Porous carbon material and method for producing the same, and composite and method for producing the same

Abstract

This invention provides a method for producing porous carbon materials from nitrogen-containing carbon materials in a simple and safe manner. [Solution] A method for producing a porous carbon material, comprising: a first step of heat-treating diaminomaleonitrile in an inert gas to obtain a nitrogen-containing carbon material; and a second step of alkali-activating the obtained nitrogen-containing carbon material to obtain a porous carbon material.

Description

[Technical Field]

[0001] The present invention relates to a method for producing porous carbon materials, and more particularly to the positive electrode of a lithium-sulfur battery and materials used for the positive electrode. [Background technology]

[0002] Porous carbon materials, such as activated carbon, have been used in liquid-phase wastewater treatment, decolorization, and purification, as well as in gas-phase air purification, deodorization, gas separation, and solvent recovery, taking advantage of their adsorption and separation properties. Furthermore, porous carbon materials are also used as catalyst supports or as active materials in electric double-layer capacitors, utilizing their porosity.

[0003] Incidentally, lithium-ion secondary batteries, which use an organic electrolyte of lithium salt as a non-aqueous electrolyte, are lightweight and have high energy density, and are used as power sources for mobile phones and laptops. In the future, they are also expected to be used as power sources for mobile devices such as electric vehicles, exemplified by hybrid cars. For this reason, in recent years, there has been an increasing demand for batteries that have a higher capacity density than current lithium-ion secondary batteries and are also low-cost.

[0004] Generally, metal oxides containing Co, Ni, Mn, Fe, and Li are used as the positive electrode active material for lithium-ion secondary batteries. However, there is a growing development of lithium-sulfur batteries, which use sulfur as the positive electrode active material instead of these, as it has an extremely high theoretical capacity density and is also low cost.

[0005] A lithium-sulfur battery is a secondary battery that uses a sulfur-based compound as the positive electrode active material and a material that allows for the insertion and removal of metal ions, such as metallic lithium or lithium ions, such as silicon, as the negative electrode active material.

[0006] The oxidation-reduction reaction of sulfur is reversible, and due to the large number of reaction electrons, it has a theoretical capacity of 1672 mAh / g, which is about 10 times higher than that of a metal oxide cathode. However, the main reason why lithium-sulfur batteries have not been put into practical use is that there are several problems with the cathode active material composed of lithium and sulfur.

[0007] One reason for this is that the positive electrode active material, which consists of lithium and sulfur, has poor electrical conductivity and can only transfer electrons in the parts near the surface. This results in a low utilization rate of sulfur, which is involved in the electrochemical oxidation-reduction reaction, and thus a lower battery capacity.

[0008] Furthermore, due to the reaction during charging and discharging, the formula Li2S x Lithium polysulfide, represented by {where x is 4 to 8} in the formula, dissolves into the electrolyte, causing the loss of sulfur present on the positive electrode. This leads to a problem of reduced capacity in lithium-sulfur batteries. To prevent this reduction in lithium-sulfur battery capacity, Li2S x It is desirable to stop the elution process.

[0009] As a technology to solve these problems, a method has been proposed to composite conductive materials, namely carbon materials, with sulfur. While conductive polymers are also conductive materials, carbon materials are attracting attention because they have higher conductivity, superior structural stability, and flame retardancy compared to conductive polymers.

[0010] As a method for compounding carbon materials and sulfur, for example, a technique has been disclosed in which sulfur and / or sulfur compound particles with a particle size of 75 μm or less, and predetermined carbon fine particles having a hollow structure are used as raw materials, and these are compounded by mechanofusion to form a composite structure in which sulfur and / or sulfur compound particles form a core and a layer of carbon fine particles has a surface, and this composite is used as a cathode material (Patent Documents 1 and 2).

[0011] Patent Document 3 discloses a method of dissolving sodium thiosulfate in water, adding acetic anhydride to form an aqueous solution, adding Ketjen black and mixing, mixing by a mechanochemical method while remaining in the aqueous solution, removing the supernatant by centrifugation and filtration, and using the residue as a Ketjen black-sulfur nanoparticle composite for the positive electrode of a lithium-sulfur battery. Patent Document 3 also discloses a method of reacting by applying strong shear stress and centrifugal force to composite carbon materials and sulfur.

[0012] Non-Patent Document 1 discloses that a composite material composed of activated carbon containing 31% by mass of sulfur was prepared by heating sulfur and activated carbon at 155°C for 5 hours.

[0013] Patent Document 4 discloses a technique in which azurmic acid obtained by polymerizing hydrocyanic acid is heat-treated in an inert gas atmosphere to obtain a nitrogen-containing carbon material, and then alkali-activated, and the resulting porous carbon material is combined with sulfur. The composite material containing the carbon material and sulfur obtained using the nitrogen-containing porous carbon material described in Patent Document 4 has a large sulfur loading of 54 to 62% by mass, and when used as a positive electrode active material of a lithium-sulfur battery, it has the advantages of a large capacity per mass of sulfur and a large capacity per positive electrode material containing sulfur and the carbon material.

Prior Art Documents

Patent Documents

Summary of the Invention

Problems to be Solved by the Invention

[0016] However, in a lithium-sulfur battery using a positive electrode material obtained by mixing a carbon material and sulfur using the mechanochemical method described in Patent Documents 1 and 2, most of the sulfur is deposited on the outer surface of the carbon material or exists as free sulfur. Therefore, in addition to the problems of low sulfur utilization rate and small capacity per sulfur mass, there is a problem that the cycle characteristics are extremely low due to sulfur elution.

[0017] The method described in Patent Document 3 has problems of small capacity per sulfur mass and low sulfur content of 22.9% by mass. Therefore, the capacity per mass of the positive electrode material containing sulfur and the carbon material is small. In addition, the cycle characteristics of a lithium-sulfur battery containing the composite obtained by this method as a positive electrode material are also insufficient (Figure 10 of Patent Document 3).

[0018] A lithium-sulfur battery using the positive electrode material described in Non-Patent Document 1 has a relatively large capacity per sulfur mass, but since the sulfur content is 31% by mass, there is a problem that the capacity per mass of the positive electrode material containing sulfur and the carbon material is low. In addition, the manufacturing method of the positive electrode material described in Non-Patent Document 1 is also complicated.

[0019] The porous carbon material described in Patent Document 4 has a large sulfur loading amount. When used as a positive electrode active material of a lithium-sulfur battery, the capacity per sulfur mass and the capacity per positive electrode material containing sulfur and the carbon material are large, and the cycle characteristics using a tetraglyme (G4) solvent in the electrolyte are excellent. However, the porous carbon material described in Patent Document 4 is produced by a process of polymerizing azurmic acid, which is a nitrogen-containing carbon material, with toxic hydrocyanic acid. Therefore, there are problems that it is difficult to handle and it is difficult to develop a process for obtaining a nitrogen-containing carbon material.

[0020] This disclosure has been made in view of the above circumstances, and aims to provide a method for producing porous carbon materials that can be easily and safely manufactured from nitrogen-containing carbon materials. [Means for solving the problem]

[0021] In other words, examples of embodiments of the present disclosure are as follows: (1) A method for producing a porous carbon material, comprising: a first step of heat-treating diaminomaleonitrile in an inert gas to obtain a nitrogen-containing carbon material; and a second step of alkali-activating the nitrogen-containing carbon material to obtain a porous carbon material. (2) The method for producing a porous carbon material according to (1), wherein the nitrogen content of the nitrogen-containing carbon material is 2 to 45 atomic concentration percent. (3) A method for producing a porous carbon material according to (1) or (2), wherein the temperature of the heat treatment in the inert gas is 600°C to 1200°C. (4) A method for producing a porous carbon material according to any one of (1) to (3), wherein the temperature of the alkali activation heat treatment is 600°C to 1200°C. (5) A method for producing a porous carbon material according to any one of (1) to (4), wherein the porous carbon material is used as the positive electrode of a lithium sulfur battery. (6) A porous carbon material manufactured by any of the manufacturing methods described in (1) to (5). (7) In the laser Raman spectrum diagram, wavenumbers 1250-1385 cm⁻¹ -1 Between these peaks, P1 and wavenumbers of 1550-1620 cm -1 A porous carbon material having at least two major peaks, peak P2, between P1 and P2, and having a ratio (L / H1) of the height L from the baseline of the minimum point M between P1 and P2 to the height H1 from the baseline of P1 being 0.40 to 0.85. (8)(6) or (7) A composite of a porous carbon material and sulfur. (9) A composite of the porous carbon material described in (8) and sulfur, wherein the sulfur content is 40 to 90% by mass. (10) A composite of the porous carbon material described in (8) or (9) and sulfur, used as the positive electrode of a lithium-sulfur battery. (11) A method for producing a composite of a porous carbon material and sulfur according to any one of (8) to (10), comprising a mixing step of mixing a porous carbon material with sulfur heated above its melting point. A positive electrode for a lithium-sulfur battery containing a composite of a porous carbon material and sulfur as described in any of (12)(8) to (10). A lithium sulfur battery including the positive electrode described in (13)(12). [Effects of the Invention]

