JP7102831B2 - Positive electrode and lithium ion secondary battery - Google Patents

Description

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

Lithium-ion secondary batteries are lighter and have higher capacity than nickel-cadmium batteries, nickel-metal hydride batteries, and the like, and are therefore widely used as power sources for portable electronic devices. It is also a promising candidate for power sources installed in hybrid vehicles and electric vehicles. With the recent miniaturization and higher functionality of portable electronic devices, it is expected that the capacity of lithium ion secondary batteries, which are the power sources for these devices, will be further increased.

In particular, lithium ion secondary batteries using a layered lithium-containing transition metal composite oxide as a positive electrode active material are now widely used because they can have a higher energy density than conventional batteries. ing. However, the current lithium-ion secondary batteries are desired to have various characteristics improved due to diversification of applications.

One of the problems of the lithium ion secondary battery using the lithium-containing transition metal composite oxide having a layered structure is to improve the output characteristics. Various methods have been tried for the purpose of improving the output characteristics, and one of them is to use graphene or multi-layer graphene for the positive electrode.

For example, Patent Document 1 discloses a positive electrode material for a lithium secondary battery in which a granular positive electrode active material is coated with graphene to improve electron conductivity and ionic conductivity.

JP-A-2017-135105

However, further improvement in output characteristics is required.

The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a positive electrode and an ion secondary battery capable of improving output characteristics.

The present inventor has made extensive studies in order to solve the above problems.
The present inventor has found that the output characteristics of a lithium ion secondary battery can be improved when predetermined fine particles are attached to the surface of graphene or multilayer graphene contained in the positive electrode active material layer.
That is, in order to solve the above problems, the following means are provided.

(1) The positive electrode according to the first aspect has a positive electrode current collector and a positive electrode active material layer provided on the surface of the positive electrode current collector, and the positive electrode active material layer has the following composition formula. The positive electrode active material includes the positive electrode active material represented by (1) and graphene or multilayer graphene, and the positive electrode active material is composed of particles having a particle size of 1/2 or less of the average particle size Da of the graphene or the multilayer graphene. The particle group A1 is formed, and at least a part of the particle group A1 is attached to the surface of the graphene or the multilayer graphene.
Li _a Ni _b Co _c Mn _d (M) _e O ₂ (1)
(However, M indicates at least one selected from the group consisting of Al, Si, Zr, Ti, Fe, Mg, Nb, Ba and V, and 1.90 ≦ (a + b + c + d + e) ≦ 2.2, 0 <a. ≦ 1.3, 0.5 ≦ b ≦ 1.0, 0 ≦ c ≦ 1.0, 0 ≦ d ≦ 0.7, 0 ≦ e ≦ 0.2)

(2) In the positive electrode according to the above aspect, the positive electrode active material further has a particle group A2 composed of particles having a particle size larger than the average particle size Da of the graphene or the multilayer graphene, and the particle group A1. At least a part may be present between the graphene or the multilayer graphene and the particle group A2.

(3) In the positive electrode according to the above aspect, the average particle diameter Db of the adhered particles adhering to the surface of the graphene or the multilayer graphene may be 3 μm or less.

(4) In the positive electrode according to the above aspect, the average particle size Da of the graphene or the multilayer graphene may be 3 to 25 μm.

(5) In the positive electrode according to the above aspect, the average particle size Da of the graphene or the multilayer graphene and the average particle size Db of the adhered particles adhering to the surface of the graphene or the multilayer graphene have Da / Db ≧ 2. May be satisfied.

(6) The lithium ion secondary battery according to the second aspect has a positive electrode according to the above aspect.

In the positive electrode according to the above aspect, predetermined fine particles are attached to the surface of graphene or multilayer graphene contained in the positive electrode active material layer, so that the output characteristics of the lithium ion secondary battery are improved.

It is sectional drawing of the lithium ion secondary battery which concerns on this embodiment. It is a scanning electron microscope (SEM) image of the positive electrode active material which concerns on this embodiment.

Hereinafter, the present embodiment will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may be enlarged for convenience in order to make the features of the present invention easy to understand, and the dimensional ratios of the respective components may differ from the actual ones. be. The materials, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited thereto, and the present invention can be appropriately modified without changing the gist thereof.

[Lithium-ion secondary battery]
FIG. 1 is a schematic cross-sectional view of the lithium ion secondary battery 100 according to the present embodiment. The lithium ion secondary battery 100 shown in FIG. 1 mainly includes a laminate 40, an exterior body 50 that houses the laminate 40 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 40. Although not shown, the electrolytic solution is housed in the exterior body 50 together with the laminated body 40.

