WO2019017380A1 - Hybrid capacitor - Google Patents

Abstract

This hybrid capacitor is a hybrid capacitor in which the discharge capacity retention rate can maintain 80% or more for 1000 hours or more in a constant current constant voltage continuous charge test at 60 ° C. and 3.5 V, and is a positive electrode side and a negative electrode side The current collector is an aluminum material, the aluminum material is coated with an amorphous carbon film, the thickness of the amorphous carbon film is 60 nm or more and 300 nm or less, and the positive electrode contains graphite as a positive electrode active material, The negative electrode contains activated carbon as a negative electrode active material, and the activated carbon contains nitrogen.

Description

Hybrid capacitor

The present invention relates to a hybrid capacitor.
Priority is claimed on Japanese Patent Application No. 2017-139523, filed July 18, 2017, the content of which is incorporated herein by reference.

Conventionally, an electric double layer capacitor (for example, refer to patent documents 1) and a secondary battery are known as art of storing electric energy. Electric double layer capacitors (EDLCs) have much longer life, safety and power density than secondary batteries. However, the electric double layer capacitor has a problem that the energy density (volume energy density) is lower than that of the secondary battery.
Here, the energy (E) stored in the electric double layer capacitor is expressed as E = 1/2 × C × V ² using the capacitance (C) of the capacitor and the applied voltage (V), and the energy is It is proportional to the capacitance and the square of the applied voltage. Therefore, in order to improve the energy density of the electric double layer capacitor, a technique for improving the capacitance and the applied voltage of the electric double layer capacitor has been proposed.

As a technique for improving the capacitance of the electric double layer capacitor, there is known a technique for increasing the specific surface area of the activated carbon constituting the electrode of the electric double layer capacitor. Currently known activated carbons have a specific surface area of 1000 m ² / g to 2500 m ² / g. In an electric double layer capacitor using such activated carbon as an electrode, an organic electrolytic solution in which a quaternary ammonium salt is dissolved in an organic solvent, an aqueous electrolytic solution such as sulfuric acid, or the like is used as an electrolytic solution.
Since the organic electrolytic solution has a wide usable voltage range, the applied voltage can be increased, and the energy density can be improved.

A lithium ion capacitor is known as a capacitor whose applied voltage is improved utilizing the principle of an electric double layer capacitor. A lithium ion capacitor is used that uses graphite or carbon that can intercalate and deintercalate lithium ions as the negative electrode and uses activated carbon equivalent to the electrode material of an electric double layer capacitor that can adsorb and desorb electrolyte ions to the positive electrode. ing. In addition, as to the positive electrode or the negative electrode, an active carbon equivalent to the electrode material of the electric double layer capacitor is used, and the other electrode using a metal oxide or a conductive polymer as an electrode causing a faradic reaction is as follows: It is called a hybrid capacitor. In the lithium ion capacitor, among the electrodes constituting the electric double layer capacitor, the negative electrode is made of graphite, hard carbon or the like which is a negative electrode material of a lithium ion secondary battery, and lithium ions are inserted in the graphite or hard carbon It is an electrode. The lithium ion capacitor is characterized in that the applied voltage is larger than that of a general electric double layer capacitor, ie, one in which both electrodes are made of activated carbon.

However, when graphite is used for the electrode, there is a problem that propylene carbonate, which is known as a solvent for an electrolytic solution, can not be used. When graphite is used for the electrode, propylene carbonate is electrolyzed, and a decomposition product of propylene carbonate adheres to the surface of the graphite, thereby reducing the reversibility of lithium ions. Propylene carbonate is a solvent that can operate even at low temperatures. When propylene carbonate is applied to the electric double layer capacitor, the electric double layer capacitor can operate even at -40 ° C. Therefore, in lithium ion capacitors, hard carbon in which propylene carbonate is not easily decomposed is used as an electrode material. However, hard carbon has a lower capacity per electrode volume than graphite, and a lower voltage than that of graphite (a noble potential). Therefore, there is a problem that the energy density of the lithium ion capacitor becomes low.

When importance is attached to low temperature characteristics, it is difficult to further increase the energy density of a lithium ion capacitor which is difficult to use high capacity graphite for the negative electrode. Furthermore, in the lithium ion capacitor, copper foil is used as the current collector in the same manner as the negative electrode of the lithium ion secondary battery, and therefore copper is eluted to cause a short circuit when overdischarge is performed at 2 V or less. There are problems such as a decrease in discharge capacity. Therefore, lithium ion capacitors have problems such as limited usage as compared to electric double layer capacitors that can discharge to 0 V.

