Positive electrode active material particle mixture for lithium ion secondary batteries

Abstract

To provide a positive electrode active material particle mixture for a lithium-ion secondary battery that is a mixture of particles such as LiMn2O4 and particles such as LiMnxFe1-xPO4 and that is capable of sufficiently enhancing discharge capacity per unit volume while maintaining excellent cycle characteristics.SOLUTION: Provided is a positive electrode active material particle mixture for a lithium-ion secondary battery that is a mixture of particles A and particles B. The particles A are represented by the following formula (a): LiM1aMnbO4 and have an average particle diameter of 1 to 15 μm. The particles B are represented by the following formula (b): LifMngFehM2xPO4 and have an average particle diameter of 50 to 500 nm, and has a surface covered with carbon at a coating ratio equal to or more than 5% and less than 70%. A mass ratio (A:B) of the particles A and the particles B in the mixture is 90:10 to 50:50.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material particle mixture for a lithium ion secondary battery that increases discharge capacity per unit volume while maintaining excellent cycle characteristics. [Background technology]

[0002] Secondary batteries such as lithium-ion secondary batteries are used in a wide range of fields, including mobile phones, digital cameras, laptop computers, hybrid vehicles, and electric vehicles. As a cathode material for these lithium-ion secondary batteries, particles with a spinel structure such as LiMn2O4 are useful as materials that greatly improve rate characteristics, but ensuring sufficient cycle characteristics remains a challenge. For this reason, various developments have been carried out to date that utilize particles such as LiMn2O4 in combination with other particles in an attempt to further improve battery characteristics.

[0003] For example, Patent Document 1 discloses a cathode formed by mixing lithium metal oxide consisting of primary particles aggregated into secondary particles of a specific size with lithium metal phosphate, in an attempt to improve cycle life, safety, etc. Also, Patent Document 2 discloses a positive electroactive material containing a nanoscale lithium iron manganate compound and a lithium metal oxide such as LiMn2O4, in an attempt to provide a battery material with improved thermal stability and high energy density. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2016-524307 [Patent Document 2] Special Publication No. 2014-524133 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when obtaining an electrode material by mixing particles such as LiMn2O4 with fine particles of LiMn 1 , , , , f , 1 , Fe 1-x PO4 and the like, even with the technology described in the above patent document, there is still room for improvement in improving the cycle characteristics, and there has not yet been sufficient consideration regarding increasing the discharge capacity per unit volume.

[0006] Therefore, an object of the present invention is to provide a positive electrode active material particle mixture for a lithium ion secondary battery that can sufficiently increase the discharge capacity per unit volume while maintaining excellent cycle characteristics while being a mixture of particles such as LiMn2O4 and LiMn x Fe 1-x PO4 and the like.

Means for Solving the Problems

[0007] As a result of intensive studies to solve the above problems, the present inventor has found that a mixture of particles having a spinel structure such as LiMn2O and lithium iron manganese phosphate particles in which carbon is coated at a specific coating rate in a specific mass ratio can effectively increase the discharge capacity per unit volume while ensuring good cycle characteristics. A positive electrode active material particle mixture for a lithium ion secondary battery was obtained.

[0008] That is, the present invention provides the following formula (a): LiM 1 a Mn b O4 ··· (a) (In formula (a), M 1 represents one or more elements selected from Ni, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, Cu, and Si. a and b satisfy 0 ≦ a ≦ 0.1, 0 < b ≦ 2, and (valence of M 1 ) × a + (valence of Mn) × b = 7.)<0000J200>Particles A represented by and having an average particle size of 1 μm to 15 μm, and The following formula (b): Li f Mng Fe h M 2 x PO4···(b) (In formula (b), M 2 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x satisfy 0 < f ≤ 1.2, 0.3 ≤ g ≤ 1.2, 0.2 ≤ h ≤ 1.2, 0 ≤ x ≤ 0.3, and f + (valence of Mn) × g + (valence of Fe) × h + (valence of M 2 ) × x = 3.) represented by, having an average particle size of 50 nm to 500 nm, and being coated with carbon on the surface at a coating rate of 5% or more and less than 70% to form particle B and a mixture thereof, providing a positive electrode active material particle mixture for a lithium ion secondary battery in which the mass ratio (A:B) of particle A to particle B in the mixture is 90:10 to 50:50.

Advantages of the Invention

[0009] According to the positive electrode active material particle mixture for a lithium ion secondary battery of the present invention, it is possible to realize a lithium ion secondary battery that effectively increases the discharge capacity per unit volume while maintaining excellent cycle characteristics.

Brief Description of the Drawings

[0010] [Figure 1] SEM image showing a part of particle B1 obtained in Production Example 4. [Figure 2] SEM image showing a part of particle B6 obtained in Production Example 9.

