JP2011026188A - Alkali metal titanate compound, process of producing the same, electrode-active material including the alkali metal titanate compound, and storage device using the electrode-active material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a novel alkali metal titanate compound and a process of producing the same. <P>SOLUTION: The novel compound has a chemical composition expressed by the general formula: H<SB>2-x</SB>M<SB>x</SB>Ti<SB>12</SB>O<SB>25</SB>(0<x≤2; M represents an alkali metal atom; when x=2, Na is excluded from M) and preferably a one-dimensional tunnel structure. The compound is synthesized by reacting a compound having a chemical composition expressed by the general formula: H<SB>2</SB>Ti<SB>12</SB>O<SB>25</SB>and an alkali metal compound. A storage device using an electrode containing an electrode-active material produced from the compound as its constitutional member has an excellent charge/discharge cycle characteristic and expectedly a high capacity. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a novel alkali metal titanate compound and a method for producing the same. The present invention also relates to an electrode active material containing the alkali metal titanate compound and an electricity storage device using the electrode active material.

Lithium secondary batteries have been rapidly spreading in recent years because of their excellent cycle characteristics. As an electrode active material of a lithium secondary battery, in particular, a negative electrode active material, a lithium-titanium composite oxide having a high energy density and excellent rate characteristics is widespread, while a discharge potential is high and safety is excellent. Titanate compounds are also attracting attention. For example, a spinel type represented by Li ₄ Ti ₅ O ₁₂ (Patent Document 1, Non-Patent Document 1), a Ramsdelite type represented by Li ₂ Ti ₃ O ₇ (Patent Document 2, Non-Patent Document 2), etc. It is represented by lithium titanate or H _x Li _y-x Ti _z O ₄ (0 <x ≦ y, 0.8 ≦ y ≦ 2.7, 1.3 ≦ z ≦ 2.2) (Patent Document 3). A technique using lithium hydrogen titanate as an electrode active material is known. _{Alternatively,} titanate compounds represented by H _{2 Ti} 12 _{O 25} (Patent Document _{4), H x M y Ti} 1.73 O z (0.5 ≦ x + y ≦ 1.07,0 ≦ y / (x + y) ≦ 0.2, 3.85 ≦ z ≦ 4.0, M is an alkali metal other than Li (Patent Document 5), A ₂ Ti ₃ O ₇ (A is at least one selected from Na, Li, and H) (Patent) A technique using an alkali metal titanate compound or the like represented by literature 6) is also known.

JP 2002-270175 A JP 2004-221523 A International Publication WO99 / 003784 Pamphlet International Publication WO2008 / 111465 Pamphlet JP 2007-220406 A JP 2007-243233 A

G.X.Wang, D.H.Bradhurst, S.X.Dou, H.K.Liu, Journal of Power Sources, 83 (1999) P156-161 Jie Shu, Electrochimica Acta, 54 (2009) P2869-2876

  An object of this invention is to provide the novel alkali metal titanate compound which can be used for an electrode active material.

As a result of extensive research, the present inventors have expressed (Formula 1) H _2-x M _x Ti ₁₂ O ₂₅ (0 <x ≦ 2, M represents an alkali metal element, where x = 2 In this case, the present inventors have found a novel compound having a chemical composition of M (except for Na), and found that excellent battery characteristics can be obtained when this compound is used as an active material of an electricity storage device, thereby completing the present invention. .

That is, the present invention
(1) the chemical composition of the general formula (the 0 <x ≦ 2, M is representative of the alkali metal element, provided that when the x = 2 M excluding Na) (Equation _{1) H 2-x M x} Ti 12 O 25 A compound that takes
(2) the chemical composition of the general formula (the 0 <x ≦ 2, M is representative of the alkali metal element, provided that when the x = 2 M excluding Na) (Equation _{1) H 2-x M x} Ti 12 O 25 A compound having a one-dimensional tunnel structure as a characteristic of the crystal structure,
(3) the chemical composition of the general formula (the 0 <x ≦ 2, M is representative of the alkali metal element, provided that when the x = 2 M excluding Na) (Equation _{1) H 2-x M x} Ti 12 O 25 As a characteristic of the crystal structure, it has a one-dimensional tunnel structure, belongs to the monoclinic system, and the peak waveform of the X-ray diffraction pattern is similar to that of H ₂ Ti ₁₂ O _25, and the position of each peak Is a compound in which H ₂ Ti ₁₂ O ₂₅ is shifted to a high angle side or a low angle side,
(4) the chemical composition of the general formula (the 0 <x ≦ 2, M is representative of the alkali metal element, provided that when the x = 2 M excluding Na) (Equation _{1) H 2-x M x} Ti 12 O 25 An electrode active material for an electricity storage device made from a compound taking
(5) An electricity storage device including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode or the negative electrode contains the electrode active material according to (4) above,
It is.

  When the alkali metal titanate compound of the present invention is used as an electrode active material, an electricity storage device having excellent battery characteristics can be obtained.

