JP2016056038A - Carbon material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon material useful as the electrode active substance of a nonaqueous electrolyte secondary battery.SOLUTION: Provided is a carbon material in which the interlayer distance din the C axis direction calculated from the diffraction peak from the carbon (002) face by an X-ray wide-angle diffraction method is 0.35 to 0.38 nm, and crystallite thickness Lis 1.1 to 1.8 nm using carbon particles produced by being left under a supercritical liquid using a cyclic compound containing oxygen as a hetero atom as starting raw material.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a carbon material useful as an electrode material or the like.

  As an electrode constituent material of primary and secondary batteries represented by lithium batteries, sodium batteries and electric double layer capacitors, carbon materials having a graphene structure composed of a hexagonal surface of carbon represented by graphite are widely used. Yes. It is known that these carbon materials can improve the capacity characteristics and input / output characteristics of the battery by controlling the microstructure. For example, in order to manufacture a carbon material having high input / output characteristics, a carbon material manufacturing method in which a carbon raw material such as pitch is heat-treated at a high temperature of 1000 to 3000 ° C. is widely known. Patent Document 1 discloses that carbon particles having a controlled fine structure can be obtained by coating a small spherical graphite powder with an oxygen-containing organic substance and performing a heat treatment at a temperature of 800 to 1300 ° C.

Patent No. 3091943 JP 2010-254513 A JP 2013-043807 A

Negative electrode active material of the carbon particles of Patent Document 1, the C-axis direction of the interlayer distance _{d 002} is 0.35Nm～0.37Nm, crystallite thickness _{L C002} is 0.8Nm～1.0Nm, such as lithium-ion batteries When used as, it has a crystal structure capable of realizing a relatively high capacity. However, such a carbon material has such a structure in the surface layer covered with small spherical graphite powder, and there is room for improvement in increasing the capacity of the battery.

  Therefore, an object of the present invention is to provide a carbon material useful as an electrode material such as an electrode active material in order to solve the above-described problems of the conventional technology.

In order to solve the above-mentioned problems, the present invention uses a cyclic compound containing oxygen as a heteroatom (hereinafter, such a compound may be simply referred to as “oxygen-containing cyclic compound”) as a starting material. The carbon material produced | generated by putting under a supercritical fluid is provided. Such a carbon material has an interlayer distance d ₀₀₂ in the C-axis direction calculated from a diffraction peak from the carbon (002) plane by an X-ray wide angle diffraction method of 0.35 nm or more and 0.38 nm or less, and a crystallite thickness L _C002. Is 1.1 nm or more and 1.8 nm or less.

According to such a configuration, a carbon material having a unique crystal structure in which the crystallite thickness L _C002 is extended in the C-axis direction by supercritical fluid processing is provided. Furthermore, by performing a heat treatment at a relatively low temperature of about 1000 ° C. or less (more preferably about 500 ° C. to 800 ° C., for example, a temperature range of about 600 ° C. to 700 ° C.), the electric resistance is reduced, and the battery A carbon material that realizes a high capacity suitable as an electrode active material is also provided.

In a preferred embodiment of the carbon material disclosed herein, the oxygen-containing cyclic compound is any one or more compounds selected from sucrose and derivatives thereof.
According to such a configuration, oxygen atoms can be easily introduced into the carbon network surface constituting the carbon material, and the above-described unique crystal structure can be suitably formed. Due to the presence of oxygen atoms, such a carbon material has characteristics (typically, electrical conductivity and capacity characteristics) as an electrode active material, for example, when further heat treatment is performed in addition to the supercritical fluid treatment. Can be improved. Therefore, the present invention can also provide a battery having a higher capacity (typically a nonaqueous electrolyte secondary battery), an electric double layer capacitor, or the like containing such a carbon material as an electrode active material.

