JP6870712B2 - 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 are also widely used as a power source for mobile devices such as mobile phones and laptop computers and hybrid cars. With the development of these fields, lithium ion secondary batteries are required to have higher performance.

One of its performance is the rate characteristic. The rate characteristic is a charge / discharge characteristic when charging / discharging at high speed. For example, Patent Document 1 describes that the composite oxide formed by the sol-gel method has a large BET specific surface area and enables a rapid insertion / release reaction of lithium ions.

Further, for example, Patent Document 2 describes that the rate characteristics of a lithium ion secondary battery are improved by using secondary particles in which crystalline carbon particles are bonded to each other using carbon as a binder.

Further, for example, Patent Document 3 describes that by carbonizing a polymer in which metal ions are dispersed at the molecular level, an active material in which metal-containing particles are supported can be obtained, and the rate characteristics are improved.

Japanese Unexamined Patent Publication No. 10-32230 Japanese Unexamined Patent Publication No. 2006-024374 Japanese Unexamined Patent Publication No. 2008-251221

Further improvement of rate characteristics is required by improving the performance of the lithium ion secondary battery.

The present disclosure has been made in view of the above problems, and an object of the present disclosure is to provide a negative electrode and a lithium ion secondary battery having improved input / output characteristics.

The present inventors have found that when the active material or the conductive auxiliary agent is aggregated flatly, the rate characteristics of the lithium ion secondary battery are improved even when the thickness of the active material layer is thin.
That is, in order to solve the above problems, the following means are provided.

(1) The negative electrode according to the first aspect includes a current collector and an active material layer in contact with at least one surface of the current collector, and the average thickness of the active material layer is the average thickness of the current collector. The active material layer is 0.8 times or more and 2.0 times or less, the active material layer has a plurality of active materials and a plurality of conductive auxiliary agents, and the active material layer is a part of the plurality of active materials or the said. The aggregate has an agglomerate in which a part of a plurality of conductive auxiliary agents is aggregated, and the agglomerate has a peripheral length of 5 μm or more and 62 μm or less on a cut surface intersecting the surface on which the active material layer spreads. It contains flat aggregates having a flatness of 0.33 or more and 0.80 or less, which is obtained by dividing the minor axis length on the cut surface by the major axis length.

(2) The negative electrode according to the above aspect may have one or more and five or less agglomerates in a width of 300 μm on a cut surface intersecting the surface on which the active material layer spreads.

(3) The long axis of the flat aggregate in the negative electrode according to the above aspect may be oriented in the in-plane direction.

(4) In the negative electrode according to the above aspect, the agglomerates are an active material agglomerate in which a part of the plurality of active materials is aggregated and a conductive auxiliary agent agglomerate in which a part of the plurality of conductive auxiliary agents is aggregated. The average particle size of the active material aggregate may be three times or more the average particle size of the active material.

(5) In the negative electrode according to the above aspect, the agglomerates are an active material agglomerate in which a part of the plurality of active materials is aggregated and a conductive auxiliary agent agglomerate in which a part of the plurality of conductive auxiliary agents is aggregated. The average particle size of the conductive auxiliary agent aggregate may be 40 times or more the average particle size of the conductive auxiliary agent.

(6) In the negative electrode according to the above aspect, the average particle size of the active material may be 100 nm or more and 500 nm or less.

(7) In the negative electrode according to the above aspect, the average particle size of the conductive auxiliary agent may be 20 nm or more and 40 nm or less.

(8) In the negative electrode according to the above aspect, the mass% of the active material is 70% by mass or more and 85% by mass or less, the mass% of the conductive auxiliary agent is 3% by mass or more and 10% by mass or less, and the mass of the binder. % May be 10% by mass or more and 20% by mass or less.

(9) The lithium ion secondary battery according to the second aspect includes a negative electrode according to the above aspect, a positive electrode facing the negative electrode, and a non-aqueous electrolyte connecting the negative electrode and the positive electrode.

