JP5725456B2 - Method for producing positive electrode active material for lithium ion secondary battery - Google Patents

Description

  The present invention relates to a method for producing a lithium iron phosphate-based positive electrode active material useful as a positive electrode material for a lithium ion secondary battery.

Secondary batteries used for portable electronic devices, hybrid cars, electric cars, and the like have been developed, and lithium ion secondary batteries are particularly widely known. The lithium ion battery basically includes a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator. LiCoO ₂ is widely used as a positive electrode material, and LiNiO ₂ , LiMn ₂ O _{4 and the} like have been developed. However, these lithium metal oxides have a problem that the capacity is low although the voltage is high.

  On the other hand, recently, it has been proposed to use a phosphate compound such as lithium iron phosphate having an olivine structure for the positive electrode (Patent Document 1). However, this method of synthesizing lithium iron phosphate is a solid phase method, which requires firing and pulverization in an inert gas atmosphere, and the operation is complicated.

  Therefore, attempts have been made to produce lithium iron phosphate by a hydrothermal reaction (Patent Documents 2 and 3, Non-Patent Document 1). In these methods, a lithium compound, an iron compound, and a phosphoric acid compound are hydrothermally reacted in a pressure resistant vessel.

Japanese Patent Laid-Open No. 9-171827 JP 2002-151082 A JP 2004-95385 A

Electrochemistry Communications 3 (2001) 505-508

As the iron compound in the method for producing lithium iron phosphate by hydrothermal reaction, iron chloride, iron oxalate, ferrous phosphate, and the like are used, and it is described that iron sulfate can also be used.
However, when cheap by-product ferrous sulfate obtained as a by-product of titanium oxide as iron sulfate is used as a raw material, the obtained lithium iron phosphate crystals are not uniform, and lithium ions obtained due to that It was found that the discharge characteristics of the secondary battery deteriorate.
Therefore, the subject of this invention is providing the new manufacturing method of lithium iron phosphate useful as a positive electrode active material for lithium ion secondary batteries using cheap byproduct ferrous sulfate.

  Therefore, the present inventor tried hydrothermal synthesis of lithium iron phosphate under various conditions using by-product ferrous sulfate as a raw material. Ti concentration and Ca concentration as impurities in by-product ferrous sulfate It is found that the particles of lithium iron phosphate obtained can be made fine and uniform by adjusting the discharge characteristics of lithium ion secondary batteries using the lithium iron phosphate. The present invention has been completed.

That is, in the present invention, after mixing a lithium compound, a phosphoric acid compound and water, a hydrothermal reaction is performed by adding by-product ferrous sulfate having a Ti content of less than 1000 mg / kg and a Ca content of less than 500 mg / kg. A feature of the present invention is to provide a method for producing lithium iron phosphate.
In the present invention, after mixing a lithium compound, a phosphoric acid compound and water, a by-product ferrous sulfate salt having a Ti content of less than 1000 mg / kg and a Ca content of less than 500 mg / kg, and a carbon source are added to the water. The present invention provides a method for producing a lithium iron phosphate-based positive electrode active material, characterized by performing a thermal reaction and then firing in an inert gas or a reducing atmosphere.
Moreover, this invention provides the lithium ion secondary battery which contains the lithium iron phosphate obtained by said manufacturing method as a positive electrode material.

  According to the method of the present invention, high-purity lithium iron phosphate having a fine and uniform particle size can be obtained in high yield by using inexpensive by-product ferrous sulfate. When lithium iron phosphate obtained by the method of the present invention is used as a positive electrode material, a lithium ion secondary battery having a high capacity and excellent charge / discharge characteristics can be obtained.

It shows an SEM image of LiFePO ₄ obtained in Example 1. It shows an SEM image of LiFePO ₄ obtained in Example 2. It shows an SEM image of LiFePO ₄ obtained in Comparative Example 1. It shows an SEM image of LiFePO ₄ obtained in Comparative Example 2. It shows an SEM image of LiFePO ₄ obtained in Comparative Example 3. It shows an SEM image of LiFePO ₄ obtained in Reference Example. The charging / discharging characteristic of the battery produced in Example 3 is shown.

  In the method for producing lithium iron phosphate of the present invention, first, a lithium compound, a phosphate compound and water are mixed.

