JPH11126600A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery which can work with good charge-discharge cycle characteristics, by coating a carbon material on a negative electrode current collector to a large thickness so that no exfoliation will be made. SOLUTION: A lithium ion secondary battery is composed of a negative electrode 2 formed by coating a negative electrode black mixture to a negative electrode current collector and a positive electrode 1 formed by coating a positive electrode black mixture to a positive electrode current collector, wherein the negative electrode black mixture is prepared from scale-form graphite (A), carbon material (B) with the mean particle size below 20 μm, and a binding agent (C), and when the coating film of the negative electrode black mixture is to be formed on the negative electrode current collector, the coating film structure is divided into an upper layer and a lower layer, and it is arranged so that the carbon material (B) content by wt. of the lower layer is greater than the carbon material (B) content by wt. of the whole coating film including the upper and lower layers.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium ion secondary battery having particularly excellent charge / discharge cycle characteristics.

[0002]

2. Description of the Related Art Lithium-ion batteries have recently been used in portable electronic devices such as mobile phones and notebook computers because they are smaller, lighter and have higher energy density than conventional secondary batteries typified by nickel cadmium batteries. Secondary batteries are used. Lithium-ion secondary batteries are also expected to be used as large-capacity power supplies for use in electric vehicles, electric power storage, and the like, and research for their practical use is being actively conducted.

As a positive electrode of a lithium ion secondary battery, a positive electrode current collector such as an aluminum foil coated with a lithium-containing metal composite oxide typified by lithium cobalt oxide (LiCoO2) is used. Lithium cobaltate has a structure in which lithium ions are inserted between layers formed of one cobalt atom and two oxygen atoms.

On the other hand, as a negative electrode of a lithium ion secondary battery, a negative electrode current collector such as a copper foil coated with a negative electrode mixture obtained by mixing carbon such as graphite and a binder is coated. Carbon has a layer structure similar to that of the lithium cobalt oxide described above, although the degree varies depending on the type thereof, and lithium ions extracted from the lithium cobalt oxide of the positive electrode during charging are inserted (doped) between carbon layers of the negative electrode. ) Is done. Conversely, at the time of discharge, lithium ions are extracted from the carbon layer (dedoped) and move again to the lithium cobalt oxide layer of the positive electrode.

Therefore, as the amount of carbon in the negative electrode mixture increases, more lithium can be doped, and the charge / discharge cycle characteristics of the battery can be improved.

[0006]

However, the carbon material in the negative electrode mixture coated on the negative electrode current collector expands and contracts repeatedly due to doping and undoping of lithium during charging and discharging.
Since the film shrinks, the coating film is easily peeled off from the negative electrode current collector, and thus the amount of the carbon material contained in the negative electrode mixture and the thickness of the coating film are limited. Increasing the amount of the binder can strengthen the connection between the negative electrode current collector and the carbon material or between the carbon materials, but if the amount of the binder is increased too much, the electrochemical characteristics of the electrode will be affected. On the contrary, the charge / discharge cycle characteristics are deteriorated.

The present invention has been made in view of such circumstances, and an object of the present invention is to obtain a sufficient charge / discharge cycle characteristic by forming a thick coating on a negative electrode current collector so as not to peel off a carbon material. To provide a lithium-ion secondary battery that can be used.

[0008]

According to the present invention, there is provided a negative electrode comprising a negative electrode current collector coated with a negative electrode mixture, and a positive electrode current collector coated with a positive electrode mixture. In a lithium ion secondary battery having a positive electrode, the negative electrode mixture is flaky graphite (A), a carbon material (B) having an average particle size smaller than that of the flaky graphite (A), and a binder ( C), and the negative electrode mixture is coated on the negative electrode current collector in two layers, an upper layer coating film and a lower layer coating film, and the weight content of the carbon material (B) in the lower layer coating film Was set to be larger than the weight content of the carbon material (B) in the entire coating film including the upper coating film and the lower coating film (claim 1).

