JP2013193927A - Method of producing particulate mixture, particulate mixture, lithium ion secondary battery positive electrode active material, lithium ion secondary battery, and aqueous solution used in method of producing the particulate mixture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and the like of producing a particulate mixture capable of continuously synthesizing lithium iron phosphate having small particle diameter and uniform element distribution at low cost.SOLUTION: Preferably, a divalent iron chloride, which is a divalent iron, is used for an iron source. A phosphorous acid is used for a phosphorus source. The phosphorous acid has strong reduction characteristics. When a divalent iron is oxidized to a trivalent iron, it might precipitate hydroxides. Therefore, the phosphorous acid is applied to control progress of oxidation of iron. That is, the phosphorous acid serves as the phosphorus source and functions as an antioxidant for iron. For a lithium source, a lithium hydroxide is desirable because it is inexpensive and includes no corrosive substance such as chlorides.

Description

  The present invention relates to a method for producing a fine particle mixture which is a precursor of a positive electrode active material for a lithium ion secondary battery.

  In recent years, with the increasing mobility and functionality of electronic devices, secondary batteries, which are driving power sources, have become one of the most important components. In particular, lithium-ion secondary batteries replace the conventional NiCd batteries and Ni-hydrogen batteries due to the high energy density obtained from the high voltages of the positive electrode active material and the negative electrode active material used. Occupies a position.

However, lithium ion secondary batteries using a combination of a lithium cobaltate (LiCoO ₂ ) -based positive electrode active material and a graphite-based carbon-based negative electrode active material, which are used in current lithium ion batteries, The power consumption of high-function, high-load electronic components cannot be sufficiently supplied, and the required performance cannot be satisfied as a portable power source.

  Further, since lithium cobaltate uses cobalt, which is a rare metal, there are significant resource constraints, high costs, and a problem in price stability. Further, since lithium cobaltate releases a large amount of oxygen at a high temperature of 180 ° C. or higher, there is a possibility that explosion occurs during abnormal heat generation or short-circuiting of the battery.

For this reason, lithium transition metal lithium having an olivine structure such as lithium iron phosphate (LiFePO ₄ ) has been attracting attention as a material that satisfies resource, cost, and safety aspects.

  As a method for synthesizing lithium iron phosphate, a method called a solid phase method is known. For example, the raw material of the dried positive electrode active material is mixed in a dry state for about 1 hour to 1 day using a planetary ball mill or the like, or the positive electrode active material is mixed with an organic solvent such as alcohols, ketones, tetrahydrofuran, Alternatively, a calcined precursor is obtained by adding water and mixing in a wet manner.

  Thus, after mixing the substance used as the raw material of a positive electrode active material and obtaining a baking precursor, after baking once at 300-450 degreeC for several hours, it takes out from a furnace, the temporary A predetermined amount of a conductive carbon precursor is added to the fired product, and further, the second stage firing is performed in a predetermined atmosphere for several hours (for example, Patent Document 1).

  That is, the solid phase method is a method in which powders of a lithium source, an iron source, and a phosphorus source are mixed and fired in an inert atmosphere. This method has a problem that the composition of the product is not as intended unless the firing conditions are properly selected, and it is difficult to control the particle size.

  As a method for synthesizing lithium iron phosphate, a hydrothermal synthesis method utilizing hydrothermal synthesis in a liquid phase is also known. For example, a solution or suspension containing an electron conductive substance or a precursor of the electron conductive substance is sprayed and heated. These raw materials are dissolved or dispersed in a solvent to form a uniform solution or suspension, and the solution or suspension is sprayed and heated in an inert atmosphere or a reducing atmosphere.

  Although the heating temperature at the time of spraying changes with raw materials to be used, it is preferably 500 to 1000 ° C, more preferably 650 to 900 ° C. Since the electronically conductive substance or the precursor of the electronically conductive substance that has been uniformly dispersed in the solution or suspension exists in the droplets uniformly, when the secondary particles are formed, Secondary particles can be obtained that are incorporated in a state of being uniformly present in the secondary particles and in which an electron conductive substance is interposed between the primary particles (for example, Patent Document 2).

  That is, the hydrothermal synthesis method is performed in the presence of high-temperature and high-pressure hot water. A product with high purity is obtained at a much lower temperature than in the solid phase method. However, the control of the particle size is performed according to the preparation conditions such as the reaction temperature and time, but the reproducibility of the control of the particle size is poor and it is difficult to control the particle size.