The negative electrode and the lithium ion secondary battery according to the above aspect are excellent in input / output characteristics.

It is a schematic diagram of the lithium ion secondary battery which concerns on 1st Embodiment. It is sectional drawing of the negative electrode which concerns on 1st Embodiment.

Hereinafter, embodiments will be described in detail with reference to the drawings as appropriate. In the drawings used in the following description, the featured portion may be enlarged for convenience in order to make the feature easy to understand, and the dimensional ratio of each component may be different from the actual one. 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 view of a lithium ion secondary battery according to the first embodiment. The lithium ion secondary battery 100 shown in FIG. 1 includes a power generation element 40, an exterior body 50, and a non-aqueous electrolytic solution (not shown). The exterior body 50 covers the periphery of the power generation element 40. The power generation element 40 is connected to the outside by a pair of connected terminals 60 and 62. The non-aqueous electrolyte solution is housed in the exterior body 50.

(Power generation element)
The power generation element 40 includes a positive electrode 20, a negative electrode 30, and a separator 10.

<Negative electrode>
FIG. 2 is a schematic cross-sectional view of the negative electrode of the lithium ion secondary battery according to the first embodiment. FIG. 2 is a cross-sectional view of the negative electrode 30 cut along a plane that intersects (substantially orthogonally) the in-plane direction in which the negative electrode current collector 32 spreads. The negative electrode 30 has, for example, a negative electrode current collector 32 and a negative electrode active material layer 34.

[Negative electrode current collector]
The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 is, for example, a thin metal plate such as aluminum, copper, nickel, titanium, or stainless steel. The average thickness of the negative electrode current collector 32 is, for example, 10 μm or more and 30 μm or less.

[Negative electrode active material layer]
The negative electrode active material layer 34 is in contact with at least one surface of the negative electrode current collector 32. The negative electrode active material layer 34 is in contact with both sides of the negative electrode current collector 32, for example. The negative electrode active material layer 34 spreads in the in-plane direction along the negative electrode current collector 32.

The average thickness of the negative electrode active material layer 34 is 0.8 times or more and 2.0 times or less, preferably 0.9 times or more and 1.5 times or less of the average thickness of the negative electrode current collector 32. The average thickness of the negative electrode active material layer 34 is, for example, 8 μm or more and 30 μm or less. The average thickness of the negative electrode active material layer 34 is an average value obtained by measuring the thickness of the negative electrode active material layer 34 at 10 different points. A system in which the average thickness of the negative electrode active material layer 34 is thin tends to have a low energy density. Further, in a system in which the average thickness of the negative electrode active material layer 34 is thin, it is difficult to secure a conductive path for electrons and lithium ions, and the rate characteristics tend to be low.

The negative electrode active material layer 34 has a plurality of negative electrode active materials and a plurality of conductive auxiliaries. The negative electrode active material layer 34 may have a binder, if necessary.

The negative electrode active material layer 34 contains, for example, a negative electrode active material, a conductive auxiliary agent, and a binder in a predetermined ratio. For example, the mass% of the negative electrode active material of the negative electrode active material layer 34 is 70% by mass or more and 85% by mass or less, the mass% of the conductive auxiliary agent is 3% by mass or more and 10% by mass or less, and the mass% of the binder is 10% by mass. It may be 20% by mass or more and 20% by mass or less.

The negative electrode active material layer 34 is divided into a dispersion portion 341 and an agglomerate 342. The dispersion portion 341 is a region in which the negative electrode active material and the conductive auxiliary agent are dispersed. For example, in the dispersion section 341, the negative electrode active material and the conductive auxiliary agent are uniformly dispersed. The agglomerate 342 is an agglomerate of the negative electrode active material or the conductive auxiliary agent.

