JPH05198317A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To improve the charge efficiency and the cycle life by using the electrolyte, in which phthalide or derivertive is added, as the organic solvent, in which lithium salt is dissolved, or the ion conductive high molecular compound. CONSTITUTION:At the time of manufacturing a positive electrode 4, manganese dioxide at 85 parts by weight, acetylene black as the conductive material at 10 parts by weight, polytetrafluoroethylene as the binder at 5 parts by weight are kneaded to be formed into a sheet, and thereafter, a positive electrode 4 at a desired diameter is punched out from a mold, and the high temperature vacuum drying is performed to make the positive electrode 4 pressure-contact to a positive electrode current collector 1. Next, at the time of manufacturing a negative electrode 5, a metal lithium foil at a desired diameter is punched out, and it is made to pressure-contact to a negative electrode current collector 2. Next, as the electrolyte to be filled, the electrolyte obtained by dissolving lithium perchlorate as the solute in the mixture solvent of propylene carbonate and dimethoxyethane at 1mol/l is used, and phthalide as the addition agent is dissolved in this electrolyte at 0.1mol/l, and a separator 6 is impregnated with this electrolyte.

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 that operates reversibly at ambient temperature, and more particularly to improvement of electrolyte.

[0002]

2. Description of the Related Art Recent microelectronics is
As represented by a memory backup power supply for various electronic devices, it has been made smaller, lighter, and thinner with the integration of batteries in electronic devices and integration with electronics electronics and circuits, and has a high energy density. Batteries are desired. In the field of primary batteries, small and lightweight batteries such as lithium batteries have already been put into practical use, but their fields of use are limited. Therefore, conventional lead batteries, nickel-
As a battery that replaces the cadmium battery, a lithium secondary battery that uses a non-aqueous electrolyte that can be made smaller and lighter is drawing attention, but one that satisfies the practical physical properties such as the cycle characteristics and self-discharge characteristics of the electrode active material. It is still being investigated by many research institutes because it has not been found.

What is particularly important as a problem of this type of secondary battery is that the negative electrode active material, lithium, grows in a dendritic manner on the surface of the negative electrode during charging, thereby coming into contact with the positive electrode and causing a short circuit inside the battery. Alternatively, the deposition in the form of mossy causes the loss of lithium and the like, resulting in a very short charge / discharge cycle. This is because, when lithium is ionized and eluted during discharging, the surface of the negative electrode becomes uneven, and lithium tends to be concentrated on the convex portion during subsequent charging. Further, these precipitated lithium particles are highly active because they are in the form of fine particles having a large surface area. Therefore, they react with the organic electrolytic solution to decompose the electrolytic solution and deteriorate the electrolyte, resulting in extremely poor cycle characteristics.

As a countermeasure against this, it has already been proposed to use a lithium alloy for the negative electrode in Japanese Patent Laid-Open Publication No. 52-5423. However, since the strength of the alloy is low as represented by a lithium-aluminum alloy, it is difficult to improve the cycle characteristics because the electrodes are cracked or miniaturized due to repeated charging and discharging. The same applies to other lithium alloys. Although various additives have been proposed recently, the charge / discharge efficiency is still low and unsatisfactory.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to provide a lithium secondary battery having improved charge / discharge efficiency and cycle life.

[0006]

In order to achieve the above object, the present invention is characterized by using an electrolyte obtained by adding a phthalide or a derivative thereof to an organic solvent or an ion conductive polymer compound in which a lithium salt is dissolved. And a lithium secondary battery.

