-sulfur battery capable of forming a battery with improved charge and discharge capacity, and a method for producing the same.

[0006] Solutions to the problem

[0007] The first embodiment is a method for producing a carbon material for a lithium-sulfur battery, the method comprising: subjecting a mixture containing a first carbon material, a dispersant and a liquid medium to a wet pulverization treatment to obtain a slurry; removing at least a portion of the liquid medium from the slurry to obtain a second carbon material; subjecting the second carbon material to a heat treatment to obtain a third carbon material; and subjecting the third carbon material to an activation treatment to obtain a fourth carbon material.

[0008] The second embodiment is a carbon material for a lithium-sulfur battery, wherein the average primary particle size is 1.5 μm or less, the peak pore size is 1.0 nm or more and 5.0 nm or less, the pore volume is 0.6 ml / g or more, and the 90% particle size D in the cumulative particle size distribution based on volume is 90 Relative to 10% particle size D 10 Ratio D 90 / D 10 More than 60.

[0009] The third embodiment is a positive electrode material for a lithium-sulfur battery, comprising the carbon material for a lithium-sulfur battery according to the second embodiment and sulfur.

[0010] A fourth embodiment is a positive electrode for a lithium-sulfur battery, comprising a current collector and a positive electrode active material layer disposed on the current collector and containing the positive electrode material for a lithium-sulfur battery according to the third embodiment.

[0011] A fifth embodiment is a positive electrode for a lithium-sulfur battery, comprising a current collector and a positive electrode active material layer disposed on the current collector. The positive electrode active material layer comprises a carbon material for a lithium-sulfur battery and a sulfur-containing positive electrode material for a lithium-sulfur battery having an average primary particle size of 1.5 μm or less, and a porosity of 10.0% or less.

[0012] Effects of the Invention

[0013] According to one embodiment of the present disclosure, a carbon material for a lithium-sulfur battery capable of forming a battery having improved charge and discharge capacity and a method for producing the same can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a diagram showing an example of a scanning electron microscope (SEM) image of the carbon material of Example 1.

[0015] Figure 2 This is a diagram showing an example of a scanning electron microscope (SEM) image of the carbon material of Example 2.

[0016] Figure 3 This is a diagram showing an example of a scanning electron microscope (SEM) image of the carbon material of Example 3.

[0017] Figure 4 This is a diagram showing an example of a scanning electron microscope (SEM) image of the carbon material of Example 4.

[0018] Figure 5 This is a diagram showing an example of a scanning electron microscope (SEM) image of the carbon material of Example 5.

[0019] Figure 6 This is a diagram showing an example of a scanning electron microscope (SEM) image of the carbon material of Comparative Example 1.

[0020] Figure 7 This is a diagram showing an example of a scanning electron microscope (SEM) image of the carbon material of Comparative Example 2.

[0021] Figure 8 This is a diagram showing an example of a scanning electron microscope (SEM) image of the carbon material of Comparative Example 3.

[0022] Fig. 9 This is a diagram showing an example of a scanning electron microscope (SEM) image of the carbon material of Comparative Example 4.

[0023] Fig.10 This is a graph showing the particle size distribution of the carbon material of Example 1.

[0024] Fig.11 3 is a graph showing the particle size distribution of the carbon material of Comparative Example 1.

[0025] Fig.12 3 is a graph showing the particle size distribution of the carbon material of Comparative Example 2.

[0026] Fig.13 This is a diagram showing an example of a cross-sectional SEM image of the positive electrode of Example 1.

[0027] Fig.14 This is a diagram showing an example of a cross-sectional SEM image of the positive electrode of Example 2.

[0028] Fig.15 This is a diagram showing an example of a cross-sectional SEM image of the positive electrode of Example 3.

[0029] Fig.16 This is a diagram showing an example of a cross-sectional SEM image of the positive electrode of Example 4.

[0030] Fig.17 This is a diagram showing an example of a cross-sectional SEM image of the positive electrode of Example 5.

[0031] Fig.18 This is a diagram showing an example of a cross-sectional SEM image of the positive electrode of Comparative Example 1.

[0032] Fig.19 This is a diagram showing an example of a cross-sectional SEM image of the positive electrode of Comparative Example 2.

[0033] Fig. 20 This is a diagram showing an example of a cross-sectional SEM image of the positive electrode of Comparative Example 3.

[0034] Fig.21 This is a diagram showing an example of a cross-sectional SEM image of the positive electrode of Comparative Example 4. DETAILED DESCRIPTION

[0035] In this specification, the term "process" is not only an independent process, but also included in this term as long as the intended purpose of the process can be achieved when it cannot be clearly distinguished from other processes. In addition, when there are multiple substances corresponding to each component in the composition, unless otherwise specified, the content of each component in the composition represents the total amount of the multiple substances present in the composition. Furthermore, the upper and lower limits of the numerical ranges recorded in this specification can be arbitrarily selected and combined with the numerical values ​​exemplified as numerical ranges. Below, the embodiments of the present invention are described based on the pictures. However, the carbon material for lithium-sulfur battery and the method for producing the same illustrated in the embodiments shown below are for the purpose of concretizing the technical idea of ​​the present invention, and the present invention is not limited to the carbon material for lithium-sulfur battery and the method for producing the same illustrated below.

[0036] Method for producing carbon material for lithium-sulfur battery

[0037] The method for producing a carbon material for a lithium-sulfur battery (hereinafter sometimes simply referred to as "carbon material") includes: a first step of wet-grinding a mixture containing a first carbon material, a dispersant, and a liquid medium to obtain a slurry; a second step of removing at least a portion of the liquid medium from the slurry to obtain a second carbon material; a third step of heat-treating the second carbon material to obtain a third carbon material; and a fourth step of activating the third carbon material to obtain a fourth carbon material. The fourth carbon material obtained may include, for example, the intended carbon material for a lithium-sulfur battery, or may be a carbon material for a lithium-sulfur battery.

[0038] By wet-crushing the first carbon material as a raw material in the presence of a dispersant, and activating the third carbon material obtained by heat treatment after drying, a fourth carbon material having a smaller particle size, a wider particle size distribution, and an increased pore volume than the first carbon material as a raw material can be obtained. When the obtained carbon material is used to form an electrode, a dense active material layer can be formed. In addition, the peak pore diameter exists in a specified range, and by increasing the pore volume, the reactivity of sulfur as an active material is improved, and the sulfur dissolution accompanying charging and discharging can be suppressed. It is believed that the charge and discharge capacity of the lithium-sulfur battery can be increased.

[0039] In the first step, a mixture including a first carbon material, a dispersant, and a liquid medium is subjected to a wet pulverization treatment to obtain a slurry.

[0040] In the cumulative particle size distribution based on volume, the 50% particle size D of the first carbon material is 50 For example, the particle size may be 1 μm or more and 1000 μm or less, preferably 2 μm or more or 4 μm or more, and preferably 100 μm or less or 10 μm or less. The 50% particle size D of the first carbon material 50 It refers to the particle size corresponding to 50% of the cumulative volume from the smaller particle size side in the volume-based cumulative particle size distribution, and is measured, for example, using a laser diffraction particle size distribution analyzer under wet conditions.

