JP2012151087A - Negative electrode material for lithium ion secondary battery, method for manufacturing the same, negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode material for a lithium ion secondary battery enabling high density and excellent in electrolyte permeability.SOLUTION: A negative electrode material for a lithium ion secondary battery contains a carbon particle mixture made of at least two kinds of carbon particle groups having different average fracture strengths. Selected from all the carbon particle groups before mixing to constitute the carbon particle mixture, a first carbon particle group has the lowest average fracture strength and a second carbon particle group has the highest average fracture strength. The ratio of the average fracture strength of the second carbon particle group to that of the first carbon particle group is in the range of 1.5 to 50.

Description

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

  Lithium-ion secondary batteries are lighter and have higher input / output characteristics than other secondary batteries such as nickel metal hydride batteries and lead-acid batteries. It is attracting attention as an output power source.

Examples of the negative electrode active material used in the lithium ion secondary battery include graphite and amorphous carbon. With the recent increase in capacity, the development of a negative electrode material close to 372 mAh / g, which is the theoretical discharge capacity of graphite, is also progressing (see, for example, Patent Document 1).
In order to further increase the capacity, it is necessary to fill the active material with a high density or press the electrode with a high load (pressurization) to increase the volume density of the electrode. In this connection, for example, attempts have been made to change the particle size distribution in order to fill the active material with a high density (see, for example, Patent Document 2), but this is still insufficient for increasing the capacity, and further, It is necessary to press the load (pressurize) to increase the density of the electrode.

Japanese Patent No. 4448279 JP 2005-340025 A

However, when the electrode density is increased by high-load (pressurization) pressing on the electrode, the permeability of the electrolytic solution to the electrode may decrease. Since the insertion / extraction reaction of lithium ions to / from the electrode material is performed through the electrolytic solution, a decrease in the permeability of the electrolytic solution may cause a decrease in battery characteristics.
The present invention provides a negative electrode material for a lithium ion secondary battery that can be densified and has excellent electrolyte permeability, a method for producing the same, and a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery using the same. The task is to do.

Specific means for solving the above problems are as follows.
<1> A carbon particle mixture composed of at least two types of carbon particle groups having different average fracture strengths. Among all the carbon particle groups before mixing constituting the carbon particle mixture, the first average fracture strength is the smallest. The ratio of the average fracture strength of the second carbon particle group to the average fracture strength of the first carbon particle group is 1.5 or more and 50 for the carbon particle group and the second carbon particle group having the largest average fracture strength. The negative electrode material for lithium ion secondary batteries which is the following.

<2> The lithium ion secondary battery according to <1>, wherein the ratio of the content of the second carbon particle group to the content of the first carbon particle group is 1% by mass or more and 50% by mass or less. Negative electrode material.

<3> The carbon particle mixture is an isotropic pressure-treated product with a pressure of 4.9 × 10 ⁶ Pa or more and 4.9 × 10 ⁸ Pa or less, and the lithium ion two according to <1> or <2>. Negative electrode material for secondary batteries.

<4> The volume average particle diameter (D50) of the first carbon particle group is 1 μm or more and 40 μm or less, and the volume average particle diameter (D50) of the second carbon particle group is 1 μm or more and 40 μm or less. Any one of <1> to <3>, wherein the ratio of the volume average particle diameter (D50) of the second carbon particle group to the volume average particle diameter (D50) of the carbon particle group is 1 or more and 50 or less. The negative electrode material for lithium ion secondary batteries as described.

<5> A step of mixing at least two types of carbon particle groups having different average breaking strengths to obtain a carbon particle mixture, and among all the carbon particle groups before mixing, the first carbon having the smallest average breaking strength For the particle group and the second carbon particle group having the maximum average breaking strength, the ratio of the average breaking strength of the second carbon particle group to the average breaking strength of the first carbon particle group is 1.5 or more and 50 or less. A method for producing a negative electrode material for a lithium ion secondary battery.

<6> The lithium ion secondary battery according to <5>, further including a step of isotropically pressurizing the carbon particle mixture at a pressure of 4.9 × 10 ⁶ Pa to 4.9 × 10 ⁸ Pa. Manufacturing method of negative electrode material.

<7> A lithium including a current collector and a negative electrode material layer provided on the current collector and including the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4. Negative electrode for ion secondary battery.

<8> A lithium ion secondary battery comprising the lithium ion secondary battery negative electrode according to <7>, a positive electrode, and an electrolyte.

  ADVANTAGE OF THE INVENTION According to this invention, the negative electrode material for lithium ion secondary batteries which can be densified and is excellent in electrolyte solution permeability, its manufacturing method, and the negative electrode and lithium ion secondary battery for lithium ion secondary batteries using the same Can be provided.

6 is a diagram illustrating an example of an SEM image showing the entire cross section of a negative electrode according to Comparative Example 1. FIG. 6 is a diagram illustrating an example of an enlarged view of a part of a cross section of a negative electrode according to Comparative Example 1. FIG. It is a figure which shows an example of the SEM image which shows the whole negative electrode cross section concerning Example 7. FIG. FIG. 9 is a diagram illustrating an example of an enlarged view of a part of a cross section of a negative electrode according to Example 7.

The negative electrode material for a lithium ion secondary battery of the present invention (hereinafter also simply referred to as “negative electrode material”) includes a carbon particle mixture composed of at least two types of carbon particle groups having different average fracture strengths, and constitutes the carbon particle mixture. For the first carbon particle group having the smallest average fracture strength and the second carbon particle group having the largest average fracture strength selected from all the carbon particle groups before mixing, the average fracture of the first carbon particle group The ratio of the average fracture strength of the second carbon particle group to the strength is 1.5 or more and 50 or less.
The carbon particle mixture includes at least a first carbon particle group and a second carbon particle group having an average fracture strength different from that of the first carbon particle group, and includes other carbon particle groups as necessary. .

  By having such a configuration for the negative electrode material, the electrolyte permeability when the electrode is formed is improved. Further, even when the electrode material is pressed and pressed to form a densified electrode, the permeability of the electrolytic solution is maintained. This is because, for example, when the electrode material is pressed and pressed, the soft carbon particles having a low fracture strength are deformed, but the hard carbon particles having a high fracture strength are not easily deformed. As a result, it can be considered that the permeability of the electrolytic solution in the negative electrode formed by press-pressing the negative electrode material is maintained.

In the present invention, the ratio of the average fracture strength of the second carbon particle group to the average fracture strength of the first carbon particle group (hereinafter also referred to as “average fracture strength ratio”) is 1.5 to 50. It is preferably 1.7 or more and 40 or less, more preferably 1.7 or more and 30 or less, still more preferably 1.7 or more and 25 or less, and particularly preferably 15 or more and 25 or less.
When the average fracture strength ratio is less than 1.5, the entire carbon particles contained in the negative electrode material are easily deformed during pressure pressing, and the voids between the carbon particles through which the electrolyte solution permeates are blocked, and the electrolyte solution permeates. May decrease. Further, if the average fracture strength ratio exceeds 50, the deformation of the carbon particles contained in the first carbon particle group having a small average fracture strength becomes larger, and as a result, the orientation of the entire carbon particles deteriorates, and the input / output characteristics. May decrease.

