JP2023035188A - Lithium titanate powder, electrode using the same, and electric storage device - Google Patents

Abstract

To provide lithium titanate powder, which is used as an electrode material for an electric storage device and is excellent in maintaining electrode density, volumetric energy density, rate characteristics, initial low-temperature input characteristics, and charge rate characteristics after storage at a high temperature in a balanced manner, and provide an electrode using the same and an electric storage device.SOLUTION: Lithium titanate powder consists mainly of Li4Ti5O12, wherein aluminum (Al) is locally present on the surface of primary particles of the lithium titanate powder, the aluminum content of the lithium titanate powder is 0.2 mass%, and the following formula (I) is satisfied: log10(D90)-log10(D10)<0.8 (I) (D90 denotes a particle size at the point where the cumulative volume distribution of particle size is 90% in the particle size distribution, and D10 denotes a particle size at the point where the cumulative volume distribution of particle size is 10% in the particle size distribution).SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a lithium titanate powder suitable as an electrode material or the like for an electric storage device, an electrode using the same, and an electric storage device.

In recent years, various materials have been studied as electrode materials for electric storage devices. Among them, lithium titanate is attracting attention as an active material for electric storage devices for electric vehicles such as HEV, PHEV, and BEV because of its excellent input/output characteristics when used as an active material.

A power storage device for an electric vehicle is required to have a high energy density from the viewpoint of improving fuel efficiency or power consumption. In addition, stability over a wide temperature range from high to low temperatures is also important. For example, in cold regions in winter, it is required to guarantee operation performance at -20°C below freezing. Lithium titanate can be an optimal electrode material candidate for these applications because of its excellent initial input/output characteristics. On the other hand, since the temperature inside the vehicle reaches 60° C. or higher in summer, it is required that there be no safety problems and that the performance does not deteriorate even at high temperatures. Electricity storage devices containing lithium titanate have problems with storage stability such as gas generation when used at high temperatures of 60°C or higher for a long period of time. known to change. Therefore, it is desired to develop a lithium titanate that maintains excellent initial input/output characteristics while increasing the energy density of an electricity storage device, and that suppresses gas generation and resistance increase after long-term high-temperature operation.

In Patent Document 1, the specific surface area equivalent diameter DBET calculated from the specific surface area obtained by the BET method and the crystallite diameter _DX are specified within a certain range, and M (M is Mg, Zn, Al, Ga , or at least one metal element selected from In), and the element is localized near the surface of the lithium titanate particle. Lithium titanate powder is disclosed. According to Patent Document 1, when applied as an electrode material for an electricity storage device, titanium exhibits a large initial charge/discharge capacity, excellent input/output characteristics, and extremely high charge/discharge capacity at an extremely low temperature of -30°C. A lithium oxide powder is disclosed.

Patent Document 2 discloses a lithium titanate using anatase-type titanium dioxide having a purity of 95% by mass or more as a raw material, which contains Li ₄ Ti ₅ O ₁₂ as a main component and has an average particle size of 0.5 to 1.5%. 5 μm, the maximum particle size is 25 μm or less, and the SD value ((D84%−D16%)/2) indicated by the particle size distribution is 0.5 μm or less. is disclosed. Patent Literature 2 discloses an electricity storage device capable of increasing the discharge capacity during high-current discharge by controlling the particle size and particle size distribution.

Patent Document 3 discloses a lithium-titanium composite oxide containing primary particles coated with aluminum. According to Patent Document 3, it is disclosed that there is an effect of suppressing the decomposition of the electrolytic solution and the generation of gas due to residual lithium.

Japanese Patent No. 5790894 JP-A-2003-137547 Japanese Patent Publication No. 2021-516208

However, although the electric storage device in which lithium titanate is applied as a negative electrode material in Patent Document 1 exhibits excellent input/output characteristics in the initial stage, there is no description of electrode density, and knowledge regarding both initial input/output characteristics and volumetric energy density is not provided. is also not shown. The advantage of increasing the electrode density is that the battery itself can be made lighter and smaller, so the proportion of the battery in electronic devices and vehicles will decrease, and the degree of freedom in designing electronic devices will be greatly improved. In the case of applications, there is no need to secure battery space, and it leads to an improvement in energy density per unit volume, which leads to the extension of the driving distance of electric vehicles and the securing of installation space for storage batteries. etc. In addition, it was found that it is necessary to further suppress the amount of gas generated after high-temperature storage and to suppress the increase in resistance after the cycle test.

In the lithium titanate powder of Patent Document 2, in which the particle size and particle size distribution are specified within a certain range, there is no description of suppression of gas generation and resistance increase during high-temperature operation, and input/output characteristics at low temperature. It was found that there is a problem in coexistence with density.

Furthermore, although Patent Document 3 describes rate characteristics and the amount of gas generated after high-temperature storage, there is no description about resistance increase after high-temperature storage or battery performance at low temperatures, and knowledge about improving electrode density and coexistence. is not shown.

From the above points, in the electricity storage devices using the negative electrode active materials and electrodes of Patent Documents 1 to 3, the initial excellent input/output characteristics are maintained while improving the electrode density of the negative electrode, which is directly linked to the energy density. However, it is not possible to simultaneously suppress the generation of gas and the increase in resistance during high-temperature operation.

Therefore, in the present invention, it is used as an electrode material for an electricity storage device, and is excellent in maintaining well-balanced electrode density, volumetric energy density, rate characteristics, initial low temperature input characteristics, and charge rate characteristics after high temperature storage. A battery with an electrode density of 1.98 g/cm ³ or more, a volumetric energy density of 334 mAh/g or more, a 50C rate characteristic of 74% or more, an initial low temperature input characteristic of 46% or more, and a charge rate characteristic of 93% or more after high temperature storage. An object of the present invention is to provide a lithium titanate powder having properties, an electrode using the same, and an electric storage device.

As a result of various studies aimed at achieving the above-mentioned object, the present inventors have found that the firing temperature of the lithium titanate powder, the crushing process in the post-treatment step after firing, the amount of aluminum (Al) added in the surface treatment step, etc. By controlling, a lithium titanate powder having a specific particle size distribution and having an Al content of 0.2% by mass or more locally present on the surface of the primary particles was found. The electricity storage device in which the lithium titanate powder is applied as an electrode material is excellent in maintaining electrode density, volumetric energy density, rate characteristics, initial low temperature input characteristics, and charge rate characteristics after high temperature storage in a well-balanced manner. For example, the electrode density is 1.98 g/cm ³ or more, the volume energy density is 334 mAh/g or more, the 50C rate characteristic is 74% or more, the initial low temperature input characteristic is 46% or more, and the charge rate characteristic after high temperature storage is 93% or more. The present invention was completed by discovering that the battery characteristics were different. That is, the present invention relates to the following matters.

(1) A lithium titanate powder containing Li ₄ Ti ₅ O ₁₂ as a main component, wherein aluminum (Al) is locally present on the surface of the primary particles of the lithium titanate powder, and the aluminum of the lithium titanate powder A lithium titanate powder having a content of 0.2% by mass or more and satisfying the following formula (I).
_log10 ( _D90 ) - _log10 ( _D10 ) < 0.8 (I)
(D ₉₀ indicates the particle size at which the cumulative volume distribution of the particle size of the primary particles is 90% in the particle size distribution, and D ₁₀ indicates the cumulative volume distribution of the particle size of the primary particles in the particle size distribution of 10%. indicates the particle size at the point where

(2) The lithium titanate powder according to (1) above, which satisfies the following formula (II).
_log10 ( _D90 ) - _log10 ( _D10 ) < 0.7 (II)
(D ₉₀ indicates the particle size at which the cumulative volume distribution of the particle size of the primary particles is 90% in the particle size distribution, and D ₁₀ indicates the cumulative volume distribution of the particle size of the primary particles in the particle size distribution of 10%. indicates the particle size at the point where

(3) The lithium titanate powder according to (1) or (2) above, wherein the particle size _D50 of the primary particles corresponding to 50% of the cumulative volume in the particle size distribution is 0.5 μm or more.

(4) The lithium titanate powder according to (3) above, which contains primary particles and secondary particles of lithium titanate and satisfies the following formula (III).
D ₅₀ of primary particles/D ₅₀ of secondary particles ≤ 0.05 (III)

(5) The lithium titanate powder according to (4) above, wherein the lithium titanate powder has a specific surface area of 6 m ² /g or more.

(6) An electrode comprising the lithium titanate powder according to any one of (1) to (5).

(7) An electricity storage device comprising the electrode according to (6) above.

According to the present invention, the electrode density, volume energy density, rate characteristics, initial low temperature input characteristics, and charge rate characteristics after high temperature storage are excellently maintained in a well-balanced manner ^. As described above, as an electrode material for an electricity storage device having battery characteristics such as a volume energy density of 334 mAh/g or more, a 50C rate characteristic of 74% or more, an initial low temperature input characteristic of 46% or more, and a charge rate characteristic of 93% or more after high temperature storage. A suitable lithium titanate powder, an electrode using the same, and an electric storage device can be provided.

