Carbon material, method for manufacturing the same, and electrode active material

Abstract

This invention provides a carbon material with excellent filling properties and a large specific surface area. The carbon material has a sphericity greater than 0.83 and a specific surface area of ​​400 m². 2 / g or more. Carbon materials can be manufactured by the following method, which includes: preparing materials containing a specific surface area of ​​50m² or more. 2 A suspension of first carbon particles of / g or more, a water-soluble binder, and water; granules are obtained from the suspension; and the granules are heat-treated to obtain second carbon particles.

Description

[0001] This application is a divisional application of Chinese invention application No. 202280016610.3, filed on February 17, 2022, entitled "Carbon Materials, Methods for Manufacturing Them and Electrode Active Substances". Technical Field

[0002] This disclosure relates to carbon materials, methods for manufacturing them, and electrode active materials. Background Technology

[0003] Lithium-sulfur batteries are high-energy-density rechargeable batteries, for example, those using mesoporous sulfur-carbon composites with sulfur arranged within the mesopores. As lighter than conventional lithium-ion batteries, lithium-sulfur batteries are expected to be used in applications such as large power equipment like electric vehicles and aerospace applications like drones, where high cycle capacity retention is required.

[0004] For example, Japanese Patent Application Publication No. 2019-513673 discloses a carbon-sulfur composite with a layered porous structure and a method for manufacturing the same. Additionally, Japanese Patent Application Publication No. 2016-141592 discloses porous carbon-based spherical particles and a method for manufacturing the same. Furthermore, Japanese Patent Application Publication No. 2014-42910 discloses spherical carbon catalyst granules and a method for manufacturing the same. Summary of the Invention

[0005] The problem that the invention aims to solve

[0006] One aspect of this disclosure is to provide carbon materials with excellent filling properties and large specific surface area.

[0007] Methods for solving problems

[0008] The first method involves a carbon material with a sphericity greater than 0.83 and a specific surface area of ​​400 m². 2 / g or more. The second method is a method for manufacturing a carbon material, the method comprising: preparing a material containing a specific surface area of ​​50m² or more. 2 A suspension of first carbon particles (at least / g), a water-soluble binder, and water; granules obtained from the suspension; and second carbon particles obtained by heat treatment of the granules, wherein the second carbon particles have a sphericity greater than 0.83 and a specific surface area of ​​400 m². 2 / g or more.

[0009] The third method is an electrode active material that contains the aforementioned carbon material.

[0010] The effects of the invention

[0011] According to one aspect of this disclosure, carbon materials with excellent filling properties and large specific surface area can be provided. Attached Figure Description

[0012] Figure 1 This is an example of a scanning electron microscope (SEM) image of the carbon material obtained in Example 1.

[0013] Figure 2 This is an example of a cross-sectional SEM image of the carbon material obtained in Example 1.

[0014] Figure 3 This is an example of a SEM image of a commercially available porous carbon material.

[0015] Figure 4 This is an example of a cross-sectional SEM image of a commercially available porous carbon material.

[0016] Figure 5 This is an example of a SEM image of the carbon material obtained in Example 2.

[0017] Figure 6 This is an example of a cross-sectional SEM image of the carbon material obtained in Example 2. Detailed Implementation

[0018] In this specification, the term "process" is not limited to independent processes; it also includes processes that achieve their intended purpose, even when they cannot be clearly distinguished from others. Furthermore, regarding the content of each component in a composition, if multiple substances belonging to each component are present in the composition, unless otherwise specified, it refers to the total amount of those substances present in the composition. Additionally, the upper and lower limits of the numerical ranges described in this specification can be arbitrarily selected and combined from the numerical values ​​exemplified as numerical ranges. Hereinafter, embodiments of the present invention will be described based on the accompanying drawings. The embodiments shown below are carbon materials, methods for manufacturing them, and electrode active materials used to embody the technical concept of the present invention; however, the present invention is not limited to the carbon materials, methods for manufacturing them, and electrode active materials shown below.

[0019] carbon materials

[0020] The carbon material in this embodiment has a sphericity greater than 0.83 and a specific surface area of ​​400 m². 2 / g or more. The carbon material can be porous spherical particles or secondary particles composed of aggregates of multiple primary particles. For porous carbon materials with a sphericity of a given value or higher, for example, when imparting a desired functional component, the imparting properties of each particle tend to become uniform, reducing the unevenness of the imparted amount between particles. Therefore, for example, the effect of the functional component can be made more stable over time. Furthermore, by increasing the specific surface area and the number of reaction sites per unit mass, the desired effect can be achieved sufficiently with a small amount.

[0021] The roundness of carbon materials can be greater than 0.83, preferably greater than 0.86, more preferably greater than 0.9, or greater than 0.93. It should be noted that the upper limit for roundness is 1. Roundness is an index representing the degree of roundness of the particle's outline shape; the closer it is to a circle, the closer it is to 1. Roundness can be determined, for example, by viewing the particle's outline shape from above. For roundness, the diameter of a circle with the same area as the particle's outline shape is defined as the equivalent circle diameter d. HA In this case, the major axis D of a smooth approximation ellipse can be used. e The major axis D of the smooth approximate ellipse is then calculated. e It is calculated based on the ratio (length-to-short axis ratio λ) obtained by dividing the major axis of the carbon material's profile by its minor axis, and the area of ​​the particle image of the carbon material's profile. Circularity is defined as the equivalent circle diameter d. HA The ratio of the major axis De to the smooth approximate ellipse (d) HA / D e The major axis De of the smooth approximate ellipse is calculated based on the area and aspect ratio of the particle image of the carbon material's contour shape.

