JP2012146763A - Electrochemical capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrochemical capacitor having high output characteristics and high energy characteristics which could not be attained until now.SOLUTION: A titanium source such as titanium alkoxide and a lithium source such as lithium acetate are used as the starting material, predetermined carbon is added during a reaction process, and a dispersion of the precursor of lithium titanate nanoparticles and carbon is produced by mechanochemical reaction. A carbon having oxygen deficiency of 5 to 20 nm, and carrying nitrogen-doped lithium titanate nanoparticles with high dispersion is produced by heating the dispersion in a nitrogen atmosphere. An electrochemical capacitor is configured by using an electrode containing that carbon as a negative electrode, and using alkali activation active carbon as a positive electrode.

Description

  The present invention relates to an electrochemical capacitor using lithium titanate nanoparticles as a negative electrode and a polarizable electrode as a positive electrode.

  Conventionally, lithium titanate has been used as a lithium storage / release active material for electrochemical element negative electrodes. Further, a wet method (see Patent Document 1) is known as a method for forming lithium titanate particles having excellent crystallinity, but there is a problem that output characteristics are not sufficient.

  Therefore, as shown in Patent Document 2, the present applicants presented a method for promoting and controlling the reaction in an unprecedented liquid phase reaction, and further produced lithium titanate nanoparticles prepared using this reaction method. A patent application was filed for an electrochemical capacitor using an electrode containing carbon that was highly dispersed and supported.

JP 2000-36441 A JP 2008-270795 A

  However, in recent years, there has been a demand for an electrochemical capacitor having a higher energy density than the conventional electrochemical capacitor as described above.

  The present invention has been proposed in order to solve the above-described problems of the prior art, and an object thereof is to provide an electrochemical capacitor having a higher energy density.

  In order to achieve the above object, the electrochemical capacitor of the present invention uses an electrode containing carbon having oxygen deficiency and highly dispersed support of nitrogen-doped lithium titanate nanoparticles as an anode, and an alkali-activated activated carbon. Is used for the positive electrode.

  According to the electrochemical capacitor of the present invention having the above-described configuration, the oxygen deficiency portion in the carbon used for the negative electrode becomes a lithium adsorption / desorption portion, and further, nitrogen is doped to increase the electrical conductivity and output characteristics. The energy density is increased by using a high-capacity activated carbon as the positive electrode material. As a result, an electrochemical capacitor having unprecedented high output characteristics and high energy density can be realized.

  Further, the electrochemical capacitor of the present invention is the lithium titanate nanoparticles produced by applying shear stress and centrifugal force to a reactant containing a reaction inhibitor in a swirling reactor in the course of a chemical reaction. The lithium titanate nanoparticles are highly dispersed and supported on the carbon by heating in a nitrogen atmosphere the carbon dispersed by applying shear stress and centrifugal force in a swirling reactor. It is characterized by.

  In the electrochemical capacitor of the present invention, the carbon may apply a shear stress and a centrifugal force to a reactant containing a reaction inhibitor in a rotating reactor in the course of a chemical reaction to promote and control the chemical reaction. After that, it is formed by a reaction method characterized by heating in a nitrogen atmosphere.

  In addition, the electrochemical capacitor of the present invention promotes a chemical reaction by applying shear stress and centrifugal force to the reactant and carbon containing a reaction inhibitor in a rotating reactor in which the carbon is in the course of a chemical reaction, It is formed by a reaction method in which the product and carbon are dispersed at the same time as the control.

  In the electrochemical capacitor of the present invention, the carbon generates a thin film containing a reaction product containing a reaction inhibitor in a swirling reactor, and applies a shear stress and a centrifugal force to the thin film to promote a chemical reaction. It is formed by a reaction method characterized by controlling.

  According to the present invention, the oxygen-deficient portion becomes a lithium adsorption / desorption portion by using, as the negative electrode, an electrode containing carbon in which lithium titanate nanoparticles having oxygen deficiency and doped with nitrogen are highly dispersed and supported. Further, doping with nitrogen increases the electrical conductivity, improves the output characteristics, and increases the energy density by using high-capacity activated carbon for the positive electrode material. Thereby, an electrochemical capacitor having a higher energy density can be provided.

