JP6013435B2 - ELECTRODE ACTIVE MATERIAL, ITS MANUFACTURING METHOD, AND ELECTRIC STORAGE DEVICE USING THE ELECTRODE ACTIVE MATERIAL - Google Patents

Description

  The present invention relates to an electrode active material comprising a titanium compound and a method for producing the same. The present invention also relates to an electricity storage device using the electrode active material.

In recent years, lithium secondary batteries have been rapidly spread mainly in portable devices because of their light weight and high energy density. As an electrode active material for a lithium secondary battery, a lithium-titanium composite oxide or titanic acid compound having a high discharge potential and excellent safety has been attracting attention. For example, a lithium-titanium composite oxide such as a spinel type represented by Li ₄ Ti ₅ O ₁₂ (Patent Document 1) and a ramsdellite type represented by Li ₂ Ti ₃ O ₇ (Patent Document 2), or H _x Li Lithium hydrogen titanate represented by _y-x Ti _z O ₄ (0 <x ≦ y, 0.8 ≦ y ≦ 2.7, 1.3 ≦ z ≦ 2.2) (Patent Document 3) is used as an electrode. Techniques used for active materials are 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 a compound represented by Document 6) or the like, or a mixture of a titanic acid compound having a crystal structure similar to the bronze type and a spinel type lithium titanium composite oxide (Patent Document 7) is also known. In addition, a negative electrode active material includes a spinel type lithium titanium composite oxide layer on the side in contact with the current collector and a ramsdellite type lithium titanium composite oxide or anatase type titanium oxide layer on the side facing the separator, respectively. The nonaqueous electrolyte battery used (Patent Document 8) has also been proposed.

JP 2002-270175 A JP-A-11-283624 International Publication WO99 / 003784 Pamphlet International Publication WO2008 / 111465 Pamphlet JP 2007-220406 A JP 2007-243233 A International Publication WO2009 / 028530 Pamphlet JP 2009-81049 A

As described above, there are various types of electrode active materials, but each has superiority or inferiority in electric capacity, high rate charge / discharge characteristics, charge / discharge cycle characteristics, and the like. For example, a spinel type lithium titanium composite oxide represented by Li ₄ Ti ₅ O ₁₂ is excellent in charge / discharge cycle characteristics, but the battery capacity is not high. In addition, the Ramsdelite-type lithium titanium composite oxide represented by Li ₂ Ti ₃ O ₇ has a high theoretical capacity, but the electric capacity that can be taken out in an actual battery system is not sufficient, and the charge / discharge cycle characteristics are inferior. As a problem, the synthesis requires a high-temperature heating step exceeding 1000 ° C. Although the titanic acid compound represented by H ₂ Ti ₁₂ O ₂₅ has a high electric capacity, it seems that improvement of high rate charge / discharge characteristics is required particularly for application to high-power applications that have been actively studied in recent years. It has become. Therefore, at present, development of power storage devices is being studied for various applications, but depending on the performance most important for each application, electrode active materials are being selected while sacrificing other performances. . As a solution to this problem, for example, the technique of Patent Document 8 has been proposed.

  Here, the technique of Patent Document 8 is a device in which the arrangement of each active material inside the battery is devised in order to take advantage of the individual active materials, and is capable of improving the total balance between capacity and cycle characteristics. Technology. However, the electric capacity is not as high as when using a ramsdellite-type lithium-titanium composite oxide alone, and the cycle characteristics are not as good as when using a spinel-type lithium-titanium composite oxide alone. There is a problem of becoming.

  An object of this invention is to provide the electrode active material which shows the outstanding battery characteristic, for example, an electrical capacity, and a high rate charge / discharge characteristic.

  As a result of intensive studies, the present inventors have developed an electrode active material containing at least a lithium titanium compound phase having a spinel crystal structure and a specific titanium compound phase having a one-dimensional tunnel structure as an active material for an electricity storage device. Compared to the theoretical battery capacity and high rate charge / discharge capacity calculated from the capacity and high rate charge / discharge capacity of spinel type lithium titanium compound and the capacity and high rate charge / discharge capacity of one-dimensional tunnel structure titanium compound The inventors have found that the present invention has been completed by showing high battery values, sometimes showing battery characteristics equivalent to or exceeding the battery characteristics of each compound when used alone as an active material.

That is, the present invention
(1) An electrode active material having at least two phases of a lithium titanium compound phase having a spinel crystal structure and a titanium compound phase having a one-dimensional tunnel structure represented by (Formula 1) as a general formula,
(Equation _{1) H 2-X A X} Ti n O 2n + 1 (A is an alkali metal, X is a real number satisfying 0 ≦ X ≦ 2, n is an integer satisfying n ≧ 6.)
(2) The electrode active material according to the above (1), wherein the titanium compound phase having the one-dimensional tunnel structure is any one or two or more of 6, 8, 12, 18, and 24 in Formula 1. Yes,
(3) An electricity storage device using the electrode active material according to (1) or (2) as a positive electrode or a negative electrode,
(4) Production of an electrode active material having a step of mixing a lithium titanium compound-containing material having a spinel crystal structure with a titanium compound-containing material having a one-dimensional tunnel structure represented by Formula 1 as a general formula Is the way
(5) A method for producing an electrode active material comprising a step of firing the mixture produced in (4),
It is.

