JP6400250B1 - Lithium titanate having no variation in battery characteristics, lithium ion secondary battery using the same, and manufacturing method thereof - Google Patents

Abstract

Production of lithium titanate and lithium titanate with stable crystal structure, almost no loss of electric capacity during high-speed charge / discharge of lithium secondary battery, higher discharge capacity, excellent charge / discharge cycle characteristics, and excellent safety The intermediate for manufacturing, lithium titanate, and the manufacturing method of the intermediate for manufacturing lithium titanate are provided.
When the F center value by ESR spectrum measurement is 1.0 × 10 ¹⁵ (pieces / g) or more, no trivalent titanium is present, and the color is expressed in the L ^* a ^* b ^* color system , 70 <L ^* <90, -1 <a ^* <1, 0 <b ^* <5 in the range of 400 ° C. to 600 ° C. in an atmosphere containing an oxidizing gas. Lithium titanate produced by the above process has a standard deviation of 3.5 or less in standard deviation when used as an electrode active material of an ion secondary battery. It has the characteristic of being.

Description

  The present invention relates to a lithium titanate suitable for use as an electrode of a lithium ion secondary battery, an intermediate for producing lithium titanate, a lithium titanate, and a method for producing a lithium titanate intermediate.

In recent years, lithium ion secondary batteries are rapidly spreading as secondary batteries having excellent cycle characteristics. Among the electrode active materials of lithium ion secondary batteries, particularly as the negative electrode active material, an alkali metal titanate compound excellent in safety, a titanium compound having a ramsdellite structure, and the like are attracting attention. In particular, spinel type lithium titanate has a theoretical capacity of 175 mAh / g, which is relatively suitable for a negative electrode active material, and has a feature that it has excellent cycle characteristics because of its small volume expansion during charging and discharging. .
The spinel type lithium titanate is produced by calcining a mixture of one or two or more lithium compounds and titanium oxide among lithium carbonate, lithium hydroxide, lithium nitrate and lithium oxide at 670 ° C. or higher and lower than 800 ° C. Preparing a composition composed of TiO ₂ and Li ₂ TiO ₃ or a composition composed of TiO ₂ , Li ₂ TiO ₃ and Li ₄ Ti ₅ O ₁₂ , and then subjecting to the main firing, or the above There has been proposed a method of performing main firing at 800 ° C. to 950 ° C. in an atmosphere having an oxygen gas partial pressure of 1 Pa or less after calcination (Patent Documents 1 and 2).
Lithium titanate obtained by such a method has a high discharge capacity and excellent charge / discharge cycle characteristics when used as an electrode active material of a lithium ion secondary battery. There is a need for lithium titanate with excellent discharge cycle characteristics.

On the other _hand, Li ₄ Ti ₅ in the synthesis of _{O 12} was calcined in an atmosphere containing a reducing _agent, Li ₄ Ti ₅ is oxygen starved of _{O 12 Li 4 Ti 5 O 12} -X lithium titanate intermediate consisting form of Has been proposed to be fired in an oxidizing gas atmosphere and used as an active material for battery materials (Patent Documents 3, 4, and 5).
However, this lithium titanate has oxygen defects due to its inherently stable spinel structure, so that the valence of titanium in the lithium titanate is changed from tetravalent (Ti ⁴⁺ ) to trivalent (T ³⁺ ). It is possible that Originally, the valence of stable titanium is tetravalent, and the trivalence of titanium is unstable and difficult to handle in an air atmosphere at normal temperature and pressure. Therefore, in the battery characteristic in the negative electrode active material of the lithium ion secondary battery obtained by re-oxidizing these or these, the product varies.
In addition, this method of obtaining lithium titanate uses a reducing agent such as hydrogen, hydrocarbon, or carbon monoxide to create a reducing atmosphere, and requires careful attention regarding explosion during preparation. Yes, it is dangerous work.

