Positive electrode active material particles for lithium ion secondary batteries and method for producing same

Abstract

To provide positive electrode active material particles for a lithium ion secondary battery and a manufacturing method of the same, capable of effectively improving the cycle characteristics of the lithium ion secondary battery.SOLUTION: In positive electrode active material particles for a lithium ion secondary battery, the surface of lithium-based polyanion particles X represented by LiaMnbFecMxPO4 is coated with carbon, (M indicates 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 satisfies 0<a≤1.2, 0≤b≤1.2, 0≤c≤1.2, 0≤x≤0.3, and b+c≠0, and indicates a number that satisfies a+(Mn valence)×b+(Fe valence)×c+(M valence)×x=3), and the surface of the lithium-based polyanion particles X has a carbon coating rate of 5% or more and less than 70%, and the maximum thickness of the carbon coating of 5 nm or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a positive electrode active material particle for a lithium ion secondary battery capable of effectively improving the cycle characteristics of the lithium ion secondary battery, and a method for manufacturing the same.

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, so-called lithium-based polyanion particles such as LiMn x Fe 1-x PO 4 are regarded as promising. On the other hand, such lithium-based polyanion particles have low conductivity and still need improvement in order to sufficiently enhance the battery characteristics in the obtained lithium ion secondary battery. Therefore, various developments have been made conventionally.

[0003] For example, Patent Document 1 discloses an electrode material for a lithium ion battery comprising agglomerated particles in which the surfaces of particles such as LiMn x Fe 1-x PO 4 are coated with a carbonaceous film, and the coating rate of the surface of the primary particles with the carbonaceous film is 80% or more, aiming to improve the discharge capacity and output characteristics. Further, Patent Document 2 discloses an electrode material for a lithium ion secondary battery comprising inorganic particles represented by LiFe x Mn 1-x-y M y PO 4 and a carbonaceous film covering 50% or more of the surface of the inorganic particles, and having a peak of a specific micropore size distribution, and attempts have been made to increase the charge-discharge capacity and activation energy.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, as for the lithium-based polyanion particles having a carbonaceous film as described in the above patent documents, since carbon present on the surface of such particles may react with the electrolyte during charge and discharge to generate by-products, it has been found by the present inventors that it can be a factor that hinders the improvement of cycle characteristics.

[0006] Therefore, an object of the present invention is to provide a positive electrode active material particle for a lithium ion secondary battery and a method for producing the same, which can effectively improve the cycle characteristics of the lithium ion secondary battery.

Means for Solving the Problems

[0007] Then, as a result of intensive studies to solve the above problems, the present inventor has found that by coating carbon on the surface of lithium-based polyanion particles under specific conditions, positive electrode active material particles for a lithium ion secondary battery that can exhibit excellent cycle characteristics can be obtained in the resulting lithium ion secondary battery.

[0008] That is, the present invention provides the following formula (X): Li a Mn b Fe c M x PO 4 ···(X) (In formula (X), 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. ) The surface of the lithium-based polyanion particle X represented by the formula (I) is coated with carbon, The present invention provides positive electrode active material particles for lithium ion secondary batteries, in which the carbon coverage on the surface of lithium-based polyanion particles X is 5% or more and less than 70%, and the maximum thickness of the carbon coating is 5 nm or more.

[0009] The present invention also relates to a method for producing ... polymerizable compound comprising the steps of (I) to (IV): (I) A step of adding a lithium compound, a metal compound including at least a manganese compound and / or an iron compound, a phosphate compound, and water to obtain slurry water a, and then subjecting the slurry water to a hydrothermal reaction to obtain preliminary particles x of lithium-based polyanion particles X. (II) A step of adding the obtained preliminary particles x of the lithium-based polyanion particles X, a carbon coating agent B, a carbon coating inhibitor C, and water to obtain a slurry water b. (III) A step of subjecting the obtained slurry water b to spray drying to obtain granules Y. (IV) A step of firing the obtained granules Y Equipped with The carbon coating agent B is one or more carbon materials selected from saccharides, and The present invention provides a method for producing positive electrode active material particles for a lithium ion secondary battery, wherein the carbon coating inhibitor C is one or more carbon materials selected from polyols other than saccharides, amines, and amides. Effect of the Invention

[0010] According to the positive electrode active material particles for a lithium ion secondary battery of the present invention, a lithium ion secondary battery having effectively improved cycle characteristics can be realized. [Brief description of the drawings]

[0011] [Figure 1] 1 is a TEM photograph of the surface of a particle of a positive electrode active material for a lithium ion secondary battery obtained in Example 2. xn represents the circumferential length of the surface of a particle X that is not coated with carbon, and xc represents the circumferential length of the surface of a particle X that is coated with carbon. [Diagram 2]This is a TEM photograph of the surface of the positive electrode active material particles for a lithium ion secondary battery obtained in Comparative Example 2. xn represents the perimeter of the surface of particle X not coated with carbon, and xc represents the perimeter of the surface of particle X coated with carbon.