[0022] According to this disclosure, porous carbon materials can be manufactured simply and safely from nitrogen-containing carbon materials. Furthermore, according to this disclosure, a composite of a porous carbon material with a high sulfur-supporting content and sulfur is provided. In addition, according to this disclosure, a positive electrode for a lithium-sulfur battery with a high discharge capacity per unit mass of sulfur and a high discharge capacity per unit of positive electrode material containing sulfur and carbon material, and a lithium-sulfur battery using said positive electrode can be provided. [Brief explanation of the drawing]

[0023] [Figure 1] This is a schematic diagram of the method for manufacturing the porous carbon material of this embodiment. [Figure 2] This is a schematic diagram of the method for producing the composite of porous carbon material and sulfur according to this embodiment. [Figure 3] This diagram illustrates a method for calculating peak heights (H1 and H2), minimum value height (L), and full width at half maximum from the laser Raman spectrum of the porous carbon material of this embodiment. [Figure 4] This is a graph of thermogravimetric analysis (TG) results when measuring the sulfur content of a composite material of porous carbon material and sulfur. [Figure 5] This graph shows the pore distribution of the porous carbon material obtained in Example 1. [Figure 6]Figure 6(a) is an FE-SEM image of diaminomaleonitrile (DAMN). Figure 6(b) is an FE-SEM image of the nitrogen-containing carbon material obtained in Production Example 1. Figure 6(c) is an FE-SEM image of the nitrogen-containing carbon material obtained in Production Example 2. Figure 6(d) is an FE-SEM image of the nitrogen-containing carbon material obtained in Production Example 3. [Figure 7] This graph shows the pore distribution of the porous carbon material obtained in Example 2. [Figure 8] Figure 8(a) is an FE-SEM image of the porous carbon material obtained in Example 2. Figure 8(b) is an FE-SEM image of the porous carbon material obtained in Example 3. [Figure 9] Figure 9(a) is a STEM image of the porous carbon material obtained in Example 2. Figure 9(b) is a STEM image of the porous carbon material obtained in Example 3. [Figure 10] This is a laser Raman spectrum diagram of the porous carbon material obtained in Example 2. [Figure 11] This is a graph of thermogravimetric analysis (TG) results when the sulfur content of the composite of porous carbon material and sulfur in Example 2 was measured. [Figure 12] These are the charge-discharge curves for the porous carbon material obtained in Example 2 at cycles 1, 2, 10, 30, 50, and 100. [Figure 13] This graph shows the pore distribution of the porous carbon material obtained in Example 3. [Figure 14] This is a laser Raman spectrum diagram of the porous carbon material obtained in Example 3. [Figure 15] This is a graph of thermogravimetric analysis (TG) results when the sulfur content of the composite of porous carbon material and sulfur obtained in Example 3 was measured. [Figure 16] These are the charge-discharge curves for the 1st, 2nd, 10th, 30th, 50th, and 100th cycles of the porous carbon material obtained in Example 3. [Figure 17] This graph shows the cycle characteristics of the discharge capacity and Coulomb efficiency of the porous carbon materials obtained in Example 2 and Example 3. [Figure 18] This graph shows the pore distribution of the porous carbon material obtained in Example 4. [Figure 19] This graph shows the pore distribution of the porous carbon material obtained in Example 5. [Figure 20] Figure 20(a) is an FE-SEM image of the 3DAl current collector cellmet used as the positive electrode current collector of the laminate cell fabricated in Example 6. Figure 20(b) shows the shape of the laminate cell fabricated in Example 6. [Figure 21] These are the charge-discharge curves for the first and second cycles of the laminate cell prepared in Example 6. [Modes for carrying out the invention]

[0024] Hereinafter, embodiments for carrying out the present invention (hereinafter simply referred to as "this embodiment") will be described in detail, with reference to the drawings as necessary. This embodiment is an example for explaining the present invention, and the present invention is not limited to this embodiment. That is, the present invention can be modified in various ways without departing from its essence.

[0025] (A) Porous carbon material The present inventors have discovered that a method for producing a porous carbon material (hereinafter referred to as the method for producing a porous carbon material of this embodiment) includes a first step of heat-treating diaminomaleonitrile in an inert gas to obtain a nitrogen-containing carbon material, and a second step of alkali-activating the nitrogen-containing carbon material to obtain a porous carbon material. Surprisingly, they have found that a nitrogen-containing carbon material can be produced simply and safely, and that a porous carbon material can be obtained from this nitrogen-containing carbon material, resulting in a composite of a porous carbon material with a high sulfur-supporting capacity and sulfur. The method for manufacturing the porous carbon material of this embodiment is shown with reference to Figure 1.

[0026] Although the reason is not entirely clear, it is thought that nitrogen-containing carbon materials obtained by heat-treating diaminomaleonitrile in an inert gas have a high nitrogen content. Therefore, when pores are formed by alkali activation, the nitrogen portion is preferentially removed, resulting in the preferential formation of micropores. This leads to a larger pore volume and the acquisition of a porous carbon material with a large sulfur-supporting capacity. It is also thought that the structure obtained through these effects exhibits excellent performance as a positive electrode material for lithium-sulfur batteries.

[0027] In this embodiment, the composite of porous carbon material and sulfur has many micropores and few mesopores, which suppresses the elution of sulfur into the electrolyte. Furthermore, the increased amount of sulfur available for electron transfer due to the filling of pores increases the sulfur utilization rate, leading to increased capacity and improved cycle characteristics. In this disclosure, in the pore distribution diagram obtained by the quenched solid-state density function method (QSDFT), pores with a diameter of 2 nm or less are called micropores, pores with a diameter greater than 2 nm and less than or equal to 50 nm are called mesopores, and pores with a diameter greater than 50 nm are called macropores.

[0028] The nitrogen-containing carbon material used in this embodiment is a carbon material containing nitrogen. The raw material for the nitrogen-containing carbon material in this embodiment is diaminomaleonitrile (DAMN). There are no particular restrictions on the diaminomaleonitrile (DAMN) used in the method for producing the porous carbon material in this embodiment; for example, diaminomaleonitrile manufactured by Tokyo Chemical Industry Co., Ltd. can be used.

[0029] The method for producing the porous carbon material of this embodiment includes a first step of obtaining a nitrogen-containing carbon material by heat-treating diaminomaleonitrile in an inert gas, and a second step of obtaining a porous carbon material by alkali-activating the nitrogen-containing carbon material. The porous carbon material of this embodiment can be obtained by the method for producing the porous carbon material of this embodiment. The porous carbon material of this embodiment is preferably used as the positive electrode of a lithium-sulfur battery.

[0030] The first step, which involves heat-treating diaminomaleonitrile in an inert gas to obtain a nitrogen-containing carbon material, will be described. The conditions for heat-treating diaminomaleonitrile in an inert gas include using a rotary furnace, tunnel furnace, tubular furnace (e.g., horizontal tubular furnace), box furnace, and fluidized bed furnace, and performing the heat treatment under an inert gas atmosphere. The temperature of the heat treatment is not particularly limited, but is preferably 600 to 1200°C, more preferably 700 to 1050°C, and even more preferably 750 to 1000°C.

[0031] The above-mentioned inert gases are not limited to the following gases, but examples include nitrogen, argon (Ar), helium, and neon. The inert gas atmosphere may also be under reduced pressure, that is, a pressure environment lower than atmospheric pressure. Among these, nitrogen gas is preferred as the inert gas. The inert gas atmosphere may be one in which the inert gas is stationary or circulating, but circulating gas is preferred. The oxygen concentration in the inert gas is preferably 5% or less, more preferably 1% or less, and particularly preferably 0.1% or less. The heat treatment time is preferably 10 seconds to 100 hours, more preferably 5 minutes to 10 hours, even more preferably 15 minutes to 5 hours, and particularly preferably 30 minutes to 2 hours. The pressure during heat treatment, when using an inert gas, is preferably 0.01 to 5 MPa, more preferably 0.05 to 1 MPa, even more preferably 0.08 to 0.3 MPa, and particularly preferably 0.09 to 0.15 MPa.

