CN112424118A - Method for producing monolithic mesophase graphitized article - Google Patents
Method for producing monolithic mesophase graphitized article Download PDFInfo
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- CN112424118A CN112424118A CN202080003288.1A CN202080003288A CN112424118A CN 112424118 A CN112424118 A CN 112424118A CN 202080003288 A CN202080003288 A CN 202080003288A CN 112424118 A CN112424118 A CN 112424118A
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- mesophase
- product
- mass
- negative electrode
- firing
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- 238000004519 manufacturing process Methods 0.000 title claims abstract description 28
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- 238000010438 heat treatment Methods 0.000 claims abstract description 44
- 238000005087 graphitization Methods 0.000 claims abstract description 42
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- 229910052721 tungsten Inorganic materials 0.000 description 1
- 239000002759 woven fabric Substances 0.000 description 1
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Abstract
The present invention provides a method for producing an overall mesophase graphite product which can provide a negative electrode material for a lithium ion secondary battery having a high discharge capacity per unit mass and a low electrode expansion rate during continuous charge and discharge. The method for producing an overall mesophase graphitized product of the present invention comprises the steps of: a heat treatment step of heat-treating tar and/or pitch having a crude QI (quinoline insoluble substance) content of 1 mass% or less to obtain an overall mesophase; a non-melting step of obtaining a non-melted product of the entire intermediate phase by not melting the entire intermediate phase obtained in the heat treatment step; a firing step of adding a graphitization catalyst to the bulk mesophase infusible processed product obtained in the infusible step, and then firing the product to obtain a bulk mesophase fired product; and a graphitization step of graphitizing the fired bulk mesophase product obtained in the firing step to obtain a bulk mesophase graphitized product.
Description
Technical Field
The present invention relates to a method for producing a bulk mesophase graphite (バルクメソフェーズ).
Background
In recent years, with the miniaturization and high performance of electronic devices, there has been an increasing demand for improving the energy density of batteries. In particular, lithium ion secondary batteries are attracting attention because they can form a high voltage as compared with other secondary batteries, and therefore can realize a high energy density.
A lithium ion secondary battery has a negative electrode, a positive electrode, and an electrolyte (nonaqueous electrolyte) as main components. Lithium ions move between the negative electrode and the positive electrode through the electrolyte during the discharge process and the charge process, and become a secondary battery. The negative electrode is generally composed of a negative electrode material (active material) bonded to a current collector made of a copper foil with a binder. Generally, a carbon material is used as the anode material. As such a carbon material, graphite excellent in charge and discharge characteristics and exhibiting high discharge capacity and potential flatness is widely used.
Lithium ion secondary batteries mounted in recent portable electronic devices are required to have excellent quick charging and quick discharging properties, and also to have a low expansion rate of electrodes even when charging and discharging are repeated.
In general, the cause of expansion of a lithium ion secondary battery during charging is considered to be the orientation of graphite during the production of an electrode or the generation of gas by the reaction of an electrolyte solution at the graphite edge face, and these are problems that are difficult to avoid with natural graphite having high crystallinity and easily forming a flat shape.
On the other hand, it is considered that the fine crystallites of the artificial graphite such as spherulites and a bulk mesophase are randomly oriented and hence contribute to the reduction of the swelling.
Representative examples of the bulk mesophase graphite-based negative electrode material are described below.
A spherical graphite powder obtained by subjecting pulverized bulk mesophase pitch to heat treatment in air to oxidize only the surface layer and then heat-treating it in an inert gas atmosphere (patent document 1).
However, there is an increasing demand for higher capacity of lithium ion secondary batteries, and one of the problems is that the capacity is relatively low because the bulk mesophase spherulitic graphite has a higher proportion of as-grown QI (quinoline insoluble) and has lower crystallinity than the mesophase spherulitic graphite.
One of the methods for solving this problem is a method in which pitch having a low content of native QI (quinoline insoluble) is used as a raw material, and typical examples of the overall mesophase spherulites using this pitch as a raw material are as follows.
A graphite powder obtained by graphitizing a bulk mesophase carbon obtained by heat-treating tar or pitch having a crude QI (quinoline insoluble content) of 0.3 mass% or less (patent document 2).
Documents of the prior art
Patent document
Patent document 1: japanese laid-open patent publication No. 10-139410
Patent document 2: japanese laid-open patent publication No. 2001-316105
Disclosure of Invention
Problems to be solved by the invention
However, it is generally considered that if the capacity is increased, the expansion rate of the negative electrode material for a lithium ion secondary battery increases, and it is considered to be a difficult problem to achieve both the increase in the capacity and the decrease in the expansion rate.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for producing an overall mesophase graphite product that can obtain a negative electrode material for a lithium ion secondary battery having a high discharge capacity per unit mass and a low electrode expansion rate during continuous charge and discharge.
Means for solving the problems
The present invention provides the following [1] to [3 ].
[1] A method of making monolithic mesophase graphites comprising:
a heat treatment step of heat-treating tar and/or pitch having a quinoline insoluble (first QI) content of 1 mass% or less to obtain an overall mesophase;
a non-melting step of obtaining a non-melted product of the entire intermediate phase by not melting the entire intermediate phase obtained in the heat treatment step;
a firing step of adding a graphitization catalyst to the bulk mesophase infusible processed product obtained in the infusible step, and then firing the product to obtain a bulk mesophase fired product; and
and a graphitization step of graphitizing the bulk mesophase fired product obtained in the firing step to obtain a bulk mesophase graphitized product.
[2]According to [1]The method for producing a monolithic mesophase graphitized product, wherein the graphitization catalyst is only Fe2O3Or only Al2O3。
[3] The method for producing a bulk mesophase graphitized product according to any one of [1] and [2], wherein the amount of the graphitization catalyst added is 0.5 to 30 mass% in terms of a metal element or a metalloid element.
