TW201917750A - Hybrid capacitor - Google Patents

Abstract

A hybrid capacitor is able to maintain a discharge capacity retention rate of at least 80% or more for at least 1000 hours or more in a constant current-constant voltage continuous discharge test at 3.5 V and 60 DEG C, and includes: a positive electrode side current collector and a negative electrode side current collector both made of aluminum material, where the aluminum material is coated with an amorphous carbon film having a thickness ranging from 60 nm and 300 nm; a positive electrode including graphite as a positive electrode active material; and a negative electrode including an activated carbon as a negative electrode active material; and the activated carbon including nitrogen.

Description

   Hybrid capacitor   

The invention relates to a hybrid capacitor.

This application claims priority based on Japanese Patent Application No. 2017-139523 filed in Japan on July 18, 2017, the contents of which are incorporated herein by reference.

Conventionally, as a technology for storing electric energy, an electric double-layer capacitor (for example, refer to Patent Document 1) or a secondary battery is known. Electric double layer capacitors (EDLCs) have far better life, safety and output density than secondary batteries. However, the electric double-layer capacitor has a lower energy density (volume energy density) than the secondary battery, which is a problem.

Here, the energy (E) stored in the electric double-layer capacitor is represented by the capacitance (C) and the applied voltage (V) of the capacitor as E = 1/2 × C × V ² . The square of the voltage is proportional. Therefore, in order to improve the energy density of the electric double-layer capacitor, a technique for increasing the electrostatic capacity or the applied voltage of the electric double-layer capacitor is proposed.

As a technique for increasing the electrostatic capacity of an electric double-layer capacitor, a technique of increasing the specific surface area of activated carbon constituting an electrode of the electric double-layer capacitor is known. Today, known activated carbon has a specific surface area of 1000 m ² / g to 2500 m ² / g. An electric double-layer capacitor using such activated carbon as an electrode uses an organic electrolytic solution in which a quaternary ammonium salt is dissolved in an organic solvent, or an aqueous electrolytic solution such as sulfuric acid as the electrolytic solution.

Since the organic electrolyte can be used in a wide range of voltages, the applied voltage can be increased, so the energy density can be increased.

Lithium-ion capacitors are known as a technique that uses the principle of an electric double-layer capacitor to increase the voltage applied to the electric double-layer capacitor. The use of graphite or carbon that can insert and desorb lithium ions in the negative electrode and the use of activated carbon equivalent to the electrode material of an electric double-layer capacitor that can absorb and desorb electrolyte ions is called a lithium ion capacitor. In addition, the use of activated carbon equivalent to the electrode material of an electric double-layer capacitor for either the positive electrode or the negative electrode, and the use of a metal oxide or a conductive polymer as the electrode for the Faraday reaction at the other electrode are called hybrid capacitors. (Hybrid capacitor). Regarding lithium ion capacitors, the negative electrode in the electrodes constituting the electric double-layer capacitor is composed of graphite or carbon black, etc. as the negative electrode material of the lithium ion secondary battery, and lithium ions are embedded in the graphite or carbon black. Lithium-ion capacitors are characterized by a larger applied voltage than ordinary electric double-layer capacitors, that is, those whose poles are made of activated carbon.

However, when graphite is used for the electrode, it is impossible to use propylene carbonate as the electrolytic solution. When graphite is used as an electrode, propylene carbonate is electrolyzed, and a decomposition product of propylene carbonate is attached to the graphite surface, thereby reducing the reversibility of lithium ions. Propylene carbonate is a solvent that can operate at low temperatures. When propylene carbonate is used for an electric double-layer capacitor, the electric double-layer capacitor can also operate at -40 ° C. Therefore, in a lithium ion capacitor, hard carbon that is not easily decomposed by propylene carbonate is used for the electrode system. However, compared with graphite, hard carbon has a lower capacity per unit volume and a lower voltage than graphite (which becomes an expensive potential). Therefore, lithium ion capacitors have problems such as a decrease in energy density.

