Titanium-containing oxide powder, electrodes using the same, and energy storage devices

Abstract

To provide a titanium-containing oxide powder that is used as an electrode material of a power storage device and allows for improved cycle characteristics while maintaining initial discharge capacity and rate characteristics, and an electrode and a power storage device each including the titanium-containing oxide powder.SOLUTION: A titanium-containing oxide powder includes titanium-containing oxide particles represented by the general formula Li4Ti5O12 or Ti1-x / 2 Nb2O7-x (0≤X<2). The titanium-containing oxide particles have their surfaces at least partially coated with polyimide.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a titanium-containing oxide powder suitable as an electrode material for energy storage devices, an electrode using the same, and an energy storage device. [Background technology]

[0002] In recent years, various materials have been studied as electrode materials for energy storage devices. Among them, Li4Ti5O 12 Lithium titanate, represented by [formula], is attracting attention as a negative electrode active material for auxiliary power supply devices in electric vehicles such as HEVs, PHEVs, and BEVs, due to its excellent input / output characteristics, especially in the low-temperature range, when used as an active material.

[0003] Furthermore, high energy density is required for main power supply devices in electric vehicles from the perspective of improving energy efficiency. Although lithium titanate has excellent input / output characteristics, its energy density remains at 175 mAh / g, leaving challenges for further energy increases. Therefore, there is a growing trend to utilize niobium-titanium-containing oxides, mainly niobium titanate represented by the general formula TiNb2O7, which has a high energy density of 380 mAh / g, as a negative electrode active material.

[0004] High energy density is required for energy storage devices in electric vehicles from the perspective of improving fuel efficiency or energy consumption. In addition, stability over a wide temperature range from high to low temperatures is also important. Specifically, this includes initial input / output characteristics and suppression of gas generation at high temperatures. When titanium-containing oxides, including lithium titanate, are used in energy storage devices, prolonged use at high temperatures above 60°C presents challenges in storage stability, such as gas generation, which can lead to an increase in battery resistance and changes in long-term input / output characteristics. Therefore, there is a need for the development of titanium-containing oxides that can increase the energy density of energy storage devices while maintaining excellent initial input / output characteristics, and suppress gas generation and resistance increase after long-term high-temperature operation.

[0005] Patent Document 1 discloses polyimide-coated active material particles obtained by coating active material particles, which serve as a material for a negative electrode of a battery, with a polyimide layer derived from a monomeric polyimide precursor. It is stated that forming a negative electrode of a battery using the active material particles can suppress a decrease in cycle characteristics when the battery is repeatedly charged and discharged.

[0006] Patent Document 2 discloses negative electrode active material particles for a battery that are enclosed by a thin layer of ultra-high molecular weight high-elasticity polymer. It is stated that forming a negative electrode of a battery using the active material particles can suppress a decrease in the cycle characteristics of the battery. [Prior Art Literature] [Patent Literature]

[0007] [Patent Document 1] Japanese Unexamined Patent Publication No. 2016-157652 [Patent Document 2] Japanese National Publication of International Patent Application No. 2020-513138 [Summary of the Invention] [Problem to be Solved by the Invention]

[0008] Incidentally, the active material of Patent Document 1 uses silicon powder (silicon) as the negative electrode material, and an improvement in the cycle characteristics of the battery is shown. However, power storage devices are also required to have output performance at high rates and to prevent battery swelling caused by gas generation. Patent Document 1 does not disclose anything regarding this point. As described above, particularly for titanium-containing oxide active materials, suppression of gas generation and resistance increase after high-temperature operation is regarded as important.

[0009] The active material of Patent Document 2 also uses Si nanoparticles, graphite and the like as negative electrode materials, and an improvement in the initial capacity and cycle characteristics of the battery is shown, but there is no description of findings regarding performance other than the above.

[0010] For the reasons stated above, energy storage devices using negative electrode active materials and electrodes described in Patent Documents 1 and 2 cannot simultaneously improve cycle performance and discharge rate characteristics when using titanium-containing oxide active materials, nor can they suppress gas generation at high temperatures (hereinafter referred to as temperatures of 50°C or higher).

[0011] Therefore, the present invention aims to provide a titanium-containing oxide powder that can be used as an electrode material for energy storage devices and can improve cycle characteristics while maintaining initial discharge capacity and rate characteristics, an electrode using the same, and an energy storage device. [Means for solving the problem]

[0012] As a result of various studies to achieve the above objective, the inventors discovered a titanium-containing oxide powder with a polyimide surface. They found that an energy storage device using this titanium-containing oxide powder as an active material in an electrode material can improve cycle characteristics while maintaining initial discharge capacity and rate characteristics, thus completing the present invention. In other words, the present invention relates to the following matters.

[0013] (1) General formula Li4Ti5O 12 or action 1-x / 2 Nb2O 7-x A titanium-containing oxide powder comprising titanium-containing oxide particles represented by (0 ≤ X < 2), wherein at least a portion of the surface of the titanium-containing oxide particles is coated with polyimide. (2) The titanium-containing oxide powder according to (1), characterized in that the polyimide present on the surface of the particles in the titanium-containing oxide is a polyimide derived from a polymer-type total aromatic polyimide precursor. (3) The titanium-containing oxide powder according to (1) or (2), characterized in that the content (mass%) of polyimide present on the surface of the particles is 0.01 to 1.5. (4) General formula Ti 1-x / 2 Nb2O 7-xThe titanium-containing oxide powder according to any one of (1) to (3), wherein the titanium-containing oxide represented by (0≦X<2) is TiNb₂O₇. (5) An electrode comprising the titanium-containing oxide powder according to any one of (1) to (4). (6) An electricity storage device comprising the electrode according to (5). Effects of the Invention

[0014] According to the present invention, there can be provided a titanium-containing oxide powder suitable as an electrode material for an electricity storage device that can improve cycle characteristics while maintaining initial discharge capacity and rate characteristics, an electrode using the same, and an electricity storage device. Mode for Carrying Out the Invention

[0015] [Titanium-containing oxide powder] The titanium-containing oxide powder of the present invention has the general formula Li₄Ti₅O 12 or Ti 1-x / 2 Nb₂O 7-x A powder comprising titanium-containing oxide particles represented by (0≦X<2), wherein at least a part of the surface of said titanium-containing oxide particles is coated with polyimide, which is a titanium-containing oxide powder characterized by the above.

[0016] <Li₄Ti₅O 12 lithium titanate powder containing as a main component> The titanium-containing oxide powder of the present invention may contain, as titanium-containing oxide particles, one having Li₄Ti₅O 12 as a main component. In this case, it contains Li₄Ti₅O 12 as a main component, and may contain crystalline components and / or amorphous components other than Li₄Ti₅O 12 within a range where the effects of the present invention can be obtained. The term "main component" means that the ratio of the intensity of the main peak of Li₄Ti₅O 12 among diffraction peaks measured by X-ray diffraction method is 90% or more. In the lithium titanate powder of the present invention, among diffraction peaks measured by X-ray diffraction method, Li₄Ti₅O12 The percentage of the main peak intensity is more preferably 92% or higher, and even more preferably 95% or higher. Li4Ti5O 12 Other components include 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, depending on the raw materials and synthesis conditions during its synthesis, can be anatase-type titanium dioxide, rutile-type titanium dioxide, and lithium titanates with different chemical formulas such as Li2TiO3 and Li 0.6 Ti 3.4 O8, etc. may be included as the crystalline component. The lithium titanate powder of the present invention contains these Li4Ti5O 12 Other crystalline components, especially Li 0.6 Ti 3.4 The lower the proportion of O8 generation, the better the charging characteristics and charge / discharge capacity of the energy storage device. Among the diffraction peaks measured by X-ray diffraction, Li4Ti5O 12 When the intensity of the main peak of is set to 100, it is particularly preferable that the sum of the intensity of the main peak of anatase-type titanium dioxide, the intensity of the main peak of rutile-type titanium dioxide, and the intensity corresponding to the main peak of Li2TiO3 calculated by multiplying the peak intensity corresponding to the (-133) plane of Li2TiO3 by 100 / 80 is 5 or less. Here, Li4Ti5O 12 The main peak is Li4Ti5O in PDF card 00-049-0207 of ICDD (PDF2010). 12 This peak corresponds to the diffraction peak attributed to the (111) plane (2θ=18.33). The main peak of anatase-type titanium dioxide corresponds to the diffraction peak attributed to the (101) plane (2θ=25.42) in PDF card 01-070-6826. The main peak of rutile-type titanium dioxide corresponds to the diffraction peak attributed to the (110) plane (2θ=27.44) in PDF card 01-070-7347. The peak corresponding to the (-133) plane of Li2TiO3 corresponds to the diffraction peak attributed to the (-133) plane (2θ=43.58) of Li2TiO3 in PDF card 00-033-0831. Li 0.6 Ti3.4 The main peak of O8 corresponds to the diffraction peak attributed to the (101) plane (2θ=19.98) in PDF card 01-070-2732. "ICDD" stands for International Centre for Diffraction Data, and "PDF" stands for Powder Diffraction File.