  The average fracture strengths of the first carbon particle group and the second carbon particle group are each measured using a micro compression tester (for example, MCT-W500 manufactured by Shimadzu Corporation). Specifically, ten carbon particles having a particle diameter of 0.9 to 1.1 times the volume average particle diameter (D50) are selected from each of the first carbon particles and the second carbon particles. For each of the 10 selected carbon particles, the breaking strength is measured using a micro compression tester, and the average breaking strength is given as an arithmetic average value of the measured values.

Further, the volume average particle diameter (D50) of the first carbon particle group and the second carbon particle group is a particle whose accumulation is 50% when a volume cumulative distribution curve is drawn from the small diameter side in each particle diameter distribution. Given as diameter.
The volume average particle size (D50) can be measured with a laser diffraction particle size distribution analyzer (for example, SALD-3000J manufactured by Shimadzu Corporation) by dispersing a sample in purified water containing a surfactant. it can.

  For carbon particles having a particle diameter of 0.9 to 1.1 times the volume average particle diameter (D50), observe the carbon particles using an optical microscope (500 times) and select the corresponding carbon particles. Can be obtained at The particle diameter of each carbon particle is given as the long diameter of the carbon particle. Specifically, the carbon particles are observed with an optical microscope (500 times), and the maximum value of the distance between two parallel planes circumscribing the carbon particles is defined as the major axis of the carbon particles.

Further, the average fracture strength of the first carbon particle group and the average fracture strength of the second carbon particle group are not particularly limited as long as the average fracture strength ratio is satisfied, but the average fracture strength of the first carbon particle group is 1 MPa or more. Preferably, the average fracture strength of the second carbon particle group is 1.7 MPa or more and 150 MPa or less, the average fracture strength of the first carbon particle group is 2 MPa or more and 30 MPa or less, The average fracture strength of the carbon particle group is more preferably 50 MPa or more and 120 MPa or less.
When the average fracture strength of the first carbon particles is 1 MPa or more, the adjustment to the target density at the time of pressure pressing becomes easy, and the productivity is improved. Further, when the average fracture strength of the first carbon particles is 88 MPa or less, the pressure applied during pressure pressing can be reduced, and the density can be easily increased. On the other hand, when the average breaking strength of the second carbon particles is 1.7 MPa or more, the effect of improving the electrolyte permeability can be expected even when the first particles have a breaking strength of about 1 MPa. In addition, when the average fracture strength of the second carbon particles is 150 MPa or less, an increase in pressure required at the time of pressure pressing is suppressed, and high density is facilitated.

  Further, the volume average particle diameter of the first carbon particle group and the volume average particle diameter of the second carbon particle group are not particularly limited, but the volume average particle diameter of the first carbon particle group is 1 μm or more and 40 μm or less, The volume average particle diameter of the second carbon particle group is preferably 1 μm or more and 40 μm or less, the volume average particle diameter of the first carbon particle group is more than 15 μm and 30 μm or less, and the second carbon particle group The volume average particle diameter is more preferably 2 μm or more and 15 μm or less, the volume average particle diameter of the first carbon particle group is 20 μm or more and 25 μm or less, and the volume average particle diameter of the second carbon particle group is 5 μm or more. More preferably, it is 15 μm or less.

When the volume average particle diameter of the first carbon particle group is 40 μm or less, unevenness on the electrode surface is suppressed, the battery reliability is improved, and the diffusion distance of Li from the carbon particle surface to the inside is shortened. Therefore, the input / output characteristics of the lithium ion secondary battery tend to be improved. On the other hand, when the volume average particle diameter of the first carbon particle group is 1 μm or more, the specific surface area can be suppressed from increasing, the decrease in the initial charge / discharge efficiency in the lithium ion secondary battery can be suppressed, and the contact between the particles can be prevented. This improves the input / output characteristics.
Further, when the volume average particle diameter of the second carbon particle group is 1 μm or more, the specific surface area can be suppressed from increasing, the decrease in the initial charge / discharge efficiency in the lithium ion secondary battery can be suppressed, and the contact between the particles is good. The input / output characteristics are improved. On the other hand, when the volume average particle diameter of the second carbon particle group is 40 μm or less, the occurrence of unevenness on the electrode surface is suppressed and the battery reliability is improved, and the diffusion distance of Li from the carbon particle surface to the inside is increased. Since it becomes shorter, the input / output characteristics of the lithium ion secondary battery tend to be improved.

Further, the maximum particle size of the first carbon particle group and the maximum particle size of the second carbon particle group are not particularly limited, but from the viewpoint of electrode thinning, input / output characteristics and high rate cycle characteristics, The maximum particle diameter of the carbon particle group is 5 μm or more and 70 μm or less, the maximum particle diameter of the second carbon particle group is preferably 2 μm or more and 50 μm or less, and the maximum particle diameter of the first carbon particle group is 10 μm or more. More preferably, the maximum particle diameter of the second carbon particle group is 5 μm or more and 40 μm or less.
The maximum particle diameter means a particle diameter D99.9 that is 99.9% cumulative when a volume cumulative distribution is drawn from the small diameter side in the particle diameter distribution.

The ratio of the volume average particle diameter of the second carbon particle group to the volume average particle diameter of the first carbon particle group (hereinafter also referred to as “average particle diameter ratio”) is not particularly limited, but is 1 or more and 50 or less. It is preferable that it is 1.2 or more and 30 or less, more preferably 2 or more and 20 or less.
When the average particle size ratio is 1 or more, deformation of the carbon particles constituting the first carbon particle group during pressure pressing is suppressed, voids are easily maintained, and electrolyte permeability is improved. In addition, when the average particle size ratio is 50 or less, the carbon particles constituting the second carbon particle group are suppressed from being buried in the carbon particles constituting the first carbon particle group, and the voids are easily maintained. Improves liquid permeability.

  The negative electrode material for a lithium ion secondary battery has a volume average particle diameter of the first carbon particle group of 1 to 40 μm and a volume average of the second carbon particle group from the viewpoint of electrolyte permeability and battery characteristics. It is preferable that the particle diameter is 1 μm or more and 40 μm or less, the average particle diameter ratio is 1 or more and 50 or less, the volume average particle diameter of the first carbon particle group is more than 10 μm and 30 μm or less, and the second carbon More preferably, the volume average particle diameter of the particle group is 2 μm or more and 10 μm or less, and the average particle diameter ratio is 1.2 or more and 30 or less, and the volume average particle diameter of the first carbon particle group is more than 10 μm and 30 μm. More preferably, the volume average particle diameter of the second carbon particle group is 2 μm or more and 10 μm or less, and the average particle diameter ratio is 2 or more and 20 or less.

