Activated carbon and its manufacturing method

Abstract

To provide activated carbon capable of suppressing the occurrence of internal short circuits. [Solution] Powdered activated carbon, in which the number of metal particles having a short side length of 45 μm or more among the metal particles remaining on a sieve obtained by sieving the powdered activated carbon using a sieve with a mesh size of 32 μm conforming to JIS Z 8801 is 30% or less of the total number of metal particles remaining on the sieve.

Description

[Technical Field]

[0001] The present invention relates to activated carbon and a method for producing the same. [Background technology]

[0002] In recent years, the increasing sophistication of mobile devices and the spread of hybrid and electric vehicles have led to an increasing demand for high-performance energy storage devices. In particular, electric double-layer capacitors (EDLCs) and lithium-ion capacitors (LiCs), which are capable of instantaneously charging and discharging large amounts of power, are important devices, and porous carbon materials such as activated carbon, which have an extremely large specific surface area and excellent conductivity, are widely used as electrode materials for these devices.

[0003] To ensure the performance of high-performance energy storage devices, particularly their long-term reliability and lifespan, it is essential to minimize the content of impurities contained in activated carbon, the electrode active material, particularly alkali metals (Na, K, etc.) and conductive metals such as iron (Fe), copper (Cu), nickel (Ni), and chromium (Cr). This is because metals remaining in activated carbon can cause unintended side reactions such as gas generation and increased resistance at the interface with the electrolyte, which can lead to performance degradation, shortened lifespan, or even short circuits. However, in the chemical activation method, a typical method for producing activated carbon, alkali metal compounds such as potassium hydroxide (KOH) and sodium hydroxide (NaOH) are often used as activators. This method tends to leave alkali metals inside the activated carbon, particularly in its micropores, after the activation process, making it extremely difficult to completely remove them through the subsequent cleaning process alone. Meanwhile, there are concerns that conductive metals such as iron, copper, nickel, and chromium may remain in the activated carbon due to contamination from raw materials or the materials used in various equipment. As a result, efforts have been made to strengthen acid cleaning and other methods to remove these impurities, but the removal of remaining or contaminated metals after cleaning has been difficult, posing a challenge to their application in energy storage devices.

[0004] Against this background, there has been a demand for the development of advanced purification technologies to reduce residual metals. In recent years, technologies have been developed that combine electrodialysis and special acid treatment to reduce the alkali metal concentration in activated carbon to a level significantly lower than conventional levels, for example, to 50 ppm or less (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2020-531405 Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, there has been an increasing demand for smaller energy storage devices with higher energy density, and in response to this demand, efforts are being made to improve the performance of component parts, such as thinner electrodes and higher-density separators. However, the inventors have found that even if the amount of residual metal in the activated carbon is reduced or the electrodes are made thinner as in the past, the narrow gap between the electrodes and the separator can still cause unexpected internal short circuits, which can lead to an increase in the defective rate during production. An object of the present invention is to provide activated carbon that can suppress the occurrence of internal short circuits, dendrites, and the like. [Means for solving the problem]

[0007] [1] Powdered activated carbon, The powdered activated carbon is sieved using a sieve with a mesh size of 32 μm conforming to JIS Z 8801 to obtain metal particles remaining on the sieve, and the number of metal particles having a short side length of 45 μm or more is 30% or less of the total number of metal particles remaining on the sieve. [2] The powdered activated carbon has an average particle size of 20 μm or less, [1] The powdered activated carbon according to [1], wherein the number of metal particles having a short side of 30 μm or more among the metal particles on the sieve is 70% or less of the total number of metal particles remaining on the sieve. [3] The powdered activated carbon has a specific surface area of ​​1700 to 3500 m 2 / g of the powdered activated carbon according to [1] or [2]. [4] Among the metal particles contained in the activated carbon, the product of the long and short sides on the observation plane is 2025 μm 2 The powdered activated carbon according to any one of [1] to [3], wherein the number of metal particles having a particle size of 100 or more is 50% or less of the total number of metal particles remaining on the sieve. [5] The powdered activated carbon according to any one of [1] to [4], wherein the powdered activated carbon is alkali-activated carbon. [6] An electrode material using the powdered activated carbon according to any one of [1] to [5]. [7] An electricity storage device using the electrode material described in [6].

[0008] [8] A method for producing powdered activated carbon, wherein the number of metal particles having a short side of 45 μm or more among the metal particles remaining on the sieve obtained by sieving powdered activated carbon using a sieve with a mesh size of 32 μm conforming to JIS Z 8801 is 30% or less of the total number of metal particles remaining on the sieve, a step of carbonizing the carbonaceous material; a step of activating the carbonized product obtained by the carbonization treatment; a step of performing a pulverization treatment at least once before the carbonization treatment step, between the carbonization treatment step and the activation treatment step, or after the activation treatment step; a step of sieving the powdered activated carbon either after the activation treatment step or after the pulverization treatment step following the activation treatment step; A method for producing powdered activated carbon having the formula: [9] The method for producing powdered activated carbon according to [8], wherein the activation treatment step is an alkali activation treatment.

