JP2021141168A - Activated carbon for power storage device electrode - Google Patents

Abstract

To provide an activated carbon that has high capacitance at normal temperature and low temperature, can maintain high capacitance even in a low temperature environment, and is suitable as an electrode material for a power storage device that generates a small amount of gas.SOLUTION: In an activated carbon for a power storage device electrode, the mesopore volume is 0.3 mL/g or more, and the molar ratio of acidic functional groups to basic functional groups is more than 2.0.SELECTED DRAWING: Figure 1

Description

The present invention relates to activated carbon for a power storage device electrode.

As a power storage device, a lithium ion capacitor and an electrochemical capacitor such as an electric double layer capacitor; a secondary battery such as a lithium ion battery, a fuel cell, and an air battery are known. The fields of use of power storage devices are diverse. For example, electrochemical capacitors are expanding their applications as the storage capacity increases, and are used as main power sources, auxiliary power sources, or regenerative power storage devices for various products. Further, for example, an electrochemical capacitor is attracting attention as an energy device such as being used as a natural energy buffer for photovoltaic power generation.

As the applications of power storage devices expand, various performance improvements are required. As an example, it has been pointed out that an electrochemical capacitor has a high temperature dependence, and its capacitance decreases when it is used in a low temperature environment (about -30 ° C to -40 ° C). As a measure to improve the capacitance in a low temperature environment, improvement of activated carbon, which is an electrode material of a power storage device, is being studied. For example, Patent Document 1 discloses a technique capable of maintaining a sufficient capacity even when used in a low temperature environment.

Japanese Unexamined Patent Publication No. 2018-011015

An object of the present invention is to provide activated carbon suitable as an electrode material for realizing a power storage device having a high capacitance at normal temperature and low temperature and capable of maintaining a high capacitance even in a low temperature environment. ..

[1] The present invention is an activated carbon for a power storage device electrode having a mesopore volume of 0.3 mL / g or more and a molar ratio of acidic functional groups to basic functional groups of more than 2.0.

[2] The present invention also includes an electrode for a power storage device using the activated carbon of the above [1] as a preferred embodiment.

[3] The present invention also includes an electric double layer capacitor or a lithium ion capacitor provided with the electrode of the above [2] as a preferred embodiment.
[4] The present invention also includes a lithium-sulfur secondary battery provided with the electrode of the above [2] as a preferred implementation.

According to the present invention, it is possible to provide activated carbon which has a high capacitance at normal temperature and low temperature and is suitable as an electrode material of a power storage device capable of maintaining a high capacitance even in a low temperature environment. Further, the power storage device provided with the electrode for the power storage device using the activated carbon of the present invention exerts the above effect.

FIG. 1 is a graph showing the relationship between the heat treatment temperature after activation and the amount of acidic functional groups. FIG. 2 is a graph showing the relationship between the heat treatment temperature after activation and the amount of basic functional groups. FIG. 3 is a graph showing the relationship between the heat treatment temperature after activation and the total amount of acidic / basic functional groups. FIG. 4 is a schematic view of an electric double layer capacitor.

The activated carbon for a power storage device electrode of the present invention has a mesopore volume of 0.3 mL / g or more and a molar ratio of acidic functional groups to basic functional groups of more than 2.0. By appropriately controlling the mesopore volume of activated carbon and the molar ratio of acidic functional groups to basic functional groups, the storage device has a high capacitance at room temperature (25 ° C) (hereinafter referred to as room temperature capacitance). In addition to having a high capacitance (hereinafter referred to as low-temperature capacitance), it can maintain a high capacitance (hereinafter referred to as low-temperature capacitance) even in a low-temperature environment (-30 ° C). Each value of the present invention is a value based on the measurement method described in the examples.

