JP5916007B2 - Method for producing composite of lithium titanate and carbon nanofiber - Google Patents

Description

The present invention relates to the production how lithium titanate complex.

  At present, carbon materials that store and release lithium are used as electrodes of lithium batteries, but there is a risk of decomposition of the electrolyte because the negative potential is smaller than the reductive decomposition potential of hydrogen. Therefore, use of lithium titanate having a negative potential larger than the reductive decomposition potential of hydrogen has been studied (for example, see Patent Documents 1 and 2).

The chemical formula of this lithium titanate is Li ₄ Ti ₅ O ₁₂ . Lithium titanate is a lithium ion intercalating material that can release and store energy by insertion and removal of Li ⁺ as represented by the following formula (1).
[Formula 1]
Li ₄ Ti ₅ O ₁₂ + 3Li + + 3e ⁻ → Li ₇ Ti ₅ O ₁₂ (1)

This lithium titanate belongs to the space group Fd3m and has a spinel structure as shown in FIG. In addition, Li ⁺ lithium titanate after insertion is the space group Fm3m
And has a rock salt type structure as shown in FIG.

Since the lattice constant of lithium titanate is 8.358Å, the volume change rate due to Li ⁺ insertion / extraction is 0.2%. That is, when Li ⁺ is inserted, the positions of Li and Ti at the 16d site of lithium titanate and the position of O at the 32e site do not change, and only Li at the 8a site moves. Therefore, the Li ₄ Ti ₅ O ₁₂ crystal structure does not collapse even if Li ⁺ is repeatedly inserted and desorbed, so that when used as an electrode material, an energy storage device having excellent cycleability is obtained.

JP 2007-160151 A JP 2008-270795 A

On the other hand, the theoretical capacity of lithium titanate is 175 mAhg ⁻¹ , and Li ⁺ insertion / desorption occurs at a potential of 1.55 V with respect to the redox potential of metallic lithium. Lithium titanate has an electrical conductivity of 10 −13 Scm ⁻¹ and a Li ⁺ diffusion coefficient of 10 −11 cm ² s ⁻¹ and is very small.

For this reason, when used as an energy storage device for high power applications requiring high electrical conductivity and a large Li ⁺ diffusion coefficient, it is necessary to overcome the problems of electrical conductivity and Li ⁺ diffusion coefficient.

In order to increase the electrical conductivity and the Li ⁺ diffusion coefficient, there are the following methods (1) and (2).
(1) Nanoparticulation of lithium titanate (2) Composite of lithium titanate and conductive additive

(1) Making nanoparticles of Li ₄ Ti ₅ O _{12 If} the electrical conductivity and Li ⁺ diffusion coefficient of lithium titanate are small, titanium can be reduced by shortening the Li ⁺ diffusion distance and electron transfer distance in lithium titanate. The electrical conductivity and Li ⁺ diffusion coefficient of lithium acid can be increased. However, when the lithium titanate particles have a size of the order of nm, the ratio of surface atoms to the entire particles increases. The surface of this particle has an irregular structure compared to the inside of the particle and is unstable, and a particle of nm order size is unstable. Therefore, nm order size particles tend to become stable, i.e., aggregate to try to reduce the surface area. Therefore, there is a limit to the formation of nanoparticles in order to overcome the low Li ⁺ diffusion distance and electron transfer distance.

(2) Composite of lithium titanate and conductive auxiliary agent Since Li ₄ Ti ₅ O ₁₂ has a small electric conductivity, when an electron moves other than lithium titanate, a material having a high electric conductivity is defined as an electron transfer path. Just do it. Therefore, by covering the periphery of lithium titanate with a material having high electrical conductivity such as a noble metal or carbon, the electrical conductivity and Li ⁺ diffusion coefficient of lithium titanate can be increased. However, if the particle size of the lithium titanate is large, the inside of the lithium titanate does not participate in the electrochemical reaction, so that the fundamental problem is not solved.

  The present invention has been proposed in order to solve the problems of the prior art as described above, and (1) nano-particle formation of lithium titanate (2) composite of lithium titanate and a conductive auxiliary agent Are used simultaneously. That is, an object of the present invention is to provide a composite of lithium titanate and carbon nanofibers in which lithium titanate and carbon nanofibers are combined and a carbon film is formed on the surface of lithium titanate, and a method for producing the same. Another object of the present invention is to provide an electrode and an electrochemical device using the composite.