Embodiments for Carrying Out the Invention

[0012] Hereinafter, the present invention will be described in detail. The positive electrode active material particles for a lithium ion secondary battery of the present invention have the following formula (X): Li a Mn b Fe c M x PO 4 ···(X) (In formula (X), 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.) The surface of the lithium-based polyanion particle X represented by the formula is coated with carbon, On the surface of the lithium-based polyanion particle X, the carbon coating rate is 5% or more and less than 70%, and the maximum thickness of the carbon coating is 5 nm or more.

[0013] That is, the positive electrode active material particles for a lithium ion secondary battery of the present invention are particles composed of particle X in which carbon is unevenly distributed and coated on the surface of the lithium-based polyanion particle X (hereinafter also referred to as "particle X") that constitutes such particles, while showing a value in a limited range as low as 5% or more and less than 70% of the carbon coating rate on the surface, and showing a high value of 5 nm or more for the maximum thickness of the carbon coating in particle X. If such positive electrode active material particles for a lithium ion secondary battery are used as a positive electrode material, during charge and discharge of the resulting lithium ion secondary battery, generation of by-products due to the reaction between the electrolytic solution and carbon present on the particle surface can be effectively suppressed, and the cycle characteristics can be effectively improved.

[0014] The “lithium-based polyanion particles X (particles X)” are particles (secondary particles) formed by aggregation of precursor particles x of lithium-based polyanion particles X (hereinafter also referred to as “precursor particles x”) corresponding to so-called primary particles of particles X. Such precursor particles x are particles obtained by step (I) in the method for producing positive electrode active material particles for a lithium ion secondary battery of the present invention described below. Therefore, the “surface of particles X” corresponds to the outermost surface of precursor particles x, excluding the surface where precursor particles x are joined or bonded to each other by aggregation existing inside particles X among the surfaces of precursor particles x, regardless of the presence or absence of carbon coating.

[0015] Particles X constituting the positive electrode active material particles for a lithium ion secondary battery of the present invention are represented by the following formula (X): Li a Mn b Fe c M x PO 4 ···(X) (In formula (X), 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.) It is represented by.

[0016] The particle X represented by the above formula (X) is an olivine-type lithium transition metal phosphate compound containing manganese (Mn) or iron (Fe) as at least a so-called transition metal. In formula (X), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd, and from the viewpoint of improving cycle characteristics, Mg, Al, Ti, Zn, Co, Sr or Zr is preferable. In addition, a, b, c, and x in the above formula (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. As for the lithium-based polyanion particles represented by the above formula (X), from the viewpoint of improving cycle characteristics, for a, 0.6 ≤ a ≤ 1.2 is preferable, 0.65 ≤ a ≤ 1.15 is more preferable, and 0.7 ≤ a ≤ 1.1 is even more preferable. For b, 0.4 ≤ b ≤ 0.8 is preferable. For c, 0.2 ≤ c ≤ 0.6 is preferable. For x, 0 ≤ z ≤ 0.2 is preferable, 0 ≤ z ≤ 0.15 is more preferable, and 0 ≤ x ≤ 0.1 is even more preferable.

[0017] Specifically, for example, LiMnPO 4 、LiMn 0.3 Fe 0.7 PO 4 、LiMn 0.4 Fe 0.6 PO 4 、LiMn 0.45 Fe 0.55 PO 4 、LiMn 0.7 Fe 0.3 PO 4 、LiMn 0.9 Fe 0.1 PO 4 、LiMn 0.8 Fe 0.2 PO 4 、LiMn 0.75 Fe 0.15 Mg 0.1 PO 4 、LiMn 0.75 Fe 0.19 Zr 0.03 PO 4, LiMn 0.6 Fe 0.4 PO 4 , LiMn 0.5 Fe 0.5 PO 4 , Li 1.2 Mn 0.63 Fe 0.27 PO 4 , Li 0.6 Mn 0.84 Fe 0.36 PO 4 Among them, LiMn 0.4 Fe 0.6 PO 4 , LiMn 0.45 Fe 0.55 PO 4 , LiMn 0.7 Fe 0.3 PO 4 , LiMn 0.8 Fe 0.2 PO 4 , LiMn 0.6 Fe 0.4 PO 4 , Li 1.2 Mn 0.63 Fe 0.27 PO 4 is preferred.

[0018] The particle X represented by the above formula (X) has a surface coated with carbon. The carbon is formed by carbonizing a carbon coating agent B, which is one or more carbon materials selected from saccharides, to carbon, which coats the surface of the particle X. The carbon coating agent B is one or more carbon materials selected from saccharides. Specific examples of the carbon coating agent B include one or more selected from monosaccharides such as glucose, fructose, galactose, mannose, etc.; disaccharides such as maltose, sucrose, cellobiose, etc.; polysaccharides such as starch, dextrin, cellulose, etc.; and polysaccharide nanofibers such as cellulose nanofibers, lignocellulose nanofibers, chitin nanofibers, chitosan nanofibers, etc. Among these, cellulose nanofibers, lignocellulose nanofibers, chitin nanofibers, and chitosan nanofibers are preferred from the viewpoint of effectively suppressing the deterioration of the electronic conductive path and contributing to improving the cycle characteristics of the resulting battery.

