Patent Application
20240136589
This application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2022-0137504, filed on Oct. 24, 2022, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.
The following disclosure relates to a lithium air battery. The following disclosure may provide a lithium air battery that reduces a discharge product that deteriorates performance of the lithium air battery and exhibits improved cyclability at a low temperature.
Lithium-ion secondary batteries have recently been in the spotlight, but due to a low energy density, many studies have been conducted on lithium air batteries as a type of battery with a higher energy density. The capacity of the lithium air battery is currently expected to have an energy density three to five times higher than that of the lithium-ion battery at a material level. However, the lithium air battery has poor cyclability for two reasons. This is because, first, a superoxide radical (O2⋅−), which is an intermediate of lithium peroxide (Li2O2), deteriorates a carbon-based electrode and an electrolyte solution, and second, lithium carbonate (Li2CO3), which is a by-product, has a high charge potential of 4.5 V (vs. Li/Li+) or less, causing unwanted electrochemical oxidation. Although a redox mediator and an accelerator appear to suppress such side reactions during the initial cycle, the cyclability is not significantly improved.
Meanwhile, a peroxodicarbonate (C2O62−) dianion may be formed as an intermediate of Li2CO3 in an electrolyte solution containing a non-aqueous solvent such as dimethyl sulfoxide (DMSO) or tetraglyme. Such a soluble material is less reactive than O2⋅− and is oxidized at a low charge potential (3.75 V or less). Therefore, C2O62− is a preferred discharge product over O2⋅−, Li2O2, and Li2CO3. Accordingly, CO2 gas is mixed with O2 to form the intermediate in the electrolyte, such that more excellent cyclability is exhibited than in a Li—O2 battery alone. However, as C2O62− is easily converted into Li2CO3 at room temperature, deposition of Li2CO3 increases rapidly. A second charge plateau (4.5 V or less) is related to oxidation of Li2CO3. This plateau appears clearly after the first cycle, and the first plateau associated with C2O62− quickly disappears. This is due to incomplete decomposition of Li2CO3 during charging. As a result, the capacity of the Li—O2/CO2 battery is rapidly reduced due to accumulation of Li2CO3, and the cyclability is poor.
When a solvent having a low conductivity is used as the electrolyte solution of the battery, the first charge plateau may be extended, but the low ionic conductivity increases voltage polarization and ultimately results in poor cyclability of the lithium air battery.
An embodiment of the present disclosure is directed to providing a lithium air battery that reduces a discharge product that deteriorates performance of the lithium air battery and exhibits improved cyclability at a low temperature.
In one general aspect, a lithium air battery includes: an anode including lithium; a cathode located so as to face the anode and using oxygen as a cathode active material; an electrolyte solution located between the anode and the cathode and containing a solvent and a lithium salt; and carbon dioxide, wherein the solvent is a heteroaliphatic solvent having a dielectric constant of 30 or more and containing a nitrogen atom or a sulfur atom. The solvent may have 2 to 8 carbon atoms.
The solvent may be at least one selected from the group consisting of dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), acetonitrile, and nitromethane.
The solvent may be an amide-based compound.
The lithium air battery may be operated at a temperature that is equal to or higher than a melting point of the solvent and is 5° C. or lower.
The lithium air battery may be operated at −20 to 0° C.
The carbon dioxide may be supplied to one side of the cathode as a mixed gas mixed with oxygen.
A volume ratio of the oxygen to the carbon dioxide in the mixed gas may be 5:5 to 9:1.
The carbon dioxide may be dissolved in the electrolyte solution.
A Gutmann donor number (DN) of the solvent may be 25 to 30.
The lithium air battery may further include a separator located between the cathode and the anode.
The cathode may include a carbon body.
The lithium salt may include at least one selected from the group consisting of LiNO3, LiNO2, LiC2F6NO4S2 (LiTFSI), LiCF3SO3 (LiOTf), LiF2NO4S2 (LiFSI), LiC4F10NO4S2 (LiBETI), LiPF6, LiBF4, LiSbF6, LiAsF6, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiCl, LiI, LiBr, and LiB(C2O4)2.
The lithium salt may include LiNO3.
In another general aspect, an electrolyte composition for a lithium air battery includes: a lithium salt; a heteroaliphatic solvent having a dielectric constant of 30 or more and containing a nitrogen atom or a sulfur atom; and dissolved carbon dioxide.
