JP5731732B2 - Carbon-coated graphite negative electrode material for lithium ion secondary battery, production method thereof, negative electrode for lithium ion secondary battery using the negative electrode material, and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a carbon-coated graphite negative electrode material for a lithium ion secondary battery, a production method thereof, a negative electrode for a lithium ion secondary battery using the negative electrode material, and a lithium ion secondary battery. More specifically, a carbon-coated graphite negative electrode material for lithium ion secondary batteries excellent in discharge load characteristics, cycle characteristics, and charging characteristics, suitable for use in portable electronic devices, electric vehicles, power storage, etc., a method for producing the same, The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery using the negative electrode material.

  The graphite particles used as the negative electrode material include, for example, natural graphite particles, coke, organic polymer materials, artificial graphite particles graphitized pitch, etc., and these natural graphite particles and artificial graphite particles are pulverized There are graphite particles.

  These graphite particles are mixed with an organic binder and a solvent as a negative electrode material to form a graphite paste. This graphite paste is applied to the surface of the copper foil, and the solvent is dried and molded to form a lithium ion secondary battery. It is used as a negative electrode. For example, it is known that by using graphite as the negative electrode material, the problem of internal short circuit caused by lithium dendrite, which occurs when lithium metal is used as the negative electrode material, is solved, and cycle characteristics are improved ( For example, see Patent Document 1).

  However, natural graphite particles in which graphite crystals are developed and artificial graphite particles obtained by graphitizing coke have a weaker bonding force between crystal layers in the c-axis direction than the bonding in the crystal plane direction. By the pulverization to prepare a particle size suitable for the above, the bonds between the graphite layers are broken, and so-called scaly graphite particles having a large aspect ratio are obtained.

  Since the scaly graphite particles have a large aspect ratio, the scaly graphite particles are oriented in the surface direction of the current collector when kneaded with a binder and applied to the current collector to produce an electrode. Distortion in the c-axis direction caused by repeated insertion and extraction of lithium into and from the porous particles causes destruction of the inside of the electrode, resulting in a problem that cycle characteristics are deteriorated, and discharge load characteristics tend to be deteriorated.

  Furthermore, since the scaly graphite particles having a large aspect ratio have a large specific surface area, there is a problem that adhesion with the current collector is poor and a large amount of binder is required. If the adhesion with the current collector is poor, there is a problem that the current collecting effect is lowered and the discharge capacity, discharge load characteristics, cycle characteristics, etc. are lowered.

In addition, scaly graphite particles having a large specific surface area are liable to cause decomposition of the electrolytic solution, and a lithium ion secondary battery using the same has a problem that the irreversible capacity of the first cycle is large.
Furthermore, scaly graphite particles having a large specific surface area have low thermal stability in a state where lithium ions are occluded, and there is a problem in safety when used as a negative electrode material for lithium ion secondary batteries. Therefore, there is a demand for graphitic particles that can improve discharge load characteristics, cycle characteristics, and irreversible capacity in the first cycle.

  As a solution to the above requirement, there has been proposed a graphite particle (hereinafter referred to as “non-oriented graphite particle”) in which flat particles are aggregated or bonded so that a plurality of orientation planes are non-parallel. (For example, refer to Patent Document 2). The lithium ion secondary battery using the non-oriented graphite particles as a negative electrode material has a high charge / discharge capacity and is excellent in charge / discharge load characteristics, cycle characteristics, and charge / discharge efficiency in the first cycle. It can be suitably used for a battery.

  However, in the non-oriented graphite particles, a highly conductive coating (SEI) that is said to be generated on the surface of the negative electrode material in the initial stage of charging is affected by the charge / discharge capacity. Due to the resistance of SEI), there is a problem that the charge / discharge capacity (charge load characteristics) is low. In order to solve this problem, a negative electrode material in which the aspect ratio and pore volume of the non-oriented graphite particles are defined and a carbon layer is coated thereon has been proposed (see, for example, Patent Document 3). However, since charging characteristics, particularly charging performance at low temperatures, are not fully satisfactory, better charging characteristics and further charging performance at low temperatures are required for the negative electrode material for lithium ion secondary batteries.

  In addition, it has been proposed to coat the surface of graphite particles with low crystalline carbon (see, for example, Patent Document 4). Specifically, a negative electrode material using a mixture of a carbonaceous material as a nucleus, a carbonaceous material particle having a multiphase structure composed of a carbon surface layer covering the core, and a carbonaceous material particle having a single phase structure is used as a lithium ion secondary battery. A lithium ion secondary battery having a large electrode capacity and improved charge / discharge cycle characteristics has been proposed. However, the negative electrode material using a mixture of carbonaceous material particles having a multiphase structure and carbonaceous material particles having a single phase structure does not sufficiently improve the irreversible capacity and charge / discharge characteristics of the first cycle.

Japanese Examined Patent Publication No. 62-023433 JP-A-10-158005 International Publication No. 2005/024980 Pamphlet Japanese Patent Laid-Open No. 05-307777

  The present invention provides a negative electrode material for a lithium ion secondary battery that maintains the characteristics of non-oriented graphite particles and improves high-speed charge / discharge load characteristics and low-temperature charge performance, a method for producing the same, and lithium ion secondary using the negative electrode material. An object of the present invention is to provide a negative electrode for a secondary battery and a lithium ion secondary battery.

As a result of examining the surface of the non-oriented graphite particles covered with low crystalline carbon, the present inventors have found that the charge load characteristics are improved.
However, even if the surface of non-oriented graphite particles is simply coated with low crystalline carbon, the low crystalline carbon itself has a large irreversible capacity, so the initial charge / discharge efficiency is reduced due to the large irreversible capacity of the first cycle. Will occur. In addition, the problem is that the particles become hard due to the low crystalline carbon, which causes easy peeling at the electrode after pressing, the characteristics of the non-oriented graphite particles are lost, and the characteristics of the obtained lithium ion secondary battery are It turns out that it falls.
Therefore, the inventors of the present invention have studied sharply to reduce the irreversible capacity, avoid peeling at the electrode after pressing, and to improve the low-temperature charging performance. It was coated with a carbon layer by increasing the affinity with lithium ions, and by setting the ratio of the carbon layer covering the surface of non-oriented graphite particles and the nitrogen element concentration of the entire negative electrode material to a specific value. The inventors have found that the graphite particles can be prevented from becoming too hard, and arrived at the carbon-coated graphite negative electrode material for lithium ion secondary batteries of the present invention, which can solve the above problems.