(A) is the figure of carbon material A1-A4 which concerns on one Embodiment, (B) is the figure which illustrated the X-ray-diffraction pattern of conventional carbon material B1-B4. It is a graph which shows the charging / discharging capacity | capacitance of the nonaqueous electrolyte secondary battery which used carbon material A2-A4 which concerns on one Embodiment, and conventional carbon material B2-B4 for the negative electrode active material.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

The carbon material disclosed here has an interlayer distance d ₀₀₂ in the C-axis direction calculated from a diffraction peak from the carbon (002) plane by an X-ray wide angle diffraction method of 0.35 nm to 0.38 nm. The thickness _LC002 is characterized by being 1.1 nm or more and 1.8 nm or less. The interlayer distance d ₀₀₂ in the C-axis direction is thus large, for example, when the carbon material is used as an electrode material of a secondary battery that uses lithium as a charge carrier. It can be said that the storage site of a certain Li ion is increased, and the capacity of the secondary battery can be increased. Further, a large crystallite thickness L _{C002 in the} C-axis direction means that the crystallinity is high in the C-axis direction. Such a carbon material typically has a particulate form, and the crystal structure is not uniformly constructed over the entire particle. For example, the carbon network surface is randomly arranged. Can be configured. Therefore, although not particularly limited, typically, the true density measured by the density meter can be realized as 1.7 or less (for example, 1.2 or more and 1.7 or less).

  The carbon material having such a crystal structure is not particularly limited, but can be suitably produced by, for example, the following method using a reaction field with a supercritical fluid. This manufacturing method is also preferable in that it can be carried out in a range in which all steps (which may include the heat treatment described below in addition to the treatment with the supercritical fluid) do not exceed 1000 ° C., preferably in the range of about 600 ° C. or less. Here, as the starting material, a cyclic compound containing oxygen (O) as a hetero atom, that is, an oxygen-containing cyclic compound is used, and the starting material is placed in the reaction field by the supercritical fluid. Hereinafter, the characteristic of the carbon material disclosed here will be described in detail while describing a preferred embodiment of the method for producing the carbon material disclosed herein.

  As a starting material, an oxygen-containing cyclic compound is used. Here, the cyclic compound is a general term for organic compounds having a cyclic structure based on a carbon skeleton, and in the present invention, a compound in which at least one of the rings of the carbon skeleton is constituted by an oxygen atom. (Heterocyclic compound) is used as a starting material. The oxygen-containing cyclic compound may be a monocyclic compound in which one ring exists in one molecule or a polycyclic compound in which two or more rings exist. In addition, an aromatic cyclic compound having a conjugated unsaturated ring structure (for example, it may be a monocyclic or polycyclic aromatic compound having oxygen as a heteroatom in an aromatic ring), saturated or unsaturated having no aromaticity. It may be an alicyclic compound containing one or more saturated carbocycles (for example, it may be a monocyclic or polycyclic alicyclic compound having oxygen as a heteroatom in the carbocycle). Preferable is a polycyclic compound having two or more rings in one molecule. There is no restriction | limiting in the number of atoms which comprise this ring structure, For example, it may be a compound which has a small ring, a medium ring, or a large ring. Typically, an oxygen-containing cyclic compound having about 1 to 10 members can be considered. By using a material having such a ring structure as a starting material, a carbon material having a more developed carbon network surface (which may be a graphene structure) can be suitably obtained.

The above compound as a starting material is preferably liquid or solid at normal temperature and normal pressure (typically 25 ° C., 1 atm). Specific examples of such oxygen-containing cyclic compounds include, for example, α-acetolactone, β-propiolactone, γ-butyrolactone, δ-valerolactone, ethylene oxide, propylene oxide, epoxide, glycidol, acetolactone, furan, 2 -Methyltetrahydrofuran, oxetane, diketene, tetrahydropyran, sotron, ellagic acid, coumarin, oxazole, telomestatin, tegafur, benzopyran, topiramate, pederin, tannin, flavonol, holoturin, sucrose and the like and their derivatives. Only one kind of such oxygen-containing cyclic compounds may be used, or two or more kinds may be used in combination. Further, as exemplified by the above oxazole, telomestatin, tegafur and the like, a compound containing a hetero atom other than oxygen in the ring structure may be used. Such heteroatoms include nitrogen (N), sulfur (S), boron (B) and the like. As a starting material of the technology disclosed herein, sucrose (C ₁₂ H ₂₂ O ₁₁ ) and derivatives thereof represented by the following general formula are particularly preferable examples.