  When an electrode active material containing at least a lithium titanium compound phase having a spinel crystal structure and a specific titanium compound phase having a one-dimensional tunnel structure according to the present invention is used, the characteristics when each is used alone as an active material An electric storage device having characteristics equivalent to or exceeding the above can be obtained.

3 is an X-ray powder diffraction diagram of H ₂ Ti ₁₂ O ₂₅ obtained in Experiment 1. FIG. _{2 is} a scanning electron micrograph of H ₂ Ti ₁₂ O ₂₅ obtained in Experiment 1. FIG. 2 is an X-ray powder diffraction diagram of Li ₄ Ti ₅ O ₁₂ obtained in Experiment 1. FIG. 2 is an X-ray powder diffraction diagram of Sample D of Experiment 1. FIG. 6 is a graph showing a current load characteristic evaluation result of Experiment 1;

The present invention may be described as a lithium titanium compound phase having a spinel crystal structure and a titanium compound phase having a one-dimensional tunnel structure represented by the above formula 1 as a general formula (hereinafter referred to as “tunnel structure titanium compound”). ), An electrode active material having at least two phases. Although the lithium titanium compound having a spinel crystal structure is not particularly limited, an example thereof includes a lithium titanium composite oxide represented by Li ₄ Ti ₅ O ₁₂ (formula 2). Taking the compound of formula 2 as an example, if it is a compound phase represented by a general formula such as formula 2, a part of lithium, titanium, or oxygen may be substituted with other elements, Not only a stoichiometric composition but also a non-stoichiometric composition in which some elements are deficient or excessive. The lithium-titanium complex acid compound of Formula 2 has an X-ray diffraction pattern peak of 2θ of 18.5 °, 35.7 °, 43.3 °, 47.4 in powder X-ray diffraction measurement (using CuKα ray). It exists at least at the positions of 5 °, 57.3 °, 62.9 °, and 66.1 ° (all of which have an error of about ± 0.5 °). Note that, when substituted with a different element, a slight shift is observed at the peak position.

A titanium compound having a one-dimensional tunnel structure represented by Formula 1 as a general formula has a structure in which a TiO6 octahedron is connected through edge sharing and vertex sharing, and a hydrogen or alkali It is a compound in which metal elements can be inserted in a large amount by one-dimensionally arranging in the tunnel structure. Examples of such titanic acid compounds include titanic acid compounds represented by general formulas such as H ₂ Ti ₁₂ O ₂₅ (formula 3), H _1.5 Li _0.5 Ti ₆ O ₁₃ , and Li ₂ Ti ₁₈ O _37. Can be mentioned. In the present invention, the value of n in Formula 1 is preferably n = 6, 8, 12, 18, 24, and more preferably n = 12,18. There is no particular limitation on the one-dimensional tunnel structure, but a tunnel structure surrounded by two sides of two TiO6 octahedrons connected by a shared edge and two sides connected by a shared vertex (a so-called 2 × 1 tunnel structure) It is preferable that at least a tunnel structure having a size other than () exists. For example, when n = 12, there are two types of tunnel structures represented by so-called 2 × 1 and 3 × 1, and when n = 8, a 4 × 1 tunnel structure is 3 × There is one tunnel structure. Examples of the alkali metal A include lithium, sodium, and potassium, and lithium is particularly preferable. As long as it is represented by a general formula such as Formula 1, a part of hydrogen, titanium, or oxygen may be substituted with another element, and not only a stoichiometric composition but also a part of A non-stoichiometric composition in which elements are deficient or excessive may be used. The titanic acid compound of Formula 3 is represented by the space group P2 ₁ / m, and in the powder X-ray diffraction measurement (using CuKα ray), the peak of the X-ray diffraction pattern is 2θ = 14.0 °, 24.8 °, 28.7 °, 30.3 °, 31.1 °, 43.5 °, 44.5 °, 48.6 °, 57.6 °, and 59.0 ° (all have an error of ± 0) About 5 °). Note that, when substituted with a different element, a slight shift is observed at the peak position. In addition, in the X-ray diffraction pattern of the titanate compound having a structure similar to the bronze type (Patent Document 7), there are peaks at 2θ = 23.9 ° and 33.4 °. Is not observed in the X-ray diffraction pattern of the titanate compound of formula 3. For this reason, both are clearly distinguished.

