Lithium titanate powder, electrodes using the same, and power storage devices

Abstract

To provide lithium titanate powder, which is used as an electrode material for a power storage device, can increase an electrode density, and has excellent charge-discharge rate characteristics and cycle characteristics, an electrode using the same, and a power storage device.SOLUTION: Lithium titanate powder contains Li4Ti5O12 as a main component, in which one or more of M1 (M1 is a metallic element of Group 2, Group 12, Group 13, or Group 14, excluding Ti, or molybdenum element) exist in a localized manner, on a surface of a primary particle of the lithium titanate powder, and the lithium titanate powder satisfies the following equations (I) and (II): D50≥1.7 μm (I) and DBET≥0.7 μm (II) (where D50 denotes a particle size at a point where a cumulative volume distribution of the particle size of the primary particle is 50% in the particle size distribution, and DBET denotes a specific surface area equivalent diameter calculated from a specific surface area obtained by the BET method).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a lithium titanate powder suitable as an electrode material for an electricity storage device, an electrode using the same, and an electricity storage device. [Background technology]

[0002] In recent years, various materials have been researched as electrode materials for energy storage devices. Among them, 12 Lithium titanate represented by the formula (I) has been attracting attention as a negative electrode active material for auxiliary power supply devices for electric vehicles (HEVs, PHEVs, BEVs) because of its excellent input / output characteristics, especially in the low temperature range, when used as an active material.

[0003] Energy storage devices for electric vehicles require high energy density to improve fuel economy and power consumption. Higher energy density could lead to longer driving ranges and free up space for storage battery installation, potentially accelerating the widespread adoption of various types of electric vehicles. Furthermore, energy storage devices for electric vehicles must be stable over a wide temperature range, from high to low. For example, if the performance of an energy storage device deteriorates during long-term use in midsummer, the fuel economy or power consumption of the electric vehicle may deteriorate in winter. In this regard, specific characteristics required for energy storage devices include improved initial and long-term input / output characteristics. When lithium titanate is used in energy storage devices, it is known that long-term use at temperatures above 60°C can cause storage stability issues, such as gas generation, which can lead to increased battery resistance and changes in long-term input / output characteristics. Therefore, there is a need for lithium titanate that has excellent initial and long-term input / output characteristics while increasing the energy density of energy storage devices.

[0004] Patent Document 1 describes a specific surface area equivalent diameter D calculated from the specific surface area obtained by the BET method. BET and crystallite diameter D Xis specified within a certain range, and contains at least one localized element selected from M (M is at least one metal element selected from Mg, Zn, Al, Ga, or In), the element being localized near the surface of the lithium titanate particles. Patent Document 1 discloses a lithium titanate powder that, when used as an electrode material for an electricity storage device, not only has a large initial charge / discharge capacity and excellent input / output characteristics, but also exhibits an extremely high charge / discharge capacity at an extremely low temperature of -30°C.

[0005] Patent Document 2 discloses a lithium-titanium composite oxide containing aluminum-coated primary particles, which is effective in suppressing decomposition of the electrolyte and generation of gas due to residual lithium.

[0006] Patent Document 3 describes a method for producing a zeolite with a BET specific surface area of ​​3 m 2 / g~10m 2 / g and a volume-based average particle size measured by laser diffraction is 0.5 μm to 20 μm. According to Patent Document 3, by controlling the average pore size (μm) and the cumulative pore volume (mL / g) to be in the same range, it is possible to increase the capacity retention rate of an electricity storage device during rapid charge and discharge. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 5790894 [Patent Document 2] Special Publication No. 2021-516208 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-24723 Summary of the Invention [Problem to be solved by the invention]

[0008] However, although the electricity storage device disclosed in Patent Document 1, which uses lithium titanate as a negative electrode material, exhibits excellent input / output characteristics initially, there is no description regarding electrode density, nor is there any knowledge regarding the compatibility of initial input / output characteristics and volumetric energy density. Note that an advantage of increasing electrode density is the improvement of the energy density per unit volume or unit weight of the electricity storage device itself.

[0009] Regarding the lithium titanate powder in Patent Document 2, although there is a description regarding rate characteristics and the amount of gas generated after high-temperature storage, there is no description regarding an increase in resistance after cycling, and no findings regarding compatibility with improved electrode density are presented.

[0010] In the electricity storage device in which lithium titanate is used as the negative electrode material in Patent Document 3, although there is a description regarding the capacity retention rate during rapid charge and discharge, there is no information regarding the long-term battery characteristics.

[0011] For these reasons, it has been difficult to apply lithium titanate powder, which can increase the electrode density of the negative electrode, which is directly linked to the energy density, while maintaining cycle characteristics and achieving long-term battery characteristics, to an electricity storage device.

[0012] Therefore, the present invention aims to provide a lithium titanate powder that can be used as an electrode material for an electricity storage device and that achieves high electrode density, thereby achieving a well-balanced and excellent volumetric energy density, cycle characteristics, and charge rate characteristics at high temperatures, as well as an electrode and an electricity storage device using the same. [Means for solving the problem]

[0013] The present inventors conducted extensive research to achieve the above-mentioned object using a simple manufacturing method, and discovered a lithium titanate powder in which specific metal elements are present on the particle surface by subjecting lithium titanate powder, whose physical properties, such as particle size, are controlled within specific ranges, to a surface treatment process. They also discovered that an energy storage device using this lithium titanate powder as an active material in an electrode material achieves high electrode density, resulting in a well-balanced and excellent volumetric energy density, cycle characteristics, and high-temperature charge rate characteristics, and thus completed the present invention. Specifically, the present invention relates to the following:

[0014] (1) Li4Ti5O 12 a lithium titanate powder mainly composed of the above, characterized in that one or more M1 (M1 is a metal element of Group 2, Group 12, Group 13, or Group 14 excluding Ti, or molybdenum element) is localized on the surface of the primary particles of the lithium titanate powder, and the lithium titanate powder satisfies the following formulas (I) and (II): D 50 ≧ 1.7μm (I) D BET ≧ 0.7μm (II) (In the above formula, D 50 D indicates the particle size at which the cumulative volume distribution of the particle size of primary particles is 50% in the particle size distribution. BET indicates the diameter equivalent to the specific surface area calculated from the specific surface area determined by the BET method. (2) The lithium titanate powder according to (1) above, characterized in that the lithium titanate powder satisfies the following formula (III): D 50 ≧ 5.0μm (III) (In the above formula, D 50 indicates the particle size at which the cumulative volume distribution of the particle size of primary particles is 50% in the particle size distribution. (3) The lithium titanate powder according to any one of (1) and (2) above, characterized in that the element M1 present on the particle surface in the lithium titanate powder includes at least one element selected from the group consisting of Al, Mg, Ca, Sr, Zn, Ga, Ge, In, and Mo. (4) The lithium titanate powder has a relative density of 60% and a conductivity of 1.0 × 10 at 25°C. -6 The lithium titanate powder according to (3) above, characterized in that its specific surface area is less than S / cm. (5) The lithium titanate powder according to any one of (3) or (4) above, characterized in that the absolute value of the zeta potential of the lithium titanate powder at 25°C measured by electrophoresis is greater than 20 mV. (6) The lithium titanate powder according to any one of (3) to (5) above, characterized in that in a surface analysis using X-ray photoelectron spectroscopy, the ratio of the concentration of element M1 present on the surface of the lithium titanate powder particles to the concentration of O (oxygen) atoms (ratio of M1 atomic concentration / O atomic concentration (%)) is less than 15%. (7) An electrode comprising the lithium titanate powder according to any one of (1) to (6) above as an active material. (8) An electricity storage device comprising the electrode according to (7) above. [Effects of the Invention]

[0015] According to the present invention, by achieving high electrode density, it is possible to provide a lithium titanate powder suitable as an electrode material for an electricity storage device, which has excellent volumetric energy density, cycle characteristics, and charge rate characteristics at high temperatures, as well as an electrode and an electricity storage device using the same. DETAILED DESCRIPTION OF THE INVENTION

[0016] [Lithium titanate powder of the present invention] The lithium titanate powder of the present invention is Li4Ti5O 12 The term "lithium titanate powder" refers to a lithium titanate powder having as its main component one or more of M1 (M1 is a metal element of Group 2, Group 12, Group 13, or Group 14 excluding Ti, or molybdenum element) localized on the primary particle surface of the lithium titanate powder, and characterized in that the following formulas (I) and (II) are satisfied: D 50 ≧ 1.7μm (I) D BET ≧ 0.7μm (II) (In the above formula, D 50 D indicates the particle size at which the cumulative volume distribution of the particle size of primary particles is 50% in the particle size distribution. BET indicates the diameter equivalent to the specific surface area calculated from the specific surface area determined by the BET method.