The agglomerate 342 has a peripheral length of 5 μm or more and 62 μm or less. The perimeter of the agglomerates 342 is the perimeter of the agglomerates 342 on the cut surface that intersects (substantially orthogonally) the surface on which the negative electrode active material layer 34 spreads. The agglomerate 342 preferably contains an agglutinin having a circumference of 15 μm or more, more preferably 20 μm or more, and further preferably 30 μm or more.

Aggregates 342 include flat aggregates. Flat agglomerates are agglomerates having anisotropy in shape. The presence or absence of flat aggregates is determined by five images of different cut surfaces obtained by cutting the negative electrode active material layer 34 at equal intervals along a first direction intersecting (approximately orthogonal) with the surface on which the negative electrode active material layer 34 spreads, and a first. Judgment is made based on five images of different cut surfaces obtained by cutting the negative electrode active material layer 34 at equal intervals along a second direction orthogonal to the direction of 1.

The flat agglomerates have a flatness of 0.33 or more and 0.80 or less, which is obtained by dividing the minor axis length by the major axis length on any cut surface that intersects (approximately orthogonally) the surface on which the negative electrode active material layer 34 spreads. Is. The minor axis length and the major axis length of the flat aggregate are the length in the major axis direction and the length in the minor axis direction of the smallest ellipse containing the flat aggregate. The shape of the flat agglomerates may be amorphous. It is preferable that all the aggregates 342 are flat aggregates.

The flat agglomerates are preferably oriented in the in-plane direction. “Oriented in the in-plane direction” means that the inclination angle of the major axis with respect to the in-plane direction in which the negative electrode active material layer 34 spreads is within 45 degrees. Further, it is preferable that the long axis direction of the flat agglomerates substantially coincides with the in-plane direction. The fact that the long axis direction of the flat agglomerates substantially coincides with the in-plane direction means that the inclination angle of the flat agglomerates with respect to the xy plane in the long axis direction is 10 degrees or less.

The negative electrode active material layer 34 contains one or more agglomerates 342. The negative electrode active material layer 34 may have only the active material agglomerate as the agglomerate 342, may have only the conductive auxiliary agent agglomerate, and has both the active material agglomerate and the conductive auxiliary agent agglomerate. You may.

It is preferable that the number of agglomerates 342 is 1 or more and 5 or less in a width of 300 μm on a cut surface that intersects (substantially orthogonally) the surface on which the negative electrode active material layer 34 spreads. The number of agglomerates 342 can be measured with a scanning electron microscope (SEM) and is calculated as an average value of 10 images.

The agglomerate 342 is divided into an active material agglomerate in which the negative electrode active material is aggregated and a conductive auxiliary agent agglomerate in which the conductive auxiliary agent is aggregated. The active material aggregate mainly contains a negative electrode active material. The active material aggregate basically consists of only the negative electrode active material, but may contain a conductive auxiliary agent or a binder as an impurity. The active material aggregate contains 90% or more of the negative electrode active material. The conductive auxiliary agent agglomerate mainly contains a conductive auxiliary agent. The conductive auxiliary agent agglomerate basically consists of only the conductive auxiliary agent, but may contain a negative electrode active material or a binder as an impurity. The conductive auxiliary agent aggregate contains 90% or more of the conductive auxiliary agent.

The average particle size of the active material aggregate is, for example, three times or more the average particle size of the negative electrode active material. The active material aggregate preferably contains an average particle size of 6 times or more the average particle size of the negative electrode active material, and preferably contains an average particle size of 10 times or more the average particle size of the negative electrode active material. More preferred.

The average particle size of the conductive auxiliary agent aggregate is, for example, 40 times or more the average particle size of the conductive auxiliary agent. The conductive auxiliary agent aggregate preferably contains an average particle size of 60 times or more the average particle size of the conductive auxiliary agent, and includes an average particle size of 80 times or more the average particle size of the conductive auxiliary agent. Is more preferable.