In the laminated body 40, the positive electrode 20 and the negative electrode 30 are arranged so as to face each other with the separator 10 interposed therebetween. The positive electrode 20 has a positive electrode active material layer 24 provided on a plate-shaped (film-shaped) positive electrode current collector 22. The negative electrode 30 has a negative electrode active material layer 34 provided on a plate-shaped (film-shaped) negative electrode current collector 32.

The positive electrode active material layer 24 and the negative electrode active material layer 34 are in contact with both sides of the separator 10, respectively. Leads 62 and 60 are connected to the ends of the positive electrode current collector 22 and the negative electrode current collector 32, respectively, and the ends of the leads 60 and 62 extend to the outside of the exterior body 50. In FIG. 1, a case where one laminated body 40 is provided in the exterior body 50 is illustrated, but a plurality of laminated bodies 40 may be laminated.

"Positive electrode"
The positive electrode 20 has a positive electrode current collector 22 and a positive electrode active material layer 24 provided on the surface of the positive electrode current collector 22.

(Positive current collector)
The positive electrode current collector 22 may be any conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

(Positive electrode active material layer)
The positive electrode active material layer 24 contains the positive electrode active material represented by the following composition formula (1), graphene or multi-layer graphene, and may contain a binder if necessary. As can be seen from the composition formula (1), the positive electrode active material contains Li and Ni, and graphene or multi-layer graphene plays the role of a positive electrode conductive material.
Li _a Ni _b Co _c Mn _d (M) _e O ₂ (1)
(However, M indicates at least one selected from the group consisting of Al, Si, Zr, Ti, Fe, Mg, Nb, Ba and V, and 1.90 ≦ (a + b + c + d + e) ≦ 2.2, 0 <a. ≦ 1.3, 0.5 ≦ b ≦ 1.0, 0 ≦ c ≦ 1.0, 0 ≦ d ≦ 0.7, 0 ≦ e ≦ 0.2)

The term "multilayer graphene" means graphene having a laminated structure of about 50 layers or less, and particularly includes graphene having a laminated structure of about 2 to 30 layers.

FIG. 2 shows a scanning electron microscope (SEM) image of the powder for the positive electrode active material layer according to the present embodiment. The positive electrode active material has a particle group A1 composed of particles having a particle size of 1/2 or less of the average particle size Da of graphene or multilayer graphene G contained in the positive electrode active material layer 24.

The particle size of graphene or multilayer graphene G and the positive electrode active material can be determined by measuring with SEM. Specifically, it is calculated as follows. First, 10 images of secondary electron images and backscattered electron images at a magnification of 1000 times the cross section of the positive electrode are taken. Since the binder appears black in the secondary electron image, the image is binarized and the positive electrode active material and graphene or multi-layer graphene G are extracted. Next, the backscattered electron image is binarized, and the positive electrode active material and graphene or multi-layer graphene G are extracted, respectively. The particle size of all the extracted particles is measured by image processing. The same operation is performed on 10 images, and the particle size of graphene or multilayer graphene G and the particle size of the positive electrode active material are determined. The graphene or multi-layer graphene G has a flat shape, and the particle size means the width (major diameter) of graphene or multi-layer graphene G orthogonal to the stacking direction of graphene or multi-layer graphene G.

Alternatively, the particle size of graphene or multilayer graphene G and the positive electrode active material may be measured after separating graphene and the positive electrode active material from the positive electrode. Specifically, the positive electrode taken out from the battery is put in a solvent for dissolving the binder and stirred, and the positive electrode constituent material released in the solvent is taken out by filtration. Then, by observing the graphene or multilayer graphene G separated from the positive electrode constituent material and the positive electrode active material particles with an electron microscope, the particle size of the graphene or multilayer graphene G and the positive electrode active material can be determined.

The particle group A1 is composed of particles having a particle diameter of 1/2 or less of the average particle diameter Da of graphene or multilayer graphene G determined by the above measurement. The composition of the particles contained in the particle group A1 may be the same as or different from the positive electrode active material (particle group A2 described later) not included in the particle group A1 as long as the composition formula (1) is satisfied.