[0017] <Li4Ti5O 12 Lithium titanate powder containing The lithium titanate powder of the present invention is Li4Ti5O 12 The main component is Li4Ti5O within the range in which the effects of the present invention can be obtained. 12 The main component is a crystalline component and / or amorphous component other than Li4Ti5O among the diffraction peaks measured by X-ray diffraction. 12 The lithium titanate powder of the present invention has a diffraction peak intensity ratio of 90% or more of the main peak of Li4Ti5O among the diffraction peaks measured by X-ray diffraction. 12 The ratio of the intensity of the main peak of Li4Ti5O is more preferably 92% or more, and even more preferably 95% or more. 12 The other components are the sum of the main peak intensity due to the crystalline component and the maximum intensity of the halo pattern due to the amorphous component. In particular, the lithium titanate powder of the present invention is composed of anatase type titanium dioxide, rutile type titanium dioxide, and lithium titanates with different chemical formulas, Li2TiO3, Li 0.6 Ti 3.4 O8, etc. as the crystalline component. The lithium titanate powder of the present invention may contain these Li4Ti5O 12 Crystalline components other than Li 0.6 Ti 3.4 The lower the rate of O8 generation, the better the charging characteristics and charge / discharge capacity of the energy storage device. 12When the intensity of the main peak of anatase titanium dioxide is taken as 100, it is particularly preferable that the sum of the intensity of the main peak of anatase titanium dioxide, the intensity of the main peak of rutile titanium dioxide, and the intensity corresponding to the main peak of Li2TiO3 calculated by multiplying the peak intensity corresponding to the (-133) plane of Li2TiO3 by 100 / 80 is 5 or less. 12 The main peak of Li4Ti5O in ICDD (PDF2010) PDF card 00-049-0207 is 12 The main peak of anatase titanium dioxide is a peak corresponding to the diffraction peak attributable to the (111) plane (2θ=18.33) of PDF card 01-070-6826. The main peak of rutile titanium dioxide is a peak corresponding to the diffraction peak attributable to the (110) plane (2θ=27.44) of PDF card 01-070-7347. The peak corresponding to the (-133) plane of Li2TiO3 is a peak corresponding to the diffraction peak attributable to the (-133) plane of Li2TiO3 (2θ=43.58) of PDF card 00-033-0831. Li 0.6 Ti 3.4 The main peak of O8 corresponds to the diffraction peak attributable to the (101) plane (2θ = 19.98) on PDF card 01-070-2732. Note that "ICDD" stands for International Centre for Diffraction Data, and "PDF" stands for Powder Diffraction File.

[0018] <Metal element M1> The lithium titanate powder of the present invention has at least one metal element M1 (M1 is a metal element of Group 2, Group 12, Group 13, or Group 14 excluding Ti, or molybdenum) present on the surface of the primary particles. The presence of metal element M1 means that metal element M1 is detected by inductively coupled plasma atomic emission spectroscopy (ICP-AES) or X-ray fluorescence spectroscopy (XRF) of the lithium titanate powder of the present invention. The lower limit of the amount detectable by inductively coupled plasma atomic emission spectroscopy is usually 0.001 mass%.

[0019] <Content of metal element M1> The content (mass%) of the metal element M1 in the lithium titanate powder of the present invention, as determined by X-ray fluorescence analysis (XRF), may be 0.01 to 0.8. When the content of the metal element M1 is within this range, an electricity storage device excellent in volumetric energy density, cycle characteristics, and high-temperature charge rate characteristics can be obtained. The content is preferably 0.05 to 0.5, more preferably 0.1 to 0.3, even more preferably 0.1 to 0.25, and particularly preferably 0.1 to 0.2.

[0020] Furthermore, in the lithium titanate powder of the present invention, the metal element M1 is localized and present in greater amounts in the surface regions than in the internal regions of the lithium titanate particles constituting the powder. That is, the metal element M1 is present on the surface of the lithium titanate particles, and more specifically, the metal element M1 is localized and present in greater amounts in the surface regions than in the internal regions of the lithium titanate particles. For example, in cross-sectional analysis of the lithium titanate particles using a scanning transmission electron microscope, the metal element M1 may be present in greater amounts in the so-called near-surface region, extending from the surface to a depth of about 20 nm as measured by energy dispersive X-ray spectroscopy. It is preferable that the metal element M1 is not detected at a depth of 100 nm from the surface. That is, when measured by energy dispersive X-ray spectroscopy, the metal element M1 is below the detectable amount. The lower limit of the detectable amount in energy dispersive X-ray spectroscopy is typically 0.5 atm%, although this value varies depending on the element and state being measured. Other surface analysis techniques include X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). For example, in the case of surface analysis using X-ray photoelectron spectroscopy, the ratio of the concentration of metal element M1 to the concentration of O (oxygen) atoms (M1 atom concentration / O atom concentration ratio (%)) present on the surface of the lithium titanate powder particles of the present invention is preferably less than 25%, more preferably less than 15%.

[0021] <Specific examples of metal element M1> In the lithium titanate powder, the metal element M1 present on the surface of the lithium titanate particles is at least one of metal elements of Group 2, Group 12, Group 13, or Group 14 (excluding Ti), or molybdenum. More specifically, it preferably contains at least one selected from the group consisting of Al, Mg, Ca, Sr, Zn, Ga, Ge, In, and Mo. Among these, Al and Mo are preferred, and Al is more preferred. Two or more of these metal elements may be contained. When two or more metal elements M1 are contained, a combination of Al and Mo is preferred. This is because the lithium titanate powder of the present invention contains these elements in a state localized on the particle surface, thereby providing an electricity storage device with high energy density, excellent cycle characteristics, and excellent charge rate characteristics at high temperatures.

[0022] <Inclusion of additional heterogeneous elements> The lithium titanate powder of the present invention may further contain a different element other than Ti, and particularly preferably contains at least one element selected from the group consisting of Nb, B, W, and S. Of these, S is particularly preferred. It is presumed that the lithium titanate powder of the present invention contains such a different element together with element M1, thereby improving the ionic conductivity of the surface of the lithium titanate powder compared to when element M1 is contained alone.

[0023] <Specific surface area> The specific surface area (hereinafter sometimes referred to as SSA) of the lithium titanate powder of the present invention is the surface area per unit mass when nitrogen is used as the adsorption gas. The measurement method will be explained in the examples below.

[0024] The lithium titanate powder of the present invention has a specific surface area of ​​3.0 m 2 / g or less is sufficient, and 2.7m 2 / g or less is preferable, and 2.0m 2 The lower limit of the specific surface area is not particularly limited, but is preferably 0.3 m 2 / g or more.

[0025] <D 50 > D of the lithium titanate powder of the present invention 50 is an index of the volume median particle size. 50 "means the particle size at which the cumulative volume frequency calculated from the volume fraction determined by laser diffraction / scattering particle size distribution measurement reaches 50% when calculated from the smallest particle size. The measurement method will be explained in the examples below.

[0026] The lithium titanate powder of the present invention may be in the form of primary particles or secondary particles formed by agglomeration of primary particles. When primary particles made of lithium titanate particles contain secondary particles formed by agglomeration, some of the secondary particles may not form secondary particles and may be in the form of primary particles themselves.

[0027] When the lithium titanate powder of the present invention is a secondary particle, the D of the secondary particle 50 From the viewpoint of improving the electrode density, the lower limit is preferably 11 μm or more, more preferably 12 μm or more, and even more preferably 13 μm or more. 50 The upper limit of the D of the secondary particles is preferably 50 μm or less, more preferably 30 μm or less. 50 is the D before crushing process (ultrasonication with an ultrasonic device). 50 Represents.

[0028] D of the primary particles of the lithium titanate powder of the present invention 50 (D of primary particles 50 is the particle size at which the cumulative volume distribution of the particle size of primary particles reaches 50% in laser diffraction / scattering particle size distribution measurement. From the viewpoint of achieving high electrode density and improving volumetric energy density, D 50 The lower limit of the D of the primary particles is 1.7 μm or more, preferably more than 1.7 μm, more preferably 2.0 μm or more, even more preferably 2.5 μm or more, still more preferably 5.0 μm or more, and particularly preferably 7.0 μm or more. 50 The upper limit of D of the primary particles is preferably 15 μm or less, more preferably 10 μm or less, and even more preferably 8 μm or less. 50is the D after crushing process (ultrasonication with an ultrasonic device). 50 The lithium titanate powder may contain 15% to 30% primary particles having a primary particle diameter of less than 1.7 μm, 15% to 45% primary particles having a primary particle diameter of less than 2.0 μm, 15% to 45% primary particles having a primary particle diameter of less than 2.5 μm, 15% to 45% primary particles having a primary particle diameter of less than 5.0 μm, or 15% to 45% primary particles having a primary particle diameter of less than 7.0 μm. The lithium titanate powder may contain 45% to 75% primary particles having a primary particle diameter of more than 15 μm, 25% to 75% primary particles having a primary particle diameter of more than 10 μm, or 25% to 80% primary particles having a primary particle diameter of more than 8 μm.

