JP6650656B2 - Graphite material and manufacturing method thereof - Google Patents

Description

  The present invention relates to a graphite material suitable for a negative electrode of a lithium ion secondary battery having excellent large current load characteristics and DC resistance characteristics, and a method for producing the same.

  In a lithium ion secondary battery, generally, a lithium salt such as lithium cobalt oxide is used as a positive electrode active material, and a carbonaceous material such as graphite is used as a negative electrode active material. Graphite includes natural graphite and artificial graphite.

Among them, natural graphite has an advantage that it can be obtained at low cost. However, since the surface of natural graphite was active, a large amount of gas was generated at the time of initial charging, the initial efficiency was low, and the cycle characteristics were not good. In order to solve these problems, Patent Literature 1 discloses a method of coating artificial carbon on the surface of spherical natural graphite.
On the other hand, regarding artificial graphite, Patent Document 2 discloses a graphitized mesocarbon sphere.

Artificial graphite typified by petroleum pitch, coal pitch, petroleum coke, and graphitized coal coke can be obtained at relatively low prices. However, acicular coke having good crystallinity tends to be scaly and easily oriented. In order to solve this problem, Patent Document 3 discloses a graphite material in which a plurality of flat graphite particles are aggregated or bonded so that the orientation planes are non-parallel.
Patent Document 4 discloses a negative electrode material using so-called hard carbon or amorphous carbon.

Patent Document 5 discloses a carbon fiber heated in a vacuum with a strong oxidizing agent.
Patent Literature 6 discloses a graphite material which has been mixed with a Li compound in advance and then subjected to a heating and firing treatment.
Patent Document 7 discloses that a graphite material is immersed in a solution containing a nonmetallic organic compound.

Patent No. 3534391 (US6632569 B1) JP-A-4-190555 Patent No. 3361510 (US6344296 B1) JP-A-7-320740 (US5587255A) JP-A-5-299074 JP-A-5-135802 JP-A-10-270045 (CA2230948 A1)

Materials manufactured by the methods described in Patent Literatures 1 to 3 can cope with the electric capacity at a low current density and the medium-term cycle characteristics when a battery is used in a mobile application. It is very difficult to cope with electric capacity at a large current density and long-term cycle characteristics when used.
The negative electrode material described in Patent Document 4 has an extremely low volume energy density and is very expensive, and is therefore used only for some special large batteries.
In the method described in Patent Document 5, a strong oxidizing agent is used, and heating is performed at a high temperature, which involves danger in the production process.
In the method described in Patent Document 6, the cost is high because a compound containing a large amount of Li is used. Further, the treated graphite material has a low electric capacity and poor cycle performance, and thus is insufficient for use in a secondary battery.
The method described in Patent Document 7 has insufficient electric capacity.

  The present inventors have conducted intensive studies in order to solve the above-described problems, and as a result, have found a graphite material suitable for producing a negative electrode of a lithium ion secondary battery having excellent large current load characteristics and excellent DC resistance characteristics, and a method for producing the same. Heading, the present invention has been completed.

That is, the present invention relates to the following (1) to (7) graphite materials and (8) to (10).
(1) The surface carbon atom specific concentration measured by X-ray photoelectron spectroscopy (XPS) is 98.3% or more, and the R value (RE) and graphite of the graphite edge surface measured by Raman spectroscopy using an argon laser A graphite material whose R value (RB) on the basal surface satisfies the following requirements (a) and (b).
(A) 0.19 ≦ RE ≦ 0.54
(B) 0.10 ≦ RB ≦ 0.14
(2) The graphite material according to the above item 1, wherein the average spacing d002 of the (002) planes measured by the X-ray diffraction method is 0.3356 nm to 0.3375 nm.
(3) The graphite material according to the above item 1 or 2, wherein the BET specific surface area is 1.0 m ² / g or more and 8.0 m ² / g or less.
(4) The graphite material according to any one of (1) to (3) above, wherein the 50% particle diameter D50 in the volume-based cumulative particle size distribution is 2 μm to 55 μm.
(5) A carbon material for a battery electrode comprising the graphite material according to any one of the above items 1 to 4.
(6) A secondary battery including the carbon material for battery electrodes according to the above (5).
(7) A method for producing a graphite material comprising a step of mixing a solution containing a compound for surface treatment and raw graphite particles, and a step of performing a heat treatment in an inert atmosphere, wherein the graphite material uses an argon laser. A method for producing a graphite material in which the R value (RE) of the graphite edge surface and the R value (RB) of the graphite basal surface measured by Raman spectroscopy satisfy the following requirements (c) and (d).
(C) (RE of graphite material / RE of raw graphite particles) is 1.1 to 4.0.
(D) (RB of graphite material / RB of raw graphite particles) 1.2 to 2.0
(8) The method for producing a graphite material according to the above (7), wherein the mixing molar ratio of the compound for surface treatment and the raw graphite particles is 1: 1000 to 1: 100.
(9) The method for producing a graphite material according to the above (7) or (8), wherein the heat treatment temperature is 400 ° C. or more and 1300 ° C. or less.
(10) The compound for surface treatment is a transition metal chloride, a transition metal nitrate, a transition metal sulfate, a transition metal phosphate, nitric acid, sulfuric acid, phosphoric acid, or a mixture thereof (excluding a mixture of nitric acid and sulfuric acid) 10.) The method for producing a graphite material according to any one of the above items 7 to 9, wherein

