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

Abstract

The present invention pertains to a method for manufacturing a positive electrode active material for a lithium-ion secondary battery, the method, although simple, being capable of widening the ranges of selection of raw materials to be used, while using a hydrothermal reaction.　That is, the present invention pertains to a method for manufacturing a positive electrode active material which is for a lithium-ion secondary battery and which is represented by formula (A) of LiaMnbFecMxPO4, the method comprising the following steps (I)-(IV): (I) a step for adding a lithium compound and a metal source including at least a manganese source and / or an iron source to a mixed solution of a phosphoric acid compound, a sulfuric acid, and water and performing stirring to obtain an aqueous solution i; (II) a step for adding a pH adjuster x to the obtained aqueous solution i and performing stirring to obtain a mixed solution ii having a pH of at least 10; (III) a step for adding a pH adjuster y to the obtained mixed solution ii and performing stirring to obtain a mixed solution iii having a pH of 6-8; and (IV) a step for subjecting the obtained mixed solution iii to a hydrothermal reaction.

Description

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

[0001] The present invention relates to a method for producing a positive electrode active material for a lithium ion secondary battery, which can use a wide variety of raw materials regardless of their quality.

[0002] Positive electrode active materials composed of compounds such as Li(Fe,Mn)PO4 with an olivine structure are known to be highly useful as positive electrode materials for lithium-ion secondary batteries. To obtain such positive electrode active materials, a hydrothermal reaction-based manufacturing method is often used, which can easily achieve particle size reduction and process simplification.

[0003] In order to achieve further improvements in this situation, various production methods utilizing hydrothermal reactions have been developed. For example, Patent Document 1 discloses a production method in which a slurry having adjusted viscosity and pH is prepared using particles after a hydrothermal reaction, and the slurry is then granulated.

[0004] JP 2013-149602 A

[0005] However, the technology described in the above patent document not only requires a separate step of granulating the slurry, but also inevitably requires the use of high-quality raw materials, so there is still ample room for improvement.

[0006] Therefore, the present invention relates to a method for producing a positive electrode active material for a lithium ion secondary battery, which utilizes a hydrothermal reaction, is simple, and yet allows for a wide range of choices for the raw materials used.

[0007] Therefore, the present inventors have conducted extensive research to solve the above problems, and as a result have discovered a manufacturing method that can obtain a positive electrode active material for a lithium ion secondary battery without being limited to using high-quality raw materials, by adjusting the pH of the mixed solution in two stages in advance before subjecting it to a hydrothermal reaction.

[0008] That is, the present invention provides a compound represented by the following formula (A): a Mn b Fe c M xPO4...(A) (in formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd; a, b, c, and x represent numbers that satisfy 0<a≦1.2, 0≦b≦1.2, 0≦c≦1.2, 0≦x≦0.3, and b+c≠0, and that satisfy a+(valence of Mn)×b+(valence of Fe)×c+(valence of M)×x=3), the method comprising the following steps (I) to (IV): (I) adding a lithium compound and a metal source containing at least a manganese source and / or an iron source to a mixed solution of a phosphoric acid compound, sulfuric acid, and water, and stirring the mixture to obtain an aqueous solution i; (II) adding a pH adjuster x to the obtained aqueous solution i and stirring the mixture to obtain a mixed solution ii with a pH of 10 or higher; (III) adding a pH adjuster y to the obtained mixed solution ii and stirring the mixture to obtain a mixed solution iii with a pH of 6 to 8; and (IV) subjecting the obtained mixed solution iii to a hydrothermal reaction.

[0009] According to the method for producing a positive electrode active material for a lithium ion secondary battery of the present invention, by utilizing a hydrothermal reaction and adjusting the pH of the mixed solution over two stages, it is possible to achieve a simple production method without a process for obtaining an intermediate substance, and it is possible to actively use not only high-quality raw materials but also inexpensive raw materials, thereby achieving effective cost reduction.

[0010] FIG. 1 is a pattern diagram showing the analysis results of an XRD pattern.

[0011] The present invention is a method for producing a positive electrode active material for a lithium ion secondary battery represented by the following formula (A): Li a Mn b Fe c M xPO4 ... (A) (In formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd. a, b, c, and x represent numbers that satisfy 0<a≦1.2, 0≦b≦1.2, 0≦c≦1.2, 0≦x≦0.3, and b+c≠0, and that satisfy a+(valence of Mn)×b+(valence of Fe)×c+(valence of M)×x=3.)