[0017] <General formula Ti 1-x / 2 Nb2O 7-x Niobium-titanium composite oxide powder represented by (0 ≤ X < 2) The titanium-containing oxide powder of the present invention comprises titanium-containing oxide particles, with the general formula being Ti 1-x / 2 Nb2O 7-x The material may contain niobium-titanium composite oxides represented by (0≦X<2). Specific examples of compounds include TiNb2O7, a niobium-titanium composite oxide capable of intercalating and releasing Li and Na ions. From the viewpoint of improving cycle characteristics while maintaining initial discharge capacity and rate characteristics, the inclusion of TiNb2O7 is preferable. The niobium-titanium composite oxide may also contain a titanium oxide phase derived from the synthesis raw materials (e.g., rutile-type TiO2, TiO, etc.). In the case of niobium-titanium composite oxides, the ratio of moles of Nb to moles of Ti (Nb / Ti ratio) is preferably in the range of 1.5 to 2.5, and more preferably in the range of 1.8 to 2.2. Within this range, the electronic conductivity of the niobium-titanium composite oxide is improved, and its rate characteristics are excellent.

[0018] While there are no restrictions on the crystal system of the niobium-titanium composite oxide of the present invention, lithium titanate is generally cubic, and niobium-titanium composite oxide is generally monoclinic. In the case of the monoclinic type, the aspect ratio tends to be large, but from the viewpoint of improving electrode density, it is preferable that it be in the range of 1.0 to 4.0.

[0019] <Polyimide> The polyimide of this invention is a polymer having imide bonds (-O=CNC=O-) in its main chain as repeating units. Typically, it refers to a polyimide in which acidic dianhydrides and diamines are directly and repeatedly linked by imide bonds.

[0020] <Polyimide coating> In this invention, polyimide coating refers to a state in which all or part of the surface of titanium-containing oxide particles is coated with the polyimide. Specifically, it refers to the detection of polyimide-derived components when the titanium-containing oxide powder of this invention is analyzed using a scanning electron microscope, Auger electron spectroscopy, or Fourier transform infrared spectroscopy, etc.

[0021] <Polyimide derived from polymer-type all-aromatic polyimide precursors> In the present invention, the polyimide used for coating the titanium-containing oxide particles is not particularly limited, but it is preferable to use a polyimide derived from a polymer-type all-aromatic polyimide precursor because of its significant effect. The polyimide precursor is more preferably water-soluble. An example of a polymer-type all-aromatic polyimide precursor is one produced by hydrolyzing polyamic acid (polyamic acid obtained by reacting tetracarboxylic dianhydride with a diamine in an organic solvent) obtained in an organic solvent, adding it to water to obtain polyamic acid powder, further grinding and washing the polyamic acid powder in hot water, and then mixing it with water and a specific amine compound such as 2-methylaminodiethanol to obtain an aqueous polyamic acid salt composition. Examples of tetracarboxylic dianhydrides used in this method include benzophenone-3,3′,4,4′-tetracarboxylic dianhydride, diphenylsulfone-3,3′,4,4′-tetracarboxylic dianhydride, 2,2-bis(4-phthalic anhydride)propane, oxy-bis(4-phthalic anhydride), 4,4′-(hexafluoroisopropylidene)phthalic dianhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride, 3,3″,4,4″-p-terphenyltetracarboxylic dianhydride, diphenyl ether-3,3′,4,4′-tetracarboxylic dianhydride, pyromellitic dianhydride, trifluoromethylpyromellitic dianhydride, and bis(trifluoromethyl)pyromellitic dianhydride. On the other hand, as the aromatic diamine component, any aromatic diamine, for example, o-tolidine, p-phenylenediamine, 2,4-diaminodiphenyl ether, 4,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl sulfone, 4,4′-diaminodiphenylmethane, 4,4′-diaminodiphenyl sulfide, 4,4′-diaminoterphenyl, 1,5-diaminonaphthalene, 4,4′-bis(p-aminophenoxy)biphenyl, 4,4′-bis(m-aminophenoxy)diphenyl sulfone, 2,2-bis(4-(p-aminophenoxy)phenyl)propane, 3,3′-dimethyl-4,4′-diaminodiphenylmethane, 2,7-diaminofluorene, 3,3′-dimethoxybenzidine, m-phenylenedimine, 2,2-bis(4-(p-aminophenoxyphenyl)hexafluor Examples include olopropane, 2,6-diaminoanthraquinone, 1,4-diaminodurene, 2,6-aminotoluene, 2,5-diaminotoluene, 4,4′-diaminobenzophenone, 4,4′-bis(p-aminophenoxy)diphenylsulfone, benzoguanamine, 2,7-diaminonaphthalene, 3,4-diaminotoluene, m-xylenediamine, p-xylenediamine, 4,4′-dithiodianiline, and o-phenylenediamine. In addition to aromatic diamines, alicyclic diamines such as diaminopolysiloxane and norbornanediamine, and aliphatic diamines such as ethylenediamine, hexamethylenediamine, heptamethylenediamine, octamethylenediamine, 1,10-decanediamine, and 1,12-dodecanediamine can also be used. These may be used individually or in combination of two or more.

[0022] Another method involves separating and obtaining a polyimide precursor from a reaction mixture of polyamic acid (polyamic acid) obtained in an organic solvent with 1,2-dimethylimidazole and / or 1-methyl-2-ethylimidazole. In this method, the tetracarboxylic acid component can be 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 2,3,3',4'-biphenyltetracarboxylic acid dianhydride, pyromellitic acid dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 2,2'-bis(3,4-dicarboxyphenyl)propane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, etc. Furthermore, as aromatic diamine components, any aromatic diamine can be used, such as p-phenylenediamine, 4,4'-diaminodiphenyl ether, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenylethane, 4,4'-diaminodiphenylmethane, 2,2'-bis[4-(4-aminophenoxy)phenyl]propane, 2,2'-bis[4-(4-aminophenoxy)phenyl]1,1,1,3,3,3-hexafluoropropane Examples include bis[4-(4-aminophenoxy)phenyl]ether and bis[4-(3-aminophenoxy)phenyl]sulfone, but preferably 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, as an alicyclic diamine component, for example diaminopolysiloxane, norbornanediamine, and as an aliphatic diamine, for example hexamethylenediamine, heptamethylenediamine, octamethylenediamine, 1,10-decanediamine, 1,12-dodecanediamine, etc. can be used.

[0023] Furthermore, a method is used in which, using water as the reaction solvent, a tetracarboxylic dianhydride is reacted with a diamine having a solubility of 0.1 g / L or more in water at 25°C in the presence of imidazoles to obtain an aqueous solution composition of the polyimide precursor. Aromatic diamines used in this method include p-phenylenediamine, m-phenylenediamine, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 2,4-toluenediamine, 3,3'-dihydroxy-4,4'-diaminobiphenyl, bis(4-amino-3 carboxyphenyl)methane, etc., as alicyclic diamine components, for example, diaminopolysiloxane, norbornanediamine, and as aliphatic diamines, for example, hexamethylenediamine, heptamethylenediamine, octamethylenediamine, 1,10-decanediamine, 1,12-dodecanediamine. The term "polyimide" refers to a polyimide obtained by imidizing such a polyimide precursor. Polymer-type all-aromatic polyimide precursors include both all-aromatic polyimide precursors and partially aliphatic polyimide precursors. From the viewpoint of improving rate properties, polyimides derived from polymer-type all-aromatic polyimide precursors that do not contain all-aliphatic polyimide precursors are preferred. More preferred are polyimides derived from polymer-type all-aromatic polyimide precursors that do not contain partially aliphatic polyimide precursors.