When the graphite material of the present invention is used as a negative electrode material of a lithium ion secondary battery, a battery having excellent electric capacity, coulomb efficiency, cycle characteristics, and energy density, particularly a large current load characteristic and a low DC resistance value can be obtained. .
Further, the graphite material of the present invention is excellent in economic efficiency and mass productivity, and can be manufactured by a safe method.

  The graphite material according to one embodiment of the present invention preferably has a surface carbon atom specific concentration measured by X-ray photoelectron spectroscopy (XPS) of 98.3% or more. More preferably, it is 98.7% or more, more preferably 99.1% or more. If the surface carbon atom ratio is 98.3% or more, the amount of extraneous elements on the surface of the graphite material is small, so that the electrochemical side reaction is reduced.

The graphite material according to one embodiment of the present invention has an R value (RE) of a graphite edge surface and an R value (RB) of a graphite basal surface measured by Raman spectroscopy using an argon laser satisfying the following requirements (a) and (B) is satisfied.
(A) 0.19 ≦ RE ≦ 0.54
(B) 0.10 ≦ RB ≦ 0.14
R value is a graph showing the ratio (ID / IG) of the intensity of the G peak appearing at the center wavenumber 1574～1576Cm ^-1 intensity of D peak appearing at the center wavenumber 1344～1348Cm ^-1 for (IG) (ID) .

In Raman spectroscopy, multipoint measurement is performed, and the intensity ratio (ID / IG) is calculated from the G peak intensity and the D peak intensity of the five-point averaged spectra of the edge surface and the basal surface, respectively. It is possible to calculate RE, which is the value, and RB, which is the R value of the basal plane. The graphite material according to one embodiment of the present invention has an R value of the edge surface and an RB value of the basal surface of 0.19 ≦ RE ≦ 0.54 and 0.10 ≦ RB ≦ 0.14. Is preferably 0.20 ≦ RE ≦ 0.54 and 0.11 ≦ RB ≦ 0.14, more preferably 0.25 ≦ RE ≦ 0.54 and 0.12 ≦ RB ≦ 0. More preferably, it is 14.
When RE and RB are within such ranges, the insertion positions of Li ions increase, and the electrochemical reaction speed increases.

The graphite material in a preferred embodiment of the present invention preferably has a 50% cumulative diameter (D50) in a volume-based cumulative particle size distribution measured in a solvent using a laser diffraction type particle size distribution analyzer of 2 μm or more and 55 μm or less. The thickness is more preferably 3 μm or more and 40 μm or less, and most preferably 5 μm or more and 35 μm or less.
As a laser diffraction type particle size distribution measuring device, for example, Master Sizer (registered trademark) manufactured by Malvern can be used.

  Further, it is preferable that the graphite material in a preferred embodiment of the present invention does not substantially contain particles having a particle size of 0.5 μm or less. Particles of 0.5 μm or less have large active points on the surface and reduce the initial efficiency of the battery. Here, substantially not containing means that particles having a particle size of 0.5 μm or less are 0.1% by mass or less. The content of particles having a particle size of 0.5 μm or less can be measured by a laser diffraction type particle size distribution analyzer as described above.