  As a method for synthesizing lithium iron phosphate, there is a spray pyrolysis method. The spray pyrolysis method generates fine mist from a mixed solution of a carbon-containing compound, a lithium-containing compound, an iron-containing compound, and a phosphorus-containing compound, and thermally decomposes by heating while circulating the generated fine mist, and carbon A lithium iron phosphate powder containing carbon by producing a fine powder composed of a lithium iron phosphate precursor containing carbon and heating and firing the produced fine powder in an inert gas-hydrogen mixed gas atmosphere It is a method of generating a body. (See Patent Document 3)

Japanese Patent No. 4822416 JP 2004-14340 A JP 2009-070666 A

  However, lithium iron phosphate has a problem of low charge / discharge speed because of its low electrical conductivity and poor lithium ion diffusibility due to its structure.

  Therefore, lithium iron phosphate having a smaller particle size is required. If the particle size is small, the conductive path through the lithium iron phosphate may be short even if the electrical conductivity of the lithium iron phosphate itself is low. Moreover, if the particle size is small, the diffusion distance is shortened, and it is considered that high-speed charge / discharge can be handled.

In addition, in a layered structure such as LiCoO ₂ and a spinel structure such as LiMnO ₂ , the diffusion direction of lithium ions during charge and discharge is two-dimensional or three-dimensional, but olivine including lithium iron phosphate is used. In the structure, the diffusion direction of lithium ions is one-dimensional. Therefore, if the composition inside the lithium iron phosphate particles is not uniform, the diffusion of lithium ions is hindered, and only a portion of the lithium iron phosphate constituting the particles can participate in charging and discharging, resulting in a reduction in capacity.

  Furthermore, both the solid-phase method and hydrothermal synthesis method basically use a batch reactor and a small-scale reactor, and there is a need for a method that can synthesize lithium iron phosphate on a large scale on a continuous basis. .

  Moreover, in the above-mentioned spray pyrolysis method, the pyrolysis temperature is 500 to 900 ° C. (Patent Document 2 claim 2), and further the pyrolysis time is 10 seconds or more (calculated from Paragraph 0026 of Patent Document 2). there were.

  On the other hand, there is a spray combustion method as a method at a higher temperature in a shorter time. The spray combustion method is a high temperature (for example, 1000 to 3000 ° C., usually around 2000 ° C.), and the combustion time is also short (several milliseconds). For this reason, the obtained fine particle mixture (active material precursor) has a small particle size, and each particle is independent.

  Further, in the above-described spray pyrolysis method, since carbon is contained in the pyrolysis process, it is necessary to add hydrogen gas, which is a reducing gas, in the firing process. On the other hand, in the spray combustion method, since a carbon source is added after the production process of fine particles by the spray combustion method, a reducing carbon source can be used. Therefore, it is not necessary to use a reducing gas in the firing step.

  Here, in the spray pyrolysis method, the carrier gas of the mist is only an inert gas, but in the spray combustion method, the carrier gas contains a combustible gas and burns the droplets of the raw material solution. Usually, in the spray combustion method, a kerosene-based solution has been used as the raw material solution. By burning such a solution, a fine particle mixture of a desired metal is formed.

  However, the method using such a kerosene-based solution generates a large amount of heat during combustion. Therefore, since operation | movement etc. need cooling, the continuous driving | operation was difficult. In addition, operating costs such as exhaust cooling increase. In addition, since kerosene-based solvents are expensive, raw material costs increase. Furthermore, when the viscosity of the kerosene solvent increases, it becomes difficult to obtain fine particles.

  The present invention has been made in view of the above-described problems, and includes a method for producing a fine particle mixture capable of synthesizing lithium iron phosphate having a small particle size and uniform element distribution continuously and at low cost. The purpose is to provide.

In order to achieve the above-described object, a first invention is a method for producing a fine particle mixture that is a precursor of a positive electrode active material for a lithium ion secondary battery, and includes a lithium source, a phosphorus source, and an iron source in the fine particle mixture. An aqueous solution is used, the phosphorus source is phosphorous acid, the aqueous solution contains Li ions, P ions, and divalent Fe ions in substantially the same ratio, and the aqueous solution contains a combustible gas and a combustion-supporting gas. A method of producing a fine particle mixture, wherein the fine particle mixture is produced by spraying into a flame formed by mixing and burning. As a result, a fine particle compound in the vicinity of the stoichiometric composition of LiFePO ₄ can be obtained.

  The iron source is preferably divalent iron chloride.

  The lithium source is lithium hydroxide, and the aqueous solution is preferably formed by mixing phosphorous acid and a divalent iron chloride aqueous solution and then adding the lithium hydroxide aqueous solution and nitric acid.

