JP4809617B2 - Negative electrode material for lithium ion secondary battery, method for producing the same, negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Description

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

Among various secondary batteries, lithium ion secondary batteries having excellent characteristics such as high voltage and high energy density are widely used as power sources for electronic devices. In recent years, miniaturization or performance enhancement of electronic devices has rapidly progressed, and there is an increasing demand for further higher energy density of lithium ion secondary batteries.
Currently, lithium ion secondary batteries generally use LiCoO _{2 for} the positive electrode and graphite for the negative electrode. However, although the graphite negative electrode is excellent in reversibility of charge and discharge, its discharge capacity has already reached a value close to the theoretical value of 372 mAh / g corresponding to the intercalation compound LiC ₆ in order to achieve further higher energy density. Needs to develop a negative electrode material having a larger discharge capacity than graphite.

Although metallic lithium has the highest discharge capacity as a negative electrode material, there is a problem that lithium is deposited in a dendritic state during charging, the negative electrode is deteriorated, and the charge / discharge cycle is shortened. In addition, lithium deposited in a dendrite shape may penetrate the separator and reach the positive electrode, causing a short circuit.
Therefore, a metal or a metal compound that forms an alloy with lithium has been studied as a negative electrode material that replaces metallic lithium. These alloy negative electrodes have discharge capacities far surpassing that of graphite, although they do not reach metal lithium. However, active materials are pulverized and peeled off due to volume expansion accompanying alloying, and cycle characteristics at a practical level have not yet been obtained.

In order to improve the defects of the alloy negative electrode, a composite of a metal or a metal compound and a graphite material and / or a carbonaceous material has been studied.
Patent Document 1 discloses a composite material in which a metal or a metal compound and a graphite material are bonded or coated with a carbonaceous material. However, when the carbonaceous material penetrates into the inside of the composite material, it is not possible to secure voids for buffering the expansion around the metal or metal compound, which may lead to deterioration of cycle characteristics due to destruction of the composite particle structure. There is.

  Patent Document 2 discloses a negative electrode material in which composite particles obtained by coating silicon phase particles with a solid solution containing silicon or a phase of an intermetallic compound are further electrically joined with fibrous carbon. However, when the carbonaceous material is not coated and a large amount of fibrous carbon is exposed on the surface, the charge / discharge efficiency may be reduced due to the large specific surface area of the fibrous carbon. Furthermore, if many particles containing a silicon phase are exposed on the surface, the particles may be peeled off during charge / discharge, leading to a decrease in cycle characteristics.

  Patent Document 3 discloses composite carbon particles containing a graphite part, an amorphous carbon part, and silicon. However, an organosilicon compound is used as a silicon source in producing the composite carbon particles. For this reason, silicon is included in the amorphous carbonaceous material, and voids for buffering expansion are not formed around the silicon, which may cause deterioration of cycle characteristics due to destruction of the composite carbon particle structure.

Furthermore, Patent Document 4 discloses a ratio of an electrode material composed of composite particles containing an element that can be alloyed with lithium and a conductive material, the material containing an element that can be alloyed with lithium. Is 30% by mass or more and 80% by mass or less with respect to the total mass of the composite particles, the composite particles have voids inside, and the bulk density of the composite particles is D1 (g / cm ³ ). When the true density of the composite particles is D2 (g / cm ³ ) and the void volume occupancy (%) of the composite particles is Vs = (1-1.35 × D1 / D2) × 100, Vs Is described for an electrode material characterized by being 35% or more and 70% or less.

This electrode material has a void inside the composite particle so as to absorb expansion during charging of a material containing an element that can be alloyed with lithium, but a material containing an element that can be alloyed with lithium Since the ratio of the ratio is as large as 30% by mass or more and 80% by mass or less with respect to the total mass of the composite particles, the void volume occupancy of 35% or more and 70% or less may not be able to absorb expansion and cycle characteristics may deteriorate. .
Japanese Patent No. 3369589 JP 2000-173612 A JP 2000-203818 A JP 2003-303588 A

The present inventor presumes that the above-described conventional graphite composite particles cannot absorb the expansion of metal particles capable of forming an alloy with lithium, and therefore, when used as a negative electrode material, the cycle characteristics deteriorate. As a result of intensive studies, the composite particles in which the metal particles are held by the fibrous graphite material and further coated at least partially with the carbonaceous material can absorb the expansion of the metal at the time of alloy formation. It has been found that powdering and peeling can be prevented and conductivity can be maintained, and the present invention has been completed.
The present invention has been made in view of the above knowledge, and when used as a negative electrode material for a lithium ion secondary battery, the negative electrode has a large discharge capacity and excellent cycle characteristics and initial charge / discharge efficiency. It is an object to provide a material, a negative electrode, and a lithium ion secondary battery using the material.

The present invention includes the following (1) to (1 2 ).

(1) At least a part of the outer surface of the composite particles having a structure in which the fibrous graphite material is entangled with metal particles that can be alloyed with lithium and sandwiches the metal particles, and voids exist, is a carbonaceous material. Ri Oh coated composite particles coated, the metal particles, the content of the fibrous graphitized material and the carbonaceous material is, in wt% relative to the total weight of the coated complex particles, the metal particles / the fibrous Negative electrode material for lithium ion secondary battery , wherein graphite material / carbonaceous material = 1 or more and less than 30/30 to 95/4 to 50 .

( 2 ) At least a part of the outer surface of the composite particle having a structure in which the fibrous graphite material is entangled with metal particles that can be alloyed with lithium and sandwiches the metal particles and there is a void, is a carbonaceous material. Coated composite particles, wherein the coated composite particles further contain a non-fibrous graphite material, and the metal particles, the fibrous graphite material, the non-fibrous graphite material, and the carbonaceous material The metal particles / the fibrous graphite material / the non-fibrous graphite material / the carbonaceous material = 1 or more and less than 30/28 to 75/2, in mass% with respect to the total mass of the coated composite particles. The negative electrode material for lithium ion secondary batteries which is -20 / 4-50 .

( 3 ) The negative electrode material for a lithium ion secondary battery as described in ( 2 ) above, wherein the non-fibrous graphite material is dispersed and held inside the structure of the coated composite particles.
( 4 ) The lithium ion secondary battery according to ( 2 ) above, wherein at least a part of the outer surface of the coated composite particle is coated with the carbonaceous material and the non-fibrous graphite material. Negative electrode material .

( 5 ) The negative electrode material for a lithium ion secondary battery according to any one of the above (1) to ( 4 ), wherein the covering composite particles have a specific surface area of 10 m ² / g or less.

( 6 ) Mixing and stirring treatment (mixing and / or stirring treatment) of metal particles that can be alloyed with lithium and fibrous graphite material to form composite particles, and then precursors of the composite particles and carbonaceous material The coated composite particles according to the above (1 ) are obtained by mixing with a body and heat-treating, and a method for producing a negative electrode material for a lithium ion secondary battery.

( 7 ) After mixing and stirring treatment (mixing and / or stirring treatment) of metal particles that can be alloyed with lithium, fibrous graphite material, and non-fibrous graphite material, the composite particles and carbon A method for producing a negative electrode material for a lithium ion secondary battery, wherein the coated composite particles according to the above ( 2) or (3 ) are obtained by mixing with a precursor of a porous material and heat-treating.

( 8 ) Mixing and stirring treatment (mixing and / or stirring treatment) of metal particles that can be alloyed with lithium and fibrous graphite material to form composite particles, and then the composite particles and the precursor of the carbonaceous material A method for producing a negative electrode material for a lithium ion secondary battery, comprising mixing a body and a non-fibrous graphite material and heat-treating to obtain coated composite particles according to ( 2) or (4 ) above.

( 9 ) The method for producing a negative electrode material for a lithium ion secondary battery according to any one of the above ( 6 ) to ( 8 ), wherein the mixing / stirring treatment is a mechanochemical treatment.