[0024] <Polyimide content> The polyimide content (mass%) is expressed as the mass of polyimide present on the surface of titanium-containing oxide particles relative to the total mass of the titanium-containing oxide powder. The calculation method will be described later, but it is calculated from the polymer concentration of each polyimide precursor solution added and the mass of the titanium-containing oxide powder, and is suitable if it is between 0.01% and 1.5% by mass. If the polyimide content is within this range, an energy storage device with improved rate characteristics and cycle characteristics can be obtained. Preferably, it is between 0.02% and 1.0% by mass, and more preferably between 0.5% and 1.0% by mass.

[0025] The imidization rate of polyimide present on the surface of titanium-containing oxide particles is preferably 70-100%, more preferably 85-100%, and even more preferably 95-100%. By setting the imidization rate within the above range, the initial discharge capacity and charge rate characteristics can be further improved. The imidization rate of polyimide can be adjusted by controlling the imidization reaction conditions, such as the reaction temperature.

[0026] <Specific surface area> The specific surface area of ​​the titanium-containing oxide powder of the present invention refers to the surface area per unit mass, using nitrogen as the adsorbent gas. The measurement method will be explained in the examples described later.

[0027] The titanium-containing oxide powder of the present invention has a specific surface area of ​​8.0 m². 2 It is sufficient if it is less than / g, 7.0m 2 Preferably less than / g, and 6.4m 2 Less than / g is preferable.

[0028] <d50> In the present invention, D50 of the titanium-containing oxide powder is an indicator of the median volume particle size. It refers to the particle size at which the cumulative volume frequency calculated from the volume fractions obtained by laser diffraction-scattering particle size distribution measurement accumulates to 50% when calculated from the smallest particle size. The measurement method will be explained in the examples described later.

[0029] The titanium-containing oxide powder of the present invention may consist of primary particles or secondary particles formed by aggregation of primary particles. When it contains secondary particles formed by aggregation of primary particles consisting of titanium-containing oxide particles, some of these secondary particles may not be formed, but rather remain in the form of primary particles themselves.

[0030] When the titanium-containing oxide powder of the present invention is in the form of secondary particles, the lower limit of the D50 of the secondary particles is preferably 11 μm or more, more preferably 12 μm or more, and even more preferably 13 μm or more, from the viewpoint of improving electrode density. 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. Note that the D50 of the secondary particles refers to the D50 before the crushing treatment (applying ultrasound with an ultrasonic device).

[0031] In the titanium-containing oxide powder of the present invention, from the viewpoint of achieving both discharge rate characteristics and cycle characteristics, the lower limit of the D50 of the primary particles should be 0.6 μm or more, preferably 0.7 μm or more. The upper limit of the D50 of the primary particles should be 10 μm or less, preferably 7 μm or less, and more preferably 5 μm or less. Note that the D50 of the primary particles refers to the D50 after crushing treatment (ultrasonic treatment with an ultrasonic device). Furthermore, the titanium-containing oxide powder may contain 15% to 30% of primary particles with a primary particle diameter of less than 0.6 μm, and 15% to 45% of primary particles with a primary particle diameter of less than 0.7 μm. The titanium-containing oxide powder may also contain 0.1% to 10% of primary particles with a primary particle diameter exceeding 10 μm, 0.1% to 15% of primary particles exceeding 7 μm, and 0.1% to 20% of primary particles exceeding 5 μm.

[0032] [Method for producing titanium-containing oxide powder] Below, an example of a method for producing the titanium-containing oxide powder of the present invention will be described, divided into a raw material preparation step, a calcination step, and a surface treatment step. However, the method for producing the titanium-containing oxide powder of the present invention is not limited thereto.

[0033] <Preparation process of raw materials> First, the starting materials are mixed. The raw materials for lithium titanate powder consist of titanium and lithium. As lithium raw materials, lithium compounds such as lithium hydroxide monohydrate, lithium oxide, lithium bicarbonate, and lithium carbonate are used. As titanium raw materials, titanium compounds such as anatase-type titanium dioxide and rutile-type titanium dioxide are used. In the case of niobium titanium composite oxide, an oxide or salt containing Ti and Nb is used as the starting material. When other additive elements are included in the niobium titanium composite oxide, the salt used as the starting material is preferably a salt that decomposes at a relatively low melting point to produce an oxide, such as a hydroxide salt, carbonate, or nitrate.

[0034] There are no particular restrictions on the method of mixing the raw materials; either wet or dry mixing is acceptable. For example, a Henschel mixer, ultrasonic dispersion device, homomixer, mortar and pestle, 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.

[0035] <Firing Process> Next, the mixture obtained above is calcined. From the viewpoint of controlling the specific surface area and crystallite size of the powder obtained by calcination, the primary particle size of the powder, and the amount of impurities from the furnace material, it is preferable to calcinate at a high temperature for a short time. Calcination is carried out in the temperature range of 500 to 1300°C, more preferably in the range of 700 to 1100°C. By performing the calcination at a temperature of 1100°C or lower, general-purpose equipment can be used.

[0036] The firing method is not particularly limited as long as it can be fired under the aforementioned conditions. Usable firing methods include fixed-bed furnaces, roller hearth furnaces, mesh belt furnaces, fluidized-bed furnaces, and rotary kilns. However, for efficient firing in a short time, roller hearth furnaces, mesh belt furnaces, and rotary kilns are preferred. In particular, rotary kilns are preferred because they do not require a container for the mixture, allow for continuous feeding of the mixture during firing, and provide a uniform thermal history to the fired material, resulting in a homogeneous oxide.

[0037] <Crushing process> Methods for crushing titanium-containing oxide powder after firing include hammer mills, ball mills, jet mills, vibratory mills, and bead mills, with bead mills being particularly preferred. When using a bead mill, any of the following crushing methods can be employed: wet crushing with circulation, wet crushing in batches, dry crushing with circulation, or dry crushing in batches. However, uniform crushing is preferable, and in this respect, wet crushing with circulation is preferred. The circulation conditions should be determined considering the firing temperature in the firing process, etc., but for example, by adjusting the circulation conditions, the log of the primary particles of the titanium-containing oxide powder can be adjusted. 10 (D90)-log 10 The value of (D10) can be suitably controlled. For wet crushing, the calcined titanium-containing oxide powder is added to water or an alcohol solvent and mixed in a slurry state. As the alcohol solvent, methanol, ethanol, isopropyl alcohol, etc., which have a boiling point of 100°C or less are preferred because they are easy to remove. Furthermore, from an industrial standpoint, water is preferred because it is easy to recover and dispose of.

[0038] Regarding the amount of solvent, it is preferable that the titanium-containing oxide powder after calcination is uniformly dispersed in the solvent. For this purpose, the slurry viscosity is preferably 8000 cP or less, more preferably 5000 cP or less, and even more preferably 3000 cP or less.

[0039] The circulation processing time (number of pulverization passes by circulation processing) for wet crushing is not particularly limited as long as the crystallinity of the titanium-containing oxide does not decrease and adversely affect battery performance, but the log of primary particles 10 (D90)-log 10 It is preferable to determine this according to the value of (D10). More preferably, log 10 (D90)-log 10 A range of (D10) < 0.7 is preferred. (Note that D90 represents the particle size at which the cumulative volume distribution of primary particles reaches 90% in the particle size distribution, and D10 represents the particle size at which the cumulative volume distribution of primary particles reaches 10% in the particle size distribution.)

[0040] <Surface treatment process> Next, the titanium-containing oxide obtained above is subjected to surface treatment. The titanium-containing oxide of the present invention is characterized by the presence of polyimide on the surface of the particles, preferably polyimide derived from a polymer-type all-aromatic polyimide precursor on the surface of the particles, which can improve rate characteristics and cycle characteristics when applied as a negative electrode material for batteries. The titanium-containing oxide powder of the present invention can be manufactured appropriately and relatively easily by employing the following surface treatment steps. In the following, the surface treatment steps will be explained using the case in which polyimide derived from a polymer-type all-aromatic polyimide precursor is present on the surface of the particles as an example.