As a capacitor of a new concept, a capacitor utilizing a reaction of inserting and desorbing electrolyte ions between graphite layers using graphite as a positive electrode active material instead of activated carbon has been developed (see, for example, Patent Document 2). According to Patent Document 2, in a conventional electric double layer capacitor using activated carbon as a positive electrode active material, decomposition of the electrolytic solution occurs and gas is generated when a voltage exceeding 2.5 V is applied to the positive electrode. It is described that capacitors of a new concept using graphite do not cause decomposition of the electrolytic solution even at a charge voltage of 3.5 V, and can operate at a higher voltage than conventional electric double layer capacitors using activated carbon as a positive electrode active material . The cycle characteristics, low temperature characteristics, and output characteristics are also equal to or higher than those of the conventional electric double layer capacitor. The specific surface area of graphite is several hundredths of the specific surface area of activated carbon, and the difference in the electrolytic solution decomposition action is attributed to the difference in the large specific surface area.

A new concept capacitor using graphite as a positive electrode active material is not sufficiently durable, so its commercialization has been hampered, but the technology using an aluminum material coated with an amorphous carbon film as a current collector (patented Reference 3) shows that high temperature durability performance can be improved to a practical level. The capacitor of this new concept is a capacitor that uses the reaction of inserting and desorbing electrolyte ions between the layers of graphite in the positive electrode, and although not strictly an electric double layer capacitor, in Patent Document 3, in a broad sense, electrical It is called a double layer capacitor.

Here, the durability test is usually performed by an accelerated test (high temperature durability test, charge and discharge cycle test) by raising the temperature. The test can be performed by the method according to the "durability (high temperature continuous rated voltage application) test" described in JIS D 1401: 2009. It is said that increasing the temperature from room temperature to 10 ° C. approximately doubles the degradation rate. As a high temperature durability test, for example, the battery is held (continuously charged) in a constant temperature bath of 60 ° C. for 2000 hours at a predetermined voltage (3 V or more in the present invention), and then returned to room temperature to perform charge / discharge. There is a test to measure the volume. After this high temperature durability test, it is considered desirable that the discharge capacity retention ratio be 80% or more of the initial discharge capacity.

Patent Document 4 discloses that doping of activated carbon with nitrogen can increase the withstand voltage of EDLC. Further, Non-Patent Document 1 discloses that nitrogen-doped activated carbon is used as a catalyst for removing SO _{2 in} exhaust gas.

JP, 2011-046584, A JP, 2010-040180, A International Publication No. 2017/216960 JP, 2013-026484, A

Carbon 41 (2003) 1925-1932

A hybrid capacitor using graphite as a positive electrode active material and activated carbon as a negative electrode active material is required to further increase energy density. In that case, since the capacity of the activated carbon used for the negative electrode active material is smaller than the capacity of the graphite used for the positive electrode active material, the negative electrode capacity governs the capacity at the time of charge and discharge of the cell. In order to increase the capacity of the negative electrode, it is effective to lower the negative electrode reduction potential, but if the reduction potential is too low, decomposition of the electrolytic solution generates gas or the activated carbon surface is covered with the electrolytic solution decomposition product There is a problem such as a decrease in capacity due to a decrease in specific surface area due to the above, or degradation due to decomposition of the activated carbon itself.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a hybrid capacitor with high energy density and excellent high temperature durability performance by achieving high capacity and high voltage.

As a result of repeated investigations to solve the above problems, the inventors of the present invention have found that, in a hybrid capacitor using graphite as a positive electrode active material and activated carbon as a negative electrode active material, activated carbon doped with nitrogen in the active material of the negative electrode The reduction potential of the negative electrode can be lowered to increase the capacity and voltage of the negative electrode, thereby achieving high capacity and high voltage of the entire cell of the hybrid capacitor, and high energy density of the cell. It has been found that it is possible to improve the durability and high temperature durability performance. At that time, when etched aluminum widely used in EDLC is used as a current collector, the etched aluminum may be corroded below a certain negative electrode reduction potential, and an aluminum material coated with an amorphous carbon film is used. It was also confirmed that it is preferable to use in combination.

In order to solve the above-mentioned subject, the present invention provides the following means.

(1) The hybrid capacitor according to one aspect of the present invention is a hybrid capacitor in which the discharge capacity retention rate can maintain 80% or more in a constant current constant voltage continuous charge test at 60 ° C. and 3.5 V for 1000 hours or more. The current collectors on the positive electrode side and the negative electrode side are aluminum materials, and the aluminum material is coated with an amorphous carbon film, and the thickness of the amorphous carbon film is 60 nm or more and 300 nm or less. Is characterized in that it contains graphite as a positive electrode active material, the negative electrode contains activated carbon as a negative electrode active material, and the activated carbon contains nitrogen.

