KR20240023997A - Electrochemical Analysis Method of Recognizing the Interfacial Layer with depleted water on Positively Electrified Electrode in water-in-salt electrolyte solutions - Google Patents

Abstract

An electrochemical analysis method that recognizes the formation of a hydrophobic interface layer of a positive potential electrode using chlorine ions in a battery system containing a salt-in-water electrolyte and an electrode, wherein the salt-in-water electrolyte includes an aqueous solvent and a salt, and the weight of the aqueous solvent The ratio of the weight of salt to is more than 1, and the above analysis method involves adding a solution containing chlorine ions to a water-in-salt electrolyte, and when two types of redox reactions related to chlorine ions are confirmed under a voltage applied, it is applied to a positive potential electrode. It relates to an electrochemical analysis method that recognizes the hydrophobic interface layer of a positive potential electrode in a salt-in-water electrolyte solution, which determines that a hydrophobic interface layer has been formed.

Description

Electrochemical Analysis Method of Recognizing the Interfacial Layer with depleted water on Positively Electrified Electrode in water-in-salt electrolyte solutions}

This relates to an electrochemical analysis method that recognizes the water-excluding interfacial layer of the positive potential electrode in a salt-in-water electrolyte.

Water-in-Salt Electrolyte (WiSE) is an aqueous medium containing highly concentrated electrolytes and is considered a promising electrolyte for various battery systems. In order to increase the kinetic overpotential of the water oxidation reaction in a salt-in-water electrolyte system, it is important for a hydrophobic interfacial layer (IFL) to be formed on the surface of a positively electrified electrode. In other words, the presence or absence of a hydrophobic interfacial layer formed at the interface between the positive potential electrode and the electrolyte solution can be said to be a critical factor in the operation of the salt-in-water electrolyte system. Accordingly, it can be said that it is an important task to recognize the nature of the interfacial layer formed at the interface between the positive potential electrode and the electrolyte solution.

The interface layer (IFL) is a double layer formed by arranging ions, etc. on an electrode to which a voltage is applied, and is a layer that disappears when the potential difference disappears. This is a distinct concept from the previously studied SEI (Solid Electrolyte Interphase). SEI is an ionic conductive and electronic insulating layer formed by decomposition (reduction) of the electrolyte during the initial charging and discharging process of the battery. It is a type of film that remains even when the potential difference disappears, that is, even when the voltage applied to the electrode disappears. Although many analyzes have been conducted on stable layers such as SEI, the interfacial layer exists only under voltage-applied conditions and its thickness is expected to be less than 1 nm, so there are limits to its practical detection, and existing studies are also based on theoretical methods. Access is limited. In Non-Patent Document 1, an attempt was made to analyze the interfacial layer through atomic force microscopy (AFM) and surface-enhanced infrared absorption spectroscopy, but these methods are difficult to analyze and intuitively confirm, and are limited to ultra-small electrodes. There are limitations to the method.

[Non-patent Document 1] CS Appl. Energy Mater. 2020, 3, 8, 8086-8094

A simple, intuitive, and economical electrochemical analysis method is provided as a method to identify the water-excluding hydrophobic interface layer of the positive potential electrode in a salt-in-water electrolyte system.

In one embodiment, it is an electrochemical analysis method that recognizes whether a hydrophobic interface layer is formed on a positive potential electrode using chloride ions in a battery system including a salt-in-water electrolyte and an electrode, wherein the salt-in-water electrolyte is an aqueous solvent and salt. and the ratio of the weight of salt to the weight of the aqueous solvent is 1 or more. The analysis method includes adding a solution containing chlorine ions to a water-in-salt electrolyte, and performing two types of redox associated with chlorine ions under voltage. An electrochemical analysis method is provided to recognize the hydrophobic interface layer of the positive potential electrode in the salt-in-water electrolyte solution, which determines that the hydrophobic interface layer of the positive potential electrode has been formed when the reaction is confirmed.