  The Li ion concentration, the P ion concentration, and the divalent Fe ion concentration in the aqueous solution are preferably 0.5 mol / L or more and 1.0 mol / L or less.

The fine particle mixture includes a fine particle compound having a composition in the vicinity of the stoichiometric composition of LiFePO ₄ .

  According to the first invention, since each metal source constituting the fine particle mixture is dissolved in the aqueous solution, the calorific value at the time of combustion is suppressed as compared with the conventional kerosene-based solution. Therefore, continuous operation is possible. Moreover, since it is aqueous solution, the cost of a raw material can also be reduced. Moreover, since aqueous solution has low viscosity, it becomes possible to make a particle size smaller.

  Further, in iron hydroxide, of divalent iron and trivalent iron, trivalent iron has a smaller solubility product of iron ions and hydroxide ions. For this reason, trivalent iron tends to cause a precipitate of hydroxide. On the other hand, in this invention, since phosphorous acid is used as a phosphorus source, the oxidation of iron can be suppressed by the strong reducibility of phosphorous acid. Therefore, precipitation of iron hydroxide can be suppressed.

  In addition, by using divalent iron chloride that is divalent as the iron source, the iron source is inexpensive and the above-described precipitation of iron hydroxide can be suppressed.

  In addition, by using lithium hydroxide as a lithium source, for example, the amount of corrosive chloride is suppressed as compared with the case of using lithium chloride, and at the same time, the formation of hydroxide is suppressed by adding nitric acid. be able to.

  Further, by setting each of Li ion, P ion, and divalent Fe ion to 0.5 mol / L or more, each metal ion concentration becomes sufficient, and the production efficiency of the fine particle mixture is high. Moreover, since each ion is 1.0 mol / L or less, it can prevent that solid remains in aqueous solution. Therefore, a uniform and small particle size mixture can be obtained, and there is no fear of nozzle clogging.

Also, particulate mixture obtained, to have a composition near the stoichiometric composition of the LiFePO _4, by subsequent firing, it is possible to obtain LiFePO ₄ lithium ion secondary battery positive electrode active material.

The second invention is a fine particle mixture containing a fine particle compound having a composition in the vicinity of the stoichiometric composition of LiFePO ₄ produced by the production method according to the first invention. The composition in the vicinity of the stoichiometric composition means that when the composition of the fine particle compound is shifted from the stoichiometric composition of LiFePO ₄ by assuming that the composition of each element in the stoichiometric composition is 100%, ± It is a fine particle mixture satisfying the range of 95 to 105% within 5%.

  According to a third aspect of the present invention, there is provided a positive electrode active material for a lithium ion secondary battery obtained by firing a fine particle mixture obtained by the method for producing a fine particle mixture according to the first invention together with a carbon source.

  Moreover, 4th invention is a lithium ion secondary battery using the lithium ion secondary battery positive electrode active material concerning 3rd invention.

  According to the second to fourth inventions, a lithium ion secondary battery positive electrode active material having a uniform and small particle size can be obtained without using hydrogen gas, which is a reducing gas, in the firing step. Therefore, a secondary battery positive electrode active material having a large capacity and a lithium ion secondary battery using the same can be obtained.

  A fifth invention is an aqueous solution used in a method for producing a fine particle mixture, which is a precursor of a positive electrode active material for a lithium ion secondary battery, comprising phosphorous acid as a phosphorus source, lithium hydroxide as a lithium source, and two as an iron source. The aqueous solution contains a valent iron chloride and nitric acid, and is an aqueous solution used in a method for producing a fine particle mixture characterized by containing Li ions, P ions, and divalent Fe ions in substantially the same ratio.

  According to the fifth aspect of the present invention, it is inexpensive, and it is possible to efficiently generate a fine particle mixture of lithium iron phosphate that is a precursor of a positive electrode active material of a lithium ion secondary battery with a small particle size by spray combustion. Become.

  INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a method for producing a fine particle mixture capable of synthesizing lithium iron phosphate having a uniform element distribution and a small particle diameter continuously and at low cost.

1 is a schematic view of a fine particle production apparatus 1 used in a spray combustion method for producing a fine particle mixture according to the present invention. The figure which shows the manufacturing process of the aqueous solution used for the spray combustion method.

  Embodiments according to the present invention will be described below. FIG. 1 is a diagram showing a fine particle production apparatus for producing a fine particle mixture by a spray combustion method. The fine particle manufacturing apparatus 1 includes an aqueous solution supply unit 3, a combustion gas supply unit 5, an air supply unit 7, a mixing nozzle 9, a reaction vessel 11, a filter 13, and the like.

