JP4152279B2 - Negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery.

  Lithium ion secondary batteries having excellent characteristics such as high voltage and high energy density are now widely used as power sources for electronic devices. As a negative electrode material of a lithium ion secondary battery, graphite having excellent charge / discharge characteristics and high discharge capacity and potential flatness has become the mainstream ([Patent Document 1]). The graphite (graphite particles) used as the negative electrode material includes graphite particles such as natural graphite and artificial graphite, and mesophase-based graphite particles obtained by heat-treating mesophase spherules using tar and pitch as raw materials. Can be mentioned.

  In recent years, with the downsizing and high performance of electronic devices, it has been required to improve rapid charge / discharge characteristics and cycle characteristics without deteriorating discharge capacity and initial charge / discharge efficiency with respect to graphite as a negative electrode material. ing. However, the natural graphite as the negative electrode material has a high discharge capacity, but is easily oriented when the negative electrode is formed due to the scale-like shape. Therefore, the electrolytic solution does not penetrate into the inside of the negative electrode, and the lithium ion storage / release reaction can be performed only on the surface of the negative electrode, so that the rapid charge / discharge characteristics are deteriorated. Furthermore, metallic lithium deposited on the negative electrode surface causes deterioration of cycle characteristics and safety.

  On the other hand, graphite particles obtained by heat-treating mesophase pitch, particularly graphitized particles of mesophase spherules, have a spherical shape or a shape close to a spherical shape, and are randomly laminated when forming the negative electrode. Therefore, the electrolyte solution penetrates into the inside of the negative electrode, and the occlusion / release reaction of lithium ions is performed uniformly, so that good cycle characteristics and rapid charge / discharge characteristics are exhibited. However, in the case of spherical particles, the contact between the particles and the current collector is smaller than that of the scaly particles, so that there is a problem that the electron conductivity is lowered, and there is still a need for improvement in this respect.

  As an improvement measure when using scaly particles, in Patent Document 2, the average particle size of the surface layer of the negative electrode is 1 to 10 μm, and the average particle size of the layer in contact with the current collector of the negative electrode is 10 to 30 μm. An electrode for a non-aqueous electrolyte secondary battery that achieves both electrolyte permeability and electronic conductivity is disclosed. However, the scaly particles having a small particle diameter have a very large specific surface area, and the initial charge / discharge efficiency and safety are reduced by increasing the contact area with the electrolytic solution.

  As an improvement measure in the case of using spherical particles, Patent Document 3 discloses a non-aqueous electrolyte solution containing scaly graphite in the negative electrode active material layer in contact with the current collector and containing spherical graphite in the active material layer on the negative electrode surface. A secondary battery electrode is disclosed. However, when different types of graphite are applied in two layers, the adhesiveness of the joint is deteriorated, and there arises a problem of peeling during repeated charging and discharging.

Japanese Examined Patent Publication No. 62-23433 JP-A-10-27600 JP-A-9-213372

  An object of the present invention is to provide a negative electrode for a lithium ion secondary battery capable of obtaining a lithium ion secondary battery exhibiting excellent rapid charge / discharge characteristics without reducing discharge capacity, initial charge / discharge efficiency and safety, and An object of the present invention is to provide a lithium ion secondary battery that exhibits the above characteristics using a negative electrode.

  The present invention uses an electrode containing graphite particles, especially spherical graphitized particles as an active material, and having different particle diameters of graphite particles in the thickness direction of the active material layer as a negative electrode for a lithium ion secondary battery. It was achieved by knowing that the problem could be solved.

The present invention is a negative electrode for a lithium ion secondary battery comprising an active material layer containing graphite particles as an active material, and a current collector that holds the active material layer, on the current collector side of the active material layer the average particle diameter of the graphite particles that contain the rather smaller than the average particle diameter of the graphite particles contained in the negative electrode surface side of the active material layer, the graphite particles contained in the current collector side of the active material layer The graphite has an average particle size of 1 to 20 μm, the average particle size of the graphite particles contained on the negative electrode surface side of the active material layer is more than 20 μm and 40 μm or less, and the graphite contained on the current collector side of the active material layer The negative electrode for a lithium ion secondary battery is characterized in that the ratio of the solid particles to the entire graphite particles of the active material layer is 10 to 30% by mass .

