CN1319195C - Composite particle and, utilizing the same, negative electrode material for lithium-ion secondary battery, negative electrode and lithium-ion secondary battery - Google Patents

Abstract

Composite particles comprising a metal alloyable with lithium and a graphite material and/or carbon material, wherein at least portion of the metal is in contact with at least one material selected from the group consisting of graphite materials and carbon materials and wherein the ratio of void around the metal to total void is 20% or more. There are further provided, utilizing the composite particles, a negative electrode material for lithium ion secondary battery, negative electrode and lithium ion secondary battery. When these composite particles are used in a negative electrode material for lithium ion secondary battery, there can be accomplished three characteristics of large discharge capacity, excellent cycle performance and excellent initial charge discharge efficiency among the properties of lithium ion secondary battery.

Description

Composite particle, negative electrode material for lithium ion secondary battery using same, negative electrode, and lithium ion secondary battery

technical field

The present invention relates to composite particles formed by compounding a metal that can be alloyed with lithium, graphite material and/or carbon material, a negative electrode material for a lithium ion secondary battery using the composite particle, a negative electrode, and a lithium ion secondary battery.

Background technique

Lithium-ion secondary batteries are widely used as power sources for electronic devices because they have higher voltage and higher energy density than other secondary batteries. In recent years, the miniaturization and high performance of electronic devices have rapidly progressed, and there is an increasing demand for further improvement in the energy density of lithium-ion secondary batteries.

Now, Li-ion secondary batteries usually use _LiCoO2 as the positive electrode and graphite as the negative electrode. However, although graphite electrodes are excellent in reversibility of charge and discharge, their discharge capacity has already reached a value close to the theoretical value (372 mAh/g) corresponding to an interlayer compound (LiC ₆ ). Therefore, in order to further increase the energy density of the battery, it is necessary to develop an anode material with a larger discharge capacity than graphite.

Metal lithium ions have the largest discharge capacity as negative electrode materials. However, since lithium is precipitated in a dendrite form during charging, the negative electrode is deteriorated, and there is a problem that the charge-discharge cycle of the battery is short. In addition, the dendrite-precipitated lithium penetrates through the separator and reaches the positive electrode, and the battery may be short-circuited.

Therefore, metals or metal compounds that form alloys with lithium are studied instead of lithium ions as negative electrode materials. The discharge capacity of these alloy anodes is not as good as that of metallic lithium, but far exceeds that of graphite. However, since the alloying volume expands, the active material is pulverized and peeled off, and the cycle properties of the lithium-ion secondary battery are not yet practical.

In order to solve the shortcomings of the aforementioned alloy negative electrodes, negative electrodes formed by composites of metals or metal compounds and graphite materials and/or carbon materials have been researched and developed.

In order to absorb the expansion caused by alloying, it is effective to have voids in the composite material. However, if there are too many voids, the strength and conductivity of the composite material will be low. That is to say, the void volume of the composite material has an antinomy relationship with the damage resistance and electrical conductivity of the material itself, and it is very difficult to satisfy the two evenly.

For example, Japanese Patent No. 3369589 discloses that a composite material composed of a metal substance capable of forming an alloy with an alkali metal such as lithium, a graphite material, and a carbon material can be used as an electrode material. In this composite material, the carbon material acts to bind or cover the metallic substance and the graphite material. The ratio ID/IG (R value) of the D-band 1360 cm ⁻¹ peak intensity ID to the G-band 1580 cm ⁻¹ peak intensity IG of the surface of the carbon material measured by Raman spectroscopy using an argon laser was 0.4 or more. This indicates that the carbon material is not graphitized. However, even with such a composite material, the carbon material penetrates into the composite material, so it is difficult to avoid the destruction of the composite material due to the volume expansion when the metal substance is alloyed with lithium, resulting in a decrease in periodicity.

On the other hand, Japanese Unexamined Patent Publication No. 2000-173612 discloses a negative electrode material in which fibrous carbon is immobilized on a part or the entire surface of silicon-containing particles. This technology ensures electrical conductivity between silicon particles by fibrous carbon even when silicon particles shrink during discharge. However, although this structure can maintain conductivity, it cannot absorb the expansion of the metal that occurs during charging, resulting in a decrease in periodicity.

Japanese Patent No. 3466576 discloses a negative electrode material in which porous particles composed of silicon-containing particles and carbon-containing particles are covered with carbon. In addition, the carbon-containing particles correspond to a graphite material. In the example of this technology, although the negative electrode material can be positively porous, the structural damage of the negative electrode material will occur due to the volume expansion during alloying of silicon and lithium, and satisfactory periodic properties cannot be obtained. In addition, since the carbon-containing particles (graphite material) are as small as 1 μm or less, the electrolytic solution is likely to undergo a decomposition reaction, and the initial charge and discharge efficiency is also low.

As described above, in the prior art, it was difficult to make both absorption of expansion and maintenance of electrical conductivity excellent.

Based on the above-mentioned technical background, the present invention provides a lithium ion secondary battery in which composite particles containing three components of a metal that can be alloyed with lithium, a graphite material, and a carbon material are used for the lithium ion secondary battery. When used as the negative electrode material of the secondary battery, the discharge capacity is large, and excellent cycle properties and excellent initial charge-discharge rate can be achieved. In other words, the object of the present invention is to provide novel composite particles capable of satisfying these three performances of lithium ion secondary batteries, negative electrode materials for lithium ion secondary batteries using the composite particles, negative electrodes, and lithium ion secondary batteries.

Contents of the invention

The present invention provides a composite particle containing a metal, a graphite material, and a carbon material, wherein at least a part of a metal alloyed with lithium and at least one material selected from a graphite material and a carbon material can be in contact, and the voids around the metal It is 20 vol% or more with respect to all voids. In addition, the graphite material of the composite particles is preferably at least one material selected from flaky graphite and fibrous graphite.

In addition, in these composite particles, when the graphite material is in the form of scales, it is preferable that the ratio of the peak intensity in the D-band to the peak intensity in the G-band in the Raman spectrum is less than 0.4.

In addition, among these composite particles, the graphite material is preferably fibrous graphite whose average interplanar spacing d ₀₀₂ in X-ray diffraction is 0.34 nm or less.

In any of the composite particles described above, it is preferable that at least a part of the metal is in contact with the fibrous graphite material, and at least a part of their outer surfaces is covered with a carbon material. In addition, it is more preferable that the composite particles further contain flaky graphite.

In addition, the metal in any of the composite particles described above is preferably silicon.

In addition, the average particle diameter of the metal in any of the composite particles described above is preferably 0.01 to 10 μm.

In addition, the metal of any of the composite particles described above is preferably amorphous.

In addition, the specific surface area of any of the composite particles described above is preferably 20 m ² /g or less.

In addition, the average particle diameter of any one of the above-mentioned composite particles is preferably 1 to 50 μm.

In addition, the present application also provides a negative electrode material for a lithium ion secondary battery containing any of the composite particles described above. In addition, a negative electrode for a lithium ion secondary battery using the negative electrode material for a lithium ion secondary battery is also provided. Moreover, the lithium ion secondary battery using this negative electrode for lithium ion secondary batteries is also provided.

In addition, the present application also provides an invention of a composite particle, which is a composite particle that integrates a metal that can be alloyed with lithium and a graphite material through a carbon material, and is characterized in that the composite particle has voids , and the ratio of voids around the metal to all voids in the composite particle is 20% or more.

Description of drawings

FIG. 1 is a schematic cross-sectional view showing the structure of a coin-type evaluation battery used in a charge-discharge test.

Fig. 2 is a schematic cross-sectional view of composite particles exemplified in Example 1 of the present invention.

Detailed ways

Hereinafter, the present invention will be described in more detail.

