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

Abstract

To provide positive electrode active material particles for lithium ion secondary batteries that can effectively suppress side reactions during charge / discharge cycles of the lithium ion secondary batteries and effectively improve the cycle characteristics of the lithium ion secondary batteries, and a method for manufacturing the same.SOLUTION: Lithium-based polyanion particles for a secondary battery positive electrode active material are expressed by the following formula (A): LiaMnbFecMxPO4 (In formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd. a, b, c, and x represent numerical values satisfying 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), carbon derived from cellulose nanofibers is carried thereon, the contents of Mn and Fe increase from the center to the surface, and a pore volume in a pore diameter range of 0.5 nm to 20 nm ranges from 0.005 cm3 / g to 0.035 cm3 / g.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a positive electrode active material particle for a lithium ion secondary battery exhibiting good cycle characteristics and a method for producing 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, lithium-based polyanion particles such as LiMn x Fe 1-x PO4 are regarded as promising. On the other hand, since lithium-based polyanion particles have low conductivity and still need improvement to sufficiently enhance the battery characteristics in the obtained lithium ion secondary battery, various developments have been made conventionally.

[0003] For example, Patent Document 1 discloses an electrode material that is an aggregate of electrode active materials (such as Li a A x M y PO4, etc.) coated with a specific carbonaceous film. By using an ionic organic substance as a carbon source, the particle growth and sintering of the electrode active material are suppressed, and the cycle characteristics are improved. Patent Document 2 also discloses a composite particle containing a carbon material such as a fibrous carbon material or a chain-like carbon material and a lithium-containing phosphate (such as LiMn x Fe 1-x PO4, etc.) and having pores leading to the outside of the composite particle starting from the carbon material. Attempts have been made to improve the rate characteristics of the battery and enhance the cycle characteristics and the like.

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, in the electrode active material described in Patent Document 1 and the composite particles described in Patent Document 2, when producing a positive electrode, they are likely to absorb liquid agents such as solvents and binders used for preparing the composite material layer, and the adhesive strength of the composite material layer to the current collector decreases, which highly likely causes peeling of the composite material layer during charge and discharge cycles of the lithium-ion secondary battery. There is still room for improvement to sufficiently enhance the 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 manufacturing the same, which can effectively suppress side reactions during charge and discharge cycles of the lithium-ion secondary battery and effectively improve the cycle characteristics of the lithium-ion secondary battery.

Means for Solving the Problems

[0007] Therefore, as a result of intensive studies to solve the above problems, the present inventor has found that specific lithium-based polyanion particles in which the contents of Mn and Fe increase from the center to the surface can effectively improve the cycle characteristics in the obtained lithium-ion secondary battery.

[0008] That is, the present invention provides the following formula (A): Li a Mn b Fe c M x PO4···(A) (In formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd. a, b, c, and x satisfy 0 < a ≤ 1.2, 0 ≤ b ≤ 1.2, 0 ≤ c ≤ 1.2, 0 ≤ x ≤ 0.3, and b + c ≠ 0, and a + (valence of Mn) × b + (valence of Fe) × c + (valence of M) × x = 3.) It is a lithium-based polyanion particle represented by , in which carbon derived from cellulose nanofibers is supported, and the contents of Mn and Fe increase from the center toward the surface. The pore volume in the pore diameter range of 0.5 nm to 20 nm is 0.005 cm 3 / g to 0.035 cm 3 / g, and it provides lithium-based polyanion particles for a positive electrode active material of a secondary battery.

[0009] Further, the present invention includes the following steps (I) to (IV): (I) A step of mixing a lithium compound, a phosphoric acid compound, and water, or mixing lithium phosphate particles and water to obtain preliminary particles i. (II) A step of adding the obtained preliminary particles i, cellulose nanofibers, and water to obtain slurry water a, and then subjecting it to spray drying to obtain granulated bodies ii. (III) A step of firing the obtained granulated bodies ii to obtain a composite iii. (IV) A step of adding the obtained composite iii, a metal compound containing a manganese compound and / or an iron compound, a phosphoric acid compound, a wetting agent, and water to obtain slurry water b, and then subjecting it to a hydrothermal reaction to obtain a composite iv. It comprises The method for producing the lithium-based polyanion particles for a positive electrode active material of a secondary battery, wherein the addition amount of the wetting agent in step (IV) is 0.01 part by mass to 4 parts by mass with respect to 100 parts by mass of the composite iii.

Effect of the Invention

[0010] For the lithium-based polyanion particles for a positive electrode active material of a secondary battery of the present invention, since the absorption of liquid agents such as solvents and binders used in the preparation of the composite layer is effectively suppressed, a positive electrode material capable of effectively improving the cycle characteristics of a lithium-ion secondary battery can be realized.

Brief Description of the Drawings

[0011]

Figure 1

Mode for Carrying Out the Invention

[0012] Hereinafter, the present invention will be described in detail. The lithium-based polyanion particles for the positive electrode active material of the secondary battery of the present invention have the following formula (A): Li a Mn b Fe c M x PO4···(A) (In formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd. a, b, c, and x satisfy 0 < a ≤ 1.2, 0 ≤ b ≤ 1.2, 0 ≤ c ≤ 1.2, 0 ≤ x ≤ 0.3, and b + c ≠ 0, and represent numbers that satisfy a + (valence of Mn) × b + (valence of Fe) × c + (valence of M) × x = 3.) It is a lithium-based polyanion particle represented by the formula, in which carbon derived from cellulose nanofibers is supported, and the contents of Mn and Fe increase from the center toward the surface, and the pore volume in the pore diameter range of 0.5 nm to 20 nm is 0.005 cm 3 / g to 0.035 cm 3 / g.

