JP2015060642A - Silicon oxide-graphite composite particle and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: composite particles capable of improving charge/discharge cycle characteristics of a nonaqueous electrolyte secondary battery, such as a lithium ion secondary battery, furthermore; and a method for producing the composite particles.SOLUTION: Silicon oxide-graphite composite particles according to the present invention include a plurality of scale-like graphite particles and SiOx particles (where 0<x≤0.9). The plurality of scale-like graphite particles are disposed in layers. The SiOx particles are inserted between the plurality of scale-like graphite particles. When an electrode having an electrode density of 1.70±0.02 g/cmis manufactured from the silicon oxide-graphite composite particles, the ratio of the "intensity I(110) of the peak attributed to the (110) plane" to the "intensity I(004) of the peak attributed to the (004) plane" in an X-ray diffraction image of the electrode is preferably within a range of 0.0010 or more and 0.0300 or less.

Description

  The present invention relates to silicon oxide graphite composite particles and a method for producing the same.

  Conventionally, graphite, silicon, tin particles and the like are generally used as a negative electrode active material of a lithium ion secondary battery. Among these negative electrode active materials, silicon particles are particularly attracting attention because a negative electrode having a high discharge capacity can be produced. However, the silicon particles have an extremely large volume change of about 4 times due to insertion and extraction of lithium ions. For this reason, when charging / discharging is repeated with respect to the battery which uses a silicon particle as a negative electrode active material, the conductive network of a silicon particle will collapse gradually, As a result, the discharge capacity of a battery will fall.

Therefore, in recent years, for the purpose of coexistence of “improvement of discharge capacity” and “suppression of decrease in discharge capacity due to charge / discharge cycle” of the negative electrode of a lithium ion secondary battery, Proposed. As such silicon graphite composite particles, for example, “D, measured by Raman spectroscopy using an argon laser, containing silicon, scaly graphite and carbonaceous material, the content of carbonaceous material being less than 20% by mass. Composite graphite particles in which the ratio ID / IG (R value) of the band 1360 cm ⁻¹ peak intensity ID and the G band 1580 cm ⁻¹ peak intensity IG is less than 0.4 (for example, see JP-A-2005-243508) “Silicon particles comprising a silicon particle, a graphite material, and a carbonaceous material, subjected to a treatment for imparting a compressive force and a shearing force, and having a coating made of the carbonaceous material on at least a part of the surface; And the like (for example, refer to Japanese Patent Application Laid-Open No. 2008-235247).

JP 2005-243508 A JP 2008-235247 A

  However, it is difficult to say that the charge / discharge cycle characteristics of a lithium ion secondary battery using the above-described silicon graphite composite particles as a negative electrode active material are sufficient.

  The subject of this invention is providing the composite particle which can further improve the charging / discharging cycling characteristics of nonaqueous electrolyte secondary batteries, such as a lithium ion secondary battery, and its manufacturing method.

The silicon oxide graphite composite particles according to one aspect of the present invention include a plurality of scaly graphite particles and SiOx particles (where 0 <x ≦ 0.9). Incidentally, the SiOx particles are preferably a wet pulverized product having a specific surface area of 40 m ² / g or more. In addition, it is preferable that this specific surface area is 200 m < ² > / g or less. By the way, in the past, attempts have been made to improve cycle characteristics by pulverizing and reducing silicon particles. If the particle size of the silicon particles is large, cracks and the like occur in the silicon particles due to expansion and contraction during charge and discharge, and a new surface is generated in the silicon particles for each cycle, and the new surface reacts with the electrolyte solution, and the electrolyte solution for each cycle. This is because cycle deterioration occurs due to consumption of silicon, but such a problem does not occur if the particle size of the silicon particles is reduced from the beginning. This can be estimated from the low charge / discharge efficiency during the cycle. When the particle size of the silicon particles is reduced, the specific surface area of the silicon particles increases and the oxidization of the new surface proceeds, and the silicon particles have a mixed structure of a silicon phase and a silicon dioxide phase. In the present application, such silicon particles are referred to as “silicon oxide particles” or “SiOx particles”. In the present application, “x” of SiOx is in the range of more than 0 and not more than 0.9 as described above. A plurality of scaly graphite particles are arranged in layers. The plurality of scaly graphite particles are preferably oriented in the same direction or substantially the same direction. The SiOx particles are sandwiched between a plurality of scaly graphite particles.

  As a result of intensive studies, the inventors of the present application have clarified that the above-described silicon oxide graphite composite particles can further improve the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery. The inventors of the present application presume this cause as follows.

  When forming an electrode from the electrode mixture slurry containing the silicon oxide graphite composite particles according to the present invention, the silicon oxide graphite composite particles are laminated so that the lamination direction of the silicon oxide graphite composite particles is along the electrode thickness direction. As a result, the electrode has, for example, a repeating layer of ... // graphite layer / SiOx particle layer / graphite layer // graphite layer / SiOx particle layer / graphite layer // ... along the electrode thickness direction. (In the above description, the symbol “//” indicates the boundary line between the particles, and “/” indicates the boundary line of the layer in the silicon oxide graphite composite particle). With such an electrode structure, the volume change of the silicon oxide graphite composite particles is concentrated in the electrode thickness direction. And in the inside of a battery, the force which compresses an electrode along the direction perpendicular | vertical to an electrode is always provided. For this reason, the electrode using the silicon oxide graphite composite particles as an electrode active material is prevented from being collapsed by its compressive force, and further improves the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery. (Normally, since there are voids in the electrode, it is difficult to suppress the collapse of the electrode when the volume of the silicon oxide-graphite composite particles changes in all directions).

  In the above-mentioned silicon oxide graphite composite particles, it is preferable that the SiOx particles are sandwiched between a plurality of scaly graphite particles and the SiOx particles are adhered to the outer surface of the outermost scaly graphite particles by non-graphitic carbon. . By making the silicon oxide graphite composite particles into such a structure, the content of SiOx particles in the silicon oxide graphite composite particles can be increased, and as a result, a non-aqueous electrolyte secondary such as a lithium ion secondary battery can be increased. It is because it can contribute to the improvement of the discharge capacity and charge capacity of the battery.