[10] The sieving process is a wet sieving process, The activated carbon has a slurry concentration of 0.5 to 40% by mass, and The method for producing powdered activated carbon according to [8] or [9], wherein the slurry is wet-sieved using a sieve with a mesh size of 38 μm or less. [Effects of the Invention]

[0009] The activated carbon of the present invention can solve the above problems, and in particular, in an electrode material using the activated carbon of the present invention, the occurrence of an internal short circuit can be effectively suppressed. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a filtration treatment flow diagram in an example. [Figure 2] FIG. 1 is a schematic diagram illustrating a filtration device in an embodiment. [Figure 3] Figure 3 is a graph plotting the size of metal particles contained in the filtration residue. [Figure 4] Figure 4 is a photograph of metal particles contained in the residue on the sieve. DETAILED DESCRIPTION OF THE INVENTION

[0011] The inventors conducted a detailed study on the causes of internal short circuits in electrode materials and discovered that the distance between the electrodes and separators becomes shorter as a result of thinner electrodes and separators, and that foreign matter, which was not a problem in the past, increases the risk of short circuits. In particular, it was found that even when the metal concentration in the activated carbon is low, metallic foreign matter with large particle size and electrical conductivity can penetrate the separator and form a conductive path between the electrodes, becoming a major cause of short circuits. Furthermore, the inventors have discovered that many of these metallic foreign matters originate from contamination introduced from manufacturing equipment used in downstream processes such as heat treatment after activation and cleaning of activated carbon, such as stainless steel (SUS) grinders and piping, and are primarily metal particles such as Fe, Cr, and Ni. Furthermore, it has become clear that these metal particles are difficult to remove by the washing process after activation treatment, which is typically carried out in the manufacturing process, since they become mixed in or adhere to the activated carbon after the washing process. As a result, they remain in the activated carbon as metal foreign matter. Therefore, the inventors have come to the realization that effectively reducing metal particles (metallic foreign matter) originating from the manufacturing equipment and having a relatively large particle size is extremely important in suppressing internal short circuits in capacitors, and have thus completed the present invention.

[0012] In the present disclosure, powdered activated carbon refers to an aggregate of numerous powdered activated carbon particles, and metal particles are present as impurities in this aggregate. In this disclosure, metal particles refer to independent particles that exist separately from powdered activated carbon particles, as opposed to metals chemically supported on the surface of activated carbon particles or metal components adsorbed or encapsulated within the micropores of activated carbon.

[0013] The powdered activated carbon of the present disclosure is obtained by sieving using a sieve with 32 μm mesh size conforming to JIS Z 8801, and among the metal particles on the sieve (hereinafter sometimes referred to as total metal particles), the number of metal particles with a short side length of 45 μm or more is 30% or less of the total number of metal particles. These coarse metal particles can cause short circuits in energy storage devices. Specifically, in electrode sheets made from powdered activated carbon containing coarse metal particles, the long sides of the coarse metal particles tend to be oriented in the direction of the inner surface (lengthwise direction) of the electrode sheet, and the short sides in the direction of the thickness, depending on the manufacturing process and coating conditions. As a result, the coarse metal particles tend to apply high localized pressure to the separator or generate piercing stress, increasing the risk of short circuits by forming conductive paths between the electrodes.

[0014] Therefore, in order to suppress internal short circuits in electricity storage devices such as electric double layer capacitors, it is effective to control the amount (number) of coarse metal particles contained in powdered activated carbon. In the present disclosure, the number of metal particles with short sides of 45 μm or more contained in powdered activated carbon (powdered activated carbon and metal particles mixed therein, the same applies hereinafter) is preferably as small as possible, and is 30% or less of the total number of metal particles, preferably 25% or less, more preferably 20% or less, even more preferably 10% or less, even more preferably 5% or less, and most preferably 0%.

[0015] Although the dimension of the long side of the coarse metal particles is not particularly limited, if the long side is extremely long, a stable conductive path is likely to be formed inside the electrode, which may increase the risk of short circuiting. In one embodiment, the long side of the metal particles is preferably 60 μm or less, more preferably 55 μm or less, and even more preferably 45 μm or less. In one embodiment, the number of metal particles with long sides, preferably metal particles with long sides of 45 μm or more, is preferably as small as possible, and is preferably 73% or less, more preferably 50% or less, even more preferably 40% or less, even more preferably 20% or less, and most preferably 0% of the total number of metal particles.

[0016] The metal particles contained in activated carbon are mainly derived from the constituent materials such as stainless steel (SUS) used in manufacturing equipment such as crushers and piping, but are not limited to this. These metal particles are often particles containing transition metals, particularly transition metal elements belonging to the fourth period of the periodic table. Specifically, examples include particles of simple metals such as iron (Fe), chromium (Cr), and nickel (Ni), or alloy particles containing these as the main components. Furthermore, metal particles containing manganese (Mn), molybdenum (Mo), copper (Cu), and the like may also be mixed in. In particular, the inclusion of conductive metal particles such as Fe, Ni, Cr, and Cu may cause short circuits, so it is preferable to suppress the content of these metal particles.

[0017] Metal particles can cause defects such as a decrease in electrode performance and short circuits, so it is desirable that the amount of metal particles contained in the activated carbon is as small as possible. In one embodiment, the total content of metal particles, preferably iron (Fe), copper (Cu), nickel (Ni), and chromium (Cr), contained in the activated carbon (powdered activated carbon and metal particles mixed therein) is preferably 5000 ppm or less, more preferably 400 ppm or less, even more preferably 300 ppm or less, and even more preferably 200 ppm or less. The metal particles referred to here are not limited to large metal particles (those with a short side of 45 μm or more), but also include smaller metal particles.

