JPH08203501A - Lithium secondary battery - Google Patents

Abstract

(57) [Summary] [Purpose] There is little capacity deterioration due to repeated charging and discharging,
Provided is a lithium secondary battery having excellent charge / discharge cycle characteristics. A lithium secondary battery having an electrode formed with a coating film containing a binder and a lithium-containing composite oxide or a substance capable of being doped or undoped with lithium ions on a conductive substrate has the above-mentioned coating film formed thereon. The surface shape of the subsequent conductive substrate has a maximum recess depth Rv of 0.5 μm.
Or more, 2 μm or less, preferably 0.7 μm or more, 1.5
It is less than μm.

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 on which a coating film containing an active material is formed 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 the same. A lithium secondary battery using a spirally wound electrode body produced by spirally winding with
It is characterized by high energy density per unit volume and energy density per unit weight.

[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, the adhesiveness between the active material-containing coating film and the metal foil as the conductive substrate gradually decreases, 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, JP-A-5-6766 discloses
The surface roughness of the metal foil is 0.15 μm in terms of center line average roughness Ra.
It has been proposed to roughen the surface. This is intended 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 area with the active material-containing coating film. is there.

However, in this method, it is difficult to fill the paint to the bottoms of the recesses on the surface of the roughened metal foil, and therefore, the results are in spite of increasing the surface area of the metal foil by roughening. Not up.

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

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 due to charge and discharge, or cracks or breaks in the conductive substrate. 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

[0008]

According to the present invention, the surface shape of a conductive substrate after a coating film containing an active material or the like is formed has a maximum concave depth Rv of 0.5 μm or more and 2 μm or less. Thereby, even when the charge and discharge are repeated several hundred times or more, it is possible to suppress the peeling of the active material-containing coating film from the conductive substrate and the occurrence of cracks or fractures due to mechanical fatigue of the conductive substrate. The present invention has been completed by finding that a lithium secondary battery having excellent discharge cycle characteristics can be obtained.

That is, in the present invention, the contact area between the conductive substrate and the active material-containing coating film is set by setting the surface shape of the conductive substrate after forming the coating film to 0.5 μm or more at the maximum concave depth Rv. To suppress peeling of the active material-containing coating film from the conductive substrate due to charging / discharging, and by setting the maximum recess depth Rv to 2 μm or less, the mechanical properties of the conductive substrate due to charging / discharging can be reduced. The present invention provides a lithium secondary battery that suppresses the occurrence of cracks and fractures due to fatigue, reduces capacity deterioration due to charge and discharge, and has excellent charge and discharge cycle characteristics.

As described above, the surface shape of the conductive substrate after the active material-containing coating film is formed has a maximum recess depth Rv of 0.
To make it 5 μm or more and 2 μm or less, for example, first,
The average film thickness of the coating film after drying the coating material containing the active material on the surface of the conductive substrate is 0.5 to 50 μm, preferably 1 to 10 μm.
After coating so as to form a coating film of the first layer (lower layer), a pressure step is performed by a roll press machine. As a result, the active material particles in the coating film are pressed against the surface of the conductive substrate to perform embossing, and the contact area between the coating film and the conductive substrate is widened. Then, in order to make the coating film have a required thickness, a coating material containing an active material is applied again on the first layer (lower layer) and dried to form an active material-containing coating film having a required thickness on the conductive substrate. Form on top.

In the electrode thus manufactured, the coating material such as the active material is press-fitted into the recess of the conductive substrate, and the contact area between the conductive substrate and the active material-containing coating is wide. Therefore, the adhesiveness between the conductive substrate and the active material-containing coating film is excellent, and the peeling or dropping of the active material-containing coating film from the conductive substrate due to repeated charging / discharging is suppressed. Since there is virtually no reduction in thickness, the conductive substrate is tough against mechanical fatigue due to repeated stress on the conductive substrate during charge and discharge, and cracks and fractures in the conductive substrate due to accumulation of mechanical fatigue. Since the occurrence of
A lithium secondary battery having excellent charge / discharge cycle characteristics can be obtained.