Japanese Patent Publication No. 2000-302547 Japanese Patent Publication No. 2001-213623 Japanese Patent Publication No. 2011-520552 Patent publication 2010-517223 JP, 2006-50156, A

The present invention solves these problems of the prior art, has a stable crystal structure, has almost no loss of capacitance during high-speed charge / discharge of a lithium secondary battery, has a higher discharge capacity, and a charge / discharge cycle. It aims at providing the manufacturing method of the lithium titanate which is excellent in a characteristic, and is excellent in safety, the intermediate for lithium titanate manufacture, the lithium titanate, and the intermediate for lithium titanate manufacture.
In particular, an object of the present invention is to obtain lithium titanate with little variation in electric capacity of charge / discharge characteristics when used as an electrode active material of a lithium ion secondary battery.

The inventors of the present invention are the inventors of the invention described in Patent Document 5, and in the course of further research on the invention, focused on the following phenomenon.
In the invention described in Patent Document 5, a mixture of a titanium-based compound and a lithium-based compound is baked in an inert gas atmosphere to form a lithium titanate intermediate having oxygen vacancies. It is an invention of recalcining to obtain lithium titanate. The lithium titanate intermediate obtained in this invention was visually in a state in which a blue portion and a gray portion were mixed, but when left untreated, the blue portion gradually changed to gray.
Therefore, this color change was considered that the blue portion was oxidized and turned gray, so that in the firing process of the lithium titanate intermediate having oxygen deficiency in an inert gas atmosphere, a specific oxidation treatment was performed. To obtain a specific gray lithium titanate intermediate. The lithium titanate obtained by refiring this intermediate in an oxidizing gas atmosphere has a smaller variation in the electric capacity of a battery using the lithium titanate as an electrode active material than the conventional one. It turns out that
The present invention has been obtained as a result of such research.

This phenomenon is not construed to limit the present invention, but is assumed to be physically explained as follows.
That is, a lithium titanate intermediate in which oxygen vacancies are generated is obtained by firing the raw material for forming lithium titanate in an inert gas. When oxygen deficiency occurs in the lithium titanate crystal, the lithium titanate crystal is considered to maintain electrical neutrality in the following two modes.
(1) Electrical neutralization by Ti ³⁺ This is because a part of the tetravalent cation atoms (Ti ⁴⁺ ) surrounding the oxygen-deficient site is reduced to trivalent Ti ^3+. Thus, this is a mode in which the electrical neutrality is maintained as a whole. The state where the blue part and the gray part are mixed is visually observed. However, in this embodiment, since the trivalent titanium ions try to return to a stable tetravalent state, the crystallographically unstable state is obtained.
(2) Neutralization by trapping electrons This is a mode in which two electrons corresponding to the valence of oxygen atoms are trapped at the sites of the deficient oxygen atoms, thereby maintaining electrical neutrality. In this embodiment, the color is generally gray. Since there is no unstable trivalent titanium ion, the crystallographically stable state.
Anion lattice defect sites in the crystal, such as sites lacking oxygen atoms, are particularly called F centers.
These two modes can be distinguished by observing the ESR spectrum. That is, the embodiment of (1) can be identified by detecting a trivalent Ti ³⁺ peak, and the embodiment of (2) does not detect the Ti ³⁺ peak and the F center concentration is 1.0. × 10 Specified by ¹⁵ / g or more. In addition, F center can be measured by the signal intensity of g value 2.002 in ESR.
Here, the concentration of F center is defined as the number of oxygen result sites (units / g) per unit weight of the lithium titanate intermediate.
Eventually, the lithium titanate intermediate of Patent Document 1 includes both aspects (1) and (2), but the present invention is only the aspect (2). It became clear from the result of Comparative Example 2.

The present invention was obtained on the basis of such findings, and in a firing step with an inert gas for producing a lithium titanate intermediate, in a cooling step after firing, or after firing and cooling, 100 ° C. or higher It is characterized by being exposed to an atmosphere containing an oxidizing gas in a temperature range of 200 ° C. or lower.
As a result, among the oxygen vacancies formed during firing with the inert gas, the lithium titanate having the above-described aspect (1) disappeared by the supply of oxygen, and only the oxygen vacancy of the aspect (2) was left. An intermediate is formed.

The lithium titanate obtained in the present invention is stable and has little variation, and when used as an electrode active material, the variation in the 10C cycle characteristics of the electric capacity of the charge / discharge characteristics is small compared to the conventional one. Obtained.
In addition, since the oxidation treatment is performed during cooling after firing or after firing and cooling, the firing time can be shortened.

