JP2016058247A - Electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode for a lithium ion secondary battery of high output characteristics, by ensuring the electron conductivity and lithium ion conductivity sufficiently in an active material layer, in the electrode for a lithium ion secondary battery requiring a high output, and to provide a lithium ion secondary battery.SOLUTION: In the electrode 10 for a lithium ion secondary battery where an active material layer 5, containing a granular active material 4, a binder and a conductive aid, is laminated on the surface of a collector 1, the active material layer 5 has a low porosity area (density of active material is high) 3 and a high porosity area (density of active material is low) 2 in the surface direction of the collector 1, where the layer thickness of the high porosity area 2 is in a range of 1-10 times of the average grain size of the granular active material, and the difference of density of the low porosity area and high porosity area is in a range of 0.2-0.7 g/cm.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an electrode for a lithium ion secondary battery, and more particularly to an electrode for a lithium ion secondary battery that can improve output characteristics and a lithium ion secondary battery using the same.

  Lithium ion secondary batteries, which are already widely used as power sources for mobile devices such as notebook personal computers and mobile phones, have recently attracted attention in new fields such as being mounted on hybrid vehicles (xEV) and electric vehicles (EV). Gathered. In particular, in recent years, the number of hybrid vehicles has increased and the market has grown, and various performances are also required for lithium ion secondary batteries according to their characteristics. For example, in addition to cost competitiveness, a μHEV battery is particularly important for high-speed charge / discharge performance, that is, high output density in order to efficiently recover regenerative energy.

A lithium ion secondary battery is generally composed of a positive electrode in which a positive electrode active material layer containing a lithium-containing metal oxide capable of reversibly occluding and releasing lithium ions is formed on a current collector, and a negative electrode active material layer containing carbon or a silicon material And a negative electrode formed on a current collector facing each other through a separator which is an insulator, and stored in a battery case such as a metal can or a laminate pack, and there are several types of carbonates It is prepared by adding and impregnating a mixed organic solvent with a nonaqueous electrolytic solution in which a lithium salt such as lithium hexafluorophosphate (LiPF ₆ ) is dissolved.

  The method of housing the electrode and separator in the battery exterior material varies depending on the shape of the exterior material.For example, a laminate pack is a method of stacking electrodes and separators punched out to a certain standard. There are methods such as winding them alternately.

  Conventionally, when manufacturing an electrode of a lithium ion secondary battery, an active material slurry containing an active material, a binder, a conductive assistant, and the like is prepared, and the active material slurry is uniformly applied to the surface of the current collector. A material layer is formed.

  In a battery including an electrode manufactured by such a conventional technique, when charging / discharging is performed with an output not so high, the charging / discharging reaction can proceed uniformly even in an active material layer having a uniform composition. However, for a charge / discharge reaction at a higher output as required for an in-vehicle battery, an active material layer having a uniform composition produced by a conventional method can be used to convert the current from the current collector to the active material. There is a problem that the electron conductivity and the conduction of lithium ions to the electrolytic solution or the active material layer are not compatible, and a sufficient charge / discharge reaction does not proceed.

  From the above viewpoints, a charge / discharge reaction can proceed sufficiently even when the active material layer having a thinner film thickness has a higher output. However, in the electrode for a lithium ion secondary battery housed in the battery pack, the electrolyte solution is mainly impregnated from the end of the electrode, so the electrode thickness is reduced in order to increase the battery capacity. As such, the electronic conductivity from the current collector to the active material and the conduction of lithium ions to the electrolyte or active material layer are not compatible.

  In order to improve the electron conductivity from the current collector to the active material, it is effective to increase the density of the active material layer and increase the number of contacts between the current collector, the active material, and the conductive additive. When the pore volume is reduced, it becomes difficult for the electrolytic solution to impregnate the active material layer. On the other hand, if the hole volume is sufficiently secured, not only the electron conductivity of the active material layer is deteriorated, but also the adhesiveness to the current collector is lowered and the cycle characteristics are lowered. Density decreases.

  Therefore, as in Patent Document 1, for example, in a negative electrode formed of a carbonaceous material, a porosity distribution is provided in the thickness direction, an energy density is secured in an internal high bulk density portion, and electrolysis is performed in an external low bulk density portion. Liquid permeability can be ensured.