The solvent may have 2 to 8 carbon atoms.
The solvent may be at least one selected from the group consisting of dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), acetonitrile, and nitromethane.
The solvent may be an amide-based compound.
The solvent may be dimethylacetamide (DMAc).
The lithium salt may include at least one selected from the group consisting of LiNO3, LiNO2, LiC2F6NO4S2 (LiTFSI), LiCF3SO3 (LiOTf), LiF2NO4S2 (LiFSI), LiC4F10NO4S2 (LiBETI), LiPF6, LiBF4, LiSbF6, LiAsF6, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiCl, LiI, LiBr, and LiB(C2O4)2.
The lithium salt may include LiNO3.
Other features and aspects will be apparent from the following detailed description, the drawings, and the claims.
Technical terms and scientific terms used in the present specification have the general meaning understood by those skilled in the art to which the present disclosure pertains unless otherwise defined, and a description for the known function and configuration unnecessarily obscuring the gist of the present disclosure will be omitted in the following description.
In addition, unless the context clearly indicates otherwise, the singular forms used in the present specification may be intended to include the plural forms.
In addition, a numerical range used in the present specification includes upper and lower limits and all values within these limits, increments logically derived from a form and span of a defined range, all double limited values, and all possible combinations of the upper and lower limits in the numerical range defined in different forms. Unless otherwise specifically defined in the specification of the present disclosure, values out of the numerical range that may occur due to experimental errors or rounded values also fall within the defined numerical range.
The expression “comprise(s)” described in the present specification is intended to be an open-ended transitional phrase having an equivalent meaning to “include(s)”, “contain(s)”, “have (has)”, or “are (is) characterized by”, and does not exclude elements, materials, or steps, all of which are not further recited herein.
Hereinafter, a lithium air battery of the present disclosure will be described in detail.
The present disclosure provides a lithium air battery including: an anode including lithium; a cathode located so as to face the anode and using oxygen as a cathode active material; an electrolyte solution located between the anode and the cathode and containing a solvent and a lithium salt; and carbon dioxide, wherein the solvent is a heteroaliphatic solvent having a dielectric constant of 30 or more and containing a nitrogen atom or a sulfur atom.
In an exemplary embodiment, the solvent may have a dielectric constant of 30 or more or 35 or more. In addition, the solvent may have a dielectric constant of 70 or less, 60 or less, 50 or less, 45 or less, or 40 or less. The dielectric constant of the solvent may be 30 to 50, specifically, 30 to 40, and more specifically, 35 to 40. When a solvent having a high dielectric constant is used as an electrolyte solution in a battery, cyclability may be improved. This is because such a solvent exhibits a high ionic conductivity at a low temperature and a first charge plateau proportion may increase during cycling.
In an exemplary embodiment, the solvent may contain a nitrogen atom or a sulfur atom, but is not limited thereto. Specifically, the solvent may contain a nitrogen atom.
In an exemplary embodiment, the solvent may have 1 to 8 carbon atoms, specifically, 2 to 8 carbon atoms, more specifically, 2 to 7 carbon atoms, more specifically, 3 to 6 carbon atoms, and still more specifically, 4 or 5 carbon atoms.
In an exemplary embodiment, the solvent may be at least one selected from the group consisting of dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), acetonitrile, and nitromethane, but is not limited thereto.
In an exemplary embodiment, the solvent may be an amide-based compound, and specifically, the solvent may be dimethylacetamide (DMAc). When a dimethylacetamide (DMAc) solvent is used, the cyclability may be improved. This is because such a solvent exhibits a high ionic conductivity at a low temperature and the first charge plateau proportion may increase during cycling.
In an exemplary embodiment, it is preferable that the lithium air battery is operated at a low temperature region. According to an exemplary embodiment, the lithium air battery may be operated at a temperature that is equal to or higher than a melting point of the solvent and is 5° C. or lower. Specifically, the lithium air battery may be operated at a temperature that is equal to or higher than the melting point of the solvent and is 0° C. or lower, and more specifically, the lithium air battery may be operated at a temperature that is equal to or higher than the melting point of the solvent and is −5° C. or lower. As the lithium air battery is operated at the low temperature range as described above, C2O62−, which is a discharge product, may be stabilized, and therefore, an effect of extending the first charge plateau may be achieved, which is preferable.