[0032] The nitrogen content in the nitrogen-containing carbon material is measured by energy-dispersive X-ray spectroscopy (EDX), and in detail by the method described in the examples. The nitrogen content in the nitrogen-containing carbon material is preferably 2 to 45 atomic concentrations. The lower limit of the nitrogen content in the nitrogen-containing carbon material is preferably 2 atomic concentrations or more, more preferably 3 atomic concentrations or more, even more preferably 5 atomic concentrations or more, and particularly preferably 10 atomic concentrations or more. The upper limit of the nitrogen content in the nitrogen-containing carbon material is preferably 45 atomic concentrations or less, more preferably 40 atomic concentrations or less, even more preferably 35 atomic concentrations or less, and particularly preferably 30 atomic concentrations or less. When a nitrogen-containing carbon material with a nitrogen content of 2 atomic concentrations or more is used, micropores tend to develop easily. When a nitrogen-containing carbon material with a nitrogen content of 45 atomic concentrations or less is used, the carbon structure tends to develop easily.

[0033] The nitrogen-containing carbon material according to this embodiment is a material in which, in the X-ray diffraction pattern (XRD) obtained using CuKα rays as an X-ray source, the main peak is located at a position of 22.5 to 26.8° with respect to the diffraction angle (2θ), originating from the (002) plane. In addition, the nitrogen-containing carbon material according to this embodiment shows a weak peak at a position of 43.0 to 46.0°, usually at 44.0 to 45.5°.

[0034] XRD measurements are performed using the following settings: tube voltage: 20kV, tube current: 40mA, spectroscopic crystal: present, scattering slit: 8.0mm, divergence slit: 2 / 3°, receiving slit: open, scan speed: 40.0° / min, sampling width: 0.02°, and scanning method: 2θ / θ method.

[0035] The second step, in which a porous carbon material is obtained by alkali activation of a nitrogen-containing carbon material, will be described. In this disclosure, activation refers to a treatment for making a nitrogen-containing carbon material porous. In this embodiment, alkaline activation is preferred because it makes it easier to obtain a porous carbon material with a large specific surface area, high microporosity, and large pore volume.

[0036] In this embodiment, alkali activation is performed by mixing a nitrogen-containing carbon material with an alkali and then heat-treating it. In this disclosure, alkali refers to a basic substance. Examples of alkalis include alkali metal salts, alkaline earth metal salts, and ammonia, but alkali metal salts and alkaline earth metal salts are preferred, and alkali metal salts are particularly preferred. Examples of alkali metal salts include alkali metal hydroxides such as potassium hydroxide and sodium hydroxide, alkali metal carbonates such as potassium carbonate (K2CO3) and sodium carbonate, and alkali metal sulfates such as potassium sulfate and sodium sulfate. Examples of alkaline earth metal salts include alkaline earth metal hydrochlorides such as zinc chloride and calcium chloride. One of these may be used, or a mixture of two or more. Preferably, potassium hydroxide, sodium hydroxide, potassium carbonate, and sodium carbonate are used, with potassium carbonate being particularly preferred.

[0037] The amount of alkali used in relation to the nitrogen-containing carbon material is not particularly limited, but the alkali / nitrogen-containing carbon material (mass ratio) is preferably 0.1 to 10. The lower limit of the alkali / nitrogen-containing carbon material (mass ratio) is more preferably 0.5 or higher, and particularly preferably 1 or higher. The upper limit of the alkali / nitrogen-containing carbon material (mass ratio) is more preferably 5 or lower, and particularly preferably 3 or lower. If the alkali / nitrogen-containing carbon material (mass ratio) is too low, pore development tends to be poor, and if the alkali / nitrogen-containing carbon material (mass ratio) is too high, overactivation occurs, and the destruction of the pore walls tends to progress.

[0038] An inert gas atmosphere is preferred for alkali activation. Examples of inert gases include nitrogen, argon (Ar), helium, and neon.

[0039] The alkaline activation treatment is not particularly limited, but is preferably carried out at a temperature of 600 to 1200°C, more preferably 700 to 1000°C, and even more preferably 750 to 950°C. If the temperature of the alkaline activation treatment is too low, the activation will not proceed sufficiently, and if it is too high, problems such as pore contraction and severe corrosion of the activation device may occur.

[0040] The duration of the alkaline activation treatment is not particularly limited, but is preferably 10 minutes to 50 hours, more preferably 30 minutes to 10 hours, and most preferably 1 to 5 hours.

[0041] The pressure used for alkaline activation treatment is usually atmospheric pressure, but it can also be performed under increased or decreased pressure.

[0042] As the activation furnace, rotary furnaces, tunnel furnaces, tubular furnaces (for example, horizontal tubular furnaces), box furnaces, and fluidized bed furnaces can be used.

[0043] After the alkaline activation is complete, it is preferable to include a washing step in which the metal components used in the alkaline activation are washed with water, neutralized with hydrochloric acid, sulfuric acid, nitric acid, etc., and then washed again with water to wash away the acid.

[0044] After the washing process, the washed product may be subjected to solid-liquid separation treatment such as filtration, and then dried to obtain a porous carbon material. The drying process is carried out, for example, by drying the filtered product overnight at 100°C under reduced pressure.

[0045] The nitrogen and oxygen content of the porous carbon material in this embodiment is measured by EDX. The nitrogen and oxygen content of the porous carbon material in this embodiment is measured using EDX with a Hitachi High-Technologies Corp., SU-1000, with an electron gun filament acceleration voltage of 15kV and a working distance of 15mm. The nitrogen content of the porous carbon material of this embodiment is preferably 0 to 15 atomic concentration %, more preferably 0.001 to 10 atomic concentration %, and particularly preferably 0.01 to 5 atomic concentration %. The oxygen content of the porous carbon material of this embodiment is preferably 2 to 11 atomic concentration %, more preferably 4 to 11 atomic concentration %.

[0046] The specific surface area of the porous carbon material of this embodiment is preferably 1000 to 3800 m 2 / g, more preferably 1150 to 3600 m 2 / g, and particularly preferably 1200 to 3100 m 2 / g. The specific surface area of the porous carbon material of this embodiment is measured by the BET method of the nitrogen adsorption and desorption method, and specifically, it is measured by the method described in the examples.

[0047] The pore volume of the porous carbon material of this embodiment is preferably 0.50 to 2.50 cm 3 / g, more preferably 0.56 to 2.50 cm 3 / g, and particularly preferably 1.0 to 2.1 cm 3 / g. The pore volume of the porous carbon material of this embodiment is measured by the quenched solid density functional theory method (QSDFT) of the nitrogen adsorption and desorption method. In QSDFT, calculations are performed using the slit model (Horvath-Kawazoe (HK) Method).

[0048] The porous carbon material of this embodiment has at least two main peaks, namely peak P1 (referred to as the D band) between wavenumbers 1250 to 1385 cm -1 and peak P2 (referred to as the G band) between wavenumbers 1550 to 1620 cm -1 in the laser Raman spectrum diagram with a wavenumber of 800 to 2000 cm -1 , and also has the ratio (L / H1) of the height L from the baseline of the minimum point M between P1 and P2 to the height H1 from the baseline of P1 and the ratio (L / H2) of the height L from the baseline of the minimum point M between P1 and P2 to the height H2 from the baseline of P2. From the viewpoint of further increasing the discharge capacity per unit sulfur mass and the discharge capacity per cathode material, and further achieving even better cycle characteristics, L / H1 is preferably 0.40 to 0.85.

[0049] L / H1 is more preferably 0.50 to 0.76, and even more preferably 0.55 to 0.72.

[0050] In this embodiment, the laser Raman spectrum is measured under the following conditions. LD-pumped solid-state laser λ: 532nm 47mW measurement conditions, 532nm Irradiation 2.34mW Exposure time: 10 seconds Total 10 times Grating 600G / mm

[0051] In this disclosure, P1 and P2 are defined as having a Raman shift in the laser Raman spectrum diagram between wavenumbers 1200 and 1700 cm⁻¹. -1 These are the two main peaks between them. P1 is at wavenumbers 1250–1385 cm. -1 The peak is between these two points, and P2 corresponds to wavenumbers 1550-1620 cm. -1 This is the peak during that period.