When low-temperature characteristics are important, it is difficult to further increase the energy density of lithium ion capacitors in which it is difficult to use high-capacity graphite for the negative electrode. In addition, in lithium-ion capacitors, copper foil is used for the current collector in the same way as the negative electrode of a lithium-ion secondary battery. When an over-discharge of 2 V or less is performed, copper will dissolve out and cause a short circuit, resulting in a problem of reduced discharge capacity . Therefore, compared with an electric double-layer capacitor capable of discharging to 0 V, there is a problem that the method of using the lithium ion capacitor is limited.

As a capacitor of a new concept, a capacitor has been developed using graphite as a positive electrode active material instead of activated carbon, and a reaction in which graphite ions are intercalated and detached from electrolyte ions (for example, refer to Patent Document 2). Patent Document 2 describes that a conventional electric double-layer capacitor using activated carbon for a positive electrode active material will decompose the electrolytic solution to generate a gas if a voltage exceeding 2.5 V is applied to the positive electrode. The electric double-layer capacitor does not decompose the electrolyte even at a charging voltage of 3.5V, and can operate at a higher voltage than the conventional electric double-layer capacitor using activated carbon as the positive electrode active material. Regarding the cycle characteristics or low-temperature characteristics, the 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. The difference in the decomposition of the electrolyte is caused by such a large difference in specific surface area.

In a capacitor of a new concept using graphite as a positive electrode active material, practical applications are hindered due to insufficient durability, but it has been found that by using an aluminum material covered with an amorphous carbon film as a current collector technology (see Patent Literature) 3), can improve the high temperature durability to a practical level. This new concept capacitor is a capacitor that uses a reaction in which the electrolyte ions are intercalated between the graphite layers on the positive electrode, and this is not strictly an electric double layer capacitor, but in Patent Document 3, it is in a broad sense It is called an electric double-layer capacitor.

Here, the durability test is generally performed by an accelerated test (high-temperature durability test, charge-discharge cycle test) to increase the temperature. This test can be performed according to the method of "durability (high-temperature continuous rated voltage application) test" described in JIS D 1401: 2009. If the temperature is increased from room temperature by 10 ° C, the deterioration rate will be approximately doubled. For a high-temperature durability test, for example, it is maintained (continuously charged) in a constant temperature bath at 60 ° C. for a predetermined voltage (3V or more in the present invention) for 2000 hours, and then returned to room temperature for charge and discharge, and a test for measuring the discharge capacity at this time. After this high-temperature durability test, it is preferable that the discharge capacity maintenance ratio with respect to the initial discharge capacity is 80% or more.

Patent Document 4 discloses that the withstand voltage of EDLC can be increased by doping nitrogen in activated carbon. In addition, Non-Patent Document 1 discloses doping nitrogen in activated carbon as a catalyst for removing SO ₂ in exhaust gas.

[Prior technical literature]

[Patent Literature]

Patent Document 1: Japanese Patent Application Laid-Open No. 2011-046584

Patent Document 2: Japanese Patent Application Laid-Open No. 2010-040180

Patent Document 3: International Publication No. 2017/216960

Patent Document 4: Japanese Patent Laid-Open No. 2013-026484

[Non-patent literature]

Non-Patent Document 1: Carbon 41 (2003) 1925-1932

Hybrid capacitors using graphite as a positive electrode active material and activated carbon as a negative electrode active material need to have a higher energy density. In this case, since the capacity of the activated carbon for the negative electrode active material is smaller than the capacity of the graphite for the positive electrode active material, the capacity at the time of charge and discharge of the battery depends on the capacity of the negative electrode. In order to increase the capacity of the negative electrode, it is effective to reduce the reduction potential of the negative electrode, but when the reduction potential is too low, it has: the generation of gas due to the decomposition of the electrolyte, or the specific surface area is reduced because the surface of the activated carbon is covered by the product of the electrolyte decomposition And problems associated with a decrease in capacity or degradation due to decomposition of activated carbon itself.

In view of the above circumstances, an object of the present invention is to provide a hybrid capacitor having high energy density and excellent high-temperature durability by realizing high capacity and high voltage.

The inventors have repeatedly researched and solved the above problems. As a result, in a hybrid capacitor using graphite as a positive electrode active material and activated carbon as a negative electrode active material, a negative electrode is realized by reducing the reduction potential of the negative electrode by using a negative electrode active material doped with nitrogen. High capacity and high voltage. As a result, it has been found that it is possible to increase the capacity and voltage of the entire battery of the hybrid capacitor to increase the energy density of the battery and improve the high-temperature durability. At this time, if the etched aluminum widely used in EDLC is used as a current collector, the etched aluminum may corrode below a certain negative electrode reduction potential, and it has also been confirmed that it is preferable to use a combination of aluminum material covered with an amorphous carbon film .

