JP5119662B2 - Negative electrode and battery using the same - Google Patents

Description

  The present invention relates to a negative electrode including mesophase graphite spherules and a battery using the same.

  In recent years, many portable electronic devices such as a camera-integrated VTR (videotape recorder), a mobile phone, or a laptop computer have appeared, and their size and weight have been reduced. Accordingly, as a portable power source for these electronic devices, research and development for improving the energy density of batteries, particularly secondary batteries, are being actively promoted.

  Among them, lithium ion secondary batteries using a carbon material for the negative electrode are known (see, for example, Patent Documents 1 to 6), and these batteries are lead batteries or nickel which are conventional aqueous electrolyte secondary batteries. Since a large energy density can be obtained as compared with a cadmium battery, it is highly expected.

On the other hand, in order to increase the battery capacity without changing the size of the secondary battery, the electrode material is filled with high density, or the electrode material is subjected to high load (pressurization) pressing, and the volume density of the electrode material is reduced. It is known that it should be higher. In addition, it is known that mesophase graphite spherules having high compressive fracture strength may be used as the carbon material of the negative electrode in order to prevent the particles of the electrode material from being broken by the high load press (for example, Patent Document 7). reference.). Since mesophase graphite spherules are spherulites, the specific surface area is relatively small, and even if the ratio used is high, it is difficult to cause a decrease in adhesive strength and a decrease in charge / discharge efficiency due to electrolyte decomposition.
JP-A-57-208079 JP 58-93176 A JP 58-192266 A JP 62-90863 A JP-A-62-212666 JP-A-2-66856 JP-A-7-272725

  However, in order to increase the capacity of the battery, when trying to fill mesophase graphite spherules with high density, the mesophase graphite spherules have high compressive fracture strength (particle hardness), so there is a gap between adjacent particles. Therefore, there is a problem that it cannot be filled with high density. Therefore, instead of mesophase graphite spherules, for example, when scaly graphite with a large discharge capacity or pulverized and expanded graphite is packed at a high density, graphite materials with a large discharge capacity generally have a low compressive fracture strength, and when pressed In addition, the graphite material near the electrode surface is crushed and fills the gaps through which the electrolyte permeates, so that the problem arises that the charge / discharge capacity of the electrode is lowered and the capacity of the battery cannot be increased.

  In general, when the discharge capacity is increased, the graphite material used for the negative electrode has a narrower graphite surface interval and a higher graphitization degree. For example, a graphite material with a high degree of graphitization and a large discharge capacity, such as natural graphite, has a low particle hardness, so when trying to fill it with high density, the graphite itself near the electrode surface is crushed, making it difficult to contribute to charge and discharge. Therefore, it is difficult to obtain a high capacity electrode. On the other hand, mesophase graphite spheres with high particle hardness have a relatively low discharge capacity, and the mesophase graphite spheres are not easily crushed and the gaps between particles cannot be filled with high density. It is difficult to obtain.

  As a means for solving the limitation of increasing the capacity of these electrodes, an electrode in which mesophase graphite spheres having a high particle hardness and a graphite material having a low particle hardness are mixed can be considered. When graphite having high discharge capacity and low particle hardness is added to mesophase graphite spherules having high particle hardness, graphite having low particle hardness is filled in the gaps between the mesophase graphite spherules having high particle hardness and can be filled with high density. Furthermore, since graphite particles with high particle hardness exist in the vicinity of the electrode surface, it is possible to avoid deterioration of battery characteristics due to the crushing of the electrode surface, thereby obtaining a high capacity negative electrode filled with high discharge capacity graphite at high density. However, graphite materials with high discharge capacity and low particle hardness have a large specific surface area, unlike spherulites such as mesophase graphite spherules. It is easy to invite a decline.

  On the other hand, a high discharge capacity can be obtained by increasing the graphitization temperature of the mesophase graphite microspheres or adjusting the graphitization process. However, by increasing the graphitization temperature, the particle hardness decreases, and there is a problem that the graphite itself in the vicinity of the electrode surface is crushed when trying to fill with high density.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a negative electrode having a high volume density, a large discharge capacity, and excellent cycle characteristics, and a battery using the negative electrode.

  In order to solve the above problems, the negative electrode of the present invention uses, as a negative electrode active material, mixed graphite of mesophase graphite spherules having high particle hardness and mesophase graphite spherules having low particle hardness. That is, the negative electrode of the present invention is a negative electrode including a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, and the negative electrode active material layer is a small mesophase graphite small particle having different particle hardness. A negative electrode active material containing mixed graphite of sphere A and mesophase graphite small sphere B and a binder containing a fluorine-based polymer binder resin.

The negative electrode active material layer has a volume density of 1.80 g / cm ³ or more. The mesophase graphite spherule A has a specific surface area of 0.3 to 0.7 m 2 / g, a fracture strength of 75 MPa or more, a graphite surface spacing d002 of 0.3359 nm or more, and a discharge capacity of 335 to 352 mAh / g. The mesophase graphite small sphere B has a specific surface area of 0.9 to 2.0 m 2 / g, a fracture strength of 70 MPa or less, a graphite surface interval d002 of less than 0.3359 nm, and a discharge capacity of 355 to 365 mAh / g. Furthermore, the mixing ratio of the mesophase graphite microspheres A and the mesophase graphite microspheres B is 40:60 to 60:40 in terms of mass ratio.

  According to the negative electrode of the present invention, a mixed graphite of mesophase graphite spheres with high particle hardness and mesophase graphite spheres with low particle hardness is used as the negative electrode active material, so that the volume density of the negative electrode active material is increased and the discharge capacity is increased. As a result, excellent cycle characteristics can be obtained. Therefore, by using the negative electrode of the present invention, a battery having high charge / discharge capacity and excellent cycle characteristics can be produced.

  Hereinafter, the best mode for carrying out the present invention will be described with reference to the drawings, but the present invention is not limited to the following mode. A battery using the negative electrode according to the present invention will be described together with the negative electrode.

  FIG. 1 schematically shows a configuration of a negative electrode 10 according to an embodiment of the present invention. The negative electrode 10 includes, for example, a negative electrode current collector 11 having a pair of opposed surfaces, and a negative electrode active material layer 12 provided on one surface of the negative electrode current collector 11. Although not shown, the negative electrode active material layers 12 may be provided on both surfaces of the negative electrode current collector 11.

  The negative electrode current collector 11 preferably has good electrochemical stability, electrical conductivity, and mechanical strength, and is made of a metal material such as copper, nickel, and stainless steel.

  The negative electrode active material layer 12 includes a carbon material capable of inserting and extracting lithium or the like as the negative electrode active material. In the present embodiment, mixed graphite of mesophase graphite small spheres 12A and mesophase graphite small spheres 12B is used as the carbon material.

