JP2018163742A - Positive electrode material for lithium ion secondary battery, positive electrode for lithium ion secondary battery, lithium ion secondary battery, and method for evaluating positive electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode material for a lithium ion secondary battery, which has optimal coatability of a carbonaceous coating film.SOLUTION: The positive electrode material for a lithium ion secondary battery is an active material particle including: a center particle expressed by a general formula of LiFeMPO(0.95≤x≤1.1, 0.8≤y≤1.1, 0≤z≤0.2, and M is at least one kind selected from a group of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and a rare earth element); and a carbonaceous coating film that coats a surface of the center particle. A specific surface ratio Δb/BET is 0.25 or more and 0.35 or less (Δbis a difference between a chromaticity bin a Labcolorimetric system after heating the active material particle for a 72-hour under an air atmosphere and at 180°C and the chromaticity bbefore heating the active material particle).SELECTED DRAWING: None

Description

  The present invention relates to a positive electrode material for a lithium ion secondary battery, a positive electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a method for evaluating a positive electrode material for a lithium ion secondary battery.

In recent years, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have been proposed and put into practical use as batteries that are expected to be reduced in size, weight, and capacity. A lithium ion secondary battery is composed of a positive electrode and a negative electrode having a property capable of reversibly removing and inserting lithium ions, and a non-aqueous electrolyte.
Generally as a negative electrode active material of the negative electrode material of a lithium ion secondary battery, the carbon-type material or the Li containing metal oxide which has a property which can reversibly insert and remove lithium ion is used. Examples of such a Li-containing metal oxide include lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

On the other hand, as a positive electrode of a lithium ion secondary battery, a positive electrode material mixture including a positive electrode material and a binder is used. As the positive electrode active material, for example, a Li-containing metal oxide having a property capable of reversibly removing and inserting lithium ions such as lithium iron phosphate (LiFePO ₄ ) is used. And the positive electrode of a lithium ion secondary battery is formed by apply | coating this positive electrode material mixture on the surface of metal foil called an electrode collector.

  A non-aqueous solvent is used for the electrolyte of the lithium ion secondary battery. As the non-aqueous solvent, a positive electrode active material that is oxidized and reduced at a high potential and a negative electrode active material that is oxidized and reduced at a low potential can be applied. Thereby, the lithium ion secondary battery which has a high voltage is realizable.

  Such a lithium ion secondary battery is lighter and smaller than conventional secondary batteries such as a lead battery, a nickel cadmium battery, and a nickel metal hydride battery, and has high energy. Therefore, lithium ion secondary batteries are used not only as small power sources used for portable electronic devices such as portable telephones and laptop personal computers, but also as stationary emergency large power sources.

  In recent years, improvement in performance of lithium ion secondary batteries has been demanded and various studies have been made. For example, when a lithium ion secondary battery using a positive electrode active material having a low electronic conductivity such as lithium iron phosphate is used in a high current density region, the electron conductivity of the positive electrode active material is improved in order to improve performance. Is required. In response to such physical property requirements, a technique of coating the surface of the positive electrode active material with a carbonaceous material (hereinafter, sometimes referred to as “carbonaceous coating”) is known (for example, Patent Document 1). To 3). As a method for coating the surface of the positive electrode active material with a carbonaceous film, a method of mixing the positive electrode active material and a carbon source and firing the mixture in an inert atmosphere or a reducing atmosphere is known.

  As the coverage of the carbonaceous film on the surface of lithium iron phosphate increases, the electron conductivity tends to improve and the input / output characteristics tend to improve. However, since the carbonaceous film also has a side surface that inhibits the movement of lithium ions, it is also known that the input / output characteristics deteriorate if the coverage is too high (see, for example, Patent Document 4).

JP 2009-004371 A JP 2011-046161 A JP 2012-104290 A Japanese Patent No. 5822017

  The optimal coating state of the carbonaceous film to improve the input / output characteristics of lithium iron phosphate can increase the electronic conductivity of lithium iron phosphate as much as possible while preventing the migration of lithium ions too much. It is the state of such a coverage.

However, it is not easy to accurately and simply evaluate the coverage of the carbonaceous film. For example, in Patent Document 4, carbon coverage is determined by surface analysis using a transmission electron microscope (TEM) and an energy dispersive X-ray analyzer (Energy Dispersive X-ray microanalyzer, EDX). The percentage is calculated. This surface analysis cannot detect a fine uncovered region of the carbonaceous coating layer, and therefore the detection accuracy is low. In addition to this surface analysis, a method for evaluating the coverage of a carbonaceous film from the area ratio between the peak area derived from the carbonaceous film obtained by Raman spectroscopy and the PO ₄ peak area not coated with carbon is also known. ing. This method has low accuracy because the carbonaceous film is oxidized and decomposed by the energy of the beam light.
Furthermore, the coverage of the carbonaceous film is also affected by the coverage, film thickness and uniformity thereof (irregularity state), pores in the carbonaceous film, crystallinity of the carbonaceous film, and the like.

  For the above reasons, it is difficult to accurately evaluate the coverage of the carbonaceous film on the surface of lithium iron phosphate. Further, the optimal coverage of the carbonaceous film for improving the input / output characteristics has been unknown.

  The present invention has been made in view of the above circumstances, and is a lithium ion secondary battery positive electrode material having an optimal carbonaceous coating property, and a lithium ion secondary battery containing the lithium ion secondary battery positive electrode material. A positive electrode for a secondary battery, a lithium ion secondary battery provided with the positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery capable of accurately and simply evaluating the coverage of a carbonaceous film on the surface of lithium iron phosphate It aims at providing the evaluation method of the positive electrode material for secondary batteries.

As a result of intensive studies to solve the above problems, the present inventors have found that the general formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1). , 0 ≦ z ≦ 0.2, where M is at least selected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge and rare earth elements By evaluating the coverage of the carbonaceous film in the active material particles including the center particle represented by 1 type) and the carbonaceous film covering the surface of the center particle by Δb ^* / BET specific surface area ratio, Δb ^* By providing a / BET specific surface area ratio of 0.25 or more and 0.35 or less, it is possible to provide a positive electrode material for a lithium ion secondary battery having an optimal carbonaceous film covering property for improving input / output characteristics. As a result, the present invention has been completed.

The positive electrode material for a lithium ion secondary battery of the present invention has a general formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1, 0 ≦ z ≦ 0. 2, wherein M is a center represented by Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and a rare earth element. Active material particles comprising particles and a carbonaceous film covering the surface of the central particle, wherein a Δb ^* / BET specific surface area ratio is 0.25 or more and 0.35 or less (Δb ^* : the active material particles characterized in that the atmosphere is at 180 ° C. the difference between the chromaticity b ^* and the active material chromaticity before heating of the particles b ^* in 72 hours while heating to L ^* a ^* b ^* color system after) And

  The positive electrode for a lithium ion secondary battery of the present invention is a positive electrode for a lithium ion secondary battery comprising an electrode current collector and a positive electrode mixture layer formed on the electrode current collector, wherein the positive electrode The mixture layer contains the positive electrode material for a lithium ion secondary battery of the present invention.

  The lithium ion secondary battery of the present invention includes the positive electrode for a lithium ion secondary battery of the present invention.

In addition, as a result of intensive studies to solve the above problems, the present inventors, after heating lithium iron phosphate particles coated with a carbonaceous film at 180 ° C. for 72 hours in an air atmosphere L ^* a ^* b ^* Value obtained by dividing the difference between the chromaticity b ^* in the color system and the chromaticity b ^* before heating of the lithium iron phosphate particles by the BET specific surface area before heating of the lithium iron phosphate particles ( Hereinafter, the present invention has been completed by finding that the coverage of lithium iron phosphate particles by a carbonaceous film can be accurately evaluated by “Δb ^* / BET specific surface area ratio”.