In order to solve the above problems, the present invention provides the following means.

(1) One aspect of the present invention relates to a hybrid capacitor that is capable of maintaining a discharge capacity retention rate of 80% or more for 1000 hours or more in a constant-charge constant-voltage and constant-voltage test at 60 ° C and 3.5V. The hybrid capacitor includes: a positive-side current collector and a negative-side current collector, both of which are made of aluminum material, wherein the aluminum material is covered by an amorphous carbon film, and the thickness of the amorphous carbon film is Above 60 nm and below 300 nm; a positive electrode including graphite as a positive electrode active material; and a negative electrode including activated carbon as a negative electrode active material, and the active carbon includes nitrogen.

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

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

According to the present invention, a hybrid capacitor having high energy density and excellent high-temperature durability can be provided.

FIG. 1 is a graph showing the discharge characteristics (discharge capacity improvement rate when a constant-current constant-voltage continuous charge test is performed at 60 ° C. when the nitrogen doping treatment time is changed) of the hybrid capacitor fabricated in Example 2.

Fig. 2 shows the discharge characteristics (discharge capacity improvement rate during a constant-current constant-voltage continuous charging test at 60 ° C when the charging voltage changes in a high-voltage region when the charging voltage changes in a high-voltage region) chart.

Hereinafter, the structure of a hybrid capacitor to which the present invention is applied will be described using drawings. In the drawings used in the following description, in order to easily understand the features, there are cases where the feature portions are enlarged for convenience, and the size ratio of each component is not always the same as the actual one. In addition, the materials, dimensions, and the like exemplified in the following description are merely examples, and the present invention is not limited thereto, and can be implemented by appropriately changing within a range in which effects are exhibited.

A hybrid capacitor according to an embodiment of the present invention is a hybrid capacitor capable of maintaining a discharge capacity retention rate of 80% or more for 1,000 hours or more in a constant-current and constant-voltage constant-charge test at 60 ° C and 3.5V. It is equipped with a positive electrode, a negative electrode, an electrolytic solution and a separator. A positive electrode side current collector and a negative electrode side current collector are made of aluminum material, the aluminum material is covered with an amorphous carbon thin film, and the thickness of the amorphous carbon thin 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 uses activated carbon as a negative electrode active material, a nitrogen doping treatment is applied to the activated carbon, the functional groups on the surface of the activated carbon are replaced by nitrogen, and the activated carbon contains nitrogen.

The positive electrode is formed by forming a positive electrode active material layer on a current collector (a 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 a conductive material in an amount as needed to the positive electrode current collector and drying it.

Binders, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene-propylene-diene rubber, styrene-butadiene, acrylic, olefin-based, carboxymethyl cellulose can be used (CMC) is a single or a mixture of two or more.

The conductive material is not particularly limited as long as the conductivity of the positive electrode active material is good, and a known conductive material can be used. For example, carbon black, carbon fiber (including carbon nanotube (CNT), VGCF (registered trademark), etc., and not limited to carbon nanotube) can be used.

The positive electrode current collector may be an aluminum material with improved corrosion resistance, such as an aluminum material coated with an amorphous carbon thin film. The aluminum material may be covered only with an amorphous carbon film, or a conductive carbon layer may be provided between the amorphous carbon film and the positive electrode active material.

As the base material, the aluminum material used for general current collector applications can be used. The shape of aluminum can be made into foil, sheet, film, net, etc. As the current collector, aluminum foil is preferably used. In addition to this aluminum material, etched aluminum described later can be used in addition to flat ones.

When the aluminum material is foil, sheet, or film, its thickness is not particularly limited, but when the battery itself has the same size, the thinner the more active material that can be enclosed in the battery case, but the disadvantage of reduced strength , Need to choose the appropriate thickness. The actual thickness is preferably 10 μm to 40 μm , and more preferably 15 μm to 30 μm . When the thickness is less than 10 μm , the aluminum material may be cracked or cracked in the step of roughening the surface of the aluminum material or in other manufacturing steps.

