JP6306836B2 - Carbon materials for hybrid capacitors - Google Patents

Description

  The present invention relates to a carbon material. More specifically, the present invention relates to a carbon material used for an electrode of a hybrid capacitor (for example, a lithium ion capacitor).

Hybrid capacitors (asymmetric capacitors) such as lithium-ion capacitors have higher energy density than electric double layer capacitors and are superior in output density and durability to secondary batteries. It is used suitably for the use of.
In such a hybrid capacitor, further energy density and output density are being studied as part of higher performance. Patent documents 1 to 4 can be cited as conventional techniques related to this. For example, Patent Document 1 describes that a lithium ion capacitor excellent in energy density and output density can be realized by using a carbon material having many lactone-like carboxyl functional groups for the positive electrode.

JP 2007-180437 A JP 2008-060479 A JP 2010-054341 A JP 2013-021234 A

  However, according to the study by the present inventors, when the conventional carbon material as described above is used, the durability and reliability of the battery are lowered as a contradiction to the increase in energy density and output density. was there. Specifically, as the charge / discharge cycle is repeated, the nonaqueous electrolytic solution may be decomposed, or the lactone-like carboxyl functional group may be detached from the surface of the carbon material to generate gas in the battery. Therefore, it is required to improve battery characteristics (for example, energy density) without significantly reducing (preferably maintaining) durability (for example, cycle stability) and reliability.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a carbon material for a hybrid capacitor having both high energy density and good durability. Another related object is to provide a hybrid capacitor (eg, a lithium ion capacitor having such a material as a positive electrode) provided with such a material as a positive electrode or a negative electrode.

The present inventors give a surface functional group having higher stability (in other words, low reactivity with a non-aqueous electrolyte and less detachment from the surface of the carbon material) to the surface of the carbon material. Therefore, we thought that the above problem could be solved. And earnest examination was repeated and the present invention was completed.
According to the present invention, a carbon material used for an electrode of a hybrid capacitor is provided. Such a carbon material satisfies both of the following conditions (1) and (2).
(1) The volatile content when heat-treated at 850 ° C. in an inert atmosphere is 3% by mass or more and 7% by mass or less.
(2) The amount of acidic functional groups is 0.1 mmol / g or more. Here, at least a part of the acidic functional group contains a nitrogen atom.

By using a carbon material having an acidic functional group amount of 0.1 mmol / g or more and a volatile content at 850 ° C. in an inert atmosphere of 3% by mass or more, the pseudo capacity derived from the acidic functional group is increased. Or the wettability with respect to a non-aqueous electrolyte improves, and an energy density and an output density can be improved compared with the past.
Further, according to the study by the present inventors, an amide group (for example, —C (═O) —NH ₂ ), an imide group (—C (═O) —NH—C (═O) —), —NHC ( An acidic functional group containing a nitrogen atom such as ═O) O— (hereinafter sometimes simply referred to as “nitrogen-containing group”) is a hydroxyl group (—OH), a carboxyl group (—C (═O) OH) or the like. Compared with the general acidic functional group, it can be firmly bonded to the surface of the carbon material. Therefore, by using a carbon material having an acidic functional group containing a nitrogen atom and having a volatile content at 850 ° C. of 7% by mass or less under an inert atmosphere, the functional group reacts with the nonaqueous electrolytic solution. Alternatively, desorption from the surface of the carbon material can be suppressed. Therefore, it is possible to achieve the same durability as when a carbon material having a small amount of acidic functional groups is used.
As described above, according to the invention disclosed herein, it is possible to realize a hybrid capacitor having both high energy density and good durability. Although there are a wide variety of carbon materials with different raw materials (carbon precursors) and properties, the technology disclosed here can be applied to all carbon materials, as will be apparent from the examples described later. It is possible.

In this specification, “volatile matter when heat-treated at 850 ° C.” means that the sample is heat-treated at 850 ° C. in a nitrogen atmosphere, and the mass before and after firing is expressed by the following formula: volatile matter (%) = [(before firing Mass) − (mass after firing)] / (mass before firing) × 100. Further, in this specification, “amount of acidic functional group” refers to the amount of surface functional groups measured using a sodium ethoxide (CH ₃ ONa) aqueous solution as a basic reagent according to the Boehm method (neutralization titration method). Whether or not the acidic functional group contains a nitrogen atom can be confirmed by, for example, general CHN elemental analysis or X-ray photoelectron spectroscopy (XPS).