FIG. 3 is a schematic diagram showing a crystal structure of H _2−x M _x Ti ₁₂ O ₂₅ (0 <x ≦ 2, M represents an alkali metal element, where M is excluded Na when x = 2) of the present invention. . _{2 is} an X-ray powder diffraction pattern of known H ₂ Ti ₁₂ O ₂₅ . 4 is an X-ray powder diffraction _pattern of H _1.49 Li _0.52 Ti ₁₂ O ₂₅ (Sample C) of the present invention obtained in Example 3. FIG. 4 is an X-ray powder diffraction _pattern of H _1.57 Li _0.42 Ti ₁₂ O ₂₅ (sample D) of the present invention obtained in Example 4. FIG. The characteristics of the X-ray powder diffraction pattern of H _2-x Li _x Ti ₁₂ O ₂₅ of the present invention are compared with known H ₂ Ti ₁₂ O ₂₅ .

New alkali metal titanate compounds of the present invention (Formula _{1) H 2-x M x} Ti 12 O 25 ( the 0 <x ≦ 2, M represents an alkali metal element, provided that when the x = 2 M excluding Na) Preferably has a one-dimensional tunnel structure as a characteristic of its crystal structure.
More preferably, the compound of formula 1 belongs to the monoclinic system, and the peak waveform of the X-ray diffraction pattern is similar to that of H ₂ Ti ₁₂ O _25, and the position of each peak is from H ₂ Ti ₁₂ O ₂₅ . Shifted to the high angle side or the low angle side. Note that “the peak waveform is similar” means a characteristic peak of H ₂ Ti ₁₂ O ₂₅ , that is, 2Θ is 14.0 ° and 24.8 ° in X-ray powder diffraction measurement (using CuK _α- ray). , 28.7 °, 31.1 °, 43.5 °, 44.5 °, 48.6 °, 57.6 °, peaks that can be assigned to the high angle side or low angle side Having at a shifted position. Each peak intensity or intensity ratio between peaks may be different.
Furthermore, the compound of Formula 1 can be used as an electrode active material of an electrode material for an electricity storage device.

The one-dimensional tunnel structure of the novel alkali metal titanate compound of Formula 1 of the present invention preferably has a tunnel structure by a skeleton structure constructed by connecting TiO ₆ octahedrons as shown in FIG. Means having two types of tunnel spaces of different sizes. With this crystal structure, a large amount of lithium ions can be occluded in the tunnel, and since a one-dimensional conduction path is secured, ions can easily move in the tunnel direction. And is preferable as an electrode active material. A part of the tunnel structure may be blocked.

  Furthermore, since the compound of Formula 1 of the present invention has a tunnel structure, it can be used as an adsorbent, a catalyst, and the like in addition to the electrode active material. Examples of the alkali metal element represented by M in Formula 1 include lithium, sodium, potassium, rubidium, cesium, and the like, which can be appropriately selected depending on the intended use. In particular, when used as an electrode active material, it is preferable that the alkali metal element is lithium, because better battery characteristics are easily obtained.

The average particle diameter (median diameter by laser scattering method) of the compound of Formula 1 is not particularly limited, but is usually in the range of 0.05 to 10 μm, and more preferably in the range of 0.1 to 2 μm. . The particle shape is not particularly limited, such as an isotropic shape such as a spherical shape or a polyhedral shape, an anisotropic shape such as a rod shape or a plate shape, or an indefinite shape. When the primary particles are aggregated into secondary particles, powder characteristics such as fluidity, adhesion, and filling properties are improved, and battery characteristics such as cycle characteristics are improved when used as an electrode active material. Therefore, it is preferable. The secondary particles in the present invention are in a state in which the primary particles are firmly bonded to each other, and are not easily disintegrated by industrial operations such as normal mixing, pulverization, filtration, washing, transport, weighing, bagging, and deposition. Most of them remain as secondary particles. The average particle diameter (median diameter by laser scattering method) of the secondary particles is preferably in the range of 0.1 to 20 μm. Although the specific surface area (BET method) is not particularly limited, preferably in the range of 0.1 to 100 m ² / g, more preferably in the range of 1 to 100 m ² / g. The particle shape is not limited as in the case of the primary particle, and various shapes can be used.

The particle surfaces of the primary particles or secondary particles of the compound of Formula 1 are coated with at least one selected from inorganic compounds such as carbon, silica, and alumina, and organic compounds such as surfactants and coupling agents. Also good. These coating species can be coated as a single species, or can be laminated as two or more species, or can be coated as a mixture or composite. Especially, when coated with carbon, the electrical conductivity is improved. When using as, it is preferable. Coating amount of carbon, the compound of formula 1 in terms of TiO _2, 0.05 to 10 wt% is preferable in C terms. If it is less than this range, the desired electrical conductivity cannot be obtained, while if it is more, the characteristics deteriorate. A more preferable content is in the range of 0.1 to 5% by weight. The carbon content can be analyzed by a CHN analysis method, a high frequency combustion method, or the like. Alternatively, a different element other than titanium, alkali metal, hydrogen, and oxygen can be contained in the crystal lattice by doping, etc., as long as the crystal form is not inhibited.