[0012] In the above formula (A), a is preferably 0.6≦a≦1.2, more preferably 0.65≦a≦1.15, and even more preferably 0.7≦a≦1.1. b is preferably 0.4≦b≦0.8. c is preferably 0.2≦c≦0.6. x may be 0≦x≦0.2, or may be 0≦x≦0.15, or may be 0≦x≦0.1. Furthermore, from the viewpoint of further increasing the discharge capacity, M may be Mg, Al, Ti, Zn, Nb, Co, Zr, or Gd.

[0013] Specifically, for example, LiMnPO4, LiFePO4, LiMn 0.3 Fe 0.7 PO4, LiMn 0.4 Fe 0.6 PO4, LiMn 0.45 Fe 0.55 PO4, LiMn 0.7 Fe 0.3 PO4, LiMn 0.9 Fe 0.1 PO4, LiMn 0.8 Fe 0.2 PO4, LiMn 0.75 Fe 0.15 Mg 0.1 PO4, LiMn 0.75 Fe 0.19 Zr 0.03 PO4, LiMn 0.6 Fe 0.4 PO4, LiMn 0.5 Fe 0.5 PO4, Li 1.2 Mn 0.63 Fe 0.27 PO4, Li 0.6 Mn 0.84 Fe 0.36 PO4, etc. Among them, LiMn0.8 Fe 0.2 PO4, LiMn 0.4 Fe 0.6 PO4, LiMn 0.45 Fe 0.55 PO4, LiMn 0.7 Fe 0.3 PO4, LiMn 0.6 Fe 0.4 PO4, Li 1.2 Mn 0.63 Fe 0.27 PO4 is preferred.

[0014] The method for producing a positive electrode active material for a lithium ion secondary battery of the present invention is a method for producing a positive electrode active material for a lithium ion secondary battery represented by formula (A) above, and includes the following steps (I) to (IV): (I) adding a lithium compound and a metal source including at least a manganese source and / or an iron source to a mixed solution of a phosphoric acid compound, sulfuric acid, and water, and stirring the mixture to obtain an aqueous solution i; (II) adding a pH adjuster x to the obtained aqueous solution i and stirring the mixture to obtain a mixed solution ii having a pH of 10 or more; (III) adding a pH adjuster y to the obtained mixed solution ii and stirring the mixture to obtain a mixed solution iii having a pH of 6 to 8; and (IV) subjecting the obtained mixed solution iii to a hydrothermal reaction.

[0015] Step (I) is a step of adding a lithium compound and a metal source containing at least a manganese source and / or an iron source to a mixed solution of a phosphoric acid compound, sulfuric acid, and water, and stirring the mixture to obtain an aqueous solution i.

[0016] Examples of phosphoric acid compounds that can be used include orthophosphoric acid (H3PO4, phosphoric acid), metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, ammonium phosphate, ammonium hydrogen phosphate, etc. Among these, phosphoric acid is preferably used.

[0017] As the sulfuric acid that can be used, concentrated sulfuric acid is preferable from the viewpoint of increasing the solubility of the phosphoric acid compound, and concentrated sulfuric acid having a concentration of 96% by mass or more is more preferable.

[0018] In step (I), the phosphoric acid compound, sulfuric acid, and water are first mixed to prepare a mixed solution. The mixing time is not particularly limited as long as the phosphoric acid compound is well dissolved, but for example, the mixing time is preferably 10 minutes or more, more preferably 10 minutes to 2 hours. From the viewpoint of efficiently and effectively performing the subsequent steps, the solid content of the mixed solution obtained here is preferably 0.1% by mass to 20% by mass, more preferably 1% by mass to 15% by mass, and even more preferably 3% by mass to 10% by mass, based on 100% by mass of the mixed solution. Note that, when the raw materials used themselves contain water, such as when hydrates or the like are used as raw materials, the solid content of the mixed solution excludes this amount, and can be calculated by converting it into the total amount of the raw materials used.

[0019] Next, in step (I), a lithium compound and a metal source containing at least a manganese source and / or an iron source are added to the above mixed solution and stirred to obtain an aqueous solution i.

[0020] Usable lithium compounds include hydroxides (e.g., LiOH.H2O, LiOH), carbonates, sulfates, and acetates, with hydroxides being preferred.