The evaluation method of the positive electrode material for a lithium ion secondary battery of the present invention is expressed by the general formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1, 0 ≦ z ≦ 0.2, where M is at least one selected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and rare earth elements) The color in the L ^* a ^* b ^* color system after heating the active material particle containing the center particle represented and the carbonaceous film which coat | covers the surface of this center particle | grain at 72 degreeC in air | atmosphere atmosphere for 72 hours The coverage of the center particle by the carbonaceous film is evaluated by a value obtained by dividing the difference between the degree b ^* and the chromaticity b ^* of the center particle before heating by the BET specific surface area of the active material particle before heating. It is characterized by doing.

According to the positive electrode material for a lithium ion secondary battery of the present invention, since the Δb ^* / BET specific surface area ratio is 0.25 or more and 0.35 or less, it is possible to minimize the inhibition of lithium ion migration and It is possible to provide a positive electrode material for a lithium ion secondary battery that can maximize the electronic conductivity of the substance (center particle) and can improve the input / output characteristics.

  According to the positive electrode for a lithium ion secondary battery of the present invention, since it contains the positive electrode material for a lithium ion secondary battery of the present invention, a lithium ion secondary battery having high energy density and excellent input / output characteristics is obtained. It is done.

  According to the lithium ion secondary battery of the present invention, since the positive electrode for a lithium ion secondary battery of the present invention is provided, a lithium ion secondary battery having high energy density and excellent input / output characteristics can be obtained.

According to the method for evaluating a positive electrode material for a lithium ion secondary battery of the present invention, by measuring the Δb ^* / BET specific surface area ratio, the covering property of the conductive carbonaceous film can be accurately and easily evaluated. It becomes possible to adjust the coverage of the carbonaceous film suitable for the positive electrode active material for the secondary battery.

In Examples 1-3 and Comparative Examples 1-3, it is a figure which shows the relationship between (DELTA ⁾ b ^* / BET specific surface area ratio and 10C discharge capacity. In Examples 4-6 and Comparative Examples 4-6, it is a figure which shows the relationship between (DELTA ⁾ b ^* / BET specific surface area ratio and 10C discharge capacity. In Examples 7-9 and Comparative Examples 7-9, it is a figure which shows the relationship between (DELTA ⁾ b ^* / BET specific surface area ratio and 10C discharge capacity.

Embodiments of an evaluation method for a positive electrode material for a lithium ion secondary battery, a positive electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a positive electrode material for a lithium ion secondary battery of the present invention will be described.
Note that this embodiment is specifically described in order to better understand the gist of the invention, and does not limit the present invention unless otherwise specified.

[Positive electrode material for lithium ion secondary batteries]
The positive electrode material for a lithium ion secondary battery of the present embodiment (hereinafter sometimes referred to as “positive electrode material”) has a general formula of Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0). .8 ≦ y ≦ 1.1, 0 ≦ z ≦ 0.2, where M is Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and rare earth elements Active material particles comprising a center particle represented by at least one selected from the group consisting of: and a carbonaceous film covering the surface of the center particle, wherein the Δb ^* / BET specific surface area ratio is 0.25. And not more than 0.35 (Δb ^* : L ^* a ^* b ^* chromaticity b ^* in the color system and heating of the active material particles after heating the active material particles at 180 ° C. for 72 hours in the atmosphere. Difference from the previous chromaticity b ^* ).

The average secondary particle diameter of the aggregated particles obtained by aggregating a plurality of the central particles (primary particles) is preferably 0.5 μm or more and 20 μm or less, and more preferably 0.7 μm or more and 10 μm or less.
When the average secondary particle diameter of the aggregated particles is 0.5 μm or more, a positive electrode material, a conductive additive, a binder resin (binder), and a solvent are mixed to obtain a positive electrode material for a lithium ion secondary battery. The compounding quantity of the conductive support agent and binder at the time of preparing a paste can be suppressed. As a result, the battery capacity of the lithium ion secondary battery per unit mass of the positive electrode mixture layer can be increased. On the other hand, when the average secondary particle size of the aggregated particles is 20 μm or less, the dispersibility and uniformity of the conductive additive and the binder in the positive electrode mixture layer can be improved. As a result, the lithium ion secondary battery using the positive electrode material for lithium ion secondary batteries can increase the discharge capacity in high-speed charge / discharge.

    The shape of the agglomerated particles is not particularly limited, but is preferably spherical, particularly true spherical. When the aggregated particles are spherical, the amount of solvent in preparing the positive electrode material paste can be reduced by mixing the positive electrode material for a lithium ion secondary battery, a binder resin (binder), and a solvent. it can. Moreover, since the shape of the aggregated particles is spherical, it is easy to apply the positive electrode material paste to the electrode current collector. Furthermore, if the shape of the aggregated particles is spherical, the amount of binder resin (binder) in the positive electrode material paste can be minimized. As a result, the filling amount of the positive electrode material for lithium ion secondary battery per unit volume of the positive electrode using the positive electrode material for lithium ion secondary battery is increased, and a high capacity lithium ion secondary battery is obtained.

  Here, the average particle diameter is a volume average particle diameter. The average primary particle diameter of the primary particles of the positive electrode material can be measured using a laser diffraction scattering type particle size distribution measuring apparatus or the like. Alternatively, a plurality of primary particles observed with a scanning electron microscope (SEM) may be arbitrarily selected, and the average particle size of the primary particles may be calculated.

The amount of carbon contained in the positive electrode material for a lithium ion secondary battery, that is, the amount of carbon forming the carbonaceous film is preferably 0.1 parts by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the center particle. More preferably, it is 0.6 parts by mass or more and 3 parts by mass or less.
When the carbon content is 0.1 parts by mass or more, the discharge capacity at the high-speed charge / discharge rate of the lithium ion secondary battery is increased, and sufficient charge / discharge rate performance can be realized. On the other hand, it can suppress that the battery capacity of the lithium ion secondary battery per unit mass of positive electrode material falls more than necessary as carbon content is 10 mass parts or less.

The film thickness of the carbonaceous film is preferably 1.0 nm or more and 10.0 nm or less, and the average film thickness of the carbonaceous film is preferably 2.0 nm or more and 7.0 nm or less.
When the average film thickness of the carbonaceous film is 2.0 nm or more, the sum of the electron transfer resistance in the carbonaceous film is difficult to increase, the increase in the internal resistance of the battery is suppressed, and the voltage drop at the high-speed charge / discharge rate is reduced. Can be prevented. On the other hand, when the average film thickness of the carbonaceous film is 7.0 nm or less, steric hindrance when lithium ions diffuse in the carbonaceous film is suppressed, and the migration resistance of lithium ions is reduced. As a result, an increase in the internal resistance of the battery is suppressed, and a voltage drop at a high speed charge / discharge rate can be prevented.
Moreover, it is easy to keep the average film thickness of a carbonaceous film in 2.0 nm or more as the film thickness of a carbonaceous film is 1.0 nm or more. On the other hand, when the film thickness of the carbonaceous film is 10.0 nm or less, it is easy to suppress the average film thickness of the carbonaceous film to 7.0 nm or less.
The film thickness of the carbonaceous film can be measured using a transmission electron microscope.

The density of the carbonaceous film, calculated by the carbon content of the carbonaceous film, is preferably 0.3 g / cm ³ or more and 1.5 g / cm ³ or less, preferably 0.4 g / cm ³ or more and 1.0 g / cm. More preferably, it is not more than cm ³ .
When the density of the carbonaceous film calculated by the carbon content of the carbonaceous film is 0.3 g / cm ³ or more, the carbonaceous film exhibits sufficient electronic conductivity. On the other hand, when the density of the carbonaceous film is 1.5 g / cm ³ or less, a small amount of graphite microcrystals having a layered structure is present in the carbonaceous film, so that when lithium ions diffuse in the carbonaceous film. No steric hindrance due to graphite microcrystals. Thereby, lithium ion movement resistance does not become high. As a result, the internal resistance of the lithium ion secondary battery does not increase, and the voltage drop at the high-speed charge / discharge rate of the lithium ion secondary battery does not occur.

