JP6138554B2 - COMPOSITE MATERIAL, METHOD FOR PRODUCING THE COMPOSITE MATERIAL, LITHIUM ION SECONDARY BATTERY AND ELECTROCHEMICAL CAPACITOR USING THE COMPOSITE MATERIAL - Google Patents

Description

The present invention relates to a composite material of spinel-type lithium titanate (Li _{4 + x} Ti ₅ O ₁₂ (0 ≦ x ≦ 3)) (hereinafter simply referred to as “lithium titanate”) and conductive carbon. In addition, the present invention relates to a composite material substantially free of titanium oxide that is easily produced as a by-product during the production of lithium titanate and a method for producing the same. The present invention also relates to a lithium ion secondary battery and an electrochemical capacitor using the composite material.

Lithium ion secondary batteries using non-aqueous electrolytes with high energy density are widely used as power sources for information devices such as cellular phones and notebook computers. A lithium ion secondary battery using a current non-aqueous electrolyte includes lithium cobalt oxide (LiCoO ₂ ) as a positive electrode active material, graphite as a negative electrode active material, and a lithium salt such as lithium hexafluorophosphate (LiPF ₆ ). The mainstream is an electrolyte obtained by dissolving a solution in a non-aqueous solvent such as ethylene carbonate or propylene carbonate. However, regarding the graphite of the negative electrode, when lithium ions are inserted into the graphite, a solid electrolyte interface (SEI) film with poor thermal stability is easily formed on the surface of the graphite due to the decomposition reaction of the electrolytic solution, and lithium dendride is likely to precipitate. There was a problem.

  In order to solve this problem, it has been proposed to use lithium titanate as a negative electrode material instead of graphite. Lithium titanate does not easily form an SEI film, lithium dendride does not precipitate, and there is almost no volume change when lithium ions are inserted into or extracted from lithium titanate. An ion secondary battery is expected to be obtained. However, when producing lithium titanate by mixing the raw material titanium compound (titanium source) and lithium compound (lithium source) and firing them, rutile titanium oxide that lowers battery performance is produced as a by-product. There was a problem that it was easy. Further, since the obtained lithium titanate generally has an agglomerated structure, there is a problem that the capacity of a battery using this material as a negative electrode is low, and the capacity is greatly reduced in discharging with a large current.

  By the way, the applicant, in Patent Document 1 (Japanese Patent Laid-Open No. 2007-160151), introduced a reaction liquid containing a raw material of metal oxide and conductive carbon into a swirlable reactor, and then installed the reactor. Rotating and applying a shear stress and centrifugal force to the reaction solution to disperse the conductive carbon to advance the chemical reaction, depositing the metal oxide precursor on the conductive carbon with good dispersibility, and then the obtained metal By heating the conductive carbon on which the oxide precursor is supported and converting the metal oxide precursor to a metal oxide on the conductive carbon, the metal oxide nanoparticles are supported on the surface of the conductive carbon. Has proposed a method of manufacturing composite materials. This production method can also be suitably applied to the production of a composite material of lithium titanate nanoparticles and conductive carbon. The term “nanoparticle” means a particle having a diameter of 100 nm or less in the case of a spherical particle, and a diameter (short axis) of a particle cross section in the case of a needle-like, tubular or fibrous particle of 100 nm or less. Means particles. The nanoparticles may be primary particles or secondary particles.

  Patent Document 2 (Japanese Patent Application Laid-Open No. 2008-270795) and Patent Document 3 (Japanese Patent Application Laid-Open No. 2011-213556) disclose lithium titanate nanoparticles and conductive carbon obtained by improving the production method described in Patent Document 1. And a method for producing a composite material. When titanium alkoxide is used as the titanium source, a chemical reaction in which shear stress and centrifugal force are applied depending on the reaction conditions is too rapid, and a product in which titanium oxide nanoparticles, not lithium titanate, are mainly supported on conductive carbon Problem arises. Patent Document 2 discloses a method of adding acetic acid or the like forming a complex with titanium alkoxide as a reaction inhibitor to the reaction solution in order to solve this problem. Patent Document 3 discloses a lithium titanate doped with nitrogen by carrying a heat treatment in a nitrogen atmosphere after supporting a lithium titanate precursor on conductive carbon by a chemical reaction in which shear stress and centrifugal force are applied. A method for producing a composite material comprising a plurality of nanoparticles is disclosed. During the heat treatment in a nitrogen atmosphere, oxygen defects are generated in the lithium titanate due to the reducing action of carbon, and the oxygen defects are doped with nitrogen. This nitrogen-doped lithium titanate has high electrical conductivity, and lithium ions can easily enter and exit between the crystal lattices of lithium titanate.