It is a figure which shows the analysis result by X-ray photoelectron spectroscopy (XPS) which shows the oxygen defect of the electrode material 1 and the electrode material 2. FIG. It is a figure which shows the analysis result by X-ray photoelectron spectroscopy (XPS) which shows N dope of the electrode material 1 and the electrode material 2. FIG. It is a figure which shows the result of having performed charging / discharging measurement about the electrochemical capacitor cell of an Example and a comparative example.

  The form for implementing this invention is demonstrated below.

(1) Carbon used for negative electrode The method for producing carbon used for the negative electrode of the electrochemical capacitor according to the present invention is the same mechanochemical reaction as the method shown in Patent Document 2 previously filed by the present applicant. In the course of a chemical reaction, a chemical reaction is promoted and controlled by applying shear stress and centrifugal force to the reactants and reaction inhibitor in a rotating reactor, and then heated in a nitrogen atmosphere. Is.

  That is, as shown in Patent Document 2, a reactant and a reaction inhibitor are introduced into an inner cylinder of a reactor including an outer cylinder having a claw plate at an opening and a rotating inner cylinder having a through hole. By turning the inner cylinder, the reactant inside the inner cylinder is moved by the centrifugal force to the inner wall of the outer cylinder through the through hole of the inner cylinder. At this time, the reaction product collides with the inner wall of the outer cylinder by the centrifugal force of the inner cylinder, and forms a thin film and slides up to the upper part of the inner wall. In this state, both the shear stress between the inner wall and the centrifugal force from the inner cylinder are simultaneously applied to the reactant, and a large mechanical energy is applied to the thin-film reactant.

  This mechanical energy seems to be converted into chemical energy necessary for the reaction, so-called activation energy, but the reaction proceeds in a shorter time, and a dispersion of lithium titanate nanoparticle precursor and carbon is generated. The Thereafter, by heating in a nitrogen atmosphere, oxygen deficiency occurs in lithium titanate due to the reducing action of carbon. By doping the deficient portion with nitrogen, carbon having a desired oxygen deficiency and carrying highly dispersed lithium titanate nanoparticles doped with nitrogen can be generated.

  In this reaction, since the mechanical energy applied to the reaction product is large when it is in the form of a thin film, the thickness of the thin film is 5 mm or less, preferably 2.5 mm or less, more preferably 1.0 mm or less. The thickness of the thin film can be set according to the width of the dam plate and the amount of the reaction solution.

This reaction method is considered to be realized by the mechanical energy of the shear stress and centrifugal force applied to the reactant, but this shear stress and centrifugal force are generated by the centrifugal force applied to the reactant in the inner cylinder. . Thus, the centrifugal force applied to the reactants in the inner cylinder necessary for the present invention, 1500 N (kgms ^-2) or more, preferably 60000N (kgms ^-2) or more, more preferably 270000N (kgms ^-2) or .

  Subsequently, a method for producing carbon used in the above-described negative electrode will be described more specifically. That is, in order to obtain the lithium titanate nanoparticles, for example, using a titanium source such as titanium alkoxide, a lithium source such as lithium acetate as a starting material, and adding a predetermined carbon in the reaction process, the mechanochemical reaction Thus, a precursor of lithium titanate nanoparticles and a dispersion of carbon are generated. By heating this dispersion in a nitrogen atmosphere, carbon having a high dispersion of lithium titanate nanoparticles having an oxygen deficiency of 5 to 20 nm and doped with nitrogen is generated. Examples of the titanium source include titanium alkoxide, and examples of the lithium source include lithium acetate, lithium nitrate, lithium carbonate, and lithium hydroxide.

  That is, the above-mentioned metal salt, a reaction inhibitor such as acetic acid, and predetermined carbon are introduced into the inner cylinder of the reactor, and the inner cylinder is turned to mix and disperse the metal salt, the reaction inhibitor and carbon. Further, while turning the inner cylinder, a catalyst such as sodium hydroxide is added to proceed with hydrolysis and condensation reaction to generate a lithium titanate precursor, and this lithium titanate precursor and carbon are mixed in a dispersed state. . Upon completion of the reaction, a lithium titanate nanoparticle precursor and a dispersion of carbon can be produced. Further, by heating in a nitrogen atmosphere, the lithium titanate nanoparticles doped with nitrogen having oxygen deficiency can be increased. Dispersed and supported carbon can be formed.

