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

Abstract

To provide a lithium titanate powder which is used as an electrode material for a power storage device, has excellent charge rate characteristics, has high initial efficiency, can suppress the amount of gas generated during high-temperature operation, and can further increase the electrode density; an electrode using the same; and a power storage device.SOLUTION: A lithium titanate powder comprises Li4Ti5O12 as a main component in which molybdenum (Mo) is composed of secondary particles formed by aggregating primary particles localized in the vicinity of a particle surface. The lithium titanate powder has a specific surface area of 6.2 m2 / g to 6.8 m2 / g and has a D50 of 0.6 μm to 0.8 μm for the primary particles of which a volume accumulation is equivalent to 50% in a volume reference particle size distribution by a laser diffraction scattering method.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] In recent years, various materials have been studied as electrode materials for energy storage devices. Among these, lithium titanate has attracted attention as an active material for energy storage devices in electric vehicles (HEVs, PHEVs, and BEVs) due to its excellent input / output characteristics.

[0003] It is not uncommon for the temperature inside a car to exceed 60°C in the summer. Therefore, energy storage devices for electric vehicles must be safe and perform well even at high temperatures. However, when energy storage devices containing lithium titanate are used at temperatures above 60°C, maintaining cycle stability becomes more difficult, and their charge / discharge capacity deteriorates with each charge / discharge cycle. Furthermore, during the initial stage of operation, water adsorbed to the surface hydroxyl groups can be reduced and decomposed, generating hydrogen gas. Furthermore, repeated charge / discharge cycles can cause electrochemical side reactions in lithium titanate, decomposing the organic solvents contained in the electrolyte and generating gases such as carbon monoxide, carbon dioxide, methane, and hydrogen (hereinafter referred to as decomposition gases). These decomposition gases can cause the energy storage device to expand and pose safety concerns. Therefore, there is a need for lithium titanate that can reduce the amount of gas generation while maintaining the battery performance during high-temperature operation of the energy storage device.

[0004] Patent Document 1 describes a specific surface area of ​​4m 2 / g or more, and contains at least one localized element selected from boron (B), Ln (Ln is at least one metal element selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Hb, Er, Tm, Yb, Lu, Y, and Sc), and M1 (M1 is at least one metal element selected from W and Mo), wherein the localized elements, boron (B), Ln, and M1, are localized near the surfaces of the lithium titanate particles that constitute the lithium titanate powder. Patent Document 1 reports that when used as an electrode material for an electricity storage device, the resulting lithium titanate powder has a large charge / discharge capacity and can suppress gas generation during high-temperature operation.

[0005] Patent Document 2 discloses a negative electrode active material containing lithium-titanium composite oxide particles having an average pore diameter of 50 Å to 500 Å, and a nonaqueous electrolyte battery and battery pack using the same. According to Patent Document 2, it is possible to obtain a negative electrode active material, a nonaqueous electrolyte battery, and a battery pack that are excellent in large current characteristics and charge / discharge cycle characteristics.

[0006] Patent Document 3 discloses lithium titanate granules having a grinding degree Zd, represented by the following formula 1, of 2 or more, where D50 is the volume median particle size, D50,1 is the D50 before grinding, and D50,2 is the D50 after grinding. (Formula 1) Zd=D50,1 / D50,2 According to Patent Document 3, lithium titanate can be easily pulverized when mixed with a binder to produce an electrode, and easily dispersible lithium titanate can be obtained.

[0007] Patent Document 4 describes a method in which primary particles aggregate to form spherical secondary particles with a particle size of 1 μm to 50 μm, and a specific surface area of ​​0.5 m 2 / g~10m 2 / g, with the main component being Li 4 / 3 Ti 5 / 3Patent Document 4 discloses a lithium-titanium composite oxide characterized by being composed of O4. According to Patent Document 4, it is said that a battery having a large charge / discharge capacity, excellent cycle stability, and safety can be obtained.

[0008] Patent Document 5 discloses an active material used as a positive electrode active material or a negative electrode active material for non-aqueous electrolyte secondary batteries, which is obtained by mixing lithium titanate with an active material obtained by adding at least one additive element selected from the group consisting of Al (aluminum), B (boron), Nb (niobium), Ti (titanium), and W (tungsten) to MoO2 (molybdenum dioxide).Patent Document 5 states that an active material for non-aqueous electrolyte secondary batteries, which has a large battery capacity and can suppress an increase in internal resistance after a storage test, and a non-aqueous electrolyte secondary battery using the same can be obtained.

[0009] Patent Document 6 discloses an electrode for a non-aqueous electrolyte secondary battery, comprising a sheet-like current collector and an active material layer attached to the surface of the current collector, the active material layer comprising two or more active materials, the active material layer including at least one first active material that absorbs or desorbs lithium ions at a first potential, and at least one second active material that absorbs or desorbs lithium ions at two operating voltages: a voltage region identical to the first potential and a second potential higher than the first potential, the difference between the first potential and the second potential being 0.2 V or more, and the ratio of the electrochemical capacity of the second active material at the voltage region identical to the first potential to the electrochemical capacity at the second potential being 3.3 or less. Patent Document 6 claims that a high-energy non-aqueous electrolyte secondary battery electrode with excellent input characteristics in low-temperature environments can be obtained. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] International Publication No. 2018 / 110708 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-18883 [Patent Document 3] International Publication No. 2014 / 196462 [Patent Document 4] Japanese Patent Application Laid-Open No. 2001-192208 [Patent Document 5] Japanese Patent Application Laid-Open No. 2009-99522 [Patent Document 6] Japanese Patent Application Laid-Open No. 2015-46218 Summary of the Invention [Problem to be solved by the invention]

[0011] However, in the energy storage device described in Patent Document 1, in which lithium titanate is used as the negative electrode material, the amount of gas generated during high-temperature operation can be suppressed in a battery with a large charge / discharge capacity, but when a coated electrode is produced, it is difficult to increase the electrode density, resulting in a problem of reduced energy density as a battery. The advantages of increasing electrode density include making the battery itself lighter and smaller, thereby reducing the proportion of the battery in electronic devices or on-board devices, and significantly improving the design freedom of electronic devices; in the case of large battery applications, there is no need to reserve space for the battery, which leads to an increase in energy density per unit area, leading to an extension of the driving range of electric vehicles and the creation of more space for the storage battery; and reducing the contact resistance of the electrodes inside the battery.

[0012] The lithium titanate powder in Patent Document 2, which specifies the average pore diameter and pH value, confirms its agglomeration state (determining whether it is a primary particle or a secondary particle), but does not specify a specific particle size. This does not lead to suppression of gas generation during long-term cycling or improvement of electrode density. Furthermore, the lithium titanate powder in Patent Document 3 is easily pulverized when mixed with a binder to prepare an electrode, which makes densification difficult during electrode preparation, posing a challenge to improving electrode density. Patent Document 4 also discloses that specifying the specific surface area and particle size of the primary and secondary particles of the lithium titanate powder improves the coatability of the electrode. However, as with the lithium titanate powders in Patent Documents 2 and 3, no findings are provided on achieving both suppression of gas generation during long-term cycling and improvement of electrode density and rate characteristics.

[0013] Furthermore, Patent Documents 5 and 6 disclose active materials and electrodes for non-aqueous electrolyte secondary batteries that contain a metal oxide, including molybdenum (Mo) oxide and lithium titanate, as the active material. However, there is no description of specific particle size control, and no findings whatsoever about suppressing gas generation during long-term cycles or improving electrode density.

[0014] For the reasons mentioned above, in the electricity storage devices using the negative electrode active materials of Patent Documents 2 to 5 and the electrode of Patent Document 6, it is not possible to simultaneously improve the electrode density and suppress the amount of gas generation during high-temperature operation or to improve the rate characteristics.

[0015] Therefore, the present invention provides a lithium titanate (hereinafter referred to as LTO or Li4Ti5O) that can be used as an electrode material for electricity storage devices, has excellent charge rate characteristics, high initial efficiency, can suppress gas generation during high-temperature operation, and can increase electrode density. 12 The present invention aims to provide a powder (sometimes referred to as "aluminum-based nanoparticles"), an electrode using the powder, and an electricity storage device. [Means for solving the problem]

[0016] As a result of extensive investigations to achieve the above-mentioned object, the inventors discovered that by controlling the drying temperature, heat treatment temperature, and holding time during the granulation process of lithium titanate powder, a lithium titanate powder can be produced that has a specific range of specific surface area and volume median particle size (D50), particularly the D50 of the primary particles, and that contains a specific metal element, with the specific metal element being localized near the surface of the primary particles within the lithium titanate particles. The inventors discovered that an energy storage device using this lithium titanate powder as an electrode material has excellent charge rate characteristics, high initial efficiency, reduced gas generation during high-temperature operation, and increased electrode density, and thus completed the present invention. Specifically, the present invention relates to the following:

[0017] (1) Li4Ti5O 12 The lithium titanate powder has a specific surface area of ​​6.2 m2 and contains secondary particles formed by aggregation of primary particles in which molybdenum (Mo) is localized near the surface of the primary particles. 2 / g~6.8m 2 / g, and the D50 of primary particles corresponding to a cumulative volume of 50% in a volume-based particle size distribution determined by a laser diffraction scattering method is 0.6 μm to 0.8 μm.

[0018] (2) The lithium titanate powder according to (1), wherein the D50 of the secondary particles of the lithium titanate powder is 11 μm to 20 μm.

[0019] (3) The lithium titanate powder according to (1) or (2), characterized in that the molybdenum content (mass %) determined by X-ray fluorescence analysis (XRF) is 0.01 to 0.8.

[0020] (4) The lithium titanate powder according to any one of (1) to (3), wherein the lithium titanate powder has a pH of 11.2 or less.

[0021] (5) The valence of molybdenum localized on the surface of the lithium titanate powder is Mo 6+ , and Mo 5+The lithium titanate powder according to any one of (1) to (4), characterized in that the following are mixed:

[0022] (6) The valence of molybdenum localized on the surface of the lithium titanate powder is Mo 6+ , Mo 5+ , and Mo 4+ The lithium titanate powder according to any one of (1) to (5), characterized in that:

[0023] (7) The valence of molybdenum present locally on the surface of the lithium titanate powder, Mo 6+ The area ratio of Mo 6+ , Mo 5+ , and Mo 4+ The lithium titanate powder according to (6), wherein the area ratio of the above is 80% or less relative to 100% of the total area ratio of the above.