[0013] That is, the lithium-based polyanion particles for the positive electrode active material of the secondary battery of the present invention are particles in which carbon derived from cellulose nanofibers is firmly supported, the contents of Mn and Fe increase from the center toward the surface, and as shown by having a specific pore volume in the above specific pore diameter range, the abundance of Mn and Fe gradually decreases from the vicinity of the surface toward the vicinity of the center, that is, particles having a surface with a higher abundance of Mn and Fe than the center. Thus, in the lithium-based polyanion particles for a secondary battery cathode active material of the present invention, as the primary particles constituting such particles carry carbon derived from cellulose nanofibers and Mn and Fe are densely present as they approach the surface of the lithium-based polyanion particles corresponding to secondary particles, which are the secondary battery cathode active material, they have a surface property that is difficult to absorb liquid agents such as solvents and binders used in the preparation of the composite layer, and it is considered that the cycle characteristics of the lithium-ion secondary battery are effectively improved.

[0014] The lithium-based polyanion particles for a secondary battery cathode active material of the present invention are represented by the following formula (A): Li a Mn b Fe c M x PO4···(A) (In formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd. a, b, c, and x satisfy 0 < a ≤ 1.2, 0 ≤ b ≤ 1.2, 0 ≤ c ≤ 1.2, 0 ≤ x ≤ 0.3, and b + c ≠ 0, and represent numbers that satisfy a + (valence of Mn) × b + (valence of Fe) × c + (valence of M) × x = 3.) It is represented by.

[0015] That is, the lithium-based polyanion particles for a secondary battery positive electrode represented by the above formula (A) are lithium-based polyanion particles for a secondary battery positive electrode containing at least manganese (Mn) or iron (Fe) (hereinafter, also referred to as "particles (A)"), and are secondary particles formed by aggregation of primary particles. In formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd, preferably Mg or Zr. a satisfies 0 < a ≤ 1.2, preferably 0.6 ≤ a ≤ 1.2, more preferably 0.65 ≤ a ≤ 1.15, and even more preferably 0.7 ≤ a ≤ 1.1. b satisfies 0 ≤ b ≤ 1.2, preferably 0.25 ≤ b ≤ 0.8, more preferably 0.35 ≤ b ≤ 0.8. c satisfies 0 ≤ c ≤ 1.2, preferably 0.2 ≤ c ≤ 0.75, more preferably 0.2 ≤ c ≤ 0.65. And b and c satisfy b + c ≠ 0. x satisfies 0 ≤ x ≤ 0.3, preferably 0 ≤ x ≤ 0.15, more preferably 0 ≤ x ≤ 0.1. And a, b, c, and x are numbers that satisfy a + (valence of Mn) × b + (valence of Fe) × c + (valence of M) × x = 3. More specifically, examples of the particles (A) represented by the above formula (A) include, for example, LiMnPO4, LiFePO4, LiMn 0.8 Fe 0.2 PO4, LiMn 0.7 Fe 0.3 PO4, LiMn 0.1 Fe 0.9 PO4, LiMn 0.8 Fe 0.1 Mg 0.1 PO4, LiMn 0.8 Fe 0.1 Zr 0.05 PO4, etc.

[0016] The lithium-based polyanion particles for a secondary battery positive electrode active material of the present invention are represented by the above formula (A) and are supported by carbon derived from cellulose nanofibers. Such cellulose nanofibers are carbonized to become carbon, which is supported on the surface of the primary particles so as to fill a part of the gaps between the primary particles constituting the particles (A), and is also firmly supported on the surface of the particles (A). Cellulose nanofibers are a skeletal component that accounts for about 50% of all plant cell walls, and are lightweight and high-strength fibers that can be obtained by defibrating plant fibers constituting such cell walls to the nanosize. The fiber diameter of such cellulose nanofibers is 1 nm to 1000 nm, and it also has good dispersibility in water. Further, in the cellulose molecular chains constituting the cellulose nanofibers, a periodic structure by carbon is formed, so that this is carbonized and firmly supported on the particles (A). While the abundance of Mn and Fe is reduced from the surface toward the center, excellent battery characteristics can be ensured for the entire particles (A).

[0017] In addition, the particles (A) may carry carbon derived from a water-soluble carbon material together with carbon derived from cellulose nanofibers. Such a water-soluble carbon material, like cellulose nanofibers, is carbonized to become carbon, which is carried on the surface of the primary particles so as to fill a part of the gaps between the primary particles constituting the particles (A), and is also firmly carried on the surface of the particles (A). Examples of such water-soluble carbon materials include one or more selected from saccharides, polyols, polyethers, and organic acids. More specifically, for example, monosaccharides such as glucose, fructose, galactose, mannose; disaccharides such as maltose, sucrose, cellobiose; polysaccharides such as starch, dextrin; polyols and polyethers such as ethylene glycol, propylene glycol, diethylene glycol, polyethylene glycol, butanediol, propanediol, polyvinyl alcohol, glycerin; and organic acids such as citric acid, tartaric acid, ascorbic acid. Among them, from the viewpoint of enhancing solubility and dispersibility in a solvent and effectively functioning as a carbon material, glucose, fructose, sucrose, and dextrin are preferred, and glucose is more preferred.

[0018] The amount of carbon derived from cellulose nanofibers, which is carried on the above particles (A) and is carbonized, that is, the amount of carbon derived from cellulose nanofibers carried, is preferably 0.9% by mass to 1.4% by mass, more preferably 0.8% by mass to 1.3% by mass, and still more preferably 0.9% by mass to 1.2% by mass in 100% by mass of the lithium-based polyanion particles for secondary battery positive electrode active material of the present invention. The amount of carbon derived from cellulose nanofibers (carried amount) present in the particles (A) is determined by measurement using a carbon-sulfur analyzer.