The formation of a hydrophobic interface layer excluding water on the positive potential electrode is an important factor in determining the stable operation of the salt-in-water electrolyte battery system. Therefore, it can be said that it is an essential task to recognize the nature of the interfacial layer formed in the manufacturing and research process of the battery system, especially in relation to water. The analysis method according to one embodiment is a method that can recognize the nature of the interface layer formed on the positive potential electrode in the salt-in-water electrolyte battery system. Specifically, water is excluded from the interface between the positive potential electrode and the salt-in-water electrolyte solution. It is an electrochemical analysis method that can determine whether a hydrophobic interface layer has been formed. This analysis method is a very simple and intuitive method and can be used in various battery systems using salt-in-water electrolyte.

Figure 1 is a graph of Cyclic Voltammetry (CV) analysis in LiTFSI electrolyte solutions at concentrations of 0.5 m and 12 m for Example 1 (GC electrode) and Reference Example 1 (Pt electrode).
Figure 2 is a CV graph analyzed after adding 50 mM HCl to electrolyte solutions of various molar concentrations prepared in Example 1 (GC electrode) and Reference Example 1 (Pt electrode).
Figure 3 is a diagram schematically showing the difference in the redox reaction of chlorine ions in the interface layer depending on the potential difference (0V on the left and 2V on the right), and the graph at the top right of each figure represents the peak related to chlorine ions. This is a CV graph.
Figure 4 shows the linear sweep potential associated with the Oxygen Evolution Reaction (OER) in LiTFSI electrolyte at 0.5 m and 12 m concentrations for Example 1 (GC electrode) and Reference Example 1 (Pt electrode). Voltammetry (LSV) analysis graph.
Figure 5 is a CV graph analyzed after adding HBr to the 12 m salt-in-water electrolyte solution of Example 1 (GC electrode) and Reference Example 1 (Pt electrode).
Figure 6 is a CV graph analyzed after adding HI to the 12 m salt-in-water electrolyte solution of Example 1 (GC electrode) and Reference Example 1 (Pt electrode).
Figure 7 shows X-ray photoelectron spectroscopy (XPS) analysis after electrochemical treatment of the GC electrode of Example 1 in a 12 m salt-in-water electrolyte containing halides (Cl ^- , Br ^- , and I ^- ). ) is the result.
^Figure 8 is a graph comparing the peak intensities _of F, N, _S , X ^- ,

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention may be implemented in many different forms and is not limited to the embodiments described herein.

Hereinafter, “a combination thereof” means a mixture of constituents, a laminate, a composite, a copolymer, an alloy, a blend, a reaction product, etc.

Here, terms such as “comprise,” “comprise,” or “have” are understood to indicate the presence of implemented features, numbers, steps, components, or a combination thereof, and one or more other features or It should be understood that this does not exclude in advance the possibility of the presence or addition of numbers, steps, components, or combinations thereof.

In the drawing, the thickness is enlarged to clearly express various layers and areas. When a part of a layer, membrane, region, plate, etc. is said to be “on” or “on” another part, this includes not only cases where it is “directly above” another part, but also cases where there is another part in between. Conversely, when a part is said to be “right on top” of another part, it means that there is no other part in between. Additionally, “layer” includes not only the shape formed on the entire surface when observed in plan view, but also the shape formed on some surfaces.

Here, “or” is not interpreted in an exclusive sense; for example, “A or B” is interpreted as including A, B, A+B, etc.

In one embodiment, an electrochemical analysis method is provided to recognize whether a hydrophobic interface layer of a positive potential electrode is formed using chlorine ions in a battery system including a salt-in-water electrolyte and an electrode.

Here, the battery system is a concept that encompasses electrochemical devices, and can be interpreted as a concept that includes secondary batteries such as lithium secondary batteries and lead acid batteries, redox flow batteries, fuel cells, capacitors, and electrolysis devices.

