JP2019067596A - Method for manufacturing electrode material for lithium ion secondary battery - Google Patents

Abstract

To provide: an electrode material for a lithium ion secondary battery, which can reduce DC resistance when the lithium ion secondary battery is charged/discharged; an electrode for the lithium ion secondary battery, which is arranged by use of the electrode material; and the lithium ion secondary battery using the electrode.SOLUTION: An electrode material for a lithium ion secondary battery according to the present invention comprises spherical secondary particles produced by granulation of primary particles of an electrode active substance consisting of a transition metal lithium phosphate compound of an olivine structure. The spherical secondary particles each have: a surface layer portion ranging from the spherical secondary particle surface to a depth of 10% of a particle diameter of the spherical secondary particle; and a body portion inside the surface layer portion. A ratio (Carbon amount of the surface layer portion/Carbon amount of the body portion) of an amount of carbon in the surface layer portion to an amount of carbon in the body portion is 0.90 or more and 1.30 or less. An electrode for the lithium ion secondary battery according to the present invention comprises the electrode material for the lithium ion secondary battery. The lithium ion secondary battery according to the present invention comprises the electrode for the lithium ion secondary battery.SELECTED DRAWING: None

Description

  The present invention relates to an electrode material for a lithium ion secondary battery, an electrode using the electrode material, and a lithium ion secondary battery using the electrode.

Lithium ion secondary batteries have higher energy density and power density than lead batteries and nickel hydrogen batteries, and are used in various applications such as small-sized electronic devices such as smartphones, home backup power supplies, and electric tools. In addition, commercialization of large-capacity lithium ion secondary batteries for renewable energy storage such as solar power generation and wind power generation is in progress.
The lithium ion secondary battery includes a positive electrode, a negative electrode, an electrolytic solution, and a separator. The electrode material constituting the positive electrode has a property capable of reversibly de-inserting lithium ions such as lithium cobaltate (LiCoO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium iron phosphate (LiFePO ₄ ), etc. Lithium-containing metal oxides are used, and improvements are being considered from various viewpoints such as high capacity, long life, improved safety, and low cost of batteries.

Lithium iron phosphate (LiFePO ₄ ), which is the electrode material, is a material that is easy to reduce in cost because it is rich in resources and uses inexpensive iron. Lithium iron phosphate has excellent safety that is not outstanding in oxide-based cathode materials represented by lithium cobaltate, such as having excellent safety because there is no oxygen release at high temperature due to strong covalent bond of phosphorus and oxygen Have.
On the other hand, lithium iron phosphate is inferior in the input / output characteristics to the oxide-based positive electrode material because the diffusivity of Li ions and the electron conductivity are low. Since this characteristic difference becomes more pronounced when the battery operating temperature is low, lithium iron phosphate is considered to be unsuitable for in-vehicle applications such as hybrid vehicles where high input / output characteristics are required in the low temperature region. It has

LiMPO ₄ (M is a metal element) having an olivine structure typified by lithium iron phosphate has a low Li ion diffusion property and low electron conductivity, and thus the primary particles of LiMPO ₄ are made finer and individual primary particles thereof The charge and discharge characteristics can be improved by coating the surface with a conductive carbonaceous film.
On the other hand, since the finely divided LiMPO ₄ has a large specific surface area, it requires thickening of the electrode mixture slurry and a large amount of binder, so primary particles coated with a carbonaceous film are granulated to form secondary particles. It is general to improve the properties of the electrode mixture slurry by setting it as above.

  For example, a slurry prepared by dispersing the positive electrode active material in water is sprayed in a high temperature atmosphere, for example, at a temperature of 100 ° C. to 250 ° C., in an atmosphere at a temperature equal to or higher than the boiling point of the solvent. Electrode materials produced by dry-mixing the obtained granulated dried product and the powder of the carbonaceous film precursor and baking the mixture obtained by dry-mixing are known as prior art. (See, for example, Patent Document 1). As a result, an electrode material is produced, which comprises aggregated particles formed by aggregating a plurality of primary particles covered with the carbonaceous film. By using this electrode material, a lithium ion secondary battery with improved charge and discharge characteristics can be obtained.

Patent No. 6176381

  In order to improve the charge and discharge characteristics of the lithium ion secondary battery, it is preferable that the direct current resistance of the lithium ion secondary battery be low. Although the direct current resistance of the lithium ion secondary battery using the electrode material of patent document 1 is low, the electrode material which can further reduce the direct current resistance of a lithium ion secondary battery is desired. Therefore, the present invention provides an electrode material for a lithium ion secondary battery capable of reducing direct current resistance during charge and discharge of the lithium ion secondary battery, an electrode for a lithium ion secondary battery using the electrode material, and the electrode An object of the present invention is to provide a lithium ion secondary battery used.

MEANS TO SOLVE THE PROBLEM The present inventors discovered that the said subject could be solved by the following invention, as a result of earnestly examining in order to solve said subject.
[1] It has spherical secondary particles formed by granulating primary particles of electrode active material consisting of transition metal lithium phosphate compound of olivine structure, and spherical secondary particles are spherical secondary particles from the surface of spherical secondary particles The ratio of the amount of carbon in the surface layer to the amount of carbon in the main body (the amount of carbon in the surface layer / the amount of carbon in the main body) The electrode material for lithium ion secondary batteries which is 0.90 or more and 1.30 or less.
[2] The electrode material for a lithium ion secondary battery according to the above [1], wherein the primary particles further have a carbonaceous film coating the electrode active material, and the average particle diameter of the primary particles is 10 nm or more and 500 nm or less.
[3] When the cumulative volume percentage of the particle size distribution of the spherical secondary particles is 10%, the particle diameter is 0.1 μm or more and 3.0 μm or less, and the cumulative volume percentage of the particle size distribution of the spherical secondary particles is 90% The electrode material for lithium ion secondary batteries as described in said [1] or [2] whose particle diameter of these is 3.0 micrometers-18.0 micrometers.
[4] The electrode material for a lithium ion secondary battery according to any one of the above [1] to [3], wherein the carbon content of the spherical secondary particles is 0.8% by mass to 1.6% by mass. .
[5] The electrode material for a lithium ion secondary battery according to any one of the above [1] to [4], wherein an oil absorption amount using N-methyl-2-pyrrolidone (NMP) is 50 ml / 100 g or less.
[6] An electrode for a lithium ion secondary battery using the electrode material for a lithium ion secondary battery according to any one of the above [1] to [5].
[7] A lithium ion secondary battery using the electrode for a lithium ion secondary battery according to the above [6].