[0024] (8) An electrode comprising the lithium titanate powder according to any one of (1) to (7).

[0025] (9) An electricity storage device comprising the electrode according to (8). [Effects of the Invention]

[0026] According to the present invention, it is possible to provide a lithium titanate powder suitable as an electrode material for an electricity storage device, which has excellent charge rate characteristics, high initial efficiency, and reduced gas generation during high-temperature operation while maintaining high electrode density, as well as an electrode and an electricity storage device using the same. [Brief explanation of the drawings]

[0027] [Figure 1] Figure 1 shows an XPS spectrum measuring the valence distribution of Mo contained in lithium titanate powder whose main component is Li4Ti5O12. DETAILED DESCRIPTION OF THE INVENTION

[0028] [Lithium titanate powder of the present invention] The lithium titanate powder of the present invention is Li4Ti5O 12 The lithium titanate powder is mainly composed of molybdenum, and contains secondary particles formed by aggregation of primary particles in which molybdenum is localized near the particle surface. The specific surface area is 6.2 m 2 / g~6.8m 2 / g and the D50 of the primary particles is 0.6 μm to 0.8 μm.

[0029] <Li4Ti5O 12 Lithium titanate powder containing The lithium titanate powder of the present invention is Li4Ti5O 12 The main component is Li4Ti5O within the range in which the effects of the present invention can be obtained. 12 The term "main component" means that the diffraction peaks measured by X-ray diffraction method are those of Li4Ti5O 12 The lithium titanate powder of the present invention has a diffraction peak intensity ratio of 90% or more of the main peak of Li4Ti5O among the diffraction peaks measured by X-ray diffraction. 12 The ratio of the intensity of the main peak of Li4Ti5O is preferably 92% or more, and more preferably 95% or more. 12 The component other than the crystalline component is the sum of the intensity of the main peak due to the crystalline component and the maximum intensity of the halo pattern due to the amorphous component. The lithium titanate powder of the present invention may contain anatase-type titanium dioxide, rutile-type titanium dioxide, and Li2TiO3, which is a lithium titanate with a different chemical formula, as the crystalline component due to the raw materials used in its synthesis. The lithium titanate powder of the present invention contains these Li4Ti5O 12 The lower the proportion of crystalline components other than Li4Ti5O, the more improved the charge rate characteristics and charge / discharge capacity of the power storage device. 12When the intensity of the main peak of anatase titanium dioxide is taken as 100, it is particularly preferable that the sum of the intensity of the main peak of anatase titanium dioxide, the intensity of the main peak of rutile titanium dioxide, and the intensity corresponding to the main peak of Li2TiO3 calculated by multiplying the peak intensity corresponding to the (-133) plane of Li2TiO3 by 100 / 80 is 5 or less. 12 The main peak of Li4Ti5O in ICDD (PDF2010) PDF card 00-049-0207 is 12 The main peak of anatase titanium dioxide corresponds to the diffraction peak attributable to the (111) plane (2θ = 18.33) of the anatase type titanium dioxide. The main peak of rutile titanium dioxide corresponds to the diffraction peak attributable to the (101) plane (2θ = 25.42) of the anatase type titanium dioxide on PDF card 01-070-6826. The main peak of rutile titanium dioxide corresponds to the diffraction peak attributable to the (110) plane (2θ = 27.44) of the anatase type titanium dioxide on PDF card 01-070-7347. The peak attributable to the (-133) plane of Li2TiO3 corresponds to the diffraction peak attributable to the (-133) plane of Li2TiO3 on PDF card 00-033-0831. Note that "ICDD" stands for International Centre for Diffraction Data, and "PDF" stands for Powder Diffraction File.

[0030] <Molybdenum content> The lithium titanate powder of the present invention contains molybdenum. "Containing molybdenum" means that molybdenum is detected in the lithium titanate powder of the present invention by inductively coupled plasma atomic emission spectrometry (ICP-AES). The lower limit of the amount of molybdenum detected by inductively coupled plasma atomic emission spectrometry is typically 0.001% by mass.

[0031] <Molybdenum content> The molybdenum content (mass%) of the lithium titanate powder of the present invention, as determined by X-ray fluorescence analysis (XRF), is preferably 0.01 to 0.8. When the molybdenum content is within this range, when applied to an electricity storage device, the resulting electricity storage device has excellent charge rate characteristics, high initial efficiency, and reduced gas generation during high-temperature operation. From the viewpoint of further reducing gas generation during high-temperature operation of the electricity storage device, the molybdenum content is preferably 0.1 or more, more preferably 0.2 or more. Furthermore, from the viewpoint of increasing the initial efficiency of the electricity storage device, the molybdenum content is preferably 0.6 or less, more preferably 0.3 or less.

[0032] Furthermore, in the lithium titanate powder of the present invention, molybdenum is localized and contained in greater amounts in the surface regions of the lithium titanate particles constituting the powder than in the internal regions. In cross-sectional analysis of the lithium titanate particles constituting the lithium titanate powder using a scanning transmission electron microscope, it is sufficient that the molybdenum content is high in the so-called near-surface region, extending to a depth of about 5 nm from the surface of the lithium titanate particles as measured by energy dispersive X-ray spectroscopy. Preferably, no molybdenum is detected at a depth of 100 nm. That is, when measured by energy dispersive X-ray spectroscopy, molybdenum is not detected when the amount is below the detectable amount. The lower limit of the detectable amount in energy dispersive X-ray spectroscopy measurement varies depending on the element and state being measured, but is typically 0.5 atm%.

[0033] <Containment of heterogeneous element M> The lithium titanate powder of the present invention preferably contains at least one element selected from B, Mg, Zn, Al, Ga, W, S, or In as a further different element (hereinafter referred to as element M). This is because the lithium titanate powder of the present invention further improves the efficiency and rate characteristics during high-temperature charge / discharge by containing element M together with molybdenum. It is more preferable that element M is Al. It is presumed that this is because the lithium titanate powder of the present invention contains element M together with molybdenum, which adjusts the electronic conductivity of the surface of the lithium titanate powder and reduces the electrical resistance more than when molybdenum is contained alone.

[0034] The content (mass%) of element M determined by X-ray fluorescence analysis (XRF) is preferably 0.01 to 1.0. From the viewpoint of increasing the charge / discharge capacity of the electricity storage device, the content is preferably 0.03 to 0.8, more preferably 0.1 to 0.6.

[0035] <Ratio of element M to molybdenum (element M (mass%) / molybdenum (mass%))> In the lithium titanate powder of the present invention, the mass ratio of element M to molybdenum (element M (mass%) / molybdenum (mass%)) is preferably 0.3 to 30, more preferably 0.45 to 20, and even more preferably 0.8 to 15. In particular, when element M is Al, in order to achieve both the effect of suppressing the amount of gas generation and the effect of rate characteristics, the mass ratio of Al to molybdenum (Al (mass%) / molybdenum (mass%)) is preferably 0.3 to 2, and even more preferably 0.5 to 1.5.

[0036] <Moisture content> The moisture content (25°C to 350°C) of the lithium titanate powder of the present invention measured by the Karl Fischer method (hereinafter sometimes referred to as the moisture content at 25°C to 350°C) is preferably 5000 ppm or less. Here, the moisture content (25°C to 350°C) of the lithium titanate powder of the present invention measured by the Karl Fischer method refers to the total moisture content obtained by heating the lithium titanate powder of the present invention from 25°C to 200°C under nitrogen flow and maintaining it at 200°C for one hour, followed by heating it from 200°C to 350°C under nitrogen flow and maintaining it at 350°C for one hour. A moisture content of 5000 ppm or less is preferable because it provides good handleability during electrode coating when used as an electrode material for an electrical storage device. The moisture content measurement method will be described later in the section <Moisture Content Measurement by Karl Fischer Method>. The moisture content measured by the Karl Fischer method (25°C to 350°C) includes both moisture physically adsorbed and moisture chemically adsorbed on the lithium titanate powder of the present invention. Generally, in lithium titanate powder, measurement by the Karl Fischer method is difficult in the temperature range above 350°C, and moisture is hardly detected by other methods (e.g., pyrolysis gas chromatography-mass spectrometry). From the viewpoint of further suppressing the amount of gas generated during high-temperature operation of the power storage device, the moisture content measured by the Karl Fischer method (25°C to 350°C) is more preferably 1000 ppm or less, and particularly preferably 600 ppm or less.

[0037] Here, the moisture content (200°C to 350°C) of the lithium titanate powder of the present invention measured by the Karl Fischer method refers to the amount of moisture obtained from the start of heating at 200°C to the end of holding at 350°C, out of the moisture content (25°C to 350°C). The moisture contained in lithium titanate includes physically adsorbed moisture and chemically adsorbed moisture, as described above. It is presumed that most of the moisture present on the surface of the lithium titanate is likely desorbed by 200°C and included in the moisture content (25°C to 200°C) measured by the Karl Fischer method. Here, the moisture content (25°C to 200°C) of the lithium titanate powder of the present invention measured by the Karl Fischer method refers to the amount of moisture released from the lithium titanate powder of the present invention when the lithium titanate powder of the present invention is heated from 25°C to 200°C under a nitrogen flow and held at 200°C for 1 hour, measured by the Karl Fischer method. Furthermore, since the fabrication of a typical electricity storage device involves a step of drying the electrodes, the moisture content (25°C to 200°C) measured by the Karl Fischer method is almost entirely released during this drying step. Therefore, it is believed that the moisture that affects the electricity storage device is not present on the surface of the lithium titanate particles, but is present mainly inside the particles, where it is difficult to remove during this drying step. Therefore, it is believed that the moisture present inside the particles and that substantially affects the electricity storage device is mostly included in the moisture content (200°C to 350°C) measured by the Karl Fischer method. From the above perspective, the moisture content (200°C to 350°C) measured by the Karl Fischer method is more preferably 300 ppm or less, and particularly preferably 150 ppm or less. The lower limit of the moisture content (200°C to 350°C) measured by the Karl Fischer method is not particularly limited, and in some cases, it may be below the detection limit of the measuring device (i.e., it can be determined to be essentially 0 ppm).

