Gel polymer electrolyte for lithium-air battery and lithium-air battery

Abstract

The present disclosure relates to a gel polymer electrolyte for a lithium-air battery including a specific amount of a zwitterionic salt and a lithium-air battery including the same and thus having an extended battery life by suppressing the volatilization of the electrolyte and imparting lithium-air battery interface stability by suppressing the formation of dendrites at the lithium anode and suppressing side reactions between the lithium anode and the liquid electrolyte. Furthermore, the use of the zwitterionic salt can increase the lithium ion transference number, thereby increasing the life of the battery.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a gel polymer electrolyte including a zwitterionic salt for a lithium air battery and a lithium air battery including the same and thus having a prolonged battery life. BACKGROUND

[0002] A lithium air battery is a secondary battery composed of a lithium anode, an electrolyte, and an air cathode in which a reversible electrochemical reaction can occur between oxygen and lithium ions. The lithium air battery has a high energy density of 11000 Wh / kg in theory, is inexpensive, environmentally friendly, and safe compared to existing lithium ion batteries, and thus research and development on using the same as an energy source for an electric vehicle are actively being conducted.

[0003] However, in order to commercialize such a lithium air battery, there are still problems to be solved. When lithium metal is used as an anode, it reacts with an electrolyte or impurities in the battery, thereby forming a passivation layer. The layer causes a local current density difference, which eventually leads to uneven current distribution during charging and at the same time, formation of lithium dendrites. In addition, unlike conventional secondary batteries, since the lithium air battery is driven in an open system, a liquid electrolyte solvent used is evaporated, thereby greatly increasing the internal resistance of the battery, which shortens the life of the battery.

[0004] When an organic liquid electrolyte is used as an electrolyte for a lithium air battery, the basic electrochemical properties of the organic liquid electrolyte are excellent, but the liquid electrolyte is evaporated during charging and discharging, thereby causing a sharp decrease in battery performance. In addition, when an organic liquid electrolyte is used, since the organic liquid electrolyte has high reactivity, a side reaction can occur at the lithium anode, and lithium dendrites can be formed during charging and discharging, thus undesirably decreasing the charge-discharge efficiency, eventually reducing the life of the lithium ion battery.

[0005] In order to solve these problems, a method of coating an organic-inorganic hybrid film including an inorganic material having high mechanical strength on lithium metal has been proposed, but the method has disadvantages in that the interfacial resistance of lithium metal is large and the inorganic solid electrolyte can react with lithium metal. In addition, when a linear polymer is coated on lithium metal, a side reaction with an electrolyte can be suppressed, but the protective layer is easily detached from lithium metal, and it is difficult to suppress the formation of lithium dendrites due to poor mechanical physical properties.

[0006] Therefore, there is a need to develop an electrolyte that suppresses the evaporation of a liquid electrolyte and imparts stability to the interface of a lithium air battery by suppressing the formation of dendrites at a lithium anode and suppressing side reactions between the lithium anode and the liquid electrolyte. SUMMARY

[0007] Accordingly, the disclosure is directed to solving the problems encountered in the related art, and specific objects of the disclosure are as follows.

[0008] An object of the disclosure is to provide a gel polymer electrolyte for a lithium air battery including a specific amount of a zwitterionic salt and a lithium air battery including the same.

[0009] The objects of the disclosure are not limited to the foregoing objects, and can be clearly understood by the following description and can be realized by the ways described in the claims and combinations thereof.

[0010] One aspect of the disclosure provides a gel polymer electrolyte for a lithium air battery, the gel polymer electrolyte including: a complex polymer resin including a linear polymer and a crosslinking agent, an ionic liquid, a lithium salt, a zwitterionic salt, an initiator, and an organic solvent.

[0011] The gel polymer electrolyte can include: 4 to 6 wt% of the complex polymer resin, 7 to 10 wt% of the ionic liquid, 2 to 5 wt% of the lithium salt, 0.15 to 0.40 wt% of the zwitterionic salt, 0.02 to 0.04 wt% of the initiator, and 80 to 85 wt% of the organic solvent.

[0012] The complex polymer resin can include 85 to 95 wt% of the linear polymer and 5 to 15 wt% of the crosslinking agent.

[0013] The linear polymer can be at least one copolymer or mixture selected from the group consisting of polymethyl methacrylate, polystyrene, polyethylene oxide, polytetrafluoroethylene, perfluorosulfonic acid, and polyurethane.

[0014] The crosslinking agent can include at least one selected from the group consisting of divinylbenzene (DVB), triethylene glycol diacrylate (TEGDA), and polyethylene glycol diacrylate (PEGDA).

[0015] The ionic liquid can include a salt including at least one cation selected from the group consisting of imidazolium, pyrrolidinium, piperidinium, ammonium, phosphonium, sulfonium, pyridinium, pyrazolium, triazolium, thiazolium, oxazolium, pyridazinium, pyrimidinium, and pyrazinium, and at least one anion selected from the group consisting of acetate, bis(fluorosulfonyl)imide, bis(fluoromethanesulfonyl)imide, bromide, chloride, iodide, dicyanamide, hexafluorophosphate, tetrafluoroborate, and hydrosulfate.

[0016] The lithium salt can include at least one selected from the group consisting of LiTFSI, LiFSI, LiBF4, LiPF6, LiClO4, LiAsF6, LiDFOB, and LiBOB.

