JP2011216746A - Electrochemical capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an unprecedented electrochemical capacitor having high output characteristics.SOLUTION: In the electrochemical capacitor, an electrode containing carbon, where lithium titanate nanoparticles including oxygen vacancies and doped with nitrogen are highly dispersed and carried, is used for a negative electrode, a polarizable electrode is used for a positive electrode, and an electrolyte including lithium salt and quaternary ammonium salt is used. In the electrochemical capacitor, an oxygen vacancy unit becomes an adsorbing and desorbing unit of lithium, further doping nitrogen increases electric conductivity, output characteristics are improved, and mixing quaternary ammonium salt to lithium salt changes solvation structure to increase a diffusion speed of lithium ions, thereby possibly further improving output characteristics, and hence achieving the electrochemical capacitor having unprecedented high output characteristics.

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, the present applicants provide a method for accelerating and controlling the reaction in an unprecedented liquid phase reaction, and further containing carbon in which lithium titanate nanoparticles prepared using this reaction method are highly dispersed and supported. In order to provide an electrochemical capacitor using the electrode to be applied, a patent application was filed in Japanese Patent Application Laid-Open No. 2008-86009 (see Patent Document 2).
JP 2000-36441 A JP 2008-86009 A

  However, in the electrochemical capacitor described in the specification of the prior application as described above, there is a demand for an electrochemical capacitor having further excellent output characteristics, and the object of the present invention is an electrochemical that can meet this requirement. It is to provide a capacitor and a manufacturing method thereof.

  In order to achieve the above object, an electrochemical capacitor according to the present invention uses an electrode containing carbon having oxygen deficiency and high-dispersion support of nitrogen-doped lithium titanate nanoparticles as a negative electrode. Is used for the positive electrode, and an electrolytic solution containing a lithium salt and a quaternary ammonium salt is used.

  In addition, in the course of a chemical reaction, the carbon is a lithium titanate nanoparticle formed by applying shear stress and centrifugal force to a reactant containing a reaction inhibitor in a swirling reactor, and shearing in the swirling reactor. Carbon dispersed by applying stress and centrifugal force is heated in a nitrogen atmosphere, and the lithium titanate nanoparticles are supported in a highly dispersed manner.

  Then, in the course of the chemical reaction, the carbon is heated in a nitrogen atmosphere after applying a shear stress and centrifugal force to the reactant containing the reaction inhibitor in the swirling reactor to promote and control the chemical reaction. It was formed by the reaction method characterized by this.

  In the course of the chemical reaction of the carbon, a shearing stress and centrifugal force are applied to the reactant containing the reaction inhibitor and carbon in the swirling reactor to promote and control the chemical reaction, while at the same time dispersing the product and carbon. It was formed by the reaction method to be made.

  A reaction method characterized by generating a thin film containing a reaction product containing a reaction inhibitor in a reactor in which the carbon rotates, and applying a shear stress and a centrifugal force to the thin film to promote and control a chemical reaction. It is formed.

  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. It is a reaction method in which a chemical reaction is promoted and controlled by applying a shear stress and a centrifugal force.

  The thin film has a thickness of 5 mm or less.

The centrifugal force applied to the reaction product in the inner cylinder of the reactor is 1500 N (kgms ⁻² ) or more.

  The chemical reaction is a hydrolysis reaction and / or a condensation reaction of a metal salt.

  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, improving the output characteristics, and mixing the quaternary ammonium salt with the lithium salt changes the solvation structure of the solvent and increases the diffusion rate of lithium ions. The output characteristics are further improved. Thus, the structure of the present application can realize an electrochemical capacitor having unprecedented high output characteristics.

The graph which shows that the oxygen deficient spinel structure exists in the composite_body | complex of lithium titanate nanoparticle and carbon of this invention. The graph which shows that a titanium-nitrogen bond exists in the composite_body | complex of lithium titanate nanoparticle and carbon of this invention. The figure which shows the rate characteristic of the electrochemical capacitor of this invention.

  Hereinafter, modes for carrying out the present invention will be described.

  The reaction method for producing the negative electrode used in the present invention is a mechanochemical reaction similar to the method shown in Patent Document 2 previously filed by the present applicant and the like, and in the reactor that rotates in the course of the chemical reaction. In this method, shear reaction and centrifugal force are applied to the reactant and reaction inhibitor to promote and control the chemical reaction, and then heated in a nitrogen atmosphere.