[0019] The lithium-based polyanion particles for secondary battery positive electrode active material of the present invention have a pore volume in the pore diameter range of 0.5 nm to 20 nm of 0.005 cm 3 / g to 0.035 cm 3 / g. Thus, the lithium-based polyanion particles for a secondary battery cathode active material of the present invention have a specific shape in which the primary particles constituting the particles (A) are densely present, in order to have a surface property that is difficult to absorb liquid agents such as solvents and binders used in the preparation of the composite layer. Therefore, the lithium-based polyanion particles for a secondary battery cathode active material of the present invention contain an appropriate amount of Mn and Fe as a whole, while the abundance of Mn and Fe is reduced from the surface toward the center, and can have a surface property that is difficult to absorb liquid agents such as solvents and binders used in the preparation of the composite layer.

[0020] The lithium-based polyanion particles for a secondary battery cathode active material of the present invention have a pore volume in the pore diameter range of 0.5 nm to 20 nm of 0.005 cm 3 / g to 0.035 cm 3 / g, preferably 0.005 cm 3 / g to 0.030 cm 3 / g, more preferably 0.005 cm 3 / g to 0.025 cm 3 / g, from the viewpoint of ensuring the expression of excellent battery characteristics. Note that the pore volume in the pore diameter range of 0.5 nm to 20 nm means the value measured by the BET multipoint method.

[0021] Specifically, as shown in FIG. 1, the lithium-based polyanion particles for a secondary battery cathode active material of the present invention are divided into a cross-section X including the center 2 of the lithium-based polyanion particles and a cross-section Y including the surface of the lithium-based polyanion particles with a circle 5 formed by connecting points 4 that bisect the radius 3 of the cross-section 1 as a boundary passing through the center 2 of the cross-section 1 of the lithium-based polyanion particles. When divided, the ratio (x / A) of the content x of Mn and Fe in the cross-section X to the content A of Mn and Fe in the lithium-based polyanion particles is 0.85 to 0.99, and the ratio (y / A) of the content y of Mn and Fe in the cross-section Y to the content A of Mn and Fe in the lithium-based polyanion particles is preferably 1.01 to 1.15. Thus, in the cross-section 1 of the lithium-based polyanion particles for the positive electrode active material of the present invention, with respect to the contents A of Mn and Fe in the lithium-based polyanion particles, the contents y of Mn and Fe in the cross-section Y including the surface are higher than the contents x of Mn and Fe in the cross-section X including the center 2. As approaching the surface of the lithium-based polyanion particles for the positive electrode active material of the secondary battery, Mn and Fe are densely present, and it has a surface property that is difficult to absorb liquid agents such as solvents and binders used in the preparation of the composite material layer.

[0022] In addition, the contents of Mn and Fe in the lithium-based polyanion particles for the positive electrode active material of the secondary battery mean the total amounts (mass %) of Mn and Fe when the total amounts of Mn, Fe, and P analyzed by X-ray fluorescence analysis (XRF) are 100 mass %. Also, the contents of Mn and Fe in the cross-section X and the cross-section Y mean the total amounts (mass %) of Mn and Fe when the total amounts of Mn, Fe, and P analyzed by the energy dispersive X-ray analyzer (EDX) of the scanning electron microscope (SEM) are 100 mass %.

[0023] The ratio (x / A) of the content x of Mn and Fe in the cross-section X to the content A of Mn and Fe in the lithium-based polyanion particles is preferably 0.85 to 0.99, more preferably 0.85 to 0.97, and even more preferably 0.85 to 0.95. Also, the ratio (y / A) of the content y of Mn and Fe in the cross-section Y to the content A of Mn and Fe in the lithium-based polyanion particles is preferably 1.01 to 1.15, more preferably 1.03 to 1.15, and even more preferably 1.05 to 1.15.

[0024] From the viewpoint of effectively improving the cycle characteristics in the obtained battery, the average particle size of the lithium-based polyanion particles for the positive electrode active material of the present invention is preferably 10 μm to 40 μm, more preferably 10 μm to 35 μm, and even more preferably 10 μm to 30 μm. Note that the average particle size is D obtained from the volume-based particle size distribution based on the laser diffraction / scattering method. 50It means the value (particle size at 50% cumulative (median diameter)).

[0025] The tap density of the lithium-based polyanion particles for the positive electrode active material of the secondary battery of the present invention is preferably 1.0 g / cm 3 ~1.9 g / cm 3 from the viewpoint of ensuring the expression of excellent battery characteristics as a whole of the particles (A), while Mn and Fe are densely present from the center to the surface, and more preferably 1.1 g / cm 3 ~1.9 g / cm 3 and even more preferably 1.2 g / cm 3 ~1.9 g / cm 3 is. The tap density means the "tapped bulk density" measured by the method specified in JIS R 1628 "Method for Measuring the Bulk Density of Fine Ceramic Powders".

[0026] The BET specific surface area of the lithium-based polyanion particles for the positive electrode active material of the secondary battery of the present invention is preferably 10 m 2 / g to 23 m 2 / g from the viewpoint of ensuring the expression of excellent battery characteristics as a whole of the particles (A), while the contents of Mn and Fe increase from the center to the surface, and more preferably 10 m 2 / g to 20 m 2 / g, and even more preferably 10 m 2 / g to 17 m 2 / g. The BET specific surface area can be measured, for example, using a flow-type specific surface area automatic measuring device (FlowSorbIII2305, manufactured by Shimadzu Corporation) under the condition of using a nitrogen-helium mixed gas containing 30% nitrogen.