(2) In the hybrid capacitor according to the above aspect, the activated carbon may be subjected to a nitrogen doping process.

(3) In the hybrid capacitor according to the above aspect, the ratio of nitrogen to carbon in the activated carbon may be 1.0 at% or more and 4.0 at% or less.

According to the present invention, it is possible to provide a hybrid capacitor with high energy density and excellent high temperature durability.

It is a graph which shows the discharge characteristic (The discharge capacity improvement rate at the time of doing the constant current constant voltage continuous charge test in 60 degreeC when changing nitrogen doping treatment time) when the hybrid capacitor produced in Example 2 is changed. It is a graph which shows the discharge characteristic (The discharge capacity improvement rate at the time of performing the constant current constant voltage continuous charge test in 60 degreeC when changing a charge voltage in a high voltage area) when the hybrid capacitor produced in Example 4 is changed. .

Hereinafter, a hybrid capacitor according to an embodiment to which the present invention is applied will be described in detail with reference to the drawings. In the drawings used in the following description, in order to make the features easy to understand, the features that are the features may be enlarged for the sake of convenience, and the dimensional ratio of each component may be limited to the same as the actual Absent. In addition, the materials, dimensions, and the like exemplified in the following description are merely examples, and the present invention is not limited to them, and can be appropriately modified and implemented within the scope of achieving the effects.

A hybrid capacitor according to an embodiment of the present invention is a hybrid capacitor capable of maintaining a discharge capacity maintenance ratio of 80% or more for 1000 hours or more in a constant current constant voltage continuous charge test at 60 ° C. and 3.5 V, A negative electrode, an electrolytic solution, and a separator are provided. The current collectors on the positive electrode side and the negative electrode side are an aluminum material, and the aluminum material is coated with an amorphous carbon film, and the thickness of the amorphous carbon film is 60 nm or more and 300 nm or less. The positive electrode contains graphite as a positive electrode active material, activated carbon is used as a negative electrode active material, activated carbon is subjected to nitrogen doping treatment, functional groups on the surface of activated carbon are replaced with nitrogen, and activated carbon contains nitrogen.

The positive electrode has a positive electrode active material layer formed on a current collector (current collector on the positive electrode side).
The positive electrode active material layer can be formed by applying a paste-like positive electrode material containing a binder and an amount of a conductive material as required on a current collector on the positive electrode side, and drying it.

As the binder, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, acrylic type, olefin type, carboxymethyl type (CMC) type alone or two types The above mixed system can be used.

The conductive material is also not particularly limited as long as it can improve the conductivity of the positive electrode active material layer, and known conductive materials can be used. For example, carbon black, carbon fibers (including carbon nanotubes (CNT), VGCF (registered trademark) and the like, and not limited to carbon nanotubes) can be used.

As the current collector on the positive electrode side, an aluminum material having improved corrosion resistance, for example, an aluminum material coated with an amorphous carbon film can be used. The aluminum material may be coated only with the amorphous carbon film, or a conductive carbon layer may be provided between the amorphous carbon film and the positive electrode active material.
As an aluminum material which is a substrate, an aluminum material generally used for current collector application can be used. The shape of the aluminum material can be in the form of foil, sheet, film, mesh or the like. An aluminum foil can be suitably used as the current collector. Moreover, you may use the etched aluminum mentioned later other than a plain thing as an aluminum material.

The thickness in the case where the aluminum material is a foil, sheet or film is not particularly limited. However, when the size of the cell itself is the same, there is an advantage that more active material to be contained in the cell case can be enclosed as the cell size is thinner. In order to do so, select the appropriate thickness. The actual thickness is preferably 10 μm to 40 μm, and more preferably 15 μm to 30 μm. If the thickness is less than 10 μm, the aluminum material may be broken or cracked during the step of roughening the surface of the aluminum material or other manufacturing steps.

Etched aluminum may be used as the aluminum material coated with the amorphous carbon film.
Etched aluminum is one that has been roughened by etching. The etching is generally performed by immersion (chemical etching) in an acid solution such as hydrochloric acid or electrolysis (electrochemical etching) using aluminum as an anode in an acid solution such as hydrochloric acid. In the case of electrochemical etching, the etching shape differs depending on the current waveform at the time of electrolysis, the composition of the solution, the temperature and the like, so that it can be selected from the viewpoint of capacitor performance.