Carbon loading relative to the specific surface area of the primary particles of the central particles constituting the positive electrode material for a lithium ion secondary battery (“[carbon loading] / [specific surface area of primary particles of the central particles]”; hereinafter “carbon loading ratio” Is preferably 0.01 g / m ² or more and 0.5 g / m ² or less, and more preferably 0.03 g / m ² or more and 0.3 g / m ² or less.
When the carbon loading ratio is 0.01 g / m ² or more, the discharge capacity at the high-speed charge / discharge rate of the lithium ion secondary battery is increased, and sufficient charge / discharge rate performance can be realized. On the other hand, it can suppress that the battery capacity of the lithium ion secondary battery per unit mass of positive electrode material falls more than necessary that a carbon carrying amount ratio is 0.5 g / m < ² > or less.

The BET specific surface area of the positive electrode material for a lithium ion secondary battery is preferably 5 m ² / g or more and 30 m ² / g or less.
When the BET specific surface area is 5 m ² / g or more, coarsening of the positive electrode material can be suppressed, and the diffusion rate of lithium ions in the particles can be increased. Thereby, the battery characteristic of a lithium ion secondary battery can be improved. On the other hand, when the BET specific surface area is 30 m ² / g or less, the positive electrode density in the positive electrode including the positive electrode material for a lithium ion secondary battery of the present embodiment can be increased. Therefore, a lithium ion secondary battery having a high energy density can be provided.

The Δb ^* / BET specific surface area ratio of the positive electrode material for a lithium ion secondary battery is 0.25 or more and 0.35 or less, and preferably 0.27 or more and 0.33 or less.
When the Δb ^* / BET specific surface area ratio is 0.25 or more, the carbonaceous film does not excessively inhibit the movement of lithium ions, so that the input / output characteristics of the lithium ion secondary battery can be improved. On the other hand, when the Δb ^* / BET specific surface area ratio is 0.35 or less, the electron conductivity is sufficiently secured, so that the input / output characteristics of the lithium ion secondary battery can be improved. If the Δb ^* / BET specific surface area ratio is in the range of 0.25 or more and 0.35 or less, only one of the electron conduction speed and the lithium ion conduction speed in a trade-off relationship is not extremely slow, Speed balance is the best. Therefore, a carbonaceous film having the most excellent input / output characteristics can be obtained.

(Center particle)
The central particles constituting the positive electrode material for a lithium ion secondary battery of the present embodiment have a general formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1, 0 ≦ z ≦ 0.2, where M is at least one selected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and rare earth elements A positive electrode active material represented by
The rare earth elements are 15 elements of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu which are lanthanum series.

Examples of the compound represented by the general formula Li _x Fe _y M _z PO ₄ include LiFePO ₄ , LiFe _0.95 Mg _0.05 PO ₄ , Li _0.95 Fe _0.95 Al _0.05 PO _{4 and the} like. It is done. Among these, LiFePO ₄ is preferable because it does not include a metal that does not exhibit electrochemical activity and has the highest theoretical energy density.

The average primary particle diameter of the primary particles of the central particles constituting the positive electrode material for a lithium ion secondary battery of the present embodiment is preferably 5 nm or more and 800 m or less, and more preferably 20 nm or more and 500 nm or less.
When the average primary particle diameter of the primary particles of the central particles is 5 nm or more, the surface of the primary particles of the central particles can be sufficiently covered with the carbonaceous film. And the discharge capacity in the high-speed charge / discharge of a lithium ion secondary battery can be made high, and sufficient charge / discharge performance can be implement | achieved. On the other hand, when the average primary particle diameter of the primary particles of the central particles is 800 nm or less, the internal resistance of the primary particles of the central particles can be reduced. And the discharge capacity in the high-speed charge / discharge of a lithium ion secondary battery can be made high.

(Carbonaceous film)
The carbonaceous coating covers the surface of the central particle.
By covering the surface of the center particle with a carbonaceous film, the electron conductivity of the positive electrode material for a lithium ion secondary battery can be improved.

The thickness of the carbonaceous film is preferably 0.2 nm or more and 10 nm or less, and more preferably 0.5 nm or more and 4 nm or less.
When the thickness of the carbonaceous film is 0.2 nm or more, it can be suppressed that a film having a desired resistance value cannot be formed because the thickness of the carbonaceous film is too thin. And the electroconductivity as a positive electrode material for lithium ion secondary batteries is securable. On the other hand, it can suppress that the battery capacity per unit mass of the positive electrode material for lithium ion secondary batteries falls that the thickness of a carbonaceous film is 10 nm or less.
Further, when the thickness of the carbonaceous film is 0.2 nm or more and 10 nm or less, the positive electrode material for a lithium ion secondary battery is easily packed most closely, so the positive electrode material for a lithium ion secondary battery per unit volume in the positive electrode The amount of filling increases. As a result, the positive electrode density can be increased, and a high-capacity lithium ion secondary battery can be obtained.

According to the positive electrode material for a lithium ion secondary battery of the present embodiment, the general formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1, 0 ≦ z ≦ 0.2, where M is at least one selected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and rare earth elements) Active material particles including a center particle represented and a carbonaceous film covering a surface of the center particle, wherein a ratio of Δb ^* / BET specific surface area is 0.25 or more and 0.35 or less (Δb ^* : the active material the material particles in an air atmosphere, the difference) between the chromaticity b ^* and the active material chromaticity before heating of the particles b ^* at 72 hours heated after the L ^* a ^* b ^* color system at 180 ° C. As a result, the electron transfer of the positive electrode active material is minimized while inhibiting the movement of lithium ions. Can be increased sex to maximize, can provide a positive electrode material for a lithium ion secondary battery can be improved input-output characteristic.

[Method for producing positive electrode material for lithium ion secondary battery]
"Method for producing active material particles"
The method for producing active material particles in the present embodiment includes, for example, a production process of central particles and central particle precursors, and at least one central particle raw material selected from the group consisting of central particles and central particle precursors, Mixing an organic compound that is a carbonaceous film precursor, water, and ammonia (NH ₃ ) water to prepare a slurry, drying the slurry, and firing the resulting dried product in a non-oxidizing atmosphere And a firing step.

(Manufacturing process of center particle and center particle precursor)
General formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1, 0 ≦ z ≦ 0.2, where M is Mg, Ca, Co, As a method for producing a compound (central particle) represented by Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and a rare earth element, a solid phase is used. Conventional methods such as a method, a liquid phase method, and a gas phase method are used. _{Examples of the} Li _x Fe _y M _z PO ₄ obtained by such a method include particles (hereinafter sometimes referred to as “Li _x Fe _y M _z PO ₄ particles”).

For example, Li _x Fe _y M _z PO ₄ particles are obtained by hydrothermal synthesis of a slurry mixture obtained by mixing a Li source, a Fe source, a P source, water, and an M source as required. Is obtained. According to hydrothermal synthesis, Li _x Fe _y M _z PO ₄ is produced as a precipitate in water. The resulting precipitate may be a precursor of Li _x Fe _y M _z PO ₄ . In this _case, by firing the precursor of _{Li x Fe y M z PO 4} , Li x Fe y M z PO 4 particles of interest are obtained.
It is preferable to use a pressure-resistant airtight container for this hydrothermal synthesis.

Here, examples of the Li source include lithium salts such as lithium acetate (LiCH ₃ COO) and lithium chloride (LiCl), lithium hydroxide (LiOH), and the like. Among these, it is preferable to use at least one selected from the group consisting of lithium acetate, lithium chloride, and lithium hydroxide as the Li source.

Examples of the Fe source include iron chloride, iron carboxylate, and iron sulfate. Examples of the Fe source include divalent iron salts such as iron chloride (II) (FeCl ₂ ), iron acetate (II) (Fe (CH ₃ COO) ₂ ), and iron (II) sulfate (FeSO ₄ ). It is done. Among these, as the Fe source, it is preferable to use at least one selected from the group consisting of iron (II) chloride, iron (II) acetate, and iron (II) sulfate.

  M source includes chloride, carboxylic acid containing at least one selected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge and rare earth elements Examples thereof include salts and sulfates.

