KR102602749B1 - carbon coated NMTVP nano-composite cathode material and method for manufacturing cathode material as sodium ion batteries - Google Patents

Abstract

The present invention relates to anode material technology for sodium ion batteries. More specifically, it not only improves eco-friendliness and economic efficiency by reducing the vanadium content, but also improves the electrochemical properties of sodium ion batteries containing this as an anode. The present invention relates to a carbon-coated NMTVP nanocomposite containing a compound of a novel composition, a manufacturing method thereof, and an application product containing the same.

Description

Carbon coated NMTVP nano-composite cathode material and method for manufacturing cathode material as sodium ion batteries}

The present invention relates to anode material technology for sodium ion batteries. More specifically, it not only improves eco-friendliness and economic efficiency by reducing the vanadium content, but also improves the electrochemical properties of sodium ion batteries containing this as an anode. The present invention relates to a carbon-coated NMTVP nanocomposite containing a compound of a novel composition, a manufacturing method thereof, and an application product containing the same.

Sodium-ion batteries (SIB) are currently one of the most studied energy storage devices after lithium-ion batteries (LIB). SIB was first introduced as an electronic device in the 1980s. However, the successful commercialization of LIBs due to their high performance in the early 1980s delayed the development of SIBs. However, research on SIBs accelerated significantly in the 21st century when researchers realized that available lithium resources were not meeting the growing demand for LIBs, especially with grid scale requirements. SIB initially made its entry into electronics as a potential LIB replacement due to the low cost and vast availability of sodium resources. However, it is still significantly inferior to the well-established LIB in terms of operating voltage specific energy and electrochemical stability.

Researchers are making extensive efforts to solve the above problems and are developing anode materials suitable for SIB, including olivine, sodium super-ionic conductor (NASICON) structure, Prussian blue compound, mixed polyanion material, and layered oxides.

Among the electrode materials established for SIB, phosphate with a NASICON structure has been studied extensively due to its highly stable crystal structure. Among the anode materials with a NASICON structure, the one most studied for SIB is Na ₃ V ₂ (PO ₄ ) ₃ (NVP), which has a high theoretical capacity of 110 mAh g ^-1 and an operating potential of 3.3-3.4 V. . NVP is an ideal anode for SIB, but its expensive and hazardous V (vanadium) content limits its widespread application. To overcome this problem, Goodenough's group replaced the environmentally friendly V component in the structure with a transition metal ion. Through systematic research by various research groups, a number of NASICON type anodes for SIB were successfully synthesized through cation exchange, including Na ₃ MnTi(PO ₄ ) ₃ , Na ₄ MnV(PO ₄ ) ₃ , and Na ₃ FeV( PO ₄ ) ₃ , Na ₃ MnZr(PO ₄ ) ₃ , Na _3.5 Mn _0.5 V _1.5 (PO ₄ ) ₃ and Na ₂ VTi(PO ₄ ) ₃ . However, these anode materials are still insufficient in terms of capacity and average operating potential for large-scale energy storage devices.

First-principles calculations based on density functional theory (DFT) and machine-learning (ML) are emerging as potential tools to accelerate materials discovery. Compared to conventional methods of discovering potential candidates through trial and error, DFT and ML approaches were found to be effective and time-efficient. The DFT method reconstructs the Schrödinger equation of ground state energy into electron density by implementing several approximations. DFT methods can predict material properties, including physical, chemical, and electrical properties. In the case of ML, a computer learns the patterns present in a data set without being explicitly programmed, and with the help of algorithms can derive relationships in the data and predict target outputs and, in this case, material properties. Therefore, ML techniques can be a very powerful screening tool for material selection in a short period of time. Additionally, combined ML-DFT methods have been demonstrated to predict multiple properties and are leading to the discovery of special materials.

Therefore, there is an urgent need to develop technology for a potential low-cost NASICON type anode material for high-energy sodium-ion batteries that solves the above-mentioned problems.

Domestic Patent Registration No. 10-0809570

As a result of numerous studies, the present inventors completed the present invention by developing a technology related to carbon-coated NMTVP nanocomposite that can improve the electrochemical properties of NVMP, an anode material for sodium ion batteries.

Therefore, the purpose of the present invention is to present a practical ML-DFT-experimental procedure to find a new anode material for SIB, and to produce a carbon-coated NMTVP nanocomposite with excellent electrochemical sodium storage properties of a new composition determined using this and its preparation. It provides a method.

Another object of the present invention is to provide application products such as a positive electrode with excellent electrochemical properties by including a carbon-coated NMTVP nanocomposite with excellent electrochemical sodium storage properties and a sodium ion battery including the positive electrode.

The object of the present invention is not limited to the object mentioned above, and even if not explicitly mentioned, the object of the invention that can be recognized by a person of ordinary skill in the art from the description of the detailed description of the invention described later may also naturally be included. .

In order to achieve the object of the present invention described above, the present invention first provides a NMTVP nanocomposite containing a compound represented by the following formula.

Na _(4-x) MnTi _x V _{(1-x) (} PO ₄ ) ₃

Here, 0 < x ≤ 0.5 is a real number.

In a preferred embodiment, the NMTVP nanocomposite made of the above compound has three-dimensional porosity.

In a preferred embodiment, when it has the three-dimensional porosity, its specific surface area is 100 m ² g ^-1 or more.

In a preferred embodiment, the compound is Na _3.5 MnTi _0.5 V _0.5 (PO ₄ ) ₃ .

In addition, the present invention includes the steps of dissolving sodium salt, manganese salt, and titanium precursor in a solvent to prepare reaction solution 1; Preparing reaction solution 2 by adding vanadium salt to deionized water and then adding organic acid as a reducing agent; Adding reaction solution 2 to reaction solution 1 and then adding phosphoric acid to prepare reaction solution 3; After heating the reaction solution 3, igniting the heated reaction solution 3 to induce a self-extinguishing combustion reaction to obtain a combustion precipitate; and heat-treating the combustion precipitate in an inert gas atmosphere. A method for producing a carbon-coated NMTVP nanocomposite is provided.

In a preferred embodiment, the reaction solution 3 contains sodium: manganese: titanium: vanadium in a molar ratio of 4-x:1:x:1-x (x is a real number where 0<x≤0.5).

In a preferred embodiment, the sodium salt is any one selected from the group consisting of sodium hydroxide, sodium nitrate, sodium sulfide, and sodium acetate, and the manganese salt is any one selected from the group consisting of manganese nitrate, manganese sulfate, and manganese acetate. One is, the titanium precursor is titanium isopropoxide or titanium nitrate.

In a preferred embodiment, the solvent is a polyol.

In a preferred embodiment, the vanadium salt and the organic acid are contained in a molar ratio of 1:2 to 1:3 in reaction solution 2.

In a preferred embodiment, the vanadium salt is any one selected from the group consisting of ammonium vanadate, vanadium acetate, vanadium nitrate, and vanadium sulfate, and the organic acid is any one selected from the group consisting of oxalic acid, acetic acid, and citric acid.

In a preferred embodiment, the temperature of the heated reaction solution 3 is 200°C to 450°C.

In a preferred embodiment, the heat treatment is performed at 600°C to 900°C for 1 hour to 12 hours.

In addition, the present invention provides a NMTVP anode material manufactured by any of the above-described manufacturing methods or a positive electrode containing any of the above-described NMTVP anode materials.

Additionally, the present invention provides a Na-ion battery including the above-described positive electrode.

