WO2018198377A1 - Lithium ion secondary battery negative electrode material, lithium ion secondary battery negative electrode, and lithium ion secondary battery - Google Patents

Abstract

This lithium ion secondary battery negative electrode material satisfies the following (1) to (4) in a particle size distribution based on volume. (1) A particle diameter (D10) obtained when an accumulation from the smaller diameter side reaches 10 % is 5 μm to 14 μm. (2) A particle diameter (D50) obtained when an accumulation from the smaller diameter side reaches 50 % is 15 μm to 27 μm. (3) A particle diameter (D90) obtained when an accumulation from the smaller diameter side reaches 90 % is 20 μm to 55 μm. (4) An accumulation value Q3 obtained by accumulating particle diameters of 9.516 μm or less is 4 % to 30 %.

Description

Negative electrode material for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary battery

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

In recent years, with the spread of portable electronic devices such as smartphones, improvements in characteristics such as higher capacity and compactness, longer life to accommodate electric vehicles and power storage applications, and shortened charging time (improved rapid charging characteristics) Is required for lithium ion secondary batteries. For example, Patent Document 1 uses a graphite particle having a secondary particle structure in which a plurality of flat primary particles are aggregated or bonded so that their orientation planes are non-parallel as a negative electrode active material. The discharge cycle characteristics are improved.

Japanese Patent Laid-Open No. 10-158005

While the usage scenes of lithium ion secondary batteries are diversifying, if the temperature of the usage environment is low in a lithium ion secondary battery, the charge capacity maintenance rate decreases, and stable charging performance may not be obtained. Therefore, development of a lithium ion secondary battery having excellent charge capacity stability is desired.

The present invention provides a negative electrode material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery capable of obtaining a lithium ion secondary battery having excellent charging capacity stability. Let it be an issue.

Specific means for solving the above problems include the following embodiments.
<1> A negative electrode material for a lithium ion secondary battery satisfying the following (1) to (4) in a volume-based particle size distribution.
(1) The particle diameter (D10) when the accumulation from the small diameter side is 10% is 5 μm to 14 μm. (2) The particle diameter (D50) when the accumulation from the small diameter side is 50% is 15 μm to 27 μm. (3) The particle diameter (D90) when the accumulation from the small diameter side is 90% is 20 μm to 55 μm. (4) The integrated value Q3 of the particle diameter of 9.516 μm or less is 4% to 30%. .
<2> The negative electrode material for a lithium ion secondary battery according to <1>, comprising particles in which a plurality of flat graphite particles are aggregated or bonded so that their orientation planes are non-parallel.
<3> The negative electrode material for a lithium ion secondary battery according to <1> or <2>, wherein an interlayer distance d (002) of the graphite crystal determined by X-ray diffraction measurement using CuKα rays is 3.38 mm or less. .
<4> The specific surface area by BET method of nitrogen gas adsorption is ^{1.0m 2 /g~5.0m 2 / g, <} 1> ~ for lithium ion secondary battery according to any one of <3> Negative electrode material.
<5> The negative electrode material for a lithium ion secondary battery according to any one of <1> to <4>, wherein the true specific gravity is 2.22 or more.
<6> A lithium ion having a current collector and a negative electrode material layer including the negative electrode material for a lithium ion secondary battery according to any one of <1> to <5> formed on the current collector. Negative electrode for secondary battery.
<7> A lithium ion secondary battery having the negative electrode for a lithium ion secondary battery according to <6>.

ADVANTAGE OF THE INVENTION According to this invention, the negative electrode material for lithium ion secondary batteries which can obtain the lithium ion secondary battery excellent in stability of charge capacity, the negative electrode for lithium ion secondary batteries, and a lithium ion secondary battery are provided. The

Hereinafter, examples of embodiments of the negative electrode material for a lithium ion secondary battery, the negative electrode for a lithium ion secondary battery, and the lithium ion secondary battery according to the present disclosure will be described in detail. However, the present disclosure is not limited to the following embodiment. In the following embodiments, the components (including element steps and the like) are not essential unless otherwise specified. The same applies to numerical values and ranges thereof, and the scope of the present disclosure is not limited.

In the present disclosure, the term “process” includes a process that is independent of other processes and includes the process if the purpose of the process is achieved even if it cannot be clearly distinguished from the other processes. .
In the present disclosure, numerical ranges indicated using “to” include numerical values described before and after “to” as the minimum value and the maximum value, respectively.
In the numerical ranges described stepwise in the present disclosure, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerical description. . Further, in the numerical ranges described in the present disclosure, the upper limit value or the lower limit value of the numerical range may be replaced with the values shown in the examples.
In the present disclosure, the content rate or content of each component in the composition is such that when there are a plurality of substances corresponding to each component in the composition, the plurality of substances present in the composition unless otherwise specified. Means the total content or content.
In the present disclosure, the particle size of each component in the composition is a mixture of the plurality of types of particles present in the composition unless there is a specific indication when there are a plurality of types of particles corresponding to each component in the composition. Means the value of
In the present disclosure, the term “layer” or “film” includes only a part of the region in addition to the case where the layer or film is formed over the entire region. The case where it is formed is also included.
In the present disclosure, the term “lamination” indicates that layers are stacked, and two or more layers may be combined, or two or more layers may be detachable.