[0022] Circularity = d HA / D e

[0023] Specifically, for 10 to 30 carbon material particles whose entire outline can be identified, image processing software is used to determine the equivalent circle diameter, which is the diameter of a circle with the same area as the particle image area of ​​the carbon material's outline shape. Next, the major and minor axes are determined based on the outline shape, and the ratio of the major axis to the minor axis is calculated. The major axis of a smoothed approximate ellipse is obtained by dividing the product of the particle image area and the ratio of the major axis by pi (2 times the square root of pi). The roundness of each particle is then calculated by dividing the equivalent circle diameter by the major axis of the smoothed approximate ellipse. The roundness of the carbon material can be determined as their arithmetic mean.

[0024] The specific surface area of ​​carbon materials can be 400 m² 2 / g or more, preferably 500m 2 / g or more, preferably 600mg 2 / g or more, or 650m 2 / g or higher. The upper limit for specific surface area can be, for example, 3000 m². 2 / g or less, preferably 2500m 2 / g or less, or 2000m 2The specific surface area of ​​carbon materials can be determined by the BET method based on the BET (Brunauer-Emmett-Teller) theory. Specifically, for example, it can be determined by the one-point method using nitrogen.

[0025] For carbon materials, the pore volume can be as low as 1.5 cm³ for example, pores with a diameter of less than 0.2 μm. 3 / g or more and 3cm 3 / g or less, preferably 1.6cm 3 / g or more, or 1.7cm 3 / g or more, and preferably 2.8cm 3 / g or less, or 2.4cm 3 / g or less. By ensuring that the pore volume, with a pore diameter of 0.2 μm or less, is within the above-mentioned range, it is sometimes possible to increase the amount of functional component imparted per unit weight of carbon when imparting functional components to carbon materials. The pore volume can be determined, for example, by mercury porosimetry.

[0026] For the pore diameter of carbon materials, for example, the peak pore diameter of 0.2 μm or less can be 1 nm or more and 150 nm or less, preferably 120 nm or less, or 100 nm or less, and more preferably 2 nm or more, or 20 nm or more. By making the pore diameter of the carbon material within the above range, when imparting functional components to the carbon material, it is sometimes possible to make the functional components more easily impregnated into the pores of the carbon material. In particular, when adding sulfur, there is a tendency to add it more efficiently. The pore diameter of carbon materials can be determined, for example, using mercury porosimetry.

[0027] Carbon materials can be composed of secondary particles formed by aggregates of multiple primary particles. The average particle size of the primary particles constituting the secondary particles can be, for example, 1 nm or more and 200 nm or less, preferably 5 nm or more, or 10 nm or more, and more preferably 150 nm or less, or 100 nm or less. By keeping the average particle size of the primary particles within the above range, it is sometimes easier to form carbon materials with finer pores and smaller pore diameters. The average particle size of the primary particles can, for example, be the average particle size D observed under an electron microscope. SEMThe average particle size of primary particles observed by electron microscopy can be determined as follows: Using a scanning electron microscope (SEM), the primary particles constituting secondary particles are observed at magnifications ranging from 20,000x to 100,000x, based on their particle size. Thirty primary particles with identifiable profiles are selected. Based on the profiles of the selected primary particles, the sphere conversion diameter is calculated using image processing software. The average particle size of the primary particles observed by electron microscopy is then calculated as the arithmetic mean of the obtained sphere conversion diameters. In one embodiment, primary particles may have particles with smaller average particle sizes attached to their surfaces. In another embodiment, primary particles may be an aggregate of particles with smaller average particle sizes than primary particles. The average particle size of particles with smaller average particle sizes than the aforementioned primary particles can be determined by electron microscopy observation in the same manner as described above. Identifying the profile of primary particles means being able to trace the entire profile of the primary particles on the image.

[0028] For secondary particles, the 50% particle size D in the cumulative particle size distribution based on volume is... 50 Compared to the average particle size D observed by electron microscopy SEM The ratio of D 50 / D SEM For example, it can be 10 or higher. Ratio D 50 / D SEM For example, it can be 10 or more and 2000 or less, preferably 50 or more, and more preferably 100 or more. Additionally, the ratio D... 50 / D SEM Preferably, it is 1000 or less, more preferably 400 or less. By making the ratio D... 50 / D SEM Within the aforementioned range, the particle intensity of secondary particles sometimes increases.

[0029] The primary particles can be hollow particles. When the primary particles are hollow, their porosity can be, for example, 10% or more, preferably 20% or more, 40% or more, 50% or more, 60% or more, 70% or more, or 80% or more. The upper limit of the porosity can be, for example, 95% or less. When the primary particles are hollow, there is a tendency for the specific surface area of ​​the carbon material to increase further.

[0030] The volume average particle size of the carbon material can be, for example, 1 μm or more and 30 μm or less, preferably 2 μm or more or 3 μm or more, and more preferably 25 μm or less or 20 μm or less. When the volume average particle size of the carbon material is within the above range, it exhibits good flowability. For example, when manufacturing electrodes for secondary batteries, particle inhomogeneity is suppressed, and there is a tendency to manufacture homogeneous electrodes, sometimes further improving durability and output characteristics. Here, the volume average particle size is the 50% particle size D corresponding to the cumulative 50% from the smallest particle size side of the cumulative particle size distribution based on volume. 50 The cumulative particle size distribution on a volume basis can be determined under wet conditions using a laser diffraction particle size distribution measuring device.