Examples of the P source include phosphoric acid compounds such as phosphoric acid (H ₃ PO ₄ ), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), and diammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ). . Among these, as the P source, it is preferable to use at least one selected from the group consisting of phosphoric acid, ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

(Slurry preparation process)
By the slurry preparation step, an organic compound that is a precursor of the carbonaceous film is interposed between the center particles, and they are uniformly mixed, so that the surface of the center particles can be uniformly coated with the organic compound.
Furthermore, the organic compound which coat | covers the surface of center particle | grains carbonizes by a baking process, The active material particle (positive electrode material) containing the center particle | grains by which the carbonaceous film was uniformly coat | covered is obtained.

  The organic compound used in the method for producing active material particles in the present embodiment is not particularly limited as long as it is a compound that can form a carbonaceous film on the surface of the center particle. Examples of such organic compounds include polyvinyl alcohol (PVA), polyvinyl pyrrolidone, cellulose, starch, gelatin, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, polyacrylic acid, polystyrene sulfonic acid, polyacrylamide, and polyvinyl acetate. Glucose, fructose, galactose, mannose, maltose, sucrose, lactose, glycogen, pectin, alginic acid, glucomannan, chitin, hyaluronic acid, chondroitin, agarose, polyether, ethylene glycol and other dihydric alcohols, glycerin and other trihydric alcohols Etc.

In the slurry preparation step, a uniform slurry is prepared by dissolving or dispersing the center particle raw material, the organic compound, and aqueous ammonia in water.
When these raw materials are dissolved or dispersed in water, a dispersant can be added.
The method for dissolving or dispersing the central particle raw material and the organic compound in water is not particularly limited as long as the central particle raw material is dispersed in water and the organic compound is dissolved or dispersed in water. As such a method, for example, a method using a medium agitation type dispersion device that agitates medium particles at high speed, such as a planetary ball mill, a vibration ball mill, a bead mill, a paint shaker, and an attritor, is preferable.

  When the central particle raw material and the organic compound are dissolved or dispersed in water, the central particle raw material is dispersed as primary particles in water, and then stirred so that the organic compound is added to water and dissolved or dispersed. It is preferable. In this way, the surface of the primary particles of the central particle raw material is easily coated with the organic compound. Thereby, the organic compound is uniformly arranged on the surface of the primary particles of the central particle raw material, and as a result, the surface of the primary particles of the central particle is covered with the carbonaceous film derived from the organic compound.

  By adjusting the amount of ammonia water input in the slurry preparation process, the pH of the slurry is adjusted, the degree of oxidation of iron ions in the center particle raw material is adjusted, and consumed in the reduction of iron ions at the center particle surface in the firing process The carbon amount to be adjusted can be adjusted to control the coverage of the carbonaceous film. The amount of ammonia water added is preferably 3 mol% or less with respect to the total molar amount (100 mol%) of the center particle raw material. When the amount of ammonia water added is 3 mol% or less, the production of iron impurities due to excessive oxidation of the center particle surface can be suppressed.

(Baking process)
Next, the slurry prepared in the slurry preparation step is sprayed and dried in a high temperature atmosphere, for example, in an atmosphere of 70 ° C. or higher and 250 ° C. or lower.
Next, the obtained dried product is calcined in a non-oxidizing atmosphere, preferably at a temperature of 500 ° C. or more and 1000 ° C. or less, more preferably at a temperature of 600 ° C. or more and 1000 ° C. or less for 0.1 hours or more and 40 hours or less. To do.

As the non-oxidizing atmosphere, an atmosphere made of an inert gas such as nitrogen (N ₂ ) or argon (Ar) is preferable. When it is desired to further suppress the oxidation of the dried product, a reducing atmosphere containing about several volume% of a reducing gas such as hydrogen (H ₂ ) is preferable. Further, for the purpose of removing organic components evaporated in the non-oxidizing atmosphere at the time of firing, a flammable gas such as oxygen (O ₂ ) or a combustible gas may be introduced into the non-oxidizing atmosphere.

  Here, by setting the firing temperature to 500 ° C. or higher, the decomposition and reaction of the organic compound contained in the dried product can easily proceed sufficiently, and the organic compound can be easily carbonized. As a result, it is easy to prevent the decomposition product of the high-resistance organic compound from being formed in the obtained aggregated particles. On the other hand, by setting the firing temperature to 1000 ° C. or less, lithium (Li) in the center particle raw material is difficult to evaporate, and the center particle is prevented from growing beyond its intended size. As a result, when a lithium ion secondary battery including a positive electrode including the positive electrode material of the present embodiment is manufactured, it is possible to prevent the discharge capacity at a high speed charge / discharge rate from being lowered, and lithium having sufficient charge / discharge rate performance. An ion secondary battery can be realized.

  As described above, active material particles composed of aggregated particles in which the surfaces of the primary particles of the center particles are coated with carbon (carbonaceous film) generated by thermal decomposition of the organic compound in the dried product are obtained.

[Positive electrode for lithium ion secondary battery]
The positive electrode for a lithium ion secondary battery of the present embodiment (hereinafter sometimes simply referred to as “positive electrode”) includes the positive electrode material for a lithium ion secondary battery of the present embodiment. More specifically, the positive electrode of the present embodiment includes an electrode current collector made of a metal foil and a positive electrode mixture layer formed on the electrode current collector, and the positive electrode mixture layer is the present embodiment. The positive electrode material for lithium ion secondary batteries is contained. That is, the positive electrode of the present embodiment is obtained by forming a positive electrode mixture layer on one main surface of the electrode current collector using the positive electrode material for a lithium ion secondary battery of the present embodiment.

  Since the positive electrode for the lithium ion secondary battery of the present embodiment includes the positive electrode material for the lithium ion secondary battery of the present embodiment, the lithium ion secondary battery using the positive electrode for the lithium ion secondary battery of the present embodiment is High energy density and excellent input / output characteristics.

[Method for producing positive electrode for lithium ion secondary battery]
The manufacturing method of the positive electrode for lithium ion secondary batteries of this embodiment is a method that can form a positive electrode mixture layer on one main surface of an electrode current collector using the positive electrode material for lithium ion secondary batteries of this embodiment. If there is no particular limitation. As a manufacturing method of the positive electrode of this embodiment, the following method is mentioned, for example.
First, the positive electrode material for a lithium ion secondary battery of the present embodiment, a binder composed of a binder resin, and a solvent are mixed to prepare a positive electrode material paste. Under the present circumstances, you may add conductive support agents, such as carbon black, to the positive electrode material paste in this embodiment as needed.

"Binder"
As the binder, that is, a binder resin, for example, polytetrafluoroethylene (PTFE) resin, polyvinylidene fluoride (PVdF) resin, fluororubber, and the like are preferably used.

The blending amount of the binder used in preparing the positive electrode material paste is not particularly limited. For example, it is 1 part by mass or more and 30 parts by mass or less with respect to 100 parts by mass of the positive electrode material for a lithium ion secondary battery. Is preferably 3 parts by mass or more and 20 parts by mass or less.
When the blending amount of the binder is 1 part by mass or more, the binding property between the positive electrode mixture layer and the electrode current collector can be sufficiently increased. Thereby, it is possible to prevent the positive electrode mixture layer from cracking or falling off during the rolling of the positive electrode mixture layer. Moreover, in the charging / discharging process of a lithium ion secondary battery, it can suppress that a positive mix layer peels from an electrode electrical power collector, and battery capacity and a charging / discharging rate fall. On the other hand, when the blending amount of the binder is 30 parts by mass or less, the internal resistance of the positive electrode material for a lithium ion secondary battery is reduced, and the battery capacity at a high-speed charge / discharge rate can be suppressed from decreasing.

"Conductive aid"
Although it does not specifically limit as a conductive support agent, For example, the group which consists of fibrous carbon, such as acetylene black (AB), ketjen black, furnace black, a vapor growth carbon fiber (VGCF; Vapor Growth Carbon Fiber), and a carbon nanotube At least one selected from is used.

