Catalyst for a lithium-air battery, lithium-air battery comprising the same, and vehicle comprising the same

Abstract

Catalyst for a lithium-air battery, containing: a redox mediator dissolved in an electrolyte, wherein an ionization energy of the redox mediator is about 5.5 to 7.5 eV in vacuum, and where the redox mediator is 5,10-dimethylphenazine (DMPZ), 1,5-naphthalenediamine (NDA), 4,N,N-trimethylaniline (TMA) or 1-phenylpyrrolidine (PPD).

Description

Technical field

[0001] The present invention relates to a catalyst, and more specifically, to a soluble catalyst for a lithium-air battery. The soluble catalyst may contain a redox mediator (RM) that has an ionization energy of about 5.5 to 7.5 eV in vacuum or an oxidation potential of about 3.0 to 4.0 V and is readily soluble in an electrolyte without reacting with the electrolyte. In addition, the redox mediator (RM) in its native state occupies a HOMO level (the energy level of the highest occupied orbital of the molecule) that is below the formation energy of lithium peroxide (Li 2 O 2 ), but as close as possible to the formation energy, and a HOMO level in an oxidized state (RM + ), which lies above the HOMO level of the electrolyte. background

[0002] Problems such as fossil fuel depletion, environmental pollution, and global warming are increasing at an ever-increasing rate. While a new, renewable energy source has been developed to address these issues, it has so far failed to deliver significant results. Accordingly, there is growing interest in energy storage technology, particularly in the form of batteries.

[0003] One such development has been achieved with a lithium-ion battery. However, due to their low energy density, conventional lithium-ion batteries are inadequate replacements for fossil fuels. In addition, metal-air batteries, such as lithium-air batteries, have recently been actively developed.

[0004] The lithium-air battery uses oxygen, drawn from the air, as the active material without any restrictions. Theoretically, a lithium-air battery can achieve a sufficiently high energy density, for example, of approximately 3,200 Wh / kg, which is about ten times that of a conventional lithium-ion battery. Because oxygen is used as the active material, the lithium-air battery is also environmentally friendly.

[0005] However, a conventional lithium-air battery has the disadvantage of a short lifespan and overvoltage due to strong polarization. When the battery is discharged, lithium peroxide (Li 2 O 2), whose ionic conductivity is low. When lithium peroxide covers the air electrode (cathode) of a lithium-air battery, strong polarization occurs, and energy efficiency therefore decreases significantly. An important aspect of research into lithium-air batteries may therefore be the development of a suitable catalyst that decomposes lithium peroxide.

[0006] One of the early prior art lithium-air batteries used a solid catalyst that generated oxygen. However, because the catalyst was in a solid state, it was difficult to bring it into contact with lithium peroxide, which was also in a solid state. Since the catalyst had little activity, the above-described problem could not be solved. Recently, active research has been conducted on a soluble catalyst. Because a soluble catalyst can move freely in the electrolyte, it can more easily come into contact with the lithium peroxide.

[0007] The soluble catalyst contains a redox mediator that decomposes lithium peroxide while itself being oxidized and reduced. For example, tetrathiafulvalene (TTF) has been proposed in the prior art as a redox mediator for the soluble catalyst of a lithium-air battery. The present inventors also cited lithium iodide (LI) as another alternative redox mediator. Accordingly, the present inventors focused their research on developing a novel and improved soluble catalyst that could solve the problems of a lithium-air battery, and thus arrived at the present invention.

[0008] The information disclosed in the "Background" section above is intended only to clarify the background of the invention and may therefore contain information that does not constitute prior art as is already known to a person of ordinary skill in the art in this country.

[0009] Chen et al. (Chen, Yuhui et al., Charging a Li-O 2 Battery using a redox mediator, Nature Chemistry, Vol. 5, 2013, No. 6, pp. 489-494, ISSN 1755-4330) also disclose tetrathiafulvalene (TTF) as a redox mediator with an ionization energy of 6.34 eV in vacuum. It is shown that the use of a redox mediator enables recharging at rates impossible for a cell without the mediator. Summary of Revelation

[0010] The present invention provides a soluble catalyst that can effectively decompose lithium peroxide generated during the discharge of a lithium-air battery. In addition, the present invention provides a compound that can be used as a soluble catalyst for a lithium-air battery.

