Ionic Liquid Electrolyte for Lithium-Air Secondary Battery with Redox Mediator Function and Lithium-Air Secondary Battery

Abstract

To provide an ionic liquid electrolyte for a lithium-air secondary battery that achieves RM solubilization in an ionic liquid and can achieve both long-term stability and high energy efficiency.SOLUTION: An ionic liquid electrolyte for a lithium-air secondary battery according to the present invention is composed by dissolving a modified redox mediator in which a Li salt is complexed in an ionic liquid, and imparts redox mediator capability.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an ionic liquid electrolyte for a lithium-air secondary battery and a lithium-air secondary battery. More specifically, the present invention relates to an ionic electrolyte for a lithium-air secondary battery having a redox mediator function that promotes the oxidative decomposition of lithium peroxide Li2O2 generated at the air electrode during discharge, and a lithium-air secondary battery using the same.

Background Art

[0002] In order to solve global energy-related problems such as depletion of fossil fuels and environmental pollution, research and development of large-capacity energy storage devices that enable efficient storage and consumption of sustainable natural energy have been actively carried out in recent years. Among them, the lithium-air secondary battery is a secondary battery that uses oxygen abundantly present in the atmosphere as a positive electrode active material and lithium metal as a negative electrode active material, and is expected to have an energy density far exceeding that of current lithium-ion batteries. It is one of the next-generation secondary batteries expected to be put into practical use.

[0003] However, due to the structure of the lithium-air secondary battery, the air electrode is provided with oxygen introduction holes (air holes), and there is a problem that the electrolyte solvent volatilizes with the cycle. Although the volatilization of the electrolyte can be avoided by using a non-volatile ionic liquid as the solvent of the electrolyte solution, a lithium-air secondary battery using an ionic liquid-based electrolyte generally has a large overvoltage during charge and discharge (low energy efficiency) and a small capacity itself. Thus, the lithium-air secondary battery has many problems in electrode structure, charge and discharge reaction, and electrolyte, and has not yet been put into practical use.

[0004] Among battery materials, electrolytes are one of the particularly important key materials that determine battery performance. In lithium-air secondary batteries, ether-based electrolytes that are chemically stable against oxygen radicals as reaction intermediates and lithium peroxide as a discharge product have been most intensively studied. However, due to the special nature of the battery structure of lithium-air secondary batteries, where oxygen in the air is used as a cathode active material, it is not possible to apply volatile general-purpose organic solvents to the electrolytes of lithium-air secondary batteries. Therefore, recently, non-volatile ionic liquids have attracted attention. It has been reported that applying ionic liquids to electrolytes exhibits battery characteristics superior to those of organic solvent electrolytes.

[0005] The present inventors also applied an ionic liquid electrolyte to a lithium-air secondary battery and compared the effects on battery characteristics with an organic solvent electrolyte. As a result, while the capacity was not exhibited at all after one month of assembly with the organic solvent electrolyte, the ionic liquid electrolyte was found to have better long-term stability than the battery system using the organic solvent electrolyte, such as being able to be charged and discharged even one month after battery assembly. On the other hand, a major problem is the low energy efficiency, such as a large voltage being required for the oxidative decomposition (charging process) of lithium peroxide Li2O2.

[0006] The theoretical oxidation-reduction potential of lithium peroxide Li2O2 is 2.97 V vs. Li +Although it is calculated as / Li, when the charge-discharge test is actually performed, the discharge voltage is 2.7 V and the charge voltage is 4 V or more, and a particularly large overvoltage occurs during charging. As an approach to suppressing the charging overvoltage, the use of a substance in which a redox reaction occurs around 3.5 V, a redox mediator (hereinafter sometimes referred to as RM), has been studied (for example, Non-Patent Documents 1 to 3). This makes it possible to suppress the charging voltage. RM includes those composed of organic compounds and those composed of inorganic compounds. Organic-based RM has excellent solubility in electrolytes but many have low chemical stability and are reduced by contact with lithium metal. On the other hand, as inorganic-based RM, LiNO3, LiBr, CsBr, etc. have been studied in recent years. These have low solubility in organic solvents and also tend to have low dissociation degrees in electrolytes, but it has been reported that they not only promote the decomposition of Li2O2 but also have an effect on the cycle stability of lithium metal. By adding an inorganic-based RM with excellent chemical stability to an ionic liquid electrolyte, the coexistence of long-term stability and high energy efficiency is expected, but inorganic-based RM hardly dissolves in ionic liquids, and it is very difficult to impart RM ability to ionic liquids.