ADVANTAGEOUS EFFECTS OF INVENTION
The present invention can provide a method for producing an overall mesophase graphite product that can provide a negative electrode material for a lithium ion secondary battery having a high discharge capacity per unit mass and a low electrode expansion rate during continuous charge and discharge.
Drawings
Fig. 1 is a sectional view schematically showing the structure of a button-type evaluation battery used in a charge-discharge test in the examples.
Description of the symbols
1 external cover
2 working electrode (cathode)
3 outer can
4 pairs of electrodes (anode)
5 partition board
6 insulating pad
7a, 7b current collector
Detailed Description
In the present invention, when a range is expressed by using "to", both ends of the range include "to". For example, the range of A to B includes A and B.
The present invention will be specifically described below.
[ bulk mesophase graphites ]
The overall mesophase graphites obtained by the process of the present invention are artificial graphite particles with a dense interior of the particles. The average particle diameter (in terms of volume) of the entire mesophase graphite product is not particularly limited, but is preferably 2 to 20 μm, more preferably 10 to 20 μm, and still more preferably 15 to 20 μm. When the particle size is less than 2 μm, initial charge/discharge efficiency may be lowered. When it exceeds 20 μm, the continuous expansion ratio increases.
The bulk mesophase graphitized product preferably has high crystallinity, with an average interplanar spacing d002Preferably less than 0.3360nm, more preferably 0.3359nm or less, and most preferably 0.3358nm or less.
The specific surface area of the whole mesophase graphitized product is preferably 2.0-10.0 m2A more preferable range is 5.0 to 10.0 m/g2A more preferable range is 6.5 to 10.0 m/g2(ii) g, most preferably 7.0 to 10.0m2/g。
The aspect ratio of the overall mesophase graphitized product is preferably as close to 1.0 as possible, i.e., close to spherical.
[ Process for producing Whole mesophase graphitized Material ]
The method for producing a non-graphitizable carbon material of the present invention (hereinafter, also simply referred to as "the method for producing the present invention") includes the steps of: a heat treatment step of heat-treating tar and/or pitch having a crude QI (quinoline insoluble substance) content of 1 mass% or less to obtain an overall mesophase; a non-melting step of obtaining a non-melted product of the entire intermediate phase by not melting the entire intermediate phase obtained in the heat treatment step; a firing step of adding a graphitization catalyst to the bulk mesophase infusible processed product obtained in the infusible step, and then firing the product to obtain a bulk mesophase fired product; and a graphitization step of graphitizing the bulk mesophase fired product obtained in the firing step to obtain a bulk mesophase graphitized product.
The production method of the present invention will be described in detail below.
[ Heat treatment Process ]
In the heat treatment step, tar and/or pitch having a native QI (quinoline insoluble) content of 1 mass% or less is heat-treated to obtain an overall mesophase.
When tar and/or pitch are heat-treated, aromatic components are condensed, and these condensates are accumulated to form a spherical shape called mesophase microspheres. Upon further heating, the mesophase microspheres merge with each other to form an integral mesophase. The crystal structure of the entire mesophase is random, and is effective for reducing the aspect ratio after pulverization.
The crude QI (quinoline insoluble matter) of tar and/or pitch as a raw material is free carbon (about 1 μm or less in particle diameter) as gas phase-forming carbon generated when coal is dry distilled.
The reason why the raw QI (quinoline insoluble) content of the tar and/or pitch as the raw material is 1 mass% or less is that the raw QI (quinoline insoluble) in the raw material becomes a factor inhibiting the growth of graphite crystals and causes a decrease in capacity, and is set to prevent such a situation.
The method for measuring the content of native QI (quinoline insoluble matter) is more preferably 15.1 (filtration method) of JIS K2425: 2006 (test method for creosote, refined tar and tar pitch).
(Tar, Pitch)
Coal-based tar and/or pitch is used as the tar and/or pitch of the raw material. As the raw material, either tar or pitch may be used alone, or both may be used in combination.
(atmosphere of Heat treatment)
The gas atmosphere during the heat treatment (heat treatment gas atmosphere) is not particularly limited, and may be either a non-oxidizing gas atmosphere (including a non-active gas atmosphere and a reducing gas atmosphere) or an oxidizing gas atmosphere, and is preferably a non-oxidizing gas atmosphere or a slightly oxidizing gas atmosphere.
(Heat treatment pressure)
The pressure (heat treatment pressure) at the time of heat treatment is not particularly limited, and may be any of reduced pressure, normal pressure, or increased pressure.
(Heat treatment temperature)
The temperature (heat treatment temperature) during the heat treatment is not particularly limited, and is preferably 250 to 400 ℃. When the heat treatment temperature is 250 ℃ or higher, the polycondensation reaction of the aromatic hydrocarbon compound proceeds rapidly, and the formation of the mesophase microspheres does not require a long time, and therefore, it is practical. When the heat treatment temperature is 400 ℃ or lower, the polycondensation reaction of the aromatic hydrocarbon compound does not become too fast, and the production of mesophase microspheres is industrially easily controlled. In addition, when the heat treatment temperature is 250 to 400 ℃, the balance between the production rate of the mesophase microbeads and the ease of controlling the production thereof is excellent.
(Heat treatment time)
The time during the heat treatment (heat treatment time) until the formation of the entire mesophase is not particularly limited. In the heat treatment step, the heat treatment may be performed in a plurality of steps. In the case where the heat treatment is performed in a plurality of divided portions, the heat treatment time is the total of the respective treatment times of the divided heat treatments.
[ non-melting Process ]
In the non-melting step, the entire mesophase obtained in the heat treatment step is not melted to obtain an entire mesophase non-melted processed product.
Since the entire mesophase obtained in the heat treatment step has a residual meltability, it is pulverized by a pulverizer (atomizer) or the like, shaped into a block having an average particle diameter of 2 to 25 μm by pressing, and then heated in air at 275 to 500 ℃, preferably 280 to 450 ℃, to oxidize the particle surface without melting.