[0019] The carbon coverage rate on the surface of the particle X is 5% or more and less than 70%, preferably 5% or more and less than 50%, more preferably 10% to 48%, and even more preferably 15% to 45%, from the viewpoint of effectively improving cycle characteristics by unevenly distributing carbon on the surface of the particle X while controlling the value within a low, limited range.

[0020] The "carbon coverage (%)" on the surface of particle X means a value determined by the following method. Specifically, as shown in FIG. 1, first, the surface of particle X is observed in one field of view of a positive electrode active material particle for a lithium ion secondary battery by TEM electron microscopy, and the surface of particle X not coated with carbon and the surface of particle X coated with carbon are identified. Next, the "perimeter xn of the surface of particle X not coated with carbon" and the "perimeter xc of the surface of particle X coated with carbon" in the field of view are measured, and the sum of xn and xc is determined as the "total perimeter xA of the surface of particle X." The obtained values ​​of "total circumference xA of the surface of particle X" and "circumference xc of the surface of particle X coated with carbon" are inserted into the following formula (1) to calculate the carbon coverage (%) in one visual field, and the values ​​obtained in 50 visual fields are averaged to obtain the carbon coverage (%) on the surface of particle X. Carbon coverage (%) = [(perimeter xc of the surface of the carbon-coated particle X) / (Total perimeter of the surface of particle X xA)] × 100 (1)

[0021] The maximum thickness of the carbon coating on the surface of the particle X is 5 nm or more, preferably 8 nm or more, more preferably 10 nm or more, and even more preferably 15 nm or more, from the viewpoint of effectively suppressing the generation of by-products during charging and discharging of the lithium ion secondary battery and effectively improving the cycle characteristics by controlling the carbon coating rate to a low, limited range and distributing carbon unevenly on the surface of the particle X. The upper limit is not particularly limited, but is preferably 100 nm or less.

[0022] The "maximum thickness of carbon coating" on the surface of particle X means a value obtained by the following method. Specifically, first, the surface of particle X is observed in one visual field of a lithium ion secondary battery positive electrode active material particle by TEM electron microscope observation, and a point P on the surface of the lithium ion secondary battery positive electrode active material particle that is recognized to be the thickest carbon coating in the visual field is identified. Next, a straight line perpendicular to the tangent line including point P is drawn, and an intersection Q where this line intersects with the surface of particle X is identified, and the distance between point P and intersection Q is measured as the carbon coating thickness (nm) in one visual field, and the values ​​obtained in 50 visual fields are averaged to obtain the maximum thickness (nm) of the carbon coating on the surface of particle X. A carbon coating thickness of less than 1 nm is the measurement limit in TEM electron microscope observation.

[0023] The carbon content in the positive electrode active material particles for lithium ion secondary batteries of the present invention corresponds to the content of carbon formed by carbonizing the carbon coating agent B, and is preferably 0.5% by mass to 5.0% by mass, more preferably 0.6% by mass to 5.0% by mass, and even more preferably 0.7% by mass to 5.0% by mass in the positive electrode active material particles for lithium ion secondary batteries of the present invention.

[0024] The carbon contained in the positive electrode active material particles for a lithium ion secondary battery corresponds to the carbon formed by carbonizing the carbon coating agent B present on the surface of the particle X, i.e., the amount of carbon coating agent B converted into atoms, and can be determined by measurement using a carbon / sulfur analyzer.

[0025] The BET specific surface area of ​​the positive electrode active material particles for a lithium ion secondary battery of the present invention is preferably 15.0 m from the viewpoint of contributing to improvement of cycle characteristics. 2 / g~25.0m 2 / g, more preferably 17.0m 2 / g~22.0m 2 / g, and more preferably 18.0m 2 / g~20.0m 2 / g.

[0026] The conductivity of the positive electrode active material particles for a lithium ion secondary battery of the present invention is preferably 1.0×10 -07 S / cm or more, more preferably 5.0×10 -07 S / cm~1.0×10 -02 S / cm, and more preferably 1.0×10 -06 S / cm~1.0×10 -02 S / cm.

[0027] The positive electrode active material particles for a lithium ion secondary battery of the present invention are prepared by the following steps (I) to (IV): (I) A step of adding a lithium compound, a metal compound including at least a manganese compound and / or an iron compound, a phosphate compound, and water to obtain slurry water a, and then subjecting the slurry to a hydrothermal reaction to obtain preliminary particles x of lithium-based polyanion particles X. (II) A step of adding the obtained preliminary particles x of the lithium-based polyanion particles X, a carbon coating agent B, a carbon coating inhibitor C, and water to obtain a slurry water b. (III) A step of subjecting the obtained slurry water b to spray drying to obtain granules Y. (IV) A step of firing the obtained granules Y Equipped with The carbon coating agent B is one or more carbon materials selected from saccharides, and The carbon-coated inhibitor C can be obtained by a production method in which one or more carbon materials selected from polyols other than saccharides, amines, and amides.