The compound represented by Formula 1 of the present invention is obtained by a production method including a step of reacting a compound having a chemical composition of (Formula 2) H ₂ Ti ₁₂ O ₂₅ as a general formula with an alkali metal compound. In order to react the compound of formula 2 and the alkali metal compound, a method of contacting them by mixing them in a liquid phase may be used, or they may be contacted and heated by mixing them in a solid phase. . When the reaction is performed in the liquid phase, the reaction is preferably performed in a slurry, and more preferably slurried using an aqueous medium. When using an aqueous medium, it is preferable to use water-soluble alkali metal compounds such as alkali metal hydroxides and carbonates. When the reaction is carried out in the solid phase, it is considered that upon heating, the alkali metal compound becomes a molten salt and reacts with the compound of formula 2. As the alkali metal compound, an alkali metal nitrate, chloride, etc. having a relatively low melting point is considered. It is preferable to use a physical salt or a sulfate. Although heating temperature is suitably set with the alkali metal compound to be used, the range of 150-400 degreeC is preferable normally. The range of the value of x in Formula 1 is determined by controlling the amount of hydrogen ion substitution by the alkali metal compound. For example, if a part of hydrogen ions contained in the compound of Formula 2 is replaced with lithium ions and the range of x in Formula 1 is adjusted to 0 <x <2, H _2x Li _x Ti ₁₂ as a general formula A compound having a chemical composition of O ₂₅ (0 <x <2) is obtained. Alternatively, in order to obtain a compound in which x in formula 1 is 2, that is, a compound other than sodium titanate having a chemical composition of Na ₂ Ti ₁₂ O ₂₅ as a general formula, all of the hydrogen ions may be replaced.

In order to obtain a compound represented by (formula 1 ′) H _2-x MxTi ₁₂ O ₂₅ (0 <x <2, M represents an alkali metal), an alkali metal compound having a reaction equivalent or more than the compound of formula 2 After the reaction, once all of the hydrogen ions contained in the compound of formula 2 are replaced with alkali metal ions, some of the alkali metal ions contained in the obtained compound may be replaced with hydrogen ions. This method is more preferable because it is easy to control x in Formula 1 within the range of 0 <x <2. In this method, all hydrogen ions contained in the compound of formula 2 are substituted as a general formula (formula 3) M _x Ti ₁₂ O ₂₅ (x ≧ 2, M represents an alkali metal element) It is considered as a compound that takes Li ₂ Ti ₁₂ O ₂₅ , K ₂ Ti ₁₂ O ₂₅ or the like may be used as the compound represented by Formula 3, or when x in composition formula 3 is 2, an alkali metal element represented by M A compound in which is sodium can also be used. The reason why x in formula 3 can take a value larger than 2 is presumed that the compound of formula 2 has a one-dimensional tunnel structure and excess alkali metal ions are taken into the crystal structure. The There is no upper limit to the value of x, but a value of 3 or less is preferable because the structure of the compound of Formula 3 is difficult to change. Substitution of alkali metal ions and hydrogen ions can be performed by reacting the compound of formula 3 with an acidic compound and / or water. Since alkali metal ions contained in this compound are highly reactive, a substitution reaction occurs even when water is used. As the acidic compound, use of an inorganic acid such as hydrochloric acid, sulfuric acid, nitric acid, or hydrofluoric acid is preferable because the reaction can easily proceed, and hydrochloric acid or sulfuric acid can be advantageously implemented industrially. The concentration of the acidic compound is not particularly limited, but the free acid concentration is preferably 2 N or less. Although there is no restriction | limiting in particular in reaction temperature, It is preferable to carry out under the temperature of the range of less than 100 degreeC in which the structure of the compound of Formula 3 cannot change easily. The degree of substitution with hydrogen ions is determined when a substitution reaction is performed using an acidic compound, for example, by controlling the amount of acidic compound used, and when water is used, for example, the amount of water used, temperature, reaction time, etc. It can control by controlling.

  The obtained compound of the formula 1 is washed, solid-liquid separated if necessary, and then dried. In particular, washing is preferable because the remaining alkali metal compound can be removed. Examples of such a cleaning medium include water and polar nonaqueous solvents. Examples of the polar non-aqueous solvent include alcohols such as ethanol and methanol, and ketones such as acetone. Industrially, ethanol is preferable. Or you may grind | pulverize in the range which does not impair the effect of this invention using a well-known apparatus according to the grade of aggregation of particle | grains.

