JP2010267629A - Negative electrode for lithium secondary battery, and lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high capacity lithium secondary battery with improved energy density per volume of a secondary battery by preventing deterioration of rapid charge and discharge characteristics and cycle characteristics when a negative electrode density of the lithium secondary battery is increased. <P>SOLUTION: The negative electrode for a lithium secondary battery includes a mixture layer containing graphite particles and organic binder on a collector. A diffraction intensity ratio (002)/(110) measured by X-ray diffraction of the mixture layer of the negative electrode for the lithium secondary battery is ≤500. The lithium secondary battery includes the negative electrode for the lithium secondary battery and a positive electrode containing a lithium compound. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery. More particularly, the present invention relates to a lithium secondary battery having a high capacity and excellent quick charge / discharge characteristics and cycle characteristics suitable for use in portable equipment, electric vehicles, power storage, and the like, and a negative electrode for obtaining the lithium secondary battery.

  The negative electrode of a conventional lithium secondary battery includes, for example, natural graphite particles, artificial graphite particles graphitized with coke, organic polymer materials, artificial graphite particles graphitized with pitch, graphite particles obtained by pulverizing these, and mesophase carbon. Examples include graphitized spheroidal graphite. These graphite particles are mixed with an organic binder and an organic solvent to form a graphite paste. This graphite paste is applied to the surface of a copper foil, and the solvent is dried to be used as a negative electrode for a lithium secondary battery. .

  For example, as disclosed in Patent Document 1, the use of graphite for the negative electrode eliminates the problem of content short circuit due to lithium dendrite and improves cycle characteristics.

However, natural graphite, which has developed graphite crystals, has weaker bond strength between crystal layers in the C-axis direction than the bond in the crystal plane direction. It becomes what is called scale-like graphite particles. Since scaly graphite has a large aspect ratio, when graphite is kneaded with a binder and applied to a current collector to produce an electrode, the scaly graphite particles are oriented in the surface direction of the current collector, resulting in a charge / discharge capacity. In addition to the rapid deterioration of the rapid charge / discharge characteristics, there is a problem in that the internal characteristics of the electrode are broken due to expansion and contraction in the C-axis direction caused by repeated insertion and extraction of lithium into and from the graphite crystal, resulting in deterioration of cycle characteristics. In addition, when the negative electrode density is 1.45 g / cm ³ or more, lithium is difficult to be occluded / released in the negative electrode graphite, and there is a problem that rapid charge / discharge characteristics, discharge capacity per weight of the negative electrode, and cycle characteristics are deteriorated. .

  On the other hand, the lithium secondary battery can be expected to increase the energy density per volume by increasing the negative electrode density. Therefore, in order to improve the energy density per volume of the lithium secondary battery, there is a demand for a negative electrode in which rapid charge / discharge characteristics and cycle characteristics are less deteriorated when the negative electrode density is increased.

Japanese Examined Patent Publication No. 62-23433

  In view of the above problems, the present invention provides a negative electrode suitable for a lithium secondary battery excellent in rapid charge / discharge characteristics and cycle characteristics, and further provides a negative electrode suitable for a high capacity lithium secondary battery.

(1) The present invention is a negative electrode for a lithium secondary battery having a mixture layer comprising graphite particles and an organic binder on a current collector, and diffraction measured by X-ray diffraction of the mixture layer The present invention relates to a negative electrode for a lithium secondary battery having an intensity ratio (002) / (110) of 500 or less.

(2) Moreover, this invention is a negative electrode for lithium secondary batteries as described in said (1) whose density of the mixture layer containing a graphite particle and an organic type binder is 1.50-1.95g / cm < ³ >. About.

(3) Further, the present invention provides the lithium according to the above (1) or (2), wherein the graphite particles have an average particle size of 1 to 100 μm and the crystallite size Lc (002) in the C-axis direction of the crystal is 500 angstroms or more. The present invention relates to a negative electrode for a secondary battery.

(4) Moreover, this invention relates to the lithium secondary battery which has the negative electrode for lithium secondary batteries of this invention in any one of said (1)-(3), and the positive electrode containing a lithium compound.

(5) Furthermore, the present invention relates to the lithium secondary battery according to (4), wherein the lithium compound contains at least Ni.

  According to the present invention, a negative electrode for a lithium secondary battery excellent in cycle characteristics and rapid discharge characteristics can be obtained, which is suitable for use in a high capacity lithium secondary battery.

FIG. 1 is a partially sectional schematic front view showing an example of a lithium secondary battery of the present invention. FIG. 2 is a schematic diagram of a lithium secondary battery used in the measurement of charge / discharge capacity and discharge capacity retention rate in an example of the present invention.