In a preferred embodiment, the amount of the acidic functional group is 0.7 mmol / g or less. According to the study by the present inventors, the pseudo-capacitance (capacitance) derived from the functional group increases as the acidic functional group amount (typically nitrogen-containing group amount) on the surface of the carbon material increases, and / or The wettability with respect to the non-aqueous electrolyte improves. However, there is practically a limit, and the above range is preferable from the viewpoint of workability and cost.
In another preferred embodiment, the carbon material disclosed herein has a BET specific surface area of 100 m ² / g or less. Alternatively, the BET specific surface area may be 1000 m ² / g or more. In this specification, the “BET specific surface area” means a gas adsorption amount measured by a gas adsorption method (fixed capacity adsorption method) using nitrogen (N ₂ ) gas as an adsorbate, and a BET method (for example, BET method). This is the value analyzed by the multipoint method.

  The carbon material having the properties as described above can be produced by, for example, mixing and reacting a carbon source material and a nitrogen-containing group-imparting agent (for example, an aqueous nitric acid solution), followed by heat treatment. By reacting the carbon source material with the nitrogen-containing group imparting agent, an acidic functional group containing a nitrogen atom (nitrogen-containing group) can be suitably imparted to the surface of the carbon source material. Thereafter, the imparted functional group can be firmly fixed (stabilized) on the surface of the carbon source material by heat treatment. Thereby, the carbon material provided with the above characteristics can be produced suitably. In the present specification, the “carbon source material” refers to all carbon-based (carbonaceous) materials before the application of the nitrogen-containing group and the heat treatment, such as a carbon precursor (raw material), a carbide, an activated material, and the like. It is a term encompassing.

  Moreover, according to this invention, a hybrid capacitor (for example, lithium ion capacitor) provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte is provided. Such a hybrid capacitor is characterized in that the positive electrode or the negative electrode (typically the positive electrode) is provided with the carbon material as described above. As a result, it is possible to realize a hybrid capacitor that can achieve both energy density (for example, volume energy density) and durability (for example, cycle characteristics) at a high level.

It is a flowchart of the manufacturing method which concerns on one Embodiment. It is the graph which plotted the electrostatic capacitance (relative value) which concerns on one Embodiment with respect to the amount of acidic functional groups (mmol / g). It is the graph which plotted the capacity | capacitance maintenance factor (%) of the cycle characteristic test which concerns on one Embodiment with respect to 850 degreeC volatile matter (%). It is the graph which plotted the electrostatic capacitance (relative value) which concerns on other one Embodiment with respect to the amount of acidic functional groups (mmol / g). It is the graph which plotted the capacity | capacitance maintenance factor (%) of the cycle characteristic test which concerns on other one Embodiment with respect to 850 degreeC volatile matter (%).

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for implementation can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

  The carbon material disclosed here is a carbon material for forming an electrode of a hybrid capacitor (for example, a positive electrode of a lithium ion capacitor). Such a carbon material has a volatile content of 3% by mass or more (for example, 3.1% by mass or more, preferably 3.4% by mass or more) when heat-treated at 850 ° C. in an inert atmosphere, and 7% by mass or less. (For example, 6.5% by mass or less, preferably 6.2% by mass or less). Thereby, both energy density and durability can be achieved at a high level. In addition, as shown also in the Example mentioned later, the said volatile matter of a general carbon material (typically the carbon material which is not performing provision of a nitrogen-containing group and heat processing as disclosed here) Usually, it is about 1-2% by mass.

  The carbon material disclosed herein has an acidic functional group amount of 0.1 mmol / g or more (for example, 0.12 mmol / g or more, preferably 0.15 mmol / g or more), and the acidic functional group. At least a part of these contains a nitrogen atom. Thereby, it is possible to realize a higher energy density and output density than conventional ones. The upper limit of the amount of the acidic functional group is not particularly limited, but is 0.7 mmol / g or less (for example, 0.6 mmol / g or less, preferably 0.56 mmol / g or less) from the viewpoint of cost effectiveness and work efficiency. Is preferred.

The BET specific surface area of the carbon material disclosed herein is not particularly limited, but can be, for example, 100 m ² / g or less (typically ^{20 to} 100 m ² / g, for example, 30 to 60 m ² / g). As a result, a hybrid capacitor that is particularly excellent in volume energy density can be realized. Alternatively, the BET specific surface area of the carbon material can be 1000 m ² / g or more (typically 1000 to 3000 m ² / g, for example 1500 to 2500 m ² / g). As a result, a hybrid capacitor with particularly excellent input / output density can be realized.

