JP7654971B2 - Lithium titanate powder, electrode sheet using same, and power storage device - Google Patents

Description

The present invention relates to lithium titanate powder suitable as an electrode material for electricity storage devices, an electrode sheet using the same, and an electricity storage device.

In recent years, various materials have been researched as electrode materials for electricity storage devices. Among these, lithium titanate has attracted attention as an active material for electricity storage devices for electric vehicles such as HEVs, PHEVs, and BEVs because of its excellent input/output characteristics when used as an active material.

A high energy density is required for an electric vehicle storage device from the viewpoint of improving fuel efficiency or power consumption. In addition, stability over a wide temperature range is also important. For example, it is not uncommon for the temperature inside a car to reach 60°C or higher in the summer. Therefore, it is required that an electric vehicle storage device does not have any safety problems or performance degradation even at high temperatures. However, it is known that when an electric storage device containing lithium titanate is used for a long period of time at a high temperature of 60°C or higher, it becomes more difficult to maintain storage stability and cycle stability, which causes an increase in the resistance of the battery. In addition, when charging and discharging are repeated, the lithium titanate causes an electrochemical side reaction, which decomposes the organic solvent contained in the electrolyte, and may generate gases such as carbon monoxide, carbon dioxide, methane, and hydrogen (hereinafter referred to as decomposition gases). Therefore, there is a need for the development of lithium titanate that suppresses gas generation and resistance increase while increasing the energy density of the electric storage device and maintaining the battery characteristics during high-temperature operation.

Patent Document 1 discloses a lithium titanate powder having a specific surface area of 4 m ² /g or more, containing at least one localized element selected from boron (B), Ln (Ln is at least one metal element selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Hb, Er, Tm, Yb, Lu, Y, and Sc), and M1 (M1 is at least one metal element selected from W and Mo), and characterized in that the localized elements boron (B), Ln, and M1 are localized and present in the vicinity of the surface of the lithium titanate particles constituting the lithium titanate powder. Patent Document 1 discloses a lithium titanate powder that, when applied as an electrode material for an electricity storage device, has a large charge/discharge capacity and can suppress the amount of gas generated during high-temperature operation.

Patent Document 2 discloses a lithium titanate powder that is a spherical secondary particle body containing fibrous carbon with an average diameter of 5 to 40 nm and an average aspect ratio of 10 to 1000, contains fibrous carbon with an average circularity of 90% or more, and has a specified average compressive strength and 1-methyl-2-pyrrolidone oil absorption of the spherical secondary particles. Patent Document 2 discloses an electricity storage device that has excellent charge/discharge cycle characteristics in a high-temperature environment and generates a small amount of gas after charge/discharge cycles.

Patent Document 3 discloses a lithium titanium composite oxide in which primary particles are aggregated to form spherical secondary particles, the circularity of the particle projection image of the secondary particles and the oil absorption amount are within specific ranges, and the main component is Li4 _/ 3Ti5 _{/ 3O4} . Patent Document 3 discloses a lithium secondary battery that is easy to handle during electrode production and has good coatability onto a current collector, has little expansion of the electrodes used, and has excellent characteristics.

Patent Document 4 discloses a positive electrode material for lithium ion secondary batteries that is composed of spherical granules made of secondary particles in which the carbon is interposed between primary particles of a positive electrode active material having an olivine structure, and non-spherical agglomerates in which the carbon is interposed between the primary particles, and that specifies the mass, particle size, and N-methyl-2-pyrrolidone oil absorption of the covering carbon. Patent Document 4 discloses that a positive electrode material for lithium ion secondary batteries that has excellent input/output characteristics and high electrode density can be obtained.

International Publication No. 2018/110708 International Publication No. 2017/047755 JP 2001-192208 A JP 2020-140827 A

However, in the energy storage device in which the lithium titanate of Patent Document 1 is used as the negative electrode material, the amount of gas generated during high-temperature operation can be suppressed in a battery with a large charge/discharge capacity, but when a coated electrode is created, there is a problem that the electrode density is difficult to increase, resulting in a decrease in the energy density of the battery. The advantages of increasing the electrode density include making the battery itself lighter and smaller, reducing the proportion of the battery in electronic devices and vehicles, greatly improving the design freedom of electronic devices, eliminating the need to secure space for batteries in large battery applications, which leads to an increase in energy density per unit volume, leading to an extension of the driving distance of electric vehicles and the securing of installation space for storage batteries, and reducing the contact resistance of the electrodes inside the battery.

In the lithium titanate powder of Patent Document 2, which contains fibrous carbon and specifies the average compressive strength of spherical secondary particles, it has been confirmed that the cycle characteristics are improved by ensuring the permeability of the non-aqueous electrolyte, but there is no mention of an increase in resistance during long-term storage in a charged state, and it has been found that there are issues with achieving compatibility with volumetric energy density.

Furthermore, Patent Document 3 does not include any description of the increase in resistance during long-term storage in a charged state, and does not provide any knowledge regarding compatibility with improved electrode density. Furthermore, Patent Document 4 describes the positive electrode active material, and does not provide any knowledge regarding the increase in resistance during long-term cycle gas or storage in a charged state.

For these reasons, in the energy storage devices using the negative electrode active materials and electrodes of Patent Documents 1 to 4, it is not possible to simultaneously improve the electrode density of the negative electrode, which is directly linked to the energy density, and suppress gas generation and resistance increase during high-temperature operation.

The present invention aims to provide lithium titanate powder that can be used as an electrode material for an electricity storage device, has excellent initial discharge capacity, rate characteristics, electrode density, and volumetric energy density, and can suppress the amount of gas generated and the increase in resistance value during high-temperature operation, an electrode sheet using the same, and an electricity storage device.

The inventors conducted various studies to achieve the above-mentioned object, and as a result, discovered a lithium titanate powder in which the concentration of a specific metal element on the particle surface and inside the particle is within a specific range and the concentration is within a specific range by controlling the raw material mixing ratio in the raw material preparation process, the firing temperature in the firing process, and the amount of additives and heat treatment temperature in the surface treatment process when manufacturing the lithium titanate powder, and exhibits a specific range of 1-methyl-2-pyrrolidone oil absorption. They discovered that an electricity storage device in which this lithium titanate powder is used as an electrode material is excellent in initial discharge capacity, rate characteristics, electrode density, and volumetric energy density, and furthermore, can suppress the amount of gas generation and increase in resistance value during high-temperature operation, and thus completed the present invention. That is, the present invention relates to the following items.

(1) A lithium titanate powder mainly composed of Li ₄ Ti ₅ O ₁₂ , characterized in that the lithium titanate powder contains primary particles and secondary particles of lithium titanate, has a 1-methyl-2-pyrrolidone oil absorption of 71 mL/100 g or less, and has molybdenum (Mo) and aluminum (Al) present on the surfaces of the primary particles.

(2) The lithium titanate powder according to (1), characterized in that the lithium titanate powder has a specific surface area of 6.1 m ² /g or less.

(3) The lithium titanate powder according to (1) or (2), characterized in that the particle size D50 of the primary particles corresponding to a cumulative volume of 50% in the volume-based particle size distribution measured by the laser diffraction scattering method is 0.7 μm or more.

(4) The lithium titanate powder according to any one of (1) to (3), characterized in that the particle size D50 of secondary particles corresponding to a cumulative volume of 50% in the volume-based particle size distribution measured by the laser diffraction scattering method is 11 μm or more.

(5) An electrode sheet comprising the lithium titanate powder described in any one of (1) to (4).

(6) An electricity storage device comprising the electrode sheet described in (5).

The present invention can provide lithium titanate powder suitable as an electrode material for an electricity storage device, which has excellent initial discharge capacity, rate characteristics, electrode density, and volumetric energy density, and further suppresses the amount of gas generated and the increase in resistance value during high-temperature operation, as well as an electrode sheet and an electricity storage device using the same.

FIG. 1 is an XPS spectrum obtained by measuring the valence distribution of Mo contained in lithium titanate powder containing Li ₄ Ti ₅ O ₁₂ as the main component.

[Lithium titanate powder of the present invention]
The lithium titanate powder of the present invention is a lithium titanate powder mainly composed of Li ₄ Ti ₅ O ₁₂ , which contains primary particles and secondary particles of lithium titanate, has a 1-methyl-2-pyrrolidone oil absorption of 71 mL/100 g or less, and has Mo and Al present on the surfaces of the primary particles.