[ firing Process ]
In the firing step, a graphitization catalyst is added to the bulk mesophase infusible treated product obtained in the infusible step, and then fired to obtain a bulk mesophase fired product.
As the graphitization catalyst, for example, there can be used: alkali metals such as Na and K, alkaline earth metals such as Mg and Ca, transition metals such as Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir and Pt, metals such as Al and Ge, and metalloid such as B, Si, and they may be compounds such as oxides, hydroxides, carbides, nitrides, chlorides and sulfides. These metals and metal compounds may be used alone or in combination of two or more. Further, a metal and a metal compound may be mixed and used. Among them, iron oxide (hematite, Fe) is preferable2O3) Ferrosilicon (Ferrosilicon), boron, silicon, oxides of Al (Al)2O3) And the like.
The graphitization catalyst added to the bulk mesophase non-melted treatment is preferably of the same size as the bulk mesophase non-melted treatment. The average particle diameter of the entire mesophase infusible-processed product is 2 to 25 μm, and therefore the graphitization catalyst is also preferably as large as that.
The amount of the graphitization catalyst added is preferably in the range of 0.5 to 30 mass%, more preferably 1.0 to 25 mass%, further preferably 4.0 to 15 mass%, and most preferably 4.0 to 13.5 mass% in terms of the metal element or metalloid element.
Preferably, the graphitization catalyst is dispersed and adhered to the outer surface of the bulk mesophase infusible processed article by adding the graphitization catalyst to the bulk mesophase infusible processed article, and performing mechanical stirring, applying vibration such as ultrasonic waves, or introducing gas to move the bulk mesophase infusible processed article.
(atmosphere of firing treatment gas)
The atmosphere during firing (firing atmosphere) is a non-oxidizing atmosphere. The non-oxidizing gas atmosphere is not particularly limited, and for example, an inert gas atmosphere based on an inert gas such as nitrogen or argon, or a reducing gas atmosphere based on hydrogen or the like can be used. When the entire mesophase is subjected to a firing treatment in an oxidizing gas atmosphere such as air (atmosphere), carbon is burned and turns into ash.
(temperature of calcination treatment)
The temperature during firing treatment (firing treatment temperature) is 500 to 1300 ℃. When the firing temperature is less than 500 ℃, carbonization becomes slow and sufficient carbonization may not be achieved. When the firing treatment temperature exceeds 1300 ℃, there is a risk that the added graphitization catalyst volatilizes, and the effect of adding the catalyst is reduced.
(time of calcination treatment)
The time during the firing treatment (firing treatment time) is not particularly limited until the entire mesophase is carbonized.
[ graphitization step ]
In the graphitization step, the bulk mesophase fired product obtained in the firing step is graphitized to obtain a bulk mesophase graphitized product.
Herein, graphitization means an operation of heating the entire mesophase fired product to form a graphite structure, and is an operation of heating the whole mesophase fired product at a temperature in the range of 1500 to 3300 ℃. As a method for carrying out graphitization, a known high temperature furnace such as an acheson furnace can be used. In this case, graphitization is preferably performed in a non-oxidizing gas atmosphere. The non-oxidizing gas atmosphere is not particularly limited, and for example, an inert gas atmosphere based on an inert gas such as nitrogen or argon, or a reducing gas atmosphere based on hydrogen or the like can be used. The graphitization is preferably performed at 2500 ℃ or higher, and more preferably 2800 ℃ or higher.
In the case where the entire mesophase fired product obtained in the firing step is fused together, the whole mesophase fired product is pulverized by a pulverizer or the like, shaped into a block having an average particle diameter of 2 to 25 μm by pressing, and graphitized.
Next, a lithium ion secondary battery (hereinafter, also referred to as "lithium ion secondary battery of the present invention") used as a negative electrode material using the entire mesophase graphite product of the present invention will be described.
[ lithium ion Secondary Battery ]
The lithium ion secondary battery conforms to the elements of a general secondary battery. That is, the electrolyte solution, the negative electrode, and the positive electrode are main battery components, and these components are enclosed in, for example, a battery can. The negative electrode and the positive electrode each function as a carrier for lithium ions, and lithium ions are desorbed from the negative electrode during charging.
[ negative electrode ]
The production of the negative electrode is not limited as long as the entire mesophase graphite product of the present invention is used as a negative electrode material, and can be carried out according to a usual method for producing a negative electrode, as long as a chemically and electrochemically stable negative electrode can be obtained.
The negative electrode can be produced by using a negative electrode mixture obtained by adding a binder to the entire mesophase graphite product of the present invention. As the binder, a binder having chemical stability and electrochemical stability to an electrolyte is preferably used, and for example, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyethylene, polyvinyl alcohol, styrene-butadiene rubber, carboxymethyl cellulose, and the like can be used. They may also be used in combination. The binder is preferably 1 to 20 mass% of the total amount of the negative electrode mixture.
For the production of the negative electrode, N-methylpyrrolidone, dimethylformamide, water, alcohol, or the like, which is a solvent generally used for the production of the negative electrode, can be used.
The negative electrode can be produced, for example, as follows: the negative electrode mixture is dispersed in a solvent to prepare a paste-like negative electrode mixture, and then the negative electrode mixture is applied to one or both surfaces of a current collector and dried to prepare the negative electrode mixture. This makes it possible to obtain a negative electrode in which the negative electrode mixture layer (active material layer) is uniformly and firmly bonded to the current collector.
More specifically, for example, the entire mesophase graphite product of the present invention, a fluororesin powder or a water dispersion agent of styrene-butadiene rubber and a solvent are mixed to prepare a slurry, and then the slurry is stirred and mixed by using a known stirrer, mixer, kneader or the like to prepare a negative electrode mixture paste. When the negative electrode mixture layer is applied to a current collector and dried, the negative electrode mixture layer is uniformly and firmly bonded to the current collector. The thickness of the negative electrode mixture layer is 10 to 200 μm, preferably 30 to 100 μm.
