Niobium-coated lithium manganese oxide powder and its manufacturing method

Abstract

To provide lithium manganese oxide powder being useful as a cathode active material of a lithium secondary battery, making it possible to curb the elution of Mn of the lithium manganese oxide powder and sufficiently improving cycle characteristics.MEANS FOR SOLVING THE PROBLEM: Niobium-coated lithium manganese oxide powder has a coating layer including niobium (Nb) in at least a part of a surface thereof, with a niobium content of 0.05 mass% or more and 3.0 mass% or less. The powder contains particles in a ratio of 50% or more in number, the particles having 1.2 or less of a variation coefficient of an atomic percentage ratio of Nb / Mn.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a niobium-coated lithium manganese oxide powder useful as a positive electrode active material, etc., and a method for producing the same. [Background technology]

[0002] As technological development and demand for mobile devices increases, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries have become common and are widely used due to their high energy density and voltage, long cycle life, and low self-discharge rate.

[0003] Lithium transition metal composite oxides are used as the positive electrode active material in lithium secondary batteries, and among these, the lithium nickel cobalt manganese composite metal oxide LiNixCoyMnzO2(x+y+z=1) (hereinafter sometimes referred to as "NCM") is the most commonly used, due to its high operating voltage and excellent capacity characteristics. However, NCM has issues with its thermal properties due to the instability of its crystal structure caused by delithiation, and it is expensive, so there are limitations to its mass use as a power source in fields such as electric vehicles.

[0004] Other materials that have been developed include lithium manganese oxides (LiMnO2 or LiMn2O4, etc.), lithium iron phosphate compounds (LiFePO4, etc.), and lithium nickel composite metal oxides (LiNiO2, etc.). Of these, lithium manganese oxides have the advantages of excellent thermal stability and output characteristics, as well as low cost, but the Mn 3+ This causes structural deformation (Jahn-Teller distortion), and when the trace moisture in the battery reacts with the electrolyte at high temperatures, HF is formed, causing Mn to dissolve out, which causes problems with the cycle characteristics.

[0005] In view of such problems, Patent Document 1 discloses a technique for forming a coating layer containing one or more coating elements selected from the group consisting of Al, Ti, W, B, F, P, Mg, Ni, Co, Fe, Cr, V, Cu, Ca, Zn, Zr, Nb, Mo, Sr, Sb, Bi, Si, and S on the surface of lithium manganese oxide (powder). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 2020-525990 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the technology described in Patent Document 1 cannot sufficiently suppress the elution of Mn from the lithium manganese oxide, and there is a problem in that when the lithium manganese oxide is used as a positive electrode active material for a lithium secondary battery, sufficient cycle characteristics cannot be obtained. Therefore, an object of the present invention is to provide a lithium manganese oxide that is useful as a positive electrode active material for lithium secondary batteries, capable of suppressing the elution of Mn from the lithium manganese oxide and sufficiently improving the cycle characteristics. [Means for solving the problem]

[0008] As a result of extensive research aimed at solving the above problems, the inventors discovered that when a lithium manganese oxide powder has a coating layer containing a compound derived from niobic acid and a basic organic compound, a coating layer of Nb oxide with little Nb segregation can be obtained, improving cycle characteristics, and thus conceived the present invention.

[0009] That is, the present invention provides a niobium-coated lithium manganese oxide powder having a coating layer containing niobium (Nb) on at least a portion of its surface, wherein the niobium content is 0.05% by mass or more and 3.0% by mass or less, and the niobium content is 50% or more by number of particles having a coefficient of variation of the Nb / Mn atomic percentage ratio of 1.2 or less.

[0010] The present invention also provides a method for producing niobium-coated lithium manganese oxide powder, which includes the steps of mixing lithium manganese oxide with a niobate compound dispersion and adhering the niobate compound dispersion to the surface of the lithium manganese oxide, wherein the niobate compound dispersion is obtained by dispersing or dissolving part or all of the niobate compound in a mixed solvent containing a lower alcohol and water as main solvents and a basic organic compound solvent.