[0041] The specific surface area of ​​the first carbon material may be, for example, 100 m 2 / g or more, preferably 500m 2 / g or more, 1000m 2 / g or above, or 1500m 2 The upper limit of the specific surface area is, for example, 4000 m 2 / g or below, or 1800m 2 The specific surface area of ​​the first carbon material is measured by the BET method based on the BET (Brunauer Emmett Teller) theory. Specifically, for example, it is measured by a multipoint method using nitrogen gas.

[0042] The peak pore diameter of the first carbon material can be, for example, 1.0 nm or more and 5.0 nm or less, preferably 1.4 nm or more, 1.6 nm or more, or 2.0 nm or more, and preferably 4.0 nm or less, 3.0 nm or less, or 2.5 nm or less. When the peak pore diameter of the first carbon material is within the above range, there is a tendency to obtain a fourth carbon material having a peak pore diameter that improves the charge and discharge capacity of the lithium-sulfur battery. Here, the peak pore diameter refers to the pore diameter showing the maximum peak in the pore distribution curve obtained by nitrogen adsorption and desorption measurement based on the quenched solid density functional theory (QSDFT method).

[0043] The pore volume of the first carbon material can be, for example, 0.1 ml / g or more and 10 ml / g or less, preferably 0.3 ml / g or more, or 0.5 ml / g or more, and preferably 5 ml / g or less, 1 ml / g or less, or 0.8 ml / g or less. The pore volume of pores having a diameter of 1.4 nm or more and 5.0 nm or less is particularly preferably within the above range. When the pore volume of the first carbon material is within the above range, there is a tendency to obtain a fourth carbon material having a peak pore diameter that improves the charge and discharge capacity of the lithium-sulfur battery. The pore volume of the first carbon material is measured, for example, by nitrogen adsorption and desorption measurement.

[0044] The carbon content in the first carbon material may be, for example, 80% by mass or more, preferably 90% by mass or more, or 95% by mass or more, and may be substantially composed of only carbon. The term “substantially” here means that other elements that are inevitably mixed in are not excluded.

[0045] Specific examples of the first carbon material include activated carbon, carbon black, etc. The first carbon material may contain at least activated carbon.

[0046] The content of the first carbon material in the mixture relative to the total mass of the mixture may be, for example, 1% by mass or more and 50% by mass or less, preferably 2% by mass or more, or 3% by mass or more, and preferably 10% by mass or less, or 5% by mass or less. When the content of the first carbon particles is within the above range, wet pulverization can be performed more efficiently.

[0047] As the dispersant constituting the mixture, there can be mentioned: thickeners such as polyvinyl alcohol, polyethylene glycol, carboxymethyl cellulose, carboxyethyl cellulose, starch, regenerated cellulose, ethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, and hydroxypropyl cellulose; surfactants such as alkyl sulfate salts, alkylbenzene sulfonic acid, fat-soluble alkylbenzene sulfonic acid, α-olefin sulfonic acid, sodium alkylbenzene sulfonate, fat-soluble alkylbenzene sulfonate, and α-olefin sulfonate. The number average molecular weight of polyvinyl alcohol can be, for example, 900 to 100,000. Preferably, it is 5,000 to 15,000. The saponification degree of polyvinyl alcohol can be, for example, 50 mol% or more, preferably 60 mol% or more, 70 mol% or more, or 80 mol% or more.

[0048] The dispersant in the mixture may contain at least a thickener. Since the dispersant contains a thickener, there is a tendency that wet grinding can be performed more efficiently. The content of the dispersant in the mixture, relative to the total mass of the mixture, can be, for example, 0.5% by mass or more and 10% by mass or less, preferably 1% by mass or more, or 1.5% by mass or more, and preferably 5% by mass or less, or 3% by mass or less. In addition, relative to the total mass of the first carbon material, the content of the dispersant can be, for example, 10% by mass or more and 100% by mass or less, preferably 30% by mass or more, or 40% by mass or more, and preferably 80% by mass or less, or 60% by mass or less. When the content of the dispersant is within the above range, wet grinding can be performed more efficiently.

[0049] The liquid medium constituting the mixture may contain at least water. In addition to water, the liquid medium may contain a water-soluble organic solvent as required. Examples of the water-soluble organic solvent include: alcohol solvents such as methanol, ethanol, and isopropanol; nitrile solvents such as acetonitrile; ketone solvents such as acetone and methyl ethyl ketone; amide solvents such as N-methyl-2-pyrrolidone and N,N-dimethylformamide. The content of water in the liquid medium relative to the total volume of the liquid medium may be, for example, 10% by volume or more, preferably 50% by volume or more, or 90% by volume or more, and may actually be only water.

[0050] The mixture may further contain other components as needed. As other components, for example, pH adjusters, surfactants, metal oxides, etc. may be mentioned. As pH adjusters, inorganic acids such as hydrochloric acid, nitric acid, phosphoric acid, and sulfuric acid may be mentioned; organic acids such as acetic acid, citric acid, and oxalic acid may be mentioned; inorganic bases such as alkali metal hydroxides and alkaline earth metal hydroxides; ammonia, etc. may be mentioned. As metal oxides, oxides containing transition metals such as titanium oxide and tungsten oxide may be mentioned. In addition, metal oxides may also contain compounds complexed with alkali metals, alkaline earth metals, etc. As complexed compounds, for example, lithium titanate, lithium tungstate, etc. may be mentioned. The mixture may contain an organic acid as a pH adjuster. When a pH adjuster is contained in the mixture, its content may be appropriately selected according to the intended pH.

[0051] The mixture can be prepared by mixing other components such as the first carbon material, the dispersant, the liquid medium, and a pH adjuster as needed. As a mixing method, a commonly used mixing method such as a shearing mixer such as a bead mill, a ball mill, a homogenizer, a planetary mixer, etc. can be used. The mixing method can also be combined with a wet pulverization process.

[0052] The wet pulverization of the mixture can be carried out using, for example, a bead mill, a ball mill, etc. When the wet pulverization is carried out using a bead mill, beads are added to the mixture. As the material of the added beads, zirconium oxide, etc. can be cited. The particle size of the beads can be, for example, 0.1 mm or more and 3 mm or less, preferably 0.5 mm or more, or 1.5 mm or less. In the pulverization process, beads with different particle sizes can be used in combination. For example, the particle size ratio when using two kinds of beads with different particle sizes, that is, the ratio of beads with small particle sizes to beads with large particle sizes, can be, for example, 0.1 or more and 0.9 or less, preferably 0.5 or more, or 0.7 or less. In addition, when using two kinds of beads with different particle sizes, the mixing ratio (large particle size beads: small particle size beads) based on volume can be, for example, from 10:1 to 1:10, or from 2:1 to 1:2. The ratio of the total volume of the beads to the total volume of the mixture may be, for example, 0.1 or more and 1 or less, preferably 0.5 or more or 0.7 or less.

[0053] The time of the wet pulverization treatment may be, for example, 1 hour or more and 200 hours or less, preferably 10 hours or more, or 20 hours or more, and preferably 130 hours or less, 90 hours or less, or 50 hours or less. The ambient temperature during the wet pulverization treatment may be, for example, 15° C. or more and 40° C. or less, preferably 20° C. or more, or 30° C. or less. The atmosphere of the wet pulverization treatment may be, for example, an air atmosphere.