"solvent"
The solvent used for the positive electrode material paste containing the positive electrode material for a lithium ion secondary battery of the present embodiment is appropriately selected according to the properties of the binder. By appropriately selecting the solvent, the positive electrode material paste can be easily applied to an object to be coated such as an electrode current collector.
Examples of the solvent include water; alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol, pentanol, hexanol, octanol, and diacetone alcohol; ethyl acetate, butyl acetate, and lactic acid. Esters such as ethyl, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate and γ-butyrolactone; diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol monobutyl ether ( Butyl cellosolve), diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, etc. Ethers; Ketones such as acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), acetylacetone and cyclohexanone; Amides such as dimethylformamide, N, N-dimethylacetoacetamide and N-methyl-2-pyrrolidone (NMP) And glycols such as ethylene glycol, diethylene glycol and propylene glycol. These solvents may be used alone or in a combination of two or more.

The content of the solvent in the positive electrode material paste is 50% by mass or more and 70% by mass or less when the total mass of the positive electrode material for the lithium ion secondary battery, the binder, and the solvent of the present embodiment is 100% by mass. It is preferable that it is 55 mass% or more and 65 mass% or less.
When the content of the solvent in the positive electrode material paste is within the above range, a positive electrode material paste having excellent positive electrode forming properties and excellent battery characteristics can be obtained.

  The method of mixing the positive electrode material for a lithium ion secondary battery, the binder, the conductive additive, and the solvent of the present embodiment is not particularly limited as long as these components can be mixed uniformly. Examples thereof include a mixing method using a kneading machine such as a ball mill, a sand mill, a planetary (planetary) mixer, a paint shaker, and a homogenizer.

The positive electrode material paste is applied to one main surface of the electrode current collector to form a coating film, and then the coating film is dried, and a coating film made of a mixture of the positive electrode material and the binder is formed on one main surface. A formed electrode current collector is obtained.
Thereafter, the coating is pressure-bonded and dried to obtain a positive electrode having a positive electrode mixture layer on one main surface of the electrode current collector.

[Lithium ion secondary battery]
The lithium ion secondary battery of the present embodiment includes a positive electrode, a negative electrode, and a nonaqueous electrolyte, and the positive electrode is the positive electrode for the lithium ion secondary battery of the present embodiment. Specifically, the lithium ion secondary battery of the present embodiment includes the positive electrode for the lithium ion secondary battery of the present embodiment as a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte.
In the lithium ion secondary battery of this embodiment, the negative electrode, the nonaqueous electrolyte, and the separator are not particularly limited.

"Negative electrode"
As the negative electrode, for example, a metal Li, natural graphite, carbon materials such as hard carbon, include those containing an anode material such as Li alloys and _{Li 4 Ti 5 O 12, Si} (Li 4.4 Si).

"Nonaqueous electrolyte"
As the non-aqueous electrolyte, for example, ethylene carbonate (ethylene carbonate; EC) and ethyl methyl carbonate (ethyl methyl carbonate; EMC) are mixed at a volume ratio of 1: 1, and the resulting mixed solvent is mixed. lithium hexafluorophosphate (LiPF _6), for example, those dissolved at a concentration of 1 mol / dm ^3.

"Separator"
For example, porous propylene can be used as the separator.
A solid electrolyte may be used instead of the nonaqueous electrolyte and the separator.

  Since the lithium ion secondary battery of the present embodiment includes the positive electrode for the lithium ion secondary battery of the present embodiment as the positive electrode, it has a high energy density and excellent input / output characteristics.

[Method for evaluating positive electrode material for lithium ion secondary battery]
The evaluation method of the positive electrode material for a lithium ion secondary battery according to the present embodiment is expressed by the general formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1, 0 ≦ z ≦ 0.2, where M is at least one selected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and rare earth elements) In the L ^* a ^* b ^* color system after heating the active material particles containing the center particle represented by the above and the carbonaceous film covering the surface of the center particle at 180 ° C. for 72 hours in the air atmosphere The difference (Δb ^* ) between the chromaticity b ^* and the chromaticity b ^* of the center particle before heating is divided by the BET specific surface area of the active material particles before heating (Δb ^* / BET specific surface area ratio). The covering property of the center particle by the quality coating is evaluated.

When the iron iron phosphate particles coated with the carbonaceous film are heated at 180 ° C. for 72 hours in the air, the lithium iron phosphate particles not coated with the carbonaceous film (general formula Li _x Fe _y M _z PO ₄ Fe ions are oxidized in the surface region of the central particle represented by In response to the oxidation of Fe ions, the yellowness of the surface region of lithium iron phosphate not covered with a carbonaceous coating increases. The variation range of the yellowness changes according to the coverage of the carbonaceous film. When the coverage of the carbonaceous coating is high, the variation range of the yellowness becomes small. On the other hand, when the coverage of the carbonaceous film is low, the variation range of yellowness increases. The variation range of yellowness can be evaluated by the variation range of chromaticity b ^* in the L ^* a ^* b ^* color system. Further, since the oxidation reaction of Fe ions in the lithium iron phosphate particles occurs in the surface region of the lithium iron phosphate particles, the size of the BET specific surface area of the lithium iron phosphate particles and the variation range of the yellowness are proportional. Therefore, by dividing the fluctuation range of the chromaticity b ^* by the BET specific surface area, a value suggesting the coverage of the carbonaceous film per specific surface area of the lithium iron phosphate particles can be obtained.

The measuring method of the BET specific surface area of the active material particles was measured by a one-point method and a relative pressure of 0.29 (P / P ₀ ) using a measuring device (trade name: HM model-1208, manufactured by Mountec).

L ^* a ^* b ^* chromaticity b ^* in the color system of the active material particles is a reflected light twice field using a spectroscopic colorimeter (model number: SE-2000, manufactured by Nippon Denshoku Industries Co., Ltd.) and a D65 light source. Obtained by measurement. When measuring the chromaticity b ^* of the positive electrode material for a lithium ion secondary battery, the positive electrode material to be measured was placed on a petri dish without unevenness, and the chromaticity b ^{* of the} positive electrode material was measured.

In the evaluation method of the positive electrode material for a lithium ion secondary battery of the present embodiment, the smaller the value of Δb ^* / BET specific surface area ratio, the higher the coverage of the lithium iron phosphate particles by the carbonaceous film and the better the electronic conductivity. It can be said that. On the other hand, it can be said that the larger the value of Δb ^* / BET specific surface area ratio, the lower the covering property of the lithium iron phosphate particles by the carbonaceous film, and the better the lithium ion conductivity.

  In the evaluation method of the positive electrode material for a lithium ion secondary battery of the present embodiment, the heating condition of the lithium iron phosphate particles coated with the carbonaceous film can oxidize Fe ions without the carbonaceous film being oxidized and decomposed. It is necessary to make it a condition. When the heating temperature is 180 ° C., the carbonaceous film is hardly oxidized and decomposed, and Fe ions existing in the surface region of the lithium iron phosphate particles can be sufficiently oxidized. Therefore, it is possible to accurately evaluate the coverage of the lithium iron phosphate particles by the carbonaceous film.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further more concretely, this invention is not limited to a following example.

[Example 1]
"Synthesis of cathode material for lithium ion secondary battery"
Water was added to 2 mol of lithium phosphate (Li ₃ PO ₄ ) and 2 mol of iron (II) sulfate (FeSO ₄ ), and the mixture was mixed so that the total amount became 4 L to prepare a uniform slurry mixture. .
Subsequently, this mixture was accommodated in a pressure-resistant airtight container having a capacity of 8 L, and hydrothermal synthesis was performed at 200 ° C. for 12 hours to produce a precipitate.
Next, the precipitate was washed with water to obtain a cake-like positive electrode active material precursor.
Next, 150 g of the positive electrode active material precursor (in terms of solid content), 20 g of polyethylene glycol as an organic compound, 0.58 g of NH ₃ water (28%) as a carbonaceous coating coverage adjuster, and pure water Then, these mixtures were subjected to a dispersion treatment for 1 hour in a bead mill using zirconia balls having a diameter of 5 mm as medium particles to prepare a uniform slurry. At this time, the amount of pure water was adjusted so that the ratio when the slurry mass was the denominator and the mass of the positive electrode active material precursor was the numerator was 0.4.
Next, this slurry was sprayed into an air atmosphere at 200 ° C. and dried to obtain aggregated particles of a precursor of the positive electrode material coated with an organic substance having an average particle size of 8.3 μm.
Next, the obtained dry powder (aggregated particles of the precursor of the positive electrode material) was baked at 780 ° C. for 1.5 hours in a nitrogen atmosphere to obtain a positive electrode material 1 having an average particle diameter of 8.3 μm. .

