JPH08329928A - Lithium secondary battery - Google Patents

Abstract

PURPOSE: To provide a lithium secondary battery having an excellent charging and discharging cycle property by making a coating formed on a conductive substrate be a multilayer coating and a lowermost layer neighboring the substrate contain electron transmission assisting particles projected to an upper layer. CONSTITUTION: A lithium secondary battery has an electrode produced by forming a coating containing lithium-containing composite oxide and a binder on a conductive substrate 1. The coating is made to be a multilayer coating and electron transmission assisting particles 2a (e.g. aluminum powder) with particle size D larger than the average thickness δ of the outermost layer 2 are contained in the outermost layer 2. The electron transmission assisting particles 2a are well brought into contact with the surface of the conductive substrate 1, projected to the upper layer 3 formed on the outermost layer 2, and well brought into contact with an electron emitting material contained in the upper layer 3, so that the conduction between the upper layer 3 and the conductive substrate 1 is kept well. As a result, a lithium secondary battery having small capacity deterioration by repeated charging and discharging and having an excellent charging and discharging cycle property can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery having excellent charge / discharge cycle characteristics.

[0002]

2. Description of the Related Art Generally, an electrode having a coating film containing an active material and the like is applied together with a separator by applying a coating material in which an active material, a binder and the like are dispersed in an organic solvent to a metal foil as a conductive substrate and drying. A battery using a spirally wound electrode body manufactured by spirally winding has a feature that the energy density per unit capacity and the energy density per unit weight are high.

[0003]

However, in the lithium secondary battery having the coating film type electrode, the coating film containing the active material repeats expansion and contraction due to charging and discharging, so that charging and discharging are repeated about 100 times. Then, there is a problem that the adhesiveness between the active material-containing coating film and the metal foil as the conductive substrate is gradually reduced, thereby deteriorating the capacity of the battery.

Therefore, in order to improve the adhesion between the coating film containing the active material and the metal foil as the conductive substrate, JP-A-5-6 is used.
In 766 publication, the surface roughness of the metal foil is defined as the center line average roughness R
It has been proposed that a should be roughened to 0.15 μm or more. This is to prevent the active material-containing coating film from falling off or peeling from the metal foil by roughening the surface of the metal foil and increasing the contact surface between the active material-containing coating film and the metal foil. To do.

However, in this method, it is difficult to fill the bottom of the recess of the surface of the roughened metal foil with the paint, and therefore, the result is satisfactory even if the surface area of the metal foil is increased by roughening. Not not.

When the surface of the metal foil having a total thickness of about 20 μm or less is roughened to have a center line average roughness Ra of 0.15 μm or more, the depth of the unevenness on the surface of the metal foil is approximately the total thickness of the metal foil. Since it occupies 10%, when the battery is charged and discharged several hundreds of times or more, mechanical fatigue accumulates in the metal foil as the conductive substrate, causing cracks and fractures, which deteriorates the battery capacity. There was a problem of doing.

Further, in Japanese Patent Laid-Open No. 63-1212247, regarding the negative electrode, the ratio of the binder on the surface side of the coating film containing carbon as an active material and the binder in contact with the metal foil is more It has been proposed to form so as to be large. This is intended to increase the adhesive force between the coating film and the metal foil without lowering the electron conductivity by configuring a large amount of binder at the interface of the coating film in contact with the metal foil.

However, in this method, since the active material densities in the coating film are different, there is a difference in expansion and contraction due to charging and discharging, and mechanical stress is generated in the coating film. for that reason,
When the battery is charged and discharged hundreds of times or more, a part of the chain structure formed by the carbon particles in the coating film is cut, which causes a problem that the capacity of the battery is deteriorated. In particular, a metal foil as a conductive substrate, a lower layer film with a large proportion of binder,
In a multi-layered coating film in which an upper layer film having a large proportion of active material is formed on the lower layer film, there is an interface where the active material density is greatly different between the lower layer film and the upper layer film, and therefore repeated charge and discharge. As a result, the coating film is apt to be cracked or ruptured, resulting in a rapid decrease in battery capacity.

