JP7743904B2 - Niobium-containing oxide powder, electrode using same, and power storage device - Google Patents

Description

The present invention relates to niobium-containing oxide powder suitable as an electrode material for electricity storage devices, electrodes using the same, and electricity storage devices.

Electric vehicle energy storage devices require high energy density in order to improve fuel economy and power consumption, and lithium-ion batteries in particular are widely used as power sources for electric vehicles and power storage. Various materials are being researched as electrode materials for lithium-ion batteries, and while lithium titanate has excellent input/output characteristics, its energy density remains at 175 mAh/g, leaving challenges in achieving even higher energy. Therefore, as an alternative candidate, there is a movement to use niobium-containing oxides, primarily niobium titanate, which has a high energy density of 380 mAh/g, as anode materials.

In addition, currently commercially available lithium-ion batteries use electrolytes containing flammable organic solvents, making it necessary to install safety devices to suppress temperature increases in the event of a short circuit, as well as short-circuit prevention structures. Under these circumstances, all-solid-state secondary batteries that use inorganic solid electrolytes instead of organic electrolytes are attracting attention. Because the positive and negative electrodes and electrolytes of all-solid-state secondary batteries are all solid, they have the potential to significantly improve the safety and reliability issues associated with batteries that use organic electrolytes. Furthermore, because safety devices can be simplified, higher energy densities are possible, making them promising for use in electric vehicles, large-scale storage batteries, and other applications.

Non-Patent Documents 1 and 2 disclose porous Ti 2 Nb 14 O 39 and Ti ₂ Nb 14 O ₃₉ produced by a solid-state method, which are niobium-titanium composite oxides and are composed of primary particles with a particle size range of ₂₀ to 200 nm, produced by a solvothermal method, and Ti ₂ Nb ₁₄ O ₃₉ produced by a solid-state method, _and report that when used as an electrode material for an electricity storage device, Ti ₂ Nb 14 O ₃₉ produced by a solvothermal method in particular has excellent rate characteristics. Furthermore, Patent Document 1 discloses TiNb ₂ O ₇ and TiNb ₁₀ O ₂₉ containing a trivalent metal produced by a solvothermal method, and reports that they have smaller capacity loss with cycling than oxides that do not contain a trivalent metal.

Small 2017, 13, 1702903 Journal of Colloid and Interface Science 608 (2022) 90－102

International Publication No. 2014/122376

However, when the porous Ti ₂ Nb ₁₄ O ₃₉ described in Non-Patent Documents 1 and 2, which is composed of 20-200 nm primary particles, is used as a negative electrode material, it contains a large amount of conductive additive to ensure conductivity even within the secondary particles. As a result, the active material content in the negative electrode mixture is reduced, resulting in a low energy density per negative electrode. Furthermore, the very large specific surface area significantly reduces the initial efficiency. Furthermore, when such porous particles are applied to all-solid-state batteries, the solid electrolyte cannot contact the interior of the secondary particles, making it impossible to obtain sufficient capacity. Furthermore, when the Ti ₂ Nb ₁₄ O ₃₉ prepared by the solid-state method described in Non-Patent Document 1 is evaluated in a negative electrode with a higher active material content and a lower conductive additive content, there is a problem of reduced charge/discharge rate characteristics. Although battery performance was improved by controlling the particle size and specific surface area of _{Ti 2} Nb ₁₄ O ₃₉ , the results were not fully satisfactory. On the other hand, in the batteries using _TiNb2O7 or _Ti2Nb10O29 containing trivalent metals described in Patent Document 1, although the _rate characteristics were improved _, there was a problem that the battery capacity (battery capacity) significantly decreased with cycling. After various investigations into the cause of this, it was found _{that TiNb2O7} or _Ti2Nb10O29 containing _{trivalent metals} have a large change in electrode thickness with charge _and discharge, resulting in a decrease in cycle performance. The advantage of increasing battery capacity is that it leads to an improvement in energy density per unit weight or unit area (unit volume), which can extend the driving range of electric vehicles and secure installation space for storage batteries.

For these reasons, the energy density, discharge rate characteristics, charge rate characteristics, and electrode thickness changes associated with charge and discharge of energy storage devices using the negative electrode active materials of Non-Patent Document 1, Non-Patent Document 2, and Patent Document 1 are not fully satisfactory, and further improvements are required.

The present invention therefore aims to provide a niobium-containing oxide powder that can be used as an electrode material for non-aqueous electrolyte electricity storage devices, and that exhibits excellent rate characteristics while maintaining initial discharge capacity in liquid-based lithium-ion secondary batteries, a small rate of change in electrode thickness during charge and discharge, and excellent initial discharge capacity and rate characteristics in all-solid-state batteries; an electrode for an electricity storage device using the same; and an electricity storage device using the same.

As a result of extensive investigations to achieve the above-mentioned object, the present inventors have found that by using, as an electrode material, a niobium-containing oxide powder in which part of the Ti in a _{niobium -} containing oxide such as _Ti2Nb14O39 is substituted with a different element, it is possible to obtain an electricity storage device that is excellent in initial discharge capacity and rate characteristics and that further has a small rate of change in electrode thickness with charge and discharge, and have completed the present invention. Such an effect, in particular the effect of being able to reduce the change in electrode thickness with charge and discharge, is not described or suggested at all in the above-mentioned Non-Patent Documents 1 and 2 and Patent Document 1. That is, the present invention relates to the following matters.

(1) A niobium-containing oxide powder satisfying the following formula (I) or (II):
A _a Ti _2-x (M ¹ _0.5+v M ² _0.5-v ) _x Nb ₁₄ O _39±z (I)
A _a Ti _2-y (M ¹ _0.67+w M ³ _0.33-w ) _y Nb ₁₄ O _39±z (II)
(In the formula, A is at least one element selected from Li and Na, ^M1 is at least one element selected from Nb, Ta, and V, ^M2 is at least one element selected from Al, Ga, Fe, and Cr, ^M3 is at least one element selected from the group consisting of Mg, Ca, Cu, and Zn, and 0≦a≦6, −0.05≦v≦0.05, −0.05≦w≦0.05, 0<x<2, 0<y<2, 0≦z≦1)
(2) The niobium-containing oxide powder according to (1), characterized in that D50 of primary particles corresponding to 50% cumulative volume in a volume-based particle size distribution measured by a laser diffraction scattering method of the niobium-containing oxide powder is 0.25 μm or more and 4.5 μm or less.
(3) The niobium-containing oxide powder according to (1) or (2), wherein the specific surface area of the niobium-containing oxide powder is 2.3 m ² /g or more and 10 m ² /g or less.
(4) An electrode for a non-aqueous electrolyte electricity storage device, characterized by containing the niobium-containing oxide powder according to any one of (1) to (3).
(5) A negative electrode active material composition for a non-aqueous electrolyte storage device, comprising the niobium-containing oxide powder according to any one of (1) to (3) and an inorganic solid electrolyte.
(6) An electricity storage device comprising the electrode according to (4).
(7) An all-solid-state secondary battery comprising a positive electrode layer, a negative electrode layer, and a solid electrolyte layer, wherein the negative electrode layer is a layer containing the negative electrode active material composition according to (5).

