JP2016186877A - Olivine-type positive electrode active material and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an olivine-type positive electrode active material that exhibits excellent battery characteristics of high capacity and high energy density, and the positive electrode active material.SOLUTION: In a method for producing an olivine-type positive electrode active material, powder carbon such as acetylene black and graphite is added during a pulverizing and mixing process to be dispersed uniformly into raw materials and to be granulated so that the inside of secondary particles is coated with carbon and then both the inside and the surface of the secondary participles are coated with carbon by coating the vicinity of the surfaces of the secondary particles with carbon by means of a method that decomposes an organic compound under an inert atmosphere.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an olivine-type positive electrode active material and a method for producing the same.

Lithium secondary batteries are lightweight and have high energy density, so they are widely used in small batteries such as mobile phones, notebook computers, and other IT equipment. LiCoO ₂ and LiCo _1/3 are mainly used for these applications. Layered rock salt compound positive electrode active materials such as Ni _1/3 Mn _1/3 O ₂ and LiNiO ₂ are used.

  With the development and popularization of IT equipment, the demand is still growing on a global scale. In addition to these small batteries, industrial large batteries can also be used for hybrid vehicles (HEV), plug-in hybrid vehicles (PHEV), electric vehicles (EV), power leveling, power storage, and more. The demand is expected to expand in the direction, and research and development are also actively conducted.

Under such circumstances, high safety, long life, high output, and low cost are required for the positive electrode active material as a problem for full-scale commercialization of large industrial batteries. Among them, as a material exhibiting high safety and excellent cycle performance, an olivine-type positive electrode active material has attracted attention as an alternative positive electrode active material such as LiCoO ₂ or LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ . .

  The olivine-type positive electrode active material has a theoretical capacity of about 170 mAh / g and all O is covalently bonded to P. Therefore, even if the battery generates heat, there is a risk of ignition without releasing oxygen. In addition, the structure is stable due to the skeleton of phosphoric acid, and therefore, the electrode is not easily deteriorated even after repeated charge and discharge, and the cycle life is long.

The most common material among the olivine-type positive electrode active materials is lithium iron phosphate. However, since the charge / discharge potential is 3.4 V (vs. Li / Li ⁺ ), which is lower than that of the conventional lithium ion secondary battery, the energy density is low. Lithium manganese phosphate (LiMnPO ₄ ) is 4.1 V against Li metal, which is equivalent to the layered rock salt compound positive electrode active material, and is expected to have high energy density. Collecting. In addition, materials exhibiting a charge / discharge potential of 4.5 V or higher, such as LiCoPO ₄ and LiNiPO _4, are also included in the olivine-type positive electrode active material, and may be used for batteries with higher energy density in the future.

The olivine-type positive electrode active material generally has a problem that it is difficult to obtain a practical charge / discharge capacity because the electron conductivity and Li ion conductivity are lower than those of the conventional layered rock salt compound positive electrode active material. . In response to this problem, countermeasures for reducing resistance such as shortening of the movement distance of electrons and lithium ions in the particles by miniaturization of LiMnPO ₄ particles (Patent Document 1) and imparting conductivity by treatment of carbon or the like are provided. A process (patent document 2) is included.

The olivine-type positive electrode active material is generally synthesized by a solid-phase synthesis method in which transition metal oxides, oxalates, phosphates, etc., and a phosphorus source and a lithium source are pulverized and mixed with a ball mill or the like and fired. is there. At this time, since mixing for a long time is required to uniformly mix the raw materials, the productivity is low. Therefore, NH ₄ MPO ₄ · H ₂ O (M = one or more elements from Mg, Fe, Mn, Co, and Ni) co-precipitated with transition metal and phosphorus has been made uniform at the atomic level. It has attracted attention as a raw material (Patent Document 3).

NH ₄ MPO ₄ · H ₂ O is easily refined to the submicron order in the pulverization process, and maintains the fine particle size until the final product. It is beneficial. However NH ₄ MPO ₄ · H ₂ O because handling is very difficult to miniaturized, in industrial mass production scale it is difficult to handle. Therefore, a granulation step is required to obtain a secondary particle size of 1 to 75 μm using a spray dryer.

  Usually, since the pulverization is performed in a solvent such as water and ethanol, it is efficient to perform the spray drying process immediately after the raw materials are dispersed and mixed. The granulation and drying steps are performed at the same time, followed by a firing step in which an olivine-type positive electrode active material is generated.

  As described above, in order to obtain a higher capacity than the originally non-conductive olivine type positive electrode active material, it is necessary to impart conductivity by carbon coating treatment. The carbon coating treatment is generally performed by mixing an organic compound such as a saccharide, a polymer compound, or a coal dry product with an olivine-type positive electrode active material and baking it in an inert gas. In general, it is preferable that the carbon coating treatment uniformly covers the surface of the primary particles. For this purpose, it is desirable to divide the firing for synthesizing the olivine-type positive electrode active material and the firing for the carbon coating treatment into two stages (Patent Document 4).

  However, after the secondary particles are granulated by spray drying, if the conventional carbon coating treatment is performed on the calcined olivine-type positive electrode active material, the carbon coating concentrates only on the surface of the secondary particles, and the inside of the granulated particles There is a problem that the particles inside the particles do not contribute to charging / discharging and the charge / discharge capacity decreases without being uniformly carbon-coated to the primary particles.