[0052] From the viewpoint of further increasing the discharge capacity per unit sulfur mass and the discharge capacity per cathode material, and further achieving even better cycle characteristics, L / H2 is preferably 0.50 to 0.85, more preferably 0.60 to 0.80, and even more preferably 0.70 to 0.76.

[0053] The half-width of peak P1 is 70-150cm. -1 Preferably, 90-125cm -1 More preferably, and even more preferably, 105-120 cm -1 That is the case.

[0054] The half-width of peak P2 is 20-100 cm. -1 Preferably, 40-70cm-1 More preferably, and even more preferably, 55-65 cm -1 That is the case.

[0055] Figure 3 is a laser Raman spectrum diagram of the porous carbon material of this embodiment, and is an explanatory diagram of the method for calculating H1, H2, and L from such a spectrum.

[0056] Note that Figure 3 is not limited in any way to the laser Raman spectrum obtained from the porous carbon material of this embodiment.

[0057] In Figure 3, B1 is 800-1250 cm. -1 This is the minimum intensity value, and B2 corresponds to 1700-1900 cm. -1 This is the minimum intensity value between the specified ranges.

[0058] In this embodiment, the baseline in the laser Raman spectrum diagram is the straight line connecting B1 and B2.

[0059] Next, C1 and C2 shown in Figure 3 are the intersections of the baseline and the perpendiculars drawn from peaks P1 and P2 to the Raman shift axis (horizontal axis), respectively.

[0060] D is the intersection of the baseline and the perpendicular line drawn from the minimum intensity value M between peaks P1 and P2 to the Raman shift axis (horizontal axis), and the height L is the length from M to the intersection of the perpendicular line drawn to the Raman shift axis and the baseline. Specifically, L is the length of the line segment MD in the laser Raman spectrum diagram illustrated in Figure 3.

[0061] On the other hand, height H1 is the length from P1 to the intersection of the perpendicular line drawn from the Raman shift axis and the baseline. In the laser Raman spectrum diagram illustrated in Figure 3, the length of the line segment P1C1 corresponds to height H1.

[0062] Height H2 is the length from P2 to the intersection of the perpendicular line drawn from the Raman shift axis and the baseline. In the laser Raman spectrum diagram illustrated in Figure 3, the length of the line segment P2C2 corresponds to height H2. Let M1 be the intersection point of the line segment drawn parallel to the horizontal axis from M and the line segment P1C1, and let N1 be the midpoint of the line segment P1M1. Let S1 and S2 be the intersection points of the line segment drawn parallel to the horizontal axis from N1 and the Raman shift diagram, and the length of the line segment S1S2 is the full width at half maximum of the peak P1. Let M2 be the intersection point of the line segment drawn parallel to the horizontal axis from M and the line segment P2C2, and let N2 be the midpoint of the line segment P2M2. Let S3 and S4 be the intersection points of the line segment drawn parallel to the horizontal axis from N2 and the Raman shift diagram, and the length of the line segment S3S4 is the full width at half maximum of the peak P2.

[0063] (B) Composite of porous carbon material and sulfur A composite of porous carbon material and sulfur will be described. The composite of porous carbon material and sulfur in this embodiment will be referred to with reference to Figure 2. The composite of porous carbon material and sulfur contains sulfur within the porous carbon material. The sulfur content is preferably 40-90% by mass, based on 100% by mass of the composite. The lower limit of the sulfur content is more preferably 50% by mass or more, even more preferably 60% by mass or more, and particularly preferably 62% by mass or more. The upper limit of the sulfur content is more preferably 85% by mass or less. In this disclosure, "high sulfur load" means that the sulfur content of the composite of porous carbon material and sulfur is 68% by mass or more.

[0064] Here, the sulfur content in the composite is the value calculated by the formula: Sulfur content in the composite = ((Mass of sulfur in the composite) / (Mass of the composite of porous carbon material and sulfur)) × 100. The sulfur content (mass%) in a composite of porous carbon material and sulfur is measured by thermogravimetric analysis (TG) after preparing a sample using the following method, taking advantage of the fact that sulfur incorporated into micropores is difficult to evaporate, while sulfur outside the micropores is easily evaporated.

[0065] After mixing porous carbon material and sulfur, the mixture is kept in a sealed container at 155°C for 5 hours to melt the sulfur, which then fills the pores of the porous carbon material by capillary action. The temperature is then raised to 300°C and kept there for 2 hours to evaporate and remove any sulfur remaining on the surface of the porous carbon material. The mixture is then air-cooled to room temperature, the container is opened, and the composite of porous carbon material and sulfur is taken out as a sample. Next, the sulfur content (mass%) in the composite of porous carbon material and sulfur is measured using thermogravimetric analysis (TG). Specifically, a Shimadzu DTG-60AH is used, with 10 mg of the sample placed in the cell, and the measurement is performed under the following conditions: measurement gas Ar, flow rate 50 ml / min, starting temperature 30°C, heating rate 5°C / min, and upper temperature limit 600°C.

[0066] Using Figure 4, which shows an example of TG obtained under the above conditions for a composite of porous carbon material and sulfur, we will explain how to calculate the sulfur content from the obtained TG. In the TG graph, the mass decreases with increasing temperature, but a point appears in the 300°C to 500°C region where the slope of the tangent line to the graph changes discontinuously. If the initial mass % is 100 and the mass % at the point where the slope of the tangent line to the graph changes discontinuously is X, then the sulfur content Y is defined as Y = 100 - X. In Figure 4, X = 42, so Y = 58, and the sulfur content is calculated to be 58 mass %.

[0067] A method for producing a composite of porous carbon material and sulfur includes a step of mixing the porous carbon material with sulfur heated above its melting point (mixing step). Examples of methods for producing a composite of porous carbon material and sulfur include the following:

[0068] A porous carbon material and sulfur powder are mixed together. Next, the material is heated above the melting point of sulfur to melt it, and the sulfur is impregnated into the pores of the porous carbon material by capillary action. The melting temperature should be 107°C or higher, which is the melting point of sulfur, but preferably 130°C or higher, and particularly preferably 150°C or higher. Furthermore, the melting temperature should preferably be 200°C or lower, more preferably 180°C or lower, and particularly preferably 170°C or lower. From the viewpoint of suppressing the volatilization of sulfur, it is preferable to impregnate the pores with sulfur in a sealed container. The reaction time is preferably 0.1 to 100 hours, more preferably 0.5 to 20 hours, and particularly preferably 1 to 10 hours. The atmosphere may be air, or an inert gas such as nitrogen, argon, helium, and neon.

[0069] Next, the porous carbon material that has been treated with sulfur impregnation is heated to remove any sulfur that has not been impregnated into the pores. The heating temperature is preferably 250°C or higher, more preferably 270°C or higher, and particularly preferably 290°C or higher. The heating temperature is preferably 500°C or lower, preferably 400°C or lower, and particularly preferably 330°C or lower. The container may be either a sealed or open container. In the case of a sealed container, it is preferable that the temperature of the top of the container is lower than that of the heating section. The atmosphere may be air, or an inert gas such as nitrogen, argon, helium, and neon, but air is preferred from the viewpoint of ease of operation.

[0070] When mixing porous carbon material and sulfur, the respective masses are calculated using equation (1) to determine the theoretical sulfur content A (mass%). The resulting theoretical sulfur content A (mass%) is then applied to equation (2) to calculate the amount of sulfur M (sulfur). The constant B represents the excess amount relative to the theoretical sulfur content A (mass%). The constant B is between 0.1 and 20, preferably between 0.5 and 5, and particularly preferably between 1 and 3. In this embodiment, B=2 is used.

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[0071] In one embodiment, the porous carbon material of this embodiment is a porous carbon material manufactured by the manufacturing method of this embodiment. In one embodiment, the porous carbon material of this embodiment exhibits a laser Raman spectrum with wavenumbers of 1250 to 1385 cm⁻¹. -1 Between these peaks, P1 and wavenumbers of 1550-1620 cm -1 This is a porous carbon material having at least two major peaks, peak P2, between P1 and P2, and having a ratio (L / H1) of the height L from the baseline of the minimum point M between P1 and P2 to the height H1 from the baseline of P1 being 0.40 to 0.85.