The present invention provides niobium-containing oxide powder, an electrode for a storage battery device using the same, and a storage battery device that exhibits excellent charge-discharge rate characteristics while maintaining initial discharge capacity in a liquid lithium-ion secondary battery for a non-aqueous electrolyte storage device, and exhibits minimal change in electrode thickness during charge-discharge. It also provides an excellent initial discharge capacity and rate characteristics in an all-solid-state battery.

[Niobium-containing oxide powder of the present invention]
The niobium-containing oxide powder of the present invention is a niobium-containing oxide powder that satisfies the following formula (I) or (II).
A _a Ti _2-x (M ¹ _0.5+v M ² _0.5-v ) _x Nb ₁₄ O _39±z (I)
A _a Ti _2-y (M ¹ _0.67+w M ³ _0.33-w ) _y Nb ₁₄ O _39±z (II)
(In the formula, A is at least one element selected from Li and Na, ^M1 is at least one element selected from Nb, Ta, and V, ^M2 is at least one element selected from Al, Ga, Fe, and Cr, ^M3 is at least one element selected from the group consisting of Mg, Ca, Cu, and Zn, and 0≦a≦6, −0.05≦v≦0.05, −0.05≦w≦0.05, 0<x<2, 0<y<2, 0≦z≦1)
The niobium-containing oxide powder of the present invention is suitable for use in electrodes of nonaqueous electrolyte electricity storage devices. That is, the niobium-containing oxide powder of the present invention is suitable as a niobium-containing oxide powder for electrodes of nonaqueous electrolyte electricity storage devices. The nonaqueous electrolyte may be any electrolyte other than an aqueous electrolyte, and is not particularly limited, but examples thereof include nonaqueous electrolytic solutions and solid electrolytes.

<Niobium-containing oxide powder represented by the general formula A _a Ti _2-x (M ¹ _0.5+v M ² _0.5-v ) _x Nb ₁₄ O _39±z and the general formula A _a Ti _2-y (M ¹ _0.67+w M ³ _0.33-w ) _y Nb ₁₄ O _39±z >
The niobium-containing oxide powder of the present invention has a general formula A _a Ti _2-x (M ¹ _0.5+v M ² _0.5-v ) _x Nb ₁₄ O _39±z or a general formula A _a Ti _2-y (M ¹ _0.67+w M ³ _0.33-w ) _y Nb ₁₄ O _39±z (wherein A is at least one element selected from Li and Na, M ¹ is at least one element selected from Nb, Ta and V, M ² is at least one element selected from Al, Ga, Fe and Cr, and M ³ is at least one element selected from the group consisting of Mg, Ca, Cu, and Zn, and contains a niobium-containing oxide powder expressed by 0≦a≦6, −0.05≦v≦0.05, −0.05≦w≦0.05, 0<x<2, 0<y<2, 0≦z≦1). The niobium-containing oxide may partially contain a titanium oxide phase (e.g., rutile-type TiO ₂ , TiO, etc.) derived from the synthesis raw material.

The A is not particularly limited as long as it is at least one element selected from Li and Na, but is preferably Li.
The ^M1 is not particularly limited as long as it is at least one element selected from Nb, Ta, and V, but is preferably Nb.

The ^M2 is not particularly limited as long as it is at least one element selected from Al, Ga, Fe, and Cr, but is preferably Al, Fe, and Cr, more preferably Al and Fe, and even more preferably Al.

The ^M3 is not particularly limited as long as it is at least one element selected from the group consisting of Mg, Ca, Cu, and Zn, but is preferably Mg, Cu, and Zn, more preferably Cu and Zn, and even more preferably Cu.

In the general formula A _a Ti _2-x (M ¹ _0.5+v M ² _0.5-v ) _x Nb ₁₄ O 39 _±z and the general formula A _a Ti _2-y (M ¹ _0.67+w M ³ _0.33-w ) _y Nb ₁₄ O _39±z , 0≦a≦6, preferably 0≦a≦2, more preferably 0≦a≦0.5, and even more preferably a=0. Also, -0.05≦v≦0.05, preferably -0.01≦v≦0.01, and more preferably v=0. Also, -0.05≦w≦0.05, preferably -0.01≦w≦0.01, and more preferably w=0. Furthermore, 0<x<2, preferably 0<x≦1.3, more preferably 0<x≦1, even more preferably 0<x≦0.9, and even more preferably 0<x≦0.5. Furthermore, 0<y<2, preferably 0<y≦1.4, more preferably 0<y≦0.9, and even more preferably 0<y≦0.6. Furthermore, 0≦z≦1, preferably 0≦z≦0.5, more preferably 0≦z≦0.2, and even more preferably z=0. In the general formula A _a Ti _2-y (M ¹ _0.67+w M ³ _0.33-w ) _y Nb ₁₄ O _39±z , the ratio of M ¹ to M ³ is M ¹ :M ³ =0.67+w:0.33-w, but it may also be M ¹ :M ³ =2/3+w:1/3-w. That is, in this case, when w=0, M ¹ :M ³ =2/3:1/3 (that is, A _a Ti _2-y (M ¹ _2/3 M ³ _1/3 ) _y Nb ₁₄ O _39±z ).

<Containing at least one metal element selected from the group consisting of Mo and Ce>
The niobium-containing oxide powder of the present invention may contain at least one metal element selected from the group consisting of Mo and Ce. "Containing at least one metal element selected from the group consisting of Mo and Ce" means that the at least one metal element selected from the group consisting of Mo and Ce is localized and present on the surface of the niobium-containing oxide particles constituting the powder, more specifically, that at least one metal element selected from the group consisting of Mo and Ce is localized and present in greater amounts in the surface region than in the internal region of the niobium-containing oxide particles. Furthermore, this means that at least one metal element selected from the group consisting of Mo and Ce is detected in inductively coupled plasma atomic emission spectroscopy (ICP-AES) or X-ray fluorescence spectroscopy (XRF) of the niobium oxide powder of the present invention. The lower limit of the amount detected by inductively coupled plasma atomic emission spectroscopy is typically 0.001% by mass. Mo and Ce may both be contained on the particle surface of the niobium-containing oxide powder. From the viewpoint of improving initial discharge capacity and rate characteristics, it is preferable to contain Mo.

The content (mass%) of at least one metal element selected from the group consisting of Mo and Ce in the niobium-containing oxide powder of the present invention, as determined by X-ray fluorescence analysis (XRF), may be 0.01 to 2.0. This range of content of at least one metal element selected from the group consisting of the metal elements Mo and Ce results in a secondary battery with excellent initial discharge capacity and rate characteristics. A content of 0.01 to 1.7 is preferred, and from the perspective of further improving charge rate characteristics, a content of 0.015 to 1.5 is more preferred, 0.04 to 1.3 is even more preferred, 0.07 to 1.25 is even more preferred, and 0.2 to 1.1 is particularly preferred. However, when Mo and Ce are simultaneously contained on the particle surfaces of the niobium-containing oxide powder, the content (mass%) refers to the combined content of the two metal elements.