[0017] The zwitterionic salt can include at least one cation selected from the group consisting of imidazolium, pyrrolidinium, piperidinium, and ammonium, and at least one anion selected from the group consisting of sulfonate, carboxylate, and phosphite.

[0018] The initiator can include at least one selected from the group consisting of 2,2'-azobis(2-methylpropionitrile) (AIBN), benzoyl peroxide, and lauryl peroxide.

[0019] The organic solvent can include at least one selected from the group consisting of tetrahydrofuran (THF), acetonitrile (ACN), and dimethyl carbonate (DMC).

[0020] Another aspect of the present disclosure provides a lithium-air battery including: an anode including lithium; a cathode using oxygen as a cathode active material; the above-described gel polymer electrolyte disposed between the anode and the cathode; and a separation film located on the gel polymer electrolyte.

[0021] The separation film can include at least one selected from the group consisting of an olefin resin (polyethylene, polypropylene, etc.), glass fiber, fluororesin, ester resin, and cellulose nonwoven fabric.

[0022] The present disclosure provides a gel polymer electrolyte for a lithium-air battery including a specific amount of a zwitterionic salt and a lithium-air battery including the same. Since the lithium-air battery according to the present disclosure includes a gel polymer electrolyte for a lithium-air battery, volatilization of the electrolyte can be suppressed, and in addition, the lithium-air battery can be imparted with interfacial stability by suppressing the formation of dendrites at the lithium anode and suppressing side reactions between the lithium anode and the liquid electrolyte. Furthermore, the zwitterionic salt can be included in the lithium-air battery, and thus the number of lithium ion transfers is increased, thereby increasing the life of the battery.

[0023] Effects of the present disclosure are not limited to the aforementioned and should be understood to include all effects that can be reasonably expected from the following description. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is a schematic cross-sectional view illustrating a lithium-air battery according to an embodiment of the present disclosure;

[0025] Figure 2A is a scanning electron microscope photograph showing a fracture surface of a gel polymer electrolyte for a lithium-air battery manufactured according to Example 1-1 of the present disclosure;

[0026] Figure 2B is an EDX elemental analysis image showing F (fluorine);

[0027] Figure 2C is an EDX elemental analysis image showing N (nitrogen);

[0028] Figure 3 is a graph showing an FT-IR spectrum of a gel polymer electrolyte manufactured according to Embodiment 1-1 of the present disclosure;

[0029] Figure 4 is a graph showing ion conductivity of lithium air batteries manufactured according to Comparative Example 1, Comparative Example 2-2, Comparative Example 3-2, Comparative Example 4-2, and Embodiment 1-2 of the present disclosure;

[0030] Figure 5A is a DC current graph of a lithium air battery manufactured according to Embodiment 1-2 of the present disclosure;

[0031] Figure 5B is an AC impedance spectrum graph of a lithium air battery manufactured according to Embodiment 1-2 of the present disclosure;

[0032] Figure 6A and Figure 6B is a linear potential scan graph of a gel polymer electrolyte manufactured according to Embodiment 1-1 of the present disclosure and an ionic liquid electrolyte included in Comparative Example 1, in which Figure 6A is a graph showing a curve of values from 3.0 to 6.0 on the x-axis, while Figure 6B is a graph showing a curve of values from 0.0 to 1.5 on the x-axis;

[0033] Figure 7A is an AC impedance spectrum graph of a lithium air battery manufactured according to Comparative Example 1 of the present disclosure;

[0034] Figure 7B is an AC impedance spectrum graph of a lithium air battery manufactured according to Comparative Example 2-2 of the present disclosure;

[0035] Figure 7C is an AC impedance spectrum graph of a lithium air battery manufactured according to Embodiment 1-2 of the present disclosure;

[0036] Figure 8 is a graph showing a DC polarization curve of lithium air batteries manufactured according to Comparative Example 1, Comparative Example 2-2, and Embodiment 1-2 of the present disclosure;

[0037] Figure 9 is an FT-IR spectrum graph (1680 to 1780 cm -1 ) of a gel polymer electrolyte according to Comparative Example 2-1 and Embodiment 1-1 of the present disclosure;

[0038] Figure 10A is an FT-IR spectrum graph (565 to 585 cm -1);

[0039] Figure 10B is a graph of FT-IR spectra of gel polymer electrolytes according to Comparative Example 2-2 and Example 1-2 (735 to 755 cm -1 );

[0040] Figure 11 is a graph showing charge-discharge curves of each cycle of a lithium air battery manufactured according to Comparative Example 1 of the present disclosure;

[0041] Figure 12 is a graph showing charge-discharge curves of each cycle of a lithium air battery manufactured according to Example 1-2 of the present disclosure;

[0042] Figure 13 is a graph showing charge-discharge curves of each cycle of a lithium air battery manufactured according to Comparative Example 2-2 of the present disclosure;

[0043] Figure 14A is a graph showing charge-discharge curves of each cycle of a lithium air battery manufactured according to Comparative Example 3-2 of the present disclosure; and

[0044] Figure 14B is a graph showing charge-discharge curves of each cycle of a lithium air battery manufactured according to Comparative Example 4-2 of the present disclosure. DETAILED DESCRIPTION

[0045] The above and other objects, features and advantages of the present disclosure will be more clearly understood from the following preferred embodiments taken in conjunction with the accompanying drawings. However, the present disclosure is not limited to the embodiments disclosed herein and can be modified in various forms. The embodiments are intended to explain the present disclosure thoroughly and to convey the spirit of the present disclosure to those skilled in the art.