The water-in-salt electrolyte solution contains an aqueous solvent and a salt and can be said to be a high-concentration aqueous electrolyte. Specifically, it means that the ratio of the weight of the salt to the weight of the aqueous solvent is 1 or more. In the water-in-salt electrolyte solution, the ratio of the weight of salt to the weight of the aqueous solvent may be 1 or more and 30 or less, or 1 or more and 20 or less. In a salt-in-water electrolyte, the relative number of water molecules decreases and the number of ions increases, highlighting the interaction between positive and negative ions, increasing the energy barrier for the redox reaction of water, and expanding the electrochemical stability window. This salt-in-water electrolyte is considered to be an electrolyte that can overcome the limitations of existing aqueous electrolytes.

The aqueous solvent may include water, an alcohol-based solvent, or a combination thereof, and may, for example, mean a solvent containing 80% by volume or more of water. The salt is a substance that dissolves in a solvent and acts as a source of ions and promotes the movement of ions between the anode and the cathode, and can be said to be a substance that enables the operation of the battery system. The salt can be applied without limitation as long as it can form a water-in-salt electrolyte. For example, it may be an alkali metal salt such as sodium salt or lithium salt, and an example is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). ), lithium trifluoromethanesulfonate (LiOTf), or a combination thereof. The molal concentration of the water-in-salt electrolyte may vary depending on the type of salt applied, and may be, for example, 5 m to 50 m, 8 m to 40 m, or 10 m to 30 m.

Hereinafter, the interfacial layer (IFL) refers to a layer formed on the surface of a positive potential electrode, that is, at the interface between a positive potential electrode and an electrolyte, and should be understood as a different concept from SEI, which has been reported to be formed on the surface of a conventional negative potential electrode. The positively electrified electrode may be expressed as a positively charged electrode, and may be an anode or a cathode, for example. The positive potential electrode may be, for example, an electrode to which a potential of +1 V or more is applied, and may specifically be an electrode to which a potential of 1 V to 5 V is applied.

The interfacial layer is a layer that exists only under voltage conditions and disappears when the potential difference disappears. Its thickness is very thin, at the level of 10 nm or less or 1 nm, so it was almost impossible to capture it, so it is difficult to know what properties the interface layer has. There was a limitation in that complex analysis methods had to be used to recognize or determine whether a hydrophobic interfacial layer excluding water had been formed. On the other hand, the analysis method according to one embodiment can be said to be a method that can very easily and intuitively determine whether a hydrophobic interface layer has been formed using chlorine ions. According to one embodiment, a hydrophobic interfacial layer recognition method involves adding a solution containing chlorine ions to a battery system including a salt-in-water electrolyte and an electrode, and confirming two types of redox reactions related to chlorine ions when a voltage is applied. This can be said to be a method of determining that a hydrophobic interface layer has been formed on the positive potential electrode.

Here, the two types of redox reactions related to chlorine ions can be specifically expressed as Scheme 1 and Scheme 2 below.

[Scheme 1]

Cl ₃ ^- + 2e ^- ⇔ 3Cl ^-

[Scheme 2]

3Cl ₂ + 2e ^- ⇔ 2Cl ₃ ^-

The electrochemical analysis method for confirming the above two types of redox reactions may, for example, use Cyclic Voltammetry (CV) or Chronoamperometry, but is not limited thereto. For example, if two types of redox peaks appear in the CV analysis graph, it can be determined that a hydrophobic interfacial layer excluding water has been formed. Additionally, if a section in which the current density value is maintained as the voltage increases appears twice in the chronoamperometric graph, it can be seen that the above two types of redox reactions have occurred, and accordingly, it can be determined that a hydrophobic interface layer has been formed.

In general, chlorine ions in organic solvents undergo two types of redox reactions shown in Reaction Formula 1 and Reaction Formula 2 (Cl ^- ↔ Cl ₃ ^- ↔ Cl ₂ ). On the other hand, since Cl ₃ ^- is not stabilized in an aqueous solution, only one type of redox reaction occurs (Cl ^- ↔ Cl ₂ ), and since the water-in-salt electrolyte is also an aqueous solution, it is confirmed that only one type of redox reaction occurs. However, when a specific electrode is applied to a salt-in-water electrolyte system, the detection of two types of redox reactions of chlorine ions under voltage is interpreted to mean that chlorine ions are associated with a hydrophobic interface layer, and accordingly, water It can be judged that an excluded hydrophobic interface layer has been formed. This will be explained in detail in the examples below to prove this.