In order to achieve the above object, the lithium titanate and carbon nanofiber composite of the present invention has a carbon film composed of sucrose formed on the surface of lithium titanate , and the carbon film is 10 ⁻ It is characterized by adding sucrose in a proportion of ⁴ wt% to 10 ⁻³ wt% .

  Moreover, it is also one aspect | mode of this invention that the thickness of the said carbon film is 1-2 nm.

It is also an embodiment of the present invention that the composition ratio of lithium titanate and carbon nanofiber is 80 to 70:20 to 30 by weight .

  An electrode formed by molding the composite of lithium titanate and carbon nanofiber using a binder is also an embodiment of the present invention.

  An electrochemical element using the electrode is also an embodiment of the present invention.

  A method for producing lithium titanate and carbon nanofibers is also an embodiment of the present invention.

According to the present invention, in the composite of lithium titanate and carbon nanofiber, excellent electrical conductivity of Li ₄ Ti ₅ O ₁₂ , Li ⁺ diffusion coefficient, and large capacity charge / discharge characteristics can be expressed.

It is a schematic diagram which shows the structure of sucrose. It is a TEM image of carbon nanofiber. It is a perspective view which shows an example of the reactor used for the manufacturing method of this invention. It is the block diagram which showed the operation | movement procedure in the case of forming the carbon film by a sucrose on the surface of lithium titanate and carbon nanofiber in embodiment of this invention. It is the graph showing the result of the TG measurement of the comparative example 3 composite_body | complex in embodiment of this invention. It is a figure showing the result of the crystal structure analysis by XRD (X-ray powder diffraction method) performed with respect to the composite body of Examples 1, 2 and Comparative Examples 1-3 in the embodiment of the present invention. It is the figure which showed the crystallite size of (111), (311), (400) plane of the composite_body | complex of Examples 1, 2 and Comparative Examples 1-3 in embodiment of this invention. It is the graph which showed the charging / discharging test result of the composite_body | complex of Comparative Examples 1-3 in embodiment of this invention. It is the graph which showed the charging / discharging test result of the composite_body | complex of Example 1, 2 in embodiment of this invention. It is a diagram showing the _Li 4 _Ti 5 _{O 12} near HR-TEM image of the composite of Comparative Example 3 in the embodiment of the present invention. It is a figure showing the HR-TEM image of CNF periphery of the composite_body | complex of the comparative example 3 in embodiment of this invention. It is a diagram showing the _Li 4 _Ti 5 _{O 12} near HR-TEM image of the composite of Comparative Example 2 in an embodiment of the present invention. It is a figure showing the HR-TEM image of CNF periphery of the composite_body | complex of the comparative example 2 in embodiment of this invention. It is a diagram showing the _Li 4 _Ti 5 _{O 12} near HR-TEM image of the composite of Example 1 in an embodiment of the present invention. It is the block diagram which showed the operation | movement procedure in the case of forming the carbon film by a sucrose on the surface of the lithium titanate in embodiment of this invention. It is the graph showing the result of TG measurement of the composite_body | complex of Example 4 in embodiment of this invention. It is a figure showing the result of the crystal structure analysis by XRD (X-ray powder diffraction method) performed with respect to the composite_body | complex of Example 3 and 4 in embodiment of this invention. It is the figure which showed the crystallite size of (111), (311), (400) plane of the composite_body | complex of Example 3, 4 in embodiment of this invention. It is the graph which showed the charging / discharging test result of the composite_body | complex of Example 2, 4 in embodiment of this invention. It is the graph which showed the charging / discharging test result of the composite_body | complex of Example 1, 3 in embodiment of this invention. It is a schematic diagram which shows the spinel type structure of lithium titanate. It is a schematic diagram which shows the rock salt type | mold structure of lithium titanate.

  Embodiments for carrying out the present invention will be described below. In addition, this invention is not limited to embodiment described below.

