KR100589308B1 - Negative active material for lithium secondary battery and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a carbonaceous material and a lithium secondary battery including the same, wherein the carbonaceous material includes silicon and carbon around graphite particles having a plane spacing d002 of (002) plane of less than 0.337 nm by X-ray wide-angle diffraction. Composite particles containing and having a smaller particle size than the graphite particles are dispersed and covered, and the graphite particles and the composite particles are covered with an amorphous carbon film having a surface spacing d002 of 0.37 nm or more. In the composite particle, a conductive carbon material is disposed around Si fine particles made of crystalline silicon, and the Si fine particles and the conductive carbon material are coated with a hard carbon film or a conductive polymer film.

The present invention provides a carbonaceous material having high charge and discharge capacity and at the same time excellent cycle characteristics.

Carbonaceous material, lithium secondary battery, graphite, negative electrode

Description

Negative active material for lithium secondary battery and lithium secondary battery comprising same TECHNICAL FIELD OF LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME

1 is a cross-sectional schematic diagram showing an example of a carbonaceous material according to an embodiment of the present invention.

2 is a cross-sectional schematic diagram showing an example of a carbonaceous material according to an embodiment of the present invention.

3 is a cross-sectional schematic diagram showing an example of a carbonaceous material according to an embodiment of the present invention.

4 is a schematic cross-sectional view showing an example of a carbonaceous material according to an embodiment of the present invention.

Figure 5 is a schematic cross-sectional view showing an example of the composite particles contained in the carbonaceous material according to an embodiment of the present invention.

* Description of the symbols for the main parts of the drawings *

1 carbonaceous material 2 graphite particles

3: composite particle 4: amorphous carbon film

5: Si fine particles 6: Conductive carbon material

7: hard carbon film

[TECHNICAL FIELD OF THE INVENTION]

The present invention relates to a carbonaceous material and a lithium secondary battery including the same, and more particularly, to a carbonaceous material having a high charge and discharge capacity and excellent cycle characteristics and a lithium secondary battery including the same.

[Private Technology]

In order to meet the needs of portable electronic devices, which are leading to small size, light weight, and high performance, high capacity of lithium secondary batteries has emerged as an urgent problem.

However, graphite, which is one of the negative electrode active materials of a lithium secondary battery, has a theoretical capacity of 372 mAh / g. However, in order to obtain a higher capacity negative electrode active material, graphite has been developed to replace amorphous carbon materials or carbon materials. It is necessary to proceed.

Silicon and its compound have been examined conventionally as a novel material which can replace graphite. Silicon and its compounds are known to form an alloy with lithium and exhibit a larger capacitance than graphite.

Therefore, recently, as a negative electrode material of a lithium secondary battery, (1) a material in which a silicon compound powder is simply mixed with graphite, (2) a fine powder silicon compound or the like is chemically fixed on the graphite surface by using a silane coupling agent or the like, (3) A material in which a graphite carbon material and a metal material such as Si is bonded or coated with an amorphous carbon material has been proposed.

However, since the graphite and silicon compound are not necessarily in close contact with each other in the above-mentioned (1) graphite, the silicon compound powder is graphite when the graphite expands and shrinks as the charge and discharge cycle proceeds. Since the silicon compound itself has low electron conductivity, the silicon compound is not sufficiently used as a negative electrode active material, and there is a problem that the cycle characteristics of the lithium secondary battery are lowered.

In addition, the above-mentioned material (2) chemically fixing finely powdered silicon compound on the graphite surface by using a silane coupling agent or the like is maintained in a state in which the silicon compound is in close contact with the graphite during the initial charge and discharge cycle. It functions as a negative electrode active material, but as the charge and discharge cycle progresses, the silicon compound itself expands as the alloy with lithium forms, thereby destroying the bond by the silane coupling agent, thereby releasing the silicon compound from the graphite, and the silicon compound as the negative electrode. There is a problem that the cycle characteristics of the lithium secondary battery may be lowered because it cannot be sufficiently used as an active material. In addition, the silane coupling treatment may not be performed homogeneously at the time of manufacturing the negative electrode material, and there is a problem that a negative electrode material of stable quality cannot be easily produced.

In addition, (3) the material which combines or coats the graphite-based carbon material and metal materials such as Si with an amorphous carbon material is (2) chemically fixing finely divided silicon compounds or the like on the graphite surface using a silane coupling agent or the like. There is the same problem as the above problem of the material. That is, as the charge and discharge cycle proceeds, the bond by the amorphous carbon material is broken by the expansion of the metal material itself due to the alloy formation with lithium, and the metal material is released from the graphite carbon material, so that the metal material is sufficiently used as the negative electrode active material. There was a problem that it was not used and the cycle characteristics were lowered.

The present invention has been made in view of the above problems, and an object thereof is to provide a carbonaceous material having high charge and discharge capacity and excellent cycle characteristics.

Moreover, an object of this invention is to provide the lithium secondary battery containing the said carbonaceous material.

In order to achieve the above object, the carbonaceous material of the present invention has the following configuration. In the carbonaceous material of the present invention, composite particles containing silicon and carbon and having a smaller particle size than the graphite particles are dispersed and disposed around graphite particles having an interplanar spacing d002 of the (002) plane by X-ray wide-angle diffraction of less than 0.337 nm. And the graphite particles and the composite particles are covered with an amorphous carbon film having a surface spacing d002 of 0.37 nm or more, wherein the composite particles are disposed with a conductive carbon material around Si fine particles made of crystalline silicon, and the Si fine particles and the conductive A carbon material is covered with a hard carbon film or a conductive polymer film.

