JP2011082054A - Coke for negative electrode carbon material, anode carbon material, and lithium ion battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode carbon material capable of restraining deterioration of capacity to the utmost and maintaining capacity at a high level, even with charge and discharge of a battery repeated at large current. <P>SOLUTION: The negative electrode carbon material for a lithium ion battery is prepared by heat-treating coke at about 2,100-2,700°C, wherein a cross-sectioned optical structure of the coke observed with a polarizing microscope is of an anisotropic structure formed with a fine-mosaic structure and a flow structure and an area ratio of both structures is: the former/the latter=10/90 to 70/30. In this carbon material, an average particle diameter (D50) is 6-20 μm, a specific surface area measured with a BET method is 3 m<SP>2</SP>/g or below, an interlayer distance d value (002) in X-ray diffraction is 0.3360-0.3395 nm, and a thickness La of a crystallite in an α-axis direction is 60-300 nm. The lithium ion battery having this negative electrode carbon material is suitable for a lithium ion battery for a hybrid vehicle or the like. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to coke useful as a precursor of a negative electrode carbon material, a negative electrode carbon material obtained using the coke, a method for producing the same, and a lithium ion battery including the negative electrode carbon material.

  Lithium-ion batteries using carbon materials as negative electrode materials, metal chalcogenides and metal oxides as positive electrode materials, and electrolytes in which various salts are dissolved in an aprotic organic solvent are used as high-energy density batteries. It is attracting attention as a kind of and is actively researched.

  Currently used negative electrode materials for lithium ion batteries can be broadly classified into two types, graphite materials and non-graphitizable carbons, based on the crystalline state of the final product. The graphite material is artificial graphite obtained by firing a carbon precursor called natural graphitizable carbon, which is easy to develop natural graphite or graphite crystals, at a temperature of 2800 ° C. or higher, and is characterized by high graphite crystallinity. . On the other hand, non-graphitizable carbon is obtained by firing a non-graphitizable carbon precursor, which is difficult to develop crystals, at around 1000 ° C., and is characterized by low graphite crystallinity. When the former is used as a negative electrode material of a lithium ion battery, it has an advantage that a high capacity density is obtained and a change in potential accompanying the release of lithium ions is small. On the other hand, when the latter is used as a negative electrode material for a lithium ion battery, the rapid charge / discharge characteristics (input / output characteristics) are superior to those of graphite materials, but the high capacity density cannot be obtained and the irreversible capacity is large. Have

  In a consumer-use small lithium ion battery (for example, a battery for a mobile device such as a mobile phone or a notebook personal computer), a graphite material having a high capacity density is generally used as a negative electrode material. Among graphite materials, artificial graphite has been the mainstream in the past, but from the viewpoint of cost, it has been proposed in Japanese Patent Application Laid-Open No. 04-368778 (Patent Document 1), Japanese Patent Application Laid-Open No. 04-370662 (Patent Document 2), and the like. By using the technology, graphite materials in which the surface of natural graphite particles is modified are becoming mainstream.

  In recent years, lithium ion batteries have been studied for application as power sources for electric tools and hybrid vehicles. In particular, when used as a power source for automobiles, input / output characteristics corresponding to rapid acceleration and deceleration are emphasized. When using a lithium ion battery for such an application, the characteristics required of the battery are different from those for the mobile device described above, so the graphite material having high crystallinity sufficiently satisfies the required performance. I can't.

  Therefore, it has been studied to use non-graphitizable carbon as a negative electrode material in lithium ion batteries for applications such as electric tools and hybrid vehicles. However, the non-graphitizable carbon has a problem that even if the input / output characteristics are satisfied, a high capacity density cannot be obtained and the irreversible capacity is large. Moreover, it is inferior to the graphite material which modified the surface of the natural graphite particle also in terms of cost.

  On the other hand, Japanese Patent Application Laid-Open No. 2004-335132 (Patent Document 3) proposes to use carbon in the course of graphitization process obtained by heat-treating a graphitizable carbon precursor at a low temperature as a negative electrode material. ing. This document describes a carbon material in which the c-axis direction crystallite thickness Lc is 20 nm or more and less than 60 nm and the a-axis direction crystallite thickness La is smaller than Lc. However, the carbon material having such parameters has insufficient crystal development and a very low discharge capacity.

  Similarly, Japanese Unexamined Patent Application Publication No. 2005-135905 (Patent Document 4) and Japanese Unexamined Patent Application Publication No. 2006-140138 (Patent Document 5) propose carbon materials that are in the process of graphitization. However, these carbon materials also have a problem that the discharge capacity is extremely low as in Patent Document 3.

  Japanese Patent Laid-Open No. 2002-270169 (Patent Document 6) discloses a method of adding boron as a graphitization catalyst to a carbon precursor. However, this method also causes a problem that impurities derived from the graphitization catalyst are mixed into the negative electrode carbon material.

  Furthermore, Japanese Patent Application Laid-Open No. 2007-149424 (Patent Document 7) discloses a method of heat-treating mesophase microspheres in a temperature range of more than 1500 ° C. and less than 2500 ° C. However, the graphitizable carbon obtained by this method has an interlayer distance d value (002) in X-ray diffraction exceeding 0.3395 nm, insufficient crystal development, and a very low discharge capacity.

