JP2018097935A - Carbonaceous material, lithium secondary battery, and method of producing carbonaceous material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve input/output characteristics in charge/discharge furthermore.SOLUTION: A lithium secondary battery 20 includes: a positive electrode 22 having a positive electrode active material absorbing and desorbing lithium ions; a negative electrode 23 having a carbonaceous material as a negative electrode active material; and an ion-conductive medium 27 interposed between the positive electrode 22 and the negative electrode 23 and conducting lithium ions. The carbonaceous material serving as an active material includes a nuclear graphite particle having a basal surface and an edge surface, the basal surface having defects introduced therein, and the edge surface having an open end.SELECTED DRAWING: Figure 3

Description

  The present disclosure, which is an invention disclosed in the present specification, relates to a carbonaceous material, a lithium secondary battery, and a method for producing the carbonaceous material.

  Conventionally, lithium secondary batteries using non-aqueous electrolytes not only provide high voltage and high energy density, but also can be reduced in size and weight, so they have already been used in information communication equipment such as personal computers and mobile phones. It has been put into practical use. In recent years, it is also used as a power source mounted on electric vehicles and hybrid electric vehicles due to resource problems and environmental problems. The lithium secondary battery is composed of a lithium transition metal composite oxide as a positive electrode active material, a carbonaceous material as a negative electrode active material, and a lithium salt dissolved in an organic solvent as an electrolytic solution.

  When such a lithium secondary battery is used in a vehicle, high input characteristics with a regenerative brake and quick charge characteristics for charging in a short time may be required. At the time of charging, lithium ions desorbed from the positive electrode active material are inserted into graphite as the negative electrode active material through the electrolytic solution. However, when the lithium ion insertion reaction into the graphite is performed at a high rate, the diffusion of lithium ions from the surface of the graphite particles into the interior becomes rate-limiting, and metallic lithium may be deposited on the graphite surface. For this reason, it has been proposed to improve the input / output and charge / discharge efficiency by coating the surface of graphite particles with pitch, an organic compound or a polymer compound and firing or graphitizing (for example, Patent Documents 1 to 4). Etc.)

JP 2014-99419 A JP 2014-89975 A JP 2012-216545 A JP 2013-219023 A

  However, as described in Patent Documents 1 to 4 above, by providing a coating layer on the surface of the graphite particles, the input / output characteristics and the charge / discharge efficiency durability may be improved, but this is an essential solution. It was not enough and still not enough. Generally, when the amount of the coating layer is increased, the input / output characteristics and durability characteristics are saturated, and the charge / discharge efficiency tends to decrease. On the other hand, when surface roughening treatment or the like is performed on graphite, storage durability may be reduced.

  This indication is made in view of such a subject, and it aims at providing the carbonaceous material, lithium secondary battery, and carbonaceous material which can improve input-output characteristics more.

  As a result of diligent research to achieve the above-described object, the present inventors can improve the input / output characteristics in charge and discharge when heat treatment is performed on graphite using an aromatic carboxylic acid. The inventors have found what can be done and have completed the present invention.

That is, the carbonaceous material of the present disclosure is
A carbonaceous material used as an electrode active material,
It includes nuclear graphite particles having a basal surface and an edge surface, defects are introduced into the basal surface, and the edge surface is open.

The lithium secondary battery of the present disclosure is
An electrode containing the carbonaceous material described above as an active material;
An ion conducting medium for conducting lithium ions;
It is equipped with.

The method for producing the carbonaceous material of the present disclosure includes:
A method for producing a carbonaceous material used for an electrode active material,
A nuclear graphite treatment process in which an aromatic carboxylic acid is added to a nuclear graphite particle having a base surface and an edge surface and heated in an inert atmosphere;
Is included.

  The carbonaceous material, lithium secondary battery, and carbonaceous material of the present disclosure can further improve input / output characteristics in charge / discharge. The reason why such an effect is obtained is presumed as follows. For example, when nuclear graphite particles are heat-treated with an aromatic carboxylic acid, it is presumed that defects are introduced into the base surface, thereby making it easier to occlude and release carrier ions. In addition, there may be a clogged portion on the edge surface that is the carbon layered end, but when the nuclear graphite particles are heat-treated with aromatic carboxylic acid, such a clogged site opens, It is presumed that the ions of the carrier are easily occluded and released. Therefore, in a lithium secondary battery using this carbonaceous material as an electrode active material, the input / output characteristics in charge / discharge can be further improved, for example, by reducing the reaction resistance at a low temperature such as −30 ° C. It is guessed.