[0028] The step (I) of the production method of the present invention is a step of obtaining a slurry water a by adding a lithium compound, a metal compound containing at least a manganese compound and / or an iron compound, a phosphate compound, and water, and then subjecting the slurry water a to a hydrothermal reaction to obtain preliminary particles x of the lithium-based polyanion particles X.

[0029] The lithium compounds that can be used include hydroxides (e.g., LiOH·H 2 O, LiOH), carbonates, sulfates, and acetates. Of these, hydroxides are preferred. Examples of manganese compounds that can be used include manganese acetate, manganese nitrate, manganese sulfate, etc. These may be used alone or in combination of two or more. Among these, manganese sulfate is preferred from the viewpoint of improving battery characteristics. Examples of the iron compound that can be used include iron acetate, iron nitrate, iron sulfate, etc. These may be used alone or in combination of two or more. Among them, iron sulfate is preferred from the viewpoint of improving the battery characteristics. In addition to these manganese compounds and iron compounds, compounds of metals other than manganese compounds and iron compounds (M: M has the same meaning as M in formula (X)) may be used. The phosphoric acid compounds that can be used include orthophosphoric acid (H 3 PO 4 Examples of suitable phosphoric acid include phosphoric acid, metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, ammonium phosphate, ammonium hydrogen phosphate, etc. Among these, it is preferable to use phosphoric acid, and it is preferable to use it as an aqueous solution having a concentration of 70% by mass to 90% by mass.

[0030] More specifically, the step (I) includes a step (i-1) of mixing a phosphoric acid compound with a slurry water a' containing a lithium compound to obtain a precursor A'; A step (i-2) of subjecting the obtained precursor A' and a slurry water a containing at least a metal compound including a manganese compound or an iron compound to a hydrothermal reaction to obtain preliminary particles x of the lithium-based polyanion particles X. It is preferable to provide:

[0031] In the step (i-1), the content of the lithium compound in the slurry water a' is preferably 5 parts by mass to 50 parts by mass, and more preferably 7 parts by mass to 45 parts by mass, relative to 100 parts by mass of water. It is preferable to stir the slurry water a' before adding the phosphoric acid compound to the slurry water a'. The stirring time of the slurry water a' is preferably 1 to 15 minutes, more preferably 3 to 10 minutes. The temperature of the slurry water a' is preferably 20°C to 90°C, more preferably 20°C to 70°C.

[0032] In the step (I), when mixing the phosphoric acid with the slurry water a', it is preferable to drop the phosphoric acid while stirring the slurry water. The dropping speed of the phosphoric acid into the slurry water a' is preferably 15 mL / min to 50 mL / min, more preferably 20 mL / min to 45 mL / min, and even more preferably 28 mL / min to 40 mL / min. The stirring time of the slurry water a' while dropping the phosphoric acid is preferably 0.5 hours to 24 hours, and more preferably 3 hours to 12 hours. The stirring speed of the slurry water a' while dropping the phosphoric acid is preferably 200 rpm to 700 rpm, more preferably 250 rpm to 600 rpm, and even more preferably 300 rpm to 500 rpm. When the slurry water a' is stirred, it is preferable to cool the slurry water a' to a temperature equal to or lower than the boiling point of the slurry water a'. Specifically, it is preferable to cool the slurry water a' to a temperature of 80°C or lower, and more preferably to a temperature of 20°C to 60°C.

[0033] The slurry water a' after mixing with the phosphoric acid compound preferably contains 2.0 to 4.0 moles of lithium, more preferably 2.0 to 3.1 moles, per mole of phosphoric acid, and the lithium compound and phosphoric acid compound may be used in such amounts. More specifically, the slurry water a' after mixing with the phosphoric acid compound preferably contains 2.7 to 3.3 moles of lithium, more preferably 2.8 to 3.1 moles, per mole of phosphoric acid.

[0034] By purging nitrogen into the slurry water a' after mixing with the phosphoric acid compound, the reaction in the slurry water is completed, and precursor A' of the preliminary particles x that constitute the particles represented by (A) above is obtained as a slurry. When nitrogen is purged, the reaction can be allowed to proceed in a state where the dissolved oxygen concentration in the slurry water a' is reduced, and the dissolved oxygen concentration of the resulting slurry water containing precursor A' is also effectively reduced, so that oxidation of the metal compound to be added in the next step can be suppressed. In the slurry water a' containing precursor A', the precursor of the particles represented by (A) above exists as fine dispersed particles. Precursor A' is a mixture of lithium triphosphate (Li 3 PO 4 ) is obtained.

[0035] Next, in step (i-2), the precursor A' obtained in step (i-1) and the slurry water a containing metal compounds including at least a manganese compound and an iron compound are subjected to a hydrothermal reaction to obtain preliminary particles x of the lithium-based polyanion particles X.