[0027] The lithium-based polyanion particles for the positive electrode active material of the secondary battery of the present invention are the following steps (I) to (IV): (I) A step of mixing a lithium compound, a phosphoric acid compound, and water, or mixing lithium phosphate particles and water to obtain preliminary particles i (II) After adding the obtained preliminary particles i, cellulose nanofibers, and water to obtain slurry water a, the slurry water a is subjected to spray drying to obtain granulated particles ii. (III) The obtained granulated particles ii are fired to obtain composite iii. (IV) After adding the obtained composite iii, a metal compound containing a manganese compound and / or an iron compound, a phosphoric acid compound, a wetting agent, and water to obtain slurry water b, the slurry water b is subjected to a hydrothermal reaction to obtain composite iv. It includes It can be obtained by a production method in which the addition amount of the wetting agent in step (IV) is 0.01 part by mass to 4 parts by mass with respect to 100 parts by mass of the composite iii.

[0028] Thus, first, by passing through step (I), preliminary particles i, which are lithium phosphate particles serving as the nuclei of the primary particles constituting particle (A), are formed in the slurry, or lithium phosphate particles and water are mixed to prepare a slurry containing preliminary particles i. Then, by passing through steps (II) to (III), composite iii in which carbon derived from cellulose nanofibers is carried on the periphery of preliminary particles i and Mn and Fe are appropriately present can be obtained. Thereafter, by passing through step (IV), with the intervention of the wetting agent, primary particles are formed while allowing Mn and Fe to penetrate into the composite iii, and further, these primary particles aggregate to form composite iv, and lithium-based polyanion particles for a secondary battery positive electrode active material having a surface property that is difficult to absorb liquid agents such as solvents and binders used for preparing the composite material layer of the lithium ion secondary battery can be obtained.

[0029] Step (I) included in the production method of the present invention is a step of mixing a lithium compound, a phosphoric acid compound, and water, or mixing lithium phosphate particles and water to obtain preliminary particles i.

[0030] In step (I), when mixing a lithium compound, a phosphoric acid compound, and water, examples of the lithium compound include hydroxides (for example, LiOH·H2O, LiOH), carbonates, sulfates, and acetates. Among them, hydroxides are preferable. Examples of the phosphoric acid compound include orthophosphoric acid (H3PO4, 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 preferably used as an aqueous solution with a concentration of 70% by mass to 90% by mass. In the slurry water obtained by mixing a lithium compound, a phosphoric acid compound, and water, the content of the lithium compound per 100 parts by mass of water is preferably 5 parts by mass to 50 parts by mass, more preferably 7 parts by mass to 45 parts by mass.

[0031] Furthermore, in step (I), when using phosphoric acid as the phosphoric acid compound, when mixing phosphoric acid into the slurry water, it is preferable to dropwise add phosphoric acid while stirring the slurry water. The dropping rate of phosphoric acid into the above slurry water 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. Also, the stirring time of the slurry water while dropping phosphoric acid is preferably 0.5 hours to 24 hours, more preferably 3 hours to 12 hours. Furthermore, the stirring speed of the slurry water while dropping 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. Also, when using phosphoric acid as the phosphoric acid compound and mixing it, it is preferable to dropwise add phosphoric acid while stirring the slurry water. By dropwise adding phosphoric acid to the mixed solution little by little, the reaction proceeds well in the slurry water, and the preliminary particles i are generated while being uniformly dispersed in the slurry water, and it is possible to effectively suppress the unnecessary aggregation of such preliminary particles i. When stirring the slurry water, it is further preferable to cool it to a temperature below the boiling point temperature of the slurry water. Specifically, it is preferably cooled to 80°C or lower, and more preferably cooled to 20°C to 60°C.

[0032] The slurry water after mixing the phosphate compound preferably contains 2.7 to 3.3 moles of lithium per 1 mole of phosphoric acid, more preferably 2.8 to 3.1 moles of lithium. The above lithium compound and phosphate compound may be used so that such an amount is obtained.

[0033] By purging nitrogen into the slurry water after mixing the phosphate compound, the reaction in such a mixed solution is completed, and preliminary particles i, which are lithium phosphate particles as the nuclei of the primary particles constituting the particles (A), can be obtained as a slurry. When nitrogen is purged, the reaction can proceed in a state where the dissolved oxygen concentration in the slurry water is reduced, and the dissolved oxygen concentration of the slurry containing the obtained composite X is also effectively reduced. Therefore, the oxidation of the metal salt added in the subsequent step (IV) can be suppressed. Such preliminary particles i are obtained as fine dispersed particles of lithium phosphate (Li3PO4). The obtained preliminary particles i may be isolated by filtering, washing with water, and drying, or may be used as a slurry containing the preliminary particles i as they are in the subsequent step (II). Freeze drying or vacuum drying is used as the drying means.

[0034] In step (I), when mixing lithium phosphate particles and water, the preliminary particles i are obtained as a slurry containing them. In the subsequent step (II), the slurry containing such preliminary particles i may be used as the preliminary particles i obtained in step (I) as they are. In this case, the content of the preliminary particles i in the slurry is preferably 3% by mass to 50% by mass, more preferably 5% by mass to 45% by mass.

[0035] Step (II) is a step of adding the preliminary particles i obtained in step (I), cellulose nanofibers, and water to obtain slurry water a, and then subjecting it to spray drying to obtain granulated bodies ii. Here, by using cellulose nanofibers and going through subsequent steps, the cellulose nanofibers can be carbonized, and carbon derived from the cellulose nanofibers can be supported on the preliminary particles i corresponding to the nuclei of the primary particles constituting the particles (A). The cellulose nanofibers that can be used in step (II) are as described above.

[0036] The content of cellulose nanofibers in slurry water a is preferably 0.5 parts by mass to 5 parts by mass, more preferably 0.5 parts by mass to 4.5 parts by mass, and still more preferably 0.5 parts by mass to 4.0 parts by mass, in terms of the amount of residue after carbonization treatment, per 100 parts by mass of water.

