ELECTROLYTE AND MANUFACTURING METHOD OF GREEN AMMONIA FROM Li-MEDIATRED NITROGEN REDUCTION USING THE SAME

Abstract

The present disclosure provides an electrolyte including a lithium compound, a proton donor, and a sulfur compound.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Korean Patent Application No. 10-2023-0164927 filed on Nov. 23, 2023, the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an electrolyte, and more particularly, to an electrolyte including a sulfur compound, and to a manufacturing method of ammonia, and more particularly, to a manufacturing method of green ammonia from Li-mediated nitrogen reduction using a Li-mediated nitrogen reduction reaction.

BACKGROUND ART

Ammonia is the second most produced major chemical in the world, and has recently attracted attention as a powerful high-density hydrogen carrier due to its high hydrogen content (17.6 wt %) and high energy density (6.25 kWh/kg).

Currently, the Haber-Bosch process is commonly used to manufacture ammonia, but has some problems in that it consumes a huge amount of energy (about 1% of global energy consumption) and greenhouse gases (about 1.4% of global emissions) due to the strict process conditions (300 to 500° C., up to 200 bar).

Recently, the electrochemical process has been attracting attention as a very promising solution for green ammonia production due to its mild operating conditions that do not emit carbon dioxide and can operate on renewable energy.

Nitrogen (N₂) gas, which is abundant in the air, acts as an optimal raw material for electrochemical ammonia production, but there is a problem in that a strong triple bond (−941 kJ/mol) of nitrogen should be broken.

Therefore, there is a need to develop a method that can solve environmental concerns, pursue global carbon neutrality, and manufacture ammonia with a simpler process.

DISCLOSURE

Technical Problem

The present disclosure attempts to provide an electrolyte capable of manufacturing ammonia through a simple process using an electrochemical method, and a manufacturing method of ammonia.

Technical Solution

According to an aspect of the present disclosure, an electrolyte includes a lithium compound, a proton donor, and a sulfur compound.

According to another aspect of the present disclosure, a manufacturing method of ammonia includes: filling an electrolyte including a lithium compound, a proton donor, and a sulfur compound into a reactor, and positioning a working electrode and a counter electrode to contact the electrolyte; applying a current to the working electrode and the counter electrode to form a lithium layer and a solid electrolyte interface (SEI) layer on one surface of the working electrode; forming lithium nitride on the lithium layer by reacting with nitrogen supplied into the reactor; and obtaining ammonia by reacting the lithium nitride with the proton donor.

Advantageous Effect

According to the electrolyte and the manufacturing method of ammonia using the same according to the present disclosure, it is possible to manufacture ammonia through a simple process using an electrochemical method.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a manufacturing method of Li-mediated green ammonia according to some exemplary embodiments of the present disclosure.

FIG. 2 is a diagram illustrating a gas system of a manufacturing method of ammonia according to some exemplary embodiments of the present disclosure.

FIG. 3 is a diagram illustrating an electrochemical reactor of the manufacturing method of ammonia according to some exemplary embodiments of the present disclosure.

FIGS. 4 and 5 are diagrams illustrating results of cyclic voltammetry analysis according to an experiment on the effect of sulfur compounds on a Li-mediated green ammonia synthesis process.

FIG. 6 is a diagram illustrating a standard addition amount using a colorimetric method calibration curve for NH₄⁺—N according to the experiment on the effect of the sulfur compounds on the Li-mediated ammonia synthesis process.

FIG. 7 is a diagram illustrating an ion chromatography calibration curve for NH₄⁺—N according to the experiment on the effect of the sulfur compounds on the Li-mediated ammonia synthesis process.

FIG. 8 is a diagram illustrating an NMR calibration curve for NH₄⁺—N according to the experiment on the effect of the sulfur compounds on the Li-mediated ammonia synthesis process.

FIG. 9 is a diagram illustrating the results of NMR analysis on a progress of ammonia production according to the experiment on the effect of the sulfur compounds on the Li-mediated ammonia synthesis process.

FIGS. 10 and 11 are diagrams illustrating the IC spectrum and NMR results of a control experiment on the effect of the sulfur compounds on the Li-mediated ammonia synthesis process.

FIG. 12 is a diagram illustrating the results of evaluating Faradaic efficiency (F.E.) and an NH₃ production rate according to the experiment on the effect of the sulfur compounds on the Li-mediated ammonia synthesis process.

FIG. 13 is a diagram illustrating a voltage profile according to an experiment on the effect of the sulfur compounds on the Li-mediated ammonia synthesis process.

FIG. 14 is a diagram illustrating potential profiles of a working electrode and a counter electrode according to the experiment on the effect of the sulfur compounds on the Li-mediated ammonia synthesis process.

FIG. 15 is a diagram illustrating the potential profiles of the working electrode and the counter electrode according to the experiment on the effect of the sulfur compounds on the Li-mediated ammonia synthesis process.

FIG. 16 is a diagram illustrating a change in a physical structure of a solid electrolyte interface (SEI) layer according to an experiment on a morphology of the SEI layer.