  The composite of lithium titanate and carbon nanofiber according to this embodiment includes a titanium source (hereinafter referred to as Ti source), a lithium source containing sucrose (hereinafter referred to as Li source), and a carbon nanofiber (hereinafter referred to as CNF). Is added, subjected to ultra-centrifugal force processing (hereinafter referred to as UC treatment), which is one of mechanochemical reactions, the product is vacuum-dried, and then calcined.

(Titanium source)
As the titanium source, titanium (IV) tetrabutoxy monomer (Ti [O (CH ₂ ) ₃ CH ₃ ] ₄ , Wako Pure Chemical Industries, Ltd., first grade) can be used. As other than titanium (IV) tetrabutoxy monomer, isopropyl alkoxide, titanium chloride and the like can be used. The Ti source solution is prepared by adding acetic acid (CH3COOH, manufactured by Kanto Chemical Co., Ltd., special grade) as a chelating reagent, and isopropyl alcohol ((CH ₃ ) ₂ CHOH, manufactured by Wako Pure Chemical Industries, Ltd., for organic synthesis) as a solvent. .

(Lithium source)
Lithium acetate (CH3COOLi, manufactured by Wako Pure Chemical Industries, Ltd., special grade) can be used as the lithium source. As a lithium source other than lithium acetate, lithium hydroxide, lithium carbonate, or the like can be used. The Li source solution was prepared by dissolving lithium acetate in a mixed solution of distilled water, acetic acid and isopropyl alcohol.

(sucrose)
Sucrose to be added to the lithium source is a disaccharide in which glucose and fructose are glycosidically bonded. Sucrose has the chemical formula C ₁₂ H ₂₂ O ₁₁
It has the structure of FIG. The physical properties of sucrose have a density of 1.587 gcm ⁻³ , a melting point of 186 ° C., a solubility in water of 211.5 g (100 ml, 20 ° C.), and in a broad sense, correspond to a polyalcohol.

  Sucrose is added to the Li source, and a UC treatment is performed together with the Ti source and CNF, thereby forming a carbon film on the surface of lithium titanate and the surface of CNF. In addition, amorphous carbon is formed inside the composite. In particular, the carbon film formed on the surface of lithium titanate suppresses the growth of lithium titanate crystals. At this time, the thickness of the carbon film is preferably 1 to 2 nm.

The addition amount of sucrose at this time is 10 ⁻⁴ wt% or more and 10 ⁻³ wt% or less with respect to lithium titanate. The proportion of ^{sucrose, 10-4} less than wt% and _Li 4 _Ti carbon film on 5 _{O 12} is insufficient, _Li 4 _Ti 5 _{O 12} precursor having no a _Li 4 _Ti 5 _{O 12} precursor having the carbon film Will be mixed. In the Li ₄ Ti ₅ O ₁₂ precursor having no carbon film, more Ti source and Li source react than in the additive-free system, and larger particles are generated. This is because the capacity is reduced by the large particles.
Further, if the ratio of sucrose to be added is more than 10 ⁻³ wt, formation of amorphous carbon in the entire composite will interfere with Li ⁺ diffusion and a thick carbon film will be formed on CNF. As a result, electrical conductivity is lost. Thereby, the utilization factor and rate characteristic of lithium titanate are lowered.

(Carbon nanofiber)
By adding predetermined CNF during the reaction process, a complex of lithium titanate and CNF can be obtained. CNF is a multi-walled carbon nanotube (CNT) that includes fibers with a diameter of several hundred nm. The CNF of this example used a diameter of 15-30 nm as shown in the TEM image shown in FIG. This CNF has an electrical conductivity of 25 Ω ⁻¹ cm ⁻¹ and a specific surface area of 322 m ² g ⁻¹ , and is produced by catalytic chemical vapor deposition (CCVD) using carbon monoxide and a raw material. It is.

  In this example, carbon nanofibers are added at a ratio in the range of 20 to 30 wt% with respect to lithium titanate. If the ratio of the carbon nanofibers added to the lithium titanate is less than 20 wt%, the lithium titanate supported on the carbon nanofibers is reduced, and the capacity and rate characteristics are lowered. This is because when the proportion of the carbon nanofibers exceeds 30 wt%, the lithium titanate decreases and the capacity decreases.

(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.