(1 0 ) The method for producing a negative electrode material for a lithium ion secondary battery according to any one of the above ( 6 ) to ( 9 ), wherein the temperature of the heat treatment is 600 ° C. or higher and lower than 1300 ° C.

(1 1 ) A negative electrode for a lithium ion secondary battery comprising the negative electrode material for a lithium ion secondary battery according to any one of the above (1) to ( 5 ).

(1 2 ) A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery described in (1 1 ) above.

  A negative electrode and a lithium ion secondary battery produced using the negative electrode material for a lithium ion secondary battery of the present invention have a high discharge capacity, and are excellent in initial charge / discharge efficiency and cycle characteristics. Therefore, the lithium ion secondary battery using the negative electrode material of the present invention satisfies the recent demand for higher energy density, and is effective for downsizing and higher performance of electronic devices to be mounted.

The present invention provides a coated composite particle in which at least a part of a composite particle having a structure in which a fibrous graphite material holds metal particles that can be alloyed with lithium is coated with a carbonaceous material, preferably the coated composite A negative electrode material for a lithium ion secondary battery, wherein the particles are coated composite particles further containing a non-fibrous graphite material. Moreover, this invention is the manufacturing method, it is a negative electrode for lithium ion secondary batteries containing this negative electrode material, and is a lithium ion secondary battery using this negative electrode.
The present invention will be described below.

(Coated composite particles)
The coated composite particle used in the present invention is a coated composite particle in which at least a part of a composite particle having a structure in which a fibrous graphite material holds metal particles that can be alloyed with lithium is coated with a carbonaceous material. It is. The coated composite particles preferably have a plurality of large and small voids dispersed therein. In particular, it is preferable that voids exist around the metal particles. If there are voids inside the coated composite particles, the expansion during charging can be absorbed, and the cycle characteristics can be improved. Since the specific surface area of the fibrous graphite material is large, when mixing the precursor of the carbonaceous material having fluidity, the fluid precursor is mainly adsorbed on the surface of the fibrous carbonaceous material, It is based on the phenomenon that it is difficult to penetrate inside. The shape of the coated composite particles is not specified, and the size is not particularly limited. The specific surface area of the coated composite particles is preferably 10 m ² / g or less. When the specific surface area exceeds 10 m ² / g, when the coated composite particles are used as a negative electrode material of a lithium ion secondary battery, the charge / discharge efficiency of the secondary battery may be lowered.

  A preferable composition of the main component of the coated composite particle of the present invention is metal particle / fibrous graphite material / carbonaceous material (coating material) = 1 or more and less than 30/30 to 95 in mass% with respect to the total mass of the coated composite particle. It is the range of / 4-50, Preferably, it is the range of 1-20 / 30-95 / 4-50, More preferably, it is the range of 2-10 / 55-93 / 5-35. When the proportion of the metal particles is less than the range, when the coated composite particles are used for the negative electrode material for a lithium ion secondary battery, the increase in the discharge capacity of the secondary battery may be small. If the number is increased, the improvement of the cycle characteristics of the secondary battery may be small. Further, when the ratio of the fibrous graphite material deviates from the above range, the cycle characteristics may not be sufficiently improved. Further, if the proportion of the carbonaceous material deviates from the range, it may not be said that the charge / discharge efficiency and cycle characteristics of the secondary battery are sufficiently improved.

  When the coated composite particles of the present invention further contain a non-fibrous graphite material, the mass composition of the metal particles / fibrous graphite material / non-fibrous graphite material / carbonaceous material (of the coated composite particles) % By mass relative to the total mass) is 1 or more and less than 30/28 to 75/2 to 20/4 to 50, and preferably 1 to 20/28 to 75/2 to 20/4 to 50. More preferred is 2 to 10/45 to 88/5 to 10/5 to 35. When the ratio of the metal particles is less than the range, when the coated composite particles are used as a negative electrode material for a lithium ion secondary battery, the increase in discharge capacity of the secondary battery may be small. If it increases, the improvement of the cycle characteristics of the secondary battery may be small. Further, when the ratio of the fibrous graphite material deviates from the above range, the cycle characteristics may not be sufficiently improved. If the ratio of non-fibrous graphite material is less than the range, the improvement of the charge / discharge efficiency of the secondary battery may be small. Conversely, if the ratio is higher than the range, the improvement of the cycle characteristics of the secondary battery may be small. is there. Furthermore, if the proportion of the carbonaceous material deviates from the range, it may not be said that the charge / discharge efficiency and cycle characteristics of the secondary battery are sufficiently improved.

The mass composition of the coated composite particles is a value converted into a concentration as a metal by conducting elemental analysis by emission spectroscopy after ashing the coated composite particles.
The ratio between the fibrous graphite material and the carbonaceous material is determined by taking a cross-section of the coated composite particle using a polarizing microscope at a magnification of 1000 times, and from the difference in appearance derived from high and low crystallinity of any 10 particles. It is the average value of the area ratio measured visually by the fibrous graphite material and the carbonaceous material inside the composite particles. In addition, the area ratio which a fibrous graphite material and a carbonaceous material occupy can be calculated | required by adjusting the slice of the cross section of a coating composite particle, and observing using a transmission electron microscope. Here, the area ratio between the fibrous graphite material and the carbonaceous material is obtained, but since there is no significant difference in the density between the fibrous graphite material and the carbonaceous material, the area ratio obtained as described above is determined in the present invention. Consider it as a mass ratio.

  Furthermore, in the case of containing a non-fibrous graphite material, in the polarizing microscope photograph of the coated composite particle cross section, the fibrous graphite material and the non-fibrous graphite material are observed separately from the difference in appearance derived from the shape, The ratio of non-fibrous graphite material is determined from the area ratio by the same method as described above.

In the negative electrode material for a lithium ion secondary battery of the present invention, it is confirmed as follows that at least a part of the composite particles are coated with a carbonaceous material and that there are voids inside the coated composite particles. can do.
That at least a part of the coated composite particles is coated with the carbonaceous material can be confirmed by a polarizing micrograph (magnification 1000 times) of the cross section of the coated composite particles. The carbonaceous material and the graphite material can be observed separately as described above. The presence of voids inside can be confirmed by a scanning electron micrograph (400 magnifications) of the cross section of the coated composite particles.

  In the coated composite particles of the present invention, the metal particles are retained by the fibrous graphite material, and at least a part of the composite particles are coated with the carbonaceous material, so that the metal particles are prevented from falling off from the coated composite particles. Even when charging and discharging are repeated, the conductivity of the negative electrode material is maintained. Further, since the metal particles are held by the fibrous graphite material, preferably, an appropriate void is formed in the coated composite particles, and the expansion of the metal particles accompanying charge / discharge can be buffered, and the coated composite particles Structural destruction of the particles is suppressed. As a result, it is estimated that the cycle characteristics of the lithium ion secondary battery are improved. Further, it is presumed that the carbonaceous material covers at least a part of the coated composite particles, thereby suppressing the exposure of the fibrous graphite material and improving the initial charge / discharge efficiency.

(Metal that can be alloyed with lithium)
The reason why the metal that can be alloyed with lithium is blended is to achieve a high capacity lithium ion secondary battery using a graphite material as a negative electrode material. Metals that can be alloyed with lithium are Al, Pb, Zn, Sn, Bi, In, Mg, Ga, Cd, Ag, Si, B, Au, Pt, Pd, Sb, Ge, Ni, etc. Two or more kinds of alloys may be used. The alloy may further contain elements other than those described above. In addition, a part of the metal may be a compound such as an oxide, nitride, or carbide. Preferred metals are silicon and tin, which can obtain a lithium ion secondary battery having a particularly high capacity and are relatively easily available, and silicon that is effective for higher capacity is particularly preferred. The metal may be in a crystalline or amorphous state.
In the present invention, the metal is used as particles. The shape of the metal particles is not particularly limited, but may be granular, spherical, plate-like, scale-like, needle-like or thread-like. The average particle diameter of the metal particles is preferably 10 μm or less, and more preferably 0.1 to 5 μm. When the average particle diameter of the metal particles exceeds 10 μm, the improvement of the cycle characteristics of the lithium ion secondary battery may be small. Here, the average particle diameter means a particle diameter at which the cumulative frequency measured by a laser diffraction particle size meter is 50% by volume.