  Incidentally, when an electrode is formed from an electrode mixture slurry containing the above-mentioned silicon oxide graphite composite particles, the electrode has, for example, along the electrode thickness direction ... // SiOx particle layer / graphite layer / SiOx particle Layer / graphite layer / SiOx particle layer // SiOx particle layer / graphite layer / SiOx particle layer / graphite layer / SiOx particle layer // ... (in the above, the symbol “//” , Indicates a boundary line between the particles, and “/” indicates a boundary line of the layer in the silicon oxide graphite composite particle).

  In the above-mentioned silicon oxide graphite composite particles, the mass ratio of the scaly graphite particles, the SiOx particles, and the non-graphitic carbon is preferably 97 to 55: 1 to 30: 2 to 15, and 97 to 70: 1 to 15 : More preferably, it is 2-15. Here, the notation of “97 to 55” means 97 or less and 55 or more, and the notation of “1 to 30” means 1 or more and 30 or less (the same applies hereinafter). This is because if the composition of the silicon oxide graphite composite particles is as described above, an electrode having an excellent balance of discharge capacity, charge / discharge efficiency, and charge / discharge cycle characteristics can be formed.

When an electrode having an electrode density of 1.70 ± 0.02 g / cm ³ was prepared from the above-mentioned silicon oxide graphite composite particles, the peak intensity I attributed to the graphite (004) plane in the X-ray diffraction image of the electrode The ratio of “peak intensity I (110) attributed to the graphite (110) plane” to (004) ”is preferably within the range of 0.0010 to 0.0300. This is because if the silicon oxide graphite composite particles satisfy this condition, the degree of orientation of the scaly graphite particles in the electrode becomes good, and the above-described effects can be enjoyed more efficiently.

  In the above-mentioned silicon oxide graphite composite particles, the ratio of the major axis length to the length in the stacking direction of the scaly graphite particles (so-called aspect ratio) is preferably in the range of 1.5 or more and 10 or less. More preferably, it is in the range of 5 or more and 5 or less. This is because if the above-mentioned silicon oxide graphite composite particles satisfy this condition, the degree of orientation of the scaly graphite particles in the electrode becomes good, and the above-described effects can be enjoyed more efficiently.

  The method for producing silicon oxide graphite composite particles according to one aspect of the present invention includes a primary composite particle preparation step, a mixed powder preparation step, and a heating step. The primary composite particle preparation step and the mixed powder preparation step are performed by a dry method. In the primary composite particle preparation step, primary composite particles are prepared by applying compressive force and shear force to the mixed particles of SiOx particles (where 0 <x ≦ 0.9) and scaly graphite particles. In this primary composite particle preparation step, it is preferable that a mechanochemical (registered trademark) treatment is performed on the mixed particles of SiOx particles and scaly graphite particles. In the mixed powder preparation step, the primary composite particles and the solid non-graphitic carbon raw material are mixed to prepare a mixed powder. In the heating step, the mixed powder is heated. As a result, the non-graphitic carbon raw material is melted and adhered to the primary composite particles, and the non-graphitic carbon raw material is further converted into non-graphitic carbon.

  By the method for producing silicon oxide graphite composite particles, the above-mentioned silicon oxide graphite composite particles are produced. That is, this silicon oxide graphite composite particle can express the above-mentioned effect.

  The method for producing silicon oxide-graphite composite particles according to another aspect of the present invention includes an intermediate composite particle preparation step and a heating step. The intermediate composite particle preparation step is performed by a dry method. In the intermediate composite particle preparation step, the mixture is compressed to a mixture of SiOx particles (where 0 <x ≦ 0.9), flaky graphite particles and solid non-graphitic carbon raw material at a temperature equal to or higher than the softening point of the non-graphitic carbon raw material. An intermediate composite particle is prepared by applying a force and a shearing force. In this intermediate composite particle preparation step, it is preferable that a mechanochemical (registered trademark) treatment is performed on a mixture of SiOx particles, scaly graphite particles, and a solid non-graphitic carbon raw material. This is because the melted non-graphitic carbon raw material plays the role of an adhesive and increases the number of layers of scaly graphite particles and SiOx particles under the condition where compressive force acts. In the heating step, the intermediate composite particles are heated. As a result, the non-graphitic carbon raw material is converted to non-graphitic carbon.

  By the method for producing silicon oxide graphite composite particles, the above-mentioned silicon oxide graphite composite particles are produced. That is, this silicon oxide graphite composite particle can express the above-mentioned effect.

  The above-described method for producing silicon oxide graphite composite particles preferably further includes a wet pulverization step, a mixing step, and a drying step. In the wet pulverization step, the silicon powder is wet pulverized to prepare a SiOx particle slurry. In the mixing step, scaly graphite particles are mixed with the SiOx particle slurry to prepare a mixed slurry. In the drying step, the mixed slurry is dried to prepare a mixed powder. Note that drying may be performed to such an extent that the solvent used in the wet pulverization can be handled as a powder without completely evaporating. Then, in the intermediate composite particle preparation step, the composite powder and the solid non-graphitic carbon raw material are given compressive force and shear force at a temperature equal to or higher than the softening point of the non-graphitic carbon raw material, so that the intermediate composite particle is Prepared.

  The above-mentioned silicon oxide graphite composite particles can be used as an active material that constitutes an electrode, particularly an electrode of a nonaqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery here is represented by a lithium ion secondary battery.

1 is a schematic side view of silicon oxide graphite composite particles according to an embodiment of the present invention. It is a figure which represents typically the structure of the electrode formed in the silicon oxide graphite composite particle which concerns on embodiment of this invention. 4 is a reflected electron image photograph of a cross section of a silicon oxide graphite composite particle according to Example 2. FIG. In the photograph, the gray area indicates the scaly graphite particles, and the white area indicates the silicon oxide particles.