[0046] It will be understood that the terms "comprise", "include", "have" and the like used in this specification specify the presence of stated features, integers, steps, operations, elements, components and / or combinations thereof but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0047] Unless otherwise specifically detailed, all numbers, values and / or expressions representing quantities of ingredients, reaction conditions, polymer compositions, and mixtures used herein are to be, and are hereby, interpreted as approximations based on the concept is that the term "about" modifying the base number characterizes the approximate precision of the value, the approximate precision of the value that would be achieved if the various underlying factors were taken into account in arriving at the value, and other factors. Furthermore, when disclosing numerical ranges, the range is continuous, and unless otherwise specified, includes all values from the lowest value to the highest value of the range. Further, when such a range includes both integers and fractional values, unless otherwise specified, all integers within the range are included.

[0048] Gel polymer electrolyte for lithium-air battery

[0049] According to embodiments of the present disclosure, a gel polymer electrolyte for a lithium-air battery can include a composite polymer resin containing a linear polymer and a cross-linking agent, an ionic liquid, a lithium salt, a zwitterionic salt, an initiator, and an organic solvent.

[0050] The gel polymer electrolyte for a lithium-air battery according to the present disclosure can include 4 to 6 wt% of a composite polymer resin, 7 to 10 wt% of an ionic liquid, 2 to 5 wt% of a lithium salt, 0.15 to 0.40 wt% of a zwitterionic salt, 0.02 to 0.04 wt% of an initiator, and 80 to 85 wt% of an organic solvent.

[0051] In preferred embodiments of the present disclosure, the gel polymer electrolyte for a lithium-air battery is in a gel form having a three-dimensional network structure including a zwitterionic salt, has an ionic conductivity of 3.5 x 10 -4 to 5.9 x 10 -4 The three-dimensional network structure including the zwitterionic salt can suppress the evaporation of a liquid electrolyte from the lithium-air battery, and can suppress dendrite growth at a lithium anode and side reactions with the electrolyte.

[0052] The content of each component of the gel polymer electrolyte for a lithium-air battery according to the present disclosure will be described below and is expressed based on 100 wt% of the gel polymer electrolyte. If the basis of the gel polymer electrolyte is changed, a new basis will generally be proposed, so that a person skilled in the art will clearly understand the described basis of the content.

[0053] (1) Composite polymer resin

[0054] In embodiments of the present disclosure, the composite polymer resin is not particularly limited as long as it contains a polymer capable of undergoing a cross-linking reaction with a cross-linking agent contained therein and thus can serve as a base resin of the gel polymer electrolyte for a lithium-air battery.

[0055] The polymer according to the present disclosure can include a conventional polymer that can be used in the present disclosure, for example, a linear polymer, a branched polymer, etc. Although not limited to a specific polymer, a linear polymer is preferred, which is stable to a discharge product such as Li2O2, has high wettability and compatibility with an electrolyte, and is capable of effectively forming a three-dimensional network structure.

[0056] The linear polymer according to the present disclosure can be a conventional linear polymer that can be used in the present disclosure, for example, at least one copolymer or mixture selected from the group consisting of polymethyl methacrylate, polystyrene, polyethylene oxide, polytetrafluoroethylene, perfluorosulfonic acid, and polyurethane. Although not limited to a specific linear polymer, methyl methacrylate is preferred, which is inexpensive and stable to a superoxide anion radical generated during discharge, has high compatibility with an ionic liquid due to a polymer polar group, and dissociates a lithium salt, thus being capable of contributing to an increase in ionic conductivity.

[0057] The linear polymer according to the present disclosure can have a weight average molecular weight (Mw) of from 20,000 to 1,000,000 g / mol. If the weight average molecular weight (Mw) of the linear polymer is less than 20,000 g / mol, desired physical properties cannot be obtained. On the other hand, if the weight average molecular weight (Mw) of the linear polymer exceeds 1,000,000 g / mol, high ionic conductivity cannot be obtained. Preferably, the weight average molecular weight (Mw) of the linear polymer is in the range of 100,000 to 500,000 g / mol, and more preferably, the weight average molecular weight (Mw) of the linear polymer is in the range of 340,000 to 360,000 g / mol.

[0058] The amount of the linear polymer according to the present disclosure is 85 to 95 wt% based on the total amount of the composite polymer resin being 100 wt%. If the amount of the linear polymer is less than 85 wt%, desired physical properties cannot be obtained, and ionic conductivity can decrease due to an increase in crosslinking density. On the other hand, if the amount of the linear polymer exceeds 95 wt%, high ionic conductivity cannot be obtained, and desired mechanical physical properties cannot be obtained due to low crosslinking density.

[0059] The cross-linking agent according to the present disclosure can include a conventional cross-linking agent that can be used in the present disclosure, for example, at least one selected from the group consisting of divinylbenzene (DVB) having a polymerizable vinyl group, tri(ethylene glycol)diacrylate (TEGDA), and poly(ethylene glycol)diacrylate (PEGDA). Although not limited to a specific cross-linking agent, divinylbenzene (DVB) is preferred, as polystyrene having a cross-linked structure obtained by a cross-linking reaction has excellent mechanical physical properties in divinylbenzene (DVB), and divinylbenzene (DVB) has high chemical resistance and electrochemical stability.