  The synthetic raw material of lithium iron phosphate is basically a divalent iron compound, a lithium compound and a phosphate compound. In the present invention, a mixture of a lithium compound, a phosphate compound and water is prepared first. Finally, adding by-product ferrous sulfate as a divalent iron compound is important for preventing side reactions and allowing the reaction to proceed easily. When the by-product ferrous sulfate, phosphate compound and water are mixed first, a carbon source is added to this, and a lithium compound is added after introducing nitrogen gas, condensation occurs during the reaction, which makes stirring impossible. A special stirring device is required. On the other hand, when by-product ferrous sulfate, lithium compound and water are mixed first, carbon source is added, and nitrogen gas is introduced and then phosphoric acid compound is added, stirring becomes difficult due to excessive foaming and condensation occurs . In contrast, when the raw materials are added in the order as in the present invention, no agglomeration occurs, stirring is easy, and the reaction proceeds smoothly.

  Examples of the lithium compound used as a raw material include lithium metal salts such as lithium fluoride, lithium chloride, lithium bromide, and lithium iodide, lithium hydroxide, lithium carbonate, etc., but it is inexpensive to use lithium carbonate. It is preferable in a certain point.

  As the phosphoric acid compound, orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, ammonium phosphate, ammonium hydrogen phosphate and the like are used.

  The amount of the lithium compound and phosphate compound used is preferably 1.5: 1 to 3.7: 1 in terms of a molar ratio of lithium ion and phosphate ion, and is approximately 2.5: 1 to 3.3: 1. Is more preferable.

  The amount of water used is preferably 10 to 50 mol, more preferably 13 to 30 mol with respect to 1 mol of phosphorus ion of the phosphoric acid compound, from the viewpoints of solubility of the raw material compound, easiness of stirring, synthesis efficiency, and the like. 15-20 mol is particularly preferable.

  The order of addition of the lithium compound, phosphate compound and water is not particularly limited, and the mixing time of these raw materials is not limited. These raw materials may be mixed at room temperature, for example, 10 to 35 ° C.

  In the method of the present invention, before adding the by-product ferrous sulfate, a base is added to the mixture of lithium compound, phosphate compound and water to adjust the pH after addition of the divalent iron compound to 3-6. You can also By adjusting the pH by the addition of the base, high-purity lithium iron phosphate can be obtained efficiently even if the amount of the lithium compound used as a raw material is not excessively large.

The base to be used may be any base that can adjust the pH of the mixture after addition of the divalent iron compound to 3 to 6, but is required to be non-metallic and is preferably ammonia or organic amine. Examples of organic amines include alkylamines, dialkylamines, trialkylamines, alkanolamines, dialkanolamines, trialkanolamines, cyclic amines such as piperidine, pyrrolidine, and morpholine, and aromatic amines such as pyridine. Of these bases, ammonia is particularly preferred.
The amount of these bases used is preferably such that the pH of the mixture after addition of the divalent iron compound is 3 to 6, more preferably 3 to 5.

  In the method of the present invention, it is preferable to introduce nitrogen gas into the mixture before adding the by-product ferrous sulfate. The introduction of nitrogen gas is preferable from the viewpoint of reducing the amount of dissolved oxygen in the reaction solution, preventing oxidation of by-product ferrous sulfate added later, and reducing the amount of antioxidant added. The amount of nitrogen gas introduced is preferably carried out until the dissolved oxygen concentration in the solution is 1.0 mg / L or less, more preferably 0.5 mg / L or less. Nitrogen gas is preferably introduced by bubbling nitrogen gas into the solution.

  In the method of the present invention, an antioxidant may be added at this point in order to prevent oxidation of by-product ferrous sulfate added later. The antioxidant may be added at the time of introduction of nitrogen gas and at that time, or before, during or after these operations. Examples of the antioxidant include ascorbic acid, ascorbic acid ester, ascorbate, isoascorbic acid, aldehydes, hydrogen gas, sulfite and the like. The amount of these antioxidants to be used is preferably 0.001 mol to 0.1 mol, and more preferably 0.005 mol to 0.05 mol, per 1 mol of iron ions of the divalent iron compound.