[Lithium titanate powder of the present invention]
The lithium titanate powder of the present invention is a lithium titanate powder containing Li ₄ Ti ₅ O ₁₂ as a main component, aluminum (Al) is locally present on the surface of the primary particles of the lithium titanate powder, and the A lithium titanate powder having an aluminum content of 0.2% by mass or more and satisfying the following formula (I).
_log10 ( _D90 ) - _log10 ( _D10 ) < 0.8 (I)
(In addition, _D90 indicates the particle size at the point where the cumulative volume distribution of the particle size is 90% in the particle size distribution, and _D10 is the particle size at the point where the cumulative volume distribution of the particle size is 10% in the particle size distribution. indicates.)

<Lithium titanate powder containing Li ₄ Ti ₅ O ₁₂ as a main component>
The lithium titanate powder of the present invention contains Li ₄ Ti ₅ O ₁₂ as a main component, and contains a crystalline component and/or an amorphous component other than Li ₄ Ti ₅ O ₁₂ to the extent that the effects of the present invention can be obtained. can be done. The term "main component" means that the main peak of Li ₄ Ti ₅ O ₁₂ accounts for 90% or more of the diffraction peaks measured by the X-ray diffraction method. In the lithium titanate powder of the present invention, the ratio of the intensity of the main peak of Li ₄ Ti ₅ O ₁₂ among the diffraction peaks measured by the X-ray diffraction method is more preferably 92% or more, more preferably 95% or more. It is even more preferable to have The component other than Li ₄ Ti ₅ O ₁₂ is the sum of the intensity of the main peak 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 Li ₂ TiO _{3 and} Li ₀ , which are lithium titanates having different chemical formulas, due to raw materials and synthesis conditions at the time of synthesis. _.6 Ti _3.4 O _{8 ,} etc. may be included as said crystalline component. In the lithium titanate powder of the present invention, the lower the proportion of crystalline components other than Li ₄ Ti ₅ O ₁₂ , particularly Li _0.6 Ti _3.4 O ₈ , the better the charging characteristics of the electricity storage device and the better the charging performance. Discharge capacity can be improved. Among the diffraction peaks measured by the X-ray diffraction method, when the intensity of the main peak of Li ₄ Ti ₅ O ₁₂ is 100, the intensity of the main peak of anatase-type titanium dioxide and the main peak intensity of rutile-type titanium dioxide and the intensity corresponding to the main peak of Li ₂ TiO ₃ calculated by multiplying the peak intensity corresponding to the (−133) plane of Li ₂ TiO ₃ by 100/80 is particularly preferably 5 or less. Here, the main peak of Li ₄ Ti ₅ O ₁₂ is diffraction attributed to the (111) plane (2θ=18.33) of Li ₄ Ti ₅ O ₁₂ in PDF card 00-049-0207 of ICDD (PDF2010). It is the peak corresponding to the peak. The main peak of anatase titanium dioxide is a peak corresponding to the diffraction peak attributed to the (101) plane (2θ=25.42) in PDF card 01-070-6826. The main peak of rutile-type titanium dioxide is a peak corresponding to the diffraction peak attributed to the (110) plane (2θ=27.44) in PDF card 01-070-7347. The peak corresponding to the (−133) plane of Li ₂ TiO ₃ corresponds to the diffraction peak attributed to the (−133) plane (2θ=43.58) of Li ₂ TiO ₃ in PDF card 00-033-0831. peak. The main peak of Li _0.6 Ti _3.4 O ₈ is a peak corresponding to the diffraction peak attributed to the (101) plane (2θ=19.98) in PDF card 01-070-2732. "ICDD" is an abbreviation for International Center for Diffraction Data, and "PDF" is an abbreviation for Powder Diffraction File.

<Containing aluminum (Al)>
The lithium titanate powder of the present invention contains Al on the surfaces of primary particles. The content of Al means that Al is detected in the inductively coupled plasma emission spectroscopy (ICP-AES) or X-ray fluorescence spectroscopy (XRF) of the niobium oxide powder of the present invention. The lower limit of the amount detected by inductively coupled plasma emission spectrometry is usually 0.001% by mass, and the lower limit of the amount detected by fluorescent X-ray analysis is usually 0.001% by mass.

<Content of aluminum (Al)>
The Al content of the lithium titanate powder of the present invention determined by X-ray fluorescence analysis (XRF) may be 0.2% by mass or more. It should be noted that the upper limit value is preferably 3% by mass or less because the larger the content, the lower the initial capacity. When the content is within this range, an electricity storage device having excellent electrode density, volumetric energy density, rate characteristics, initial low temperature input characteristics, and suppression of gas generation during high temperature storage can be obtained. It is more preferably 0.25% by mass or more and 2% by mass or less, still more preferably 0.27% by mass or more and 1% by mass or less, and particularly preferably 0.27% by mass or more and 0.4% by mass or less. In addition, content represents the ratio of the mass which Al contains with respect to the mass of the whole lithium titanate powder.

<Inclusion of further dissimilar elements>
The lithium titanate powder of the present invention may contain further dissimilar elements other than aluminum (Al), and in particular, may contain at least one element selected from the element group consisting of B, Mo and W. preferable. Among these, Mo is particularly preferred. It is speculated that the lithium titanate powder of the present invention contains such a dissimilar element together with aluminum (Al), so that the modification of the powder surface progresses more than when aluminum (Al) is contained alone. The amount generated is suppressed.

<Mass ratio of molybdenum (Mo) and aluminum (Al) (Mo (mass%)/Al (mass%))>
In the case of the above lithium titanate powder, it is preferable that the surfaces of the primary particles contain Mo and Al, and the respective mass ratio (Mo (mass %)/Al (mass %)) is 0.01 or more and 80 or less. If the mass ratio is within this range, an electricity storage device with excellent rate characteristics, initial low-temperature input characteristics, and suppression of gas generation after high-temperature storage can be obtained. It is more preferably 0.2 or more and 6 or less, still more preferably 0.3 or more and 4 or less, and particularly preferably 0.5 or more and 1.5 or less.

In addition, in the lithium titanate powder of the present invention, more Al is contained in the surface than in the interior of the primary particles of lithium titanate contained in the powder. In the cross-sectional analysis of the primary particles of lithium titanate using a scanning transmission electron microscope, the so-called surface from the surface of the primary particles of lithium titanate to a depth of about 20 nm measured by energy dispersive X-ray spectroscopy A large amount of Al should be contained in the neighboring region, and it is preferable that Al is not detected at a depth position of about 100 nm. Al is preferably fixed on the surface of the primary particles in a chemically bonded state. When Al is present in such a state, an electricity storage device having excellent electrode density, volumetric energy density, rate characteristics, initial low temperature input characteristics, and suppression of gas generation during high temperature storage can be obtained. The lower limit of the detectable amount in measurement by energy dispersive X-ray spectroscopy varies depending on the element to be measured and the state, but is usually 0.5 atm %. Therefore, at a depth position of about 100 nm, Al may be detected in a range of 0.5 atm % or less.

Although it is not clear as to why the electrode density, volume energy density, rate characteristics, initial low temperature input characteristics, and suppression of gas generation after high temperature storage are excellent, the lithium titanate particle surface and the non-aqueous electrolyte (solid electrolyte It is thought that the local presence of Al at the interface with the particles (including .

The lithium titanate powder of the present invention may contain at least one element selected from Mg, Zn, Ga, sulfur (S), or In as a further dissimilar element. This is because the lithium titanate powder of the present invention contains these dissimilar elements together with Al (or together with Mo and/or Al) to further increase the efficiency and rate characteristics in high-temperature charging and discharging. Among these, sulfur (S) is preferred. The lithium titanate powder of the present invention contains the element S together with Al (or together with Mo and/or Al) to adjust the electronic conductivity of the surface of the primary particles of the lithium titanate powder and suppress the electrical resistance. presumed to be for

<Valence of Molybdenum (Mo)>
Regarding the valence of molybdenum (Mo), by X-ray photoelectron spectroscopy (XPS) of the lithium titanate primary particles contained in the lithium titanate powder of the present invention, peaks derived from Mo (hexavalent, pentavalent and tetravalent) The ratio of the peak area of Mo (pentavalent and tetravalent) to the total area is preferably 30% or less, more preferably 20% or less. If the peak area ratio of Mo (pentavalent and tetravalent) is 30% or less, the side reaction of Mo is suppressed during the initial charge and discharge, and the initial discharge capacity, rate characteristics, and suppression of gas generation after high temperature storage. It is possible to obtain an electricity storage device excellent in As for Mo, the lithium titanate powder of the present invention preferably contains more Mo in the surface than in the interior of the primary particles of lithium titanate contained in the powder.