[0021] Examples of manganese sources that can be used include one or more manganese compounds selected from metal oxalates, metal sulfates, metal chlorides, and hydrates thereof, or metallic manganese. Furthermore, as the metallic manganese, manganese-containing ores can be used, and they are not limited to concentrates, and low-grade crude ores can also be used. Such ores can be suitably used in the present invention as long as they have a manganese content of 40% to 100%. More specifically, the manganese content may be, for example, 80% to 100%, 60% to 80%, or 40% to 60%. The manganese content refers to a value indicating the content of manganese compounds in the ore. Examples of such manganese-containing ores include one or more selected from manganese oxide, manganese metal, rhodochrosite, hausmannite, penwithite, braunsite, pyrolusite, rhodonite, tephroite, and bementite.

[0022] Examples of iron sources that can be used include one or more iron compounds selected from metal oxalates, metal sulfates, metal chlorides, and hydrates thereof, or metallic iron. Furthermore, iron-containing ores can be used as metallic iron, and are not limited to concentrates, and low-grade crude ores can also be used. Such ores can be suitably used in the present invention as long as they have an iron content of 40% to 100%. More specifically, the iron content may be, for example, 80% to 100%, 60% to 80%, or 40% to 60%. The iron content refers to a value indicating the content of iron compounds in the ore. Examples of such iron-containing ores include one or more selected from iron oxide ore, hydrous iron oxide ore, iron carbonate, and pyrite sulfide. Manganous iron ore can also be used as a mineral containing both manganese and iron.

[0023] Furthermore, a metal (M: M has the same meaning as M in formula (A)) source other than these manganese sources and iron sources may be used as appropriate, and an ore containing a metal (M) may also be used, similar to the manganese source and iron source.

[0024] When the lithium compound and the metal source containing at least a manganese source and / or an iron source are added and stirred, the stirring time is preferably 1 minute to 24 hours, more preferably 5 minutes to 10 hours, and even more preferably 10 minutes to 1 hour, from the viewpoint of satisfactory dissolution of these added raw materials.

[0025] Furthermore, when adding the lithium compound and the metal source containing at least a manganese source and / or an iron source, they may be added all at once. However, from the viewpoint of efficiently and effectively performing the subsequent steps, and preferably from the viewpoint of further increasing the solubility of each of these raw materials to obtain an aqueous solution i in which all of the raw materials are completely dissolved, it is desirable to add the raw materials sequentially, stirring each raw material before adding the next raw material. For example, when adding the lithium compound, manganese source, and iron source in this order, the stirring time after adding the lithium compound is preferably 0.5 to 24 hours, more preferably 1 to 2 hours. Next, the manganese source is added thereto, and the stirring time is preferably 0.16 to 24 hours, more preferably 1 to 2 hours. Then, the stirring time after adding the iron source is preferably 0.16 to 24 hours, more preferably 1 to 2 hours.

[0026] The aqueous solution i preferably contains 2.0 to 4.0 moles, more preferably 2.0 to 3.1 moles, of lithium per mole of phosphoric acid, and each raw material may be added appropriately so as to achieve such amounts.

[0027] Furthermore, from the viewpoint of efficiently and effectively undergoing the subsequent steps, the solid content in the aqueous solution i is preferably 10% by mass to 30% by mass, and more preferably 15% by mass to 25% by mass, based on 100% by mass of the aqueous solution i.

[0028] In step (I), in many conventional production methods, the obtained aqueous solution i is subjected to a treatment such as purging with nitrogen to form trilithium phosphate (Li3PO4) as an intermediate, which is then recovered and subjected to a hydrothermal reaction. However, in the present invention, the aqueous solution i is subjected to a hydrothermal reaction (step (IV)) after passing through the subsequent steps (II) to (III) without recovering any intermediates such as trilithium phosphate (Li3PO4).

[0029] However, when raw materials containing unnecessary impurities such as the above-mentioned ores are used as the manganese source or iron source, it is preferable to subject the aqueous solution i to a treatment for removing these impurities, such as filtration, before proceeding to step (II).

[0030] Step (II) is a step in which a pH adjuster x is added to the aqueous solution i obtained in step (I) and the mixture is stirred to obtain a mixed solution ii having a pH of 10 or more. That is, step (II) is the first step of a two-step process for adjusting the pH of the mixed solution before subjecting it to a hydrothermal reaction (step (IV)).