[0031] Carbon materials can be carbon materials with narrow particle size distributions. Regarding particle size distribution, the 90% particle size D in the cumulative particle size distribution based on volume is considered. 90 With 10% particle size D 10 Difference divided by 50% of particle size D 50 The obtained value ((D) 90 -D 10 ) / D 50 The particle size distribution can be 6 or less, preferably 4 or less, and more preferably 2 or less. The lower limit of the particle size distribution is, for example, 0.05 or more. The ratio (D) represents the particle size distribution. 90 -D 10 ) / D 50 This is an index representing the deviation in particle size among the individual particles in the particle group constituting the carbon material. The smaller the value, the smaller the deviation in particle size. When the particle size distribution of the carbon material is within the above range, functional components containing other elements are easily and uniformly attached to the surface of the carbon material.

[0032] Carbon materials may contain other elements besides carbon. Examples of other elements include typical elements such as nitrogen and oxygen, and transition metal elements. The carbon content of a carbon material can be, for example, 70% by mass or more, preferably 80% by mass or more, or 90% by mass or more, and can be substantially composed of only carbon. Here, "substantially" means that the unavoidable inclusion of other elements is not excluded.

[0033] Applications of carbon materials include common adsorbents, negative electrode materials for lithium-ion batteries, electrode materials for capacitors, sulfur carriers in positive electrode materials for lithium-sulfur batteries, and catalyst carriers for air batteries and fuel cells.

[0034] Methods for manufacturing carbon materials

[0035] The aforementioned carbon material can be manufactured, for example, by the following manufacturing method. The manufacturing method of the carbon material may include: a preparation step, preparing materials containing a specific surface area of ​​50 m² / g.2 A suspension of first carbon particles (at least / g), a water-soluble binder, and water is prepared; a granulation process is performed to obtain granules from the suspension; a heat treatment process is then performed to heat-treat the granules to obtain second carbon particles. The obtained second carbon particles have a sphericity greater than 0.83 and a specific surface area of ​​400 m². 2 / g or more constitutes the target carbon material.

[0036] After a first carbon particle with a given specific surface area is aggregated using a water-soluble binder to form a granule, it is heat-treated to produce a second carbon particle as an aggregate of the first carbon particles. This allows for the efficient production of carbon materials with a given sphericity and a given specific surface area.

[0037] In the preparation process, a suspension containing first carbon particles, a water-soluble binder, and water is prepared. The suspension is prepared as a dispersion in a liquid medium containing water, in which the first carbon particles are dispersed. The dispersion state of the first carbon particles in the suspension can be that each first carbon particle is dispersed independently, or it can be dispersed as both individual particles and aggregates of particles. Furthermore, the dispersion state of the first carbon particles in the suspension is such that the suspension can be supplied for spray drying.

[0038] The specific surface area of ​​the first carbon particle can be, for example, 50 m². 2 / g or more, preferably 200m 2 / g or more, preferably 400mg 2 / g or more, or up to 500m 2 / g or higher. The upper limit for specific surface area can be, for example, 2000 m². 2 / g or less. The specific surface area of ​​the first carbon particle can be determined by the BET method described above.

[0039] The average particle size of the first carbon particle can be, for example, 1 nm or more and 200 nm or less, preferably 5 nm or more, or 10 nm or more, and preferably 150 nm or less, or 100 nm or less. The average particle size of the first carbon particle can be determined, for example, by observation with an electron microscope.

[0040] The first carbon particle can be either a hollow particle or a solid particle; from the perspective of the specific surface area of ​​carbon materials, it can be a hollow particle. Here, a hollow particle refers to a particle with cavities inside. The porosity of a hollow particle can be, for example, 10% or more, preferably 20% or more, 40% or more, 50% or more, 60% or more, 70% or more, or 80% or more. The upper limit of the porosity can be, for example, 95% or less. Conversely, a solid particle refers to a particle that is essentially filled with carbon atoms and has no cavities inside.

[0041] The carbon content of the first carbon particle can be, for example, 70% by mass or more, preferably 80% by mass or more, or 85% by mass or more, and can be substantially composed of carbon alone. Here, "substantially" means that the unavoidable inclusion of other elements is not excluded.

[0042] The first carbon particle can be specifically categorized as: acetylene black, furnace black, channel black, pyrolysis carbon black, Ketjen black, and other carbon blacks. Alternatively, carbon materials obtained by micronizing carbon materials obtained by heating organic compounds in an inert gas atmosphere can be used. The first carbon particle may contain at least one of these carbon blacks, or may contain at least Ketjen black. The first carbon particle may consist of one type of carbon black, or may contain two or more types of carbon black.

[0043] Relative to the total mass of the suspension, the content of the first carbon particles in the suspension can be, for example, 0.1% by mass or more and 50% by mass or less, preferably 0.2% by mass or more, or 0.5% by mass or more, and more preferably 30% by mass or less, or 20% by mass or less. When the content of the first carbon particles is within the above range, a better suspension state can be obtained, granulation processes such as spray drying can be performed more efficiently, and there is a tendency to easily obtain the desired granules.