  By the method described in Patent Documents 1 to 3, a composite material in which lithium titanate nanoparticles having a large specific surface area are attached on conductive carbon can be obtained. And the lithium ion secondary battery which used this composite material for the negative electrode shows the high discharge capacity, and also shows the outstanding rate characteristic in which the fall of the capacity | capacitance was suppressed also in the discharge by a large current. These documents also describe that this composite material is also suitable for the negative electrode of an electrochemical capacitor.

JP 2007-160151 A JP 2008-270795 A JP 2011-213556 A

  However, as a result of producing a composite material in which lithium titanate nanoparticles adhered to conductive carbon by the method described in Patent Documents 1 to 3 and analyzing by X-ray powder diffraction, a small amount of titanium oxide was mixed. I found out. And the content of titanium oxide in a composite material became remarkable, so that the manufacturing amount per time of composite material was increased, or the density | concentration of the titanium source and lithium source in a reaction liquid was increased. Since the mixed titanium oxide deteriorates the performance of the lithium ion secondary battery and the electrochemical capacitor, it is desired to produce a composite material not containing titanium oxide.

  Therefore, an object of the present invention is to provide a method for producing a composite material of lithium titanate nanoparticles and conductive carbon, and a method for producing a composite material substantially free of titanium oxide.

  As a result of intensive studies based on the reaction methods disclosed in Patent Documents 1 to 3, the inventors supported a lithium titanate precursor on conductive carbon by a chemical reaction in which shear stress and centrifugal force were applied. Then, it discovered that the said objective was achieved by performing baking in two steps.

  Accordingly, the present invention first comprises a composite comprising conductive carbon powder and spinel-type lithium titanate nanoparticles adhering to the conductive carbon powder, and substantially free of titanium oxide. Provide material. The term “substantially free of titanium oxide” means that no diffraction peak of titanium oxide is observed in the X-ray powder diffractogram of the composite material.

The composite material of the present invention is
a) a preparation step of introducing a reaction solution containing a lithium source, a titanium source and a conductive carbon powder into a swirlable reactor;
b) A supporting step of supporting the lithium titanate precursor in a highly dispersed state on the conductive carbon powder by rotating the reactor and applying shear stress and centrifugal force to the reaction solution.
c) By conducting the primary firing of the conductive carbon powder carrying the lithium titanate precursor at a temperature of 200 to 500 ° C, followed by secondary firing at a temperature of 600 to 950 ° C in vacuum or nitrogen, It can be obtained by the method for producing a composite material of the present invention, comprising a firing step of converting the lithium titanate precursor into spinel-type lithium titanate nanoparticles on the conductive carbon powder.

In the supporting step, by applying shear stress and centrifugal force to the reaction liquid by swirling the reactor, the conductive carbon powder is dispersed and the titanium source hydrolysis reaction, condensation reaction, and subsequent lithium source The lithium titanate precursor composed of a composite of lithium and titanium oxide is supported on the conductive carbon powder. The centrifugal force applied to the reaction solution by the rotation of the reactor is a centrifugal force in a range generally referred to as “ultracentrifugal force”, and preferably a centrifugal force of 1500 kgms ⁻² or more. With this range of centrifugal force, the lithium titanate precursor is supported on the conductive carbon powder as fine particles of uniform size. In the present specification, the process of applying shear stress and centrifugal force to the reaction solution in a swirling reactor may be referred to as “ultracentrifugation process”. As a reactor, it consists of the concentric cylinder of the outer cylinder and the inner cylinder described in FIG. 1 of patent document 1 (Unexamined-Japanese-Patent No. 2007-160151). A reactor in which a plate is disposed at the opening of the outer cylinder is preferably used. The description regarding the reactor in Patent Document 1 is incorporated herein by reference. In the reactor, the distance between the inner cylinder outer wall surface and the outer cylinder inner wall surface is preferably 5 mm or less, and more preferably 2.5 mm or less.

  As described above, when the titanium source is a titanium alkoxide, the chemical reaction in which shear stress and centrifugal force are applied depending on the reaction conditions is too rapid, and the titanium oxide nanoparticles rather than lithium titanate are mainly conductive. A product supported on carbon is obtained. In order to solve this problem, it is preferable to include a reaction inhibitor that forms a complex with titanium alkoxide in the reaction solution.

In the method for producing a composite material of the present invention, Li ₂ TiO ₃ and titanium oxide are generated from the lithium titanate precursor supported on the conductive carbon powder by the primary firing in the firing step. Since the lithium titanate precursor is supported in a highly dispersed state on the conductive carbon powder, fine Li ₂ TiO ₃ and titanium oxide are uniformly formed on the conductive carbon powder after the primary firing. Then, by secondary firing, spinel-type lithium titanate nanoparticles are generated from finely distributed Li ₂ TiO ₃ and titanium oxide. It was found that the titanium oxide in the composite material finally obtained can be substantially eliminated by passing through a step of generating Li ₂ TiO ₃ and titanium oxide from the lithium titanate precursor.

