JP6584975B2 - Carbon material for negative electrode of lithium ion secondary battery, negative electrode of lithium ion secondary battery, and method for producing lithium ion secondary battery - Google Patents

Description

  The present invention relates to a carbon material for a negative electrode of a lithium ion secondary battery, a negative electrode of a lithium ion secondary battery, and a method for producing a lithium ion secondary battery.

  In recent years, with the miniaturization and high performance of electronic devices, there is an increasing demand for higher energy density of batteries. Among these, lithium ion secondary batteries are attracting attention because of their high energy density and high voltage.

  As the material for the negative electrode of the lithium ion secondary battery, it is common to use a carbon material that can occlude and release lithium ions. A wide variety of structures, structures, and forms such as graphite structures and turbulent structures are known as carbon materials. Depending on these various structures, structures, and forms, the operating voltage during charging and discharging, etc. Electrode performance varies greatly. Among them, graphite that exhibits a high discharge capacity and potential flatness is currently used in many cases.

It has been reported that graphite materials are more likely to stably form intercalation compounds with lithium as the crystalline graphite structure develops, and that a large amount of lithium is inserted between the layers of the carbon network, resulting in high discharge capacity. ing. Various layer structures are formed depending on the amount of lithium inserted, and in a region where they coexist, they are flat and have a high potential close to lithium metal. Therefore, when an assembled battery is used, it is possible to obtain a high output. Generally, the theoretical capacity (limit value) of a carbon anode material is an ideal graphite intercalation compound of graphite and lithium. The discharge capacity when LiC ₆ is formed is 372 mAh / g.

  On the other hand, in recent years, high-rate characteristics have been required for high power output due to high functionality for power sources such as in-vehicle use, power tools, and even portable devices. For this, there is a great expectation for improvement of the negative electrode for inserting and extracting lithium.

  As a lithium ion secondary battery negative electrode material for a hybrid vehicle that particularly requires high rate characteristics, there is hard carbon. The hard carbon means “non-graphitizable carbon” and is a material in which the carbon network surface is not developed with respect to the above-described graphite material. Therefore, it is suitable for lithium occlusion / desorption at a high rate because the pores between the carbon network surfaces and between the network surface structures are wider than graphite.

  In recent years, soft carbon has attracted attention as a material that is expected to have the same high-rate characteristics as hard carbon by stopping what is originally graphitized at a firing temperature of around 1000 ° C rather than non-graphitizable to hard carbon. Has been. Unlike hard carbon, soft carbon does not require the use of expensive non-graphitizing raw materials such as phenolic resins, and tar pitch raw materials that are inherently graphitized do not need to be graphitized by oxidation reaction, etc. The low manufacturing cost is also an important point.

For this soft carbon, various materials and manufacturing methods have been proposed.
Soft carbon has a capacity of 300 mAh / g or less, which is significantly lower than that of the graphite material, and the density when formed into an electrode cannot be increased to 1.2 g / cm ³ or more. There is. Therefore, in recent years, development of a negative electrode material in which a graphite material and soft carbon are blended has been actively studied.

  In Patent Document 1, in claim 1, “a negative electrode material for a lithium secondary battery containing two or more kinds of carbon materials, at least one of the carbon materials is selected from a specific group of highly crystalline graphite, and at least one of the other carbon materials. One is selected from a specific low crystalline carbon group, and describes a non-aqueous electrolyte secondary battery in which low crystalline carbon is fused to high crystalline graphite. However, there is no description as to whether or not low-crystalline carbon that is not fused to high-crystalline graphite exists in the final product.

For example, Patent Document 2 describes a mixed carbon negative electrode composed of a mixture of natural graphite and soft carbon in order to provide a lithium secondary battery having a large capacity and excellent both cycle characteristics and high rate discharge characteristics. Has been. However, the negative electrode material of Patent Document 1 is a mixture of natural graphite and soft carbon and is not fired, and its characteristics have been studied only when combined with a specific positive electrode.
Further, in Patent Document 3, “a negative electrode material for a lithium secondary battery obtained through a process of heat-treating a graphite-based carbon material in an atmosphere containing a pyrolysis product having a softening point of 150 to 300 ° C.”. Are listed.
However, a carbon material obtained by firing a graphite-based carbon material and a pitch having a softening point of 300 ° C. or lower has a limited improvement in battery characteristics of the carbon material obtained because the pitch softening point is low.

  Patent Document 4 describes a negative electrode material for a lithium secondary battery obtained by firing a mixture of a carbon material raw material (raw material A) and a carbon material (carbon material B). In paragraphs [0022] and [0023], “the carbon material B is a graphite-based carbon material, the raw material A is an isotropic pitch, specifically, the softening point is more preferably 210 ° C. or higher, and usually 340 It is possible to use an isotropic pitch of not more than ° C., more preferably not more than 290 ° C. ”. The softening point of the isotropic pitch to be used is more preferably 290 ° C. or less, and in the examples, only the isotropic pitch having a softening point of 280 ° C. is used. In addition, in paragraph [0037], “when producing a negative electrode material for use in a large-sized lithium secondary battery, the raw material A has an isotropic pitch, particularly an isotropic whose softening point is in the range of 150 to 300 ° C. It is described that it is a preferable embodiment in which the pitch is used and the mixture of the raw material A and the carbon material B is infusible before firing. The infusibilization temperature is described as 150 to 300 ° C. (Claims 5 and 6).