Such starting materials are present under supercritical fluid. The supercritical fluid is not particularly limited, and may be a supercritical fluid such as water, saturated hydrocarbons such as carbon dioxide, methane, and ethane, alkenes such as ethylene and propylene, and lower alcohols such as methanol and ethanol. Preferably, it is water or carbon dioxide. Since each critical condition is different, a typical carbon material manufacturing method using a water supercritical fluid (ie, supercritical water) will be described below as an example. The same applies to other critical fluids.
That is, the starting material is treated with hydrogen peroxide and water under conditions of a temperature of 300 ° C. to 600 ° C. and a pressure of 22 MPa or more (for example, 22 to 100 MPa). The treatment conditions (temperature and pressure) are the supercritical state of water (critical point of water: 374 ° C. and 22.1 MPa, a high temperature and high pressure state) or the subcritical state of water (the above temperature and pressure range). , A state below the critical point of water). More preferably, it may be in a supercritical state at 374 ° C. or higher and 22.1 MPa or higher. Under such treatment conditions (in other words, in a critical field with supercritical water or subcritical water), by introducing a mixture containing an oxygen-containing cyclic compound, hydrogen peroxide and water, supercritical water or subcritical water In this way, the oxygen-containing cyclic compound can be uniformly dissolved to form a homogeneous system of the oxygen-containing cyclic compound. Further, hydrogen peroxide is thermally decomposed by such a critical field to generate hydroxy radicals. Hydroxyl radicals decompose and polymerize oxygen-containing cyclic compounds as starting materials (typically dissociation of carbon atom bonds) in the temperature range of 300 ° C. to 600 ° C., which is sufficiently low from the viewpoint of decomposition of organic substances. The oxygen-containing cyclic compound can be converted into a carbon material having a carbon network structure. Moreover, the conversion to carbon material (carbonization) can be further promoted by using the starting material as described above.

Specific manufacturing conditions for the treatment with such a supercritical fluid can be appropriately determined with reference to Patent Documents 1 and 2, for example. That is, the amount of hydrogen peroxide (H ₂ O ₂ ) to be reacted with the oxygenated cyclic compound typically degrades the oxygenated cyclic compound completely (eg, the oxygenated cyclic compound Less than half the number of moles M required for complete oxidative decomposition to carbon dioxide and water (typically 1/2 to 1/50, for example 1/5 to 1/20) It is preferable. The amount of water depends on, for example, the configuration of the critical field generator to be used (typically the capacity of the container for generating supercritical water) so that supercritical water or subcritical water can be formed. Can be determined as appropriate. That is, for example, since supercritical water has a density close to that of a liquid and a viscosity close to that of a gas, it has both the properties of a liquid having high solubility and the properties of a gas having good diffusibility and permeability. In addition, for example, supercritical water is in a state where the cohesive force and diffusive force between molecules are balanced, so if there are solute (oxygen-containing cyclic compound) molecules with different intermolecular forces, On the other hand, the solvation effect that the solvent molecules strongly solvate is expressed, and the radical reaction can be actively advanced. From these perspectives, the amount of water is sufficient to include the oxygenated cyclic compound (in other words, sufficient to provide a supercritical field for the oxygenated cyclic compound) or supercritical water or subcritical water. If it can prepare, it will not restrict | limit in particular. As described above, even when the amount of water is appropriately adjusted, the carbon material having the crystal structure can be efficiently formed.