In addition, in this invention, "periphery" shows the positional relationship of the composite particle with respect to graphite particle, and means "surface or surface vicinity" of graphite particle.

 In addition, "around" shows the positional relationship of the electroconductive carbon material with respect to Si microparticles | fine-particles, and means "surface or surface vicinity" of Si microparticles | fine-particles.

In addition, "dispersion arrangement" means a state where a plurality of composite particles are dispersed with each other without agglomeration and are positioned on the surface of the graphite particles or slightly spaced apart from the surface.

In addition, "coating" means the state which covers a to-be-covered particle completely and couples a to-be-covered particle. In this case, the particles to be coated may not necessarily be in direct contact with each other.

Specifically, the graphite particles and the composite particles which cover the graphite particles and the composite particles with the amorphous carbon film are completely covered with the amorphous carbon film to bond the graphite particles and the composite particles, or the composite particles are arranged in the amorphous carbon film to the surface of the graphite particles. It means to be close.

Similarly, covering the Si fine particles and the conductive carbon material with the hard carbon film or the conductive polymer film completely covers the Si fine particles and the conductive carbon material with the hard carbon film or the conductive polymer film to bond the Si fine particles with the conductive carbon material or the hard carbon film. Alternatively, it means that the conductive carbon material is disposed in the conductive polymer film to approach the surface of the Si fine particles.

In such carbonaceous materials, since the graphite particles and the Si fine particles occlude Li, the charge and discharge capacity is improved compared with the case where the graphite particles are used alone.

Moreover, electroconductivity of Si microparticles | fine-particles can be improved apparently by arrange | positioning a conductive carbon material around Si microparticles | fine-particles which are high specific resistance with respect to graphite particle | grains.

In addition, by covering the Si fine particles with a hard carbon film or a conductive polymer film, volume expansion and contraction of the Si fine particles due to occlusion and release of Li are mechanically suppressed.

In addition, when the graphite particles and the composite particles are covered with an amorphous carbon film, the graphite particles do not directly contact the electrolyte solution, so that decomposition of the electrolyte solution is suppressed, and the composite particles do not drop off from the graphite particles and Si due to volume expansion due to filling Prevents micronization of particulates.

As described above, as the carbonaceous material of the present invention, the charge and discharge capacity is increased, and the microparticles of the Si fine particles due to the volume expansion of the Si fine particles, the dropping of the composite particles, and the volume expansion due to the filling are suppressed, and the cycle characteristics are suppressed. Can be prevented from deteriorating.

In addition, in the carbonaceous material of the present invention, the particle diameter of the graphite particles is in the range of 2 µm to 70 µm, the particle diameter of the composite particles is in the range of 50 nm to 2 µm, and the film thickness of the amorphous carbon film is It is characterized by being in the range of 50 nm or more and 5 micrometers or less.

If the particle diameter of the graphite particles is less than 2 μm, the particle diameter of the graphite particles is relatively smaller than the particle diameter of the composite particles, and it is not preferable because the composite particles are difficult to adhere uniformly to the surface of the graphite particles, and the particle diameter is 70 μm or more. It is not preferable because the back surface is inferior in adhesion to the current collector and also in the voids in the electrode.

In addition, the particle diameter of the composite particles is preferably 50 nm or more and 2 μm or less, preferably 50 nm or more and 500 nm or less. In order to disperse and arrange the composite particles on the surface of the graphite particles, the particle size of the composite particles may be It is because it is necessary to set it as 2 micrometers or less which is the minimum particle diameter, and when the particle diameter is 500 nm or less, the volume change of the composite particle by expansion and contraction can be made small. In addition, when the particle diameter is 50 nm or less, the separation of the crystal structure of the Si fine particles contained in the composite particles becomes large, which is not preferable because the amount of Li occlusion decreases.

If the film thickness of the amorphous carbon film is less than 50 nm, the graphite particles may not be completely covered by the amorphous carbon film, and the dropping of the composite particles from the graphite particles cannot be prevented, and at the same time, electrolytic solution decomposition can be prevented. It is not preferable because there is a possibility that it may be absent, and when the film thickness exceeds 5 µm, lithium ions do not reach the graphite particles, and the storage amount of Li ions decreases and the charge / discharge capacity decreases.

In addition, the carbonaceous material of the present invention has a particle diameter of the Si fine particles in the range of 10 nm or more and less than 2 nm, the specific resistance of the conductive carbon material is 10 ^-4 Ω · m or less, the bending strength of the hard carbon film or conductive polymer film Is 500 kg / cm 2 or more and the film thickness is 10 nm or more and 1 m or less.

The particle diameter of the Si fine particles is 10 nm or more to prevent the collapse of the crystal structure of the Si fine particles and to improve the amount of Li occlusion. The particle size of the particle diameter is less than 2 m, the particle diameter of the composite particles is the minimum particle diameter of the graphite particles. It is for making it smaller than 2 micrometers of phosphorus.

The specific resistance of the conductive carbon material is set to 10 ⁻⁴ Ω · m or less in order to impart sufficient conductivity to the Si fine particles.

In addition, the bending strength of the hard carbon film or the conductive polymer film is set to 500 kg / cm 2 or more to reduce the volume change by mechanically pressing the expansion and contraction of the Si fine particles caused by the occlusion and release of Li. The film thickness is not less than 10 nm and not more than 1 µm because undesirably reducing the binding force between the conductive carbon material and the Si fine particles and suppressing the volume expansion of the composite particles when the film thickness is less than 10 nm is undesirable. When the thickness is 1 µm or more, lithium ions do not reach the Si fine particles and the charge and discharge capacity is lowered, which is not preferable.

