KR102652684B1 - Anode material for sodium metal battery and the method for manufacturing the same - Google Patents

Abstract

In this specification, a crystalline graphitic carbon structure; And an amorphous carbon structure located on at least a portion of the crystalline graphite carbon structure, wherein the amorphous carbon structure is doped with a heterogeneous element. An anode material for a sodium metal battery is provided.

Description

Anode material for sodium metal battery and method of manufacturing the same {ANODE MATERIAL FOR SODIUM METAL BATTERY AND THE METHOD FOR MANUFACTURING THE SAME}

This specification relates to anode materials for sodium metal batteries and methods for manufacturing the same.

Rechargeable sodium batteries (RSB) are considered one of the promising power sources for future grid systems because they use sodium ions, which are abundant in nature and have high redox activity. Meanwhile, sodium metal has attracted considerable attention as an active anode material for sodium metal batteries (SMB) due to its high electrochemical properties, such as low redox potential (-2.714 V - standard hydrogen electrode) and high theoretical specific capacity (~1,165 mA ^hg -1). However, metal anodes are problematic in that they have problems such as low coulombic efficiency (CE) and safety hazards due to dendrite metal growth. Accordingly, research has been conducted aimed at inducing uniform metal growth and minimizing by-products generated from side reactions.

Meanwhile, sodium ion batteries require the development of suitable anode materials because the interlayer insertion-based mechanism used in existing lithium ion batteries is not applicable due to sodium ions being larger and heavier than lithium.

Among these new anode active materials, research is being conducted on sodium metal batteries that use sodium metal as an anode due to their high theoretical capacity and low redox potential. Additionally, while previous studies have identified the influence of heteroatoms such as oxygen or nitrogen on carbon-based electrode materials, little research has been conducted on functionalized carbon-based sodium metal anodes, and these functional groups have been shown to form sodium metal nuclei. The impact on is unclear. Additionally, it is also important to note that carbon materials with an amorphous microstructure and rich in heteroatoms are prone to wear under repeated cycling. If the metal deposition is non-uniform when the activated carbon surface is passivated during the cycling process, the sodium metal anode may electrically short-circuit, exposing it to safety hazards. Therefore, designs based on functionalized carbon electrodes require better cycling stability. do.

On the other hand, sodium metal anodes can be problematic in that there is a large gap between theoretical and feasible energy densities. This means that even if the CE decreases by just 1%, the capacity can decrease to 36% of the initial capacity after 100 cycles, and from this, excess sodium metal may be needed to make up for the loss during the cycling process.

Therefore, while excessive metal loading is an essential factor in maintaining a long cycle life, it seriously reduces the specific energy density of SMB, making it very difficult to achieve both high specific energy density and long cycle life. have

Accordingly, embodiments according to the present invention seek to solve the problem of functionalized carbon-based electrode materials that are prone to wear during repetitive cycling.

Additionally, embodiments according to the present invention seek to solve the problem of a serious decrease in specific energy density when loading excess metal in order to maintain a long cycle life.

In order to achieve the above-described object, in one embodiment according to the present invention, a crystalline graphitic carbon structure; and an amorphous carbon structure located on at least a portion of the crystalline graphite carbon structure, wherein the amorphous carbon structure is doped with a heterogeneous element.

In one embodiment, the amorphous carbon structure may be doped with one or more atoms of nitrogen (N) and oxygen (O).

In one embodiment, the amorphous carbon structure may be doped with nitrogen (N) and oxygen (O) atoms.

In one embodiment, the amorphous carbon structure may include one or more of pyridinic-N and pyridonic-N structures.

In one embodiment, the anode material may have a specific surface area of 150 m ² g ^-1 or more.

In one embodiment, the anode material may have a ratio I _D /I _G of the D-band peak intensity (I _D ) and the G-band peak intensity (I _G ) measured in a Raman spectroscopic spectrum of 0.1 to 0.3.

In one embodiment, the anode material may have a mesoporous pore structure with a nitrogen adsorption isotherm defined by IUPAC of TYPE IV.

In one embodiment according to the present invention, a sodium secondary battery including the anode material for a sodium metal battery described above is provided.

In one embodiment, the anode material for a sodium metal battery may be pre-loaded with sodium metal and may contain excess sodium metal.

In another embodiment according to the present invention, preparing a carbon material; heat treating the carbon material; and functionalizing the carbon material with a heterogeneous element by treating the carbon material with one or more heterogeneous precursors. It provides a method of manufacturing an anode material for a sodium metal battery, including a.

In one embodiment, the carbon material may be one or more selected from the group consisting of CNT, RGO, and carbon black.

In one embodiment, the fine crystal structure of the carbon material can be controlled by controlling the heat treatment temperature profile in the heat treatment step.

In one embodiment, the heat treatment step may be performed in a temperature range of 1500 to 2800 °C.

In one embodiment, the heterogeneous precursor may include one or more of a nitrogen (N)-containing precursor and an oxygen (O)-containing precursor.

Accordingly, the anode material for a sodium metal battery according to an embodiment of the present invention provides a sodium metal anode with low overvoltage and excellent lifespan characteristics by controlling the crystallinity and surface properties of the highly crystalline carbon nanotube (CNT)-based anode material. You can.

In addition, the anode material for a sodium metal battery according to an embodiment of the present invention can exhibit high electrochemical performance with a Coulombic efficiency of 99.9% or more and an overvoltage equivalent to 5 mV by introducing nitrogen and oxygen among various surface functional groups into highly crystalline CNTs. , it can exhibit excellent lifespan characteristics by maintaining a coulombic efficiency of over 99.9% and a stable structure during over 1,000 charging and discharging processes.

In addition, the anode material for a sodium metal battery according to an embodiment of the present invention exhibits high electrochemical performance, and can compensate for the sodium of the cathode consumed during the charging and discharging process at the anode, enabling capacity maintenance for a long time and exceeding that of conventional sodium. It can have higher energy density and power density than ion-based energy storage devices.

