JP6555050B2 - Negative electrode material for lithium ion secondary battery, negative electrode material slurry for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Description

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

  Lithium ion secondary batteries have higher energy density than other secondary batteries such as nickel-cadmium batteries, nickel-hydrogen batteries, or lead-acid batteries. For this reason, it is used as a power source for portable electronic products such as mobile phones and portable electronic devices.

  Recent trends in the development of lithium ion secondary batteries include higher capacity and downsizing with the spread of smartphones, and longer life for electric vehicles and power storage applications. For this reason, high capacity | capacitance by the high density of a negative electrode and the outstanding charging / discharging cycling characteristics are calculated | required. As a negative electrode material for obtaining the above characteristics, carbon materials with high crystallinity such as artificial graphite and spherical natural graphite obtained by spheroidizing flaky natural graphite have attracted attention.

  In artificial graphite, as disclosed in Patent Document 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 negative electrode active materials. By using it as a substance, the charge / discharge cycle characteristics are improved.

In the lithium ion secondary battery, the energy density per volume can be increased by increasing the electrode density of the negative electrode as described above. However, when the electrode density of the negative electrode is increased, the permeability of the electrolytic solution into the negative electrode material layer is lowered, so that the charge / discharge capacity is lowered and the charge / discharge cycle characteristics are liable to be caused. In particular, when a strong press is applied such that the electrode density of the negative electrode exceeds 1.7 g / cm ³ , the anisotropy of the graphite crystal increases, so the thickness of the electrode due to repeated insertion and extraction of lithium ions into the graphite particles. The expansion coefficient and contraction ratio in the vertical direction are increased, the delamination between the particles proceeds, and the charge / discharge cycle characteristics are deteriorated.

  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. However, since the reactivity with the electrolytic solution is high and the permeability of the electrolytic solution is low, there is room for improvement in the initial charge / discharge efficiency and the high-speed charge / discharge efficiency. Further, when the electrode density of the negative electrode is increased, the particles are oriented in the direction along the current collector, and the expansion coefficient of the electrode is increased. As a result, the charge / discharge cycle characteristics are degraded.

As another carbon material used as a negative electrode material, in Patent Document 2, spherical or fine structure oriented graphitized particles obtained by graphitizing mesophase microspheres extracted from mesophase pitch, In addition, a carbon fiber having a lamellar type or a Brooks-Tailor type orientation has been proposed. However, the former graphitized particles have a low discharge capacity, and the latter carbon fibers are difficult to increase in density so that the electrode density of the negative electrode exceeds 1.7 g / cm ^3. However, there is a problem that a short circuit easily occurs.

JP-A-10-158005 Japanese Patent No. 2,637,305

  The present invention has been made in view of the above circumstances, and has a high discharge capacity and excellent electrolyte permeability into the negative electrode material layer even when the electrode density is increased. A negative electrode material for a lithium ion secondary battery capable of obtaining a lithium ion secondary battery having a low expansion coefficient and excellent charge / discharge cycle characteristics, a negative electrode material slurry for a lithium ion secondary battery, and a lithium ion secondary battery It is an object to provide a negative electrode and a lithium ion secondary battery.

Specific means for solving the above problems include the following embodiments.
<1> A plurality of flat graphite particles include composite particles that are aggregated or bonded so that the orientation planes are non-parallel,
The graphite crystal interlayer distance d (002) determined by X-ray diffraction measurement using CuKα rays is 3.38 mm or less,
According to the X-ray diffraction measurement using CuKα rays, the diffraction peak corresponding to the (101) plane of rhombohedral graphite and the diffraction peak corresponding to the (012) plane were not observed,
Pellet density of 1.40g ^/ cm ³ ~1.65g ^/ cm 3 and a negative electrode material for a lithium ion secondary battery.

<2> The negative electrode material for a lithium ion secondary battery according to <1>, wherein the composite particles further include spherical graphite particles.

<3> The specific surface area by BET method of nitrogen gas adsorption is ^{1.0m 2 /g~3.5m 2 / g <1} > or negative electrode material for a lithium ion secondary battery according to <2>.

<4> The negative electrode material for a lithium ion secondary battery according to any one of <1> to <3>, wherein the true specific gravity is 2.22 or more.

<5> A negative electrode material slurry for a lithium ion secondary battery, comprising the negative electrode material for a lithium ion secondary battery according to any one of <1> to <4>, an organic binder, and a solvent.

<6> A lithium ion secondary battery 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 <4> formed on the current collector. Negative electrode for secondary battery.

<7> A lithium ion secondary battery having a positive electrode, an electrolyte, and the negative electrode for a lithium ion secondary battery according to <6>.

  According to the present invention, even when the electrode density is increased, the anode has a high discharge capacity, excellent permeability of the electrolyte into the anode material layer, low electrode expansion rate, and charge / discharge. A negative electrode material for a lithium ion secondary battery capable of obtaining a lithium ion secondary battery having excellent cycle characteristics, a negative electrode material slurry for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery Can be provided.

It is a figure which shows an example of the scanning electron microscope (SEM) image of the composite particle contained in the negative electrode material for lithium ion secondary batteries. It is a figure which shows the other example of the scanning electron microscope (SEM) image of the composite particle contained in the negative electrode material for lithium ion secondary batteries. It is a figure which shows the diffraction profile obtained by performing X-ray diffraction measurement about the negative electrode material for lithium ion secondary batteries of Example 1. It is a figure which shows the diffraction profile obtained by performing a X-ray-diffraction measurement about the negative electrode material for lithium ion secondary batteries of Example 2. It is a figure which shows the diffraction profile obtained by performing a X-ray-diffraction measurement about the negative electrode material for lithium ion secondary batteries of Example 3. FIG. It is a figure which shows the diffraction profile obtained by performing a X-ray-diffraction measurement about the negative electrode material for lithium ion secondary batteries of Example 4. It is a figure which shows the diffraction profile obtained by performing a X-ray-diffraction measurement about the negative electrode material for lithium ion secondary batteries of Example 5. FIG. It is a figure which shows the diffraction profile obtained by performing a X-ray-diffraction measurement about the negative electrode material for lithium ion secondary batteries of Example 6. It is a figure which shows the diffraction profile obtained by performing a X-ray-diffraction measurement about the negative electrode material for lithium ion secondary batteries of Example 7. FIG. It is a figure which shows the diffraction profile obtained by performing X-ray-diffraction measurement about the negative electrode material for lithium ion secondary batteries of the comparative example 1. It is a figure which shows the diffraction profile obtained by performing an X-ray-diffraction measurement about the negative electrode material for lithium ion secondary batteries of the comparative example 2. It is a figure which shows the diffraction profile obtained by performing X-ray-diffraction measurement about the negative electrode material for lithium ion secondary batteries of the comparative example 3. It is a figure which shows the diffraction profile obtained by performing a X-ray-diffraction measurement about the negative electrode material for lithium ion secondary batteries of the comparative example 4. It is a figure which shows the diffraction profile obtained by performing a X-ray-diffraction measurement about the negative electrode material for lithium ion secondary batteries of the comparative example 5. It is a figure which shows the diffraction profile obtained by performing a X-ray-diffraction measurement about the negative electrode material for lithium ion secondary batteries of the comparative example 6. It is a figure which shows the diffraction profile obtained by performing a X-ray-diffraction measurement about the negative electrode material for lithium ion secondary batteries of the comparative example 7.