As described above, the flaky graphite (A) and the carbon material (B) having an average particle size smaller than that of the flaky graphite (A) are used as the carbon component of the negative electrode mixture, When coating on the anode, it is divided into two layers, an upper layer and a lower layer. The lower layer coating film adjacent to the negative electrode current collector contains a large amount of the carbon material (B) having a small particle size, and the scale having a large particle size correspondingly. When the graphite is reduced and unevenly distributed, not only does the amount of the binder in the lower layer coating increase and the adhesiveness with the negative electrode current collector improves, but also the particles bind strongly. In addition, since the carbon material (B) having a small particle size is contained in a large amount, the amount of expansion and contraction of the lower coating film during charging and discharging is reduced, thereby reducing distortion and making it difficult to peel off from the negative electrode current collector. .
Therefore, compared to the case where the flaky graphite (A), the carbon material (B), and the binder (C) are uniformly kneaded without being unevenly distributed in the thickness direction and coated in a single layer, the negative electrode current collection is performed. The negative electrode mixture can be coated thickly on the body, so that the amount of carbon in the negative electrode can be increased to improve the charge / discharge cycle characteristics. Here, the flaky graphite, the ab plane of graphite is larger than the c-axis direction,
In other words, it is graphite having a plate-like shape like mica.
In addition, flaky graphite usually has such a shape, and some artificial graphite has such a shape. On the other hand, the carbon material is graphite having a general rock shape.

The weight content of the carbon material (B) in the lower coating film is at least 119% of the weight content of the carbon material (B) in the entire coating film including the upper coating film and the lower coating film. It is preferably 168% or less (claim 2).

Further, it is preferable that the thickness of the lower coating film is 10% or more and 60% or less of the total thickness of the coating film including the upper coating film and the lower coating film. Item 3).

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail. In the following description, the weight content is
The amount of the substance P contained in the substance Q is compared by weight, and this is expressed as a ratio. For example, when the substance Q is composed of x (g weight) of the substance X and y (g weight) of the substance Y, the weight content of the substance X in the substance Q is x / (x +
y).

[Configuration of Lithium Ion Secondary Battery] FIG.
Represents AA (1) used in Examples and Comparative Examples described later.
FIG. 4 is a longitudinal sectional view of a 4.5-mm × 50-mm) wound lithium ion secondary battery. In this figure, 1 is a positive electrode sheet, 2 is a negative electrode sheet, 3 is a porous film separator made of polypropylene, 4 is a positive electrode lead made of titanium, 5 is a negative electrode lead made of nickel, 6 is a sealing plate made of stainless steel, 7
Is a positive electrode cap / positive terminal plate made of aluminum, 8 is an insulating bottom plate made of polypropylene, 9 is an insulating gasket made of polypropylene, 10 is a safety valve, and 11 is a case.

The positive electrode sheet 1 is obtained by coating a positive electrode mixture on both surfaces of a 30 μm-thick aluminum foil prepared as a positive electrode current collector, further drying and rolling, and then cutting into a sheet. Further, a part of the positive electrode mixture was scraped off perpendicularly to the longitudinal direction of the positive electrode sheet 1 to expose the current collector, and the positive electrode lead 4 was spot-welded thereto.

The negative electrode sheet 2 is formed by coating a negative electrode mixture on both surfaces of a copper foil prepared as a negative electrode current collector, drying the mixture, and cutting it into a sheet. In addition, the negative electrode sheet 2
A portion of the negative electrode mixture was scraped off perpendicularly to the longitudinal direction to expose the current collector, and the negative electrode lead 5 was spot-welded thereto.

The positive electrode sheet 1 and the negative electrode sheet 2 are spirally wound with a separator 11 interposed therebetween.
Has been inserted. Further, the other end of the positive electrode lead 4 welded to the positive electrode sheet 1 is spot-welded to the sealing plate 6, and the other end of the negative electrode lead 5 spot-welded to the negative electrode sheet 2 is connected to the case 11 serving as the negative electrode terminal. Spot welded to the center of the circular bottom. A hole is formed in the insulating bottom plate 8 so as to have the same area as the space generated at the time of winding.

The positive electrode terminal plate 7 is welded to the sealing plate 6 in advance. The safety valve 10 is for releasing a gas to the outside by breaking a part of the battery when the internal pressure of the battery rises due to an abnormality in the battery.

Further, in the case, an electrolytic solution prepared by dissolving LiPF6 to 1 mol / l in a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1 (2.3 ml) was prepared. ) Injected.