  In the negative electrode for a lithium ion secondary battery of the present invention, it is preferable that the graphite particles include mesophase small sphere graphitized particles.

  The present invention also provides a lithium ion secondary battery using any one of the negative electrodes for lithium ion secondary batteries.

  When the negative electrode for a lithium ion secondary battery of the present invention is used, a lithium ion secondary battery exhibiting excellent rapid charge / discharge characteristics can be obtained without reducing the discharge capacity and initial charge / discharge efficiency. In addition, the lithium ion secondary battery of the present invention has a high discharge capacity, a high initial charge / discharge efficiency, and an excellent rapid charge / discharge characteristic. Therefore, it is effective for downsizing and improving the performance of various electronic devices equipped with the lithium ion secondary battery. It is.

The present invention is described in further detail below.
In general, a lithium ion secondary battery includes a nonaqueous electrolyte, a negative electrode, and a positive electrode as main battery components, and these elements are enclosed in a battery can, for example. The negative electrode and the positive electrode each act as a lithium ion carrier. The battery mechanism is such that lithium ions are occluded in the negative electrode during charging, and lithium ions are released from the negative electrode during discharging.

(Negative electrode)
The negative electrode for a lithium ion secondary battery of the present invention comprises graphite particles as an essential component as an active material, an active material layer containing the active material, and a current collector that holds the active material layer. In particular, the graphite particles preferably include mesophase small sphere graphitized particles. In addition to graphitized particles of mesophase spherules, those obtained by processing and granulating natural graphite, and those coated with a carbonaceous material such as resin or pitch can also be used.

In the present invention, the shape of the graphite particles is not particularly limited, and those having a spherical shape, a block shape, a scale shape and the like can be used. Of these, spherical ones are preferred. Here, the term “spherical” refers to those in which the ratio of the major axis of the graphite particles to the minor axis orthogonal thereto (aspect ratio: major axis / minor axis) is 2 or less, particularly preferably 1.5 or less.
When the graphite particles are spherical, an appropriate gap can be secured in the negative electrode, and the permeability of the electrolytic solution is not lowered. As a result, rapid charge / discharge characteristics are improved. Moreover, since the content of the active material can be increased while maintaining the specific surface area small by using the spherical particles, the initial charge / discharge efficiency is not lowered.

  An example of a method for producing graphitized particles of mesophase spherules is as follows. Optically anisotropic spherules produced in pitch when heat treating coal-based or petroleum-based pitches at 350 to 450 ° C. After the heat treatment at 350 to 600 ° C. under flow, the heat treatment (firing and graphitization) is finally performed at a temperature of 1000 ° C. or higher, preferably 2000 to 3200 ° C.

  In the negative electrode for a lithium ion secondary battery of the present invention, the average particle size of the graphite particles contained in the current collector side of the active material layer is included in the anti-current collector side of the active material layer, that is, the negative electrode surface side. It is necessary to be smaller than the average particle size of the graphite particles. That is, by arranging particles having a smaller particle size on the current collector side of the active material layer than on the negative electrode surface side, the adhesion is increased by increasing the contact between the graphite particles and the current collector, and the electron conductivity And quick charge / discharge characteristics are improved. In addition, the negative electrode surface side particles have a larger specific particle size than the current collector side particles, so the specific surface area is small, the contact area with the electrolyte solution is small, and the initial charge / discharge efficiency is reduced due to the decomposition reaction of the electrolyte solution. And a decrease in safety can be suppressed. Further, since the graphite particles themselves are not subjected to any special treatment that changes their properties, the high discharge capacity derived from the graphite is not impaired. Here, the current collector side is preferably in the range of at least 10 μm from the surface of the active material layer in contact with the current collector, and the negative electrode surface side is preferably in the range of at least 10 μm from the negative electrode surface. The average particle diameter of the graphite particles included in this range can be compared by observing the cross section of the active material layer using a scanning electron microscope.