In the present invention, the surface shape of the conductive substrate after forming the coating film is 0.5 at the maximum concave depth Rv as described above.
It is necessary that the thickness is not less than μm and not more than 2 μm, and it is particularly preferable that the maximum recess depth Rv is not less than 0.7 μm and not more than 1.5 μm. That is, when the maximum recess depth Rv is less than 0.5 μm, the adhesion between the active material-containing coating film and the conductive substrate gradually decreases due to repeated charging / discharging, which deteriorates the battery capacity. When the maximum recess depth Rv exceeds 2 μm, mechanical fatigue accumulates in the conductive substrate during repeated charging and discharging several hundreds of times, cracks and fractures occur in the conductive substrate, and the battery capacity increases. to degrade.

In the present invention, the surface shape of the conductive substrate is processed during the coating film forming step, so that the surface roughness before forming the coating film is, for example, 0.15 μ in terms of the center line average roughness Ra.
It may be smooth and less than m. Rather, smoother ones are more tough against mechanical fatigue and are preferable.

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

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

The positive electrode comprises the above positive electrode active material, and an electron conduction aid such as flaky graphite or acetylene black.
For example, a binder such as polyvinylidene fluoride (PVDF) or polytetrafluoroethylene, a solvent (for example, N
-Methylpyrrolidone) is added and mixed to prepare a paint, and the paint is applied to a conductive substrate and dried.

The negative electrode active material is a material that can be doped or undoped with lithium, and metallic lithium or a lithium-containing compound is used as the negative electrode active material. Examples of the lithium-containing compound include lithium alloys and others. Examples of the lithium alloy include lithium-aluminum, lithium-lead, lithium-bismuth, lithium-indium, lithium-gallium, lithium-indium-gallium and the like. 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 manufacture,
When it acts as a negative electrode, it becomes a state containing lithium by a chemical means, an electrochemical means, or the like. As described above, the negative electrode active material may be any material that can be doped with lithium (loaded) or undoped (released).

The negative electrode is obtained by mixing the above-mentioned negative electrode active material (which is made into powder, if necessary) with a binder such as polyvinylidene fluoride or polytetrafluoroethylene, and mixing them. A solvent (for example, N-
(Methylpyrrolidone) is added and mixed to prepare a paint, and the paint is applied to the conductive substrate and dried.

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, 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 electrolyte solution in which one or more electrolytes such as LiClO ₄ , LiPF ₆ , LiBF ₄ and the like are dissolved is used.

As the separator, for example, a thickness of 10
A microporous polypropylene film or a microporous polyethylene film having an aperture ratio of 30 to 70% at 50 μm can be used.

The battery is made of, for example, nickel-plated iron or stainless steel, which is formed by spirally winding a spirally wound electrode body with a separator interposed between the sheet-shaped positive electrode and the sheet-shaped negative electrode. It is manufactured by inserting the battery into a battery case and sealing it. In addition, an explosion-proof mechanism for preventing the battery from bursting under high pressure is usually incorporated into the battery by discharging the gas generated inside the battery to the outside of the battery when it rises to a certain pressure.

The charger used for charging the battery is preferably one that can be charged by controlling the maximum current with a constant voltage. Further, in order to ensure reliability, it is usually preferable to use a battery pack provided with a protection circuit such as a fuse when an excessive current flows.

In the present invention, the maximum concave depth Rv of the surface of the conductive substrate is determined by the optical interference type three-dimensional surface shape measuring device RST.
(Made by WYKO, USA) 20x objective head (WYK, USA)
(Manufactured by O Co., Ltd.), and the three-dimensional shape data measured at a spatial sampling interval of 0.96 μm in a 250 μm square area on the surface to be measured is calculated by the following method.

(A) Maximum recess depth Rv After performing inclination correction, spherical surface correction and cylindrical surface correction on the three-dimensional shape data, the hole-shaped recess 2 from the average surface 20 shown in FIG.
The maximum depth Rv of 1 is obtained. The inclination correction, the spherical surface correction, and the cylindrical surface correction are performed by the following procedures.