[0044] A water-soluble binder is any substance that is soluble in water and capable of binding the first carbon particles together in a dry state. Regarding the water-soluble binder's solubility in water, a solubility of 100g of pure water at 25°C can be, for example, 1g or more and 500g or less, preferably 10g or more, more preferably 50g or more, and more preferably 450g or less, more preferably 400g or less. Compared to non-water-soluble binders, water-soluble binders tend to adhere more uniformly to the surface of the first carbon particles when dry. Furthermore, when the granules manufactured using the water-soluble binder are heat-treated, it is possible to produce particle shapes with high sphericity and easily obtain carbon materials with large surface areas. Additionally, when the solubility in water is within the above-mentioned range, it is sometimes possible to maintain adhesion to the first carbon particles and easily increase the specific surface area.

[0045] Water-soluble binders can specifically include, for example, monosaccharides such as glucose, disaccharides such as sucrose, and polysaccharides; water-soluble polymers such as polyvinyl alcohol and polyethylene glycol; and water-soluble surfactants that uniformly disperse the first carbon particle. A water-soluble binder may contain at least one selected from sugars, water-soluble polymers, and water-soluble surfactants. A single water-soluble binder may be used, or two or more may be used in combination.

[0046] Relative to the total mass of the suspension, the content of the water-soluble binder in the suspension can be, for example, 0.1% by mass or more and 50% by mass or less, preferably 0.2% by mass or more, or 0.5% by mass or more, and more preferably 20% by mass or less, or 10% by mass or less. Furthermore, the content ratio of the water-soluble binder relative to the first carbon particle can be, for example, 0.05% by more and 10% or less, preferably 0.1% or more, and more preferably 5% or less. When the content of the water-soluble binder is within the above range, a better suspension state can be obtained, granulation processes such as spray drying can be performed more efficiently, and there is a tendency to easily obtain the desired granules.

[0047] The liquid medium constituting the suspension contains at least water. In addition to water, the liquid medium may further contain water-soluble organic solvents as needed. Examples of water-soluble organic solvents include: alcohols such as methanol, ethanol, propanol, and isopropanol; ketones such as acetone and methyl ethyl ketone; and nitrile solvents such as acetonitrile. The water content in the liquid medium can be, for example, 50% by mass or more, preferably 80% by mass or more, and can be substantially only water.

[0048] The suspension may further include other components as needed. Examples of such components include pH adjusters, surfactants, and metal oxides. Examples of pH adjusters include: inorganic acids such as hydrochloric acid, nitric acid, phosphoric acid, and sulfuric acid; organic acids such as acetic acid, citric acid, and oxalic acid; inorganic bases such as alkali metal hydroxides and alkaline earth metal hydroxides; and ammonia. Examples of metal oxides include: oxides containing transition metals such as titanium oxide and tungsten oxide. Furthermore, metal oxides may include compounds formed by complexation with alkali metals and alkaline earth metals. Examples of such complexed compounds include: lithium titanate and lithium tungstate. The suspension may contain organic acids as pH adjusters. When a pH adjuster is included in the suspension, its content can be appropriately selected according to the target pH.

[0049] Suspensions can be prepared, for example, by mixing first carbon particles, a water-soluble binder, a liquid medium containing water, and other components such as a pH adjuster added as needed. Commonly used mixing methods can be employed, such as bead mills, ball mills, homogenizers, planetary mixers, and shear mixers.

[0050] In the granulation process, a suspension is granulated to obtain granules. Granulation can be, for example, spray drying. Granulation is not limited to spray drying; for example, it can be a method of adding an agglomerating agent to a suspension for granulation and then drying after filtration, or a method of emulsifying and agglomerating a suspension for granulation and then drying after filtration. The granules are formed by binding multiple first-carbon particles together using a water-soluble binder. It should be noted that spray drying refers to the process of using gas flow to disperse a suspension into small-diameter particles and then drying them in order to obtain a particulate product. Therefore, as a drying device that can be used for spray drying in the granulation process, a device with both spraying and drying functions can be selected. Examples of such drying devices include, for example, airflow drying devices, fluidized bed drying devices, and so on, represented by so-called spray drying devices.

[0051] The conditions for spray drying can be appropriately selected based on factors such as the particle size of the target granules. For example, the drying temperature in spray drying can be above 80°C and below 150°C.

[0052] In spray drying, a gas is supplied to the drying device for spraying and drying the suspension. The supplied gas can be, for example, atmospheric air, or inert gases such as nitrogen or argon. When using an inert gas, spray drying is easily performed regardless of the type of liquid medium in the suspension.

[0053] In the heat treatment process, carbon material, which serves as the second carbon particle, is obtained by heat-treating the granules obtained in the granulation process. In the heat treatment process, a water-soluble binder contained in the granules binds the first carbon particles together by carbonization, for example, to form the second carbon particles. Alternatively, the heat treatment process can be performed in the same process as the granulation process described above. For example, according to spray pyrolysis methods, the heat treatment process can be included as part of the granulation process.

[0054] The temperature in the heat treatment process can be, for example, above 400°C and below 2000°C, preferably above 500°C, or above 550°C, and more preferably below 1500°C, or below 1300°C. The heat treatment time can be, for example, above 3 hours and below 48 hours, preferably above 4 hours, and more preferably below 24 hours. The gas atmosphere for heat treatment can be a non-reactive gas such as nitrogen or argon. The oxygen concentration in the gas atmosphere for heat treatment can be below 20% by volume, preferably below 1% by volume.

