JPWO2018198377A1 - Anode material for lithium ion secondary battery, anode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

A negative electrode material for a lithium ion secondary battery that satisfies 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 becomes 10% is 5 μm to 14 μm. (2) The particle diameter (D50) when the accumulation from the small diameter side becomes 50% is 15 μm to 27 μm. (3) The particle diameter (D90) when the accumulation from the small diameter side becomes 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%. .

Description

  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 downsizing, longer life for electric vehicles and power storage applications, and shorter charging times (improving rapid charging characteristics) have been made. Is required for lithium ion secondary batteries. For example, in Patent Literature 1, graphite particles 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 are used as a negative electrode active material. The discharge cycle characteristics are improved.

JP-A-10-158005

  While the use scene of the lithium ion secondary battery is diversifying, in the case of the lithium ion secondary battery, if the temperature of the use environment is low, the charge capacity retention rate decreases, and stable charge 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 charge capacity stability. Make it 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 becomes 10% is 5 μm to 14 μm. (2) The particle diameter (D50) when the accumulation from the small diameter side becomes 50% is 15 μm to 27 μm. (3) The particle diameter (D90) when the accumulation from the small diameter side becomes 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>, wherein the negative electrode material includes particles in which a plurality of flat graphite particles are aggregated or bonded such that their orientation planes are non-parallel.
<3> The negative electrode material for a lithium ion secondary battery according to <1> or <2>, wherein the interlayer distance d (002) of the graphite crystal determined by X-ray diffraction measurement using CuKα radiation is 3.38 ° 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> Anode 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> 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, the negative electrode for lithium ion secondary batteries, and the lithium ion secondary battery which can obtain the lithium ion secondary battery which is excellent in charge capacity stability are provided. You.

  Hereinafter, an example of an embodiment of 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 according to the present disclosure will be described in detail. However, the present disclosure is not limited to the following embodiments. In the following embodiments, the components (including the element steps and the like) are not essential unless otherwise specified. The same applies to numerical values and their ranges, and does not limit the scope of the present disclosure.

In the present disclosure, the term "step" includes, in addition to a step independent of other steps, even if the purpose of the step is achieved even if it cannot be clearly distinguished from the other steps, the step is also included. .
In the present disclosure, the numerical ranges indicated by using “to” include the numerical values described before and after “to” as the minimum value and the maximum value, respectively.
In the numerical ranges described in stages in the present disclosure, the upper limit or lower limit described in one numerical range may be replaced with the upper limit or lower limit of the numerical range described in other stages. . Further, in the numerical range described in the present disclosure, the upper limit or the lower limit of the numerical range may be replaced with the value shown in the embodiment.
In the present disclosure, the content or content of each component in the composition is, when there are a plurality of substances corresponding to each component in the composition, unless otherwise specified, the plurality of substances present in the composition Means the total content or content.
In the present disclosure, the particle diameter of each component in the composition, when there are a plurality of types of particles corresponding to each component in the composition, unless otherwise specified, for a mixture of the plurality of types of particles present in the composition Means the value of
In the present disclosure, the term "layer" or "film" means that when a region where the layer or film is present is observed, in addition to a case where the layer or film is formed over the entire region, only a part of the region The case where it is formed is also included.
In the present disclosure, the term "lamination" refers to stacking layers, where two or more layers may be joined, or two or more layers may be removable.

<Negative electrode material for lithium ion secondary batteries>
A negative electrode material for a lithium ion secondary battery (hereinafter, also simply referred to as a “negative electrode material”) according to an embodiment of the present disclosure satisfies 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 becomes 10% is 5 μm to 14 μm. (2) The particle diameter (D50) when the accumulation from the small diameter side becomes 50% is 15 μm to 27 μm. (3) The particle diameter (D90) when the accumulation from the small diameter side becomes 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%. .

  The present inventors have found that a lithium ion secondary battery obtained by using a negative electrode material satisfying the above conditions has a high charge capacity maintenance ratio even under a low-temperature environment and has excellent charge performance stability. Although the reason is not clear, for example, since the path for the electrolyte solution to move in the negative electrode material is sufficiently secured, even if the viscosity of the electrolyte solution increases due to the temperature drop, the electrolyte solution is likely to move. It is presumed.