Inclination correction After calculating a plane (called an average plane) that minimizes the square root of the deviation in the height direction from the three-dimensional data at each coordinate, the average plane is calculated from the height of the three-dimensional data. Divide the height.

Spherical Surface Correction After calculating a spherical surface that minimizes the square root of the deviation in the height direction from the three-dimensional shape data at each coordinate after tilt correction, the height of the spherical surface is calculated from the height of the three-dimensional data. Divide by.

Cylindrical Surface Correction After calculating a cylindrical surface having a central axis parallel to the average surface such that the square root of the deviation in the height direction from the three-dimensional shape data at each coordinate after spherical surface correction is minimized, The height of the cylindrical surface is divided from the height of the three-dimensional data.

[0028]

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 Synthesis of Lithium Nickel Oxide Lithium hydroxide (LiOH.H ₂ O) and nickel (III) oxide (NiO ₃ ) were heat-treated to use lithium nickel oxide (LiNiO ₂ ) as a positive electrode active material. , But the ratio of Li to Ni is slightly deviated from the stoichiometric composition) was synthesized. The above synthesis was performed as follows.

Li / hydroxide and nickel oxide
After weighing so that the ratio of Ni was 1.05 (molar ratio), they were mixed while being crushed 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 water, handling such as pulverization was performed in an atmosphere of argon gas.

Preparation of Positive Electrode First, a positive electrode coating material 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 positive electrode coating composition is prepared by first dissolving polyvinylidene fluoride in N-methylpyrrolidone (hereinafter abbreviated as "NMP"), adding lithium nickel oxide and flaky graphite, and mixing them. Further, NMP was added to adjust the viscosity to 10,000 mPa · s.

The obtained positive electrode coating material was applied to one surface of aluminum having a thickness of 20 μm to form a first layer (lower layer) using an applicator having a clearance of 30 μm. Then 100-120 on the hot plate
It was dried at ° C. Similarly, the positive electrode coating material was applied to the other surface of the aluminum foil and vacuum dried at 100 ° C. for 8 hours.
The average film thickness of the first layer after drying was about 8 μm. This electrode body was roll-pressed at a pressure of 23 kg / cm ² to prepare an undercoat substrate. Next, on both surfaces of this undercoat substrate, an applicator with a clearance of 300 μm was used to apply a coating material for the positive electrode in the same procedure as the above-mentioned first layer, followed by drying and roll pressing to form a coating film. A sheet-shaped positive electrode was produced.

Preparation of Negative Electrode The following materials were used as the negative electrode material. Artificial graphite (synthesized at 2800 ° C) 90 parts by weight Polyvinylidene fluoride 10 parts by weight

NMP was added to the above negative electrode material to prepare a negative electrode coating material in the same manner as the positive electrode coating material, and the thickness was 18 μm.
In the same manner as in the case of producing the positive electrode, the copper foil was sequentially applied in multiple layers on both sides to produce a sheet-like negative electrode having a total thickness of 250 μm. In addition,
The weight ratio of the positive electrode and negative electrode active materials was 2: 1 and the average film thickness of the first layer (lower layer) after drying was about 8 μm.

Electrolyte solution LiPF ₆ was added to a mixed solution of ethylene carbonate and ethylmethyl carbonate (volume ratio 1: 1) at 1 mol / mol.
1 was dissolved to prepare an organic electrolytic solution.

Assembling a Cylindrical 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 into 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 strip separator made of a microporous polypropylene film having a 5 mm opening ratio of 50% 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 The lead body was welded. And
The can body manufactured through these steps is vacuum dried at 60 ° C. for 10 hours, then 2 ml of the electrolytic solution is injected in a dry atmosphere, and the container is sealed to form a cylindrical R5 battery (outer diameter: :
14.95 mm, height: 39.7 mm) was produced.

Explaining the battery shown in FIG. 1, 1 is the positive electrode and 2 is the negative electrode. However, in FIG. 1, in order to avoid complication, a metal foil or the like as a conductive substrate used for manufacturing the positive electrode 1 and the negative electrode 2 is not shown. And 3 is a separator and 4 is an electrolytic solution.