[0011] The present invention also provides a method for producing niobium-coated lithium manganese oxide powder, comprising the steps of mixing lithium manganese oxide with a niobate compound dispersion and adhering the niobate compound dispersion to the surface of the lithium manganese oxide, wherein the niobate compound dispersion is obtained by dispersing or dissolving part or all of the niobate compound in an amine compound solvent selected from at least one of primary amines, secondary amines, tertiary amines, and quaternary ammonium compounds, with a lower alcohol and water as main solvents, and containing 5.0 mass% or more of water, the content of the amine compound solvent being 0.50 or more in molar ratio relative to the content of niobium (Nb), and the lower alcohol accounting for 70 mass% or more of the components other than components derived from the niobate compound, components derived from the amine compound solvent, and water. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a lithium manganese oxide powder that is useful as a positive electrode active material for lithium secondary batteries, which can suppress the elution of Mn from the lithium manganese oxide powder and sufficiently improve the cycle characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described below based on preferred embodiments thereof. In the niobium-coated lithium manganese oxide powder of the present invention, the surfaces of the lithium manganese oxide particles are coated with a coating layer containing niobium. The niobium content is 0.05% by mass or more and 3.0% by mass or less, preferably 0.07% by mass or more and 2.0% by mass or less, and more preferably 0.10% by mass or more and 1.8% by mass or less, relative to the niobium-coated lithium manganese oxide. When the niobium content is within this range, a sufficient amount of niobium-containing coating layer is formed on the surface of the lithium manganese oxide powder, making it possible to suppress the elution of Mn from the lithium manganese oxide powder. The niobium is derived from a polyacid such as niobic acid in the production method described below.

[0014] The niobium content was measured using an ICP emission spectrometer (SPS3520V manufactured by Hitachi High-Tech Science).

[0015] The niobium-containing coating layer may contain lithium in addition to niobium. The niobium-containing coating layer may be present over the entire surface of the lithium manganese oxide particle, or may be present only over a portion of the surface.

[0016] In the niobium-coated lithium manganese oxide powder of the present invention, the surfaces of the lithium manganese oxide particles are preferably uniformly coated with a niobium-containing coating layer. Considering the Nb / Mn atomic percentage ratio as a measure of uniform coating, the niobium-coated lithium manganese oxide of the present invention contains particles with a coefficient of variation of the Nb / Mn atomic percentage ratio of 1.2 or less, preferably 1.1 or less, and more preferably 1.0 or less, in a proportion of 50% or more, preferably 55% or more, and more preferably 60% or more by number. This improves the uniformity of Nb in the coating layer formed on the surface of the lithium manganese oxide powder, thereby more effectively suppressing the elution of Mn from the lithium manganese oxide powder. The smaller the coefficient of variation, the more preferable; for example, the lower limit can be set to 0.1. Similarly, the higher the upper limit of the particle content ratio of the coefficient of variation, the more preferable; for example, the upper limit can be set to 95%.

[0017] The coefficient of variation of the atomic percentage ratio of Nb / Mn is the ratio of the Nb and Mn concentrations (at%) at 10 measurement points, which are obtained by measuring the Nb and Mn concentrations at 10 points per particle using an analytical scanning electron microscope (SEM-EDS), and is calculated by dividing the standard deviation by the average.

[0018] SEM-EDX was performed using an SEM (Hitachi High-Tech SU8200) and an EDX (Brukar FQ-EDX) under the following conditions. Acceleration voltage: 15 keV Emission current: 30μA Probe current: High Condenser lens: 1 ·Magnification: 5000x ·Analysis method: Multi-point analysis Detectable elements: Mn, Nb

[0019] Furthermore, in the present invention, the average coefficient of variation of the Nb / Mn atomic percentage ratio is preferably 1.2 or less, more preferably 1.1 or less, and particularly preferably 1.0 or less. This further improves the uniformity of Nb in the coating layer formed on the surface of the lithium manganese oxide powder, making it possible to more effectively suppress the elution of Mn from the lithium manganese oxide powder. The lower limit of this average value is preferably as low as possible, and can be set to, for example, 0.1.

[0020] The average value of the coefficient of variation was determined by randomly selecting 10 particles, calculating the coefficient of variation for each particle, and then averaging the coefficients of variation for the 10 particles.