The present invention relates to the following matters.
(1) A carbon-coated graphite negative electrode material having graphite particles as nuclei and a carbon layer covering the graphite particles,
The core graphite particles have a structure in which a plurality of flat particles are assembled or bonded non-parallel to each other,
R value is the intensity ratio of the peak intensity (IG) in the peak intensity in the range of 1300～1400Cm ^-1 as measured by Raman spectroscopy and (ID) in a range of 1560~1650cm ^-1 (ID / IG) is zero .3 or less,
A carbon-coated graphite negative electrode material for a lithium ion secondary battery, wherein the nitrogen element concentration on the surface of the carbon-coated graphite negative electrode material measured by X-ray photoelectron spectroscopy (XPS) is 1.5 to 10 at%.

(2) The carbon-coated graphite negative electrode material for a lithium ion secondary battery as described in (1) above, wherein a ratio (mass ratio) of carbon to the graphite particles is 0.001 to 0.02.
(3) The carbon-coated graphite negative electrode material has an average particle diameter of 10 to 30 μm, a true specific gravity of 2.10 or more, a specific surface area by nitrogen gas adsorption of 0.5 to 10 m ² / g, and is measured by a Raman spectrum. the R value is the intensity ratio of the peak intensity (IG) in the peak intensity in the range of 1300～1400Cm ^-1 and (ID) in a range of 1560~1650cm ^-1 (ID / IG) is 0.3 or less ( The carbon-coated graphite negative electrode material for lithium ion secondary batteries according to 1) or (2).

(4) Agglomerated structure in which a plurality of flat particles are assembled or bonded non-parallel to each other, and graphite particles having an aspect ratio of 5 or less are mixed into a mixed solution of a nitrogen-containing polymer compound and a solvent for dissolving the same. Dispersing and mixing,
Removing the solvent to produce graphite particles coated with the nitrogen-containing polymer compound;
Calcining the graphite particles coated with the nitrogen-containing polymer compound to obtain a carbon-coated graphite negative electrode material for a lithium ion secondary battery, and carbon-coated graphite for a lithium ion secondary battery Manufacturing method of negative electrode material.

(5) The carbon-coated graphite negative electrode material for a lithium ion secondary battery according to any one of (1) to (3) or the carbon for a lithium ion secondary battery produced by the production method according to (4). A graphite negative electrode for a lithium ion secondary battery using a coated graphite negative electrode material.
(6) A lithium ion secondary battery using the graphite negative electrode for lithium ion secondary batteries according to (5).

  The graphite negative electrode material for a lithium ion secondary battery according to the present invention is excellent in discharge capacity, charge / discharge efficiency, and charge load characteristics. Therefore, a lithium ion secondary battery using the graphite negative electrode material is a portable electronic device, Suitable for automobiles, power storage and the like.

The negative electrode material for a lithium ion secondary battery of the present invention is a carbon-coated graphite negative electrode material having graphite particles as cores and a carbon layer covering the graphite particles,
The graphite particles serving as the nucleus have a structure in which a plurality of flat particles are assembled or bonded non-parallel to each other, and have a peak intensity (ID) in a range of 1300 to 1400 cm ⁻¹ measured by a Raman spectrum. ) And the R value (ID / IG) which is the intensity ratio of the peak intensity (IG) in the range of 1560 to 1650 cm ⁻¹ is 0.3 or less,
The nitrogen element concentration on the surface of the carbon-coated graphite negative electrode material measured by X-ray photoelectron spectroscopy (XPS) is 1.5 to 10 at%.
Moreover, it is preferable that the ratio (mass ratio) of the carbon with respect to the said graphite particle is 0.001-0.02.

When the carbon ratio of the carbon layer is less than 0.001 with respect to the graphite particles, the improvement width of the charge load characteristics tends to be small. On the other hand, when the carbon ratio of the carbon layer exceeds 0.02, the initial charge / discharge efficiency tends to decrease. Further, the particles of the carbon-coated graphite negative electrode material become hard, and the spring back due to elastic deformation becomes large when the electrode is pressed, and the electrode is likely to be peeled after pressing. The carbon ratio is more preferably 0.005 to 0.018, still more preferably 0.008 to 0.016.
The carbon ratio of the carbon layer can be calculated from the carbonization rate of the carbon layer precursor described later and the mass of the carbon layer precursor coated on the graphite particles. The “carbon layer precursor” refers to a nitrogen-containing polymer compound that becomes a carbon layer, and details will be described later.
The carbonization rate of the precursor of the carbon layer is measured as follows. The carbonization rate is measured when the nitrogen-containing polymer compound alone that is the precursor of the carbon layer is heated to 900 ° C. at 20 ° C./h in a nitrogen stream and held for 1 hour.
In the present invention, the “carbon ratio” is the mass ratio of the carbon content of the graphite particles to the carbon content of the carbonized nitrogen-containing polymer compound in the carbon layer.
The carbon-coated graphite negative electrode material of the present invention has a surface nitrogen element concentration of 1.5 to 10 at%. More preferably, it is 1.5-8 at%. When the concentration of nitrogen element on the surface of the carbon-coated graphite negative electrode material is less than 1.5 at%, the proportion of the carbon layer is small, so that it is greatly affected by the coating (SEI) generated on the surface of the negative electrode material at the initial stage of charging. There is a tendency for characteristics to deteriorate. Moreover, when it exceeds 10 at%, since the ratio of a carbon layer will be too large, there exists a tendency for a first time charge / discharge efficiency to fall.
As a method for measuring the nitrogen element concentration on the surface of the carbon-coated graphite negative electrode material, it is preferable to use X-ray photoelectron spectroscopy (XPS). In the present invention, for example, “AXIS165” manufactured by Shimadzu Corporation / Crates Analytical Co., Ltd. can be used.

