JP2012124116A - 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 realizing a lithium ion secondary battery having excellent cycle characteristics and a high level of safety, and inhibited from expanding with charges.SOLUTION: The negative electrode material for a lithium ion secondary battery contains composite particles made by compositing the surfaces of first particles composed of graphite having circularity of 0.60 to 1.00 with second particles containing silicon using a carbonaceous material.

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.

As mobile devices such as mobile phones and notebook computers become more sophisticated, there is a strong demand for higher capacity lithium ion secondary batteries. At present, graphite is mainly used as the negative electrode material for lithium ion secondary batteries.However, in order to further increase the capacity, elements with high theoretical capacity and capable of occluding and releasing lithium ions (hereinafter referred to as `` specific elements ''). ", And those containing the specific element are also called" specific element bodies ") are being actively developed.
As the specific element, silicon, tin, lead, aluminum and the like are well known. Among them, silicon and silicon oxide have advantages such as higher capacity, lower cost, and better workability than those made of other specific elements, and research on negative electrode materials using them is particularly active.

On the other hand, these specific element bodies are known to undergo large volume expansion when alloyed by charging. Such volume expansion makes the specific element bodies themselves finer, and further, the structure of the negative electrode material using these elements is broken and the conductivity is cut. Therefore, it has been a problem that the capacity is remarkably lowered with the passage of cycles.
In response to this problem, a technique has been proposed in which a specific element body is made into fine particles and combined with graphite using a carbonaceous material or a resin. In such composite particles, even if the specific element is alloyed with Li and the conductivity can be ensured by graphite or a carbonaceous material even if it is refined, the cycle characteristics can be significantly improved as compared with the case where the specific element body alone is used as a negative electrode material. Are known.
However, the expansion during Li alloying may still destroy the composite particle structure, cutting the conductivity in the composite particles and failing to obtain sufficient cycle characteristics, mainly for the purpose of absorbing and mitigating this expansion. In addition, studies focusing on the introduction of voids into the composite particles have been actively conducted (for example, see Patent Documents 1 to 4).

Japanese Patent No. 3466576 JP 2006-228640 A Japanese Patent No. 39955050 Japanese Patent No. 3998753

  The powdered negative electrode material containing the composite particles as described above is generally used after being applied to a current collector and adjusting the electrode density by a roll press or the like. However, the composite particles containing many voids as described above have poor compressibility at the time of roll pressing and the electrode density is low, so that a sufficient capacity increasing effect cannot be obtained when a lithium ion secondary battery is configured. There is a case. On the other hand, when the density is increased by roll pressing at high pressure, the voids in the composite particles are almost crushed, so that the absorption and relaxation action of expansion due to the voids may be reduced, and the improvement effect of cycle characteristics may be reduced.

In addition, when preparing composite particles having voids as described above, a specific element fine particle is used with a large amount of carbonaceous material such as fine graphite, and a void forming material is added as necessary to form composite particles. It is common to do. However, such particles generally have a high specific surface area, and since they contain a large amount of low crystalline carbon, the charge / discharge efficiency is lowered, and the increase in capacity as a battery may be insufficient.
Furthermore, in the composite particles as described above, the specific element fine particles are also distributed inside the composite particles. In such a case, the composite particles may expand excessively due to a synergistic action because the composite particles expand while forming a space in the interior as the specific element fine particles present inside the composite particles expand. For this reason, the composite particles, further the negative electrode, and the expansion amount thereof become larger than the expansion amount of the specific element fine particles themselves, and as a result, the battery cell may expand, which may cause a problem in safety.

  This invention is made | formed in view of the above conventional trouble, and makes it a subject to achieve the following objectives. That is, an object of the present invention is to provide a lithium ion secondary battery having excellent cycle characteristics. Also, it is possible to provide a lithium ion secondary battery negative electrode material capable of constituting a lithium ion secondary battery excellent in both cycle characteristics and safety and suppressing expansion due to charging, and a lithium ion secondary battery negative electrode. With the goal.

Specific means for solving the above problems are as follows.
<1> Composite particles in which second particles containing silicon atoms are compounded with a carbonaceous material so as to be unevenly distributed on the surface of the first particles that are graphite having a circularity of 0.60 to 1.00 A negative electrode material for a lithium ion secondary battery.

<2> 1/8 of the length of the short axis perpendicular to the midpoint of the long axis centered on the midpoint of the long axis when the cross section of the composite particle is observed Included in the inner region from the outer periphery of the composite particle to the inner depth of 1/8 the length of the short axis with respect to the content of silicon atoms contained in the inner region of the circle whose radius is the length of The negative electrode material for a lithium ion secondary battery according to <1>, wherein the ratio of the content of silicon atoms is 2 or more.

<3> The lithium ion according to <2>, wherein the ratio of the content of silicon atoms contained in the inner region of the circle to the total content of silicon atoms contained in the cross section of the composite particle is 0.2 or less. Secondary battery negative electrode material.

<4> The negative electrode material for a lithium ion secondary battery according to any one of <1> to <3>, wherein the volume average particle diameter of the first particles is 5 μm or more and 40 μm or less.

<5> The content of the carbonaceous substance is 1% by mass or more and 10% by mass or less in the entire composite particle, and the carbonaceous substance is an organic carbonized product, and any one of the items <1> to <4> The negative electrode material for lithium ion secondary batteries as described in the item.

<6> The negative electrode material for a lithium ion secondary battery according to any one of <1> to <5>, further including a conductive material.

<7> A first particle which is graphite having a circularity of 0.60 to 1.00 and a second particle containing a silicon atom are combined using a carbonaceous material, and the volume average particle diameter is the above-mentioned The lithium ion according to any one of <1> to <6>, which includes a step of obtaining composite particles that are 1.0 to 1.3 times the volume average particle diameter of the first particles. A method for producing a negative electrode material for a secondary battery.

<8> A negative electrode for a lithium ion secondary battery, comprising the negative electrode material for a lithium ion secondary battery according to any one of <1> to <6>.

<9> A lithium ion secondary battery comprising the lithium ion secondary battery negative electrode according to <8>, a positive electrode, and an electrolyte.

  According to the present invention, a lithium ion secondary battery excellent in both cycle characteristics and safety can be provided. In addition, it is possible to provide a lithium ion secondary battery negative electrode material capable of constituting a lithium ion secondary battery having excellent cycle characteristics and suppressing expansion due to charging, and a lithium ion secondary battery negative electrode.

It is a figure which shows an example of the cross section of the composite particle concerning Example 2 of this invention. It is a figure which shows an example of the center part of the cross section of the composite particle concerning Example 2 concerning this invention. It is a figure which shows an example of the surface part of the cross section of the composite particle concerning Example 2 concerning this invention. It is a figure which shows an example of the cross section of the composite particle concerning the comparative example 2 of this invention. It is a figure which shows an example of the center part of the cross section of the composite particle concerning the comparative example 2 concerning this invention. It is a figure which shows an example of the surface part of the cross section of the composite particle concerning the comparative example 2 concerning this invention.

