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

Abstract

This invention relates to a method for producing a positive electrode active material for lithium-ion secondary batteries that can efficiently utilize the added carbon source and effectively improve the battery properties of lithium-ion secondary batteries. [Solution] Formula: Li a Mn b Fe c M x A method for producing a positive electrode active material for a lithium-ion secondary battery, represented as PO4 and having carbon supported on it, comprising the steps of: (I) mixing a lithium compound, a metal compound containing at least a manganese compound and / or an iron compound, a phosphoric acid compound, and water to obtain slurry water a, and then subjecting it to a hydrothermal reaction to obtain preliminary particles x; (II) adding the obtained preliminary particles x, a conductive carbon material, and water to obtain slurry water b; (III) drying the obtained slurry water b to obtain particles Y; and (IV) calcining the obtained particles Y under an inert gas atmosphere, wherein step (II) and / or step (IV) includes the step of adding a carbonization accelerator.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a positive electrode active material for a lithium-ion secondary battery, which can efficiently support carbon on the positive electrode active material for a lithium-ion secondary battery and improve the battery characteristics of the lithium-ion secondary battery.

Background Art

[0002] Secondary batteries such as lithium-ion secondary batteries are used in a wide range of fields such as mobile phones, digital cameras, notebook PCs, hybrid vehicles, and electric vehicles. As a positive electrode material for such lithium-ion secondary batteries, due to its high safety and large capacity, LiMn , , , , ,

[0004] , , Fe 1-x Positive electrode active materials such as PO4 are regarded as promising. On the other hand, since such positive electrode active materials also have the characteristic of low conductivity, in order to ensure the battery characteristics in a lithium-ion secondary battery, various attempts have been made to obtain a positive electrode active material using various carbon sources.

[0003] For example, in Patent Document 1, granulated bodies for a positive electrode of a lithium secondary battery using fibrous carbon such as carbon nanotubes are disclosed, and attempts have been made to form a good conductive path to increase the battery capacity and the like. Further, in Patent Document 2, the discharge capacity and cycle characteristics in a lithium-ion battery are improved by an electrode active material using an ionic organic substance or the like as a carbon source. Furthermore, in Patent Document 3, a positive electrode active material for a secondary battery in which particles derived from vegetable proteins containing carbon or the like are supported is disclosed, and attempts have been made to exhibit excellent cycle characteristics in a lithium-ion secondary battery.

Prior Art Documents

Patent Documents

[0004] <00001​​​​​​​​​​​Japanese Patent Publication No. 2020-102321 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Incidentally, these carbon sources are carbonized through a calcination process to obtain a positive electrode active material on which carbon is supported as the final product. However, in this calcination process, it is not possible to completely avoid the volatilization of some of the added carbon source as CO2 or CO, or decomposition into other organic matter. As a result, the amount of carbon supported in the final product is less than the amount of carbon atoms equivalent to the carbon source added during the process. However, none of the aforementioned patent documents address these points, and there is still 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 that can efficiently utilize the added carbon source and effectively improve the battery properties of the lithium-ion secondary battery. [Means for solving the problem]

[0007] Therefore, the inventors conducted diligent research to solve the above problems and have found a method for producing a positive electrode active material for lithium-ion secondary batteries that effectively suppresses unwanted reactions while increasing the carbonization efficiency of the added carbon source by including a step of adding a carbonization accelerator.

[0008] In other words, the present invention relates to 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 satisfy 0 < a ≤ 1.2, 0 ≤ b ≤ 1.2, 0 ≤ c ≤ 1.2, 0 ≤ x ≤ 0.3, and b + c ≠ 0, and represent numbers that satisfy a + (valence of Mn) × b + (valence of Fe) × c + (valence of M) × x = 3.) A method for producing a positive electrode active material for a lithium ion secondary battery, which is represented by and has carbon supported thereon, comprising the following steps (I) to (IV): (I) A step of mixing a lithium compound, a metal compound containing at least a manganese compound and / or an iron compound, a phosphoric acid compound, and water to obtain slurry water a, and then subjecting it to a hydrothermal reaction to obtain preliminary particles x (II) A step of adding the obtained preliminary particles x, a conductive carbon material, and water to obtain slurry water b (III) A step of drying the obtained slurry water b to obtain particles Y (IV) A step of firing the obtained particles Y in an inert gas atmosphere comprising A method for producing a positive electrode active material for a lithium ion secondary battery, which includes a step of adding a carbonization accelerator in step (II) and / or step (IV).