[0018] The average particle size (D50) of the powdered activated carbon is not particularly limited as it is selected appropriately depending on the application, but may be in the following ranges. In one embodiment, the average particle size (D50) of the powdered activated carbon is preferably 20 μm or less, more preferably 15 μm or less, even more preferably 10 μm or less, and even more preferably 6.0 μm or less. The lower limit may be, for example, 0.1 μm or more, or 1.0 μm or more. Such a particle size allows a large external specific surface area to be obtained when used as an electrode, which contributes to improving electrochemical performance such as capacitance. Furthermore, in powdered activated carbon having an average particle size (D50) within the above range, the presence of conductive metal particles, particularly metal particles with a short side of 30 μm or more (hereinafter referred to as semi-coarse metal particles), may increase the risk of short circuits in the power storage device. Therefore, it is effective to suppress the occurrence of internal short circuits and dendrites by making the number of these semi-coarse metal particles preferably 70% or less, more preferably 50% or less, even more preferably 30% or less, even more preferably 10% or less, and most preferably 0% of the total number of metal particles remaining on the sieve. In other words, as powdered activated carbon becomes finer, it becomes more important to control the amount of coarse or semi-coarse metal particles that are mixed in, and by appropriately controlling the amount of these particles present, product reliability and safety can be improved.

[0019] In one embodiment, the product of the long and short sides of the metal particles contained in the activated carbon on the observation plane is 2025 μm.2 It is preferable that the number of metal particles having a size equal to or larger than this (hereinafter referred to as large-area metal particles) is 50% or less of the total number of metal particles remaining on the sieve. These large-area metal particles are large and conductive, forming conductive paths between the electrode and the separator, increasing the risk of internal short circuits. In particular, when the projected area of ​​the particles is large, the force pushing up the separator locally increases, which can lead to punctures or penetrations. In one embodiment, the geometric two-dimensional projected area (long side × short side) of the metal particles is used as the evaluation criterion, and in particular, 2025 μm 2 Reducing the number of these metal particles to 50% or less contributes to improving the reliability of energy storage devices and reducing the risk of internal short circuits. 2 The number of the above particles is more preferably 40% or less, more preferably 30% or less, even more preferably 20% or less, even more preferably 10% or less, and most preferably 0% of the total number of metal particles.

[0020] In this disclosure, the "long side" and "short side" of a metal particle are defined based on a two-dimensional projection image of the metal particle on an observation plane. Specifically, the shape of the metal particles contained in activated carbon is observed using a scanning electron microscope (SEM), and the projected outline of each particle is recognized from the image. The smallest rectangle circumscribing the obtained particle outline (circumscribing rectangle) is assumed, and the long side direction of this rectangle is defined as the "long side" of the metal particle, and the short side direction is defined as the "short side." Even if the particle shape is irregular, the dimensional direction of the object to be measured can be determined by applying the circumscribing rectangle to the particle outline. Furthermore, the dimensions of the long side and short side are measured in μm units using image analysis software. In this case, the long side and short side are defined as the maximum extension direction on the observation plane and the direction perpendicular to it, regardless of the particle orientation or tilt.

[0021] specific surface area The larger the specific surface area of ​​powdered activated carbon, the more it contributes to an increase in capacitance when used as an electrode, so the specific surface area is a suitable design factor. The specific surface area of ​​the powdered activated carbon of the present disclosure is preferably 1700 to 3500 m 2 / g, more preferably 1900 to 3200m 2 / g, more preferably 2000 to 3000m 2 / g.

[0022] Total Pore Volume The larger the total pore volume of powdered activated carbon, the more it contributes to an increase in capacitance as an electrode material. The total pore volume of the powdered activated carbon is preferably 0.5 to 5.0 mL / g, more preferably 0.7 to 4.0 mL / g, and even more preferably 0.9 to 3.0 mL / g.

[0023] Micropore Volume The larger the micropore volume of the powdered activated carbon, the more the amount of electrolyte ions that can be adsorbed, which contributes to improving the capacitance. The micropore volume of the powdered activated carbon is preferably 0.1 to 4.0 mL / g, more preferably 0.5 to 3.0 mL / g, and even more preferably 0.7 to 2.0 mL / g.

[0024] Average pore size The average pore diameter can be adjusted appropriately depending on the type and design of the device. A smaller average pore diameter improves the electrode density and increases the capacitance per volume. On the other hand, a larger average pore diameter improves the diffusion of electrolyte ions and contributes to lower resistance. In one embodiment, the average particle size is preferably 0.8 to 4.0 nm, more preferably 1.0 to 3.0 nm, and even more preferably 1.5 to 2.5 nm.

[0025] Total acidic functional group amount In one embodiment, the total amount of acidic functional groups in the powdered activated carbon can be adjusted appropriately depending on the design of the device it is used in. For example, when used in an electric double layer capacitor, a larger total amount of acidic functional groups leads to a decrease in durability. Here, the acidic functional group is a group that can react with a basic reagent, and specifically includes a hydroxy group (R-OH group), a carboxy group (R-COOH group), an ester group (R-OCO group) such as a lactone group, and a carbonyl group (R=O group) such as an α,β-unsaturated carbonyl group. The total amount of acidic functional groups is the sum of these acidic functional groups and is measured by titration. The total amount of acidic functional groups in the powdered activated carbon is preferably 0.01 meq / g or more, more preferably 0.1 meq / g or more, even more preferably 0.2 meq / g or more, and is preferably 3.0 meq / g or less, more preferably 1.5 meq / g or less, even more preferably 1.0 meq / g or less.

[0026] impurities The presence of alkali metals such as lithium (Li), sodium (Na), and potassium (K) in activated carbon can reduce cycle life and safety. Therefore, the lower the alkali metal content in activated carbon (powdered activated carbon and metal particles mixed therein), the better. The total content of alkali metals is preferably 1200 ppm or less, more preferably 1000 ppm or less, even more preferably 500 ppm or less, and even more preferably 250 ppm or less. The lower limit of the alkali metal content is not particularly limited, and may be 0 ppm or may be in a range exceeding 0 ppm.

[0027] Alkaline activation The powdered activated carbon of the present disclosure is preferably produced by an alkali activation method. Powdered activated carbon obtained by alkali activation treatment tends to have a pore structure that contributes to improving capacitance, compared to activated carbon obtained by steam activation. A suitable example of the alkali activation method is the alkali activation treatment step described below.