[0072] (C) Positive electrode of a lithium-sulfur battery In this embodiment, the composite of porous carbon material and sulfur obtained as described above can be used as the positive electrode of a lithium-sulfur battery.

[0073] The positive electrode of the lithium-sulfur battery in this embodiment comprises a composite of the porous carbon material and sulfur, and optionally a binder, a conductive additive, a dispersant, and a current collector. The composite of the porous carbon material and sulfur mainly functions as an active material, but the positive electrode of the lithium-sulfur battery in this embodiment may also contain active materials other than the composite of the porous carbon material and sulfur.

[0074] The binder used in the positive electrode of this embodiment is not particularly limited and includes, for example, polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, ethylene-tetrafluoroethylene copolymer, polychlorotrifluoroethylene, ethylene-chlorotrifluoroethylene copolymer, polyvinyl fluoride, vinylidene fluoride-hexafluoropropylene-based fluororubber, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber, vinylidene fluoride-pentafluoropropylene-based fluororubber, vinylidene Examples include vinylidene fluoride-based fluororubbers such as fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubbers, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene-based fluororubbers, vinylidene fluoride-chlorotrifluoroethylene-based fluororubbers, tetrafluoroethylene-propylene-based fluororubbers, tetrafluoroethylene-perfluoroalkyl vinyl ether-based fluororubbers, thermoplastic fluororubbers, polyethylene, polypropylene, styrene-butadiene rubber (SBR), isoprene rubber, butadiene rubber, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, and aqueous binders. These may be used individually or in any combination and ratio of two or more types.

[0075] The binder content used in the positive electrode of this embodiment is preferably 1 to 20% by mass relative to the active material, and more preferably 2 to 10% by mass. If the binder content is 1% by mass or more, the strength of the positive electrode becomes sufficient, and if the binder content is 20% by mass or less, the increase in electrical resistance or decrease in capacitance can be further suppressed.

[0076] Examples of conductive additives used in the positive electrode of this embodiment include carbon black, acetylene black, and carbon nanotubes. These may be used individually or in any combination and ratio of two or more. The content of the conductive additive is preferably 1 to 20% by mass, and more preferably 2 to 10% by mass, relative to the active material.

[0077] The current collector used in the positive electrode of this embodiment is not particularly limited as long as it enables electrical connection of the electrodes, and examples of its material include aluminum, titanium, zirconium, hafnium, niobium, tantalum, and alloys containing these metals. Among these, a selection from the group consisting of aluminum, titanium, tantalum, and alloys containing these metals is preferred, and aluminum is particularly preferred. These metals or alloys can be used as the current collector of the positive electrode in the form of foil, perforated foil, or mesh. In addition, porous materials can be used as the current collector. Examples of porous materials include 3D Al current collector cellmet, porous metal (foamed metal), and carbon paper.

[0078] The thickness of the current collector is preferably 1 to 100 μm. A thickness of 1 μm or more is preferable because it can maintain the shape and strength of the positive electrode. On the other hand, a thickness of 100 μm or less is preferable because it results in an appropriate weight and volume as an energy storage element, and tends to have high performance per unit weight and volume. If there is a hole in the current collector, the portion with the hole should be excluded when measuring the thickness.

[0079] Next, the manufacturing method for the positive electrode of the lithium-sulfur battery of this embodiment will be described. The method for manufacturing the positive electrode of the lithium-sulfur battery in this embodiment is not particularly limited as long as it is a known method, except that a composite of a porous carbon material and sulfur is used as the active material. For example, one method for manufacturing the positive electrode of a lithium-sulfur battery is to add a binder, a conductive additive and a solvent to a composite of a porous carbon material and sulfur to form a slurry, apply or fill the slurry onto a current collector, dry it, and then press it to increase its density.

[0080] The thickness of the positive electrode layer laminated onto the current collector is preferably around 20 to 400 μm. A positive electrode layer thickness of 20 μm or more is preferable because it increases the proportion of active material to the total battery amount, and tends to increase the energy density. On the other hand, a positive electrode layer thickness of 400 μm or less is preferable because it reduces the resistance inside the electrode, and tends to increase the power density.

[0081] The solvent used in the method for manufacturing the positive electrode of the lithium-sulfur battery in this embodiment may be either water or an organic solvent. Examples of organic solvents include N-methylpyrrolidone, methyl ethyl ketone, cyclohexanone, isophorone, N,N-dimethylformamide, and N,N-dimethyl. The solvent is preferably water.

[0082] The above slurry is prepared by kneading using a dispersion device such as a stirrer, pressure kneader, ball mill, and super sand mill.

[0083] Furthermore, a dispersant to adjust the viscosity may be added to the slurry. Examples of dispersants include carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, polyacrylic acid (salt), oxide starch, phosphorylated starch, and casein. These may be used individually or in any combination and ratio of two or more.

[0084] The method for applying the slurry to the current collector is not particularly limited, but known methods include, for example, the doctor blade method, metal mask printing method, electrostatic coating method, dip coating method, spray coating method, roll coating method, gravure coating method, and screen printing method. After applying the slurry, the slurry is dried, and if necessary, a rolling process is performed using a flat plate press, calender roll, etc. The electrode material slurry, formed into a sheet shape, pellet shape, etc., is then integrated with the current collector by known methods such as rolls, presses, or a combination thereof to obtain the positive electrode of a lithium-sulfur battery.

[0085] A lithium-sulfur battery equipped with the lithium-sulfur battery positive electrode obtained as described above has excellent cycle characteristics, large capacity, input / output characteristics, and Coulomb efficiency.

[0086] (D) Lithium sulfur battery The following describes an example of a lithium-sulfur battery equipped with the positive electrode for a lithium-sulfur battery according to this embodiment. However, the materials that can be used in the lithium-sulfur battery, the manufacturing method of the lithium-sulfur battery, etc., are not limited to the specific examples below.

[0087] The lithium-sulfur battery of this embodiment is not particularly limited in its configuration other than having the positive electrode of the lithium-sulfur battery of this embodiment as the positive electrode, and components other than the positive electrode of the lithium-sulfur battery may be known. For example, this lithium-sulfur battery comprises a separator, a positive electrode for the lithium-sulfur battery of this embodiment arranged opposite each other via the separator, a negative electrode, an electrolyte in contact with them, and an outer casing. This lithium-sulfur battery has an electrode body including the negative electrode and positive electrode arranged as described above, and an outer casing that houses the electrode body, and can be obtained by injecting the electrolyte into the space inside the outer casing and sealing it.

[0088] The negative electrode can be manufactured in the same manner as the positive electrode, by forming a negative electrode layer containing negative electrode active material on a current collector.

[0089] The negative electrode current collector is not particularly limited as long as it enables electrical connection of the electrodes, and its material can be, for example, copper, aluminum, nickel, titanium, and stainless steel. Of these, copper is preferred from the viewpoint of ease of processing into a thin film and cost. The shape of the current collector can be, for example, foil, perforated foil, or mesh. In addition, porous materials such as porous metal (foamed metal) and carbon paper can also be used as current collectors.

[0090] The negative electrode active material can be any material containing lithium, including metallic lithium or lithium alloys, as well as lithium oxides, lithium composite oxides, lithium sulfides, and lithium composite sulfides. Examples of lithium alloys include alloys of lithium with aluminum or silicon, tin, magnesium, indium, calcium, etc. The negative electrode active material is preferably metallic lithium or an alloy of silicon and lithium.

[0091] The negative electrode may be metallic lithium or a lithium alloy itself, or it may be formed by mixing a negative electrode active material with a conductive material such as graphite, acetylene black, or Ketjenblack and a binder, adding a suitable solvent to form a paste-like negative electrode material, which is then applied to the surface of the current collector, dried, and compressed as needed to increase the electrode density.

[0092] The electrolyte is prepared by dissolving a lithium-containing electrolyte in a non-aqueous solvent. Examples of lithium-containing electrolytes include LiPF6, LiClO4, LiBF4, LiAsF6, LiCF(CF3)5, LiCF2(CF3)4, LiCF3(CF3)3, LiCF4(CF3)2, LiCF5(CF3), LiCF3(C2F5)3, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(C2F5CO)2, LiI, LiAlCl4, LiBC4O8, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium bis(fluorosulfonyl)imide (LiFSI). These can be used individually or in mixtures of two or more. Among these, LiPF6, LiFSI, and LiTFSI are preferred. LiFSI and LiTFSI are particularly preferred. Furthermore, the lithium salt concentrations are preferably 0.1 to 3.0 mol / L, and more preferably 0.5 to 2.0 mol / L.