FIG. 17 is a diagram illustrating the potential profile according to the adjustment of a sulfur-added amount.

FIGS. 18 to 22 are diagrams illustrating an XPS spectrum of the SEI layer according to an experiment on a chemical composition of the SEI layer.

FIGS. 23 and 24 are diagrams illustrating additional depth profiling data of the XPS spectrum of the SEI layer according to the experiment on the chemical composition of the SEI layer.

FIG. 25 is a diagram illustrating XRD analysis data of the chemical composition of the SEI layer according to an addition of sulfur.

FIGS. 26 and 27 are diagrams illustrating the results of TOF-SIMS analysis according to the experiment on the chemical composition of the SEI layer. FIG. 28 is a diagram illustrating the NMR results according to electrolyte analysis and mechanism study.

FIG. 29 is a diagram illustrating the physical structure of the SEI layer according to the morphology experiment of the SEI layer after the reaction.

FIG. 30 is a diagram illustrating an energy level according to the electrolyte analysis and mechanism study.

FIG. 31 is a diagram illustrating a UV-vis absorption spectrum according to the electrolyte analysis and mechanism study.

FIG. 32 is a diagram illustrating an EIS spectrum according to the electrolyte analysis and mechanism study.

FIG. 33 is a diagram illustrating the physical structure of the SEI layer according to the morphology experiment after the reaction for an electrolyte to which sulfur is not added.

FIG. 34 is a diagram illustrating a process model of a manufacturing method of ammonia according to some exemplary embodiments of the present disclosure.

MODE FOR INVENTION

Hereinafter, an exemplary embodiment of the present disclosure is described in detail. However, examples are provided by way of example, and the present disclosure is not limited by examples, but is defined by only the scope of claims to be described below.

Throughout the present specification, when any member is referred to as being positioned “on” another member, it includes not only a case in which any member and another member are in contact with each other, but also a case in which the other member is interposed between any member and another member.

In the present disclosure, unless explicitly described to the contrary, “comprising” any components will be understood to imply the inclusion of other components rather than the exclusion of any other components.

An aspect of the present disclosure relates to an electrolyte including a lithium compound, a proton donor, and a sulfur compound.

The electrolyte includes the lithium compound, the proton donor, and the sulfur compound.

The sulfur is generally recognized as a catalyst poisoning element, but has a lithium storage effect. Specifically, the sulfur compound may increase uniformity, chemical resistance, and mechanical performance of a lithium layer by forming lithium sulfate/sulfide within a solid electrolyte interface (SEI) layer.

Without being limited by theory, since the sulfur has a low oxidation potential, a low reduction potential, and a simple chemical structure, when the sulfur is added to the electrolyte in an amount of 0.05 wt % or less, it brings about a significant change in the electrolyte, thereby changing a physicochemical structure of the SEI layer, which will be described later.

In one exemplary embodiment of the present disclosure, the sulfur compound may be included in an amount of 0.005 to 0.5 vol % with respect to the total volume of the electrolyte.

Preferably, the sulfur compound is included in an amount of 0.01 to 0.3 vol % with respect to the total volume of the electrolyte.

In another exemplary embodiment of the present disclosure, the sulfur compound may be included in an amount of 0.03 to 0.1 vol % with respect to the total volume of the electrolyte.

In another exemplary embodiment of the present disclosure, the sulfur compound may be included in an amount of 0.03 to 0.05 vol % with respect to the total volume of the electrolyte.

When the sulfur compound is included within the range, it is preferable that lithium ion conductivity and electronic insulation properties may be improved while using the sulfur compound at a minimum to form a uniform lithium layer and suppress an electrolyte decomposition, thereby significantly improving the stability of an ammonia manufacturing process.

The proton donor may be any suitable material capable of donating a proton in the electrochemical reactor.

The proton donor may be provided as an acidic, neutral or alkaline aqueous solution, and the proton donor may alternatively be provided by H₂ oxidation. In short, hydrogen may be considered as a source of protons.

Specifically, the proton donor may be ethanol.

More specifically, the proton donor may be an H⁺ donor.

The proton donor may be included in an amount of 0.05 to 5 vol %, preferably 0.5 to 2 vol %, more preferably 0.5 to 1.5 vol %, and most preferably 0.8 to 1.2 vol % with respect to the total volume of the electrolyte.

When the proton donor is included within the range, the effect of donating the proton within the reactor may be sufficiently achieved, which is preferable.

In another exemplary embodiment of the present disclosure, the lithium compound may include at least one selected from the group consisting of LiClO₄, LiBF₄, and LiTFSI.

Specifically, the lithium compound may be LiClO₄.

When the lithium compound includes LiClO₄, it is preferable because it has an economical advantage over other lithium compounds.

In another exemplary embodiment of the present disclosure, the sulfur compound may include (CH₃)₂S(DMS).