[0028] The method for producing powdered activated carbon of the present disclosure is described below, but the production method of the present disclosure is not limited to the specific production method described below as long as the desired physical properties as described above can be obtained. Furthermore, the production conditions described below are examples of preferred ranges, and it is desirable to appropriately adjust the conditions for each step depending on the physical properties to be obtained.

[0029] Activating ingredients The activated carbon of the present disclosure preferably uses a carbonaceous material, more preferably a carbonized material thereof, as the activation raw material. The activation raw material may be a known carbonaceous material or its carbide, such as non-graphitizable carbon, graphitizable carbon, or a mixture thereof. Examples of non-graphitizable carbon include wood, sawdust, charcoal, coconut shell, cellulose-based fibers, and synthetic resins (such as phenolic resins). Examples of graphitizable carbon include mesophase pitch, pitch coke, petroleum coke, coal coke, needle coke, polyvinyl chloride, polyimide, and polyacrylonitrile. These carbonaceous materials may be used alone or in combination of two or more. Particularly preferred carbonaceous materials are mixtures of hardly graphitizable carbon and easily graphitizable carbon, with phenolic resins or petroleum coke being particularly preferred.

[0030] Carbonization The carbonization treatment of the carbonaceous material is a heat treatment carried out in an atmosphere of an inert gas such as nitrogen, and is preferably carried out by holding the material at a temperature in the range of 400° C. to 1000° C. for 0.5 to 10 hours, for example. Carbonization proceeds through such treatment, and a carbonized product suitable for activation treatment can be obtained.

[0031] Activation treatment process The activation treatment for the carbonized material may be any treatment suitable for obtaining activated carbon having a desired pore structure, and examples thereof include gas activation treatment and alkali activation treatment. For the gas activation treatment, water vapor, air, carbon dioxide, oxygen, combustion gas, and mixtures of these gases can be used. Although water vapor is used as an example below, the same applies to other activation gases such as carbon dioxide. The conditions for the steam activation treatment are not particularly limited, and examples include a steam concentration of 40 to 100 Vol %, a steam partial pressure of 40 kPa or more, an activation treatment temperature of 850 to 1500° C., and a heating holding time of 0.1 to 10 hours.

[0032] The alkali activation treatment is a process in which an activator containing an alkali metal compound is mixed with an activation raw material and heated in an inert gas atmosphere to obtain activated carbon. An alkali metal compound can be used as the alkali activator, and specific examples include alkali metal hydroxides such as potassium hydroxide and sodium hydroxide; alkali metal carbonates such as potassium carbonate and sodium carbonate; and alkali metal sulfates such as potassium sulfate and sodium sulfate. Alkali metal hydroxides are preferred, and potassium hydroxide is more preferred.

[0033] Increasing the mass ratio of the activation raw material to the alkali activator (alkali activator / activation raw material) promotes the activation reaction, contributing to the development of the specific surface area and pore structure. In one embodiment, the upper limit of the mass ratio may be appropriately limited from the viewpoint of ensuring the mechanical strength of the activated carbon. The mass ratio of the activation raw material to the alkali activator is preferably 0.5 to 10.0, more preferably 1.0 to 5.0, and even more preferably 2.0 to 4.0.

[0034] The alkali activation treatment is preferably carried out in an atmosphere of an inert gas such as argon, helium, or nitrogen. It is preferable to increase the alkali activation treatment temperature from the viewpoint of increasing the specific surface area and the amount of acidic functional groups, but if the temperature is too high, the amount of acidic functional groups may decrease and the strength of the activated carbon may decrease. The alkali activation treatment temperature is preferably 450 to 1200°C, more preferably 500 to 1000°C, even more preferably 550 to 850°C, and even more preferably 600 to 800°C. The activation treatment time is preferably 0.5 to 24 hours, more preferably 1.0 to 12 hours. The temperature rise rate during activation treatment is preferably 1 to 20°C / min, more preferably 5 to 15°C / min.

[0035] Post-processing In the present disclosure, the activated carbon after the activation treatment may be subjected to post-treatment steps such as washing, drying, heat treatment, and pulverization, if necessary. These post-treatment steps can be carried out in any order, for example, the order of the heat treatment and the grinding treatment can be reversed.

[0036] Cleaning process In one embodiment, in the method for producing powdered activated carbon, a washing treatment is preferably carried out after the activation treatment. In one embodiment, the washing treatment can be aimed at reducing or removing alkali metal compounds (eg, KOH or NaOH) remaining on the activated carbon after activation. The washing treatment may involve at least one of water washing and acid washing, and more preferably both. The order of water washing and acid washing and the number of times each washing is performed are not particularly limited, but it is more preferable to perform water washing multiple times after acid washing.

[0037] Pretreatment process (water washing process) The water washing treatment step is a step that is carried out as needed after the alkali activation treatment, and is carried out for the purpose of removing alkali metals remaining in the activated carbon prior to the subsequent acid washing step. In this step, a method is preferred in which activated carbon is added to water, stirred or dispersed as necessary, and then collected by filtration. Examples of the stirring or dispersion method include mechanical stirring, gas blowing, ultrasonic irradiation, and boiling with heat. The temperature of the water used during water washing is not particularly limited, but is preferably 20°C or higher from the viewpoint of increasing the alkali metal removal efficiency, and is 100°C or lower, more preferably 95°C or lower, under normal pressure from the viewpoint of suppressing water evaporation loss.