As described above, in the past metal-graphite (carbon) type composite material, reduction in periodic properties due to expansion when metal and lithium in the negative electrode are formed and alloyed cannot be avoided. Therefore, the present inventors studied a negative electrode that can absorb the expanded structure while maintaining the conductivity of the negative electrode. As a result, the conductivity of the negative electrode cannot be maintained only by increasing the total voids of the composite particles. However, if voids that can absorb the expansion are formed around the constituent metals, it has been found that powdering and powdering of the composite particles can be prevented while maintaining the conductivity of the negative electrode. Peel off, so far the present invention is completed.

(composite particles)

The composite particle of the present invention is a composite particle containing a metal, a graphite material, and a carbon material, wherein at least a part of the metal that can be alloyed with lithium is in contact with at least one material selected from the graphite material and the carbon material, and the voids around the metal are It is 20 vol% or more with respect to all voids.

In the composite particle, at least a portion of the metal is in contact with the graphite material or the carbon material, or both, and the void surrounding the metal is also in contact with at least a portion of the metal surface. Usually, the composite particle is dispersed to contain a plurality of metal particles, and dispersed to contain a plurality of voids of indeterminate size.

In the present invention, the ratio of voids around the metal (hereinafter also referred to as peripheral voids) to all voids in the composite particle is 20 vol% or more. When it is less than 20 vol%, the expansion when the metal and lithium are alloyed cannot be absorbed. A preferable peripheral porosity is 40 vol% or more, and a more effective peripheral porosity is 50 vol% or more. In addition, the upper limit of the void ratio around the metal may be 100% of the theory. At this time, all the voids in the composite particles are in a state of being in contact with the metal part. This application does not exclude such cases. However, for ordinary particles of the present invention, an appropriate upper limit of the peripheral void ratio is 80 to 90 vol%.

In addition, in the composite particles of the present invention, the ratio of the total porosity to the total volume is preferably 3 to 50 vol%. Usually, if it is 3 vol% or more, the volume expansion caused by alloying can be sufficiently absorbed; if it is 50 vol% or less, the strength of composite particles can be sufficiently ensured, and it is particularly preferably 30 to 50 vol%.

The volume of all voids in the composite particle of the present invention can be obtained, for example, by measuring the composite particle whose cross-section is exposed by crushing with a mercury porosimeter. Furthermore, from this, the porosity (volume ratio) of the entire composite particle can be calculated.

The ratio of the voids of the surrounding metal to the voids of the entire composite particle of the present invention was obtained by the following method. Select any 50 composite particles with a scanning electron microscope, and take cross-sectional photos with a magnification of 400 times. The total value of the total void area of each composite particle and the total value of the surrounding voids of each metal were obtained from the cross-sectional photograph. Using the values of these 50 composite particles, the ratio (area ratio) of the void area around the metal to the total void area was obtained, and the arithmetic mean value per one composite particle was used as the void ratio around the metal in the present invention. From the cross-sectional photograph, it can be judged whether at least one part of the metal is in contact with graphite material and/or carbon material.

In addition, the weight composition of the composite particles is a value converted into a metal concentration by performing an elemental analysis by emission spectrometry when the composite particles are incinerated for metals.

For graphite materials and carbon materials, the section of composite particles is magnified to 1000 times using a polarizing microscope, and then photographed, with any 10 particles as targets, and judged according to the difference in appearance caused by the level of crystallinity. In addition, the ratio of both is the average value of the ratio of the area occupied by the graphite material and the carbon material inside the particle.

In addition, the area ratio occupied by the graphite material and the carbon material can be determined by observation using a transmission electron microscope after preparing a cross-sectional sheet of the composite particle. Here, although the area ratio of the graphite material and the carbon material is obtained, since there is no large difference in density between the graphite material and the carbon material, in the present invention, the area ratio obtained above is approximately the same as the weight ratio.

In the composite particle of the present invention, since the aforementioned voids exist around the aforementioned metal, it is possible to improve the cycle properties of the lithium ion secondary battery. This is because the expansion of the metal is absorbed by the voids during charging, thereby suppressing the structural destruction of the negative electrode material containing the composite particles. That is, for example, even when the metal itself is pulverized, the shape of the composite particles as a whole of the negative electrode material can be maintained, and the contact between the composite particles can be maintained without impairing the current collection property. Therefore, it is inferred that the decrease in periodicity can be suppressed.

In the composite particle of the present invention, it is preferable that the graphite material constituting the composite particle is in the form of scales or fibers.

If the graphite material is in the form of scales, it is easy to form voids in the composite particles, and in particular, periodic properties and the like can be improved. In addition, when the graphite material of the composite particle of the present invention is in the form of scales, the ratio (ID/IG) of the peak intensity (ID) in the D-band to the peak intensity (IG) in the G-band in the Raman spectrum of the composite particle is Preferably less than 0.4. When the Raman spectrum of the composite particle is measured using an argon laser with a wavelength of 514.5nm, the outer surface of the composite particle can be judged from the ratio (ID/IG) of the peak intensity (ID) in the D-band to the peak intensity (IG) in the G-band crystallinity. This ID/IG ratio is generally referred to as "R value", and the R value of the composite particles of the present invention is preferably less than 0.4. In addition, the G-band is usually observed at 1580 ^-1 , and the D-band is observed at 1360 ^-1 , and they are observed in the range of ±20 ^-1 depending on the measurement error. Composite particles having an R value of less than 0.4 can be obtained by satisfying the aforementioned structure of the composite particles. Such composite particles are particularly preferable since they have high surface crystallinity and excellent periodicity and initial charge-discharge efficiency. Moreover, the more preferable range of R value is 0.15-0.38, More preferably, it is 0.2-0.3.

In addition, the case where the R value shows 0.4 or more is, for example, a case where a graphite material other than flaky graphite is used as the graphite material, and the edge surface of the graphite is exposed to the outer surface.

On the other hand, if the graphite material is fibrous, the electrical conductivity in the composite particle is improved, especially the periodicity and the like are improved. In addition, in the composite particles of the present invention, when the graphite material is fibrous, the average lattice spacing d ₀₀₂ of X-ray diffraction of the fibrous graphite is preferably 0.34 nm or less. Such fibrous graphite is particularly preferable because of its high crystallinity and high discharge capacity. In addition, the measurement of lattice plane spacing is to use CuKα rays as the X-ray source and high-purity silicon as the standard substance to measure the diffraction peak of the (002) plane of the graphitic material, and calculate d ₀₀₂ from the peak position. The calculation method is based on the Gakushin method (measurement method established by the 117th Committee of the Japan Society for the Promotion of Science), specifically "Carbon Fiber" (by Otani Sugiro, pp. 733-742 (1986), modern compilations The value measured by the method described by the company).

In addition, in the composite particle of the present invention, it is preferable that at least a part of the metal is in contact with the fibrous graphite material, and at least a part of their outer surfaces are covered with a carbon material. The "covering" mentioned here means that the periphery of the composite particle, that is, part or all of the outer surface is covered with carbon material. Therefore, as long as this requirement is satisfied, the part of the carbon material can enter the three-dimensional interior formed by the fibrous graphite and contact the metal. For example, it also includes a structure in which metal is stored in complex fibrous graphite and the outer surface is covered with a carbon material. Such composite particles are particularly preferable since they maintain excellent electrical conductivity and periodic properties while maintaining voids for absorbing expansion. In addition, the composite particles more preferably contain flaky graphite. The reason is that flake graphite is easy to form voids and has a smaller specific surface area than fibrous graphite, which can improve cycle properties and initial charge and discharge efficiency. This flaky graphite enters the composite particle in a state of surrounding the fibrous graphite holding the metal.