For example, if a hydrophobic interface layer excluding water is not formed, Cl ₃ ^- ions cannot be stabilized in an aqueous salt-in-water electrolyte, and therefore only one type of redox reaction of Cl ^- ↔ Cl ₂ occurs, and thus Accordingly, only one type of redox peak appears in the CV graph, and only one section in which the current density is maintained as the voltage rises appears in the chronoamperometric graph. On the other hand, if a hydrophobic interfacial layer excluding water is formed at the interface between the positive potential electrode and the salt-in-water electrolyte, Cl ₃ ^- ions can be stabilized by the hydrophobic interfacial layer, and thus the two types of Reaction Formula 1 and Reaction Formula 2 It is understood that a redox reaction occurs. In this case, two types of redox peaks appear in the CV graph, and it is confirmed that two current density maintenance sections appear in the chronoamperometry graph. The analysis method according to one embodiment uses this principle to simply and accurately determine whether a hydrophobic interface layer has been formed on the positive potential electrode by observing the redox trend after adding chlorine ions to the salt-in-water electrolyte system. It can be said to be an electrochemical analysis method.

The formation of a hydrophobic interface layer on a positive potential electrode can be an indicator of the performance of an aqueous battery, and the analysis method according to one embodiment is a simple method of checking the presence or absence of a hydrophobic interface layer and can be used in various ways in aqueous battery systems. .

As described above, the interface layer is formed under voltage, so for example, the analysis method may be performed with a potential of +1V or more applied, for example, 1V to 5V, or 1V. It may be analyzed in a voltage range of 3V to 3V.

The interface layer may be said to be a double layer formed by arranging the anions and cations of the salt. As the voltage applied to the electrode increases, the degree of alignment of the ions increases, forming an interface layer. As a result, water molecules withdraw from the interface, and water can be expressed as being depleted or excluded from the interface layer. The strong interaction between the interface layer and chlorine ions can also be proven by XPS, and this will be explained in detail in the examples below. In the analysis method according to one embodiment, when two types of redox reactions related to chlorine ions are confirmed, it means that a hydrophobic interface layer is formed, and in this case, the interface layer contains anions and cations of salt and chlorine ions. can do.

The thickness of the interfacial layer may be 0.1 nm to 10 nm, for example, 0.1 nm to 5 nm, 0.1 nm to 3 nm, or 0.1 nm to 1 nm.

The solution containing the chlorine ion may be, for example, an aqueous solution containing a chlorine salt, such as an aqueous solution containing HCl, LiCl, NaCl, KCl, MgCl ₂ , CaCl ₂ , or a combination thereof. The solution containing chlorine ions may further contain HClO ₄ or the like to adjust pH. Additionally, the molar concentration of the solution containing the chlorine ion is not particularly limited, but may be, for example, 10 mM to 100 mM, 20 mM to 80 mM, or 30 mM to 70 mM.

Hereinafter, examples, comparative examples, and evaluation examples of the present invention will be described. The following examples are only examples of the present invention, and the present invention is not limited to the following examples.

1. CV evaluation for Pt electrode and GC electrode

[Example 1]

A glassy carbon (GC) macrodist electrode with a diameter of 3 mm is prepared as a working electrode, Ag/AgCl (1M KCl) is prepared as a reference electrode, and a Pt wire is prepared as a counter electrode. Prepare aqueous electrolyte solutions of various concentrations by adding LiTFSI to the distilled water solvent to have molar concentrations of 0.5 m, 6 m, 12 m, and 18 m. Here, the electrolyte solution having a molal concentration of 5 m or more corresponds to the water-in-salt electrolyte solution. Prepare the battery by putting the three electrodes and electrolyte into the battery case.

[Example 2]

A battery was prepared in the same manner as Example 1, except that LiOTf was used instead of LiTFSI.

