JPWO2014092141A1 - Negative electrode material for lithium ion secondary battery, negative electrode sheet for lithium ion secondary battery, and lithium secondary battery - Google Patents

Abstract

The interplanar spacing (d (002)) of the (002) plane of the graphite structure measured by powder X-ray diffraction method is in the range of 0.335 to 0.339 nm, and the cumulative volume frequency of the particle size distribution measured by laser diffraction method Is an artificial graphite (A) having a particle diameter (D50) of 4 to 10 μm, and d (002) is 0.340 nm or more, D50 is 7 to 17 μm, and D50 is the artificial graphite ( A negative electrode material for a lithium ion secondary battery, comprising a mixture of a carbon material (B) larger than D50 of A).

Description

  The present invention relates to a negative electrode material that can be a lithium ion secondary battery excellent in high-speed charging characteristics while maintaining high output and high energy density. Moreover, this invention relates to the negative electrode sheet for lithium ion secondary batteries using the negative electrode material, and a lithium secondary battery.

Lithium ion secondary batteries are used as power sources for portable electronic devices, and in recent years, they are also used as power sources for electric tools and electric vehicles. In electric vehicles such as battery electric vehicles (BEV) and hybrid electric vehicles (HEV), maintaining a high charge / discharge cycle characteristic for more than 10 years and a large current load characteristic sufficient to drive a high-power motor And a high volumetric energy density is required to extend the cruising range. In particular, a plug-in hybrid vehicle (PHEV) has a smaller battery capacity than EV and has to be trickle charged by driving a motor with a low-capacity battery.
Conventionally, carbon-based materials such as graphite have been mainly used as the negative electrode material, but recently, metal-based negative electrode materials have also been developed. However, there are still problems such as cycle life and stability, and many problems still remain.

Carbon-based materials can be broadly divided into graphite materials with high crystallinity and amorphous carbon materials with low crystallinity, both of which can be used for negative electrode active materials because they can undergo lithium insertion and desorption reactions. it can.
Graphite materials include natural graphite and artificial graphite. Natural graphite is known to be spherically granulated. For example, Patent Document 1 describes a graphite material obtained by coating artificial carbon with the surface of natural graphite formed into a spherical shape. Lithium ion secondary batteries using this graphite material have some performance required as a power source for portable electronic devices, but they have sufficiently reached the performance required as a power source for electric vehicles and power tools. Not. A highly crystalline graphite material is not sufficient in charge characteristics in spite of stable cycle characteristics. When rapid charge / discharge is performed, the insertion / extraction reaction of lithium ions on the negative electrode active material side is not in time, the battery voltage suddenly reaches the lower limit or upper limit, and the reaction does not proceed further. Is remarkable for highly crystalline graphite materials. However, high crystalline graphite materials are widely used as negative electrode materials at present because of the capacity of graphite, such as the theoretical battery capacity phase, and the stability of cycle characteristics.

  Amorphous carbon materials are known to be able to be used in rapid charge / discharge because they can be charged from a low potential region that cannot be charged with graphite. However, they have a drawback that cycle deterioration is remarkable, irreversible capacity is large and capacity is small.

  In view of the above background, research and development of materials having both characteristics, such as combining an amorphous material and a highly crystalline graphite material, has been actively conducted, and various techniques have been proposed.

Patent Document 2 discloses a technique in which a carbon material serving as a core material is immersed in tar or pitch and dried or heat-treated at 900 to 1300 ° C.
Patent Document 3 discloses a technique in which a carbon precursor such as pitch is mixed on the surface of graphite particles obtained by granulating natural graphite or scaly artificial graphite, and fired in a temperature range of 700 to 2800 ° C. in an inert gas atmosphere. Has been.
In Patent Document 4, spherical graphite particles obtained by granulating spheroidized graphite having d (002) of 0.3356 nm, R value of around 0.07, and Lc of about 50 nm by mechanical external force, phenol resin, etc. It is disclosed that composite graphite particles formed by coating a heated carbide of the above resin are used as a negative electrode active material. The composite graphite particles are obtained by performing pretreatment for carbonization at 1000 ° C. in a nitrogen atmosphere and carbonizing at 3000 ° C.
Patent Document 5 discloses that a mixed carbon material obtained by mixing a graphite-based carbon material having an average particle size of 15 μm and a low crystalline carbon surface and a low crystalline carbon having an average particle size of 10 μm is used as the negative electrode active material. Yes.

JP 2005-285633 A Japanese Patent No. 2976299 (EP0861804A) Japanese Patent No. 3193342 (EP0917228A) Japanese Patent Laid-Open No. 2004-210634 JP 2006-338777 A

None of the carbon materials described in Patent Documents 1 to 4 have sufficient charging characteristics. Moreover, the cycle characteristics were insufficient.
The carbon material described in Patent Document 5 has good low-temperature characteristics, but has insufficient discharge characteristics.

