JP2012204150A - Method of producing electrode active material and electrode active material, electrode, and battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of producing an electrode active material, by which an electrode active material can be obtained with high yield and also at low manufacturing cost, the electrode active material being high in discharge capacity, excellent in discharge characteristics and also excellent in long-term reliability, and including a lithium-iron-phosphorus composite oxide particle having a surface coated with a carbonaceous material, and to provide an electrode active material, an electrode, and a battery.SOLUTION: In a method of producing an electrode active material of the present invention, ferrous phosphate hydrate (Fe(PO)-8HO), lithium phosphate (LiPO) subjected to dispersion treatment using a polymer dispersant with water as medium, and water are mixed to form an aqueous mixture, and a surface of a lithium-iron-phosphorus composite oxide particle which is produced by performing pressing and heating treatment to the aqueous mixture in a sealed state is coated with a carbonaceous material.

Description

  The present invention relates to a method for producing an electrode active material, an electrode active material, an electrode, and a battery, and in particular, a method for producing an electrode active material suitable for use as a positive electrode material for a battery and further a positive electrode material for a lithium ion battery. The present invention relates to an electrode active material obtained by this manufacturing method, an electrode using this electrode active material as a positive electrode material, and a battery using this electrode as a positive electrode.

In recent years, non-aqueous electrolyte secondary batteries such as lithium ion batteries have been proposed and put into practical use as batteries that are expected to be reduced in size, weight, and capacity.
This lithium ion battery includes a positive electrode and a negative electrode having a property capable of reversibly removing and inserting lithium ions, and a non-aqueous electrolyte.
As a negative electrode material of this lithium ion battery, as a negative electrode active material, generally a Li-containing metal oxide having a property capable of reversibly removing and inserting lithium ions such as a carbon-based material or lithium titanate (Li ₄ Ti ₅ O ₁₂ ). Things are used.

On the other hand, as a positive electrode material of a lithium ion battery, as a positive electrode active material, a Li-containing metal oxide having a property capable of reversibly removing and inserting lithium ions, such as lithium iron phosphate (LiFePO ₄ ), a conductive auxiliary agent, and An electrode material mixture containing a binder or the like is used. And this positive electrode material mixture is apply | coated to the surface of metal foil called a collector, and the positive electrode of a lithium ion battery is formed.

  Such lithium ion batteries are lighter, smaller and have higher energy than secondary batteries such as conventional lead batteries, nickel cadmium batteries, nickel hydride batteries, etc., such as portable telephones, notebook personal computers, etc. It is used as a power source for portable electronic devices. In recent years, lithium ion batteries have also been studied as high-output power sources for electric vehicles, hybrid vehicles, electric tools, and the like. High-speed charge / discharge characteristics are required for the electrode active materials of batteries used as these high-output power supplies.

However, an electrode active material, for example, an electrode material including a lithium-containing metal oxide having a property capable of reversibly removing and inserting lithium ions has a problem of poor electron conductivity, and is therefore an electrode active material. Lithium iron-phosphorus complex oxide carbon composite in which the surface of lithium iron phosphate particles is covered with an organic component that is a carbon source and then carbonized to form a carbonaceous film on the surface of lithium iron phosphate particles A body has been proposed (see Patent Documents 1 to 3).
This lithium iron phosphate is produced by a solid phase reaction between lithium phosphate and ferrous phosphate hydrate.

JP 2003-292308 A JP 2003-292309 A Japanese Patent No. 4176804

By the way, in the conventional lithium iron phosphorus composite oxide carbon composite, since lithium iron phosphate particles are produced by a solid phase reaction between lithium phosphate and ferrous phosphate hydrate, the specific surface area is 9 m ^2. There is a problem that coarse particles of less than / g are easily generated.
When coarse particles having a specific surface area of 9 m ² / g or less are generated in the lithium iron phosphate particles, the discharge capacity of the lithium ion battery produced using the lithium iron phosphate particles is lowered and the discharge characteristics are also lowered. As a result, long-term reliability may be reduced.