  The breaking strength is St (Sx) = 2.8P / (π × d × d) (St (Sx): breaking strength [Pa], P: test force [N], d: particle diameter [mm]) and It is a defined value. The breaking strength can be measured using, for example, Shimadzu small compression tester MCT-W500. The fracture strength of the mesophase graphite microspheres 12A is in the range of 75 MPa or more, preferably in the range of 75 MPa or more and 120 MPa or less. If the fracture strength of the mesophase graphite spherules 12A is less than 75 MPa, the graphite particles are brittle, and the vicinity of the surface of the electrode is concentrated and crushed by the press, leading to a decrease in electrolyte impregnation property, a decrease in lithium ion migration, and the like. This is because the battery characteristics are extremely lowered at the electrode density. In addition, if the fracture strength of the mesophase graphite spherules 12A is greater than 120 MPa, the graphite particles are too hard, so that the electrode density does not increase even when pressed, and a high electrode density battery cannot be obtained. In this case, if it is forced to crush with a press, damage to the current collector increases, and adverse effects such as electrode breakage during cycling appear.

  Further, the fracture strength of the mesophase graphite spherules 12B is in the range of 70 MPa or less, preferably in the range of 1 MPa or more and 70 MPa or less. If the fracture strength of the mesophase graphite spherules 12B is greater than 70 MPa, the graphite particles are too hard, so the mesophase graphite spherules 12B are not efficiently filled between the particles of the mesophase graphite spherules 12A, and the electrode density increases even when pressed. Therefore, a battery having a high electrode density cannot be obtained. Furthermore, if it is forced to crush with a press, damage to the current collector will increase, resulting in effects such as electrode breakage during cycling. Further, if the fracture strength of the mesophase graphite microspheres 12B is less than 1 MPa, the graphite particles are too brittle, so that they are locally crushed to a high density and the edges of the graphite itself are buried, resulting in a decrease in capacity and lithium ion migration. This is because the battery characteristics are extremely lowered when the electrode density is high.

  The particle hardness can be compared based on the value of the breaking strength. If the breaking strength is high, the particle hardness is high. Therefore, the particle hardness of the mesophase graphite spherules 12A and the mesophase graphite spherules 12B is preferably set so that the mesophase graphite spherules 12A> the mesophase graphite spherules 12B. By mixing the mesophase graphite spherules 12A having high particle hardness and the mesophase graphite spherules 12B having low particle hardness, the mesophase graphite spherules 12B having low particle hardness are filled in the gaps between the mesophase graphite spherules 12A having high particle hardness. This is because it can be filled with high density. Further, the presence of the mesophase graphite microspheres A having a high particle hardness in the vicinity of the electrode surface can prevent deterioration of battery characteristics due to the collapse of the electrode surface, and a high-capacity negative electrode filled with high density can be obtained.

The specific surface area is a value based on the N ₂ gas BET (Brunauer, Emmett, Teller) method, and can be measured using, for example, a specific surface area measuring device (manufactured by Shimadzu Corporation). The specific surface area of the mesophase graphite spherules 12A is in the range of 0.3 to 0.7 m ² / g. The specific surface area of the mesophase graphite spherules 12B is in the range of 0.9 to 2.0 m ² / g. By setting the specific surface areas of the mesophase graphite spherules 12A and mesophase graphite spherules 12B within this range, even if the content of these mesophase graphite spherules in the negative electrode active material is increased, the adhesive strength is reduced, and the electrolytic solution is decomposed. This is because the charging / discharging efficiency is not reduced by the above.

The graphite plane distance d ₀₀₂ is the (002) plane distance measured by the X-ray wide angle diffraction method. Graphite surface spacing d ₀₀₂ represents the degree of graphitization of the carbon material. The interval between the graphite surfaces of the mesophase graphite spherules 12A is d ₀₀₂ : 0.3359 nm or more. Further, the graphite plane distance of mesophase graphite globules 12B is _d 002: to less than 0.3359Nm. It is because it can be filled efficiently. The graphite surface spacing can be adjusted by the graphitization temperature in the process of producing mesophase graphite spherules, and the graphitization temperature is preferably set to mesophase graphite spherules 12A <mesophase graphite spherules 12B.

  The discharge capacity of the mesophase graphite microspheres 12A is in the range of 335 to 352 mAh / g, preferably in the range of 340 to 350 mAh / g. The discharge capacity of the mesophase graphite small spheres B is in the range of 355 to 365 mAh / g, preferably in the range of 358 to 363 mAh / g. This is because a high capacity electrode can be obtained by mixing the mesophase graphite small spheres 12A having a low discharge capacity and the mesophase graphite small spheres 12B having a high discharge capacity at a specific mixing ratio.

The median diameter D _50, a cumulative value of 50 wt% particle diameter in the cumulative particle size distribution can be measured by using, for example, a laser diffraction particle size distribution analyzer (manufactured by Shimadzu Corporation). The median diameter of the mesophase graphite spherule A and the mesophase graphite spherule B is preferably in the range of D ₅₀ : 17 to 23 μm. This is because the adhesive strength is strong, the decomposition with the electrolytic solution is small, and good battery characteristics can be obtained.

  The true specific gravity refers to the specific gravity of the portion excluding the liquid and gas contained in the sample (see JIS Z8807). The true specific gravity can be measured by a method according to JIS Z8807 using a solid sample pulverized into a powder, and can be measured using, for example, a powder measuring instrument MAT-7000 (manufactured by Seishin Enterprise). The true specific gravity of the mesophase graphite spherule A and the mesophase graphite spherule B is preferably set to mesophase graphite spherule A <mesophase graphite spherule B. It is because it can be filled efficiently.

  The mesophase graphite small spheres 12A and the mesophase graphite small spheres 12B may be mixed graphite of spherulites of the mesophase graphite small spheres and pulverized bodies thereof. This is because mesophase graphite spherules are basically spherulites, but may be partially damaged in the manufacturing process such as pressing. Therefore, the spherulites mentioned here are spherulites and parts lacking from the spherulites, and have a volume of 50% or more of the original spherulites. The spherulites are pulverized and have a volume of less than 50% of the original spherulite volume. When the spherulites and pulverized bodies of the mesophase graphite small spheres 12A are mixed, the mixing ratio is preferably 30:70 to 100: 0 in mass ratio. When the ratio of the spherulites of the mesophase graphite spherules 12A is less than 30% by mass, the interparticle gaps of the mesophase graphite spherules A, which are the hard spherulites as the base, are reduced, and the mesophase graphite spherules B that are brittle are efficiently filled. It is because it is not possible.

  Moreover, when the spherulites of the mesophase graphite small spheres 12B and the pulverized bodies are mixed, the mixing ratio is preferably 0: 100 to 70:30. When the ratio of the spherulites of the mesophase graphite spherules 12B is more than 70% by mass, the spherulites of the mesophase graphite spherules B that are brittle are forcibly crushed in the interstitial space between the spherulites of the hard mesophase graphite spherules A that are the base. This is because adverse effects such as cracking of the particles remarkably appear, leading to a decrease in the electrolyte impregnation property.