Japanese Patent Laid-Open No. 04-368778 (Claims) Japanese Patent Laid-Open No. 04-370662 (Claims) JP 2004-335132 A (claims, paragraphs [0031] [0032], examples) Japanese Patent Laying-Open No. 2005-135905 (Claims) JP 2006-140138 A (Claims, paragraph [0052]) JP 2002-270169 (Claims) JP 2007-149424 A (Claims, paragraph [0022], Examples)

  Accordingly, an object of the present invention is to provide a precursor of a negative electrode carbon material that can suppress a decrease in capacity as much as possible and maintain a high level of capacity even when the battery is repeatedly charged and discharged with a large current, and a negative electrode carbon obtained using the precursor. It is providing the raw material, its manufacturing method, and the lithium ion battery provided with the said negative electrode carbon material.

  Another object of the present invention is to achieve both diffusion and retention of lithium ions, a precursor of a negative electrode carbon material suitable for a lithium ion battery for a hybrid vehicle, a negative electrode carbon material obtained using the same, a method for producing the same, and It is providing the lithium ion battery provided with the said negative electrode carbon material.

  As a result of intensive studies to achieve the above-mentioned problems, the present inventors have used a precursor having a specific structure and performed a specific heat treatment, whereby an X-ray diffraction parameter is different from that of a conventional graphite material. Could be manufactured. The negative electrode material obtained by this method was a material that could be charged and discharged with a large current without drastically reducing the capacity density. That is, the present inventors have found that the negative electrode material having such characteristics is suitable as a negative electrode material for lithium ion batteries in recent years in which input / output characteristics are important, and is different from the graphite material and non-graphitizable carbon. The present invention was completed by finding that it has excellent performance.

  That is, the coke of the present invention is a precursor of a negative electrode carbon material for a lithium ion battery, and an optical structure of a cross section observed with a polarizing microscope is an anisotropic structure formed by a fine mosaic structure and a flow structure. In addition, the area ratio of both tissues is the former / the latter = 10/90 to 70/30. The fine mosaic structure may be a structure composed of an assembly of fine regions having an average diameter of 10 μm or less, and the flow structure may be a structure composed of a continuous phase exceeding the average diameter of 10 μm. The area ratio between the fine mosaic structure and the flow structure may be about the former / the latter = 15/85 to 50/50. When the coke of the present invention is heat-treated at 1100 ° C., the half-value width (Δ2θ) of the 002 diffraction line pattern of X-ray diffraction may be about 2.0 to 6.0 °. Furthermore, when heat-treated at 1100 ° C., the R value measured by Raman spectroscopy may be about 2.0 to 3.0.

  The negative electrode carbon material for a lithium ion battery of the present invention may be a carbon material that satisfies the following characteristics (1) to (4).

(1) Average particle diameter (D50) is 6 to 20 μm
(2) Specific surface area measured by BET method is 3 m ² / g or less (3) Interlayer distance d value (002) in X-ray diffraction is 0.3360 to 0.3395 nm
(4) The crystallite thickness La in the a-axis direction is 60 to 300 nm.

  In the negative electrode carbon material of the present invention, the intensity ratio I (101) / I (100) between the (101) diffraction peak and the (100) diffraction peak in X-ray diffraction may be about 0.7 to 1.5. . Such a negative electrode carbon material can be obtained by heat-treating the coke at, for example, about 2100 to 2700 ° C. In the present invention, carbon containing a large amount of a turbulent structure (a turbulent structure with a low three-dimensional regularity) in which the stacking order of the carbon network surface is disturbed to some extent by firing at such a temperature using the coke. Can be a material.

  Furthermore, the present invention includes a lithium ion battery (for example, a lithium ion battery for a hybrid vehicle) provided with the negative electrode carbon material.

  The negative electrode carbon material of the present invention is obtained by using a specific coke, has a specific controlled crystallinity, and is a carbon material that contains a larger number of turbostratic structures than ordinary graphite. Charging / discharging with current is possible. In particular, even when the battery is repeatedly charged and discharged with a large current, the reduction in capacity can be suppressed as much as possible, and the capacity can be maintained at a high level. Therefore, it is useful as a constituent material of a lithium ion battery (particularly, a lithium ion battery for a hybrid vehicle) such as a power source for automobiles that requires both lithium ion diffusion and retention.

FIG. 1 is a polarizing micrograph of coke A used in the examples. FIG. 2 is a polarizing micrograph of coke B used in the examples. FIG. 3 is a polarizing micrograph of coke C used in the examples. FIG. 4 is a polarizing micrograph of coke D used in the examples.

[Carbon material precursor (coke)]
The coke of this invention is a precursor of the negative electrode carbon material for lithium ion batteries, and the optical structure of the cross section observed with the polarization microscope has the anisotropic structure formed with the fine mosaic structure and the flow structure. As long as such coke has the anisotropic structure, the production method is not particularly limited, but commercially available needle coke and gilsonite coke do not have the anisotropic structure, and are usually described later. Further, it can be obtained by adjusting the manufacturing method.

  Coke is obtained by pyrolyzing (coking) a carbonaceous raw material in a temperature range of 350 to 600 ° C. using a coking apparatus such as a delayed coker (delayed coke). As said carbonaceous raw material, the raw material which has hydrocarbons, such as coal pitch, petroleum pitch, an aromatic compound, and resin, as a main component is used, for example.

