JPH03219570A - Cylindrical lithium secondary battery - Google Patents

Abstract

PURPOSE:To obtain good cycle characteristics and high rate discharge characteristics by putting a collector of a thin foil between powders of positive pole active material and sheets of mixed agent, and providing a highly conductive layer on the surface of the sheet of the mixed agent in contact with the collector. CONSTITUTION:A collector of aluminum foil, for example, put between sheets of positive pole mixed agent is used for a positive pole thin layer sheet. For the positive pole mixed agent sheets, powders of positive pole active material of LiV3O2 is used for example, powders of acetylene black is used for conductivity assisting agent, and these are mixed and formed into a sheet using binding agent of Teflon binder. This positive pole mixed agent sheet is used having a conductive layer of a higher conductivity than the positive pole mixed agent sheet applied on one side.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a cylindrical lithium secondary battery using lithium as a negative electrode. The present invention relates to a cylindrical lithium secondary battery having a cylindrical lithium secondary battery.

(Prior art) In recent years, nonaqueous electrolyte batteries that use alkali metals such as lithium as negative electrode active materials have attracted attention because they exhibit high voltage, high energy density, and excellent self-discharge properties. Secondary batteries that can be discharged, especially cylindrical high-capacity secondary batteries, have been expected to be used as the main power source for various electronic devices such as portable devices.

Conventionally, these cylindrical batteries have a structure in which a thin positive electrode plate and a negative electrode plate are formed in a spiral shape with a separator interposed between them, which have a large electrode surface area that participates in battery reactions, so that a large discharge current can be extracted. It was preferred and adopted.

This is true in the case of a reaction mechanism in which lithium ions move reversibly during charging and discharging, such as in lithium secondary batteries.
The smaller the current value per unit area, that is, the smaller the amount of ion movement, the higher the rate of discharge, and the lower the current density during charging, which resulted in better charge-discharge cycle characteristics.

Conventionally, for the positive electrode plate of this type of non-aqueous electrolyte battery, a sheet formed by mixing and kneading active material powder, a conductive additive, and a binder has been used.

A further improvement was to use a highly conductive metal as a current collector and integrate it with the positive electrode plate in order to reduce the resistance in the longitudinal direction of the positive electrode plate or to ensure uniform battery reactions. .

In the case of secondary batteries, the general method was to apply the above mixture to a metal mesh such as expanded metal as a positive electrode plate, compress it with a roll, and embed it.

In the case of secondary batteries, as mentioned above, it is necessary to reduce the current value per unit area in order to improve charge-discharge characteristics and high-rate discharge characteristics, and it is desirable to have as large an electrode area as possible. Therefore, a method is used in which a thin foil such as aluminum or stainless steel with a thickness of about 10 to 50 μm is used as the current collector, and a thinly rolled positive electrode mixture sheet is brought into contact with the current collector and rolled up to integrate the foil. There were many (found).

As a manufacturing method of this positive electrode, for example, Japanese Patent Application Laid-Open No. 62-16065
According to the description in Publication No. 6, a conductive medium containing polyacrylic acid or a copolymer of acrylic acid and acrylic ester as a binder and carbon powder as a conductive filler is placed on a metal foil that also serves as a positive electrode current collector. There is a method for manufacturing a positive electrode in which a positive electrode mixture layer is formed on the conductive coating film, and then dried.

(Problems to be Solved by the Invention) A battery in which a sheet-like positive electrode plate is assembled directly as a spiral structure electrode has a drawback in that the positive electrode plate does not have conductivity in the longitudinal direction and is almost impossible to discharge.

In the case of primary batteries, when the positive electrode is made of expanding metal, the metal mesh needs to be made somewhat thick to ensure durability during coating and compression, and depending on the coating method, the thickness of the positive electrode plate also increases, making it difficult to wrap it in a spiral. In this case, the length of the electrode becomes relatively short and the surface area of the electrode also tends to become narrow, which is not preferable.

When the present inventors investigated a method for integrating a positive electrode mixture sheet with a current collector suitable for secondary batteries, it became clear that the following difficulties existed.