The mixing ratio of the mesophase graphite small spheres 12A and the mesophase graphite small spheres 12B in the negative electrode active material layer 12 is a mass ratio in the range of 40:60 to 60:40, preferably in the range of 45:55 to 55:45. To do. By setting the mixing ratio within this range, the mesophase graphite spherule B having a low compressive fracture strength is a mesophase graphite spherule having a high compressive fracture strength even when a pressure press is used when producing a negative electrode. This is because the volume density of the negative electrode active material layer can be increased because it exists in the gap of 12A. Further, when the mixing ratio of the mesophase graphite spherules 12B is higher than 60% by mass, for example, when the volume density of the negative electrode active material layer is set to a range of 1.80 g / cm ³ or more, the vicinity of the electrode surface during high load pressing. The mesophase graphite spherules B of the active material layer are crushed to fill the voids through which the electrolyte solution penetrates, and the battery characteristics are improved by decreasing the permeability, burying the edge portion, and aligning the basal plane in the in-plane direction of the electrode. This is because it is greatly reduced.

The negative electrode active material layer 12 contains a fluorine-based polymer binder resin as a binder. Since the fluorine-based polymer binder resin has a small so-called spring back and can be heated and pressed at a high temperature as compared with the rubber-based binder resin, the volume density of the negative electrode active material layer 12 is 1.80 g / cm ³ or more. This is because the density can be increased. Moreover, since it has swelling property with respect to the electrolytic solution, it has excellent lithium ion permeability and high battery characteristics can be obtained. Examples of the fluorine-based polymer binder resin include polyvinylidene fluoride (PVdF), PVdF copolymer (for example, PVdF-HFP, PVdF-CTFE, PVdF-PTFE, and the like).

  The negative electrode 10 can be manufactured as follows, for example. First, a negative electrode mixture was prepared by mixing mesophase graphite small spheres 12A and mesophase graphite small spheres 12B and a binder composed of a fluorine-based polymer binder resin. It is made to disperse | distribute to solvents, such as, and it is set as a paste-form negative mix slurry. Subsequently, after applying this negative electrode mixture slurry to the negative electrode current collector 11 and drying the solvent, the negative electrode 10 is formed by compression molding using a roll press machine or the like.

  The negative electrode 10 is used, for example, in a secondary battery having a laminate film as an exterior as follows. FIG. 2 shows an exploded view of the structure of such a secondary battery. In the secondary battery, a wound electrode body 20 to which a positive electrode lead 21 and a negative electrode lead 22 are attached is enclosed in a film-shaped exterior member 30.

  The positive electrode lead 21 and the negative electrode lead 22 are directed from the inside of the exterior member 30 to the outside, and are led out in the same direction, for example. The positive electrode lead 21 and the negative electrode lead 22 are made of a metal material such as aluminum, copper, nickel, and stainless steel, respectively, and each have a thin plate shape or a mesh shape.

  The exterior member 30 is made of, for example, a rectangular laminate film in which a nylon film, an aluminum foil, and a polyethylene film are bonded together in this order. The exterior member 30 is disposed, for example, so that the polyethylene film side and the wound electrode body 20 face each other, and the outer edge portions are in close contact with each other by fusion or an adhesive.

  The exterior member 30 may be made of a laminated film having another structure, a polymer film such as polypropylene, or a metal film instead of the above-described laminated film.

  FIG. 3 shows a cross-sectional structure taken along line II of the spirally wound electrode body 20 shown in FIG. The wound electrode body 20 is formed by laminating a positive electrode 23 and a negative electrode 10 with a separator 24 interposed therebetween, and is wound. The outermost peripheral portion is protected by a protective tape 25.

  The negative electrode 10 has the above-described configuration, and includes, for example, a negative electrode current collector 11 and a negative electrode active material layer 12 provided on both surfaces or one surface of the negative electrode current collector 11. As a result, high capacity, excellent large current discharge characteristics, low temperature discharge characteristics, and the like can be obtained. The negative electrode current collector 11 has an exposed portion where the negative electrode active material layer 11 is not provided at one end in the longitudinal direction, and a negative electrode lead 22 is attached to the exposed portion. In FIG. 3, the negative electrode active material layer 12 is represented as being formed on both surfaces of the negative electrode current collector 11.

  The positive electrode 23 includes, for example, a positive electrode current collector 23A and a positive electrode active material layer 23B provided on both surfaces or one surface of the positive electrode current collector 23A. The positive electrode current collector 23A has an exposed portion where the positive electrode active material layer 23B is not provided at one end in the longitudinal direction, and the positive electrode lead 21 is attached to the exposed portion. The positive electrode current collector 23A is made of, for example, a metal material such as an aluminum foil, a nickel foil, and a stainless steel foil.

The positive electrode active material layer 23B includes, as the positive electrode active material, for example, any one or more of positive electrode materials capable of inserting and extracting lithium, which is an electrode reactant, and, if necessary, such as graphite. A conductive agent and a binder such as polyvinylidene fluoride (PVdF) may be included. As the positive electrode material, for example, lithium-containing compounds such as lithium oxide, lithium sulfide, and an intercalation compound containing lithium are suitable, and a plurality of these may be mixed and used. In order to increase the energy density, a lithium composite oxide represented by the general formula Li _X MO ₂ or an intercalation compound containing lithium is particularly preferable. Here, M is preferably one or more transition metals, and specifically, at least one of cobalt, nickel, manganese, iron, aluminum, vanadium, and titanium is preferable. Further, x varies depending on the charge / discharge state of the battery, and is usually a value within a range of 0.05 ≦ x ≦ 1.10. Furthermore, lithium manganate (LiMn ₂ O ₄ ) having a spinel type crystal structure and lithium iron phosphate (LiFePO ₄ ) having an olivine type crystal structure are preferable because a high energy density can be obtained.

  Examples of the positive electrode material include other metal compounds and polymer materials. Examples of other metal compounds include oxides such as titanium oxide, vanadium oxide, and manganese dioxide, or disulfides such as titanium sulfide and molybdenum sulfide, and examples of the polymer material include polyaniline and polythiophene. It is done.

  The positive electrode mixture layer includes, for example, a conductive agent, and may further include a binder as necessary. Examples of the conductive agent include carbon materials such as graphite, carbon black, and ketjen black. In addition to the carbon material, a metal material, a conductive polymer material, or the like may be used as long as the material has conductivity. Any one kind of conductive agent may be used, or a plurality of kinds may be used in combination. Examples of the binder include synthetic rubbers such as styrene butadiene rubber, fluorine rubber and ethylene propylene diene rubber, and polymer materials such as polyvinylidene fluoride (PVdF) and PVdF copolymers. Any one kind of binder may be used, or a plurality of kinds may be mixed and used.