JP 2002-151082 A JP 2003-292308 A Japanese Patent No. 5120523 Japanese Patent No. 5464322

In view of the above-mentioned problems of the prior art, the object of the present invention is to use the NH ₄ MPO ₄ · H ₂ O as a raw material for the olivine-type positive electrode active material while granulating the secondary particles. Provided a method for producing an olivine-type positive electrode active material that is coated with carbon up to primary particles and exhibits excellent battery characteristics of high capacity and high energy density, and a lithium secondary battery having good characteristics using the positive electrode active material There is to do.

  In order to achieve the above object, the present inventors have intensively studied the olivine-type positive electrode active material coated with carbon up to the primary particles inside the secondary particles while granulating the secondary particles. A method of adding powdered carbon such as acetylene black or graphite, uniformly dispersing it in the raw material, granulating it and coating the carbon inside the secondary particles, and then decomposing the organic compound in an inert atmosphere Thus, it was found that by covering the surface of the secondary particles with carbon near the surface of the secondary particles, high charge / discharge capacity can be obtained by covering both the inside and the surface of the secondary particles. The present invention has been completed based on these findings.

  The first invention of the present invention is a method for producing a positive electrode active material for a lithium secondary battery, and is a raw material M (one or more elements selected from M = Mg, Fe, Mn, Co, Ni) A first step of obtaining a slurry by pulverizing and mixing a coprecipitation compound obtained by coprecipitation of a compound and a phosphorus compound, a lithium compound, and carbon powder in a solvent, and secondly obtaining a granulated powder by granulating and drying the slurry A step, a third step of obtaining the positive electrode active material for a lithium secondary battery by calcining the granulated powder at a temperature and atmosphere at which the positive electrode active material for the lithium secondary battery is produced, and carbon in the positive electrode active material for the lithium secondary battery A method for producing a positive electrode active material for a carbon-coated lithium secondary battery, comprising a fourth step of mixing precursor materials and re-baking at a temperature and atmosphere where the carbon precursor materials are thermally decomposed.

The second aspect of the present invention, the coprecipitation compound NH ₄ MPO ₄ · H ₂ O wherein a is (M = Mg, Fe, Mn , Co, one or more elements from Ni) to It is a manufacturing method of the positive electrode active material for lithium secondary batteries.

  A third aspect of the present invention is the lithium secondary battery, wherein the carbon powder is one or more of acetylene black, ketjen black, vapor grown carbon, graphite, activated carbon, and carbon nanotube. It is a manufacturing method of the positive electrode active material.

  A fourth invention of the present invention is a method for producing a positive electrode active material for a lithium secondary battery, wherein the carbon powder has an average primary particle diameter of 30 to 200 nm.

  A fifth invention of the present invention is a method for producing a positive electrode active material for a lithium secondary battery, wherein a dispersant is added so that carbon powder is dispersed in the solvent.

  A sixth invention of the present invention is a method for producing a positive electrode active material for a lithium secondary battery, wherein the average particle size of various raw materials is pulverized to 50 nm to 1000 nm in the pulverization and mixing process.

  A seventh invention of the present invention is a method for producing a positive electrode active material for a lithium secondary battery, wherein an average secondary particle diameter is granulated to 1 to 75 μm in the granulation process.

  An eighth invention of the present invention is a positive electrode active material for a lithium secondary battery produced by the production method according to the first to seventh inventions, wherein 0.5 to 6.0 wt% of the active material particle surface is formed. Lithium secondary battery characterized in that carbon generated by thermal decomposition of a carbon precursor material is supported, and 20 to 65% of the total amount of carbon contained in the active material is supplied by powdered carbon. Cathode active material.

  A ninth invention of the present invention is a positive electrode active material for a lithium secondary battery, characterized in that the discharge capacity is 140 mAh / g or more and the average discharge voltage is 3.70 V or more.

A tenth aspect of the present invention is a positive electrode active material for a lithium secondary battery characterized by having an olivine-structure crystal as measured by XRD.

According to the present invention, it is possible to obtain an olivine-type positive electrode active material coated with carbon up to primary particles inside secondary particles while granulating the secondary particles, and a lithium secondary battery obtained using the positive electrode active material Shows excellent battery characteristics such as high capacity, high energy density and high charge / discharge efficiency, and its industrial value is extremely large.

Charge / discharge curves of secondary batteries using positive electrode active materials according to examples and comparative examples of the present invention

Hereinafter, the present invention will be described in detail. NH is when described as coprecipitated compound ₄ MPO ₄ · H ₂ O will be described an example of using a (M = Mg, Fe, Mn , Co, one or more elements selected from Ni), limited to it Absent.
(1) First Step NH ₄ MPO ₄ .H ₂ O (M = one or more elements selected from Mg, Fe, Mn, Co, Ni) used as a raw material for the positive electrode active material for a secondary battery of the present invention Is a mixture solution for preparing a mixed solution of divalent Mg ions, Mn ions, Fe ions, Co ions, Ni ions and phosphate ions, and ammonia dropwise into the reaction vessel in which the solvent is stirred, The pH is adjusted to a range of 7 to 9 and coprecipitated, and the solid content is filtered, washed and dried.