[0060] The amount of the cross-linking agent according to the present disclosure can be 5 to 15 wt% based on the total amount of 100 wt% of the composite polymer resin. If the amount of the cross-linking agent is less than 5 wt%, the cross-linking reaction can not occur sufficiently, so that it is difficult to form a three-dimensional network structure, and thus it can be impossible to obtain the desired mechanical physical properties. On the other hand, if the amount of the cross-linking agent exceeds 15 wt%, the number of ion migrations is reduced due to an increase in cross-linking density, undesirably reducing the ionic conductivity.

[0061] The amount of the composite polymer resin including the linear polymer and the cross-linking agent can be 4 to 6 wt% based on the total amount of 100 wt% of the gel polymer electrolyte. If the amount of the composite polymer resin is less than 4 wt%, the volume of the composite polymer resin in the polymer solution is small, and thus it is not possible to sufficiently fill the pores in the separation membrane after the evaporation of the organic solvent. On the other hand, if the amount of the composite polymer resin exceeds 6 wt%, a thick coating layer of the polymer electrolyte is generated, undesirably increasing the cell resistance.

[0062] (2) Zwitterionic salt

[0063] The zwitterionic salt according to the embodiments of the present disclosure is not particularly limited, as long as the zwitterionic salt includes both an anionic functional group and a cationic functional group.

[0064] The zwitterionic salt according to the present disclosure can include a conventional zwitterionic salt that can be used in the present disclosure, for example, a zwitterionic salt capable of promoting ion dissociation in a gel polymer electrolyte or a polymer functional group for a lithium-air battery and including at least one cation selected from the group consisting of imidazolium, pyrrolidinium, piperidinium, and ammonium, and at least one anion selected from the group consisting of sulfonate, carboxylate, and phosphite. Although not limited to a zwitterionic salt having a specific combination, N-methyl-N-(propanesulfonate) pyrrolidinium (MPS P) is preferred, which promotes ion dissociation to increase ionic conductivity and is capable of improving interfacial properties between a lithium anode and an electrolyte.

[0065] The amount of the zwitterionic salt can be 0.15 to 0.40 wt% based on the total amount of the gel polymer electrolyte being 100 wt%. If the amount of the zwitterionic salt is less than 0.15 wt%, the interfacial properties between the lithium anode and the electrolyte cannot be improved. On the other hand, if the amount of the zwitterionic salt exceeds 0.40 wt%, an increase in viscosity of the electrolyte due to an increase in the number of molecules decreases ionic conductivity, which can result in a decrease in performance due to an increase in overvoltage.

[0066] The gel polymer electrolyte for a lithium-air battery according to the present disclosure can include 0.15 to 0.40 wt% of a zwitterionic salt, thereby not only suppressing volatilization of the electrolyte but also imparting stability to the lithium-air battery interface by suppressing the formation of dendrites at the lithium anode and suppressing side reactions between the lithium anode and the liquid electrolyte. In addition, the zwitterionic salt can be included to thereby increase the number of lithium ion transfers, thereby increasing the life of the battery.

[0067] (3) Ionic liquid and lithium salt

[0068] The ionic liquid according to the embodiment of the present disclosure is not particularly limited as long as it includes an ionic salt formed by ionic bonding of an organic cation with an organic or inorganic anion.

[0069] The lithium salt according to the present disclosure is not particularly limited as long as it is mixed with the ionic liquid and thus can act as a plasticizer to impart flexibility to the polymer.

[0070] The ionic liquid according to the present disclosure can include a conventional ionic liquid that can be used in the present disclosure, for example, an ionic salt consisting of at least one cation selected from the group consisting of imidazolium, pyrrolidinium, piperidinium, ammonium, phosphonium, sulfur, pyridinium, pyrazolium, triazolium, thiazolium, oxazolium, pyridazinium, pyrimidinium, and pyrazinium, and at least one anion selected from the group consisting of acetate, bis(fluorosulfonyl)imide, bis(fluoromethanesulfonyl)imide, bromide, chloride, iodide, dicyanamide, hexafluorophosphate, tetrafluoroborate, and hydrosulfate. Although not limited to a specific ionic salt, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIm-TFSI) is preferred, which imparts flexibility to the polymer through interaction with the lithium salt and helps to increase ionic conductivity by trapping the electrolyte in a crosslinked network structure through a crosslinking reaction.

[0071] The lithium salt according to the present disclosure can include a conventional lithium salt that can be used in the present disclosure, for example, at least one selected from the group consisting of LiTFSI, LiFSI, LiBF4, LiPF6, LiClO4, LiAsF6, LiDFOB, and LiBOB. Although not limited to a specific lithium salt, LiTFSI is preferred, which imparts flexibility to the polymer through interaction with the ionic liquid and helps to increase ionic conductivity by trapping the electrolyte in a crosslinked network structure through a crosslinking reaction.

[0072] The amount of the ionic liquid can be 7 to 10 wt% based on the total amount of 100 wt% of the gel polymer electrolyte. If the amount of the ionic liquid is less than 7 wt%, the ionic conductivity of the gel polymer electrolyte can decrease. On the other hand, if the amount of the ionic liquid exceeds 10 wt%, the mechanical physical properties of the polymer electrolyte can decrease due to plasticization.