  For example, the following two reasons can be considered as the reason that the path of the electrolyte in the negative electrode material is sufficiently ensured. First, since the negative electrode material contains the small-diameter particles and the large-diameter particles in a ratio satisfying the above-mentioned particle size distribution conditions, the pressure applied for increasing the density of the negative electrode is higher than when the ratio of the small-diameter particles is larger than this. Is likely to be transmitted to the entire negative electrode, and it is conceivable that variation in the degree of crushing of particles between the vicinity of the negative electrode surface and the inside of the negative electrode is suppressed. Further, it is conceivable that the number of electrolyte solution paths is sufficiently ensured compared to the case where the ratio of 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 may be 18 μm to 23 μm. In some embodiments, 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 analyzer (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 the mixture was irradiated with ultrasonic waves for 30 seconds to be dispersed.

The particle size distribution of the negative electrode material is obtained by dividing the range of particle diameters from 0.1 μm to 2000 μm by 50 in logarithmic ratio. For example, particle size, obtains the ^{n = (2000 / 0.1) 1/50} , 0.1 × n, 0.1 × n 2, ···, obtained from 0.1 × ^{n 50.} The total value of the relative particle amounts in each particle size range in the range of 0.1 μm to 2000 μm is 100 (%). Specifically, the range of the particle diameter 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. Further, it 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, and the plurality of flat graphite particles may be included. May be particles that are deformed so that their orientation planes are non-parallel. Examples of the 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 flaky natural graphite.

(Composite particles)
The flat graphite particles contained in the composite particles are non-spherical particles having shape anisotropy. Examples of the flat graphite particles include graphite particles having a shape such as a scale, a scale, and a partial lump. The flat graphite particles may have an aspect ratio represented by A / B of 1.2 to 5 when the length in the major axis direction is A and the length in the minor axis direction is B, 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 projection image of the graphite particles observed using a microscope, a two parallel tangents circumscribing the outer periphery of the graphite particles, by selecting the tangent line a ₁ and tangential a ₂ where the distance is maximum , the distance between the tangent line a ₁ and the tangent a ₂ of major axis length a. In addition, the length of the longest line segment perpendicular to the long axis and connecting two points on the outline of the projected image of the graphite particles is defined as the length B in the short axis direction.

The non-parallel orientation plane of the flat graphite particles means that the plane (orientation plane) parallel to the plane having the largest cross-sectional area of the flat graphite particles is 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. Since a plurality of flat graphite particles are gathered or bonded in a state where the orientation planes are not parallel to each other, it is possible to suppress an increase in the orientation of the particles on the electrode, and to 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 such that the orientation planes of the flat graphite particles are parallel.

  The state in which a plurality of flat graphite particles are aggregated or bonded refers to a state in which two or more flat graphite particles are aggregated or bonded. Bonding refers to a state in which the particles are chemically bonded directly or via a carbon material. The term “assembly” refers to a state in which particles are not chemically bonded to each other, but maintain the shape as an aggregate due to the shape and the like. The flat graphite particles may be aggregated or bonded via a carbon material. The carbon material may be, for example, graphite obtained by graphitizing a binder such as tar or pitch in a firing step. From the viewpoint of mechanical strength, two or more flat graphite particles may be bonded via a carbon material. 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 may be 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 connection. 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. Above all, artificial graphite, natural graphite, or graphite obtained from coke has high crystallinity and becomes soft particles, and thus tends to easily increase the density of the negative electrode.

The composite particles may further include spherical graphite particles. In general, spherical graphite particles have a higher density than flat graphite particles, so that the composite particles include spherical graphite particles, so that the density of the negative electrode material can be increased. The pressure can be reduced. As a result, the flat graphite particles are suppressed from being oriented in the direction along the surface of the current collector, and the movement of lithium ions tends to be more favorable. 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 electrolyte into the negative electrode material layer is further increased, and the discharge capacity and the charge capacity are reduced. 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 material. The carbon material may be, for example, graphite obtained by graphitizing a binder such as tar or pitch in a firing step. Whether or not the composite particles contain 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, the number may be three or more, or ten 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 a 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 spheroidizing treatment so that a high tap density can be achieved. Spherical natural graphite has a strong peeling strength, and has a feature that it is hard to be peeled off from the current collector even when the electrode is pressed with a strong force.By using composite particles containing spherical graphite particles, it has a stronger peeling strength. 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 the 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 manufactured using a negative electrode material, for example, after embedding a sample electrode (described later) or an electrode to be observed in an epoxy resin, mirror-polishing the electrode, A method of observing the cross section with a scanning electron microscope (for example, VE-7800, manufactured by KEYENCE CORPORATION), and forming and scanning an electrode cross section using an ion milling apparatus (for example, E-3500, manufactured by Hitachi High Technology Co., Ltd.) Observation method using a scanning electron microscope (for example, VE-7800, manufactured by KEYENCE CORPORATION).