(carbon)
Examples of the carbon used here include carbon blacks such as ketjen black and acetylene black, carbon nanotubes, carbon nanohorns, amorphous carbon, carbon fibers, natural graphite, artificial graphite, activated carbon, mesoporous carbon, and the like. Composite materials can also be used.

(solvent)
As the solvent, alcohols, water, or a mixed solvent thereof can be used. For example, a mixed solvent in which acetic acid and lithium acetate are dissolved in a mixture of isopropanol and water can be used.

(Reaction inhibitor)
As described in Patent Document 2, it is preferable to add a predetermined compound that forms a complex with the titanium source as a reaction inhibitor to a predetermined titanium source to which the mechanochemical reaction is applied. Thereby, it can suppress and control that a chemical reaction promotes too much. That is, the reaction can be suppressed and controlled by adding 1 to 3 mol of a predetermined compound such as acetic acid that forms a complex with the titanium source to 1 mol of the titanium source to form a complex. .

  Substances that can be used as reaction inhibitors, in other words, substances that can form a complex with the titanium source include acetic acid, citric acid, succinic acid, formic acid, lactic acid, tartaric acid, fumaric acid, and succinic acid. And complexing agents typified by carboxylic acids such as propionic acid and repric acid, aminopolycarboxylic acids such as EDTA, and amino alcohols such as triethanolamine.

(Baking process)
It was found that crystallization of lithium titanate proceeds well by rapid heating from room temperature to 700 to 900 ° C. in the step of firing the precursor of the obtained lithium titanate nanoparticles. Below this temperature, good progress of crystallization cannot be obtained, and when this temperature is exceeded, lithium titanate with good energy storage characteristics cannot be obtained due to phase transition.

(Action / Effect)
The carbon in which the lithium titanate nanoparticles obtained as described above are highly dispersed and supported is kneaded and molded with a binder, and can be used as an electrode of an electrochemical element, that is, an electrode for storing electrical energy. Showed high output characteristics and high capacity characteristics.

  The reason is that oxygen deficiency occurs when the lithium titanate nanoparticle precursor and carbon dispersion are heated in a nitrogen atmosphere, and lithium is occluded and desorbed to improve capacity and output characteristics. In addition, it is considered that this oxygen deficiency site is doped with nitrogen to improve the electrical conductivity of lithium titanate and improve the output characteristics.

(2) Electrochemical capacitor The electrochemical capacitor according to the present invention uses the electrode formed as described above as a negative electrode, and uses alkali activated carbon as a positive electrode. Details will be described below.

(Negative electrode)
The negative electrode is formed by mixing carbon kneaded with the above-described lithium titanate nanoparticles in a highly dispersed manner with a binder, kneading, and forming into a sheet shape, which is joined to a current collector. As the binder, polyvinylidene fluoride, polytetrafluoroethylene, or the like is used. Alternatively, a mixture of carbon and binder may be applied on a current collector by a doctor blade method or the like and dried.

(Positive electrode)
Alkaline activated activated carbon used as the positive electrode is made of wood raw material such as coconut husk, coal / petroleum raw material such as pitch, various polymers carbonized at high temperature, and activated using an alkali catalyst such as potassium hydroxide or sodium hydroxide. It is a carbon material having a highly developed pore structure obtained by performing the above.

  The positive electrode is formed by mixing a binder with a mixture of the alkali-activated activated carbon powder and the conductive material, kneading, forming the sheet, and bonding it to a current collector. As the binder, polyvinylidene fluoride, polytetrafluoroethylene, or the like is used. Alternatively, a mixture of activated carbon powder, conductive material powder and binder coated on a current collector by a doctor blade method or the like and dried may be used. Examples of the activated carbon include palm, phenol resin, petroleum coke, and the like, and examples of the conductive material include conductive carbon black or graphite.

(Electrolyte)
Lithium salts used in the electrolyte include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiCF ₃ SO ₃ , LiC (SO ₂ CF ₃ ). ₃ and one or more selected from the group consisting of LiPF ₃ (C ₂ F ₅ ) ₃ can be used.