The negative electrode material for a lithium ion secondary battery, the negative electrode sheet for a lithium ion secondary battery using the negative electrode material, and the lithium secondary battery in a preferred embodiment of the present invention are as follows.
[1] The (002) plane spacing (d (002)) of the graphite structure measured by powder X-ray diffraction method is in the range of 0.335 to 0.339 nm, and the particle size distribution measured by laser diffraction method Artificial graphite (A) having a particle diameter (D50) of 4 to 10 μm with a volume cumulative frequency of 50%, and d (002) of 0.340 nm or more, D50 of 7 to 17 μm, and D50 Carbon material larger than D50 of artificial graphite (A) (B)
The negative electrode material for lithium ion secondary batteries characterized by including the mixture of these.
[2] The negative electrode material for a lithium ion secondary battery as described in 1 above, wherein the composition ratio between the artificial graphite (A) and the carbon material (B) is in the range of 8: 2 to 2: 8 in terms of mass ratio.
[3] The negative electrode material for a lithium ion secondary battery as described in 1 above, wherein the artificial graphite (A) is obtained by heat treating petroleum-based and / or coal-based coke at 2500 ° C. or higher.
[4] The negative electrode material for a lithium ion secondary battery as described in 1 above, wherein the artificial graphite (A) comprises particles having a carbon coating layer on the surface of artificial graphite particles as a core material.
[5] The negative electrode material for a lithium ion secondary battery as described in 4 above, wherein the artificial graphite as the core material is obtained by heat treating petroleum-based and / or coal-based coke at 2500 ° C. or higher.
[6] of the coating layer, the intensity ratio of the peak intensity (I _G) with a peak intensity in the range of 1300～1400Cm ^-1 as measured by Raman spectroscopy and (I _D) in the range of 1580～1620Cm ^-1 6. The negative electrode material for a lithium ion secondary battery according to 4 or 5 above, wherein I _D / I _G (R value) is 0.1 or more.
[7] The negative electrode for a lithium ion secondary battery according to any one of 4-6, wherein the amount of the coating layer is 0.05 to 10 parts by mass when the artificial graphite as a core material is 100 parts by mass. Wood.
[8] The above-mentioned 4-7, wherein the artificial graphite (A) is obtained by attaching an organic compound to particles of artificial graphite as a core material and then heat-treating at a temperature of 500 ° C. or higher and 2000 ° C. or lower. The negative electrode material for lithium ion secondary batteries in any one.
[9] The organic compound is at least one compound selected from the group consisting of petroleum pitch, coal pitch, phenol resin, polyvinyl alcohol resin, furan resin, cellulose resin, polystyrene resin, polyimide resin, and epoxy resin. 9. The negative electrode material for a lithium ion secondary battery as described in 8 above.
[10] The negative electrode material for a lithium ion secondary battery as described in 1 above, wherein the artificial graphite (A) has a BET specific surface area of 0.5 to ⁵ m ² / g.
[11] The negative electrode material for a lithium ion secondary battery as described in 1 above, wherein the carbon material (B) is hard carbon and / or soft carbon.
[12] Obtained by applying a negative electrode paste containing the negative electrode material for a lithium ion secondary battery according to any one of the above 1-11, a binder, and a dispersion medium onto a collector foil, drying, and pressing. A negative electrode sheet for a lithium ion secondary battery.
[13] The negative electrode sheet for a lithium ion secondary battery as described in 12 above, wherein the density of the negative electrode portion other than the current collector foil is 1.1 g / cm ³ or more and 1.6 g / cm ³ or less.
[14] A lithium ion battery comprising the negative electrode sheet for a lithium ion secondary battery as described in 12 or 13 above as a constituent element.
[15] The lithium ion as described in 14 above, wherein a non-aqueous electrolyte solution and / or a non-aqueous polymer electrolyte is used, and propylene carbonate is contained in the non-aqueous solvent used in the non-aqueous electrolyte solution and / or the non-aqueous polymer electrolyte. battery.

  By using a material containing a mixture of specific artificial graphite and a specific carbon material as a negative electrode material for a lithium secondary battery, a lithium secondary battery excellent in charge / discharge characteristics and cycle characteristics and its performance balance can be obtained. it can. Further, the cycle characteristics can be further improved by using the artificial graphite having a carbon coating layer on the surface of the graphite particles.

The charge curve of 5C charge of the battery obtained in Examples 1-3.

<Anode material for lithium ion secondary battery>
The negative electrode material for a lithium ion secondary battery according to a preferred embodiment of the present invention includes artificial graphite (A) and a carbon material having a larger average particle diameter (D50, see below) and lower crystallinity than the artificial graphite (A). A mixture with (B) is included.

[Artificial graphite (A)]
The artificial graphite (A) preferably has a (002) plane spacing (d (002)) in the range of 0.335 to 0.339 nm as measured by powder X-ray diffraction. A more preferable d (002) is in the range of 0.335 to 0.337 nm. When d (002) increases, the discharge capacity decreases and the true density also decreases, so that the battery has a low energy density. Moreover, since the crystallinity is low and the conductivity is inferior, the discharge characteristics tend to decrease. Further, the thickness (Lc) of the crystallite in the c-axis direction is preferably 50 nm or more.

Artificial graphite (A) has a particle diameter (D50) (sometimes referred to as an average particle diameter in this specification) having a volume cumulative frequency of 50% as measured by a laser diffraction method of 4 to 10 μm. Is preferred. D50 is more preferably 4 to 8 μm, still more preferably 4 to 6 μm. When D50 is in the above range, lithium ions can efficiently react with the electrolytic solution and exhibit excellent discharge characteristics, and the capacity and cycle characteristics can be maintained high. If D50 is too small, the number of particles that cannot efficiently participate in the electrochemical reaction with lithium ions increases, and the capacity and cycle characteristics tend to be reduced. On the other hand, if D50 is too large, the contact area with the electrolytic solution becomes small, and the output characteristics tend to be lowered.
The artificial graphite (A) preferably has 90% or more of particles in the range of 4 to 10 μm in the number-based cumulative particle size distribution in the particle size distribution measurement by laser diffraction method. Since graphite in the above range efficiently reacts with the electrolytic solution, it exhibits excellent charge / discharge characteristics.