The compound of the formula 2 used in the production method of the present invention includes a compound having a general composition of (formula 4) M ₂ Ti _y O _{2y + 1} (y> 2, M represents an alkali metal element) and an acidic compound. After the reaction, it is obtained by washing, solid-liquid separation and drying as appropriate, followed by heat dehydration at a temperature in the range of 150 to 400 ° C. Examples of the alkali metal element represented by M in Formula 4 include lithium, sodium, potassium, rubidium, cesium, and the like. Among them, sodium, potassium, and cesium are preferable because they can be implemented industrially advantageously. The value of y in Formula 4 is not particularly limited as long as it is greater than 2, but is preferably an integer in the range of 3 to 5. _{Specifically, Na 2 Ti 3 O 7,} K 2 Ti 4 O 9, Cs 2 Ti 5 O 11 and the like. As the acidic compound, use of an inorganic acid such as hydrochloric acid, sulfuric acid, nitric acid, or hydrofluoric acid is preferable because the reaction can easily proceed, and hydrochloric acid or sulfuric acid can be advantageously implemented industrially. Although there is no restriction | limiting in particular in the quantity and density | concentration of an acidic compound, It is preferable to make the density | concentration of a free acid 2 N or less above the reaction equivalent of the alkali metal ion contained in the compound of Formula 4. Although there is no restriction | limiting in particular in reaction temperature, It is preferable to carry out at the temperature of the range of less than 100 degreeC in which the structure of the compound of Formula 4 cannot change easily. It is preferable to granulate the compound of Formula 4 in advance because the reactivity with an acidic compound is increased, and it is industrially preferable to use spray drying for granulation. A fluidized furnace or a stationary furnace rotary kiln can be used for the heat dehydration, and the heating atmosphere may be either air or an inert gas.

The compound of Formula 4 is obtained by mixing a titanium compound and an alkali metal compound in a dry or wet manner and then baking. As the titanium compound, inorganic titanium salts such as titanium oxide and titanium chloride, and organic titanium compounds such as titanium alkoxide can be used, and alkali metal carbonates such as alkali metal carbonates and hydroxides can be used. Can be used. Among these, it is preferable to use titanium oxide and alkali metal carbonate. In the present invention, the titanium oxide is a compound including a compound of titanium and oxygen and a hydrogen-containing compound, a hydrate or a hydrate thereof. For example, in addition to titanium oxide such as titanium dioxide (TiO ₂ ), Examples thereof include titanic acid compounds such as metatitanic acid (H ₂ TiO ₃ ) and orthotitanic acid (H ₄ TiO ₄ ), and one or more selected from these can be used. Further, it may be a crystalline compound or amorphous, and in the case of crystallinity, it may be any of rutile type, anatase type, brookite type, etc. I do not receive it. The firing temperature is preferably in the range of 600 to 1000 ° C. When the temperature is lower than this range, the reaction hardly proceeds. When the temperature is higher than this range, the products are easily sintered. A more preferable range is 700 to 900 ° C. In order to accelerate the reaction and suppress the sintering of the product, the firing can be repeated twice or more. For firing, a known firing furnace such as a fluidized furnace, a stationary furnace, a rotary kiln, or a tunnel kiln can be used. As the firing atmosphere, air and a non-oxidizing atmosphere can be appropriately selected. The firing equipment is appropriately selected according to the firing temperature and the like. After firing, pulverization may be performed according to the degree of sintering. The pulverization is dry using an impact pulverizer such as a hammer mill, a pin mill, or a centrifugal pulverizer, a grinding pulverizer such as a roller mill, a compression pulverizer such as a roll crusher or a jaw crusher, or an airflow pulverizer such as a jet mill. It may be carried out by a wet method using a sand mill, a ball mill, a pot mill or the like.

In order to produce secondary particles of the compound of formula 1, in the step of reacting the compound of formula 2 and the alkali metal compound, (1) whether the compound of formula 2 and the alkali metal compound are reacted together after granulation (2) The compound of Formula 2 may be granulated and reacted with an alkali metal compound, or (3) the step of granulating after reacting the compound of Formula 2 with an alkali metal compound may be included. Examples of granulation include dry granulation, stirring granulation, compaction granulation, and the like, and dry granulation is preferable because the particle diameter and shape of secondary particles can be easily adjusted. In dry granulation, a slurry containing the compound of formula 2, an alkali metal compound or a reaction product thereof is dehydrated, dried and pulverized; the slurry is dehydrated, molded and dried; and the slurry is spray dried. Among them, spray drying is industrially preferable. When secondary particles are used, substitution of hydrogen ions contained in the compound of Formula 2 with alkali metal ions is promoted, so that not only improvement in powder characteristics and battery characteristics but also Li ₂ Ti ₁₂ in which all hydrogen ions are substituted. This is a preferable method for obtaining a compound of formula 3 such as O ₂₅ and K ₂ Ti ₁₂ O ₂₅ . When the obtained secondary particles are heat-treated at a temperature of 200 to 500 ° C., substitution of hydrogen ions is further promoted, so that the compound of formula 3 such as Li ₂ Ti ₁₂ O ₂₅ , K ₂ Ti ₁₂ O _25, etc. This is a more preferable method for production. In order to obtain the secondary particles of the compound of the formula 1 ′ by adjusting the range of x in the formula 1 to 0 <x <2, the secondary particles of the compound of the formula 3 are obtained by the methods (1) to (3). Then, it is preferable to react with an acidic compound and / or water to replace some of the alkali metal ions and hydrogen ions contained in the compound of Formula 3 so that the value of x can be easily controlled.