<Lithium titanate powder containing Li ₄ Ti ₅ O ₁₂ as the main component>
The lithium titanate powder of the present invention is mainly composed of Li ₄ Ti ₅ O ₁₂ , and may contain crystalline components and/or amorphous components other than Li ₄ Ti ₅ O ₁₂ within the range in which the effects of the present invention can be obtained. The main component means that the ratio of the intensity of the main peak of Li ₄ Ti ₅ O ₁₂ among the diffraction peaks measured by X-ray diffraction is 90% or more. 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 X-ray diffraction is more preferably 92% or more, and even more preferably 95% or more. 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 may contain, as the crystalline components, anatase type titanium dioxide, rutile type titanium dioxide, and lithium titanates having different chemical formulas, such as _{Li2TiO3 and Li0.6Ti3.4O8 ,} depending on the raw materials and synthesis conditions used during synthesis. _The lower the occurrence rate of crystalline components other than _Li4Ti5O12 , particularly _Li0.6Ti3.4O8 , the more the lithium _{titanate powder} of the present invention can improve the charging characteristics and charge _/ discharge capacity _of an electricity storage device. Among the diffraction peaks measured by X-ray diffraction, when the intensity of the main peak of Li ₄ Ti ₅ O ₁₂ is taken as 100, 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 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 a peak corresponding to the diffraction peak belonging to the (111) plane (2θ=18.33) of Li ₄ Ti ₅ O ₁₂ in PDF card 00-049-0207 of ICDD (PDF2010). The main peak of anatase titanium dioxide is a peak corresponding to the diffraction peak attributable to the (101) plane (2θ = 25.42) in 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) in PDF card 01-070-7347. The peak corresponding to the (-133) plane of Li ₂ TiO ₃ is a peak corresponding to the diffraction peak attributable to the (-133) plane (2θ = 43.58) of Li ₂ TiO ₃ in PDF card 00-033-0831. The main peak of Li _0.6 Ti _3.4 O ₈ is a peak corresponding to the diffraction peak attributable to the (101) plane (2θ = 19.98) in PDF card 01-070-2732. It should be noted that "ICDD" is an abbreviation for International Centre for Diffraction Data, and "PDF" is an abbreviation for Powder Diffraction File.

<Content of Molybdenum (Mo) and Aluminum (Al)>
The lithium titanate powder of the present invention contains Mo and Al on the surface of the primary particles. The term "containing each of the metal elements" means that Mo and Al are detected by inductively coupled plasma atomic emission spectrometry (ICP-AES) of the lithium titanate powder of the present invention. The lower limit of the amount detected by inductively coupled plasma atomic emission spectrometry is usually 0.001 mass%.

<Content of Molybdenum (Mo) and Aluminum (Al)>
The content (mass%) of Mo and Al in the lithium titanate powder of the present invention determined by X-ray fluorescence analysis (XRF) is 0.01 or more and 0.8 or less. If the content of each of the metal elements is within this range, an electric storage device excellent in initial discharge capacity, rate characteristics, electrode density, and volumetric energy density, and further suppresses the amount of gas generated and the increase in resistance value during high-temperature operation can be obtained. More preferably, it is 0.1 or more and 0.6 or less, and even more preferably, it is 0.1 or more and 0.4 or less. The content refers to the mass ratio of Mo and Al contained in the total mass of the lithium titanate powder.

<Mass ratio of molybdenum (Mo) to aluminum (Al) (Mo (mass%)/Al (mass%)>
The lithium titanate powder of the present invention contains Mo and Al on the surface of the primary particles, and the 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 excellent in initial discharge capacity, rate characteristics, electrode density, and volumetric energy density, and further suppresses the amount of gas generation and the increase in resistance value during high temperature operation can be obtained. The mass ratio is more preferably 0.2 or more and 6 or less, and even more preferably 0.3 or more and 4 or less.

In addition, in the lithium titanate powder of the present invention, Mo and Al are contained more on the surface than inside the primary particles of lithium titanate contained in the powder. In a cross-sectional analysis of the primary particles of lithium titanate using a scanning transmission electron microscope, it is sufficient that Mo is contained more in the so-called near-surface region from the surface of the primary particles of lithium titanate to a depth of about 5 nm, and it is sufficient that Al is contained more in the so-called near-surface region from the surface to a depth of about 20 nm, and it is preferable that Mo and Al are not detected at a depth of about 100 nm. It is preferable that Mo and Al are fixed in a state of being chemically bonded to the surface of the primary particles. When Mo and Al are present in such a state, a power storage device is obtained that is excellent in initial discharge capacity, rate characteristics, electrode density, and volumetric energy density, and further suppresses the amount of gas generation and the increase in resistance value during high-temperature operation. The lower limit of the detection amount in the measurement by energy dispersive X-ray spectroscopy is usually 0.5 atm%, although the value varies depending on the element and state to be measured. Therefore, at a depth of about 100 nm, Mo and Al may each be detected in the range of 0.5 atm% or less.

<Inclusion of Different Element M1>
The lithium titanate powder of the present invention may contain at least one element selected from B, Mg, Zn, Ga, W, S, or In as a further hetero element (hereinafter referred to as element M1). The lithium titanate powder of the present invention further improves the efficiency and rate characteristics at high temperature charge and discharge by containing these elements M1 together with Mo and Al. The element M1 is preferably sulfur (S). It is presumed that the lithium titanate powder of the present invention contains the element S together with Mo and Al, so that the electronic conductivity of the surface of the primary particles of the lithium titanate powder is adjusted, thereby suppressing the electrical resistance.

The content (mass%) of element M1 determined by X-ray fluorescence analysis (XRF) is 0.01 to 1.0. From the viewpoint of improving the efficiency and rate characteristics of the power storage device during high-temperature charge/discharge, the content is preferably 0.03 to 0.8, and more preferably 0.1 to 0.6. When element M1 is S, the content is preferably 0.1 to 0.5, and more preferably 0.15 to 0.48.

<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 71 ml/100 g or less. If the NMP oil absorption is within this range, it is possible to obtain an electricity storage device that improves the electrode density, initial discharge capacity, volumetric energy density, and rate characteristics, and suppresses the amount of gas generation and the increase in resistance value during high-temperature operation. It is preferably 70 ml/100 g, more preferably 69 ml/100 g. Here, the unit ml/100 g represents the amount of NMP absorption (unit is ml) per 100 g of the lithium titanate powder of the present invention. The reason why the above effect can be obtained is not necessarily clear, but it is presumed that NMP is a solvent used in electrode preparation, and by appropriately controlling the NMP oil absorption of the lithium titanate powder, it contributes to efficient adhesion with the negative electrode active material, conductive assistant, and binder, and high-density electrode processability can be realized.

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

In the lithium titanate constituting the lithium titanate powder of the present invention, if the specific surface area is 6.1 ^m2 /g or less, it is possible to obtain an electricity storage device that is excellent in initial discharge capacity, rate characteristics, electrode density, and volumetric energy density, and further suppresses the amount of gas generated and the increase in resistance value during high-temperature operation. The specific surface area of the lithium titanate powder is more preferably 6.0 ^m2 /g or less, and even more preferably 5.7 ^m2 /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 the particle size at which the cumulative volume frequency calculated from the volume fraction obtained by laser diffraction/scattering type particle size distribution measurement is 50% when calculated from the smallest particle size. The measurement method will be described in the examples below.

<Primary and secondary particles>
The lithium titanate powder of the present invention includes primary particles of lithium titanate and secondary particles formed by aggregation of the primary particles. The primary particles refer to individual particles that constitute the lithium titanate powder and cannot be further divided by a crushing treatment.

From the viewpoint of improving electrode density, volumetric energy density, and rate characteristics, 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, and more preferably 12.4 μm or more. Also, it is 20 μm or less, preferably 18 μm or less, and more preferably 14 μm or less. Note that the D50 of the secondary particles represents the D50 before the crushing process (ultrasonic application with an ultrasonic irradiator).

From the viewpoint of improving the electrode density, volumetric energy density, and rate characteristics, the D50 of the primary particles of the lithium titanate powder of the present invention is 0.7 μm or more, preferably 0.73 μm or more, and more preferably 0.75 μm or more. Also, it is 3 μm or less, preferably 2 μm or less, and more preferably 1.5 μm or less. The D50 of the primary particles represents the D50 after a crushing process using an ultrasonic irradiator. Also, the lithium titanate powder may contain a cumulative volume frequency of primary particles having a primary particle diameter of less than 0.7 μm in the range of 10% to 50%, a cumulative volume frequency of primary particles having a primary particle diameter of less than 0.73 μm in the range of 15% to 55%, and a cumulative volume frequency of primary particles having a primary particle diameter of less than 0.75 μm in the range of 10% to 60%. Furthermore, the cumulative volume frequency of primary particles larger than 3 μm may be in the range of 50% to 90%, the cumulative volume frequency of primary particles larger than 2 μm may be in the range of 45% to 85%, and the cumulative volume frequency of primary particles larger than 1.5 μm may be in the range of 40% to 80%.