As an example, in a cross-sectional analysis of the niobium-containing oxide particles using a scanning transmission electron microscope, it is sufficient that at least one metal element selected from the group consisting of Mo and Ce is contained in a large amount in the so-called near-surface region from the surface of the niobium-containing oxide particles to a depth of about 20 nm as measured by energy dispersive X-ray spectroscopy. At a depth of 20 nm from the surface of the niobium-containing oxide particles, at least one metal element selected from the group consisting of Mo and Ce is detected, while at a depth of 100 nm from the surface, it is preferable that Mo and Ce are not detected. In such a state, it can be determined that at least one metal element selected from the group consisting of Mo and Ce is localized on the surface of the niobium-containing oxide particles. That is, when measured by energy dispersive X-ray spectroscopy, it means that the amount is below the amount detected by the measurement. The lower limit of the amount detected by energy dispersive X-ray spectroscopy varies depending on the element and state to be measured, but is usually 0.5 atm%. Other surface analysis techniques include X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES).
In the present invention, the form of the at least one metal element selected from the group consisting of Mo and Ce that is localized on the surface of the niobium-containing oxide particle is not particularly limited, and it is sufficient that the at least one metal element selected from the group consisting of Mo element and Ce element is localized on the surface, and the at least one metal element may be in a metallic state or in the form of a metal compound such as a metal oxide.

<D50>
The D50 of the primary particles of the niobium-containing oxide 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 determined by laser diffraction/scattering particle size distribution measurement reaches 50% when calculated from the smallest particle size. The measurement method will be explained in the examples below.

The niobium-containing oxide powder of the present invention may be in the form of primary particles or secondary particles formed by agglomeration of primary particles. When primary particles made of niobium-containing oxide particles contain secondary particles formed by agglomeration, some of them may not form secondary particles and may remain in the form of primary particles themselves.

When the niobium-containing oxide powder of the present invention is in the form of secondary particles, from the viewpoint of improving electrode density, the lower limit of the D50 of the secondary particles is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 9 μm or more. Furthermore, the upper limit of the D50 of the secondary particles is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 14 μm or less.

From the viewpoint of improving the initial discharge capacity and rate characteristics in liquid-type lithium-ion secondary batteries of nonaqueous electrolyte storage devices, and also the initial discharge capacity and rate characteristics in all-solid-state batteries, the D50 of the primary particles of the niobium-containing oxide powder of the present invention is preferably 0.25 μm or more, more preferably 0.4 μm or more, and even more preferably 0.6 μm or more. The upper limit of the D50 of the primary particles is preferably 4.5 μm or less, more preferably 4 μm or less, even more preferably 3 μm or less, even more preferably 2 μm or less, and particularly preferably 1.8 μm or less. The niobium-containing oxide powder may contain primary particles of less than 0.4 μm, or may contain primary particles of greater than 4.5 μm.

<D90>
In terms of improving the initial discharge capacity and rate characteristics of liquid-based lithium-ion secondary batteries in nonaqueous electrolyte storage devices, and the initial discharge capacity and rate characteristics of all-solid-state batteries, the D90 of the primary particles of the niobium-containing oxide powder of the present invention is preferably 0.8 μm or more, more preferably 1.0 μm or more, and even more preferably 1.2 μm or more. The upper limit of the D90 of the primary particles is preferably 20 μm or less, more preferably 15 μm or less, even more preferably 10 μm or less, even more preferably 5 μm or less, and particularly preferably 3 μm or less. The D90 of the primary particles of the niobium-containing oxide powder refers to the particle size at which the cumulative volume frequency calculated from the volume fraction determined by laser diffraction/scattering particle size distribution measurement is 90% when calculated from the smallest particle size. The measurement method will be described in the Examples below.

<D10>
From the viewpoint of improving the initial discharge capacity and rate characteristics of a liquid-based lithium-ion secondary battery in a nonaqueous electrolyte storage device and the initial discharge capacity and rate characteristics of an all-solid-state battery, the D10 of the primary particles of the niobium-containing oxide powder of the present invention has a lower limit of preferably 0.05 μm or more, more preferably 0.1 μm or more, and even more preferably 0.2 μm or more. The upper limit of the D10 of the primary particles is preferably 0.9 μm or less, more preferably 0.8 μm or less, even more preferably 0.7 μm or less, and even more preferably 0.6 μm or less. The D10 of the primary particles of the niobium-containing oxide powder refers to the particle size at which the cumulative volume frequency calculated from the volume fraction determined by laser diffraction/scattering particle size distribution measurement is 10% from the smallest particle size. The measurement method will be described in the Examples below.

<Specific surface area>
The specific surface area of the niobium-containing oxide powder of the present invention is the surface area per unit mass when nitrogen is used as an adsorption gas. The measurement method will be explained in the examples below.

The niobium-containing oxide powder of the present invention has an upper limit of the specific surface area of 10 m ² /g or less, preferably 8.0 m ² /g or less, more preferably 6.0 m ² /g or less, and even more preferably 4.0 m ² /g or less. On the other hand, the lower limit of the specific surface area is 1.5 ^{m 2} /g or more, preferably 2.0 m ² /g or more, more preferably 2.1 m ² /g or more, even more preferably 2.4 m ² /g or more, and even more preferably 3.0 m ² /g or more. Within the above ranges, a non-aqueous electrolyte electricity storage device having excellent initial discharge capacity and rate characteristics can be obtained in a liquid lithium ion secondary battery, and an all-solid-state battery having excellent initial discharge capacity and rate characteristics can be obtained.

The reason why the use of a niobium-containing oxide in which a portion of the Ti in Ti ₂ Nb ₁₄ O ₃₉ is substituted with a different element exhibits such remarkable effects is unclear, but it is thought to be as follows. Generally, negative electrode active materials, such as niobium-containing oxide powders such as Ti ₂ Nb ₁₄ O ₃₉ , exhibit capacity by absorbing and releasing lithium ions via an electrolyte solution or solid electrolyte. Therefore, the contact area between the negative electrode active material and a non-aqueous electrolyte, such as an electrolyte solution or solid electrolyte, and the diffusion of lithium ions and electrons within the active material particles are extremely important. Furthermore, in electrodes using niobium oxide active materials with low electronic conductivity, volume changes associated with charge and discharge of the active material in the negative electrode composite layer gradually cause the contact points with the conductive additive to disappear, significantly reducing cycle performance. In the niobium oxide of the present invention, the substitution of a portion of Ti with a different element is believed to improve the diffusion of lithium ions and electrons within the active material particles, and furthermore, the unique crystal structure of Ti ₂ Nb ₁₄ O ₃₉ , in which a portion of all cation sites are vacant, is believed to reduce volume change during charge and discharge. As a result, it is believed that battery characteristics were obtained that could not be achieved with Ti ₂ Nb ₁₄ O ₃₉ in which a portion of Ti is not substituted with a different element, or with TiNb ₂ O ₇ or Ti ₂ Nb ₁₀ O _29, which contain a trivalent metal and exhibit large volume change during charge and discharge as described in Patent Document 1.

[Method of producing niobium-containing oxide powder of the present invention]
An example of the method for producing the niobium-containing oxide powder of the present invention will be explained below, dividing it into a raw material preparation step and a firing step, but the method for producing the niobium-containing oxide powder of the present invention is not limited thereto.