  As the graphite particles serving as the core, it is preferable that the artificial graphite is a block because the characteristics (cycleability, discharge load characteristics, etc.) of the lithium ion secondary battery using the obtained graphite particles can be improved. In particular, the massive artificial graphite particles are preferably graphite particles having a structure in which a plurality of flat particles are aggregated or bonded non-parallel to each other (hereinafter also referred to as “non-oriented graphite particles”). When the surface of such non-oriented graphite particles is coated with a carbon layer containing nitrogen element, more excellent cycle characteristics, discharge load characteristics, and charge characteristics can be achieved as a negative electrode material.

In the present invention, the “flat particle” means a shape having a major axis and a minor axis and is not a perfect sphere. For example, those having a shape such as a scale shape, a scale shape, or a partial lump shape are included. In the present invention, a flat particle refers to a particle unit observed when observed with, for example, a scanning electron microscope (SEM).
In addition, “the plurality of flat particles are not parallel to each other” means that the flat surfaces of the respective flat particles, in other words, the surfaces that are closest to the flat surface are the alignment surfaces, and the plurality of flat primary particles A state in which the particles are aggregated without aligning their orientation planes in a certain direction to form graphite particles.

In the graphite particles, the flat particles are aggregated or bonded non-parallel to each other. The bond is a carbonaceous material in which the flat particles are graphitized with a graphitizable binder for graphite particles, which will be described later. It means a state of being chemically bonded via. The term “aggregate” refers to a state in which the particles are not chemically bonded but the shape of the aggregate is maintained due to the shape and the like. From the viewpoint of mechanical strength, those bonded are preferable.
The massive structure refers to a structure in which flat particles are gathered or bonded non-parallel as described above. In one carbon particle, the number of flat particles aggregated or bonded is preferably 3 or more. Although there is no restriction | limiting in particular as a magnitude | size of each flat particle, It is preferable that it is 2/3 or less of the average particle diameter of the graphite particle | grains which these aggregated or couple | bonded.

The non-oriented graphite particles in the present invention preferably have an aspect ratio of 5 or less.
The aspect ratio is represented by A / B, where A is the length in the major axis direction of the non-oriented graphite particles and B is the length in the minor axis direction. The aspect ratio in the present invention is obtained by enlarging non-oriented graphite particles with an electron microscope, arbitrarily selecting 20 non-oriented graphite particles, measuring A / B, and taking the average value. is there.
When determining the major and minor axes of non-oriented graphite particles, enlarge the non-oriented graphite particles with a scanning electron microscope (SEM) and observe the non-oriented graphite particles from various directions. Then, taking into consideration the three-dimensional characteristics of the non-oriented graphite particles, the length in the major axis direction of the non-oriented graphite particles is determined as A and the length in the minor axis direction is determined as B.
For example, when the non-oriented graphite particles are approximately spherical, such as spherical, spherical, and massive, the longest part of the non-oriented graphite particles projected in the two-dimensional field of view in the SEM image Is the long axis A, and the longest portion orthogonal to the long diameter is the short axis B.
Further, when the non-oriented graphite particles are thin, flat, and have a thickness direction such as a scale shape, a plate shape, or a block shape, the minor axis B is the thickness of the particle. In the case of non-oriented graphite particles such as rod-like and needle-like, the long axis A is the length of the non-oriented graphite particles, and the short axis B is the rod-like (or needle-like) non-oriented graphite. It becomes the thickness of the particle. In addition, for example, when the shape of non-oriented graphite particles is changed by applying mechanical force, etc., the non-oriented graphite particles are observed from various directions and the tertiary of non-oriented graphite particles is observed. Considering the original characteristics, the values of the major axis A and the major axis B are determined as described above after approximately determining the shape of the non-oriented graphite particles.

  The non-oriented graphite particles in the present invention preferably have a true specific gravity of 2.2 or more, a Raman spectrum peak ratio of preferably 0.05 or less, and a nitrogen element concentration of 0.1 at% or less.

Non-oriented graphite particles can be produced, for example, by the method disclosed in JP-A-10-158005. Specifically, it is as follows.
That is, the non-oriented graphite particles are mixed by adding a graphitization catalyst to a mixture of a flat graphitizable aggregate or flat graphite and a graphitizable graphite particle binder, and firing. It is obtained by graphitization to obtain a graphitized product and pulverization.
As the graphitizable aggregate, various cokes such as fluid coke and needle coke can be used.
Further, already aggregated flat aggregates such as natural graphite and artificial graphite may be used.
The particle size of the aggregate or graphite is preferably smaller than the particle size after pulverizing the graphitized material.

As the binder for graphitizable particles that can be graphitized, various pitches and tars such as coal, petroleum, and artificial can be used.
As the graphitization catalyst, iron, nickel, titanium, boron and the like, carbides thereof, oxides, nitrides, and the like can be used.

  The graphitization catalyst is preferably added in an amount of 1 to 50 parts by mass with respect to 100 parts by mass of the aggregate of graphitizable aggregate or graphite and a graphitizable binder for graphite particles. If the amount is less than 1 part by mass, the development of the graphite particle crystals tends to deteriorate, and the charge / discharge capacity tends to decrease. On the other hand, when it exceeds 50 mass parts, it will become difficult to mix uniformly and workability | operativity will fall.