The number of particles in the particle group A1 is preferably 3% or more, more preferably 5% or more, of the number of particles of the positive electrode active material. By adopting such a number of particles, it is possible to sufficiently secure an electron conduction path between graphene or multilayer graphene G and the positive electrode active material. The number of particles in the particle group A1 is preferably 30% or less, more preferably 15% or less of the number of particles of the positive electrode active material. By adopting such a number of particles, deterioration of the positive electrode active material layer 24 can be suppressed during the charge / discharge cycle of the lithium ion secondary battery 100. That is, the cycle characteristics of the lithium ion secondary battery can be improved.

Conventionally, small crushed particles may be mixed as a positive electrode active material, but in order to prevent deterioration of cycle characteristics, small crushed particles are usually removed. Therefore, in the conventional positive electrode active material layer, the amount of small positive electrode active material particles having an average particle diameter of 3 μm or less is less than 3%, and usually less than 1%. That is, unless a small positive electrode active material is intentionally added, the number of particles in the particle group A1 does not exceed 3% of the total number of particles of the positive electrode active material.

At least a part of the particle group A1 is attached to the surface of graphene or multi-layer graphene G. Hereinafter, the positive electrode active material adhering to the surface of the graphene or the multilayer graphene G is referred to as adhering particles F. The composition of the adhered particles F may be the same as or different from the positive electrode active material (particle group A2 described later) not included in the particle group A1 as long as the composition formula (1) is satisfied.

By adhering a relatively small positive electrode active material to the surface of graphene or multilayer graphene G, the output characteristics of the lithium ion secondary battery can be enhanced. The reason for this is not completely clear, but it can be thought of as follows. That is, since a small positive electrode active material containing Ni is attached to the surface of graphene or multi-layer graphene G, a positive electrode active material containing Ni that is not attached to the surface of graphene or multi-layer graphene G and graphene Alternatively, the affinity with the multilayer graphene G is enhanced. As a result, it can be understood that it has become easier to secure an electron conduction path between graphene and the positive electrode active material.

The average particle diameter Db of the adhered particles F is preferably 3 μm or less, and more preferably 2 μm or less. The average particle size Db of the attached particles F can be obtained by averaging the particle sizes of any 20 attached particles F confirmed in the SEM image. The particle size of the attached particles F overlaps with graphene or multi-layer graphene G in the SEM image, and is difficult to measure by extraction by binarization. Therefore, the adhered particles F are measured directly from the SEM image as described above. By selecting such a Db, the adherent particles F can suitably secure an electron conduction path between graphene or multilayer graphene G and the positive electrode active material.

The positive electrode active material further has a particle group A2 composed of particles having a particle size larger than the average particle size Da of graphene or multilayer graphene G, and at least a part of the particle group A1 is graphene or multilayer graphene G and a particle group. It is preferable that it exists between A2 and A2.

Since at least a part of the particle group A1 is present between graphene or multilayer graphene G and the particle group A2, the output characteristics of the lithium ion secondary battery can be improved. The reason for this is not completely clear, but it can be thought of as follows. The particle swarm A1 with a large particle size occludes and releases lithium, and is responsible for the main reaction of the battery. It can be understood that the electrons generated there are conducted via the particle group A2 (particularly adherent particles) having a high affinity with graphene or multi-layer graphene G to improve the output characteristics of the lithium ion secondary battery.

The relatively small positive electrode active material particles adhering to the surface of graphene or multilayer graphene G are preferably adjacent to the relatively large positive electrode active material particles not adhering to the surface of graphene or multilayer graphene G. .. In this case, the relatively small positive electrode active material particles adhering to the surface of graphene or multilayer graphene G are in contact with graphene or multilayer graphene G and relatively large positive electrode active material particles. Therefore, electron conduction between graphene or multilayer graphene G and a relatively large positive electrode active material becomes easier.

The average particle size Da of graphene or multilayer graphene G is preferably 3 to 25 μm, more preferably 5 to 20 μm, and even more preferably 10 to 15 μm. By adopting such Da, it is possible to suitably secure an electron conduction path between graphene or multilayer graphene G and the positive electrode active material while maintaining the flatness of the positive electrode active material layer 24.

The average particle size Da of graphene or multilayer graphene G and the average particle size Db of adherent particles F preferably satisfy Da / Db ≧ 2, and more preferably Da / Db ≧ 5. When Da and Db satisfy such a relationship, the positive electrode active material can be suitably adhered to graphene or the multilayer graphene G, and the electron conduction path in the positive electrode active material layer 24 can be suitably secured. ..

