Porous carbon material for lithium-sulfur battery and method for producing same

Abstract

The present disclosure provides a porous carbon material from which it is possible to fabricate a lithium-sulfur battery having a large weight energy density. The present disclosure relates to a porous carbon material for a lithium-sulfur battery, in which: according to a heat-generated gas mass analysis, the ratio of the desorption amount of CO2 desorbed, at a temperature increase rate of 10°C / minute, in a temperature range from 400°C to 1000°C relative to the ratio of the desorption amount of CO2 desorbed in a temperature range from 30°C to 1000°C is 85% or more; and the total acidic functional group amount measured by the Boehm method is 0.3 mmol / g or more.

Description

Porous carbon material for lithium-sulfur batteries and method for producing the same

[0001] The present disclosure relates to a porous carbon material for lithium-sulfur batteries and a method for producing the same.

[0002] Lithium-sulfur batteries are secondary batteries with high energy density per unit mass, and for example, mesoporous sulfur-carbon composites, in which sulfur is arranged within mesopores, are used. Lithium-sulfur batteries are lighter than conventional lithium-ion batteries, and are expected to be used in applications such as large power equipment like electric vehicles and aviation applications like drones. In particular, improvements in cycle capacity retention and charge / discharge capacity are required for these applications.

[0003] Patent Document 1 proposes a positive electrode material that is a composite of lithium sulfide and a carbon material. Patent Documents 2 and 3 propose inventions that introduce functional groups into a porous carbon material to prevent deposition on the negative electrode.

[0004] Japanese Patent Application Laid-Open No. 2015-220225 International Publication No. 2018 / 030616 International Publication No. 2022 / 186151

[0005] An object of the present disclosure is to provide a porous carbon material that can be used to fabricate lithium-sulfur batteries having high gravimetric energy density.

[0006] The porous carbon material for a lithium-sulfur battery according to one embodiment of the present disclosure has a CO desorbed rate of 10°C / min in a temperature range of 30°C to 1000°C when measured by thermal evolved gas mass spectrometry. ２ CO desorbed in the temperature range of 400°C to 1000°C relative to the amount of CO desorbed ２ The ratio of the amount of the released acidic functional groups is 85% or more, and the total amount of the acidic functional groups measured by the Boehm method is 0.3 mmol / g or more.

[0007] A positive electrode material for a lithium-sulfur battery according to one embodiment of the present disclosure contains the carbon material for a lithium-sulfur battery according to the present disclosure and a material containing elemental sulfur.

[0008] A method for producing a positive electrode material for a lithium-sulfur battery according to one embodiment of the present disclosure includes a step of mixing the carbon material for a lithium-sulfur battery according to the present disclosure with a material containing elemental sulfur, and a step of heating the resulting mixture.

[0009] According to the porous carbon material for lithium-sulfur batteries of the present disclosure, a lithium-sulfur battery having a high weight energy density can be obtained.

[0010] 1 is a diagram showing the pore size distribution and cumulative particle size distribution of the porous carbon material produced in Comparative Example 1. FIG. 2 is a diagram showing the pore size distribution and cumulative particle size distribution of the porous carbon material produced in Example 2. FIG. 3 is a diagram showing the pore size distribution and cumulative particle size distribution of the porous carbon material produced in Example 5.

[0011] Hereinafter, embodiments of the present disclosure will be described in detail. However, the embodiments described below are examples for embodying the technical ideas of the present disclosure, and the present disclosure is not limited to the following. In this specification, the term "process" includes not only an independent process, but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

[0012] <<Porous Carbon Material for Lithium-Sulfur Batteries>> The porous carbon material for lithium-sulfur batteries of the present disclosure is a material that exhibits a high carbon content of CO desorbed in a temperature range of 30°C to 1000°C at a heating rate of 10°C / min when measured by thermally evolved gas mass spectrometry (TPD-MS). ２ CO desorbed in the temperature range of 400°C to 1000°C relative to the amount of CO desorbed ２ The ratio of the amount of the released acidic functional groups is 85% or more, and the total amount of the acidic functional groups measured by the Boehm method is 0.3 mmol / g or more.

[0013] The porous carbon material used in the porous carbon material for lithium-sulfur batteries of the present disclosure is not particularly limited, and examples thereof include activated carbon such as microporous carbon, mesoporous carbon, macroporous carbon, and micromesoporous carbon, as well as ketjen black, acetylene black, carbon nanotubes, multilayer graphene, and graphite. These carbon materials may aggregate to form secondary particles. These porous carbon materials may be used alone or in combination of two or more.