The negative electrode mixture layer may be produced as follows: the entire mesophase graphitized product of the present invention is dry-mixed with resin powder such as polyethylene or polyvinyl alcohol, and then hot-pressed in a mold to produce the final product. Among them, in dry mixing, a large amount of binder is required to obtain sufficient negative electrode strength, and when the amount of binder is too large, the discharge capacity and the rapid charge-discharge efficiency may be reduced.
After the negative electrode mixture layer is formed, the adhesion strength between the negative electrode mixture layer and the current collector can be further improved by pressure bonding such as pressing.
From the viewpoint of increasing the volume capacity of the negative electrode, the density of the negative electrode mixture layer is preferably 1.60g/cm3Above, 1.70g/cm is particularly preferable3The above.
The shape of the current collector used for the negative electrode is not particularly limited, and is preferably a foil-like, mesh-like or net-like material such as a metal net. As the material of the current collector, copper, stainless steel, nickel, or the like is preferable. In the case of foil, the thickness of the current collector is preferably 5 to 20 μm.
(degree of orientation of negative electrode)
The degree of orientation of the negative electrode can be quantitatively evaluated by X-ray diffraction, and the measurement method thereof will be described below.
Adjusting the density of the negative electrode mixture layer to 1.60 to 1.80g/cm3Is punched into 2cm2The negative electrode mixture layer is attached to a glass plate so as to face upward. When this sample is irradiated with X-rays and diffracted, a diffraction peak corresponding to the crystal plane of graphite appears. Among the plurality of diffraction peaks, the ratio I004/I110 of the peak intensity I004 at about 54.6 ° derived from the (004) plane to the peak intensity I110 at about 77.4 ° derived from the (110) plane was used as an index of the orientation degree. The lower the degree of orientation of the negative electrode, the smaller the expansion rate of the negative electrode during charging, and the more excellent the permeability and fluidity of the electrolyte, the better the quick charging, quick discharging, and cycle characteristics of the lithium ion secondary battery.
The density of the negative electrode mixture layer is 1.60 to 1.80g/cm3In the case of (2), the degree of orientation of the negative electrode (I004/I110) is preferably 10 or less.
[ Positive electrode ]
The positive electrode can be formed by applying, for example, a positive electrode mixture made of a positive electrode material, a binder, and a conductive material to the surface of a current collector. As a material of the positive electrode (positive electrode active material), a lithium compound can be used, and it is preferable to select a material capable of occluding/desorbing a sufficient amount of lithium. For example, transition metal oxides, transition metal chalcogenides, vanadium oxides, other lithium compounds, compounds of formula M, containing lithium may be usedXMo6OS8-Y(wherein X is a number in the range of 0. ltoreq. X.ltoreq.4, Y is a number in the range of 0. ltoreq. Y.ltoreq.1, and M is at least one transition metal element), activated carbon fiber, and the like. The vanadium oxide is V2O5、V6O13、V2O4、V3O8And the like.
The transition metal composite oxide containing lithium is a composite oxide of lithium and a transition metal, and may be a composite oxide in which lithium and two or more transition metals are solid-dissolved. The composite oxides may be used alone or in combination of two or more. The transition metal composite oxide containing lithium is specifically LiM11-XM2XO2(wherein X is a number in the range of 0. ltoreq. X.ltoreq.1, and M1 and M2 are at least one transition metal element) or LiM11-YM2YO4(wherein Y is a number in the range of 0. ltoreq. Y.ltoreq.1, and M1 and M2 are at least one transition metal element). The transition metal elements represented by M1 and M2 are Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc., preferably Co, Mn, Cr, Ti, V, Fe, Al, etc. A preferred specific example is LiCoO2、LiNiO2、LiMnO2、LiNi0.9Co0.1O2、LiNi0.5Co0.5O2And the like.
The lithium-containing transition metal oxide is obtained, for example, by: lithium, transition metal oxides, hydroxides, salts, and the like are used as starting materials, and these starting materials are mixed according to the composition of a desired metal oxide, and fired at a temperature of 600 to 1000 ℃ in an oxygen atmosphere.
The positive electrode active material may be a lithium compound alone or two or more kinds of lithium compounds may be used in combination. In addition, a basic carbonate such as lithium carbonate may be added to the positive electrode.
The positive electrode can be produced, for example, as follows: a positive electrode mixture layer is formed by applying a positive electrode mixture made of a lithium compound, a binder, and a conductive material for imparting conductivity to the positive electrode to one surface or both surfaces of a current collector to form a positive electrode mixture layer. As the binder, the same binder as used for producing the negative electrode can be used. As the conductive material, a carbon material such as graphite or carbon black is used.
In the same manner as in the negative electrode, the positive electrode mixture may be dispersed in a solvent, and the paste-like positive electrode mixture may be applied to a current collector and dried to form a positive electrode mixture layer, or the positive electrode mixture layer may be formed and then pressed by pressing or the like. This allows the positive electrode mixture layer to be uniformly and firmly bonded to the current collector.
The shape of the current collector is not particularly limited, but is preferably a foil shape, a mesh shape such as a metal mesh, or the like. The current collector is made of aluminum, stainless steel, nickel, or the like. When the film is in the form of foil, the thickness is preferably 10 to 40 μm.
[ non-aqueous electrolyte ]
The nonaqueous electrolyte (electrolytic solution) is an electrolyte salt used in a general nonaqueous electrolytic solution. As the electrolyte salt, for example, LiPF can be used6、LiBF4、LiAsF6、LiClO4、LiB(C6H5)4、LiCl、LiBr、LiCF3SO3、LiCH3SO3、LiN(CF3SO2)2、LiC(CF3SO2)3、LiN(CF3CH2OSO2)2、LiN(CF3CF2OSO2)2、LiN(HCF2CF2CH2OSO2)2、LiN[(CF3)2CHOSO2]2、LiB[C6H3(CF3)2]4、LiAlCl4、LiSiF5And the like lithium salts. From oxidative stabilityFrom the viewpoint of the above, LiPF is particularly preferable6、LiBF4。
The concentration of the electrolyte salt in the nonaqueous electrolyte (electrolyte) is preferably 0.1 to 5mol/L, and more preferably 0.5 to 3 mol/L.