  The particle size distribution can be adjusted by grinding and classification. Examples of the pulverizer include a hammer mill, a jaw crusher, and a collision pulverizer. The classification can be performed by an airflow classification method or a sieve classification method. Examples of the air classifier include a turbo classifier and a turboplex.

The artificial graphite (A) has a BET specific surface area of preferably 0.5 to ⁵ m ² / g, more preferably 0.5 to 3.5 m ² / g. When the BET specific surface area is in the above range, the coulombic efficiency is good, and the graphite having a good balance between cycle and output characteristics is obtained. If the BET specific surface area is too large, the surface activity of the particles increases, the Coulomb efficiency decreases due to decomposition of the electrolyte, and the cycle characteristics tend to decrease. On the other hand, if the BET specific surface area is too small, the contact area with the electrolytic solution decreases, and the output characteristics tend to deteriorate.

  The artificial graphite (A) may have a carbon coating layer on the particle surface. As the core material in that case, the same material as the artificial graphite (A) described above can be used. By using artificial graphite (A) (hereinafter sometimes referred to as composite graphite) composed of a core material and a coating layer, input / output characteristics can be improved.

The coating layer of the composite graphite, the intensity of the peak intensity in the range of the peak intensity (I _D) and 1580～1620Cm ^-1 in the range of 1300～1400Cm ^-1 as measured by Raman spectroscopy spectra (I _G) Those composed of carbon having a ratio I _D / I _G (R value) of 0.1 or more are preferred. A more preferable R value is 0.20 or more. By providing a surface layer having a large R value, lithium ions can be easily inserted into and removed from the graphite layer, and quick chargeability when used as an electrode material for a secondary battery is improved. In addition, it shows that crystallinity is so low that R value is large.

The carbon raw material used for the production of artificial graphite (A) has a weight loss by heating (for example, volatile content of hydrocarbon accompanying carbonization) of 5 to 20% by mass when heated from 300 ° C. to 1200 ° C. in an inert atmosphere. It is preferable. If the amount of heat loss is small, the particle shape tends to be plate-like after pulverization, and the pulverized surface (edge portion) is exposed, and the specific surface area tends to increase and side reactions tend to increase. On the other hand, if the amount of heat loss is large, many particles are bound in the process of graphitization, which tends to affect the yield.
The carbon raw material having such a heat loss is selected from petroleum pitch coke or coal pitch coke. In particular, the carbon raw material for artificial graphite (A) is preferably selected from raw coke which is a kind of petroleum coke. Since raw coke is undeveloped, it becomes spherical when pulverized and its specific surface area tends to be small. Accordingly, the carbon raw material is preferably non-needle-like coke.
Petroleum coke is a black, porous solid residue obtained by cracking or cracking distillation of petroleum or bituminous oil. Petroleum coke includes fluid coke and delayed coke depending on the coking method. However, fluid coke is in the form of powder and is not used for much as it is used for private fuel in refineries, and what is generally called petroleum coke is delayed coke. Delayed coke includes raw coke and calcined coke. Raw coke is the raw coke collected from the coking device, and calcined coke is further baked to remove volatiles. Since raw coke has a high possibility of causing a dust explosion, in order to obtain fine-grained petroleum coke, raw coke was calcined to remove volatile matter and then pulverized. Conventionally, calcined coke has been generally used for electrodes and the like. Since raw coke has less ash than coal coke, it has only been used for carbon materials in casting industry, coke for casting, coke for alloy iron, and the like.

  Next, this carbon raw material is pulverized. A known jet mill, hammer mill, roller mill, pin mill, vibration mill or the like is used for pulverizing the carbon raw material. The carbon raw material is preferably pulverized with a carbon raw material having a thermal history as low as possible. When pulverization is performed with a carbon raw material having a low heat history, the carbon raw material is easily pulverized, and the crack direction at the time of crushing becomes almost random and the aspect ratio tends to be small. In addition, the probability that the edge portion exposed to the pulverized surface is repaired in the subsequent heating process is increased, and there is an effect that side reactions during charging and discharging can be reduced.

  The pulverized carbon raw material can be fired at a low temperature of about 500 to 1200 ° C. in a non-oxidizing atmosphere before being graphitized. By this low-temperature firing, gas generation in the next graphitization treatment can be reduced, and since the bulk density can be lowered, the graphitization treatment cost can be reduced.

The graphitization treatment of the pulverized carbon raw material is desirably performed in an atmosphere in which the carbon raw material is not easily oxidized. For example, a heat treatment method in an atmosphere such as argon gas, a heat treatment method in an Atchison furnace (non-oxidizing graphitization process), and the like can be given. Of these, the non-oxidizing graphitization process is preferable from the viewpoint of cost.
The lower limit of the temperature in the graphitization treatment is usually 2000 ° C, preferably 2500 ° C, more preferably 2900 ° C, and most preferably 3000 ° C. The upper limit of the temperature in the graphitization treatment is not particularly limited, but is preferably 3300 ° C. from the viewpoint that a high discharge capacity is easily obtained.