<pH>
In the lithium titanate powder of the present invention, the pH is preferably 10.0 to 11.5, more preferably 10.5 to 11.4, and even more preferably 10.7 to 11.3. Within this range, an electricity storage device can be obtained in which the amount of gas generated and the increase in resistance value during high-temperature operation are suppressed without impairing the electrode density and rate characteristics of the electricity storage device. Although this is only speculation, it is assumed that the pH of the lithium titanate powder of the present invention is influenced by the surface state, particularly the valence of molybdenum. The pH in the present invention refers to a value measured by dissolving the lithium titanate powder in water.

<Valence of Molybdenum (Mo)>
In the manufacturing method described in the specification of Patent Document 1, the valence of Mo is not specified. According to X-ray photoelectron spectroscopy (XPS) of the lithium titanate primary particles contained in the lithium titanate powder of the present invention, the ratio of the peak area of Mo (pentavalent and tetravalent) to the total peak area derived from Mo (hexavalent, pentavalent and tetravalent) is preferably 30% or less, more preferably 20% or less. If the ratio of the peak area of Mo (pentavalent and tetravalent) is 30% or less, the side reaction of Mo is suppressed during the initial charge and discharge, and an electricity storage device excellent in initial discharge capacity, rate characteristics, electrode density and volumetric energy density, and further suppressed gas generation amount and resistance value increase during high temperature operation can be obtained.

The reason why the gas generation amount and the increase in resistance value are suppressed is not clear, but it is believed that the presence of Mo at the interface between the lithium titanate particle surface and the non-aqueous electrolyte (including solid electrolyte) allows Mo to efficiently convert the decomposition products generated at the particle surface into other substances, thereby suppressing the increase in the overall gas generation amount and resistance components. In addition, the formation of a Mo diffusion layer present on the lithium titanate particle surface protects the active points of Li ₄ Ti ₅ O ₁₂ caused by the decomposition of the electrolyte, and therefore it is believed that the gas generation amount due to the decomposition of the electrolyte can be further suppressed.

<Periodic Table>
The periodic table in the present invention refers to a long-form periodic table of elements based on the provisions of IUPAC (International Union of Pure and Applied Chemistry).

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

<Raw material preparation process>
The raw materials for the lithium titanate powder of the present invention are composed of a titanium raw material and a lithium raw material. As the titanium raw material, titanium compounds such as anatase type titanium dioxide and rutile type titanium dioxide are used. It is preferable that the titanium raw material reacts easily with the lithium raw material in a short time, and from that viewpoint, anatase type titanium dioxide is preferable. In order to sufficiently react the raw materials by firing for a short time, the D50 of the titanium raw material is preferably 5 μm or less.

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

The ratio of titanium raw material and lithium raw material is preferably such that the atomic ratio Li/Ti of Li to Ti is 0.81 or more, and more preferably 0.83 or more. If the ratio is too low, it may promote the generation of certain impurity phases in the lithium titanate powder obtained after firing, which may have a detrimental effect on the battery characteristics.

In the present invention, when the mixture of the above raw materials is fired in a short time, it is preferable to prepare the mixed powder that constitutes the mixture before firing so that D95 in the particle size distribution curve measured by a laser diffraction/scattering type particle size distribution measuring device is 5 μm or less. Here, D95 refers to the particle size at which the cumulative volume frequency calculated by volume fraction is 95% when integrated from the smallest particle size.

The following methods can be used to prepare the mixture. In the first method, the raw materials are mixed and pulverized at the same time. In the second method, the raw materials are pulverized until the D95 after mixing is 5 μm or less, and then these are mixed or lightly pulverized while being mixed. In the third method, the raw materials are produced into powders consisting of fine particles by a method such as crystallization, classified as necessary, and then mixed or lightly pulverized while being mixed. Among them, in the first method, the method of mixing the raw materials and pulverizing them at the same time is an industrially advantageous method since it requires fewer steps. A conductive agent may also be added at the same time.

In any of the first to third methods, there is no particular restriction on the method of mixing the raw materials, and either wet mixing or dry mixing may be used. For example, a Henschel mixer, ultrasonic dispersion device, homomixer, mortar, ball mill, centrifugal ball mill, planetary ball mill, vibrating ball mill, attritor-type high-speed ball mill, bead mill, roll mill, etc. can be used.

If the mixture obtained by any of the first to third methods is a mixed powder, it can be directly subjected to the next firing step. If the mixture is a mixed slurry consisting of the mixed powder, it can be directly subjected to the next firing step after drying the mixed slurry using a rotary evaporator or the like. If firing is performed using a rotary kiln furnace, the mixed slurry can be directly subjected to the furnace.

<Firing 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 derived from the furnace tube, firing at a high temperature and for a short time is preferable. From this viewpoint, the maximum temperature during firing is preferably 1100°C or less, more preferably 1000°C or less, and even more preferably 960°C or less. From the viewpoint of reducing the proportion of specific impurity phases and increasing the crystallinity of lithium titanate, the maximum temperature during firing is preferably 800°C or more, more preferably 810°C or more. Similarly, from the above viewpoint, the holding time at the maximum 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 select a shorter holding time. Similarly, from the viewpoint of increasing the crystallite size obtained by firing, it is preferable to particularly shorten the residence time at 700°C to 800°C during the temperature rise process during firing, for example, within 15 minutes.

The firing method is not particularly limited as long as it can be performed under the above conditions. Examples of firing methods 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. However, for efficient firing in a short time, roller hearth firing furnaces, mesh belt firing furnaces, and rotary kiln firing furnaces are preferred. When using a roller hearth firing furnace or mesh belt firing furnace in which the mixture is placed in a sagger and fired, it is preferable to keep the amount of mixture placed in the sagger small in order to ensure uniformity in the temperature distribution of the mixture during firing and to ensure a constant quality of the resulting lithium titanate.

A rotary kiln-type firing furnace is particularly preferred for producing the lithium titanate powder of the present invention because it does not require a container to hold the mixture, the mixture can be continuously fed while firing, the thermal history of the fired material is uniform, and homogeneous lithium titanate can be obtained.

The atmosphere during firing is not particularly limited, regardless of the firing furnace, as long as it is an atmosphere that can remove desorbed moisture and carbon dioxide gas. Usually, an air atmosphere using compressed air is used, but an oxygen, nitrogen, or hydrogen atmosphere may also be used.

Although the lithium titanate powder after firing has slight agglomerations, it does not need to be pulverized to destroy the particles, and therefore after firing, it is sufficient to perform disintegration or classification to the extent that disintegration is broken down as necessary. If disintegration is only performed to the extent that disintegration is broken down without pulverization, the high crystallinity of the lithium titanate powder after firing is maintained even after that.

<Surface treatment process>
The lithium titanate powder of the present invention is a lithium titanate powder containing molybdenum and aluminum, and when used as an electrode material for an electric storage device, it can provide an effect of suppressing gas generation and an increase in resistance value during high-temperature operation. The lithium titanate powder of the present invention can be produced by adding a compound containing molybdenum and aluminum (hereinafter, sometimes referred to as a treatment agent) in the firing step, but more preferably, the lithium titanate powder of the present invention can be produced by the following surface treatment step or the like.

The lithium titanate powder before surface treatment obtained by the above steps (hereinafter, this may be referred to as the lithium titanate powder of the substrate. Also, hereinafter, the lithium titanate particles constituting the lithium titanate powder of the substrate may be referred to as the lithium titanate particles of the substrate) is mixed with treatment agent 1 and treatment agent 2, and is preferably heat-treated.

Compounds containing molybdenum (treatment agent 1) are not particularly limited, but examples thereof include oxides, hydroxides, sulfates, nitrates, fluorides, organic compounds, and metal salt compounds containing molybdenum. 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. In order to uniformly diffuse molybdenum on the particle surface of the lithium titanate powder, it is suitable to use the wet method described below. In this case, it is preferable to dissolve a molybdenum-containing compound that is soluble in a solvent in the solvent and mix it with the lithium titanate powder of the base material. In terms of suppressing the amount of gas generation, lithium molybdate containing molybdenum is preferable. Regarding the valence of molybdenum, even if it is added in a hexavalent state as a raw material, it is assumed that when chemical bonds are formed on the LTO surface or inside the crystal, dangling bonds exist and the valence is partially 4 to 5.

The amount of the molybdenum-containing compound (treatment agent 1) added may be any amount as long as the molybdenum content in the lithium titanate powder falls within the range of the present invention. For example, when lithium molybdate (Li ₂ MoO ₄ ) is used as treatment agent 1, from the viewpoint of achieving excellent initial discharge capacity, rate characteristics, electrode density, and volumetric energy density of the power storage device and further suppressing the amount of gas generated and increase in resistance value during high-temperature operation, treatment agent 1 may be added to the lithium titanate powder of the base material in a ratio of 0.12 mmol/100 g to 28.8 mmol/100 g of LTO, more preferably 0.5 mmol/100 g to 15.5 mmol/100 g of LTO, and particularly preferably 1.0 mmol/100 g to 10.0 mmol/100 g of LTO.