1A and 1B show FE-SEM images of P-CNT and NO-S-CNT, respectively, according to an embodiment of the present invention.
Figures 1c and 1d show FE-TEM images of P-CNT and NO-S-CNT, respectively, according to an embodiment of the present invention.
Figure 1e shows Raman spectra of P-CNT and NO-S-CNT according to an embodiment of the present invention.
Figure 1f shows the XRD patterns of P-CNT and NO-S-CNT according to an embodiment of the present invention.
Figure 1g shows a nitrogen adsorption/desorption adiabatic curve graph of P-CNT and NO-S-CNT according to an embodiment of the present invention.
Figure 1h shows pore size distribution data of P-CNT and NO-S-CNT according to an embodiment of the present invention.
Figure 1I is an XPS graph of NO-S-CNT according to an embodiment of the present invention, showing C 1s, O 1s, and O 1s spectra, respectively.
Figures 2a and 2b show XPS spectra of O 1s and N 1s of O-CNT and N-CNT, respectively, among anode samples for sodium metal batteries according to an embodiment of the present invention.
Figures 2c and 2d show XPS spectra of O 1s and N 1s of NO-CNT among anode samples for sodium metal batteries according to an embodiment of the present invention, respectively.
Figures 2e and 2f show the nitrogen atom ratio and C/O and C/N ratios of anode samples for sodium metal batteries according to an embodiment of the present invention, respectively.
Figure 3A shows the initial galvanostatic discharge profile in anode samples for sodium metal batteries according to an embodiment of the present invention.
Figure 3b shows a comparison of the coulombic efficiencies of anode samples for sodium metal batteries according to embodiments of the present invention.
Figures 3c and 3d compare the cycling performance of anode samples for sodium metal batteries according to embodiments of the present invention.
Figure 4 shows (a) Al foil, (b) P-CNT, (c) O-CNT, (d) N-CNT, and (e) NO-CNT among anode samples for sodium metal batteries according to an embodiment of the present invention. , and (f) the contact angles of electrolyte droplets on NO-S-CNT are compared.
Figure 5a shows Raman spectra of anode samples for sodium metal batteries according to an embodiment of the present invention.
Figure 5b shows an XRD pattern of anode samples for a sodium metal battery according to an embodiment of the present invention.
Figure 5c shows a nitrogen adsorption/desorption adiabatic curve graph of anode samples for sodium metal batteries according to an embodiment of the present invention.
Figure 5d shows pore size distribution data of anode samples for sodium metal batteries according to an embodiment of the present invention.
Figure 6 schematically shows the chemical adsorption behavior of sodium ions in the pyridone-N structure in anode samples for sodium metal batteries according to an embodiment of the present invention.
7A to 7E show ex-situ FE-SEM images of anode samples for sodium metal batteries according to embodiments of the present invention.
Figure 7f shows an ex-situ FE-SEM image of the Al foil.
Figures 7g and 7h show FE-SEM images of NO-CNT and NO-S-CNT after 300 cycles among anode samples for sodium metal batteries according to an embodiment of the present invention, respectively.
Figures 7i and 7j show FE-TEM images of NO-S-CNT and NO-CNT after 300 cycles among anode samples for sodium metal batteries according to an embodiment of the present invention, respectively.
Figures 7k and 7l compare the relative atomic masses of NO-S-CNT and NO-CNT obtained from ex-situ XPS depth profiles.
Figures 7m and 7n show the ex-situ EIS profiles of NO-S-CNT and NO-CNT among anode samples for sodium metal batteries according to embodiments of the present invention at various cycles, respectively.
Figure 8 shows ex-situ energy dispersive X-ray spectroscopy (EDS) mapping data from anode samples for sodium metal batteries according to an embodiment of the present invention.
Figure 9a shows the deposition/dissolution cycle performance of sodium metal at 2 mA cm ^-2 in a symmetric cell applying NO-S-CNT according to an embodiment of the present invention.
Figure 9b shows the deposition/dissolution cycle performance of sodium metal at 4 mA cm ^-2 in a symmetric cell applying NO-S-CNT according to an embodiment of the present invention.
Figures 10a to 10c show the electrochemical performance of Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) cathode.
Figure 11 shows galvanostatic charge/discharge profiles of Na/NO-S-CNT (blue) and Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) cathode (red) in a full cell.
Figure 12a shows a comparison of constant current charge/discharge profiles at 0.1 A g ^-1 of anode samples for sodium metal batteries according to an embodiment of the present invention.
Figure 12b shows a comparison of the cycling performance at 1.0 A g ^-1 of anode samples for sodium metal batteries according to an embodiment of the present invention.
Figure 12c shows anode samples for sodium metal batteries according to an embodiment of the present invention.
Figure 12d shows a comparison of Ragone plots of anode samples for sodium metal batteries and various energy storage devices according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in more detail.

The embodiments of the present invention disclosed in the text are illustrative only for illustrative purposes, and the embodiments of the present invention may be implemented in various forms and should not be construed as limited to the embodiments described in the text. .

The present invention can make various changes and take various forms, and the embodiments are not intended to limit the present invention to the specific disclosed form, but all changes, equivalents, or substitutes included in the spirit and technical scope of the present invention. It should be understood as including.

Singular expressions include plural expressions unless the context clearly dictates otherwise. In this application, terms such as “comprise” or “have” are intended to designate the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, but are not intended to indicate the presence of one or more other features. It should be understood that this does not exclude in advance the possibility of the existence or addition of elements, numbers, steps, operations, components, parts, or combinations thereof.

Anode material for sodium metal batteries

To achieve the above-described object, in an exemplary embodiment according to the present invention, a crystalline graphitic carbon structure; and an amorphous carbon structure located on at least a portion of the crystalline graphite carbon structure, wherein the amorphous carbon structure is doped with a heterogeneous element.