  If spray drying is performed, a spray dryer such as a disk type, a pressure nozzle type, a two-fluid nozzle type, or a four-fluid nozzle type can be appropriately selected depending on the properties and processing capacity of the slurry. . The secondary particle size can be controlled, for example, by adjusting the solid content concentration in the slurry, or by rotating the number of rotations of the disk in the case of the above-described disk type, and spraying in the case of a pressure nozzle type, two-fluid nozzle type, four-fluid nozzle type, etc. This can be done by controlling the size of the sprayed droplets by adjusting the pressure and nozzle diameter. As the drying temperature, the inlet temperature is preferably in the range of 150 to 250 ° C, and the outlet temperature is preferably in the range of 70 to 120 ° C. An appropriate amount of an organic binder may be added when the slurry has a low viscosity and is difficult to granulate, or in order to further facilitate the control of the particle diameter. Examples of the organic binder used include (1) vinyl compounds (polyvinyl alcohol, polyvinyl pyrrolidone, etc.), (2) cellulose compounds (hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, ethyl cellulose, etc.), and (3) protein compounds ( Gelatin, gum arabic, casein, sodium caseinate, ammonium caseinate, etc.), (4) acrylic acid compounds (sodium polyacrylate, ammonium polyacrylate, etc.), (5) natural polymer compounds (starch, dextrin, agar) , Sodium alginate, etc.), (6) synthetic polymer compounds (polyethylene glycol, etc.), etc., and at least one selected from these can be used. Especially, what does not contain inorganic components, such as soda, is more preferable because it is easily decomposed and volatilized by heat treatment.

  In addition, an electricity storage device using an electrode containing the compound of Formula 1 of the present invention as an electrode active material as a constituent member has a high capacity and a reversible lithium insertion / extraction reaction, and has high reliability. It is an electric storage device that can be expected.

Specific examples of the electricity storage device include a lithium battery, a capacitor, and the like, which include an electrode, a counter electrode, a separator, and an electrolyte solution. The electrode includes a conductive material such as carbon black and a fluororesin as the electrode active material. It can be obtained by adding a binder such as or the like, and forming or coating the electrode substrate as appropriate. In the case of a lithium battery, the electrode active material can be used for a positive electrode, and metallic lithium, a lithium alloy, or a carbon-based material such as graphite can be used as a counter electrode. Alternatively, the electrode active material is used as a negative electrode, and a lithium / transition metal composite oxide such as a lithium / manganese composite oxide, a lithium / cobalt composite oxide, a lithium / nickel composite oxide, a lithium / vanadine composite oxide, Olivine type compounds such as lithium, iron, and complex phosphate compounds can be used. In the case of a capacitor, an asymmetric capacitor using the electrode active material and graphite can be used. As the separator, a porous polyethylene film or the like is used, and as the electrolyte, a solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, 1,2-dimethoxyethane, LiPF ₆ , LiClO ₄ , LiCF ₃ SO is used. ₃ , conventional materials such as those in which lithium salts such as LiN (CF ₃ SO ₂ ) ₂ and LiBF ₄ are dissolved can be used. The structure of the electricity storage device is not particularly limited, except for the electrode active material, and other well-known ones can be used.

  Examples of the present invention are shown below, but these do not limit the present invention.

Example 1
1284 g of pure water was added to 1000 g of commercially available rutile type high-purity titanium dioxide (PT-301: manufactured by Ishihara Sangyo) and 451.1 g of sodium carbonate, and the mixture was stirred to form a slurry. This slurry was spray-dried under conditions of an inlet temperature of 200 ° C. and an outlet temperature of 70 to 90 ° C. using a spray dryer (MDL-050C type: manufactured by Fujisaki Electric). The obtained spray-dried product was heated and fired at 800 ° C. for 10 hours in the atmosphere using an electric furnace to obtain a product having a median diameter of 5.5 μm.