[Reference Example 1]

A battery was prepared in the same manner as Example 1, except that a Pt macrodist electrode with a diameter of 2 mm was used as the working electrode.

[Evaluation Example 1]

Batteries using 0.5 m and 12 m LiTFSI electrolytes were prepared in Example 1 and Reference Example 1, respectively, and cyclic voltammetry (CV) analysis was performed on them, and the results are shown in FIG. 1. Referring to Figure 1, for the GC electrode of Example 1 at a low molality of 0.5 m, the anodic current density ( j ) associated with the oxygen evolution reaction (OER) increased rapidly at 1.85 V, which was significantly higher than that of Pt in Reference Example 1. It is 0.41 V higher than that of the electrode, which is due to the higher kinetic barrier to OER at the GC electrode. At 12 m, which corresponds to the salt-in-water electrolyte, the anodic overpotential of OER increased compared to 0.5 m in both Example 1 and Reference Example 1, which is the same result as previously known.

2. Analysis of redox reaction of chlorine ions in salt-in-water electrolyte system

Each battery was prepared by adding 50 mM HCl to the electrolyte solutions of various molal concentrations prepared in Example 1 and Reference Example 1, and then CV analysis was performed, and the results are shown in FIG. 2. CV analysis was performed using DigiElch Professional v6.F software (ElchSoft.com). Here, instead of 50mM HCl, it is also possible to use a mixture of 25mM HCl and 25mM HClO ₄ or a mixture of 50mM HCl and 25mM LiCl. The left side of Figure 2 shows the analysis results for the Pt electrode of Reference Example 1, in which only two voltage-current peaks corresponding to one redox were observed from 0.5 m to 18 m. This corresponds to the electro-oxidation reaction of Cl ^- to Cl ₂ and the electro-reduction reaction of Cl ₂ to Cl ^- , respectively. The right side of Figure 2 shows the analysis results for the GC electrode of Example 1. At 0.5 m, only an increase graph due to OER was seen, but at 6 m, a new peak appeared at 1.72V, and in the salt-in-water electrolyte system at 12 m and 18 m, two types of redox peaks appeared in the forward scan. This corresponds to two types of redox reactions in which Cl ^- is oxidized to Cl ₃ ^- (Scheme 1) and subsequently oxidized to Cl ₂ (Scheme 2). This result is in contrast to the fact that only one redox peak (Cl ^- /Cl ₂ ) was obtained from the Pt electrode.

In the salt-in-water electrolyte system, unlike the Pt electrode, the GC electrode contains Cl ^- strongly in the hydrophobic interface layer, and Cl ₃ ^- is found to be thermodynamically stabilized within the hydrophobic interface layer. Accordingly, it can be confirmed that the interface layer of the Pt electrode is not hydrophobic, that is, that a hydrophobic interface layer is not formed in the Pt electrode. In contrast, the interface layer of the GC electrode is hydrophobic, that is, the GC electrode has It can be experimentally confirmed that a hydrophobic interface layer that excludes water has been formed. Accordingly, by using chlorine ions in the salt-in-water electrolyte system, it can be confirmed whether a hydrophobic interfacial layer has been formed on the positive potential electrode.

3. Voltage-driven Cl representing the interfacial layer in a salt-in-water electrolyte system. ^- /Cl ₃ ^- /Cl ₂ redox reaction

According to existing research, the interfacial layer will contain Li ⁺ , TFSI ^- and water molecules, and for example, it is thought that a layered [Li(H ₂ O) _x ] ⁺ -[TFSI] ^- network will be formed. When a positive potential of +2 V is applied, TFSI ^- will first be adsorbed to the electrode surface, and then Li ⁺ will exist above the anions. Unlike these ions, water molecules will withdraw from the LiTFSI-induced interfacial layer. And as the potential increases, the cation-anion network in the interface layer will become more aligned, and as the degree of alignment increases, the negatively charged Cl (Cl ^- , Cl ₃ ^- ) interacts more strongly with [Li(H ₂ O) _x ] ⁺ It will be done. Electrochemical oxidation of Cl ^- occurs at the GC electrode starting from 2.1 V (vs.PZC), and in this potential region, the cation-anion network becomes more aligned, water molecules withdraw, and the affinity of Cl ^- and Cl ₃ ^- with the interface layer increases. It is thought that the sensitivity increases, and accordingly, two types of redox peaks related to chlorine ions are observed in voltammetric analysis.