  The aqueous solution supply unit 3 is a part for supplying an aqueous solution as a raw material. The aqueous solution supply unit 3 may have an aqueous solution tank or the like, and may be connected to an aqueous solution production line so that the aqueous solution can be continuously supplied. The details of the aqueous solution used will be described later.

  The combustion gas supply unit 5 supplies a hydrocarbon-based combustible gas such as a paraffinic hydrocarbon gas such as methane, ethane, propane, or butane, or an olefinic hydrocarbon gas such as ethylene, propylene, or butylene. is there. Moreover, the air supply part 7 is a site | part which supplies the air (or oxygen) which is a combustion support gas.

  Each substance supplied from the aqueous solution supply unit 3, the combustion gas supply unit 5 and the air supply unit 7 is sent to the mixing nozzle 9 (in the direction of arrow A in the figure) and mixed by the mixing nozzle 9, and fine particles are synthesized. The Note that an aqueous solution may be separately sprayed on a burner using a combustible gas and a combustion-supporting gas. Moreover, it is preferable to supply the mixed solution of a lithium source, an iron source, and a phosphorus source in the form of mist droplets having a diameter of 20 μm or less.

  In the reaction vessel 11 sprayed with the aqueous solution as the raw material, the metals in the raw material aqueous solution react to form a fine particle mixture. In addition, as such a spray combustion method, a VAD (Vapor-phase Axial Deposition) method etc. are mentioned as a suitable example. The temperature of these flames varies depending on the mixing ratio of the combustible gas and the combustion-supporting gas and the addition ratio of the constituent raw materials, but is usually between 1000 and 3000 ° C., particularly about 1500 to 2500 ° C. Is preferred. The reaction vessel 11 is filled with an inert gas in advance.

  The produced fine particle mixture can be recovered from the exhaust gas by the filter 13. It can also be generated around the core rod as follows. A silica or silicon core rod (also called a seed rod) is installed in the reactor, and a lithium source, an iron source, and a phosphorus source are supplied together with the flame raw material into the oxyhydrogen flame or propane flame that is blown onto the core rod. When hydrolyzed or oxidized, mainly nano-order fine particles are generated and attached to the surface of the core rod. These generated fine particles are collected and, if necessary, filtered or sieved to remove impurities and coarse aggregates. The fine particle mixture thus obtained is composed of fine particles mainly having an amorphous nano-scale particle size and amorphous.

  In the spray combustion method, which is a method for producing a fine particle mixture according to the present invention, the fine particle mixture that can be produced is mainly amorphous and has a small particle size. Furthermore, in the spray combustion method, a large amount of synthesis is possible in a short time compared to the conventional hydrothermal synthesis method and solid phase method, and a homogeneous fine particle mixture can be obtained at low cost.

  The obtained fine particle mixture mainly comprises oxides of lithium, iron, phosphorus, and amorphous fine particles of lithium iron phosphate. Furthermore, some of them include a crystal component of a lithium iron phosphate compound. The composition of the fine particle mixture is uniform, the shape of the fine particle mixture is substantially spherical, and the average aspect ratio (major axis / minor axis) of the particles is 1.5 or less, preferably 1.2 or less, more preferably 1.1. It is as follows. The particle size of the fine particle mixture is in the range of 5 to 200 nm.

  It should be noted that the fact that the particle is substantially spherical does not mean that the particle shape is a geometrically strict spherical or elliptical sphere, and the surface of the particle is generally a smooth curved surface even if there are a few protrusions. It only has to be configured.

  Next, an aqueous solution that is a raw material solution used in the present invention will be described. As the iron source, iron chloride, iron hydroxide, iron oxide and the like can be used, but it is desirable to use divalent iron.

  In an aqueous solution, trivalent iron ions are smaller than when the solubility product with hydroxide ions is divalent. For this reason, trivalent iron ions may cause a precipitate of hydroxide. Such precipitates are undesirable because they cause nozzle clogging and hinder the formation of a fine particle mixture with a desired element ratio. Therefore, in consideration of cost and handleability, divalent iron oxalate (ferrous oxalate), divalent iron acetate (ferrous acetate), divalent iron oxide (ferrous oxide), divalent However, divalent iron chloride is particularly desirable in terms of cost.

  Phosphorous acid is used as the phosphorus source. Phosphorous acid has a strong reducibility. As described above, when divalent iron is oxidized to be trivalent, a precipitate of hydroxide may be formed. Therefore, phosphorous acid is applied in the present invention to suppress the progress of iron oxidation. That is, phosphorous acid serves as a phosphorus source and functions as an iron antioxidant.

