CN107534148B - Carbonaceous coated graphite particles for negative electrode material of lithium ion secondary battery, negative electrode of lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Carbonaceous coated graphite particles for negative electrode material of lithium ion secondary battery, negative electrode of lithium ion secondary battery, and lithium ion secondary battery Download PDF

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CN107534148B
CN107534148B CN201680025230.0A CN201680025230A CN107534148B CN 107534148 B CN107534148 B CN 107534148B CN 201680025230 A CN201680025230 A CN 201680025230A CN 107534148 B CN107534148 B CN 107534148B
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negative electrode
secondary battery
lithium ion
ion secondary
carbonaceous
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CN107534148A (en
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间所靖
江口邦彦
盐出哲夫
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JFE Chemical Corp
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/362—Composites
    • H01M4/366—Composites as layered products
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/583—Carbonaceous material, e.g. graphite-intercalation compounds or CFx
    • H01M4/587—Carbonaceous material, e.g. graphite-intercalation compounds or CFx for inserting or intercalating light metals
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Abstract

The present invention provides a negative electrode material which can obtain excellent battery characteristics when used as a negative electrode material for a lithium ion secondary battery. A carbonaceous coated graphite particle for a negative electrode material of a lithium ion secondary battery, which comprises a carbonaceous material on at least a part of the surface of a graphite particle formed by anisotropically pressurizing spherical and/or ellipsoidal graphite, wherein the carbonaceous coated graphite particle satisfies the following (1) to (3). (1) The content of the carbonaceous material is 0.1-3.0 parts by mass relative to 100 parts by mass of the graphite particles formed by anisotropic pressurization in the carbonaceous coated graphite particles. (2) The volume of pores having a pore diameter of 1.1 μm or less as measured by a mercury porosimeter is 0.100mL/g or less, and the ratio of the volume of pores having a pore diameter of 0.54 μm or less to the volume of pores having a pore diameter of 1.1 μm or less is 80% or more. (3) Dibutyl phthalate (DBP) oil absorption of 40.0mL/100g or less.

Description

Carbonaceous coated graphite particles for negative electrode material of lithium ion secondary battery, negative electrode of lithium ion secondary battery, and lithium ion secondary battery
Technical Field
The present invention relates to carbonaceous coated graphite particles for a negative electrode material of a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery using the negative electrode.
Background
Lithium ion secondary batteries are widely mounted in portable electronic devices, and are also beginning to be used in hybrid vehicles and electric vehicles. Under such circumstances, lithium ion secondary batteries are required to have higher capacity, higher rate charge/discharge characteristics, and better cycle characteristics.
A lithium ion secondary battery has a negative electrode, a positive electrode, and a nonaqueous electrolyte as main components, and functions as a secondary battery by causing lithium ions to migrate between the negative electrode and the positive electrode during a discharge process and a charge process. At present, graphite is widely used as the negative electrode material. Graphite is roughly classified into natural graphite and artificial graphite. Natural graphite has the advantages of high crystallinity and high capacity, but has the disadvantage that the particles are oriented in one direction in the electrode due to the flake shape, and thus the high-rate charge-discharge characteristics and the cycle characteristics are poor.
In order to compensate for this drawback, a large number of materials have been proposed in which flake-shaped graphite is processed into a spherical shape and further subjected to a surface coating treatment. The spheroidized natural graphite has a surface on which many edge surfaces having high reactivity with an electrolyte are exposed, and the purpose of coating is to seal the edge surfaces and suppress side reactions. In recent years, with the increase in size of portable devices and the like, further increase in energy density of batteries has been demanded, and along with this, further increase in density of negative electrodes has also been demanded. However, in the conventional coated natural graphite, the strength of the coating layer is insufficient, and the coating layer is cracked or cracked due to the densification, and as a result, there is a problem that the initial efficiency, the cycle characteristics, and the like are lowered.
In contrast, patent document 1 discloses a method for producing a negative electrode material for a lithium ion secondary battery, which is characterized by isotropically pressurizing a spheroidized graphite. Patent document 2 discloses a graphite material for a lithium ion secondary battery, which is characterized in that a coating layer containing a carbide is formed on the surface of compressed graphite particles obtained by compressing natural graphite spheroidized particles and/or natural graphite agglomerated particles.
These documents describe: as an effect of using graphite particles having high density and high isotropy as a negative electrode material, the voids between the graphite particles are widened at the same negative electrode density, so that the liquid permeability of the electrolytic solution can be improved, and even if a negative electrode is produced by press molding, the crystal structure of graphite is not easily oriented, and the liquid permeability of the electrolytic solution is not impaired.
However, even when these methods are used, the effect of improving the battery performance is not sufficient at present.
Documents of the prior art
Patent document
Patent document 1: japanese patent No. 4499498
Patent document 2: japanese patent laid-open publication No. 2011-60465
Disclosure of Invention
Problems to be solved by the invention
The present invention has been made in view of the above circumstances. That is, an object of the present invention is to provide a negative electrode material which can obtain excellent battery characteristics even when used as a negative electrode material for a lithium ion secondary battery. Further, it is an object to provide a method for producing the negative electrode material, a negative electrode using the negative electrode material, and a lithium ion secondary battery using the negative electrode. Here, the excellent battery characteristics mean high discharge capacity, high initial charge-discharge efficiency, high-rate charge-discharge characteristics, and excellent cycle characteristics.
Means for solving the problems
The present invention is a carbonaceous coated graphite particle for a negative electrode material of a lithium ion secondary battery, characterized in that it is a carbonaceous coated graphite particle having a spherical or ellipsoidal graphite particle as a core material, the core material being a graphite core material formed by anisotropic pressurization, and having a coating layer made of carbonaceous material on at least a part of the surface of the core material.