"Production of lithium ion secondary battery"
The mass of the positive electrode material 1, polyvinylidene fluoride (PVdF) as a binder, and acetylene black (AB) as a conductive additive in N-methyl-2-pyrrolidinone (NMP) as a solvent In addition, the positive electrode material 1: AB: PVdF = 90: 5: 5 was added, these were mixed, and using a kneader (trade name: Awatori Nertaro, manufactured by Shinky Corp.), revolution of 2000 rpm The mixture was kneaded under conditions for 10 minutes to prepare a positive electrode material paste (for positive electrode).
This positive electrode material paste (for positive electrode) is applied to the surface of an aluminum foil (electrode current collector) having a thickness of 30 μm to form a coating film, the coating film is dried, and a positive electrode mixture layer is formed on the surface of the aluminum foil. Formed.
Thereafter, the positive electrode mixture layer was pressurized at a pressure of 58.84 MPa to produce the positive electrode 1 of Example 1.

Lithium metal was disposed as a negative electrode with respect to the positive electrode 1, and a separator made of porous polypropylene was disposed between the positive electrode 1 and the negative electrode to obtain a battery member 1.
On the other hand, ethylene carbonate and diethyl carbonate were mixed at 1: 1 (mass ratio), and a 1 mol / L LiPF ₆ solution was further added to prepare an electrolyte solution 1 having lithium ion conductivity.
Next, the battery member 1 was immersed in the electrolyte solution 1 to produce the lithium ion secondary battery 1 of Example 1.

[Example 2]
A positive electrode material 2 of Example 2 was obtained in the same manner as in Example 1 except that the amount of NH ₃ water added was 0.72 g.
A lithium ion secondary battery 2 of Example 2 was produced in the same manner as Example 1 except that the positive electrode material 2 was used.

[Example 3]
A positive electrode material 3 of Example 3 was obtained in the same manner as in Example 1 except that the amount of NH ₃ water added was 0.87 g.
A lithium ion secondary battery 3 of Example 3 was produced in the same manner as Example 1 except that the positive electrode material 3 was used.

[Example 4]
"Synthesis of cathode material for lithium ion secondary battery"
Water was added to 2 mol of lithium phosphate (Li ₃ PO ₄ ) and 2 mol of iron (II) sulfate (FeSO ₄ ), and the mixture was mixed so that the total amount became 4 L to prepare a uniform slurry mixture. .
Subsequently, this mixture was accommodated in a pressure-resistant airtight container having a capacity of 8 L, and hydrothermal synthesis was performed at 170 ° C. for 8 hours to produce a precipitate.
Next, the precipitate was washed with water to obtain a cake-like positive electrode active material precursor.
Next, 150 g of the positive electrode active material precursor (in terms of solid content), 20 g of polyethylene glycol as an organic compound, 0.72 g of NH ₃ water (28%) as a carbonaceous coating coverage adjuster, and pure water Then, these mixtures were subjected to a dispersion treatment for 1 hour in a bead mill using zirconia balls having a diameter of 5 mm as medium particles to prepare a uniform slurry. At this time, the amount of pure water was adjusted so that the ratio when the slurry mass was the denominator and the mass of the precursor of the positive electrode active material was the numerator was 0.35.
Next, the slurry was sprayed into an air atmosphere at 200 ° C. and dried to obtain aggregated particles of a precursor of the positive electrode material coated with an organic substance having an average particle diameter of 9.7 μm.
Next, the obtained dry powder (aggregated particles of the precursor of the positive electrode material) was baked at 760 ° C. for 1.0 hour in a nitrogen atmosphere to obtain a positive electrode material 4 having an average particle diameter of 9.7 μm. .

"Production of lithium ion secondary battery"
The mass of the positive electrode material 4, polyvinylidene fluoride (PVdF) as a binder, and acetylene black (AB) as a conductive auxiliary agent in N-methyl-2-pyrrolidinone (NMP) as a solvent in the paste. In addition, the positive electrode material 4: AB: PVdF = 90: 5: 5 was added, and these were mixed, and using a kneader (trade name: Awatori Nerita, manufactured by Shinky Corp.), revolution of 2000 rpm The mixture was kneaded under conditions for 10 minutes to prepare a positive electrode material paste (for positive electrode).
This positive electrode material paste (for positive electrode) is applied to the surface of an aluminum foil (electrode current collector) having a thickness of 30 μm to form a coating film, the coating film is dried, and a positive electrode mixture layer is formed on the surface of the aluminum foil. Formed.
Thereafter, the positive electrode mixture layer was pressurized at a pressure of 58.84 MPa, and the positive electrode 4 of Example 4 was produced.

Lithium metal was disposed as a negative electrode with respect to the positive electrode 4, and a separator made of porous polypropylene was disposed between the positive electrode 4 and the negative electrode to obtain a battery member 4.
On the other hand, ethylene carbonate and diethyl carbonate were mixed at 1: 1 (mass ratio), and a 1 mol / L LiPF ₆ solution was further added to produce an electrolyte solution 4 having lithium ion conductivity.
Next, the battery member 4 was immersed in the electrolyte solution 4 to produce the lithium ion secondary battery 4 of Example 4.

[Example 5]
A positive electrode material 5 of Example 5 was obtained in the same manner as in Example 4 except that the amount of NH ₃ water added was 0.87 g.
A lithium ion secondary battery 5 of Example 5 was produced in the same manner as Example 4 except that the positive electrode material 5 was used.

[Example 6]
A positive electrode material 6 of Example 6 was obtained in the same manner as in Example 4 except that the amount of NH ₃ water added was 1.01 g.
A lithium ion secondary battery 6 of Example 6 was produced in the same manner as Example 4 except that the positive electrode material 6 was used.

[Example 7]
"Synthesis of cathode material for lithium ion secondary battery"
Water was added to 2 mol of lithium phosphate (Li ₃ PO ₄ ) and 2 mol of iron (II) sulfate (FeSO ₄ ), and the mixture was mixed so that the total amount became 4 L to prepare a uniform slurry mixture. .
Subsequently, this mixture was accommodated in a pressure-resistant airtight container having a capacity of 8 L, and hydrothermal synthesis was performed at 140 ° C. for 2 hours to produce a precipitate.
Next, the precipitate was washed with water to obtain a cake-like positive electrode active material precursor.
Next, 150 g (converted to solid content) of this positive electrode active material precursor, 20 g of polyethylene glycol as an organic compound, 0.87 g of NH ₃ water (28%) as a carbonaceous coating coverage adjuster, and pure water Then, these mixtures were subjected to a dispersion treatment for 4 hours in a bead mill using zirconia balls having a diameter of 5 mm as medium particles to prepare a uniform slurry. At this time, the amount of pure water was adjusted so that the ratio when the slurry mass was the denominator and the mass of the positive electrode active material precursor was the numerator was 0.30.
Next, the slurry was sprayed into an air atmosphere at 200 ° C. and dried to obtain aggregated particles of a precursor of the positive electrode material coated with an organic substance having an average particle diameter of 11.2 μm.
Next, the obtained dry powder (aggregated particles of the precursor of the positive electrode material) was baked at 700 ° C. for 1.0 hour in a nitrogen atmosphere to obtain a positive electrode material 7 having an average particle diameter of 11.2 μm. .