[0029] <D BET > D of the lithium titanate powder of the present invention BET is the diameter equivalent to the specific surface area calculated from the specific surface area determined by the BET method. From the viewpoint of achieving a high electrode density and improving the volumetric energy density, D of the lithium titanate powder of the present invention is BET The D of the lithium titanate powder is 0.7 μm or more, preferably 1 μm or more, more preferably 1.5 μm or more, further preferably 2 μm or more, and particularly preferably 2.5 μm or more. BET The upper limit of D of the lithium titanate powder is 5 μm or less, preferably 4 μm or less, and more preferably 3.5 μm or less. BET can be determined by the method described in the Examples below.

[0030] [Method of manufacturing lithium titanate powder of the present invention] An example of the method for producing the lithium titanate powder of the present invention will be explained below, dividing it into a raw material preparation step, a firing step, and a surface treatment step, but the method for producing the lithium titanate powder of the present invention is not limited thereto.

[0031] <Raw material preparation process> First, the starting materials are mixed. The raw materials for lithium titanate powder consist of a titanium raw material and a lithium raw material. The lithium raw material may be a lithium compound such as lithium hydroxide monohydrate, lithium oxide, lithium bicarbonate, or lithium carbonate. The titanium raw material may be a titanium compound such as anatase titanium dioxide or rutile titanium dioxide. For example, the D of the lithium titanate powder can be adjusted by adjusting the average particle size of the titanium raw material and the lithium raw material used. 50 and D BET can be suitably controlled.

[0032] The method for mixing the raw materials is not particularly limited, and either wet mixing or dry mixing may be used. For example, a Henschel mixer, an ultrasonic disperser, a homomixer, a mortar, a ball mill, a centrifugal ball mill, a planetary ball mill, a vibrating ball mill, an attritor-type high-speed ball mill, a bead mill, a roll mill, etc. may be used.

[0033] <Firing process> Next, the mixture obtained above is fired. From the viewpoints of controlling the specific surface area and crystallite size of the powder obtained by firing, controlling the primary particle size of the powder, and reducing the amount of impurities from furnace materials, etc., firing at a high temperature for a short time is preferred. Firing is carried out in a temperature range of 500 to 1300°C, more preferably in a range of 700 to 1100°C. By setting the firing temperature at 1100°C or less, general-purpose equipment can be used.

[0034] The calcination method is not particularly limited as long as it is a method that can perform calcination under the above conditions. Usable calcination methods include fixed-bed calcination furnaces, roller hearth calcination furnaces, mesh belt calcination furnaces, fluidized bed calcination furnaces, and rotary kiln calcination furnaces. However, for efficient calcination in a short time, roller hearth calcination furnaces, mesh belt calcination furnaces, and rotary kiln calcination furnaces are preferred. In particular, rotary kiln calcination furnaces are particularly preferred because they do not require a container to contain the mixture, can be calcined while continuously adding the mixture, and can provide a uniform thermal history to the calcined material, thereby enabling the production of homogeneous oxides.

[0035] <Crushing process> Methods for crushing the lithium titanate powder after calcination include hammer mills, ball mills, jet mills, vibration mills, and bead mills, with bead mills being particularly preferred. When a bead mill is used, any of the following methods can be adopted for crushing: wet crushing circulation treatment, wet crushing batch treatment, dry crushing circulation treatment, and dry crushing batch treatment. It is preferable to perform crushing uniformly, and in this respect, wet crushing circulation treatment is preferred. The circulation conditions can be determined taking into consideration the firing temperature in the firing step, and for example, by adjusting the circulation conditions, the log of the primary particles of the lithium titanate powder can be reduced. 10 (D 90 )-log 10 (D 10 ) value can be suitably controlled. In wet crushing, the calcined lithium titanate powder is introduced into a water or alcohol solvent and mixed in a slurry state. As the alcohol solvent, those with a boiling point of 100°C or less, such as methanol, ethanol, and isopropyl alcohol, are preferred because they are easy to remove. Furthermore, from an industrial perspective, water solvents are preferred because of the ease of recovery and disposal.

[0036] The amount of solvent is preferably such that the lithium titanate powder after firing is uniformly dispersed in the solvent, and for this purpose the slurry viscosity is preferably 8000 cP or less, more preferably 5000 cP or less, and even more preferably 3000 cP or less.

[0037] The circulation treatment time of wet crushing (the number of crushing passes by circulation treatment) is not particularly limited as long as the crystallinity of lithium titanate does not decrease and the battery performance is not adversely affected. 10 (D 90 )-log 10 (D 10 It is preferable to determine the value of log 10 (D 90 )-log 10 (D 10 ) < 0.7 is preferable. 90D indicates the particle size at which the cumulative volume distribution of primary particles is 90% in the particle size distribution. 10 indicates the particle size at which the cumulative volume distribution of primary particles is 10% in the particle size distribution.

[0038] <Surface treatment process> Next, the lithium titanate obtained above is subjected to a surface treatment. The lithium titanate of the present invention is characterized by the presence of one or more M1 (M1 is a metal element of Group 2, Group 12, Group 13, or Group 14 excluding Ti, or molybdenum) on the particle surface. When used as a battery anode material, it can improve the volumetric energy density, cycle characteristics, and high-temperature charge rate characteristics in a balanced manner. The lithium titanate powder of the present invention can also be produced by adding a compound containing the metal element M1 (hereinafter sometimes referred to as a treating agent) in the firing step. However, it is more preferable to produce the lithium titanate powder of the present invention by the following surface treatment step. In particular, by employing the following surface treatment step, the metal element M1 can be localized on the surface of the lithium titanate particles appropriately and relatively simply.

[0039] There are no particular limitations on the method for mixing the lithium titanate powder as the base material with the compound containing the metal element M1, and either wet mixing or dry mixing can be used. However, it is preferable to uniformly disperse the compound containing the metal element M1 on the surfaces of the particles constituting the lithium titanate powder as the base material, and in this respect, wet mixing is preferred.

[0040] In wet mixing, the treatment agent and the base lithium titanate powder are placed in a water or alcohol solvent and mixed in a slurry state. Alcohol solvents with a boiling point of 100°C or less, such as methanol, ethanol, and isopropyl alcohol, are preferred because they are easy to remove. Furthermore, water solvents are industrially preferred because of their ease of recovery and disposal.

[0041] Compounds (treatment agents) containing the metal element M1 (M1 is a metal element of Groups 2, 12, 13, or 14 excluding Ti, or molybdenum) are not particularly limited, and examples thereof include oxides, phosphates, hydroxides, sulfates, nitrates, fluorides, chlorides, organic compounds, and metal salt compounds such as ammonium salts and phosphates. Specifically, when the metal element M1 is Al, examples of compounds containing Al include aluminum oxide, aluminum phosphate, aluminum hydroxide, aluminum sulfate, aluminum nitrate, aluminum fluoride, aluminum chloride, aluminum acetate, aluminum ammonium sulfate, and aluminum alkoxides, with aluminum sulfate and its hydrates being preferred. When the metal element M1 is Mg, examples of compounds containing Al include magnesium oxide, magnesium phosphate, magnesium hydroxide, magnesium sulfate, magnesium nitrate, magnesium fluoride, magnesium chloride, magnesium acetate, magnesium ammonium phosphate, and magnesium alkoxides, with magnesium sulfate and its hydrates being preferred. When the metal element M1 is Mo, specific examples include molybdenum oxide, molybdenum trioxide, molybdenum trioxide hydrate, molybdenum boride, molybdophosphoric acid, molybdenum disilicide, molybdenum chloride, molybdenum sulfide, molybdosilicic acid hydrate, sodium molybdenum oxide, molybdenum carbide, molybdenum acetate dimer, lithium molybdate, sodium molybdate, potassium molybdate, calcium molybdate, magnesium molybdate, manganese molybdate, and ammonium molybdate, and among these, molybdenum trioxide, molybdenum trioxide hydrate, molybdenum chloride, molybdenum sulfide, and lithium molybdate are preferred.

[0042] The amount of the compound containing the metal element M1 added may be any amount as long as the amount of the metal element M1 in the lithium titanate falls within the range of the present invention, but it is preferably added in a proportion of 0.01% by mass or more, more preferably 0.1% by mass or more, relative to the lithium titanate powder of the substrate, from the viewpoint of improving battery performance. It is also preferably added in a proportion of 10% by mass or less, more preferably 8% by mass or less. If the amount added is too large, there is a concern that the battery capacity may decrease.