In the production process of the negative electrode for a lithium ion secondary battery, press molding is performed to achieve a designed electrode density. In general, this press molding uses a roll press machine to apply anisotropic pressure to the negative electrode material constituting the electrode. The present invention has been made in view of the above-mentioned point that by applying an anisotropic pressure treatment to a negative electrode material in advance, deformation of particles and damage to a coating layer accompanying the deformation caused by press molding in producing a negative electrode are suppressed, and high initial charge-discharge efficiency, high-rate charge-discharge characteristics, and cycle characteristics can be maintained even when an electrode density is made high.
Effects of the invention
The carbonaceous coated graphite particles of the present invention satisfy good discharge capacity, initial charge-discharge efficiency, high-rate charge-discharge characteristics, and cycle characteristics at the same time as a negative electrode material for a lithium ion secondary battery. Therefore, the lithium ion secondary battery formed using the negative electrode material of the present invention satisfies the recent demand for higher energy and higher density of the battery, and is useful for downsizing and improving performance of the mounted device.
Drawings
Fig. 1 is a cross-sectional view of an evaluation battery for evaluating battery characteristics of the negative electrode of the present invention.
Detailed Description
The present invention will be described more specifically below.
1. Graphite particles
[ spherical and/or ellipsoidal graphite ]
The graphite particles used as the raw material of the graphite particles of the present invention are spherical or ellipsoidal graphite particles having an average particle diameter of 1 to 50 μm, preferably an average aspect ratio of 5 or less, and an average particle diameter of 5 to 30 μm. More preferably, the average aspect ratio is 2 or less and the average specific surface area is 10m2A ratio of 8m or less in particular2The ratio of the carbon atoms to the carbon atoms is less than g.
Commercially available spherical or ellipsoidal natural graphite particles can also be used. In the case of natural graphite particles having a shape other than spherical or ellipsoidal, for example, flaky graphite particles, natural flaky graphite particles are granulated and spheroidized by a mechanical external force to produce spherical graphite particles. Examples of the method of processing into a spherical or ellipsoidal shape include: a method of mixing a plurality of flaky graphite particles in the presence of a granulation aid such as an adhesive or a resin, a method of applying a mechanical external force to a plurality of flaky graphite particles without using an adhesive, a combination of the two methods, or the like. However, the most preferable method is a method of granulating into a spherical shape by applying a mechanical external force without using a granulation aid. The mechanical external force means that the flake graphite can be mechanically crushed and granulated to be spheroidized. As the pulverizing device for the flake graphite, for example, a pressure kneader, a kneader such as a twin-roll mill, a rotary ball mill, カウンタジェットミル (reverse jet mill) (manufactured by sikalim corporation), カレントジェット (manufactured by riqing エンジニアリング corporation), and the like can be used.
Since the surface of the pulverized product has an acute angle portion, the pulverized product can be used after being granulated and spheroidized. Examples of the granulation and spheroidization device for the pulverized product include a granulator such as granuerex (manufactured by フロイント industries), ニューグラマシン (refreshing corporation), アグロマスター (manufactured by Mikroo corporation), a shearing and compression processing device such as ハイブリダイゼーション (hybrid system) (manufactured by Nara machinery, Kabushiki corporation), メカノマイクロス (manufactured by Nara machinery, Kabushiki corporation), メカノフュージョンシステム (mechanical fusion system) (manufactured by Mikroo corporation).
Lc which is a measurement value of X-ray diffraction of the graphite particle of the present invention is preferably 40nm or more, and La is preferably 40nm or more. Here, Lc represents the size Lc (002) of crystallites in the c-axis direction of the graphite structure, and La represents the size La (110) of crystallites in the a-axis direction. Preferably 1360cm in d002 of 0.337nm or less as measured by Raman spectroscopy using an argon laser-1Peak intensity (I)1360) And 1580cm-1Peak intensity (I)1580) Ratio of (A to (B))1360/I1580(R value) is 0.06-0.30 and 1580cm-1The half-peak width of the spectral band is 10-60.
[ graphite particles formed by anisotropic pressure ]
The graphite particles of the present invention are formed by anisotropically pressurizing the spherical or ellipsoidal graphite. As a result, the graphite particles of the present invention have an orientation property in which the density is high in the direction in which anisotropic pressure is applied and the density is low in the direction perpendicular to the direction.
The graphite particle of the present invention preferably has a pore volume of 500nm or less as measured by mercury intrusion measurement of 0.100mL/g or less, or a pore volume of 100 to 200nm as measured by mercury intrusion measurement of 0.02mL/g or less. If the amount exceeds this range, the binder in the production of the negative electrode may penetrate into the pores, and the electrode peel strength may be reduced.
2. Carbonaceous coated graphite particles
The carbonaceous coated graphite particles for a negative electrode material of a lithium ion secondary battery of the present invention have a carbonaceous material on at least a part of the surface of the graphite particles formed by the anisotropic pressure application. That is, at least a part of the surface of the graphite particle is coated with a carbonaceous material. The production method is not limited, and it is preferable that the carbonaceous precursor is used as a raw material, and at least a part of the surface of the graphite particle is coated with a carbonaceous material by a production method described later. Examples of the precursor of the carbonaceous material to be used include pitch and/or resin. Specifically, examples of the heavy oils, particularly tar pitches, include coal tar, gas tar oil, medium tar oil, heavy tar oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen-bridged petroleum pitch, heavy oil, and the like. Examples of the resin include thermoplastic resins such as polyvinyl alcohol and polyacrylic acid, and thermosetting resins such as phenol resin and furan resin. It is preferable that the resin is not contained, and only tar pitch is contained, which is advantageous in cost. Any of the above-exemplified precursors of the carbonaceous material can be used, but 80 mass% or more of coal tar pitch is particularly preferable.