In addition, the carbonaceous material for a lithium secondary battery of the present invention is characterized in that the content of the composite particles is 1% by weight or more and 25% by weight or less. When the content of the composite particles is less than 1% by weight, it is not preferable because the charge / discharge capacity higher than that in which only the carbon material is used as the active material can be obtained. On the other hand, when the content of the composite particles exceeds 25% by weight, the contribution of the carbon material portion decreases, and since the voltage increases from the initial discharge to the reaction potential of Si, it is not preferable, and the distance between the composite particles is narrow. It is not preferable because it is highly reagglomerated to cause volume expansion and contraction by the Si fine particles and the cycle characteristics are lowered.

Next, the lithium secondary battery of the present invention is characterized by comprising the above carbonaceous material.

Such a lithium secondary battery has at least a positive electrode, an electrolyte, and a negative electrode having the above negative electrode material, and has various shapes such as a cylindrical shape, a square shape, a coin shape, and a sheet shape. The lithium secondary battery of the present invention is not limited to the above embodiment, but may be formed in other forms.

According to such a lithium secondary battery, a lithium secondary battery having high energy density and excellent cycle characteristics can be constituted.

Embodiments of the present invention will be described below with reference to the drawings.

1-4 is a cross-sectional schematic diagram of the carbonaceous material for lithium secondary batteries of this invention. In this carbonaceous material, composite particles are dispersed and disposed around graphite particles, and graphite particles and composite particles are covered with an amorphous carbon film.

Here, "around" shows the positional relationship of composite particles with respect to graphite particles, and means "on or near surface" of the graphite particles. That is, the state in which the composite particles are bonded to the surface of the graphite particles and the composite particles are separated from the surface of the graphite particles and are positioned around the graphite particles.

"Dispersed arrangement" means a state in which a plurality of composite particles are dispersed from each other and bonded to the surface of the graphite particles or located slightly apart from the surface. The composite particles may be in contact with each other to such an extent that they do not aggregate.

"Coating" means a state in which the particles to be covered are completely covered and the particles to be coated are bonded to each other. In this case, the particles to be coated may not necessarily be in direct contact with each other.

Specifically, coating the graphite particles and the composite particles with the amorphous carbon film completely covers the graphite and the composite particles with the amorphous carbon film to bond the graphite particles and the composite particles, or arranges the composite particles in the amorphous carbon film to approach the graphite particle surface. Means that.

Therefore, the carbonaceous material of the present invention includes various forms as shown below.

For example, the carbonaceous material 1 shown in FIG. 1 is bonded to the surface of the graphite particles 2 in a state in which the plurality of composite particles 3 are dispersed with each other, and the amorphous carbon film 4 is formed of the composite particles 3. The graphite particles 2 and the composite particles 3 are formed to have a uniform film thickness smaller than the particle diameter of.

In addition, the carbonaceous material 1 shown in FIG. 2 is bonded to the surfaces of the plurality of graphite particles 2 in a state in which the plurality of composite particles 3 are dispersed with each other, such that the amorphous carbon film 4 has the composite particles 3. It is formed so as to cover the graphite particles 2 and the composite particles 3 to have a uniform film thickness larger than the particle diameter of. The amorphous carbon film 4 combines the plurality of graphite particles 2. 2 shows a state in which two or three graphite particles 2 are bonded by an amorphous carbon film 4, but the present invention is not limited thereto, and four or more graphite particles 2 are bonded by an amorphous carbon film 4. It may be.

In addition, the carbonaceous material 1 shown in FIG. 3 is bonded to the surface of the graphite particles 2 in a state in which a plurality of composite particles 3 are dispersed with each other, and the graphite particles 2 and the graphite particles 2 are bonded to the amorphous carbon film 4. It is comprised by covering the composite particle 3. The film thickness of the amorphous carbon film 4 shown in FIG. 3 is nonuniform, for example, as a part which covers only the graphite particle 2, it is set larger than the particle diameter of the composite particle 3, and as a part which covers the composite particle 3 is shown. It is set smaller than the particle diameter of the composite particle 3.

In addition, the carbonaceous material 1 shown in FIG. 4 is bonded to the surface of the graphite particles 2 in a state in which a plurality of composite particles 3 are dispersed with each other, such that the amorphous carbon film 4 has the graphite particles 2 and the composite particles. It is comprised by covering (3). The film thickness of the amorphous carbon film 4 shown in FIG. 4 is nonuniform, for example, as a part which covers only the graphite particle 2, it is set larger than the particle diameter of the composite particle 3, and the part which covers the composite particle 3 is shown. It is set smaller than the particle size of the composite grain | particle 3, and the surface of the amorphous carbon film 4 is formed in the smooth surface without unevenness | corrugation regardless of the shape of the composite grain | particle 3. As shown in FIG.

The carbonaceous material of the present invention is not limited to that shown in Figs. 1 to 4, and may be any one as long as the definition of the term is satisfied.

As the graphite particles 2 contained in the carbonaceous material, those having an interplanar spacing d002 of the (002) plane by X-ray wide-angle diffraction are preferably 0.335 nm or more and less than 0.337 nm, and more preferably 0.335 nm or more and less than 0.337 nm. desirable.

An interplanar spacing d002 of 0.337 nm or more is not preferable because the crystallinity of the graphite particles decreases, the initial irreversible capacity increases markedly, and the electron conductivity of the graphite particles decreases.