  According to the present invention, an electrode material for a lithium ion secondary battery capable of reducing direct current resistance during charge and discharge of the lithium ion secondary battery, an electrode for a lithium ion secondary battery using the electrode material, and the electrode The lithium ion secondary battery used can be provided.

An embodiment of the electrode material for a lithium ion secondary battery of the present invention will be described.
In addition, this embodiment is concretely described in order to understand the meaning of invention better, and unless there is particular specification, it does not limit this invention.

<Electrode material for lithium ion secondary battery>
The electrode material for a lithium ion secondary battery according to this embodiment has spherical secondary particles formed by granulating primary particles of an electrode active material composed of a transition metal lithium phosphate compound having an olivine structure, and the spherical secondary particles are The surface portion from the surface of the spherical secondary particles to a depth of 10% of the particle diameter of the spherical secondary particles and the main portion inside from the surface portion, and the ratio of the amount of carbon in the surface portion to the amount of carbon in the main portion It is characterized in that (the surface layer carbon amount / the main body carbon amount) is 0.90 or more and 1.30 or less.

The electrode active material used in the electrode material for a lithium ion secondary battery of this embodiment is made of a transition metal lithium phosphate compound having an olivine structure. The transition metal lithium phosphate compound having an olivine structure is preferably an electrode active material represented by the following general formula (1) from the viewpoint of high discharge capacity and high energy density.
Li _x A _y D _z PO ₄ (1)
(Wherein A is at least one selected from the group consisting of Co, Mn, Ni, Fe, Cu and Cr, D is Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si , Ge, Sc and Y, at least one selected from the group consisting of 0.9 <x <1.1, 0 <y ≦ 1, 0 ≦ z <1, 0.9 <y + z <1.1 .)

Here, for A, Co, Mn, Ni and Fe are preferable, and Fe is more preferable.
As for D, Mg, Ca, Sr, Ba, Ti, Zn and Al are preferable. When the electrode active material contains these elements, it is possible to obtain a positive electrode mixture layer capable of realizing high discharge potential and high safety. Moreover, it is preferable as a material to be selected because it is abundant in resources.

  The electrode material for a lithium ion secondary battery of the present embodiment has spherical secondary particles formed by granulating primary particles of the electrode active material. The content of the spherical secondary particles in the electrode material for a lithium ion secondary battery according to this embodiment is preferably 80% by mass or more, more preferably 90% by mass or more, still more preferably 95% by mass or more, and still more preferably 97 It is mass% or more. The upper limit of the range of the content of the spherical secondary particles in the electrode material for a lithium ion secondary battery of the present embodiment is 100% by mass.

  The average particle diameter of the primary particles of the electrode active material is preferably 10 nm to 500 nm, more preferably 20 nm to 400 nm, still more preferably 20 nm to 300 nm, and still more preferably 30 nm to 200 nm. is there. When the average particle diameter of the primary particles of the electrode active material is 10 nm or more, the increase in the mass of carbon required by the increase in the specific surface area of the primary particles of the electrode active material is suppressed, and charge and discharge per unit mass of electrode material The reduction of the capacity can be suppressed. In addition, the surface of the primary particles of the electrode active material can be easily covered uniformly with the carbonaceous film. As a result, in the lithium ion secondary battery using the electrode material for a lithium ion secondary battery of the present embodiment, the discharge capacity at high speed charge and discharge becomes large, and sufficient charge and discharge performance can be realized. In addition, when the average particle diameter of the primary particles of the electrode active material is 500 nm or less, the lithium ion diffusion resistance and the migration resistance of electrons in the primary particles of the electrode active material can be suppressed from increasing. As a result, in the lithium ion secondary battery using the electrode material for a lithium ion secondary battery of the present embodiment, the discharge capacity in high-speed charge and discharge can be increased.

  Here, the average particle size is the number average particle size. The average particle size of the primary particles of the electrode active material of the present embodiment randomly selects 100 primary particles, and the major and minor axes of the individual primary particles are measured with a scanning electron microscope (SEM; Scanning Electron Microscope) Can be measured as the average value.

  The average secondary particle diameter of spherical secondary particles formed by granulating primary particles of the electrode active material is preferably 0.5 μm to 15 μm, and more preferably 1.0 μm to 10 μm. A positive electrode material for a lithium ion secondary battery by mixing a positive electrode material, a conductive support agent, a binder resin (binder) and a solvent as the average secondary particle diameter of the spherical secondary particles is 0.5 μm or more The compounding quantity of the conductive support agent at the time of preparing a paste and a binder can be restrained, and the battery capacity of the lithium ion secondary battery per unit mass of the positive electrode mixture layer for lithium ion secondary batteries is increased. Can. On the other hand, when the average secondary particle diameter of the spherical secondary particles is 15 μm or less, the dispersibility and uniformity of the conductive aid and the binder in the positive electrode mixture layer can be enhanced. As a result, in the lithium ion secondary battery using the electrode material for a lithium ion secondary battery of the present embodiment, the discharge capacity in high-speed charge and discharge can be increased.

In the present embodiment, all granulated particles and aggregated particles other than the primary particles present singly (present without aggregation) are spherical secondary particles.
Here, the average secondary particle size is a volume average particle size. The average secondary particle diameter of the spherical secondary particles can be measured using a laser diffraction scattering type particle size distribution measuring device or the like.