  The present invention has been made to solve the above problems, and has a high discharge capacity, excellent discharge characteristics, and long-term reliability. It is an object to provide an electrode active material manufacturing method, an electrode active material, an electrode, and a battery that can be obtained in high yield, and can be obtained at a low production cost. And

As a result of intensive studies to solve the above problems, the present inventors have found that a ferrous phosphate hydrated salt (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O) and a polymer dispersion using water as a medium Lithium phosphate (Li ₃ PO ₄ ) that has been dispersed using an agent and water are mixed to form an aqueous mixture, and this aqueous mixture is subjected to pressure heat treatment in a sealed state to produce lithium If the surface of the iron-phosphorus composite oxide particles is coated with a carbonaceous material, an electrode active material having a high discharge capacity, excellent discharge characteristics, and excellent long-term reliability can be obtained in a high yield. It was found that the manufacturing cost was low, and the present invention was completed.

That is, the method for producing an electrode active material according to the present invention includes a dispersion treatment using ferrous phosphate hydrate (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O) and a polymeric dispersant using water as a medium. Lithium phosphate-based composite oxide particles produced by mixing the applied lithium phosphate (Li ₃ PO ₄ ) and water to form an aqueous mixture, and subjecting the aqueous mixture to pressure heat treatment in a sealed state The surface of is coated with a carbonaceous material.

Another method for producing the electrode active material according to the present invention includes a first step in which lithium phosphate (Li ₃ PO ₄ ) is dispersed in water as a medium using a polymer dispersant to form an aqueous slurry, An aqueous slurry, ferrous phosphate hydrate (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O), and a reducing agent are mixed to form an aqueous mixture, and this aqueous mixture is subjected to pressure heat treatment in a sealed state. And a second step of producing lithium iron phosphorus-based composite oxide particles, a third step of mixing the lithium iron phosphorus-based composite oxide particles and the carbonaceous material precursor to form a mixture, and spraying the mixture A fourth step of obtaining a granulated body by drying; and a lithium iron phosphorus composite oxide carbon composite in which the granulated body is fired and a surface of the lithium iron phosphorus composite oxide particles is coated with a carbonaceous material A fifth step of obtaining a body, To.

The polymer dispersant is preferably polyacrylic acid.
The ratio of the lithium iron-phosphorus composite oxide to be produced by setting the addition amount of the polymer dispersant to 0.1 mass% or more and 10 mass% or less with respect to the solid content mass of the lithium phosphate. It is preferable to control the surface area to 10 m ² / g or more and 20 m ² / g or less.

The electrode active material of the present invention is a lithium phosphate that has been subjected to a dispersion treatment using a ferrous phosphate hydrate (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O) and water as a medium and a polymer dispersant. The surface of the lithium iron phosphorus composite oxide particles to be produced is coated with a carbonaceous material by subjecting an aqueous mixture containing (Li ₃ PO ₄ ) and water to pressure heat treatment in a sealed state. an electrode active material, the specific surface area of the lithium-iron-phosphorus compound oxide is characterized in that 10 m 2 / ^g or more and is not more than 20 m 2 / ^g.

The electrode of the present invention is characterized by using the electrode active material of the present invention for a positive electrode.
The battery of the present invention is characterized by using the electrode of the present invention as a positive electrode.

According to the method for producing an electrode active material of the present invention, a dispersion treatment is performed using a ferrous phosphate hydrated salt (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O) and a polymer dispersant using water as a medium. Lithium phosphate-based composite oxide particles produced by mixing the applied lithium phosphate (Li ₃ PO ₄ ) and water to form an aqueous mixture, and subjecting the aqueous mixture to pressure heat treatment in a sealed state Is coated with a carbonaceous material, so that the specific surface area of the lithium iron phosphorus composite oxide is 10 m ² / g or more and 20 m ² / g or less, and has a high discharge capacity, excellent discharge characteristics, and long-term It is possible to produce an electrode active material that is excellent in overall reliability with a high yield. In addition, the manufacturing cost is low.