  The separator 24 separates the positive electrode and the negative electrode and allows lithium ions to pass through while preventing a short circuit of current due to contact between the two electrodes. This separator is composed of, for example, a porous film made of synthetic resin such as polytetrafluoroethylene, polypropylene, and polyethylene, or a porous film made of ceramic, and has a structure in which these plural kinds of porous films are laminated. Also good. Among these, a porous film made of polyolefin is preferable because it is excellent in short-circuit preventing effect and can improve the safety of the battery due to the shutdown effect. In particular, polyethylene is preferable as a material constituting the separator 24 because a shutdown effect can be obtained within a range of 100 ° C. or more and 160 ° C. or less and excellent in electrochemical stability. Polypropylene is also preferable, and any other resin having chemical stability can be used by being copolymerized or blended with polyethylene or polypropylene.

  The electrolytic solution impregnated in the separator 24 includes a liquid solvent, for example, a nonaqueous solvent such as an organic solvent, an electrolyte salt dissolved in the nonaqueous solvent, and an additive as necessary. The liquid non-aqueous solvent is made of, for example, a non-aqueous compound and has an intrinsic viscosity at 25 ° C. of 10.0 mPa · s or less. In addition, the intrinsic viscosity in a state where the electrolyte salt is dissolved may be 10.0 mPa · s or less. When a solvent is formed by mixing a plurality of types of nonaqueous compounds, the intrinsic viscosity in the mixed state is What is necessary is just 10.0 mPa * s or less.

  Examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4 -Methyl-1,3-dioxolane, methyl acetate, methyl propionate, ethyl propionate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, 3-methoxypropyronitrile, N, N-dimethylformamide, N-methylpyrrolidinone, N-methyloxazolidinone, nitromethane, nitroethane, sulfolane, dimethyl sulfoxide, phosphoric acid Examples include trimethyl and triethyl phosphate.

  In particular, in order to achieve excellent charge / discharge capacity characteristics and charge / discharge cycle characteristics, for example, at least one of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, vinylene carbonate, dimethyl carbonate and ethyl methyl carbonate, and ethylene sulfite. It is preferable that 1 type is included.

  In addition, the non-aqueous solvent may contain a room temperature molten salt. Among them, those having a quaternary ammonium salt structure composed of a quaternary ammonium cation and a fluorine atom-containing anion are preferable. For example, a salt structure comprising at least one ammonium cation selected from the following ammonium cation group and at least one anion selected from the following anion group can be mentioned.

  Examples of the ammonium cation group include pyrrolium cation, pyridinium cation, imidazolium cation, pyrazolium cation, benzimidazolium cation, indolium cation, carbazolium cation, quinolinium cation, pyrrolidinium cation, and piperidinium. Nium cation, piperazinium cation, alkylammonium cation (including those substituted with a hydrocarbon group having 1 to 30 carbon atoms, hydroxyalkyl, and alkoxyalkyl). In addition, all include those in which a hydrocarbon group having 1 to 10 carbon atoms, a hydroxyalkyl group, and an alkoxyalkyl group are bonded to N and / or a ring.

Examples of the anion group include BF ₄ , PF ₆ , C _n F _{2n + 1} CO ₂ (where n is an integer of 1 to 4), C _n F _{2n + 1} SO ₃ (where n is an integer of 1 to 4), (FSO ₂ ) ₂ N, (CF ₃ SO ₂ ) ₂ N, (C ₂ F ₅ SO ₂ ) ₂ N, (CF ₃ SO ₂ ) ₃ C, CF ₃ SO ₂ —N—COCF ₃ , R—SO ₂ —N— SO ₂ CF ₃ (R is an aliphatic group), and _{ArSO 2 -N-SO 2 CF 3} (Ar is an aromatic group).

Examples of the lithium salt include lithium hexafluorophosphate (LiPF ₆ ), lithium bis (pentafluoroethanesulfonyl) imide, lithium perchlorate (LiClO ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), and tetrafluoride. Lithium borate (LiBF ₄ ), lithium trifluoromethanesulfonate (LiSO ₃ CF ₃ ), lithium bis (trifluoromethanesulfonyl) imide (LiN [SO ₂ (CF ₃ )] ₂ ), tris (trifluoromethanesulfonyl) methyllithium ( LiC [SO ₂ (CF ₃ )] ₃ ), lithium chloride (LiCl), and lithium salts such as lithium bromide (LiBr). In order to obtain characteristics according to the purpose as the electrolyte salt, any one or more of these lithium salts may be contained.

  Instead of the electrolytic solution, a gel electrolyte in which an electrolytic solution is held in a polymer compound may be used. Examples of the polymer compound include polyacrylonitrile, polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and polyhexafluoropropylene, polytetrafluoroethylene, polyhexafluoropropylene, polyethylene oxide, polypropylene oxide, polyphosphazene, poly Examples include siloxane, polyvinyl acetate, polyvinyl alcohol, polymethyl methacrylate, polyacrylic acid, polymethacrylic acid, styrene-butadiene rubber, nitrile-butadiene rubber, polystyrene, and polycarbonate. In particular, from the viewpoint of electrochemical stability, it is desirable to use a polymer compound having a structure such as polyacrylonitrile, polyvinylidene fluoride, polyhexafluoropropylene, and polyethylene oxide. In addition, although the addition amount of the high molecular compound with respect to electrolyte solution changes also with both compatibility, it is preferable to add the high molecular compound equivalent to 5-50 mass% of electrolyte solution normally.

  The electrode material thus obtained can be used as an electrode for various batteries, and the type of the battery is not particularly limited, but is preferably used as an electrode for a secondary battery. As a particularly preferable secondary battery, for example, a secondary battery using a non-aqueous electrolyte containing an alkali metal salt such as lithium perchlorate, lithium borofluoride, and lithium hexafluorophosphate can be given.

  In the present embodiment, the method for producing an electrode sheet by applying a positive electrode material and a negative electrode material to a current collector is not particularly limited. However, due to the nature of the present invention, the electrode sheet is dispersed in a solvent together with a binder and a conductive agent. In general, a method in which a solution is applied and dried, or an active material is attached to a current collector by using a conductive binder or a mixture of a conductive agent and a binder is used. In addition, the current collector in the present embodiment can use, for example, metal in a foil shape, a net shape, a lath shape, or the like, but is not limited to these forms.

  The present invention has been described with reference to the embodiment. However, the present invention is not limited to the above embodiment, and various modifications can be made. For example, in the above embodiment, the case where the positive electrode and the negative electrode are wound has been described. However, a plurality of positive electrodes and negative electrodes may be stacked or folded. Furthermore, the negative electrode of the present invention can be applied to, for example, a battery of a cylindrical type, a square type, a coin type, a button type or the like using a can as an exterior member. Furthermore, the negative electrode of the present invention can be applied not only to secondary batteries but also to primary batteries.

  Hereinafter, specific examples of the present invention will be described in detail.