An aluminum material coated with an amorphous carbon film may also be etched aluminum.

Etching aluminum is a roughening process by etching. Generally, etching can be performed by immersing in an acid solution such as hydrochloric acid (chemical etching), or electrolytically (electrochemical etching) using aluminum as an anode in an acid solution such as hydrochloric acid. In electrochemical etching, the shape of the etching is different because of the current waveform, the composition of the solution, and the temperature during electrolysis, so it can be selected from the viewpoint of capacitor performance.

The aluminum material can be used on either a surface with or without a passivation layer. When a passivation film of a natural oxide film is formed on the aluminum material, an amorphous carbon passivation layer may be disposed on the natural oxide film, or, for example, the natural oxide layer may be removed by argon sputtering and then provided.

The natural oxide film on aluminum is a passive film, which has the advantage of not being easily eroded by the electrolyte. On the other hand, it is also related to the increase of the collector resistance. Therefore, from the viewpoint of reducing the collector resistance, An oxide film is preferred.

In this specification, an amorphous carbon thin film refers to an amorphous carbon film or a hydrogenated carbon film, and includes a diamond-like carbon (DLC) film, a carbon hard film, an amorphous carbon (aC) film, and a hydrogenated amorphous carbon. (aC: H) film and the like. As a method for forming the amorphous carbon thin film, a known method such as a plasma CVD method using a hydrocarbon-based gas, a sputtering evaporation method, an ion plating method, and a vacuum arc evaporation method can be used. In addition, the amorphous carbon thin film preferably has a degree of conductivity that functions as a current collector.

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

The thickness of the amorphous carbon thin film is preferably 60 nm or more and 300 nm or less. If the thickness of the amorphous carbon thin film is less than 60 nm, the thickness of the amorphous carbon thin film is too small, and the corrosion of the current collector in the constant current and constant voltage continuous charging test cannot be sufficiently suppressed. If it is too thick, the amorphous carbon thin film becomes a resistor, and the resistance to the active material layer increases. Therefore, it is preferable to appropriately select a suitable thickness. The thickness of the amorphous carbon thin film is preferably 80 nm or more and 300 nm or less, and more preferably 120 nm or more and 300 nm or less. When an amorphous carbon thin film is formed by a plasma CVD method using a hydrocarbon-based gas, the thickness of the amorphous carbon thin film can be controlled by the energy injected into the aluminum material, specifically, by applying a voltage, a time, and a temperature.

The positive electrode active material used in the hybrid capacitor of this embodiment is one containing graphite. As the graphite, any of artificial graphite and natural graphite can be used. In addition, natural graphite is known to be scaly and earthy. Natural graphite is obtained by pulverizing the excavated raw ore and repeatedly performing a beneficiation method called a suspension beneficiation method. The artificial graphite is produced, for example, by a graphitization step of firing a carbon material at a high temperature. More specifically, for example, a binder such as pitch can be added to the raw coke for molding, and the firing can be performed by heating to about 1300 ° C, then impregnating the primary fired product with the asphalt resin, and then performing the high temperature approaching 3000 ° C. It is obtained by secondary firing. Alternatively, a coating having carbon on the surface of the graphite particles may be used.

The crystal structure of graphite can be roughly divided into a hexagonal crystal having a layer structure composed of ABAB and a rhombohedral crystal having a layer structure composed of ABCABC. Depending on the conditions, these structures are singular or mixed, but any crystal structure or mixed state can be used. For example, the erythrite graphite and carbon Japan used in the examples described later ( ‧ ‧ ) The ratio of rhombohedral crystals of graphite made by KS-6 (trade name) of the joint-stock company is 26%. The ratio is 0%.

The graphite used in this embodiment has a different mechanism for displaying electrostatic capacity than the activated carbon used in the conventional EDLC. In the case of activated carbon, with a large specific surface area, electrolyte ions are adsorbed and desorbed on its surface to generate an electrostatic capacity. On the other hand, in the case of graphite, the electrostatic capacity is exhibited by intercalation and deintercalation (intercalation-deintercalation) of anions which are electrolyte ions between the graphite layers. Due to this difference, the hybrid capacitor using graphite according to the present embodiment is referred to as an electric double-layer capacitor in the broad sense in Patent Document 3, but is different from an EDLC using activated carbon having an electric double layer.