  The negative electrode for a lithium secondary battery of the present invention is a negative electrode for a lithium secondary battery having a mixture layer comprising graphite particles and an organic binder on a current collector, the graphite particles and the organic binder A diffraction intensity ratio (002) / (110) measured by X-ray diffraction of the mixture layer containing is 500 or less. The diffraction intensity ratio (002) / (110) is preferably in the range of 10 to 500, more preferably 10 to 400, still more preferably 10 to 300, and particularly preferably 50 to 200. When the diffraction intensity ratio (002) / (110) exceeds 500, the rapid charge / discharge characteristics and cycle characteristics of the lithium secondary battery to be manufactured are deteriorated.

  Here, the diffraction intensity ratio (002) / (110) of the mixture layer of graphite particles and organic binder is a mixture of graphite particles and organic binder by X-ray diffraction using CuKα rays as an X-ray source. The surface of the layer is measured, and the intensity of the (002) plane diffraction peak detected near the diffraction angle 2θ = 26 to 27 degrees and the (110) plane diffraction peak detected near the diffraction angle 2θ = 70 to 80 degrees From the following equation (1).

(002) Plane diffraction peak intensity / (110) plane diffraction peak intensity (1) Equation (1) Note that setting the diffraction intensity ratio (002) / (110) to 500 or less means, for example, the particle size of graphite particles It is possible to adjust the press pressure at the time of producing the negative electrode, the thermal expansion coefficient of the raw material of the graphite particles, and the like as appropriate. It can also be adjusted by appropriately controlling the number of pores inside the graphite particles, etc., thereby suppressing the shape change or breakage of the particles when producing the negative electrode.

In the negative electrode for a lithium secondary battery of the present invention, the density of the mixture layer containing the graphite particles and the organic binder on the current collector is preferably 1.50 to 1.95 g / cm ³ . The density is more preferably 1.55~1.90g / cm ^3, more preferably ^{1.60~1.85g / cm 3, 1.65~1.80g / cm} 3 is particularly preferred.

By increasing the density of the mixture layer comprising the graphite particles and the organic binder on the current collector in the negative electrode of the present invention, the energy density per volume of the lithium secondary battery obtained using this negative electrode can be reduced. Can be bigger. When the density of the mixture layer containing the graphite particles and the organic binder is less than 1.50 g / cm ³ , the energy density per volume of the obtained lithium secondary battery tends to be small. On the other hand, when the density of the mixture layer containing the graphite particles and the organic binder exceeds 1.95 g / cm ³ , the pouring property of the electrolytic solution when producing a lithium secondary battery tends to deteriorate. In addition, there is a tendency that rapid charge / discharge characteristics and cycle characteristics of a lithium secondary battery to be manufactured are deteriorated.

  Here, the density of the mixture layer containing the graphite particles and the organic binder can be calculated from the measured values of the weight and volume of the mixture layer containing the graphite particles and the organic binder.

  The density of the mixture layer containing the graphite particles and the binder after the integration can be appropriately adjusted by, for example, the pressure at the time of integral molding, the clearance of an apparatus such as a roll press, or the like.

  The crystallite size Lc (002) in the C-axis direction of the graphite particles used in the present invention is preferably 500 angstroms or more, more preferably 800 angstroms or more, and particularly preferably 1000 to 10,000 angstroms. When the crystallite size Lc (002) in the C-axis direction is less than 500 angstroms, the discharge capacity tends to be small.

  Further, the interlayer distance d (002) of the graphite particle crystals is preferably 3.38 angstroms or less, more preferably 3.37 angstroms or less, and even more preferably 3.36 angstroms or less. Moreover, the one close | similar to a perfect graphite structure is preferable. When the interlayer distance d (002) between crystals exceeds 3.38 angstroms, the discharge capacity tends to decrease. The Lc (002) and d (002) can be measured by X-ray wide angle diffraction.

  The graphite particles used for the negative electrode for a lithium secondary battery of the present invention have a diffraction intensity ratio (002) / (110) of 500 measured by X-ray diffraction of the negative electrode graphite particles and the organic binder mixture layer. Any material can be used as long as it can be set as follows.For example, scaly graphite, spheroidal graphite, graphite whose particle shape is modified by mechanical treatment, or a mixture of a plurality of materials can be used. It is preferable to use secondary graphite particles in which a plurality of particles are aggregated or bonded so that their orientation planes are non-parallel. These are used alone or in combination of two or more.

  In the present invention, the flat particle means a shape having a major axis and a minor axis and is not completely spherical. For example, those having a shape such as a scale shape, a scale shape, or a part of a lump shape are included. In graphite particles, the orientation planes of a plurality of flat particles are non-parallel. In other words, the flat surfaces in the shape of each particle, in other words, the plane closest to the plane is the orientation plane, and each of the plurality of flat particles is The orientation planes are assembled or bonded without being aligned in a certain direction to form graphite particles.