As the aluminum material, either one having a passivation layer on the surface or one not having it can be used. When an aluminum material has a passivation film which is a natural oxide film formed on its surface, an amorphous carbon film layer may be provided on the natural oxide film, or a natural oxide film may be sputtered with argon, for example. It may be provided after removal by
The natural oxide film on the aluminum material is a passive film and has the advantage that it is not easily eroded by the electrolyte itself, but it leads to an increase in the resistance of the current collector, so in terms of reducing the resistance of the current collector It is better to have no natural oxide film.

In the present specification, the amorphous carbon film is an amorphous carbon film or a hydrogenated carbon film, and is a diamond like carbon (DLC) film, a hard carbon film, an amorphous carbon (a-C) film, a hydrogenation Amorphous carbon (aC: H) film etc. are included. As a method of forming the amorphous carbon film, known methods such as plasma CVD method using hydrocarbon gas, sputter deposition method, ion plating method, vacuum arc deposition method and the like can be used. The amorphous carbon film preferably has such conductivity as to function as a current collector.

Among the exemplified amorphous carbon film materials, diamond-like carbon is a material having an amorphous structure in which both diamond bonds (sp ³ ) and graphite bonds (sp ² ) are mixed, and has high chemical resistance. However, since the conductivity is low for use in the film of the current collector, it is preferable to dope boron or nitrogen in order to enhance the conductivity.

The thickness of the amorphous carbon film is preferably 60 nm or more and 300 nm or less. When the film thickness of the amorphous carbon film is less than 60 nm, the coating effect of the amorphous carbon film becomes small, and the corrosion of the current collector in the constant current constant voltage continuous charge test can not be sufficiently suppressed. If the thickness is more than 300 nm and the thickness is too large, the amorphous carbon film becomes a resistor to increase the resistance to the active material layer, so an appropriate thickness is appropriately selected. The thickness of the amorphous carbon film is more preferably 80 nm or more and 300 nm or less, and further preferably 120 nm or more and 300 nm or less. When an amorphous carbon film is formed by plasma CVD using a hydrocarbon gas, the thickness of the amorphous carbon film is controlled by the energy injected into the aluminum material, specifically by the applied voltage, applied time, and temperature. can do.

The positive electrode active material used in the hybrid capacitor of the present embodiment contains graphite. As graphite, any of artificial graphite and natural graphite can be used. As natural graphite, scaly and earthy ones are known. Natural graphite is obtained by crushing mined raw ore and repeating benefaction called flotation. In addition, artificial graphite is produced, for example, through a graphitization process in which a carbon material is fired at high temperature. More specifically, for example, a binder such as pitch is added to coke as a raw material and molded, and primary baking is performed by heating to around 1300 ° C., and then the primary baked product is impregnated with pitch resin and further 3000 ° C. It can be obtained by secondary firing at a near high temperature. Further, it is also possible to use one in which the surface of a graphite particle is coated with carbon.

Further, the crystal structure of graphite is roughly divided into a hexagonal structure of a layer structure consisting of ABAB and a rhombohedral crystal of a layer structure consisting of ABCABC. Depending on the conditions, these structures may be present alone or in a mixed state, but any crystal structure or a mixed state may be used. For example, the graphite of Immers GC Japan Ltd. KS-6 (trade name) used in the examples described later has a rhombohedral crystal ratio of 26%, and meso graphite which is an artificial graphite manufactured by Osaka Gas Chemical Co., Ltd. Carbon microbeads (MCMB) have a rhombohedral ratio of 0%.

Graphite used in the present embodiment differs from activated carbon used in the conventional EDLC in the mechanism of expression of capacitance. In the case of activated carbon, taking advantage of the fact that the specific surface area is large, adsorption and desorption of electrolyte ions are performed on the surface thereof to express electrostatic capacity. On the other hand, in the case of graphite, an anion, which is an electrolyte ion, is intercalated / deintercalated between the layers to develop capacitance. From such a difference, the hybrid capacitor using the graphite according to this embodiment is called an electric double layer capacitor in a broad sense in Patent Document 3, but it is distinguished from EDLC using activated carbon having an electric double layer. It is a thing.

Since the current collector used in the present embodiment has an amorphous carbon film on the surface of the aluminum material, the contact of the aluminum material with the electrolytic solution is prevented to prevent the corrosion of the current collector by the electrolytic solution. Can.

The negative electrode has a negative electrode active material layer formed on a current collector (current collector on the negative electrode side).
The negative electrode active material layer mainly applies a paste-like negative electrode material containing a negative electrode active material, a binder, and an optional amount of a conductive material on the current collector on the negative electrode side, and It can be formed.