Figure 3 is a diagram schematically showing the difference in the redox reaction of chlorine ions in the interface layer according to the difference in potential to aid understanding. The left side of Figure 3 shows that in a salt-in-water electrolyte system, positive and negative ions are not aligned on the electrode surface at no potential or low potential (vs. PZC), and only one type of chlorine ion redox reaction (Cl ^- /Cl ₂ ) occurs. It shows what is happening. The right side of Figure 3 shows that at an electrode with a positive potential of about +2 V, positive and negative ions are aligned in the interface layer, and two types of redox reactions of Cl ^- /Cl ₃ ^- /Cl ₂ occur in the interface layer. It is showing.

4. Evaluation of OER overpotential changes through LSV analysis

As the concentration of the electrolyte increases and approaches the salt-in-water electrolyte, the anodic overpotential due to OER increases, confirming the phenomenon of water being excluded from the interfacial layer. Figure 4 is a graph of linear sweep voltammetry (LSV) analysis related to OER at Pt and GC electrodes in 0.5 m and 12 m LiTFSI aqueous solutions. In Figure 4, the black graph on the left is the Pt electrode result, the red graph on the right is the GC electrode result, the dotted line graph is the result at 0.5 m concentration, and the solid line graph is the analysis result at 12 m concentration.

Referring to Figure 4, as the molal concentration of salt increased, the overpotential due to OER increased by 0.34 V for the Pt electrode, while it increased by 0.52 V for the GC electrode. In other words, the overpotential that increased at the GC electrode as it became a salt-in-water electrolyte was 1.5 times that of the Pt electrode. This shows that the interfacial layer at the GC electrode functions as a larger kinetic barrier to OER.

5. Comparison with other halogen ions (Br, I)

[Comparative Example 1]

As in Example 1, the GC electrode was set to the operating voltage and a 12 m salt-in-water LiTFSI electrolyte was used, but CV analysis was performed by adding 50 mM HBr instead of HCl to the electrolyte. Similarly, as in Reference Example 1, a Pt electrode is used and a 12 m salt-in-water LiTFSI electrolyte solution is used, but CV analysis is performed by adding 50 mM HBr instead of HCl. In Figure 5, the black graph on the left is the Pt electrode, and the red graph on the right is the CV analysis result for the GC electrode.

Referring to Figure 5, it can be seen that two voltage-current peaks corresponding to one redox reaction also appeared in the GC electrode, which is similar to the graph of the Pt electrode. This means that Br ₃ ^- is unstable on both the Pt electrode and the GC electrode. In other words, even though a hydrophobic interface layer was formed at the GC electrode, the Br ₃ ^- ion was not stabilized, so the Br ^- /Br ₃ ^- /Br ₂ redox reaction was not captured, and a type of Br ^- /Br ₂ It is confirmed that only the oxidation-reduction reaction occurs. Therefore, unlike chlorine ions, it is impossible to confirm whether a hydrophobic interface layer has been formed using bromine ions. This is believed to be because the degree of stabilization in aprotic solvents varies depending on the type of halide ion, and the degree of stabilization varies in the order of I ₃ ^- < Br ₃ ^- < Cl ₃ ^- .

[Comparative Example 2]

As in Example 1, a GC electrode was used and a 12 m salt-in-water LiTFSI electrolyte was used, but CV analysis was performed by adding 50 mM HI instead of HCl to the electrolyte. Similarly, as in Reference Example 1, a Pt electrode is used and 12 m salt-in-water LiTFSI electrolyte is used, but CV analysis is performed by adding 50 mM HI instead of HCl. In Figure 6, the black graph on the left is the Pt electrode, and the red graph on the right is the CV analysis result for the GC electrode.