The shape of the composite particles of the present invention is indeterminate and not particularly limited. The average particle diameter of the composite particles is preferably 1 μm to 50 μm. The reason is that, within this range, sufficient contacts exist between the composite particles when the electrode is produced, electrical conductivity can be ensured, and the periodicity is particularly excellent. Preferably, it is 3 μm to 30 μm. In addition, generally, the size preferable for general use as a negative electrode material is about 3 to 50 μm.

The specific surface area of the composite particles is preferably 20 m ² /g or less. The reason is that the reaction area with the electrolytic solution is limited, and the initial charge and discharge efficiency is excellent. More preferably, it is 0.5m ² /g to 20m ² /g. Particularly preferably, it is 1 m ² /g to 10 m ² /g. The specific surface area was measured by nitrogen adsorption BET method.

The average aspect ratio of the composite particles of the present invention is preferably 5 or less, particularly preferably 3 or less.

(metals that can be alloyed with lithium)

Metals that can be alloyed with lithium include, for example, Al, Pb, Zn, Sn, Bi, In, Mg, Ga, Cd, Ag, Si, B, Au, Pt, Pd, Sb, Ge, Ni, and the like. Alternatively, an alloy of two or more of these metals may be used. The alloy may further contain elements other than the above. In addition, part or all of the metal may be compounds such as oxides, nitrides, and carbides. Preferable metals are silicon (Si) and tin (Sn), particularly preferably silicon. In addition, the metal may be crystalline or non-crystalline, and is more preferably non-crystalline. The reason is that the expansion occurs isotropically and has little effect on the composite particles.

The shape of the metal is not particularly limited, and may be any of granular, spherical, flake, scale, needle, filament, and the like. It may exist in the form of a thin film on the surface of graphite material and carbon material. Among them, granular or spherical particles are preferable.

The average particle diameter of the metal is preferably 0.01 μm to 10 μm. If it is 0.01 μm or more, the dispersibility of the metal is sufficient. On the other hand, if it is 10 μm or less, the expansion of the metal is easily absorbed. It is particularly preferably 1 μm or less. Here, the average particle size refers to the particle size at which the cumulative frequency measured by a laser diffraction particle size meter is 50% in volume percentage.

The metal preferably enters the interior of the composite particle rather than intercalating on the outer surface. If the metal exists inside, it is easy to ensure the contact with the graphite material and/or carbon material, improve the electrical conductivity, and exhibit a high capacity corresponding to the amount of metal added.

(carbon material)

The carbon material is conductive and is an indispensable component as a substance that binds or covers metal and graphite materials, and is produced by heat-treating the precursor at a final temperature of less than 1500°C. In this application, carbon materials are also referred to as carbonaceous materials. The carbon material is not limited as long as it contains substantially no volatile components, has electrical conductivity, and can absorb or desorb lithium ions. The type of precursor of the carbon material is not limited, and in the present application, it is preferable to use two or more precursors having different char residue ratios of the carbonized carbon material. Here, the carbon residue rate is the residual component when the carbon is fixed to 800°C according to the carbon fixation method of JIS K2425, and the above-mentioned substantially all carbonization is expressed in percentage. The difference in carbon residue rate refers to that the difference between the carbon residue rates of several carbon materials is more than several %, preferably more than 10%.

Examples of carbon material precursors include coal tar, light tar, tar middle fraction, heavy tar, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen crosslinked petroleum pitch, heavy oil, etc. Petroleum-based or fuel-based tar pitch, or thermoplastic resins such as polyvinyl alcohol, phenolic resins, urea resins, maleic acid resins, coumarone resins, xylene resins, furan resins, and other thermosetting resins. From the viewpoint of simultaneously suppressing a decrease in discharge capacity, etc., tar pitches are particularly preferable.

Two or more precursors having different carbon residue rates, for example, phenolic resin with a carbon residue rate of 10-50% and coal tar pitch with a carbon residue rate of 50-90% can be used.

The precursor of the carbon material (carbon material A) having a relatively low carbon residue rate mainly plays a role in forming the voids around the metal because many voids are generated in the carbon material after heating. On the other hand, the precursor of the carbon material (carbon material B) with a relatively high carbon residue rate has fewer voids in the heated carbon material and can form a dense carbon material, so it mainly plays the role of forming the outermost layer of composite particles. , enclosing the role of composite particles. As a result, the irreversible capacity of a lithium ion secondary battery using a negative electrode material containing it may decrease (initial charge and discharge efficiency improves). Therefore, in the production process of the composite particle of the present invention, it is preferable to first mix the precursor of the carbon material A with the metal and the graphite material to form a composite, and then mix the precursor of the carbon material B to form a composite.

(graphite material)

The graphite material is not particularly limited as long as it can absorb and release lithium ions. Specifically, part or all of the material is formed of graphite. For example, artificial graphite and natural graphite obtained by heat-treating (graphitizing) tar oil and pitch at a final temperature of 1500°C or higher. In this application, graphite material is also referred to as graphitic material. More specifically, there can be exemplified mesophase baked bodies and mesophase spheres obtained by heat-treating and polycondensing a carbon material having an easily graphitizable property such as petroleum or coal tar pitch. Cokes can also be obtained by graphitization treatment at 1500°C or higher, preferably 2800 to 3300°C.

The shape of the graphite material may be any of spherical shape, block shape, sheet shape, scale shape, fiber shape and the like. In particular, scale-like or nearly scale-like substances and fibrous substances are preferable. The reason for this is as described above. In addition, various mixtures, granules, coverings, and laminates mentioned above may also be used. In addition, various chemical treatments, heat treatments, oxidation treatments, physical treatments, etc. in liquid phase, gas phase, and solid phase may be used.

The average particle size of the graphite material is 1 to 30 μm, particularly preferably 3 to 15 μm. The reason is that composite particles having the aforementioned appropriate average particle diameter can be easily produced.

In the composite particle of the present invention, when a flaky graphite material is used, the flaky graphite material is preferably arranged randomly. In particular, it is preferable to arrange them in a cabbage shape or in a concentric circle shape. The base surface (the surface perpendicular to the edge surface) of the flaky graphite preferably faces the outer surface side of the composite particle, and more preferably the base surface is partially exposed to the outer surface of the composite particle.

In the composite particle of the present invention, when a fibrous graphite material is used, the fibrous graphite material may be in an aggregated state, or may be in an aggregated, de-agglomerated, and dispersed state, and is particularly preferably in a cotton-like aggregated state containing metal particles. Since the fibrous graphite material has a relatively large specific surface area, when a fluid carbon material precursor is mixed with the composite particles, the fluid precursor is adsorbed on the surface of the fibrous carbon material constituting the composite particles, and it is difficult to It penetrates into the composite particle and has the effect of easily securing voids inside the covered composite particle.

The fibrous graphite material can be obtained by heat-treating its precursor at a final temperature of 1500-3300°C. The precursor is not limited as long as a fibrous graphite material can be obtained, but a fibrous carbon material that can be graphitized is particularly preferable. For example, carbon nanofibers, carbon nanotubes, vapor phase growth carbon fibers, etc. are mentioned. The precursor preferably has a minor axis length (diameter) of 1 to 500 nm, particularly preferably 10 to 200 nm. In addition, the aspect ratio of the precursor is preferably 5 or more, particularly preferably 10-300. Here, the aspect ratio refers to fiber length/short axis length.

(Manufacture of Composite Particles)

Hereinafter, the method for producing the composite particles of the present invention will be exemplified. In the method of the present invention, at least a plurality of carbon material precursors that can be alloyed with lithium, graphite materials, and carbon residue ratios relatively different are used as raw materials. That is, for example, mixing a metal and a graphite material that can be alloyed with lithium and a precursor (precursor A) of a carbon material (carbon material A) with a relatively low carbon residue rate, in the resulting composite particles , and then mix the precursor (precursor B) of a carbon material (carbon material B) with a relatively high carbon residue rate, and heat it. In this production method, the heat treatment is preferably performed at a temperature at which the carbon material A and the carbon material B of the composite particles are substantially free of volatile matter.