<1-methyl-2-pyrrolidone oil absorption>
The 1-methyl-2-pyrrolidone (hereinafter sometimes referred to as NMP) oil absorption of the lithium titanate powder of the present invention is desirably 80 ml/100 g or less. If the NMP oil absorption is within this range, it is possible to obtain an electricity storage device that is excellent in electrode density, volumetric energy density, rate characteristics, and suppression of gas generation during high-temperature storage. More preferably 75 ml/100 g or less, still more preferably 71 ml/100 g or less. Here, the unit of ml/100 g represents the absorption amount of NMP (unit: ml) per 100 g of the lithium titanate powder of the present invention. Although the reason why the above effect can be obtained is not necessarily clear, NMP is a solvent used for electrode preparation, and by appropriately controlling the NMP oil absorption amount of lithium titanate powder, negative electrode active material, conductive aid , contributes to efficient adhesion with the binder, and it is presumed that high-density electrode workability can be realized.

<Specific surface area>
The specific surface area of the lithium titanate powder of the present invention is the adsorption area per unit mass when nitrogen is used as the adsorption gas. A measuring method will be described in Examples described later.

The lithium titanate constituting the lithium titanate powder of the present invention may have a specific surface area of 4.0 m ² /g or more, thereby obtaining an electricity storage device excellent in initial discharge capacity and rate characteristics. is preferably 4.7 m ² /g or more, more preferably 6.0 m ² /g or more. On the other hand, the specific surface area of the lithium titanate powder is more preferably 9.0 m ² /g or less, more preferably 7.0 m ² /g or less.

_<D50>
The _D50 of the lithium titanate powder of the present invention is an index of the volume median particle size. It means a particle size at which the cumulative volume frequency calculated from the volume fraction obtained by laser diffraction/scattering particle size distribution measurement is integrated from the smaller particle size to 50%. A measuring method will be described in Examples described later.

<Primary particles and secondary particles>
The lithium titanate powder of the present invention includes primary particles of lithium titanate and secondary particles obtained by agglomeration of the primary particles. Primary particles represent individual particles that make up the lithium titanate powder and cannot be further divided by the crushing process.

_{The D50} of the secondary particles of the lithium titanate powder of the present invention is 11 μm or more, preferably 12 μm or more, from the viewpoint of improving rate characteristics and initial low-temperature input characteristics. 12.4 μm or more is more preferable. In addition, _D50 of the secondary particles is 20 μm or less, preferably 18 μm or less, more preferably 14 μm or less. In addition, _D50 of a secondary particle represents _D50 before crushing treatment (applying ultrasonic waves with an ultrasonic wave irradiator).

_{The D50} of the primary particles of the lithium titanate powder of the present invention is 0.5 μm or more, more preferably 0.6 μm or more, from the viewpoint of improving electrode density and volume energy density. . The _D50 of the primary particles is 3 μm or less, preferably 2 μm or less, more preferably 1.5 μm or less. The _D50 of the primary particles represents the _D50 after the crushing treatment by the ultrasonic irradiation device. In addition, the lithium titanate powder may contain a cumulative volume frequency of primary particles having a primary particle diameter of less than 0.5 μm in the range of 10% to 50%, and a cumulative volume frequency of primary particles having a primary particle diameter of less than 0.6 μm is 15%. % to 55%. Furthermore, the cumulative volume frequency of primary particles exceeding 3 μm may be in the range of 50% to 90%, and the cumulative volume frequency of primary particles exceeding 2 μm may be in the range of 50% to 90%.

The values calculated by log ₁₀ (D ₉₀ ) - log ₁₀ (D ₁₀ ) of the primary particles of the lithium titanate powder of the present invention are the electrode density, volume energy density, initial low temperature input characteristics, and after high temperature storage. From the viewpoint of improving charge rate characteristics, it is less than 0.8, preferably less than 0.7, more preferably less than 0.6, and even more preferably less than 0.5. Also, the lower limit value is preferably larger than 0.3, more preferably larger than 0.4. This is because when the value is small, the battery performance may deteriorate.

_D90 indicates the particle size at the point where the cumulative volume distribution of the particle size of the primary particles is 90% in the particle size distribution, and _D10 is the point at which the cumulative volume distribution of the particle size of the primary particles is 10% in the particle size distribution. More specifically, for the primary particles of lithium titanate powder, the cumulative volume frequency calculated by the volume fraction obtained by laser diffraction/scattering particle size distribution measurement is integrated from the smaller particle size. _D10 is the particle size that becomes 10% of the total, and _D90 is the particle size that becomes 90% of the total from the smaller particle size. Here, D ₉₀ is the D ₉₀ of the primary particles, and represents the D ₉₀ after performing the crushing treatment with the ultrasonic irradiation device, D ₁₀ is the D ₁₀ of the primary particles, ultrasonic irradiation It represents _D10 after the crushing treatment by the instrument.

Further, log ₁₀ (D ₉₀ ) is the log value at D ₉₀ of the primary particles, which is a log value with the base of 10, and its unit is μm. So, for example, when D ₉₀ =10 μm, the value of log ₁₀ (D ₉₀ ) is 1, and when D ₉₀ =1 μm, the value of log 10 (D ₉₀ ) is 0. Similarly, log ₁₀ (D ₁₀ ) is the log value at D ₁₀ of the primary particles, which is a base 10 log value, and its unit is μm.

_{The D10} of the primary particles of the lithium titanate powder described above is 0.3 μm or more, more preferably 0.4 μm or more. Also, the _D90 of the primary particles is 4 μm or less, more preferably 3 μm or less.

In view of the above description, the ratio of the _D50 of the primary particles to the secondary particles of the lithium titanate powder ( _D50 of the primary particles/ _D50 of the secondary particles) is preferably 0.06 or less, and is 0.05 or less. is more preferable. This is because if it exceeds 0.06, any of the electrode density, the initial low temperature input characteristics, and the volumetric energy density may deteriorate.

<Water content>
The water 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 water content at 25° C. to 350° C.) is preferably 5000 ppm or less. . Here, the water content (25° C. to 350° C.) of the lithium titanate powder measured by the Karl Fischer method of the present invention means that the lithium titanate powder of the present invention is heated from 25° C. to 200° C. under nitrogen flow. The amount of water obtained by heating to 200° C. for 1 hour, and the water content by heating from 200° C. to 350° C. under nitrogen flow until 350° C. for 1 hour. is the amount of water obtained by summing If it is 5000 ppm or less, it is preferable because the handleability at the time of electrode coating is good when it is applied as an electrode material for an electricity storage device. The water content (25° C. to 350° C.) measured by the Karl Fischer method includes both physically adsorbed water and chemically adsorbed water on the lithium titanate powder of the present invention. . Generally, in lithium titanate powder, it is difficult to measure by the Karl Fischer method in the region exceeding 350° C., and moisture is hardly detected by other methods (for example, pyrolysis gas chromatography mass spectrometry). From the viewpoint of further suppressing the amount of gas generated during high-temperature operation of the electricity storage device, the moisture content (25° C. to 350° C.) measured by the Karl Fischer method is more preferably 1000 ppm or less, particularly preferably 600 ppm or less.

Here, the moisture content (200° C. to 350° C.) of the lithium titanate powder measured by the Karl Fischer method of the present invention means the moisture content (25° C. to 350° C.) from the start of heating at 200° C. It is the amount of moisture obtained until the completion of holding at 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 it is probably desorbed and is included in the water 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 measured by the Karl Fischer method of the present invention means that the lithium titanate powder of the present invention is heated from 25° C. to 200° C. under nitrogen flow. When heated and held at 200°C for 1 hour, the amount of water released from the lithium titanate powder of the present invention from the start of heating to the completion of holding at 200°C is measured by the Karl Fischer method. That is. In addition, since there is a step of drying the electrodes in the production of a normal electricity storage device, the water content (25° C. to 200° C.) measured by the Karl Fischer method is almost released in such a drying step. . Therefore, it is considered that the moisture that affects the electric storage device is not the surface of the lithium titanate particles but the moisture present inside the particles that is difficult to remove in such a drying process. Therefore, it is considered that most of the moisture that exists inside the particles and that substantially affects the electricity storage device is included in the moisture content (200° C. to 350° C.) measured by the Karl Fischer method. From the above viewpoint, the water content (200° C. to 350° C.) measured by the Karl Fischer method is more preferably 300 ppm or less, particularly preferably 150 ppm or less. In addition, the lower limit of the water content (200 ° C. to 350 ° C.) measured by the Karl Fischer method is not particularly limited, and in some cases, when it is below the detection limit of the measuring device (substantially 0 ppm In some cases.

[Method for producing lithium titanate powder of the present invention]
An example of the method for producing the lithium titanate powder of the present invention will be described below by dividing it into a raw material preparation step, a firing step, and a surface treatment step. Not limited.

<Raw material preparation process>
The raw material of the lithium titanate powder of the present invention consists of a titanium raw material and a lithium raw material. Titanium compounds such as anatase-type titanium dioxide and rutile-type titanium dioxide are used as titanium raw materials.

Lithium compounds such as lithium hydroxide monohydrate, lithium oxide, lithium hydrogen carbonate, and lithium carbonate are used as lithium raw materials.

In the preparation step, a mixture is obtained by mixing the above raw materials. As a method for preparing the mixture, there is a method in which raw materials are blended and then pulverized at the same time as mixing.