The number distribution of the particle size of the particle group A1 preferably has a peak at half or less of the average particle size Da of graphene or multi-layer graphene G. In this case, the number distribution of the particle size of the entire positive electrode active material has two or more peaks. When the particle group A1 has such a particle size distribution, it is possible to suitably secure an electron conduction path between graphene or multilayer graphene G and the positive electrode active material.

(Positive binder)
A known binder can be used. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoro Fluororesin such as ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF) can be mentioned.

In addition to the above, as binders, for example, vinylidene fluoride-hexafluoropropylene-based fluorine rubber (VDF-HFP-based fluorine rubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluorine rubber (VDF-HFP-TFE-based) Fluoro-rubber), vinylidene fluoride-pentafluoropropylene-based fluorine rubber (VDF-PFP-based fluorine rubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluorine rubber (VDF-PFP-TFE-based fluorine rubber), vinylidene fluo Vinylidene fluoride-based fluorine such as ride-perfluoromethyl vinyl ether-tetrafluoroethylene-based fluorine rubber (VDF-PFMVE-TFE-based fluorine rubber), vinylidene fluoride-chlorotrifluoroethylene-based fluorine rubber (VDF-CTFE-based fluorine rubber) Rubber may be used.

Further, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron-conducting conductive polymer include polyacetylene and the like. In this case, since the binder also functions as a conductive material, it is not necessary to add the conductive material. As the ionic conductive polymer, for example, one having ionic conductivity such as lithium ion can be used, and for example, a polymer compound (polyether-based polymer compound such as polyethylene oxide and polypropylene oxide) can be used. , Polyphosphazene, etc.) and a lithium salt such as LiClO ₄ , LiBF ₄ , LiPF ₆ , or an alkali metal salt mainly composed of lithium, and the like. Examples of the polymerization initiator used for the compounding include a photopolymerization initiator or a thermal polymerization initiator compatible with the above-mentioned monomers.

In addition, as the binder, for example, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamideimide resin, acrylic resin or the like may be used.

The composition ratio of the positive electrode active material in the positive electrode active material layer 24 is preferably 80% or more and 90% or less in terms of mass ratio. The composition ratio of the conductive material in the positive electrode active material layer 24 is preferably 0.5% or more and 10% or less in terms of mass ratio, and the composition ratio of the binder in the positive electrode active material layer 24 is 0.5% by mass ratio. It is preferably 10% or more.

"Negative electrode"
The negative electrode 30 has a negative electrode current collector 32 and a negative electrode active material layer 34 provided on the surface of the negative electrode current collector.

(Negative electrode current collector)
The negative electrode current collector 32 may be any conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used. The negative electrode current collector 32 is preferably not alloyed with lithium, and copper is particularly preferable. The thickness of the negative electrode current collector 32 is preferably 6 to 30 μm.

(Negative electrode active material layer)
The negative electrode active material layer 34 has a negative electrode active material and a negative electrode binder, and has a conductive material if necessary.

(Negative electrode active material)
As the negative electrode active material, a negative electrode active material used in a known non-aqueous electrolytic solution secondary battery can be used. Examples of the negative electrode active material include alkali or alkaline earth metals such as metallic lithium, graphite capable of storing and releasing ions (natural graphite, artificial graphite), carbon nanotubes, non-graphitizable carbon, easily graphitized carbon, and low temperature. Amorphous, mainly composed of carbon materials such as calcined carbon, metals that can be combined with metals such as lithium such as aluminum, silicon, and tin, and oxides such as SiO _x (0 <x <2) and tin dioxide. Examples thereof include particles containing a compound, lithium titanate (Li ₄ Ti ₅ O ₁₂ ) and the like.

(Negative electrode conductive material)
Examples of the conductive material include carbon powder such as carbon black, carbon nanotube, carbon material, metal fine powder such as copper, nickel, stainless steel and iron, a mixture of carbon material and metal fine powder, and conductive oxide such as ITO. Can be used. Among these, a carbon material such as carbon black is preferable. If sufficient conductivity can be ensured only with the active material, it is not necessary to contain the conductive auxiliary agent.

(Negative electrode binder)
The binder used for the negative electrode can be the same as that used for the positive electrode. Moreover, you may use an aqueous binder as a negative electrode binder. As the water-based binder, for example, styrene-butadiene rubber (SBR) can be used.