[0031] The pH adjuster x used is preferably sodium hydroxide. The amount of pH adjuster x added is not particularly limited as long as a mixed solution ii having the above-mentioned predetermined pH is obtained, but is, for example, preferably 1.05 to 2.25 parts by mass, more preferably 1.5 to 2.25 parts by mass, and even more preferably 1.5 to 1.8 parts by mass per part by mass of the solid content of aqueous solution i. The pH of the resulting mixed solution ii is 10 or higher, preferably 10 to 14, and more preferably 10.5 to 11.5.

[0032] The stirring time after the addition of the pH adjuster x is preferably 10 minutes to 4 hours, more preferably 10 minutes to 1 hour.

[0033] Step (III) is a step of adding a pH adjuster y to the mixed solution ii obtained in step (II) and stirring the mixture to obtain a mixed solution iii having a pH of 6 to 8. That is, step (III) is the second step of two steps of adjusting the pH of the mixed solution before subjecting it to a hydrothermal reaction (step (IV)).

[0034] The pH adjuster y used is preferably sulfuric acid, and from the viewpoint of ensuring that the raw materials remain completely dissolved, concentrated sulfuric acid is more preferable, and concentrated sulfuric acid with a concentration of 96% by mass or more is even more preferable. The amount of pH adjuster y added is not particularly limited as long as a mixed solution iii having the above-mentioned predetermined pH is obtained, but for example, it is preferably 1.25 to 2.7 parts by mass, more preferably 1.8 to 2.7 parts by mass, and even more preferably 1.8 to 2.2 parts by mass per part by mass of the solid content of mixed solution ii.

[0035] When adding the pH adjuster y to the mixed solution ii, it is preferable to add the pH adjuster y dropwise from the viewpoint of ensuring that the raw materials remain completely dissolved. When adding the pH adjuster y dropwise, the rate of addition is preferably 0.1 mL / min to 50 mL / min, more preferably 0.1 mL / min to 20 mL / min, and even more preferably 0.1 mL / min to 5 mL / min.

[0036] It is also preferable to stir while adding the pH adjuster y dropwise. The time required from the start of adding the pH adjuster y dropwise and / or stirring to the end of adding the pH adjuster y dropwise and / or stirring is preferably 5 minutes to 3 hours, more preferably 10 minutes to 2 hours, and even more preferably 1 hour to 2 hours.

[0037] The pH of the resulting mixed solution iii is 6 to 8, preferably 6.2 to 7.8, and more preferably 6.6 to 7.2.

[0038] Furthermore, the difference between the pH of the mixed solution ii obtained in the above step (II) and the pH of the mixed solution iii obtained in the above step (III) (pH of mixed solution ii - pH of mixed solution iii) is preferably 4.2 to 5.8, more preferably 4.8 to 5.4, from the viewpoint of favorably promoting the hydrothermal reaction in the subsequent step (IV).

[0039] Step (IV) is a step of subjecting the mixed solution iii obtained in step (III) to a hydrothermal reaction. The hydrothermal reaction may be carried out at a temperature of 100°C or higher, preferably 130°C to 180°C. The hydrothermal reaction is preferably carried out in a pressure-resistant vessel. When the reaction is carried out at 130°C to 180°C, the pressure is preferably 0.3 MPa to 0.9 MPa, and when the reaction is carried out at 140°C to 160°C, the pressure is preferably 0.3 MPa to 0.6 MPa. The hydrothermal reaction time is preferably 0.1 hours to 48 hours, more preferably 0.2 hours to 24 hours. After the hydrothermal reaction, the product is preferably isolated by filtration, washing with water, and drying. The drying method used may be freeze drying or vacuum drying.

[0040] After isolating the product, it is preferable to calcinate it to obtain a positive electrode active material for a lithium ion secondary battery. The calcination conditions are preferably a reducing atmosphere or an inert atmosphere, the calcination temperature is preferably 500°C to 1000°C, more preferably 550°C to 900°C, and the calcination time is preferably 0.5 hours to 12 hours, more preferably 1 hour to 6 hours.

[0041] The obtained positive electrode active material for a lithium ion secondary battery can be used as it is as a positive electrode material to construct a lithium ion secondary battery according to a conventional method. Specifically, for example, the positive electrode active material for a lithium ion secondary battery is kneaded with acetylene black, ketjen black, polyvinylidene fluoride, N-methyl-2-pyrrolidone, or the like to prepare a positive electrode slurry, which is then applied to a current collector and press-molded to form a positive electrode.

[0042] The lithium ion secondary battery to which such a positive electrode can be applied is not particularly limited as long as it essentially comprises a positive electrode, a negative electrode, an electrolyte solution, and a separator, or a positive electrode, a negative electrode, and a solid electrolyte.