When the chemical composition of the obtained sample was analyzed by ICP emission analysis (trade name ICPS-7500, manufactured by Shimadzu Corporation), Na: Ti = 1.99: 3.00 (analysis error 0.04 of each element) And was valid in the chemical formula of Na ₂ Ti ₃ O ₇ . Furthermore, an X-ray powder diffractometer (trade name RINT2550V, manufactured by Rigaku) has revealed that the crystal has a monoclinic system and a single phase with a crystal structure of the space group P2 ₁ / m having good crystallinity. . Further, when the lattice constant was determined by the least square method using each index and its surface spacing, the following values were obtained, which were in good agreement with the values of known Na ₂ Ti ₃ O ₇ .
a = 9.131 mm (within 0.001 mm error)
b = 3.804 mm (within 0.001 mm error)
c = 8.569mm (within 0.001mm error)
β = 101.60 ° (error within 0.01 °)

4310 g of pure water was added to 1077 g of the obtained Na ₂ Ti ₃ O ₇ , and wet pulverization was performed with Dino Mill (MULTI LAB type: manufactured by Shinmaru Enterprise) to obtain a dispersion slurry having a median diameter of 1.2 μm. To 4848 g of this slurry, 739 g of 64% sulfuric acid was added and reacted for 5 hours at 60 ° C. with stirring, followed by washing with filtered water. Pure water was added to the filter cake to make 3370 g, and then redispersed. 481 g of 64% sulfuric acid was added, and the mixture was reacted for 5 hours at 80 ° C. with stirring, washed with filtered water and dried to obtain a product.

When the chemical composition of the obtained sample was analyzed by ICP emission spectrometry, sodium was not detected, and the chemical formula of H ₂ Ti ₃ O ₇ which was almost completely proton-exchanged was appropriate. Furthermore, it was revealed by an X-ray powder diffractometer that the crystal has a monoclinic system and a single phase of H ₂ Ti ₃ O ₇ having a crystal structure of the space group C2 / m having good crystallinity. Further, when the lattice constant was determined by the least square method using each index and its surface spacing, the following value was obtained, which was in good agreement with the known H ₂ Ti ₃ O ₇ value.
a = 16.032 mm (within 0.001 mm error)
b = 3.751 mm (within 0.001 mm error)
c = 9.194 mm (error within 0.001 mm)
β = 101.44 ° (within 0.01 ° error)

When the particle shape of H ₂ Ti ₃ O ₇ obtained in this way was examined with a scanning electron microscope (SEM) (manufactured by JEOL, trade name JSM-5400), Na ₂ Ti ₃ O _{7 as} a starting material was used. It had an isotropic shape of about 1 micron square with the shape of

300 g of the obtained H ₂ Ti ₃ O ₇ was dehydrated by heating at 260 ° C. for 10 hours in the atmosphere using an electric furnace to obtain a product.

About the obtained sample, X-ray diffraction data was measured with an X-ray powder diffractometer, and a monoclinic system having a good crystallinity and a single crystal structure of H ₂ Ti ₁₂ O ₂₅ having a crystal structure of space group P2 / m. It became clear that it was one phase. The X-ray diffraction pattern is shown in FIG. In addition, when the lattice constant was determined by the least square method using each index and its surface spacing, the following values were obtained, which agreed well with the value of H ₂ Ti ₁₂ O ₂₅ disclosed in International Publication WO2008 / 111465. It was.
a = 14.82 mm (within error of 0.01 mm)
b = 3.890 mm (error within 0.001 mm)
c = 9.866 mm (within 0.08 mm error)
β = 1111.06 ° (within 0.08 ° error)

Moreover, the validity of the chemical composition, where the heat loss in the temperature range of 250 to 600 ° C. samples were measured using a differential thermal balance, heat loss was calculated on the assumption that corresponding to structural water, H ₂ Ti The chemical composition of ₁₂ O ₂₅ was confirmed to be reasonable.

The particle shape of H ₂ Ti ₁₂ O ₂₅ thus obtained was examined by a scanning electron microscope (SEM). As a result, Na ₂ Ti ₃ O _{7 as} a starting material and H ₂ Ti ₃ O _{7 as} a precursor were used. It had an isotropic shape of about 1 micron square with the shape of

1 liter of pure water and 13.1 g of lithium hydroxide monohydrate were added to 153 g of the obtained H ₂ Ti ₁₂ O ₂₅ and reacted at 60 ° C. for 3 hours while stirring. Thereafter, this slurry is spray-dried under the same conditions as Na ₂ Ti ₃ O ₇ and further heat-treated at a temperature of 350 ° C. for 10 hours to obtain a compound of formula 1 of the present invention having a median diameter of 5.3 μm. It was. (Sample A)

Example 2
To 100 g of the sample A obtained in Example 1, 900 ml of pure water was added and reacted at 60 ° C. for 3 hours. Thereafter, it was washed with filtered water and dried at 100 ° C. to obtain the compound of formula 1 of the present invention. (Sample B)

Example 3
10 g of lithium nitrate was mixed with 0.5 g of H ₂ Ti ₁₂ O ₂₅ obtained in Example 1, and the mixture was heated and reacted at 270 ° C. for 5 hours using an electric furnace. After the reaction, the mixture was cooled, washed thoroughly with pure water to remove excess lithium nitrate, washed with filtered water, and dried at 100 ° C. to obtain the compound of formula 1 of the present invention. (Sample C)

Example 4
In Example 3, the compound of Formula 1 of the present invention was obtained in the same manner as in Example 3 except that the heating time was 20 hours. (Sample D)