Next, ground and mixed with the NH ₄ MPO ₄ · H ₂ O and a lithium salt and carbon powder in a solvent. Lithium compound and NH ₄ MPO ₄ · H ₂ O, as lithium manganese iron composite phosphate represented by the following general formula (2) is obtained, which is mixed, the ratio of Li to M in a molar ratio (Li / M) is preferably 0.95 to 1.05.

General formula: LiMPO ₄ (2)
(Wherein M = one or more elements from Mg, Fe, Mn, Co, Ni).

The lithium salt is not particularly limited, and one or more general lithium salts such as lithium hydroxide, lithium carbonate, and lithium acetate can be used.

  Water, alcohols, and the like can be used as a solvent for pulverization, but it is preferable to use water in view of the cost of the solvent and vaporization at the time of drying in a subsequent granulation step.

The carbon powder is not particularly limited as long as it is conductive such as acetylene black, ketjen black, vapor-grown carbon, graphite, activated carbon, and carbon nanotube. The average primary particle size of the carbon powder is preferably 5 to 200 nm, particularly preferably 10 to 50 nm. When the average primary particle diameter is less than 5 nm, the specific surface area increases, the slurry viscosity during pulverization increases, and dispersion becomes difficult. Further, the average primary particle size exceeds 200 nm, for the purposes of particle size than NH ₄ MPO ₄ · H ₂ O after grinding is increased, to cover the NH ₄ MPO ₄ · H ₂ O surface carbon Is not appropriate.

  The average primary particle diameter here refers to observing particles with an electron microscope or the like, measuring the diameters of 100 randomly selected particles, and taking their arithmetic average. Moreover, an average secondary particle diameter means the volume average particle diameter obtained from the measurement by the particle size distribution analyzer of particle | grains.

  The amount of carbon powder added is preferably in the range of 20 to 65% of the carbon contained in the final product. If it is less than 20%, a sufficient charge / discharge capacity can be obtained without ensuring a conductive path due to carbon inside the particles. I can't. If it is 65% or more, the charge / discharge capacity decreases due to excessive carbon that does not contribute to charge / discharge.

  In general, carbon powder is hydrophobic and does not disperse uniformly when an aqueous solvent is used for grinding. Therefore, a dispersant can be added so that the carbon powder is dispersed in the aqueous solvent. The dispersant to be used is not particularly limited, but it is desirable to use a nonionic surfactant such as polyvinyl alcohol, gelatin or carboxymethylcellulose which does not bring in other elements than those constituting the olivine type positive electrode active material. There is no problem in using a dispersant even in the case of an organic solvent.

  Since many dispersants themselves have carbon, they are thermally decomposed in a later firing step to become part of the coated carbon. However, if the generation reaction of the olivine-type positive electrode active material and the carbon pyrolysis reaction are performed in parallel, the resulting coated carbon layer is not preferable. Therefore, the amount of the dispersant added for dispersing the powdered carbon is preferably 17 wt% or less of the powdered carbon, although it depends on the type.

The pulverization and mixing method is not particularly limited as long as a pulverization mixer that can pulverize NH ₄ MPO ₄ · H ₂ O to an average particle size of 50 nm to 1000 nm and sufficiently mix a lithium compound and carbon powder is used. A ball mill, planetary ball mill, bead mill, or the like using a grinding medium such as alumina or zirconia sphere in a solvent can be used.

If NH ₄ finer average particle MPO ₄ · H ₂ O to 50nm, it is possible to obtain sufficiently good discharge capacity, grinding of less than 50nm only increases the time it takes meaningless grinding step Because there is, it is not appropriate. NH ₄ When MPO ₄ · H ₂ O average particle exceeds 1000 nm, the charge-discharge capacity reaction interface is insufficient final cathode active material is inappropriate for might become caught.
Crushing and mixing are not necessarily performed at the same time, but are preferably performed simultaneously from the viewpoint of productivity.

(2) Second Step In the second step, the slurry in which NH ₄ MPO ₄ · H ₂ O, lithium compound, and carbon powder obtained in the first step are dispersed is granulated while drying.

  The average secondary particle diameter of the granulated particles is preferably 1 to 75 μm, and more preferably 3 to 50 μm. If it is less than 1 μm, there is little contribution to the improvement of handling by granulation. If it exceeds 75 μm, it will be difficult to produce a battery in a later step.

The granulation method is not particularly limited as long as secondary particles having a target particle diameter can be obtained from the slurry. Specific examples include a spray dryer and a medium fluidized dryer.

(3) Third Step In the third step, the granulated powder obtained in the second step is heat-treated at a temperature at which the lithium secondary battery positive electrode material is generated.

  In the case of an olivine-type lithium secondary battery positive electrode active material, heat treatment is performed at 200 to 500 ° C, preferably 300 to 400 ° C. When the heat treatment temperature is less than 200 ° C., lithium carbonate as a reaction raw material may remain. When the heat treatment temperature exceeds 500 ° C., sintering of the particles proceeds and coarse particles are generated, and finally obtained positive electrode The charge / discharge capacity of the active material is reduced.