In a preferred embodiment, it has a reversible capacity of 135 mAh g ^-1 at 0.25 C, an average discharge potential of 3.42 V, and a specific energy of 462 Wh kg ^-1 .

In a preferred embodiment, an operating potential of more than 4.08 V can be supplied after 100 cycles.

The carbon-coated NMTVP nanocomposite of the present invention described above not only preserves the crystal structure but also has excellent electrochemical sodium storage properties through multiple redox pairs, so when used as an anode material, it can amplify the practical capacity and functional voltage of the anode. there is.

In addition, the carbon-coated NMTVP nanocomposite manufacturing method of the present invention can improve electrochemical sodium storage properties by providing three-dimensional porosity through controlling crystal growth.

In addition, the applied product of the present invention has excellent electrochemical properties by including a carbon-coated NMTVP nanocomposite with a new structure and composition as an anode material.

These technical effects of the present invention are not limited to the scope mentioned above, and even if not explicitly mentioned, the effects of the invention that can be recognized by a person of ordinary skill in the art from the description of the specific contents for implementing the invention described later. Of course it is included.

Figure 1 is a scatter plot showing the ML-predicted formation energy per atom versus the ML-predicted formation energy per atom calculated using the algorithms: (a) XG Boost, (b) Random Forest, (c) Bayesian Ridge, and (d) SVM.
In Figure 2a, a is a scatter plot of the DFT formation energy compared to the formation energy predicted by different algorithms used in this work, and b is the XRD Rietveld refinement pattern (internal fit value) of Na _3.5 MnV _0.5 Ti _0.5 (PO ₄ ) ₃ . Figure 2b is the structural model of NMTVP, and Figure 2c is the DFT predicted structures: (a) Na _3.5 MnV _0.5 Al _0.5 (PO ₄ ) ₃ , (b) Na _3.5 MnV _0.5 Fe _0.5 (PO ₄ ) ₃ , and (c) ) Na _3.5 MnV _0.5 Zr _0.5 (PO ₄ ) ₃ . Figure 2d is an enlarged view of part of the SEM image of NMTVP/C, and Figure 2e is a FE-SEM image of NMTVP/C at (a) low magnification and (b) high magnification. Figure 2f is a HRTEM image of NMTVP/C at (e) low magnification and (f) high magnification, and Figure 2g is a brightfield image of the corresponding elements (Na, Mn, V, Ti, P, O, C) is.
In Figure 3, a is a thermogravimetric analysis graph of NMTVP/C, and b is a Raman spectrum.
In Figure 4, a is the XPS profile for V 2p of NMTVP/C, and b is the XPS profile for Mn 2p.
In Figure 5, a to e are the .
Figure ^6a shows the CV profile of the Na _3.5 MnV _0.5 Ti _0.5 (PO ₄ ) ₃ anode at 0.1 mV s -1 . Figure 6b is a multi-scan CV profile at various scan rates, b in Figure 6c is a graph of the contribution ratio of diffusion limited capacity and capacitance at 0.1 mV s-1, and c is a bar chart of the capacity contribution ratio at various scan rates. .
In Figure 7a, b shows the charge/discharge pattern of the Na _3.5 MnV _0.5 Ti _0.5 (PO ₄ ) ₃ anode at a rate of 110 mA g ^-1 , and c shows the periodic pattern required at a rate of 110 mA g ^-1 . will be. Figure 7b shows the selected charge/discharge pattern for NMTVP/C, where a is 110 mA g -1, b is the rate capacity curve, c is 1500 mA g ^-1 and d is 2000 mA g ^-1 . Figure 7c is the rate capacity profile at various current ratios. In Figure 7d, e and f are periodicity performance graphs at rates of 1500 mA g ^-1 and 2000 mA g ^-1 , respectively.
Figure 7e is a Ragone plot of the Na _3.5 MnV _0.5 Ti _0.5 (PO ₄ ) ₃ anode and various polycation electrodes used in the literature. The kinetic characteristics of the Na _3.5 MnV _0.5 Ti _0.5 (PO ₄ ) ₃ anode are shown, and h in Figure 7f is the voltage profile during GITT at a current density of 20 mAg ^-1 , and the electrochemical Na ⁺ (de) insertion process (i.e.) Sodium ion chemical diffusion coefficient calculated from GITT curves during (i) charging and (j) discharging.
A in Figure 8 shows the sodium storage characteristics of the Na _3.5 MnV _0.5 Ti _0.5 (PO ₄ ) ₃ anode, and the charge/discharge profile of the battery cycling between 4.15-.5V; b is the charge/discharge conditions, c to e are Ex situ XANES spectra for the NMTVP/C anode at the edge (c ) Mn-K edge, (d) V-edge and (e) Ti-k.
Figure 9a is the in situ XRD pattern within the selected scanning area (a) 19–22°, (b) 22–40°, and Figure 9b is the in situ
Figure 10a schematically shows the electrochemical mechanism that occurs at the NMTVP anode during Na (de)insertion, b in Figure 10b shows the half-cell charge/discharge profile, and c in Figure 10b shows the high voltage of the full cell configuration. This is a graph showing the operating potential (3.3 V) and excellent discharge capacity (152 mAh g ^-1 ), Figure 10c shows the results when various applied currents were applied to NMTVP//HC, and Figure 10d shows the sodium This is a graph comparing the performance of the ion battery and NMTVP//HC, where f in Figure 10e shows the CV profile of the NMTVP//hardened carbon full battery, and g in Figure 10e shows the performance at a current density of 150 mA g ^-1 . This is a graph of the results of measuring battery performance. Figure 10f is a photograph showing the NMTVP//HC full battery connected to a digital device to verify commercialization, and Figure 10g shows the results of power supply to an LED bulb and an electronic digital clock. It's a photo.

The terms used in the present invention are merely used to describe specific embodiments and are not intended to limit the present invention. 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 description of the invention, but are intended to indicate the presence of one or more other It should be understood that this does not exclude in advance the presence or addition of features, numbers, steps, operations, components, parts, or combinations thereof.

Terms such as first, second, etc. may be used to describe various components, but the components should not be limited by the terms. The above terms are used only for the purpose of distinguishing one component from another. For example, a first component may be referred to as a second component, and similarly, the second component may also be referred to as a first component without departing from the scope of the present invention.

Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by a person of ordinary skill in the technical field to which the present invention pertains. Terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with the meaning in the context of the related technology, and unless clearly defined in the present invention, should not be interpreted in an idealized or excessively formal sense. No.

The term “three-dimensional porosity” or “three-dimensional porous structure” used in the present invention means that the overall shape has a three-dimensional shape like a cotton candy, and a large number of pores with a diameter of 100 nm or less are uniformly distributed from the inside to the surface. It refers to a structure whose surface is formed to have a bumpy and curved shape due to a large number of pores.

When interpreting a component, it is interpreted to include the margin of error even if there is no separate explicit description. In particular, when terms of degree such as "about", "substantially", etc. are used, they may be interpreted as being used at or close to that value when manufacturing and material tolerances inherent in the stated meaning are presented. .

In the case of a description of a temporal relationship, for example, if a temporal relationship is described as ‘after’, ‘after’, ‘after’, ‘before’, etc., ‘immediately’ or ‘directly’ Even non-consecutive cases are included unless ' is used.

Hereinafter, the technical configuration of the present invention will be described in detail with reference to the attached drawings and preferred embodiments.