When the sulfur compound includes (CH₃)₂S, a lithium layer having a high density and a uniform shape like a thin film may be obtained, and thus, the shape of the SEI layer may be changed to a mesh-like structure. Therefore, it is preferable because the stability of a Li—NRR process may be greatly improved.

The lithium compound may be included in an amount of 0.3 to 3M, preferably 0.3 to 2M, more preferably 0.3 to 1M, and most preferably 0.4 to 0.6M with respect to the total volume of the electrolyte.

When the lithium compound is included within the range, it is preferable because the uniform lithium layer may be formed.

The electrolyte may include the balance solvent.

The solvent may be used without limitation as long as it is a solvent commonly used in the art.

For example, the solvent may include at least one selected from the group consisting of acetone, acetonitrile, ethyl alcohol, methyl alcohol, isopropyl alcohol (iPrOH), N-methylpyrrolidinone (NMP), tetrahydrofuran (THF), N,N-dimethyl-formamide (DMF), N,N-dimethylacetamide (DMA), and hexamethylphosphoramide (HMPA).

Specifically, the solvent may be tetrahydrofuran. When the solvent is tetrahydrofuran, it is preferable because it exhibits excellent solubility for the lithium compound compared to other solvents.

In another exemplary embodiment of the present disclosure, the electrolyte may be for manufacturing ammonia.

The electrolyte according to the present disclosure may be used not only in electrochemical devices such as secondary batteries, capacitors, etc., but may also be usefully used for manufacturing ammonia using a Li-mediated nitrogen reduction method, in particular.

Another aspect of the present disclosure relates to a manufacturing method of ammonia, including: filling an electrolyte including a lithium compound, a proton donor, and a sulfur compound into a reactor, and positioning a working electrode and a counter electrode to contact the electrolyte; applying a current to the working electrode and the counter electrode to form a lithium layer and an SEI layer on one surface of the working electrode; forming lithium nitride on the lithium layer by reacting with nitrogen supplied into the reactor; and obtaining ammonia by reacting the lithium nitride with the proton donor.

The electrolyte may be applied to the above-described contents.

In short, another aspect of the present disclosure relates to a manufacturing method of ammonia, including: filling the above-described electrolyte and positioning a working electrode and a counter electrode to contact the electrolyte; applying a current to the working electrode and the counter electrode to form a lithium layer and an SEI layer on one surface of the working electrode; forming lithium nitride on the lithium layer by reacting with nitrogen supplied into the reactor; and obtaining ammonia by reacting the lithium nitride with the proton donor.

A manufacturing method of ammonia according to some exemplary embodiments of the present disclosure is illustrated in FIG. 1. The manufacturing method of ammonia according to the present disclosure has the advantage of improving the manufacturing stability of ammonia and increasing the ammonia production yield by changing the physicochemical structure of the SEI layer by controlling the reaction conditions.

The manufacturing method of ammonia according to the present disclosure includes filling the electrolyte including the lithium compound, the proton donor, and the sulfur compound into the reactor, and positioning the working electrode and the counter electrode to contact the electrolyte.

The reactor may be, specifically, an electrochemical reactor (see FIG. 3).

Specifically, the manufacturing method of ammonia according to the present disclosure may use a Li-mediated nitrogen reduction reaction (Li—NRR). More specifically, the manufacturing method of ammonia according to the present disclosure may use a Li-mediated nitrogen reduction reaction with added sulfur.

The Li-mediated nitrogen reduction reaction may proceed through three steps as follows.

Among these, the most energy-intensive process is lithium deposition, which consumes most of the electrical energy. Thereafter, through a chemical reaction, a triple bond of nitrogen is broken by the strong reducing ability of lithium metal, thereby forming lithium nitride. Finally, the ammonia may be produced by accepting hydrogen through protonation.

During the lithium deposition, the SEI is formed by a decomposition reaction of a lithium source, a proton source, and an organic solvent in an electrolytic solution.

The chemical stability and mechanical strength of the SEI layer affect an additional electron consumption decomposition reaction and the stability of the SEI layer during the lithium deposition.

Therefore, in the present disclosure, sulfur is added to the SEI layer using the sulfur compound, thereby increasing the chemical stability as well as the mechanical strength, which may improve the stability of the ammonia production process may be improved and increase the ammonia production yield.

The working electrode may be an electrode commonly used in the art, such as gold (Au), molybdenum (Mo), palladium (Pd), silicon (Si), or carbon (C).

Specifically, the working electrode may be a Mo electrode.

The counter electrode may be an electrode commonly used in the art, such as platinum (Pt), titanium (Ti), aluminum (Al), or graphite.

Specifically, the counter electrode may be a Pt electrode.

The present disclosure includes applying a current to the working electrode and the counter electrode to form the lithium layer and the SEI layer on one surface of the working electrode.

The applied current may be, for example −2 mA/cm² to −8 mA/cm², preferably −3 mA/cm² to −6 mA/cm², and more preferably −3 mA/cm² to −5 mA/cm².

When the range of the applied current satisfies the range, it is preferable that the energy is minimized and the lithium layer and the SEI layer are easily formed.

