JP2010226116A - Compound material of nanoparticles of lithium titanate and carbon, manufacturing method thereof, electrode material made of the compound, electrode using the electrode material, and electrochemical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compound material in which nanoparticles of lithium titanate with a diameter of 5-20 nm are highly dispersed and deposited in carbon. <P>SOLUTION: In this method, crystallization of titanium oxide including titanium is advanced by quickly heating, in a vacuum, compound material powder in which a precursor of lithium titanate is highly dispersed and deposited in Ketjen black, so that nanoparticles of lithium titanate are highly dispersed and deposited in Ketjen black, wherein the precursor of lithium titanate is formed by a mechanochemical reaction, giving shear stress and centrifugal force on reactants in a revolving reactor, in this case reaction inhibitor is added. In the quickly heating process, a compound material of the precursor of lithium titanate and carbon is heated from a room temperature to 800°C within one minute in a vacuum. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a composite of lithium titanate nanoparticles and carbon, a method for producing the composite, an electrode material comprising the composite, an electrode using the electrode material, and an electrochemical device.

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 the reaction in an unprecedented liquid phase reaction, and further, the metal oxide nanoparticles prepared by using this reaction method and the electrode material for an electrochemical device. A patent application was previously filed as Japanese Patent Application No. 2005-356845 for the purpose of providing carbon in which metal oxide nanoparticles are highly dispersed and supported and an electrochemical device using this electrode.

JP 2000-36441 A

However, for example, when a titanium alkoxide is used as a metal salt for causing a hydrolysis reaction using the mechanochemical reaction described in the specification of the prior application as described above, the reaction is too early, and lithium titanate is used. When manufacturing, titanium oxide was formed, and there was a problem that lithium titanate could not be manufactured. In addition, there is a problem that it is difficult to support lithium titanate on carbon in the state of fine particles.

The present invention has been proposed to solve the problems of the prior art as described above, and its purpose is to make lithium titanate nanoparticles finely divided by advancing crystallization of a metal oxide. An object of the present invention is to provide a composite of carbon and carbon, a method for producing such a composite, an electrode material comprising the composite, an electrode using the electrode material, and an electrochemical device.

As a result of intensive studies to solve the above-mentioned problems, the present inventors, based on the idea as described below, based on the composite of lithium titanate nanoparticles and carbon of the present invention, its production method, and this composite Thus, the present inventors have completed an electrode material, an electrode using the electrode material, and an electrochemical element.

(Reaction inhibitor)
That is, by adding a predetermined compound that forms a complex with the metal alkoxide as a reaction inhibitor to the predetermined metal alkoxide to which the mechanochemical reaction described in the specification of the prior application is applied, the chemical reaction is promoted too much. It has been found that this can be suppressed.

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 metal alkoxide to 1 mol of the metal alkoxide to form a complex. I understood that I could do it. It is to be noted that a metal-oxide composite nano-particle, for example, a lithium-titanium oxide composite nano-particle, which is a precursor of lithium titanate, is produced by firing this reaction. Thus, a crystal of lithium titanate is obtained.

Thus, by adding a predetermined compound such as acetic acid as a reaction inhibitor, it is possible to suppress the chemical reaction from being promoted too much because the predetermined compound such as acetic acid can form a stable complex with the metal alkoxide. It is thought that it is for forming.

Examples of substances that can form a complex with a metal alkoxide include acetic acid, citric acid, succinic acid, formic acid, lactic acid, tartaric acid, fumaric acid, succinic acid, propionic acid, carboxylic acid such as propionic acid, and EDTA. Examples include complexing agents represented by amino alcohols such as aminopolycarboxylic acid and triethanolamine.

(Baking process)
Moreover, in the baking process of the obtained lithium titanate precursor, rapid heating from room temperature to 800 ° C., rapid cooling to 50 ° C., and rapid heating and rapid cooling are performed within 1 minute to prevent aggregation of lithium titanate. It was found that nanoparticles having a small particle size were formed.