The firing is preferably performed in an atmosphere in which the mixture is not easily oxidized. Examples of such an atmosphere include a method of firing in a nitrogen atmosphere, argon gas, and vacuum.
The graphitization temperature is preferably 2000 ° C. or higher, more preferably 2500 ° C. or higher, and further preferably 2800 ° C. or higher. When the graphitization temperature is less than 2000 ° C., the development of graphite crystals deteriorates and the graphitization catalyst tends to remain in the produced graphite particles, and the obtained graphite particles were used as a lithium ion secondary battery. At the same time, there is a tendency that the charge / discharge capacity decreases, the cycle characteristics decrease, and the safety decreases.

Next, the graphitized material obtained is pulverized to obtain non-oriented graphite particles. The method for pulverizing the graphitized material is not particularly limited, and known methods such as a jet mill, a vibration mill, a pin mill, and a hammer mill can be used. The average particle diameter (median diameter) after pulverization is preferably 10 to 30 μm. The average particle size can be adjusted by obtaining particles having a desired size using, for example, a pulverizer or a sieve.
The average particle size is measured as follows, for example. Using a laser diffraction particle size distribution device “SALD-3000” manufactured by Shimadzu Corporation, the particle size at 50% D is defined as the average particle size.
In order to set the aspect ratio of the non-oriented graphite particles to 5 or less, the pulverization method may be adjusted such that extreme pulverization is not performed.
In the present invention, the true specific gravity of the non-oriented graphite particles is preferably 2.2 or more, the Raman spectrum peak ratio is preferably 0.05 or less, and the nitrogen element concentration is preferably 0.1 at% or less.
In order to set the true specific gravity of the non-oriented graphite particles to 2.2 or more, heat treatment may be performed at 2000 ° C. or more.
In order to set the Raman spectrum peak ratio to 0.05 or less, for example, it may be adjusted by controlling the graphitization temperature of graphite serving as a nucleus, the coating amount of the carbon layer, and the ratio of carbon.
In order to set the nitrogen element concentration to 0.1 at% or less, for example, it may be adjusted by controlling the carbon layer coverage and the carbon ratio.

The carbon-coated graphite negative electrode material for a lithium ion secondary battery of the present invention is prepared by dispersing and mixing the non-oriented graphite particles serving as nuclei in a solution in which a nitrogen-containing polymer compound is dissolved, and then removing the solvent. A carbon-coated graphite negative electrode material coated with a carbon layer can be prepared by preparing graphite particles coated with a nitrogen-containing polymer compound serving as a precursor of the carbon layer and firing the particles.
In order to adjust the nitrogen element concentration on the surface of the carbon-coated graphite negative electrode material to 1.5 to 10 at%, specifically, the carbon ratio of the carbon layer and the coating amount of the carbon layer may be adjusted.
In addition, in order for the carbon ratio (mass ratio) to the non-oriented graphite particles that are nuclei to be 0.001 to 0.02, the carbonization rate of the nitrogen-containing polymer compound that is the precursor of the carbon layer is considered. The amount of the nitrogen-containing polymer compound coated on the non-oriented graphite particles may be adjusted as appropriate.

  Nitrogen-containing polymer compounds used in the present invention include polyacrylonitrile, urethane resin, polyimide, polyamide, phenol resin, epoxy resin, urea resin, poly (meth) acrylate, etc., and nitrogen-containing as a curing agent. And a high molecular compound containing a nitrogen atom.

  In the present invention, the nitrogen-containing polymer compound is used as a solution in order to coat the surface of graphite particles serving as a nucleus as a precursor of a carbon layer. The solvent used at this time is not particularly limited as long as it dissolves the polymer compound. For example, water, tetrahydrofuran, toluene, xylene, benzene, quinoline, pyridine, N-methyl-2-pyrrolidone and the like can be used alone or in combination. Moreover, the mixture (creosote oil) of the liquid substance of the comparatively low boiling point produced | generated in the case of coal dry distillation can also be used.

  The non-oriented graphite particles serving as nuclei were dispersed and mixed in a solution in which the nitrogen-containing polymer compound was dissolved, and then the solvent was removed, and the solution was coated with the nitrogen-containing polymer compound that is the precursor of the carbon layer. Graphite particles are produced. When water is used as a solvent, it is preferable to add a surfactant in order to promote the dispersion of the graphite particles in the solution and improve the adhesion between the nitrogen-containing polymer compound and the graphite particles.

The removal of the solvent can be performed by heating in a normal pressure or reduced pressure atmosphere. The temperature at the time of removing the solvent is preferably 200 ° C. or lower when the atmosphere is air. Above 200 ° C, oxygen and nitrogen-containing polymer and solvent in the atmosphere (especially when creosote oil is used)
Reaction, the amount of carbon produced by firing fluctuates, and the carbon layer becomes more porous, the characteristic range of the present invention as a negative electrode material (nitrogen element concentration on the surface of the carbon-coated graphite negative electrode material, the intensity ratio R of the Raman spectrum) Value, the carbon ratio of the carbon layer to the graphite particles), and the desired characteristics may not be achieved.

  After removing the solvent, the graphite particles coated with the nitrogen-containing polymer compound as a carbon layer precursor are then fired, and the nitrogen-containing polymer compound is carbonized to obtain carbon-coated graphite particles coated with the carbon layer. obtain. Prior to this firing, the graphite particles coated with the nitrogen-containing polymer compound may be pre-fired by heating at a temperature of 150 to 300 ° C. For example, when polyacrylonitrile is used, the carbon ratio of the carbon layer can be increased by such heat treatment, and the crystallinity of the carbon layer can be improved.

  Firing of the graphite particles coated with the nitrogen-containing polymer compound is preferably performed in a non-oxidizing atmosphere. Examples of such an atmosphere include an inert gas atmosphere such as nitrogen, argon, and helium, a vacuum atmosphere, and a circulated combustion exhaust gas atmosphere.