Modes for Carrying Out the Invention

[0011] Hereinafter, the present invention will be described in detail. The positive electrode active material particle mixture for a lithium ion secondary battery of the present invention (hereinafter, also simply referred to as "the particle mixture of the present invention") has the following formula (a): LiM1 a Mn b O4···(a) (In formula (a), M 1 represents one or more elements selected from Ni, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, Cu, and Si. a and b satisfy 0 ≦ a ≦ 0.1, 0 < b ≦ 2, and (valence of M 1 × a + (valence of Mn) × b = 7).) Particles A represented by the formula and having an average particle diameter of 1 μm to 15 μm, and The following formula (b): Li f Mn g Fe h M 2 x PO4···(b) (In formula (b), M 2 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x satisfy 0 < f ≦ 1.2, 0.3 ≦ g ≦ 1.2, 0.2 ≦ h ≦ 1.2, 0 ≦ x ≦ 0.3, and f + (valence of Mn) × g + (valence of Fe) × h + (valence of M 2 × x = 3).)<了 Particles B represented by the formula, having an average particle diameter of 50 nm to 500 nm, and having carbon coated on the surface at a coating rate of 5% or more and less than 70% A mixture with, The mass ratio (A:B) of particles A and particles B in the mixture is 90:10 to 50:50.

[0012] Thus, the particle mixture of the present invention is composed of particle A having an appropriate average particle size represented by a specific formula and nanoparticle B having a limited range where the carbon coating rate on the surface is 5% or more and less than 70%, and is a mixture in which these particles maintain a limited mass ratio. In the particle mixture of the present invention, together with such particle A, nanoparticle B with a low carbon coating rate on the surface is mixed, which facilitates the presence of nanoparticle B in the gaps between particle A and increases the electrode density. By being such a particle mixture in which such particles are mixed, it becomes possible to effectively suppress the deposition of by-products on the particle surface while effectively suppressing the deterioration of the electrode structure, and in the obtained lithium-ion secondary battery, it is possible to improve the discharge capacity per unit volume while ensuring good cycle characteristics.

[0013] Particle A constituting the particle mixture of the present invention is represented by the following formula (a): LiM 1 a Mn b O4···(a) (In formula (a), M 1 represents one or more elements selected from Ni, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, Cu, and Si. a and b are numbers satisfying 0 ≦ a ≦ 0.1, 0 < b ≦ 2, and (valence of M 1 ) × a + (valence of Mn) × b = 7.) and is represented by and has an average particle size of 1 μm to 15 μm.

[0014] Particle A represented by the above formula (a) is a particle having a spinel structure, a primary particle having an appropriate average particle size, or a secondary particle having an appropriate average particle size formed by aggregation of primary particles. By forming a particle mixture with a specific mass ratio together with nanoparticle B of the microparticles described below, it is possible to effectively increase the discharge capacity per unit volume while ensuring good cycle characteristics.

[0015] Specific examples of particle A represented by the above formula (a) include LiMn2O4, LiNi 0.5Mn 1.5 O4, LiCoMnO4, LiCrMnO4, LiFeMnO4, LiAlMnO4, LiCu 0.5 Mn 1.5 Among them, LiMn2O4 is preferred.

[0016] The average particle size of the particles A is 1 μm to 15 μm, preferably 2 μm to 15 μm, and more preferably 4 μm to 15 μm, from the viewpoint of ensuring excellent cycle characteristics and ease of handling. Here, the "average particle size" of particle A is the D obtained from the volume-based particle size distribution based on the laser diffraction / scattering method. 50 The values ​​are the particle diameters (median diameters) at 50% of the cumulative particle size.

[0017] The particles A can be obtained, for example, by a production method including steps of adding a lithium compound and a manganese compound, pulverizing and mixing them in a ball mill, calcining, and then calcining. Examples of the lithium compound to be used include hydroxides (e.g., LiOH·H2O, LiOH), carbonates, acetates, and nitrates. Of these, carbonates are preferred. Examples of manganese compounds include manganese acetate, manganese nitrate, and manganese oxide. These may be used alone or in combination of two or more. Among these, manganese oxide is preferred from the viewpoint of improving battery characteristics. In addition to these lithium compounds and manganese compounds, metals other than these compounds (M 1 ) compounds may also be used. When firing, it is preferable to first calcinate at 250°C to 700°C, preferably 300°C to 600°C, for 1 to 15 hours, preferably 6 to 12 hours, and then calcinate at 500°C to 1000°C, preferably 600°C to 900°C, for 3 to 30 hours, preferably 12 to 24 hours. After calcination, it is preferable to crush the material before calcining. After calcination, the particle size may be controlled by appropriate granulation.

[0018] Particle B constituting the particle mixture of the present invention is represented by the following formula (b): Li f Mn g Fe h M 2 x PO4···(b) (In formula (b), M 2 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x satisfy 0 < f ≤ 1.2, 0.3 ≤ g ≤ 1.2, 0.2 ≤ h ≤ 1.2, 0 ≤ x ≤ 0.3, and f + (valence of Mn) × g + (valence of Fe) × h + (valence of M 2 ) × x = 3.) It is represented by , has an average particle size of 50 nm to 500 nm, and is coated with carbon on the surface at a coating rate of 5% or more and less than 70%.