The BET specific surface area (SBET) is measured by a general method of measuring the amount of gas adsorbed and desorbed per unit mass. As the measuring device, for example, NOVA-1200 can be used. BET specific surface area (SBET) is preferably 1.0 m ² / g or more and 8.0 m ² / g or less, more preferably 1.2 m ² / g or more and 7.0 m ² / g or less, and 1.5 m ² / g or more. Most preferred is 4.5 m ² / g or less. When the SBET is in this range, the area in contact with the electrolyte is secured without excessive use of the binder, lithium is smoothly inserted and desorbed, and the reaction resistance of the battery is reduced. Can be.

  The graphite material preferably has an average spacing d002 between (002) planes measured by an X-ray diffraction method of 0.3356 nm or more and 0.3375 nm or less. The thickness Lc of the crystal in the C-axis direction is preferably 30 nm or more and 1000 nm or less, more preferably 100 nm or less, and particularly preferably 50 nm or more and 100 nm or less. The thickness La of the crystallite in the a-axis direction is preferably 100 nm or more. By setting the content in such a range, a site to which the active material is doped is sufficiently obtained, and the edge portion of the crystallite is not too large, so that the decomposition of the electrolytic solution is further suppressed.

d002, La and Lc can be measured by a known method using the powder X-ray diffraction (XRD) method (Michio Inagaki, "Carbon", 1963, No. 36, pp. 25-34, Iwashita et al. , Carbon vol.42 (2004), p.701-714).
When the average plane distance d002 is in the range of 0.3356 nm to 0.3375 nm, the crystallinity of graphite is high, and the space in which lithium ions can be intercalated increases.

[Production method of graphite material]
In the method for producing a graphite material according to one embodiment of the present invention, a raw material graphite particle is mixed with a solution containing a compound for introducing defects into the surface of the raw graphite particle (abbreviated as “surface treatment compound”). This is a method for producing a graphite material, including a step of performing a heat treatment in an inert atmosphere. By adopting such a process, defects are introduced into the surface of the raw graphite particles.
When defects are introduced into the surface of the raw graphite particles, it is preferable to control the amount of defects introduced into the edge surface and the basal surface of graphite within a certain range.

Raman spectroscopy is used as an index for measuring the amount of defects introduced into the edge surface and the basal surface of graphite. The raw graphite particles and the graphite material are subjected to multi-point measurement by Raman spectroscopy, and the intensity ratio (ID / IG) is calculated from the G peak intensity and the D peak intensity of the five-point averaged spectra of the edge surface and the basal surface. Thus, it is possible to calculate RE, which is the R value of the edge surface, and RB, which is the R value of the basal surface.
R value is a graph showing the ratio (ID / IG) of the intensity of the G peak appearing at the center wavenumber 1574～1576Cm ^-1 intensity of D peak appearing at the center wavenumber 1344～1348Cm ^-1 for (IG) (ID) .

Regarding the R value of the edge surface, (RE of graphite material / RE of raw graphite particles) is preferably 1.1 to 4.0, more preferably 1.5 to 4.0, and 1.0. 8-4.0 is more preferable.
Regarding the RB that is the R value of the basal surface, (RB of graphite material / RB of raw graphite particles) is preferably 1.2 to 2.0, more preferably 1.4 to 2.0, and 1.6. -2.0 is more preferable.

Preferably, (RB of graphite material / RB of raw graphite particles) is 1.2 to 2.0, and (RE of graphite material / RE of raw graphite particles) is 1.1 to 4.0.
More preferably, (RB of graphite material / RB of raw graphite particles) is 1.4 to 2.0, and (RE of graphite material / RE of raw graphite particles) is 1.5 to 4.0. .
More preferably, (RB of graphite material / RB of raw graphite particles) is 1.6 to 2.0, and (RE of graphite material / RE of raw graphite particles) is 1.8 to 4.0. .

  When each R value is controlled so as to be in the above range, appropriate defects are introduced into the surface of graphite, the insertion position of Li ions is increased, and the speed of the electrochemical reaction is increased. There is no effect and the long-term cycle characteristics are good.