  The particle diameter (D10) when the cumulative percentage of particle size distribution of the spherical secondary particles is 10% is preferably 0.1 μm or more and 3.0 μm or less, more preferably 0.5 μm or more and 3.0 μm or less, More preferably, it is 1.0 μm or more and 2.8 μm or less. The positive electrode material, the conductive support agent, the binder resin (binder) and the solvent having a particle diameter (D10) of 0.1 μm or more and 3.0 μm or less when the cumulative percentage of particle size distribution of the spherical secondary particles is 10% And the ratio of fine spherical secondary particles to increase the blending amount of the conductive additive and the binder when preparing the positive electrode material paste for a lithium ion secondary battery by mixing The DC resistance during charge and discharge of the lithium ion secondary battery can be reduced. The particle size distribution of the spherical secondary particles can be measured using a laser diffraction scattering type particle size distribution measuring apparatus or the like.

  The particle diameter (D90) when the cumulative percentage of particle size distribution of the spherical secondary particles is 90% is preferably 3.0 μm or more and 18.0 μm or less, and more preferably 4.0 μm or more and 17.5 μm or less. More preferably, they are 5.0 micrometers or more and 17.0 micrometers or less. It forms using the electrode material for lithium ion secondary batteries of this embodiment that the particle diameter (D10) in case the accumulation percentage of the particle size distribution of spherical secondary particles is 90% is 3.0 micrometers or more and 18.0 micrometers or less While being hard to produce an unevenness | corrugation on the surface of the electrode which carried out, the direct current resistance at the time of charging / discharging of a lithium ion secondary battery can be reduced.

The primary particles of the electrode active material further have a carbonaceous coating that covers the electrode active material. The carbonaceous film is for imparting desired electron conductivity to the primary particles.
The carbonaceous film is a pyrolytic carbonaceous film derived from a heat-treated organic compound, and the thickness of the carbonaceous film is preferably 0.5 nm or more and 5.0 nm or less, and 1.0 nm or more and 3.0 nm It is more preferable that If the thickness of the carbonaceous film is 0.5 nm or more, the thickness of the carbonaceous film does not become too thin, and a film having a desired resistance value can be formed. As a result, the conductivity is improved, and the conductivity as an electrode material can be secured. On the other hand, when the thickness of the carbonaceous film is 5.0 nm or less, reduction in battery activity, for example, battery capacity per unit mass of the electrode material can be suppressed.

  The carbon content of the spherical secondary particles is preferably 0.8% by mass or more and 1.6% by mass or less, more preferably 0.8% by mass or more and 1.3% by mass or less, and still more preferably 0. 9 mass% or more and 1.1 mass% or less. When the carbon content is 0.8% by mass or more, the conductivity as an electrode material can be secured, and when a lithium ion secondary battery is formed, the discharge capacity at a high-speed charge and discharge rate becomes large, and sufficient Charge / discharge rate performance can be realized. On the other hand, when the carbon content is 1.6% by mass or less, the amount of carbon does not increase excessively, and the battery capacity of the lithium ion secondary battery per unit mass of the electrode material for a lithium ion secondary battery decreases more than necessary Can be suppressed. Moreover, the direct current resistance at the time of charge / discharge of a lithium ion secondary battery can be reduced as a carbon content rate is 0.8 mass% or more and 1.6 mass% or less. In addition, the carbon content rate of spherical secondary particles can be measured by the method as described in an Example. Moreover, this carbon content rate is a carbon content rate in terms of carbon atoms.

  The coverage of the carbonaceous film with respect to the electrode active material is preferably 60% or more, and more preferably 80% or more. The covering effect of a carbonaceous film is fully acquired because the coverage of a carbonaceous film is 60% or more. The film thickness of the carbonaceous film can be measured using a transmission electron microscope.

The density of the carbonaceous film, which is calculated by the carbon content constituting the carbonaceous film, is preferably 0.3 g / cm ³ or more and 1.5 g / cm ³ or less, more preferably 0.4 g / cm ³ or more and 1.0 g or more / Cm ³ or less. The density of the carbonaceous film, which is calculated by the carbon content of the carbonaceous film, is the mass per unit volume of the carbonaceous film, assuming that the carbonaceous film is composed of only carbon.
If the density of the carbonaceous film is 0.3 g / cm ³ or more, the carbonaceous film exhibits sufficient electron conductivity. On the other hand, when the density of the carbonaceous film is 1.5 g / cm ³ or less, since the content of the crystallites of graphite having a layered structure in the carbonaceous film is small, Li ions diffuse in the carbonaceous film. There is no steric hindrance due to graphite crystallites. As a result, the charge transfer resistance does not increase. As a result, the internal resistance of the lithium ion secondary battery does not rise, and the voltage drop at the high-speed charge and discharge rate of the lithium ion secondary battery does not occur.

  When the spherical secondary particles are composed of a surface portion from the surface of the spherical secondary particles to a depth of 10% of the particle diameter of the spherical secondary particles, and a main portion inside the surface portions, carbon in the main portion The ratio of the amount of carbon in the surface layer to the amount (the amount of carbon in the surface layer / the amount of carbon in the main body) is 0.90 or more and 1.30 or less, preferably 1.00 or more and 1.30 or less, more preferably 1. 10 or more and 1.30 or less, more preferably 1.20 or more and 1.30 or less. When the ratio of the carbon amount in the surface layer to the carbon amount in the main body (the surface carbon) / the carbon in the main body is 0.90 or more and 1.30 or less, direct current resistance during charging and discharging of the lithium ion secondary battery is It can be reduced. The ratio of the amount of carbon in the surface layer to the amount of carbon in the main body of the spherical secondary particles (the amount of carbon in the surface layer / the amount of carbon in the main body) can be measured by the method described in the Examples. Moreover, these carbon amounts are carbon amounts in terms of carbon atoms.

The oil absorption of the spherical secondary particles using N-methyl-2-pyrrolidone (NMP) is preferably 50 ml / 100 g or less, more preferably 48 ml / 100 g or less, and still more preferably 45 ml / 100 g or less . When the NMP oil absorption is 50 ml / 100 g or less, thickening of the electrode paste can be suppressed, and dispersion of the conductive additive and the binder can be facilitated. The lower limit value of the oil absorption amount using NMP is not particularly limited, and is, for example, 20 ml / 100 g.
Moreover, the said NMP oil absorption can be measured by the method as described in an Example.