The treatment in the supercritical fluid described above depends on the amount of starting material and the like, the formation conditions of the critical field, etc., but it cannot be generally stated, but it can be set as 1 to 10 hours as an approximate guide. As described above, since the reaction by the hydroxy radical occurs rapidly and continuously, the target carbon material can be sufficiently formed from the organic substance by setting it to about 1 hour or more, for example, about 3 hours or more. Further, it can be about 10 hours or less, for example, about 8 hours. In addition, after the treatment in such a critical field, it is preferable to rapidly cool the produced carbon material to a temperature of at least 100 ° C. within 10 minutes. By performing such rapid cooling, crystals of the formed carbon material can be randomly oriented, and it is preferable because the interlayer distance can be increased. Accordingly, the interlayer distance _{d 002} in the C-axis direction calculated from the diffraction peak of carbon (002) plane by X-ray wide angle diffraction method, for example, on the order or 0.36nm or less 0.35 nm, a crystallite thickness L _C002 is much more 1.8nm or less 1.5 nm, it is possible to particularly crystallite thickness _{L C002} to obtain a high carbon material. Such a carbon material may have, for example, an average particle size of about 1 μm to 100 μm (typically 1 μm to 20 μm). In addition, the average particle diameter regarding the carbon material in this specification is an arithmetic average of equivalent circle diameters measured for 100 or more particles based on observation with a microscope with an appropriate magnification (for example, an electron microscope such as a scanning electron microscope). Value.

  Note that the carbon material that has been treated with the above supercritical fluid has oxygen atoms introduced into its crystal structure (typically within the carbon network surface), and has a relatively low electrical conductivity. obtain. Such heteroatoms (including heteroatoms such as oxygen, nitrogen, boron, sulfur, etc.) can be introduced into the carbon network surface by, for example, an X-ray photoelectron spectroscopy (XPS) method or the like. This can be confirmed by examining the bonding state of the constituent elements. The amount of oxygen introduced is not particularly limited, but is preferably 5 atm% or more. According to XPS analysis, typically, carbon atoms (C), oxygen atoms (O), nitrogen atoms (N), etc. should be identified and quantified from the peaks attributed to C1s, N1s and O1s and their intensities. Can do.

Therefore, in the technology disclosed herein, by subjecting the carbon material that has been treated with the supercritical fluid (hereinafter, also referred to as precursor carbon material) to heat treatment to desorb such oxygen atoms, For example, it is possible to obtain a carbon material (hereinafter also referred to as a heat-treated carbon material) that is superior in electrical characteristics such as electrical conductivity. The temperature of the heat treatment is, for example, more than 100 ° C. and 1000 ° C. or less (for example, less than 1000 ° C., preferably about 400 ° C. to 800 ° C., more preferably about 400 ° C. to 700 ° C., still more preferably about 500 ° C. to 600 ° C. ). If the heat treatment temperature is too low, it may be difficult to obtain the effect of improving electrical conductivity, or the treatment time may be prolonged, resulting in a decrease in productivity. On the other hand, when the heat treatment temperature is too high or too long, the characteristic crystal structure described above is impaired, or an effect of improving cycle characteristics when such a heat treated carbon material is used as an electrode active material is obtained. It is not preferable because it becomes difficult to be done. In addition, this heat treatment can be performed in an inert gas atmosphere including, for example, neon (Ne) gas, argon (Ar) gas, nitrogen (N ₂ ), or a mixed gas thereof. The atmospheric pressure during the heat treatment may be ordinary pressure (atmospheric pressure), or may be increased or reduced. From the viewpoint of ease of operation and equipment costs, it is usually preferable to perform the heat treatment at normal pressure. The heat treatment as described above may cause a slight change in the crystal structure of the precursor carbon material. For example, the interlayer distance d ₀₀₂ in the C-axis direction calculated from the diffraction peak from the carbon (002) plane by the X-ray wide angle diffraction method is about 0.35 nm to _0.38 nm and the crystallite thickness L _C002 Is a heat-treated carbon material having a thickness of about 1.1 nm to 1.6 nm. For example, when the heat-treated carbon material is used as an electrode active material or the like, it has a high capacity as compared with a conventional carbon material, and may have good cycle characteristics. Specifically, for example, the initial capacity may be as large as 600 to 1200 mAh / g as compared to 100 to 300 mAh / g of a normal carbon material.