Mesopore volume The activated carbon of the present invention has a mesopore volume of 0.3 mL / g or more. When the molar ratio of the acidic functional group to the basic functional group is satisfied and the mesopore volume is increased, it is possible to achieve both an improvement in the moving speed of the electrolyte ion in the pore and an increase in the capacitance in a well-balanced manner. Further, as the mesopore volume is increased, the volume development at low temperature does not become rate-determining, and the volume retention rate can be increased. As a result, the activated carbon of the present invention is effective in achieving a high normal temperature capacitance and a high low temperature capacitance. Further, by appropriately controlling the mesopore volume and the molar ratio of the acidic functional groups, it also contributes to the suppression of the decrease of the following maintenance rate (1) and / or the following maintenance rate (2) (hereinafter, (1), ( 2) are sometimes collectively referred to as "low temperature capacitance retention rate").
(1) Retention rate of capacitance per volume in a low temperature environment (= low temperature capacitance (F / cm ³ ) / normal temperature capacitance (F / cm ³ ) x 100)
(2) Capacitance retention rate per mass in a low temperature environment (= low temperature capacitance (F / g) / normal temperature capacitance (F / g) x 100)

The mesopore volume of the activated carbon of the present invention is preferably 0.5 mL / g or more, more preferably more than 0.6 mL / g, still more preferably 0.65 mL / g or more, still more preferably 0.7 mL / g or more. It is preferably 1.5 mL / g or less, more preferably 1.25 mL / g or less, and further preferably 1.0 mL / g or less.

Molar ratio of acidic functional group to basic functional group The activated carbon of the present invention has a molar ratio of acidic functional group to basic functional group of more than 2.0. Even if the mesopore volume is satisfied, the low temperature capacity decreases when the molar ratio of the acidic functional group to the basic functional group decreases. On the other hand, if the molar ratio of the amount of acidic functional groups to the basic functional groups is increased and the mesopore volume is appropriately controlled, the activated carbon of the present invention can increase the room temperature capacitance of the power storage device and suppress the decrease in low temperature capacitance. Contribute to.

The molar ratio of acidic functional groups (amount of acidic functional groups / amount of basic functional groups) to the basic functional groups of the activated carbon of the present invention is more than 2.0, preferably 2.1 or more, more preferably 2.5 or more. It is more preferably 2.8 or more, preferably 5.0 or less, more preferably 4.5 or less, still more preferably 4.0 or less.

Acidic Functional Group, Basic Functional Group It is also a preferred embodiment of the present invention to satisfy at least one of the following acidic functional group amount and basic functional group amount, more preferably both. When the amount of acidic functional group and / or the amount of basic functional group is within an appropriate range, the effect is further exhibited by increasing the room temperature capacitance and achieving a high low temperature capacitance.

The amount of acidic functional groups of the activated carbon of the present invention is preferably 0.1 meq / g or more, more preferably 0.3 meq / g or more, still more preferably 0.5 meq / g or more, and preferably 2.0 meq / g. Below, it is more preferably 1.5 meq / g or less, still more preferably 1.0 meq / g or less, still more preferably 0.8 meq / g or less.

The amount of basic functional groups of the activated carbon of the present invention is preferably 0.1 meq / g or more, more preferably 0.15 meq / g or more, still more preferably 0.2 meq / g or more, and preferably 1.0 meq / g or more. It is g or less, more preferably 0.8 meq / g or less, still more preferably 0.5 meq / g or less, still more preferably 0.3 meq / g or less.

Total Functional Group Amount It is also a preferred embodiment of the present invention to satisfy the following total functional group amount. When the total amount of the acidic functional group and the basic functional group is within an appropriate range, it is more effective in suppressing the decrease in the low temperature capacitance.

The total amount of the acidic functional group and the basic functional group of the activated carbon of the present invention is preferably 0.2 meq / g or more, more preferably 0.5 meq / g or more, still more preferably 0.6 meq / g or more, still more preferably. Is 0.7 meq / g or more, preferably 2.0 meq / g or less, more preferably 1.8 meq / g or less, still more preferably 1.5 meq / g or less.

Specific Surface Area Satisfying the following specific surface area is also a preferred embodiment of the present invention. The larger the specific surface area of the activated carbon, the more the capacitance contributes to the increase and the ^{capacitance per volume (F / cm 3) of the power storage device.}

The specific surface area of the activated carbon of the present invention is preferably 1500 m ² / g or more, more preferably 2000 m ² / g or more, more preferably 2500 m ² / g or more, more further preferably 2800 m ² / g or more, preferably 4000m ^{It is 2} / g or less, more preferably 3800 m ² / g or less, ^{still more preferably 3500 m 2} / g or less.