[Preparation of Examples and Comparative Examples] Next, using the lithium ion secondary battery having the above-described structure, batteries of the following Examples and Comparative Examples were prepared. In preparing a battery, flaky graphite (A) as a negative electrode mixture, a carbon material (B) having an average particle size of 6 μm, and polyvinylidene fluoride (hereinafter abbreviated as PVDF) are dissolved in N-methylpyrroline. 50: 50: 5 with the prepared binder (C)
(However, the ratio of PVDF is the weight ratio of the solid content), and the same amount was prepared in each Example and each Comparative Example.

As the positive electrode mixture, a composite oxide (LiCoO
2), a conductive material (carbon powder), and a binder composed of an aqueous dispersion of polytetrafluoroethylene (PTFE) in a weight of 100: 10: 10 (the ratio of the aqueous dispersion is the solid content) The mixture was mixed in a ratio, and water was added to the mixture, and the mixture was kneaded to prepare a paste.
In all Examples and Comparative Examples, the coating of the positive electrode mixture on the positive electrode current collector was performed in a single layer.

(Example 1) All of the negative electrode mixtures prepared for this example were used for the lower layer coating and the upper layer coating.
Negative electrode mixtures were prepared, and each was coated on the negative electrode current collector with the same thickness. The weight ratio of the flaky graphite (A), the carbon material (B), and the binder (C) is 2 for the negative electrode mixture for the lower layer coating film.
5: 75: 6. The negative electrode mixture for the upper layer coating film is flake graphite (A), carbon material (B), binder (C)
Was 75: 25: 4.

Example 2 Using all the negative electrode mixtures prepared for the present example, two negative electrode mixtures for the lower layer coating and the upper layer coating were used.
Negative electrode mixtures were prepared, and each was coated on the negative electrode current collector in the same thickness as in Example 1. The weight ratio of the flaky graphite (A), the carbon material (B), and the binder (C) was 40: 60: 6 in the negative electrode mixture for the lower layer coating film. In the negative electrode mixture for the upper layer coating film, flaky graphite (A), carbon material (B), and binder (C) were adjusted to have a weight ratio of 60: 40: 4.

(Example 3) All of the negative electrode mixtures prepared for this example were used for the lower layer coating and the upper layer coating.
Negative electrode mixtures were prepared, and each was coated on the negative electrode current collector in the same thickness as in Example 1. The weight ratio of the flaky graphite (A), the carbon material (B), and the binder (C) was adjusted to 15: 85: 6 in the negative electrode mixture for the lower layer coating film. In the negative electrode mixture for the upper layer coating film, flake graphite (A), carbon material (B), and binder (C) were adjusted to have a weight ratio of 85: 15: 4.

(Comparative Example 1) The flake graphite (A), the carbon material (B), and the binder ( C) was mixed at a weight ratio of 50: 50: 5 with a negative electrode mixture prepared on both surfaces in a thickness equal to the entire thickness of the coating film including the upper and lower coating films in Example 1. Coated.

(Comparative Example 2) Two kinds of negative electrode mixtures for the lower layer coating film and the upper layer coating film were prepared, and each was coated on the negative electrode current collector in the same thickness as in Example 1. did. The weight ratio of the flaky graphite (A), the carbon material (B), and the binder (C) was set to 50: 50: 6 for the negative electrode mixture for the lower layer coating film. The negative electrode mixture for the upper layer coating film was composed of flaky graphite (A), carbon material (B), and binder (C) in a ratio of 50:50:
4 by weight.

(Comparative Example 3) All of the negative electrode mixtures prepared for this comparative example were used for the lower layer coating and the upper layer coating.
Negative electrode mixtures were prepared, and each was coated on the negative electrode current collector in the same thickness as in Example 1. The weight ratio of the flaky graphite (A), the carbon material (B), and the binder (C) was set to 45: 55: 6 in the negative electrode mixture for the lower layer coating film. In the negative electrode mixture for the upper layer coating film, the flaky graphite (A), the carbon material (B), and the binder (C) were adjusted to have a weight ratio of 55: 45: 4.