JP 7-326343 A Japanese Patent No. 3776230 JP 2000-243398 A JP 2000-58051 A

  For the mixed negative electrode material of surface-modified graphite material and soft carbon, which has been actively used in recent years, a carbon material for negative electrode having excellent cycle characteristics and high rate characteristics can be obtained when used for the negative electrode of a lithium secondary battery. The present invention provides a manufacturing method that greatly reduces the processing capacity and cost.

  In order to obtain the above materials, it is conceivable to select and mix appropriate surface-modified graphite and soft carbon in the conventional method. However, in this method, the surface-modified graphite and the soft carbon obtained after the surface-modifying step and the firing step for obtaining the surface-modified graphite and the raw material adjustment and the firing step including the particle size adjustment of the soft carbon are appropriately used. It is necessary to mix at a proper ratio. Therefore, complicated processes and matching costs are required. In addition, in order to exert the characteristics of the soft carbon to be mixed at this time, there is a problem that when the particle size is reduced by pulverization in the raw material adjustment step, the bulk density of the powder is lowered, and the processing efficiency in the firing step is greatly lowered. .

  Therefore, the present invention provides a carbon material for a negative electrode of a lithium ion secondary battery, which is obtained by a simple process excellent in cost from a mixed carbon powder of surface-modified graphite powder and soft carbon excellent in cycle characteristics and high rate characteristics. It is an object of the present invention to provide a manufacturing method, a negative electrode for a lithium ion secondary battery using the obtained carbon material, and a method for manufacturing a lithium ion secondary battery.

As a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention have a softening point of over 300 ° C. and 400 ° C. or less and a D ₅₀ particle size of less than 10 μm with respect to 100 parts by mass of the graphite material powder. The manufacturing method which mixes 0.1-20 mass parts of pitches, and bakes in the temperature range of 700-1500 degreeC was invented. According to this manufacturing method, a mixed powder of the soft carbon particles and the carbon-coated graphite material powder whose surface is coated with carbon is obtained. While the volatile matter and pyrolysis gas generated by firing at a high softening point pitch are adsorbed and precipitated on the surface of the graphite material, the high softening point pitch does not melt because of its high softening point and remains as soft carbon particles. Therefore, it can be known that a mixed carbon powder of surface modified graphite powder and soft carbon with excellent cycle characteristics and high rate characteristics can be obtained in a simple process with excellent cost, for use as a negative electrode for lithium ion secondary batteries. The present invention has been completed.

That is, the present invention provides the following (1) to (3).
(1) For 100 parts by mass of graphite material powder, 0.1 to 20 parts by mass of a pitch having a softening point of more than 300 ° C. and not more than 400 ° C. and a D ₅₀ particle diameter of less than 10 μm is mixed with a mixer, and mixed powder A mixing step to obtain,
The mixed powder obtained in the mixing step is fired in a temperature range of 700 to 1500 ° C. in an inert atmosphere, and the pitch of the soft carbon particles obtained by firing and the surface of the graphite material powder is coated with carbon. A method for producing a carbon material for a negative electrode of a lithium ion secondary battery, comprising a firing step of obtaining a mixed powder with the carbon-coated graphite material powder thus prepared.
(2) A lithium ion secondary battery comprising a step of mixing a carbon material for a negative electrode of a lithium ion secondary battery obtained by the production method described in (1) above and a binder, and applying the mixture to a current collector to produce a negative electrode. Manufacturing method of negative electrode.
(3) A method for producing a lithium ion secondary battery in which the negative electrode, the positive electrode, the electrolyte, and the separator according to (2) are disposed in combination.

  According to the present invention, a carbon material for a negative electrode of a lithium ion secondary battery, which is obtained by a simple process excellent in cost from a mixed carbon powder of surface-modified graphite powder and soft carbon particles excellent in cycle characteristics and high rate characteristics The manufacturing method of this, the lithium ion secondary battery negative electrode, and the manufacturing method of a lithium ion secondary battery can be provided.

FIG. 1 is a cross-sectional view showing a coin-type secondary battery for evaluation. FIG. 2 is an SEM (scanning electron microscope) photograph of the powder surface obtained in Example 1. FIG. 3 is a SEM (scanning electron microscope) photograph of the powder cross section obtained in Example 1.

  Hereinafter, the manufacturing method of the carbon material for lithium ion secondary battery negative electrodes of this invention, the carbon material for lithium ion secondary battery negative electrodes, the lithium ion secondary battery negative electrode, and the manufacturing method of a lithium ion secondary battery are demonstrated in detail.

