Method for manufacturing positive electrode material for lithium-ion secondary battery

Abstract

This method for manufacturing a positive electrode material for a lithium-ion secondary battery includes: a slurry preparation step (S10) in which a precursor slurry is obtained; a granulation step (S20) in which precursor particles are obtained from the precursor slurry; and a sintering step (S30) in which the precursor particles are sintered. The precursor slurry includes lithium phosphate, a manganese-containing phosphate compound, an iron oxide, and a dispersion medium such that the proportion of the mole ratio of manganese relative to the total of the mole ratio of manganese and the mole ratio of iron is at least 70% and no greater than 90%. The manganese-containing phosphate compound is Mn54, Mn32, or a mixture thereof. In the slurry preparation step (S10), a pulverization step (S11) is performed in which the lithium phosphate and the manganese-containing phosphate compound are pulverized together with the iron oxide.

Description

Method for producing positive electrode material for lithium-ion secondary battery

[0001] The present disclosure relates to a method for producing a positive electrode material for a lithium-ion secondary battery.

[0002] Patent Document 1 describes an olivine-type lithium iron phosphate (LiFePO ４ The olivine-type structure active material, typified by olivine-type lithium iron phosphate, is a positive electrode active material with excellent thermal stability.

[0003] Japanese Patent Application Laid-Open No. 2019-185920

[0004] Olivine-type lithium manganese iron phosphate (LiMnFePO ４ ) is a material in which part of the iron constituting olivine-type lithium iron phosphate is replaced with manganese. A positive electrode active material using olivine-type lithium manganese iron phosphate can increase battery capacity compared to a positive electrode active material using olivine-type lithium iron phosphate. However, as the proportion of manganese in olivine-type lithium manganese iron phosphate increases, the diffusibility of lithium ions within the solid (hereinafter referred to as solid-state diffusion) decreases. Therefore, when olivine-type lithium manganese iron phosphate is used as the positive electrode active material, the resistance of the positive electrode increases, resulting in an increase in the resistance of the positive electrode.

[0005] The resistance within a particle of a positive electrode active material is positively correlated with the product of the diffusion rate within the solid of the material constituting the particle and the primary particle size of the particle. Therefore, in the case of a positive electrode active material made of granules of olivine-type lithium manganese iron phosphate, it is thought that reducing the primary particle size of the granules can suppress the increase in resistance caused by the inclusion of manganese.

[0006] In one aspect of the present disclosure, there is provided a method for producing a cathode material for a lithium-ion secondary battery. The method includes a slurry preparation step of obtaining a precursor slurry containing lithium phosphate, a manganese-containing phosphate compound, iron oxide, and a dispersion medium, a granulation step of spray-drying the precursor slurry to obtain precursor particles as aggregates, and a calcination step of calcining the precursor particles to obtain olivine-type lithium manganese iron phosphate. In the precursor slurry, the molar ratio of manganese to the total molar ratio of manganese and iron is 70% or more and 90% or less. The manganese-containing phosphate compound is Mn ５ (HPO ４ ) ２ (P.O. ４ ) ２ (H ２ O) ４ , Mn ５ (HPO ４ ) ２ (P.O. ４ ) ２ , Mn ３ (P.O. ４ ) ２ ・3H ２ O, Mn ３ (P.O. ４ ) ２ or a mixture thereof. The slurry preparation step includes a grinding step of grinding the manganese-containing phosphate compound together with iron oxide.

[0007] In the method for producing a positive electrode material for a lithium ion secondary battery, the slurry preparation step includes a first reaction of producing the lithium phosphate by reacting a lithium compound with phosphoric acid in the dispersion medium, and a second reaction of producing the manganese-containing phosphate compound by reacting a manganese compound with phosphoric acid in the dispersion medium.

[0008] In the slurry preparation step of the method for producing a positive electrode material for a lithium ion secondary battery, the lithium phosphate and the manganese-containing phosphate compound are mixed with iron oxide after the first reaction and the second reaction.

[0009] In the method for producing a positive electrode material for a lithium ion secondary battery, at least one of the first reaction and the second reaction is a reaction accompanied by the generation of carbon dioxide gas, and after the first reaction and the second reaction, the lithium phosphate and the manganese-containing phosphate compound are mixed with at least one component other than the lithium phosphate and the manganese-containing phosphate compound contained in the precursor slurry.

[0010] In the first reaction of the method for producing a positive electrode material for a lithium ion secondary battery, a metal compound other than lithium is added, and the amount of the metal compound added is such that the ratio (Li / MA) of the molar ratio of lithium (Li) to the sum (MA) of the molar ratios of the metal components constituting the metal compound is 1.0 or more and 1.1 or less, and the ratio (P / MA) of the molar ratio of phosphorus (P) to the sum (MA) of the molar ratios of the metal components constituting the metal compound is 1.0 or more and 1.1 or less.

[0011] In the method for producing a positive electrode material for a lithium ion secondary battery, the precursor slurry contains a carbon source. In the method for producing a positive electrode material for a lithium ion secondary battery, the precursor slurry contains a carboxylic acid, and the grinding step is performed in the presence of the carboxylic acid.

[0012] In the method for producing a positive electrode material for a lithium ion secondary battery, the precursor slurry contains a titanium compound. In the method for producing a positive electrode material for a lithium ion secondary battery, the precursor slurry contains a magnesium compound.

[0013] According to the present invention, granulated bodies of olivine-type lithium manganese iron phosphate having a high manganese content and a small primary particle size can be obtained.

[0014] 1 is a micrograph of a cross section of a granule, 2 is a schematic diagram showing the structure of a primary particle in a cross section of a granule, and 3 is a flowchart of a method for producing a positive electrode material.

[0015] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Hereinafter, a positive electrode material for a lithium ion secondary battery and an electrode of the lithium ion secondary battery may be abbreviated to a positive electrode material and an electrode, respectively.

[0016] <Cathode Material> The cathode material produced by the production method of this embodiment will be described with reference to Figures 1 and 2. The cathode material is used as a cathode active material capable of absorbing and releasing charge carriers such as lithium ions in, for example, a cathode for a lithium ion secondary battery.

[0017] The positive electrode material includes granules 10 made of olivine-type lithium manganese iron phosphate (hereinafter referred to as LMFP). The positive electrode material may be made of only the granules, or may contain components other than the granules 10 as necessary.

[0018] An example of a granule 10 shown in Fig. 1 has a core 11 made of LMFP and a carbon coating 12 formed on the surface of the core 11. The LMFP constituting the core 11 is represented by the general formula Li ａ Mn ｘ Fe ｙ Me ｚ P.O. ４ It is a polyanionic compound having an olivine structure represented by the general formula Li ａ Mn ｘ Fe ｙ Me ｚ P.O. ４ In the formula, Me is at least one element selected from the group consisting of Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te, and Mo.

[0019] General formula Li ａ Mn ｘ Fe ｙ Me ｚ P.O. ４ In the formula, a, x, y, and z are numbers that satisfy 0<a<2, 0<x+y+z<2, 0.7≦x / (x+y)≦0.9, 0.1≦y / (x+y)≦0.3, and 0≦z≦0.1. a is preferably a number that satisfies (x+y+z)≦a≦1.1(x+y+z).