[0038] <Specific surface area> The specific surface area of ​​the lithium titanate 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.

[0039] The lithium titanate constituting the lithium titanate powder of the present invention has a specific surface area of ​​6.2 m 2 / g~6.8m 2 If the lower limit is 6.3 m / g, it is possible to obtain an electricity storage device in which the electrode density, initial efficiency, and rate characteristics are all satisfied and the amount of gas generated during high-temperature operation is suppressed. 2 / g or more is more preferable, and 6.4m 2 The upper limit is more preferably 6.7 m / g or more. 2 / g or less is more preferable, and 6.6m 2 / g or less is more preferable.

[0040] <d50> The D50 of the lithium titanate powder of the present invention is an index of the volume median particle size. It 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 reaches 50% when calculated from the smallest particle size. The measurement method will be explained in the examples below.

[0041] <Secondary particles formed by agglomeration of primary particles> The lithium titanate powder of the present invention contains secondary particles formed by aggregation of primary particles made of lithium titanate, but some of the secondary particles may not form secondary particles and may be in the form of primary particles themselves.

[0042] From the viewpoint of improving electrode density, the lower limit of D50 of the secondary particles of the lithium titanate powder of the present invention is preferably 11 μm or more, more preferably 12 μm or more, and even more preferably 13 μm or more. Furthermore, the upper limit of D50 is preferably 20 μm or less, more preferably 18 μm or less, particularly preferably 14 μm or less, and most preferably 13.6 μm or less. Note that D50 of the secondary particles represents D50 before crushing treatment (ultrasonic application with an ultrasonic device).

[0043] In the lithium titanate primary particles contained in the lithium titanate powder of the present invention, there is a gradient in molybdenum concentration between the surface and the interior of the primary particles, with the molybdenum concentration being high on the surface (for example, the so-called near-surface region extending to a depth of about 5 nm from the surface of the primary particles) and preferably no molybdenum being present in the interior (for example, a position 100 nm from the surface of the primary particles toward the interior), and molybdenum is preferably fixed in a state chemically bonded to the surface of the primary particles. When molybdenum is present in this state, an electricity storage device can be obtained in which the amount of gas generation during high-temperature operation is suppressed without impairing the charge rate characteristics and initial efficiency of the electricity storage device.

[0044] From the viewpoint of both improving electrode density and achieving rate characteristics, the lower limit of D50 of the primary particles of the lithium titanate powder of the present invention is preferably 0.6 μm or more, more preferably 0.65 μm or more, and particularly preferably 0.69 μm or more. The upper limit of D50 is preferably 0.8 μm or less, more preferably 0.75 μm or less, and particularly preferably 0.7 μm or less. The D50 of the primary particles represents the D50 after a crushing treatment (using an ultrasonic device). The lithium titanate powder may contain primary particles with a primary particle diameter of less than 0.6 μm in a cumulative volume frequency range of 15% to 30% and primary particles with a primary particle diameter of more than 0.8 μm in a cumulative volume frequency range of 45% to 75%.

[0045] In the lithium titanate powder of the present invention, the lower limit of the pH is preferably 10.0 or higher, more preferably 10.5 or higher. The upper limit of the pH is preferably 11.2 or lower, more preferably 11.1 or lower, and even more preferably 10.9 or lower. Within this range, an electricity storage device can be obtained in which the amount of gas generated during high-temperature operation is suppressed without impairing the charge rate characteristics and initial efficiency of the electricity storage device. Although this is merely speculation, it is assumed that the pH of the particles is affected by the surface condition, particularly the valence of molybdenum.

[0046] In the manufacturing method described in the specification of Patent Document 1, the valence of molybdenum is not specified. However, in the lithium titanate primary particles constituting the lithium titanate powder of the present invention, the valence of molybdenum localized on the surface of the lithium titanate powder is Mo 6+ (hexavalent), and Mo 5+ (pentavalent) is sufficient, and Mo 6+ (hexavalent), Mo 5+ (pentavalent), and Mo 4+ It is more preferable that molybdenum (hexavalent) is mixed in. In addition, in the lithium titanate primary particles constituting the lithium titanate powder of the present invention, the area ratio of molybdenum (hexavalent) (Mo 6+ The area ratio of Mo 6+ , Mo 5+ , and Mo 4+ The area ratio of molybdenum (hexavalent) is preferably 80% or less, more preferably 75% or less, relative to a total of 100%. The lower limit is not particularly limited, but is preferably 60% or more, more preferably 65% ​​or more. When the area ratio of molybdenum (hexavalent) is 80% or less, side reactions of molybdenum are suppressed during the initial charge / discharge, and an electricity storage device can be obtained in which the amount of gas generated during high-temperature operation is suppressed without impairing the charge rate characteristics and initial efficiency of the electricity storage device.

[0047] The reason for the suppression of gas generation is unclear, but it is thought that the presence of molybdenum near the interface between the lithium titanate particle surface and the electrolyte allows the molybdenum to efficiently convert the decomposition gas generated near the particle surface into other substances, thereby suppressing the overall gas generation. In addition, the formation of a localized molybdenum diffusion layer near the lithium titanate particle surface prevents the decomposition of the electrolyte, resulting in the conversion of Li4Ti5O 12 It is believed that the amount of gas generated by decomposition of the electrolyte is further suppressed by protecting the active sites of the electrolyte.

[0048] [Method of manufacturing lithium titanate powder of the present invention] An example of the method for producing the lithium titanate powder of the present invention will be explained below, dividing it into a raw material preparation step, a firing step, and a surface treatment step, but the method for producing the lithium titanate powder of the present invention is not limited thereto.

[0049] <Raw material preparation process> The raw materials for the lithium titanate powder of the present invention consist of a titanium raw material and a lithium raw material. Titanium compounds such as anatase-type titanium dioxide and rutile-type titanium dioxide are used as the titanium raw material. It is preferable that the titanium raw material reacts easily with the lithium raw material in a short time, and from that perspective, anatase-type titanium dioxide is preferred. To ensure sufficient reaction of the raw materials in a short firing time, the D50 of the titanium raw material is preferably 2 μm or less.

[0050] As the lithium raw material, lithium compounds such as lithium hydroxide monohydrate, lithium oxide, lithium hydrogen carbonate, and lithium carbonate are used.

[0051] In the present invention, when a mixture of the above raw materials is fired in a short 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 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 calculated from the smallest particle size.

[0052] The following methods can be used to prepare the mixture. The first method is to mix the raw materials and then pulverize them simultaneously. The second method is to pulverize the raw materials until the D95 after mixing is 5 μm or less, and then mix them or mix them while lightly pulverizing them. The third method is to produce powders consisting of fine particles from the raw materials by a method such as crystallization, classify them as necessary, and then mix them or mix them while lightly pulverizing them. Among these, in the first method, the method of mixing the raw materials and pulverizing them simultaneously is an industrially advantageous method because it requires fewer steps. A conductive agent may also be added at the same time.

[0053] In any of the first to third methods, the method for mixing the raw materials is not particularly limited, and either wet mixing or dry mixing may be used. For example, a Henschel mixer, an ultrasonic disperser, a homomixer, a mortar, a ball mill, a centrifugal ball mill, a planetary ball mill, a vibrating ball mill, an attritor-type high-speed ball mill, a bead mill, a roll mill, etc. may be used.

[0054] When the obtained mixture is a mixed powder, it can be directly subjected to the next firing step. When the mixture is a mixed slurry consisting of the mixed powder, the mixed slurry can be dried using a rotary evaporator or the like before being subjected to the next firing step. When firing is performed using a rotary kiln, the mixed slurry can be directly subjected to the furnace.

[0055] <Firing process> The resulting mixture is then fired. From the viewpoint of increasing the specific surface area and crystallite size of the powder obtained by firing, firing at a high temperature for a short time is preferable. From this viewpoint, the maximum temperature during firing is preferably 1100°C or less, more preferably 1000°C or less, and even more preferably 900°C or less. From the viewpoint of reducing the proportion of specific impurity phases and increasing the crystallinity of lithium titanate, the maximum temperature during firing is preferably 800°C or more, more preferably 810°C or more. Similarly, from the above viewpoint, the holding time at the maximum temperature during firing is preferably 2 minutes to 60 minutes, more preferably 5 minutes to 45 minutes, and even more preferably 5 minutes to 30 minutes. When the maximum temperature during firing is high, it is preferable to select a shorter holding time. Similarly, from the viewpoint of increasing the crystallite size obtained by firing, it is preferable to particularly shorten the residence time at 700°C to 800°C during the temperature rise process during firing, for example, preferably within 15 minutes.

[0056] The calcination method is not particularly limited as long as it can be performed under these 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. When using a roller hearth calcination furnace or mesh belt calcination furnace in which the mixture is placed in a sagger and calcined, it is preferable to place a small amount of the mixture in the sagger in order to ensure uniformity in the temperature distribution of the mixture during calcination and to obtain a consistent quality lithium titanate.

[0057] A rotary kiln-type firing furnace is particularly preferred for producing the lithium titanate powder of the present invention because it does not require a container to contain the mixture, allows firing while continuously adding the mixture, and provides a uniform thermal history to the fired material, allowing for the production of homogeneous lithium titanate.

[0058] The atmosphere during firing is not particularly limited, regardless of the firing furnace, as long as it can remove the desorbed moisture and carbon dioxide gas. Usually, an air atmosphere using compressed air is used, but an oxygen, nitrogen, or hydrogen atmosphere may also be used.

[0059] Although the lithium titanate powder after firing has some slight agglomeration, it does not need to be pulverized to destroy the particles, and therefore, after firing, it is sufficient to perform disintegration or classification to the extent that the agglomeration is broken down as needed. If only disintegration to the extent that the agglomeration is broken down without pulverization, the high crystallinity of the lithium titanate powder after firing can be maintained even after that.