[0043] It is preferable to perform a heat treatment after the surface treatment. The heat treatment temperature is a temperature at which the metal element diffuses into at least the surface region of the lithium titanate particles constituting the lithium titanate powder substrate, and at which a significant decrease in the specific surface area due to sintering of the lithium titanate substrate does not occur. The upper limit of the heat treatment temperature is preferably 900°C or lower, more preferably 700°C or lower. The lower limit of the heat treatment temperature is preferably 300°C or higher, more preferably 400°C or higher. The heat treatment time is preferably 0.1 to 8 hours, more preferably 0.5 to 5 hours. The temperature and time at which the metal element diffuses into at least the surface region of the lithium titanate powder substrate should be appropriately set, as the reactivity varies depending on the compound containing the metal element. The heating method for the heat treatment is not particularly limited. Examples of heat treatment furnaces that can be used include fixed-bed firing furnaces, roller hearth firing furnaces, mesh belt firing furnaces, fluidized-bed firing furnaces, and rotary kiln firing furnaces. The atmosphere during the heat treatment may be either an air atmosphere or an inert atmosphere such as a nitrogen atmosphere. In particular, when a metal salt compound is used for the surface treatment, an air atmosphere is preferred because anion species are easily removed from the particle surface.

[0044] The lithium titanate powder obtained after the heat treatment as described above may have slight agglomerations, but does not need to be pulverized to destroy the particles. Therefore, after the heat treatment, it is sufficient to perform crushing or classification to the extent that agglomerations are broken down, if necessary.

[0045] The lithium titanate powder of the present invention may be mixed with a treatment agent in a surface treatment step, granulated, and then heat-treated to form a powder containing secondary particles formed by aggregation of primary particles. Any method for granulation may be used as long as it can produce secondary particles, but a spray dryer is preferred because it can process large quantities.

[0046] <Moisture content> The moisture content (25°C to 350°C) of the lithium titanate powder of the present invention measured by the Karl Fischer method (hereinafter sometimes referred to as the moisture content at 25°C to 350°C) is preferably 5000 ppm or less. Here, the moisture content (25°C to 350°C) of the lithium titanate powder of the present invention measured by the Karl Fischer method refers to the total moisture content obtained by heating the lithium titanate powder of the present invention from 25°C to 200°C under nitrogen flow and maintaining it at 200°C for one hour, followed by heating it from 200°C to 350°C under nitrogen flow and maintaining it at 350°C for one hour. A moisture content of 5000 ppm or less is preferable because it provides good handleability during electrode coating when used as an electrode material for an electrical storage device. The moisture content measurement method will be described later in the section <Moisture Content Measurement by Karl Fischer Method>. The moisture content measured by the Karl Fischer method (25°C to 350°C) includes both moisture physically adsorbed and moisture chemically adsorbed on the lithium titanate powder of the present invention. Generally, in lithium titanate powder, measurement by the Karl Fischer method is difficult in the temperature range above 350°C, and moisture is hardly detected by other methods (e.g., pyrolysis gas chromatography-mass spectrometry). From the viewpoint of further suppressing the amount of gas generated during high-temperature operation of the power storage device, the moisture content measured by the Karl Fischer method (25°C to 350°C) is preferably 1000 ppm or less, and more preferably 600 ppm or less.

[0047] Here, the moisture content (200°C to 350°C) of the lithium titanate powder of the present invention measured by the Karl Fischer method refers to the amount of moisture obtained from the start of heating at 200°C to the end of holding at 350°C, out of the moisture content (25°C to 350°C). The moisture contained in lithium titanate includes physically adsorbed moisture and chemically adsorbed moisture, as described above. It is presumed that most of the moisture present on the surface of the lithium titanate is likely desorbed by 200°C and included in the moisture content (25°C to 200°C) measured by the Karl Fischer method. Here, the moisture content (25°C to 200°C) of the lithium titanate powder of the present invention measured by the Karl Fischer method refers to the amount of moisture released from the lithium titanate powder of the present invention when the lithium titanate powder of the present invention is heated from 25°C to 200°C under a nitrogen flow and held at 200°C for 1 hour, measured by the Karl Fischer method. Furthermore, since the fabrication of a typical electricity storage device involves a step of drying the electrodes, the moisture content (25°C to 200°C) measured by the Karl Fischer method is almost entirely released during this drying step. Therefore, it is believed that the moisture that affects the electricity storage device is not present on the surface of the lithium titanate particles, but is present mainly inside the particles, where it is difficult to remove during this drying step. Therefore, it is believed that the moisture present inside the particles and that substantially affects the electricity storage device is mostly included in the moisture content (200°C to 350°C) measured by the Karl Fischer method. From the above perspective, the moisture content (200°C to 350°C) measured by the Karl Fischer method is more preferably 300 ppm or less, and particularly preferably 150 ppm or less. The lower limit of the moisture content (200°C to 350°C) measured by the Karl Fischer method is not particularly limited, and in some cases, it may be below the detection limit of the measuring device (i.e., it can be determined to be essentially 0 ppm).

[0048] <Absolute value of zeta potential and pH by electrophoresis> The absolute value of the zeta potential at the starting point of the lithium titanate powder of the present invention is preferably 15 mV or more, more preferably more than 20 mV. The upper limit of the zeta potential is not particularly limited, but is preferably 60 mV or less, more preferably 50 mV or less. The zeta potential represents the potential difference between the sliding surface in the electric double layer and a portion sufficiently distant from the interface. This potential difference is the potential difference between the Li on the surface of the lithium titanate powder. + It is presumed that this affects the permeability. The pH of the lithium titanate powder of the present invention is preferably 11 or less, and more preferably 8 or less. The lower limit is not particularly limited, but is preferably 6 or more.

[0049] <Conductivity at 25°C when compressed to a relative density of 60%> The lithium titanate powder of the present invention has a conductivity of 1.0×10 at 25° C. when compressed to a relative density of 60% (i.e., compressed to a density of 60% of the true density). -6 S / cm or less, preferably 9 × 10 -7 S / cm or less, preferably 5×10 -7 The lower limit of the conductivity is not particularly limited, but is preferably 1×10 -9 S / cm or more. By setting the conductivity in this range, the charge rate characteristics can be improved. If the conductivity is too low, it will affect the electron acceptance of the lithium titanate powder during charge and discharge, and will also have a negative effect on the battery characteristics.

[0050] [Active material] The active material of the present invention contains the lithium titanate powder of the present invention. It may contain one or more substances other than the lithium titanate powder of the present invention. Examples of other substances include carbon materials (pyrolytic carbons, cokes, graphites (artificial graphite, natural graphite, etc.), organic polymer compound combustion bodies, carbon fibers), tin and tin compounds, silicon and silicon compounds, and lithium-containing metal oxides.

[0051] [Energy storage devices] The electricity storage device of the present invention is a device that includes an electrode containing the active substance material of the present invention and stores and releases energy by utilizing the intercalation and deintercalation of lithium ions into such an electrode, and examples thereof include a hybrid capacitor and a lithium battery.

[0052] [Lithium battery] The lithium battery of the present invention is a general term for lithium primary batteries and lithium secondary batteries. In this specification, the term lithium secondary battery is used to conceptually include so-called lithium ion secondary batteries and all-solid-state lithium ion secondary batteries.

[0053] The lithium battery is composed of a positive electrode, a negative electrode, and a non-aqueous electrolyte solution in which an electrolyte salt is dissolved in a non-aqueous solvent, or a solid electrolyte, and the active material of the present invention can be used as an electrode material. The active material of the present invention is usually used in the form of an electrode sheet for the lithium battery. This active material may be used as either a positive electrode active material or a negative electrode active material, but the following description will be given of its use as a negative electrode active material.

[0054] <Negative electrode> The negative electrode has a negative electrode layer containing a negative electrode active material (the active material of the present invention), a conductive agent, and a binder on one or both sides of a negative electrode current collector. This negative electrode layer is usually in the form of an electrode. In the case of a negative electrode current collector having pores such as a porous body, the negative electrode layer contains the negative electrode active material (the active material of the present invention), a conductive agent, and a binder in the pores.

[0055] The conductive agent for the negative electrode is not particularly limited as long as it is an electron-conductive material that does not undergo chemical changes. Examples include graphites such as natural graphite (e.g., flake graphite) and artificial graphite, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black, and carbon nanotubes such as single-phase carbon nanotubes, multi-walled carbon nanotubes (graphite layers in a multi-layered concentric cylindrical shape) (non-fishbone-shaped), cup-stacked carbon nanotubes (fishbone-shaped), nodular carbon nanofibers (non-fishbone structure), and platelet-type carbon nanofibers (playing card-shaped). Graphites, carbon blacks, and carbon nanotubes may also be used in combination.