The average particle diameter of the carbon material disclosed herein is not particularly limited, but may be, for example, submicron to micron order (typically 0.1 to 10 μm, for example, 3 to 10 μm). Thereby, more excellent characteristics (for example, at least one of high energy density, high output density, and high durability) can be realized. In the present specification, the “average particle size” means a particle size corresponding to 50% cumulative from the fine particle side in the volume-based particle size distribution obtained by the particle size distribution measurement based on the laser diffraction / light scattering method. D ₅₀ particle diameter, also referred to as median diameter).

Such a carbon material can be produced, for example, by mixing a carbon source substance and a nitrogen-containing group-imparting agent and reacting them for a certain period of time, followed by heat treatment in a high temperature environment.
FIG. 1 is a flowchart showing a carbon material manufacturing method according to an embodiment. The manufacturing method shown in FIG. 1 includes the following steps: (S10) carbonization step; (S20) activation step; (S30) nitrogen-containing group application step; and (S40) heat treatment step. The production method disclosed herein is characterized by subjecting a carbon source material (activator) to a nitrogen-containing group-imparting agent, followed by heat treatment. Therefore, it is not specifically limited about other processes, It can carry out similarly to a conventionally well-known method. Hereinafter, each process is demonstrated in order.

  In the carbonization step (S10), for example, carbon precursors (raw materials) such as coconut shells, coal, coke, pitch, and resin are carbonized and activated, and processing such as pulverization and sieving is performed as necessary. Although the said carbonization and activation depend on the kind etc. of carbon precursor, it can be performed by carrying out dry distillation (heat processing) for about 1 to 10 hours at 500-1000 degreeC, for example by inert atmosphere. Carbonization and activation may be performed simultaneously or in stages. Thereby, a carbide can be obtained. In addition, this process can also be abbreviate | omitted when purchasing and using the carbon source material (carbide) already carbonized and activated. The average particle size of the carbide after the carbonization step is preferably about 0.1 to 5 μm, for example, from the viewpoint of performing uniform activation in the activation step described later.

  In the activation step (S20), activation treatment is performed on the obtained carbide by a gas activation method, a chemical activation method, or the like. The gas activation can be performed, for example, by subjecting the obtained carbide to dry distillation (heat treatment) at 500 to 1000 ° C. for about 1 to 10 hours in the presence of water vapor, oxygen, carbon dioxide and the like. In addition, chemical activation is performed by, for example, mixing the obtained carbide with an alkali chemical containing an alkali metal such as sodium or potassium (for example, potassium hydroxide, sodium hydroxide, potassium carbonate, etc.), and then in an inert atmosphere. It can be carried out by dry distillation (heat treatment) at ˜1000 ° C. for about 1 to 10 hours. At this time, the ratio of the carbide and the alkaline chemical is, for example, preferably about 1: 1 to 1:10 (typically 1: 1 to 1: 5) as an equivalent ratio. When using a monovalent alkali salt such as potassium hydroxide, the ratio of carbide to alkaline chemical is about 1: 1 to 1:10 (typically 1: 1 to 1: 5) in mass ratio. It is good to. Thereby, a uniform activation product can be obtained. When the alkali activation is employed, typically, in a room temperature environment (for example, 20 to 25 ° C.), the activated material obtained above is washed with pure water or an acidic solution (for example, a hydrochloric acid aqueous solution or a sulfuric acid aqueous solution) Remove remaining alkali components. As a result, a high-quality activator with few impurities can be obtained. In addition, when purchasing and using already activated carbon source material (activation material), this process can also be abbreviate | omitted.

In the nitrogen-containing group application step (S30), the activated material obtained above is typically used in a temperature environment higher than room temperature (typically 30 ° C or higher, for example 50 ° C or higher, preferably 70 ° C or higher). By exposing to a nitrogen-containing group imparting agent, a nitrogen-containing group is imparted to the surface of the activation product.
The nitrogen-containing group-imparting agent only needs to have at least a nitrogen atom, and one or more nitrogen-containing compounds can be appropriately used. The form of the nitrogen-containing group-imparting agent may be solid or liquid, or may be a gas (gas). Specific examples include nitrogen-containing inorganic compounds such as nitric acid, ammonium hydroxide, ammonium nitrate, and ammonia gas; nitrogen-containing organic compounds such as imidazole and pyridine. Among these, nitric acid or nitrate having high reaction activity can be preferably used. Further, the method for imparting a nitrogen-containing group is not particularly limited, and depending on the form of the nitrogen-containing group imparting agent to be used, a conventionally known liquid phase reaction method, a heat treatment method in a nitrogen-containing gas environment, a nitrogen-containing compound and A method such as a co-carbonization method can be appropriately employed. Alternatively, a conventionally known nitrogen plasma treatment may be performed on the surface of the activation material.