[0011] The object of the present invention is not limited to the aspects mentioned above. The object of the present invention will become clear from the following description and can be implemented by means of the description and a combination thereof described in the claims.

[0012] In order to achieve the above-mentioned objects, the present invention may be constructed as follows.

[0013] In one aspect, the present invention provides a catalyst for a lithium-air battery. The catalyst of the present invention may, in particular, be soluble in an electrolyte used in the lithium-air battery. The term "soluble catalyst" as used herein refers to a catalyst or catalyst composition that is soluble in various liquid or semi-solid electrolytes (e.g., a gel electrolyte in a solid state), a solution of an electrolyte, or a solution containing an electrolyte. The electrolyte may, in addition, contain various solvents used in the art for manufacturing a battery (e.g., a lithium-air battery), such as an aprotic solvent (a non-aqueous or organic solvent that is either polar or non-polar), a protic or aqueous solvent, or mixtures thereof.In the following, the catalyst may refer to a soluble catalyst unless otherwise stated.

[0014] The catalyst contains a redox mediator (RM) that has an ionization energy of approximately 5.5 to 7.5 eV in vacuum and is dissolved in an electrolyte. As used herein, the term "redox mediator" refers to a molecule or compound that can accept or donate electrons and thereby promote the transfer of electrons in a matrix or system, e.g., the catalyst in a lithium-air battery.

[0015] The catalyst for the lithium-air battery according to the present invention may contain a redox mediator whose oxidation potential is greater than an equilibrium potential of Li 2 O 2 is.

[0016] The oxidation potential of the redox mediator can be approximately 3.0 to 4.0 V.

[0017] The electrolyte can be triethylene glycol dimethyl ether (TEGDME) or dimethyl sulfoxide (DMSO).

[0018] The redox mediator can be an organic compound that does not react with the electrolyte.

[0019] The energy level of the highest occupied molecular orbital (HOMO) of the redox mediator molecule can preferably be below the formation energy of Li 2 O 2 lay.

[0020] The HOMO level of the redox mediator in the oxidized state may preferably be above the HOMO level of the electrolyte.

[0021] The redox mediator is selected from the group consisting of 5,10-dimethylphenazine (DMPZ), 1,5-naphthalenediamine (NDA), 4,N,N-trimethylaniline (TMA) and 1-phenylpyrrolidine (PPD).

[0022] In another aspect, the present invention provides a lithium-air battery containing the catalyst as described herein.

[0023] In addition, a vehicle is provided having the lithium-air battery containing the catalyst as described herein.

[0024] According to various exemplary embodiments, the present invention can provide a catalyst that can effectively decompose lithium peroxide generated during discharging of a lithium-air battery, thus reducing the extent of polarization. Furthermore, compounds that can act as a catalyst, ie, as a soluble catalyst, can be provided.

[0025] The effect of the present invention is not limited to the effects described above. It should be understood that the effect of the present invention includes all effects that can be derived from the following description.

[0026] Further aspects and preferred embodiments of the invention are explained below. Short description of the characters