<Configuration of silicon oxide graphite composite particles>
As shown in FIGS. 1 and 2, silicon oxide graphite composite particles 100 according to an embodiment of the present invention mainly include SiOx particles 110, scaly graphite particles 120, and non-graphitic carbon (not shown). Consists of However, x is in the range of more than 0 and not more than 0.9, and preferably in the range of more than 0 and not more than 0.7. In this silicon oxide graphite composite particle 100, as shown in FIGS. 1 and 2, the SiOx particles 110 are sandwiched between the plurality of scaly graphite particles 120, and the outer surface of the scaly graphite particles 120 as the outermost layer has SiOx. Particles 110 adhere (see FIG. 1). Hereinafter, each component of the silicon oxide graphite composite particle 100 will be described in detail.

(1) SiOx Particles The SiOx particles 110 are sandwiched between a plurality of scaly graphite particles 120 and adhere to the outer surface of the scaly graphite particles 120 as the outermost layer of the silicon graphite composite particles 100 (see FIGS. 1 and 2). . The SiOx particles 110 are preferably as small as possible. This is because it is possible to disperse the stress caused by the volume change accompanying the insertion / release of lithium ions. Specifically, the particle diameter (that is, the median diameter) at a volume fraction of 50% is preferably 2 μm or less, more preferably 500 nm or less, further preferably 200 nm or less, and 100 nm or less. It is particularly preferred.

(2) Scale-like graphite particles The scale-like graphite particles 120 are arranged in layers and sandwich the SiOx particles 110 as described above (see FIG. 1). The scaly graphite particles 120 may be any of natural graphite particles, artificial graphite particles, and quiche graphite particles, but are preferably natural graphite particles from the viewpoint of economical efficiency and securing discharge capacity. As the scale-like graphite particles 120, the above-mentioned mixture of graphite particles may be used. The scaly graphite particles 120 previously heat-treated at a high temperature may be used as the scaly graphite particles. The particle size (that is, the median diameter) of the scaly graphite particles 120 when the volume fraction is 50% is preferably 5 μm or more and 30 μm or less. The scaly graphite particles 120 preferably have an aspect ratio of 3 or more and 50 or less. In the embodiment of the present invention, the scaly graphite particles 120 are preferably flexible, highly crystalline, and easily deformable when sandwiching the SiOx particles 110. Therefore, the hexagonal mesh plane spacing d002 of the scaly graphite particles 120 used in the embodiment of the present invention is preferably in the range of 0.3354 nm or more and 0.3370 nm or less, and the pellet density is 1.80 g / cm. it is preferably in the range of ³ to 2.00 g / cm ³ or less.

(3) Non-graphitic carbon Non-graphitic carbon causes the SiOx particles 110 to adhere to the scaly graphite particles 120. Non-graphitic carbon is at least one of amorphous carbon and turbostratic carbon. Here, “amorphous carbon” has short-range order (several atoms to tens of atoms order), but does not have long-range order (several hundreds to thousands of atoms order). Refers to carbon. Here, “turbulent structure carbon” refers to carbon composed of carbon atoms having a turbulent structure parallel to the hexagonal network plane direction but having no crystallographic regularity in the three-dimensional direction. In the X-ray diffraction pattern, hkl diffraction lines corresponding to the 101 plane and the 103 plane do not appear. However, since the silicon oxide graphite composite particles 100 according to the embodiment of the present invention have strong diffraction lines of graphite as a base material, it is difficult to confirm the existence of the turbostratic structure carbon by X-ray diffraction. For this reason, it is preferable that the turbostratic structure carbon is confirmed with a transmission electron microscope (TEM) or the like.

  This turbostratic carbon is obtained by firing a non-graphitic carbon raw material. In the embodiment of the present invention, the non-graphitic carbon raw material is a solid non-graphitic carbon raw material, for example, an organic compound such as petroleum pitch powder, coal pitch powder, and thermoplastic resin powder. . The raw material for non-graphitic carbon may be a mixture of the aforementioned powders. Among these, pitch powder is particularly preferable. This is because the pitch powder is melted and carbonized in the temperature raising process, and as a result, the SiOx particles 110 can be suitably fixed to the scaly graphite particles 120. Pitch powder is preferable from the viewpoint of low irreversible capacity even when fired at low temperature. As an example of the heat treatment conditions in the firing, the heat treatment temperature may be in the range of 800 ° C to 1200 ° C. This heat treatment time is appropriately determined in consideration of the heat treatment temperature and the characteristics of the non-graphitic carbon raw material, and is typically about 1 hour. The atmosphere during the heat treatment is preferably a non-oxidizing atmosphere (inert gas atmosphere, vacuum atmosphere), and a nitrogen atmosphere is preferred from an economic viewpoint. Amorphous carbon can be formed, for example, by a vapor phase method such as a vacuum deposition method or a plasma CVD method.

<Characteristics of silicon oxide graphite composite particles>
In the silicon oxide graphite composite particles 100 according to the embodiment of the present invention, the mass ratio of the above-mentioned SiOx particles 110, the scaly graphite particles 120, and the non-graphitic carbon is 1 to 30:97 to 55: 2 to 15. It is preferable that it is 1-15: 97-70: 2-15. By setting the silicon oxide graphite composite particles 100 to this composition, the SiOx particles 110 can be firmly fixed to the outer surface of the scaly graphite particles 120 of the outermost layer of the silicon oxide graphite composite particles 100, and the electrodes This is because the discharge capacity, charge / discharge efficiency, and charge / discharge cycle characteristics can be optimized during production.

  The particle diameter (that is, the median diameter) of the silicon oxide graphite composite particles 100 according to the embodiment of the present invention when the volume fraction is 50% is preferably in the range of 10 μm to 35 μm. This is because, when the particle diameter is within this range, the charge / discharge efficiency and the charge / discharge cycle characteristics can be optimized during electrode production.