However, the present invention is not limited to the embodiments described herein and may be embodied in other forms. Like reference numerals used to describe the invention throughout the specification represent like elements.

The technical feature of the present invention is to present a practical ML-DFT-experiment procedure to find a new anode material for SIB, and to use this to determine a NMTVP nanocomposite with excellent electrochemical sodium storage properties of a new composition and the NMTVP nanocomposite using the traditional A method of manufacturing NMTVP nanocomposites with a new structure by controlling crystal growth using a modified thermal synthesis process rather than a thermal synthesis process and giving three-dimensional porosity through this, and excellent electrochemical properties by including the NMTVP nanocomposite as an anode material. It is found in applied products such as Na-ion batteries with special properties.

In other words, the present invention presents a practical ML-DFT-experiment procedure to find new anode materials for SIB. First, since crystal stability is an important factor in the study of new materials, ML techniques were used to determine the proposed electrode materials, i.e. previously We predicted the crystal stability of various unexplored Naxicon structural materials. This structure was then verified using DFT. In particular, the presence of Mn, Ti, and V included in the NMTVP nanocomposite with the following chemical formula not only preserves the crystal structure, but also preserves a large number of (Ti ³⁺ /Ti ⁴⁺ , V It was verified that the practical capacity and functional voltage of the anode can be amplified through redox pairs ( ³⁺ /V ⁴⁺ and V ⁴⁺ /V ⁵⁺ ).

Accordingly, the NMTVP nanocomposite of the present invention includes a compound represented by the following formula.

Na _(4-x) MnTi _x V _(1-x) (PO ₄ ) ₃

Here, 0 < x ≤ 0.5 is a real number.

The NMTVP nanocomposite made of the above compounds can have three-dimensional porosity. As shown, as an example, it has an overall shape like cotton candy and its surface has a shape formed by overlapping a large number of fine and thin petals, so the specific surface area is This effect is maximized.

When it has three-dimensional porosity like this, its specific surface area is at least 100 m ² g ^-1 or more, so it can be seen that the specific surface area of the conventionally known nanoparticle NVMP is more than 5 times larger than that of 20 m ² g ^-1 or less. In particular, the cathode material in which the compound is Na _3.5 MnTi _0.5 V _0.5 (PO ₄ ) ₃ has excellent sodium storage properties, and thus electrochemical properties can be improved.

Next, the method for producing a carbon-coated NMTVP nanocomposite of the present invention includes preparing reaction solution 1 by dissolving sodium salt, manganese salt, and titanium precursor in a solvent; Preparing reaction solution 2 by adding vanadium salt to deionized water and then adding organic acid as a reducing agent; Adding reaction solution 2 to reaction solution 1 and then adding phosphoric acid to prepare reaction solution 3; After heating the reaction solution 3, igniting the heated reaction solution 3 to induce a self-extinguishing combustion reaction to obtain a combustion precipitate; and heat-treating the combustion deposit in an inert gas atmosphere.

Here, the final reaction solution 3 contains sodium: manganese: titanium: vanadium in a molar ratio of 4-x:1:x:1-x, where x is 0<x≤0.5, which was experimentally determined when x exceeds 0.5. This is because the problem of voltage and capacity reduction occurs. The solvent used in reaction solution 1 is not limited as long as it can dissolve the sodium salt, manganese salt, and titanium precursor, but as an example, polyol may be used, such as ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, It may be any one selected from the group consisting of tetraethylene glycol. For example, if a TTEG solvent with three glycol groups is used, it can have various advantages because it has a higher boiling point and viscosity compared to a single ethylene glycol solvent. That is, it promotes the strong influence of TTEG on the formation of chelate complexes with transition metals, and the high boiling point of TTEG ensures sustained polyol combustion over an apparently long period of time, which can promote the formation of hierarchical or secondary particles, and long chain networks. This is because the TTEG solvent with can serve as a carbon-enriched backbone for active NVMP particles.

The sodium salt contains sodium ions, and any salt that can be dissolved in the solvent used in reaction solution 1 can be used. As an example, any salt selected from the group consisting of sodium hydroxide, sodium nitrate, sodium sulfide, and sodium acetate. It can be. The manganese salt also contains manganese ions, and any salt that can be dissolved in the solvent used in reaction solution 1 can be used. In one embodiment, it may be any one selected from the group consisting of manganese nitrate, manganese sulfate, and manganese acetate. . The titanium precursor contains titanium ions and can be any material that can be dissolved in the solvent used in reaction solution 1. As an example, it may be any one selected from the group consisting of titanium isopropoxide and titanium nitrate. .

In reaction solution 2, vanadium salt and organic acid may be included in a molar ratio of 1:2 to 1:3. The content of vanadium salt:organic acid is experimentally determined. If it exceeds 1:3, excessive carbon is generated and performance is impaired. This is because if it is less than 1:3, there is a problem that less carbon is formed and the conductivity is lowered. The vanadium salt contains vanadium ions, and any salt that can be dissolved in deionized water, which is the solvent used in reaction solution 2, can be used. As an example, a group consisting of ammonium vanadate, vanadium acetate, vanadium nitrate, and vanadium sulfate. It may be any one selected from . The organic acid may be any known organic acid that can be used as a reducing agent. In one embodiment, it may be any one selected from the group consisting of oxalic acid, acetic acid, and citric acid.

Reaction Solution 3 is prepared by mixing Reaction Solution 1 and Reaction Solution 2 and then adding phosphoric acid. The content of phosphoric acid in Reaction Solution 3 may be three times the amount of vanadium.

The temperature of heated reaction solution 3 may be 200°C to 450°C. If it is less than 200°C, ignition does not occur well when ignited with a torch, etc., and if it exceeds 450°C, the solvent contained in reaction solution 3 may spontaneously ignite. Because there is.

Heat treatment can be performed at 600°C to 900°C for 1 to 12 hours. The heat treatment temperature and time are also determined experimentally. If the temperature is below 600°C, there is a problem of not being able to obtain a crystalline phase, and if it exceeds 900°C, impurities are generated. may occur. Heat treatment time can be adjusted depending on the heat treatment temperature.

Next, the positive electrode containing the NMTVP nanocomposite of the present invention has excellent electrochemical properties because it is composed of a positive electrode material containing a compound represented by the above-mentioned chemical formula, and the Na-ion battery containing the same is composed of the known nanostructure NVMP. As can be seen in the experimental example described later, compared to a battery including a positive electrode, the electrochemical properties are superior.

Example

NMTVP nanocomposite was synthesized using the modified thermal synthesis method according to the present invention as follows.

1. Steps to prepare reaction solution 1

3.5 mmol sodium nitrate (Sigma-Aldrich, 99%), 1 mmol manganese nitrate (Sigma-Aldrich, 97%), and 0.5 mmol titanium isopropoxide (Sigma-Aldrich, 99.99%) in 100 mL of tetraethylene glycol. ) was dissolved to prepare reaction solution 1.

2. Steps to prepare reaction solution 2

Reaction solution 2 was obtained by dissolving 0.5 mmol of ammonium vanadate (JUNSEI, 99%) in a solution of 5 mL of deionized water and 1 mmol of reducing agent oxalic acid (DAEJUNG, 99.5%). Acids promote the reduction of V ⁵⁺ to V ³⁺ .

3. Steps to prepare reaction solution 3

Reaction solution 2 was added to reaction solution 1, then 3 mmol phosphoric acid (Daejung, 85%) was added and mixed well to obtain reaction solution 3, a homogeneous solution.