  This bond refers to a state in which the particles are chemically bonded, for example, through carbonaceous carbonized binder such as pitch, tar, etc. However, it refers to a state in which the shape is maintained as an aggregate in the process of producing the negative electrode due to the shape and the like. From the viewpoint of mechanical strength, those bonded are preferable.

  The graphite particles used in the present invention preferably have an aspect ratio of 5 or less, more preferably 1.2 to 5, more preferably 1.2 to 3, and more preferably 1.3 to 2. 5 is particularly preferable. The graphite particles having an aspect ratio of 5 or less may be secondary particles in which a plurality of primary particles are aggregated or combined, and one particle is mechanically applied so that the aspect ratio is 5 or less. The shape may be changed, and further, a combination of these may be used.

  If this aspect ratio exceeds 5, the diffraction intensity ratio (002) / (110) measured by X-ray diffraction of the mixture layer containing the graphite particles of the negative electrode and the organic binder tends to increase. As a result, the rapid charge / discharge characteristics and cycle characteristics of the obtained lithium secondary battery tend to deteriorate. On the other hand, when the aspect ratio is less than 1.2, the contact area between the particles decreases, and the conductivity of the negative electrode to be produced tends to decrease.

  The aspect ratio is represented by A / B, where A is the length in the major axis direction of the graphite particles and B is the length in the minor axis direction. In the present invention, the aspect ratio is obtained by magnifying graphite particles with an electron microscope, arbitrarily selecting 10 graphite particles, measuring A / B while changing the observation angle of the electron microscope, and taking the average value. is there. When the graphite particles have a thickness direction such as a scale shape, a plate shape, or a block shape, the thickness is defined as a length B in the minor axis direction.

  The graphite particles used in the present invention are preferably those capable of withstanding mechanical treatment with little change in shape or destruction of the graphite particles in the process of producing the negative electrode. X-ray diffraction of the mixture layer containing the graphite particles of the negative electrode and the organic binder due to the increase in specific surface area or the orientation of the graphite particles on the electrode when the shape change or destruction of the graphite particles occurs As a result, the charge / discharge efficiency, thermal stability, rapid charge / discharge characteristics and cycle characteristics of the obtained lithium secondary battery are degraded. Tend to.

  In the case where the graphite particles exist as an aggregate or a combination of a plurality of particles, the primary particles of the graphite particles refer to particle units that are recognized when observed with a scanning electron microscope (SEM) or the like. Further, the secondary particles refer to lumps in which the primary particles are aggregated or bonded.

  In one secondary particle, the number of flat primary particles aggregated or bonded is preferably 3 or more, more preferably 5 or more. The size of each flat primary particle is preferably 1 to 100 μm, more preferably 5 to 80 μm, and even more preferably 5 to 50 μm, and these are aggregates or bonds. The average particle size of the secondary particles is preferably 2/3 or less. Further, the aspect ratio of each flat primary particle is preferably 100 or less, more preferably 50 or less, and further preferably 20 or less. A preferable lower limit of the aspect ratio of the primary particles is 1.2, and it is preferably not spherical.

Furthermore, the secondary particles preferably have a specific surface area of 8 m ² / g or less, more preferably 5 m ² / g or less. When graphite particles having a secondary particle specific surface area of 8 m ² / g or less are used for the negative electrode, the rapid charge / discharge characteristics and cycle characteristics of the obtained lithium secondary battery can be improved. The irreversible capacity can be reduced. When the specific surface area exceeds 8 m ² / g, the irreversible capacity of the first cycle of the obtained lithium secondary battery tends to be large, the energy density is small, and many binders are used when producing a negative electrode. Tend to be needed. The specific surface area is more preferably 1.5 to ⁵ m ² / g, more preferably ^{2 to 5} m ² / g, from the viewpoint that the rapid charge / discharge characteristics, cycle characteristics, and the like of the obtained lithium secondary battery are further favorable. Is particularly preferred. The specific surface area can be measured by, for example, the BET method using nitrogen gas adsorption.

  The method for producing the negative electrode for a lithium secondary battery of the present invention is not particularly limited. For example, at least a graphitizable aggregate or graphite and a graphitizable binder are mixed and pulverized, and then the pulverized product and graphitized are mixed. 1 to 50% by weight of catalyst is mixed and calcined to obtain graphite particles, and then the organic particles and a solvent are added to and mixed with the graphite particles, and the mixture is applied to a current collector and dried. Then, after removing the solvent, it can be manufactured by pressurizing and integrating.