According to another method for producing an electrode active material of the present invention, the first step is to form a water-based slurry by subjecting lithium phosphate (Li ₃ PO ₄ ) to a dispersion treatment using water as a medium and a polymer-based dispersant. The aqueous slurry, ferrous phosphate hydrate (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O), and a reducing agent are mixed to form an aqueous mixture, and this aqueous mixture is heated under pressure in a sealed state. A second step of performing treatment to produce lithium iron phosphorus-based composite oxide particles, a third step of mixing the lithium iron phosphorus-based composite oxide particles and the carbonaceous material precursor, and making the mixture, and the mixture A fourth step of obtaining a granulated body by spray-drying, and a lithium iron phosphorus based composite oxide obtained by firing the granulated body and coating the surface of the lithium iron phosphorus based composite oxide particles with a carbonaceous material A fifth step of obtaining a carbon composite, High discharge capacity, excellent discharge characteristics, the electrode active material is superior long-term reliability can be manufactured at high yield. In addition, the manufacturing cost is low.

According to the electrode active material of the present invention, phosphorous that has been subjected to dispersion treatment using a ferrous phosphate hydrate (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O) and water as a medium and a polymer dispersant. The specific surface area of the lithium iron-phosphorus composite oxide particles to be produced is 10 m ² / g or more by subjecting an aqueous mixture containing lithium acid (Li ₃ PO ₄ ) and water to pressure heat treatment in a sealed state. Since it is 20 m ² / g or less, the discharge capacity can be increased, and the discharge characteristics and long-term reliability are also excellent.

According to the electrode of the present invention, since the electrode active material of the present invention is used for the positive electrode, the discharge capacity, discharge characteristics, and long-term reliability of the positive electrode can be improved.
According to the battery of the present invention, since the electrode of the present invention is used as the positive electrode, the discharge capacity, discharge characteristics, and long-term reliability of the battery can be improved.

A method for producing an electrode active material, an electrode active material, an electrode, and a battery according to the present invention will be described.
This embodiment is specifically described for better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

The method for producing an electrode active material according to the present embodiment includes a dispersion treatment using ferrous phosphate hydrate (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O) and a polymer-based dispersant using water as a medium. Lithium phosphate (Li ₃ PO ₄ ) and water are mixed to form an aqueous mixture, and the aqueous mixture is subjected to pressure and heat treatment in a sealed state to produce lithium iron phosphorus-based composite oxide particles. In this method, the surface is coated with a carbonaceous material.

Next, the manufacturing method of the electrode active material of this embodiment is demonstrated in detail.
[First step]
In this process, lithium phosphate (Li ₃ PO ₄ ) is dispersed in water as a medium using a polymer dispersant to form an aqueous slurry.

Examples of the polymer dispersant include polyacrylic acid, polyvinyl alcohol, polyethyl acrylate, polymethyl acrylate, polyethylene glycol, and polypropylene glycol.
The addition amount of the polymer dispersant is preferably 0.1% by mass or more and 10% by mass or less, more preferably 0.5% by mass or more and 3% by mass or less, based on the mass of the lithium phosphate solid content. is there.
Here, if the addition amount of the polymeric dispersant is less than 0.1% by mass, the lithium phosphate cannot be sufficiently drowned. On the other hand, if the addition amount exceeds 10% by mass, an excessive polymer type is added. Since the dispersant remains on the surface of the lithium iron phosphate particles produced in the post-process and inhibits crystallization of the lithium iron phosphate in the post-pressurizing heat treatment stage, it is not preferable.

  The medium is basically water, but in order to improve the dispersibility of the polymer dispersant, an organic solvent other than water, such as methanol, ethanol, 1-propanol, 2-propanol, butanol, pentanol, hexanol, is used. Alcohols such as octanol and diacetone alcohol, esters such as ethyl acetate, butyl acetate, ethyl lactate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and γ-butyrolactone may be added.

The content of lithium phosphate in the aqueous slurry is preferably 30% by mass or more and 70% by mass or less, and more preferably 40% by mass or more and 60% by mass or less.
Here, if the content of lithium phosphate is less than 30% by mass, the concentration of lithium phosphate is too thin, and as a result, the dispersion treatment effect by the polymer dispersant is reduced, which is not preferable. When the lithium content exceeds 70% by mass, the concentration of lithium phosphate becomes too high, and the dispersion treatment effect by the polymer dispersant is also lowered, which is not preferable.

The specific surface area of the lithium iron phosphorus composite oxide to be produced by setting the addition amount of the polymer dispersant to 0.1 mass% or more and 10 mass% or less with respect to the mass of the solid content of lithium phosphate. Can be controlled to 10 m ² / g or more and 20 m ² / g or less.