Explanatory drawing of the nuclear graphite particle 10 contained in a carbonaceous material. Explanatory drawing of a nuclear graphite process and a coating process. FIG. 2 is a schematic diagram illustrating an example of a lithium secondary battery 20. CO ₂ - relationship diagram between the temperature and the heating mass reduction rate in the heat mass measurement.

  Next, the carbonaceous material, the manufacturing method thereof, and the lithium secondary battery of the present disclosure will be described with reference to the drawings. This carbonaceous material is used for an electrode active material, and includes nuclear graphite particles having a basal surface and an edge surface. Defects are introduced into the basal surface, and the edge surface is open. Is. That is, the basal surface and the edge surface of the carbonaceous material are activated. FIG. 1 is an explanatory diagram of nuclear graphite particles 10 contained in a carbonaceous material. The nuclear graphite particle 10 has a basal surface 11 that is a surface of a laminated carbon layer and an edge surface 12 that is a surface on the end side of the laminated carbon layer. Defects 13 are introduced into the basal surface 11 of the nuclear graphite particle 10, and the edge surface 12 is open. When a carbonaceous material containing nuclear graphite particles 10 is used as an active material of a lithium secondary battery, lithium ions of carriers are inserted and desorbed from the edge surface 12. Further, in the nuclear graphite particles 10, lithium ions can be inserted and desorbed from the defects 13 on the basal surface 11. The nuclear graphite particles 10 may be subjected to nuclear graphite treatment with an aromatic carboxylic acid, which will be described in detail later. Further, this graphite 10 is preferable because the edge surface 12 is more open and the diffusion resistance of lithium ions is small.

  Examples of the nuclear graphite particles 10 contained in the carbonaceous material include natural graphite (scale-like graphite, scale-like graphite) and artificial graphite. Further, the nuclear graphite particles 10 may be those that have been subjected to mechanical shape control. For example, the nuclear graphite particles 10 may be those obtained by taking corners of the graphite particles, rounding them into a spherical shape, or pulverizing them. Such shape control is considered to suppress selective orientation and inhibit lithium ion insertion and desorption.

This carbonaceous material may have a maximum value in the profile of the heating mass reduction rate (mass% / ° C.) in the thermal mass (TG) measurement under the CO ₂ atmosphere. In this thermal mass (TG) measurement, ₂ to ³ mg of carbonaceous material powder is placed in a platinum pan, and CO ₂ is flowed at a flow rate of 250 cm ³ / min, while 20 ° C./min from room temperature (20 ° C.) to 700 ° C. From 700 ° C. to 950 ° C., the heating rate is 2 ° C./min. In this thermal mass measurement, α-alumina is used as a standard substance. The heating mass reduction rate (mass% / ° C.) is calculated by differentiating the obtained mass change curve which is measurement data in the range of 700 ° C. or more and 950 ° C. or less. This carbonaceous material has a maximum value in this profile. The carbonaceous material may have a maximum value of 0.001 (% by mass / ° C.) or more in thermal mass measurement. Moreover, this carbonaceous material is good also as a thing with the said maximum value being 0.005 (mass% / degreeC) or less in a thermal mass measurement. Moreover, this carbonaceous material is good also as what exists in the range whose said maximum value is 850 degreeC or more and 950 degrees C or less in a thermal mass measurement. Since the surface structure of nuclear graphite affects the rate at which CO ₂ reacts with graphite, for example, the presence or absence of defects on the basal surface and the state of the edge surface, the mass loss rate by heating (mass% / ° C) profile is assumed to have a maximum value. Therefore, when there is a maximum value in the profile of the heating mass reduction rate (mass% / ° C.), as described above, there are defects 13 on the basal surface 11 and many of the edge surfaces 12 are open. I can judge.

  This carbonaceous material may be one in which the surface of the nuclear graphite particle 10 is coated with an amorphous carbon layer. Such a structure is preferable because an increase in specific surface area can be further suppressed. The amorphous carbon layer preferably covers the entire surface of the nuclear graphite particle 10 uniformly. In this way, it can be considered that a material with less variation can be obtained and the electrode capacity can be controlled more appropriately. The thickness of the amorphous carbon layer is not particularly limited, but is preferably in the range of 10 nm to 200 nm, and more preferably in the range of 20 nm to 150 nm. Within this range, lithium ions can be easily inserted and removed, and the specific surface area can be controlled to an appropriate value. The raw material and production method of amorphous carbon are not particularly limited. For example, heavy carbon such as coal-based or petroleum-based tar, pitch, and asphalt may be carbonized.