Moreover, the particle diameter of graphite particle has the preferable range of 2 micrometers or more and 70 micrometers or less. When the particle diameter of the graphite particles 2 is less than 2 μm, the particle diameter of the graphite particles 2 becomes relatively smaller than that of the composite particles 3, so that the composite particles 3 are uniformly formed on the surface of the graphite particles 2. It is not preferable because it becomes difficult to adhere, and if the particle diameter is 70 µm or more, the adhesion between the carbonaceous material and the current collector is lowered, and the voids in the electrode of the battery also become large, which is not preferable.

Next, the amorphous carbon film 4 covers the graphite particles 2 and the composite particles 3, as shown in FIGS. 1 to 4, and attaches the composite particles 3 to the surface of the graphite particles 2. I'm making it. As shown in FIG. 2, this amorphous carbon film 4 also serves to bond the graphite particles 2 together.

The amorphous carbon film 4 is heat-treated at least one or more materials selected from the group consisting of thermoplastic resins, thermosetting resins, vinyl resins, cellulose resins, phenolic resins, coal-based pitch materials, petroleum-based pitch materials, tar-based materials, and the like. It is obtained by graphitization, which is relatively non-graphitized, is amorphous, and has a surface spacing d002 of 0.37 nm or more. Since the amorphous carbon film 4 is amorphous, there is no fear that the organic electrolyte will decompose even if it touches the amorphous carbon film 4, and the surface spacing d002 of the amorphous carbon film 4, which can increase the charge and discharge efficiency of the carbonaceous material 1, is If the thickness is less than 0.37 nm, the crystallinity of the amorphous carbon film 4 is improved, which is close to the graphite structure, and may decompose the organic electrolyte solution.

In addition, since the composite particles 3 are positioned on the surface of the graphite particles 2 by the amorphous carbon film 4, the composite particles 3 having relatively high resistivity are prevented from being released from the graphite particles 2, It is possible to prevent the generation of the composite particles (3) that do not contribute to the charge and discharge reaction.

The amorphous carbon film 4 dissolves the thermoplastic resin, the thermosetting resin, and the like in a solvent, and also introduces the graphite particles 2 to precipitate the thermoplastic resin, the thermosetting resin, and the like on the surface of the graphite particles 2. In addition, since the graphite particles 2 can be completely covered with the graphite particles 2, the density of the graphite particles 2 and the composite particles 3 and the lithium ions are easily reduced because the density is relatively low and lithium ions are easily permeable. It does not inhibit the reaction.

It is preferable that the film thickness of the amorphous carbon film 4 is 50 nm or more and 5 micrometers or less. If the film thickness is less than 50 nm, the graphite particles 2 may not be completely covered, and the composite particles 3 may fall off from the graphite particles 2, which is not preferable. If the film thickness is 5 µm or more, the irreversible capacity increases. It is not preferable because it is.

Next, as shown in FIG. 5, the conductive particles 3 are disposed around the Si fine particles 5, and the Si fine particles 5 and the conductive carbon material 6 are formed of a hard carbon film (as shown in FIG. 5). 7) or it is covered with a conductive polymer film.

Here, "around" shows the positional relationship of the electroconductive carbon material 6 with respect to Si microparticles | fine-particles 5, and means "surface or surface vicinity" of Si microparticles | fine-particles. That is, the state in which the conductive carbon material 6 is bonded to the surface of the Si fine particles 5 and the conductive carbon material 6 are separated from the surface of the Si fine particles 5 are located around the Si fine particles 5. .

In addition, coating the Si fine particles 5 and the conductive carbon material 6 with the hard carbon film 7 or the conductive polymer film means that the Si fine particles 5 and the conductive carbon material 6 are coated with the hard carbon film 7 or the conductive polymer film. Covering the silicon fine particles 5 and the conductive carbon material 6 completely, or arranging the conductive carbon material 6 in the hard carbon film 7 or the conductive polymer film and bringing the conductive carbon material 6 into close proximity to the surface of the Si fine particles 5. .

The range of 50 nm or more and 2 micrometers or less is preferable, and, as for the particle diameter of the composite particle 3, the range of 50 nm or more and 500 nm or less is more preferable.

In order to disperse and arrange the composite particles 3 on the surface of the graphite particles 2, the particle diameter of the composite particles 3 is 2 μm or less. The particle size of the composite particles 3 is 2, which is the minimum particle size of the graphite particles 2. This is because it is necessary to set the particle size to 500 nm or less, and when the particle diameter is 500 nm or less, the volume change due to expansion and contraction of the Si fine particles 5 due to occlusion and release of lithium can be reduced. The lower limit of the particle size is 50 nm or more because the crystal structure of the Si fine particles 5 contained in the composite particles 3 becomes larger when the particle size is 50 nm or less, and the lithium occlusion amount is lowered, thereby reducing the charge and discharge capacity. Because there is.

Si microparticles | fine-particles 5 consist of crystalline silicon, and are a thing of the range whose particle diameter is 10 nm or more and less than 2 micrometers.

Silicon is an element which forms an alloy with lithium, and when lithium ion acts on the Si microparticles | fine-particles 5 which consist of this silicon, lithium inserts into the space | gap part of the surface of Si microparticles 5 or the inside of Si microparticles | fine-particles 5 To form an alloy, whereby the Si fine particles 5 themselves expand. Here, when the crystallinity of the Si microparticles | fine-particles 5 falls, since alloy formation ability with lithium falls and a charge / discharge capacity falls, it is unpreferable.