In an exemplary embodiment, the lithium air battery may be operated at 5° C. or lower, 0° C. or lower, −5° C. or lower, −10° C. or lower, −15° C. or lower, -20° C. or lower, −25° C. or lower, or −30° C. or lower, and the lithium air battery may be operated, without limitation, at 0° C. or higher, −5° C. or higher, −10° C. or higher, −15° C. or higher, −20° C. or higher, −25° C. or higher, −30° C. or higher, −35° C. or higher, or −40° C. or higher.
In an exemplary embodiment, the lithium air battery may be operated at −40° C. to 5° C., specifically, −30° C. to 5° C., more specifically, −20° C. to 5° C., more specifically, −20° C. to 0° C., more specifically, −15° C. to 0° C., and still more specifically, −10° C. to 0° C.
In an exemplary embodiment, the carbon dioxide included in the lithium air battery may be supplied to one side of the cathode as a mixed gas mixed with oxygen. As a more specific example, the mixed gas may be supplied to one side of the cathode in the form of natural convection or forced convection, but is not limited thereto. As the mixed gas is supplied to one side of the cathode, oxygen is used as an active material in the cathode, and carbon dioxide is dissolved in the solvent, such that oxygen comes into contact with carbon dioxide, and C2O62−, which is a discharge product, may be produced during discharging.
In an exemplary embodiment, a volume ratio of the oxygen to the carbon dioxide in the mixed gas may be 5:5 to 9.9:0.1, 6:4 to 9.9:0.1, 5:5 to 9.5:0.5, 6.5:3.5 to 9.5:0.5, 5:5 to 9:1, or 7:3 to 9:1.
In another exemplary embodiment, the carbon dioxide may be dissolved in the electrolyte solution. As the carbon dioxide is dissolved in the solvent contained in the electrolyte solution at a high concentration, C2O62−, which is a discharge product as described above, may be produced in the electrolyte solution. Accordingly, the lithium air battery may be operated even when only oxygen or air is supplied to the one side of the cathode. However, since the carbon dioxide dissolved in the solvent is lost, a modified example in which a mixed gas containing carbon dioxide is supplementally supplied to one side of the cathode is also included in the present disclosure.
In an exemplary embodiment, a Gutmann donor number (DN) of the solvent may be 22 or more, 23 or more, 24 or more, 25 or more, 26 or more, or 27 or more. In addition, the Gutmann donor number (DN) of the solvent may be 35 or less, 34 or less, 33 or less, 32 or less, 31 or less, 30 or less, 29 or less, or 28 or less. In an exemplary embodiment, the Gutmann donor number (DN) of the solvent may be 22 to 35, specifically, 25 to 30, and more specifically, 26 to 28. When a solvent having a high Gutmann donor number is used as the electrolyte solution of the battery, a high ionic conductivity may be exhibited at a low temperature, and thus, the cyclability of the battery may be improved.
In an exemplary embodiment, the lithium air battery may further include a separator located between the cathode and the anode. The separator may include at least one selected from the group consisting of a polyolefin-based resin such as polyethylene or polypropylene; a fluorine-based resin such as polytetrafluoroethylene or polyvinylidene fluoride; a polyamide-based resin; and a polyimide-based resin such as polyamideimide or polyimide, but is not limited thereto. In addition, the separator may include inorganic fiber or inorganic powder such as glass fiber, mica, alumina, or silica without limitation.
In an exemplary embodiment, the cathode may include a carbon body. The carbon body may include at least one selected from the group consisting of fullerene, graphite, graphene, carbon black, Ketjenblack, a carbon fiber, and a carbon nanotube, but is not limited thereto. Specifically, the carbon body may be a carbon nanotube.
In an exemplary embodiment, the lithium salt may include at least one selected from the group consisting of LiNO3, LiNO2, LiC2F6NO4S2 (LiTFSI), LiCF3SO3 (LiOTf), LiF2NO4S2 (LiFSI), LiC4F10NO4S2 (LiBETI), LiPF6, LiBF4, LiSbF6, LiAsF6, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiCl, LiI, LiBr, and LiB(C2O4)2, but is not limited thereto. Preferably, the lithium salt may include LiNO3 or LiC2F6NO4S2(LiTFSI). More preferably, the lithium salt may include LiNO3. As LiNO3 is included as a lithium salt, the anode including lithium is stabilized and other side reactions are suppressed, which may be preferable.