  In such a coking process, the molecular weight of the residue generally increases due to thermal decomposition and polycondensation of the raw material mainly composed of hydrocarbon. In particular, among hydrocarbons, a thermally stable aromatic hydrocarbon is converted into a huge aromatic hydrocarbon having a planar structure by polycondensation. Furthermore, huge aromatic hydrocarbons having a planar structure form a laminated structure with each other by intermolecular force, whereby the optically isotropic raw material is changed to anisotropic raw coke. At this time, if a large amount of elements other than carbon and hydrogen called hetero elements are present in the raw material, the aromatic hydrocarbon becomes a steric hindrance when forming a planar structure, or becomes an obstructive factor for forming a laminated structure. In addition, the viscosity of the residue during the reaction is an important factor governing the fluidity of the aromatic hydrocarbon. If the viscosity is high, the fluidity of the aromatic hydrocarbon is lowered, so that a fully developed laminated structure cannot be formed. . Since it is difficult to measure the viscosity of the residue during the reaction, the toluene insoluble matter (TI) of the raw material is generally used as an index for estimating the viscosity of the residue during the reaction. As the TI increases, the viscosity of the residue during the reaction increases, and raw coke with insufficient development of the laminated structure can be obtained.

  That is, in order to produce the coke of the present invention, it is preferable to adjust the heteroelement content and TI of the carbonaceous raw material. That is, the carbonaceous raw material used in the present invention contains, for example, a nitrogen content of 0.1% by weight or more (particularly 0.5 to 2.0% by weight) in order to prevent the coke crystallinity from becoming excessively high. %) And a raw material containing 5% by weight or more (particularly 10 to 15% by weight) of TI may be used. In order to control the nitrogen content, a mixture of the pitches may be used as a raw material. For example, it is known that coal-based pitch has more nitrogen than petroleum-based pitch. Therefore, the nitrogen content can be increased by mixing coal pitch with petroleum pitch. Moreover, since TI increases by removal of volatile components by distillation or heat treatment and can be reduced by separation means such as solvent extraction or filtration, the properties of the raw material may be controlled by combining these treatments.

  Next, calcined coke is obtained by heating (calcining) raw coke in a temperature range of 800 to 1400 ° C. by a calsiner (calcination apparatus) such as a rotary kiln. As coke which is a precursor of the negative electrode carbon material manufactured by this invention, you may use any coke of raw coke and calcined coke.

  In order to produce the coke of the present invention, in addition to the adjustment of the carbonaceous raw material, the coke structure is adjusted by adjusting the heating temperature such as the calcination temperature, the heating time, and the type of the atmospheric gas. Also good.

  In the coke of the present invention, the optical structure observed using a polarizing microscope has an anisotropic structure formed by a fine mosaic structure and a flow structure. Such an optical structure has a magnification of about 200, for example. It can be easily identified (or observed) by observing with a double polarization microscope. That is, in the coke of the present invention, all the structures are composed of an anisotropic structure, and this anisotropic structure has a mosaic structure in which a fine structure having a size of 10 μm or less (for example, about 1.0 to 10 μm) is mosaic. A flow structure (continuous structure or block structure) constituting a discontinuous fine mosaic structure (honeycomb structure or network structure) and a continuous phase having a size of more than 10 μm (for example, more than 10 μm and less than 500 μm). And can be observed as two types of regions. In addition, the said size shows the average diameter of a long diameter and a short diameter, when a fine mosaic structure | tissue and a flow structure | tissue are areas of an anisotropic shape.

Specifically, as a method of calculating the ratio of the fine mosaic structure contained in the optical structure of coke, the coke is embedded in a resin, the surface is polished, and then observed with a polarizing microscope having an observation magnification of about 200 times. About 10 images are taken into a computer as digital images having an area of 44 μm × 34 μm per observation image, and divided into squares of 2 μm × 2 μm on the computer. The divided grids are visually classified into fine mosaic structures and flow structures, and 1250 or more grids (5000 μm ² or more) are inspected to obtain the number of grids recognized as fine mosaic structures. The area ratio of the mosaic structure is calculated.

  In the coke of the present invention, the area ratio between the fine mosaic structure and the flow structure is, for example, the former / the latter = 10/90 to 70/30, preferably 15/85 to 60/40 (for example, 15/85 to 50 / 50), more preferably about 20/80 to 40/60 (particularly 25/75 to 35/65). The negative carbon material obtained from coke with an area ratio of the fine mosaic structure that is too small is that the graphite crystal structure develops too much, so it is difficult for lithium ions to enter and exit, and the laminated structure is sufficiently developed as described above. For this reason, when a battery is produced using this negative electrode carbon material, the lamination direction of graphite is easily arranged in parallel with the electrode sheet. Furthermore, since lithium ions are more likely to come in and out of randomly arranged graphite, the high-speed charge / discharge characteristics of such a battery deteriorate. On the other hand, if the area ratio of the fine mosaic structure is too large, the negative electrode carbon material obtained by heat treatment contains many interfaces of the fine mosaic structure, and there are many paths for lithium ions, but the number of occlusion sites is reduced, and charge / discharge is performed. The capacity of is reduced.

  As described above, when the area ratio of the fine mosaic structure and the fine mosaic structure increase, the capacity retention rate and the initial capacity decrease, and conversely, when the area ratio decreases, the initial capacity increases and the capacity maintenance rate increases. There is a relationship where a drop occurs. That is, in order to satisfy the contradictory characteristics of the capacity retention rate and the initial capacity, the area ratio of both tissues needs to be in the above range. In particular, in order to maintain a capacity retention rate of 90% or more while maintaining the initial capacity described in the examples described later at 230 mAh / g or more, the ratio between the two is fine mosaic structure / flow structure = 20 / 80- You may adjust to about 40/60 (especially 25 / 75-30 / 70).