<Raw material preparation process>
First, the starting materials are mixed. The starting materials include an oxide containing Ti and Nb, as well as an oxide or salt containing at least one element selected from the group consisting of Ta, V, Al, Ga, Fe, Cr, Mg, Ca, Cu, and Zn. These are mixed in a stoichiometric ratio to achieve the target composition of the present invention. The salt used is preferably a salt that decomposes at a relatively low melting point to produce an oxide, such as a hydroxide, carbonate, or nitrate. Furthermore, to reduce the primary particle size, it is preferable to use powder as the starting material with an average primary particle size of 2 μm or less, preferably 0.5 μm or less.

There are no particular restrictions on the method for mixing the raw materials, and either wet or dry mixing may be used. For example, a Henschel mixer, ultrasonic disperser, 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.

<Firing process>
Next, the mixture obtained above is fired. Firing is preferably carried out in a temperature range of 500 to 1200°C, more preferably in a range of 700 to 1150°C, and even more preferably in a range of 900 to 1150°C. By setting the firing temperature at 1150°C or less, general-purpose equipment can be used. When firing the mixture for a short period of time, it is preferable to prepare the mixed powder constituting the mixture before firing so that the D95 value in the particle size distribution curve measured with a laser diffraction/scattering particle size distribution analyzer 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.

There are no particular limitations on the calcination method, as long as it can be performed under the above conditions. Usable calcination methods include fixed-bed calcination furnaces, roller hearth calcination furnaces, mesh belt calcination furnaces, fluidized bed calcination furnaces, and rotary kiln calcination furnaces. However, for efficient calcination in a short time, roller hearth calcination furnaces, mesh belt calcination furnaces, and rotary kiln calcination furnaces are preferred. In particular, rotary kiln calcination furnaces are particularly preferred for producing the niobium-containing oxide powder of the present invention because they do not require a container to hold the mixture, can be used to calcinate the mixture while continuously adding it, and provide a uniform thermal history to the calcined material, resulting in the production of a homogeneous oxide.

<Surface treatment process>
The niobium-containing oxide obtained above may be subjected to a surface treatment. By localizing at least one metal element selected from the group consisting of Mo and Ce on the surface of the particles constituting the niobium-containing oxide powder of the present invention, when the powder is used as a battery anode material, a dense anode layer can be formed and excellent charge rate characteristics can be imparted. In the calcination step, a compound containing at least one metal element selected from the group consisting of Mo and Ce (hereinafter, sometimes referred to as a treating agent) can be added to produce a surface-treated niobium-containing oxide powder of the present invention. However, it is more preferable to produce a surface-treated niobium-containing oxide powder of the present invention by the following surface treatment step. In particular, by employing the following surface treatment step, it is possible to appropriately and relatively simply make the surface of the niobium-containing oxide particles have at least one metal element selected from the group consisting of Mo and Ce present.

There are no particular restrictions on the method for mixing the niobium-containing oxide powder base material with the compound containing at least one metal element selected from the group consisting of Mo and Ce, and either wet mixing or dry mixing can be used. However, it is preferable to uniformly disperse the compound containing at least one metal element selected from the group consisting of Mo and Ce on the surfaces of the particles that make up the niobium-containing oxide powder base material, and in this respect, wet mixing is preferred.

Wet mixing involves adding the treatment agent and the niobium-containing oxide powder of the base material to a water or alcohol solvent and mixing them in a slurry state. Alcohol solvents with a boiling point of 100°C or less, such as methanol, ethanol, or isopropyl alcohol, are preferred because they are easy to remove. Furthermore, from an industrial perspective, water solvents are preferred because of their ease of recovery and disposal.

Compounds (treatment agents) containing at least one metal element selected from the group consisting of Mo and Ce include, but are not limited to, oxides, phosphates, hydroxides, sulfates, nitrates, fluorides, chlorides, organic compounds, and metal salt compounds such as ammonium salts and phosphates. Specific examples of Mo compounds include molybdenum oxide, molybdenum trioxide, molybdenum trioxide hydrate, molybdenum boride, molybdophosphoric acid, molybdenum disilicide, molybdenum chloride, molybdenum sulfide, molybdenum silicic 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. Among these, molybdenum trioxide, molybdenum trioxide hydrate, molybdenum chloride, molybdenum sulfide, and lithium molybdate are preferred. Examples of Ce compounds include cerium oxide, cerium hydroxide, cerium fluoride, cerium sulfate, cerium nitrate, cerium carbonate, cerium acetate, cerium oxalate, cerium chloride, cerium boride, and cerium phosphate, with cerium sulfate and its hydrates being particularly preferred.

The amount of the compound containing at least one metal element selected from the group consisting of Mo and Ce may be any amount as long as the amount of the at least one metal element selected from the group consisting of Mo and Ce in the niobium-containing oxide falls within the range of the present invention. However, it is sufficient to add the compound in a proportion of 0.03% by mass or more, preferably 0.05% by mass or more, relative to the niobium-containing oxide powder of the base material. It is also sufficient to add the compound in a proportion of 12% by mass or less, preferably 10% by mass or less, and more preferably 8% by mass or less, relative to the niobium-containing oxide powder of the base material.

It is preferable to perform heat treatment after the surface treatment. The heat treatment temperature is a temperature at which the at least one metal element selected from the group consisting of Mo and Ce diffuses into at least the surface region of the niobium-containing oxide particles constituting the niobium-containing oxide powder substrate, and at which a significant decrease in the specific surface area due to sintering of the niobium-containing oxide substrate does not occur. 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 0.5 to 5 hours. The temperature and time at which the at least one metal element selected from the group consisting of Mo and Ce diffuses into at least the surface region of the niobium-containing oxide powder substrate should be set appropriately, as the reactivity varies depending on the compound containing the at least one metal element selected from the group consisting of Mo and Ce. The heating method used in the heat treatment is not particularly limited. Usable heat treatment furnaces include fixed-bed furnaces, roller hearth furnaces, mesh belt furnaces, fluidized-bed furnaces, and rotary kiln furnaces. The atmosphere during heat treatment may be either air or an inert atmosphere such as a nitrogen atmosphere. In particular, when a metal salt compound is used for surface treatment, an air atmosphere is preferred, as it facilitates the removal of anionic species from the particle surface.

The heat-treated niobium-containing oxide powder obtained as described above is preferably subjected to a crushing treatment as needed to bring the specific surface area within the above range. The crushing treatment can be carried out using, for example, a ball mill, and the conditions for the crushing treatment are not particularly limited as long as they allow the specific surface area to fall within the above range. Furthermore, classification may be carried out in addition to the crushing treatment.

The niobium-containing oxide powder of the present invention may be granulated and heat-treated to form a powder containing secondary particles formed by aggregation of primary particles. Any granulation method can be used as long as it produces secondary particles, but a spray dryer is preferred because it can process large quantities.