[0041] First, the titanium-containing oxide powder base material and a polymer-type fully aromatic polyimide precursor (polyamic acid solution) are mixed and added as a surface treatment agent and then dried. There are no particular restrictions on the mixing method; either wet mixing or dry mixing can be used. However, it is preferable to uniformly disperse the polymer-type fully aromatic polyimide precursor on the surface of the particles constituting the titanium-containing oxide powder base material, and in this respect, wet mixing is preferred. There are also no particular restrictions on the drying method; for example, evaporation to dryness using a constant temperature bath can be used.

[0042] For wet mixing, a polymer-type fully aromatic polyimide precursor (polyamic acid solution) and the titanium-containing oxide powder of the base material are added to water or an alcohol solvent and mixed in a slurry state. As for the alcohol solvent, methanol, ethanol, isopropyl alcohol, etc., which have a boiling point of 100°C or less are preferred because they are easy to remove. Furthermore, from an industrial standpoint, water is preferred because it is easy to recover and dispose of.

[0043] The amount of the polymer-type total aromatic polyimide precursor (polyamic acid solution) added can be any amount as long as the amount of polyimide on the titanium-containing oxide surface falls within the range of the present invention. For example, when using a polyamic acid solution with a polymer concentration of 10% by mass, it is sufficient to add it in a ratio of 0.1% by mass or more relative to the titanium-containing oxide powder of the base material. It is also preferable to add it in a ratio of 20% by mass or less relative to the titanium-containing oxide powder of the base material, and more preferably 15% by mass or less. If the amount added is too small, polyimide will not form on the titanium-containing oxide surface, and no improvement in battery performance will be observed. Conversely, if the amount added is too large, polyimide will be excessively formed on the titanium-containing oxide surface, which can lead to a decrease in rate characteristics.

[0044] It is preferable to perform heat treatment after the above surface treatment. The heat treatment temperature should be a temperature at which polyimide derived from a polymer-type fully aromatic polyimide precursor is formed on the particle surface of the titanium-containing oxide powder of the substrate (imidation temperature), and at a temperature at which a significant reduction in specific surface area does not occur due to sintering of the titanium-containing oxide of the substrate. The upper limit of the heat treatment temperature is preferably 700°C or less, and more preferably 600°C or less. The lower limit of the heat treatment temperature is preferably 100°C or more, and more preferably 200°C or more. The heat treatment time is preferably 0.1 hours to 8 hours, and more preferably 0.5 hours to 5 hours. Furthermore, the heating method in the heat treatment is not particularly limited. Examples of usable heat treatment furnaces include fixed-bed kilns, roller hearth kilns, mesh belt kilns, fluidized bed kilns, and rotary kilns. The atmosphere during heat treatment can be either an air atmosphere or an inert atmosphere such as a nitrogen atmosphere.

[0045] The titanium-containing oxide powder obtained after heat treatment as described above shows slight aggregation, but does not require grinding that would destroy the particles. Therefore, after heat treatment, it is sufficient to perform crushing or classification to break down the aggregation as needed.

[0046] The titanium-containing oxide powder of the present invention may be granulated and heat-treated after adding and mixing a compound containing a dissimilar metal element other than Ti or Nb in a surface treatment step to obtain a powder containing secondary particles formed by the aggregation of primary particles. The dissimilar metal element may be one or more selected from the group consisting of elements of Group 2, Group 12, or Group 13 of the periodic table; metallic elements of Group 14 of the periodic table; and molybdenum elements. Specifically, one or more selected from the group consisting of Al, Mg, Ca, Sr, Zn, Ga, Ge, In, and Mo. Granulation can be carried out by any method as long as secondary particles are produced, but a spray dryer is preferred because it can process large quantities.

[0047] To reduce the moisture content of the titanium-containing oxide powder of the present invention, dew point control may be performed during the heat treatment process. Since moisture from the atmosphere will be adsorbed onto the powder if it is exposed to the air after heat treatment, 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 powder after heat treatment may be classified as needed to bring the particles within the desired maximum particle size range. When dew point control is performed during the heat treatment process, it is preferable to seal the titanium-containing oxide powder in an aluminum laminate bag or the like before exposing it to an environment outside of dew point control. Even under dew point control, grinding the titanium-containing oxide powder after heat treatment makes it easier for moisture to be absorbed from the crushed surface, increasing the moisture content of the powder; therefore, it is preferable not to grind the powder after heat treatment. The temperature and holding time within a specific range of heat treatment conditions greatly affect the secondary particle morphology and surface treatment process. The heat treatment temperature is preferably 450°C or higher, and preferably below 550°C. This is because exceeding 550°C significantly reduces the specific surface area, drastically decreasing battery performance, particularly rate characteristics. Furthermore, a holding time of one hour or more is preferable, as a shorter holding time is presumed to increase the moisture content of the powder and also affect the surface condition of the particles.

[0048] [Active material] The active material of the present invention contains the titanium-containing oxide powder of the present invention. The active material of the present invention may also contain one or more substances other than the titanium-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 products, carbon fibers], tin and tin compounds, silicon and silicon compounds, and metal oxides containing lithium.

[0049] [Energy storage devices] The energy storage device of the present invention comprises electrodes containing the active material of the present invention, and is a device that stores and releases energy by utilizing the intercalation and deintercalation of lithium ions to such electrodes, and examples include hybrid capacitors and lithium batteries.

[0050] [Lithium battery] The lithium batteries of this invention refer collectively to lithium primary batteries and lithium secondary batteries. Furthermore, in this specification, the term lithium secondary battery is used to include so-called lithium-ion secondary batteries and all-solid-state lithium-ion secondary batteries.

[0051] The lithium battery described above 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, etc., but 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 described above. This active material may be used as either a positive electrode active material or a negative electrode active material, but the case in which it is used as a negative electrode active material will be described below.

[0052] <Negative electrode> The negative electrode has a negative electrode layer on one or both sides of the negative electrode current collector, which contains a negative electrode active material (the active material of the present invention), a conductive agent, and a binder. This negative electrode layer is usually in the form of an electrode. In the case of a negative electrode current collector having pores, such as a porous material, the negative electrode layer containing the negative electrode active material (the active material of the present invention), a conductive agent, and a binder is contained within the pores.

[0053] The conductive agent for the negative electrode is not particularly limited as long as it is an electron-conducting material that does not undergo chemical changes. Examples include graphites such as natural graphite (flaky graphite, etc.) 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 (multi-walled concentric cylindrical graphite layer) (non-fishbone-shaped), cup-type carbon nanotubes (fishbone-shaped), segmented carbon nanofibers (non-fishbone structure), and platelet-type carbon nanofibers (playing card-shaped). Furthermore, graphites, carbon blacks, and carbon nanotubes may be used in appropriate mixtures.

[0054] The amount of conductive agent to be added varies depending on the specific surface area of ​​the active material and the type and combination of conductive agents, so optimization should be performed. Preferably, it is 0.1% to 10% by mass in the negative electrode layer, and more preferably 0.5% to 5% by mass. Below 0.1% by mass, the conductivity of the negative electrode layer cannot be ensured, and above 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 high capacity applications. The conductive agent may be added during electrode fabrication, or the conductive agent 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.

[0055] Examples of binders for the negative electrode include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR), and carboxymethylcellulose (CMC). While not particularly limited, the molecular weight of polyvinylidene fluoride is preferably 20,000 to 1,000,000. From the viewpoint of ensuring the bonding properties of the negative electrode layer, it is preferably 25,000 or more, more preferably 30,000 or more, and even more preferably 50,000 or more. From the viewpoint of ensuring conductivity without hindering contact between the active material and the conductive agent, it is preferably 500,000 or less. In particular, the specific surface area of ​​the active material is 10 m². 2 If the amount is greater than or equal to 1g, the molecular weight is preferably 100,000 or more.

[0056] 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, but is preferably 0.2% to 15% by mass in the negative electrode layer. From the viewpoint of improving bonding properties 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 not reducing the active material ratio and thus reducing the discharge capacity of the energy 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.

[0057] <Positive electrode> The positive electrode has a positive electrode layer on one or both sides of the positive electrode current collector, which includes a positive electrode active material, a conductive agent, and a binder.