As the aggregate that can be graphitized, for example, coke, a carbide of resin, or the like can be used, but a powder material that can be graphitized is preferable. Among these, coke powder that is easily graphitized such as needle coke is preferable. The coke used as the aggregate preferably has a coefficient of thermal expansion of 0.9 × 10 ^{−6 to} 7.0 × 10 ⁻⁶ / ° C., and 1.0 × 10 ^{−6 to} 6.5 × 10 ⁻⁶ / ° C. More preferably, 1.2 × 10 ^{−6 to} 6.0 × 10 ⁻⁶ / ° C., and even more preferably 2.0 × 10 ^{−6 to} 6.0 × 10 ⁻⁶ / ° C. preferable. When the thermal expansion coefficient is less than 0.9 × 10 ⁻⁶ / ° C., X-ray diffraction of a mixture layer containing graphite particles and an organic binder on the current collector of the negative electrode for a lithium secondary battery to be produced The measured diffraction intensity ratio (002) / (110) tends to increase. Moreover, when a thermal expansion coefficient exceeds 7.0 * 10 < ^-6 > / ^degreeC , there exists a tendency for the charging / discharging capacity | capacitance of the lithium secondary battery to produce to fall. Moreover, as graphite, natural graphite powder, artificial graphite powder, etc. can be used, for example, but is preferably in powder form. The particle size of the graphitizable aggregate or graphite is preferably smaller than the particle size of the graphite particles to be produced, the average particle size is more preferably 1 to 80 μm, more preferably 1 to 50 μm, and more preferably 5 to 50 μm. It is particularly preferable if it exists. The aspect ratio of the graphitizable aggregate or graphite is preferably from 1.2 to 500, more preferably from 1.5 to 300, and even more preferably from 1.5 to 100. A range of ˜50 is particularly preferable. Here, the aspect ratio measurement is performed by the same method as described above. When the aspect ratio of the graphitizable aggregate or graphite exceeds 500, the diffraction intensity ratio (002) / (110) measured by X-ray diffraction of the mixture layer containing the graphite particles of the negative electrode and the organic binder. ) Tends to increase, and if it is less than 1.2, the discharge capacity per graphite particle weight tends to decrease.

  As the binder, for example, organic materials such as tar, pitch, thermosetting resin, and thermoplastic resin are preferable. The blending amount of the binder is preferably 5 to 80% by weight, more preferably 10 to 80% by weight, and further preferably 20 to 80% by weight based on the graphitizable aggregate or graphite. It is preferable to add 30 to 80% by weight. If the amount of the binder is too large or too small, the aspect ratio and specific surface area of the graphite particles to be produced tend to increase. The method of mixing the aggregate that can be graphitized or graphite and the binder is not particularly limited, and can be performed using, for example, a kneader. Moreover, it is preferable to mix at the temperature more than the softening point of a binder. Specifically, when the binder is pitch, tar or the like, 50 to 300 ° C is preferable, and when the binder is a thermosetting resin, 20 to 180 ° C is preferable.

  Next, the mixture is pulverized, and the pulverized product and the graphitization catalyst are mixed. The particle size of the pulverized product is preferably 1 to 100 μm, more preferably 5 to 80 μm, further preferably 5 to 50 μm, and particularly preferably 10 to 30 μm.

  If the particle size of the pulverized product exceeds 100 μm, the specific surface area of the obtained graphite particles tends to increase, and if it is less than 1 μm, the obtained negative electrode graphite particles and an organic binder (002) The / (110) ratio tends to increase. The volatile matter of the pulverized product is preferably 0.5 to 50% by weight, more preferably 1 to 30% by weight, and even more preferably 5 to 20% by weight. The volatile matter can be determined from, for example, a weight reduction value when the pulverized product is heated at 800 ° C. for 10 minutes. The graphitization catalyst to be mixed with the pulverized product is not particularly limited as long as it has a function as a graphitization catalyst. For example, metals such as iron, nickel, titanium, silicon and boron, carbides and oxides thereof Graphitization catalysts such as can be used. Of these, iron or silicon compounds are preferred. The chemical structure of the compound is preferably a carbide. The addition amount of these graphitization catalysts is preferably 1 to 50% by weight, more preferably 5 to 30% by weight, when the total amount of the pulverized product mixed with the graphitization catalyst and the graphitization catalyst is 100% by weight. 7 to 20% by weight is more preferable. If the amount of the graphitization catalyst is less than 1% by weight, not only will the development of the crystals of the graphite particles produced worsen, but the specific surface area tends to increase. On the other hand, if it exceeds 50% by weight, the graphitization catalyst tends to remain in the produced graphite particles. The graphitization catalyst used is preferably in the form of a powder, preferably in the form of a powder having an average particle size of 0.1 to 200 μm, more preferably 1 to 100 μm, and particularly preferably 1 to 50 μm.