Dew point control may be performed during the heat treatment process to reduce the moisture content of the niobium-containing oxide powder of the present invention. Because the heat-treated powder will absorb moisture from the atmosphere if exposed to the air, 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 needed to achieve the desired maximum particle size range. When dew point control is performed during the heat treatment process, it is preferable to seal the niobium-containing oxide powder of the present invention in an aluminum-laminated bag or similar and then place it in an environment outside of dew point control. Even under dew point control, pulverizing the heat-treated niobium-containing oxide powder makes it more likely to absorb moisture from the crushed surfaces, increasing the moisture content of the powder. Therefore, it is preferable not to pulverize the powder after heat treatment. Regarding heat treatment conditions, the temperature and holding time within specific ranges significantly affect the secondary particle morphology and surface treatment process. The heat treatment temperature is preferably 450°C or higher, and less than 550°C. This is because if the heat treatment temperature exceeds 550°C, the specific surface area will decrease significantly, resulting in a significant decrease in battery performance, especially rate characteristics. Furthermore, a holding time of one hour or more is preferable, as a short holding time is thought to increase the amount of moisture contained in the powder and also affect the particle surface condition.

[Active material]
The active material of the present invention contains the niobium-containing oxide powder of the present invention. It may contain one or more substances other than the niobium-containing oxide powder of the present invention. Examples of other substances include carbon materials (pyrolytic carbons, cokes, graphites (artificial graphite, natural graphite, etc.), organic polymer compound combustion bodies, and carbon fibers), _tin and tin compounds, silicon and silicon compounds, and lithium-containing metal oxides. In particular, lithium-containing metal oxides include lithium _titanate mainly composed of _Li4Ti5O12 .

[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 thereof include a hybrid capacitor and a lithium battery.

[Hybrid capacitor]
The hybrid capacitor is a device that uses, as a positive electrode, an active material that generates capacitance by physical adsorption, such as activated carbon, similar to the electrode material of an electric double layer capacitor, an active material that generates capacitance by physical adsorption and intercalation/deintercalation, such as graphite, or an active material that generates capacitance by redox, such as a conductive polymer, and uses, as a negative electrode, the active material of the present invention. 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 to conceptually include so-called lithium ion secondary batteries and all-solid-state lithium ion secondary batteries.

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

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

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

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

Examples of binders for the negative electrode include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR), and carboxymethyl cellulose (CMC).

The amount of binder added should be optimized, as it varies depending on the specific surface area of the active material and the type and combination of conductive agents. It is preferably 0.2% to 15% by mass in the negative electrode layer. From the viewpoint of improving binding and ensuring the strength of the negative electrode 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 power storage device per unit mass and unit volume of the negative electrode 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, and materials coated with carbon, nickel, titanium, or silver. The surfaces of these materials may also be oxidized, or the negative electrode current collector surface may be roughened by surface treatment.

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 porous negative electrode current collector having pores, the paint is prepared by uniformly mixing the negative electrode active material (including the active material of the present invention), conductive agent, and binder in a solvent, and is then pressed into the pores of the current collector to fill them, or by immersing the porous current collector in the paint to allow the paint to diffuse into the pores, followed by drying and compression.

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 planetary mixer or other kneader in which the stirring rod revolves while rotating within the kneading vessel, a twin-screw extrusion kneader, a planetary stirring degasser, a bead mill, a high-speed rotary mixer, and a powder suction continuous dissolution and dispersion device. Furthermore, the manufacturing process may be divided into steps depending on the solids concentration, and these devices may be used separately.

Uniform mixing of the negative electrode active material (the active material of the present invention), conductive agent, and binder in the solvent requires optimization, as 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 within the kneading vessel, a twin-screw extrusion kneader, or a planetary agitation degassing device, it is preferable to divide the manufacturing process into steps based on the solids concentration, knead the materials at a high solids concentration, and then gradually reduce the solids concentration to adjust the viscosity of the paint. A high solids concentration is preferably 60% to 90% by mass, more preferably 60% to 80% by mass. A solids concentration of 60% by mass or higher is preferred because it provides sufficient shear force, while a solids concentration of 90% by mass or lower is preferred because it reduces the load on the equipment, and 80% by mass or lower is even more preferred.

The mixing procedure is not particularly limited, but examples include a method in which the negative electrode active material, conductive agent, and binder are mixed simultaneously in a solvent; a method in which the conductive agent and binder are mixed in a solvent in advance and then the negative electrode active material is added and mixed; and a method in which a negative electrode active material slurry, conductive agent slurry, and binder solution are prepared in advance and then mixed. Of these, to achieve uniform dispersion, the method in which the conductive agent and binder are mixed in a solvent in advance and then the negative electrode active material is added and mixed, and the method in which a negative electrode active material slurry, conductive agent slurry, and binder solution are prepared in advance and then mixed, are preferred.

An organic solvent can be used as the solvent. Examples of organic solvents include aprotic organic solvents such as 1-methyl-2-pyrrolidone, dimethylacetamide, and dimethylformamide, used alone or in combination, with 1-methyl-2-pyrrolidone being preferred.

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 positive electrode layer containing a positive electrode active material, a conductive agent, and a binder on one or both surfaces of a positive electrode current collector.

As the positive electrode active material, a material capable of absorbing and releasing lithium is used. Examples of the active material include composite metal oxides containing cobalt, manganese, and nickel with lithium, 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 LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiCo _1-x Ni _x O ₂ (0.01<x<1), LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , and LiNi _1/2 Mn _3/2 O _4. These lithium composite oxides may be partially substituted with other elements, such as by substituting at least one element such as B, Nb, Sn, Mg, Fe, Ti, Al, Zr, Cr, V, Ga, Zn, Cu, Bi, Mo, or La for the cobalt, manganese, or nickel, or by substituting at least one element such as S or F for the oxygen, or by coating with a compound containing these other elements. Examples of lithium-containing olivine-type phosphates include LiFePO ₄ , LiCoPO ₄ , LiNiPO ₄ , LiMnPO ₄ , and LiFe _1- xMxPO ₄ (wherein M is at least one element selected from Co, Ni, Mn, Cu, Zn, and Cd, and x satisfies 0≦x≦0.5).

Conductive agents and binders for the positive electrode can be similar to those for the negative electrode. Examples of the positive electrode current collector include aluminum, stainless steel, nickel, titanium, baked carbon, and aluminum or stainless steel surfaces treated with carbon, nickel, titanium, or silver. The surfaces of these materials can be oxidized, or the positive electrode current collector surface can be roughened by surface treatment.

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

The electrolyte salt used is one that dissolves in a non-aqueous electrolyte, and examples thereof include inorganic lithium salts such as LiPF ₆ , LiBF ₄ , LiPO ₂ F ₂ , LiN(SO ₂ F) ₂ and LiClO ₄ , LiN(SO ₂ CF ₃ ) ₂ , LiN(SO ₂ C ₂ F ₅ ) ₂ , LiCF ₃ SO ₃ , LiC(SO ₂ CF ₃ ) ₃ , LiPF ₄ (CF ₃ ) ₂ , LiPF ₃ (C ₂ F ₅ ) ₃ , LiPF ₃ (CF ₃ ) ₃ , LiPF ₃ (iso-C ₃ F ₇ ) ₃ and LiPF ₅ (iso-C ₃ F _{7 ) .} 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 ) 2 (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.

The concentration at which all of these electrolyte salts are dissolved and used in the non-aqueous solvent is generally 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 to encompass chain carbonates and chain carboxylic acid 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 collectively referred to as "DFEC"), vinylene carbonate (VC), vinylethylene carbonate (VEC), and 4-ethynyl-1,3-dioxolan-2-one (EEC). One or more selected from the group consisting of 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 perspective of improving the charge rate characteristics of the power storage device and suppressing the amount of gas generated during high-temperature operation, and one or more cyclic carbonates having an alkylene chain selected from propylene carbonate, 1,2-butylene carbonate, and 2,3-butylene carbonate are even more preferred.