  The compound for graphite surface treatment can be selected from transition metal chlorides, transition metal nitrates, transition metal sulfates, transition metal phosphates, nitric acid, sulfuric acid, and phosphoric acid. These compounds may be used alone or as a mixture of two or more. However, the use of a mixed acid obtained by mixing nitric acid and sulfuric acid is excluded. This is because a mixed acid of nitric acid and sulfuric acid is too oxidative and is not suitable for the surface treatment of the present invention. Further, as the transition metal, for example, iron, cobalt, nickel, copper, and zinc can be used.

The molar ratio of the compound for graphite surface treatment and the raw graphite particles is preferably 1: 1000 to 1: 100 in the ratio of compound: graphite. More preferably, it is 1: 800 to 1: 100, and still more preferably, it is 1: 500 to 1: 100.
The solvent of the solution preferably contains water and a volatile organic solvent. The boiling point of the volatile organic solvent is preferably from 50 to 85 ° C. More preferably, it is 55 to 80C, and most preferably 60 to 80C.

  After mixing the solution containing the compound and the raw graphite particles, it is preferable to dry the mixture. The drying temperature is preferably from 70 to 120C, more preferably from 80 to 110C, even more preferably from 90 to 110C.

The mixture is heat treated in an inert atmosphere. The heat treatment temperature is preferably from 400 to 1300C, more preferably from 500 to 1100C, even more preferably from 500 to 1000C.
The heat treatment time is preferably 0.5 to 5 hours, more preferably 1 to 4 hours, and still more preferably 1.5 to 3 hours.
The graphite material obtained after the heat treatment is preferably washed, filtered, and dried.

[Carbon materials for battery electrodes]
The carbon material for a battery electrode in a preferred embodiment of the present invention comprises the above graphite material. When the graphite material is used as a carbon material for a battery electrode, a DC electrode with reduced DC resistance and an improved charge / discharge rate can be obtained while maintaining high capacity, high coulomb efficiency, and high cycle characteristics.
As the carbon material for a battery electrode, for example, it can be used as a negative electrode active material or a negative electrode conductivity imparting material of a lithium ion secondary battery.
As the carbon material for a battery electrode in a preferred embodiment of the present invention, another graphite material and the above graphite material may be mixed or used, or the above graphite material alone may be used.

[Electrode paste]
A paste for an electrode according to a preferred embodiment of the present invention comprises the carbon material for a battery electrode and a binder. This electrode paste is obtained by kneading the carbon material for a battery electrode and a binder. For kneading, a known device such as a ribbon mixer, a screw type kneader, a Spartan Luiser, a Loedige mixer, a planetary mixer, and a universal mixer can be used. The electrode paste can be formed into a shape such as a sheet or a pellet.

Examples of the binder used for the electrode paste include known binders such as a fluorine-based polymer such as polyvinylidene fluoride and polytetrafluoroethylene, and a rubber-based binder such as SBR (styrene butadiene rubber).
The amount of the binder used is suitably from 1 to 30 parts by mass, preferably from 3 to 20 parts by mass, per 100 parts by mass of the carbon material for battery electrodes.

  When kneading, a solvent can be used. Examples of the solvent include known solvents suitable for each binder, such as toluene and N-methylpyrrolidone in the case of a fluoropolymer; water and the like in the case of SBR; and dimethylformamide and isopropanol. In the case of a binder using water as a solvent, it is preferable to use a thickener in combination. The amount of the solvent is adjusted so as to have a viscosity that allows easy application to the current collector.

[electrode]
The electrode in a preferred embodiment of the present invention is formed of a molded product of the electrode paste. The electrode is obtained, for example, by applying the electrode paste on a current collector, drying and pressing.

  Examples of the current collector include foils such as aluminum, nickel, copper, and stainless steel, and meshes. The coating thickness of the paste is usually 50 to 200 μm. If the coating thickness is too large, it may not be possible to accommodate the negative electrode in a standardized battery container. The method for applying the paste is not particularly limited, and examples thereof include a method of applying the paste using a doctor blade, a bar coater, or the like, and then forming the paste using a roll press.

Examples of the pressure forming method include forming methods such as roll pressing and press pressing. The pressure at the time of pressure molding is preferably about 100 to 300 MPa (about 1 to 3 t / cm ² ). As the electrode density of the electrodes increases, the battery capacity per volume usually increases. However, when the electrode density is too high, the cycle characteristics usually deteriorate. When the electrode paste according to the preferred embodiment of the present invention is used, even if the electrode density is increased, the deterioration of the cycle characteristics is small, so that an electrode having a high electrode density can be obtained. The maximum value of the electrode density of the electrode obtained by using this electrode paste is usually 1.7 to 1.9 g / cm ³ . The electrode thus obtained is suitable for a negative electrode of a battery, particularly a negative electrode of a secondary battery.