Total Pore Volume Satisfying the following total pore volume is also a preferred embodiment of the present invention. The larger the total pore volume of activated carbon, the more it contributes to the increase in room temperature capacitance. On the other hand, the upper limit of the total pore volume may be limited from the viewpoint of ensuring the capacitance per volume.

The total pore volume of the activated carbon of the present invention is preferably 0.5 mL / g or more, more preferably 1.0 mL / g or more, still more preferably 1.5 mL / g or more, preferably 5.0 mL / g or more. Below, it is more preferably 4.0 mL / g or less, still more preferably 3.0 mL / g or less.

Micropore Volume Satisfying the following micropore volume is also a preferred embodiment of the present invention. The larger the micropore volume of activated carbon, the more it contributes to the increase of capacitance. On the other hand, the upper limit of the micropore volume may be limited in consideration of the balance between the micropore volume and the mesopore volume.

The micropore volume of the activated carbon is preferably 0.1 mL / g or more, more preferably 0.5 mL / g or more, still more preferably 1.0 mL / g or more, preferably 4.0 mL / g or less, more preferably. It is 3.0 mL / g or less, more preferably 2.0 mL / g or less.

Ratio of mesopore volume It is also a preferred embodiment of the present invention to satisfy the following mesopore volume ratio. The larger the ratio of the mesopore volume, the better the movement of electrolyte ions in the pores, the less rate-determining the capacity development at low temperature, the suppression of the decrease in low temperature capacitance, and the maintenance of high low temperature capacity. .. On the other hand, from the viewpoint of ensuring a sufficient normal temperature capacitance, the upper limit of the ratio of the mesopore volume may be limited.

The ratio of the mesopore volume of the activated carbon of the present invention (the mesopore volume to the total of the micropore volume and the mesopore volume) is preferably 30% or more, more preferably 35% or more, still more preferably 40% or more. It is preferably 70% or less, more preferably 60% or less, still more preferably 55% or less.

Average Pore Diameter Satisfying the following average pore diameter is also a preferred embodiment of the present invention. By increasing the average pore diameter of the activated carbon, the movement of electrolyte ions in the pores becomes good, the decrease in low-temperature capacitance can be suppressed, and high low-temperature capacitance can be maintained. On the other hand, the upper limit of the average pore diameter may be limited in consideration of ensuring the bulk density.

The average pore diameter of the activated carbon of the present invention is preferably 1.50 nm or more, more preferably 1.80 nm or more, still more preferably 2.00 nm or more, preferably 2.60 nm or less, more preferably 2.50 nm or less. More preferably, it is 2.40 nm or less.

Average particle size It is also a preferred embodiment of the present invention to satisfy the following average particle size. Reducing the average particle size of activated carbon contributes to improving the current density. On the other hand, the lower limit of the average particle size may be limited in consideration of handleability.

_{The average particle size (D 50} ) of the activated carbon of the present invention is preferably 10 μm or less, more preferably 6.0 μm or less, still more preferably 5.5 μm or less, preferably 0.5 μm or more, and more preferably 1. It is 0 μm or more, more preferably 2.0 μm or more.

Hereinafter, a suitable method for producing the activated carbon of the present invention will be described. The method for producing activated carbon of the present invention is not limited to the following production conditions, and it is sufficient that activated carbon can be produced by appropriately modifying it to satisfy the above-mentioned predetermined requirements.

Activation raw material The activated carbon of the present invention preferably uses a carbonaceous substance, more preferably a carbide of the carbonaceous substance, as an activation raw material. Non-graphitizable carbon such as wood, shavings, charcoal, coconut husk, cellulose fiber, synthetic resin (for example, phenol resin) as an activating raw material; easy graphite such as mesophase pitch, petroleum pitch coke, petroleum coke, needle coke, polyacrylonitrile, etc. Chemical carbon; and mixtures thereof and the like are exemplified. These activation raw materials may be used alone or in combination of two or more. The carbonization treatment of the carbonaceous substance may be carried out by heat-treating the carbonaceous substance in an inert gas such as nitrogen, argon or helium, for example, holding the carbonaceous substance at 400 ° C. to 1000 ° C. for 1 hour to 3 hours.

Primary pulverization treatment It is also a preferable production method of the present invention to perform a primary pulverization treatment of the activated raw material. When the activation raw material is first pulverized and then subjected to the alkali activation treatment, activated carbon having desired physical properties can be easily obtained. The activated raw material may be crushed using a known crusher. The lower limit of the activated raw material after crushing is not particularly limited.

