WO2015146900A1 - 負極炭素材料、負極炭素材料の製造方法、リチウム二次電池用負極およびリチウム二次電池 - Google Patents
負極炭素材料、負極炭素材料の製造方法、リチウム二次電池用負極およびリチウム二次電池 Download PDFInfo
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Definitions
- the present invention relates to a method for producing a negative electrode carbon material for a lithium secondary battery. Moreover, it is related with the negative electrode carbon material for lithium secondary batteries, the negative electrode for lithium secondary batteries, and a lithium secondary battery.
- Lithium secondary batteries have been widely put into practical use as batteries for small electronic devices such as notebook computers and mobile phones because of their advantages such as high energy density, low self-discharge and excellent long-term reliability. In recent years, advanced functions of electronic devices and use in electric vehicles have progressed, and development of lithium secondary batteries with higher performance has been demanded.
- carbon materials are generally used as negative electrode active materials for lithium secondary batteries, and various carbon materials have been proposed for improving battery performance.
- Carbon materials include high crystalline carbon such as natural graphite and artificial graphite, low crystalline carbon such as graphitizable carbon (soft carbon) and non-graphitizable carbon (hard carbon), and amorphous carbon (amorphous). Carbon) is known. It is known that graphite, which is highly crystalline carbon, is excellent in reactivity with Li ions and has a capacity close to the theoretical capacity value. On the other hand, the highly crystalline carbon easily reacts with propylene carbonate (PC), which is frequently used as a solvent for the electrolytic solution, thereby causing a decrease in cycle characteristics due to deterioration of the electrolytic solution.
- PC propylene carbonate
- Low crystalline carbon and amorphous carbon have a theoretical capacity value that is higher than the theoretical capacity value of graphite, but the reactivity with Li ions is low and long-time charging is required. Lower. On the other hand, the reactivity with PC is low and the deterioration of the electrolytic solution is small. Therefore, a composite carbon material combining graphite and amorphous carbon (including low crystalline carbon) has been proposed.
- Patent Document 1 discloses a negative electrode active material in which amorphous carbon is adhered to the surface of graphite particles.
- the graphite particles are oxidized to generate oxygen-containing functional groups on the surface of the graphite particles, and the surface of the graphite particles is roughened. It is disclosed.
- Patent Document 1 discloses a method in which air oxidation is performed at a temperature of 200 ° C. to 700 ° C., and heat treatment is performed at 300 ° C. to 700 ° C. after alkali is attached to the surface of the graphite particles.
- An object of the present invention is to solve the above-described problems, that is, a negative electrode carbon material from which a lithium secondary battery with improved capacity characteristics can be obtained, and a negative electrode for a lithium secondary battery and a lithium secondary battery using the same. Is to provide.
- the second heat treatment is performed in an inert gas atmosphere, so that the greatly reduced capacity is reduced before the first heat treatment. It was found that it recovered to near the capacity value of graphite. Moreover, it discovered that a capacity
- a first heat treatment step of heat-treating graphite particles in an oxidizing atmosphere, and subsequent to the first heat treatment step, is higher than the first heat treatment step in an inert gas atmosphere.
- a method for producing a negative electrode carbon material for a lithium secondary battery which includes a second heat treatment step in which heat treatment is performed at a temperature.
- a negative electrode carbon material for a lithium secondary battery comprising graphite particles having a surface coated with a first amorphous carbon film, the graphite film being formed from the first amorphous carbon film.
- a negative electrode carbon material characterized by having continuous pores in a particle surface layer.
- the negative electrode for lithium secondary batteries containing the said negative electrode carbon material or the negative electrode carbon material manufactured by said method is provided.
- a lithium secondary battery including the negative electrode is provided.
- a negative electrode carbon material from which a lithium secondary battery having a large initial capacity and improved battery characteristics such as rate characteristics can be obtained, and a negative electrode for a lithium secondary battery and a lithium secondary using the same A battery can be provided.
- the Rutherford backscattering analysis result of the negative electrode carbon material obtained in Comparative Example 2 and Example 4 is shown.
- the Raman spectrum results of the negative electrode carbon materials obtained in Comparative Example 2 and Examples 4 to 6 are shown.
- 3 shows FT-IR spectra of negative electrode carbon materials obtained in Comparative Example 2 and Examples 4 to 6.
- the charge / discharge curves of the negative electrode carbon materials obtained in Comparative Example 2 and Examples 4 to 6 are shown.
- the charge rate characteristics of the negative electrode carbon material obtained in Comparative Example 2 and Examples 4 to 6 at an application amount of 50 g / m 2 are shown.
- the discharge rate characteristics when the coating amount of the negative electrode carbon material obtained in Comparative Example 2 and Examples 4 to 6 is 50 g / m 2 are shown.
- the charge rate characteristics when the coating amount of the negative electrode carbon material obtained in Comparative Example 2 and Examples 4 to 6 is 100 g / m 2 are shown.
- the discharge rate characteristics when the coating amount of the negative electrode carbon material obtained in Comparative Example 2 and Examples 4 to 6 is 100 g / m 2 are shown.
- the cycle characteristics of 1C charge-0.1C discharge of the negative electrode carbon materials obtained in Comparative Example 2 and Examples 4 to 6 are shown.
- the cycle characteristics of 3C charge-0.1C discharge of the negative electrode carbon materials obtained in Comparative Example 2 and Examples 4 to 6 are shown.
- the manufacturing method of the negative electrode carbon material for lithium secondary batteries of this invention is demonstrated.
- the 1st heat processing which heats graphite in an oxidizing atmosphere is performed.
- the heat treatment in an oxidizing atmosphere is performed at a temperature lower than the ignition temperature of graphite. If ignition occurs, the temperature cannot be controlled by combustion, and the oxidation process itself becomes difficult.