In this specification, the term “process” is not limited to an independent process, and is included in the term if the intended action of the process is achieved even when it cannot be clearly distinguished from other processes. .
In the present specification, a numerical range indicated by using “to” indicates a range including the numerical values described before and after “to” as the minimum value and the maximum value, respectively.

<Anode material for lithium ion secondary battery>
The negative electrode material for lithium ion secondary batteries of the present invention (hereinafter also simply referred to as “negative electrode material”) is unevenly distributed on the surface of the first particles that are graphite having a circularity of 0.60 to 1.00. The second particles containing silicon atoms include at least one kind of composite particles composited with a carbonaceous material, and include other components as necessary.

In the composite particles, the second particles are unevenly distributed on the surface of the first particles. That is, there are more second particles near the surface than inside the composite particles.
In such a composite particle, when the cross section thereof is observed, the center of the long axis which is the maximum length of the composite particle is the center, and the length of the short axis perpendicular to the midpoint of the long axis is The inner side from the outer periphery of the composite particle to the inner side to the depth of 1/8 of the length of the short axis with respect to the content of silicon atoms contained in the inner region of the circle having a radius of 1/8. The ratio of the content of silicon atoms contained in the region is preferably 2 or more.

By being in such a composite state, the volume expansion of the second particles during charging occurs almost in the vicinity of the surface of the composite particles and hardly occurs inside the composite particles, thereby suppressing excessive expansion of the composite particles themselves. Can do.
A lithium ion secondary battery comprising a negative electrode for a lithium ion secondary battery formed using a negative electrode material for a lithium ion secondary battery containing such composite particles is excellent in cycle characteristics and safety, and further has a battery. Excellent capacity and charge / discharge efficiency.

[Composite particles]
In the composite particles, the first particles and the second particles are composited with a carbonaceous material. Here, “composite” means that a plurality of different elements are integrated.
The composite particles in the present invention are those in which at least the first particles and the second particles are integrated, and a plurality of second particles are integrated with the first particles to constitute independent particles. It is preferable.
As a specific aspect of the composite in the composite particle, a carbonaceous substance exists between the first particle and the second particle, and the second particle adheres to the surface of the first particle and is integrated. The first particle and the second particle are in direct contact, and the carbonaceous material is in contact with both the first particle and the second particle, so that the second particle is on the surface of the first particle. The mode which the particle | grains adhere and are integrated is mentioned. That is, the carbonaceous material has a function of connecting and integrating the first particles and the second particles.

  The composite state of the first particles and the second particles in the composite particles is preferably determined as follows, for example. Furthermore, a negative electrode material for a lithium ion secondary battery including composite particles that satisfies the following requirements is included in the scope of the present invention.

  The composite state of the composite particles is judged from the distribution state of the second particles by observing the cross section of the composite particles. The method for observing the cross section of the composite particle is not particularly limited. For example, a slurry containing composite particles and an organic binder as described below is prepared, and this is applied and dried to produce a coated electrode. The cross section of the obtained coated electrode is processed by a focused ion beam (FIB), ion milling, or the like to prepare a sample from which the composite particles are cut. Examples thereof include a method of observing the cross section of the composite particle thus obtained with a scanning electron microscope (SEM), a scanning ion microscope (SIM), or the like.

In the composite particle cross section observed by the method as described above, a composite particle satisfying the following conditions (a) and (b) is set as a target particle for the composite state determination. In addition, the definition of the major axis and the minor axis in the cross-sectional observation of (a) and (b) will be described later.
(A) Particle size The particle size of the composite particles is approximately the same as the volume average particle size (50% D) measured by a laser diffraction particle size distribution analyzer. Specifically, target particles are composite particles having a ratio of the length of the major axis in the cross-sectional observation of the composite particles to the volume average particle diameter of 1.0 to 1.2.
(B) Particle state The broken composite particles and the broken composite particles are excluded because they are not suitable for the judgment of the composite state. That is, a composite particle in which the number of intersections between the major axis and the minor axis in the cross-sectional observation of the composite particle and the outer periphery of the composite particle are both 2 is a target particle.

The cross section of the composite particle corresponding to the condition of the target particle of the composite state determination is observed, and the long axis center and the short axis length in the cross section of the composite particle are selected as follows.
Two parallel tangents circumscribing the outer periphery of the composite particle, the tangent m ₁ and the tangent m ₂ having the maximum distance are selected. The distance between the tangent line m ₁ and the tangent line m ₂ is the maximum length of the composite particle, that is, the length of the long axis.
However, the length of the long axis in the cross section of the composite particle is 70% or more with respect to the maximum length of the composite particle obtained by observing the entire image of the composite particle with a scanning electron microscope (SEM) or the like. Is preferably selected. In other words, the cross section of the composite particle is preferably selected so as to include the length of the long axis of the composite particle itself or the length close to that of the composite particle as a whole in three dimensions. Note that the length of the long axis of the composite particle itself is given as a distance between two parallel planes circumscribing the composite particle and having the maximum distance.

Next, on the cross section of the composite particle, two parallel tangent lines n ₁ and tangent line n ₂ that are orthogonal to the tangent line m ₁ and the tangent line m ₂ and circumscribe the outer periphery of the composite particle are selected.
A straight line parallel to the tangent n _1, a distance equal to the straight line of the distance to the tangent n ₂ to the tangent n ₁ is the major axis of the composite particles. Intersections between the major axis and the tangent line m ₁ and tangent line m ₂ are defined as intersection point P ₁ and intersection point P ₂ , respectively, and the midpoint of the line segment connecting intersection point P ₁ and intersection point P ₂ is defined as the midpoint of the major axis. A straight line passing through the midpoint of this long axis and orthogonal to the long axis is taken as the short axis. The distance between the two intersections Q ₁ and intersection Q ₂ between the minor axis and the outer periphery of the composite particles to the length of the minor axis.

Next, on the cross section of the composite particle, a circle having a radius R of 1/8 of the length of the short axis as a center is drawn, and the inner region of the circle is defined as the center of the composite particle. Part.
On the other hand, on the cross section of the composite particle, the inner region from the outer periphery to the depth of the length R is defined as the surface portion of the composite particle.
Here, when an overlap portion is generated between the center portion and the surface portion, it is excluded from the target particles for the composite state determination.

The central portion and the surface portion of the composite particle determined as described above are observed using an SEM, and an element contained in the observed region is applied by applying an X-ray spectrometer to the observed region. Quantitative analysis. Using the element mass concentration thus obtained, the conditions of the following composite state are evaluated, and composite particles satisfying the conditions are identified as constituting the negative electrode material for a lithium ion secondary battery of the present invention.
The X-ray spectrometer is not particularly limited as long as the elements contained in the observation region can be quantified. For example, an energy dispersion type (EDX) and a wavelength dispersion type (WDX) can be used.