Referring to Figure 6, the CV shapes were found to be the same for two different types of Pt and GC electrodes. In other words, even though a hydrophobic interface layer was formed at the GC electrode, the I ₃ ^- ion was not stabilized, so the I ^- /I ₃ ^- /I ₂ redox reaction was not captured, and a type of I ^- /I ₂ It is confirmed that only the oxidation-reduction reaction occurs. Therefore, it can be seen that it is impossible to confirm whether a hydrophobic interface layer has been formed through iodine ions.

In conclusion, it can be seen that it is impossible to recognize the hydrophobic interface layer in the salt-in-water electrolyte system using bromine ions and iodine ions among halide ions. Chlorine ions are the only ones that exhibit two types of redox reactions in the hydrophobic interface layer, and therefore, the formation of a hydrophobic interface layer can be confirmed only through chlorine ions.

6. Ex-situ XPS evaluation to demonstrate the interaction between chloride ions and the interfacial layer

The interaction of Cl ^- and Cl ₃ ^- with the interfacial layer can also be confirmed through XPS analysis. Treat the GC electrode electrochemically by applying a constant potential for 1000 s in 12 m LiTFSI salt-in-water electrolyte containing 50 mM halide (Cl ^- , Br ^- , or I ^- ). Afterwards, XPS analysis (Thermo Scientific Co. Nexsa; monochromatic Al Kα X-ray source 1486.7 eV) was performed on each electrode, and the results are shown in Figure 7.

For electrodes that have been electrochemically treated in a salt-in-water electrolyte solution containing each halide, peaks in the spectrum for each element are analyzed to conduct qualitative and quantitative analysis of the elements remaining in the electrode. The top row of Figure 7 shows the C 1s spectrum results according to halide type. According to this, C-Cl peaks appear at 286.8 eV and 288.5 eV, but C-Br and CI peaks are not observed. This qualitatively proves that among various halides, only chlorine ions cause strong interaction with the GC electrode. In addition, the Cl 2p spectrum can confirm the presence of chlorine ions previously identified in the C 1s spectrum, and the relative peaks appearing at 197.4 eV, 199.8 eV, and 201.5 eV, corresponding to Cl ^- , Cl ₃ ^- , and Cl _2, respectively. Through the intensity, it can be seen that Cl ₃ ^- is the form that most stably exists in the electrode. When bromine ions and iodine ions are used, when confirmed through Br 3d and I 3d spectra, respectively, peaks of Br ₃ ^- and Br ₂ are observed at 68.2 eV and 70.0 eV, and peaks of 618.5 eV and 620.6 eV (I 3d _{5/ 2} ), 630.0 eV, 632.1 eV (I 3d _3/2 ) I ₃ ^- and I ₂ peaks are observed, but when comparing their intensities, they are negligible compared to the peaks related to chlorine ions.

In order to quantitatively analyze each element, peak intensities were compared, and the results are shown in Figure 8. In order to compensate for errors that may occur depending on the electrode sample, the intensity of the CC peak appearing at 284 eV in the C 1s spectrum at each electrode is used as the background intensity, and the intensity of each halide-related peak is compared against this to create a histogram for each halide. , and the intensities of F, N, and S elements resulting from TFSI ^- anions, which are commonly contained in all electrolytes, are also shown in a histogram. Referring to Figure 8, the F, N, and S element peaks all show the greatest intensity at the electrode treated with an electrolyte solution containing chlorine ions rather than bromine and iodine, and among several halide peaks, chlorine (Cl ^- , Cl ₃ ^- , Cl ₂ ) The peak shows the greatest intensity.

In summary, compared to bromine ions and iodine ions, only chlorine ions interact strongly with the interface layer of the electrode, leaving traces on the electrode for XPS measurement and analysis even when no voltage is applied.

Although the preferred embodiments have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements made by those skilled in the art using the basic concept defined in the following claims are also within the scope of the present invention. It belongs.