In order to prevent the mixed carbon powder from burning, the heat treatment is preferably performed in an inert gas such as N ₂ , Ar, or He, or a mixed gas of an inert gas and hydrogen gas. The hydrogen gas content is preferably up to 20% by volume. If it exceeds 20% by volume, the reduction potential becomes excessive, Ni, Co, Fe, and Mn are reduced and deposited as a metal in the material, which is not preferable.

As a furnace used for heat treatment, for example, a general heat treatment furnace / firing furnace such as a batch furnace, a roller hearth kiln, a pusher furnace, a rotary kiln, or a fluidized bed furnace can be used.

(4) Fourth Step In the fourth step, in order to coat the lithium secondary battery positive electrode active material obtained by the third in addition to the carbon powder more uniformly and impart conductivity, a carbon precursor is added. Mix.

  The carbon precursor is not particularly limited as long as it is graphitized by firing to become a conductive carbonaceous material. Common hydrocarbons such as sucrose, coal distillate, ascorbic acid and others, A wide variety of organic compounds that produce carbonaceous materials by decomposition can be used.

  The carbon precursor is added so that the carbon content remaining in the final product from the precursor is 0.5 to 6.0 wt%. If it is 0.5 wt% or less, a conductive path by carbon is not secured, and the charge / discharge capacity as a battery is reduced. If it is 6.0 wt% or more, the specific surface area increases, and handling at the time of producing a battery is deteriorated.

  The mixing is not particularly limited as long as the lithium secondary battery positive electrode active material and the carbon precursor are uniformly mixed. If the mixing force is excessive, the granulated secondary particles may be pulverized, which is not preferable. Specifically, a dry mill using a shaker mixer, alumina, zirconia sphere, water, ethanol, etc., dissolve and disperse the lithium secondary battery positive electrode active material and C precursor, and heat the solvent while stirring to dry the solvent. The method of making it, etc. are mentioned.

  A lithium secondary battery positive electrode active material mixed with a carbon precursor is calcined at 600 to 800 ° C., preferably 600 to 700 ° C. in an inert or reducing atmosphere, whereby the carbon precursor is thermally decomposed and becomes conductive. Becomes good carbon and covers the surface of the positive electrode active material of the lithium secondary battery.

  When the firing temperature is less than 600 ° C., graphitization of carbon does not proceed and sufficient conductivity cannot be obtained for the positive electrode active material. On the other hand, when the firing temperature exceeds 800 ° C., the sintering of the particles proceeds and becomes coarse, and the conductivity of the positive electrode active material is lowered.

In the carbon coating step, as in the case of the firing step, a general heat treatment furnace / firing furnace such as a batch furnace, a roller hearth kiln, a pusher furnace, a rotary kiln, or a fluidized bed furnace can be used.

(5) Positive electrode active material In the positive electrode active material for a lithium secondary battery of the present invention, carbon generated by thermal decomposition of 0.5 to 6.0 wt% of a carbon precursor material is supported on the surface of the active material particles, and Of the total amount of carbon contained in the active material, 20 to 65% is supplied by powdered carbon. That is, the carbon powder contained in the positive electrode active material particles is approximately 0.13 to 11 wt%.

Here, the carbon powder is not particularly limited as long as it is conductive such as acetylene black, ketjen black, vapor-grown carbon, graphite, activated carbon, and carbon nanotube. The average primary particle size of the carbon powder is preferably 5 to 200 nm, and more preferably 10 to 50 nm. When the average primary particle diameter is less than 5 nm, the specific surface area of the carbon powder increases, so that the slurry viscosity during pulverization increases, and as a result, dispersion becomes difficult. On the other hand, if the average primary particle diameter exceeds 200 nm, the particle diameter is too large to cover the surface of the NH ₄ MPO ₄ .H ₂ O after pulverization, which is not appropriate.

The carbon powder contained in the positive electrode active material particles is 0.13 to 11 wt%, preferably 0.4 to 8.0 wt%. If the carbon powder is less than 0.13 wt%, sufficient conductivity in the positive electrode active material cannot be obtained, and if it exceeds 11 wt%, the charge / discharge capacity is reduced due to excess carbon that does not contribute to charge / discharge.

(5) Lithium Secondary Battery The lithium secondary battery according to the present invention is composed of the same constituent elements as a general lithium secondary battery, such as a positive electrode, a negative electrode, and a nonaqueous electrolyte.

Hereinafter, the embodiment of the lithium secondary battery of the present invention will be described in detail by dividing into its components, uses, etc., but the following embodiments are merely examples, and the lithium secondary battery of the present invention is In addition to the embodiments described in the present specification, various modifications and improvements can be made based on the knowledge of those skilled in the art.

(A) Positive electrode A positive electrode is formed from the positive electrode compound material containing the positive electrode active material of this invention, the electrically conductive material, and the binder.

  Specifically, a powdered positive electrode active material and a conductive material are mixed, a binder is added thereto, and if necessary, a solvent for viscosity adjustment is further added to adjust the positive electrode mixture paste, For example, the positive electrode mixture paste is applied to the surface of a current collector made of aluminum foil, dried, and pressurized as necessary to produce a sheet-like positive electrode.