  The aspect ratio of silicon oxide graphite composite particles 100 according to the present embodiment, that is, the long axis length (corresponding to “H” in FIG. 1) in the stacking direction of scaly graphite particles 120 (“ W ”) is preferably in the range of 1.5 or more and 10 or less, more preferably in the range of 1.5 or more and 8 or less, and in the range of 1.5 or more and 6 or less. More preferably, it is particularly preferably within the range of 1.5 to 5. This is because when the aspect ratio is within this range, the charge / discharge cycle characteristics can be optimized and the electrode can be easily produced.

When an electrode having an electrode density of 1.70 ± 0.02 g / cm ³ was produced from the silicon oxide-graphite composite particles 100 according to the present embodiment (see FIG. 2), the X-ray diffraction image of the electrode 200 shows “(004 The ratio of the peak intensity I (110) attributed to the (110) plane is preferably 0.0300 or less, and 0.0200 or less. More preferably, it is more preferably 0.0150 or less, and particularly preferably 0.0100 or less. This is because if this silicon oxide graphite composite particle 100 can satisfy this condition, the degree of orientation of the scaly graphite particles 120 in the electrode will be good, and the above-described effects can be enjoyed more efficiently. In FIG. 2, reference numeral 210 denotes an active material layer, and reference numeral 220 denotes a current collector.

<Manufacture of silicon oxide graphite composite particles>
Silicon oxide graphite composite particles 100 according to an embodiment of the present invention are manufactured by any of the following manufacturing methods.

(1) First Manufacturing Method In the first manufacturing method, silicon oxide graphite composite particles 100 are manufactured through a primary composite particle preparation step, a mixed powder preparation step, and a heating step. The primary composite particle preparation step and the mixed powder preparation step are performed in a dry state.

  In the primary composite particle preparation step, a compression force and a shear force are applied to the mixed particles of the SiOx particles 110 and the scaly graphite particles 120 by a process such as a mechanochemical (registered trademark) process or a mechanofusion (registered trademark) process. Composite particles are prepared. At this time, the mixed particles of the SiOx particles 110 and the scaly graphite particles 120 may be introduced into the mechanochemical system and the mechanofusion system, or the SiOx particles 110 and the scaly graphite particles 120 are sequentially added to the mechanochemical system and the mechanochemical system, respectively. After being introduced into the fusion system, a process such as a mechanochemical (registered trademark) process or a mechanofusion (registered trademark) process may be performed while mixing both particles. In the primary composite particles, the SiOx particles 110 are attached to the surface of the scaly graphite particles 120 with a weak force.

  In the mixed powder preparation step, the primary composite particles and the solid non-graphitic carbon raw material are solid-phase mixed to prepare a mixed powder.

  The method for mixing the primary composite particles and the solid non-graphitic carbon raw material in the mixed powder preparation step is not particularly limited as long as the method can uniformly mix the particles without destroying them. For example, there is a method using an ordinary mixer. Examples of the mixer include a rotating container type mixer, a fixed container type mixer, an airflow type mixer, and a high-speed flow type mixer. Examples of the rotating container type mixer include a V blender.

  In the heating step, the mixed powder is heat-treated at a temperature of 800 ° C. or higher and 1200 ° C. or lower in a non-oxidizing atmosphere (inert gas atmosphere, vacuum atmosphere, etc.). As a result, the non-graphitic carbon raw material is melted and adhered to the primary composite particles, and the non-graphitic carbon raw material is converted to non-graphitic carbon, whereby the target silicon oxide graphite composite particles 100 are obtained. By setting the heating temperature to 1200 ° C. or lower, the amount of silicon carbide (SiC) produced can be suppressed, so that an electrode excellent in discharge capacity can be formed. By setting the heating temperature to 800 ° C. or higher, an electrode having excellent charge / discharge efficiency can be formed. Thus, the electrode excellent in the balance of discharge capacity and charging / discharging efficiency can be formed as heating temperature is the said range.

(2) Second Manufacturing Method In the second manufacturing method, silicon oxide graphite composite particles 100 are manufactured through an intermediate composite particle preparation step and a heating step. The intermediate composite particle preparation step is performed in a dry state.

  In the intermediate composite particle preparation step, the mixture of the SiOx particles 110, the scaly graphite particles 120 and the solid non-graphitic carbon raw material is processed into a non-graphitic carbon raw material by a process such as a mechanochemical (registered trademark) process or a mechanofusion (registered trademark) process. Intermediate composite particles are prepared by applying compressive force and shear force at a temperature equal to or higher than the softening point of the graphitic carbon raw material. At this time, the melted non-graphitic carbon raw material serves as an adhesive to increase the number of layers of the scaly graphite particles 120 and the SiOx particles 110 under a situation where a compressive force acts. At this time, the mixture of the SiOx particles 110, the scaly graphite particles 120 and the solid non-graphitic carbon raw material may be put into a mechanochemical system or a mechanofusion system, or the SiOx particles 110, the scaly graphite particles 120 and After each solid non-graphitic carbon raw material is put into the mechanochemical system and mechanofusion system in sequence, the mechanochemical (registered trademark) processing, mechanofusion (registered trademark) processing, etc. can be performed while mixing the particles. Good.

  In the heating step, the mixture is heat-treated at a temperature of 800 ° C. to 1200 ° C. in a non-oxidizing atmosphere (inert gas atmosphere, vacuum atmosphere, etc.). As a result, the non-graphitic carbon raw material is converted to non-graphitic carbon, and the target silicon oxide graphite composite particles 100 are obtained. By setting the heating temperature to 1200 ° C. or lower, the amount of silicon carbide (SiC) produced can be suppressed, so that an electrode excellent in discharge capacity can be formed. By setting the heating temperature to 800 ° C. or higher, an electrode having excellent charge / discharge efficiency can be formed. Thus, the electrode excellent in the balance of discharge capacity and charging / discharging efficiency can be formed as heating temperature is the said range.