  It is preferable not to crush or pulverize the resulting artificial graphite after the graphitization treatment. If the powder is crushed or pulverized after the graphitization treatment, the smooth surface may be damaged and the performance may be deteriorated.

The obtained artificial graphite can be used as a negative electrode material as it is. Further, the artificial graphite can be used as a core material, and the surface thereof can be coated with a carbon material and combined to be used as a negative electrode material.
The compounding can be performed according to a known method. For example, stirring is performed while spraying an organic compound on the obtained artificial graphite. Further, the artificial graphite and the organic compound can be mixed with an apparatus such as a hybridizer manufactured by Nara Machinery, and then can be naturally adhered to the surface of the graphite at the stage of heat treatment to be combined.

  A preferable coating layer is obtained by heat-treating an organic compound at 200 ° C. or higher and 3000 ° C. or lower, preferably 500 ° C. or higher and 2000 ° C. or lower. If the final heat treatment temperature is too low, carbonization is not sufficiently completed and hydrogen or oxygen remains, which may adversely affect battery characteristics. Further, if the treatment temperature is too high, the crystallization of graphite may proceed excessively and the charging characteristics may be lowered.

The organic compound is not particularly limited, but isotropic pitch, anisotropic pitch, resin, resin precursor or monomer is preferable. When a resin precursor or monomer is used, it is preferable to polymerize the resin precursor or monomer to make a resin. Suitable organic compounds include at least one compound selected from the group consisting of petroleum pitch, coal pitch, phenol resin, polyvinyl alcohol resin, furan resin, cellulose resin, polystyrene resin, polyimide resin and epoxy resin. It is done.
The amount of the organic compound used is preferably 0.1 to 10 parts by mass with respect to 100 parts by mass of artificial graphite.

  The heat treatment is preferably performed in a non-oxidizing atmosphere. Examples of the non-oxidizing atmosphere include an atmosphere filled with an inert gas such as argon gas or nitrogen gas.

It is preferable to crush after the heat treatment. By the heat treatment, the artificial graphite is fused and becomes a lump, so that it is atomized for use as an electrode active material.
Since the thickness of the coating layer is on the order of nm, the particle diameter of the composite artificial graphite is almost equal to the particle diameter of the core material. That is, D50 of the composite graphite is preferably 4 to 10 μm, more preferably 4 to 8 μm, and further preferably 4 to 6 μm.

As for the composite graphite, it is desirable that 90% or more of the total number of particles is in the range of 4 to 10 μm, as in the case of the artificial graphite (A) that is not composited.
Also, the d (002) and BET specific surface area of the composite graphite are preferably in the same range as that of the uncomposited artificial graphite (A).

  When the coating layer is provided, the ratio between the artificial graphite as the core material and the coating layer is not particularly limited, but when the core material is 100 parts by mass, the coating layer is preferably 0.05 to 10 parts by mass. 1-10 mass parts is more preferable. If the amount of the coating layer is too small, the effect based on the formation of the coating layer cannot be obtained. If the amount of the coating layer is too large, the battery capacity may be reduced. The amount of the coating layer can be calculated from the amount when an organic compound is used.

[Carbon material (B)]
The carbon material (B) preferably has d (002) of 0.340 nm or more. Further preferred d (002) is 0.342 nm or more. When d (002) becomes narrower than 0.340 nm, the acceptability of lithium ions decreases, and the charging efficiency decreases. Lc is preferably 10 nm or less.

The carbon material (B) preferably has a D50 of 7 to 17 μm. When D50 is in the above range, lithium ions can efficiently react with the electrolytic solution and exhibit excellent discharge characteristics, and the capacity and cycle characteristics can be maintained high. If D50 is too small, the specific surface area becomes large, so that the reaction active point with the electrolytic solution increases, leading to a decrease in the initial efficiency. When D50 is too large, the contact area with the electrolytic solution becomes small, and the resistance value and the input / output characteristics become small.
Moreover, it is preferable that D50 of a carbon material (B) is larger than D50 of the said artificial graphite (A).

The upper limit value of the BET specific surface area of the carbon material (B) is preferably 7 m ² / g, more preferably 6 m ² / g. The lower limit value of the BET specific surface area is preferably 0.5 m ² / g, more preferably 1.0 m ² / g. If the BET specific surface area is too large, the contact area with the electrolytic solution increases, so the irreversible capacity tends to be large and the cycle characteristics tend to deteriorate. Moreover, the mixture (slurry) containing a carbon material (B) with a large specific surface area has a high viscosity, and there exists a tendency for applicability | paintability to fall. On the contrary, if the BET specific surface area is too small, the electrolytic solution and the reaction area become small, and the charge / discharge characteristics tend to be small.

As the carbon material (B), both graphitizable carbon (soft carbon) and non-graphitizable carbon (hard carbon) can be used.
The raw material for the carbon material (B) is coal-based or petroleum-based raw coke, calcined coke, resin, resin precursor, or monomer. When using a resin precursor and a monomer, it is preferable to polymerize it into a resin. Suitable resins include phenol resins, polyvinyl alcohol resins, furan resins, cellulose resins, polystyrene resins, polyimide resins, and epoxy resins, and these can be used alone or in combination.