  The electrode active material of the present invention refers to an electrode active material in which at least two phases of a lithium titanium compound phase having a spinel crystal structure and a tunnel structure titanium compound phase exist as separate phases, and powder X-ray diffraction measurement Can be confirmed. As described later, the shape of the electrode active material is preferably particulate. However, both phases may coexist in the primary particles, and the primary particles composed of each single phase coexist in both phases. Secondary particles or aggregates may be formed, and primary particles, secondary particles, aggregates, etc., each consisting of a single phase may exist independently, one phase exists as primary particles, and the other The phase may be present as secondary particles, and the mixture may be a mixture without any bonding state between the lithium titanium compound and the tunnel structure titanium compound. In addition, it is preferable that a primary particle consists of each single phase. Of course, it does not exclude the presence of another phase other than the two phases. The secondary particles in the present invention are in a state in which the primary particles are firmly bonded to each other, and easily disintegrated in industrial operations such as normal mixing, pulverization, filtration, water washing, transportation, weighing, bagging, and deposition. Most of them remain as secondary particles.

The electrode active material is preferably in the form of particles, and the average particle size (represented by the average primary particle size by electron microscopy and the major axis length in the case of an anisotropic shape) is not particularly limited. However, the range of 0.05 to 100 μm is usually preferable, and the range of 0.1 to 20 μm is more preferable. The particle shape is not limited, but isotropic shape such as spherical shape, polyhedral shape, isotropic shape such as needle shape, rod shape, plate shape, indefinite shape, etc. Powder characteristics such as fluidity, adhesion, and filling properties are improved, and when used as an electrode active material, battery characteristics such as cycle characteristics are also improved, which is preferable. The average particle diameter (median diameter by laser scattering method) of the electrode active material particles containing 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, still more preferably in the range of 2~30m ² / g . The particle shape is not limited as in the case of the primary particles, and various shapes can be used.

  The abundance ratio of the lithium titanium compound phase having the spinel crystal structure and the tunnel structure titanium compound phase is not particularly limited, and any ratio may be used as long as the synergistic effect of the present invention can be obtained by the coexistence of two phases. The mass ratio is preferably 0.1: 9.9 to 9.9: 0.1, and more preferably 3: 7 to 7: 3.

  The reason why the electrode active material of the present invention exhibits excellent battery characteristics is not clear, but a spinel-type crystal structure lithium titanium compound is an excellent material for inserting and desorbing lithium ions, and a large amount of tunnel structure titanium compound. The present inventors consider that the lithium ion is occluded in the tunnel and is excellent in capacity characteristics, and thus is caused by a synergistic effect due to the coexistence of these materials.

The particle surfaces of the primary particles or secondary particles of the electrode active material 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. Note that the term “coating” as used herein includes not only a layer that completely covers the surface but also a state in which the substance is scattered in an island shape on the surface. 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, for each compound of 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, lithium, hydrogen, and oxygen can be contained in the crystal lattice by doping, etc., as long as the crystal form is not inhibited.

  Next, the present invention includes a step of mixing the inclusion of a lithium titanium compound having a spinel crystal structure with the inclusion of a titanium compound having a one-dimensional tunnel structure represented by the above-described formula 1 as a general formula. It is a manufacturing method of an electrode active material.

The tunnel structure titanium compound represented by Formula 1 can be obtained by a known method. For example, by reacting a compound having a chemical composition of M ₂ Ti _y O _{2y + 1} (M represents an alkali metal element and y is an integer greater than 2) (formula 4) with an acidic compound, and then dehydrating by heating can get. 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. 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 compound of Formula 4 is obtained by mixing a titanium compound and an alkali metal compound in a desired ratio in a dry or wet manner, and then firing. 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. As the alkali metal compound, lithium, sodium, potassium, rubidium, cesium and the like can be used. Alkali metal carbonates, hydroxides, and the like can be used. Among these, it is preferable to use titanium oxide and alkali metal carbonate. In the present invention, the titanium oxide means a compound containing titanium and oxygen and a hydrogen-containing compound, a hydrate or a hydrate thereof. For example, titanium oxide such as titanium dioxide (TiO ₂ ) In addition, metatitanic acid (TiO ₂ · H ₂ O), orthotitanic acid (TiO ₂ · 2H ₂ O), peroxotitanic acid, and the like can be used, 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. You may grind | pulverize as needed after baking. 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.

Hereinafter, the manufacturing method will be described by taking the case of n = 12 in Formula 1 (Formula 3) as an example. The titanic acid compound of the formula 3 is reacted with a compound having the chemical composition of the formula 4 and an acidic compound, and then appropriately washed, solid-liquid separated and dried, and heated and dehydrated at a temperature in the range of 150 to 400 ° C. Can be obtained. 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, inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid can be used. 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 the reaction temperature of an acidic compound and the compound of Formula 4, 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 does not change easily. For heat dehydration, a known firing furnace such as a fluidized furnace, a stationary furnace, a rotary kiln, a tunnel kiln can be used, and the heating atmosphere may be either in the air or in an inert gas. A suitable heat dehydration temperature depends on the compound type of the formula 4 used, but for example, when Na ₂ Ti ₃ O ₇ and a sulfuric acid aqueous solution are used, the crystal of the compound of the formula 3 is in the range of 230 to 270 ° C. The properties are likely to be high and a single phase is easily obtained. When the temperature is outside the range of 230 to 270 ° C., other phases are likely to exist in addition to the H ₂ Ti ₁₂ O ₂₅ phase.