[0056] The amount of conductive agent added varies depending on the specific surface area of ​​the active material and the type and combination of conductive agents, and therefore should be optimized. However, it is preferably 0.1% to 10% by mass, and more preferably 0.5% to 5% by mass, in the negative electrode layer. If it is less than 0.1% by mass, the conductivity of the negative electrode layer cannot be ensured. If it exceeds 10% by mass, the active material ratio decreases, resulting in insufficient discharge capacity of the power storage device per unit mass and unit volume of the negative electrode layer, making it unsuitable for achieving high capacity. The conductive agent may be added during electrode preparation, or it may be coated on the active material itself. This is because coating with a conductive agent such as carbon fiber can further improve the conductivity of the negative electrode layer.

[0057] Examples of binders for the negative electrode include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR), and carboxymethyl cellulose (CMC). While not particularly limited, the molecular weight of polyvinylidene fluoride is preferably 20,000 to 1,000,000. From the viewpoint of ensuring the binding of the negative electrode layer, it is preferably 25,000 or more, more preferably 30,000 or more, and even more preferably 50,000 or more. From the viewpoint of ensuring conductivity without interfering with the contact between the active material and the conductive agent, it is preferably 500,000 or less. In particular, when the specific surface area of ​​the active material is 10 m 2 / g or more, the molecular weight is preferably 100,000 or more.

[0058] The amount of binder added varies depending on the specific surface area of ​​the active material and the type and combination of conductive agents, and therefore should be optimized, but is preferably 0.2% by mass to 15% by mass in the negative electrode layer. From the viewpoint of enhancing binding properties and ensuring the strength of the negative electrode layer, the amount is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more. From the viewpoint of preventing a decrease in the active material ratio and a decrease in the discharge capacity of the electricity storage device per unit mass and unit volume of the negative electrode layer, the amount is preferably 10% by mass or less, and more preferably 5% by mass or less.

[0059] <Positive electrode> The positive electrode has a positive electrode layer containing a positive electrode active material, a conductive agent, and a binder on one or both surfaces of a positive electrode current collector.

[0060] As the positive electrode active material, a material capable of absorbing and releasing lithium is used, and examples of the active material include composite metal oxides containing lithium and containing cobalt, manganese, or nickel, and lithium-containing olivine-type phosphates. These positive electrode active materials can be used alone or in combination of two or more. Examples of such composite metal oxides include LiCoO2, LiMn2O4, LiNiO2, and LiCo 1-x Ni xO2(0.01 <x<1)、LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, LiNi 1 / 2 Mn 3 / 2 O4, etc., and these lithium composite oxides may be partially substituted with other elements, such as partially substituting cobalt, manganese, and nickel with at least one element such as B, Nb, Sn, Mg, Fe, Ti, Al, Zr, Cr, V, Ga, Zn, Cu, Bi, Mo, and La, partially substituting O with S or F, or coating with a compound containing these other elements. Examples of lithium-containing olivine-type phosphates include LiFePO4, LiCoPO4, LiNiPO4, LiMnPO4, and LiFe 1-x Examples include MxPO4 (M is at least one selected from Co, Ni, Mn, Cu, Zn, and Cd, and x is 0≦x≦0.5).

[0061] The conductive agent and binder for the positive electrode may be the same as those for the negative electrode. Examples of the positive electrode current collector include aluminum, stainless steel, nickel, titanium, baked carbon, and aluminum or stainless steel surfaces treated with carbon, nickel, titanium, or silver. The surface of these materials may be oxidized, or the positive electrode current collector surface may be roughened by surface treatment. Examples of the current collector form include sheets, nets, foils, films, punched materials, laths, porous materials, foams, fiber groups, and nonwoven fabric molded bodies.

[0062] <Nonaqueous electrolyte> The non-aqueous electrolyte is a non-aqueous solvent in which an electrolyte salt is dissolved. There are no particular limitations on the non-aqueous electrolyte, and various types can be used.

[0063] The electrolyte salt used is one that dissolves in a non-aqueous electrolyte. Examples of the electrolyte salt include inorganic lithium salts such as LiPF, LiBF, LiPO, LiN(SOF), and LiClO; lithium salts containing a chain-like fluorinated alkyl group such as LiN(SOCF), LiN(SOCF) , LiCFSO, LiC(SOCF), LiPF(CF), LiPF(CF) , LiPF(CF) , LiPF(CF) , LiPF(iso-CF) , and LiPF(iso-CF); lithium salts containing a cyclic fluorinated alkylene chain such as (CF)(SO)NLi and (CF)(SO)NLi; and lithium salts having an oxalate complex as the anion, such as lithium bis[oxalate-O,O']borate and lithium difluoro[oxalate-O,O']borate. Among these, particularly preferred electrolyte salts are LiPF6, LiBF4, LiPO2F2, and LiN(SO2F)2, with LiPF6 being the most preferred. These electrolyte salts can be used alone or in combination of two or more. A preferred combination of these electrolyte salts is one in which the non-aqueous electrolyte solution contains LiPF6 and at least one lithium salt selected from LiBF4, LiPO2F2, and LiN(SO2F)2.

[0064] On the other hand, examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, chain esters, ethers, amides, phosphate esters, sulfones, lactones, nitriles, and S=O bond-containing compounds, and the non-aqueous solvent preferably contains a cyclic carbonate. Note that the term "chain ester" is used as a concept including chain carbonates and chain carboxylic acid esters.

[0065] Examples of the cyclic carbonate include one or more selected from ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 4-fluoro-1,3-dioxolan-2-one (FEC), trans- or cis-4,5-difluoro-1,3-dioxolan-2-one (hereinafter, both are collectively referred to as "DFEC"), vinylene carbonate (VC), vinylethylene carbonate (VEC), and 4-ethynyl-1,3-dioxolan-2-one (EEC). One or more selected from ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 4-fluoro-1,3-dioxolan-2-one, and 4-ethynyl-1,3-dioxolan-2-one (EEC) are more preferred from the viewpoint of improving the charge rate characteristics of the power storage device and suppressing the amount of gas generated during high-temperature operation, and one or more cyclic carbonates having an alkylene chain selected from propylene carbonate, 1,2-butylene carbonate, and 2,3-butylene carbonate are even more preferred. The proportion of the cyclic carbonate having an alkylene chain in all cyclic carbonates is preferably 55% to 100% by volume, and more preferably 60% to 90% by volume.

[0066] Suitable examples of the chain ester include one or more asymmetric chain carbonates selected from methyl ethyl carbonate (MEC), methyl propyl carbonate (MPC), methyl isopropyl carbonate (MIPC), methyl butyl carbonate, and ethyl propyl carbonate; one or more symmetric chain carbonates selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, and dibutyl carbonate; pivalic acid esters such as methyl pivalate, ethyl pivalate, and propyl pivalate; and one or more chain carboxylic acid esters selected from methyl propionate, ethyl propionate, propyl propionate, methyl acetate, and ethyl acetate (EA).

[0067] <Lithium battery structure> The structure of the lithium battery of the present invention is not particularly limited, and examples include a coin battery having a positive electrode, a negative electrode, and a single-layer or multi-layer separator, and further, a cylindrical battery or a prismatic battery having a positive electrode, a negative electrode, and a roll-shaped separator.

[0068] The separator is an insulating thin film having high ion permeability and a predetermined mechanical strength. Examples include polyethylene, polypropylene, cellulose paper, glass fiber paper, polyethylene terephthalate, and polyimide microporous membranes. Multilayer membranes formed by combining two or more of these materials can also be used. The surface of these separators can also be coated with resins such as PVDF, silicone resins, and rubber-based resins, or particles of metal oxides such as aluminum oxide, silicon dioxide, and magnesium oxide. The pore size of the separator may be within a range generally useful for batteries, for example, 0.01 μm to 10 μm. The thickness of the separator may be within a range generally used for batteries, for example, 5 μm to 300 μm.

[0069] <Solid electrolyte> A solid electrolyte is a solid electrolyte capable of ion migration. In particular, inorganic solid electrolytes are solid in a steady state and are not typically dissociated or liberated into cations and anions. Inorganic solid electrolytes are not particularly limited as long as they have the conductivity of metal ions belonging to Group 1 of the periodic table, but generally have almost no electronic conductivity. Representative examples of inorganic solid electrolytes include (A) sulfide inorganic solid electrolytes and (B) oxide inorganic solid electrolytes. Sulfide solid electrolytes are particularly preferred because they have high ionic conductivity and can be formed into dense compacts with few grain boundaries simply by applying pressure at room temperature. The periodic table in this specification refers to the long-period periodic table of elements established by IUPAC (International Union of Pure and Applied Chemistry).