5 is a battery case made of stainless steel,
The battery case 5 also serves as a negative electrode terminal. An insulator 6 made of a polytetrafluoroethylene sheet is arranged at the bottom of the battery case 5, and an insulator 7 made of a polytetrafluoroethylene sheet is also arranged at the inner peripheral part of the battery case 5. The spiral electrode body including the negative electrode 2 and the separator 3, the electrolytic solution 4, and the like are contained in the battery case 5.

Reference numeral 8 is a stainless steel sealing plate, and a gas vent hole 8a is provided at the center of the sealing plate 8. Reference numeral 9 is a polypropylene-made annular packing, 10 is a flexible thin plate made of titanium, and 11 is an annular heat-deformable member made of polypropylene.

The thermal deformation member 11 acts to change the breaking pressure of the flexible thin plate 10 by being deformed by the temperature.

Reference numeral 12 is a nickel-plated terminal plate made of rolled steel. The terminal plate 12 is provided with a cutting edge 12a and a gas discharge hole 12b. When the internal pressure rises and the flexible thin plate 10 is deformed due to the increase in the internal pressure, the cutting blade 12a breaks the flexible thin plate 10 to discharge the gas inside the battery from the gas discharge hole 12b to the outside of the battery. In addition, the battery is designed to be prevented from being broken under high pressure.

13 is an insulating packing, 14 is a lead body made of aluminum, the lead body 14 electrically connects the positive electrode 1 and the sealing plate 8, and the terminal plate 12 is in contact with the sealing plate 8. Acts as a positive electrode terminal. Reference numeral 15 is an aluminum lead body that electrically connects the negative electrode 2 and the battery case 5.

Comparative Example 1 The formation of the first layer (lower layer) in Example 1 was omitted, and a coating film was formed using an applicator with a clearance of 325 μm. It was made.

Comparative Example 2 Instead of the aluminum foil and the copper foil used in Example 1, an aluminum foil having a surface roughened by immersing in an aqueous hydrochloric acid solution having a concentration of 20% by weight and an aqueous sulfuric acid solution having a concentration of 20% by weight were used. A copper foil whose surface is roughened by immersion is used, and the formation of the first layer (lower layer) in Example 1 is omitted, and the clearance 325 is used.
A coating film was formed using a μm applicator, and the R5 type battery was manufactured in the same procedure as in Example 1 except for the above.

With respect to the batteries of Example 1 and Comparative Examples 1 and 2 produced as described above, the following charge / discharge cycle test was carried out to evaluate the capacity deterioration (capacity retention rate in charge / discharge cycle) due to repeated charging / discharging. Examined. In addition, the maximum concave depth Rv of the surface of the sheet-shaped positive electrode and sheet-shaped negative electrode metal foils produced in Example 1 and Comparative Examples 1 and 2 was measured before and after coating film formation. The maximum concave depth Rv of the surface after coating film formation is measured by cutting each sheet-like electrode into small pieces of about 5 cm square, ultrasonically cleaning the pieces with NMP, and removing the coating film. The measurement was performed by measuring the maximum concave depth Rv of the surface of those metal foils.

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 Table 1 and FIG.

[0048]

[Table 1]

As is clear from the results shown in Table 1 and FIG. 2, the battery of Example 1 has a higher capacity retention rate after repeated charge / discharge cycles than the batteries of Comparative Examples 1 and 2, and is 500 Even after repeated charging and discharging, there was little capacity deterioration.

[0050]

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 showing an example of a lithium secondary battery of the present invention.

FIG. 2 is a diagram showing charge / discharge cycle characteristics of the battery of Example 1 and the batteries of Comparative Examples 1 and 2.

FIG. 3 is a diagram schematically showing an average surface and hole-shaped concave portions when measuring the maximum concave depth Rv of the surface of the conductive substrate in the present invention.

[Explanation of symbols]

 1 Positive electrode 2 Negative electrode 3 Separator 4 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 or undoped with lithium ions is formed on a conductive substrate, wherein the coating film is formed. The surface shape of the conductive substrate after being subjected to the heat treatment has a maximum concave depth Rv of 0.5 μ.
A lithium secondary battery having a thickness of m or more and 2 μm or less.