4. Step of obtaining combustion deposits

Reaction solution 3 was poured into an aluminum boat, heated to 450°C, and stored on a hot plate. Afterwards, the heated reaction solution 3 was ignited using a torch to activate an ultra-fast self-extinguishing combustion reaction and obtain a combustion precipitate.

5. Heat treatment step

Carbon-coated NMTVP nanocomposite (NMTVP/C) was obtained by collecting combustion deposits and heat-treating them at 600°C for 10 hours in Ar atmosphere.

Experiment method

1. Structural and physical properties

The crystal structure was identified using a 3D high-resolution X-ray diffractometer (Empyrean, PANalytical, The Netherlands). Powder X-ray diffraction (PXRD) patterns were obtained using a Shimadzu X-ray diffractometer (Cu Kα radiation, λ = 1.4506 Å). The morphology of the synthesized materials was observed via field emission scanning electron microscopy using a S-4700 Hitachi with an energy-dispersive X-ray detector (KBSI or other validation required). Additionally, a field emission transmission electron microscope (20kV, Philips Tecnai F20, KBSI Chonnam National University) with selected-area electron diffraction was used to analyze the lattice pattern. The elemental surface oxidation states of NMTVP/C samples were determined using X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific Multilab 2000) using an Al Kα X-ray source. Calibration was performed with the C 1s binding energy at 284.6 eV. Raman spectra were acquired using a JASCO laser Raman spectrometer NRS-5100 series to confirm the carbon properties of the samples. Inductively coupled plasma-optical emission spectroscopy was used using a PerkinElmer 4300 DV analyzer to determine the elemental composition of the samples.

2. Electrochemical characterization:

For half-cell assembly, a cathode was fabricated by mixing 70% of the active material, 20% of Ketjen Black, and 10% of polyacrylic acid, adding 5 wt% of the resulting mixture and N-methyl-2-pyrrolidone solvent, and casting the slurry onto aluminum foil. . Afterwards, the electrodes were dried in vacuum at 80 °C, and the dried foil was hot pressed between stainless steel rollers at 120 °C and punched into round disks with a diameter of 14 mm. CR-2032 coin-type cells were fabricated using Na metal as the reference/counter electrode and glass fiber as the separator in an Ar-filled glovebox. The electrolyte used was 1 M NaPF6 in ethylene carbonate/diethylene carbonate with 5% fluoroethylene carbonate additive. The cells were aged overnight before testing to ensure complete absorption of the electrolyte into the electrodes. A full cell was constructed using a CR-2032 coin-type cell, and Na _3.5 MnTi _0.5 V _0.5 (PO ₄ ) ₃ was used as the anode and hard carbon as the cathode. The hard carbon cathode is chemically soaked prior to assembly by mating with a metallic sodium anode (electrolyte packed in between) for one hour before being recovered and used in the entire cell structure. And overall cell system balance is achieved by adjusting the anode/cathode capacity ratio to about 1.2. The same electrolyte and separator used in the half-cell structure were used in the full-cell assembly, and the rectifier was performed using a BTS 2004H model (NAGANO KEIKI Co., LTD, Ota-ku, Tokyo, Japan) battery test system. Charge/discharge measurements at various current rates between 4.15 and 1.5V. Cyclic voltammetry (CV) and GITT studies were performed using devices from BIO-Logic Science Instruments.

3. In situ XRD study:

In situ XRD examination of NMTVP/C samples during galvanostatic cycling was performed using an in-house designed in situ cell. The in situ XRD setup includes a special split test cell used with a beryllium window for X-ray transmission. For in situ cells, the electrodes were made by mixing 70% active material, 20% Ketjen black, and 10% Teflonized acetylene black binder, then pressing it onto an aluminum mesh and applying it on top of beryllium. (Be) It serves as a current collector and X-ray transmission window. Metallic sodium disks were used as counter electrodes, Whatman glass fibers were used as separators, and 1 M NaPF6 in ethylene carbonate/diethylene carbonate with 5% fluoroethylene carbonate additive was used as electrolyte. In situ cells were galvanically charged and discharged at 10 mA g ⁻¹ current density between 4.15–1.5 V. In situ XRD output was recorded using a laboratory diffractometer using Mo Kα radiation in reflection geometry. Each array took 2 minutes to record a pattern. The recorded Mo Kα radiation pattern was converted to Cu Kα radiation pattern using the inbuild X'Pert Highscore Plus program.

4. Machine learning:

The machine learning method was implemented in a Python program using the Jupyter Notebook interface. We searched the Materials Project database for formation energies per atom using matminer.data_retrieval.retrieve_MP.1. The criteria used for data retrieval were “nelements” with values 1, 2, 3, and 4. Models were trained using XG Boost (XGB), Random Forest (RF), Bayesian Ridge (BR), and Support Vector Machine (SVM) regression algorithms. For the train-test split, we used a test size of 0.33 and a random state of 42. Before implementing the algorithm, the dataset was scaled and normalized using StandardScaler and Normalizer, respectively.

5. First principles calculations

The Quantum-Espresso package with projector augmented wave (PAW) pseudopotential and Perdew-Burke-Ernzerhof (PBE) exchange correlation functions was used to perform first-principles calculations based on density functional theory (DFT). The values set for the plane wave-based kinetic energy and charge density cutoffs were 30 Ry (408 eV) and 120 Ry (1632 eV). Using the “vc-relax” function and Broyden-Fletcher-Goldfarb-Shanno (BFGS), the lattice parameters and the 123 atoms in each structure can be optimized. The convergence threshold was set to 1.0E-06.

Experimental Example 1

In the study of new electrode materials for SIB applications, the proposed platform consists of three parts, including ML model prediction, DFT calculation, and experimental verification. First, the crystal stability of the proposed structure was predicted using an ML model. The available data set of formation energies per atom for ML was obtained from the Materials Project (MP). Then, the proposed structure was optimized using DFT calculations, and finally, an attempt was made to synthesize the selected material and identify its properties. From the MP database, 35 209 entries were obtained. After cleaning the data by removing duplicates, 24 695 pieces of data were finally obtained. For data functionalization, 176 descriptors from Matminer were used. Four algorithms implemented in Scikit-Learn were mostly used to train the ML model: XG Boost (XGB), Random Forest (RF), Bayesian Ridge (BR), and Support Vector Machine (SVM) regression. The effectiveness of the ML model was evaluated using the coefficient of determination (R2) and root mean square error (RMSE). Figure 1 illustrates the test results of each ML model. SVM regression performs better than XGB, RF, and BR, with R2 and RMSE values of 0.927 and 0.25, respectively. R2 values exceeding 0.9 can be a sign of a good model, especially for predicting formation energies.20 Additional optimization of the SVR algorithm was performed using GridSearchCV (see a in Figure 2a). The optimized C and gamma values were found to be 1000 and 0.1, respectively. After implementing the optimized C and gamma values in the SVM research model, the test results were improved to R2 and RMSE values of 0.944 and 0.22, respectively (see Figure 2a and b). Accordingly, the SVM model was selected to predict the crystal stability of the newly proposed NIB anode material. In particular, we focused on NASICON derived compounds and thus four new compounds were identified, namely Na _3.5 MnTi _0.5 V _0.5 (PO ₄ ) ₃ , Na _3.5 MnV _0.5 Fe _0.5 (PO ₄ ) ₃ , Na _3.5 MnV _0.5 Zr _0.5 (PO ₄ ) ₃ , and Na _3.5 MnV _0.5 Al _0.5 (PO ₄ ) ₃ were proposed and predicted. Na _3.5 MnTi _0.5 V _0.5 (PO ₄ ) ₃ , Na _3.5 MnV _0.5 Fe _0.5 (PO ₄ ) ₃ , Na _3.5 MnV _0.5 Zr _0.5 (PO ₄ ) ₃ , and Na _3.5 MnV _0.5 Al _0.5 predicted using the ML model. The formation energies per atom of (PO ₄ ) ₃ were -1.99, -1.86, -1.96, and -1.82 eV, respectively. These results indicate that the proposed compounds are predicted to have good crystal stability and that Na _3.5 MnTi _0.5 V _0.5 (PO ₄ ) ₃ is the most stable. We went to DFT calculations and verified the proposed structures. Note that although ML models can quantitatively predict crystal stability, allowing a more accurate assessment of the degree to which the NASICON structure is maintained, DFT calculations are still required. The loose structures of the proposed compounds are shown in Figures 2b and 2c. The lattice parameters of the structures of the proposed compounds obtained from these DFT calculations are shown in Table 1.

