JPH11265708A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery having high adhesion strength of the boundary face between an electrode coating film and a collector without damaging battery capacity. SOLUTION: In this lithium ion secondary battery carbon material is used for a negative electrode, lithium oxide containing manganese, for a positive electrode, and after coating solution of high hinder concentration is applied to a collector and is dried, coating solution of low binder concentration is applied thereon and is dried to form an electrode so as to be used as the positive electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium ion secondary battery.

[0002]

2. Description of the Related Art In recent years, along with the development of electronic portable devices, there has been a remarkable progress in the development of batteries as driving sources thereof. Among them, lithium ion secondary batteries have attracted particular attention because of their high energy density. Equipment miniaturization,
Because it is possible to reduce the weight, lithium ion secondary batteries are
Recently, it has been widely used in portable devices such as a camera-integrated VTR and a mobile phone. Currently, generally available lithium ion secondary batteries use a lithium composite oxide containing a transition metal such as cobalt, nickel, and manganese as a positive electrode active material, a carbon material as a negative electrode active material, and a lithium gap between both electrodes. A mechanism for charging and discharging by moving ions is adopted.

It is known that the lithium composite oxide used as a positive electrode active material behaves differently as a battery positive electrode depending on the contained transition metal, and the positive electrode active materials of lithium ion secondary batteries currently on the market include: Lithium cobaltate is used because it has a good balance between capacity, battery characteristics, and safety. However, it has been pointed out that cobalt contained in lithium cobaltate has a scarce resource reserve, and there is a potential supply anxiety. Also, the lithium cobaltate positive electrode induces lithium metal deposition on the negative electrode when the battery is overcharged,
Includes danger of overheating and ignition. Therefore, for the purpose of further enhancing the safety of lithium ion secondary batteries, manganese-based positive electrodes, which are considered to be superior to cobalt in safety, have been actively developed (for example, Japanese Patent Application Laid-Open No. 55-100224). JP, JP-A-58-220362,
JP-A-63-114065). Batteries using lithium manganate for the positive electrode are inferior to lithium cobalt oxide and lithium nickel oxide in terms of battery capacity, but have greater stability at overcharge and at high temperatures. Danger is reduced. This leads to simplification of the charge / discharge protection circuit which has been indispensable when using a lithium ion secondary battery, and also leads to cost advantages. Manganese is also rich in resources,
There is little worry about supply anxiety.

[0004]

However, a battery using lithium manganate for the positive electrode has the following problems. The positive electrode using lithium manganate as the active material has lower adhesive strength at the interface between the coating film and the current collector than the positive electrode based on cobalt lithium oxide, which has been conventionally used as the positive electrode active material for lithium ion secondary batteries. However, a sufficient production yield cannot be obtained in the production process of the positive electrode. Also, the battery performance of the obtained battery has a large rate of causing a capacity reduction. These phenomena are mainly caused by peeling of the interface between the coating film and the current collector. The details of the reason why the adhesive strength at the interface between the coating film and the current collector is low in a positive electrode using lithium manganate as an active material are not clear, but generally, the adhesive strength tends to increase as the binder concentration in the coating film increases. Can be seen. However, increasing the binder concentration to keep the bonding strength high is not preferable because it leads to a decrease in the active material content of the whole battery and results in a decrease in the battery capacity. In order to solve these two conflicting problems, only the binder concentration near the interface between the coating film and the current collector should be kept high, and the remaining coating film portion should have a binder concentration sufficient to maintain the current collecting property. good. However, in order to adjust the binder concentration in the thickness direction of the coating film, it is necessary to carefully examine the drying conditions of the coating film and make strict adjustments in order to obtain a desired concentration distribution. In addition, this method may not always be able to be adjusted to an arbitrary concentration, and this method is not practical.

[0005]

The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have found that the following method is effective, and have completed the present invention. That is, in the present invention, in a lithium ion secondary battery using a carbon material for the negative electrode and a lithium oxide containing manganese for the positive electrode, a coating solution having a high binder concentration is applied to the current collector, dried, and then placed thereon. A lithium ion secondary battery characterized in that an electrode formed by applying and drying a coating solution having a low binder concentration is used as a positive electrode.

A feature of the present invention is that when a positive electrode is produced, a coating solution having a high binder concentration is applied to a current collector and dried,
Apply a coating solution with a low binder concentration on it and dry it,
It is to form a positive electrode. By using this method, only the binder concentration near the interface between the coating film and the current collector can be kept high, and the remaining coating film portion can have a binder concentration sufficient to maintain the current collecting property. As a result, the adhesive strength at the interface between the coating film and the current collector can be increased without impairing the battery capacity.

The present invention relates to a lithium ion secondary battery using a carbon material for a negative electrode and a lithium oxide containing manganese for a positive electrode. As the carbon material used for the negative electrode, a carbon material currently used generally as a negative electrode of a lithium ion secondary battery, such as natural graphite, artificial graphite, soft carbon, and hard carbon, or a mixture thereof can be used. Lithium oxide containing manganese used for the positive electrode includes lithium manganate having a stoichiometric composition, lithium manganate or cobalt having a non-stoichiometric composition in which the composition ratio of manganese and lithium is shifted from the stoichiometric composition, Nickel, magnesium,
Examples thereof include lithium manganate to which an element other than manganese such as chromium and aluminum is added.