(UC processing)
The UC process used in the present embodiment is a process using a mechanochemical reaction. In the mechanochemical reaction, in the course of the chemical reaction, shearing stress and centrifugal force are applied to the reactants in a rotating reactor to promote the chemical reaction.

  This reaction method can be performed using, for example, a reactor as shown in FIG. As shown in FIG. 3, the reactor includes an outer cylinder 1 having a cough plate 1-2 at an opening, and an inner cylinder 2 having a through hole 2-1 and swirling. By putting the reactant into the inner cylinder 2 of this reactor and turning the inner cylinder 2, the reactant inside the inner cylinder 2 passes through the through-hole 2-1 of the inner cylinder 2 by the centrifugal force. Move to the inner wall 1-3. At this time, the reaction product collides with the inner wall 1-3 of the outer cylinder 1 by the centrifugal force of the inner cylinder 2, and forms a thin film and slides up to the upper part of the inner wall 1-3. In this state, both the shear stress between the inner wall 1-3 and the centrifugal force from the inner cylinder 2 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 required for the reaction, so-called activation energy, but the reaction proceeds in a short time.

  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 reactant.

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 embodiment is 1500 N (kgms ^-2) or more, preferably 60000N (kgms ^-2) or more, more preferably 270000N (kgms ^-2) or .

  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.

(Drying / Baking treatment)
In this embodiment, after obtaining a composite of lithium titanate and carbon nanofibers by a mechanochemical reaction, the composite is dried and fired in vacuum, whereby an electrode or an electrochemical element using the composite is used. Improve the capacity and output characteristics.

  That is, in this embodiment, in the drying process of the composite, drying is performed in a vacuum at a temperature of 80 ° C. for 17 hours. Thereafter, in the baking step, baking is performed in a vacuum at a temperature of 900 ° C. for 3 minutes. As a result, aggregation of the nanoparticles is suppressed, and the carbon nanofibers are supported in a highly dispersed manner, thereby improving capacity and rate characteristics.

(electrode)
The composite of lithium titanate and carbon nanofibers obtained by the present embodiment can be kneaded and molded with a binder to form an electrode for an electrochemical element, that is, an electrode for storing electrical energy. Shows output characteristics and high capacity characteristics.

(Electrochemical element)
An electrochemical element that can use this electrode is an electrochemical capacitor or a battery that uses an electrolytic solution containing a metal ion such as lithium or magnesium. That is, the electrode of the present embodiment can occlude and desorb metal ions, and operates as a negative electrode and a positive electrode. Therefore, an electrochemical capacitor or a battery can be configured by using an electrolytic solution containing metal ions and using, as a counter electrode, activated carbon, lithium titanate or a metal oxide that occludes and desorbs metal ions.

[First characteristic comparison]
This characteristic comparison is a characteristic comparison in the process of forming a carbon film with sucrose on the surfaces of lithium titanate and carbon nanofibers. FIG. 4 is a diagram showing the process of comparing the characteristics, and the composites of Examples 1 and 2 and Comparative Examples 1 to 3 were synthesized according to the figure.

In Example 1, the ratio of sucrose added to the solution on the Li source side is 10 ⁻³ wt%, and in Example 2, the ratio of sucrose added to the solution on the Li source side is 10 ⁻⁴ wt%. In Comparative Example 1, the ratio of sucrose added to the solution on the Li source side is 10 ⁻² wt%. In Comparative Example 2, the ratio of sucrose added to the solution on the Li source side is 10 ⁻¹ wt%. In Comparative Example 3, the ratio of sucrose added to the solution on the Li source side is 2 wt%.

  In this characteristic comparison, a Ti source solution was prepared by adding 5 mol of tetrabutoxytitanium to 300 mol of isopropyl alcohol.