[0054] When a bead mill is used in the wet pulverization of the mixture, the beads can be removed after the wet pulverization to obtain a slurry. The volume average particle size of the carbon material contained in the obtained slurry can be, for example, 1 μm or more and 10 μm or less, preferably 2 μm or more, or 7 μm or less. The volume average particle size here is the 50% particle size D corresponding to the cumulative volume from the small particle size side in the cumulative particle size distribution based on volume when the cumulative volume is 50%. 50 .

[0055] In the second step, the second carbon material is obtained by a drying treatment to remove at least a portion of the liquid medium from the slurry. The liquid medium can be removed from the slurry by, for example, a heating drying treatment, a spray drying treatment, a coagulant addition treatment, or the like.

[0056] The heat drying treatment can be, for example, after filtering the slurry to remove a part of the liquid medium to obtain a wet cake, the wet cake is heat treated. The filtration of the slurry can be performed, for example, by suction filtration. By filtering to remove the liquid medium, for example, more than 90% by volume of the liquid medium contained in the slurry can be removed, preferably more than 95% by volume. When removing the liquid medium by filtering, a cleaning treatment can be performed with a liquid medium such as water as needed.

[0057] The heat treatment temperature of the wet cake can be, for example, 50° C. to 200° C., preferably 100° C. or more, or 150° C. or less. In addition, the heat treatment atmosphere can be, for example, an air atmosphere. The heat treatment of the wet cake can be performed, for example, using a constant temperature dryer. In the heat drying treatment, crushing treatment, classification treatment, etc. can be performed after the heat treatment as needed.

[0058] Spray drying is a drying process that utilizes the flow of gas to disperse the slurry into particles of small diameter and simultaneously dry it to obtain a particulate product. Therefore, as a drying device that can be used in the spray drying process, a drying device having a spray function and a drying function can be selected. As such a drying device, for example, various drying devices such as an airflow drying device represented by a so-called spray drying device and a fluidized bed drying device can be cited. The drying temperature in the spray drying process can be, for example, 80°C or more and 150°C or less, preferably 90°C or more, or 130°C or less.

[0059] In the spray drying process, gas is supplied to the drying device for spraying and drying the slurry. The type of gas supplied may be, for example, the atmosphere or an inert gas such as a rare gas such as nitrogen or argon. When an inert gas is used, the spray drying process becomes easier regardless of the liquid medium of the slurry.

[0060] The drying treatment in the second step may include at least spray drying. That is, the second carbon material obtained in the second step may include a spray-dried product. By including the spray-dried product, the second carbon material tends to have improved handleability of the dried product.

[0061] The average primary particle size of the second carbon material obtained by the second step can be, for example, less than 1.5 μm, preferably less than 1.4 μm, or less than 1.3 μm, and preferably more than 0.4 μm, or more than 0.5 μm. When the average primary particle size of the second carbon material is within the above range, there is a tendency to obtain a fourth carbon material having an average primary particle size that improves the charge and discharge capacity of the lithium-sulfur battery. It should be noted that the average primary particle size of the second carbon material can be determined from a scanning electron microscope (SEM) image. The particle size of multiple primary particles can be measured and calculated as an arithmetic mean. Specifically, a plurality of (for example, 200) primary particles whose contours can be confirmed are selected on the SEM image, and the particle size of the primary particles is determined by the longest distance (maximum Feret diameter) between two points on the contour of the selected primary particles. The average primary particle size is calculated by the arithmetic mean of the particle size of the measured primary particles. Here, being able to confirm the contour means that the contour of the primary particle can be traced all around on the SEM image.

[0062] In the third step, the second carbon material is heat-treated to obtain the third carbon material. The heat treatment in the third step may include: a heating step of heating to a given heat treatment temperature, a temperature holding step of maintaining a given heat treatment temperature, and a cooling step of cooling from a given heat treatment temperature. In the heating step, for example, the temperature is raised from room temperature to a given heat treatment temperature. The heating rate may be, for example, 3°C / minute or more and 8°C / minute or less. In the temperature holding step, a given heat treatment temperature is maintained for a long time within a given heat treatment time. The heat treatment temperature in the third step may be, for example, 600°C or more and 1500°C or less, preferably 800°C or more, or 900°C or more, and preferably 1200°C or less, or 1100°C or less. The heat treatment time may be, for example, 1 hour or more and 10 hours or less, preferably 3 hours or more, or 4 hours or more, and preferably 7 hours or less, or 6 hours or less. In the cooling step, for example, the temperature is cooled from a given heat treatment temperature to, for example, room temperature. The cooling rate may be, for example, 0.1°C / minute or more and 50°C / minute or less.

[0063] The atmosphere of the heat treatment may be an atmosphere of an inert gas such as a rare gas such as nitrogen or argon. The concentration of oxygen in the atmosphere of the heat treatment may be 20% by volume or less, preferably 1% by volume or less.

[0064] The heat treatment in the third step can be performed using, for example, a tube furnace, a box furnace, a roller kiln, a rotary furnace, or the like.

[0065] In the fourth step, the third carbon material is activated to obtain a fourth carbon material. Generally speaking, the activation treatment of carbon materials refers to the process of gasifying volatile components, carbon atoms, etc. in the carbon material at high temperature using gases, chemicals, etc. to give it a developed microporous structure. The activation treatment includes a gas activation method using an activating gas and a chemical activation method using an activating chemical. Examples of the gas used in the gas activation method include: water vapor, carbon dioxide, air, oxygen, etc. In addition, examples of the activator used in the chemical activation method include: alkali metal hydroxides such as potassium hydroxide and sodium hydroxide; and metal halides such as zinc chloride.

[0066] The temperature of the activation treatment may be, for example, 650° C. to 900° C., preferably 700° C. to 720° C., and preferably 850° C. to 800° C. The time of the activation treatment may be, for example, 30 minutes to 5 hours, preferably 1 hour to 1.5 hours, and preferably 3 hours to 4 hours.

[0067] From the viewpoint of increasing the pore volume, the activation treatment applicable to the fourth step may include, for example, a chemical activation method, preferably a chemical activation method using an alkali metal hydroxide. When the activation treatment of the third carbon material includes a chemical activation method, the activation treatment in the fourth step may include: a mixing step of mixing the third carbon material and an activator to obtain a mixture, and an activation step of heat-treating the obtained mixture to obtain a fourth carbon material.

[0068] In the mixing step, the third carbon material and the activator are mixed to obtain a mixture. In the mixture, the mass ratio of the activator to the third carbon material (activator / third carbon material) can be, for example, 1 or more, preferably 2 or more, or 3 or more, and preferably 6 or less, or 5 or less. As a mixing method of the third carbon material and the activator, a mechanical mixing method can be cited.

[0069] In the activation process, a mixture containing a third carbon material and an activator is heat-treated to obtain a fourth carbon material. The activation process may include heating to a given activation treatment temperature, maintaining a given activation treatment temperature, and cooling from a given activation treatment temperature. The rate of heating to the activation treatment temperature may be, for example, 8°C / minute or more and 18°C / minute or less. The activation treatment temperature may be, for example, 650°C or more and 900°C or less, preferably 700°C or more, or 720°C or more, and preferably 850°C or less, or 800°C or less. The activation treatment time may be, for example, 30 minutes or more and 5 hours or less, preferably 1 hour or more, or 1.5 hours or more, and preferably 4 hours or less, or 3 hours or less. The rate of cooling from the activation treatment temperature may be, for example, 0.1°C / minute or more and 50°C / minute or less.