[0037] The solid content concentration of slurry water a is preferably 40% by mass to 70% by mass, more preferably 45% by mass to 70% by mass, and still more preferably 50% by mass to 70% by mass. When the preliminary particles i obtained in step (I) are used as a slurry, the amount of water added in step (II) may be appropriately adjusted so that the solid content concentration of slurry water a becomes such an amount.

[0038] It is preferable to stir slurry water a in advance after adding water and before subjecting it to spray drying. The stirring time of such slurry water a is preferably 3 minutes to 60 minutes, more preferably 5 minutes to 30 minutes. Also, the temperature of slurry water a is preferably 10°C to 60°C, more preferably 20°C to 40°C.

[0039] Next, the obtained slurry water a is subjected to spray drying to obtain granulated particles ii. In spray drying, the operating conditions may be appropriately set according to the apparatus used. For example, as the treatment conditions in a micro mist 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. Also, the volume 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.

[0040] Step (III) is a step of firing the granulated product ii obtained in step (II) to obtain the composite iii. The firing conditions for such step (III) are preferably in a reducing atmosphere or an inert atmosphere. The firing temperature is preferably 500°C to 800°C, more preferably 550°C to 750°C. The firing time is preferably 0.5 hour to 12 hours, more preferably 1 hour to 6 hours.

[0041] Step (IV) is a step of adding the composite iii obtained in step (III), a metal compound containing a manganese compound and / or an iron compound, a phosphoric acid compound, a wetting agent, and water to obtain slurry water b, and then drying to obtain the composite iv. The addition amount of the wetting agent is 0.01 part by mass to 4 parts by mass with respect to 100 parts by mass of the composite iii. Thereby, Mn and Fe can be densely present around the composite iii, and the content of Mn and Fe can be increased from the center to the surface of the composite iv. By using a specific amount of the wetting agent, primary particles can be formed while allowing Mn and Fe to penetrate into the composite iii, and the aggregation of the primary particles can be effectively promoted.

[0042] The phosphoric acid compound may be the same as that used in step (I). As the metal compound containing a manganese compound and / or an iron compound, in addition to the manganese compound and the iron compound, a metal (M) compound can be used. Examples of the manganese compound 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 them, from the viewpoint of improving battery characteristics, manganese sulfate is preferable. 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, from the viewpoint of improving battery characteristics, iron sulfate is preferable. Examples of the metal (M) compound that can be used include sulfates, nitrates, acetates, etc. containing a metal synonymous with M in the above formula (A). Among them, from the viewpoint of improving battery characteristics, sulfates are preferable.

[0043] The addition amount of the metal compound containing a manganese compound and / or an iron compound is preferably 200 parts by mass to 600 parts by mass, more preferably 250 parts by mass to 550 parts by mass, and still more preferably 300 parts by mass to 500 parts by mass with respect to 100 parts by mass of the composite iii. Further, the molar ratio of the manganese compound to the iron compound used (manganese compound: iron compound) is preferably 80:20 to 20:80, more preferably 80:20 to 30:70, and still more preferably 80:20 to 35:65. The addition amount of the phosphoric acid compound is preferably 50 parts by mass to 300 parts by mass, more preferably 70 parts by mass to 250 parts by mass, and still more preferably 90 parts by mass to 200 parts by mass with respect to 100 parts by mass of the composite iii.

[0044] Examples of the wetting agent include one or more selected from polyether ester amine, polyether phosphate ester amine, and polyether phosphate ester. Among them, polyether ester amine is preferable from the viewpoint of effectively promoting the dense aggregation of primary particles and increasing the contents of Mn and Fe from the center to the surface. The addition amount of the wetting agent is 0.01 parts by mass to 4 parts by mass, preferably 0.1 parts by mass to 4 parts by mass, and more preferably 0.5 parts by mass to 4 parts by mass with respect to 100 parts by mass of the composite iii. Note that, by passing through the step (IV) described later, the added wetting agent will be burned out and will not remain in the obtained particles (A).

[0045] In step (IV), from the viewpoint of fully exerting the effect of adding the wetting agent and effectively obtaining the particles (A) in which the contents of Mn and Fe increase from the center to the surface, the addition order of the composite iii, the metal compound containing a manganese compound and / or an iron compound, the phosphoric acid compound, the wetting agent, and water is preferably to first add the wetting agent and water, then add the composite iii, and then add the manganese compound and / or the iron compound and the phosphoric acid compound to obtain the slurry water b.

[0046] The solid content concentration of the slurry water b is preferably 5% by mass to 50% by mass, more preferably 8% by mass to 45% by mass, and still more preferably 10% by mass to 40% by mass. In addition, the conditions for stirring the slurry water b before subjecting it to the hydrothermal reaction are the same as the conditions for stirring the slurry water a before subjecting it to spray drying in step (II), and can be appropriately selected.

[0047] Next, in step (IV), the obtained slurry water b is subjected to a hydrothermal reaction to obtain the composite iv. The hydrothermal reaction may be at 100°C or higher, preferably 130°C to 200°C. The hydrothermal reaction is preferably carried out in a pressure-resistant container. When the reaction is carried out at 130°C to 200°C, the pressure at this time is preferably 0.3 MPa to 1.6 MPa, and when the reaction is carried out at 140°C to 160°C, the pressure is preferably 0.3 MPa to 0.6 MPa. The hydrothermal reaction time is preferably 0.1 hour to 48 hours, and more preferably 0.2 hour to 24 hours. The obtained composite iv is isolated by filtration, washing with water, and drying, and obtained as the lithium-based polyanion particles for the positive electrode active material of the secondary battery of the present invention. As the drying means, freeze drying or vacuum drying is used.