  In a preferred embodiment, nitric acid is used as the nitrogen-containing group imparting agent. In another preferred embodiment, a liquid phase reaction method is used. By using the liquid phase reaction method, the nitrogen-containing group can be imparted more uniformly to the surface of the activated material. As a specific procedure, first, an aqueous nitric acid solution having a suitable concentration is prepared. The concentration of nitric acid in the aqueous nitric acid solution may be adjusted to about 1 to 10 mol / L (for example, 2 to 6 mol / L), although it depends on the type of carbon source material (in this case, activated material) that imparts a nitrogen-containing group. Next, the activated material obtained above is immersed in an aqueous nitric acid solution and held at a temperature higher than room temperature (typically 30 ° C. or higher, for example, 30 to 110 ° C., preferably 50 to 100 ° C.) for a certain time. The ratio of the activation product in the aqueous nitric acid solution is 0.1 to 20 masses when the total of the nitric acid aqueous solution and the activation product is 100% by mass, for example, from the viewpoint of uniformly imparting nitrogen-containing groups to the surface of the activation product. % Or less (for example, 5 to 20% by mass). Moreover, although it changes with temperature etc., the time (reaction time) hold | maintained at the said temperature can be made into about 10 minutes-10 hours (typically 30 minutes-5 hours), for example. According to such an embodiment, the amount of acidic functional group (nitrogen-containing group) can be adjusted according to the concentration and holding time of the aqueous nitric acid solution, and an optimal amount of functional group is stably imparted to the surface of the carbon source material. be able to. Moreover, since the functional group can be stably imparted in a relatively short time and / or with a small amount of energy, it is preferable from the viewpoint of work efficiency, cost, and reliability. The carbon source material after imparting the nitrogen-containing group is typically weakly acidic to neutral in a room temperature environment (for example, 20 to 25 ° C.) using pure water or an acidic solution (for example, hydrochloric acid aqueous solution or sulfuric acid aqueous solution). Wash until (for example, pH≈5-7). As a result, a high-quality carbon source material with few impurities can be obtained.

  In the heat treatment step (S40), the carbon material after imparting the nitrogen-containing group is heat-treated in a high temperature environment. Thereby, the provided acidic functional group (nitrogen-containing group) can be immobilized on the surface of the carbon source material. The temperature of the heat treatment is preferably typically 300 ° C. or higher (for example, 400 ° C. or higher, particularly 450 ° C. or higher) from the viewpoint of stably immobilizing the nitrogen-containing group on the surface of the carbon source material. The upper limit temperature of the heat treatment is typically 700 ° C. or lower (for example, 600 ° C. or lower, particularly 550 ° C. or lower) from the viewpoint of suitably leaving the nitrogen-containing group on the surface of the carbon source material. The heat treatment time may be, for example, 1 to 24 hours (typically 5 to 20 hours).

  In addition, although the manufacturing method which performs provision (S30) of nitrogen-containing group and heat processing (S40) after activation (S20) was shown here, provision of a nitrogen-containing group and heat processing can also be performed before activation, for example. That is, the application of the nitrogen-containing group and the heat treatment may be performed at any stage of the carbon material manufacturing process.

  According to the invention disclosed herein, a hybrid capacitor including the above-described carbon material as an electrode (typically a positive electrode) is provided. Such a hybrid capacitor can have a higher pseudocapacitance derived from surface functional groups and an improved capacitance (for example, volume energy density) as compared with a conventional carbon material. Further, even if it is repeated a plurality of times, there is little decrease in capacitance, and it has high durability (for example, cycle characteristics) that can exhibit high performance over a long period of time. Therefore, taking advantage of such characteristics, it can be preferably used in applications where high energy density and high durability are required.