The negative electrode for a lithium ion secondary battery of the present invention described above can be produced as follows. That is, after applying a negative electrode mixture paste in which a binder and a solvent described later are added to graphite particles having an average particle diameter of 1 to 20 μm, particularly preferably 1 to 15 μm, on one or both surfaces of the current collector, the average particle diameter superimposed 20μm greater, 40 [mu] m or less, particularly preferably by applying a negative electrode mixture paste containing 20μm greater, 35 [mu] m below graphite particles, drying, after pressurizing, molded into a predetermined size.

When the average particle diameter of the graphite particles applied to one or both sides of the current collector is smaller than 1 μm, the specific surface area increases, and a large amount of solvent is required when preparing the negative electrode mixture paste described later. May be significantly reduced. Further, the initial charge / discharge efficiency is lowered, which is disadvantageous in terms of safety. When the average particle diameter exceeds 20 μm, it is difficult to ensure sufficient contact with the current collector, and the rapid charge / discharge characteristics may be deteriorated.
Further, when the average particle diameter of the graphite particles to be further applied is less than 20 μm, the penetration of the electrolytic solution is hindered, and the rapid charge / discharge efficiency may be lowered. Moreover, when this average particle diameter exceeds 40 micrometers, when producing the negative mix paste mentioned later, there exists a possibility that the particle | grains which are easy to settle increase and it may become difficult to produce the negative electrode with which the particle | grains were disperse | distributed uniformly. In this production method, the average particle diameter means a particle diameter at which the cumulative frequency of the particle size distribution measured by the laser diffraction method is 50% by volume.

  In the present invention, the thickness of the entire active material layer is preferably 50 to 150 μm. In addition, as described above, the active material layer formed by applying a negative electrode mixture paste containing graphite particles having a small average particle diameter on a current collector and drying is 5 to 50 μm, and the average particle diameter is The active material layer formed by further applying a negative electrode mixture paste containing large graphite particles is preferably applied so as to have a thickness of 20 to 100 μm. Further, it may have a gradient particle size distribution between the two layers of this double structure.

Furthermore, in the above-described method for producing a negative electrode for a lithium ion secondary battery, the ratio of the active material layer of the graphite particles having a small average particle diameter applied on the current collector to the entire graphite particles is 10 to 30% by mass. To be manufactured . If this ratio is less than 10% by mass, it is difficult to ensure a sufficient contact between the graphite particles and the current collector, and if it exceeds 30% by mass, the penetration of the electrolytic solution is hindered. Leads to.

The production of the negative electrode can be performed according to a normal molding method, but is not limited as long as it is a method capable of obtaining a chemically and electrochemically stable negative electrode.
Moreover, the negative electrode mixture which added the binder to the said graphite particle can be used at the time of manufacture of a negative electrode. As the binder, those having chemical stability and electrochemical stability with respect to the electrolyte are preferably used. For example, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyethylene, polyvinyl alcohol, For example, carboxymethylcellulose is used. These can also be used together. In general, the binder is preferably used in an amount of about 1 to 20% by mass in the total amount of the negative electrode mixture.

As a specific example of manufacturing a negative electrode, a negative electrode mixture is prepared by mixing the graphite particles as an active material with a binder, and this negative electrode mixture is usually applied to one or both sides of a current collector. Thus, an active material layer can be formed to form a negative electrode.
For production of the negative electrode, a known solvent for producing the negative electrode can be used. That is, when a solvent is added to the negative electrode mixture to form a negative electrode mixture paste, which is applied to the current collector and dried, the active material layer is uniformly and firmly adhered to the current collector. More specifically, for example, after mixing and kneading the graphite particles and fluorine-based resin powder such as polytetrafluoroethylene in a solvent such as isopropyl alcohol, the obtained negative electrode mixture paste is applied, What is necessary is just to dry.