Example 5
In Example 3, the compound of formula 1 of the present invention was obtained in the same manner as in Example 3 except that ethanol was used instead of pure water for the washing after the reaction and cooling, and the heating time was 20 hours. . (Sample E)

Comparative Example 1
The compound of formula 2 H ₂ Ti ₁₂ O ₂₅ obtained as an intermediate product in Example 1 was used as a comparison target. (Sample F)

Evaluation 1: Confirmation of crystallinity and measurement of lattice constant The compound (sample C) obtained in Example 3 and the compound (sample D) obtained in Example 4 were subjected to X-ray diffraction using an X-ray powder diffractometer. The data was measured, the crystal was monoclinic with good crystallinity, and the peak waveform of the X-ray diffraction pattern was similar to H ₂ Ti ₁₂ O _25, and the position of each peak was H ₂ Ti ₁₂ O _25. It became clear that it shifted to the higher angle side or the lower angle side. The powder X-ray diffraction pattern at this time is shown in FIGS. Further, the peaks having a remarkable difference from the peak positions of known H ₂ Ti ₁₂ O ₂₅ are compared in FIG. It was confirmed that the peak positions in Sample C and Sample D were greatly shifted to the high angle side as compared with known H ₂ Ti ₁₂ O ₂₅ . Such an X-ray diffraction pattern was not consistent with known compounds, and was found to be a new substance.

Evaluation 2: Confirmation of crystal structure Further, when the crystal structure analysis was performed by the powder X-ray structure analysis method (using the program RIEtan 2000) using the intensity data measured by the powder X-ray diffractometer, Na ₂ which is a known substance was used. It was revealed that it has a one-dimensional tunnel structure having the same skeleton structure as Ti ₁₂ O ₂₅ .

In addition, the crystal structure which has been clarified by analysis is shown in FIG. It was clarified that two kinds of tunnel spaces with different sizes were formed by the skeletal structure constructed by the TiO ₆ octahedron.

Evaluation 3: Confirmation of composition The compounds (samples A to E) obtained in Examples 1 to 5 were dissolved in hydrofluoric acid, and the contents of titanium and lithium were measured by ICP analysis. Moreover, the heating loss of these samples in the temperature range of 250-600 degreeC was measured using the differential thermal balance, and the value of x in a composition formula 1 was computed on the assumption that a heating loss corresponded to structural water. The results are shown in Table 1.

Evaluation 4: Evaluation of charge / discharge characteristics (1)
Lithium secondary batteries were prepared using the compounds (samples A to D) obtained in Examples 1 to 4 as electrode active materials, and their charge / discharge characteristics were evaluated. The battery configuration and measurement conditions will be described.

  Each sample, acetylene black powder as a conductive agent, and polytetrafluoroethylene resin as a binder are mixed at a weight ratio of 5: 4: 1, kneaded in a mortar, and molded into a circle with a diameter of 10 mm. It was in a pellet form. The weight of the pellet was 10 mg. An aluminum mesh cut to a diameter of 10 mm was placed on this pellet and pressed at 9 MPa to obtain a working electrode.

This working electrode was vacuum-dried at a temperature of 100 ° C. for 4 hours, and then incorporated as a positive electrode in a sealable coin-type evaluation cell in a glove box having a dew point of −70 ° C. or lower. The evaluation cell used was made of stainless steel (SUS316) and had an outer diameter of 20 mm and a height of 3.2 mm. As the negative electrode, a metal lithium having a thickness of 0.5 mm formed into a circle having a diameter of 12 mm was used. As the non-aqueous electrolyte, a mixed solution of ethylene carbonate and dimethyl carbonate (mixed in a volume ratio of 1: 2) in which LiPF ₆ was dissolved at a concentration of 1 mol / liter was used.

  The working electrode was placed in the lower can of the evaluation cell, a porous polypropylene film was placed thereon as a separator, and a non-aqueous electrolyte was dropped from above. Further, a negative electrode, a 0.5 mm-thickness spacer for adjusting the thickness, and a spring (both made of SUS316) were placed thereon, and an upper can with a polypropylene gasket was put on the outer peripheral edge portion and sealed.

  The charge / discharge capacity was measured at a constant current at room temperature with the voltage range set to 1.0 to 2.5 V and the charge / discharge current set to 0.2 mA. The charge / discharge capacity at the second and 30th cycles was measured, and (discharge capacity at the 30th cycle / discharge capacity at the second cycle) × 100 was defined as the cycle characteristics. The larger this value, the better the cycle characteristics. The results are shown in Table 2. It can be seen that the electrode active material using the alkali metal titanate compound of the present invention has excellent cycle characteristics and a large charge / discharge capacity.