[0048] The lithium-based polyanion particles for the positive electrode active material of the secondary battery of the present invention are materials used as the positive electrode active material of a lithium-ion secondary battery. Specifically, for example, after kneading the lithium-based polyanion particles for the positive electrode active material of the present invention with acetylene black, ketjen black, polyvinylidene fluoride, N-methyl-2-pyrrolidone, etc. to prepare a positive electrode slurry, it is coated on a current collector, and then press-molded to produce a positive electrode. If it is the lithium-based polyanion particles for the positive electrode active material of the secondary battery of the present invention, the contents of Mn and Fe on the surface are higher than those at the center, and it has a surface property that is difficult to absorb liquid agents such as solvents and binders used in the preparation of the composite layer of the lithium-ion secondary battery. Therefore, it can exhibit excellent adhesive strength of the composite layer to the current collector, effectively suppress the peeling of the composite layer during the charge and discharge cycle of the obtained lithium-ion secondary battery, and effectively improve the cycle characteristics.

[0049] As a lithium-ion secondary battery to which a positive electrode obtained by using the lithium-based polyanion particles for a positive electrode active material of the present invention can be applied, there is no particular limitation as long as it essentially includes a positive electrode, a negative electrode, an electrolytic solution, and a separator, or a positive electrode, a negative electrode, and a solid electrolyte.

[0050] Here, regarding the negative electrode, as long as it can occlude lithium ions during charging and release them during discharging, its material composition is not particularly limited, and a known material composition can be used. For example, lithium metal, graphite, silicon-based (Si, SiOx), lithium titanate, or a carbon material such as amorphous carbon can be used. And it is preferable to use an electrode formed of an intercalation material capable of electrochemically occluding and releasing lithium ions, particularly a carbon material. Furthermore, two or more of the above negative electrode materials may be used in combination. For example, a combination of graphite and silicon-based can be used.

[0051] The electrolytic solution is obtained by dissolving a supporting salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent usually used in the electrolytic solution of a lithium-ion secondary battery. For example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, oxolane compounds, etc. can be used.

[0052] The type of the supporting salt is not particularly limited, but it is preferably at least one of inorganic salts selected from LiPF6, LiBF4, LiClO4, and LiAsF6, derivatives of the inorganic salts, organic salts selected from LiSO3CF3, LiC(SO3CF3)2, and LiN(SO3CF3)2, LiN(SO2C2F5)2, and LiN(SO2CF3)(SO2C4F9), and derivatives of the organic salts.

[0053] The separator serves to electrically insulate the positive electrode and the negative electrode and hold the electrolytic solution. For example, a porous synthetic resin film, particularly a porous film of a polyolefin-based polymer (polyethylene, polypropylene) can be used.

[0054] The solid electrolyte electrically insulates the positive electrode and the negative electrode and exhibits high lithium ion conductivity. For example, La 0.51 Li 0.34 TiO 2.94 , Li 1.3 Al 0.3 Ti 1.7 (PO4)3, Li7La3Zr2O 12 , 50Li4SiO4·50Li3BO3, Li 2.9 PO 3.3 N 0.46 , Li 3.6 Si 0.6 P 0.4 , Li 1.07 Al 0.69 Ti 1.46 (PO4)3, Li 1.5 Al 0.5 Ge 1.5 (PO4)3, Li 10 GeP2S 12 , Li 3.25 Ge 0.25 P 0.75 S4, 30Li2S·26B2S3·44LiI, 63Li2S·36SiS2·1Li3PO4, 57Li2S·38SiS2·5Li4SiO4, 70Li2S·30P2S5, 50Li2S·50GeS2, Li7P3S 11 , Li 3.25 P 0.95 S4 may be used.

[0055] The shape of the lithium ion secondary battery having the above configuration is not particularly limited, and may be various shapes such as coin type, cylindrical type, rectangular type, or an irregular shape enclosed in a laminated exterior body.

Examples

[0056] Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to these examples. Note that the carbon equivalent amount (loading amount) and the average particle size of the obtained lithium-based polyanion particles were measured by the following method.

[0057] 《Carbon atom equivalent amount (loading amount)》 Using a carbon and sulfur analyzer (EMIA-220V2, manufactured by Horiba, Ltd.), the carbon atom equivalent amount (loading amount) of the obtained lithium-based polyanion particles was measured.

[0058] 《Average particle size》 The particle size distribution of the obtained lithium-based polyanion particles was measured using a laser diffraction apparatus (Microtrac MT3000II, manufactured by MicrotracBEL), and the average particle size (D 50 ) was determined.

[0059] [Example 1] 1272 g of LiOH·H2O and 4 L of water were mixed to obtain a slurry. Next, while maintaining the temperature of the obtained slurry at 25°C and stirring for 3 minutes, 1153 g of an 85% aqueous phosphoric acid solution was added dropwise at 35 mL / min, and the mixture was stirred at a speed of 400 rpm for 12 hours to obtain a slurry containing preliminary particles i-1 (Li3PO4). After filtering the obtained slurry, it was washed with water and dried with warm air at 80°C for 12 hours to isolate the preliminary particles i-1. 1158 g of the obtained preliminary particles i-1 were taken, 3 L of water was added thereto, and subsequently 2114 g of cellulose nanofibers (Cellish KY-100G, manufactured by Daicel Finechem, fiber diameter 4 nm to 100 nm) was added to obtain slurry a1. The obtained slurry a1 was dispersed using an ultrasonic stirrer (T25, manufactured by IKA) for 2 minutes to make the whole uniformly colored, and then spray-dried (nozzle air flow rate 52 L / min, supply air temperature 180°C) using a spray dryer (MDL-050M, manufactured by Fujisaki Electric Co., Ltd.) to obtain granulated particles ii-1.