Since the current collector used in this embodiment has an amorphous carbon film on the surface of the aluminum material, the aluminum material is prevented from contacting the electrolyte, and the current collector can be prevented from being corroded by the electrolyte.

The negative electrode is formed by forming a negative electrode active material layer on a current collector (negative electrode side current collector).

The negative electrode active material layer can be formed by applying a slurry negative electrode material mainly containing a negative electrode active material, a binder, and a conductive material in an amount as needed, to the negative electrode current collector.

As the negative electrode active material, activated carbon of a carbonaceous material capable of desorbing and desorbing cations of electrolyte ions can be used. The nitrogen contained in the activated carbon is preferably doped in the activated carbon by a nitrogen doping treatment.

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

Binders, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene-propylene-diene rubber, styrene-butadiene, acrylic, olefin-based, carboxymethyl cellulose can be used (CMC) is a single or a mixture of two or more.

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

For the nitrogen-doping treatment of the activated carbon, a known treatment may be used, but it may also be performed by, for example, exposing the surface of the negative electrode to ammonia gas, or exposing the negative electrode to a negative electrode as disclosed in Patent Document 4. The surface is exposed to ammonium carbamate with high temperature treatment, or is synthesized by adding a material as a nitrogen source to a raw material when producing activated carbon.

As the nitrogen-doping treatment device, an electric furnace or a rotary kiln device for uniformly contacting activated carbon with activated carbon is suitable. The processing temperature is preferably in the range of 600 ° C to 900 ° C. If the processing temperature is too low, the nitrogen doping reaction is difficult to proceed. Conversely, if the temperature exceeds 900 ° C, the pores of the activated carbon itself will shrink, and the specific surface area of the activated carbon will decrease, resulting in a decrease in capacity, which is not good. If the processing temperature is 900 ° C or lower, there is no concern that the pores will shrink as they approach the upper limit temperature of the activated carbon production process.

In nitrogen doping, the ratio of nitrogen to carbon (N / C ratio) to be doped varies depending on the 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, and particularly preferably 2.0 at % To 3.0at%. If the doping amount of nitrogen is too small, the effect of reducing the reduction potential is reduced. On the other hand, if it is too much, the capacity as an activated carbon decreases, and therefore, it is used under optimal conditions in the above range in combination with a graphite positive electrode. The ratio of nitrogen to carbon (N / C ratio) can be measured by a combustion method or an X-ray photoelectron spectroscopy (XPS).

In the combustion method, by burning a sample, nitrogen in the sample is vaporized to form NOx gas, and then it is reduced to N ₂ gas. In addition, carbon is vaporized to CO gas or CO ₂ gas, and the obtained N ₂ gas and CO The gas or CO ₂ gas was quantified by chromatography (detector: TCD), respectively. 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.

The electrolytic solution may be an organic electrolytic solution using an organic solvent. The electrolyte contains electrolyte ions that can be desorbed and attached to the electrode. The type of electrolyte ions is preferably as small as possible. Specifically, an ammonium salt or a phosphorus salt, or an ionic liquid, a lithium salt, or the like can be used.

As the ammonium salt, tetraethylammonium (TEA) salt, triethylammonium (TEMA) salt, or the like can be used. As the phosphorus salt, a spiro compound having two five-membered rings can be used.

The type of the ionic liquid is not particularly limited, and considering the ease of movement of electrolyte ions, a material having as low a viscosity as possible or high conductivity (conductivity) is preferred. Specific examples of the cation constituting the ionic liquid include an imidazolium ion and a pyridinium ion. Examples of the imidazolium ion include 1-ethyl-3-methylimidazolium (EMIm) ion, and 1-methyl-1-propylpyrrolinium (1-methyl-1- propyl-pyrrolizinium (MPPy) ion, 1-methyl-1-propyl-piperizinium (MPPi) ion, and the like. In addition, lithium tetrafluoroborate LiBF ₄ , lithium hexafluorophosphate LiPF ₆ and the like can be used as the lithium salt.

Examples of the pyridinium ion include a 1-ethylpyridium ion, and a 1-buthylpyridnium ion.