  The sample electrode has a solid content of, for example, a mixture of 98 parts by mass of the negative electrode material, 1 part by mass of a styrene butadiene resin as a binder, and 1 part by mass of carboxymethyl cellulose as a thickener, and has a viscosity at 25 ° C. of 1500 mPa · s to 2500 mPa · s to prepare a dispersion by adding water, and applying the dispersion to a thickness of about 70 μm (at the time of coating) 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 include flat graphite particles or spherical graphite particles that do not form composite particles, in addition to the composite particles.

(Distance between graphite crystals d (002))
In the negative electrode material, the interlayer distance d (002) of graphite crystals determined by X-ray diffraction measurement using CuKα radiation is 3.38 ° or less, and may be 3.37 ° or less, or 3.36 ° or less. Is also good. When the interlayer distance d (002) of the graphite crystal is 3.38 ° or less, the amount of lithium ions that can be inserted or removed between the carbon hexagonal mesh planes increases, and the discharge capacity tends to be improved. The lower limit of the interlayer distance d (002) of the graphite crystal is not particularly limited, but the theoretical value of d (002) of the pure graphite crystal is usually about 3.35 °.

  More specifically, the interlayer distance d (002) of the graphite crystal is determined by irradiating the negative electrode material with X-rays (CuKα rays) and measuring the diffraction line with a goniometer, and the diffraction angle 2θ is 24 degrees or more. 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.
-Measuring equipment and conditions-
X-ray diffractometer: MultiFlex, Rigaku Corporation Goniometer: MultiFlex goniometer (without shutter)
Attachment: Standard sample holder Monochromator: Fixed monochromator Scanning mode: 2θ / θ
Scan type: continuous Output: 40kV, 40mA
Divergence slit: 1 degree Scattering slit: 1 degree Receiving slit: 0.30 mm
Monochrome receiving slit: 0.8mm
Measuring 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 sample obtained by filling a negative electrode material in a measurement cell and performing a heating pretreatment at 200 ° C. while degassing in a vacuum is charged with nitrogen gas using a gas adsorption device (for example, ASAP2010, manufactured by Shimadzu Corporation). Adsorb. 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 set in the above range by adjusting the average particle diameter, for example. The specific surface area tends to increase as the average particle diameter decreases.

(True specific gravity)
The negative electrode material may have a true specific gravity of 2.22 or more, or may have a specific gravity of 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 increases, and the capacity tends to be easily increased. Further, when the true specific gravity is 2.22 or more, the crystallinity of the graphite is increased, so that the reactivity with the electrolytic solution is reduced, 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 displacement method using a specific gravity bottle (JIS R 7212-1995).

(Production method of negative electrode material)
The method for producing the negative electrode for a lithium secondary battery is not particularly limited, but can be produced, for example, as follows. After at least graphitizable aggregate or graphite and a graphitizable binder are mixed and pulverized, the pulverized material and the graphitizing catalyst are mixed and calcined to obtain graphite particles. Next, an organic binder and a solvent are added to the graphite particles and mixed to obtain a mixture. The mixture can be applied to a current collector, dried to remove the solvent, and then pressurized to be integrated.

  As the graphitizable aggregate, for example, coke, carbide of resin, and the like can be used, but there is no particular limitation. The graphitizable aggregate is preferably in the form of particles, and more preferably coke particles such as needle coke which are easily graphitized. As the 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 size of the graphitizable aggregate or graphite is preferably smaller than the particle size of the graphite particles to be produced, the average particle size is more preferably 1 μm to 80 μm, further preferably 1 μm to 50 μm, It is particularly preferred that it is 5 μm to 50 μm. The aspect ratio of the graphitizable aggregate or graphite particles is preferably from 1.2 to 500, more preferably from 1.5 to 300, and even more preferably from 1.5 to 100. It is particularly preferably 2 to 50. Here, the measurement of the aspect ratio is performed in the same manner as described above. When the aspect ratio of the graphitizable aggregate or graphite particles increases, the diffraction intensity ratio (002) / (110) measured by X-ray diffraction of the negative electrode after pressing and integration tends to increase, When it is 1.2 or more, the discharge capacity per graphite particle mass tends to be sufficiently ensured. The average particle size of the graphitizable aggregate or graphite is measured in the same manner as the average particle size (D50) of the negative electrode material.