[Method for producing carbon material for negative electrode of lithium ion secondary battery]
The method for producing a carbon material for a negative electrode of a lithium ion secondary battery according to the present invention (hereinafter also simply referred to as “the production method of the present invention”) has a softening point of more than 300 ° C. and 400 with respect to 100 parts by mass of graphite material powder. The mixing step of mixing 0.1 to 20 parts by mass of a pitch of less than 10 μm with a D ₅₀ particle size of less than 10 μm to obtain a mixed powder, and the mixed powder obtained in the mixing step under an inert atmosphere, A firing step of obtaining a mixed powder of soft carbon particles obtained by firing in the temperature range of 700 to 1500 ° C. and obtained by firing the pitch, and carbon-coated graphite material powder having the surface of the graphite material powder coated with carbon. And have.
Hereinafter, each step will be described.

<Mixing process>
(pitch)
The pitch of the raw material used in the present invention is a high softening point pitch having a softening point of more than 300 ° C. and 400 ° C. or less and a D ₅₀ particle diameter of less than 10 μm.
High softening point pitch, a particle size in advance tar and / or tar pitch and heat treatment was adjusted so that the particle size D ₅₀ of less than 10 [mu] m, the softening point of the pitch 300 ° C. greater than the 400 ° C. or less You may heat-process to raise. In addition, after heat treatment for increasing the softening point of the pitch to above 300 ° C. and below 400 ° C., the particle size of D ₅₀ may be adjusted to be less than 10 μm, or the particle size may be adjusted while performing the heat treatment. You can also. Of course, you may choose from pitches that have already been manufactured.

<Heat treatment>
When tar and / or tar pitch are heat-treated, the aromatic hydrocarbon compound that is a component of tar and tar pitch is polymerized by polycondensation reaction, and spherical mesocarbon spherules are precipitated in the pitch matrix. Come. When the heat treatment is further advanced, the mesocarbon spherules grow and coalesce and cannot maintain a spherical shape, and are deformed to generate a high softening point pitch. In addition, you may divide and heat-process in multiple times in the high softening point pitch production | generation process.
In the obtained high softening point pitch, the ratio of the high softening point pitch in a specific range is 50% by mass or more. For example, when the high softening point pitch at a softening point of 250 ° C. or higher is 50% by mass or higher, the pitch is called a high softening point pitch at a softening point of 250 ° C. or higher.

(Tar, tar pitch)
As tars and tar pitches, coal-based and petroleum-based tars and tar pitches are used. Each of tar and tar pitch can be used alone or in combination of two or more.

(Heat treatment atmosphere)
The atmosphere during the heat treatment (heat treatment atmosphere) is not particularly limited, and may be either a non-oxidizing atmosphere (including an inert atmosphere and a reducing atmosphere) or an oxidizing atmosphere, but an inert atmosphere. Is preferred. The inert atmosphere is not particularly limited, and for example, an inert atmosphere with an inert gas such as nitrogen gas or argon gas can be used. It may be a non-oxidizing atmosphere or some oxidizing atmosphere.

(Heat treatment pressure)
The pressure during the heat treatment (heat treatment pressure) is not particularly limited and may be any of reduced pressure, normal pressure or increased pressure, but the polycondensation reaction of the aromatic hydrocarbon compound is promoted, and as a result, the high softening point. In order to improve the production speed of the pitch, it is preferable to perform the heat treatment under pressure conditions.

(Heat treatment temperature)
The temperature during the heat treatment (heat treatment temperature) is particularly limited as long as it is within the range from the start of the production of mesophase from the pitch until the carbonization proceeds to a softening point of more than 300 ° C and less than 400 ° C. However, it is preferably 200 ° C. or higher, more preferably 250 to 500 ° C., and further preferably 280 to 450 ° C. When the temperature of the heat treatment is 250 ° C. or higher, the polycondensation reaction of the aromatic hydrocarbon compound proceeds, and it takes a long time to generate the mesophase, which is realistic. In addition, when the heating temperature is 500 ° C. or less, since the carbonization is advanced and the bulking is started, it is possible to control the softening point of the pitch to be produced industrially. When the heating temperature is 250 ° C. or more and 500 ° C. or less, the balance between the production rate of mesophase and the ease of control of the production is excellent, and when 280 ° C. or more and 450 ° C. or less, the balance is more excellent, and when 300 ° C. or more and less than 400 ° C., it is even better. .