In addition, the present disclosure provides an electrolyte composition for a lithium air battery, the electrolyte composition including: a lithium salt; a heteroaliphatic solvent having a dielectric constant of 30 or more and containing a nitrogen atom or a sulfur atom; and dissolved carbon dioxide.
In an exemplary embodiment, the solvent may have a dielectric constant of 30 or more or 35 or more. In addition, the solvent may have a dielectric constant of 70 or less, 60 or less, 50 or less, 45 or less, or 40 or less. The dielectric constant of the solvent may be 30 to 50, specifically, 30 to 40, and more specifically, 35 to 40. When a solvent having a high dielectric constant is used as an electrolyte solution in a battery, cyclability of the battery may be improved. This is because such a solvent exhibits a high ionic conductivity at a low temperature and a first charge plateau proportion may increase during cycling.
In an exemplary embodiment, the solvent may contain a nitrogen atom or a sulfur atom, but is not limited thereto. Specifically, the solvent may contain a nitrogen atom.
In an exemplary embodiment, the solvent may have 1 to 8 carbon atoms, specifically, 2 to 8 carbon atoms, more specifically, 2 to 7 carbon atoms, more specifically, 3 to 6 carbon atoms, and still more specifically, 4 or 5 carbon atoms.
In an exemplary embodiment, the solvent may be at least one selected from the group consisting of dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), acetonitrile, and nitromethane, but is not limited thereto.
In an exemplary embodiment, the solvent may be an amide-based compound, and specifically, the solvent may be dimethylacetamide (DMAc). When a dimethylacetamide (DMAc) solvent is used, the cyclability may be improved. This is because such a solvent exhibits a high ionic conductivity at a low temperature and the first charge plateau proportion may increase during cycling.
In an exemplary embodiment, the lithium salt may include at least one selected from the group consisting of LiNO3, LiNO2, LiC2F6NO4S2 (LiTFSI), LiCF3SO3 (LiOTf), LiF2NO4S2 (LiFSI), LiC4FiONO4S2 (LiBETI), LiPF6, LiBF4, LiSbF6, LiAsF6, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiCl, LiI, LiBr, and LiB(C2O4)2 , but is not limited thereto. Preferably, the lithium salt may include LiNO3 or LiC2F6NO4S2 (LiTFSI). More preferably, the lithium salt may include LiNO3. As LiNO3 is included as a lithium salt, the anode including lithium is stabilized and other side reactions are suppressed, which may be preferable. Hereinafter, the present disclosure will be described in more detail with reference
to Examples and Comparative Examples. However, the following Examples and Comparative Examples are only examples for describing the present disclosure in more detail, and the present disclosure is not limited by the following Examples and Comparative Examples. In the present disclosure, unless otherwise specified, all temperatures refer to a unit of ° C., and the amount of composition used refers to a unit of wt %.
As a solvent contained in an electrolyte, tetraethylene glycol dimethyl ether (tetraglyme, Sigma Aldrich), 1,2-dimethoxyethane (DME, Sigma Aldrich), dimethyl sulfoxide (DMSO, Sigma Aldrich), and N,N-dimethylacetamide (DMAc, Sigma Aldrich) were used, and lithium bistriflimide (LiTFSI, Sigma Aldrich) and lithium nitrate (LiNO3, Sigma Aldrich) were used as lithium salts. A binder-free carbon nanotube (CNT) film was prepared as a working electrode. A metallic lithium foil (thickness: 50 μm, Honjo) and CNT (JENO 20A, >95% purity, JEIO) were used as an anode and a cathode, respectively. Polypropylene Celgard (Celgard 2500) and glass fiber (GF/C, Whatman) or GB100R glass fiber film (Adventec) were used together as separators.
In order to synthesize potassium peroxodicarbonate (K2C2O6), a hydrogen peroxide solution (H2O2, 34.5 to 36.5%, Sigma Aldrich) and potassium hydroxide (KOH, 99.99%, Sigma Aldrich) were used. 8.5 mL of H2O2 was cooled with stirring at −20° C. Separately, 7 g of KOH was dissolved in 5.5 mL of deionized water, and this KOH solution was slowly added dropwise to the H2O2 solution at −20° C. When the mixed solution became transparent, the solution was stirred and CO2 gas was strongly bubbled. When a light yellow product was precipitated, the powder was separated by filtration from the solution, and the separated powder was washed with cold ethyl alcohol (cooled to −10° C. before use, Samchun) and diethyl ether (cooled to −10° C. before use, Sigma Aldrich). The resulting powder was dried in a vacuum oven for 1 hour, and the dried powder was sealed in an Ar state before stored in an incubator at −10° C.