- the ignition temperature varies depending on the composition of graphite
- the first heat treatment can usually be selected from a temperature range of 400 to 900 ° C. under normal pressure.
- the heat treatment time is in the range of about 30 minutes to 10 hours.
- the oxidizing atmosphere include oxygen, carbon dioxide, and air. Also, the oxygen concentration and pressure can be adjusted as appropriate.
- a channel is formed on the graphite surface as described later.
- the description of the present embodiment will describe a method for producing a negative electrode carbon material including first and second heat treatment steps with the surface of the graphite particles exposed.
- the second heat treatment is performed in an inert gas atmosphere.
- the second heat treatment is performed at a temperature higher than that of the first heat treatment, and can usually be selected from a temperature range of 800 ° C. to 1400 ° C. under normal pressure.
- the second heat treatment is performed in a state where the channel formed by the first heat treatment is exposed.
- the heat treatment time ranges from about 1 hour to 10 hours.
- a rare gas atmosphere such as Ar or a nitrogen gas atmosphere can be used.
- After the second heat treatment it can be cleaned by washing with water and drying.
- the first and second heat treatment steps can be continuously performed in the same heating furnace.
- the oxidizing atmosphere in the first heat treatment step is replaced with an inert gas and then heated to the second heat treatment temperature.
- Two heating furnaces can be arranged in succession and performed separately. Furthermore, a certain amount of time may be allowed between the first heat treatment step and the second heat treatment step as long as the surface state of the formed channel is not affected, and another step such as washing and drying is interposed. But it ’s okay.
- raw material graphite used in the present embodiment natural graphite or artificial graphite can be used.
- artificial graphite a normal product obtained by graphitizing coke or the like can be used.
- a graphitized mesophase microsphere also called mesocarbon microbead (MCMB)
- MCMB mesocarbon microbead
- Artificial graphite that has been heat-treated in the range of 2000 to 3200 ° C. can also be used.
- Such raw material graphite can be used in the form of particles in terms of filling efficiency, mixing properties, moldability, and the like. Examples of the particle shape include a spherical shape, an elliptical spherical shape, and a scale shape (flakes).
- a general spheroidizing treatment may be performed.
- the average particle size of the raw material graphite is preferably 1 ⁇ m or more, more preferably 2 ⁇ m or more, further preferably 5 ⁇ m or more, in terms of input / output characteristics, from the viewpoint of suppressing side reactions during charge / discharge and suppressing reduction in charge / discharge efficiency. From the viewpoint of electrode production (such as the smoothness of the electrode surface), it is preferably 40 ⁇ m or less, more preferably 35 ⁇ m or less, and even more preferably 30 ⁇ m or less.
- the average particle diameter means the particle diameter (median diameter: D 50 ) at an integrated value of 50% in the particle size distribution (volume basis) by the laser diffraction scattering method.
- the weight loss due to the first heat treatment is not large, and the particle size distribution is comparable after the first heat treatment.
- BET specific surface area of the raw material graphite is preferably less than 10m 2 / g, 5m 2 / g or less is more preferable.
- a plurality of grooves are formed on the surface of the graphite particles by the first and second heat treatments.
- the channel includes one formed by being connected to each other.
- the channel is formed at various depths from the surface of the graphite particles, and more preferably formed from the surface of the graphene laminated structure (hereinafter referred to as graphene layer) to a certain depth inside.
- These channels can pass lithium ions (Li ions) and function as a Li ion path (Li path) into the graphene layer.
- the Li path of Li ions into the graphene layer is almost limited to the path from the edge surface side of the graphene layer, and the distance to the back of the graphene layer (the center in the plane direction of the graphene layer) is As a result, the input characteristics deteriorated as the amount of reaction with lithium increased.
- a channel functioning as a Li path can be formed on the graphene layer plane (basal plane), so the Li path increases, The path to the back is shortened. As a result, the input characteristics of the lithium secondary battery can be improved.
- Such a channel is preferably formed through several layers of graphene, more preferably at least three channels deep from the surface to the inside, and at least 5 from the surface to the inside. More preferably, the channel is formed at the depth of the layer, and a channel reaching the depth of more layers (for example, 10 layers or more) can be formed. Further, since graphite is an aggregate of a plurality of graphene layers, a channel can be formed so as to partially penetrate the plurality of graphene layers. By forming such a channel, a Li path that reaches the inside of the graphene stacking direction (a direction perpendicular to the graphene layer plane) is formed, and input characteristics can be further improved. The depth of the channel in the plane of the internal graphene layer (which may be a vacancy state) can be observed by an electron microscope such as TEM or SEM by cutting the negative electrode carbon material by various methods to obtain a cross section. .
- FIG. 1 is a diagram schematically illustrating the present invention, where (a) is a schematic cross-sectional view of untreated graphite, and (b) is a schematic cross-sectional view of graphite that has been heat-treated according to the present invention.
- graphite is in the form of cabbage in which graphene is folded several times as shown in FIG.
- fine pores are formed in the surface layer (basal surface) of the graphite particles, the micropores are etched by heat treatment in an oxidizing atmosphere to form large pores (FIG. 1B).
- the hole may be formed with a surface channel 1 formed on the basal surface and a perforation 2 (also referred to as blind hole or drill hole) formed inside.
- the surface channel 1 and the perforations 2 may be continuous.
- the opening widths of these channels are not particularly limited as long as lithium ions can pass through and the characteristics of graphite are not greatly deteriorated by channel formation, but are preferably from nanometer size to micrometer size.
- the nanometer size means several nm to several tens of nm (less than 50 nm) including 1 nm
- the micrometer size means several ⁇ m to several tens of ⁇ m (less than 50 ⁇ m) including 1 ⁇ m.