  Hereinafter, examples of the negative electrode material for a lithium ion secondary battery, the negative electrode material slurry for a lithium ion secondary battery, the negative electrode for a lithium ion secondary battery, and the lithium ion secondary battery of the present embodiment will be described in detail. However, the present invention is not limited to the following embodiments. 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 present invention is not limited thereto.

In this specification, 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. It is.
In the present specification, the numerical ranges indicated by using “to” include numerical values described before and after “to” as the minimum value and the maximum value, respectively.
In the present specification, the content of each component in the composition is the sum of the plurality of substances present in the composition unless there is a specific indication when there are a plurality of substances corresponding to each component in the composition. It means the content rate of.
In the present specification, the particle diameter 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 this specification, the term “layer” refers to the case where the layer is formed only in a part of the region in addition to the case where the layer is formed over the entire region. Is also included.
In this specification, 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>
In the negative electrode material for a lithium ion secondary battery of the present embodiment (hereinafter also simply referred to as “negative electrode material”), a plurality of flat graphite particles are aggregated or bonded so that their orientation planes are non-parallel. The interlayer distance d (002) of the graphite crystal obtained by X-ray diffraction measurement using CuKα rays including the composite particles is 3.38 mm or less, and the rhombohedral graphite of the rhombohedral graphite is measured by X-ray diffraction measurement using CuKα rays. (101) without being diffracted peaks corresponding to diffraction peak and (012) plane corresponding observed in surface, pellet density of ^{1.40g / cm 3 ~1.65g / cm 3} .

  By using the negative electrode material of the present embodiment, even if the electrode density increasing treatment of the negative electrode is performed, it has a high discharge capacity, excellent permeability of the electrolyte solution into the negative electrode material layer, and a low expansion coefficient of the electrode. And the lithium ion secondary battery which is excellent in charging / discharging cycling characteristics can be obtained. In addition, when the negative electrode material of the present embodiment is used, it is excellent in the permeability of the electrolytic solution into the negative electrode material layer. Therefore, even when the negative electrode is subjected to a high electrode density treatment, the internal resistance of the battery can be suppressed, and the charge / discharge efficiency Thus, a lithium ion secondary battery excellent in safety, low temperature characteristics, and charge / discharge load characteristics can be obtained.

(Composite particles)
The negative electrode material of this embodiment includes composite particles in which a plurality of flat graphite particles are aggregated or bonded so that their orientation planes are non-parallel.

The flat graphite particles of this embodiment 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 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 is obtained by observing graphite particles with a microscope, arbitrarily selecting 100 graphite particles, measuring A / B, and taking the average value. In the observation of the aspect ratio, 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. In addition, two parallel tangents circumscribing the outer periphery of the graphite particle, the tangent line b ₁ and the tangent line b ₂ having the smallest distance are selected, and the distance between the tangent line b ₁ and the tangent line b ₂ is determined. The length B in the minor axis direction is assumed.

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. Multiple flat graphite particles are assembled or bonded with their orientation planes being non-parallel to each other, thereby suppressing the increase in the orientation of the particles on the electrode and reducing electrode expansion due to charge / discharge. , Excellent charge / discharge cycle characteristics tend to be obtained.
In addition, the negative electrode material of this embodiment 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.

  The state in which flat graphite particles are aggregated or bonded refers to a state in which two or more flat graphite particles are aggregated or bonded. The bond means 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 step. 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 in the composite particles of the present embodiment may be 3 or more, or 10 or more.

The average particle diameter of the flat graphite particles may be 50 μm or less, 25 μm or less, or 15 μm or less from the viewpoint of easy aggregation or bonding. The average particle size of the flat graphite particles may be 1 μm or more. The average particle diameter can be measured by a laser diffraction particle size distribution measuring device, and is a particle diameter (D50) when the integration from the small diameter side is 50% in the volume-based particle size distribution.
In addition, an average particle diameter can be measured on condition of the following using the laser diffraction particle size distribution measuring apparatus (for example, SALD-3000J, Shimadzu Corporation make).
Absorbance: 0.05-0.20
Sonic: 1 to 3 minutes

  The flat graphite particles and their raw materials are not particularly limited, and examples thereof include artificial graphite, scaly natural graphite, scaly natural graphite, coke, resin, tar, and pitch. Among them, graphite obtained from artificial graphite, natural graphite, or coke has high crystallinity and becomes soft particles, so that the density of the negative electrode tends to be increased.

The composite particles of this embodiment may further contain 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 good. 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 increased, and the discharge capacity and charge / discharge are reduced. Cycle characteristics tend to improve.

  When the composite particles of this embodiment 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 step. 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 of the present embodiment include spherical graphite particles, the total number of flat graphite particles and spherical graphite particles 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 of the spherical graphite particles is not particularly limited, and may be 5 μm to 40 μm, 8 μm to 35 μm, or 10 μm to 30 μm. The average particle diameter can be measured by a laser diffraction particle size distribution measuring device, and is a particle diameter (D50) when the integration from the small diameter side is 50% in the volume-based particle size distribution. The average particle diameter of the spherical graphite particles can be measured in the same manner as the average particle diameter of the flat graphite particles.

Saturated tapping density of the graphite particles spherical is not particularly ^limited, may be ^{0.8g / cm 3 ~1.1g / cm 3} , even 0.9g / cm ³ ~1.05g / cm 3 Good. The saturation tap density can be measured by a known method. Preferably, using a packing density measuring device (for example, KRS-406, manufactured by Kuramochi Scientific Instruments Co., Ltd.), 100 mL of spherical graphite particles are put into a measuring cylinder, and a tap (from a predetermined height to a measuring cylinder is reached until the density is saturated. Drop) and calculate.

The circularity of the spherical graphite particles may be 0.70 or more, or 0.85 or more. Some spherical graphite particles are deformed by mechanical force during the production process of the negative electrode material. However, the higher the circularity of the spherical graphite particles contained in the negative electrode material, the lower the orientation as the negative electrode material, and the higher the permeability of the electrolyte solution into the negative electrode material layer. In particular, in the case of a lithium ion secondary battery used at low temperatures, the viscosity of the electrolyte solution tends to be high, but by using spherical graphite particles with a high degree of circularity for the negative electrode material, discharge capacity and charge / discharge cycle characteristics Tend to improve.
An example of a method for increasing the circularity of the spherical graphite particles contained in the negative electrode material of the present embodiment is to use spherical graphite particles having a high circularity as a raw material. The degree of circularity is measured for a portion of spherical graphite particles contained in the composite particles.

The degree of circularity of the spherical graphite particles can be determined by the following formula by taking a photograph of a cross section of the spherical graphite particles.
Circularity = (perimeter of equivalent circle) / (perimeter of cross-sectional image of spherical graphite particles)
Here, the “equivalent circle” is a circle having the same area as the cross-sectional image of the spherical graphite particles. The peripheral length of the cross-sectional image of the spherical graphite particles is the length of the outline of the cross-sectional image of the captured spherical graphite particles.
In the present specification, the circularity is determined by enlarging the cross section of the spherical graphite particles with a scanning electron microscope to a magnification of 1000 times, arbitrarily selecting 10 spherical graphite particles, and individually spherical graphite particles by the above method. This is a value obtained by measuring the degree of circularity and taking the average.