Hereinafter, as a preferred embodiment, the present invention will be specifically described by taking a hybrid capacitor (for example, a lithium ion capacitor) including the carbon material disclosed herein as a positive electrode. In addition, the shape, constituent material, manufacturing process, etc. of the said hybrid capacitor may be the same as that of the past, except using the carbon material disclosed here for a positive electrode.
The hybrid capacitor disclosed herein includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. Such a hybrid capacitor can be constructed, for example, by producing a positive electrode by a conventionally known method using the carbon material, and then housing the positive electrode, the negative electrode, and a nonaqueous electrolytic solution in a case. The lithium ion capacitor may further include a pre-doping metallic lithium foil in addition to the positive electrode, the negative electrode, and the non-aqueous electrolyte.

  The positive electrode of the hybrid capacitor disclosed herein includes at least the carbon material as described above. Such a positive electrode is typically a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, the positive electrode including the carbon material as a positive electrode active material, a conductive material, and a binder. An active material layer is provided. As the positive electrode current collector, a conductive member made of a metal having good conductivity (for example, aluminum or stainless steel) can be suitably used. As the conductive material, an easily graphitizable carbon material such as carbon black (for example, acetylene black or ketjen black) can be suitably used. As the binder, polymer materials such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), and polyethylene oxide (PEO) can be suitably used.

  The negative electrode of the hybrid capacitor disclosed herein is typically a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, and includes a negative electrode active material, a conductive material, and a binder. A negative electrode active material layer is provided. As the negative electrode current collector, a conductive material made of a metal having good conductivity (for example, copper or stainless steel) can be suitably used. As the negative electrode active material, carbon materials such as graphite (hard graphite), conductive polymer materials such as polyacene organic semiconductors, metal oxide materials such as lithium titanate, etc. (lithium ions) In the capacitor, graphite) can be preferably used. As the conductive material and the binder, for example, the same material as the positive electrode can be appropriately included.

  Moreover, in the typical one aspect | mode, the separator as an insulating layer is arrange | positioned between the positive electrode and the negative electrode. As the separator, a porous resin sheet made of a resin such as polyethylene (PE) or polypropylene (PP); a nonwoven fabric;

The non-aqueous electrolyte of the hybrid capacitor disclosed herein is typically in a form in which a supporting salt is contained in a non-aqueous solvent. Alternatively, the liquid non-aqueous electrolyte may be added with a polymer to form a solid (typically a so-called gel). As the supporting salt, lithium salts such as LiPF ₆ and LiBF ₄ can be preferably used. As the non-aqueous solvent, aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones and lactones can be used. Of these, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) can be preferably used. Moreover, various additives can also be added to the electrolytic solution as necessary.

  In addition, although the case where the above carbon materials are used for a positive electrode (used as a positive electrode active material) was shown here, when using such a carbon material for a negative electrode (used as a negative electrode active material), as a positive electrode active material, A material capable of inserting and extracting charge carriers (for example, lithium ions) is employed. Specifically, conductive polymer compound materials such as polythiophene and polyanions; metal oxide materials such as lithium cobaltate, manganese oxide, and ruthenium oxide; and the like can be used.

  Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to the specific examples.

<I. When coal-based coke is used as the carbon precursor>
[Manufacture of carbon materials]
In this example, coal-based coke (BET specific surface area: approximately 0 to 10 m ² / g) was used as a carbon precursor (raw material).
<Carbon material A>
The coal-based coke was carbonized (carbonized) at 860 ° C. for 1 hour in an inert atmosphere, mixed with 4 times equivalent amount of potassium hydroxide as an alkaline chemical, and the resulting mixture was mixed at 800 ° C. under an inert atmosphere. The alkali was activated for 4 hours. The obtained activation product was dispersed in water, and then washed with water and acid to remove the alkali component. Here, washing was performed until the concentration of the remaining potassium (K) became 5000 ppm or less. The activated product after washing was dried and then reacted with a 2N (2 mol / L) aqueous nitric acid solution heated to 80 ° C. for 1 hour to give a nitrogen-containing group to the surface of the activated product. After the reaction, it was washed with water until the pH became approximately 5 to 7, dried, and then pulverized with a vibration mill to adjust to an average particle size of 3 μm. This pulverized product was heat-treated at 500 ° C. for 10 hours in an inert atmosphere to obtain a carbon material A.
<Carbon material B>
A carbon material B was obtained in the same manner as the carbon material A except that the heat treatment (500 ° C., 10 hours) was not performed.
<Carbon material C>
A carbon material C was obtained following the carbon material A except that the nitrogen-containing group was not added. That is, in the same manner as the carbon material A, carbonization, alkali activation, and washing were performed, followed by pulverization with a vibration mill to adjust the average particle size to 3 μm. The pulverized product was heat treated with rhodium (Rh) as a reaction catalyst in a 3% hydrogen atmosphere at 750 ° C. for 8 hours to reduce the amount of acidic functional groups on the surface of the activated material, thereby obtaining a carbon material C.
The manufacturing method of the produced carbon materials A to C is summarized in Table 1 below. Moreover, the following physical-property measurements were performed about carbon material AC.