Note that by replacing part of the H of the compound represented by Formula 3 with an alkali _metal, the compound of _{H 2-x A x Ti 12} O 25 (A is an alkali metal) (Equation 5) can be prepared by reacting a compound of formula 3 It is obtained by a production method including a step of reacting with an alkali metal compound. As the alkali metal compound, carbonates or hydroxides of alkali metals such as lithium, sodium, potassium, rubidium, and cesium can be used. In order to react the compound of Formula 3 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 the alkali metal compound becomes a molten salt by heating and reacts with the compound of formula 3, and as the alkali metal compound, an alkali metal nitrate or chloride having a relatively low melting point can be used. 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 5 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 3 is replaced with lithium ions and the range of x in formula 5 is adjusted to 0 <x ≦ 2, the general formula H _2−x Li _x Ti ₁₂ A compound having a chemical composition of O ₂₅ (0 <x ≦ 2) is obtained.

  The obtained compound of the formula 5 is washed, solid-liquid separated if necessary, and then dried. 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.

  In order to produce secondary particles of the titanic acid compound of formula 3, in the step of reacting the compound of formula 4 and the acidic compound, (1) whether the compound of formula 4 is further granulated and then reacted with the acidic compound. Or (2) a step of granulating after reacting the compound of formula 4 with an acidic compound may be included. Moreover, in order to produce the secondary particles of titanic acid of formula 5, in the step of reacting the titanic acid compound of formula 3 and the alkali metal compound, (1) a titanic acid compound of formula 3 and an alkali metal compound are further added. Either after reacting after granulation, (2) granulating the titanate compound of formula 3 and then reacting with the alkali metal compound, or (3) reacting the titanate compound of formula 3 with the alkali metal compound. The process of granulating should just 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. Dry granulation includes methods such as dehydrating a slurry containing a raw material compound or reaction product, drying and pulverizing; dehydrating the slurry, molding and drying; spray drying the slurry, and the like. Of these, spray drying is industrially preferable.

A compound of H ₂ Ti ₈ O _{17 and} a compound of H ₂ Ti ₆ O ₁₃ can also be prepared from compounds of formula 4, such as K ₂ Ti ₄ O ₉ or Na ₂ Ti ₃ O ₇ as raw materials. _The compound of H _{2 Ti} 18 _{O 37} is obtained by sintering at a temperature of titanate compounds of Formula 3 further 300 to 600 ° C.. These compounds can also be used by substituting a part of hydrogen with an alkali metal or granulated, like the titanic acid compound of formula 3.

  The lithium titanium compound having the spinel crystal structure of Formula 2 is obtained by mixing a titanium compound and a lithium compound in a desired ratio in a dry or wet manner and then firing the mixture. As the titanium compound, the same titanium compound as described in paragraph (0020), which can be used for preparing the compound of formula 4, can be used. 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 650 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 the firing, the above-mentioned known firing furnace 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. For the pulverization, the aforementioned pulverizer and pulverization method can be employed.

  In order to produce secondary particles of a lithium titanium compound having a spinel type crystal structure of Formula 2, in the step of reacting the lithium compound and the titanium compound, (1) reaction after the granulation of the lithium compound and the titanium compound together Or (2) granulating the titanium compound to react with the lithium compound, or (3) granulating the lithium compound after reacting with the titanium compound. For granulation, the above granulation methods can be used as appropriate, but spray drying is industrially preferable.

  The lithium titanium compound-containing material having a spinel crystal structure and the tunnel structure titanium compound-containing material can be mixed by any method, for example, a method of mixing each compound particle by a dry method or a wet method. Dry mixing can be performed by, for example, using a dry pulverizer such as a fluid energy pulverizer or an impact pulverizer, or a high-speed stirrer such as a Henschel mixer or a supermixer, and stirring and mixing them. In the wet mixing, for example, both compounds may be dispersed in a slurry and mixed through a wet pulverizer such as a sand mill, a ball mill, a pot mill, or a dyno mill. In some cases, the mixed slurry may be spray-dried by a spray dryer such as spray-drying. Moreover, the said process may be performed by preparing the electrode mixture slurry mixed with the binder, the electrically conductive material, etc. for each compound individually, and mixing both electrode mixture slurries.

  In addition, in the production of the electrode active material of the present invention, it is preferable that the mixture is fired after the mixing because the high rate charge / discharge characteristics when used in an electricity storage device are improved. The firing temperature is preferably in the range of 150 to 400 ° C. When the firing temperature is high, the phase transition of the tunnel structure titanium compound occurs, and the improvement in battery characteristics when used in an electricity storage device is reduced. A suitable firing temperature depends on the value of x in the formula 1 (alkali metal substitution amount), but if it is a titanic acid compound of the formula 3, a more preferable range is 230 to 270 ° C. The firing time is not particularly limited, but about 5 hours is industrially preferable. In order to accelerate the reaction, the firing can be repeated twice or more. For the firing, the above-mentioned known firing furnace 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. For the pulverization, the aforementioned pulverizer and pulverization method can be employed.