(Heat treatment time)
The time for the heat treatment (heat treatment time) is until a high softening point pitch is generated, and is not particularly limited. In the case where the heat treatment is performed in a plurality of times during the high softening point pitch generation step, the heat treatment time is the sum of the respective treatment times of the divided heat treatments.
The resulting pitch has a softening point greater than 300 ° C. Since the present invention uses a high softening point pitch having a softening point of more than 300 ° C., there is an advantage that it can be stored with a stable quality for a long period of time with respect to the low softening point pitch with little change over time during raw material storage. The softening point is preferably more than 320 ° C to 400 ° C or less, more preferably more than 340 ° C to 400 ° C or less. The softening point is measured in the heating step using suggested thermal analysis.
When the softening point of the raw material pitch is low at 300 ° C. or lower, it melts when mixed and fired with the graphite material, so that the particles are fused together, and pulverization after firing is necessary to obtain a preferable particle size. Moreover, the surface of the graphite material mixed by melting will be excessively covered. These factors result in inferior initial efficiency of the battery and high rate characteristics in the aqueous binder.

(Crushing process)
The high softening point pitch obtained above is pulverized so that the D ₅₀ particle diameter is less than 10 μm in order to exhibit the high rate characteristics of the battery. The method of the pulverization treatment is not particularly limited, and for example, high-speed pulverization and / or shear pulverization can be performed for pulverization. After the pulverization treatment, if necessary, classification may be performed by light pulverization or sieving. Particle size after pulverization (pulverized average particle _{diameter), D 50} particle size of preferably less than 8μm, 0.1~6μm more and more preferably 1~4μm is preferred. In the present invention, the average particle size is a particle size (D ₅₀ ) at which the cumulative frequency of the laser diffraction particle size distribution meter is 50% in terms of volume distribution.

(Graphite material powder)
The graphite material powder used in the present invention is not particularly limited. A part or all of which is made of graphite, for example, natural graphite, and artificial graphite obtained by finally heat-treating tar and pitch at 1500 ° C. or higher. Specifically, mesophase fired bodies obtained by heat-condensing petroleum-based and coal-based tar pitches called carbonitizable carbon materials and subjected to polycondensation, and cokes are graphite at 1500 ° C. or higher, preferably 2800-3300 ° C. It can be obtained by the chemical treatment. Examples include spherical or ellipsoidal average particle size of 1 to 50 μm, specific surface area of 1 to ²⁰ m ² / g, preferably average aspect ratio of 5 or less, and average particle size of 5 to 30 μm. it can.
Commercially available natural graphite particles or artificial graphite particles processed into spherical or ellipsoidal shapes can also be used. In the case of graphite particles having a shape other than spherical or ellipsoidal shape, for example, flaky graphite particles, the flaky graphite may be granulated and spheroidized by mechanical external force to obtain spherical graphite particles. The method of processing into a spherical or ellipsoidal shape is, for example, a method of mixing a plurality of scaly graphites in the presence of a granulating aid such as an adhesive or a resin, without using an adhesive for a plurality of scaly graphites. The method of applying mechanical external force, the combined use of both, etc. are mentioned. However, the most preferable method is to apply a mechanical external force to granulate into a spherical shape without using a granulating aid. The mechanical external force is mechanically pulverizing and granulating, and scaly graphite can be granulated and spheroidized. Examples of the flaky graphite crusher include a pressure kneader, a kneader such as a two-roll mill, a rotating ball mill, a counter jet mill (manufactured by Hosokawa Micron Corporation), a current jet (manufactured by Nisshin Engineering Co., Ltd.), and the like. A grinding device can be used.

Lc, which is a measured value of X-ray diffraction of the graphite particles used in the present invention, is preferably 40 nm or more, and La is preferably 40 nm or more. Here, Lc is the crystallite size Lc (002) in the c-axis direction of the graphite structure, and La is the crystallite size La (110) in the a-axis direction. d002 is 0.337nm or less, the ratio I1360 / I1580 (R value) of 1360 cm ^-1 peak intensity measured by Raman spectroscopy using an argon laser (I1360) and 1580 cm ^-1 peak intensity (I1580) is from 0.06 to 0 It is preferable that the half widths of the .30 and 1580 cm ⁻¹ bands are 10 to 60.

<Mixing process>
Mixing process, with respect to the graphite material powder 100 parts by weight, with a softening point of 300 ° C. Ultra 400 ° C. or _less, and _{D 50} particle size of the pitch of less than 10μm was mixed 0.1-20 parts by weight, to obtain a mixed powder It is a process.
When firing the mixed powder of the high softening point pitch and the graphite material obtained above after pulverization, the dry powder is granulated with any type of mixer such as a Nauter mixer, a Henschel mixer, a grinder, a grinder, etc. Is mixed under conditions that do not cause excessive destruction or deformation.
The mixing ratio between the high softening point pitch after pulverization and the graphite material is not unconditionally determined because it depends on the softening point of the high softening point pitch, but considering the amount of remaining soft carbon particles, If the high softening point pitch of 0.1 parts by mass or more is not mixed, the intended effect of the present invention cannot be obtained. Moreover, if the high softening point pitch exceeds 20 parts by mass with respect to 100 parts by mass of the graphite material, the graphite material is excessively coated to impair the high rate characteristics, and the amount of remaining soft carbon also becomes excessive and the capacity is reduced. In addition, the press compressibility when the lithium ion negative electrode is formed decreases. The mixing ratio of the high softening point pitch with respect to 100 parts by mass of the graphite material is 0.1 to 20 parts by mass, preferably 3 to 20 parts by mass, and more preferably 4 to 15 parts by mass.