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

  An example of a scanning electron microscope image of the composite particles contained in the negative electrode material of this embodiment is shown in FIG. Composite particles (portions indicated by solid lines in the figure) are formed by a plurality of flat graphite particles that are aggregated or bonded so that the orientation planes are non-parallel.

  Another example of the scanning electron microscope image of the composite particles contained in the negative electrode material of this embodiment is shown in FIG. The part shown with the dotted line in a figure is a spherical graphite particle. Composite particles (parts indicated by solid lines in the figure) are formed by spherical graphite particles and a plurality of flat graphite particles that are aggregated or bonded so that the orientation planes around them are non-parallel. Yes.

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

(Distance between graphite crystal layers d (002))
In the negative electrode material of the present embodiment, the interlayer distance d (002) of the graphite crystal determined by X-ray diffraction measurement using CuKα rays is 3.38 mm or less, may be 3.37 mm or less, and may be 3.36 mm. It may be the following. 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.

  In the X-ray diffraction measurement using CuKα rays, the measurement range of 10 ° ≦ 2θ ≦ 35 ° can be used, and the measurement conditions for rhombohedral graphite peaks described later can be employed.

(Diffraction peak corresponding to (101) plane of rhombohedral graphite and diffraction peak corresponding to (012) plane)
Graphite includes hexagonal graphite and rhombohedral graphite. Hexagonal graphite has a diffraction angle 2θ in the range of 41.7 ° to 42.7 ° ((100) plane) and in the range of 43.7 ° to 44.7 ° by X-ray diffraction measurement using CuKα rays ( A diffraction peak appears on the (101) plane. The rhombohedral graphite has a diffraction angle 2θ in the range of 42.8 degrees to 43.8 degrees ((101) plane) and in the range of 45.5 degrees to 46.5 degrees by X-ray diffraction measurement using CuKα rays. A diffraction peak appears at ((012) plane). Therefore, graphite usually has two or four diffraction peaks in the range of diffraction angle 2θ of 40 degrees to 50 degrees.

  The hexagonal graphite has a so-called AB-type laminated structure in which layers of carbon hexagonal network planar structures are moved in parallel (2/3, 1/3) and stacked. On the other hand, rhombohedral graphite is a so-called ABC type laminate in which the hexagonal network plane structure of carbon is first (2/3, 1/3) translated and then (1/3, 2/3) translated and stacked. It has a structure. The rhombohedral structure is formed by lattice strain generated when hexagonal graphite is pulverized.

  In the negative electrode material of this embodiment, the diffraction peak corresponding to the (101) plane and the diffraction peak corresponding to the (012) plane of rhombohedral graphite are not observed by X-ray diffraction measurement using CuKα rays. That is, the negative electrode material of the present embodiment is substantially made of hexagonal graphite. When such a negative electrode material is used for a lithium ion secondary battery, the degree of graphitization is high, so a high discharge capacity is exhibited, and since the lattice distortion is very small, the reaction with the electrolytic solution is suppressed. Efficiency tends to improve and charge / discharge cycle characteristics tend to improve.

X-ray diffraction measurement using CuKα rays can be performed under the following conditions.
-Measurement equipment and conditions-
X-ray diffractometer: MultiFlex, Rigaku Corporation goniometer: MultiFlex goniometer (without shutter)
Attachment: Standard specimen holder Monochromator: Fixed monochromator Scanning mode: 2θ / θ
Scan type: Continuous output: 40 kV, 40 mA
Divergence slit: 1 degree scattering slit: 1 degree light receiving slit: 0.30 mm
Monochrome light receiving slit: 0.8mm
Measurement range: 41 degrees ≤ 2θ ≤ 47.5 degrees Sampling width: 0.01 degrees

(Pellet density)
The negative electrode material of the present embodiment, the pellet density of ^{1.40g / cm 3 ~1.65g / cm 3} , may be ^{1.45g / cm 3 ~1.65g / cm 3} . When the pellet density is 1.65 g / cm ³ or less, it is possible to suppress a decrease in the amount of voids due to the deformation of the graphite particles when the negative electrode is densified by applying a press. It tends to penetrate into the entire material layer and tends to improve charge / discharge cycle characteristics. Further, when the pellet density is 1.40 g / cm ³ or more, the graphite particles themselves are easily deformed, and even when a strong press is applied so that the electrode density of the negative electrode exceeds 1.7 g / cm ^3. Further, the collapse of the particles themselves is suppressed, and the deterioration of the charge / discharge characteristics and the charge / discharge cycle characteristics tends to be suppressed.

In addition, a pellet density is obtained by calculating | requiring the volume density of the tablet after throwing 1.0g of negative electrode materials into a tablet molding machine (tablet bottom area: 1.327cm < ² >) and applying the pressure of 1000 kg for 30 second.

(Specific surface area)
The negative electrode material of the present embodiment has a specific surface area by the BET method of nitrogen gas adsorption may be ^{1.0m 2 /g~3.5m 2 / g, 1.0m} 2 /g~3.0m 2 / g It may be.
The specific surface area is an index indicating the area of the interface with the electrolytic solution. That is, when the value of the specific surface area is 3.5 m ² / g or less, the area of the interface between the negative electrode material and the electrolytic solution is not too large, the increase in the reaction area in the decomposition reaction of the electrolytic solution is suppressed, and gas generation is suppressed. The expansion characteristics of the electrode and the initial charge / discharge efficiency tend to be good. Further, when the specific surface area value is 1.0 m ² / g or more, the current density per unit area does not increase rapidly, and the load is suppressed, so that charge / discharge efficiency, charge acceptance, rapid charge / discharge characteristics, etc. Tends to be good.

  The specific surface area can be measured 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, and a specific surface area is calculated.

  The specific surface area of the negative electrode material of this embodiment can be made into the said range by adjusting an average particle diameter, for example. The specific surface area tends to increase as the average particle size decreases.

(True specific gravity)
The negative electrode material of this embodiment 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 of the present embodiment 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.

(Average particle diameter (median diameter))
The average particle diameter (median diameter) of the negative electrode material of this embodiment is not particularly limited. From the viewpoint of the influence on the orientation and the permeability of the electrolytic solution, the average particle diameter of the negative electrode material may be 10 μm to 30 μm or 10 μm to 25 μm. The average particle diameter can be measured by a laser diffraction particle size distribution measuring device, and is a particle diameter (D50) when the integration from the small diameter side is 50% in the volume-based particle size distribution. The average particle diameter of the spherical graphite particles can be measured in the same manner as the average particle diameter of the flat graphite particles. In addition, the average particle diameter of the negative electrode material of this embodiment is an average value including the composite particles and the graphite particles not forming the composite particles.

  As a method of measuring the average particle diameter when the negative electrode is manufactured using the negative electrode material of the present embodiment, a sample electrode is prepared, the electrode is embedded in an epoxy resin, and then mirror-polished to scan the cross section of the electrode. A method of observing with a microscope (for example, VE-7800, manufactured by Keyence Corporation), a cross section of an electrode using an ion milling device (for example, E-3500, manufactured by Hitachi High-Technology Corporation), and a scanning electron microscope (for example, , VE-7800, manufactured by Keyence Corporation), and the like. The average particle size in this case is the median value of 100 particle sizes arbitrarily selected from composite particles and graphite particles not forming composite particles.