[0027] The above and other features of the present invention will now be described with reference to exemplary embodiments shown in the accompanying figures, which are provided herein for illustrative purposes only and are therefore not intended to limit the present invention in any way. In the figures: The Fig.1A shows the measurement result of an FT-IR (Fourier transform infrared) spectrum used to investigate the stability of the exemplified compound NDA as a redox mediator when the redox mediator and an electrolyte according to an exemplified embodiment of the present invention were mixed together; the Fig. 1B shows the measurement result of an FT-IR (Fourier transform infrared) spectrum, with which the stability of the exemplified compound TMA as a redox mediator was investigated when the redox mediator and an electrolyte according to an exemplified embodiment of the present invention were mixed together; the Fig.1C shows the measurement result of an FT-IR (Fourier transform infrared) spectrum used to investigate the stability of the exemplified compound PPD as a redox mediator when the redox mediator and an electrolyte according to an exemplified embodiment of the present invention were mixed together; the Fig. 1D shows the measurement result of an FT-IR (Fourier transform infrared) spectrum, with which the stability of the exemplified compound NC as a redox mediator was investigated when the redox mediator and an electrolyte according to an exemplified embodiment of the present invention were mixed together; the Fig.1E shows the measurement result of an FT-IR (Fourier transform infrared) spectrum used to investigate the stability of the exemplified compound DMPZ as a redox mediator when the redox mediator and an electrolyte according to an exemplified embodiment of the present invention were mixed together; the Fig. 2 shows the results of the evaluation of the charging and discharging of an exemplary cell into which an exemplary soluble catalyst containing an exemplary redox mediator according to an exemplary embodiment of the present invention has been introduced; the Fig.3 shows the measurement results of the HOMO / LUMO levels of an exemplary redox mediator according to an exemplary embodiment of the present invention when triethylene glycol dimethyl ether (TEGDME) is used as the exemplary electrolyte; the Fig. 4 shows the measurement results of the HOMO / LUMO levels of an exemplary redox mediator according to an exemplary embodiment of the present invention when dimethyl sulfoxide (DMSO) is used as the exemplary electrolyte; the Fig. 5 shows the result of an X-ray diffraction (XRD) analysis of an exemplary cell into which an exemplary soluble catalyst containing 5,10-dimethylphenazine (DMPZ) according to an exemplary embodiment of the present invention was introduced, wherein the DMPZ is used as an example of a redox mediator; and the Fig. 6 shows the recording of an investigation with an exemplary cathode (air electrode) of the Fig. 5 shows an exemplary cell according to an exemplary embodiment of the present invention under a scanning electron microscope (SEM analysis).

[0028] It should be understood that the accompanying figures are not necessarily to scale, but present a somewhat simplified representation of various exemplary features illustrating the principles underlying the invention. The specific features for carrying out the present invention as disclosed herein, including, for example, particular dimensions, orientations, positions, and shapes, will be determined in part by the conditions and circumstances of the particular application and use contemplated. In the figures, reference numerals designate the same or equivalent parts of the present invention. Detailed description

[0029] Reference will now be made in detail to various exemplary embodiments of the present invention, which are shown by way of example in the accompanying figures and described below. While the invention will be described with reference to exemplary embodiments, it is to be understood that the present description is not intended to limit the invention to these exemplary embodiments. Rather, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents, and other embodiments which may be included within the true spirit and scope of the present invention as defined by the appended claims.

[0030] The description of generally known relationships or functions may be omitted if it can be assumed that this would tend to obscure the core message of the present invention.

[0031] The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It is further to be understood that the terms “comprises” and / or “comprising,” when used in this specification, indicate the presence of recited features, indicia, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, indicia, steps, operations, elements, components, and / or groups thereof.As used herein, the term “and / or” includes any and all combinations of one or more of the related listed items.

[0032] Unless expressly stated or clear from the context, the term "about" as used herein should be understood as being within a range of tolerances normal in science, for example, within 2 standard deviations of the mean. "About" can be understood as being within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless the context clearly indicates otherwise, all numerical values ​​stated herein are deemed to include the term "about."

[0033] It should be understood that the term "vehicle" or "vehicular" or other similar term, as used herein, includes motor vehicles in general, such as passenger cars, including sports utility vehicles (SUVs), buses, trucks, various commercial vehicles, watercraft, including a variety of boats and ships, aircraft, and the like, as well as hybrid vehicles, electric vehicles, plug-in hybrid-electric vehicles, hydrogen-powered vehicles, and other vehicles powered by alternative fuels (e.g., fuels derived from a source other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example, a vehicle that runs on both gasoline and electricity.

[0034] A lithium-air battery is a battery arrangement in which lithium is used as the anode and oxygen from the air serves as the active material for the cathode (air electrode). The oxidation and reduction of lithium occurs at the anode, and the externally supplied oxygen is reduced and oxidized at the cathode.