Prior Art Documents

Non-Patent Documents

[0007]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0008] The present invention has been made in view of such problems of the prior art, and an object thereof is to provide an ionic liquid electrolyte for a lithium-air secondary battery capable of realizing solubilization of RM in an ionic liquid and achieving both long-term stability and high energy efficiency, and a lithium-air secondary battery using the same.

Means for Solving the Problems

[0009] As a result of intensive studies to solve the above problems, the present inventors have found that solubilization of RM in an ionic liquid can be realized by modifying poorly soluble inorganic RM, and have completed the present invention.

[0010] That is, according to the present invention, there is provided an ionic liquid electrolyte for a lithium-air secondary battery imparted with a redox mediator function, characterized in that a modified redox mediator in which a Li salt is complexed is dissolved in the ionic liquid.

[0011] In the present invention, the Li salt is preferably at least one selected from the group consisting of LiNO3, LiBr, and LiI.

[0012] Further, the modified redox mediator is preferably a complex formed by adding at least one ligand selected from the group consisting of glymes, sulfoxides, nitriles, and imidazoles, and further at least one ligand selected from the group consisting of tetraglyme (G4), triglyme (G3), diglyme (G2), dimethyl sulfoxide (DMSO), acetonitrile (ACN), succinonitrile (SN), and methylimidazole (MeIm).

[0013] Regarding the ionic liquid, it preferably consists of at least one of a phosphonium cation, an ammonium cation, a sulfonium cation having a chain structure, or at least one of a nitrogen (N)-containing heterocyclic cation, and further, at least one cation selected from triethyl-methoxymethylphosphonium, ethyl-dimethyl-propylammonium, triethylsulfonium, N-methyl-N-butylpiperidinium, N-methyl-N-butylpyrrolidinium, 1-methyl-3-butylimidazolium, N-butylpyridinium, and N-methyl-butylmorpholinium, and an anion. Regarding the anion here, a bis(trifluoromethylsulfonyl)amine anion is preferable.

[0014] Furthermore, in the present invention, a novel lithium-air secondary battery is provided, which is characterized by using an ionic liquid electrolyte for a lithium-air secondary battery having the above-described characteristics.

Advantages of the Invention

[0015] According to the present invention, by adopting the above configuration, compared with existing organic solvent-based electrolytes, an ionic liquid electrolyte for a lithium-air secondary battery that is hardly volatile and has an overvoltage suppression effect equivalent to that of an organic solvent-based electrolyte by imparting RM ability, and a lithium-air secondary battery using the same are provided. Therefore, the present invention realizes the solubilization of RM in an ionic liquid and enables both long-term stability and high energy efficiency.

Brief Description of the Drawings

[0016]

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Mode for Carrying Out the Invention

[0017] Hereinafter, the ionic liquid electrolyte for a lithium-air secondary battery of the present invention will be described in detail based on embodiments.

[0018] The ionic liquid electrolyte for a lithium-air secondary battery of the present invention is characterized in that a modified redox mediator in which a Li salt is complexed is dissolved in an ionic liquid, and it is provided with redox mediator (RM) ability.

[0019] That is, as described above, the redox mediator (RM) means a substance in which an oxidation-reduction reaction occurs around 3.5V. In the present invention, a Li salt is used as the redox mediator, and this is adopted as a modified redox mediator in which it is complexed. Regarding the Li salts here, inorganic salts of Li can be cited as typical ones.

[0020] The ionic liquid electrolyte for a lithium-air secondary battery of the present invention realizes solubilization of RM in an ionic liquid by modifying hardly soluble inorganic RM. Originally, inorganic RM has an extremely low solubility, which is attributed to the high Lewis acidity of metal ions. Metal ions have a high surface charge density and interact strongly with counter anions. On the other hand, an ionic liquid is composed of weakly coordinating ions both for cations and anions, and the solvation interaction by the ionic species constituting the ionic liquid cannot exceed the ionic interaction in inorganic RM, so the solubility of inorganic RM is low. Therefore, the inventors of the present invention tried to complex metal ions with ligands to reduce their Lewis acidity. As a result, the solubility of inorganic RM in the ionic liquid was improved. As a result of applying the electrolyte of the present invention to a lithium-air secondary battery, the charging overvoltage was dramatically reduced as in the case of an organic solvent-based electrolyte, and an improvement in energy efficiency was achieved.