The size of the activating raw material after the primary pulverization is preferably 15 mm or less, more preferably 10 mm or less, preferably 0.20 mm or more, and more preferably 0.30 mm or more.

Alkali activation treatment step The alkali activation treatment of the present invention is a step of mixing an activator containing an alkali metal compound and an activation raw material and heating them in an inert gas to obtain activated charcoal. The alkali activator is preferably potassium hydroxide.

Increasing the mass ratio of the activating raw material to the alkaline activating agent (alkali activating agent / activating raw material) contributes to the promotion of activation of activated carbon. On the other hand, the upper limit of the mass ratio may be limited from the viewpoint of ensuring the strength of the activated carbon.

The mass ratio of the activating raw material to the alkaline activating agent in the production method of the present invention is preferably 0.5 or more, more preferably 1.5 or more, still more preferably 2.0 or more, and preferably 4.5 or less. , More preferably 4.0 or less, still more preferably 3.5 or less.

The activation treatment is preferably heated and retained in two steps as a condition for achieving the predetermined mesopore volume and the molar ratio of the acidic functional group to the predetermined basic functional group. For example, a mixture of an activator and an activator is inserted into a heating furnace, the temperature inside the furnace is raised to about 350 to 450 ° C., and then held at that temperature for 10 minutes to 1 hour (1 step), followed by 700 ° C. The temperature may be raised to ~ 900 ° C. and maintained at that temperature for 0.1 to 3.5 hours (two steps). The heating and holding time of the second stage is made longer than the heating and holding time of the first stage. The atmosphere during the activation treatment is preferably an atmosphere of an inert gas such as argon, helium, or nitrogen.

Further, in the present invention, after the activation treatment, a washing treatment, a drying treatment, a heat treatment, a crushing treatment and the like may be performed as necessary. The order of these treatments is not limited, and for example, after the washing treatment and the drying treatment, the pulverization treatment may be performed and then the heat treatment may be performed, or after the cleaning treatment, the heat treatment may be performed and then the pulverization treatment may be performed.

Cleaning Step The method for producing activated carbon of the present invention preferably carries out the following cleaning treatment. The cleaning treatment is a step of reducing or removing the alkali metal compound remaining in the activated carbon after the activation treatment. The washing of the activated carbon is to perform at least one of water washing and acid washing, more preferably both. The order and number of washings when both water washing and acid washing are performed are not limited. After acid washing, it is preferable to wash with water multiple times.

Water washing The water washing method is not particularly limited. For example, it is preferable to put activated carbon into water, stir and disperse it as necessary, and then filter it. Stirring and dispersion can be performed by mechanical stirring, gas blowing, ultrasonic irradiation, and heating and boiling. The water temperature at the time of washing with water is not particularly limited, and may be, for example, 30 ° C. or higher. Further, the stirring and dispersion times are not particularly limited, and for example, it is preferably performed for 0.5 hours or more.

Acid cleaning Acid cleaning is cleaning performed using a cleaning solution containing an inorganic acid, an organic acid, or the like. Alkali hydroxide metal used as an alkali activator can be efficiently removed from activated carbon by acid cleaning. The solvent of the cleaning liquid is not limited, and for example, water is suitable. Examples of the inorganic acid that can be used for acid cleaning include hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, carbonic acid and the like. These inorganic acids may be used alone or in combination of two or more. The concentration of the inorganic acid in the cleaning liquid is preferably 0.5 mol / L or more, more preferably 1.0 mol / L or more, preferably 3.5 mol / L or less, and more preferably 3.0 mol / L or less. .. The acid cleaning method is not limited, but it is preferable that, for example, activated carbon and an inorganic acid cleaning solution are mixed and stirred at a temperature of 50 ° C. to 100 ° C. for 30 minutes to 120 minutes.

Examples of organic acids that can be used for acid cleaning include formic acid, oxalic acid, malonic acid, succinic acid, acetic acid, and propionic acid. These organic acids may be used alone or in combination of two or more. The concentration of the organic acid in the cleaning liquid is preferably 1 vol% or more, more preferably 2 vol% or more, still more preferably 5 vol% or more, preferably 100 vol% or less, more preferably 80 vol% or less, still more preferably 60 vol% or less. Is. The acid cleaning method is not limited, but it is preferable to mix activated carbon and an organic acid cleaning solution and stir at a temperature of 20 ° C. to 80 ° C. for 1 minute to 120 minutes.