Effect of the Invention

[0009] According to the production method of the present invention, volatilization such as CO2 and CO and decomposition into unnecessary organic substances induced through the firing step can be effectively suppressed, and while effectively increasing the carbonization efficiency of the added carbon source, a positive electrode active material for a lithium ion secondary battery can be obtained, which can greatly contribute to the realization of a lithium ion secondary battery that exhibits excellent battery physical properties.

Mode for Carrying Out the Invention

[0010] Hereinafter, the present invention will be described in detail. In the present invention, the "carbonization efficiency" means, in the positive electrode active material for a lithium ion secondary battery obtained by the production method of the present invention (hereinafter also referred to as "active material (A)"), the mass ratio (%) of the carbon carried on the active material (A) after the conductive carbon material added as a carbon source during the process is carbonized, based on the mass of the carbon atom equivalent of the conductive carbon material. Here, the carbon atom equivalent of the added conductive carbon material can be calculated from the added amount of the conductive carbon material, and the amount of carbon carried on the active material (A) can be determined by measurement using a carbon-sulfur analyzer. In addition, the preliminary particles x used in step (II) in the present invention are considered to be finally obtained in their entirety as the active material (A) through the present invention, and the mass of the active material (A) does not include the mass of the carbon carried thereon.

[0011] The production method of the positive electrode active material for a lithium ion secondary battery of the present invention is a production method for obtaining an active material (A) represented by the following formula (A) and carrying carbon. 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 satisfy 0 < a ≤ 1.2, 0 ≤ b ≤ 1.2, 0 ≤ c ≤ 1.2, 0 ≤ x ≤ 0.3, and b + c ≠ 0, and a + (valence of Mn) × b + (valence of Fe) × c + (valence of M) × x = 3.)

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

[0013] Examples of such active materials (A) include, for example, LiMnPO4, LiFePO4, and 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 Examples include PO4, among others, LiFePO4 and LiMn. 0.8 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, Li1.2 Mn 0.63 Fe 0.27 PO4 is preferred, LiMn 0.4 Fe 0.6 PO4, LiMn 0.7 Fe 0.3 PO 4、 Li 1.2 Mn 0.63 Fe 0.27 PO4 is preferable.

[0014] The active material (A) obtained by the present invention has carbon supported on it. This carbon is carbon obtained by carbonizing the conductive carbon material used in its manufacture. From the viewpoint of ensuring excellent battery properties in the resulting lithium-ion secondary battery, the amount of carbon supported is preferably 0.5 to 3 parts by mass, more preferably 0.9 to 2.5 parts by mass, and even more preferably 1.0 to 2.0 parts by mass, per 100 parts by mass of the active material (A).

[0015] The present invention relates to a method for producing a positive electrode active material for a lithium-ion secondary battery, which is represented by the above formula (A) and has carbon supported on it, and comprises the following steps (I) to (IV): (I) A step of mixing a lithium compound, a metal compound containing at least a manganese compound and / or an iron compound, a phosphate compound, and water to obtain slurry water a, and then subjecting it to a hydrothermal reaction to obtain preliminary particles x. (II) Steps to obtain slurry water b by adding the obtained preliminary particles x, conductive carbon material, and water. (III) Step of drying the obtained slurry water b to obtain particles Y. (IV) A step of calcining the obtained particles Y under an inert gas atmosphere. Equipped with, Step (II) and / or step (IV) include the step of adding a carbonization accelerator.

[0016] With the manufacturing method of the present invention, when the added conductive carbon material is carbonized through the firing process and supported as carbon on the active material (A), the carbonization accelerator acts effectively to effectively suppress the volatilization of CO2 and CO, as well as decomposition into unwanted organic matter during the process, and significantly increase the carbonization efficiency of the conductive carbon material.

[0017] Step (I) is a step in which a lithium compound, a metal compound containing at least a manganese compound and / or an iron compound, a phosphate compound, and water are mixed to obtain slurry water a, and then subjected to a hydrothermal reaction to obtain preliminary particles x.

[0018] Examples of lithium compounds that can be used include hydroxides (e.g., LiOH·H2O, LiOH), carbonates, sulfates, and acetates. Among these, hydroxides are preferred.

[0019] Examples of manganese compounds that can be used include one or more of the following: metal oxalate salts, metal sulfate salts, metal chlorides, and hydrates thereof. Among these, metal sulfate salts and their hydrates are preferred.