At this time, the suitable composition of metal, graphite material and carbon material is in the range of metal/graphite material/carbon material=1-50wt%/30-95wt%/4-50wt% in weight percentage. If the composition ratio is within this range, when a negative electrode material containing the composite particles is used in a lithium ion secondary battery, the discharge capacity of the battery can be improved, or the effect of improving the cycle properties of the battery can be obtained. Preferably, it is mixed in the range of 2 to 30 wt %/60 to 93 wt %/5 to 30 wt %. Specifically, it is the range of metal/graphite material/carbon material A/carbon material B=1～50wt%/35～95wt%/2～50wt%/2～40wt%, preferably 2～30wt%/60～93wt% /3～30wt%/2～30wt%. However, in the final product, the carbon materials A and B derived from the precursors A and B cannot be distinguished.

The precursor is heat-treated and carbonized at a temperature of 600° C. or higher, preferably 800° C. or higher, to impart conductivity to the carbon material. This heat treatment can be divided into several stages and performed multiple times, and can also be performed in the presence of a catalyst. In addition, it can also be performed in either an oxidizing atmosphere or a non-oxidizing atmosphere.

However, when silicon is used as the metal, since carbon and silicon react to form SiC at 1500°C or higher, the heating temperature is preferably lower than 1500°C. Usually, it is preferably 1000 to 1200°C. In addition, it is preferable to mix using an appropriate dispersant. The dispersant is preferably removed at a temperature below which it does not decompose after softening the precursor A or precursor B.

In addition, at any stage before and after heat treatment, it is preferable to adjust the particle size by appropriate pulverization, sieving, classification and removal of fine powder, and the like. In addition, by performing heat treatment at a relatively low temperature, the operation of rotating the composite and the operation of imparting high shear force can be applied while the composite has flexibility. In this way, the composite has a nearly spherical shape, and it is particularly preferable when flaky graphite is used as one of the graphite materials because the flaky graphite is easily arranged concentrically. As an apparatus capable of performing such an operation, GRANUREX (manufactured by Front Industry Co., Ltd.), agitator granulator (Nyu-Gramashin) (manufactured by Seishin Co., Ltd.), vibration granulator (Agromaster) (manufactured by Hosokawa Micron Co., Ltd.) can be used. Compression shears such as granulators such as roll crushers, hybrid systems (manufactured by Nara Machinery Manufacturing Co., Ltd.), mechanical microsystems (manufactured by Nara Machinery Manufacturing Co., Ltd.), mechanical melting systems (manufactured by Hosukawa Micron Co., Ltd.) Cutting processing equipment, etc.

In addition, the outer surface of the composite body can be coated with multiple layers of the same or different carbon material precursors before the final heat treatment.

As another production method, a method in which a precursor of a carbon material is adhered to a graphite material in advance, mixed with the metal, and then heat-treated can be exemplified. Alternatively, a method of embedding or covering the metal in a graphite material, mixing it with a carbon material precursor, and heat-treating it may be used. At this time, as a method of adhering the metal or the organic compound of the metal to the graphite material in the gas phase, PVD (Physical Vapor) such as vacuum evaporation, sputtering, ion plating, molecular beam epitaxy, etc. Deposition) method, atmospheric pressure CVD (Chemical Vapor Depositon) method, decompression CVD method, plasma CVD method, MO (Magneto-optic) CVD method, optical CVD method and other CVD methods.

In addition, when flaky graphite is used, after spheroidizing the flaky graphite in advance, injecting a liquid mixture of a precursor of a carbon material and a metal into the void, impregnating it, mixing the precursor of a carbon material, and then heat-treating it may be used.

In addition, when fibrous graphite is used, a method of heat-treating after mixing the carbon material precursor after integrating the metal and the fibrous graphite may be employed. In the integration of this metal and this fibrous graphite and the subsequent heat treatment stage, etc., it can coexist with flaky graphite. As a method of integrating the metal and the fibrous graphite, for example, mechanochemical treatment that imparts mechanical energy such as compression, shear, impact, and friction, and adding the metal particles to an organic solvent in which the fibrous graphite material is dispersed After that, the method of removing the organic solvent, etc.

When using the composite particles of the present invention to manufacture negative electrode materials and negative electrodes, they can coexist with conductive materials, modified materials, additives, etc. commonly used in the production of negative electrode materials. For example, various graphite materials such as natural graphite, artificial graphite, graphitized mesophase fired body, graphitized mesophase fibrous body, carbon materials such as amorphous hard carbon, and conductive auxiliary materials such as carbon black and vapor-phase grown carbon fiber can be added. , Organic substances such as phenolic resin, metals such as silicon, metal compounds such as tin oxide. The total amount of these additions is usually 0.1 to 50% by weight relative to the composite particles.

The present invention relates to a negative electrode material for a lithium ion secondary battery containing the aforementioned composite particles, and a lithium ion secondary battery using the negative electrode material.

(negative electrode)

The negative electrode for a lithium ion secondary battery of the present invention can be produced by a common negative electrode forming method, and there is no limitation as long as a stable negative electrode can be obtained chemically or electrochemically. When producing a negative electrode, it is preferable to use a negative electrode mixture prepared in advance by adding a binder to the composite particles of the present invention. Substances which exhibit chemical and electrochemical stability to the electrolyte are preferred as binders. For example, powders of fluorine-based resins such as polytetrafluoroethylene and polyvinylidene fluoride, powders of resins such as polyethylene and polyvinyl alcohol, carboxymethylcellulose, and the like can be used. They can also be used simultaneously. The binder is usually used in a ratio of about 1 to 20 wt % in the total amount of the negative electrode mixture.

In a more specific example, first, the composite particles of the present invention are adjusted to a desired particle size by classification or the like, mixed with a binder, and the resulting mixture is dispersed in a solvent to form a paste to prepare a negative electrode mixture. That is, the composite particles and binder of the present invention are mixed with solvents such as water, isopropyl alcohol, N-methylpyrrolidone, and dimethylformamide to obtain a slurry, and a known mixer, mixer, or kneading machine is used to obtain a slurry. Stir and mix the obtained slurry with an agent, a kneader, etc. to prepare a paste. This paste is applied to one or both surfaces of a current collector sheet, and dried to obtain a negative electrode to which the negative electrode mixture is adhered uniformly and firmly. The film thickness of the negative electrode mixture layer is 10 to 200 μm, preferably 20 to 100 μm.

In addition, the negative electrode of the present invention can also be manufactured by dry-mixing the composite particles of the present invention with resin powders such as polyethylene and polyvinyl alcohol, and then hot-pressing molding in a mold.

After the negative electrode mixture layer is formed, performing hot extrusion or other pressure welding can further increase the bonding strength between the negative electrode mixture layer and the current collector.

The shape of the current collector used in negative electrode production is not particularly limited. A foil shape, a mesh shape, etc. are preferable. A metal mesh etc. are mentioned as a mesh-like thing. The material as the current collector is preferably copper, stainless steel, nickel, or the like. The thickness of the current collector is preferably about 5 to 20 μm when it is in the form of a foil.

In addition, the negative electrode of the present invention can further add natural graphite layer graphite materials, carbon materials such as amorphous hard carbon, and organic substances such as phenolic resins in the composite particles containing metals that can be alloyed with lithium, graphite materials, and carbon materials. , silicon and other metals, tin oxide and other metal compounds, etc.