(Fibrous graphite material)
The fibrous graphite material has a fibrous shape and is made of graphite, so that it has conductivity and can occlude / release lithium ions. The fibrous graphite material may be in an agglomerated state or a dispersed state in which the agglomeration is released, but it is particularly preferable that the fibrous graphite material is in an agglomerated state so as to enclose the metal particles.
Further, since the fibrous graphite material has a large specific surface area, when the carbonaceous material precursor having fluidity is mixed with the composite particles, the fluidic precursor forms the fibrous carbonaceous material constituting the composite particles. It adsorbs on the surface of the material and does not easily penetrate into the composite particles, and it is easy to ensure voids inside the coated composite particles.
The fibrous graphite material can be obtained by finally heat-treating the precursor at 1500 to 3300 ° C. The precursor may be any material as long as a fibrous graphite material can be obtained, and a graphitizable fibrous carbonaceous material is particularly preferable. Examples include carbon nanofibers, carbon nanotubes, and vapor grown carbon fibers. The precursor preferably has a minor axis length (diameter) of 1 to 500 nm, particularly 10 to 200 nm. The aspect ratio of the precursor is preferably 5 or more, particularly 10 to 300. Here, the aspect ratio means fiber length / short axis length.
The fibrous graphite material may be subjected to various chemical treatments in the liquid phase, gas phase, and solid phase, heat treatment, oxidation treatment, physical treatment, and the like.

(Non-fibrous graphite material)
The coated composite particles of the present invention preferably further contain a graphite material (non-fibrous graphite material) other than the fibrous graphite material. The non-fibrous graphite material is further used because the non-fibrous graphite material has a smaller specific surface area than the fibrous graphite material and can improve charge / discharge efficiency. The fibrous graphite material and the non-fibrous graphite material may be used integrally as the composite particles, or may be used by mixing with the composite particles and a precursor of the carbonaceous material. The non-fibrous graphite material is not particularly limited as long as it can occlude and release lithium ions. A part or all of which is formed of graphite, for example, artificial graphite or natural graphite obtained by finally heat treating (graphitizing) tar pitches at 1500 ° C. or higher. Specifically, mesophase fired bodies, mesophase spherules, and cokes obtained by heat-treating and polycondensing easily graphitizable carbon materials such as petroleum-based or coal-based tar pitches are 1500 ° C. or higher, preferably 2800- And those graphitized at 3300 ° C.
The shape of the non-fibrous graphite material may be other than the fiber shape, and may be a spherical shape, a block shape, a plate shape, a scale shape, etc., but a scale shape or a shape close to a scale shape is preferable. This is because it is easy to secure a contact between the coated composite particles or in the particles, and the conductivity is further improved. Moreover, the above-mentioned various mixtures, granulated products, coatings, and laminates may be used. Further, it may be subjected to various chemical treatments in the liquid phase, gas phase, and solid phase, heat treatment, oxidation treatment, physical treatment and the like. The average particle size of the non-fibrous graphite material is preferably 1 to 30 μm, particularly preferably 3 to 15 μm.

(Carbonaceous material)
The carbonaceous material used in the present invention has conductivity and is an essential component for covering at least a part of the composite particles in which the metal particles are held by the fibrous graphite material. The carbonaceous material is used in order to reduce the specific surface area mainly derived from the fibrous graphite material or non-fibrous graphite material and improve the charge / discharge efficiency by covering at least a part of the composite particles. It is. The carbonaceous material can be obtained by heat-treating the precursor. The type of the precursor of the carbonaceous material is not limited, but tar pitches and / or resins are preferable. Specifically, coal or tar tar pitches such as coal tar, tar light oil, tar medium oil, tar heavy oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen-crosslinked petroleum pitch, heavy oil Etc. Examples of the resins include thermoplastic resins such as polyvinyl alcohol, phenol resins, and furan resins. Preferred precursors are coal tar pitch, phenol resin and the like.
As will be described later, the heat treatment temperature of the precursor is 600 ° C. or higher, preferably 800 ° C. or higher.

(Method for producing coated composite particles)
The coated composite particles of the present invention are produced by a method of mixing and heat-treating a precursor of a carbonaceous material to composite particles in which metal particles that can be alloyed with lithium are held by a fibrous graphite material.
That is, for the production of the coated composite particles, a method of mixing and stirring (mixing and / or stirring) the metal particles and the fibrous graphite material, particularly preferably compressing, shearing, colliding, and frictioning them. For example, a method of applying mechanical energy such as a so-called mechanochemical treatment, a method of removing the organic solvent after the metal particles are introduced into the organic solvent in which the fibrous graphite material is dispersed, or the like is used. Particularly preferred is a mechanochemical treatment.

Mechanochemical treatment refers to a treatment in which a compressive force and a shear force are applied simultaneously to a graphite material or the like. Although the shearing force and compressive force are usually larger than general stirring force, these mechanical stresses are preferably applied to the surface of the graphite material, and preferably do not destroy the particle skeleton of the graphite material. When the skeleton is destroyed, when used as a negative electrode material, the irreversible capacity tends to increase. In general, the shearing force and compressive force are preferably such that the reduction rate of the average particle diameter of the graphite material by mechanochemical treatment is suppressed to 20% or less.
As long as the mechanochemical treatment apparatus is an apparatus capable of simultaneously applying a shearing force and a compressive force to the graphite material and the metal particles, the type and structure of the apparatus are not particularly limited. For example, a kneader such as a pressure kneader or two rolls, a rotating ball mill, a high-speed impact dry compounding device such as a hybridization system (manufactured by Nara Machinery Co., Ltd.), a mechano micro system (manufactured by Nara Machinery Co., Ltd.) ), A compression shear type dry powder compounding device such as Mechano-Fusion System (Hosokawa Micron Co., Ltd.) can be used.

Among them, an apparatus that applies a shearing force and a compressing force simultaneously using a rotational speed difference is preferable. Specifically, it has a rotating drum (rotating rotor), an internal member (inner piece) having a rotation speed different from that of the drum, and a circulation mechanism (for example, a blade for circulation) of the graphite material and the metal particles. Using a device (mechano-fusion system), while applying centrifugal force to the graphite material and the metal particles supplied between the rotating drum and the internal member, the internal member is caused by a speed difference from the rotating drum. The mechanochemical treatment is preferably performed by repeatedly applying a shearing force and a compressive force simultaneously.
Further, by passing the graphite material and the metal particles between a fixed drum (stator) and a rotating rotor rotating at a high speed, shear force and compressive force due to a speed difference between the fixed drum and the rotating rotor can be obtained. An apparatus (hybridization system) for simultaneously applying the material and the metal particles is also preferable.

  The conditions of the mechanochemical treatment differ depending on the apparatus used, and cannot be said unconditionally. For example, in the case of a mechanofusion system, the peripheral speed difference between the rotating drum and the internal member is 5 to 50 m / s. It is preferable that the distance is 1 to 100 mm and the processing time is 3 to 90 min. In the case of a hybridization system, it is preferable that the peripheral speed difference between the fixed drum and the rotating rotor is 10 to 100 m / s, and the processing time is 30 s to 10 min.