[0038] Acid cleaning process The acid washing step is a step in which the activated carbon that has been subjected to alkali activation treatment is treated with a washing liquid containing an inorganic acid to remove alkali metal components remaining in the activated carbon. Examples of inorganic acids include hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, and carbonic acid, but are not limited to these. Hydrogen acids (e.g., hydrochloric acid, hydrofluoric acid), oxygen acids (e.g., sulfuric acid, nitric acid, phosphoric acid, and perchloric acid) can be appropriately selected and used. Hydrochloric acid is particularly preferred. The solvent for the cleaning solution is not limited, but an aqueous solution of an inorganic acid using water is preferred. The inorganic acid concentration in this aqueous inorganic acid solution is not particularly limited, but from the viewpoint of balancing production costs and removal efficiency, it is desirable to adjust the concentration to contain 10 to 100 parts by mass of inorganic acid per 100 parts by mass of activated carbon after alkali activation treatment. The temperature of the inorganic acid cleaning solution is not particularly limited, but from the viewpoint of suppressing volatilization of the inorganic acid while increasing the removal efficiency, it is preferable to perform cleaning at a temperature of 50°C or higher, preferably 100°C or lower, and more preferably 85°C or lower.

[0039] Post-processing process (water washing process) In one embodiment, after the activated carbon that has undergone the alkali activation treatment is washed with an acid, a step of further washing the activated carbon with water (hereinafter referred to as a water washing step) may be carried out. In one embodiment, the purpose of this water washing step may be to remove inorganic acids, basic substances, or their neutralization reaction products (salts) remaining in the activated carbon. The temperature of the water used for washing is not particularly limited, but from the viewpoint of enhancing the removal efficiency, it is preferably 30°C or higher, more preferably 50°C or higher, and even more preferably 60°C or higher. On the other hand, in order to suppress loss of water due to evaporation, the temperature is preferably 100°C or lower, more preferably 95°C or lower, under normal pressure. The water used in this step may contain other components as long as they do not interfere with the objective of the step. For example, the water may contain alcohols such as methanol and ethanol as components that can be easily removed from the activated carbon by heating.

[0040] Drying process In one embodiment, it is preferable to carry out a drying treatment step as needed. In one embodiment, the drying treatment may be performed for the purpose of removing moisture from activated carbon that is in a water-containing state after the washing treatment or acid treatment, and the drying method itself is not particularly limited as long as the activated carbon is sufficiently dried. In one embodiment, the drying treatment is preferably carried out at a temperature of 50° C. to 180° C. for about 0.5 hours to 48 hours. Drying can be carried out by known methods such as natural drying, warm air drying, hot air drying, vacuum drying, far infrared drying, etc.

[0041] Heat Treatment Process In one embodiment, a heat treatment step may be carried out as needed to adjust the amount of functional groups present on the surface of the activated carbon. The activated carbon may be heat-treated in an inert gas atmosphere to promote decomposition and release of the acidic functional groups, thereby controlling the desired composition of surface functional groups. In one embodiment, this treatment is expected to have the effect of reducing the amount of acidic functional groups and relatively increasing the amount of basic functional groups. The heat treatment is carried out in an inert gas atmosphere such as argon, nitrogen, or helium. The heat treatment temperature is not particularly limited, but in one embodiment, in order to promote effective decomposition of the acidic functional groups, it is preferably 300°C to 1200°C, more preferably 400°C to 1100°C, and even more preferably 500°C to 900°C. In addition, the heating time is not particularly limited, but in one embodiment, by optimizing it together with the heating temperature, the desired functional group configuration can be stably obtained. For example, the heating time is preferably 1 minute to 10 hours, more preferably 5 minutes to 8 hours, and even more preferably 10 minutes to 4 hours.

[0042] Crushing process In one embodiment, it is preferable to carry out a milling process as needed to obtain a desired particle size distribution or average particle size. The pulverization step can be carried out at any stage before or after the activation step, and can be carried out at any step, for example, before or after the carbonization step, or after the activation step. Furthermore, it can be carried out after any of the post-treatment steps such as washing, drying, heat treatment, etc. For example, the heat treatment may be carried out after washing and drying, or the heat treatment may be carried out after washing, and then the pulverization may be carried out. The activated carbon is preferably pulverized appropriately so that the average particle size thereof becomes a predetermined size. In one embodiment, the average particle size (D50) may be adjusted to, for example, about 0.1 to 20 μm. In one embodiment, the pulverization conditions may be appropriately set so as to obtain powdered activated carbon with a highly precisely controlled particle size.

[0043] The means for pulverizing the activated carbon is not limited. As the pulverizing means, various known pulverizing devices can be used, such as a jet mill, ball mill, disk mill, bead mill, rod mill, roller mill, hammer mill, pin mill, aeroform mill, and blade mill. In particular, a ball mill is preferable because it is simple and easy to use. The grinding method may be either wet or dry. Furthermore, in order to adjust the particle size of the activated carbon, classification treatment may be carried out in combination as necessary, and as a means for this, a stainless steel sieve, a cyclone-type classifier, or the like can be used.

[0044] Screening process In the present disclosure, a screening process is performed as the final process. The term "final process" used here refers to a case where no other processing steps are performed after the screening process. In one embodiment, for example, the sieving treatment may be performed after the pulverization treatment after the post-treatment step, or may be performed immediately after the post-treatment step if the pulverization treatment is omitted. That is, this treatment is a step performed on the powdered activated carbon that will be the final product in order to ensure the product quality. In one embodiment, the sieving treatment may be performed as the final step, immediately before the shipping step, such as bagging. In one embodiment, in addition to the sieving treatment as the final step, it can also be carried out at any stage during the manufacturing process, for example, after the grinding treatment before the activation treatment.