A freestanding CNT film was prepared by a filtration method. 0.12 g of CNT powder was dispersed in isopropyl alcohol for 20 minutes using a tip ultrasonicator. The dispersed CNTs were peeled off by filtration through glass fiber (5C, Adventec). All battery components were dried in a vacuum oven at 80° C. overnight prior to battery assembly. A Li—O2/CO2 battery was assembled using a metallic Li foil (diameter: 12 mm, thickness: 50 μm) as an anode, Celgard 2500 (diameter: 26 mm, thickness: 50 μm) or GF/C as a separator (diameter: 19 mm, thickness: 260 μm), a freestanding CNT film (diameter: 12 mm, thickness: 45 μm, mass: 1.2 mg) as a cathode, and a 0.5 M LiNO3/DMAc electrolyte solution (150 μm) in an Ar-filled glove box (MOTek, O2≤0.7 ppm, H2O≈0 ppm). Contents of moisture of 0.5 M LiNO3/DMAc were about 15 and 49 ppm, respectively (Karl Fischer titration, Metrohm). When a DMSO electrolyte solution was used, the GF/C separator was replaced with GB100R glass fiber for a battery test.
After the assembly, O2 and CO2 gasses (purities of both gases: 99.999%, Daesung Industrial Gases) were simultaneously introduced at flow rates of 90 and 10 sccm, respectively, for 15 minutes to provide 90/10 v/v %. For several OEMS tests, metallic Li was replaced with LiFePO4 (LFP). LFP slurry was prepared by mixing LFP powder (carbon coating, Prayon), SuperP (Timcal), and polyvinylidene fluoride (PVdF, Arkema) in N-methyl-2-pyrrolidone (NMP, Sigma Aldrich, 99%) at a weight ratio of 8:1:1 using a mixer (THINKY Corporation, ARE-310). This slurry was cast on a titanium foil (Welcos, 99.5%) using a doctor blade method and then dried in a vacuum oven at 100° C. for 60 hours. The areal mass load, thickness, and diameter of the LFP electrode were 8 mg·cm−2 or less, 790 μm, and 14 mm, respectively.
Electrochemical measurement was performed using a battery cycler (Versa Stat3 (AMETEK) or WPG100e (WonATech)). The battery was rest in an incubator for 2 hours, and then the battery was operated in an incubator maintained at −10° C. (IN604, Yamato). When the battery was operated at 25° C., the incubator was replaced with an incubator (INE800, Yamato). The first plateau proportion and the number of cycles of the battery were measured and shown in Table 1.
A process was performed in the same manner as that of Example 1, except that the temperature was changed to 0° C.
A process was performed in the same manner as that of Example 1, except that the temperature was changed to −5° C.
A process was performed in the same manner as that of Example 1, except that the temperature was changed to −15° C.
A process was performed in the same manner as that of Example 1, except that the temperature was changed to 25° C.
A process was performed in the same manner as that of Example 1, except that 0.5 M LiTFSI/tetraglyme was used as an electrolyte solution.
A process was performed in the same manner as that of Example 1, except that 0.5 M LiTFSI/tetraglyme was used as an electrolyte solution and the temperature was changed to 0° C.
A process was performed in the same manner as that of Example 1, except that 0.5 M LiTFSI/tetraglyme was used as an electrolyte solution and the temperature was changed to 25° C.
A process was performed in the same manner as that of Example 1, except that the volume ratio of O2 gas to CO2 gas was changed to 70:30.
A process was performed in the same manner as that of Example 5, except that the temperature was changed to 0° C.
A process was performed in the same manner as that of Example 5, except that the temperature was changed to −5° C.
A process was performed in the same manner as that of Example 5, except that the temperature was changed to −15° C.
A process was performed in the same manner as that of Example 5, except that the temperature was changed to 25° C.
A process was performed in the same manner as that of Example 5, except that 0.5 M LiTFSI/tetraglyme was used as an electrolyte solution.
A process was performed in the same manner as that of Example 5, except that 0.5 M LiTFSI/tetraglyme was used as an electrolyte solution and the temperature was changed to 0° C.
A process was performed in the same manner as that of Example 5, except that 0.5 M LiTFSI/tetraglyme was used as an electrolyte solution and the temperature was changed to 25° C.