  The conductive material is for ensuring the electrical conductivity of the positive electrode, and for example, a material obtained by mixing one or more carbon material powders such as carbon black, acetylene black, and graphite can be used. .

  The binder plays a role of anchoring the active material particles. For example, a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, and fluorine rubber, a thermoplastic resin such as polypropylene and polyethylene, and other suitable materials. Can be used. If necessary, an organic solvent such as N-methyl-2-pyrrolidone is used as a solvent to be added to the positive electrode mixture, that is, as a solvent for dispersing the active material, conductive material, activated carbon, and dissolving the binder. it can. Activated carbon can also be added to increase the electric double layer capacity.

  Such a positive electrode active material, a conductive material, and a binder are mixed, and if necessary, activated carbon and a solvent are added and kneaded to prepare a positive electrode mixture paste.

  Each mixing ratio in the positive electrode mixture can also be an important factor that determines the performance of the lithium ion secondary battery. When the total solid content (meaning excluding the solvent) of the positive electrode mixture is 100% by mass, the positive electrode active material is 60 to 95% by mass, and the conductive material is 1 to 5 like the positive electrode of a general lithium secondary battery. It is desirable that 20% by mass and the binder be 1 to 20% by mass.

For example, the above-mentioned positive electrode mixture paste sufficiently kneaded is applied to the surface of a metal foil current collector such as aluminum, dried to disperse the solvent, and then, if necessary, a roll to increase the electrode density By compressing with a press or the like, the positive electrode can be formed into a sheet. The sheet-like positive electrode can be cut into an appropriate size according to the intended battery and used for battery production.

(B) Negative electrode For the negative electrode, metallic lithium, lithium alloy, or the like, and a negative electrode mixture made by mixing a binder with a negative electrode active material capable of inserting and extracting lithium ions and adding a suitable solvent to form a paste. , And applied to the surface of a current collector of a metal foil such as copper, dried, and compressed to increase the electrode density as necessary. At this time, as the negative electrode active material, for example, a fired organic compound such as natural graphite, artificial graphite, or a phenol resin, or a powdery carbon material such as coke can be used. In this case, the negative electrode binder is a fluorine-containing resin such as polyvinylidene fluoride as in the positive electrode, and the negative electrode active material and the binder are organic solvents such as N-methyl-2-pyrrolidone. A solvent can be used.

(C) Separator A separator is sandwiched and loaded between the positive electrode and the negative electrode. The separator separates the positive electrode and the negative electrode and retains the electrolyte, and a thin microporous film such as polyethylene or polypropylene can be used.

(D) Nonaqueous electrolyte The nonaqueous electrolyte is obtained by dissolving a lithium salt as a supporting salt in an organic solvent. Examples of the organic solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate, chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate, and tetrahydrofuran, 2- One kind selected from ether compounds such as methyltetrahydrofuran and dimethoxyethane, sulfur compounds such as ethylmethylsulfone and butanesultone, phosphorous compounds such as triethyl phosphate, triethyl phosphate and trioctyl phosphate alone, or two or more kinds It can be used by mixing.

As the supporting salt, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiASF ₆ , LiN (CF ₃ SO ₂ ) ₂ , and complex salts thereof can be used. Further, the non-aqueous electrolyte may contain a radical scavenger, a surfactant, a flame retardant, and the like.

  The lithium secondary battery of the present invention configured as described above can have various shapes such as a cylindrical type and a stacked type. Even if any shape is adopted, the positive electrode and the negative electrode are laminated via a separator to form an electrode body, and between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal, A current collecting lead or the like is used for connection, the electrode body is impregnated with the nonaqueous electrolyte, and the battery case is sealed to complete the battery.

  The lithium secondary battery of the present invention includes a positive electrode using the positive electrode active material for a lithium secondary battery of the present invention as a positive electrode material, and is charged and discharged at a potential of 3.0 to 4.5 V. It is possible to industrially realize a lithium secondary battery that is extremely safer than conventional lithium metal composite oxides and also has a high capacity.

Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. In addition, the chemical analysis method of the metal used in the Example, the X-ray diffraction, and the evaluation method of battery capacity are as follows.
(1) Analysis of metals:
An ICP emission analysis method (Varian, 725ES) was used for ICP emission analysis.
(2) X-ray diffraction:
Using a powder X-ray diffractometer (manufactured by PANalytical, X'Prt PRO), the obtained positive electrode active material was measured by powder X-ray diffraction using Cu-Kα rays.
(3) Measurement of particle size distribution:
The average secondary particle size of the slurry, the granulated particles and the positive electrode active material was measured using a particle size distribution measuring device (Nikkiso Co., Ltd., Microtrac MT3000II).

The primary particle size of the powder carbon and the positive electrode active material is obtained by observing the powder using a scanning electron microscope (manufactured by JEOL, JSM-7001F) and measuring the size of 100 randomly selected particles. It was.
(4) Carbon content:
The amount of carbon formed on the surface of the lithium silicate compound was measured by a high frequency combustion infrared absorption method using a carbon analyzer (CS-600) manufactured by LECO. In addition, 0.2 g of the positive electrode active material was burned in an oxygen stream in a high-frequency furnace using a copper and iron tip as a combustion aid, and the carbon that became CO _{2 at} that time was measured by the amount of infrared energy. .