  In this reaction method, as shown in Patent Document 2, a reactant and a reaction inhibitor are placed inside an inner cylinder of a reactor which includes an outer cylinder having a claw plate at an opening and a rotating inner cylinder having a through hole. When the inner cylinder is turned and the inner cylinder is swung, the reactant inside the inner cylinder moves to the inner wall of the outer cylinder through the through hole of the inner cylinder by the centrifugal force. 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 is considered to be converted into chemical energy necessary for the reaction, so-called activation energy, but the reaction proceeds in a short time to produce a dispersion of lithium titanate nanoparticle precursor and carbon. Thereafter, by heating in a nitrogen atmosphere, oxygen deficiency occurs in the lithium titanate due to the reducing action of carbon, and this deficient portion is doped with nitrogen, has oxygen deficiency, and is doped with nitrogen titanate lithium titanate nanoparticles Can be produced in a highly dispersed state.

  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 the centrifugal force applied to the reactant, and the shear stress and the 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 is 1500 N (kgms ^-2) or more, preferably 60000N (kgms ^-2) or more, more preferably 270000N (kgms ^-2) or more.

  In this reaction method, mechanical energy of both shear stress and centrifugal force is applied to the reactant at the same time, which seems to be due to the conversion of this energy into chemical energy. Can be promoted.

  As the reactant of the lithium titanate nanoparticles according to the present invention, for example, a titanium source such as titanium alkoxide or a lithium source such as lithium acetate is used as a starting material, and a predetermined carbon is added in the reaction process, whereby the mechano A chemical reaction produces a dispersion of lithium titanate nanoparticles precursor and carbon. By heating this carbon in a nitrogen atmosphere, carbon in which the lithium titanate nanoparticles of the present invention having 5 to 20 nm of oxygen vacancies and doped with nitrogen are highly dispersed and supported 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, a metal salt, the above reaction inhibitor, and a predetermined carbon are introduced into the inner cylinder of the reactor, and the inner cylinder is rotated to mix and disperse the metal salt, the above 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 produce a lithium titanate precursor, and this lithium titanate precursor and carbon are mixed in a dispersed state. To do. 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.

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

  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.

  As described in Patent Document 2, a predetermined compound that forms a complex with the titanium source can be added 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 forming a complex with the titanium source to 1 mol of the titanium source to form a complex. I understood. It is to be noted that the lithium titanate nanoparticle precursor, which is a precursor of lithium titanate nanoparticles, is produced by this reaction, and a lithium titanate crystal is obtained by firing this. .

  Thus, by adding a predetermined compound such as acetic acid as a reaction inhibitor, the chemical reaction can be prevented from being promoted too much because the predetermined compound such as acetic acid can form a stable complex with the titanium source. It is thought that it is for forming.

  Substances that can form complexes with the titanium source include acetic acid, citric acid, succinic acid, formic acid, lactic acid, tartaric acid, fumaric acid, succinic acid, propionic acid, amino acids such as repric acid, and aminopolyesters such as EDTA. Examples include complexing agents represented by amino alcohols such as carboxylic acid and triethanolamine.

  In the present invention, when a lithium titanate nanoparticle precursor and a carbon dispersion are heated in a nitrogen atmosphere, oxygen deficiency occurs, and lithium is occluded and desorbed at this site. It is thought that this is due to a mechanism in which the oxygen deficient site is doped with nitrogen, the electrical conductivity of lithium titanate is improved, and the output characteristics are improved.

  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.

  The carbon in which the lithium titanate nanoparticles obtained by the present invention are highly dispersed and supported can be kneaded and molded with a binder to form an electrode of an electrochemical element, that is, an electrode for storing electrical energy. Shows output characteristics and high capacity characteristics.

  The electrochemical capacitor of the present invention is formed using the electrode formed as described above as a negative electrode, a polarizable electrode as a positive electrode, and an electrolyte containing a lithium salt and a quaternary ammonium salt.

  The negative electrode is formed by mixing carbon kneaded with the above-mentioned lithium titanate nanoparticles in a highly dispersed manner and a binder, kneading the mixture, forming it into a sheet, and bonding it to a current collector. Polyvinylidene fluoride, polytetrafluoroethylene or the like is used as the binder. Alternatively, a mixture of carbon and binder coated on a current collector by a doctor blade method or the like and dried may be used.