  The maximum temperature for firing is preferably 700 to 1400 ° C, more preferably 800 to 1200 ° C, and still more preferably 850 to 1100 ° C. Below 700 ° C., when used as a negative electrode material, the irreversible capacity of the first cycle tends to increase. On the other hand, when the temperature exceeds 1400 ° C., the nitrogen atoms contained are desorbed, and the nitrogen element concentration of the carbon-coated graphite negative electrode material is remarkably reduced.

  The carbon-coated graphite particles produced as described above are subjected to a pulverization treatment, a classification treatment, and a sieving treatment as necessary, whereby the negative electrode material for a lithium secondary battery of the present invention can be obtained.

The carbon-coated graphite negative electrode material for a lithium ion secondary battery of the present invention has an average particle size of 10 to 30 μm, a true specific gravity of 2.10 or more, a specific surface area of 0.5 to 10 m ² / g by nitrogen gas adsorption, and a Raman spectrum. in R value is the intensity ratio of the peak intensity in the range of 1300～1400Cm ^-1 measured (ID) and the peak intensity in the range of ^{1560~1650cm -1 (IG) (ID /} IG) is 0.3 or less It is characterized by being.

The average particle diameter is a value measured as 50% D (median diameter) using a laser diffraction particle size distribution analyzer (for example, “SALD-3000” manufactured by Shimadzu Corporation). When the average particle diameter of the carbon-coated graphite negative electrode material is less than 10 μm, the specific surface area tends to increase and the initial charge / discharge efficiency tends to decrease. On the other hand, when the average particle size of the carbon-coated graphite negative electrode material exceeds 30 μm, unevenness is likely to occur on the electrode surface due to the size of the particle size, which may cause a short circuit of the battery.
The average particle diameter can be adjusted by obtaining particles having a desired size using a pulverizer or a sieve.

When the true specific gravity of the carbon-coated graphite negative electrode material is less than 2.10, it means that graphitization has not progressed sufficiently, and the discharge capacity tends to decrease. In order to adjust the true specific gravity, for example, heat treatment may be performed at 2000 ° C. or higher.
The true specific gravity can be measured by a butanol substitution method using a butanol pycnometer.
The specific surface area is calculated according to the BET method by measuring the nitrogen adsorption amount at the liquid nitrogen temperature. When the specific surface area of the carbon-coated graphite negative electrode material is less than 0.5 m ² / g, the carbon layer of the graphite particles is generally excessive, and the irreversible capacity of the first cycle tends to increase and the electrode adhesion tends to decrease. On the other hand, the specific surface area exceeding 10 m ² / g is seen when the carbon layer is made porous for some reason, which increases the irreversible capacity of the first cycle because the crystallinity of the carbon layer is lowered. This is not preferable.

The R value is an intensity ratio of a peak intensity (IG) in the peak intensity in the range of 1300～1400Cm ^-1 as measured by Raman spectroscopy and (ID) in a range of 1560~1650cm ^-1 (ID / IG) is there.
During the Raman spectrum measured with an argon laser beam having a wavelength of 5145 Å, a peak IG ranging 1580～1620Cm ^-1 is highly crystalline carbon, the peak ID in the range of 1350 ^-1 corresponds to the low crystalline carbon .

In the present invention, the ratio of these peak heights (R = ID / IG) is preferably 0.3 or less, and more preferably 0.25 or less. When the R value exceeds 0.3, the carbon layer is excessive, the irreversible capacity of the first cycle increases, and the electrode adhesion tends to decrease.
The R value is measured using, for example, “NRS-2100” manufactured by JASCO Corporation, an argon laser output of 10 mW, a spectroscope F signal, an incident slit width of 800 μm, an integration count of 2 times, and an exposure time of 30 seconds. The ID is measured and calculated.
In order to make the R value 0.3 or less, the carbon ratio of the carbon layer of the graphite particles to the graphite particles (mass ratio) may be 0.001 to 0.02.

The carbon-coated graphite negative electrode material for a negative electrode of a lithium ion secondary battery in the present invention is mixed with an organic binder, a solvent such as a solvent or water, and if necessary, a thickener, and is applied to a current collector to apply the solvent or water. It can be set as the negative electrode for lithium secondary batteries by drying and pressure-molding.
Next, the lithium ion secondary battery negative electrode will be described. The carbon-coated graphite negative electrode material of the present invention is generally kneaded with an organic binder and a solvent, and formed into a sheet shape, a pellet shape, or the like.

  Examples of organic binders include ethylenic polymers such as styrene-butadiene copolymer, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, and hydroxyethyl (meth) acrylate. Saturated carboxylic acid esters, ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid, and polymer compounds having high ionic conductivity can be used.

As the polymer compound having a high ionic conductivity, polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile and the like can be used.
The content of the organic binder is preferably 1 to 20 parts by mass with respect to 100 parts by mass of the raw material mixture of the negative electrode material and the organic binder and the like.

Furthermore, as the thickener of the negative electrode slurry for adjusting the viscosity, carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, casein, etc. It is also preferable to use it together with a system binder.
Although there is no restriction | limiting in particular as a solvent used for mixing of an organic type binder, N-methylpyrrolidone, dimethylacetamide, dimethylformamide, (gamma) -butyrolactone, etc. are used.

The negative electrode material is kneaded with an organic binder, a solvent, etc., and after adjusting the viscosity with the above thickener, this is applied to, for example, a current collector, dried, and pressure-molded to form the current collector. It is made into the negative electrode for lithium ion secondary batteries by integrating.
The preferable electrode density of the negative electrode for lithium ion secondary batteries of the present invention is 1.6 to 1.9 g / cm ³ , more preferably 1.65 to 1.85 / cm ³ .
As the current collector, for example, a foil such as nickel or copper, a mesh, or the like can be used. The integration can be performed by a molding method such as a roll or a press.

The negative electrode thus obtained can be made into a lithium ion secondary battery by, for example, disposing the positive electrode opposite to each other with a separator interposed therebetween and injecting an electrolytic solution. It is within the scope of the present invention.
The lithium ion secondary battery of the present invention is superior in quick charge / discharge characteristics, cycle characteristics, and charging characteristics, and has a small irreversible capacity and safety compared to a lithium ion secondary battery using a conventional carbon material as a negative electrode. It will be excellent.