A lithium ion secondary battery using these active materials is composed of a positive electrode, a negative electrode, a separator, an electrolytic solution, and other battery can members, and is charged and discharged repeatedly by moving lithium ions between the positive electrode and the negative electrode. Point to. Various types of battery cans are conceivable, such as a cylindrical type, a square type, a thin square type, and a coin type. In order to manufacture the positive electrode of the above-described lithium ion secondary battery, the following process is usually performed. The active material, filler, and binder are mixed with an appropriate solvent to form a slurry-like liquid, which is applied on a current collector and dried. In the present invention, carbons such as carbon black, acetylene black, natural graphite, artificial graphite, soft carbon, hard carbon, and pitch coke are preferable as the filler.
As the binder, a fluorine-based binder containing a polymer such as polyvinylidene fluoride, polytetrafluoroethylene, or polyhexafluoropropylene or a copolymer thereof as a main component is recommended. Although the solvent for dispersing these is not particularly limited, N-methylpyrrolidone is used as a solvent for uniformly dispersing the active material, the filler, and the binder and hardly changing the properties of the liquid. After the prepared coating liquid is uniformly applied to a current collector, the solvent is evaporated by heating and dried. As the current collector, it is preferable to use an aluminum foil having sufficient conductivity and being electrically and chemically stable.

It is a feature of the present invention that a coating liquid is further uniformly applied on the positive electrode coating film obtained as described above and dried again to obtain a final electrode. At this time, the binder concentration of the first application liquid is increased, and the binder concentration of the second application liquid is decreased. The binder concentration of the coating liquid applied for the first time is 2 to 20% by weight based on the active material, as a concentration range for maintaining a sufficient adhesive strength between the current collector and the coating film.
Preferably, the content is 5 to 10% by weight. The binder concentration of the second coating liquid is not particularly limited as long as the structure of the coating film can be maintained, and is preferably about 1 to 5% by weight in a range lower than the binder concentration of the first coating liquid. Is done. The application amount of the first application liquid and the application amount of the second application liquid may be determined in consideration of the thickness of the coating film obtained after drying, but it is necessary to maintain sufficient adhesive strength. The range that does not decrease the capacity of the positive electrode while
It is recommended that the ratio of the first application amount to the second application amount be in the range of 1:10 to 1: 3. The coating film thus obtained is dried again to obtain a final positive electrode coating film. When the cross section of the coating film obtained in this manner is analyzed using an analyzing means such as a fluorescent X-ray, information on the film thickness direction distribution of the coating film can be obtained for fluorine contained in the binder. The obtained positive electrode coating film has a feature that the concentration of fluorine derived from the binder near the current collector is higher than other portions. This indicates that a sufficient amount of the binder is distributed in the vicinity of the interface with the current collector, whereby sufficient adhesive strength can be exhibited as a positive electrode.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to specific examples.

[0011]

Example 1 Lithium manganate, polyvinylidene fluoride, graphite, and N-methylpyrrolidone were added in a weight ratio of 100.
Parts, 5 parts, 7 parts, and 150 parts, thereby obtaining a slurry-like coating liquid 1. This was applied to an aluminum foil having a thickness of 15 microns using a coating applicator having a clearance of 50 microns, and then dried at 120 ° C for 40 minutes. Then lithium manganate,
A slurry-like coating liquid 2 in which polyvinylidene fluoride, graphite, and N-methylpyrrolidone were mixed at a weight ratio of 100 parts, 2 parts, 7 parts, and 60 parts was prepared, and a coating applicator having a clearance of 50 microns was used. And coated on the coating film formed with the coating solution 1 and dried at 120 ° C. for 20 minutes to obtain a positive electrode coating film.

Next, the adhesive strength at the interface between the positive electrode coating film and the current collector was evaluated as follows. The obtained positive electrode was cut into a size of 2 cm × 10 cm, and an adhesive tape having a thickness of 0.2 mm was adhered to the entire surface on the coating film side. When the pressure-sensitive adhesive tape was peeled off from the current collector in this state, the pressure-sensitive adhesive tape was peeled off from the current collector with the coating applied thereto, and an interface between the current collector and the coating appeared.
In this way, the adhesive tape and the current collector with the coating film were respectively fixed to a SU-52 type tensile tester manufactured by Imada Seisakusho, and a tensile test was performed at a tensile speed of 10 mm / min. The maximum value was taken as the adhesive strength at the interface. The adhesive strength of the positive electrode obtained by this method was 10.5 g, which was sufficient for a positive electrode coating film.

[0013]

Examples 2 to 5 Positive electrode coating films were prepared and evaluated in the same manner as in Example 1 except that the composition of each component in the coating liquid 1 was as shown in Table 1. Table 1 shows the results. The adhesive strength of the positive electrode obtained by this method was sufficient as a positive electrode coating film.

[0014]

[Comparative Example] Lithium manganate, polyvinylidene fluoride, graphite, N-methylpyrrolidone in a weight ratio of 100
Parts, 2 parts, 7 parts, and 60 parts of a slurry-like coating liquid 1 were prepared, applied to a current collector using a coating applicator having a clearance of 75 microns, and dried at 120 ° C. for 20 minutes. Then, a positive electrode coating film was obtained. When the evaluation of the adhesive strength was performed in the same manner as in the example, it was 3.5 g, and it was not possible to obtain sufficient adhesive strength as the positive electrode coating film.

[0015]

[Table 1]

[0016]

According to the present invention, it is possible to obtain a coating film which maintains a sufficient current collecting property without reducing the yield during the production of the positive electrode.

Claims (1)

[Claims] In a lithium ion secondary battery using a carbon material for a negative electrode and a lithium oxide containing manganese for a positive electrode, a coating solution having a high binder concentration is applied to a current collector, dried, and then placed on the current collector. A lithium ion secondary battery characterized in that an electrode formed by applying and drying a coating solution having a low binder concentration is used as a positive electrode.