[0035] The following chemical formulas 1 and 2 show the reactions taking place at the anode and cathode when discharging the lithium-air battery. [Chemical Formula 1] (anode): Li → Li + + e - [Chemical Formula 2] (Cathode): 2Li + + O 2 + 2e - → Li 2 O 2

[0036] The lithium metal of the anode is oxidized, producing lithium ions and electrons. The lithium ions migrate through the electrolyte, and the electrons migrate through external wires or current collectors to the cathode. Because the cathode is porous, air can flow into the cathode from outside. The oxygen contained in the outside air is reduced by the electrons at the cathode, forming Li. 2 O 2 .

[0037] The charging process is the exact opposite of the reactions described above. In other words, as shown in the following chemical formula 3, the Li 2 O 2 decomposes at the cathode and lithium ions and electrons are formed. [Chemical Formula 3] (cathode) Li 2 O 2 →2Li + + O 2 + 2e -

[0038] If the Li 2 O 2is not decomposed properly, the energy efficiency may decrease due to the high polarization mentioned above.

[0039] In one aspect, the soluble catalyst for the lithium-air battery (hereinafter referred to as “soluble catalyst”) according to the present invention may comprise the Li 2 O 2 effectively decompose by promoting the reaction according to chemical formula 3. Accordingly, the energy efficiency of the battery can be increased and its service life can be extended.

[0040] The soluble catalyst can promote the reaction according to chemical formula 3 with the help of the redox mediator (RM).

[0041] The mechanism of the redox mediator is briefly shown below. [Chemical Formula 4] 1. 2RM → 2RM + + 2e - 2. Li 2 O 2 + 2RM + → 2 Li + + O 2 + 2RM

[0042] When the lithium-air battery is charged, the redox mediator can be oxidized first. Since the redox mediator (2RM + ) is dissolved in the electrolyte in an oxidized state, it can be more easily mixed with the Li 2 O 2 In the oxidized state, the redox mediator can then transfer electrons from the Li 2 O 2 As a result, Li 2 O 2 effective in 2Li + and O 2 decomposed and the redox mediator in the oxidized state can be reduced and return to its original state.

[0043] In order for the mechanism described above to proceed smoothly or alternatively the decomposition of Li 2 O 2 favored, the redox mediator must be the Li 2 O 2 chemically decompose. The oxidation potential (or oxidation voltage) of the redox mediator must therefore be greater than the equilibrium potential of Li2 O 2 Due to the high overpotential (e.g. more than about 1.5 V) during the electrochemical decomposition of Li 2 O 2 the redox mediator can then be oxidized earlier than the Li 2 O 2 .

[0044] However, if the difference between the oxidation potential and the equilibrium potential of Li 2 O 2 increases significantly, the charging and discharging efficiency (the discharge / charge voltage) may decrease because the voltage applied to the battery increases. The oxidation potential of the redox mediator must be greater than the equilibrium potential of Li 2 O 2 , whereby the oxidation potential of the redox mediator in a given range is preferably almost as large as the equilibrium potential of the Li 2 O 2 can be.

[0045] Since the equilibrium potential of Li 2 O 2is about 2.96 V, the oxidation potential of the redox mediator can in particular be in a range of about 3.0 V to 4.0 V.

[0046] The above-mentioned condition can be described using the ionization energy, since the mechanism according to chemical formula 4 can start when the redox mediator is oxidized, or - in other words - when the redox mediator releases electrons.

[0047] In the prior art, materials such as tetrathiafulvalene (TTF), ferrocene, and tetramethyl-p-phenylenediamine (TMPD) have been proposed as redox mediators. The ionization energies of these compounds in vacuum can be, for example, 6.8 eV, 6.7 eV, and 6.7 eV, respectively. The redox mediator can act as a soluble catalyst when the ionization energy is between 5.5 eV and 7.5 eV.

[0048] Accordingly, the present inventors have fabricated a lithium-air battery in which various organic compounds having an oxidation potential of about 3.0 V to 4.0 V or having an ionization energy of about 5.5 eV to 7.5 eV in vacuum were used as redox mediators.

[0049] As a result, the present invention can provide a soluble catalyst that can effectively decompose the lithium peroxide generated during the discharge of a lithium-air battery, thus reducing the extent of polarization. In addition, compounds that can function as a soluble catalyst can also be provided. Examples

[0050] The following examples illustrate the invention and are not intended to limit it.