- the opening width is preferably 10 nm or more, more preferably 50 nm or more, and further preferably 100 nm or more.
- the opening width is preferably 1 ⁇ m or less, more preferably 800 nm or less, and even more preferably 500 nm or less.
- opening width means the width of the channel in the short direction.
- the channel is preferably formed over the entire surface of the graphite particle, and the more uniform the distribution is.
- the channel opening width, distribution, and the like can be controlled by heat treatment conditions such as temperature, time, and oxygen concentration in the first heat treatment.
- the channels formed on the graphite surface in this way are different from the voids inherent to graphite (voids between primary particles, defects, voids and cracks near edges). Even when ordinary graphite having such inherent voids is used for the negative electrode, the input characteristics of the lithium secondary battery are low. Moreover, even if the surface of the graphite is roughened (for example, a treatment in which graphite is immersed in an alkali solution and then irradiated with ultrasonic waves), and the graphite after such treatment is used for the negative electrode, the input characteristics of the lithium secondary battery are as follows. Low.
- the second heat treatment in an inert gas can restore the capacity characteristics and improve the characteristics of the lithium secondary battery. it can.
- the negative electrode carbon material after channel formation according to the present embodiment can have a structure and physical properties corresponding to the raw material graphite.
- the plane spacing d 002 of (002) plane of the negative electrode carbon material according to the present embodiment is less 0.340 nm, more preferably at 0.338 or less, d 002 theoretical value of the graphite is 0.3354
- d 002 of the negative electrode carbon material according to the present embodiment is preferably in the range of 0.3354 to 0.340 nm.
- This d 002 can be obtained by X-ray diffraction (XRD).
- Lc is preferably 50 nm or more, and more preferably 100 nm or more.
- the manufacturing method according to the present embodiment may include a step of forming a metal that can be alloyed with lithium (Li) or an oxide thereof on the graphite surface after the first and second heat treatment steps.
- This metal or metal oxide can react with lithium and is electrochemically active in charging / discharging of a lithium secondary battery.
- a metal or metal oxide at least one metal selected from the group consisting of Si, Ge, Sn, Pb, Al, Ga, In, and Mg, or an oxide thereof can be used.
- the reaction capacity can be increased by forming such a metal or metal oxide.
- the metal or metal oxide when a metal or metal oxide is formed around the channel, the metal or metal oxide can be strongly bonded to the graphene layer in the periphery of the channel, and the Li reaction site having excellent reversibility increases.
- the reaction capacity can be improved.
- Examples of such a metal or metal oxide forming method include CVD, sputtering, electrolytic plating, electroless plating, and hydrothermal synthesis.
- the metal or metal oxide content is preferably 0.1 to 30% by mass with respect to graphite. If the content is too small, a sufficient content effect cannot be obtained. If the content is too large, the influence of volume expansion / contraction during charging / discharging of the metal or metal oxide is large, and the negative electrode carbon material is likely to deteriorate. .
- the negative electrode carbon material according to the present embodiment can be coated with amorphous carbon.
- Amorphous carbon can suppress side reactions between graphite and the electrolytic solution, improve charge / discharge efficiency, and increase reaction capacity.
- the negative electrode carbon material in which the metal which can be alloyed with the above-mentioned lithium (Li), or its oxide was formed in the surface can also be coat
- Examples of the method for coating the negative electrode carbon material with amorphous carbon include hydrothermal synthesis, CVD, and sputtering.
- the amorphous carbon coating can be performed after the first and second heat treatments.
- the amorphous carbon coating by the hydrothermal synthesis method can be performed, for example, as follows. First, the powder of the negative electrode carbon material in which the void
- Various sugar solutions can be used as the carbon precursor solution, and an aqueous sucrose solution is particularly preferable. The sucrose concentration of this aqueous solution can be set to 0.1 to 6M, and the immersion time can be set to 1 minute to 24 hours.
- the heat treatment can be performed at 400 to 1200 ° C. for 0.5 to 24 hours in an inert atmosphere such as nitrogen or argon.
- the first and second heat treatments can be performed after the graphite is coated with amorphous carbon (referred to as first amorphous carbon).
- first amorphous carbon amorphous carbon
- graphite having high crystallinity when graphite having high crystallinity is compared with amorphous carbon, graphite is more easily oxidized.
- the first amorphous carbon serves as a protective layer, and almost no channel is formed when graphite is oxidized as it is.
- first, vacancies are formed in the first amorphous carbon film, and the graphite exposed in the vacancies is further oxidized to extend the vacancies inside the graphite. Therefore, a continuous hole is formed in the first amorphous carbon film and graphite.
- a negative electrode carbon material for a lithium secondary battery comprising graphite particles having a surface coated with a first amorphous carbon film, wherein the first amorphous carbon film is transformed into graphite.
- a negative electrode carbon material characterized by having continuous pores in a particle surface layer.
- the first amorphous carbon may not completely cover the graphite surface.
- the graphite in the uncoated part is oxidized as it is, and a plurality of channels are formed in the graphite in the uncoated part.
- the first heat treatment oxidation treatment
- the first heat treatment is performed at 500 ° C. or higher, more preferably 600 ° C. or higher, particularly 700 ° C. or higher.
- the upper limit temperature is up to 900 ° C.
- the second heat treatment is performed in the range of 800 to 1400 ° C. as described above.
- the vacancies thus formed become a Li path into the graphene layer as in the above-described channel, and the input characteristics can be further improved.
- the first amorphous carbon can further increase the reaction capacity while suppressing side reactions with the electrolytic solution.