[0036] The molar ratio of the manganese compound to the iron compound (manganese compound:iron compound) used is preferably 3:7 to 9:1, more preferably 4:6 to 8:2, and further preferably 5:5 to 7:3. The total amount of these metal compounds added is preferably 0.99 mol to 1.01 mol, and more preferably 0.995 mol to 1.005 mol, per mol of phosphate ions contained in the slurry water A.

[0037] The amount of water used in the hydrothermal reaction is preferably 10 mol to 50 mol, and more preferably 12.5 mol to 45 mol, per mol of phosphate ions contained in the slurry water a, from the viewpoints of the solubility of the metal compound, ease of stirring, synthesis efficiency, and the like.

[0038] The order of addition of the manganese compound, iron compound, and metal (M) compound is not particularly limited. In addition to adding these metal compounds, an antioxidant may be added as necessary. Examples of such antioxidants include sodium sulfite (Na2 SO 3 ), Sodium hydrosulfite (Na 2 S 2 O 4 The amount of the antioxidant added is preferably 0.01 mol to 1 mol, and more preferably 0.03 mol to 0.5 mol, per 1 mol of the total of the manganese compound, the iron compound, and the metal (M) compound used as needed.

[0039] The content of precursor A' in the slurry water a obtained by adding a manganese compound and / or an iron compound, and optionally a metal compound including a metal (M) compound, and optionally an antioxidant, etc., is preferably 10 mass% to 50 mass%, more preferably 15 mass% to 45 mass%, and even more preferably 20 mass% to 40 mass%.

[0040] The hydrothermal reaction may be carried out at 100° C. or higher, preferably at 130° C. to 180° C. The hydrothermal reaction is preferably carried out in a pressure-resistant vessel, and 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. The obtained preliminary particles x are isolated by filtering, washing with water, and drying, and the drying method may be freeze drying or vacuum drying.

[0041] The step (II) of the production method of the present invention is a step of obtaining a slurry water b by adding the preliminary particles x of the lithium-based polyanion particles X obtained in the step (I), the carbon coating agent B, the carbon coating inhibitor C, and water. As described above, the carbon coating agent B is one or more carbon materials selected from saccharides, and specifically, the same materials as those described above can be used. Among them, it is preferable to use cellulose nanofibers, lignocellulose nanofibers, chitin nanofibers, or chitosan nanofibers, from the viewpoint of effectively suppressing the deterioration of the electronic conductive path and contributing to improving the cycle characteristics of the resulting battery.

[0042] The carbon coating inhibitor C is one or more carbon materials selected from polyols other than sugars, amines, and amides, i.e., one or more carbon materials selected from polyols, amines, and amides other than carbon coating agent B. In this way, by adding the carbon coating inhibitor C together with the carbon coating agent B in step (II), as the subsequent steps (III) to (IV) proceed, the carbon coating inhibitor C coats a part of the surface of the particle X while appropriately inhibiting the coating of the carbon coating agent B, making it possible to control the carbon coating rate and maximum thickness of the carbon coating on the surface of the particle X within the above range. In addition, by undergoing the step (IV) described below, the carbon coating agent B is carbonized and coated as carbon on the surface of the particle X, while the carbon coating inhibitor C is burned off and does not remain in the positive electrode active material particles for a lithium ion secondary battery.

[0043] Specific examples of polyols other than sugars include polyols having two hydroxy groups, such as polyethylene glycols having a mass average molecular weight of 1000 or less and polypropylene glycols having a mass average molecular weight of 2000 or less; and polyether polyols having three or more hydroxy groups, such as mass average molecular weight of 3000 or less. Among these, polyols having a volatilization temperature of 170°C to 400°C are preferred, and more specific examples thereof include polyols having two hydroxy groups, such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, butanediol, pentanediol, hexanediol, hexanetriol, heptanediol, heptanetriol, octanediol, octanetriol, nonanediol, nonanetriol, decanediol, decanetriol, and dodecanediol; Examples of the polyol include polyols having three or more hydroxy groups, such as glycerin, trimethylolpropane, pentaerythritol, and dipentaerythritol.

[0044] The amines and amides preferably have a volatilization temperature of 170°C or higher. Specific examples of the amines and amides include aliphatic amines such as diethanolamine and triethanolamine, heterocyclic amines such as imidazole, amides such as formamide and acetamide, polyamides such as polyacrylamide and poly-N-vinylacetamide, and fatty acid amides such as oleic acid amide and stearic acid amide.

[0045] As such a carbon coating inhibitor C, ethylene glycol, propylene glycol, glycerin, triethanolamine, or oleic acid amide is preferable.

[0046] The order of addition of the preliminary particles x of the lithium-based polyanion particles X, the carbon coating agent B, the carbon coating inhibitor C, and the water is not particularly limited, and they may be added all at once, or the preliminary particles x and water may be mixed first and then the carbon coating agent B and the carbon coating inhibitor C may be added simultaneously, or the preliminary particles x, water, and the carbon coating inhibitor C may be mixed first and then the carbon coating agent B is added, but it is preferable to add them all at once.