The aluminum-containing compound (treatment agent 2) is not particularly limited, but examples thereof include molybdenum oxides, hydroxides, sulfate compounds, nitrate compounds, fluorides, organic compounds, and metal salt compounds containing molybdenum. Specific examples of compounds containing Al include aluminum acetate, aluminum fluoride, and aluminum sulfate. Among these, aluminum sulfate, its hydrate, and aluminum fluoride are preferred.

The amount of the aluminum-containing compound (treatment agent 2) added may be any amount as long as the molybdenum content in the lithium titanate powder falls within the range of the present invention. However, for example, when aluminum sulfate 14-18 hydrate (Al ₂ (SO ₄ ) _{3.14-18H 2} O) is used, from the viewpoint of achieving excellent initial discharge capacity, rate characteristics, electrode density, and volumetric energy density of the power storage device and further suppressing the amount of gas generated and increase in resistance value during high-temperature operation, treatment agent 2 may be added to the lithium titanate powder of the substrate in a ratio of 0.16 mmol/100 g to 63.5 mmol/100 g of LTO, more preferably 3.2 mmol/100 g to 31.7 mmol/100 g of LTO, and particularly preferably 4.0 mmol/100 g to 15.9 mmol/100 g of LTO.

There is no particular restriction on the method of mixing the lithium titanate powder substrate with the molybdenum-containing compound (treatment agent 1), the aluminum-containing compound (treatment agent 2), and the compound containing element M (treatment agent 3), and either wet mixing or dry mixing can be used, but it is preferable to uniformly disperse the molybdenum-containing compound (treatment agent 1) or the aluminum-containing compound (treatment agent 2) on the surface of the lithium titanate particles substrate, and in this respect, wet mixing is preferred.

For dry mixing, for example, a paint mixer, a Henschel mixer, an ultrasonic dispersing device, 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. can be used.

For wet mixing, treatment agent 1, treatment agent 2, and the lithium titanate powder of the substrate are added to 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. Also, from an industrial perspective, water solvents are preferred because they are easy to recover and dispose of.

The amount of solvent is not a problem if the treatment agent 1, the treatment agent 2, and the lithium titanate particles of the substrate are sufficiently wetted, but it is preferable that the treatment agent 1, the treatment agent 2, and the lithium titanate particles of the substrate are uniformly dispersed in the solvent. For this purpose, the amount of the treatment agent 1 or the treatment agent 2 dissolved in the solvent is preferably 50% or more of the total amount of the treatment agent 1 or the treatment agent 2 put into the solvent. Since the amount of the treatment agent 1 or the treatment agent 2 dissolved in the solvent increases as the temperature increases, it is preferable to mix the lithium titanate powder of the substrate and the treatment agent 1 or the treatment agent 2 in the solvent while heating. Furthermore, since the amount of the solvent can be reduced by heating, the method of mixing while heating is suitable for industrial use. The temperature during mixing is preferably 40°C to 100°C, more preferably 60°C to 100°C.

In the case of wet mixing, although it depends on the heat treatment method, it is preferable to remove the solvent before the heat treatment performed after the mixing process. When removing the solvent, it is difficult to control the specific surface area and D50 of the primary particles and secondary particles by simply evaporating the solvent to dryness. Methods for evaporating the solvent to dryness include a method of heating and evaporating the slurry while stirring it with a stirring blade, a method using a drying device that can dry while stirring, such as a conical dryer, and a method using a spray dryer, but the method using a spray dryer is preferable. The reason is that, unlike conventional evaporation to dryness, the conditions of the spray dryer have a significant effect on the secondary particle morphology and the surface treatment process. The drying temperature of the spray dryer is preferably 180°C to 300°C, more preferably 200°C to 270°C, and even more preferably 215°C to 270°C. Since the powder property range of the secondary particles varies depending on the specific condition of the drying temperature, it is possible to increase the initial discharge capacity, rate characteristics, electrode density, and volumetric energy density as a result.

It is preferable to perform a heat treatment after mixing the lithium titanate powder of the substrate with the treatment agent 1 and further with the treatment agent 2. The heat treatment temperature is a temperature at which molybdenum or further element M diffuses into at least the surface region of the lithium titanate particles of the substrate, and at which the specific surface area does not decrease significantly due to sintering of the lithium titanate of the substrate. 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 more, more preferably 400°C or more. The heat treatment time is preferably 0.1 to 8 hours, more preferably 1 to 5 hours. The temperature and time at which element M diffuses into at least the surface region of the lithium titanate particles of the substrate should be set appropriately because the reactivity differs depending on the compound containing element M.

The heating method in 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. When a metal salt compound containing molybdenum or element M is used as the compound containing molybdenum or element M (treatment agent 2), an air atmosphere is preferred because it is easy to remove anion species from the particle surface.

Although the lithium titanate powder obtained after the heat treatment as described above has some degree of agglomeration, it 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.

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

In order to reduce the amount of moisture contained in the lithium titanate powder of the present invention, dew point control may be performed in the heat treatment process. If the heat-treated powder is exposed to the air as it is, the amount of moisture contained in the powder will increase, so it is preferable to handle the powder in a dew point controlled environment during cooling in the heat treatment furnace and after heat treatment. The heat-treated powder may be classified as necessary to make the particles within the desired maximum particle size range. When dew point control is performed in the heat treatment process, it is preferable to seal the lithium titanate powder of the invention in an aluminum laminate bag or the like and then place it in an environment outside of dew point control. Even under dew point control, if the lithium titanate powder after heat treatment is pulverized, it becomes easier to absorb moisture from the crushed surface, and the amount of moisture contained in the powder increases, so it is preferable not to pulverize it when heat treatment is performed. As for the heat treatment conditions, the temperature and holding time in a specific range greatly affect the secondary particle morphology and the surface treatment process. The heat treatment temperature is preferably 450°C or higher, and preferably less than 550°C. This is because if the heat treatment temperature exceeds 550°C, the specific surface area will be significantly reduced, and the battery performance, especially the rate characteristics, will be significantly reduced. Furthermore, a holding time of 11 hours or more is preferable, because if the heating time is short, not only will the amount of moisture in the powder increase, but it is thought that this will also affect the state of the particle surfaces.

[Active material]
The active material used in the electrode sheet 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 the 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.

[Electricity storage device]
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 of such a device include a hybrid capacitor and a lithium battery.

[Hybrid Capacitor]
The hybrid capacitor is a device in which an active material such as activated carbon, which forms a capacitance by physical adsorption similar to the electrode material of an electric double layer capacitor, an active material such as graphite, which forms a capacitance by physical adsorption and intercalation/deintercalation, or an active material such as a conductive polymer, which forms a capacitance by redox, is used for the positive electrode, and the active material of the present invention is used for the negative electrode. The active material of the present invention is usually used in the form of an electrode sheet for the hybrid capacitor.

[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 as a concept including so-called lithium ion secondary batteries and all-solid-state lithium secondary 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, 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 describes its use as a negative electrode active material.

<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 sheet. In the case of a negative electrode current collector having pores such as a porous body, the mixture layer contains a negative electrode active material (the active material of the present invention), a conductive agent, and a binder in the pores. When this mixture layer is used in an all-solid-state battery, the mixture layer may further contain a solid electrolyte material.

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. For example, graphites such as natural graphite (e.g., flake graphite), artificial graphite, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black, carbon nanotubes such as single-phase carbon nanotubes, multi-walled carbon nanotubes (graphite layers are multi-layered concentric cylinders) (non-fishbone-shaped), cup-stacked carbon nanotubes (fishbone-shaped), nodular carbon nanofibers (non-fishbone structure), and platelet-type carbon nanofibers (trump-shaped), and the like can be mentioned. In addition, graphites, carbon blacks, and carbon nanotubes may be appropriately mixed and used. Although not particularly limited, the specific surface area of carbon blacks is preferably 30 m ² /g to 3000 ^{m 2} /g, and more preferably 50 m ² /g to 2000 m ² /g. The specific surface area of the graphite is preferably 30 m ² /g to 600 ^{m 2} /g, and more preferably 50 m ² /g to 500 m ² /g. The aspect ratio of the carbon nanotubes is 2 to 150, preferably 2 to 100, and more preferably 2 to 50.

The amount of conductive agent added should be optimized because it varies depending on the specific surface area of the active material and the type and combination of conductive agents. It is preferably 0.1% to 10% by mass, and more preferably 0.5% to 5% by mass in the mixture layer. If it is less than 0.1% by mass, the conductivity of the mixture layer cannot be ensured, and if it exceeds 10% by mass, the active material ratio decreases and the discharge capacity of the electricity storage device per unit mass and unit volume of the mixture layer becomes insufficient, making it unsuitable for achieving high capacity.