[0073] In addition, the amount of the lithium salt can be 2 to 5 wt% based on the total amount of 100 wt% of the gel polymer electrolyte. If the amount of the lithium salt is less than 2 wt%, the battery reaction cannot be effectively performed due to insufficient concentration of lithium ions in the electrolyte. On the other hand, if the amount of the lithium salt exceeds 5 wt%, the ionic conductivity can decrease due to increased viscosity.

[0074] (4) Initiator

[0075] The initiator according to the embodiment of the present disclosure is not particularly limited as long as it can initiate a polymerization chain reaction.

[0076] The initiator according to the present disclosure can include a conventional initiator that can be used in the present disclosure, for example, at least one selected from the group consisting of 2,2'azobis(2-methylpropionitrile) (AIBN), benzoyl peroxide, and lauroyl peroxide. Although not limited to a specific initiator, AIBN is preferred, which can effectively initiate a polymerization chain reaction.

[0077] The amount of the initiator can be 0.02 to 0.04 wt% based on the total amount of 100 wt% of the gel polymer electrolyte. If the amount of the initiator is less than 0.02 wt%, the reaction rate is low and unreacted material can remain. On the other hand, if the amount of the initiator exceeds 0.04 wt%, the remaining initiator can participate in a side reaction.

[0078] (5) Organic solvent

[0079] The organic solvent according to the embodiment of the present disclosure is not particularly limited as long as it creates an environment in which a polymerization reaction can occur and does not react with other ingredients.

[0080] The organic solvent according to the present disclosure can include a conventional organic solvent that can be used in the present disclosure, for example, at least one selected from the group consisting of tetrahydrofuran (THF), acetonitrile (ACN), and dimethyl carbonate (DMC). Although not limited to a specific organic solvent, THF is preferred, which well dissolves the components, has low reactivity with lithium metal, and has high volatility and low boiling point, and thus can be easily dried.

[0081] The amount of the organic solvent can be 80 to 85 wt% based on the total amount of 100 wt% of the gel polymer electrolyte. If the amount of the organic solvent is less than 80 wt%, a thick separation film is formed due to the high viscosity of the polymer solution, resulting in low ionic conductivity. On the other hand, if the amount of the organic solvent exceeds 85 wt%, the relative amount of the composite polymer resin is low, making it difficult to uniformly fill the pores in the separation film after the organic solvent is dried.

[0082] Lithium-air battery

[0083] Figure 1 is a schematic cross-sectional view illustrating a lithium-air battery 1 according to the present disclosure. Referring to Figure 1The lithium air battery 1 includes an anode 10, a cathode 20, a gel polymer electrolyte 30 for a lithium air battery disposed between the anode and the cathode, and a separation film 40 on the gel polymer electrolyte.

[0084] The anode 10 and the cathode 20 according to the present disclosure can be conventional known anodes and cathodes that can be used in the present disclosure. Although not limited to a specific anode and cathode, the anode preferably includes lithium, and the cathode can use oxygen as a cathode active material.

[0085] The gel polymer electrolyte 30 according to the present disclosure can be disposed between the anode and the cathode, and preferably formed on the anode. The gel polymer electrolyte for a lithium air battery according to the present disclosure can be the same as or different from the above description. Specifically, due to a three-dimensional network structure including a zwitterionic salt, the gel polymer electrolyte according to the present disclosure can be firmly attached to the surface of the anode, and in addition, a liquid electrolyte is well fixed in a polymer matrix so that organic solvent evaporation can be prevented and formation of lithium dendrites during charging and discharging can be effectively suppressed. Furthermore, a side reaction between the anode and the electrolyte can be suppressed.

[0086] The separation film 40 according to the present disclosure can be formed on the gel polymer electrolyte and the cathode. The separation film can be a conventional known separation film that can be used in the present disclosure, for example, at least one selected from the group consisting of an olefin resin (polyethylene, polypropylene, etc.), glass fiber, a fluororesin (polyvinylidene fluoride, polytetrafluoroethylene, etc.), an ester resin (polyethylene terephthalate, etc.), and a cellulose nonwoven fabric. Although not limited to a specific ingredient, a polyethylene separation film is preferred, which exhibits excellent chemical resistance and mechanical physical properties and has a low electrical resistance due to its thinness.

[0087] The lithium air battery is configured such that an organic-inorganic hybrid gel polymer electrolyte layer is formed on the anode, and a separation film is stacked on the organic-inorganic hybrid gel polymer electrolyte layer to form a double layer structure, so that the chemically crosslinked gel polymer electrolyte can prevent evaporation of the liquid electrolyte and can suppress growth of lithium dendrites and a side reaction. Therefore, by the above structural advantages, the life of the lithium air battery can be greatly increased.

[0088] A better understanding of the present disclosure will be given by the following examples, which are intended to be illustrative only and are not to be construed as limiting the scope of the present disclosure.