<Baking process>
The resulting mixture is then fired. From the viewpoint of increasing the specific surface area of the powder obtained by firing, increasing the crystallite size and the primary particle size of the powder, and reducing the amount of impurities from the furnace material, firing at a high temperature for a short time is preferable. From this point of view, the maximum temperature during firing is preferably 1100° C. or lower, more preferably 1000° C. or lower, and even more preferably 960° C. or lower. From the viewpoint of reducing the proportion of impurities and increasing the crystallinity of lithium titanate, the maximum temperature during firing is preferably 800° C. or higher, more preferably 840° C. or higher, and still more preferably 860° C. or higher. . Similarly, from the above point of view, the retention time at the highest temperature during firing is preferably 2 minutes to 60 minutes, more preferably 5 minutes to 45 minutes, and even more preferably 5 minutes to 30 minutes. When the maximum temperature during firing is high, it is preferable to choose a shorter holding time. Similarly, from the viewpoint of increasing the crystallite size obtained by firing, it is preferable to shorten the residence time at 700° C. to 800° C. in the heating process during firing, preferably within 15 minutes.

The firing method is not particularly limited as long as it can be fired under the above conditions. Available firing methods include a fixed bed firing furnace, a roller hearth firing furnace, a mesh belt firing furnace, a fluidized bed firing furnace, and a rotary kiln firing furnace. However, for efficient firing in a short time, a roller hearth type firing furnace, a mesh belt type firing furnace, and a rotary kiln type firing furnace are preferable. When using a roller hearth type firing furnace in which the mixture is placed in a sagger and fired, or a mesh belt type firing furnace, the quality of the lithium titanate obtained is kept constant by ensuring the uniformity of the temperature distribution of the mixture during firing. In order to achieve this, it is preferable that the amount of the mixture contained in the sagger is small.

<Crushing process>
Lithium titanate powder after sintering needs to be pulverized. Methods for pulverizing the fired material recovered from the fired material recovery side include hammer mills, ball mills, jet mills, vibration mills, bead mills, and the like, and bead mills are particularly preferred. When a bead mill is used, any of wet crushing circulation processing, wet crushing batch processing, dry crushing circulation processing, and dry crushing batch processing shall be adopted. However, it is preferable to perform uniform crushing, and in that respect, wet crushing circulating treatment is preferable. The circulation conditions may be determined in consideration _of the _{sintering temperature} in the _sintering step. ) can be suitably controlled. For wet pulverization, the lithium titanate powder after calcining is put into water or an alcohol solvent and mixed in a slurry state. As the alcohol solvent, an alcohol solvent having a boiling point of 100° C. or less such as methanol, ethanol, or isopropyl alcohol is preferable because the solvent can be easily removed. Moreover, from the standpoint of ease of recovery and disposal, an aqueous solvent is industrially preferred.

As for the amount of the solvent, it is preferable that the lithium titanate powder after firing is uniformly dispersed in the solvent. For this purpose, the slurry viscosity is preferably 8000 cP or less, more preferably 5000 cP or less, and still more preferably 3000 cP. It is below.

The circulation treatment time of wet pulverization (the number of pulverization passes by the circulation treatment) is not particularly limited as long as the crystallinity of the lithium titanate does not deteriorate and the battery performance is not adversely affected, but the primary particles of the lithium titanate powder is preferably determined according to the value of log ₁₀ (D ₉₀ )−log ₁₀ (D ₁₀ ).

<Surface treatment process>
The lithium titanate powder of the present invention is an aluminum-containing lithium titanate powder, and when applied as an electrode material for an electricity storage device, it exhibits electrode density, volume energy density, rate characteristics, initial low temperature input characteristics, and high temperature storage. It is possible to impart an excellent effect in suppressing the amount of gas generated at the time. In the firing step, the lithium titanate powder of the present invention can be produced by adding a compound containing a dissimilar element such as aluminum (hereinafter sometimes referred to as a treating agent). The lithium titanate powder of the present invention can be produced by such a surface treatment step as described above.

Lithium titanate powder before surface treatment obtained by the above steps (hereinafter sometimes referred to as base material lithium titanate powder. Also, hereinafter, lithium titanate particles constituting the base material lithium titanate powder may be referred to as base material lithium titanate particles) are mixed with a treating agent and preferably heat-treated.

The aluminum-containing compound (treatment agent) is not particularly limited, but examples include aluminum oxides, hydroxides, sulfate compounds, nitrate compounds, fluorides, organic compounds, and aluminum-containing metal salt compounds. mentioned. Specific examples of Al-containing compounds include aluminum acetate, aluminum fluoride, and aluminum sulfate. Among them, aluminum sulfate, its hydrates, and aluminum fluoride are preferred.

The amount of the aluminum-containing compound (treatment agent) to be added may be any amount as long as the content of aluminum in the lithium titanate powder falls within the range of the present invention. When the hydrate (Al ₂ (SO ₄ ) _{3.14-18H 2} O) is used, the electrode density, volumetric energy density, rate characteristics, initial low temperature input characteristics, and gas during high temperature storage of the electricity storage device From the viewpoint of having an excellent property of suppressing the amount generated, the treatment agent may be added to the lithium titanate powder of the base material at a ratio of 0.16 mmol/LTO 100 g or more and 63.5 mmol/LTO 100 g or less, more preferably 3. 2 mmol/100 g of LTO to 31.7 mmol/100 g of LTO, and particularly preferably 4.0 mmol/100 g of LTO to 15.9 mmol/100 g of LTO. 1.0 mmol of aluminum sulfate 14-18 hydrate corresponds to 0.34 g, although it depends on the ratio of hydrates. Here, "mmol/100 g of LTO" is the amount (mmol) of the treating agent per 100 g of the base material lithium titanate powder (the same applies hereinafter).

In addition to Al, when Mo is also localized on the surface of the primary particles of the lithium titanate powder, in the surface treatment step, in addition to the aluminum-containing compound, a molybdenum-containing compound is also added. It is preferable to use
The molybdenum-containing compound (Mo-containing treatment agent) is not particularly limited, but examples include molybdenum oxides, hydroxides, sulfate compounds, nitrate compounds, fluorides, organic compounds, and molybdenum-containing metal salts. compound. Specifically, molybdenum oxide, molybdenum trioxide, molybdenum trioxide hydrate, molybdenum boride, phosphomolybdic acid, molybdenum disilicide, molybdenum chloride, molybdenum sulfide, silicomolybdic acid hydrate, sodium molybdenum oxide, carbonization Molybdenum, molybdenum acetate dimer, lithium molybdate, sodium molybdate, potassium molybdate, calcium molybdate, magnesium molybdate, manganese molybdate, ammonium molybdate, and the like. In order to uniformly diffuse molybdenum on the particle surface of the lithium titanate powder, it is suitable to use the wet method described later. In that case, a molybdenum-containing compound soluble in a solvent is added to the solvent. It is preferably dissolved and mixed with the substrate lithium titanate powder. Lithium molybdate containing molybdenum is preferable from the viewpoint of suppressing the amount of gas generated. Regarding the valence of molybdenum, even if it is added in a hexavalent state as a raw material, dangling bonds exist when forming chemical bonds on the LTO surface or inside the crystal, and it partially takes a valence of 4 to 5. assumed to be

The amount of the molybdenum-containing compound (Mo-containing treatment agent) may be any amount as long as the content of molybdenum in the lithium titanate powder falls within the scope of the present invention. When lithium molybdate (Li ₂ MoO ₄ ) is used for the base material, the rate characteristics of the electric storage device, the initial low temperature input characteristics, and the characteristics excellent in suppressing the amount of gas generated after high temperature storage are used. The Mo-containing treatment agent may be added to the lithium titanate powder at a ratio of 0.12 mmol/LTO100 g or more and 28.8 mmol/LTO100 g or less, more preferably 0.5 mmol/LTO100 g or more and 15.5 mmol/LTO100 g or less. A particularly preferable ratio is 1.0 mmol/100 g of LTO or more and 10.0 mmol/100 g or less of LTO. 1.0 mmol of lithium molybdate corresponds to 0.17 g.

In addition, in the surface treatment step, a B-containing compound or a W-containing compound may be used as necessary.
There is no particular limitation on the method of mixing the substrate lithium titanate powder with the compound containing aluminum, the compound containing molybdenum used as necessary, the compound containing B, and the compound containing W. Either a wet mixing method or a dry mixing method can be employed, but it is preferable to uniformly disperse the aluminum-containing compound and the molybdenum-containing compound on the surface of the lithium titanate particles of the base material. In that respect, wet mixing is preferred.

It is preferable to perform heat treatment after mixing the lithium titanate powder of the substrate and the treating agent. The heat treatment temperature is a temperature at which aluminum or other elements diffuse into at least the surface region of the lithium titanate particles of the substrate, and the lithium titanate of the substrate is sintered to significantly increase the specific surface area. A temperature at which no significant reduction occurs is good. The upper limit of the heat treatment temperature is preferably 700° C. or less, more preferably 600° C. or less. 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 hour to 8 hours, more preferably 1 hour to 5 hours. The temperature and time at which the element M diffuses into at least the surface region of the lithium titanate particles of the base material are preferably set as appropriate, since the reactivity varies depending on the compound containing the element M.