  In this production method, it is preferable that a wet pulverization step, a mixing step, and a drying step are provided before the start of the intermediate composite particle preparation step. In the wet pulverization step, the silicon powder is wet pulverized to prepare a SiOx particle slurry. In the mixing step, scaly graphite particles are mixed with the SiOx particle slurry to prepare a mixed slurry. In the drying step, the mixed slurry is dried to prepare a mixed powder. Then, in the intermediate composite particle preparation step, the composite powder and the solid non-graphitic carbon raw material are given compressive force and shear force at a temperature equal to or higher than the softening point of the non-graphitic carbon raw material, so that the intermediate composite particle is Prepared. In this case, drying may be performed to such an extent that it can be handled as a powder without completely evaporating the solvent used for wet grinding.

<Characteristics of silicon oxide graphite composite particles>
When the silicon oxide graphite composite particles 100 according to the embodiment of the present invention are used as an electrode active material of a nonaqueous electrolyte secondary battery, their charge / discharge cycle characteristics can be further improved.

<Examples and Comparative Examples>
Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples.

<Manufacture of silicon oxide graphite composite particles>
(1) Grinding of silicon particles For 1 hour, silicon powder (specific surface area 20 m ² / g) was ground with a bead mill to prepare a ground slurry. At this time, ethanol was used as the solvent and zirconia balls were used as the media. At this time, the new surface of the silicon powder is oxidized to become a SiOx powder. Only this pulverized slurry was naturally dried in the air to recover the SiOx powder, and the specific surface area of the SiOx powder was determined by the BET one-point method using a canter soap manufactured by Yuasa Ionics Co., Ltd. The surface area was 40 m ² / g (see Table 1).

(2) Preparation of Mixed Slurry Scale-like natural graphite powder (manufactured by Chuetsu Graphite Industry Co., Ltd., average particle size: 10 μm, d002: 0.3357 nm, pellet density: 1.82 g / cm ³ ) The mixture was put into a machine and the above-mentioned pulverized slurry was added and mixed while rotating the machine to prepare a mixed slurry. At this time, the scaly natural graphite powder and the pulverized slurry were mixed so that the mass ratio of the scaly natural graphite powder to the SiOx powder was 84: 7.

In addition, the pellet density of scaly natural graphite powder is calculated | required with the following method.
1.00 g of scaly natural graphite powder is filled into a 15 mm diameter mold, and the mold is pressurized with a uniaxial press at a pressure of 8.7 kN for 5 seconds, and then the pressure is reduced to 0.15 kN. The displacement of the upper punch at that time is read. The pressing speed is 10 mm / second. In addition, without filling the mold with scaly natural graphite powder, the mold was pressurized to a pressure of 8.7 kN with the same axis press, and then the pressure was reduced to 0.15 kN. Obtain the upper punch displacement. This displacement is used as a reference. Then, the difference between the displacement of the upper punch and the reference displacement at the time of filling the scaly natural graphite powder is obtained as the sample thickness, and the compression density, that is, the pellet density is calculated from this thickness.

(3) Preparation of intermediate composite particles After the mixed slurry was naturally dried to obtain a mixed powder, 91 parts by mass of the mixed powder and 18 parts by mass of a coal-based pitch powder (softening point 86 ° C., average particle size 20 μm, 1000 The residual carbon ratio after heating at 50 ° C. was introduced into a circulation mechano-fusion system (AMS-mini manufactured by Hosokawa Micron Corporation) in which the gap between the rotor and the inner piece was 1 mm. Then, while adjusting the temperature of the circulation type mechanofusion system to 90 ° C. to 120 ° C., the mixed powder was mechanochemically treated at 7000 rpm for 15 minutes to prepare intermediate composite particles.

(4) Heat treatment of coal-based pitch powder Next, the intermediate composite particles were put into a graphite crucible, and then the intermediate composite particles were heated in a nitrogen stream at a temperature of 1000 ° C. for 1 hour to remove the coal-based pitch powder. Converted to graphitic carbon.

(5) Crushing process Finally, the intermediate composite particles after the heat treatment were pulverized until 98% by mass or more passed through a sieve having an opening of 75 μm to obtain the target silicon oxide graphite composite particles. The mass ratio of the scaly natural graphite powder, the SiOx powder (silicon oxide powder), and the heat-treated product of pitch in the silicon oxide graphite composite particles was 84: 7: 9 (see Table 1). Here, all the weight changes before and after firing were due to pitch.

<Characteristic evaluation of silicon oxide graphite composite particles>
(1) Measurement of oxidation degree of silicon particles After silicon oxide graphite composite particles were acid-decomposed, the residue was melted to prepare a sample. And the elemental analysis of the sample was performed using the emission-spectral-analysis apparatus, and silicon atom content and zirconium atom content were calculated | required. Zirconium atoms are mixed by wear of zirconia balls (made of zirconium dioxide, ZrO ₂ ). Further, another silicon oxide-graphite composite particle was melted by an inert gas transport melting method to prepare a sample, and then the oxygen atom content of the sample was determined using an infrared absorption method. Then, the composition ratio of oxygen atoms to silicon atoms (ie, “x” of SiOx) was determined from the value obtained by subtracting the oxygen atom content of zirconium dioxide from the oxygen atom content and the silicon atom content. In the silicon oxide graphite composite particles according to this example, the composition ratio was 0.41 (see Table 1).
(2) Measurement of particle diameter The volume-based particle size distribution of the silicon oxide graphite composite particles was measured by a light scattering diffraction method using a laser diffraction / scattering particle size distribution analyzer (LA-910, manufactured by Horiba, Ltd.). Thereafter, the particle size (median diameter) at a volume fraction of 50% was determined using the obtained particle size distribution. As a result, the particle diameter was 21 μm (see Table 1).