Next, the carbonaceous material precursor is mixed with the composite particles and heat treated to carbonize, and at least a part of the composite particles is coated with a carbonaceous material to produce coated composite particles. Mixing of the precursor and heat treatment may be performed simultaneously. The heat treatment is performed at a temperature of 600 ° C. or higher, preferably 800 ° C. or higher, so that the precursor of the carbonaceous material is carbonized and conductivity is imparted to the carbonaceous material. However, when silicon particles are used as the metal particles, when silicon is 1300 ° C. or higher, it reacts with carbon to produce SiC, so the heat treatment temperature must be lower than 1300 ° C., and 1000 ° C. or higher and lower than 1300 ° C. Preferably there is.
The heat treatment may be performed in a single stage using a general mixing apparatus such as a biaxial heating kneader, or may be performed a plurality of times in stages. The precursor of the carbonaceous material is supplied after being melted or dispersed or dissolved in a medium. The heat treatment may be performed in the presence of a catalyst. The medium is preferably removed after the heat treatment at a temperature at which the precursor of the carbonaceous material does not soften or decompose.

The coated composite particles of the present invention are heat-treated by mixing a precursor of a carbonaceous material with composite particles in which metal particles that can be alloyed with lithium are held by a fibrous graphite material and a non-fibrous graphite material. It is manufactured by. That is, when the metal particles that can be alloyed with lithium and the fibrous graphite material are integrated, the non-fibrous graphite material is also coexisted and integrated into composite particles, which are manufactured in the same manner as in the above manufacturing method.
Furthermore, the coated composite particle of the present invention is obtained by mixing a precursor of a carbonaceous material and a non-fibrous graphite material into a composite particle in which metal particles that can be alloyed with lithium are held and integrated with a fibrous graphite material. Manufactured by a heat treatment method. That is, in the said manufacturing method, when mixing the precursor of a carbonaceous material as a composite particle, it manufactures similarly to the said manufacturing method except mixing a non-fibrous graphite material simultaneously.

(Negative electrode)
The production of the negative electrode of the present invention is carried out according to a conventionally known production method of a negative electrode. A paste-like negative electrode mixture is prepared from the coated composite particles, a binder and a solvent, and this is prepared on one side of a current collector or A method of applying and drying on both sides to form a negative electrode mixture layer is preferable. When the negative electrode mixture layer is formed and then pressure bonding such as press working is performed, the adhesive strength between the negative electrode mixture layer and the current collector can be further increased. The layer thickness of the negative electrode mixture is 10 to 200 μm, preferably 20 to 200 μm.

  The negative electrode mixture paste is prepared using a known stirrer, mixer, kneader, kneader or the like. Specifically, the fibrous graphite material holding the metal particles and the other graphite material are adjusted to an appropriate particle size by classification or the like, and these are combined with a binder and / or solvent using a mixer. Can be prepared by mixing.

As the binder, those having chemical stability and electrochemical stability with respect to the electrolyte are preferable. In addition to the organic binder that is dissolved and / or dispersed in the organic solvent, the binder is dissolved in the aqueous solvent. A wide range of water-based binders are / are dispersed. For example, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene, resins such as polyethylene and polyvinyl alcohol, and rubbers such as carboxymethyl cellulose and styrene butadiene rubber are used. It is particularly preferable to use an aqueous binder such as These can also be used together. In general, the binder is preferably used at a ratio of 0.5 to 20% by mass in the total amount of the negative electrode mixture.
As the solvent, a normal solvent used for the preparation of the negative electrode mixture is used, but the solvent itself is preferably used as the binder. Specific examples include N-methylpyrrolidone, dimethylformamide, water, alcohol and the like. Use of an aqueous solvent is preferable from the viewpoint of environmental pollution and safety.

  The shape of the current collector used for the negative electrode is not particularly limited, but a foil or a net-like material such as a mesh or expanded metal is used. Examples of the material for the current collector include copper, stainless steel, and nickel. In the case of a foil, the thickness of the current collector is preferably 5 to 20 μm.

  Moreover, if necessary, the coated composite particles and resin powder such as polyethylene and polyvinyl alcohol can be dry-mixed together with other graphite materials to form a negative electrode according to a normal molding method. For example, the negative electrode can be formed by hot press molding the mixture in a mold.

  When the negative electrode material / negative electrode is prepared using the coated composite particles, a conductive material, a modifier, an additive, and the like that are usually used for the preparation of the negative electrode material may coexist. For example, natural graphite, artificial graphite, carbon black, vapor grown carbon fiber, low crystalline carbon particles, or a graphitized product thereof may be added. Although these addition amounts cannot be generally stated, the total amount is 0.1 to 50% by mass.

(Lithium ion secondary battery)
A lithium ion secondary battery usually has a negative electrode, a positive electrode, and a non-aqueous electrolyte as main battery components. Each of the positive electrode and the negative electrode is a lithium ion carrier. Lithium ions are occluded in the negative electrode during charging and from the negative electrode during discharging. It depends on the battery mechanism to be detached.
The constituent elements of the lithium ion secondary battery of the present invention are not particularly limited except that the above coated composite particles are used. Other battery components such as a positive electrode, an electrolyte, and a separator conform to the components of a general lithium ion secondary battery.

(Positive electrode)
The positive electrode is formed, for example, by applying a positive electrode material composed of a positive electrode material, a binder, and a conductive agent to the surface of the current collector. It is preferable to select a positive electrode material (positive electrode active material) that can occlude / release a sufficient amount of lithium. Examples of positive electrode active materials include lithium-containing transition metal oxides, transition metal chalcogenides, vanadium oxides (V ₂ O ₅ , V ₆ O ₁₃ , V ₂ O ₄ , V ₃ O _8, etc.) and lithium compounds such as lithium compounds thereof. Containing compound, general formula M _X Mo ₆ S _8-y (wherein M is at least one transition metal element, X is a number in the range of 0 ≦ X ≦ 4, Y is 0 ≦ Y ≦ 1) The chevrel phase compound, activated carbon, activated carbon fiber, etc. which are represented can be used. The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and may be a solid solution of lithium and two or more transition metals.

Specifically, the lithium-containing transition metal oxide is LiM ¹ _1-p M ² _p O ₂ (wherein M ¹ and M ² are at least one transition metal element, and p is 0 ≦ p ≦ 1) LiM ¹ _1-q M ² _q O ₄ (wherein M ¹ and M ² are at least one transition metal element, and q is a number in the range of 0 ≦ q ≦ 1). Indicated by
Transition metals represented by M, M ¹ and M ² are Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc., preferably Co, Fe, Mn, Cr, Ti, V, Al and the like. Preferred examples include 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.
The lithium-containing transition metal oxide is prepared by mixing lithium and a transition metal oxide or salt as starting materials, for example, and mixing these starting materials according to the composition of the desired metal oxide, and in an oxygen atmosphere, 600 to 1000 It can be obtained by firing at a temperature of ° C. The starting material is not limited to oxides or salts, but may be hydroxides.

In the present invention, the positive electrode active material may be used alone or in combination of two or more. For example, an alkali carbonate such as lithium carbonate can be added to the positive electrode material.
In order to form a positive electrode using such a positive electrode material, for example, a positive electrode mixture comprising a positive electrode material, a binder, and a conductive agent for imparting conductivity to the electrode is applied to both surfaces of the current collector. An agent layer is formed. As the binder, for example, a carbon material, graphite or carbon black is used.

The shape of the current collector used for the positive electrode is not particularly limited, but a foil or a net-like material such as a mesh or expanded metal is used. Examples of the material for the current collector include aluminum, copper, stainless steel, and nickel. In the case of a foil, the thickness of the current collector is preferably 10 to 40 μm.
In the case of the positive electrode, as in the case of the negative electrode, the positive electrode mixture is dispersed in a solvent to form a paste, and the paste-like negative electrode mixture is applied to a current collector and dried to form a positive electrode mixture layer. In addition, after forming the positive electrode mixture layer, pressure bonding such as press pressing may be further performed. Thereby, the positive electrode mixture layer is uniformly and firmly bonded to the current collector.