<Anode material for lithium ion secondary battery>
A negative electrode material for a lithium ion secondary battery (hereinafter also simply referred to as “negative electrode material”) according to an embodiment of the present disclosure satisfies the following (1) to (4) in the volume-based particle size distribution.
(1) The particle diameter (D10) when the accumulation from the small diameter side is 10% is 5 μm to 14 μm. (2) The particle diameter (D50) when the accumulation from the small diameter side is 50% is 15 μm to 27 μm. (3) The particle diameter (D90) when the accumulation from the small diameter side is 90% is 20 μm to 55 μm. (4) The integrated value Q3 of the particle diameter of 9.516 μm or less is 4% to 30%. .

As a result of studies by the present inventors, it has been found that a lithium ion secondary battery obtained using a negative electrode material satisfying the above conditions has a high charge capacity maintenance rate even in a low temperature environment and is excellent in charging performance stability. The reason for this is not clear, but for example, there is a sufficient path for the electrolyte solution to move through the negative electrode material, so the electrolyte solution can easily move even if the viscosity of the electrolyte solution increases due to a temperature drop. It is guessed.

As the reason why the electrolyte solution path in the negative electrode material is sufficiently secured, for example, there are the following two reasons. First, since the negative electrode material contains small-sized particles and large-sized particles in a ratio that satisfies the above particle size distribution, the pressure applied to increase the density of the negative electrode compared to the case where the ratio of small-sized particles is larger than this. Is easily transmitted to the whole negative electrode, and it is considered that variation in the degree of particle collapse between the vicinity of the negative electrode surface and inside the negative electrode is suppressed. Further, it is conceivable that the number of paths of the electrolytic solution is sufficiently ensured as compared with the case where the ratio of the large diameter particles is larger than this. However, these assumptions do not limit the present disclosure.

In one embodiment, D10 of the negative electrode material may be 5 μm to 10 μm, or may be 6 μm to 10 μm. In one embodiment, the D50 of the negative electrode material may be 17 μm to 25 μm, or 18 μm to 23 μm. In one embodiment, D90 of the negative electrode material may be 30 μm to 50 μm, or may be 35 μm to 47 μm. In one embodiment, the integrated value Q3 when the particle diameter of the negative electrode material is 9.516 μm or less may be 4% to 20%, or may be 5 μm to 15 μm. In one embodiment, the difference between D10 and D90 of the negative electrode material may be 20 μm to 50 μm, or may be 25 μm to 40 μm.

In the present disclosure, the volume-based particle size distribution of the negative electrode material is a value measured using a laser diffraction particle size distribution measuring apparatus (for example, SALD-3000J, manufactured by Shimadzu Corporation). The measurement was performed after the sample was mixed with ion-exchanged water to which a surfactant (polyoxyethylene (20) sorbitan monolaurate) was added and dispersed by irradiation with ultrasonic waves for 30 seconds.

The particle size distribution of the negative electrode material is obtained by dividing the range of the particle diameter of 0.1 μm to 2000 μm into 50 logarithmic ratios. For example, the particle diameter is obtained from 0.1 × n, 0.1 × n ² ,..., 0.1 × n ⁵⁰ by obtaining n = (2000 / 0.1) ^1/50 . The total value of the relative particle amount in each particle size range in the range of 0.1 μm to 2000 μm is 100 (%). Specifically, the particle size range of 0.1 μm to 2000 μm was divided into 50 as shown in Table 1 below.

The negative electrode material may be a carbon material. In addition, a plurality of flat graphite particles may include particles (hereinafter also referred to as composite particles) in which a plurality of flat graphite particles are aggregated or bonded so that their orientation planes are non-parallel. However, the particles may be deformed so that their orientation planes are non-parallel. Examples of particles in which a plurality of flat graphite particles are deformed so that their orientation planes are non-parallel include spherical natural graphite obtained by spheroidizing a plurality of scaly natural graphites.

(Composite particles)
The flat graphite particles contained in the composite particles are non-spherical particles having anisotropy in shape. Examples of the flat graphite particles include graphite particles having a shape such as a scale shape, a scale shape, or a partial lump shape. The flat graphite particles may have an aspect ratio of 1.2 to 5 represented by A / B, where A is the length in the major axis direction and B is the length in the minor axis direction. It may be 1.3 to 3. The aspect ratio in the present disclosure is obtained by observing graphite particles with a microscope, arbitrarily selecting 100 graphite particles, measuring A / B, and taking the average value.
The length A in the major axis direction and the length B in the minor axis direction are measured as follows.
That is, in the projected image of the graphite particles observed using a microscope, two parallel tangents circumscribing the outer periphery of the graphite particles, the tangent line a ₁ and the tangent line a ₂ having the maximum distance are selected. The distance between the tangent line a ₁ and the tangent line a ₂ is a length A in the major axis direction. Further, the length of the longest line segment connecting the two points on the contour line of the projected image of the graphite particles is defined as the length B in the minor axis direction.

The orientation plane of the flat graphite particles being non-parallel means that planes (alignment planes) parallel to the plane having the largest cross-sectional area of the flat graphite particles are not aligned in a certain direction. Whether or not the orientation planes of the flat graphite particles are non-parallel to each other can be confirmed by microscopic observation. A plurality of flat graphite particles that are assembled or bonded with their orientation planes being non-parallel to each other can suppress an increase in the orientation of the particles on the electrode, and can reduce electrode expansion due to charge and discharge. , Excellent charge / discharge cycle characteristics tend to be obtained.
The negative electrode material may partially include a structure in which a plurality of flat graphite particles are aggregated or bonded so that the orientation planes of the flat graphite particles are parallel to each other.