(Metal alkoxide)
As the metal alkoxide to which the reaction inhibitor according to the present invention can be applied, in addition to the above titanium alkoxide, a metal alkoxide having a reaction rate constant of 10 ⁻⁵ mol ⁻¹ sec ⁻¹ or more for the hydrolysis reaction of the metal alkoxide. Can be mentioned. Examples of such metals include tin, zirconia, and cesium.

(carbon)
In addition, by adding predetermined carbon during the reaction process, carbon in which 5 to 20 nm lithium titanate is highly dispersed and supported can be obtained. 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 cause hydrolysis and condensation reaction to proceed to produce a metal oxide, and the metal oxide and carbon are mixed in a dispersed state. Along with the completion of the reaction, carbon in which metal oxide nanoparticles are supported in a highly dispersed state 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 fibers, natural graphite, artificial graphite, activated carbon, mesoporous carbon, and the like. Composite materials can also be used.

Further, the carbon in which the metal oxide nanoparticles obtained according to the present invention are highly dispersed and supported may be calcined, kneaded with a binder, molded, and then an electrode of an electrochemical element, that is, an electrode for storing electric energy. However, this electrode exhibits high output characteristics and high capacity characteristics.

Here, the electrochemical element which can use this electrode is an electrochemical capacitor and a battery using an electrolytic solution containing lithium ions. That is, the electrode of the present invention can occlude and desorb lithium ions and operates as a negative electrode. Therefore, an electrochemical capacitor and a battery can be configured by using an electrolytic solution containing lithium ions and using, as a counter electrode, activated carbon, carbon that occludes and desorbs lithium, and the like.

(Mechanochemical reaction)
The reaction method used in the present invention is a mechanochemical reaction similar to the method shown in the above specification previously filed by the applicant of the present application, and is performed in a rotating reactor in the course of the chemical reaction. A chemical reaction is promoted by applying shear stress and centrifugal force to an object.

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.

And in order to promote such a chemical reaction, the reactor which consists of a concentric cylinder of an outer cylinder and an inner cylinder, is equipped with a through-hole in the side surface of an inner cylinder, and arrange | positions a slat in the opening part of an outer cylinder. In this, the reactant in the inner cylinder is moved to the inner wall surface of the outer cylinder through the through-hole of the inner cylinder by centrifugal force due to the turning of the inner cylinder, and a thin film containing the reactant is generated on the inner wall surface of the outer cylinder. This can be realized by applying shear stress and centrifugal force.

According to the present invention, in the baking process of the carbon carrying the precursor of the metal oxide nanoparticles, it is possible to prevent aggregation of the metal oxide nanoparticles by rapid heating, and the nanoparticles having a small particle diameter It is formed.

3 is a drawing-substituting photograph showing a TEM image of the composite of Example 1. FIG. The figure which shows the cyclic voltammogram about an Example and a comparative example. The figure which shows the result of the charging / discharging test using the electrode which concerns on this invention. The figure which shows the relationship between charging potential and an irreversible capacity | capacitance. The figure which shows the relationship between carbon amount and an irreversible capacity | capacitance. The figure which shows the relationship between a carbon amount and a capacity | capacitance reduction rate. The figure which shows the relationship between baking temperature and a capacity | capacitance. The figure which shows the relationship between energy density and power density.

Hereinafter, the present invention will be described more specifically with reference to examples.

(1) Composite powder composed of lithium titanate and ketjen black (Example 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 (Ketjen Black International Co., Ltd., trade name: ketjen black EC600JD, porosity 78 Vol.%, Primary particle size 40 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 composite powder in which the obtained lithium titanate precursor is highly dispersed and supported on ketjen black is rapidly heated and rapidly cooled in room temperature → 800 ° C. → 50 ° C. within one minute in a vacuum to contain lithium. Crystallization of the titanium oxide was advanced to obtain a composite powder in which lithium titanate nanoparticles were highly dispersed and supported on Ketjen Black.

A TEM image of the composite thus obtained is shown in FIG. In FIG. 1, it can be seen that 5 to 20 nm lithium titanate nanoparticles are highly dispersed and supported on Ketjen Black.