(Compound condition)
The ratio of the content of silicon atoms contained in the surface portion of the composite particle to the content of silicon atoms contained in the central portion (surface portion / center portion) is preferably 2 or more, and preferably 3 or more. More preferably, it is 5 or more. This means that the second particles containing silicon atoms are preferably unevenly distributed on the surface of the composite particles.
The ratio is the ratio of the content of silicon atoms to the total content of carbon atoms, oxygen atoms and silicon atoms in the central portion (Si / (C + O + Si)), and the total content of carbon atoms, oxygen atoms and silicon atoms in the surface portion. The ratio of the content of silicon atoms with respect to the amount is obtained and calculated as these ratios.
Specifically, for example, when quantitative analysis is performed by EDX, if only quantitative analysis is performed for carbon atoms, oxygen atoms, and silicon atoms, the mass concentration of silicon atoms in the central portion and the surface portion is simply compared, A ratio can be obtained.
When the ratio is 2 or more, the expansion of the composite particles can be suppressed, and good cycle characteristics can be obtained.

  The ratio of the content of silicon atoms to the total content of carbon atoms, oxygen atoms and silicon atoms in the central portion and the surface portion is specifically the length of one side in each of the central portion and the surface portion. Three square areas that are 1/5 to 1/2 of R are selected so as not to overlap as much as possible. About the selected square area | region, the content ratio of the silicon atom with respect to a carbon atom, an oxygen atom, and a silicon atom is measured, respectively, and it calculates as an arithmetic average value of those measured values.

In the present invention, the ratio of the content of silicon atoms contained in the central portion to the total content of silicon atoms contained in the cross section of the composite particles is preferably 0.2 or less, preferably 0.15 or less. More preferably. This means that silicon atoms are not substantially present in the central portion of the composite particle.
The total content of silicon atoms contained in the cross section of the composite particle and the content of silicon atoms contained in the central portion can be obtained in the same manner as described above.

  In the present invention, the condition of the composite state is evaluated for 10 composite particles satisfying the above conditions (a) and (b), and the present invention is used when 3 or more, preferably 5 or more composite particles satisfy the conditions. It can be judged that this is a negative electrode material for a lithium ion secondary battery.

  Examples of a method for configuring the composite state of the composite particles in the present invention as described above include a method of manufacturing composite particles by a method of manufacturing composite particles as described below.

(First particle)
The negative electrode material for a lithium ion secondary battery of the present invention includes at least one first particle that is graphite having a circularity of 0.60 to 1.00.
The first particles are made of graphite and have a circularity of 0.60 to 1.00. By being such a form, it can suppress that each composite particle which comprises a negative electrode material will orient in a surface direction in the case of the pressurization at the time of electrode formation. Thereby, it is easy to transfer Li ions in the composite particles, and a battery having excellent rate characteristics can be configured.
The circularity is preferably 0.60 to 0.95, more preferably 0.65 to 0.90, and still more preferably 0.70 to 0.90, from the viewpoint of particle orientation control. When the degree of circularity is less than 0.60, the composite particles are oriented in the plane direction in the press during electrode formation, and the rate characteristics tend to be lowered. On the other hand, 1.00 is a perfect circle and is the upper limit.

Here, the circularity is the circumference measured from the projected image of the first particle as the circumference calculated as the circle equivalent diameter, which is the diameter of a circle having the same area as the projected area of the first particle. It is a numerical value obtained by dividing by the length (length of the contour line), and is obtained by the following formula. The circularity is 1.00 for a perfect circle.
Circularity = (perimeter of equivalent circle) / (perimeter of particle cross-sectional image)
Specifically, the circularity is measured by observing an image magnified 1000 times with a scanning electron microscope, arbitrarily selecting 10 first particles, and measuring the circularity of each carbon particle by the above method. The average circularity calculated as the arithmetic average value. The circularity, the circumference of the equivalent circle, and the circumference of the projected image of the particle can be obtained by commercially available image analysis software.

  The form and shape of the first particles are not particularly limited as long as the circularity is 0.60 to 1.00. Examples of the form include single particles composed of one particle, and granulated particles formed by granulating a plurality of primary particles. In addition, the shape may be a spherical particle if it is a single particle. Examples of the granulated particles include various shapes such as a spherical shape or a porous shape.

  Regarding the form of the first particles, granulated particles are preferable to single particles from the viewpoint of rate characteristics when a battery is constructed. This is because, for example, particles formed by granulating a plurality of graphite can more easily suppress the orientation of the particles in the plane direction when pressed to increase the density when forming the electrode. It can be considered that the rate characteristics are improved because the transfer and reception of Li ions in the particles is more efficient.

The shape of the granulated particles is preferably porous rather than spherical from the viewpoint of rate characteristics when a battery is constructed. For example, in the case of porous granulated particles, the presence of the internal space makes it easier for Li ions to diffuse, so it can be considered that the rate characteristics are improved.
Among these, porous particles having a small porosity so that the second particles do not enter the center of the granulated particles are preferable. By using such granulated particles, a high tap density can be achieved when composite particles are formed, and a high volume capacity can be achieved because the electrode density of the formed electrodes is improved.

The first particles preferably have an average interplanar spacing (d ₀₀₂ ) of 0.335 nm to 0.338 nm obtained by measurement based on the Gakushin method. Examples of the graphite that satisfies this include artificial graphite, natural graphite, graphitized MCMB (mesophase carbon microbeads), and the like.

From the viewpoint of battery capacity, the average interplanar spacing (d ₀₀₂ ) is more preferably 0.335 nm to 0.337 nm, and further preferably 0.335 nm to 0.336 nm. When the average interval is 0.338 nm or less, the crystallinity as graphite is high, and both battery capacity and charge / discharge efficiency tend to be improved. On the other hand, since the theoretical value of the graphite crystal is 0.335 nm, the battery capacity and the charge / discharge efficiency tend to be improved closer to this value.

  The volume average particle diameter (50% D) of the first particles is not particularly limited, but is preferably larger than the second particles described later, preferably 5 μm to 40 μm, and preferably 5 μm to 35 μm. More preferably, it is more preferably 7 μm to 30 μm, further preferably 10 μm to 30 μm.

When the volume average particle diameter is 5 μm or more, the specific surface area is prevented from becoming too large, and the initial charge / discharge efficiency is improved. Further, the electrode density is further improved, and a high capacity lithium ion secondary battery can be obtained. On the other hand, when the volume average particle diameter is 40 μm or less, electrode characteristics such as rate characteristics tend to be improved.
The volume average particle diameter of the first particles is measured under normal conditions using a laser diffraction particle size distribution measuring device.

The first particles can be obtained, for example, as powdered carbon products commercially available from various companies.
In addition, flaky graphite having a circularity of less than 0.60 can be spheroidized using a commonly used spheroidizing method of graphite to obtain a circularity of 0.60 to 1.00. . Furthermore, the 1st particle | grains which consist of a plurality of particles may be prepared by granulating the graphite particles so that the circularity is 0.60 to 1.00 using a granulation method in which graphite particles are usually used.
Examples of the spheronization treatment include a treatment method such as a mechanochemical method. Examples of the granulation method include treatment methods such as a fluidized bed granulation method, a spray granulation method, and a stirring granulation method.