In the firing step, the primary firing atmosphere and firing time are not strictly limited, but the secondary firing needs to be performed in a vacuum or nitrogen in order to prevent the loss of conductive carbon. About baking temperature, primary baking is the range of 200-500 degreeC, and secondary baking is the range of 600-950 degreeC. Within this range, good lithium titanate can be obtained, and good capacity and rate characteristics can be obtained. If the firing temperature in the primary firing is less than 200 ° C., the conversion from the lithium titanate precursor to Li ₂ TiO ₃ and titanium oxide is not preferable, and if the firing temperature in the primary firing exceeds 500 ° C., the lithium titanate precursor Lithium titanate is generated without undergoing conversion from the body to Li ₂ TiO ₃ and titanium oxide, and when oxygen is contained in the firing atmosphere, the conductive carbon powder is lost. This is not preferable because it cannot be ignored. When the secondary firing temperature is less than 600 ° C., conversion from Li ₂ TiO ₃ and titanium oxide to lithium titanate is not preferable, and when the secondary firing temperature exceeds 950 ° C., lithium titanate aggregates. This is not preferable because the particle size exceeds the size of the nanoparticles.

  Since secondary firing is firing at a high temperature, long-time firing is not preferable because lithium titanate aggregates and the particle size increases (see Example 2 of Patent Document 2). This increase in particle size can also occur in the temperature raising process up to the temperature of secondary firing and the temperature lowering process after secondary firing. In order to suppress the increase in the particle size, in the method for producing a composite material of the present invention, in the firing step, the temperature raising process from 500 ° C. to the temperature of the secondary firing is performed for 5 minutes or less in vacuum or nitrogen. It is preferable to carry out the second baking in the range of 1 to 20 minutes, and perform the temperature lowering process up to 500 ° C. after the second baking in a vacuum or in nitrogen for a time of 5 minutes or less. Within this range, good lithium titanate without aggregation can be obtained, and good rate characteristics can be obtained.

  In the composite material of the present invention, when the lithium titanate in the composite material has an oxygen defect and the oxygen defect is doped with nitrogen, the nitrogen-doped lithium titanate has a high electrical conductivity and a crystal lattice. It is preferable because lithium ions can easily enter and leave the space. In the firing step in the method for producing a composite material of the present invention, the temperature raising process from 500 ° C. to the secondary firing temperature, the secondary firing, and the temperature lowering process to 500 ° C. after the secondary firing are performed in nitrogen. A composite material containing nitrogen-doped lithium titanate can be suitably obtained.

  The composite material of the present invention is suitable for a negative electrode of a lithium ion secondary battery and an electrochemical capacitor because it provides an electrode exhibiting a high discharge capacity and excellent rate characteristics. Therefore, the present invention also provides a lithium ion secondary battery having a negative electrode having an active material layer containing the composite material of the present invention, and an electrochemical capacitor having a negative electrode having an active material layer containing the composite material of the present invention. ,I will provide a.

  By the composite material manufacturing method of the present invention, a composite of spinel-type lithium titanate nanoparticles and conductive carbon substantially free of titanium oxide that adversely affects the performance of lithium ion secondary batteries and electrochemical capacitors A material is obtained. By using this composite material, a lithium ion secondary battery and an electrochemical capacitor having a high discharge capacity and excellent rate characteristics can be obtained.

It is an X-ray powder diffractogram about the composite material manufactured through primary baking, and the composite material manufactured without performing primary baking. It is a figure which shows the rate characteristic about the half battery using the composite material manufactured through primary baking, and the half battery using the composite material manufactured without passing through primary baking. It is the figure which showed the rate characteristic in the form of the capacity | capacitance maintenance factor about the half battery using the composite material manufactured through primary baking, and the half battery using the composite material manufactured without passing through primary baking. .

(A) Composite Material and Method for Producing the Same The composite material of the present invention includes a conductive carbon powder and spinel-type lithium titanate nanoparticles attached to the conductive carbon powder, and substantially contains titanium oxide. Not included. This composite material
a) a preparation step of introducing a reaction solution containing a lithium source, a titanium source and a conductive carbon powder into a swirlable reactor;
b) A supporting step of supporting the lithium titanate precursor in a highly dispersed state on the conductive carbon powder by rotating the reactor and applying shear stress and centrifugal force to the reaction solution.
c) By conducting the primary firing of the conductive carbon powder carrying the lithium titanate precursor at a temperature of 200 to 500 ° C, followed by secondary firing at a temperature of 600 to 950 ° C in vacuum or nitrogen, It can be produced by a method including a firing step of converting the lithium titanate precursor into spinel type lithium titanate nanoparticles on the conductive carbon powder. Hereinafter, each step will be described in detail.

a) Preparation Step In the preparation step, at least a lithium source and a titanium source that are raw materials for lithium titanate and a conductive carbon powder for adhering lithium titanate are added to the solvent, and the lithium source and the titanium source are used as a solvent. To obtain a reaction solution.