  The coke of the present invention has a peak width Δ2θ (half-value width of the 002 diffraction line figure of X-ray diffraction) that is half the maximum value of the 002 diffraction peak that appears at 15 to 35 ° when X-ray diffraction is measured. For example, it is about 2.0 to 6.0 °, preferably 2.5 to 5.5 °, more preferably about 2.5 to 5.0 ° (particularly 3.0 to 4.5 °). If Δ2θ is too small, the crystal develops too much due to heat treatment, and the rate at which lithium ions can be diffused decreases due to an increase in the number of occlusion sites of lithium ions. Decreases. On the other hand, if Δ2θ is too large, the crystallinity of the negative electrode carbon material obtained by heat treatment decreases, and the lithium ion storage sites are few, so the charge / discharge capacity decreases.

  In the present invention, in the measurement of Δ2θ of coke, the coke is once heat-treated at 1100 ° C. in order to compare the crystal structure after heat-treatment at the same temperature. Furthermore, the 002 diffraction pattern by X-rays is measured in the range of 2θ of 5 to 40 ° with an X-ray diffractometer (manufactured by Rigaku Corporation, model: RINT 2500), and the 002 diffraction peak appearing at 15 to 35 ° is analyzed. . An interval between peak positions that is a half value with respect to the maximum value of the peak is obtained by calculation as a peak width Δ2θ.

  In the coke of the present invention, the R value of Raman spectrum analysis (R value measured by Raman spectroscopy) is, for example, 1.9 to 3.0, preferably 2.0 to 3.0, and more preferably 2.0. It is about -2.8 (especially 2.0-2.5). If the R value is too low, the crystal develops too much due to the heat treatment, and the high-speed charge / discharge characteristics of the battery deteriorate. On the other hand, when the R value is too high, the charge / discharge capacity of the battery decreases due to the decrease in crystallinity.

In addition, the R value is also the same as that of the pretreatment of the X-ray measurement, using a micro Raman spectroscope (manufactured by JEOL Ltd., JRS-SYS1000 type) for coke heat-treated at 1100 ° C. Measure under the conditions and obtain by the following method. That is, the spectrum obtained by the measurement is derived from D band present in the vicinity of 1360 cm ^-1 peak, the peak derived from G band exists in the vicinity of the peak and 1580 cm ^-1 derived from D 'band present in the vicinity of 1600 cm ^-1 The R value is determined from the ratio (D / G) between the peak intensity of the D band and the peak intensity of the G band.

Excitation light: Ar ion laser 514.4 nm
Excitation light output: 20 mW
Objective mirror magnification: 50 times Integration count: 1 time Irradiation time: 120 seconds Measuring method: 180 ° backscattering Measurement area: 800 to 2000 cm ⁻¹
Wave number calibration: Silicon.

[Negative carbon material]
The negative electrode carbon material of the present invention can be obtained by heat-treating or baking the coke. The heat treatment can usually be performed in a graphitization furnace, and the graphitization furnace is not particularly limited as long as it can reach a predetermined temperature. For example, an Acheson furnace, direct current graphitization A furnace, a vacuum furnace, etc. can be illustrated. In many cases, the pulverized product is pulverized by a pulverizer (ball mill, hammer mill, etc.) before heat treatment, and then crushed and classified after heat treatment to obtain a final product.

  The heat treatment temperature (or final temperature reached) is, for example, about 2100 to 2700 ° C., preferably about 2100 to 2500 ° C. (for example, 2100 to 2400 ° C.), more preferably about 2150 to 2400 ° C. (particularly about 2200 to 2350 ° C.). If the heat treatment temperature is too high, the degree of graphitization cannot be controlled within the above range, and graphite is substantially generated, so that the charge / discharge characteristics at a large current are greatly reduced. On the other hand, if the heat treatment temperature is too low, the degree of graphitization cannot be controlled within the above range, the crystal development becomes insufficient, and the lithium ion storage sites rapidly decrease, resulting in a significant decrease in discharge capacity.

  The negative electrode carbon material of the present invention is formed by using the coke to adjust the firing temperature and heat-treat, thereby forming a turbulent structure in which the stacking order of the carbon network surface is disturbed to some extent (100 and 101 diffraction lines are separated in the X-ray profile). Carbon having a structure that does not) facilitates the diffusion of lithium, enables charge and discharge with a large current, and improves the capacity maintenance rate of charge and discharge. The negative electrode carbon material having such characteristics includes (1) average particle diameter (D50), (2) specific surface area measured by BET method, (3) interlayer distance d value in X-ray diffraction (002), (4 ) The thickness La of the crystallites in the a-axis direction satisfies the following numerical ranges, respectively. The negative electrode carbon material of the present invention is a novel carbon material satisfying the conflicting characteristics of diffusion and retention of lithium ions, because the graphite structure and the turbulent layer structure exist appropriately by satisfying these characteristics. is there.