  As the lithium source, lithium inorganic acid salts such as lithium chloride, lithium hydroxide, lithium carbonate, lithium acetate, lithium nitrate, lithium bromide, lithium phosphate, and lithium sulfate can be used. In particular, lithium hydroxide is preferable because it is inexpensive and does not contain corrosive substances such as chloride.

  FIG. 2 is a diagram showing an example of producing an aqueous solution used in the method for producing a fine particle mixture which is a precursor of the positive electrode active material of the lithium ion secondary battery of the present invention. First, phosphorous acid is added to a divalent iron chloride aqueous solution (step 101). Thereby, the oxidation of iron is suppressed.

  Next, an aqueous lithium hydroxide solution and nitric acid are added (step 102). Nitric acid makes the aqueous solution acidic. This is because if the hydroxide ions are present in excess, the above-described hydroxide precipitation may occur.

In addition, the ratio of each metal ion in an iron source, a phosphorus source, and a lithium source is adjusted so that it may become substantially the same. This is the target metal ratio of LiFePO ₄ . On the other hand, when Li is present in excess, the capacity per weight is reduced and the cost is increased. On the other hand, if Li is insufficient, the initial charge capacity as a lithium ion secondary battery is reduced.

  Moreover, when Fe is excessive, it will become trivalent because P is consumed, and it will become easy to produce a deposit. In addition, iron-rich compounds other than the target compound or iron oxides may be generated, which may inhibit the charge / discharge reaction. On the other hand, when Fe is insufficient, the amount of active material produced is reduced. To do.

  On the other hand, if P is excessive, the raw material cost increases and other phosphoric acid compounds are produced, resulting in a decrease in capacity. On the other hand, when P is insufficient, the amount of active material produced decreases. Moreover, the same harmful effect as the case where Fe and Li are relatively excessive arises.

  Therefore, it is desirable that the ion ratio of each metal is substantially the same. In addition, if each element is in the range of 95% to 105% with respect to the stoichiometric ratio (same molar ratio) of each element, even if there is a slight increase / decrease in the element, the above-described adverse effects are small.

The concentration of each metal ion is desirably 0.5 mol / L or more and 1.0 mol / L or less. When the ion concentration is less than 0.5 mol / L, the concentration of the metal source with respect to the solution is thin, so that the production efficiency is inferior. When the concentration of each metal ion is below the above range, Li, Fe, and P ions are insufficient, and the production efficiency of the fine particle mixture containing the LiFePO ₄ compound is lowered.

  On the other hand, if the ion concentration exceeds 1.0 mol / L, solid components may remain due to insufficient dissolution. Remaining solid components cause nozzle clogging and are undesirable because the above-mentioned ion ratio changes. Therefore, the concentration of each metal ion is desirably 0.5 mol / L or more and 1.0 mol / L or less.

Next, a method for producing a positive electrode active material material from the obtained fine particle mixture will be described. The fine particle mixture obtained by the above method contains a fine particle compound having a composition in the vicinity of the stoichiometric composition of LiFePO ₄ . An active material aggregate is obtained by firing the fine particle mixture in an atmosphere filled with an inert gas. Moreover, the amorphous compound contained in the fine particle mixture or the active material or the oxide form mixture is changed into a crystalline form compound of olivine type lithium iron phosphate mainly by firing. The inert gas atmosphere can prevent the carbon source from burning during firing and the positive electrode active material from being oxidized.

  Nitrogen gas, argon gas, neon gas, helium gas, carbon dioxide gas, etc. can be used as the inert gas. In addition, in order to increase the conductivity of the product after heat treatment, an organic compound that is a conductive carbon source such as polyhydric alcohol such as polyvinyl alcohol, saccharide such as sucrose, or carbon black is aggregated before the heat treatment. In addition to firing, it can be fired. Polyvinyl alcohol is particularly preferable because it serves as a binder for the fine particle mixture before firing and can reduce the iron component during firing.

  A coating or supporting treatment with carbon is performed in the same firing step together with the crystallization of the fine particle mixture. As the heat treatment conditions, a fired product having a desired crystallinity and particle size can be appropriately obtained by combining a temperature of 300 to 900 ° C. and a treatment time of 0.5 to 10 hours. Excessive heat load due to high temperature or prolonged heat treatment should produce a coarse single crystal and should be avoided. Heat treatment conditions that can suppress the size of the crystallites as small as possible under the heating conditions that can obtain the desired crystalline or microcrystalline lithium iron phosphate compound are desirable.