This carbonaceous material preferably has a BET specific surface area of 7.0 m ² / g or less due to Kr at liquid nitrogen temperature. Since this specific surface area greatly affects the initial charge / discharge efficiency and the high-temperature storage durability, it is desirable to suppress the specific surface area to 7.0 m ² / g or less. The specific surface area is preferably 2.9 m ² / g or more, and more preferably 3.0 m ² / g or more. The carbonaceous material preferably has an average particle size (D50%) of 1 μm to 100 μm, more preferably 3 μm to 30 μm, and still more preferably 5 μm to 25 μm. The true specific gravity is preferably 1.5 to 2.3, and more preferably 2.0 to 2.3.

  Next, the manufacturing method of this carbonaceous material is demonstrated. This manufacturing method is a method for manufacturing a carbonaceous material used for an electrode active material, and includes a nuclear graphite processing step for performing nuclear graphite processing. This manufacturing method may further include a coating step of coating the surface of the nuclear graphite particles after the nuclear graphite treatment with amorphous carbon. 2A and 2B are explanatory diagrams of nuclear graphite treatment and coating treatment. FIG. 2A is a diagram of graphite 10 before nuclear graphite treatment, FIG. 2B is a diagram of graphite 10 after nuclear graphite treatment, FIG. (C) is the figure of the graphite 10 after a coating process.

(Nuclear graphite treatment process)
In this step, a nuclear graphite treatment is performed in which an aromatic carboxylic acid is added to the nuclear graphite particles 10 having the base surface 11 and the edge surface 12 and heated in an inert atmosphere. As shown in FIG. 2A, the nuclear graphite particles 10 before the nuclear graphite treatment may have a closed portion 14 on the edge surface 12 in which the carbon layers are closed in the manufacturing process. The blocking portion 14 can be a diffusion resistance of lithium ions. In this nuclear graphite treatment, the closed portion 14 is removed to open the edge surface 12 and a hole-like defect 13 is introduced into the base surface 11. Since this aromatic carboxylic acid is mixed with nuclear graphite particles and subjected to heat treatment, it is desirable that the aromatic carboxylic acid is a substance that is solid at room temperature. In this step, natural graphite (scale-like graphite, scale-like graphite), artificial graphite, or the like can be used as the raw material nuclear graphite particles, for example, as described above. Further, the nuclear graphite particles may be subjected to mechanical shape control. The core graphite particles of the raw material preferably have an average particle size of 1 μm or more and 100 μm or less, more preferably 3 μm or more and 30 μm or less, and further preferably 5 μm or more and 25 μm or less. The average particle diameter is a value obtained by measuring the diameter of particles using an image observed with a scanning electron microscope (SEM) and calculating the average. In this step, as the aromatic carboxylic acid, for example, a compound having a structure having two or more aromatic ring structures and carboxylic acids may be used. The aromatic ring structure may be, for example, a structure in which one or two or more benzene rings are bonded, or a structure in which two or more benzene rings are condensed. Examples of such aromatic carboxylic acids include phthalic acid, isophthalic acid, terephthalic acid, benzoic acid, pyridinecarboxylic acid, biphenyldicarboxylic acid, naphthalenedicarboxylic acid, piperidinecarboxylic acid, anthracenecarboxylic acid, and anhydrides of the above compounds. Such as things. Since the carbon layer of the nuclear graphite particle 10 includes an aromatic ring structure and is more preferably eroded by carboxylic acid, it is presumed that aromatic carboxylic acid is preferable for the nuclear graphite treatment. In this treatment, one kind of aromatic carboxylic acid may be used, or two or more kinds may be mixed and used.