[0020] Examples of iron compounds that can be used include one or more of the following: metal oxalate salts, metal sulfate salts, metal chlorides, and hydrates thereof. Among these, metal sulfate salts and their hydrates are preferred. In addition, metal compounds other than these manganese and iron compounds (M: M is synonymous with M in formula (A)) may also be used.

[0021] Examples of usable phosphoric acid compounds include orthophosphoric acid (H3PO4, phosphoric acid), metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, ammonium phosphate, and ammonium hydrogen phosphate. Among these, phosphoric acid is preferred, and it is preferable to use it as an aqueous solution with a concentration of 70% to 90% by mass.

[0022] The slurry water a obtained by mixing a lithium compound, a metal compound containing at least a manganese compound and / or an iron compound, a phosphate compound, and water preferably contains 2.0 to 4.0 moles of lithium, more preferably 2.0 to 3.1 moles, per mole of phosphate, and each raw material may be added as appropriate to achieve these amounts.

[0023] Furthermore, nitrogen gas may be purged from the slurry water a after the addition of each of the above raw materials. When nitrogen gas is purged, the reaction can proceed with a reduced dissolved oxygen concentration in the slurry water a, effectively suppressing the oxidation of the metal compound in step (I) while forming trilithium phosphate (Li3PO4), a precursor of the active material (A), as fine dispersed particles.

[0024] Furthermore, it is preferable to pre-mix the obtained slurry water a before subjecting it to the subsequent hydrothermal reaction. The mixing time is preferably 0.25 to 24 hours, and more preferably 0.5 to 15 hours. In addition, it is preferable to mix by ultrasonic stirring.

[0025] Next, slurry water a is subjected to a hydrothermal reaction to obtain preliminary particles x of the active material (A). The hydrothermal reaction can take place at temperatures above 100°C, with 130°C to 200°C being preferred. The hydrothermal reaction is preferably carried out in a pressure vessel. When the reaction is carried out at 130°C to 200°C, the pressure is preferably 0.3 MPa to 1.6 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, and more preferably 0.2 hours to 24 hours.

[0026] The amount of water used when subjecting the reaction to the hydrothermal reaction is preferably 10 to 50 moles, and more preferably 12.5 to 45 moles, per mole of phosphate ions contained in slurry water a, from the viewpoint of solubility of the metal compound, ease of stirring, and efficiency of synthesis. The obtained preliminary particles x are isolated by filtering, washing with water, and drying. Freeze-drying and vacuum drying are used as drying methods.

[0027] Step (II) is a step in which the preliminary particles x obtained in step (I), conductive carbon material, and water are added to obtain slurry water b.

[0028] Examples of conductive carbon materials that can be used include, specifically, monosaccharides such as glucose, fructose, galactose, and mannose; disaccharides such as maltose, sucrose, and cellobiose; polysaccharides such as starch, dextrin, and cellulose; nanofibers of polysaccharides such as cellulose nanofibers, lignocellulose nanofibers, chitin nanofibers, and chitosan nanofibers; polyols and polyethers such as ethylene glycol, propylene glycol, diethylene glycol, polyethylene glycol, butanediol, propanediol, polyvinyl alcohol, and glycerin; and one or more organic acids such as citric acid, tartaric acid, and ascorbic acid. In particular, from the viewpoint of enhancing solubility and dispersibility in the solvent to enable effective function as a carbon material, fully enjoying the effects of the carbonization accelerator, and firmly supporting the carbon on the resulting active material to effectively suppress the elution of unwanted carbon, one or more selected from monosaccharides, polysaccharides, and polysaccharide nanofibers are preferred, one or more selected from glucose, cellulose, and cellulose nanofibers are more preferred, and it is even more preferable to include cellulose nanofibers as at least one conductive carbon material.

[0029] The amount of conductive carbon material added can be adjusted as appropriate so that the amount of carbon supported in the resulting active material (A) is the desired amount, while fully enjoying the effects of the carbonization accelerator. For every 100 parts by mass of pre-particles x, the amount is preferably 1.0 to 5.0 parts by mass, more preferably 1.5 to 4.0 parts by mass, and even more preferably 1.8 to 3.0 parts by mass in terms of carbon atoms.