[0014] By surface treating the porous carbon material, functional groups are formed on the surface of the carbon material. When the carbon material with the formed functional groups is heated, the functional groups present on the surface decompose depending on the heating temperature, and CO, CO ２ Examples of functional groups that generate such decomposition gases include carboxyl groups, functional groups having a lactone ring, acid anhydride groups, phenolic hydroxyl groups, carbonyl groups, ether groups, and groups having a quinone structure. For example, carboxyl groups generate CO at temperatures between 100°C and 400°C, and functional groups having a lactone ring generate CO at temperatures between 190°C and 650°C. ２ The acid anhydride group decomposes at 350 to 620°C, and CO ２ At the same time, CO is produced. On the other hand, phenolic hydroxyl groups decompose at 600 to 700°C, producing CO and water. Carbonyl groups decompose at 700 to 980°C, ether groups at 700°C, and groups with a quinone structure decompose at 700 to 980°C, producing CO. The type of functional group in the carbon material can be estimated from the desorption temperature and decomposition gas components. CO desorbs in the temperature range of 30 to 1000°C. ２ The amount of CO released is a physical property related to the amount of carboxyl groups, functional groups having lactone rings, and acid anhydride groups present, and the amount of CO released in the temperature range of 400°C to 1000°C ２ The amount of CO released is a physical property related to the amount of functional groups having lactone rings and acid anhydride groups present. ２ The ratio of the amount of elimination of the above represents the proportion of functional groups having a lactone ring and acid anhydride groups present.

[0015] CO desorbed in the temperature range from 30°C to 1000°C ２ CO desorbed in the temperature range of 400°C to 1000°C relative to the amount of CO desorbed ２ The ratio of the amount of CO desorbed at low temperatures is 85% or more, preferably 90% or more, and more preferably 95% or more. If the ratio is less than 85%, the proportion of functional groups with weak acidity, such as lactone rings, among the functional groups present on the surface of the carbon material decreases, and the reverse Coulomb efficiency tends to decrease. If this ratio is high, the proportion of CO desorbed at low temperatures is low. ２ CO desorbed at high temperatures ２This means that there are fewer carboxyl groups, which are strongly acidic, and more groups, which have lactone rings or the like, which are weakly acidic. It is thought that the acidic functional groups capture Li ions in the electrolyte, and then the Li ions are conducted into the sulfur. When the functional groups are weakly acidic, the Li ions are smoothly conducted from the functional groups to the sulfur, whereas when the functional groups are strongly acidic, the Li ions are strongly adsorbed by the functional groups, and the Li ion conduction is weak. Therefore, a higher proportion of weakly acidic functional groups is preferable because it increases the reverse Coulomb efficiency.

[0016] The total amount of acidic functional groups is measured by the Boehm method. The Boehm method is a method in which an alkali is added to a sample to cause a reaction, and the alkali concentration after the reaction is back-titrated with an acid to quantify the amount of acidic functional groups present on the sample surface. The total amount of acidic functional groups is determined by the aforementioned CO, CO ２ This is a physical property related to the amount of carboxyl groups, phenolic hydroxyl groups, lactone groups, and acid anhydride groups present among the functional groups that generate decomposition gases such as those mentioned above.

[0017] The total amount of all acidic functional groups is 0.3 mmol / g or more, preferably 0.5 mmol / g or more, and more preferably 1.0 mmol / g or more. The upper limit is not particularly limited, but 4 mmol / g or less is preferable. If it is less than 0.3 mmol / g, the initial discharge capacity tends to be low. This is thought to be because if the total amount of acidic functional groups is large, the amount of Li that is irreversibly adsorbed decreases.

[0018] Specific CO ２The porous carbon material for lithium-sulfur batteries of the present disclosure, having a ratio of the amount of desorption of the acidic functional groups and a specific total amount of acidic functional groups, can be prepared by performing an oxidation treatment, if necessary. Examples of oxidation treatment methods include heating in the atmosphere. The heat treatment conditions required to obtain a carbon material having the functional group state of the present disclosure vary greatly depending on the original surface functional group state of the base carbon material. The heating temperature is not particularly limited, but is preferably 100°C or higher and 500°C or lower, and more preferably 200°C or higher and 400°C or lower. Temperatures below 100°C do not increase the targeted amount of acidic functional groups, while temperatures above 400°C tend to result in oxygen combustion of carbon, resulting in significant weight loss. The heating time is also not particularly limited, but is preferably 2 hours or higher and 24 hours or lower, and more preferably 3 hours or higher and 12 hours or lower. Heating for less than 2 hours results in a small amount of modified acidic functional groups, resulting in little impact on battery characteristics. On the other hand, heat treatment in the atmosphere for a long period of time exceeding 24 hours only slightly increases the amount of acidic functional groups, which is undesirable from the viewpoint of low yield.