(5) Battery capacity evaluation:
About the obtained positive electrode active material, the coin type battery was produced in the following procedures, and the charge / discharge capacity of the battery was measured and evaluated.

  The positive electrode active material was mixed with 33 wt% of acetylene black as a conductive material, 17 wt% of polyvinylidene fluoride (PVDF) as a binder, and an N-methylpyrrolidone (NMP) solution, and the positive electrode active material was 50 wt% —the conductive material was 33 wt%. -A PVDF 17 wt% mixture was obtained.

This mixture was applied onto an aluminum foil, dried at 80 ° C., punched to an electrode size of 11 mmφ, and pressed at a press pressure of 98 MPa (1.0 tonf / cm ² ) to produce an electrode.

Using this electrode as a positive electrode, a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DEC) containing metal Li as a negative electrode and 1 mol / L of electrolyte LiPF ₆ as an electrolyte in a glove box (EC: DEC = volume ratio) 7: 3) A C2023 coin battery was produced using a glass separator as the separator.

The battery was charged and discharged under the conditions of charge 0.2 mA / cm ² , 4.5 V, rest 60 minutes, discharge 0.2 mA / cm ² , 2.0 V, 25 ° C., and the discharge capacity at the first cycle was While used as an evaluation value, the initial charge / discharge efficiency (discharge capacity / charge capacity) was determined. Moreover, the average discharge voltage was calculated | required from the voltage curve measured at the time of charging / discharging.

[Example 1]
Iron sulfate heptahydrate (special grade by Wako Co., Ltd .: purity 99.5% by mass) 7.5 mol (2095.4 g) and manganese sulfate n-hydrate (Chuo Denko: purity 99.9% by mass) 22.5 mol (3807.2 g) and 30 mol (3458.4 g) of phosphoric acid (manufactured by Wako Co., Ltd .: purity 85.0% by mass or more) were dissolved by stirring in ion-exchange water for 1 hour with a stirrer, and the volume was adjusted to 30 L. A raw material solution was obtained. Moreover, 25 mass% ammonia aqueous solution was used as pH adjustment solution.

  1 L of pure water was put into a 50 L separable flask equipped with a stirrer, and the mixture was stirred for 30 minutes while the inside was replaced with nitrogen. The raw material solution was added at a rate of 10 mL per second while the pH adjustment solution was connected to a pH controller and the pH was controlled at 8.0 to 8.2. After completion of dropping, stirring was continued for 30 minutes while replacing the separable flask with nitrogen to complete the coprecipitation reaction. The slurry after the reaction was subjected to solid-liquid separation by suction filtration, and then washed with pure water twice with pure water.

  After washing with water, it was dried under vacuum at 120 ° C. for 24 hours to obtain ammonium manganese iron phosphate.

  2000 g of ammonium manganese iron phosphate, 408.2 g of lithium carbonate (Kanto Chemical Co., Ltd. deer special grade: 99.0 mass%), 22.0 g of powdery carbon (Denka Black HS-100) having an average primary particle size of 48 nm, PVA3. 66 g and 10 L of ion exchange water were stirred with a stirrer all day and night to make a slurry. The slurry was passed through a nanograin mill (NGL-2, manufactured by Asada Tekko) and pulverized. As a grinding medium, 5 kg of φ0.3 mm zirconia balls were used, and the inside of the vessel was stirred at a peripheral speed of 12 m / s. The particle size distribution of the slurry was measured, and it was confirmed that the average secondary particle size was 500 nm or less.

  The slurry after pulverization was dried using a micro mist dryer (manufactured by Fujisaki Electric, model number MDL-050M), and secondary particles composed of each pulverized raw material were granulated. Granulation was performed by using a three-fluid nozzle and drying while spraying the slurry with 100 L / min of air. The apparatus inlet temperature was 130 ° C., and the amount of slurry fed was adjusted so that the exhaust temperature was 60 to 70 ° C. When the particle size distribution of the obtained granulated particles was measured, the average secondary particle size was 4.3 μm.

  The granulated particles obtained using an electric furnace were fired. The mixture was heated at 10 ° C./min while purging a mixed gas of 98 vol% nitrogen and 2 vol% hydrogen into the furnace at a flow rate of 1 L / min, and then calcined at 350 ° C. for 5 hours. When the fired product was analyzed by X-ray diffraction, it was identified as an olivine type lithium manganese iron composite phosphate single phase. The carbon content was measured and found to be 1.3 wt%.

  Take 30 g of the above lithium manganese iron complex phosphate and 1.23 g of dextrin n hydrate (Kanto Chemical Co., Ltd.) in a 100 ml plastic container and use a tumbler mixer (manufactured by Shinmaru Enterprises, model number T2F type). Mix for 10 minutes at 100 rpm. The mixture was baked for 5 hours at 650 ° C. with a temperature rising rate of 10 ° C./min while purging the mixed gas of 98 vol% nitrogen + 2 vol% hydrogen into the furnace at a flow rate of 1 L / min using an electric furnace. A positive electrode active material was obtained.

  The composition of Li: Mn: Fe: P of this positive electrode active material was 1.00: 0.75: 0.25: 1.00 in terms of molar ratio, and the carbon content was 2.2 mass%. It can be said that the carbon produced by pyrolysis of dextrin is 40.9% of the total coated carbon.