Examples of the binder for the negative electrode include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP), styrene and butadiene copolymer (SBR), acrylonitrile and butadiene copolymer (NBR), and carboxymethyl cellulose (CMC). Although not particularly limited, the molecular weight of polyvinylidene fluoride is preferably 20,000 to 1,000,000. From the viewpoint of ensuring the binding property of the mixture 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.

The amount of binder added should be optimized because it varies depending on the specific surface area of the active material and the type and combination of conductive agents, but is preferably 0.2% by mass to 15% by mass in the mixture layer. From the viewpoint of increasing the binding property and ensuring the strength of the mixture layer, it 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 mixture layer, it is preferably 10% by mass or less, and more preferably 5% by mass or less.

Examples of the negative electrode current collector include aluminum, stainless steel, nickel, copper, titanium, baked carbon, or those whose surfaces are coated with carbon, nickel, titanium, or silver. The surfaces of these materials may be oxidized, or the surface of the negative electrode current collector may be roughened by surface treatment. Examples of the form of the negative electrode current collector include a sheet, net, foil, film, punched material, lath, porous material, foam, fiber group, and nonwoven fabric molded body. Porous aluminum is preferred as the form of the negative electrode current collector. The porosity of the porous aluminum is 80% or more and 95% or less, and preferably 85% or more.

The negative electrode can be produced by uniformly mixing the negative electrode active material (including the active material of the present invention), conductive agent, and binder in a solvent to form a paint, which is then applied to the negative electrode current collector, dried, and compressed. In the case of a negative electrode current collector that is porous or the like and has pores, the paint is pressed into the pores of the current collector to fill them, or the current collector having pores is immersed in the paint to diffuse the pores, and then dried and compressed.

Methods for uniformly mixing the negative electrode active material (the active material of the present invention), conductive agent, and binder in a solvent to produce a paint include, for example, a kneader in which a stirring rod revolves while rotating inside a kneading vessel such as a planetary mixer, a twin-screw extrusion type kneader, a planetary type stirring and defoaming device, a bead mill, a high-speed rotating type mixer, and a powder suction continuous dissolution and dispersion device. In addition, the manufacturing process may be divided into steps according to the solid content concentration, and these devices may be used separately.

In order to mix the negative electrode active material (the active material material of the present invention), the conductive agent, and the binder uniformly in the solvent, optimization is required since it depends on the specific surface area of the active material, the type of conductive agent, the type of binder, and the combination of these. However, when using a kneader such as a planetary mixer in which the stirring rod revolves while rotating in a kneading container, a twin-screw extrusion type kneader, or a planetary stirring and degassing device, it is preferable to divide the manufacturing process according to the solid content concentration, knead the mixture in a state where the solid content concentration is high, and then gradually reduce the solid content concentration to adjust the viscosity of the paint. The high solid content concentration is preferably 60% to 90% by mass, and more preferably 60% to 80% by mass. If it is 60% by mass or more, it is preferable because shear force can be obtained, if it is 90% by mass or less, it is preferable because the load on the device is reduced, and if it is 80% by mass or less, it is more preferable.

The mixing procedure is not particularly limited, but examples include a method of simultaneously mixing the negative electrode active material, the conductive agent, and the binder in a solvent, a method of mixing the conductive agent and the binder in a solvent in advance and then adding and mixing the negative electrode active material, and a method of preparing a negative electrode active material slurry, a conductive agent slurry, and a binder solution in advance and then mixing them. Among these, in order to disperse them uniformly, the method of mixing the conductive agent and the binder in a solvent in advance and then adding and mixing the negative electrode active material, and the method of preparing a negative electrode active material slurry, a conductive agent slurry, and a binder solution in advance and then mixing them are preferred.

As the solvent, an organic solvent can be used. Examples of the organic solvent include aprotic organic solvents such as 1-methyl-2-pyrrolidone, dimethylacetamide, and dimethylformamide, either alone or in combination of two or more kinds, and preferably 1-methyl-2-pyrrolidone.

When using an organic solvent, it is preferable to dissolve the binder in the organic solvent before use.

<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 absorbing and releasing lithium is used. For example, examples of the active material include composite metal oxides containing lithium and cobalt, manganese, or nickel, and lithium-containing olivine-type phosphates. These positive electrode active materials may be used alone or in combination of two or more. Examples of such composite metal oxides include _LiCoO2 , _LiMn2O4 , _{LiNiO2 ,} LiCo1 _{-xNixO2 (0.01<x<1), LiCo1/3Ni1 / 3Mn1 / 3O2 ,} LiNi1 _{/ 2Mn3 / 2O4} , and the like. These lithium composite oxides may be partially substituted with other elements, and the cobalt, manganese, and nickel may be partially substituted with at least one element such as B, Nb, Sn, Mg, Fe, Ti, Al, Zr, Cr, V, Ga, Zn, Cu, Bi, Mo, La, etc., or the O may be partially substituted with S or F, or the oxide may be coated with a compound containing these other elements. Examples of lithium-containing olivine-type phosphates include _LiFePO4 , LiCoPO4, _LiNiPO4 , _LiMnPO4 , LiFe1 _{-xMxPO4 (} wherein M is at least one selected from Co, _Ni , Mn, Cu, Zn, and Cd, and x is 0≦x≦0.5).

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 surface-treated with carbon, nickel, titanium, or silver. The surface of these materials may be oxidized, or the positive electrode current collector surface may be unevenly formed by surface treatment. Examples of the form of the current collector include a sheet, net, foil, film, punched, lath, porous body, foam, fiber group, and nonwoven fabric molded body.

<Non-aqueous electrolyte>
The non-aqueous electrolyte is a non-aqueous solvent having an electrolyte salt dissolved therein. There are no particular limitations on the type of non-aqueous electrolyte, and various types can be used.

_The electrolyte salt used is one that dissolves in a non-aqueous electrolyte, and examples of _the electrolyte salt include inorganic lithium salts such as _LiPF6 , _LiBF4 , _LiPO2F2 , _LiN ( _SO2F ) ₂ , and LiClO4 _{, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiCF3SO3, LiC(SO2CF3)3 , LiPF4 ( CF3 ) 2 , LiPF3 ( C2F5 ) 3 , LiPF3 ( CF3} ) ₃ , _LiPF3 (iso _{- C3F7 ) 3} , and _LiPF5 ( _iso - _{C3F7 ) .} Examples of the electrolyte salt include lithium salts containing a chain-like fluorinated alkyl group such as (CF ₂ ) ₂ (SO ₂ ) ₂ NLi and (CF ₂ ) ₃ (SO ₂ ) ₂ NLi, lithium salts containing a cyclic fluorinated alkylene chain such as (CF 2 ) 3 (SO 2 ) 2 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 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 alone or in combination of two or more. A suitable combination of these electrolyte salts is one in which the non-aqueous electrolyte contains _LiPF6 and further contains at least one lithium salt selected _{from LiBF4} , _LiPO2F2 , and LiN( _SO2F ) ₂ .

The concentration at which all of these electrolyte salts are dissolved and used in the nonaqueous solvent is usually preferably 0.3 M or more, more preferably 0.5 M or more, and even more preferably 0.7 M or more. The upper limit is preferably 2.5 M or less, more preferably 2.0 M or less, and even more preferably 1.5 M or less.

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 it is preferable to include cyclic carbonates. The term "chain ester" is used as a concept including chain carbonates and chain carboxylate esters.

Examples of cyclic carbonates 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). , 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 more preferred are one or more cyclic carbonates having an alkylene chain selected from propylene carbonate, 1,2-butylene carbonate, and 2,3-butylene carbonate. The proportion of cyclic carbonates having alkylene chains in all cyclic carbonates is preferably 55% to 100% by volume, and more preferably 60% to 90% by volume.

Therefore, the non-aqueous electrolyte is preferably prepared by dissolving an electrolyte salt containing at least one lithium salt selected from LiPF ₆ , LiBF 4 , LiPO ₂ F ₂ , and LiN(SO ₂ F) 2 in a non-aqueous solvent containing one or more cyclic carbonates 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 . The cyclic carbonate is more preferably one or more cyclic carbonates having an alkylene chain selected from propylene carbonate, 1,2-butylene carbonate, and 2,3-butylene carbonate.