  The negative electrode material for a lithium ion secondary battery is a first carbon particle having a volume average particle diameter of 1 μm to 40 μm and an average breaking strength of 1 MPa to 88 MPa from the viewpoint of electrolyte permeability and battery characteristics. Preferably obtained by mixing at least a second carbon particle group having a volume average particle diameter of 1 μm or more and 40 μm or less and an average breaking strength of 1.7 MPa or more and 150 MPa or less. The first carbon particle group having an average fracture strength of 2 MPa to 30 MPa and a volume average particle diameter of 2 μm to 10 μm, and an average fracture strength of 50 MPa to 125 MPa. More preferably, it is obtained by mixing at least a certain second carbon particle group, and the volume average particle diameter is 15 μm or more and 25 μm or less. The first carbon particle group having an average breaking strength of 2.5 MPa or more and 15 MPa or less, and the second carbon particle group having a volume average particle diameter of 2.5 μm or more and 7 μm or less and an average breaking strength of 75 MPa or more and 110 MPa or less. More preferably, it is obtained by mixing at least a carbon particle group.

Examples of the carbon particles used in the present invention include natural graphite (scale-like, spherical, etc.), artificial graphite, amorphous carbon (hard carbon, soft carbon), etc. Any combination may be used as long as it is selected.
These carbon particles can be appropriately selected and used from natural graphite, artificial graphite, amorphous carbon and the like that are usually used in the art. Further, selected from composite particles in which carbon particles and other materials (for example, metals, metal oxides, etc.) are combined, or surface modified particles whose surface has been modified to some extent by the other materials. Also good.

  In the present invention, from the viewpoint of electrolyte permeability, the material of the first carbon particle group (carbon particles having a low breaking strength) is mainly selected from natural graphite and artificial graphite, and the second carbon particles (having a high breaking strength). The material of the carbon particles) is preferably selected mainly from natural graphite and amorphous carbon, the material of the first carbon particles is mainly selected from natural graphite and artificial graphite, and the material of the second carbon particles is mainly non- More preferably, it is selected from crystalline carbon, more preferably the first carbon particles are selected from spherical natural graphite and the second carbon particles are selected from amorphous carbon. Here, “mainly” means that the main component (50% by mass or more) constituting each carbon particle is the material, and it is not necessary that all of the material is the material, and the amount is preferably It is 70 mass% or more, More preferably, it is 80 mass% or more, Most preferably, it is 90 mass% or more. This amount may be observed with an optical microscope (500 times), and a certain number of particles having a corresponding particle diameter may be arbitrarily selected (for example, 10) to determine the material.

  The negative electrode material for a lithium ion battery of the present invention is selected from natural graphite and artificial graphite, has a volume average particle diameter of 1 μm to 40 μm, and an average fracture strength of 1 MPa to 88 MPa, Obtained by mixing at least a second carbon particle group selected from natural graphite and amorphous carbon and having a volume average particle diameter of 1 μm to 40 μm and an average fracture strength of 1.7 MPa to 150 MPa. A first carbon particle group selected from natural graphite and artificial graphite, having a volume average particle diameter of more than 10 μm and 30 μm or less, and an average fracture strength of 2 MPa or more and 30 MPa or less, and amorphous carbon Second carbon particles having a volume average particle diameter of 2 μm to 10 μm and an average breaking strength of 50 MPa to 125 MPa More preferably, the first carbon particles selected from natural graphite and having a volume average particle diameter of 15 μm to 25 μm and an average fracture strength of 2.5 MPa to 15 MPa. And at least a second carbon particle group selected from amorphous carbon and having a volume average particle diameter of 2.5 μm to 7 μm and an average fracture strength of 75 MPa to 110 MPa. It is more preferable.

The content ratio of the first carbon particle group and the second carbon particle group in the negative electrode material for a lithium ion secondary battery is not particularly limited, but from the viewpoint of electrolyte permeability, the first carbon particle group and the second carbon particle group are not limited. Ratio of content of second carbon particle group to total content of carbon particle group of (second carbon particle group / (first carbon particle group + second carbon particle group), hereinafter, “content ratio” Is preferably 1% by mass or more and 50% by mass or less, more preferably 5% by mass or more and 50% by mass or less, and still more preferably 10% by mass or more and 50% by mass or less.
When the content ratio is 1% by mass or more, voids between the carbon particles are sufficiently formed, and the permeation path of the electrolytic solution can be effectively maintained. In addition, when the content ratio is 50% by mass or less, deformation of the carbon particles constituting the second carbon particle group having a high average fracture strength when pressed with pressure can be suppressed, and deterioration of permeability can be suppressed. .

  The carbon particle mixture containing the first carbon particle group and the second carbon particle group has an average particle diameter ratio of 1 to 50 in terms of electrolyte permeability and battery characteristics, and the content ratio Is preferably 1% by mass or more and 50% by mass or less, more preferably the average particle size ratio is 1.2 or more and 30 or less, and the content rate is 5% by mass or more and 20% by mass or less. More preferably, the ratio is 2 or more and 20 or less, and the content ratio is 5 mass% or more and 20 mass% or less.

From the viewpoint of electrolyte permeability, the carbon particle mixture contained in the negative electrode material for a lithium ion secondary battery is 4.9 × 10 ⁶ Pa (50 kgf / cm ² ) or more and 4.9 × 10 ⁸ Pa (5000 kgf / cm). it is preferable that the isotropic pressure treatment at a pressure of ²⁾ below, isotropic pressure below ^{9.8 × 10 6 Pa (100kgf /} cm 2) or more ^{1.96 × 10 8 Pa (2000kgf /} cm 2) More preferably, it is pressure-treated.
Details of the isotropic pressure treatment will be described later.

  Moreover, the carbon particle mixture in the present invention includes a first carbon particle group having a volume average particle diameter of 1 μm or more and 40 μm or less and an average breaking strength of 1 MPa or more and 88 MPa or less from the viewpoint of electrolyte permeability and battery characteristics. A second carbon particle group having a volume average particle diameter of 1 μm or more and 40 μm or less and an average fracture strength of 1.7 MPa or more and 150 MPa or less, the total of the first carbon particle group and the second carbon particle group It is preferably obtained by mixing so that the content ratio of the second carbon particle group with respect to the content is 1% by mass or more and 50% by mass or less, and the volume average particle diameter is more than 10 μm and 30 μm or less, and the average fracture A first carbon particle group having a strength of 2 MPa to 30 MPa, a volume average particle diameter of 2 μm to 10 μm, and an average fracture strength of 50 MPa to 125 MPa. The second carbon particle group so that the content ratio of the second carbon particle group to the total content of the first carbon particle group and the second carbon particle group is 5% by mass or more and 20% by mass or less. More preferably, it is obtained by mixing.

  The negative electrode material for a lithium ion secondary battery of the present invention contains a carbon particle mixture, but the shape of the particle size distribution is not particularly limited, with the particle size as the horizontal axis and the appearance frequency as the vertical axis, and the single peak particle size It may be a distribution or a particle size distribution having a plurality of peaks. From the viewpoint of electrolyte solution permeability, a particle size distribution having a plurality of peaks is preferable, and a particle size distribution having two peaks is more preferable.