Specifically, the crystalline graphite carbon structure may have a highly aligned graphite structure, unlike general carbon materials, such as CNTs, which have an irregular graphite lattice structure in which CNTs are stacked in a somewhat distorted manner. When such an aligned graphite structure is applied as an anode material, the clogging time of the porous structure is significantly delayed, preventing battery poisoning, thereby enabling a longer cycle life.

In an exemplary embodiment, the carbon structures may be derived from one or more carbon materials selected from the group consisting of CNT, RGO, and carbon black.

In an exemplary embodiment, the amorphous carbon structure may be doped with one or more atoms of nitrogen (N) and oxygen (O). Preferably, the amorphous carbon structure may be doped with nitrogen (N) and oxygen (O) atoms. Additionally, the nitrogen (N) and oxygen (O) atoms may tend to be doped at the edges of the carbon structure rather than at the base of the amorphous carbon structure. In this way, the surface characteristics and electrochemical performance of the anode material may vary depending on the doping of heterogeneous elements. For example, the oxygen atom can serve as a redox host for sodium ion storage.

In an exemplary embodiment, the amorphous carbon structure may include one or more of pyridinic-N and pyridonic-N structures. For example, when the amount of oxygen atom doping is low, the pyridine-N structure may be superior to the pyridone-N structure based on an oxygen functional group. On the other hand, when oxygen is more abundant, the pyridone-N structure appears more dominant.

In particular, pyridine nitrogen, which has an extra pair of electrons, can act as an electron-rich donor with a full p-orbital and can strongly adsorb Na ions of the Lewis phase through acid-base action. Therefore, when it has a carbon structure functionalized with such heteroatoms, it can act as an active site that guides the metallic Na nucleus. Additionally, the oxygen and nitrogen atoms of the pyridone nitrogen structure can combine with Na ions through synergistic interaction.

In an exemplary embodiment, the anode material may have a C/N element ratio and a C/O element ratio of 30 or less. In the above range, the ratio of O and N may be high, and through this, excellent electrochemical performance may be exhibited.

In an exemplary embodiment, the crystalline graphitic carbon structure can have an aspect ratio of 100 to 250. By having such a high aspect ratio, the space between CNTs increases, suppressing volume expansion when sodium is reduced to metal, and increasing the specific surface area, which has the effect of reducing the current speed flowing in the actual electrode. For example, if there is a difference in specific surface area of about 100 times, it can be reduced from 1 mA per unit area to 0.01 mA per unit area.

In an exemplary embodiment, the anode material may have a specific surface area of 150 m ² g ^-1 or more. If the specific surface area is less than 150 m ² g ^-1 , it may be difficult to solve the problem of volume expansion due to the small specific surface area or small internal pores.

In an exemplary embodiment, the anode material may have a ratio I _D /I _G of the D-band peak intensity (I _D ) and the G-band peak intensity (I _G ) measured in a Raman spectroscopic spectrum of 0.1 to 0.3. Meanwhile, in the case of carbon materials such as CNTs, the intensity ratio of the D band and the G band is similar, while the intensity ratio of the D band to the G band in the anode material is 0.1 to 0.3, for example, about 0.22, which is about 5 times different, and D The band and G band may be separate and very narrow.

In an exemplary embodiment, the anode material may have a mesoporous pore structure with a nitrogen adsorption isotherm defined by IUPAC of TYPE IV. Additionally, the carbon structure may have a tubular morphology.

In one embodiment according to the present invention, a sodium secondary battery including the anode material for a sodium metal battery described above is provided.

In an exemplary embodiment, the anode material may be pre-loaded with the sodium metal to contain excess sodium metal. Specifically, the excess sodium metal may have a loading amount of 0.12 mg cm ^-2 to 0.60 mg cm ^-2 .

Method for manufacturing anode material for sodium metal battery

In another embodiment according to the present invention, preparing a carbon material; heat treating the carbon material; and functionalizing the carbon material with a heterogeneous element by treating the carbon material with one or more heterogeneous precursors. It provides a method of manufacturing an anode material for a sodium metal battery, including a.

First, a carbon material can be prepared, and the carbon material may be one or more selected from the group consisting of CNT and carbon black.

Next, the carbon material can be heat treated.

In an exemplary embodiment, the fine crystal structure of the carbon material can be controlled by controlling the heat treatment temperature profile in the heat treatment step. Specifically, a crystalline graphite carbon structure can be formed through heat treatment, and a strong fine carbon structure can be obtained, resulting in a longer cycle life.

In an exemplary embodiment, the heat treatment step may be performed in a temperature range of 1500 to 2800 °C. If the heat treatment temperature is less than 1500 ℃, the development of the crystalline graphitic carbon structure is small, and the defective portion of the carbon is heavily doped in the process of introducing heterogeneous elements, so the carbon decomposes during the charging and discharging process, and the lifespan characteristics may be poor. 2800 ℃ If it is exceeded, the degree of development of the graphite carbon structure is too high and there are few defects in the carbon, making it difficult to introduce heterogeneous elements, and uniform growth of sodium metal may be difficult due to poor wetting with the electrolyte.

Next, the carbon material can be functionalized with a heterogeneous element by treating it with one or more heterogeneous precursors. Specifically, one or more heterogeneous precursors can be heat treated to induce heterogeneous elemental functional groups. At this time, heat treatment can be performed at a temperature of approximately 500°C.

In an exemplary embodiment, the heterogeneous precursor may include one or more of a nitrogen (N)-containing precursor and an oxygen (O)-containing precursor.

Example

Hereinafter, the configuration and effects of the present invention will be described in more detail through examples. However, these examples are provided only for illustrative purposes to aid understanding of the present invention, and the scope and scope of the present invention are not limited by the following examples.