Claims (13)

An electrochemical analysis method that recognizes the formation of a hydrophobic interface layer on a positive potential electrode using chlorine ions in a battery system including a salt-in-water electrolyte and electrodes, comprising:
The water-in-salt electrolyte solution includes an aqueous solvent and a salt, and the ratio of the weight of the salt to the weight of the aqueous solvent is 1 or more,
In the above analysis method, a solution containing chlorine ions is added to a salt-in-water electrolyte solution, and when two types of redox reactions related to chloride ions are confirmed under a voltage applied, it is determined that a hydrophobic interface layer has been formed on the positive potential electrode. , an electrochemical analysis method that recognizes the hydrophobic interfacial layer of a positive potential electrode in a salt-in-water electrolyte solution. In paragraph 1:
The above two types of redox reactions are represented by Scheme 1 and Scheme 2 below, an electrochemical analysis method that recognizes the hydrophobic interface layer of a positive potential electrode in a water-in-salt electrolyte solution.
[Scheme 1]
Cl ₃ ^- + 2e ^- ⇔ 3Cl ^-
[Scheme 2]
3Cl ₂ + 2e ^- ⇔ 2Cl ₃ ^- In paragraph 1:
Confirming the above two types of redox reactions can be done by performing cyclic voltammetry analysis to identify two types of redox peaks, or by performing chronoamperometry analysis according to the voltage increase. An electrochemical analysis method that recognizes the hydrophobic interface layer of a positive potential electrode in a salt-in-water electrolyte solution, which confirms that the section where the current density is maintained appears twice. In paragraph 1:
The positive potential electrode is an electrode to which a potential of +1 V to +5 V is applied, an electrochemical analysis method that recognizes the hydrophobic interface layer of the positive potential electrode in the salt-in-water electrolyte solution. In paragraph 1:
An electrochemical method for recognizing the hydrophobic interface layer of a positive potential electrode in a salt-in-water electrolyte solution, in which the hydrophobic interface layer exists only when voltage is applied or a potential difference occurs. In paragraph 1:
An electrochemical analysis method for recognizing a hydrophobic interface layer of a positive potential electrode in a water-in-salt electrolyte solution, wherein the hydrophobic interface layer is ordered by anions and cations of the salt. In paragraph 1:
An electrochemical analysis method for recognizing a hydrophobic interface layer of a positive potential electrode in a salt-in-water electrolyte solution, wherein the hydrophobic interface layer has a thickness of 0.1 nm to 10 nm. In paragraph 1:
When the above two types of redox reactions are confirmed, the hydrophobic interfacial layer includes anions and cations of the salt and chlorine ions. An electrochemical analysis method for recognizing the hydrophobic interfacial layer of a positive potential electrode in a water-in-salt electrolyte. In paragraph 1:
An electrochemical analysis method for recognizing the hydrophobic interface layer of a positive potential electrode in the water-in-salt electrolyte, wherein the molal concentration of the water-in-salt electrolyte is 5 m to 50 m. In paragraph 1:
In the water-in-salt electrolyte, the salt is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiOTf), or a combination thereof, and the hydrophobicity of the positive potential electrode in the water-in-salt electrolyte is Electrochemical analysis method to recognize the interfacial layer. In paragraph 1:
The solution containing chlorine ions is an aqueous solution containing HCl, LiCl, NaCl, KCl, MgCl ₂ , CaCl ₂ , or a combination thereof. An electrochemical analysis method that recognizes the hydrophobic interface layer of a positive potential electrode in a water-in-salt electrolyte. In paragraph 1:
An electrochemical analysis method for recognizing the hydrophobic interface layer of a positive potential electrode in a water-in-salt electrolyte solution, wherein the molar concentration of the solution containing chlorine ions is 10mM to 100mM. In paragraph 1:
The battery system is a lithium secondary battery, lead acid battery, redox flow battery, fuel cell, capacitor, or electrolysis device. An electrochemical analysis method that recognizes the hydrophobic interfacial layer of a positive potential electrode in a salt-in-water electrolyte.