The specific surface area of the spherical secondary particles determined by the BET method is preferably 10 m ² / g or more and 25 m ² / g or less, more preferably 10 m ² / g or more and 20 m ² / g or less. When the specific surface area is 10 m ² / g or more, resistance to diffusion of Li ions and movement of electrons in the primary particles of the electrode material are reduced. Therefore, the internal resistance can be reduced, and the output characteristics can be improved. On the other hand, when the specific surface area is 25 m ² / g or less, the specific surface area of the electrode material does not increase excessively, the required carbon mass is suppressed, and the battery capacity of the lithium ion secondary battery per unit mass of the electrode material It can be improved.
In addition, the said specific surface area can be measured by BET method using a specific surface area meter (For example, the Microtrac bell corporation | Co., Ltd. make, brand name: BELSORP-mini).

[Method of producing electrode material]
The method for producing an electrode material for a lithium ion secondary battery according to the present embodiment includes, for example, at least a step of producing an electrode active material and an electrode active material precursor, and an electrode active material and an electrode active material precursor A slurry preparation step of preparing a slurry by mixing one kind of electrode active material raw material and water, and a granulating step of obtaining a granulated product by adding an aggregation holding agent to the slurry obtained in the above step The granulated product obtained in the step is dry-mixed with an organic compound which is a carbonaceous film precursor, and the obtained mixture is fired in a non-oxidative atmosphere.

(Step of manufacturing electrode active material and electrode active material precursor)
It does not specifically limit as a manufacturing process of an electrode active material and an electrode active material precursor (Hereafter, an electrode active material and an electrode active material precursor may be called an electrode active material raw material), For example, this electrode active material and When the electrode active material precursor is represented by the general formula (1), conventional methods such as a solid phase method, a liquid phase method, and a gas phase method can be used. The _Li x _A y _D z PO ₄ obtained in this way, for example, those of the particulate (hereinafter, may be referred to as _"Li x _A y _M z PO ₄ particles".) Can be mentioned.
Li _x A _y D _z PO ₄ particles are prepared, for example, by hydrothermally synthesizing a slurry-like mixture obtained by mixing Li source, A source, P source, water, and optionally D source. It is obtained by According to hydrothermal synthesis, Li _x A _y D _z PO ₄ forms as a precipitate in water. The obtained precipitate may be a precursor of Li _x A _y D _z PO ₄ . In this case, the target Li _x A _y D _z PO ₄ particles can be obtained by firing the Li _x A _y D _z PO ₄ precursor.
It is preferable to use a pressure-resistant closed vessel for the hydrothermal synthesis.

Here, as the Li source, lithium salts such as lithium acetate (LiCH ₃ COO) and lithium chloride (LiCl), lithium hydroxide (LiOH) and the like can be mentioned. Among these, as the Li source, at least one selected from the group consisting of lithium acetate, lithium chloride and lithium hydroxide is preferably used.

As the source A, chlorides, carboxylates, sulfates and the like containing at least one selected from the group consisting of Co, Mn, Ni, Fe, Cu and Cr can be mentioned. For example, when A in Li _x A _y D _z PO ₄ is Fe, iron (II) chloride (FeCl ₂ ), iron acetate (II) (Fe (CH ₃ COO) ₂ ), iron sulfate can be used as the Fe source. (II) divalent iron salts such as (FeSO ₄ ). 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.

  As the D source, chloride, carboxylate containing at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc and Y And sulfates.

Examples of P sources 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)
At this process, the electrode active material raw material obtained at the said process is disperse | distributed to water, and a uniform slurry is prepared. When the electrode active material material is dispersed in water, a dispersant can also be added. The method for dispersing the electrode active material in water is not particularly limited. For example, a method using a medium stirring type dispersing device for stirring medium particles such as a planetary ball mill, a vibrating ball mill, a bead mill, a paint shaker, and attritor at high speed is used. preferable.

  When preparing the slurry, the particle diameter (D10) when the cumulative percentage of particle size distribution of the electrode active material material in the slurry is 10% is preferably 0.1 μm or more and 3.0 μm or less, more preferably 0. The particle diameter (D90) when the cumulative percentage of the particle size distribution of the electrode active material material in the slurry is 90% is controlled so as to be 5 μm to 3.0 μm, more preferably 1.0 μm to 2.8 μm. It is preferable to control so as to be preferably 3.0 μm or more and 18.0 μm or less, more preferably 4.0 μm or more and 17.5 μm or less, and still more preferably 5.0 μm or more and 17.0 μm or less. As a result, the particle size distribution of the electrode active material raw material in the slurry is broadened, the density of the obtained spherical secondary particles can be increased, and the effects of the present invention can be exhibited more.

  In addition, when preparing the slurry, the cumulative percentage of the particle size distribution of the electrode active material in the slurry is 90 relative to the particle diameter (D10) when the cumulative percentage of the particle size distribution of the electrode active material in the slurry is 10%. The ratio (D90 / D10) of the particle diameter (D90) at% may be controlled to be 1 or more and 30 or less. By setting (D90 / D10) in the above range, the particle size distribution of the electrode active material raw material in the slurry becomes wide, the density of the obtained particles can be increased, and the effect of the present invention can be exhibited.

  The dispersion conditions of the slurry can be adjusted, for example, by the concentration of the electrode active material in the slurry, the stirring speed, the stirring time, and the like.