[0070] The atmosphere of the activation step may be an atmosphere of an inert gas such as a rare gas such as nitrogen or argon. The oxygen concentration in the atmosphere of the activation step may be 20% by volume or less, preferably 1% by volume or less.

[0071] The fourth step may include a preliminary heat treatment step before the activation step. In the preliminary heat treatment step, for example, at a temperature of 250°C or more and less than 650°C, a mixture containing a third carbon material and an activator is heat treated to obtain a preliminary heat-treated product. The preliminary heat treatment step may include: heating to a given preliminary heat treatment temperature, maintaining a given preliminary heat treatment temperature, and cooling from a given preliminary heat treatment temperature. The rate of heating to the preliminary heat treatment temperature may be, for example, 5°C / minute or more and 20°C / minute or 8°C or more and 15°C or less. The preliminary heat treatment temperature is preferably 300°C or more, or 350°C or more, and preferably 550°C or less, or 450°C or less. The preliminary heat treatment time may be, for example, 10 minutes or more and 3 hours or less, preferably 30 minutes or more, or 50 minutes or more, and preferably 2 hours or less, or 1.5 hours or less. The rate of cooling from the preliminary heat treatment temperature may be, for example, 0.1°C / minute or more and 50°C / minute or less.

[0072] The atmosphere of the preliminary heat treatment step may be an atmosphere of an inert gas such as a rare gas such as nitrogen or argon. The oxygen concentration in the atmosphere of the preliminary heat treatment may be 20% by volume or less, preferably 1% by volume or less.

[0073] The fourth carbon material obtained in the fourth step may be subjected to washing treatment, crushing treatment, classification treatment, etc. as necessary.

[0074] The washing treatment may include, for example, contacting the fourth carbon material obtained in the activation step with an acidic solution, and contacting the fourth carbon material after contacting the acidic solution with a liquid medium containing water.

[0075] The contact between the fourth carbon material and the acidic solution can be carried out, for example, by mixing the fourth carbon material and the acidic solution. The temperature at which the fourth carbon material and the acidic solution are in contact can be, for example, above 15°C and below 40°C. The contact time can be, for example, above 1 minute and below 1 hour. The acidic solution can contain an acidic compound and a liquid medium containing at least water. Examples of the acidic compound include hydrochloric acid, nitric acid, and the like. The concentration of the acidic compound in the acidic solution can be, for example, above 0.1 mol / l and below 2 mol / l, preferably above 0.5 mol / l, or below 1.5 mol / l. At least a portion of the acidic solution can be removed from the fourth carbon material after contact with the acidic solution by filtration or the like.

[0076] The contact between the fourth carbon material and the aqueous liquid medium can be carried out by mixing the fourth carbon material and the aqueous liquid medium. The temperature at which the fourth carbon material and the liquid medium are in contact can be, for example, above 15°C and below 40°C. The liquid medium can contain water, or can be substantially water. Here, "substantially" means that components other than water that is inevitably mixed in are not excluded, and the water content in the liquid medium can be, for example, above 90% by mass, or above 95% by mass. The contact between the fourth carbon material and the aqueous liquid medium can be carried out, for example, by passing the aqueous liquid medium through the fourth carbon material maintained on the funnel. At this time, the contact between the fourth carbon material and the aqueous liquid medium can be carried out by passing the liquid medium before the pH of the filtrate is, for example, above 5 and below 7, preferably before it becomes above 6.

[0077] The fourth carbon material after the washing treatment may be dried. The drying temperature in the drying treatment may be, for example, 120° C. or more and 160° C. or less. The drying time may be, for example, 10 hours or more and 24 hours or less.

[0078] Carbon materials for lithium-sulfur batteries

[0079] The carbon material for lithium-sulfur battery has an average primary particle size of 1.5 μm or less, a peak pore size of 1.0 nm or more and 5.0 nm or less, a pore volume of 0.6 ml / g or more, and a 90% particle size D corresponding to 90% of the cumulative volume from the small particle size side in the cumulative particle size distribution based on volume 90 Relative to the 10% particle size D corresponding to 10% cumulative volume 10 Ratio D 90 / D 10 More than 60.

[0080] Carbon materials for lithium-sulfur batteries can improve the charge and discharge capacity of applicable lithium-sulfur batteries by reducing the average primary particle size to less than 1.5 μm and increasing the pore volume to more than 0.6 ml / g. For example, it can be considered as follows. Since the average primary particle size is small, the diffusion distance of lithium ions in the pores becomes shorter, which can reduce the internal resistance caused by charging and discharging. In addition, since the pore volume is large, there is space for the sulfur in the pores to expand and contract, and it becomes difficult for sulfur to dissolve in the electrolyte during charging and discharging, so the charge and discharge reaction can be carried out efficiently. Furthermore, due to D 90 / D 10 The particle size distribution is wide, and when the positive electrode active material layer is formed, the small particles enter the gaps between the particles, so the porosity of the positive electrode active material layer becomes lower and the filling property of the particles is enhanced. Therefore, it is believed that the charge and discharge capacity of the lithium-sulfur battery can be improved.

[0081] The average primary particle size of the carbon material for lithium-sulfur batteries may be, for example, 1.5 μm or less, preferably 1.4 μm or less, or 1.3 μm or less, and more preferably 1.0 μm or less, or 0.9 μm or less. The lower limit of the average primary particle size may be, for example, 0.5 μm or more, 0.6 μm or more, or 0.8 μm or more. The average primary particle size of the carbon material for lithium-sulfur batteries is calculated in the same manner as the average primary particle size of the second carbon material.

[0082] The peak pore diameter of the carbon material for lithium-sulfur batteries can be, for example, 1.0 nm or more and 5.0 nm or less, preferably 1.4 nm or more, or 2.0 nm or more, and preferably 4.0 nm or less, 3.5 nm or less, 3.0 nm or less, or 2.5 nm or less. When the peak pore diameter of the carbon material for lithium-sulfur batteries is within the above range, sulfur becomes difficult to dissolve from the electrolyte, and there is a tendency for the charge and discharge capacity to increase. The peak pore diameter of the carbon material for lithium-sulfur batteries is measured by nitrogen adsorption and desorption measurement as described above.

[0083] The pore volume of the carbon material for lithium-sulfur batteries can be, for example, 0.6 ml / g or more, preferably 0.7 ml / g or more, 0.8 ml / g or more, or 0.9 ml / g or more, and preferably 1.4 ml / g or less, 1.3 ml / g or less, or 1.1 ml / g or less. Particularly preferred are pores having a diameter of 1.4 nm or more and 5.0 nm or less and a pore volume within the above range. When the pore volume of the carbon material for lithium-sulfur batteries is within the above range, sulfur can obtain sufficient volume for expansion and contraction when sulfur is enclosed in the pores, and there is a tendency for the charge and discharge capacity to increase. The pore volume of the carbon material for lithium-sulfur batteries is measured by nitrogen adsorption and desorption measurement as described above.

[0084] The specific surface area of ​​the carbon material for lithium-sulfur batteries can be, for example, 1000 m 2 / g or more, preferably 1300m 2 / g or more, 1400m 2 / g or more, 1500m 2 / g or more, 1800m 2 / g or above, or 2000m 2 The upper limit of the specific surface area can be, for example, 2500 m 2 / g or below, or 2200m 2 / g or less.