[0089] Example 1-1: Manufacturing gel polymer electrolyte for lithium-air battery

[0090] A gel polymer electrolyte was manufactured under the following conditions. Specifically, to manufacture a composite polymer resin, 0.18 g of 0.2 M PMMA polymer (average molecular weight: 350000) was prepared as a linear polymer and 0.02 g of DVB was prepared as a crosslinking agent. In addition, 0.01 g of N-methyl-N-(propane sulfonate) pyrrolidinium (MPSP) was prepared as a zwitter ionic salt including an anionic functional group and a cationic functional group, and the above ingredients were dispersed in 3 g of a THF solvent as an organic solvent. Then, 0.133 g of LiTFSI as a lithium salt, 0.33 g of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EMIm-TFSI) as an ionic liquid, and 0.001 g of AIBN as an initiator were mixed. Then, a PE separation film was placed on a release paper, 100 μl of the mixed solution was dropped on the PE separation film, and due to the surface tension between the solution and the release paper, 100 μl of the mixed solution was uniformly distributed only on the separation film, and then heat crosslinking was performed at 70°C for 12 hours in a vacuum oven. To minimize the influence of water and active gases in the atmosphere, all manufacturing procedures were performed in a glove box filled with inert argon gas.

[0091] Example 1-2: Manufacturing lithium-air battery including gel polymer electrolyte

[0092] A lithium air battery was manufactured using the gel polymer electrolyte manufactured in Example 1-1, using lithium metal as an anode and an air electrode using air, i.e., oxygen, as a cathode active material as a cathode, and using a polyethylene separation film as a separation film. Here, to wet the cathode, 0.5 M LiTFSI was dissolved in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EMIm-TFSI) as an ionic liquid, and the amount of 0.5 M LiTFSI was 30 μl. An air electrode as a cathode was manufactured by uniformly coating conductive carbon and polytetrafluoroethylene (PTFE) as a binder on a gas diffusion layer (GDL).

[0093] Comparative Example 1: Manufacturing lithium-air battery including ionic liquid electrolyte

[0094] A lithium-air battery was manufactured in the same manner as in Example 1-2, except that a polyethylene separator film (thickness: 9 μm) was impregnated with 40 μl of an ionic liquid electrolyte manufactured by dissolving 0.5 M LiTFSI in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EMIm-TFSI) as an ionic liquid instead of the gel polymer electrolyte manufactured according to Example 1-1.

[0095] Comparative Example 2-1: Manufacturing gel polymer electrolyte for lithium-air battery not including zwitterionic salt

[0096] A gel polymer electrolyte for a lithium-air battery was manufactured in the same manner as in Example 1-1, except that a zwitterionic salt was not included.

[0097] Comparative Example 2-2: Manufacturing lithium-air battery including gel polymer electrolyte not containing zwitterionic salt

[0098] A lithium-air battery was manufactured in the same manner as in Example 1-2, except that the gel polymer electrolyte manufactured according to Comparative Example 2-1 was used.

[0099] Comparative Example 3-1: Manufacturing gel polymer electrolyte for lithium-air battery using different amount of zwitterionic salt

[0100] A gel polymer electrolyte for a lithium-air battery was manufactured in the same manner as in Example 1-1, except that 0.005 g of a 0.1 M zwitterionic salt was used.

[0101] Comparative Example 3-2: Manufacturing lithium-air battery including gel polymer electrolyte using different amount of zwitterionic salt

[0102] A lithium-air battery was manufactured in the same manner as in Example 1-2, except that the gel polymer electrolyte manufactured according to Comparative Example 3-1 was used.

[0103] Comparative Example 4-1: Manufacturing gel polymer electrolyte for lithium-air battery using different amount of zwitterionic salt

[0104] A gel polymer electrolyte for a lithium-air battery was manufactured in the same manner as in Example 1-1, except that 0.015 g of a 0.3 M zwitterionic salt was used.

[0105] Comparative Example 4-2: Manufacturing lithium-air battery including gel polymer electrolyte using different amount of zwitterionic salt

[0106] A lithium-air battery was manufactured in the same manner as in Example 1-2, except that the gel polymer electrolyte manufactured according to Comparative Example 4-1 was used.

[0107] Test Example 1: Evaluating distribution of gel polymer electrolyte for lithium-air battery and whether reactive residue remains Vinyl

[0108] The gel polymer electrolyte of Example 1-1 was imaged and evaluated to determine whether a vinyl group remained.

[0109] Reference Figures 2A to 2C, the chemically cross-linked gel polymer electrolyte is uniformly distributed on the separation membrane. Referring to Figure 3 , based on the FT-IR spectrum results, no double bond peak of the cross-linking agent was observed near 1400 and 1620 cm -1 after cross-linking, indicating that the cross-linking reaction was carried out without unreacted materials.

[0110] Test Example 2: Evaluating ion conductivity and lithium ion migration of lithium-air battery including gel polymer electrolyte Figure 4

[0111] The ion conductivity (S / m) and lithium ion transference number (t Li+ ) of Example 1-2, Comparative Example 1, Comparative Example 2-2, Comparative Example 3-2, and Comparative Example 4-2 are shown in Table 1 and Ion conductivity (S / cm) below.

[0112] Table 1

[0113] Comparative Example 1 Lithium ion transference number (t Li+ ) Comparative Example 2-2 3.2 x 10 -3 ]] 0.21 Comparative Example 3-2 3.1 x 10 -4 ]] 0.30 Example 1-2 5.9 x 10 -4 ]] 0.31 Comparative Example 4-2 4.9 x 10 -4 ]] 0.34 Figure 5A 3.5 x 10 -4 ]] 0.35

[0114] * Ion conductivity: The polymer electrolyte manufactured according to the above examples was placed between two SUS blocking electrodes to manufacture a coin cell, and the ion conductivity of the polymer electrolyte was calculated using , where R is the resistance value measured using an AC impedance method, and l and A are the thickness and area of the polymer electrolyte, respectively.