The average particle size of the agglomerates is the average value of the minor axis length and the major axis length on the cut surface measured by a scanning electron microscope (SEM), and is obtained as the average value of the agglomerates in 10 images.

The negative electrode active material is a compound that can occlude and release ions. The negative electrode active material is, for example, a negative electrode active material used in a known lithium ion secondary battery. The negative electrode active material is, for example, a carbon material. The carbon material is, for example, graphite (natural graphite, artificial graphite) capable of storing and releasing ions, carbon nanotubes, non-graphitizable carbon, easily graphitized carbon, and low-temperature calcined carbon. The average particle size of the negative electrode active material is, for example, 100 nm or more and 500 nm or less.

The conductive auxiliary agent enhances the electron conductivity between the negative electrode active materials in the negative electrode active material layer 34. The conductive auxiliary agent is, for example, carbon powder such as carbon black, carbon nanotube, carbon material, metal fine powder such as copper, nickel, stainless steel, iron, a mixture of carbon material and metal fine powder, and conductive oxide such as ITO. .. The conductive auxiliary agent is preferably a carbon material such as carbon black.

The binder binds the active materials together. A known binder can be used. The binder is, for example, a fluororesin. Fluororesin is, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), Ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinylidene fluoride (PVF) and the like.

In addition to the above, the binder is, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based). Fluororesin), Vinylidene Fluoride-Pentafluoropropylene Fluororesin (VDF-PFP Fluororubber), Vinylidene Fluoride-Pentafluoropropylene-Tetrafluoroethylene Fluororesin (VDF-PFP-TFE Fluororubber), Vinylidene Fluoro Vinylidene fluoride-based fluoropolymers such as Ride-Perfluoromethyl Vinyl Ether-Tetrafluoroethylene Fluororesin (VDF-PFMVE-TFE Fluororesin) and Vinylidene Fluoride-Chlorotrifluoroethylene Fluororesin (VDF-CTFE Fluororesin) It may be rubber. Further, the binder may be, for example, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin or the like.

<Positive electrode>
As shown in FIG. 1, the positive electrode 20 has, for example, a positive electrode current collector 22 and a positive electrode active material layer 24.

[Positive current collector]
The positive electrode current collector 22 is, for example, a conductive plate material. The positive electrode current collector 22 has the same configuration as the negative electrode current collector 32, for example.

[Positive electrode active material layer]
The positive electrode active material layer 24 includes, for example, a positive electrode active material, a conductive auxiliary agent, and a binder. When sufficient conductivity can be ensured only by the positive electrode active material, the positive electrode active material layer 24 does not have to contain the conductive material.

The positive electrode active material is an electrode active material capable of reversibly advancing the occlusion and release of lithium ions, the desorption and insertion (intercalation) of lithium ions, or the doping and dedoping of lithium ions and counter anions. Can be used.

The positive electrode active material is, for example, a composite metal oxide. Composite metal oxides, for example, lithium cobalt oxide (LiCoO _2), lithium nickelate (LiNiO _2), lithium manganate (LiMnO _2), lithium manganese spinel (LiMn ₂ O _4), and the general formula: LiNi _x Co _y Mn _z M _a O ₂ compound (in the general formula x + y + z + a = 1,0 ≦ x <1,0 ≦ y <1,0 ≦ z <1,0 ≦ a <1, M is Al, Mg, Nb, One or more elements selected from Ti, Cu, Zn, and Cr), lithium vanadium compound (LiV ₂ O ₅ ), olivine type LiMPO ₄ (where M is Co, Ni, Mn, Fe, Mg, Nb, Ti). , Al, showing one or more elements or VO selected from Zr), lithium titanate _{(Li 4 Ti 5 O 12)} , in _{LiNi x Co y Al z O 2} (0.9 <x + y + z <1.1) is there. The positive electrode active material may be an organic substance. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

As the conductive auxiliary agent and the binder, the same material as the conductive auxiliary agent and the binder in the negative electrode active material layer 34 can be used.