(Li-ion secondary battery)

Lithium-ion secondary batteries usually use negative electrodes, positive electrodes and non-aqueous electrolytes as the main components of the battery. Since the positive electrode and the negative electrode each serve as carriers of lithium ions, they have a structure in which lithium ions are adsorbed to the negative electrode during charging and released from the negative electrode during discharging.

The lithium ion secondary battery of the present invention is not particularly limited except that the negative electrode material of the present invention is used as the negative electrode material, and other battery components such as positive electrodes, electrolytes, and separators can be prepared according to common lithium ion secondary battery components.

(positive electrode)

The positive electrode can be formed, for example, by applying a positive electrode mixture formed of a positive electrode material, a binder, and a conductive agent onto the surface of a current collector. The material of the positive electrode (positive electrode active material) is preferably selected from a material that can absorb and detach a sufficient amount of lithium, which can be lithium-containing compounds such as lithium-containing transition metal oxides, transition metal chalcogenides, vanadium oxides, and lithium compounds thereof. Chevrel phase compound and activated carbon represented by the formula MxMo ₆ S _8-Y (wherein, M is at least one transition metal element, X is 0≤X≤4, and Y is a value within the range of 0≤Y≤1) , activated carbon fiber, etc. Vanadium oxides are those represented by V ₂ O ₅ , V ₆ O ₁₃ , V ₂ O ₄ , and V ₃ O ₈ .

The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and may be a substance in which lithium and two or more transition metals form a homogeneous phase. Composite oxides may be used alone or in combination of two or more. Lithium-containing transition metal oxides, specifically LiM ¹ _1-x M ² _x O ₂ (wherein, M ¹ and M ² are at least one transition metal element, and X is in the range of 0≤X≤1 numerical value), or LiM ¹ _2-Y M ² _Y O ₄ (wherein, M ¹ and M ² are at least one transition metal element, and Y is a numerical value within the range of 0≤Y≤2).

The transition metal elements represented by M ¹ and M ² are Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc., preferably Co, Fe, Mn, Ti, Cr, V, Al wait. Preferable examples are LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiNi _0.9 Co _0.1 O ₂ , LiNi _0.5 Mn _0.5 O ₂ and the like.

Lithium-containing transition metal oxides, such as lithium, transition metal oxides, hydroxides, salts, etc., are used as starting materials, and these starting materials are mixed according to the composition of the desired metal oxides, and passed under an oxygen atmosphere. , Calcined at a temperature of 600-1000°C.

As the positive electrode active material, the aforementioned compounds may be used alone, or two or more of them may be used in combination. For example, carbonates such as lithium carbonate can be added to the positive electrode. In addition, various additives such as conventionally known conductive agents and binders can be appropriately used when forming the positive electrode.

The positive electrode can be produced by applying a positive electrode mixture formed by the aforementioned positive electrode material, a binder, and a conductive agent for imparting conductivity to the positive electrode on both surfaces of a collector to form a positive electrode mixture layer. As the binder, the same substance as that used in the production of the negative electrode can be used. Known substances such as graphitized substances and carbon black can be used as the conductive agent.

The shape of the current collector is not particularly limited, and a mesh-like material such as a foil, a mesh, or a metal mesh can be used. The material of the current collector is aluminum, stainless steel, nickel, or the like. The thickness is suitably 10 to 40 μm.

The positive electrode can also be the same as the negative electrode, the positive electrode mixture is dispersed in the solvent to become a paste, and the paste-like positive electrode mixture is coated on the current collector and dried to form a positive electrode mixture layer; it is also possible to form the positive electrode mixture layer After that, extrusion pressure isobaric welding is carried out. Thus, the positive electrode mixture layer can be uniformly and firmly adhered to the current collector.

(non-aqueous electrolyte)

The nonaqueous electrolyte used as the lithium ion secondary battery of the present invention is an electrolyte salt used in a common nonaqueous electrolytic solution, for example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiB(C ₆ H ₅ ), LiCl, LiBr, LiCF ₃ SO ₃ , LiCH ₃ SO ₃ , LiN(CF ₃ SO ₂ ) ₂ , LiC(CF ₃ SO ₂ ) ₃ , LiN(CF ₃ CH ₂ OSO ₂ ) ₂ , LiN(CF ₃ CF ₂ OSO ₂ ) ₂ , LiN(HCF ₂ CF ₂ CH ₂ OSO ₂ ) ₂ , LiN((CF ₃ ) ₂ CHOSO ₂ ) ₂ , LiB[C ₆ H ₃ (CF ₃ ) ₂ ] ₄ , LiAlCl ₄ , LiSiF ₆ and other lithium Salt. In particular, LiPF ₆ and LiBF ₄ are preferably used from the viewpoint of oxidation stability.

The concentration of the electrolyte salt in the electrolyte is preferably 0.1 to 5 mol/l, more preferably 0.5 to 3.0 mol/l.

Carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1, 2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, anisole, di Ethers such as ethyl ether, sulfolanes such as sulfolane and methyl sulfolane, nitriles such as acetonitrile, chloroacetonitrile and propionitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N -Methylpyrrolidone, ethyl acetate, trimethylorthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidine Ketones, ethylene glycol, dimethyl sulfide and other aprotic organic solvents.

When a nonaqueous electrolyte is used as a polymer electrolyte such as a polymer solid electrolyte or a polymer gel electrolyte, a polymer compound gelled by a plasticizer (nonaqueous electrolyte) can be used as a matrix. As the polymer compound of the matrix, ether resins such as polyethylene oxide and its cross-linked body, polymethacrylate resin, polyacrylate resin, polyvinylidene fluoride (PVDF) and vinylidene fluoride- Fluorine-based resins such as hexafluoropropylene copolymers are used alone or in combination.

Among them, fluorine-based resins such as polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymer are preferably used from the viewpoint of oxidation-reduction stability and the like.

As the plasticizer used, the aforementioned electrolyte salt and nonaqueous solvent can be used. In the case of a polymer gel electrolyte, the electrolyte salt concentration in the nonaqueous electrolytic solution as a plasticizer is preferably 0.1 to 5 mol/l, more preferably 0.5 to 2.0 mol/l.

The manufacture of the polymer electrolyte is not particularly limited, and examples include a method of mixing a polymer compound constituting a matrix, a lithium salt, and a nonaqueous solvent (plasticizer), heating, and melting and dissolving the polymer compound; After dissolving the lithium salt and the non-aqueous solvent in the organic solvent for mixing, the method of evaporating the organic solvent for mixing; mixing the mixed monomer, lithium salt and the non-aqueous solvent, and irradiating the mixture with ultraviolet rays, electron beams or molecular beams, A method of polymerizing a polymerizable monomer to obtain a polymer compound, etc.

The proportion of the non-aqueous solvent of the polymer electrolyte is preferably 10 to 90% by weight, more preferably 30 to 80% by weight. If it is less than 10% by weight, the electrical conductivity will be low; if it exceeds 90% by weight, the mechanical strength will be weak, making it difficult to form a film.

(partition)

In the lithium ion secondary battery of the present invention, a separator can also be used. The material and structure of the separator are not particularly limited, and examples thereof include woven fabrics, nonwoven fabrics, and microporous membranes made of synthetic resins. Microporous membranes made of synthetic resins are suitable, and particularly polyolefin-based microporous membranes are suitable in terms of thickness, membrane strength, and membrane resistance. Specifically, it is a microporous film made of polyethylene and polypropylene, or a microporous film formed by combining them.

In the lithium ion secondary battery of the present invention, since the mesophase spheres with no end faces exposed are used as the carbon material for the negative electrode, a gel electrolyte can also be used.