[0085] 50% particle size D of carbon material for lithium-sulfur battery in cumulative particle size distribution based on volume 50 For example, it may be 2 μm or more and 10 μm or less, preferably 2.5 μm or more or 4 μm or more, and preferably 8 μm or less or 7 μm or less.

[0086] 10% particle size D of carbon materials for lithium-sulfur batteries 10 For example, it may be 1 μm or less, preferably 0.9 μm or less, or 0.8 μm or less, more preferably 0.6 μm or less, 0.4 μm or less, or 0.2 μm or less. It is also preferably 0.1 μm or more. 10% particle size D 10 When the particle size is within the above range, when the positive electrode active material layer is formed, the filling property of the particles is enhanced when the small particles enter the gaps between the particles. 90 For example, the thickness may be 50 μm or more, preferably 70 μm or more, or 80 μm or more, and preferably 160 μm or less, or 170 μm or less.

[0087] 90% particle size D of carbon materials for lithium-sulfur batteries 90 Relative to 10% particle size D 10 Ratio D 90 / D 10 For example, it may be 60 or more, preferably 500 or more, or 600 or more. 90 / D 10 The upper limit of may be, for example, 1300 or less, or 1200 or less. The cumulative particle size distribution based on volume is measured under wet conditions using a laser diffraction particle size distribution measuring device. 10 , 50% particle size D 50 and 90% particle size D 90 Each was measured as a particle size corresponding to 10%, 50%, and 90% of the cumulative volume.

[0088] The carbon material for lithium-sulfur batteries contains at least carbon. The carbon content in the carbon material for lithium-sulfur batteries may be, for example, 70% by mass or more, preferably 80% by mass or more, or 90% by mass or more, and may be substantially composed of only carbon. Here, "substantially" means that other elements that are inevitably mixed are not excluded.

[0089] Carbon materials for lithium-sulfur batteries may further contain other elements in addition to carbon. As other elements, for example, nitrogen, oxygen, typical elements represented by alkali metals, transition metal elements, etc. can be cited. As alkali metal elements, for example, potassium can be cited. Alkali metals such as potassium contained in carbon materials can be derived from carbon raw materials, activators, etc. The content of potassium varies according to raw materials, activation treatment conditions, cleaning conditions, etc., for example, it can be more than 1500ppm, or more than 3000ppm. In addition, it can be less than 10000ppm, or less than 6000ppm. In addition, zirconium can be cited as a transition metal element. Transition metal elements such as zirconium contained in carbon materials can be derived from beads, containers, etc. used in wet pulverization. The content of zirconium has a tendency to increase in proportion to the processing time of the bead mill. It can be considered that this is because the zirconium constituting the beads is attached to the carbon material. Its content can be more than 20ppm, or more than 100ppm, and can also be less than 400ppm, or less than 200ppm. The content of the metal element in the carbon material can be measured by, for example, inductively coupled plasma emission spectrometry (ICP).

[0090] Cathode materials for lithium-sulfur batteries

[0091] The positive electrode material for lithium-sulfur battery includes the carbon material for lithium-sulfur battery described above (hereinafter referred to as "carbon material") and a material containing sulfur attached to the carbon material. The material containing sulfur can be attached to the voids in the carbon material. The carbon material can increase the battery capacity by increasing the specific surface area and pore volume.

[0092] As the material containing sulfur element, for example, simple sulfur, polysulfide generated during the charge and discharge process of lithium-sulfur battery, etc. As the material containing sulfur element, specifically, S8, Li2S8, Li2S6, Li2S4, Li2S2, etc. can be mentioned, and simple sulfur (S8) is preferred from the viewpoint of yield.

[0093] The content of the material containing sulfur element arranged in the voids of the carbon material of the sulfur-carbon composite, which is the ratio of the mass of the material containing sulfur element to the total mass of the material containing carbon material and sulfur element, can be, for example, 25 mass % or more and 95 mass % or less, preferably 45 mass % or more and 55 mass % or less. When the content of the material containing sulfur element is within this range, the capacity loss of the sulfur-carbon composite is reduced.

[0094] Lithium-sulfur battery cathode

[0095] The positive electrode for lithium-sulfur battery (hereinafter referred to as "positive electrode") comprises a current collector and a positive electrode active material layer disposed on the current collector. The positive electrode active material layer may contain the positive electrode material for lithium-sulfur battery described above. The positive electrode is manufactured by, for example, the following method: an electrode composition comprising the positive electrode material for lithium-sulfur battery described above, a liquid medium, a binder, and a conductive aid is applied to the current collector, followed by drying and pressure molding to form the positive electrode active material layer on the current collector.

[0096] Depending on the purpose, the liquid medium can use an organic solvent, water, etc. As an organic solvent, for example, there can be mentioned: amide solvents such as N-methyl-2-pyrrolidone (NMP); ketone solvents such as diisopropyl ketone, diisobutyl ketone, methyl ethyl ketone; hydrocarbon solvents such as heptane, ether solvents such as tetrahydrofuran, ethylene glycol dimethyl ether, and dioxolane; amine solvents such as diethylenetriamine; ester solvents, etc. The organic solvent can be used alone or in combination of two or more. Relative to the total mass of the electrode composition, the content of the liquid medium can be, for example, 10% by mass or more and 90% by mass or less.

[0097] Binder is a material that helps the attachment of positive electrode active material and conductive aid, etc., and the attachment of electrode composition to collector. As examples of binders, polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, various copolymers, etc. can be cited. Relative to the gross mass of the electrode composition, the content of the binder can be, for example, 0.05% by mass or more and 50% by mass or less.

[0098] The conductive aid is a material that enhances the conductivity of the positive electrode composition layer. Examples of the conductive aid include: graphite such as modified graphene, natural graphite, and artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; carbon materials such as graphene and carbon nanotubes, etc. The content of the conductive aid may be, for example, 0.5% by mass or more and 30% by mass or less relative to the total mass of the electrode composition.

[0099] Examples of current collectors include: metals such as copper, stainless steel, aluminum, nickel, and titanium; composite materials in which the surface of copper, stainless steel, etc. is treated with carbon, nickel, titanium, silver, etc.; and carbon foils. When manufacturing lightweight lithium-sulfur batteries, for example, aluminum and carbon foils are preferred as lightweight current collectors. The current collector can improve the adhesion of the positive electrode composition layer, etc. by forming fine irregularities on its surface. In addition, it can be in various forms such as a film, a sheet, a foil, a mesh, a porous body, a foam, and a non-woven fabric. The thickness of the current collector can be, for example, not less than 3 μm and not more than 500 μm.

[0100] The positive electrode active material layer formed of the positive electrode material for lithium-sulfur battery as described above can be formed as a dense layer with few voids. The porosity of the positive electrode active material layer in the positive electrode can be, for example, 10.0% or less, preferably 9.0% or less, 6.0% or less, 5.0% or less, or 3.0% or less. The lower limit of the porosity of the positive electrode active material layer can be, for example, 1% or more. The porosity of the positive electrode active material layer is calculated by specifying a given range from a cross-sectional SEM image of the positive electrode active material layer and measuring the area ratio of the void portion in an image analysis software.