(Production of electrodes)
The negative electrode was produced as follows. A negative electrode mixture was prepared by mixing 92% by mass of graphite powder and 8% by mass of polyvinylidene fluoride (PVdF) or styrene butadiene rubber (SBR) as a binder. Next, this negative electrode mixture was mixed with N as a solvent. -A paste-like negative electrode mixture slurry was prepared by dispersing in methyl-2-pyrrolidone. Next, the negative electrode mixture slurry was applied to a negative electrode current collector made of a strip-shaped copper foil having a thickness of 10 μm and dried to prepare a negative electrode. The coating thickness on one side of the negative electrode active material was 65 μm.

  The positive electrode was produced as follows. As the positive electrode, a lithium-cobalt composite oxide having a 50% particle size of 15 μm was used as the positive electrode active material. 95% by mass of this lithium-cobalt composite oxide powder and 5% by mass of lithium carbonate powder are mixed, 94% by mass of this mixture, 3% by mass of ketjen black as a conductive agent, and polyvinylidene fluoride as a binder. A positive electrode mixture was prepared by mixing 3% by mass. Next, this positive electrode mixture is dispersed in N-methyl-2-pyrrolidone as a solvent to form a paste-like positive electrode mixture slurry, which is uniformly applied to both surfaces of a positive electrode current collector made of a strip-shaped aluminum foil having a thickness of 20 μm. It dried and compression-molded, the positive mix layer was formed, and the positive electrode was produced. The coating thickness on one side of the positive electrode active material was 70 μm.

(Production of coin cell)
A coin cell having a diameter of 20 mm and a thickness of 3.2 mm (cell structure: counter electrode / Li metal; separator / polyethylene porous membrane; electrolyte / mixed solvent of ethylene carbonate and diethyl carbonate) A solution prepared by dissolving LiPF ₆ at a ratio of 1 mol / dm ³ in [1: 1 (volume ratio)] (current collector / copper foil) was produced.

(Production of cylindrical can battery)
FIG. 4 shows a cross-sectional structure of the cylindrical can battery according to this example. As shown in FIG. 4, the cylindrical can battery has a substantially hollow cylindrical battery can 31 (can inner diameter 17.3 mm, inner length 55.5 mm), and a strip-like positive electrode 41 and a negative electrode 42 having the above composition. The wound electrode body 40 is wound around a separator 43 made of a stretched microporous polyethylene film having a thickness of 25 μm. The wound electrode body 40 was a jelly roll-type wound electrode body having an outer diameter of 17 mm formed by laminating a separator, a negative electrode, a separator, and a positive electrode in this order and then winding them many times. The battery can 31 is made of iron (Fe) plated with nickel (Ni), and has one end closed and the other end open. Inside the battery can 11, a pair of insulating plates 32 and 33 are arranged perpendicular to the winding peripheral surface so as to sandwich the winding electrode body 40.

A battery lid 34 is attached to the open end of the battery can 31 by caulking through a gasket 37, and the inside of the battery can 31 is sealed. The battery lid 34 is made of the same material as the battery can 31. The gasket 37 is made of an insulating material, and the surface is coated with asphalt. For example, a center pin 44 is inserted in the center of the wound electrode body 40. A positive electrode lead 45 made of aluminum is electrically connected to the battery lid 34 by welding to the safety mechanism 35 to the positive electrode 41 of the wound electrode body 40, and a negative electrode lead made of nickel to the negative electrode 42. 46 is connected, and the negative electrode lead 46 is electrically connected to the battery can 31 by welding. Subsequently, an electrolytic solution was injected into the battery can. As the electrolytic solution, a solution in which LiPF ₆ was dissolved at a ratio of 1 mol / dm ³ in a mixed solvent of ethylene carbonate and diethyl carbonate [1: 1 (volume ratio)] was used.

(Evaluation method)
The graphite material used for the negative electrode active material of an Example and the evaluation method of a battery are demonstrated.
(1) Measurement of specific surface area The specific surface area was measured using a specific surface area measuring device (manufactured by Shimadzu Corporation). As the preliminary drying, the negative electrode active material was heated to 350 ° C. and exposed to a nitrogen gas flow for 30 minutes. Thereafter, the specific surface area was measured by BET1 point method at a relative pressure of _P / _P 0 = 0.3 by nitrogen gas adsorption.

(2) Measurement of breaking strength The breaking strength is St (Sx) (St (Sx) = 2.8P / (π × d × d) St (Sx): breaking strength [Pa], P: testing force [N] , D: particle diameter [mm]). The breaking strength was measured using a micro compression tester MCT-W500 (manufactured by Shimadzu Corporation).

(3) Measurement graphite spacing _{d 002} of the graphite spacing _{d 002} is measured using a powder X-ray diffractometer (manufactured by Rigaku). First, graphite powder was filled in a holder, and an X-ray diffraction pattern was obtained using CuKα rays monochromatized by a graphite monochromator as a radiation source. Next, the Lorentz / polarization factor, the absorption factor, and the atomic scattering factor were corrected for the obtained (002) plane diffraction pattern of carbon. Subsequently, the peak position of the diffraction pattern is determined by the midpoint of the part where the parallel line drawn parallel to the background at the position of the peak height 2/3 from the background is delimited by the diffraction peak, and the diffraction angle θ and did. After that, this diffraction angle θ is corrected with the diffraction angle obtained by using the diffraction peak of the (111) plane of the high-purity silicon powder for standard material, and the 002 plane graphite plane spacing d is determined by the Bragg formula shown in Equation 1. ₀₀₂ was determined. Here, the wavelength λ of the CuKα ray was 0.15418 nm.
[Formula 1] d ₀₀₂ = λ / 2 sin θ

(4) Measurement median diameter _{D 50} of the median diameter _{D 50} was measured using a powder measuring device MAT-7000 a (manufactured by Seishin Enterprises).

(5) Measurement of discharge capacity using a coin cell battery Using the negative electrode and lithium metal as a counter electrode, charging was performed until the equilibrium potential became 5 mV with respect to lithium at a constant current of 0.1 C, and further from the start of charging. A coin cell battery charged at a constant voltage at the same potential until the time reached 20 hours was prepared. The discharge capacity when this battery was discharged at a constant current of 0.1 C until the equilibrium potential was 1.5 V with respect to lithium was measured and used as the discharge capacity (mAh) of the coin cell. Here, 0.1 C is a current value at which the theoretical capacity can be discharged in 10 hours.

(6) Measurement of discharge capacity using cylindrical can battery First, the cylindrical can battery is charged at a constant current of 0.2 C until the battery voltage reaches 4.2 V, and further, the total charging time from the start of charging. The battery was charged at a constant voltage of 4.2 V until 10 hours. Next, the discharge capacity when this cylindrical battery was discharged at a constant current of 0.2 C until the battery voltage reached 3.0 V was measured, and this was used as the discharge capacity (mAh) of the cylindrical can battery. Here, 0.2 C is a current value at which the theoretical capacity can be discharged in 5 hours.