Drying Treatment The method for producing activated carbon of the present invention preferably carries out the following drying treatment. The drying treatment only needs to be able to dry the activated carbon, and the drying conditions are not limited. The drying treatment is preferably carried out, for example, at 50 ° C. to 180 ° C. for about 0.5 hours to 24 hours.

Crushing Treatment The method for producing activated carbon of the present invention preferably carries out the following crushing treatment. It is preferable to appropriately pulverize the activated carbon so that the average particle size becomes the above-mentioned predetermined size. The means for crushing activated carbon is not limited. Examples of the crushing means include a jet mill, a disc mill, a ball mill, and a bead mill.

Heat treatment In the method for producing activated carbon of the present invention, it is preferable to carry out the following heat treatment. The amount of functional groups on the surface of activated carbon can be adjusted by heat treatment. The heat treatment is performed in an atmosphere of an inert gas such as argon, nitrogen or helium. The heat treatment temperature is not limited, but the higher the heat treatment temperature, the lower the amount of acidic functional groups. Therefore, it is preferable to appropriately control the heat treatment temperature in the range of, for example, 400 ° C. or higher and 1200 ° C. or lower so that the molar ratio of the acidic functional group to the basic functional group becomes the above-mentioned predetermined value.

The activated carbon of the present invention is suitable as an activated carbon for a power storage device electrode. The activated carbon of the present invention can be used as an electrode material for a power storage device. Therefore, the present invention can provide an electrode for a power storage device using the activated carbon.

The electrode for the power storage device may contain the activated carbon of the present invention, and a known material can be used as the other electrode material. For example, the electrode for a power storage device may be composed of the activated carbon of the present invention, a binder, and preferably a conductivity-imparting agent. As the binder, a fluorine-based polymer compound such as polytetrafluoroethylene or polyvinylidene fluoride, carboxymethyl cellulose, styrene-butadiene rubber, petroleum pitch, phenol resin and the like can be used. Further, as the conductivity-imparting agent, acetylene black, ketjen black and the like can be used. For the electrode for the power storage device, for example, a paste was prepared by further adding a solvent to the electrode material obtained by kneading the activated charcoal, the conductivity-imparting agent, and the binder, and this paste was applied to a current collector plate such as aluminum foil. After that, the solvent is dried and removed.

The electrode for a power storage device using the activated charcoal of the present invention is particularly suitable for an electrochemical capacitor such as a lithium ion capacitor or an electric double layer capacitor; and various power storage devices such as a lithium sulfur secondary battery provided with the electrode. For example, an electric double layer capacitor generally has an electrode, an electrolytic solution, and a separator as its main components, and has a structure in which a separator is arranged between a pair of electrodes. Can be configured. Examples of the electrolytic solution include an electrolytic solution in which an amidine salt is dissolved in an organic solvent such as propylene carbonate, ethylene carbonate, and methyl ethyl carbonate; an electrolytic solution in which a quaternary ammonium salt of perchloric acid is dissolved; quaternary ammonium, lithium, and the like. Examples thereof include an electrolytic solution in which a boron tetrafluoride salt and a phosphorus hexafluoride salt of an alkali metal are dissolved; an electrolytic solution in which a quaternary phosphonium salt is dissolved. Examples of the separator include non-woven fabrics, cloths, and micropore films containing cellulose, glass fibers, or polyolefins such as polyethylene and polypropylene as main components.