[0021] Fig. 1 schematically shows an example of the structure of a general lithium-air secondary battery using a liquid electrolyte. In this lithium-air secondary battery, Li metal is used for the negative electrode 1, a carbon electrode is used for the positive electrode 2, a separator 3 is provided between the negative electrode 1 and the positive electrode 2, and a liquid electrolyte 4 is filled. The negative electrode reaction, positive electrode reaction, and overall reaction of this lithium-air secondary battery are as shown below Fig. 1.

[0022] Figure 2 shows the charge-discharge test results of a lithium-air secondary battery using various electrolytes. In the upper left figure (a) of Figure 2, an organic solvent (tetraglyme: G4) + RM electrolyte is used; in the upper right figure (b), an ionic liquid (IL) (pyrrolidinium IL) + RM electrolyte is used; and in the lower left figure (c), an ionic liquid (pyrrolidinium IL) + modified RM electrolyte is used as the electrolyte for each test result. For the charge-discharge test, a CR232 coin cell with the configuration shown in Figure 3 was used as the electrochemical measurement cell. This CR232 coin cell is composed of an upper lid 11, a carbon electrode 12, a separator 13 + electrolyte solution 14, a metallic Li foil 15, and a lower lid 16. The measurement conditions are the same as those in the examples described below.

[0023] As shown in Figures 2(a) and 2(b), in the coin cells using an organic solvent + RM electrolyte and an ionic liquid + RM electrolyte as the electrolyte, RM hardly dissolved, and even in the coin cells where RM ability was exhibited, the cycle characteristics were extremely low. As shown in Figure 2(c), it was confirmed that by modifying RM, the solubility in the ionic liquid was dramatically improved, and an ionic liquid-based electrolyte showing stable charge-discharge characteristics was realized.

[0024] In the ionic liquid electrolyte for a lithium-air secondary battery of the present invention, a Li salt is used as the inorganic salt-based RM. As particularly preferred Li salts, at least one selected from the group consisting of LiNO3, LiBr, and LiI can be mentioned. By mixing LiBr and LiI with Br2 and I2 molecules in G4, low-melting complexes such as [Li(G4)]Br3 and [Li(G4)]I are formed. Since these Li salts can be dissolved in the ionic liquid at a high concentration, they can be preferably used as the modified inorganic-based RM. The concentration of the Li salt can be, for example, 0.1 to 0.4 M, and as a specific example, it can be 0.2 M, but it is not limited thereto.

[0025] In the ionic liquid electrolyte for a lithium-air secondary battery of the present invention, various combinations of ions can be considered for the ionic liquid, including those known so far. As an example of the anion, TFSA (bis(trifluoromethylsulfonyl)amine anion) is preferably exemplified. Of course, it is not limited to this. The cation can also be various, but preferably, a phosphonium cation, an ammonium cation, or a sulfonium cation having a chain structure is mentioned. Also, an N (nitrogen)-containing heterocyclic cation is preferably mentioned. Table 1 shows examples of preferred cations among these.

[0026]

Table 1

[0027] In the ionic liquid electrolyte for a lithium-air secondary battery of the present invention, for the modified redox mediator, it is preferable that at least one of saccharides, sulfoxides, nitriles, and imidazoles be used as a ligand for complexing the above-mentioned Li: lithium salt. For example, in the modified redox mediator of the present invention, it is possible to preferably use a complex formed by adding at least one ligand selected from the group consisting of tetraglyme (G4), triglyme (G3), diglyme (G2), dimethyl sulfoxide (DMSO), acetonitrile (ACN), succinonitrile (SN), and methylimidazole (MeIm). Specifically, it is exemplified that at least one selected from the group consisting of the seven types shown in Table 2 is used.

[0028]

Table 2

[0029] In the present invention, the mixing ratio (content ratio) of the ligand and the Li salt (RM) is not limited, but for example, the following can be used as a guide. In the case of tetraglyme (G4), 1:1 (4.44%) For triglyme (G3), 1:1 (3.56%) For diglyme (G2), 1:2 (5.36%) For dimethyl sulfoxide (DMSO), 1:6 (9.36%) For acetonitrile (ACN), 1:6 (4.92%) For methylimidazole (MeIm), 1:6 (9.84%) For succinonitrile (SN), 1:3 (4.80%) It is important to add the minimum amount of ligand required for complex formation, complex the Li salt (RM), and facilitate dissolution in the ionic liquid (IL).