(7) Measurement of discharge capacity retention rate (%) after 100 cycles Cycle characteristics (%) were measured using a cylindrical can battery. The discharge capacity maintenance rate (%) after 100 cycles was determined as the capacity maintenance rate of the discharge capacity at 100 cycles with respect to the discharge capacity at 1 cycle. In this case, charging for one cycle is performed at a constant current of 1.0 C until the battery voltage reaches 4.2 V, and further, constant voltage charging at 4.2 V until the total charging time from the start of charging reaches 3 hours. did. Discharge was performed at a constant current of 1.0 C until the battery voltage reached 3.0V. Here, 1.0 C is a current value at which the theoretical capacity can be discharged in one hour.

(8) Measurement of volume density The volume density was measured using a cylindrical can battery for the volume density of the negative electrode coating film calculated from the amounts of the graphite-based active material, the binder, and the carbon-based conductive agent of the negative electrode.

[Examples 1-1 to 1-3]
In Examples 1-1 to 1-3, the mesophase system A1 was used as the mesophase graphite spherule A having a high particle hardness, and the mesophase system B1 was used as the mesophase graphite spherule B having a low particle hardness. Table 1 shows the results of evaluating the physical properties of mesophase system A1 and mesophase system B1. The discharge capacity (mAh / g) in Table 1 is measured using a coin cell produced by the above method.

  In Examples 1-1 to 1-3, the mixed graphite in which the mixing ratio (mass ratio) of the mesophase system A1 and the mesophase system B1 is within the range of 40:60 to 60:40 as shown in Table 2 is the negative electrode. A cylindrical can battery was produced using the active material. Moreover, in Comparative Examples 1-1 to 1-4, the mass ratio of the mesophase system A1 to the total of the mesophase system A1 and the mesophase system B1 is 100% by mass, 70% by mass, 30% by mass, and 0% by mass. A cylindrical can battery was produced by changing the process to.

  In Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-4, polyvinylidene fluoride (PVdF), which is a fluorine-based polymer binder resin, was used as a negative electrode binder. In Comparative Example 1-5, rubber-based styrene butadiene rubber (SBR) was used as a binder for the negative electrode. In Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-5, the amount of the binder was set to 5% by mass of the total solid content. Here, the total solid content refers to a portion of the negative electrode excluding the current collector.

  Table 2 shows the results of evaluating the physical properties of the cylindrical can batteries of Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-5.

(Example 1-1)
In Example 1-1, mixed graphite obtained by mixing mesophase system A1 and mesophase system B1 at a mass ratio of 60:40 was used as the negative electrode active material. PVdF was used as the binder for the negative electrode, and the total solid content was 5% by mass. The electrode was pressed with a gauge pressure of 220 kgf / cm ² (1 kgf = 9.80665 N) and a press temperature of 130 ° C. As a result of evaluating the physical properties of the cylindrical can battery, as shown in Table 2, the volume density was 1.83 g / cm ³ . The discharge capacity was 2212 mAh, and the discharge capacity retention rate after 100 cycles was as high as 92.6%.

(Example 1-2)
In Example 1-2, the physical properties of the cylindrical can battery were the same as Example 1-1 except that mixed graphite obtained by mixing mesophase system A1 and mesophase system B1 at a mass ratio of 50:50 was used as the negative electrode active material. Evaluated. As a result, as shown in Table 2, the volume density after pressing was 1.83 g / cm ³ . The discharge capacity was 2220 mAh, and the discharge capacity retention rate after 100 cycles was 91.1%, indicating a high cycle characteristic.

(Example 1-3)
In Example 1-3, the physical properties of the cylindrical can battery were the same as Example 1-1 except that mixed graphite obtained by mixing mesophase system A1 and mesophase system B1 at a mass ratio of 40:60 was used as the negative electrode active material. Evaluated. As a result, as shown in Table 2, the volume density after pressing was 1.83 g / cm ³ . The discharge capacity was 2231 mAh, and the discharge capacity retention rate after 100 cycles was as high as 89.8%.

(Comparative Example 1-1)
In Comparative Example 1-1, the physical properties of a cylindrical can battery were the same as in Example 1-1 except that mixed graphite obtained by mixing mesophase system A1 and mesophase system B1 at a mass ratio of 100: 0 was used as the negative electrode active material. Evaluated. As a result, as shown in Table 2, the volume density after pressing showed a low value of 1.70 g / cm ³ . Further, the discharge capacity is as low as about 2103 mAh, which is the conventional battery capacity level, and it has been found that a particularly high capacity battery cannot be realized. Further, when mixed graphite obtained by mixing mesophase system A1 and mesophase system B1 at a mass ratio of 100: 0 is used as the negative electrode active material and the volume density after pressing is 1.83 g / cm ³ , the adhesive strength is reduced. And could not be used as a battery.

(Comparative Example 1-2)
In Comparative Example 1-2, the physical properties of the cylindrical can battery were the same as Example 1-1, except that mixed graphite obtained by mixing mesophase system A1 and mesophase system B1 at a mass ratio of 70:30 was used as the negative electrode active material. Evaluated. As a result, the volume density after pressing showed a low value of 1.79 g / cm ³ . Further, the discharge capacity was about 2142 mAh, which is lower than that of Example 1-4, which was a conventional battery capacity level, and it was found that a particularly high capacity battery could not be realized.

(Comparative Example 1-3)
In Comparative Example 1-3, the physical properties of the cylindrical can battery were the same as Example 1-1, except that mixed graphite obtained by mixing mesophase system A1 and mesophase system B1 at a mass ratio of 30:70 was used as the negative electrode active material. Evaluated. As a result, as shown in Table 2, the volume density after pressing was 1.83 g / cm ³ . The discharge capacity was 2239 mAh, and the capacity retention after 100 cycles showed a low cycle characteristic of 73.5%.

(Comparative Example 1-4)
In Comparative Example 1-4, the physical properties of the cylindrical can battery were the same as in Example 1-1 except that mixed graphite obtained by mixing mesophase system A1 and mesophase system B1 at a mass ratio of 0: 100 was used as the negative electrode active material. Evaluated. As a result, as shown in Table 2, the volume density after pressing was 1.83 g / cm ³ . The discharge capacity was 2254 mAh, and the capacity retention after 100 cycles showed a low cycle characteristic of 25.3%.

(Comparative Example 1-5)
In Comparative Example 1-5, mixed graphite obtained by mixing mesophase system A1 and mesophase system B1 at a mass ratio of 50:50 was used as a negative electrode active material, rubber-based SBR was used as a binder, and the press temperature was changed. Except for the above, the physical properties of the cylindrical can battery were evaluated in the same manner as in Example 1-1. As a result, as shown in Table 2, the volume density after pressing showed a low value of 1.60 g / cm ³ . Further, the discharge capacity was about 1972 mAh, which was lower than that of Example 1-4, which was a conventional battery capacity level, and it was found that a particularly high capacity battery could not be realized.