(Granulation process)
At this process, a granulated material is manufactured from the electrode active material raw material in a slurry.
At the time of granulation, it is preferable to softly agglomerate primary particles in the slurry. Thereby, the collapse of the spherical secondary particles can be suppressed, and the content of the secondary particles in the slurry can be 18% or more in number ratio to the total number of particles including all single particles and spherical secondary particles. Can. As a method of promoting the soft aggregation of primary particles and suppressing the disintegration of the granulated material, in the present embodiment, the aggregation holding agent is added to the slurry in the granulation step. Here, the aggregation support means a compound that promotes aggregation of primary particles and retains the shape of secondary particles in which the primary particles are aggregated.
As said method, the method of adding and mixing organic acids, such as a citric acid, polyacrylic acid, ascorbic acid, etc. as an aggregation holding | maintenance agent in a slurry, etc. are mentioned, for example. By lowering the pH of the slurry with an organic acid, aggregation of primary particles is promoted, and after granulation, secondary particles in which primary particles are more compacted can be formed, and the strength of secondary particles can be enhanced.
The reason why the organic acid is selected is that it is preferable that the organic acid be carbonized as a part of the carbonaceous film without remaining as an impurity in the later-described firing step. However, it is difficult to make a carbonaceous film by itself because it is difficult to carbonize residual organic acids, and it is not preferable to add a large amount because it is difficult to form a good carbonaceous film.
Moreover, you may use the carbonization catalyst for promoting carbonization of an organic compound in the baking process mentioned later.

  The compounding amount of the aggregation support agent is preferably 0.1 to 2.0% by mass, more preferably 0.2 to 1.5% by mass in terms of solid content with respect to the electrode active material raw material. By setting it as 0.1 mass% or more, disintegration of a granulated body can be suppressed. By setting the content to 2.0% by mass or less, an increase in the amount of carbon derived from the aggregation holding agent can be suppressed. If the amount of carbon is large, the lithium ion conductivity may be poor.

  In addition, spherical secondary particles can be obtained by preparing the concentration of the electrode active material material contained in the slurry to be preferably 15 to 80% by mass, and more preferably 20 to 70% by mass.

Next, the mixture obtained above is in an atmosphere having an atmosphere temperature of less than the boiling point of the solvent, preferably 55 ° C. to less than 100 ° C., more preferably 55 ° C. to 95 ° C., more preferably 60 ° C. to 90 ° C. Spray in and let dry. Thereby, in the electrode material for a lithium ion secondary battery, the ratio of the amount of carbon in the surface layer to the amount of carbon in the main portion (the amount of carbon in the surface layer / the amount of carbon in the body) is 0.90 or more and 1.30 or less, preferably 1 .00 or more and 1.30 or less, more preferably 1.10 or more and 1.30 or less, and further preferably 1.20 or more and 1.30 or less.
In the electrode material for lithium ion secondary batteries that the atmosphere temperature at which the mixture is sprayed and dried is equal to or higher than the boiling point of the solvent, the ratio of the amount of carbon in the surface portion to the amount of carbon in the main portion (surface portion carbon The value of amount / body part carbon amount) is 1.35 or more.

When the spherical secondary particles are prepared by granulating primary particles by spray drying, the fine primary particles in the droplets move to the surface of the droplets as the water moves to the surface of the droplets. Thereby, the primary particle density in the surface layer portion of the spherical secondary particles is increased. In addition, when organic matter is contained in the droplet, the viscosity increases with the increase of the organic matter concentration in the surface layer with drying of the droplet, and a shell structure derived from the organic matter is formed in the surface layer. An increase in primary particle density in the surface layer causes sintering of primary particles due to high particle adhesion at the time of firing, resulting in deterioration of properties. In addition, the shell structure derived from the organic matter causes hollowing of spherical secondary particles, which causes a decrease in energy density. Therefore, the increase in the carbonaceous coverage of the surface portion of the spherical secondary particles causes a decrease in the permeability of lithium, which causes a decrease in the electron conductivity. For this reason, in the electrode material for a lithium ion secondary battery, when the ratio of the amount of carbon in the surface layer to the amount of carbon in the main body is large, it is considered that the direct current resistance during charge and discharge of the lithium ion secondary battery is high.
In the present embodiment, the mixture is sprayed and dried within the above-described range of the ambient temperature, thereby suppressing an increase in primary particle density in the surface layer portion of the spherical secondary particles and, at the same time, preventing the surface layer portion of the spherical secondary particles. It can suppress that the shell structure derived from organic matter is formed.

In addition, crushing of the spherical secondary particles can suppress the decrease in the adhesion of the primary particles, but if the crushing strength is too strong, the carbonaceous film of the primary particles peels off the surface of the primary particles, and the electron conductivity is descend. The decrease in electron conductivity causes the decrease in input / output characteristics and the capacity after charge and discharge cycles. On the other hand, when the crushing strength is too weak, voids remain in the electrode, and the energy density per unit volume decreases.
According to this embodiment, the ratio of the amount of carbon in the surface layer portion to the amount of carbon in the main body portion can be reduced without crushing the spherical secondary particles.

  In addition, the conditions for spraying, for example, the concentration of the electrode active material in the slurry, the spraying pressure and speed, and the conditions for drying after spraying, for example, the temperature of drying air and the internal pressure of the apparatus are appropriately adjusted. Thereby, spherical secondary particles having an average particle diameter in the above range can be obtained.

It should be noted that the longer the atmosphere temperature becomes lower than the boiling point of the solvent in the slurry, the longer it takes to dry the sprayed droplets, so the resulting dried product shrinks sufficiently during the time required for this drying. It will be. As a result, the dried product sprayed and dried at an atmospheric temperature lower than the boiling point of the solvent in the slurry tends to have a solid structure.
Moreover, since the drying time of the sprayed droplet becomes shorter as the ambient temperature becomes higher than the boiling point of the solvent in the slurry, the resulting dried product can not shrink sufficiently. This makes it easier for the dried product to have a porous or hollow structure.

(Firing process)
At this process, the granulated material obtained at the said process is baked by non-oxidative atmosphere.
First, before firing, an organic compound which is a carbonaceous film precursor is dry mixed with the granulated product.
The organic compound is not particularly limited as long as it is a compound capable of forming a carbonaceous film on the surface of the positive electrode active material. For example, polyvinyl alcohol (PVA), polyvinyl pyrrolidone, cellulose, starch, gelatin, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose , Hydroxyethyl cellulose, polystyrene sulfonic acid, polyacrylamide, polyvinyl acetate, glucose, fructose, galactose, mannose, maltose, sucrose, lactose, glycogen, pectin, alginic acid, glucomannan, chitin, hyaluronic acid, chondroitin, agarose, polyether, Examples thereof include dihydric alcohols and trihydric alcohols. However, what is contained in the organic acid used at the above-mentioned granulation process is excluded. These may use only 1 type and may mix and use 2 or more types.