  As the solvent, any solvent that does not adversely influence the reaction can be used without particular limitation, and water, methanol, ethanol, isopropyl alcohol, and the like can be suitably used. Two or more solvents may be mixed and used.

  As the lithium source and the titanium source, a compound that can be dissolved in the solvent can be used without any particular limitation. Examples of the lithium source include lithium acetate, lithium hydroxide, lithium nitrate, lithium carbonate, and lithium sulfate. As the titanium source, alkoxides such as methoxide, ethoxide, isopropoxide, and butoxide of titanium are suitable, but other than these, organic metal salts such as acetate, and inorganic metal salts such as chloride and nitrate are exemplified. A single compound may be used for each of the lithium source and the titanium source, or two or more compounds may be mixed and used. The atomic ratio of lithium in the lithium source to titanium in the titanium source is selected to be greater than 0.8 and is preferably approximately 1.

  As the conductive carbon powder, any conductive carbon powder can be used without particular limitation. Examples include carbon black such as ketjen black, acetylene black, channel black, fullerene, carbon nanotube, carbon nanofiber, amorphous carbon, carbon fiber, natural graphite, artificial graphite, graphitized ketjen black, activated carbon, mesoporous carbon And so on. Also, vapor grown carbon fiber can be used. These carbon powders may be used alone or in combination of two or more. When the specific surface area of the carbon powder is large, the dispersibility of lithium titanate in the final product can be improved, and the content of lithium titanate in the composite material of the final product can be increased. Nanoparticles are preferred.

  When the titanium source is a titanium alkoxide, for example, a complex with a titanium alkoxide such as acetic acid, citric acid, succinic acid, formic acid, lactic acid, tartaric acid, fumaric acid, succinic acid, propionic acid, repric acid, EDTA, triethanolamine, etc. The compound to be formed is preferably included in the reaction solution as a reaction inhibitor for the purpose of suppressing the hydrolysis reaction rate of titanium alkoxide.

  As the reactor capable of swirling, any reactor can be used as long as it can apply a supercentrifugal force to the reaction solution, but it is described in FIG. 1 of Patent Document 1 (Japanese Patent Laid-Open No. 2007-160151). A reactor having a concentric cylinder of an outer cylinder and an inner cylinder, in which a through-hole is provided in a side surface of the rotatable inner cylinder, and a slat plate is disposed at an opening of the outer cylinder is preferably used. . Hereinafter, the form which uses this suitable reactor is demonstrated.

  A reaction solution for reaction in an ultracentrifugal force field is introduced into the inner cylinder of the reactor. A reaction liquid prepared in advance may be introduced into the inner cylinder, or may be introduced by preparing the reaction liquid in the inner cylinder. It is only necessary to obtain a reaction solution in which a lithium source, a titanium source, and a reaction inhibitor as required are dissolved in a solvent and a conductive carbon powder is dispersed in the solution. There is no limitation on the order of addition of the reaction inhibitor and the conductive carbon powder according to the conditions. Add the titanium source and conductive carbon powder to the solvent, stir the resulting liquid to disperse the conductive carbon powder in the liquid, then add the lithium source and water as needed to the solvent containing water for hydrolysis It is preferable to mix a solution in which the reaction inhibitor is dissolved. This is because by increasing the dispersibility of the conductive carbon powder, the lithium titanate precursor is more uniformly and finely supported on the conductive carbon powder in the following supporting step. Stirring may be performed using a stirring means such as a homogenizer, or when preparing a reaction liquid in the reactor, the stirring may be performed by rotating the reactor.

b) Supporting step In the supporting step, the reactor is swirled to apply shear stress and centrifugal force to the reaction solution, thereby dispersing the conductive carbon powder, followed by hydrolysis reaction, condensation reaction and subsequent titanium source. The reaction with the lithium source is promoted, and a lithium titanate precursor composed of a composite of lithium and titanium oxide is supported on the conductive carbon powder.

Conversion from a titanium source and a lithium source to a lithium titanate precursor and loading of the lithium titanate precursor onto a conductive carbon powder is realized by mechanical energy of shear stress and centrifugal force applied to the reaction solution. However, the shear stress and the centrifugal force are generated by the centrifugal force applied to the reaction liquid by the rotation of the inner cylinder. The centrifugal force applied to the reaction solution in the inner cylinder is a centrifugal force in a range generally referred to as “ultracentrifugal force”, and is generally 1500 kgms ⁻² or more, preferably 70000 kgms ⁻² or more, particularly preferably 270000 kgms ⁻² or more. .