[0019] The porous carbon material may have a peak pore diameter of, for example, 0.5 nm to 10 nm, 1.5 nm to 2.0 nm, or 2.5 nm or more, or 8 nm to 6 nm, or 4 nm to 4 nm. When the peak pore diameter of the carbon material for lithium-sulfur batteries is within the above range, sulfur is less likely to dissolve into the electrolyte, and the charge / discharge capacity tends to be increased. Here, the peak pore diameter refers to the pore diameter showing the largest peak in a pore distribution curve obtained by quenched solid density functional theory (QSDFT) from a nitrogen adsorption / desorption curve obtained by nitrogen gas adsorption / desorption measurement.

[0020] The pore volume of the carbon material for lithium-sulfur batteries may 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 2.0 ml / g or less, 1.8 ml / g or less, or 1.6 ml / g or less. In particular, the pore volume of pores having a diameter of 1.4 nm or more and 5.0 nm or less is preferably within the above range. When the pore volume of the carbon material for lithium-sulfur batteries is within the above range, sufficient volume is obtained for sulfur to expand and contract when sulfur is encapsulated in the pores, and charge / discharge capacity tends to be increased. The pore volume of the carbon material for lithium-sulfur batteries is measured by the nitrogen gas adsorption / desorption measurement described above.

[0021] The pore diameter at 70% cumulative volume obtained by the QSDFT method of the porous carbon material may be 1 nm or more and 5 nm or less, preferably 2 nm or more and 4 nm or less. The pore diameter at 80% cumulative volume may be 1 nm or more and 5 nm or less, preferably 2 nm or more and 4 nm or less. Furthermore, the volume fraction of pores having a pore diameter of 1.5 nm or more and 4 nm or less is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more. Furthermore, the proportion of pores having a pore diameter of 1.5 nm or more and 5 nm or less is preferably 70% or more on a volume basis, and the proportion of pores having a pore diameter of 2 nm or more and 4 nm or less is also preferably 60% or more on a volume basis. Within the above-mentioned range, elution of intermediates from the pores tends to be reduced, improving cycle characteristics. Here, the QSDFT method is a quenched solid density functional method, and the pore distribution is calculated by applying the QSDFT method to the measured nitrogen adsorption / desorption curve, and a predetermined cumulative volume pore diameter is calculated.

[0022] The specific surface area of ​​the porous carbon material for a lithium-sulfur battery of the present disclosure is 100 m ２ / g or more 4500m ２ The lower limit of the specific surface area is 200 m ２ / g or more is more preferable, and 500m ２ / g or more is more preferable. The specific surface area may be calculated, for example, by a gas adsorption method. Alternatively, a value measured by a single-point method or a multi-point method using nitrogen gas based on the Brunauer, Emmett, Teller (BET) theory from a nitrogen adsorption / desorption curve obtained by a nitrogen adsorption / desorption measurement method may be used. In general, porous carbon materials with a large specific surface area tend to have a high charge / discharge capacity for sulfur-carbon composites. Alternatively, the specific surface area calculated by the QSDFT method described above may be used as the specific surface area.

[0023] The porous carbon material may be a secondary particle formed by an aggregation of primary particles, or may be a secondary particle having interconnected pores in which voids are connected internally. The 50% particle size D50 in the cumulative particle size distribution based on volume of the secondary particles of the porous carbon material may be 0.1 μm or more and 100 μm or less, preferably 1 μm or more, and more preferably 2 μm or more. The volume average particle size of the porous carbon material is preferably 50 μm or less, more preferably 30 μm or less. D50 is measured, for example, using a laser diffraction particle size distribution analyzer.

[0024] The primary particles constituting the secondary particles of the porous carbon material may be hollow particles. The particle size of the primary particles constituting the secondary particles may be, for example, 1 nm or more and 100 μm or less. The particle size of the primary particles is preferably 2 nm or more, more preferably 5 nm or more. The particle size of the primary particles is preferably 100 μm or less, more preferably 30 μm or less. The particle size of the primary particles is determined, for example, by calculating the sphere-equivalent diameter from the contour lengths of 20 primary particles for each of 10 selected secondary particles in an image observed using a scanning electron microscope (SEM), a transmission electron microscope (TEM), or a scanning transmission electron microscope (STEM), and then calculating the arithmetic average of these diameters to determine the particle size of the primary particles.