  The compounding ratio of the organic compound to the electrode active material raw material is 1.0 part by mass or more and 5.0 parts by mass or less in the compounding ratio obtained from the organic compound to 100 parts by mass of the active material obtained from the electrode active material Is preferred. The actual blending amount varies depending on the carbonization amount by heating carbonization (the type of carbon source and carbonization conditions), but is about 1.3 parts by mass to 4.5 parts by mass.

  Then, the mixture obtained by the above-mentioned dry mixing is preferably subjected to non-oxidizing atmosphere at a temperature of preferably 650 ° C. or more and 1000 ° C. or less, more preferably 700 ° C. or more and 900 ° C. or less for 0.1 hour or more Bake for 40 hours or less.

As the non-oxidizing atmosphere, an atmosphere composed 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 mixture, a reducing atmosphere containing about several volume% of a reducing gas such as hydrogen (H ₂ ) is preferable. Further, for the purpose of removing the organic components evaporated in the non-oxidizing atmosphere at the time of firing, a combustion supporting gas such as oxygen (O ₂ ) or a flammable gas may be introduced into the non-oxidizing atmosphere.

  Here, when the baking temperature is set to 650 ° C. or more, the decomposition and reaction of the organic compound contained in the mixture can easily proceed sufficiently, and the carbonization of the organic compound can be sufficiently performed. As a result, it is easy to prevent the formation of decomposition products of highly resistant organic compounds in the obtained particles. On the other hand, by setting the firing temperature to 1000 ° C. or less, lithium (Li) in the electrode active material raw material is difficult to evaporate, and the electrode active material is suppressed from growing to a size larger than a target size. As a result, when a lithium ion secondary battery provided with a positive electrode including the electrode material of the present embodiment is manufactured, it is possible to prevent the decrease in discharge capacity at a high speed charge and discharge rate, and lithium having sufficient charge and discharge rate performance. An ion secondary battery can be realized.

[Electrode for lithium ion secondary battery]
The lithium ion secondary battery electrode of the present embodiment uses the electrode material for a lithium ion secondary battery of the present embodiment.
For example, the electrode for a lithium ion secondary battery of the present embodiment is produced as follows.

In order to produce the lithium ion secondary battery electrode of the present embodiment, the electrode material for lithium ion secondary battery of the present embodiment, a binder comprising a binder resin, and a solvent are mixed to form an electrode. Prepare paint or paste for electrode formation. At this time, if necessary, a conductive aid such as carbon black, acetylene black, graphite, ketjen black, natural graphite, artificial graphite or the like may be added.
As a binder, that is, a binder resin, for example, polytetrafluoroethylene (PTFE) resin, polyvinylidene fluoride (PVdF) resin, fluororubber and the like are suitably used.
Although the compounding ratio of the electrode material for lithium ion secondary batteries of this embodiment and binder resin is not specifically limited, For example, 1 mass part-30 mass parts, preferably 3 of binder resin are 100 mass parts of electrode materials. A mass part will be 20 mass parts.

The solvent used for the electrode-forming paint or electrode-forming paste may be appropriately selected according to the nature of the binder resin.
For example, alcohols such as water, methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol, pentanol, hexanol, octanol, diacetone alcohol, ethyl acetate, butyl acetate, ethyl lactate, propylene glycol Esters such as monomethyl ether acetate, propylene glycol monoethyl ether acetate, γ-butyrolactone, diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol monobutyl ether (butyl cellosolve ), Ethers such as diethylene glycol monomethyl ether and diethylene glycol monoethyl ether, acetone, Ketones such as methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), acetylacetone, cyclohexanone, dimethylformamide, amides such as N, N-dimethylacetoacetamide, N-methylpyrrolidone, glycols such as ethylene glycol, diethylene glycol, propylene glycol And the like. These may use only 1 type and may mix and use 2 or more types.

Subsequently, a paint for electrode formation or a paste for electrode formation was applied to one side of an aluminum foil and then dried to form a coating film made of a mixture of the above electrode material and a binder resin on one side. Obtain an aluminum foil.
Next, the coated film is pressure-bonded and dried, and a current collector (electrode) having an electrode material layer on one surface of an aluminum foil is produced.
In this way, it is possible to manufacture an electrode capable of reducing the direct current resistance of the lithium ion secondary battery and increasing the discharge capacity.

Lithium-ion rechargeable battery
The lithium ion secondary battery of the present embodiment includes the electrode for a lithium ion secondary battery of the present embodiment as a positive electrode (hereinafter sometimes referred to as a positive electrode), a negative electrode, and an electrolyte. The positive electrode has a positive electrode mixture layer formed using the above-described electrode material.

[Negative electrode]
Examples of the negative electrode include those containing a carbon material such as metal Li, natural graphite and hard carbon, and a negative electrode material such as Li alloy and Li ₄ Ti ₅ O ₁₂ and Si (Li _4.4 Si).

〔Electrolytes〕
The electrolyte is not particularly limited, but is preferably a non-aqueous electrolyte. For example, ethylene carbonate (ethylene carbonate; EC) and ethyl methyl carbonate (ethyl methyl carbonate; EMC) become 1: 1 in volume ratio were mixed so, the resulting mixture lithium hexafluorophosphate in a solvent (LiPF _6), for example, those obtained by dissolving at a concentration 1 mol / dm ^3.

[Separator]
The positive electrode and the negative electrode can be opposed via a separator. For example, porous propylene can be used as a separator.
Also, instead of the non-aqueous electrolyte and the separator, a solid electrolyte may be used.

  Since the lithium ion secondary battery of the present embodiment uses the electrode formed of the electrode material for a lithium ion secondary battery of the present embodiment as a positive electrode, the DC resistance during charge and discharge of the lithium ion secondary battery is reduced. Charge and discharge characteristics can be improved.

  Hereinafter, the present invention will be specifically described by way of examples and comparative examples. In addition, this invention is not limited to the form as described in an Example.