  The sample electrode can be produced, for example, as follows. First, water is added to a mixture of 98 parts by mass of the negative electrode material, 1 part by mass of styrene butadiene rubber as a binder, and 1 part by mass of carboxymethyl cellulose as a thickener to prepare a dispersion. The amount of water added is adjusted so that the viscosity of the dispersion at 25 ° C. is 1500 mPa · s to 2500 mPa · s. A sample electrode can be prepared by 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 and then drying at 110 ° C. for 1 hour.

<Method for producing negative electrode material>
The manufacturing method of the negative electrode material of this embodiment is not particularly limited. Hereinafter, an example of the manufacturing method of the negative electrode material of this embodiment is demonstrated.
The method for producing a negative electrode material of the present embodiment includes, for example, a step of obtaining a mixture containing at least one selected from the group consisting of graphitizable aggregates and graphite and a graphitizable binder (hereinafter referred to as “step”). (Also referred to as “(a)”), a step of firing the mixture obtained in step (a) (hereinafter also referred to as “step (b)”), and a step of pulverizing the fired product obtained in step (b). (Hereinafter also referred to as “step (c)”), a step of obtaining a mixture containing the pulverized product obtained in step (c) and a graphitization catalyst (hereinafter also referred to as “step (d)”), A step of firing the mixture obtained in the step (d) (hereinafter also referred to as “step (e)”) and a step of grinding the fired product obtained in the step (e) (hereinafter referred to as “step (f)”). Also called).

In the step (a), a mixture containing at least one selected from the group consisting of a graphitizable aggregate and graphite and a graphitizable binder is obtained.
Examples of the aggregate that can be graphitized include coke such as fluid coke, needle coke, and mosaic coke. The aggregate that can be graphitized is not particularly limited as long as it is powdery. For example, coke powder that is easily graphitized such as needle coke may be used.
Examples of graphite include flaky artificial graphite, flaky natural graphite, flaky natural graphite, spherical artificial graphite, and spherical natural graphite. The graphite is not particularly limited as long as it is powdered.
Examples of the graphitizable binder include coal-based, petroleum-based and artificial pitches and tars, thermoplastic resins, thermosetting resins, and the like.

  As described above, the mixture of step (a) may contain graphite. According to this production method, when the raw material is graphitized by firing, heavy metals, magnetic foreign substances, and impurities contained in the raw material are removed by high heat, so even if natural graphite or the like is used as the raw material, Processing, washing, etc. can be omitted. Thereby, a manufacturing cost can be reduced and a highly safe negative electrode material can be provided. Furthermore, by using graphite as at least a part of the raw material, the production cost can be reduced by reducing the amount of graphitization catalyst required for graphitization of the raw material, shortening the firing time for graphitization, and the like. As a result, a cheaper negative electrode material can be provided while using expensive artificial graphite. Moreover, the binder component used for manufacture of a negative electrode material can be reduced.

  When the mixture of the step (a) includes graphite, the content of graphite may be 40 parts by mass to 72 parts by mass, or 52 parts by mass to 65 parts by mass with respect to 100 parts by mass of the mixture. 55 parts by mass to 60 parts by mass. When the graphite content is in the above range, even when the negative electrode is subjected to a high electrode density treatment, it tends to exhibit high discharge capacity, low electrode expansion coefficient, and excellent charge / discharge cycle characteristics.

  The content of the graphitizable binder may be 10 to 30 parts by mass or 15 to 25 parts by mass with respect to 100 parts by mass of the mixture in the step (a). By setting the content of the graphitizable binder in an appropriate range, it is possible to suppress the specific surface area of the flat graphite particles obtained by graphitization from becoming too large. Furthermore, the discharge capacity of the flat graphite particles obtained by graphitizing the binder is smaller than the theoretical discharge capacity of graphite. Therefore, by setting the content of the graphitizable binder in the above range, a high discharge capacity can be obtained. There exists a tendency to obtain the negative electrode material which can implement | achieve the lithium ion secondary battery which has.

  There is no restriction | limiting in particular in the mixing method for obtaining the mixture of a process (a), For example, it can mix using a kneader etc. Mixing may be performed at a temperature above the softening point of the graphitizable binder. Specifically, when the graphitizable binder is pitch, tar, etc., it may be mixed at a temperature of 50 ° C. to 300 ° C., and when it is a thermosetting resin, a temperature of 20 ° C. to 100 ° C. May be mixed.

  In step (b), the mixture obtained in step (a) is baked. Firing is preferably performed in an atmosphere in which the mixture obtained in step (a) is not easily oxidized. Examples thereof include a method of firing in a nitrogen atmosphere, argon gas, or vacuum. The firing temperature may be 600 ° C. or higher, or 600 ° C. to 1500 ° C. By this firing, the organic component contained in the mixture obtained in step (a) can be removed.

  In the step (b) and the step (e) to be described later, it is preferable that the mixture to be fired is not subjected to a molding treatment such as pressurization and is fired in a graphite container or the like. By not performing the molding treatment, the pulverization strength when pulverizing the fired product can be lowered, and as a result, an increase in specific surface area can be suppressed. If the increase in the specific surface area is suppressed, the increase in the reaction area with the electrolytic solution can be suppressed, so that the reduction in charge / discharge efficiency and the generation of gas due to the decomposition of the electrolytic solution tend to be suppressed. In addition, as a result of reducing the pulverization strength when pulverizing the fired product, the occurrence of lattice distortion is suppressed, and the diffraction peak corresponding to the (101) plane of rhombohedral graphite and the diffraction corresponding to the (012) plane. Peak expression is suppressed. By suppressing the formation of rhombohedral graphite, the reactivity with the electrolytic solution is suppressed, and the decrease in charge / discharge efficiency tends to be suppressed.

Further, by not performing the molding process, the thermal conductivity of the composition or the mixture is suppressed as compared with the case of performing the molding process, and as a result, the crystallinity of the obtained negative electrode material is moderately suppressed, resulting in excessively soft particles. And the pellet density of the negative electrode material is easily made 1.65 g / cm ³ or less. When the pellet density is 1.65 g / cm ³ or less, the orientation of the particles when the electrode is pressed is suppressed, and the expansion coefficient of the electrode tends to decrease. In particular, when the negative electrode is subjected to a high electrode density treatment, the orientation of the particles can be suppressed. As a result, the permeability of the electrolytic solution is improved, and a high discharge capacity tends to be maintained. Furthermore, an increase in the internal resistance of the battery is suppressed, and the charge / discharge cycle characteristics tend to be improved.

In step (c), the fired product obtained in step (b) is pulverized. There is no restriction | limiting in particular in the grinding | pulverization method of a baked product. For example, it can be performed by a known method using a jet mill, a vibration mill, a pin mill, a hammer mill or the like. The average particle size (median diameter) of the pulverized product after pulverization may be 100 μm or less, or 10 μm to 50 μm. In the step (c), it is preferable that the negative electrode material is pulverized so as to be slightly smaller than a desired average particle diameter. The reason is that if the pulverized product is graphitized in the step (d), the particles are bound to each other by the graphitization catalyst and the particle size may be increased.
After pulverization, the pulverized product may be sieved. There is no restriction | limiting in particular in the method of sieving, For example, it can carry out by a known method using a vibration sieve, a rotary dry sieve, etc.