(Nonaqueous electrolyte)
In the lithium ion secondary battery of the present invention, a polymer electrolyte such as a solid electrolyte or a gel electrolyte can be used as a nonaqueous electrolyte in addition to a liquid electrolyte.
Non-aqueous electrolyte used in the lithium ion secondary battery of the present invention is an electrolyte salt used in the conventional non-aqueous electrolyte, _{specifically, 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 ₃ 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, include lithium salts such as LiSiF _6. In particular, LiPF ₆ and LiBF ₄ are preferable from the viewpoint of oxidation stability.

Examples of the solvent for making the non-aqueous electrolyte include those used as solvents for ordinary non-aqueous electrolytes. Specifically, carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butrolactone, 1,3-dioxofuran, 4-methyl-1,3-dioxolane, ethers such as anisole and diethyl ether, thioethers such as sulfolane and methylsulfolane, nitriles such as acetonitrile, chloronitrile and propionitrile, trimethyl borate, tetrasilicate Methyl, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethylorthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydro Thiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, may be used an aprotic organic solvent such as sulfite, dimethyl sulfite. Preferably the concentration of the electrolyte salt in the electrolytic solution is ^{0.1~5mol / dm 3 (0.1~5mol / l} ), 0.5~3.0mol / dm 3 (0.5~3.0mol / l) is more preferable.

  The method for producing the polymer electrolyte is not particularly limited. For example, the polymer compound, lithium salt and non-aqueous solvent (plasticizer) constituting the matrix are mixed and heated to melt and dissolve the polymer compound. A method in which a polymer compound, a lithium compound, and a non-aqueous solvent are dissolved in an organic solvent for use, and then the organic solvent for mixing is evaporated. A polymerizable monomer, a lithium salt, and a non-aqueous solvent are mixed. Or the method of irradiating with a molecular beam etc. and polymerizing can be mentioned. The ratio of the nonaqueous solvent in the polymer electrolyte is preferably 10 to 90% by mass, and more preferably 30 to 80% by mass. If it is less than 10% by mass, the electrical conductivity will be low, and if it exceeds 90% by mass, the mechanical strength will be weak and film formation will be difficult.

Examples of the polymer electrolyte include ether polymers such as polyethylene oxide and cross-linked polymers thereof, polymethacrylate polymers, polyacrylate polymers, polyvinylidene fluoride, and vinylidene fluoride-hexafluoropropylene copolymers. These resins can be used alone or in combination. Among these, it is preferable to use a fluorine-based resin such as polyvinylidene fluoride or vinylidene fluoride-hexafluoropropylene copolymer from the viewpoint of oxidation-reduction stability. In the case of the polymer gel electrolyte, the concentration of the electrolyte salt in the electrolytic solution as a plasticizer is preferably 0.1 to 5 mol / dm ³ (0.1 to 5 mol / l), and preferably 0.5 to 2.0 mol / l. dm and more preferably ^{3 (0.5~2.0mol} / l).

Since the lithium ion secondary battery of the present invention uses the coated composite particles, a gel electrolyte can be used.
A lithium ion secondary battery using a gel electrolyte is composed of a negative electrode containing the coated composite particles, a positive electrode, and a gel electrolyte. For example, the negative electrode, the gel electrolyte, and the positive electrode are stacked in this order and accommodated in the battery outer packaging material. In addition to this, a gel electrolyte may be further arranged outside the negative electrode and the positive electrode. In the gel electrolyte lithium ion secondary battery using the negative electrode material of the present invention, the gel electrolyte can contain propylene carbonate. In general, propylene carbonate has a strong electrolysis reaction with respect to the graphite material, but since the decomposition reactivity with the negative electrode material of the present invention is low, the irreversible capacity in the first cycle can be kept small.

(Separator)
In the lithium ion secondary battery of the present invention, a separator can also be used. Although a separator is not specifically limited, For example, a woven fabric, a nonwoven fabric, a synthetic resin microporous film, etc. are mentioned. A microporous membrane made of synthetic resin is preferred, and among these, a microporous membrane made of polyolefin is preferred from the viewpoint of thickness, membrane strength, membrane resistance, and the like. Specifically, it is a microporous film made of polyethylene and polypropylene, or a microporous film in which these are combined.

  The structure of the lithium ion secondary battery of the present invention is arbitrary, and is not particularly limited with respect to its shape and form, and may be cylindrical, rectangular, depending on the application, mounted equipment, required charge / discharge capacity, etc. A coin type, a button type, or the like can be arbitrarily selected. In order to obtain a sealed nonaqueous electrolyte battery with higher safety, it is preferable to include a means for detecting an increase in the internal pressure of the battery and shutting off the current when there is an abnormality such as overcharging. In the case of a polymer solid electrolyte or a polymer gel electrolyte battery, it can also have a structure enclosed in an aluminum laminate film.

The present invention will be specifically described with reference to Examples and Comparative Examples, but the present invention is not limited to these. In Examples and Comparative Examples, button-type secondary batteries for evaluation having the configuration shown in FIG. 1 were produced and evaluated. The battery can be manufactured according to a known method based on the gist of the present invention. In the evaluation battery, the working electrode was expressed as a negative electrode, and the counter electrode was expressed as a positive electrode.
In the examples and comparative examples, the particle size (particle size) of the composite particles is a particle size at which the cumulative frequency measured with a laser diffraction particle size meter is 50% by volume. The fiber length and short axis length of the fibrous carbonaceous material were measured by taking 100 images at a magnification with which the shape of each material can be recognized with a scanning electron microscope, and the average value was obtained. The average aspect ratio was the average value of the ratios of the fiber length / short axis length. The structure and shape of the composite particles coated with the carbonaceous material were observed with a scanning electron microscope. The specific surface area of the composite particles coated with the carbonaceous material was measured by the BET method in which nitrogen gas is adsorbed.

(Example 1)
92.7 parts by mass of graphitized vapor-grown carbon fiber (Showa Denko KK, VGCF, short axis length 150 nm, average aspect ratio about 50) and silicon particles (manufactured by High Purity Chemical Laboratory Co., Ltd.) Then, 7.3 parts by mass of an average particle size of 2 μm) was mixed and put into a mechano-fusion system (manufactured by Hosokawa Micron Co., Ltd.) to give mechanical energy and perform mechanochemical treatment. That is, composite particles in which silicon particles are sandwiched between vapor-grown carbon fibers by repeatedly applying compressive force and shearing force under conditions of a peripheral speed of the rotating drum of 20 m / s, a processing time of 30 min, and a distance of 5 mm between the rotating drum and the internal member. Got.

  Next, a coal tar pitch solution prepared by mixing 30 g of coal tar pitch (manufactured by JFE Chemical Co., Ltd., residual carbon ratio 60%) with 300 g of tar middle oil (manufactured by JFE Chemical Co., Ltd.) and the composite particles, The mixture was kneaded at 200 ° C. for 1 h using a shaft heating kneader. At that time, the solid content ratio was adjusted to be coal tar pitch: composite particles = 42: 58. After kneading, the solvent tar oil was removed from the kneaded product under vacuum to obtain composite particles coated 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, substantially all of the volatiles were removed. The residual carbon ratio is based on the fixed carbon method of JIS K2425, and is expressed as a percentage, which is the remainder when heated to 800 ° C. and substantially completely carbonized.

The composite particles coated with the carbonaceous material were spherical, the average particle diameter was 10 μm, and the specific surface area was 5.2 m ² / g.
The carbonaceous material covers the outer surface of the composite particles, the silicon particles are sandwiched between the vapor-grown carbon fibers, and a large number of voids are dispersed throughout the composite particles. confirmed. The mass composition of silicon / fibrous graphite material / carbonaceous material of the obtained coated composite particles was 5.1 / 64.6 / 30.3.
Note that the ratio of silicon in the coated composite particles was determined by the above-described emission spectroscopy. The ratio between the fibrous graphite material and the carbonaceous material was determined by the method using the polarizing microscope described above.