  The amount of the aromatic carboxylic acid added is, for example, preferably 1% by mass or more and more preferably 5% by mass or more with respect to the total mass of the nuclear graphite particles and the aromatic carboxylic acid. Moreover, this addition amount is preferably 20% by mass or less, and more preferably 15% by mass or less. If the addition amount is within this range, it is suitable for nuclear graphite treatment. Examples of the inert atmosphere in which this treatment is performed include a rare gas atmosphere and a nitrogen atmosphere. Among these, an Ar atmosphere is more preferable in consideration of the influence on the nuclear graphite itself. Further, the heating temperature may be a temperature range in which the defect 13 on the base surface 11 can be formed and the blocking portion 14 can be removed. For example, a range of 700 ° C. to 1100 ° C. is preferable, and 850 ° C. to 950 ° C. More preferably, it is the range. When the temperature is 700 ° C. or higher, the above-described defect formation and occlusion removal can be sufficiently performed, and when the temperature is 1100 ° C. or lower, the influence on the structure of the nuclear graphite particles 10 can be further reduced. What is necessary is just to select a heating time suitably, for example, it is preferable that it is 1 hour or more, and it is more preferable that it is 5 hours or more. Further, this heating time is preferably 24 hours or less, and more preferably 12 hours or less. When the heating time is 1 hour or longer, the above defect formation and occlusion removal can be sufficiently performed, and when the heating time is 24 hours or shorter, the production efficiency of the carbonaceous material is improved. By performing such treatment, the nuclear graphite particles 10 after the nuclear graphite treatment are obtained (see FIG. 2B).

(Coating process)
In this step, a coating process is performed in which the surface of the nuclear graphite particles 10 after the nuclear graphite process is coated with the amorphous carbon layer 15. In the coating process, by adding an amorphous carbon raw material to the nuclear graphite particles 10 and heating them in an inert atmosphere, the melted component and the vaporized component adhere to the surface of the graphite particles. Alternatively, amorphous carbon may be formed on the surface of the nuclear graphite particles 10 by a method combining the vapor phase method. Alternatively, amorphous carbon may be formed on the surface of the nuclear graphite particles 10 by a vapor phase method (for example, CVD or sputtering). Alternatively, a solid phase method may be used in which an amorphous carbon raw material is added to the nuclear graphite particles 10 and heated. Here, a combination of a solid phase method and a gas phase method will be mainly described. Examples of the raw material for amorphous carbon include coal-based or petroleum-based tar, pitch, and asphalt. The amount of the amorphous carbon raw material added may be, for example, an empirically obtained amount capable of forming a uniform amorphous carbon layer 15 having a predetermined thickness on the surface of the nuclear graphite particle 10, and may be the obtained value. . The thickness of the amorphous carbon layer 15 is preferably in the range described above, for example. About half of the added amount of amorphous carbon is considered to remain on the nuclear graphite particles 10. The remaining amorphous carbon on the nuclear graphite particles 10 is preferably in the range of 1% by mass to 10% by mass, and more preferably in the range of 4% by mass to 8% by mass. For this reason, the addition amount of the raw material of amorphous carbon is preferably 2% by mass or more, and more preferably 8% by mass or more with respect to the total mass of the nuclear graphite particles and the raw material of amorphous carbon. Moreover, this addition amount is preferably 20% by mass or less, and more preferably 16% by mass or less. If the addition amount is within this range, it is suitable for coating treatment.

  Examples of the inert atmosphere in which this treatment is performed include a rare gas atmosphere and a nitrogen atmosphere, and among these, an Ar atmosphere is more preferable. The heating temperature may be a temperature range in which this raw material is converted to amorphous carbon, and may be, for example, a range of 600 ° C. or higher and 2000 ° C. or lower, or a range of 900 ° C. or higher and 1200 ° C. or lower. What is necessary is just to select a heating time suitably, for example, it is preferable that it is 1 hour or more, and it is more preferable that it is 5 hours or more. Further, this heating time is preferably 24 hours or less, and more preferably 12 hours or less. When the heating time is 1 hour or longer, the amorphous carbon layer 15 can be sufficiently formed, and when it is 24 hours or shorter, the production efficiency of the carbonaceous material is improved. By performing such a treatment, a carbonaceous material including the nuclear graphite particles 10 coated on the amorphous carbon layer 15 is obtained (see FIG. 2C).

  A lithium secondary battery of the present disclosure includes an electrode including the above-described carbonaceous material as an active material, and an ion conductive medium that conducts lithium ions. This lithium secondary battery may be a lithium ion secondary battery in which an electrode including a carbonaceous material as an active material is used as a negative electrode. That is, the lithium secondary battery includes a positive electrode having a positive electrode active material that occludes and releases lithium ions, a negative electrode having the above-described carbonaceous material that occludes and releases lithium ions as a negative electrode active material, and a positive electrode and a negative electrode. And an ion conduction medium that conducts lithium ions interposed therebetween.