"Production of lithium ion secondary battery"
A mass of the positive electrode material 7, polyvinylidene fluoride (PVdF) as a binder, and acetylene black (AB) as a conductive auxiliary agent in N-methyl-2-pyrrolidinone (NMP) as a solvent. In addition, the positive electrode material 7: AB: PVdF = 90: 5: 5 was added, and these were mixed, and using a kneader (trade name: Awatori Nerita, manufactured by Shinky Corp.), revolution of 2000 rpm The mixture was kneaded under conditions for 10 minutes to prepare a positive electrode material paste (for positive electrode).
This positive electrode material paste (for positive electrode) is applied to the surface of an aluminum foil (electrode current collector) having a thickness of 30 μm to form a coating film, the coating film is dried, and a positive electrode mixture layer is formed on the surface of the aluminum foil. Formed.
Thereafter, the positive electrode mixture layer was pressurized at a pressure of 58.84 MPa to produce the positive electrode 7 of Example 7.

Lithium metal was disposed as a negative electrode with respect to the positive electrode 7, and a separator made of porous polypropylene was disposed between the positive electrode 7 and the negative electrode to obtain a battery member 7.
On the other hand, ethylene carbonate and diethyl carbonate were mixed at 1: 1 (mass ratio), and a 1 mol / L LiPF ₆ solution was further added to produce an electrolyte solution 7 having lithium ion conductivity.
Next, the battery member 7 was immersed in the electrolyte solution 7 to produce the lithium ion secondary battery 1 of Example 7.

[Example 8]
A positive electrode material 8 of Example 8 was obtained in the same manner as in Example 7 except that the amount of NH ₃ water added was 1.01 g.
A lithium ion secondary battery 8 of Example 8 was produced in the same manner as Example 7 except that the positive electrode material 8 was used.

[Example 9]
A positive electrode material 9 of Example 9 was obtained in the same manner as in Example 7 except that the amount of NH ₃ water added was 1.16 g.
A lithium ion secondary battery 9 of Example 9 was produced in the same manner as Example 7 except that the positive electrode material 9 was used.

"Comparative Example 1"
A positive electrode material 11 of Comparative Example 1 was obtained in the same manner as in Example 1 except that the amount of NH ₃ water added was 0.29 g.
A lithium ion secondary battery 11 of Comparative Example 1 was produced in the same manner as in Example 1 except that the positive electrode material 11 was used.

"Comparative Example 2"
A positive electrode material 12 of Comparative Example 2 was obtained in the same manner as in Example 1 except that the amount of NH ₃ water added was 1.01 g.
A lithium ion secondary battery 12 of Comparative Example 2 was produced in the same manner as in Example 1 except that the positive electrode material 12 was used.

“Comparative Example 3”
A positive electrode material 13 of Comparative Example 3 was obtained in the same manner as in Example 1 except that the input amount of NH ₃ water was 1.16 g.
A lithium ion secondary battery 13 of Comparative Example 3 was produced in the same manner as in Example 1 except that the positive electrode material 13 was used.

“Comparative Example 4”
A positive electrode material 14 of Comparative Example 4 was obtained in the same manner as in Example 4 except that the input amount of NH ₃ water was 0.29 g.
A lithium ion secondary battery 14 of Comparative Example 4 was produced in the same manner as Example 4 except that the positive electrode material 14 was used.

“Comparative Example 5”
A positive electrode material 15 of Comparative Example 5 was obtained in the same manner as in Example 4 except that the amount of NH ₃ water added was 0.58 g.
A lithium ion secondary battery 15 of Comparative Example 5 was produced in the same manner as Example 4 except that the positive electrode material 15 was used.

“Comparative Example 6”
A positive electrode material 16 of Comparative Example 6 was obtained in the same manner as in Example 4 except that the input amount of NH ₃ water was 1.16 g.
A lithium ion secondary battery 16 of Comparative Example 6 was produced in the same manner as Example 4 except that the positive electrode material 16 was used.

“Comparative Example 7”
A positive electrode material 17 of Comparative Example 7 was obtained in the same manner as in Example 7 except that the amount of NH ₃ water added was 0.29 g.
A lithium ion secondary battery 17 of Comparative Example 7 was produced in the same manner as Example 7 except that the positive electrode material 17 was used.

“Comparative Example 8”
A positive electrode material 18 of Comparative Example 8 was obtained in the same manner as in Example 7 except that the amount of NH ₃ water added was 0.58 g.
A lithium ion secondary battery 18 of Comparative Example 8 was produced in the same manner as Example 7 except that the positive electrode material 18 was used.

"Comparative Example 9"
A positive electrode material 19 of Comparative Example 9 was obtained in the same manner as in Example 7 except that the amount of NH ₃ water added was 0.72 g.
A lithium ion secondary battery 19 of Comparative Example 9 was produced in the same manner as in Example 7 except that the positive electrode material 19 was used.

[Evaluation of cathode material for lithium ion secondary battery and lithium ion secondary battery]
The positive electrode materials for lithium ion secondary batteries and the lithium ion secondary batteries of Examples 1 to 9 and Comparative Examples 1 to 9 were evaluated as follows.

(1) Specific surface area The BET specific surface area of the positive electrode material for a lithium ion secondary battery is determined by a one-point method, a relative pressure of 0.29 (P / P) using a measuring device (trade name: HM model-1208, manufactured by Mountec). ₀ ).

(2) Carbon content The carbon content of the positive electrode material for a lithium ion secondary battery was measured using a carbon sulfur analyzer (trade name: EMIA-220V, manufactured by Horiba, Ltd.).

(3) Chromaticity b ^*
L ^* a ^* b ^* chromaticity b ^* in the color system of a positive electrode material for a lithium ion secondary battery was measured using a spectroscopic colorimeter (model number: SE-2000, manufactured by Nippon Denshoku Industries Co., Ltd.) and a D65 light source. Obtained by two-field measurement of reflected light. When measuring the chromaticity b ^* of the positive electrode material for a lithium ion secondary battery, the positive electrode material to be measured was placed on a petri dish without unevenness, and the chromaticity b ^{* of the} positive electrode material was measured.

(4) 10C discharge capacity The 10C discharge capacity of the lithium ion secondary battery is the battery voltage at a current value of 10C after being charged at constant current until the battery voltage reaches 4.0V at a current value of 1C under an environment of 25 ° C. Is the discharge capacity when discharged until 2.0V.
In Examples 1 to 3 and Comparative Examples 1 to 3 having a large primary particle size, when the 10C discharge capacity is 100 mAh / g or more, the input / output characteristics are ○, and when the primary particle diameter is less than 100 mAh / g, X was evaluated.
In Examples 4 to 6 and Comparative Examples 4 to 6 in which the primary particle size is medium, when the 10C discharge capacity is 105 mAh / g or more, the input / output characteristics are ○, and when the 10C discharge capacity is less than 105 mAh / g, the input / output characteristics are ×. evaluated.
In Examples 7 to 9 and Comparative Examples 7 to 9 having a small primary particle size, when the 10C discharge capacity is 103 mAh / g or more, the input / output characteristics are ○, and when the 10 C discharge capacity is less than 103 mAh / g, the input / output characteristics are ×. evaluated.
The reason why the judgment of quality is changed depending on the primary particle diameter is that the contact area with the electrolyte and the lithium diffusion distance in the primary particle bulk differ depending on the primary particle diameter, and therefore the absolute value of the 10C discharge capacity differs. It is.

"Evaluation results"
Table 1 shows the evaluation results of the positive electrode materials for lithium ion secondary batteries and the lithium ion secondary batteries of Examples 1 to 9 and Comparative Examples 1 to 9.
In addition, the carbon amount shown in Table 1 shows the quantity (mass part) of the carbon which forms a carbonaceous film with respect to 100 mass parts of positive electrode active materials. In addition, FIG. 1 (primary particle size: large size), FIG. 2 (plots of Δb ^* / BET specific surface area ratio, which is a measure of the coverage of the carbonaceous film, and 10 C discharge capacity, which is a measure of input / output characteristics, are plotted. Primary particle diameter: medium size) and FIG. 3 (primary particle diameter: small size).