[0093] Non-aqueous solvents can be appropriately selected from, for example, carbonates, ethers, ketones, lactones, nitriles, amines, amides, sulfur compounds, halogenated hydrocarbons, esters, nitro compounds, phosphate ester compounds, sulfolane hydrocarbons, and ionic liquids. Among these, carbonates such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC), and fluoroethylene carbonate (FEC), ethers such as dimethoxyethane (DME), triglyme (G3), and tetraglyme (G4), cyclic ethers such as dioxolane (DOL) and tetrahydrofuran, 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)-propane (HFE), sulfolane (SL), and mixtures thereof are preferred. Particularly preferred is a solvent containing carbonates, and especially preferred is a mixture of (FEC) and (HFE).

[0094] The separator is not particularly limited in material or shape, as long as it is one used in a typical lithium-sulfur battery. This separator separates the positive electrode and the negative electrode so that they do not physically come into contact, and it is preferable that it has high ion permeability. Examples of separators include synthetic resin microporous membranes, woven fabrics, and nonwoven fabrics, with synthetic resin microporous membranes being preferred. Examples of materials for the synthetic resin microporous membrane include polyolefins such as polyethylene, polypropylene, and polybutene, as well as polyamide, cellulose acetate, nitrocellulose, polysulfone, polyacrylonitrile, and polyvinylidene fluoride. Among these, polyethylene and polypropylene are preferred. Note that if the lithium sulfur battery being manufactured is constructed so that the positive and negative electrodes are not in direct contact, a separator is not required.

[0095] The structure of the electrode body of the lithium-sulfur battery in this embodiment is not particularly limited, but it is common to have a structure in which the positive electrode, negative electrode, and a separator provided as needed are wound in a flat spiral shape to form a wound electrode plate group, or these are stacked as flat plates to form a stacked electrode plate group, and these electrode plate groups are sealed in an outer casing.

[0096] The outer casing can be made of a metal can, laminate film, or the like. Aluminum is preferred for the metal can. In this case, for example, the metal can is used as the negative terminal, and the metal lid, crimped to the metal can with an insulator in between, is used as the positive terminal. The electrode body and the terminals are electrically connected by electrode tabs.

[0097] Furthermore, the laminate film is preferably a film made by laminating a metal foil and a resin film, and an example is a three-layer structure consisting of an outer resin film, metal foil, and an inner resin film. The outer resin film is for preventing damage to the metal foil due to contact, etc., and resins such as nylon or polyester can be suitably used. The metal foil is for preventing the permeation of moisture and gas, and foils such as copper, aluminum, and stainless steel can be suitably used. The inner resin film protects the metal foil from the electrolyte solution housed inside and is for melt sealing during heat sealing, and polyolefins, acid-modified polyolefins, etc. can be suitably used.

[0098] When using a laminate film enclosure, the edges of the laminate film should be sealed with the ends of the electrode terminals extended into the external space of the enclosure. Heat sealing is preferred as the sealing method.

[0099] The lithium-sulfur battery of this embodiment may take the form of a paper battery, a button battery, a coin battery, a stacked battery, or a cylindrical battery. [Examples]

[0100] The embodiment will be described in more detail below with reference to examples, but these are illustrative and the embodiment is not limited thereto. Therefore, those skilled in the art can implement this embodiment by making various modifications to the examples shown below, and such modifications are included in the claims of this application.

[0101] <Analysis method> (Methods for measuring specific surface area, pore volume, and pore distribution) Using Quantachrome's AUTOSORB iQ, the specific surface area was measured by the BET method, and the pore distribution and pore volume were measured using the slit model of the quenched solid-state density function method (QSDFT). Specifically, the samples were degassed under vacuum at 100°C for 3 hours before measurement. The nitrogen adsorption isotherm was measured at liquid nitrogen temperature to determine the specific surface area, pore volume, and pore distribution of the porous carbon material.

[0102] (Measurement method using energy-dispersive X-ray fluorescence (EDX)) The carbon, nitrogen, and oxygen content of nitrogen-containing carbon materials and porous carbon materials was measured by energy-dispersive X-ray spectroscopy (EDX). A Hitachi High-Technologies Corp., SU-1000 was used, with an electron gun filament acceleration voltage of 10kV and a working distance of 15mm.

[0103] (Field Emission Scanning Electron Microscope (FE-SEM)) Measurement Method A high-resolution field emission scanning electron microscope (Hitachi High-Technologies Corp. SU-9000) was used. The powder sample was fixed to an aluminum sample stage with conductive carbon tape, introduced into the observation chamber, and secondary electron images were captured at an accelerating voltage of 1 kV.

[0104] (Scanning Transmission Electron Microscopy (STEM)) Measurement Method A high-resolution field emission scanning electron microscope (Hitachi High-Technologies Corp. SU-9000) was used. The sample, dispersed in ethanol, was attached to a microgrid and introduced into the observation room. Bright-field STEM images were then captured at an accelerating voltage of 30 kV.

[0105] (Method for measuring sulfur content) To measure the sulfur content in a composite of porous carbon material and sulfur, samples were prepared using the following method, taking advantage of the fact that sulfur incorporated into micropores is difficult to evaporate, while sulfur outside the micropores evaporates easily. After mixing porous carbon material and sulfur, the mixture was kept in a sealed container at 155°C for 5 hours to melt the sulfur, which then filled the pores of the porous carbon material by capillary action. The temperature was then raised to 300°C and kept there for 2 hours to evaporate and remove any sulfur remaining on the surface of the porous carbon material. The mixture was then air-cooled to room temperature, the container was opened, and the composite of porous carbon material and sulfur was removed and used as a sample. Next, the sulfur content in the composite of porous carbon material and sulfur was measured using thermogravimetric analysis (TG). Specifically, a Shimadzu DTG-60AH was used, with 10 mg of the sample placed in the cell, and measurements were performed under the following conditions: measurement gas Ar, flow rate 50 ml / min, starting temperature 30°C, heating rate 5°C / min, and upper temperature limit 600°C.

[0106] <Manufacturing of lithium-sulfur batteries> (Fabrication of the positive electrode of a lithium-sulfur battery) Carbon nanotubes (CNT, manufactured by Kusumoto Chemicals, Ltd., Tubal) were used as conductive additives, carboxymethylcellulose (CMC, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., F-AG) as a dispersant, and styrene-butadiene rubber (SBR, manufactured by JSR Co., TRD2001) as a binder. SBR (binding agent) was dissolved in pure water to prepare a 48% by mass dispersion. CNT (conductive additive) was added to this dispersion, followed by the addition of the porous carbon material and sulfur composite described later (composites in the examples and comparative examples: active material), and then CMC (dispersant). CMC was used in the form of a 2% by mass aqueous solution.

[0107] A composite material consisting of CNTs, CMCs, and SBRs in a mass ratio of 93:0.7:2.8:3.5 was mixed and stirred using a rotation / revolution mixer (Awatori Rentaro, Thinky Co., ARE-310) to obtain a slurry. The obtained slurry was coated onto etched aluminum current collector foil (20 μm thick, Hosen Corp.) to create a sheet-like electrode. The coating method used was the doctor blade method, and the applicator gap was 200 μm. After coating, the electrode sheet was placed on a hot plate and heated at 40°C for 1 hour to remove any remaining moisture. Then, a pressing process was performed to increase the electrode density. The pressing was carried out using a roll press (HSTK-2022, Hosen Co.) with the linear pressure set to 2t. After pressing, the electrodes were punched out into 12mm diameter discs. To completely remove moisture from the fabricated electrodes, they were dried overnight at 50°C under reduced pressure. The sulfur load of the fabricated positive electrode was 3.5mg / cm³. 2 That was the case.

[0108] (Fabrication of negative electrodes for lithium-sulfur batteries) In an atmospheric environment with a dew point of -40°C or lower, a 30 μm thick Li foil was cut to a 13 mm diameter and used as the negative electrode.