  The embodiment using the preferred reactor having the outer cylinder and the inner cylinder described above will be described. When the inner cylinder of the reactor into which the reaction liquid has been introduced is swung, the centrifugal force generated by the swirling of the inner cylinder causes The reaction liquid moves to the outer cylinder through the through hole, and the reaction liquid between the outer wall of the inner cylinder and the inner wall of the outer cylinder becomes a thin film and slides up to the upper part of the inner wall of the outer cylinder. As a result, shear stress and centrifugal force are applied to this thin film, and this mechanical energy seems to be converted into chemical energy necessary for the reaction, so-called activation energy, but the reaction proceeds in a short time.

  In the above reaction, the narrower the distance between the outer wall surface of the inner cylinder and the inner wall surface of the outer cylinder, the more preferable mechanical energy can be applied to the thin film. The distance between the inner cylinder outer wall surface and the outer cylinder inner wall surface is preferably 5 mm or less, more preferably 2.5 mm or less, and particularly preferably 1.0 mm or less. The distance between the inner cylinder outer wall surface and the outer cylinder inner wall surface can be set by the width of the reactor plate and the amount of the reaction liquid introduced into the reactor.

  The turning time of the inner cylinder is not strictly limited and varies depending on the amount of the reaction solution and the turning speed (centrifugal force) of the inner cylinder, but is generally in the range of 0.5 to 10 minutes. . After completion of the reaction, the inner cylinder stops turning, and the conductive carbon powder carrying the lithium titanate precursor is recovered and dried.

  In this supporting step, by using a rotating reactor, the conductive carbon powder is highly dispersed in the reaction solution, and the lithium titanate precursor is supported in a highly dispersed state on the highly dispersed conductive carbon.

c) Firing step In the firing step, the conductive carbon powder carrying the lithium titanate precursor obtained in the carrying step is first calcined at a temperature of 200 to 500 ° C, and then in vacuum or in nitrogen in 600 to 950. Secondary firing at a temperature of ° C.

The primary firing is performed at a temperature in the range of 200 to 500 ° C. By primary firing, the lithium titanate precursor supported on the conductive carbon powder is converted into Li ₂ TiO ₃ and titanium oxide.

The atmosphere for the primary firing is not particularly limited, and may be firing in a vacuum, firing in nitrogen, or firing in an oxygen-containing atmosphere such as air. Although there is no special limitation in the time of primary baking, generally it is the range of 30 minutes-10 hours. If the firing temperature in the primary firing is less than 200 ° C., the conversion from the lithium titanate precursor to Li ₂ TiO ₃ and titanium oxide is not preferable, and if the firing temperature in the primary firing exceeds 500 ° C., the lithium titanate precursor It is not preferable because lithium titanate is produced without undergoing conversion from the body to Li ₂ TiO ₃ and titanium oxide, and it is not preferable because the loss of the conductive carbon powder cannot be ignored if oxygen is contained in the firing atmosphere. Absent.

In the supporting step, since the lithium titanate precursor is supported in a highly dispersed state on the conductive carbon powder, fine Li ₂ TiO ₃ and titanium oxide are uniformly formed on the conductive carbon powder after the primary firing. .

Secondary baking is performed after completion | finish of primary baking. Although secondary baking may be performed immediately after primary baking, a cooling process may be provided after primary baking, and then secondary baking may be performed. Fine Li ₂ TiO ₃ and titanium oxide uniformly formed on the conductive carbon powder by the primary firing are converted into spinel-type lithium titanate nanoparticles on the conductive carbon powder by the secondary firing.

The secondary firing needs to be performed in vacuum or nitrogen in order to prevent disappearance of the conductive carbon. The firing temperature is selected in the range of 600 to 950 ° C. When the secondary firing temperature is less than 600 ° C., conversion from Li ₂ TiO ₃ and titanium oxide to lithium titanate is not preferable, and when the secondary firing temperature exceeds 950 ° C., lithium titanate aggregates. Then, the particle size exceeds the size of the nanoparticles, which is not preferable.

  Since secondary firing is firing at a high temperature, performing firing for a long time is not preferable because lithium titanate aggregates and the particle size increases. This increase in particle size can also occur in the temperature raising process up to the temperature of secondary firing and the temperature lowering process after secondary firing. Therefore, in order to suppress this agglomeration, the temperature raising process from 500 ° C. to the temperature of the secondary firing is performed in a vacuum or nitrogen in a time of 5 minutes or less, and the secondary firing is performed in a time of 1 to 20 minutes. The temperature lowering process up to 500 ° C. after the secondary firing is preferably performed in vacuum or nitrogen for a time of 5 minutes or less.