[0051] The selected organic compounds are 5,10-dimethylphenazine (hereinafter referred to as DMPZ), 1,5-naphthalenediamine (hereinafter referred to as NDA), 4,N,N-trimethylaniline (hereinafter referred to as TMA), 1-phenylpyrrolidine (hereinafter referred to as PPD) and may include naphthacene (hereinafter referred to as NC) and the physical properties of these compounds are given in Table 1 below. Table 1 name Structural formula Ionization energy in vacuum [eV]* Oxidation potential in the electrolyte (TEGDME) [V] TTF 6,34 3,42 Lithium iodide - not known 3,6 NDA 6,51 3,64 TMA 6,79 3,81 PPD 6,84 3,89 NC 6,55 3,83 DMPZ 5,89 3,10 * Currently calculated value Production of the lithium-air battery cell

[0052] The cathode (air electrode) was made by applying a mixture of Ketjen carbon black and polytetrafluoroethylene (PTFE) to a nickel mesh (Ni mesh). Lithium metal was used as the anode.

[0053] The cell was fabricated by laminating the lithium metal (anode), separator, and cathode in sequence. 1 M LiTFSI-TEGDME (lithium bis(trifluoromethane)sulfonamide in tetraethylene glycol dimethyl ether) was used as the electrolyte. Assessment of the stability of the redox mediator

[0054] The soluble catalyst containing the redox mediator used herein can be dissolved in the electrolyte in which it is to be introduced into the cell. The redox mediator is therefore dissolved in the electrolyte. Furthermore, the redox mediator must not react with the electrolyte but must remain stable in it.

[0055] After NDA, TMA, PPD, NC, and DMPZ were mixed with the electrolyte at the specified concentration, the stability was investigated using FT-IR analysis (a Fourier transform infrared spectrum). The results are shown in the Fig. 1A-1E shown.

[0056] As in the Fig. 1A-1E, the peaks of NDA ( Fig. 1A), TMA ( Fig. 1B), PPD ( Fig. 1C), NC ( Fig. 1D) and DMPZ ( Fig. 1E) when the redox mediator and the electrolyte were mixed together, indicating that NDA, TMA, PPD, NC, and DMPZ were each dissolved in the electrolyte. Furthermore, the peaks did not change over a period of 10 days, indicating that NDA, TMA, PPD, NC, and DMPZ did not react with the electrolyte but remained stable. Assessment of charging and discharging of a lithium-air battery cell

[0057] The cell was discharged to a capacity limit of 1 mAh and then charged using a specified current of 0.2 mA / cm 2 was applied. The charging and discharging voltages ranged from 2.0 V to 4.5 V. A continuous supply of high-purity oxygen was established inside the cell.

[0058] The result of the charging and discharging assessment is shown in the Fig. 2 shown.

[0059] A cell in which no soluble catalyst containing a redox mediator was added to the electrolyte served as a reference. The graphs show the charging and discharging evaluation results of the cell in which a soluble catalyst containing NDA, TMA, PPD, NC, and DMPZ was added to the electrolyte.

[0060] As in the Fig. As shown in Figure 2, strong polarization occurred in the reference and a large overcharge of about 1.5 V was observed.

[0061] Secondly, it can be seen that in a cell containing the soluble catalyst, the overcharging described above could be significantly reduced. When the cell is charged, NDA, TMA, PPD, and DMPZ contained in the soluble catalyst can inhibit the decomposition of the Li 2 O2 In particular, the cell containing the DMPZ was most effective at reducing polarization.

[0062] However, when the cell contained NC, it discharged only inconsistently. Since the cell is discharged in an atmosphere of high-purity oxygen and the NC is light- and air-sensitive, a side reaction can occur. The NC may therefore be the least suitable for use as a redox mediator. Measurement of the HOMO / LUMO level of the redox mediator

[0063] As in the Fig. 2, DMPZ Li 2 O 2 As described above, the extent to which the decomposition reaction of Li 2 O 2 favored (i.e., the activity of the catalyst) may vary depending on the type of redox mediator used.