(Comparative Example 1)
Ketjen black carrying a highly dispersed lithium titanate precursor prepared in the same manner as in Example 1 is baked at 800 ° C. for 12 hours to advance crystallization of titanium oxide containing lithium. A ketjen black composite carrying lithium acid was obtained.

SEM observation of the ketjen black composite on which lithium titanate thus obtained was supported revealed that aggregated particles of lithium titanate of 0.5 μm to 2 μm were generated.

(Comparative Example 2)
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. While stirring titanium alkoxide and isopropyl alcohol with a stirrer, the mixed solvent was added dropwise and stirred for 5 hours. The produced lithium titanate precursor was dried in vacuum at 80 ° C. for 17 hours to obtain lithium-containing titanium oxide powder.

The obtained lithium titanate precursor was baked at 800 ° C. for 12 hours in an oxygen atmosphere to promote crystallization of lithium-containing titanium oxide to obtain a lithium titanate powder.

When the lithium titanate powder thus obtained was observed with an SEM, lithium titanate particles of 0.5 μm to 2 μm were produced.

(Comparative Example 3)
When it was produced by the same method as in Example 1 without using acetic acid, hydrolysis progressed when the materials were mixed, and the composite could not be produced by ultracentrifugation.

(Measurement result)
The composite powder obtained in Example 1, Comparative Example 1 and Comparative Example 2 was put into a SUS mesh welded on a SUS plate to obtain an electrode. A separator, a counter electrode, and a Li foil were placed on the electrode as a reference electrode, and 1M LiPF ₆ EC / DEC was infiltrated as an electrolyte to obtain a cell.

The working electrode is a SUS mesh welded SUS plate having a composite of the above cells, and a voltage range of 1.3 to 2.0 Vvs. Cyclic voltammograms were measured by sweeping at Li / Li ⁺ and a sweep rate of 0.1 mVsec ⁻¹ . The measured cyclic voltammogram is shown in FIG. Further, when the peak potential difference was measured for Example 1, Comparative Example 1 and Comparative Example 2, the results shown in Table 1 were obtained.

As is clear from FIG. 2, it was found that Example 1 had a sharp CV peak. This is presumably because the output characteristics of the electrodes were improved because the output characteristics of the nanoparticles themselves were improved or the output characteristics were improved by improving the crystallinity. Further, as is clear from Table 1, it was found that Example 1 has a high rate characteristic.

(2) Regarding the electrochemical capacitor using the electrode of the present invention, an electrode using lithium titanate and ketjen black obtained by the above mechanochemical reaction is used as the negative electrode, and a polarizable electrode such as activated carbon is used as the positive electrode. Thus, it was found that an electrochemical capacitor containing a lithium salt in an electrolytic solution in which lithium dendride is not generated can be obtained. Details will be described below.

The negative electrode of an electrochemical capacitor that contains a lithium salt in a normal electrolyte uses a carbon material that can store and release lithium. However, since the potential at which lithium dendrites are generated is close to the potential at carbon, lithium dendrites May occur. On the other hand, since the potential of the lithium titanate according to the present invention is about 1.5 V higher than the potential at which lithium dendride is generated, dendride is not generated.

However, when a charge / discharge test was performed using the electrode of the present application, results as shown in FIG. 3 were obtained. That is, when charging was performed from start to A and discharging was performed, only D was discharged, and it was found to have irreversible capacity. In addition, since lithium titanate alone has no irreversible capacity, it was considered that carbon was the cause.

Next, the irreversible capacity | capacitance at the time of charging to A, B, and C was shown in FIG. As can be seen from the figure, the smaller the charge capacity, the smaller the irreversible capacity. The horizontal axis in FIG. 4 represents the charge capacity converted to “x” of Li _{4 + x} Ti ₅ O ₁₂ .

Further, when the relationship between the amount of carbon in the negative electrode of the present application electrode and the irreversible capacity was examined, the results shown in FIG. 5 were obtained. Further, when the relationship between the amount of carbon and the capacity reduction rate was examined, results as shown in FIG. 6 were obtained. From these results, it was found that the content of carbon is preferably 30 to 50 wt%, more preferably 35 to 45 wt%.