  A negative electrode mixture paste obtained by mixing the graphite particles with a fluorine-based resin powder such as polyvinylidene fluoride or a water-soluble binder such as carboxymethyl cellulose with a solvent such as N-methylpyrrolidone, dimethylformaldehyde or water, alcohol, etc. This can also be applied.

Also, the graphite particles and resin powder such as polyethylene and polyvinyl alcohol can be dry mixed and hot press molded in a mold.
When pressurization such as pressing is performed after forming the active material layer, the adhesive strength between the active material layer and the current collector can be further increased. Furthermore, the negative electrode of the present invention can be obtained by molding into a desired negative electrode shape after pressurization.

  The shape of the current collector used for the negative electrode is not particularly limited, and examples thereof include a foil shape or a net shape such as a mesh or an expanded metal. Examples of the material for the current collector include copper, stainless steel, and nickel. In the case of a foil shape, the thickness of the current collector is preferably about 5 to 20 μm.

Moreover, this invention is also a lithium ion secondary battery formed using the said negative electrode for lithium ion secondary batteries.
The lithium ion secondary battery of the present invention is not particularly limited except that the negative electrode is used, and other battery components conform to the elements of a general lithium ion secondary battery.

(Positive electrode)
As the positive electrode material (positive electrode active material) used for the lithium ion secondary battery of the present invention, a lithium compound is used, but it is preferable to select a material that can occlude and release a sufficient amount of lithium. Such lithium compounds include lithium-containing transition metal oxides, transition metal chalcogenides, vanadium oxides and lithium-containing compounds such as lithium compounds thereof, general formula M ¹ _x M ² O ₆ S _8-y (where X is A number of 0 ≦ X ≦ 4, Y is a number of 0 ≦ Y ≦ 1, and M ¹ and M ² represent a metal such as a transition metal), activated carbon, activated carbon fiber, and the like. Examples of the vanadium oxide include those represented by V ₂ O ₅ , V ₆ O ₁₃ , V ₂ O ₄ , and V ₃ O ₈ .

The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and may be a composite oxide in which lithium and two or more transition metals are dissolved. The composite oxide may be used alone or in combination of two or more. Specifically, the lithium-containing transition metal oxide is LiM ¹ _1-x M ² _x O ₂ (wherein X is a number _satisfying 0 ≦ X ≦ 1, and M ¹ and M ² are at least one kind of transition metal element) Or LiM ¹ _2-y M ² _y O ₄ (wherein Y is a number satisfying 0 ≦ Y ≦ 1, and M ¹ and M ² are at least one transition metal element). In the formula, transition metal elements represented by M ¹ and M ² are Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn and the like, and preferred specific examples are LiCoO ₂ and LiNiO _2. LiMnO ₂ , LiNi _0.9 Co _0.1 O ₂ , LiNi _0.5 Mn _0.5 O ₂ and the like.

  The lithium-containing transition metal oxide is, for example, lithium, a transition metal oxide, salt or hydroxide as a starting material, and these starting materials are mixed according to the composition of the desired metal oxide, and an oxygen-containing atmosphere It can be obtained by firing at a temperature of 600 to 1000 ° C. below.

  In the lithium ion secondary battery of the present invention, the positive electrode active material may be used alone or in combination of two or more. Moreover, carbonates, such as lithium carbonate, can also be added in a positive electrode.

  The positive electrode can be obtained, for example, by forming a positive electrode mixture layer by applying a positive electrode mixture comprising the above compound, a binder and a conductive agent to one or both sides of a current collector. As the binder, any of those exemplified for the negative electrode can be used. As the conductive agent, a carbon material such as graphite or carbon black is used.