[0019] Particle B represented by the above formula (b) is an olivine-type lithium transition metal phosphate compound containing at least both manganese (Mn) and iron (Fe) as transition metals. It is a nanoparticle, and a specific amount of carbon exists on the surface of particle B at a specific coating rate. Since the particle mixture of the present invention is a mixture in which such particle B is interposed in the gaps between particle A while maintaining a specific mass ratio with particle A, it is possible to effectively improve the discharge capacity per unit volume while effectively maintaining high cycle characteristics.

[0020] Regarding the particle B, from the viewpoint of the average discharge voltage, f is preferably 0.6≦f≦1.2, more preferably 0.65≦f≦1.15, and even more preferably 0.7≦f≦1.1. g is preferably 0.3≦g≦0.8, more preferably 0.35≦g≦0.75, and even more preferably 0.4≦g≦0.7. h is preferably 0.2≦h≦0.8, more preferably 0.25≦h≦0.75, and even more preferably 0.3≦h≦0.7. x is preferably 0≦x≦0.2, more preferably 0≦x≦0.15, and even more preferably 0≦x≦0.1. g / h is the molar ratio of Mn and Fe constituting the so-called particle B, and is preferably 0.2≦g / h≦5.0, more preferably 0.3≦g / h≦4.5, and even more preferably 0.4≦g / h≦4.0.

[0021] Specifically, for example, LiMn 0.2 Fe 0.8 PO4, LiMn 0.3 Fe 0.7 PO4, LiMn 0.4 Fe 0.6 PO4, LiMn 0.45 Fe 0.55 PO4, LiMn 0.6 Fe 0.4 PO4, LiMn 0.7 Fe 0.3 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 0.5 Fe 0.5 PO4, Li 1.2 Mn 0.63 Fe 0.27 PO4, Li 0.6 Mn 0.84 Fe 0.36 PO4, etc. Among them, LiMn 0.45 Fe 0.55 PO4, LiMn 0.7 Fe 0.3 PO4, LiMn 0.8 Fe 0.2PO4 is preferred.

[0022] The average particle size of particles B represented by the above formula (b) is 50 nm to 500 nm, preferably 50 nm to 400 nm, and more preferably 50 nm to 300 nm, from the viewpoints of effectively promoting the insertion and desorption of lithium ions, efficiently interposing them in the gaps between particles A, and ease of handling. Here, the "average particle size" of particles B means the average particle size of 100 particles observed under SEM.

[0023] The surfaces of the particles B are coated with carbon at a coverage of 5% or more but less than 70% in order to improve the discharge capacity per unit volume while maintaining good cycle characteristics. This effectively prevents the generation of by-products due to a reaction between the electrolyte and the carbon present on the surfaces of the particles B in a particle mixture in which the particles A and B are mixed at a specific mass ratio, thereby maintaining excellent cycle characteristics. Furthermore, while the fine particles B are interposed between the particles A, the specific amount of carbon coated on the surfaces of the particles B is appropriately unevenly or scattered in the gaps between the particles A, thereby enabling the carbon-interposed particle network to be more effective. This effectively prevents the deterioration of the electronic conductive path, effectively increasing the discharge capacity per unit volume while maintaining excellent cycle characteristics.

[0024] The surface of particle B is coated with carbon. This carbon is obtained by carbonizing one or more carbon materials selected from sugars to form carbon, which coats the surface of particle B. While being carbonized to form carbon, the one or more carbon materials selected from sugars (also referred to as "carbon coating agent X" as described below) are efficiently coated on the surface of particle B due to the presence of one or more carbon materials other than sugars selected from polyols, amines, and amides (also referred to as "carbon coating inhibitor Y" as described below). The carbon coated on the surface of particle B is carbon derived from one or more carbon materials selected from sugars, while one or more carbon materials selected from polyols, amines, and amides other than sugars do not remain on particle B.

[0025] Specific examples of the one or more carbon materials selected from sugars include one or more selected from monosaccharides such as glucose, fructose, galactose, and mannose; disaccharides such as maltose, sucrose, and cellobiose; polysaccharides such as starch, dextrin, and cellulose; and polysaccharide nanofibers such as cellulose nanofibers, lignocellulose nanofibers, chitin nanofibers, and chitosan nanofibers. 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 rate characteristics.

[0026] The carbon content in particles B corresponds to the content of carbon obtained by carbonizing the carbon material, and is preferably 0.5% by mass to 5.0% by mass in particles B, more preferably 0.6% by mass to 4.0% by mass, and more preferably 0.7% by mass to 3.0% by mass.

[0027] The carbon contained in particle B is carbon formed by carbonizing the above-mentioned carbon material present on the surface of particle B, i.e., corresponds to the atomic equivalent of the above-mentioned carbon material, and can be determined by measurement using a carbon / sulfur analyzer. The amount of particles B whose surfaces are coated with carbon includes the amount of carbon formed by carbonizing the carbon material.

[0028] The carbon coverage on the surface of particle B is 5% or more and less than 70%, preferably 10% to 48%, and more preferably 15% to 45%, from the viewpoint of effectively suppressing the generation of by-products due to the reaction between the electrolyte and the carbon present on the surface of particle B while controlling this value within a limited range and maintaining excellent cycle characteristics.