Example

[0030] Hereinafter, the present invention will be described in more detail based on examples. In the following examples, for the ionic liquid (IL), the anion is in combination with bis(trifluoromethylsulfonyl)amine anion, and for the cation, triethyl-methoxymethylphosphonium, ethyl-dimethyl-propylammonium, triethylsulfonium, N-methyl-N-butylpiperidinium, N-methyl-N-butylpyrrolidinium, 1-methyl-3-butylimidazolium, N-butylpyridinium, and N-methyl-butylmorpholinium are abbreviated as phosphonium, ammonium, sulfonium, piperidinium, pyrrolidinium, imidazolium, pyridinium, and morpholinium, respectively.

[0031] [Example 1] 0.2 M LiNO3 (Fuji Film Wako Pure Chemical Industries, Ltd.: purity 99.99%) as the Li salt, tetraglyme (G4) as the ligand, and phosphonium, sulfonium, pyridinium, and imidazolium as the ionic liquid (IL) were mixed and stirred at 60 °C for 8 hours to prepare an electrolyte of RM + ligand / ionic liquid (IL).

[0032] As shown in Figure 3, as the positive electrode, a carbon electrode (loading amount: 0.50 mg·cm -2 , electrode area 2 cm-2 ) Using a metallic Li foil (φ = 16 mm) as the negative electrode, Whatman / GF as the separator, and RM + ligand / ionic liquid (IL) (dropwise addition amount: 25 μl·cm -2 ) as the electrolyte, an electrochemical measurement cell (CR2032 coin cell) was fabricated.

[0033] Using this electrochemical measurement cell, a constant current discharge test was conducted under the following conditions using an electrochemical measurement device (HOKUTO DENKO HIR-1010mSM8A).

[0034] Measurement conditions: Discharge current 50 μAcm -2 Limited charge-discharge capacity: 0.5 mAhcm -2 Charge-discharge time: 20 h Voltage range: 2.0 - 4.3 V Atmosphere: Dry air Measurement temperature: 25 °C Electrode area: 2 cm 2 Amount of electrolyte: 25 μlcm -2 Carbon loading: 0.5 mgcm -2

[0035] The results of the charge-discharge test are shown in Figure 4. From these results, it was confirmed that the ionic liquid electrolyte for the lithium-air secondary battery of Example 1 was sufficiently endowed with redox mediator ability.

[0036] [Example 2] As the Li salt, 0.2 M LiNO3 (FUJIFILM Wako Pure Chemical Corporation: purity 99.99%), as the ligand, acetanilide (ACN), and as the ionic liquid (IL), sulfonium, ammonium, and imidazolium were mixed respectively, and stirred at 60 °C for 8 hours to prepare an electrolyte of RM + ligand / ionic liquid (IL).

[0037] Similar to Example 1, using the above RM + ligand / ionic liquid (IL) as the electrolyte (dropwise addition amount: 25 μl·cm -2Using [it], an electrochemical measurement cell (CR2032 coin cell) was fabricated, and a constant current discharge test was conducted under the same conditions as in Example 1.

[0038] The results of the charge-discharge test are shown in Fig. 5. From these results, it was confirmed that the ionic liquid electrolyte for the lithium-air secondary battery of Example 2 was imparted with sufficient redox mediator ability.

[0039] [Example 3] 0.2 M LiNO3 (Fuji Film Wako Pure Chemical Industries, Ltd.: purity 99.99%) as the Li salt, dimethyl sulfoxide (DMSO) as the ligand, and phosphonium, sulfonium, pyrrolidinium, ammonium, and imidazolium as the ionic liquids (ILs) were mixed respectively, stirred at 60 °C for 8 hours, and an electrolyte solution of RM + ligand / ionic liquid (IL) was prepared.

[0040] Similar to Example 1, using the above RM + ligand / ionic liquid (IL) (dropwise addition amount 25 μl·cm -2 ) as the electrolyte solution, an electrochemical measurement cell (CR2032 coin cell) was fabricated, and a constant current discharge test was conducted under the same conditions as in Example 1.

[0041] The results of the charge-discharge test are shown in Fig. 6. From these results, it was confirmed that the ionic liquid electrolyte for the lithium-air secondary battery of Example 3 was imparted with sufficient redox mediator ability.

[0042] [Example 4] 0.2 M LiBr (Sigma-A: purity 99.99%) as the Li salt, tetraglyme (G4) as the ligand, and phosphonium, sulfonium, pyrrolidinium, pyridinium, ammonium, and imidazolium as the ionic liquids (ILs) were mixed respectively, stirred at 60 °C for 8 hours, and an electrolyte solution of RM + ligand / ionic liquid (IL) was prepared.