[0055] Heat treatment can be carried out using, for example, box furnaces, roller kilns, rotary kilns, etc.

[0056] Electrode active material

[0057] The electrode active material is composed of the aforementioned carbon material. The carbon material has a large specific surface area and a highly spherical shape. Therefore, when the carbon material is used in the electrode active material, it exhibits excellent filling properties.

[0058] Electrode active materials containing carbon can be used in applications such as negative electrode active materials for lithium-ion batteries, electrode active materials for electric double-layer capacitors, and fuel cells. Alternatively, for example, carbon materials can be loaded with sulfur-containing materials to create positive electrode active materials for lithium-sulfur batteries.

[0059] Positive electrode active material for lithium-sulfur batteries

[0060] The positive electrode active material for lithium-sulfur batteries comprises the aforementioned carbon material and a material containing sulfur that adheres to the carbon material. The sulfur-containing material can adhere to the porous surface of the carbon material. The carbon material has a large specific surface area, thus increasing the battery capacity. Furthermore, by maintaining a given sphericity, the sulfur-containing material adheres more uniformly, suppressing uneven adhesion between particles and improving the battery's cycle characteristics. In addition, it exhibits excellent fillability during electrode formation.

[0061] Materials containing sulfur include, for example, elemental sulfur and polysulfides generated during the charging and discharging process of lithium-sulfur batteries. Specific examples of sulfur-containing materials include S8, Li2S8, Li2S6, Li2S4, and Li2S2. From the perspective of material utilization, elemental sulfur (S8) is preferred.

[0062] Regarding the content of sulfur-containing material disposed in the voids of carbon material in the sulfur-carbon composite, the ratio of the mass of sulfur-containing material to the total mass of carbon material and sulfur-containing material is, for example, 25% by mass or more and 95% by mass or less, preferably 50% by mass or more and 90% by mass or less, and more preferably 67% by mass or more and 80% by mass or less. It should be noted that "67% by mass" refers to the content when the sulfur to carbon mixing ratio is set to 2:1. Furthermore, the content of sulfur-containing material can be, for example, 25% by mass or more, preferably 50% by mass or more, or 67% by mass or more, and for example, 95% by mass or less, preferably 90% by mass or less, or 80% by mass or less. When the content of sulfur-containing material is within these ranges, the capacity of the sulfur-carbon composite decreases.

[0063] The coefficient of variation (CV) of the sulfur-to-carbon ratio in the sulfur-carbon complex is, for example, less than 0.64, preferably less than 0.6 or 0.5, and more preferably less than 0.4. The lower limit of the coefficient of variation can be, for example, 0.05 or higher. The coefficient of variation (CV) of the sulfur-to-carbon ratio is obtained by dividing the average value t1 of the sulfur-to-carbon ratio of any 50 particles of the sulfur-carbon complex by the standard deviation σ1 of the ratio (σ1 / t1). It can be considered that a coefficient of variation of the sulfur-to-carbon ratio within the above range indicates that the deviation in sulfur content among the particles of the sulfur-carbon complex is small, and that the sulfur-carbon complex is homogeneous.

[0064] For the calculation of the coefficient of variation, the average ratio of sulfur to carbon detected amounts is calculated by selecting any 50 particles of the sulfur-carbon complex and taking the arithmetic mean of the detected amounts of each particle. The standard deviation of the sulfur to carbon detected amounts can be calculated from the obtained average value and the detected amounts of each particle. The detected amounts of the sulfur-carbon complex can be determined, for example, using a scanning electron microscope (SEM) / energy-dispersive X-ray analysis (EDX) device.

[0065] Positive electrode for lithium-sulfur batteries

[0066] A positive electrode for lithium-sulfur batteries (hereinafter also referred to as "positive electrode") comprises a current collector and a positive electrode composition layer, wherein the positive electrode composition layer is disposed on the current collector and comprises the aforementioned positive electrode active material for lithium-sulfur batteries. The positive electrode can be manufactured by coating an electrode composition comprising the aforementioned positive electrode active material, liquid medium, binder, and conductive additives onto the current collector, followed by drying and pressure molding.

[0067] The liquid medium can be an organic solvent or water, depending on the application. Examples of organic solvents include amide solvents such as N-methyl-2-pyrrolidone (NMP), ketone solvents such as diisopropyl ketone, diisobutyl ketone, and methyl ethyl ketone, hydrocarbon solvents such as heptane, ether solvents such as tetrahydrofuran, dimethoxyethane, and dioxolane, amine solvents such as diethylenetriamine, and ester solvents. One organic solvent can be used alone, or two or more can be used in combination. The content of the liquid medium relative to the total mass of the electrode composition can be, for example, 10% by mass or more and 90% by mass or less.

[0068] The binder is, for example, a material that facilitates the adhesion of the positive electrode active material and conductive additives, as well as the adhesion of the electrode composition to the current collector. Examples of binders include: polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers. The binder content relative to the total mass of the electrode composition can be, for example, 0.05% by mass or more and 50% by mass or less.