Examples of the anion constituting the ionic liquid include BF ₄ ion, PF ₆ ion, [(CF ₃ SO ₂ ) ₂ N] ion, FSI (bis (fluorosulfonyl) imide; bis (fluorosulfonyl) imide) ion, and TFSI ( Bis (trifluorosulfonyl) imine; bis (trifluorosulfonyl) imide).

As the solvent, acetonitrile, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethylphosphonium, ethylisopropylphosphonium, ethyl carbonate, fluoroethylene carbonate, and gamma butyrolactone can be used. , Cyclobutane, N, N-dimethylformamide, or dimethylmethane, etc., alone or in a mixed solvent.

The separator is preferably a cellulose-based paper separator, a glass fiber separator, or a microporous film such as polyethylene or polypropylene, for reasons such as preventing the positive and negative electrodes from being short-circuited or ensuring the electrolyte storage stability.

As described above, in the hybrid capacitor according to the embodiment, a nitrogen-containing activated carbon is used as a negative electrode active material, and it is preferable to use a nitrogen doping treatment to use a nitrogen-doped activated carbon. When the activated carbon is subjected to a nitrogen doping treatment, the functional groups present on the surface of the activated carbon are replaced with nitrogen. For example, when using an activated carbon having a functional group on the surface as a negative electrode active material, if the electrode potential in the direction of a low potential (reduction side) is increased (in the case of a battery, the direction in which the battery voltage increases), The functional group reacts with the electrolyte solution to generate an organic decomposition product or a decomposition gas. When the generated organic decomposition product accumulates on the surface of the activated carbon and 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. In contrast, when nitrogen doping is performed, even if the electrode potential of the negative electrode increases in the direction of a low potential (reduction side), since the functional group that reacts with the electrolytic solution is replaced by nitrogen, the generation of a decomposition gas or an organic decomposition product can be suppressed. . Therefore, regarding the hybrid capacitor of the present embodiment, a high capacity of the negative electrode is achieved by using a nitrogen-containing activated carbon in which a functional group on the surface is replaced with nitrogen as a negative electrode active material with a nitrogen doping treatment to reduce the reduction potential of the negative electrode. In order to increase the capacity and voltage of the entire battery of the hybrid capacitor using graphite as a positive electrode active material, the purpose is to increase the capacity and voltage of the battery in order to achieve high energy density of the battery and improve high-temperature durability.

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

    (Example)    

In the following, the effects of the present invention will be made clearer by examples. It should be noted that the present invention is not limited to the following embodiments, but may be implemented by appropriate changes within a range in which effects are exhibited.

[Example 1]

10 grams of activated carbon (trade name: YP50F) manufactured by Kureha was weighed out as a negative electrode active material, and then placed in a benchtop rotary kiln device manufactured by Takasago Industrial Co., Ltd. while flowing nitrogen at a flow rate of 5 liters / minute while The activated carbon was heated to 800 ° C. Next, this nitrogen gas was replaced with ammonia gas, and a heat treatment was performed at 800 ° C. for 20 minutes while flowing at a flow rate of 5 liters / minute. Next, this ammonia gas was replaced with nitrogen gas, and the activated carbon cooled to room temperature was taken out of the apparatus. It is to be noted that the nitrogen doping treatment time refers to a time in which an ammonia gas flow is allowed to pass through the activated carbon while maintaining the temperature at 800 ° C. (heat treatment) after the temperature rise.

According to the combustion method, the N / C ratio (ratio of nitrogen to carbon) of the extracted activated carbon was calculated to be 2.1 at%. It should be mentioned that nitrogen is doped with activated carbon functional groups. Functional groups exist only on the surface of activated carbon, but in the combustion method, the numerical calculation method is based on the measurement of the entire activated carbon as opposed to the activated carbon surface. As an object.

Nitrogen-containing activated carbon (hereinafter sometimes referred to as "nitrogen-doped activated carbon"), acetylene black, and polyvinylidene fluoride were replaced with nitrogen by replacing the functional groups on the surface with nitrogen by nitrogen doping treatment by the above treatment: After weighing at a ratio of 10:10 weight percent concentration (wt%), the slurry obtained by dissolving and mixing with N-methylpyrrolidone was coated on a DLC-coated aluminum foil (thickness: 20 μm) using a doctor blade as a negative electrode.