[0025] The aspect ratio of the porous carbon material for lithium-sulfur batteries of the present disclosure is preferably 1 or more and 10 or less, and more preferably 5 or less. The closer the aspect ratio is to 1, the higher the sphericity is, and the more uniform the reaction between lithium and sulfur progresses within the particles, making the material suitable for use as a porous carbon material for lithium-sulfur batteries.

[0026] <<Positive Electrode Material for Lithium-Sulfur Batteries>> The positive electrode material for lithium-sulfur batteries of the present disclosure is characterized by containing the carbon material for lithium-sulfur batteries of the present disclosure described above and a material containing elemental sulfur.

[0027] Materials containing sulfur element include not only elemental sulfur but also polysulfides generated during the charge and discharge process of lithium-sulfur batteries. ２ S ８ , Li ２ S ６ , Li ２ S ４ , Li ２ S ２ Among these, elemental sulfur is preferred from the viewpoint of yield.

[0028] The content of the porous carbon material in the positive electrode material for lithium-sulfur batteries may be, 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. Note that "67% by mass" refers to the content when the mixture ratio of the material containing elemental sulfur to the carbon material is 2:1. When the content of the material containing elemental sulfur is within these ranges, the capacity loss of the sulfur-carbon composite is reduced.

[0029] The positive electrode material for a lithium-sulfur battery according to the present disclosure is disposed on a current collector to form a positive electrode for the lithium-sulfur battery. The positive electrode material for a lithium-sulfur battery according to the present disclosure is mixed with a liquid medium, a binder, a conductive additive, etc., as needed, and the resulting mixture is applied to a current collector, followed by drying and pressure molding to form a positive electrode active material layer on the current collector, thereby producing a positive electrode.

[0030] The liquid medium may be an organic solvent, water, or the like, depending on the application. Examples of organic solvents include amide-based solvents such as N-methyl-2-pyrrolidone (NMP), ketone-based solvents such as diisopropyl ketone, diisobutyl ketone, and methyl ethyl ketone, hydrocarbon-based solvents such as heptane, ether-based solvents such as tetrahydrofuran, dimethoxyethane, and dioxolane, amine-based solvents such as diethylenetriamine, and ester-based solvents. The organic solvents may be used alone or in combination of two or more. The content of the liquid medium may be, for example, 10% by mass or more and 90% by mass or less with respect to the total mass of the electrode composition.

[0031] The binder is a material that aids in adhesion between the positive electrode active material and the conductive additive, and in 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-butylene rubber, fluororubber, and various copolymers. The binder content may be, for example, 0.05% by mass or more and 50% by mass or less relative to the total mass of the electrode composition.

[0032] The conductive additive is, for example, a material that improves the electrical conductivity of the positive electrode composition layer. Examples of the conductive additive 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; and carbon materials such as graphene and carbon nanotubes. The content of the conductive additive may be, for example, 0.5% by mass or more and 30% by mass or less with respect to the total mass of the electrode composition.

[0033] Examples of current collectors include metals such as copper, stainless steel, aluminum, nickel, and titanium; composite materials in which the surface of copper or stainless steel is surface-treated with carbon, nickel, titanium, silver, or the like; and carbon foil. When manufacturing a lightweight lithium-sulfur battery, for example, aluminum and carbon foil are preferred as lightweight current collectors. The current collector can also have fine irregularities on its surface to increase the adhesive strength of the positive electrode composition layer, etc. Various forms are possible, such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics. The thickness of the current collector may be, for example, 3 μm or more and 500 μm or less.

[0034] Lithium-sulfur secondary battery The lithium-sulfur battery includes the above-mentioned positive electrode for a lithium-sulfur battery. The lithium-sulfur battery includes a positive electrode for a lithium-sulfur battery, a negative electrode, an electrolyte disposed between the positive electrode and the negative electrode, and the like. The lithium-sulfur battery may also include a separator as necessary. The electrolyte may be contained in the positive electrode, negative electrode, and separator for the lithium-sulfur battery.