Material a b c α β γ NMVAP 8.98768 8.99277 21.47619 89.9142 90.0737 119.9723 NMVFP 9.42037 9.42182 22.50501 89.9948 90.0156 120.0059 NMVTP 9.47602 9.48161 22.50751 90.0992 89.8899 120.0612 NMVZP 9.30002 9.29997 22.56087 90.0031 90.0042 119.9801

It can be observed that all structures can preserve the NASICON properties. However, Na _3.5 MnV _0.5 Zr _0.5 (PO ₄ ) ₃ , and Na _3.5 MnV _0.5 Al _0.5 (PO ₄ ) ₃ experienced more distortion than Na _3.5 MnTi _0.5 V _0.5 (PO ₄ ) ₃ and Na _3.5 MnV _0.5 Fe. It showed a unit cell that was relatively narrower than _0.5 (PO ₄ ) ₃ . Previous reports indicate that slightly larger channels in the structure are desirable for SIB applications. Therefore, the Na _3.5 MnTi _0.5 V _0.5 (PO ₄ ) ₃ structure was the subject of further synthesis and characterization. The proposed structure is expected to be promising for NIB systems.

Experimental Example 2

The NASICON method NMTVP nanocomposite was synthesized through a modified rapid thermal synthesis process as in the examples. A combination of XRD and Rietveld refinement was adopted to determine the crystal structure of NMTVP nanocomposite. Bragg diffraction is (012), (104), (113), (006), (024), (211), (122), (030), (220), (131), (134), (042) , (404), (321), (232), (140), (143), (502), (330), (422) and ( 51 1), with a space of R-3c according to Xpert Highscore. It is well classified as a trigonal symmetry with groups. From structural elaboration, the lattice parameters of NMTVP were calculated as a = b = 8.85431 Å, c = 21.68460 Å, and V = 1472.285 Å (see Table 2).

These values are in good agreement with those reported in previous studies. In addition, the adequacy of the diagram (box in d in Figure 2a) confirms the validity of the refinement results, and the results of the structural framework of Rietveld refinement are presented in Table 2. Table 2 shows the crystallographic data of NMTVP powder obtained from Rietveld refinement.

Element Wyckoff Positions S.O.F. Biso x y z Na 0 0 0 0.99641 2.64602 Na 0.6425 0 0.25 0.81685 1.51747 Mn 0 0 0.14901 0.5 One V 0 0 0.14901 0.25 One P 0.298 0 0.25 One One O 0.0136 0.209 0.1932 One One O 0.1863 0.1721 0.0852 One One Ti 0 0 0.14901 0.25 0.5 Rwp = 10.977, Rp = 10.303, Rexp = 8.606, GoF = 1.62 a = b = 8.85431 Å, c = 21.6846 Å; α = β =90˚, γ = 120˚

The detailed microstructure of NMTVP synthesized by the modified thermal synthesis method of the present invention was observed through SEM (see Figures 2d and 2e) and TEM (see Figure 2f). NMTVP exhibits a flake-like shape with a size ranging from 100 to 500 nm. The high-resolution TEM image of the single thin section in F of Figure 2f shows a highly crystalline lattice pattern with a lattice spacing of 0.61 nm, which is attributed to the (012) plane of the trigonal crystal system, as previously shown by the PXRD pattern. . TEM/energy-dispersive

Experimental Example 3

Thermogravimetric analysis (TGA) was performed on NMTVP/C to determine the precise C content. The corresponding TGA diagram is shown in a of Figure 3. The amount of C in the NMTVP/C material was found to be 4.73%. Raman spectroscopy was performed to explain the properties of C coated in the NMTVP/C material, and the results are shown in Figure 3(b). The Raman spectrum consists of two characteristic peaks at 1349.71 cm ^-1 (D-band) and 1592.84 cm ^-1 (G-band). The sp ³ :sp ² ratio (i.e. ID/IG ratio) was determined to be 0.85, indicating that C in the sample was amorphous in nature.

Experimental Example 4

Additionally, XPS was employed to determine the chemical composition and oxidation state of transition metals in the NMTVP sample. The V 2P profile shown in a in Figure 4 shows two main peaks with binding energies of 519.2 and 522.8 eV, which correspond to the 2p3/2 and 2p1/2 orbital energy steps, respectively, separated by 3.6 eV. This is attributed to the trivalent nature of V in the NMTVP sample. The Mn 2P profile shown in b in Figure 4 shows peak values at 642.33 and 653.4 eV, which correspond to the 2p _3/2 and 2p _1/2 orbital energy steps, respectively, separated by 11.07 eV. This indicates that the binding energy of Mn is ²⁺ . Additionally, the paramagnetic metallic state appears at 645.9 and 656.6 eV in the form of a rearrangement satellite. The Ti 2P XPS profile shown in a of Figure 5 shows peak values at 459.4 and 464.9 eV with an energy separation of 5.5 eV. This represents the tetravalent nature of Ti. ^{The C} 1s The small peak at 289.1 eV represents COO bonds condensed at the surface. The O 1s The Na 1s and P 2p XPS profiles shown in d and e of Figure 5, respectively, show distinct peak values at 1071.5 eV and 133.1 eV, indicating the presence of Na and P in the sample. The results shown in f in Figure 5, which summarizes these observations, indicate the purity of the NMTVP sample.