In step (d), a mixture containing the pulverized material obtained in step (c) and the graphitization catalyst is obtained. Examples of the graphitization catalyst include substances having a graphitization catalytic action such as silicon, iron, nickel, titanium, and boron, and carbides, oxides, nitrides, and the like of these substances.
There is no particular limitation on the mixing method of the pulverized product obtained in step (c) and the graphitization catalyst, and at least the graphitization catalyst is present inside the particle surface or on the particle surface before firing for graphitization. Any mixing method may be used.

  The content of the graphitization catalyst in the mixture may be 1 part by mass to 15 parts by mass with respect to 100 parts by mass of the mixture. When the amount of the graphitization catalyst is 1 part by mass or more, the development of graphite crystals tends to be good, and the discharge capacity of the lithium ion secondary battery tends to be improved.

  In step (e), the mixture obtained in step (d) is baked. The firing temperature is not particularly limited as long as it is a temperature at which a graphitizable component can be graphitized. The firing temperature may be, for example, 2000 ° C. or higher, 2500 ° C. or higher, or 2800 ° C. or higher. Further, the firing temperature may be 3200 ° C. or lower. If the calcination temperature is 2000 ° C. or more, the crystal changes, the graphite crystal develops better, and the amount of the graphitization catalyst remaining after the calcination tends to decrease (that is, the increase in the ash content is suppressed). is there. As a result, the charge / discharge capacity and charge / discharge cycle characteristics of the lithium ion secondary battery tend to be improved. Moreover, it can suppress that a part of graphite sublimates that a calcination temperature is 3200 degrees C or less.

In the step (f), the fired product obtained in the step (e) is pulverized. There is no restriction | limiting in particular in the grinding | pulverization method of a baked product. For example, it can be performed by a known method using a jet mill, a vibration mill, a pin mill, a hammer mill or the like. The average particle size (median diameter) of the pulverized product after pulverization may be 100 μm or less, or 10 μm to 50 μm.
After pulverization, the pulverized product may be sieved. There is no restriction | limiting in particular in the method of sieving, For example, it can carry out by a known method using a vibration sieve, a rotary dry sieve, etc.

The negative electrode material of the present embodiment may include 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.
In the negative electrode material of the present embodiment, part or all of the surface of the composite particle may be coated with a low crystalline carbon material. By covering part or all of the surface of the composite particles with a low crystalline carbon material, it is possible to improve input / output characteristics such as rapid charging.

<Anode material slurry for lithium ion secondary battery>
The negative electrode material slurry for a lithium ion secondary battery of the present embodiment (hereinafter also simply referred to as “negative electrode material slurry”) includes the negative electrode material of the present embodiment, an organic binder, and a solvent.

There is no restriction | limiting in particular in the organic binder contained in the negative electrode material slurry of this embodiment. Examples of the organic binder include styrene-butadiene rubber; ethylenically unsaturated carboxylic acid ester (methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, hydroxyethyl (meth)) Acrylates) and (meth) acrylic copolymers derived from ethylenically unsaturated carboxylic acids (acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, etc.); polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin , Polymer compounds such as polyphosphazene, polyacrylonitrile, polyimide, and polyamideimide.
(Meth) acrylate means acrylate or methacrylate, and (meth) acrylonitrile means acrylonitrile or methacrylonitrile.

  There is no restriction | limiting in particular in the solvent contained in the negative electrode material slurry of this embodiment. Examples of the solvent include organic solvents such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide, and γ-butyrolactone.

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

<Anode for lithium ion secondary battery>
A negative electrode for a lithium ion secondary battery of the present embodiment (hereinafter also referred to as “negative electrode”) has a current collector and a negative electrode material layer including the negative electrode material of the present embodiment formed on the current collector. .

  The material and shape of the current collector are not particularly limited. Examples of the current collector include a belt-like foil, a belt-like perforated foil, a belt-like mesh made of a metal or an alloy such as aluminum, copper, nickel, titanium, and stainless steel. As the current collector, porous materials such as porous metal (foamed metal) and carbon paper can also be used.

  The method for forming the negative electrode material layer including the negative electrode material of the present embodiment on the current collector is not particularly limited. For example, the negative electrode material layer is formed on the current collector 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. Can be formed. 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.

  The negative electrode obtained by forming the negative electrode material layer on the current collector may be heat-treated depending on the type of the organic binder used. 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 can 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.

Before performing the heat treatment, the negative electrode may be pressed (pressure treatment). The electrode density can be adjusted by the pressure treatment. Electrode density ^may be ^{1.5g / cm 3 ~1.9g / cm 3} , may be ^{1.6g / cm 3 ~1.8g / cm 3} . As the electrode density is higher, the volume capacity is improved, the adhesion of the negative electrode material layer to the current collector is improved, and the charge / discharge cycle characteristics tend to be improved.

<Lithium ion secondary battery>
The lithium ion secondary battery of this embodiment includes a positive electrode, an electrolyte, and the negative electrode described above. The lithium ion secondary battery can be configured, for example, such that 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. Furthermore, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), and their double oxides (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: Co, Ni, Mn, Fe ), Conductive polymer (polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, etc.), porous carbon, etc. More than one species can be used in combination. 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.

  As a separator, the nonwoven fabric, cloth, microporous film, and those combination which have polyolefins, such as polyethylene and a polypropylene, as a main component are mentioned, for example. 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 an electrolyte, lithium salts such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF ₃ , ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentanone, sulfolane, 3 -Methyl sulfolane, 2,4-dimethyl sulfolane, 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 Butylethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxy A so-called organic electrolytic solution in which solan, methyl acetate, ethyl acetate and the like are dissolved in a non-aqueous solvent containing one kind alone or a combination of two or more kinds can be used. Among these, an electrolytic solution containing fluoroethylene carbonate is preferable because it has a tendency to form a stable SEI (solid electrolyte interface) on the surface of the negative electrode material, and the charge / discharge cycle characteristics are remarkably improved.

  The form of the lithium ion secondary battery of the present embodiment is not particularly limited, and examples thereof include paper batteries, button batteries, coin batteries, stacked batteries, cylindrical batteries, and prismatic batteries. 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 present invention will be described in more detail based on examples. The present invention is not limited to the following examples.