As the negative electrode active material, activated carbon which is a carbonaceous material capable of adsorbing and desorbing a cation which is an electrolyte ion can be used. Activated carbon contains nitrogen. The nitrogen contained in the activated carbon is preferably doped into the activated carbon by a nitrogen doping process.

As the current collector of the negative electrode, similarly to the current collector on the positive electrode side, an aluminum material having improved corrosion resistance, for example, an aluminum material coated with an amorphous carbon film can be used. The aluminum material may be coated only with the amorphous carbon film, or a conductive carbon layer may be provided between the amorphous carbon film and the negative electrode active material.

As the binder, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, acrylic type, olefin type, carboxymethyl type (CMC) type alone or two types The above mixed system can be used.

The conductive material is not particularly limited as long as it can improve the conductivity of the negative electrode active material layer, and a known conductive material can be used. For example, carbon black, carbon fibers (including carbon nanotubes (CNT), VGCF (registered trademark) and the like, and not limited to carbon nanotubes) can be used.

For nitrogen doping treatment to activated carbon, known treatments can be used. For example, the surface of the negative electrode is exposed to ammonia gas, exposed to ammonium carbamate as disclosed in Patent Document 4, and high temperature treatment is performed. It can carry out by adding and synthesize | combining the material which becomes a nitrogen source to the raw material at the time of producing or manufacturing activated carbon.

As a nitrogen doping treatment apparatus, an electric furnace, in particular, a rotary kiln apparatus is preferable in order to uniformly bring ammonia gas or the like into contact with activated carbon to treat it. The processing temperature is preferably in the range of 600 ° C. to 900 ° C. If the treatment temperature is too low, the nitrogen doping reaction is difficult to proceed, and conversely, if the temperature is higher than 900 ° C., the pores of the activated carbon shrink, which causes the specific surface area of the activated carbon to decrease, which is not preferable. If the treatment temperature is 900 ° C. or less, there is no concern that the pores will shrink since the temperature is near the upper limit temperature of the manufacturing process of activated carbon.

In nitrogen doping, the ratio of nitrogen to carbon to be doped (N / C ratio) changes depending on processing temperature, gas flow rate, concentration, processing time, and the like. The N / C ratio is preferably 0.7 at% (atomic composition percentage) or more, more preferably 1.0 at% or more and 4.0 at% or less, still more preferably 1.5 at% or more and 3.0 at% or less, particularly preferably Is 2.0 at% or more and 3.0 at% or less. If the nitrogen doping amount is too small, the effect of lowering the reduction potential is reduced, and if it is too large, the capacity of the activated carbon is reduced. Therefore, the combination with the graphite positive electrode is used under the optimum condition in the above range. The ratio of nitrogen to carbon (N / C ratio) can be determined by a combustion method or X-ray photoelectron spectroscopy (XPS).

In the combustion method, nitrogen in the sample is NOx gasified by burning the sample and then reduced to N ₂ gas, and carbon is gasified to CO gas or CO ₂ gas, and the obtained N ₂ gas and CO _{2 are} obtained. This is a method of quantifying gas or CO ₂ gas by chromatography (detector: TCD). X-ray photoelectron spectroscopy is a method of analyzing the composition of elements (N and C) constituting the surface of a sample by irradiating the surface of the sample with X-rays and measuring the kinetic energy of photoelectrons emitted from the surface of the sample. It is.

As the electrolytic solution, an organic electrolytic solution using an organic solvent can be used. The electrolyte contains electrolyte ions that can be adsorbed to and desorbed from the electrode. The electrolyte ion is preferably as small as possible. Specifically, ammonium salts, phosphonium salts, ionic liquids, lithium salts and the like can be used.

As an ammonium salt, tetraethyl ammonium (TEA) salt, triethyl ammonium (TEMA) salt, etc. can be used. Moreover, as a phosphonium salt, a spiro compound having two five-membered rings can be used.

The type of ionic liquid is not particularly limited, but from the viewpoint of facilitating the movement of electrolyte ions, a material having as low a viscosity as possible and having a high conductivity (conductivity) is preferable. As a cation which comprises an ionic liquid, an imidazolium ion, a pyridinium ion, etc. are mentioned, for example. As an imidazolium ion, for example, 1-ethyl-3-methylimidazolium (1-ethyl-3-methylimidazolium) (EMIm) ion, 1-methyl-1-propylpyrrolidinium (1-methyl-1-propylpyrrolidinium) Examples include (MPPy) ion, 1-methyl-1-propylpiperidinium (MPPi) ion, and the like. Further, as the lithium salt, lithium tetrafluoroborate LiBF ₄ , lithium hexafluorophosphate LiPF ₆ or the like can be used.