When tetraglyme was used as the electrolyte solution, although a relatively long first charge plateau was exhibited, the cyclability was poor. On the other hand, in the case where DMAc was used as the electrolyte solution, when O2:CO2 was 90:10, 100 cycles were performed at −10° C. These results indicate that the cyclability may be improved when peroxo-linked C2O62− is formed at a low temperature. In addition, it was found that Li2CO3 accumulation hardly occurred in the DMAc-based battery (
A gas generation profile during a charge process was measured in a self-produced OEMS. The assembled Li—O2/CO2 battery described above was first discharged to 1,000 mAh·gc−1. Subsequently, the cell was purged with Ar gas (99.999%, Daesung Industrial Gases), and a headspace of the battery was changed from an Ar-filled glove box to an OEMS compatible storage (Tomcell). These batteries were connected to the OEMS, the charge process was performed at 1,000 mAh·gc−1, and a change in partial pressure of generated gas was observed. The generated gas was collected and analyzed every hour using a mass spectrometer (RGA200, Stanford Research Systems). A preloaded K2C2O6 electrode was prepared by adding 0.0022 g of as-synthesized K2C2O6 powder to a freestanding CNT electrode in an Ar-filled glove box. This electrode was charged at 10° C. with OEMS analysis.
An ionic conductivity of the electrolyte solution was measured at a desired temperature using an ionic conductivity battery (Tera leader®). The cell was maintained with a fixed distance between two stainless steel electrodes of 0.2 cm, and the diameter and the area of the electrode was 14 mm and 1.539 cm2, respectively. An assembled battery filled with an electrolyte solution was transferred to an incubator and stabilized at a desired temperature at least 2 hours before measurement. An electrochemical impedance spectroscopy (EIS) test was investigated using VersaStat3 at a DC voltage amplitude of 10 mV and a frequency window of 105-1 Hz. The ionic conductivity of each electrolyte solution was estimated using the following equation.
Here, σ is an ionic conductivity, R is a solution resistance, A is an electrode area,
and L is a distance between two electrodes.
For ex-situ analysis, the tested Li—O2/CO2 battery was purged with Ar gas and disassembled in an Ar-filled glove box. The CNT electrode was immersed in diglyme (99.5% anhydrous, Sigma Aldrich) for at least 10 minutes and rinsed three times with fresh diglyme. The diglyme solvent was dried using a 4 Å molecular sieve for longer than a day, and this process was repeated three times before use (H2O<15 ppm). Thereafter, the CNT electrode was dried in a vacuum chamber at room temperature overnight. A GF/C glass fiber separator used for Raman analysis was collected immediately after the cell test without a washing process. X-ray diffraction (XRD, Rigaku Dmax 2000 diffractometer) analysis at KAIST Analysis Center for Research Advancement (KARA) was performed using Ni-filtered Cu-Kα radiation (λ=1.5418 Å). Fourier-transform infrared spectroscopy (FTIR, Nicolet i S50 spectrometer, Thermo Fisher Scientific) analysis was investigated using a portable glove box system purged with Ar gas. The sample was mixed with CsI powder (>99.99% purity, Kanto Chemical Co.), and pellets were prepared in an Ar-filled glove box. The Raman analysis was performed using a Raman spectrometer (Acton Spectra Pro 2300i) with He/Ne laser (633 nm), an integrated microscope (Olympus BX 43, objective×50, NA=0.55), and a thermoelectrically cooled 1,024×127 pixel charge-coupled device (CCD) detector (ANDOR, DV401A-BV). A holographic grid was 1,200 grooves mm−1. KARA's scanning electron microscope (SEM, Magellan400, FEI company) was used to image the CNT electrode.
Confirmation of Discharge Product in Li—O2/CO2 Battery
Chemical/electrochemical properties of the Li—O2/CO2 battery were verified by online electrochemical mass spectrometry (OEMS) and were compared with those of the Li—O2 battery at 25° C. Metallic Li and a binder-free CNT film were used as an anode and a cathode, respectively. Gas in which a volume ratio of O2:CO2 was 90:10 and a 0.5 M LiTF SI/tetraglyme electrolyte solution were included in the fully sealed battery. After a discharge process at 25° C., two potential plateaus at 3.75 V and 4.45 V were observed at a current density of 50 mA·gc−1 and a capacity of 1,000 mAh·gc−1 , where gc represents the mass of the CNT electrode (
The fact that only O2 is not generated in the Li—O2/CO2 battery indicates that Li2O2 may be ignored after discharge. X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectra consistently confirmed a faint Li2O2 signal at the CNT electrode at the end of discharge (
Li2O2(s)+CO2(g)→Li2CO3(s)+0.5 O2(g) (1)
After 50% state of charge (SOC) in the Li—O2/CO2 battery, XRD showed crystalline Li2CO3 with reflections of −112, 020, and −311 at 2θ at 33.9°, 35.8°, and 36.6°, respectively.