<Separator>
The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 isolates the positive electrode 20 and the negative electrode 30 and prevents a short circuit between the positive electrode 20 and the negative electrode 30. The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is selected from, for example, a monolayer of a film made of polyolefin such as polyethylene or polypropylene, a stretched film of a laminate or a mixture of the above resins, or a group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene and polypropylene. Examples thereof include fibrous polypropylene made of at least one constituent material. The separator 10 may be, for example, a solid electrolyte. The solid electrolyte is, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte.

(Terminal)
The terminals 60 and 62 are connected to the positive electrode 20 and the negative electrode 30, respectively. The terminal 60 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 62 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60 and 62 are responsible for electrical connection with the outside. The terminals 60 and 62 are formed of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing. It is preferable to protect the terminals 60 and 62 with insulating tape in order to prevent a short circuit.

(Exterior body)
The exterior body 50 seals the power generation element 40 and the non-aqueous electrolytic solution inside. The exterior body 50 suppresses leakage of the non-aqueous electrolytic solution to the outside and invasion of water or the like from the outside into the lithium ion secondary battery 100.

The exterior body 50 has, for example, as shown in FIG. 1, a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52. The exterior body 50 is a metal laminate film in which a metal foil 52 is coated with a polymer film (resin layer 54) from both sides.

As the metal foil 52, for example, an aluminum foil can be used. A polymer film such as polypropylene can be used for the resin layer 54. The material constituting the resin layer 54 may be different between the inside and the outside. For example, a polymer having a high melting point, for example, polyethylene terephthalate (PET), polyamide (PA) or the like is used as the outer material, and polyethylene (PE), polypropylene (PP) or the like is used as the material of the inner polymer film. be able to.

(Non-aqueous electrolyte)
The non-aqueous electrolytic solution is sealed in the exterior body 50 and impregnated in the power generation element 40.
The non-aqueous electrolyte solution has, for example, a non-aqueous solvent and an electrolyte. The electrolyte is dissolved in a non-aqueous solvent.

The non-aqueous solvent contains, for example, a cyclic carbonate and a chain carbonate. Cyclic carbonate solvates the electrolyte. Cyclic carbonates are, for example, ethylene carbonate, propylene carbonate and butylene carbonate. The cyclic carbonate preferably contains at least propylene carbonate. The chain carbonate reduces the viscosity of the cyclic carbonate. The chain carbonate is, for example, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate. The non-aqueous solvent may also include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane and the like. ..

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

The electrolyte is, for example, a lithium salt. Electrolytes include, for example, 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 , 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 the like. One type of lithium salt may be used alone, or two or more types may be used in combination. From the viewpoint of the degree of ionization, the electrolyte preferably contains _{LiPF 6.}

"Manufacturing method of lithium ion secondary battery"
First, the negative electrode 30 is manufactured. As the negative electrode current collector 32, for example, a commercially available one is used. A paste-like negative electrode slurry is applied to at least one surface of the negative electrode current collector 32. The coating method is not particularly limited. For example, the slit die coat method and the doctor blade method can be mentioned. The negative electrode slurry is a paste obtained by mixing a negative electrode active material, a conductive auxiliary agent, a binder, and a solvent. Further, an aggregate of the negative electrode active material or the conductive auxiliary agent is added to the negative electrode slurry. The agglutinate is obtained by compounding a negative electrode active material or a conductive auxiliary agent. The compounding process is, for example, a mechanical alloying method. Agglomerates are obtained by mechanically mixing a plurality of negative electrode active materials or a plurality of conductive auxiliaries.

Then, the solvent in the negative electrode slurry is removed. The removal method is not particularly limited. For example, the negative electrode current collector 32 coated with the negative electrode slurry is dried in an atmosphere of 80 ° C. to 150 ° C. Next, the obtained coating film is pressed to increase the density of the negative electrode active material layer 34, whereby the negative electrode 30 is obtained. As the pressing means, for example, a roll press machine, a hydrostatic press machine or the like can be used.