[Second step]
The above aqueous slurry, ferrous phosphate hydrate (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O), and a reducing agent are mixed to form an aqueous mixture, and this aqueous mixture is heated under pressure in a sealed state. This is a step of performing hydrothermal synthesis by performing treatment to produce lithium iron phosphorus-based composite oxide particles such as lithium iron phosphate (LiFePO ₄ ) particles.
As a reducing agent, lithium phosphate (Li ₃ PO ₄ ) and ferrous phosphate hydrate (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O) are reduced to produce lithium iron phosphorus-based composite oxide particles. Any material can be used as long as it is not particularly limited, and examples thereof include sulfite, glucose, fructose, maltose, and lactose.

This hydrothermal synthesis is performed at 0.2 to 4.0 MPa, preferably 0.4 to 2.5 MPa, 120 to 250 ° C., preferably 150 to 220 ° C. for 1 to 24 hours, preferably Perform for 1 to 10 hours.
Thereby, lithium iron phosphorus complex oxide particles having a specific surface area of 10 m ² / g or more and 20 m ² / g or less are generated.

The lithium iron phosphorus based composite oxide particles may be crystalline particles, may be amorphous particles, or may be particles in which crystalline and amorphous are mixed.
Here, the reason why amorphous particles may be used is that amorphous lithium iron-phosphorus composite oxide particles are used when firing in a non-oxidizing atmosphere of 500 ° C. or higher and 1000 ° C. or lower in the subsequent step. Is because it crystallizes.

The average particle diameter of primary particles of the lithium iron phosphorus composite oxide particles is preferably 0.01 μm or more and 20 μm or less, more preferably 0.02 μm or more and 5 μm or less.
Here, if the average particle diameter of the primary particles is less than 0.01 μm, it is difficult to sufficiently cover the surfaces of the primary particles with thin film carbon, and the discharge capacity at a high-speed charge / discharge rate is low, which is sufficient. It becomes difficult to realize the charge / discharge rate performance. On the other hand, when the average particle diameter of the primary particles exceeds 20 μm, the internal resistance of the primary particles increases, and as a result, the discharge capacity at the high-speed charge / discharge rate becomes insufficient.

The shape of the lithium iron phosphorus-based composite oxide particles is not particularly limited, but since it is easy to produce an electrode material composed of spherical, particularly spherical, secondary particles, the shape of the lithium iron phosphorus-based composite oxide particles is also A spherical shape, particularly a true spherical shape is preferred.
Here, the reason why the shape of the lithium iron phosphorus composite oxide particles is preferably spherical is that the electrode material, the binder resin (binder), and the solvent are mixed to prepare the positive electrode preparation paste. This is because the amount of the solvent used can be reduced and the application of the positive electrode paste to the current collector is facilitated.

  Furthermore, if the shape of the lithium iron phosphorus composite oxide particles is spherical, the surface area of the lithium iron phosphorus composite oxide particles is minimized, and the amount of binder resin (binder) added to the electrode material mixture This is because the internal resistance of the positive electrode obtained can be reduced. Furthermore, since the closest packing is easy, the amount of the positive electrode material to be filled per unit volume is increased, so that the electrode density can be increased. Therefore, a high capacity lithium ion battery can be provided.

[Third step]
In this step, lithium iron phosphorus composite oxide particles and a carbonaceous material precursor are mixed to form a mixture.
The mass ratio of the lithium iron phosphorus-based composite oxide particles to the carbonaceous material precursor is such that, when the amount of the carbonaceous material precursor is converted into the amount of carbon, carbon is contained in 100 parts by mass of the lithium iron phosphorus-based composite oxide particles. Is preferably 0.1 parts by mass or more and 30 parts by mass or less, more preferably 0.5 parts by mass or more and 2 parts by mass or less.

  Here, when the mass ratio of carbon is less than 0.1 parts by mass, the discharge capacity at the high-speed charge / discharge rate is low, and it is difficult to realize sufficient charge / discharge rate performance. On the other hand, when the mass ratio of carbon exceeds 30 parts by mass, the mass ratio of the lithium iron phosphorus composite oxide particles becomes low. As a result, when a lithium ion battery is formed, the capacity of the battery becomes low.