(Comparative Example 4) Using all the negative electrode mixtures prepared for this comparative example, two
Various kinds of negative electrode mixtures were prepared, and each was coated on the negative electrode current collector in the same thickness as in Example 1. The weight ratio of the flaky graphite (A), the carbon material (B), and the binder (C) was set to 10: 90: 6 in the negative electrode mixture for the lower layer coating film. In the negative electrode mixture for the upper layer coating film, the flake graphite (A), the carbon material (B), and the binder (C) were adjusted to have a weight ratio of 90: 10: 4.

(Comparative Example 5) All of the negative electrode mixtures prepared for this comparative example were used for the lower layer coating film and the upper layer coating film.
Various kinds of negative electrode mixtures were prepared, and each was coated on the negative electrode current collector in the same thickness as in Example 1. The weight ratio of the flaky graphite (A), the carbon material (B), and the binder (C) was 5: 95: 6 in the negative electrode mixture for the lower layer coating film. In the negative electrode mixture for the upper layer coating film, the flaky graphite (A), the carbon material (B), and the binder (C) were adjusted to have a weight ratio of 95: 5: 4.

[Test of the Batteries Prepared] The batteries of the examples and the comparative examples prepared above were charged to 4.1 V at a constant current of 40 mA in a room temperature environment as a first cycle and then charged to 400 V.
The battery was discharged at a constant current of mA until the battery voltage reached 2.8 V, and the initial discharge capacity (mAh) was measured.

Thereafter, charge and discharge were repeated 100 cycles, and the discharge capacity (mAh) at the 100th cycle was examined. Further, the discharge capacity at the 100th cycle was divided by the initial discharge capacity to obtain a charge / discharge cycle retention rate. Table 1 shows the above results.

[0031]

[Table 1]

As is apparent from Table 1, the charge / discharge cycle capacity retention ratio in Comparative Example 1 in which the negative electrode mixture was coated in a single layer was considerably lower than in Examples 1 to 3 and Comparative Examples 2 to 5. Value. From this, it is understood that the charge / discharge cycle capacity retention ratio is more improved in the case of two layers than in the case of a single layer.

Further, from the results of Examples 1 to 3, the weight ratio of the carbon material (B) contained in the negative electrode mixture for the lower coating film was calculated by the weight of the carbon material (B) contained in the negative electrode mixture for the upper coating film. When the ratio is larger than the ratio, in particular, the weight ratio of the flaky graphite (A), the carbon material (B), and the binder (C) is from 15: 85: 6 (Example 3) to 40: 60: 6 (Example). It is understood that in the case of 2), a high charge / discharge cycle capacity retention rate can be obtained.

Here, the weight content of the carbon material (B) contained in the negative electrode mixture of the lower coating film in the case of Example 3 was calculated to be 85 / (15 + 85 + 6) [≒ 80.1%]. In the case of Example 2, the weight content of the carbon material (B) contained in the negative electrode mixture of the lower coating film was 60 /
(60 + 40 + 6) [＋ 56.6%] is calculated.

On the other hand, looking at the total weight ratio of the lower layer coating film and the upper layer coating film, Examples 1 to 3 show that the flake graphite (A), the carbon material (B) and the binder (C) The blending ratios are the same, 100: 100: 10, and the weight content of the carbon material (B) is 100/210 [≒ 47.6%], so that the carbon material in the entire coating film is the same. Assuming that the weight content of (B) is a reference value, in Example 3, (85/1
06) / (100/210) ≒ 1.68, which means that the carbon material (B) containing 68% of the reference value is included. Similarly, in Example 2, (60/106) /
(100/210) ≒ 1.19, which includes the carbon material (B) which is 19% higher than the reference value.

That is, the weight content of the carbon material (B) in the lower coating film is more than the weight content of the carbon material (B) in the entire coating film including the upper coating film and the lower coating film. In other words, when it is 119% or more and 168% or less, the charge / discharge cycle capacity retention rate is improved.

[Relationship Between Thickness of Lower Coating Film and Charge / Discharge Cycle Capacity Retention Ratio] Next, the relationship between the thickness of the lower coating film and the corresponding charge / discharge cycle capacity retention ratio was examined. In this test, the weight ratio of the negative electrode mixture for the lower layer coating composed of flaky graphite (A), carbon material (B) and binder (C) was 2
5: 75: 6 according to Example 1 above, and the weight ratio according to Example 2 above is 40: 60: 6, and further, the weight ratio is 15: 85: 6. 3 was performed for each of them.