A heating method in the heat treatment is not particularly limited. Usable heat treatment furnaces include a fixed bed furnace, a roller hearth furnace, a mesh belt furnace, a fluidized bed furnace, and a rotary kiln furnace. The atmosphere during heat treatment may be either an air atmosphere or an inert atmosphere such as a nitrogen atmosphere. When molybdenum or a metal salt compound containing other elements is used as the compound (treatment agent) containing aluminum or other elements, an atmospheric atmosphere is preferred in which anion species are easily removed from the particle surfaces.

The lithium titanate powder of the present invention may be mixed with a treating agent in the surface treatment step, then granulated and heat-treated to obtain a powder containing secondary particles in which primary particles are agglomerated. Any method may be used for granulation as long as secondary particles can be produced, but a spray dryer is preferable because it can process a large amount.

In order to reduce the amount of water contained in the lithium titanate powder of the present invention, the dew point may be controlled in the heat treatment step. If the heat-treated powder is exposed to the atmosphere as it is, the amount of moisture contained in the powder increases. Therefore, it is preferable to handle the powder in an environment where the dew point is controlled during cooling in the heat treatment furnace and after the heat treatment. The heat-treated powder may be classified as necessary to bring the particles into the desired maximum particle size range. When the dew point is controlled in the heat treatment step, it is preferable to seal the lithium titanate powder of the invention in an aluminum laminate bag or the like and then put it in an environment outside the dew point control. Even under dew point control, when lithium titanate powder is pulverized after heat treatment, moisture is easily absorbed from the crushed surface, and the amount of water contained in the powder increases. is preferred. As heat treatment conditions, the temperature and holding time within specific ranges greatly affect the secondary particle morphology and the surface treatment process. The heat treatment temperature is preferably 450°C or higher, and preferably lower than 550°C. This is because if the heat treatment temperature exceeds 550° C., the specific surface area is greatly reduced, and the battery performance, particularly the rate characteristics, is greatly deteriorated. The retention time is preferably 11 hours or more. If the heating time is short, the amount of water contained in the powder increases, and it is presumed that the surface state of the particles and the interfacial reaction with the electrolyte or solid electrolyte are also affected. is.

[Active material]
The active material used for the electrode 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. Other substances include, for example, 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. is used.

[Power storage device]
The electric storage device of the present invention comprises an electrode containing the active material of the present invention, and is a device that stores and releases energy by utilizing intercalation and de-intercalation of lithium ions in such an electrode. , for example, hybrid capacitors and lithium batteries.

The lithium battery is composed of a positive electrode, a negative electrode, and a non-aqueous electrolyte in which an electrolyte salt is dissolved in a non-aqueous solvent, a solid electrolyte, or the like, 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 the electrode of the lithium battery. Although this active material may be used as either a positive electrode active material or a negative electrode active material, the case where it is used as a negative electrode active material will be described below.

<Negative Electrode>
The negative electrode has a mixture 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 the negative electrode current collector. This mixture 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 pores have a mixture layer containing a negative electrode active material (the active material of the present invention), a conductive agent, and a binder. When this mixture layer is used in an all-solid battery, the mixture layer may further contain a solid electrolyte material.

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

As the positive electrode active material, a material capable of intercalating and deintercalating lithium is used. For example, the active material may be a composite metal oxide with lithium containing cobalt, manganese or nickel, or a lithium-containing olivine-type phosphate. These positive electrode active materials can be used singly or in combination of two or more. Examples of such composite metal oxides include LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiCo _1-x Ni _x O ₂ (0.01<x<1), LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , LiNi _1/2 Mn _3/2 O ₄ and the like, and a portion of these lithium composite oxides may be substituted with other elements, and cobalt, manganese, and nickel may be partially replaced with B , Nb, Sn, Mg, Fe, Ti, Al, Zr, Cr, V, Ga, Zn, Cu, Bi, Mo, La, etc., or substitute a part of O with S or It can be substituted with F or coated with a compound containing these other elements. Lithium-containing olivine-type phosphates include, for example, LiFePO ₄ , LiCoPO ₄ , LiNiPO ₄ , LiMnPO ₄ , LiFe _1-x MxPO ₄ (M is at least one selected from Co, Ni, Mn, Cu, Zn, and Cd). 1 type, and x is 0≦x≦0.5.) and the like.

Examples of the positive electrode conductive agent and the binder include those similar to those for the negative electrode. Examples of the positive electrode current collector include aluminum, stainless steel, nickel, titanium, calcined carbon, and aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver. The surface of these materials may be oxidized, or the surface of the positive electrode current collector may be roughened by surface treatment. Examples of forms of current collectors include sheets, nets, foils, films, punched materials, laths, porous bodies, foams, fibers, nonwoven fabrics, and the like.

<Non-aqueous electrolyte>
The non-aqueous electrolyte is obtained by dissolving an electrolyte salt in a non-aqueous solvent. The non-aqueous electrolyte is not particularly limited, and various types can be used.

_As the electrolyte salt _{, one} that _dissolves in a non _{- aqueous} electrolyte _is used _{. CF3} ) ₂ , LiN( _SO2C2F5 ) ₂ , _LiCF3SO3 , _{LiC ( SO2CF3} ) _{3 , LiPF4} ( _CF3 ) ₂ , _{LiPF3 ( C2F5} ) _{3 , LiPF3} (CF ₃ ) ₃ , LiPF ₃ (iso-C ₃ F ₇ ) ₃ , LiPF ₅ (iso-C ₃ F ₇ ) and other lithium salts containing chain-like fluorinated alkyl groups, and (CF ₂ ) ₂ ( Lithium salts containing cyclic alkylene fluoride chains such as SO ₂ ) ₂ NLi, (CF ₂ ) ₃ (SO ₂ ) ₂ NLi, lithium bis[oxalate-O,O′]borate and difluoro[oxalate-O, Lithium salts having an anion of an oxalate complex such as lithium O'] borate are exemplified. Among these, particularly preferred electrolyte salts are LiPF ₆ , LiBF ₄ , LiPO ₂ F ₂ and LiN(SO ₂ F) ₂ , and the most preferred electrolyte salt is LiPF ₆ . These electrolyte salts can be used singly or in combination of two or more. A suitable combination of these electrolyte salts includes LiPF ₆ and at least one lithium salt selected from LiBF ₄ , LiPO ₂ F ₂ , and LiN(SO ₂ F) ₂ in the non-aqueous electrolyte. It is preferable if it is contained in

On the other hand, examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, chain esters, ethers, amides, phosphoric acid esters, sulfones, lactones, nitriles, S=O bond-containing compounds, etc., and include cyclic carbonates. is preferred. The term "chain ester" is used as a concept including chain carbonates and chain carboxylic acid esters.

Cyclic carbonates include 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-dioxolane-2-one (hereinafter collectively referred to as "DFEC"), vinylene carbonate (VC), vinylethylene carbonate (VEC), and 4-ethynyl- One or more selected from 1,3-dioxolan-2-one (EEC), ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 4-fluoro-1, One or more selected from 3-dioxolan-2-one and 4-ethynyl-1,3-dioxolan-2-one (EEC) improves the charge rate characteristics of the power storage device and suppresses the amount of gas generated during high-temperature operation. From this point of view, one or more cyclic carbonates having an alkylene chain selected from propylene carbonate, 1,2-butylene carbonate, and 2,3-butylene carbonate are more preferable. The ratio of the cyclic carbonate having an alkylene chain in the total cyclic carbonate is preferably 55% by volume to 100% by volume, more preferably 60% by volume to 90% by volume.

Chain esters 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. , dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, and dibutyl carbonate, one or more symmetric chain carbonates selected from dibutyl carbonate, methyl pivalate, ethyl pivalate, pivalic acid such as propyl pivalate One or more chain carboxylic acid esters selected from esters, methyl propionate, ethyl propionate, propyl propionate, methyl acetate, and ethyl acetate (EA) are suitable.

<Structure of Lithium Battery>
The structure of the lithium battery of the present invention is not particularly limited, and includes a coin battery having a positive electrode, a negative electrode, and a single-layer or multi-layer separator, and a cylindrical battery having a positive electrode, a negative electrode, and a rolled separator. A battery, a rectangular battery, etc. are mentioned as an example.

As the separator, an insulating thin film having a high ion permeability and a predetermined mechanical strength is used. For example, polyethylene, polypropylene, cellulose paper, glass fiber paper, polyethylene terephthalate, polyimide microporous membrane, etc., can be used. Moreover, the surfaces of these separators can be coated with resins such as PVDF, silicone resins and rubber resins, and particles of metal oxides such as aluminum oxide, silicon dioxide and magnesium oxide. The pore size of the separator may generally be in a range useful for batteries, and is, for example, 0.01 μm to 10 μm. The thickness of the separator may be within the range for general batteries, for example, 5 μm to 300 μm.