  In order to produce secondary particles of a lithium titanium compound having a spinel crystal structure and an electrode active material containing at least a tunnel structure titanium compound, in the step of mixing both compounds, (1) tunnel structure titanium compound and spinel type Or after mixing together with a lithium titanium compound having a crystal structure, or (2) granulating one of a tunnel structure titanium compound or a lithium titanium compound having a spinel crystal structure and then mixing with the other compound, (3) Granulation after mixing with tunnel structure titanium compound and lithium titanium compound having spinel crystal structure, or (4) tunnel structure titanium compound precursor (after reacting compound of formula 4 with acidic compound) Before heating and dehydration) and a lithium titanium compound having a spinel crystal structure. Heating dehydration from the rear granulation, it is sufficient that contain. 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 a tunnel structure titanium compound and a lithium titanium compound having a spinel crystal structure 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.

  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 to be used include (1) vinyl compounds (polyvinyl alcohol, polyvinyl pyrrolidone, etc.), (2) cellulose compounds (hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, ethyl cellulose, etc.), (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 electrode active material of the present invention as a constituent member has battery characteristics, particularly high capacity, excellent high-rate charge / discharge characteristics, and reversible lithium insertion / extraction reaction. Therefore, it is an electricity storage device that can be expected to have high reliability.

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 is obtained by adding a binder such as or the like, and forming or coating the electrode substrate appropriately. 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 known material for the positive electrode, for example, lithium such as lithium / manganese composite oxide, lithium / cobalt composite oxide, lithium / nickel composite oxide, lithium / vanadine composite oxide Transition metal complex oxides and olivine compounds such as lithium / iron / 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. For example, a porous polyethylene film or the like can be used as the separator. As an electrolyte, a solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, 1,2-dimethoxyethane, LiPF ₆ , LiClO ₄ , LiCF ₃ SO _{3 is used.} Conventional materials such as an electrolytic solution in which a lithium salt such as LiN (CF ₃ SO ₂ ) ₂ or LiBF ₄ is dissolved, a solid electrolyte, or a molten salt 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.

Experiment 1
(Production of raw material H ₂ Ti ₁₂ O ₂₅ )
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 Co., Ltd.). 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 sample having a median diameter of 5.5 μm.

When the chemical composition analysis of the obtained sample by ICP emission analysis (ICPS-7500: manufactured by Shimadzu Corporation) was performed, it was Na: Ti = 1.99: 3.00. Further, it is a single phase of Na ₂ Ti ₃ O ₇ having a monoclinic system and a crystal structure of space group P2 ₁ / m having good crystallinity by an X-ray powder diffractometer (RINT2550V: manufactured by Rigaku Corporation). It became clear.

4210 g of pure water was added to 1077 g of the obtained Na ₂ Ti ₃ O _{7 to} make a slurry, 657 g of 64% sulfuric acid was added, and the mixture was reacted for 5 hours at 60 ° C. with stirring, and then washed with filtered water. Pure water was added to the filter cake to make 3326 g, and then re-dispersed. 34 g of 64% sulfuric acid was added, reacted for 5 hours at 70 ° C. with stirring, washed with filtered water and dried to obtain a sample.

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.

The particle shape of the H ₂ Ti ₃ O ₇ obtained in this way was examined with a scanning electron microscope (SEM) (JSM-5400: manufactured by JEOL Ltd.). It had 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 Sample A.

The obtained sample A, the loss on heat in the temperature range of 250 to 600 ° C., measured using a differential thermal balance, heat loss is was calculated by assuming that corresponds to the structure water, of H ₂ Ti ₁₂ O ₂₅ The chemical composition was confirmed to be reasonable. In addition, X-ray diffraction data is measured by an X-ray powder diffractometer, and it is a single phase of H ₂ Ti ₁₂ O ₂₅ having a monoclinic system and a crystal structure of space group P2 / m having good crystallinity. It became clear. The X-ray diffraction pattern is shown in FIG.

A scanning electron microscope (SEM) photograph of the H ₂ Ti ₁₂ O ₂₅ particles thus obtained is shown in FIG. It had an isotropic or rod-like shape on the order of submicron to micron, in which the shape of the precursor H ₂ Ti ₃ O ₇ was almost retained.