  Similarly to the negative electrode, the positive electrode mixture may be formed into a positive electrode mixture paste by dispersing the positive electrode mixture in a solvent, and this positive electrode mixture paste may be applied to a current collector and dried to form a positive electrode mixture layer. In addition, after forming the positive electrode mixture layer, pressurization such as pressing may be further performed. Thereby, the positive electrode mixture layer is uniformly and firmly bonded to the current collector.

  The shape of the current collector is not particularly limited, and a foil shape or a net shape such as a mesh or an expandable metal is used. Examples of the material of the current collector include aluminum foil, stainless steel foil, and nickel foil. The thickness is preferably 10 to 40 μm.

(Electrolytes)
The electrolyte used in the lithium ion secondary battery of the present invention is an organic electrolyte composed of a solvent and an electrolyte salt, or a polymer electrolyte composed of a polymer compound and an electrolyte salt. As the electrolyte salt, _{e.g., LiPF 6, LiBF 4, LiAsF} 6, LiClO 4, LiB (C 6 H 5), LiCl, LiBr, LiCF 3 SO 3, LiCH 3 SO 3, LiN (CF 3 SO 2) 2, LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ CH ₂ OSO ₂ ) ₂ , LiN (CF ₃ CF ₃ OSO ₂ ) ₂ , LiN (HCF ₂ CF ₂ CH ₂ OSO ₂ ) ₂ , LiN ((CF ₃ ) ₂ CHOSO ₂ ) ₂ , LiB [(C ₆ H ₃ (CF ₃ ) ₂ ] ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiAlCl ₄ , LiSiF ₆ and other lithium salts are used. In particular, LiPF ₆ and LiBF ₄ are preferred from the viewpoint of oxidation stability, and the electrolyte salt concentration in the organic electrolyte is preferably 0.1 to 5 mol / l, and preferably 0.5 to 3.0 mol / l. l better to be Arbitrariness.

  Examples of the organic electrolyte solvent include carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and diethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran. , Γ-butyrolactone, 1,3-dioxofuran, 4-methyl-1,3-dioxolane, ethers such as anisole and diethyl ether, thioethers such as sulfolane and methylsulfolane, nitriles such as acetonitrile, chloronitrile and propionitrile, boric acid Trimethyl, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzo bromide Le, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazoline, ethylene glycol, can be used sulfite, aprotic organic solvents such as dimethyl sulfite.

  When the nonaqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte or a polymer gel electrolyte, a polymer gelled with a plasticizer (nonaqueous electrolyte) is used as a matrix. Examples of the polymer compound constituting the matrix include ether polymer compounds such as poly (ethylene oxide) and cross-linked products thereof, methacrylate polymer compounds such as polymethacrylate, acrylate polymer compounds such as polyacrylate, and polyvinylidene. Fluorine polymer compounds such as fluoride (PVDF) and vinylidene fluoride-hexafluoropropylene copolymer are preferred. These can also be mixed and used. From the viewpoint of oxidation-reduction stability, a fluorine-based polymer compound such as PVDF or vinylidene fluoride-hexafluoropropylene copolymer is particularly preferable.

  A plasticizer is blended in the polymer solid electrolyte or the polymer gel electrolyte. As the plasticizer, the electrolyte salt and the non-aqueous solvent can be used. In the case of a polymer gel electrolyte, the concentration of the electrolyte salt that is a plasticizer in the non-aqueous electrolyte is preferably 0.1 to 5 mol / l, more preferably 0.5 to 2.0 mol / l. .

The method for producing the solid electrolyte is not particularly limited. For example, the polymer compound constituting the matrix, the lithium salt and the nonaqueous solvent (plasticizer) are mixed and heated to melt the polymer compound. Method of evaporating organic solvent after dissolving molecular compound, lithium salt and non-aqueous solvent (plasticizer), and mixing of polymerizable monomer, lithium salt and non-aqueous solvent (plasticizer) as raw material for polymer electrolyte And a method of producing a polymer electrolyte by polymerizing the mixture by irradiating the mixture with ultraviolet rays, electron beams or molecular beams.
Moreover, 10-90 mass% is preferable and, as for the addition rate of the nonaqueous solvent (plasticizer) in the said solid electrolyte, 30-80 mass% is more preferable. If it is less than 10% by mass, the electrical conductivity will be low, and if it exceeds 90% by mass, the mechanical strength will be weak and film formation will be difficult.