The above-mentioned DLC-coated aluminum foil (hereinafter sometimes referred to as "DLC-coated aluminum foil") is a negative-electrode-side current collector and corresponds to an aluminum material covered with an amorphous carbon thin film. The manufacturing method of DLC-coated aluminum foil is to remove the natural oxide film on the surface of the aluminum foil by argon sputtering on the aluminum foil with a purity of 99.99%, and then discharge the plasma near the aluminum surface in a mixed gas of methane, acetylene and nitrogen to generate plasma The DLC film is generated by applying a negative pulse voltage to the aluminum material. Here, the thickness of the DLC film on the DLC-coated aluminum foil was measured with a probe type surface shape measuring device DektakXT made by BRUKER, and the result was 135 nm.

Will Iris graphite and carbon Japan ( ‧ ‧ ) Graphite (commercial name: KS-6), acetylene black, and polyvinylidene fluoride manufactured by the joint stock company are weighed at a weight percentage concentration (wt%) of 80:10:10, and dissolved and mixed with N-methylpyrrolidone A positive electrode active material slurry was obtained, and the slurry was applied to a DLC-coated aluminum foil (thickness: 20 μm ), which was the same as the negative electrode, using a doctor blade to prepare a positive electrode.

Next, the positive electrode and the negative electrode were punched into a disk shape with a diameter of 16 mm, and vacuum-dried at 150 ° C. for 24 hours, and then moved to an argon glove working box. These were laminated through a paper separator manufactured by Nippon Kogyo Paper Industry Co., Ltd. and 0.1 mL of an electrolytic solution (wherein the electrolytic solution used 1M TEA-BF ₄ (tetraethylammonium tetrafluoroborate) as an electrolyte, and SL + DMS (Sulfolane + dimethyl sulfide) as a solvent), and a 2032 type button battery was produced in an argon glove working box.

For the obtained battery, a charge-discharge test device BTS2004 made by Nagano Co., Ltd. was used in a constant-temperature bath at 20 ° C to conduct a charge-discharge test at a current density of 0.4 mA / cm ² in a voltage range of 0 to 3.5 V. Discharge capacity before constant current and constant voltage charging test. In addition, the upper limit of the applied voltage can be applied to 3.5V in Example 1, Example 2 (described later), and Example 3 (described later) using nitrogen-doped activated carbon as a negative electrode active material, but In Comparative Example 1 (which will be described later) using an activated carbon that has not been subjected to a nitrogen doping treatment as a negative electrode active material, it was measured up to 2.5V.

Next, a continuous charge test (constant current and constant voltage continuous charge test) was performed in a constant temperature bath at 60 ° C. with a charge current of 0.4 mA / cm ² and a voltage of 3.5 V using a charge and discharge test device. Specifically, after charging is stopped for a predetermined time during the charging period, and the battery is transferred to a 25 ° C constant temperature bath, the current density of 0.4 mA / cm ² and the charging voltage in the range of 0V to 3.5V are similar to the above-mentioned charge-discharge test. Next, 5 charge and discharge tests were performed to obtain the discharge capacity. After that, return to the thermostat at 60 ° C and restart the continuous charging test, and perform the test until the total of the continuous charging test time reaches 2000 hours.

The resulting improvement in discharge capacity is shown in Table 1. The improvement rate of the discharge capacity is defined as the life of the charge time after the constant capacity constant-voltage constant-charge test is 80% or less of the discharge capacity before the constant-current constant-voltage continuous charge test is started. The life time (2050 hours) in Comparative Example 1 was normalized to 100. That is, Comparative Example 1 in a case where an activated carbon that has not been subjected to a nitrogen doping treatment is used as a negative electrode active material is standardized to 100.

[Example 2]

In addition to changing the nitrogen doping treatment time of the activated carbon (the time to maintain the ammonia gas flow after heating and maintain it at 800 ° C) from 5 minutes to 120 minutes, the N / C ratio of activated carbon (nitrogen to carbon Except for the ratio), the same 2032-type button battery as in Example 1 was produced, and the same evaluation was performed.

The obtained discharge capacity improvement rate is shown in the graph in FIG. 1. The abscissa of the graph represents the N / C ratio (the ratio of nitrogen to carbon, at%: atomic composition percentage), and the ordinate of the graph represents the discharge capacity improvement rate (%).