The state where a plurality of flat graphite particles are aggregated or bonded means a state where two or more flat graphite particles are aggregated or bonded. The bond refers to a state in which the particles are chemically bonded directly or via a carbon substance. In addition, the term “aggregate” refers to a state in which the particles are not chemically bonded but the shape as an aggregate is maintained due to the shape or the like. The flat graphite particles may be aggregated or bonded via a carbon substance. The carbon material may be, for example, graphite obtained by graphitizing a binder such as tar or pitch in the firing process. From the viewpoint of mechanical strength, it may be in a state where two or more flat graphite particles are bonded via a carbon substance. Whether or not the flat graphite particles are aggregated or bonded can be confirmed, for example, by observation with a scanning electron microscope.

The total number of flat graphite particles contained in one composite particle may be 3 or more, or 10 or more.

The average particle diameter (D50) of the flat graphite particles is not particularly limited. For example, it can be selected from the range of 1 μm to 25 μm from the viewpoint of easy assembly or coupling. The average particle diameter (D50) of the flat graphite particles is measured in the same manner as D50 of the negative electrode material.

The material of the flat graphite particles and the raw material thereof is not particularly limited, and examples thereof include artificial graphite, scaly natural graphite, scaly natural graphite, coke, resin, tar, and pitch. Among these, graphite obtained from artificial graphite, natural graphite, or coke has high crystallinity and becomes soft particles, and thus tends to increase the density of the negative electrode.

The composite particles may further include spherical graphite particles. In general, since spherical graphite particles are denser than flat graphite particles, the composite particles contain spherical graphite particles, so that the density of the negative electrode material can be increased and added during densification treatment. The pressure can be reduced. As a result, the orientation of the flat graphite particles in the direction along the surface of the current collector is suppressed, and the movement of lithium ions tends to be better. In particular, when the electrode density of the negative electrode exceeds 1.7 g / cm ³ , by suppressing the orientation of the flat graphite particles, the permeability of the electrolytic solution into the negative electrode material layer is further increased, and the discharge capacity and charge capacity are increased. The discharge cycle characteristics tend to be further improved.

When the composite particles include spherical graphite particles, the flat graphite particles and the spherical graphite particles may be aggregated or bonded via a carbon substance. The carbon material may be, for example, graphite obtained by graphitizing a binder such as tar or pitch in the firing process. Whether or not the composite particles include spherical graphite particles can be confirmed, for example, by observation with a scanning electron microscope.

When the composite particles include spherical graphite particles, the total number of flat graphite particles and spherical graphite particles contained in one composite particle is not particularly limited. For example, it may be 3 or more, or 10 or more.

Examples of the spherical graphite particles include spherical artificial graphite and spherical natural graphite. From the viewpoint of obtaining a sufficient saturated tap density as the negative electrode material, the spherical graphite particles may be high-density graphite particles. Specifically, it may be spherical natural graphite that has been subjected to a particle spheroidization treatment so as to increase the tap density. Spherical natural graphite has the advantage that it has a high peel strength and is difficult to peel off from the current collector even if the electrode is pressed with a strong force, so it has a stronger peel strength by using composite particles containing spherical graphite particles. A negative electrode material tends to be obtained.

The average particle diameter (D50) of the spherical graphite particles is not particularly limited. For example, it can be selected from a range of 5 μm to 30 μm. The average particle diameter (D50) of the spherical graphite particles is measured in the same manner as D50 of the negative electrode material.

As a method of observing a cross-sectional image of spherical graphite particles when a negative electrode is produced using a negative electrode material, for example, a sample electrode (described later) or an electrode to be observed is embedded in an epoxy resin, and then mirror-polished to an electrode. A method of observing a cross section with a scanning electron microscope (for example, VE-7800, manufactured by Keyence Corporation), an electrode milling apparatus (for example, E-3500, manufactured by Hitachi High Technology Co., Ltd.), and preparing an electrode cross section for scanning. And a method of observation with a scanning electron microscope (for example, VE-7800, manufactured by Keyence Corporation).

The sample electrode has, for example, a mixture of 98 parts by mass of a negative electrode material, 1 part by mass of styrene butadiene resin as a binder, and 1 part by mass of carboxymethyl cellulose as a thickener, and the viscosity of this mixture at 25 ° C. is 1500 mPa · After adding water so as to be s-2500 mPa · s to prepare a dispersion, the dispersion is applied to a thickness of about 70 μm (at the time of application) on a copper foil having a thickness of 10 μm, It can be produced by drying at 120 ° C. for 1 hour.

The negative electrode material may contain, in addition to the composite particles, flat graphite particles or spherical graphite particles that do not form composite particles.

(Distance between graphite crystal layers d (002))
The negative electrode material has an interlayer distance d (002) of graphite crystals determined by X-ray diffraction measurement using CuKα rays of 3.38 mm or less, may be 3.37 mm or less, and is 3.36 mm or less. Also good. When the interlayer distance d (002) of the graphite crystal is 3.38 mm or less, the amount of lithium ions that can be inserted or removed between the hexagonal planes of carbon increases, and the discharge capacity tends to be improved. Although there is no particular limitation on the lower limit value of the interlayer distance d (002) of the graphite crystal, the theoretical value of d (002) of the pure graphite crystal is usually about 3.35 mm.