(Second particle)
The negative electrode material for a lithium ion secondary battery of the present invention contains at least one kind of second particles containing silicon atoms. The second particles are not particularly limited as long as they contain silicon atoms. For example, particles containing silicon, particles containing a silicon compound such as silicon oxide, and the like can be given. From the viewpoint of battery capacity, particles containing silicon or silicon oxide are preferable, and particles substantially consisting of silicon or particles consisting essentially of silicon oxide are more preferable.
Here, “substantially” means that impurities inevitably mixed are allowed, and the content of impurities is preferably 10% by mass or less.

The volume average particle diameter of the second particles is not particularly limited, but preferably has a volume average particle diameter smaller than the volume average particle diameter of the first particles, and the volume average particle diameter is 0.01 μm to 5 μm. More preferably, 0.03 to 3 μm is more preferable, 0.05 to 2 μm is further preferable, and 0.1 to 1 μm is particularly preferable.
When the volume average particle diameter of the second particles is 0.01 μm or more, the second particles can be obtained with good productivity, excellent handleability, and can be combined on the surface of the first particles. Can be done efficiently. On the other hand, when the volume average particle diameter is 5 μm or less, it is possible to efficiently combine on the surface of the first particle, and to suppress the expansion of the second particle during charging. The cycle characteristics tend to be improved.

  The silicon oxide is generally represented by SiOx. The range of x is preferably 0.8 ≦ x ≦ 1.6, more preferably 0.9 ≦ x ≦ 1.5, and still more preferably 1.0 ≦ x ≦ 1.4. Manufacture and acquisition are easy because x is 0.8 or more. On the other hand, when x is 1.6 or less, it is possible to suppress an excessive increase in the silicon dioxide portion in the silicon oxide, the diffusion of lithium ions in the silicon oxide is promoted, and the rate characteristics tend to be improved. .

The ratio of the volume average particle diameter of the second particles to the volume average particle diameter of the first particles (the particle diameter of the second particles / the particle diameter of the first particles) is not particularly limited, but cycle characteristics and battery capacity In view of the above, it is preferably 0.0003 to 0.2, and more preferably 0.001 to 0.1.
In the observation of the cross section of the composite particle, the ratio of the long axis length of the second particle to the long axis length of the composite particle (the long axis length of the second particle / the long axis length of the composite particle) In the following, “long axis length ratio”) is preferably 0.0003 to 0.2 and more preferably 0.001 to 0.1 from the viewpoint of cycle characteristics and battery capacity. preferable. Further, when the cross section of 10 composite particles is observed, it is preferable that 5 or more composite particles satisfy this condition, and it is particularly preferable that all particles satisfy this condition.
The major axis length of the second particle is determined in the same manner as the major axis length of the composite particle. When there are a plurality of second particles, the arithmetic average value of the lengths of the major axes of three arbitrarily selected second particles is used.

Further, the content of the second particles contained in the composite particles can be appropriately selected according to the purpose, but from the viewpoint of cycle characteristics and battery capacity, it is 0.5% by mass to 20% by mass in the whole composite particles. Preferably, 1% by mass to 15% by mass is more preferable, and 2% by mass to 10% by mass is more preferable. Battery capacity improves because content of the 2nd particle is 0.5 mass% or more. Moreover, cycling characteristics improve more because it is 20 mass% or less.
Furthermore, the ratio of the content of the second particles to the content of the first particles in the composite particles (the content of the second particles / the content of the first particles) can be appropriately selected according to the purpose, but the cycle From the viewpoint of characteristics and battery capacity, it is preferably 0.005 to 0.3, more preferably 0.01 to 0.25 on a mass basis.

(Carbonaceous material)
The composite particles in the present invention are formed by combining the first particles and the second particles with at least one kind of carbonaceous material. The carbonaceous substance is not particularly limited as long as it is an organic substance as a precursor and is carbonized by heat treatment or the like, and there are no particular restrictions on the type of organic substance used as the precursor, the history of heat treatment, the structure of the carbonaceous substance, and the like.
Examples of the organic material include polymer compounds such as phenol resin and styrene resin, and carbonizable solid materials such as pitch. These can be used as a binder at the time of compounding in a dissolved or solid state.
The composite particles according to the present invention can be obtained by compositing the first particles and the second particles with a precursor of a carbonaceous material and then carbonizing the precursor.

About content of the carbonaceous substance in the said composite particle, it is preferable that it is 1 mass%-10 mass% in the whole composite particle, 1 mass%-8 mass% are more preferable, 2 mass%-8 mass% are still more. 2% by mass to 6% by mass is particularly preferable.
When the amount of the carbonaceous material is 10% by mass or less, the content of amorphous carbon can be suppressed, and the first-time charge / discharge efficiency can be suppressed from decreasing. Further, in the step of producing composite particles, binding between the composite particles can be suppressed, and an increase in particle diameter can be suppressed. On the other hand, when it is 1% by mass or more, cycle characteristics tend to be improved. This can be considered, for example, because the second particles are likely to be efficiently combined with the surface of the first particles.

  Further, the content ratio of the carbonaceous material to the second particle (carbonaceous material B / second particle) is not particularly limited as long as the first particle and the second particle can be combined. For example, from the viewpoint of cycle characteristics and battery capacity, it is preferably 0.1 to 10 and more preferably 0.3 to 5 on a mass basis.

The volume average particle diameter (50% D) of the composite particles in the present invention is not particularly limited. For example, it is preferably 5 μm to 40 μm, more preferably 5 μm to 35 μm, still more preferably 7 μm to 30 μm, and particularly preferably 10 μm to 30 μm.
When the volume average particle diameter of the composite particles is 5 μm or more, an increase in specific surface area can be suppressed, and the initial charge / discharge efficiency is improved. In addition, the electrode density can be easily increased, and the capacity of the lithium ion secondary battery can be increased. On the other hand, when the volume average particle diameter is 40 μm or less, electrode characteristics such as rate characteristics tend to be improved.

  The ratio of the volume average particle diameter of the composite particles to the volume average particle diameter of the first particles (composite particle diameter / first particle diameter) is not particularly limited. From the viewpoint of cycle characteristics and battery capacity, it is preferably 1.0 to 1.3, more preferably 1.01 to 1.25, still more preferably 1.03 to 1.20, and 1.05 to 1.15. Is particularly preferred.

The volume average particle diameter of the composite particles is measured under normal conditions using a laser diffraction particle size distribution measuring apparatus.
Moreover, in the manufacturing method of the composite particle mentioned later, it can control by selecting crushing conditions suitably.

The tap density of the composite particles in the present invention is not particularly limited. For example, preferably a ^{0.6g / cm 3 ~1.2g / cm 3} , 0.7g / cm 3 ~1.2g / cm 3 more ^{preferably, 0.8g / cm 3 ~1.15g / cm} 3 but more ^preferably, particularly preferably ^{0.9g / cm 3 ~1.1g / cm 3} .
Cycle characteristics are improved by being 0.7 g / cm ³ or more. Moreover, the compressibility at the time of pressing at the time of forming a negative electrode is improved, a high electrode density is achieved, and a battery with a higher capacity can be obtained. On the other hand, the battery characteristic deterioration can be suppressed by being 1.2 g / cm ³ or less. This may be because, for example, the particle diameter of the composite particles or the density of the composite particles themselves affects the exchange and diffusion of Li ions.
The tap density of the composite particles is measured according to JIS standard R1628.