  In the method of the present invention, the carbon source may be added after the hydrothermal reaction, but it is preferable to add it before the hydrothermal reaction from the viewpoint of obtaining a uniform positive electrode active material. Here, examples of the carbon source include glucose, fructose, polyethylene glycol, polyvinyl alcohol, carboxymethylcellulose, saccharose, starch, dextrin, and citric acid, and starch and glucose are particularly preferable from the viewpoint of low cost. The amount of carbon source used is 0 with respect to the weight of the mixture of lithium compound and phosphate compound, by-product ferrous sulfate and water, from the viewpoint of charge / discharge characteristics when the obtained lithium iron phosphate is used as the positive electrode material. 0.1 wt% to 15 wt% is preferable, 0.5 wt% to 10 wt% is more preferable, and 1.5 wt% to 5 wt% is particularly preferable.

  The order of adding the base, introducing nitrogen gas, and adding the carbon source is not limited and may be simultaneous.

  In the method of the present invention, by-product ferrous sulfate having a Ti content of less than 1000 mg / kg and a Ca content of less than 500 mg / kg is added to the mixture and subjected to a hydrothermal reaction. The by-product ferrous sulfate used is ferrous sulfate by-produced during the production of titanium oxide, and the Ti content and Ca content are adjusted to the above ranges. Here, when using by-product ferrous sulfate having a Ti content of 1000 mg / kg or more or a Ca content of 500 mg / kg or more, the crystals of lithium iron phosphate obtained in any case are uniform. In addition, the particle size becomes large. The same applies when either the Ti content or the Ca content exceeds the above numerical value. The amount of by-product ferrous sulfate used is preferably Li / Fe = 1.5 to 4.0 in terms of obtaining high-purity lithium iron phosphate, and more preferably Li / Fe = 2. 5 to 3.3. The pH of the mixture after the addition of the divalent iron compound is desirably 3 to 6 due to the addition of the base.

  In the method of the present invention, the mixture is preferably stirred from the viewpoint of obtaining lithium iron phosphate having a fine and uniform particle size, and more preferably mixed for 30 minutes or more. In mixing, it is preferable to stir. The stirring time is 30 minutes or more, preferably 30 to 120 minutes, and more preferably 60 to 120 minutes. The stirring reaction may be performed at room temperature, and is preferably performed at 10 to 35 ° C. Stirring can be performed by ordinary stirring means such as propeller stirring and pump circulation stirring.

  The mixture is then subjected to a hydrothermal reaction. In the hydrothermal reaction, since water is present in the reaction mixture, the reaction mixture may be sealed in a pressure vessel and heated to 150 ° C or higher. A more preferable reaction temperature is 150 to 200 ° C, and further preferably 180 to 200 ° C. The pressure only needs to be sealed and heated in a pressure-resistant container, and is theoretically about 1.0 to 1.5 MPa. The heating time is preferably 1 to 10 hours, more preferably 2 to 5 hours. In addition, it is preferable to stir the reaction liquid during the hydrothermal reaction.

  After completion of the hydrothermal reaction, the produced lithium iron phosphate is preferably collected by filtration and washed. Washing is preferably performed with water using a filtration device having a cake washing function. The obtained crystals are dried if necessary. Examples of the drying means include spray drying, vacuum drying, freeze drying and the like.

The obtained lithium iron phosphate becomes a lithium iron phosphate useful as a positive electrode material by firing in an inert gas or a reducing atmosphere. If the carbon source is not added before the hydrothermal reaction, the carbon source is added here. Examples of the inert gas include Ar and N ₂ . In addition, hydrogen gas may be introduced to obtain a reducing atmosphere. Firing conditions are preferably 600 ° C. or higher, more preferably 600 to 900 ° C., and particularly preferably 600 to 800 ° C. The firing time is preferably 0.5 hours to 5 hours, more preferably 1 hour to 3 hours.

The lithium iron phosphate obtained by the method of the present invention has a chemical composition represented by LiFePO ₄ and is useful as a positive electrode active material because it is coated with carbon. The obtained lithium iron phosphate has a feature that the average particle size is as fine as 1 μm or less and the particle size distribution is narrow. The average particle size calculated from the SEM image is 1000 nm or less, and the particle size distribution is preferably 100 to 800 nm, more preferably 200 to 600 nm, and particularly preferably 300 to 500 nm. The average particle size is preferably 600 nm or less, and particularly preferably 500 nm or less.