The positive electrode of this lithium secondary battery is, for example, a mixture of a positive electrode active material, a conductive material, and a binder, and an appropriate solvent is added to form a paste-like positive electrode mixture on the surface of the current collector. However, it may be compressed to increase the electrode density as necessary. As the positive electrode active material, a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, or the like can be used. Specifically, transition metal sulfides such as TiS ₂ , TiS ₃ , MoS ₃ , and FeS _2, and the basic composition formula are Li _(1-x) MnO ₂ (0 <x <1, etc., the same shall apply hereinafter) and Li _{(1 -x)} Lithium-manganese composite oxides such as Mn ₂ O _4, lithium cobalt composite oxides whose basic composition formula is Li _(1-x) CoO _2, etc., basic composition formulas such as Li _(1-x) NiO ₂ lithium nickel composite oxide and a _{Li (1-x) Ni a} Co b Mn c O 2 (a + b + c = 1) lithium-nickel-cobalt-manganese composite oxide, and the like basic formula, LiV ₂ O ₃ the basic formula Or a transition metal oxide having a basic composition formula of V ₂ O ₅ or the like can be used. Of these, lithium transition metal composite oxides such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiV ₂ O ₃ are preferable. The “basic composition formula” is intended to include other elements. The conductive material is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance of the positive electrode. For example, acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, (Nickel, aluminum, silver, gold, etc.) can be used. Among these, as the conductive material, carbon black and acetylene black are preferable from the viewpoints of electron conductivity and coatability. The binder serves to bind the active material particles and the conductive material particles. For example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), a fluorine-containing resin such as fluororubber, polypropylene, Thermoplastic resins such as polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. In addition, an aqueous dispersion of cellulose or styrene butadiene rubber (SBR), which is an aqueous binder, can also be used. Examples of the solvent for dispersing the positive electrode active material, the conductive material, and the binder include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, and N, N-dimethylaminopropylamine. Organic solvents such as ethylene oxide and tetrahydrofuran can be used. Moreover, a dispersing agent, a thickener, etc. may be added to water, and an active material may be slurried with latex, such as SBR. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more. Examples of the application method include roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, and the like, and any of these can be used to obtain an arbitrary thickness and shape. Current collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, and aluminum, copper, etc. for the purpose of improving adhesion, conductivity, and oxidation resistance. A surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group. The thickness of the current collector is, for example, 1 to 500 μm.

  The negative electrode of the lithium secondary battery may be formed, for example, by adhering a negative electrode active material of a carbonaceous material and a current collector, or by mixing a negative electrode active material of a carbonaceous material and a binder. A paste-like negative electrode mixture obtained by adding an appropriate solvent may be applied to the surface of the current collector and dried, and may be compressed to increase the electrode density as necessary. The carbonaceous material mentioned above shall be used as a negative electrode active material. In addition, as the binder and the solvent used for the negative electrode, those exemplified for the positive electrode can be used. The negative electrode current collector includes copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., as well as improved adhesion, conductivity and reduction resistance. For the purpose, for example, a copper surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. The shape of the current collector can be the same as that of the positive electrode.

The negative electrode of the lithium secondary battery preferably has a reaction resistance value at −30 ° C. of 1200Ω or less, more preferably 1000Ω or less, and still more preferably 800Ω or less. Further, in this negative electrode, the reaction resistance value at −30 ° C. is preferably lower, but may be, for example, 500Ω or more. The reaction resistance value of the negative electrode is a value obtained by performing impedance measurement on a symmetrical cell in which two negative electrodes that have been charged with constant current and constant voltage at 0.005 V are opposed to each other. This impedance measurement is performed by measuring ⁵ mV at −30 ° C. and a frequency range of 10 ^{5 to} 0.002 Hz, and fitting the arc portion of the obtained impedance spectrum using a parallel circuit of a resistor R and a capacitance C. The resistance value R (Ω) is calculated.

  As the ion conduction medium of the lithium secondary battery, a non-aqueous electrolyte solution containing a supporting salt, a non-aqueous gel electrolyte solution, or the like can be used. Examples of the solvent for the nonaqueous electrolytic solution include carbonates, esters, ethers, nitriles, furans, sulfolanes and dioxolanes, and these can be used alone or in combination. Specifically, as carbonates, cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t -Chain carbonates such as butyl carbonate, di-i-propyl carbonate, t-butyl-i-propyl carbonate, cyclic esters such as γ-butyllactone and γ-valerolactone, methyl formate, methyl acetate, ethyl acetate, Chain esters such as methyl butyrate, ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; Examples include furans such as lan, methyltetrahydrofuran, sulfolanes such as sulfolane and tetramethylsulfolane, and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, the combination of cyclic carbonates and chain carbonates is preferable. According to this combination, not only the cycle characteristics representing the battery characteristics in repeated charge and discharge are excellent, but also the viscosity of the electrolyte, the electric capacity of the obtained battery, the battery output, etc. should be balanced. it can. The cyclic carbonates are considered to have a relatively high relative dielectric constant and increase the dielectric constant of the electrolytic solution, and the chain carbonates are considered to suppress the viscosity of the electrolytic solution.

Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiSbF ₆ , LiSiF ₆ , LiAlF ₄ , LiSCN, LiClO. ₄ , LiCl, LiF, LiBr, LiI, LiAlCl _{4 and the} like. Among these, from the group consisting of inorganic salts such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , and organic salts such as LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) _3. It is preferable from the viewpoint of electrical characteristics to use a combination of one or two or more selected salts. The supporting salt preferably has a concentration in the non-aqueous electrolyte of 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less. When the concentration for dissolving the supporting salt is 0.1 mol / L or more, a sufficient current density can be obtained, and when it is 5 mol / L or less, the electrolytic solution can be made more stable. Moreover, you may add flame retardants, such as a phosphorus type and a halogen type, to this non-aqueous electrolyte.

  Further, instead of the liquid ion conducting medium, a solid ion conducting polymer may be used as the ion conducting medium. As the ion conductive polymer, for example, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, vinylidene fluoride and a supporting salt can be used. Further, an ion conductive polymer and a non-aqueous electrolyte can be used in combination. In addition to the ion conductive polymer, an inorganic solid electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, an inorganic solid powder bound by an organic binder, or the like can be used as the ion conductive medium.

  This lithium secondary battery may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the lithium secondary battery. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, or a thin fine olefin resin such as polyethylene or polypropylene is used. A porous membrane is mentioned. These may be used alone or in combination.

  The shape of the lithium secondary battery is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a rectangular type. Moreover, you may apply to the large sized thing etc. which are used for an electric vehicle etc. FIG. 3 is a schematic diagram illustrating an example of the lithium secondary battery 20 of the present disclosure. The lithium secondary battery 20 includes a cup-shaped battery case 21, a positive electrode 22 having a positive electrode active material and provided at a lower portion of the battery case 21, a negative electrode active material having a separator 24 with respect to the positive electrode 22. A negative electrode 23 provided at a position facing each other, a gasket 25 formed of an insulating material, and a sealing plate 26 disposed in the opening of the battery case 21 and sealing the battery case 21 via the gasket 25. I have. In the lithium secondary battery 20, an ion conductive medium 27 is filled in a space between the positive electrode 22 and the negative electrode 23. This negative electrode 23 contains the carbonaceous material mentioned above as a negative electrode active material.

  In the carbonaceous material detailed above, the manufacturing method thereof, and the lithium secondary battery using the carbonaceous material, the input / output characteristics in charge / discharge can be further improved. The reason why such an effect is obtained is presumed as follows. For example, when the nuclear graphite particles 10 are heat-treated with an aromatic carboxylic acid, it is presumed that defects 13 are introduced into the base surface 11 so that the lithium ions of the carrier are easily occluded and released. Further, the edge surface 12 which is the end portion of the carbon layer of the nuclear graphite particle 10 may have a blocked portion (blocking portion 14). When the nuclear graphite particle 10 is heat-treated with an aromatic carboxylic acid, It is presumed that the opening of such a closed portion 14 makes it easier to occlude and release lithium ions of the carrier. Therefore, in a lithium secondary battery using this carbonaceous material as an electrode active material, the input / output characteristics in charge / discharge can be further improved, for example, by reducing the reaction resistance at a low temperature such as −30 ° C. It is guessed.

  Note that the carbonaceous material, lithium secondary battery, and carbonaceous material of the present disclosure are not limited to the above-described embodiments, and can be implemented in various modes as long as they belong to the technical scope of the present disclosure. Not too long.

  Hereinafter, an example in which the lithium secondary battery of the present disclosure is specifically manufactured will be described as an experimental example. Experimental examples 1 and 2 correspond to examples, and experimental examples 3 and 4 correspond to comparative examples.