[Substance with conductivity]
The negative electrode material for a lithium ion secondary battery of the present invention preferably contains at least one material having conductivity in addition to the composite particles.
Examples of the conductive material include carbon black, graphite, coke, carbon fiber, and carbon nanotube.
In addition, the type and shape of the conductive material can be appropriately selected according to the purpose. For example, graphite and the like are preferable from the viewpoint of capacity and efficiency, and carbon fibers and carbon nanotubes are preferable because the conductivity between the composite particles can be secured with a small amount.

  The content of the conductive substance in the negative electrode material for a lithium ion secondary battery can be appropriately selected according to the purpose. For example, from the viewpoint of capacity, it is preferably 0.1% by mass to 20% by mass in the negative electrode material for a lithium ion secondary battery, and more preferably 0.5% by mass to 10% by mass. On the other hand, from the viewpoint of the cycle, 20% by mass to 95% by mass is preferable, and 50% by mass to 90% by mass is more preferable.

[Method for producing negative electrode material for lithium ion secondary battery]
The method for producing the negative electrode material for lithium ion secondary batteries is not particularly limited as long as the negative electrode material for lithium ion secondary batteries containing the composite particles can be produced. For example, it includes a step of obtaining the composite particles and other steps as necessary.
In the present invention, the step of obtaining the composite particles includes first particles which are graphite having a circularity of 0.60 to 1.00 and silicon atoms from the viewpoint of battery cycle characteristics and an expansion coefficient of the negative electrode material. The second particles are combined with a carbonaceous material to obtain composite particles having a volume average particle size of 1.0 to 1.3 times the volume average particle size of the first particles. It is preferable to include a process.
By producing composite particles in such a process, the second particles can be unevenly distributed on the surface of the first particles. Furthermore, the content of silicon atoms in the surface portion of the composite particle can be made twice or more that of the central portion. Furthermore, it can be set as the state which a silicon atom does not exist substantially in the center part of a composite particle.

  Specifically, in the step of obtaining the composite particles, the first particles which are graphite having a circularity of 0.60 to 1.00 and the second particles containing silicon are mixed with carbonaceous material carbon. A step of compounding the material precursor, a step of firing the composite obtained by the compounding to obtain a mass, and applying a shearing force to the mass, A composite particle having a volume average particle size of 1.0 to 1.3 times the volume average particle size, wherein the first particles and the second particles are combined with the carbonaceous material. It is preferable to include the process of obtaining.

(Composite)
The composite of the first particle, the second particle, and the carbonaceous material precursor can realize the composition ratio of the first particle, the second particle, and the carbonaceous material B in the composite particle obtained by this manufacturing method. There is no particular limitation as long as these components are combined at a proper quantitative ratio. By the present composite, a composite including the first particles, the second particles, and the carbonaceous material precursor is obtained. Note that the composite obtained in the composite process is obtained by integrating the second particle and the carbonaceous material precursor on the surface of the first particle, and the carbonaceous material precursor is not carbonized. Carbonized composite.

The second particles and the carbon are formed so that the second particles, the first particles, and the carbonaceous material precursor are formed into composite particles without forming the composite particles with the carbonaceous material precursor alone. The active substance precursor is preferably mixed with the first particles in a state of being dissolved or dispersed in the dispersion medium.
As a dispersion medium used in the case of a dispersion, an organic solvent is preferably used. Thereby, for example, oxidation of the second particles can be suppressed. When the carbonaceous material precursor is a solid, it is preferably dissolved in the organic solvent. The organic solvent to be used is not particularly limited. For example, when pitch or the like is used as the carbonaceous material precursor, an aromatic hydrocarbon solvent such as toluene or methylnaphthalene having solubility in this is preferable.

In order to uniformly combine the cohesive second particles and the carbonaceous material precursor on the surface of the first particles, the second particles and the carbonaceous material precursor are highly concentrated in the dispersion medium. It is preferably dispersed. The dispersion method is not particularly limited, but it is preferable to ultrasonically disperse the second particles, the carbonaceous material precursor, and the dispersion medium because a more uniform dispersion can be obtained.
In addition, when obtaining a dispersion, you may mix a 1st particle | grain simultaneously. The dispersion method in that case is not particularly limited as long as the first particles are not pulverized during dispersion. For example, dispersion can be performed using a stirring homogenizer, a bead mill, a ball mill, or the like.

  When the carbonaceous material precursor is converted into a carbonized product by the baking treatment, the mass thereof decreases. Therefore, it is preferable to measure the carbonization rate in advance for the amount of the carbonaceous material precursor in the composite, and use an amount corresponding to the amount of remaining carbon in the composite particles for the composite treatment.

The amount of the second particles is one of the factors that determine the capacity of the lithium ion secondary battery configured using the negative electrode material according to the present invention. Therefore, it is preferable to appropriately determine the amount of the second particles used for compositing according to the target volume.
Specifically, it is preferable to select appropriately such that the content of the second particles in the composite particles is in the range described above.

The amount of the first particles can be any amount, and for example, it is preferable to select appropriately within a range of 60 to 99% of the total mass of the composite particles.

As a specific method for combining the first particles, the second particles, and the carbonaceous material precursor, for example, when the dispersion and the first particles are combined, the dispersion is performed with a heatable kneader. The method of volatilizing the organic solvent while mixing the product and the first particles and compositing, or the method of mixing the first particles in the dispersion in advance and spray-drying the compound to compound It is done.
In these compounding methods, it is preferable to mix in the state of paste or slurry so that the first particles and the dispersion are uniformly mixed.

(Baking process)
In the firing step, the composite product obtained in the composite step is fired. By this firing treatment, the carbonaceous material precursor becomes a carbonized product. When silicon oxide is contained in the second particles, for example, the silicon oxide is disproportionated to form a structure in which silicon microcrystals are dispersed in the silicon oxide.
By the firing treatment, the composite product is fired to obtain a lump. Here, the lump is an aggregate of composite particles having first particles, second particles, and a carbonaceous material.

The firing treatment is preferably performed in an inert atmosphere from the viewpoint of suppressing oxidation, and nitrogen and argon are suitable as the inert atmosphere.
The baking treatment conditions are not particularly limited, but it is preferable to hold at about 200 ° C. for a certain time, volatilize the residual solvent completely, and then raise the temperature to the target temperature.
About baking temperature, 800-1200 degreeC is preferable, 850-1200 degreeC is more preferable, 900-1200 degreeC is further more preferable. By setting the firing temperature to 800 ° C. or higher, carbonization of the carbonaceous material precursor proceeds sufficiently, and the initial charge / discharge efficiency tends to be improved. On the other hand, by setting the firing temperature to 1200 ° C. or less, silicon carbide can be suppressed, and a decrease in battery capacity tends to be suppressed. Further, the growth of the silicon dioxide portion in the silicon oxide can be suppressed, and the inhibition of diffusion of lithium ions and the deterioration of rate characteristics in the silicon oxide can be suppressed.