[0047] The amount of carbon coating agent B added is, in terms of carbon atom (mass of carbon atoms contained in carbon coating agent B to be added), preferably 0.5 to 30 parts by mass, more preferably 1.0 to 25 parts by mass, and even more preferably 1.5 to 20 parts by mass, per 100 parts by mass of preliminary particles of lithium-based polyanion particles X.

[0048] The amount of carbon-coated inhibitor C added is preferably 1 to 20 parts by mass, more preferably 2 to 18 parts by mass, and even more preferably 3 to 15 parts by mass, per 100 parts by mass of the preliminary particles of lithium-based polyanion particles X.

[0049] The mass ratio (B / C) of the amount of carbon coating agent B added, calculated as carbon atoms, to the amount of carbon coating inhibitor C added is preferably 0.05 to 3.00, more preferably 0.10 to 2.00, and further preferably 0.15 to 1.50.

[0050] The solid content concentration of the slurry water b is preferably 30% by mass to 70% by mass, more preferably 35% by mass to 65% by mass, and further preferably 40% by mass to 60% by mass.

[0051] After adding water, it is preferable to pre-stir the slurry water b before proceeding to step (III). The stirring time of 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.

[0052] The step (III) of the production method of the present invention is a step of spray-drying the slurry water b obtained in the step (II) to obtain granules Y. As a result, the preliminary particles x aggregate to form particles X, and as the subsequent step (IV) passes, a carbon-coating inhibitor C is coated on a part of the surface of the particles X, which appropriately inhibits the coating of the carbon coating agent B, and the carbon coating rate and the maximum thickness of the carbon coating on the surface of the particles X can be controlled within the above-mentioned ranges.

[0053] In step (III), the operating conditions for spray drying may be appropriately set depending on the device used. For example, as the treatment conditions for a micromist dryer (MDL-050M manufactured by Fujisaki Electric Co., Ltd.) equipped with a four-fluid nozzle, the hot air temperature is preferably 110°C to 300°C, more preferably 150°C to 250°C. In addition, the ratio of the supply amount of hot air to the supply amount of slurry water (supply amount of hot air / supply amount of slurry water) is preferably 500 to 10,000, more preferably 1,000 to 9,000.

[0054] The step (IV) of the production method of the present invention is a step of firing the granules Y obtained in the step (III). As a result, the carbon-coating inhibitor C is burned off on the surface of the particles X formed by agglomeration of the preliminary particles x, while the carbon coating agent B is carbonized and coated on the surface of the particles X. The firing conditions in step (IV) are preferably 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.

[0055] The positive electrode active material particles for lithium ion secondary batteries of the present invention are materials used as positive electrode active materials for lithium ion secondary batteries. Specifically, for example, the positive electrode active material particles for lithium ion secondary batteries of the present invention are kneaded with acetylene black, ketjen black, polyvinylidene fluoride, N-methyl-2-pyrrolidone, etc. to prepare a positive electrode slurry, which is then applied to a current collector and press-molded to prepare a positive electrode. The lithium ion secondary battery to which the positive electrode obtained using the positive electrode active material particles for lithium ion secondary batteries of the present invention can be applied is not particularly limited as long as it essentially comprises a positive electrode, a negative electrode, an electrolyte, and a separator, or a positive electrode, a negative electrode, and a solid electrolyte.

[0056] The positive electrode active material particle for a lithium ion secondary battery of the present invention is constituted by particles X having a large carbon coating thickness while maintaining an appropriate carbon coating rate, and carbon is present on the surface in an appropriately uneven distribution, so that a highly useful positive electrode capable of effectively improving cycle characteristics can be obtained.

[0057] 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 (Si, SiOx), lithium titanate, or carbon materials such as amorphous carbon can be used. It is preferable to use an electrode formed of an intercalation 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, and for example, a combination of graphite and silicon-based materials can be used.

[0058] 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 that is usually used in electrolytes for lithium ion secondary batteries, and examples of the organic solvent that can be used include carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, and oxolane compounds.

[0059] The supporting salt is not particularly limited in type, but is preferably LiPF 6 , LiBF 4 , LiClO 4 and LiAsF 6 an inorganic salt selected from the group consisting of a derivative of said 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 at least one of the derivatives of said organic salts.

[0060] The separator serves to electrically insulate the positive and negative electrodes and to retain the electrolyte, and may be, for example, a porous synthetic resin film, particularly a porous film of a polyolefin polymer (polyethylene, polypropylene).