[0060] The obtained granule ii-1 was calcined at 650 °C for 30 minutes in an argon-hydrogen atmosphere (hydrogen concentration 3%) to obtain a composite iii-1. 585 g of the obtained composite iii-1 was taken, added to a mixed solution containing 0.06 g of polyether ester amine (Disparon DA-234, manufactured by Enomoto Kasei Co., Ltd.) (corresponding to 0.01 part by mass based on 100 parts by mass of the composite iii-1) and 5 L of water, and then 2415 g of MnSO4·5H2O, 1251 g of FeSO4·7H2O, and 1153 g of an 85% phosphoric acid aqueous solution were added to obtain slurry water b1. Next, the obtained slurry water b1 was put into an autoclave and subjected to a hydrothermal reaction at 170 °C for 2 hours. The pressure in the autoclave was 0.3 MPa. After the hydrothermal reaction, the generated crystals were filtered and then washed with 12 parts by mass of water per 1 part by mass of the crystals. The washed crystals were freeze-dried at -50 °C for 12 hours to obtain lithium-based polyanion particles (LiMn 0.7 Fe 0.3 PO4, carbon loading = 1.1% by mass, average particle size: 15 μm).

[0061] [Example 2] Lithium-based polyanion particles (LiMn 0.7 Fe 0.3 PO4, carbon loading = 1.1% by mass, average particle size: 15 μm) were obtained in the same manner as in Example 1, except that the addition amount of the polyether ester amine was 0.78 g (corresponding to 0.13 part by mass based on 100 parts by mass of the composite iii-1).

[0062] [Example 3] Lithium-based polyanion particles (LiMn 0.7 Fe 0.3 PO4, carbon loading = 1.1% by mass, average particle size: 14 μm) were obtained in the same manner as in Example 1, except that the addition amount of the polyether ester amine was 3.9 g (corresponding to 0.65 part by mass based on 100 parts by mass of the composite iii-1).

[0063] [Example 4] Except that the addition amount of the polyether ester amine was 7.8 g (equivalent to 1.3 parts by mass based on 100 parts by mass of the composite iii-1), in the same manner as in Example 1, lithium-based polyanion particles (LiMn 0.7 Fe 0.3 PO4, carbon loading = 1.1% by mass, average particle size: 14 μm) were obtained.

[0064] [Example 5] Except that the addition amount of the polyether ester amine was 23.4 g (equivalent to 4 parts by mass based on 100 parts by mass of the composite iii-1), in the same manner as in Example 1, lithium-based polyanion particles (LiMn 0.7 Fe 0.3 PO4, carbon loading = 1.1% by mass, average particle size: 13 μm) were obtained.

[0065] [Comparative Example 1] Except that no polyether ester amine was added, in the same manner as in Example 1, lithium-based polyanion particles (LiMn 0.7 Fe 0.3 PO4, carbon loading = 1.1% by mass, average particle size: 16 μm) were obtained.

[0066] 《Measurement of Pore Volume in the Pore Size Range of 0.5 nm to 20 nm》 Using each of the obtained lithium-based polyanion particles, the pore volume (cm 3 / g) in the pore size range of 0.5 nm to 20 nm was measured by the BET multipoint method using BELSORP MINI X (manufactured by Shimadzu Corporation). The results are shown in Table 1.

[0067] 《Calculation of the Ratio (x / A) of the Contents x of Mn and Fe in Cross-Section X to the Contents A of Mn and Fe in the Lithium-Based Polyanion Particles, and the Ratio (y / A) of the Contents y of Mn and Fe in Cross-Section Y to the Contents A of Mn and Fe in the Lithium-Based Polyanion Particles》 Using each of the obtained lithium-based polyanion particles, A was calculated as the total amount of Mn and Fe (mass %) when the total amount of Mn, Fe, and P analyzed by XRF (ZSX Primus IV, manufactured by Rigaku Corporation) was set to 100 mass %. Also, using each of the obtained lithium-based polyanion particles, a cross-section passing through the center of the particle was cut out with a laser cutter, and x and y were calculated as the total amount of Mn and Fe (mass %) when the total amount of Mn, Fe, and P analyzed from the average value in 10 fields of 500 nm × 500 nm by SEM-EDX (JSM-7900F, manufactured by JEOL Ltd.) was set to 100 mass %. The results are shown in Table 1.

[0068] 《Evaluation of the Presence or Absence of Impurities》 Using each of the obtained lithium-based polyanion particles, after obtaining an X-ray diffraction pattern at 2θ = 10 to 80° using an X-ray diffractometer (D8 ADVANCE, manufactured by BRUKER Corporation), the presence or absence of impurities was evaluated based on the presence or absence of a peak derived from Li3PO4 at 2θ = 23.8°. The results are shown in Table 1.

[0069] 《Calculation of the Adsorbed Water Content》 Using each of the obtained lithium-based polyanion particles, in accordance with K 5101-13-2:2004 (Method for Measuring Oil Absorption), the adsorbed water content (mass %) was determined using an aqueous solution of 0.5 wt.% carboxymethyl cellulose instead of linseed oil. The results are shown in Table 1.