[0021] The niobium-coated lithium manganese oxide powder of the present invention may also contain carbon (C). The carbon content is preferably 200 ppm or more and 12,000 ppm or less, more preferably 250 ppm or more and 10,000 ppm or less, and particularly preferably 300 ppm or more and 8,000 ppm or less, relative to the niobium-coated lithium manganese oxide.

[0022] The carbon amount was measured using a carbon analyzer (for example, EMIA-20P manufactured by Horiba Ltd.).

[0023] In the present invention, it is presumed that, since the niobium-coated lithium manganese oxide powder contains carbon derived from a basic organic compound in the above-mentioned proportion, a polyacid such as niobic acid and the basic organic compound form a compound that is uniformly distributed on the surface of the lithium manganese oxide powder to form a coating layer. Therefore, it is presumed that segregation of niobium in the coating layer can be suppressed, and a coating layer in which the niobium is uniformly distributed can be formed, thereby suppressing the elution of Mn from the lithium manganese oxide powder.

[0024] Conventionally, for example, in the method described in Patent Document 1, when forming a coating layer containing a coating element such as Nb, a wet method using a solvent such as alcohol or a dry method such as a grinder mixing method is used, and a basic organic compound solvent as described above is not used. Therefore, since a compound of a polyacid and a basic organic compound is not formed in the coating layer, segregation of the coating element occurs in the coating layer, which is presumed to result in elution of Mn from the lithium manganese oxide powder.

[0025] In addition, the niobium-coated lithium manganese oxide powder has a volume cumulative particle size D at 50% cumulative volume by laser diffraction scattering particle size distribution measurement method. 50 However, it is preferably 4.0 μm or more and 18 μm or less, and more preferably 5.0 μm or more and 17 μm or less.

[0026] Next, a method for producing the niobium-coated lithium manganese oxide powder of the present invention will be described. First, a method for producing lithium manganese oxide will be described. The lithium raw material, manganese raw material, and raw materials for the additive elements are weighed and mixed to obtain a mixed raw material. The obtained mixed raw material is filled into a firing container. The firing container is then placed in an electric furnace equipped with a mechanism for venting gases generated during the firing reaction, and firing is performed by adjusting the heating conditions, holding temperature, and temperature decreasing conditions. The fired powder obtained by firing is crushed and sieved, and the powder that falls through the sieve is collected to obtain lithium manganese oxide powder.

[0027] The preparation of the dispersion is described below. A niobic acid compound dispersion is prepared by dispersing or dissolving a part or all of a niobic acid compound in a mixed solvent containing a lower alcohol and water as the main solvent and at least one amine compound solvent selected from primary amines, secondary amines, tertiary amines, and quaternary ammonium compounds. This niobic acid compound dispersion contains 5.0 mass% or more of water, the content of the amine compound solvent relative to the content of niobium (Nb) is 0.50 or more in molar ratio, and lower alcohol accounts for 70 mass% or more of the components contained other than the components derived from the niobic acid compound, the components derived from the amine compound solvent, and water.

[0028] The niobic acid compound may be any compound containing niobic acid. For example, a compound consisting of Nb, O, and H, or a compound consisting of Nb, O, H, and N, may be mentioned. Among them, ammonium niobate or a salt thereof is preferable from the viewpoint of excellent dispersibility or solubility in a mixed solvent. However, the compound is not limited to these.

[0029] The lower alcohol refers to an alcohol having 5 or less carbon atoms and can be composed of a monohydric alcohol or a polyhydric alcohol. Examples of the lower alcohol include, but are not limited to, methanol, ethanol, propanol, isopropyl alcohol, butanol, ethylene glycol, propylene glycol, and glycerin.

[0030] Any water can be used, such as tap water, distilled water, ion-exchanged water, or RO water. From the viewpoint of dispersibility or solubility, the water content is 5.0% by mass or more, preferably 8.0% by mass or more, more preferably 9.0% by mass or more, and particularly preferably 10% by mass or more. The upper limit of the water content is necessarily determined by the lower limit of the lower alcohol content, which will be described below.

[0031] Amine compound solvents include alkylamines (including quaternary alkylammonium compounds), choline ([(CH3)3NCH2CH2OH] + ), choline hydroxide ([(CH3)3NCH2CH2OH] + OH - ) can be mentioned.