On the other hand, it was prepared by dissolving 5 mol of lithium acetate in a mixed solution of 50 mol of distilled water, 9 mol of acetic acid and 36 mol of isopropyl alcohol as a Li source solution. In order to prepare the composites of Examples 1 and 2 and Comparative Examples 1 to 3, sucrose was used as a Li source at a ratio of 10 ⁻² wt%, 10 ⁻³ wt%, and 10 ⁻⁴ wt% with respect to lithium titanate. Added to. At this time, in order to control the addition amount, an aqueous sucrose solution was prepared and dropped as follows.
(1) Example 1: When adding 10 ⁻³ wt% sucrose, 56.2 μL of an aqueous solution prepared by dissolving 0.0178 g of sucrose in 100 mL of distilled water was added dropwise to the lithium titanate raw material solution.
(2) Example 2: When adding 10 ⁻⁴ wt% of sucrose, 10.0 μL of an aqueous solution prepared by dissolving 0.0025 g of sucrose in 25 mL of distilled water was added dropwise to the lithium titanate raw material solution.
(3) Comparative Example 1: When 10 ⁻² wt% sucrose was added, 10.1 μL of an aqueous solution prepared by dissolving 0.1978 g of sucrose in 20 mL of distilled water was added dropwise to the lithium titanate raw material solution.
(4) Comparative Example 2: When 10 ⁻¹ wt% sucrose was added, sucrose at a ratio of 10 ⁻¹ wt% with respect to lithium titanate was added to the UC reaction raw material solution.
(5) Comparative Example 3: When 2 wt% sucrose was added, sucrose at a ratio of 2 wt% with respect to lithium titanate was added to the UC reaction raw material solution.

The Li source solution was added to the Ti source solution prepared as described above, and CNF was further added to the raw material solution at a weight ratio of Li ₄ Ti ₅ O ₁₂ : CNF = 70: 30. This raw material solution was subjected to UC treatment in an ultracentrifugal force field to produce a precursor. Then, after the obtained precursor was dried under vacuum for 17 hours, the composites of Examples 1 and 2 and Comparative Examples 1 to 3 were obtained by firing for 3 minutes at 900 ° C. under vacuum.

  For the composites of Examples 1 and 2 and Comparative Examples 1 to 3, analysis of crystal structure by XRD (X-ray powder diffraction method), calculation of thermal characteristics and carbon ratio by TG (thermogravimetric analysis), and HR -Identification of the crystal system by TEM (transmission electron microscope) and observation by STEM (scanning transmission electron microscope).

[Calculation of thermal characteristics and carbon ratio by TG]
Li ₄ Ti ₅ O ₁₂ in the composite
In order to confirm the composition ratio of CNF and CNF, TG measurement was performed. The atmosphere was under air (N ₂ + O ₂ ), and the measurement temperature was 30-900 ° C. Heating rate is 20 ° C
The temperature was held at min ⁻¹ and 900 ° C. for 10 minutes.

FIG. 5 shows a TG of a composite obtained by adding sucrose at a ratio of 2 wt% to the lithium titanate of Comparative Example 3.
It is a graph showing the result of a measurement. From the figure, even when the sucrose addition rate is 2 wt% which is the largest among the composites of Examples 1 and 2 and Comparative Examples 1 to 3, the composition ratio is Li ₄ Ti ₅ O ₁₂ / CNF = 70: It was almost consistent with 30. From this result, it can be seen that the composition can be controlled even in the composites of Examples 1 and 2 and Comparative Examples 1 and 2 in which the amount of sucrose added is smaller than that in Comparative Example 3.

[Crystal structure analysis by XRD]
XRD measurement was performed to identify the crystal structure. From the XRD results, the crystallite size d [nm] was determined from Scherrer's formula (2) shown below.
d = (Kλ) / (β cosθ) (2)
K is the structure factor, λ is the X-ray wavelength, β is the half-width

FIG. 6 is a diagram showing the results of crystal structure analysis by XRD (X-ray powder diffraction method) performed on the composites of Examples 1 and 2 and Comparative Examples 1 to 3. According to this figure, in Examples 1 and 2 and Comparative Examples 1 to 3, except that a peak of the carbon derived from at about 25 °, does not contain impurities such as anatase type TiO ₂ and rutile-type TiO ₂ Li It can be seen that ₄ Ti ₅ O ₁₂ is formed.

  FIG. 7 shows crystallite sizes in the (111), (311), and (400) planes obtained from the XRD results and Scherrer's equation for the crystallite sizes in the composites of Examples 1 and 2 and Comparative Examples 1 to 3. It is the figure which showed the size. From this figure, it can be seen that the crystallite size is smaller in the complex added with sucrose than in the case where sucrose is not added.