  Further, in Patent Document 2, in a non-aqueous electrolyte secondary battery including a positive and negative electrode plate in which an active material mixture is applied to a current collector, at least one of the positive and negative electrode plates is the active material mixture. It has a density change part where the density changes from one side of the surface direction to the other side at a substantially constant rate, and the active material ratio increases in the high density part, so that the energy density can be improved and the low density. In this part, a gap impregnated with the non-aqueous electrolyte is secured, and the input / output characteristics can be improved.

JP-A-8-138650 JP2009-259502A

  However, in Patent Document 1, since the porosity distribution is provided only in the thickness direction, the impregnation with the electrolytic solution in the electrode in-plane direction is not studied, and the output characteristics are not sufficiently ensured. In Patent Document 2, the impregnation property of the electrolyte solution inside and outside the wound electrode is examined, but the electrolyte solution impregnation property in the short side direction of the electrode is not studied. Therefore, ensuring the output characteristics is not sufficient.

  Accordingly, the present invention provides an electrode for a lithium ion secondary battery that has sufficient output and lithium ion conductivity in an active material layer for a lithium ion secondary battery electrode that requires a high output. And it aims at providing a lithium ion secondary battery.

  As a result of intensive studies, the inventor has formed portions with different porosity in the surface direction of the active material layer, thereby improving the electron conductivity and lithium ion conductivity in the electrode surface direction, respectively. The inventors have found that the problem can be solved and have completed the present invention.

The invention according to claim 1 of the present invention is an electrode for a lithium ion secondary battery in which an active material layer containing a granular active material, a binder and a conductive additive is laminated on the surface of a current collector,
The active material layer is
Having a low porosity region (region where the density of the active material is high) and a high porosity region (region where the density of the active material is low) in the surface direction of the current collector,
In the range where the layer thickness of the high porosity region is 1 to 10 times the average particle size of the granular active material,
In addition, the lithium ion secondary battery electrode is characterized in that a difference in density between the low porosity region and the high porosity region is in a range of 0.2 to 0.7 g / cm ³ .

  The invention of claim 2 is characterized in that the high porosity region is formed continuously from the end in the surface direction of the current collector to the center. It is an electrode for a secondary battery.

  The invention according to claim 3 is the lithium ion according to claim 1 or 2, wherein the area of the high porosity region occupying the outermost surface of the active material layer is smaller than the area of the low porosity region. It is an electrode for a secondary battery.

  Further, the invention of claim 4 is characterized in that an active material layer composed of the low porosity region and the high porosity region having different area ratios in a thickness direction of the current collector is laminated in multiple layers. It is an electrode for lithium ion secondary batteries in any one of Claims 1-3.

  The invention according to claim 5 is the lithium according to any one of claims 1 to 4, wherein an interval between the low porosity regions adjacent to the left and right and up and down of the high porosity region is 1 mm or less. It is an electrode for an ion secondary battery.

  The invention according to claim 6 is the electrode for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the granular active material has an average particle size of 15 μm or less.

  A seventh aspect of the present invention is a lithium ion secondary battery using the lithium ion secondary battery electrode according to any one of the first to sixth aspects.

  The invention of claim 8 is characterized in that the electrode for a lithium ion secondary battery is wound and has a high porosity region continuous from the center of the electrode surface toward the electrode end perpendicular to the winding direction. The lithium ion secondary battery according to claim 7, wherein:

  According to claim 1 of the present invention, by setting the layer thickness of the high porosity region of the active material layer to a range of 1 to 10 times the average particle size (D50) of the granular active material, excellent electron conduction is achieved. Characteristics and output characteristics. When the layer thickness exceeds 10 times, the distance from the current collector to the active material is increased, and the above-described effects are hardly obtained. In addition, the average particle diameter (D50) as used in the field of this invention is a median diameter, and when a powder is divided into two from a certain particle diameter, it means the diameter from which a larger side becomes equal to a smaller side.

  Further, by forming a low porosity region and a high porosity region in the surface direction of the current collector, in the low porosity region (region where the density of the active material is high), there is no contact between the current collector and the active material. In addition, in the high porosity region (region where the density of the active material is low), the penetration of the electrolytic solution can be improved. By both of these effects, the conductivity of electrons and lithium ions can be improved and high output characteristics can be obtained.