(Preparation of negative electrode mixture paste)
90% by mass of the composite particles coated with the carbonized product and 10% by mass of polyvinylidene fluoride are placed in an N-methylpyrrolidone solvent, and are stirred and mixed at 2000 rpm for 3 minutes using a homomixer. A paste was prepared.

(Production of working electrode)
The negative electrode mixture paste was applied to a copper foil with a uniform thickness, and the solvent N-methylpyrrolidone was volatilized at 90 ° C. in a vacuum and dried. The obtained negative electrode mixture layer was pressurized by a hand press. The current collector copper foil and the negative electrode mixture layer were punched into a cylindrical shape having a diameter of 15.5 mm, and a working electrode composed of a copper foil (thickness 16 μm) and a negative electrode mixture (thickness 50 μm) adhered to the copper foil was produced.

(Production of counter electrode)
The lithium foil was pressed against the current collector nickel net and punched into a cylindrical shape with a diameter of 15.5 mm to produce a counter electrode made of lithium foil (thickness 0.5 μm) in close contact with the nickel net.

(Electrolyte / Separator)
LiPF ₆ was dissolved at a concentration of 1 mol / dm ^{3 in a} mixed solvent obtained by mixing 33 vol% of ethylene carbonate and 67 vol% of methyl ethyl carbonate to prepare a nonaqueous electrolytic solution. The obtained nonaqueous electrolytic solution was impregnated into a polypropylene porous sheet (thickness: 20 μm) to produce a separator impregnated with the electrolytic solution.

(Production of evaluation battery)
As an evaluation battery, a button-type secondary battery shown in FIG. 1 was produced by the following procedure.
The separator 5 impregnated with an electrolytic solution was sandwiched between the working electrode 2 in close contact with the current collector 7b and the counter electrode 4 in close contact with the current collector 7a. Then, the exterior cup 1 and the exterior cup 3 were match | combined so that the collector 7b side of the working electrode 2 might be accommodated in the exterior cup 1 and the exterior can 3 from the collector 7a side of the counter electrode 4. FIG. In that case, the insulating gasket 6 was interposed in the peripheral part of the exterior cup 1 and the exterior can 3, and the both peripheral parts were crimped | contacted and adhered.

The evaluation battery was subjected to the following charge / discharge test at a temperature of 25 ° C., and the charge / discharge capacity, initial charge / discharge efficiency, and cycle characteristics were calculated. The charge / discharge characteristics (discharge capacity, initial charge / discharge efficiency, and cycle characteristics) are shown in Table 2.
(Discharge capacity and initial charge / discharge efficiency)
Constant current charging is performed until the circuit voltage reaches 0 mV at a current value of 0.9 mA, and the circuit voltage is 0 mV.
Switch to constant voltage charging until the current value reaches 20μA. Continued charging. The charging capacity was determined from the amount of electricity applied during that time. After that, it rested for 120 hours. Next, constant current discharge was performed at a current value of 0.9 mA until the circuit voltage reached 1.5 V, and the discharge capacity was obtained from the energization amount during this period. The initial charge / discharge efficiency was calculated from the following equation. In this test, the process of occluding lithium into the graphite particles was charged and the process of detaching was defined as discharge.
Initial charge / discharge efficiency (%) = (first cycle discharge capacity / first cycle charge capacity)
× 100

(Cycle characteristics)
In addition to these evaluation tests, after performing constant current charging at a current value of 4.0 mA until the circuit voltage reaches 0 mV, switching to constant voltage charging and continuing charging until the current value reaches 20 μm. For 120 minutes. Next, constant current discharge was performed until the circuit voltage reached 1.5 V at a current value of 4.0 mA. This charging / discharging was repeated 20 cycles. The discharge capacities at the 1st and 20th cycles were determined, and the cycle characteristics were calculated from the following equation.
Cycle characteristics (%) = (discharge capacity of 20th cycle / discharge capacity of 1st cycle)
× 100

(Example 2)
In Example 1, the same methods and conditions as in Example 1 except that carbonized nanotubes (diameter 10 nm, average aspect ratio of about 300) were used instead of vapor-grown carbon fibers as the fibrous graphite material. Thus, composite particles composed of silicon particles and carbon nanotubes were prepared, and then composite particles coated with carbonized carbon of coal tar pitch were prepared. The obtained coated composite particles were spherical, the average particle size was 12 μm, and the specific surface area was 6.1 m ² / g. It was confirmed that the outer surface of the coated composite particles was uniformly coated, silicon particles were entangled between carbon nanotubes, and a large number of voids were dispersed throughout the interior of the composite particles. The mass composition of silicon / fibrous graphite material / carbonaceous material of the coated composite particles was 5.1 / 64.6 / 30.3.

  In Example 1, as the coated composite particles, except for using the composite particles based on graphitized carbon nanotubes instead of the composite particles based on vapor-grown carbon fibers, the same method and conditions as in Example 1, A negative electrode mixture paste, a negative electrode material, a working electrode, and an evaluation battery were prepared. The charge / discharge characteristics of the evaluation battery were measured by the same method and conditions as in Example 1. The results are shown in Table 2.

(Example 3)
In Example 1, when the coal tar pitch solution and the composite particles were kneaded at 200 ° C. for 1 h using a biaxial heating kneader, scaly natural graphite (manufactured by Chuetsu Graphite Industries Co., Ltd., average particle diameter: 5 μm) Was added in the same manner and under the same conditions as in Example 1 except that coal tar pitch: composite particles: scale-like natural graphite = 34: 60: 6 was added as a solid content ratio. Subsequently, firing was performed to obtain coated composite particles. The obtained coated composite particles were agglomerated, the average particle diameter was 12 μm, and the specific surface area was 5.3 m ² / g.
The carbonaceous material and scaly natural graphite cover the outer surface of the composite particle, silicon particles are entangled between vapor-grown carbon fibers, and many large and small voids are dispersed throughout the composite particle. It was confirmed that it was formed. The mass composition of silicon / fibrous graphite material / flaky graphite material / carbonaceous material of the obtained coated composite particles was 5.1 / 64.8 / 6.5 / 23.6.

  In Example 1, as the coated composite particles, except for using the coated composite particles obtained by adding scaly natural graphite to the composite particles based on vapor-grown carbon fibers, the same method and conditions as in Example 1, A negative electrode mixture paste, a working electrode, and an evaluation battery were prepared. The charge / discharge characteristics of the evaluation battery were measured in the same manner as in Example 1. The results are shown in Table 2.

Example 4
In Example 3, the composite particles were prepared in the same manner and under the same conditions as in Example 1 except that a mesophase small sphere graphitized material was used instead of scaly natural graphite and pulverized so that the average particle size was 3 μm. It was produced and subsequently fired to obtain coated composite particles. The obtained coated composite particles were agglomerated, the average particle diameter was 10 μm, and the specific surface area was 4.0 m ² / g.
The carbonaceous material and graphitized mesophase spherules coat the outer surface of the composite particle, silicon particles are sandwiched between vapor-grown carbon fibers, and a large number of large and small voids are formed inside the composite particle. It was confirmed that it was dispersed throughout. The mass composition of silicon / fibrous graphite material / non-fibrous graphite material / carbonaceous material of the obtained coated composite particles was 5.1 / 64.8 / 6.5 / 23.6.
In Example 1, the same method and conditions as in Example 1 were used except that the coated composite particles obtained by adding graphitized mesophase spherules to the composite particles based on vapor-grown carbon fibers were used as the coated composite particles. Thus, a negative electrode mixture paste, a working electrode, and an evaluation battery were produced. The charge / discharge characteristics of the evaluation battery were measured in the same manner as in Example 1. The results are shown in Table 2.