[0020] Also, the general formula Li ａ Mn ｘ Fe ｙ Me ｚ P.O. ４The molar ratio of P constituting the polyanionic compound having an olivine structure represented by the formula (1) is defined as "b." In this case, b, x, y, and z are preferably values ​​that satisfy the relationship (x + y + z) ≦ b ≦ 1.1 (x + y + z). In other words, x, y, and z are preferably values ​​that satisfy the relationship 1 / 1.1 ≦ x + y + z ≦ 1.

[0021] Specific examples of the ranges of x, y, and z when z = 0 include x + y = 1, 0.7 ≤ x ≤ 0.9, and 0.1 ≤ y ≤ 0.3. Specific examples of the ranges of x, y, and z when z is not 0 include 0.9 ≤ x + y < 1, 0.63 ≤ x < 0.9, 0.09 ≤ y < 0.3, and 0 < z ≤ 0.1. LMFP preferably contains at least one of Mg and Ti as an element constituting Me. The olivine-type lithium manganese iron phosphate constituting the core 11 may be one type or two or more types.

[0022] Fig. 2 schematically shows the structure of the area indicated by arrow A in Fig. 1. As shown in Fig. 2, the core 11 has a structure in which primary particles 11a of LMFP are aggregated. The core 11 also has pores 11b formed between the multiple primary particles 11a.

[0023] The particle diameter of the primary particles 11a is, for example, 100 nm or less, preferably 70 nm or less, and more preferably 50 nm or less. The particle diameter of the primary particles 11a is, for example, 20 nm or more. The particle diameter of the primary particles 11a is a value determined by small angle X-ray scattering (SAXS). More specifically, the particle diameter of the primary particles 11a is a value defined as the average particle diameter (D50) obtained from the particle diameter distribution of the primary particles 11a, assuming that the primary particles 11a are spherical, after which the particle diameter distribution is determined. Unless otherwise specified, the particle diameter values ​​described below are values ​​determined by the same small angle X-ray scattering method as above.

[0024] The pore volume of the core 11 is not particularly limited, but is preferably small. For example, the pore volume of the core 11 is 0.24 cm ３ / g or less, preferably 0.2 cm ３ / g or less, more preferably 0.15 cm ３ / g or less. The pore volume is, for example, 0.05 cm ３ / g or more. The pore volume can be determined by nitrogen adsorption / desorption measurement of the granules 10 and the Barrett Joyner Hallenda (BJH) method. The pore volume means the total pore volume in the range of pore diameters of 300 nm or less.

[0025] The carbon content in the granules 10 is, for example, 1.8% by mass or more, preferably 1.9% by mass or more, and more preferably 2.0% by mass or more. The carbon content is, for example, 3.0% by mass or less, and preferably 2.7% by mass or less. The carbon content can be measured using a carbon / sulfur analyzer (CS meter). The carbon coating 12 of the granules 10 may be omitted.

[0026] The LMFP content in the granules 10 is, for example, 95% by mass or more, and preferably 96% by mass or more. The content is, for example, 99% by mass or less, and preferably 98% by mass or less.

[0027] The granules 10 may contain other components in addition to LMFP and carbon. In this case, the content of the other components is, for example, 2 mass% or less. The average particle size (D50) of the granules 10 is, for example, 3.0 μm or more, preferably 5.0 μm or more. The average particle size (D50) of the granules 10 is, for example, 30 μm or less, preferably 20 μm or less. The average particle size of the granules 10 can be measured, for example, using a laser diffraction particle size analyzer.

[0028] <Method for manufacturing positive electrode material> Next, a method for manufacturing a positive electrode material will be described. As shown in Fig. 3, the method for manufacturing granules 10, which are positive electrode material, includes a slurry preparation step S10, a granulation step S20, and a firing step S30. The granules 10 are manufactured by sequentially undergoing the slurry preparation step S10, the granulation step S20, and the firing step S30.

[0029] (Slurry Preparation Step) The slurry preparation step S10 is a step of obtaining a precursor slurry containing lithium phosphate, a manganese-containing phosphate compound, iron oxide, and a dispersion medium. The slurry preparation step S10 includes a grinding step S11 of grinding the lithium phosphate and the manganese-containing phosphate compound together with the iron oxide.

[0030] First, the components contained in the precursor slurry obtained in the slurry preparation step S10 will be described. The components that the precursor slurry may contain can be roughly divided into components that form LMFP, a dispersion medium, and other components.

[0031] [Components forming LMFP] Lithium phosphate (Li ３ P.O. ４ ) is a Li source for forming LMFP. The lithium phosphate may be a commercially available product or may be produced in the slurry preparation step S10. The lithium phosphate can be produced, for example, by reacting a lithium-containing compound with phosphoric acid (hereinafter referred to as the first reaction). The first reaction will be described in detail later.

[0032] The manganese-containing phosphate compound is a source of Mn and a source of phosphate for forming LMFP. ５ (HPO ４ ) ２ (P.O. ４ ) ２ (H ２ O) ４ , Mn ５ (HPO ４ ) ２ (P.O. ４ ) ２ , Mn ３ (P.O. ４ ) ２ ・3H ２ O, Mn ３ (P.O. ４ ) ２ , or a mixture thereof.

[0033] Mn ５ (HPO ４ ) ２ (P.O. ４ ) ２ is Mn ５ (HPO４ ) ２ (P.O. ４ ) ２ (H ２ O) ４ It is an anhydrous form in which the water molecules that make up the hydrate have been removed from Mn ５ (HPO ４ ) ２ (P.O. ４ ) ２ and Mn ５ (HPO ４ ) ２ (P.O. ４ ) ２ (H ２ O) ４ These may be collectively referred to as Mn54. ３ (P.O. ４ ) ２ is Mn ３ (P.O. ４ ) ２ ・3H ２ It is an anhydrous form in which the water molecules that make up the hydrate are removed from Mn ３ (P.O. ４ ) ２ ・3H ２ O and Mn ３ (P.O. ４ ) ２ These may be collectively referred to as Mn32.

[0034] The manganese-containing phosphate compound is preferably Mn54. When the manganese-containing phosphate compound is Mn54, the amount of LiH ２ P.O. ４ The generation of by-products such as ammonium hydroxide, ammonium nitrate, and ammonium hydroxide can be suppressed. Furthermore, when Mn54 is a hydrate, the proportion of water molecules constituting the hydrate is smaller than that of Mn32. Therefore, when Mn54 is used, the amount of water volatilized in the firing step S30 is reduced. This makes it possible to obtain granules 10 with a small pore volume. Furthermore, when Mn54 is used, the primary particle size of the granules 10 can be made smaller due to the small size of the crystal particles. This point will be described in detail later.

[0035] The manganese-containing phosphate compound may be a commercially available product or may be produced in the slurry preparation step S10. The manganese-containing phosphate compound can be produced, for example, by reacting the manganese-containing compound with phosphoric acid (hereinafter referred to as the second reaction). The second reaction will be described in detail later.

[0036] The iron oxide is a source of Fe for forming the LMFP, and is also a component included in the milling step S11 to mill the manganese-containing phosphate compound more finely. As the iron oxide, a substance harder than the manganese-containing phosphate compound is selected. Since the manganese-containing phosphate compounds Mn54 and Mn32 have a Mohs hardness of "5", a substance with a Mohs hardness of more than "5" is selected as the iron oxide. For example, Fe ２ O ３ (Mohs hardness 5.5), Fe ３ O ４ (Mohs hardness: 6.0) The precursor slurry may contain one type of iron oxide or two or more types of iron oxide.