In addition, the particle size of the Si fine particles 5 is 10 nm or more in order to prevent the collapse of the crystal structure of the Si fine particles 5 and improve the amount of Li occlusion. This is because the particle diameter of the c) should be smaller than 2 µm, which is the minimum particle diameter of the graphite particles 2.

The conductive carbon material 6 is disposed on or near the surface of the Si fine particles 5. In FIG. 5, the conductive carbon material 6 is disposed around the Si fine particles 5, but the conductive carbon material 6 is disposed. The shape of the raw material 6 is not limited to the particles, and may be various forms such as film, layer, and fiber. The electroconductive carbon material 6 is arrange | positioned on the surface of Si microparticles | fine-particles 5 which are semiconductors, and give an apparent conductivity to Si microparticles | fine-particles 5. The specific resistance of this conductive carbon material 6 is preferably in the range of 10 ⁻⁴ Ω · m or less. When the resistivity is 10 ⁻⁴ Ω · m or more, the apparent conductivity of the Si fine particles 5 is lowered, and the charge and discharge reaction of lithium ions to the Si fine particles 5 does not proceed smoothly, and the charge and discharge capacity of the carbonaceous material is improved. It is not preferable because it cannot be made. Examples of the conductive carbon material 6 include carbon black, Ketjen black, and vapor-grown carbon fiber (VGCF).

The hard carbon film 7 covers the Si fine particles 5 and the conductive carbon material 6 and arranges the conductive carbon material 6 on the surface of the Si fine particles 5. In addition, instead of the hard carbon film 7, the conductive polymer film may cover the Si fine particles 5 and the conductive carbon material 6 and the conductive carbon material 6 may be disposed on the surface of the Si fine particles 5.

The hard carbon film 7 is obtained by firing polyvinyl alcohol, a phenol resin, or the like. The conductive polymer membrane is obtained by drying conductive polymers such as ionic polymer materials such as polyaniline dissolved in a solvent and polyacetylene materials such as phenylacetylene.

In addition, both the hard carbon film 7 and the conductive polymer film have a bending strength of 500 kg / cm 2 or more and a film thickness of 10 nm or more and 1 μm or less.

The hard carbon film 7 or the conductive polymer film is for preventing the glass of the composite particles 3 from the graphite particles 2 caused by the expansion and contraction of the Si fine particles 5 due to the charge / discharge reaction of lithium ions. The expansion and contraction of the Si fine particles 5 are mechanically suppressed. Therefore, it is preferable that the bending strength of the hard carbon film 7 or the conductive polymer film is 500 kg / cm 2 or more. If the bending strength is less than 500 kg / cm 2, the expansion and contraction of the Si fine particles 5 may not be mechanically pressed, and the composite particles 3 may be released from the graphite particles 2, which is not preferable.

Moreover, when the film thickness of the hard carbon film 7 or the conductive polymer film is less than 10 nm, the binding force between the conductive carbon dog 6 and the Si fine particles 5 is lowered, and the effect of suppressing the volume expansion of the composite particles 3 is reduced. Not desirable In addition, the film thickness of 1 µm or more is not preferable because it causes an increase in irreversible capacity due to amorphous carbon.

In addition, when the conductive polymer film is used, high conductivity can be imparted to the Si fine particles, the apparent conductivity of the Si fine particles 5 can be increased, and the charge and discharge reaction of lithium ions to the Si fine particles 5 can be smoothly progressed. The charge and discharge capacity of the carbonaceous material can be improved.

And it is preferable that content of said composite particle 3 in the carbonaceous material of this invention is 1 weight% or more and 25 weight% or less. When the content of the composite particles 3 is less than 1% by weight, an improved discharge capacity cannot be obtained as compared with the case where only the carbon material is used as the active material, which is not preferable. In addition, if the content exceeds 25% by weight, the contribution of the carbon material portion is small, reaching the reaction potential of Si at the beginning of discharge, and the average voltage of the battery is lowered, which is not preferable, and the distance between the composite particles 3 becomes narrow and It is unpreferable because it aggregates and volume expansion and contraction with respect to Si microparticles | fine-particles 5 occur easily, and a cycling characteristic falls.

When the carbonaceous material 1 reacts with lithium ions, lithium ions are mainly occluded with graphite particles 2 and are combined with the Si fine particles 5 to form an alloy. The conductive carbon material 6 adheres to the surface of the Si fine particles 5, the conductivity is apparently high, and lithium ions are easily alloyed with the Si fine particles 5.

At this time, the volume of the graphite particles 2 and the Si fine particles 5 expands, but since the Si fine particles 5 are covered with the hard carbon film 7 or the conductive polymer film, the volume expansion is mechanically suppressed and the Si fine particles ( The composite particles 3 including 5) can be dissociated from the graphite particles 2 and can contribute to the charge and discharge reaction of the Si fine particles 5, so that the charge and discharge capacity of the carbonaceous material 1 can be achieved. This does not deteriorate.

In addition, the graphite particles 2 and the composite particles 3 are covered with the amorphous carbon film 4 so that the graphite particles 2 do not directly contact the organic electrolyte, and decomposition of the organic electrolyte is suppressed. In addition, the fine particles of Si particles 5 due to volume expansion due to filling are prevented without the composite particles 3 falling off from the graphite particles 2.

Therefore, according to the carbonaceous material 1, the fine particles of the Si fine particles 5 due to the increase in charge and discharge capacity and the volume expansion due to the volume expansion of the Si fine particles 5 and the dropping and filling of the composite particles 3 are obtained. It is possible to suppress the deterioration in cycle characteristics by suppressing this.