- the opening size of these vacancies is not particularly limited as long as lithium ions can pass therethrough and the characteristics of the carbon material are not greatly deteriorated by vacancy formation, but it is preferably 10 nm or more, preferably 50 nm or more. More preferably, it is more preferably 100 nm or more. Further, from the viewpoint of not deteriorating the characteristics of the carbon material, the opening size is preferably 1 ⁇ m or less, more preferably 800 nm or less, and even more preferably 500 nm or less.
- the “opening size” means the maximum length (maximum opening size) of the opening, and corresponds to the diameter of a circle having the smallest area that can accommodate the outline of the opening.
- the opening size (minimum opening size) corresponding to the diameter of the circle with the maximum area that can exist inside the outline of the hole opening is also preferably 10 nm or more, and preferably 50 nm or more. More preferably, it is more preferably 100 nm or more.
- the depth of the vacancies is preferably 3 or more layers of graphene on the graphite surface, more preferably 5 or more layers, as in the above-described channel.
- vacancies that penetrate through a plurality of graphene layers may be formed inside.
- the negative electrode carbon material having vacancies continuous from the first amorphous carbon film to the graphite particle surface layer may be formed by forming a metal that can be alloyed with lithium (Li) or an oxide thereof. Good.
- a negative electrode carbon material having continuous pores, or a negative electrode carbon material in which such a metal or oxide thereof is formed may be further coated with an amorphous carbon (second amorphous carbon) film.
- the negative electrode carbon material described above can be applied to a negative electrode active material of a lithium ion secondary battery.
- a lithium ion secondary battery with improved input characteristics can be provided. it can.
- a negative electrode for a lithium ion secondary battery can be produced, for example, by forming a negative electrode active material layer containing a negative electrode active material and a binder made of this negative electrode carbon material on a negative electrode current collector.
- This negative electrode active material layer can be formed by a general slurry coating method.
- a negative electrode can be obtained by preparing a slurry containing a negative electrode active material, a binder, and a solvent, applying the slurry onto a negative electrode current collector, drying, and pressing as necessary.
- Examples of the method for applying the negative electrode slurry include a doctor blade method, a die coater method, and a dip coating method.
- a negative electrode can be obtained by forming a thin film of aluminum, nickel, or an alloy thereof as a current collector by a method such as vapor deposition or sputtering.
- the binder for the negative electrode is not particularly limited, but polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene.
- NMP N-methyl-2-pyrrolidone
- water carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, and polyvinyl alcohol can be used as a thickener.
- the content of the binder for the negative electrode is preferably in the range of 0.1 to 30 parts by mass with respect to 100 parts by mass of the negative electrode active material, from the viewpoints of binding force and energy density that are in a trade-off relationship.
- the range of 0.5 to 25 parts by mass is more preferable, and the range of 1 to 20 parts by mass is more preferable.
- the negative electrode current collector is not particularly limited, but copper, nickel, stainless steel, molybdenum, tungsten, tantalum and an alloy containing two or more of these are preferable from the viewpoint of electrochemical stability.
- Examples of the shape include foil, flat plate, and mesh.
- the lithium ion secondary battery by embodiment of this invention contains the said negative electrode, a positive electrode, and electrolyte.
- a positive electrode for example, a slurry containing a positive electrode active material, a binder, and a solvent (and a conductive auxiliary material if necessary) is prepared, applied to the positive electrode current collector, dried, and pressurized as necessary.
- a positive electrode active material layer can be formed on the positive electrode current collector.
- lithium complex oxide lithium iron phosphate, etc.
- the lithium composite oxide include lithium manganate (LiMn 2 O 4 ); lithium cobaltate (LiCoO 2 ); lithium nickelate (LiNiO 2 ); and at least part of the manganese, cobalt, and nickel portions of these lithium compounds.
- lithium composite oxides may be used individually by 1 type, and 2 or more types may be mixed and used for them.
- the average particle diameter of the positive electrode active material for example, a positive electrode active material having an average particle diameter in the range of 0.1 to 50 ⁇ m can be used from the viewpoint of reactivity with the electrolytic solution, rate characteristics, and the like.
- a positive electrode active material having a particle diameter in the range of 1 to 30 ⁇ m, more preferably an average particle diameter in the range of 5 to 25 ⁇ m can be used.
- the average particle diameter means the particle diameter (median diameter: D 50 ) at an integrated value of 50% in the particle size distribution (volume basis) by the laser diffraction scattering method.
- the binder for the positive electrode is not particularly limited, but the same binder as that for the negative electrode can be used. Among these, polyvinylidene fluoride is preferable from the viewpoint of versatility and low cost.
- the content of the binder for the positive electrode is preferably in the range of 1 to 25 parts by mass with respect to 100 parts by mass of the positive electrode active material from the viewpoint of the binding force and energy density which are in a trade-off relationship. The range of 2 to 10 parts by mass is more preferable.
- binders other than polyvinylidene fluoride (PVdF) vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer rubber, polytetrafluoroethylene, polypropylene, Examples include polyethylene, polyimide, and polyamideimide.
- NMP N-methyl-2-pyrrolidone
- the positive electrode current collector is not particularly limited, but from the viewpoint of electrochemical stability, for example, aluminum, titanium, tantalum, stainless steel (SUS), other valve metals, or alloys thereof are used. Can be used. Examples of the shape include foil, flat plate, and mesh. In particular, an aluminum foil can be suitably used.
- a conductive auxiliary material may be added for the purpose of reducing the impedance.
- the conductive auxiliary material include carbonaceous fine particles such as graphite, carbon black, and acetylene black.
- a nonaqueous electrolytic solution in which a lithium salt is dissolved in one or two or more nonaqueous solvents can be used.
- cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC); Dimethyl carbonate (DMC), Chain carbonates such as diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and dipropyl carbonate (DPC); aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate; ⁇ -lactones such as ⁇ -butyrolactone Chain ethers such as 1,2-ethoxyethane (DEE) and ethoxymethoxyethane (EME); and cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofur
- non-aqueous solvents include dimethyl sulfoxide, 1,3-dioxolane, dioxolane derivatives, void muamide, acetamide, dimethyl void muamide, acetonitrile, propionitrile, nitromethane, ethyl monoglyme, phosphate triester, trimethoxymethane , Sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethyl ether, 1,3-propane sultone, anisole, N-methylpyrrolidone An aprotic organic solvent such as can also be used.
- lithium salt dissolved in the nonaqueous solvent is not particularly limited, for example LiPF 6, LiAsF 6, LiAlCl 4 , LiClO 4, LiBF 4, LiSbF 6, LiCF 3 SO 3, LiCF 3 CO 2, Li (CF 3 SO 2 ) 2 , LiN (CF 3 SO 2 ) 2 , and lithium bisoxalatoborate are included.
- These lithium salts can be used individually by 1 type or in combination of 2 or more types.
- a polymer electrolyte may be used instead of the non-aqueous electrolyte solution.
- a separator can be provided between the positive electrode and the negative electrode.
- a porous film, a woven fabric, or a nonwoven fabric made of a polyolefin such as polypropylene or polyethylene, a fluororesin such as polyvinylidene fluoride, polyimide, or the like can be used.
- Battery shapes include cylindrical, square, coin type, button type, and laminate type.
- a laminate type it is preferable to use a laminate film as an exterior body that accommodates a positive electrode, a separator, a negative electrode, and an electrolyte.
- the laminate film includes a resin base material, a metal foil layer, and a heat seal layer (sealant).
- the resin base material include polyester and nylon
- examples of the metal foil layer include aluminum, an aluminum alloy, and a titanium foil.
- the material for the heat welding layer include thermoplastic polymer materials such as polyethylene, polypropylene, and polyethylene terephthalate.
- the resin base material layer and the metal foil layer are not limited to one layer, and may be two or more layers. From the viewpoint of versatility and cost, an aluminum laminate film is preferable.
- the positive electrode, the negative electrode, and the separator disposed between them are accommodated in an outer container made of a laminate film or the like, and an electrolyte is injected and sealed.
- a structure in which an electrode group in which a plurality of electrode pairs are stacked can be accommodated.
- Natural graphite powder (spherical graphite) having an average particle size of 20 ⁇ m and a specific surface area of 5 m 2 / g was subjected to only the first heat treatment step in air at 480 ° C. for 1 hour to obtain a negative electrode carbon material.
- Example 1 A natural graphite powder (spherical graphite) having an average particle size of 20 ⁇ m and a specific surface area of 5 m 2 / g is subjected to a first heat treatment step in air at 480 ° C. for 1 hour, and then in a nitrogen atmosphere at 1000 ° C. for 4 hours. The heat treatment process of 2 was implemented and the negative electrode carbon material was obtained.
- a first heat treatment step in air at 480 ° C. for 1 hour, and then in a nitrogen atmosphere at 1000 ° C. for 4 hours.
- the heat treatment process of 2 was implemented and the negative electrode carbon material was obtained.
- Example 2 Amorphous carbon was coated by CVD (acetylene gas, 620 ° C., 20 minutes) on natural graphite powder (spherical graphite) having an average particle size of 24 ⁇ m and a specific surface area of 5 m 2 / g. Thereafter, a first heat treatment step was performed in air at 650 ° C. for 1 hour, and subsequently, a second heat treatment step was performed in nitrogen atmosphere at 1000 ° C. for 4 hours to obtain a negative electrode carbon material.
- CVD acetylene gas, 620 ° C., 20 minutes
- Example 3 Amorphous carbon was coated by CVD (acetylene gas, 620 ° C., 20 minutes) on natural graphite powder (spherical graphite) having an average particle size of 24 ⁇ m and a specific surface area of 5 m 2 / g. Thereafter, a first heat treatment step was performed in air at 700 ° C. for 1 hour, and then a second heat treatment step was performed in nitrogen atmosphere at 1000 ° C. for 4 hours to obtain a negative electrode carbon material.
- CVD acetylene gas, 620 ° C., 20 minutes
- Comparative Example 1 A natural graphite powder having an average particle size of 20 ⁇ m and a specific surface area of 5 m 2 / g as in Example 1 was prepared and used as a negative electrode carbon material as it was.
- FIG. 4 shows an SEM image of the negative electrode carbon material that has been coated with the amorphous carbon film of Example 2 and then subjected to the first and second heat treatments. Compared with the case of FIG. 3, it can be seen that almost no channels are formed on the surface, and the vacancies formed in the amorphous carbon film further extend continuously to the surface layer of the graphite particles.
- the slurry was applied on a copper foil, dried and rolled, and then cut into 22 ⁇ 25 mm to obtain an electrode.
- This electrode was used as a working electrode (negative electrode) and combined with a counter electrode (positive electrode) Li foil across a separator to obtain a laminate.
- the potential of the working electrode with respect to the counter electrode was charged to 0 V (Li was inserted into the working electrode) and discharged to 1.5 V (Li was desorbed from the working electrode).
- the current value at the time of charging / discharging is 1C for the current flowing through the discharge capacity of the working electrode in 1 hour, and charging / discharging in the first and second cycles is 0.1C charging-0.1C discharging.
- Charge / discharge characteristics include initial discharge capacity (discharge capacity at the first cycle), initial efficiency (discharge capacity at the first cycle / charge capacity at the first cycle), and charge rate characteristics (discharge capacity at the third cycle / 2nd cycle). Discharge capacity). The results are shown in Table 1.