Example 1: Preparation of CNT-based electrode material ('NO-S-CNT')

Pristine CNTs (Hanwha Chemical) were purified by impregnating them in a 30% by weight nitric acid solution and sonicating them for 3 hours. The purified pristine CNT ('P-CNT') was vacuum filtered with distilled water and ethanol, washed several times, and dried in a vacuum oven at 30°C. Then, it was heated to 2,800°C in a graphite furnace (ThermVac, South Korea) under Ar gas atmosphere. In the heating process, various heating rates were applied: from room temperature to 1800°C at a rate of 10°C min ^-1 , from 1800°C to 2400°C at a rate of 5°C min ^-1 , and from 2400°C to 2800°C at a rate of 3°C min ^-1. , Finally, after heat treatment at 2,800°C for 2 hours, the furnace was naturally cooled to room temperature to obtain highly crystalline CNTs.

The resulting CNTs (1g) were mixed with a surfactant ( ^Triton . For this heat treatment, the temperature was raised from room temperature to 500°C at a heating rate of 10°C min ^-1 and maintained for 2 hours. After cooling the pipe to room temperature, the prepared 'NO-S-CNT' was washed several times by vacuum filtration with distilled water and ethanol and stored in a vacuum oven at 30°C.

Example 2: Preparation of CNT-based electrode material ('N-CNT')

N-CNT samples were prepared in the same manner as in Example 1, except that they were obtained by heating P-CNTs with melamine at 500°C for 2 hours without a high temperature heat treatment process (2800°C).

Example 3: Preparation of CNT-based electrode material ('O-CNT')

The O-CNT sample was prepared in the same manner as in Example 1, except that the O-CNT sample was obtained by heating the P-CNT with Tiriton

Example 4: Preparation of CNT-based electrode material ('NO-CNT')

The NO-CNT sample was prepared in the same manner as in Example 1, except that it was obtained by heating P-CNT with melanin and Tyriton X-100 at 500°C for 2 hours without a high temperature heat treatment process (2800°C).

Experimental Example 1: Structural analysis of anode material

FE-TEM and FE-SEM analysis

The P-CNTs and NO-S-CNTs of the examples were dispersed in a porous alumina sheet, and the external morphology of the samples was analyzed using a field emission scanning electron microscope (FE-SEM, S-4300SE, Hitachi, Japan).

Figure 1a shows a P-CNT, which has a bending ratio of 0.55 and a sustained length of 0.55 due to inherent defects such as the pentagon-heptagon couple, which can cause permanent deformation of straight CNTs. It can be seen that it has a serpentine shape of about 230 nm.

Figure 1b shows NO-S-CNT, and it can be seen that the P-CNT maintains its unique form with a high aspect ratio even though the P-CNT is treated at a high temperature of 2,800°C during the NO-S-CNT manufacturing process. .

In this way, although P-CNTs and NO-S-CNTs are similar in shape, the carbon microstructure of each sample was analyzed by FE-TEM (JEM2100F, JEOL, Japan), and as a result, there were differences in microstructure (Figure 1c). and 1d.

The field emission transmission electron microscopy (FE-TEM) image of the P-CNT in Figure 1c shows that the P-CNT has an irregular graphitic lattice composed of somewhat distorted stacked layers, whereas the NO-S-CNT in Figure 1d shows a highly ordered graphite lattice. It can be confirmed that it has a graphitic structure. Additionally, a structure in which amorphous carbon containing nitrogen and oxygen partially coats the surface of this aligned graphite structure can also be observed.

Raman spectroscopy and XRD analysis

The carbon microstructure of P-CNT and NO-S-CNT samples was additionally analyzed through Raman spectroscopy (LabRAm HR Evolution, HORIBA) and X-ray diffraction analysis (XRD, Rigaku, MiniFlex).

In the Raman spectroscopy in Figure 1e, a pair of D and G bands with similar intensity ratios (I _D /I _G = ~1.02) were identified for the P-CNT sample. On the other hand, in the case of the NO-S-CNT sample, the intensity ratio of the D band to the G band is I _D /I _G = ~0.22, which is about 5 times different, and the D and G bands are separated from each other and have a very narrow width. You can check that.

Applying these results to the Tuinstra and Koenig equations, the average lateral polyhexagonal carbon crystal size (La) of the P-CNT sample and the NO-S-CNT sample is ~4.3 and ~19.8 nm, respectively, which shows that the two samples have a large difference in crystal plane size. It can be seen that it has . In addition, the NO-S-CNT sample has a relatively sharper and larger 2D band than the P-CNT sample at about 2,680 cm ^-1 , which confirms that it has an aligned three-dimensional graphite structure.

Meanwhile, despite the large difference in La values of the P-CNT and NO-S-CNT samples, the . On the other hand, the graphite (100) peak is further developed during the heating process, which is in good agreement with the Raman spectroscopy results described above.

Nitrogen adsorption and desorption isotherms

Nitrogen adsorption and desorption isotherm curves were analyzed for the porosity and surface properties of the prepared samples using a porosity analyzer (ASAP 2020, Micromeritics, USA).

From the isotherm curve in Figure 1g, the amount of adsorbed nitrogen increased rapidly at relative pressure sections above 0.8, revealing a mesoporous structure corresponding to Type-IV of IUPAC. The hysteresis roof between the adsorption and desorption curves shows regularly arranged H1 type uniform spheres, which is because CNTs have a tubular morphology. The isothermal curves show that their morphologies and porosity are similar, which can also be confirmed through the pore size distribution data in Figure 1h. The specific surface areas of P-CNT and NO-S-CNT were 208.9 and 194.6 m ² g ^-1 , respectively. Although the specific surface area decreased by about 9% due to high temperature heating, NO-S-CNT still had a high aspect ratio nanostructure and It can be seen that it has a high specific surface area.