[Measurement of the amount of acidic functional groups]
The amount of acidic functional groups of the carbon materials A to C was measured according to the Boehm method. Specifically, 2 g of the carbon material previously vacuum-dried at 110 ° C. for 2 hours was weighed and transferred to a flask. 20 ml of a 0.1 mol / L sodium ethoxide aqueous solution was added thereto, and the mixture was stirred for 3 minutes in a centrifuge at a rotational speed of 2000 rpm. Next, in an environment of 25 ° C., it was subjected to an ultrasonic machine for 20 minutes to sufficiently react the carbon material and the chemical. Next, the carbon material was filtered off, 5 ml was collected from the filtrate, back titrated with 0.05 mol / L hydrochloric acid aqueous solution, and the hydrochloric acid titration amount b was measured using methyl orange as an indicator. As a blank test, an aqueous hydrochloric acid solution was dropped into 20 ml of an aqueous sodium ethoxide solution, and a hydrochloric acid titration amount a was measured using methyl orange as an indicator in the same manner as described above. And the amount of acidic functional groups was computed by the following formula (1). The results are shown in Table 1.

  As is clear from Table 1, the carbon material C that was not subjected to nitric acid treatment (addition of nitrogen-containing groups) and heat-treated at 750 ° C. in a hydrogen atmosphere had the smallest amount of acidic functional groups of 0.01 mmol / g. It was. Moreover, in the carbon material B which was not heat-treated, the most acidic functional groups remained at 0.21 mmol / g.

[Measurement of volatile matter]
The volatile components of the carbon materials A to C were calculated by heat-treating a predetermined amount at 850 ° C. in a nitrogen atmosphere and measuring mass changes before and after firing. Specifically, first, 1 g of a carbon material vacuum-dried at 110 ° C. for 2 hours in advance was weighed and baked at 850 ° C. for 1 hour in a nitrogen atmosphere, and the mass after baking was measured. And volatile matter (%) was computed from the following formula (2). The results are shown in Table 1.

As is apparent from Table 1, the carbon material C that was not subjected to nitric acid treatment (addition of nitrogen-containing groups) and heat-treated at 750 ° C. in a hydrogen atmosphere had the smallest volatile content of 1%.
Moreover, the carbon material B which was not heat-treated had the largest volatile content of 9%. This is presumably because the amount of acidic functional groups was large and the functional groups were not immobilized (stabilized) on the surface of the carbon material because no heat treatment was performed.

[Measurement of BET specific surface area measurement]
The BET specific surface areas of the carbon materials A to C were measured by a nitrogen adsorption method using Quadrasorb manufactured by Quantachrome. The results are shown in Table 1.
As is clear from Table 1, the BET specific surface areas of the carbon materials A to C were approximately 30 to 60 m ² / g.

[Construction of tripolar cell]
<Example 1>
An evaluation cell was constructed using the carbon material A produced above as the positive electrode active material.
Specifically, carbon material A as a positive electrode active material, carbon black (CB) as a conductive agent, and polytetrafluoroethylene (PTFE) as a binder, the mass ratio of these materials is carbon material A: CB: PTFE = 81.8: 9.1: 9.1 was weighed and kneaded while adjusting the viscosity with ion-exchanged water to prepare a positive electrode active material slurry. This slurry was applied onto a current collector, dried, and then compressed with a roll press to obtain a sheet-like positive electrode. The sheet-like positive electrode was punched into a size of 2 cm ² with a punching machine and vacuum-dried at 120 ° C. for 12 hours to obtain a positive electrode. And using this positive electrode, the 3 pole type coin cell was built in the glove box. Specifically, the above-prepared positive electrode is used as a working electrode, a metal lithium foil is used as a counter electrode and a reference electrode, and a non-aqueous electrolyte (here, ethylene carbonate (EC) and diethyl carbonate (DEC) is 50:50). A tripolar coin cell (Example 1) was constructed by enclosing it in a non-aqueous solvent containing a volume ratio of 1 and a LiPF ₆ as a supporting salt dissolved at a concentration of 1 mol / L. did.
<Example 2>
A tripolar coin cell (Example 2) was constructed in the same manner as in Example 1 except that the carbon material B produced above was used as the positive electrode active material.
<Example 3>
A tripolar coin cell (Example 3) was constructed in the same manner as in Example 1 except that the carbon material C prepared above was used as the positive electrode active material.