Hereinafter, although the structural example of the lithium ion battery which used the carbon material (typically heat-treated carbon material) disclosed here for a negative electrode active material is demonstrated, the usage aspect of the said carbon material is not the intention limited to these. .
The lithium ion battery disclosed herein has a form in which a positive electrode and a negative electrode each having an electrode active material capable of reversibly occluding and releasing lithium are housed in a container together with a non-aqueous electrolyte. The negative electrode includes any of the carbon materials disclosed herein as an active material (negative electrode active material). For example, a negative electrode in a form in which the carbon material is fixed to a current collector made of copper or the same alloy together with a binder and a conductive material used as necessary can be preferably used. Examples of the binder include styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC). As the negative electrode active material, a known material that can be used as a negative electrode active material of a general lithium ion battery may be used together with any of the carbon materials disclosed herein.

  As the positive electrode, a material in which a suitable positive electrode active material is fixed to a current collector made of aluminum or an aluminum alloy together with a binder and, if necessary, a conductive material can be preferably used. As the positive electrode active material, an oxide-based active material having a layered structure, an oxide-based active material having a spinel structure, and the like used for a positive electrode of a general lithium ion battery can be preferably used. Typical examples of such active materials include lithium transition metal oxides such as lithium cobalt oxides, lithium nickel oxides, and lithium manganese oxides. Examples of the conductive material include carbon materials such as carbon black (for example, acetylene black) and graphite powder, and conductive metal powder such as nickel powder. As the binder, the same binder as that for the negative electrode can be used.

As the electrolyte interposed between the positive electrode and the negative electrode, a liquid electrolyte (electrolytic solution) containing a nonaqueous solvent and a lithium salt (supporting electrolyte) that can be dissolved in the solvent can be preferably used. It may be a solid (gel) electrolyte in which a polymer is added to such a liquid electrolyte. As the non-aqueous solvent, aprotic solvents such as carbonates, esters and ethers can be particularly preferably used. For example, a mixed solvent such as ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) can be used. As the supporting electrolyte, one or more selected from lithium salts such as LiPF ₆ , LiBF ₄ , LiN (SO ₂ CF ₃ ) _2, or the like, about 0.1 mol / L to 5 mol / L (for example, about 0.8 mol) are used. / L to 1.5 mol / L).

  A lithium ion battery is constructed by housing the positive electrode and the negative electrode together with an electrolyte in a suitable container (a metal or resin casing, a bag made of a laminate film, or the like). A separator may be interposed between the positive electrode and the negative electrode. As a separator, the thing similar to the separator used for a general lithium ion battery can be used, For example, the thing of the laminated structure which consists of polyethylene (PE) and a polypropylene (PP) can be used. The shape (outer shape of the container) of the lithium ion battery is not particularly limited, and may be, for example, a cylindrical shape, a rectangular shape, a coin shape, or the like.

  Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to the specific examples.

[Manufacture of carbon materials]
(Sample A1)
Using sucrose (C ₁₂ H ₂₂ O ₁₁ , almost transparent granular crystals) as a starting material, a carbon material was produced using a supercritical fluid. That is, sucrose as a starting material (carbon and oxygen source) in a supercritical high-pressure vessel (made by pressure-resistant glass industry) made of nickel alloy (Hastelloy C-22) having an internal volume of 10.8 mL. 13 g (15.0 mmol), 30% hydrogen peroxide solution 2.00 g (18.2 mmol H ₂ O ₂ is included. This amount is the number of moles required for complete oxidative decomposition of the above amount of sucrose. corresponding to about 1/12 of M _P.) and ion exchanged water was placed 3g. And this container was covered and it hold | maintained for 3 hours under the conditions (temperature supercritical conditions) of temperature 400 degreeC and pressure 22.1 MPa. Thereafter, the high-pressure vessel was rapidly cooled, and after confirming that the inside of the vessel had returned to room temperature and normal pressure, the contents were taken out. The content was dried after suction filtration and washing with ion exchange water. This obtained black powder A1 (carbon material) as a product.