XPS analysis

The porosity and surface properties of the prepared samples were analyzed using XPS (PHI 5700 ESCA, Chanhassen, USA).

In Figure 1i, the deconvoluted C1s spectrum shows several chemical structures, such as sp2 C=C, sp3 C-C, C-O/C-N, and C=O bonds, where the aromatic carbon-carbon double bond is the main chemical structure.

The deconvoluted O1s spectrum in Figure 1j shows the oxygen bond group mainly composed of two structures: C-O and C=O bonds.

The C/O atomic ratio is 17.2, which gives NO-S-CNT hydrophilicity and shows excellent wettability in glyme-based electrolytes. In addition, NO-S-CNT contains nitrogen functional groups with a C/N atomic ratio of 25.3, mainly pyridinic-N and pyridonic-N with an N-oxide group. N) structure (Figure 1k).

Oxygen and nitrogen functional groups were derived from Triton X-100 and melamine, respectively, through heat treatment (500°C).

NO-S-CNTs with similar oxygen or nitrogen contents were introduced onto the surface of O-CNTs (C/O atomic ratio 18.2) or N-CNTs (C/N atomic ratio 29.5), respectively (Figures 2a and 2b). The oxygen composition of O-CNTs was similar to that of NO-S-CNTs, but the nitrogen composition of N-CNTs was very different from that of NO-S-CNTs. N-CNTs are mainly composed of a pyridinic-N structure and have much smaller amounts of pyridonic-N and N-oxide groups (Figure 2b).

Considering that N-CNTs have a low oxygen content, the pyridine-N structure appears to be dominant because the pyridone-N structure is based on oxygen functional groups. Additionally, oxygen and nitrogen co-doped CNTs (NO-CNTs) were prepared from P-CNTs without a high-temperature heating process, and the surface properties were mainly composed of pyridone-N groups (Figures 2c and 2d).

Given that most oxygen and nitrogen atoms tend to dope the edge regions of the poly-hexagonal carbon structure rather than the basal plane, pyrids in oxygen-rich NO-S-CNTs (Figure 1k) and NO-CNTs (Figure S1d) Don-N Gi appears. Figure 2e shows the relative proportions of nitrogen functional groups in NO-S-CNT, NO-CNT and N-CNT. P-CNTs have low oxygen content corresponding to C/O and C/N ratios of ∼67.2 and ∼76.4, respectively. Figure 2f shows the C/O and C/N ratios for all samples.

Surface characterization through electrochemical analysis

The influence of different surface properties on a series of CNT-based electrode materials was investigated through electrochemical analysis. The CNT-based samples of the examples were coated on Al foil, and their electrochemical performance was tested through a galvanostatic sodium metal deposition/dissolution process with a cutoff discharge capacity of 0.5 mAh cm ^-2 , and their electrochemical performance was compared to that of the Al foil. Comparison confirmed the effect of specific heteroatom configurations (Figure 3).

Figure 3a shows the first rectification discharge profiles of Al foil, P-CNT, N-CNT, O-CNT and NO-CNT, showing different voltage overshooting (VO) values resulting from sodium metal nucleation polarization. The VO value of the Al foil was ~30 mV, which significantly decreased up to ~18 mV when P-CNT was introduced due to the expansion of the effective surface area (~208.9 m ² g ^-1 ). When introducing 0.1 mg P-CNTs into 1 cm ^-2 of Al foil, the specific surface area of P-CNTs was calculated to be at least 200 times higher than that of Al foil.

Meanwhile, Figure 4 shows the contact angle data of P-CNT with respect to the electrolyte droplet. In P-CNT, the droplet was absorbed after several seconds, while in the case of Al foil, the initial contact angle was maintained, confirming electrolyte affinity. (Figure 4). This shows that P-CNTs have a much higher active surface area.

Additionally, the VO value further decreased when doped with hetero atoms, showing ~9 mV and ~8 mV for O-CNT and N-CNT, respectively (Figure 3a). Since O-CNTs and N-CNTs have similar material properties to P-CNTs except for surface properties, it can be seen that the reduction in the nucleation overpotential effect is due to the oxygen or nitrogen functional groups (Figure 5).

Specifically, according to Equation 1 below, which is the classical nucleation equation, it can be seen that the size of the electrodeposited Na nuclei is closely related to the nucleation overpotential.

[Equation 1]

Here, △Gnucleation is the Gibbs energy required to form a spherical nucleus of radius r, △GV is the free energy change per volume, is the surface energy of the Na electrolyte interface. The nucleation overvoltage is related to △GV by equation (2) below, and the critical radius is inversely proportional to the nucleation overvoltage by equation (3) below.

[Equation 2]

[Equation 3]

where r _crit is the critical radius, V _m is the molar volume of Na, F is Faraday's constant and η _n is the nucleation overpotential.

Through Equation 3 above, it can be seen that the critical radius greatly depends on the surface energy between the electrolyte and sodium metal. In contrast, the existing nucleation theory did not consider the influence of substrates that strongly interact with sodium metal.

Additionally, oxygen functional groups in carbon-based materials are well known as redox hosts for sodium ion storage and are capable of Na-O bonding in the electrochemical voltage window. According to Lewis acid-base theory, the interaction between oxygen and Na atoms is stronger than the Na bond because the electronegativity of oxygen (3.44) is higher than that of Na (0.93). The sodium nucleation process with smaller surface tension proceeds along the surface of O-CNTs (surface coating) because O-CNTs have more Na-O binding sites on the top surface with higher binding energy to the precursor nuclei. Due to the higher electronegativity of nitrogen (3.04), Na can have a nucleation process similar to that of oxygen atoms.

In particular, the pyridine nitrogen, which has an extra pair of electrons, is an electron-rich donor with a packed p-orbital that naturally acts as a Lewis base site, and is expected to strongly adsorb Na ions of the Lewis phase through acid-base action. Therefore, oxygen or nitrogen atoms can be uniformly deposited on the surface of functionalized CNTs and function as active sites to guide metallic Na nuclei.