A lithium ion secondary battery using a gel electrolyte can be composed of a negative electrode containing the aforementioned composite particles, a positive electrode, and a gel electrolyte. For example, a negative electrode, a gel electrolyte, and a positive electrode are sequentially laminated and stored in a battery outer packaging material. In addition, in addition to this, a gel electrolyte may be arranged outside the negative electrode and the positive electrode.

In addition, the lithium ion secondary battery of the present invention can be of any structure, and its shape and form are not particularly limited, and it can be cylindrical, cubic, coin, etc. according to its use, mounting equipment, required charge and discharge capacity, etc. Type, button type, etc. can be selected arbitrarily. In order to obtain a sealed non-aqueous electrolyte battery with higher safety, it is preferable to use a battery with a device capable of sensing a rise in the internal pressure of the battery and blocking the current when an abnormality such as overcharge occurs. In the case of a polymer solid electrolyte battery and a polymer gel electrolyte battery, a laminated film encapsulation structure may also be used.

Example

Next, although an Example and a comparative example demonstrate this invention more concretely, this invention is not limited by these. In addition, in the examples and the comparative examples, the coin-type secondary battery for the evaluation of manufacturing the structure shown in FIG. 1 was evaluated. An actual battery is manufactured according to a known method for the purpose of the present invention. In this battery for evaluation, the working electrode is referred to as a negative electrode, and the counter electrode is referred to as a positive electrode.

In Examples and Comparative Examples, the carbon residue rate of the precursor of the carbon material was measured as follows according to the fixed carbon method of JIS K2425.

Measure 1g of carbon material into the crucible, without the lid, and heat in an electric furnace at 430°C for 30 minutes. Afterwards, as a double crucible, it was heated in an electric furnace at 800°C for 30 minutes to remove volatile components, and the percentage of residual components was used as the carbon residue rate.

The average particle diameter of the composite particles is measured using a laser diffraction particle size distribution meter (manufactured by Seishin Co., Ltd., LS-5000), and is a particle diameter at which the cumulative frequency is 50% in volume percentage.

The porosity of the entire composite particle is calculated as a ratio to the volume of the entire composite particle after measuring the volume of all voids using a mercury porosimeter.

The specific surface area was determined by the BET method of nitrogen gas adsorption.

The lattice plane spacing d ₀₀₂ of X-ray diffraction is determined according to the aforementioned method.

The ratio of the voids around the metal to the total voids of the composite particle was obtained by calculating the area ratio of the two-dimensional void region from the observation of the cross section of the particle with a scanning electron microscope, and the average of the measurement results in the cross section of 50 composite particles was used. value. Here, if a void exists in direct contact with a part of the surface of the metal, it is defined as a void around the metal.

In addition, the proportion of the metal in the composite particles was determined by the above-mentioned emission spectrometry. The ratio of the graphite material and the carbon material was determined by the method using the aforementioned polarizing microscope.

In addition, the R value of Raman spectroscopy was analyzed using a laser Raman spectroscopy analyzer (NR-1800: manufactured by JASCO Co., Ltd.), with an excitation light of 514.5 nm argon ion laser and an irradiation area of 50 μmφ. The intensity of the ^-1 peak is ID, and the intensity of the peak at 1580cm ^-1 in the G band is the ratio ID/IG of IG.

(Example 1)

(Manufacture of Composite Particles)

Metal silicon powder (manufactured by High Purity Chemical Research Institute Co., Ltd., average particle size 2 μm) was dispersed in an ethanol solution of phenolic resin (manufactured by Sumitomo Becklite Co., Ltd., carbon residue rate 50%) to form a slurry, and a biaxial The kneader was heated, and the slurry was kneaded with natural graphite (manufactured by Chuetsu Graphite Co., Ltd., average particle diameter: 10 μm) at 150° C. for 1 hour to obtain a kneaded product. At this time, the weight percentages of the prepared solid components are 18wt% of phenolic resin, 6wt% of silicon powder, and 76wt% of natural graphite. In addition, the solid component mentioned in this application is a substance which shows a solid state at normal temperature before preparation of a solution.

Then, a middle fraction of tar was mixed with coal tar pitch (manufactured by JFE Chemical Co., Ltd., carbon residue rate: 60%) to prepare a coal tar pitch solution. Using a biaxial heating kneader, the solution and the kneaded product were kneaded at 200° C. for 1 hour. At this time, the weight percentages of the solid content were prepared to be 30 wt% of coal tar pitch and 70 wt% of the kneaded product. After kneading, the solvent in the kneaded product was removed in vacuo.

The obtained kneaded product was coarsely pulverized, and then heated at 1000° C. for 10 hours to make the kneaded product substantially free of volatile matter. That is, phenolic resin and coal tar pitch were carbonized. The average particle diameter of the obtained composite particles was 15 μm. The weight percentage of each constituent material in the obtained composite particle, the porosity of the composite particle as a whole, and the ratio of voids around the metal to the total void of the composite particle were measured, and the results are shown in Table 1-1 and Table 1-2.

In addition, using a scanning electron microscope, the structure inside the particle was observed from the cross section of the composite particle. The results are shown graphically in FIG. 2 . Thus, it can be judged that at least a part of metal silicon 12 that can be alloyed with lithium is in contact with graphite material 11 and/or carbon material 13 , and the voids around the metal are also in contact with at least a part of the surface of metal 12 . In addition, the mark 14 is an edge surface of a graphite material, and the mark 15 is a base surface of a graphite material.

(Manufacture of negative electrode mixture paste)

Add 90wt% of the aforementioned composite particles and 10wt% polyvinylidene fluoride to N-methylpyrrolidone, and use a homomixer to stir and mix at 2000rpm for 30 minutes to prepare an organic solvent negative electrode mixture.

(Manufacture of working electrode (negative electrode))

The above-mentioned negative electrode mixture paste is coated on the copper foil with a uniform thickness, the solvent is evaporated in vacuum at 90°C, dried, and the negative electrode mixture layer is pressed by hand extrusion. The copper foil and the negative electrode mixture layer were drawn into a cylindrical shape with a diameter of 15.5 mm, and a working electrode (negative electrode) formed of a current collector and the negative electrode mixture in close contact with the current collector was produced.

(manufacturing of electrodes)

The lithium metal foil was extruded on the nickel mesh, drawn into a cylindrical shape with a diameter of 15.5 mm, and a current collector formed of the nickel mesh and a counter electrode formed of the lithium metal foil closely attached to the current collector were produced.

(Electrolyte, Separator)

LiPF ₆ was dissolved in a solvent mixed with 33% by weight of ethylene carbonate and 67% by weight of ethyl methyl carbonate so as to have a concentration of 1 mol/dm ³ to prepare a non-aqueous electrolytic solution. The obtained non-aqueous electrolytic solution was impregnated into a polypropylene porous body to manufacture an electrolytic solution-impregnated separator.

(evaluation battery)

A coin-type secondary battery shown in FIG. 1 was manufactured as an evaluation battery.

The separator 5 impregnated with the electrolytic solution is sandwiched between the negative electrode 2 of the dense current collector 7b and the positive electrode 4 of the dense current collector 7a, and laminated. Thereafter, the negative electrode current collector 7 b side was stored in the exterior cup 1 , and the positive electrode current collector 7 a side was stored in the exterior can 3 , thereby combining the exterior cup 1 and the exterior can 3 . At this time, at the edges of the outer cup 1 and the outer can 3, the insulating gasket 6 is used to rivet and seal the two edges.

The evaluation battery was subjected to the following charge and discharge experiments at a temperature of 25° C., and the discharge capacity, initial charge and discharge efficiency, and cycle properties were calculated. The evaluation results are shown in Table 2.