[Charge / discharge test results]
In the charge / discharge test, the rate on the discharge side (the side on which Li ⁺ is inserted into Li ₄ Ti ₅ O ₁₂ ) is fixed at 10 C, and the charge rate is changed to 10, 30, 60, 120, 180, 300 C, Measurements were made three times at a rate. The measurement potential is 1-3 Vvs. The range was Li / Li ⁺ .

FIG. 8 shows the charge / discharge test results of the composites of Comparative Examples 1 to 3. As described in the crystal structure analysis by XRD, the complex with sucrose added has a smaller crystallite size than the complex without sucrose added. Nevertheless, it can be seen from FIG. 8 that the capacity is lower than the complex without sucrose added. That is, in the composites of Comparative Examples 1 to 3, the utilization rate of Li ₄ Ti ₅ O ₁₂ decreases at all rates.

On the other hand, FIG. 9 shows the charge / discharge test results of the composites of Examples 1 and 2. FIG. 9 shows that the composite of Example 1 has a larger capacity than the composite without sucrose added at all rates. It can also be seen that the composite of Example 2 has a larger capacity than the composite without sucrose on the low rate side. That is, by adding sucrose at a ratio of 10 ⁻⁴ wt% to 10 ⁻³ wt%, the utilization rate of Li ₄ Ti ₅ O ₁₂ is improved, and the capacity is increased. In particular, the addition of sucrose in a proportion of 10 ⁻³ wt% improves the capacity at all rates.

[Identification and morphology observation of crystal system by HR-TEM]
10 to 14 are photographs showing the crystal structures of the composites of Example 1 and Comparative Examples 2 and 3. FIG. The crystal structure of the composite was observed by using HR-TEM (high resolution transmission electron microscope).

10 and 11 are views showing HR-TEM images around the Li ₄ Ti ₅ O ₁₂ and CNF of the composite of Comparative Example 3. FIG. 12 and 13 are views showing TEM images of the composite of Comparative Example 2 around Li ₄ Ti ₅ O ₁₂ and CNF.

10 and ₁₂ , it can be seen that a carbon film is formed on the surface of Li ₄ Ti ₅ O ₁₂ . The growth of the crystal of Li ₄ Ti ₅ O ₁₂ is suppressed by this carbon film. Also, from FIGS. 11 and 13, it can be seen that large amorphous carbon is formed on the entire composite. Thereby, the diffusion of Li ⁺ is disturbed, and the utilization factor of Li ₄ Ti ₅ O ₁₂ is lowered.

On the other hand, FIG. 14 is a diagram showing an HR-TEM image around Li ₄ Ti ₅ O ₁₂ and CNF of the composite of Example 1. FIG. 14 shows that a carbon film is formed on the surface of Li ₄ Ti ₅ O ₁₂ . That is, like the comparative examples 2 and 3, the growth of the crystal of Li ₄ Ti ₅ O ₁₂ is suppressed. However, the thickness of the carbon film on the surface of Li ₄ Ti ₅ O ₁₂ is only about 1 to 2 nm, which is significantly thinner than Comparative Examples 2 and 3. Moreover, it turns out that the amorphous carbon with respect to the whole composite is not formed. As the carbon film on CNF becomes thinner, the utilization ratio of Li ₄ Ti ₅ O ₁₂ is improved.

[Summary]
As described above, by the addition of _sucrose, crystal growth of the Li ₄ Ti ₅ carbon film on the surface of the _{O 12} is formed _Li 4 _Ti 5 _{O 12} is suppressed. However, when the sucrose addition rate is 10 ⁻² wt% or more, the carbon film on CNF becomes thick enough to lose the electrical conductivity inherent to CNF. Further, since amorphous carbon is formed over the entire complex and Li ⁺ diffusion is hindered, the utilization rate of Li ₄ Ti ₅ O ₁₂ is lower than that of the complex not added with sucrose.

On the other hand, when the sucrose addition rate was further reduced to 10 ⁻³ wt%, amorphous carbon was not formed on the entire composite, and the carbon film on CNF was also thin. As a result, the utilization rate of Li ₄ Ti ₅ O ₁₂ is improved as compared with the sucrose-free system.