A C—O vibration band of Li2CO3 appeared in FTIR. This indicates the continuity of Li2CO3 deposition during the charge process. These Li2CO3-related signals were attenuated at 100% SOC during the first cycle. Therefore, in the OEMS analysis (
The reaction of O2 and CO2 should be considered to identify the first charge-plateau reaction. CO2 undergoes a chemical reaction with O2⋅− supplied through O2 reduction (Formula 2). Soluble anionic materials such as peroxo-monocarbonate (CO4⋅− and peroxo-dicarbonate (C2O62−) are produced before Li2CO3 is produced via an electrochemical (EC) route (Formulas 3 to 5).
O2(g)+e−→O2·− (2)
O2·−+CO2(g)→CO4·− (3)
CO4·−+O2·−+CO2(g)→C2O62−+O2(g) (4)
C2O62−+4 Li+ +2 O2·−→2 Li2CO3(s)+2 O2(g) (5)
In the series of chemical reactions, C2O62− and Li2CO3 are assumed to be the main discharge products. In particular, since a reversible thermodynamic potential of C2O62− (3.27 V) is lower than that of Li2CO3 (3.82 V), C2O62− may be oxidized in the first charge plateau.
In order to confirm the presence of C2O62− in the Li—O2/CO2 battery, K2C2O6 powder was synthesized and gas generation during electrochemical oxidation was analyzed. Light orange K2C2O6 powder was prepared at −20° C. from a mixture of KOH, hydrogen peroxide (H2O2), and CO2 gas. A peroxo-linked C2O62− structure in the K2C2O6 powder was confirmed by Raman and FTIR spectra (
Temperature Influence on C2O62− Stability
Inhibiting the conversion of C2O62− into Li2CO3 is a key point of reducing the charge potential and improving the cyclability of the Li—O2/CO2 battery. A high charge potential is required for Li2CO3 to oxidize, which unfortunately decomposes both the CNT electrode and the electrolyte solution. In addition, Li2CO3 is partially decomposed even at 4.5 V, and insulating Li2CO3 is accumulated on the CNT electrode. Therefore, a constant current test was performed at a temperature of lower than 25° C. to preserve C2O62− species in the Li—O2/CO2 battery. When the temperature was lowered from 25° C. to −10° C., the first charge plateau became longer while the second charge plateau became shorter (
The proportion of the first plateau was estimated to be 47% or less at 25° C. and reached 86% or less at 0° C. during the charge process. It was shown that O2 and CO2 were simultaneously generated in the first charging plateau, which was extended at 10° C., and the stability of C2O62− at a low temperature was confirmed (
Such a poor electrochemical reaction is due to the high viscosity of the tetraglyme solvent. The conductivity of 0.5 M LiTF SI/tetraglyme was 2.175 (±0.015) mS·cm−1 at 25° C. and decreased to 0.73 (±0.01) mS·cm−1 at −10° C. (see Table 2).
In the Li—O2/CO2 battery, the actual ionic conductivity of the electrolyte solution should be lower than the measured value because C2O62− is dissolved in the solution. In terms of cyclability, at 25° C., the Li—O2/CO2 battery was able to perform 30 cycles at 50 mA·gc−1 and 500 mAh·gc−1 (
Influence of Electrolyte Solution on C2O62− Stability
In order to balance the C2O62− stability and the ionic conductivity of the electrolyte solution, other solvents such as dimethoxyethane (DME), dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO) were considered. A charge profile was confirmed at 25° C. in a Li—O2/CO2 battery using a LiFePO4 (LFP) electrode instead of metallic Li. In all the batteries, similar discharge potentials of 2.7 to 2.9 V compared to Li/Li+ were shown in a constant current profile, but the proportions of the first charge plateau to the total capacity were different. DME (and tetraglyme) showed 40 to 50% of the first plateau during the charge process, whereas DMAc and DMSO showed 4% or less and less than 1%, respectively. These results indicate that solvent properties also affect the stability of C2O62−. This trend is related to the dielectric constant (c) and Gutmann donor number (DN) of the solvent (see Table 3).