[0032] The alkylamine (including quaternary alkylammonium compounds) can be one having 1 to 4 alkyl groups. Among them, those having 1, 2, or 4 alkyl groups are preferred, and those having 1 or 4 are even more preferred. When there are 2 to 4 alkyl groups, the 2 to 4 alkyl groups may all be the same or may include some different alkyl groups. From the viewpoint of solubility, the alkyl group of the alkylamine is preferably one having 1 to 6 carbon atoms, and among these, those having 4 or less carbon atoms are preferred, among these, those having 3 or less carbon atoms are even more preferred, and among these, those having 2 or less carbon atoms are even more preferred.

[0033] Specific examples of the alkylamine include methylamine, dimethylamine, trimethylamine, tetramethylammonium hydroxide (TMAH), ethylamine, methylethylamine, diethylamine, triethylamine, methyldiethylamine, dimethylethylamine, tetraethylammonium hydroxide (TEAH), tetrabutylammonium hydroxide (TBAH), n-propylamine, di-n-propylamine, tri-n-propylamine, iso-propylamine, di-iso-propylamine, tri-iso-propylamine, n-butylamine, di-n-butylamine, tri-n-butylamine, iso-butylamine, di-iso-butylamine, tri-iso-butylamine, tert-butylamine, n-pentaamine, and n-hexaamine. Among these, from the viewpoint of solubility, methylamine, dimethylamine, tetramethylammonium hydroxide (TMAH), ethylamine, methylethylamine, diethylamine, and tetraethylammonium hydroxide (TEAH) are preferred, and among these, methylamine, ethylamine, tetramethylammonium hydroxide (TMAH), and tetraethylammonium hydroxide (TEAH) are more preferred, with tetramethylammonium hydroxide (TMAH) being most preferred.

[0034] From the viewpoint of improving the dispersibility or solubility of the niobic acid compound in the mixed solvent, the content of the amine compound solvent is, in terms of a molar ratio relative to the content of niobium (Nb), 0.50 or more, preferably 0.60 or more, more preferably 0.80 or more, and particularly preferably 1.0 or more. The upper limit of the content of the amine compound solvent is necessarily determined by the lower limit of the content of the lower alcohol described below.

[0035] For the same reason, the Nb content of the dispersion is preferably 0.1% by mass or more and 40% by mass or less, more preferably 0.5% by mass or more and 35% by mass or less, and particularly preferably 1.0% by mass or more and 30% by mass or less, calculated as Nb2O5.

[0036] The Nb content of the dispersion can be measured by evaporating the dispersion to dryness and baking it at 1000°C for 4 hours to produce NbO, and then calculating the amount of niobate compound-derived components contained in the dispersion from the mass of the NbO. It can also be measured by ICP atomic emission spectroscopy (inductively coupled plasma atomic emission spectroscopy, ICP-OES / ICP-AES). The amine compound solvent content can be measured by ion chromatography, liquid chromatography, gas chromatography, capillary electrophoresis, or the like.

[0037] Of the components contained in the dispersion other than the components derived from the niobic acid compound, the components derived from the amine compound solvent, and water, 70% by mass or more of the components are lower alcohols, and preferably 80% by mass or more of the components are lower alcohols. This improves the dispersibility or solubility of the niobic acid compound in lower alcohols. The upper limit of the lower alcohol content is not particularly limited, but is, for example, 90% by mass. The lower limit of the lower alcohol content is also not particularly limited, but is, for example, 30% by mass.

[0038] In the above-mentioned niobic acid compound dispersion, the niobic acid compound is uniformly dispersed or dissolved in the mixed solvent, and therefore the light transmittance at a wavelength of 400 nm is preferably 65% ​​or more, more preferably 70% or more, and particularly preferably 80% or more.

[0039] Next, the niobate compound dispersion is mixed with lithium manganese oxide powder to adhere the niobate compound dispersion to the surface of the lithium manganese oxide powder. The mixing time is, for example, from 30 seconds to 1 minute. The mixing can be performed multiple times.