The contents of the negative electrode active material, the conductive material and the binder in the negative electrode active material layer are not particularly limited. The composition ratio of the negative electrode active material in the negative electrode active material layer is preferably 70% or more and 99% or less, and more preferably 90% or more and 98% or less in terms of mass ratio. The composition ratio of the conductive material in the negative electrode active material layer is preferably 0% or more and 20% or less in terms of mass ratio, and the composition ratio of the binder in the negative electrode active material layer is 1% or more and 30% or less in terms of mass ratio. Is preferable.

By setting the contents of the negative electrode active material and the binder within the above range, it is possible to prevent the amount of the binder from being too small to form a strong negative electrode active material layer. In addition, the amount of binder that does not contribute to the electric capacity increases, and the tendency that it becomes difficult to obtain a sufficient volumetric energy density can be suppressed.

"Separator"
The separator 10 may be formed from an electrically insulating porous structure, for example, a monolayer of a film made of polyethylene, polypropylene or polyolefin, a laminated body or a stretched film of a mixture of the above resins, or cellulose, polyester and Examples thereof include fibrous nonwoven fabrics made of at least one constituent material selected from the group consisting of polypropylene.

"Electrolytic solution"
As the electrolytic solution, an electrolyte solution containing a lithium salt (an aqueous electrolyte solution, an electrolyte solution using an organic solvent) can be used. However, since the decomposition voltage of the aqueous electrolyte solution is electrochemically low, the withstand voltage during charging is low and limited. Therefore, it is preferable that the electrolyte solution uses an organic solvent (non-aqueous electrolyte solution).

The non-aqueous electrolyte solution has an electrolyte dissolved in a non-aqueous solvent, and may contain a cyclic carbonate and a chain carbonate as the non-aqueous solvent.

As the cyclic carbonate, one capable of solvating an electrolyte can be used. For example, ethylene carbonate, propylene carbonate, butylene carbonate and the like can be used.

The chain carbonate can reduce the viscosity of the cyclic carbonate. For example, diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate can be mentioned. In addition, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane and the like may be mixed and used.

The ratio of cyclic carbonate to chain carbonate in the non-aqueous solvent is preferably 1: 9 to 1: 1 in volume.

Examples of the electrolyte include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , and LiN (CF ₃ CF). ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , LiBOB and other lithium salts can be used. One of these lithium salts may be used alone, or two or more thereof may be used in combination. In particular, from the viewpoint of the degree of ionization, it is preferable to contain LiPF ₆ .

When dissolving LiPF ₆ in a non-aqueous solvent, it is preferable to adjust the concentration of the electrolyte in the non-aqueous electrolyte solution to 0.5 to 2.0 mol / L. When the concentration of the electrolyte is 0.5 mol / L or more, the lithium ion concentration of the non-aqueous electrolyte solution can be sufficiently secured, and a sufficient capacity can be easily obtained during charging / discharging. Further, by suppressing the concentration of the electrolyte to 2.0 mol / L or less, it is possible to suppress an increase in the viscosity of the non-aqueous electrolyte solution, to sufficiently secure the mobility of lithium ions, and to obtain a sufficient capacity during charging and discharging. It will be easier.

Even when LiPF ₆ is mixed with other electrolytes, it is preferable to adjust the lithium ion concentration in the non-aqueous electrolyte solution to 0.5 to 2.0 mol / L, and the lithium ion concentration from LiPF ₆ is 50 mol% thereof. It is more preferable that the above is included.

"Exterior body"
The exterior body 50 seals the laminate 40 and the electrolytic solution inside the exterior body 50. The exterior body 50 is not particularly limited as long as it can suppress leakage of the electrolytic solution to the outside and invasion of moisture or the like into the inside of the lithium ion secondary battery 100 from the outside.

For example, as the exterior body 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52, and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, for example, polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferable.

"Lead"
The leads 60 and 62 are formed of a conductive material such as aluminum. Then, the leads 60 and 62 are welded to the positive electrode current collector 22 and the negative electrode current collector 32, respectively, by a known method, and between the positive electrode active material layer 24 of the positive electrode 20 and the negative electrode active material layer 34 of the negative electrode 30. With the separator 10 sandwiched, it is inserted into the exterior body 50 together with the electrolytic solution to seal the entrance of the exterior body 50.

[Manufacturing method of lithium ion secondary battery]
Hereinafter, a method for manufacturing the lithium ion secondary battery 100 will be specifically described.