[Battery, secondary battery]
A battery or a secondary battery can be formed by using the electrode as a component (preferably a negative electrode).
A battery or a secondary battery according to a preferred embodiment of the present invention will be described with reference to a specific example of a lithium ion secondary battery. A lithium ion secondary battery has a structure in which a positive electrode and a negative electrode are immersed in an electrolytic solution or an electrolyte. The electrode in the preferred embodiment of the present invention is used for the negative electrode.

  Known positive electrode active materials can be used for the positive electrode of the lithium ion secondary battery. For example, a lithium-containing transition metal oxide can be adopted, and preferably contains mainly lithium and at least one transition metal element selected from Ti, V, Cr, Mn, Fe, Co, Ni, Mo and W. Compounds that are oxides and have a molar ratio of lithium to the transition metal element of 0.3 to 2.2 can be employed.

In a lithium ion secondary battery, a separator is sometimes provided between a positive electrode and a negative electrode. Examples of the separator include a nonwoven fabric, a cloth, a microporous film, or a combination thereof, containing a polyolefin such as polyethylene or polypropylene as a main component.
As the electrolyte and the electrolyte, known organic electrolytes, inorganic solid electrolytes, and polymer solid electrolytes can be used.

  Hereinafter, the present invention will be described more specifically by showing typical examples. These are merely examples for explanation, and the present invention is not limited to these.

Physical property measurement and battery evaluation in Examples and Comparative Examples were performed as follows.
(1) Surface carbon atom concentration (atomic%)
X-ray photoelectron spectroscopy (XPS) measurement using Quantara II (registered trademark) (manufactured by ULVAC-PHI) to determine the carbon atom concentration (atomic%) in the surface region (abbreviated as “surface carbon atom concentration”). Can be. The measurement conditions were as follows: X-ray source: Al monochrome (25 W, 15 kV), analysis area: 100 μm (Spot), electron / ion neutralizing gun: ON, photoelectron take-out angle: 45 degrees. The binding energy scan range was 0-1100 eV. The XPS spectrum obtained by the measurement is subjected to data processing using data analysis software for XPS (PHI MultiPak (registered trademark) manufactured by ULVAC-PHI, Inc.). The surface carbon concentration was calculated as a relative ratio (atomic%) of the amount of

(2) Raman spectroscopy The measurement was performed using JASCO NRS-5100 manufactured by JASCO Corporation at a center wave number of 2082.24 cm ^-1 , an excitation wavelength of 532.36 nm, and a laser intensity of 0.8 mW. IG is the intensity of the G peak appearing at the center wave number of 1574 to 1576 cm ^-1 . ID is the intensity of the D peak appearing at the center wave number of 1344 to 1348 cm ^-1 . Multipoint measurement was performed on the sample, and the R value (intensity ratio ID / IG) was calculated from the G and D peak intensities of the five-point averaged spectrum on each of the edge surface and the basal surface.
(3) D50
D50, which is the 50% particle size in the volume-based cumulative particle size distribution, was determined by using a Malvern Mastersizer (registered trademark) as a laser diffraction type particle size distribution measuring device.
(4) BET specific surface area (SBET)
The BET specific surface area (SBET) was calculated from the amount of adsorption and desorption of nitrogen gas using a specific surface area measuring device NOVA-1200 (manufactured by Yuasa Ionics Inc.).

(5) Battery evaluation using coin cell a) Preparation of paste:
0.1 part by mass of an aqueous solution containing 2% by mass of SBR manufactured by JSR Corporation was added to 1 part by mass of the graphite material, and the mixture was kneaded with a planetary mixer to obtain a base stock solution.
b) Electrode preparation:
After adjusting the viscosity by adding water to the stock solution of the main agent, the solution was applied on a high-purity copper foil to a thickness of 150 μm using a doctor blade. This was vacuum-dried at 70 ° C. for 1 hour and punched out to 16 mmφ. The punched electrode was sandwiched between super-steel press plates, and pressed so that the pressing pressure applied to the electrode was about 100 to 300 MPa (about 1 to 3 t / cm ² ). Then, it dried at 120 degreeC for 12 hours with the vacuum dryer, and was set as the electrode for evaluation.