First, according to the first embodiment,
Using the negative electrode mixture for the lower layer coating and the negative electrode mixture for the upper layer coating used in Example 1, the thickness of the lower layer coating was 10 times the total thickness including the upper layer and the lower layer coating. % (Example 4), 25% (Example 5), 60% (Example 6), 5%
(Comparative Example 7), 65% (Comparative Example 8), and 70% (Comparative Example 9) batteries were produced. The structure other than the thicknesses of the lower and upper coating films is exactly the same as the battery prepared in Example 1.

Each of the batteries of Examples 4 to 6, the batteries of Comparative Examples 7 to 9, and the battery of Example 1 prepared as described above was subjected to the first cycle at room temperature in the environment of 4 cycles.
After charging to 4.1 V with a constant current of 0 mA, 400 mA
Was discharged until the battery voltage reached 2.8 V, and the initial discharge capacity (mAh) was measured.

Thereafter, charging and discharging were repeated 100 cycles, and the discharge capacity (mAh) at the 100th cycle was examined. Further, the discharge capacity at the 100th cycle was divided by the initial discharge capacity to determine a charge / discharge cycle retention rate. Table 2 shows the above results.

[0041]

[Table 2]

Next, according to the second embodiment,
Using the negative electrode mixture for the lower layer coating and the negative electrode mixture for the upper layer coating used in Example 2, the thickness of the lower layer coating was 10 times the total thickness including the upper layer and the lower layer coating. % (Example 7), 25% (Example 8), 60% (Example 9), 5%
Batteries of (Comparative Example 10), 65% (Comparative Example 11), and 70% (Comparative Example 12) were prepared. The configuration other than the thicknesses of the lower and upper coating films is exactly the same as that of the battery prepared in Example 2.

Each of the batteries of Examples 7 to 9, the batteries of Comparative Examples 10 to 12 and the battery of Example 2 prepared as described above was subjected to a constant current of 40 mA as a first cycle under a room temperature environment. 400m after charging to 1V
The battery was discharged at a constant current of A until the battery voltage reached 2.8 V, and the initial discharge capacity (mAh) was measured.

Thereafter, the charge and discharge were repeated 100 cycles, and the discharge capacity (mAh) at the 100th cycle was examined. Further, the discharge capacity at the 100th cycle was divided by the initial discharge capacity to determine a charge / discharge cycle retention rate. Table 3 shows the above results.

[0045]

[Table 3]

Further, according to the third embodiment,
Using the negative electrode mixture for the lower layer coating and the negative electrode mixture for the upper layer coating used in Example 3, the thickness of the lower layer coating was 10 times the total thickness including the upper layer and the lower layer coating. % (Example 1
0), 25% (Example 11), 60% (Example 12),
5% (Comparative Example 13), 65% (Comparative Example 14), 70%
Batteries referred to as (Comparative Example 15) were prepared. The configuration other than the thicknesses of the lower and upper coating films is exactly the same as that of the battery prepared in Example 2.

The batteries of Examples 10 to 12, the batteries of Comparative Examples 13 to 15, and the Example 3 prepared as described above.
After charging each battery to 4.1 V at a constant current of 40 mA in a room temperature environment as a first cycle,
The battery was discharged at a constant current of mA until the battery voltage reached 2.8 V, and the initial discharge capacity (mAh) was measured.

Thereafter, the charge and discharge were repeated 100 cycles, and the discharge capacity (mAh) at the 100th cycle was examined. Further, the discharge capacity at the 100th cycle was divided by the initial discharge capacity to determine a charge / discharge cycle retention rate. Table 4 shows the above results.

[0049]

[Table 4]

As shown in Tables 2 to 4, the batteries of Examples 1 and 4 to 6, the batteries of Examples 2 and 7 to 9, the batteries of Examples 3 and 9, 1
In the case of batteries 0 to 12, that is, when the ratio of the lower coating film to the total thickness of the coating film is in the range of 10% to 60%, a high initial discharge capacity and a high charge / discharge cycle retention ratio can be obtained.

In the case of Comparative Examples 7, 10, and 12,
It is considered that the thickness of the lower layer coating film is too thin to ensure sufficient adhesion to the current collector, so that the coating film is peeled off and a sufficient charge / discharge cycle maintenance ratio cannot be obtained.