A paint is prepared by mixing a negative electrode active material, a binder and a solvent. If necessary, a conductive material may be further added. As the solvent, for example, water, N-methyl-2-pyrrolidone, N, N-dimethylformamide and the like can be used. The composition ratio of the negative electrode active material, the conductive material, and the binder is preferably 70 wt% to 90 wt%: 0.1 wt% to 10 wt%: 0.1 wt% to 30 wt% in terms of mass ratio. These mass ratios are adjusted so as to be 100 wt% as a whole.

The method of mixing these components constituting the paint is not particularly limited, and the mixing order is also not particularly limited. The above paint is applied to the negative electrode current collector 32. The coating method is not particularly limited, and a method usually adopted when producing an electrode can be used. For example, the slit die coat method and the doctor blade method can be mentioned.

As for the positive electrode, a paint is applied onto the positive electrode current collector 22 in the same manner as the negative electrode using a positive electrode active material having a desired particle size distribution. A positive electrode active material having a desired particle size distribution can be prepared by mixing a positive electrode active material having a controlled particle size. Further, the positive electrode active material having a specific particle size may be mixed while applying a shearing force to prepare a particle group A1 having a small particle size and a particle group A2 having a large particle size.

Subsequently, the solvent in the paint applied on the positive electrode current collector 22 and the negative electrode current collector 32 is removed. The removal method is not particularly limited. For example, the positive electrode current collector 22 and the negative electrode current collector 32 coated with the paint may be dried in an atmosphere of 80 ° C. to 150 ° C.

Then, the electrodes on which the positive electrode active material layer 24 and the negative electrode active material layer 34 are formed in this way are pressed by a roll press device or the like, if necessary.

Next, the positive electrode 20 having the positive electrode active material layer 24, the negative electrode 30 having the negative electrode active material layer 34, the separator 10 interposed between the positive electrode and the negative electrode, and the electrolytic solution are sealed in the exterior body 50.

For example, the positive electrode 20, the negative electrode 30, and the separator 10 are laminated, and the laminated body 40 is put into a bag-shaped exterior body 50 prepared in advance.

Finally, the lithium ion secondary battery is produced by injecting the electrolytic solution into the exterior body 50. Instead of injecting the electrolytic solution into the exterior body, the laminated body 40 may be impregnated with the electrolytic solution.

Although the embodiments of the present invention have been described in detail with reference to the drawings, the configurations and combinations thereof in the respective embodiments are examples, and the configurations are added or omitted within a range not deviating from the gist of the present invention. , Replacement, and other changes are possible.

"Example 1"
(Preparation of negative electrode)
94% by weight lithium-ion battery grade graphite (negative electrode active material), 2% by weight acetylene black (conductive aid), 4% by weight PVDF (binder), and N-methyl-2-pyrrolidone (solvent). And were mixed and dispersed to prepare a paste-like negative electrode slurry. The negative electrode slurry was applied to one surface of an electric field copper foil having a thickness of 10 μm so that the coating amount was 6.1 mg / cm ² . After coating, it was dried at 100 ° C. to remove the solvent to form a negative electrode active material layer. Then, the negative electrode active material layer was pressure-molded by a roll press to prepare a negative electrode according to Example 1.

(Preparation of positive electrode)
LiNi _0.8 Co _0.15 Al _0.05 O ₂ having an average particle diameter of 25 μm and LiNi _0.8 Co _0.15 Al _0.05 O ₂ having an average particle diameter of 2.0 μm were combined at 95: 5. The positive electrode active material was prepared by mixing in a number ratio. The prepared positive electrode active material, graphene containing multi-layer graphene prepared as a conductive material, and polyvinylidene fluoride (PVdF) prepared as a binder were mixed to prepare a positive electrode mixture. The average particle size of graphene containing the multilayer graphene used as the conductive material was 10 μm.

The mass ratio of the positive electrode active material, the conductive material, and the binder was 90: 5: 5. This positive electrode mixture was dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode mixture coating material. Then, it was applied to one surface of an aluminum foil having a thickness of 20 μm so as to have the calculated weight per unit area of the positive electrode. After coating, it was dried at 100 ° C. to remove the solvent to form a positive electrode active material layer. Then, the positive electrode active material layer was pressure-molded by a roll press to prepare a positive electrode according to Example 1. When a part of the prepared positive electrode was confirmed by cross-section SEM, a part of LiNi _0.8 Co _0.15 Al _0.05 O ₂ having an average particle diameter of 2.0 μm adhered to the surface of graphene containing multi-layer graphene. It was confirmed that

(Manufacturing of lithium-ion secondary battery for evaluation Full cell)
The prepared negative electrode and the positive electrode were alternately laminated via a polypropylene separator having a thickness of 16 μm, and three negative electrodes and two positive electrodes were laminated to prepare a laminated body. Further, in the negative electrode of the laminated body, a nickel negative electrode lead was attached to the protruding end of the copper foil not provided with the negative electrode active material layer. Further, in the positive electrode of the laminated body, an aluminum positive electrode lead was attached to the protruding end of the aluminum foil not provided with the positive electrode active material layer by an ultrasonic welding machine.