Examples of pyridinium ions include 1-ethylpyridinium ion, 1-butylpyridinium ion and the like.

As an anion which comprises an ionic liquid, BF ₄ ion, PF ₆ ion, [(CF ₃ SO ₂ ) ₂ N] ion, FSI (bis (fluoro sulfonyl) imide, bis (fluoro sulfonyl) imide) ion, TFSI (bis ( And trifluoromethylsulfonyl) imide, bis (trifluoromethylsulfonyl) imide) ion and the like.

As the solvent, acetonitrile or propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl sulfone, ethyl isopropyl sulfone, ethyl carbonate, fluoroethylene carbonate, γ-butyrolactone, sulfolane, N, N-dimethylformamide, dimethyl sulfoxide or the like alone or Mixed solvents can be used.

As the separator, a cellulose-based paper-like separator, a glass fiber separator, a microporous film of polyethylene or polypropylene, or the like is preferable from the viewpoints of preventing short circuit of the positive electrode and the negative electrode and securing electrolyte retention.

As described above, in the hybrid capacitor according to the present embodiment, activated carbon containing nitrogen as the negative electrode active material, preferably activated carbon doped with nitrogen by nitrogen doping treatment is used. When nitrogen doping treatment is performed on activated carbon, functional groups present on the activated carbon surface are replaced with nitrogen. For example, in the case of using activated carbon having a functional group on the surface as a negative electrode active material, if the electrode potential is increased in the direction of the basic potential (reduction side) (the cell voltage is increased when viewed from the cell) Direction), the functional group reacts with the electrolytic solution to generate an organic decomposition product or decomposition gas. The generated organic decomposition product accumulates on the activated carbon surface, and when the surface of the activated carbon is covered with the organic decomposition product, the specific surface area of the activated carbon decreases and the capacity of the negative electrode decreases. Further, the electrolytic solution in the electrode and the separator is pushed out by the generated decomposition gas, and the charge and discharge capacity is reduced. On the other hand, when nitrogen doping is performed, the functional group that reacts with the electrolytic solution is substituted by nitrogen even if the electrode potential of the negative electrode is raised to the direction of the basic potential (reduction side). And the formation of organic decomposition products can be suppressed. Therefore, the hybrid capacitor according to the present embodiment is subjected to nitrogen doping treatment, the functional group on the surface is replaced with nitrogen, and nitrogen-containing activated carbon is used as the negative electrode active material to lower the reduction potential of the negative electrode. By achieving high capacity and high voltage, it is possible to achieve high capacity and high voltage of the entire cell of the hybrid capacitor using graphite as a positive electrode active material, and to achieve high energy density of the cell and high temperature durability performance. It is a thing.

In the present embodiment, the negative electrode using nitrogen-doped activated carbon using an aluminum foil coated with an amorphous carbon film is not limited to use in a hybrid capacitor. The negative electrode using the nitrogen-doped activated carbon can also be used as an electrode of an EDLC by using a nitrogen-doped activated carbon, a nitrogen-doped activated carbon, or the like as the positive electrode.

The effects of the present invention will be made more clear by the following examples. In addition, this invention is not limited to a following example, It can change suitably and can implement in the range with the effect.

Example 1
After weighing 10 g of activated carbon (trade name: YP50F) manufactured by Kureha Co., Ltd. as a negative electrode active material, it was set in a desktop rotary kiln apparatus manufactured by Takasago Kogyo Co., Ltd. The temperature of the activated carbon was raised to 800 ° C. while flowing nitrogen gas at a flow rate of 5 L / min. Next, this nitrogen gas was replaced with ammonia gas, and heat treatment was performed at 800 ° C. for 20 minutes while flowing at a flow rate of 5 L / min. Next, the ammonia gas was replaced with nitrogen gas, and the activated carbon cooled to room temperature was taken out of the apparatus. The nitrogen doping treatment time refers to a time during which the temperature is raised (temperature treatment) while maintaining the ammonia gas flow (heat treatment) while flowing ammonia gas, and the activated carbon is nitrogen-doped.
It was 2.1 at% when the N / C ratio (ratio of nitrogen to carbon) of the taken-out active carbon was computed by the combustion method. In addition, although it is a functional group of activated carbon which is doped with nitrogen, and the functional group exists only on the surface of activated carbon, it measures not the surface of activated carbon but the whole of activated carbon by the combustion method and calculates numerical values. It is a method.