[0070] The sulfide inorganic solid electrolyte may be amorphous glass, crystallized glass, or a crystalline material. Specific examples of the sulfide inorganic solid electrolyte include, but are not limited to, the following combinations: Li2S-P2S5, Li2S-P2S5-Al2S3, Li2S-GeS2, Li2S-Ga2S3, Li2S-GeS2-Ga2S3, Li2S-GeS2-P2S5, Li2S -GeS2-Sb2S5, Li2S-GeS2-Al2S3, Li2S-SiS2, Li2S-Al2S3, Li2S-SiS2-Al2S3, Li2S-SiS2-P2S5, Li 10 GeP2S 12 .

[0071] Among these combinations, LPS glass and LPS glass ceramics made by combining Li2S-P2S5 are preferred.Other sulfide inorganic solid electrolytes that are suitable include argerodite-type solid electrolytes such as Li6PS5Cl and Li6PS5Br.

[0072] Oxide-based inorganic solid electrolytes contain oxygen atoms and are made of metals belonging to Group 1 of the periodic table. Preferably, the material has both ionic conductivity and electronic insulation.

[0073] Examples of oxide inorganic solid electrolytes include Lithium super ionic conductor (LISICON) type crystal structures. 3.5 Zn 0.25 GeO4, La with perovskite crystal structure 0.55 Li 0.35 TiO3, LiTi2P3O with NASICON (sodium super ionic conductor) type crystal structure 12 , Li7La3Zr2O with a garnet-type crystal structure 12 (LLZ), lithium phosphate (Li3PO4), lithium phosphate in which some of the oxygen in the lithium phosphate is replaced with nitrogen, LiPON, Li3BO3-Li2SO4, Li2O-B2O3-P2O5, Li2O-SiO2, and Li6BaLa2Ta2O12 Suitable examples include:

[0074] The volume-average particle size of the inorganic solid electrolyte is not particularly limited, but is preferably 0.01 μm or more, more preferably 0.1 μm or more. The upper limit is preferably 100 μm or less, more preferably 50 μm or less. These may be contained in the electrode layer of the positive electrode or negative electrode, and further, the conductive agent and binder exemplified as the conductive agent and binder used to form the negative electrode may be appropriately contained in the electrode layer. [Example]

[0075] Next, the present invention will be explained in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples and includes various combinations that can be easily inferred from the gist of the invention.

[0076] [Example 1-1] <Raw material preparation process> For the raw powder, Li2CO3 (average particle size 4.6 μm) and anatase TiO2 (average particle size 5.3 μm) were weighed and mixed so that the atomic ratio of Li to Ti (Li / Ti) was 0.83. This mixed powder was heat-treated at 960°C for 5 hours. Powder X-ray diffraction measurements were performed on the resulting fired powder sample. Crystal structure analysis using the Rietveld method confirmed that lithium titanate (ICDD (PDF2010) PDF card 00-049-0207) had been synthesized.

[0077] <Surface treatment process> Ion-exchanged water was added to the crushed fired powder and stirred to a solids concentration of 40% by mass. 0.22% by mass of aluminum sulfate hexahydrate (Al2(SO4)3·16H2O) was added to 100 g of crushed fired powder as a treatment agent (equivalent to 0.02% by mass of Al content in lithium titanate powder) to prepare a mixed slurry. This mixed slurry was mixed for 3 hours using a paint shaker, then dried and heat-treated at 500°C in an air atmosphere to produce lithium titanate powder (hereinafter referred to as LTO) according to Example 1-1. The D of LTO in this example 50 was calculated from the particle size distribution curve measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT3300EXII, manufactured by Nikkiso Co., Ltd.), and was found to be 5.2 μm. In Example 1-1, Al was introduced as the metal element M1 through the surface treatment process, and therefore Al as the metal element M1 was localized near the surface of the primary particles of lithium titanate (the same applies to Examples 1-2, 1-3, 2-1 to 2-4, and 3-1 described below).

[0078] [Example 1-2] The LTO of Example 1-2 was produced in the same manner as in Example 1-1, except that in the surface treatment step, 1.6 mass% of aluminum sulfate hexahydrate (Al2(SO4)3·16H2O) was added as a treatment agent (equivalent to an Al content of 0.16 mass% in the LTO powder). 50 was 5.3 μm.

[0079] [Examples 1-3] The LTO of Example 1-3 was produced in the same manner as in Example 1-1, except that in the surface treatment process, 3.2 mass% of aluminum sulfate hexahydrate (Al2(SO4)3·16H2O) (equivalent to 0.32 mass% Al content of the LTO powder) was added as a treatment agent, and the LTO was dried and heat-treated at 550°C in an air atmosphere. 50 was 5.3 μm.

[0080] [Comparative Example 1-1] Except for not performing the surface treatment step, LTO according to Comparative Example 1-1 was produced in the same manner as in Example 1-1. The D of the LTO 50 was 5.7 μm.

[0081] [Comparative Example 1-2] Except for using anatase-type TiO2 with an average particle size of 0.5 μm as the raw material powder, LTO according to Comparative Example 1-2 was produced in the same manner as in Example 1-2. The D of the LTO 50 was 0.7 μm.

[0082] [Comparative Example 1-3] Except for not performing the surface treatment step, LTO according to Comparative Example 1-3 was produced in the same manner as in Comparative Example 1-2. The D of the LTO 50 was 0.7 μm.

[0083] [Measurement of Powder Physical Properties] The various physical properties of the LTO powders of each example and comparative example were measured as follows.

[0084] [Measurement of BET Specific Surface Area (SSA)] The BET specific surface area (m 2 / g) of the LTO powders of each example and comparative example was measured using a fully automatic BET specific surface area measuring device (manufactured by Mountech Co., Ltd., product name "Macsorb HM model-1208"), and nitrogen gas was used as the adsorption gas. 0.5 g of the measurement sample powder was weighed, placed in a φ12 standard cell (HM1201-031), degassed under vacuum at 100 °C for 0.5 hours, and then measured by the BET single-point method. The results are shown in Table 1.

[0085] [D BET > The D of the LTO powders of each example and comparative example BET was determined from the following formula assuming that all the particles constituting the powder were spheres of the same diameter. The results are shown in Table 1. D BET = 6 / (ρ S ×S) Here, ρ S is the true density of lithium titanate (g / cc), and S is the BET specific surface area (m 2 / g).

[0086] <Measurement of Conductivity> Weighed 1 g of the LTO powder of each example and comparative example, packed it into a metal cell with a diameter of 1 cm, then used a uniaxial press to form the powder into a shape. When the relative density reached 60% (2.07 g / cm 3 ), the conductivity of the LTO powder was measured by the two-probe method using a multimeter (manufactured by Toyo Technica Co., Ltd., product name "KEITHLEY2000 Digital Multimeter"). The results are shown in Table 1.

[0087] <XRD Measurement> As the measuring device, an X-ray diffractometer (manufactured by Rigaku Corporation, model RINT-TTR-III) using CuKα rays was used. The measurement conditions for X-ray diffraction measurement were as follows: measurement angle range (2θ): 10° to 90°, step interval: 0.02°, measurement time: 0.25 seconds / step, radiation source: CuKα rays, tube voltage: 50 kV, current: 300 mA. As a result of the measurement, it was confirmed that it was lithium titanate (PDF card 00-049-0207 of ICDD (PDF2010)).

[0088] [Evaluation of Battery Characteristics] Coin-type batteries and laminate-type batteries were fabricated using the LTO of each example and comparative example, and their battery characteristics were evaluated. The evaluation results are shown in Table 1.

[0089] <Fabrication of Negative Electrode Sheet> The negative electrode sheets were prepared as follows in a room controlled at a room temperature of 25°C and a dew point of -20°C or less. A coating material was prepared by mixing 90% by mass of LTO from each example and comparative example as the active material, 5% by mass of acetylene black as a conductive agent, and 5% by mass of polyvinylidene fluoride as a binder as follows: Polyvinylidene fluoride, acetylene black, and 1-methyl-2-pyrrolidone, which had been previously dissolved in 1-methyl-2-pyrrolidone, were mixed using a planetary mixer / deaerator. LTO was then added, and the total solids concentration was adjusted to 64% by mass. The mixture was then mixed using a planetary mixer / deaerator. 1-methyl-2-pyrrolidone was then added, and the total solids concentration was adjusted to 50% by mass. The resulting coating material was applied to aluminum foil and dried to prepare a single-sided negative electrode sheet for use in the coin battery described below, and a double-sided negative electrode sheet for use in the laminate battery described below. The target coating weight is 7.5 mg / cm 2 It was decided.

[0090] <Measurement of electrode density> The negative electrode single-sided sheet coated in the above manner was pressed with a roll press (roll φ60 × 150 mm, press pressure equivalent to 40 MPa), and the density of the active material layer was measured as "electrode density." The evaluation results are shown in Table 1. A high electrode density is desirable because it allows more active material to be packed into a given volume, increasing the capacity that can be used as a battery. The results are shown in Table 1.