[0035] Anode: Any known material may be used as the anode of the lithium-sulfur battery. Examples of anode materials include Li metal, Li-Si alloy, Li-Al alloy, Li-In alloy, and lithium titanate (e.g., Li ４ Ti ５ O １２ and Li ２ TiO ３ ), lithium titanium composite oxide (e.g., Li ４ Ti ５ -xMn ｘ O １２ ;0<x≦0.3), LiC ｘ (x≦6), etc. In these negative electrode materials, a part of the lithium may be substituted with other alkali metals. Examples of the negative electrode material include Li metal, Li-Si alloy, Li-Al alloy, Li-In alloy, Li ｘ C (x≦6) is preferred. These materials allow a high voltage to be extracted from the lithium-sulfur battery.

[0036] Separator The separator may be made of a known material, such as porous polyethylene, polypropylene, etc. Also, a known separator may be used after being coated.

[0037] The electrolyte may contain a lithium salt and may be appropriately selected from lithium salts used in conventional lithium ion batteries. The lithium salt may contain, for example, an anion having a fluorine element. Specific examples of lithium salts containing an anion having a fluorine element include LiPF ６ , LiBF ４ , LiSbF ６ , LiAsF ６ , LiSO ３ CF ３ , LiN(SO ２ CF ３ ) ２ (LiTFSI), etc. The electrolyte may be lithium nitrate, LiClO ４ The electrolyte may contain a lithium salt not containing fluorine element, such as: One of these may be used alone, or two or more may be used in combination.

[0038] The electrolyte may contain an organic solvent. Examples of the organic solvent include carbonate-based solvents, ether-based solvents, ester-based solvents, amide-based solvents, nitrile-based solvents, and sulfur-containing solvents. Alternatively, organic solvents in which some of the elements of the above organic solvents have been substituted with fluorine may also be used. Examples of the organic solvent include carbonate-based solvents such as propionate carbonate, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, fluoroethylene carbonate, and vinylene carbonate; ether-based solvents such as 1,3-dioxolane, 1,2-dimethoxyethane, 1,3-dimethoxypropane, 2,2,3,3-tetrafluoropropyl difluoromethyl ether and tetrahydrofuran; ester-based solvents such as methyl formate, methyl acetate, and γ-butyrolactone; amide-based solvents such as N,N-dimethylacetamide and N,N-dimethylformamide; and sulfur-containing solvents such as sulfolane, dimethyl sulfoxide, and 1,3-propane sultone.

[0039] <<Method for producing a positive electrode material for a lithium-sulfur battery>> The method for producing a positive electrode material for a lithium-sulfur battery of the present disclosure is characterized by including a step of mixing the carbon material for a lithium-sulfur battery of the present disclosure with a material containing elemental sulfur, and a step of heating the resulting mixture.

[0040] The method for mixing the carbon material for a lithium-sulfur battery with the material containing elemental sulfur is not particularly limited, and any known method can be used.

[0041] The heating temperature of the obtained mixture is not particularly limited, but is preferably 110°C or higher and 450°C or lower, and more preferably 150°C or higher and 400°C or lower. If the temperature is lower than 110°C, the sulfur is below the melting point of sulfur and therefore does not penetrate into the pores of the porous carbon. If the temperature exceeds 450°C, the sulfur is above the boiling point of sulfur and therefore does not penetrate into the pores of the porous carbon. The heating time is also not particularly limited, but is preferably 1 hour or higher and 24 hours or lower, and more preferably 2 hours or higher and 20 hours or lower.

[0042] Examples will be described below. Unless otherwise specified, "%" is by mass.

[0043] Comparative Examples 1, 4, and 5, and Examples 1, 2, 4, 6, 7, and 9 Porous carbon materials of the comparative examples and examples were prepared by placing 10 g of a porous carbon material (manufactured by AT Electrode Co., Ltd., AP20-0001HC) in an alumina crucible and heat-treating it in an air atmosphere under the conditions shown in Table 1. Note that as the porous carbon material of Comparative Example 1, the porous carbon material (manufactured by AT Electrode Co., Ltd., AP20-0001HC) was used as is without heat-treating it.

[0044] Comparative Examples 2 to 3, Example 5 100 g of nitric acid having the concentration shown in Table 1 and 10 g of a porous carbon material (manufactured by AT Electrode Co., Ltd., AP20-0001HC) were transferred to a glass beaker and stirred for 2 hours at room temperature. After stirring, the mixture was filtered, washed with pure water, and dried at 150°C for 15 hours to produce the porous carbon materials of Comparative Examples 2 to 3. For Example 5, an additional heat treatment was performed at 350°C for 5 hours.