In the case of DMAc, the first charge plateau was short at 25° C., but the viscosity and the melting point were low, and thus a charge/discharge cycle was performed at a low temperature. The ionic conductivity of 0.5 M LiNO3/DMAc was 8.695 (±0.265) mS·cm−1 at 25° C. and 5.735 (±0.135) mS·cm−1 at −10° C., which was higher than that of the tetraglyme electrolyte solution (see Table 2). Therefore, a 0.5 M LiNO3 salt was used in DMAc to stabilize the Li electrode surface against DMAc.
The Li—O2/CO2 battery including 0.5 M LiNO3/DMAc showed temperature-dependent charge behavior (
Despite the similar temperature-related charge behavior, in the case of the tetraglyme solvent, a longer first plateau capacity than DMAc was provided at the same temperature (
The electrochemical process that depends on the electrolyte solution is related to the dielectric constant (c) and the Gutmann donor number (DN) of the electrolyte solution. DMAc having a high dielectric constant (c) and Gutmann donor number (DN) further solvates Li+. Therefore, free O2·− reacts with CO4·− (Formula 6), and the subsequent reaction of CO42− with CO2 forms peroxo-linked C2O62− (CO2OOCO22−, Formula 7 and
CO4·−+O2·−→CO42−+O2 (6)
CO42−+CO2→C2O62− (peroxo-linked) (7)
In comparison, tetraglyme having a low dielectric constant (c) is not preferable for forming highly charged CO42− and C2O62− (
CO4·−+Li+→LiCO4· (8)
LiCO4·+O2·−+CO2→LiC2O6− (anhydride-linked)+O2 (9)
Since an extended first charge plateau appears when tetraglyme is used, a stable anhydride-linked LiC2O6− may be inferred in the Li—O2/CO2 battery. However, the DFT calculation shows more excellent stability for peroxo-linked C2O62− than for anhydride-linked LiC2O6−− for Li2CO3. ΔG for peroxo-linked C2O62− and anhydride-linked LiC2O6− to become Li2CO3 were measured to be −32 and −42.6 kcal/mol, respectively. Although the exact reason is not yet known, it is presumed that LiC2O6− in the anhydride form is slowly converted into Li2CO3 through structural rearrangement, in contrast to C2O62− in the peroxo form, which undergoes simple O—O link cleavage.
Improved Cyclability of Li—O2/CO2 Battery Including 0.5 M LiNO3/DMAc at −10° C.
At 50 mA·gc−1 and 500 mAh·gc−1, the Li—O2/CO2 battery including 0.5 M LiNO3/DMAc was stably operated for 100 cycles at −10° C. (
In an additional cycle test under harsh conditions, an important factor determining Li2CO3 decomposition has been identified. When the O2/CO2 ratio was set to a volume ratio of 50:50, the capacity began to decrease after the 10th cycle at −10° C. (
As set forth above, the lithium air battery according to the present disclosure reduces a discharge product such as Li2CO3, which deteriorates the performance of the lithium air battery, and exhibits excellent cyclability at a low temperature. Specifically, in the lithium air battery according to the present disclosure, a solvent having a high ionic conductivity at a low temperature is used as the electrolyte solution of the battery, such that the first charge plateau proportion increases, thereby improving the cyclability.
Further, in the lithium air battery according to the present disclosure, almost no solid product deposited on the electrode of the battery is generated after the cycle is performed, and Li2CO3 is completely decomposed during the second plateau charge process, such that significantly excellent cyclability is exhibited.
Further, as the lithium air battery according to the present disclosure is operated at a low temperature, C2O62−, which is an intermediate derived from CO2, may be significantly stabilized, the first charge plateau in which O2 and CO2 gases are generated simultaneously may be extended, and significantly excellent cyclability may be realized.
Although the exemplary embodiments of the present disclosure have been described hereinabove, various modifications and equivalents of the present disclosure are possible, and the exemplary embodiments may be appropriately modified and similarly applied. Therefore, contents described do not limit the scope of the present disclosure as defined by the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2022-0137504 | Oct 2022 | KR | national |