[0115] * Lithium ion transference number: The lithium ion transference number was calculated using Li+ = I S (ΔV - I0R0) / I0(ΔV - I S R S ), where I0and R0are the initial DC current and interfacial resistance value in Figure 4 , and I S and R S are the DC current and interfacial resistance values when the steady state is reached, respectively.

[0116] Referring to Table 1 and Figure 5A , Figure 5B , and Figure 9 , the lithium-air battery including the gel polymer electrolyte containing the zwitterionic salt (Example 1-2) exhibited further improved ion conductivity and lithium ion transference number. Referring to Figure 10A , Figure 10B , and Test Example 3: Evaluating electrochemical stability of gel polymer electrolyte and ionic liquid electrolyte , it can be confirmed that the ion conductivity improvement effect is as follows: in the FT-IR spectrum, when the zwitterionic salt is added, the amount of lithium ions dissociated from the polymer C=O functional group increases, and the dissociation of the lithium salt LiTFSI is promoted, thereby increasing the free Li +ion and TFSI - the amount of ion.

[0117] Figure 6A

[0118] To compare the electrochemical stability of the gel polymer electrolyte manufactured according to Example 1-1 with the existing ionic liquid electrolyte included in Comparative Example 2, the current value was measured according to the applied voltage by Linear Sweep Voltammetry (LSV) (the scan rate was 1 mV / s). Figure 6B and Figure 6A The results of the measurement are shown in Table 1.

[0119] Figure 6B and Figure 6A are linear potential sweep graphs of the gel polymer electrolyte manufactured according to Example 1-1 and the ionic liquid electrolyte included in Comparative Example 2. In Table 1, Figure 6B and Test Example 4: Evaluating interface of lithium metal and electrolyte In Table 1, the liquid electrolyte included in Comparative Example 2 exhibited a reduction stability of 0.5 V and an oxidation stability of 5 V, while the gel polymer electrolyte manufactured according to Example 1-1 exhibited a reduction stability of 0 V and an oxidation stability of 5.2 V or more. This is because the ionic liquid electrolyte is trapped in the polymer matrix, and thus the number of molecules capable of reacting with lithium metal is reduced. As a result, it can be confirmed that the organic-inorganic hybrid gel polymer electrolyte can be used instead of the existing liquid electrolyte and is easy to apply to a lithium air battery requiring a predetermined level of reduction stability and oxidation stability.

[0120] Figure 7A

[0121] To measure the change in the interfacial resistance of the electrode and the electrolyte over time, the AC impedance spectrum of the lithium air battery of each of Comparative Example 2, Comparative Example 3-2, and Example 1-2 was obtained. Figure 7B and Figures 7A to 7C The results are shown in Table 2.

[0122] Referring to Table 2, Test Example 5: DC polarization of lithium symmetric cell The initial interfacial resistance of the battery manufactured using the gel polymer electrolyte including the zwitterionic salt was large compared to the battery using the ionic liquid electrolyte, but the degree of increase in the interfacial resistance over time was small. This result is because the side reaction of the lithium electrode with the ionic liquid and the lithium salt was inhibited by the gel polymer electrolyte in contact with the lithium electrode. It can be confirmed that the gel polymer electrolyte effectively improves the interfacial stability between the lithium electrode and the electrolyte.

[0123] Figure 8

[0124] The voltage change of the battery was measured while applying a predetermined amount of current to the lithium air battery of each of Comparative Example 1, Comparative Example 2-2, and Example 1-2 repeatedly, thereby evaluating the polarization of the battery, the results of which are shown in Test Example 6: Evaluating charge and discharge of lithium-air battery . The current density was 0.25 mA / cm 2 , and the total charge amount was 1.0 mAh / cm 2 . In the initial stage, the overvoltage of the battery including the gel polymer electrolyte not containing the zwitterionic salt sharply increased at the beginning of the cycle. This is because the lithium ion reached the limit of migration due to high electrode electrolyte interface resistance and low ionic conductivity. In contrast, the battery including the ionic liquid electrolyte initially exhibited the lowest overvoltage, but the side reaction with the lithium electrode continuously occurred due to poor reduction stability, so the overvoltage sharply increased as the battery voltage changed. At the same time, in the battery including the gel polymer electrolyte containing the zwitterionic salt, the initial overvoltage was large compared to the battery including the ionic liquid electrolyte, but the overvoltage did not significantly increase for a long time, and stable charge-discharge behavior was exhibited. Therefore, it can be confirmed that stable interface resistance is maintained at the lithium electrode, the redox reaction of lithium occurs, and the growth of lithium dendrites is suppressed.

[0125] Figure 11

[0126] To evaluate the charge-discharge characteristics of the lithium air battery of Example 1-2, as well as each of Comparative Example 1, Comparative Example 2-2, Comparative Example 3-2, and Comparative Example 4-2, a constant current of 0.25 mA / cm 2 was applied in an oxygen environment, and the charge-discharge voltage of each cycle was measured, the results of which are shown in Figure 12 , Figure 13 , Figure 14A , Figure 14B , and Figure 11 .