In addition, if the density of the low porosity region (region where the density of the active material is high) is too high, there may be a region where the infiltration of the electrolyte is inhibited and the charge / discharge reaction does not proceed. If the density of the region where the density of the substance is low) is too low, the adhesion may be insufficient and the cycle characteristics may deteriorate. As a result of intensive studies, the inventors have made the difference in density between the low porosity region and the high porosity region within the range of 0.2 to 0.7 g / cm ³ , so that the cycle characteristics do not deteriorate. It has been found that an optimal charge / discharge reaction can be performed.

  According to claim 2 of the present invention, the high porosity region is continuously formed from the end in the surface direction of the current collector to the center, so that the electrolyte enters the center from the electrode end. Thus, sufficient electron and lithium ion conductivity can be obtained even at the center of the electrode, and stable output characteristics can be obtained.

  In addition, according to claim 3, by making the area of the high porosity region occupying the outermost surface of the active material layer smaller than the area of the low porosity region, the entire active material layer adheres to the current collector. Therefore, the cycle characteristics can be kept at a high level, and the deterioration of the cycle characteristics can be prevented.

According to claim 4 of the present invention, the active material layer composed of the low porosity region and the high porosity region having different area ratios in a thickness direction of the current collector is laminated in multiple layers, It becomes easier for the electrolytic solution to enter from the outside to the inside of the active material layer, and sufficient electron and lithium ion conductivity is obtained even at the center of the electrode, so that stable output characteristics can be obtained.

  Further, according to claim 5, by setting the interval between the low porosity regions adjacent to the left and right and up and down of the high porosity region to 1 mm or less, the adhesion of the entire active material layer to the current collector is high. The level can be maintained, and the deterioration of cycle characteristics can be prevented.

  In addition, according to claim 6, by setting the average particle size (D50) of the granular active material to 15 μm or less, the stable active material having no defects such as stripes even when the active material layer is thinned. A material layer can be formed.

  As described above, according to the present invention, an electrode for a lithium ion secondary battery with high output characteristics and sufficient electron conductivity and lithium ion conductivity, and a lithium ion secondary battery using the same are provided. can do.

1 is a schematic diagram 1 of an electrode 10 for a secondary battery showing an example of the present invention. (A) The cross-sectional schematic diagram of the electrode 10 for secondary batteries which shows an example of this invention. (B) A schematic plan view of the above. Schematic 2 of the electrode 10 for secondary batteries which shows an example of this invention. Schematic 3 of the electrode 10 for secondary batteries which shows an example of this invention. The cross-sectional schematic diagram of the cylindrical lithium ion secondary battery 20 to which this invention is applied. The discharge capacity maintenance rate of the secondary battery to which the present invention is applied.

  Below, the electrode for lithium ion secondary batteries which concerns on embodiment of this invention is demonstrated. The embodiments of the present invention are not limited to the embodiments described below, and modifications such as design changes can be added based on the knowledge of those skilled in the art. Embodiments to which is added can also be included in the scope of the embodiments of the present invention.

  FIG. 1 is a schematic view 1 of an electrode 10 for a lithium ion secondary battery showing an example of the present invention. The active material layer 5 of the electrode for a lithium ion secondary battery formed on the current collector 1 is formed of a composition containing the active material 4, a binder, and a conductive additive, and has a low porosity region (active material layer). A region having a high density (hereinafter referred to as a high density portion) 3 and a high porosity region (a region having a low active material density, hereinafter referred to as a low density portion) 2 are configured.

Wherein it is preferably about 1.0~2.8g / cm ³ as the density of the less dense portion 2, preferably about 1.5~3.5g / cm ³ as the density of the dense portion 3. The optimum value of each density differs depending on the active material, that is, whether it is a positive electrode or a negative electrode.

If the density of the low-density portion 2 is too low, the adhesion may be deteriorated and cycle characteristics may be deteriorated. On the other hand, if the density of the high-density portion 3 is too high, impregnation with the electrolytic solution is hindered, and a region where the charge / discharge reaction does not proceed is formed. On the other hand, if the difference in density between the two is too small, the effects of the invention cannot be obtained sufficiently. As a result of intensive studies, the inventors have obtained a result that the difference is preferably in the range of 0.2 to 0.7 g / cm ³ .