(Discharge capacity·Initial charge and discharge efficiency)

At a current value of 0.9mA, charge at a constant current until the loop voltage reaches 0mV, switch to constant voltage charging when the loop voltage reaches 0mV, and continue charging until the current value reaches 20μA. The charging capacity was obtained from the amount of current flow therebetween. Afterwards, a 120-minute break. Then, at a current value of 0.9 mA, constant current discharge was performed until the circuit voltage reached 1.5 V, and the discharge capacity was obtained from the amount of conduction therebetween. The initial charge and discharge efficiency was calculated from the following formula. In addition, in this experiment, the process of adsorbing lithium ions to the graphite particles was regarded as charging, and the process of desorbing lithium ions was regarded as discharging.

Initial charge and discharge efficiency (%) = (discharge capacity of the first cycle / charge capacity of the first cycle) × 100

(periodic nature)

Carry out constant current charging at a current value of 4.0mA until the loop voltage reaches 0mV, switch to constant voltage charging, continue charging until the current value is 20μA, and then stop for 120 minutes. Then, under the current value of 4.0mA, constant current discharge is carried out until the loop voltage reaches 1.5V. Repeat charge and discharge 20 times. Use the following formula to calculate periodic properties.

Cycle properties = (discharge capacity of the 20th cycle/discharge capacity of the first cycle) × 100

Table 2 shows the evaluation results of battery properties (discharge capacity, initial charge-discharge efficiency, and cycle properties).

As shown in Table 2, the evaluation battery obtained by using the composite particles of Example 1 as the working electrode exhibited a high discharge capacity and high initial charge and discharge efficiency. In addition, excellent periodic properties are exhibited.

(Example 2)

Except in Example 1, the metal silicon powder is dispersed in the ethanol solution of the phenolic resin to form a slurry, and when the slurry and natural graphite are kneaded at 150° C. for 1 hour using a biaxial heating kneader, the preparation 1. Obtain a kneaded product so that the weight percentage of solid components is 20.4wt% of phenolic resin, 6.7wt% of silicon powder, and 72.9wt% of natural graphite. In addition, continue to use the biaxial heating kneader, mix the coal tar pitch solution and the kneaded product at 200° C. for 1 hour, and the weight percentage of the prepared solid component is 36.9 wt % for the coal tar pitch, and the kneaded product is 63.1 wt%. Composite particles were produced under the same method and conditions as in Example 1 except for this. Then, negative electrode mixture paste, working electrode, counter electrode, electrolyte solution, separator, and evaluation battery were produced in the same manner as in Example 1. The charge and discharge properties of the evaluation battery were evaluated.

(Example 3)

Composite particles were produced under the same method and conditions as in Example 1, except that in Example 1, artificial graphite (average particle diameter: 10 μm) obtained by graphitizing bulk coke was used instead of flaky natural graphite. Then, negative electrode mixture paste, working electrode, counter electrode, electrolyte solution, separator, and evaluation battery were produced in the same manner as in Example 1. The charge and discharge properties of the evaluation battery were evaluated.

(Example 4)

92.7wt% graphitized vapor phase growth carbon fiber (manufactured by Showa Denko Co., Ltd., VGCF, minor axis length 150nm, average aspect ratio about 50) and 7.3wt% silicon particles (High Purity Chemical Research Institute Co., Ltd. Production, evaluation particle size 2 μm) were mixed, put into a mechanical melting system (manufactured by Hoskawa Micron Co., Ltd.), and mechanical energy was applied to perform mechanochemical treatment. That is, under the condition that the peripheral speed of the rotating drum is 20m/s, the processing time is 30 minutes, and the distance between the rotating drum and the internal parts is 5mm, compressive force and shearing force are repeatedly applied to obtain silicon particles sandwiched by vapor-phase grown carbon fibers. live composite particles.

Then, 300 g of coal tar pitch (manufactured by JFE Chemical Co., Ltd., 60% carbon residue rate) was mixed with 300 g of middle fraction of tar (manufactured by JFE Chemical Co., Ltd.) to prepare a coal tar pitch solution, which was kneaded using biaxial heating machine, and the produced coal tar pitch solution and the composite particles were kneaded at 200° C. for 1 hour. At this time, the weight percentages of the solid content were adjusted so that the coal tar pitch was 42 wt%, and the composite particles were 58 wt%. After kneading, the solvent tar middle fraction was removed from the kneaded product in a vacuum to obtain composite particles covered with coal tar pitch. The obtained composite particles were coarsely pulverized, and then fired at 1000° C. for 10 hours to obtain coated composite particles. During firing, the volatile components are completely removed. The composite particles covered with this carbon material were spherical, had an average particle diameter of 10 μm, and a specific surface area of 5.2 m ² /g.

It was confirmed that the carbon material covered the outer surface of the composite particles, the silicon particles were intertwined and sandwiched by the vapor-phase-grown carbon fibers, and many voids were dispersed throughout the interior of the composite particles. The constituent weight percentages of the obtained covered composite particles were 5.1 wt% of silicon, 64.6 wt% of fibrous graphite material, and 30.3 wt% of carbon material.

Then, negative electrode mixture paste, working electrode, counter electrode, electrolyte solution, separator, and evaluation battery were produced in the same manner as in Example 1. The charge and discharge properties of the evaluation battery were evaluated.

(Example 5)

In Example 1, when the coal tar pitch solution, graphite material, and carbon material were kneaded at 200° C. for 1 hour using a biaxial heating kneader, flake-like natural graphite (manufactured by Chuetsu Graphite Industry Co., Ltd., average particle size 5 μm), the percentage adjusted to solid content is 34wt% of coal tar pitch, 60wt% of composite particles, and 6wt% of natural graphite. Except for this, composite particles were produced under the same method and conditions as in Example 1, and then fired to obtain coated composite particles. The obtained coated composite particles were massive, with an average particle diameter of 12 μm and a specific surface area of 5.3 m ² /g.

It was confirmed that the carbon material covered the outer surface of the composite particles, the silicon particles were intertwined and sandwiched by vapor-phase-grown carbon fibers, natural graphite was arranged around them, and many voids were dispersed throughout the interior of the composite particles. The weight percentages of constituents in the obtained coated composite particles were 5.1 wt% for silicon, 64.8 wt% for fibrous graphite, 6.5 wt% for flaky graphite material, and 23.6 wt% for carbon material.

Then, negative electrode mixture paste, working electrode, counter electrode, electrolyte solution, separator, and evaluation battery were produced in the same manner as in Example 1. The charge and discharge properties of the evaluation battery were evaluated.

(Example 6)

Except in Example 4, when the coal tar pitch solution and the composite particles were kneaded at 200° C. for 1 hour using a biaxial heating kneader, the weight percentage of the solid content was adjusted so that the coal tar pitch was 10 wt%, and the composite particles were Except for 90 wt%, composite particles were produced by the same method and conditions as in Example 3.

Then, negative electrode mixture paste, working electrode, counter electrode, electrolyte solution, separator, and evaluation battery were produced in the same manner as in Example 1. The charge and discharge properties of the evaluation battery were evaluated.

(Example 7)

In Example 1, when using tin powder (manufactured by Aldrich, average particle diameter 1 μm) instead of silicon powder, in the ethanol solution of phenolic resin, when mixing with natural graphite, the weight percentage that is prepared as solid component is: phenolic resin is 18wt %, tin powder is 26.7wt%, and natural graphite is 55.3wt%. Composite particles were produced by the same method and conditions as in Example 1 except for this. Then, negative electrode mixture paste, working electrode, counter electrode, electrolyte solution, separator, and evaluation battery were produced in the same manner as in Example 1. The charge and discharge properties of the evaluation battery were evaluated.