  The polarizable electrode used as the positive electrode is formed by mixing a binder with a mixture of activated carbon powder and a conductive material, kneading, forming into a sheet, and bonding this to a current collector. Polyvinylidene fluoride, polytetrafluoroethylene or the like is used as the binder. 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 activated carbon raw material activation method include a steam activation method and a molten alkali activation method. Examples of the conductive material include conductive carbon black or graphite.

Lithium salts used for 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 LiPF ₃ (C ₂ F ₅ ) ₃ can be used.

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

  Examples of the solvent used in the electrolytic solution include those listed below. 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.

(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. A body powder was obtained.

  The obtained composite powder 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 in which lithium acid nanoparticles were highly dispersed and supported on ketjen black was obtained.

(Electrode material 2)
In the electrode material 1, instead of rapid heating to 800 ° C. in a nitrogen atmosphere, rapid heating to 800 ° C. in a vacuum is performed, and similarly, a composite powder in which lithium titanate nanoparticles are highly dispersed and supported on ketjen black Got.

  The analysis results of XPS_O 1s and XPS_N 1s of the above composite powder are shown in FIGS. In addition, the analysis result of the conventional sintered body of lithium titanate is added and shown. According to the XPS_O 1s analysis results, the electrode material 1 has O 1s binding energy peaks 533 to 534 eV indicating a spectrum derived from oxygen deficiency, and the electrode material 2 has a spectrum derived from a normal oxide. A peak 530 eV of binding energy shown is confirmed. Further, according to the XPS_N 1s analysis result, in the electrode material 1, a peak 396 eV of N 1s bond energy indicating Ti—N bond is detected, and it is confirmed that nitrogen is doped. On the other hand, in the electrode material 2, the peak of N1s bond energy is not confirmed at 369 eV, and it is confirmed that no Ti-N bond exists, that is, nitrogen doping is not performed.

(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 (made by Showa Denko, VGCF-S) 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 activated carbon (manufactured by Kuraray Chemical Co., Ltd., YP-17), 1 part by weight of PTFE binder (polytetrafluoroethylene), and 1 part by weight of ketjen black as a conductive material are kneaded, rolled and sheeted Formed. 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 cellulosic separator in a beaker into which 1 M LiBF ₄ , +0.5 M tetraethylmethylammonium tetrafluoroborate, and a propylene carbonate solution were injected as an electrolytic solution, to produce an electrochemical capacitor cell.

(Comparative Example 1)
In the examples, an electrochemical capacitor cell was prepared in the same manner as in the examples except that 1M LiBF ₄ , + 0.5M tetraethylmethylammonium tetrafluoroborate, and propylene carbonate solution were used instead of 1M LiBF ₄ and propylene carbonate solution. did.

(Comparative Example 2)
An electrochemical capacitor cell was produced in the same manner as in Comparative Example 1 except that the electrode material 2 was used instead of the electrode material 1 in Comparative Example 1.

  The rate characteristics of the above electrochemical capacitor cell were measured. The results are shown in FIG. As shown in FIG. 3, the capacity retention rate at 400C of the example of the present application is about twice that of Comparative Example 1 and about four times that of Comparative Example 2, and the effect of the present application is clear.

Claims (9)

Electrolysis that contains lithium salt and quaternary ammonium salt using an electrode containing carbon that has highly oxygen-deficient and nitrogen-doped lithium titanate nanoparticles highly dispersed and supported as a negative electrode and a polarizable electrode as a positive electrode An electrochemical capacitor characterized by using a liquid. 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 electrochemical capacitor according to claim 1, wherein carbon dispersed by applying centrifugal force is heated in a nitrogen atmosphere, and the lithium titanate nanoparticles are supported in a highly dispersed state. In the course of a chemical reaction, the carbon is heated in a nitrogen atmosphere after applying shear stress and centrifugal force to the reactant containing the reaction inhibitor in the swirling reactor to promote and control the chemical reaction. The electrochemical capacitor according to claim 1, formed by a characteristic reaction method. In the course of the chemical reaction of the carbon, a shearing stress and centrifugal force are applied to the reactant containing the reaction inhibitor and carbon in the swirling reactor to promote and control the chemical reaction, while at the same time dispersing the product and carbon. The electrochemical capacitor according to claim 3, formed by a reaction method. A reaction method characterized by generating a thin film containing a reaction product containing a reaction inhibitor in a reactor in which the carbon rotates, and applying a shear stress and a centrifugal force to the thin film to promote and control a chemical reaction. The formed electrochemical capacitor according to claim 3 or claim 4. 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, which is a reaction method in which 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 as a reaction method 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.