Samples co-doped with nitrogen and oxygen (NO-CNT and NO-S-CNT) showed lower VO values (~5 mV and ~4 mV, respectively) than samples doped with single oxygen or nitrogen.

As shown in Figures 2d and 1k, NO-CNT and NO-S-CNT samples mainly have pyridone structures, which corresponds to the main difference from N-CNTs, which are composed of pyridine nitrogen functional groups (Figure 2b).

Because the material properties of double heteroatom doped samples, such as specific surface area, pore structure, and wettability to electrolytes, are similar to O-CNTs and N-CNTs, double heteroatoms play an important role in terms of electrochemical performance (Figure 4 and 5). Given that the oxygen and nitrogen atoms of the pyridone structure can bind Na ions by synergistic interactions, it is possible that Na can bind strongly to the two heteroatoms in the high-voltage section (Figure 6). Homogeneous Na metal deposition can be achieved with lower nucleation overpotential due to synergistic interactions with the precursor nuclei. These results support that surface functional groups as well as active surface area for metal nucleation play an important role in Na metal nucleation overpotential.

Experimental Example 2: Half-cell analysis

The CE values of the examples were tested several times at different current rates using at least 10 cells for each sample.

Figure 3b shows the average CE values between the 10th and 100th cycles. Heteroatom-doped CNT samples showed significantly higher average CE values and lower cell-to-cell variation than Al foil and P-CNTs at all areal current rates. In particular, NO-S-CNT and NO-CNT showed a high average CE value of ≥99.9% at 1.0 mA cm ^-2 , which was significantly higher than that of P-CNT (~99.2, ~99.6, and ~99.5%) corresponds to

In Figure 3b, N-CNTs and O-CNTs were each shown at the same area current rate, and the CE values of all samples decreased as the current rate decreased from 1.0 to 0.2 mA cm ^-2 and the current rate decreased from 1.0 to 6.0 mA. It decreased as cm ^-2 increased, and the reason why the CE value decreases at such a low current rate is because more side reactions occur during the sodium metal deposition/dissolution cycle.

When the current is reduced to a negative voltage, electrons with more energy are transferred to the electrolyte to form a solid-electrolyte-interface (SEI) layer. Since the reduction reaction of the electrolyte depends on the reaction time, the lower the current rate, the lower the current rate. It may cause further electrolyte decomposition reactions.

On the other hand, higher current rates induce more diffusion-controlled reactions than kinetically-controlled reactions and grow uneven sodium metal with higher surface areas. Therefore, electrolyte decomposition may occur more at high current rates.

On the other hand, the CE value in NO-S-CNT has a significantly low standard deviation (Figure 3b), and as a result of this low cell-to-cell variation, it can have high reliability and reproducibility. Unlike Al foil and P-CNT, which have large standard deviations (1.0–3.0% and 0.5–2.0%), functionalized CNT samples exhibit significantly lower cell-to-cell variation values (0.2–1.3%) in overall average CE values. . Additionally, NO-S-CNTs showed optimal performance with a cell-to-cell variation of ≤0.6% in all given area current ranges and showed excellent effects in terms of sodium metal nucleation and growth (Figure 3b).

Figures 7a-7f show the surface morphology of sodium metal deposited on P-CNT, O-CNT, N-CNT, NO-CNT, NO-S-CNT, and Al foil after sodium metal deposition through ex-situ FE-SEM. analyzed. All heteroatom-doped CNT samples showed a uniform sodium metal morphology consisting of several film-like sheets (Figures 7a-7e), and ex-situ energy dispersive X-ray spectroscopy revealed that the film-like sheets were deposited. It was confirmed that it was mainly sodium metal (Figure 8). In contrast to this film morphology, the Al foil exhibited numerous dendritic sodium metals grown randomly in a high aspect ratio form (Figure 7f), which, if unevenly deposited, can promote electrolyte decomposition and moss-like by-products, resulting in low CE values. there is.

In Figure 3c, the cycling performance of the sodium metal plating/stripping process for P-CNT, O-CNT, N-CNT, and NO-CNT was tested at 1 mA cm ^-2 with a cutoff capacity of 0.5 mA h cm ^-2 .

Referring to Figure 3d, the cycling stability of the heteroatom-doped sample was superior to that of P-CNT. Specifically, P-CNTs showed cycle collapse after 200 cycles, whereas O-CNTs, N-CNTs, and NO-CNTs survived 300 to 350 cycles in the repetitive cycling process. Meanwhile, NO-S-CNTs can be operated for more than 1,000 cycles and have a high average CE value of ~99.9% at 1, 2, and 4 mA cm ^-2 between 20 and 1,000 cycles.

In addition, cycling performance was further confirmed through a symmetric cell fabricated using two identical NO-S-CNT-based anodes (Figures 3d and 9). In symmetric cell tests, very stable cycling behavior was observed and the initial overvoltage was well maintained for 1,000 cycles at different current rates. Given the similarity of surface properties between NO-CNTs and NO-S-CNTs, it can be seen that heteroatoms are not a critical factor in determining cycling performance.

ex-situ analysis

In order to clarify the specific cause of better cycling performance in NO-S-CNT, ex-situ analysis was performed on NO-CNT and NO-S-CNT after 300 repeated cycles, and the results are shown in Figures 7g to 7n. do.

Ex-situ FE-SEM images of the two samples taken under desalting conditions up to 1.0 V after the 300th cycle show different morphological changes during the cycling process (Figures 7g and 7h).

Referring to Figure 7g, in the case of NO-CNT, a large amount of moss-like by-products were confirmed on the surface, and most pores were clogged with discharged by-products.