The effect of nitrogen doping began to appear at 0.7 at%, and the effect increased when it was above 1.0 at%, and remained constant above 2.0 at%. From this result, it is found that the N / C ratio is most preferably 2.0 at% or more and 3.0 at% or less.

[Example 3]

A 2032-type button battery was prepared and evaluated in the same manner as in Example 1, except that artificial graphite (trade name: MCMB6-10) manufactured by Osaka Gas Chemical Co., Ltd. was used as the positive electrode active material.

[Comparative Example 1]

A 2032 type button battery similar to Example 1 was prepared except that activated carbon (product name: MSP-20) without nitrogen doping treatment manufactured by Kansai Thermochemical Co., Ltd. was used as a negative electrode active material, and the same was performed. Evaluation.

Compared with Comparative Example 1 using activated carbon without nitrogen doping treatment as the negative electrode active material, Example 1 using nitrogen doped activated carbon as the negative electrode active material improved the discharge capacity retention rate by 8%. Furthermore, in addition to the use of nitrogen-doped activated carbon as the negative electrode active material, Example 3, which uses artificial graphite without rhombic crystals as the positive electrode active material, shows that the discharge capacity maintenance rate is increased by 7%, so in any case The effect of the nitrogen doping process can be confirmed.

[Example 4]

The same evaluation as in Example 1 was performed except that the range of the charging voltage for the 2000-hour continuous charging test (constant current and constant voltage continuous charging test) was 3.6V to 4.0V.

From the results, the energy (Wh) was calculated from the obtained discharge capacity and the average discharge voltage, and the results are shown in Table 2. Table 2 shows values obtained by normalizing the energy of Example 4 by Comparative Example 1. At this time, the result of Comparative Example 1 was normalized to 100.

The improvement rate of the discharge capacity obtained by the continuous charge test (constant current and constant voltage continuous charge test) is shown in FIG. 2. The horizontal axis of the graph represents the charging voltage (V) in the continuous charging test, and the vertical axis of the graph represents the improvement rate (%) of the discharge capacity.

As the effect of nitrogen doping, it is expected to reduce the following cases: the case of gas generated due to the decomposition of the electrolytic solution; the case where the surface of the activated carbon is covered with the decomposition products of the electrolyte to reduce the specific surface area and the capacity; or the activated carbon The degradation caused by the decomposition itself also reduces the reduction potential. In particular, since deterioration at high temperatures is significant, the change in charging voltage was evaluated in a 2000-hour continuous charging test at 60 ° C.

As a result, as shown in FIG. 2, the improvement rate of the discharge capacity was increased as compared with Comparative Example 1 in which activated carbon that was not doped with nitrogen was used as a negative electrode active material, until the charging voltage reached 3.8V. After that, although the discharge capacity was reduced until the charging voltage reached 4.0 V, the discharge capacity retention rate was still higher than that of Comparative Example 1.

Based on these results, it has been found that even if the reduction potential develops in a decreasing direction by nitrogen doping, it is effective to increase the withstand voltage by suppressing the decomposition of the electrolytic solution or suppressing the decomposition of the activated carbon itself. Based on Figure 2 and based on the results in Table 2, it was found that the withstand voltage can be increased to 3.8 V that maximizes the discharge capacity retention rate.

Claims (3)

    A hybrid capacitor capable of maintaining a discharge capacity retention rate of more than 80% for more than 1000 hours in a constant-current and constant-voltage continuous charging test at 60 ° C and 3.5V. The hybrid capacitor includes a positive-side current collector and a negative electrode. The side current collectors are all made of aluminum; the aluminum is covered by an amorphous carbon film; the thickness of the amorphous carbon film is more than 60 nm and less than 300 nm; a positive electrode includes graphite As a positive electrode active material; and a negative electrode, including activated carbon as a negative electrode active material; wherein the active carbon includes nitrogen.         The hybrid capacitor according to item 1 of the patent application scope, wherein a nitrogen doping treatment is applied to the activated carbon.         The hybrid capacitor according to item 1 or 2 of the scope of patent application, wherein the ratio of nitrogen to carbon in the activated carbon is 1.0at% or more and 4.0at% or less.