<Baking process>
In the firing step, the high softening point pitch obtained in the high softening point pitch volatile content adjusting step is fired at 700 to 1500 ° C. in an inert atmosphere to obtain a carbon material for a lithium ion secondary battery negative electrode (“of the present invention”). This is a step of obtaining a “carbon material”. Preferably more than 900 degreeC-1400 degreeC, More preferably, more than 1000 degreeC-1300 degreeC is still more preferable.
In the production method of the present invention, the infusibilization step of heat treatment at a lower temperature than the baking step is not performed before the baking step. In the production method of the present invention, it is known that when the infusibilization step is performed, battery characteristics having a negative electrode using the obtained carbon material for a negative electrode of a lithium ion secondary battery are deteriorated, as will be described later in a comparative example.

<Baking treatment>
In the production method of the present invention, the baking treatment is performed at 700 to 1500 ° C., and it is not necessary to perform the treatment at a high temperature of about 2000 to 3000 ° C. unlike the graphitization treatment, so that a high temperature heating furnace is unnecessary. In addition, since the pitch is set to a high softening point in advance, it is preferable not to perform preliminary heat treatment (infusibilization treatment) at a temperature lower than 700 ° C., for example, 300 to 600 ° C., before the firing treatment.

(Baking treatment atmosphere)
The atmosphere during the baking treatment (firing treatment atmosphere) is an inert atmosphere. The inert atmosphere is not particularly limited, and for example, an inert atmosphere with an inert gas such as nitrogen gas or argon gas can be used.

(Baking temperature)
The temperature during the firing treatment (firing treatment temperature) is 700 to 1500 ° C. When the firing temperature is lower than 700 ° C., the carbonization is slow, and in some cases, the carbonization cannot be sufficiently performed. In addition, when the firing temperature is higher than 1500 ° C., the high-rate characteristics may be deteriorated when an aqueous binder is used.

(Baking time)
The time during the firing treatment (firing treatment time) is until the high softening point pitch is carbonized, and is not particularly limited.

[Carbon material for negative electrode of lithium ion secondary battery]
The carbon material for a negative electrode of a lithium ion secondary battery of the present invention (hereinafter also simply referred to as “the carbon material of the present invention”) includes specific soft carbon particles obtained by the above-described production method of the present invention, and the graphite material powder. This is a mixed powder with a carbon-coated graphite material powder coated with carbon on its surface.
The carbon material of the present invention can be used as a carbon material for a negative electrode of a lithium ion secondary battery without performing a heat treatment (graphitization treatment) at 2000 to 3000 ° C. in a non-oxidizing atmosphere. The mixed powder after firing is a mixed powder of specific soft carbon particles and carbon-coated graphite material powder whose surface of the graphite material powder is carbon-coated. ) Can be seen by observing soft carbon particles and graphite material powder coated with surface carbon.

(Average particle size of negative electrode material)
The average particle diameter of the carbon material of the present invention is not particularly limited, but the bulk density is high, a higher packing density can be obtained when the electrode is used, and the electrode thickness is usually 100 μm or less. For the reason, 2 to 30 μm is preferable, and 3 to 20 μm is more preferable particularly in a lithium ion secondary battery that requires high rate characteristics. In addition, the average particle diameter of the carbon material of the present invention is a particle diameter (D ₅₀ ) at which the cumulative frequency of the laser diffraction particle size distribution meter is 50% by volume.

(Specific surface area of negative electrode material)
The specific surface area of the carbon material of the present invention is not particularly limited, but if it is too large, the safety of the lithium ion secondary battery may be lowered, and therefore it is preferably 20 m ² / g or less, 0.3 ~10.0m more preferably ² / g, in order to exhibit more excellent high-rate characteristics, more preferably 2.0~5.0m ² / g. Here, the specific surface area of the negative electrode material is a nitrogen gas adsorption BET specific surface area.

[Lithium ion secondary battery]
A lithium ion secondary battery using the carbon material of the present invention (hereinafter also referred to as “lithium ion secondary battery of the present invention”) will be described. Moreover, the lithium ion secondary battery negative electrode using the carbon material of this invention is also demonstrated.
Lithium ion secondary batteries usually have a negative electrode, a positive electrode, and a non-aqueous electrolyte as the main battery components, and the positive and negative electrodes are each composed of a layered or clustered material capable of occluding lithium ions. The entry and exit are performed between the layers. This is a battery mechanism in which lithium ions are doped into the negative electrode during charging and are dedoped from the negative electrode during discharging.
The lithium ion secondary battery of the present invention is not particularly limited except that the carbon material of the present invention is used, and other battery components conform to the elements of a general lithium ion secondary battery.