In another exemplary embodiment of the present disclosure, the SEI layer may include lithium and sulfur.

Since the SEI layer according to the present disclosure includes lithium and sulfur, it may be easy to accurately control the structure and functional characteristics of the SEI layer according to the purpose, similarly to the case where oxygen or fluorine is induced in the SEI layer. In addition, since the sulfur compound may be used in a small amount compared to the case where the oxygen is induced or the fluorine is induced, it is economically preferable, and since the sulfur compound may be added in a liquid form, there is an advantage that it may be formed through a relatively simple process.

In another exemplary embodiment of the present disclosure, the SEI layer may include at least one selected from the group consisting of LiSO_x and Li₂S.

In another exemplary embodiment of the present disclosure, the SEI layer may further include at least one selected from the group consisting of Li₂O and Li₂CO₃.

Specifically, the SEI layer according to the present disclosure may include a lithium compound and a sulfur compound included in the electrolyte, or a compound including lithium and sulfur derived from the solvent such as tetrahydrofuran. For example, the compounds of LiSO_x and Li₂S may be derived from the lithium compound and the sulfur compound, and the compounds of Li₂O and Li₂CO₃ may be derived from the lithium compound and the tetrahydrofuran.

Since the SEI layer according to the present disclosure is the sulfur-derived SEI layer, the SEI layer has the advantage of improving lithium ion conductivity and electronic insulation properties, enabling uniform lithium plating, and suppressing electrolyte decomposition. As a result, it has the advantage of greatly improving the stability of the Li—NRR process, that is, the stability during the ammonia manufacturing process.

In another exemplary embodiment of the present disclosure, the SEI layer may be a porous or in mesh form.

The SEI layer according to the present disclosure has the porous or mesh form, which means that the lithium layer, specifically, the lithium particles of the lithium layer, are formed in a larger size, and a larger amount of lithium may participate in the reaction (see FIG. 34).

In another exemplary embodiment of the present disclosure, the SEI layer may include pores having an average diameter of 0.5 to 5 μm, preferably 0.5 to 3 μm, more preferably 0.5 to 2 μm, and most preferably 0.8 to 1.2 μm.

When the average diameter of the SEI layer satisfies the range, it means that the lithium particles in the lithium layer are formed large, and accordingly, the production efficiency of ammonia using the lithium particles may be excellent, which is preferable.

The manufacturing method of ammonia according to the present disclosure includes forming lithium nitride in the lithium layer by reacting with nitrogen supplied to the reactor; and obtaining ammonia by reacting the lithium nitride with the proton donor.

The method of supplying nitrogen is not particularly limited in the present disclosure.

For example, the nitrogen may be supplied by supplying nitrogen at a pressure of 10 to 30 bar, specifically 15 to 20 bar, through a nitrogen supply line including a purifier capable of removing impurities.

When the nitrogen is supplied to the reactor, the triple bond of the nitrogen is decomposed by the strong reducing ability of the lithium metal in the lithium layer, and as this reacts with the lithium metal, the lithium in the lithium layer is transformed into lithium nitride (see FIG. 1).

The modified lithium nitride reacts with the proton donor in the electrolyte to transform into ammonia, thereby obtaining the ammonia.

The manufacturing method of ammonia according to the present disclosure has an advantage of being able to increase the selectivity of ammonia production using an electrolyte including sulfur, specifically, a sulfur compound by using the excellent Li—NRR process in terms of the environment.

In addition, the electrochemical cell potential in the base electrolyte stably increases from 12 V to 30 V after operation for 10 hours, whereas the electrolyte including the sulfur compound according to the present disclosure is fixed at 10 V and is constant even for 20 consecutive hours. The consistent electrochemical cell potential may lower the power consumption of the Li—NRR process, and when the concentration of the electrolyte including the sulfur compound is optimal, the Faradaic efficiency may increase by 10% or more. Therefore, there is an advantage of being able to improve the Li—NRR process.

The following examples and comparative examples will be described in more detail. However, the following Examples are only exemplary embodiments of the present disclosure, and the present disclosure is not limited to the following Examples.

Effect of Sulfur Compound on Lithium-Mediated Ammonia Synthesis Process

Electrochemical lithium nitrogen reduction reaction (Li—NRR) was performed using pressurized N₂ gas (20 bar) under ambient conditions by using a custom-manufactured single-compartment electrochemical reactor (see FIGS. 2 and 3).

Lithium perchlorate (LiClO₄) was used as a lithium raw material, the balance tetrahydrofuran (THF) was used as an organic solvent, and ethanol was used at 0.1 vol % as a proton donor, with respect to the total volume of the electrolyte.

Secondary methyl sulfide (DMS, (CH₃)₂S) was introduced to investigate the effect of the sulfur compound-added electrolyte.