Therefore, the present invention solves the problems in the conventional lithium secondary battery as described above, and peels the coating film containing the active material from the conductive substrate or cracks or breaks in the conductive substrate due to charging and discharging. It is an object of the present invention to provide a lithium secondary battery having excellent charge / discharge cycle characteristics by suppressing the occurrence of

[0010]

The present invention has been made as a result of various studies in order to achieve the above-mentioned object, and has at least two active material-containing coating films formed on a conductive substrate. Charge and discharge by configuring the lowermost layer film that is composed of a multilayer film and is in contact with the conductive substrate so as to include a binder and an electron conduction aid having a particle size larger than the average film thickness of the lowermost film. Even when the above is repeated several hundred times or more, a lithium secondary battery having excellent charge / discharge cycle characteristics can be obtained by suppressing the peeling of the coating film from the conductive substrate and the occurrence of cracks and fractures due to mechanical fatigue of the conductive substrate. It was made possible.

That is, in the present invention, the active material-containing coating film formed on the conductive substrate is composed of a multilayer film of at least two layers, and the lowermost layer film in contact with the conductive substrate is mainly composed of a binder and an electron conduction promoter. Since it is made of a material, the adhesive strength with the conductive substrate can be increased. On the other hand, in the two adjacent layers such as the boundary between the lowermost layer film and the upper layer film formed thereon, the binders of the respective layers are firmly adhered to each other, so that the mechanical strength of the multilayer film is deteriorated. There is no.

The above-mentioned lowermost layer film is mainly formed of a binder and an electron conduction aid and does not contain an active material or contains a small amount of active material, so that the energy density of the battery is lowered. Therefore, it is preferable to reduce the volume ratio of the lowermost layer film to the total thickness of the coating film, and the average thickness of the lowermost layer film is preferably 1/10 or less of the total thickness of the coating film at most. The thinner the film, the more preferable. However, if it is too thin, sufficient adhesion strength with the conductive substrate cannot be obtained, so the thickness of the lowermost layer film is 0.04.
It is preferably not less than μm, particularly preferably not less than 0.05 μm and not more than 15 μm.

The role of the lowermost layer film will be described in more detail with reference to the drawings. As shown in FIG. 1, in the lowermost layer film 2, electrons having a particle size D larger than the average film thickness δ of the electrons are formed. Since the conductive auxiliary material particles 2a are included, the electron conductive auxiliary material particles 2a are in good contact with the surface of the conductive substrate 1, and
Since a part of the electron conduction auxiliary material particles 2a protrudes into the upper layer film 3 formed on the lowermost layer film 2 and comes into good contact with the electron conduction auxiliary material contained in the upper layer film 3, the upper layer film 3 The conductivity between the conductive substrate 1 and the conductive substrate 1 is kept good.

Since the lowermost layer film 2 is mainly formed of a binder and an electron conduction aid, the lowermost layer film 2 is less expanded and contracted due to charging and discharging than the upper layer film 3, so that the conductive substrate 1 has a mechanical property. Almost no mechanical stress is applied. Further, the lowermost layer film 2 functions as a buffer layer for mechanical stress even if the upper layer film 3 containing an active material expands or contracts by charging and discharging, and reduces the mechanical fatigue of the conductive substrate 1.

On the other hand, expansion and contraction of the upper layer film 3 causes mechanical stress between the upper layer film 3 and the conductive substrate 1, but the electron conduction aid particles 2a protruding from the lowermost layer film 2 into the upper layer film 3 are wedged. To prevent the coating film from cracking or breaking.

The electron conduction aid particles 2a having a particle diameter D larger than the average film thickness δ of the lowermost layer film 2 are preferably contained in the lowermost layer film 2 as primary particles. It is also effective if secondary particles are contained in the lowermost layer film 2 in a form having a secondary particle diameter D larger than the average film thickness δ thereof.

Further, the lowermost layer film 2 contains the electron conduction assisting material particles 2a having a particle diameter D larger than the average film thickness δ thereof. Alternatively, the electron conduction aid particles 2a having a small particle size may be included.

The electron conduction aid used for the lowermost layer film is not particularly limited, but aluminum powder,
In addition to metal powders such as nickel powder and copper powder, powders of materials having good electric conductivity such as carbon powder generally used for batteries can be used.