[0030] Furthermore, step (II) may include a step of adding a carbonization accelerator. That is, in step (II), a carbonization accelerator may be further added to the slurry water b. Specifically, carbonization accelerators can be classified into water-soluble carbonization accelerators S1 and water-insoluble carbonization accelerators S2, depending on their solubility in water. In other words, the carbonization accelerators that can be used are one or more water-soluble carbonization accelerators S1 selected from lithium hydroxide, sodium hydroxide, and potassium hydroxide, which have a solubility in water at 20°C of 0.1% by mass or more (amount of solubility in a 100% by mass saturated aqueous solution at 20°C of 0.1% by mass or more); Examples of water-insoluble carbonization accelerators S2 include one or more selected from magnesium hydroxide, barium hydroxide, and manganese oxide, which have a solubility in water at 20°C of less than 0.1% by mass (amount dissolved in 100% by mass of saturated aqueous solution at 20°C of less than 0.1% by mass). Among these carbonization accelerators, water-soluble carbonization accelerator S1 is preferred, and lithium hydroxide is more preferred, from the viewpoint of fully enjoying the effects of the carbonization accelerator.

[0031] However, the carbonization accelerator that can be added in step (II) is a water-insoluble carbonization accelerator S2, which has lower solubility in water, from the viewpoint of dispersibility in slurry water b, etc. That is, step (II) may include a step of adding a water-insoluble carbonization accelerator S2 as a carbonization accelerator, but it does not include a step of adding a water-soluble carbonization accelerator S1.

[0032] The amount of water-insoluble carbonization accelerator S2 added is the total amount including the carbonization accelerator added in step (IV) described later, and is preferably 0.5 to 10 parts by mass, more preferably 1 to 8 parts by mass, and even more preferably 3 to 6 parts by mass, based on 100 parts by mass of carbon atoms of the added conductive carbon material present in slurry water b.

[0033] The solid content concentration of slurry water b is preferably 30% to 70% by mass, more preferably 35% to 65% by mass, and even more preferably 40% to 60% by mass.

[0034] The obtained slurry water b is preferably stirred before proceeding to step (III). The stirring time for the slurry water b is preferably 1 to 30 minutes, more preferably 5 to 20 minutes. The temperature of the slurry water b is preferably 10°C to 50°C, more preferably 15°C to 35°C.

[0035] Step (III) is the step of drying the slurry water b obtained in step (III) to obtain particles Y. Drying methods include spray drying, hot air drying, freeze drying, and vacuum drying. Among these, spray drying is preferred from the viewpoint of firmly supporting carbon on the resulting active material. In spray drying, the operating conditions can be set appropriately according to the equipment used. It is preferable to stir the slurry water b before spray drying. The stirring time for the slurry water b is preferably 3 to 60 minutes, and more preferably 5 to 30 minutes. The temperature of the slurry water b is preferably 10°C to 60°C, and more preferably 20°C to 40°C. Furthermore, when using spray drying, for example, with a micro-mist dryer equipped with four fluid nozzles (MDL-050M manufactured by Fujisaki Electric Co., Ltd.), the processing conditions are preferably such that the hot air temperature is 110°C to 300°C, and more preferably 150°C to 250°C. In addition, the volume ratio of the hot air supply amount to the slurry water supply amount (hot air supply amount / slurry water supply amount) is preferably 500 to 10000, and more preferably 1000 to 9000.

[0036] Step (IV) is a process in which the particles Y obtained in step (III) are calcined in an inert gas atmosphere. Step (IV) may also include a step of adding a carbonization accelerator, in which case the carbonization accelerator should be added to the particles Y before firing. The carbonization accelerator that can be used here may be a water-soluble carbonization accelerator S1, a water-insoluble carbonization accelerator S2, or both a water-soluble carbonization accelerator S1 and a water-insoluble carbonization accelerator S2 may be added. However, if step (II) does not include the step of adding a carbonization accelerator (water-insoluble carbonization accelerator S2), then step (IV) will include the step of adding a carbonization accelerator (water-soluble carbonization accelerator S1 and / or water-insoluble carbonization accelerator S2). In other words, the manufacturing method of the present invention includes a step of adding a carbonization accelerator in either step (II) or step (IV), or both. In this case, if a carbonization accelerator is added to the dried particles Y, it is preferable to mix the particles Y and the carbonization accelerator beforehand before firing. For such mixing, a rotary container type mixer, agitator type mixer, airflow type mixer, dry ball mill mixer, etc., can be used.