(Production of Li ₄ Ti ₅ O ₁₂ )
125 g of rutile type and anatase type mixed crystal titanium dioxide (PT-401M: manufactured by Ishihara Sangyo Co., Ltd.) was added and dispersed in 340 ml of a 4.5 mol / liter lithium hydroxide aqueous solution. While stirring the slurry, the liquid temperature was kept at 80 ° C., and 250 ml of an aqueous slurry in which 25 g of orthotitanic acid was dispersed in terms of TiO 2 was added to obtain a slurry containing a titanium compound and a lithium compound. This slurry is spray-dried under conditions of an inlet temperature of 190 ° C. and an outlet temperature of 80 ° C. using a spray dryer (GB210-B type: manufactured by Yamato Kagaku Co., Ltd.) to obtain a dry granulated product, and then dry granulation The product was calcined at 700 ° C. in the atmosphere for 3 hours to obtain Sample B.

The obtained sample B, the X-ray powder diffractometer, measuring the X-ray diffraction data, with a good crystallinity was confirmed to be a single phase of Li ₄ Ti ₅ O ₁₂ having a spinel type crystal structure . The X-ray diffraction pattern is shown in FIG.

When the particle shape of the Li ₄ Ti ₅ O ₁₂ obtained in this way was examined by a scanning electron microscope (SEM), primary particles of about 0.01 to 1 μm aggregated in a spherical or polyhedral shape and were secondary. Particles were formed, the median diameter was 5.7 μm, and the specific surface area measured by the BET method was 11 m ² / g.

(Production of mixture)
50 g of pure water was added to 7 g of H ₂ Ti ₁₂ O ₂₅ thus obtained and 3 g of Li ₄ Ti ₅ O ₁₂ , and ultrasonic dispersion was performed for 5 minutes to prepare a slurry. This slurry was put into a dryer at 100 ° C. to evaporate pure water to obtain an evaporated dry product. Subsequently, this evaporated and dried product was calcined in the electric furnace at 260 ° C. for 5 hours in the air to obtain Sample C. Further, sample D was prepared in the same procedure except that 5 g of H ₂ Ti ₁₂ O ₂₅ , 5 g of Li ₄ Ti ₅ O ₁₂ and 50 g of pure water were used, 3 g of H ₂ Ti ₁₂ O ₂₅ , Li ₄ Ti ₅ O Sample E was obtained in the same procedure except that ₁₂ was changed to 7 g and pure water was changed to 50 g. The X-ray diffraction pattern of Sample D is shown in FIG. From FIG. 4, it can be seen that in Sample D, the H ₂ Ti ₁₂ O ₂₅ phase and the Li ₄ Ti ₅ O ₁₂ phase exist as separate phases. Samples C to E of this experiment are active materials of the present invention, which are Examples 1 to 3, respectively. Samples A and B which are raw materials of the present invention are Comparative Examples 1 and 2, respectively.

(Production of electricity storage device)
Samples A to E obtained by the above procedure as the electrode active material were mixed with acetylene black powder (DENKA BLACK (registered trademark) (powder): manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive agent and polyfluoride as a binder. It is mixed with vinylidene chloride (PVdF) resin (KF polymer L- # 1120: Solvent N-methyl-2-pyrrolidone manufactured by Kureha Co., Ltd.) at a weight ratio of 100: 10: 10 (PVdF is the resin content), and the solid content concentration is 30 %, N-methyl-2-pyrrolidone was added, and the mixture was mixed at 2000 rpm for 15 minutes with a rotation / revolution mixer (Awatori Netaro ARE-310: manufactured by Sinky Corporation) to prepare a paste. This paste was applied onto an aluminum foil (IN30-H18 (20 μm): manufactured by Fuji Processing Paper Co., Ltd.), dried for 10 minutes at a temperature of 120 ° C., punched into a circle with a diameter of 12 mm, pressed at 17 MPa, did. The coating amount (coating thickness) was adjusted so that the active material weight of the electrode cut into a diameter of 12 mm was 2.5 mg.

  This electrode was vacuum-dried at 120 ° C. for 4 hours, and then incorporated as a positive electrode in a sealable coin cell in a glove box having a dew point of −70 ° C. or lower. A coin type cell made of stainless steel (SUS316), having an outer diameter of 20 mm and a height of 3.2 mm was used. As the negative electrode, a metal lithium having a thickness of 0.5 mm (lithium foil: manufactured by Honjo Metal Co., Ltd.) formed into a circle having a diameter of 12 mm was used. A mixed solution (volume ratio 1: 2) of ethylene carbonate and dimethyl carbonate in which LiPF6 was dissolved at a concentration of 1 mol / liter was used as the non-aqueous electrolyte.

  The electrode was placed in the lower can of the coin-type cell, a porous polypropylene film was placed thereon as a separator, and a nonaqueous electrolyte was dropped from above. Furthermore, a negative electrode, a 0.5 mm thick spacer for adjusting the thickness, and a wave washer (both made of SUS316) are placed thereon, and the nonaqueous electrolyte is dripped over the top so that the upper can with a polypropylene gasket is attached. The outer peripheral edge portion was covered and sealed to obtain an electricity storage device (devices c to e) of the present invention and comparative electricity storage devices (devices a and b). In addition, a CR2032 parts set (Hosen Co., Ltd.) was used for battery members such as the above-mentioned 0.5 mm thick spacers and wave washers.