  The obtained active material aggregate can then be made into fine particles again by subjecting to a mortar, ball mill or other pulverizing means, and the positive electrode active material of the present invention that can suffice with a Li ion intercalation host is obtained.

  Most of the crystallized lithium iron phosphate compound contained in the obtained positive electrode active material is microcrystalline, but there is also a “microcrystalline” state containing an amorphous component in part. A microcrystal is, for example, a state in which fine particles composed of a plurality of crystallites are covered with an amorphous component, or a state in which fine crystals are present in an amorphous component matrix, A state in which an amorphous component exists between them.

  Since the positive electrode active material according to the present invention has a small particle size, the conduction path of Li ions or electrons in a single crystal or polycrystalline particle is short, and the ionic conductivity and electron conductivity are excellent. The barrier can be lowered.

  In the present invention, as described above, it is preferable that the lithium iron phosphate fine particles as the positive electrode active material are at least partially coated with carbon or at least partially supported with carbon. The carbon coat is to coat the surface of particles with carbon, and the carbon support is to contain carbon in the particles. The carbon coating or carbon support increases the electrical conductivity of the material, provides a conductive path to lithium phosphate transition metal fine particles, and improves the electrode characteristics when used for the positive electrode.

  From the positive electrode active material obtained as described above, a positive electrode can be formed as follows. First, the positive electrode active material powder coated or supported with carbon is added to an aqueous solvent or organic solvent matrix to form a slurry. This is applied on one or both sides of a current collector such as an aluminum alloy foil containing 95% by weight or more of aluminum, and dried to evaporate and dry the solvent.

  In addition, to the powder of the positive electrode active material, a conductive material such as carbon black is further added as necessary, and a binder such as polytetrafluoroethylene, polyvinylidene fluoride, and polyimide, or a dispersant such as butadiene rubber, Alternatively, a thickener such as carboxymethylcellulose or a cellulose derivative may be added. In this case, the mixture is added to an aqueous solvent or organic solvent matrix to form a slurry. Thus, a positive electrode can be obtained.

  At this time, in order to improve the application property of the slurry, the adhesion between the current collector and the active material, and the current collection, granulation was performed by a spray dry method using a positive electrode active material and a carbon source, and then fired. Secondary particles can be used in the slurry instead of the active material. The granulated secondary particle lump becomes a large lump of about 0.5 to 20 μm, which greatly improves the slurry coating property and further improves the characteristics and life of the battery electrode. As the slurry used for the spray drying method, either an aqueous solvent or a non-aqueous solvent can be used.

  Furthermore, in a positive electrode in which a slurry containing a positive electrode active material is coated on a current collector such as an aluminum alloy foil, the current collector surface roughness of the active material layer forming surface is Japanese Industrial Standard (JIS B 0601-1994). It is desirable that the ten-point average roughness Rz specified in (1) is 0.5 μm or more. The adhesiveness between the formed active material layer and the current collector is excellent, the electron conductivity accompanying the insertion and release of Li ions and the current collecting power to the current collector are increased, and the cycle life of charge / discharge is improved.

  In addition, at the interface between the current collector and the active material layer formed on the current collector, when the main component of the current collector is at least a kind of diffusion bonded state diffused to the active material layer, the current collector and the active material Interfacial bondability with the material is improved. Therefore, resistance to changes in volume and crystal structure in the charge / discharge cycle increases, so that the cycle life is improved. It is even better when the current collector surface roughness condition is also satisfied. According to sufficient firing conditions that can volatilize the solvent, the current collector component diffuses into the active material layer, resulting in an interfacial state having mutual components, excellent adhesion, and volume change due to the entry and exit of Li ions even after repeated charge and discharge Withstands and improves cycle life.

  In addition, in order to obtain a high-capacity lithium ion secondary battery using the obtained positive electrode, various materials such as a negative electrode, an electrolytic solution, a separator, and a battery case using a conventionally known negative electrode active material are not particularly limited. Can be used.

  According to the present invention, lithium iron phosphate having a small particle size and uniform element distribution can be synthesized continuously and at low cost using the spray combustion method. In particular, since the raw material is an aqueous solution, it is excellent in handleability as compared with a kerosene system, and there is no problem such as cooling of the apparatus during production. In addition, since the organic material is not included in the raw material solution, impurities in the generated substance can be reduced.

  Further, since the aqueous solution has a low viscosity, the particle size of the obtained fine particle mixture can be reduced. Therefore, the distance traveled by lithium ions and electrons is small, the ion conductivity and the electron conductivity are excellent, the active material can participate in charging and discharging efficiently, and charging and discharging can be performed at high speed.