Specifically, the interlayer distance d (002) of the graphite crystal is determined by the diffraction profile obtained by irradiating the negative electrode material with X-rays (CuKα rays) and measuring the diffraction lines with a goniometer. From the diffraction peak corresponding to the d (002) plane appearing in the range of 26 degrees, it can be calculated using the Bragg equation.

The details of the X-ray diffraction measurement using CuKα rays are as follows.
-Measurement equipment and conditions-
X-ray diffractometer: MultiFlex, manufactured by Rigaku Corporation Goniometer: MultiFlex goniometer (without shutter)
Attachment: Standard specimen holder Monochromator: Fixed monochromator Scanning mode: 2θ / θ
Scan type: Continuous Output: 40kV, 40mA
Diverging slit: 1 degree Scattering slit: 1 degree Receiving slit: 0.30 mm
Monochrome light receiving slit: 0.8mm
Measurement range: 0 degrees ≤ 2θ ≤ 35 degrees Sampling width: 0.01 degrees

(Specific surface area)
Negative electrode material has a specific surface area by the BET method of nitrogen gas adsorption may be 1.0m ² /g~5.0m ² / ^g, even 3.0m ² /g~4.5m 2 / g Good.

The measurement of the specific surface area can be performed by the following method. For example, a negative electrode material is filled in a measurement cell, and a sample obtained by performing preheating treatment at 200 ° C. while vacuum degassing is used to apply nitrogen gas using a gas adsorption device (for example, ASAP2010, manufactured by Shimadzu Corporation). Adsorb. A BET analysis is performed on the obtained sample by a five-point method to calculate a specific surface area.

The specific surface area of the negative electrode material can be adjusted to the above range by adjusting the average particle diameter, for example. In addition, it exists in the tendency for a specific surface area to become large, so that an average particle diameter is small.

(True specific gravity)
The negative electrode material may have a true specific gravity of 2.22 or more, or 2.22 to 2.27. When the true specific gravity is 2.22 or more, the charge / discharge capacity per unit volume of the lithium ion secondary battery is increased, and the capacity tends to be increased. Further, when the true specific gravity is 2.22 or more, the crystallinity of graphite increases, and as a result, the reactivity with the electrolytic solution decreases and the initial charge / discharge efficiency tends to be improved.

Examples of the method for setting the true specific gravity of the negative electrode material to 2.22 or more include a method using natural graphite having high crystallinity and a method using artificial graphite having high crystallinity. In order to increase the crystallinity of graphite, for example, heat treatment may be performed at a temperature of 2000 ° C. or higher.
The true specific gravity can be measured by a butanol substitution method (JIS R 7212-1995) using a specific gravity bottle.

(Method for producing negative electrode material)
Although the manufacturing method in particular of the negative electrode for lithium secondary batteries is not restrict | limited, For example, it can manufacture as follows. At least a graphitizable aggregate or graphite and a graphitizable binder are mixed and pulverized, and then the pulverized product and a graphitization catalyst are mixed and fired to obtain graphite particles. Next, an organic binder and a solvent are added to the graphite particles and mixed to obtain a mixture. This mixture can be applied to a current collector, dried to remove the solvent, and then pressurized to be integrated.

As aggregates that can be graphitized, for example, coke, resin carbide, etc. can be used, but there is no particular limitation. The graphitizable aggregate is preferably in the form of particles, more preferably coke particles such as needle coke that are easily graphitized. As graphite, for example, natural graphite, artificial graphite and the like can be used, but there is no particular limitation. The graphite is preferably in the form of particles.

The particle diameter of the graphitizable aggregate or graphite is preferably smaller than the particle diameter of the graphite particles to be produced, the average particle diameter is more preferably 1 μm to 80 μm, still more preferably 1 μm to 50 μm, A thickness of 5 μm to 50 μm is particularly preferable. Further, the aspect ratio of the graphitizable aggregate or graphite particles is preferably 1.2 to 500, more preferably 1.5 to 300, and further preferably 1.5 to 100. It is preferably 2 to 50, particularly preferably. Here, the aspect ratio is measured by the same method as described above. When the aspect ratio of graphitizable aggregate or graphite particles increases, the diffraction intensity ratio (002) / (110) measured by X-ray diffraction of the negative electrode after pressurization and integration tends to increase. If it is 1.2 or more, the discharge capacity per graphite particle mass tends to be sufficiently secured. The average particle diameter of the graphitizable aggregate or graphite is measured in the same manner as the average particle diameter (D50) of the negative electrode material.

Examples of the binder include tar, pitch, and organic materials (thermosetting resin, thermoplastic resin, etc.). The blending amount of the binder is preferably 5% by mass to 80% by mass, more preferably 10% by mass to 80% by mass, and more preferably 20% by mass to 80% by mass with respect to the graphitizable aggregate or graphite. %, More preferably 30% by mass to 80% by mass. When the amount of the binder is within the above range, the aspect ratio and specific surface area of the graphite particles to be produced tend to be easily controlled within a desired range. The method of mixing the aggregate that can be graphitized or graphite and the binder is not particularly limited, and can be performed using, for example, a kneader. The mixing is preferably performed at a temperature equal to or higher than the softening point of the binder. Specifically, for example, when the binder is pitch, tar or the like, 50 ° C. to 300 ° C. is preferable, and when the binder is a thermosetting resin, 20 ° C. to 180 ° C. is preferable.