[0037] The amount of carbonization accelerator added is the total amount of the carbonization accelerator added in step (II) above, and is preferably 0.5 to 20 parts by mass, more preferably 1 to 18 parts by mass, and even more preferably 3 to 15 parts by mass, per 100 parts by mass of carbon atoms of the conductive carbon material added in step (II).

[0038] Thus, in the manufacturing method of the present invention, the carbon accelerator added in step (II) and / or step (IV) acts effectively to suppress unwanted reactions, while the conductive carbon material is efficiently carbonized, resulting in an active material (A) supported on the surface of particle Y with high carbonization efficiency.

[0039] The firing conditions in step (IV) are preferably in a reducing atmosphere or an inert atmosphere, the firing temperature is preferably 500°C to 1000°C, more preferably 550°C to 900°C, and the firing time is preferably 0.5 hours to 12 hours, more preferably 1 hour to 6 hours.

[0040] The active material (A) obtained by the manufacturing method of the present invention can be used to construct a lithium-ion secondary battery according to conventional methods. Specifically, for example, the active material (A) is kneaded with acetylene black, Ketjenblack, polyvinylidene fluoride, N-methyl-2-pyrrolidone, etc. to prepare a positive electrode slurry, which is then coated onto a current collector and subsequently press-molded to produce a positive electrode. A lithium-ion secondary battery to which a positive electrode obtained using the active material (A) can be applied is not particularly limited as long as it has a positive electrode, a negative electrode, an electrolyte, and a separator, or a positive electrode, a negative electrode, and a solid electrolyte as essential components.

[0041] Here, the negative electrode is not particularly limited in its material composition as long as it can absorb lithium ions during charging and release them during discharge; known material compositions can be used. For example, lithium metal, graphite, silicon-based materials (Si, SiOx), lithium titanate, or amorphous carbon materials can be used. It is preferable to use an electrode formed of an intercalate material capable of electrochemically absorbing and releasing lithium ions, especially a carbon material. Furthermore, two or more of the above negative electrode materials may be used in combination; for example, a combination of graphite and silicon-based materials can be used.

[0042] The electrolyte is prepared by dissolving a support salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent commonly used in the electrolyte of lithium-ion secondary batteries. For example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, oxolane compounds, etc., can be used.

[0043] The supporting salt is not particularly limited in type, but is preferably at least one of the following: an inorganic salt selected from LiPF6, LiBF4, LiClO4, and LiAsF6; a derivative of the inorganic salt; an organic salt selected from LiSO3CF3, LiC(SO3CF3)2, LiN(SO3CF3)2, LiN(SO2C2F5)2, and LiN(SO2CF3)(SO2C4F9); and a derivative of the organic salt.

[0044] The separator serves to electrically insulate the positive and negative electrodes and to hold the electrolyte. For example, a porous synthetic resin membrane, particularly a porous membrane made of polyolefin polymers (polyethylene, polypropylene), can be used.

[0045] Solid electrolytes electrically insulate the positive and negative electrodes and exhibit high lithium-ion conductivity. For example, La 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 PO 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 Use S4.

[0046] The shape of the lithium-ion secondary battery having the above configuration is not particularly limited and may be various shapes such as coin-shaped, cylindrical, or prismatic, or it may be an irregular shape enclosed in a laminate casing. [Examples]

[0047] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples. The measurements and evaluations were carried out according to the following methods. The results are shown in Table 1.

[0048] [Example 1] Slurry water a1 was obtained by mixing 1272 g of LiOH·H2O with 4 L of water. Next, while stirring the obtained slurry water a1 at 25°C for 3 minutes, 1153 g of 85% phosphoric acid aqueous solution was added dropwise at a rate of 35 mL / min, and the mixture was stirred at a speed of 400 rpm for 12 hours to obtain slurry water a2 containing Li3PO4. The obtained slurry water a2 was purged with nitrogen to adjust the dissolved oxygen concentration of slurry water a2 to 0.5 mg / L. Then, 1688 g of MnSO4·5H2O and 834 g of FeSO4·7H2O were added to the total volume of slurry water a2 to obtain slurry water a3. The molar ratio (manganese compound:iron compound) of the added MnSO4 and FeSO4 was 70:30.