[0040] Next, the niobium compound dispersion liquid adhered to the surface of the lithium manganese oxide powder is dried, for example, at 100°C to 250°C for 120 to 1440 minutes. At this time, it is presumed that the polyacid such as niobic acid and the amine compound form a compound that is uniformly distributed on the surface of the lithium manganese oxide powder to form a coating layer. As a result, a niobium-coated lithium manganese oxide powder can be obtained in which the elution of Mn from the lithium manganese oxide powder is suppressed. Note that natural drying may be used instead of drying at the above temperature.

[0041] The dried niobium-coated lithium manganese oxide powder can be crushed appropriately to break up agglomerates and have a predetermined particle size.

[0042] (Characteristics / Applications) The niobium-coated lithium manganese oxide powder of the present invention can be effectively used as a positive electrode active material for lithium batteries after being crushed and classified as necessary. For example, a positive electrode mixture can be produced by mixing this lithium manganese acid powder with a conductive material such as carbon black and a binder such as Teflon (registered trademark). A lithium secondary battery can be constructed using this positive electrode mixture for the positive electrode, a material capable of absorbing and desorbing lithium, such as lithium or carbon, for the negative electrode, and a non-aqueous electrolyte in which a lithium salt such as lithium hexafluorophosphate (LiPF6) is dissolved in a mixed solvent such as ethylene carbonate-dimethyl carbonate. However, this is not intended to limit the battery to such a configuration.

[0043] Lithium batteries having the niobium-coated lithium manganese oxide powder of the present invention as a positive electrode active material can be used particularly as a positive electrode active material for stationary storage batteries and large lithium batteries used as motor drive power sources mounted on electric vehicles (EVs) and hybrid electric vehicles (HEVs). [Example]

[0044] [evaluation] (chemical analysis values) (Nb content) Measurement was performed using an ICP emission spectrometer (Hitachi High-Tech Science SPS3520V). <Condition> A mixture of niobium-coated lithium manganese oxide powder and a reagent made by mixing sodium peroxide and sodium carbonate in a 2:1 ratio was melted with a burner, and a solution was made by impregnating it with an aqueous hydrochloric acid solution. This was introduced into the above-mentioned apparatus and the amount of Nb was measured. (Li, Mn, Mg, Al element content) The amounts of Li, Mg, and Al were measured using an ICP optical emission spectrometer (Hitachi High-Tech Science SPS3520V). The amount of Mn was measured by oxidation titration using potassium permanganate solution. <Condition> Li, Mg, Al The sample was added to hydrochloric acid and decomposed to prepare an aqueous solution, which was then introduced into the above-mentioned apparatus to measure the amount of each element. Mn The sample was acid-decomposed in sulfuric acid to prepare an aqueous solution, and the Mn content was measured by titration using potassium permanganate solution after adding an indicator.

[0045] (C amount) The measurement was carried out using a carbon analyzer (Horiba, Ltd., EMIA-20P). <Condition> The sample was mixed with tungsten powder and introduced into the above-mentioned device (high-frequency heating and combustion in an oxygen stream, infrared absorption method), and the carbon content was measured.

[0046] (Coefficient of variation of atomic percentage ratio of Nb / Mn) The coefficient of variation of the atomic percentage ratio of Nb / Mn is the variation of the atomic percentage ratio of Nb / Mn at 10 measurement points, in which the concentrations of Nb and Mn were measured at 10 points for one particle using an analytical scanning electron microscope (SEM-EDX), and was calculated by dividing the standard deviation of the atomic percentage ratios at 10 points by the average. Note that SEM-EDX was performed under the conditions described in paragraph

[0018] .

[0047] (Number ratio of variation coefficient of Nb / Mn atomic percentage ratio) Ten particles were randomly selected, and the coefficient of variation was calculated for each particle, and then the proportion of particles with a coefficient of variation of 1.2 or less was calculated. (Average value of coefficient of variation of atomic percentage ratio of Nb / Mn) Ten particles were randomly selected, and the coefficient of variation was calculated for each particle, and then these coefficients of variation were averaged over the 10 particles.