The negative electrode material for a lithium ion secondary battery has a particle diameter of 0.9 times or more and 1.1 times or less of a particle diameter D90 of 90% when a volume cumulative distribution is drawn from the small diameter side in the particle diameter distribution. Ratio of the breaking strength (P _D10 ) of the carbon particle B having a particle diameter of 0.9 times or more and 1.1 times or less of the particle diameter D10 that is 10% cumulative with respect to the breaking strength (P _D90 ) of the carbon particles A having (P _D10 / P _D90 , hereinafter also referred to as “destructive strength ratio”) is preferably 1.7 or more, 75 or less, more preferably 2 or more and 60 or less, and further preferably 20 or more and 50 or less. .
When the ratio of the breaking strength (P _D10 / P _D90 ) is within the above range, sufficient electrolyte solution permeability can be obtained. This can be considered because, for example, when the negative electrode material is pressure-pressed, voids are easily maintained.
For the fracture strengths P _D90 and P _D10 , 10 carbon particles each having a particle diameter of 0.9 to 1.1 times the particle diameter D90 or D10 are selected, and each of the 10 carbon particles is minute. The fracture strength is measured using a compression tester, and each of the measured values is obtained as an arithmetic average value.

In the present invention, the particle diameter D90 and the particle diameter D10 are not particularly limited, but the particle diameter D90 is preferably 20 μm or more and 50 μm or less, the particle diameter D10 is preferably 1 μm or more and 30 μm or less, and the particle diameter D90 is 25 μm or more and 40 μm or less. And it is more preferable that the particle diameter D10 is 5 micrometers or more and 20 micrometers or less.
The ratio of the particle diameter D10 to the particle diameter D90 (particle diameter D10 / particle diameter D90) is not particularly limited, but is preferably 0.1 to 0.8, and more preferably 0.2 to 0.6. preferable.

The volume average particle diameter (D50) of the negative electrode material for lithium ion secondary batteries of the present invention is not particularly limited, and can be, for example, 1 μm or more and 40 μm or less, preferably 3 μm or more and 30 μm or less, and more preferably 5 μm or more and 25 μm or less. preferable.
When the volume average particle size is 1 μm or more, the increase in specific surface area is suppressed, the initial charge / discharge efficiency of the lithium ion secondary battery is improved, and the voids between the particles are sufficiently maintained to improve the input / output characteristics. To do. On the other hand, when the volume average particle diameter is 40 μm or less, unevenness is suppressed on the formed electrode surface, thereby improving the reliability of the battery and reducing the diffusion distance of Li from the surface of the carbon particles to the inside. The input / output characteristics of the lithium ion secondary battery tend to be improved.

The maximum particle diameter Dmax of the negative electrode material for a lithium ion secondary battery of the present invention is not particularly limited, and can be, for example, from 10 μm to 70 μm, preferably from 30 μm to 65 μm, and preferably from 30 μm to 45 μm. It is more preferable.
When the maximum particle diameter Dmax is 70 μm or less, the electrode can be thinned, and input / output characteristics and high-rate cycle characteristics are improved.

The negative electrode material for lithium ion secondary batteries, in a profile obtained by laser Raman spectroscopy of the excitation wavelength 532 nm, Ig the intensity of a peak appearing the intensity of the peak appearing in the vicinity of 1360 cm ^-1 Id, around 1580 cm ^-1 When the intensity ratio Id / Ig between the two peaks is an R value, the R value is preferably 0.10 or more and 1.5 or less, and more preferably 0.15 or more and 1.0 or less.
When the R value is 0.10 or more, there is a tendency that the life characteristics and the input / output characteristics are excellent, and when it is 1.5 or less, an increase in irreversible capacity tends to be suppressed.

Here, the peak in the vicinity of 1360 cm ^{−1 is} a peak usually identified as corresponding to the amorphous structure of carbon, for example, a peak observed at 1300 cm ^{−1 to} 1400 cm ⁻¹ . Also a peak around 1580 cm ^-1, generally a peak identified as corresponding to the graphite crystal structure, means a peak observed for example ^{1530cm -1 ~1630cm} -1.
Incidentally, R value Raman spectrum measuring apparatus (e.g., manufactured by JASCO Corporation NSR-1000 type, excitation wavelength 532 nm) using the entire measuring range ^{(830cm -1 ~1940cm} -1) can be calculated as a baseline .

The negative electrode material for a lithium ion secondary battery preferably has a tap density of 0.3 g / cm ³ or more and 3.0 g / cm ³ or less, and 0.5 g / cm ³ or more and 2.0 g / cm ³ or less. More preferably.
When the tap density is 0.3 g / cm ³ or more, the amount of the organic binder when producing the negative electrode can be suppressed, and the energy density of the formed lithium ion secondary battery tends to increase.

For example, the tap density tends to increase as the volume average particle diameter of the negative electrode material is increased, and this property can be used to set the tap density within the above range. Incidentally, the tap density in the present invention, a sample powder 100 cm ³ was slowly poured into a measuring cylinder of volume 100 cm ^3, and the stoppered graduated cylinder, the sample after the measuring cylinder was dropped 250 times from a height of 5cm It means a value determined from the mass and volume of the powder.

In the present invention, the average fracture strength of the first carbon particle group is 1 MPa or more and 88 MPa or less, the average fracture strength of the second carbon particle group is 1.7 MPa or more and 150 MPa or less, the first carbon particle group and The content ratio of the second carbon particle group to the total content of the second carbon particle group is 1% by mass or more and 50% by mass or less, and the tap density is 0.3 g / cm ³ or more and 3.0 g / cm ³ or less. The average fracture strength of the first carbon particle group is 2 MPa or more and 30 MPa or less, the average fracture strength of the second carbon particle group is 50 MPa or more and 120 MPa or less, and the first carbon particle group and the first carbon particle group The content ratio of the second carbon particle group to the total content of the second carbon particle group is 5% by mass or more and 20% by mass or less, and the tap density is 0.5 g / cm ³ or more and 2.0 g / cm ³ or less. There is Is more preferable.

In the present invention, the average particle size ratio (second carbon particle group / first carbon particle group) is 1 or more and 50 or less, and the average fracture strength ratio (second carbon particle group / first carbon particle). Group) is 1.5 or more and 50 or less, the tap density is preferably 0.3 g / cm ³ or more and 3.0 g / cm ³ or less, the average particle size ratio is 2 or more and 20 or less, and the average fracture More preferably, the strength ratio is 1.7 to 40 and the tap density is 0.5 g / cm ^{3 to} 2.0 g / cm ³ .

The negative electrode material for a lithium ion secondary battery may contain other components as necessary in addition to the carbon particle mixture including at least the first carbon particle group and the second carbon particle group. Examples of other components include an organic binder and a conductive auxiliary material described later.
In the negative electrode material for a lithium ion secondary battery, the content ratio of the mixture of carbon particles including the first carbon particle group and the second carbon particle group can be, for example, 75% by mass or more, and 90% by mass or more. Preferably there is.