[0045] In one embodiment, the sieving process can be carried out in either a dry or wet manner, but is preferably a wet sieving process. In wet sieving, powdered activated carbon is dispersed in a liquid medium such as water, which prevents clogging of the sieve due to static electricity and agglomeration, enabling high-precision separation that is difficult to achieve with dry sieving. In particular, foreign matter with a high specific gravity and a tendency to agglomerate, such as metal particles, can be effectively separated and removed based on particle size. Furthermore, wet processing reduces impact and friction in the dispersion medium, which has the advantage of preventing activated carbon from being crushed compared to dry processing, preventing a decrease in specific surface area.

[0046] There are no particular limitations on the equipment used for the wet sieving treatment, and any equipment can be used as long as it is capable of treating the powdered activated carbon slurry with a sieve mesh having a predetermined mesh size. Examples of such devices include wet vibrating sieves, ultrasonic sieves, rotary sieves, tumbler sieves, centrifugal sieves, etc. These devices are preferably equipped with a mechanism for preventing clogging of the sieve (ultrasonic oscillation, brush, air nozzle, etc.) and a slurry supply / discharge mechanism. Wet sieving may be performed by dispersing or disaggregating powdered activated carbon containing coarse metal particles in a dispersing medium such as water to form a slurry (dispersion liquid), and passing this slurry through a sieve mesh. The concentration of powdered activated carbon in the slurry (slurry concentration) may be set in consideration of the balance between treatment efficiency and sieving accuracy. In one embodiment, when the slurry concentration is high and the mesh size is small, the amount of activated carbon remaining on the sieve screen increases, which may result in a decrease in yield. In one embodiment, the slurry concentration is preferably 0.5 to 50% by mass, more preferably 5 to 45% by mass, and even more preferably 10 to 40% by mass.

[0047] In one embodiment, the mesh size of the sieve (screen mesh) is preferably selected so that large metal particles to be removed (for example, particles with a short side of 45 μm or more) do not pass through. For example, the mesh size may be 75 μm or less, preferably 53 μm or less, more preferably 45 μm or less, and even more preferably 38 μm or less, but depending on the size of the long side, coarse metal particles with short sides of 45 μm or more can be removed. From the viewpoint of more efficiently removing coarse metal particles with short sides of 45 μm or more, the mesh size may be preferably 32 μm or less, more preferably 30 μm or less, and even more preferably 25 μm or less. In one embodiment, in a square sieve with 32 μm openings conforming to JIS Z 8801, the diagonal length of the openings is approximately 45 μm. Therefore, metal particles with short sides of 45 μm or more are unlikely to pass through even when oriented obliquely with respect to the opening direction, and are likely to be captured on the sieve. In one embodiment, from the viewpoint of removing metal particles with a short side of 30 μm or more (the semi-coarse metal particles), the mesh size may be preferably 21 μm or less, more preferably 19 μm or less, and even more preferably 10 μm or less. In one embodiment, sieving may be performed multiple times using sieves with the same mesh size. By performing sieving multiple times, the number of coarse metal particles can be further reduced. In one embodiment, multiple sieving may be performed using sieves with different size openings. In one embodiment, the mesh size of the sieve mesh can be based on the nominal dimensions of standard sieves specified in JIS Z 8801-1 (metal mesh sieves):2019. By using a sieve with appropriate mesh size, it is possible to effectively prevent large metal particles from being mixed into the final powdered activated carbon product, thereby contributing to improving the reliability and safety of the product.

[0048] The powdered activated carbon of the present disclosure has a significantly reduced content of coarse metal particles that cause internal short circuits, and is therefore suitable as an electrode material, particularly as an electrode material for electricity storage devices. In one embodiment, specific applications include the following devices or materials: activated carbon material for the positive electrode of a lithium ion capacitor (LIC), electrode material for the positive or negative electrode of an electric double layer capacitor (EDLC), negative electrode material (particularly a conductive additive or structural stabilizer for hard carbon) of a lithium ion secondary battery, a sulfur support or conductive additive in the positive electrode mixture of a lithium-sulfur secondary battery, a catalyst support for an air battery, etc. Although not limited to these applications, the powdered activated carbon exhibits extremely excellent performance as an electrode active material in capacitors, and therefore includes electrode materials containing the powdered activated carbon, as well as electricity storage devices such as capacitors using the same.

[0049] Examples of capacitors using a material for an electricity storage device capacitor that uses the powdered activated carbon of the present disclosure as an electrode material include lithium ion capacitors, electric double layer capacitors, and lithium-sulfur secondary batteries. In one embodiment, a lithium ion capacitor includes a negative electrode capable of absorbing and releasing lithium ions, a positive electrode made of activated carbon that operates primarily by electric double layer capacitance, and a non-aqueous electrolyte solution having lithium ion conductivity and a separator. In one embodiment, powdered activated carbon is particularly suitable as a positive electrode active material for lithium ion capacitors due to its high purity and large specific surface area.

[0050] In one embodiment, an electric double layer capacitor (EDLC) is an electricity storage device that mainly comprises a pair of activated carbon electrodes, an electrolyte, and a separator, and generally has a housing form such as a coin type, a wound type, or a laminate type. The powdered activated carbon of the present disclosure is also extremely suitable for use as an electrode active material for the positive or negative electrode in EDLCs, since it can achieve high capacitance and high reliability.

[0051] In one embodiment, the lithium-sulfur secondary battery comprises a sulfur-containing positive electrode active material, a negative electrode made of lithium metal or an alloy thereof, and a separator that holds an electrolyte. Due to its developed pore structure and large specific surface area, the powdered activated carbon of the present disclosure functions as a carrier that supports sulfur within the pores in the positive electrode material, and is also suitable as a conductive aid that increases the conductivity of the positive electrode mixture. [Example]

[0052] The powdered activated carbon according to the present disclosure will be explained in more detail below with reference to examples, but the present disclosure is not limited to the following examples, and it is of course possible to make appropriate modifications within the scope of the above and below-described aims, and all such modifications are within the technical scope of the present disclosure. In the following, unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass."