The carbonaceous material is, for example. It can manufacture as follows.

The carbonaceous material is produced by a process of producing composite particles and a process of mixing graphite particles with the obtained composite particles and coating them with an amorphous carbon film.

First, in the process of manufacturing a composite factor, the Si fine particles and the conductive carbon material are mixed by dry mixing or wet mixing. In the case of wet mixing, it is preferable to use a dispersion medium such as isopropyl alcohol, acetone, ethanol or water.

Next, the polymer material is dissolved in a suitable solvent, the mixture of Si fine particles and conductive carbon material is mixed with this solution, and then the solvent is removed. By removing a solvent, the composite particle precursor which coat | covered the polymer film in Si fine particle and conductive carbon material is formed.

In addition, it is preferable to use a thermoplastic resin, a thermosetting resin, a vinyl resin, a cellulose resin, a phenol resin, etc. as said polymer material, and also a coal pitch material, a petroleum pitch material, a tar type material, etc. can be used. In particular, it is preferable to use a phenol resin.

Next, the composite particle precursor is heat treated to carbonize the polymer film to form a hard carbon film. The heat treatment is preferably performed in a vacuum atmosphere or in an inert gas atmosphere, the heat treatment temperature is preferably in the range of 800 ° C. to 1200 ° C., and the heat treatment time is preferably 120 minutes or more.

When the heat treatment is performed in a vacuum atmosphere or an inert gas atmosphere, oxidation of the polymer film can be prevented and a good hard carbon film can be formed.

In addition, when the heat treatment temperature is lower than 800 ° C, carbonization is not completely performed, the specific resistance of the hard carbon film is high, and insertion and desorption of lithium ions is difficult to be performed. It is not preferable because silicon is produced. Similarly, if the heat treatment time is less than 120 minutes, it is not preferable because a uniform hard carbon film cannot be formed.

Further, in addition to the above method, a conductive polymer such as polyaniline is dissolved in a suitable solvent, a mixture of Si fine particles and a conductive carbon material is mixed with this solution, and then dried, and a composite factor having the conductive polymer film coated on the Si fine particles and the conductive carbon material is dried. It may be formed.

In this way, composite particles are obtained.

In the next step, graphite particles are mixed with the obtained composite particles by dry mixing or wet mixing. In the case of wet mixing, it is preferable to use a dispersion medium such as isopropyl alcohol, acetone, ethanol or water.

Next, a separate polymer material is dissolved in a suitable solvent, a mixture of composite particles and graphite particles is mixed with this solution, and then the solvent is removed. By removing the solvent, a carbonaceous material precursor having a polymer film coated on the composite particles and the graphite particles is formed.

In addition, it is preferable to use a thermoplastic resin, a thermosetting resin, a vinyl resin, a cellulose resin, a phenol resin, etc. as said polymer material, and also a coal pitch material, a petroleum pitch material, a tar type material, etc. can also be used. In particular, it is preferable to use a phenol resin.

Next, the polymer film is carbonized by heat treatment of the carbonaceous material precursor to form an amorphous carbon film. The heat treatment is preferably performed in a vacuum atmosphere or in an inert gas atmosphere, the heat treatment temperature is preferably in the range of 800 ° C or more and 1200 ° C or less, and the heat treatment time is preferably performed in 120 minutes or more.

When the heat treatment is performed in a vacuum atmosphere or inert gas atmosphere, oxidation of the polymer film is prevented and a good amorphous carbon film can be formed.

In addition, when the heat treatment temperature is lower than 800 ° C., the temperature is low so that carbonization is not completely performed, the specific resistance of the amorphous carbon film is high, and insertion and desorption of lithium ions is difficult to be performed. The carbonization is not preferable because silicon carbide (SiC) is produced and graphitization of the polymer film proceeds to decrease the strength of the carbon film. Similarly, if the heat treatment time is less than 120 minutes, it is not preferable because a uniform amorphous carbon film cannot be formed.

In this way, the carbonaceous material of the present invention is obtained.

A lithium secondary battery can be comprised by the negative electrode which has the said carbonaceous material, the positive electrode which can occlude and discharge | release lithium, and an organic electrolyte.

Examples of the positive electrode include a positive electrode material, an organic disulfide compound, an organic polysulfide compound, and the like, which can occlude and release lithium, such as LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , LiFeO ₂ , V ₂ O ₅ , TiS, and MoS. It can be illustrated that including the anode material of.

As a specific example of the positive electrode or the negative electrode, the binder material and the conductive material may be mixed with the positive electrode material or the carbonaceous material, and these may be applied to a current collector made of metal foil or metal net and molded into a sheet.

As an organic electrolyte, the organic electrolyte solution in which lithium salt is melt | dissolved in an aprotic solvent can be illustrated.

Examples of aprotic solvents include propylene carbonate, ethylene carbonate, butylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyllactone, dioxolane, 4-methyldioxolane and N , N-dimethylformamide, dimethylacetamide, dimethyl sulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, dimethyl carbonate, methyl isopropyl carbonate, ethyl butyl carbonate, Aprotic solvents, such as dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate, diethylene glycol, and dimethyl ether, or the mixed solvent which mixed 2 or more types of these solvent can be illustrated, Especially a propylene carbonate, ethylene carbonate, Dimethyl carbonate, methyl ethyl carbonate, die It is preferable to include any one of the butyl carbonates.

Lithium salts include LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiN (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ) (where x and y are natural numbers), one or two selected from the group consisting of LiCl, LiI, and the like What mixes the above lithium salt can be illustrated, It is preferable that especially any one of LiPF ₆ and LiBF ₄ is included.