The lithium titanate of this invention and the method of manufacturing this are demonstrated along the manufacturing process.
The lithium titanate of the present invention has an F center value by ESR spectrum measurement of 1.0 × 10 ¹⁵ or more, and when the color is expressed in L ^* a ^* b ^* color system, 70 <L ^* < 90, -1 <a ^* <1, 0 <b ^* <5 is produced by subjecting the lithium titanate intermediate to a heat treatment of 400 ° C. or more and 600 ° C. or less in an atmosphere containing an oxidizing gas. The

1 Preparation of Lithium Titanate Intermediate The titanium-based compound used as a raw material is preferably a rutile type titanium oxide, anatase type titanium oxide, metatitanic acid, orthotitanic acid, or a mixture thereof. Specifically, it is desirable that Fe, Al, Si, and Na contained as impurities are each less than 20 ppm and that Cl is less than 500 ppm. Desirably, Fe, Al, Si and Na are each less than 10 ppm, and Cl is less than 100 ppm, more desirably less than 50 ppm. When titanium oxide is used as the titanium raw material, the specific surface area is 5 m ² / g or more, preferably 10 m ² / g or more, more preferably 15 m ² / g or more.
The lithium compound is preferably lithium hydroxide or lithium carbonate. Further, the average particle size (measured by laser diffraction method) is desirably 0.01 to 100 μm, and in particular, in the case of lithium carbonate, it is 50 μm or less, preferably 5 μm or less, more preferably 0.5 μm or less.
The lithium titanium-based compound can be used as Li ₂ TiO ₃ , Li ₄ Ti ₅ O 1 ₂ , Li ₂ Ti ₆ O ₁₃ , Li ₂ Ti ₈ O ₁₆ , or a mixture thereof, and preferably has high purity. The purity is preferably 99.0% by mass or more, more preferably 99.5% by mass or more, and Fe, Al, Si and Na contained as impurities are each less than 20 ppm, and Cl is preferably less than 500 ppm. . More preferably, Fe, Al, Si and Na are each less than 10 ppm, and Cl is less than 100 ppm, more preferably less than 50 ppm.

In synthesizing the lithium titanate intermediate of the present invention, first, the above lithium compound, titanium compound and lithium titanium compound are mixed with a target value of Li / Ti ratio (atomic ratio) of lithium titanate, for example, 0.68 to 0. According to a value selected from the range of .82, both raw materials are weighed and then mixed with water or a slurry of 10 to 50% by mass of an aqueous medium, and then dried by heating or spray drying.
For mixing these raw materials, a vibration mill, a ball mill or the like is appropriately used. This mixed powder is left in a bulk state, or compressed at a pressure of about 0.5 t / cm ² and subjected to firing as a molded body, or the mixed powder is 10 to 10 in water or a medium such as an aqueous medium. After 50 wt% slurry is sufficiently mixed and dried by heating or spray drying, it is formed into a bulk form or compressed in the same manner and subjected to firing.

The lithium titanate intermediate can be produced, for example, by firing by the following method.
(1) A mixture containing a titanium-based compound and a lithium-based compound and, if necessary, a lithium-titanium-based compound is fired in a temperature range of 120 ° C. or higher and 200 ° C. or lower while being cooled in an inert gas atmosphere. The product is treated in an atmosphere containing an oxidizing gas.
(2) A mixture containing a titanium-based compound and a lithium-based compound, and if necessary, a lithium titanium-based compound is fired in an inert gas atmosphere and then cooled to room temperature. Heat treatment is performed at a temperature of ℃ or less.
Further, after finishing the treatment in the oxidizing gas atmosphere, it may be crushed and pulverized using a hammer mill, a pin mill or the like, if necessary.