In particular, it is preferable to use a non-aqueous electrolyte solution in which the concentration of the total electrolyte salt is 0.5M to 2.0M, the electrolyte salt contains at least LiPF ₆ , and further contains at least one lithium salt selected from 0.001M to 1M LiBF ₄ , LiPO ₂ F ₂ , and LiN(SO ₂ F) _2. If the ratio of lithium salts other than _{LiPF 6} in the non-aqueous solvent is 0.001M or more, the effect of improving the charge rate characteristics of the power storage device and suppressing the amount of gas generation during high-temperature operation is easily exhibited, and if it is 1.0M or less, there is little concern that the effect of improving the charge rate characteristics of the power storage device and suppressing the amount of gas generation during high-temperature operation will decrease, so this is preferable. The ratio of lithium salts other than LiPF ₆ in the non-aqueous solvent is preferably 0.01M or more, particularly preferably 0.03M or more, and most preferably 0.04M or more. The upper limit is preferably 0.8M or less, more preferably 0.6M or less, and particularly preferably 0.4M or less.

In addition, the non-aqueous solvents are preferably used in combination to achieve appropriate physical properties. Examples of such combinations include a combination of a cyclic carbonate and a chain carbonate, a combination of a cyclic carbonate, a chain carbonate, and a lactone, a combination of a cyclic carbonate, a chain carbonate, and an ether, a combination of a cyclic carbonate, a chain carbonate, and a chain ester, a combination of a cyclic carbonate, a chain carbonate, and a nitrile, and a combination of a cyclic carbonate, a chain carbonate, and an S=O bond-containing compound.

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).

Among the chain esters, chain esters having a methyl group selected from dimethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, methyl propionate, methyl acetate, and ethyl acetate (EA) are preferred, and chain carbonates having a methyl group are particularly preferred.

When chain carbonates are used, it is preferable to use two or more types. It is more preferable that both symmetric chain carbonates and asymmetric chain carbonates are included, and it is even more preferable that the content of symmetric chain carbonates is higher than that of asymmetric chain carbonates.

The content of the chain ester is not particularly limited, but is preferably in the range of 60% to 90% by volume relative to the total volume of the nonaqueous solvent. If the content is 60% by volume or more, the viscosity of the nonaqueous electrolyte will not become too high, and if it is 90% by volume or less, the electrical conductivity of the nonaqueous electrolyte will decrease, which will reduce the improvement of the charge rate characteristics of the electricity storage device and the effect of suppressing the amount of gas generation during high-temperature operation, so the above range is preferable.

The volume ratio of the symmetric chain carbonate in the chain carbonate is preferably 51% by volume or more, more preferably 55% by volume or more. The upper limit is more preferably 95% by volume or less, and even more preferably 85% by volume or less. It is particularly preferable that the symmetric chain carbonate contains dimethyl carbonate. It is also more preferable that the asymmetric chain carbonate has a methyl group, and methyl ethyl carbonate is particularly preferable. In the above case, the charging rate characteristics of the power storage device are improved, and the effect of suppressing the amount of gas generation during high-temperature operation is improved, which is preferable.

The ratio of cyclic carbonate to chain ester is preferably 10:90 to 45:55 (volume ratio), more preferably 15:85 to 40:60, and particularly preferably 20:80 to 35:65, from the viewpoint of improving the charging rate characteristics of the power storage device and suppressing the amount of gas generated during high-temperature operation.

<Structure of lithium battery>
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 square battery having a positive electrode, a negative electrode, and a roll-shaped separator.

As the separator, an insulating thin film having a large ion permeability and a certain mechanical strength is used. Examples include polyethylene, polypropylene, cellulose paper, glass fiber paper, polyethylene terephthalate, and polyimide microporous membranes, and multi-layer membranes consisting of a combination of two or more types can also be used. The surfaces 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 in a range that is generally useful for batteries, for example, 0.01 μm to 10 μm. The thickness of the separator may be in a range that is generally useful for batteries, for example, 5 μm to 300 μm.

<Solid electrolyte>
A solid electrolyte is a solid electrolyte that can move ions inside. In particular, inorganic solid electrolytes are solid in a stationary state, and therefore are not usually dissociated or liberated into cations and anions. The inorganic solid electrolyte is not particularly limited as long as it has the conductivity of metal ions belonging to the first group of the periodic table, and generally has almost no electronic conductivity. Representative examples of inorganic solid electrolytes include sulfide inorganic solid electrolytes and oxide inorganic solid electrolytes. In particular, sulfide solid electrolytes are preferably used because they have high ionic conductivity and can form a dense molded body with few grain boundaries by simply applying pressure at room temperature.

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.
Li ₂ S-P ₂ S ₅ , Li ₂ S-P ₂ S ₅ -Al ₂ S ₃ , Li ₂ S-GeS ₂ , Li ₂ S-Ga ₂ S ₃ , Li ₂ S-GeS ₂ -Ga ₂ S ₃ , Li ₂ S-GeS ₂ -P ₂ S ₅ , _Li2S -GeS2 _{-Sb2S5 , Li2S -} GeS2- _{Al2S3 , Li2S} - _SiS2 , _Li2S - _Al2S3 , _Li2S - _{SiS2 - Al2S3} , _{Li2S - SiS 2} -P ₂ S ₅ , Li _{10 GeP2S12 .}

Among the above combinations, preferred are LPS glass and LPS glass ceramics produced by combining Li ₂ S—P ₂ S _5. Other suitable sulfide inorganic solid electrolytes include argerodite-type solid electrolytes such as Li ₆ PS ₅ Cl and Li ₆ PS ₅ Br.

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

Examples of oxide inorganic solid electrolytes include Li _3.5 Zn _0.25 GeO ₄ having a LISICON (lithium super ionic conductor) type crystal structure, La _0.55 Li _0.35 TiO ₃ having a perovskite type crystal structure, LiTi ₂ P ₃ O ₁₂ having a NASICON (sodium super ionic 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 ), LiPON in which part of the oxygen in lithium phosphate is replaced with nitrogen, Li ₃ BO ₃ -Li ₂ SO ₄ , and Li ₂ O-B ₂ O ₃ Preferred examples thereof include —P ₂ O ₅ , Li ₂ O—SiO ₂ , and Li ₆ BaLa ₂ Ta ₂ O ₁₂ .

The volume average particle size of the inorganic solid electrolyte is not particularly limited, but is preferably 0.01 μm or more, and more preferably 0.1 μm or more. The upper limit is preferably 100 μm or less, and more preferably 50 μm or less.

Next, the present invention will be described 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 spirit of the invention.

[Example 1-1]
<Raw material preparation process>
Li ₂ CO ₃ (average particle size 4.6 μm) and anatase type TiO ₂ (specific surface area 10 m ² /g) were weighed to obtain a raw material powder with an atomic ratio of Li to Ti (Li/Ti) of 0.83, and ion-exchanged water was added to the raw material powder so that the solid content of the slurry was 41 mass % and stirred to prepare a raw material mixed slurry. This raw material mixed slurry was wet mixed and pulverized using a bead mill (Willi & Bachofen, model: Dyno Mill KD-20BC, agitator material: polyurethane, vessel inner surface material: zirconia) filled with zirconia beads (outer diameter: 0.65 mm) to 80 volume % in a vessel, and processed at an agitator peripheral speed of 13 m/s, a slurry feed rate of 55 kg/hr, and while controlling the vessel inner pressure to 0.02 to 0.03 MPa.

<Firing process>
The obtained mixed slurry was introduced into the furnace core tube from the raw material supply side of the 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 fired. At this time, the inclination angle of the furnace core tube from the horizontal direction was 2.5 degrees, the rotation speed of the furnace core tube was 20 rpm, the flow rate of nitrogen introduced into the furnace core tube from the fired product recovery side was 20 L/min, the heating temperature of the furnace core tube was 600 ° C. on the raw material supply side, 880 ° C. in the center, and 880 ° C. on the fired product recovery side, and the holding time of the fired product at 880 ° C. was 30 minutes.

<Post-processing step>
The sintered product recovered from the sintered product recovery side of the furnace tube was pulverized using a hammer mill (Dalton, AIIW-5 type) under the conditions of a screen opening of 0.5 mm, a rotation speed of 8,000 rpm, and a powder feed rate of 25 kg/hr.

<Surface treatment process>
Ion-exchanged water was added to the crushed fired powder so that the solid content of the slurry was 40% by mass, and the mixture was stirred. 2.75 mmol of lithium molybdate (Li ₂ MoO ₄ ) as treatment agent 1 and 5.08 mmol of aluminum sulfate 16-hydrate (Al ₂ (SO ₄ ) ₃ ·16H ₂ O) as treatment agent 2 were added to 100 g of the crushed fired powder 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 25,000 rpm and a drying temperature of 235°C, and granulated. Next, the powder that passed through the sieve was placed in an alumina sagger and heat-treated at 500°C for 1 hour in a mesh belt conveying continuous furnace equipped with a collection box at the outlet side, the temperature of which was controlled to be -20°C or lower at 25°C. The heat-treated powder was cooled in the recovery box and classified using a sieve (screen opening: 53 μm). The powder that passed through the sieve was collected in an aluminum laminate bag and sealed, and then removed from the recovery box to produce lithium titanate powder.