FIG. 4 is an image showing the results of cyclic voltammetry analysis based on lithium reduction potential. It is the results of cyclic voltammetry analysis of a base electrolyte (blue) (0.5 M LiClO₄ with 1% EtOH, hereinafter referred to as the “base electrolyte”) without added sulfur compound before the electrochemical reaction and a sulfur compound-added electrolyte (green) (0.5 M LiClO₄ with 1% EtOH, 0.05 vol % of dimethyl sulfide in THF, hereinafter referred to as the “sulfur-added electrolyte”). A Mo foil was used as the working electrode, a Pt foil as the counter electrode, and a Pt wire was used as a pseudo-reference electrode known as a reference electrode method in an organic solvent electrolyte to confirm an apparent potential of lithium.

FIG. 5 is an image showing the results of cyclic voltammetry analysis for a ferrocene-added electrolyte (0.5 M LiClO₄ with 1% EtOH and 0.05 vol % of dimethyl sulfide in THF) (red) and an electrolyte (black) without added ferrocene to confirm the apparent potential of lithium. The Mo foil was used as the working electrode, the Pt foil was used as the counter electrode, and the Pt wire was used as the pseudo-reference electrode to confirm reversible Fc/Fc⁺.

Table 1 below shows pH values of the base electrolyte (0.5 M LiClO₄ with 1% EtOH) and the sulfur-added electrolyte (0.5 M LiClO₄ with 1% EtOH and 0.05 vol % of dimethyl sulfide in THF).

	TABLE 1
	Electrolyte	Base electrolyte	S-added electrolyte
	pH value	3.92	3.56

In order to ensure the validity of the ammonia production through the electrochemical nitrogen reduction, caution was needed due to potential positive errors. To ensure the accuracy, a series of control experiments were first conducted to verify the amount of ammonia manufactured. The quantitative characteristics were cross-verified using indophenol titration, ion chromatography (IC), and nuclear magnetic resonance (NMR) analysis techniques.

The results are shown in FIGS. 6 to 8 using a constant current density of 4 mA/cm², Mo as the working electrode, Pt as the counter/reference electrode, and sulfur-added electrolyte consisting of 0.5 M LiClO₄ in THF, 1 vol % of ethanol, and 0.05 vol % of DMS in THF.

Specifically, FIG. 6 is a diagram illustrating the standard addition amount using the colorimetric method calibration curve for NH₄⁺—N. FIG. 7 is a diagram illustrating the ion chromatography calibration curve for NH₄⁺—N,

and FIG. 8 is a diagram illustrating the NMR calibration curve for NH₄⁺—N.

Referring to FIGS. 6 to 8, the ammonia concentration and the corresponding Faradaic efficiency showed values within the experimental error range within the range of IB (40.2%), IC (45.9%), and NMR (43.1%).

The NMR analysis (see FIG. 9) shows the progress of the ammonia production during different reaction periods under the same conditions of mutual verification. The intensity of a triplet peak associated with ¹⁴NH₄⁺, which is associated with a triplet peak of a characteristic 53 Hz spacing, increased in proportion to a reaction time from 1 to 12 h. To confirm the source of nitrogen, isotope experiments were performed using a mixture of ¹⁴N₂ and ¹⁵N₂ isotope gases. When reacting 2:1 and 4:1 (¹⁴N₂: ¹⁵N₂) mixture gases, a separate doublet peak with a 73 Hz spacing corresponding to ¹⁵NH₄⁺ was clearly observed at the corresponding area ratio. In addition, it was confirmed that no ammonia was produced in the Ar environment without N₂ supply (see FIG. 10). Referring to FIG. 11, it may be clearly confirmed that ammonia is produced from the supplied nitrogen.

To investigate the effect of sulfur addition, the volume percentage of DMS in the base electrolyte solution including 0.5 M LiClO₄ and 1 vol % of ethanol was varied. The Faradaic efficiency (F.E.) was evaluated by a method of monitoring a voltage profile under constant current conditions using the chrono-potentiometry technique, and the results were shown in FIG. 12.

Each reaction was maintained at a constant current density of −4 mA/cm² for 1 h, and the charge transferred and the corresponding ammonia production during this period were used to calculate the ammonia conversion efficiency, the Faradaic efficiency. Looking at the contents in detail, the product of current and time, which represents the total charge, was used in a denominator, and the number of moles of ammonia produced and the Faraday constant were used in a numerator.

Referring to FIG. 12, the F.E. of about 36±4.0% and the NH₃ production rate of 305±34 μg cm⁻²·h⁻¹ were shown without DMS using the base electrolyte.

As the DMS amount added increases, the F.E. reached a peak of about 46±6% and the NH₃ production rate of 390±50 μg cm⁻²·h⁻¹ at 0.05 addition amount (see FIG. 12). The subsequent increase in DMS concentration caused a gradual decrease in the F.E., which represents the optimal DMS dosage for improving the ammonia selectivity.

The greater effect of sulfur addition was shown in the improved stability of the cell potential. FIGS. 13 and 14 show the potential profiles of the working electrode (W.E.) and counter electrode (C.E.). When the base electrolyte was used, a continuous and significant increase in the cell potential occurred during the operation time during the Li—NRR, while the introduction of sulfur maintained a constant and stable cell potential of 10 V. The F.E. values remained similar for both electrolytes.