Then, this laminated body was inserted into the outer body of the aluminum laminated film and heat-sealed except for one peripheral portion to form a closed portion. Inside the exterior, a non-aqueous electrolytic solution containing a solvent containing EC, EMC and DEC in a volume ratio of 3: 5: 2 and 1.5 M (mol / L) of LiPF ₆ as a lithium salt. And injected. Then, the remaining one place was sealed with a heat seal while reducing the pressure with a vacuum sealer to prepare a lithium ion secondary battery (full cell).

(Examples 2 to 6)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the positive electrode active material layer was prepared by changing the size of the conductive material and the particle size distribution of the positive electrode active material. The positive electrode active material changes the particle size distribution by mixing a material having an average particle size of 25 μm and a material having an average particle size of 2.0 to 4.0 μm in a number ratio of 95: 5. rice field. Also in Examples 2 to 6, it was confirmed that a part of the positive electrode active material having an average particle size of 2.0 to 4.0 μm was attached to the surface of graphene containing the multilayer graphene.

(Examples 7 and 8)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the composition of the positive electrode active material used was changed. Also in Examples 7 and 8, it was confirmed that a part of the positive electrode active material having an average particle size of 2.0 μm was attached to the surface of graphene containing the multilayer graphene.

(Example 9)
LiNi _0.8 Co _0.15 Al _0.05 O ₂ having an average particle diameter of 25 μm and LiNi _0.8 Co _0.1 Mn _0.1 O ₂ having an average particle diameter of 2.0 μm are mixed to activate the positive electrode. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the substance was prepared. Also in Example 9, it was confirmed that a part of the positive electrode active material having an average particle size of 2.0 μm was attached to the surface of graphene containing the multilayer graphene.

(Example 10)
LiNi _0.8 Co _0.15 Al _0.05 O ₂ having an average particle diameter of 25 μm and LiNi _0.9 Co _0.08 Al _0.02 O ₂ having an average particle diameter of 2.0 μm are mixed to activate the positive electrode. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the substance was prepared. Also in Example 10, it was confirmed that a part of the positive electrode active material having an average particle size of 2.0 μm was attached to the surface of graphene containing the multilayer graphene.

(Comparative Example 1)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the positive electrode active material was prepared without adding LiNi _0.8 Co _0.15 Al _0.05 O ₂ having an average particle size of 2 μm. When the cross-sectional SEM image of the positive electrode of Comparative Example 1 was confirmed, those having an average particle size of less than half the average particle size of the graphene containing the multilayer graphene were 1% of the total number of particles of the positive electrode active material.

(Comparative Example 2)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that LiCoO ₂ was used as the positive electrode active material. When the cross-sectional SEM image of the positive electrode of Comparative Example 2 was confirmed, those having an average particle size of less than half the average particle size of the graphene containing the multilayer graphene were 1% of the total number of particles of the positive electrode active material.

The prepared lithium ion secondary battery was evaluated by the following method.

(Measurement of particle size of particle group A1, particle group A2, and conductive material)
With respect to the lithium ion secondary batteries produced in Examples and Comparative Examples, a cross section of the positive electrode was obtained using "IM4000" of Hitachi High-Technologies Corporation, and 10 SEM images of the positive electrode cross section were taken at a magnification of 1000 times. From the photographed SEM image, the particle size Da of graphene or multilayer graphene and the particle size of the positive electrode active material particles were calculated. Further, any 20 adherent particles adhering to graphene or multi-layer graphene were measured, and the average value thereof was taken as Db.