The method of bringing a positive electrode mixture sheet into contact with a thin metal foil to establish conduction allows the positive electrode plate to be made considerably thinner. However, when a positive electrode mixture is supported on a metal mesh such as expanded metal, it maintains very good conductivity due to a kind of anchor effect, but unlike that, when using a thin metal foil and a positive electrode mixture sheet, Since this is a contact between two planes, it is quite difficult to maintain it. In other words, in the case of lithium batteries,
As the positive electrode active material repeatedly expands and contracts during charge/discharge cycles, even if it exhibits good conductivity initially, in the middle to final stages of discharge, electrolyte may leak between the positive electrode mixture sheet and the metal foil current collector. It is also easy for the particles to invade and form a kind of insulating layer, making it impossible to maintain sufficient conduction, resulting in a decrease in discharge capacity and inability to discharge at a large current.

Furthermore, the method of providing a conductive layer on the positive electrode current collector using a polyacrylic acid derivative as a binder also has the disadvantage that the coating of the conductive layer peels off at the end of discharge.

The present invention was made in view of the above-mentioned circumstances, and aims to optimally integrate a positive electrode mixture sheet and a foil-like current collector in order to exhibit sufficient characteristics as a lithium secondary battery. The aim is to improve battery performance by

(Means for Solving the Problems) In order to achieve the above object, the present inventors have conducted various studies and found that a thin metal foil is used as a current collector in a cylindrical lithium battery, and the current collector is made of a positive electrode active material. It has a structure in which it is sandwiched between a mixture sheet formed from powder, a conductive additive, and a binder.
The problem was solved by providing a layer having higher conductivity than the positive electrode mixture on the surface of the mixture sheet in contact with the current collector.

The cylindrical lithium battery of the present invention achieves excellent cycle characteristics and high rate discharge toxicity by having the above structure.

(Function) Poor conductivity between the foil-like current collector and the positive electrode mixture sheet is caused by an increase in the contact resistance between them. This is a necessary condition.

The currently used combination is that the conductive additive is active material 1
Generally, the amount of binder is 5 to 15 parts based on 00, and 5 to 10 parts of binder, which is a sufficient amount to effectively use the active material in battery reactions. When it comes into contact with metal foil, the interfacial resistance is large and the high rate discharge performance becomes very poor.

Therefore, conductivity can be improved by providing a layer with higher conductivity than the mixture composition on the surface of the positive electrode mixture that is in contact with the current collector, for example, a layer containing a very large amount of conductive additive, allowing the electrolyte to invade both interfaces. However, the decrease can be kept small. The electrical resistivity of this conductive layer is preferably 175 or less, particularly 1710 or less, relative to the resistivity of the positive electrode mixture.

(Embodiment) The cylindrical lithium battery of the present invention uses, as the positive electrode thin layer sheet, a current collector such as metal foil, for example, aluminum foil, which is sandwiched between positive electrode mixture sheets on both sides.

The positive electrode mixture sheet is a sheet made by mixing a positive electrode active material such as LIV30□ powder, acetylene black powder as a conductive agent, and a Teflon binder as a binder. Then, on one side of this positive electrode mixture sheet, that is, the side in contact with the current collector, a conductive layer having higher conductivity than the positive electrode mixture sheet, for example, made of graphite and acrylic silicone resin, is used. There is. The thickness of the positive electrode mixture sheet used here is 100 to 150 μm, but the thickness of the conductive layer provided is 2
~20μm, preferably about 5~10μm is sufficient, and this layer is preferably composed of a conductive agent such as carbon and a binder such as acrylic silicone resin, and the layer is formed by combining the conductive agent and resin. It is preferable to use a method such as applying a mixed paste-like substance onto a positive electrode mixture sheet. Further, as the current collector, aluminum or stainless steel foil with a thickness of 10 to 30 μm is used.

In addition, as the positive electrode material, Te 1; tV20s, Vso+a, L
iVJe, MnL, metal oxides such as Mo0a, TiS
2. Metal sulfides such as Mo5a, conductive polymers such as polyaniline, etc. can be used.

The negative electrode material contains lithium, and specifically includes lithium metal, alloys of lithium with aluminum, indium, tin, lead, bismuth, cadmium, zinc, and the like. Among these, lithium metal and lithium-aluminum alloy are particularly preferably used.