- the charge rate characteristics are slightly improved by subjecting the graphite particles to an oxidation heat treatment (first heat treatment), but the initial capacity is greatly reduced (Reference Example 1). Thereafter, by performing heat treatment (second heat treatment) in a nitrogen gas atmosphere, the initial capacity is restored to near the level before oxidation (Comparative Example 1), and the charge rate characteristics are greatly improved (Example 1). Further, by performing the first and second heat treatments after the amorphous carbon coating, it is possible to provide a negative electrode carbon material that is further excellent in initial capacity and excellent in charge rate characteristics (Examples 2 and 3).
- Comparative Example 2 A natural graphite powder having an average particle size of 20 ⁇ m and a specific surface area of 5 m 2 / g as in Example 1 was prepared, and amorphous carbon (about 10 nm thick) was formed on this natural graphite powder by CVD (acetylene gas, 620 ° C., 20 minutes). ). This was used as a negative electrode carbon material.
- FIG. 6 shows SEM images of the negative electrode carbon materials obtained in Examples 4 to 6.
- FIG. 7 shows a high-magnification SEM image of Example 6.
- FIG. 7A the channel state of the basal surface is confirmed, and in FIG. 7B, holes penetrating the plurality of graphene layers are confirmed.
- Table 2 below shows the specific surface area, pore volume, and average pore diameter according to the nitrogen gas adsorption method (BET method).
- Table 3 shows the specific surface area, pore volume, and average pore diameter measured by the mercury intrusion method.
- the nitrogen gas adsorption method microscopic pore states from micropores (2 nm or less) to mesopores (100 nm or less) can be confirmed, and can be measured according to, for example, ISO 15901-2 (JIS Z8831-2).
- the mercury intrusion method can confirm the macroscopic pore state of macropores (over 100 nm), and can be measured according to, for example, ISO 15901-1.
- the specific surface area increases as more pores (holes) are formed on the surface of the carbon material as seen in activated carbon.
- the pore volume is increased and the average pore diameter is also increased.
- the BET specific surface area tends to decrease as the air oxidation temperature increases. This is because smaller micropores are crushed by surface oxidation to form larger mesopores or even larger macropores.
- the carbon material of Comparative Example 2 has a large proportion of micropores, the coulomb efficiency and cycle performance were adversely affected. However, these problems can be solved by reducing the micropores.
- FIG. 8 shows the ratio of oxygen atoms in the depth direction by Rutherford Backscattering Spectrometry (RBS) on the carbon material surfaces of Comparative Example 2 and Example 4.
- RBS Rutherford Backscattering Spectrometry
- the depth from the surface containing oxygen of 0.001 atomic% or more is about 28 nm in Comparative Example 2, whereas the oxygen distribution is shallow at about 23 nm in Example 4.
- the depth from the surface containing oxygen of 0.001 atomic% or more is preferably 25 nm or less, and oxygen is contained in a range of 1 nm or more in consideration of natural oxidation. If it is this range, initial stage charge-and-discharge efficiency will improve.
- Table 4 shows the results of elemental composition analysis on the surface of the carbon materials of Comparative Example 2, Examples 5 and 6 by X-ray photoelectron spectroscopy (XPS).
- the amount of oxygen on the surface of the carbon material is reduced to 1.1 atomic% or less by air oxidation at 750 ° C. or higher and nitrogen heat treatment.
- the Raman spectrum result of the obtained carbon material is shown in FIG.
- the intensity ratio I D / IG ratio (R value) of the D peak reflecting irregularity with respect to the G peak reflecting the graphite structure of graphite decreases after the surface oxidation treatment, and the crystallinity is improved. It is shown that.
- Table 5 shows the measurement results of the number of localized electrons and the number of carriers on the surface of the carbon material by electron spin resonance (ESR). Localized electrons increased due to air oxidation and nitrogen heat treatment, and carriers decreased. This is considered to be because crystallinity was improved as the micropores were decreased.
- ESR electron spin resonance
- Fig. 10 shows the FT-IR spectrum of each carbon material. There was no significant change in peak intensity or peak position before and after the surface oxidation treatment. This indicates that any functional group advantageous for Coulomb efficiency and cycle performance was not added to the surface of the carbon material by the surface oxidation treatment.
- FIG. 11 shows a charge / discharge curve when each carbon material is used for the negative electrode.
- the surface oxidation treatment improves the discharge capacity and Coulomb efficiency (CE) over the raw carbon material (Comparative Example 2).
- the discharge capacity is higher during the second discharge than during the first discharge, and the shift width increases as the surface oxidation treatment temperature increases.
- 12 and 13 show the charge rate characteristics and discharge rate characteristics of each carbon material when the negative electrode is formed with a coating amount of 50 g / m 2 , respectively.
- 14 and 15 show the charge rate characteristics and discharge rate characteristics of each carbon material when the negative electrode is formed at a coating amount of 100 g / m 2 , respectively.
- Table 6 shows typical rate characteristics (1C charge-0.1C discharge, 6C charge-0.1C discharge, 10C charge-0.1C discharge). Each rate characteristic (capacity maintenance ratio) is a relative value when 0.1 C charge-0.1 C discharge is defined as 100%.
- FIG. 16 and 17 show the cycle characteristics of each carbon material.
- FIG. 16 shows the cycle characteristics of 1C charge-0.1C discharge
- FIG. 17 shows the cycle characteristics of 3C charge-0.1C discharge.
- Amorphous carbon-coated natural graphite has been confirmed to be a negative electrode carbon material having excellent capacity, CE, and rate characteristics when the I D / IG ratio (R value) is 0.15 or less.
- Comparative Example 3 Flaked artificial graphite (particle size of about 15 ⁇ m) was used as the carbon material. An amorphous carbon film is not formed.
- Table 1 shows (1C discharge, 4C charge-0.1C discharge, 6C charge-0.1C discharge, 10C charge-0.1C discharge).