The nonaqueous electrolyte may be a liquid or a solid or gel-like polymer electrolyte.
As the solvent constituting the nonaqueous electrolyte (electrolytic solution), there can be used: carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate and diethyl carbonate, ethers such as 1, 1-or 1, 2-dimethoxyethane, 1, 2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, gamma-butyrolactone, 1, 3-dioxolane, 4-methyl-1, 3-dioxolane, anisole and diethyl ether, thioethers such as sulfolane and methylsulfolane, nitriles such as acetonitrile, chloronitrile and propionitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide and 3-methyl-2-
Aprotic organic solvents such as oxazolidinone, ethylene glycol, and dimethyl sulfite.
When a polymer electrolyte is used, a polymer compound gelled with a plasticizer (nonaqueous electrolyte solution) is preferably used as a matrix. The polymer compound constituting the matrix may be used alone or in combination of: and fluorine-based polymer compounds such as polyethylene oxide, crosslinked ether-based polymer compounds thereof, polymethacrylate-based polymer compounds, polyacrylate-based polymer compounds, polyvinylidene fluoride, and vinylidene fluoride-hexafluoropropylene copolymers. Particularly, fluorine-based polymer compounds such as polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymers are preferably used.
A plasticizer (nonaqueous electrolyte solution) is mixed in the polymer solid electrolyte or the polymer gel electrolyte, and as an electrolyte salt and a solvent constituting the plasticizer (nonaqueous electrolyte solution), an electrolyte salt and a solvent usable in the nonaqueous electrolyte (electrolyte solution) can be used. In the case of a polymer gel electrolyte, the concentration of the electrolyte salt in the plasticizer (nonaqueous electrolyte solution) is preferably 0.1 to 5mol/L, and more preferably 0.5 to 2 mol/L.
The method for producing the polymer solid electrolyte is not particularly limited, and examples thereof include the following methods: a method of mixing a polymer compound constituting a matrix, a lithium salt, and a plasticizer (nonaqueous electrolyte solution), and heating the mixture to melt the polymer compound; a method in which a polymer compound, a lithium salt, and a plasticizer (nonaqueous electrolyte solution) are dissolved in an organic solvent for mixing, and then the organic solvent for mixing is evaporated; a method in which a polymerizable monomer, a lithium salt, and a plasticizer (nonaqueous electrolyte solution) are mixed, and the mixture is irradiated with ultraviolet rays, electron beams, molecular rays, or the like to polymerize the polymerizable monomer and obtain a polymer compound.
The proportion of the plasticizer (nonaqueous electrolyte solution) in the polymer solid electrolyte is preferably 10 to 90% by mass, more preferably 30 to 80% by mass. When the amount is less than 10% by mass, the conductivity is lowered, and when the amount exceeds 90% by mass, the mechanical strength is weakened, and the film formation becomes difficult.
In the lithium ion secondary battery, a separator may also be used.
The material of the separator is not particularly limited, and examples thereof include woven fabric, nonwoven fabric, and microporous film made of synthetic resin. The microporous membrane made of a synthetic resin is suitable, and a polyolefin microporous membrane is suitable from the viewpoint of the thickness, the membrane strength, and the membrane resistance. Specifically, a microporous membrane made of polyethylene or polypropylene, or a microporous membrane obtained by combining these.
The lithium ion secondary battery can be produced by, for example, stacking a negative electrode, a nonaqueous electrolyte, and a positive electrode in this order, and housing the stacked materials in an outer package of the battery.
The nonaqueous electrolyte may be disposed outside the negative electrode and the positive electrode.
The structure of the lithium ion secondary battery is not particularly limited, and the shape and form thereof are not particularly limited, and may be arbitrarily selected from a cylindrical type, a rectangular type, a coin type, a button type, and the like, depending on the application, the mounted equipment, the required charge/discharge capacity, and the like. In order to obtain a sealed nonaqueous electrolyte battery with higher safety, it is preferable to provide a device for sensing an increase in the internal pressure of the battery and cutting off the current when an abnormality such as overcharge occurs. In the case of a polymer electrolyte battery, the structure may be such that the battery is enclosed in a laminate film.
Examples
The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to these examples.
In the examples and comparative examples, button-type secondary batteries for evaluation having a structure as shown in fig. 1 were produced and evaluated. The cell may be made according to known methods for the purposes of the present invention.
Native QI (quinoline insoluble) content, average particle diameter, specific surface area, interplanar spacing d002The measurement was performed by the following method.
Native QI (quinoline insoluble) content: the measurement was carried out in accordance with JIS K2425 (15.1 by filtration).
Average particle size: the cumulative particle size of the particle size distribution measured by a laser diffraction particle size distribution analyzer (LMS2000e, SEISHIN ENTERPRISE) was 50% by volume percentage of the particle size.
Specific surface area: pre-dried at 50 ℃ and nitrogen was introduced for 30 minutes, and then measured by the BET1 point method based on nitrogen adsorption (MONOSORB, manufactured by Quantachrome Instruments Japan g.k.). Specifically, the BET specific surface area is measured by a method for measuring the specific surface area of a powder (solid) adsorbed by gas according to JIS Z8830: 2013.
Interplanar spacing d002: the X-ray source used CuK α rays and the standard substance used high-purity silicon, and the diffraction peak of the (002) plane was measured for the coated graphite particles, and calculation was performed from the peak position. The calculation method is based on the chemical vibration method, and a specific method is described in JIS R7651:2007 "method for measuring lattice constant of carbon material and size of crystallite".