  The lithium iron phosphate-based positive electrode active material obtained by the method of the present invention is useful as a positive electrode material for lithium ion secondary batteries because the particle size is fine and uniform. Next, a lithium ion secondary battery containing the lithium iron phosphate positive electrode active material obtained by the method of the present invention as a positive electrode material will be described.

  The lithium ion secondary battery to which the positive electrode material of the present invention can be applied is not particularly limited as long as it has a positive electrode, a negative electrode, an electrolytic solution, and a separator as essential components.

  Here, as long as lithium ions can be occluded at the time of charging and released at the time of discharging, the material structure is not particularly limited, and a known material structure can be used. For example, a carbon material such as lithium metal, graphite, or amorphous carbon. It is preferable to use an electrode formed of an intercalating material capable of electrochemically inserting and extracting lithium, particularly a carbon material.

  The electrolytic solution is obtained by dissolving a supporting salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent that is usually used for an electrolyte solution of a lithium ion secondary battery. For example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones An oxolane compound or the like can be used.

The type of the supporting salt is not particularly limited, but an inorganic salt selected from LiPF ₆ , LiBF ₄ , LiClO ₄ and LiAsF ₆ , a derivative of the inorganic salt, LiSO ₃ CF ₃ , LiC (SO ₃ CF ₃ ) ₂ and LiN (SO ₃ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ and LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), and organic salt derivatives It is preferable that it is at least 1 sort of.

  The separator plays a role of electrically insulating the positive electrode and the negative electrode and holding the electrolytic solution. For example, a porous synthetic resin film, particularly a polyolefin polymer (polyethylene, polypropylene) porous film may be used.

  EXAMPLES Next, although an Example is given and this invention is demonstrated further in detail, this invention is not limited to this.

Example 1
19.9 g of Li ₂ CO ₃ , 17.6 g of H ₃ PO ₄ and 60.0 g of water were mixed. To this, 5.1 g of glucose was added, and then nitrogen gas was bubbled to confirm that the dissolved oxygen concentration was less than 0.1 mg / L. This was mixed with by-product ferrous sulfate (Ti content 500 mg / kg, Ca content 50 mg / kg) (FeSO ₄ .7H ₂ O) 50.0 g and mixed with a propeller stirrer at 23 ± 2 ° C. for 60 minutes. Stir.
The mixture stirred for 60 minutes was placed in an autoclave and heated at 200 ° C. for 3 hours. Stirring was continued during heating. The internal pressure of the autoclave was 1.5 MPa. The formed crystals were filtered and then washed with water. The crystals were vacuum dried at 60 ° C. and 1 Torr. The obtained crystal was baked at 700 ° C. for 1 hour in a tubular electric furnace in which 3% of hydrogen was introduced into argon gas to obtain a fine powder of lithium iron phosphate. The SEM image of the obtained powder is shown in FIG. The particle diameter of the obtained lithium iron phosphate was in the range of 300 to 500 nm, and it was confirmed that high purity lithium iron phosphate was obtained.

Example 2
19.9 g of Li ₂ CO ₃ , 17.6 g of H ₃ PO ₄ and 60.0 g of water were mixed. To this, 5.1 g of glucose was added, and then nitrogen gas was bubbled to confirm that the dissolved oxygen concentration was less than 0.1 mg / L. This was mixed with by-product ferrous sulfate (Ti content 200 mg / kg, Ca content 100 mg / kg) (FeSO ₄ .7H ₂ O) 50.0 g, and a propeller type stirring device at 23 ± 2 ° C. for 60 minutes. Stir.
The mixture stirred for 60 minutes was placed in an autoclave and heated at 200 ° C. for 3 hours. Stirring was continued during heating. The internal pressure of the autoclave was 1.5 MPa. The formed crystals were filtered and then washed with water. The crystals were vacuum dried at 60 ° C. and 1 Torr. The obtained crystal was baked at 700 ° C. for 1 hour in a tubular electric furnace in which 3% of hydrogen was introduced into argon gas to obtain a fine powder of lithium iron phosphate. The SEM image of the obtained powder is shown in FIG. The particle diameter of the obtained lithium iron phosphate was in the range of 300 to 1000 nm, and it was confirmed that high purity lithium iron phosphate was obtained.