(Separator)
In the lithium ion secondary battery of the present invention, a separator can be used. Although the material of a separator is not specifically limited, A woven fabric, a nonwoven fabric, a synthetic resin microporous film, etc. are illustrated. A synthetic resin microporous membrane is preferred, and among these, a polyolefin-based microporous membrane is preferred from the standpoints of film thickness, membrane strength, membrane resistance, and the like. Specifically, it is a microporous film made of polyethylene and polypropylene, or a microporous film in which these are combined.

In the lithium ion secondary battery of the present invention, a gel electrolyte can be used when mesophase spherules that are not exposed at the end face of the layered structure called the graphite edge face are used as the negative electrode graphite material.
The gel electrolyte secondary battery is configured by laminating the negative electrode containing the graphite particles, the positive electrode, and the gel electrolyte in the order of, for example, the negative electrode, the gel electrolyte, and the positive electrode, and accommodating them in the battery exterior material. . Further, a gel electrolyte may be disposed outside the negative electrode and the positive electrode. In particular, in the gel electrolyte secondary battery used for the negative electrode of the present invention, the gel electrolyte can contain propylene carbonate. In general, propylene carbonate has a strong electrolysis reaction with respect to graphite, but has a low decomposition reactivity with respect to the negative electrode of the present invention.

  Furthermore, the structure of the lithium ion secondary battery of the present invention is not particularly limited, and is not particularly limited with respect to its shape and form, depending on the application, mounted equipment, required charge / discharge capacity, etc., cylindrical type, Any shape or form of a square shape, a coin shape, or a button shape may be used. In order to obtain a sealed nonaqueous electrolyte battery with higher safety, it is preferable to provide means for detecting an increase in the internal pressure of the battery and shutting off the current when an abnormality such as overcharge occurs. In the case of a polymer solid electrolyte battery or a polymer gel electrolyte battery, a structure enclosed in a laminate film can also be used.

Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples. In addition, an Example and a comparative example are experiment in the evaluation battery for single electrode evaluation by which the battery system was comprised by the working electrode (negative electrode) containing a graphite particle, and the counter electrode (positive electrode) which consists of lithium foil. A real battery can be manufactured according to a well-known method based on the objective of this invention.
In addition, the average particle diameter of the used graphite particle | grains was made into the particle diameter from which the cumulative frequency of the particle size distribution measured with the laser diffraction type particle size distribution meter becomes 50% by volume as mentioned above. In addition, the aspect ratio of the graphite particles used was 300 times that of the graphite particles and image processing was performed using an image analyzer (manufactured by Toyobo Co., Ltd.). The average value of the ratio (ratio of the major axis to the minor axis in the orthogonal direction) was used.

(Example 1)
(Production of working electrode (negative electrode))
Mesophase spherules (manufactured by JFE Chemical Co., Ltd., KMFC) were heated to 3000 ° C. and graphitized to obtain graphite particle powder. The powder was classified and the particle size was adjusted to obtain two types of graphite particles having an aspect ratio of 1.1 and different average particle sizes (10 μm and 23 μm). Polyvinylidene fluoride was mixed as a binder so that the content of the binder was 4% by mass to each obtained graphite particle powder, and a solvent N-methylpyrrolidone was further added thereto, and a homomixer was used. Then, two kinds of organic solvent-based negative electrode mixture pastes were prepared by stirring for 30 minutes at a rotational speed of 2000 rpm.