Further, the lithium-sulfur secondary battery can adopt various known configurations. For example, it includes a positive electrode having a positive electrode active material containing sulfur, a negative electrode active material containing lithium, and a separator arranged between the positive electrode and the negative electrode to hold an electrolytic solution. The electrolytic solution of the lithium-sulfur secondary battery is also not particularly limited, but it is preferable to use a solution in which a lithium salt is dissolved in a solvent. As the lithium salt, for example, LiPF ₆ , LiClO ₄ , LiBF ₄ , Li (CF ₃ SO ₂ ) ₂ N, Li (C ₂ F ₅ SO ₂ ) ₂ N, LiNO _{3 and the} like may be used alone or in combination of two or more. The solvent of the electrolytic solution is not particularly limited, but for example, dimethoxyethane (DME), triglime, tetraglime, dioxolane (DOL), tetrahydrofuran, a mixture thereof; 1-methyl-3-propylimidazolium bis (trifluorosulfonyl) imide, An ionic liquid such as 1-ethyl-3-butylimidazolium tetrafluoroborate may be used alone or in combination of two or more. As the separator, a non-woven fabric usually used for a secondary battery or a permeable separator made of another porous material can be used.

By using the activated carbon of the present invention for the electrode, it is possible to achieve an increase in the room temperature capacitance of various power storage devices and a suppression of a decrease in the low temperature capacitance. Further, the power storage device of the present invention can be used as a power source for various mobile devices, a standby power source for home appliances, an optical communication UPS, a power source for electric vehicles, and the like. Examples of the power source include a main power source, an auxiliary power source, a regenerative power storage device, and a natural energy buffer.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples as well as the present invention, and appropriate modifications are made to the extent that it can be adapted to the gist of the above and the following. Of course, it is possible to carry out, and all of them are included in the technical scope of the present invention.

Example 1
Activation raw material Petroleum pitch coke crushed to 6 mm or less was used as the activation raw material.

Activation treatment The mixture was mixed so that the ratio (KOH / C) of this activation raw material to the 48% potassium hydroxide aqueous solution was 3.5. Next, the mixture was heated to 400 ° C. at a heating rate of 10 ° C./min under a nitrogen atmosphere, held at 400 ° C. for 30 minutes, then heated to 800 ° C. at a heating rate of 10 ° C./min, and at 800 ° C. It was held for 2 hours to obtain activated carbon.

Cleaning treatment 2 L of pure water (60 ° C.) was added to the obtained activated carbon, boiled for 1 hour, and then filtered. After filtration, washing and dehydration were repeated using pure water (60 ° C.) until the pH of the activated carbon became 9 or less.
Then, 1.7 L of pure water (60 ° C.) and 0.3 L of 35% hydrochloric acid were added to the activated carbon, boiled for 1 hour, and then filtered.
After adding 2 L of pure water (60 ° C.) to the activated carbon after filtration and boiling for 1 hour, filtration was performed. After filtration, 2 L of pure water (60 ° C.) was added and boiled for 1 hour, and then filtration was performed. Then, washing and dehydration were repeated using pure water (60 ° C.) until the pH became 6.5 or higher.

Crushing Treatment The obtained activated carbon was crushed with a counter jet mill (200 AFG manufactured by Hosokawa Micron Co., Ltd.) until the average particle size became 4 to 6 μm to obtain activated carbon A.

Example 2
Activated carbon A was placed in a muffle furnace (manufactured by Koyo Thermo Co., Ltd.), heated to a furnace temperature of 450 ° C. at a heating rate of 10 ° C./min under nitrogen flow (2 L / min), and maintained at 450 ° C. for 2 hours. Then, it was allowed to cool to room temperature to obtain activated carbon B.

Reference example 1
Activated carbon C was obtained in the same manner as in Example 2 except that the temperature inside the furnace was changed to 800 ° C.

Reference example 2
Activated carbon A is placed in an elevating furnace (manufactured by Toyo Advantech Co., Ltd.), and the temperature is raised to 1200 ° C. at a heating rate (4.2 ° C./min) under nitrogen flow (2 L / min) for 2 hours at 1200 ° C. The mixture was retained and then allowed to cool to room temperature to obtain activated carbon D.

The obtained activated carbon was evaluated according to the following criteria.