Specifically, FIG. 15 is a diagram illustrating the potential profile of the working electrode with a high current density of 6 mA/cm² (b) 8 mA/cm² in the base electrolyte (0.5 M LiClO₄ with 1% of EtOH in THF) under 20 bar N₂.

These results highlight the significant improvement in the stability of the Li—NRR process achieved by adding DMS, which means the reduced energy consumption and the possibility of ammonia production. Maintaining the stable cell potential directly leads to the reduction in the power consumption of the ammonia production using the Li—NRR reaction.

The fundamental reason for this positive effect of the DMS using the Li—NRR is inferred from the significant changes in the physicochemical properties of the SEI.

Influence of Sulfur Addition: Morphology of SEI Layer

Scanning electron microscopy (SEM) analysis was used to investigate the physical properties of the SEI layer. In this case, for the preparation of the SEM samples of the SEI layer, the electrode surface was washed several times with tetrahydrofuran (THF) to remove solvent-soluble components such as lithium metal and lithium salts.

FIG. 16 is a comparison diagram of SEI layers obtained from the base electrolyte and the sulfur-added electrolyte. a of FIG. 16 illustrates a dense film-like structure of the SEI formed after the reaction for 1 hour from the base electrolyte. Since the SEI layer originates from the lithium coating, this film-like SEI structure has fine particle-sized, well-dispersed Li-coating, which are highlighted. The low-magnification SEM image in b of FIG. 16 shows that the Mo working electrode is exposed between the SEI layers, which was confirmed by the energy-dispersive X-ray spectroscopy (EDS) results of Mo element (see c of FIG. 16). This suggests the non-uniform lithium coating during the coating process.

It may be seen that the shape of the SEI changes significantly in the presence of the sulfur-added electrolyte. d of FIG. 16 illustrates a mesh-like SEI shape produced from the sulfur-added electrolyte, and illustrates large-sized Li precipitates. The low-magnification SEM image (see e of FIG. 16) confirms a mesh-like porous SEI layer formed uniformly over the electrode surface. The uniform EDS signal corresponding to Mo element is due to the uniformly distributed porous SEI layer. This result strongly suggests that the sulfur addition was optimally controlled to improve the uniformity of the lithium coating.

In addition, when analyzing the physical structure of the SEI layer after the reaction for 10 hours, a denser and vertically aligned film-like SEI layer was observed for the base electrolyte (see FIG. 33). In contrast, the SEI layer continued to maintain the mesh-like shape even after the reaction for 10 hours in the case of the sulfur-added electrolyte (see FIG. 29). This observation emphasizes the consistent effect of the addition of the dimethyl sulfide (DMS) to promote the particle growth during the lithium deposition, thereby improving the physical stability of the SEI layer.

Influence of Sulfur Addition: Chemical Composition of SEI Layer

The SEI layer is formed at the electrode interface during the lithium plating through the decomposition reaction of the electrolyte, and its chemical structure is greatly influenced by the composition of the electrolyte. The X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition of the SEI layer. The samples were analyzed in an Ar environment in a glove box to avoid air exposure.

FIG. 17 is a diagram illustrating the potential profile according to the adjustment of a sulfur-added amount. FIGS. 18 to 22 are data comparing the XPS spectrum between the base electrolyte and the SEI derived from the sulfur-added electrolyte. Additional depth profiling XPS data of each sample are shown in FIGS. 23 and 24.

In the case of the SEI layer produced through the base electrolyte, the Li 1s spectrum (FIG. 18) has a relatively broad peak centered around 55.7 eV, which indicates the presence of multiple Li compound bonding states. Among these states, LiClOn was mainly identified around −56.5 eV, which was confirmed through the O 1 s spectrum (−533.2 eV, FIG. 3B) and the Cl 2 p spectrum (−202 eV, FIG. 3C). In addition, Li₂CO₃ and Li₂O were detected and assigned through the Li 1 s, O 1 s, and C 1 s spectra. The X-ray diffraction (XRD) analysis of the base electrolyte sample confirmed the presence of Li₂CO₃(²⁰²) and Li₂CO₃(002) phases and Li₂CO₃ 020 and Li₂CO₃(³¹³) phases. In the reaction with the sulfur-added electrolytes, the Li 1 s spectrum is shifted to lower binding energy, which indicates the formation of more reduced Li compounds within the SEI. In addition, the presence of Li₂SO₄ and Li₂S was also confirmed by the S 2 p spectrum (FIG. 21).

The sulfur addition also significantly reduced a C—Cl peak of Cl 2 p (FIG. 20) and a C═O peak of C 1 s (FIG. 22). These peaks are known to be associated with electrolyte decomposition products and may lead to the system instability, especially when LiClO₄ is used as the lithium source. This finding highlights how the sulfur addition further reduces the electrolyte decomposition and contributes to the system stabilization.