[0037] Here, the precursor slurry may be, if necessary, a compound having the general formula Li ａ Mn ｘ Fe ｙ Me ｚ P.O. ４ The precursor slurry may contain a metal source (hereinafter referred to as an optional metal source) for forming "Me" of LMFP represented by the formula (I). Examples of the optional metal source include a Co source, a Ni source, a Cu source, a Mg source, a Zn source, a V source, a Ca source, a Sr source, a Ba source, a Ti source, an Al source, a Si source, a B source, a Te source, and a Mo source. Specific examples of the optional metal source include metal oxides, metal hydroxides, and metal salts of various metal elements. The optional metal source is preferably a metal oxide. In this case, volume shrinkage during firing in the firing step S30 can be suppressed. The optional metal source contained in the precursor slurry may be one type, or two or more types.

[0038] The precursor slurry preferably contains an Mg source as an optional metal source. In this case, the resistance of the granules 10 can be reduced. Examples of the Mg source include magnesium compounds such as magnesium oxide, magnesium acetate, magnesium carbonate, and magnesium hydroxide. The Mg source is preferably magnesium oxide. In this case, volumetric shrinkage during firing in the firing step S30 can be suppressed.

[0039] The precursor slurry preferably contains a Ti source as an optional metal source. In this case, the primary particle size of the granules 10 can be further reduced. Examples of the Ti source include titanium compounds such as titanium oxide and titanium nitride. The Ti source is preferably titanium oxide. In this case, volumetric shrinkage during firing in the firing step S30 can be suppressed.

[0040] The contents of the lithium phosphate, manganese-containing phosphate compound, iron oxide, and optional metal source in the precursor slurry are adjusted so that the molar ratios of the respective metal elements satisfy a specific relationship.

[0041] The manganese-containing phosphate compound and iron oxide are contained in amounts such that the molar ratio of manganese to the total molar ratio of manganese and iron in the precursor slurry is 70% or more and 90% or less. The molar ratio of manganese is preferably 75% or more, more preferably 78% or more. The molar ratio of manganese is preferably 85% or less, more preferably 83% or less.

[0042] The content of lithium phosphate is preferably such that the ratio (Li / T) of the molar ratio of lithium (Li) to the total molar ratio (T) of the specific metal elements in the precursor slurry is 1.00 or more and 1.10 or less. The specific metal elements are manganese contained in the manganese-containing phosphate compound, iron contained in the iron oxide, and metal elements contained in the optional metal source. In other words, the general formula Li ａ Mn ｘ Fe ｙ Me ｚ P.O. ４ Mn, Fe, and Me are metal elements that constitute LMFP represented by the formula:

[0043] When an Mg source is contained, the content of the Mg source is, for example, an amount such that the ratio (Mg / T) of the molar ratio of magnesium (Mg) to the total molar ratio (T) of the specific metal elements in the precursor slurry is 0.0001 or more and 0.05 or less. The molar ratio of magnesium is preferably 0.005 or more, more preferably 0.01 or more. The molar ratio of magnesium is preferably 0.045 or less, more preferably 0.04 or less.

[0044] When a Ti source is contained, the content of the Ti source is, for example, an amount such that the ratio (Ti / T) of the molar ratio of titanium (Ti) to the total molar ratio (T) of the specific metal elements in the precursor slurry is 0.0001 or more and 0.005 or less. The molar ratio of titanium is preferably 0.005 or more, more preferably 0.01 or more. The molar ratio of titanium is preferably 0.045 or less, more preferably 0.04 or less.

[0045] In the precursor slurry, each of the above components forming the LMFP is contained in a powder state. The particle diameter of the lithium phosphate contained in the precursor slurry is, for example, 50 nm or less, preferably 40 nm or less, and more preferably 30 nm or less. The particle diameter of the lithium phosphate is, for example, 5 nm or more. The lithium phosphate becomes a powder with the above particle diameter by undergoing a pulverization process described below.

[0046] The manganese-containing phosphate compound contained in the precursor slurry has a particle size of, for example, 100 nm or less, preferably 70 nm or less, and more preferably 50 nm or less. The manganese-containing phosphate compound has a particle size of, for example, 10 nm or more. The manganese-containing phosphate compound is converted into a powder having the above particle size by undergoing a pulverization process described below.

[0047] The particle size of the iron oxide contained in the precursor slurry is, for example, 100 nm or less, preferably 70 nm or less, and more preferably 50 nm or less. The particle size of the iron oxide is, for example, 10 nm or more. The iron oxide becomes a powder of the above particle size by undergoing a pulverization step described below.

[0048] The particle size of the optional metal source contained in the precursor slurry is, for example, 300 nm or less, preferably 200 nm or less, and more preferably 100 nm or less, and the particle size of the optional metal source is, for example, 10 nm or more.

[0049] [Dispersion Medium] The dispersion medium is water or a mixed solvent of water and a non-aqueous solvent. While the water is not particularly limited, examples include ion-exchanged water, which is water treated with an ion exchange resin, and ultrapure water, which is water treated with a reverse osmosis membrane water purification system. Examples of non-aqueous solvents constituting the mixed solvent include water-miscible solvents such as lower alcohols, acetone, tetrahydrofuran, ethylene glycol, N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, acetonitrile, and dimethyl sulfoxide. One non-aqueous solvent may be used alone, or two or more may be used in combination. The volume ratio of water in the mixed solvent is preferably, for example, 50% by volume or more and 99.9% by volume or less, and more preferably 60% by volume or more and 99% by volume or less. The content of the aqueous solvent in the precursor slurry is not particularly limited, but is, for example, an amount such that the solids ratio is 20% by volume or more and 50% by volume or less.

[0050] [Other Components] The precursor slurry may contain a carbon source for forming the carbon coating 12. For example, an organic compound can be used as the carbon source. Examples of organic compounds include glucose, fructose, galactose, mannose, maltose, sucrose, lactose, glycogen, pectin, alginic acid, glucomannan, chitin, hyaluronic acid, chondroitin, agarose, polyether, polyhydric alcohol, polyvinyl alcohol, polyvinylpyrrolidone, cellulose, starch, gelatin, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, polyacrylic acid, polystyrene sulfonic acid, polyacrylamide, and polyvinyl acetate. Examples of polyhydric alcohols include polyethylene glycol, polypropylene glycol, polyglycerin, and glycerin. The carbon source may be used alone or in combination of two or more.

[0051] The carbon source is contained in the precursor slurry so that the mass of carbon in the carbon source (when a carboxylic acid described below is contained, the total mass including the mass of carbon in the carboxylic acid) is a specific amount. The carbon source content is, for example, 3 parts by mass or more and 30 parts by mass or less when the total mass of the components forming LMFP in the precursor slurry is 100 parts by mass.

[0052] The precursor slurry may contain a carboxylic acid as an optional component. In this case, the pore volume of the granules 10 can be reduced. Details will be described later in the section describing the pulverization process. Examples of carboxylic acids include citric acid, formic acid, acetic acid, propionic acid, butanoic acid, pentanoic acid, malic acid, fumaric acid, tartaric acid, ascorbic acid, gluconic acid, and polyacrylic acid. One type of carboxylic acid may be used alone, or two or more types may be used in combination. Of these carboxylic acids, citric acid is preferably used.