[0070] 《Evaluation of Battery Characteristics (Cycle Characteristics)》 Using each of the obtained lithium-based polyanion particles, first, a positive electrode for a lithium-ion secondary battery was fabricated. Specifically, each of the obtained particles, Ketjen black, and polyvinylidene fluoride were mixed at a blending ratio of 90:5:5 by mass, and N-methyl-2-pyrrolidone was added thereto and thoroughly kneaded to prepare a positive electrode slurry. The positive electrode slurry was applied to a current collector made of an aluminum foil with a thickness of 20 μm using a coater, and vacuum drying was performed at 80°C for 12 hours. Thereafter, the current collector coated with the positive electrode slurry was pressed at 20 kN using a roll press and punched into a disk shape with a diameter of 14 mm to obtain a positive electrode. Next, a coin-type secondary battery was constructed using the above positive electrode. As the negative electrode, a lithium foil punched out to φ15 mm was used. As the electrolytic solution, a solution in which LiPF6 was dissolved at a concentration of 1 mol / L in a mixed solvent obtained by mixing ethylene carbonate and ethyl methyl carbonate at a volume ratio of 3:7 was used. As the separator, a polymer porous film was used. These battery components were incorporated and housed by a conventional method in an atmosphere with a dew point of -50°C or lower to obtain a coin-type secondary battery (CR-2032). Using the obtained secondary battery, the cycle characteristics were evaluated. Specifically, constant current charging was performed at a current density of 170 mA / g and a voltage of 4.5 V, and constant current discharging was performed at a current density of 85 mA / g and a cut-off voltage of 2.0 V, and the discharge capacity at a current density of 85 mA / g (0.5 CA) was determined. Further, a 100-cycle repetition test under the same charge-discharge conditions was performed, and the capacity retention rate (%) was determined by the following formula (y). Note that all charge-discharge tests were performed at 30°C. Capacity retention rate (%) =(Discharge capacity after 100 cycles) / (Discharge capacity after 1 cycle)×100 ··· (y) The results are shown in Table 1.

[0071]

Table 1

Explanation of symbols

[0072] 1: Cross-section passing through the center 2 of the lithium-based polyanion particles 2: Center of the lithium-based polyanion particles 3: Radius of the cross-section 4: Point bisecting the radius 3 of the cross-section 5: Circle formed by the continuous points 4 bisecting the radius 3 of the cross-section X: Cross-section X including the center 2 of the lithium-based polyanion particles Y: Cross-section Y including the surface of the lithium-based polyanion particles

Claims

1. The following formula (A): Li a Mn b Fe c M x PO 4 ... (A) (In formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd. a, b, c, and x satisfy 0 < a ≤ 1.2, 0 ≤ b ≤ 1.2, 0 ≤ c ≤ 1.2, 0 ≤ x ≤ 0.3, and b + c ≠ 0, and a + (valence of Mn)×b + (valence of Fe)×c + (valence of M)×x = 3. ) A lithium-based polyanion particle represented by the formula, in which carbon derived from cellulose nanofibers is supported, and the contents of Mn and Fe increase from the center toward the surface, Lithium-based polyanion particles for a positive electrode active material of a secondary battery, having a pore volume in a pore diameter range of 0.5 nm to 20 nm of 0.005 cm 3 / g to 0.035 cm 3 / g.

2. When a cross-section passing through the center of the lithium-based polyanion particle is divided into two parts, a cross-section X including the center of the lithium-based polyanion particle and a cross-section Y including the surface of the lithium-based polyanion particle, with a circle formed by connecting the points that bisect the radius of the cross-section as the boundary, The ratio (x / A) of the contents x (mass%) of Mn and Fe when the total amount of Mn, Fe, and P in cross-section X is 100 mass% to the contents A (mass%) of Mn and Fe when the total amount of Mn, Fe, and P in the lithium-based polyanion particle is 100 mass% is 0.85 to 0.99, and the ratio (y / A) of the contents y (mass%) of Mn and Fe when the total amount of Mn, Fe, and P in cross-section Y is 100 mass% to the contents A (mass%) of Mn and Fe when the total amount of Mn, Fe, and P in the lithium-based polyanion particle is 100 mass% is 1.01 to 1.

15. The lithium-based polyanion particles for a positive electrode active material of a secondary battery according to Claim 1.

3. The lithium-based polyanion particles for a positive electrode active material of a secondary battery according to Claim 1 or 2, having an average particle diameter of 10 μm to 40 μm.

4. The tap density is 1.0 g / cm 3 ~1.9 g / cm 3 and the BET specific surface area is 10 m 2 / g to 23 m 2 / g, and the lithium-based polyanion particles for a secondary battery positive electrode active material according to any one of claims 1 to 3.

5. The lithium-based polyanion particles for a positive electrode active material of a secondary battery according to any one of Claims 1 to 4, having a carbon loading amount derived from cellulose nanofibers of 0.9 mass% to 1.4 mass%.

6. The following steps (I) to (IV): (I) A step of mixing a lithium compound, a phosphoric acid compound, and water, or mixing lithium phosphate particles and water to obtain preliminary particles i (II) A step of adding the obtained preliminary particles i, cellulose nanofibers, and water to obtain slurry water a, and then subjecting the slurry water a to spray drying to obtain granulated particles ii (III) A step of firing the obtained granulated particles ii to obtain a composite iii (IV) A step of subjecting the obtained composite iii to a heat treatment at a temperature of 600°C to 1000°C for 1 hour to 5 hours to obtain the lithium-based polyanion particles for a positive electrode active material of a secondary battery (IV) After adding the obtained complex iii, a metal compound containing a manganese compound and / or an iron compound, a phosphoric acid compound, a wetting agent, and water to obtain slurry water b, subjecting it to a hydrothermal reaction to obtain complex iv comprising The method for producing lithium-based polyanion particles for a secondary battery positive electrode active material according to any one of claims 1 to 5, wherein the addition amount of the wetting agent in step (IV) is 0.01 parts by mass to 4 parts by mass with respect to 100 parts by mass of the complex iii. **Claim 7** The method for producing lithium-based polyanion particles for a secondary battery positive electrode active material according to claim 6, wherein in step (IV), first, a wetting agent and water are added, then the complex iii is added, and then a manganese compound and / or an iron compound and a phosphoric acid compound are added to obtain slurry water b. **Claim 8** The method for producing lithium-based polyanion particles for a secondary battery positive electrode active material according to claim 6 or 7, wherein the wetting agent is one or more selected from polyether ester amine, polyether phosphate ester amine, and polyether phosphate ester.

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