[Example 1]
(1) 16 parts by mass of coke powder having an average particle diameter of 14 μm, 64 parts by mass of spherical natural graphite (circularity 0.90) having an average particle diameter of 16 μm, and 20 parts by mass of tar pitch are mixed and stirred at 100 ° C. for 1 hour. A mixture was obtained. Next, this mixture was placed in a graphite container and fired at 1000 ° C. in a nitrogen atmosphere. Then, it grind | pulverized using the hammer mill, sieved, and obtained the ground material with an average particle diameter of 17 micrometers. 100 parts by mass of a pulverized product having an average particle size of 17 μm and 12 parts by mass of silicon carbide as a graphitization catalyst are mixed, and the mixture is placed in a graphite vessel and baked at 2800 ° C. Turned into. The obtained fired product (graphitized powder) was pulverized with a hammer mill and sieved to obtain graphite powder of Example 1 (a negative electrode material for a lithium ion secondary battery).
Table 1 shows the results of measuring the average particle diameter, specific surface area, pellet density, interlayer distance d (002) of graphite crystals, and true specific gravity of the graphite powder obtained above. Moreover, the result of having measured the diffraction peak corresponding to the (101) plane of rhombohedral graphite and the diffraction peak corresponding to the (012) plane by X-ray diffraction measurement using CuKα rays is shown in FIG. Each measurement was performed by the method described above.

When the graphite powder of Example 1 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. Furthermore, in the graphite powder of Example 1, composite particles containing spherical graphite particles were also observed.
Further, as can be seen from FIG. 3, in the graphite powder of Example 1, a diffraction peak corresponding to the (101) plane of rhombohedral graphite and a diffraction peak corresponding to the (012) plane were not observed.

(2) Kneading 98 parts by mass of the graphite powder 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 (CMC 2200, manufactured by Daicel Corporation) A slurry was prepared. This slurry is applied to a current collector (copper foil having a thickness of 10 μm), dried in air at 105 ° C. for 1 hour, and the applied material (active material) becomes an electrode density of 1.70 g / cm ³ by a roll press. A negative electrode for a lithium ion secondary battery was produced by integrating with a current collector under conditions.

  As an indicator of the electrolyte penetration of the negative electrode for lithium ion secondary batteries, the electrolyte penetration time was measured by the following method. The negative electrode with a shorter electrolyte solution penetration time is superior in electrolyte solution permeability. The measurement results are shown in Table 1.

<Electrolyte penetration time>
A sample electrode having a sample mass of 15.4 mg, an electrode area of 1.54 cm ² and an electrode density of 1.70 g / cm ³ was produced using the above negative electrode for a lithium ion secondary battery. 1 μL of propylene carbonate electrolytic solution was taken in a syringe and dropped entirely onto the surface of the prepared sample electrode. The time (seconds) from immediately after dropping until all the propylene carbonate electrolyte was absorbed into the electrode (time until the propylene carbonate electrolyte on the electrode surface could no longer be confirmed visually) was measured, and was defined as the electrolyte penetration time. .

(3) 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.

  The charge capacity, discharge capacity, charge / discharge efficiency, and expansion coefficient of the lithium ion secondary battery were measured by the methods shown below. The measurement results are shown in Table 1.

<Charge capacity and discharge capacity>
Measurement of charge / discharge capacity (initial charge / discharge capacity) 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 , charge condition: constant current charge The test was performed under the conditions of 0.434 mA, constant voltage charge 0 V (Li / Li ⁺ ), cut current 0.043 mA, discharge conditions: constant current discharge 0.434 mA, cut voltage 1.5 V (Li / Li ⁺ ).
The discharge capacity was measured according to the above charging conditions and discharging conditions.

<Charge / discharge efficiency>
The charge / discharge efficiency was defined as the ratio (%) of the discharge capacity value to the charge capacity value in the charge / discharge measurement at the first cycle.

<Expansion coefficient>
In order to obtain the expansion coefficient of the negative electrode, first, two cycles of charge / discharge were performed under the conditions of measuring the charge capacity and discharge capacity. Thereafter, the battery measurement was terminated at the charge rate of 100% in the third cycle charge. The expansion coefficient was defined as the ratio (%) of the thickness of the negative electrode in a state where the charging rate was 100% with respect to the thickness of the negative electrode before the start of charge / discharge.

(4) In order to perform the charge / discharge cycle test, lithium cobalt oxide having a sample mass of 35.9 mg and an electrode area of 1.54 cm ² was used for the positive electrode. The negative electrode for lithium ion secondary batteries described above was used as the negative electrode under the conditions of sample mass: 20.0 mg, electrode area: 2.00 cm ² , and electrode density: 1.70 g / cm ³ . A lithium ion secondary battery (2016 type coin cell) was produced using the positive electrode and the negative 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 SUS316L plate having a thickness of 300 μm and a diameter of 16 mm and a SUS316L wave washer having a thickness of 250 μm and a diameter of 15 mm were used.

  The discharge capacity maintenance rate after 200 cycles of the lithium ion secondary battery was measured by the method shown below. The measurement results are shown in Table 1.

<Discharge capacity maintenance ratio after 200 cycles>
Using the above lithium ion secondary battery, a 200-cycle charge / discharge test was performed at a measurement temperature of 25 ° C. under the following conditions. The discharge capacity retention rate after 200 cycles was the ratio (%) of the discharge capacity at the 200th cycle to the discharge capacity at the 4th cycle.
(1st to 3rd cycles)
Charging conditions: constant current charge 0.49 mA, constant voltage charge 4.2 V, cut current 0.049 mA
Discharge conditions: constant current discharge 0.49 mA, cut voltage 2.75 V
(4th to 100th cycle)
Charging conditions: constant current charge 4.9 mA, constant voltage charge 4.2 V, cut current 0.049 mA
Discharge conditions: constant current discharge 4.9 mA, cut voltage 2.75 V
(101st cycle)
Charging conditions: constant current charge 0.49 mA, constant voltage charge 4.2 V, cut current 0.049 mA
Discharge conditions: constant current discharge 0.49 mA, cut voltage 2.75 V
(102nd to 200th cycles)
Charging conditions: constant current charge 4.9 mA, constant voltage charge 4.2 V, cut current 0.049 mA
Discharge conditions: constant current discharge 4.9 mA, cut voltage 2.75 V

[Example 2]
28 parts by mass of coke powder having an average particle diameter of 14 μm, 52 parts by mass of spherical natural graphite (circularity 0.80) having an average particle diameter of 21 μm, and 20 parts by mass of tar pitch are mixed and stirred at 100 ° C. for 1 hour. Obtained. Next, this mixture was put in a graphite vessel and baked at 1000 ° C. in a nitrogen atmosphere, and then pulverized using a hammer mill and sieved to obtain a pulverized product having an average particle diameter of 23 μm. 100 parts by mass of the pulverized product having an average particle size of 23 μm and 12 parts by mass of silicon carbide were mixed, and the mixture was put into a graphite container and baked at 2800 ° C. to graphitize the graphitizable component. The obtained fired product (graphitized powder) was pulverized with a hammer mill and sieved to obtain a graphite powder of Example 2 (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 were produced in the same manner as in Example 1, and measurements were performed in the same manner as in Example 1. The results are shown in Table 1 and FIG.

When the graphite powder of Example 2 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. Furthermore, in the graphite powder of Example 2, composite particles containing spherical graphite particles were also observed.
As can be seen from FIG. 4, in the graphite powder of Example 2, the diffraction peak corresponding to the (101) plane of rhombohedral graphite and the diffraction peak corresponding to the (012) plane were not observed.