[0064] To illustrate the situation described above using the ionization energy, the HOMO level (the energy level of the highest occupied orbital of the molecule) and the LUMO level (the energy level of the lowest unoccupied orbital of the molecule) of NDA, TMA, PPD, NC and DMPZ were measured in the electrolyte and the results are shown in the Fig. 3 and Fig. 4 shown.

[0065] The Fig. 3 shows the result when triethylene glycol dimethyl ether (TEGDME) was used as electrolyte and the Fig. Figure 4 shows the result when dimethyl sulfoxide (DMSO) was used as electrolyte.

[0066] The energy levels of Li 2 O 2 in the Fig. 3 and Fig. 4 give the formation energy of Li 2 O 2 The formation energy of Li 2 O 2 denotes the standard formation energy of Li 2 O 2(at a standard condition with a temperature of about 25 °C and a pressure of about 1 atm) during the reaction at the cathode, as represented by the chemical formula 2 (2Li + + O 2 + 2e - → Li 2 O 2 ) is shown.

[0067] As in the Fig. 3 and Fig. As shown in Figure 4, the HOMO levels of NDA, TMA, PPD, NC and DMPZ in the pristine state (RM) are below the formation energy of Li 2 O 2 .

[0068] The HOMO level of the redox mediator in the pristine state (RM) can be considered as the primary ionization energy of the redox mediator in the presence of the electrolyte. Therefore, if the HOMO level is below the formation energy of Li 2 O 2 the redox mediator can accordingly reduce the Li 2 O 2chemically decompose. Due to the high overpotential (e.g. more than about 1.5 V) during the electrochemical decomposition of Li 2 O 2 the redox mediator can be released earlier than the Li 2 O 2 oxidized. To prevent the decomposition reaction of Li 2 O 2 Therefore, as described above, the redox mediator should be oxidized first and the Li 2 O 2 chemically decompose (see chemical formula 4).

[0069] In order for the redox mediator to fulfil the corresponding function, the HOMO level in the original state (RM) should therefore be below the formation energy of Li 2 O 2 lay.

[0070] However, since the charging voltage in the present cell increases as the HOMO level decreases, if the HOMO level is very low, the charging and discharging efficiency (discharge voltage / charging voltage) may decrease. Therefore, a redox mediator whose ionization energy (the primary ionization energy in the electrolyte and the HOMO level in the original state) can be selected in a range of approximately 4.5 V to 6.0 V, or more particularly, in a range of approximately 4.5 V to 5.0 V.

[0071] As in the Fig. 3 and Fig. 4, the HOMO levels of TMA and PPD in the oxidized state (RM + ) lower than the HOMO level of the electrolyte (TEGDME and DMSO).

[0072] Accordingly, TMA and PPD in the oxidized state (RM + ) are more likely to accept or withdraw electrons from the electrolyte than from Li 2 O 2 . To the Li 2 O 2to decompose, RM + with Li 2 O 2 come into contact to transfer electrons from the Li 2 O 2 as shown in chemical formula 4. Since the HOMO levels of TMA and PPD in the oxidized state (RM + ) are below the HOMO level of the electrolyte, TMA and PPD can be added to the Li 2 O 2 no electrons or from the Li 2 O 2 but instead absorb electrons from the electrolyte and then return to their original state (RM).

[0073] The effectiveness of TMA and PPD as redox mediators may therefore be insufficient. The HOMO level of the redox mediator in the oxidized state (RM + ) should therefore be higher than the HOMO level of the electrolyte. Selection of the redox mediator and observation of the actual decomposition of Li 2 O 2

[0074] The redox mediator according to various exemplary embodiments of the present invention is described below. 1) The redox mediator according to the present invention has an ionization energy in vacuum of about 5.5 to 7.5 eV and may have an oxidation potential of about 3.0 to 4.0 V. 2) The redox mediator according to the present invention is highly soluble in the electrolyte, but cannot react with the electrolyte. 3) The HOMO level of the redox mediator according to various exemplary embodiments of the present invention in the original state (RM) may be lower than the formation energy of Li 2 O 2 , but preferably as large as the formation energy. 4) The HOMO level of the redox mediator according to various exemplary embodiments of the present invention in the oxidized state (RM +) can be larger than the HOMO level of the electrolyte.