  When this positive electrode active material was observed with a scanning electron microscope (SEM) (manufactured by JEOL, JSM-7001F), the primary particle size was 50 to 400 nm. In the particle size distribution measurement, the average secondary particle size was 4.4 μm.

A secondary battery was fabricated using this positive electrode active material, and the battery was evaluated. The initial charge capacity was 155 mAh / g, the initial discharge capacity was 149 mAh / g, the initial efficiency was 96%, and the average discharge voltage was 3.78 V. there were. The evaluation results of the positive electrode active material are summarized in Table 1, and the charge / discharge curves for battery evaluation are summarized in FIG.

[Example 2]
The amount of powdered carbon added at the time of initial slurry preparation is 16.5 g, the amount of PVA is 2.7 g, and the amount of dextrin n hydrate added to 30 g of lithium manganese iron composite phosphate after firing is 1. A sample was prepared in the same manner as in Example 1 except that the amount was 81 g.

  When the particle size distribution of the granulated particles was measured, the average secondary particle size was 4.7 μm.

  When the granulated product after firing was analyzed by X-ray diffraction, it was identified as an olivine type lithium manganese iron composite phosphate single phase. The carbon content was measured and found to be 1.0 wt%.

  The composition of Li: Mn: Fe: P of this positive electrode active material was 1.00: 0.75: 0.25: 1.00 in terms of molar ratio, and the carbon content was 2.3 mass%. Carbon generated by thermal decomposition of dextrin can be said to be 56.5% of the total carbon coated.

  When this positive electrode active material was observed with a scanning electron microscope (SEM), the primary particle size was 50 to 400 nm. Further, in the particle size distribution measurement of the positive electrode active material, the average secondary particle size was 4.9 μm.

When the battery evaluation of the positive electrode active material was performed, the initial charge capacity was 150 mAh / g, the initial discharge capacity was 143 mAh / g, the initial efficiency was 95%, and the average discharge voltage was 3.77 V. The evaluation results of the positive electrode active material are summarized in Table 1, and the charge / discharge curves for battery evaluation are summarized in FIG.

[Example 3]
The amount of powdered carbon added during the initial slurry preparation is 6.8 g, the amount of PVA is 1.13 g, and the amount of dextrin n hydrate added to 30 g of lithium manganese iron composite phosphate after firing is 2. A sample was prepared in the same manner as in Example 1 except that the amount was 05 g.

  When the particle size distribution of the granulated particles was measured, the average secondary particle size was 4.8 μm.

  When the granulated product after firing was analyzed by X-ray diffraction, it was identified as an olivine type lithium manganese iron composite phosphate single phase. The carbon content was measured and found to be 0.4 wt%.

  The composition of Li: Mn: Fe: P of this positive electrode active material was 1.00: 0.75: 0.25: 1.00 in terms of molar ratio, and the carbon content was 1.9% by mass. Carbon generated by pyrolysis of dextrin can be said to be 78.9% of the total carbon coated.

  When this positive electrode active material was observed with a scanning electron microscope (SEM), the primary particle size was 50 to 400 nm. Further, in the particle size distribution measurement of the positive electrode active material, the average secondary particle size was 4.8 μm.

When the battery evaluation of the positive electrode active material was performed, the initial charge capacity was 150 mAh / g, the initial discharge capacity was 144 mAh / g, the initial efficiency was 96%, and the average discharge voltage was 3.71V. The evaluation results of the positive electrode active material are summarized in Table 1, and the charge / discharge curves for battery evaluation are summarized in FIG.

[Comparative Example 1]
The amount of powdery carbon added at the time of initial slurry preparation was 37.2 g, the amount of PVA was 6.20 g, and dextrin n hydrate was not added to 30 g of lithium manganese iron composite phosphate after firing. A sample was prepared in the same manner as in 1.

  When the particle size distribution of the granulated particles was measured, the average secondary particle size was 4.5 μm.

  When the granulated product after firing was analyzed by X-ray diffraction, it was identified as an olivine type lithium manganese iron composite phosphate single phase.

  The composition of Li: Mn: Fe: P of this positive electrode active material was 1.00: 0.75: 0.25: 1.00 in terms of molar ratio, and the carbon content was 2.2 mass%. Since the carbon produced by pyrolysis of dextrin is not added, the carbon produced by pyrolysis of dextrin is 0% of the total coated carbon.

  When this positive electrode active material was observed with a scanning electron microscope (SEM), the primary particle diameter was 100 to 500 nm. Further, in the particle size distribution measurement of the positive electrode active material, the average secondary particle size was 4.8 μm.

When the battery evaluation of the positive electrode active material was performed, the initial charge capacity was 64 mAh / g, the initial discharge capacity was 32 mAh / g, the initial efficiency was 50%, and the average discharge voltage was 2.88V. The evaluation results of the positive electrode active material are summarized in Table 1, and the charge / discharge curves for battery evaluation are summarized in FIG.