  About the electrical storage device (device ae), after aging for 3 hours after cell preparation, the conditioning which charges / discharges 2 cycles at 0.25C was performed. The discharge capacity at the second cycle of conditioning was taken as the initial capacity. Thereafter, the discharge capacity (mAh / g) was measured with various current amounts. The measurement was performed with the voltage range set to 1 to 2.5 V, the charging current set to 0.25 C, and the discharging current set to the range 0.25 C to 50 C. The environmental temperature was 25 ° C. In addition, 1C means a current value that can be completely discharged in 1 hour after full charge. In this evaluation, 0.4 mA is equivalent to 1 C current value in the device b and 0.5 mA in other devices. Measurements were made.

  Table 1 shows the measurement results of the comparative example. It can be seen that Sample A is excellent in capacity characteristics and Sample B is excellent in high rate discharge capacity.

  Table 2 shows a weighted average from the measurement results of Comparative Example 1 and Comparative Example 2 and the mixing ratio of samples A and B at each current value of 70:30 (capacity of sample A × mixing ratio of sample A (0.7) + sample. (Capacitance of B × mixing ratio of sample B (0.3)), calculated capacity of sample C (mAh / g), and actual capacity of device c (mAh / g). In addition, the calculated value of the 1C current value in Table 2 is calculated using the actual measurement value of Comparative Example 1 and Comparative Example 2 and the mixing ratio 70:30 of Samples A and B (1C current value of Comparative Example 1 (0.5 mA). ) × mixing ratio of sample A (0.7) + 1C current value of comparative example 2 (0.4 mA) × mixing ratio of sample B (0.3)). The device c shows a discharge capacity exceeding the calculated value at any current value, and further, at a current value of 0.25 to 2C, the discharge capacity exceeding the devices a and b using the samples A and B alone, respectively. As shown, an unexpected effect was observed.

  Table 3 shows the measurement results of Comparative Example 1 and Comparative Example 2 at each current value and the weighted average (mixing ratio of sample A × mixing ratio of sample A (0.5) + sample) from the mixing ratio of samples A and B of 50:50. (Capacitance of B × mixing ratio of sample B (0.5)), calculated capacity of sample D (mAh / g), and actual capacity of device d (mAh / g). Further, the calculated value of the 1C current value in Table 3 was calculated using the actual measurement value of Comparative Example 1 and Comparative Example 2 and the mixing ratio 50:50 of Samples A and B (1C current value of Comparative Example 1 (0.5 mA). ) × Sample A blending ratio (0.5) + 1C current value of Comparative Example 2 (0.4 mA) × Sample B blending ratio (0.5)). The device d shows a discharge capacity that exceeds the calculated value at any current value, and further shows a discharge capacity that exceeds the devices a and b at a current value of 0.5 to 3 C, which has an unexpected effect. Admitted.

  Table 4 shows the measurement result of Comparative Example 1 and Comparative Example 2 at each current value and the weighted average (mixing ratio of sample A × mixing ratio of sample A (0.3) + sample) from the mixing ratio of samples A and B of 30:70. The calculated capacity (mAh / g) of the sample E and the actual measured capacity (mAh / g) of the device e, which are obtained by multiplying the volume of B by the mixing ratio of the sample B (0.7). Further, the calculated value of the 1C current value in Table 4 is calculated using the actually measured value of Comparative Example 1 and Comparative Example 2 and the mixing ratio 30:70 of Samples A and B (1C current value of Comparative Example 1 (0.5 mA). ) × mixing ratio of sample A (0.3) + 1C current value of comparative example 2 (0.4 mA) × mixing ratio of sample B (0.7)). The device e showed a discharge capacity exceeding the calculated value at any current value, and an unexpected effect was recognized.

  FIG. 5 is a graph showing the actual current values plotted on the x-axis for the above results. As is clear from FIG. 5, the devices c to e using the electrode active materials of the present invention of Examples 1 to 3 have a discharge capacity equivalent to that of the device b excellent in high rate discharge capacity even at a current value of 100 mA or more. It can be seen that the high-rate discharge capacity is excellent. Furthermore, in the capacity at the time of low-rate discharge, the device c and the device d have a tunnel structure titanium which is characterized by a high capacity, even though the sample B which is a spinel type crystal structure lithium titanium compound having a low capacity is blended. It can be seen that the discharge capacity exceeds the device a using the compound.

Experiment 2
In the preparation of Sample D, Sample F was obtained in the same procedure as Sample D, except that the evaporated and dried product was not fired in an electric furnace. Sample F of this experiment is the active material of the present invention, and is Example 4. Further, using Sample F and Sample D prepared in Experiment 1, an electricity storage device d ′ and an electricity storage device f were prepared in the same procedure as in Experiment 1.

  The high rate discharge capacity was measured in the same procedure as in Experiment 1 except that the discharge current was set in the range of 0.25C to 5C using the electricity storage devices d 'and f. Table 5 shows the ratio of discharge capacity (capacity maintenance ratio) at each discharge current value when the discharge capacity at 0.25 C is 100.