[Synthesis of electrode material for lithium ion secondary battery]
Example 1
Li: Fe: P = 3: 1: 1 in molar ratio of lithium phosphate (Li ₃ PO ₄ ) as Li source and P source and iron (II) sulfate (FeSO ₄ ) as Fe source Mixed. Furthermore, the distilled water for preparation was mixed, and the raw material slurry of 600 ml was prepared.
Next, the raw material slurry is accommodated in a pressure tight container, hydrothermally synthesized at 180 ° C. for 2 hours, cooled to room temperature (25 ° C.), and the cake-like electrode active precipitated in the container Material particles (primary particles) were obtained. The electrode active material particles were sufficiently washed with distilled water several times, and then the electrode active material particles and distilled water were mixed so that the concentration of the electrode active material particles was 60 mass%, to prepare a suspended slurry.
This suspended slurry is charged into a sand mill together with zirconia balls having a diameter of 0.1 mm, and a cumulative volume percentage of 10% of a particle diameter (D90) of a cumulative percentage of 90% in cumulative particle size distribution of electrode active material particles in the suspended slurry Dispersion treatment was performed while adjusting the processing time of the sand mill so that the ratio (D90 / D10) to the particle diameter (D10) was 2.
Subsequently, a glucose aqueous solution adjusted to 30 mass% in advance is mixed with the dispersion-treated slurry in an amount of 1.0 mass% in terms of glucose solid content with respect to the electrode active material particles, and the concentration of electrode active material particles in the slurry is 50 Distilled water was mixed so as to be% by mass, and then it was sprayed and dried in an air atmosphere of 60 ° C. with a gas-liquid ratio of 10000 to obtain a granulated and dried product (spherical secondary particles) of electrode active material particles.
Then, 3.5% by mass of the obtained granulated dried material is dry mixed with the electrode active material particles in a dry state, and heat treatment is performed at 750 ° C. for 1 hour in an inert atmosphere to obtain electrode active material particles. A carbonaceous film was formed on the surface of the above to prepare an electrode material for a lithium ion secondary battery of Example 1.

(Example 2)
The electrode material for a lithium ion secondary battery of Example 2 was produced in the same manner as the electrode material for a lithium ion secondary battery of Example 1 except that the temperature in the air atmosphere for spraying and drying the slurry was 70 ° C. did.

(Example 3)
The electrode material for a lithium ion secondary battery of Example 3 was produced in the same manner as the electrode material for a lithium ion secondary battery of Example 1 except that the temperature in the air atmosphere for spraying and drying the slurry was 80 ° C. did.

(Example 4)
A positive electrode material for a lithium ion secondary battery of Example 4 was produced in the same manner as the electrode material for a lithium ion secondary battery of Example 1 except that the temperature in the air atmosphere for spraying and drying the slurry was 90 ° C. did.

(Comparative example 1)
The electrode material for a lithium ion secondary battery of Comparative Example 1 was produced in the same manner as the electrode material for a lithium ion secondary battery of Example 1 except that the temperature in the air atmosphere for spraying and drying the slurry was 100 ° C. did.

(Comparative example 2)
The electrode material for a lithium ion secondary battery of Comparative Example 2 was produced in the same manner as the electrode material for a lithium ion secondary battery of Example 1 except that the temperature in the air atmosphere for spraying and drying the slurry was 50 ° C. did. However, at the time of drying, the dried matter adhered to the wall of the drying apparatus and the dried matter due to moisture, and did not become granules.

[Evaluation of electrode material]
The obtained electrode material was evaluated by the following method. The results are shown in Table 1.

(1) Ratio of carbon content of surface layer part and main body part of spherical secondary particles and average particle diameter of primary particles Embed spherical secondary particles in epoxy resin, use ion milling, and cross section of spherical secondary particles It was molded to be exposed. Then, using a scanning electron microscope, the cross section of the spherical secondary particle was observed.
In scanning electron microscopy, the backscattered electron image and the secondary electron image in the same field of view are observed, and the primary particle and carbon site in the secondary particle are separated from the composition and shape, and the area of the carbon site in the image is an image Calculated by analysis.
In 100 spherical secondary particles, the area of the carbon site in the surface layer portion of the spherical secondary particles (range of the depth of 10% of the particle diameter from the surface) was calculated by image analysis. Moreover, the area of the carbon site | part in the main-body part of spherical secondary particle was computed by image analysis. Then, the area of the carbon site of the surface layer was divided by the area of the carbon site of the main body to calculate the ratio of the amount of carbon in the surface layer to the amount of carbon in the main body (surface carbon area / body carbon content). The ratio of the amount of carbon in the surface layer to the amount of carbon in the main part (the amount of carbon in the surface part / the amount of carbon in the main part) was calculated for 100 spherical secondary particles, and the average value of 100 spherical secondary particles was calculated for each. The ratio of the amount of carbon in the surface layer to the amount of carbon in the main portion of the example and the comparative example (the amount of carbon in the surface portion / the amount of carbon in the main portion) was used.
In addition, the particle diameter of primary particles observed in the cross section of 100 spherical secondary particles was calculated by image analysis, and the average value was taken as the average primary particle diameter.

(2) Particle size (D10) and particle size (D90) of spherical secondary particles
First, 40 g of pure water as a dispersion, 0.12 g of polyvinyl pyrrolidone (PVP), and 0.04 g of an electrode material as a sample powder were weighed in a 70 mL mayonnaise bottle. The mayonnaise bottle was manually shaken about 10 times to adjust the sample powder and the dispersion.
Next, the mixed solution of the sample powder and the dispersion is subjected to ultrasonic treatment for 2 minutes under the conditions of Output 5 and 50% pulse using an ultrasonic homogenizer (trade name: SONIFIER 450, manufactured by BRANSON) to well use the sample powder as a dispersion solution. It was dispersed.
The particle size distribution of spherical secondary particles in the well-dispersed mixed solution is measured using a measuring device (trade name: LA-950V2, manufactured by Horiba, Ltd.), and the results show that the particle size distribution of spherical secondary particles is The particle diameter (D10) when the cumulative volume percentage was 10% and the particle diameter (D90) when the cumulative volume percentage of the particle size distribution of the spherical secondary particles was 90% were calculated.