Next, the positive electrode 20 is manufactured. The positive electrode 20 can be manufactured in the same manner as the negative electrode 30. A paste-like positive electrode slurry is applied to at least one surface of the positive electrode current collector 22. The positive electrode slurry is a paste obtained by mixing a positive electrode active material, a binder, a conductive additive and a solvent. The positive electrode 20 is obtained by applying the positive electrode slurry to the positive electrode current collector 22 and drying it.

Next, the separator 10 is laminated so that the separator 10 is located between the produced positive electrode 20 and the negative electrode 30, and the power generation element 40 is produced. When the power generation element 40 is a wound body, the positive electrode 20, the negative electrode 30, and the separator 10 are wound around one end side as an axis.

Finally, the power generation element 40 is enclosed in the exterior body 50. The non-aqueous electrolyte solution is injected into the exterior body 50. The non-aqueous electrolytic solution is impregnated into the power generation element 40 by reducing the pressure, heating, or the like after injecting the non-aqueous electrolytic solution. By applying heat or the like to seal the exterior body 50, the lithium ion secondary battery 100 can be obtained.

The lithium ion secondary battery according to the first embodiment has improved rate characteristics because the negative electrode active material contains flat aggregates, as shown in Examples described later.

The cause of this is not clear, but it is thought that the presence of flat agglomerates of a predetermined size may improve at least one of the lithium ion conductivity and the electron conductivity in the negative electrode active material layer 34. Be done.

The rate characteristic is a capacity retention rate at the time of rapid discharge based on the 0.2C discharge capacity after 0.2C CCCV charging. The higher the rate characteristic, the more the battery performance can be fully exhibited in rapid discharge. Lithium ion conductivity and electron conductivity in the active material layer have a great influence on the rate characteristics.

The active material aggregate mainly consists of the active material. In the active material aggregate, the active materials are in direct contact with each other without a binder, and it is expected that the lithium ion conductivity is improved. Further, the conductive auxiliary agent aggregate is mainly composed of the conductive auxiliary agent. In the agglomerates of conductive auxiliaries, the conductive auxiliaries are in direct contact with each other without a binder, and it is expected that the electron conductivity is improved.

Further, it is expected that the predetermined flat shape of the agglomerates facilitates improvement of at least one of the lithium ion conductivity and the electron conductivity in the active material layer. For electrons or lithium ions, the flat agglomerates tend to move in the long axis direction. Therefore, it is considered that the long axis direction of the flat agglomerates serves as a trunk path for electrons or lithium ions, facilitating the movement of electrons or lithium ions. When the long axis direction of the flat agglomerates is oriented in the in-plane direction, a path is secured in the in-plane direction, and the rate characteristics are further improved.

On the other hand, if the size of the agglomerates becomes too large, the rate characteristics will deteriorate. It is presumed that this is because if the agglomerates are too large, the dispersibility in the negative electrode active material layer 34 becomes too low and the resistance in the negative electrode active material layer 34 increases.

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"
First, a negative electrode was prepared. For the negative electrode, a negative electrode active material, a conductive auxiliary agent, and a binder were mixed to prepare a negative electrode mixture. The negative electrode active material was graphite having an average particle size of 500 nm, the conductive additive was carbon black having an average particle size of 20 nm, and the binder was polyvinylidene fluoride (PVDF). The mass ratio of the negative electrode active material, the conductive auxiliary agent, and the binder was 65:13:22. The negative electrode active material and the conductive auxiliary agent were kneaded with a raft machine to prepare a negative electrode mixture. This negative electrode mixture was dispersed in N-methyl-2-pyrrolidone to prepare a negative electrode slurry. Then, a negative electrode slurry was applied to one surface of a copper foil having a thickness of 10 μm. After coating, it was dried at 100 ° C. to remove the solvent to form a negative electrode active material layer. The thickness of the negative electrode active material layer was 15 μm, which was 1.5 times the thickness of the negative electrode current collector.