(Embodiment 8)

In Example 1, silicon powder was pulverized by a ball mill so that the average particle size was 0.5 μm. Composite particles were produced under the same method and conditions as in Example 1 except for this. Then, negative electrode mixture paste, working electrode, counter electrode, electrolyte solution, separator, and evaluation battery were produced in the same manner as in Example 1. The charge and discharge properties of the evaluation battery were evaluated.

(Example 9)

In Example 1, a sand mill was used to pulverize silicon powder with water as a dispersant, and silicon powder with an average particle diameter of 0.3 μm was used. Composite particles were produced by the same method and conditions as in Example 1 except for this. From X-ray diffraction measurement, it was confirmed that the pulverized silicon powder was amorphous. Then, negative electrode mixture paste, working electrode, counter electrode, electrolyte solution, separator, and evaluation battery were produced in the same manner as in Example 1. The charge and discharge properties of the evaluation battery were evaluated.

(comparative example 1)

The metal silicon powder, flake-like natural graphite and coal tar pitch used in Example 1 are prepared to have a weight percentage of solid components of 3.8wt%, 38.5wt%, and 57.7wt%, respectively, with the tar middle fraction as a solvent, by double After kneading is performed while heating the kneader with the shaft, the kneaded product is heated, the solvent is removed, and the kneaded product is dried. The obtained kneaded product was pulverized and fired at 1000°C for 10 hours to produce composite particles. Then, under the same method and conditions as in Example 1, a lithium ion secondary battery was produced using the composite particles, negative electrode, and nonaqueous electrolyte. The discharge capacity, initial charge-discharge efficiency, and cycle properties of the battery were measured in the same manner as in Example 1, and the evaluation results are shown in Table 2.

(comparative example 2)

In Comparative Example 1, the metal silicon powder, flaky natural graphite and coal tar pitch were adjusted so that the weight percentages of solid components were 3.7wt%, 38.5wt%, and 62.5wt%, respectively. Composite particles were produced by the same method and conditions as in Comparative Example 1 except for this. Then, under the same method and conditions as in Example 1, a lithium ion secondary battery was produced using the composite particles, negative electrode, and nonaqueous electrolyte. The discharge capacity, initial charge-discharge efficiency, and cycle properties of the battery were measured in the same manner as in Example 1, and the evaluation results are shown in Table 2.

In Comparative Examples 1 and 2, in which there were no voids around the silicon particles, high initial charge-discharge efficiency and cycle properties could not be obtained. This is considered to be because the expansion of the silicon particles during charging destroys the structure of the composite particles, resulting in a decrease in conductivity and separation of the active material from the current collector.

Table 1-1

Composition and composition of composite particles Metal     Graphite material Carbon materials type Average particle size (μm) Composition (wt%) type Composition (wt%) Precursor species Composition (wt%) Example 1 Crystalline silicon 2 5.1   Flake natural graphite 65.1 Phenolic resin coal tar pitch 29.8 Example 2 Crystalline silicon 2 5.1   Flake natural graphite 55.1 Phenolic resin coal tar pitch 39.8 Example 3 Crystalline silicon 2 5.1 Lump artificial graphite 65.1 Phenolic resin coal tar pitch 29.8 Example 4 Crystalline silicon 2 5.1 Fibrous graphite 64.6 Coal tar pitch 30.3 Example 5 Crystalline silicon 2 5.1 Fibrous graphite Flake natural graphite 64.86.5 Coal tar pitch 23.6 Example 6 Crystalline silicon 2 6.8 Fibrous graphite 86.9 Coal tar pitch 6.3 Example 7 Crystalline tin 1 twenty three   Flake natural graphite 47.2 Phenolic resin coal tar pitch 29.8 Example 8 Crystalline silicon 0.5 5.1   Flake natural graphite 65.1 Phenolic resin coal tar pitch 29.8 Example 9 amorphous silicon 0.3 5.1   Flake natural graphite 65.1 Phenolic resin coal tar pitch 29.8 Comparative example 1 Crystalline silicon 2 5   Flake natural graphite 50 Coal tar pitch 45 Comparative example 2 Crystalline silicon 2 5   Flake natural graphite 45 Coal tar pitch 50

Table 1-2

Physical properties of composite particles Average particle size (μm) Specific surface area (m ² /g)   Raman Spectroscopy (R Value) Overall porosity (%) The surrounding void ratio of the metal (%) The lattice plane spacing of fibrous graphite (nm) Example 1 15 4 0.29 25 55 - Example 2 15 5 0.28 twenty two 25 - Example 3 13 5 0.45 26 58 - Example 4 10 14 0.3 30 64 0.3366 Example 5 12 9 0.35 33 60 0.3366 Example 6 10 twenty three 0.32 32 55 0..3366 Example 7 15 4 0.29 25 55 - Example 8 13 6 0.3 28 48 - Example 9 14 7 0.31 31 50 - Comparative example 1 15 5 0.32 30 15 - Comparative example 2 15 5 0.34 32 10 -

Table 2

Discharge capacity (mAh/g)   Initial charge and discharge efficiency (%) Periodic nature (%) Example 1 487 87 90 Example 2 485 86 88 Example 3 480 88 87 Example 4 477 90 93 Example 5 480 92 94 Example 6 478 88 87 Example 7 486 87 87 Example 8 487 88 91 Example 9 487 87 92 Comparative example 1 475 84 76 Comparative example 2 474 83 72

A lithium ion secondary battery using the negative electrode material containing the composite particles of the present invention as the negative electrode has a large discharge capacity, excellent initial charge and discharge efficiency and cycle properties. Therefore, the lithium ion secondary battery formed using the negative electrode material of the present invention satisfies the demand for higher energy density in recent years, and is effective in downsizing and improving the performance of devices mounted thereon. In addition, since the composite particles of the present invention can be produced using materials that have been used as materials for composite particles in the past, there are advantages in that the materials are easy to obtain and the material cost is low.

Claims (15)

1. A composite particle containing metal, graphite material and carbon material, wherein at least a part of the metal alloyed with lithium and at least one material selected from graphite material and carbon material can be contacted, and the metal surrounding The void is 20 vol% or more with respect to all voids. 2. The composite particle according to claim 1, wherein the graphite material is at least one material selected from flaky graphite and fibrous graphite. 3. The composite particle according to claim 1, wherein the graphite material is in the form of scales, and the ratio of the peak intensity of the D-band to the peak intensity of the G-band in the Raman spectrum is less than 0.4. 4. The composite particle according to claim 1, wherein the graphite material is fibrous graphite whose average lattice plane spacing d ₀₀₂ in X-ray diffraction is 0.34 nm or less. 5. The composite particle according to claim 1, wherein at least one part of the metal is in contact with the fibrous graphite material, and at least one part of their outer surfaces is covered with a carbon material. 6. The composite particle according to claim 5, wherein the composite particle further contains flaky graphite. 7. The composite particle of claim 1, wherein the metal is replaced with silicon. 8. The composite particle according to claim 1, wherein the average particle diameter of the metal is 0.01-10 μm. 9. The composite particle of claim 1, wherein the metal is amorphous. 10. The composite particle according to claim 1, wherein the composite particle has a specific surface area of 20 m ² /g or less. 11. The composite particle according to claim 1, wherein the average particle diameter of the composite particle is 1-50 μm. 12. A negative electrode material for a lithium ion secondary battery comprising the composite particles according to any one of claims 1 to 11. 13. A negative electrode for lithium ion secondary batteries using the negative electrode material for lithium ion secondary batteries according to claim 12. 14. A lithium ion secondary battery using the negative electrode for lithium ion secondary batteries according to claim 13. 15. A composite particle, the composite particle is a composite particle that integrates a metal that can be alloyed with lithium and a graphite material through a carbonaceous material, and is characterized in that: the composite particle has a gap, and relative to the composite particle The ratio of all the voids and the voids around the metal is 20% or more.

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