(Samples A2 to A4)
Further, a part of the powder A1 obtained above was fractionated and subjected to heat treatment for 2 hours at 600 ° C., 800 ° C. and 1000 ° C. in an atmospheric pressure Ar gas atmosphere. As a result, powders A2 to A4 (carbon material) subjected to supercritical fluid treatment and additional heat treatment were obtained, respectively. In addition, when the true density of these materials was measured, it was 1.4 to 1.55 g · cm ³ .

(Samples B1 to B4)
For comparison, the same sucrose used for the preparation of sample A1 was subjected to a heat treatment of holding at 400 ° C., 600 ° C., 800 ° C. and 1000 ° C. for 2 hours in an atmospheric pressure Ar gas atmosphere. This obtained black powder B1-B4, respectively.

[XRD analysis]
The powders A1 to A4 and the powders B1 to B4 obtained above were analyzed for crystal structures by X-ray diffraction (XRD) analysis. Samples for measurement were obtained by uniformly mixing each of the above powders with carbon powder standard silicon for carbon analysis at a ratio of 20% by mass, and a glass having a sample window size of 15 mm × 20 mm and a depth of 0.2 mm. The sample holder was filled uniformly. For the measurement, an XRD analyzer (manufactured by Rigaku Corporation, RINT2200) was used, and the measurement was performed under the following measurement conditions by the θ-2θ method in accordance with JIS R7651: 2007. Specifically, a CuKα ray is used as the radiation source, the applied voltage and current to the radiation source are 40 kV and 30 mA, a curved graphite spectrometer is used as the monochromator, the divergence slit is variable, and the light receiving slit width is 0. The condition was 15 mm, and a scintillation counter was used as the detector. Further, the measurement was performed by continuously scanning a scanning range of 5 to 60 ° under the conditions of a sampling width of 0.010 ° and a scanning speed of 2 ° / min.

The C diffraction peak profile obtained by the measurement is shown in FIG. 1A shows diffraction patterns of the powders A1 to A4 in order from the bottom, and FIG. 1B shows diffraction patterns of the powders B1 to B4 in order from the bottom. In all the samples, C 002 diffraction lines were observed in the vicinity of 2θ of 24 °, and broad diffraction lines were observed in the vicinity of 44 ° of some samples. It is considered that the diffraction peak near 44 ° is an overlap of the diffraction peak from the carbon (100) plane and the diffraction peak from the (101) plane. Therefore, using the structural analysis software (Rigaku Co., Ltd., PDXL), the peak position and full width at half maximum (FWHM) of the diffraction line from the carbon (002) plane are also used. According to the 03 mode, the average interplanar spacing d ₀₀₂ and the crystallite thickness Lc ₀₀₂ were calculated. Further, based on the (10) diffraction peak near 44 °, the peak position, full width at half maximum, and crystallite spread (crystallite diameter La) were calculated. In general, La, which is the width (expansion, crystallite diameter) in the a-axis direction of the crystallite, is calculated from the diffraction peak from the (110) plane. However, here, the diffraction peak from the (100) plane and the diffraction peak from the (101) plane considered to have overlapped (10) were calculated based on the 03 mode of the Gakushin method. These results are shown in Table 1 below.