[0069] Conductive additives are materials that improve the conductivity of the positive electrode composition layer. Examples of conductive additives include: modified graphene, natural graphite, artificial graphite, etc.; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and pyrolysis carbon black; conductive fibers such as carbon fibers and metal fibers; and carbon materials such as graphene and carbon nanotubes. The content of the conductive additive relative to the total mass of the electrode composition can be, for example, 0.5% by mass or more and 30% by mass or less.

[0070] Examples of current collectors include metals such as copper, stainless steel, aluminum, nickel, and titanium; composite materials made by surface-treating copper, stainless steel, etc., with carbon, nickel, titanium, silver, etc.; and carbon foil. In the manufacture of lightweight lithium-sulfur batteries, aluminum and carbon foil are preferred as lightweight current collectors. The adhesion of the positive electrode composition layer, etc., can also be improved by forming fine irregularities on its surface. Furthermore, it can take various forms such as a film, sheet, foil, mesh, porous body, foam, or nonwoven fabric. The thickness of the current collector can be, for example, 3 μm or more and 500 μm or less.

[0071] Lithium-sulfur batteries

[0072] A lithium-sulfur battery includes the aforementioned positive electrode for lithium-sulfur batteries. A lithium-sulfur battery is constructed by including a positive electrode, a negative electrode, and an electrolyte disposed between the positive and negative electrodes. A separator may be included in a lithium-sulfur battery as needed. The electrolyte may be contained within the positive electrode, negative electrode, and separator.

[0073] negative electrode

[0074] As the negative electrode constituting a lithium-sulfur battery, known negative electrodes can be used. Examples of negative electrode materials include Li metal, Li-Si alloys, Li-Al alloys, Li-In alloys, and lithium titanate (e.g., Li₄Ti₅O₂). 12 and LiTi2O4), lithium-titanium composite oxides (e.g., Li4Ti 5-x Mn x O 12 ; 0 < x ≤ 0.3), Lix C(x≤6), etc. A portion of the lithium in these anode materials can be replaced with other alkali metals. Preferred anode materials include Li metal, Li-Si alloy, Li-Al alloy, Li-In alloy, and Li... x C(x≤6), etc. If these materials are used, high voltage can be obtained from lithium-sulfur batteries.

[0075] diaphragm

[0076] The diaphragm can be made of known materials, such as porous polyethylene and polypropylene. Alternatively, it can be used by coating known diaphragms.

[0077] electrolytes

[0078] The electrolyte only needs to contain a lithium salt, which can be appropriately selected from the lithium salts used in conventional lithium-ion batteries. For example, the lithium salt can contain anions containing fluorine. Specific examples of lithium salts containing fluorine anions include: LiPF6, LiBF4, LiSbF6, LiAsF6, LiSO3CF3, and LiN(SO2CF3)2 (LiTFSI). Alternatively, the electrolyte can contain fluorine-free lithium salts such as lithium nitrate and LiClO4. One of these electrolytes can be used alone, or in combination of two or more. The electrolyte can contain an organic solvent. Suitable organic solvents include carbonate solvents, ether solvents, ester solvents, amide solvents, nitrile solvents, and sulfur-containing solvents. Additionally, organic solvents in which some atoms of the above-mentioned organic solvents are replaced with fluorine atoms can also be used. Examples of organic solvents include: carbonate solvents such as propylene carbonate, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate; ether solvents such as 1,3-dioxolane, 1,2-dimethoxyethane, 1,3-dimethoxypropane, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, and tetrahydrofuran; ester solvents such as methyl formate, methyl acetate, and γ-butyrolactone; amide solvents such as N,N-dimethylacetamide and N,N-dimethylformamide; and sulfur-containing solvents such as sulfolane, dimethyl sulfoxide, and 1,3-propanesulfonic acid lactone.

[0079] Example

[0080] The present invention will be specifically described below through embodiments, but the present invention is not limited to these embodiments.

[0081] (Example 1)

[0082] Preparation of carbon materials

[0083] 10g of Ketjenblack (manufactured by Lion Corporation; Carbon ECP600JD), 5g of sucrose, and 2.5g of citric acid were mixed with water to a final volume of 1000g, yielding a mixture. 400g of zirconia beads (Φ0.65mm) and the mixture were added to a 2L plastic bottle and wet-dispersed overnight to obtain a slurry 1 in suspension form. The slurry 1 was fed with atmospheric air as a supply gas and sprayed using a spray dryer to obtain carbon precursor 1 as a granulator. Carbon precursor 1 was heat-treated at 700°C for 10 hours in a nitrogen atmosphere to obtain carbon material 1. It should be noted that the average particle size of the Ketjenblack used was 34nm, and the specific surface area was 1270m². 2 / g.

[0084] Preparation of sulfur-carbon complex

[0085] 4.5 g of sulfur (98%; FUJIFILM Wako Pure Chemical) and 1.5 g of carbon material 1 were mixed to obtain a mixture. The resulting mixture was heated at 150°C for 3 hours in a heat-resistant and pressure-resistant container to obtain a sulfur-carbon composite with sulfur adhering to the carbon material. The sulfur content in the voids of the carbon material in the sulfur-carbon composite was 75% by mass.

[0086] (Comparative Example 1)

[0087] 4.5 g of sulfur (98%; FUJIFILM Wako Pure Chemical) and 1.5 g of commercially available porous carbon material CNovel (registered trademark) (manufactured by Toyo Carbon Co., Ltd.) were mixed to obtain a mixture. The resulting mixture was heated at 170°C for 15 hours in a heat-resistant and pressure-resistant container to obtain a sulfur-carbon composite with sulfur adhering to the carbon material. The sulfur content in the pores of the carbon material in the sulfur-carbon composite was 75% by mass.