[0048] (Lithium-ion secondary battery capacity retention cycle characteristics) Lithium-ion secondary battery structure 20.0 g of positive electrode active material, 1.13 g of acetylene black (carbon black: manufactured by Denki Kagaku Kogyo), and 11.23 g of a solution (manufactured by Kureha) of 12 wt% PVDF (Teflon binder) dissolved in NMP (N-methylpyrrolidone) were accurately weighed out, 10 g of NMP (manufactured by Kanto Chemical) was added, and the mixture was thoroughly mixed to prepare a paste. This paste was placed on an aluminum foil current collector, and coated with an applicator adjusted to a gap of 250 μm. After drying in a vacuum at 120 °C for 24 hours, it was punched out with a φ16 mm hole and cut into 4 t / cm. 2 The cathode was then pressed and densified at 120°C for 120 minutes or more to form a positive electrode. Just before battery fabrication, the cathode was vacuum dried at 120°C for 120 minutes or more to remove any adhering moisture and incorporate the cathode into the battery. The average weight of the 16mm diameter aluminum foil was calculated in advance, and the weight of the aluminum foil was subtracted from the weight of the cathode to determine the weight of the cathode composite. The content of the positive electrode active material was also calculated from the mixing ratio of the positive electrode active material, acetylene black, and PVDF. The negative electrode was a 20mm diameter x 1.0mm thick metallic Li. These materials were used to fabricate TOMCELL (registered trademark) electrochemical evaluation cells.

[0049] The positive electrode was placed at the center of the inside of the organic electrolyte-resistant stainless steel lower body of the electrochemical evaluation cell. A microporous polypropylene resin separator impregnated with the electrolyte was placed on top of this positive electrode and secured with a Teflon spacer. A negative electrode made of metallic Li was placed on top of the separator, and a spacer that also served as the negative electrode terminal was placed on top of it. An organic electrolyte-resistant stainless steel upper body was then placed on top of it and tightened with screws to seal the battery.

[0050] The electrolyte used was a solvent made by mixing EC (ethylene carbonate) and DMC (dimethyl carbonate) in a volume ratio of 3:7, with 1 mol / L of LiPF6 dissolved therein as a solute.

[0051] (Battery characteristic evaluation) Using the electrochemical cell prepared as described above, the battery characteristics were determined by the method described below.

[0052] Initial activation process The battery cell was placed in an environmental tester set to an environmental temperature of 25°C for charging and discharging, and prepared for charging and discharging. After leaving the cell to cool to the environmental temperature for 8 hours, the charge and discharge range was set to 3.0V to 4.3V, and two cycles of constant current and constant voltage charging at 0.1C and constant current discharging at 0.1C were performed, followed by constant current and constant voltage charging at 0.1C. ·Capacity retention cycle characteristics The battery cell was placed in an environmental tester set to an ambient temperature of 45°C, and after leaving it for 8 hours to allow the cell temperature to return to the ambient temperature, 50 cycles of 1.0 C constant current discharge and 0.5 C constant current / constant voltage charge were performed with a charge / discharge range of 3.0 V to 4.3 V. The discharge capacity at the 50th cycle relative to the discharge capacity at the 5th cycle when charge / discharge had stabilized was defined as the capacity retention rate.

[0053] [Example 1] 100 g of niobium pentoxide was dissolved in 200 g of 55 mass % hydrofluoric acid aqueous solution, and 1830 mL of ion-exchanged water was added to obtain a niobium fluoride aqueous solution containing 50 g / L of niobium calculated as Nb2O5 (Nb2O5 = 4.69 mass %). 400 mL of this niobium fluoride aqueous solution was added to 1 L of ammonia water (NH3 concentration 25 mass %) over a period of less than 1 minute to obtain an ammonium niobate reaction solution (pH 11).

[0054] Next, the reaction solution was decanted using a centrifuge and washed until the amount of free fluorine in the supernatant was 100 mg / L or less, thereby obtaining a fluorine-removed niobic acid compound-containing material. The water content of this niobic acid compound-containing material was 35.3 mass%. In this niobic acid compound-containing material, the molar ratio of the ammonium ion content to the niobium (Nb) content (NH 4+ The Nb content was calculated by drying a portion of the hydrated niobic acid compound at 110°C for 24 hours, then firing it at 1000°C for 4 hours to generate Nb2O5, calculating the Nb2O5 concentration contained in the niobic acid compound from its mass, and then calculating the Nb content from this Nb2O5 concentration.