A negative electrode mixture paste containing graphite particles having an average particle diameter of 10 μm was applied on a current collector (copper foil), and then a negative electrode mixture paste containing graphite particles having an average particle diameter of 23 μm was further applied. . Thereafter, the solvent was volatilized at 90 ° C. in vacuum and dried to form a negative electrode mixture layer. Thereafter, pressure is applied by a hand press, a copper foil and an active material layer are punched into a cylindrical shape having a diameter of 15.5 mm, and a working electrode comprising a current collector and an active material layer having a thickness of 80 μm held by the current collector (Negative electrode) was produced. The ratio of the graphite particles in the active material layer (current collector side) containing graphite particles having an average particle diameter of 10 μm to the graphite particles in the entire active material layer was set to 15% by mass.
By observing the cross section of the active material layer after pressurization with a scanning electron microscope (magnification 500 times), the thickness of the entire active material layer and the maximum particle size of the graphite particles are up to 10 μm on the current collector side and 50 μm on the negative electrode surface side. Each was measured. The average value of 10 visual fields of the maximum particle diameter of the graphite particles was defined as the average particle diameter.

(Preparation of counter electrode (positive electrode))
The lithium metal foil was pressed against a nickel net and punched into a circular shape with a diameter of 15.5 mm to produce a current collector made of nickel net and a counter electrode made of lithium metal foil in close contact with the current collector.

(Electrolyte, separator)
LiPF ₆ was dissolved at a concentration of 1 mol / dm ^{3 in a} mixed solvent obtained by mixing 33 vol% ethylene carbonate and 67 vol% methyl ethyl carbonate to prepare a non-aqueous electrolyte. The obtained nonaqueous electrolytic solution was impregnated into a polypropylene porous body to produce a separator impregnated with the electrolytic solution.

(Evaluation battery)
A button-type secondary battery shown in FIG. 1 was produced as an evaluation battery.
The exterior cup 1 and the exterior can 3 have a sealed structure that is caulked with an insulating gasket 6 at the peripheral edge thereof. From the side of the outer can 3, the disc-shaped counter electrode (positive electrode) 9 made of nickel net, the lithium foil 4, the separator 5 impregnated with the electrolyte, the active material layer 2, and the copper foil A disc-shaped working electrode (negative electrode) 8 made of a current collector 7b is laminated.

In the evaluation battery, the separator 5 impregnated with the electrolytic solution was laminated between the working electrode (negative electrode) 8 and the counter electrode (positive electrode) 9, and then the working electrode (negative electrode) 8 was placed in the exterior cup 1 with the counter electrode ( The positive electrode) 9 was accommodated in the outer can 3, the outer cup 1 and the outer can 3 were combined, and the outer cup 1 and the outer can 3 were caulked and sealed with an insulating gasket 6.
The evaluation battery is a battery composed of a working electrode (negative electrode) 8 containing graphite particles that can be used as a negative electrode active material and a counter electrode (positive electrode) 9 in an actual battery.

(Charge / discharge test)
The evaluation battery was subjected to the following charge / discharge test at a temperature of 25 ° C.
The constant current charging was performed until the circuit voltage reached 0 mV at a current value of 0.9 mA. When the circuit voltage reached 0 mV, switching was made to constant voltage charging, and the charging was continued until the current value reached 20 μA. The charging capacity was determined from the amount of electricity applied during that time. Then, it rested for 120 minutes. Next, constant current discharge was performed until the circuit voltage reached 1.5 V at a current value of 0.9 mA, and the discharge capacity was determined from the amount of current applied during this period. This was the first cycle. The initial charge / discharge efficiency was calculated from the following equation. In this test, the process of occluding lithium ions in the graphite particles was charged and the process of detaching was regarded as discharge.

Initial charge / discharge efficiency (%) = (first cycle discharge capacity / first cycle charge capacity) ×
100
Here, the charge / discharge rate at which 100% of the discharge capacity is charged or discharged in one hour is referred to as 1C. That is, 0.1 C if it takes 10 hours to charge or discharge 100% of the discharge capacity, and 2 C if it takes 30 minutes.
Charging was performed at a rate of 0.5 C, and quick charging efficiency was calculated from the following equation.
Rapid charge efficiency (%) = (constant current charge capacity at 0.5 C / discharge capacity of the first cycle) × 100
Moreover, discharge was performed at a rate of 1.5 C, and the rapid discharge efficiency was calculated from the following equation.
Rapid discharge efficiency (%) = (constant current discharge capacity at 1.5 C / first cycle discharge capacity) × 100
Table 1 shows the evaluation results for battery characteristics (discharge capacity, initial charge / discharge efficiency, rapid charge efficiency, and rapid discharge efficiency).