[0127] Referring to Figure 12 , in the charge-discharge curve of the lithium air battery of Comparative Example 1, the overvoltage gradually increased as the cycle progressed, so the battery capacity was not effectively exhibited.

[0128] Referring to Figure 13 , in the charge-discharge curve of the lithium air battery including the gel polymer electrolyte containing the zwitterionic salt (Example 1-2), the overvoltage was low compared to the lithium air battery using the ionic liquid electrolyte, and the life was improved by about 15 cycles based on a cut-off capacity of 2.5 mAh / cm 2 .

[0129] Referring to Figure 14AIn the charge-discharge curve of the lithium-air battery including the gel polymer electrolyte not containing the zwitterionic salt (Comparative Example 2-2), the overvoltage gradually increased from the 5th cycle.

[0130] Referring to Figure 14B and ​ Compared to the lithium-air battery including the gel polymer electrolyte containing the zwitterionic salt (Example 1-2), the lifespan of the lithium-air battery was reduced in the charge-discharge curve of the lithium-air battery including the gel polymer electrolyte using various amounts of the zwitterionic salt (Comparative Examples 3-2 and 4-2). Specifically, in the lithium-air battery of Comparative Example 3-2, a smaller amount of the zwitterionic salt than Example 1-2 was used, and the ionic conductivity was high, but the amount of the zwitterionic salt was small, and thus the interface characteristics were not effectively improved. In contrast, in the lithium-air battery of Comparative Example 4-2, a larger amount of the zwitterionic salt than Example 1-2 was used, and the electrolyte viscosity increased due to the increased number of molecules in the additive, and thus the ionic conductivity was reduced, and the battery lifespan was not increased. Accordingly, it can be confirmed that the lithium-air battery including the gel polymer electrolyte containing an appropriate amount of the zwitterionic salt (Example 1-2) exhibited the best performance.

[0131] Although the preferred embodiments of the present disclosure have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the present disclosure disclosed in the appended claims.

Claims

1. A gel polymer electrolyte for a lithium-air battery, comprising: a composite polymer resin including a linear polymer and a crosslinking agent; an ionic liquid; a lithium salt; a zwitterionic salt; an initiator; and an organic solvent, wherein the gel polymer electrolyte includes 0.15 to 0.40 wt% of the zwitterionic salt, wherein the zwitterionic salt includes at least one cation selected from the group consisting of imidazolium, pyrrolidinium, piperidinium, and ammonium, and at least one anion selected from the group consisting of sulfonate, carboxylate, and phosphite.

2. The gel polymer electrolyte of claim 1, comprising: 4 to 6 wt% of the composite polymer resin; 7 to 10 wt% of the ionic liquid; 2 to 5 wt% of the lithium salt; 0.02 to 0.04 wt% of the initiator; 80 to 85 wt% of the organic solvent.

3. The gel polymer electrolyte of claim 1, wherein the composite polymer resin includes 85 to 95 wt% of the linear polymer and 5 to 15 wt% of the crosslinking agent.

4. The gel polymer electrolyte of claim 1, wherein the linear polymer is at least one copolymer or mixture selected from the group consisting of polymethyl methacrylate, polystyrene, polyethylene oxide, polytetrafluoroethylene, perfluorosulfonic acid, and polyurethane.

5. The gel polymer electrolyte of claim 1, wherein the crosslinking agent includes at least one selected from the group consisting of divinylbenzene (DVB), triethylene glycol diacrylate (TEGDA), and polyethylene glycol diacrylate (PEGDA).

6. The gel polymer electrolyte of claim 1, wherein the ionic liquid includes a salt including: at least one cation selected from the group consisting of imidazolium, pyrrolidinium, piperidinium, ammonium, phosphorus, sulfur, pyridinium, pyrazolium, triazolium, thiazolium, oxazolium, pyridazinium, pyrimidinium, and pyrazinium; and at least one anion selected from the group consisting of acetate, bis(fluorosulfonyl)imide, bis(fluoromethanesulfonyl)imide, bromide, chloride, iodide, dicyanamide, hexafluorophosphate, tetrafluoroborate, and hydrosulfate.

7. The gel polymer electrolyte of claim 1, wherein the lithium salt includes at least one selected from the group consisting of LiTFSI, LiFSI, LiBF4, LiPF6, LiClO4, LiAsF6, LiDFOB, and LiBOB.

8. The gel polymer electrolyte of claim 1, wherein the initiator includes at least one selected from the group consisting of 2,2'azobis(2-methylpropionitrile) (AIBN), benzoyl peroxide, and lauryl peroxide.

9. The gel polymer electrolyte of claim 1, wherein the organic solvent includes at least one selected from the group consisting of tetrahydrofuran (THF), acetonitrile (ACN), and dimethyl carbonate (DMC).

10. A lithium-air battery, comprising: an anode including lithium; a cathode using oxygen as a cathode active material; ​ The gel polymer electrolyte of claim 1, disposed between the anode and the cathode; and a separation membrane, located on the gel polymer electrolyte.

11. The lithium-air battery of claim 10, wherein the separation membrane comprises at least one selected from the group consisting of an olefin resin, a glass fiber, a fluorine resin, an ester resin, and a cellulose nonwoven fabric.

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