From the results of Table 1 and FIG. 1, when Examples 1 to 3 and Comparative Examples 1 to 3 are compared, Example 1 in which the Δb ^* / BET specific surface area ratio is in the range of 0.25 to 0.35. -3 showed the input-output characteristic which was 10C discharge capacity more than 100 mAh / g compared with Comparative Examples 1-3. In particular, the 10C discharge capacity which was the most excellent in Example 2 in which Δb ^* / BET specific surface area ratio was 0.31 was shown.
Further, from the results of Table 1 and FIG. 2, when Examples 4 to 6 and Comparative Examples 4 to 6 are compared, the Δb ^* / BET specific surface area ratio is in the range of 0.25 or more and 0.35 or less. Examples 4-6 showed the input-output characteristic which 10C discharge capacity was 105 mAh / g or more excellent compared with Comparative Examples 4-6. In particular, the most excellent 10 C discharge capacity was shown in Example 5 in which the Δb ^* / BET specific surface area ratio was 0.30.
From the results of Table 1 and FIG. 3, when Examples 7 to 9 and Comparative Examples 7 to 9 are compared, Example 7 in which the Δb ^* / BET specific surface area ratio is in the range of 0.25 or more and 0.35 or less. Nos. 9 to 9 showed excellent input / output characteristics with a 10 C discharge capacity of 103 mAh / g or more as compared with Comparative Examples 7 to 9. In particular, the 10C discharge capacity that was most excellent in Example 8 in which the Δb ^* / BET specific surface area ratio was 0.30 was shown.
From the results of Table 1, FIG. 1, FIG. 2 and FIG. 3, regardless of the size of the primary particle diameter, excellent input / output characteristics are obtained when the Δb ^* / BET specific surface area ratio is in the range of 0.25 to 0.35. In particular, it was suggested that excellent input / output characteristics are exhibited when the Δb ^* / BET specific surface area ratio is in the range of 0.27 to 0.33. In Examples 1 to 9, since an appropriate carbonaceous film covering property that does not inhibit the movement of lithium ions was achieved while ensuring the necessary electron conduction path of the positive electrode active material, such excellent input / output was achieved. It is inferred that the characteristics were obtained.

  The lithium ion secondary battery using the positive electrode material for the lithium ion secondary battery of the present invention is excellent in energy density and input / output characteristics, and thus greatly improves the reliability of lithium ion secondary batteries including mobile applications. Can contribute.

As a result of intensive studies to solve the above problems, the present inventors have found that the general formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1). , 0 ≦ z ≦ 0.2, where M is at least selected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge and rare earth elements The coverage of the carbonaceous film in the active material particles including the center particle represented by 1 type) and the carbonaceous film formed by carbonizing the organic compound that coats the surface of the center particle is expressed as Δb ^* / BET specific surface area ratio. The positive electrode for a lithium ion secondary battery having an optimal carbonaceous film covering property for improving input / output characteristics by making the Δb ^* / BET specific surface area ratio not less than 0.25 and not more than 0.35 by evaluating Find that material can be provided, book The invention has been completed.

The positive electrode material for a lithium ion secondary battery of the present invention has a general formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1, 0 ≦ z ≦ 0. 2, wherein M is a center represented by Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and a rare earth element. An active material particle comprising particles and a carbonaceous film formed by carbonizing an organic compound that coats the surface of the center particle, wherein a Δb ^* / BET specific surface area ratio is 0.25 or more and 0.35 or less (Δb ^*: the active material particles in an air atmosphere, the chromaticity ^{b *} and the active material before heating the particles chromaticity ^{b *} in 72 hours while heating and the ^L * ^a * ^{b *} color system after at 180 ° C. Difference).

The evaluation method of the positive electrode material for a lithium ion secondary battery of the present invention is expressed by the general formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1, 0 ≦ z ≦ 0.2, where M is at least one selected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and rare earth elements) L ^* a ^* after heating the active material particle containing the center particle | grains represented, and the carbonaceous film which the organic compound which coat | covers the surface of the center particle | grains carbonizes for 72 hours at 180 degreeC in air | atmosphere ^. The difference between the chromaticity b ^* in the b ^* color system and the chromaticity b ^* of the central particle before heating is divided by the BET specific surface area of the active material particle before heating, and the carbonaceous coating film It is characterized by evaluating the covering property of the center particle.

As a result of intensive studies to solve the above problems, the present inventors have found that the general formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1). , 0 ≦ z ≦ 0.2, where M is at least selected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge and rare earth elements The coverage of the carbonaceous film in the active material particles including the center particle represented by 1 type) and the carbonaceous film formed by carbonizing the organic compound that coats the surface of the center particle is expressed as Δb ^* / BET specific surface area ratio. By evaluating, the Δb ^* / BET specific surface area ratio is 0.25 or more and 0.35 or less , and the BET specific surface area is 5 m ² / g or more and 30 m ² / g or less. Lithium Ion with Excellent Carbonaceous Coating It found that it is possible to provide a positive electrode material for the next cell, and completed the present invention.

The positive electrode material for a lithium ion secondary battery of the present invention has a general formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1, 0 ≦ z ≦ 0. 2, wherein M is a center represented by Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and a rare earth element. An active material particle comprising particles and a carbonaceous film formed by carbonizing an organic compound that coats the surface of the center particle, wherein a Δb ^* / BET specific surface area ratio is 0.25 or more and 0.35 or less (Δb ^*: the active material particles in an air atmosphere, the chromaticity ^{b *} and the active material before heating the particles chromaticity ^{b *} in 72 hours while heating and the ^L * ^a * ^{b *} color system after at 180 ° C. Difference) , that the BET specific surface area is 5 m ² / g or more and 30 m ² / g or less. Features.

Claims (7)

General formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1, 0 ≦ z ≦ 0.2, where M is Mg, Ca, Co, And at least one selected from the group consisting of Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and rare earth elements), and carbonaceous material covering the surface of the central particle Active material particles comprising a coating,
Δb ^* / BET specific surface area ratio is 0.25 or more and 0.35 or less (Δb ^* : in the L ^* a ^* b ^* color system after heating the active material particles at 180 ° C. for 72 hours in the air atmosphere. chromaticity b ^* and the positive electrode material for a lithium ion secondary battery which is a active difference between previous chromaticity b ^* heating of material particles). The positive electrode material for a lithium ion secondary battery according to claim 1, wherein the central particle is LiFePO ₄ . The BET specific surface area is 5 m ² / g or more and 30 m ² / g or less, and the amount of carbon forming the carbonaceous film is 0.6 parts by mass or more and 3 parts by mass or less with respect to 100 parts by mass of the center particle. The positive electrode material for a lithium ion secondary battery according to claim 1 or 2. A positive electrode for a lithium ion secondary battery comprising: an electrode current collector; and a positive electrode mixture layer formed on the electrode current collector,
The said positive mix layer contains the positive electrode material for lithium ion secondary batteries of any one of Claims 1-3, The positive electrode for lithium ion secondary batteries characterized by the above-mentioned.   A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to claim 4. General formula Li _x Fe _y M _z PO ₄ (0.95 ≦ x ≦ 1.1, 0.8 ≦ y ≦ 1.1, 0 ≦ z ≦ 0.2, where M is Mg, Ca, Co, And at least one selected from the group consisting of Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and rare earth elements), and carbonaceous material covering the surface of the central particle the active material particles comprising a coating, an air atmosphere before heating the chromaticity b ^* and the central particle at 72 hours heated after the L ^* a ^* b ^* color system at 180 ° C. and chromaticity b ^* Evaluation of the positive electrode material for a lithium ion secondary battery, wherein the coverage of the center particle by the carbonaceous film is evaluated by the value obtained by dividing the difference by the BET specific surface area before heating of the active material particles Method. The method for evaluating a positive electrode material for a lithium ion secondary battery according to claim 6, wherein the central particle is LiFePO ₄ .

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