In particular, it is preferable to use a non-aqueous electrolyte solution having a total electrolyte salt concentration of 0.5 M to 2.0 M, containing at least _LiPF6 as the electrolyte salt, and further containing 0.001 M to 1 M of at _least one lithium salt selected from _LiBF4 , _LiPO2F2 , and LiN( _SO2F ) ₂ . When the proportion of lithium salts other than LiPF6 in the non-aqueous solvent is 0.001 M or more, the charge rate characteristics of the electricity storage device and the effect of suppressing the amount of gas generation during high-temperature operation are easily exhibited, and when it is 1.0 _M or less, there is little concern about a decrease in the effect of improving the charge rate characteristics of the electricity storage device and the effect of suppressing the amount of gas generation during high-temperature operation, which is preferable.

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).

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

The separator is made of an insulating thin film with high ion permeability and a certain level of mechanical strength. Examples include polyethylene, polypropylene, cellulose paper, glass fiber paper, polyethylene terephthalate, and polyimide microporous membranes, and multilayer membranes made by combining two or more types can also be used. The surface of these separators can also be coated with resins such as PVDF, silicone resins, and rubber-based resins, or with particles of metal oxides such as aluminum oxide, silicon dioxide, and magnesium oxide.

<Negative electrode active material composition>
The negative electrode active material composition of the present invention includes the niobium-containing oxide powder of the present invention and an inorganic solid electrolyte having conductivity for metal ions belonging to Group 1 of the periodic table. The content of the inorganic solid electrolyte is not particularly limited, but may be 1% by mass or more, preferably 5% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more in the active material composition. A higher content of the inorganic solid electrolyte is preferable because it facilitates contact between the niobium-containing oxide powder and the solid electrolyte. Furthermore, since a too high content of the inorganic solid electrolyte reduces the battery capacity of the all-solid-state secondary battery, the content is preferably 70% by mass or less, and more preferably 60% by mass or less. Generally, a low content of the inorganic solid electrolyte is preferable to increase the battery capacity of the all-solid-state secondary battery. However, a low content of the inorganic solid electrolyte makes it difficult to achieve contact between the niobium-containing oxide powder and the solid electrolyte. By using the niobium-containing oxide powder used in the negative electrode active material composition of the present invention, satisfactory contact between the niobium-containing oxide powder and the solid electrolyte can be achieved even when the content of the inorganic solid electrolyte is low. The niobium-containing oxide powder and inorganic solid electrolyte of the present invention may contain one or more substances other than the niobium-containing oxide powder and inorganic solid electrolyte. Examples of other substances include carbon materials (pyrolytic carbons, cokes, graphites (artificial graphite, natural graphite, etc.), organic polymer compound combustion bodies, carbon fibers), tin and tin compounds, silicon and silicon compounds, _and lithium-containing metal oxides. In particular, lithium-containing metal oxides include lithium _titanate mainly composed of _Li4Ti5O12 .

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

<Solid electrolyte>
A solid electrolyte is a solid electrolyte capable of transferring ions therein. In particular, inorganic solid electrolytes are solid in a steady state and 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 Group 1 of the periodic table, but generally has almost no electronic conductivity. Representative examples of inorganic solid electrolytes include (A) sulfide inorganic solid electrolytes, (B) oxide inorganic solid electrolytes, and (C) chloride inorganic solid electrolytes. In particular, sulfide inorganic solid electrolytes are preferred because they have high ionic conductivity and can be formed into dense compacts with few grain boundaries simply by applying pressure at room temperature. The periodic table referred to here refers to the long-period version of the periodic table.

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 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 include —P ₂ O ₅ , Li ₂ O—SiO ₂ , and Li ₆ BaLa ₂ Ta ₂ O ₁₂ .

Suitable chloride inorganic solid electrolytes include Li ₃ ScCl ₆ , LiAlCl ₄ , and Ln _1-X A _X OCl _1-X (wherein Ln represents any rare earth element, A represents an alkaline earth metal, and X satisfies 0<X<1).

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 explained in more detail using examples and comparative examples, but it should be noted that the present invention is not limited to the following examples and encompasses various combinations that can be easily inferred from the spirit of the invention.

(Lithium-ion secondary battery)
[Example 1-1]
<Raw material preparation process>
Nb ₂ O ₅ (average particle size 0.2 μm), anatase TiO ₂ (specific surface area 10 m ² /g), and Al ₂ O ₃ were weighed and mixed in a molar ratio of 78.5:21.2:0.3. This mixed powder was calcined at 1,050°C for 12 hours. Powder X-ray diffraction measurement was performed on the resulting calcined powder sample at a sampling interval of 0.01° and a scan rate of 2°/min. No peaks derived from the starting materials were observed, confirming that the reaction had proceeded completely. Crystal structure analysis of the synthesized sample and X-ray fluorescence analysis (XRF) using an X-ray fluorescence analyzer (manufactured by SII Technology Inc., product name "SPS5100") confirmed that the synthesized sample was the desired niobium-containing oxide (Ti _1.9 Nb _14.05 Al _0.05 O ₃₉ ).

<Crushing process>
Zirconia beads of φ2.0 mm were added to the obtained fired powder sample, and the mixture was subjected to a crushing treatment using a ball mill, thereby producing a niobium-containing oxide powder according to Example 1-1 in which the particle size distribution of the primary particles was adjusted.

[Examples 1-2 to 1-12]
Niobium-containing oxide powders according to Examples 1-2 to 1-12 were produced in the same manner as in Example 1-1, except that the raw materials and mixing ratios were changed in the raw material preparation step so as to obtain the composition formulas shown in Tables 1 and 2.

[Comparative Example 1-1]
Nb ₂ O ₅ and anatase-type TiO ₂ were weighed and mixed in a molar ratio of 7:2. This mixed powder was simply fired at 1100°C for 12 hours, and a niobium-containing oxide powder according to Comparative Example 1-1 was produced, in which the crushing treatment step was not carried out as in Non-Patent Document 1.

[Reference example 1-1]
A niobium-containing oxide powder according to Reference Example 1-1 was produced in the same manner as in Example 1-1, except that the molar ratio of Nb ₂ O ₅ to anatase-type TiO ₂ was changed to 7:2.

[Measurement of powder properties]
The various physical properties of the niobium-containing oxide powders of the Examples, Comparative Examples, and Reference Examples were measured as follows.

<Measurement of specific surface area>
The specific surface area (m ² /g) of the niobium-containing oxide powder of each Example, Comparative Example, and Reference Example was measured using a fully automatic BET specific surface area measuring device (manufactured by Mountec Co., Ltd., product name "Macsorb HM model-1208"), and nitrogen gas was used as the adsorption gas. 0.5 g of the measurement sample powder was weighed 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: Dry Laser Diffraction Scattering Method>
The D50 of the primary particles of the niobium-containing oxide powders of each Example, Comparative Example, and Reference Example was calculated from a particle size distribution curve measured using a laser diffraction/scattering particle size distribution analyzer (Microtrac MT3300EXII, manufactured by Nikkiso Co., Ltd.). 50 mg of sample was placed in a container containing 50 ml of ion-exchanged water as a measurement solvent, and the measurement solvent was added until the transmittance of the slurry fell within the appropriate range (the range displayed by the green bar on the device), and particle size distribution measurement was performed. The D50 of the primary particles of the powder was calculated from the obtained particle size distribution curve.