(Negative electrode)
When producing a negative electrode, the negative electrode mixture which added the binder (binder) to the carbon material of this invention mentioned above or the mixed negative electrode material containing the carbon material of this invention is used. As the binder, either a water-based binder or a non-aqueous (organic solvent-based) binder can be used, but it is preferable to use a water-based binder because the environmental load during the production of the negative electrode is low. The water-based binder is a binder using a water-soluble polymer, and examples thereof include styrene-butadiene copolymer (SBR), polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), and polyacrylic acid (PAA). These aqueous binders can be used alone or in combination of two or more. Non-aqueous (organic solvent) binder is a binder using a polymer that is insoluble in water and dissolved in an organic solvent (for example, dimethylformamide). For example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), Examples include polyethylene (PE). These non-aqueous binders can be used alone or in combination of two or more. In general, the binder is preferably used in an amount of about 1 to 20% by mass in the total amount of the negative electrode mixture.

  A normal solvent for preparing a negative electrode can be used for preparing the negative electrode. When the negative electrode mixture is dispersed in a solvent and made into a paste, and then applied to the current collector and dried, the negative electrode mixture layer is uniformly and firmly adhered to the current collector. More specifically, for example, the particles of the carbon material of the present invention and a binder are mixed with a solvent such as water or alcohol to form a slurry, and then kneaded with a kneader or a mixer to prepare a paste. If this paste is applied to one or both sides of the current collector and dried, a negative electrode in which the negative electrode mixture layer is uniformly bonded can be obtained.

  After the negative electrode mixture layer is formed, the adhesive strength between the negative electrode mixture layer and the current collector can be further increased by pressure bonding such as pressurization. The shape of the current collector used for the negative electrode is not particularly limited, and for example, a foil-like one or a net-like one such as a mesh or an expanded metal is used. Examples of the material for the current collector include copper, stainless steel, and nickel. For example, in the case of a foil, the thickness of the current collector is preferably about 5 to 20 μm.

(Positive electrode)
The positive electrode material (positive electrode active material) is a composite oxide of lithium and a transition metal, and may be a solid solution of lithium and two or more transition metals. Specifically, the lithium-containing transition metal oxide is LiM (1) _1-p M (2) _p O ₂ (wherein P is a numerical value in the range of 0 ≦ p ≦ 1, M (1), M (2) is composed of at least one transition metal element), or LiM (1) _2-q M (2) _q O ₄ (wherein q is a numerical value in the range of 0 ≦ q ≦ 1, M (1 ), M (2) is composed of at least one transition metal element). Examples of the transition metal element represented by M include Co, Ni, Mn, Cr, Ti, V, Fe, Zn, In, and Sn, and Co, Ni, Fe, Mn, Ti, and Cr are preferable.
Such a lithium-containing transition metal oxide includes, for example, Li, a transition metal oxide or salt as a starting material, these starting materials are mixed according to the composition, and fired in an oxygen atmosphere at a temperature range of 600 to 1300 ° C. Can be obtained. Note that the starting materials are not limited to oxides or salts, and can be synthesized from hydroxides or the like.

As a method of forming a positive electrode using such a positive electrode material, for example, a paste-like positive electrode mixture paint comprising a positive electrode material, a binder and a conductive agent is applied to one or both sides of a current collector. An agent layer is formed. As the binder, those exemplified for the negative electrode can be used. As the conductive agent, for example, a fine carbon material, a fibrous carbon material, graphite, or carbon black can be used. The shape of the current collector is not particularly limited, and the same shape as the negative electrode is used. As the material for the current collector, aluminum, nickel, stainless steel and the like can be usually used.
In forming the above-described negative electrode and positive electrode, various conventionally known additives such as a conductive agent and a binder can be appropriately used.

(Electrolytes)
As the electrolyte, a normal nonaqueous electrolyte containing a lithium salt such as LiPF ₆ or LiBF ₄ as the electrolyte salt is used. As this non-aqueous solvent, an aprotic organic solvent such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and diethyl carbonate can be used.

(Separator, cell case, other members)
In the lithium ion secondary battery of the present invention, a polypropylene or polyethylene microporous film or a layered structure thereof, or a separator such as a nonwoven fabric is usually used. Moreover, the cell structure of the lithium ion secondary battery of this invention is arbitrary, and it does not specifically limit about the shape and form, For example, it can select arbitrarily from a cylindrical shape, a square shape, and a coin shape.

  Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to these.