[0007]

[Operation] It is considered that the reason why the charge / discharge efficiency of lithium decreases is that the lithium is electrochemically inactivated by the reduction reaction of the solvent with lithium and that the deposited lithium is removed. Therefore, in order to improve the charge / discharge efficiency of lithium in the electrolytic solution, it is necessary to change the state of the interface between the lithium electrode and the electrolytic solution to control the reaction between the solvent and lithium and the growth of dendrite. Therefore, the above-mentioned configuration is adopted to improve the charge / discharge efficiency of lithium. Although the reason is not clear, (1) the additive is adsorbed on the lithium electrode and suppresses the reaction between the solvent and lithium. (2) The additive reacts with lithium to form L on the lithium electrode surface.
An i ⁺ ion conductive protective film is formed. By this protective film, the exchange of lithium ions between the solvent and the lithium electrode is performed through the film, so that the direct reaction with the solvent is suppressed. It is considered that there are two points.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. (Example) FIG. 1 is a cross-sectional view of a lithium secondary battery using an electrolytic solution containing manganese dioxide for the positive electrode, metallic lithium for the negative electrode, and a non-aqueous solvent, and 1 is a case which also serves as a positive electrode current collector, Reference numeral 2 is a sealing plate that serves as a negative electrode current collector, 3 is a polypropylene gasket that insulates the case and the sealing plate, 4 is a positive electrode, 5 is a negative electrode, and 6 is a separator.

Next, the manufacturing method will be described. First,
To prepare the positive electrode 4, 85 parts by weight of manganese dioxide, 10 parts by weight of acetylene black as a conductive material, and 5 parts by weight of polytetrafluoroethylene as a binder were kneaded to form a sheet having a thickness of 0.7 mm. It was punched out to a diameter of 15 mm. Then, it was vacuum-dried at high temperature and pressure-bonded to the positive electrode current collector 1. Next, in order to form the negative electrode 5, a metal lithium foil having a thickness of 0.4 mm was punched into a diameter of 16 mm and pressure-bonded to the negative electrode current collector 2. Next, a polypropylene microporous film was placed as the separator 6 between the positive electrode 4 and the negative electrode 5 described above. The electrolytic solution is propylene carbonate and dimethoxyethane 1: 1:
1 mol of lithium perchlorate as a solute in 1 mixed solvent
A battery was prepared by using phthalide as an additive in an amount of 0.1 mol / l and dissolving it in the separator 6 described above. The following test was carried out on the battery A thus manufactured. Cycle test Test temperature: 25 ° C Charge: Constant current 0.5mA, Final voltage 3.5V Discharge: Constant current 1.0mA, Final voltage 2.4V

(Comparative Example) A battery B of Comparative Example was prepared in the same manner as in Example without adding phthalide as an additive to the electrolytic solution, and a test was conducted in the same manner.

FIG. 2 shows the result of the cycle test. As is clear from FIG. 2, the capacity of the battery A is less decreased than that of the battery B, and the effect of the present invention is demonstrated.
The organic solvent to which the additive according to the present invention can be added is not particularly limited, and for example, propylene carbonate, ethylene carbonate, diethyl carbonate, γ
-Butyrolactone, sulfolane, 1,3-dimethyl-2
A high dielectric constant solvent represented by imidazolidinone, and tetrahydrofuran, 2-methyltetrahydrofuran,
1,2-dimethoxyethane, 1,3-dioxolane, 4
There are low viscosity solvents represented by -methyl-1,3-dioxolane. You may use the electrolyte solution using the solvent of 1 or more types among these. Further, regarding a lithium salt as a solute, for example, LiClO ₄ , LiSCN, LiB
F ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ C
Examples thereof include inorganic ion salts such as O ₂ and organic ion salts such as lithium dodecylbenzenesulfonate. Two or more kinds of these ionic compounds may be used in combination.

[0012]

INDUSTRIAL APPLICABILITY As described above, the present invention can provide a lithium secondary battery with improved charge / discharge efficiency and cycle life, and therefore its industrial value is extremely large.

[Brief description of drawings]

FIG. 1 is a vertical sectional view showing an embodiment of the present invention.

FIG. 2 is a graph showing a comparison of cycle characteristics between an example of the present invention and a conventional comparative example.

Claims (1)

[Claims] 1. A lithium secondary battery using an electrolyte prepared by adding phthalide or a derivative thereof to an organic solvent or an ion conductive polymer compound in which an inorganic compound is dissolved.