[Evaluation of Battery Characteristics]
The niobium-containing oxide powders of the Examples, Comparative Examples, and Reference Examples were used to fabricate coin-type batteries, and the battery characteristics thereof were evaluated. The evaluation results are shown in Table 1.

<Preparation of negative electrode sheet>
The negative electrode sheet was prepared as follows in a room controlled at a room temperature of 25°C and a dew point of -20°C or lower. The niobium-containing oxide powder of each Example and Comparative Example was removed from the aluminum laminate bag in a room controlled at a temperature of 25°C and a dew point of -20°C or lower. A coating material was prepared by mixing the removed niobium-containing oxide powder of each Example and Comparative Example as an active material at 85% by mass, acetylene black as a conductive agent at 10% by mass, and polyvinylidene fluoride as a binder at 5% by mass. The obtained coating material was applied to aluminum foil and dried to prepare a single-sided negative electrode sheet for use in a coin battery described below.

<Preparation of Electrolyte>
The electrolyte solution used in the battery for characteristic evaluation was prepared as follows: In an argon glove box controlled at a temperature of 25°C and a dew point of −70°C or less, a non-aqueous solvent of ethylene carbonate (EC):dimethyl carbonate (DMC) = 1:2 (volume ratio) was prepared, and _LiPF6 was dissolved in the non-aqueous solvent as an electrolyte salt to a concentration of 1 M to prepare the coin battery electrolyte solution described below.

<Preparation of coin battery>
The negative electrode single-sided sheet prepared by the above method was punched 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 prepared evaluation electrode 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 ADVANTEC GA-100 and Whatman GF/C), and the nonaqueous electrolyte prepared by the method described above in <Preparation of Electrolyte Solution> was added and sealed to prepare a 2032-type coin battery.

<Initial battery characteristics: initial discharge capacity, initial efficiency>
In a thermostatic chamber at 25°C, the coin battery prepared by the method described above in <Preparation of Coin Battery> was charged to 1 V at a current density of 0.2 mA/ ^cm2 , with charging in the direction in which Li was absorbed into the evaluation electrode, and then subjected to constant-current, constant-voltage charging at 1 V until the charging current reached a current density of 0.05 mA/ ^cm2 . Thereafter, constant-current discharging was performed at a current density of ^0.2 mA/cm2 to 3 V. The obtained discharge capacity (mAh) was divided by the mass of the niobium-containing oxide to determine the initial discharge capacity (mAh/g), and the discharge capacity (mAh) was divided by the charge capacity (mAh) to determine the initial efficiency (%).

<Measurement of 10C rate charge/discharge rate>
The battery was charged to 1 V at a current equivalent to 10 C of the initial discharge capacity, and then subjected to constant-current/constant-voltage charging at 1 V until the charging current reached a current density of 0.05 C. This was followed by constant-current discharging at 10 C to 3 V. The 10 C rate charge rate (%) was calculated by dividing the capacity obtained from the 10 C constant-current charge by the initial discharge capacity, and the 10 C rate discharge rate (%) was calculated by dividing the capacity obtained from the 10 C constant-current discharge by the initial discharge capacity. The results are shown in Tables 1 and 2. If the niobium-containing oxide has high 10 C rate characteristics, when it is used as an electrode material for an energy storage device, improved discharge rate characteristics of the energy storage device can be expected. The "C" in 1 C represents the current value during charging and discharging. For example, 1 C refers to the current value at which the theoretical capacity can be fully discharged (or fully charged) in 1/1 hour, while 0.1 C refers to the current value at which the theoretical capacity can be fully discharged (or fully charged) in 1/0.1 hour.

<Evaluation results>
The electrodes using the niobium-containing oxide powders of Examples 1-1 to 1-12 had better 10 C charge rate and 10 C discharge rate than the electrodes using the niobium-containing oxide powders of Comparative Example 1-1 and Reference Example 1-1 in which part of Ti was not substituted with a different element, and it was found that they had better rate characteristics.

<Measurement of electrode thickness change rate due to charge/discharge>
In a thermostatic chamber at 25 ° C., a coin battery prepared by the method described above in <Preparation of Coin Battery> was charged to 0.8 V at a current density of 0.2 mA / cm ² , with charging in the direction in which Li was absorbed into the evaluation electrode. Further constant-current, constant-voltage charging was performed at 0.8 V until the charging current reached a current density of 0.05 mA / cm ² , followed by constant-current discharging at a current density of 0.2 mA / cm ² to 2 V. Thereafter, the battery was again charged to 0.8 V at a current density of 0.2 mA / cm ² , and further constant-current, constant-voltage charging was performed at 0.8 V until the charging current reached a current density of 0.05 mA / cm ^{2 .} The coin battery was then disassembled, and the charged negative electrode sheet was removed and its thickness was measured. The thickness of the negative electrode sheet after charging to 0.8 V was divided by the thickness of the negative electrode sheet before the coin battery was prepared to calculate the rate of change in electrode thickness during charge and discharge.

<Evaluation results>
The electrode thickness change rate during charge and discharge of the electrode using the niobium-containing oxide powder of Example 1-1 was 10.5%, and the thickness change rate during charge and discharge of the electrode using Ti ₂ Nb ₁₀ O ₂₉ having a trivalent metal described in Patent Document 1 was 13.4%, indicating that the electrode using the niobium-containing oxide powder of the present invention has a smaller thickness change during charge and discharge than the electrode using the niobium-containing oxide powder described in Patent Document 1. This effect is not described at all in Non-Patent Document 1, Non-Patent Document 2, and Patent Document 1. Furthermore, from the results of Example 1-1, it is considered that the effect of suppressing the electrode thickness change rate during charge and discharge of the electrode can be similarly obtained in Examples 1-2 to 1-12 other than Example 1-1.

(All-solid-state secondary battery)
[Example 2-1]
In a glove box under an argon atmosphere, the niobium-containing oxide of Example 1-3, the sulfide solid electrolyte Li ₆ PS ₅ Cl powder (volume average particle size: 6 μm obtained using a laser diffraction/scattering particle size distribution analyzer), and the conductive agent were weighed out to a mass ratio of niobium-containing oxide:Li ₆ PS ₅ Cl:conductive agent = 60:40:6, and the mixture was stirred and mixed in an agate mortar and a planetary ball mill to obtain the negative electrode active material composition of Example 2-1. The obtained negative electrode active material composition was pressed (360 MPa) at room temperature for 10 minutes to produce a pellet (molded body) with a diameter of 10 mm and a thickness of approximately 0.7 mm. A pellet-shaped electrode containing this negative electrode active material composition, a pellet-shaped solid electrolyte layer (LPS glass with a molar ratio of _Li2S : _P2S5 = _75:25 ) as a separator layer, and a lithium-indium alloy foil as a counter electrode were laminated in this order, and the laminate was sandwiched between stainless steel current collectors to prepare an all-solid-state secondary battery, and the battery characteristics were evaluated. The evaluation results are shown in Table 3.