[0053] (Evaluation method) Measurement of impurity content Approximately 1 g of sample was placed in a conical beaker with 10 ml of 97% sulfuric acid and 5 ml of 60% nitric acid and heated on a hot plate for decomposition. Nitric acid was added in small amounts as the decomposition progressed, as necessary, and heating was continued until the sample was completely decomposed. The resulting decomposition solution was filtered using Class 5 C filter paper (a qualitative filter paper with a medium filtration rate according to JIS standards), and the filtrate was diluted with pure water in a measuring flask to a total volume of 100 ml to prepare the test solution. Using this test solution, iron (Fe), copper (Cu), nickel (Ni), chromium (Cr), and alkali metals (K) were quantified using an ICP emission spectrometer (ICAP6000 manufactured by Thermo Fisher).

[0054] Particle size The average particle size was evaluated based on particle size distribution data measured using a laser diffraction particle size distribution analyzer (e.g., SALD-2200 manufactured by Shimadzu Corporation). Based on the volume-based cumulative frequency curve obtained from the measurement results, the particle sizes corresponding to cumulative frequencies of 10%, 50%, and 90%, respectively, i.e., D10, D50, and D90, were determined.

[0055] Pore ​​characteristics specific surface area After drying the sample (0.2 g) in a vacuum at 250°C, the amount of nitrogen gas adsorbed was measured in a liquid nitrogen atmosphere (-196°C) using a specific surface area / pore size distribution analyzer (Shimadzu Micromeritics ASAP-2420) to obtain a nitrogen adsorption isotherm. The specific surface area (m 2 / g) was calculated.

[0056] Total Pore Volume The total pore volume (mL / g) was determined as the adsorption amount at a relative pressure P / P0 = 0.93 based on data obtained from the nitrogen adsorption isotherm.

[0057] Average pore size Assuming that the pores of the activated carbon are cylindrical, the calculation was carried out using the following formula. Average pore diameter (nm) = 4 x total pore volume / specific surface area x 1000

[0058] Observation of residue on sieve: The "sieve residues" collected from residues 1 to 3 in Figure 1 were transferred to a stainless steel base (diameter 9.8 mm) with carbon tape (manufactured by Nissin EM Co., Ltd.) attached as a conductive fixing material, and used as observation samples. The obtained samples were observed using a scanning electron microscope (SEM: Hitachi High-Tech SU3800) to evaluate the morphology and size (long and short sides) of the residues. Furthermore, the elemental composition of the residue was analyzed using an energy dispersive X-ray analyzer (EDS: Oxford Instruments Explore 30) in the same field of view. The aspect ratio of the particle was calculated as the value obtained by dividing the length of the short side by the length of the long side (short side / long side), and the particle area was calculated as the product of the long side and the short side on the observation plane (long side x short side).

[0059] FIG. 1 is a flow chart showing the number of wet sieving treatments performed in the comparative example and each example, and the flow of the resulting over-sieve residue and under-sieve component.

[0060] No sieving Carbonized paper phenolic resin laminate was used as raw material, and potassium hydroxide (KOH) was added as an activator. After that, the carbonized paper was heated at 800℃ under nitrogen atmosphere to obtain alkali-activated carbon. The obtained activated carbon was washed with warm water and an inorganic acid, and then heat-treated and pulverized in this order. In the pulverization treatment, after primary pulverization using a counter jet mill, the average particle size (D50) was adjusted to 2.5 to 3.5 μm using a ball mill, to obtain powdered activated carbon 1 (Comparative Example 1).

[0061] First screening process Powdered activated carbon 1 was used as a sample, and 75 g of this sample and 300 g of pure water were placed in a 500 mL polypropylene bottle. Two bottles containing the sample were prepared under the same conditions. Each bottle was set on a rotary stirring stand (ANZ-10D, ANZ-10S manufactured by Nitto Kagaku Co., Ltd.) and stirred at a rotation speed of 300 rpm for 60 minutes. Thereafter, ultrasonic waves were applied for about 30 seconds using an ultrasonic cleaner (AU-508CB manufactured by Aiwa Corporation) to prepare an activated carbon slurry. The concentration of this slurry was set to 20 mass % based on the following calculation formula. Slurry concentration (mass%) = activated carbon amount / (activated carbon amount + pure water amount) × 100 The obtained slurry was suction filtered using a sieve with a mesh size of 32 μm (JIS Z 8801-1:2019 compliant) according to the wet sieving flow shown in FIG. The sieve used had a mesh size of 32 μm, a wire diameter of 26 μm, a twill weave (T), an inner diameter of 200 mm, a height of 60 mm, and a depth of 45 mm, manufactured by Iida Seisakusho Co., Ltd. A portion of the filtrate that passed through the sieve was collected and dried at 115°C until it reached a constant weight, thereby obtaining powdered activated carbon 2 (Example 1). The residue captured on the sieve was washed with ethanol, dried at 80°C until it reached a constant weight, and collected as the residue on the sieve (residue 1) and stored. Residue 1 indicates the coarse metal particles contained in powdered activated carbon 1 of Comparative Example 1 (unsieved), and the coarse metal particles contained in powdered activated carbon 2 of Example 1 are shown as residue 2 below.

