JP4195179B2 - Method for producing negative electrode material for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a high performance negative electrode material for a lithium ion secondary battery , and a lithium ion secondary battery using the negative electrode material obtained by the production method .
[0002]
[Prior art]
<Anode material for secondary battery>
In recent years, with the demand for downsizing and weight reduction of electronic devices, there is an urgent need to develop high-capacity secondary batteries that replace lead-acid batteries and nickel-cadmium batteries. Lithium ion secondary batteries using a carbon material as a negative electrode material have attracted attention and are put into practical use because of their high energy density, safety, and excellent cycle characteristics. Graphite, in particular, has the feature that when it is used as a negative electrode material for a lithium secondary battery, it has a feature that a large potential can be taken out because the potential is flat, and the purpose of lithium ion secondary batteries including mobile phones Widely used in
[0003]
However, in general, in a negative electrode material of a lithium ion secondary battery, lithium charged at the first charge / discharge is not completely discharged, and an irreversible capacity appears. This irreversible capacity is considered to result from the electrolytic reaction of the electrolytic solution on the negative electrode material surface, and has a large correlation with the reactivity of the negative electrode material surface. That is, the lower the reactivity of the negative electrode material surface, the smaller the irreversible capacity. Suppressing the reaction between the electrolytic solution and the negative electrode material surface is effective as one measure for improving not only irreversible capacity but also various battery characteristics such as load characteristics, charging characteristics, and cycle characteristics.
[0004]
<Coating of graphite particle surface>
In order to suppress the reaction between the negative electrode material surface and the electrolyte, various surface treatment methods for coating the surface of graphite with low crystalline carbon, which has a low reactivity with the electrolyte, are proposed. Has been.
[0005]
(A) Japanese Patent Application Laid-Open No. 9-213328 discloses a composite carbon obtained by adhering 0.1 to 12 parts by weight of an organic carbide as a residual carbon amount with respect to 100 parts by weight of the graphitic carbonaceous material on the surface of the graphitic carbonaceous material. An electrode material for a non-aqueous solvent secondary battery made of a material is shown. Examples of the organic substance are heavy oil, natural polymer, thermosetting resin, and the like. The operation is performed by slurrying the graphitic carbonaceous material and the organic material with a solvent, and then performing degassing and volatilization of the solvent, followed by heat treatment.
[0006]
(B) JP-A-10-12241 discloses a graphite-carbon composite material comprising a graphite particle nucleus and a carbon layer covering the surface of the graphite particle by a chemical vapor deposition method. A negative electrode material for a lithium ion secondary battery having a specific surface area of 1 m ² / g or less and an equilibrium adsorbed water content of 0.3 wt% or less is shown. Examples of pyrolytic carbon sources for chemical vapor deposition include benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, diphenyl, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, phenanthrene, gas light oil, creo Sort oil, anthracene oil, naphtha cracked tar oil, acetylene, ethylene, propylene, isopropylene, butadiene and the like.
[0007]
(C) In JP-A-10-284080, an amorphous carbon-coated graphite-based carbonaceous material obtained by coating the surface of a graphitic carbonaceous material with a carbonizable organic material, firing, and pulverizing is acidic or A lithium ion secondary battery using a carbonaceous material treated with an alkaline solution as a negative electrode is shown. Examples of organic substances that can be carbonized include coal tar pitch, coal-based heavy oil, petroleum-based heavy oil, aromatic hydrocarbons, various synthetic resins, natural polymers, and the like.
[0008]
(D) In JP-A-11-54123, a lump of graphite powder is used as a nucleus, and the surface of the nucleus is coated with a carbon precursor and then fired in a temperature range of 700 to 2800 ° C. in an inert gas atmosphere. A nonaqueous electrolyte secondary battery using a carbonaceous powder having a multilayer structure in which a surface layer of a carbonaceous material is formed is shown. Examples of the carbon precursor are organic substances such as coal tar pitch, coal-based heavy oil, petroleum-based heavy oil, various synthetic resins, and natural polymers. The carbon precursor is coated on the graphite powder by dissolving and diluting an organic substance in a solvent and attaching it to the surface of the graphite particle nucleus. In this publication, reference is made to JP-A-6-295725, JP-A-7-134988, and JP-A-5-307959 in the description of the prior art.