[0043] Here, the negative electrode is not particularly limited in terms of its material composition, and any known material composition can be used as long as it can absorb lithium ions during charging and release them during discharging. For example, lithium metal, graphite, silicon-based materials (Si, SiOx), lithium titanate, or carbon materials such as amorphous carbon can be used. It is preferable to use an electrode formed of an intercalating material capable of electrochemically absorbing and releasing lithium ions, particularly a carbon material. Furthermore, two or more of the above negative electrode materials may be used in combination, such as a combination of graphite and silicon-based materials.

[0044] The electrolyte solution is prepared by dissolving a supporting salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent typically used in electrolyte solutions for lithium ion secondary batteries, and examples thereof include carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, and oxolane compounds.

[0045] The supporting salt is not particularly limited in type, but is preferably at least one of an inorganic salt selected from LiPF, LiBF, LiClO, and LiAsF, a derivative of the inorganic salt, an organic salt selected from LiSOCF, LiC(SOCF), and LiN(SOCF), LiN(SOCF) and LiN(SOCF)(SOCF), and a derivative of the organic salt.

[0046] The separator serves to electrically insulate the positive and negative electrodes and retain the electrolyte, and may be, for example, a porous synthetic resin film, particularly a porous film made of a polyolefin polymer (polyethylene, polypropylene).

[0047] The solid electrolyte electrically insulates the positive and negative electrodes and exhibits high lithium ion conductivity. 0.51 Li 0.34 TiO 2.94 , Li 1.3 Al 0.3 Ti 1.7 (PO4)3, Li7La3Zr2O 12 , 50Li4SiO4・50Li3BO3, Li 2.9 P.O. 3.3 N 0.46 , Li 3.6 Si 0.6 P 0.4 O4, Li 1.07 Al 0.69 Ti 1.46 (PO4)3, Li 1.5 Al 0.5 Ge 1.5 (PO4)3, Li 10 GeP2S 12 , Li 3.25 Ge 0.25 P 0.75 S4, 30Li2S・26B2S3・44LiI, 63Li2S・36SiS2・1Li3PO4, 57Li2S・38SiS2・5Li4SiO4, 70Li2S・30P2S5, 50Li2S・50GeS2, Li7P3S 11 , Li 3.25 P 0.95 S4 can be used.

[0048] The shape of the lithium ion secondary battery having the above-described configuration is not particularly limited, and may be various shapes such as a coin shape, a cylindrical shape, a square shape, or an irregular shape enclosed in a laminate outer casing.

[0049] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[0050] Example 1: 26.17 mL of phosphoric acid was added to 342.5 mL of water, and then 50 mL of 96% concentrated sulfuric acid was added thereto and mixed for 0.5 hours to prepare mixed solution a. 14.52 g of lithium hydroxide was then added to the resulting mixed solution a, and the mixture was stirred for 10 minutes until the lithium hydroxide was completely dissolved, to obtain mixed solution a'. 10.74 g of iron sulfate was then added to the resulting mixed solution a', and the mixture was stirred for 20 minutes until the iron sulfate was completely dissolved. 23.66 g of manganese sulfate was then added, and the mixture was stirred for 20 minutes until the manganese sulfate was completely dissolved, to prepare aqueous solution i-1 (solid content: 25% by mass).

[0051] Sodium hydroxide (1.6 parts by mass relative to 1 part by mass of the solid content of aqueous solution i-1) was added to the resulting aqueous solution i-1 and stirred for 1 hour, yielding a mixed solution ii-1 (solid content: 22% by mass) with a pH of 12. Next, 96% sulfuric acid (1.9 parts by mass relative to 1 part by mass of the solid content of mixed solution ii-1) was added dropwise to the resulting mixed solution ii-1 at a rate of 0.3 mL / min while stirring for 1 hour, yielding a mixed solution iii-1 with a pH of 7.2. The resulting mixed solution iii-1 was subjected to a hydrothermal reaction (set temperature: 180°C, set pressure: 1.0 MPa, reaction time: 1 hour), followed by filtration and freeze-drying to obtain a positive electrode active material A-1 (Li 0.96 Mn 0.53 Fe 0.47 PO4) was obtained.