Experimental Example 5

Electrochemical evaluation was performed to observe the sodium storage properties of NMTVP/C nanocomposites in half cells. CV indicated a sweep rate of 0.1 mV s ^-1 in the voltage range between 1.5-4.2 V for Na ⁺ /Na (see Figure 6a). In the CV profile, two oxidation peaks emerged at 2.27/2.13 and 3.62/3.36 V, which are associated with Na ³⁺ /Ti ⁴⁺ and V ³⁺ /V ⁴⁺ redox pairs, respectively, from the NMTVP/C anode. ⁺ Corresponds to step-by-step extraction of ions. In particular, although Mn ²⁺ is a redox-active transition metal, electrochemical Na ⁺ (de)insertion related to Mn ²⁺ /Mn ³⁺ redox activity does not occur at the NMTVP/C anode. Instead, it activates the V ⁴⁺ /V ⁵⁺ redox activity, characterized by 4.05/4.02 V. A similar phenomenon was observed in previous studies, and it was reported that the incorporation of Mn ²⁺ involves very vigorous redox activity of V. The characteristic peaks in successive scans overlap significantly, indicating a reverse Na ⁺ (de)insertion mechanism at the NMTVP/C anode. The electrochemical dynamics of the NMTVP/C anode were further investigated by performing CV in the characteristic voltage sweep region of 0.1 to 0.5 mV s ^-1 and the results are presented in Figure 6b. The sweep rate increases from 0.1 to 0.5 mV s ^-1 and the peak values in the CV pattern are gradually amplified. Therefore, the typical redox peaks at 2.27/2.13, 3.62/3.36, and 4/3.8 V move toward low or high pressure, respectively. At a particular potential, the contribution of diffusion-limited reaction kinetics and characteristic kinetics can be quantitatively determined from the formula:

i = k ₁ v + k ₂ v ^{1 /2} , (1)

Here, k ₁ v represents the ^surface capacity process and k ₂ v ^1/2 represents the diffusion-driven redox reaction at a finite scan rate. Surface capacitance consumption data at all investigated voltages were calculated and plotted in Figure 6c-b. The surface capacity (shaded area) calculated at a rate of 0.1 mV s ^-1 corresponds to 8% of the total capacity. Figure 6c-c presents histograms of capacitive and diffusion-driven consumption at progressive refresh rates. Surface-induced responses dominate at high scan rates. Interestingly, even at high scanning rates (0.5 mV s ^-1 ), a redox reaction of more than 14.28% occurs through the diffusion-induced insertion process. This demonstrates that the anode capacity mainly arises from the diffusion-driven Na ⁺ (de)insertion process.

Experimental Example 6

The electrochemical properties of the NMTVP/C anode were further observed in the voltage region of 1.5-4.15 V at a rate of 110 mA g ^-1 and the resulting charge/discharge profiles for the first two cycles are presented in Figure 7a-b. . The charging profile shows an extraction capacity of 112 mAh g ^-1 with two strong plateaus at 3.4 and 3.92 V. This represents an extraction of 1.96 moles of Na ⁺ ions from the NMTVP anode with respect to V ³⁺ →V ⁴⁺ and V ⁴⁺ →V ⁵⁺ . Meanwhile, the discharge curve shows a discharge capacity of 116 mAh g ^-1 with two main voltage plateaus at 3.98 and 3.48 V, which is adjusted by the reduction of V ⁵⁺ →V ⁴⁺ and V ⁴⁺ →V ^{3+ +} suggests reinsertion of the ion into the NMTVP/C structure. More importantly, the inclusion of Ti provided an additional insertion route for Na ⁺ ions into the NMTVP/C framework by reduction of Ti ⁴⁺ →Ti ³⁺ .

The continuous Na ⁺ (de)insertion properties of the NMTVP/C anode were monitored for 300 cycles at a rate of 110 mA g ^-1 in the potential range of 4.15-1.5 V. The resulting periodic stability curve is shown in Figure 7a-c. After 300 cycles, the NMTVP/C anode shows a reversible capacity of 94 mAh g ^-1 with 81% capacity and maintains nearly 100% coulombic efficiency. This demonstrates the excellent sodium storage properties of the NASICON-type NMTVP/C anode even after repeated charge/discharge cycles due to its fundamental structural stability. Selected charge/discharge patterns (after 100, 200 and 300 cycles) for the NMTVP/C anode at a current density of 110 mA g ^-1 are shown in Figure 7b. The voltage plateau is maintained in successive cycles, indicating that Na+ (de)insertion into the NMTVP/C NASICON structure follows the same path. Additionally, the stable profile in the charge/discharge curve suggests that the redox properties of Ti ³⁺ /Ti ⁴⁺ , V ³⁺ /V ⁴⁺ and V ⁴⁺ /V ⁵⁺ in NMTVP/C are preserved throughout the cycle. , which is essential to preserve the energy density of the battery at 321 Wh kg ^-1 .

To evaluate the ratio performance, various current densities of 25 to 2200 mA g ^-1 were applied to the NMTVP/C anode for 4 cycles within the potential range of 4.15-1.5 V, as shown in Figure 7c. The specific discharge capacities of ^the NMTVP/C anode were 133, 120, 110, 100, 89.1, 78, 70, 65 and It was 59.2 mAh g ^-1 . Additionally, when the current density is maintained at 100 mA g ^-1 for 10 consecutive cycles, the NMTVP/C anode displays a specific reversal capacity of 99 mAh g ^-1 . The charge/discharge plot for selected current densities in the investigated voltage range is It is presented in b of Figure 7b. Interestingly, a redox plateau pattern was observed at all applied current densities in the voltage range of 4.15–1.5 V. This further confirms the structural integrity and electrochemical Na reversibility of the NMTVP/C anode, which are all indispensable benchmarks for anode materials. The excellent ratio performance at all current densities over a narrow voltage range demonstrates the electrochemical stability of the NMTVP anode, a property that is invisible to most SIB electrode materials. Therefore, the ratio performance of the NMTVP/C anode is comparable to previously reported NASICON type anodes as listed in Table 3.

Cathodes Average Voltage (V vs. Na ⁺ /Na) Capacity Cycling stability Na ₄ MnV(PO ₄ ) ₃ ⁴ 3.45 101 mAh g ^-1 at 1C
(1C= 110 mA g ^-1 ) 89% at 1C (1000 cycles) Na ₄ MnCr(PO ₄ ) ₃ ⁵ 3.53 108 mAh g ^-1 at 0.1 C
(1C=100 mA g ^-1 ) 58% at 10 C (500 cycles) Na ₃ MnTi(PO ₄ ) ₃ ⁶ 3.5 114 mAh g ^-1 at 20 C
(1C=100 mA g ^-1 ) 81.2% at 0.1 C (100 cycles) Na ₃ MnZr(PO ₄ ) ₃ ⁷ 3.5 104 mAh g ^-1 at 0.1 C
(1C= 107 mA g ^-1 ) 91% at 0.5 C (500 cycles) Na ₃ V ₂ (PO ₄ ) ₃ ⁸
3.4 114 mAh g ^-1 at 0.5 C
(1C= 110 mA g ^-1 ) 95% at 0.5 C (1000 cycles) Na ₂ VTi(PO ₄ ) ₃ ⁹
2.5 120 mAh g ^-1 at 0.5 C
(1 C= 125 mA g ^-1 ) 86% at 50 C (1000 cycles) Na _3.5 MnTi _0.5 V _0.5 (PO ₄ ) ₃
(this invention) 3.42 135 mAh g ^-1 at 25 mA g ^-1 75% at 2 A g ^-1 (8000 cycles)

The rapid Na ⁺ (de)insertion capacity of the NMTVP/C anode was also evaluated at current rates of 1500 and 2000 mA g ^-1 over a voltage range of 4.15-1.5 V over long cycles. The resulting cycle performance is shown in Figure 7d. In Figure 7d, e and f show the cycle performance at a rate of 1500 and 2000 mA g ^-1 , respectively. After providing initial discharge capacities of 87 and 84 mAh g ^-1 , reversal capacities of 70 and 63 mAh g ^-1 It can be seen that is maintained after 1500 and 6000 cycles, respectively. This means that at rates of 1500 and 2000 mA g ^-1 the dominant capacity retention is 80% and 75%, respectively. More importantly, the selected charge/discharge pattern corresponding to the corresponding ratios, as shown in c and d in Figure 7b, preserves its shape and redox peak position over long periods of time, without sacrificing the redox properties, thereby increasing the NMTVP/NMTVP of Na+ ions. C shows rapid and stable efflux/influx into the framework. It has been demonstrated that excessively small numbers of anode materials for SIB exhibit stable voltage curves at rapid test rates. As summarized in the Ragone plot (Figure 7e), the NMTVP/C anode shows the best performance among the multivalent anion-based sodium anodes compared in this study. In particular, NMTVP/C shows the highest specific energy of 461 Wh kg ^-1 at a specific power of 85.5 W kg ^-1 (based on the mass of the anode material).