(Capacity retention rate measurement test)
For the lithium ion secondary batteries produced in Examples and Comparative Examples, the capacity retention rate was measured in an environment of 25 ° C. using a secondary battery charge / discharge test device (manufactured by Hokuto Denko Co., Ltd.). The capacity retention rate was evaluated with a voltage range of 4.2 V to 3.0 V, 1C = 3500 mAh per full cell design capacity, and a 5C capacity retention rate (%). The 5C capacity retention rate is the ratio of the discharge capacity at the time of 5C constant current discharge to the 0.2C discharge capacity based on the discharge capacity at the time of 0.2C constant current discharge, and is represented by the following equation (1).
(5C capacity retention rate (%)) = (Discharge capacity during 5C constant current discharge) / (Discharge capacity during 0.2C constant current discharge) x 100 ... (1)

The higher the 5C capacity retention rate, the better the quick charge characteristics, and the better the output characteristics of the lithium ion secondary battery.

Table 1 shows the evaluation results of the lithium ion secondary batteries produced in Examples and Comparative Examples.

In Examples 1 to 10 and Comparative Example 2, it was confirmed from the SEM image that the positive electrode active material adhering to the conductive material was also in contact with the particles of the particle group A2.

As can be seen from the results in Table 1, the capacity retention rate of the lithium ion secondary battery was remarkably improved by setting the composition and size of the positive electrode active material and the conductive material to a specific combination.

10 Separator 20 Positive electrode 22 Positive electrode current collector 24 Positive electrode active material layer 30 Negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Laminated body 50 Exterior body 60, 62 Lead 100 Lithium ion secondary battery

Claims (7)

A positive electrode having a positive electrode current collector and a positive electrode active material layer provided on the surface of the positive electrode current collector.
The positive electrode active material layer contains a positive electrode active material represented by the composition formula (1) and graphene or multilayer graphene.
The positive electrode active material is larger than the particle group A1 composed of particles having a particle size of half or less of the average particle size Da of the graphene or the multilayer graphene and the average particle size Da of the graphene or the multilayer graphene. It has a particle group A2 composed of particles having a particle diameter, and has.
The number distribution of the particle size of the entire positive electrode active material is based on the peak existing in the range of half or less of the average particle size Da of the graphene or the multilayer graphene and the average particle size Da of the graphene or the multilayer graphene. Has two or more peaks , including a peak that also exists in a large range ,
At least a part of the particle swarm A1 is attached to the surface of the graphene or the multilayer graphene.
The average particle size Da of the graphene or the multilayer graphene and the particle size of the positive electrode active material are average value of the particle size obtained by image processing 10 scanning electron microscope images.
The particle size of each image is determined by binarizing the secondary electron image to extract the graphene or the multilayer graphene and the positive electrode active material from the image, and binarizing the backscattered electron image to obtain the graphene or the multilayer graphene. Obtained by extracting each of the positive electrode active materials,
A positive electrode having an average particle diameter Da of the graphene or the multilayer graphene having a major axis orthogonal to the stacking direction of the graphene or the multilayer graphene.
Li _a Ni _b Co _c Mn _d (M) _e O ₂ (1)
(However, M indicates at least one selected from the group consisting of Al, Si, Zr, Ti, Fe, Mg, Nb, Ba and V, and 1.90 ≦ (a + b + c + d + e) ≦ 2.2, 0 <a. ≦ 1.3, 0.5 ≦ b ≦ 1.0, 0 ≦ c ≦ 1.0, 0 ≦ d ≦ 0.7, 0 ≦ e ≦ 0.2) The positive electrode according to claim 1, wherein at least a part of the particle group A1 exists between the graphene or the multilayer graphene and the particle group A2. The average particle diameter Db of the adhered particles adhering to the surface of the graphene or the multilayer graphene is 3 μm or less.
The positive electrode according to claim 1 or 2, wherein the average particle size Db of the attached particles is obtained by averaging the particle sizes of any 20 attached particles confirmed in a scanning electron microscope image. The positive electrode according to any one of claims 1 to 3, wherein the graphene or the multilayer graphene has an average particle size Da of 3 to 25 μm. The average particle size Da of the graphene or the multilayer graphene and the average particle size Db of the adhered particles adhering to the surface of the graphene or the multilayer graphene satisfy Da / Db ≧ 2.
The item according to any one of claims 1 to 4, wherein the average particle size Db of the attached particles is obtained by averaging the particle sizes of any 20 attached particles confirmed in the scanning electron microscope image. Positive electrode. The positive electrode according to any one of claims 1 to 5, wherein the number of particles in the particle group A1 is 3% or more and 30% or less of the number of particles of the positive electrode active material. A lithium ion secondary battery comprising the positive electrode according to any one of claims 1 to 6.

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