[0109] (Manufacturing of lithium-sulfur batteries) A lithium-sulfur battery was fabricated in an Ar atmosphere with a dew point of -40°C or lower using the following procedure. The positive and negative electrodes prepared according to the above specifications were used, along with bis(trifluorosulfonyl)imide (LiTFSI) as the lithium-containing electrolyte and a mixed solvent of fluoroethylene carbonate (FEC) / 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)-propane (HFE) as the non-aqueous solvent. The electrolyte was prepared by mixing FEC and HFE in a 1:1 volume ratio, dissolving LiTFSI in the FEC and HFE mixed solvent, and adjusting the electrolyte to a concentration of 1.0 mol / L LiTFSI / mixed solvent (FEC+HFE). A polyolefin microporous membrane was used as the separator. The positive electrode, separator, and negative electrode were arranged in a stacked manner within the cell, and the electrolyte was then injected to fabricate a lithium-sulfur battery.

[0110] <Charge / Discharge Test> A constant current charge-discharge test was conducted using the active material (positive electrode) as the working electrode. The charging mode was set to constant current (CC is an abbreviation for constant current) mode for both charging and discharging. The set current density was 167.2 mA / g (a current density of 1672 mA / g is defined as 1C; hereinafter, 167.2 mA / g will be referred to as 0.1C). The cutoff voltage was set with a lower limit of 1.0V and an upper limit of 3.0V. The test was conducted in an environment of 25°C. The charging and discharging capacities are defined per unit mass of sulfur. The Coulomb efficiency was calculated using the following formula. Coulomb efficiency (%) = ((Charging capacity (mAh / g)) / (Discharging capacity (mAh / g))) × 100

[0111] <Manufacturing Example 1: Manufacturing of Nitrogen-Containing Carbon Materials> 10 g of diaminomaleonitrile (DAMN) (manufactured by Tokyo Chemical Industry Co., Ltd.) was weighed onto an alumina board and placed in the center of a quartz tube. Next, this quartz tube was placed in a horizontal tubular furnace and heated to 800°C under an argon (Ar) atmosphere with a gas flow rate of 0.5 L / min and a heating rate of 10°C / min, and held for 1 hour to produce the nitrogen-containing carbon material of Production Example 1. The weighing and handling of DAMN were carried out in a normal chemical laboratory (temperature 25°C). The obtained nitrogen-containing carbon material was placed in a tungsten carbide container, and zirconia balls with a diameter of 0.8-1.0 mmφ were added in a ratio of 1:20 (by mass) to the nitrogen-containing carbon material. The mixture was then ground using a mixer mill (Retsch GmbH, MM500nano) at 35 Hz for 1 hour.

[0112] [Example 1] <Manufacturing of porous carbon materials> 4 g of K2CO3 and pure water were mixed to prepare a saturated aqueous solution. 2 g of the nitrogen-containing carbon material of Production Example 1 was mixed into this saturated aqueous solution, transferred to an alumina board, and dried overnight at 100°C. Then, this alumina board was placed in the center of a quartz tube. This was placed in a horizontal tubular furnace and activated by heating to 800°C at a flow rate of 0.5 L / min and a heating rate of 10°C / min under an Ar atmosphere and holding for 1 hour. After natural cooling, the obtained powder was taken out, put into 150 mL of pure water, and hydrochloric acid was dropped while stirring at 100°C to perform a neutralization treatment. This mixed solution was suction filtered and washed with pure water until the filtrate became neutral. Then, it was dried overnight at 100°C under reduced pressure to produce the porous carbon material of Example 1. The main production conditions are shown in Table 1.

[0113] <EDX analysis> DAMN, the results of Production Example 1 and Example 1 are shown in Table 2.

[0114] <Measurement of specific surface area, pore volume, and pore distribution> The results are shown in Table 3. The results of the pore distribution are shown in FIG. 5.

[0115] <Measurement of FE-SEM> The FE-SEM image of DAMN is shown in FIG. 6(a). The FE-SEM image of the nitrogen-containing carbon material of Production Example 1 is shown in FIG. 6(b).

[0116] <Method for producing a composite of a porous carbon material and sulfur> Formula (1):

Number

Number

[0117] After mixing 0.3 g of the porous carbon material and 0.86 g of sulfur by mass, the sulfur was melted by holding in a sealed container with an inner diameter of 20 mm and a height of 60 mm at 155 °C for 5 hours, and filled into the pores of the porous carbon material by capillary action. Then, the temperature was raised to 300 °C as it was and held for 2 hours to evaporate and remove the sulfur remaining on the surface of the porous carbon material, air-cooled to room temperature, and the container was opened to obtain a composite of the porous carbon material and sulfur (the composite of Example 1). All the steps of the composite formation were carried out under the atmosphere. Table 3 shows the results of measuring the sulfur content of the obtained composite of the porous carbon material and sulfur by thermogravimetric analysis (TG).

[0118] <Charge and discharge test> The charge and discharge test was carried out at 0.1 C. Table 4 shows the test results of the charge and discharge.

[0119] <Production Example 2: Production of nitrogen-containing carbon material> A nitrogen-containing carbon material of Production Example 2 was obtained in the same manner as the nitrogen-containing carbon material of Production Example 1, except for the production conditions of the nitrogen-containing carbon material shown in Table 1.

[0120] <FE-SEM measurement> The FE-SEM image of the nitrogen-containing carbon material of Production Example 2 is shown in Fig. 6(c).

[0121] <EDX analysis> The results of Production Example 2 are shown in Table 2

[0122] [Example 2] <Production of porous carbon material> A porous carbon material of Example 2 was obtained in the same manner as the porous carbon material of Example 1, except for the production conditions of the porous carbon material shown in Table 3.

[0123] <EDX analysis> The results of Example 2 are shown in Table 2.

[0124] <Measurement of specific surface area, pore volume, and pore distribution> The results of Example 2 are shown in Table 3. The results of the pore distribution of Example 2 are shown in Fig. 7.

[0125] <FE-SEM measurement> The FE-SEM image of the porous carbon material of Example 2 is shown in Fig. 8(a).

[0126] <STEM measurement> The STEM image of the porous carbon material of Example 2 is shown in Fig. 9(a).

[0127] <Raman analysis> The Raman measurement results of the porous carbon material of Example 2 are shown in Fig. 10, and the analysis results are shown in Table 5.

[0128] <Method for manufacturing composite of porous carbon material and sulfur> A composite of a porous carbon material and sulfur (the composite of Example 2) was obtained in the same manner as the composite of Example 1, except for the manufacturing conditions of the composite of the porous carbon material and sulfur shown in Table 3. The results of measuring the sulfur content of the obtained composite of the porous carbon material and sulfur by thermogravimetric analysis (TG) are shown in Table 3 and Fig. 11.

[0129] <Charge-discharge test> A charge-discharge test was carried out at 0.1C. The test results of charge-discharge are shown in Table 4, Fig. 12 and Fig. 17.

[0130] [Example 3] <Manufacture of porous carbon material> A porous carbon material of Example 3 was obtained in the same manner as the manufacturing conditions of the porous carbon material of Example 1, except for using Production Example 2 as the nitrogen-containing carbon material.

[0131] <EDX analysis> The results of Example 3 are shown in Table 2.

[0132] <Measurement of specific surface area, pore volume and pore distribution> The results of Example 3 are shown in Table 3. The results of the pore distribution are shown in Fig. 13.

[0133] <FE-SEM measurement > The FE-SEM image of the porous carbon material of Example 3 is shown in Fig. 8(b).

[0134] <Measurement of STEM> The STEM image of the porous carbon material of Example 3 is shown in Fig. 9(b).

[0135] <Raman analysis> The Raman measurement results of the porous carbon material are shown in Fig. 14, and the analysis results are shown in Table 5.

[0136] <Method for producing composite of porous carbon material and sulfur> A composite of a porous carbon material and sulfur (the composite of Example 3) was obtained in the same manner as the composite of Example 1, except for the production conditions of the composite of the porous carbon material and sulfur shown in Table 3. The results of measuring the sulfur content of the obtained composite of the porous carbon material and sulfur by thermogravimetric analysis (TG) are shown in Table 3 and Fig. 15.

[0137] <Charge-discharge test> A charge-discharge test was carried out at 0.1C. The test results of charge-discharge are shown in Table 4, Fig. 16 and Fig. 17.

[0138] [Example 4] <Production of porous carbon material> A porous carbon material of Example 4 was obtained in the same manner as the porous carbon material of Example 3, except for the production conditions of the porous carbon material shown in Table 3.

[0139] <EDX analysis> The results of Example 4 are shown in Table 2.

[0140] <Measurement of specific surface area, pore volume and pore distribution> The results of Example 4 are shown in Table 3. The results of the pore distribution are shown in Fig. 18.