[Comparative Example 1-1]
In the sintering step, the heating temperature of the furnace tube was 840°C in the center and 840°C on the sintered product recovery side, the holding time of the sintered product at 840°C was 30 minutes, and in the surface treatment step, treatment agent 1 was not added, but aluminum sulfate 16-hydrate ( _Al2 ( _SO4 ) _3.16H2O ) was added as treatment agent 2 as shown in Table 1. Except for this _, lithium titanate powder was produced in the same manner as in Example 1.

[Comparative Example 1-2]
In the firing step, the heating temperature of the furnace tube was set to 840°C in the center and 840°C on the fired product recovery side, the holding time of the fired product at 840°C was set to 30 minutes, and aluminum sulfate hexadecahydrate (Al ₂ (SO ₄ ) _{3.16H 2} O) was added as shown in Table 1. Except for this, lithium titanate powder was produced in the same manner as in Example 1-1.

[Comparative Example 1-3]
A lithium titanate powder was produced in the same manner as in Comparative Example 1-2, except that neither Treatment Agent 1 nor Treatment Agent 2 was added in the surface treatment step.

[Measurement of Metal Element Content]
The contents of metal elements contained in the lithium titanate powders of Examples 1-1, 2-1 to 2-5 and Comparative Examples 1-1 to 1-3, and 2-1 to 2-3 (hereinafter sometimes referred to as the lithium titanate powders of each Example or Comparative Example) were measured as follows.

<X-ray fluorescence analysis (XRF): Identification of molybdenum, aluminum, and impurity content>
Using an X-ray fluorescence spectrometer (manufactured by SII Technology Co., Ltd., product name "SPS5100"), the elements contained in the lithium titanate powder of each Example and Comparative Example were quantitatively analyzed. The measurement sample was prepared by filling a precisely weighed sample with nitric acid and hydrofluoric acid, sealing it, irradiating it with microwaves to heat decompose it, and then adding ultrapure water to a constant volume to use as a test liquid. The amount of metal impurities was calculated from the mass ratio of the content of the elements obtained by ICP-AES. The amount of metal impurities is the total content of iron (Fe), chromium (Cr), and nickel (Ni).

[Measurement of powder properties]
Various physical properties of the lithium titanate powders of the respective Examples and Comparative Examples were measured as follows.

<Measurement of specific surface area>
The specific surface area ( ^m2 /g) of the lithium titanate powder in each Example and Comparative Example was measured using a fully automatic BET specific surface area measurement device (manufactured by Mountec Co., Ltd., product name "Macsorb HM model-1208") and nitrogen gas as the adsorption gas. 0.5 g of the measurement sample powder was weighed out and placed in a φ12 standard cell (HM1201-031), and degassed under vacuum at 100°C for 0.5 hours, and then measured by the BET single point method.

<Calculation of D50 of primary particles and secondary particles: Dry laser diffraction scattering method>
The D50 of the lithium titanate powder in each example and comparative example was calculated from the particle size distribution curve measured using a laser diffraction/scattering type particle size distribution measuring instrument (Microtrack MT3300EXII, manufactured by Nikkiso Co., Ltd.). 50 mg of the sample was put into a container containing 50 ml of ion-exchanged water as a measurement solvent, and the container was shaken by hand until the powder was visually uniformly dispersed in the measurement solvent, and the container was placed in a measurement cell and measured. The crushing process was performed by applying ultrasonic waves (30 W, 3 s) using an ultrasonic device in the device. Furthermore, the measurement solvent was added until the transmittance of the slurry was within the appropriate range (the range displayed by the green bar of the device), and the particle size distribution was measured. From the obtained particle size distribution curve, the D50 of the mixed powder before and after crushing was calculated. Note that the D50 before crushing corresponds to the D50 of the secondary particles, and the D50 after crushing corresponds to the D50 of the primary particles.

<Measurement of 1-methyl-2-pyrrolidone oil absorption>
The 1-methyl-2-pyrrolidone oil absorption of the lithium titanate powder in each Example and Comparative Example was measured using an Absorbtometer C (manufactured by Brabender). 20 g of lithium titanate powder was weighed and charged into the device casing, and then the stirring blade was rotated to make it uniform. Then, 1-methyl-2-pyrrolidone was added at a drop rate of 4.0 ml/min at a rotation speed of 125 rpm, and automatic operation was started. The torque increased and then decreased, and the measurement was terminated when the torque value reached 0 mNm. The oil absorption corresponding to the maximum torque value was measured from the obtained graph.

<pH Measurement>
The pH of the lithium titanate powder in each of the Examples and Comparative Examples was measured using a pH meter D-51 (manufactured by Horiba, Ltd.) A mixed solution was prepared by adding 5.0 g of lithium titanate powder and 45.0 g of pure water in a nitrogen atmosphere, and the pH of the mixed solution was measured after stirring for 5 minutes at a liquid temperature of 25°C.

<X-ray diffraction measurement>
X-ray diffraction measurements were performed on the lithium titanate powders of each Example and Comparative Example. An X-ray diffractometer using CuKα radiation (MiniFlex600 model, manufactured by Rigaku Corporation) was used as the measuring device. The measurement conditions for the X-ray diffraction measurements were as follows: measurement angle range (2θ): 10° to 90°, step interval: 0.01°, measurement time: 0.25 seconds/step, radiation source: CuKα radiation, tube voltage: 40 V, current: 15 mA. The structure was refined by Rietveld analysis, and the presence or absence of Li _0.6 Ti _3.4 O ₈ was confirmed as a crystalline component other than Li ₄ Ti ₅ O ₁₂ .

[Evaluation of battery characteristics]
The lithium titanate powders of the Examples and Comparative Examples were used to fabricate coin-type batteries, and the battery characteristics were evaluated. The evaluation results are shown in Tables 1 and 3.

<Preparation of negative electrode sheet>
The negative electrode sheet was prepared as follows in a room controlled at room temperature of 25 ° C. and a dew point of -20 ° C. or less. The lithium titanate powder of each example was taken out of 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 in a ratio of 90 mass% as an active material, 5 mass% as acetylene black as a conductive agent, and 5 mass% as polyvinylidene fluoride as a binder to prepare a paint. Polyvinylidene fluoride, acetylene black, and 1-methyl-2-pyrrolidone dissolved in 1-methyl-2-pyrrolidone in advance were mixed in a planetary stirring and defoaming device, and then lithium titanate powder was added, and the total solid concentration was adjusted to 64 mass%, and mixed in a planetary stirring and defoaming device. Then, 1-methyl-2-pyrrolidone was added, and the total solid concentration was adjusted to 50 mass%, and mixed in a planetary stirring and defoaming device to prepare a paint. The obtained coating material was applied onto an aluminum foil and dried to prepare a negative electrode single-sided sheet for a coin battery described later and a negative electrode double-sided sheet for a laminate battery described later. The target coating weight was 7.5 mg/ ^cm2 .

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

<Preparation of Positive Electrode Sheet>
A positive electrode single-sided sheet was prepared in the same manner as described in <Preparation of Negative Electrode Sheet> above, 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.

<Preparation of Electrolyte Solution>
The electrolyte 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 LiPF ₆ was dissolved in this as an electrolyte salt to a concentration of 1M to prepare the electrolyte for the coin battery described below. Similarly, a non-aqueous solvent of propylene carbonate (PC): diethyl carbonate (DEC) = 1: 2 (volume ratio) was prepared, and LiPF ₆ was dissolved in this as an electrolyte salt to a concentration of 1.3M to prepare the electrolyte for the laminate battery described below.

<Preparation of coin battery>
The negative electrode single-sided sheet prepared by the above method was punched out into a circle with a diameter of 14 mm, pressed at a pressure of 2 t/ ^cm2 , and then vacuum dried at 120°C for 5 hours to prepare an evaluation electrode. The evaluation electrode prepared and metallic lithium (molded into a circle with a thickness of 0.5 mm and a diameter of 16 mm) were placed opposite each other via glass filters (one each of GA-100 manufactured by ADVANTEC and GF/C manufactured by Whatman), 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.

<Preparation of Laminated Battery>
The laminated battery was prepared as follows in a room controlled at a room temperature of 25°C and a dew point of -40°C or less. The negative electrode double-sided sheet was pressed at a pressure of 2t/ ^cm2 , and then a negative electrode having a lead wire connection portion was prepared. The positive electrode double-sided sheet was pressed at a pressure of 2t/ ^cm2 , and then a positive electrode having a lead wire connection portion was prepared. The prepared negative electrode and positive electrode were vacuum-dried at 150°C for 12 hours. The vacuum-dried positive electrode and negative electrode were faced to each other via a separator (UPZ210, manufactured by Ube Industries), laminated, and aluminum foil leads were connected to the positive electrode and negative electrode, respectively. The laminated battery electrolyte prepared by the method described in the above <Preparation of electrolyte> was added and the laminated battery was vacuum-sealed with an aluminum laminate to prepare a laminated battery for evaluation. At this time, the capacity of the battery was 350mAh, and the ratio of the capacity of the negative electrode to the capacity of the positive electrode (negative electrode capacity/positive electrode capacity) was 1.2.