[Example 3]
40 parts by mass of coke powder having an average particle diameter of 14 μm, 20 parts by mass of spherical natural graphite (circularity 0.91) having an average particle diameter of 10 μm, 20 parts by mass of scaly natural graphite having an average particle diameter of 14 μm, and 20 parts by mass of tar pitch. Mix and stir at 100 ° C. for 1 hour to obtain a mixture. The mixture was then placed in a graphite container and fired at 1000 ° C. in a nitrogen atmosphere, and then pulverized using a hammer mill and sieved to obtain a pulverized product having an average particle diameter of 17 μm. 100 parts by mass of the pulverized product having an average particle diameter of 17 μm and 12 parts by mass of silicon carbide were mixed, and the mixture was placed in a graphite container and baked at 2800 ° C. to graphitize the graphitizable component. The obtained fired product (graphitized powder) was pulverized with a hammer mill and sieved to obtain a graphite powder of Example 3 (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 were produced in the same manner as in Example 1, and measurements were performed in the same manner as in Example 1. The results are shown in Table 1 and FIG.

When the graphite powder of Example 3 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. Furthermore, in the graphite powder of Example 3, composite particles containing spherical graphite particles were also observed.
Further, as can be seen from FIG. 5, in the graphite powder of Example 3, a diffraction peak corresponding to the (101) plane of rhombohedral graphite and a diffraction peak corresponding to the (012) plane were not observed.

[Example 4]
20 parts by mass of coke powder having an average particle size of 14 μm, 30 parts by mass of spherical natural graphite having an average particle size of 10 μm (circularity of 0.91), 30 parts by mass of spherical natural graphite having an average particle size of 16 μm (circularity of 0.90), and 20 parts by mass of tar pitch was mixed and stirred at 100 ° C. for 1 hour to obtain a mixture. Next, this mixture was put in a graphite container and fired at 1000 ° C. in a nitrogen atmosphere, and then pulverized using a hammer mill and sieved to obtain a pulverized product having an average particle diameter of 13 μm. 100 parts by mass of the pulverized product having an average particle size of 13 μm and 12 parts by mass of silicon carbide were mixed, and the mixture was placed in a graphite container and fired at 2800 ° C. to graphitize the graphitizable component. The obtained fired product (graphitized powder) was pulverized by a hammer mill and sieved to obtain a graphite powder of Example 4 (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 were produced in the same manner as in Example 1, and measurements were performed in the same manner as in Example 1. The results are shown in Table 1 and FIG.

In addition, when the graphite powder of Example 4 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. Furthermore, in the graphite powder of Example 4, composite particles containing spherical graphite particles were also observed.
Further, as can be seen from FIG. 6, in the graphite powder of Example 4, a diffraction peak corresponding to the (101) plane of rhombohedral graphite and a diffraction peak corresponding to the (012) plane were not observed.

[Example 5]
20 parts by mass of coke powder having an average particle diameter of 14 μm, 48 parts by mass of spherical natural graphite (roundness 0.90) having an average particle diameter of 16 μm, 30 parts by mass of scaly natural graphite having an average particle diameter of 8 μm, and 20 parts by mass of tar pitch. Mix and stir at 100 ° C. for 1 hour to obtain a mixture. Next, this mixture was put in a graphite container and baked at 1000 ° C. in a nitrogen atmosphere, and then pulverized using a hammer mill and sieved to obtain a pulverized product having an average particle diameter of 18 μm. 100 parts by mass of a pulverized product having an average particle diameter of 18 μm and 12 parts by mass of silicon carbide were mixed, and the mixture was placed in a graphite container and baked at 2800 ° C. to graphitize the graphitizable component. The fired product (graphitized powder) obtained was pulverized with a hammer mill and sieved to obtain graphite powder of Example 5 (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 were produced in the same manner as in Example 1, and measurements were performed in the same manner as in Example 1. The results are shown in Table 1 and FIG.

When the graphite powder of Example 5 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. Furthermore, in the graphite powder of Example 5, composite particles containing spherical graphite particles were also observed.
As can be seen from FIG. 7, in the graphite powder of Example 5, the diffraction peak corresponding to the (101) plane of rhombohedral graphite and the diffraction peak corresponding to the (012) plane were not observed.

[Example 6]
28 parts by mass of coke powder having an average particle diameter of 14 μm, 52 parts by mass of scaly natural graphite having an average particle diameter of 8 μm, and 20 parts by mass of tar pitch were mixed and stirred at 100 ° C. for 1 hour to obtain a mixture. The mixture was then placed in a graphite container and fired at 1000 ° C. in a nitrogen atmosphere, and then pulverized using a hammer mill and sieved to obtain a pulverized product having an average particle diameter of 17 μm. 100 parts by mass of the pulverized product having an average particle diameter of 17 μm and 12 parts by mass of silicon carbide were mixed, and the mixture was placed in a graphite container and baked at 2800 ° C. to graphitize the graphitizable component. The obtained fired product (graphitized powder) was pulverized with a hammer mill and sieved to obtain a graphite powder of Example 6 (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 were produced in the same manner as in Example 1, and measurements were performed in the same manner as in Example 1. The results are shown in Table 1 and FIG.

When the graphite powder of Example 6 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.
Further, as can be seen from FIG. 8, in the graphite powder of Example 6, the diffraction peak corresponding to the (101) plane of rhombohedral graphite and the diffraction peak corresponding to the (012) plane were not observed.

[Example 7]
16 parts by mass of coke powder having an average particle diameter of 14 μm, 64 parts by mass of spherical natural graphite (circularity 0.80) having an average particle diameter of 21 μm, and 20 parts by mass of tar pitch are mixed and stirred at 100 ° C. for 1 hour. Obtained. Next, this mixture was put in a graphite container and fired at 1000 ° C. in a nitrogen atmosphere, and then pulverized using a hammer mill and sieved to obtain a pulverized product having an average particle diameter of 22 μm. The pulverized product having an average particle size of 22 μm was put in a graphite container and baked at 2800 ° C. to graphitize the graphitizable component. The obtained fired product (graphitized powder) was pulverized with a hammer mill and sieved to obtain graphite powder of Example 7 (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 were produced in the same manner as in Example 1, and measurements were performed in the same manner as in Example 1. The results are shown in Table 1 and FIG.

When the graphite powder of Example 7 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. Furthermore, in the graphite powder of Example 7, composite particles containing spherical graphite particles were also observed.
Further, as can be seen from FIG. 9, in the graphite powder of Example 7, the diffraction peak corresponding to the (101) plane of rhombohedral graphite and the diffraction peak corresponding to the (012) plane were not observed.

[Comparative Example 1]
Only spherical natural graphite used in Example 1 was filled in a graphite container and fired at 2800 ° C. in a nitrogen atmosphere to obtain graphite powder of Comparative Example 1 (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 were produced in the same manner as in Example 1, and measurements were performed in the same manner as in Example 1. The results are shown in Table 1 and FIG.

In addition, when the graphite powder of Comparative Example 1 was observed with a scanning electron microscope, the composite particles in which a plurality of flat graphite particles were assembled or bonded so that the orientation planes were not parallel were not included. .
Further, as can be seen from FIG. 10, in the graphite powder of Comparative Example 1, the diffraction peak corresponding to the (101) plane of rhombohedral graphite and the diffraction peak corresponding to the (012) plane were not observed.

[Comparative Example 2]
Only a scaly natural graphite having an average particle size of 9 μm was filled in a graphite container and fired at 2800 ° C. in a nitrogen atmosphere to obtain graphite powder of Comparative Example 2 (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 were produced in the same manner as in Example 1, and measurements were performed in the same manner as in Example 1. The results are shown in Table 1 and FIG.