(example 1)
[ preparation of bulk mesophase graphitized Material ]
(Heat treatment Process)
The coal tar pitch (the original QI (quinoline insoluble) content was less than 0.01 mass%) was heated to 400 ℃ for 12 hours in an inert gas atmosphere to perform heat treatment, and then, naturally cooled to normal temperature in an inert gas atmosphere to obtain an integral mesophase.
(non-melting Process)
The obtained bulk mesophase was pulverized by a pulverizer and shaped into a block having an average particle diameter of 15 μm by compression.
Subsequently, the surface was oxidized by heat treatment at 280 ℃ for 15 minutes in air, and a non-melted material (non-melted processed product) was obtained.
(firing Process)
10 mass% (7 mass% in terms of Fe) of hematite as a graphitization catalyst was added to the non-molten material (non-molten processed product), and the hematite was dispersed and attached to the outer surface of the non-molten material (non-molten processed product) by a mechanical melting apparatus (manufactured by HOSOKAWA MICRON corporation).
Next, firing was performed at 1000 ℃ in a nitrogen atmosphere to obtain a bulk mesophase fired product. When the powder was melt-bonded during firing, it was pulverized again and shaped into a block having an average particle size of 15 μm.
(graphitization step)
And graphitizing the integral intermediate phase firing product at 3150 ℃ in a non-oxidizing gas atmosphere to prepare the integral intermediate phase graphitized product.
[ preparation of negative electrode mixture ]
98 parts by mass of the entire mesophase graphite product obtained in the above-described step as a negative electrode material, 1 part by mass of carboxymethyl cellulose as a binder, and 1 part by mass of styrene-butadiene rubber were added to water and stirred to prepare a negative electrode mixture paste.
[ production of working electrode (negative electrode) ]
The negative electrode mixture paste obtained in the above-mentioned step was applied to a copper foil having a thickness of 16 μm in a uniform thickness, and the dispersion medium was further evaporated in vacuum at 90 ℃ to dry the paste. Then, using a manual press 12kN/cm2The negative electrode mixture applied to the copper foil was pressed (120MPa), and further punched into a circular shape having a diameter of 15.5mm, thereby producing a working electrode (negative electrode) having a negative electrode mixture layer (thickness 60 μm) adhered to the copper foil. The density of the negative electrode mixture layer was 1.6g/cm3. The working electrode was not stretched, deformed, and the current collector was not dented as viewed in cross section.
[ production of counter electrode (Positive electrode) ]
A lithium metal foil was pressed against a nickel mesh and punched out into a circular shape having a diameter of 15.5mm, thereby producing a current collector formed of a nickel mesh and a counter electrode (positive electrode) formed of a lithium metal foil (having a thickness of 0.5mm) adhered to the current collector.
[ electrolyte solution and separator ]
Mixing LiPF6The resulting solution was dissolved in a mixed solvent of 33 vol% of ethylene carbonate and 67 vol% of ethyl methyl carbonate to a concentration of 1mol/L to prepare a nonaqueous electrolytic solution. The obtained nonaqueous electrolyte was impregnated into a polypropylene porous body (thickness: 20 μm) to prepare a separator impregnated with the nonaqueous electrolyte.
[ production of evaluation Battery ]
As an evaluation battery, a button-type secondary battery shown in fig. 1 was produced.
The insulating gasket 6 is sandwiched between the outer lid 1 and the outer can 3 at their edges, and both edges are caulked and sealed. A current collector 7a formed of a nickel mesh, a cylindrical counter electrode (positive electrode) 4 formed of a lithium foil, a separator 5 impregnated with an electrolyte, a disk-shaped working electrode (negative electrode) 2 formed of a negative electrode mixture, and a current collector 7b formed of a copper foil are stacked in this order from the inner surface of the outer can 3 to produce a battery.
The evaluation battery was produced as follows: the separator 5 impregnated with the electrolyte solution is laminated between the working electrode 2 which is closely attached to the current collector 7b and the counter electrode 4 which is closely attached to the current collecting material 7a, the working electrode 2 is housed in the outer cover 1, the counter electrode 4 is housed in the outer can 3, the outer cover 1 and the outer can 3 are joined, the insulating gasket 6 is inserted into the edge portions of the outer cover 1 and the outer can 3, and both edge portions are caulked and sealed.
In an actual battery, the evaluation battery was a battery composed of a working electrode 2 containing an overall mesophase graphite compound usable as a negative electrode active material and a counter electrode 4 formed of a lithium metal foil.
(Charge and discharge test)
The obtained evaluation battery was subjected to the following charge and discharge tests at a temperature of 25 ℃ to evaluate the discharge capacity per unit mass, the initial charge and discharge efficiency, the rapid charge rate, the rapid discharge rate, and the continuous expansion rate of the electrode. The evaluation results are shown in table 1.
[ discharge Capacity per unit mass ]
After the constant current charging of 0.9mA was carried out until the circuit voltage reached 0mV, the charging was switched to the constant voltage charging, and the charging was continued until the current value became 20. mu.A. The charge capacity per unit mass is obtained from the amount of current supplied during this period. Then, stop for 120 minutes. Then, constant current discharge was performed at a current value of 0.9mA until the circuit voltage reached 1.5V, and the discharge capacity per unit mass was obtained from the amount of current supplied during this period. This was taken as the 1 st cycle. The initial charge-discharge efficiency was calculated from the charge capacity and discharge capacity in the 1 st cycle by the following equation.
Initial charge-discharge efficiency (%) (discharge capacity/charge capacity) × 100
In this test, a process of occluding lithium ions in the negative electrode material is referred to as charging, and a process of desorbing lithium ions from the negative electrode material is referred to as discharging.
[ Rapid Charge Rate ]
After the 1 st cycle, a fast charge is then performed in the 2 nd cycle.
The current value was set to 4.5mA which was 5 times that of the 1 st cycle, and constant current charging was performed until the circuit voltage reached 0mV, and the constant current charging capacity was obtained, and the rapid charging rate was calculated from the following equation.