Next, the mixture is fired and graphitized, and before firing, the mixture of the pulverized product and the graphitized catalyst may be formed into a predetermined shape by a press or the like and then fired. The molding pressure in this case is preferably about 1 to 300 MPa. Firing is preferably performed under conditions where the mixture is not easily oxidized, and examples thereof include a method of firing in a nitrogen atmosphere, an argon atmosphere, a vacuum, and a self-volatile atmosphere. The graphitization temperature is preferably 2000 ° C or higher, more preferably 2500 ° C or higher, further preferably 2700 ° C or higher, and particularly preferably 2800 to 3200 ° C. When the graphitization temperature is low, the development of graphite crystals tends to be poor, the discharge capacity tends to be low, and the added graphitization catalyst tends to remain in the graphite particles produced. When a large amount of the graphitization catalyst remains in the graphite particles produced, the discharge capacity per graphite particle weight tends to decrease. If the graphitization temperature is too high, the graphite tends to sublime. Firing, if performed in the molding prepared by molding into a predetermined shape by pressing or the like, the apparent density is preferably 1.65 g / cm ³ or less of the molded product after graphitization, more preferably if 1.55 g / cm ³ or less , more preferably if 1.50 g / cm ³ or less, particularly preferred if the 1.45 g / cm ³ or less. Further, the lower limit is preferably 1.00 g / cm ³ or more. When the apparent density of the molded product after graphitization exceeds 1.65 g / cm ³ , the specific surface area of the produced graphite particles tends to increase. Further, when the apparent density of the molded product after graphitization is less than 1.00 g / cm ³ , the (002) / (110) ratio of the mixture layer containing the graphite particles of the negative electrode and the organic binder obtained. Not only tends to increase, but also tends to reduce the handleability of the molded product after graphitization. Furthermore, the weight of the furnace packed during graphitization tends to decrease and the graphitization efficiency tends to deteriorate. In addition, the apparent density of the molded product after graphitization can be calculated from the measured values of the weight and volume of the molded product after graphitization. The apparent density of the molded product after graphitization can be changed, for example, by appropriately adjusting the particle size of the pulverized product mixed with the graphitization catalyst and the pressure when molding into a predetermined shape by a press or the like. it can.

  Subsequently, it grind | pulverizes and it is set as the graphite particle which adjusts a particle size and forms a negative electrode. The pulverization method is not particularly limited, and examples thereof include an impact pulverization method such as a jet mill, a hammer mill, and a pin mill. The average particle size of the graphite particles after pulverization is preferably 1 to 100 μm, more preferably 5 to 50 μm, and particularly preferably 10 to 30 μm. When the average particle diameter exceeds 100 μm, the surface of the negative electrode to be manufactured is likely to be uneven, and as a result, the lithium secondary battery to be manufactured tends to be micro-short-circuited and the cycle characteristics tend to deteriorate.

  In the present invention, the average particle diameter can be measured by, for example, a laser diffraction particle size distribution meter.

  The obtained graphite particles were kneaded with an organic binder and a solvent to prepare a mixture, the viscosity was adjusted as appropriate, and then applied to a current collector and dried, and then the current collector and pressure were applied. The negative electrode is integrated.

The negative electrode may have a layer or the like that adheres both the current collector and the mixture layer containing the graphite particles and the organic binder within a range not impairing the effects of the present invention. .

  Examples of the organic binder include polyethylene, polypropylene, ethylene propylene terpolymer, butadiene rubber, styrene butadiene rubber, butyl rubber, and a polymer compound having high ionic conductivity. These are used alone or in combination of two or more.

  Examples of the polymer compound having a large ion conductivity include polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphasphazene, polyacrylonitrile, and the like.

  The mixing ratio of the graphite particles and the organic binder is preferably 0.5 to 20 parts by weight of the organic binder with respect to 100 parts by weight of the graphite particles.

  There is no restriction | limiting in particular as a solvent, For example, N-methyl- 2-pyrrolidone, a dimethylformamide, isopropanol, water etc. are mentioned. When water is used as the solvent, it is preferable to use a thickener together. There is no restriction | limiting in particular in the quantity of a solvent, Although it should just be able to adjust to a desired viscosity, It is preferable to use 30-70 weight part with respect to 100 weight part of mixtures. These are used alone or in combination of two or more.

  As the current collector, for example, a metal current collector such as a foil or mesh of nickel, copper or the like can be used. The integration can be performed by a molding method such as a roll or a press, or a combination of them may be integrated. The pressure during the integration is preferably about 1 to 200 MPa.

  The negative electrode thus obtained is used for a lithium secondary battery. The lithium secondary battery of the present invention comprises a positive electrode containing a lithium compound and the negative electrode of the present invention. For example, the positive electrode and the negative electrode are arranged to face each other with a separator interposed therebetween and an electrolyte is injected. Compared to a lithium secondary battery using a conventional negative electrode, it has a high capacity and excellent cycle characteristics and rapid charge / discharge characteristics.