[0141] <Method for producing composite of porous carbon material and sulfur> A composite of a porous carbon material and sulfur (the composite of Example 4) was obtained in the same manner as the composite of Example 1, except for the production conditions of the composite of the porous carbon material and sulfur shown in Table 3. Table 3 shows the results of measuring the sulfur content of the composite of the obtained porous carbon material and sulfur by thermogravimetric analysis (TG).

[0142] <Production Example 3: Production of Nitrogen-Containing Carbon Material> A nitrogen-containing carbon material of Production Example 3 was obtained in the same manner as the nitrogen-containing carbon material of Production Example 1, except for the production conditions of the nitrogen-containing carbon material shown in Table 1.

[0143] The FE-SEM image of the nitrogen-containing carbon material of Production Example 3 is shown in Fig. 6(d).

[0144] <EDX Analysis> The results of Production Example 3 are shown in Table 2.

[0145] [Example 5][[ID=2I]] <Production of Porous Carbon Material> A porous carbon material of Example 5 was obtained in the same manner as the production conditions of the porous carbon material of Example 2, except for using Production Example 3 as the nitrogen-containing carbon material.

[0146] <EDX Analysis> The results of Example 5 are shown in Table 2.

[0147] <Measurement of Specific Surface Area, Pore Volume, and Pore Distribution> The results of Example 5 are shown in Table 3. The results of the pore distribution are shown in Fig. 19.

[0148] <Production Method of Composite of Porous Carbon Material and Sulfur> A composite of a porous carbon material and sulfur (composite of Example 5) was obtained in the same manner as the composite of Example 1, except for the production conditions of the composite of the porous carbon material and sulfur shown in Table 3. Table 3 shows the results of measuring the sulfur content of the composite of the obtained porous carbon material and sulfur by thermogravimetric analysis (TG).

[0149] [Comparative Example 1] (Production of Azulmic Acid (AZA)) An aqueous solution was prepared in a container by dissolving 150 g of hydrogen cyanide in 350 g of water. While stirring this solution, 120 g of a 25% by mass aqueous ammonia solution was added over 10 minutes, and the resulting mixture was heated to 35°C. Polymerization of hydrogen cyanide began, and a dark brown polymer started to precipitate, and the temperature gradually rose to 45°C. Two hours after polymerization began, a 30% by mass aqueous hydrogen cyanide solution was added at a rate of 200 g / hour, and 800 g was added over 4 hours. During the addition of the hydrogen cyanide solution, the container was cooled to maintain the reaction temperature at 50°C. The polymerization reaction solution was stirred at this temperature for 100 hours. The resulting black precipitate was separated by filtration. The yield of the precipitate at this time was 96% by mass relative to the total amount of hydrogen cyanide used. After washing the separated precipitate with water, it was dried in a drying oven at 120°C for 4 hours to obtain azulmic acid (AZA).

[0150] For the production and polymerization of hydrogen cyanide, a special experimental environment was established by installing a glove box in a fume hood, and further installing an exhaust system in the glove box. During the experiment, it was necessary to wear a direct-connection gas mask and handle hydrogen cyanide through gloves inside the glove box, making the handling of azurmic acid during production particularly difficult.

[0151] <Manufacturing of nitrogen-containing carbon materials> As shown in Table 1, the nitrogen-containing carbon material of Comparative Example 1 was obtained in the same manner as the nitrogen-containing carbon material of Production Example 1, except that the raw material for the nitrogen-containing carbon material was changed to azulmic acid.

[0152] <Manufacturing of porous carbon materials> A porous carbon material of Comparative Example 1 was obtained in the same manner as in Example 1, except that Comparative Example 1 was used as the nitrogen-containing carbon material.

[0153] <Measurement of specific surface area, pore volume, and pore distribution> The results for Comparative Example 1 are shown in Table 3.

[0154] <Method for producing a composite of porous carbon material and sulfur> A composite of porous carbon material and sulfur (composite of Comparative Example 1) was obtained in the same manner as the composite of Example 1, except for the manufacturing conditions of the composite of porous carbon material and sulfur shown in Table 3. Table 3 shows the results of thermogravimetric analysis (TG) measuring the sulfur content of the resulting porous carbon material and sulfur composite.

[0155] <Charge / Discharge Test> A charge-discharge test was conducted at 0.1C. The test results are shown in Table 4.

[0156] [Example 6] <Preparation of laminated cells> A laminate cell was fabricated using the porous carbon material obtained in Example 3. A lithium-sulfur battery was fabricated in the same manner as in Example 3, except that a 3D Al current collector (product name: Cellmet, manufactured by Sumitomo Electric Industries, Ltd.) shown in Figure 20(a) was used as the positive electrode current collector, and the laminate cell was fabricated by cutting it into the shape shown in Figure 20(b) and attaching tabs. The sulfur load was 8.36 mg / cm³. 2 The ratio of electrolyte (E) to sulfur (S) (E / S) was set to 3.0 μL / mg (sulfur).

[0157] <Charge / Discharge Test> Charge and discharge tests were conducted on the laminate cell at 0.1C. The test results for the first and second charge and discharge cycles are shown in Figure 21. The gravimetric energy density of the laminate cell (excluding the weight of the outer casing and tab, with the negative electrode calculated using theoretical values) was calculated to be 286 Wh / kg.

[0158] These examples demonstrate that this embodiment can easily and safely produce porous carbon materials from nitrogen-containing carbon materials. Furthermore, it is evident that it is possible to provide composites of porous carbon materials and sulfur with large pore volume, particularly micropore volume, and high sulfur-supporting capacity. Additionally, it is shown that, as a positive electrode for lithium-sulfur batteries, it exhibits a large discharge capacity per unit of sulfur mass, a large discharge capacity per unit of positive electrode material containing sulfur and carbon material, and good cycle characteristics.

[0159] [Table 1]

[0160] [Table 2]

[0161] [Table 3]

[0162] [Table 4]

[0163] [Table 5] [Explanation of Symbols]

[0164] 10 Ni Tabs 11 Al Tabs 12 Negative electrode 13 Positive electrode [Industrial applicability]

[0165] This invention provides a simple and safe method for producing porous carbon material from nitrogen-containing carbon material. The porous carbon material of this invention has industrial potential as a composite with sulfur. Furthermore, the composite of the porous carbon material of this invention and sulfur has industrial potential as a positive electrode for lithium-sulfur batteries.

Claims

1. A method for producing a porous carbon material, comprising: a first step of heat-treating diaminomaleonitrile in an inert gas to obtain a nitrogen-containing carbon material; and a second step of alkali-activating the nitrogen-containing carbon material to obtain a porous carbon material.

2. A method for producing a porous carbon material according to claim 1, wherein the nitrogen content of the nitrogen-containing carbon material is 2 to 45 atomic concentration percent.

3. A method for producing a porous carbon material according to claim 1 or 2, wherein the temperature of the heat treatment in the inert gas is 600°C to 1200°C.

4. A method for producing a porous carbon material according to claim 1 or 2, wherein the temperature of the alkali activation heat treatment is 600°C to 1200°C.

5. The method for producing a porous carbon material according to claim 1 or 2, wherein the porous carbon material is used as the positive electrode of a lithium sulfur battery.

6. A porous carbon material manufactured by the manufacturing method described in claim 1.

7. In the laser Raman spectrum diagram, wavenumbers 1250–1385 cm⁻¹ －１ Between these peaks is P1, with wavenumbers of 1550-1620 cm. －１ A porous carbon material having at least two major peaks, peak P2, between P1 and P2, and having a ratio (L / H1) of the height L from the baseline of the minimum point M between P1 and P2 to the height H1 from the baseline of P1 of 0.40 to 0.

85.

8. A composite of a porous carbon material and sulfur according to claim 6 or 7.

9. The composite of a porous carbon material and sulfur according to claim 8, wherein the sulfur content is 40 to 90% by mass.

10. A composite of a porous carbon material and sulfur according to claim 8, used as the positive electrode of a lithium-sulfur battery.

11. A method for producing a composite of a porous carbon material and sulfur according to claim 9, comprising a mixing step of mixing a porous carbon material with sulfur heated above its melting point.

12. A positive electrode for a lithium-sulfur battery, comprising a composite of the porous carbon material and sulfur as described in claim 8.

13. A lithium-sulfur battery comprising the positive electrode described in claim 12.

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