(Examples 2 and 3, Comparative Examples 3 and 4 )
In Example 1, the average particle diameter of the graphite particles of the negative electrode mixture paste containing graphite particles having a small average particle diameter, the average particle diameter of the graphite particles of the negative electrode mixture paste containing graphite particles having a large average particle diameter The negative electrode mixture paste was the same as in Example 1, except that the ratio of the negative electrode mixture paste containing graphite particles having a small average particle diameter to the total graphite particles of the graphite particles was changed as shown in Table 1. And a negative electrode and a lithium ion secondary battery were produced. Table 1 shows the evaluation results for the battery characteristics.

(Example 4 )
Instead of mesophase spheres, petroleum-based raw coke particles were mechanically processed to prepare spheres. This was oxidized in air at 350 ° C., heated to 3000 ° C. and graphitized to obtain spherical artificial graphite powder. The powder was classified and the particle size was adjusted to obtain two types of graphite particles having an aspect ratio of 1.3 and different average particle sizes (13 μm and 28 μm). Using this, the ratio of the graphite particles in the active material layer (current collector side) containing graphite particles having an average particle diameter of 13 μm to the graphite particles in the entire active material layer is changed as shown in Table 1. Prepared a negative electrode mixture paste in the same manner as in Example 1 to produce a negative electrode and a lithium ion secondary battery. Table 1 shows the evaluation results for the battery characteristics.

(Comparative Examples 1-2)
In Example 1, as shown in Table 1, the used graphite particles are changed so that the average particle diameter of the current-side graphite particles is the same as or larger than the average particle diameter of the surface-side graphite particles. Except for the above, a negative electrode mixture paste was prepared in the same manner as in Example 1, and a negative electrode and a lithium ion secondary battery were produced. Table 1 shows the evaluation results for the battery characteristics.

The lithium ion secondary batteries of Examples 1 to 4 have a high discharge capacity because the average particle diameter of the graphite particles on the current collector side is smaller than the average particle diameter of the graphite particles on the surface side, and high initial charge / discharge It turns out that it has high rapid charge / discharge efficiency compared with the comparative examples 1 and 2 with efficiency.

It is sectional drawing which shows the structure of the button type evaluation battery for using for a charging / discharging test.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Exterior cup 2 Active material layer 3 Exterior can 4 Lithium foil 5 Separator 6 Insulating gasket 7a, 7b Current collector 8 Working electrode (negative electrode)
9 Counter electrode (positive electrode)

Claims (4)

A negative electrode for a lithium ion secondary battery comprising an active material layer containing graphite particles as an active material and a current collector that holds the active material layer, the graphite contained on the current collector side of the active material layer the average particle diameter of quality particles, an average particle diameter of the negative electrode surface rather smaller than the average particle diameter of the graphite particles contained, the graphite particles contained in the current collector side of the active material layer of the active material layer 1 to 20 μm, the average particle diameter of the graphite particles contained on the negative electrode surface side of the active material layer is more than 20 μm and 40 μm or less, and the graphite particles contained on the current collector side of the active material layer, A negative electrode for a lithium ion secondary battery, wherein the ratio of the active material layer to the entire graphite particles is 10 to 30% by mass . The negative electrode for a lithium ion secondary battery according to claim 1, wherein the active material layer has a thickness of 50 to 150 μm . Claim 1 or negative electrode for a lithium ion secondary battery according to 2 wherein the graphite particles is characterized in that it comprises a graphite product particles mesophase microspheres. A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to claim 1.

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