  The secondary firing is preferably performed in nitrogen. During secondary firing in nitrogen, oxygen deficiency occurs in lithium titanate due to the reducing action of carbon, and this oxygen defect is doped with nitrogen. This nitrogen-doped lithium titanate has high electrical conductivity, and lithium ions can easily enter and exit between the crystal lattices of lithium titanate. This nitrogen-doped lithium titanate is suitably obtained by performing all of the temperature rising process from 500 ° C. to the temperature of the secondary firing, the secondary firing, and the temperature lowering process to 500 ° C. after the secondary firing in nitrogen. Can be obtained.

By passing through the step of generating Li ₂ TiO ₃ and titanium oxide from the lithium titanate precursor by primary firing, the titanium oxide in the composite material obtained after the secondary firing can be substantially eliminated. Even if the conductive carbon powder carrying the lithium titanate precursor obtained in the carrying step is fired in nitrogen or vacuum at a temperature of 600 to 950 ° C. without undergoing primary firing, titanium oxide is mixed. Can only be obtained.

(B) Use of composite material The composite material of this invention is suitable for the negative electrode of a lithium ion secondary battery. Therefore, the present invention also provides a negative electrode having an active material layer containing the composite material of the present invention, a positive electrode including an active material layer containing a positive electrode active material capable of inserting and removing lithium ions, and a negative electrode and a positive electrode. A lithium ion secondary battery including a separator that holds a non-aqueous electrolyte disposed between the two is provided.

  The active material layer for the negative electrode is obtained by dispersing the composite material of the present invention in a solvent in which a binder is dissolved, if necessary, and applying the obtained dispersion on a current collector by a doctor blade method or the like, followed by drying. Can be created. Further, the obtained dispersion may be formed into a predetermined shape and may be pressure-bonded on the current collector.

  As the current collector, a conductive material such as platinum, gold, nickel, aluminum, titanium, steel, or carbon can be used. As the shape of the current collector, any shape such as a film shape, a foil shape, a plate shape, a net shape, an expanded metal shape, and a cylindrical shape can be adopted.

  As the binder, known binders such as polytetrafluoroethylene, polyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylene copolymer, polyvinyl fluoride, and carboxymethyl cellulose are used. The content of the binder is preferably 1 to 30% by mass with respect to the total amount of the mixed material. If it is 1% by mass or less, the strength of the active material layer is not sufficient, and if it is 30% by mass or more, disadvantages such as a decrease in the discharge capacity of the negative electrode and an excessive internal resistance occur.

As the positive electrode, a positive electrode provided with an active material layer containing a known positive electrode active material can be used without any particular limitation. Examples of the positive electrode active material include, for example, lithium and transition metal composite oxides such as LiMn ₂ O ₄ , LiMnO ₂ , LiV ₃ O ₅ , LiNiO ₂ , and LiCoO ₂ , sulfides such as TiS ₂ and MoS ₂ , Selenides such as NbSe ₃ , transition metal oxides such as Cr ₃ O ₈ , V ₂ O ₅ , V ₅ O ₁₃ , VO ₂ , Cr ₂ O ₅ , MnO ₂ , TiO ₂ , and MoV ₂ O ₈ , polyfluorene In addition, conductive polymers such as polythiophene, polyaniline, and polyparaphenylene can be used.

  The active material layer for the positive electrode is dispersed on the current collector by the doctor blade method or the like by dispersing the positive electrode active material and the conductive agent in a solvent in which a binder is dissolved as necessary. And can be made by drying. Further, the obtained dispersion may be formed into a predetermined shape and may be pressure-bonded on the current collector.

  Regarding the current collector and binder, the description of the current collector and binder for the negative electrode also applies to the positive electrode. As the conductive agent, carbon powder such as carbon black, natural graphite, and artificial graphite can be used.

  As the separator, for example, a polyolefin fiber nonwoven fabric or a glass fiber nonwoven fabric is preferably used. As the electrolytic solution retained in the separator, an electrolytic solution in which an electrolyte is dissolved in a non-aqueous solvent is used, and a known non-aqueous electrolytic solution can be used without any particular limitation.

  As the solvent for the non-aqueous electrolyte, electrochemically stable ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, sulfolane, 3-methyl sulfolane, γ-butyrolactone, acetonitrile and dimethoxyethane, N-methyl-2-pyrrolidone, dimethylformamide or a mixture thereof can be preferably used.