The binder for the lowermost layer film is not particularly limited, but examples thereof include polyvinyl alcohol, ethylene / propylene / diene terpolymer, styrene / butadiene rubber, polyvinylidene fluoride, and polytetrafluoro. Ethylene, a tetrafluoroethylene-hexafluoropropylene copolymer or the like is used.

The lowermost layer film is prepared by adding a solvent such as N-methylpyrrolidone, toluene or xylene to the above-mentioned electron conduction aid and binder and mixing them, and coating the paint on a conductive substrate. Formed by drying.

This lowermost layer film can be applied to the production of coating films for both positive and negative electrodes, but the types and composition ratios of materials such as electron conduction aids and binders should be the same for both positive and negative electrodes. It may be different or different.

In the present invention, as the conductive substrate for the positive electrode, for example, a metal foil obtained by processing aluminum, stainless steel, titanium or the like into a thin plate is used. On the other hand, as the conductive substrate for the negative electrode, for example, a metal foil obtained by processing copper or the like into a thin plate shape is used.

In the present invention, as the positive electrode active material, for example, lithium cobalt oxide, lithium manganese oxide or the like can be used alone or as a mixture or solid solution of a lithium-containing composite metal oxide composed of lithium ions and a specific metal. .

In the positive electrode, the positive electrode active material is added to an electron conduction aid such as phosphorus (scaly) flake graphite or carbon black, a binder such as polyvinylidene fluoride or polytetrafluoroethylene, and N-methyl. A coating material is prepared by adding and mixing a solvent such as pyrrolidone, toluene and xylene, and the coating material is applied onto the lowermost layer film and dried to form an upper layer film.

As the binder for forming the upper layer film of this positive electrode, similar to the binder for the lowermost layer film, for example, polyvinyl alcohol, ethylene.propylene.
Diene terpolymer, styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, etc. can be used, and are the same as those used for the lowermost layer film. May be used, or different ones may be used.

The negative electrode active material is a material capable of doping (incorporating) and undoping (releasing) lithium, and lithium metal or a lithium-containing compound is used as the negative electrode active material. The lithium-containing compounds include lithium alloys and others. Examples of the lithium alloy include alloys of lithium with other metals such as lithium-aluminum, lithium-lead, lithium-bismuth, lithium-indium, lithium-gallium, and lithium-indium-gallium. Examples of lithium-containing compounds other than lithium alloys include carbon materials having a turbostratic structure and graphite. Some of these do not contain lithium at the time of production, but when they act as a negative electrode, they are in a state of containing lithium by chemical means or electrochemical means.

The negative electrode is formed by adding the above-mentioned negative electrode active material (which is made into powder, if necessary) to a binder such as polyvinylidene fluoride or polytetrafluoroethylene and a solvent such as N-methylpyrrolidone, toluene or xylene. Is prepared by coating the lowermost layer film with the coating composition and drying it to form an upper layer film.

As the binder for forming the upper layer film of this negative electrode, similar to the binder for the lowermost layer film, for example, polyvinyl alcohol, ethylene.propylene.
Diene terpolymer, styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, etc. can be used, and are the same as those used for the lowermost layer film. May be used, or different ones may be used.

In the present invention, any coating method such as an extrusion coater, a reverse roller or a doctor blade may be used as the coating method of the coating material. It is advisable to employ a combination of the above methods, for example, a simultaneous multi-layer coating method (wet-on-wet method) or a sequential multi-layer coating method (wet-on-dry method).

For example, in the case of forming a coating film by a sequential multi-layer coating method, first, for the lowermost layer film, a binder and an electron conduction aid are mixed in an appropriate solvent to prepare a coating material, and the coating material is used as a conductive substrate. The average film thickness after drying is about 1 μm
It is formed by applying and drying the following thin film. Then, thereafter, an upper layer film containing an active material is formed to a predetermined film thickness on the lowermost layer film by the same method as in the case of forming the lowermost layer film.

Since the lowermost layer film does not directly aim at the electrochemical reaction, it is not necessary to provide voids for the passage of ions, so that the lowermost layer film can be highly filled, whereby the adhesive strength with the surface of the conductive substrate is increased. Can be further increased.