<Measurement of initial discharge capacity, volumetric energy density, and rate characteristics>
In a thermostatic chamber at 25°C, the coin-type battery prepared by the method described above in <Preparation of coin battery> was charged to 1V at a current density of 0.2mA/ ^cm2 with the direction in which Li is absorbed in the evaluation electrode as charging, and then constant current constant voltage charging was performed at 1V until the charging current reached a current density of 0.05mA/ ^cm2 , and then constant current discharging was performed for three cycles at a current density of 0.2mA/ ^cm2 to 2V. The discharge capacity (mAh) at the third cycle was divided by the weight of lithium titanate to obtain the initial discharge capacity (mAh/g). The volumetric energy density was obtained by multiplying this initial discharge capacity by the electrode density obtained above. A high volumetric energy density increases the capacity available per certain volume as a battery, which is preferable in terms of miniaturization of the battery. Next, the battery was charged to 1V at a current equivalent to 1C of the initial discharge capacity, and then discharged to 2V at a current of 5C to obtain the 5C discharge capacity. The rate characteristic (%) was calculated by dividing the 5C discharge capacity by the initial discharge capacity. If the 5C rate capacity percentage of the lithium titanate powder is high, it is expected that the charge rate characteristic of the power storage device will be improved when it is used as an electrode material for the power storage device. The evaluation results are shown in Table 1. The C in 1C represents the current value when charging and discharging. For example, 1C refers to the current value at which the theoretical capacity can be fully discharged (or fully charged) in 1/1 hour, and 0.1C refers to the current value at which the theoretical capacity can be fully discharged (or fully charged) in 1/0.1 hour.

<Measurement of gas generation amount after 50 cycles at 60°C>
In a thermostatic chamber at 25° C., the laminate battery prepared by the method described above in <Preparation of Laminate Battery> was charged to 2.75 V at a current of 0.2 C, and further subjected to constant-current constant-voltage charging in which the battery was charged at 2.75 V until the charging current became 0.05 C, and then constant-current discharging was repeated three cycles in which the battery was discharged to 1.4 V at a current of 0.2 C. Thereafter, the volume of the laminate battery was measured by the Archimedes method, and this was taken as the initial volume of the laminate battery (hereinafter sometimes referred to as the initial volume).

Next, in a thermostatic chamber at 60°C, the battery was charged to 2.75V at a current of 1C, and then further charged at 2.75V until the charging current became 0.05C, followed by constant-current constant-voltage charging, in which the battery was discharged to 1.4V at a current of 1C for 50 cycles, in a cycle test.

After 50 cycles of the cycle test, the volume of the laminated battery was measured by Archimedes' method, and this was taken as the volume of the laminated battery after the cycle test (hereinafter, sometimes referred to as the volume after the cycle test). The amount of gas generated (ml) was calculated by subtracting the initial volume from the volume after the cycle test, and converted to the amount of gas generated (ml/Ah) after 50 cycles of the test. The relative values when the amount of gas generated in Comparative Example 1-2 was taken as 100% are shown in Table 1. (In this specification, this is sometimes referred to as the amount of gas generated (%) at 60°C cycle.)

<Measurement of battery resistance after long-term storage at 60° C.>
A laminate battery prepared by the method described above in <Preparation of Laminated Battery> was charged to 2.75 V at a current of 0.2 C in a thermostatic chamber at 25° C., and then a storage test was carried out for 30 days in a thermostatic chamber at 60° C. After the storage test was completed, the battery was removed and AC impedance was measured in a thermostatic chamber at 25° C. The resistance value (Ω) corresponding to a frequency of 1 kHz was read. Table 1 shows the relative values when the resistance value after storage in Comparative Example 1-2 is taken as 100%. (In this specification, this may be referred to as resistance value % after storage at 60° C.)

<Evaluation Results>
The electrode using the lithium titanate powder of Example 1-1 was excellent in initial discharge capacity, rate characteristics, electrode density, and volume energy density, and further, the amount of gas generated and the increase in resistance value during high-temperature operation could be significantly suppressed. On the other hand, when lithium titanate powder synthesized by the same manufacturing method as in Example 1-1 but not containing either molybdenum or aluminum on the surface was used (Comparative Examples 1-1 and 1-3), the amount of gas generated and the resistance value during high-temperature operation increased. In addition, when lithium titanate powder containing molybdenum and aluminum on the surface but synthesized by a manufacturing method different from that of Example 1-1 and having a 1-methyl-2-pyrrolidone oil absorption of more than 71 mL/100 g was used (Comparative Example 1-2), the electrode density became small and the volume energy density became low. From this result, it was found that lithium titanate containing molybdenum and aluminum and exhibiting a specific 1-methyl-2-pyrrolidone oil absorption exhibited particularly good battery characteristics. It is presumed that the performance has improved significantly due to the change in oil absorption reflecting the change in the surface condition, rather than relying solely on the D50 of the primary particles or the D50 of the secondary particles.

<X-ray photoelectron spectroscopy (XPS) analysis results>
The valence distribution of Mo present on the primary particle surface was measured for the lithium titanate powders of Example 1-1 and Comparative Example 1-2 using a scanning X-ray photoelectron spectrometer PHI5000 manufactured by ULVAC-PHI. After each sample was sampled on an Al plate, measurements were performed using an X-ray source AlKα (monochrome, 1486.6 eV, 50 W), an analysis area of 200 μmφ, and a charge neutralization mechanism (electron gun + Ar ions). The results of peak separation calculations (using a peak separation program on MATLAB (registered trademark)) for the obtained Mo3d spectrum are shown below.

It was confirmed that there was a difference in the valence distribution ratio of Mo ⁶⁺ , Mo ⁵⁺ , and Mo ⁴⁺ between the lithium titanate powders of Example 1-1 and Comparative Example 1-2. An example of peak separation relating to the valence distribution of Mo is shown in FIG.

[Effect of manufacturing process conditions for lithium titanate powder]
[Examples 2-1 to 2-5, Comparative Examples 2-1 to 2-3]
A lithium titanate powder was produced in the same manner as in Example 1-1, except that the raw material preparation, surface treatment step, and secondary particle formation step were carried out under the conditions shown in Table 3.

<Evaluation Results>
It was found that the electrodes using the lithium titanate powders of Examples 2-1 to 2-5 contain molybdenum and aluminum on the surface, and by controlling the 1-methyl-2-pyrrolidone oil absorption to a specific range, the electrode density, initial discharge capacity, volumetric energy density, and rate characteristics can be improved, and the battery characteristics can be maintained. On the other hand, the lithium titanate powders of Comparative Examples 2-1 to 2-3 exhibited low electrode density characteristics, and did not achieve an improvement in volumetric energy density. In particular, it was found that high electrode density was exhibited when the specific surface area was 6.1 m ² /g or less, the D50 of the primary particles was 0.7 μm or more, and the D50 of the secondary particles was 11 μm or more. The long-term characteristics of the batteries of Examples 2-1 to 2-5 showed the same tendency as Example 1-1.

The lithium titanate powder obtained by the present invention is useful as an electrode active material for lithium ion secondary batteries because it improves electrode density and volumetric energy density and suppresses the amount of gas generated and the increase in resistance value during high-temperature operation. Furthermore, lithium ion secondary batteries using this lithium titanate as an electrode active material are capable of stable high-speed charging and discharging, and are useful as power storage devices such 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 (6)

A lithium titanate powder mainly composed of Li ₄ Ti ₅ O ₁₂ , the lithium titanate powder comprising primary particles and secondary particles of lithium titanate, a 1-methyl-2-pyrrolidone oil absorption of 57 mL/100 g to 71 mL/100 g , and molybdenum (Mo) and aluminum (Al) present on the surfaces of the primary particles. 2. The lithium titanate powder according to claim 1, wherein the specific surface area of the lithium titanate powder is 6.1 ^{m 2} /g or less. The lithium titanate powder according to claim 1 or 2, characterized in that the particle size D50 of the primary particles corresponding to 50% cumulative volume in the volume-based particle size distribution measured by the laser diffraction scattering method is 0.7 μm or more. The lithium titanate powder according to any one of claims 1 to 3, characterized in that the particle size D50 of secondary particles corresponding to a cumulative volume of 50% in the volume-based particle size distribution measured by the laser diffraction scattering method is 11 μm or more. An electrode sheet comprising the lithium titanate powder according to any one of claims 1 to 4. An electricity storage device comprising the electrode sheet according to claim 5.

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