Next, the mixture is pulverized, and the obtained pulverized product and the graphitization catalyst are mixed. The particle size of the pulverized product is preferably 1 μm to 100 μm, more preferably 5 μm to 80 μm, still more preferably 5 μm to 50 μm, and particularly preferably 10 μm to 30 μm. When the particle diameter of the pulverized product is 100 μm or less, the specific surface area of the obtained graphite particles tends not to be too large, and when it is 1 μm or more, the (002) / (110) ratio of the obtained negative electrode is too large. There is no tendency.

The proportion of volatile matter in the pulverized product is preferably 0.5% by mass to 50% by mass of the entire pulverized product, more preferably 1% by mass to 30% by mass, and 5% by mass to 20% by mass. More preferably. The proportion of volatile matter can be determined from the mass reduction rate when the pulverized product is heated at 800 ° C. for 10 minutes.

The graphitization catalyst mixed with the pulverized product is not particularly limited as long as it has a function as a graphitization catalyst. For example, metals or semimetals such as iron, nickel, titanium, silicon, and boron, compounds containing these (carbides, oxides, and the like) can be used. Of these, compounds containing iron or silicon are preferred. The chemical structure of the compound is preferably a carbide. The graphitization catalyst is preferably in the form of particles, more preferably in the form of particles having an average particle diameter of 0.1 μm to 200 μm, more preferably in the form of particles having an average particle diameter of 1 μm to 100 μm, Particularly preferred is a particulate form having a diameter of 1 μm to 50 μm. The average particle diameter of the graphitization catalyst is measured in the same manner as the average particle diameter (D50) of the negative electrode material.

The addition amount of the graphitization catalyst is preferably 1% by mass to 50% by mass when the total amount of the pulverized product mixed with the graphitization catalyst and the graphitization catalyst is 100% by mass, and 5% by mass to 30% by mass. More preferably, the content is 7% by mass to 20% by mass. When the amount of the graphitization catalyst is 1% by mass or more, the crystals of the produced graphite particles tend to develop well and the specific surface area tends not to be too large. When the amount is 50% by mass or less, The graphitization catalyst tends not to remain.

Next, the above mixture is fired and graphitized. Before firing, the mixture of the pulverized product and the graphitization catalyst may be formed into a predetermined shape by a press or the like. The molding pressure in this case is preferably about 1 MPa to 300 MPa. The firing is preferably performed under conditions where the mixture is not easily oxidized. For example, baking is preferably performed in a nitrogen atmosphere, an argon atmosphere, a vacuum, a self-volatile atmosphere, or the like. The graphitization temperature is preferably 2000 ° C. or higher, more preferably 2500 ° C. or higher, further preferably 2700 ° C. or higher, and particularly preferably 2800 ° C. to 3200 ° C. When the graphitization temperature is 2000 ° C. or higher, the development of graphite crystals tends to be promoted and a sufficient discharge capacity tends to be obtained, and the added graphitization catalyst tends not to remain in the graphite particles produced. is there. If the amount of graphitization catalyst remaining in the graphite particles is too large, the discharge capacity per mass of the graphite particles tends to decrease. The upper limit of the graphitization temperature is not particularly limited, but it is preferable that the graphitization does not occur.

When mixtures performing graphitization treatment while molding the apparent density of the molded product after graphitization is preferably 1.65 g / cm ³ or less, more preferably 1.55 g / cm ³ or less, more preferably 1.50 g / cm ³ or less, particularly preferably 1.45 g / cm ³ or less. When the density of the molded product after graphitization is 1.65 g / cm ³ or less, the specific surface area of the produced graphite particles tends not to be too large. The apparent density of the molded product after graphitization can be appropriately adjusted by, for example, the particle diameter of the pulverized product mixed with the graphitization catalyst, the pressure when forming into a predetermined shape by a press or the like.

After graphitization, pulverize and adjust particle size to make negative electrode material. The pulverization method is not particularly limited, and for example, an impact pulverization method such as a jet mill, a hammer mill, or a pin mill can be adopted.

If necessary, a step of attaching an organic compound to the surface of the pulverized product after the graphitization treatment and baking (hereinafter, also referred to as “coating step”) may be performed.
In the coating step, an organic compound is attached to the surface of the obtained pulverized product and fired. The organic compound attached to the surface of the pulverized product is changed to a low crystalline carbon material by attaching the organic compound to the pulverized product and baking. Thereby, a low crystalline carbon substance is coat | covered to a part or all of the surface of a ground material. Graphite having high crystallinity has a structure in which carbons having SP ² hybrid orbitals are regularly arranged, and the number of entrances and exits of lithium ions may not be sufficient. On the other hand, the low crystalline carbon material has many layers of lithium ions because it has a turbulent structure. Therefore, by covering part or all of the surface of the pulverized product with the low crystalline carbon material, the input / output characteristics such as rapid charging tend to be improved.

The method for attaching the organic compound to the surface of the pulverized product is not particularly limited. Specifically, a wet method in which a pulverized product is dispersed and mixed in a mixed solution in which an organic compound is dissolved or dispersed in a solvent and then the solvent is removed and adhered, and the pulverized product and a solid organic compound are mixed. Examples include a dry method in which mechanical energy is applied to the obtained mixture for adhesion, a method in which a mixture obtained by mixing a pulverized product and a solid organic compound is baked in an inert atmosphere, and a gas phase method such as a CVD method. It is done.