[0052] Example 2 3.35 g of metallic iron was added to the mixed solution a' obtained in Example 1, and the mixture was stirred for 1 hour until the metallic iron was completely dissolved. 7.69 g of manganese sulfate was then added, and the mixture was stirred for 1 hour until the manganese sulfate was completely dissolved. The mixture was then filtered to remove impurities, yielding an aqueous solution i-2 (solid content: 25% by mass). Sodium hydroxide (1.6 parts by mass per part by mass of the solid content of the aqueous solution i-2) was added to the resulting aqueous solution i-2, and the mixture was stirred for 1 hour, yielding a mixed solution ii-2 (solid content: 22% by mass) with a pH of 12. Next, 96% sulfuric acid (1.9 parts by mass per part by mass of the solid content of the mixed solution ii-2) was added dropwise to the resulting mixed solution ii-2 at a rate of 0.3 mL / min while stirring for 1 hour, yielding a mixed solution iii-2 with a pH of 7.2. The resulting mixed solution iii-2 was subjected to a hydrothermal reaction in the same manner as in Example 1, followed by filtration and freeze-drying to obtain a cathode active material A-2 (Li 0.96 Mn 0.65 Fe 0.35 PO4) was obtained.

[0053] [Comparative Example 1] 10.74 g of iron sulfate was added to the mixed solution a' obtained in Example 1, and the mixture was stirred for 1 hour until the iron sulfate was completely dissolved. 23.66 g of manganese sulfate was then added, and the mixture was stirred for 1 hour until the manganese sulfate was completely dissolved, to prepare an aqueous solution e-1 (solid content: 25% by mass). Sodium hydroxide (0.8 parts by mass per 1 part by mass of the solid content of the aqueous solution e-1) was added to the obtained aqueous solution e-1, and the mixture was stirred for 1 hour to obtain a mixed solution e-2 (solid content: 23% by mass) with a pH of 7.2. The obtained mixed solution e-2 was subjected to a hydrothermal reaction similar to that in Example 1, followed by filtration and freeze-drying to obtain a positive electrode active material E-2 (Li3PO4).

[0054] <<XRD Pattern Analysis>> An XRD pattern was obtained for the positive electrode active material of Example 1 obtained above using an X-ray diffractometer (D8 ADVANCE A-25, manufactured by Bruker AXS) with a CuKα target, a tube voltage of 40 kV, a tube current of 40 mA, a scanning range of 10 to 70° (2θ), a step width of 0.023°, and a scan speed of 0.13° / step. The obtained XRD pattern is shown in FIG. 1. The results in FIG. 1 show that a positive electrode active material for a lithium ion secondary battery represented by (A) above and free of impurities was obtained in Example 1.

Claims

1. The following formula (A): Li a Mn b Fe c M x PO4...(A) (in formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd; a, b, c, and x represent numbers that satisfy 0<a≦1.2, 0≦b≦1.2, 0≦c≦1.2, 0≦x≦0.3, and b+c≠0, and that satisfy a+(valence of Mn)×b+(valence of Fe)×c+(valence of M)×x=3), the method comprising the following steps (I) to (IV): (I) adding a lithium compound and a metal source containing at least a manganese source and / or an iron source to a mixed solution of a phosphoric acid compound, sulfuric acid, and water, and stirring the mixture to obtain an aqueous solution i; (II) adding a pH adjuster x to the obtained aqueous solution i and stirring the mixture to obtain a mixed solution ii with a pH of 10 or more; (III) adding a pH adjuster y to the obtained mixed solution ii and stirring the mixture to obtain a mixed solution iii with a pH of 6 to 8; and (IV) subjecting the obtained mixed solution iii to a hydrothermal reaction.

2. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 1, wherein in step (III), the pH adjuster y is added to the mixed solution ii at a dropwise rate of 0.1 mL / min to 50 mL / min.

3. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the pH adjuster x is sodium hydroxide and the pH adjuster y is sulfuric acid.

4. The method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3, wherein in step (I), the manganese source used has a manganese content of 40% to 100%, and the iron source used has an iron content of 40% to 100%.

5. The method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 4, wherein in step (I), when a lithium compound and a metal source including at least a manganese source and / or an iron source are added to a mixed solution of a phosphoric acid compound, sulfuric acid, and water and the mixture is stirred, the stirring time is 1 minute to 24 hours.

6. The method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the amount of pH adjuster x added in step (II) is 1.05 to 2.25 parts by mass per part by mass of the solid content of the aqueous solution i.

7. A method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 6, wherein the amount of pH adjuster y added in step (III) is 1.25 to 2.7 parts by mass per 1 part by mass of the solid content of the mixed solution ii.

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