As a solute of the nonaqueous electrolytic solution, a salt that generates lithium ions when dissolved in an organic electrolytic solution can be used without any particular limitation. For _{example, LiPF 6, LiBF 4, LiClO} 4, LiN (CF 3 SO 2) 2, LiCF 3 SO 3, LiC (SO 2 CF 3) 3, LiN (SO 2 C 2 F 5) 2, LiAsF 6, LiSbF 6 Or a mixture thereof can be preferably used. As a solute of the nonaqueous electrolytic solution, a quaternary ammonium salt or a quaternary phosphonium salt having a quaternary ammonium cation or a quaternary phosphonium cation can be used in addition to a salt that generates lithium ions. For example, a cation represented by R ¹ R ² R ³ R ⁴ N ⁺ or R ¹ R ² R ³ R ⁴ P ⁺ (where R ¹ , R ² , R ³ and R ⁴ are alkyl having 1 to 6 carbon atoms) Group), PF ₆ ⁻ , BF ₄ ⁻ , ClO ₄ ⁻ , N (CF ₃ SO ₃ ) ₂ ⁻ , CF ₃ SO ₃ ⁻ , C (SO ₂ CF ₃ ) ₃ ⁻ , N (SO ₂ C ₂ A salt composed of an anion composed of F ₅ ) ₂ ⁻ , AsF ₆ ^— or SbF ₆ ^— , or a mixture thereof can be suitably used.

  The negative electrode provided with the active material layer containing the composite material of the present invention can be suitably used to configure an electrochemical capacitor by changing the positive electrode in the above-described lithium ion secondary battery to a polarizable electrode. . Therefore, the present invention also includes a negative electrode having an active material layer containing the composite material of the present invention, a polarizable electrode as a positive electrode, and a separator holding a non-aqueous electrolyte disposed between the negative electrode and the positive electrode. An electrochemical capacitor is provided.

  For the polarizable electrode used as the positive electrode of the electrochemical capacitor, a carbonaceous material such as activated carbon, carbon nanotube, or mesoporous carbon can be used. Examples of the activated carbon include palm, phenol resin, petroleum coke and the like, and examples of the activated carbon raw material activation method include a steam activation method and a molten alkali activation method. The carbonaceous material and the conductive agent are dispersed in a solvent in which the binder is dissolved, and the obtained dispersion is applied onto a current collector by a doctor blade method or the like, and dried to create a polarizable electrode. be able to. Further, the obtained dispersion may be formed into a predetermined shape and may be pressure-bonded on the current collector. As the current collector, the conductive agent, and the binder, the description of the current collector, the conductive agent, and the binder for the positive electrode of the lithium ion secondary battery described above also applies to this case. Moreover, the description about the negative electrode of the lithium ion secondary battery mentioned above, a non-aqueous electrolyte, and a separator is applicable also to an electrochemical capacitor.

  By using the composite material of the present invention, a lithium ion secondary battery and an electrochemical capacitor having a high discharge capacity and excellent rate characteristics can be obtained.

  The present invention will be described with reference to the following examples, but the present invention is not limited to the following examples.

Example 0.24 g of carbon nanofibers and 2.96 g of tetraisopropoxy titanium were added to 15.62 g of isopropyl alcohol, and tetraisopropoxy titanium was dissolved in isopropyl alcohol. The mass ratio of titanium alkoxide and carbon nanofibers was selected so that the mass ratio of lithium titanate and carbon nanofibers in the composite material as the final product was about 8: 2. The obtained liquid is composed of a concentric cylinder of an outer cylinder and an inner cylinder shown in FIG. 1 of Patent Document 1 (Japanese Patent Laid-Open No. 2007-160151), and a through hole is provided on a side surface of the inner cylinder. The carbon nanofibers are introduced into the inner cylinder of a reactor in which a swash plate is arranged at the opening of the cylinder, and the inner cylinder is swung for 300 seconds so that a centrifugal force of 35000 kgms- ² is applied to the liquid. Highly dispersed.

Acetic acid 1.99 g and lithium acetate 0.62 g were dissolved in a mixed solvent of isopropyl alcohol 1.74 g and water 1.84 g. The obtained liquid was introduced into the inner cylinder of the reactor to prepare a reaction liquid. The inner cylinder is swung for 300 seconds so that a centrifugal force of 35000 kgms- ² is applied to the reaction liquid, and a thin film of the reaction liquid is formed on the inner wall of the outer cylinder, and shear stress and centrifugal force are applied to the reaction liquid. The chemical reaction was promoted, and the lithium titanate precursor was supported on the carbon nanofibers in a highly dispersed state.

  Next, the contents of the reactor were recovered, the solvent was evaporated in the atmosphere, and the mixture was further dried at 100 ° C. for 17 hours. The obtained carbon nanofiber carrying the lithium titanate precursor was primarily fired in nitrogen at 400 ° C. for 1 hour, cooled to room temperature, and then heated from room temperature to 700 ° C. in 2 minutes in nitrogen. By performing secondary firing for 3 minutes and lowering the temperature to 500 ° C. in 3 minutes, a composite material in which 5 to 20 nm nanoparticles were supported in a highly dispersed state on carbon nanofibers was obtained.