[0009]
(E) Japanese Patent Application Laid-Open No. 11-167920 relating to the applicant's application discloses that after carbon precursor and graphite material are mixed, heat treatment is performed at 1000 to 3000 ° C. in an inert gas atmosphere, and The manufacturing method of the negative electrode material for non-aqueous secondary batteries whose ratio of the heat processing thing derived from the carbon precursor to the mixture after heat processing is 1 to 70 weight% is shown. Examples of the carbon precursor are petroleum pitch, coal pitch, aromatic organic compound, polymer compound, coke and the like.
[0010]
(F) Japanese Patent Application Laid-Open No. 2000-90925 discloses a carbon material for a negative electrode comprising a fired product of a mixture of artificial graphite or natural graphite and a carbon material containing a volatile component. As carbon materials containing volatile components, some or all of the surface of carbon material particles such as artificial graphite, natural graphite, carbonized and graphitized products of mesocarbon microbeads, carbonized products and graphitized products of resins are heavy. Carbon materials coated with volatile components derived from quality oil; mesocarbon microbeads, carbon fibers, mesophase pitch, isotropic pitch, resins, and the like. In this publication, in the description of the prior art, “JP-A-4-368778, JP-A-4-370662, JP-A-5-94838, JP-A-5-121066, JP-A-9-900 No. 213328 discloses a carbon material in which the surface of graphite particles as a core material is coated with low crystalline carbon.
[0011]
[Problems to be solved by the invention]
In the prior arts (a) to (f), a carbon coating layer is provided on the surface of graphite particles by a dry method, a wet method, a liquid phase method, a gas phase method, or a partial gas phase method. Among them, the wet method or the liquid phase method has a drawback that the coating agent is easily segregated on the graphite surface, and further, the coating agent is used as a binder, and the graphite particles are likely to aggregate. When the amount of the additive added is increased, the capacity as the negative electrode material is reduced. On the other hand, the dry method, the gas phase method or the partial gas phase method is not suitable for mass production, and causes problems such as high processing costs and a decrease in capacity as a negative electrode material. have.
[0012]
Under such circumstances, the present invention reduces the irreversible capacity by reducing the reactivity with the electrolytic solution while maintaining the high charge / discharge capacity of the graphite particles, and the initial efficiency, load characteristics, charge characteristics and cycle are reduced. To provide an industrial method for producing a high-performance lithium-ion secondary battery negative electrode material having excellent characteristics, and to provide a lithium-ion secondary battery using the secondary battery negative-electrode material thus obtained. It is intended to provide.
[0013]
[Means for Solving the Problems]
The method for producing a negative electrode material for a lithium ion secondary battery of the present invention
Physically mixing 100 parts by weight of graphite particles (1) and 0.5 to 50 parts by weight of polystyrene particles (2) ;
Heat treating the mixture at a temperature equal to or higher than the thermal decomposition temperature of the polystyrene particles (2) in an inert gas atmosphere ;
By this heat treatment, the coating layer coating film layer is carbonized with to form a coating layer by the thermal decomposition component on the surface of the thermal decomposition component is diffused in the gas phase the graphite particles (1) of the polystyrene particles (2) to obtain composite particles becomes,
It is characterized by.
[0014]
The lithium ion secondary battery of the present invention is obtained by using the composite particles obtained by the above method as a negative electrode material.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in detail below.
[0016]
In the present invention, the graphite particles (1) and the polystyrene particles (2) are first physically mixed and then heat-treated at a temperature equal to or higher than the thermal decomposition temperature of the polystyrene particles (2) .