[Example 1]
<Manufacture of carbon material for lithium ion secondary battery negative electrode>
(High softening point pitch manufacturing process)
Tar was placed in a 20 liter autoclave and heat treated at 300 ° C. for about 2.5 hours to produce a high softening point pitch. The temperature and time of the heat treatment and the softening point of the obtained high softening point pitch are shown in the column of the high softening point pitch manufacturing step in Table 1.
(High softening point pitch crushing process)
The obtained high softening point pitch was pulverized to a mean particle size of 4 μm using a jet mill. Table 1 shows the volatile content of the high softening point pitch together with the average particle diameter.
(Mixing process)
100 parts by mass of spheroidized natural graphite having an average particle diameter of 15 μm (denoted as spherical sky black in the table) and 10 parts by mass of the 4 μm high softening point pitch obtained above are 10 m / s using a Nauta mixer. For 15 minutes.
(Baking process)
The mixed powder of spheroidized natural graphite and high softening point pitch obtained in the above mixing step is fired in an inert atmosphere (N ₂ ) under conditions of a temperature of 1200 ° C. and a time of 1 hr, and a lithium ion secondary battery negative electrode A carbon material to be used as a carbon material was obtained. Table 1 shows the average particle diameter of the obtained mixed carbon powder of the surface-modified graphite powder and the soft carbon, and the production conditions such as the firing atmosphere, temperature and time. 2 and 3 show SEM images of the surface and cross section of the obtained carbon material. From the figure, it can be seen that carbon-coated graphite material powder 10 and soft carbon particles 12 exist in the obtained carbon material (powder).

<Production of evaluation battery>
A planetary mixer using 98 parts by mass of the produced carbon material for the negative electrode of a lithium ion secondary battery, 1 part by mass of carboxymethylcellulose ammonium (in solid content) and 1 part by mass of carboxy-modified styrene butadiene rubber, and water as a solvent. The mixture was stirred and mixed to obtain a negative electrode mixture paste. The obtained negative electrode mixture paste was applied on a copper foil having a thickness of 15 μm and vacuum-dried at a temperature of 110 ° C. to form a negative electrode mixture layer. The formed negative electrode mixture layer was pressed by a roll press, punched into a circular shape having a diameter of 15.5 mm, and a negative electrode having a negative electrode mixture layer adhered to a current collector made of copper foil was produced.

Next, a coin-type lithium ion secondary battery shown in FIG. 1 was prepared as an evaluation battery. The evaluation battery is a battery system in which a current collector 7a, a cylindrical positive electrode 4, a separator 5 impregnated with an electrolyte, a negative electrode 2, and a current collector 7b are stacked in that order from the inner surface of the outer can 3. . In the evaluation battery, the separator 5 is sandwiched and stacked between the current collector 7b and the positive electrode (counter electrode) 4 closely attached to the current collector 7a, and then the current collector 7b is placed in the exterior cup 1 and the positive electrode 4 is placed. It is accommodated in the outer can 3, the outer cup 1 and the outer can 3 are combined, and further, an insulating gasket 6 is interposed between the outer peripheral portion of the outer cup 1 and the outer can 3, and both peripheral portions are caulked and sealed. Produced. The electrolyte was a nonaqueous electrolyte in which LiPF ₆ was dissolved at a concentration of 1 mol / L in a mixed solvent obtained by mixing ethylene carbonate (33% by volume) and methyl ethyl carbonate (67% by volume). It is. Moreover, the separator and the negative electrode were immersed in a non-aqueous electrolyte in advance and impregnated with the non-aqueous electrolyte.

<Evaluation of battery characteristics>
About the produced evaluation battery, the following charging / discharging tests were done at 25 degreeC. In this test, the process of doping (occluding) lithium ions into the negative electrode material is referred to as “charging”, and the process of dedoping (detaching) from the negative electrode material is referred to as “discharge”.

(Discharge capacity)
After constant current charging of 1.2 mA until the circuit voltage reached 1 mV, switching to constant voltage charging was continued until the current value reached 20 μA. The charging capacity (unit: mAh / g) was determined from the energization amount during that time. Then, it rested for 10 minutes.
Next, constant current discharge was performed at a current value of 1.2 mA until the circuit voltage reached 1.5 V, and the discharge capacity (unit: mAh / g) was determined from the amount of electricity supplied during this period. This was the first cycle.
Table 2 shows the obtained discharge capacity of the first cycle.

(First-time charge / discharge efficiency)
From the results of the charge / discharge test, the initial charge / discharge efficiency (unit:%) was determined by the following formula.
Initial charge / discharge efficiency = (discharge capacity of first cycle / charge capacity of first cycle) × 100
Table 2 shows the obtained initial charge / discharge efficiency.

(Cycle characteristics)
A new unused evaluation battery was prepared and the cycle characteristics were evaluated.
After 6.0 mA constant current charging was performed until the circuit voltage reached 0 mV, switching to constant voltage charging was continued until the current value reached 20 μA, and then rested for 120 minutes.
Next, constant current discharge was performed at a current value of 6.0 mA until the circuit voltage reached 1.5V. The charge / discharge was repeated 50 times, and the cycle characteristics (unit:%) were determined from the obtained discharge capacity by the following formula.
Cycle characteristics = (discharge capacity of 50th cycle / discharge capacity of 1st cycle) × 100
Table 2 shows the obtained cycle characteristics.