<Method for producing negative electrode material for lithium ion secondary battery>
The method for producing a negative electrode material for a lithium ion secondary battery includes a step of obtaining a carbon particle mixture by mixing at least two types of carbon particle groups having different average fracture strengths. For the first carbon particle group having the smallest strength and the second carbon particle group having the largest average breaking strength, the ratio of the average breaking strength of the second carbon particle group to the average breaking strength of the first carbon particle group is It is 1.5 or more and 50 or less. Moreover, the manufacturing method of the said negative electrode material for lithium ion secondary batteries may further include the other process as needed.

  The step of obtaining the carbon particle mixture includes at least a first carbon particle group exhibiting a minimum average fracture strength and a second carbon particle exhibiting a maximum average fracture strength among the carbon particle groups constituting the carbon particle mixture. If it is the process of mixing a group, it will not restrict | limit in particular, The process of further mixing another carbon particle as needed may be sufficient. As a method for mixing carbon particles, a mixing method generally used as a method for mixing powders can be applied without particular limitation.

  The details of the first carbon particle group and the second carbon particle group in the method for producing a negative electrode material for a lithium ion secondary battery are as described above, and preferred embodiments are also the same.

The method for producing a negative electrode material for a lithium ion secondary battery preferably further includes a step of binding the carbon particles constituting the first carbon particle group and the carbon particles constituting the second carbon particle group. . By binding the carbon particles constituting the first carbon particle group and the carbon particles constituting the second carbon particle group, it is possible to disperse the carbon particle mixture in a more uniform state. Improves liquid permeability.
The binding method is not particularly limited, but the binding method is not particularly limited, and after mixing the first carbon particle group and the second carbon particle group, a method of binding by performing an isotropic pressure treatment, Examples thereof include a method in which a binder or the like is further added to the mixture of the first carbon particle group and the second carbon particle group for binding.
Among these, the binding method is preferably a method of binding by performing isotropic pressure treatment from the viewpoints of electrolyte permeability and battery characteristics.

The pressure condition in the isotropic pressure treatment is not particularly limited, but from the viewpoint of electrolyte permeability, it is 4.9 × 10 ⁶ Pa (50 kgf / cm ² ) or more and 4.9 × 10 ⁸ Pa (5000 kgf / cm ² ). less it is more preferably preferably 9.8 × ¹⁰ 6 Pa or less (100kgf / cm ²⁾ or more ^{1.96 × 10 8 Pa (2000kgf /} cm 2). When the pressure condition of the isotropic pressure treatment is 4.9 × 10 ⁶ Pa or more, the electrolyte permeability is further improved. Further, when it is 4.9 × 10 ⁸ Pa or less, the degree of freedom in the production process is increased and the productivity is improved.
The isotropic pressure treatment can be performed using, for example, a hydrostatic press using a liquid such as water as a pressurizing medium or a commercially available isotropic press using air or the like using a pressure as a pressurizing medium. . Specifically, for example, after filling and sealing a mixture of the first carbon particle group and the second carbon particle group in a rubber container or the like, the container is pressurized using a commercially available isotropic press machine. Thus, an isotropic pressure treatment can be performed.

Further, the binder in the method of adding a binder or the like to the mixture of the carbon particle group A and the carbon particle group B and binding the mixture is not limited to an organic binder in a negative electrode for a lithium ion secondary battery, which will be described later. It may be an organic compound (hereinafter also referred to as “carbon precursor”).
There are no particular restrictions on the organic compounds that can be carbonized by heat treatment. For example, ethylene heavy end pitch, crude oil pitch, coal tar pitch, asphalt cracking pitch, pitch generated by pyrolyzing polyvinyl chloride, naphthalene, etc. Examples thereof include a synthetic pitch produced by polymerization in the presence of a strong acid. In addition, thermoplastic synthetic resins such as polyvinyl chloride, polyvinyl alcohol, polyvinyl acetate, and polyvinyl butyral can also be used. Furthermore, natural polymer compounds such as starch and cellulose can also be used.
The said carbon precursor can bind | conclude a mixture by baking and carbonizing in 500 to 3000 degreeC inert atmosphere.
Further, the content ratio of the binder in the method of binding by adding a binder or the like is not particularly limited. For example, it can be set as the content rate similar to the organic binder in the negative electrode for lithium ion secondary batteries mentioned later. The content ratio of the binder is based on the mass of the carbonized material remaining after the heat treatment when a carbon precursor is used as the binder.

<Anode for lithium ion secondary battery>
The negative electrode for a lithium ion secondary battery of the present invention has a current collector and a negative electrode material layer provided on the current collector and containing the negative electrode material for a lithium ion secondary battery. It is configured including other components. This makes it possible to configure a negative electrode for a lithium ion secondary battery that is high in density and excellent in electrolyte permeability.

  The negative electrode for a lithium ion secondary battery is prepared by, for example, dispersing the negative electrode material and the organic binder for the lithium ion secondary battery according to the present invention described above together with a solvent, such as a stirrer, a ball mill, a super sand mill, or a pressure kneader. A negative electrode material slurry is prepared by kneading and applied to a current collector to form a negative electrode layer, or a paste-like negative electrode material slurry is formed into a sheet shape, a pellet shape, etc. It can be obtained by integrating with the electric body.

  Although it does not specifically limit as said organic binder, For example, a styrene-butadiene copolymer; Ethylenically unsaturated carboxylic acid ester (For example, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (Meth) acrylonitrile, hydroxyethyl (meth) acrylate, etc.) and ethylenically unsaturated carboxylic acid (for example, acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, etc.) High molecular compounds such as polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide, polyamideimide;

The content ratio of the organic binder in the negative electrode layer of the lithium ion secondary battery negative electrode is 0.5 parts by mass to 20 parts by mass with respect to 100 parts by mass in total of the negative electrode material for the lithium ion secondary battery and the organic binder. It is preferable that it is 1 mass part-10 mass parts.
Adhesiveness is good when the content ratio of the organic binder is 0.5 parts by mass, and destruction of the negative electrode due to expansion / contraction during charge / discharge is suppressed. On the other hand, it can suppress that electrode resistance becomes large because it is 20 mass parts or less.

  Further, a thickener may be added to the negative electrode material slurry in order to adjust the viscosity. As the thickener, for example, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid (and its salt), oxidized starch, phosphorylated starch, casein and the like can be used.

  Moreover, you may mix a conductive support material with the said negative electrode material slurry as needed. Examples of the conductive auxiliary material include carbon black, graphite, acetylene black, or an oxide or nitride that exhibits conductivity. What is necessary is just to let the usage-amount of a conductive support material be about 0.1 mass%-20 mass% in the total mass of the lithium ion secondary battery negative electrode material of this invention.

  Further, the material and shape of the current collector are not particularly limited. For example, if a belt-like material made of aluminum, copper, nickel, titanium, stainless steel or the like in a foil shape, a punched foil shape, a mesh shape, or the like is used. Good. A porous material such as porous metal (foamed metal) or carbon paper can also be used.

The method of applying the negative electrode material slurry to the current collector is not particularly limited. For example, metal mask printing method, electrostatic coating method, dip coating method, spray coating method, roll coating method, doctor blade method, gravure coating And publicly known methods such as screen printing and the like. After the application, it is preferable to perform a rolling process using a flat plate press, a calender roll or the like, if necessary.
Further, the integration of the negative electrode material slurry formed into a sheet shape, a pellet shape, and the like with the current collector can be performed by a known method such as a roll, a press, or a combination thereof.