  These raw materials are preferably preliminarily heat-treated in an autoclave or the like and pulverized. A known jet mill, hammer mill, roller mill, pin mill, vibration mill or the like is used for pulverization.

  The pulverized material is fired at about 700 to 1500 ° C. in a non-oxidizing atmosphere. The preferred heat treatment temperature varies depending on the type of material, but if the final heat treatment temperature is too low, carbonization does not proceed sufficiently, and hydrogen, oxygen, etc. may remain and adversely affect battery performance. preferable. In the case of graphitizable carbon, if the heat treatment temperature is too high, the crystallization of graphite may proceed excessively and the charging characteristics may be deteriorated.

  Here, graphitizable carbon (soft carbon) refers to a carbon material obtained by heat-treating an easily graphitizable organic substance at 700 ° C. or more and 2000 ° C. or less. Further, non-graphitizable carbon (hard carbon) refers to a carbon material obtained by heat-treating a non-graphitizable organic substance.

[mixture]
The negative electrode material for a lithium ion secondary battery in a preferred embodiment of the present invention can be produced by mixing the artificial graphite (A) and the carbon material (B).
The mixing method is not particularly limited. For example, a high-speed chopper such as a Henschel mixer or a Spartan Luzer, a Nauter mixer, a ribbon mixer, or the like can be used to perform uniform mixing at high speed.
The mixing ratio of the artificial graphite (A) and the carbon material (B) varies depending on the required characteristics. For example, in BEV in which energy density is regarded as important, the artificial graphite (A) is preferably 30 to 80% by mass, the carbon material (B) is preferably 70 to 20% by mass, and the artificial graphite (A) is 50 to 80% by mass. More preferably, the carbon material (B) is 50 to 20% by mass. In PHEV where input characteristics are regarded as important, the artificial graphite (A) is preferably 20 to 60% by mass, the carbon material (B) is preferably 80 to 40% by mass, and the artificial graphite (A) is 20 to 40% by mass. The carbon material (B) is more preferably 80 to 60% by mass. When the proportion of the artificial graphite (A) is less than 20% by mass, the electrode density, the battery capacity, and the output density are lowered, and the cycle characteristics tend to be lowered. When the proportion of the carbon material (B) is less than 20% by mass, the lithium ion acceptability of the electrode is lowered, and a sufficient charging property improvement effect cannot be obtained.

<Lithium ion secondary battery>
[Paste for negative electrode]
The negative electrode paste in a preferred embodiment of the present invention includes the negative electrode material, a binder, and a dispersion medium. This negative electrode paste may contain a conductive additive.
This negative electrode paste can be obtained by kneading the negative electrode material, a binder, and a dispersion medium. The negative electrode paste can be formed into a sheet shape, a pellet shape, or the like.
Examples of the binder include polyethylene, polypropylene, ethylene propylene terpolymer, butadiene rubber, styrene butadiene rubber, butyl rubber, and a polymer compound having high ionic conductivity. Examples of the polymer compound having a high ionic conductivity include polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphasphazene, polyacrylonitrile and the like. The mixing ratio of the composite graphite and the binder is preferably 0.5 to 20 parts by mass of the binder with respect to 100 parts by mass of the composite graphite.

The dispersion medium is not particularly limited, and examples thereof include N-methyl-2-pyrrolidone, dimethylformamide, isopropanol, and water. In the case of a binder that uses water as a dispersion medium, it is preferable to use a thickener together. The amount of the dispersion medium is adjusted so that the viscosity is easy to apply to the current collector.
There is no restriction | limiting in particular as a conductive support agent, Acetylene black, Ketjen black, carbon fiber, etc. are mentioned. In particular, vapor grown carbon fiber having high crystallinity and high thermal conductivity is preferable. The blending amount is preferably about 0.01 to 20 parts by mass when the negative electrode material (negative electrode active material) is 100 parts by mass. By adding a conductive aid, conductivity is imparted to the electrode and the battery life is improved.

[Negative electrode]
The negative electrode paste can be obtained by applying the negative electrode paste on a current collector, drying, and press-molding.
Examples of the current collector include foils such as nickel and copper, and meshes. The method for applying the paste is not particularly limited. The coating thickness of the paste is usually 50 to 200 μm. When the coating thickness becomes too large, the negative electrode cannot be accommodated in a standardized battery container, and the internal resistance of the battery increases due to an increase in the lithium ion diffusion distance.
Examples of the pressure molding method include molding methods such as roll pressing and press pressing. The pressure during pressure molding is preferably about 100 MPa to about 300 MPa (about 1 to 3 t / cm ² ). The negative electrode thus obtained is suitable for a lithium secondary battery.
The electrode density after pressure molding is desirably 1.1 to 1.6 g / cm ³ . When the electrode density is less than 1.1 g / cm ^{3, the} battery has a small volume energy density. Conversely, when the electrode density is greater than 1.6 g / cm ³ , the voids in the electrode are reduced and the penetration of the electrolyte solution deteriorates. There is a problem that the diffusion of the liquid becomes worse and the charge / discharge characteristics become smaller.

[Lithium secondary battery]
A lithium secondary battery can be manufactured using the negative electrode as a constituent element.