[0060] <Surface treatment process> The lithium titanate powder of the present invention is a lithium titanate powder containing molybdenum, and when used as an electrode material for an electricity storage device, it can improve the charge rate characteristics and suppress the amount of gas generation during high-temperature operation. The lithium titanate powder of the present invention can be produced by adding a molybdenum-containing compound (hereinafter sometimes referred to as treatment agent 1) in the firing step. However, from the viewpoint of more effectively localizing molybdenum near the surfaces of the primary particles, it is more preferable to produce the lithium titanate powder of the present invention by the following surface treatment step.

[0061] The lithium titanate powder before surface treatment obtained by the above steps (hereinafter, this may be referred to as the base lithium titanate powder. Also, hereinafter, the lithium titanate particles that make up the base lithium titanate powder may be referred to as the base lithium titanate particles) is mixed with treatment agent 1 and preferably subjected to heat treatment.

[0062] The molybdenum-containing compound (treatment agent 1) is not particularly limited, and examples thereof include molybdenum oxides, hydroxides, sulfates, nitrates, fluorides, organic compounds, and metal salt compounds containing molybdenum. Specific examples include molybdenum oxide, molybdenum trioxide, molybdenum trioxide hydrate, molybdenum boride, molybdophosphoric acid, molybdenum disilicide, molybdenum chloride, molybdenum sulfide, 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. To uniformly diffuse molybdenum over the particle surfaces of lithium titanate powder, it is suitable to use the wet method described below. In this case, it is preferable to dissolve a molybdenum-containing compound soluble in a solvent in the solvent and mix it with the lithium titanate powder as the base material. Lithium molybdate containing molybdenum is preferred in terms of suppressing gas generation. Regarding the valence of molybdenum, even if it is added as a raw material in a hexavalent state, it is expected that dangling bonds will exist when chemical bonds are formed on the LTO surface or inside the crystal, resulting in partial tetravalent or pentavalent states.

[0063] The amount of molybdenum-containing compound (treatment agent 1) added may be any amount as long as the molybdenum content in the lithium titanate powder falls within the range of the present invention. For example, when lithium molybdate (Li2MoO4) is used as treatment agent 1, from the viewpoint of further suppressing gas generation during high-temperature operation of the energy storage device, treatment agent 1 is added at a rate of 0.02 mass% or more relative to the lithium titanate powder substrate (corresponding to an addition of 0.12 mmol% or more of treatment agent 1 per 100 g of LTO), more preferably 0.25 mass% or more. Furthermore, from the viewpoint of increasing the initial efficiency of the energy storage device, treatment agent 1 is added at a rate of 5.0 mass% or less relative to the lithium titanate powder substrate (corresponding to an addition of 29.4 mmol% or less of treatment agent 1 per 100 g of LTO), more preferably 2.7 mass% or less, even more preferably 0.9 mass% or less, and particularly preferably 0.5 mass% or less.

[0064] The lithium titanate powder of the present invention may further contain the element M. When the element M is contained, the lithium titanate powder substrate is mixed with a compound containing molybdenum (treatment agent 1) and a compound further containing the element M (hereinafter sometimes referred to as treatment agent 2) and then heat-treated. Alternatively, the powder may be mixed with treatment agent 1 and then heat-treated, and then mixed with treatment agent 2 and then heat-treated. Alternatively, the powder may be mixed with treatment agent 2 and then heat-treated, and then mixed with treatment agent 1 and then heat-treated.

[0065] The compound containing element M (treatment agent 2) may be any compound that diffuses upon heat treatment, including, for example, oxides, hydroxides, sulfates, nitrates, fluorides, organic compounds, and metal salt compounds containing element M. To uniformly diffuse element M onto the particle surfaces of lithium titanate powder, the wet method described below is suitable. In this case, it is preferable to dissolve a compound containing element M that is soluble in a solvent in the solvent and mix it with the lithium titanate powder substrate. From the viewpoint of suppressing the amount of gas generation, sulfates and fluorides containing element M are preferred.

[0066] When the element M is aluminum (Al), examples of compounds containing Al include aluminum acetate, aluminum fluoride, and aluminum sulfate. Of these, aluminum sulfate, its hydrate, and aluminum fluoride are preferred.

[0067] The amount of the compound containing element M (treatment agent 2) added may be any amount as long as the amount of element M in the lithium titanate powder falls within the range of the present invention. For example, when aluminum sulfate·14-18 hydrate (Al2(SO4)3·14-18H2O) is used, it is preferable to add it at a rate of 0.3 mass% or more relative to the lithium titanate powder substrate (corresponding to an amount of treatment agent 2 of 0.88 mmol% or more added per 100 g of LTO). It is also preferable to add it at a rate of 12 mass% or less relative to the lithium titanate powder substrate (corresponding to an amount of treatment agent 1 of 35.2 mmol% or less added per 100 g of LTO), more preferably 10 mass% or less, and particularly preferably 8 mass% or less. The preferred amount of the compound containing element M (treatment agent 2) added is determined by its relationship to the molybdenum-containing compound (treatment agent 1).

[0068] There are no particular restrictions on the method of mixing the base lithium titanate powder with the molybdenum-containing compound (treatment agent 1) and further with the compound containing element M (treatment agent 2), and either wet mixing or dry mixing can be used. However, it is preferable to uniformly disperse the molybdenum-containing compound (treatment agent 1) or further with the compound containing element M (treatment agent 2) on the surface of the base lithium titanate particles, and in this respect, wet mixing is preferred.

[0069] For dry mixing, for example, a paint mixer, a Henschel mixer, an ultrasonic disperser, a homomixer, a mortar, a ball mill, a centrifugal ball mill, a planetary ball mill, a vibrating ball mill, an attritor-type high-speed ball mill, a bead mill, a roll mill, etc. can be used.

[0070] In wet mixing, treatment agent 1, or treatment agent 2 and the lithium titanate powder substrate are added to a water or alcohol solvent and mixed in a slurry state. Alcohol solvents with a boiling point of 100°C or less, such as methanol, ethanol, or isopropyl alcohol, are preferred because they are easy to remove. Furthermore, water solvents are industrially preferred because of their ease of recovery and disposal.

[0071] The amount of solvent used can be any amount sufficient to sufficiently wet the lithium titanate particles of the substrate with treatment agent 1 or 2. However, it is preferable that treatment agent 1 or 2 and the lithium titanate particles of the substrate are uniformly dispersed in the solvent. To achieve this, the amount of solvent used is preferably such that the amount of treatment agent 1 or 2 dissolved in the solvent is 50% or more of the total amount of treatment agent 1 or 2 added to the solvent. Because the amount of treatment agent 1 or 2 dissolved in the solvent increases with increasing temperature, it is preferable to heat the mixture of the lithium titanate powder of the substrate with treatment agent 1 or 2 in the solvent. Heating also reduces the amount of solvent used, making this a commercially suitable method. The mixing temperature is preferably 40°C to 100°C, more preferably 60°C to 100°C.

[0072] In the case of wet mixing, although this depends on the heat treatment method, it is preferable to remove the solvent before the heat treatment performed after the mixing step. Simply evaporating the solvent to dryness makes it difficult to control the specific surface area and D50 of the primary and secondary particles. Methods for evaporating the solvent to dryness include heating the slurry while stirring it with a stirring blade, using a drying device capable of drying while stirring, such as a conical dryer, and using a spray dryer. The spray dryer method is preferred. This is because, unlike conventional evaporation to dryness, the conditions of the spray dryer significantly affect the secondary particle morphology and the surface treatment process. The drying temperature of the spray dryer is preferably 180°C to 300°C, more preferably 200°C to 270°C, and even more preferably 235°C to 270°C. The specific drying temperature condition determines the range of powder physical properties of the secondary particles, which results in improved electrode density and improved battery performance, particularly initial efficiency and rate characteristics.

[0073] After mixing the lithium titanate powder as the substrate with treatment agent 1, or with treatment agent 2, it is preferable to perform a heat treatment. The heat treatment temperature is a temperature at which molybdenum and / or element M diffuses into at least the surface region of the lithium titanate particles as the substrate, and at which a significant decrease in the specific surface area due to sintering of the lithium titanate particles as the substrate does not occur. The upper limit of the heat treatment temperature is preferably 700°C or lower, more preferably 600°C or lower. The lower limit of the heat treatment temperature is preferably 300°C or higher, more preferably 400°C or higher. The heat treatment time is preferably 0.1 to 8 hours, more preferably 1 to 5 hours. The temperature and time at which element M diffuses into at least the surface region of the lithium titanate particles as the substrate should be appropriately set, since the reactivity varies depending on the compound containing element M.

[0074] The heating method for the heat treatment is not particularly limited. Usable heat treatment furnaces include fixed-bed calciners, roller hearth calciners, mesh belt calciners, fluidized-bed calciners, and rotary kiln calciners. The atmosphere during the heat treatment may be either air or an inert atmosphere such as a nitrogen atmosphere. When a metal salt compound containing molybdenum or element M is used as the compound containing molybdenum or element M (treatment agent 1, treatment agent 2), an air atmosphere is preferred, as it facilitates the removal of anion species from the particle surface.

[0075] The lithium titanate powder obtained after the heat treatment as described above may have slight agglomerations, but does not need to be pulverized to destroy the particles. Therefore, after the heat treatment, it is sufficient to perform crushing or classification to the extent that agglomerations are broken down, if necessary.

[0076] The lithium titanate powder of the present invention may be mixed with a treatment agent in a surface treatment step, granulated, and then heat-treated to form a powder containing secondary particles formed by aggregation of primary particles. Any method for granulation may be used as long as it can produce secondary particles, but a spray dryer is preferred because it can process large quantities.