[0053] The content of the carboxylic acid in the precursor slurry is, for example, 1 part by mass or more and 30 parts by mass or less when the total mass of the components forming LMFP in the precursor slurry is 100 parts by mass.

[0054] The precursor slurry may contain components other than those described above, as necessary. Examples of such components include a dispersant. [Pulverization Step] The pulverization step S11 is performed on a slurry containing lithium phosphate, a manganese-containing phosphate compound, iron oxide, and a dispersion medium (hereinafter referred to as intermediate slurry). The pulverization step S11 is a step of pulverizing the lithium phosphate, the manganese-containing phosphate compound, and the iron oxide, which are solid components contained in the intermediate slurry, to reduce their particle size.

[0055] The pulverization method used in the pulverization step S11 is not particularly limited as long as it causes collisions between solid components in the intermediate slurry. Examples of the pulverization method include methods using a pulverizer such as a bead mill, hammer mill, stirring mill, jet mill, or ball mill. Among the pulverization methods, preferred are pulverization methods that crush particles by applying shear force, such as methods using a bead mill. When a bead mill is used, the beads serving as medium particles are harder than the solid components contained in the intermediate slurry, for example, harder than iron oxide. The bead diameter is, for example, 1 mm or less, preferably 0.5 mm or less. The bead diameter is, for example, 0.1 mm or more. The pulverization temperature is, for example, 10°C or more and 50°C or less.

[0056] The particle diameters of the solid components contained in the intermediate slurry before the pulverization step S11 are as follows: The particle diameter of lithium phosphate is, for example, 50 nm to 50 μm, The particle diameter of the manganese-containing phosphate compound is, for example, 50 nm to 50 μm, and The particle diameter of iron oxide is, for example, 50 nm to 10 μm.

[0057] The particle size of each solid content contained in the intermediate slurry after the pulverization step S11, expressed as a value in a mixed state of the lithium phosphate, manganese-containing phosphate compound, and iron oxide particles, is, for example, 100 nm or less, preferably 70 nm or less, and more preferably 50 nm or less. The value is, for example, 10 nm or more.

[0058] In the pulverization step S11, the lithium phosphate and the manganese-containing phosphate compound are pulverized together with the iron oxide in the dispersion medium. In this case, the lithium phosphate and the manganese-containing phosphate compound, particularly the manganese-containing phosphate compound, can be pulverized more finely based on the difference in hardness between the lithium phosphate and the manganese-containing phosphate compound and the iron oxide.

[0059] More specifically, the Mohs hardness of lithium phosphate is "4", and the Mohs hardness of manganese-containing phosphate compounds Mn54 and Mn32 is "5". ２ O ３ ) has a Mohs hardness of 5.5, and iron oxide is harder than lithium phosphate and the manganese-containing phosphate compound. In this case, by performing two-stage pulverization in the pulverization step S11, the lithium phosphate and the manganese-containing phosphate compound can be pulverized more finely.

[0060] More specifically, in the first step, shear forces, compression forces, and the like generated in a grinder such as a bead mill act on each solid in the intermediate slurry, thereby grinding the lithium phosphate, manganese-containing phosphate compound, and iron oxide. Then, as the grinding process continues, collisions occur between the ground solids in the second step. At this time, the relatively hard ground iron oxide collides with the relatively soft lithium phosphate and manganese-containing phosphate compound, further grinding the ground lithium phosphate and manganese-containing phosphate compound. In the grinding step S11, the first and second steps are performed in parallel or simultaneously.

[0061] In the pulverization step S11, the lithium phosphate, manganese-containing phosphate compound, and iron oxide are divided along the grain boundaries. Therefore, theoretically, each particle after pulverization has a size in units of a crystal particle, and the theoretical minimum size is a size corresponding to the particle diameter of the crystal particle. Therefore, the smaller the constituent crystal particles, the finer the pulverization can be performed. In the case of a manganese-containing phosphate compound, the particle diameter of the crystal particles forming the Mn54 phase is smaller than that of Mn32. Therefore, since it can be pulverized more finely, the manganese-containing phosphate compound is preferably Mn54.

[0062] Components other than lithium phosphate, manganese-containing phosphate compound, iron oxide, and dispersion medium may be mixed in advance with the intermediate slurry to be subjected to the pulverization step S11, or may be mixed with the intermediate slurry after the pulverization step S11. Alternatively, some of the components other than lithium phosphate, manganese-containing phosphate compound, iron oxide, and dispersion medium may be mixed in advance with the intermediate slurry to be subjected to the pulverization step S11, and the remainder of the components may be mixed with the intermediate slurry after the pulverization step S11. Note that components other than lithium phosphate, manganese-containing phosphate compound, iron oxide, and dispersion medium may be mixed after being pulverized in advance to a predetermined particle size, or may be pulverized in the pulverization step to a predetermined particle size.

[0063] Here, the intermediate slurry preferably contains a carboxylic acid. In other words, the pulverization step S11 is preferably carried out in the presence of a carboxylic acid. In this case, the pore volume of the granules 10 can be reduced.

[0064] This effect is believed to be due to the metal-capturing action of carboxylic acid in the slurry. In the pulverization step S11, some of the metals, such as Li, Mn, and Fe, are eluted into the slurry. The eluted metals are captured by carboxylic acid, improving the dispersibility of each solid content in the slurry. As a result, in the subsequent granulation step S20 and firing step S30, the primary particles are densely arranged, resulting in a smaller pore volume of the granules 10.

[0065] Therefore, it is preferable that the carboxylic acid used in the intermediate slurry has a high ability to capture eluted metals, particularly Li, Mn, and Fe. The bond energies of various carboxylic acids with Li, Mn, and Fe are shown in Table 1 below. The bond energy values ​​shown in Table 1 below are values ​​calculated as the difference in energy before and after the formation of a complex between the carboxylic acid and each eluted metal, and the smaller the value, the higher the ability to capture the corresponding eluted metal. The bond energy values ​​shown in Table 1 below were calculated using CAE (Computer Aided Engineering). As shown in Table 1, citric acid has the lowest bond energy with all of the eluted metals, Li, Mn, and Fe. Therefore, it is preferable to use citric acid as the carboxylic acid.

[0066]

[0067] When the intermediate slurry to be subjected to the pulverization step S11 contains all components other than the manganese-containing phosphate compound, iron oxide, and dispersion medium, a precursor slurry is obtained as the intermediate slurry after the pulverization step S11. When the intermediate slurry to be subjected to the pulverization step S11 does not contain all components other than the manganese-containing phosphate compound, iron oxide, and dispersion medium, a precursor slurry is obtained by mixing the remaining components with the intermediate slurry after the pulverization step S11.

[0068] [Specific Example of Method for Preparing Precursor Slurry] Next, a specific example of the slurry preparation step S10 will be described. First, lithium phosphate is produced by a first reaction, and a manganese-containing phosphate compound is produced by a second reaction.