[Experimental Example 1]
The nuclear graphite powder obtained by spheroidizing the scaly natural negative electrode (Nippon Graphite spheroidized graphite, CGB-10) was mixed with 4,4′-biphenyldicarboxylic acid so as to be 10% by mass, in an Ar atmosphere. Heat treatment was performed by holding at 900 ° C. for 6 hours (nuclear graphite treatment). The resulting nuclear graphite powder was mixed so that the coal tar pitch was 10% by mass, and kept at 1000 ° C. for 6 hours in an Ar atmosphere to perform heat treatment (coating treatment). In this way, the carbonaceous material powder of Experimental Example 1 was obtained in which defects serving as a lithium insertion / extraction path were introduced into the nuclear graphite and the surface was covered with an amorphous carbon layer. The obtained powder, a styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a thickener were weighed so as to have a mass ratio of 98: 1: 1 and dispersed in water as a solvent. A material paste was prepared. This negative electrode mixture paste was applied on a copper foil (thickness 10 μm) as a current collector. The current collector was vacuum dried and then subjected to a rolling process with a roll press to prepare a negative electrode sheet.

[Experiment 2]
The carbonaceous material powder of Experimental Example 2 was obtained through the same steps as in Experimental Example 1 except that the coating treatment was not performed. Using this Experimental Example 2, a negative electrode sheet was produced in the same manner as in Experimental Example 1.

[Experimental Examples 3 and 4]
The carbonaceous material powder of Experimental Example 3 was obtained through the same steps as in Experimental Example 1 except that the nuclear graphite treatment was not performed. In addition, the carbonaceous material powder of Experimental Example 4 was used without performing any nuclear graphite treatment or coating treatment. Using these Experimental Examples 3 and 4, negative electrode sheets were produced in the same manner as in Experimental Example 1.

[Thermal mass measurement (TG) in CO ₂ atmosphere]
While placing 2-3 mg of the above carbonaceous material powder in a platinum pan and flowing CO ₂ at a flow rate of 250 cm ³ / min, from room temperature (20 ° C.) to 700 ° C., 20 ° C./min, from 700 ° C. to 950 ° C. Performed thermal mass measurement at a rate of temperature increase of 2 ° C./min. In this measurement, α-alumina was used as a standard substance. The mass reduction curve (mass% / ° C.) was calculated by differentiating the mass change curve for the obtained measurement data in the range of 700 ° C. to 950 ° C.

[Specific surface area measurement by Kr-BET]
After drying 300 mg of the above-mentioned carbonaceous material powder at 150 ° C. for 1 hour, using a specific surface area measuring device (Quantochrome Autosorb), the sample is cooled with liquid nitrogen, and Kr gas adsorption is performed at the liquid nitrogen temperature using a multipoint method. The specific surface area was calculated according to the BET method.

[Battery evaluation]
The prepared negative electrode sheet was punched into a size of 2 cm ² to prepare a negative electrode / Li metal cell using an electrolyte containing lithium as a negative electrode, lithium metal as a counter electrode, and lithium salt. As the electrolytic solution, a solution obtained by dissolving 1M LiPF ₆ in a solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 3: 4: 3 was used. Using this cell, initial charge was performed at 0.005 V at a current density of 0.05 mA / cm ² , and discharge was performed at 1.5 V at the same current density. Then, after carrying out constant current constant voltage charge to 0.005V, the cell was disassembled in the glove box, and the symmetrical cell was produced by making it oppose the electric potential adjusted negative electrode similarly. The obtained symmetric cell was subjected to impedance measurement using a frequency analyzer manufactured by Solartron. The impedance is measured at −30 ° C. at 5 mV and in the frequency range of 10 ^{5 to} 0.002 Hz, and the resistance value is obtained by fitting the arc portion of the obtained impedance spectrum using a parallel circuit of a resistor R and a capacitance C. R (Ω) was calculated.

(Results and discussion)
Table 1 shows the details of the negative electrode treatment, the presence or absence of the maximum value of the CO ₂ -TG heating mass reduction rate, the Kr-BET specific surface area (m ² / g), and the negative electrode reaction resistance (Ω) at −30 ° C. FIG. 4 is a relationship diagram between the temperature (° C.) and the heating mass reduction rate (mass% / ° C.) in CO ₂ -thermal mass measurement. As shown in FIG. 4, in the thermal mass measurement under a CO ₂ atmosphere, the presence or absence of the maximum value of the heating mass reduction rate was confirmed. As a result, the experimental examples 1 and 2 subjected to the nuclear graphite treatment have a maximum value. I understood it. This is presumed that the surface structure of nuclear graphite has an effect on the rate at which CO ₂ reacts with graphite, for example, the presence or absence of defects on the basal surface and the state of the edge surface. It was. This maximum value was 0.001 (mass% / ° C.) or more and 0.005 (mass% / ° C.) or less, and was found to exist in the range of 850 ° C. or more and 950 ° C. or less.