<Solid electrolyte>
A solid electrolyte is a solid electrolyte in which ions can move. In particular, since inorganic solid electrolytes are solid in the steady state, they are usually not dissociated or released into cations and anions. The inorganic solid electrolyte is not particularly limited as long as it has conductivity of metal ions belonging to Group 1 of the periodic table, and generally has almost no electronic conductivity. Typical examples of inorganic solid electrolytes include sulfide inorganic solid electrolytes and oxide inorganic solid electrolytes. In particular, a sulfide solid electrolyte is preferably used because it has high ion conductivity and can form a dense compact with few grain boundaries only by applying pressure at room temperature. In addition, the periodic table of the present invention refers to the periodic table of long period type elements based on the regulations of IUPAC (International Union of Pure and Applied Chemistry).

The sulfide inorganic solid electrolyte may be amorphous glass, crystallized glass, or a crystalline material. Specific examples of the sulfide inorganic solid electrolyte include the following combinations, but are not particularly limited.
_{Li2SP2S5 , Li2SP2S5 - Al2S3} , _{Li2S - GeS2 , Li2S - Ga2S3} , _{Li2S - GeS2 - Ga2S3} , Li ₂ S—GeS ₂ —P ₂ S ₅ , Li ₂ S—GeS ₂ —Sb ₂ S ₅ , Li ₂ S—GeS ₂ —Al ₂ S ₃ , Li ₂ S—SiS ₂ , Li ₂ S—Al ₂ S ₃ , Li ₂ S—SiS ₂ —Al ₂ S ₃ , Li ₂ S—SiS ₂ —P ₂ S ₅ , Li ₁₀ GeP ₂ S ₁₂ .

Among the above combinations, LPS glasses and LPS glass-ceramics produced by combining Li ₂ SP ₂ S ₅ are preferred. Algerodite-type solid electrolytes such as Li ₆ PS ₅ Cl and Li ₆ PS ₅ Br are also suitable examples of sulfide inorganic solid electrolytes other than those described above.

The oxide-based inorganic solid electrolyte preferably contains oxygen atoms, has metal ion conductivity belonging to Group 1 of the periodic table, and has electronic insulation.

Examples of oxide inorganic _solid electrolytes include _{Li3.5Zn0.25GeO4} having a LISICON (lithium _superionic conductor ₎ type crystal structure, _{La0.55Li0.35TiO3} having a perovskite _type crystal structure, LiTi ₂ P ₃ O ₁₂ having a NASICON (Natrium superionic conductor) type crystal structure, Li ₇ La ₃ Zr ₂ O ₁₂ (LLZ) having a garnet type crystal structure, lithium phosphate (Li ₃ PO ₄ ), lithium phosphate _{LiPON in which part of the oxygen in the _ _ _ _ _} O ₁₂ and the like are preferably exemplified.

Although the volume average particle diameter of the inorganic solid electrolyte is not particularly limited, it 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.

Next, the present invention will be described more specifically 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 analogized from the spirit of the invention.

[Example 1-1]
<Raw material preparation process>
A raw material obtained by weighing Li ₂ CO ₃ (average particle diameter 4.6 μm) and anatase-type TiO ₂ (specific surface area 10 m ² /g) so that the atomic ratio Li/Ti of Li to Ti is 0.83. Ion-exchanged water was added to the powder and the mixture was agitated so that the solid content concentration of the slurry was 41% by mass to prepare a raw material mixture slurry. This raw material mixture slurry is processed into zirconia beads (outer diameter: 0.5 mm) using a bead mill (manufactured by Willie & Bakkofen, model: Dyno Mill KD-20BC, agitator material: polyurethane, vessel inner surface material: zirconia). 65 mm) is filled into the vessel at 80% by volume, and the raw material powder is processed at an agitator peripheral speed of 13 m / s and a slurry feed rate of 55 kg / hr while controlling the vessel internal pressure to be 0.02 to 0.03 MPa. Wet-mixed and pulverized.

<Baking process>
The obtained mixed slurry is introduced into the furnace core tube from the raw material supply side of the firing furnace using a rotary kiln type firing furnace (furnace core tube length: 4 m, furnace core tube diameter: 30 cm, external heating type) equipped with an adhesion prevention mechanism. , dried in a nitrogen atmosphere and calcined. At this time, the inclination angle of the furnace core tube from the horizontal direction is 2.5 degrees, the rotation speed of the furnace core tube is 20 rpm, and the flow rate of nitrogen introduced into the furnace core tube from the fired material recovery side is 20 L / min. The temperature was set to 600° C. on the raw material supply side, 900° C. on the central portion, and 900° C. on the fired product recovery side, and the time for holding the fired product at 900° C. was 30 minutes.

<Post-treatment process>
The fired product recovered from the fired product recovery side of the furnace tube was pulverized using a bead mill (manufactured by Imex, NVM-1.5 type) under the conditions of a flow rate of 5 L/min and a rotation speed of 2143 rpm. The processing time for allocating the total amount of slurry by the flow rate was set as a unit time for one pass, and the number of passes was set to 9 in order to enhance the pulverization of the fired material.

<Surface treatment process>
Ion-exchanged water is added to the pulverized calcined powder and stirred so that the solid content concentration of the slurry is 40% by mass, and aluminum sulfate 16 hydrate (Al ₂ (SO ₄ ) _{3.16H 2} O) was added in an amount of 3.2% by mass to prepare a mixed slurry. This mixed slurry was sprayed and dried using a spray dryer (L-8i manufactured by Okawara Kakoki Co., Ltd.) at an atomizer rotation speed of 25000 rpm and a drying temperature of 235° C., and granulated. Next, the powder passed through the sieve is placed in an alumina sagger, and a mesh belt conveying continuous furnace equipped with a collection box on the outlet side with a temperature of 25 ° C and a dew point controlled at -20 ° C or less, 1 at 500 ° C. heat treated for hours. The powder after heat treatment is cooled in the recovery box, classified with a sieve (screen opening: 53 μm), and the powder that has passed through the sieve is collected in an aluminum laminate bag and sealed, then taken out from the recovery box and lithium titanate. A powder was produced.

[Example 1-2]
In the surface treatment step, 0.48% by mass of lithium molybdate (Li ₂ MoO ₄ ) and 3.2% by mass of aluminum sulfate hexahydrate (Al ₂ (SO ₄ ) _{3.16H 2} O) are added as treatment agents. Lithium titanate powder was produced in the same manner as in Example 1-1 except for the above. Details are as described in Table 1.

[Example 1-3]
In the firing process, the heating temperature of the furnace tube is set to 600 ° C. on the raw material supply side, 920 ° C. in the center, 920 ° C. on the recovery side of the fired product, and 30 minutes of holding time at 920 ° C. in the post-treatment step. A lithium titanate powder was produced in the same manner as in Example 1-1, except that the number of pulverization passes was changed to 3. Details are as described in Table 1.

[Comparative Example 1-1]
In the firing process, the heating temperature of the furnace core tube is set to 840 ° C. in the central part, the fired product recovery side is set to 840 ° C., the time to hold the fired product at 840 ° C. is 30 minutes, and the number of crushing passes in the post-treatment process is set. Example 1- except that the amount of aluminum sulfate hexahydrate (Al ₂ (SO ₄ ) ₃ 16H ₂ O) as shown in Table 1 was added as a treatment agent in the surface treatment step. Lithium titanate powder was produced in the same manner as in 1.

[Comparative Example 1-2]
In the surface treatment step, 0.48% by mass of lithium molybdate (Li ₂ MoO ₄ ) is further added as a treatment agent, and the amount of aluminum sulfate hexahydrate (Al ₂ (SO ₄ ) _{3.16H 2} O) is added. Lithium titanate powder was produced in the same manner as in Comparative Example 1-1, except that was added as shown in Table 1.

[Comparative Example 1-3]
In the firing process, the heating temperature of the furnace core tube was set to 920 ° C. in the central part, the fired product recovery side: 920 ° C., and the time to hold the fired product at 920 ° C. for 30 minutes, and lithium molybdate as a treatment agent. Lithium titanate powder was produced in the same manner as in Comparative Example 1-1, except that 0.48% by mass of (Li ₂ MoO ₄ ) was further added.

[Comparative Example 1-4]
In the firing process, the heating temperature of the furnace core tube is set to 920°C in the central part, the fired product recovery side is set to 920°C, and the time to hold the fired product at 920°C is 30 minutes. Lithium titanate powder was produced in the same manner as in Comparative Example 1-1, except that there was no

[Comparative Example 1-5]
A lithium titanate powder was produced in the same manner as in Comparative Example 1-1, except that no treatment agent was added in the surface treatment step.

[Measurement of metal element content]
Metal elements contained in the lithium titanate powders of Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-5 (hereinafter sometimes referred to as lithium titanate powders of Examples and Comparative Examples) Content was measured as follows.

<X-ray fluorescence analysis (XRF): molybdenum, aluminum, identification of impurity content>
Elements contained in the lithium titanate powder of each example and comparative example were quantitatively analyzed using a fluorescent X-ray induction spectrometer (manufactured by SII Technology Co., Ltd., trade name "SPS5100"). Using the lithium titanate powder itself as a measurement sample, it was calculated from the mass ratio of the element content. The amount of metal impurities is the sum of the contents of iron (Fe), chromium (Cr), and nickel (Ni).