In the first example of obtaining the lithium titanate intermediate of the present invention, the fired product is treated in an atmosphere containing an oxidizing gas in the temperature range of 100 ° C. or more and 200 ° C. or less during the cooling after firing. The oxidizing gas is the same as the oxidizing gas used in the re-baking step described later, and is preferably air. When treated outside this temperature range, the lithium titanate intermediate exhibits a white or blue color, and in the battery characteristics of the negative electrode active material of the lithium ion secondary battery obtained by reoxidizing them, the electric capacity may be less. , The variation becomes large. The heating time in the atmosphere containing the oxidizing gas is preferably 1 hour or longer, and more preferably 2 hours or longer and 4 hours or shorter.
A second example of obtaining the lithium titanate intermediate of the present invention is a method in which, after firing, after cooling to room temperature, heat treatment is performed at 100 ° C. to 200 ° C. in an atmosphere containing an oxidizing gas. Similarly to the above, when treated outside this temperature range, the lithium titanate intermediate exhibits white or blue color, and in the battery characteristics of the negative electrode active material of the lithium ion secondary battery obtained by reoxidizing them, the electric capacity Or less variation. The heating time in the atmosphere containing the oxidizing gas is preferably 1 hour or longer, and more preferably 2 hours or longer and 4 hours or shorter.

The firing atmosphere used in the method for producing a lithium titanate intermediate of the present invention is an inert atmosphere. An inert atmosphere is an atmosphere that does not contain an oxidizing gas such as oxygen or ozone, or a reducing gas such as hydrogen, carbon monoxide, or hydrogen sulfide, such as a rare gas atmosphere such as argon or helium, or nitrogen. Atmosphere of these gases. Particularly preferred is a nitrogen atmosphere. The firing atmosphere is opened to an atmosphere containing an oxidizing gas, and the atmosphere containing an oxidizing gas is an atmosphere containing oxygen, ozone, or the like, and more preferably an air atmosphere.
The firing temperature can be 600 to 800 ° C., preferably 650 to 750 ° C., in an inert gas atmosphere. The heating rate is preferably 15 ° C./min, more preferably 5 ° C./min to 15 ° C./min, and more preferably 8 ° C./min to 12 ° C./min. In addition, since the temperature increase rate affects the firing time, it is necessary to set it in consideration of the balance between production efficiency and characteristics. In the method for producing lithium titanate according to the present invention, the temperature rise rate during firing can be fired at a faster rate than the preferred temperature rise rate disclosed in JP-A-2006-50156.

The obtained lithium titanate intermediate has an F center value of 1.0 × 10 ¹⁵ or more by ESR spectrum measurement, and 70 <L when the color is expressed in the L ^* a ^* b ^* color system. ^* <90, -1 <a ^* <1, 0 <b ^* <5.
The F center of the ESR spectrum measurement is detected by coloring the sample by light absorption or the like when there is a defect (defect) in the crystal structure.
The lithium titanate intermediate of the present invention has defects (defects) peculiar to the crystal structure, and shows a gray color when viewed visually. If outside of these ranges, the variation in electric capacity within the lot of charge / discharge characteristics when a lithium ion secondary battery is made increases. Preferably, it is more desirable that an F center value is 1.0 × 10 ¹⁵ or more and a signal derived from Ti ³⁺ is not confirmed.
If it is this range, the dispersion | variation in the electric capacity in the lot of the charge / discharge characteristic when it is set as a lithium ion secondary battery can be made smaller.

2 Synthesis of lithium titanate In the present invention, a lithium titanate intermediate having a specific defect obtained by the above method is heat-treated at 400 ° C. to 600 ° C. in an atmosphere containing an oxidizing gas.
Here, the oxidizing gas is oxygen, ozone, nitrous oxide, nitric oxide, nitrogen dioxide, fluorine, chlorine, chlorine dioxide, nitrogen trifluoride, chlorine trifluoride, oxygen difluoride, perchloryl fluoride. However, it is preferable to use oxygen or ozone. These oxidizing gases may be diluted with a rare gas such as argon or helium, or an inert gas such as nitrogen or a mixed gas thereof. The content of the oxidizing gas in the atmosphere containing the oxidizing gas is preferably 10 to 50% by volume, more preferably 15 to 35% by volume. The atmosphere containing the oxidizing gas is preferably a mixed gas atmosphere of oxygen and nitrogen or an air atmosphere.
The heat treatment time is preferably 30 minutes to 4 hours. By performing heat treatment and baking in an atmosphere containing an oxidizing gas, the lithium titanate intermediate, which has been gray, becomes an electrode active material of a white lithium ion secondary battery.
The furnace used for the heat treatment or the firing for obtaining the above-described lithium titanate intermediate may be a furnace capable of adjusting the atmosphere. It can be carried out in a general box furnace, tunnel furnace, conveyor furnace, kiln furnace or the like.