Referring to Figure 7h, the presence of a high-density material interferes with the mass transfer of Li ions, causing cyclic collapse, whereas it can be seen that NO-S-CNT maintains its unique shape and porous structure even under the same conditions. .

The microstructural features of NO-S-CNTs can also be confirmed through ex-situ FE-TEM observation after 300 cycles. In Figure 7i, the well-ordered graphite lattice structure of NO-S-CNTs is well maintained even after cycling. In Figure 7j, it can be seen that the lattice of NO-CNT is worn and damaged.

Also, referring to the top of Figure 7k, the ex-situ XPS depth profile of NO-CNT after the 300th cycle shows significantly higher oxygen and sodium contents. Among them, the O/C and O/Na ratios were observed to be high during etching processes longer than 3,000 seconds. These oxygen and sodium atoms mainly come from defective carbon surfaces or by-products formed during repeated cycling, so it can be seen that the carbon structure has been corroded from a large number of oxygen and sodium atoms and a large amount of by-products are accumulated on the surface as it cycles. You can.

Meanwhile, the ex-situ (Figure 7). Considering all ex-situ results, cycling decay is closely related to carbon microstructure. Deterioration of the carbon structure during the cycling process means that the inactive surface area of the CNT surface is expanded. More by-products formed on the inert carbon surface block the porous structure, causing cell poisoning. Therefore, these ex-situ results indicate that in a more ordered graphitic structure, the plugging time is significantly delayed, leading to a longer cycle life.

Electrochemical Impedance Spectroscopy (EIS)

To determine changes in electrochemical resistance during the cycling process, ex-situ electrochemical impedance spectroscopy (EIS) was performed using an impedance analyzer (ZIVE SP2, WonATech) at room temperature and in the frequency range of 1 MHz to 1 mHz (Figure 7l). ).

The EIS profile of NO-CNT has a charge transfer resistance (Rct) of ~200 Ω at the 10th cycle, with Rct values increasing with cycling to ~250, ~270, and ~320 Ω at the 50th, 100th, and 300th cycle, respectively. increased accordingly. Therefore, it can be seen that as the number of cycles increases, sodium ion and electron transport becomes more difficult.

A similar trend was observed in the EIS profile of NO-S-CNT, but the Rct value of NO-S-CNT was much smaller and the increase rate was also lower than that of NO-CNT. Specifically, the Rct value of NO-S-CNT at the 10th cycle was ~120 Ω, which increased to ~135, ~145, and ~160 Ω at the 50th, 100th, and 300th cycles, respectively.

The EIS data are consistent with ex-situ FE-SEM, FE-TEM, and XPS results, which show that carbon microstructure plays an important role in the cycling performance of carbon-based electrode materials for reversible sodium metal storage. It can be seen that

Experimental Example 3: Full cell test

Full-cell experiments were performed using an iron-based cathode, Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ).

Figure 10 shows a specific capacity of ∼110 mA h g _cathode ⁻¹ and an average voltage of ∼2.98 V at a specific current rate of 10 mA g _cathode ⁻¹ over a voltage window of 1.7 to 4.0 V in a glyme-based electrolyte (1 M NaPF6 in DEGDME). The electrochemical performance of the cathode is shown (Figure 10a).

In FIGS. 10b and S7c, the Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) cathode showed excellent rate capability and cycling stability. Specifically, in Figure 10b, the specific current rates are 0.02 A g _cathode ^-1 , 0.05 A g _cathode ^-1 , 0.1 A g _cathode ^-1 , 0.2 A g _cathode ^-1 , 0.4 A g _cathode ^-1 , 0.7 A g _cathode ^-1 , With gradual increase to 1.0 A g _cathode ^-1 , 1.5 A g _cathode ^-1 , 2.0 A g _cathode ^-1 , 4.0 A g _cathode ^-1 and 8.0 A g _cathode ^-1 , the specific capacity is 108.3 mA hg _cathode ^-1 , 106.5 mA hg _cathode ^-1 , 103.5 mA hg _cathode ^-1 , 102.5 mA hg _cathode ^-1 , 100.8 mA hg _cathode ^-1 , ^98.7 mA hg _cathode -1 , 97.1 mA hg _cathode ^-1 , 94.8 mA hg _cathode ^-1 , 92.7 mA hg _cathode ^-1 , 85.6 mA hg _cathode ^-1, and 72.1 mA hg _cathode ^-1 were well maintained, and it was confirmed that ~65% of the initial capacity was maintained even at a current rate 800 times higher. Additionally, in Figure 10c, almost 100% capacity retention was achieved after more than 500 repetitive rectified charge/discharge cycles, with an average CE value of 99.8%.

Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) cathode, which has fast and stable sodium ion storage performance, was used as the corresponding electrode of NO-S-CNT-based sodium metal anode for SMB. In each half-cell, the NO-S-CNT electrode and the Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) electrode are precycled and then assembled into SMB, causing initial side reactions due to the formation of an SEI layer and occurring at the two electrodes. Reduces other irreversible reactions. In the precycling process, sodium loss during the cycling process was compensated by varying the loading amount of excess sodium metal. Since both NO-S-CNT and Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) exhibit significantly high CE values of approximately 100%, a lower amount of excess sodium metal would lead to a longer lifetime of the anode-minimized SMB. You can.

In order to demonstrate the high performance of SMB with a low loading amount of excess sodium metal, Na-NO-S-CNT//Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) cells with different loading amounts of excess sodium metal were used. The electrochemical performance of was tested at different current rates, and the total electrode weight was calculated as the sum of the loading amounts of Na metal, Na-NO-S-CNT, and Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ). It was calculated. The active anode and cathode loading masses were ~0.14 and ~2.05 mg cm ^-2 , respectively, and the electrode area consisted of 2.0 cm ^-2 respectively (total anode and cathode loading masses: ~0.28 mg and ~4.10 mg, respectively). In the precycling process, ~0.17 mg (~0.2 mAh), ~0.34 mg (~0.4 mAh), ~0.51 mg (~0.6 mAh), ~0.68 mg (~0.8 mAh), and ~0.86 mg were added to NO-S-CNTs, respectively. (~1.0 mAh) of excess sodium metal was loaded. The specific capacity of this injected excess sodium metal content corresponded to 40 to 200% of the cathode capacity, and the total anode mass (sodium metal + NO-S-CNT) was 11 to 28% of the active cathode mass.