As a method of highly filling the lowermost layer film as described above, for example, the solvent content of the coating material for forming the lowermost layer film is reduced, and the lowermost layer film is formed by coating and drying, or In the case of the sequential multi-layer coating method, a method of applying a strong pressure press using a roll press machine after forming the lowermost layer film can be adopted.

Examples of the electrolytic solution include 1,2-dimethoxyethane, 1.2-diethoxyethane, propylene carbonate, ethylene carbonate, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, diethylene carbonate, dimethyl carbonate and ethylmethyl. In a single solvent or a mixed solvent of two or more kinds such as carbonate, for example, LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ ,
An organic electrolytic solution in which an electrolyte such as LiClO ₄ , LiPF ₆ , LiBF ₄ or the like is used alone or in which two or more kinds are dissolved is used.

The separator may have a thickness of 10 to 10, for example.
A microporous polypropylene film or a microporous polyethylene film having an opening ratio of 30 to 70% and a thickness of 50 μm can be used.

The battery is formed by, for example, a spirally wound electrode body produced by spirally winding a sheet-shaped positive electrode and a sheet-shaped negative electrode produced as described above with a separator interposed therebetween.
It is manufactured by inserting it into a battery case made of nickel-plated iron or stainless steel and sealing it. Also,
In general, the battery may be equipped with an explosion-proof mechanism for preventing the battery from bursting under high pressure by discharging the gas generated inside the battery to the outside of the battery when the gas has risen to a certain pressure.

[0036]

EXAMPLES Next, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to only those examples.

Example 1 (1) Production of Positive Electrode The production of a positive electrode will be described in the order of formation of a lowermost layer film, synthesis of lithium nickel oxide used as an active material, and formation of an upper layer film.

Formation of Bottom Layer Film A coating material for forming the bottom layer film containing aluminum powder (average particle size 3 μm) and polyvinylidene fluoride in the following proportions was prepared. Aluminum powder (average particle size 3 μm) 50 parts by weight Polyvinylidene fluoride 50 parts by weight

The coating material for forming the lowermost layer film was prepared by previously dissolving polyvinylidene fluoride in N-methylpyrrolidone, adding aluminum powder thereto, and stirring and mixing. This coating material for forming the lowermost layer film is applied onto an aluminum foil having a thickness of 20 μm and a center line average roughness Ra of 0.07 μm using an applicator with a clearance of 15 μm, and then dried at 100 to 145 ° C. did. Similarly, the coating material for forming the lowermost layer film was applied to the back surface side of the aluminum foil, and then vacuum dried at 100 ° C. for 8 hours. The sheet having the coating film thus formed on both sides of the aluminum foil was roll-pressed at a pressure of 23 kg / cm ² to form the lowermost layer film. The average film thickness of the lowermost layer film after the roll pressing was 0.9 μm.

Synthesis of Lithium Nickel Oxide Lithium hydroxide (LiOH.H ₂ O) and nickel (III) oxide (Ni ₂ O ₃ ) are heat treated to use lithium nickel oxide (usually LiNiO ₃ ) as a positive electrode active material. ₂
, But the ratio of Li and Ni is slightly deviated from the stoichiometric composition). This synthesis was performed as shown below.

Li / hydroxide is mixed with Li /
After weighing so as to have a ratio of Ni = 1 / 1.05 (molar ratio), they were pulverized and mixed in an agate mortar. This was preheated in an oxygen (O ₂ ) stream at 500 ° C. for 2 hours, then heated up to 700 ° C. at a temperature rising rate of 50 ° C./h or less, and then heated at 700 ° C. for 20 hours for firing. Since the synthesized LiNiO ₂ is weak against moisture, handling such as crushing was performed in an atmosphere of argon gas.

Formation of Upper Layer Film A coating material for forming an upper layer film containing the above-synthesized lithium nickel oxide, flaky graphite, and polyvinylidene fluoride in the following proportions was prepared. Lithium nickel oxide 91 parts by weight Flaky graphite 6 parts by weight Polyvinylidene fluoride 3 parts by weight

The above-mentioned coating material for forming the upper layer film is prepared by dissolving polyvinylidene fluoride in N-methylpyrrolidone in advance, adding lithium nickel oxide and flake graphite to the mixture, and further mixing N-methylpyrrolidone. In addition, the viscosity was adjusted to 10,000 mPa · s.