[0091] <Preparation of positive electrode sheet> A single-sided positive electrode sheet was produced in the same manner as described above in <Production of Negative Electrode Sheet>, including the ratios of the active material, conductive agent, and binder, except that lithium nickel cobalt manganese oxide powder was used as the active material.

[0092] <Preparation of electrolyte> The electrolyte used in the battery for characteristic evaluation was prepared as follows. In an argon glove box controlled at a temperature of 25°C and a dew point of -70°C or less, a nonaqueous solvent of ethylene carbonate (EC):dimethyl carbonate (DMC) = 1:2 (volume ratio) was prepared, and LiPF6 was dissolved in this as an electrolyte salt to a concentration of 1 M to prepare the electrolyte for the coin battery described below. Similarly, a nonaqueous solvent of propylene carbonate (PC):diethyl carbonate (DEC) = 1:2 (volume ratio) was prepared, and LiPF6 was dissolved in this as an electrolyte salt to a concentration of 1.3 M to prepare the electrolyte for the laminate battery described below.

[0093] <Creating a coin battery> The negative electrode single-sided sheet prepared by the above method was punched into a circle with a diameter of 14 mm and cut into 2 t / cm 2 An electrode for evaluation was prepared by pressing the electrode at a pressure of 1000 kJ / cm2 and then vacuum drying at 120°C for 5 hours. The electrode for evaluation and metallic lithium (formed into a circle with a thickness of 0.5 mm and a diameter of 16 mm) were placed opposite each other with glass filters (one each of ADVANTEC GA-100 and Whatman GF / C) interposed between them, and the nonaqueous electrolyte prepared by the method described above in <Preparation of electrolyte> was added and sealed to prepare a 2032-type coin battery.

[0094] <Initial battery characteristics 1 (coin): initial efficiency, 0.2C discharge capacity, volumetric energy density> The coin battery fabricated by the method described in the above <Coin Battery Fabrication> was placed in a thermostatic chamber at 25°C and charged at 0.2 mA / cm in the direction in which Li was absorbed into the evaluation electrode. 2 The battery was charged to 1 V at a current density of 0.05 mA / cm at 1 V. 2After constant-current, constant-voltage charging, charging was performed until a current density of 0.2 C was reached, followed by three cycles of constant-current discharging, discharging to 2 V at a current density equivalent to 0.2 C. The initial efficiency (%) was calculated by dividing the discharge capacity (mAh) at the first cycle by the charge capacity (mAh). The discharge capacity at the third cycle was divided by the weight of the LTO to calculate the 0.2 C initial discharge capacity (mAh / g). The volumetric energy density was calculated by multiplying this initial discharge capacity by the electrode density calculated above. A higher volumetric energy density increases the available capacity per given volume of the battery, which is desirable for smaller batteries. The evaluation results are shown in Table 1.

[0095] <Initial battery characteristics 2 (coin): 10C discharge retention rate, 5C-CC charge retention rate> Next, the battery was discharged from a fully charged state to 2 V at a current equivalent to 10 C of the initial discharge capacity, and the 10 C discharge capacity was calculated. The 10 C discharge capacity was divided by the 0.2 C initial discharge capacity to calculate the 10 C discharge retention rate (%). The battery was also charged to 1 V in constant current (CC) mode at a current equivalent to 5 C of the initial discharge capacity, and the 5 C CC charge capacity was calculated. The 5 C CC charge capacity was divided by the 0.2 C initial discharge capacity to calculate the 5 C CC charge retention rate (%). The results are shown in Table 1. The high high-rate charge / discharge characteristics of LTO are expected to improve the rapid charge / discharge performance of energy storage devices when used as an electrode material. Note that the "C" in 1 C refers to the current value during charging / discharging. For example, 1 C refers to the current value at which the theoretical capacity can be fully discharged (or fully charged) in 1 / 1 hour, while 0.1 C refers to the current value at which the theoretical capacity can be fully discharged (or fully charged) in 1 / 0.1 hour. The evaluation results are shown in Table 1.

[0096] <Making a laminated battery> The laminated battery was fabricated in a room controlled at a room temperature of 25°C and a dew point of -40°C or less as follows. 2 After pressing at a pressure of 2 t / cm, a negative electrode with a lead wire connection was fabricated. 2After pressing at a pressure of 1000 kJ / cm², a positive electrode with a lead wire connection was fabricated. The fabricated negative and positive electrodes were vacuum dried at 150°C for 12 hours. After vacuum drying, the positive and negative electrodes were stacked facing each other with a separator (UPZ210, manufactured by Ube Industries) in between. Aluminum foil lead wires were connected to the positive and negative electrodes, respectively. The laminated battery electrolyte prepared as described in the "Preparation of Electrolyte" section above was added, and the laminate was vacuum sealed with an aluminum laminate to fabricate a laminated battery for evaluation. The battery capacity was 30 mAh, and the ratio of the negative and positive electrode capacities (negative electrode capacity / positive electrode capacity) was 1.2. Next, as an aging step, the fabricated laminate battery was charged to 2.75 V at a current of 0.2 C in a constant temperature bath at 60°C, and then subjected to constant current / constant voltage charging at 2.75 V until the charging current became 0.05 C, followed by two cycles of constant current discharge at a current of 0.2 C to 1.4 V. Thereafter, the volume of the laminate battery was measured by Archimedes' method, and this was defined as the initial volume of the laminate battery (hereinafter sometimes referred to as the initial volume).

[0097] <High-temperature battery characteristics (laminate): cycle capacity retention rate, 3C rate charge retention rate after cycling, cycle gas generation rate> Using a laminated battery fabricated by the method described above in <Fabrication of Laminated Battery>, a constant-current, constant-voltage charge was performed in a 55°C thermostat at 2.75 V at a current of 2 C, followed by charging at 2.75 V until the charging current reached 0.05 C. This was followed by constant-current discharge at a current of 2 C to 1.4 V. This cycle was repeated for 200 cycles. After the cycle test, the laminated battery was charged to 2.75 V in constant-current (CC) mode at a current equivalent to the initial discharge capacity of 3 C, and the 3C rate charge capacity was determined. The post-cycle 3C rate charge retention (%) was calculated by dividing the post-cycle 3C charge capacity by the initial 3C charge capacity. The volume of the laminated battery after the cycle test was measured using the Archimedes method to determine the amount of cycle gas generated. The relative ratio (%) was calculated relative to the amount of gas generated by the battery of Comparative Example 1-1, which was set to 100. The evaluation results are shown in Table 1. [Table 1]

[0098] <Evaluation results> The electrodes using LTO in Examples 1-1 to 1-3 showed a well-balanced improvement in volumetric energy density due to high electrode density, cycle characteristics, and charge rate characteristics after high-temperature cycling, and also resulted in suppression of gas generation. On the other hand, the LTO in Comparative Examples 1-1 to 1-3 showed low volumetric energy density due to low electrode density, or a decrease in charge rate characteristics and an increase in gas generation after high-temperature cycling, and did not lead to an improvement in battery characteristics. Note that although there was no difference in the conductivity of LTO between the Examples and Comparative Examples, the charge rate characteristics after high-temperature cycling improved. This is due to the Li on the surface of the LTO particles, rather than the commonly-mentioned improvement in conductivity due to doping with a different metal element. + It is presumed that this is due to the improvement in ion permeability. In Table 1, the notation "aE-b" for conductivity means "a × 10 b For example, the conductivity of Example 1-1 is "1.40 × 10 -7 (S / cm)".

[0099] [Examples 2-1 to 2-3, 2-5, Comparative Example 2-1] Lithium titanate powder was produced in the same manner as in Example 1-1, except that anatase-type TiO2 (average particle size 5.8 μm) was used as the raw material powder, the mixed powder was heat-treated at 960°C for 10 hours, and aluminum sulfate hexahydrate (Al2(SO4)3·16H2O) and lithium molybdate (Li2MoO4) were added as treatment agents in the surface treatment process so that the Al or Mo content was as shown in Table 2. 50 As shown in Table 2, the thicknesses were in the range of 8.1 to 8.8 μm. Using this LTO, evaluation was carried out using a coin battery in the same manner as in Example 1-1 above. The evaluation results are shown in Table 2. In Example 2-5, Al and Mo were introduced as the metal element M1 through the surface treatment process, and therefore Al and Mo as the metal element M1 were localized near the surface of the primary particles of lithium titanate.

[0100] [Example 2-4] Lithium titanate powder was produced in the same manner as in Example 1-3, except that anatase-type TiO2 (average particle size 5.8 μm) was used as the raw material powder, the mixed powder was heat-treated at 960 °C for 10 hours, and aluminum sulfate hexahydrate (Al2(SO4)3·16H2O) was used as the treatment agent at 1.6 mass%. 50 The thickness was 8.7 μm as shown in Table 2. This LTO was used to evaluate a coin battery, and the evaluation results are shown in Table 2.