[0055] Next, 48.4 g of ethanol (water content 0.5 mass%) was added to 13.2 g of the fluorine-removed niobic acid compound-containing material and stirred, and then 8.4 g of tetramethylammonium hydroxide pentahydrate (TMAH concentration 50 mass%, water content 50 mass%) was added and stirred so that the tetramethylammonium hydroxide (TMAH) concentration was 6.0 mass%, to prepare a slurry containing 10 mass% niobium in terms of NbO. This slurry was stirred for 24 hours to obtain a niobic acid compound dispersion.

[0056] In addition to lithium carbonate and manganese oxide (Mn3O4), magnesium oxide and aluminum hydroxide were weighed so that the magnesium content in the lithium manganese oxide was 0.1% and the aluminum content was 1.2%, and these were mixed to obtain a mixed raw material. The obtained mixed raw material was placed in a firing container (alumina crucible size = length x width x height = 10 x 10 x 5 (cm)) with the ratio of open area to filling height (open area cm 2 The filling was carried out so that the filling height (cm) was 100.

[0057] The calcination reaction was carried out in a static electric furnace equipped with a mechanism for venting gases generated during the calcination reaction. The temperature was increased from room temperature to the calcination target temperature at a rate of 150°C / hr, and the calcination temperature (holding temperature) was maintained at 750°C for 20 hours. The temperature was then decreased from the holding temperature to 600°C at a rate of 20°C / hr, and the material was then allowed to cool naturally to room temperature. Temperature variation during the holding time was controlled within the range of 740°C to 760°C. The calcined powder was crushed in a mortar and sieved through a 75μm sieve. The powder remaining under the sieve was collected to obtain lithium manganese oxide powder. The composition of the resulting lithium manganese oxide was (Li = 4.2%, Mn = 57.5%, Mg = 0.1%, Al = 1.2%), with an average particle size of 14μm and a lattice constant of 8.205Å.

[0058] Next, 2.7 mL of the niobate compound dispersion was sprayed onto 150 g of the lithium manganese oxide powder having an average particle size of 14 μm obtained above using an atomizer, and the mixture was mixed for 30 seconds using a juice mixer (manufactured by Panasonic Corp.) This operation was repeated five times, resulting in a total of 13.5 mL of the niobate compound dispersion being sprayed onto the surface of the lithium manganese oxide powder.

[0059] The lithium manganese oxide powder to which the niobium acid compound dispersion liquid was sprayed and attached was then dried at 120°C for 720 minutes to produce a niobium-coated lithium manganese oxide powder. The niobium-coated lithium manganese oxide powder was then pulverized in the juice mixer to produce particles with an average particle size of 14 μm, and the above evaluations were carried out. The results are shown in Table 1.

[0060] [Example 2] 1.4 mL of the same niobate compound dispersion as in Example 1 was sprayed onto 150 g of the lithium manganese oxide powder having an average particle size of 14 μm obtained above using an atomizer, and then mixed for 30 seconds using a juice mixer (manufactured by Panasonic Corporation). This operation was repeated five times, thereby spraying a total of 7 mL of the niobate compound dispersion onto the surface of the lithium manganese oxide powder.

[0061] The lithium manganese oxide powder to which the niobium acid compound dispersion liquid was sprayed and attached was then dried at 120°C for 720 minutes to produce a niobium-coated lithium manganese oxide powder. The niobium-coated lithium manganese oxide powder was then pulverized in the juice mixer to produce particles with an average particle size of 14 μm, and the above evaluations were carried out. The results are shown in Table 1.

[0062] [Comparative Example 1] 100 g of niobium pentoxide was dissolved in 200 g of 55% aqueous hydrofluoric acid solution, and 1830 mL of ion-exchanged water was added to obtain an aqueous niobium fluoride solution containing 50 g / L of niobium calculated as Nb2O5 (Nb2O5 = 4.69 mass%). 400 mL of this niobium fluoride aqueous solution was added to 1 L of ammonia water (NH3 concentration: 25 mass %) over a period of less than 1 minute to obtain a reaction liquid (pH 11).