The obtained coating material for forming the upper layer film was applied onto the lowermost layer film of the sheet by using an applicator having a clearance of 300 μm, and thereafter, it was applied on a hot plate by 1
It was dried at 00 to 120 ° C. Similarly, the coating material for forming the upper layer film is applied on the lowermost layer film on the back surface side of the sheet, and 1
It vacuum-dried at 00 degreeC for 8 hours, and formed the upper layer film. The electrode body thus obtained was roll-pressed at a pressure of 23 kg / cm ² to prepare a sheet-like positive electrode having a total thickness of 223 μm.

(2) Preparation of Negative Electrode The preparation of the negative electrode will be described in the order of forming the lowermost layer film and the upper layer film.

Formation of Lowermost Layer Film The following materials were used as materials for forming the lowermost layer film of the negative electrode. Copper powder (average particle size 3 μm) 50 parts by weight Polyvinylidene fluoride 50 parts by weight

Using the above-mentioned material for forming the lowermost layer film, a coating material for forming the lowermost layer film of the negative electrode was prepared in the same manner as the coating material for forming the lowermost layer film of the positive electrode. The obtained coating material for forming the lowermost layer has a thickness of 18 μm and a center line average roughness Ra of 0.0
A 5 μm thick copper foil was coated on both sides by the same method as in forming the bottom layer film of the positive electrode, and dried to form a bottom layer film. The average film thickness of the lowermost layer film (however, the average film thickness after roll pressing) was 0.9 μm.

Formation of Upper Layer Film The following materials were used as materials for forming the upper layer film. Artificial graphite (synthesized at 2800 ° C) 90 parts by weight Polyvinylidene fluoride 10 parts by weight

The coating material for forming the upper layer film was prepared by adding N-methylpyrrolidone to the above material for forming the upper layer film and in the same manner as the coating material for forming the upper layer film of the positive electrode. The obtained coating material for forming an upper layer film is applied on the lowermost layer film on both sides of the sheet in the same manner as in forming the upper layer film of the positive electrode, and dried,
An upper layer film was formed to prepare a sheet-shaped negative electrode having a total thickness of 253 μm. The coating film density was adjusted so that the weight ratio of the positive electrode and negative electrode active materials was 2: 1.

(3) Electrolyte solution 1 mL of LiPF ₆ is added to a mixed solution of ethylene carbonate and ethyl methyl carbonate (mixing ratio is 1: 1 by volume).
Mol / l was dissolved to prepare an organic electrolytic solution.

(4) Assembly of tubular battery The above sheet-like positive electrode was cut into a strip having a width of 28 mm and a length of 220 mm, and the sheet-like negative electrode was formed with a width of 30 mm and a length of 260 m.
It was cut into strips of m. Then, a part of the coating film at the end of each sheet-shaped electrode is peeled off, and an aluminum lead body is resistance-welded to the exposed portion of the metal foil to obtain a thickness of 2
A band-shaped separator made of a microporous polypropylene film having a pore size of 50% and an aperture ratio of 5 μm is interposed between the sheet-like positive electrode and the sheet-like negative electrode and spirally wound to produce a spirally wound electrode body. The body was inserted into a stainless steel battery case. Then, the tip of the lead body on the negative electrode side is penetrated through the insulator and welded to the bottom of the battery case, and the insulator is inserted into the opening of the battery case to form a groove, and then the sealing plate and the positive electrode side Welded with the lead body. Then, the can body manufactured through the above-mentioned steps is subjected to 1
After vacuum-drying for 0 hours, 2 ml of the electrolytic solution was injected in a dry atmosphere, and then sealed to form a cylindrical R5 battery (outer diameter: 14.95 mm, height: 39.7 mm) having the structure shown in FIG. It was made.

The battery shown in FIG. 2 will be explained.
Is the positive electrode and 12 is the negative electrode. However, in order to avoid complication, FIG. 2 does not show a metal foil or the like as a conductive substrate used for manufacturing the positive electrode 11 and the negative electrode 12. And 13 is the above separator,
14 is the above-mentioned electrolytic solution.