The negative electrode layer formed on the current collector and the negative electrode layer integrated with the current collector are preferably heat-treated according to the organic binder used. For example, when an organic binder having polyacrylonitrile as the main skeleton is used, the temperature is 100 ° C. to 180 ° C., and when using an organic binder having polyimide, polyamideimide as the main skeleton, 150 ° C. to 450 ° C. It is preferable to heat-treat with.
This heat treatment increases the strength by removing the solvent and curing the binder, thereby improving the adhesion between the particles and between the particles and the current collector. These heat treatments are preferably 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.

Moreover, it is preferable that the negative electrode is pressure-pressed (pressure treatment) after the heat treatment. The electrode density can be adjusted by applying pressure treatment. It The negative electrode for a lithium ion secondary battery of the present invention, it is preferable that the electrode density of ^{1.0g / cm 3 ~2.0g / cm 3} , a 1.2g / cm ³ ~1.9g / cm 3 it is more ^preferable, further preferably ^{1.4g / cm 3 ~1.8g / cm 3} . As for the electrode density, the higher the volume capacity, the better the adhesion and the cycle characteristics.

<Lithium ion secondary battery>
The lithium ion secondary battery of the present invention includes the above-described negative electrode for a lithium ion secondary battery of the present invention, a positive electrode, and an electrolyte. For example, the negative electrode and the positive electrode for a lithium ion secondary battery of the present invention can be arranged so as to face each other with a separator interposed therebetween, and an electrolytic solution containing an electrolyte can be injected.

  The positive electrode can be obtained by forming a positive electrode layer on the current collector surface in the same manner as the negative electrode. In this case, the current collector may be a band-shaped material made of a metal or an alloy such as aluminum, titanium, or stainless steel in a foil shape, a punched foil shape, a mesh shape, or the like.

The positive electrode material used for the positive electrode layer is not particularly limited. For example, a metal compound, metal oxide, metal sulfide, or conductive polymer material that can be doped or intercalated with lithium ions may be used. It is not limited. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), and double oxides thereof (LiCo _x Ni _y Mn _z O ₂ , x + y + z = 1, 0 <x, 0 <y; LiNi _2-x Mn _x O ₄ , 0 <x ≦ 2), lithium manganese spinel (LiMn ₂ O ₄ ), lithium vanadium compound, V ₂ O ₅ , V ₆ O ₁₃ , VO ₂ , MnO ₂ , _{TiO 2, MoV 2 O 8,} TiS 2, V 2 S 5, VS 2, MoS 2, MoS 3, Cr 3 O 8, Cr 2 O 5, olivine-type _{LiMPO 4 (M: Co, Ni} , Mn, Fe) , Conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, porous carbon, etc. alone or in combination Can be used.

  As the separator, for example, a nonwoven fabric mainly composed of polyolefin such as polyethylene and polypropylene, cloth, microporous film, or a combination thereof can be used. In addition, when it is set as the structure where the positive electrode and negative electrode of the lithium ion secondary battery to produce are not in direct contact, it is not necessary to use a separator.

Examples of the electrolyte include lithium salts such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF _{3, which} are electrolytes, ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentacarbonate. Non, 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-methyltetrahydro A so-called organic electrolyte solution dissolved in a non-aqueous solvent of a simple substance such as furan, 1,3-dioxolane, methyl acetate, ethyl acetate or a mixture of two or more components can be used.

Although the structure of the lithium ion secondary battery of the present invention is not particularly limited, usually, a positive electrode and a negative electrode, and a separator provided as necessary, are wound into a flat spiral to form a wound electrode group, In general, these are laminated as a flat plate to form a laminated electrode plate group, or the electrode plate group is enclosed in an exterior body.
The lithium ion secondary battery of the present invention is not particularly limited, but is used as a paper-type battery, a button-type battery, a coin-type battery, a laminated battery, a cylindrical battery, a rectangular battery, or the like.

  The above-described negative electrode material for a lithium ion secondary battery according to the present invention has been described as being used for a lithium ion secondary battery. However, in general, electrochemical devices having a charge / discharge mechanism that inserts and desorbs lithium ions, such as hybrid capacitors, etc. It is also possible to apply to.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, “part” and “%” are based on mass.

<Example 1>
Using spheroidized natural graphite particles having a volume average particle diameter of 23 μm and an average fracture strength of 28 MPa, and spheroidized natural graphite particles having a volume average particle diameter of 10 μm and an average fracture strength of 51 MPa, the total content of graphite particles The negative electrode material 1 for lithium ion secondary batteries was obtained by mixing so that the content ratio of graphite particles having a volume average particle diameter of 10 μm and an average fracture strength of 51 MPa was 10% by mass.

(Measurement of fracture strength ratio)
About the negative electrode material 1 for lithium ion secondary batteries obtained above, a particle size distribution is measured using a laser diffraction type particle size distribution measuring apparatus (for example, SALD-3000J manufactured by Shimadzu Corporation), and a particle size D90 The particle diameter D10 was determined. Further, using a micro compression tester (for example, MCT-W500 manufactured by Shimadzu Corporation), the fracture strength ratio which is the ratio of the fracture strength of the carbon particles having the particle diameter D10 to the fracture strength of the carbon particles having the particle diameter D90 When (P _D10 / P _D90 ) was determined, it was 2.1.

(Tap density)
For negative electrode material 1 for a lithium ion secondary battery obtained above was slowly charged sample powder 100 cm ³ graduated cylinder volume 100 cm ^3, and the stoppered graduated cylinder, 250 Kai落the graduated cylinder from a height of 5cm The tap density was measured from the mass and volume of the sample powder after being lowered. The tap density was 1.2 g / cm ³ .

(Preparation of negative electrode for lithium ion secondary battery)
When the total solid content of the negative electrode material slurry is 100% by mass, the negative electrode material 1 for lithium ion secondary battery obtained above is mixed to 96% by mass, and acetylene black is mixed to 1% by mass as a conductive auxiliary. Further, a 5% by mass solution of PVDF (polyvinylidene fluoride, manufactured by Kureha Chemical Co., # 9305) as an organic binder was added to a solid content of 3% by mass and kneaded for 30 minutes. Next, N-methyl-2pyrrolidone (manufactured by Wako Chemical Co., Ltd.) was added so that the solid content concentration was 40 to 50% by mass and mixed for 20 minutes to prepare a paste-like negative electrode material slurry.

The obtained negative electrode material slurry was applied to an electrolytic copper foil having a thickness of 10 μm so as to be 10 mg / cm ² and dried at 80 ° C. for 15 minutes. Furthermore, it heat-processed at 105 degreeC in air | atmosphere for 1 hour, and produced the negative electrode 1 for lithium ion secondary batteries.