[0069] More specifically, phosphoric acid is added dropwise to a solution in which a lithium-containing compound and a manganese-containing compound are dissolved or dispersed in a dispersion medium containing water at normal pressure and at a temperature of 0° C. to 90° C. As a result, the lithium-containing compound reacts with the phosphoric acid to produce lithium phosphate (first reaction), and the manganese-containing compound reacts with the phosphoric acid to produce a manganese-containing phosphate compound (second reaction).

[0070] Components other than the lithium phosphate and the manganese-containing phosphate compound contained in the precursor slurry may be added in advance to the liquid in which the first and second reactions are carried out, or may be added to the reaction liquid after the first and second reactions. Note that, if iron oxide is added to the liquid in which the first and second reactions are carried out, the viscosity of the liquid increases, making it difficult to handle, and therefore it is preferable to add the iron oxide after the first and second reactions.

[0071] Examples of the lithium-containing compound used in the first reaction include lithium hydroxide, lithium carbonate, and lithium oxide. The lithium-containing compound is preferably lithium hydroxide or lithium carbonate.

[0072] In the first reaction, by adjusting the ratio of the lithium-containing compound and phosphoric acid to the metal compound other than lithium, it is possible to suppress the production of by-products and increase the production rate of lithium phosphate. Examples of the by-products include LiOH.H ２ O, LiH ２ P.O. ４ If the precursor slurry contains the by-products, the amount of water that evaporates in the firing step S30 increases, resulting in a large pore volume of the granules 10. ２ P.O. ４ This can cause the formation of coarse particles in the granulation step S20 and can also cause the particles to adhere to various devices used in the production. Therefore, it is preferable to reduce the amount of the above-mentioned by-products produced in the first reaction.

[0073] The metal compound other than lithium is the manganese compound, iron oxide, and any of the above-mentioned metal sources, such as magnesium oxide and titanium oxide, used in the second reaction. In the first reaction, the amount of the metal compound other than lithium added is an amount such that the ratio (Li / MA) of the molar ratio of lithium (Li) to the sum (MA) of the molar ratios of the metal components constituting the metal compound is 1.0 or more and 1.1 or less, and the ratio (P / MA) of the molar ratio of phosphorus (P) to the sum (MA) of the molar ratios of the metal components constituting the metal compound is 1.0 or more and 1.1 or less. By adjusting the amount of the metal compound other than lithium added relative to the lithium-containing compound and phosphoric acid so as to satisfy the above conditions, the generation of the by-products can be suppressed.

[0074] Examples of the manganese-containing compound used in the second reaction include manganese carbonate, manganese sulfate, and manganese oxalate. The manganese-containing compound is preferably manganese carbonate. Compared with other manganese-containing compounds, manganese carbonate has a relatively small particle size, so the primary particle size of the granules 10 can be made smaller by using manganese carbonate. When manganese sulfate is used, it is necessary to perform a post-treatment of washing the produced manganese-containing phosphate compound before the pulverization step S11 described below.

[0075] In the second reaction, the type of manganese-containing phosphate compound produced can be adjusted by adjusting the reaction temperature. When producing Mn54, the reaction temperature of the second reaction is preferably 60°C or higher and 90°C or lower. When producing Mn32, the reaction temperature of the second reaction is preferably 0°C or higher and lower than 60°C.

[0076] The first and second reactions may be carried out sequentially in the same system, instead of simultaneously. For example, phosphoric acid is added dropwise to a solution in which one of a lithium-containing compound and a manganese-containing compound is dissolved or dispersed in a dispersion medium, and then the other of the lithium-containing compound and the manganese-containing compound is added to the reaction solution. This results in a reaction solution containing lithium phosphate and a manganese-containing compound. It is preferable to carry out the first and second reactions simultaneously, or to carry out the second reaction after the first reaction, since this makes it easy to adjust the component ratios to suppress the generation of by-products in the first reaction.

[0077] The first reaction and the second reaction may be carried out separately in different systems. For example, the first reaction and the second reaction may be carried out in different systems, and then the reaction solution after the first reaction and the reaction solution after the second reaction are mixed. This produces a reaction solution containing lithium phosphate and a manganese-containing compound.

[0078] Next, iron oxide is mixed with the reaction solution containing lithium phosphate and the manganese-containing compound to prepare an intermediate slurry. If necessary, an optional metal source and other components are also mixed. For example, an Mg source and a Ti source are mixed as optional metal sources, and a carbon source and a carboxylic acid are mixed as other components. If the corresponding components have been added in advance in the first and second reactions, this step is omitted.

[0079] The optional metal source and other components may be mixed in advance in the reaction system in which the first and second reactions are carried out, but it is preferable to mix them after each of the first and second reactions. When at least one of the first and second reactions is a reaction accompanied by the generation of carbon dioxide gas, performing a step of mixing some component after each reaction can promote the release of the generated carbon dioxide gas from the intermediate slurry. Carbon dioxide gas is generated, for example, when at least one of the lithium compound, manganese compound, and optional metal source is a carbonate.

[0080] Another specific example of a method for preparing an intermediate slurry is described below. The predetermined temperature described in the following specific example can be adjusted as appropriate. The predetermined temperature is, for example, 0°C or higher and 90°C or lower. In addition, in the second reaction, when Mn54 is produced, the predetermined temperature is preferably 60°C or higher and 90°C or lower, and when Mn32 is produced, the predetermined temperature is preferably 0°C or higher and lower than 60°C.

[0081] Specific Example 1 is an example of adding a carboxylic acid simultaneously with phosphoric acid. First, a first slurry is prepared by dispersing a lithium-containing compound, a manganese-containing compound, iron oxide, and an optional metal source in a dispersion medium containing water. After heating the first slurry to a predetermined temperature, phosphoric acid and a carboxylic acid are added, followed by the addition of a carbon source. Next, the first slurry containing the phosphoric acid, the carboxylic acid, and the carbon source is cooled to room temperature to obtain an intermediate slurry.

[0082] Specific Example 2 is an example of adding a carboxylic acid before adding phosphoric acid. First, a first slurry is prepared by dispersing a lithium-containing compound, a manganese-containing compound, iron oxide, and an optional metal source in a dispersion medium containing water. To the first slurry heated to a predetermined temperature, a carboxylic acid is added, followed by phosphoric acid, and then a carbon source. Next, the first slurry to which phosphoric acid, carboxylic acid, and carbon source have been added is cooled to room temperature to obtain an intermediate slurry.

[0083] Specific Example 3 is an example of a case where a carboxylic acid is added to a first slurry in advance. First, a first slurry is prepared by dispersing a lithium-containing compound, a manganese-containing compound, iron oxide, an optional metal source, and a carboxylic acid in a dispersion medium containing water. Phosphoric acid is added to the first slurry heated to a predetermined temperature, and then a carbon source is added. Next, the first slurry to which phosphoric acid and the carbon source have been added is cooled to room temperature to obtain an intermediate slurry.

[0084] Specific Example 4 is an example of adding a carbon source after cooling a slurry to which phosphoric acid has been added and the first and second reactions have been carried out. First, a first slurry is prepared by dispersing a lithium-containing compound, a manganese-containing compound, iron oxide, and an optional metal source in a dispersion medium containing water. Phosphoric acid and a carboxylic acid are added to the first slurry that has been heated to a predetermined temperature. Next, the first slurry to which phosphoric acid and a carboxylic acid have been added is cooled to room temperature, and a carbon source is added to the first slurry to obtain an intermediate slurry.