  40 parts by mass of the obtained composite material was mixed with 1 part by mass of carboxymethyl cellulose, 1 part by mass of an acrylic rubber binder, and 300 parts by mass of water, and this mixture was applied to a copper foil and then dried to obtain a thickness of the active material layer. An electrode having a thickness of 10 μm was produced.

The obtained electrode and a lithium foil as a counter electrode were opposed to each other through a separator, and 1M LiBF ₄ / propylene carbonate was used as an electrolytic solution to prepare a half cell. The charge / discharge characteristics of the obtained half battery were evaluated over a wide range of current densities. Although this evaluation is an evaluation as a half battery, the same effect can be expected in all batteries using the positive electrode.

Comparative Example A carbon nanofiber carrying a lithium titanate precursor was heated from room temperature to 700 ° C. in 2 minutes in nitrogen, fired for 3 minutes (one-step firing), and cooled to 500 ° C. in 3 minutes. The procedure of the example was repeated with the exception.

FIG. 1 shows an X-ray powder diffraction diagram of the composite material obtained in the example and the composite material obtained in the comparative example. It can be seen that the composite material obtained in the comparative example, that is, the composite material obtained by one-step firing, contains titanium oxide in addition to spinel type lithium titanate (Li ₄ Ti ₅ O ₁₂ ). On the other hand, in the X-ray powder diffractogram of the composite material obtained in the examples, that is, the composite material obtained through the primary firing and the secondary firing, the diffraction peak of titanium oxide is not recognized, and the conductive carbon powder A composite material containing (carbon nanofibers) and spinel-type lithium titanate nanoparticles adhering to the conductive carbon powder and substantially free of titanium oxide has been obtained. Moreover, in order to confirm that Ti-N bond exists by nitrogen dope, the X-ray photoelectron spectroscopy (XPS) was performed about the composite material obtained in the Example and the comparative example. In any XPS spectrum of the composite material, a peak of 396 eV of N 1s bond energy indicating a Ti—N bond was observed, and it was confirmed that lithium titanate was doped with nitrogen in any composite material.

2 and 3 are diagrams showing the evaluation results of the rate characteristics for the half-cells in the examples and comparative examples. FIG. 2 shows the relationship between the capacity per Li ₄ Ti ₅ O _{12 as} the active material and the rate, and FIG. 3 shows the relationship between the capacity maintenance rate and the rate when the capacity at the rate 12C is 100%. Is shown.

  As is clear from these figures, the conductive carbon powder (carbon nanofiber) and spinel-type lithium titanate nanoparticles adhering to the conductive carbon powder are included, and titanium oxide is substantially not included. The half-cell of the example using the composite material has a higher capacity than the half-cell of the comparative example using the composite material containing titanium oxide, and the capacity reduction rate at a high rate is suppressed. It had excellent battery characteristics.

  By using the composite material of the present invention, a lithium ion secondary battery and an electrochemical capacitor having a high discharge capacity and excellent rate characteristics can be obtained.

Claims (4)

A method for producing a composite material comprising conductive carbon powder and spinel-type lithium titanate nanoparticles adhering to the conductive carbon powder, substantially free of titanium oxide ,
a) a preparation step of introducing a reaction solution containing a lithium source, a titanium source and a conductive carbon powder into a swirlable reactor;
b) A supporting step of supporting the lithium titanate precursor in a highly dispersed state on the conductive carbon powder by rotating the reactor and applying shear stress and centrifugal force to the reaction solution.
c) After first firing the conductive carbon powder carrying the lithium titanate precursor at a temperature of 200 to 500 ° C., then secondary firing at a temperature of 600 to 950 ° C. in vacuum or in nitrogen, A method for producing a composite material comprising: a firing step of converting the lithium titanate precursor into spinel-type lithium titanate nanoparticles on the conductive carbon powder. In the firing step, the temperature raising process from 500 ° C. to the temperature of the secondary firing is performed in a vacuum or nitrogen in a time of 5 minutes or less, the secondary firing is performed in a range of 1 to 20 minutes, and after the secondary firing. The method for producing a composite material according to claim 1 , wherein the temperature lowering process to 500 ° C. is performed in vacuum or in nitrogen for a period of 5 minutes or less. The spinel type lithium titanate has an oxygen defect, and the oxygen defect is doped with nitrogen,
3. The composite according to claim 1, wherein in the firing step, a temperature raising process from 500 ° C. to a secondary firing temperature, a secondary firing, and a temperature lowering process to 500 ° C. after the secondary firing are performed in nitrogen. Material manufacturing method. The titanium source contained in the said reaction liquid is a titanium alkoxide, The reaction inhibitor which forms a complex with a titanium alkoxide in the said reaction liquid is further contained, The any one of Claims 1-3 . A method for producing a composite material.

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