3 Use as an active material of a lithium ion secondary battery The lithium titanate obtained by the production method of the present invention can be used as an active material of a lithium ion secondary battery. This lithium ion secondary battery includes a negative electrode including lithium titanium oxide obtained by the above-described manufacturing method as a negative electrode active material, a positive electrode including a positive electrode active material, and a lithium ion secondary battery including a nonaqueous electrolyte, or a negative electrode A lithium ion secondary battery including a negative electrode including an active material, a positive electrode including lithium titanium oxide obtained by the above-described manufacturing method as a positive electrode active material, and a nonaqueous electrolyte. The negative electrode and the positive electrode include a current collector and an active material layer formed on the current collector, and the active material layer may include an active material, a binder, and a conductive agent manufactured according to an embodiment of the present invention. .

The current collector is not particularly limited as long as it is formed of a conductive material. For example, a foil or mesh formed of a metal such as aluminum, copper, or stainless steel can be used.
The binder plays a role of properly attaching the active materials to each other and further appropriately attaching the active materials to the current collector. Typical examples include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride. Ride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon and the like may be used, but are not limited thereto.
The conductive agent is used to impart conductivity to the electrode, and any battery can be used as long as it is an electron conductive material that does not cause a chemical change in the constructed battery. As such, natural graphite, artificial graphite, or the like may be used, or a conductive material such as a polyphenylene derivative may be mixed and used.

When the lithium titanate obtained by the production method of the present invention is used as an active material for the negative electrode, the positive electrode active material serving as a counter electrode is an oxide containing lithium and a transition metal element, or a polyanionic compound. Etc. can be used. Specifically, for example, lithium cobalt composite oxide (Li _(1-n) CoO ₂ or the like (0 <n <1, the same applies hereinafter)), lithium nickel composite oxide (Li _(1-n) NiO ₂ or the like) , Lithium manganese composite oxide (Li _(1-n) MnO ₂ , Li _(1-n) Mn ₂ O ₄ etc.), lithium iron composite phosphate (LiFePO ₄ etc.), lithium vanadium composite oxide (LiV ₂ O ₃ ) and the like.
When the lithium titanate obtained by the production method of the present invention is used as an active material for the positive electrode, an example of the negative electrode active material serving as a counter electrode is Li metal foil. When Li metal foil is used as the negative electrode, it can be used by directly crimping the current collector without using a conductive agent or a binder.

  As an electrode active material for any of the lithium ion secondary batteries, lithium titanate obtained by the production method of the present invention is used. The electrode of this lithium ion battery is, for example, a mixture of powdered lithium titanate, a conductive material and a binder, an appropriate solvent added to form a paste-like negative electrode, and then applied to the surface of the current collector and dried. If necessary, it can be compressed to increase the electrode density. The negative electrode can be used by directly pressing the current collector without using a conductive agent or a solvent.

Further, in the lithium ion secondary battery of the present invention, a non-aqueous electrolyte solution, a gel electrolyte, a solid electrolyte, or the like in which a lithium salt is dissolved in a non-aqueous organic solvent can be used, but it is preferable to use a non-aqueous electrolyte solution. .
The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move. Examples of the non-aqueous organic solvent include ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (γ-BL), diethyl carbonate (DEC), dimethyl carbonate (DMC), butylene carbonate (BC), and ethyl methyl. Examples include organic solvents used in conventional secondary batteries and capacitors such as carbonate (EMC). These may be used alone or in combination.
Lithium salt dissolves in non-aqueous organic solvent, acts as a source of lithium ions in the battery, enables basic lithium secondary battery operation, and promotes the movement of lithium ions between the positive and negative electrodes It is a substance that performs. For example, a lithium salt such as LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , Li (CF ₃ SO ₂ ) ₂ N, Li (CF ₃ SO ₃ ), LiN (C ₂ F ₅ SO ₂ ) ₂ may be used. it can. The concentration of the lithium salt is preferably 0.1 to 2.0M, and more preferably 0.8 to 1.2 mol / L.