Pore characterization (specific surface area)
After vacuum drying 0.2 g of the sample at 250 ° C., nitrogen gas in a liquid nitrogen atmosphere (-196 ° C.) using a specific surface area / pore size distribution measuring device (ASAP-2420 manufactured by Shimadzu-Micromeritics). The adsorption amount of adsorption was measured to determine the nitrogen adsorption isotherm, and the specific surface area (m ² / g) was determined by the BET method.
(Total pore volume)
The total pore volume (mL / g) was calculated from the amount of nitrogen adsorbed at relative pressure (P / P _{0) = 0.93 from the nitrogen adsorption isotherm.}

(Average pore size)
The average pore diameter was calculated based on the following formula, assuming that the pores of the activated carbon were cylindrical.
Average pore diameter (nm) = 4 x total pore volume (mL / g) / specific surface area (m ² / g) x 1000

Micropore volume, mesopore volume evaluation
(Micro pore volume)
The micropore volume was determined by differentiating the pore volume from 2 nm to 30 nm analyzed by the BJH method from the total pore volume.
Micropore volume (mL / g) = Total pore volume-2 nm to 30 nm pore volume Micropore volume ratio (%) = Micropore volume / Total pore volume x 100

(Mesopore volume)
The mesopore volume was determined from the pore volume from 2 nm to 30 nm analyzed by the BJH method.
Mesopore volume (mL / g) = Pore volume from 2 nm to 30 nm Mesopore volume ratio (%) = Mesopore volume / Total pore volume x 100

Evaluation of Acid Functional Group Amount The acidic functional group amount was determined by the Boehm method.
In determining the amount of acidic functional groups, first, the activated carbon is dried by heating it at a temperature of 115 ° C. for 1 hour or more. Next, about 2.000 g of the dried activated carbon is weighed, this is mixed with 20 mL of a sodium hydroxide aqueous solution having a concentration of 0.1 mol / L, and the obtained mixed solution is stirred for 30 minutes. Then, the activated carbon is filtered off from this mixed solution to obtain a filtrate. Then, 5 mL of the filtrate is weighed, an aqueous solution containing methyl orange at a concentration of 1 g / L is added to the filtrate, and hydrochloric acid having a concentration of 0.05 mol / L is added dropwise until the color of the mixed solution changes. do. The amount of acidic functional group is determined from the amount of hydrochloric acid required for the color of the above mixed solution to change.
Amount of acidic functional group (mmol / g) = (ab) × c × f / M × 20/5
During the ceremony
a: Blank test titration (mL)
b: Sample titration (mL)
c: Hydrochloric acid standard solution concentration (mol / L)
f: Hydrochloric acid standard solution factor M: Sample weighing amount (g)
20/5: Sorting ratio

Evaluation of basic functional groups The amount of basic functional groups was determined by the Boehm method.
In determining the amount of basic functional groups, first, the activated carbon is dried by heating it at a temperature of 115 ° C. for 1 hour or more. Next, 2.000 g of the dried activated carbon is weighed, this is mixed with 20 mL of hydrochloric acid having a concentration of 0.05 mol / L, and the obtained mixed solution is stirred for 30 minutes. Then, the activated carbon is filtered off from this mixed solution to obtain a filtrate. Then, 5 mL of the filtrate is weighed, and an aqueous solution containing phenolphthalein at a concentration of 10 g / L is added thereto as an indicator, and sodium hydroxide having a concentration of 0.05 mol / L is added until the color of this mixed solution disappears. Drop the aqueous solution. The amount of the basic functional group is determined from the amount of the sodium hydroxide aqueous solution required until the color of the above mixed solution disappears.
Basic functional group amount (mmol / g) = (ab) × c × f / M × 20/5
During the ceremony
a: Blank test titration (mL)
b: Sample titration (mL)
c: Sodium hydroxide standard solution concentration (mol / L)
f: Sodium hydroxide standard solution factor M: Sample weighing amount (g)
20/5: Sorting ratio

Average particle size The average particle size was measured using a laser diffraction type particle size distribution measuring device (SALD-2000 manufactured by Shimadzu Corporation). A volume-based cumulative frequency curve was obtained from the sample particle size distribution measurement results. Then, the particle size at the cumulative frequencies of 10% (D ₁₀ ), 50% (D ₅₀ ), and 90% (D ₉₀ ) was determined, and D ₅₀ was taken as the average particle size.

Evaluation of Electric Double Layer Capacitors Manufacture of Electric Double Layer Capacitors Electric double layer capacitors were manufactured using the produced activated carbons A to D. Specifically, polytetrafluoroethylene (PTFE) powder and acetylene black are mixed with activated carbon so that activated carbon: PTFE: acetylene black = 8: 1: 1 (mass ratio) until it becomes a paste. Kneaded. Then, it was pulverized with a mini blender and sieved with a 500 μm stainless steel sieve to make the particle size uniform. Next, using a mold having a diameter of 2.54 cm, the amount of charge was adjusted so that the thickness after pressing was 0.5 mm, and press molding was performed at a pressure of 50.4 MPa to prepare an electrode for a capacitor.