[0088] Evaluation of roundness

[0089] Evaluation samples were prepared by dispersing the carbon material 1 obtained in Example 1 and the porous carbon material used in Comparative Example 1 in epoxy resin and then curing them by heating. For the evaluation samples, cross-section processing was performed using an ion milling apparatus (Hitachi High-Tech IM4000PLUS; accelerating voltage 6kV). Electron beam images (magnification: 4000x) were taken of the cross-sections of the prepared evaluation samples using a scanning electron microscope (SEM), and 10 to 30 secondary particles whose contours could be confirmed were selected. For each selected particle, the diameter of a circle with the same area as the particle image area corresponding to the contour shape of the secondary particle was determined using image processing software (ImageJ), i.e., the equivalent circle diameter. Furthermore, based on the aforementioned contour shape, the major and minor axes were determined, and the length-to-minor ratio was calculated as the value obtained by dividing the major axis by the minor axis. The major axis of the smoothed approximate ellipse is calculated by taking twice the square root of the product of the particle image area and the aspect ratio divided by pi. The roundness of each particle is then calculated by dividing the equivalent circle diameter by the major axis of the smoothed approximate ellipse, and finally by calculating their arithmetic mean. The results are shown in Table 2. Additionally, the SEM image of carbon material 1 obtained in Example 1 is shown in... Figure 1 The cross-sectional SEM image is shown in Figure 2 Furthermore, SEM images of commercially available porous carbon materials are shown below. Figure 3 The cross-sectional SEM image is shown in Figure 4 .

[0090] Pore ​​distribution evaluation

[0091] The pore volume and peak pore diameter below 0.2 μm were determined using a mercury injection porosimeter (Anton Paar (formerly Quantachrome) POREMASTER-60).

[0092] Specific surface area

[0093] The specific surface area was determined using a BET specific surface area measuring device (Macsorb, manufactured by Mountech) via nitrogen adsorption (one-point method).

[0094] Average particle size of primary particles

[0095] Using scanning electron microscopy (SEM), SEM images of primary particles constituting secondary particles were obtained at magnifications ranging from 20,000 to 100,000 times, based on particle size. Thirty primary particles with clearly identifiable outlines were selected from the SEM images. Based on the outlines of the selected primary particles, the sphere-converted diameters were calculated using image processing software. The average particle size was then determined as the arithmetic mean of the obtained sphere-converted diameters.

[0096] Particle size distribution

[0097] The volumetric particle size distribution was determined using a laser diffraction particle size distribution apparatus (Malvern MASTERSIZER 2000). The 50% particle size D, representing the 50% of the total volumetric particle size from the smallest particle size side, was defined as the particle size distribution on a volumetric basis. 50 The volume average particle size was calculated in the form of [formula missing]. Additionally, the 10% particle size D was calculated as the cumulative volume from the smallest particle size side reaching 10% and 90%. 10 and 90% particle size D 90 , using D 90 With D 10 Difference divided by D 50 The particle size distribution is then calculated.

[0098]

[0099] As shown in Table 1, carbon material 1 exhibits higher sphericity compared to commercially available porous carbon materials. Furthermore, it also has a larger pore volume with a pore diameter of less than 0.2 μm.

[0100] Evaluation of S / C ratio deviation

[0101] After the sulfur-carbon composites obtained in the examples and comparative examples were attached to carbon black, SEM-EDX analysis was performed on any 50 particles (SEM device: JEOL-IT100, accelerating voltage 15kV; EDX device: Oxford E-MAX80, accelerating voltage 15kV). The average value and standard deviation of the detected sulfur to carbon ratio were calculated, and the deviation of each particle was evaluated.

[0102] Specifically, for each particle, the sulfur-to-carbon ratio (S / C ratio) is calculated by dividing the detected sulfur amount by the detected carbon amount. Based on the obtained ratio, the arithmetic mean t1 and standard deviation σ1 of the ratios for all particles are calculated. Then, the standard deviation σ1 is calculated using the STDEV.P function in Excel. Based on the arithmetic mean t1 and standard deviation σ1 of the obtained ratios, the coefficient of variation CV (σ1 / t1) is determined. The results are shown in Table 2.

[0103]

[0104] As shown in Table 2, compared with the sulfur-carbon composite of Comparative Example 1, the sulfur-carbon composite using carbon material 1 with high sphericity has a reduced deviation in the attached sulfur.

[0105] Positive electrode production

[0106] A slurry was prepared by dispersing and dissolving 80 parts by weight of the sulfur-carbon composite obtained in Example 1 and Comparative Example 1, 16 parts by weight of PVDF, and 4 parts by weight of carbon nanotubes in NMP. The prepared slurry was coated onto a current collector and dried to obtain a dried product. The current collector used was aluminum foil coated with carbon. The dried product was compressed using a roller press to achieve a density of 0.63 g / cm³ for the active material layer. 3 Then, it is cut to a given size, thus obtaining the positive electrode.

[0107] Preparation of non-aqueous electrolyte

[0108] The preparation of the non-aqueous electrolyte was carried out as follows. 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) were mixed in a volume ratio of 5:5. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was dissolved in the resulting mixed solvent to a concentration of 1.0 mol / L. Next, lithium nitrate (LiNO3) was dissolved to a concentration of 0.2 mol / L, thus preparing the non-aqueous electrolyte.