c) Battery fabrication:
A coin cell (counter electrode lithium cell) was produced as described below. The following operation was performed in a dry argon atmosphere having a dew point of -80 ° C or less.
A separator (polypropylene microporous film (Celgard 2400)) was laminated and sandwiched between a carbon electrode with a copper foil prepared in b) and a metal lithium foil in a cell with a screw-in lid made of polypropylene (inner diameter: about 18 mm). . An electrolyte was added thereto to prepare a test cell.
d) Electrolyte:
In a mixture of 8 parts by mass of EC (ethylene carbonate) and 12 parts by mass of DEC (diethyl carbonate), 1 mol / liter of LiPF ₆ is dissolved as an electrolyte.

e) Measurement test of discharge capacity and initial efficiency:
The test was performed using a coin cell. For charging (insertion of lithium into carbon), CC (constant current: constant current) charging was performed at 0.2 mA (0.05 C) from the rest potential to 0.002 V. Next, the voltage was switched to CV (constant volt: constant voltage) charging at 0.002 V, and stopped when the current value dropped to 25.4 μA. A constant current discharge test was performed at a current density of 0.2 mA (equivalent to 0.05 C). The test was performed in a thermostat set at 25 ° C. The discharge capacity was calculated by dividing the amount of electric discharge at 0.2 mA (0.05 C) by the mass of the active material. Further, the discharge capacity and the charge capacity in the first charge / discharge cycle were measured, and the initial efficiency was calculated from the ratio of the discharge capacity / charge capacity.

f) Charge rate test A test was performed using a coin cell. After discharging at 0.2 mA (0.05 C) and measuring the discharge capacity, charging was performed by the above method, and further, charging at 2.0 mA (0.5 C) and 4.0 mA (1.0 C). The capacities were measured, and these were divided by the charge capacity at 0.2 mA (0.05 C) to obtain the charge capacity retention rates at 0.5 C and 1.0 C.

g) Measurement test of charge / discharge cycle capacity retention ratio A test was performed using a coin cell. The 300-cycle capacity retention rate was calculated as a ratio of the 300th discharge capacity to the second discharge capacity by repeating a charge / discharge cycle of 0.5C CC charge and 0.5C CC discharge for 300 cycles.

(6) Battery evaluation by laminate cell (a) Negative electrode press The negative electrode plate manufactured in the same procedure as (5) was pressed so that the electrode density after about 18 hours was 1.70 g / cm ^3. Then, pressing by a uniaxial press was performed to produce a negative electrode. The negative electrode after pressing was dried again at 70 ° C. for 1 hour in vacuum. When the electrode density was difficult to increase, the pressure was increased to perform the pressing. However, even if the pressure was increased to a maximum of 300 MPa (about 3 t / cm ² ) and pressed for 10 seconds, the electrode density immediately after the pressing was 1.70 g / cm ^3. Was not reached, it was considered that the electrode density could not substantially reach 1.70 g / cm ³ .

(B) Preparation of Positive Electrode As the positive electrode active material, 97.5% by mass of lithium cobalt oxide (average particle size: 5 μm) and 0.5% by mass of vapor-grown carbon fiber (VGCF (registered trademark) -H, manufactured by Showa Denko KK) %, Carbon black (C45, manufactured by Imeris GC Japan, 2.0% by mass) and 3.0% by mass of polyvinylidene fluoride (PVDF) were dispersed in N-methylpyrrolidone, and the coating amount was 19.2 mg / cm ² . Thus, a positive electrode plate was prepared by coating on an aluminum foil. Thereafter, the positive electrode plate was dried at 70 ° C. for 1 hour in a vacuum. Next, the produced positive electrode plate was pressed with a roll press to increase the electrode density to 3.55 g / cm ³ to obtain a positive electrode.

(C) Production of Battery A single-layer laminate cell was produced using the produced negative electrode, positive electrode, and a polypropylene separator as the separator. As the electrolytic solution, 1 mol / L of LiPF6 dissolved in a solvent in which ethyl carbonate, ethyl methyl carbonate, and vinylene carbonate were mixed at a volume ratio of 30: 70: 1 was used. The negative electrode used was prepared in (5).
(D) Measurement of DC resistance The measurement was performed using a laminate cell. In the 50% charged state, a current having a different current value was passed, and the voltage change was plotted and calculated according to Ohm's law to calculate a DC resistance value (ohm).