(Shearing force application process)
In the shearing force application step, a shearing force is applied to the mass obtained in the firing step, and the volume average particle size is 1.0 to 1.3 times the volume average particle size of the first particles. To obtain composite particles in which the first particles and the second particles are combined with the carbonaceous material.
The lump obtained by the firing step is formed by composite particles bound together by carbonization of the carbonaceous material precursor. When a shearing force is applied to the lump, an appropriate shearing force is applied to the composite particles bound to each other, and the composite particles are separated into individual composite particles having a predetermined particle diameter. The composite particles thus obtained have a form in which many second particles are present on the surface.

The application of the shearing force is not particularly limited as long as it is an apparatus capable of applying a shearing force in which the volume average particle diameter of the composite particles is in a desired range, and is a general apparatus such as a mixer, a cutter mill, a hammer mill, a jet. It can be performed using a mill or the like.
In addition, the condition for applying the shearing force so that the volume average particle diameter of the composite particles is within a desired range varies depending on the apparatus used. For example, when a Waring mixer (7012S) manufactured by WARING is used, What is necessary is just to employ | adopt the conditions sheared over the time for 30 second-3 minutes at the rotation speed of 3000-13000 rpm.
In addition, the application of shearing force is common in the industry such as pulverization treatment or pulverization treatment as long as it is a process in which a lump is made into a state of individual composite particles forming a lump and the composite particles are not destroyed. Any of the processes used in the above may be used.

It is preferable to include a classification step for the purpose of sizing after the shearing force application step. Thereby, the composite particle which has a uniform volume average particle diameter can be obtained. For classification, for example, a sieve having an opening of 40 μm is preferably used. In the classification treatment, it is preferable to remove as much as possible 1 μm or less of fine powder.
Although the classification method is not particularly limited, for example, it can be removed by an airflow classifier.
The composite particles obtained by the classification treatment may be further heat-treated in an inert atmosphere. About heat processing conditions, it is the same as that of said baking conditions. By applying this treatment, the structure of the particle surface disturbed by pulverization can be smoothed, and the initial charge / discharge efficiency can be further improved.

The composite particles in the present invention may be coated with carbon to further form a low crystalline carbon layer. However, when the carbon coating amount is large, the initial charge / discharge efficiency may be reduced due to an increase in amorphous carbon, and therefore the amount is preferably determined appropriately so that the characteristics of the negative electrode material do not deteriorate.
Examples of the carbon coating method include a wet mixing method, a chemical vapor deposition method, and a mechanochemical method. The chemical vapor deposition method and the wet mixing method are preferable from the viewpoint that the reaction system can be uniformly controlled and the shape of the composite particles can be maintained.
There is no particular limitation on the carbon source for forming the low crystalline carbon layer, but an aliphatic hydrocarbon, an aromatic hydrocarbon, an alicyclic hydrocarbon, or the like can be used in the chemical vapor deposition method. Specific examples include methane, ethane, propane, toluene, benzene, xylene, styrene, naphthalene, cresol, anthracene, or derivatives thereof.
In the wet mixing method and the mechanochemical method, a polymer compound such as a phenol resin or a styrene resin, a carbonizable solid material such as pitch, or the like can be processed as a solid or a dissolved material.
The treatment temperature is preferably performed under the same conditions as the firing treatment conditions described above.

  Furthermore, this manufacturing method may further include the process of mixing another component as needed. Examples of other components include the above-described conductive substances (conductive auxiliary materials), binders, and the like.

<Anode for lithium ion secondary battery>
The negative electrode for a lithium ion secondary battery of the present invention includes the above-described negative electrode material for a lithium ion secondary battery of the present invention, and includes other components as necessary. Thereby, it becomes possible to constitute a lithium ion secondary battery having a high capacity and excellent cycle characteristics and safety.

  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 (henceforth "binder"), For example, styrene-butadiene copolymer; Ethylenic unsaturated carboxylic acid ester (For example, methyl (meth) acrylate, ethyl (meta)) ) Acrylate, butyl (meth) acrylate, (meth) acrylonitrile, and hydroxyethyl (meth) acrylate), and ethylenically unsaturated carboxylic acids (eg, acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, etc.) (Meth) acrylic copolymers comprising: polymer compounds such as polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide, and polyamideimide.

  These organic binders may be dispersed or dissolved in water or dissolved in an organic solvent such as N-methyl-2-pyrrolidone (NMP) depending on the respective physical properties. Among these, an organic binder whose main skeleton is polyacrylonitrile, polyimide, or polyamideimide is preferable because of its excellent adhesion, and the heat treatment temperature is as described below for an organic binder whose main skeleton is polyacrylonitrile. It is more preferable because it is low and the flexibility of the electrode is excellent. Examples of the organic binder having polyacrylonitrile as a main skeleton include, for example, a product obtained by adding acrylic acid for imparting adhesiveness and a linear ether group for imparting flexibility to a polyacrylonitrile skeleton (manufactured by Hitachi Chemical Co., Ltd., LSR7) can be used.

The content ratio of the organic binder in the negative electrode layer of the lithium ion secondary battery negative electrode is preferably 1% by mass to 30% by mass, more preferably 2% by mass to 20% by mass, and 3% by mass. More preferably, it is -15 mass%.
Adhesion is good when the content ratio of the organic binder is 1% by mass or more, 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 30 mass% or less.

  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. The usage-amount of a conductive auxiliary material should just be about 0.1 mass%-20 mass% with respect to 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, heat treatment is performed at 100 to 180 ° C., and when using an organic binder having polyimide or polyamideimide as the main skeleton at 150 to 450 ° C. It is preferable to do.
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.

In addition, the negative electrode is preferably pressed (pressurized) before the heat treatment. The electrode density can be adjusted by applying pressure treatment. The negative electrode material for a lithium ion secondary battery of the present invention, it is ^{preferably, 1.5g / cm 3 ~1.85g / cm} 3 electrode density of 1.4g ^/ cm ³ ~1.9g ^/ cm 3 more ^preferably, it is more preferably from ^{1.6g / 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 is characterized by using 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. 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 used in the present invention because of its high capacity.

  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 , ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentanone, Sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidine-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, Ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1 , 3-dioxolane, methyl acetate, ethyl acetate or the like, or a so-called organic electrolyte solution dissolved in a non-aqueous solvent of a mixture of two or more components can be used. Among them, an electrolytic solution containing fluoroethylene carbonate is preferable because stable SEI (solid electrolyte interface) tends to be formed on the surface of the negative electrode material of the present invention, and the cycle characteristics are remarkably improved.

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 is described as being used for a lithium ion secondary battery. However, in general, electrochemical devices having a charging / discharging mechanism that inserts and desorbs lithium ions, for example, a hybrid capacitor 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>
(Production of composite particles)
First, a silicon oxide powder (second particle) having a volume average particle diameter of 30 μm is volume-averaged with a bead mill (manufactured by Ashizawa Finetech: LMZ) together with methylnaphthalene and a dispersing material (manufactured by Kao Corporation: L-1820). A silicon oxide slurry was prepared by grinding to a particle size of 0.5 μm.
500 g of this silicon oxide slurry (solid content 30%), 300 g of coal tar pitch (carbonization rate 50%, carbonaceous material), and 2000 g of methylnaphthalene are placed in a SUS container and stirred. A dispersion was obtained by ultrasonic dispersion treatment for 30 minutes while circulating with a sonic homogenizer (Ginsen: GSD600HAT).