Also, Kr-BET specific surface area of the carbonaceous material, while the untreated Experiment 4 is 7.42m ² / g, and Experimental Example 2 was carried out nuclear graphite treatment 12.14m ² / g Rose. On the other hand, when the coating treatment was performed, the specific surface area decreased regardless of the presence or absence of the nuclear graphite treatment. Since the specific surface area greatly affects the initial charge / discharge efficiency and the high temperature storage durability, it is desirable to suppress the specific surface area to 7 m ² / g or less. Moreover, it was speculated that the specific surface area is more preferably 2.9 m ² / g or more. It was presumed that the pore diameter of the defects introduced into the basal surface of the nuclear graphite is preferably smaller than the thickness of the amorphous carbon layer. This is because, when the defect is filled with the amorphous carbon layer, an increase in the specific surface area is suppressed, and it is considered that the high temperature storage durability can be further improved.

  As shown in Table 1, as a result of measuring the negative electrode reaction resistance at −30 ° C. with respect to the symmetrical cells using the negative electrodes of Experimental Examples 1 to 4, the untreated Experimental Example 4 was subjected to nuclear graphite treatment and coating treatment. It has been found that the negative electrode reaction resistance is further reduced by performing at least one of them. Moreover, as shown in Experimental Example 1, it was found that the negative electrode reaction resistance can be halved without increasing the specific surface area by superimposing the nuclear graphite treatment and the coating treatment. This is because, in the graphite surface structure inside the amorphous carbon layer, defects are introduced into the basal surface or more edge surfaces are opened, so that a lithium ion diffusion path is secured, and the carbonaceous material negative electrode It was assumed that this was because the reaction at the / electrolyte interface was promoted. Thus, it was speculated that by reducing the negative electrode reaction resistance, it is possible to provide a lithium (ion) secondary battery having excellent input / output characteristics.

  Also, in nuclear graphite treatment, it is presumed that by using aromatic carboxylic acid, especially compounds having an aromatic ring structure and carboxylic acid, it is effective for introducing defects on the basal plane of graphite and opening edges. It was done. This is presumed to be because graphite contains an aromatic ring structure and is more suitable for erosion by carboxylic acid.

  In addition, this invention is not limited to the Example mentioned above at all, and as long as it belongs to the technical scope of this invention, it cannot be overemphasized that it can implement with a various aspect.

  The present invention can be used in the technical field related to secondary batteries.

  10 nuclear graphite particles, 11 basal surface, 12 edge surface, 13 defect, 14 closed portion, 15 amorphous carbon layer, 20 lithium secondary battery, 21 battery case, 22 positive electrode, 23 negative electrode, 24 separator, 25 gasket, 26 Sealing plate, 27 Ion conduction medium.

Claims (9)

A carbonaceous material used as an electrode active material,
Including nuclear graphite particles having a basal surface and an edge surface, defects are introduced into the basal surface, and the edge surface is open,
Carbonaceous material. The carbonaceous material according to claim 1, wherein the carbonaceous material has a maximum value in a profile of a heating mass reduction rate (mass% / ° C.) in thermal mass measurement under a CO ₂ atmosphere.   The carbonaceous material according to claim 2, wherein the carbonaceous material has a maximum value of 0.001 (% by mass / ° C) or more and a range of 850 ° C or more and 950 ° C or less in the thermal mass measurement.   The carbonaceous material according to any one of claims 1 to 3, wherein a surface of the nuclear graphite particle is coated with an amorphous carbon layer. The carbonaceous material according to any one of claims 1 to 4, wherein the carbonaceous material has a BET specific surface area by Kr at a liquid nitrogen temperature of 7.0 m ² / g or less. An electrode containing the carbonaceous material according to any one of claims 1 to 5 as an active material;
An ion conducting medium for conducting lithium ions;
Rechargeable lithium battery. A method for producing a carbonaceous material used for an electrode active material,
A nuclear graphite treatment process in which an aromatic carboxylic acid is added to a nuclear graphite particle having a base surface and an edge surface and heated in an inert atmosphere;
The manufacturing method of the carbonaceous material containing this.   The method for producing a carbonaceous material according to claim 7, wherein at least one of biphenyldicarboxylic acid and naphthalenedicarboxylic acid is used as the aromatic carboxylic acid in the nuclear graphite treatment step. A method for producing a carbonaceous material according to claim 8 or 9,
A coating step of coating the surface of the nuclear graphite particles after the nuclear graphite treatment with amorphous carbon;
The manufacturing method of the carbonaceous material containing this.