  Examples of the carbonaceous material precursor include polyvinyl alcohol, polyvinyl pyrrolidone, cellulose, starch, gelatin, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, polyacrylic acid, polystyrene sulfonic acid, polyacrylamide, polyvinyl acetate, glucose, and fructose. Galactose, mannose, maltose, sucrose, lactose, glycogen, pectin, alginic acid, glucomannan, chitin, hyaluronic acid, chondroitin, agarose, polyether, polyhydric alcohols. These may use only 1 type and may mix and use 2 or more types.

Here, the lithium iron phosphorus composite oxide particles and the carbonaceous material precursor are dissolved or dispersed in water to prepare a uniform aqueous slurry as a mixture thereof.
The lithium iron phosphorus composite oxide particles and the carbonaceous material precursor are dispersed in water by a method in which the lithium iron phosphorus composite oxide particles are dispersed and the carbonaceous material precursor is dissolved or dispersed. Although there is no particular limitation as long as it exists, for example, a method using a medium stirring type dispersion device that stirs medium particles at high speed, such as a planetary ball mill, a vibration ball mill, a bead mill, a paint shaker, an attritor or the like is preferable.

[Fourth process]
This is a step of obtaining a granulated body by spray drying the above uniform aqueous slurry.
Here, the above aqueous slurry is sprayed in a high temperature atmosphere, for example, in the air of 70 ° C. or more and 250 ° C. or less, and dried to produce a granulated body.
The particle diameter of the droplets when sprayed in the air is preferably 0.05 μm or more and 500 μm or less.

[Fifth step]
In this step, the granulated body is fired to obtain a lithium iron phosphorus composite oxide carbon composite in which the surfaces of the lithium iron phosphorus composite oxide particles are coated with a carbonaceous material.
Here, the above-mentioned granulated product is subjected to 500 ° C. or more and 1000 ° C. or less, preferably 600 ° C. or more and 900 ° C. or less for 1 hour or more and 24 hours or less, preferably 1 hour or more in a non-oxidizing atmosphere. Bake for 10 hours or less.
As this non-oxidizing atmosphere, an inert atmosphere containing an inert gas such as N ₂ or Ar is preferable. However, when it is desired to suppress oxidation more, a reducing gas such as H _{2 is} contained in the inert gas. A reducing atmosphere is preferred.

  Moreover, the reason for setting the firing temperature of the granulated body to 500 ° C. or more and 1000 ° C. or less is that when the firing temperature of the granulated body is less than 500 ° C., the decomposition / reaction of the carbonaceous material precursor does not proceed sufficiently. Carbonization of the carbonaceous material precursor becomes insufficient, and as a result, a high-resistance organic matter decomposition product is generated. On the other hand, when the firing temperature of the granulated body exceeds 1000 ° C., not only does Li in the lithium iron phosphorus composite oxide particles evaporate and the composition shifts, but also the grain growth of the lithium iron phosphorus composite oxide particles accelerates. However, the discharge capacity at the high-speed charge / discharge rate becomes low, and it becomes difficult to realize sufficient charge / discharge rate performance.

In this firing process, the carbonaceous material precursor attached to the surface of the lithium iron phosphorus composite oxide particles is decomposed and reacted to become a carbonaceous material, and the surface of the lithium iron phosphorus composite oxide particles is coated. Become.
As described above, a lithium iron phosphorus composite oxide carbon composite in which the surfaces of lithium iron phosphorus composite oxide particles are coated with a carbonaceous material can be obtained.

The electrode active material thus obtained is an electrode active material obtained by coating the surface of lithium iron phosphorus composite oxide particles with a carbonaceous material, and the specific surface area of the lithium iron phosphorus composite oxide is 10 m. ² / g or more and 20 m ² / g or less.
A lithium ion battery using the finely divided lithium iron phosphorus-based composite oxide as a positive electrode has a high discharge capacity, excellent discharge characteristics, and excellent long-term reliability.