<Example 2>
Using spheroidized natural graphite particles having a volume average particle diameter of 23 μm and an average fracture strength of 28 MPa, and spheroidized natural graphite particles having a volume average particle diameter of 10 μm and an average fracture strength of 51 MPa, the total content of graphite particles A negative electrode material 2 for a lithium ion secondary battery was obtained by mixing so that the content ratio of spheroidized natural graphite particles having a volume average particle diameter of 10 μm and an average breaking strength of 51 MPa was 20 mass%.

The fracture strength ratio (P _D10 / P _D90 ) of the obtained negative electrode material 2 for lithium ion secondary batteries was 2.4. The tap density was 1.1 g / cm ³ .
Moreover, the negative electrode 2 for lithium ion secondary batteries was produced like Example 1 except having used the negative electrode material 2 for lithium ion secondary batteries.

<Example 3>
Using spheroidized natural graphite particles having a volume average particle diameter of 23 μm and an average fracture strength of 28 MPa, and spheroidized natural graphite particles having a volume average particle diameter of 10 μm and an average fracture strength of 51 MPa, the total content of graphite particles The negative electrode material 3 for lithium ion secondary batteries was obtained by mixing so that the content ratio of spheroidized natural graphite particles having a volume average particle diameter of 10 μm and an average breaking strength of 51 MPa was 50 mass%.

The fracture strength ratio (P _D10 / P _D90 ) of the obtained negative electrode material 3 for a lithium ion secondary battery was 2.6. The tap density was 1.1 g / cm ³ .
Moreover, the negative electrode 3 for lithium ion secondary batteries was produced like Example 1 except having used the negative electrode material 3 for lithium ion secondary batteries.

<Example 4>
Using spherical natural graphite particles having a volume average particle diameter of 20 μm and an average breaking strength of 33 MPa, and spherical natural graphite particles having a volume average particle diameter of 15 μm and an average breaking strength of 97 MPa, the total content of the graphite particles The negative electrode material 4 for lithium ion secondary batteries was obtained by mixing so that the content ratio of spheroidized natural graphite particles having a volume average particle diameter of 15 μm and an average breaking strength of 97 MPa was 20 mass%.

The fracture strength ratio (P _D10 / P _D90 ) of the obtained negative electrode material 4 for lithium ion secondary batteries was 3.1. The tap density was 1.1 g / cm ³ .
Moreover, the negative electrode 4 for lithium ion secondary batteries was produced like Example 1 except having used the negative electrode material 4 for lithium ion secondary batteries.

<Example 5>
Using spherical natural graphite particles having a volume average particle diameter of 20 μm and an average breaking strength of 5 MPa, and flaky amorphous carbon particles having a volume average particle diameter of 5 μm and an average breaking strength of 105 MPa, graphite particles and amorphous carbon The negative electrode material 5 for a lithium ion secondary battery is obtained by mixing so that the content ratio of the flaky amorphous carbon particles having a volume average particle diameter of 5 μm and an average breaking strength of 105 MPa with respect to the total content of the particles is 20 mass%. It was.

The fracture strength ratio (P _D10 / P _D90 ) of the obtained negative electrode material 5 for a lithium ion secondary battery was 34. The tap density was 0.9 g / cm ³ .
Moreover, the negative electrode 5 for lithium ion secondary batteries was produced like Example 1 except having used the negative electrode material 5 for lithium ion secondary batteries.

<Example 6>
Using spherical natural graphite particles having a volume average particle diameter of 20 μm and an average fracture strength of 5 MPa, and flaky amorphous carbon particles having a volume average particle diameter of 5 μm and an average fracture strength of 105 MPa, graphite particles and amorphous The volume average particle diameter with respect to the total content of the carbonaceous particles was 5 μm, and the content ratio of the scaly amorphous carbon particles having an average breaking strength of 105 MPa was mixed to 10 mass%.
This is filled into a rubber container and sealed, and then the rubber container is isotropically pressed with a hydrostatic pressure press at a pressure of 9.8 × 10 ⁷ Pa (1000 kgf / cm ² ). Went. Subsequently, after crushing using a cutter mill, it was passed through a 250 mesh sieve to obtain a negative electrode material 6 for a lithium ion secondary battery.

The fracture strength ratio (P _D10 / P _D90 ) of the obtained negative electrode material 5 for a lithium ion secondary battery was 24. The tap density was 0.9 g / cm ³ .
Moreover, the negative electrode 6 for lithium ion secondary batteries was produced like Example 1 except having used the negative electrode material 6 for lithium ion secondary batteries.

<Example 7>
Using spherical natural graphite particles having a volume average particle diameter of 20 μm and an average fracture strength of 5 MPa, and flaky amorphous carbon particles having a volume average particle diameter of 5 μm and an average fracture strength of 105 MPa, graphite particles and amorphous The volume average particle diameter with respect to the total content of the carbonaceous particles was 5 μm, and the mixture was mixed so that the content ratio of the scaly amorphous carbon particles having an average breaking strength of 105 MPa was 20% by mass.
This is filled into a rubber container and sealed, and then the rubber container is isotropically pressed with a hydrostatic pressure press at a pressure of 9.8 × 10 ⁷ Pa (1000 kgf / cm ² ). Went. Subsequently, after crushing using a cutter mill, it was passed through a 250 mesh sieve to obtain a negative electrode material 7 for a lithium ion secondary battery.

The fracture strength ratio (P _D10 / P _D90 ) of the obtained negative electrode material 5 for lithium ion secondary batteries was 32.5. The tap density was 0.9 g / cm ³ .
Moreover, the negative electrode 7 for lithium ion secondary batteries was produced like Example 1 except having used the negative electrode material 7 for lithium ion secondary batteries.

<Comparative Example 1>
For a lithium ion secondary battery, the same procedure as in Example 1 was used, except that spheroidized natural graphite particles having a volume average particle diameter of 20 μm and an average breaking strength of 28 MPa were used as the negative electrode material C1 for a lithium ion secondary battery. A negative electrode C1 was produced.
Moreover, the fracture strength ratio (P _D10 / P _D90 ) of the negative electrode material C1 was 1.4. The tap density was 1.1 g / cm ³ .

<Comparative example 2>
Using spherical natural graphite particles having a volume average particle diameter of 23 μm and an average breaking strength of 28 MPa, and spherical natural graphite particles having a volume average particle diameter of 20 μm and an average breaking strength of 33 MPa, the total content of graphite particles Mixing was performed so that the content ratio of the spheroidized natural graphite particles having a volume average particle diameter of 20 μm and an average breaking strength of 33 MPa was 10% by mass to obtain a negative electrode material C2 for a lithium ion secondary battery.

The fracture strength ratio (P _D10 / P _D90 ) of the obtained negative electrode material C2 for lithium ion secondary batteries was 1.4. The tap density was 1.1 g / cm ³ .
Moreover, the negative electrode C2 for lithium ion secondary batteries was produced like Example 1 except having used the negative electrode material C2 for lithium ion secondary batteries.