[0077] Dew point control may be performed during the heat treatment process to reduce the moisture content of the lithium titanate powder of the present invention. Since the moisture content of the heat-treated powder increases if exposed to the atmosphere, 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 adjust the particles to the desired maximum particle size range. When dew point control is performed during the heat treatment process, it is preferable to seal the lithium titanate powder of the present invention in an aluminum-laminated bag or the like and then place it in an environment outside of dew point control. Even under dew point control, pulverizing the heat-treated lithium titanate 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, but less than 550°C. This is because a heat treatment temperature above 550°C significantly reduces the specific surface area, significantly degrading battery performance, particularly rate characteristics. Furthermore, the holding time is preferably 1 hour or more, because if the heating time is short, the amount of moisture contained in the powder will increase and it is thought that this will also affect the particle surface condition.

[0078] As described above, the lithium titanate powder of the present invention contains secondary particles formed by aggregation of primary particles of lithium titanate. The average compressive strength of these secondary particles is preferably 0.1 MPa or more and 3 MPa or less. When the secondary particles have an average compressive strength of 0.1 MPa or more, their application as an electrode material for an electrical storage device can reduce resistance increases and capacity losses during high-temperature charge-discharge cycles of the electrical storage device. Setting the upper limit of the average compressive strength of the secondary particles to 3 MPa or less is also effective for increasing the density of the electrode mixture layer, i.e., increasing the energy density. The upper limit is preferably 1.5 MPa or less, more preferably 1.0 MPa or less, and even more preferably 0.50 MPa or less. From the above perspective, the lower limit of the average compressive strength is preferably 0.2 MPa or more, more preferably 0.23 MPa or more. The average compressive strength of the secondary particles can be determined by measuring the compressive strength of each of a predetermined number of secondary particles contained in the lithium titanate powder of the present invention, calculating the average compressive strength of the obtained predetermined number of secondary particles, and using the average compressive strength as the average compressive strength of the secondary particles constituting the lithium titanate powder of the present invention. A specific method for measuring the average compressive strength of secondary particles may be measurement using a microcompression tester.

[0079] [Active material] The active material of the present invention contains the lithium titanate powder of the present invention. It may contain one or more substances other than the lithium titanate powder of the present invention. Examples of other substances include carbon materials (pyrolytic carbons, cokes, graphites (artificial graphite, natural graphite, etc.), organic polymer compound combustion products, and carbon fibers), tin and tin compounds, silicon and silicon compounds.

[0080] [Energy storage devices] The electricity storage device of the present invention is a device that includes an electrode containing the active substance material of the present invention and stores and releases energy by utilizing the intercalation and deintercalation of lithium ions into such an electrode, and examples thereof include a hybrid capacitor and a lithium battery.

[0081] [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 as an electrode of the hybrid capacitor.

[0082] [Lithium battery] The lithium battery of the present invention is a general term for lithium primary batteries and lithium secondary batteries. In this specification, the term lithium secondary battery is used as a concept that also includes so-called lithium ion secondary batteries.

[0083] 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, 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 as an electrode of the lithium battery. This active material may be used as either a positive electrode active material or a negative electrode active material, but the following description will be given of the case where it is used as a negative electrode active material.

[0084] <Negative electrode> The negative electrode has a mixture layer containing a negative electrode active material (the active material of the present invention), a conductive agent, and a binder on one or both sides of a negative electrode current collector. This mixture layer is usually treated as an electrode. In the case of a negative electrode current collector that is porous or has pores, the mixture layer contains the negative electrode active material (the active material of the present invention), a conductive agent, and a binder in the pores.

[0085] The conductive agent for the negative electrode is not particularly limited as long as it is an electron-conductive material that does not undergo chemical changes. Examples include graphites such as natural graphite (e.g., flake graphite) and artificial graphite; carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; and carbon nanotubes such as single-phase carbon nanotubes, multi-walled carbon nanotubes (graphite layers in a multi-layered concentric cylindrical shape) (non-fishbone-shaped), cup-stacked carbon nanotubes (fishbone-shaped), nodular carbon nanofibers (non-fishbone structure), and platelet-type carbon nanofibers (playing card-shaped). Graphites, carbon blacks, and carbon nanotubes may be appropriately mixed and used. Although not particularly limited, the specific surface area of ​​the carbon black is preferably 30 m. 2 / g~3000m 2 / g, and more preferably 50m 2 / g~2000m 2 The specific surface area of ​​the graphite is preferably 30 m / g. 2 / g~600m 2 / g, and more preferably 50m 2 / g~500m 2 The aspect ratio of the carbon nanotubes is 2-150, preferably 2-100, and more preferably 2-50.

[0086] The amount of conductive agent added should be optimized because it varies depending on the specific surface area of ​​the active material and the type and combination of conductive agents, but is preferably 0.1% by mass to 10% by mass, and more preferably 0.5% by mass to 5% by mass in the mixture layer. If it is less than 0.1% by mass, the conductivity of the mixture layer cannot be ensured, and if it exceeds 10% by mass, the active material ratio decreases, and the discharge capacity of the electricity storage device per unit mass and unit volume of the mixture layer becomes insufficient, making it unsuitable for achieving high capacity.

[0087] Examples of binders for the negative electrode include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR), and carboxymethyl cellulose (CMC). While not particularly limited, the molecular weight of polyvinylidene fluoride is preferably 20,000 to 1,000,000. From the viewpoint of ensuring the binding property of the mixture layer, it is preferably 25,000 or more, more preferably 30,000 or more, and even more preferably 50,000 or more. From the viewpoint of ensuring conductivity without interfering with the contact between the active material and the conductive agent, it is preferably 500,000 or less. In particular, when the specific surface area of ​​the active material is 10 m 2 / g or more, the molecular weight is preferably 100,000 or more.

[0088] The amount of binder added varies depending on the specific surface area of ​​the active material and the type and combination of conductive agents, and therefore should be optimized, but is preferably 0.2% by mass to 15% by mass in the mixture layer. From the viewpoint of enhancing binding properties and ensuring the strength of the mixture layer, the amount is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more. From the viewpoint of preventing a decrease in the active material ratio and a decrease in the discharge capacity of the electricity storage device per unit mass and unit volume of the mixture layer, the amount is preferably 10% by mass or less, and more preferably 5% by mass or less.

[0089] Examples of the negative electrode current collector include aluminum, stainless steel, nickel, copper, titanium, calcined carbon, and those coated with carbon, nickel, titanium, or silver. The surface of these materials may be oxidized, or the negative electrode current collector surface may be roughened by surface treatment. Examples of the negative electrode current collector include a sheet, net, foil, film, punched material, lath, porous material, foam, fiber group, and nonwoven fabric molded body. Porous aluminum is preferred as the negative electrode current collector. The porosity of the porous aluminum is 80% or more and 95% or less, preferably 85% or more.

[0090] The negative electrode can be produced by uniformly mixing a negative electrode active material (including the active material of the present invention), a conductive agent, and a 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 a negative electrode active material (including the active material of the present invention), a conductive agent, and a binder in a solvent, and is then pressed into the pores of the current collector to fill the pores, or by immersing a porous current collector in the paint to allow the paint to diffuse into the pores, followed by drying and compression.

[0091] As a method for uniformly mixing the negative electrode active material (the active material of the present invention), the conductive agent, and the binder in a solvent to prepare a paint, for example, a kneader of the type in which a stirring rod revolves while rotating within a kneading vessel such as a planetary mixer, a twin-screw extrusion kneader, a planetary stirring degassing device, a bead mill, a high-speed rotary mixer, a powder suction continuous dissolution and dispersion device, etc. Alternatively, the manufacturing process may be divided into steps depending on the solid content concentration, and these devices may be used separately.

[0092] Uniform mixing of the negative electrode active material (the active material of the present invention), conductive agent, and binder in a solvent requires optimization because it depends on the specific surface area of ​​the active material, the type of conductive agent, the type of binder, and the combination thereof. However, when using a kneader such as a planetary mixer, in which the stirring rod revolves while rotating within a kneading vessel, a twin-screw extrusion kneader, or a planetary stirring and degassing device, it is preferable to divide the production process into steps based on the solid content, knead the mixture at a high solid content, and then gradually reduce the solid content to adjust the viscosity of the coating material. A high solid content is preferably 60% to 90% by mass, more preferably 60% to 80% by mass. A solid content of 60% by mass or more is preferable because it provides sufficient shear force, while a solid content of 90% by mass or less is preferable because it reduces the load on the device, and 80% by mass or less is even more preferable.

[0093] The mixing procedure is not particularly limited, but examples include a method of simultaneously mixing the negative electrode active material, the conductive agent, and the binder in a solvent, a method of previously mixing the conductive agent and the binder in a solvent and then adding and mixing the negative electrode active material, a method of previously preparing a negative electrode active material slurry, a conductive agent slurry, and a binder solution and then mixing them, etc. Among these, in order to achieve uniform dispersion, a method of previously mixing the conductive agent and the binder in a solvent and then adding and mixing the negative electrode active material, and a method of previously preparing a negative electrode active material slurry, a conductive agent slurry, and a binder solution and then mixing them are preferred.

[0094] As the solvent, an organic solvent can be used, and examples of the organic solvent include aprotic organic solvents such as 1-methyl-2-pyrrolidone, dimethylacetamide, and dimethylformamide, used alone or in combination of two or more kinds, and preferably 1-methyl-2-pyrrolidone.

[0095] When an organic solvent is used as the solvent, it is preferable to dissolve the binder in the organic solvent before use.

[0096] <Positive electrode> The positive electrode has a mixture layer containing a positive electrode active material, a conductive agent, and a binder on one or both surfaces of a positive electrode current collector.

[0097] As the positive electrode active material, a material capable of absorbing and releasing lithium is used, and examples of the active material include composite metal oxides containing cobalt, manganese, and nickel with lithium, and lithium-containing olivine-type phosphates. These positive electrode active materials can be used alone or in combination of two or more. Examples of such composite metal oxides include LiCoO2, LiMn2O4, LiNiO2, and LiCo 1-x Ni x O2(0.01 <x<1)、LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, LiNi 1 / 2 Mn 3 / 2 O4, etc., and these lithium composite oxides may be partially substituted with other elements, such as partially substituting cobalt, manganese, and nickel with at least one element such as B, Nb, Sn, Mg, Fe, Ti, Al, Zr, Cr, V, Ga, Zn, Cu, Bi, Mo, and La, partially substituting O with S or F, or coating with a compound containing these other elements. Examples of lithium-containing olivine-type phosphates include LiFePO4, LiCoPO4, LiNiPO4, LiMnPO4, and LiFe 1-x Examples include MxPO4 (M is at least one selected from Co, Ni, Mn, Cu, Zn, and Cd, and x is 0≦x≦0.5).