In the carbonaceous coated graphite particle of the present invention, the content of the carbonaceous material is 0.1 to 3.0 parts by mass per 100 parts by mass of the graphite particles in the carbonaceous coated graphite particle. When the content of the carbonaceous material is less than 0.1 part by mass, it is difficult to completely coat the active graphite edge face, and the initial charge-discharge efficiency may be lowered. On the other hand, if it exceeds 3.0 parts by mass, the proportion of the carbon material having a relatively low discharge capacity is too high, and the discharge capacity of the carbonaceous coated graphite particles is lowered. In addition, when the ratio of raw materials (thermosetting resins and pitch) for forming the carbonaceous material is large, the particles are easily fused in the coating step and the subsequent heat treatment step, and a part of the carbonaceous material layer of the finally obtained carbonaceous coated graphite particles is cracked or peeled off, and the initial charge-discharge efficiency may be lowered. The content of the carbonaceous material is preferably 0.3 to 3.0 parts by mass, and more preferably 1.0 to 3.0 parts by mass, per 100 parts by mass of the graphite particles in the carbonaceous coated graphite particles. The content of the carbonaceous material may be within the above range on the average of the entire carbonaceous coated graphite particles. All of the particles do not need to be in the above range, and some particles other than the above range may be contained.
The carbonaceous coated graphite particles of the present invention have a pore volume of 1.1 μm or less in pore diameter as measured by a mercury porosimeter of 0.100mL/g or less, and a ratio of the pore volume of 0.54 μm or less in pore diameter to the pore volume of 1.1 μm or less in pore diameter of 80% or more. When the pore volume of pores having a pore diameter of 1.1 μm or less exceeds 0.100mL/g, a binder in the production of a negative electrode penetrates into the pores, and the electrode peel strength may be lowered, resulting in a reduction in cycle characteristics. When the ratio of the pore volume of pore diameter 0.54 μm or less to the pore volume of pore diameter 1.1 μm or less is less than 80%, the binder still penetrates into the pores during the production of the negative electrode, and sufficient electrode peel strength cannot be obtained, and the cycle characteristics may be degraded.
The carbonaceous coated graphite particle of the present invention has a pore volume of 1.1 μm or less in pore diameter as measured by mercury porosimetry of preferably 0.090mL/g or less, and more preferably 0.085mL/g or less.
The ratio of the pore volume of the carbonaceous coated graphite particle of the present invention having a pore diameter of 0.54 μm or less to the pore volume (pore volume having a pore diameter of 1.1 μm or less) is preferably 81% or more, and more preferably 82% or more.
Further, the carbonaceous coated graphite particles of the present invention have a dibutyl phthalate (DBP) oil absorption of 40.0mL/100g or less. If the amount exceeds this value, the binder in the production of the negative electrode penetrates into the pores, and the electrode peel strength may be reduced, resulting in a reduction in cycle characteristics.
Further, the dibutyl phthalate (DBP) oil absorption of the graphite particles of the present invention is preferably 38.0mL/100g or less, and more preferably 37.0mL/100g or less.
The average particle diameter of the carbonaceous coated graphite particles as a final product is preferably in the range of 1 to 50 μm, and more preferably in the range of 5 to 30 μm. The specific surface area measured by the BET method is preferably 6.0m2A ratio of 4.0m or less per gram2The ratio of the carbon atoms to the carbon atoms is less than g.
The carbonaceous coated graphite particles were found to have a thickness of 1360cm as measured by Raman spectroscopy using an argon laser-1Peak intensity (I)1360) And 1580cm-1Peak intensity (I)1580) Ratio of (A to (B))1360/I1580(R value) is greater than R value of graphite, preferably 0.05-0.80.
3. Method for producing carbonaceous coated graphite particles
[ pressurization step ]
The method for producing the carbonaceous coated graphite particles of the present invention is not limited, and it is preferable to first perform anisotropic pressure treatment on the spherical and/or ellipsoidal graphite. The anisotropic pressing treatment means that a pressure is applied in a specific direction, and not isotropic pressing. The isotropic pressure is applied by, for example, using a pressure medium such as gas or liquid, and in comparative examples 2 and 3 described later, cold isostatic pressing is used.
The anisotropic pressing is preferably performed from one direction or two directions. The method of anisotropic pressing is not particularly limited, and examples thereof include press molding, roll press molding, extrusion molding, and the like, and the pressing is usually carried out at normal temperature in air. The direction of the pressing force and the anisotropy is not limited, and is preferably set to a level corresponding to the pressing force and the anisotropy pressing direction in the negative electrode forming step when the carbonaceous coated graphite particles are used as a negative electrode material for a lithium ion secondary battery. The pressurizing force has an internal volume of 2000 to 3000cm3The mold is filled with the mixture at a height of 5-10 cmAnd pressurizing at a pressure of 40 to 300 MPa.
In the case where adhesion occurs during the pressure treatment, the crushing step may be conducted after the pressure treatment as needed. In the pressure treatment, carbonaceous or graphite fibers, carbonaceous precursor materials such as amorphous hard carbon, organic materials, inorganic materials, and metal materials may be added or not added. In the case of addition, the combination of the pressing direction and the non-pressing direction becomes complicated, and therefore, the pressing result is closer to isotropic pressing. In the case where no addition is made, the pressing result becomes simpler, and the difference in orientation between the pressing direction and the non-pressing direction is larger than in the case where another material is added at the time of the pressing treatment.