The XRD patterns in FIGS. 1 (A) and 1 (B) are both broad, and the d ₀₀₂ value that can be an index for graphitization is also larger than the d ₀₀₂ value (about 0.335 nm) of graphite. It was considered that none of the powders A1 to A4 and the powders B1 to B4 had a so-called turbulent structure in which the graphite structure did not develop so much, and disorder was observed in the laminated state of the carbon network surface. Therefore, it was confirmed that the carbon material obtained by subjecting sucrose to a heat treatment at 1000 ° C. or less did not progress much in the degree of graphitization. However, the d ₀₀₂ values of the powders A1 to A4 subjected to the supercritical fluid treatment are approximately 0.357 nm to 0.373 nm, which is higher than the d ₀₀₂ values of the untreated powders B1 to B4 from 0.374 nm to 0.402 nm. It can be seen that these structural features can be distinguished by being significantly smaller.

On the other hand, when attention is paid to the thickness Lc ₀₀₂ of the crystallite, the Lc ₀₀₂ values of the powders A1 to A4 subjected to the supercritical fluid treatment are all 1.1 or more, and the Lc ₀₀₂ values of the untreated powders B1 to B4. It was found to be significantly larger than (0.8038 to 1.018). In addition, the Lc ₀₀₂ values of the powders A1 to A4 are kept higher as the temperature of the additional heat treatment is lower, in other words, as the degree of the additional heat treatment is lower. This contradicts the tendency of “the crystallites become larger as heat treatment is performed at a higher temperature”, which is seen for general carbon materials and Lc ₀₀₂ values of powders B1 to B4. That is, it was confirmed that the carbon material (powder A1) obtained by supercritical fluid treatment of sucrose has a unique crystal structure that is not found in other carbon materials. It was also found that this unique crystal structure is gradually lost by heat treatment of the carbon material (powder A1).

[Production of non-aqueous electrolyte secondary battery]
Using the powders A2 to A4 and the powders B2 to B4 obtained above as the negative electrode active material, a non-aqueous electrolyte secondary battery was produced by the following procedure. That is, first, a binder in which polyvinylidene fluoride (PVDF) was dispersed in N-methylpyrrolidone (NMP) as a solvent at a ratio of 10% by mass was prepared as a binder. For the negative electrode current collector, nickel mesh (manufactured by Nilaco Corporation, purity 99%, wire diameter φ0.10, 100 mesh) was cut into a circle having a diameter of 9 mm and used.
Next, the above binder was mixed at a ratio of 10% by mass with respect to 3 to 5 mg of the negative electrode active material, and kneaded to prepare a negative electrode paste. Then the negative electrode paste was applied to the anode current collector such that 6.2 mg / cm ^2, the compressor (Shimadzu) by By pressure bonding by applying a pressure of 157MPa 5 minutes to prepare a negative electrode.
The powders A1 and B1 have a low degree of development of the graphite structure and a large amount of oxygen introduced into the carbon network surface, so that the electrical resistance is high and are not used as the negative electrode active material in this embodiment. .

As the positive electrode, a lithium foil (manufactured by Santoku Corporation) cut into a diameter of 16 mm was used. In addition, a paper separator (Nippon Advanced Paper Industries Co., Ltd., TF48-50) was used as the separator. In the electrolyte solution, 1 mol of lithium perchlorate (LiClO ₄ ) as a supporting electrolyte was mixed with a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a molar ratio of 1: 1. A non-aqueous electrolyte dissolved at a concentration of / L was used. For the construction of a nonaqueous electrolyte secondary battery for evaluation, a CR2032-type coin cell set (manufactured by Hosen Co., Ltd., cap (negative electrode), spring, spacer, gasket (insulating sealing material) and case (positive electrode) set ) Was used.

