This application claims priority to and the benefit of Korean Patent Application No. 10-2019-0052271 filed in the Korean Intellectual Property Office on May 3, 2019 and Korean Patent Application No. 10-2020-0031340 filed in the Korean Intellectual Property Office on Mar. 13, 2020, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

This relates to lithium-air secondary battery.

(b) Description of the Related Art

Currently, various types of next-generation secondary battery systems are being researched, and among them, lithium-air secondary batteries may be capable of realizing the highest capacity. Since the lithium-air secondary battery uses light air (oxygen) as a cathode active material, it has an advantage of realizing a high capacity of several times that of a lithium ion secondary battery using a heavy transition metal oxide.

In the lithium-air secondary battery, lithium ions move from a lithium metal that is an anode through an organic electrolyte to an air electrode (cathode) in a discharging process, and in the air electrode, oxygen in the air and lithium ions react to form Li2O2(lithium peroxide). Herein, the formed Li2O2is a solid so that it covers the surface of the air electrode. In a charging process, Li2O2formed in the discharge process is decomposed into lithium ions and oxygen again. Theoretically, pure Li2O2alone should be formed during the discharge and be completely decomposed during the charge, but many side-reaction products in addition to the Li2O2are formed during the discharge and are not sufficiently decomposed during the charge but are accumulated on the air electrode, and the accumulated side-reaction products bring about a sharp capacity decrease as charges and discharges proceed.

Accordingly, the lithium-air secondary battery cell has a drawback of a very deteriorated cycle-life compared with currently commercially-available secondary battery systems. In addition, since a high overvoltage is applied thereto due to additional energy for decomposing the reaction products such as Li2O2having almost no electrical conductivity and the like, much more energy is required during the charge than the energy obtained during the discharge, which causes an efficiency decrease problem.

SUMMARY OF THE INVENTION

The present invention provides a lithium-air secondary battery having improved charge and discharge characteristics by suppressing the formation of side-reaction products during discharge.

An embodiment of the present invention provides a lithium-air secondary battery including an air electrode (air cathode), a lithium anode (Li anode), and an electrolyte including a superoxide dismutase mimic catalyst (SODm).

The superoxide dismutase mimic catalyst may perform a disproportionation reaction of a superoxide (O2−) generated at the air electrode during discharge, and may convert it into oxygen (O2−) and Li2O2.

The superoxide dismutase mimic catalyst may serve to suppress side reactions of the superoxide (O2−) generated at the air electrode during discharge.

The side reaction may include a reaction to form a lithium salt other than the Li2O2.

The lithium salt other than the Li2O2may be at least one selected from Li2CO3, LiOH, LiCOOCH3, and LiCOOR (R is alkyl).

The superoxide dismutase mimic catalyst (SODm) may be at least one of an organic compound, a free radical compound, a metal complex, and a metal oxide which includes at least one functional group of a carboxyl group, a polyphenol group, and a hydroxyl group.

The SODm (superoxide dismutase mimic catalyst) may be, for example, at least one of MA-C60, 4-carboxy-TEMPO, honokiol, CeO2, OsO4, Mn texaphyrin, MnTrM-2-Corrole3+, [MnBV2−]2, [MnBVDME]2, [MnMBVDME]2, and [MnBVDT2−]2.

A concentration of the SODm (superoxide dismutase mimic catalyst) may be 0.1 mM to 100 mM based on the electrolyte.

By suppressing the formation of side-reaction products during discharge, a high-capacity lithium-air secondary battery having improved charge/discharge characteristics may be realized.