[0045] Examples 3 and 8 30 g of a carbon material (AT Electrode Co., Ltd., LN0001C) was placed in an alumina crucible and introduced into a horizontal tubular furnace. The inside of the tubular furnace was replaced with an inert atmosphere, and the temperature was raised to 1000°C. After reaching 1000°C, additional carbon dioxide was introduced. After a predetermined time had passed, the temperature was returned to room temperature, and a porous carbon material was produced. The produced porous carbon materials were subjected to heat treatment under the conditions shown in Examples 3 and 8 in Table 1, respectively.

[0046] The obtained porous carbon material was subjected to the following evaluations. The evaluation results are shown in Table 1.

[0047] <CO ２ Desorption Amount Ratio> The amount of carbon dioxide gas generated from the porous carbon material was measured using a thermal desorption photoionization mass spectrometer (TPD Type R, manufactured by Rigaku Corporation) in a temperature range from room temperature to 1000°C. Specifically, 4 mg to 15 mg of sample was collected and introduced into the glass tube of the thermal desorption photoionization mass spectrometer. He gas was passed through at 300 mL / min, and the temperature was raised to 1000°C at a heating rate of 10°C / min. The generated gas was measured using a quadrupole mass spectrometer and the profile corresponding to m / z = 44 was defined as the generated carbon dioxide gas. From the obtained profile, the ratio of the total amount of generated carbon dioxide gas to the amount of carbon dioxide gas generated between 400°C and 1000°C was calculated.

[0048] <Total Amount of Acidic Functional Groups> Measurement was performed with reference to the Boehm method (H.P. Boehm, Adzan. Catal., 16, 179 (1966)). Specifically, 50 mL of a sodium hydroxide aqueous solution (0.05 mol / L) was added to 1 g of the porous carbon material, and the mixture was stirred with a roller for 4 hours. After stirring, 10 mL of the filtrate obtained by filtration was sampled, and a sulfuric acid aqueous solution (0.025 mol / L) was added dropwise to measure the titer at which the pH reached approximately 8.4. As a blank test, 10 mL of a sodium hydroxide aqueous solution (0.05 mol / L) was sampled, and a sulfuric acid aqueous solution (0.025 mol / L) was added dropwise to measure the titer at which the pH reached approximately 8.4. The total amount of acidic functional groups was calculated using the following formula: Total amount of acidic functional groups (mmol / g) = {(a - b) x 0.025 x 2 x 50 / 10} / S, where a: sulfuric acid titration amount (mL) in the blank test, b: sulfuric acid titration amount (mL) when the sample was reacted, and S: sample weight (g).

[0049] <Peak Pore Diameter, Specific Surface Area, and Pore Volume> The volume of open pores (pore diameter 0.4 nm to 100 nm) that can accommodate gas was measured for the obtained carbon material using a surface area / pore analyzer (trade name: Nova Touch, manufactured by Anton Paar). Specifically, after dehydration treatment at 150°C for 1 hour while evacuating, the amount of nitrogen adsorption was measured at 77 K using nitrogen as an adsorbent at a pressure range of 0.0001 Torr to 760 Torr, and the amount of nitrogen adsorption was taken as the volume of the open pores (pore volume). Furthermore, the quenched solid density functional theory (QSDFT) method was applied to the obtained nitrogen adsorption / desorption curve to calculate the pore distribution. In the QSDFT method, fitting was performed using a cylindrical pore model to calculate the peak pore diameter and specific surface area. Graphs of the pore size distribution and cumulative particle size distribution for the porous carbon materials prepared in Comparative Example 1, Examples 2, and 5 are shown in Figures 1 to 3.

[0050] <D50> The volumetric particle size distribution was measured using a laser diffraction particle size distribution analyzer (MASTERSIZER 3000, manufactured by Malvern). The measurement sample was prepared as follows. 2 mL of dispersant (ADEKA Pluronic L-44; manufactured by ADEKA) was diluted with 170 mL of water solvent and dispersed using a touch mixer to obtain a diluted dispersant. An appropriate amount of porous carbon material, 2.5 mL of water, and 2.5 mL of diluted dispersant were placed in a test tube so that the laser intensity of the measurement device was within the appropriate range. The mixture was dispersed using a touch mixer for several seconds, and then subjected to ultrasonic dispersion treatment for 120 seconds to obtain a measurement sample. The volume-average particle size was calculated as the 50% particle size D50, where the volume cumulative value from the small particle size side in the volumetric particle size distribution is 50%.