The separator separated between the above pressure and negative electrodes is made of a material that can reliably prevent contact between the two electrodes and that can pass or contain the electrolyte, such as synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene. Nonwoven fabrics, porous woven fabrics, nets, etc. can be mentioned, but microporous films made of polypropylene or polyethylene with a thickness of about 20 to 50 μm are particularly preferably used.

The nonaqueous electrolyte interposed between the pressure and negative electrodes contains lithium ions, and the lithium ion source includes lithium salts, particularly LiClO4, LiBF<,
LiPF5, LiCFaSOi and LiAs
One or more electrolytes selected from F5 are preferred, and these electrolytes are usually used in a state dissolved in a solvent. In this case, one or more organic solvents selected from, but not limited to, propylene carbonate, tetrahydrofuran, ethylene carbonate, diethyl carbonate, dimethoxyethane, γ-butyrolactone, dioxolane, butylene carbonate, and dimethylformamide are used. suitable.

In addition, as for the other constituent members of the battery of the present invention, those commonly used can be used without any problem.

(Example) Vanadium oxide represented by the chemical formula LiVaOa was used as a positive electrode active material, and 10 parts of acetylene black was added as a conductive agent to LiV, Oa: 100, and 10 parts of Teflon binder was added as a binder. After kneading with an organic solvent, roll rolling to a thickness of 100 μm,
A thin layered positive electrode mixture sheet with a width of 40 mm was produced.

A conductive layer made of graphite and acrylic silicone resin was coated on one side of this mixture sheet to a thickness of about 5 μm. Furthermore, a 25 μm soft aluminum foil is used as a current collector, and two mixture sheets are sandwiched with the conductive layer side facing the current collector, and a 25 μm thick aluminum foil is placed between the two electrodes along with a 150 μm thick lithium foil.
After winding up a polypropylene microporous film of 1.5 m as a separator, it was stored in an AA-sized container. The dimensions of the positive electrode mixture at this time were 40 mm in width and 260 mm in length.

Next, as a comparative example, an AA battery was fabricated with the same configuration except that no conductive layer was provided on one side of the mixture sheet.

Regarding the above two types of batteries, the current dependence of discharge capacity and charge/discharge cycle performance were evaluated by the following method.

Current dependence of discharge capacity: 30 cycles of charging and discharging were performed.3. After charging to OV, first discharge at 100mA, then charge again and continue for 750m.
It was discharged at A. Five batteries each were evaluated for both the example and the comparative example, and the discharge capacity at each current value and the capacity retention rate at 750 mA versus 100 mA were determined.

Charge/discharge cycle performance: Upper limit voltage 3. OV, lower limit voltage 2. OV, discharge current 150
Charging and discharging were repeated up to 100 cycles under the conditions of mA and charging current of 60 mA, and the capacity retention rate with respect to the initial stage at that point was examined, and the average value of five batteries was shown.

The results are shown in Table 1.

2 As shown in Table 1, in the example in which a conductive layer was provided on the surface of the positive electrode mixture, the capacity at 100 mA was not much different from the comparative example, but at 750 mA, the discharge capacity was extremely high. Also, in terms of cycle performance, the retention rate was extremely superior to that of the comparative example.

As described above, the excellent effects of the present invention were confirmed.

(Effects) As explained above, the cylindrical lithium secondary battery of the present invention exhibits high discharge capacity and is excellent in charge and discharge characteristics and high rate discharge characteristics.

3 4

Claims (1)

[Claims] In a cylindrical lithium secondary battery in which a spiral structure electrode in which a positive electrode thin layer sheet and a negative electrode thin layer sheet containing lithium are spirally wound through a separator is housed in a battery container, a current collector made of metal foil is used. The positive electrode thin layer sheet is an assembly having a structure in which the positive electrode active material powder, a conductive agent, and a binder are sandwiched from both sides by a mixture sheet formed from the mixture sheet, and the mixture sheet surface that is in contact with the current collector is A cylindrical lithium battery characterized by providing a layer having higher conductivity than the agent.