[Example 2-2]
An all-solid-state secondary battery according to Example 2-2 was fabricated in the same manner as in Example 2-1, except that the niobium-containing oxide was changed to that of Example 1-9, and the battery characteristics were evaluated. The evaluation results are shown in Table 3.

[Example 2-3]
Ion-exchanged water was added to the niobium-containing oxide of Example 1-3 so that the solids concentration of the slurry was 30 mass %, and lithium molybdate (Li ₂ MOO ₄ ) was added as a treating agent in an amount of 0.8 mass % relative to 100 g of the niobium-containing oxide to prepare a mixed slurry. This mixed slurry was mixed for 3 hours using a paint shaker, dried at a temperature of 100°C, and then heat-treated for 1 hour at 500°C using a muffle furnace, thereby producing a niobium-containing oxide surface-treated with lithium molybdate according to Example 2-3.
An all-solid-state secondary battery according to Example 2-3 was fabricated in the same manner as in Example 2-1, except that the niobium-containing oxide surface-treated with lithium molybdate was used instead, and the battery characteristics were evaluated. The evaluation results are shown in Table 3.

[Example 2-4]
Ion-exchanged water was added to the niobium-containing oxide of Example 1-9 so that the solids concentration of the slurry was 30 mass %, and lithium molybdate (Li ₂ MOO ₄ ) was added as a treating agent in an amount of 0.8 mass % relative to 100 g of the niobium-containing oxide to prepare a mixed slurry. This mixed slurry was mixed for 3 hours using a paint shaker, dried at a temperature of 100°C, and then heat-treated for 1 hour at 500°C using a muffle furnace, thereby producing a niobium-containing oxide surface-treated with lithium molybdate according to Example 2-4.
An all-solid-state secondary battery according to Example 2-4 was fabricated in the same manner as in Example 2-1, except that the niobium-containing oxide surface-treated with lithium molybdate was used instead, and the battery characteristics were evaluated. The evaluation results are shown in Table 3.

[Reference example 2-1]
An all-solid-state secondary battery was fabricated in the same manner as in Example 2-1, except that the niobium-containing oxide of Reference Example 1-1 was used, and the battery characteristics were evaluated. The evaluation results are shown in Table 2.

[Comparative Example 2-1]
An all-solid-state secondary battery was fabricated in the same manner as in Example 2-1, except that the niobium-containing oxide of Comparative Example 1-1 was used, and the battery characteristics were evaluated. The evaluation results are shown in Table 2.

<Measurement of 1C rate characteristics>
In a thermostatic chamber at 25 ° C., the all-solid-state secondary battery prepared by the above method was charged to 0.5 V at a current equivalent to 0.05 C of the theoretical capacity of the niobium-containing oxide, with the direction of charging being such that Li was absorbed into the evaluation electrode. Further, constant current constant voltage charging was performed in which charging was performed at 0.5 V until the charging current reached a current equivalent to 0.01 C. The initial charge capacity (mAh / g) was calculated by dividing the charge capacity (mAh) by the mass of the niobium-containing oxide. Subsequently, a constant current discharge was performed in which the battery was discharged to 2 V at a current equivalent to 0.05 C. The initial discharge capacity (mAh / g) was calculated by dividing the discharge capacity (mAh) by the mass of the niobium-containing oxide. Next, the battery was charged to 0.5 V at a current equivalent to 1 C of the theoretical capacity of the niobium-containing oxide, and the 1 C charge capacity was calculated. The 1 C charge rate (%) was calculated by dividing the 0.4 C charge capacity by the initial charge capacity. Thereafter, the battery was discharged to 2 V at a current equivalent to 1 C of the theoretical capacity of the niobium-containing oxide, and the 1 C discharge capacity was determined. The 1 C discharge capacity was divided by the initial discharge capacity to calculate the 1 C discharge rate (%). The evaluation results are shown in Table 3.

<Evaluation results>
It was also found that in all-solid-state batteries, electrodes using the niobium-containing oxide powder of the present invention had better initial discharge capacity and 1C rate characteristics than electrodes using niobium-containing oxide powder in Reference Example 2-1 and Comparative Example 2-1 in which part of Ti was not substituted with a different element. It was also found that Examples 2-3 and 2-4, which were surface-treated with lithium molybdate, further improved 1C charge/discharge rate characteristics. It can be said that the niobium-containing oxides surface-treated with lithium molybdate according to Examples 2-3 and 2-4 have Mo localized on the surface due to their production method.

In Examples 1-1 to 1-12 and Examples 2-1 to 2-4, which used the niobium-containing oxide powder of the present invention in which part of the Ti was replaced with a different element, the effect of improving rate characteristics was observed regardless of the type of electricity storage device.

The niobium-containing oxide powder obtained by the present invention is useful as an electrode active material for lithium-ion batteries because it can improve rate characteristics while maintaining initial discharge capacity. Energy storage devices using this niobium-containing oxide powder as an electrode active material are useful as secondary batteries for driving or backing up various devices, such as automobiles and electronic devices, and for storing power at night in homes and offices. Providing electrode materials for lithium-ion batteries, such as nonaqueous electrolyte secondary batteries, to society can contribute to the achievement of Goal 12 (Ensure sustainable consumption and production patterns), Goal 3 (Ensure healthy lives and promote well-being for all at all ages), Goal 7 (Ensure access to affordable, reliable, sustainable, and modern energy for all), and Goal 11 (Make cities and human settlements inclusive, safe, resilient, and sustainable) out of the 17 Sustainable Development Goals (SDGs) established by the United Nations.

Claims (6)

A niobium-containing oxide powder that satisfies the following formula (I) or (II):
A _a Ti _2-x (M ¹ _0.5+v M ² _0.5-v ) _x Nb ₁₄ O _39±z (I)
A _a Ti _2-y (M ¹ _0.67+w M ³ _0.33-w ) _y Nb ₁₄ O _39±z (II)
(In the formula, A is at least one element selected from Li and Na, ^M1 is at least one element selected from Nb, Ta, and V, ^M2 is at least one element selected from Al, Ga, Fe, and Cr, ^M3 is at least one element selected from the group consisting of Mg, Ca, Cu, and Zn, and 0≦a≦6, −0.05≦v≦0.05, −0.05≦w≦0.05, 0<x<2, 0<y<2, 0≦z≦1) The niobium-containing oxide powder according to claim 1, characterized in that the D50 of primary particles corresponding to a cumulative volume of 50% in the volume-based particle size distribution of the niobium-containing oxide powder as measured by a laser diffraction scattering method is 0.25 μm or more and 4.5 μm or less. An electrode for an electricity storage device, comprising the niobium-containing oxide powder according to claim 1 or 2 as an active material. A negative electrode active material composition for a nonaqueous electrolyte storage device, comprising the niobium-containing oxide powder according to claim 1 or 2 and an inorganic solid electrolyte. A non-aqueous electrolyte electricity storage device comprising the electrode according to claim 3 . An all-solid-state secondary battery comprising a positive electrode layer, a negative electrode layer, and a solid electrolyte layer, wherein the negative electrode layer is a layer containing the negative electrode active material composition according to claim 4.