[0085] Specific Example 5 is an example of a case where the first reaction and the second reaction are carried out in separate reaction systems. First, a first reaction slurry is prepared by dispersing a lithium-containing compound, iron oxide, and an optional metal source in a dispersion medium containing water. Phosphoric acid is added to the first reaction slurry at room temperature. Separately, a second reaction slurry is prepared by dispersing a manganese-containing compound in a dispersion medium containing water. Phosphoric acid and a carboxylic acid are added to the second reaction slurry that has been heated to a predetermined temperature. Next, the second reaction slurry to which phosphoric acid and carboxylic acid have been added is cooled to room temperature, and then the first reaction slurry to which phosphoric acid has been added and a carbon source are added simultaneously or sequentially to the second reaction slurry to obtain an intermediate slurry.

[0086] Next, the obtained intermediate slurry is subjected to a pulverization step S11. Then, a precursor slurry is obtained as the intermediate slurry after the pulverization step S11. (Granulation step) The granulation step S20 is a step of obtaining precursor particles by spray-drying the precursor slurry. The precursor particles are granules formed by agglomeration of solids contained in the precursor slurry. Examples of spraying methods in spray drying include spraying methods using a disk type, a pressure nozzle, a pressure two-fluid nozzle, a pressure four-fluid nozzle, etc. The spray temperature in spray drying is, for example, 180°C or higher and 300°C or lower.

[0087] (Firing step) The firing step S30 is a step of obtaining LMFP by firing the precursor particles obtained in the granulation step S20, specifically, a step of forming a crystalline phase of LMFP. The firing step S30 is also a step of carbonizing the carbon source contained in the precursor particles to form a carbon coating 12. The firing temperature in the firing step is, for example, 500°C or higher and 750°C or lower. The firing time in the firing step is, for example, 1 hour or higher and 12 hours or lower. The atmosphere in the firing step is, for example, a non-oxidizing atmosphere. The non-oxidizing atmosphere may be, for example, nitrogen (N ２ ), an inert atmosphere such as argon (Ar), and hydrogen (H ２ ) or the like.

[0088] When the precursor slurry contains a carbon source, the carbon source contributes to lowering the oxygen partial pressure during firing, which can prevent the generation of a heterogeneous phase of LMFP in the granules 10 obtained after firing.

[0089] <Effects> Next, the operation and effect of this embodiment will be described. (1) The method for producing a cathode material for a lithium-ion secondary battery includes a slurry preparation step S10 for obtaining a precursor slurry, a granulation step S20 for spray-drying the precursor slurry to obtain precursor particles, and a calcination step S30 for calcining the precursor particles. The precursor slurry is a slurry containing lithium phosphate, a manganese-containing phosphate compound, iron oxide, and a dispersion medium such that the molar ratio of manganese to the sum of the molar ratios of manganese and iron is 70% or more and 90% or less. The manganese-containing phosphate compound is Mn54, Mn32, or a mixture thereof. In the slurry preparation step S10, an intermediate slurry containing lithium phosphate, a manganese-containing phosphate compound, iron oxide, and a dispersion medium is subjected to a milling step S11 for milling the lithium phosphate and the manganese-containing phosphate compound together with the iron oxide.

[0090] In the above configuration, in the pulverization step S11, lithium phosphate and manganese-containing phosphate compound are pulverized together with iron oxide. Iron oxide has a higher hardness than lithium phosphate and manganese-containing phosphate compound. Therefore, the lithium phosphate and manganese-containing phosphate compound are pulverized further by the iron oxide, which has also been pulverized and made fine, colliding with the pulverized lithium phosphate and manganese-containing phosphate compound. This allows the lithium phosphate and manganese-containing phosphate compound to be pulverized more finely in the pulverization step S11.

[0091] By performing the granulation step S20 and the firing step S30 using finely pulverized powders of lithium phosphate and manganese-containing phosphate compound, it is possible to reduce the primary particle size of the resulting granules 10. The small primary particle size of the granules 10 improves the intra-solid diffusion of the granules 10. As a result, when the granules 10 are used as a positive electrode material for a lithium ion secondary battery, the battery characteristics such as the capacity of the lithium ion secondary battery are improved.

[0092] (2) The slurry preparation step S10 includes a first reaction of producing lithium phosphate by reacting a lithium compound with phosphoric acid in a dispersion medium, and a second reaction of producing a manganese-containing phosphate compound by reacting a manganese compound with phosphoric acid in a dispersion medium.

[0093] According to the above-described configuration, it is easy to obtain lithium phosphate and manganese-containing phosphate compounds of any desired composition, and the production cost can be reduced compared to when commercially available lithium phosphate and manganese-containing phosphate compounds are used.

[0094] (3) In the slurry preparation step, after the first reaction and the second reaction, lithium phosphate, the manganese-containing phosphate compound, and iron oxide are mixed together. This configuration can suppress an increase in viscosity of the reaction solution after the first reaction and the second reaction.

[0095] (4) At least one of the first reaction and the second reaction is a reaction accompanied by the generation of carbon dioxide gas. After the first reaction and the second reaction, the lithium phosphate and the manganese-containing phosphate compound are mixed with at least one component other than the lithium phosphate and the manganese-containing phosphate compound contained in the precursor slurry.

[0096] According to the above-described configuration, by performing a step of mixing some component after the first and second reactions, it is possible to promote the release of generated carbon dioxide gas from the intermediate slurry, thereby suppressing the volumetric shrinkage of the granules 10 caused by the release of carbon dioxide gas from the precursor particles containing carbon dioxide gas in the firing step S30.

[0097] (5) In the first reaction, a metal compound other than lithium is added. The amount of the metal compound other than lithium added is such that the ratio (Li / MA) of the molar ratio of lithium (Li) to the sum (MA) of the molar ratios of the metal components constituting the metal compound is 1.0 or more and 1.1 or less, and the ratio (P / MA) of the molar ratio of phosphorus (P) to the sum (MA) of the molar ratios of the metal components constituting the metal compound is 1.0 or more and 1.1 or less. According to the above configuration, the production of by-products in the first reaction can be suppressed, and the production rate of lithium phosphate can be increased.

[0098] (6) The precursor slurry contains a carbon source. According to the above configuration, it is possible to produce granules 10 having a core 11 made of LMFP and a carbon coating 12 formed on the surface of the core 11. Furthermore, in the firing step S30, it is possible to reduce the oxygen partial pressure during firing. This makes it possible to prevent the generation of a different phase of LMFP in the granules 10 obtained after firing.

[0099] (7) The precursor slurry contains a carboxylic acid, and the pulverization step is performed in the presence of the carboxylic acid. This configuration reduces the pore volume of the granules 10. (8) The precursor slurry contains a titanium compound. This configuration reduces the primary particle size of the granules 10.

[0100] (9) The precursor slurry contains a magnesium compound. This configuration can reduce the resistance of the granules 10. (10) The manganese-containing phosphate compound is Mn54 or a mixture of Mn54 and Mn32. This configuration can produce granules 10 with a small pore volume. In addition, the primary particle size of the granules 10 can be made smaller.

[0101] <Modifications> This embodiment can be modified as follows: This embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.