  From these results, Comparative Examples 1-1 and 1-2 in which the mass ratio of the mesophase graphite spherule B to the total of the mesophase graphite spherule A and the mesophase graphite spherule B was lower than those of Examples 1-1 to 1-3. Then, it turned out that the volume density of a negative electrode active material layer is low with respect to Examples 1-1 to 1-3, and the discharge capacity of a cylindrical can battery is also low. This is because in Comparative Examples 1-1 and 1-2, the mass ratio of the mesophase graphite spherules B having a large discharge capacity was lowered, so that the particle hardness of the mesophase graphite spherules A having a high particle hardness was reduced between the particles. This is because the low mesophase graphite microspheres B are not efficiently filled.

  In Comparative Examples 1-3 and 1-4 in which the mass ratio of the mesophase graphite small spheres B to the total of the mesophase graphite small spheres A and the mesophase graphite small spheres B was higher than those in Examples 1-1 to 1-3, The discharge capacity of the cylindrical can battery was higher than in Examples 1-1 to 1-3, but the discharge capacity retention rate after 100 cycles was lowered. This is because in Comparative Examples 1-3 and 1-4, the mass ratio of the mesophase graphite small spheres B having a low compressive fracture strength is high, so that the mesophase graphite small spheres B near the electrode surface are crushed by the high-load press pressure. This is because, when the gaps on the electrode surface are filled, the permeability of the electrolytic solution is lowered, the effective edge portion of the graphite is buried, and the orientation of the basal plane in the in-plane direction of the electrode occurs.

  Furthermore, in Comparative Example 1-5 using a rubber-based SBR as a binder, the volume density of the negative electrode active material is lower than in Example 1-1 using a fluorine-based polymer binder resin, and the cylindrical can battery The charging capacity of became low. This is because in Comparative Example 1-5, since SBR was used as the binder, the press temperature could not be raised, and a spring bag phenomenon due to the high elastic modulus of the rubber system occurred immediately after pressing. .

  From these results, the mixing ratio of the mesophase graphite spherule A having a high particle hardness and the mesophase graphite spherule B having a low particle hardness and a high discharge capacity is set within a range of 40:60 to 60:40 by mass ratio. Thus, it was found that the volume density of the negative electrode active material layer can be increased even when a pressure press is used when manufacturing the negative electrode. This is because the graphite material B having a low compressive fracture strength enters the gaps between the mesophase graphite microspheres A having a high compressive fracture strength. In addition, by setting the mixing ratio (mass ratio) of the mesophase graphite spherule A and the mesophase graphite spherule B within the range of 40:60 to 60:40, the osmotic pressure of the electrolyte during the above press pressing Problems such as decline were improved. This is because most of the mesophase graphite spherules B are present in the gaps of the mesophase graphite spherules A, so that the ratio of the mesophase graphite spherules B crushed near the electrode surface by the press is reduced.

  That is, by setting the mixing ratio (mass ratio) of mesophase graphite spherule A and mesophase graphite spherule B within the range of 40:60 to 60:40, the charge capacity is increased while increasing the volume density of the negative electrode active material. It was found that a battery having excellent cycle characteristics can be obtained. Furthermore, it has been found that by using a fluorine-based polymer binder resin as a binder for the negative electrode, it is possible to obtain a negative electrode having a high volume density by suppressing the occurrence of a springback phenomenon when the press temperature is increased.

[Examples 2-1 to 2-4]
In Examples 2-1 to 2-4, any of mesophase systems A1 to A3 was used as mesophase graphite spherule A, and any of mesophase systems B1 to B3 was used as mesophase system graphite spherule B. In Comparative Examples 2-1 to 2-3, as the graphite material having a low particle hardness, scale-like expanded graphite 1 or 2 or spherical natural graphite was used instead of the mesophase-based graphite small spheres B.

  Table 3 shows the evaluation results of the physical properties of mesophase systems A2 and A3, mesophase systems B2 and B3, scaly increased grains 1 and 2, and spherical natural graphite. The discharge capacity (mAh / g) in Table 3 was measured using a coin cell produced by the above method.

  In Examples 2-1 to 2-4, mesophase graphite microspheres A (mesophase systems A1 to A3) and mesophase graphite microspheres B (mesophase systems B1 to B3) are shown in Table 4 so as to have a mass ratio of 50:50. A cylindrical can battery was prepared by mixing with the composition. In Comparative Examples 2-1 to 2-3, the mass ratio of mesophase graphite spherules A (mesophase system A1) and graphite material having low particle hardness (flaky expanded graphite 1-2, spherical natural graphite) is 50:50. Thus, it mixed by the composition shown in Table 4 and produced the cylindrical can battery.

  In Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-3, PVdF was used as a binder for the negative electrode, and the total solid content was 5% by mass. Table 4 shows the results of evaluating the physical properties of the cylindrical can batteries of Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-3.

(Example 2-1)
In Example 2-1, mixed graphite obtained by mixing mesophase system A1 and mesophase system B2 at a mass ratio of 50:50 was used as the negative electrode active material. The electrode was pressed at a press temperature of 130 ° C. with a gauge pressure of 215 kgf / cm ² . As a result of evaluating the physical properties of the cylindrical can battery, as shown in Table 4, the volume density was 1.83 g / cm ³ . The discharge capacity was 2210 mAh, and the discharge capacity retention rate after 100 cycles was as high as 92.0%.

(Example 2-2)
The physical properties of the cylindrical can battery were evaluated in the same manner as in Example 2-1, except that mixed graphite obtained by mixing mesophase system A1 and mesophase system B3 at a mass ratio of 50:50 was used as the negative electrode active material. As a result, as shown in Table 4, the volume density after pressing was 1.83 g / cm ³ . The discharge capacity was 2232 mAh, and the discharge capacity retention rate after 100 cycles was as high as 90.8%.

(Example 2-3)
The physical properties of the cylindrical can battery were evaluated in the same manner as in Example 2-1, except that mixed graphite obtained by mixing mesophase system A2 and mesophase system B1 at a mass ratio of 50:50 was used as the negative electrode active material. As a result, as shown in Table 4, the volume density after pressing was 1.83 g / cm ³ . Further, the discharge capacity was 2216 mAh, and the discharge capacity retention rate after 100 cycles showed a high cycle characteristic of 91.8%.

(Example 2-4)
The physical properties of the cylindrical can battery were evaluated in the same manner as in Example 2-1, except that mixed graphite obtained by mixing mesophase system A3 and mesophase system B1 at a mass ratio of 50:50 was used as the negative electrode active material. As a result, as shown in Table 4, the volume density after pressing was 1.83 g / cm ³ . The discharge capacity was 2222 mAh, and the discharge capacity retention rate after 100 cycles was 91.0%, indicating a high cycle characteristic.