[0061] 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 (PO 4 ) 3 , Li 7 La 3 Zr 2 O 12 , 50Li 4 SiO 4 50Li 3 BO 3 , Li 2.9 PO 3.3 N 0.46 , Li 3.6 S 0.6 P 0.4 O 4 , Li 1.07 Al 0.69 Ti 1.46 (PO 4 ) 3 , Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 , Li 10 GeP2 S 12 , Li 3.25 Ge 0.25 P 0.75 S 4 , 30Li 2 S·26B 2 S 3 44LiI, 63Li 2 S·36SiS 2 1Li 3 PO 4 , 57Li 2 S·38SiS 2 5Li 4 SiO 4 , 70Li 2 S·30P 2 S 5 , 50Li 2 S·50GeS 2 , Li 7 P 3 S 11 , Li 3.25 P 0.95 S 4 can be used.

[0062] The shape of the lithium ion secondary battery having the above-mentioned configuration is not particularly limited, and may be various shapes such as a coin type, a cylindrical type, a square type, or an irregular shape enclosed in a laminate exterior body. EXAMPLES

[0063] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[0064] [Example 1] LiOH H 2 1,272 g of O and 4 L of water were mixed to obtain a slurry water a1. Next, the obtained slurry water a1 was stirred for 3 minutes while maintaining the temperature at 25° C., and 1,153 g of an 85% aqueous phosphoric acid solution was added dropwise at 35 mL / min. The mixture was stirred at a speed of 400 rpm for 12 hours to obtain Li 3 PO 4 Thus, slurry water a2 containing the above was obtained. The resulting slurry water a2 was purged with nitrogen to bring the dissolved oxygen concentration of the slurry water a2 to 0.5 m g / L, and then MnSO 4 5H 2 O 1688g, FeSO 4 7H 2 834 g of MnSO was added to obtain slurry water a3. 4 and FeSO 4 The molar ratio (manganese compound:iron compound) was 70:30.

[0065] Next, the obtained slurry water a3 was charged into an autoclave and subjected to hydrothermal reaction at 170°C for 1 hour. The pressure inside the autoclave was 0.8 MPa. After the hydrothermal reaction, the resulting The crystals were filtered and then washed with 12 parts by weight of water per 1 part by weight of the crystals. Freeze-dry at -50℃ for 12 hours to obtain the preliminary particles x 1 obtained. Obtained preliminary particles x 1 1000 g of the mixture was taken, and 1 L of water and 108 g of cellulose nanofiber (Wma-10002, manufactured by Sugino Machine Co., Ltd., fiber diameter 4 to 20 nm) (100 parts by mass of preliminary particles x 1 The obtained slurry water a5 was dispersed in an ultrasonic agitator (T25, manufactured by IKA) for 1 minute to uniformly color the entire mixture, and then spray-dried using a spray dryer (MDL-050M, manufactured by Fujisaki Electric Co., Ltd.) to obtain granules Y. 1 The hot air temperature during spray drying was set to 200° C., and the ratio of the amount of hot air supplied to the amount of slurry water supplied (amount of hot air supplied / amount of slurry water supplied) was set to 2500. The obtained granule Y 1 The mixture was calcined at 700°C for 1 hour in an argon-hydrogen atmosphere (hydrogen concentration 3%) to obtain lithium-based polyanion particles (LiMn 0.7 Fe 0.3 PO 4 The aggregates obtained had an average particle size of 20 μm.

[0066] [Example 2] Instead of adding 108 g of cellulose nanofiber and 135 g of propylene glycol, 180 g of cellulose nanofiber (100 parts by mass of preliminary particles x 1 Positive electrode active material particles for lithium ion secondary batteries were obtained in the same manner as in Example 1, except that 45 g of propylene glycol was added (2.0 parts by mass, calculated as the amount of carbon atoms relative to the total mass of the carbon nanotube) and 45 g of propylene glycol were added. FIG. 1 shows a TEM photograph of the obtained positive electrode active material particles for a lithium ion secondary battery.

[0067] [Example 3] Instead of adding 108 g of cellulose nanofiber and 135 g of propylene glycol, 450 g of cellulose nanofiber (100 parts by mass of preliminary particles x 1 Positive electrode active material particles for lithium ion secondary batteries were obtained in the same manner as in Example 1, except that 45 g of propylene glycol was added (5.0 parts by mass, calculated as the amount of carbon atoms relative to the total mass of the carbon nanotube).

[0068] [Example 4] Instead of adding 108 g of cellulose nanofiber and 135 g of propylene glycol, 50 g of glucose (100 parts by mass of preliminary particles x 1 Positive electrode active material particles for lithium ion secondary batteries were obtained in the same manner as in Example 1, except that 45 g of propylene glycol was added (2.0 parts by mass, calculated as the amount of carbon atoms relative to the total mass of the carbon nanotube) and 45 g of propylene glycol were added.

[0069] [Comparative Example 1] Positive electrode active material particles for lithium ion secondary batteries were obtained in the same manner as in Example 1, except that 540 g of propylene glycol was added instead of 135 g of propylene glycol.

[0070] [Comparative Example 2] Instead of adding 108 g of cellulose nanofiber and 135 g of propylene glycol, 50 g of glucose (100 parts by mass of preliminary particles x 1Positive electrode active material particles for lithium ion secondary batteries were obtained in the same manner as in Example 1, except that propylene glycol was not added and 2.0 parts by mass, calculated as the amount of carbon atoms, was added to the mixture. FIG. 2 shows a TEM photograph of the obtained positive electrode active material particles for a lithium ion secondary battery.