Next, 2700 g of spheroidized natural graphite (first particle) having a volume average particle diameter of 20 μm is charged into a pressure kneader, the dispersion is charged therein, and methyl naphthalene is evaporated at 200 ° C. to obtain silicon oxide particles. A mixed lump made of spheroidized natural graphite complexed with a carbonaceous material was obtained.
The obtained mixed lump was baked at 900 ° C. for 2 hours in a baking furnace in a nitrogen atmosphere. The fired lump is pulverized using a Waring mixer (manufactured by WARING: 7012S) under conditions of 3100 rpm and 1 minute, and then classified with a vibrating screen having an opening of 40 μm to obtain composite particles having a volume average particle diameter of 22 μm. This was used as a negative electrode material for a lithium ion secondary battery.

  About the negative electrode material for lithium ion secondary batteries containing the composite particle obtained by the said manufacturing method, a circularity, the content rate of a silicon atom, an average surface interval, a tap density, a BET specific surface area, a volume average particle diameter ( 50% D), the content ratio of the second particles, and the major axis length ratio were evaluated. The evaluation results are shown in Table 1.

[Circularity]
For the first particles, observe an image magnified 1000 times with a scanning electron microscope, select 10 spherical graphite particles arbitrarily, and use the image analysis software of particle analysis of Sumitomo Metal Technology, individually The circularity of the carbon particles was measured and obtained as an arithmetic average value. Note that the circularity is a circumference calculated as a circle calculated from a circle-equivalent diameter, which is a diameter of a circle having the same area as the projected area of the spherical graphite particles. It is a numerical value obtained by dividing by (the length of the contour line) and is obtained by the following formula. The circularity is 1.00 for a perfect circle.
Circularity = (perimeter of equivalent circle) / (perimeter of particle cross-sectional image)

[Section observation and quantitative analysis]
For processing the electrode cross section, an ion milling device (E-3500) manufactured by Hitachi High-Tech was used. The cross section of the electrode thus processed is subjected to quantitative analysis of carbon atoms, oxygen atoms and silicon atoms using EDX (INCA Energy 350 manufactured by Oxford Instruments) while observing with SEM (S-3400N manufactured by Hitachi High-Tech), and carbon atoms, oxygen As the ratio of the content of silicon atoms to the total content of atoms and silicon atoms, the content ratio of silicon atoms in the entire composite particle, the surface portion, and the central portion was determined.
The content ratio of silicon atoms in the entire composite particles was expanded until the length of the long axis of the composite particles to be observed and the width of the observation region were substantially equal, and the content ratio of silicon atoms in the entire observation region was obtained. Further, the content ratio of silicon atoms in the surface portion and the central portion is selected so that three square regions each having a side length of 1 μm are not overlapped as much as possible in each of the surface portion and the central portion. It was calculated as an arithmetic average value of the measured values.
Further, the ratio of the silicon atoms in the surface portion to the central portion (surface / center) and the ratio of the silicon atoms in the surface portion to the entire composite particle (center / total) were also calculated.

In cross-sectional observation, the major axis length ratio was calculated as the ratio of the major axis length of the second particle to the major axis length of the composite particle. Table 1 shows values rounded to the second decimal place.
In addition, the length of the major axis of the second particle was an arithmetic average value of the length of the major axis of three arbitrarily selected second particles.

  The number of measured composite particles was 10, and in the examples, all 10 composite particles satisfied the provisions of the present invention. Therefore, each value in Table 1 is an average value of 10 pieces. In each composite particle, there were three observation regions. In each comparative example, all 10 composite particles did not satisfy the provisions of the present invention. Each value in Table 1 is also an average value of 10 pieces.

[Measurement of average spacing (d ₀₀₂ ) (XRD)]
An average surface separation (d ₀₀₂ ) was calculated based on the Gakushin method using a wide-angle X-ray diffractometer manufactured by Rigaku Corporation.

[Tap density measurement]
The tap density was measured by a method based on JIS standard R1628.

[BET specific surface area measurement]
Using a nitrogen adsorption measuring apparatus ASAP-2010 (manufactured by Shimadzu Corporation), nitrogen adsorption was measured at 5 points in a relative pressure range of 0.04 to 0.20, and a BET specific surface area was calculated by applying the BET method.

[Average particle diameter (50% D) measurement]
Using a laser diffraction particle size distribution analyzer SALD-3000J (manufactured by Shimadzu Corporation), a dispersion liquid in which the obtained composite particles are dispersed in purified water together with a surfactant is placed in a sample water tank and pumped while applying ultrasonic waves. Measured while circulating. The cumulative 50% particle size (50% D) of the obtained particle size distribution was defined as the volume average particle size.

[Content of second particles]
The content rate of the second particles contained in the composite particles was measured as follows. 3 g of the obtained composite particles were put in an alumina crucible and heat-treated at 900 ° C. for 60 hours in the atmosphere. The obtained ash was regarded as all oxidized, and the silicon and silicon oxide contents constituting the second particles were measured from the following formula.
Silicon content (%) = (Ash content × 28.09 / 60.09) / Mass of composite particles × 100
Silicon oxide content (%) = (Amount of ash × 44.09 / 6.09) / Mass of composite particles × 100

(Preparation of negative electrode for lithium ion secondary battery)
To 95 parts of the obtained composite particles, 5 parts of a resin having a main skeleton of polyacrylonitrile (manufactured by Hitachi Chemical Co., Ltd., LSR7) is added as a binder, and an appropriate amount of NMP (N-methyl-2-pyrrolidone) is added. Then, NMP was further added to prepare a slurry having a solid content of 40%.
The obtained slurry was applied to a copper foil using an applicator so that the solid content was 7 mg / cm ² , and dried in a 90 ° C. stationary operation dryer for 2 hours. After drying, it was roll-pressed under a linear pressure of 1 t / cm, and further heat-treated at 160 ° C. for 2 hours under vacuum to obtain a negative electrode for a lithium ion secondary battery. The obtained negative electrode for a lithium ion secondary battery was punched into a 14 mmφ circle and used as a sample for evaluation.

(Production of evaluation cell)
The evaluation cell was prepared by injecting an electrolytic solution with a CR2016 type coin cell facing the negative electrode and metallic lithium through a 20 μm polypropylene separator. The electrolytic solution was obtained by dissolving ethyl carbonate and methyl ethyl carbonate in a mixed solvent having a volume ratio of 3 to 7 to a concentration of 1 mol / L of LiPF ₆ and adding 1.5% by mass of vinyl carbonate thereto. Furthermore, what added 20 volume% of fluoroethylene carbonate was used.