[Positive electrode]
For the positive electrode of the lithium secondary battery, a lithium-containing transition metal oxide is usually used as the positive electrode active material, and preferably at least one selected from Ti, V, Cr, Mn, Fe, Co, Ni, Mo and W. An oxide mainly containing a transition metal element of a seed and lithium, wherein a compound having a molar ratio of lithium to the transition metal element of 0.3 to 2.2 is used, more preferably V, Cr, Mn, Fe , An oxide mainly containing at least one transition metal element selected from Co and Ni and lithium, and a compound having a molar ratio of lithium to transition metal of 0.3 to 2.2 is used. In addition, Al, Ga, In, Ge, Sn, Pb, Sb, Bi, Si, P, B, or the like may be contained in a range of less than 30 mole percent with respect to the transition metal present mainly. Among the above positive electrode active materials, the general formula Li _x MO ₂ (M is at least one of Co, Ni, Fe, and Mn, x = 0 to 1.2), or Li _y N ₂ O ₄ (N is It is preferable to use at least one material having a spinel structure represented by at least Mn and y = 0-2.

Further, the positive electrode active material _{Li y M a D 1-a} O 2 (M is Co, Ni, Fe, at least one of Mn, D is Co, Ni, Fe, Mn, Al, Zn, Cu, Mo, Ag , W, Ga, In, Sn, Pb, Sb, Sr, B, P, at least one type other than M, y = 0 to 1.2, a = 0.5 to 1.), or _{liz (N b E 1-b} ) 2 O 4 (N is Mn, E is Co, Ni, Fe, Mn, Al, Zn, Cu, Mo, Ag, W, Ga, in, Sn, Pb, Sb, Sr , B, P, at least one of materials having a spinel structure represented by b = 1 to 0.2, z = 0 to 2.) is particularly preferable.

_{Specifically, Li x CoO 2, Li x} NiO 2, Li x MnO 2, Li x Co a Ni 1-a O 2, Li x Co b V 1-b O z, Li x Co b Fe 1-b _{O 2, Li x Mn 2 O} 4, Li x Mn c Co 2-c O 4, Li x Mn c Ni 2-c O 4, Li x Mn c V 2-c O 4, Li x Mn c Fe 2- _c O ₄ (where x = 0.02 to 1.2, a = 0.1 to 0.9, b = 0.8 to 0.98, c = 1.6 to 1.96, z = 2. 01-2.3.). The most preferred lithium-containing transition metal _{oxides, Li x CoO 2, Li x} NiO 2, Li x MnO 2, Li x Co a Ni 1-a O 2, Li x Mn 2 O 4, Li x Co b V 1 _-b O _z (x = 0.02 to 1.2, a = 0.1 to 0.9, b = 0.9 to 0.98, z = 2.01 to 2.3). In addition, the value of x is a value before the start of charging / discharging, and increases / decreases by charging / discharging.

Although the average particle size of a positive electrode active material is not specifically limited, 0.1-50 micrometers is preferable. The volume of particles of 0.5 to 30 μm is preferably 95% or more. More preferably, the volume occupied by a particle group having a particle size of 3 μm or less is 18% or less of the total volume, and the volume occupied by a particle group of 15 μm or more and 25 μm or less is 18% or less of the total volume. The particle size here is calculated by the volume-based cumulative particle size distribution in the particle size distribution measurement by the laser diffraction method, and the average particle size is the particle size at a cumulative 50%.
Although the specific surface area is not particularly limited, but is preferably 0.01 to 50 m ² / g by the BET method, particularly preferably 0.2m ² / g~10m ² / g. The pH of the supernatant when 5 g of the positive electrode active material is dissolved in 100 ml of distilled water is preferably 7 or more and 12 or less.

The electrolytic solution used for the lithium secondary battery is not particularly limited. For example, lithium salts such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF ₃ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, for example, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, Examples include so-called organic electrolytes dissolved in non-aqueous solvents such as diethyl carbonate, propylene carbonate, butylene carbonate, acetonitrile, propyronitrile, dimethoxyethane, tetrahydrofuran, and γ-butyrolactone, and so-called polymer electrolytes in solid or gel form. .

  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 like. As addition amount, 0.01-5 mass% is preferable.

  The lithium secondary battery of the present invention can be provided with a separator between the positive electrode and the negative electrode. Examples of the separator include non-woven fabric, cloth, microporous film, or a combination thereof, mainly composed of polyolefin such as polyethylene and polypropylene.

  Examples of the present invention will be described below to describe the present invention more specifically. Note that these are merely illustrative examples, and the present invention is not limited by these. The physical property measurement method, material preparation method, and battery evaluation method were as follows.

<Measurement of physical properties>
(1) Specific surface area Using a specific surface area measuring instrument (NOVA 1200 manufactured by Yua Sionics Co., Ltd.), the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) was determined and measured by the BET method.

(2) Particle diameter Two cups of graphite and two drops of nonionic surfactant (TRITON (registered trademark) -X) were added to 50 ml of water and ultrasonically dispersed for 3 minutes. This dispersion was put into a laser diffraction particle size distribution measuring instrument manufactured by CILAS, the particle size distribution was measured, and a particle size range containing 90% or more of all particles on a volume basis was calculated.
The particle diameter that is 50% based on the volume calculated from this distribution was defined as the average particle diameter (D50).

(3) d (002) and Lc
D (002) and Lc were determined by powder X-ray diffraction according to the Gakushin method.