[0029] The "carbon coverage (%)" on the surface of particle B refers to a value determined by the following method. Specifically, first, the surface is observed in one field of view of particle B using a TEM electron microscope, and the surface of particle B that is not coated with carbon and the surface of particle B that is coated with carbon are identified. Next, the "perimeter xn of the surface of particle B that is not coated with carbon" and the "perimeter xc of the surface of particle B that is coated with carbon" are measured in the field of view, and xn and xc are summed to determine the "total perimeter xB of the surface of particle B." The obtained values ​​of "total circumference xB of the surface of particle B" and "circumference xc of the surface of particle B coated with carbon" are introduced 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 determine the carbon coverage (%) on the surface of particle B. In TEM electron microscope observation, the specific limit for whether the surface of particle B is coated with carbon is up to 1 nm. Carbon coverage (%) = [(perimeter of the surface of carbon-coated particle B x c) / (total perimeter of the surface of particle B x B)] × 100 (1)

[0030] The maximum thickness of the carbon coating on the surface of particle B is preferably 5 nm or more, more preferably 8 nm or more, even 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 charge / discharge of the lithium ion secondary battery and effectively maintaining good cycle characteristics by controlling the carbon coverage rate to a low, limited range and unevenly distributing carbon on the surface of particle B, and there is no particular upper limit, but it is preferably 100 nm or less.

[0031] The "maximum thickness of the carbon coating" on the surface of particle B refers to a value determined by the following method. Specifically, first, the surface of particle B is observed in one field of view of an image of the positive electrode active material particle for a lithium ion secondary battery using a TEM electron microscope, and point P on the surface of the positive electrode active material particle for a lithium ion secondary battery that is recognized to be the thickest carbon coating in that field of view is identified. Next, a line perpendicular to a tangent containing point P is drawn, and an intersection Q where this line intersects with the surface of particle B is identified. The distance between point P and intersection Q is measured as the carbon coating thickness (nm) in one field of view, and the values ​​determined in 50 fields of view are averaged to determine the maximum thickness (nm) of the carbon coating on the surface of particle B. A carbon coating thickness of less than 1 nm is the measurement limit in TEM electron microscope observation.

[0032] The conductivity of the particles B is preferably 1.0×10 -7 S / cm or more, more preferably 5.0×10 -7 S / cm~1.0×10 -2 S / cm, and more preferably 1.0×10 -6 S / cm~1.0×10 -2 S / cm.

[0033] Particles B can be obtained, for example, by the following production method. A step of coating the surface with carbon using a carbon coating agent X, which is one or more carbon materials selected from sugars, and a carbon coating inhibitor Y, which is one or more carbon materials selected from polyols, amines, and amides other than sugars. The manufacturing method includes the steps of:

[0034] More specifically, the method for producing such particles B includes the following steps (Ib) to (IIb): (Ib) A lithium compound, a metal compound containing at least a manganese compound and an iron compound, a phosphate compound, a carbon coating agent X, a carbon coating inhibitor Y, and water are added to obtain a slurry water i, which is then subjected to a hydrothermal reaction. and obtaining preliminary particles b. (IIb) A step of firing the obtained preliminary particles b in a nitrogen atmosphere Equipped with The carbon coating agent X is one or more carbon materials selected from saccharides, and In this production method, the carbon-coated inhibitor Y is one or more carbon materials selected from polyols other than sugars, amines, and amides.

[0035] The above-mentioned step (Ib) is a step of adding a lithium compound, a metal compound containing at least a manganese compound and an iron compound, a phosphate compound, a carbon coating agent X, a carbon coating inhibitor Y, and water to obtain slurry water i, and then subjecting the mixture to a hydrothermal reaction to obtain preliminary particles b. As the lithium compound, the same compounds as those used for the particles A can be used, but hydroxides and carbonates are preferred. As the manganese compound, the same one as that used for the particles A can be used. Examples of iron compounds include iron acetate, iron nitrate, and iron sulfate. These may be used alone or in combination of two or more. Among these, iron sulfate is preferred from the viewpoint of improving battery characteristics. In addition to these manganese compounds and iron compounds, metal compounds other than manganese compounds and iron compounds (M 2 ) compounds may also be used.

[0036] Examples of phosphoric acid compounds include orthophosphoric acid (H3PO4, phosphoric acid), metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, ammonium phosphate, ammonium hydrogen phosphate, etc. Among these, phosphoric acid is preferably used, and is preferably used as an aqueous solution with a concentration of 70% by mass to 90% by mass.

[0037] The carbon coating agent X is one or more carbon materials selected from the above-mentioned 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 rate characteristics of the resulting battery.

[0038] The carbon coating inhibitor Y 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 the carbon coating agent X. In this way, by adding the carbon coating inhibitor Y together with the carbon coating agent X in step (Ib), and then going through the subsequent step (IIb), the carbon coating inhibitor Y coats a portion of the surface of the particle B while adequately inhibiting the coating of the carbon coating agent X, making it possible to control the carbon coverage on the surface of the particle B within the above range. By undergoing the step (IIb) described below, the carbon coating agent X is carbonized and coated on the surface of the particles B as carbon, while the carbon coating inhibitor Y is burned away and does not remain on the particles B.