The positive electrode of the lithium-ion secondary battery in the present invention contains a lithium compound, but the material is not particularly limited. For example, LiNiO ₂ , LiCoO ₂ , LiMn ₂ O _{4 and the} like can be used alone or in combination. It is also possible to use a lithium compound in which a part of elements such as CO, Ni, Mn and the like is substituted with a different element. In view of the energy density of the lithium secondary battery produced in the present invention, a lithium compound containing at least Co is preferable, and a lithium compound containing Ni is more preferable. Further, the positive electrode used in the lithium secondary battery according to the present invention particularly preferably contains at least Co and Ni. The positive electrode containing Co and Ni may be a mixture of LiNiO ₂ and LiCoO ₂ , or may be a lithium compound substituted with Ni element and / or Co element. Usually, a lithium secondary battery using a lithium compound containing Ni as a positive electrode has a problem that the discharge voltage decreases. However, a lithium secondary battery produced by combining the positive electrode and the negative electrode according to the present invention has a discharge voltage. This is preferable because it is possible to suppress the decrease in the density and improve the energy density.

Lithium secondary batteries usually contain an electrolyte containing a lithium compound together with a positive electrode and a negative electrode. Examples of the electrolyte solution include lithium salts such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF ₃ , CH ₃ SO ₃ Li, and CF ₃ SO ₃ Li, ethylene carbonate, diethyl carbonate, dimethyl carbonate, A so-called organic electrolytic solution dissolved in a non-aqueous solvent such as methyl ethyl carbonate, propylene carbonate, acetonitrile, propyronitrile, dimethoxyethane, tetrahydrofuran, γ-butyrolactone, or a so-called polymer electrolyte in a solid or gel form can be used. These are used alone or in combination of two or more.

  Moreover, it is preferable to add a small amount of an additive that exhibits a decomposition reaction when the lithium secondary battery is initially charged to the electrolytic solution. Examples of the additive include vinylene carbonate, biphenyl, propane sultone, and the addition amount is preferably 0.01 to 5% by weight.

  As the separator, for example, a nonwoven fabric mainly composed of polyolefin such as polyethylene or polypropylene, cloth, microporous film, or a combination thereof can be used. A microporous film having a volume porosity of 80% or more is preferable in terms of rapid charge / discharge characteristics and cycle characteristics of the lithium secondary battery to be produced. Moreover, as thickness, 5-40 micrometers is preferable, 8-30 micrometers is more preferable, and 10-25 micrometers is especially preferable. When the thickness is less than 5 μm, the thermal stability of the lithium secondary battery to be produced tends to decrease, and when it exceeds 40 μm, the energy density and rapid charge / discharge characteristics tend to decrease. In addition, when it is set as the structure where the positive electrode and negative electrode of a lithium secondary battery to produce are not in direct contact, it is not necessary to use a separator.

  FIG. 1 shows a schematic diagram of a partial cross-sectional front view of an example of a cylindrical lithium secondary battery. The cylindrical lithium secondary battery shown in FIG. 1 is obtained by stacking a positive electrode 1 processed into a thin plate shape and a negative electrode 2 processed in the same manner through a separator 3 such as a polyethylene microporous membrane. It is turned and inserted into a battery can 7 made of metal or the like and sealed. The positive electrode 1 is bonded to the positive electrode lid 6 via the positive electrode tab 4, and the negative electrode 2 is bonded to the battery bottom via the negative electrode tab 5. The positive electrode lid 6 is fixed to a battery can (positive electrode can) 7 with a gasket 8.

  Examples of the present invention will be described below.

[Example 1]
50 parts by weight of coke powder having an average particle size of 10 μm and 30 parts by weight of coal tar pitch were mixed at 230 ° C. for 2 hours. Next, after pulverizing this mixture to an average particle size of 25 μm, 80 parts by weight of the pulverized product and 20 parts by weight of silicon carbide having an average particle size of 25 μm were mixed with a blender, and the mixture was put into a mold and press-molded at 100 MPa. And formed into a rectangular parallelepiped. This molded body was heat-treated at 1000 ° C. in a nitrogen atmosphere, and further heat-treated at 3000 ° C. in a nitrogen atmosphere to obtain a graphite molded body. Furthermore, this graphite molded body was pulverized to obtain graphite particles. The following measurements were performed from the obtained graphite particles. (1) Average particle diameter by laser diffraction particle size distribution meter, (2) Specific surface area by BET method, (3) Aspect ratio (average value for 10 pieces), (4) Interlayer distance d of crystals by X-ray wide angle diffraction (002) and (5) Crystallite size Lc (002) in the C-axis direction of the crystal. These measured values are shown in Table 1.