[0058] As the positive electrode active material, a material capable of intercalating and releasing lithium is used. For example, the active material may be a composite metal oxide containing cobalt, manganese, and nickel with lithium, or a lithium-containing olivine-type phosphate. These positive electrode active materials can be used individually or in combination of two or more. Examples of such composite metal oxides include LiCoO2, LiMn2O4, LiNiO2, and LiCo 1-x Ni x O2(0.01 <X<1)、LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, LiLiLi 1 / 2 Mn 3 / 2 Examples include O4, and some of these lithium composite oxides may be substituted with other elements. For example, some of the cobalt, manganese, and nickel may be 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., some of the oxygen may be substituted with S or F, or compounds containing these other elements may be coated. Examples of lithium-containing olivine-type phosphates include LiFePO4, LiCoPO4, LiNiPO4, LiMnPO4, LiFe 1-x Examples include MxPO4 (where M is at least one selected from Co, Ni, Mn, Cu, Zn, and Cd, and X is 0 ≤ X ≤ 0.5).

[0059] Examples of conductive agents and binders for the positive electrode are the same as those for the negative electrode. Examples of the positive electrode current collector include aluminum, stainless steel, nickel, titanium, calcined carbon, and aluminum or stainless steel with carbon, nickel, titanium, or silver surface treatments. The surfaces of these materials may be oxidized, and surface treatments may be used to create irregularities on the surface of the positive electrode current collector. Examples of the form of the current collector include sheets, nets, foils, films, punched materials, laths, porous materials, foams, fiber groups, and molded nonwoven fabrics.

[0060] <Nonaqueous electrolyte> A non-aqueous electrolyte is prepared by dissolving an electrolyte salt in a non-aqueous solvent. There are no particular restrictions on the non-aqueous electrolyte, and various types can be used.

[0061] The electrolyte salts used are those that dissolve in non-aqueous electrolytes. Examples include inorganic lithium salts such as LiPF6, LiBF4, LiPO2F2, LiN(SO2F)2, and LiClO4; lithium salts containing linear alkyl fluoride such as 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); lithium salts containing cyclic alkylene fluoride chains such as (CF2)2(SO2)2NLi and (CF2)3(SO2)2NLi; and lithium salts with oxalate complexes such as bis[oxalate-O,O']lithium borate and difluoro[oxalate-O,O']lithium borate as anions. Among these, the most preferred electrolyte salts are LiPF6, LiBF4, LiPO2F2, and LiN(SO2F)2, with LiPF6 being the most preferred. These electrolyte salts can be used individually or in combination of two or more. A preferred combination of these electrolyte salts is one in which LiPF6 is included, and at least one lithium salt selected from LiBF4, LiPO2F2, and LiN(SO2F)2 is also included in the non-aqueous electrolyte.

[0062] On the other hand, examples of the non-aqueous solvent include cyclic carbonates, linear carbonates, linear esters, ethers, amides, phosphate esters, sulfones, lactones, nitriles, and S=O bond-containing compounds, with the inclusion of cyclic carbonates being preferable. The term "linear ester" is used as a concept that includes linear carbonates and linear carboxylic acid esters.

[0063] 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 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 preferable from the viewpoint of improving the charge rate characteristics of the energy storage device and suppressing gas generation during high-temperature operation, and one or more cyclic carbonates having alkylene chains selected from propylene carbonate, 1,2-butylene carbonate, and 2,3-butylene carbonate are even more preferable. The proportion of cyclic carbonates having alkylene chains in the total cyclic carbonate is preferably 55% to 100% by volume, and more preferably 60% to 90% by volume.

[0064] Suitable examples of chain-like esters include one or more asymmetric chain-like 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-like carbonates selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, and dibutyl carbonate; pivalate esters such as methyl pivalate, ethyl pivalate, and propyl pivalate; and one or more chain-like carboxylic acid esters selected from methyl propionate, ethyl propionate, propyl propionate, methyl acetate, and ethyl acetate (EA).

[0065] <Structure of a lithium battery> The structure of the lithium battery of the present invention is not particularly limited, and examples include a coin cell having a positive electrode, a negative electrode, and a single-layer or multi-layer separator, as well as cylindrical batteries and prismatic batteries having a positive electrode, a negative electrode, and a roll-shaped separator.

[0066] As the separator, an insulating thin film with high ion permeability and a predetermined mechanical strength is used. Examples include polyethylene, polypropylene, cellulose paper, glass fiber paper, polyethylene terephthalate, and polyimide microporous films, and multilayer films composed of two or more of these materials can also be used. Furthermore, the surface of these separators can be coated with resins such as PVDF, silicone resin, and rubber-based resin, or with particles of metal oxides such as aluminum oxide, silicon dioxide, and magnesium oxide. The pore size of the separator can be within a range generally useful for batteries, for example, 0.01 μm to 10 μm. The thickness of the separator can be within a range generally used for batteries, for example, 5 μm to 300 μm.

[0067] <Solid electrolyte> A solid electrolyte is a solid electrolyte that can move ions within itself. In particular, inorganic solid electrolytes are solid in a steady state and therefore do not usually dissociate or liberate into cations and anions. Inorganic solid electrolytes are not particularly limited as long as they have conductivity of metal ions belonging to Group 1 of the periodic table, and generally have little to no electronic conductivity. Representative examples of inorganic solid electrolytes include (A) sulfide inorganic solid electrolytes and (B) oxide inorganic solid electrolytes. Sulfide inorganic solid electrolytes are particularly preferred because they have high ionic conductivity and can form dense molded bodies with few grain boundaries by pressurization at room temperature alone. The periodic table in this invention refers to the long-period type periodic table of elements as defined by IUPAC (International Union of Pure and Applied Chemistry).

[0068] The sulfide inorganic solid electrolyte may be amorphous glass, crystallized glass, or a crystalline material. The following combinations are preferred as the sulfide inorganic solid electrolyte, but are not particularly limited. Li2S-P2S5, Li2S-P2S5-Al2S3, Li2S-GeS2, Li2S-Ga2S3, Li2S-GeS2-Ga2S3, Li2S-GeS2-P2S5, Li2S -GeS2-Sb2S5, Li2S-GeS2-Al2S3, Li2S-SiS2, Li2S-Al2S3, Li2S-SiS2-Al2S3, Li2S-SiS2-P2S5, Li 10 GeP2S 12 .

[0069] Among the aforementioned combinations, LPS glass and LPS glass ceramics manufactured using the combination of Li2S-P2S5 are preferred. In addition, algerodite-type solid electrolytes such as Li6PS5Cl and Li6PS5Br are also preferred as sulfide inorganic solid electrolytes other than those mentioned above.

[0070] Oxide inorganic solid electrolytes contain oxygen atoms and are metals belonging to Group 1 of the periodic table. Materials that are both on-conductive and electronically insulating are preferred.

[0071] Examples of oxide inorganic solid electrolytes include Li, which has a LISICON (Lithium superionic conductor) type crystal structure. 3.5 Zn 0.25 La, which has a perovskite crystal structure, is GeO4. 0.55 Li 0.35 LiTi2P3O has a TiO3, NASICON (Natrium superionic conductor) type crystal structure. 12 Li7La3Zr2O has a garnet-type crystal structure. 12 (LLZ), lithium phosphate (Li3PO4), LiPON (Lithium Phosphate with some of the oxygen replaced by nitrogen), Li3BO3-Li2SO4, Li2O-B2O3-P2O5, Li2O-SiO2, and Li6BaLa2Ta2O 12 These are some examples of preferred materials.

[0072] The volume-average particle size of the inorganic solid electrolyte is not particularly limited, but is preferably 0.01 μm or larger, and more preferably 0.1 μm or larger. The upper limit is preferably 100 μm or less, and more preferably 50 μm or less. [Examples]

[0073] 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 encompasses various combinations that can be easily inferred from the spirit of the invention.