[0051] <Aspect Ratio> A porous carbon material (manufactured by AT Electrode Co., Ltd., AP20-0001HC) before impregnation with sulfur, which had not undergone any surface treatment, was mounted on carbon tape, and the shape of the porous carbon material was observed using an electron microscope (manufactured by Hitachi High-Tech Corporation, FLEXSEM1000II). The observation was performed under conditions of an acceleration voltage of 5 kV and a magnification of 10,000 times. Any particle whose particle shape could be confirmed (visually observed under SEM, the arithmetic mean of the major and minor diameters was D ５０ The aspect ratio of each of the 47 particles (0.1 to 1.5 times the particle size) was determined using Image J, and was found to be 1.15.

[0052] Comparative Examples 1 to 5, Examples 1, 2, 4 to 7, and 9: 8 g of the porous carbon material prepared in each of the comparative examples and examples was mixed with 12 g of sulfur powder. The resulting mixture was placed in a pressure-resistant container and heat-treated at 150°C for 3 hours. After the heat treatment, 5 g of the mixture was placed in an SUS container and heat-treated at 300°C for 3 hours to obtain the sulfur-carbon composites of each of the comparative examples and examples.

[0053] Examples 3 and 8 3 g of the porous carbon material prepared in Examples 3 and 8 and 7 g of sulfur powder were mixed. The resulting mixture was placed in a pressure-resistant container and heat-treated at 150°C for 3 hours. After the heat treatment, 5 g of the mixture was placed in an SUS container and heat-treated at 300°C for 3 hours to obtain sulfur-carbon composites using the porous carbon material prepared in Examples 3 and 8.

[0054] Preparation of Positive Electrode for Lithium-Sulfur Secondary Battery A slurry composition for a positive electrode was prepared by mixing 85 mass % of the obtained sulfur-carbon composite, 10 mass % of carbon black, and 5 mass % of styrene-butadiene rubber (SBR) / carboxymethyl cellulose (CMC) / polyvinylpyrrolidone (PVP) (ratio of 2.0 / 2.5 / 0.5).

[0055] The obtained positive electrode slurry composition was applied to a current collector and dried at 50° C. An aluminum foil coated with carbon was used as the current collector. The dried product was pressed with a roll press to a density of the active material layer of 1.0 g / cm. ３ After pressing to a desired shape, the mixture was cut to a predetermined size to prepare a positive electrode for a lithium-sulfur secondary battery.

[0056] <Reverse Coulomb Efficiency> Preparation of Non-Aqueous Electrolyte Solution Fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 71:100 to obtain a mixed solution. Lithium hexafluorophosphate (LiPF ６ ) was dissolved in the solution to a concentration of 1.0 mol / L to prepare a non-aqueous electrolyte solution.

[0057] Assembly of Lithium-Sulfur Secondary Battery A lead electrode was attached to the obtained positive electrode, and a separator made of porous polypropylene (Celgard 2400) was placed. The resultant was then housed in a bag-shaped laminate pack. After storage, the pack was vacuum dried at 50°C to remove moisture adsorbed to each component. After vacuum drying, the metallic lithium of the negative electrode placed on SUS foil was placed in the laminate pack facing the positive electrode with the porous polypropylene in between. The nonaqueous electrolyte prepared above was poured into the laminate pack, and the pack was sealed to obtain a laminate-type lithium-sulfur secondary battery for evaluation.

[0058] The obtained test battery was placed in a thermostatic chamber at 25°C, and a test to evaluate the cycle capacity retention rate was conducted at charge / discharge voltages ranging from 1.0 V to 3.0 V. The discharge current was the current value when a 0.1 C capacity was discharged. The reverse Coulombic efficiency was calculated by dividing the obtained first cycle charge capacity by the first cycle discharge capacity. The results are shown in Table 1.

[0059]

[0060] It is believed that the acidic functional group has the effect of smoothly transferring Li ions during the charge / discharge process. If the functional group has high acidity, Li ions are trapped by oxygen and become less mobile, but if the functional group has low acidity, the oxygen's trapping power weakens, promoting the transfer of Li ions. This is thought to be why the porous carbon materials for lithium-sulfur batteries prepared in the examples had higher reverse Coulombic efficiency than the porous carbon materials for lithium-sulfur batteries prepared in the comparative examples.

[0061] According to the porous carbon material for lithium-sulfur batteries of the present disclosure, a lithium-sulfur battery having a high weight energy density can be obtained, and the battery can be mounted in large power equipment applications such as electric vehicles and aviation applications such as drones.