  The average particle size was determined by using a laser diffraction particle size distribution analyzer (product name: SALD-3000, Shimadzu Corporation), and the particle size at 50% D was defined as the average particle size. The interlayer distance d (002) was measured using an X-ray diffractometer, using Cu-Kα rays as a single color with a Ni filter, and using high-purity silicon as a standard substance. The specific surface area was calculated according to the BET method by measuring the nitrogen adsorption at a liquid nitrogen temperature by a multipoint method using a product name ASAP 2010 of micromeritics.

Subsequently, 10% by weight of organic binder polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone was added to 90% by weight of the obtained graphite particles and kneaded to prepare a graphite paste. . This graphite paste was applied to a rolled copper foil having a thickness of 10 μm, further dried at 120 ° C. to remove N-methyl-2-pyrrolidone, and compressed at 10 MPa with a vertical press to obtain a sample electrode (negative electrode). It was. When the density of the mixture layer of (6) graphite particles and PVDF of this sample electrode (negative electrode) was measured, it was 1.20 g / cm ³ and the thickness was 96 μm. Using the X-ray diffractometer, the (002) and (110) diffraction peaks of the obtained graphite electrode and organic binder mixture layer of the test electrode (negative electrode) were measured, and from the peak top intensities, (7) (002 ) / (110) intensity ratio. The results are also shown in Table 1. In the X-ray diffraction of (4), (5) and (7), the X-ray source was CuKα ray / 40 KV / 20 mA, and the step width was 0.02 °.

The produced sample electrode (negative electrode) was punched out to a size of 2 cm ² and subjected to constant current charge / discharge by the three-terminal method, and the charge / discharge capacity and discharge capacity retention rate were measured as follows. FIG. 2 shows a schematic diagram of the lithium secondary battery used in this measurement. As shown in FIG. 2, the sample electrode (negative electrode) was evaluated as follows: LiPF ₆ as ethylene carbonate (EC) and methyl ethyl carbonate (MEC) as an electrolytic solution 10 in a beaker-type glass cell 9 (EC and MEC are 1 in volume ratio). : 2) The solution dissolved so as to have a concentration of 1 mol / liter is added to the mixed solvent, the sample electrode (negative electrode) 11, the separator 12 and the counter electrode (positive electrode) 13 are stacked and arranged, and the reference electrode 14 is further arranged. A model battery was produced by hanging from the top. Metal lithium was used for the counter electrode (positive electrode) 13 and the reference electrode 14, and a polyethylene microporous membrane was used for the separator 12. Using the obtained model battery, 0 V (V vs. V) at a constant current of 0.2 mA / cm ^{2 with} respect to the area of the sample electrode (negative electrode) between the sample electrode (negative electrode) 11 and the counter electrode (positive electrode) 13. Li / Li +) was charged, and a test was conducted to discharge to 1 V (V vs. Li / Li +) at a constant current of 0.2 mA / cm ² , and (8) discharge capacity per unit volume was measured.

  Furthermore, 100 cycles of charge and discharge were repeated in the same manner, and (9) the discharge capacity retention rate when the discharge capacity in the first cycle was set to 100 was measured.

Further, charged at a constant current of 0.2 mA / cm ² until 0V (V vs.Li/Li ^+), is discharged at a constant current of 6.0 mA / cm ² until 1V (V vs.Li/Li ⁺⁾ Test (10) The discharge capacity retention rate was measured when the discharge capacity when discharged at a constant current of 0.2 mA / cm ² was taken as 100.

  The measurement results are also shown in Table 1.

[Example 2]
A test electrode (negative electrode) was produced in the same manner as in Example 1 except that the density of the mixture layer of graphite particles and PVDF was changed to 1.45 g / cm ³ by changing the pressure of the vertical press to 23 MPa instead of 10 MPa. In the same manner as in Example 1, the (002) / (110) intensity ratio, the discharge capacity per unit volume, the discharge capacity retention rate after 100 cycles, and the discharge capacity maintenance at a discharge current of 6.0 mA / cm ² The rate was measured. The measurement results are also shown in Table 1.

[Example 3]
A test electrode (negative electrode) was prepared in the same manner as in Example 1 except that the density of the mixture layer of graphite particles and PVDF was 1.55 g / cm ³ by setting the pressure of the vertical press to 31 MPa. 1, the (002) / (110) intensity ratio, the discharge capacity per unit volume, the discharge capacity maintenance rate after 100 cycles, and the discharge capacity maintenance rate at a discharge current of 6.0 mA / cm ² were measured. . The measurement results are also shown in Table 1.