  Examples of the binder include tar, pitch, and organic materials (such as thermosetting resin and thermoplastic resin). The compounding 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 in the above range, the aspect ratio and the specific surface area of the graphite particles to be produced tend to be easily controlled in a desired range. The method of mixing the graphitizable aggregate 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, the temperature is preferably 50C to 300C, and when the binder is a thermosetting resin, the temperature is preferably 20C to 180C.

  Next, the above mixture is pulverized, and the obtained pulverized substance and a graphitization catalyst are mixed. The particle size of the pulverized product is preferably from 1 μm to 100 μm, more preferably from 5 μm to 80 μm, even more preferably from 5 μm to 50 μm, particularly preferably from 10 μm to 30 μm. If the particle size 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 if it is 1 μm or more, the (002) / (110) ratio of the obtained negative electrode becomes too large. There is no tendency.

  The proportion of volatile matter in the pulverized material is preferably 0.5% by mass to 50% by mass, more preferably 1% by mass to 30% by mass, and more preferably 5% by mass to 20% by mass of the whole pulverized material. Is more preferable. The volatile content is determined from the mass reduction rate when the pulverized material is heated at 800 ° C. for 10 minutes.

  The graphitization catalyst to be mixed with the pulverized material is not particularly limited as long as it has a function as a graphitization catalyst. For example, metals or metalloids such as iron, nickel, titanium, silicon, and boron, and compounds containing these (such as carbides and oxides) can be used. Among these, compounds containing iron or silicon are preferred. Further, 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, and still more preferably in the form of particles having an average particle diameter of 1 μm to 100 μm. It is particularly preferable that the particles have 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, and more preferably 5% by mass to 30% by mass, when the total amount of the pulverized material mixed with the graphitization catalyst and the graphitization catalyst is 100% by mass. Is more preferable, and it is still more preferable that it is 7 to 20 mass%. When the amount of the graphitization catalyst is 1% by mass or more, the crystal growth of the graphite particles to be produced tends to be good and the specific surface area does not tend to be too large. The graphitization catalyst tends to hardly remain.

  Next, the above-mentioned mixture is baked and graphitized. Before firing, a mixture of the pulverized product and the graphitization catalyst may be formed into a predetermined shape by pressing 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 hardly oxidized. For example, baking is preferably performed in a nitrogen atmosphere, an argon atmosphere, a vacuum, a self-volatile atmosphere, or the like. The temperature of the graphitization treatment is preferably 2000 ° C. or higher, more preferably 2500 ° C. or higher, even more 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 is promoted, and a sufficient discharge capacity tends to be obtained, and the added graphitization catalyst tends not to remain in the graphite particles to be produced. is there. If the amount of the graphitization catalyst remaining in the graphite particles is too large, the discharge capacity per graphite particle mass tends to decrease. Although the upper limit of the graphitization temperature is not particularly limited, it is preferable that the sublimation of graphite 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. If the density of the molded product after graphitization is 1.65 g / cm ³ or less, the specific surface area of the graphite particles to be produced does not tend to be too large. The apparent density of the molded product after graphitization can be appropriately adjusted by, for example, the particle size of the pulverized product mixed with the graphitization catalyst, the pressure at the time of molding into a predetermined shape by pressing, or the like.

  After the graphitization treatment, it is pulverized to adjust the particle size to obtain a 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, and a pin mill can be used.

If necessary, a step of adhering an organic compound to the surface of the pulverized product after the graphitization treatment and baking it (hereinafter, also referred to as a “coating step”) may be performed.
In the coating step, an organic compound is attached to the surface of the obtained pulverized material, followed by firing. The organic compound attached to the surface of the pulverized material changes to a low-crystalline carbon material by causing the organic compound to adhere to the pulverized material and firing. Thereby, a part or the whole of the surface of the pulverized material is coated with the low crystalline carbon material. Highly crystalline graphite 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, a low-crystalline carbon material has a turbostratic structure and thus has many entrances and exits of lithium ions. Therefore, by coating a part or the whole of the surface of the pulverized material with the low crystalline carbon material, the input / output characteristics such as quick charging tend to be improved.

  The method for attaching the organic compound to the surface of the pulverized material is not particularly limited. Specifically, in a mixed solution in which an organic compound is dissolved or dispersed in a solvent, after dispersing and mixing the pulverized material, a wet method in which the solvent is removed and adhered, and the pulverized material and a solid organic compound are mixed. Examples include a dry method in which mechanical energy is applied to the obtained mixture to adhere thereto, a method in which a mixture obtained by mixing a pulverized material and a solid organic compound is fired in an inert atmosphere, and a gas phase method such as a CVD method. Can be

  The organic compound is not particularly limited as long as it 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. One organic compound may be used alone, or two or more organic compounds may be used in combination.