[Measurement of powder properties]
Various physical properties of the lithium titanate powder of each example and comparative example were measured as follows.

<Measurement of specific surface area>
The specific surface area (m ² /g) of the lithium titanate powder of each example and comparative example was measured using a fully automatic BET specific surface area measuring device (manufactured by Mountec Co., Ltd., trade name "Macsorb HM model-1208"). 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 at 100° C. under vacuum for 0.5 hours, and then measured by the BET single-point method. Further, the diameter equivalent to the specific surface area DBET calculated from the specific surface area determined by the BET method was measured by the method described in Patent Document 1.

<Calculation of _D50 of primary particles and secondary particles: dry laser diffraction scattering method>
The _D50 of the lithium titanate powder of each example and comparative example was calculated from a particle size distribution curve measured using a laser diffraction/scattering particle size distribution analyzer (manufactured by Nikkiso Co., Ltd., Microtrac MT3300EXII). Put 50 mg of sample into a container containing 50 ml of ion-exchanged water as a measurement solvent, shake the container by hand until the powder is evenly dispersed in the measurement solvent by visual inspection, and place the container in the measurement cell. It was measured. The crushing treatment applied ultrasonic waves (30 W, 3 s) with an ultrasonic device in the apparatus. Further, a measurement solvent was added until the transmittance of the slurry fell within the appropriate range (the range indicated by the green bar on the device), and the particle size distribution was measured. _D50 of the mixed powder before and after pulverization was calculated from the obtained particle size distribution curve. The _D50 before pulverization corresponds to the _D50 of secondary particles, and the _D50 after pulverization corresponds to the _D50 of primary particles.

<Calculation of D ₉₀ , D ₁₀ , log ₁₀ (D ₉₀ )-log ₁₀ (D ₁₀ ) of primary particles: dry laser diffraction scattering method>
D ₉₀ and D ₁₀ of the lithium titanate powder of each example and comparative example were calculated from a particle size distribution curve measured using a laser diffraction/scattering particle size distribution analyzer (manufactured by Nikkiso Co., Ltd., Microtrac MT3300EXII). . Put 50 mg of sample into a container containing 50 ml of ion-exchanged water as a measurement solvent, shake the container by hand until the powder is evenly dispersed in the measurement solvent by visual inspection, and place the container in the measurement cell. It was measured. The crushing treatment applied ultrasonic waves (30 W, 3 s) with an ultrasonic device in the device. Further, a measurement solvent was added until the transmittance of the slurry fell within the appropriate range (the range indicated by the green bar on the device), and the particle size distribution was measured. From the obtained particle size distribution curve, the particle size at the point where the cumulative volume distribution is 90% and the particle size at the point where the cumulative volume distribution is 10% are obtained, and the D ₉₀ and D ₁₀ of the mixed powder after pulverization are obtained. was calculated as the D ₉₀ and D ₁₀ of the primary particles. Also, the value of log ₁₀ (D ₉₀ )−log ₁₀ (D ₁₀ ) was calculated from D ₉₀ and D ₁₀ of the obtained primary particles.

[Evaluation of battery characteristics]
Using the lithium titanate powder of each example and comparative example, coin-type batteries were produced and their battery characteristics were evaluated. Evaluation results are shown in Tables 1 and 2.

<Production of negative electrode sheet>
The negative electrode sheet was prepared as follows in a room controlled to a room temperature of 25° C. and a dew point of −20° C. or lower. The lithium titanate powder of each example was taken out from the aluminum laminate bag in a room controlled at a temperature of 25°C and a dew point of -20°C or less. The lithium titanate powder of each example taken out was mixed as follows at a ratio of 90% by mass as an active material, 5% by mass of acetylene black as a conductive agent, and 5% by mass of polyvinylidene fluoride as a binder. A paint was prepared. After mixing polyvinylidene fluoride, acetylene black and 1-methyl-2-pyrrolidone dissolved in 1-methyl-2-pyrrolidone in advance in a planetary stirring and defoaming device, lithium titanate powder is added and the total solid concentration is was adjusted to 64% by mass and mixed with a planetary stirring deaerator. Thereafter, 1-methyl-2-pyrrolidone was added to adjust the total solid content concentration to 50% by mass, and mixed with a planetary stirring deaerator to prepare a paint. The obtained paint was applied onto an aluminum foil and dried to prepare a negative electrode single-sided sheet for use in coin batteries described later and a negative electrode double-sided sheet for use in laminated batteries described later. The target basis weight for coating was 7.5 mg/cm ² .

<Measurement of electrode density>
After pressing the single-sided negative electrode sheet coated in the manner described above with a roll press machine (roll diameter 60×150 mm, press pressure equivalent to 40 MPa), the density of the active material layer was measured as "electrode density". Table 1 shows the evaluation results. When the electrode density is high, more active material can be packed per fixed volume, and as a result, the capacity that can be used as a battery increases, which is preferable.

<Preparation of positive electrode sheet>
Except for using nickel-cobalt lithium manganate powder as the active material, the same method as described in <Preparation of Negative Electrode Sheet> was used, including the ratios of the active material, the conductive agent, and the binder. , a positive electrode single-sided sheet was produced.

<Preparation of electrolytic solution>
The electrolytic solution used for 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 non-aqueous solvent of ethylene carbonate (EC): dimethyl carbonate (DMC) = 1: 2 (volume ratio) was prepared, and an electrolyte salt was added thereto. LiPF ₆ was dissolved so as to have a concentration of 1M to prepare an electrolyte solution for a coin battery, which will be described later. Similarly, a non-aqueous solvent of propylene carbonate (PC): diethyl carbonate (DEC) = 1:2 (volume ratio) was prepared, and LiPF ₆ was dissolved as an electrolyte salt in this to a concentration of 1.3M. was prepared.

<Production of coin battery>
A circle having a diameter of 14 mm was punched out from the negative electrode single-sided sheet prepared by the method described above, pressed at a pressure of 2 t/cm ² , and vacuum-dried at 120° C. for 5 hours to prepare an evaluation electrode. The prepared evaluation electrode and metal lithium (thickness 0.5 mm, diameter 16 mm circular shape) are opposed to each other via a glass filter (ADVANTEC GA-100 and Wattman GF / C). A 2032-type coin-type battery was fabricated by adding a non-aqueous electrolyte prepared by the method described in <Preparation of electrolyte solution> and sealing.

<Production of laminated battery>
A laminate battery was produced in a room controlled to a room temperature of 25° C. and a dew point of −40° C. or less as follows. After pressing the negative electrode double-sided sheet at a pressure of 2 t/cm ² , a negative electrode having a lead wire connection portion was produced. After pressing the positive electrode double-sided sheet with a pressure of 2 t/cm ² , a positive electrode having a lead wire connection portion was produced. The produced negative electrode and positive electrode were vacuum-dried at 150° C. for 12 hours. The positive electrode and the negative electrode after vacuum drying are opposed to each other with a separator (UPZ210 manufactured by Ube Industries, Ltd.) interposed therebetween. A laminated battery for evaluation was produced by adding the electrolytic solution for a laminated battery prepared by the described method and vacuum-sealing with an aluminum laminate. At this time, the capacity of the battery was 30 mAh, and the ratio of the capacities of the negative electrode and the positive electrode (negative electrode capacity/positive electrode capacity) was 1.2. Next, as an aging step, the laminate battery produced by the method described in <Preparation of Laminate Battery> was charged to 2.75 V at a current of 0.2 C in a constant temperature bath at 25 ° C. After performing constant-current constant-voltage charging at 0.75 V until the charging current reaches 0.05 C, constant-current discharging at 0.2 C to 1.4 V was repeated three cycles. After that, the volume of the laminate battery was measured by the Archimedes method to determine the initial volume of the laminate battery (hereinafter sometimes referred to as initial volume).

<Coin battery: initial discharge capacity, volumetric energy density, measurement of 50 rate characteristics>
A current of 0.2 mA/cm ² was applied to the coin-type battery produced by the method described in <Production of coin battery> above in a constant temperature bath at 25 ° C., with the direction in which Li is occluded in the evaluation electrode as charging. After charging to 1 V at a density of 1 V and then constant-current constant-voltage charging at 1 V until the charging current reaches a current density of 0.05 mA/cm ² , the battery is charged to 2 V at a current density of 0.2 mA/cm ² . Three cycles of constant current discharge were performed. The initial discharge capacity (mAh/g) was obtained by dividing the discharge capacity (mAh) at the third cycle by the mass of lithium titanate. The volume energy density was obtained by multiplying the initial discharge capacity by the electrode density obtained above. If the volume energy density is high, the capacity that can be used as a battery per fixed volume increases, which is preferable because it leads to the miniaturization of the battery. Next, after charging to 1 V with a current corresponding to 1 C of the initial discharge capacity, the battery was discharged to 2 V with a current of 50 C to obtain the 5 C discharge capacity. The rate characteristic (%) was calculated by dividing the 50C discharge capacity by the initial discharge capacity. When the lithium titanate powder has a high 50C rate capacity ratio, it can be expected to improve the charge rate characteristics of an electricity storage device when applied as an electrode material for the electricity storage device. Table 1 shows the evaluation results. Note that C in 1C represents a current value when charging and discharging. For example, 1C refers to the current value that can fully discharge (or fully charge) the theoretical capacity in 1/1 hour, and 0.1C means the current value that can fully discharge (or fully charge) the theoretical capacity in 1/0.1 hour. Point.