  Further, since phosphorous acid is used as the phosphorus source, precipitation of hydroxide due to iron oxidation can be suppressed. That is, phosphorous acid has both a function as a phosphorus source and a function of an iron antioxidant. Therefore, precipitation of iron can be suppressed and a fine particle mixture can be efficiently generated.

  As mentioned above, although embodiment of this invention was described referring an accompanying drawing, the technical scope of this invention is not influenced by embodiment mentioned above. It is obvious for those skilled in the art that various modifications or modifications can be conceived within the scope of the technical idea described in the claims, and these are naturally within the technical scope of the present invention. It is understood that it belongs.

Using various solutions containing a lithium source, an iron source and a phosphorus source, a fine particle mixture having a stoichiometric composition of Li ₂ FePO ₄ was produced by a spray combustion method. The results are shown in Table 1. The concentration of the aqueous solution used in the examples was approximately 1: 1: 1 with each element mixed (the stoichiometric composition of LiFePO ₄ , which allowed a deviation in the range of 95% to 105%). The approximate molar concentration in terms of LiFePO ₄ .

  That is, the concentration of the aqueous solution containing each ion here means that Li ions are about 0.8 mol / L, and P ions and Fe ions are about 0.8 mol / L. In addition, an ICP emission spectroscopic analyzer (inductively coupled plasma mass spectrometer: plasma is used as an ion source to give high energy of plasma to an analysis sample with respect to a mixture of spherical fine particles that is the target in the examples. Element analysis equipment that performs highly sensitive multi-element analysis that excites the component element to be analyzed and analyzes the spectral line corresponding to the wavelength of the photon emitted when the excited component element atom returns to a low energy state) Was used to examine the composition ratio.

  Further, an active material and a slurry were produced, and a positive electrode for test evaluation and a secondary battery were produced using the slurry, and the initial characteristics of the battery were evaluated as follows.

(Production of positive electrode active material and slurry)
Polyvinyl alcohol was added to the fine particle mixture having a composition in the vicinity of the stoichiometric composition obtained in the present invention so that the polyvinyl alcohol would be 10 wt% of the total, and mixed. Thereafter, the fine particle mixture was placed in a furnace filled with N ₂ gas, and baked by performing a heat treatment at 650 ° C. for 32 hours. At the time of firing, the surface of the fine particle mixture is coated with carbon to carry carbon. The fine particle mixture after firing was pulverized to obtain a positive electrode active material.

  Further, a conductive assistant (carbon black) was mixed with the active material so as to be 10% by weight, and further mixed for 5 hours using a ball mill in which the inside was replaced with nitrogen. The mixed powder and polyvinylidene fluoride (PVdF) as a binder were mixed at a weight ratio of 95: 5, and N-methyl-2-pyrrolidone (NMP) was added and sufficiently kneaded to obtain a positive electrode slurry.

(Production of positive electrode for test evaluation and secondary battery)
The positive electrode slurry was applied at a coating amount of 50 g / m ² to an aluminum foil current collector having a surface roughness Rz (JIS B 0601-1994 10-point average roughness) of 0.7 μm and a thickness of 15 μm, and at 120 ° C. It was dried for 30 minutes, rolled to a density of 2.0 g / cm ³ with a roll press, punched into a disk shape of 2 cm ² , and used as a positive electrode. Using these positive electrodes, metallic lithium for the negative electrode, and a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 in the electrolytic solution, LiPF ₆ was dissolved at a concentration of 1M, and a lithium secondary battery was used. Produced. The production atmosphere was a dew point of −50 ° C. or lower. Each electrode was used by being crimped to a battery case with a current collector. A coin-type lithium secondary battery having a diameter of 25 mm and a thickness of 1.6 mm was formed using the positive electrode, the negative electrode, the electrolyte, and the separator.

(Battery characteristics evaluation of secondary battery)
Next, test evaluation of the positive electrode active material of the present invention was performed as follows using the coin-type lithium secondary battery. Charged to 4.2 V (vs. Li / Li ⁺ ) at a test temperature of 25 ° C. and a CC-CV method at a current rate of 0.1 C, and then stopped after the current rate dropped to 0.005 C. . Thereafter, the battery was discharged at a rate of 0.1 C to 1.5 V (same as above) by the CC method, and the initial charge / discharge capacity was measured.

  In Examples 1 and 2, lithium hydroxide, divalent iron chloride, and phosphorous acid were used as a lithium source, an iron source, and a phosphorus source, respectively, and nitric acid was added to obtain aqueous solutions. In addition, Example 2 is the same as Example 1 except that lithium nitrate is used as the lithium source.