[ mixing Process ]
The resulting pressurized treatment is mixed with a carbonaceous precursor. The mixing step is not particularly limited as long as homogeneous mixing can be performed, and a known mixing method can be used. The solid graphitic particles are preferably mixed with a solid or semi-solid (including viscous liquid) carbonaceous precursor. The heavy oil is solid at normal temperature.
When a liquid carbonaceous precursor such as light tar oil or medium tar oil is mixed as a solvent, it is preferable to evaporate the solvent at a temperature of about 200 ℃ or lower in advance and perform the subsequent calcination step. The mixing ratio is such that the carbonaceous material is mixed in a ratio of the final product to 100 parts by mass of the graphite particles in a range of 0.1 to 3.0 parts by mass. The mixing may be performed together with a temperature raising step for the heating step described later. The method of heating and mixing is not particularly limited, and a twin-screw kneader or the like having a heating mechanism such as a heater or a heat medium can be exemplified. In the mixing treatment, a carbonaceous precursor material such as carbonaceous or graphitic fibers or amorphous hard carbon, an organic material, an inorganic material, or a metal material may be added. The mixing step may be performed simultaneously with the firing step described later, or the firing may be performed after mixing.
[ calcination procedure ]
And calcining the obtained mixture at 700-2200 ℃. The method of calcination treatment is not particularly limited, but calcination is preferably performed while stirring, and a method using a rotary kiln is preferable because homogeneous calcination can be performed. As for the calcination temperature, the heat treatment may be performed in a plurality of stages as long as the finally reached temperature is within the above range. The atmosphere may be either oxidizing or non-oxidizing, and both may be used in stages. Examples of the non-oxidizing atmosphere include argon, helium, and nitrogen. The calcination time is preferably 5 minutes to 30 hours. In addition, as the temperature distribution at the time of temperature rise and at the time of firing, various forms such as linear temperature rise, stepwise temperature rise in which the temperature is maintained at constant intervals, and the like can be adopted.
The method for producing carbonaceous coated graphite particles of the present invention preferably does not include a pulverization step after the calcination. Further, different types of graphite materials may be used after being attached to each other, embedded, and combined before the calcination treatment. Carbon or graphite fibers, carbon precursor materials such as amorphous hard carbon, organic materials, inorganic materials, and metal materials may be attached to, embedded in, or combined with the graphite particles of the core material.
4. Negative electrode
The present invention also provides a negative electrode for a lithium ion secondary battery containing the carbonaceous coated graphite particles, and a lithium ion secondary battery using the negative electrode.
The negative electrode for a lithium ion secondary battery of the present invention is produced by a general method for forming a negative electrode, and is not limited to any method as long as a chemically and electrochemically stable negative electrode can be obtained. In the production of the negative electrode, it is preferable to use a negative electrode mixture prepared in advance by adding a binder to the carbonaceous coated graphite particles of the present invention. The binder is preferably a substance that exhibits chemical and electrochemical stability with respect to an electrolyte, and for example, fluorine-based resin powder such as polytetrafluoroethylene or polyvinylidene fluoride, resin powder such as polyethylene or polyvinyl alcohol, carboxymethyl cellulose, or the like can be used. They may also be used in combination. The binder is generally used in a proportion of about 1 to about 20 mass% of the total amount of the negative electrode mixture (dry basis). Therefore, the carbonaceous coated graphite particles of the present invention are generally used in a proportion of 99 to 80 mass% (dry basis).
More specifically, the negative electrode material of the present invention is first adjusted to a desired particle size by classification or the like, mixed with a binder, and the resulting mixture is dispersed in a solvent to form a paste, thereby preparing a negative electrode mixture. That is, the negative electrode material of the present invention and the binder are mixed with water, a solvent such as isopropyl alcohol, N-methylpyrrolidone, and dimethylformamide, and the obtained slurry is stirred and mixed using a known stirrer, mixer, kneader, or the like to prepare a paste. The paste is applied to one or both surfaces of a current collector and dried to obtain a negative electrode to which a negative electrode material layer is uniformly and firmly adhered. The thickness of the negative electrode mixture layer is 10 to 200 μm, preferably 20 to 100 μm.
The negative electrode produced from the negative electrode mixture containing the carbonaceous coated graphite particles of the present invention is anisotropically pressurized in the state of the graphite particles, and therefore, after the negative electrode mixture layer is formed, the electrode density can be made relatively high even when press molding is not performed at the time of producing the negative electrode.
The negative electrode of the present invention may be produced by dry-mixing the negative electrode material of the present invention with a resin powder such as polyethylene or polyvinyl alcohol, and hot-press molding the mixture in a mold.
When the negative electrode mixture layer is formed and then pressure-bonded, such as by pressing, the adhesion strength between the negative electrode mixture layer and the current collector can be further improved.
The shape of the current collector used for producing the negative electrode is not particularly limited, and is preferably a foil shape, a mesh shape, a metal lath (エキスパンドメタル), or the like. 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 from about 5 μm to about 20 μm.
The negative electrode of the present invention may be mixed, encapsulated, coated or laminated with different types of graphite materials, carbonaceous materials such as amorphous hard carbon, organic materials, metals, metal compounds, and the like, as long as the object of the present invention is not impaired.
The negative electrode produced from the negative electrode mixture containing the carbonaceous coated graphite particles of the present invention is anisotropically pressurized in the state of the graphite particles, and therefore, after the negative electrode mixture layer is formed, deformation of the particles and damage of the coating layer associated therewith caused by pressing and pressurizing at the time of producing the negative electrode can be suppressed, and even if the electrode density is high, high initial charge-discharge efficiency, high-rate charge-discharge characteristics, and cycle characteristics can be maintained.