[Second characteristic comparison]
This characteristic comparison is a characteristic comparison in the process of forming a carbon film with sucrose on the surface of lithium titanate. FIG. 15 is a diagram showing the process of this characteristic comparison, and the composites of Examples 3 and 4 were synthesized according to FIG.

In Example 3, the ratio of sucrose added to the solution on the Li source side is 10 ⁻³ wt%, and in Example 4, the ratio of sucrose added to the solution on the Li source side is 10 ⁻⁴ wt%.

  In this characteristic comparison, a Ti source solution was prepared by adding 5 mol of tetrabutoxytitanium to 300 mol of isopropyl alcohol.

On the other hand, it was prepared by dissolving 5 mol of lithium acetate in a mixed solution of 50 mol of distilled water, 9 mol of acetic acid and 36 mol of isopropyl alcohol as a Li source solution. In order to prepare the composites of Examples 3 and ⁴ , sucrose was added to the Li source at a ratio of 10 ⁻³ wt% 1 and 10 ⁻⁴ wt% with respect to lithium titanate. At this time, in order to control the addition amount, an aqueous sucrose solution was prepared and dropped as follows.
(1) Example 3: When adding 10 ⁻³ wt% sucrose, 56.2 μL of an aqueous solution obtained by dissolving 0.0178 g of sucrose in 100 mL of distilled water was added dropwise to the lithium titanate raw material solution.
(2) Example 4: When adding 10 ⁻⁴ wt% of sucrose, 10.0 μL of an aqueous solution obtained by dissolving 0.0025 g of sucrose in 25 mL of distilled water was added dropwise to the lithium titanate raw material solution.

CNF was added to the Ti source solution in a weight ratio of Li ₄ Ti ₅ O ₁₂ : CNF = 70: 30 to the Ti source solution prepared as described above, and sonication was performed for 3 minutes. Thereafter, the Li source solution was added to the raw material solution. This raw material solution was subjected to UC treatment in an ultracentrifugal force field to produce a precursor. Then, after drying the obtained precursor at a temperature of 80 ° C. for 17 hours in a vacuum, the composites of Examples 1 and 2 and Comparative Examples 1 to 3 were baked for 3 minutes under a vacuum of 900 ° C. Got.

  Analysis of the crystal structure by XRD (X-ray powder diffraction method) and heat by TG (thermogravimetric analysis) for the composites of Examples 3 and 4 and the composites of Examples 1 and 2 prepared in the above characteristic comparison. Calculation of characteristics and carbon ratio, identification of the crystal system by HR-TEM (transmission electron microscope), and STEM (scanning transmission electron microscope) observation were performed.

[Calculation of thermal characteristics and carbon ratio by TG]
Li ₄ Ti ₅ O ₁₂ in the composite
In order to confirm the composition ratio of CNF and CNF, TG measurement was performed. The atmosphere was under air (N ₂ + O ₂ ), and the measurement temperature was 30-900 ° C. The temperature elevation rate was 20 ° C. min ⁻¹ and the temperature was maintained at 900 ° C. for 10 minutes.

FIG. 16 is a graph showing the results of TG measurement of the complex of Example 4. From FIG. 16, also in the composite of Example 4, the composition ratio almost coincided with the charge ratio of Li ₄ Ti ₅ O ₁₂ / CNF = 70: 30. From this result, it can be seen that the composition can be controlled even in the composite of Example 3 with the addition amount of sucrose.

[Crystal structure analysis by XRD]
FIG. 17 is a diagram showing the results of crystal structure analysis by XRD (X-ray powder diffraction method) performed on the composites of Examples 3 and 4. According to this figure, in Examples 3 and 4, there is a peak derived from carbon in the vicinity of 25 °, and Li ₄ Ti ₅ O ₁₂ containing no impurities such as anatase TiO ₂ or rutile TiO ₂ is generated. I understand that.

Further, FIG. 18 shows the crystallite size in the composites of Examples 3 and 4 obtained from each XRD result and the above formula (2) which is Scherrer's formula (111), (311), (400) planes. It is the figure which showed the crystallite size. From this figure, it can be seen that when the sucrose addition rate is 10 ⁻³ wt%, the crystallite size hardly changes after and before the synthesis scheme change. It can be seen that the crystallite size at this time is smaller than when sucrose is not added. On the other hand, when the sucrose addition rate was 10 ⁻⁴ wt%, the crystallite size was smaller after the change than before the synthesis scheme was changed. From this result, it can be seen that the crystal grain size is suppressed by changing the scheme.