(Evaluation conditions)
The evaluation cell was placed in a constant temperature bath at 25 ° C. and subjected to a cycle test. Charging was performed until the current value reached 0.2 mA at a constant voltage of 0 V after charging to 0 V with a constant current of 2 mA. The discharge was performed at a constant current of 2 mA up to a voltage value of 1.5 V. The discharge capacity and charge / discharge efficiency were the results of the initial charge / discharge test.
Moreover, after repeating charge / discharge for 5 cycles on the said conditions, the cell for evaluation was disassembled in the charged state, and the ratio of the thickness of the obtained negative electrode to the thickness at the time of negative electrode production was defined as the expansion rate.
In addition, the cycle characteristics were evaluated as the capacity retention rate by comparing the discharge capacity after 50 charge / discharge tests under the charge / discharge conditions with the initial discharge capacity.
The evaluation results are shown in Table 1.

<Example 2>
In Example 1, silicon powder (purity 99.9%) having a volume average particle diameter of 25 μm was used instead of silicon oxide powder, and the silicon slurry was pulverized to have a volume average particle diameter of 0.2 μm. Produced. Composite particles were produced in the same manner as in Example 1 except that 200 g of this silicon was subjected to ultrasonic dispersion treatment with 180 g of coal tar pitch, and the same evaluation was performed.
FIG. 1 shows an SEM image showing the entire cross section of the obtained composite particle, FIG. 2 shows an SEM image showing the central part of the cross section of the composite particle, and FIG. 3 shows a surface portion of the cross section of the composite particle. The SEM images shown are shown respectively. In addition, the arrow in FIG. 3 shows a silicon particle.
1 to 3, it can be seen that in the composite particles according to Example 2, silicon particles are present in the surface portion and no silicon particles are present in the central portion.

<Example 3>
In Example 1, composite particles were produced in the same manner as in Example 1 except that granulated particles prepared as follows were used instead of spheroidized natural graphite, and the same evaluation was performed.
-Preparation of granulated particles-
980 g of scaly graphite having a volume average particle diameter of 8 μm and 20 g of carboxymethylcellulose (Daiichi Kogyo Seiyaku: WS-C) were mixed with 3000 g of purified water with stirring. This slurry was granulated with a fluidized bed granulator (Powrec: GPCG). The granulated particles were calcined at 900 ° C. for 2 hours in a nitrogen atmosphere to obtain granulated particles having a volume average particle diameter of 24 μm.

<Example 4>
In Example 1, except that the fired lump was crushed using a jet mill (Nihon Pneumatic LJ-3) at a pulverization pressure of 0.1 MPa, a classification zone of 12 mm clearance, and using a large louver. In the same manner as in Example 1, composite particles were produced and evaluated in the same manner.

<Example 5>
In Example 1, the obtained composite particles were mixed with flaky graphite particles having an average particle diameter of 4 μm and a specific surface area of 14 m ² / g so that the total amount would be 10%, and this was mixed with the negative electrode material for a lithium ion secondary battery. Except for the above, composite particles were produced in the same manner as in Example 1, and the same evaluation was performed.

<Comparative Example 1>
In Example 1, flaky graphite having a volume average particle diameter of 8 μm was used, and ultrasonic dispersion treatment was performed in methyl naphthalene together with a silicon oxide slurry and coal tar pitch. Methyl naphthalene was evaporated from the obtained dispersion with a pressure kneader to obtain a composite. The obtained composite was fired in the same manner as described above to obtain a lump.
The obtained lump was pulverized to a volume average particle diameter of 23 μm using a jet mill (AFG manufactured by Hosokawa Micron Corporation) under the conditions of a pulverization pressure of 0.4 MPa and a classification rotor rotation speed of 1500 rpm to obtain composite particles.
A negative electrode material was produced in the same manner as in Example 1 except that the composite particles thus obtained were used, and the same evaluation was performed.

<Comparative example 2>
In Example 1, flaky graphite having a volume average particle diameter of 8 μm was used, and ultrasonic dispersion treatment was performed in methyl naphthalene together with a silicon oxide slurry and coal tar pitch. Using this dispersion, a composite was obtained using a spray dryer (manufactured by Okawara Chemical Industries Co., Ltd .: CL-8i). A twin jet nozzle was used for spraying, and the spraying conditions were a spray pressure of 0.1 MPa and a spray inlet temperature of 110 ° C.
The obtained composite was fired and ground in the same manner as described above to obtain composite particles having a volume average particle diameter of 17 μm.
A negative electrode material was produced in the same manner as in Example 1 except that the composite particles thus obtained were used, and the same evaluation was performed.

4 shows an SEM image showing the entire cross section of the obtained composite particle, FIG. 5 shows an SEM image showing the central portion of the cross section of the composite particle, and FIG. 6 shows a surface portion of the cross section of the composite particle. The SEM images shown are shown respectively. The arrows in FIGS. 5 and 6 indicate silicon particles.
4 to 6, in the composite particles according to Comparative Example 2, it can be seen that silicon particles exist in the central portion in addition to the surface portion.

  From Table 1, it can be seen that the lithium ion secondary battery configured using the negative electrode material for a lithium ion secondary battery of the present invention is excellent in cycle characteristics while suppressing expansion of the negative electrode accompanying charging. Moreover, since the expansion | swelling of the negative electrode accompanying charging is suppressed, it is excellent in safety.

Claims (9)

  Lithium containing composite particles in which the second particles containing silicon atoms are compounded with a carbonaceous material so that the second particles containing silicon atoms are unevenly distributed on the surface of the first particles that are graphite having a circularity of 0.60 to 1.00 Negative electrode material for ion secondary battery. When observing the cross section of the composite particle,
Included in the inner region of a circle centered on the midpoint of the major axis, which is the maximum length of the composite particle, and having a radius of 1/8 of the length of the minor axis orthogonal to the midpoint of the major axis The ratio of the content of silicon atoms contained in the inner region from the outer periphery of the composite particle to the depth of 1/8 the length of the short axis to the content of silicon atoms is 2 or more, The negative electrode material for lithium ion secondary batteries according to claim 1.   3. The lithium ion secondary battery according to claim 2, wherein the ratio of the content of silicon atoms contained in the inner region of the circle to the total content of silicon atoms contained in the cross section of the composite particle is 0.2 or less. Negative electrode material.   The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the volume average particle diameter of the first particles is 5 µm or more and 40 µm or less.   5. The content of the carbonaceous substance is 1% by mass or more and 10% by mass or less in the entire composite particle, and the carbonaceous substance is an organic carbonized product. Negative electrode material for lithium ion secondary batteries.   The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 5, further comprising a substance having conductivity.   The first particles, which are graphite having a circularity of 0.60 to 1.00, and the second particles containing silicon atoms are combined using a carbonaceous material, and the volume average particle diameter is the first particles. 7. The lithium ion secondary battery according to claim 1, comprising a step of obtaining composite particles that are 1.0 to 1.3 times the volume average particle diameter of the particles. Manufacturing method of negative electrode material.   The negative electrode for lithium ion secondary batteries containing the negative electrode material for lithium ion secondary batteries of any one of Claims 1-6. The lithium ion secondary battery containing the negative electrode for lithium ion secondary batteries of Claim 8, a positive electrode, and electrolyte.