[ Positive electrode ]
The positive electrode used in the lithium secondary battery of the present invention is formed by applying a positive electrode mixture composed of, for example, a positive electrode material, a binder, and a conductive agent onto the surface of a current collector. The material (positive electrode active material) of the positive electrode is preferably a material capable of occluding/desorbing a sufficient amount of lithium, and is a lithium-containing compound such as a lithium-containing transition metal oxide, a transition metal chalcogenide, a vanadium oxide, and a lithium compound thereof, and is represented by the general formula MXMo6S8-Y(wherein M is at least one transition metal element, X is a number in the range of 0. ltoreq. X.ltoreq.4, and Y is a number in the range of 0. ltoreq. Y.ltoreq.1), activated carbon fiber, and the like.
The vanadium oxide is V2O5、V6O13、V2O4、V3O8Vanadium oxide as represented.
The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and lithium and two or more transition metals may be solid-dissolved therein. The composite oxides may be used alone or in combination of two or more.
Specifically, the lithium-containing transition metal oxide is composed of LiM1 1-XM2 XO2(in the formula, M1、M2Is at least one transition metal element, X is a number in the range of 0. ltoreq. X.ltoreq.1) or LiM1 1-YM2 YO4(in the formula, M1、M2Is at least one transition metal element, and Y is a value in the range of 0. ltoreq. Y.ltoreq.1).
By M1、M2The transition metal elements are Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc., preferably Co, Fe, Mn, Ti, Cr, V, Sn, etc,Al, and the like. A preferred specific example is LiCoO2、LiNiO2、LiMnO2、LiNi0.9Co0.1O2、LiNi0.5Co0.5O2And the like.
The lithium-containing transition metal oxide can be obtained, for example, by the following method: 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 are calcined at a temperature of 600 to 1000 ℃ in an oxygen atmosphere.
The positive electrode active material may be used alone or in combination of two or more. For example, a carbon salt such as lithium carbonate may be added to the positive electrode. In addition, various additives such as conventionally known conductive agents and binders can be used as appropriate for forming the positive electrode.
[ production of Positive electrode ]
The positive electrode is produced by applying a positive electrode mixture comprising the positive electrode material, a binder, and a conductive agent for imparting conductivity to the positive electrode to both surfaces of a current collector to form positive electrode mixture layers. As the binder, the same one as used for producing the negative electrode can be used. As the conductive agent, known conductive agents such as graphite and carbon black can be used.
The shape of the current collector is not particularly limited, and a foil shape, a mesh shape such as a metal lath, or the like can be used. The current collector is made of aluminum, stainless steel, nickel, or the like. The thickness is preferably 10 to 40 μm.
In the same manner as in the negative electrode, the positive electrode may be formed into a paste by dispersing a positive electrode mixture 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. This allows the positive electrode mixture layer to be uniformly and firmly bonded to the current collector.
[ non-aqueous electrolyte ]
As the nonaqueous electrolyte used in the lithium ion secondary battery of the present invention, LiPF, which is an electrolyte salt used in a normal nonaqueous electrolyte, can be used6、LiBF4、LiAsF6、LiClO4、LiB(C6H5)、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、LiSiF6And the like lithium salts. LiPF is particularly preferable from the viewpoint of oxidation stability6、LiBF4。
The concentration of the electrolyte salt in the electrolyte is preferably 0.1 to 5.0mol/L, more preferably 0.5 to 3.0 mol/L.
The nonaqueous electrolyte may be a liquid nonaqueous electrolyte, or may be a polymer electrolyte such as a solid electrolyte or a gel electrolyte. In the former case, the nonaqueous electrolyte battery is configured as a so-called lithium ion secondary battery, and in the latter case, the nonaqueous electrolyte battery is configured as a polymer electrolyte battery such as a polymer solid electrolyte battery or a polymer gel electrolyte battery.
Examples of the solvent used for preparing the nonaqueous electrolyte solution include carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate and diethyl carbonate, ethers such as 1, 1-dimethoxyethane and 1, 2-dimethoxyethane, 1, 2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ -butyrolactone, 1, 3-dioxolane, 4-methyl-1, 3-dioxolane, anisole and diethyl ether, sulfides such as sulfolane and methylsulfolane, acetonitrile, chloronitrile (クロロニトリル), nitrile such as propionitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-.
Figure GDA0002622828930000131
Aprotic organic solvents such as oxazolidinone, ethylene glycol, and dimethyl sulfite.
Generally, the electrolyte uses a system not containing Propylene Carbonate (PC) in view of using a graphite anode material. PC is not preferable because it is likely to cause a decomposition reaction on the graphite surface, and the internal pressure of the battery is increased by gas generation, and because a large amount of decomposition reaction products (SEI film) are formed on the negative electrode material, the battery characteristics are degraded. In the carbonaceous coated graphite particles of the present invention, spherical and/or ellipsoidal graphite is anisotropically pressed and further subjected to carbonaceous coating, and therefore, the surface of the carbonaceous coated graphite particles has low reactivity with propylene carbonate, and even if propylene carbonate is contained in the electrolytic solution, the battery characteristics when used as a negative electrode material for a lithium ion secondary battery are not inferior.
When the nonaqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte or a polymer gel electrolyte, it is preferable to use a polymer gelled with a plasticizer (nonaqueous electrolytic solution) as a matrix. The polymer constituting the matrix is particularly preferably an ether-based polymer compound such as polyethylene oxide and a crosslinked product thereof, a polymethacrylate-based polymer compound, a polyacrylate-based polymer compound, a fluorine-based polymer compound such as polyvinylidene fluoride or a vinylidene fluoride-hexafluoropropylene copolymer, or the like.