FIG. 25 is a diagram illustrating XRD analysis data of the chemical composition of the SEI layer according to an addition of sulfur.

FIGS. 26 and 27 are diagram illustrating 3D mapping and depth profiles of compounds included in the sulfur-derived SEI layer using time-of-flight secondary ion mass spectrometry (TOF-SIMS).

The 3D mapping confirms the presence of Li₂SO₄⁻, Li₂S⁻, Li₂O⁻, and Li₂CO₃⁻ within the SEI layer, which is consistent with the XPS and XRD results. The depth profile of the TOF-SIMS (see FIG. 27) maintains consistent signal intensity throughout the depth along with carbon-related compounds, C⁻ and Li₂CO3⁻, while the sulfur signal becomes significantly weaker as the depth increases. This observation suggests that the effect of sulfur is particularly prominent in the surface region.

Electrolyte Analysis and Mechanism Study

The results of investigating the experimented SEI layer show that the addition of a small amount of dimethyl sulfide (DMS) to the electrolyte significantly changes the physical and chemical properties of the SEI layer. Therefore, it may be seen that this has a significant effect on the Li—NRR process. In order to understand the mechanism of this effect, the electrolyte analysis was first performed.

FIG. 28 shows the nuclear magnetic resonance (NMR) results for the base electrolyte and sulfur-added electrolyte. In both cases, the main peak corresponding to the tetrahydrofuran (THF) appears in the range of 1.6 to 2.1 ppm and the main peak corresponding to the ethanol at 1.18 ppm were shown. In the reaction for 1 hour after introducing the DMS, a new peak appears at 2.73 ppm, which indicates the presence of dimethyl sulfoxide (DMSO). This indicates that the DMS is converted into the DMSO. The lithium perchlorate (LiClO₄), known as a strong oxidizing agent, mainly induces the oxidation of the THF. This oxidation ability of LiClO₄ promotes the oxidation of the DMS to the DMSO.

To proceed further, density functional theory (DFT) calculations were performed to determine energy levels of a lowest unoccupied molecular orbital (LUMO) and a highest occupied molecular orbital (HOMO) of the DMSO, the DMS, and the THF (see FIG. 30). The calculation results illustrated in FIG. 30 show that DMSO has the lowest LUMO energy compared to other molecules. That is, among those, the DMSO is preferentially decomposed compared to other molecules to introduce an Li—SO₄ form into the SEI. This is consistent with the XPS and TOF-SIMS analysis results illustrated in FIGS. 18 to 27.

Lithium polyanion compounds such as Li₂SO₄ and Li₂CO₃ tend to decompose into Li₂S, Li₂O, and LiCx when reacting with the residual lithium metal on the electrode (see Reaction Schemes 2 and 3 below).

The previous analysis confirmed that the decomposition products are present in the SEI layer of the Li—NRR. Among those, Li₂S and Li₂O had wider band gap energies than LiC_x, which resulted in excellent electronic insulating properties. This characteristic of the electronically insulating SEI layer plays an important role in reducing the electron consumption reaction such as the solvent decomposition. Therefore, the SEI layer formed from the sulfur compound may effectively reduce the electrolyte decomposition during the Li—NRR process.

To further demonstrate that the SEI layer derived from the sulfur compound is effective in reducing the electrolyte decomposition during the Li—NRR process, it was confirmed through the UV-Vis absorption spectrum illustrated in FIG. 31. A remarkable difference is observed when comparing the base and sulfur-added electrolytes before and after 12 hours of Li—NRR process. The base electrolyte showed higher absorption throughout the spectrum than sulfur-added electrolyte, as can be seen through the spectrum, which appears to be a darker color after the reaction. This observation emphasizes the protective function of the sulfur-added SEI layer in reducing the electrolyte decomposition during the Li—NRR.

Another important property of the SEI layer in the Li—NRR is related to the lithium ion conductivity. An electrodiffusion spectrum (EIS) is used to evaluate the effect of sulfur addition on lithium ion transport. FIG. 32 is a diagram illustrating the EIS spectrum obtained from the base electrolyte and the sulfur added-electrolyte sample. A circuit model is proposed to describe the Li—NRR, which is attributed to two semicircles related to the resistance of the SEI and the charge-transfer resistance at the SEI/electrolyte interface. The resistance of the calculated bulk SEI layer was analyzed, showing that the base electrolyte exhibited a higher resistance (157.8 Ω compared to 205 Ω) than the sulfur-added electrolyte (see FIG. 32, Table 2).

	TABLE 2
			Base	S-added
	Unit	Resistance	electrolyte	electrolyte
Rs	Ω	Resistance of bulk	57	30.9
		electrolyte
RSEI		Resistance of bulk	205	157.8
		SEI layer
RSEI-2		Resistance of grain	117	68.6
		boundaries in the SEI
Rct		Resistance of charge-	101.9	38.7
		transfer at the SEI/
		electrolyte interface

This difference indicates that the sulfur addition enhances the Li⁺ ion conductivity through the SEI layer. This trend is consistent with previous studies illustrating that Li₂S has superior ion conductivity compared to Li₂O and Li₂CO₃ (see Table 3 below).