[0101] Lithium-sulfur secondary battery

[0102] The lithium-sulfur battery comprises the above-mentioned positive electrode for lithium-sulfur battery. The lithium-sulfur battery comprises a positive electrode for lithium-sulfur battery, a negative electrode, an electrolyte disposed between the positive electrode and the negative electrode, etc. The lithium-sulfur battery may comprise a separator as required. The electrolyte may be contained in the positive electrode, the negative electrode and the separator for lithium-sulfur battery.

[0103] negative electrode

[0104] As the negative electrode constituting the lithium-sulfur battery, a known material may be used. As the negative electrode material constituting the negative electrode, for example, Li metal, Li-Si alloy, Li-Al alloy, Li-In alloy, lithium titanate (for example, Li4Ti5O 12 and LiTi2O4), lithium titanium composite oxides (such as Li4Ti 5-x Mn x O 12 ; 0<x≦0.3), Li x C (x ≤ 6), etc. For these negative electrode materials, part of the lithium can be replaced by other alkali metals. As negative electrode materials, Li metal, Li-Si alloy, Li-Al alloy, Li-In alloy, Li x C(x≦6), etc. If these materials are used, high voltage can be obtained from lithium-sulfur batteries.

[0105] Isolation

[0106] The separator may be made of a known material, for example, porous polyethylene, polypropylene, etc. Alternatively, a known separator may be used after coating its surface.

[0107] Electrolytes

[0108] The electrolyte may contain a lithium salt, which may be appropriately selected from lithium salts used in conventional lithium-ion batteries. The lithium salt may, for example, contain anions containing fluorine elements. Specifically, lithium salts containing anions containing fluorine elements include: LiPF6, LiBF4, LiSbF6, LiAsF6, LiSO3CF3, LiN(SO2CF3)2(LiTFSI), etc. In addition, the electrolyte may also contain lithium salts that do not contain fluorine elements, such as lithium nitrate and LiClO4. The electrolyte may be used alone or in combination of two or more of these.

[0109] The electrolyte may contain an organic solvent. As the organic solvent, a carbonate solvent, an ether solvent, an ester solvent, an amide solvent, a nitrile solvent, a sulfur-containing solvent may be used, or an organic solvent in which a part of the elements of the above organic solvents are replaced with fluorine may be used. As the organic solvent, for example, carbonate solvents such as propylene carbonate, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, fluoroethylene carbonate, etc.; ether solvents such as 1,3-dioxolane, 1,2-dimethoxyethane, 1,3-dimethoxypropane, 2,2,3,3-tetrafluoropropyldifluoromethyl ether, tetrahydrofuran, etc.; ester solvents such as methyl formate, methyl acetate, γ-butyrolactone, etc.; amide solvents such as N,N-dimethylacetamide, N,N-dimethylformamide, etc.; sulfur-containing solvents such as cyclopentane, dimethyl sulfoxide, 1,3-propane sultone, etc.

[0110] Example

[0111] The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to these Examples.

[0112] Example 1

[0113] Preparation of carbon materials

[0114] To 20 g of activated carbon raw material (manufactured by Universal Engraving Systems Co., Ltd., KD-PWSP) as the first carbon material and 10 g of polyvinyl alcohol (PVA; manufactured by Sigma-Aldrich, molecular weight 9000 to 10000, saponification degree 80 mol%), 500 g of water was added to obtain a mixed solution. 300 ml of zirconia beads (mixed at a volume ratio of Φ0.65 mm and Φ1 mm of 1:1) and the mixed solution were added to a 2 L polyethylene bottle, and bead mill pulverization treatment was carried out at 2000 rpm for 30 hours on a roller dispersion table to obtain a slurry. The obtained slurry was supplied together with the atmosphere as the supply gas and spray-dried by a spray drying device to obtain the second carbon material. By heating the second carbon material to 1000 °C at a rate of 5 °C / min using a tubular furnace in a nitrogen atmosphere and holding for 5 hours, heat treatment was carried out to obtain the third carbon material. The third carbon material and potassium hydroxide were mixed at a mass ratio of 1:4, added to a bauxite crucible, and heated to 400 °C at a heating rate of 10 °C / min in a nitrogen atmosphere using a box furnace and held for 1 hour. Thereafter, activation treatment was carried out by heating to 750 °C at a heating rate of 10 °C / min for 2 hours to obtain the fourth carbon material. Approximately 100 ml of 1 mol / L hydrochloric acid was added to the obtained fourth carbon material for neutralization treatment. Thereafter, while performing suction filtration, it was washed with approximately 2 L of pure water until the pH of the filtrate reached about 6 to obtain a wet filter cake. The obtained wet filter cake was dried for 12 hours using a constant temperature dryer at 150 °C to obtain the carbon material of Example 1.

[0115] Example 2

[0116] The time of the bead mill pulverization treatment was changed to 60 hours, and except for this, the carbon material of Example 2 was obtained by the same method as in Example 1.

[0117] Example 3

[0118] The time of the bead mill pulverization treatment was changed to 120 hours, and except for this, the carbon material of Example 2 was obtained by the same method as in Example 1.

[0119] Example 4

[0120] After suction-filtering the slurry, instead of spray drying using a spray drying device, it was dried by heat treatment at 150 °C in an atmosphere for 24 hours to obtain the second carbon material, and except for this, the carbon material of Example 3 was obtained by the same method as in Example 1.

[0121] Example 5

[0122] The bead mill pulverization treatment time was changed to 120 hours, and after the slurry was suction filtered, it was dried by heat treatment at 150°C in an atmospheric atmosphere for 24 hours instead of spray drying using a spray dryer to obtain a second carbon material. In addition, the carbon material of Example 4 was obtained by the same method as Example 1.

[0123] Comparative Example 1

[0124] As the carbon material of Comparative Example 1, the raw material activated carbon used in Example 1 (KD-PWSP manufactured by Universal Engineering Systems, Ltd.) itself was used.

[0125] Comparative Example 2

[0126] The carbon material of Comparative Example 2 was obtained by the same method as in Example 1 except that PVA was not added during the bead mill pulverization treatment.

[0127] Comparative Example 3

[0128] The carbon material of Comparative Example 3 was obtained by the same method as in Example 2 except that PVA was not added during the bead mill pulverization treatment.

[0129] Comparative Example 4

[0130] Spherical phenolic resin (Bellpearl, R100, manufactured by Air Water Bellpearl) was heated to 600°C at a rate of 5°C / min in a nitrogen atmosphere and maintained for 5 hours to obtain a spherical carbide. Spherical carbide and potassium hydroxide were mixed at a mass ratio of 1:6, heated to 400°C at a rate of 10°C / min in a nitrogen atmosphere, maintained for 1 hour, and then activated by heating to 900°C at a rate of 10°C / min and performing a heat treatment for 2 hours to obtain a carbon material of Comparative Example 4.

[0131] Pore ​​distribution evaluation

[0132] For the carbon material obtained above, the volume of the open pores (pore diameter of 0.4 nm or more and 100 nm or less) through which the gas can enter was measured using a surface area / pore analyzer (trade name: Nova Touch, manufactured by Anton Paar). After a dehydration treatment was carried out at 150°C for 1 hour while vacuum exhausting, nitrogen was used as an adsorbent, and the nitrogen adsorption amount was measured at 77K and a pressure range of 0.0001 Torr or more and 760 Torr or less, and the nitrogen adsorption amount was taken as the volume of the open pores. In addition, the pore distribution was calculated by applying the quenched solid density functional theory (QSDFT method) to the obtained adsorption isotherm. The cylindrical pore model was used for adjustment in the QSDFT method. The pore volume in the range of pore diameter of 0.4 nm or more and 100 nm or less is shown in Table 1 as the pore volume w, and the pore volume in the range of pore diameter of 1.4 nm or more and 5.0 nm or less is shown as the pore volume n.