[0071] Carbon content of positive electrode active material particles for lithium ion secondary batteries The carbon content of the obtained positive electrode active material particles for lithium ion secondary batteries was measured using a carbon / sulfur analyzer (EMIA-220V2, manufactured by Horiba, Ltd.).

[0072] <BET specific surface area of ​​positive electrode active material particles for lithium-ion secondary batteries> The BET specific surface area of ​​the obtained positive electrode active material particles for lithium ion secondary batteries was measured using a flow type specific surface area automatic measurement device (FlowSorbIII2305, manufactured by Shimadzu Corporation) using a nitrogen-helium mixed gas containing 30% nitrogen.

[0073] <<Conductivity of Positive Electrode Active Material Particles for Lithium-Ion Secondary Batteries>> The conductivity of the obtained positive electrode active material particles for lithium ion secondary batteries was measured using a low resistivity meter (MCP-T610, manufactured by Mitsubishi Analytech Co., Ltd.) when a load of 20 kN was applied to 3.0 g of the positive electrode active material particles for lithium ion secondary batteries.

[0074] <Carbon coverage rate and maximum thickness of carbon coating on the surface of particle X> The carbon coverage and maximum thickness of the carbon coating for each of the obtained particles X were determined using a TEM (JEM-ARM200F, manufactured by JEOL Ltd.) according to the above-mentioned method. The results are shown in Table 1.

[0075] <Evaluation of battery characteristics (cycle characteristics)> The obtained positive electrode active material particles for lithium ion secondary batteries were used as positive electrode materials to prepare positive electrodes for lithium ion secondary batteries. Specifically, the obtained positive electrode active materials, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 90:5:5, and N-methyl-2-pyrrolidone was added to the mixture and thoroughly kneaded to prepare a positive electrode slurry. The positive electrode slurry was applied to a current collector made of aluminum foil with a thickness of 20 μm using a coater, and vacuum dried at 80° C. for 12 hours. Thereafter, the mixture was punched into a disk shape of φ14 mm and pressed with a hand press at 20 kN for 2 minutes to form a positive electrode.

[0076] Next, a coin-type secondary battery was constructed using the above positive electrode. A lithium foil punched to a diameter of 15 mm was used as the negative electrode. The electrolyte was a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7, with LiPF 6 The battery was dissolved at a concentration of 1 mol / L. A polymer porous 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 in a conventional manner to obtain a coin-type secondary battery (CR-2032).

[0077] The obtained coin-type secondary battery was used to determine the cycle characteristics according to the following formula (2) by repeatedly charging and discharging 50 times at 1C (170mAh / g) in an environment of 30°C using a discharge capacity measuring device (HJ-1001SD8, manufactured by Hokuto Denko Corporation). Cycle characteristics = (discharge capacity after 50 cycles) / (Maximum discharge capacity in 50 cycles) (2)

[0078] [Table 1]

Claims

1. The following formula (X): Li a Mn b Fe c M x PO 4 ...(X) (In formula (X), 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 surface of the lithium-based polyanion particle X represented by the formula (I) is coated with carbon, A method for producing positive electrode active material particles for a lithium ion secondary battery, in which a carbon coverage rate on the surface of lithium-based polyanion particles X is 5% or more and less than 70%, and the maximum thickness of the carbon coverage is 5 nm or more, comprising the steps of (I) to (IV): (I) A step of adding a lithium compound, a metal compound including at least a manganese compound and / or an iron compound, a phosphoric acid compound, and water to obtain a slurry water a, and then subjecting the slurry water to a hydrothermal reaction to obtain preliminary particles x of lithium-based polyanion particles X. (II) A step of adding the obtained preliminary particles x of the lithium-based polyanion particles X, the carbon coating agent B, the carbon coating inhibitor C, and water to obtain a slurry water b. (III) A step of subjecting the obtained slurry water b to spray drying to obtain granules Y. (IV) A step of firing the obtained granules Y Equipped with The carbon coating agent B is one or more carbon materials selected from saccharides, and A method for producing positive electrode active material particles for lithium ion secondary batteries, wherein the carbon coating inhibitor C is one or more carbon materials selected from polyols other than saccharides, amines, and amides.

2. 2. The method for producing positive electrode active material particles for lithium ion secondary batteries according to claim 1, wherein in the step (II), the amount of the carbon-coated inhibitor C added is 1 part by mass to 20 parts by mass per 100 parts by mass of the preliminary particles x of the lithium-based polyanion particles X.

3. 3. The method for producing positive electrode active material particles for lithium ion secondary batteries according to claim 1 or 2, wherein in step (II), the mass ratio (B / C) of the amount of carbon coating agent B added in terms of carbon atoms to the amount of carbon coating inhibitor C added is 0.05 to 3.00.

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