[0098] The conductive agent and binder for the positive electrode may be the same as those for the negative electrode. Examples of the positive electrode current collector include aluminum, stainless steel, nickel, titanium, baked carbon, and aluminum or stainless steel surfaces treated with carbon, nickel, titanium, or silver. The surface of these materials may be oxidized, or the positive electrode current collector surface may be roughened by surface treatment. Examples of the current collector form include sheets, nets, foils, films, punched materials, laths, porous materials, foams, fiber groups, and nonwoven fabric molded bodies.

[0099] <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.

[0100] The electrolyte salt used is one that dissolves in a non-aqueous electrolyte. Examples of the electrolyte salt include inorganic lithium salts such as LiPF, LiBF, LiPO, LiN(SOF), and LiClO; lithium salts containing a chain-like fluorinated alkyl group such as LiN(SOCF), LiN(SOCF) , LiCFSO, LiC(SOCF), LiPF(CF), LiPF(CF) , LiPF(CF) , LiPF(CF) , LiPF(iso-CF) , and LiPF(iso-CF); lithium salts containing a cyclic fluorinated alkylene chain such as (CF)(SO)NLi and (CF)(SO)NLi; and lithium salts having an oxalate complex as the anion, such as lithium bis[oxalate-O,O']borate and lithium difluoro[oxalate-O,O']borate. Among these, particularly preferred electrolyte salts are LiPF6, LiBF4, LiPO2F2, and LiN(SO2F)2, with LiPF6 being the most preferred. These electrolyte salts can be used alone or in combination of two or more. A preferred combination of these electrolyte salts is one in which the non-aqueous electrolyte solution contains LiPF6 and at least one lithium salt selected from LiBF4, LiPO2F2, and LiN(SO2F)2.

[0101] The concentration of all of these electrolyte salts dissolved in the nonaqueous solvent is usually preferably 0.3 M or more, more preferably 0.5 M or more, and even more preferably 0.7 M or more, with the upper limit being preferably 2.5 M or less, more preferably 2.0 M or less, and even more preferably 1.5 M or less.

[0102] On the other hand, examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, chain esters, ethers, amides, phosphate esters, sulfones, lactones, nitriles, and S=O bond-containing compounds, and the non-aqueous solvent preferably contains a cyclic carbonate. Note that the term "chain ester" is used as a concept including chain carbonates and chain carboxylic acid esters.

[0103] Examples of the cyclic carbonate include one or more selected from ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 4-fluoro-1,3-dioxolan-2-one (FEC), trans- or cis-4,5-difluoro-1,3-dioxolan-2-one (hereinafter, both are collectively referred to as "DFEC"), vinylene carbonate (VC), vinylethylene carbonate (VEC), and 4-ethynyl-1,3-dioxolan-2-one (EEC). One or more selected from ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 4-fluoro-1,3-dioxolan-2-one, and 4-ethynyl-1,3-dioxolan-2-one (EEC) are more preferred from the viewpoint of improving the charge rate characteristics of the power storage device and suppressing the amount of gas generated during high-temperature operation, and one or more cyclic carbonates having an alkylene chain selected from propylene carbonate, 1,2-butylene carbonate, and 2,3-butylene carbonate are even more preferred. The proportion of the cyclic carbonate having an alkylene chain in all cyclic carbonates is preferably 55% to 100% by volume, and more preferably 60% to 90% by volume.

[0104] Therefore, the non-aqueous electrolyte is preferably prepared by dissolving an electrolyte salt containing at least one lithium salt selected from LiPF, LiBF, LiPOF, and LiN(SOF) in a non-aqueous solvent containing one or more cyclic carbonates selected from ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 4-fluoro-1,3-dioxolan-2-one, and 4-ethynyl-1,3-dioxolan-2-one. The cyclic carbonate is more preferably one or more cyclic carbonates having an alkylene chain selected from propylene carbonate, 1,2-butylene carbonate, and 2,3-butylene carbonate.

[0105] In particular, it is preferable to use a non-aqueous electrolyte solution having a total electrolyte salt concentration of 0.5M to 2.0M, containing at least LiPF as the electrolyte salt, and further containing 0.001M to 1M of at least one lithium salt selected from LiBF, LiPOF, and LiN(SOF). When the proportion of lithium salts other than LiPF in the non-aqueous solvent is 0.001M or more, the charge rate characteristics of the power storage device are improved and the gas generation amount during high-temperature operation is suppressed. When the proportion is 1.0M or less, there is little concern about a decrease in the effect of improving the charge rate characteristics of the power storage device and suppressing the gas generation amount during high-temperature operation. The proportion of lithium salts other than LiPF in the non-aqueous solvent is preferably 0.01M or more, particularly preferably 0.03M or more, and most preferably 0.04M or more. The upper limit is preferably 0.8M or less, more preferably 0.6M or less, and particularly preferably 0.4M or less.

[0106] In order to achieve suitable physical properties, the non-aqueous solvents are preferably used in combination, such as 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, or a combination of a cyclic carbonate, a chain carbonate and an S═O bond-containing compound.

[0107] 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).

[0108] Among the chain esters, chain esters having a methyl group selected from dimethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, methyl propionate, methyl acetate, and ethyl acetate (EA) are preferred, and chain carbonates having a methyl group are particularly preferred.

[0109] When chain carbonates are used, it is preferable to use two or more types. It is more preferable that both symmetric chain carbonates and asymmetric chain carbonates are contained, and it is even more preferable that the content of the symmetric chain carbonate is higher than that of the asymmetric chain carbonate.

[0110] The content of the chain ester is not particularly limited, but is preferably in the range of 60% to 90% by volume relative to the total volume of the nonaqueous solvent. If the content is 60% by volume or more, the viscosity of the nonaqueous electrolyte solution will not become too high, and if it is 90% by volume or less, the electrical conductivity of the nonaqueous electrolyte solution will decrease, which will reduce the risk of reducing the effect of improving the charge rate characteristics of the electricity storage device and reducing the amount of gas generation during high-temperature operation, so the above range is preferable.

[0111] The volume ratio of the symmetric chain carbonate in the chain carbonate is preferably 51% by volume or more, more preferably 55% by volume or more. The upper limit is more preferably 95% by volume or less, and even more preferably 85% by volume or less. It is particularly preferable that the symmetric chain carbonate contains dimethyl carbonate. Furthermore, it is more preferable that the asymmetric chain carbonate has a methyl group, and methyl ethyl carbonate is particularly preferable. In the above case, the charge rate characteristics of the electricity storage device are improved and the effect of suppressing the amount of gas generation during high-temperature operation is improved, which is preferable.

[0112] The ratio of cyclic carbonate to chain ester is preferably 10:90 to 45:55 (volume ratio), more preferably 15:85 to 40:60, and particularly preferably 20:80 to 35:65, from the viewpoint of improving the charge rate characteristics of the electricity storage device and enhancing the effect of suppressing the amount of gas generated during high-temperature operation.

[0113] <Lithium battery structure> The structure of the lithium battery of the present invention is not particularly limited, and examples include a coin battery having a positive electrode, a negative electrode, and a single-layer or multi-layer separator, and further, a cylindrical battery or a prismatic battery having a positive electrode, a negative electrode, and a roll-shaped separator.

[0114] The separator is an insulating thin film having high ion permeability and a predetermined mechanical strength. Examples include polyethylene, polypropylene, cellulose paper, glass fiber paper, polyethylene terephthalate, and polyimide microporous membranes. Multilayer membranes formed by combining two or more of these materials can also be used. The surface of these separators can also be coated with resins such as PVDF, silicone resins, and rubber-based resins, or particles of metal oxides such as aluminum oxide, silicon dioxide, and magnesium oxide. The pore size of the separator may be within a range generally useful for batteries, for example, 0.01 μm to 10 μm. The thickness of the separator may be within a range generally used for batteries, for example, 5 μm to 300 μm. [Example]

[0115] Next, the present invention will be explained in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples and includes various combinations that can be easily inferred from the gist of the invention.

[0116] [Example 1-1] <Raw material preparation process> Li2CO3 (average particle size 4.6 μm) and anatase TiO2 (specific surface area 10 m) were mixed so that the atomic ratio of Li to Ti (Li / Ti) was 0.83. 2 The raw material powder was weighed out (100g / g), and ion-exchanged water was added to the resulting raw material powder to a solids concentration of 41% by mass. The mixture was then stirred to produce a raw material mixed slurry. The raw material mixed slurry was then wet-mixed and milled using a bead mill (Willi & Bachofen, Model: Dynomill KD-20BC, agitator material: polyurethane, vessel inner surface material: zirconia) with a vessel filled to 80% by volume with zirconia beads (outer diameter: 0.65 mm). The agitator peripheral speed was 13 m / s, the slurry feed rate was 55 kg / hr, and the vessel inner pressure was controlled to 0.02-0.03 MPa.

[0117] <Firing process> The resulting mixed slurry was introduced into a rotary kiln-type firing furnace (furnace tube length: 4 m, furnace tube diameter: 30 cm, external heating type) equipped with an anti-adhesion mechanism from the raw material supply side of the firing furnace, dried in a nitrogen atmosphere, and fired. The furnace tube was tilted at a 2.5° angle from the horizontal, rotated at 20 rpm, and introduced into the furnace from the sintered product recovery side at a nitrogen flow rate of 20 L / min. The heating temperatures of the furnace tube were 600°C on the raw material supply side, 840°C in the center, and 840°C on the sintered product recovery side, and the sintered product was held at 840°C for 30 minutes.

[0118] <Post-processing process> The fired material recovered from the fired material recovery side of the furnace tube was pulverized using a hammer mill (Dalton, AIIW-5 model) under the following conditions: screen opening: 0.5 mm, rotation speed: 8,000 rpm, powder feed rate: 25 kg / hr.