Examples of the quaternary ammonium salt used in the electrolyte include tetraethylammonium, tetraethylmethylammonium and the like as cations, and BF ₄ ⁻ , PF ₆ ⁻ , ClO ₄ ⁻ , AsF ₆ ⁻ , SbF ₆ ⁻ , and the like as cations. AlCl ₄ ⁻ , RfSO ₃ ⁻ , (RfSO ₂ ) ₂ N ⁻ , RfCO ₂ ⁻ (Rf is a fluoroalkyl group having 1 to 8 carbon atoms) and the like.

  Moreover, as a solvent used for electrolyte solution, the following can be used. These solvents may be used alone or in combination of two or more. For example, propylene carbonate, propylene carbonate derivative, ethylene carbonate, ethylene carbonate derivative, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, 1,3-dioxolane, dimethyl sulfoxide, sulfolane, formamide, dimethylformamide, dimethyl Acetamide, dioxolane, phosphate triester, maleic anhydride, succinic anhydride, phthalic anhydride, 1,3-propane sultone, 4,5-dihydropyran derivative, nitrobenzene, 1,3-dioxane, 1,4-dioxane, 3-methyl-2-oxazolidinone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydrofuran derivatives, It can be used non-compound, acetonitrile, nitromethane, alkoxy ethane, and toluene.

(1) About electrode material (electrode material 1)
A mixed solvent was prepared by dissolving acetic acid and lithium acetate in an amount of 1.8 mol of acetic acid and 1 mol of lithium acetate in a mixture of isopropanol and water with respect to 1 mol of titanium alkoxide. This mixed solvent, titanium alkoxide, isopropyl alcohol, ketjen black (manufactured by ketjen black international, trade name: ketjen black EC600JD, porosity 78 Vol.%, Primary particle size 34 nm, average secondary particle size 337.8 nm) Into the swirl reactor, and the inner cylinder is swirled with a centrifugal force of 66,000 N (kgms ^-2 ) for 5 minutes to form a thin film of the reactant on the inner wall of the outer cylinder. A chemical reaction was promoted by applying centrifugal force to obtain Ketjen Black carrying a highly dispersed lithium titanate precursor.

  The obtained Ketjen black carrying a highly dispersed lithium titanate precursor was dried at 80 ° C. for 17 hours in a vacuum to obtain a composite in which the lithium titanate precursor was carried highly dispersed by Ketjen black. Body powder a was obtained.

  The obtained composite powder a in which the precursor of lithium titanate is highly dispersed and supported on ketjen black is rapidly heated to 800 ° C. in a nitrogen atmosphere to promote crystallization of lithium-containing titanium oxide. A composite powder A in which nanoparticles of lithium titanate were highly dispersed and supported on ketjen black was obtained.

(Electrode material 2)
The electrode material 1 was rapidly heated to 800 ° C. in a vacuum instead of being rapidly heated to 800 ° C. in a nitrogen atmosphere, and similarly lithium titanate nanoparticles were highly dispersed and supported on Ketjen Black. A composite powder B was obtained.

(result of analysis)
The analysis results of XPS_O1s and XPS_N1s of the composite powders A and B are shown in FIGS. 1 and 2, respectively. In addition, analysis results of a conventional sintered body of lithium titanate are shown. A conventional sintered body of lithium titanate was prepared by preparing a lithium titanate precursor by a sol-gel method and sintering the precursor.

  That is, as shown in FIG. 1, according to the XPS_O1s analysis results, the electrode material 1 has O1s binding energy peaks 533 to 534 eV indicating a spectrum derived from oxygen deficiency, and the electrode material 2 has normal oxidation. A binding energy peak 530 eV showing a spectrum derived from the product was confirmed.

  Further, as shown in FIG. 2, according to the XPS_N1s analysis result, in the electrode material 1, a peak 396 eV of N1s bond energy indicating Ti—N bond is detected, and it is confirmed that nitrogen is doped. It was. On the other hand, in the electrode material 2, the peak of N1s bond energy was not confirmed at 396 eV, and it was confirmed that no Ti—N bond was present, that is, nitrogen doping was not performed.