[0062] Second screening process An activated carbon slurry was prepared using powdered activated carbon 2 in the same manner as in the first sieving treatment, and the obtained slurry was subjected to wet sieving treatment under the same conditions as in the first sieving treatment, to obtain filtrate 2. In addition, the residue on the sieve was washed and dried in the same manner as in the first sieving process, and was collected and stored as residue on the sieve (residue 2).

[0063] Third screening process Filtrate 2 was subjected to wet sieving in the same manner as in the first sieving, to obtain filtrate 3. The residue on the sieve was also washed and dried in the same manner as in the first sieving process, and was collected and stored as residue on the sieve (residue 3).

[0064] Fourth screening process Filtrate 3 was subjected to wet sieving in the same manner as in the first sieving, and the resulting filtrate 4 was dried to obtain powdered activated carbon 3 (Example 2). There was no residue (residue 4) on the sieve mesh.

[0065] 5th screening process An activated carbon slurry was prepared using powdered activated carbon 3 in the same manner as in the first sieving treatment, and the obtained slurry was subjected to wet sieving treatment under the same conditions as in the first sieving treatment, to obtain filtrate 5. There was no residue (residue 5) on the sieve mesh.

[0066] [Table 1]

[0067] [Table 2]

[0068] (1) Amount of impurities: Comparative Example 1 is powdered activated carbon 1 before sieving, Example 1 is powdered activated carbon 2 which has been sieved once, and Example 2 is powdered activated carbon 3 which has been sieved four times. The amount of impurities contained in each powdered activated carbon is shown in Table 1. There was no significant change in the concentration of each impurity element before and after wet sieving, or depending on the number of sieves used, indicating that sieving has little effect on the original chemical composition of activated carbon.

[0069] (2) Observation results of residue on sieve: The residue captured on the sieve was analyzed using SEM-EDX, and a large number of coarse metal particles with short sides of 45 μm or more, as shown in Figure 4(A)(B), were confirmed, consisting primarily of Fe and Cr.

[0070] (3) Results of screening process Figure 3 shows the size distribution (short side vs. long side) of metal particles in the residue captured in each sieving process. The metal particles captured in the first sieving process (residue 1) contained a large number of coarse metal particles with short sides of 45 μm or more (including particles with a maximum short side of approximately 90 μm and a long side of approximately 140 μm). The metal particles captured in the second sieving process (residue 2) tended to be smaller overall in size compared to the first sieving process, and the number of coarse metal particles with short sides of 45 μm or more was also clearly reduced (maximum short sides were approximately 50 μm). In the metal particles captured in the third sieving process (residue 3), the size of the metal particles became even smaller, and coarse metal particles with short sides of 45 μm or more were hardly observed. In addition, no metal particles were captured in the fourth and fifth sieving processes.

[0071] (4) Considerations and Effects These results indicate that repeated wet sieving can gradually remove coarse metal particles that could not be removed by a single sieving process, thereby more reliably reducing the coarse metal particle content in the product. In particular, it was suggested that repeated sieving three times could almost completely remove metal particles with a short side of 45 μm or more. These results demonstrate that the sieving process of the present invention is an effective means for effectively controlling coarse metal particles by adjusting the number of sieving processes according to the target cleanliness level.

Claims

1. Powdered activated carbon, The powdered activated carbon is sieved using a sieve with a mesh size of 32 μm conforming to JIS Z 8801 to obtain metal particles on the sieve, the number of metal particles having a short side length of 45 μm or more being 30% or less of the total number of metal particles remaining on the sieve.

2. The powdered activated carbon has an average particle size of 20 μm or less, 2. The powdered activated carbon according to claim 1, wherein the number of metal particles having a short side length of 30 μm or more among the metal particles on the sieve is 70% or less of the total number of metal particles remaining on the sieve.

3. The powdered activated carbon has a specific surface area of ​​1700 to 3500 m ２ 2. The powdered activated carbon according to claim 1, wherein the saturation energy of the activated carbon is 1 / g.

4. Among the metal particles contained in the activated carbon, the product of the long side and short side on the observation plane is 2025 μm ２ 2. The powdered activated carbon according to claim 1, wherein the number of metal particles having a particle size of 100 or more is 50% or less of the total number of metal particles remaining on the sieve.

5. 2. The powdered activated carbon according to claim 1, wherein the powdered activated carbon is alkali-activated carbon.

6. An electrode material using the powdered activated carbon according to any one of claims 1 to 5.

7. An electricity storage device using the electrode material according to claim 6.

8. A method for producing powdered activated carbon, comprising sieving powdered activated carbon through a sieve having a mesh size of 32 μm in accordance with JIS Z 8801, and obtaining metal particles on the sieve, wherein the number of metal particles having a short side length of 45 μm or more is 30% or less of the number of all metal particles remaining on the sieve, The method for producing the powdered activated carbon includes the steps of: a step of carbonizing the carbonaceous material; a step of activating the carbonized product obtained by the carbonization treatment; a step of performing a pulverization treatment at least once before the carbonization treatment step, between the carbonization treatment step and the activation treatment step, or after the activation treatment step; a step of sieving the powdered activated carbon either after the activation treatment step or after the pulverization treatment step following the activation treatment step; and The sieving process is a wet sieving process, A powdered activated carbon slurry having a concentration of the activated carbon of 0.5 to 50 mass % A method for producing powdered activated carbon, comprising wet sieving the slurry once or multiple times using a sieve mesh with a mesh size selected to prevent coarse metal particles with a short side of 45 μm or more from passing through.

9. The method for producing powdered activated carbon according to claim 8, wherein the activation treatment step is an alkali activation treatment.

10. The screening process includes:

9. The method for producing powdered activated carbon according to claim 8, wherein the slurry is wet-sieved using a sieve having an opening of 38 μm or less.

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