[0074] (Liquid-based lithium-ion secondary battery using lithium titanate) [Example 1-1] <Raw material preparation process> Li₂CO₃ (average particle size 4.6 μm) and anatase-type TiO₂ (specific surface area 10 m²) are used so that the atomic ratio of Li to Ti (Li / Ti) is 0.83. 2 A raw material mixture slurry was prepared by weighing the raw material powder (by g) and adding deionized water to the mixture so that the solid content concentration of the slurry was 41% by mass, and then stirring. This raw material mixture slurry was then wet-mixed and pulverized using a bead mill (Willi E. Bakkofen, model: Dynomill KD-20BC, agitator material: polyurethane, vessel inner surface material: zirconia). Zirconia beads (outer diameter: 0.65 mm) were packed into the vessel at 80% by volume, and the process was carried out with an agitator peripheral speed of 13 m / s and a slurry feed rate of 55 kg / hr, while controlling the internal pressure of the vessel to 0.02 to 0.03 MPa.

[0075] <Firing Process> The obtained mixed slurry was introduced into the furnace core from the raw material supply side of a rotary kiln-type firing furnace (furnace core length: 4 m, furnace core diameter: 30 cm, external heating type) equipped with an anti-adhesion mechanism, dried in a nitrogen atmosphere, and fired. At this time, the tilt angle of the furnace core from the horizontal direction was 2.5 degrees, the rotation speed of the furnace core was 20 rpm, and the flow rate of nitrogen introduced into the furnace core from the calcined material recovery side was 20 L / min. The heating temperature of the furnace core was set to 600°C on the raw material supply side, 900°C in the center, and 900°C on the calcined material recovery side, and the holding time of the calcined material at 900°C was 30 minutes.

[0076] <Post-processing steps> The calcined material recovered from the calcined material recovery side of the furnace tube was crushed using a bead mill (AIMEX, NVM-1.5 type) to obtain a calcined powder sample. Powder X-ray diffraction measurements were performed on the obtained calcined powder sample, and it was confirmed to be lithium titanate (ICDD (PDF2010) PDF card 00-049-0207), a titanium-containing oxide powder.

[0077] <Surface treatment process> The obtained calcined powder sample is dissolved by adding deionized water to the slurry so that the solid content concentration is 30% by mass, and stirring. As a surface treatment agent, a polyamic acid solution, which is a water-soluble polymer-type fully aromatic polyimide precursor, is obtained as UPIA. (R) -NF1001 (a polyamic acid varnish manufactured by Ube Industries, Ltd. (aromatic tetracarboxylic acid + aromatic diamine, polymer concentration 10% by mass, solvent: water)) was added at a concentration of 1% by mass to 100 g of crushed calcined powder to prepare a mixed slurry. This mixed slurry was mixed in a paint shaker for 3 hours, dried at a temperature of 60°C, and then heat-treated in a muffle furnace at 200°C for 3 hours to produce lithium titanate (hereinafter referred to as LTO) as a titanium-containing oxide powder having polyimide derived from a polymer-type total aromatic polyimide precursor on the particle surface according to Example 1-1. The imidization rate of the polyimide present on the LTO surface was measured using the method described in Japanese Patent Application Publication No. 2009-6542, which utilizes an IR-ATR as a measuring device and calculates the imidization rate using the ratio of the vibration band peak heights of each cured product and the fully cured product. The imidization rate was 99% (similar results were obtained in Examples 1-2 to 1-4 and Reference Example 1-1 described later).

[0078] [Examples 1-2 to 1-3, Reference Example 1-1] In the surface treatment process, UPIA is used as the treatment agent. (R) Except for the amount of NF1001 added as shown in Table 1, the same procedure as in Example 1-1 was followed to produce LTO according to Example 1-2, Example 1-3, and Reference Example 1-1.

[0079] [Examples 1-4] (Polymerization of a partial aliphatic polyimide precursor polyamic acid aqueous solution A) In a glass reaction vessel equipped with a stirrer and nitrogen gas inlet / outlet pipes, 349 g of water was added as a solvent. To this, 37.06 g (0.214 mol) of 1,10-decanediamine and 51.42 g (2.50 equivalents relative to the carboxyl group) of 1,2-dimethylimidazole were added and the mixture was stirred at 25°C for 1 hour to dissolve. To this solution, 62.94 g (0.214 mol) of 3,3',4,4'-biphenyltetracarboxylic dianhydride was added and the mixture was stirred at 70°C for 6 hours to obtain an aqueous polyamic acid solution A with a solid content concentration of 18.5% by mass and a solution viscosity of 6.6 Pa·sec. In the surface treatment step, 5% by mass of aqueous polyamic acid solution A was added per 100 g of crushed calcined powder, except that the LTO according to Example 1-4 was produced in the same manner as in Example 1-1.

[0080] [Comparative Example 1-1] LTO according to Comparative Example 1-1 was manufactured in the same manner as in Example 1-1, except that no surface treatment agent was added during the surface treatment process.

[0081] [Comparative Example 1-2] In the surface treatment process, an attempt was made to produce LTO according to Comparative Example 1-2 in the same manner as in Example 1-1, except that a monomer-type total aromatic polyimide precursor solution, which was a mixture of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), metaphenylenediamine (MPDA), and an aqueous solvent as described in Patent Document 1, was added at a mass of 5% each per 100g of crushed calcined powder. However, the surface treatment agent itself did not dissolve in water, and the monomer-type total aromatic polyimide precursor solution itself could not be prepared.

[0082] <Method for calculating the polyimide content> The polyimide content in the titanium-containing oxide powder of each example and comparative example was calculated from the polymer concentration of each polyimide precursor solution added and the mass of the titanium-containing oxide powder.

[0083] <Method for measuring the imidization rate> Using an IR-ATR as the measuring device, the imidization rate was calculated by utilizing the ratio of the vibrational band peak heights between each cured sample (a polyimide precursor contained on the particle surface of titanium-containing oxide powder cast onto a glass plate and treated under the same curing conditions as the powder) and the fully cured sample. Specifically, a baseline was drawn based on the peak corresponding to the imide group, and the ratio of the height of this imide group peak to the height of the peak corresponding to the benzene ring was calculated. Next, the ratio was calculated for each cured (heat-treated) sample whose imidization rate was to be determined using the same method, and the imidization rate of each cured (heat-treated) sample relative to the imidized film was calculated.

[0084] <Calculation of average particle size D50: Dry laser diffraction scattering method> The average particle size D50 of the titanium-containing oxide powders in each example and comparative example was calculated from the particle size distribution curve measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT3300EXII, manufactured by Nikkiso Co., Ltd.), and in all cases it was 1.0 μm, showing no significant difference.

[0085] <Measurement of specific surface area> The specific surface area (BET) of the titanium-containing oxide powder in each example and comparative example (m²) 2 The specific surface area ( / g) was measured using a fully automated BET specific surface area analyzer (Mountec Co., Ltd., product name "Macsorb HM model-1208"), with nitrogen gas used as the adsorption gas. 0.5g of the sample powder was weighed, placed in a φ12 standard cell (HM1201-031), degassed under vacuum at 100°C for 0.5 hours, and then measured using the BET single-point method.

[0086] [Evaluation of battery characteristics] Coin-type batteries were fabricated using LTO from each example and comparative example, and their battery characteristics were evaluated. The evaluation results are shown in Table 1.

[0087] <Fabrication of the negative electrode sheet> The negative electrode sheets were prepared in a room controlled at 25°C and a dew point of -20°C or lower as follows. A coating was prepared by mixing 90% by mass of LTO as the active material, 5% by mass of acetylene black as the conductive agent, and 5% by mass of polyvinylidene fluoride as the binder, as follows. Polyvinylidene fluoride, acetylene black, and 1-methyl-2-pyrrolidone, which had been dissolved in 1-methyl-2-pyrrolidone beforehand, were mixed in a planetary stirring and defoaming apparatus. Then, LTO was added and the mixture was adjusted to a total solid content concentration of 64% by mass, and mixed in a planetary stirring and defoaming apparatus. Subsequently, 1-methyl-2-pyrrolidone was added and the mixture was adjusted to a total solid content concentration of 50% by mass, and mixed in a planetary stirring and defoaming apparatus to prepare the coating. The obtained coating was applied to aluminum foil and dried to prepare a single-sided negative electrode sheet for use in the coin cell described later, and a double-sided negative electrode sheet for use in the laminate cell described later. The target base weight during coating is 7.5 mg / cm². 2 That's what I decided.