[0102] The intermediate slurry may not contain lithium phosphate. That is, after the pulverization step S11 is performed on the intermediate slurry not containing lithium phosphate, separately pulverized lithium phosphate is added to the intermediate slurry after the pulverization step S11. In this case, the pulverization step S11 is a step of pulverizing the manganese-containing phosphate compound together with iron oxide.

[0103] Examples that further embody the above-described embodiment will be described below. (Example 1) Lithium carbonate and manganese carbonate were dispersed in water at room temperature, and then phosphoric acid was added dropwise to produce lithium phosphate and Mn32, a manganese-containing phosphate compound. The Mn32 produced in this example was Mn ３ (P.O. ４ ) ２・3H ２ Then, Fe was added to a reaction solution containing lithium phosphate and Mn32. ２ O ３ An intermediate slurry was obtained by mixing 100% ammonium hydroxide, MgO, fructose, and citric acid.

[0104] In the intermediate slurry, the molar ratio of manganese to the sum of the molar ratio of manganese and the molar ratio of iron is 80%. In the intermediate slurry, the ratio (P / T) of the molar ratio of phosphoric acid (P) to the sum (T) of the molar ratios of specific metal elements (Mn, Fe, Mg) is 1.03. In the intermediate slurry, the ratio (Li / T) of the molar ratio of lithium (Li) to the sum (T) of the molar ratios of specific metal elements (Mn, Fe, Mg) is 1.05. In the intermediate slurry, the ratio of fructose to the components excluding water is 20% by mass. In the intermediate slurry, the ratio of citric acid to the components excluding water is 10% by mass.

[0105] Next, beads were added to the intermediate slurry, and the solid content of the intermediate slurry was pulverized using a bead mill (MSC50 manufactured by Nippon Coke Co., Ltd.) to obtain a precursor slurry. The diameter of the beads and the time for the pulverization treatment are shown in Table 2 below. The beads were ZrO ２ Beads (Mohs hardness 8) were used. The pulverization treatment using the bead mill was carried out at a peripheral speed of 15 m / s.

[0106] Next, the precursor slurry was dried and granulated using a spray dryer (drying outlet temperature: 100°C) to obtain precursor particles as aggregates. ２ The mixture was heated at 650° C. for 6 hours in an atmosphere to obtain granules of Example 1.

[0107] Example 2 Granules of Example 2 were obtained in the same manner as in Example 1, except that lithium carbonate was changed to an amount of lithium hydroxide equivalent in terms of the molar ratio of lithium.

[0108] (Example 3) Lithium carbonate and manganese carbonate were dispersed in water at 60°C, and then phosphoric acid was added dropwise to produce lithium phosphate and manganese-containing phosphate compound Mn54. The Mn54 produced in this example was Mn ５ (HPO ４ ) ２ (P.O. ４ ) ２ (H ２ O) ４ Then, Fe was added to a reaction solution containing lithium phosphate and Mn54. ２ O ３ The granules of Example 3 were obtained in the same manner as in Example 1.

[0109] Example 4 Granules of Example 4 were obtained in the same manner as in Example 3, except that lithium carbonate was changed to an amount of lithium hydroxide equivalent in terms of the molar ratio of lithium.

[0110] (Example 5) Lithium carbonate and manganese carbonate were dispersed in water at 60°C, and then phosphoric acid was added dropwise to produce lithium phosphate and manganese-containing phosphate compound Mn54. The Mn54 produced in this example was Mn ５ (HPO ４ ) ２ (P.O. ４ ) ２ (H ２ O) ４ Next, Fe is added to a reaction solution containing lithium phosphate and a manganese-containing phosphate compound. ２ O ３ , MgO, TiO ２ The granules of Example 5 were obtained by mixing the granules, fructose, and citric acid together.

[0111] In the intermediate slurry, the molar ratio of manganese to the sum of the molar ratio of manganese and the molar ratio of iron is 80%. In the intermediate slurry, the ratio (P / T) of the molar ratio of phosphoric acid (P) to the sum (T) of the molar ratios of specific metal elements (Mn, Fe, Mg, Ti) is 1.03. In the intermediate slurry, the ratio (Li / T) of the molar ratio of lithium (Li) to the sum (T) of the molar ratios of specific metal elements (Mn, Fe, Mg, Ti) is 1.05. In the intermediate slurry, the ratio of fructose to the components excluding water is 20% by mass. In the intermediate slurry, the ratio of citric acid to the components excluding water is 10% by mass.

[0112] Example 6 Granules of Example 6 were obtained in the same manner as in Example 5, except that lithium carbonate was changed to an amount of lithium hydroxide equivalent in terms of the molar ratio of lithium.

[0113] Example 7 Granules of Example 7 were obtained in the same manner as in Example 3, except that the diameter of the beads used in the pulverization treatment was changed.

[0114] Example 8 Granules of Example 8 were obtained in the same manner as in Example 4, except that the diameter of the beads used in the pulverization treatment was changed.

[0115] Example 9 Granules of Example 9 were obtained in the same manner as in Example 7, except that citric acid was not added to the intermediate slurry.

[0116] Example 10 Granules of Example 10 were obtained in the same manner as in Example 8, except that citric acid was not added to the intermediate slurry.

[0117] (Comparative Example 1) Fe ２ O ３ The amount of Fe that is equivalent to the molar ratio of iron ２ C ２ O ４ ・2H ２ Granules of Comparative Example 1 were obtained in the same manner as in Example 1, except that O was used.

[0118] (Comparative Example 2) Fe ２ O ３ The amount of Fe that is equivalent to the molar ratio of iron２ C ２ O ４ ・2H ２ Granules of Comparative Example 2 were obtained in the same manner as in Example 2, except that O was used.

[0119] (Comparative Example 3) Fe ２ O ３ The amount of FeSO equivalent to the molar ratio of iron is ４ ・7H ２ O, and manganese carbonate was changed to an amount of MnSO equivalent to the molar ratio of manganese. ４ ・5H ２ Granules of Comparative Example 3 were obtained in the same manner as in Example 2, except that O was used.

[0120] (Evaluation of Primary Particle Size of Granules) The primary particle size of the granules of each Example and Comparative Example was measured as follows. The particle size distribution of the primary particles was determined using the SAXS method, assuming that the primary particles were spherical. The average particle size (D50) obtained from the determined particle size distribution was calculated, and this value was used as the particle size of the primary particles. In addition, the particle size of the precursor particles before calcination was measured for the granules of each Example and Comparative Example. The results are shown in Table 2.

[0121] (Evaluation of pore volume of granules) The pore volume of the granules of each Example and Comparative Example was measured as follows. The pores of the granules were measured using a specific surface area / pore distribution measuring device. The cumulative pore distribution of the granules in the pore diameter range of 300 nm or less was determined by analysis using the BJH method. The cumulative pore volume calculated from the determined cumulative pore distribution was taken as the pore volume of the granules. The results are shown in Table 2.

[0122] (Evaluation of Battery Characteristics of Granules) A ​​positive electrode composite was coated on one surface of a 15 μm thick aluminum foil. The coated positive electrode composite was dried to produce a positive electrode in which a positive electrode active material layer was formed on one surface of a positive electrode current collector. As the positive electrode composite, a slurry containing the granules (positive electrode active material) of each Example and Comparative Example, acetylene black, and polyvinylidene fluoride (PVdF) in a solid content mass ratio of 90:5:5, and using water as a solvent, was used.