  In the negative electrode carbon material of the present invention, the average particle size (D50) of the carbon material may be, for example, 6 to 20 μm, preferably 6 to 18 μm, more preferably 8 to 16 μm (particularly 9 to 15 μm). If the average particle size is too small, the specific surface area becomes large and the initial efficiency is lowered. On the other hand, if the average particle size is too large, when producing a thin electrode for high output, the carbon particles protrude on the surface during pressing, or the contact resistance becomes high. The shape or form of the carbon negative electrode material is not particularly limited, and may be indeterminate, flat (or flat), flaky, granular, etc. Many.

The specific surface area (nitrogen gas adsorption method) measured by the BET method is, for example, 3 m ² / g or less (for example, 0.3 to 3 m ² / g), preferably 0.5 to ^2.5 m ² / g (for example, , 1~2.3m ² / g), more preferably 1.2~2.2m ² / g (or in particular 1.5 to 2 m ² / g) approximately. If the specific surface area is too large, the amount of the electrolytic solution decomposed at the initial stage of charging is remarkably increased, so the initial efficiency is lowered.

  The interlayer distance d value (002) in X-ray diffraction is, for example, 0.3360 to 0.3395 nm, preferably 0.3364 to 0.3395 nm, more preferably 0.3370 to 0.3395 nm (particularly 0.3376 to 0). .3395 nm). If the d value (002) is too small, crystals will develop too much and the number of lithium ion occlusion sites will increase, and the rate at which lithium ions can diffuse will decrease, resulting in poor input / output characteristics. On the other hand, if the d value (002) is too large, the lithium ion storage sites are rapidly reduced, resulting in a significant reduction in discharge capacity.

  The thickness La of the crystallite in the a-axis direction may be, for example, about 60 to 300 nm, preferably 60 to 270 nm, and more preferably about 60 to 200 nm (particularly 70 to 150 nm). If La is too small, the relative proportion of the edge surface where lithium ions enter and exit increases, and the lithium ion storage sites rapidly decrease, resulting in a significant decrease in discharge capacity. On the other hand, if La is too large, crystals will develop too much and it will be difficult for lithium ions to enter and exit, resulting in poor input / output characteristics.

  Further, the negative electrode carbon material of the present invention may have an intensity ratio I (101) / I (100) of (101) diffraction peak to (100) diffraction peak in X-ray diffraction of 0.7 to 1.5. For example, it may be about 0.7 to 1.2, preferably about 0.7 to 1.0, and more preferably about 0.7 to 0.9. If I (101) / I (100) is too small, the discharge capacity is remarkably lowered due to the decrease in the lithium ion storage sites. On the other hand, if I (101) / I (100) is too large, the crystal develops too much and the input / output characteristics deteriorate.

  As described above, the negative electrode carbon material is obtained by heat treating coke, but a carbon material satisfying the above characteristics may be selected after the heat treatment. For example, in the coke, when a heat treatment condition is selected such that the d value (002) is within a predetermined range, there is coke in which La deviates from the predetermined range or vice versa. In such a case, the negative electrode carbon material of the present invention can be obtained by selecting a carbon material having the above characteristics after the heat treatment.

[Method for producing negative electrode carbon material]
The negative electrode carbon material of the present invention can be suitably used as a constituent material of a negative electrode for a lithium ion battery (further, a lithium ion battery). The negative electrode for a lithium ion battery is, for example, a method of forming a mixture containing a negative electrode carbon material, a binder, etc., a paste containing a negative electrode carbon material, a solvent, a binder, etc., on the negative electrode current collector using a coating means (such as a doctor blade). It can be formed into an arbitrary shape depending on the application method. In forming the negative electrode, it may be combined with a terminal as necessary.

  The negative electrode current collector is not particularly limited, and a known current collector, for example, a metal foil made of a conductor such as copper, aluminum, gold, or silver can be used.

  In the production method of applying the paste, as the solvent, a solvent that can dissolve or disperse the binder is usually used, and can be selected according to the binder type. In the case of an aqueous binder, an aqueous solvent such as water or alcohol can be used, In the case of a hydrophobic binder, for example, an organic solvent such as N-methylpyrrolidone or N, N-dimethylformamide can be used. The solvents may be used alone or in combination of two or more. The amount of the solvent used is not particularly limited as long as it becomes a paste, and is, for example, about 60 to 150 parts by weight, preferably about 60 to 100 parts by weight with respect to 100 parts by weight of the negative electrode carbon material.

  Examples of the binder include a fluorine-containing resin (such as polyvinylidene fluoride and polytetrafluoroethylene), an aqueous binder (such as a water-dispersible synthetic rubber latex or an emulsion), and the like. The amount of binder used (in the case of a dispersion, the amount used in terms of solid content) is not particularly limited, and is, for example, 3 to 20 parts by weight, preferably 5 to 15 parts by weight with respect to 100 parts by weight of the negative electrode carbon material. Part, more preferably about 5 to 10 parts by weight. The method for preparing the paste is not particularly limited, and examples thereof include a method of mixing a mixed liquid (or dispersion liquid) of a binder and a solvent and a negative electrode material.

  The negative electrode carbon material of the present invention may be used in combination with another conductive material (carbonaceous material or conductive carbon material) to produce a negative electrode. The use ratio of the other conductive material is not particularly limited, but is usually about 1 to 10% by weight, preferably about 1 to 5% by weight with respect to the total amount of the negative electrode carbon material of the present invention and the other conductive material. By using another conductive material [for example, a carbonaceous material such as carbon black (for example, acetylene black, thermal black, furnace black)], conductivity as an electrode may be improved. Other conductive materials can be used alone or in combination of two or more. The other conductive material can be effectively used together with the negative electrode carbon material of the present invention, for example, by mixing the paste with the negative electrode carbon material of the present invention and a solvent and applying this paste to the negative electrode current collector. .