[0088] <Fabrication of the positive electrode sheet> Except for using lithium nickel-cobalt-manganate powder as the active material, the positive electrode single-sided sheet was prepared using the same method as described in the <Preparation of Negative Electrode Sheet> section above, including the ratios of the active material, conductive agent, and binder.

[0089] <Preparation of Electrolyte> The electrolytes used for characterization batteries were prepared as follows: A non-aqueous solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a 1:2 (volume ratio) was prepared in an argon glove box controlled at a temperature of 25°C and a dew point of -70°C or lower. LiPF6 was dissolved in this solvent as an electrolyte salt to a concentration of 1 M to prepare the electrolyte for coin cells described later. Similarly, a non-aqueous solvent of propylene carbonate (PC) and diethyl carbonate (DEC) in a 1:2 (volume ratio) was prepared, and LiPF6 was dissolved in this solvent as an electrolyte salt to a concentration of 1.3 M to prepare the electrolyte for laminated batteries described later.

[0090] <Manufacturing a coin cell (single-electrode)> The negative electrode single-sided sheet produced by the method described above was punched out into a circle with a diameter of 14 mm, at 2 t / cm². 2 After pressing under pressure, the evaluation electrodes were fabricated by vacuum drying at 120°C for 5 hours. The fabricated evaluation electrodes and metallic lithium (formed into a circular shape with a thickness of 0.5 mm and a diameter of 16 mm) were placed facing each other through a glass filter (one each of ADVANTEC GA-100 and Whatman GF / C), and a non-aqueous electrolyte prepared by the method described in <Preparation of Electrolyte> above was added and sealed to create a 2032 type coin cell.

[0091] <Fabrication of laminated batteries (full cells)> The laminated batteries were fabricated in a room controlled at 25°C and with a dew point of -40°C or lower as follows: The negative electrode double-sided sheet fabricated by the method described above was laid at 2 t / cm². 2 After press-forming with pressure, a negative electrode with a lead wire connection portion was fabricated. Next, a positive electrode single-sided sheet was fabricated at 2 t / cm². 2 After press-forming under pressure, a positive electrode with a lead wire connection portion was fabricated. The fabricated negative and positive electrodes were vacuum-dried at 150°C for 12 hours. The vacuum-dried positive and negative electrodes were placed opposite each other via a separator (UPZ210, manufactured by Ube Industries), laminated, and aluminum foil lead wires were connected to the positive and negative electrodes respectively. An electrolyte for laminate batteries, prepared using the method described in <Preparation of Electrolyte> above, was added, and the assembly was vacuum-sealed with aluminum laminate to fabricate a laminate battery for evaluation. At this time, the battery capacity was 30 mAh, and the ratio of the negative electrode capacity to the positive electrode capacity (negative electrode capacity / positive electrode capacity) was 1.2. Next, as an aging process, the laminated battery prepared using the method described in <Fabrication of Laminated Battery> above was charged to 2.75V with a current of 0.2C in a constant temperature bath at 60°C. After that, constant current constant voltage charging was performed, where the charging current was reduced to 0.05C at 2.75V, followed by two cycles of constant current discharge, where the battery was discharged to 1.4V with a current of 0.2C. Subsequently, the volume of the laminated battery was measured by the Archimedes method and defined as the initial volume of the laminated battery (hereinafter sometimes referred to as the initial volume).

[0092] <Initial battery characteristics (unipolar): Initial efficiency, 0.2C initial discharge capacity, 10C rate charge / discharge characteristics> In a constant temperature bath at 25°C, the coin-type battery prepared using the method described above in <Fabrication of Coin Cells> was charged at 0.2 mA / cm² with the direction in which Li is absorbed into the evaluation electrode being used as the charging direction. 2 The device is charged to 1V with a current density of 0.05mA / cm², and then the charging current at 1V is 0.05mA / cm². 2 Constant current and constant voltage charging was performed to charge the LTO until it reached a certain current density, followed by three cycles of constant current discharge, discharging to 2V at a current density equivalent to 0.2C. The initial efficiency (%) was calculated by dividing the discharge capacity (mAh) of the first cycle by the charge capacity (mAh). The initial discharge capacity (mAh / g) of 0.2C was calculated by dividing the discharge capacity of the third cycle by the weight of the LTO. Next, the LTO was charged to 1V with a current equivalent to the initial discharge capacity of 10C to determine the 10C rate charge capacity. The 10C rate charge capacity retention rate (%) was calculated by dividing this 10C rate charge capacity by the initial discharge capacity of 0.2C. Subsequently, constant current and constant voltage charging was performed according to the method described above, followed by discharging to 2V with a current equivalent to the initial discharge capacity of 10C to determine the 10C rate discharge capacity. The 10C rate discharge capacity retention rate (%) was calculated by dividing this 10C rate discharge capacity by the initial discharge capacity of 0.2C. The results are shown in Table 1. If LTO has high 10C rate charge / discharge characteristics, applying it as an electrode material for energy storage devices can be expected to improve the rapid charge / discharge performance of those devices. Note that 1C represents the current value during charging and discharging. For example, 1C refers to the current value that can completely discharge (or fully charge) the theoretical capacity in 1 / 1 hour, while 0.1C refers to the current value that can completely discharge (or fully charge) the theoretical capacity in 1 / 0.1 hour.

[0093] <High-temperature battery characteristics: Cycle capacity retention rate, post-cycle 10C rate discharge capacity retention rate, gas generation amount> Using the laminated batteries prepared using the method described above in "Fabrication of Laminated Batteries," the batteries were charged to 2.75V with a current of 1C in a constant temperature bath at 55°C, followed by constant current constant voltage charging at 2.75V until the charging current was reduced to 0.05C. This constant current discharge was then repeated 100 times, with the batteries discharged to 1.4V with a current of 1C. The high-temperature battery characteristics referred to here are the evaluation of battery characteristics at temperatures above 50°C, and represent an evaluation under a measurement temperature environment where battery degradation is relatively more likely to occur compared to, for example, the evaluation of battery characteristics at 25°C.

[0094] After 100 cycles, the discharge capacity was measured at 25°C when charged and discharged to 2.75V with a current equivalent to 0.2C or 10C. The cycle capacity retention rate (%) was calculated by dividing the discharge capacity at the 0.2C rate after the cycles by the initial discharge capacity at 0.2C. Similarly, the 10C rate discharge capacity retention rate (%) after the cycles was calculated by dividing the discharge capacity at the 10C rate after the cycles by the initial discharge capacity at 0.2C.

[0095] Laminate batteries, prepared using the method described in <Fabrication of Laminate Batteries> above, were charged to approximately 30% of their capacity with a current of 0.2C in a constant temperature bath at 25°C. A 30-day storage test was then conducted in a constant temperature bath at 60°C. The volume of the laminate batteries after the storage test was measured using the Archimedes method to determine the post-storage volume (hereinafter sometimes referred to as post-storage volume). The gas generation amount (ml) was calculated by subtracting the initial volume from the post-storage volume, and Table 1 shows the relative values ​​when the gas generation amount in Comparative Example 1-1 is set to 100%. [Table 1]

[0096] <Evaluation Results> The electrodes using LTO in Examples 1-1 to 1-3 and 1-4 were found to exhibit excellent high-temperature cycle characteristics and high-temperature rate characteristics while maintaining initial discharge capacity and rate characteristics, and further suppressing gas generation after high-temperature storage, by incorporating a predetermined amount of polyimide derived from polymer-type all-aromatic polyimide precursor on the surface of titanium-containing oxide particles. In particular, in the case of all-aromatic polyimide precursors that do not contain polymer-type partially aliphatic polyimide precursors, a tendency for further improvement in initial discharge rate characteristics was observed. Furthermore, even when a relatively large amount of polyimide derived from polymer-type all-aromatic polyimide precursors was included, as in Reference Example 1-1, although the initial efficiency decreased, the cycle characteristics were improved while maintaining good discharge rate characteristics. On the other hand, the LTO of Comparative Example 1-1 did not show a decrease in initial 10C rate charge / discharge characteristics or suppression of gas generation after high-temperature storage, and did not result in an improvement in battery characteristics. In addition, although an attempt was made to process the monomer-type surface treatment agent described in Patent Document 1 into an aqueous solution for coating, the surface treatment agent did not dissolve in water, and it was not possible to prepare the monomer-type all-aromatic polyimide precursor solution itself.