Moreover, in order to raise the capacity | capacitance of a lithium titanate, to reduce a charging potential, and to reduce an irreversible capacity | capacitance, when the calcination temperature was examined, the result as shown in FIG. 7 was obtained. As can be seen from the figure, the capacity increased from 900 to 1000 ° C. and increased to the theoretical capacity. In order to increase the capacity, it is preferable to use ketjen black having a large specific surface area.

Based on these results, an electrochemical capacitor was fabricated with a firing temperature of 900 ° C. and a ketjen black content of 40 wt%, and the characteristics were evaluated. The results shown in FIG. 8 were obtained. As is clear from the figure, it was found that an electrochemical capacitor having a higher energy density can be obtained as compared with an electric double layer capacitor using activated carbon for both electrodes.

(Example 2)
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 (Ketjen Black International Co., Ltd., trade name: ketjen black EC600JD, porosity 78 Vol.%, Primary particle size 40 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 composite powder in which the obtained lithium titanate precursor is highly dispersed and supported on ketjen black is rapidly heated and rapidly cooled in room temperature → 800 ° C. → 50 ° C. within one minute in a vacuum to contain lithium. Crystallization of the titanium oxide was advanced to obtain a composite powder in which lithium titanate nanoparticles were highly dispersed and supported on Ketjen Black.

7 parts by weight of the composite powder obtained as described above, 2 parts by weight of binder (polytetrafluoroethylene), and 1 part by weight of carbon nanofiber (VGCF-S, manufactured by Showa Denko) as a conductive material. Kneaded and rolled to form a sheet. This sheet was vacuum dried and then joined to a copper foil to obtain a negative electrode.

Further, 8 parts by weight of activated carbon (manufactured by Kuraray Chemical Co., Ltd., YP-17), 1 part by weight of binder (polytetrafluoroethylene) and 1 part by weight of ketjen black as a conductive material are kneaded and rolled to obtain a sheet. Formed. This sheet was vacuum dried and then joined to an aluminum foil to form a positive electrode.

(Comparative Example 4)
8 parts by weight of activated carbon (manufactured by Kuraray Chemical Co., YP-17), 1 part by weight of binder (polytetrafluoroethylene) and 1 part by weight of ketjen black as a conductive material were kneaded and rolled to form a sheet. . This sheet was vacuum dried and then joined to an aluminum foil to form a positive electrode and a negative electrode.

These electrodes were made to face each other through a cellulosic separator in a beaker into which LiPF ₄ and a propylene carbonate solution had been injected to produce a cell. When these cells were subjected to a charge / discharge test at a constant current and measured for energy density and power density, results as shown in FIG. 8 were obtained.

As apparent from FIG. 8, it was found that the energy density was improved as compared with the electric double layer capacitor using the conventional activated carbon.

Claims (8)

  In the swirling reactor, the chemical reaction is promoted by applying shear stress and centrifugal force to the solution containing lithium titanate nanoparticles raw material, ketjen black and reaction inhibitor, and the precursor of lithium titanate is ketjen black The composite powder supported in a highly dispersed state is rapidly heated to promote crystallization of titanium oxide containing lithium, and the lithium titanate nanoparticles are supported in a highly dispersed state on Ketjen Black. A method for producing a composite of lithium titanate nanoparticles and carbon.   The method for producing a composite of lithium titanate nanoparticles and carbon according to claim 1, wherein the rapid heating and rapid cooling treatment is performed in a vacuum.   2. The composite of lithium titanate nanoparticles and carbon according to claim 1, wherein the rapid heat treatment changes the composite in a vacuum from room temperature to 800 ° C. within 1 minute. Production method.   The method for producing a composite of lithium titanate nanoparticles and carbon according to claim 1, 2 or 3, wherein the titanate nanoparticles have a particle size of 5 nm to 20 nm.   A composite of lithium titanate nanoparticles and carbon produced by the method according to any one of claims 1 to 4.   An electrode material comprising the composite according to claim 5.   The electrode obtained by shape | molding, after mixing the electrode material of Claim 6 with a binder.   An electrochemical device using the electrode according to claim 7.