[0043] Similar to Example 1, using the above RM + ligand / ionic liquid (IL) (dropwise addition amount 25 μl·cm -2Using [it], an electrochemical measurement cell (CR2032 coin cell) was fabricated, and a constant current discharge test was conducted under the same conditions as in Example 1.

[0044] The results of the charge-discharge test are shown in Fig. 7. From these results, it was confirmed that the ionic liquid electrolyte for the lithium-air secondary battery of Example 4 was imparted with sufficient redox mediator ability.

[0045] [Example 5] 0.2 M LiBr (Sigma-A: purity 99.99%) as the Li salt, acetanilide (ACN) as the ligand, and phosphonium, sulfonium, pyrrolidinium, pyridinium, ammonium, and imidazolium as the ionic liquids (ILs) were mixed and stirred at 60 °C for 8 hours to prepare an electrolyte of RM + ligand / ionic liquid (IL).

[0046] Similar to Example 1, using the above RM + ligand / ionic liquid (IL) as the electrolyte (dropwise addition amount 25 μl·cm -2 ), an electrochemical measurement cell (CR2032 coin cell) was fabricated, and a constant current discharge test was conducted under the same conditions as in Example 1.

[0047] The results of the charge-discharge test are shown in Fig. 8. From these results, it was confirmed that the ionic liquid electrolyte for the lithium-air secondary battery of Example 5 was imparted with sufficient redox mediator ability.

[0048] [Example 6] 0.2 M LiBr (Sigma-A: purity 99.99%) as the Li salt, dimethyl sulfoxide (DMSO) as the ligand, and phosphonium, sulfonium, pyrrolidinium, pyridinium, ammonium, and imidazolium as the ionic liquids (ILs) were mixed and stirred at 60 °C for 8 hours to prepare an electrolyte of RM + ligand / ionic liquid (IL).

[0049] Similar to Example 1, using the above RM + ligand / ionic liquid (IL) as the electrolyte (dropwise addition amount 25 μl·cm -2Using [it], an electrochemical measurement cell (CR2032 coin cell) was fabricated, and a constant current discharge test was conducted under the same conditions as in Example 1.

[0050] The results of the charge-discharge test are shown in Fig. 9. From these results, it was confirmed that the ionic liquid electrolyte for the lithium-air secondary battery of Example 6 was imparted with sufficient redox mediator ability.

[0051] [Example 7] 0.2 M LiI (Fuji Film Wako Pure Chemical Industries, Ltd.: purity 99.99%) as the Li salt, tetraglyme (G4) as the ligand, and phosphonium, sulfonium, pyrrolidinium, pyridinium, ammonium, and imidazolium as the ionic liquids (ILs) were mixed and stirred at 60 °C for 8 hours to prepare an electrolyte of RM + ligand / ionic liquid (IL).

[0052] Similar to Example 1, using the above RM + ligand / ionic liquid (IL) (dropwise addition amount 25 μl·cm -2 ) as the electrolyte, an electrochemical measurement cell (CR2032 coin cell) was fabricated, and a constant current discharge test was conducted under the same conditions as in Example 1.

[0053] The results of the charge-discharge test are shown in Fig. 10. From these results, it was confirmed that the ionic liquid electrolyte for the lithium-air secondary battery of Example 7 was imparted with sufficient redox mediator ability.

[0054] [Example 8] 0.2 M LiI (Fuji Film Wako Pure Chemical Industries, Ltd.: purity 99.99%) as the Li salt, acetanilide (ACN) as the ligand, and sulfonium, pyridinium, and ammonium as the ionic liquids (ILs) were mixed and stirred at 60 °C for 8 hours to prepare an electrolyte of RM + ligand / ionic liquid (IL).

[0055] Similar to Example 1, using the above RM + ligand / ionic liquid (IL) (dropwise addition amount 25 μl·cm -2Using [it], an electrochemical measurement cell (CR2032 coin cell) was fabricated, and a constant current discharge test was conducted under the same conditions as in Example 1.

[0056] The results of the charge-discharge test are shown in Fig. 11. From these results, it was confirmed that the ionic liquid electrolyte for the lithium-air secondary battery of Example 8 was imparted with sufficient redox mediator ability.

[0057] [Example 9] 0.2 M LiI (Fuji Film Wako Pure Chemical Industries, Ltd.: purity 99.99%) as the Li salt, dimethyl sulfoxide (DMSO) as the ligand, and sulfonium, pyrrolidinium, ammonium, and imidazolium as ionic liquids (ILs) were mixed and stirred at 60 °C for 8 hours to prepare an electrolyte solution of RM + ligand / ionic liquid (IL).