Nitrogen doping treatment is performed by the above treatment, functional groups on the surface are substituted with nitrogen, and nitrogen-containing activated carbon (hereinafter sometimes referred to as "nitrogen-doped activated carbon"), acetylene black, polyvinylidene fluoride, weight percent concentration ( The paste obtained by weighing so that the ratio of wt%) becomes 80:10:10 and dissolving and mixing with N-methyl pyrrolidone is applied on a DLC coated aluminum foil (thickness 20 μm) using a doctor blade The coated material was used as a negative electrode.

The DLC coated aluminum foil (hereinafter sometimes referred to as “DLC coated aluminum foil”) is a current collector on the negative electrode side and corresponds to an aluminum material coated with an amorphous carbon film. As a manufacturing method of DLC coated aluminum foil, after removing the natural oxide film of the aluminum foil surface by argon sputtering to aluminum foil of purity 99.99%, mixed gas of methane, acetylene and nitrogen in the vicinity of the aluminum surface A discharge plasma was generated therein, and a negative bias voltage was applied to the aluminum material to form a DLC film. Here, the thickness of the DLC film on the aluminum foil coated (coated) with DLC was measured using a stylus type surface shape measuring instrument Dektak XT manufactured by BRUKER and found to be 135 nm.

Weigh the weight of the mixture (weight percent concentration (wt%)) to 80:10:10 as the positive electrode active material: Graphite (trade name: KS-6), acetylene black, polyvinylidene fluoride manufactured by Immers Japan Ltd. The paste obtained by dissolving and mixing with N-methylpyrrolidone is coated with a doctor blade on the same DLC-coated aluminum foil (20 μm in thickness) as that used for the negative electrode, and the positive electrode did.

Next, the positive electrode and the negative electrode punched into a disk shape having a diameter of 16 mm were vacuum dried at 150 ° C. for 24 hours, and then moved to an argon glove box. These are laminated through Nippon Paper Industries Co., Ltd. paper separator (trade name: TF40-30), 1M TEA-BF ₄ (tetraethylammonium tetrafluoride borate) in the electrolyte, SL + DMS (sulfolane (Sulfolane) in the solvent 0.1 mL of an electrolytic solution using (a) + (dimethyl sulfide) was added to prepare a 2032 coin cell in an argon glove box.

With respect to the obtained cell, charge and discharge in the range of 0.4 mA / cm ^{2 at} a current density of 0 V to 3.5 V in a thermostatic chamber at 25 ° C. using a charge and discharge test apparatus BTS 2004 manufactured by Nagano Ltd. A test was conducted to measure the discharge capacity before the constant current constant voltage continuous charge test. As to the upper limit of the applied voltage, in Example 1, Example 2 (described later) and Example 3 (described later) in which nitrogen-doped activated carbon was used as the negative electrode active material, the voltage could be applied up to 3.5 V, but nitrogen doping treatment was applied. In the comparative example 1 (after-mentioned) which used the activated carbon which is not carried out as a negative electrode active material, it measured by 2.5 V.
Next, using a charge / discharge test apparatus, a continuous charge test (constant current constant voltage continuous charge test) was performed for 2000 hours at a current density of 0.4 mA / cm ² and a voltage of 3.5 V in a constant temperature bath at 60 ° C. . Specifically, after charging is stopped for a predetermined time and the cell is transferred to a 25 ° C. constant temperature bath, the current density of 0.4 mA / cm ² , 0 V to 3 as in the charge / discharge test described above. The discharge capacity was obtained by performing the charge and discharge test five times at a charge voltage in the range of 5 V. Then, it returned to a 60 degreeC thermostat and restarted a continuous charge test, and the test was implemented until the total of continuous charge test time became 2000 hours.
The resulting discharge capacity improvement rates are shown in Table 1. The discharge capacity improvement rate refers to the charge time when the discharge capacity maintenance rate after the constant current constant voltage continuous charge test is 80% or less of the discharge capacity before the start of the constant current constant voltage continuous charge test as the life, as described later. The time (2050 hours) to reach the end of life in Comparative Example 1 was standardized to be 100. That is, the case where the activated carbon which was not subjected to the nitrogen doping treatment of Comparative Example 1 was used as the negative electrode active material was standardized as 100.