  In the lithium ion secondary battery, a separator may be present between the positive electrode and the negative electrode depending on the type of the lithium ion secondary battery. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the lithium ion secondary battery. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, or an olefin resin such as polyethylene or polypropylene can be used. A thin microporous membrane can be mentioned. These may be used alone or in combination.

  The lithium ion secondary battery produced by the above-described method can measure charge / discharge characteristics using a charge / discharge device (for example, HJ1001SD manufactured by Hokuto Denko Co., Ltd.). When the lithium titanate obtained in this step is used as an electrode, the standard deviation (σ) of the electric capacity of the lithium ion secondary battery obtained with the same lot of lithium titanate is 3.5 or less, more preferably It can be 3.0 or less.

Ｎ_sam ： 実試料中のスピン数［個］
Ｎ_ref ： 標準試料中のスピン数
Ｓ ： 実試料のスピン量子数
ｓ ： 標準試料のスピン量子数
Ｉ_sam ： 実試料の信号強度
Ｉ_ref ： 標準試料の信号強度
ここで、信号強度とは、測定で得られる微分型のスペクトルを1回積分して算出されるピーク面積を意味する。
また、標準試料は、ダメージを与えたポリエチレンを使用した。
２）色差の測定
（１）装置：測色色差計；ＺＥ ２０００（日本電色工業）
（２）測定条件
測定方法 ：刺激値直読方法
分光感度 ：ＹＺ方式
測光方式 ：ダブルビーム方式（交照測光方式）
光源 ：ハロゲンランプ
照明受光条件： ０°−ｄ法 ＪＩＳ Ｚ −８７２２に準拠
照射径 ：３０ｍｍφThe Example of this invention is shown. However, the present invention is not construed as being limited to these examples.
The ESR spectrum measurement and the color difference measurement were performed by the following method.
1) ESR spectrum measurement (20K)
(1) Model: Elexsys E580 (BRUKER)
(2) Measurement conditions Measurement temperature 20 K
Central magnetic field 3428 G vicinity Magnetic field sweep range 2500 G
Modulation 100 kHz, 5 G
Microwave 9.44 GHz, 0.026 mW
Sweep time 167.77 s × 3-6 times
Time constant 163.84 ms
Number of data points 2048 points
Cavity TE011, cylindrical type conversion formula (F center value)
A value obtained by dividing N _sam calculated by the following equation by the weight of the material was defined as an F center value (pieces / g).

N _sam : Number of spins in real sample [pieces]
N _ref : Number of spins in the standard sample
S: Spin quantum number of real sample
s: spin quantum number of standard sample
I _sam : Signal strength of actual sample
I _ref : Signal strength of standard sample
Here, the signal intensity means a peak area calculated by integrating a differential spectrum obtained by measurement once.
Moreover, the damaged polyethylene was used for the standard sample.
2) Measurement of color difference
(1) Apparatus: Colorimetric color difference meter; ZE 2000 (Nippon Denshoku Industries)
(2) Measurement conditions Measurement method: Stimulus value direct reading method Spectral sensitivity: YZ method Metering method: Double beam method (crossing metering method)
Light source: Halogen lamp Light receiving condition: 0 ° -d method Conforms to JIS Z-8722 Irradiation diameter: 30 mmφ