[0039] Specific examples of polyols other than sugars include polyols having two hydroxy groups, such as polyethylene glycols having a mass average molecular weight of 1,000 or less and polypropylene glycols having a mass average molecular weight of 2,000 or less, and polyether polyols having three or more hydroxy groups, such as polyether polyols having a mass average molecular weight of 3,000 or less. Of these, polyols having a volatilization temperature of 170°C to 400°C are preferred, more specifically, 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 include polyols having three or more hydroxy groups, such as glycerin, trimethylolpropane, pentaerythritol, and dipentaerythritol.

[0040] The amines and amides preferably have a volatilization temperature of 170°C or higher, and specific examples thereof 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.

[0041] Such carbon coating inhibitor Y is preferably ethylene glycol, propylene glycol, glycerin, triethanolamine, or oleic acid amide.

[0042] The order of addition of the lithium compound, metal compound containing at least a manganese compound and an iron compound, phosphate compound, carbon coating agent X, carbon coating inhibitor Y, and water is not particularly limited, and they may be added all at once, or the metal compound, phosphate compound, and water may be mixed first and then carbon coating agent X and carbon coating inhibitor Y may be added simultaneously, or the metal compound, phosphate compound, water, and carbon coating inhibitor Y may be mixed first and then carbon coating agent X may be added, but adding them all at once is preferred.

[0043] The amounts of metal compounds including lithium compounds, manganese compounds and iron compounds, and phosphate compounds used in the slurry water i may be determined appropriately depending on the composition of the desired particles B, and may be prepared according to a conventional method.

[0044] The amount of carbon coating agent X added may be appropriately adjusted so that the carbon content in particles B falls within the above range, taking into account the carbon atom equivalent amount of carbon coating agent X, from the viewpoint of controlling the carbon coverage rate on the surface of particles B within the above range. Furthermore, from the viewpoint of controlling the carbon coverage rate on the surface of particle B within the above range, the amount of carbon coating inhibitor Y added is preferably 0.05 to 3.00, more preferably 0.10 to 2.00, and even more preferably 0.15 to 1.50, as a mass ratio (X / Y) of the amount of carbon coating agent X added in terms of carbon atoms to the amount of carbon coating inhibitor Y added.

[0045] The solid content concentration of the slurry water i is preferably 20 to 80 parts by mass, more preferably 30 to 70 parts by mass, and even more preferably 40 to 60 parts by mass.

[0046] After adding water, it is preferable to pre-stir the slurry water i before proceeding to step (IIb). The stirring time of the slurry water i is preferably 1 to 30 minutes, more preferably 5 to 20 minutes. The temperature of the slurry water ii is preferably 10 to 50°C, more preferably 15 to 35°C.

[0047] Next, the obtained slurry water i is subjected to a hydrothermal reaction to obtain preliminary particles b. 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 i, from the viewpoints of the solubility of the metal compound, ease of stirring, synthesis efficiency, and the like.

[0048] From the viewpoint of controlling the average particle size of particles B within the above range, the hydrothermal reaction is preferably carried out at 100°C to 300°C, more preferably 120°C to 250°C. The hydrothermal reaction is preferably carried out in a pressure-resistant vessel, and when the reaction is carried out at 100°C to 300°C, the pressure is preferably 0.1 MPa to 10 MPa, and when the reaction is carried out at 130°C to 250°C, the pressure is preferably 0.2 MPa to 4.0 MPa. From the viewpoint of controlling the average particle size of particles B within the above range, the hydrothermal reaction time is preferably 0.2 hours to 30 hours, more preferably 0.5 hours to 10 hours. The obtained preliminary particles b are preferably filtered and then washed with water.

[0049] The step (IIb) is a step of firing the preliminary particles b obtained in the step (Ib) in a nitrogen atmosphere, which allows the carbon coating inhibitor Y to coat a portion of the surface of the preliminary particles b while appropriately inhibiting the coating of the carbon coating agent X, thereby forming particles B whose surface carbon coverage is controlled within the above range. By undergoing step (IIb), the carbon coating agent X is carbonized and coated on the surface of the particles B as carbon, while the carbon coating inhibitor Y is burned off and does not remain on the particles B.

[0050] From the viewpoint of controlling the average particle size of particles B within the above range and carbonizing the carbon, the firing temperature is 400° C. to 1100° C., preferably 500° C. to 900° C., and more preferably 600° C. to 800° C. The firing time is preferably 0.5 hours to 30 hours, more preferably 0.5 hours to 20 hours, and even more preferably 0.7 hours to 10 hours.

[0051] The positive electrode active material particle mixture for a lithium ion secondary battery of the present invention is a particle mixture in which particles A and particles B are mixed together. In the particle mixture of the present invention, the mass ratio (A:B) of particles A to particles B is 90:10 to 50:50, preferably 90:10 to 55:45, and more preferably 90:10 to 60:40, from the viewpoint of uniformly mixing particles A and particles B and appropriately distributing or scattering the carbon coating particles B in the gaps between the particles, thereby effectively improving the rate characteristics.