[0049] Next, the obtained slurry water a3 was placed in an autoclave and a hydrothermal reaction was carried out at 170°C for 1 hour. The pressure inside the autoclave was 0.8 MPa. After the hydrothermal reaction, the resulting crystals were filtered and then washed with 12 parts by mass of water per 1 part by mass of crystals. The washed crystals were freeze-dried at -50°C for 12 hours to obtain preliminary particles x1. 1000g of the obtained preliminary particles x1 was taken out, and 1L of water and 225g of cellulose nanofiber (FD200L, manufactured by Daicel Mirise, solid content concentration 20% by weight) as a conductive carbon material (2.0 parts by mass in terms of carbon atoms per 100 parts by mass of preliminary particles x1) were added and mixed to obtain slurry water a5. The obtained slurry water a5 was dispersed in an ultrasonic stirrer (T25, manufactured by IKA) for 1 minute to uniformly color the entire mixture, and then spray dried using a spray drying device (MDL-050M, manufactured by Fujisaki Electric Co., Ltd.) to obtain granules Y1. The hot air temperature during spray drying was set to 200°C, and the ratio of the hot air supply amount to the slurry water supply amount (hot air supply amount / slurry water supply amount) was set to 2500.

[0050] To the obtained granules Y1, 0.12 g of LiOH (0.6 parts by mass per 100 parts by mass of carbon atoms in cellulose nanofibers) was added as a carbonization accelerator and mixed. Then, under an argon hydrogen atmosphere (hydrogen concentration 3%), it was calcined at 700°C for 1 hour to produce an active material (A) (LiMn) on which carbon was supported. 0.7 Fe 0.3 We obtained PO4 (average particle size 20 μm). The obtained active material (A) was measured using a carbon-sulfur analyzer (EMIA-220V2, manufactured by Horiba, Ltd.) to determine the amount of carbon (parts by mass) per 100 parts by mass of active material (A).

[0051] Using the obtained measurement values, the carbonization efficiency (%) was calculated according to the following formula α. Carbonization efficiency (%) = {(Amount of carbon supported per 100 parts by mass of active material (A) (parts by mass)) / (Amount of carbon atoms in the conductive carbon material relative to 100 parts by mass of spare particle x1) Addition amount (parts by mass))}×100...(α)

[0052] [Example 2] Active material (A) was obtained in the same manner as in Example 1, except that 0.40 g of LiOH (2.0 parts by mass per 100 parts by mass of cellulose nanofiber carbon atoms) was added to the obtained granule Y1 as a carbonization accelerator. The obtained active material (A) was measured in the same manner as in Example 1 to determine the amount of carbon supported per 100 parts by mass (parts by mass) of active material (A), and the carbonization efficiency (%).

[0053] [Example 3] Active material (A) was obtained in the same manner as in Example 1, except that 1.00 g of LiOH (5.0 parts by mass per 100 parts by mass of cellulose nanofiber carbon atoms) was added to the obtained granule Y1 as a carbonization accelerator. The obtained active material (A) was measured in the same manner as in Example 1 to determine the amount of carbon supported per 100 parts by mass (parts by mass) of active material (A), and the carbonization efficiency (%).

[0054] [Example 4] Active material (A) was obtained in the same manner as in Example 1, except that 1.80 g of LiOH (9.0 parts by mass per 100 parts by mass of cellulose nanofiber carbon atoms) was added to the obtained granule Y1 as a carbonization accelerator. The obtained active material (A) was measured in the same manner as in Example 1 to determine the amount of carbon supported per 100 parts by mass (parts by mass) of active material (A), and the carbonization efficiency (%).

[0055] [Example 5] Active material (A) was obtained in the same manner as in Example 1, except that 3.00 g of LiOH (15.0 parts by mass per 100 parts by mass of carbon atoms in cellulose nanofibers) was added to the obtained granule Y1 as a carbonization accelerator. The obtained active material (A) was measured in the same manner as in Example 1 to determine the amount of carbon supported per 100 parts by mass (parts by mass) of active material (A), and the carbonization efficiency (%).

[0056] [Example 6] Active material (A) was obtained in the same manner as in Example 1, except that 21.00 g of Mg(OH) (5.0 parts by mass per 100 parts by mass of carbon atoms in cellulose nanofibers) was added to the obtained granule Y1 as a carbonization accelerator instead of LiOH. The obtained active material (A) was measured in the same manner as in Example 1 to determine the amount of carbon supported per 100 parts by mass (parts by mass) of active material (A), and the carbonization efficiency (%).