The configuration of the active material layer 5 will be described in detail. Fig.2 (a) is a cross-sectional schematic diagram of the electrode 10 for secondary batteries which shows an example of this invention. The film thickness of the active material layer 5 is 10 times or less the average particle diameter (D50) of the active material 4. If it is larger than 10 times, the distance from the current collector 1 to the active material in the vicinity of the surface of the active material layer 5 becomes long, and sufficient electron conductivity may not be ensured, and the output characteristics may deteriorate.

  FIG. 2B is a schematic plan view of the secondary battery electrode 10 showing an example of the present invention. The active material layer 5 is composed of a low-density portion 2 and a high-density portion 3, and if the low-density portion 2 is continuously formed from the end in the surface direction of the active material layer 5 to the center, the low-density portion 2 is formed. The shape of is not particularly limited. For example, as shown in FIG. 1, a low-density portion 2 that is continuous from one end to the other end may be formed, and the low-density portion may be formed as shown in the schematic diagrams of FIGS. 3 and 4. It may be formed. If the low-density part 2 is not continuously formed from the end in the plane direction to the center, the impregnation with the electrolyte is insufficient, and sufficient lithium ion conductivity is not ensured at the center of the electrode. May decrease.

  The area of the low density portion 2 constituting the active material layer 5 is not particularly limited as long as it is smaller than the area of the high density portion 3. When the area of the low-density portion 2 is equal to or greater than the area of the high-density portion 3, the adhesiveness of the active material layer 5 as a whole decreases and the cycle characteristics deteriorate, or the electronic conductivity of the active material layer 5 as a whole May decrease and output characteristics may deteriorate.

  It is desirable that the interval between the high-density portions 3 adjacent to each other with the low-density portion 2 constituting the active material layer 5 is 1 mm or less, and in consideration of securing sufficient adhesion and electronic conductivity and obtaining sufficient output characteristics, Preferably it is 10 times or less of the film thickness.

  In the active material layer 5 of the present invention, a slurry obtained by adding a solvent to a mixture of the active material 4, a binder, a conductive additive, and the like is applied to the current collector 1 and dried, and then partially pressed to make the pressed portion highly dense. It can be formed by using the portion 3. The solvent is not particularly limited as long as it can dissolve the binder resin, and examples thereof include organic solvents such as N-methylpyrrolidone and N, N-dimethylformamide, and water.

  As the current collector 1, a material conventionally used as a current collector material for a secondary battery may be appropriately employed. For example, aluminum, nickel, copper, iron, stainless steel (SUS), titanium and the like can be mentioned, and it is preferable to select in consideration of battery operating potential and electronic conductivity applied to the current collector. The general thickness of the current collector 1 is about 8 to 30 μm.

  When a positive electrode active material is used as the active material 4, any material that can occlude and release lithium can be used, and known positive electrode active materials for lithium ion secondary batteries can be used. For example, lithium manganese oxide, lithium nickel oxide, lithium cobalt oxide, lithium iron oxide and lithium nickel manganese oxide, lithium nickel cobalt oxide, lithium nickel manganese cobalt oxide, lithium transition metal phosphate compound, etc. are used. be able to. Note that a plurality of the above active materials may be mixed and used as the positive electrode active material.

  When a negative electrode active material is used as the active material 4, any material that can occlude and release lithium can be used, and a known negative electrode active material for a lithium ion secondary battery can be used. For example, carbon materials such as graphite-based carbon materials, hard carbon, soft carbon, activated carbon, lithium metal oxides such as lithium titanium oxide, Li alloy metals such as silicon and tin can be used. Note that a plurality of the above active materials may be mixed and used as the negative electrode active material.

  The average particle diameter (D50) of the active material 4 is desirably 15 μm or less. When the thickness is 15 μm or more, when the thickness of the active material layer 5 is reduced, it is difficult to apply and form the active material layer 5 without defects such as streaks and particle clogging, and the charge / discharge performance may deteriorate. . On the other hand, when the film thickness is increased to avoid streaks and particle clogging, the distance from the current collector 1 to the active material near the surface of the active material layer 5 is increased, and sufficient electron conductivity is ensured. In some cases, the output characteristics may deteriorate.