[0123] A positive electrode half-cell was fabricated using the fabricated positive electrode. A separator was sandwiched between a positive electrode (evaluation electrode) formed by cutting the positive electrode into a 25 mm square and a negative electrode formed by cutting a 200 μm thick lithium metal foil into a 27 mm square to form an electrode body battery. The electrode body battery was housed in a laminate exterior, and a nonaqueous electrolyte was poured into it and sealed, to obtain a half-cell for electrochemical testing. A glass filter manufactured by Hoechst Celanese was used as the separator. The nonaqueous electrolyte used was a nonaqueous electrolyte prepared by dissolving lithium hexafluorophosphate to a concentration of 1 M in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:3:4.

[0124] The prepared positive electrode half cell was charged at 25° C. and a constant current of 0.05 C up to 4.3 V and then discharged down to 3.0 V. The discharge capacity at this time was measured. The results are shown in Table 2.

[0125]

[0126] The values ​​in parentheses in Table 2 are the Mohs hardness of the substance listed above the value. (Regarding Examples 1 and 2 and Comparative Examples 1 to 3) Examples 1 and 2 are examples in which an Fe source harder than Mn32, a manganese-containing phosphate compound, was used, while Comparative Examples 1 to 3 are examples in which an Fe source softer than Mn32 was used. As shown in Table 2, the primary particle sizes before and after firing are significantly smaller in Examples 1 and 2 than in Comparative Examples 1 to 3. Furthermore, the discharge capacity when the granules of Examples 1 and 2 are used as the positive electrode material is higher than when the granules of Comparative Examples 1 to 3 are used.

[0127] These results show that when a manganese-containing phosphate compound is pulverized, it can be pulverized more finely by pulverizing it together with an Fe source that is harder than the manganese-containing phosphate compound. Furthermore, it can be seen that the primary particle size of the resulting granules can be reduced by granulating and firing precursor particles using a more finely pulverized manganese-containing phosphate compound powder. It is believed that the small primary particle size of the granules improves the diffusion within the solid, resulting in an improved discharge capacity.

[0128] Furthermore, the pore volumes of the granules of Examples 1 and 2 were smaller than those of Comparative Examples 1 to 3. This result is believed to be due to the use of a more finely pulverized powder of the manganese-containing phosphate compound to granulate and sinter the precursor particles. The pore volume of the granules is believed not to have a significant effect on the discharge capacity. The reason for this will be explained later.

[0129] (Regarding Examples 3 and 4) Examples 3 and 4 are examples in which the manganese-containing phosphate compound is Mn54. The primary particle sizes before and after calcination in Examples 3 and 4 are smaller than those in Examples 1 and 2. Therefore, it can be seen that even when the manganese-containing phosphate compound is Mn54, the effect of reducing the primary particle size of the granules can be obtained. Furthermore, the pore volumes of Examples 3 and 4 are significantly reduced compared to Examples 1 and 2. These results show that using Mn54 as the manganese-containing phosphate compound is effective when producing granules with a small pore volume.

[0130] (Regarding Examples 5 to 6) Examples 5 to 6 are examples in which titanium oxide was added as a Ti source. The primary particle size and pore volume before and after firing in Examples 5 to 6 were smaller than those in Examples 3 to 4 in which titanium oxide was not added. Furthermore, the discharge capacity in Examples 5 to 6 was higher than that in Examples 3 to 4. These results demonstrate that adding titanium oxide is effective when producing granules with a small primary particle size or a small pore volume.

[0131] (Regarding Examples 7 to 10) Examples 7 to 8 are examples in which citric acid was added, and Examples 9 to 10 are examples in which citric acid was not added. The pore volume of the granules of Examples 7 to 8 was significantly reduced compared to Examples 9 to 10. This result shows that adding citric acid is effective when producing granules with a small pore volume. Furthermore, the discharge capacity of Examples 7 to 8 was similar to that of Examples 9 to 10. This result shows that the pore volume of the granules does not have a significant effect on the discharge capacity.

[0132] DESCRIPTION OF SYMBOLS 10... granules 11... core 11a... primary particles 11b... pores 12... carbon coating

Claims

1. A method for producing lithium manganese iron phosphate comprising the steps of: preparing a precursor slurry containing lithium phosphate, a manganese-containing phosphate compound, iron oxide, and a dispersion medium; granulating the precursor slurry to obtain precursor particles as aggregates; and calcining the precursor particles to obtain olivine-type lithium manganese iron phosphate, wherein the molar ratio of manganese in the precursor slurry is 70% or more and 90% or less of the total molar ratio of manganese and iron, and the manganese-containing phosphate compound is Mn ５ (H.P.O. ４ ) ２ (P.O. ４ ) ２ (H ２ O) ４ , Mn ５ (H.P.O. ４ ) ２ (P.O. ４ ) ２ , Mn ３ (P.O. ４ ) ２ ・3H ２ O, Mn ３ (P.O. ４ ) ２ or a mixture thereof, and the slurry preparation step includes a grinding step of grinding the manganese-containing phosphate compound together with iron oxide.

2. The method for producing a positive electrode material for a lithium ion secondary battery as described in claim 1, wherein the slurry preparation step includes a first reaction of producing the lithium phosphate by reacting a lithium compound with phosphoric acid in the dispersion medium, and a second reaction of producing the manganese-containing phosphate compound by reacting a manganese compound with phosphoric acid in the dispersion medium.

3. The method for producing a positive electrode material for a lithium ion secondary battery as described in claim 2, wherein in the slurry preparation step, after the first reaction and after the second reaction, the lithium phosphate and the manganese-containing phosphate compound are mixed with iron oxide.

4. A method for producing a positive electrode material for a lithium ion secondary battery as described in claim 2 or 3, wherein at least one of the first reaction and the second reaction is a reaction accompanied by the generation of carbon dioxide gas, and after the first reaction and after the second reaction, the lithium phosphate and the manganese-containing phosphate compound are mixed with at least one component other than the lithium phosphate and the manganese-containing phosphate compound contained in the precursor slurry.

5. A method for producing a positive electrode material for a lithium ion secondary battery according to any one of claims 2 to 4, wherein in the first reaction, a metal compound other than lithium is added, and the amount of the metal compound added is such that a ratio (Li / MA) of the molar ratio of lithium (Li) to the sum (MA) of the molar ratios of metal components constituting the metal compound is 1.0 or more and 1.1 or less, and a ratio (P / MA) of the molar ratio of phosphorus (P) to the sum (MA) of the molar ratios of metal components constituting the metal compound is 1.0 or more and 1.1 or less.

6. The method for producing a positive electrode material for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the precursor slurry contains a carbon source.

7. The method for producing a positive electrode material for a lithium ion secondary battery according to any one of claims 1 to 6, wherein the precursor slurry contains a carboxylic acid, and the grinding step is carried out in the presence of the carboxylic acid.

8. The method for producing a positive electrode material for a lithium ion secondary battery according to any one of claims 1 to 7, wherein the precursor slurry contains a titanium compound.

9. The method for producing a positive electrode material for a lithium ion secondary battery according to any one of claims 1 to 8, wherein the precursor slurry contains a magnesium compound.

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