<Preparation of material>
(1) Artificial graphite (A)
Petroleum coke was used as a raw material, and pulverization was performed so that the average particle size (D50) was 5 μm. This was heat-treated at 3000 ° C. in an Atchison furnace to obtain a core material having d (002) of 0.336 nm and Lc value of 63 nm. This was mixed with 1% by mass of powdery isotropic pitch with respect to the core material, and heat-treated at 1100 ° C. in an argon atmosphere to obtain artificial graphite (A). The artificial graphite (A) had a BET specific surface area of 2.5 m ² / g and a D50 of 5 μm.
(2) Carbon material (B)
Petroleum coke was used as a raw material and pulverized to an average particle size (D50) of 15 μm. This was heat-treated at 1100 ° C. to obtain a carbon material (B) having d (002) of 0.343 nm and Lc of 4.5 nm. The BET specific surface area of the carbon material (B) was 2.6 m ² / g, and D50 was 15 μm.

<Battery evaluation>
(1) Preparation of negative electrode An aqueous dispersion containing styrene-butadiene (SBR) fine particles having a solid content ratio of 40% by adding 1.5 g of carboxymethylcellulose (CMC) as a thickener and water appropriately to 100 g of the negative electrode material and adjusting the viscosity. 3.8 g was added and stirred and mixed to prepare a slurry-like dispersion having sufficient fluidity. The prepared dispersion was applied onto a copper foil having a thickness of 20 μm using a doctor blade so as to be uniform at a thickness of 150 μm, dried on a hot plate, and then dried at 70 ° C. for 12 hours in a vacuum dryer. The density of the dried electrode was adjusted by a roll press to obtain a negative electrode for battery evaluation. The applied amount of the obtained electrode was 7 mg / cm ² , and the electrode density was 1.4 g / cm ³ .
(2) Preparation of positive electrode 90 g of LiFePO _{4, 5} g of carbon black (manufactured by TIMCAL) as a conductive auxiliary agent, and 5 g of polyvinylidene fluoride (PVdF) as a binder are stirred and mixed while appropriately adding slurry. A liquid dispersion was prepared.
The produced dispersion was applied onto a 20 μm thick aluminum foil with a roll coater, dried, and then pressure-formed with a roll press. The coating amount of the obtained positive electrode was 10 mg / cm ² , and the electrode density was 2.0 g / cm ³ .

(3) Electrolyte preparation Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 3: 7 as a nonaqueous solvent, and 1.0 mol of lithium hexafluorophosphate (LiPF ₆ ) was used as an electrolyte salt. / L dissolved was used as an electrolytic solution.

(4) Battery preparation The negative electrode and the positive electrode were punched out to obtain a negative electrode piece and a positive electrode piece having an area of 20 cm ² . An Al tab was attached to the Al foil of the positive electrode piece, and an Ni tab was attached to the Cu foil of the negative electrode piece. A polypropylene microporous film was sandwiched between a negative electrode piece and a positive electrode piece, and packed in an aluminum laminate in that state. And electrolyte solution was poured into it. Thereafter, the opening was sealed by heat sealing to produce a battery for evaluation (design capacity 25 mAh).

(5) Initial discharge capacity Charging was performed at an upper limit voltage of 3.6 V in a CC (constant current) and CV (constant voltage) mode at 5 mA and a cut-off current value of 1.25 mA.
The initial discharge capacity was measured by discharging 5 mA in CC mode at a lower limit voltage of 2.5 V.

(6) Charge / Discharge Rate Test After charging the cell with an upper limit voltage of 3.6 V and a cut-off current value of 1.25 mA with CC and CV mode at 5 mA, the lower limit voltage is 2.5 V and 5 C (about 125 mA) is discharged with CC mode. The energy density (mWh) at 5C was calculated.
In addition, the cell is discharged at 5 mA in CC mode with a lower limit voltage of 2.5 V, charged at 5 C in CC mode with an upper limit voltage of 3.6 V, and 5 C with reference to 0.2 C charge capacity (0.2 C = about 5 mA). The ratio of charge capacity was calculated.
(5C charge capacity (%))
= (5C charge capacity (mAh)) / (0.2C charge capacity (mAh)) x 100

(7) Measurement of cycle characteristics Charging was performed at a cutoff current value of 1.25 mA at 2C (2C = about 50 mA) in CC and CV modes with an upper limit voltage of 3.6 V.
2C discharge was performed in CC mode with a lower limit voltage of 2.5V.
Under the above conditions, 500 cycles of charge and discharge were repeated.
The discharge capacity at 500 cycles was measured. The ratio of the discharge capacity at 500 cycles to the initial discharge capacity was calculated and used as the discharge capacity retention rate.
(Discharge capacity maintenance rate after 500 cycles (%))
= (Discharge capacity at 500 cycles) / (initial discharge capacity) x 100

Example 1
70 parts by mass of artificial graphite (A) and 30 parts by mass of the carbon material (B) were placed in a Fuji Panda Spartan Luther and mixed for 5 minutes to obtain a negative electrode material. The obtained negative electrode material had a D50 of 8.2 μm and a BET specific surface area of 2.6 m ² / g. An electrode and a battery cell were produced using this negative electrode material, and the battery characteristics were evaluated. The results are shown in Table 1.