As described above, according to the method for producing an electrode active material of the present embodiment, lithium phosphate (Li ₃ PO ₄ ) is subjected to a dispersion treatment using a polymer dispersant using water as a medium, and an aqueous slurry and The first step, the aqueous slurry, ferrous phosphate hydrate (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O), and a reducing agent are mixed to form an aqueous mixture, and the aqueous mixture is sealed A third step of mixing the second step of producing a lithium iron phosphorus-based composite oxide particle and the lithium iron phosphorus-based composite oxide particle and the carbonaceous material precursor by performing a pressure heat treatment at And a fourth step of obtaining a granulated product by spray-drying the mixture, and calcining the granulated product and coating the surface of the lithium iron phosphorus-based composite oxide particles with a carbonaceous material. First to obtain a phosphorus-based composite oxide carbon composite Because it contains a step, a high discharge capacity, excellent discharge characteristics, the electrode active material is superior long-term reliability can be manufactured at high yield. In addition, the manufacturing cost is low.

According to the electrode active material of this embodiment, ferrous phosphate hydrated salt (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O) and water were used as a medium to perform dispersion treatment using a polymer dispersant. The specific surface area of the lithium iron phosphorus-based composite oxide particles to be produced is 10 m ² / g or more by subjecting an aqueous mixture containing lithium phosphate (Li ₃ PO ₄ ) and water to pressure heat treatment in a sealed state. And since it was 20 m < ² > / g or less, discharge capacity can be increased and the discharge characteristic and long-term reliability are also excellent.

  According to the lithium ion battery of this embodiment, since the electrode active material of this embodiment is used for the positive electrode, the discharge capacity, discharge characteristics, and long-term reliability of the lithium ion battery can be improved.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention concretely, this invention is not limited by these Examples.

"Example 1"
1% by mass of polyacrylic acid was added to lithium phosphate (Li ₃ PO ₄ ) and mixed for 8 hours by a ball mill to obtain an aqueous slurry.
Next, an aqueous slurry containing lithium phosphate and ferrous phosphate hydrated salt (Fe ₃ (PO ₄ ) ₂ .8H ₂ O) are set so that the ratio of Li, Fe and P is 1: 1: 1. After mixing, the mixture was put into an autoclave and subjected to heat and pressure treatment at 190 ° C. for 3 hours under the condition of 1 MPa.

Next, the slurry was taken out from the autoclave, and polyvinyl alcohol serving as conductive carbon was added to the slurry so as to be 1% by mass with respect to the solid content of the slurry, followed by mixing with a ball mill for 8 hours.
Subsequently, the obtained mixture was spray-dried to obtain a reaction precursor.
This reaction precursor was calcined in a nitrogen atmosphere at 700 ° C., which is the maximum holding temperature, for 1 hour and cooled to room temperature to obtain lithium iron phosphate coated with conductive carbon of Example 1.

"Example 2"
2% by mass of polyacrylic acid was added to lithium phosphate (Li ₃ PO ₄ ) and mixed for 8 hours by a ball mill to obtain an aqueous slurry.
Next, an aqueous slurry containing lithium phosphate and ferrous phosphate hydrated salt (Fe ₃ (PO ₄ ) ₂ .8H ₂ O) are set so that the ratio of Li, Fe and P is 1: 1: 1. After mixing, the mixture was put into an autoclave and subjected to heat and pressure treatment at 190 ° C. for 3 hours under the condition of 1 MPa.

Next, the slurry was taken out from the autoclave, and polyvinyl alcohol serving as conductive carbon was added to the slurry so as to be 1% by mass with respect to the solid content of the slurry, followed by mixing with a ball mill for 8 hours.
Subsequently, the obtained mixture was spray-dried to obtain a reaction precursor.
This reaction precursor was calcined in a nitrogen atmosphere at 700 ° C., the maximum holding temperature, for 1 hour and cooled to room temperature to obtain lithium iron phosphate coated with conductive carbon of Example 2.

“Comparative Example 1”
0.1% by mass of polyacrylic acid was added to lithium phosphate (Li ₃ PO ₄ ) and mixed for 8 hours by a ball mill to obtain an aqueous slurry.
Next, an aqueous slurry containing lithium phosphate and ferrous phosphate hydrated salt (Fe ₃ (PO ₄ ) ₂ .8H ₂ O) are set so that the ratio of Li, Fe and P is 1: 1: 1. After mixing, the mixture was put into an autoclave and subjected to heat and pressure treatment at 190 ° C. for 3 hours under the condition of 1 MPa.