The cross section of the negative electrode produced under the same conditions was confirmed by SEM. As a result, one active material agglomerate and one conductive auxiliary agent agglomerate were present within a width range of 300 μm. The active material aggregate had a peripheral length of 30 μm and a flatness of 0.5. The conductive auxiliary agent aggregate had a peripheral length of 40 μm and a flatness of 0.8.

Then, a positive electrode was prepared. For the positive electrode, a positive electrode active material, a conductive material, and a binder were mixed to prepare a positive electrode mixture. The positive electrode active material was nickel-manganese-cobalt (NCA), the conductive material was carbon black, and the binder was polyvinylidene fluoride (PVDF). The positive electrode active material, the conductive material, and the binder had a mass ratio of 95: 2: 3. This positive electrode mixture was dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode slurry. Then, a positive electrode slurry was applied to one surface of an aluminum foil having a thickness of 15 μm. After coating, it was dried at 100 ° C. to remove the solvent to form a positive electrode active material layer.

(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 10 μm, and 6 negative electrodes and 5 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 laminated film and heat-sealed except for one peripheral portion to form a closed portion. A non-aqueous electrolyte solution was injected into the outer body. The non-aqueous electrolyte solution was prepared by adding _{1.0 M (mol / L) of LiPF 6} as a lithium salt to a solvent having a volume ratio of ethylene carbonate (EC) and diethyl carbonate (DEC) of 3: 7. did. 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).

The rate characteristics were measured using a secondary battery charge / discharge test device. The rate characteristics were evaluated with a voltage range of 4.2 V to 3.0 V, 1 C = 1000 mAh per full cell design capacity, and a 3C discharge capacity retention rate (%). The 3C discharge capacity retention rate is the discharge capacity when CCCV charging (constant current constant voltage charging, termination current value is 0.05C) is performed at a current value of 0.2C and discharged at a current value of 0.2C. It is the ratio of the discharge capacity when CCCV charging (constant current constant voltage charging, termination current value is 0.05C) is performed with a current value of 0.2C and discharged at 3C, and is represented by the following equation (1).
(3C capacity retention rate (%)) = (0.2CCCV charge / 3C discharge capacity) / (0.2CCCV charge / 0.2C discharge capacity) x 100 ... (1)

"Examples 2 to 6, Comparative Examples 1 to 5"
In Examples 2 to 6 and Comparative Examples 1 to 5, the peripheral length, flatness and abundance of the negative electrode active material and the conductive auxiliary agent, the time for kneading with a rake, and the negative electrode active material layer for the negative electrode current collector The difference from Example 1 is that the thickness is changed. The agglomerates were prepared by stirring the negative electrode active material and the conductive auxiliary agent with a rake. The aggregates were sized using a sieve. The classified aggregates were observed by SEM, and it was confirmed that the peripheral length and the flatness were different. The aggregates separated by this were mixed with each sample. The peripheral length and flatness of the active material agglomerates and the conductive additive agglomerates changed due to the difference in kneading time.

The conditions and measurement results of Examples 1 to 6 and Comparative Examples 1 to 5 are summarized in Table 1. In Table 1, the thickness ratio is a value obtained by dividing the thickness of the negative electrode active material layer by the thickness of the negative electrode current collector. The capacity retention rate is the above-mentioned 3C capacity retention rate and shows rate characteristics.

"Examples 7 to 9"
In Examples 7 to 9, the number of active material aggregates in the negative electrode active material layer was changed from that of Example 1. The number of active material aggregates was adjusted by adding 1.3 × (n-1)% of the active material aggregates having a circumference of 30 μm with respect to the solid content when preparing the negative electrode slurry. Here, n corresponds to the number of active material aggregates. The number of aggregates in the table is the sum of the number of active material aggregates and the number of conductive aid aggregates.