[0097] [Examples 2-1 to 2-2] In the surface treatment process, UPIA is used as the surface treatment agent. (R) Except for the addition of NF1001 (as shown in Table 2) and the heat treatment temperature (350°C or 250°C), the same procedure as in Example 1-1 was followed to produce LTOs according to Example 2-1 and Example 2-2. The initial battery characteristics (monoelectrode) were evaluated in the same manner as in Example 1-1. The results, along with the data from Example 1-3 and Comparative Example 1-1, are shown in Table 2. [Table 2]

[0098] <Evaluation Results> The results from Examples 2-1 to 2-2, and from 1-3 using LTO, showed that the charge rate characteristics improved when the imidization rate was within a predetermined numerical range. Furthermore, the high-temperature battery characteristics of Examples 2-1 and 2-2 showed performance equivalent to that of Example 1-3.

[0099] (Liquid-based lithium-ion secondary battery using niobium titanate) <Raw material preparation process> Nb2O5 (average particle size 0.2 μm) and anatase-type TiO2 (specific surface area 10 m²) 2 The powder was weighed ( / g) in a molar ratio of 1:1 and mixed. This mixed powder was heat-treated at 1000°C for 5 hours. Powder X-ray diffraction measurements were performed on the resulting calcined powder sample under conditions of a sampling interval of 0.01° and a scan speed of 2° / min. Crystal structure analysis results by the Rietveld method confirmed that the synthesized sample was the target niobium-titanium composite oxide, niobium titanate (TiNb2O7: Titanium niobium oxide, ICDD (PDF2010) PDF card 01-077-1374).

[0100] [Example 3] Using the calcined powder obtained above as the base, and in the surface treatment process, UPIA as the surface treatment agent (R) Except for the amount of -NF1001 added shown in Table 3, the procedure was the same as in Example 1-1, and niobium titanate (hereinafter referred to as TNO) was produced as a titanium-containing oxide powder having a polyimide derived from a water-soluble polymer-type total aromatic polyimide precursor on the particle surface. The imidization rate of the polyimide present on the TNO surface was measured using an IR-ATR as a measuring device, and the imidization rate was calculated by using the ratio of the vibration band peak heights of each cured product and the fully cured product. The imidization rate was found to be 99%.

[0101] [Comparative Example 3] TNO according to Comparative Example 3 was produced in the same manner as in Example 3, except that no surface treatment agent was added during the surface treatment process.

[0102] [Evaluation of battery characteristics] Coin-type batteries were fabricated in each example and comparative example using the same procedure as in Example 1-1, except for the use of TNO, and their battery characteristics were evaluated. The evaluation results are shown in Table 3.

[0103] <Battery characteristics: electrode density, 0.2C initial discharge capacity, 2C rate discharge capacity retention rate, cycle capacity retention rate> The mass and thickness of the electrode sheets fabricated using TNO in each example and comparative example were measured, and the electrode density was calculated. Furthermore, the same tests as in Example 1-1 were performed using the fabricated coin batteries to calculate the 0.2C initial discharge capacity and the 2C rate discharge capacity retention rate.

[0104] The cycle capacity retention rate was calculated by performing constant current / constant voltage charging, where the battery was charged to a current equivalent to 0.05C at 0.8V, followed by constant current discharge, where the battery was discharged to 2V at a current equivalent to the initial discharge capacity of 0.5C. This process was repeated for a total of 15 cycles. The discharge capacity after 15 cycles was divided by the initial discharge capacity to determine the discharge capacity retention rate (%).

[0105] [Table 3]

[0106] <Evaluation Results> The electrode using TNO in Example 3 was found to have superior cycle characteristics compared to Comparative Example 3, without impairing electrode density or initial discharge characteristics, by incorporating a predetermined amount of polyimide derived from a polymer-type total aromatic polyimide precursor on the surface of titanium-containing oxide particles.

[0107] (All-solid-state secondary battery) [Example 4] In a glove box under an argon atmosphere, TNO (niobium titanate having polyimide derived from a polymer-type fully aromatic polyimide precursor on the particle surface) and Li6PS5Cl powder (volume-average particle size obtained using a laser diffraction / scattering particle size distribution analyzer: 6 μm), which is a sulfide inorganic solid electrolyte, were weighed in a mass ratio of TNO:Li6PS5Cl = 60:40 and mixed in an agate mortar. Next, zirconia balls (3 mm in diameter, 20 g) were placed in an 80 mL zirconia pot, and the mixed powder was added. Then, this pot was set in a planetary ball mill and stirred at a rotation speed of 200 rpm for 15 minutes to obtain the anode active material composition of Example 4. The obtained anode active material composition was pressed at room temperature for 10 minutes (360 MPa) to produce pellets (molded bodies) 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 fabricate an all-solid-state secondary battery, and its battery characteristics were evaluated. The results are shown in Table 4.

[0108] [Comparative Example 4] An all-solid-state secondary battery was fabricated in the same manner as in Example 4, except that TNO (niobium titanate without surface treatment agent) was used in Comparative Example 3, and the battery characteristics were evaluated. The results are shown in Table 4.

[0109] <Measurement of charge rate characteristics> In a constant temperature bath at 25°C, the all-solid-state secondary battery prepared using the method described above was charged to 0.5V with a current equivalent to 0.05C of the theoretical capacity of the TNO, with the direction in which Li is absorbed into the evaluation electrode considered as charging. Then, constant current constant voltage charging was performed at 0.5V until the charging current was equivalent to 0.01C, and then constant current discharge was performed to 2V with a current equivalent to 0.05C. The initial discharge capacity (mAh / g) was calculated by dividing the discharge capacity (mAh) by the mass of the TNO. Next, the battery was charged to 0.5V with a current equivalent to 0.4C of the theoretical capacity of the TNO, and then discharged to 2V with a current of 0.05C to determine the 0.4C charge capacity (mAh / g). The rate characteristic (%) was calculated by dividing the 0.4C charge capacity by the initial discharge capacity. The charge rate characteristic was examined relative to the value of Comparative Example 3, which was set to 100%. The evaluation results are shown in Table 4.

[0110] [Table 4]

[0111] In the all-solid-state secondary battery system, the electrode containing the negative electrode layer using TNO in Example 4 was found to have excellent charge rate characteristics due to the inclusion of polyimide derived from a polymer-type all-aromatic polyimide precursor on the particle surface. [Industrial applicability]

[0112] The polyimide-coated titanium-containing oxide powder obtained in this invention can improve initial charge-discharge rate characteristics, and further improve long-term cycle characteristics and storage characteristics, making it useful as an electrode active material for lithium-ion secondary batteries. Furthermore, lithium-ion secondary batteries using this polyimide-coated titanium-containing oxide powder as an electrode active material can perform stable, high-speed charge-discharge, making them useful as secondary batteries for driving or backing up various devices such as automobiles and electronic devices, and for storing electricity at night in homes and offices.

Claims

1. General formula Li ４ Ti ５ O １２ Or Ti １－ｘ／２ Nb ２ O ７－ｘ A method for producing a titanium-containing oxide powder comprising titanium-containing oxide particles represented by (0 ≤ X < 2), wherein at least a portion of the surface of the titanium-containing oxide particles is coated with polyimide, and the polyimide content (mass%) present on the surface of the particles is 0.01 to 1.5, A method for producing titanium-containing oxide powder, characterized by comprising the steps of mixing the titanium-containing oxide particles and a water-soluble polyimide precursor in an aqueous solvent, drying and heat-treating the mixture to form a coating of the polyimide on at least a portion of the surface of the titanium-containing oxide particles.

2. General formula Ti １－ｘ／２ Nb ２ O ７－ｘ The titanium-containing oxide represented by (0≦X<2) is TiNb ２ O ７ The method for producing titanium-containing oxide powder according to claim 1, characterized in that:

3. The method for producing titanium-containing oxide powder according to claim 1, wherein the polyimide precursor is a water-soluble polymer-type total aromatic polyimide precursor.

4. A method for producing an electrode, characterized by obtaining a titanium-containing oxide powder by the method described in any one of claims 1 to 3, and using the titanium-containing oxide powder.

5. A method for manufacturing an energy storage device, characterized by obtaining an electrode by the method described in claim 4 and using the electrode.

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