  In Table 5, the high-rate discharge capacity measurement of the devices d 'and f using the sample D that was baked after mixing the samples A and B was performed with a 1C current value of 0.5 mA. On the other hand, in the calculated value of the 1C current value in Table 5, the 1C current value is 0.45 mA (1C current value of sample A (0.5 mA) in Table 1 × mixing ratio of sample A (0.5) + Table 1) 1C current value of sample B (0.4 mA) × mixing ratio of sample B (0.5)). Therefore, in the devices d 'and f, compared with the calculated value, a high load of 10% or more is applied at each measured current value (a large current of + 10% or more is flowing). Nevertheless, it can be seen that a sufficiently high discharge characteristic can be obtained even with the device f. Further, as is apparent from the measurement result of the device d ', it can be seen that the high rate discharge characteristics are further improved by performing the firing after mixing.

Experiment 3
The effect of the present invention was verified using a lithium titanium compound having a spinel crystal structure obtained by a production method different from Experiment 1.

(Production of Li ₄ Ti ₅ O ₁₂ )
187 g of lithium carbonate powder and 500 g of rutile titanium dioxide (CR-EL: manufactured by Ishihara Sangyo) were mixed at 1800 rpm for 10 minutes with a Henschel mixer. This mixture was baked in an electric furnace at a temperature of 940 ° C. for 3 hours in the atmosphere to obtain a sample G.

The obtained sample G, the X-ray powder diffractometer, measuring the X-ray diffraction data, with a good crystallinity was confirmed to be a single phase of Li ₄ Ti ₅ O ₁₂ having a spinel type crystal structure . When the particle shape was examined with a scanning electron microscope (SEM), it was found to be amorphous particles having a size of submicron to micron order.

(Production of mixture)
50 g of pure water was added to 5 g of H ₂ Ti ₁₂ O ₂₅ (Sample A) obtained in the same manner as in Experiment 1 and 5 g of Li ₄ Ti ₅ O ₁₂ (Sample G) obtained in Experiment 3 above. Sonic dispersion was performed for 5 minutes to prepare a slurry. This slurry was put into a dryer at 100 ° C. to evaporate pure water to obtain an evaporated dry product. Subsequently, this evaporated and dried product was baked in the electric furnace at 260 ° C. for 5 hours in the air, and sample H was obtained. Sample H of this experiment is the active material of the present invention, and is Example 5. Sample G is Comparative Example 3.

  Samples A, G, and H were used as electrode active materials to prepare lithium secondary batteries, and their charge / discharge cycle 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 out to a diameter of 10 mm was superimposed on this pellet and pressed at 9 MPa to form an electrode.

This 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 electrode was placed in the lower can of the evaluation cell, a porous polypropylene film was placed thereon as a separator, and a nonaqueous 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 devices produced were devices a, g, and h, respectively.

  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 capacities at the second cycle, 10, 20, and 30 cycles were measured, and (discharge capacity at each 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 6. It can be seen that the device h using the active material H of the present invention in Example 5 exhibits cycle characteristics equivalent to or higher than those of the devices a and g of the comparative example.

Experiment 4
The high-temperature cycle characteristics were evaluated in the same manner as in Experiment 3 except that the charge / discharge in Experiment 3 was performed at 60 ° C. The results are shown in Table 7. It can be seen that the device h using the active material H of the present invention in Example 5 exhibits cycle characteristics equivalent to or higher than those of the devices a and g of the comparative example.

In addition, a device was fabricated using Samples A, G, and H in the same manner as in Experiment 1, and the 0.25C discharge capacity ratio of the 50C discharge capacity was measured. As a result, Sample A was 18% and Sample G was 23%. On the other hand, Sample H in which both were mixed showed 38%, and an unexpected effect was observed.

  The electrode active material of the present invention comprises a mixture of a specific titanium compound having a one-dimensional tunnel structure and a lithium titanium compound having a spinel type crystal structure, which exceeds the battery characteristics when each is used alone as an active material. It shows battery characteristics such as initial capacity and high rate charge / discharge capacity, and is an excellent material. 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.

Claims (3)

A step of mixing Li ₄ Ti ₅ O ₁₂ having a spinel crystal structure with a titanium compound having a one-dimensional tunnel structure represented by H ₂ Ti ₁₂ O ₂₅ as a general formula;
Next, the _two phases of the Li ₄ Ti ₅ O ₁₂ phase and the titanium compound phase represented by the general formula H ₂ Ti ₁₂ O ₂₅ , including a step of firing the mixture at a temperature of 150 to 400 ° C. And the ratio is from 3: 7 to 7: 3 by mass ratio .   The method for producing an electrode active material according to claim 1, wherein the firing is performed in an atmosphere in the air.   The manufacturing method of the electrode active material of Claim 1 or 2 which performs said baking at the temperature of 230-270 degreeC.

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