[0017]
As the graphite particles (1), natural graphite or artificial graphite is used. These may be pulverized by an appropriate method or may be subjected to modification such as spheroidization. An example of spheroidization modification is disclosed in Japanese Patent Application Laid-Open No. 11-263612 filed by the present applicant. The average particle diameter (D50) of the graphite particles (1) is not particularly limited, but it is practically about 1 to 60 μm, preferably about 10 to 50 μm.
[0018]
Polystyrene particles (2) are used as the counterpart particles of the graphite particles (1). This is because using polystyrene and using the polystyrene as particles is optimal for the purpose of the present invention in terms of the effect of suppressing the reaction with the electrolytic solution, economy, and the like.
[0019]
In order to increase the mixing efficiency with the graphite particles (1), the average particle diameter of the polystyrene particles (2) is often 20 mm or less, preferably 10 mm or less. The lower limit of the particle size is often about 0.1 μm, especially about 1 μm.
[0020]
The mixing ratio of the graphite particles (1) and polystyrene particles (2), graphite particles (1) with respect to 100 parts by weight, so that the proportion of polystyrene particles (2) is 0.5 to 50 parts by weight. When the amount of the polystyrene particles (2) mixed with the graphite particles (1) is less than this range , the coating layer (and further the coating layer is formed by the pyrolysis component of the polystyrene particles (2) on the surface of the graphite particles (1). Since the formation amount of the carbonized coating layer) is insufficient, the reaction with the electrolytic solution is not sufficiently suppressed. On the other hand, when the amount of the polystyrene particles (2) mixed with the graphite particles (1) is larger than this range , the ratio of the graphite particles (1) contributing to high capacity / high energy density becomes small, so that the battery capacity decreases. Come.
[0021]
Heat treatment of a physical mixture of graphite particles (1) and polystyrene particles (2) is made by thermal decomposition temperature or more of polystyrene particles (2). The heat treatment at this time is performed in a carbonization temperature region or a graphitization temperature region depending on required characteristics of the obtained composite particles as a negative electrode material for a lithium ion secondary battery. The carbonization temperature region is about 500 to 2000 ° C, preferably about 600 to 1300 ° C. The graphitization temperature region is about 2000 to 3000 ° C, preferably about 2500 to 3000 ° C. When the heat treatment temperature is too low, the formation of a coating layer (and a coating layer obtained by carbonizing the coating layer ) on the graphite particles (1) of the pyrolysis component of the polystyrene particles (2) becomes insufficient, while the heat treatment temperature is low. If it is too high, it is disadvantageous in terms of equipment and power requirements.
[0022]
The above heat treatment, N _2, Ar, He, carried out in an inert gas atmosphere such as CO _2. At this time, it is preferable to carry out in a sealed atmosphere in which pyrolysis components generated by pyrolysis of the polystyrene particles (2) are retained and the surfaces of the graphite particles (1) are easily covered .
[0023]
By the above-described method, target composite particles having a small irreversible capacity and excellent initial efficiency, load characteristics, charge characteristics, and cycle characteristics can be obtained.
[0024]
In the present invention, the composite particles thus obtained are used as a negative electrode material for a lithium ion secondary battery.
[0025]
As the cathode material in a lithium ion secondary battery, reforming _{MnO 2, LiCoO 2, LiNiO 2} , LiNi 1-y Co y O 2, LiMnO 2, LiMn 2 O 4, LiFeO 2 and the like are used. As the electrolytic solution, an organic solvent such as ethylene carbonate, or a mixed solvent of the organic solvent and a low boiling point solvent such as dimethyl carbonate, diethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxymethane, ethoxymethoxyethane In addition, a solution in which an electrolyte solute such as LiPF ₆ , LiBF ₄ , LiClO ₄ , or LiCF ₃ SO ₃ is dissolved is used.