NASICON method The Na migration dynamics and phase transition of the NMTVP/C anode were observed by performing GITT at 20 mA g ^-1 with 10 min intervals between each pulse. The anode was rested for 2 h to obtain a quasi-equilibrium potential, and irradiation was performed in the potential range of 4.2-1.5 V. The resulting titration curve for the first cycle is shown in h of Figure 7f. The sodium diffusion coefficient can be calculated using Fick's second law from the transient voltage determined from the GITT curve:

, (2)

here, is the sodium chemical diffusion coefficient (cm ² s ^-1 ) and and A are the mass loading (g), molar volume (cm ³ mol ^-1 ), molecular weight (g mol ^-1 ), and sum of surface area (cm ² ) of the NMTVP/C anode, respectively. τ(s) is the time applied to the current pulse, and ΔEs and ΔEτ are the total difference between the cell voltage and the steady-state voltage fluctuation resulting from the persistent pulse in a single-step GITT irradiation, respectively. Continuous fluctuations of the equilibrium voltage indicate a single-state reaction mechanism of Na+ (de)insertion between 4.15 and 1.5 V (h in Figure 7f). The diffusion coefficients corresponding to the charging and discharging process are shown in i and j of Figure 7f. The calculated diffusion coefficients range from 3.96×10 ^-8 to 6.25×10 ^-9 and 5.5×10 ^-8 to 9.1×10 ^-9 for the extraction and insertion removal processes, respectively. This indicates rapid Na+ flux at the NMTVP/C anode. In particular, the rise and fall of the diffusion coefficient during the charging and discharging process in the voltage range of 2.27/2.13, 3.62/3.36 and 3.9/3.8 V, respectively, Ti ³⁺ /Ti ⁴⁺ , V ³⁺ /V ⁴⁺ and V ⁴⁺ This suggests that the redox activity of /V ⁵⁺ causes reverse Na + extraction and insertion at the NMTVP/C anode. The advanced Na+ diffusivity contributes to the rapid sodium storage properties of the NMTVP/C anode, resulting in excellent rate properties and rapid current probing capabilities.

Experimental Example 7

The expected peaks can be assigned as the (104), (113), (006), (024), (211), (122) and (030) Bragg planes of the upper NMTVP anode. (006) By itself, virtually no changes during charge and discharge conditions were performed. In ^- situ The designed cell is initially charged to 4.15 V and discharged to 1.5 V at a current density of 20 mA g ^-1 . The in situ Interestingly, when Na+ extraction started, the (113), (024), (211), (122), and (030) planes of NMTVP/C moved slightly toward higher angles. This indicates that the initial sodium charge was caused by single-state sodium extraction with the V ³⁺ →V ⁴⁺ redox dominating. However, the (113), (024), (122), and (030) planes disappear at 23.32°, 28.24°, 31.72°, and 34.77°, respectively, and new peaks appear at 23.93°, 28.92°, 32.72°, and 36.11°, respectively. It is interesting to see that this occurs, indicating a two-state reaction occurring in the V ⁴⁺ → V ⁵⁺ transition. More interestingly, the peaks at 23.93°, 28.92°, 32.72°, and 36.11° move to higher 2θ angles without interruption, and no additional peaks occur, indicating that the solid solution reaction is dominant in the NMTVP/C anode. Meanwhile, the (104) diffraction pattern alone splits into two peaks that arise sharply during the charging process. Therefore, the appearance of new peaks and disappearance of dominant peaks during the charging process suggests that further Na ⁺ extraction was suppressed by a two-state transition based on the V ⁴⁺ →V ⁵⁺ redox type. As the discharge process began, all peak values began to return to their original state. In particular, after discharging, the (104) peak value that underwent division begins to be integrated into the original peak value indicated by the open-circuit voltage (OCV). Additionally, the new peaks at 23.93°, 28.92°, 32.72°, and 36.11° disappear back to lower 2θ angles. And, the peak values corresponding to the (113), (024), (122), and (030) planes reappear at their original positions, indicating that the original frame is restored at the end of the first cycle. This strongly suggests that a reversible phase transition occurs between 4.15 and 1.5 V. _{Further exploration of the} in _situ

The above experimental results show that all peak values undergo significant shifts and then gradually return to their original states, indicating that the charge/discharge process is highly reversible. A change in peak position indicates a single-state solid solution reaction, whereas the appearance of a new peak or disappearance of an existing peak indicates a two-state reaction. Therefore, it can be seen that the NMTVP/C anode later undergoes a reversible solid solution and two-state transition during the electrochemical Na ⁺ (de)insertion process, resulting in a well-established voltage configuration.

To confirm the results obtained from the field study on the sodium storage mode of the NMTVP/C anode, ex situ XRD measurements were performed. The in situ XRD patterns at various charge/discharge depths show patterns equivalent to those of the in situ (030) occurrence/disappearance of peak value) (see Figure 9 a and b). Therefore, the correlation between in situ and ex situ observations clearly confirms solid solution and two-state processes. In order to obtain an in-depth understanding of the volume changes during the Na ⁺ (de)insertion process, Le Bail fitting is performed using the Xpert Highscore program. As illustrated in Figure 8, the unit cell volume gradually shrinks during the insertion and removal period and expands during the reinsertion process. The overall volume change (after charge/discharge period) is estimated to be about 4.7% (ΔV/VParent), close to that of Na ₃ V ₂ (PO ₄ ) ₃ but smaller than that of Na ₄ MnV(PO ₄ ) ₃ . The fairly small positive volume change indicates excellent rate performance of the NMTVP/C anode currently under investigation.

To understand the influence of transition metal ions during the Na ⁺ (de)insertion process, in situ X-ray absorption near-edge structure (XANES) spectra of NMTVP/C were obtained (Figure 8). c to e). The Mn K-edge absorption spectra at different charging and discharging condition depths along the pristine state are shown in c in Figure 8. The primordial Mn K-edge absorption energy at 6548 eV is due to the Mn ²⁺ state. In particular, the Mn K-edge spectrum maintains its position even after different electrochemical processes (OCV/charge/discharge). This shows that the Mn component in the NMTVP anode is not involved in the electrochemical Na ⁺ (de)insertion process. Meanwhile, the V K-edge spectrum shifts to higher energy levels after (or during) the initial charging process. The V K-edge spectrum is located above the pristine state spectrum (d in Figure 8), indicating that most of the V in the crystal structure has reached the high valence state (V ⁴⁺ ). This confirms that the initial sodium extraction is amplified by V ³⁺ →V ⁴⁺ oxidation. After discharge to 1.5 V, the energy returns to a stage close to that of the initial material, indicating reinsertion of Na ⁺ ions with respect to V ⁴⁺ →V ³⁺ decrease. It is important to note that V5+ was not observed in this study, confirming the presence of a small amount of V5 ⁺ in the charged electrode as in previous studies. During initial charging at 4.15 V, the Ti K-edge spectrum showed no obvious change in edge energy localization (e in Figure 8). In particular, the XANES spectrum of Ti after the initial charging process remains the same as before the electrochemical process, indicating that Ti exists in the 4+ state in the pristine and charged states. Interestingly, when the anode is discharged to 1.5 V, the Ti K-edge absorption shifts to a lower energy state due to the insertion of sodium ions into the NMTVP/C structure. This indicates that sodium insertion into the NMTVP/C structure is further driven by reduction of Ti ⁴⁺ →Ti ³⁺ . From the XANES observations, we can conclude that V ³⁺ /V ⁴⁺ and Ti ³⁺ /Ti ⁴⁺ redox pairs are the main driving forces behind the electrochemical Na ⁺ (de)insertion process at the NMTVP anode.