[0052] The positive electrode active material particle mixture for a lithium ion secondary battery of the present invention can be obtained, for example, by the following production method. Particle A represented by (a) above, a carbon coating agent X, which is one or more carbon materials selected from saccharides, and a carbon coating inhibitor Y, which is one or more carbon materials selected from polyols, amines, and amides other than saccharides, and a particle B, which is represented by the above formula (b), whose surface is coated with carbon; A process of mixing particles A and particles B so that the mass ratio (A:B) is 90:10 to 50:50. Equipped with.

[0053] To mix particles A and particles B, a conventional mixing method may be used. For example, particles A and particles B adjusted to satisfy the above-mentioned predetermined mass ratio can be added and mixed using a Henschel mixer. Alternatively, a dry mixing device other than the Henschel mixer may be used for mixing, or the ingredients may be mixed manually using a pestle and mortar. Among these, it is preferable to mix particles A and particles B using a Henschel mixer, since it is possible to control the mixing conditions to avoid unnecessary pulverization of particles A and particles B by appropriately setting the mixing conditions.

[0054] The positive electrode active material particle mixture for a lithium ion secondary battery of the present invention is highly useful as a positive electrode material for a lithium ion secondary battery, and a lithium ion secondary battery manufactured by utilizing such a particle mixture can exhibit a high discharge capacity per unit volume while maintaining good cycle characteristics.

[0055] For example, specifically, the manufacturing method of a lithium ion secondary battery is represented by the following formula (a): LiM 1 a Mn b O4···(a) (In formula (a), M 1 represents one or more elements selected from Ni, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, Cu, and Si. a and b are numbers satisfying 0 ≦ a ≦ 0.1, 0 < b ≦ 2, and (valence of M 1 ) × a + (valence of Mn) × b = 7. Particles A represented by the formula and having an average particle diameter of 1 μm to 15 μm, and the following formula (b): Li f Mn g Fe h M 2 x PO4···(b) (In formula (b), M 2 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x are numbers satisfying 0 < f ≦ 1.2, 0.3 ≦ g ≦ 1.2, 0.2 ≦ h ≦ 1.2, 0 ≦ x ≦ 0.3, and f + (valence of Mn) × g + (valence of Fe) × h + (valence of M 2 ) × x = 3. Particles B represented by the formula, having an average particle diameter of 50 nm to 500 nm, and having carbon coated on the surface at a coating rate of 5% or more and less than 70% are blended so that the mass ratio (A:B) of particles A to particles B is 90:10 to 50:50, and a conductive assistant and a binder are further blended to prepare a positive electrode paste. This process is included.

[0056] The particles A and B are as described above. The particles A and B may be blended so that the mass ratio (A:B) of particles A to particles B is 90:10 to 50:50, preferably 90:10 to 55:45, and more preferably 90:10 to 60:40. When blending particles A and particles B, they may be blended in advance as a mixture in the above mass ratio (A:B), that is, the above particle mixture that has been produced in advance may be blended as is.

[0057] Then, a conductive additive and a binder are further added to prepare a positive electrode paste. The conductive additive may be carbon black such as acetylene black or ketjen black, etc. The binder may be polyvinylidene fluoride or styrene butadiene rubber (SBR), etc. Furthermore, a solvent such as N-methyl-2-pyrrolidone may be added. A positive electrode paste is prepared by adding and mixing the particles A and B, or a particle mixture, with a conductive additive, a binder, and a solvent, etc. These may be mixed all at once, or may be mixed sequentially as appropriate.

[0058] The positive electrode paste obtained through these steps is then applied to a current collector such as aluminum foil, and then pressed using a roller press or the like, followed by drying to obtain a positive electrode. This positive electrode is composed of particles A having an appropriate average particle size and particles B whose surfaces are coated with carbon, mixed together while maintaining a limited mass ratio, and therefore can provide a highly useful positive electrode that can effectively increase the discharge capacity per unit volume while effectively maintaining good cycle characteristics.

[0059] The obtained positive electrode can be further combined with a negative electrode, an electrolyte solution, and a separator, or a negative electrode and a solid electrolyte, to form a lithium ion secondary battery.

[0060] The negative electrode is not particularly limited in terms of material composition, and any known material composition can be used as long as it can absorb lithium ions during charging and release them during discharging. For example, lithium metal, graphite, silicon-based materials (Si, SiOx), lithium titanate, or carbon materials such as amorphous carbon can be used. It is preferable to use an electrode formed of an intercalating material capable of electrochemically absorbing and releasing lithium ions, particularly a carbon material. Furthermore, two or more of the above negative electrode materials may be used in combination, such as a combination of graphite and silicon-based materials.

[0061] The electrolyte solution is prepared by dissolving a supporting salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent typically used in electrolyte solutions for lithium ion secondary batteries, and examples thereof include carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, and oxolane compounds.