[0063] Next, the reaction liquid was decanted using a centrifuge and washed until the amount of free fluorine was 100 mg / L or less, thereby obtaining a fluorine-removed niobic acid compound-containing material. Next, the fluorine-free niobate compound-containing material was diluted with pure water to obtain a slurry. A portion of this slurry was dried at 110°C for 24 hours and then calcined at 1000°C for 4 hours to produce Nb2O5, and the Nb2O5 concentration in the slurry was calculated from its weight. Pure water was added to the slurry, and then a 50% methylamine aqueous solution was added to adjust the methylamine concentration to 2.0% by mass, resulting in a slurry with an Nb2O5 solids concentration of 10.0% by mass. This slurry was stirred for 48 hours to obtain an aqueous dispersion of the niobic acid compound.

[0064] Next, 2.0 mL of the niobate compound dispersion was sprayed onto 150 g of the lithium manganese oxide powder having an average particle size of 14 μm obtained above using an atomizer, and then mixed for 30 seconds using a juice mixer (manufactured by Panasonic Corp.) This operation was repeated five times, thereby spraying a total of 10 mL of the niobate compound dispersion onto the surface of the lithium manganese oxide powder.

[0065] The lithium manganese oxide powder to which the niobium acid compound dispersion liquid was sprayed and attached was then dried at 120°C for 720 minutes to produce a niobium-coated lithium manganese oxide powder. The niobium-coated lithium manganese oxide powder was then pulverized in the juice mixer to produce particles with an average particle size of 14 μm, and the above evaluations were carried out. The results are shown in Table 1.

[0066] Comparative Example 2 1.1 g of niobium pentoxide powder was added to 150 g of the lithium manganese oxide powder with an average particle size of 14 μm obtained above, and the mixture was mixed for 30 seconds in a juice mixer (manufactured by Panasonic) five times, thereby adhering niobium pentoxide to the surface of the lithium manganese oxide powder.

[0067] The lithium manganese oxide powder with niobium pentoxide attached was then dried at 300°C for 720 minutes to produce a niobium-coated lithium manganese oxide powder, which was then pulverized in the juice mixer to produce particles with an average particle size of 14 μm, and the above evaluations were carried out. The results are shown in Table 1.

[0068] [Table 1]

[0069] As is clear from Table 1, in Examples 1 and 2, the Nb content is in the range of 0.05% by mass to 3.0% by mass, and the coefficient of variation of the Nb / Mn atomic percentage ratio is 1.2 or less. As a result, the capacity retention rate after 50 cycles is high, at least 99.5%.

[0070] On the other hand, in the comparative example in which the Nb content was in the range of 0.05 mass % or more and 3.0 mass % or less and the coefficient of variation of the Nb / Mn atomic percentage ratio exceeded 1.2, it was found that the capacity retention rate after 50 cycles decreased to 96.7% or less.

[0071] Furthermore, in the Examples, an organo solution was used as the dispersion liquid, which provided good wettability and compatibility with the lithium-manganese composite oxide, resulting in the distribution of Nb (small Nb / Mn variation coefficient), whereas in Comparative Example 1, an aqueous solution was used as the dispersion liquid, which did not provide good wettability and compatibility with the lithium-manganese composite oxide, resulting in poor distribution of Nb. Carbon (C) is a constituent element of the amine compound and is considered to be part of the coating layer containing a compound derived from niobic acid and the amine compound. It is believed that the presence of carbon (C) results in a coating layer of Nb oxide with even less Nb segregation.

[0072] From the above, it can be seen that the niobium-coated lithium manganese oxide powder according to the present invention can suppress the elution of Mn from the lithium manganese oxide powder and provide the lithium manganese oxide powder useful as a positive electrode active material for lithium secondary batteries that can sufficiently improve the cycle characteristics.

Claims

1. A niobium-coated lithium manganese oxide powder having a coating layer containing niobium (Nb) on at least a portion of its surface, The niobium content is 0.05% by mass or more and 3.0% by mass or less, A niobium-coated lithium manganese oxide powder containing particles having a coefficient of variation of the atomic percentage ratio of Nb / Mn of 1.2 or less in a proportion of 50% or more on a number basis.

2. 2. The niobium-coated lithium manganese oxide powder according to claim 1, wherein the average coefficient of variation of the Nb / Mn atomic percentage ratio is 1.2 or less.

3. The niobium-coated lithium manganese oxide powder according to claim 1 or 2, wherein the carbon (C) content is 200 ppm or more and 12,000 ppm or less.

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