Example 1
20.00 g of artificial graphite A and 10 ml of a water-ethanol mixed solution containing 0.757 g of zinc chloride (volume ratio of water to ethanol 1: 1) are mixed (molar ratio of graphite (carbon) to zinc chloride is 300: 1). And dried in an oven at 90 ° C. for 7 hours. The mixture was heat-treated at 500 ° C. for 3 hours in a nitrogen atmosphere. The heat-treated sample was washed with a 0.1 M HCl aqueous solution and distilled water, filtered, and dried. Thereafter, physical property measurement and battery evaluation were performed.

Example 2:
20.00 g of artificial graphite A and 10 ml of a water-ethanol mixed solution (volume ratio of water to ethanol 1: 1) containing 0.540 g of nickel chloride are mixed (molar ratio of graphite (carbon) to nickel chloride is 400: 1). And dried in an oven at 90 ° C. for 7 hours. The mixture was heat-treated at 700 ° C. for 3 hours in a nitrogen atmosphere. The heat-treated sample was washed with a 0.1 M HCl aqueous solution and distilled water, filtered, and dried. Thereafter, physical property measurement and battery evaluation were performed.

Example 3
A mixture of 20.00 g of artificial graphite A and 10 ml of a water-ethanol mixed solution (volume ratio of water to ethanol 1: 1) containing 1.292 g of cobalt sulfate (molar ratio of graphite (carbon) to cobalt sulfate 200: 1) And dried in an oven at 90 ° C. for 7 hours. The mixture was heat-treated at 800 ° C. for 3 hours in a nitrogen atmosphere. The heat-treated sample was washed with a 0.1 M HCl aqueous solution and distilled water, filtered, and dried. Thereafter, physical property measurement and battery evaluation were performed.

Example 4:
20.00 g of artificial graphite A and 10 ml of a water-ethanol mixed solution containing 0.817 g of phosphoric acid (volume ratio of water to ethanol 1: 1) were mixed (molar ratio of graphite (carbon) to phosphoric acid was 200: 1). Other than that, it processed similarly to Example 3. Thereafter, physical property measurement and battery evaluation were performed.

Example 5:
20.00 g of artificial graphite A and 10 ml of a water-ethanol mixed solution (volume ratio of water and ethanol 1: 1) containing 1.633 g of phosphoric acid were mixed (molar ratio of graphite (carbon) and phosphoric acid was 100: 1). Other than that, it processed similarly to Example 3. Thereafter, physical property measurement and battery evaluation were performed.

Example 6:
20.00 g of artificial graphite B and 10 ml of a water-ethanol mixed solution containing 0.181 g of phosphoric acid (volume ratio of water to ethanol 1: 1) were mixed (the molar ratio of graphite (carbon) to phosphoric acid was 900: 1). Other than that, it processed similarly to Example 3. Then, physical property measurement and battery evaluation were performed.

Example 7:
20.00 g of artificial graphite C and 10 ml of a water-ethanol mixed solution containing 0.327 g of phosphoric acid (volume ratio of water to ethanol 1: 1) were mixed (molar ratio of graphite (carbon) to phosphoric acid was 500: 1). Other than that, it processed similarly to Example 3. Then, physical property measurement and battery evaluation were performed.

Comparative Example 1:
For the untreated artificial graphite A, physical property measurement and battery evaluation were performed.

Comparative Example 2:
For the untreated artificial graphite B, physical property measurement and battery evaluation were performed.

Comparative Example 3:
10.00 g of artificial graphite A was immersed in 50 ml of a mixed acid of sulfuric acid and nitric acid (2M sulfuric acid: 2M nitric acid = 1: 1, volume ratio) for 30 minutes, filtered with pure water, washed, and then heated in vacuum (800 ° C., 30 minutes). 10 ⁻³ torr). Thereafter, physical property measurement and battery evaluation were performed.

Comparative Example 4:
10.00 g of artificial graphite A was added to 15 ml of a 0.1 M aqueous solution of lithium hydroxide (molar ratio of graphite (carbon) to lithium hydroxide = 560: 1), stirred at room temperature for 1 hour, and then water was filtered under reduced pressure. The residue was calcined at 400 ° C. for 5 hours under an argon gas atmosphere. Thereafter, physical property measurement and battery evaluation were performed.