Fast charge rate (%) × 100 (constant current charge capacity in 2 nd cycle/discharge capacity in 1 st cycle) × 100
[ Rapid discharge Rate ]
Using the other evaluation cells, rapid discharge was performed after the 1 st cycle, followed by the 2 nd cycle. After the 1 st cycle, the charging was performed in the same manner as in the 1 st cycle, and then, the constant current discharge was performed until the circuit voltage reached 1.5V, with the current value being 18mA which is 20 times the current value in the 1 st cycle. The discharge capacity per unit mass was obtained from the amount of current applied during this period, and the rapid discharge rate was calculated by the following equation.
Rapid discharge rate (%) × 100 (discharge capacity in 2 nd cycle/discharge capacity in 1 st cycle) × 100
[ electrode expansion ratio ]
An evaluation battery different from the evaluation battery in which the discharge capacity per unit mass, the rapid charge rate, and the rapid discharge rate were evaluated was produced, and the following evaluations were performed.
After the constant current charging at 0.1C was carried out until the circuit voltage reached 4.2V, the charging was switched to the constant voltage charging, and the charging was continued until the current value became 20 μ a, and the operation was stopped for 10 minutes. Then, constant current discharge was performed at a current value of 0.1C until the circuit voltage reached 3.0V. Then, after the constant current charging at 0.2C was carried out until the voltage reached 4.2V, the constant voltage charging was switched to, and the charging was continued until the current value became 20. mu.A, and the operation was stopped for 10 minutes. Then, constant current discharge was performed at a current value of 0.2C until the circuit voltage reached 3.0V. Then, after the constant current charging at 0.5C was carried out until the voltage reached 4.2V, the constant voltage charging was switched to, and the charging was continued until the current value became 20. mu.A, and the operation was stopped for 10 minutes. Then, constant current discharge was performed at a current value of 0.5C until the circuit voltage reached 3.0V. This was repeated for a total of 10 cycles, and the expansion ratio was calculated from the obtained expanded thickness using the following formula.
Electrode expansion ratio (%) { (electrode thickness at the time of 12-cycle charging) - (electrode thickness at the start of initial cycle) }/(electrode thickness at the start of initial cycle) × 100
[ degree of orientation ]
X-ray diffraction analysis was performed on the same electrode as the working electrode (negative electrode) used for the evaluation battery, and the ratio I004/I110 of the peak intensity I004 derived from the (004) plane at about 54.6 ° to the peak intensity I110 derived from the (110) plane at about 77.4 ° was measured as the degree of orientation.
As shown in table 1, the evaluation battery obtained using the negative electrode material of example 1 in the working electrode exhibited a high discharge capacity per unit mass.
(example 2)
A bulk mesophase graphitized material was prepared in the same manner as in example 1 except that 5 mass% (3.5 mass% in terms of Fe) of hematite was added instead of 10 mass% hematite in example 1, and evaluated in the same manner as in example 1. The results are shown in Table 1.
(example 3)
A bulk mesophase graphitized material was prepared in the same manner as in example 1 except that 20 mass% (14.0 mass% in terms of Fe) of hematite was added instead of 10 mass% hematite in example 1, and evaluated in the same manner as in example 1.
(example 4)
A bulk mesophase graphitized material was prepared in the same manner as in example 1, except that in example 1, graphitization was performed at 3000 ℃ instead of graphitization at 3150 ℃, and evaluated in the same manner as in example 1. The results are shown in Table 1.
(example 5)
A bulk mesophase graphitized material was prepared in the same manner as in example 1 except that 10 mass% of Si was added instead of 10 mass% of hematite in example 1, and evaluated in the same manner as in example 1. The results are shown in Table 1.
(example 6)
A bulk mesophase graphitized material was prepared in the same manner as in example 1, except that 5 mass% of hematite (3.5 mass% in terms of Fe) and 5 mass% of Si (7.5 mass% in terms of Fe and the total amount of Si) were added instead of 10 mass% of hematite in example 1, and evaluated in the same manner as in example 1. The results are shown in Table 1.
(example 7)
In example 1, Al was added2O3Except that 10 mass% (7.16 mass% in terms of Al) was added instead of 10 mass% of hematite, the same procedure as in example 1 was repeatedBulk mesophase graphites were prepared in the same manner and evaluated in the same manner as in example 1. The results are shown in Table 1.
Comparative example 1
[ preparation of bulk mesophase graphitized Material ]
In example 1, a bulk mesophase graphitized material was prepared without using a graphitization catalyst. That is, the coal tar pitch (the content of native QI (quinoline insoluble) is less than 0.01 mass%) was subjected to heat treatment by raising the temperature to 400 ℃ for 12 hours in an inert gas atmosphere, and then naturally cooled to normal temperature in an inert gas atmosphere to obtain an overall mesophase.
The obtained bulk mesophase was pulverized by a pulverizer to be shaped into a block having an average particle size of 15 μm.
Subsequently, the surface was oxidized by heat treatment at 280 ℃ for 15 minutes in air, and a non-melted material (non-melted processed product) was obtained.
The non-melted material (non-melted processed product) was fired at 1000 ℃ in a nitrogen atmosphere to obtain an integral mesophase fired product. When the powder was melt-bonded during firing, it was pulverized again and shaped into a block having an average particle size of 15 μm.
The entire mesophase fired product is graphitized at a high temperature of 3100 ℃ or higher in a non-oxidizing gas atmosphere to prepare an entire mesophase graphitized product.
[ evaluation ]
The density of the negative electrode mixture layer was adjusted to 1.6g/cm in the same manner as in example 13Working electrodes were produced, and evaluation batteries were produced. The same charge/discharge test as in example 1 was carried out, and the evaluation results of the battery characteristics are shown in table 1.