[0026]
<Action>
In the present invention, graphite particles (1) and polystyrene particles (2) are physically mixed, and the mixture is heat-treated at a temperature equal to or higher than the thermal decomposition temperature of polystyrene particles (2) in an inert gas atmosphere . Composite particles are obtained in which a coating layer (and a coating layer obtained by carbonizing the coating layer) of the polystyrene particles (2) is formed on the surface of the graphite particles (1).
[0027]
The pyrolysis component of the polystyrene particles (2) produced during the heating process diffuses to the corners of the graphite particles (1) in the gas phase, reacts with and adheres to active parts of the graphite surface, and is thin and uniform. A thick coating layer is formed. Then, this is further heated to a predetermined temperature and subjected to heat treatment, whereby the coating layer is carbonized to form a stable carbon layer.
[0028]
In the composite particles thus obtained, the active sites (particularly the edge surfaces) of the graphite particles (1) are covered with the coating layer, so that the reaction with the electrolytic solution is suppressed. In addition, the bulk density as particles is improved and the angle of repose is reduced. When this is used as a negative electrode material, the excellent properties as graphite are maintained, and the reactivity with the electrolyte is lowered. Thus, a negative electrode plate having a small irreversible capacity and excellent initial efficiency, load characteristics, charging characteristics and cycle characteristics can be obtained.
[0029]
Instead of polystyrene particles (2), for example, polymer particles that leave carbides during pyrolysis, such as phenol resin, the carbides remain after heat treatment, and the graphite particles stick or a lump of carbides is formed. The intended purpose cannot be fully achieved, and when a gas or liquid such as ethylene or styrene (monomer) is used, the reaction and attachment with the active part of the graphite surface becomes insufficient, and it is still the case. The purpose of the period cannot be fully fulfilled. Polystyrene select as the polymer, and when using it as a particle, it is the best result is obtained as the negative electrode material for a lithium ion secondary battery.
[0030]
The charge / discharge reaction of the lithium ion secondary battery is as follows, and lithium ions travel between the positive electrode and the negative electrode. The reaction from the left side to the right side is a charge reaction, and the reaction from the right side to the left side is a discharge reaction.
6C + LiCoO ₂ = C ₆ Li + CoO ₂
[0031]
【Example】
The following examples further illustrate the invention. Hereinafter, “parts” means parts by weight.
[0032]
[Production of composite particles]
Composite particles were produced as follows.
[0033]
Example 1 (Reference Example)
100 parts of natural graphite particles having an average particle diameter of 22 μm and 10 parts of polyethylene particles having a particle diameter of 3 mm were physically mixed with each other and heat-treated at 800 ° C. for 2 hours in a nitrogen atmosphere. By this heat treatment, 101.1 parts of composite particles in which a coating layer formed of the pyrolytic component of polyethylene particles was formed on the surface of the graphite particles were obtained.
[0034]
Example 2
Example 1 100 parts of natural graphite particles having an average particle diameter of 22 μm used in Reference Example and 10 parts of polystyrene particles having a particle diameter of 3 mm were mixed and heat-treated at 800 ° C. for 2 hours in a nitrogen atmosphere. By this heat treatment, 101.8 parts of composite particles in which a coating layer of pyrolytic components of polystyrene particles was formed on the surface of the graphite particles were obtained.
[0035]
Example 3
Example 1 100 parts of natural graphite particles having an average particle diameter of 22 μm used in Reference Example and 2 parts of polystyrene particles having a particle diameter of 3 mm were mixed and heat-treated at 800 ° C. for 2 hours in a nitrogen atmosphere. By this heat treatment, 100.4 parts of composite particles in which a coating layer made of a pyrolysis component of polystyrene particles was formed on the surface of the graphite particles were obtained.
[0036]
Comparative Example 1
The natural graphite particles having an average particle diameter of 22 μm used in Example 1 (Reference Example) were used as they were without heat treatment.
[0037]
[Measurement of particle diameter, evaluation of electrode performance]
(Measurement of particle size of graphite particles)
Measurement was performed using a “SALD-2000” laser diffraction particle size distribution measuring apparatus manufactured by Shimadzu Corporation, and an average particle diameter (D50) was determined.