<Laminate battery: measurement of initial IMP (impedance)>
A negative electrode sheet was taken out from the laminated battery produced by the method described in <Preparation of Laminate Battery> above to prepare a symmetric cell in which both electrodes were negative electrodes. Using this cell, IMP (impedance) measurement was performed at −20° C. and 0.01 to 10 MHz, and the IMP value (Ω) at −20° C. from a specific frequency (1 Hz) was obtained.

<Laminate battery: measurement of initial −30° C. low temperature input characteristics>
Using the laminate battery produced by the method described in <Preparation of laminate battery> above, at 25 ° C. and −30 ° C., CC (constant current) and a current value equivalent to 1 C were charged to 2.75 V. Charge capacity was measured. By introducing this value into the following equation, the low temperature input characteristics at -30°C were calculated.

<Laminate battery: Measurement of -30°C low temperature input characteristics after 400 cycles at 55°C>
Using the laminate battery produced by the method described in <Preparation of Laminate Battery> above, in a constant temperature chamber at 55 ° C., it was charged with a current of 1 C to 2.75 V, and further the charging current was 0 at 2.75 V. After performing constant-current and constant-voltage charging until a current of 0.05 C was performed, constant-current discharging was performed by discharging to 1.4 V at a current of 1 C, which was repeated 400 cycles.

After 400 cycles, the charge capacity was measured when charging to 2.75 V at −30° C. with CC (constant current) and a current value corresponding to 1C. By introducing this value into the above (Equation 1), the low temperature input characteristics at -30°C were calculated.

<Laminate battery: Amount of gas generated after high temperature storage at 60°C and charge rate characteristics>
In a constant temperature bath at 25 ° C., the laminate battery produced by the method described in <Preparation of Laminate Battery> above was charged to 2.75 V at a current of 0.2 C, and then placed in a constant temperature bath at 60 ° C. A storage test for 30 days was performed. After the storage test, the volume of the laminated battery was measured by the Archimedes method, and the post-storage volume (hereinafter sometimes referred to as the post-storage volume) was measured. By subtracting the initial volume from the post-storage volume, the amount of gas generated (ml) was obtained. Also, using the battery after the storage test, the charge capacity when charged at current values of 0.5 C and 0.2 C was obtained, and the charge rate characteristics were calculated from the ratio. Table 1 shows the results.

<Evaluation results>
The electrodes using the lithium titanate powders of Examples 1-1 to 1-3 maintain well-balanced electrode density, volumetric energy density, rate characteristics, initial low temperature input characteristics, and charge rate characteristics after high temperature storage. Excellent result. On the other hand, when lithium titanate powders were experimentally prepared in different firing temperatures, post-treatment steps, or surface treatment steps from Examples 1-1 to 1-3 (Comparative Examples 1-1 to 1-5), the electrode density This resulted in a decrease in volumetric energy density, a decrease in low-temperature characteristics, and an increase in gas generation and resistance during high-temperature operation. In particular, titanic acid with a log ₁₀ (D ₉₀ )-log ₁₀ (D ₁₀ ) value of greater than 0.8 for primary particles that contain aluminum on the surface but are trial-produced at different firing temperatures and post-treatment steps than in Example 1-1. When lithium powder was used (Comparative Example 1-1), although the value of the equivalent specific surface area DBET calculated from the specific surface area obtained by the BET method was almost the same as in Example 1-1, the electrode density As a result, the volumetric energy density decreased and the amount of gas generated after storage increased. Further, when comparing Example 1-1 and Example 1-3, it was found that rate characteristics and low-temperature input characteristics could be further improved by adjusting the firing temperature and the number of crushing passes in the post-treatment process.

[Influence of manufacturing process conditions of lithium titanate powder]
[Examples 2-1 to 2-3]
Lithium titanate powder was produced in the same manner as in Example 1-2 except that the firing process and post-treatment process were performed under the conditions shown in Table 2, and evaluation was performed using coin batteries and laminate batteries.

<Evaluation results>
The electrodes using the lithium titanate powders of Examples 2-1 to 2-3 tended to be excellent in maintaining well-balanced electrode density, volume energy density, rate characteristics, and initial low-temperature input characteristics. Do you get it. In particular, it was found that the electrode density and initial low temperature input characteristics can be further improved by adjusting the number of crushing passes in the post-treatment process.

(all-solid secondary battery)
[Example 3-1]
In a glove box under an argon atmosphere, the lithium titanate powder of Example 1-2 and the Li ₆ PS ₅ Cl powder, which is a sulfide solid electrolyte (obtained using a laser diffraction/scattering particle size distribution analyzer). The volume average particle diameter obtained: 6 μm) was weighed so that the mass ratio of lithium titanate powder: Li ₆ PS ₅ Cl=60:40, and mixed in an agate mortar. Next, zirconia balls (diameter 3 mm, 20 g) were put into an 80 mL zirconia pot, and the mixed powder was put thereinto. After that, this pot was set in a planetary ball mill, and stirring was continued for 15 minutes at a rotation speed of 200 rpm to obtain a negative electrode active material composition of Example 2-1. The obtained negative electrode active material composition was pressed (360 MPa) for 10 minutes at room temperature to prepare a pellet (molded body) having a diameter of 10 mm and a thickness of about 0.7 mm. A pellet-shaped electrode containing this negative electrode active material composition, a pellet-shaped solid electrolyte layer (LPS glass having a molar ratio of Li ₂ S:P ₂ S ₅ =75:25) as a separator layer, and lithium indium as a counter electrode The alloy foils are laminated in this order, and the laminate is sandwiched between stainless steel current collectors to produce an all-solid secondary battery. properties were evaluated. As a result, a discharge capacity of 133 mAh/g was confirmed for the first time, and it was confirmed that the material works effectively as an active material for an all-solid secondary battery.

[Comparative Example 3-1]
An all-solid secondary battery was produced in the same manner as in Example 3-1 above, except that the lithium titanate powder of Comparative Example 1-5 was used, and battery characteristics were evaluated. As a result, a discharge capacity of 118 mAh/g was confirmed for the first time, resulting in a discharge capacity inferior to that of Example 3-1.

The lithium titanate powder obtained in the present invention is excellent in maintaining well-balanced electrode density, volumetric energy density, rate characteristics, initial low temperature input characteristics, and charge rate characteristics after high temperature storage. It has battery characteristics of 1.98 g/cm ³ or more, volume energy density of 334 mAh/g or more, 50C rate characteristics of 74% or more, initial low temperature input characteristics of 46% or more, and charge rate characteristics after high temperature storage of 93% or more. Therefore, it is useful as an electrode active material for lithium ion secondary batteries, and lithium ion secondary batteries using this lithium titanate as an electrode active material can charge and discharge stably at high speed. It is useful as a power storage device such as a secondary battery for driving or backing up various devices and for storing power at night in homes and offices.

Claims (7)

A lithium titanate powder containing Li ₄ Ti ₅ O ₁₂ as a main component, wherein aluminum (Al) is locally present on the surface of primary particles of the lithium titanate powder, and the aluminum content of the lithium titanate powder is A lithium titanate powder having a content of 0.2% by mass or more and satisfying the following formula (I).
_log10 ( _D90 ) - _log10 ( _D10 ) < 0.8 (I)
(D ₉₀ indicates the particle size at which the cumulative volume distribution of the particle size of the primary particles is 90% in the particle size distribution, and D ₁₀ indicates the cumulative volume distribution of the particle size of the primary particles in the particle size distribution of 10%. indicates the particle size at the point where 2. The lithium titanate powder according to claim 1, wherein the lithium titanate powder satisfies the following formula (II).
_log10 ( _D90 ) - _log10 ( _D10 ) < 0.7 (II)
(D ₉₀ indicates the particle size at which the cumulative volume distribution of the particle size of the primary particles is 90% in the particle size distribution, and D ₁₀ indicates the cumulative volume distribution of the particle size of the primary particles in the particle size distribution of 10%. indicates the particle size at the point where 3. The lithium titanate powder according to claim 1 or 2, wherein the particle size _D50 of the primary particles corresponding to 50% volume accumulation in the particle size distribution is 0.5 µm or more. 4. The lithium titanate powder according to claim 3, wherein the lithium titanate powder contains primary particles and secondary particles of lithium titanate and satisfies the following formula (III).
D ₅₀ of primary particles/D ₅₀ of secondary particles ≤ 0.05 (III) The lithium titanate powder according to claim 4, wherein the lithium titanate powder has a specific surface area of 6 ^m2 /g or more. An electrode comprising the lithium titanate powder according to any one of claims 1 to 5. An electricity storage device comprising the electrode according to claim 6 .