[0133] Specific surface area

[0134] The specific surface area was measured by a nitrogen adsorption method (multipoint method) using a surface area / pore analysis device (trade name: Nova Touch, manufactured by Anton Paar). The results are shown in Table 1.

[0135] Primary particle size

[0136] Using a scanning electron microscope (SEM), SEM images that can confirm the outlines of the primary particles that constitute the secondary particles were obtained within a magnification range of 1000 to 10,000 times according to the particle size. 200 primary particles whose outlines can be confirmed were selected on the SEM image, and the longest distance (maximum Feret's diameter) between any two points on the outline of the selected primary particles was calculated. The arithmetic mean of the maximum Feret's diameter obtained was used as the average particle size of the primary particles, and the primary particle size was calculated. The results are shown in Table 1. It should be noted that the SEM images of the carbon materials obtained in Examples 1 to 5 and Comparative Examples 1 to 4 are shown in Figures 1 to 9 .

[0137] Particle size distribution

[0138] The particle size distribution based on volume was measured using a laser diffraction particle size distribution device (MASTERSIZER 2000 manufactured by Malvern). The preparation of the measurement sample was carried out as described below. 2 ml of the dispersant (ADEKA Pluronic L-44; manufactured by ADEKA) was diluted with 170 ml of an aqueous solvent, and dispersed with a touch mixer to obtain a diluted dispersant. In order to make the laser intensity of the measuring device within an appropriate range, an appropriate amount of carbon material, 2.5 ml of water and 2.5 ml of the diluted dispersant were added to a test tube, and after being dispersed within a few seconds with a touch mixer, it was dispersed for 120 seconds using ultrasound to obtain a measurement sample. The 50% particle size D, which is the volume cumulative value from the small particle size side, is 50% in the particle size distribution based on volume. 50 The volume average particle size was calculated. In addition, the 10% particle size D was calculated as the value at which the volume cumulative value from the small particle size side becomes 10% and 90%. 10 and 90% particle size D 90 , calculated D 90 Divide by D 10 The particle size distribution (D 90 / D 10 The results are shown in Table 1. In addition, the particle size distribution curves of the carbon materials of Example 1, Comparative Example 1 and Comparative Example 2 are shown in Figure 10 to Figure 12 .

[0139] Analysis of metal element content

[0140] The contents of potassium (K) and zirconium (Zr) in the carbon materials of Examples 1 to 5 and Comparative Examples 1 to 2 were measured by inductively coupled plasma emission spectrometry (ICP). Specifically, the carbon materials were heated and decomposed by nitric acid in a microwave sample digestion device, and then standard samples using K and Zr were measured at room temperature using an ICP emission spectrometer (Optima 8300) manufactured by Perkin Elmer. The results are shown in Table 1.

[0141] [Table 1]

[0142]

[0143] The longer the pulverization time, the smaller the primary particle size. In addition, by adding PVA, the primary particle size of the raw activated carbon can be effectively reduced. Furthermore, when PVA is added, D 10 The value of becomes smaller, and the particle size distribution D 90 / D 10 Particle size distribution D 90 / D 10When the diameter is larger, the porosity of the positive electrode active material layer described below becomes smaller, and the filling property of the particles is enhanced. After the carbon materials of the examples and comparative examples are activated by KOH, the pore volume around 2 nm increases.

[0144] Depend on Fig.10 , 11 As shown in Figures 1 and 12, compared with the carbon materials of Comparative Examples 1 and 2, the peak of the overall particle size through the bead mill moves to the small particle size side. Furthermore, compared with the carbon material of Example 1, Comparative Example 2 shows that by adding PVA, a small particle peak appears near 0.1 μm. Therefore, it can be considered that the small particles are effectively blocked between the particles, so that the porosity of the electrode plate decreases. It should be noted that the carbon materials of Examples 2 to 5 also have a small particle peak near 0.1 μm.

[0145] Preparation of sulfur-carbon composites

[0146] The carbon material obtained in each example and comparative example and sulfur (99.99%; Kojun Chemical Laboratory Co., Ltd.) were mixed at a mass ratio of 1:1. The obtained mixture was heated at 155°C for 3 hours in a heat-resistant and pressure-resistant container. Then, the obtained mixture was placed in a stainless steel container and sealed, and heated at 300°C for 24 hours to obtain sulfur-carbon composites.

[0147] Preparation of positive electrode

[0148] To a mixture of 30 parts by mass of the sulfur-carbon composite obtained above, 3.6 parts by mass of acetylene black, 0.71 parts by mass of styrene-butadiene rubber (SBR), and 2 parts by mass of carboxymethyl cellulose (CMC), 0.89 parts by mass of polyvinyl pyrrolidone (PVP) and 64 parts by mass of water were added, dispersed and dissolved, and a slurry was prepared. The ratio of the amount of the sulphur-carbon composite to the amount of the acetylene black ... 2 ) containing 1.6 mg of sulfur, the prepared slurry was coated on the collector and dried, and the coated electrode plate was cut into a size of 3 cm×5 cm and pressed at 3.7 tons (pressure of about 24 MPa) using a roller press to obtain a positive electrode.

[0149] Determination of the Porosity of the Positive Electrode

[0150] Cross-sectional SEM image acquisition of the positive electrode

[0151] After the positive electrode obtained above was cut by ion beam profiling (CP), the reflected electron image of the cross section of the positive electrode active material layer was photographed by scanning electron microscopy (SEM). The photographic conditions were: acceleration voltage 5.00 kV, working distance (WD) 6 mm to 8 mm, and magnification 1000 times. The cross-sectional SEM images of the positive electrodes obtained in Examples 1 to 5 and Comparative Examples 1 to 4 are shown in Figures 13 to 21 .

[0152] The porosity was calculated from the cross-sectional SEM image of the positive electrode obtained above by the following method. Specifically, the image was processed as follows using free software Image J Fiji.

[0153] (1) The captured cross-sectional SEM image was imported into Image J, and the range was specified as 30 μm × 90 μm.

[0154] (2) The brightness of the grayscale image is converted to binary (0 or 255) by Threshold operation. The brightness threshold is set to 75. Based on this operation, the gap part (black) becomes 255, and the part other than the gap (white) becomes 0.

[0155] (3) The percentage of the number of pixels in the gap divided by the sum of the number of pixels in the gap and the number of pixels other than the gap was calculated and taken as the void ratio. The results are shown in Table 2.

[0156] Lithium-sulfur secondary battery

[0157] Fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1:1 to obtain a mixed solvent. Lithium hexafluorophosphate (LiPF6) was added to the obtained mixed solvent and dissolved until the concentration of the mixed solvent became 1.0 mol / L, thereby preparing a non-aqueous electrolyte.

[0158] A lead electrode was installed on the positive electrode obtained above, and a separator made of porous polypropylene (Celgard 2400) was arranged, and they were placed in a bag-shaped laminated package. After storage, it was vacuum dried at 50°C to remove the moisture adsorbed in each component. After vacuum drying, the metal lithium of the negative electrode arranged on the stainless steel (SUS) foil was arranged in the laminated package opposite to the positive electrode through the porous polypropylene, and a non-aqueous electrolyte was injected and sealed to obtain a laminated lithium-sulfur secondary battery as an evaluation battery. The following battery characteristics were evaluated using the obtained evaluation battery.