(3) Carbon Content of Spherical Secondary Particles The carbon content of the electrode material was measured using a carbon-sulfur analyzer (trade name: EMIA-220V, manufactured by Horiba, Ltd.).

(4) Oil absorption using N-methyl-2-pyrrolidone (NMP) (NMP oil absorption)
The oil absorption using N-methyl-2-pyrrolidone (NMP) was measured by replacing linseed oil with NMP according to a method according to JIS K 5101-13 (refined linseed oil method).

(5) Direct current resistance during charge and discharge of lithium ion secondary battery [Production of positive electrode]
The obtained electrode material, polyvinylidene fluoride (PVdF) as a binder, and acetylene black (AB) as a conductive aid are mixed so that the mass ratio becomes 90: 5: 5, and N-methyl as a solvent -2-Pyrrolidone (NMP) was added to impart fluidity to make a slurry.
Next, this slurry was applied onto a 30 μm thick aluminum (Al) foil (current collector) and dried. Then, it cut out in strip shape of application width 40 mm, and it pressurized with applied roll pressure 5 t / 250 mm with the roll press machine, and produced the positive electrode of the lithium ion secondary battery.

[Fabrication of lithium ion secondary battery]
The produced positive electrode and a commercially available negative electrode made of natural graphite were punched out to a predetermined size, current collecting tabs were welded to each, and placed in an aluminum laminate film through a separator made of a porous polypropylene film. An electrolyte solution having a concentration of 1 mol / dm ³ LiPF ₆ / EC: DEC = 50: 50 (vol%) was injected into the above and sealed to prepare a lithium ion secondary battery for battery characteristic evaluation.

[Evaluation of lithium ion secondary battery]
The direct current resistance at the time of charge and discharge of the obtained lithium ion secondary battery was measured by the following method.
The direct current resistance was measured using a lithium ion secondary battery whose charge depth was adjusted to 50% (SOC 50%) at a constant current of a charge rate of 0.1 C at an environmental temperature of 0 ° C. Currents of 1C, 3C, 5C and 10C rates are alternately applied to the charge side and the discharge side for 10 seconds each for 10 seconds for each rate in a lithium ion secondary battery adjusted to SOC 50% at room temperature (25 ° C.) The current value in seconds is plotted on the horizontal axis, and the voltage value is plotted on the vertical axis to calculate the slope of the approximate straight line by the least squares method, and the value obtained by multiplying the mass of the active material used for the positive electrode is "charge side = input “DCR”, “discharge side = output DCR”. In addition, the idle time for 10 minutes was each provided at the time of the electricity supply direction change by each electric current, and the time of the change of an electricity supply current.

[Evaluation result of electrode material]
The evaluation results of the electrode materials of Examples 1 to 4 and Comparative Example 1 are shown in Table 1. In addition, about the electrode material of the comparative example 2, since spherical secondary particle was not obtained, evaluation of the electrode material was not able to be performed.

(Evaluation results)
The direct current resistance at the time of charge and discharge of the lithium ion secondary battery produced using the electrode material for lithium ion secondary batteries of Examples 1 to 4 was produced using the electrode material for lithium ion secondary batteries of Comparative Example 1 Compared with the direct current resistance at the time of charge and discharge of a lithium ion secondary battery, it was low. From this, when the ratio of the carbon amount in the surface layer to the carbon amount in the main body (the surface carbon) / the carbon in the main body is 0.90 or more and 1.30 or less, the charge and discharge of the lithium ion secondary battery It turned out that the direct current resistance of can be reduced.

The particle diameter (D90) when the cumulative percentage of particle size distribution of the spherical secondary particles is 90% is preferably 3.0 μm or more and 18.0 μm or less, and more preferably 4.0 μm or more and 17.5 μm or less. More preferably, they are 5.0 micrometers or more and 17.0 micrometers or less. It forms using the electrode material for lithium ion secondary batteries of this embodiment that the particle diameter ( D90 ) in case the accumulation percentage of the particle size distribution of spherical secondary particles is 90% is 3.0 micrometers or more and 18.0 micrometers or less. While being hard to produce an unevenness | corrugation on the surface of the electrode which carried out, the direct current resistance at the time of charging / discharging of a lithium ion secondary battery can be reduced.

Claims (7)

It has spherical secondary particles formed by granulating primary particles of an electrode active material comprising a transition metal lithium phosphate compound having an olivine structure,
The spherical secondary particle comprises a surface portion from the surface of the spherical secondary particle to a depth of 10% of the particle diameter of the spherical secondary particle, and a main portion inside the surface portion.
An electrode material for a lithium ion secondary battery, wherein the ratio of the carbon amount in the surface layer to the carbon amount in the main part (the surface carbon) / the main part carbon is 0.90 to 1.30. The primary particle further comprises a carbonaceous film covering the electrode active material,
The electrode material for a lithium ion secondary battery according to claim 1, wherein an average particle diameter of the primary particles is 10 nm or more and 500 nm or less. The particle size is 0.1 μm to 3.0 μm when the cumulative volume percentage of the particle size distribution of the spherical secondary particles is 10%,
The electrode material for a lithium ion secondary battery according to claim 1 or 2, wherein the particle diameter is 3.0 μm or more and 18.0 μm or less when the cumulative volume percentage of the particle size distribution of the spherical secondary particles is 90%.   The carbon content of the spherical secondary particles is 0.8% by mass or more and 1.6% by mass or less, The electrode material for a lithium ion secondary battery according to any one of claims 1 to 3.   The oil absorption amount using N-methyl-2-pyrrolidone (NMP) is 50 ml / 100 g or less, The electrode material for lithium ion secondary batteries of any one of Claims 1-4.   The electrode for lithium ion secondary batteries using the electrode material for lithium ion secondary batteries of any one of Claims 1-5.   A lithium ion secondary battery using the electrode for a lithium ion secondary battery according to claim 6.