Experimental Example 8

Based on these observations, the electrochemical mechanism that occurs at the NMTVP anode during Na (de)insertion can be understood focusing on Figure 10a. Na _3.5 MnTi _0.5 V _0.5 (PO ₄ ) ₃ Extraction of the possible 2 moles of Na ⁺ from the anode results in the formation of Na _1.5 MnTi _0.5 V _0.5 (PO ₄ ) ₃ as an intermediate state after the first charge. After the first discharge process is completed, Na _3.5 MnTi _0.5 V _0.5 (PO ₄ ) ₃ is obtained again. In order to commercialize the battery, a sodium ion battery was manufactured and demonstrated using an additional NMTVP anode and a hydrogenated carbon (HC) cathode. Chemical sodium ion line replacement of the hardened carbon cathode is performed prior to cell assembly, and complete cell system balancing is achieved by adjusting the anode/cathode capacity ratio to approximately 1.2. The half-cell charge/discharge profile in b of Figure 10b shows the high voltage contribution of the NMTVP anode, resulting in a high operating potential (3.3 V) and excellent discharge capacity (152 mAh g ^-1 ) of the full cell configuration at 10 mA g ^-1. It shows the low-voltage Na insertion property of the hardened carbon anode (c in Figure 10b). The rate capability gain of NMTVP//HC is excellent, with cycles of 224, 126,114,102 and 100 mAh g ^-1 at different rates of 10, 20, 40, 80 and 100 mA g ^-1 , and the NMTVP//HC full cell is actually strong. It indicates comprehensiveness in application. The voltage-power density of NMTVP//HC is plotted in Figure 10d, comparing the performance of NMTVP//HC with the sodium ion batteries reported to date. In particular, it can be seen that the NMTVP//HC full cell energy density of the present invention significantly exceeds the performance of the so-called sodium complete device. Using cyclic voltammetry, the profile of the NMTVP//HC full cell under experimental conditions of 0.1 mV s ^-1 showed that oxidation sites occurred at 4.1, 3.5, and 2.1 V, and reduction sites occurred at 4.05, 3.3, and 1.8 V ( F in Figure 10e). Additionally, battery performance was measured at a current density of 150 mA g ^-1 (g in Figure 10e), and a discharge capacity of 50 mA hg ^-1 was still supplied after 100 cycles. To verify commercialization, the NMTVP//HC full cell was connected to a digital device and operated (Figure 10f). The NMTVP//HC full cell can supply a very high operating potential of 4.09 V even after 100 cycles (Figure 10e-f), which is very important for powering LED bulbs and electronic digital clocks (Figure 10g).

As can be seen from the above-described experimental results, the present invention manufactures NMTVP nanocomposite as a NASICON type anode material for SIB, and employs in situ XRD, ex situ XRD ^, ex situ The (de)insertion mechanism was explained. The in situ and ex situ results, combined with electrochemical observations, show that electrochemical diffusion of Na+ ions into the NASICON structure supplies three reversible redox pairs - Ti ³⁺ /Ti ⁴⁺ , supplying an average sodium (de)insertion potential of 3.42 V. , V ³⁺ /V ⁴⁺ and V ⁴⁺ /V ⁵⁺ -. A specific energy of 461 Wh kg ^-1 was achieved at a specific power of 85.5 W Kg ^-1 , indicating the suitability of the NMTVP/C anode for SIB.

Therefore, the NMTVP nanocomposite of the present invention was obtained by considering the sodium ion substitution properties by employing experiments and density functional theory calculations to predict the design of a wide range of high energy density anodes for SIB, and a sodium ion full battery containing it as an anode. The experimental results on the prototype are expected to promote the use of the NASICON-style NMTVP anode as a representative anode material for SIB in the near future.

Although the present invention has been illustrated and described with preferred embodiments as discussed above, it is not limited to the above-described embodiments and is intended to be used by those skilled in the art without departing from the spirit of the invention. Various changes and modifications will be possible.

Claims (16)

delete delete delete delete Preparing reaction solution 1 by dissolving sodium salt, manganese salt, and titanium precursor in a solvent;
Preparing reaction solution 2 by adding vanadium salt to deionized water and then adding organic acid as a reducing agent;
Adding reaction solution 2 to reaction solution 1 and then adding phosphoric acid to prepare reaction solution 3;
After heating the reaction solution 3, igniting the heated reaction solution 3 to induce a self-extinguishing combustion reaction to obtain a combustion precipitate; and
A method for producing a carbon-coated NMTVP nanocomposite comprising: heat-treating the combustion precipitate in an inert gas atmosphere to obtain a carbon-coated NMTVP nanocomposite containing a compound represented by the following formula.
[Chemical formula]
Na _(4-x) MnTi _x V _(1-x) (PO ₄ ) ₃
Here, 0 < x ≤ 0.5 is a real number.
According to claim 5,
The reaction solution 3 is a carbon-coated NMTVP nanocomposite characterized in that it contains sodium: manganese: titanium: vanadium in a molar ratio of 4-x:1:x:1-x (x is a real number of 0<x≤0.5) Manufacturing method.
According to claim 5,
The sodium salt is any one selected from the group consisting of sodium hydroxide, sodium nitrate, sodium sulfide, and sodium acetate, and the manganese salt is any one selected from the group consisting of manganese nitrate, manganese sulfate, and manganese acetate, and the titanium precursor A method for producing a carbon-coated NMTVP nanocomposite, characterized in that any one selected from the group consisting of titanium isopropoxide and titanium nitrate.
According to claim 5,
A method for producing a carbon-coated NMTVP nanocomposite, characterized in that the solvent is polyol.
According to claim 5,
A method for producing a carbon-coated NMTVP nanocomposite, characterized in that the vanadium salt and the organic acid are contained in a molar ratio of 1:2 to 1:3 in the reaction solution 2.
According to clause 9,
The vanadium salt is any one selected from the group consisting of ammonium vanadate, vanadium acetate, vanadium nitrate, and vanadium sulfate, and the organic acid is any one selected from the group consisting of oxalic acid, acetic acid, and citric acid. NMTVP nanocomposite manufacturing method.
According to claim 5,
A method for producing a carbon-coated NMTVP nanocomposite, characterized in that the temperature of the heated reaction solution 3 is 200 ℃ to 450 ℃.
According to claim 5,
A method for producing a carbon-coated NMTVP nanocomposite, characterized in that the heat treatment is performed at 600 ℃ to 900 ℃ for 1 to 12 hours.
delete delete delete delete