In addition, when the graphite powder of Comparative Example 2 was observed with a scanning electron microscope, the composite particles in which a plurality of flat graphite particles were assembled or bonded so that the orientation planes were not parallel were not included. .
As can be seen from FIG. 11, in the graphite powder of Comparative Example 2, the diffraction peak corresponding to the (101) plane of rhombohedral graphite and the diffraction peak corresponding to the (012) plane were not observed.

[Comparative Example 3]
Only a coke powder having an average particle size of 14 μm was filled in a graphite container and fired at 2800 ° C. in a nitrogen atmosphere to obtain a graphite powder of Comparative Example 3 (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 were produced in the same manner as in Example 1, and measurements were performed in the same manner as in Example 1. The results are shown in Table 1 and FIG.

In addition, when the graphite powder of Comparative Example 3 was observed with a scanning electron microscope, the composite particles in which a plurality of flat graphite particles were assembled or bonded so that the orientation planes were not parallel were not included. .
As can be seen from FIG. 12, in the graphite powder of Comparative Example 3, the diffraction peak corresponding to the (101) plane of rhombohedral graphite and the diffraction peak corresponding to the (012) plane were not observed.

[Comparative Example 4]
71.2 parts by mass of coke powder having an average particle size of 14 μm, 5.2 parts by mass of pitch, and 23.6 parts by mass of coal tar were mixed and stirred at 100 ° C. for 1 hour to obtain a mixture. Next, this mixture was put in a graphite container and fired at 1000 ° C. in a nitrogen atmosphere, and then pulverized using a hammer mill and sieved to obtain a pulverized product having an average particle diameter of 34 μm. 100 parts by mass of a pulverized product having an average particle size of 34 μm and 12 parts by mass of silicon carbide were mixed, and the mixture was placed in a graphite container and baked at 2800 ° C. to graphitize the graphitizable component. The obtained fired product (graphitized powder) was pulverized with a hammer mill and sieved to obtain a graphite powder (a negative electrode material for a lithium ion secondary battery) of Comparative Example 4.
A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery were produced in the same manner as in Example 1, and measurements were performed in the same manner as in Example 1. The results are shown in Table 1 and FIG.

In addition, when the graphite powder of Comparative Example 4 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.
As can be seen from FIG. 13, in the graphite powder of Comparative Example 4, the diffraction peak corresponding to the (101) plane of rhombohedral graphite and the diffraction peak corresponding to the (012) plane were not observed.

[Comparative Example 5]
48 parts by mass of coke powder having an average particle size of 14 μm, 32 parts by mass of tar, and 20 parts by mass of silicon carbide were heated and mixed at 100 ° C. for 1 hour, and the resulting mixture was pulverized. Next, the pulverized product was pressure-molded into pellets, fired at 900 ° C. in nitrogen, fired at 2800 ° C. using a graphitization furnace, and graphitized. The obtained fired product was pulverized with a hammer mill and sieved to obtain a graphite powder of Comparative Example 5 (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 were produced in the same manner as in Example 1, and measurements were performed in the same manner as in Example 1. The results are shown in Table 1 and FIG.

When the graphite powder of Comparative Example 5 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.
As can be seen from FIG. 14, in the graphite powder of Comparative Example 5, a diffraction peak corresponding to the (101) plane of rhombohedral graphite and a diffraction peak corresponding to the (012) plane were observed.

[Comparative Example 6]
43 parts by mass of coke powder having an average particle size of 14 μm, 18.5 parts by mass of tar pitch, 18.5 parts by mass of silicon carbide, and 20 parts by mass of spherical natural graphite (roundness 0.92) were heated and mixed at 100 ° C. for 1 hour. The resulting mixture was ground. Next, the pulverized product was pressure-molded into pellets, fired at 900 ° C. in a nitrogen atmosphere, fired at 2800 ° C. using a graphitization furnace, and graphitized. The obtained fired product was pulverized with a hammer mill and sieved to obtain a graphite powder of Comparative Example 6 (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 were produced in the same manner as in Example 1, and measurements were performed in the same manner as in Example 1. The results are shown in Table 1 and FIG.

When the graphite powder of Comparative Example 6 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.
As can be seen from FIG. 15, in the graphite powder of Comparative Example 6, a diffraction peak corresponding to the (101) plane of rhombohedral graphite and a diffraction peak corresponding to the (012) plane were observed.

[Comparative Example 7]
90 parts by weight of spherical natural graphite having an average particle diameter of 16 μm and 10 parts by weight of tar pitch used in Example 1 were mixed and fired at 1000 ° C. in a nitrogen atmosphere. Thereafter, the fired product obtained was pulverized with a hammer mill and sieved to obtain a graphite powder of Comparative Example 7 (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 were produced in the same manner as in Example 1, and measurements were performed in the same manner as in Example 1. The results are shown in Table 1 and FIG.

In addition, when the graphite powder of Comparative Example 7 was observed with a scanning electron microscope, the composite particles in which a plurality of flat graphite particles were assembled or bonded so that the orientation planes were not parallel were not included. .
As can be seen from FIG. 16, in the graphite powder of Comparative Example 7, a diffraction peak corresponding to the (101) plane of rhombohedral graphite and a diffraction peak corresponding to the (012) plane were observed.

As shown in Table 1, in Examples 1 to 7, even if the electrode density increasing treatment of the negative electrode was performed, the discharge capacity was high, the electrolyte solution was excellent in permeability into the negative electrode material layer, and the electrode A lithium ion secondary battery having a low expansion coefficient and excellent charge / discharge cycle characteristics could be obtained.
On the other hand, the comparative examples 1-7 were inferior to the Example in any one of discharge capacity, the permeability of the electrolyte solution in the negative electrode material layer, the charge / discharge cycle characteristics, and the expansion coefficient of the electrode.

Claims (7)

A plurality of flat graphite particles include composite particles that are assembled or bonded so that the orientation planes are non-parallel,
The graphite crystal interlayer distance d (002) determined by X-ray diffraction measurement using CuKα rays is 3.38 mm or less,
According to the X-ray diffraction measurement using CuKα rays, the diffraction peak corresponding to the (101) plane of rhombohedral graphite and the diffraction peak corresponding to the (012) plane were not observed,
Pellet density of 1.40g ^/ cm ³ ~1.65g ^/ cm 3 and a negative electrode material for a lithium ion secondary battery.   The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the composite particles further include spherical graphite particles. The specific surface area by BET method of nitrogen gas adsorption 1.0m ² /g~3.5m ² / g and is claim 1 or negative electrode material for a lithium ion secondary battery according to claim 2.   The true specific gravity is 2.22 or more. The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 3.   The negative electrode material slurry for lithium ion secondary batteries containing the negative electrode material for lithium ion secondary batteries of any one of Claims 1-4, the organic binder, and a solvent.   A lithium ion secondary battery comprising: a current collector; and a negative electrode material layer comprising the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4 formed on the current collector. Negative electrode.   The lithium ion secondary battery which has a positive electrode, electrolyte, and the negative electrode for lithium ion secondary batteries of Claim 6.