Reference numeral 15 is a battery case made of stainless steel, and this battery case 15 also serves as a negative electrode terminal. An insulator 16 made of a polytetrafluoroethylene sheet is arranged at the bottom of the battery case 15, and an insulator 17 made of a polytetrafluoroethylene sheet is also arranged at the inner periphery of the battery case 15. A spiral electrode body composed of the negative electrode 12 and the separator 13 and the electrolytic solution 1
4 and the like are housed in the battery case 15.

Reference numeral 18 is a stainless steel sealing plate, and a gas vent hole 18a is provided at the center of the sealing plate 18. 19 is polypropylene ring packing, 2
Reference numeral 0 is a flexible thin plate made of titanium, and reference numeral 21 is an annular heat-deformable member made of polypropylene.

The above-mentioned thermal deformation member 21 acts to change the breaking pressure of the flexible thin plate 20 by being deformed by the temperature.

Reference numeral 22 denotes a nickel-plated rolled steel terminal plate. The terminal plate 22 is provided with a cutting edge 22a and a gas discharge hole 22b. When the internal pressure rises and the flexible thin plate 20 is deformed due to the increase in the internal pressure, the cutting blade 22a destroys the flexible thin plate 20, and the gas inside the battery is discharged to the outside of the battery from the gas discharge hole 22b. In addition, the battery is designed to be prevented from being broken under high pressure.

Reference numeral 23 is an insulator, and 24 is a lead body made of aluminum. The lead body 24 electrically connects the positive electrode 11 and the sealing plate 18, and the terminal plate 22 is the sealing plate 1.
The contact with 8 acts as a positive electrode terminal. Reference numeral 25 denotes an aluminum lead body that electrically connects the negative electrode 12 and the battery case 15.

Example 2 Instead of the aluminum powder in the material for forming the lowermost layer film of the positive electrode of Example 1, carbon powder having an average particle size of 3 μm was used, and copper powder in the material for forming the lowermost layer film of the negative electrode was used. Instead, an R5 type battery was produced in the same manner as in Example 1 except that carbon powder having an average particle size of 3 μm was used.

Example 3 Carbon powder having an average particle size of 0.15 μm was used in place of the aluminum powder in the material for forming the lowermost layer film of the positive electrode of Example 1, and otherwise the same as in Example 1. A lowermost layer film was formed in the same manner as in Example 1 except that a coating material for forming the lower layer film was prepared and the coating material for forming the lowermost layer film was applied using a wire bar coater having a clearance of 5 μm. The average film thickness of the lowermost layer film (however, the average film thickness after roll pressing) was 0.08 μm. Then, a positive electrode was produced in the same manner as in Example 1 except that this lowermost layer film was used.

Further, in place of the copper powder in the material for forming the lowermost layer film of the negative electrode of Example 1, the average particle size is 0.15 μm.
Except that the coating powder for forming the lowermost layer film was prepared and the coating material for forming the lowermost layer film was applied using a wire bar coater having a clearance of 5 μm. A lowermost layer film was formed in the same manner as in Example 1. The average film thickness of the lowermost layer film (however, the average film thickness after roll pressing) was 0.08 μm. Then, a negative electrode was made in the same manner as in Example 1 except that this lowermost layer film was used, and an R5 type battery was made in the same manner as in Example 1 except that those positive electrode and negative electrode were used. .

Comparative Example 1 Instead of the aluminum powder having an average particle size of 3 μm in the material for forming the lowermost layer film of the positive electrode of Example 1, an aluminum powder having an average particle size of 0.5 μm was used, and the lowermost layer film of the negative electrode was used. An R5 type battery was produced in the same manner as in Example 1 except that copper powder having an average particle size of 0.5 μm was used instead of the copper powder having an average particle size of 3 μm in the forming material.

Comparative Example 2 A carbon powder having an average particle diameter of 0.1 μm was used instead of the carbon powder having an average particle diameter of 3 μm in the material for forming the lowermost layer film of the positive electrode and the negative electrode of Example 2. An R5-type battery was produced in the same manner as in Example 2.