The amount of the paste applied to the negative electrode current collector is not particularly limited, and is usually about 5 to 15 mg / cm ² , preferably about 7 to 13 mg / cm ² . The thickness of the film applied to the negative electrode current collector (the film thickness of the paste) is, for example, about 40 to 100 μm, preferably about 40 to 80 μm, and more preferably about 40 to 60 μm.

  The method for applying the paste is not particularly limited, and a conventional method such as a roll coater, an air knife coater, a blade coater, a rod coater, a reverse coater, a bar coater, a comma coater, a dip squeeze coater, a die coater, a gravure coater, Examples include a micro gravure coater and a silk screen coater method.

  Note that after the application, the negative electrode current collector may be subjected to a drying treatment. The drying method is not particularly limited, and vacuum drying, hot air, far infrared rays, microwaves, or the like may be used in addition to natural drying.

[Lithium ion battery]
As described above, the negative electrode carbon material of the present invention can constitute a lithium ion battery as a constituent material of the negative electrode. In particular, the negative electrode carbon material of the present invention is suitable for a lithium ion battery for enabling repeated charging and discharging with a large current. The lithium ion battery is a combination of the negative electrode (negative electrode containing the negative electrode carbon material of the present invention), a positive electrode capable of occluding and releasing lithium, and an electrolyte solution, and further a separator (a porous polypropylene nonwoven fabric or a porous polyethylene nonwoven fabric). Polyolefin-based porous membranes, carbonaceous separators, etc.), current collectors, gaskets, sealing plates, cases, and other battery components, and can be manufactured by assembling in a conventional manner. For details of the method of assembling the lithium ion battery, for example, the method described in JP-A-7-249411 can be referred to.

A positive electrode in particular is not restrict | limited, A well-known positive electrode can be used, For example, it can comprise with a positive electrode collector, a positive electrode active material, a electrically conductive material etc. Examples of the positive electrode current collector include a metal foil made of a conductor such as aluminum, copper, gold, and silver. As the positive electrode active material, for example, a lithium composite oxide (such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _{4 having} a spinel structure) or the like can be used.

Examples of the electrolytic solution include propylene carbonate, ethylene carbonate, butylene carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, 4-methyldioxolane, sulfolane, 1,2-dimethoxy. Examples thereof include aprotic solvents such as ethane, dimethyl sulfoxide, acetonitrile, N, N-dimethylformamide, diethylene glycol, and dimethyl ether. The electrolyte also includes a solution in which a salt that generates an anion that is difficult to solvate, such as LiPF ₆ , LiClO ₄ , LiBF ₄ , is dissolved in these aprotic solvents. The electrolyte solutions may be used alone or in combination of two or more. Preferred electrolytes include ether solvents that are stable even in a strong reducing atmosphere, such as tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, 4-methyldioxolane, and aprotic solvents (preferably two types). The above mixed solvent) includes a solution in which the exemplified salt is dissolved.

  Note that the lithium ion battery can have any shape or form such as a cylindrical shape, a square shape, or a button shape.

  Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples. In the following examples, “part” or “%” is based on weight unless otherwise specified, and the evaluation methods for coke and carbon materials obtained in Examples and Comparative Examples are shown below.

[Method for calculating area ratio of fine mosaic structure contained in coke]
Coke is embedded in resin, the surface is polished, and then observed with a polarizing microscope having an observation magnification of about 200 times. About 10 images are taken into a computer as digital images having an area of 44 μm × 34 μm per observation image, and divided into squares of 2 μm × 2 μm on the computer. The divided squares are visually classified into fine mosaic structures and the others, and 1250 or more squares (5000 μm ² or more) are inspected to obtain the number of squares recognized as fine mosaic tissues. Calculate the area ratio of the tissue.

[Calculation method for Δ2θ of coke]
In order to compare the crystal structure after heat treatment at the same temperature, the coke heat treated at 1100 ° C. is analyzed by X-ray diffraction. The 002 diffraction pattern by X-rays is measured with an X-ray diffractometer (manufactured by Rigaku Corporation, model: RINT2500) in the range of 2θ of 5 to 40 °, and the 002 diffraction peak appearing at 15 to 35 ° is analyzed. An interval between peak positions that is a half value with respect to the maximum value of the peak is obtained by calculation as a peak width Δ2θ.

[Coke R value calculation method]
Coke heat-treated at 1100 ° C. is measured with a microscopic Raman spectroscope (manufactured by JEOL Ltd., JRS-SYS1000 type) under the following measurement conditions.

Excitation light: Ar ion laser 514.4 nm
Excitation light output: 20 mW
Objective mirror magnification: 50 times Integration count: 1 time Irradiation time: 120 seconds Measuring method: 180 ° backscattering Measurement area: 800 to 2000 cm ⁻¹
Wave number calibration: Silicon

A spectrum obtained by measuring a peak derived from D band present in the vicinity of 1360 cm ^-1, the three peaks from G band present in peak and 1580cm around ^-1 from D 'band present in the vicinity of 1600 cm ^-1 Separately, the R value is obtained from the ratio (D / G) of the peak intensity of the D band and the peak intensity of the G band.