Next, the slurry was taken out from the autoclave, and polyvinyl alcohol serving as conductive carbon was added to the slurry so as to be 1% by mass with respect to the solid content of the slurry, followed by mixing with a ball mill for 8 hours.
Subsequently, the obtained mixture was spray-dried to obtain a reaction precursor.
This reaction precursor was calcined at 700 ° C., which is the maximum holding temperature, under a nitrogen atmosphere for 1 hour and allowed to cool to room temperature to obtain lithium iron phosphate coated with conductive carbon of Comparative Example 1.

"Comparative Example 2"
10% by mass of polyacrylic acid was added to lithium phosphate (Li ₃ PO ₄ ) and mixed for 8 hours by a ball mill to obtain an aqueous slurry.
Next, an aqueous slurry containing lithium phosphate and ferrous phosphate hydrated salt (Fe ₃ (PO ₄ ) ₂ .8H ₂ O) are set so that the ratio of Li, Fe and P is 1: 1: 1. After mixing, the mixture was put into an autoclave and subjected to heat and pressure treatment at 190 ° C. for 3 hours under the condition of 1 MPa.

Next, the slurry was taken out from the autoclave, and polyvinyl alcohol serving as conductive carbon was added to the slurry so as to be 1% by mass with respect to the solid content of the slurry, followed by mixing with a ball mill for 8 hours.
Subsequently, the obtained mixture was spray-dried to obtain a reaction precursor.
This reaction precursor was calcined at 700 ° C., which is the maximum holding temperature, under a nitrogen atmosphere for 1 hour and allowed to cool to room temperature to obtain lithium iron phosphate coated with conductive carbon of Comparative Example 2.

"Measurement of specific surface area"
The specific surface areas of Examples 1 to 2 and Comparative Examples 1 to 2 and the lithium iron phosphate coated with the conductive carbon were measured using a bell soap (manufactured by Nippon Bell Co., Ltd.). The measurement results are shown in Table 1.

“Production of lithium-ion batteries”
Each positive electrode of Examples 1-2 and Comparative Examples 1-2 was produced.
Here, 90 parts by mass of lithium iron phosphate coated with conductive carbon obtained in each of Examples 1 and 2 and Comparative Examples 1 and 2, 5 parts by mass of graphite powder as a conductive aid, and polyfluoride as a binder 5 parts by mass of vinylidene (PVdF) was mixed to prepare positive electrode agents for Examples 1-2 and Comparative Examples 1-2.

Subsequently, this positive electrode agent was dispersed in N-methyl-2-pyrrolidinone (NMP) to prepare kneaded pastes of Examples 1-2 and Comparative Examples 1-2.
Next, these kneaded pastes were applied onto an aluminum (Al) foil having a thickness of 30 μm and dried. Then, it crimped | bonded so that it might become a predetermined density, and it punched in the disk shape of diameter 15mm using the molding machine, and obtained each positive electrode plate of Examples 1-2 and Comparative Examples 1-2.

On the other hand, a commercially available metal lithium foil was used for the negative electrode, a porous polypropylene film was used for the separator, and a 1 mol / L LiPF ₆ solution was used for the nonaqueous electrolyte solution as the nonaqueous electrolyte. As a solvent for this LiPF ₆ solution, a solvent having a ratio of ethylene carbonate to diethyl carbonate of 1: 1 was used.
Then, using the positive electrode plate, the metal lithium foil (negative electrode) and the LiPF ₆ solution prepared as described above, and the 2016 type coin cell, each of the lithium ion batteries of Examples 1-2 and Comparative Examples 1-2 was prepared. Produced.

"Performance evaluation of lithium-ion battery"
The lithium ion batteries of Examples 1-2 and Comparative Examples 1-2 were operated at room temperature (25 ° C.), and the initial discharge capacity was measured.
Table 1 shows the initial discharge capacities (mAh / g) of Examples 1-2 and Comparative Examples 1-2.