(Comparative Example 2-1)
A cylindrical can battery was evaluated in the same manner as in Example 2-1, except that mixed graphite obtained by mixing mesophase system A1 and scale-like expanded graphite 1 at a mass ratio of 50:50 was used as the negative electrode active material. As a result, as shown in Table 4, the volume density after pressing was 1.83 g / cm ³ . Moreover, since the adhesive strength is lowered due to the large specific surface area of the scale-like expanded graphite 1, the amount of the binder had to be increased from 5% by mass to 13% by mass of the total solid content. As a result, the discharge capacity was as low as 2129 mAh, and the discharge capacity retention rate after 100 cycles showed a low cycle characteristic of 75.6% due to the increase in the binder that inhibits lithium ion migration.

(Comparative Example 2-2)
A cylindrical can battery was evaluated in the same manner as in Example 2-1, except that mixed graphite obtained by mixing mesophase system A1 and scale-like expanded graphite 2 at a mass ratio of 50:50 was used as the negative electrode active material. As a result, as shown in Table 4, the volume density after pressing was 1.83 g / cm ³ . Moreover, since the specific surface area of the scale-like granulated graphite 2 was large, the adhesive strength was lowered, and the amount of the binder had to be increased from 5% by mass to 15% by mass of the total solid content. As a result, the discharge capacity was as low as 2122 mAh, and the discharge capacity retention rate after 100 cycles showed a low cycle characteristic of 80.2% due to the increase in the binder that inhibits lithium ion migration.

(Comparative Example 2-3)
A cylindrical can battery was evaluated in the same manner as in Example 2-1, except that mixed graphite obtained by mixing mesophase system A1 and spherical natural graphite at a mass ratio of 50:50 was used as the negative electrode active material. As a result, as shown in Table 4, the volume density after pressing was 1.83 g / cm ³ . Further, since the specific surface area of spherical natural graphite is large, the adhesive strength is lowered, and the amount of the binder must be increased from 5% by mass to 17% by mass of the total solid content. As a result, the discharge capacity was as low as 2110 mAh, and the discharge capacity retention rate after 100 cycles showed a low cycle characteristic of 69.3% due to an increase in the amount of the binder that inhibits lithium ion migration.

  From these results, Comparative Example 2 in which mesophase graphite spherule A and graphite material having a larger specific surface area than that of mesophase graphite spherule B (scale-like expanded graphite, spherical natural graphite) were mixed at a mass ratio of 50:50. In -1 to 2-3, the amount of the binder is inevitably larger than that of Examples 2-1 to 2-4 in which the mesophase graphite spheres A and the mesophase graphite spheres B are mixed at a mass ratio of 50:50. It was found that the battery capacity and the discharge capacity maintenance rate were reduced. That is, it was found that even when the mesophase graphite microsphere A having a high particle hardness and a graphite material having a high discharge capacity and a low particle hardness were mixed, the battery capacity was lowered if the specific surface area of the graphite material was too large.

  Although the present invention has been described with reference to the examples, it is needless to say that the present invention is not limited to the above-described examples, and various modifications are possible.

It is sectional drawing which represents typically the structure of the negative electrode which concerns on one Embodiment of this invention. It is a disassembled perspective view showing the structure of the secondary battery using the negative electrode shown in FIG. It is sectional drawing showing the structure along the II line | wire of the winding electrode body shown in FIG. It is sectional drawing showing the structure of the cylindrical can battery which concerns on the Example of this invention.

Explanation of symbols

10 ·· Negative electrode, 11 ·· Negative electrode current collector, 12 ·· Negative electrode active material layer, 12A ·· Mesophase graphite spherules, 12B ·· Mesophase graphite spherules, 20 ·· Winded electrode bodies, 21 ·· Positive lead , 22 .. Negative electrode lead, 23... Positive electrode, 23 A... Positive electrode current collector, 23 B .. Positive electrode active material layer, 24 .. Separator, 25 .. Protective tape, 30. , 32 .. Insulating plate, 33 .. Insulating plate, 34 .. Battery cover, 35 .. Safety mechanism, 37 .. Gasket, 40 .. Winding electrode body, 41 .. Positive electrode, 42 .. Negative electrode, 43.・ Separator, 44 ・ ・ Center pin, 45 ・ ・ Positive lead, 46 ・ ・ Negative lead

Claims (6)

A negative electrode comprising a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector,
The negative electrode active material layer includes a negative electrode active material containing mixed graphite of mesophase graphite small spheres A and mesophase graphite small spheres B, and a binder containing a fluorine-based polymer binder resin.
The negative electrode active material layer has a volume density of 1.80 g / cm ³ or more,
The mesophase graphite microsphere A has a specific surface area of 0.3 to 0.7 m ² / g, a fracture strength of 75 MPa or more, a graphite surface interval d ₀₀₂ : 0.3359 nm or more, and a discharge capacity of 335 to 352 mAh / g.
The mesophase graphite microspheres B have a specific surface area of 0.9 to 2.0 m ² / g, a fracture strength of 70 MPa or less, a graphite surface spacing d ₀₀₂ : less than 0.3359 nm, a discharge capacity of 355 to 365 mAh / g,
A negative electrode, wherein a mixing ratio of the mesophase graphite spherules A and the mesophase graphite spherules B is 40:60 to 60:40 by mass ratio. 2. The negative electrode according to claim 1, wherein a median diameter of the mesophase graphite spherule A and the mesophase graphite spherule B is D ₅₀ : 17 to 23 μm.   2. The negative electrode according to claim 1, wherein a true specific gravity of the mesophase graphite spherule A and the mesophase graphite spherule B is mesophase graphite spherule A <mesophase graphite spherule B. 3. A battery comprising an electrolyte together with a positive electrode and a negative electrode,
The negative electrode includes a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector,
The negative electrode active material layer includes a negative electrode active material containing mixed graphite of mesophase graphite small spheres A and mesophase graphite small spheres B, and a binder containing a fluorine-based polymer binder resin.
The negative electrode active material layer has a volume density of 1.80 g / cm ³ or more,
The mesophase graphite microsphere A has a specific surface area of 0.3 to 0.7 m ² / g, a fracture strength of 75 MPa or more, a graphite surface interval d ₀₀₂ : 0.3359 nm or more, and a discharge capacity of 335 to 352 mAh / g.
The mesophase graphite microspheres B have a specific surface area of 0.9 to 2.0 m ² / g, a fracture strength of 70 MPa or less, a graphite surface spacing d ₀₀₂ : less than 0.3359 nm, a discharge capacity of 355 to 365 mAh / g,
A battery, wherein a mixing ratio of the mesophase graphite spherule A and the mesophase graphite spherule B is 40:60 to 60:40 by mass ratio. 5. The battery according to claim 4, wherein a median diameter of the mesophase graphite microspheres A and the mesophase graphite microspheres B is D ₅₀ : 17 to 23 μm. The battery according to claim 4, wherein the true specific gravity of the mesophase graphite spherule A and the mesophase graphite spherule B is mesophase graphite spherule A <mesophase graphite spherule B.

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