Example 2
The nitrogen doping treatment time to activated carbon (time after temperature rising and holding at 800 ° C while flowing ammonia gas) was changed from 5 minutes to 120 minutes, and the N / C ratio (ratio of nitrogen to carbon) of activated carbon was changed A 2032 coin cell similar to that of Example 1 was manufactured except for the above, and the same evaluation was performed.
The resulting discharge capacity improvement rate is shown in the graph of FIG. The horizontal axis of the graph shows the N / C ratio (ratio of nitrogen to carbon, at%: atomic composition percentage), and the vertical axis of the graph shows the discharge capacity improvement rate (%).

The effect of nitrogen doping starts to appear at 0.7 at%, the effect increases at 1.0 at% or more, and becomes constant at 2.0 at% or more. From this result, it is understood that the N / C ratio is optimal at 2.0 at% or more and 3.0 at% or less.

Example 3
A 2032-type coin cell similar to that of Example 1 was produced except that artificial graphite (trade name: MCMB 6-10) manufactured by Osaka Gas Chemical Co., Ltd. was used as a positive electrode active material, and the same evaluation was performed.

Comparative Example 1
A 2032 coin cell similar to that of Example 1 was produced except that activated carbon (trade name: MSP-20) not subjected to nitrogen doping treatment manufactured by Kansai Thermochemical Co., Ltd. was used as the negative electrode active material, and the same I made an evaluation.

Example 1 using nitrogen-doped activated carbon as the negative electrode active material was able to improve the discharge capacity retention rate by 8% as compared to Comparative Example 1 using activated carbon not subjected to nitrogen doping treatment as the negative electrode active material. In addition to using nitrogen-doped activated carbon as the negative electrode active material, the discharge capacity retention ratio is 7% as compared with Comparative Example 1 also in Example 3 in which artificial graphite containing no rhombohedral crystals is used as the positive electrode active material. It could be improved and in any case the effect of nitrogen doping could be confirmed.

Example 4
The same evaluation as in Example 1 was performed except that the range of the charging voltage in the 2000-hour continuous charging test (constant current constant voltage continuous charging test) was 3.6 V to 4.0 V.
The result of having calculated energy (Wh) from the discharge capacity and average discharge voltage which were obtained as a result is shown in Table 2. In Table 2, the value which normalized the energy of Example 4 by the comparative example 1 was shown. Under the present circumstances, the result of comparative example 1 was standardized as 100.
Moreover, the discharge capacity improvement rate obtained as a result of a continuous charging test (constant current constant voltage continuous charging test) is shown in FIG. The horizontal axis of the graph indicates the charge voltage (V) in the continuous charge test, and the vertical axis of the graph indicates the discharge capacity improvement rate (%).

As an effect of nitrogen doping, a gas is generated by the decomposition of the electrolyte, the capacity is reduced with the decrease of the specific surface area by covering the activated carbon surface with the decomposition product of the electrolyte, or the deterioration due to the decomposition of the activated carbon itself is reduced Even if the potential is lowered, it is expected to be small, and the deterioration is remarkable particularly at high temperatures. Therefore, evaluation was made to change the charging voltage in a 2000-hour continuous charging test at 60 ° C.
As a result, as shown in FIG. 2, the rate of discharge capacity improvement with respect to Comparative Example 1 in which activated carbon without nitrogen doping treatment was used as the negative electrode active material up to a charge voltage of 3.8 V increased. Thereafter, the discharge capacity decreased toward 4.0 V, but the discharge capacity retention rate higher than that of Comparative Example 1 is shown.
From these results, it was found that even if the reduction potential is enlarged in the ordinary direction by nitrogen doping, the withstand voltage is improved by the effect of suppressing the decomposition of the electrolyte and the decomposition of the activated carbon itself. Based on the results in FIG. 2, it is understood from the results in Table 2 that the withstand voltage can be increased to 3.8 V at which the discharge capacity retention rate is maximum.

Claims (3)

A hybrid capacitor in which the time during which a discharge capacity retention rate can maintain 80% or more in a constant current constant voltage continuous charge test at 60 ° C. and 3.5 V is 1000 hours or more,
The current collectors on the positive electrode side and the negative electrode side are aluminum materials, and
The aluminum material is coated with an amorphous carbon film,
The thickness of the amorphous carbon film is 60 nm or more and 300 nm or less,
The positive electrode contains graphite as a positive electrode active material,
The negative electrode contains activated carbon as a negative electrode active material,
The said activated carbon contains nitrogen, The hybrid capacitor characterized by the above-mentioned. The hybrid capacitor according to claim 1, wherein the activated carbon is subjected to a nitrogen doping process. The hybrid capacitor according to claim 1, wherein the activated carbon has a nitrogen to carbon ratio of 1.0 at% or more and 4.0 at% or less.

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