[0038]
(Electrode performance evaluation)
100 parts by weight of the negative electrode material, 3 parts by weight of polyvinylidene fluoride as a binder, and an appropriate amount of N-methylpyrrolidone as a solvent were mixed and stirred uniformly in a liquid phase. The obtained slurry was applied onto a copper foil, dried, and then pressure-formed with a press machine to prepare a negative electrode plate, followed by vacuum drying at 150 ° C. for 6 hours. A lithium electrode pressed onto a stainless steel plate was used as a counter electrode through a separator to assemble a bipolar cell. The assembly is performed in a dry box adjusted to a moisture value of 20 ppm or less, and the electrolyte is 1M-LiPF ₆ / (EC + DEC (1: 1)), that is, 1: 1 by volume ratio of ethylene carbonate and diethyl carbonate. A solution obtained by dissolving LiPF ₆ at a ratio of 1M in a mixed solvent was used.
[0039]
Charging was carried out until the current value reached 0.01 mA / cm ² at a constant potential of 0 V after charging at a constant current value of 0.2 mA / cm ² (0.05 C) until it reached 0 V. Discharging was performed until the voltage reached 1 V at a current value of 0.2 mA / cm ² . By the first charge capacity and discharge capacity of each sample,
Initial efficiency (%) = 100 × discharge capacity / charge capacity was calculated.
[0040]
The load characteristic is the ratio (%) of the discharge capacity discharged in 30 minutes to the discharge capacity discharged in 10 hours.
[0041]
The charge characteristic is a ratio (%) of the constant current charge capacity charged in 1 hour to the constant current charge capacity charged in 10 hours.
[0042]
[Characteristics of negative electrode material and battery electrode performance using the same]
Example 1 (Reference Example), the electrode performance characteristics and a battery using the negative electrode material of Example 2-3 and Comparative Example 1 shown in Table 1.
[0043]
[Table 1]

Property Electrode Performance
Particle size Coverage Discharge capacity Initial efficiency Load characteristics Charging characteristics
( μm ) (part) (mAh / g) (%) (%) (%)
Comparative Example 1 22 - 366 91.7 90 54
Example 1 (Reference Example) 22 1.1 366 92.5 92 67
Example 2 22 1.8 366 92.7 94 70
Example 3 22 0.4 366 93.0 92 68
[0044]
From Table 1, it can be seen that in Examples 2 to 3 , the initial efficiency, load characteristics and charging characteristics are improved while maintaining the particle diameter and discharge capacity of the original graphite particles. That is, the irreversible capacity is reduced by suppressing the reaction between the electrolytic solution and the graphite surface while maintaining the high capacity of the graphite material.
[0045]
【The invention's effect】
The negative electrode material for a lithium ion secondary battery obtained by the production method of the present invention has low irreversible capacity and low initial efficiency by reducing the reactivity with the electrolyte while maintaining the high charge / discharge capacity of the graphite particles. It has high performance with excellent load characteristics, charging characteristics and cycle characteristics.
[0046]
In addition, the negative electrode material thus obtained can be easily manufactured simply by heat-treating a physical mixture of graphite particles and polystyrene particles that are specific polymer particles. This is also advantageous.

Claims (2)

Physically mixing 100 parts by weight of graphite particles (1) and 0.5 to 50 parts by weight of polystyrene particles (2) ;
Heat treating the mixture at a temperature equal to or higher than the thermal decomposition temperature of the polystyrene particles (2) in an inert gas atmosphere ;
By this heat treatment, the coating layer coating film layer is carbonized with to form a coating layer by the thermal decomposition component on the surface of the thermal decomposition component is diffused in the gas phase the graphite particles (1) of the polystyrene particles (2) to obtain composite particles becomes,
A method for producing a negative electrode material for a lithium ion secondary battery. A lithium ion secondary battery using the composite particles obtained by the method of claim 1 as a negative electrode material.