The obtained capacitor electrode was dried under vacuum conditions at 200 ° C. for 1 hour, and then an electrolytic solution (1M tetraethylammonium tetrafluoroborate propylene carbonate solution) was used as the electrode in a glove box through which nitrogen gas was circulated. Vacuum impregnated. This electrode was used to assemble an electric double layer capacitor as shown in FIG. In the electric double layer capacitor shown in FIG. 4, a separator (manufactured by Celgard, "Cellguard (registered trademark) # 3501") 1 impregnated with the electrolytic solution is sandwiched between the capacitor electrodes 2, and the electrodes are surrounded by an O-ring 3. After that, it was further sandwiched between aluminum plates 4 as a current collecting plate.

The charging / discharging terminal of the capacitance charging / discharging device (“ETAC (registered trademark) Ver. 4.4” manufactured by Kusumoto Kasei Co., Ltd.) is connected to the current collecting plate of the electric double layer capacitor, and the voltage between the collecting plates is 2. A constant current charge of 40 mA was performed until the voltage reached 5 V, and then charging was performed at a constant voltage of 2.5 V for 30 minutes. After charging, the electric double layer capacitor was discharged with a constant current (discharge current 10 mA). At this time, the discharge times t1 and t2 required for the voltage between the current collector plates to reach V1 and V2 were measured, and the capacitance was determined using the following formula (1). The mass reference capacitance (F / g) is calculated by dividing the obtained capacitance by the mass of activated carbon in the electrode material layer of the capacitor electrode, and divided by the total volume of the electrode material layer of the capacitor electrode. The volume-based capacitance (F / cm ³ ) was calculated. Moreover, the internal resistance was calculated using the following formula (2). The capacitance and internal resistance were measured at temperatures of 25 ° C. and −30 ° C.

During the ceremony
I: 10 (mA)
t1: Discharge time (sec) required for the electric double layer capacitor voltage to reach V1
t2: Discharge time (sec) required for the electric double layer capacitor voltage to reach V2
V0: Discharge start voltage V1: 2.0 (V)
V2: 1.5 (V)

The following can be seen from Table 1, Table 2, and FIGS. 1 to 3. As shown in FIGS. 1 to 3, when the heat treatment temperature after the activation treatment is raised, the amount of acidic functional groups of the activated carbon decreases, but the amount of basic functional groups tends to increase. Further, as the heat treatment temperature increases, the total amount of the acidic functional group amount and the basic functional group amount tends to decrease. From this, it can be seen that appropriate control of the heat treatment temperature is important in order to appropriately control the surface functional groups of the activated carbon.

Activated carbons A and B, which satisfy all the requirements of the present invention, have large both normal temperature capacitance and low temperature capacitance, and activated carbon B in particular has a high retention rate of capacitance per volume. From this, it can be seen that the electric double layer capacitor can increase the normal temperature capacitance and the low temperature capacitance by using the activated carbon of the present invention.
On the other hand, the activated carbons C and D, which did not satisfy the molar ratio of the acidic functional group to the basic functional group, had a small normal temperature capacitance and a small low temperature capacitance.

From the above results, the activated carbons A to D were excellent in all of the capacitance retention rate per mass, the capacitance retention rate per volume (hereinafter referred to as the capacitance retention rate), and the amount of gas generated. Activated carbons C and D, which do not satisfy all the requirements of the invention, had small normal temperature capacitance and low temperature capacitance.

1: Separator 2: Capacitor electrode 3: O-ring 4: Aluminum plate 5: Polytetrafluoroethylene plate 6: Stainless steel plate

Claims (4)

Activated carbon for power storage device electrodes having a mesopore volume of 0.3 mL / g or more and a molar ratio of acidic functional groups to basic functional groups of more than 2.0. The electrode for a power storage device using the activated carbon according to claim 1. An electric double layer capacitor or a lithium ion capacitor having the electrode according to claim 2. A lithium-sulfur secondary battery comprising the electrode according to claim 2.

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