[0062] The supporting salt is not particularly limited in type, but is preferably at least one of inorganic salts selected from LiPF6, LiBF4, LiClO4, and LiAsF6, derivatives of these inorganic salts, organic salts selected from LiSO3CF3, LiC(SO3CF3)2, LiN(SO3CF3)2, LiN(SO2C2F5)2, and LiN(SO2CF3)(SO2C4F9), and derivatives of these organic salts.

[0063] The separator serves to electrically insulate the positive and negative electrodes and retain the electrolyte solution, and may be, for example, a porous synthetic resin film, particularly a porous film of a polyolefin polymer (polyethylene, polypropylene).

[0064] The solid electrolyte electrically insulates the positive and negative electrodes and exhibits high lithium ion conductivity. 0.51 Li 0.34 TiO 2.94 , Li 1.3 Al 0.3 Ti 1.7(PO4)3, Li7La3Zr2O 12 , 50Li4SiO4·50Li3BO3, Li 2.9 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 Just use S4.

[0065] The shape of the lithium ion secondary battery having the above-described configuration is not particularly limited, and may be various shapes such as a coin shape, a cylindrical shape, a square shape, or an irregular shape enclosed in a laminate outer casing. [Example]

[0066] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[0067] <Measuring average particle size> The average particle size of particle A was determined by measuring the particle size distribution using a laser diffraction device (Microtrac MT3000II, manufactured by MicrotracBEL). 50 The values ​​(μm) were measured (particle transmittance: transmission, particle shape: non-spherical, particle refractive index: 1.52, solvent: ethanol, solvent refractive index: 1.36). The average particle size of particles B was determined by measuring the particle sizes of 100 particles observed using an SEM (JSM-7001F, manufactured by JEOL Ltd.), and taking the average value (μm) of the particle sizes.

[0068] <Carbon content of particle B> The carbon content of the obtained particles B was measured using a carbon / sulfur analyzer (EMIA-220V2, manufactured by Horiba, Ltd.).

[0069] <Carbon coating rate and maximum thickness of carbon coating on the surface of particle B> Using a TEM (JEM-ARM200F, manufactured by JEOL Ltd.), the carbon coating rate and the maximum thickness of the carbon coating for the obtained particles B were determined according to the above-mentioned method.

[0070] [Production Example 1: Production of Particles A1] 173.9 g of manganese dioxide and 36.9 g of lithium carbonate were mixed and crushed in a ball mill so that the molar ratio of Mn:Li was 2:1, and then the mixture was calcined at 500°C for 6 hours in an air atmosphere and crushed. Then, the mixture was calcined at 800°C for 12 hours in an air atmosphere to obtain particles A1 (LiMn2O4) (average particle size: 4.7 μm).

[0071] [Production Example 2: Production of Particles A2] The particles A1 obtained in Production Example 1 were pulverized using a ball mill and then fired in an air atmosphere at 800°C for 1 hour to obtain particles A2 (LiMn2O4) (average particle size: 0.5µm).

[0072] [Production Example 3: Production of Particles A3] Particles A3 (LiMn2O4) (average particle size: 24.3 μm) were obtained in the same manner as in Production Example 1, except that the mixture was pre-baked at 800° C. for 24 hours in an air atmosphere, crushed, and then main-baked at 900° C. for 36 hours in an air atmosphere.

[0073] [Production Example 4: Production of Particles B1] Mix 12.72 g of LiOH·H2O and 40 mL of water to make slurry water i 1 The resulting slurry water i1 While maintaining the temperature at 25°C and stirring for 3 minutes, 11.53 g of an 85% aqueous solution of phosphoric acid was added dropwise at a rate of 35 mL / min. Subsequently, taking into consideration the carbon content in the resulting particles B1, 10.72 g of cellulose nanofiber (Wma-10002, manufactured by Sugino Machine Co., Ltd., fiber diameter 4 to 20 nm) as a carbon coating agent X and 2.65 g of propylene glycol as a carbon coating inhibitor Y (equivalent carbon atom amount of cellulose nanofiber / added amount of propylene glycol (X / Y) = 0.36) were added all at once, and the mixture was stirred at a speed of 400 rpm for 12 hours to obtain a slurry water containing Li3PO4. 2 obtained. The obtained slurry water i 2 Nitrogen purging was carried out to the slurry water i 2 After setting the dissolved oxygen concentration to 0.5 mg / L, the slurry water i 2 Add 16.88g of MnSO4·5H2O and 8.34g of FeSO4·7H2O to the total amount of water to make slurry. 3 The molar ratio of the added MnSO4 to FeSO4 (manganese compound:iron compound) was 70:30. Then, the obtained slurry water i 3 was placed in an autoclave and subjected to a hydrothermal reaction 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 part by mass of the crystals. The washed crystals were freeze-dried at -50°C for 12 hours to obtain preliminary particles b1. The obtained preliminary particles b1 were fired at 700°C for 1 hour in a nitrogen atmosphere to form particles B1 (composition: LiMn 0.7 Fe 0.3 PO4) was obtained. FIG. 1 shows an SEM photograph of the obtained particles B1 (measuring device: JEM-ARM200F (manufactured by JEOL Ltd.), magnification: ×2,000,000).