  In Comparative Example 1, as compared with Example 1, the phosphorous source was ammonium dihydrogen phosphate, and ascorbic acid was added instead of nitric acid. In Comparative Example 2, ascorbic acid is not added to Comparative Example 1. Comparative Example 3 uses lithium naphthenate, iron 2-ethylhexanoate, and ethyl phosphonoacetate as the lithium source, iron source, and phosphorus source, respectively.

Table 1 shows the evaluation results of the manufacturability and the composition range of the fine particle mixture, and the initial characteristics of the prototype secondary battery. Here, the evaluation of “manufacturability” is indicated by “◯” when the manufacturability is good, “Δ” when manufacturable but not manufacturable, and “×” when not manufacturable. "The composition of the particulate mixture", "○" and those included in the range of 95% to 105% of the composition ratio of the stoichiometric composition of LiFePO _4, the composition of each element constituting the particulate mixture of LiFePO ₄ Chemical What was not included in the range of 95% to 105% of the composition ratio of the stoichiometric composition was represented by “x”, but the product that was not manufacturable and could not produce the fine particle mixture was composition analysis of the fine particle mixture The capacity of the secondary battery was not measured.

  From the results, in Example 1, precipitates or the like were not generated in the solution, and a spherical fine particle mixture could be obtained by the spray combustion method. On the other hand, in Comparative Example 1, solid components were confirmed after 3 days. For this reason, spraying could not be performed. Further, in Comparative Example 2, a solid component was confirmed and spraying could not be performed immediately after generation. In Comparative Example 3, although no solid component was observed, the amount of heat during combustion was too large, so that a spherical fine particle mixture could be produced, but continuous production was difficult and productivity was poor. It was.

As shown in Table 1, when the composition ratio of the spherical fine particles obtained in Example 1 was examined, the composition of each element constituting LiFePO ₄ was 95% to 105% of the stoichiometric composition ratio of LiFePO ₄ . Was in range.
Further, as a result of trial manufacture of a secondary battery having a positive electrode material using an active material manufactured using the spherical fine particle mixture of the present invention under the above-described conditions and evaluating initial characteristics of the secondary battery, The battery characteristics of the material exceeded 160 mAh / g, and it was confirmed that there was no practical problem.

DESCRIPTION OF SYMBOLS 1 ......... Particle production apparatus 3 ......... Aqueous solution supply part 5 ......... Combustion gas supply part 7 ......... Air supply part 9 ......... Mixing nozzle 11 ......... Reaction vessel 13 ......... Filter

Claims (9)

A method for producing a fine particle mixture, which is a precursor of a positive electrode active material for a lithium ion secondary battery,
Using an aqueous solution containing a lithium source, a phosphorus source, and an iron source in a fine particle mixture,
The phosphorus source is phosphorous acid;
The aqueous solution contains Li ions, P ions, and divalent Fe ions in substantially the same ratio,
A method for producing a fine particle mixture, wherein the fine particle mixture is produced by spraying the aqueous solution into a flame.   2. The method for producing a fine particle mixture according to claim 1, wherein the iron source is divalent iron chloride. The lithium source is lithium hydroxide;
3. The method for producing a fine particle mixture according to claim 2, wherein the aqueous solution is produced by mixing phosphorous acid and a divalent iron chloride aqueous solution and then adding a lithium hydroxide aqueous solution and nitric acid.   The concentration of the Li ion, the concentration of the P ion, and the concentration of the divalent Fe ion in the aqueous solution are 0.5 mol / L or more and 1.0 mol / L or less, A method for producing the fine particle mixture according to claim 2. 5. The method for producing a fine particle mixture according to claim 1, wherein the fine particle mixture contains a fine particle compound having a composition in the vicinity of the stoichiometric composition of LiFePO ₄ . A fine particle mixture containing a fine particle compound having a composition in the vicinity of the stoichiometric composition of LiFePO ₄ produced by the production method according to claim 1.   A lithium ion secondary battery positive electrode active material obtained by firing a fine particle mixture obtained by the method for producing a fine particle mixture according to any one of claims 1 to 5 together with a carbon source.   The lithium ion secondary battery using the lithium ion secondary battery positive electrode active material of Claim 7. An aqueous solution used in a method for producing a fine particle mixture that is a precursor of a lithium ion secondary battery positive electrode active material,
Phosphorous acid as the phosphorus source, lithium hydroxide as the lithium source, divalent iron chloride and nitric acid as the iron source,
The aqueous solution contains a Li ion, a P ion, and a divalent Fe ion in substantially the same ratio, and is an aqueous solution used in a method for producing a fine particle mixture.

Images