A plasticizer may be added to the polymer solid electrolyte or the polymer gel electrolyte, and the electrolyte salt or the nonaqueous solvent may be used as the plasticizer. In the case of a polymer gel electrolyte, the concentration of the electrolyte salt in the nonaqueous electrolytic solution as a plasticizer is preferably 0.1 to 5.0mol/L, and more preferably 0.5 to 2.0 mol/L.
The method for producing the polymer solid electrolyte is not particularly limited, and examples thereof include: a method of mixing and heating a polymer compound constituting a matrix, a lithium salt, and a nonaqueous solvent (plasticizer) to melt the polymer compound; a method in which a polymer compound, a lithium salt, and a nonaqueous solvent (plasticizer) are dissolved in an organic solvent, and then the organic solvent for mixing is evaporated; a method in which a polymerizable monomer, a lithium salt, and a nonaqueous solvent (plasticizer) are mixed, and the mixture is irradiated with ultraviolet rays, electron beams, molecular beams, or the like to polymerize the polymerizable monomer and obtain a polymer.
Here, the proportion of the nonaqueous solvent (plasticizer) in the solid electrolyte is preferably 10 to 90% by mass, and more preferably 30 to 80% by mass. When the amount is less than 10% by mass, the electrical conductivity decreases, and when the amount exceeds 90% by mass, the mechanical strength decreases, and film formation becomes difficult.
[ partition board ]
In the lithium ion secondary battery of the present invention, 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 material of the separator is preferably a microporous membrane made of a synthetic resin, and particularly, a polyolefin microporous membrane is preferable in view of thickness, membrane strength, and membrane resistance. Specifically, a microporous membrane made of polyethylene and polypropylene, or a microporous membrane obtained by combining these, or the like is preferable.
[ production of lithium ion Secondary Battery ]
The lithium ion secondary battery of the present invention is configured by stacking the negative electrode, the positive electrode, and the nonaqueous electrolyte in this order, for example, in the order of the negative electrode, the nonaqueous electrolyte, and the positive electrode, and housing them in a battery casing. Further, a nonaqueous electrolyte may be disposed outside the negative electrode and the positive electrode.
The lithium ion secondary battery of the present invention is not particularly limited in structure, shape, and form, and may be arbitrarily selected from cylindrical, rectangular, coin, button, and the like according to the application, the device to be mounted, the required charge/discharge capacity, and the like. In order to obtain a sealed nonaqueous electrolyte battery with higher safety, it is preferable to use a configuration including means for sensing a rise in the internal pressure of the battery and interrupting the current when an abnormality such as overcharge occurs.
When the lithium ion secondary battery is a polymer solid electrolyte battery or a polymer gel electrolyte battery, the lithium ion secondary battery may be configured to be enclosed in a laminate film.
Examples
The present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples. In the following examples and comparative examples, as shown in fig. 1, a button-type secondary battery for single-pole evaluation was prepared and evaluated, the button-type secondary battery being composed of a current collector (negative electrode) 7b having the negative electrode mixture 2 of the present invention adhered to at least a part of the surface thereof, and a counter electrode (positive electrode) 4 composed of a lithium foil. The actual battery can be fabricated according to a known method based on the concept of the present invention.
The physical properties described in the present specification were measured by the following methods.
1) Average particle diameter (μm): the cumulative frequency of the particle size distribution measured by a laser diffraction particle size distribution meter reached a particle size of 50% by volume percentage.
2) Proportion of carbon (%): the same thermal history as that of the carbonaceous coated graphite particles was applied to the simple substance of the raw material (including many cases) of the carbonaceous precursor to prepare a carbide of the carbonaceous simple substance, and the residual carbon ratio of the raw material was determined. The ratio of the carbonaceous material in the carbonaceous coated graphite particles was calculated by conversion from the obtained residual carbon ratio.
3) Pore volume (mL/g): the pore size and pore volume were measured by mercury intrusion porosimetry to calculate the total pore volume of pores having a pore size of 1.1 μm or less and 0.54 μm or less.
4) DBP oil absorption (mL/100 g): 40g of the measurement material was charged in accordance with JIS K6217, and the measurement was carried out under conditions of a dropping speed of 4 mL/min and a rotation speed of 125rpm until the maximum value of the torque was confirmed. In the range of the period of the maximum torque from the start of the measurement, the amount of oil added when the torque of 70% of the maximum torque is exhibited is converted into 100g of the material, and the DBP oil absorption is calculated.
(example 1)
[ production of carbonaceous coated graphite particles as negative electrode Material ]
The natural graphite particles processed into spherical shapes having an average particle diameter of 15 μm were anisotropically pressed at 50MPa using a press. After crushing the mixture so that the average particle diameter became 15 μm, an oil-in-tar solution of coal tar pitch (carbon residue ratio: 50%) was added so that the solid content ratio became 2.0 parts by mass per 100 parts by mass, and the mixture was heated to 150 ℃ in a twin-screw kneader and mixed for 60 minutes. The resultant mixture was subjected to a heat treatment at 1300 ℃ for 3 hours using a tube furnace under a nitrogen gas flow of 2L/min, thereby obtaining a final product.
[ preparation of negative electrode mixture paste ]
98 parts by mass of the above 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) ]
Coating the negative electrode mixture paste on a copper foil with a uniform thickness, volatilizing the solvent at 90 ℃ in vacuum, drying, and pressing the negative electrode mixture layer with a roll press to adjust the electrode density to 1.70g/cm3. The copper foil and the negative electrode mixture layer were punched out into a cylindrical shape having a diameter of 15.5mm, and a working electrode (negative electrode) comprising a current collector and a negative electrode mixture laminated on the current collector was produced.