  The electrode for a lithium ion secondary battery of the present invention may contain a conductive additive. As the conductive aid, carbon black, natural graphite, artificial graphite, metal oxides such as titanium oxide and ruthenium oxide, metal fibers, and the like can be used. Among these, carbon black having a structure structure is preferable, and furnace black, ketjen black, and acetylene black (AB), which are one of them, are particularly preferable. A mixed system of carbon black and other conductive assistants such as vapor grown carbon fiber (VGCF) is also preferably used.

  The content of the conductive assistant is preferably 1% by weight or more and less than 90% by weight with respect to the weight of the active material. If it is less than 1% by weight, conductivity may be insufficient and electrode resistance may increase. If it is 90% by weight or more, the amount of active material may be insufficient and the lithium storage capacity may decrease.

  The electrode for a lithium ion secondary battery of the present invention may contain a binder. The binder is not particularly limited as long as the mixture of the active material and the conductive additive can be adhered to the current collector, and examples thereof include fluorine-containing binders such as polyvinylidene fluoride and polytetrafluoroethylene, and synthetic rubber binders.

  The binder contained in the electrode for a lithium ion secondary battery of the present invention is desirably 3% by weight or more and 40% by weight or less based on the total weight of the active material. When the amount is less than 3% by weight, sufficient binding cannot be achieved, and when the amount is more than 40% by weight, the capacity per electrode volume is greatly reduced. More preferably, it is 3 to 25% by weight.

  FIG. 5 is a schematic cross-sectional view of a cylindrical lithium ion secondary battery 20 to which the present invention is applied. With reference to FIG. 5, the lithium ion secondary battery which concerns on embodiment of this invention is demonstrated.

  A cylindrical lithium ion secondary battery 20 to which the present invention is applied has a bottomed cylindrical battery can 11 made of iron and plated with nickel as a battery container. In the battery can 11, a positive electrode 10 a, a negative electrode 10 b, and a separator 12 formed in a band shape are wound and accommodated in a spiral cross section.

  The separator 12 is disposed between the positive electrode 10a and the negative electrode 10b arranged to face each other, and the positive electrode 10a and the negative electrode 10b are electrically insulated by the separator 12. As the separator 12, for example, a microporous film made of polyolefin such as polyethylene or polypropylene, a microporous film made of aromatic polyamide resin, a nonwoven fabric, a porous resin coat containing inorganic ceramic powder, or the like can be used.

  Above the wound group of the positive electrode 10a, the negative electrode 10b, and the separator 12, a ribbon-shaped aluminum positive electrode tab terminal 13 having one end fixed to the positive electrode current collector 1a is led out. The other end of the positive electrode tab terminal 13 is ultrasonically welded to the lower surface of a disk-shaped upper lid 14 that is disposed on the upper portion of the battery can 11 and serves as a positive electrode external terminal. On the other hand, a ribbon-like nickel negative electrode tab terminal 15 with one end fixed to the negative electrode current collector 1b is led out below the winding group. The other end of the negative electrode tab terminal 13 is joined to the inner bottom surface of the battery can 11 by resistance welding. That is, the positive electrode tab terminal 13 and the negative electrode tab terminal 15 are led out to the opposite sides from both end surfaces of the wound group, respectively. Although not shown, resin insulating plates are arranged on both the upper and lower sides of the wound group, respectively, and an insulating coating is also applied to the entire outer peripheral surface of the wound group.

Grooving is applied to the upper part of the battery can 11. The upper lid 14 is caulked and fixed to the upper portion of the battery can 11 via a gasket 16 designed to fit into the grooving portion. For this reason, the inside of the lithium ion secondary battery 20 is sealed. Gasket 16
Is a member for preventing a short circuit and preventing leakage of the electrolytic solution, and an insulating material such as polypropylene can be used.

In addition, the battery can 11 is filled with a non-aqueous electrolyte composed of a solvent and an electrolyte. Examples of the solvent for the electrolyte used in the lithium ion secondary battery of the present invention include low-viscosity chain carbonates such as dimethyl carbonate and diethyl carbonate, and cyclic carbonates having a high dielectric constant such as ethylene carbonate, propylene carbonate, and butylene carbonate. Γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, methyl propionate, vinylene carbonate, dimethylformamide, sulfolane, and mixed solvents thereof. it can.