Figure 12a shows galvanostatic charge/discharge profiles of full cells with different sodium metal contents, where the anode and cathode profiles are shown in Figure 11. The full-cell profile was similar to the cathode profile of the half-cell configuration (Figure 10), but the specific capacity decreased with increasing anode mass. Through anode mass-controlled full-cell testing, it was confirmed that the cycle life of SMB could be systematically extended due to excess sodium metal content.

When NO-S-CNTs were applied without excess sodium metal, the capacity of the anode-free SMB decreased rapidly even though both the anode and cathode had high CE values, and the capacity retention of the anode-free SMB was 5 cycles; It was 94.2%, 80.8%, and 75.2% of initial capacity at 10 and 15 cycles, respectively (Figure 12b). This shows that extra sodium metal is essential to maintain stable cycling even when high-performance electrode pairs with nearly 100% CE values are used in SMB.

When excess sodium metal of ~0.2 mAh, ~0.4 mAh, ~0.6 mAh, ~0.8 mAh and ~1.0 mAh is loaded into the NO-S-CNT, Na/NO-S-CNT//Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) SMB showed significantly improved cycling performance maintained for 125 cycles, 235 cycles, 370 cycles, 650 cycles, and 1000 cycles, respectively. The cycling performance was calculated when capacity retention reached up to 75% of initial capacity. In particular, the SMB containing excess sodium of ~1.0 mA h was able to operate for more than 1,000 cycles with a capacity retention of 75.2% of the initial capacity. Additionally, despite the increased metal content, the rate capabilities of SMB were slightly affected over the current density range of 0.1 to 8 Ag ^-1 , indicating excellent power performance (FIG. 12c).

Figure 12d shows the relationship between specific energy and power density in a Ragone plot along with previously reported results. The energy density of SMB with different anodes varies depending on the loading amount of excess sodium metal, and the SMB loaded with 0.2 mAh ^-1 Na had a high specific energy density of ~289 Wh kg ^-1 . In particular, the 1.0 mAh ^-1 Na loading SMB sample showed the lowest energy density of ~214.7 Wh kg ^-1 at 295.87 W kg ^-1 , but the specific energy density was lower than that of FeS _{2 - x} Se _x //NVP, SnPSe ₃ @G //NVP, FeS ₂ //NVP, FeP@OCF//NVP, NMTiO ₂ //NG, VN//AC, and S-HIC cells still showed much higher values compared to previous studies.

As seen in the SMB test with excess anode, specific energy density and cycling stability could be obtained by controlling the excess sodium metal loading. In other words, the electrochemical performance of SMB can be controlled through excess sodium metal loading, which can be adjusted depending on the targeted application. Considering its excellent electrochemical performance and low price, SMB according to embodiments of the present invention can not only be applied to large-scale energy storage devices for grid systems, but also be competitive in the EV and drone markets.

The embodiments of the present invention described above should not be construed as limiting the technical idea of the present invention. The scope of protection of the present invention is limited only by the matters stated in the claims, and those skilled in the art can improve and change the technical idea of the present invention into various forms. Accordingly, such improvements and changes will fall within the scope of protection of the present invention as long as they are obvious to those skilled in the art.

Claims (15)

Crystalline graphitic carbon structure; and
An amorphous carbon structure located on at least a portion of the crystalline graphite carbon structure,
The amorphous carbon structure is doped with nitrogen (N) and oxygen (O) atoms,
The amorphous carbon structure includes a pyridinic-N structure and a pyridonic-N structure. Compared to the pyridinic-N structure, the pyridonic-N structure Contains a large amount of
An anode material for a sodium metal battery having a ratio I _D /I _G of 0.1 to 0.3 between the D-band peak intensity (I _D ) and the G-band peak intensity (I _G ) measured in a Raman spectroscopic spectrum. According to paragraph 1,
The anode material is an anode material for a sodium metal battery having a specific surface area of 150 m ² g ^-1 or more. According to paragraph 1,
The anode material is an anode material for a sodium metal battery having a C/N element ratio and a C/O element ratio of 30 or less. According to paragraph 1,
The anode material is an anode material for a sodium metal battery that has a mesoporous pore structure with a nitrogen adsorption isotherm defined by IUPAC of TYPE Ⅳ. A sodium secondary battery comprising the anode material for a sodium metal battery of any one of claims 1 to 4. According to clause 5,
The anode material for the sodium metal battery is pre-loaded with sodium metal and contains excess sodium metal. Preparing a carbon material;
heat treating the carbon material; and
The method of manufacturing an anode material for a sodium metal battery according to any one of claims 1 to 4, comprising the step of functionalizing the carbon material with a heterogeneous element by treating the carbon material with one or more heterogeneous precursors. In clause 7,
A method of manufacturing an anode material for a sodium metal battery, wherein the carbon material is at least one selected from the group consisting of CNT, RGO, and carbon black. In clause 7,
In the heat treatment step, a method of manufacturing an anode material for a sodium metal battery, wherein the fine crystal structure of the carbon material is controlled by controlling the heat treatment temperature profile. In clause 7,
The heat treatment step is performed in a temperature range of 1500 to 2800 ° C. In clause 7,
The heterogeneous precursor is a method of manufacturing an anode material for a sodium metal battery, including one or more precursors of a nitrogen (N)-containing precursor and an oxygen (O)-containing precursor. delete delete delete delete