(Example 5)
In Example 1, except that the mass ratio of vapor-grown carbon fiber and silicon particles subjected to mechanochemical treatment was 95.7: 4.3, coating with coal tar pitch in the same manner and conditions as in Example 1 The composite particles thus prepared were produced, followed by firing to obtain coated composite particles. The obtained coated composite particles were spherical, the average particle diameter was 10 μm, and the specific surface area was 5.0 m ² / g.
The carbonaceous material covers the outer surface of the composite particles, and silicon particles are sandwiched between vapor-grown carbon fibers, and a large number of voids are dispersed throughout the interior of the composite particles. confirmed. The mass composition of silicon / fibrous graphite material / carbonaceous material of the coated composite particles was 3.0 / 66.7 / 30.3.
A negative electrode mixture paste, a working electrode, and an evaluation battery were produced in the same manner and in the same manner as in Example 1, except that the coated composite particles with different amounts of silicon were used. The charge / discharge characteristics of the evaluation battery were measured in the same manner as in Example 1. The results are shown in Table 2.

(Example 6)
Example 1 is the same as Example 1 except that tin particles are used instead of silicon particles, and the mass ratio of vapor-grown carbon fibers to be subjected to mechanochemical treatment and tin particles is 94.2: 5.8. Composite particles coated with coal tar pitch were produced by the method and conditions, followed by firing to obtain coated composite particles. The obtained coated composite particles were spherical, the average particle diameter was 9 μm, and the specific surface area was 4.8 m ² / g.
The carbonaceous material covers the outer surface of the composite particle, tin particles are sandwiched between vapor-grown carbon fibers, and a large number of voids are dispersed throughout the composite particle. confirmed. The mass composition of tin / fibrous graphite material / carbonaceous material of the coated composite particles was 4.0 / 65.7 / 30.3.
A negative electrode mixture paste, a working electrode, and an evaluation battery were produced in the same manner and in the same manner as in Example 1 except that coated composite particles in which silicon was changed to tin were used in Example 1. The charge / discharge characteristics of the evaluation battery were measured in the same manner as in Example 1. The results are shown in Table 2.

(Example 7)
In Example 1, instead of the coal tar pitch solution, a phenol resin solution prepared by mixing 50 g of phenol resin (manufactured by JFE Chemical Co., Ltd., residual carbon ratio 40%) with 200 g of ethanol was used. When kneading the composite particles with a biaxial heating kneader, the same method as in Example 1 except that the solid content ratio is phenol resin: composite particles = 50: 50 and the kneading conditions are 150 ° C. and 1 h. The composite particles coated with the phenol resin were prepared under the above conditions, followed by firing to obtain the coated composite particles. The obtained coated composite particles were spherical, the average particle diameter was 10 μm, and the specific surface area was 5.1 m ² / g.
The carbonaceous material covers the outer surface of the composite particles, and silicon particles are sandwiched between vapor-grown carbon fibers, and a large number of voids are dispersed throughout the interior of the composite particles. confirmed. The mass composition of silicon / fibrous graphite material / carbonaceous material of the coated composite particles was 5.2 / 66.2 / 28.6.
A negative electrode mixture paste, a working electrode, and an evaluation battery were produced in the same manner and in the same manner as in Example 1 except that coated composite particles in which the coal tar pitch was changed to a phenol resin were used in Example 1. The charge / discharge characteristics of the evaluation battery were measured in the same manner as in Example 1. The results are shown in Table 2.

(Example 8)
In Example 1, when the coal tar pitch solution and the composite particles were kneaded at 200 ° C. for 1 h using a biaxial heating kneader, the solid content ratio was set to coal tar pitch: composite particles = 30: 70, In the same manner and conditions as in Example 1, composite particles coated with coal tar pitch were produced, followed by firing to obtain coated composite particles. The obtained coated composite particles were spherical, the average particle diameter was 10 μm, and the specific surface area was 5.5 m ² / g.
The carbonaceous material covers the outer surface of the composite particles, and silicon particles are sandwiched between vapor-grown carbon fibers, and a large number of voids are dispersed throughout the interior of the composite particles. confirmed. The mass composition of silicon / fibrous graphite material / carbonaceous material of the coated composite particles was 5.8 / 73.7 / 20.5.
A negative electrode mixture paste, a working electrode, and an evaluation battery were produced in the same manner and in the same manner as in Example 1 except that the coated composite particles in which the blend amount of coal tar pitch was changed in Example 1. The charge / discharge characteristics of the evaluation battery were measured in the same manner as in Example 1. The results are shown in Table 2.

Example 9
In Example 1, in addition to 84.3 parts by mass of graphitized vapor-grown carbon fiber and 6.6 parts by mass of silicon particles, the sample to be subjected to mechanochemical treatment was used in addition to the scaly natural graphite of Example 3. A composite particle coated with coal tar pitch was produced under the same method and conditions as in Example 1 except that the amount was 1 part by mass, followed by firing to obtain a coated composite particle. The obtained coated composite particles were spherical, the average particle diameter was 11 μm, and the specific surface area was 5.2 m ² / g.
The carbonaceous material covers the outer surface of the composite particles, and scaly graphite is dispersed and held in a structure in which silicon particles are sandwiched between vapor-grown carbon fibers, and a large number of voids are formed. It was confirmed that it was dispersed throughout the interior of the composite particles. The mass composition of silicon / fibrous graphite material / flaky graphite material / carbonaceous material of the coated composite particles was 5.1 / 64.8 / 6.5 / 23.6.
A negative electrode mixture paste, working electrode, and evaluation battery were prepared in the same manner and in the same manner as in Example 1 except that coated composite particles obtained by adding scaly graphite material to a sample subjected to mechanochemical treatment in Example 1 were used. Produced. The charge / discharge characteristics of the evaluation battery were measured in the same manner as in Example 1. The results are shown in Table 2.

(Comparative Example 1)
In Example 1, silicon particles were obtained in the same manner and under the same conditions as in Example 1 except that scaly natural graphite (manufactured by Chuetsu Graphite Industry Co., Ltd., average particle size: 10 μm) was used instead of vapor-grown carbon fiber. Then, composite particles containing the coal tar were coated, and coated composite particles coated with coal tar pitch were prepared and baked to obtain particles. The obtained particles were spherical, the average particle size was 10 μm, and the specific surface area was 3.1 m ² / g.
Silicon particles and natural graphite were present inside the particles, and the carbonized product of coal tar pitch covered the outer surface of the particles, but almost no voids were confirmed. The mass composition of silicon / flaky graphite material / carbonaceous material of the particles was 5.1 / 64.6 / 30.3.

  Example 1 is the same as Example 1 except that the particles obtained by using scaly natural graphite instead of the composite particles based on vapor-grown carbon fibers are used as the composite particles coated with the carbonaceous material. The negative electrode mixture paste, the working electrode, and the evaluation battery were prepared by various methods and conditions. The charge / discharge characteristics of the evaluation battery were measured in the same manner as in Example 1. The results are shown in Table 2.

(Comparative Example 2)
In Example 1, graphitized vapor-grown carbon fiber (manufactured by Showa Denko KK, VGCF, short axis length 150 nm, average aspect ratio of about 50) and silicon particles (manufactured by High Purity Chemical Laboratory Co., Ltd.) The mixture ratio is 92.7 parts by mass: 7.3 parts by mass to 95 parts by mass: 5 parts by mass, and only the mechanochemical treatment is performed under the same method and conditions as in Example 1. Particles were prepared and coarsely pulverized to obtain particles having no carbonaceous material coating. The obtained particles were massive, the average particle diameter was 8 μm, and the specific surface area was 50 m ² / g.
In the particles, silicon particles were sandwiched between vapor-grown carbon fibers, and a large number of voids were dispersed throughout the interior of the particles, but it was confirmed that there was no carbonaceous material coating. The particles had a mass ratio of silicon / graphitized vapor grown carbon fiber = 5.1 / 94.9.

  In Example 1, a negative electrode mixture paste, a working electrode, and an evaluation battery were produced in the same manner and under the same conditions as in Example 1 except that the particles having no carbonaceous material coating were used instead of the coated composite particles. did. The charge / discharge characteristics of the evaluation battery were measured in the same manner as in Example 1. The results are shown in Table 2.