  Table 1 shows the results of evaluating the physical properties of coke, which is a precursor of the carbon negative electrode material. Furthermore, the result (photograph) which observed the optical structure of coke AD shown in Table 1 with the polarizing microscope is shown in FIGS. 1-4, respectively.

[X-ray diffraction measurement method and analysis method of negative electrode carbon material]
In Examples and Comparative Examples, the crystal structure of the negative electrode carbon material was evaluated as follows using an X-ray wide-angle diffractometer (manufactured by Rigaku Corporation, model: RINT 2500).

  First, weigh about 0.1 g of the negative electrode carbon material pulverized to 200 mesh or less, and put it in a glass sample holder (35 × 50 × 1 mm) equipped with a recess (20 × 16 × 0.5 mm) for receiving the sample. Consolidation with a simple glass plate. After consolidation, a sample with a flat surface was set in the apparatus and measured. The measurement conditions are as follows.

Tube voltage: 40 kV
Tube current: 200 mA
Gonio radius: 185mm
Scanning axis: 2θ / θ
Step: 0.02
Counting time: 1 second Measuring method: FT
Repeat count: 1 time

  The X-ray scanning range is 10 to 90 °, and the intensity I (100) of the (100) diffraction line peak seen near 42.5 ° and the (101) diffraction line peak seen near 44.5 ° The intensity I (101) / I (100) was obtained by examining the intensity I (101).

  The crystallite size was measured according to the Gakushin method. As standard silicon, NIST650b Silicon Powder XRD Spacing (US Department of Commerce of National Institute of Technology and Rhizo System) is used as analysis software, and Carbon Analysis is used as analysis software. d (002) and La (110) were calculated.

[Measurement of particle size of anode carbon material]
Using a particle size analyzer (Nikkiso Co., Ltd., Microtrac HRA), the particle size distribution and average particle size D50 of the particles were measured.

[Measurement of specific surface area of negative electrode material]
The specific surface area was measured using a nitrogen adsorption BET specific surface area measuring apparatus (manufactured by Canterchrome, NOVA2000).

[Production of negative electrode body]
To the obtained negative electrode carbon material, 5% polyvinylidene fluoride was added, and N-methyl-2-pyrrolidone was mixed as a solvent to form a slurry. Then, the slurry was apply | coated on copper foil using the doctor blade. The obtained laminate was dried and then roll pressed to produce an electrode having a thickness of 40 μm after roll pressing. The electrode thus obtained was vacuum-dried at 110 ° C. for 6 hours.

[Production of battery]
In addition to the obtained negative electrode body, a Li metal foil was used as a counter electrode, and a solution obtained by dissolving LiPF ₆ at a ratio of 1 mol / L in a mixed solvent of ethylene carbonate and ethyl methyl carbonate (volume ratio 1: 2) as an electrolyte solution. A laminate cell was prepared using a polypropylene nonwoven fabric as a separator.

Example 1
Coke A shown in Table 1 was pulverized to an average particle size (D50) of 10 μm, and the pulverized product was heat-treated in a self-generated gas atmosphere at a temperature of 2170 ° C. to prepare a negative electrode carbon material to produce a lithium ion battery. .

Example 2 and Comparative Examples 1-2
A negative electrode carbon material was prepared in the same manner as in Example 1 except that cokes B to D shown in Table 1 were used instead of coke A, and a lithium ion battery was produced.

Examples 3-6 and Comparative Examples 3-4
A negative electrode carbon material was prepared in the same manner as in Example 1 except that the heat treatment temperature was changed to the temperature shown in Table 2, and a lithium ion battery was produced.

Examples 7-8 and Comparative Examples 5-6
A negative electrode carbon material was prepared in the same manner as in Example 1 except that the average particle size (D50) of the pulverized coke A was changed to the particle size shown in Table 2, and a lithium ion battery was produced.

Examples 9-10
A negative electrode carbon material was prepared in the same manner as in Example 3 except that the average particle size (D50) of the pulverized coke A was changed to the particle size shown in Table 2, and a lithium ion battery was produced.

Example 11
A negative electrode carbon material was prepared in the same manner as in Example 6 except that the average particle size (D50) of the pulverized coke A was 20 μm, and a lithium ion battery was produced.

Comparative Example 7
A lithium ion battery was produced in the same manner as in Example 1 except that mesocarbon microbeads (MCMB) having an average particle size of 12 μm were used instead of coke.

Comparative Example 8
A lithium ion battery was produced in the same manner as in Example 1 except that natural graphite having an average particle size of 12 μm was used in place of the heat-treated coke and the heat treatment was performed at a temperature of 2800 ° C.

[Measurement of battery characteristics]
The charge / discharge characteristics of the lithium ion batteries obtained in Examples 1 to 11 and Comparative Examples 1 to 8 were measured as follows.

First, when the lithium ion batteries of Examples 1 to 11 and Comparative Examples 1 to 8 were fully charged with a constant current and a constant voltage at 0.3 C (0.27 mA / cm ² ) under an environmental temperature (25 ° C.). The capacity is the charge capacity. After charging, the battery was discharged at 0.3 C to 1.2 V, the discharge capacity at that time was defined as the initial capacity, and the ratio of the initial capacity to the charge capacity was determined as the initial efficiency (initial charge / discharge efficiency). Then, using the same cell, fully charged with constant current + constant voltage at 0.3 C, discharged to 1.2 V at 10 C (9.0 mA / cm ² ), and the ratio of the discharge capacity at that time to the initial capacity was The capacity retention rate (%) is shown in Table 2 together with the X-ray diffraction parameters of the negative electrode carbon material.