(3) Battery characteristics evaluation (3-1) Electrode preparation AB (acetylene black), CMC (carboxymethylcellulose sodium) powder, and aqueous dispersion of SBR (styrene-butadiene rubber) on the above-described silicon oxide graphite composite particles Then, water was blended to obtain an electrode mixture slurry. Here, AB is a conductive additive, and CMC and SBR are binders. The compounding ratio of the silicon oxide graphite composite particles, AB, CMC and SBR was 97.0: 1.0: 1.0: 1.0 by mass ratio. And this electrode mixture slurry was apply | coated by the doctor blade method on the 17-micrometer-thick copper foil (current collector) (the coating amount was 7-8 mg / cm < ² >). After drying the coating solution to obtain a coating film, the coating film was punched into a disk shape having a diameter of 13 mm. Then, the disk was pressed by a press molding machine to produce an electrode having an electrode density of 1.70 ± 0.02 g / cm ³ . In addition, the electrode density of the obtained electrode is obtained by measuring the thickness of the disk (part excluding the copper foil) and measuring the volume by measuring the thickness with a micrometer.

(3-2) Battery Production The electrode assembly was produced by disposing the above electrode and the counter Li metal foil on both sides of the polyolefin separator. And the electrolyte solution was inject | poured into the inside of the electrode assembly, and the coin-type non-aqueous test cell of the cell size 2016 was produced. The composition of the electrolytic solution was ethylene carbonate (EC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC): vinylene carbonate (VC): fluoroethylene carbonate (FEC): LiPF ₆ = 23: 4: 48: 1 : 8:16 (mass ratio).

(3-3) Evaluation of Discharge Capacity, Initial Charge / Discharge Efficiency, Battery Deterioration Level, and Charge / Discharge Cycle In this non-aqueous test cell, first, with a current value of 0.33 mA, the potential difference is 0 (zero) V with respect to the counter electrode. After performing constant current dope (equivalent to insertion of lithium ions into the electrode, charging of the lithium ion secondary battery) until it becomes, continue to dope the counter electrode at a constant voltage until 5 μA while maintaining 0 V. The doping capacity was measured. Next, dedoping (corresponding to detachment of lithium ions from the electrode and discharging of the lithium ion secondary battery) was performed at a constant current of 0.33 mA until the potential difference became 1.5 V, and the dedoping capacity was measured. The doping capacity and dedoping capacity at this time correspond to the charging capacity and discharging capacity when this electrode is used as the negative electrode of the lithium ion secondary battery, and these were used as charging capacity and discharging capacity. The discharge capacity of the non-aqueous test cell according to this example was 472 mAh / g (see Table 1). Since the ratio of the dedoping capacity / doping capacity at this time corresponds to the ratio of the discharge capacity / charge capacity of the lithium ion secondary battery at the first time, this ratio was defined as the initial charge / discharge efficiency. The initial charge and discharge efficiency of the non-aqueous test cell according to this example was 88.5% (see Table 1).

  The cycle characteristics were measured using a coin-type non-aqueous test cell configured in the same manner as described above. In this test cell, after the second cycle, after doping with a constant current of 1.33 mA until the potential difference becomes 5 mV with respect to the counter electrode (corresponding to charging), while maintaining 5 mV, with a constant voltage until 50 μA is reached. Continued dope. Next, dedoping was performed at a constant current of 1.33 mA until the potential difference became 1.5 V (corresponding to discharge), and the dedoping capacity was measured. The dedope capacity at this time was defined as the discharge capacity.

  Doping and dedoping are repeated 31 times under the same conditions as described above, the average value of the charge / discharge efficiency in the 10th to 15th cycles is determined and the degree of battery deterioration is evaluated. The ratio (capacity maintenance ratio) of “discharge capacity at the time of dedoping at the 31st cycle” to “capacity” was determined to evaluate cycle characteristics. If the average value of the charge / discharge efficiency from the 10th cycle to the 15th cycle is smaller than 100%, the electrode is broken or the active material and the electrolytic solution react between the cycles. Further, if the capacity retention rate is 90% or more, it can be considered as a practical battery. In addition, the average value of the charge / discharge efficiency of the 10th cycle to the 15th cycle of the non-aqueous test cell according to the present example was 99.6%, and the capacity retention rate was 96.1% (see Table 1). .

(4) Measurement of degree of orientation of scaly natural graphite particles in silicon oxide graphite composite particles The degree of orientation of scaly natural graphite particles in silicon oxide graphite composite particles uses a reflection diffraction type powder X-ray diffraction method. Is required. Specifically, the pressed disk-like electrode produced in the above “(3-1) Electrode production” is fixed to a non-reflective plate with a double-sided tape, and RINT-1200V made by Rigaku is used to make copper (Cu). Is measured by irradiating a disk electrode with CuKα rays at a tube voltage of 40 kV and a tube current of 30 mA. Thereafter, the peaks are separated to obtain a powder X-ray diffraction spectrum by CuKα1 rays. The intensities of the (004) plane diffraction peak with 2θ in the range of 52 to 57 ° and the (110) plane diffraction peak with 2θ in the range of 75 to 80 ° are determined. Then, the degree of orientation of the scaly natural graphite particles in the silicon oxide graphite composite particles is calculated by dividing the diffraction peak intensity of the (110) plane by the diffraction peak intensity of the (004) plane. The degree of orientation of the scaly natural graphite particles in the silicon oxide graphite composite particles according to this example was 0.0075 (see Table 1). Note that the smaller the degree of orientation, the higher the orientation of the scaly natural graphite particles in the silicon oxide graphite composite particles.

(5) Measurement of aspect ratio After embedding the pre-pressurized disk-shaped electrode produced in the above "(3-1) Electrode production", the resin was cut and the cut surface was polished. Then, the cut surface (electrode cross section) was observed with an optical microscope, the dimensions of 50 silicon oxide graphite composite particles were measured, and the aspect ratio (of the scaly graphite particles in FIG. 1) was measured for each silicon oxide graphite composite particle. The ratio of the major axis length W to the length H in the stacking direction is calculated. The average of the aspect ratios of the 50 silicon oxide graphite composite particles was defined as the aspect ratio of the silicon oxide graphite composite particles. The aspect ratio of the silicon oxide graphite composite particles according to this example was 2.4.