[Capacitance measurement]
The cells of Examples 1 to 3 constructed above were charged at a constant current (CC) up to 4.8 V at a current density of ² mA / cm ² in a temperature environment of 25 ° C., and then at the same voltage for 1 hour. After performing constant voltage (CV) charging, constant current (CC) discharging was performed to 3.0 V at a current density of ² mA / cm ² . Subsequently, charge / discharge of 3 cycles was performed in the following pattern, and the CCCV discharge capacity at the 3rd cycle was set as the initial capacitance (F).
(1) CC charging is performed up to 4.4 V at a current density of ² mA / cm ² , and then CV charging is performed for 5 minutes at the same voltage.
(2) Perform CC discharge to 3.0 V at a current density of ² mA / cm ² .
Then, the initial electrostatic capacity was divided by the volume of the carbon material, and the electrostatic capacity per 1 ml of the carbon material (F / ml) was calculated. The results are shown in Table 2 together with the physical property values of the carbon material. Moreover, the graph which plotted the electrostatic capacitance with respect to the amount of acidic functional groups is shown in FIG.

  As is clear from Table 2 and FIG. 2, the more the acidic functional group, the higher the capacitance. For example, in Examples 1 and 2, the capacitance was improved by about 15 to 20% compared to Example 3. This is thought to be due to an increase in pseudo capacity derived from acidic functional groups.

[Charge / discharge cycle test]
The cells of Examples 1 to 3 were further charged and discharged for 200 cycles under the same conditions as those for measuring the capacitance. Then, the discharge capacity at the 200th cycle relative to the initial discharge capacity was calculated as a capacity retention rate (%). The results are shown in Table 2. Moreover, the graph which plotted the capacity | capacitance maintenance factor with respect to 850 degreeC volatile matter is shown in FIG.

  As apparent from Table 2 and FIG. 3, the cell of Example 2 developed only about 25% of the initial capacity after 200 cycles of charge and discharge. This is probably because the acidic functional group on the surface of the carbon material was not fixed (stabilized), and the acidic functional group was desorbed by charge / discharge, leading to decomposition of the non-aqueous electrolyte and generation of gas. . On the other hand, the cells of Examples 1 and 3 achieved a capacity retention rate of 85% even after 200 cycles, and were found to be excellent in durability.

<II. When coconut-based activated carbon is used as the carbon precursor>
[Manufacture of carbon materials]
In this example, coconut-based activated carbon (BET specific surface area: 2000-2500 m ² / g) was used as the carbon precursor (raw material).
<Carbon material D>
First, the coconut-based activated carbon was activated with water vapor so that the BET specific surface area was adjusted to 2200 m ² / g. Next, the obtained activation product was reacted with a 6N (6 mol / L) nitric acid aqueous solution heated to 80 ° C. for 1 hour to give a nitrogen-containing group to the surface of the activation product. After the reaction, it was washed with water until the pH was about 5 to 7, dried, and then pulverized with a vibration mill to adjust the average particle size to 3 μm. This pulverized product was heat-treated in an inert atmosphere at 500 ° C. for 10 hours to obtain a carbon material D.
<Carbon material E>
A carbon material E was obtained in the same manner as the carbon material D except that a 2N (2 mol / L) nitric acid aqueous solution was used when the nitrogen-containing group was imparted.
<Carbon material F>
A carbon material F was obtained in the same manner as the carbon material E except that the heat treatment (500 ° C., 10 hours) was not performed.
<Carbon material G>
A carbon material G was obtained in the same manner as the carbon material D except that the heat treatment (500 ° C., 10 hours) was not performed.
<Carbon material H>
A carbon material H was obtained following the carbon material D except that the heat treatment conditions were changed. That is, after steam activation was performed in the same manner as the carbon material D, it was pulverized with a vibration mill and adjusted to an average particle size of 5 μm. This pulverized product was heat-treated with rhodium (Rh) as a reaction catalyst in a 3% hydrogen atmosphere at 750 ° C. for 8 hours to obtain a carbon material H.
The manufacturing methods of the produced carbon materials D to H are summarized in Table 3 below. Further, regarding the carbon materials D to H, I. The physical properties were measured by the same method. The results are shown in Table 3.