The electrolyte contained in the electrolytic solution is not particularly limited, and examples thereof include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiI, LiAlCl _4, and mixtures thereof. It is done. A lithium salt obtained by mixing one or more of LiBF ₄ and LiPF ₆ is preferable.

  Hereinafter, the electrode for a lithium ion secondary battery and the manufacturing method thereof according to the present invention will be described with reference to specific examples and comparative examples. In addition, this invention is not restrict | limited by the following Example.

(Example 1)
<Preparation of positive electrode>
Lithium manganese oxide having an average particle diameter (D50) of 12 μm as the positive electrode active material, acetylene black as the conductive auxiliary agent, and polyvinylidene fluoride (PVdF) as the binder were mixed in a ratio of 88: 7: 5, respectively, using a planetary mixer. After kneading, an appropriate amount of N-methyl-2-pyrrolidone as a solvent was added to adjust the viscosity to obtain a positive electrode slurry for a lithium ion secondary battery.

  As the positive electrode current collector, an aluminum foil (20 μm thickness) was used, and the positive electrode slurry prepared above was applied thereon with a die coater so that the coating film became 50 μm after drying, and dried to obtain a coating film. . Next, a quartz mold was placed on the positive electrode coating film and roll-pressed to complete the positive electrode. The mold used had a shape in which grooves having a width of 100 μm and a depth of 30 μm were formed at intervals of 200 μm in the long side direction of the electrode. As a result of confirming the film thickness of the obtained positive electrode with an electron microscope, the film thickness of the low-density part was 33 μm, and the film thickness of the high-density part was 28 μm.

<Production of negative electrode>
Next, natural graphite as a negative electrode active material, acetylene black as a conductive additive, styrene butadiene rubber as a binder, and carboxymethyl cellulose as a thickener are mixed in a ratio of 90: 8: 1: 1, kneaded with a disper, As described above, an appropriate amount of pure water was added to adjust the viscosity to obtain a negative electrode slurry for a lithium ion secondary battery.

  The obtained negative electrode slurry was applied to the negative electrode current collector with a die coater and dried to obtain a coating film. A copper foil (10 μm thick) was used as the negative electrode current collector. The negative electrode active material layer was applied with the basis weight adjusted to 1.1 times the capacity of the positive electrode active material layer.

<Production of cell>
The obtained positive electrode and negative electrode are wound with a separator (model number 2200, manufactured by Celgard) facing each other, tabbed, sealed in a battery can, and then injected with the following electrolyte solution to obtain a lithium ion secondary battery. Produced. In addition, LiPF6 was added so that it might become 1M to the mixed solution of ethylene carbonate (EC) and diethyl carbonate (DMC) 3: 7 (volume ratio) as electrolyte solution, and also 2 weight% of vinylene carbonate (VC) was added. I used something.

(Example 2)
As a mold, a groove having a width of 100 μm and a depth of 30 μm exists in one position corresponding to the center in the short side direction of the electrode, and has a length from the center groove to a position corresponding to the end portion in the short side direction of the electrode. A lithium ion secondary battery was produced in the same manner as in Example 1 except that a positive electrode was produced using a mold having a shape in which grooves having a thickness of 30 μm were formed at intervals of 200 μm. The film thickness of the low density portion was 33 μm, and the film thickness of the high density portion was 28 μm.

(Example 3)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that a positive electrode was produced using a mold having a shape in which grooves having a width of 500 μm and a depth of 30 μm were formed at intervals of 1000 μm. The film thickness of the low density portion was 33 μm, and the film thickness of the high density portion was 28 μm.

Example 4
A lithium ion secondary battery was fabricated in the same manner as in Example 1 except that a positive electrode was produced using a mold having a shape in which grooves having a width of 100 μm and a depth of 30 μm were formed at intervals of 200 μm in the short side direction of the electrode. Produced. The film thickness of the low density portion was 33 μm, and the film thickness of the high density portion was 28 μm.

(Comparative Example 1)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the positive electrode was produced by pressing without using a mold. The active material layer had a thickness of 28 μm.

(Comparative Example 2)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the positive electrode slurry was dried and then applied so that the coating film became 150 μm to produce a positive electrode. The film thickness of the low density portion was 105 μm, and the film thickness of the high density portion was 85 μm.