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Abstract
Description
本実施形態の製造方法では、黒鉛を酸化雰囲気中で加熱する第1の熱処理を行う。なお、酸化雰囲気下での熱処理は、黒鉛の発火温度未満の温度で行う。発火してしまうと、燃焼により温度制御ができなくなり、酸化処理自体が困難となる。黒鉛の組成により発火温度は種々異なるが、通常、第1の熱処理は常圧下では400~900℃の温度範囲から選択できる。また、熱処理時間は30分から10時間程度の範囲である。酸化雰囲気としては、酸素、二酸化炭素、空気などが挙げられる。又、酸素濃度や圧力を適宜調整することもできる。黒鉛粒子の表面が露出した状態での第1の熱処理では、黒鉛表面に後述するようにチャネルが形成される。以下、本実施形態の説明は黒鉛粒子の表面が露出した状態での第1及び第2の熱処理工程を含む負極炭素材料の製造方法について説明する。
正極は、例えば、正極活物質、結着剤及び溶媒(さらに必要により導電補助材)を含むスラリーを調製し、これを正極集電体上に塗布し、乾燥し、必要に応じて加圧することにより、正極集電体上に正極活物質層を形成することにより作製できる。
平均粒径20μm、比表面積5m2/gの天然黒鉛粉末(球形黒鉛)を空気中、480℃で1時間第1の熱処理工程のみを実施して、負極炭素材料を得た。
平均粒径20μm、比表面積5m2/gの天然黒鉛粉末(球形黒鉛)を空気中、480℃で1時間第1の熱処理工程を実施し、続いて、窒素雰囲気中、1000℃で4時間第2の熱処理工程を実施して、負極炭素材料を得た。
平均粒径24μm、比表面積5m2/gの天然黒鉛粉末(球形黒鉛)にCVD(アセチレンガス、620℃、20分)にて非晶質炭素を被覆した。その後、空気中、650℃で1時間第1の熱処理工程を実施し、続いて、窒素雰囲気中、1000℃で4時間第2の熱処理工程を実施して、負極炭素材料を得た。
平均粒径24μm、比表面積5m2/gの天然黒鉛粉末(球形黒鉛)にCVD(アセチレンガス、620℃、20分)にて非晶質炭素を被覆した。その後、空気中、700℃で1時間第1の熱処理工程を実施し、続いて、窒素雰囲気中、1000℃で4時間第2の熱処理工程を実施して、負極炭素材料を得た。
実施例1と同じ平均粒径20μm、比表面積5m2/gの天然黒鉛粉末を用意し、そのまま負極炭素材料として用いた。
参考例1において、酸化処理前に、真空ろ過により分離した黒鉛粉末(比較例1)を、走査型電子顕微鏡(SEM)で観察した。そのSEM画像を図2に示す。また、酸化処理後のSEM画像を図3に示す。図2と図3の比較から分かるように、黒鉛粒子表面全体に複数のチャネルが形成されていることが分かる。なお、実施例1の負極炭素材料について同様にSEM画像を観察したが、参考例1とほぼ同じであり、チャネルの開口幅に変化は見られなかった。
X線回折法(XRD)により、実施例1および参考例1の黒鉛粉末の結晶構造を測定した。得られたXRDパターンを図5に示す。この図が示すように、第2の熱処理によって黒鉛粒子の結晶性が向上している。また、部分拡大図に示すように、ピークが低角度側にシフトしている。これは、グラフェンの層間距離が、第2の熱処理によって広がったことを示す。一般的には層間距離は大きいほど、入力レート特性は向上する。なお、実施例1のXRDパターンは便宜的にベースラインを引き上げて表示している。
負極炭素材料と導電剤(カーボンブラック)と結着剤(PVdF)を、負極炭素材料:導電剤:結着剤=92:1:7の質量比率で混合し、NMPに分散させてスラリーを作製した。このスラリーを銅箔上に塗布し、乾燥、圧延した後、22×25mmに切り出して電極を得た。この電極を作用極(負極)とし、セパレータを挟んで対極(正極)のLi箔と組み合わせて積層体を得た。この積層体と電解液(1MのLiPF6を含むECとDECの混合溶液、容量比EC/DEC=3/7)をアルミラミネート容器内に封入し、電池を作製した。
実施例1と同じ平均粒径20μm、比表面積5m2/gの天然黒鉛粉末を用意し、この天然黒鉛粉末にCVD(アセチレンガス、620℃、20分)にて非晶質炭素(約10nm厚)を被覆した。これを負極炭素材料として用いた。
比較例2で得られた非晶質炭素被覆天然黒鉛粒子を、O2:N2=1:4(容量比)の雰囲気中650℃で1時間第1の熱処理工程を実施し、続いて、窒素雰囲気中、1000℃で4時間第2の熱処理工程を実施して、負極炭素材料を得た。
比較例2で得られた非晶質炭素被覆天然黒鉛粒子を、O2:N2=1:4(容量比)の雰囲気中750℃で1時間の第1の熱処理工程を実施し、続いて、窒素雰囲気中、1000℃で4時間の第2の熱処理工程を実施して、負極炭素材料を得た。
比較例2で得られた非晶質炭素被覆天然黒鉛粒子を、O2:N2=1:4(容量比)の雰囲気中850℃で1時間の第1の熱処理工程を実施し、続いて、窒素雰囲気中、1000℃で4時間の第2の熱処理工程を実施して、負極炭素材料を得た。
非晶質炭素被覆天然黒鉛においては、ID/IG比(R値)が0.15以下であれば、容量、CE、レート特性に優れた負極炭素材料となることが確認されている。
フレーク状人造黒鉛(粒径約15μm)を炭素材料として用いた。非晶質炭素被膜は形成していない。
比較例3のフレーク状人造黒鉛を用いて、O2:N2=1:4(容量比)の雰囲気中650℃で1時間第1の熱処理工程を実施し、続いて、窒素雰囲気中、1000℃で4時間第2の熱処理工程を実施して、負極炭素材料を得た。
比較例3のフレーク状人造黒鉛を用いて、O2:N2=1:4(容量比)の雰囲気中850℃で1時間第1の熱処理工程を実施し、続いて、窒素雰囲気中、1000℃で4時間第2の熱処理工程を実施して、負極炭素材料を得た。
この出願は、2014年3月26日に出願された日本出願特願2014-63287を基礎とする優先権を主張し、その開示の全てをここに取り込む。