[0119] <Surface treatment process> The crushed calcined powder was mixed with ion-exchanged water to a solids concentration of 30% by mass. Lithium molybdate (Li2MoO4) was added as treatment agent 1 (1.38 mmol per 100 g of crushed calcined powder (LTO powder)), and aluminum sulfate hexahydrate (Al2(SO4)3·16H2O) was added as treatment agent 2 (10.15 mmol per 100 g of crushed calcined powder (LTO powder)). This mixture was spray-dried and granulated using a spray dryer (Okawara Kakoki Co., Ltd., L-8i) at 25,000 rpm and 200°C. The sieved powder was then placed in an alumina sagger and heat-treated for 1 hour at 500°C in a continuous furnace equipped with a mesh belt conveyor and a collection box at the outlet, where the dew point was controlled to below -20°C. The heat-treated powder was cooled in the recovery box and classified using a sieve (screen opening: 53 μm). The powder that passed through the sieve was collected in an aluminum laminated bag and sealed, and then removed from the recovery box to produce the lithium titanate powder of Example 1.

[0120] [Examples 1-2, 1-3, and 1-4] Lithium titanate powders according to Examples 1-2, 1-3, and 1-4 were produced in the same manner as in Example 1, except that in the surface treatment step, the amount of lithium molybdate (Li2MoO4) added as treatment agent 1 was as shown in Table 1.

[0121] [Comparative Example 1-1] A lithium titanate powder according to Comparative Example 1-1 was produced in the same manner as in Example 1-1, except that lithium molybdate (Li2MoO4) as treating agent 1 was not added in the surface treatment step.

[0122] [Comparative Example 1-2] The surface treatment process was carried out by evaporation to dryness, similar to the manufacturing method described in Patent Document 1. The treatment conditions were as follows: ion-exchanged water was added to the crushed calcined powder so that the solids concentration of the slurry was 30% by mass, and the mixture was stirred. 2.75 mmol of lithium molybdate (Li2MoO4) was added as treatment agent 1 per 100 g of crushed calcined powder (LTO powder), and 10.15 mmol of aluminum sulfate hexahydrate (Al2(SO4)3·16H2O) was added as treatment agent 2 per 100 g of crushed calcined powder to prepare a mixed slurry. This mixed slurry was heated to 200°C with stirring and dried. The resulting dried powder was then sieved and heat-treated in the same manner as in Example 1-1, producing the lithium titanate powder of Comparative Example 1-2.

[0123] [Molybdenum content measurement] The molybdenum content in the lithium titanate powders of Examples 1-1 to 1-4 and Comparative Examples 1-1 and 1-2 (hereinafter sometimes referred to as the lithium titanate powders of the respective Examples and Comparative Examples) was measured as follows.

[0124] <X-ray fluorescence analysis (XRF): Identification of molybdenum content> The elements contained in the lithium titanate powders of each example and comparative example were quantitatively analyzed using an X-ray fluorescence analyzer (manufactured by SII Technology Inc., product name "SPS5100"). The measurement sample was prepared by adding a precisely weighed sample to nitric acid and hydrofluoric acid, sealing the container, irradiating it with microwaves to thermally decompose it, and then adding ultrapure water to a constant volume to use as the test solution. The mass ratio of the element content determined by ICP-AES was calculated.

[0125] [Measurement of powder properties] The various physical properties of the lithium titanate powders of the Examples and Comparative Examples were measured as follows.

[0126] <Measurement of specific surface area (S)> The specific surface area (S) (m 2 / g) was measured using a fully automatic BET specific surface area measurement device (manufactured by Mountech Co., Ltd., trade name "Macsorb HM model-1208"), and nitrogen gas was used as the adsorption gas. 0.5 g of the measurement sample powder was weighed, placed in a φ12 standard cell (HM1201-031), degassed under vacuum at 100 °C for 0.5 hours, and then measured by the BET single-point method.

[0127] <Calculation of D50 of primary particles and secondary particles: Dry laser diffraction scattering method> The D50 of the lithium titanate powder in each example and each comparative example was calculated from the particle size distribution curve measured using a laser diffraction / scattering type particle size distribution analyzer (Microtrac MT3300EXII, manufactured by Nikkiso Co., Ltd.). 50 mg of the sample was put into a container containing 50 ml of ion-exchanged water as the measurement solvent, and the container was shaken by hand until it was visually confirmed that the powder was uniformly dispersed in the measurement solvent. Then the container was placed in the measurement cell for measurement. The crushing treatment was carried out by applying ultrasonic waves (30 W, 3 s) with an ultrasonic wave in the device. Further, the measurement solvent was added until the transmittance of the slurry was within the appropriate range (the range indicated by the green bar of the device) for particle size distribution measurement. The D50 of the mixed powder before and after crushing was calculated from the obtained particle size distribution curve. Note that the D50 before crushing corresponds to the D50 of the secondary particles, and the D50 after crushing corresponds to the D50 of the primary particles.

[0128] <Measurement of pH> The pH of the lithium titanate powder in each example and each comparative example was measured using a pH meter D-51 (manufactured by Horiba, Ltd.). A mixed solution was prepared by adding 5.0 g of lithium titanate powder and 45.0 g of pure water in a nitrogen atmosphere. After stirring for 5 minutes at a liquid temperature of 25 °C, the pH of this mixed solution was measured.

[0129] <Measurement of average compressive strength of secondary particles> The average compressive strength of the secondary particles of the lithium titanate powder in each example and comparative example was measured using a Shimadzu microcompression tester (MCT-510) equipped with a test force "measurement mode of 1 g or less." The test mode was selected as "compression test," and the compressive strength measured when the particles were compressed by 10% of the measured particle diameter was taken as the compressive strength of the secondary particles. Particles that could be clearly determined to form secondary particles were randomly selected and measured one by one, and the average of 10 points was calculated, and this average was taken as the "average compressive strength of the secondary particles."

[0130] [Evaluation of battery characteristics] Coin-type batteries were fabricated using the lithium titanate powders of each Example and Comparative Example, and their battery characteristics were evaluated. The evaluation results are shown in Table 2.

[0131] <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 lithium titanate powder of each Example and Comparative Example was removed from an aluminum laminate bag in a room controlled at a temperature of 25°C and a dew point of -20°C or lower. The removed lithium titanate powder of each Example and Comparative Example was mixed as follows to prepare a coating material: 90% by mass of the active material, 5% by mass of acetylene black as a conductive agent, and 5% by mass of polyvinylidene fluoride as a binder. Polyvinylidene fluoride, acetylene black, and 1-methyl-2-pyrrolidone, which had been previously dissolved in 1-methyl-2-pyrrolidone, were mixed using a planetary agitator / deaerator. The lithium titanate powder was then added, and the total solids concentration was adjusted to 64% by mass. The mixture was then mixed using a planetary agitator / deaerator. 1-methyl-2-pyrrolidone was then added, and the total solids concentration was adjusted to 50% by mass. The mixture was then mixed using a planetary agitator / deaerator to prepare a coating material. The resulting coating material was applied to aluminum foil and dried to prepare a single-sided negative electrode sheet for use in a coin battery (described later) and a double-sided negative electrode sheet for use in a laminate battery (described later). The target coating weight was 7.5 mg / cm. 2 It was decided.

[0132] <Measurement of electrode density> The negative electrode single-sided sheet coated as described above was pressed with a roll press (roll φ60 × 150 mm, press pressure equivalent to 40 MPa), and the density of the active material layer was measured as "electrode density." The evaluation results are shown in Table 1. A high electrode density is preferable because it allows more active material to be packed into a given volume, resulting in an increase in the capacity that can be used as a battery.

[0133] <Preparation of positive electrode sheet> A single-sided positive electrode sheet was produced in the same manner as described above in <Production of Negative Electrode Sheet>, including the ratios of the active material, conductive agent, and binder, except that lithium nickel cobalt manganese oxide powder was used as the active material.

[0134] <Preparation of electrolyte> The electrolyte used in the battery for characteristic evaluation was prepared as follows. In an argon glove box controlled at a temperature of 25°C and a dew point of -70°C or less, a nonaqueous solvent of ethylene carbonate (EC):dimethyl carbonate (DMC) = 1:2 (volume ratio) was prepared, and LiPF6 was dissolved in this as an electrolyte salt to a concentration of 1 M to prepare the electrolyte for the coin battery described below. Similarly, a nonaqueous solvent of propylene carbonate (PC):diethyl carbonate (DEC) = 1:2 (volume ratio) was prepared, and LiPF6 was dissolved in this as an electrolyte salt to a concentration of 1.3 M to prepare the electrolyte for the laminate battery described below.

[0135] <Creating a coin battery> The negative electrode single-sided sheet prepared by the above method was punched into a circle with a diameter of 14 mm and cut into 2 t / cm 2 An electrode for evaluation was prepared by pressing the electrode at a pressure of 1000 kJ / cm2 and then vacuum drying at 120°C for 5 hours. The electrode for evaluation and metallic lithium (formed into a circle with a thickness of 0.5 mm and a diameter of 16 mm) were placed opposite each other 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> was added and sealed to prepare a 2032-type coin battery.

[0136] <Making a laminated battery> The laminate battery was fabricated in a room controlled at a room temperature of 25°C and a dew point of -40°C or less as follows. 2 After pressing at a pressure of 2 t / cm, a negative electrode having a lead wire connection portion was fabricated. 2 After pressing at a pressure of 1000 kJ / cm², a positive electrode with a lead wire connection was fabricated. The fabricated negative and positive electrodes were vacuum dried at 150°C for 12 hours. After vacuum drying, the positive and negative electrodes were stacked facing each other with a separator (UPZ210, manufactured by Ube Industries) in between. Aluminum foil lead wires were connected to the positive and negative electrodes, respectively. The laminated battery electrolyte prepared as described in the "Preparation of Electrolyte" section above was added, and the laminate was vacuum sealed with an aluminum laminate to fabricate a laminated battery for evaluation. The battery capacity was 350 mAh, and the ratio of the negative and positive electrode capacities (negative electrode capacity / positive electrode capacity) was 1.2.