The following evaluation cell was constructed in a glove box (Nexus II System, manufactured by VAC) having an Ar atmosphere as an inert gas for the purpose of preventing oxidation of lithium metal used as the positive electrode. Each material used was dried at 100 ° C. for 12 hours or more with a vacuum constant temperature dryer (Tokyo Science Machinery Co., Ltd.).
That is, the positive electrode and the negative electrode prepared above were opposed to each other with a separator interposed between the positive electrode and the negative electrode, and 200 μL of the electrolytic solution was dropped. After that, the gasket, spacer, and spring attached to the coin cell set are appropriately placed in the case, covered with a cap, and the cap is sealed with a manual caulking machine (manufactured by Hosen Co., Ltd.). Type cell (non-aqueous electrolyte secondary battery) was constructed. The cell thus obtained was taken out of the glove box and allowed to stand for 12 hours or longer to allow the electrolytic solution to permeate the separator and the electrode, thereby stabilizing the potential, thereby obtaining an evaluation cell. These evaluation cells were called cells A2 to B4 and B2 to 4 corresponding to the negative electrode active material, and were subjected to the following cycle characteristics evaluation.

[Evaluation of cycle characteristics]
For cells A2-4 and B2-4, the charge capacity and discharge capacity during the charge / discharge cycle were measured. That is, using a charge / discharge device (manufactured by Nagano, BTS2004W), up to 2.8 V at a charge rate of C / 3 of the theoretical capacity of the positive electrode for each of the cells A2 to B4 and 4 in a 25 ° C. environment. After charging with a constant current (CC), the battery was discharged with a constant current (CC) of 1 C up to 0 V, thereby charging and discharging the first cycle. Subsequently, the operation of charging with a constant current (CC) to 2.8 V and the operation of discharging with a constant current to 0 V were repeated up to 10 cycles. The charge capacity and discharge capacity during the charge / discharge cycle are shown in FIG. In FIG. 2, the white legend indicates the capacity during charging, and the black legend indicates the capacity during discharging. In addition, Table 2 below shows the charge capacity, the discharge capacity, and the Coulomb efficiency at the first charge / discharge.

  As is apparent from Table 2 and FIG. 2, A2-A4, which is a carbon material obtained by supercritical fluid treatment, is used as the negative electrode active material, compared with the case where B2-B4, which is a carbon material formed only by heat treatment, is used. When used, the discharge capacity (mAh / g) measured for each triode cell is about 300 to 700 mAh / g, indicating a high capacity that can be preferably used as an electrode active material of a secondary battery. all right. This is because the temperature of the powder A1 exceeds 400 ° C and is about 1000 ° C or less (more preferably about 500 ° C to 800 ° C, for example, 500 ° C to 700 ° C, and further about 500 ° C to 600 ° C). It supports that the powder A1 can be converted into an electrode active material having excellent performance by heat treatment in an inert gas atmosphere.

  From our experience so far, in the powder A1 before the heat treatment, a large amount of oxygen atoms derived from the raw materials and the like are incorporated in the carbon network surface. The oxygen atom is desorbed from the carbon material (for example, including the form of desorbing as carbon monoxide or carbon dioxide with the carbon atom), and many fine pores are formed in the carbon material. It is thought to be due to. Thus, by applying the technique disclosed herein, a carbon material into which a large amount of oxygen is introduced by supercritical fluid processing (for example, considered to be in the range of 10 to 25 atm%) and has a unique crystal structure It was confirmed that a carbon material suitable as an electrode active material for a secondary battery can be effectively produced by heat-treating.

  As mentioned above, although this invention was demonstrated in detail, the said embodiment is only an illustration and what changed and modified the above-mentioned specific example is included in the invention disclosed here.

Claims (2)

A carbon material produced by using a cyclic compound containing oxygen as a hetero atom as a starting material, and placing the starting material in a supercritical fluid,
The interlayer distance d ₀₀₂ in the C-axis direction calculated from the diffraction peak from the carbon (002) plane by the X-ray wide angle diffraction method is 0.35 nm or more and 0.38 nm or less,
A carbon material having a crystallite thickness L _C002 of 1.1 nm to 1.8 nm. The carbon material according to claim 1, wherein the cyclic compound containing oxygen as a hetero atom is any one or more compounds selected from sucrose and derivatives thereof.

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