Further, Comparative Examples 8 to 9, Comparative Examples 11 to 12,
In the case of Comparative Examples 14 and 15, the lower layer coating film is too thick and the content of the binder is high, so that the electrode reaction is hindered and the initial discharge capacity is lower than in other examples. It is thought to be.

[Other Binders] By the way, in Examples and Comparative Examples described above, P was used as a binder.
VDF was used. Polytetrafluoroethylene (PTFE), polyacrylic acid, polyvinyl alcohol, carboxymethyl cellulose, diacetyl cellulose, hydroxydipropyl cellulose, SBR, EP as a binder
When using DM, sulfonated EPDM, fluoro rubber, polybutadiene, polyethylene oxide, etc.
The same result as in the case of using was obtained.

[0054]

According to the present invention, flaky graphite (A) and a carbon material (B) having an average particle diameter smaller than that of the flaky graphite (A) are used as the carbon component of the negative electrode mixture. The lower coating film adjacent to the negative electrode current collector contains a large amount of the carbon material (B) having a small particle size, and the scale-like graphite having a large particle size is reduced by that much. When these are unevenly distributed, not only the amount of the binder in the lower layer coating film is increased and the adhesiveness with the negative electrode current collector is improved, but also the particles are firmly bound to each other, and By containing a large amount of the carbon material (B) having a small diameter, the amount of expansion and contraction of the lower coating film during charging and discharging is reduced, and the resulting distortion is reduced, making it difficult for the negative electrode current collector to peel off. Therefore, as compared with a case where a negative electrode mixture in which the flaky graphite (A), the carbon material (B), and the binder (C) are uniformly kneaded without uneven distribution is coated in one layer, the negative electrode current collection is performed. The negative electrode mixture can be coated thickly on the body, so that the amount of carbon in the negative electrode can be increased to improve the charge / discharge cycle characteristics.

The ratio of the weight of the carbon material (B) contained in the lower coating film to the total weight of the lower coating film is determined by the ratio of the weight of the carbon material (B) contained in the entire coated negative electrode mixture to the negative electrode coating. 119% or more 1 based on the weight of the whole agent
When the content is 68% or less, a lithium ion secondary battery having good charge / discharge cycle characteristics can be obtained without the binder affecting the electrochemical characteristics of the electrode.

Further, the thickness of the lower layer coating film is 10% of the total thickness of the upper layer coating film and the lower layer coating film combined.
% To 60% or less, the adhesion to the current collector cannot be sufficiently ensured, the coating film does not peel off, and the electrode reaction is inhibited by the binder. It is possible to provide a lithium ion secondary battery having good charge-discharge cycle characteristics without any problem.

[Brief description of the drawings]

FIG. 1 is a view showing a longitudinal section of a lithium ion secondary battery according to one embodiment of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 positive electrode sheet 2 negative electrode sheet 3 separator 4 positive electrode lead 5 negative electrode lead 6 sealing plate 7 positive electrode terminal plate 8 insulating bottom plate 9 insulating gasket 10 safety valve 11 case

Claims (3)

[Claims] 1. A lithium ion secondary battery comprising: a negative electrode formed by coating a negative electrode current collector with a negative electrode mixture; and a positive electrode formed by coating a positive electrode current collector with a positive electrode mixture. The agent comprises flaky graphite (A), a carbon material (B) having an average particle size smaller than that of the flaky graphite (A), and a binder (C). The negative electrode current collector is divided into two layers, an upper layer coating film and a lower layer coating film, and the weight content of the carbon material (B) in the lower layer coating film is determined by combining the upper layer coating film and the lower layer coating film. A lithium ion secondary battery characterized in that the weight content of the carbon material (B) in the entire coating film is larger than the weight content. 2. The weight content of the carbon material (B) in the lower coating film is 1% of the weight content of the carbon material (B) in the entire coating film including the upper coating film and the lower coating film.
The lithium ion secondary battery according to claim 1, wherein the content is 19% or more and 168% or less. 3. The method according to claim 1, wherein the thickness of the lower coating film is 10% to 60% of the total thickness of the coating film including the upper coating film and the lower coating film. Item 7. The lithium ion secondary battery according to Item 1.