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

  The negative electrode material may include carbonaceous particles or occluded metal particles different from at least one of the above-described composite particles and graphite particles in shape and physical properties. 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 alloys with lithium, such as Al, Si, Ga, Ge, In, Sn, Sb, and Ag.

<Negative electrode for lithium ion secondary battery>
A negative electrode for a lithium ion secondary battery (hereinafter, also referred to as a “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 band-shaped foil, a band-shaped perforated foil, a band-shaped mesh, or the like made of a metal or alloy such as aluminum, copper, nickel, titanium, and stainless steel can be used. In addition, a porous material such as a porous metal (foam metal) or 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 may be collected by a known method such as a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method, or a screen printing method. It can be formed by being applied on top. When integrating the negative electrode material layer and the current collector, a known method such as roll, press, or a combination thereof can be used.

  As the negative electrode material composition, for example, those 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 of, for example, 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 esters (such as methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, and hydroxyethyl (meth) acrylate); (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, High molecular compounds such as polyimide and polyamide imide are exemplified. In addition, (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. For example, organic solvents such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide, γ-butyrolactone and the like can be mentioned.

  The negative electrode material composition may include a thickener for adjusting the viscosity, if necessary. Examples of the thickener 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 auxiliary as needed. Examples of the conductive assistant include carbon black, graphite, acetylene black, conductive oxides, and conductive nitrides.

  When the negative electrode material composition is provided 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 tend to be improved. The heat treatment may be performed in an inert atmosphere such as helium, argon, or nitrogen or in a vacuum atmosphere to prevent oxidation of the current collector during the treatment.

The negative electrode may be subjected to a pressure treatment (press) to increase the density. 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 electrolyte 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 band-shaped foil, a band-shaped perforated foil, a band-shaped mesh, or the like made of a metal or alloy such as aluminum, titanium, and 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 capable of doping or intercalating lithium ions. Specifically, lithium cobalt oxide (LiCoO _2), lithium nickelate (LiNiO _2), lithium manganate (LiMnO _2), and these mixed 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), lithium manganese spinel (LiMn ₂ O _4), lithium vanadium _{compounds, 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 LiMPO ₂ (M is Co, Ni, Mn Or Fe), a conductive polymer (polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, etc.), porous carbon, and the like. It is. These cathode materials can be used alone or in combination of two or more. Among them, lithium nickelate (LiNiO ₂₎ and its composite 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 nonwoven fabrics, cloths, microporous films, and combinations thereof mainly containing polyolefins such as polyethylene and polypropylene. In the case where the lithium ion secondary battery has a structure in which the positive electrode and the negative electrode do not come into contact, it is not necessary to use a separator.

As the electrolytic solution, a so-called organic electrolytic solution obtained by dissolving an electrolyte 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 . Examples of the non-aqueous solvent 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 Examples thereof include tetrahydrofuran, 1,3-dioxolan, methyl acetate, and ethyl acetate. The electrolyte and the non-aqueous solvent can be used each alone or in combination of two or more. Among them, an 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 thereof include a paper battery, a button battery, a coin battery, a stacked battery, a cylindrical battery, and a square battery. The above-described negative electrode material can be applied to all electrochemical devices such as hybrid capacitors having a charge / discharge mechanism for inserting and removing lithium ions, in addition to the lithium ion secondary battery.

  Hereinafter, the above embodiment will be described in more detail with reference to examples. The above embodiment is not limited by the following examples.

[Preparation of negative electrode 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. Next, this mixture was pulverized to an average particle diameter 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, press-molded at 100 MPa, and formed into a rectangular parallelepiped. After heat-treating this compact at 1000 ° C. in a nitrogen atmosphere, it was further heat-treated at 3000 ° C. in a nitrogen atmosphere to obtain a graphitized compact. The graphitized compact was further 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 diameter of 10 μm and 30 parts by mass of coal tar pitch were mixed at 230 ° C. for 2 hours. Next, this mixture was pulverized to an average particle diameter 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, press-molded at 100 MPa, and formed into a rectangular parallelepiped. After heat-treating this compact at 1000 ° C. in a nitrogen atmosphere, it was further heat-treated at 3000 ° C. in a nitrogen atmosphere to obtain a graphitized compact. The graphitized compact was further pulverized to obtain graphite particles (negative electrode material B).