(High rate discharge characteristics (rapid discharge characteristics))
A new unused evaluation battery was prepared and the high rate discharge characteristics were evaluated.
Following the first cycle, constant current charging of 1.2 mA was performed until the circuit voltage reached 1 mV, then switching to constant voltage charging was continued until the current value reached 20 μA. Then, it rested for 10 minutes.
Next, constant current discharge was performed at a current value of 18.0 mA until the circuit voltage reached 1.5 V, and the discharge capacity (unit: mAh / g) was determined from the amount of current supplied. From the obtained discharge capacity, the high-rate discharge characteristics (unit:%) were obtained by the following formula.
High-rate discharge characteristic = (18.0 mA discharge capacity / first cycle 1.2 mA discharge capacity) × 100
Table 2 shows the obtained high-rate discharge characteristics.
In the present specification, the rapid discharge characteristics thus obtained may be referred to as “3C / 0.2C discharge rate”.

(High rate charging characteristics (rapid charging characteristics))
A new unused evaluation battery was prepared and the high rate charging characteristics were evaluated.
Following the first cycle, a constant current charge of 6.0 mA was performed until the circuit voltage reached 1 mV, and then rested for 10 minutes. The charging capacity (unit: mAh / g) was determined from the energization amount during this period. From the obtained charge capacity, the high rate charge characteristic (unit:%) was obtained by the following equation.
High-rate charge characteristic = (6.0 mA charge capacity / first cycle 1.2 mA charge capacity) × 100
Table 2 shows the obtained high rate charging characteristics.
In the present specification, the quick charge characteristics thus obtained may be referred to as “1C / 0.2C charge rate”.

[Examples 2-9, Comparative Examples 1-6]
Examples 2 to 9 and Comparative Examples 1 to 6 were produced and evaluated in the same manner as in Example 1. Production and evaluation in the same manner as in Example 1 using the mixed carbon powder of the surface-modified graphite powder and soft carbon obtained in Table 1 for the manufacturing conditions of each example and comparative example, the physical properties of intermediate steps, etc. Went. The results are shown in Table 2.
In addition, as the graphite material powder, the same spherical natural graphite as in Example 1 was used, and the artificial graphite of Example 6 was manufactured by firing coal tar pitch as a raw material at 3000 ° C. for 3 hours in an argon atmosphere. Graphite was used.

(Evaluation of Examples and Comparative Examples)
1) In Comparative Example 1, the same raw material as in the example was manufactured under the same firing conditions except that the softening point of the pitch used was lower than 300 ° C. of the example and 250 ° C., but the softening point of the pitch was Therefore, the high rate characteristics of the obtained battery characteristics were inferior to those of the examples. On the other hand, Comparative Example 2 in which the softening point of the raw material pitch exceeded 400 ° C. was inferior in the initial charge / discharge efficiency and cycle characteristics of the obtained battery characteristics, and the high rate charge characteristics were also low. One feature of the present invention is that a high softening point pitch having a softening point of the raw material pitch of more than 300 ° C. and 400 ° C. or less is used.
2) In Comparative Example 3, the firing temperature was too high, so the cycle characteristics were inferior, and the high rate charge characteristics were also low. In Comparative Example 4, since the amount of the high softening point pitch is too large, the carbon material for negative electrode produced is inferior in battery characteristics that can be inferior in compression characteristics. In Comparative Example 5, the particle size of the high softening point pitch is large and the high rate charging characteristics are inferior.
3) According to the present invention, as a result of mixing graphite material powder and high softening point pitch and firing in a specific temperature range, graphite powder for negative electrode in which soft carbon particles and carbon-coated graphite material powder coated on the surface are present is obtained. It can be seen from the results of the examples and comparative examples that this is another feature of the present invention.

DESCRIPTION OF SYMBOLS 1 Exterior cup 2 Negative electrode 3 Exterior can 4 Positive electrode 5 Separator 6 Insulating gasket 7a Current collector 7b Current collector 10 Carbon covering graphite material powder 12 Soft carbon particle

Claims (3)

Against the graphite material powder 100 parts by weight, with softening point of 300 ° C. Ultra 400 ° C. or less, 0.1 to 20 parts by mass of pitch of less than D ₅₀ particle size of 10 [mu] m, and mixed in a mixer to obtain a mixed powder A mixing step;
The mixed powder obtained in the mixing step is fired in a temperature range of 700 to 1500 ° C. in an inert atmosphere, and the pitch of the soft carbon particles obtained by firing and the surface of the graphite material powder is coated with carbon. A method for producing a carbon material for a negative electrode of a lithium ion secondary battery, comprising a firing step of obtaining a mixed powder with the carbon-coated graphite material powder thus prepared.   A method for producing a negative electrode for a lithium ion secondary battery comprising a step of mixing a carbon material for a negative electrode for a lithium ion secondary battery obtained by the production method according to claim 1 and a binder and applying the mixture to a current collector to produce the negative electrode .   The manufacturing method of the lithium ion secondary battery arrange | positioned combining the negative electrode of Claim 2, a positive electrode, electrolyte, and a separator.