At the time of pulverization of silicon particles, silicon powder (specific surface area 20 m ² / g) is pulverized with a bead mill for 2 hours to prepare a pulverized slurry, and at the time of powder mixing, the mass ratio of flaky natural graphite powder to silicon powder is 83. : Except that it was set to 8, silicon oxide graphite composite particles were prepared in the same manner as in Example 1, and each characteristic was evaluated. The mass ratio of the scaly natural graphite powder, silicon powder (silicon oxide powder) and pitch heat-treated product in the silicon oxide graphite composite particles was 83: 8: 9 (see Table 1).

At this time, the composition ratio of oxygen atoms to silicon atoms in the SiOx particles (ie, “x”) was 0.69, and the BET specific surface area of the SiOx particles was 54 m ² / g. The aspect ratio of the silicon oxide graphite composite particles was 2.5. The particle diameter (median diameter) of the silicon oxide graphite composite particles at a volume fraction of 50% was 20 μm. The degree of orientation of the silicon oxide graphite composite particles was 0.0078. The discharge capacity of the non-aqueous test cell is 470 mAh / g, the initial charge / discharge efficiency is 87.4%, the average value of the charge / discharge efficiency from the 10th cycle to the 15th cycle is 99.7%, and the capacity is maintained. The rate was 96.8% (see Table 1).

At the time of pulverization of silicon particles, silicon powder (specific surface area 20 m ² / g) is pulverized by a bead mill for 5 hours to prepare a pulverized slurry, and at the time of powder mixing, the mass ratio of flaky natural graphite powder to silicon powder is 77. : Except that it was set to 14, silicon oxide graphite composite particles were prepared in the same manner as in Example 1, and each characteristic was evaluated. The mass ratio of the scaly natural graphite powder, silicon powder (silicon oxide powder) and pitch heat-treated product in the silicon oxide graphite composite particles was 77: 14: 9 (see Table 1).

At this time, the composition ratio of oxygen atoms to silicon atoms in the SiOx particles (ie, “x”) was 0.85, and the BET specific surface area of the SiOx particles was 82 m ² / g. The aspect ratio of the silicon oxide graphite composite particles was 2.7. The particle diameter (median diameter) of the silicon oxide graphite composite particles at a volume fraction of 50% was 20 μm. The degree of orientation of the silicon oxide graphite composite particles was 0.0081. The discharge capacity of the non-aqueous test cell is 471 mAh / g, the initial charge / discharge efficiency is 86.0%, the average value of the charge / discharge efficiency from the 10th cycle to the 15th cycle is 99.6%, and the capacity is maintained. The rate was 95.5% (see Table 1).

(Comparative Example 1)
<Production of comparative silicon oxide graphite composite particles>
(1) Grinding of silicon particles In the same manner as in Example 2, the silicon powder (specific surface area 20 m ² / g) was ground for 2 hours with a bead mill to prepare a ground slurry. The BET specific surface area of the SiOx powder obtained at this time was 54 m ² / g (see Table 1).

(2) Preparation of mixed slurry As in Example 2, scaly natural graphite powder (manufactured by Chuetsu Graphite Industry Co., Ltd., average particle size: 10 μm, d002: 0.3357 nm, pellet density: 1.82 g / cm ³ ) Was put into a Newgra machine manufactured by Seishin Co., Ltd., and the above pulverized slurry was added and mixed while rotating the machine to prepare a mixed slurry. At this time, as in Example 2, the scaly natural graphite powder and the pulverized slurry were mixed so that the mass ratio of the scaly natural graphite powder to the SiOx powder was 83: 8.

(3) Preparation of agglomerates Next, tetrahydrofuran and a coal-based pitch powder (softening point 86 ° C., average particle size 20 μm, residual carbon ratio after heating at 1000 ° C. 50%) are mixed with the above mixed slurry to prepare a dispersion. did. The dispersion was dried to obtain a dry powder. Next, after this dry powder was put into a graphite crucible, the dry powder was heated in a nitrogen stream at a temperature of 450 ° C. for 1 hour to obtain an aggregate.

(4) Preparation of intermediate composite particles Next, a circulation mechanofusion system (Hosokawa Micron Corporation) with a powder obtained by pulverizing the aggregate after heat treatment with a coffee mill with a gap of 1 mm between the rotor and the inner piece was obtained. AMS-mini). The powder was subjected to mechanochemical treatment for 15 minutes at a rotational speed of 7000 rpm by the circulation type mechanofusion system to prepare intermediate composite particles.

(5) Heat treatment of coal-based pitch powder Subsequently, the intermediate composite particles were put into a graphite crucible, and then the intermediate composite particles were heated in a nitrogen stream at a temperature of 1000 ° C. for 1 hour, Converted to non-graphitic carbon.

(6) Crushing process Finally, the intermediate composite particles after the heat treatment were pulverized until 98% by mass or more passed through a sieve having an opening of 75 μm to obtain target comparative silicon graphite composite particles. In this comparative silicon oxide graphite composite particle, the mass ratio of the scaly natural graphite powder, the SiOx powder (silicon oxide powder), and the pitch heat-treated product was 83: 8: 9 (see Table 1). Here, all the weight changes before and after firing were due to pitch.

  The characteristics of the comparative silicon graphite composite particles obtained as described above were evaluated in the same manner as in Example 1. As a result, the composition ratio of oxygen atoms to silicon atoms in the SiOx particles (ie, “x”) was 0. 69. The aspect ratio of the comparative silicon oxide graphite composite particles was 1.9. The particle diameter (median diameter) of the comparative silicon oxide graphite composite particles at a volume fraction of 50% was 30 μm. The degree of orientation of the comparative silicon oxide graphite composite particles was 0.0350. The discharge capacity of the non-aqueous test cell is 460 mAh / g, the initial charge / discharge efficiency is 86.0%, the average value of the charge / discharge efficiency from the 10th cycle to the 15th cycle is 96.0%, and the capacity is maintained. The rate was 87.1% (see Table 1).