[ production of counter electrode (Positive electrode) ]
The lithium metal foil was pressed against the nickel mesh, and punched out into a circular shape having a diameter of 15.5mm, thereby producing a counter electrode including a current collector made of a nickel mesh and a lithium metal foil (having a thickness of 0.5mm) adhered to the current collector.
[ electrolyte solution, separator ]
Make LiPF6The nonaqueous electrolytic solution was dissolved in a mixed solvent of 33 vol% of ethylene carbonate and 67 vol% of ethyl methyl carbonate to a concentration of 1 mol/L. The obtained nonaqueous electrolyte was impregnated into a polypropylene porous body (thickness: 20 μm) to prepare an electrolyte-impregnated separator.
[ production of evaluation Battery ]
A button-type secondary battery shown in fig. 1 was produced as an evaluation battery.
The outer cup 1 and the outer can 3 are sealed by caulking both peripheral portions thereof with an insulating gasket 6 interposed therebetween. The evaluation battery is a battery system in which a current collector 7a made of a nickel mesh, a cylindrical counter electrode (positive electrode) 4 made of a lithium foil, a separator 5 impregnated with an electrolyte, and a current collector (negative electrode) 7b made of a copper foil to which a negative electrode mixture 2 is attached are stacked in this order from the inner surface of the outer can 3.
The evaluation battery was produced as follows: after the separator 5 impregnated with the electrolyte solution is sandwiched and laminated between the current collector 7b and the counter electrode 4 adhered to the current collector 7a, the current collector 7b is housed in the exterior cup 1, the counter electrode 4 is housed in the exterior can 3, the exterior cup 1 and the exterior can 3 are joined together, the insulating gas cushion 6 is interposed between the peripheral portions of the exterior cup 1 and the exterior can 3, and both peripheral portions are caulked and sealed. The charge and discharge characteristics were measured by the following methods. The results are shown in table 1.
[ Charge/discharge test ]
After the constant current charging of 0.9mA was carried out until the circuit voltage reached 1mV, the charging was switched to the constant voltage charging at the time when the circuit voltage reached 1mV, and the charge capacity (unit: mAh/g) was determined from the amount of current flowing during the period when the current value reached 20. mu.A. Then, pause for 10 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 (unit: mAh/g) was determined from the amount of current passed during this period. This was taken as the 1 st cycle. Next, charge and discharge were performed in the same manner as in the 1 st cycle, with the charge current set to 1C and the discharge current set to 2C. The current values of 1C and 2C were calculated from the discharge capacity at the 1 st cycle and the active material weight of the negative electrode.
The initial charge-discharge efficiency is calculated by the following formula (1).
Initial charge-discharge efficiency (%) < 100 × ((charge capacity at cycle 1-discharge capacity at cycle 1)/discharge capacity at cycle 1) < … (1) >
The 1C charging rate is calculated by the following formula (2).
1C charging rate (%). 100 × (charging capacity of CC portion/discharging capacity of 1 st cycle in 1C current value) … (2)
In addition, the 2C discharge rate was calculated from the following formula (3).
2C discharge rate (%) of 100X (discharge capacity in 2C current value/discharge capacity at 1 st cycle) … (3)
In addition, the cycle characteristics were measured in the following manner. Constant current charging was performed at a current value of 1C until the circuit voltage reached 1mV, then, constant voltage charging was switched to, and charging was continued until a current value of 20 μ A was reached, followed by a 10 minute pause. Then, constant current discharge was performed at a current value of 2C until the circuit voltage reached 1.5V. This charge and discharge was repeated 50 times, and the cycle characteristics were calculated from the obtained discharge capacity using the following formula (4).
Cycle characteristics (%). 100 × (discharge capacity at 50 th cycle/discharge capacity at 1 st cycle) … (4)
In this test, the process of storing lithium ions in the negative electrode material was referred to as charging, and the process of desorbing lithium ions from the negative electrode material was referred to as discharging.
(example 2)
In example 1, the electrode density at the time of evaluation of battery performance was set to 1.75g/cm3Except for this, evaluation was performed in the same manner as in example 1. The evaluation results are shown in table 1.
(example 3)
Carbonaceous coated graphite particles were produced in the same manner as in example 1, and evaluation batteries were produced and evaluated in the same manner as in example 1, except that the carbonaceous coating amount was changed to the amount shown in table 1 in example 1.
(example 4)
Carbonaceous-coated graphite particles were produced in the same manner as in example 1, except that the pressure in the pressurization treatment was set to 100MPa in example 1, and the electrode density was set to 1.70g/cm3To perform the evaluation. The evaluation results are shown in table 1.
(example 5)
Carbonaceous-coated graphite particles were produced in the same manner as in example 1, except that the pressure in the pressurization treatment was set to 150MPa in example 1, and the electrode density was set to 1.70g/cm3To perform the evaluation. The evaluation results are shown in table 1.
(example 6)
Evaluation was performed in the same manner as in example 1 except that the carbonaceous coating amount in example 1 was changed to the amount shown in table 1. The evaluation results are shown in table 1.
Comparative example 1
Carbonaceous coated graphite particles were produced in the same manner as in example 1, and an evaluation battery was produced and evaluated in the same manner as in example 1, except that the pressure treatment was not performed in example 1.
Comparative example 2
A coated natural graphite material was produced in the same manner as in example 1, and an evaluation battery was produced and evaluated in the same manner as in example 1, except that the method of the pressing treatment was set to cold isostatic pressing and 50MPa was isotropically pressed in example 1.