  The material used for the positive electrode of the lithium ion secondary battery of the present invention is not particularly limited, and a lithium-containing metal composite oxide containing at least one kind of metal selected from lithium and iron, cobalt, nickel, and manganese. preferable. For example, lithium manganese composite oxide, lithium cobalt composite oxide, lithium nickel composite oxide, or the like is used. These lithium-containing composite oxides further include at least one metal selected from Al, V, Cr, Fe, Co, Sr, Mo, W, Mn, B, Mg, lithium sites, manganese, cobalt, nickel, etc. It is also possible to use a lithium-containing metal composite in which these sites are substituted.

The active material used for the positive electrode of the lithium ion secondary battery is preferably a general formula Li _x Mn _y O ₂ (x is in the range of 0.2 ≦ x ≦ 2.5, and y is in the range of 0.8 ≦ x ≦ Lithium manganese composite oxide represented by the following formula: These active materials are used alone or in combination of two or more. In addition, you may use a positive electrode active material combining a conductive support agent.

Examples of the conductive assistant include graphite particles and carbon black. These conductive assistants may be used alone or in combination of two or more.
The positive electrode includes the above positive electrode active material, the same organic binder as the organic binder used in the negative electrode, such as polyvinylidene fluoride, and N-methyl-2-pyrrolidone, which is a solvent used in the negative electrode, γ -A positive electrode slurry is prepared by mixing with a solvent such as butyl lactone, this positive electrode slurry is applied to at least one surface of a current collector such as an aluminum foil, the solvent is then removed by drying, and rolling is performed as necessary. Can be produced.

As an electrolytic _{solution, LiClO 4, LiBF 4, LiI} , LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, LiAlCl 4, LiCl, LiBr, LiB (C 2 H 5), LiCH 3 SO ₃ , LiC ₄ F ₉ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, Li {(CO ₂ ) ₂ } _{2 and} other lithium salts such as propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate Carbonates such as methyl ethyl carbonate and vinylene carbonate; lactones such as γ-butyl lactone; trimethoxymethane, 1,2-dimethoxyethane, diethyl ether, 2-ethoxyethane, tetrahydrofuran, 2-methyltetrahydrofura Ethers such as dimethyl sulfoxide; nitrogen-containing compounds such as acetonitrile, nitromethane, N-methyl-2-pyrrolidone; esters such as methyl formate, methyl acetate, butyl acetate, ethyl propionate, phosphate triester Glymes such as diglyme, triglyme and tetraglyme; ketones such as acetone, diethyl ketone, methyl ethyl ketone and methyl isobutyl ketone; sulfolanes such as sulfolane; oxazolidinones such as 3-methyl-2-oxazolidinone; 1,3-propane; A so-called organic electrolyte solution dissolved in a non-aqueous solvent such as a simple substance or a mixture of sultones such as sultone, 4-butane sultone, naphtha sultone, and the like can be used.

  As the separator, for example, a nonwoven fabric, a cloth, a microporous film, or a combination thereof, which is mainly composed of polyolefin such as polyethylene or polypropylene, can be used.

  The method for producing the lithium ion secondary battery of the present invention is not particularly limited except that the carbon-coated graphite negative electrode material of the present invention or the negative electrode for lithium ion secondary batteries of the present invention is used. It can be produced by using a material such as an electrolyte solution for a secondary battery and a separator, and utilizing a known method for producing a lithium ion secondary battery.

  Although there is no restriction | limiting in particular about the manufacturing method of a lithium ion secondary battery, All can use a well-known method. For example, first, two electrodes of a positive electrode and a negative electrode are wound through a separator made of a polyethylene microporous film. The obtained spiral wound group is inserted into a battery can, and a tab terminal previously welded to a negative electrode current collector is welded to the bottom of the battery can. Inject the electrolyte into the resulting battery can, weld the tab terminal that was previously welded to the positive electrode current collector to the battery lid, and place the lid on the top of the battery can via an insulating gasket A battery is obtained by caulking and sealing the part where the lid and the battery can are in contact.

EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not restrict | limited by these.
(Preparation of graphite particles)
100 parts by mass of coke powder having an average particle diameter of 5 μm as an aggregate of graphite particles, 40 parts by mass of tar pitch and 20 parts by mass of coal tar as a binder for graphite particles, and silicon carbide 25 having an average particle diameter of 48 μm as a graphitization catalyst Mass parts were mixed and mixed at 270 ° C. for 1 hour.

  The obtained mixture is pulverized, pressed into pellets, calcined at 900 ° C. in nitrogen, graphitized at 3000 ° C. using an Atchison furnace, pulverized using a hammer mill, passed through a 200 mesh standard sieve, and graphitic. Particles were made.

  According to the scanning electron microscope (SEM) photograph of the obtained graphite particles, the graphite particles had a structure in which a plurality of flat particles were aggregated or bonded so that the orientation planes were non-parallel. Table 1 shows the physical property values of the obtained graphite particles. The measuring method of each physical property value of the graphite particles is as follows.

(1) Average particle diameter: A laser diffraction particle size distribution measuring device “SALD-3000” manufactured by Shimadzu Corporation was used, and the particle diameter at 50% D was defined as the average particle diameter. The sample was mixed with ion-exchanged water to which a surfactant (polyoxyethylene (20) sorbitan monolaurate) was added and dispersed by irradiation with ultrasonic waves for 1 minute, followed by measurement.

(2) True specific gravity: measured by a butanol substitution method.
(3) Specific surface area: Nitrogen adsorption at liquid nitrogen temperature was measured by a multipoint method using ASAP 2010 manufactured by Micromeritics, and calculated according to the BET method.