(Test results)
In the batteries of Examples 1 to 11, the capacity retention rates were 94.2%, 94.5%, 92.7%, 87.5%, 86.8%, 85.5%, 96.3%, High output was maintained at 95.4%, 95.2%, 93.5%, and 88.0%.

  On the other hand, in the battery of Comparative Example 1, since the crystal was developed too much as compared with Example 1 that was heat-treated at the same temperature, the capacity retention rate was 86.7%, which was remarkably lowered.

  In the battery of Comparative Example 2, although the capacity retention rate was maintained at 95.2% and high output, the crystal capacity was insufficient compared with Example 1 heat-treated at the same temperature, so the initial capacity was It was 226 mAh / g, and the discharge capacity was significantly reduced.

  In the battery of Comparative Example 3, since the heat treatment temperature was low, the crystal was not sufficiently developed, so the initial capacity was 225 mAh / g, and the discharge capacity was significantly reduced.

  In the battery of Comparative Example 4, since the heat treatment temperature was high and the crystals were excessively developed, the capacity retention rate was 58.6%, and the output characteristics were significantly reduced.

  In the battery of Comparative Example 5, since the average particle size (D50) was small and the specific surface area was high, the initial efficiency was 90.7%, which was remarkably reduced.

  In the battery of Comparative Example 6, although the battery characteristics were satisfactory, the average particle size (D50) exceeded 20 μm, and some of the particles were seen during electrode production pressing.

  In the battery of Comparative Example 7 using MCMB, since the crystals were excessively developed, the initial efficiency was low and the capacity retention rate was 52.7%, which resulted in a greatly inferior output characteristic.

  Similarly, in the battery of Comparative Example 8 using natural graphite, since the crystals were excessively developed, the initial efficiency was low and the capacity retention rate was 35.8%, which resulted in greatly inferior output characteristics.

  That is, by using coke with a mosaic structure ratio, Δ2θ, and R value in specific ranges, and controlling the degree of graphitization (and crystallite size) and particle size to predetermined ranges, high charge / discharge characteristics can be achieved. It was proved to be an excellent negative electrode carbon material for lithium ion batteries.

  According to the negative electrode carbon material of the present invention, the capacity of the negative electrode can be maintained at a high level without extremely reducing the capacity even during charge / discharge with a large current. Therefore, the negative electrode carbon material and the precursor thereof of the present invention can be charged / discharged with a large current, and applications such as electronic devices, electrical devices, automobiles (hybrid vehicles, large vehicles, etc.) are required. ), Lithium-ion batteries for power storage and auxiliary power supplies (particularly lithium-ion batteries for hybrid vehicles).

Claims (12)

  A precursor of a negative electrode carbon material for a lithium ion battery, wherein an optical structure of a cross section observed with a polarizing microscope is an anisotropic structure formed by a fine mosaic structure and a flow structure, and an area ratio of both structures is Coke where the former / the latter = 10/90 to 70/30.   The coke according to claim 1, wherein the fine mosaic structure is a structure composed of an aggregate of fine regions having an average diameter of 10 µm or less, and the flow structure is a structure composed of a continuous phase having an average diameter exceeding 10 µm.   The coke according to claim 1 or 2, wherein the area ratio between the fine mosaic structure and the flow structure is the former / the latter = 15/85 to 50/50.   The coke according to any one of claims 1 to 3, wherein a half width (Δ2θ) of a 002 diffraction line figure of X-ray diffraction is 2.0 to 6.0 ° when heat-treated at 1100 ° C.   The coke according to any one of claims 1 to 4, wherein when heat-treated at 1100 ° C, the R value measured by Raman spectroscopy is 2.0 to 3.0. A negative electrode carbon material for a lithium ion battery satisfying the following characteristics (1) to (4).
(1) Average particle diameter (D50) is 6 to 20 μm
(2) Specific surface area measured by BET method is 3 m ² / g or less (3) Interlayer distance d value (002) in X-ray diffraction is 0.3360 to 0.3395 nm
(4) The crystallite thickness La in the a-axis direction is 60 to 300 nm.   The negative electrode carbon material according to claim 6, wherein an intensity ratio I (101) / I (100) between a (101) diffraction peak and a (100) diffraction peak in X-ray diffraction is 0.7 to 1.5.   The negative electrode carbon material obtained by heat-processing the coke in any one of Claims 1-5.   The negative electrode carbon material according to claim 8, wherein the heat treatment temperature is 2100 to 2700 ° C. The following characteristics (1) to (4) are satisfied, and the intensity ratio I (101) / I (100) between the (101) diffraction peak and the (100) diffraction peak in X-ray diffraction is 0.7 to 1.5. The negative electrode carbon material according to claim 8 or 9.
(1) Average particle diameter (D50) is 6 to 20 μm
(2) Specific surface area measured by BET method is 3 m ² / g or less (3) Interlayer distance d value (002) in X-ray diffraction is 0.3360 to 0.3395 nm
(4) The crystallite thickness La in the a-axis direction is 60 to 300 nm.   The lithium ion battery provided with the negative electrode carbon material in any one of Claims 6-10.   The lithium ion battery according to claim 11, which is a lithium ion battery for a hybrid vehicle.