(2) Electrochemical capacitor (Example)
1, 9 parts by weight of the electrode material obtained as described above, 1 part by weight of PVDF (polyvinylidene fluoride) binder, and 1 part by weight of carbon nanofiber (VGCF-S, manufactured by Showa Denko) as a conductive material, Were kneaded and rolled to form a sheet. This sheet was vacuum dried and then joined to a copper foil to obtain a negative electrode.

  Also, 8 parts by weight of alkali activated carbon (manufactured by Kansai Thermal Chemical Co., Ltd., MSP-20M), 1 part by weight of PTFE binder (polytetrafluoroethylene), 1 part by weight of ketjen black as a conductive material are kneaded and rolled. To form a sheet. This sheet was vacuum dried and then joined to an aluminum foil to form a positive electrode.

These electrodes were made to face each other through a cellulose-based separator in a beaker into which 1 M LiBF ₄ and a propylene carbonate solution were injected as an electrolytic solution, to produce an electrochemical capacitor cell.

(Comparative example)
Electrochemical in the same manner as in the above example except that the alkali activated charcoal (MSP-20M, manufactured by Kansai Thermochemical Co., Ltd.) used as the activated carbon in the above examples was changed to steam activated charcoal (manufactured by Kuraray Chemical Co., YP-17). A capacitor cell was produced.

(Test results)
When the electrochemical capacitor cell was discharged at a constant current from 3 V to 1.5 V and the energy density was calculated from the relationship between time and voltage, the results shown in FIG. 3 were obtained. As is apparent from the figure, the energy density of the example using the alkali activated carbon is about 22 AhL ^−1, which is about 1.23 times the energy density of the comparative example using the steam activated coal (about 18 AhL ⁻¹ ). Thus, the effect of the present application was clear.

Claims (9)

An electrode containing carbon in which lithium titanate nanoparticles having oxygen deficiency and doped with nitrogen are highly dispersed and supported is used for the negative electrode.
An electrochemical capacitor characterized in that alkali activated carbon is used for the positive electrode.   Lithium titanate nanoparticles produced by applying shear stress and centrifugal force to a reactant containing a reaction inhibitor in a swirling reactor in the course of a chemical reaction, and the shear stress in the swirling reactor. The carbon dispersed by applying a centrifugal force is heated in a nitrogen atmosphere so that the lithium titanate nanoparticles are highly dispersed and supported on the carbon. Electrochemical capacitor.   The carbon is heated in a nitrogen atmosphere after the chemical reaction is promoted and controlled by applying shear stress and centrifugal force to the reactant containing the reaction inhibitor in the swirling reactor in the course of the chemical reaction. The electrochemical capacitor according to claim 1, which is formed by a reaction method characterized by the following.   In the course of the chemical reaction, the carbon applies a shear stress and centrifugal force to the reactant containing the reaction inhibitor and the carbon in the swirling reactor to promote and control the chemical reaction. 4. The electrochemical capacitor according to claim 3, wherein the electrochemical capacitor is formed by a reaction method of dispersing.   A reaction method characterized by generating a thin film containing a reaction product containing a reaction inhibitor in a swirling reactor, and applying a shear stress and a centrifugal force to the thin film to promote and control a chemical reaction. The electrochemical capacitor according to claim 3, wherein the electrochemical capacitor is formed by:   The reactor comprises a concentric cylinder of an outer cylinder and an inner cylinder, and has a through-hole on the side surface of the inner cylinder, and a slat plate is disposed at the opening of the outer cylinder, and by centrifugal force due to the turning of the inner cylinder, The reactant containing the reaction inhibitor in the inner cylinder is moved to the inner wall surface of the outer cylinder through the through hole of the inner cylinder, and a thin film containing the reactant containing the reaction inhibitor is formed on the inner wall surface of the outer cylinder. 6. The electrochemical capacitor according to claim 5, wherein a chemical reaction is promoted and controlled by applying a shear stress and a centrifugal force.   The electrochemical capacitor according to claim 5 or 6, wherein the thin film has a thickness of 5 mm or less. The electrochemical capacitor according to claim 6 or 7, wherein a centrifugal force applied to the reactant in the inner cylinder of the reactor is 1500 N (kgms ^-2 ) or more.   The electrochemical capacitor according to any one of claims 2 to 8, wherein the chemical reaction is a hydrolysis reaction and / or a condensation reaction of a metal salt.