Comparative Example 3 Instead of the aluminum foil and the copper foil used in Example 1, aluminum was immersed in a hydrochloric acid aqueous solution having a concentration of 20% by weight to roughen the surface to a center line average roughness Ra of 0.55 μm. Using a foil and a copper foil whose surface is roughened to 0.48 μm with a center line average roughness Ra by immersing the foil in a sulfuric acid aqueous solution having a concentration of 20% by weight,
Moreover, an R5 type battery was produced in the same manner as in Example 1 except that the formation of the lowermost layer film was omitted.

Comparative Example 4 Example 1 was repeated except that the formation of the lowermost layer film of Example 1 was omitted.
An R5 type battery was prepared in the same manner as in.

The batteries of Examples 1 to 3 and Comparative Examples 1 to 4 produced as described above were subjected to the following charge / discharge cycle test to show capacity deterioration due to repeated charge / discharge (capacity retention rate in charge / discharge cycle). ) Was investigated. The results are shown in Tables 1-2 and FIG. Further, Tables 1 and 2 show the average film thickness of the lowermost layer film of the electrodes prepared in Examples 1 to 3 and Comparative Examples 1 and 2, and the average film thickness of the lowermost layer film is The cross section is 25 with a transmission electron microscope (TEM).
It is obtained by averaging the thicknesses measured by observing at 6 points.

Charge / discharge cycle test method: Charge / discharge current is C
In the case of, the R5 battery was charged / discharged at 1 C of 560 mA. Charging was carried out at a constant voltage of 4.2V with a current limiting circuit of 1C provided, and discharging was carried out until the inter-electrode voltage of the battery dropped to 2.75V. Then, such charging / discharging cycle is repeated, and the fifth, 20th, 50th, 100th, 200th, 5th charging / discharging cycles are performed.
The capacity at the 00th time was measured, and the capacity at the 5th time of the charge / discharge cycle of the battery of Example 1 was set to 100%, and the 5th, 20th, 50th, 100th, 200th, 500th charge / discharge cycles of the respective batteries were performed. The capacity retention rate for the second time was calculated. The results are shown in Tables 1-2 and FIG. In Tables 1 and 2, the positive electrode metal foil is an aluminum foil, the negative electrode metal foil is a copper foil, and Ra is a simplified representation of the centerline average roughness Ra. The positive electrode aluminum foil acts as a positive electrode conductive substrate, and the negative electrode copper foil acts as a negative electrode conductive substrate.

[0067]

[Table 1]

[0068]

[Table 2]

As is clear from the results shown in Tables 1 to 3 and FIG. 3, the batteries of Examples 1 to 3 have a capacity retention when the charge and discharge cycles are repeated, as compared with the batteries of Comparative Examples 1 to 4. The rate was high, and the capacity deterioration was small even when the charge and discharge were repeated 500 times.

[0070]

As described above, according to the present invention,
Provided is a lithium secondary battery having excellent charge / discharge cycle characteristics, in which capacity deterioration due to repeated charge / discharge is small.

[Brief description of drawings]

FIG. 1 is a cross-sectional view schematically showing a lowermost layer film of an electrode in a lithium secondary battery of the present invention and a main part around the film.

FIG. 2 is a cross-sectional view schematically showing an example of the lithium secondary battery of the present invention.

FIG. 3 is a diagram showing charge / discharge cycle characteristics of the batteries of Examples 1 to 3 and the batteries of Comparative Examples 1 to 4.

[Explanation of symbols]

 1 Conductive Substrate 2 Bottom Layer Film 2a Electron Conduction Aid Particles 3 Upper Layer Film 11 Positive Electrode 12 Negative Electrode 13 Separator 14 Electrolyte

Claims (1)

[Claims] 1. A lithium secondary battery having an electrode on which a coating film containing a binder and a lithium-containing composite oxide or a substance capable of being doped / undoped with a lithium ion is formed on a conductive substrate. At least 2
Lithium, characterized in that the lowermost layer film composed of a multi-layered film of at least one layer and in contact with the conductive substrate contains a binder and an electron conduction aid having a particle size larger than the average film thickness of the lowermost layer film. Secondary battery.