[0058] Similar to Example 1, using the above RM + ligand / ionic liquid (IL) (dropwise addition amount 25 μl·cm -2 ) as the electrolyte solution, an electrochemical measurement cell (CR2032 coin cell) was fabricated, and a constant current discharge test was conducted under the same conditions as in Example 1.

[0059] The results of the charge-discharge test are shown in Fig. 12. From these results, it was confirmed that the ionic liquid electrolyte for the lithium-air secondary battery of Example 9 was imparted with sufficient redox mediator ability.

[0060] [Example 10] Similar to Example 4, 0.2 M LiBr (Sigma-A: purity 99.99%) as the Li salt, tetraglyme (G4) as the ligand, and sulfonium were mixed and stirred at 60 °C for 8 hours to obtain RM + ligand / ionic liquid (IL).

[0061] Similar to Example 1, using the above RM + ligand / ionic liquid (IL) as the electrolyte solution, a lithium-air secondary battery was fabricated, and a charge-discharge test was conducted under the following conditions.

[0062] Measurement conditions Discharge current: 400 μA / cm -2 Limited discharge capacity: 4.0 mAh / cm -2 Charge and discharge time: 20 h Voltage range: 2.0 - 4.3 V Atmosphere: Oxygen Measurement temperature: 70 °C Electrolyte volume: 25 μl / cm -2 Carbon free-standing film

[0063] The charge and discharge results are shown in Fig. 13. Discharge current 400 μA / cm -2 , limited discharge capacity: 4.0 mAh / cm -2 Even under the conditions of, it was confirmed that the lithium-air secondary battery could perform three or more charge and discharge cycles.

Explanation of symbols

[0064] 1 Anode 2 Cathode 3 Separator 4 Liquid electrolyte 11 Upper cover 12 Carbon electrode 13 Separator 14 Electrolyte 15 Lithium metal foil 16 Lower cover

Claims

1. A modified redox mediator complexed with a Li salt is dissolved in an ionic liquid, wherein the Li salt is at least one of LiNO₃ or LiBr, and the ionic liquid electrolyte for a lithium-air secondary battery imparted with redox mediator ability is characterized thereby.

2. The modified redox mediator according to claim 1, wherein the modified redox mediator is complexed by adding at least one ligand selected from the group consisting of glymes, sulfoxides, nitriles, and imidazoles.

3. The ionic liquid electrolyte for a lithium-air secondary battery according to claim 2, wherein the ligand is at least one selected from the group consisting of tetraglyme (G4), triglyme (G3), diglyme (G2), dimethyl sulfoxide (DMSO), acetonitrile (ACN), succinonitrile (SN), and methylimidazole (MeIm).

4. The ionic liquid electrolyte for a lithium-air secondary battery according to any one of claims 1 to 3, wherein the ionic liquid consists of at least one cation selected from the group consisting of a phosphonium cation, an ammonium cation, and a sulfonium cation having a chain structure and an anion.

5. The cation is at least one selected from the group consisting of triethyl-methoxymethylphosphonium, ethyl-dimethyl-propylammonium, and triethylsulfonium and the ionic liquid electrolyte for a lithium-air secondary battery according to claim 4 is characterized thereby.

6. The ionic liquid electrolyte for a lithium-air secondary battery according to any one of claims 1 to 3, wherein the ionic liquid consists of at least one N-containing heterocyclic cation and an anion.

7. The ionic liquid electrolyte for a lithium-air secondary battery according to claim 6, wherein the N-containing heterocyclic cation is at least one selected from the group consisting of N-methyl-N-butylpiperidinium, N-methyl-N-butylpyrrolidinium, 1-methyl-3-butylimidazolium, N-butylpyridinium, and N-methyl-butylmorpholinium.

8. The ionic liquid electrolyte for a lithium-air secondary battery according to claim 4 or 6, wherein the anion is a bis(trifluoromethylsulfonyl)amine anion.

9. A lithium-air secondary battery, characterized in that the ionic liquid electrolyte for a lithium-air secondary battery according to any one of claims 1 to 8 is used.

10. The lithium-air secondary battery according to claim 9, wherein a Li metal is disposed on the negative electrode, a carbon electrode is disposed on the positive electrode, and the cell in which a separator is provided between the negative electrode and the positive electrode is filled with the ionic liquid electrolyte for a lithium-air secondary battery.

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