Example 2
A negative electrode material, an electrode and a battery cell were produced in the same manner as in Example 1 except that the amount of the artificial graphite (A) was changed to 50 parts by mass and the amount of the carbon material (B) was changed to 50 parts by mass. The obtained negative electrode material had a D50 of 9.9 μm, a BET specific surface area of 2.6 m ² / g, and the battery characteristics were as shown in Table 1.

Example 3
A negative electrode material, an electrode, and a battery cell were produced in the same manner as in Example 1 except that the amount of the artificial graphite (A) was changed to 30 parts by mass and the amount of the carbon material (B) was changed to 70 parts by mass. The obtained negative electrode material had a D50 of 11.4 μm, a BET specific surface area of 2.5 m ² / g, and the battery characteristics were as shown in Table 1.

Comparative Example 1
Except for the pulverization so that the average particle size (D50) is 16 μm, the same operation as in the method for producing artificial graphite (A) is carried out, the BET specific surface area is 1.1 m ² / g, and d (002 ) Obtained artificial graphite having 0.336 nm.
50 parts by mass of the obtained artificial graphite and 50 parts by mass of the carbon material (B) were placed in a Fujipandal Spartan Luther and mixed for 5 minutes to obtain a negative electrode material. The obtained negative electrode material had a D50 of 14.5 μm and a BET specific surface area of 1.8 m ² / g. An electrode and a battery cell were produced using this negative electrode material, and the battery characteristics were evaluated. The results are shown in Table 1.

Comparative Example 2
The battery characteristics were evaluated in the same manner as in Example 1 except that only the carbon material (B) was used as the negative electrode active material and the electrode collapsibility was poor and the electrode density of the negative electrode was 1.3 g / cm ² . The results are shown in Table 1.

  As shown in Table 1, it can be seen that by using the negative electrode material of the example, a lithium ion secondary battery having a well-balanced performance with high energy density at the time of discharge, high charging characteristics, and excellent cycle characteristics can be obtained. .

  The charge curve of 5C charge measured with the battery obtained in Examples 1-3 is shown in FIG. It can be seen that the cell can be charged from a low potential state by mixing the carbon material (B) having low crystallinity. This is considered to be due to the increase in Li ion acceptability at the initial stage of charging due to the carbon material (B), thereby enabling smooth charging.

Claims (15)

The interplanar spacing (d (002)) of the (002) plane of the graphite structure measured by powder X-ray diffraction method is in the range of 0.335 to 0.339 nm, and the cumulative volume frequency of the particle size distribution measured by laser diffraction method Of artificial graphite (A) having a particle diameter (D50) of 4 to 10 μm, and d (002) of 0.340 nm or more, D50 of 7 to 17 μm, and D50 of said artificial graphite ( Carbon material larger than D50 of A) (B)
The negative electrode material for lithium ion secondary batteries characterized by including the mixture of these.   2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein a composition ratio of the artificial graphite (A) and the carbon material (B) is in a range of 8: 2 to 2: 8 by mass ratio.   2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the artificial graphite (A) is a heat-treated petroleum-based and / or coal-based coke at 2500 ° C. or higher.   2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the artificial graphite (A) comprises particles having a carbon coating layer on the surface of artificial graphite particles as a core material.   The negative electrode material for a lithium ion secondary battery according to claim 4, wherein the artificial graphite as the core material is obtained by heat-treating petroleum-based and / or coal-based coke at 2500 ° C or higher. Wherein the coating layer, the intensity ratio of the peak intensity (I _G) with a peak intensity in the range of 1300～1400Cm ^-1 as measured by Raman spectroscopy and (I _D) in the range of 1580~1620cm ^-1 I _D / The negative electrode material for a lithium ion secondary battery according to claim 4, wherein I _G (R value) is 0.1 or more.   The negative electrode material for a lithium ion secondary battery according to any one of claims 4 to 6, wherein the amount of the coating layer is 0.05 to 10 parts by mass when the artificial graphite as a core material is 100 parts by mass.   The artificial graphite (A) is obtained by attaching an organic compound to particles of artificial graphite as a core material and then heat-treating at a temperature of 200 ° C to 3000 ° C. The negative electrode material for lithium ion secondary batteries as described in 2.   The organic compound is at least one compound selected from the group consisting of petroleum pitch, coal pitch, phenol resin, polyvinyl alcohol resin, furan resin, cellulose resin, polystyrene resin, polyimide resin, and epoxy resin. 8. The negative electrode material for a lithium ion secondary battery according to 8. ^{2. The} negative electrode material for a lithium ion secondary battery according to claim 1, wherein the artificial graphite (A) has a BET specific surface area of 0.5 to ⁵ m ² / g.   The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the carbon material (B) is hard carbon and / or soft carbon.   Lithium ions obtained by applying a negative electrode paste containing the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 11, a binder and a dispersion medium onto a current collector foil, drying, and pressing. A negative electrode sheet for a secondary battery. The negative electrode sheet for a lithium ion secondary battery according to claim 12, wherein the density of the negative electrode portion other than the current collector foil is 1.1 g / cm ³ or more and 1.6 g / cm ³ or less.   The lithium ion battery which contains the negative electrode sheet for lithium ion secondary batteries of Claim 12 or 13 as a component.   The lithium ion battery according to claim 14, wherein a nonaqueous electrolytic solution and / or a nonaqueous polymer electrolyte is used, and propylene carbonate is contained in the nonaqueous solvent used in the nonaqueous electrolytic solution and / or the nonaqueous polymer electrolyte.

Images