[Comparative Example 2]
The amount of powdery carbon added at the time of initial slurry preparation is 32.1 g, the amount of PVA is 5.35 g, and the amount of dextrin n hydrate added to 30 g of lithium manganese iron composite phosphate after firing is 0.00. A sample was prepared in the same manner as in Example 1 except that the amount was 41 g.

  When the particle size distribution of the granulated particles was measured, the average secondary particle size was 4.8 μm.

  When the granulated product after firing was analyzed by X-ray diffraction, it was identified as an olivine type lithium manganese iron composite phosphate single phase. The carbon content was measured and found to be 1.9 wt%.

  The composition of Li: Mn: Fe: P of this positive electrode active material was 1.00: 0.75: 0.25: 1.00 in terms of molar ratio, and the carbon content was 2.2 mass%. Carbon generated by pyrolysis of dextrin can be said to be 13.6% of the total carbon coated.

  When this positive electrode active material was observed with a scanning electron microscope (SEM), the primary particle diameter was 100 to 500 nm. Further, in the particle size distribution measurement of the positive electrode active material, the average secondary particle size was 4.8 μm.

When the battery evaluation of the positive electrode active material was performed, the initial charge capacity was 103 mAh / g, the initial discharge capacity was 88 mAh / g, the initial efficiency was 85%, and the average discharge voltage was 3.25V. The evaluation results of the positive electrode active material are summarized in Table 1, and the charge / discharge curves for battery evaluation are summarized in FIG.

[Comparative Example 3]
The same as in Example 1 except that powdery carbon and PVA are not added at the initial slurry preparation, and the amount of dextrin n hydrate added to 30 g of the lithium manganese iron composite phosphate after firing is 3.0 g. A sample was prepared by this method.

  When the particle size distribution of the granulated particles was measured, the average secondary particle size was 4.6 μm.

  When the granulated product after firing was analyzed by X-ray diffraction, it was identified as an olivine type lithium manganese iron composite phosphate single phase.

  The composition of Li: Mn: Fe: P of this positive electrode active material was 1.00: 0.75: 0.25: 1.00 in terms of molar ratio, and the carbon content was 2.2 mass%. Since powdered carbon is not added, it can be said that C generated by pyrolysis of dextrin is 100% of the total.

  When this positive electrode active material was observed with a scanning electron microscope (SEM), the primary particle size was 50 to 400 nm. Further, in the particle size distribution measurement of the positive electrode active material, the average secondary particle size was 4.7 μm.

When the battery evaluation of the positive electrode active material was performed, the initial charge capacity was 146 mAh / g, the initial discharge capacity was 133 mAh / g, the initial efficiency was 91%, and the average discharge voltage was 3.46V. The evaluation results of the positive electrode active material are summarized in Table 1, and the charge / discharge curves for battery evaluation are summarized in FIG.

  As is clear from the above, the method for producing a lithium secondary electron cathode active material of the present invention can secure a conductive path to the inside of the secondary particles, and the obtained lithium secondary electron cathode active material has a high capacity and a high capacity. It shows energy density, and its industrial applicability is extremely large.

Claims (10)

  A method for producing a positive electrode active material for a lithium secondary battery, wherein a raw material M (one or more elements selected from Mg, Fe, Mn, Co, Ni) and a phosphorus compound are co-precipitated. The first step of obtaining a slurry by pulverizing and mixing the coprecipitated compound, lithium compound and carbon powder obtained in a solvent, the second step of granulating and drying the slurry to obtain a granulated powder, A third step of obtaining a positive electrode active material for a lithium secondary battery by firing at a temperature and an atmosphere at which the positive electrode active material for the secondary battery is generated; a carbon precursor material is mixed with the positive electrode active material for a lithium secondary battery; A method for producing a positive electrode active material for a carbon-coated lithium secondary battery, comprising a fourth step of refiring in a temperature and atmosphere at which the body material is thermally decomposed. The coprecipitation compound NH ₄ MPO ₄ · H ₂ O lithium secondary according to claim 1, characterized in that the (M = Mg, Fe, Mn , Co, one or more elements from Ni) A method for producing a positive electrode active material for a battery.   3. The lithium secondary battery according to claim 1, wherein the carbon powder is one or more of acetylene black, ketjen black, vapor-grown carbon, graphite, activated carbon, and carbon nanotube. A method for producing a positive electrode active material.   The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein the carbon powder has an average primary particle size of 5 to 200 nm.   The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein a dispersant is added so that the carbon powder is dispersed in the solvent.   6. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein an average secondary particle diameter of various raw materials is pulverized to 50 nm to 1000 nm in the pulverization and mixing process.   The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein an average secondary particle diameter is granulated to 1 to 75 μm in the granulation process.   A positive electrode active material for a lithium secondary battery produced by the production method according to claim 1, wherein the active material particle surface is produced by thermal decomposition of a carbon precursor material of 0.5 to 6.0 wt%. A positive electrode active material for a lithium secondary battery, wherein 20 to 65% of the total amount of carbon contained in the active material is supplied by powdered carbon.   The positive electrode active material for a lithium secondary battery according to claim 8, wherein the discharge capacity is 140 mAh / g or more and the average discharge voltage is 3.70 V or more.   The positive electrode active material for a lithium secondary battery according to claim 8, wherein the positive electrode active material has an olivine structure crystal as measured by XRD.

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