Claims (23)
- 黒鉛粒子を酸化雰囲気中で熱処理する第1の熱処理工程と、
前記第1の熱処理工程に続いて、不活性ガス雰囲気中で、前記第1の熱処理工程よりも高い温度で熱処理する第2の熱処理と
を含むリチウム二次電池用負極炭素材料の製造方法。 - 前記第1の熱処理工程における加熱温度は、400~900℃の範囲であり、前記第2の熱処理工程における加熱温度は、800~1400℃の範囲である、請求項1に記載の負極炭素材料の製造方法。
- 前記第1の熱処理工程が空気中で実施され、前記第2の熱処理工程が窒素ガス雰囲気下に実施される請求項2に記載の負極炭素材料の製造方法。
- 前記第1の熱処理工程の前に、前記黒鉛粒子を第1の非晶質炭素膜で被覆する工程をさらに有する請求項1乃至3のいずれか1項に記載の負極炭素材料の製造方法。
- 前記第1の熱処理工程は、前記黒鉛粒子の表面層が露出した状態で実施される請求項1乃至3のいずれか1項に記載の負極炭素材料の製造方法。
- 前記第2の熱処理工程の後に、リチウムと合金化できる金属またはその酸化物を形成する工程を含む請求項1乃至3のいずれか1項に記載の負極炭素材料の製造方法。
- 前記第2の熱処理工程の後に、第2の非晶質炭素膜で全体を覆う工程を有する請求項1乃至3のいずれか1項に記載の負極炭素材料の製造方法。
- 前記リチウムと合金化できる金属またはその酸化物を形成する工程の後に、第2の非晶質炭素膜で全体を覆う工程を有する請求項7に記載の負極炭素材料の製造方法。
- 表面を第1の非晶質炭素膜で被覆した黒鉛粒子からなるリチウム二次電池用負極炭素材料であって、前記第1の非晶質炭素膜から黒鉛粒子表面層に連続した空孔を有することを特徴とする負極炭素材料。
- 前記空孔の開口サイズが10nm~1μmの範囲にある、請求項9に記載の負極炭素材料。
- 前記空孔は、黒鉛粒子表面層の複数のグラフェンの層を貫通して形成されている、請求項9に記載の負極炭素材料。
- 対リチウム電位が0~2Vにおける充放電において放電容量が360mAh/g以上である、請求項9乃至11のいずれか1項に記載の負極炭素材料。
- 前記第1の非晶質炭素膜及び黒鉛表面にリチウムと合金化できる金属またはその酸化物が形成された、請求項9乃至12のいずれか1項に記載の負極炭素材料。
- 前記負極炭素材料は、表面を覆う第2の非晶質炭素膜を有する請求項9乃至13のいずれか1項に記載の負極炭素材料。
- 請求項1乃至8のいずれか1項に記載の負極炭素材料の製造方法によって製造された負極炭素材料。
- 前記黒鉛粒子が、非晶質炭素膜被覆天然黒鉛球状粒子である請求項15に記載の負極炭素材料。
- 負極炭素材料のID/IG比(R値)が0.15以下である請求項16に記載の負極炭素材料。
- 前記黒鉛粒子が、フレーク状の人造黒鉛である請求項15に記載の負極炭素材料。
- 前記負極炭素材料の表面の電子スピン共鳴法により測定されるキャリア数が4.8E+18個/g以下である請求項15に記載の負極炭素材料。
- 前記負極炭素材料は、窒素吸着法によるBET比表面積が9.5m2/g未満、かつ、水銀圧入法による比表面積が3.1m2/g未満である請求項15に記載の負極炭素材料。
- 前記負極炭素材料は、ラザフォード後方散乱分析により測定される0.001atomic%以上の酸素が含まれる表面からの深さが1nm~25nmの範囲にある請求項15に記載の負極炭素材料。
- 請求項9乃至21のいずれか1項に記載の負極炭素材料を含むリチウム電池用負極。
- 請求項22に記載の負極を含むリチウム二次電池。
Priority Applications (3)
Application Number | Priority Date | Filing Date | Title |
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EP15768792.2A EP3136481B1 (en) | 2014-03-26 | 2015-03-23 | Negative electrode carbon material, method for producing negative electrode carbon material, negative electrode for lithium secondary cell, and lithium secondary cell |
US15/127,293 US10431824B2 (en) | 2014-03-26 | 2015-03-23 | Negative electrode carbon material, method for producing negative electrode carbon material, negative electrode for lithium secondary battery, and lithium secondary battery |
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JP2021015814A (ja) * | 2016-11-08 | 2021-02-12 | エルジー・ケム・リミテッド | 負極及び該負極の製造方法 |
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EP3136481A1 (en) | 2017-03-01 |
EP3136481A4 (en) | 2018-02-21 |
US20170133680A1 (en) | 2017-05-11 |
US10431824B2 (en) | 2019-10-01 |
JP6500892B2 (ja) | 2019-04-17 |
JPWO2015146900A1 (ja) | 2017-04-13 |
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