[0109] Assembly of lithium-sulfur secondary batteries

[0110] Lead electrodes were attached to the obtained positive electrode, and a separator made of porous polypropylene (SELGUARD 2400) was prepared. These were then placed in a pouch-like laminated bag. After preparation, the bags were vacuum-dried at 50°C to remove any moisture adsorbed on the components. Following vacuum drying, the lithium metal-coated porous polypropylene negative electrode, mounted on a SUS foil, was placed opposite the positive electrode in the laminated bag. A non-aqueous electrolyte was injected, and the bag was sealed to obtain a laminated lithium-sulfur secondary battery for evaluation. The battery characteristics were evaluated using the obtained evaluation battery.

[0111] Evaluation of capacity retention during charge-discharge cycles

[0112] The evaluation battery obtained as described above was placed in a constant temperature bath at 25°C, and a cycle capacity retention evaluation test was conducted. The charge and discharge voltage was in the range of 1.8V to 3.0V. The discharge current was the current value at which the 0.2C capacity was obtained. The discharge capacity Qcyc(1) (mAh / g) of the first cycle immediately after discharge was measured. Hereinafter, the charge and discharge were repeated, and the discharge capacity Qcyc(30) of the 30th cycle was measured. The capacity retention rate Pcyc (=100×Qcyc(30) / Qcyc(1)) (%) after 30 cycles was calculated by dividing the obtained Qcyc(1) by Qcyc(30). The results are shown in Table 3. It should be noted that the discharge capacity was calculated based on the conversion of sulfur per unit mass.

[0113]

[0114] For evaluation batteries containing positive electrode active materials made of carbon material 1, the cycle characteristics are improved.

[0115] (Example 2)

[0116] Preparation of carbon materials

[0117] Except for changing the supply ratio of the mixed slurry 1 to the supply gas, carbon material 2 was obtained by the same method as in Example 1.

[0118] The obtained carbon material 2 has a sphericity of 0.94 and a specific surface area of ​​885 m². 2 / g, with a volume average particle size of 9.6μm and a particle size distribution of 1.42. The SEM image of carbon material 2 obtained in Example 2 is shown below. Figure 5 The cross-sectional SEM image is shown in Figure 6 .

[0119] Powder resistance evaluation

[0120] For carbon material 2 and commercially available porous carbon material, the pressures shown in Table 4 were applied for compression molding to obtain compressed molded products. The volume of the compressed molded product was calculated by measuring the thickness of the compressed molded product under each pressure using a micrometer, and then divided by the mass of the carbon material used to calculate the density of the compressed molded product. Furthermore, the powder resistivity of the compressed molded products under each pressure was evaluated using a four-probe method (MCP-PD51 manufactured by Nittoseiko Analytech, formerly Mitsubishi Chemical Analytech). The results are shown in Table 4.

[0121]

[0122] As shown in Table 4, when compressed under the same pressure, carbon material 2, with its higher sphericity, exhibits higher filling capacity compared to the commercially available porous carbon material of the comparative example. Furthermore, when compared at the same density, carbon material 2 shows a lower powder resistivity than the commercially available porous carbon material, demonstrating superior electrical conductivity.

[0123] The disclosure of Japanese Patent Application No. 2021-029069 (filed on February 25, 2021) is incorporated herein by reference in its entirety. For all documents, patent applications, and technical standards described herein, each document, patent application, and technical standard is incorporated herein by reference to the same extent as the specific and separately described cases.

Claims

1. A positive electrode active material for lithium-sulfur batteries, comprising: Carbon materials with a sphericity greater than 0.83 and a volume average particle size of 1 μm or more and 30 μm or less; and Sulfur element attached to the carbon material.

2. The positive electrode active material for lithium-sulfur batteries according to claim 1, wherein, The carbon material comprises secondary particles consisting of a collection of multiple carbon-containing primary particles.

3. The positive electrode active material for lithium-sulfur batteries according to claim 1 or 2, wherein, The carbon material has a pore volume of 1.5 cm3 / g or more for pores having a pore diameter of 0.2 μm or less 3 / g or less and 3 cm 3 / g or less.

4. The positive electrode active material for lithium-sulfur batteries according to claim 1, wherein, The specific surface area of ​​the carbon material is 2000 m². 2 / g or less.

5. The positive electrode active material for lithium-sulfur batteries according to claim 2, wherein, The specific surface area of ​​the carbon material is 2000 m². 2 / g or less.

6. The positive electrode active material for lithium-sulfur batteries according to claim 5, wherein, The primary particles in the carbon material have an average particle size of 1 nm or more and 200 nm or less.

7. The positive electrode active material for lithium-sulfur batteries according to claim 5, wherein, The primary particles in the carbon material are hollow particles.

8. The positive electrode active material for lithium-sulfur batteries according to claim 1 or 2, wherein, The sulfur content is between 67% and 80% by mass relative to the total mass of carbon and sulfur.

9. The positive electrode active material for lithium-sulfur batteries according to claim 1 or 2, wherein, The coefficient of variation for the ratio of sulfur to carbon detected is less than 0.

64.

10. A lithium-sulfur battery comprising the positive electrode active material for lithium-sulfur batteries as described in claim 1 or 2.

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