<Evaluation>
The discharge rate test was done with respect to the lithium ion secondary battery produced by each Example and the comparative example. Charging / discharging was performed at 3.0V to 4.2V. First, as an initial discharge capacity evaluation, a constant current charge / discharge at 0.2 C was performed once, and then a discharge rate test was performed at 1 C, 5 C, 10 C, and 20 C. All charging during the discharge rate test was performed at 0.2C. FIG. 6 shows the discharge capacity maintenance rate for each discharge rate when 0.2 C is 100% for each battery.

<Comparison result>
In 1C, a big difference is not seen by Examples 1-4 and Comparative Examples 1-2, but Examples 1-4 are discharge capacity maintenance factors rather than Comparative Example 1 and Comparative Example 2 on the high output conditions of 2C or more. Was found to be good. Therefore, it is thought that Examples 1-4 which have a low-density part and a high-density part are easy to impregnate electrolyte solution compared with the comparative example 1 with a uniform in-plane porosity, and lithium ion conductivity is ensured. On the other hand, Comparative Example 2 has a low-density part and a high-density part as in Examples 1 to 4, but because the film thickness is thick, the electrolyte solution hardly penetrates in the thickness direction and sufficient lithium ion conductivity could not be ensured. Conceivable. From the above, the effect of the present invention was confirmed.

According to the present invention, in an electrode in which the film thickness of the active material layer is 10 times or less of the active material particle size (D50), a low-density portion with a relatively high porosity and a relatively low porosity in the surface direction of the active material layer. Has a small and high-density part, and the low-density part is continuously formed from the edge in the surface direction of the active material layer to the center part, so that the electron conductivity and lithium ion conductivity in the electrode surface direction are sufficient. Therefore, there is an effect that the high output characteristics are improved, and the industrial utility value is high. Therefore, the negative electrode for a lithium ion secondary battery of the present invention can be suitably used as a storage battery for a drive battery of an electric vehicle, a power storage facility for various energy, a home power storage facility, etc. that require high durability.

DESCRIPTION OF SYMBOLS 1 .... Current collector 1a ... Positive electrode current collector 1b ... Negative electrode current collector 2 .... High porosity region (region with low density of active material, low density portion)
3. Low porosity area (active material high density area, high density area)
4 ... Active material 5 ... Active material layer 10 ... Electrode for lithium ion secondary battery 10a ... Positive electrode 10b ... Negative electrode 11 ... Battery can 12 ... Separator 13 ... Positive electrode Tab terminal 14 ... Upper lid 15 ... Negative electrode tab terminal 16 ... Gasket 20 ... Lithium ion secondary battery

Claims (8)

An electrode for a lithium ion secondary battery in which an active material layer containing a granular active material, a binder, and a conductive additive is laminated on the surface of a current collector,
The active material layer is
Having a low porosity region and a high porosity region in the surface direction of the current collector,
In the range where the layer thickness of the high porosity region is 1 to 10 times the average particle size of the granular active material,
And the difference in the density of the said low-porosity area | region and the said high-porosity area | region is the range of 0.2-0.7 g / cm < ³ >, The electrode for lithium ion secondary batteries characterized by the above-mentioned.   2. The electrode for a lithium ion secondary battery according to claim 1, wherein the high porosity region is continuously formed from an end portion in a plane direction of the current collector to a central portion.   The electrode for a lithium ion secondary battery according to claim 1 or 2, wherein an area of the high porosity region occupying the outermost surface of the active material layer is smaller than an area of the low porosity region.   The active material layer which consists of the said low-porosity area | region from which the area ratio differs, and the said high porosity area | region with respect to the thickness direction of the said electrical power collector is laminated | stacked in multiple layers, The any one of Claims 1-3 characterized by the above-mentioned. An electrode for a lithium ion secondary battery according to claim 1.   The electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein an interval between the low porosity regions adjacent to the left and right and up and down of the high porosity region is 1 mm or less.   6. The electrode for a lithium ion secondary battery according to claim 1, wherein the granular active material has an average particle size of 15 [mu] m or less.   A lithium ion secondary battery using the electrode for a lithium ion secondary battery according to claim 1.   The electrode for a lithium ion secondary battery is wound, and has a high porosity region continuous from the center of the electrode surface toward the electrode end perpendicular to the winding direction. The lithium ion secondary battery described in 1.

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