Comparative Example 1
19.9 g of Li ₂ CO ₃ , 17.6 g of H ₃ PO ₄ and 60.0 g of water were mixed. To this, 5.1 g of glucose was added, and then nitrogen gas was bubbled to confirm that the dissolved oxygen concentration was less than 0.1 mg / L. This was mixed with by-product ferrous sulfate (Ti content 1500 mg / kg, Ca content 50 mg / kg) (FeSO ₄ .7H ₂ O) 50.0 g and mixed with a propeller-type stirrer at 23 ± 2 ° C. for 60 minutes. Stir.
The mixture stirred for 60 minutes was placed in an autoclave and heated at 200 ° C. for 3 hours. Stirring was continued during heating. The internal pressure of the autoclave was 1.5 MPa. The formed crystals were filtered and then washed with water. The crystals were vacuum dried at 60 ° C. and 1 Torr. The obtained crystal was baked at 700 ° C. for 1 hour in a tubular electric furnace in which 3% of hydrogen was introduced into argon gas to obtain a fine powder of lithium iron phosphate. The SEM image of the obtained powder is shown in FIG. The particle diameter of the obtained lithium iron phosphate was in the range of 1000 to 5000 nm, the particle diameter was large, and non-uniform lithium iron phosphate was obtained.

Comparative Example 2
19.9 g of Li ₂ CO ₃ , 17.6 g of H ₃ PO ₄ and 60.0 g of water were mixed. To this, 5.1 g of glucose was added, and then nitrogen gas was bubbled to confirm that the dissolved oxygen concentration was less than 0.1 mg / L. This was mixed with by-product ferrous sulfate (Ti content 200 mg / kg, Ca content 1000 mg / kg) (FeSO ₄ .7H ₂ O) 50.0 g, and the propeller type stirring device at 23 ± 2 ° C. for 60 minutes. Stir.
The mixture stirred for 60 minutes was placed in an autoclave and heated at 200 ° C. for 3 hours. Stirring was continued during heating. The internal pressure of the autoclave was 1.5 MPa. The formed crystals were filtered and then washed with water. The crystals were vacuum dried at 60 ° C. and 1 Torr. The obtained crystal was baked at 700 ° C. for 1 hour in a tubular electric furnace in which 3% of hydrogen was introduced into argon gas to obtain a fine powder of lithium iron phosphate. The SEM image of the obtained powder is shown in FIG. The particle diameter of the obtained lithium iron phosphate was in the range of 300 to 1000 nm, the particle diameter was large, and non-uniform lithium iron phosphate was obtained.

Comparative Example 3
19.9 g of Li ₂ CO ₃ , 17.6 g of H ₃ PO ₄ and 60.0 g of water were mixed. To this, 5.1 g of glucose was added, and then nitrogen gas was bubbled to confirm that the dissolved oxygen concentration was less than 0.1 mg / L. This was mixed with 50.0 g of by-product ferrous sulfate (Ti content 1500 mg / kg, Ca content 1000 mg / kg) (FeSO ₄ .7H ₂ O), and the propeller type stirring device at 23 ± 2 ° C. for 60 minutes. Stir.
The mixture stirred for 60 minutes was placed in an autoclave and heated at 200 ° C. for 3 hours. Stirring was continued during heating. The internal pressure of the autoclave was 1.5 MPa. The formed crystals were filtered and then washed with water. The crystals were vacuum dried at 60 ° C. and 1 Torr. The obtained crystal was baked at 700 ° C. for 1 hour in a tubular electric furnace in which 3% of hydrogen was introduced into argon gas to obtain a fine powder of lithium iron phosphate. An SEM image of the obtained powder is shown in FIG. The particle diameter of the obtained lithium iron phosphate was in the range of 1000 to 5000 nm, the particle diameter was large, and non-uniform lithium iron phosphate was obtained.