  According to the above results, it was found that the lithium ion batteries of Examples 1 and 2 had a higher initial discharge capacity than the lithium ion batteries of Comparative Examples 1 and 2.

In this embodiment, graphite powder is used as the conductive additive, but carbon materials such as carbon black, graphite, ketjen black, natural graphite, and artificial graphite may be used. Moreover, although evaluation is made with a battery using a commercially available metal lithium foil for the negative electrode, naturally, a negative electrode material such as natural graphite, artificial graphite, carbon material such as coke, Li ₄ Ti ₅ O ₁₂ or Li alloy is used. May be. Further, as the nonaqueous electrolyte solution as the nonaqueous electrolyte solution, a mixture of ethylene carbonate containing 1 mol / L LiPF ₆ and diethyl carbonate in 1: 1 (volume ratio) is used, but LiBF ₆ is used instead of LiPF _{6. 4} and LiClO _4, may be used propylene carbonate and diethyl carbonate in place of ethylene carbonate. A solid electrolyte may be used instead of the electrolytic solution and the separator.

In the method for producing an electrode active material according to the present invention, ferrous phosphate hydrate (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O) and water were used as a medium, and a dispersion treatment was performed using a polymer dispersant. Lithium phosphate (Li ₃ PO ₄ ) and water are mixed to form an aqueous mixture. The aqueous mixture is subjected to pressure and heat treatment in a sealed state, and the surface of the resulting lithium iron phosphorous composite oxide particles is carbonized. Lithium ion batteries can be produced in high yields by coating electrode materials with high discharge capacity, excellent discharge characteristics, and long-term reliability. In addition to being able to further improve the discharge characteristics, it can also be applied to next-generation secondary batteries that are expected to be smaller, lighter, and higher capacity. In the case of batteries, the effect is very large A.

Claims (7)

And ferrous phosphate hydrate _{(Fe 3 (PO 4) 2} · 8H 2 O), lithium phosphate subjected to dispersion treatment using a polymeric dispersing agent and water as the medium _(Li 3 PO _4), A water-based mixture is formed by mixing water, and the surface of the resulting lithium iron-phosphorus composite oxide particles is coated with a carbonaceous material by subjecting the aqueous mixture to pressure heat treatment in a sealed state. A method for producing an electrode active material. A first step in which lithium phosphate (Li ₃ PO ₄ ) is dispersed in water as a medium using a polymer dispersant to form an aqueous slurry;
The aqueous slurry, ferrous phosphate hydrate (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O), and a reducing agent are mixed to form an aqueous mixture, and this aqueous mixture is heated under pressure in a sealed state. A second step of producing lithium iron phosphorus-based composite oxide particles,
A third step of mixing the lithium iron phosphorus-based composite oxide particles and the carbonaceous material precursor to form a mixture;
A fourth step of obtaining a granulated body by spray drying the mixture;
A fifth step of obtaining the lithium iron phosphorus-based composite oxide carbon composite obtained by firing the granulated body and coating the surface of the lithium iron phosphorus-based composite oxide particles with a carbonaceous material;
A method for producing an electrode active material comprising:   The method for producing an electrode active material according to claim 1, wherein the polymer dispersant is polyacrylic acid. The ratio of the lithium iron-phosphorus composite oxide to be produced by setting the addition amount of the polymer dispersant to 0.1 mass% or more and 10 mass% or less with respect to the solid content mass of the lithium phosphate. The method for producing an electrode active material according to any one of claims 1 to 3, wherein the surface area is controlled to be 10 m ² / g or more and 20 m ² / g or less. And ferrous phosphate hydrate _{(Fe 3 (PO 4) 2} · 8H 2 O), lithium phosphate subjected to dispersion treatment using a polymeric dispersing agent and water as the medium _(Li 3 PO _4), An electrode active material obtained by coating the surface of a lithium iron phosphorus-based composite oxide particle to be produced with a carbonaceous material by subjecting an aqueous mixture containing water to a pressure heat treatment in a sealed state,
The specific surface area of the lithium-iron-phosphorus compound oxide, the electrode active material being not more than 10 m 2 / ^g or more and 20 m 2 / ^g.   An electrode comprising the electrode active material according to claim 5 as a positive electrode.   A battery comprising the electrode according to claim 6 as a positive electrode.