[0101] [Table 2]

[0102] <Evaluation results> When the LTO of Examples 2-1 to 2-5 was used, the volumetric energy density was equivalent to that of the LTO of Comparative Example 2-1, which was not subjected to the surface treatment process, and high input / output characteristics were also exhibited despite the low electrical conductivity of the LTO. 50 It was found that the battery characteristics can be improved by performing a surface treatment process even when the Li2TiO3 intensity ratio is greater than 8 μm. Furthermore, when Example 2-3 and Example 2-4 are compared, it was found that the 5C-CC charge retention rate is further improved when the Li2TiO3 intensity ratio is greater than 0.2.

[0103] <Results of zeta potential and pH measurement by electrophoresis> Regarding the LTO powders of Example 2-3, Example 2-4, Comparative Example 1-1, and Comparative Example 1-2, the absolute value (mV) of the zeta potential and pH of the LTO powders were measured by electrophoresis using a zeta potential measuring device (manufactured by Malvern, device name "Zetasizer Nano ZS"). 0.02 g of the LTO powder was weighed and placed in 200 mL of ion-exchanged water, and the measurement results under an environment of 25°C are shown in Table 3.

[0104]

Table 3

[0105] Regarding the above LTO powders, it was confirmed that there were differences in the zeta potential. It was found that when the absolute value of the zeta potential was less than 50 mV, high input-output characteristics were shown in a well-balanced manner. Furthermore, when comparing Example 2-3 and Example 2-4, it was found that when the absolute value of the zeta potential was greater than 20 mV, the 5C-CC charge retention rate was further improved. Although it is beyond the scope of speculation, due to the difference in the surface coating state, the zeta potential reflecting the ion diffusion layer was displaced, resulting in an impact on the Li + mobility on the surface of the LTO particles during rapid charging.

[0106] <Measurement Results by X-ray Photoelectron Spectroscopy> Regarding the LTO powders of Example 2-3 and Example 2-4, the Al (aluminum) and O (oxygen) atom concentrations (atomic%) on the surface of the LTO powders and the Al-O bond energy were measured from the Al2p narrow spectrum using X-ray photoelectron spectroscopy (manufactured by ULVAC-PHI, device name "PHI5000"). The X-ray source was Al Kα, 50 W, and the measurement results under the conditions of an analysis area of 1.0×0.2 mm 2 are shown in Table 4.

[0107]

Table 4

[0108] For the above LTO powder, we confirmed that there were differences in the Al and O atom concentrations and bonding states on the LTO surface. Comparing Example 2-3 and Example 2-4, we found that the 5C-CC charge retention rate was further improved when the Al atom / O atom concentration ratio was 15% or higher. Although this is only speculation, we suspect that the difference in the Al-O bonding state on the surface affected the Li+ diffusivity on the LTO particle surface during fast charging.

[0109] [Example 3-1, Comparative Example 3-1] Lithium titanate powder was produced in the same manner as in Example 1-1, except that anatase TiO2 (average particle size 1.5 μm) was used as the raw material powder, and aluminum sulfate hexahydrate (Al2(SO4)3·16H2O) was added as a treatment agent in the surface treatment process in an amount to achieve the Al content shown in Table 5. 50 The thickness was 1.7 μm as shown in Table 5. This LTO was used to evaluate coin batteries and laminate batteries, and the evaluation results are shown in Table 5.

[0110] [Table 5]

[0111] <Evaluation results> When the LTO of Example 3-1 was used, a high volumetric energy density was achieved due to the electrode density equivalent to that of the LTO of Comparative Example 3-1, which was not subjected to the surface treatment process, and high input / output characteristics were also achieved in a well-balanced manner despite the low electrical conductivity of the LTO. 50 On the other hand, it was found that even when the D of the LTO powder was as small as 1.7 μm, the battery characteristics could be improved by carrying out the surface treatment process. 50 If the value is too small, the electrode density does not increase, and the volumetric energy density tends to be low.

[0112] (All-solid-state secondary battery) [Example 4-1] In a glove box under an argon atmosphere, the LTO and sulfide solid electrolyte Li6PS5Cl powder (volume average particle size: 6 μm, measured using a laser diffraction / scattering particle size distribution analyzer) from Example 3-1 were weighed to a mass ratio of 60:40 and mixed in an agate mortar. Next, zirconia balls (3 mm diameter, 20 g) were placed in an 80 mL zirconia pot, and the mixed powder was added. The pot was then placed in a planetary ball mill and stirred at 200 rpm for 15 minutes to obtain the negative electrode active material composition of Example 4-1. The resulting negative electrode active material composition was pressed (360 MPa) at room temperature for 10 minutes to produce a pellet (molded body) with a diameter of 10 mm and a thickness of approximately 0.7 mm. A pellet-shaped electrode containing this negative electrode active material composition, a pellet-shaped solid electrolyte layer (LPS glass with a molar ratio of Li2S:P2S5 = 75:25) as a separator layer, and a lithium-indium alloy foil as a counter electrode were stacked in this order, and the stack was sandwiched between stainless steel current collectors to fabricate an all-solid-state secondary battery, and the battery characteristics were evaluated. As a result, a charge capacity of 120 mAh / g was confirmed in the initial stage, confirming that the composition works effectively as an active material for all-solid-state secondary batteries.

[0113] [Comparative Example 4-1] An all-solid-state secondary battery was fabricated in the same manner as in Example 4-1 above, except that the LTO was changed to that of Comparative Example 3-1, and the battery characteristics were evaluated. As a result, a charge capacity of only 95 mAh / g was obtained initially, resulting in a charge capacity inferior to that of Example 4-1. In other words, it was found that even in the all-solid-state secondary battery system, the electrode including the anode layer using LTO of Example 3-1 was excellent in charge capacity. [Industrial Applicability]

[0114] The lithium titanate powder obtained by the present invention is useful as an electrode active material for lithium ion secondary batteries because it can improve the charge rate characteristics and cycle characteristics at high temperatures while increasing the electrode density, which is directly related to the energy density. Furthermore, lithium ion secondary batteries using this lithium titanate powder as an electrode active material are capable of stable, high-speed charging and discharging, and are therefore useful as secondary batteries for driving or backing up various devices such as automobiles and electronic devices, and for storing power at night in homes and offices.

Claims

1. Li ４ Ti ５ O １２ a lithium titanate powder mainly composed of the above, wherein a Group 13 metal element excluding Ti is localized on the surface of a primary particle of the lithium titanate powder, or a Group 13 metal element excluding Ti and one or more of a Group 2, Group 12 or Group 14 metal element and molybdenum are localized on the surface of a primary particle of the lithium titanate powder, and the lithium titanate powder satisfies the following formulas (I) and (II): 15μm ≧ D ５０ ≧ 1.7μm (I) 5μm ≧ D ＢＥT ≧ 0.7μm (II) (In the above formula, D ５０ indicates the particle size at which the cumulative volume distribution of particle sizes of primary particles is 50% in the particle size distribution, and D ＢＥT indicates the diameter equivalent to the specific surface area calculated from the specific surface area determined by the BET method.)

2. 2. The lithium titanate powder according to claim 1, wherein the lithium titanate powder satisfies the following formula (III): D ５０ ≧ 5.0μm (III) (In the above formula, D ５０ indicates the particle size at which the cumulative volume distribution of particle sizes of primary particles is 50% in the particle size distribution.

3. 3. The lithium titanate powder according to claim 1, wherein the Group 13 metal element excluding Ti is at least one selected from the group of elements consisting of Al, Ga, and In, and the Group 2, Group 12, or Group 14 metal element or molybdenum element is at least one selected from the group of elements consisting of Mg, Ca, Sr, Zn, Ge, and Mo.

4. The lithium titanate powder is compressed to a relative density of 60%, and the electrical conductivity at 25°C is 1.0 × 10 －６ 4. The lithium titanate powder according to claim 3, wherein the specific surface area is less than 100 nm.

5. 5. The lithium titanate powder according to claim 3, wherein the absolute value of the zeta potential of the lithium titanate powder at 25°C measured by electrophoresis is greater than 20 mV and less than 50 mV.

6. The lithium titanate powder according to any one of claims 3 to 5, characterized in that, in a surface analysis using X-ray photoelectron spectroscopy, the ratio of the concentration of element M1 present on the surface of the lithium titanate powder particles to the concentration of O (oxygen) atoms (ratio of M1 atomic concentration / O atomic concentration (%)) is less than 15%.

7. An electrode comprising the lithium titanate powder according to any one of claims 1 to 6.

8. An electricity storage device comprising the electrode according to claim 7.

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