(4) Raman spectrum peak intensity ratio: measured using “NRS-2100” manufactured by JASCO Corporation, argon laser output 10 mW, spectrometer F single, incident slit width 800 μm, number of integrations 2 times, exposure time 30 seconds. Went.
(5) Aspect ratio: An image obtained by enlarging the graphite particles with a scanning electron microscope (SEM) was obtained, 20 graphite particles were arbitrarily selected, A / B was measured, and an average value thereof was taken.

Example 1
First, polyacrylonitrile was synthesized by the following method.
In a 1.0-liter separable flask equipped with a stirrer, a thermometer, and a condenser, 45.0 g of nitrile group-containing monomer acrylonitrile (manufactured by Wako Pure Chemical Industries, Ltd.), lauryl acrylate (Aldrich) under a nitrogen atmosphere 5.0 g (0.0232 mol ratio relative to 1 mol of acrylonitrile), 1.175 mg of polymerization initiator potassium persulfate (manufactured by Wako Pure Chemical Industries, Ltd.), α-methylstyrene dimer as a chain transfer agent A reaction solution was prepared by adding 135 mg (manufactured by Wako Pure Chemical Industries, Ltd.) and 450 ml of purified water (manufactured by Wako Pure Chemical Industries, Ltd.). The reaction solution was stirred at 60 ° C. for 3 hours and at 80 ° C. for 3 hours while stirring vigorously. After cooling to room temperature, the reaction solution was suction filtered, and the precipitated resin was filtered off. The filtered resin was sequentially washed with 300 ml of purified water (manufactured by Wako Pure Chemical Industries, Ltd.) and 300 ml of acetone (manufactured by Wako Pure Chemical Industries, Ltd.). The washed resin was dried for 24 hours with a vacuum tube dryer at 60 ° C./1 torr to obtain polyacrylonitrile.
900 g of N-methyl-2-pyrrolidone solution in which 10 g of polyacrylonitrile obtained above was dissolved was put in a flask equipped with a condenser, and 500 g of graphite particles shown in Table 1 were added thereto. While stirring, the mixture was heated to 200 ° C. in an oil bath and mixed for 1 hour.

  Next, the solution containing the graphite particles is transferred to a rotary evaporator to remove N-methyl-2-pyrrolidone, and further dried at 120 ° C. for 1 hour using a vacuum dryer, and coated with polyacrylonitrile as a carbon layer precursor. Graphite particles were obtained.

  The obtained polyacrylonitrile-coated graphite particles were heat-treated at 170 ° C. for 5 hours, then heated to 900 ° C. at a rate of temperature increase of 20 ° C./h under nitrogen flow and held for 1 hour. Carbon-coated graphite particles coated with a layer were obtained.

The obtained carbon-coated graphite particles were pulverized with a cutter mill and passed through a 250 mesh standard sieve to obtain a carbon-coated graphite negative electrode material 1.
In order to determine the carbon ratio of the carbon layer to the graphite particles as the nucleus, the carbonization rate was measured when polyacrylonitrile alone was heated to 900 ° C. at 20 ° C./h in a nitrogen stream and held for 1 hour. . The carbonization rate was 52%. Was calculated carbon ratios for graphite particles of this value and polyacrylonitrile coating amount of carbon layer, it was 0.01.
The average particle diameter, true specific gravity, specific surface area, R value (Raman spectrum peak ratio), and nitrogen element concentration of the carbon-coated graphite negative electrode material 1 were measured in the same manner as the above graphite particles. The results are shown in Table 2.

(Example 2)
900 g of N-methyl-2-pyrrolidone solution in which 10 g of polyimide (trade name “HCI-7000” manufactured by Hitachi Chemical Co., Ltd.) was dissolved was placed in a flask equipped with a condenser, and the graphite shown in Table 1 500 g of particles were added. While stirring, the mixture was heated to 200 ° C. in an oil bath and mixed for 1 hour.

  Next, the solution containing the graphite particles is transferred to a rotary evaporator to remove N-methyl-2-pyrrolidone, and further dried at 120 ° C. for 1 hour using a vacuum dryer, so that the polyimide is coated as a carbon layer precursor. Graphite particles were obtained.

The obtained polyimide-coated graphite particles were heated to 900 ° C. at a rate of temperature increase of 20 ° C./h under nitrogen flow, and held for 1 hour to obtain carbon-coated graphite particles.
Next, the obtained carbon-coated graphite particles were pulverized with a cutter mill and passed through a 250 mesh standard sieve to obtain a carbon-coated graphite negative electrode material 2.

In order to obtain the carbon ratio of the carbon layer to the graphite particles as the nucleus, the carbonization rate was measured when the polyimide alone was heated to 900 ° C. at 20 ° C./h in a nitrogen stream and held for 1 hour. The carbonization rate was 50%. It was calculated carbon ratios than this value, and polyimide coating amount relative to graphite particles in the carbon layer, was 0.01.
The average particle diameter, true specific gravity, specific surface area, R value (Raman spectrum peak ratio), and nitrogen element concentration of the carbon-coated graphite negative electrode material 2 were measured in the same manner as the above graphite particles. The results are shown in Table 2.