In addition, what is conventionally known as an organic electrolyte of a lithium secondary battery can also be used.

As another example of the organic electrolyte, a polymer such as polyethylene oxide (PEO), polyvinyl alcohol (PVA), or the like, in which any one of the lithium salts is mixed, or a polymer having high swellability impregnated with an organic electrolyte solution You may use an electrolyte.

In addition, the lithium secondary battery of the present invention is not limited to the positive electrode, the negative electrode, and the electrolyte, and may be provided with other members, if necessary, and may include, for example, a separator that separates the positive electrode and the negative electrode.

According to the said lithium secondary battery, since the said carbonaceous material 1 is included, a lithium secondary battery with high energy density and excellent cycling characteristics can be comprised.

Hereinafter, preferred embodiments of the present invention will be described. The following examples are only for illustrating the present invention, and the present invention is not limited to the following examples.

Preparation of Carbonaceous Materials

(Examples 1 to 4)

1 weight part of carbon black was mixed with 2 weight part of Si microparticles | fine-particles whose average particle diameter is 390-2500 nm. Most of the Si fine particles consisted of crystalline silicon, and the carbon black had a specific resistance of 10 ⁻⁴ Ω · m or less.

Next, a solution in which 10 parts by weight of the phenol resin was dissolved in isopropyl alcohol was prepared, and the mixture of the above Si fine particles and carbon black was mixed with the solution, and after stirring sufficiently, the solvent was removed. In this way, the composite particle precursor which the carbon black and phenol resin film adhered to the surface of Si microparticles | fine-particles was formed.

Next, the composite particle precursor was heat-treated at 1000 ° C. for 180 minutes in an argon gas atmosphere to carbonize the phenol resin film to form a hard carbon film having a thickness of 0.05 μm.

In addition, when the phenol resin is carbonized alone under the above conditions, the bending strength of the obtained carbide can be estimated to be about 800 kg / cm 2, so that the bending strength of the hard carbon film is about the same. In this way, composite particles were obtained.

Next, 5 parts by weight of the above composite particles were added to 95 parts by weight of natural graphite having an average particle diameter of 15 µm, and isopropyl alcohol was added to wet mix. The surface spacing d002 of the (002) plane by X-ray wide-angle diffraction of natural graphite was 0.3355 nm.

Next, an isopropyl alcohol solution containing 10 parts by weight of a phenol resin was added to the mixture of the natural graphite and the composite particles, followed by evaporation of isopropyl alcohol. In this way, a carbonaceous material precursor formed by adhering the composite particles and the phenol resin film to the surface of natural graphite was formed.

Next, the carbonaceous material precursor was fired at 1000 ° C. (1273 K) in a vacuum atmosphere to carbonize the phenol resin and form an amorphous carbon film having a thickness of 0.05 μm.

In addition, since d002 of the (002) plane of the carbide obtained when carbonizing a phenol resin alone under the above conditions is about 0.39 nm, it is estimated that the surface spacing d002 of the amorphous carbon film is also about the same. In this manner, the carbonaceous materials of Examples 1 to 4 were obtained.

(Example 5)

Instead of carbonizing the phenol resin to obtain an amorphous carbon film, carbon obtained by mixing the composite graphite and the conductive polymer film on the surface of natural graphite by evaporating acetonitrile after mixing a mixture of natural graphite and composite particles in an acetonitrile solution in which polyaniline was dissolved. A carbonaceous material was obtained in the same manner as in Example 1 except that a quality material precursor was formed. In addition, the amount of polyaniline at this time was 10 weight part with respect to the mixture of natural graphite and a composite particle.

(Comparative Example 1)

A carbonaceous material was obtained in the same manner as in Example 1 except that 5 parts by weight of the Si fine particles having a particle size of 390 nm were mixed with 95 parts by weight of natural graphite instead of the composite particles to form a phenol resin film and fired.

Fabrication of test cell for charge and discharge test

Polyvinylidene fluoride was mixed with the carbonaceous materials prepared in Examples 1 to 5, and N-methylpyrrolidone was added to prepare a slurry liquid. This slurry liquid was applied to a copper foil having a thickness of 14 μm by a doctor blade, and dried at 120 ° C. for 24 hours in a vacuum atmosphere to volatilize N-pyrrolidone. Thus, the negative electrode mixture of 100 micrometers in thickness was laminated | stacked on the copper foil. Moreover, content of polyvinylidene fluoride in negative mix was 8 weight%, and the density of negative mix was 1.5 g / cm <3> or more. And the copper foil which laminated | stacked the negative electrode mixture was drilled in the circular shape of diameter 13mm, and the negative electrode was produced.

In addition, the cathode electrode was prepared in the same manner as in Example except that polyvinylidene fluoride and N-methylpyrrolidone were added to the carbonaceous material of Comparative Example 1 and carbon black was added to prepare a slurry solution. Got. The content of polyvinylidene fluoride in the negative electrode mixture was 8% by weight and the content of carbon black was 2.5% by weight.

A separator made of a porous polypropylene film was inserted between the working electrode and the counter electrode by using the cathode electrodes of Examples 1 to 5 and Comparative Example 1 as the working electrode, and the metal lithium foil punched in a circle as the counter electrode. A coin-type test cell was prepared by dissolving LiPF ₆ at a concentration of 1 (mol / L) as a solute in a mixed solvent of dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethylene carbonate (EC). It was.