Comparative example 2
A mesophase microsphere graphitized material was prepared in the same manner as in comparative example 1 except that in comparative example 1, coal tar pitch (the content of native QI (quinoline insoluble) was 7 mass%) was used instead of coal tar pitch (the content of native QI (quinoline insoluble) was less than 0.01 mass%), and the temperature of the graphitization treatment was 3000 ℃ instead of 3150 ℃, and the evaluation was performed in the same manner as in example 1. The results are shown in Table 1.
Comparative example 3
A mesophase microsphere graphitized material was prepared in the same manner as in example 1 except that coal tar pitch (the content of native QI (quinoline insoluble) was 7 mass%) was used instead of coal tar pitch (the content of native QI (quinoline insoluble) was less than 0.01 mass%) in example 1, and the temperature of graphitization was 3000 ℃ instead of 3150 ℃, and evaluation was performed in the same manner as in example 1. The results are shown in Table 1.
Comparative example 4
[ preparation of mesophase microsphere graphitized ]
Coal tar pitch (the content of native QI (quinoline insoluble) was less than 0.01 mass%) was subjected to heating treatment at 450 ℃ for 90 minutes in an inert gas atmosphere, and 35 mass% of mesophase microspheres were produced in the pitch matrix. Then, the mesophase microbeads were dissolved and extracted using tar midle oil (tar midle oil), separated by filtration, and the resulting mesophase microbeads were dried at 120 ℃ under a nitrogen atmosphere.
This was subjected to heat treatment at 600 ℃ for 3 hours in a nitrogen atmosphere to prepare a sintered mesophase microsphere. The fired material was pulverized, filled in a graphite crucible, and graphitized at 3150 ℃ in a non-oxidizing gas atmosphere to obtain a mesophase microsphere graphitized material.
[ evaluation ]
The density of the negative electrode mixture layer was adjusted to 1.6g/cm in the same manner as in example 13Working electrodes were produced, and evaluation batteries were produced. The same charge/discharge test as in example 1 was carried out, and the evaluation results of the battery characteristics are shown in table 1.
Comparative example 5
In comparative example 4, a mesophase microsphere graphitized material was prepared in the same manner as in comparative example 4, and evaluated in the same manner as in example 1, except that 10 mass% (7 mass% in terms of Fe) of hematite as a graphitization catalyst was added to the crushed mesophase microsphere fired material, and that the hematite was dispersed and attached to the outer surface of the crushed mesophase microsphere fired material by a mechanical melting apparatus (manufactured by HOSOKAWA MICRON corporation) to graphitize the material. The results are shown in Table 1.
Comparative example 6
In comparative example 4, a mesophase microsphere graphitized material was prepared in the same manner as in comparative example 4, and evaluated in the same manner as in example 1, except that coal tar pitch (the content of native QI (quinoline insoluble) is 7 mass%) was used instead of coal tar pitch (the content of native QI (quinoline insoluble) is less than 0.01 mass%), and that 10 mass% (7 mass% in terms of Fe) of hematite as a graphitization catalyst was added to the crushed material of the calcined mesophase microsphere, and that the hematite was dispersed and attached to the outer surface of the crushed material of the calcined mesophase microsphere by a mechanical fusion apparatus (manufactured by HOSOKAWA MICRON corporation) to thereby graphitize the material. The results are shown in Table 1.
TABLE 1 (part 2)
From the results shown in table 1, it is understood that the coal tar pitch having a raw QI (quinoline insoluble content) content of less than 1 mass% is used as a raw material, and a graphitization catalyst is added and then the mixture is fired and graphitized to obtain a bulk mesophase graphitized material.
In comparative example 1 in which a bulk mesophase graphite product obtained by firing and graphitization without adding a graphitization catalyst was used as a negative electrode material, the discharge capacity per unit mass of the lithium ion secondary battery was low.
Comparative example 2, which used a raw material with a high native QI and did not use a graphitization catalyst, had a low discharge capacity, a low rapid charge rate, and a low rapid discharge rate.
Comparative example 3 using a raw material with a high native QI has a low discharge capacity, rapid charge rate, and rapid discharge rate.
The lithium ion secondary battery of comparative example 4, in which the mesophase microsphere graphite was used as the negative electrode material, had a low discharge capacity per unit mass.
Comparative example 5, in which a mesophase microspherical graphitized material was obtained by graphitizing with a graphitization catalyst and the mesophase microspherical graphitized material was used, had a low rapid charging rate.
Comparative example 6, in which pitch having a primary QI of more than 1 mass% was used as a raw material and graphitization was performed using a graphitization catalyst, showed a low discharge capacity and a low rapid charge rate.
Industrial applicability
According to the overall mesophase graphitized material of the present invention, a negative electrode material for a lithium ion secondary battery having a high discharge capacity per unit mass and a low electrode expansion rate during continuous charge and discharge can be obtained. It can be used as a negative electrode material for a lithium ion secondary battery that effectively contributes to miniaturization and high performance of a mounted device.
Claims (3)
1. A method of making monolithic mesophase graphites comprising:
a heat treatment step of heat-treating tar and/or pitch having a crude QI (quinoline insoluble substance) content of 1 mass% or less to obtain an overall mesophase;
a non-melting step of obtaining a non-melted product of the entire intermediate phase by not melting the entire intermediate phase obtained in the heat treatment step;
a firing step of adding a graphitization catalyst to the bulk mesophase infusible processed product obtained in the infusible step, and then firing the product to obtain a bulk mesophase fired product; and
and a graphitization step of graphitizing the bulk mesophase fired product obtained in the firing step to obtain a bulk mesophase graphitized product.
2. According to claimThe method for producing a monolithic mesophase graphitized article as claimed in claim 1, wherein the graphitization catalyst is only Fe2O3Or only Al2O3。
3. The method for producing a monolithic mesophase graphitized product according to claim 1 or 2, wherein the amount of the graphitization catalyst added is 0.5 to 30 mass% based on the metal element or the metalloid element.
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Application publication date: 20210226 |