(Comparative Example 3)
A coal tar pitch solution was prepared by mixing 30 g of coal tar pitch (manufactured by JFE Chemical Co., Ltd.) with 300 g of tar medium oil (manufactured by JFE Chemical Co., Ltd.). 92.7 parts by mass of the solution and graphitized vapor-grown carbon fiber (manufactured by Showa Denko KK, VGCF, short axis length 150 nm, average aspect ratio about 50) and silicon particles (High Purity Chemical Laboratory ( Co., Ltd., average particle diameter 2 μm) 7.3 parts by mass using a biaxial heating kneader, kneaded at 200 ° C. for 1 h, and then calcined at 1000 ° C. for 10 h, carbonized coal tar pitch, fibrous graphite Particles in which the material and silicon particles were integrated were produced. The obtained particles were massive, the average particle diameter was 10 μm, and the specific surface area was 20 m ² / g.
In the particles, silicon particles and vapor-grown carbon fibers were in contact with the carbonized product of coal tar pitch, and almost no voids were observed. The particles had a mass ratio of silicon / fibrous carbonaceous material / carbonaceous material = 5.1 / 64.6 / 30.3.

  In Example 1, in place of the coated composite particles prepared by mechanochemical treatment, a negative electrode mixture paste was prepared in the same manner and under the same conditions as in Example 1 except that the particles produced without using mechanochemical treatment were used. A negative electrode material, a working electrode, and an evaluation battery were prepared. The charge / discharge characteristics of the evaluation battery were measured in the same manner as in Example 1. The results are shown in Table 2.

(Comparative Example 4)
In place of the vapor-grown carbon fiber graphitized in Example 1, a composite particle is produced by the same method and conditions as in Example 1 except that the vapor-grown carbon fiber not graphitized is used. Subsequently, firing was performed to obtain coated composite particles. The obtained coated composite particles were spherical, the average particle diameter was 10 μm, and the specific surface area was 5.5 m ² / g. It was confirmed that the outer surface of the coated composite particles was uniformly coated, the silicon particles were entangled between carbon fibers, and a large number of voids were dispersed throughout the interior of the composite particles. . The mass composition of silicon / fibrous carbon material / carbonaceous material of the obtained coated composite particles was 5.1 / 64.6 / 30.3.
Example 1 except that coated composite particles based on vapor-grown carbon fibers that have not been graphitized are used as coated composite particles instead of composite particles based on vapor-grown carbon fibers that have been graphitized. A negative electrode mixture paste, a working electrode and an evaluation battery were produced in the same manner and under the same conditions as in 1. The charge / discharge characteristics of the evaluation battery were measured in the same manner as in Example 1. The results are shown in Table 2.

As shown in Table 2, the evaluation battery composed of the working electrode using the coated composite particles of the present invention of Examples 1 to 9 has a discharge capacity of 350 to 360 mAh / g when graphite, which is the current negative electrode material, is used. In comparison, even compared with the theoretical value 372 mAh / g of the intercalation compound LiC ₆ , it shows a high discharge capacity and has a high initial charge / discharge efficiency. Furthermore, it exhibits excellent cycle characteristics.
As in Comparative Example 1, an evaluation battery including a working electrode using composite particles that do not use a fibrous graphite material does not provide high initial charge / discharge efficiency and cycle characteristics. This is presumably because the structure of the composite particles was destroyed by the expansion / contraction of the silicon particles during charge / discharge, resulting in a decrease in conductivity and separation of the active material from the current collector.
As in Comparative Example 2, an evaluation battery composed of a working electrode using composite particles in which the composite particles are not coated with a carbonaceous material cannot obtain high initial charge / discharge efficiency and cycle characteristics. This is presumably because decomposition of the electrolyte solution on the surface of the fibrous graphite material or dropping of the silicon particles occurred.
As in Comparative Example 3, the action of using the integrated particles that hold coal tar as a binder without applying mechanical energy such as mechanochemical treatment to the fibrous graphite material and silicon particles. An evaluation battery composed of electrodes cannot obtain high initial charge / discharge efficiency and cycle characteristics. This is because the carbonaceous material penetrated into the interior of the particle, no void was formed around the silicon particle, and the coating of the carbonaceous material on the outer surface was insufficient due to the penetration into the interior. It can be considered that it depends.
As in Comparative Example 4, an evaluation battery including a working electrode using composite particles using a fibrous carbonaceous material that has not been graphitized does not have high discharge capacity, initial charge / discharge efficiency, and cycle characteristics. This is presumably because the carbon fiber is not graphitized and thus has low crystallinity and low conductivity.

The schematic cross section which shows the structure of the button type evaluation battery used in order to evaluate the charging / discharging characteristic of the negative electrode material of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Exterior cup 2 Working electrode 3 Exterior can 4 Counter electrode 5 Separator 6 Insulating gasket 7a, 7b Current collector

Claims (12)

At least a part of the outer surface of the composite particle having a structure in which the fibrous graphite material is entangled with metal particles that can be alloyed with lithium and sandwiches the metal particles and a void exists is coated with the carbonaceous material. Ri Oh coated composite particles are,
The content of the metal particles, the fibrous graphite material, and the carbonaceous material is mass% with respect to the total mass of the coated composite particles, and the metal particles / the fibrous graphite material / the carbonaceous material = 1 or more. The negative electrode material for lithium ion secondary batteries which is less than 30 / 30-95 / 4-50 . At least a part of the outer surface of the composite particle having a structure in which the fibrous graphite material is entangled with metal particles that can be alloyed with lithium and sandwiches the metal particles and a void exists is coated with the carbonaceous material. Coated composite particles,
The coated composite particles further contain a non-fibrous graphite material ;
Content of the metal particles, the fibrous graphite material, the non-fibrous graphite material, and the carbonaceous material is mass% with respect to the total mass of the coated composite particles, and the metal particles / the fibrous graphite material. / Non-fibrous graphite material / Carbonaceous material = 1 or more and less than 30/28 to 75/2 to 20/4 to 50, A negative electrode material for a lithium ion secondary battery. The negative electrode material for a lithium ion secondary battery according to claim 2 , wherein the non-fibrous graphite material is dispersed and held in the structure of the coated composite particles. The negative electrode material for a lithium ion secondary battery according to claim 2 , wherein at least a part of the outer surface of the coated composite particle is coated with the carbonaceous material and the non-fibrous graphite material. Anode material for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the specific surface area of the coated complex particles is less than 10 m 2 / ^g. After mixing and stirring the metal particles that can be alloyed with lithium and the fibrous graphite material, the composite particles and the carbonaceous material precursor are mixed, heat-treated, and charged. A method for producing a negative electrode material for a lithium ion secondary battery, wherein the coated composite particle according to Item 1 is obtained. Metal particles that can be alloyed with lithium, fibrous graphite material and non-fibrous graphite material are mixed and stirred to form composite particles, and then the composite particles and carbonaceous material precursor are mixed and heat-treated. And the manufacturing method of the negative electrode material for lithium ion secondary batteries characterized by obtaining the covering composite particle of Claim 2 or 3 . After mixing and stirring the metal particles that can be alloyed with lithium and the fibrous graphite material, the composite particles, the precursor of the carbonaceous material, and the non-fibrous graphite material are mixed. A method for producing a negative electrode material for a lithium ion secondary battery, wherein the coated composite particles according to claim 2 or 4 are obtained by heat treatment. The method for producing a negative electrode material for a lithium ion secondary battery according to any one of claims 6 to 8 , wherein the mixing / stirring treatment is a mechanochemical treatment. The method for producing a negative electrode material for a lithium ion secondary battery according to any one of claims 6 to 9 , wherein the temperature of the heat treatment is 600 ° C or higher and lower than 1300 ° C. A negative electrode for a lithium ion secondary battery comprising the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 5 . Lithium-ion secondary battery, which comprises using a negative electrode for a lithium ion secondary battery of claim 1 1.

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