[Example 4]
A test electrode (negative electrode) was prepared in the same manner as in Example 1 except that the density of the mixture layer of graphite particles and PVDF was changed to 1.65 g / cm ³ by setting the pressure of the vertical press to 50 MPa. 1, the (002) / (110) intensity ratio, the discharge capacity per unit volume, the discharge capacity maintenance rate after 100 cycles, and the discharge capacity maintenance rate at a discharge current of 6.0 mA / cm ² were measured. . The measurement results are also shown in Table 1.

[Example 5]
A test electrode (negative electrode) was prepared in the same manner as in Example 1 except that the density of the mixture layer of graphite particles and PVDF was changed to 1.75 g / cm ³ by setting the pressure of the vertical press to 85 MPa. 1, the (002) / (110) intensity ratio, the discharge capacity per unit volume, the discharge capacity maintenance rate after 100 cycles, and the discharge capacity maintenance rate at a discharge current of 6.0 mA / cm ² were measured. . The measurement results are also shown in Table 1.

[Example 6]
A test electrode (negative electrode) was produced in the same manner as in Example 1 except that the density of the mixture layer of graphite particles and PVDF was 1.85 g / cm ³ by setting the pressure of the vertical press to 143 MPa. 1, the (002) / (110) intensity ratio, the discharge capacity per unit volume, the discharge capacity maintenance rate after 100 cycles, and the discharge capacity maintenance rate at a discharge current of 6.0 mA / cm ² were measured. . The measurement results are also shown in Table 1.

[Example 7]
Chinese natural graphite was pulverized with a jet mill to produce scaly natural graphite particles.

The average particle diameter, specific surface area, aspect ratio, d (002), and Lc (002) measurement results of the graphite particles are also shown in Table 1. A test electrode (negative electrode) was prepared in the same manner as in Example 1 except that the density of the mixture layer of graphite particles and PVDF was changed to 1.00 g / cm ³ by using the graphite particles at a vertical pressing pressure of 2 MPa. Was made. In the same manner as in Example 1, the (002) / (110) intensity ratio, the discharge capacity per unit volume, the discharge capacity maintenance rate after 100 cycles, and the discharge capacity maintenance rate at a discharge current of 6.0 mA / cm ² were obtained. It was measured. The measurement results are also shown in Table 1.

[Comparative Example 1]
A test electrode (negative electrode) was prepared in the same manner as in Example 7 except that the density of the mixture layer of graphite particles and PVDF was 1.50 g / cm ³ by setting the pressure of the vertical press to 27 MPa. 1, the (002) / (110) intensity ratio, the discharge capacity per unit volume, the discharge capacity maintenance rate after 100 cycles, and the discharge capacity maintenance rate at a discharge current of 6.0 mA / cm ² were measured. . The measurement results are also shown in Table 1.

[Comparative Example 2]
A test electrode (negative electrode) was prepared in the same manner as in Example 7 except that the density of the mixture layer of graphite particles and PVDF was changed to 1.65 g / cm ³ by setting the pressure of the vertical press to 42 MPa. 1, the (002) / (110) intensity ratio, the discharge capacity per unit volume, the discharge capacity maintenance rate after 100 cycles, and the discharge capacity maintenance rate at a discharge current of 6.0 mA / cm ² were measured. . The measurement results are also shown in Table 1.

  As shown in Table 1, it was shown that the negative electrode for a lithium secondary battery of the present invention has a high capacity, excellent cycle characteristics and rapid discharge characteristics, and is suitable for use in a lithium secondary battery.

  According to the present invention, a negative electrode for a lithium secondary battery excellent in cycle characteristics and rapid discharge characteristics can be obtained, which is suitable for use in a high capacity lithium secondary battery.

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Positive electrode tab 5 Negative electrode tab 6 Positive electrode lid 7 Battery can 8 Gasket 9 Glass cell 10 Electrolytic solution 11 Sample electrode (negative electrode)
12 Separator 13 Counter electrode (positive electrode)
14 Reference pole

Claims (5)

A negative electrode for a lithium secondary battery having a mixture layer comprising graphite particles and an organic binder on a current collector, wherein a diffraction intensity ratio (002) / (measured by X-ray diffraction of the mixture layer) 110) The negative electrode for lithium secondary batteries whose 500 or less. The negative electrode for a lithium secondary battery according to claim 1, wherein the density of the mixture layer comprising the graphite particles and the organic binder is 15 to 1.95 g / cm ³ . 3. The negative electrode for a lithium secondary battery according to claim 1, wherein the graphite particles have an average particle diameter of 1 to 100 μm and a crystallite size Lc (002) in the C-axis direction of the crystal is 500 angstroms or more. A lithium secondary battery comprising the negative electrode for a lithium secondary battery according to any one of claims 1 to 3 and a positive electrode containing a lithium compound. The lithium secondary battery according to claim 4, wherein the lithium compound contains at least Ni.

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