[0062] The present disclosure includes the following aspects: (Item 1) In thermal evolved gas mass spectrometry, CO desorbed in a temperature range of 30°C to 1000°C at a temperature rise rate of 10°C / min. ２ CO desorbed in the temperature range of 400°C to 1000°C relative to the amount of CO desorbed ２ Item 2: A porous carbon material for a lithium-sulfur battery according to Item 1, wherein the ratio of the amount of desorption of CO is 85% or more, and the total amount of acidic functional groups measured by the Boehm method is 0.3 mmol / g or more. (Item 3) A porous carbon material for a lithium-sulfur battery according to Item 2, wherein the total amount of acidic functional groups is 0.5 mmol / g or more. (Item 4) A porous carbon material for a lithium-sulfur battery according to Item 2, wherein the total amount of acidic functional groups is 1.0 mmol / g or more. (Item 5) A porous carbon material for a lithium-sulfur battery according to Item 1, wherein the ratio of the amount of desorption of CO is 85% or more, and the total amount of acidic functional groups measured by the Boehm method is 0.3 mmol / g or more. ２ Item 5. The porous carbon material for a lithium-sulfur battery according to any one of Items 1 to 3, wherein the ratio of the amount of CO desorption is 90% or more. ２ Item 4. A porous carbon material for a lithium-sulfur battery according to Item 4, wherein the ratio of the amount of desorption is 95% or more. (Item 6) A porous carbon material for a lithium-sulfur battery according to any one of Items 1 to 5, wherein the peak pore diameter obtained by QSDFT is 0.5 nm or more and 10.0 nm or less. (Item 7) A porous carbon material for a lithium-sulfur battery according to Item 4, wherein the ratio of the amount of desorption is 95% or more. ２ / g or more. (Item 8) The porous carbon material for a lithium-sulfur battery according to any one of Items 1 to 7, having a 50% particle size D50 in a cumulative particle size distribution on a volume basis of 0.1 μm or more and 30 μm or less. (Item 9) The porous carbon material for a lithium-sulfur battery according to any one of Items 1 to 8, having an aspect ratio of 1 or more and 5 or less. (Item 10) A positive electrode material for a lithium-sulfur battery, comprising the porous carbon material for a lithium-sulfur battery according to any one of Items 1 to 9 and a material containing elemental sulfur. (Item 11) A method for producing a positive electrode material for a lithium-sulfur battery, comprising the steps of mixing the porous carbon material for a lithium-sulfur battery according to any one of Items 1 to 9 with a material containing elemental sulfur, and heating the resulting mixture.

Claims

1. In thermal evolved gas mass spectrometry, CO desorbed at a temperature rise rate of 10°C / min in the temperature range of 30°C to 1000°C ２ CO desorbed in the temperature range of 400°C to 1000°C relative to the amount of CO desorbed ２ and the total amount of acidic functional groups measured by the Boehm method is 0.3 mmol / g or more.

2. The porous carbon material for a lithium-sulfur battery according to claim 1, wherein the total amount of acidic functional groups is 0.5 mmol / g or more.

3. The porous carbon material for a lithium-sulfur battery according to claim 2, wherein the total amount of acidic functional groups is 1.0 mmol / g or more.

4. The above CO ２ The porous carbon material for a lithium-sulfur battery according to any one of claims 1 to 3, wherein the ratio of the amount of desorption of 5. The above CO ２ 5. The porous carbon material for a lithium-sulfur battery according to claim 4, wherein the ratio of the amount of desorption of the above is 95% or more.

6. The porous carbon material for lithium-sulfur batteries according to any one of claims 1 to 5, wherein the peak pore diameter obtained by QSDFT is 0.5 nm or more and 10.0 nm or less.

7. Specific surface area is 500m ２ The porous carbon material for a lithium-sulfur battery according to any one of claims 1 to 6, wherein the porous carbon material has a molecular weight of 1 / g or more.

8. The porous carbon material for a lithium-sulfur battery according to any one of claims 1 to 7, wherein the 50% particle size D50 in the cumulative particle size distribution on a volume basis is 0.1 µm or more and 30 µm or less.

9. The porous carbon material for a lithium-sulfur battery according to any one of claims 1 to 8, having an aspect ratio of 1 or more and 5 or less.

10. A positive electrode material for a lithium-sulfur battery comprising the porous carbon material for a lithium-sulfur battery according to any one of claims 1 to 9 and a material containing elemental sulfur.

11. A method for producing a positive electrode material for a lithium-sulfur battery, comprising the steps of mixing the porous carbon material for a lithium-sulfur battery according to any one of claims 1 to 9 with a material containing elemental sulfur, and heating the resulting mixture.

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