Comparative example 3
In comparative example 2, the electrode density at the time of evaluation of battery performance was set to 1.75g/cm3Except for this, evaluation was performed in the same manner as in comparative example 2. The evaluation results are shown in table 1.
Comparative example 4
Carbonaceous coated graphite particles were produced in the same manner as in example 1, and evaluation batteries were produced and evaluated in the same manner as in example 1, except that the carbonaceous coating amount was changed to the amount shown in table 1 in example 1.
Comparative example 5
Carbonaceous coated graphite particles were produced in the same manner as in example 1, and evaluation batteries were produced and evaluated in the same manner as in example 1, except that the carbonaceous coating amount was changed to the amount shown in table 1 in example 1.
Comparative example 6
Evaluation was performed in the same manner as in example 1 except that the pressure in the pressurization treatment was set to 10MPa in example 1. The evaluation results are shown in table 1.
Comparative example 7
Evaluation was performed in the same manner as in example 1 except that the pressure during the pressurization treatment was set to 10MPa and the carbonaceous coating amount was set to 3.0 parts by mass in example 1. The evaluation results are shown in table 1.
Comparative example 8
Evaluation was performed in the same manner as in example 1 except that the pressure during the pressurization treatment was set to 30MPa and the carbonaceous coating amount was set to 3.0 parts by mass in example 1. The evaluation results are shown in table 1.
Comparative example 9
Evaluation was performed in the same manner as in example 1 except that the pressure during the pressurization treatment was set to 10MPa and the carbonaceous coating amount was set to 0.15 parts by mass in example 1. The evaluation results are shown in table 1.
Figure GDA0002622828930000221
In examples 1 to 6 in which the graphite particles formed by anisotropic pressing satisfied the following (1) to (3), the discharge capacity, the initial charge-discharge efficiency, the high-rate charge-discharge characteristic, and the cycle characteristic were good. As is clear from comparison of examples 1 and 2, even if the electrode density is further increased, the characteristics can be maintained.
(1) The content of the carbonaceous material is 0.1 to 3.0 parts by mass per 100 parts by mass of the graphite particles formed by anisotropic pressing among the carbonaceous coated graphite particles.
(2) The volume of pores having a pore diameter of 1.1 μm or less as measured by a mercury porosimeter is 0.100mL/g or less, and the ratio of the volume of pores having a pore diameter of 0.54 μm or less to the volume of the pores is 80% or more.
(3) Dibutyl phthalate (DBP) oil absorption of 40.0mL/100g or less.
On the other hand, in comparative example 1 in which graphite particles were not subjected to pressure treatment, comparative examples 2 and 3 in which isotropic pressure treatment was performed, comparative example 4 in which the coating amount of carbonaceous material was less than 0.1%, comparative example 5 in which the coating amount of carbonaceous material was more than 3%, comparative example 6 in which the pore volume of 1.1 μm or less in pore diameter was more than 0.100mL/g, the ratio of the pore volume of 0.54 μm or less in pore diameter to the pore volume was less than 80% and the DBP oil absorption was more than 40.0mL/100g, comparative example 7 in which the pore volume of 1.1 μm or less in pore diameter was more than 0.1mL/g, comparative example 8 in which the ratio of the pore volume of 0.54 μm or less in pore diameter to the pore volume of 1.1 μm or less in pore diameter was less than 80%, comparative example 9 in which the pore volume of 1.1 μm or less in pore diameter was more than 0.100mL/g and the DBP oil absorption was more than 40.0mL, high discharge capacity, high initial charge-discharge efficiency, high-rate charge-discharge characteristics, and excellent cycle characteristics cannot be simultaneously achieved. In comparative examples 2 and 3 in which the isotropic pressure treatment was performed, when the electrode density was further increased, the first charge-discharge efficiency, the high-rate charge-discharge characteristic, and the cycle characteristic were observed to be decreased.
Industrial applicability
The negative electrode material containing the carbonaceous coated graphite particles of the present invention is a negative electrode material having good discharge capacity, initial charge-discharge efficiency, high-rate charge-discharge characteristics, and cycle characteristics as a negative electrode material for a lithium ion secondary battery. The negative electrode can be used for a small-sized to large-sized high-performance lithium ion secondary battery by utilizing the characteristics thereof.
Description of the symbols
1 outer cup
2 negative electrode mixture
3 outer can
4 pairs of electrodes
5 separator impregnated with electrolyte
6 insulating air cushion
7a, 7b current collector

Claims (3)

1. A carbonaceous coated graphite particle for a negative electrode material of a lithium ion secondary battery, which comprises a carbonaceous material on at least a part of the surface of a graphite particle formed by anisotropically pressurizing spherical and/or ellipsoidal graphite, wherein the carbonaceous coated graphite particle satisfies the following (1) to (3),
(1) the content of the carbonaceous material is 0.1-3.0 parts by mass relative to 100 parts by mass of the graphite particles formed by anisotropic pressurization in the carbonaceous coated graphite particles;
(2) a pore volume of 1.1 μm or less in pore diameter measured by a mercury porosimeter of 0.100mL/g or less, and a ratio of the pore volume of 0.54 μm or less in pore diameter to the pore volume of 1.1 μm or less in pore diameter of 80% or more;
(3) dibutyl phthalate (DBP) oil absorption of 40.0mL/100g or less.
2. A negative electrode for a lithium ion secondary battery, comprising the graphite particles for a negative electrode material for a lithium ion secondary battery according to claim 1.
3. A lithium ion secondary battery having the lithium ion secondary battery negative electrode according to claim 2.
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