[0075] As explained above, among NDA, TMA, PPD, NC, and DMPZ, NC may be the least suitable as a redox mediator because NC is less stable to light and oxygen and a side reaction may occur during discharge. As described above, the HOMO levels of TMA and PPD in the oxidized state (RM + ) are lower than the HOMO level of the electrolyte, such a catalyst may be insufficiently effective. As described in the Fig. 3 and Fig. As shown in Figure 4, since the HOMO level of NDA is low in the pristine state (RM), there are concerns about the increase in the charging voltage of the cell, which may lead to a decrease in the charging and discharging efficiency.

[0076] Thus, the soluble catalyst containing DMPZ as a redox mediator can be the most effective because a soluble catalyst containing DMPZ is stable in the electrolyte, no side reaction occurs, polarization and overvoltage can be largely avoided, and the charging and discharging efficiency are satisfactory.

[0077] After preparing the cell containing the soluble catalyst containing DMPZ as a redox mediator, X-ray diffraction (XRD) analysis was performed and the cell was examined under a scanning electron microscope (SEM). The results are shown in the Fig. 5 and Fig. 6. As shown in the Fig. 5, during charging and discharging of the cell Li 2 O 2 decomposed and formed.

[0078] By comparing the graph (as prepared), which shows a just prepared cell, and the graph (discharged), which shows a discharged cell, it can be confirmed that - during discharge - the peak of Li 2 O 2 appears and Li 2 O 2 is formed.

[0079] By comparing the graph (discharged), which shows a discharged cell, and the graph (recharged), which shows a recharged cell, it can be confirmed that - during charging - the peak of Li 2 O 2 disappears and the Li 2 O 2 is completely decomposed.

[0080] As in the Fig. 6, the decomposition and formation of Li 2 O 2 confirm even more clearly. It can be seen that when the lithium-air battery is discharged, Li 2 O 2ring-shaped on the surface (A) of the cathode (air electrode). After complete discharge and subsequent recharging (B) of the cell, the Li 2 O 2 besides, it has completely decomposed and can therefore no longer be detected.

[0081] As described above, the present invention provides compounds that can be used as a novel soluble catalyst. The soluble catalyst of the present invention can, in particular, effectively convert Li 2 O 2 decompose the soluble catalyst formed during the discharge of a lithium-air battery, thereby reducing polarization and increasing charging and discharging efficiency and cycle life. Furthermore, the present invention contributes to further research by setting forth and proposing conditions for use as a soluble catalyst in a lithium-air battery.

[0082] The invention has been described in detail with reference to various embodiments given by way of example.

Claims

[1] Catalyst for a lithium-air battery, containing: a redox mediator dissolved in an electrolyte, wherein the ionization energy of the redox mediator is about 5.5 to 7.5 eV in vacuum, and where the redox mediator is 5,10-dimethylphenazine (DMPZ), 1,5-naphthalenediamine (NDA), 4,N,N-trimethylaniline (TMA) or 1-phenylpyrrolidine (PPD). [2] The catalyst according to claim 1, wherein an oxidation potential of the redox mediator is greater than an equilibrium potential of lithium peroxide (Li2O2). [3] The catalyst of claim 2, wherein the oxidation potential of the redox mediator is about 3.0 to 4.0 V. [4] A catalyst according to claim 1, wherein the electrolyte is triethylene glycol dimethyl ether (TEGDME) or dimethyl sulfoxide (DMSO). [5] The catalyst of claim 1, wherein the redox mediator does not react with the electrolyte. [6] The catalyst according to claim 1, wherein the energy level of the highest occupied orbital (HOMO) of the redox mediator molecule is below a formation energy of Li2O2. [7] A catalyst according to claim 1, wherein the energy level of the highest occupied orbital (HOMO) of the redox mediator molecule in an oxidized state is above a HOMO level of the electrolyte. [8] Lithium-air battery containing: a catalyst according to claim 1. [9] A vehicle comprising a lithium-air battery according to claim 8.

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