[Construction of tripolar cell]
<Examples 4 to 8>
A tripolar coin cell (Example 4) was constructed in the same manner as in Example 1 except that the carbon material D produced above was used as the positive electrode active material.
A tripolar coin cell (Example 5) was constructed in the same manner as in Example 1 except that the carbon material E produced above was used as the positive electrode active material.
A tripolar coin cell (Example 6) was constructed in the same manner as in Example 1 except that the carbon material F produced above was used as the positive electrode active material.
A tripolar coin cell (Example 7) was constructed in the same manner as in Example 1 except that the carbon material G produced above was used as the positive electrode active material.
A tripolar coin cell (Example 8) was constructed in the same manner as in Example 1 except that the carbon material H produced above was used as the positive electrode active material.

[Capacitance measurement and charge / discharge cycle test]
The capacitance of the cells of Examples 1 to 8 constructed above was measured under a temperature environment of 25 ° C. Specifically, charging / discharging of 3 cycles was performed with the following pattern, and the CCCV discharge capacity of the 3rd cycle was made into the initial stage electrostatic capacitance (F).
(1) CC charging is performed up to 4.25 V at a current density of ² mA / cm ² , and then CV charging is performed for 5 minutes at the same voltage.
(2) Perform CC discharge to 3.0 V at a current density of ² mA / cm ² .
Then, the initial electrostatic capacity was divided by the volume of the carbon material, and the electrostatic capacity per 1 ml of the carbon material (F / ml) was calculated. The results are shown in Table 4 together with the physical property values of the carbon material. Moreover, the graph which plotted the electrostatic capacitance with respect to the amount of acidic functional groups is shown in FIG.
Furthermore, about the cell of Examples 1-8, charging / discharging of 200 cycles was repeated on the conditions similar to the time of an electrostatic capacitance measurement. Then, the discharge capacity at the 200th cycle relative to the initial discharge capacity was calculated as a capacity retention rate (%). The results are shown in Table 4. Moreover, the graph which plotted the capacity | capacitance maintenance factor with respect to 850 degreeC volatile matter is shown in FIG.

As apparent from Table 4 and FIGS. 4 and 5, the above-mentioned I.I. The same tendency as in the case of using coal-based coke was shown.
That is, in any experimental example, the following conditions (1) and (2):
(1) The volatile content when heat-treated at 850 ° C. in an inert atmosphere is 3% by mass or more and 7% by mass or less;
(2) The amount of acidic functional group is 0.1 mmol / g or more;
It was found that a hybrid capacitor (lithium ion capacitor) capable of achieving both high energy density and durability can be realized by using a carbon material satisfying the above as an electrode material.
In addition, according to the method for producing a carbon material disclosed herein, energy can be maintained without deteriorating the durability (for example, cycle characteristics) of various carbon precursors (raw materials), carbides, and activated materials. It has been found that the density (capacitance) can be improved.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

Claims (7)

A carbon material used for an electrode of a hybrid capacitor, under the following conditions:
(1) The volatile content when heat-treated at 850 ° C. in an inert atmosphere is 3% by mass or more and 7% by mass or less; and (2) The amount of the acidic functional group is 0.1 mmol / g or more. At least some of the acidic functional groups contain nitrogen atoms;
Meet the carbon material.   The carbon material according to claim 1, wherein the amount of the acidic functional group is 0.7 mmol / g or less. The carbon material of Claim 1 or 2 whose BET specific surface area measured by a nitrogen adsorption method is 100 m < ² > / g or less. The carbon material of Claim 1 or 2 whose BET specific surface area measured by a nitrogen adsorption method is 1000 m < ² > / g or more. It is a manufacturing method of the carbon material as described in any one of Claims 1-4,
A carbon source material and a nitrogen-containing group-imparting agent are mixed to impart an acidic functional group containing a nitrogen atom to the surface of the carbon source material, and then heat treated to immobilize the acidic functional group on the surface of the carbon source material. A method for producing a carbon material. The method for producing a carbon material according to claim 5 , wherein the nitrogen-containing group imparting agent is an aqueous nitric acid solution. A hybrid capacitor comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte,
It said positive electrode or said negative electrode comprises a carbon material according to any one of claims 1-4, hybrid capacitor.

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