The organic compound is not particularly limited as long as the organic compound changes into a low crystalline carbon material by firing (carbon precursor). Specific examples include petroleum pitch, naphthalene, anthracene, phenanthrolen, coal tar, phenol resin, polyvinyl alcohol, and the like. An organic compound may be used individually by 1 type, or may use 2 or more types together.

The temperature at which the pulverized product with the organic compound attached to the surface is baked is not particularly limited as long as the organic compound attached to the surface of the pulverized product is carbonized. For example, the firing temperature may be in the range of 750 ° C. to 2000 ° C. Firing is preferably performed in an inert gas atmosphere such as a nitrogen atmosphere.

The negative electrode material may contain carbonaceous particles or occluded metal particles that are different in shape and physical properties from the composite particles and graphite particles described above. Examples of the carbonaceous particles include natural graphite particles, artificial graphite particles, graphite particles coated with a low crystalline carbon material, resin-coated graphite particles, and amorphous carbon particles. Examples of the occluded metal particles include particles containing an element that forms an alloy with lithium, such as Al, Si, Ga, Ge, In, Sn, Sb, and Ag.

<Anode for lithium ion secondary battery>
A negative electrode for a lithium ion secondary battery (hereinafter, also referred to as “negative electrode”) according to an embodiment of the present disclosure includes a current collector and a negative electrode material layer including the above-described negative electrode material formed on the current collector. Have.

The material and shape of the current collector are not particularly limited. For example, a belt-shaped foil made of a metal or an alloy such as aluminum, copper, nickel, titanium, and stainless steel, a belt-shaped perforated foil, a belt-shaped mesh, and the like can be given. In addition, porous materials such as porous metal (foam metal) and carbon paper can be used as the current collector.

The method for forming the negative electrode material layer on the current collector is not particularly limited. For example, the negative electrode material composition is collected by a known method such as a metal mask printing method, electrostatic coating method, dip coating method, spray coating method, roll coating method, doctor blade method, gravure coating method, or screen printing method. It can be applied and formed on top. When integrating a negative electrode material layer and a collector, it can carry out by well-known methods, such as a roll, a press, and these combination.

As the negative electrode material composition, for example, a material containing the above-described negative electrode material, an organic binder, and a solvent can be used. The negative electrode material composition may be in a state such as a slurry or a paste.

The organic binder contained in the negative electrode material composition is not particularly limited. For example, styrene-butadiene rubber; ethylenically unsaturated carboxylic acid ester (such as methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, hydroxyethyl (meth) acrylate)) and ethylenically unsaturated (Meth) acrylic copolymers derived from saturated carboxylic acids (acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, etc.); polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, Examples thereof include polymer compounds such as polyimide and polyamideimide. (Meth) acrylate means acrylate or methacrylate, and (meth) acrylonitrile means acrylonitrile or methacrylonitrile.

The solvent contained in the negative electrode material composition is not particularly limited. Examples thereof include organic solvents such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide, and γ-butyrolactone.

The negative electrode material composition may contain a thickener for adjusting the viscosity, if necessary. Examples of the thickening material include carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, polyacrylic acid and salts thereof, oxidized starch, phosphorylated starch, and casein.

The negative electrode material composition may contain a conductive aid as necessary. Examples of the conductive aid include carbon black, graphite, acetylene black, conductive oxide, and conductive nitride.

When the negative electrode material composition is applied on the current collector to form the negative electrode material layer, heat treatment may be performed as necessary. By performing the heat treatment, the solvent is removed, the strength of the organic binder is increased by hardening, and the adhesion between the particles and between the particles and the current collector tends to be improved. The heat treatment may be performed in an inert atmosphere such as helium, argon, nitrogen, or a vacuum atmosphere in order to prevent oxidation of the current collector during the treatment.

The negative electrode may be subjected to pressure treatment (pressing) for densification. By performing the pressure treatment, the electrode density can be adjusted to a desired range. Electrode density ^may be 1.5g / cm 3 ~ 1.9g / cm 3, may be 1.6g / cm 3 ~ 1.8g / cm 3.

<Lithium ion secondary battery>
A lithium ion secondary battery according to an embodiment of the present disclosure has the above-described negative electrode. The lithium ion secondary battery may have a configuration in which, for example, a negative electrode and a positive electrode are arranged to face each other with a separator interposed therebetween, and an electrolytic solution containing an electrolyte is injected.

The positive electrode can be obtained by forming a positive electrode material layer on the current collector surface in the same manner as the negative electrode. As the current collector, a strip-shaped foil, strip-shaped punched foil, strip-shaped mesh or the like made of a metal or alloy such as aluminum, titanium, or stainless steel can be used.

The positive electrode material used for the positive electrode material layer is not particularly limited. Examples of the positive electrode material include metal compounds, metal oxides, metal sulfides, and conductive polymer materials that can be doped or intercalated with lithium ions. Specifically, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), and double oxides thereof (LiCo _x Ni _y Mn _z O ₂ , x + y + z = 1, 0 <X, 0 <y; LiNi _2-x Mn _x O ₄ , 0 <x ≦ 2), lithium manganese spinel (LiMn ₂ O ₄ ), lithium vanadium compound, V ₂ O ₅ , V ₆ O ₁₃ , VO ₂ , MnO ₂ , TiO ₂ , MoV ₂ O ₈ , TiS ₂ , V ₂ S ₅ , VS ₂ , MoS ₂ , MoS ₃ , Cr ₃ O ₈ , Cr ₂ O ₈ , olivine type LiMPO ₂ (M is Co, Ni, Mn Or Fe), conductive polymers (polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, etc.), porous carbon, etc. It is. These positive electrode materials can be used individually by 1 type or in combination of 2 or more types. Among them, lithium nickelate (LiNiO ₂ ) and its double oxide (LiCo _x Ni _y Mn _z O ₂ , x + y + z = 1, 0 <x, 0 <y; LiNi _2−x Mn _x O ₄ , 0 <x ≦ 2 ) Is suitable as a positive electrode material because of its high capacity.