[0057] [Example 7] Active material (A) was obtained in the same manner as in Example 1, except that 1.00 g of MnO2 (5.0 parts by mass per 100 parts by mass of carbon atoms in cellulose nanofibers) was added to the obtained granule Y1 instead of LiOH as a carbonization accelerator. The obtained active material (A) was measured in the same manner as in Example 1 to determine the amount of carbon supported per 100 parts by mass (parts by mass) of active material (A), and the carbonization efficiency (%).

[0058] [Comparative Example 1] Active material (A) was obtained in the same manner as in Example 1, except that a carbonization accelerator was not added to the obtained granule Y1. The obtained active material (A) was measured in the same manner as in Example 1 to determine the amount of carbon supported per 100 parts by mass (parts by mass) of active material (A), and the carbonization efficiency (%).

[0059] Evaluation of battery characteristics (rate characteristics) Each of the obtained active materials was used as a positive electrode material to fabricate a positive electrode for a lithium-ion secondary battery. Specifically, each of the obtained active materials, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 90:5:5, and N-methyl-2-pyrrolidone was added and thoroughly kneaded to prepare a positive electrode slurry. The positive electrode slurry was applied to a current collector made of 20 μm thick aluminum foil using a coating machine and vacuum dried at 80°C for 12 hours. After that, it was punched out into a φ14 mm disc shape and pressed with a hand press at 20 kN for 2 minutes to form the positive electrode.

[0060] Next, a coin-type secondary battery was constructed using the above-mentioned positive electrode. A lithium foil stamped to a diameter of φ15 mm was used as the negative electrode. For the electrolyte, a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7 was used, in which LiPF6 was dissolved at a concentration of 1 mol / L. A porous polymer film was used as the separator. These battery components were assembled and housed in an atmosphere with a dew point of -50°C or lower using a conventional method to obtain a coin-type secondary battery (CR-2032).

[0061] Next, using the obtained coin-type secondary battery, the discharge capacity (mAh / g) at 0.2C and 3C in a 30°C environment was determined using a discharge capacity measuring device (HJ-1001SD8, manufactured by Hokuto Denko Co., Ltd.), and the rate characteristic value (discharge capacity ratio (%)) was calculated using the following formula (β). Discharge capacity ratio (%) = (Discharge capacity at 3C) / (Discharge capacity at 0.2C) × 100···(β)

[0062] [Table 1]

Claims

1. Formula (A) below: Li ａ Mn ｂ Fe ｃ M ｘ 2O ４ ・・・(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 a + (valence of Mn) × b + (valence of Fe) × c + (valence of M) × x = 3.) A method for producing a positive electrode active material for a lithium-ion secondary battery, which is represented by and has carbon supported, comprising the following steps (I) to (IV): (I) A step of mixing a lithium compound, a metal compound containing at least a manganese compound and / or an iron compound, a phosphate compound, and water to obtain slurry water a, and then subjecting it to a hydrothermal reaction to obtain preliminary particles x. (II) Step of adding the obtained preliminary particles x, conductive carbon material, and water to obtain slurry water b. (III) A step of drying the obtained slurry water b to obtain particles Y. (IV) A step of calcining the obtained particles Y under an inert gas atmosphere. Equipped with, A method for producing a positive electrode active material for a lithium-ion secondary battery, comprising the step of adding a carbonization accelerator in step (II) and / or step (IV).

2. A method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 1, wherein the carbonization accelerator is one or more water-soluble carbonization accelerators S1 selected from lithium hydroxide, sodium hydroxide, and potassium hydroxide, or one or more water-insoluble carbonization accelerators S2 selected from magnesium hydroxide, barium hydroxide, and manganese oxide.

3. When the carbonization accelerator is a water-soluble carbonization accelerator S1, Regarding step (II), the step of adding the water-soluble carbonization accelerator S1 is excluded, and The method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 2, wherein step (IV) is a step of adding a water-soluble carbonization accelerator S1 to the obtained particles Y and then firing them.

4. When the carbonization accelerator is a water-insoluble carbonization accelerator S2, The method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 2 or 3, wherein step (II) is further a step of adding a water-insoluble carbonization accelerator S2 to obtain slurry water b.

5. A method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 3, wherein the amount of carbonization accelerator added is 0.5 to 20 parts by mass per 100 parts by mass of the amount of carbon atoms in the conductive carbon material added in step (II).

6. A method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 4, wherein the amount of carbonization accelerator added is 0.5 to 20 parts by mass per 100 parts by mass of the amount of carbon atoms in the conductive carbon material added in step (II).

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