[0074] [Production Example 5: Production of Particles B2] Slurry water i 1Particles B2 (LiMn 0.7 Fe 0.3 PO4) was obtained.

[0075] [Production Example 6: Production of Particles B3] Slurry water i 1 Particles B3 (LiMn 0.7 Fe 0.3 PO4) was obtained.

[0076] [Production Example 7: Production of Particles B4] Slurry water i 3 Particles B4 (LiMn 0.7 Fe 0.3 PO4) was obtained.

[0077] [Production Example 8: Production of Particles B5] Spare particle b 1 To 1 part by mass of the granules, 1 part by mass of water was added. The resulting slurry was dispersed for 1 minute using an ultrasonic agitator (T25, manufactured by IKA Corporation) 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 Z1. The hot air temperature during spray-drying was 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 2500. The obtained granules Z1 were fired at 700°C for 1 hour in a nitrogen atmosphere to obtain particles B5 (LiMn 0.7 Fe 0.3 PO4) was obtained.

[0078] [Production Example 9: Production of Particles B6] Spare particle b 1Particles B6 (LiMn 0.7 Fe 0.3 PO4) was obtained. FIG. 2 shows an SEM photograph of the obtained particles B6 (measuring device: JEM-2100F (manufactured by JEOL Ltd.), magnification: ×1,200,000).

[0079] [Examples 1 to 6, Comparative Examples 1 to 6] The obtained particles A and particles B were mixed manually using a pestle and mortar in the mass ratio (A:B) shown in Table 1 to obtain each mixture.

[0080] <Evaluation of battery characteristics (cycle characteristics and discharge capacity per unit volume)> The resulting mixtures were used as cathode materials to fabricate cathodes for lithium-ion secondary batteries. Specifically, the resulting mixtures, 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 cathode slurry. The cathode slurry was applied to a 20 μm-thick aluminum foil current collector using a coating machine and vacuum dried at 80°C for 12 hours. The resulting mixtures were then punched into 14 mm diameter disks and pressed at 16 MPa for 2 minutes using a hand press to form cathodes.

[0081] 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 1 mol / L solution of LiPF6 in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7. 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 using standard methods to obtain a coin-type secondary battery (CR-2032).

[0082] Using the obtained coin-type secondary battery, charging and discharging was repeated 50 times at 1C in an environment of 30°C using a discharge capacity measuring device (HJ-1001SD8, manufactured by Hokuto Denko Corporation), and the discharge capacity (mAh / g) was determined, and the cycle characteristic value (capacity retention rate (%)) was calculated using the following formula (x). Cycle characteristics = (discharge capacity after 50 cycles) / (discharge capacity after 1 cycle) × 100 (x) Furthermore, the electrode density was calculated by the following formula (y), and this was introduced into the following formula (z) to calculate the discharge capacity per unit volume. Electrode density (g / cm 3 )= Mass of positive electrode active material in the positive electrode (g) / electrode volume (cm 3 )(φ14mm×thickness(μm)) (y) Discharge capacity per unit volume of positive electrode at 30°C (mAh / cm 3 )= Discharge capacity at 30°C (mAh / g) x electrode density (g / cm 3 ) ···(z) The results are shown in Table 1.

[0083] [Table 1]

Claims

1. The following formula (a): LiM 1 a Mn b O 4 ・・・(a) (In formula (a), M 1 represents one or more elements selected from Ni, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, Cu, and Si; a and b are in the range of 0≦a≦0.1, 0<b≦2, and (M 1 (valence of Mn) × a + (valence of Mn) × b = 7. and particles A having an average particle size of 2 μm to 15 μm; The following formula (b): Li f Mn g Fe h M 2 x 2O 4 ・・・(b) (In formula (b), M 2 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x satisfy the following: 0<f≦1.2, 0.3≦g≦1.2, 0.2≦h≦1.2, 0≦x≦0.3, and f+(valence of Mn)×g+(valence of Fe)×h+(valence of Mn). 2 (valence of x) × x = 3. Particles B having an average particle size of 50 nm to 400 nm and having a surface covered with carbon at a coverage of 5% to 57%. and a mixture of A positive electrode active material particle mixture for a lithium ion secondary battery, wherein the mass ratio (A:B) of particles A to particles B in the mixture is 90:10 to 50:

50.

2. 2. The positive electrode active material particle mixture for a lithium ion secondary battery according to claim 1, wherein the carbon content in particle B is 0.5% by mass to 5.0% by mass.

3. 3. The positive electrode active material particle mixture for a lithium ion secondary battery according to claim 1, wherein g and h in formula (b) satisfy 0.2≦g / h≦5.

0.

4. Particles A represented by the above formula (a), a carbon coating agent X, which is one or more carbon materials selected from saccharides, and a carbon coating inhibitor Y, which is one or more carbon materials selected from polyols, amines, and amides other than saccharides, and a particle B, which is represented by the above formula (b), whose surface is coated with carbon; A step of mixing particles A and particles B so that the mass ratio (A:B) is 90:10 to 50:

50. The method for producing a positive electrode active material particle mixture for a lithium ion secondary battery according to any one of claims 1 to 3, comprising:

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