Examples of the separator include non-woven fabrics, cloths, microporous films, combinations thereof, and the like mainly composed of polyolefins such as polyethylene and polypropylene. In addition, when a lithium ion secondary battery has a structure where a positive electrode and a negative electrode do not contact, it is not necessary to use a separator.

As the electrolytic solution, a so-called organic electrolytic solution in which an electrolyte is dissolved in a non-aqueous solvent can be used. Examples of the electrolyte include lithium salts such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , and LiSO ₃ CF ₃ . Non-aqueous solvents include ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentanone, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidine- 2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyl Tetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate and the like can be mentioned. The electrolyte and the non-aqueous solvent can be used singly or in combination of two or more. Among them, the electrolytic solution containing fluoroethylene carbonate tends to form a stable SEI (solid electrolyte interface) on the surface of the negative electrode material, and is suitable for improving charge / discharge cycle characteristics.

The form of the lithium ion secondary battery is not particularly limited, and examples include a paper battery, a button battery, a coin battery, a stacked battery, a cylindrical battery, and a square battery. In addition to the lithium ion secondary battery, the negative electrode material described above can be applied to any electrochemical device such as a hybrid capacitor that has a charge / discharge mechanism that inserts and desorbs lithium ions.

Hereinafter, the embodiment will be described in more detail based on examples. The above embodiment is not limited to the following examples.

[Preparation of Anode Material A]
50 parts by mass of coke powder having an average particle size of 25 μm and 30 parts by mass of coal tar pitch were mixed at 230 ° C. for 2 hours. The mixture was then pulverized to an average particle size of 25 μm. Thereafter, 80 parts by mass of the pulverized product and 20 parts by mass of silicon carbide having an average particle diameter of 25 μm were mixed with a blender to obtain a mixture. This mixture was put into a mold and press-molded at 100 MPa to form a rectangular parallelepiped. This molded body was heat-treated at 1000 ° C. in a nitrogen atmosphere, and further heat-treated at 3000 ° C. in a nitrogen atmosphere to obtain a graphitized molded body. Further, the graphitized molded body was pulverized to obtain graphite particles (negative electrode material A).

[Preparation of negative electrode material B]
50 parts by mass of coke powder having an average particle size of 10 μm and 30 parts by mass of coal tar pitch were mixed at 230 ° C. for 2 hours. The mixture was then pulverized to an average particle size of 25 μm. Thereafter, 80 parts by mass of the pulverized product and 20 parts by mass of silicon carbide having an average particle diameter of 25 μm were mixed with a blender to obtain a mixture. This mixture was put into a mold and press-molded at 100 MPa to form a rectangular parallelepiped. This molded body was heat-treated at 1000 ° C. in a nitrogen atmosphere, and further heat-treated at 3000 ° C. in a nitrogen atmosphere to obtain a graphitized molded body. Furthermore, this graphitized shaped body was pulverized to obtain graphite particles (negative electrode material B).

[Preparation of negative electrode material C]
Negative electrode material A and negative electrode material B were mixed so that the mass ratio (negative electrode material A: negative electrode material B) was 50:50 to obtain negative electrode material C.

Table 2 shows the results of measuring D10, D50, D90 and Q3 of the negative electrode material obtained above. In addition, Table 2 shows the results of measurement of specific surface area, interlayer distance d (002) of graphite crystals, and true specific gravity. Each measurement was performed by the method described above. In addition, Table 2 shows the results of measuring the injection time by the method described later. In addition, when the negative electrode material was observed with a scanning electron microscope, a plurality of flat graphite particles contained composite particles that were assembled or bonded so that the orientation planes were non-parallel.

The injection time of the negative electrode material was measured as follows. 98 parts by mass of the negative electrode material obtained above, 1 part by mass of styrene-butadiene rubber (BM-400B, manufactured by Nippon Zeon Co., Ltd.), and 1 part by mass of carboxymethyl cellulose (CMC2200, manufactured by Daicel Corporation) were kneaded to form a slurry. A negative electrode material composition was prepared.
This was dried at 105 ° C. and ground using a mortar. Next, the pulverized powder was sieved with a 200-mesh standard sieve to prepare a measurement sample. The obtained measurement sample was tableted using a tablet molding machine (tablet bottom area: 1.327 cm ² ). Specifically, 1.0 g of a measurement sample was put into a tablet molding machine, and a pressure at which a tablet had a predetermined density (1.75 g / cm ³ ) was applied for 30 seconds. Next, an electrolytic solution (mixed solution of ethylene carbonate / ethyl methyl carbonate (volume ratio: 3/7) containing 1.0 M LiPF ₆ and vinylene carbonate (0.5% by mass)) is applied to the surface of the prepared tablet. 130 μL was dropped and the time until the electrolyte soaked was measured.