[Charge / discharge test results]
Similar to the first characteristic comparison, in the charge / discharge test, the rate on the discharge side (the side on which Li ⁺ is inserted into Li ₄ Ti ₅ O ₁₂ ) is fixed at 10 C, and the charge rate is 10, 30, 60, 120, The measurement was performed three times at each rate while changing to 180 and 300C.

FIG. 19 is a diagram illustrating rate characteristics of the second and fourth embodiments. From this figure, it can be seen that the capacity on the high rate side is improved in Example 4 in which the manufacturing process is changed. That is, in both Examples 2 and 4, the capacity is improved on the low rate side compared to the case where sucrose is not added. In particular, in Example 4, it can be seen that the utilization rate of Li ₄ Ti ₅ O ₁₂ on the high rate side is improved.
This is because a carbon film is selectively formed on Li ₄ Ti ₅ O ₁₂ by changing the synthesis scheme.

FIG. 20 is a diagram illustrating rate characteristics of the first and third embodiments. From this figure, in Examples 1 and 3, the capacity is improved on the low rate side compared to the case where sucrose is not added. However, in Example 1 , although the capacity | capacitance of the high rate side is improving rather than the case where sucrose is not added, in Example 3 , it turns out that the capacity | capacitance of the high rate side is falling.
This carbon film was present on CNF also for all supported on _Li 4 _Ti 5 _{O 12 surface,} because the carbon film of Li ₄ Ti _{5 O 12} surface becomes thicker.

[Identification and morphology observation of crystal system by HR-TEM]

FIG. 21 is a diagram showing an HR-TEM image around Li ₄ Ti ₅ O ₁₂ and CNF of the composite of Comparative Example 3. From FIG. 21, a carbon film is formed on the surface of Li ₄ Ti ₅ O ₁₂ . Further, this carbon _film, Li ₄ Ti ₅ Growth _{O 12} crystals seen to have been suppressed. Thereby, the diffusion of Li ⁺ is disturbed, and the utilization factor of Li ₄ Ti ₅ O ₁₂ is lowered.

[Summary]
As described above, by dissolving CNF in the Ti source solution first, the Ti source is preferentially supported on the CNF surface, so that sucrose to be charged later is not supported on the CNF surface. _Thus, it is possible to selectively form the carbon film on the Li ₄ Ti _{5 O 12} surface, sucrose smaller than in the case of forming a carbon film on the surface of the _CNF, the crystal growth of Li ₄ Ti _{5 O 12} Is suppressed, and the utilization rate of Li ₄ Ti ₅ O ₁₂ is improved.

DESCRIPTION OF SYMBOLS 1 ... Outer cylinder 1-2 ... Baffle 1-3 ... Inner wall 2 ... Inner cylinder 2-1 ... Through-hole

Claims (3)

In a swirling reaction vessel, a solution of lithium titanate and carbon nanofibers is reacted by applying shear stress and centrifugal force to a solution containing a titanium source, a lithium source containing sucrose, and carbon nanofibers. The composite processing to generate,
A heat treatment for heating the composite through the composite treatment in a vacuum;
A method for producing a composite of lithium titanate and carbon nanofibers having
The method for producing a composite of lithium titanate and carbon nanofiber, wherein the amount of sucrose added is 10 ⁻⁴ wt% to 10 ⁻³ wt% with respect to lithium titanate. The compounding process is
2. The lithium titanate according to claim 1 , which is a treatment in which a shearing stress and a centrifugal force are applied to the solution to which the carbon nanofibers are added and then reacted after adding a lithium source containing sucrose to the titanium source. A method for producing a composite of carbon nanofibers. The compounding process is
2. The lithium titanate according to claim 1 , wherein after adding carbon nanofibers to a titanium source, the reaction is performed by applying shear stress and centrifugal force to a solution in which a lithium source containing lithium containing sucrose is added. A method for producing a composite of carbon nanofibers.