(Comparative Example 1)
As Comparative Example 1, the graphite particles shown in Table 1 were used as the negative electrode material 1a instead of the carbon-coated graphite negative electrode material.
The average particle diameter, true specific gravity, specific surface area, R value (Raman spectrum peak ratio), and nitrogen element concentration of the negative electrode material 1a were measured in the same manner as the above graphite particles. The results are shown in Table 2.
(Comparative Example 2)
As Comparative Example 2, a negative electrode material 2a was produced in the same manner as in Example 1 except that the amount of coal tar pitch added was changed to 10 g instead of 10 g of polyacrylonitrile as the carbon layer precursor, and the solvent was changed to tetrahydrofuran.
In order to determine the carbon ratio of the carbon layer to the graphite particles as the nucleus, the carbonization rate was measured when coal tar pitch alone was heated to 900 ° C. at 20 ° C./h in a nitrogen stream and held for 1 hour. The carbonization rate was 54%. Was calculated carbon ratios for graphite particles of this value and coal tar pitch-coated amount of carbon layer, it was 0.01.
The average particle diameter, true specific gravity, specific surface area, R value (Raman spectrum peak ratio), and nitrogen element concentration of the negative electrode material 2a were measured in the same manner as the above graphite particles. The results are shown in Table 2.
(Comparative Example 3)
As Comparative Example 3, a negative electrode material 3a was produced in the same manner as in Example 2 except that the amount of coal tar pitch added was changed to 10 g instead of 10 g of polyimide as the carbon layer precursor, and the solvent was changed to tetrahydrofuran.
In order to determine the carbon ratio of the carbon layer to the graphite particles, coal tar pitch alone was heated to 900 ° C. at 20 ° C./h in a nitrogen stream, and the carbonization rate was maintained for 1 hour. The carbonization rate was 52%. Was calculated carbon ratios for graphite particles of this value and coal tar pitch-coated amount of carbon layer, it was 0.01.
The average particle diameter, true specific gravity, specific surface area, R value (Raman spectrum peak ratio), and nitrogen element concentration of the negative electrode material 3a were measured in the same manner as the above graphite particles. The results are shown in Table 2.

(Evaluation of negative electrode material)
The negative electrode materials 1, 2 and 1a to 3a obtained above were evaluated by the following methods.
(1) Preparation of negative electrode material slurry 1 part by mass of carboxymethyl cellulose (CMC2200) as a thickener with respect to 98 parts by mass of each negative electrode material 1, 2, 1a to 3a obtained in Examples 1 and 2 and Comparative Examples 1 to 3. Were mixed with 222 parts by mass of ion-exchanged water, and 1 part by mass of styrene-butadiene-rubber (“BM-400B” manufactured by Nippon Zeon Co., Ltd.) as an organic binder was added to prepare each negative electrode material slurry.

(2) Production of negative electrode Each of the negative electrode material slurries prepared as described above was continuously applied on a rolled copper foil (thickness 11 μm) and dried in a dry zone at 130 ° C. in the air at a rate of 15 cm / min. Then, it dried for 1 hour with 120 degreeC drying machine.
Next, it adjusted so that the density of an electrode might be set to 1.75 g / cm < ³ > using a roll press, and it punched in the circle of diameter 14mm ((phi)), and each negative electrode of Examples 1, 2 and Comparative Examples 1-3 was used. Obtained.

(3) Production of 2016 type coin cell About each obtained negative electrode, lithium metal is used for the counter electrode, 1M LiPF ₆ / ethylene carbonate: ethyl methyl carbonate (3: 7 volume ratio) is used for the electrolyte, and a 25 μm thick polyethylene micro separator is used for the separator. A 2016-type coin cell was produced using a copper plate having a suitable thickness as a pore film and a spacer.

(4) Charging / discharging test conditions <Evaluation of charging / discharging capacity and initial charging / discharging efficiency>
How the charge and discharge test, after the battery voltage at a constant current density of 0.434mA / cm ² at the beginning has been charged to a 0V, a constant voltage of 0V to the current density decreased to 0.043mA / cm ² Charged further. After charging, the battery was discharged after a 30-minute pause. Discharging was performed at a constant current density of 0.433 mA / cm ² until the battery voltage reached 1.5V.

  After the end of discharge, a charge / discharge test was performed again after a 30-minute pause. The initial charge / discharge efficiency was calculated as the ratio of the initial discharge capacity to the initial charge capacity ((initial discharge capacity) / (initial charge capacity) × 100).

<Evaluation of charging load characteristics>
Charging load characteristic, the ratio of the constant current charge capacity when increased from a constant current of 0.434mA / cm ² to 4.34mA / cm ² ((constant current charge capacity of 4.3 mA / cm ²⁾ / ( 0.434 mA / cm ² constant current charge capacity) × 100).

<Evaluation of low-temperature charging characteristics>
Low temperature charging characteristics, constant current charge capacity of 0.434MA / constant-current charging capacity of 25 ° C. during charging at a constant current of cm ² and 0 ℃ ratio of the constant current charge capacity during charge ((0 ℃) / (25 ℃ Constant current charge capacity) × 100).
Table 3 shows the characteristics of each negative electrode.

  As shown in Table 3, it is clear that the present invention (Examples 1 and 2) is excellent in discharge capacity, charge / discharge efficiency, and charge load characteristics.

  Further, by using the carbon-coated graphite negative electrode material for a lithium ion secondary battery negative electrode of the present invention or the negative electrode for a lithium ion secondary battery of the present invention, lithium having high capacity, excellent low-temperature charging performance, and excellent cycleability. It can be set as an ion secondary battery.

Claims (3)

Disperse and mix graphite particles having a block structure in which a plurality of flat particles are aggregated or bonded non-parallel to each other and having an aspect ratio of 5 or less in a mixed solution of a nitrogen-containing polymer compound and a solvent for dissolving the same. And a process of
Removing the solvent to produce graphite particles coated with the nitrogen-containing polymer compound;
Firing the graphite particles coated with the nitrogen-containing polymer compound, peaks at a peak intensity in the range of 1300～1400Cm ^-1 as measured by Raman spectroscopy and (ID) in a range of 1560～1650Cm ^-1 R value (ID / IG), which is the intensity ratio of intensity (IG), is 0.3 or less, and the concentration of nitrogen element on the surface measured by X-ray photoelectron spectroscopy (XPS) is 1.5 to 10 at% Obtaining a carbon-coated graphite negative electrode material,
A method for producing a carbon-coated graphite negative electrode material for a lithium ion secondary battery.   The graphite negative electrode for lithium ion secondary batteries using the graphite negative electrode material for lithium ion secondary batteries produced with the manufacturing method of Claim 1.   The lithium ion secondary battery using the graphite negative electrode for lithium ion secondary batteries of Claim 2.