Evaluation 5: Evaluation of charge / discharge characteristics (2)
For the compounds (samples A to F) obtained in Examples 1 to 5 and Comparative Example 1, lithium secondary batteries were prepared in the same manner as in Evaluation 4, and their charge / discharge characteristics were evaluated. The cycle characteristics were calculated by measuring charge / discharge capacities at the second and thirty cycles and (discharge capacity at thirty cycles / discharge capacity at the second cycle) × 100. The results are shown in Table 3. It can be confirmed that the electrode active material using the alkali metal titanate compound of the present invention is excellent in cycle characteristics.

Evaluation 6: Evaluation of charge / discharge characteristics (3)
In Evaluation 5, cycle characteristics were evaluated in the same manner as in Evaluation 5 except that charging / discharging performed at room temperature was performed in a constant temperature bath at 60 ° C. The results are shown in Table 4. It can be seen that the present invention is excellent in cycle characteristics even when charging and discharging are performed at a high temperature. In Comparative Example 1, the initial capacity is larger than that of the Example at a high temperature, but the cycle characteristics are remarkably deteriorated.

New alkali metal titanate compound _{H 2-x M x Ti 12} O 25 of the present invention (0 <x ≦ 2, M represents an alkali metal element, M except the Na case where x = 2), the one-dimensional The characteristics of the crystal structure of having a typical tunnel space are higher in capacity than the current spinel type Li ₄ Ti ₅ O ₁₂ , and are advantageous for smooth occlusion / release of lithium, and the initial charge / discharge efficiency, cycle It is an excellent material in terms of characteristics. Therefore, it is highly practical as a power storage device electrode material such as a lithium secondary battery.

  Also, the manufacturing method does not require a special apparatus, and the raw material to be used is low in price, so that a high value-added material can be manufactured at a low cost.

Furthermore, the novel alkali metal titanate compound _{H 2-x M x Ti 12} O 25 (0 <x ≦ 2, M represents an alkali metal element, provided that when the x = 2 M excludes Na) the electrode of the present invention An electricity storage device applied to an electrode material as an active material is an electricity storage device that can perform reversible lithium insertion / extraction reaction, can support a long charge / discharge cycle, and can be expected to have a high capacity.

Claims (12)

A compound having a chemical composition of the general formula (Formula 1) H _2−x M _x Ti ₁₂ O ₂₅ (0 <x ≦ 2, M represents an alkali metal element, but when x = 2, M excludes Na) . As a general formula, a chemical composition of (formula 1) H _2−x M _x Ti ₁₂ O ₂₅ (0 <x ≦ 2, M represents an alkali metal element, but when x = 2, M excludes Na), A compound having a one-dimensional tunnel structure. As a general formula, a chemical composition of (formula 1) H _2−x M _x Ti ₁₂ O ₂₅ (0 <x ≦ 2, M represents an alkali metal element, but when x = 2, M excludes Na), It has a one-dimensional tunnel structure, belongs to the monoclinic system, and the peak waveform of the X-ray diffraction pattern is similar to that of H ₂ Ti ₁₂ O _25, and the position of each peak is higher than that of H ₂ Ti ₁₂ O _25. A compound that is shifted to the side or low angle side.   The compound which is a secondary particle which aggregated the primary particle of the compound as described in any one of Claims 1-3. A compound having a chemical composition of the general formula (Formula 1) H _2−x M _x Ti ₁₂ O ₂₅ (0 <x ≦ 2, M represents an alkali metal element, but when x = 2, M excludes Na) An electrode active material for an electricity storage device.   The electrode active material according to claim 5, wherein the alkali metal element represented by M in Formula 1 is at least one selected from lithium, sodium, potassium, and cesium. Process for the preparation of a compound according to claim 1, comprising the step of reacting a formula and (Equation 2) The compound takes the chemical composition of H ₂ Ti ₁₂ O ₂₅ and an alkali metal compound.   The method for producing a compound according to claim 7, wherein the compound of formula 2 and an alkali metal compound are reacted in contact with each other in a liquid phase.   The method for producing a compound according to claim 7, wherein the compound of formula 2 and the alkali metal compound are brought into contact with each other in the solid phase and heated to react. In the step of reacting a compound having a chemical composition of (formula 2) H ₂ Ti ₁₂ O ₂₅ as a general formula with an alkali metal compound, (1) the compound of formula 2 and the alkali metal compound are both reacted after granulation. Or (2) granulating the compound of formula 2 and reacting with the alkali metal compound, or (3) granulating the compound of formula 2 and the alkali metal compound. A method for producing the compound. Secondary particles of a compound having a chemical composition of (formula 3) M _x Ti ₁₂ O ₂₅ (x ≧ 2, M represents an alkali metal element) as a general formula by any one of steps (1) to (3) 11. The method for producing a compound according to claim 10, wherein the secondary particle is reacted with an acidic compound and / or water to adjust x in the formula 1 in a range of 0 <x <2.   The electrical storage device containing a positive electrode, a negative electrode, a separator, and an electrolyte, The electrical storage device in which the said positive electrode or negative electrode contains the electrode active material of Claim 5.