Example 1
Prepare 240.9 g of lithium hydroxide as the lithium source and 573.5 g of titanium oxide powder as the titanium source, mix with ion-exchanged water so that the concentration of the raw material solids is 20% by mass, and make a slurry. As a result, 2% by mass of Kaosela 2100 (Kao Corporation) was added as a solid content. This slurry is pulverized and mixed using a ball mill, and then spray granulated with hot air at 220 ° C. using a spray dryer (manufactured by Yamato Kagaku Co., Ltd., GB210-B) to form spherical granules having an average particle size of about 10 μm. A mixed powder was obtained.
This granulated mixed powder was sufficiently replaced with nitrogen and kept at a firing temperature of 750 ° C. for 6 hours in a nitrogen atmosphere furnace with an oxygen concentration of 0.1% or less at a heating rate of 10 ° C./min. did. At that time, nitrogen was continued in the nitrogen atmosphere furnace. Thereafter, while the temperature inside the furnace was 120 ° C. or more and 200 ° C. or less during the cooling, the inside of the furnace was opened to the atmosphere and the cooling was continued. What was obtained was a gray powdery lithium titanate intermediate. When this was subjected to ESR spectrum measurement, the F center was 1.2 × 10 ¹⁶ [pieces / g], and no peak derived from Ti ³⁺ was detected. When the L ^* a ^* b ^* color system was confirmed with a color difference meter, L ^* = 79.1, a ^* = 0.6, and b ^* = 3.7.
The obtained lithium titanate intermediate was heated in an air atmosphere at 500 ° C. for 2 hours, and 1.8 g of the obtained lithium titanate was collected, 3.0 mL of N-methyl-2-pyrrolidone, polyfluorinated 0.1 g of vinylidene and 0.1 g of acetylene black were mixed using a rotating / revolving mixer to obtain a paste.
Then, the slurry was apply | coated by the thickness of 80 micrometers on aluminum foil using the doctor blade. This was dried at a temperature of 80 ° C., punched out into a circle having a diameter of 14 mm, and pressed to obtain an electrode.
With respect to the obtained electrode, an electrolytic solution in which LiPF ₆ was dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate and ethyl methyl carbonate (volume ratio 1: 1) as an anode, A coin cell was prepared using a polypropylene nonwoven fabric as a separator.
The coin cell produced above was set in a holder for charge / discharge measurement installed in a thermostat at 30 ° C., and charge / discharge characteristics were measured using a charge / discharge device (HJ1001SD, manufactured by Hokuto Denko). First, a current of 17.5 mA per 1 g of lithium titanate in the positive electrode (discharge rate: 0.1 C) was passed to discharge the battery to a voltage of 1.0 V, and the voltage was further maintained at 1.0 V for 6 hours. Discharged (initial discharge).
After the initial discharge, the battery was charged to 3.0 V with a current of 0.1 C, and then discharged twice to 1.0 V at 0.1 C. Thereafter, charging / discharging was performed under the condition of a current of 10 C (1750 mA / h per gram of lithium titanate), and an average value of the amount of current flowing at the time of discharge was converted into an amount of electricity per gram of lithium titanate. It was. Fifteen coin cells were made from the same lot of lithium titanate, and the average value and standard deviation (σ) of the electric capacity obtained at each 10C were determined. The standard deviation at this time was 1.5.
Here, 1C means the discharge rate of the battery, which means a current value when the entire battery capacity [Ah] is completely discharged at a constant current for 1 h. Therefore, 0.1 C means a current value when the entire capacity is completely discharged in 10 hours. 10C means a current value when the entire capacity is completely discharged at 0.1 h. For example, the discharge current value when the discharge rate is 10C is 100 compared with that when the discharge rate is 0.1C. The value will be twice as large.

(Example 2)
Lithium titanate intermediate was prepared under the same conditions as in Example 1 except that the lithium titanate which was allowed to cool to room temperature was not dried during cooling and was dried in air at 120 ° C. for 2 hours. Was made. As a result, gray powdery lithium titanate was obtained.
When this was subjected to ESR spectrum measurement, the F center was 2.2 × 10 ¹⁶ [pieces / g], and a peak derived from Ti ³⁺ was not detected. Further, when the L ^* a ^* b ^* color system was confirmed with a color difference meter, L ^* = 80.1, a ^* = 0.2, and b ^* = 2.9. This was used as an electrode in the same manner as in Example 1 to produce a coin cell, and the average value and standard deviation (σ) of the electric capacity obtained under the condition of a current of 10 C were obtained. The standard deviation at this time was 2.3.

(Comparative Example 1)
A lithium titanate intermediate was produced under the same conditions as in Example 1 except that the firing was started in an air atmosphere with an oxygen concentration of 21% without performing nitrogen substitution. As a result, white powdery lithium titanate was obtained.
When this was subjected to ESR spectrum measurement, neither the F center nor the peak derived from Ti ³⁺ was detected. When the L ^* a ^* b ^* color system was confirmed with a color difference meter, L ^* = 95.8, a ^* = − 0.2, and b ^* = 0.5. This was used as an electrode in the same manner as in Example 1 to produce a coin cell, and the average value and standard deviation (σ) of the electric capacity obtained under the condition of a current of 10 C were obtained. The standard deviation at this time was 4.2.