	TABLE 3
	SIE	Ionic	Young's
	components	conductivity(S cm−2)	modulus(GPa)
	Li2S	10−5	78.2-82.6
	Li2O	10−9-10−11	161.3-170.3
	Li2CO3	6.7 × 10−8	36.2-75.0

In addition, the sulfur-derived SEI layer is expected to have improved mechanical stability compared to Li₂CO₃ (36.2 to 75.0 GPa), which has high Young's modulus of Li₂S (78.2 to 82.6 GPa) and Li₂O (161.3 to 170.3 GPa). This indicates that the sulfur addition improves the mechanical toughness of SEI, thereby improving its stability.

Based on this comprehensive analysis, the manufacturing method of ammonia according to the present disclosure, namely, the Li—NRR process model based on sulfur addition, is illustrated in FIG. 34.

The left image of FIG. 34 shows the formation of the SEI composed of LiCO₃ and LiClOn due to the electrolyte decomposition during the lithium plating in the base electrolyte reaction. The lithium plating of small particles proceeds through the SEI layer with low Li+ ion conductivity, so the formation of the membrane-type SEI layer is induced through particle aggregation. This non-uniform lithium plating exposes a specific electrode surface. In addition, due to the low electronic insulation properties of the SEI layer, additional electrolyte decomposition proceeds, which reduces the overall process stability. Referring to the right image of FIG. 34, when the DMS is added to the electrolyte, the conversion into the DMSO is promoted under the strong oxidation ability of LiClO₄, which changes the physicochemical properties of the SEI. The preferential decomposition of the DMSO causes Li₂SO₄ and Li₂S to be integrated into the SEI layer, which increases the lithium ion conductivity and improves the electronic insulation properties. As a result, the lithium particle size increases and the uniformity are improved, and the additional electrolyte decomposition is suppressed, thereby increasing the process stability, improving the reaction selectivity (Faraday efficiency), and reducing the power consumption required for the ammonia production.

The present disclosure is not limited to the exemplary embodiments, but may be manufactured in a variety of different forms, and the present disclosure may be manufactured in a variety of different forms, and those of ordinary skill in the art to which the present disclosure pertains will understand that the present disclosure may be implemented in other specific forms without changing the technical spirit or essential features of the present disclosure. Therefore, it is to be understood that the exemplary embodiments described above are illustrative rather than being restrictive in all aspects.

Claims

1. An electrolyte, comprising: a lithium compound, a proton donor, and a sulfur compound.

2. The electrolyte of claim 1, wherein: the sulfur compound is included in an amount of 0.005 to 0.5 vol % with respect to a total volume of the electrolyte.

3. The electrolyte of claim 2, wherein: the sulfur compound is included in an amount of 0.03 to 0.05 vol % with respect to the total volume of the electrolyte.

4. The electrolyte of claim 1, wherein: the proton donor is included in an amount of 0.05 to 5 vol % with respect to the total volume of the electrolyte.

5. The electrolyte of claim 1, wherein: the lithium compound includes at least one selected from the group consisting of LiClO4, LiBF4, and LiTFSI.

6. The electrolyte of claim 1, wherein: the sulfur compound includes (CH3)2S.

7. The electrolyte of claim 1, wherein: the electrolyte is for manufacturing ammonia.

8. A manufacturing method of ammonia, comprising: filling an electrolyte including a lithium compound, a proton donor, and a sulfur compound into a reactor, and positioning a working electrode and a counter electrode to contact the electrolyte;applying a current to the working electrode and the counter electrode to form a lithium layer and a solid electrolyte interface (SEI) layer on one surface of the working electrode;forming lithium nitride on the lithium layer by reacting with nitrogen supplied into the reactor; andobtaining ammonia by reacting the lithium nitride with the proton donor.

9. The manufacturing method of claim 8, wherein: the sulfur compound is included in an amount of 0.005 to 0.5 vol % with respect to a total volume of the electrolyte.

10. The manufacturing method of claim 9, wherein: the sulfur compound is included in an amount of 0.03 to 0.05 vol % with respect to a total volume of the electrolyte.

11. The manufacturing method of claim 8, wherein: the SEI layer includes lithium and sulfur.

12. The manufacturing method of claim 8, wherein: the SEI layer includes at least one selected from the group consisting of LiSOx and Li2S.

13. The manufacturing method of claim 12, wherein: the SEI layer further includes at least one selected from the group consisting of Li2O and Li2CO3.

14. The manufacturing method of claim 8, wherein: the lithium compound includes at least one selected from the group consisting of LiClO4, LiBF4, and LiTFSI.

15. The manufacturing method of claim 8, wherein: the sulfur compound includes (CH3)2S.

16. The manufacturing method of claim 8, wherein: the SEI layer has a porous or mesh shape.

17. The manufacturing method of claim 16, wherein: the SEI layer includes a pore having an average diameter of 0.5 to 5 μm.