[Preparation of negative electrode material C]
Negative electrode material C was obtained by mixing negative electrode material A and negative electrode material B such that the mass ratio (negative electrode material A: negative electrode material B) was 50:50.

  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 measurement results of the specific surface area, the interlayer distance d (002) of the graphite crystal, and the true specific gravity. Each measurement was performed by the method described above. In addition, Table 2 shows the results of the injection time measured by the method described below. In addition, when the negative electrode material was observed with a scanning electron microscope, it was found that a plurality of flat graphite particles contained composite particles that were aggregated or bonded so that the orientation planes were non-parallel.

The injection time of the negative electrode material was measured as described below. 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) are kneaded to form a slurry. A negative electrode material composition was prepared.
This was dried at 105 ° C. and pulverized 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 charged 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 (a mixture of ethylene carbonate / ethyl methyl carbonate (volume ratio: 3/7) containing 1.0 M LiPF ₆ and vinylene carbonate (0.5% by mass)) was applied to the surface of the prepared tablet. 130 μL was added dropwise, and the time until the electrolyte solution permeated was measured.

[Preparation 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) are kneaded to form a slurry. A negative electrode material composition was prepared. This was applied to a current collector (a copper foil having a thickness of 10 μm) and dried at 105 ° C. for 1 hour in the air to form a negative electrode material layer. Next, pressure treatment was performed by a roll press so that the electrode density became 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 a positive electrode. As an 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 25 μm-thick polyethylene microporous membrane was used. As the spacer, a circular copper plate having a thickness of 230 μm and a diameter of 14 mm was used.

[Initial charge / discharge capacity]
The initial charge / discharge capacity (mAh / g) was measured as follows: sample weight: 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: performed under the conditions of constant current discharge 0.543 mA, cut voltage 1.5 V (Li / Li ⁺ ). Was. Table 3 shows the results.

[Initial charge / discharge efficiency]
Initial charge / discharge efficiency (%) was determined by the following equation. Table 3 shows the results.
Initial charge / discharge efficiency = Initial discharge capacity / First charge capacity x 100

[Irreversible capacity]
The irreversible capacity (mAh / g) was determined by the following equation. Table 3 shows the results.
Irreversible capacity = Initial charge capacity-Initial discharge capacity

[Charge capacity retention rate under low temperature conditions]
(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 charging conditions: constant current charging 0.543 mA, constant voltage charging Two-cycle charge / discharge was performed under the conditions of 0 V (Li / Li ⁺ ), a cut current of 0.053 mA, and CC discharge conditions: a constant current discharge of 0.543 mA, and a cut voltage of 1.5 V (Li / Li ⁺ ).
(2) Next, at a measurement temperature of 0 ° C., CC-CV charge conditions: constant current charge 0.543 mA, constant voltage charge 0 V (Li / Li ⁺ ), cut current 0.053 mA, CC discharge conditions: constant current discharge 0 One cycle charge / discharge was performed under the conditions of .543 mA and a cut voltage of 1.5 V (Li / Li ⁺ ).
(3) The capacity of the CC charge capacity (mAh / g) at 25 ° C. and 0 ° C. was determined, and the charge capacity retention rate under low temperature conditions was calculated by the following equation. Table 3 shows the results.
Charge capacity retention 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 a negative electrode material whose volume-based particle size distribution does not satisfy the above-described conditions. The charge / discharge capacity retention ratio under low-temperature conditions was higher than that of the lithium ion secondary battery of Comparative Example using, and the stability of charging performance was excellent. Further, the initial charge / discharge efficiency of the lithium ion secondary battery of the example was at the same level as that of the lithium ion secondary battery of the comparative example.

Claims (7)

A negative electrode material for a lithium ion secondary battery that satisfies 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 becomes 10% is 5 μm to 14 μm. (2) The particle diameter (D50) when the accumulation from the small diameter side becomes 50% is 15 μm to 27 μm. (3) The particle diameter (D90) when the accumulation from the small diameter side becomes 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, wherein the negative electrode material for a lithium ion secondary battery includes particles in which a plurality of flat graphite particles are aggregated or bonded such that their orientation planes are non-parallel.   3. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein an interlayer distance d (002) of graphite crystals determined by X-ray diffraction measurement using CuKα radiation is 3.38 ° or less. 4. The specific surface area by BET method of nitrogen gas adsorption is 1.0m ² /g~5.0m ² / g, a negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 3.   The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4, having a true specific gravity of 2.22 or more.   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. For negative electrode.   A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to claim 6.