Comparative Example 5:
10.00 g of artificial graphite A was mixed with 50 ml of a 0.5 M solution of phosphorus pentachloride in methylene chloride (molar ratio of graphite (carbon) to phosphorus pentachloride = 33: 1) and contacted for 12 hours. Then at 40 degrees the solvent was removed by evaporation. Thereafter, physical property measurement and battery evaluation were performed.

Comparative Example 6:
Except that 20.00 g of graphite A and 10 ml of a water-ethanol mixed solution (volume ratio of water and ethanol 1: 1) containing 4.54 g of zinc chloride were mixed (molar ratio of graphite and zinc chloride was 50: 1). Treated as in Example 1. Thereafter, physical property measurement and battery evaluation were performed.

Comparative Example 7:
Example 3 except that a solution of water and ethanol (volume ratio of water and ethanol 1: 1) containing 20.00 g of graphite B and 0.136 g of phosphoric acid was mixed (molar ratio of graphite and phosphoric acid was 1200: 1). Treated similarly. Thereafter, physical property measurement and battery evaluation were performed.

Comparative Example 8:
The same treatment as in Example 1 was performed except that the mixture of artificial graphite A and zinc chloride was heat-treated at 300 ° C. for 3 hours in a nitrogen atmosphere. Thereafter, physical property measurement and battery evaluation were performed.

Comparative Example 9:
The mixture was treated in the same manner as in Example 1 except that the mixture of artificial graphite A and zinc chloride was heat-treated at 1500 ° C. for 3 hours in a nitrogen atmosphere. Thereafter, physical property measurement and battery evaluation were performed.

  In Examples 1 to 7 and Comparative Examples 1 to 9, the physical property values of the raw graphite particles used are shown in Table 1, the processing conditions are shown in Table 2, the physical property evaluation results of the obtained graphite material are shown in Table 3, and the battery evaluation was performed. Table 4 shows the results.

From the results shown in Table 4, when the Raman R values of the edge surface and the basal surface of the graphite material are within the range described in claim 1, at least one of the Raman R values is out of the above range. It can be seen that a higher charge capacity retention ratio (at 0.5 C and 1.0 C) and a lower DC resistance value can be obtained as compared with Comparative Examples 6 to 9. In the counter electrode lithium cell, even a current of about 1 C is in a large current region.
Further, even if the Raman R value of the edge surface and the basal surface of the graphite material is within the range described in claim 1, in the case of Comparative Example 5 in which the surface carbon atom concentration is lower than 98.3%, the DC resistance is lower. It turns out that it becomes high.

  By controlling the defects on the graphite surface, charging performance and DC resistance are improved. A lithium ion secondary battery using the graphite material of the present invention as an electrode active material is small and lightweight, and has high discharge capacity and high cycle characteristics. Therefore, it is suitable for use in a wide range of fields from mobile phones to power tools and hybrid vehicles. Can be used.

Claims (4)

A method for producing a graphite material comprising a step of mixing a solution containing a compound for surface treatment and raw graphite particles, and a step of performing a heat treatment in an inert atmosphere, wherein the graphite material is subjected to Raman spectroscopy using an argon laser. A method for producing a graphite material in which the R value (RE) of the graphite edge surface and the R value (RB) of the graphite basal surface measured in the above step satisfy the following requirements (c) and (d).
(C) (RE of graphite material / RE of raw graphite particles) is 1.1 to 4.0.
(D) (RB of graphite material / RB of raw graphite particles) 1.2 to 2.0 The method for producing a graphite material according to claim 1 , wherein a mixing molar ratio of the surface treatment compound and the raw material graphite particles is 1: 1000 to 1: 100. The heat treatment temperature is, the manufacturing method of the graphite material according to claim 1 or claim 2 is 400 ° C. or higher 1300 ° C. or less. The compound for surface treatment is a transition metal chloride, a transition metal nitrate, a transition metal sulfate, a transition metal phosphate, nitric acid, sulfuric acid, phosphoric acid, or a mixture thereof (excluding a mixture of nitric acid and sulfuric acid). The method for producing a graphite material according to any one of claims 1 to 3 .