(Comparative Example 2)
A lithium titanate intermediate was produced under the same conditions as in Example 1, except that the atmosphere was not released during the cooling, and the heat treatment was not performed in the atmosphere after firing. As a result, blue powdery lithium titanate was obtained.
When this was subjected to ESR spectrum measurement, the F center was 6.1 × 10 ¹⁵ [pieces / g], and the peak derived from Ti ³⁺ was 1.6 × 10 ¹⁹ [pieces / g]. When the L ^* a ^* b ^* color system was confirmed with a color difference meter, L ^* = 74.6, a ^* = − 2.5, and b ^* = − 5.4. This was used as an electrode in the same manner as in Example 1 to produce a coin cell, and the average value and standard deviation (σ) of the electric capacity obtained under the condition of a current of 10 C were obtained. The standard deviation at this time was 3.7.

(Comparative Example 3)
During the cooling, while the furnace temperature was 300 ° C. or higher and 400 ° C. or lower, the inside of the furnace was opened to the atmosphere and the cooling was continued. An intermediate was made. As a result, gray powdery lithium titanate was obtained.
When this was subjected to ESR spectrum measurement, the F center was 6.3 × 10 ¹⁵ [pieces / g], and no peak derived from Ti ³⁺ was detected. When the L ^* a ^* b ^* color system was confirmed with a color difference meter, L ^* = 84.1, a ^* = 0.1, and b ^* = 1.9. This was used as an electrode in the same manner as in Example 1 to produce a coin cell, and the average value and standard deviation (σ) of the electric capacity obtained under the condition of a current of 10 C were obtained. The standard deviation at this time was 6.7.

The results of the above examples and comparative examples are summarized in a table as shown in [Table 1] below.

As is clear from the results of [Table 1], the lithium titanate intermediate produced under the firing conditions specified in the present invention has an F center concentration of 1.0 × 10 ¹⁵ (pieces / g) or more, And Ti ³⁺ are not detected. And the lithium titanate produced using the lithium titanate intermediate having this unique oxygen deficiency structure has a variation in electric capacity of charge / discharge characteristics when used as an electrode active material of a lithium ion secondary battery. The lithium ion secondary battery manufactured using lithium titanate having excellent electrical characteristics with a standard deviation of 3.5 or less has remarkably improved charge / discharge characteristics.

Claims (5)

When the F center value by ESR spectrum measurement is 1.0 × 10 ¹⁵ (pieces / g) or more, no trivalent titanium is present, and the color is expressed in the L ^* a ^* b ^* color system , 70 <L ^* <90, -1 <a ^* <1, 0 <b ^* <5. The F center value measured by ESR spectrum is 1.0 × 10 ¹⁵ (pieces / g) or more, trivalent titanium (Ti ³⁺ ) does not exist, and the color is L ^* a ^* b ^* color When expressed in a system, a lithium titanate intermediate in the range of 70 <L ^* <90, -1 <a ^* <1, 0 <b ^* <5 is 400 ° C. or higher and 600 ° C. in an atmosphere containing an oxidizing gas. A method for producing lithium titanate , characterized by performing a heat treatment at a temperature of ℃ or less. The lithium titanate intermediate is a mixture containing a titanium compound and a lithium compound, and optionally a lithium titanium compound, after firing in an inert gas atmosphere and cooling, in the middle of 100 ° C. to 200 ° C. The method for producing lithium titanate according to claim 2 , wherein the fired product is treated in an atmosphere containing an oxidizing gas in a temperature range of 5%. The lithium titanate intermediate is a mixture containing a titanium compound and a lithium compound, and optionally a lithium titanium compound, fired in an inert gas atmosphere, cooled to room temperature, and then in an atmosphere containing an oxidizing gas. The method for producing lithium titanate according to claim 2 , wherein a heat treatment is performed at 100 ° C. to 200 ° C. A paint containing lithium titanate obtained by the manufacturing method according to claim 2 , a binder, and a conductive agent is applied on a current collector, and the current collector and the current collector are laminated. The manufacturing method of a lithium ion secondary battery including the process of manufacturing the electrode which has the made lithium titanate layer.