The charge / discharge test was performed with the charge and discharge current density as 0.2C, the charge end voltage as 0 V (Li / Li ⁺ ) and the discharge end voltage as 1.5 V (Li / Li ⁺ ), and the results are shown in Table 1 below. It was. Table 1 shows the discharge capacity and charge / discharge efficiency in one cycle of the negative electrode material. Furthermore, the capacity ratio (2 cycles / 1 cycle) obtained by dividing the discharge capacity in two cycles by the discharge capacity in one cycle was obtained. The results are shown in Table 1.

 sample Average Particle Size (nm) of Si Fine Particles Discharge Capacity (mAh / g) (1 Cycle) Charge / discharge capacity (%) (1 cycle) Capacity ratio (%) (2 cycles / 1 cycle) Example 1 390 425 90.0 99.7 Example 2 700 440 89.7 98.9 Example 3 1060 451 87.7 98.0 Example 4 2500 456 87.7 95.0 Example 5 390 415 90.0 99.3 Comparative Example 1 390 392 85.7 95.1

As shown in Table 1, it can be seen that the discharge capacity in one cycle is higher than that of Comparative Example 1 in Examples 1 to 5. In addition, when comparing the Examples 1 to 4, it can be seen that the discharge capacity increases as the particle size of the Si fine particles increases.

It can be seen that the charge and discharge efficiency in one cycle is about 2 to 5% higher than in the case of Comparative Examples 2 in Examples 1 to 5. In addition, when comparing the Examples 1 to 4 it can be seen that the charge and discharge efficiency is increased as the particle size of the Si fine particles.

Looking at the capacity ratio, it can be seen that in the case of Examples 1 to 3 it is about 3 to 5% higher than in the case of Comparative Example 1. In addition, in Examples 1 to 3, it can be seen that the capacity ratio increases as the particle diameter of the Si fine particles decreases.

It is understood that the capacity ratio of Example 4, which is the case where the particle diameter of the Si fine particles is 2 μm or more, is low, and when the average particle size of the Si fine particles is 2 μm or more, the effect of suppressing expansion during filling is small.

In this way, the discharge capacity of one cycle is higher than that of Comparative Example 2 in the case of Examples 1 to 5, and in the case of Examples 1 to 3 and Example 5 in terms of other characteristics, It is found to be excellent, in particular, excellent in charge and discharge capacity and cycle characteristics.

In the case of Examples 1 to 3 and Example 5, since the conductive carbon material and the hard carbon film or the conductive polymer film are formed around the Si fine particles, the apparent vapor phase conductivity of the Si fine particles is improved and the Si fine particles are formed by the hard carbon film or the conductive polymer film. It is considered that the discharge capacity and the cycle characteristics are improved because the volume change of is suppressed mechanically and there is no fear that the Si fine particles are liberated from the graphite.

As described above in detail, according to the carbonaceous material of the present invention, since the graphite particles and the Si fine particles occlude Li, the charge and discharge capacity is improved as compared with the graphite particles alone. In addition, by disposing a conductive carbon material around the Si particles having high specific resistance to the graphite particles, the apparent conductivity of the Si particles is improved. In addition, by covering the Si fine particles with a hard carbon film or a conductive polymer film, the volume expansion and contraction of the Si fine particles due to the occlusion and release of Li is mechanically suppressed. In addition, by covering the graphite particles and the composite particles with an amorphous carbon film, the decomposition of the electrolyte solution is suppressed without the graphite particles directly contacting the electrolyte solution, and the composite particles do not fall off from the graphite particles, and the volume expansion by filling It prevents the micronization of the resulting Si fine particles.

As the carbonaceous material of the present invention, it is possible to increase the charge and discharge capacity, to suppress the micronization of the Si fine particles due to the volume expansion of the Si fine particles and the volume expansion due to the dropping and filling of the composite particles, thereby preventing the deterioration of the cycle characteristics. You can do it.

Moreover, according to the lithium secondary battery of the present invention, since the carbonaceous material according to the present invention is provided as a negative electrode, energy density and cycle characteristics can be improved.

Claims (5)

Composite particles containing silicon and carbon and having a smaller particle size than the graphite particles are dispersed and disposed around the graphite particles having an interplanar spacing d002 of the (002) plane by X-ray wide-angle diffraction, In addition, the graphite particles and the composite particles are covered with an amorphous carbon film having a surface spacing d002 of 0.37 nm or more, As for the said composite particle, the conductive carbon material is arrange | positioned around the Si fine particle which consists of crystalline silicon, The negative electrode active material for a lithium secondary battery, wherein the Si fine particles and the conductive carbon material are coated with a hard carbon film or a conductive polymer film. The particle diameter of the graphite particles is in the range of 2 µm or more and 70 µm or less, the particle diameter of the composite particles is in the range of 50 nm or more and 2 µm or less, and the film thickness of the amorphous carbon film is 50 nm or more. The negative electrode active material for lithium secondary batteries which is in the range of 5 micrometers or less. The particle diameter of the said Si microparticles | fine-particles is in the range of 10 nm or more and less than 2 nm, The specific resistance of the said conductive carbon material is 10 ^-4 ohm * m or less, The said hard carbon film or the conductive polymer film of Claim 1 or 2 Bending strength is 500 kg / cm <2> or more and film thickness is 10 nm-1 micrometer or less, The negative electrode active material for lithium secondary batteries characterized by the above-mentioned. The negative electrode active material for lithium secondary battery according to claim 1 or 2, wherein the content of the composite particles is 1% by weight or more and 25% by weight or less. A lithium secondary battery comprising the negative electrode active material of claim 1.

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