(Comparative Example 2)
<Production of comparative silicon oxide graphite composite particles>
(1) Grinding of silicon particles In the same manner as in Example 2, the silicon powder (specific surface area 20 m ² / g) was ground for 2 hours with a bead mill to prepare a ground slurry. The BET specific surface area of the SiOx powder obtained at this time was 54 m ² / g (see Table 1).

(2) Preparation of mixed slurry Next, a tetrahydrofuran solution of coal-based pitch powder (softening point 86 ° C., average particle size 20 μm, residual carbon ratio 50% after heating at 1000 ° C.) was mixed with the above-described pulverized slurry, Further, spheroidized natural graphite (average particle size 20 μm) was mixed with the liquid to prepare a mixed slurry.

(3) Heat treatment of coal-based pitch powder Subsequently, the mixed slurry was dried and the dried product was put into a graphite crucible, and then the dried product was heated at a temperature of 1000 ° C. for 1 hour in a nitrogen stream. The pitch powder was converted to non-graphitic carbon.

(4) Crushing process Finally, the intermediate composite particles after the heat treatment were pulverized until 98% by mass or more passed through a sieve having an opening of 75 μm to obtain target comparative silicon graphite composite particles. In this comparative silicon oxide graphite composite particle, the mass ratio of the scaly natural graphite powder, the SiOx powder (silicon oxide powder), and the pitch heat-treated product was 83: 8: 9 (see Table 1). Here, all the weight changes before and after firing were due to pitch.

  The characteristics of the comparative silicon graphite composite particles obtained as described above were evaluated in the same manner as in Example 1. As a result, the composition ratio of oxygen atoms to silicon atoms in the SiOx particles (ie, “x”) was 0. 69. The aspect ratio of the comparative silicon oxide graphite composite particles was 1.3. The comparative silicon oxide graphite composite particles had a particle size (median diameter) of 21 μm at a volume fraction of 50%. The degree of orientation of the comparative silicon oxide graphite composite particles was 0.0420. The discharge capacity of the non-aqueous test cell is 470 mAh / g, the initial charge / discharge efficiency is 88.6%, the average value of the charge / discharge efficiency from the 10th cycle to the 15th cycle is 98.2%, and the capacity is maintained. The rate was 89.0% (see Table 1).

  From the above results, when the silicon oxide graphite composite particles according to the examples of the present invention are used as a negative electrode active material of a lithium ion secondary battery, the charge / discharge cycle characteristics of the lithium ion secondary battery are effectively improved. It became clear to do.

100 Silicon oxide graphite composite particle 110 SiOx particle 120 Scale-like graphite particle 200 Electrode 210 Active material layer 220 Current collector

Claims (12)

A plurality of scaly graphite particles arranged in layers;
Silicon oxide graphite composite particles comprising SiOx particles (where 0 <x ≦ 0.9) sandwiched between the plurality of scaly graphite particles. ^2. The silicon oxide-graphite composite particles according to claim 1, wherein the SiOx particles have a specific surface area within a range of 40 m ² / g to 200 m ² / g. 3. The silicon oxide graphite according to claim 1, wherein the SiOx particles are sandwiched between the plurality of scaly graphite particles and adhered to the outer surface of the scaly graphite particles in the outermost layer by non-graphitic carbon. Composite particles. The silicon oxide-graphite composite particles according to claim 3, wherein a mass ratio of the scaly graphite particles, the SiOx particles, and the non-graphitic carbon is 97 to 55: 1 to 30: 2 to 15. “(110) plane relative to“ peak intensity I (004) attributed to (004) plane ”in the X-ray diffraction image of the electrode when an electrode density of 1.70 ± 0.02 g / cm ³ was produced 5. The silicon oxide-graphite composite particles according to claim 1, wherein the ratio of the peak intensity I (110) attributed to is in the range of 0.0010 to 0.0300. The silicon oxide graphite composite particle according to any one of claims 1 to 5, wherein a ratio of a major axis length to a length in a laminating direction of the scaly graphite particles is in a range of 1.5 or more and 10 or less. A primary composite particle preparation step of preparing primary composite particles by applying compressive force and shear force to mixed particles of SiOx particles (where 0 <x ≦ 0.9) and scaly graphite particles;
A mixed powder preparation step of preparing a mixed powder by mixing the primary composite particles and a solid non-graphitic carbon raw material;
The manufacturing method of a silicon oxide graphite composite particle provided with the heating process which heat-processes the said mixed powder. A compressive force and shear force are applied to a mixture of SiOx particles (where 0 <x ≦ 0.9), scaly graphite particles and solid non-graphitic carbon raw material at a temperature equal to or higher than the softening point of the non-graphitic carbon raw material. Intermediate composite particle preparation step for preparing intermediate composite particles,
A method for producing silicon oxide-graphite composite particles, comprising a heating step of heat-treating the intermediate composite particles. A wet pulverization step of preparing a SiOx particle slurry by wet pulverization of silicon powder;
A mixing step of mixing the scaly graphite particles with the SiOx particle slurry to prepare a mixed slurry;
And further comprising a drying step of preparing the mixed powder by drying the mixed slurry,
In the intermediate composite particle preparation step, an intermediate composite is obtained by applying a compressive force and a shear force to the mixture of the mixed powder and the solid non-graphitic carbon raw material at a temperature higher than the softening point of the non-graphitic carbon raw material. The method for producing silicon oxide graphite composite particles according to claim 8, wherein the particles are prepared.   Silicon oxide graphite composite particles obtained by the method for producing silicon oxide graphite composite particles according to claim 7.   The electrode which uses the silicon oxide graphite composite particle in any one of Claim 1, 2, 3, 4, 5, 6, and 10 as an active material.   A nonaqueous electrolyte secondary battery comprising the electrode according to claim 11.