[Production of negative electrode]
98 parts by mass of the negative electrode material obtained above, 1 part by mass of styrene-butadiene rubber (BM-400B, manufactured by Nippon Zeon Co., Ltd.), and 1 part by mass of carboxymethyl cellulose (CMC2200, manufactured by Daicel Corporation) were kneaded to form a slurry. A negative electrode material composition was prepared. This was applied to a current collector (copper foil having a thickness of 10 μm) and dried in the air at 105 ° C. for 1 hour to form a negative electrode material layer. Next, pressure treatment was performed by a roll press so that the electrode density was 1.70 g / cm ³ , and the negative electrode material layer was integrated with the current collector to produce a negative electrode.

[Production of lithium ion secondary battery]
A lithium ion secondary battery (2016 type coin cell) was produced using the negative electrode obtained above and metallic lithium as the positive electrode. As the electrolytic solution, a mixed solution of ethylene carbonate / ethyl methyl carbonate (volume ratio: 3/7) containing 1.0 M LiPF ₆ and vinylene carbonate (0.5% by mass) was used. As the separator, a polyethylene microporous film having a thickness of 25 μm was used. As the spacer, a circular copper plate having a thickness of 230 μm and a diameter of 14 mm was used.

[First charge / discharge capacity]
Measurement of initial charge / discharge capacity (mAh / g) is as follows: sample mass: 15.4 mg, electrode area: 1.54 cm ² , measurement temperature: 25 ° C., electrode density: 1.70 g / cm ³ , CC-CV charge conditions: Constant current charge 0.543 mA, constant voltage charge 0 V (Li / Li ⁺ ), cut current 0.053 mA, CC discharge condition: constant current discharge 0.543 mA, cut voltage 1.5 V (Li / Li ⁺ ) It was. The results are shown in Table 3.

[First-time charge / discharge efficiency]
The initial charge / discharge efficiency (%) was determined by the following formula. The results are shown in Table 3.
Initial charge / discharge efficiency = initial discharge capacity / initial charge capacity x 100

[Irreversible capacity]
The irreversible capacity (mAh / g) was determined by the following formula. The results are shown in Table 3.
Irreversible capacity = initial charge capacity-initial discharge capacity

[Charge capacity maintenance rate at low temperature]
(1) Sample mass: 15.4 mg, electrode area: 1.54 cm ² , electrode density: 1.70 g / cm ³ , measurement temperature: 25 ° C., CC-CV charge condition: constant current charge 0.543 mA, constant voltage charge Charge / discharge was performed for 2 cycles under the conditions of 0 V (Li / Li ⁺ ), cut current 0.053 mA, CC discharge conditions: constant current discharge 0.543 mA, cut voltage 1.5 V (Li / Li ⁺ ).
(2) Next, at a measurement temperature of 0 ° C., CC-CV charge condition: constant current charge 0.543 mA, constant voltage charge 0 V (Li / Li ⁺ ), cut current 0.053 mA, CC discharge condition: constant current discharge 0 The battery was charged and discharged for one cycle under the conditions of .543 mA and a cut voltage of 1.5 V (Li / Li ⁺ ).
(3) The capacity | capacitance of CC charge capacity (mAh / g) in 25 degreeC and 0 degreeC was calculated | required, and the charge capacity maintenance factor in low temperature conditions was computed with the following formula. The results are shown in Table 3.
Charge capacity maintenance rate under low temperature condition = 0 CC charge capacity at 0 ° C./CC charge capacity at 25 ° C. × 100

As shown in the results of Table 3, the lithium ion secondary battery of the example using the negative electrode material whose volume-based particle size distribution satisfies the above-described conditions is the negative electrode material whose volume-based particle size distribution does not satisfy the above-described conditions. Compared to the lithium ion secondary battery of the comparative example using the battery, the charge / discharge capacity maintenance rate under a low temperature condition was high, and the stability of the charging performance was excellent. In addition, the initial charge / discharge efficiency of the lithium ion secondary battery of the example was the same level as the lithium ion secondary battery of the comparative example.

Claims (7)

1. A negative electrode material for a lithium ion secondary battery satisfying the following (1) to (4) in a volume-based particle size distribution.
   (1) The particle diameter (D10) when the accumulation from the small diameter side is 10% is 5 μm to 14 μm. (2) The particle diameter (D50) when the accumulation from the small diameter side is 50% is 15 μm to 27 μm. (3) The particle diameter (D90) when the accumulation from the small diameter side is 90% is 20 μm to 55 μm. (4) The integrated value Q3 of the particle diameter of 9.516 μm or less is 4% to 30%. .
2. The negative electrode material for a lithium ion secondary battery according to claim 1, comprising particles in which a plurality of flat graphite particles are aggregated or bonded so that their orientation planes are non-parallel.
3. 3. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein an interlayer distance d (002) of the graphite crystal obtained by X-ray diffraction measurement using CuKα rays is 3.38 mm or less.
4. The specific surface area by BET method of nitrogen gas adsorption is 1.0m ² /g~5.0m ² / g, according to claim 1 negative electrode material for a lithium ion secondary battery according to any one of claims 3.
5. The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the true specific gravity is 2.22 or more.
6. A lithium ion secondary battery comprising: a current collector; and a negative electrode material layer including the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 5 formed on the current collector. Negative electrode.
7. A lithium ion secondary battery having the negative electrode for a lithium ion secondary battery according to claim 6.