The conditions and measurement results of Examples 7 to 9 are summarized in Table 2. Table 2 shows the results of Example 1, Example 3, Example 4, and Example 6 at the same time for comparison.

"Examples 10 to 13"
Examples 10 to 13 are different from Example 1 in that the average particle size of the active material is changed. The conditions and measurement results of Examples 10 to 13 are summarized in Table 3. Table 3 shows the results of Example 1 at the same time for comparison.

"Examples 14 to 17"
Examples 14 to 17 are different from Example 1 in that the average particle size of the conductive auxiliary agent is changed. The conditions and measurement results of Examples 14 to 17 are summarized in Table 4. Table 4 shows the results of Example 1 at the same time for comparison.

"Examples 18 to 22"
Examples 18 to 22 are different from Example 1 in that the composition ratios of the negative electrode active material, the conductive auxiliary agent, and the binder in the negative electrode active material layer are changed. The conditions and measurement results of Examples 18 to 22 are summarized in Table 5. Table 5 shows the results of Example 1 at the same time for comparison.

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 341 Dispersion part 342 Aggregate 40 Power generation element 50 Exterior body 52 Metal foil 54 Resin layer 60, 62 Terminal 100 Lithium Ion secondary battery

Claims (10)

A current collector and an active material layer in contact with at least one surface of the current collector are provided.
The average thickness of the active material layer is 0.8 times or more and 2.0 times or less the average thickness of the current collector.
The active material layer has a plurality of active materials and a plurality of conductive auxiliary agents, and has a plurality of active materials.
The active material layer has an agglomerate in which a part of the plurality of active materials or a part of the plurality of conductive auxiliary agents is aggregated without using a binder.
Each of the plurality of active materials has at least one of lithium ion conductivity and electron conductivity.
The aggregate has a peripheral length of 5 μm or more and 62 μm or less on the cut surface intersecting the surface on which the active material layer spreads.
The agglomerate is a negative electrode for a lithium ion secondary battery containing a flat agglomerate having a flatness of 0.33 or more and 0.80 or less obtained by dividing the minor axis length on the cut surface by the major axis length. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the cut surface intersecting the surface on which the active material layer spreads has one or more and five or less agglomerates within a width of 300 μm. The negative electrode for a lithium ion secondary battery according to claim 1 or 2, wherein the flat aggregate has a major axis oriented in the in-plane direction. The agglomerate is either an active material agglomerate in which a part of the plurality of active materials is aggregated or a conductive auxiliary agent agglomerate in which a part of the plurality of conductive auxiliary agents is aggregated.
The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the average particle size of the active material aggregate is three times or more the average particle size of the active material. The agglomerate is either an active material agglomerate in which a part of the plurality of active materials is aggregated or a conductive auxiliary agent agglomerate in which a part of the plurality of conductive auxiliary agents is aggregated.
The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the average particle size of the conductive auxiliary agent aggregate is 40 times or more the average particle size of the conductive auxiliary agent. The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the average particle size of the active material is 100 nm or more and 500 nm or less. The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 6, wherein the conductive auxiliary agent has an average particle size of 20 nm or more and 40 nm or less. The mass% of the active material is 70% by mass or more and 85% by mass or less.
The mass% of the conductive auxiliary agent is 3% by mass or more and 10% by mass or less.
The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 7, wherein the mass% of the binder is 10% by mass or more and 20% by mass or less. The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 8, wherein the active material is graphite having an average particle size of 100 nm or more and 500 nm or less. Between the negative electrode for a lithium ion secondary battery according to any one of claims 1 to 9, the positive electrode facing the negative electrode for a lithium ion secondary battery, and the negative electrode for a lithium ion secondary battery and the positive electrode. A lithium-ion secondary battery equipped with a non-aqueous electrolyte that connects the electrodes.

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