Reference Example 19.9 g of Li ₂ CO ₃ , 17.6 g of H ₃ PO ₄ and 60.0 g of water were mixed. To this, 5.1 g of glucose was added, and then nitrogen gas was bubbled to confirm that the dissolved oxygen concentration was less than 0.1 mg / L. To this, 50.0 g of ferrous sulfate as an experimental reagent (manufactured by Wako Pure Chemical Industries, Ltd.) (FeSO ₄ .7H ₂ O) was mixed and stirred at 23 ± 2 ° C. with a propeller type stirring device for 60 minutes.
The mixture stirred for 60 minutes was placed in an autoclave and heated at 200 ° C. for 3 hours. Stirring was continued during heating. The internal pressure of the autoclave was 1.5 MPa. The formed crystals were filtered and then washed with water. The crystals were vacuum dried at 60 ° C. and 1 Torr. The obtained crystal was baked at 700 ° C. for 1 hour in a tubular electric furnace in which 3% of hydrogen was introduced into argon gas to obtain a fine powder of lithium iron phosphate. The SEM image of the obtained powder is shown in FIG. The particle diameter of the obtained lithium iron phosphate was in the range of 300 to 500 nm, and it was confirmed that high purity lithium iron phosphate was obtained.

Example 3
Batteries were produced using the materials obtained in Examples 1 and 2, Comparative Examples 1 to 3 and Reference Example as positive electrode materials.
The fired products, ketjen black (conductive agent), and polyvinylidene fluoride (binder) obtained in Examples 1 and 2 and Comparative Examples 1 to 3 and Reference Example were mixed at a weight ratio of 75:15:10. After mixing, N-methyl-2-pyrrolidone was added thereto and kneaded sufficiently to prepare a positive electrode slurry. The positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 80 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a positive electrode.
Next, a coin-type lithium ion secondary battery was constructed using the positive electrode. A lithium foil punched to φ15 mm was used for the negative electrode. As the electrolytic solution, a solution obtained by dissolving LIPF ₆ at a concentration of 1 mol / l in a mixed solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 1 was used. As the separator, a known one such as a polymer porous film such as polypropylene was used. These battery components were assembled and housed in a conventional manner in an atmosphere with a dew point of −50 ° C. or lower to produce a coin-type lithium secondary battery (CR-2032).
A charge / discharge test at a constant current density was performed using the manufactured lithium ion secondary battery. Charging conditions at this time were constant current charging with a current of 0.1 CA (17 mA / g) and a voltage of 4.2 V, and discharging conditions were constant current discharging with a current of 0.1 CA and a final voltage of 2.0 V. All temperatures were 30 ° C.
FIG. 7 shows the discharge characteristics from the results of the charge / discharge test. As a result, the batteries using the positive electrode materials of Examples 1 and 2 and the reference example showed excellent charge / discharge capacities, but the charge / discharge capacities of the batteries using the materials of Comparative Examples 1 to 3 were not sufficient.
From the above results, according to the method of the present invention, fine and uniform lithium iron phosphate can be obtained by using inexpensive by-product ferrous sulfate as the divalent iron compound. Moreover, if the obtained lithium iron phosphate is used, the lithium ion secondary battery which shows the outstanding charging / discharging capacity | capacitance will be obtained.

Claims (7)

  An iron phosphate characterized by mixing a lithium compound, a phosphate compound and water and then adding a by-product ferrous sulfate having a Ti content of less than 1000 mg / kg and a Ca content of less than 500 mg / kg to cause a hydrothermal reaction. Lithium production method.   After mixing the lithium compound, phosphate compound and water, a by-product ferrous sulfate salt with a Ti content of less than 1000 mg / kg and a Ca content of less than 500 mg / kg is added, and a hydrothermal reaction is carried out. A method for producing a lithium iron phosphate-based positive electrode active material, characterized by firing in an inert gas or a reducing atmosphere.   The method according to claim 1 or 2, wherein nitrogen gas is introduced into the reaction system before the addition of the by-product ferrous sulfate.   The production method according to any one of claims 1 to 3, wherein the by-product ferrous sulfate is by-product ferrous sulfate at the time of producing titanium oxide. Hydrothermal reaction, preparation of any of claims 1-4 are intended to be carried out under the condition of 150 to 200 ° C. in a pressure vessel.   Firing conditions are 600-800 degreeC, The manufacturing method of any one of Claims 2-5.   A lithium ion secondary battery containing, as a positive electrode material, a lithium iron phosphate-based positive electrode active material obtained by the production method according to claim 2.