<Electrolytic solution permeability>
About the negative electrode material for lithium ion secondary batteries obtained above, electrolyte solution permeability was evaluated as follows. The results are shown in Table 1.
Each of the negative electrode materials for lithium ion secondary batteries obtained above is punched into a circle of 14 mmφ, pressed with a hand press, adjusted to an electrode density of 1.6 g / cm ^3, and used as an evaluation sample. did.
As an electrolytic solution, LiPF ₆ was dissolved to a concentration of 1 mol / L in a mixed solvent of 3 to 7 in a volume ratio of ethyl carbonate and methyl ethyl carbonate, and 0.5% by mass of vinyl carbonate was added thereto. It was used.
1 μL of the electrolytic solution was taken in a microsyringe, dropped on the obtained sample for evaluation, and visually observed, and the permeation time (seconds) was measured as the time until the droplet disappeared.

  Table 1 shows that the negative electrode for lithium ion secondary batteries produced using the negative electrode material for lithium ion secondary batteries of the present invention is excellent in electrolyte permeability.

<Electrochemical characteristics>
The electrochemical measurement was performed by producing a lithium ion secondary battery (coin cell) for evaluation and measuring the discharge capacity retention rate as follows.
(Preparation of positive electrode for lithium ion secondary battery)
When the total solid content of the positive electrode material slurry is 100% by mass, 94% by mass of LiCoO ₂ is mixed as the positive electrode active material, and 3% by mass of acetylene black is mixed as the conductive assistant, and further, as an organic binder. A 12% by mass solution of PVDF (polyvinylidene fluoride, manufactured by Kureha Chemical Co., # 1120) was added to a solid content of 3% by mass and kneaded for 30 minutes. Next, N-methyl-2pyrrolidone (manufactured by Wako Chemical Co., Ltd.) was added so that the solid content concentration was 50% by mass to 60% by mass and mixed for 20 minutes to prepare a paste-like positive electrode material slurry.

The obtained positive electrode material slurry was applied to an aluminum foil having a thickness of 21 μm so as to be 26 mg / cm ² and dried at 80 ° C. for 15 minutes. Further, after heat treatment at 105 ° C. for 1 hour in the air, it was punched into a circle of 14 mmφ. The electrode density was adjusted to 3.2 g / cm ³ by pressure molding with a hand press, and a positive electrode for a lithium ion secondary battery was produced.

Each of the negative electrodes for lithium ion secondary batteries obtained above was punched into a 16 mmφ circle, press molded with a hand press, and the electrode density was adjusted to 1.5 g / cm ³ , and obtained above. A CR2016 type coin cell was assembled in an argon circulation type glove box together with the positive electrode, separator and electrolyte. In the electrolyte solution, ethyl carbonate and methyl ethyl carbonate were dissolved in a mixed solvent with a volume ratio of 3 to 7 to a concentration of 1 mol / L of LiPF ₆ and 0.5% by mass of vinyl carbonate was added thereto. We used what we did. A polyethylene microporous membrane was used as the separator.

  The charging condition of the obtained coin cell was that the battery was charged to a battery voltage of 4.15 V at a constant current of 0.1 C (0.42 mA), and then the current density was 0.01 C (0.042 mA) at a battery voltage of 4.15 V. The battery was charged at a constant voltage until The discharge condition was a discharge at a constant current of 0.1 C to 2.7 V (0.42 mA). This test was performed for 4 cycles. From the fifth cycle onward, after charging to a battery voltage of 4.15 V at a constant current of 1.0 C (4.2 mA), until the current density reaches 0.01 C (0.042 mA) at a battery voltage of 4.15 V Charged at a constant voltage. As the discharge condition, a test for discharging to 2.7 V (4.2 mA) at a constant current of 1.0 C was repeated up to 300 cycles.

  The discharge capacity maintenance rate of each cycle was the ratio of the discharge capacity of each cycle to the discharge capacity of the fifth cycle. Table 2 shows the discharge capacity retention rates at the 100th cycle and the 300th cycle.

  Table 2 shows that the lithium ion secondary battery produced using the negative electrode for lithium ion secondary batteries produced using the negative electrode material for lithium ion secondary batteries of the present invention is excellent in cycle characteristics.

<Section observation>
The negative electrode materials for lithium ion secondary batteries obtained in Example 7 and Comparative Example 1 were each punched into a circle of 14 mmφ. The electrode density was adjusted to 1.7 g / cm ³ by pressure molding with a hand press, and this was used as a sample for cross-sectional observation.
About each of the samples for cross-section observation, the cross section was produced using the ion milling apparatus (Hitachi High-Tech Co., Ltd. product: E-3500). Using a scanning electron microscope (manufactured by Keyence Corporation: VE-7800), the state of particles in the cross section was observed. The SEM image of the electrode cross section of Comparative Example 1 is shown in FIGS. 1 and 2, and the SEM image of the electrode cross section of Example 7 is shown in FIGS.

  1-4, the distribution of interparticle voids is nonuniform in the electrode cross section of the negative electrode of Comparative Example 1, whereas the distribution of interparticle voids is uniform in the electrode cross section of the negative electrode of Example 7. I understand. Moreover, it turns out that the fine space | gap exists continuously between particles in the electrode cross section of the negative electrode of Example 7. FIG.

Claims (8)

A carbon particle mixture comprising at least two carbon particle groups having different average fracture strengths,
Of all the carbon particle groups before mixing constituting the carbon particle mixture, the first carbon particle group having the smallest average breaking strength and the second carbon particle group having the largest average breaking strength, A negative electrode material for a lithium ion secondary battery, wherein a ratio of an average breaking strength of the second carbon particle group to an average breaking strength of the carbon particle group is 1.5 or more and 50 or less.   2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein a ratio of the content of the second carbon particle group to the content of the first carbon particle group is 1% by mass or more and 50% by mass or less. 3. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the carbon particle mixture is an isotropically pressure-treated product with a pressure of 4.9 × 10 ⁶ Pa to 4.9 × 10 ⁸ Pa. 4. Wood.   The volume average particle diameter (D50) of the first carbon particle group is 1 μm or more and 40 μm or less, the volume average particle diameter (D50) of the second carbon particle group is 1 μm or more and 40 μm or less, and the first carbon particle group The ratio of the volume average particle diameter (D50) of said 2nd carbon particle group with respect to the volume average particle diameter (D50) of 1 or more and 50 or less, The lithium ion of any one of Claims 1-3. Secondary battery negative electrode material. Mixing at least two carbon particle groups having different average fracture strengths to obtain a carbon particle mixture,
Of all the carbon particle groups before mixing, the first carbon particle group having the smallest average breaking strength and the second carbon particle group having the largest average breaking strength, the average breaking strength of the first carbon particle group A method for producing a negative electrode material for a lithium ion secondary battery, wherein the ratio of the average breaking strength of the second carbon particle group to 1.5 to 50 is 1.5. The method for producing a negative electrode material for a lithium ion secondary battery according to claim 5, further comprising a step of subjecting the carbon particle mixture to isotropic pressure treatment at a pressure of 4.9 x 10 ⁶ Pa to 4.9 x 10 ⁸ Pa. Method.   A lithium ion secondary comprising: a current collector; and a negative electrode material layer provided on the current collector and comprising the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4. Battery negative electrode.   The lithium ion secondary battery containing the negative electrode for lithium ion secondary batteries of Claim 7, a positive electrode, and electrolyte.