[0137] <Measurement of initial discharge capacity and volumetric energy density> The coin battery fabricated by the method described in the above <Coin Battery Fabrication> was placed in a thermostatic chamber at 25°C and charged at 0.2 mA / cm in the direction in which Li was absorbed into the evaluation electrode. 2 The battery was charged to 1 V at a current density of 0.05 mA / cm at 1 V. 2 After constant current and constant voltage charging, the current density was increased to 0.2 mA / cm. 2 Three cycles of constant current discharge were performed, in which the battery was discharged to 2 V at a current density of 1000 kJ / cm. The discharge capacity (mAh) at the third cycle was divided by the mass of lithium titanate to obtain the initial discharge capacity (mAh / g). This initial discharge capacity was multiplied by the electrode density obtained above to obtain the volumetric energy density (mAh / cm). 3 A high volumetric energy density increases the capacity that can be used per given volume of the battery, which is desirable as it leads to a smaller battery.

[0138] <Initial efficiency, 50C charge rate measurement> The coin battery fabricated by the method described in the above <Coin Battery Fabrication> was placed in a thermostatic chamber at 25°C and charged at 0.2 mA / cm in the direction in which Li was absorbed into the evaluation electrode. 2 The battery was charged to 1 V at a current density of 0.05 mA / cm at 1 V. 2 After constant current and constant voltage charging, the current density was increased to 0.2 mA / cm. 2 Three cycles of constant-current discharge were performed, with the battery being discharged to 2 V at a current density of 1000 kJ / s. The initial efficiency (%) was calculated by dividing the discharge capacity at the first cycle by the charge capacity at the first cycle. A high initial efficiency is desirable because it reduces irreversible capacity during charge and discharge, thereby increasing the usable capacity of the battery. Next, the discharge capacity at the third cycle was taken as the initial capacity. The battery was then charged to 1 V at a current of 50 C, the initial capacity, and then discharged to 2 V at a current of 0.2 C. The 50 C charge capacity was calculated, and this was divided by the 0.2 C charge capacity to calculate the 50 C charge rate (%). A high 50 C charge rate of lithium titanate powder is expected to improve the charge rate characteristics of energy storage devices when used as an electrode material for such devices. The evaluation results are shown in Table 1. The "C" in 50 C represents the current value during charge and discharge. For example, 1C refers to the current value that can fully discharge (or fully charge) the theoretical capacity in 1 / 1 hour, 50C refers to the current value that can fully discharge (or fully charge) the theoretical capacity in 1 / 50 hour, and 0.2C refers to the current value that can fully discharge (or fully charge) the theoretical capacity in 1 / 0.2 hour.

[0139] <Measurement of gas generation amount after 50 cycles at 60℃> In a thermostatic chamber at 25°C, a laminate battery prepared by the method described above in <Preparation of Laminated Battery> was charged to 2.75 V at a current of 0.2 C, and then subjected to constant-current, constant-voltage charging at 2.75 V until the charging current became 0.05 C. After that, three cycles of constant-current discharging were repeated, in which the battery was discharged to 1.4 V at a current of 0.2 C. The volume of the laminate battery was then measured by Archimedes' method, and this was defined as the initial volume of the laminate battery (hereinafter sometimes referred to as the initial volume).

[0140] Next, in a thermostatic chamber at 60°C, the battery was charged to 2.75 V at a current of 1 C, and then further charged at 2.75 V until the charging current became 0.05 C. After that, the battery was discharged to 1.4 V at a current of 1 C, and this cycle test was repeated 50 times.

[0141] After 50 cycles of the cycle test, the volume of the laminated battery was measured by Archimedes' method and designated as the volume of the laminated battery after the cycle test (hereinafter, sometimes referred to as "volume after cycle test"). The initial volume was subtracted from the volume after the cycle test to determine the amount of gas generated (ml), which was converted to the amount of gas generated per 1000 mAh of battery capacity after 50 cycles (ml / Ah) (hereinafter, sometimes referred to as "60°C cycle gas generation amount").

[0142] [Table 1]

[0143] <Evaluation results> The electrodes using the lithium titanate powders of Examples 1-1 to 1-4 maintained high initial efficiency and high electrode density, and furthermore, had a high 50C charge rate and suppressed gas generation during high-temperature operation. On the other hand, when lithium titanate powder synthesized by the same production method as in Examples 1-1 to 1-4 but without molybdenum on the surface was used (Comparative Example 1-1), the 50C charge rate decreased and the gas generation amount during high-temperature operation increased. Particularly regarding the 50C charge rate, it is generally recognized that as the specific surface area of the active material increases, the area of the lithium ion insertion / desorption reaction increases, so the 50C charge rate increases. However, for the lithium titanate powder of the present invention, although the specific surface area of the active material decreases compared to the lithium titanate powder of Comparative Example 1-1 as the molybdenum content increases (Examples 1-3, 1-4), the 50C charge rate tends to improve. Regarding the gas generation amount, since it is generated by the reaction between the active material and the electrolyte, it is generally recognized that the smaller the specific surface area of the active material, the more the gas generation amount is suppressed. However, in this example, since the gas generation amount does not necessarily proportional to the specific surface area, it is important to control it within a certain range. Also, when lithium titanate powder with a molybdenum content on the surface but synthesized by a production method different from that of Examples 1-1 to 1-4 and having a specific surface area of less than 6.2 m 2 / g and a primary particle size larger than 0.8 μm was used (Comparative Example 1-2), the electrode density decreased. From these results, it was found that lithium titanate containing molybdenum and showing a specific specific surface area and D50 of primary particles exhibits particularly good battery characteristics. In Examples 1-1 to 1-4, since Mo and Al as the metal element M were introduced by the surface treatment step, Mo and Al as the metal element M were localized and present near the surface of the primary particles of lithium titanate (the same applies to Examples 2-1 to 2-5 described later).

[0144] <Results of X-ray photoelectron spectroscopy (XPS) analysis> The valence distribution of Mo localized near the surface of the primary particles was measured for the lithium titanate powders of Examples 1-1, 1-2, and 1-4 using a scanning X-ray photoelectron spectrometer (ULVAC-PHI PHI5000). Each sample was sampled on an Al plate, and measurements were performed using an AlKα X-ray source (monochrome, 1486.6 eV, 50 W), an analysis area of ​​200 μmφ, and a charge neutralization mechanism (electron gun + Ar ions). Peak separation calculations (using a peak separation program on MATLAB®) were performed on the obtained Mo 3d spectrum, and the Mo 6+ , and Mo 5+ It is a mixture of Mo 6+ , Mo 5+ , and Mo 4+ It was also confirmed that Mo existed in the mixture. 6+ , Mo 5+ , and Mo 4+ Mo 6+ The valence distribution ratio of Mo is 60-80%. 5+ and Mo 4+ It was confirmed that the total valence distribution ratio of Mo was 20 to 40%. Figure 1 shows an example of peak separation related to the Mo valence distribution.

[0145] [Influence of manufacturing process conditions for lithium titanate powder] [Examples 2-1 to 2-5, Comparative Examples 2-1 to 2-4] Lithium titanate powders according to Examples 2-1 to 2-5 and Comparative Examples 2-1 to 2-4 were produced in the same manner as in Example 1, except that the post-treatment step and surface treatment step were carried out under the conditions shown in Table 2.

[0146] [Table 2]

[0147] <Evaluation results> It was found that by incorporating molybdenum into the surface and controlling the specific surface area and primary particle D50 within a specific range, it was possible to improve all of the electrode density, initial efficiency, and 50C charge rate. On the other hand, it was found that the lithium titanate powders of Comparative Examples 2-1 to 2-5 exhibited low initial efficiency, electrode density, or 50C charge rate, resulting in an unbalanced battery performance. The post-cycle gas generation amounts of Examples 2-1 to 2-5 were equivalent to those of Example 1-2. Furthermore, the battery characteristics of electrode density, volumetric energy density, initial efficiency, and 50C charge rate of Examples 2-1 to 2-5 were not extremely low compared to Comparative Examples 1-1 to 1-2 and Comparative Examples 2-1 to 2-4, and it was found that they had an excellent balance of battery performance. [Industrial Applicability]

[0148] The lithium titanate powder obtained by the present invention has excellent charge rate characteristics, high initial efficiency, can suppress the amount of gas generation during high-temperature operation, and can also increase electrode density, making it useful as an electrode active material for lithium ion secondary batteries. Furthermore, lithium ion secondary batteries using this lithium titanate as an electrode active material are capable of stable, high-speed charging and discharging, making them 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, offices, etc.

Claims

1. Li ４ Ti ５ O １２ The lithium titanate powder has a specific surface area of ​​6.2 m and contains secondary particles formed by aggregation of primary particles in which molybdenum (Mo) and aluminum (Al) are localized near the surfaces of the primary particles. ２ / g to 6.8m ２ / g, and the D50 of primary particles corresponding to a cumulative volume of 50% in a volume-based particle size distribution measured by a laser diffraction scattering method is 0.6 μm to 0.8 μm.

2. 2. The lithium titanate powder according to claim 1, wherein the D50 of the secondary particles of the lithium titanate powder is 11 μm to 20 μm.

3. 3. The lithium titanate powder according to claim 1, wherein the molybdenum content (mass%) determined by X-ray fluorescence analysis (XRF) is 0.01 to 0.

8.

4. 4. The lithium titanate powder according to claim 1, wherein the lithium titanate powder has a pH of 11.2 or less.

5. The valence of molybdenum localized on the surface of the lithium titanate powder is Mo ６＋ , and Mo ５＋ The lithium titanate powder according to any one of claims 1 to 4, characterized in that

6. The valence of molybdenum localized on the surface of the lithium titanate powder is Mo ６＋ , Mo ５＋ , and Mo ４＋ The lithium titanate powder according to any one of claims 1 to 5, characterized in that:

7. The valence of molybdenum locally present on the surface of the lithium titanate powder is Mo ６＋ The area ratio of Mo ６＋ , Mo ５＋ , and Mo ４＋ 7. The lithium titanate powder according to claim 6, wherein the area ratio of the above is 80% or less relative to 100% of the total area ratio of the above.

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

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

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