[0159] Evaluation of charge-discharge cycle capacity retention

[0160] The evaluation battery was placed in a thermostatic chamber at 25°C, and a cycle capacity retention rate evaluation test was performed. Specifically, the test was performed within the charge and discharge voltage range of 1.0V to 3.0V. The discharge current is the current value when 0.1C capacity is taken out. The discharge capacity Qcyc(1) (mAh / g) of the first cycle just after the discharge was started was measured. The following charge and discharge were repeated, and the discharge capacity Qcyc(2) of the second cycle, the discharge capacity Qcyc(5) of the fifth cycle, and the discharge capacity Qcyc(10) of the tenth cycle were measured. The results are shown in Table 2.

[0161] For each cycle, the current value when the discharge current of 0.1C capacity was passed in the 1st to 3rd cycles, the current value when the discharge current of 0.2C capacity was passed in the 4th cycle, and the current value when the discharge current of 2.0C capacity was passed in the 5th cycle was measured, and the discharge capacity of the 4th cycle (R(0.2C)) and the 5th cycle (R(2.0C)) was measured. In addition, for the charging current, all the current values ​​when 0.1C capacity was taken out were passed. The rate characteristic R was further calculated by dividing each R(2.0C) obtained by R(0.2C) c (=100×R(2.0C) / R(0.2C))(%). The discharge capacity was calculated by conversion corresponding to the mass of sulfur. The above results are shown in Table 2.

[0162] For Examples 1 to 5, charge and discharge were performed under the above-mentioned cycle capacity retention evaluation test conditions until the 30th cycle, and the respective discharge capacities QCyC(30) were measured. Furthermore, the capacity retention rate Pcyc (=100×Qcyc(30) / Qcyc(2)) (%) after 30 cycles was calculated by dividing the obtained Qcyc(30) by Qcyc(2). The results are shown in Table 3.

[0163] [Table 2]

[0164]

[0165] As the primary particle size of the carbon material decreases, Qcyc(2) tends to increase. The Qcyc of the embodiment is larger than that of the comparative example, and the high capacity can be maintained even after the 10th cycle of charge and discharge. Furthermore, high capacity is shown even in a high-rate discharge of 2C. The smaller the primary particle size, the shorter the diffusion distance of lithium ions in the pores, and it can be considered that the battery reaction occurs effectively. In addition, the particle size distribution D in the embodiment is 90 / D 10 The particle size distribution is large, and it can be considered that the particle size distribution is wide. At this time, the particles of the carbon material can be tightly blocked, and the porosity of the plate becomes small. As a result, it can be considered that the battery reaction occurs effectively, showing a high capacity.

[0166] [Table 3]

[0167]

[0168] Particle size distribution D 90 / D 10 When the diameter increases, it is considered that the packing property of the particles is enhanced, so that the dissociation of the active materials is suppressed during repeated charge and discharge, and the capacity retention rate increases.

[0169] The entire contents disclosed in Japanese Patent Application No. 2022-073324 (filing date: April 27, 2022) are incorporated into this specification by reference. All documents, patent applications, and technical standards described in this specification are incorporated into this specification by reference to the same extent as when each document, patent application, and technical standard is specifically and separately recorded as being incorporated by reference.

Claims

1. A method for producing a carbon material for a lithium-sulfur battery, the method comprising: wet-milling a mixture comprising a first carbon material, a dispersant, and a liquid medium to obtain a slurry; removing at least a portion of the liquid medium from the slurry to obtain a second carbon material; heat-treating the second carbon material to obtain a third carbon material; as well as The third carbon material is activated to obtain a fourth carbon material.

2. The method for producing a carbon material for a lithium-sulfur battery according to claim 1, wherein: The second carbon material has an average primary particle size of 1.5 μm or less.

3. The method for producing a carbon material for a lithium-sulfur battery according to claim 1 or 2, wherein: The first carbon material is activated carbon having a peak pore diameter of 1.0 nm or more and 5.0 nm or less.

4. The method for producing a carbon material for a lithium-sulfur battery according to any one of claims 1 to 3, wherein: The second carbon material is a product obtained by spray-drying the slurry.

5. The method for producing a carbon material for a lithium-sulfur battery according to any one of claims 1 to 4, wherein: The dispersant contains a thickener.

6. The method for producing a carbon material for a lithium-sulfur battery according to any one of claims 1 to 5, wherein: The heat treatment temperature of the second carbon material in the heat treatment is 600° C. or higher and 1500° C. or lower.

7. The method for producing a carbon material for a lithium-sulfur battery according to any one of claims 1 to 6, wherein: The activation treatment includes bringing the third carbon material into contact with an activating agent at a temperature of 650° C. or higher and 900° C. or lower.

8. The method for producing a carbon material for a lithium-sulfur battery according to any one of claims 1 to 7, wherein The wet pulverization treatment is performed using a bead mill.

9. A carbon material for lithium-sulfur batteries, wherein the average primary particle size is 1.5 μm or less, the peak pore size is 1.0 nm or more and 5.0 nm or less, the pore volume is 0.6 ml / g or more, and the 90% particle size D in the cumulative particle size distribution based on volume is 90 Relative to 10% particle size D 10 Ratio D 90 / D 10 More than 60.

10. The carbon material for lithium-sulfur batteries according to claim 9, wherein The peak pore diameter is 1.5 nm or more and 4.0 nm or less.

11. The carbon material for lithium-sulfur battery according to claim 9 or 10, wherein the specific surface area is 1000 m 2 / g or above.

12. The carbon material for lithium-sulfur batteries according to any one of claims 9 to 11, wherein The 50% particle size D in the cumulative particle size distribution based on volume 50 It is not less than 2 μm and not more than 10 μm.

13. The carbon material for lithium-sulfur batteries according to any one of claims 9 to 12, wherein The 90% particle size D in the cumulative particle size distribution based on volume 90 Relative to 10% particle size D 10 Ratio D 90 / D 10 More than 500.

14. The carbon material for lithium-sulfur batteries according to any one of claims 1 to 13, wherein The 10% particle size D in the cumulative particle size distribution based on volume 10 1 μm or less, 90% particle size D 90 50 μm or more.

15. A positive electrode material for a lithium-sulfur battery, comprising: The carbon material for lithium-sulfur battery according to any one of claims 9 to 14, and sulfur.

16. A positive electrode for a lithium-sulfur battery, comprising: Current collector, and a positive electrode active material layer disposed on the current collector, in, The positive electrode active material layer comprises the positive electrode material for a lithium-sulfur battery according to claim 15 .

17. The positive electrode for a lithium-sulfur battery according to claim 15 or 16, wherein: The porosity of the positive electrode active material layer is 10.0% or less.

18. A positive electrode for a lithium-sulfur battery, wherein the porosity is less than 10.0%. The positive electrode for lithium-sulfur battery comprises: Current collector, and a positive electrode active material layer disposed on the current collector, in, The positive electrode active material layer includes a carbon material for a lithium-sulfur battery having an average primary particle size of 1.5 μm or less and a positive electrode material for a lithium-sulfur battery containing sulfur.

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