KR102599656B1 - 3-dimensional porous 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. NVMP anode material containing a compound of a new composition that can be used, and a method of manufacturing the same. It relates to a positive electrode and sodium ion battery including the same.

Description

3D porous anode material and manufacturing method thereof. Cathode and sodium ion battery including this {3-dimensional porous 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. NVMP anode material containing a compound of a new composition that can be used, and a method of manufacturing the same. It relates to a positive electrode and sodium ion battery including the same.

Among the many available energy storage devices, Sodium Ion Battery (SIB) is a favorable option for electronics in the post-Lithium Ion Battery (LIB) era. Currently, LIBs appear inadequate to meet the enormous requirements of grid-scale energy storage for the rapidly growing electronics market. Therefore, SIBs are attracting attention due to their low cost and vast availability. Due to the intercalation chemistry, acceptable redox potential (E ^ㆀ _(Na+/Na) =2.71 vs. standard hydrogen electrode, similar to lithium) and simple cell structure, the operating principle of SIB is not only inspired by the chemistry of LIB, but also almost same. For example, it is almost similar to the LIB anode and has strong electrochemical performance as sodium equivalents (Na _1.1 Li ₂ V ₂ (PO ₄ ) ₃ , Li ₂ NaV ₂ (PO ₄ ) ₃ , Na _2.4 V ₂ (PO ₄ ) ₃ There are many lithium-based intercalation compounds that show that the main bottleneck in realizing high electrochemical performance in SIB anodes is the lower ionic radius of Na ⁺ (1.02 Å) than that of Li ⁺ (0.76 Å) through extensive investigation. It was found that it was the slow diffusion kinetics of Na ⁺ ions that resulted in the rate performance.

To overcome these shortcomings, SIB has been systematically investigated over the current decade. As a result, many anodes with excellent electrochemical properties have been introduced for SIB, including layered oxides, polyanionic compounds, and Prussian blue analogs. _{Among these} , ^polyanionic compounds of the _{NASICON structure} with the _general formula _Na , Si, and As), it is recognized as a potential anode for SIB due to its unique 3D crystal structure. More importantly, NASICON-based materials exhibit extreme safety, excellent thermal stability and structural robustness. NVP, the most studied electrode material for SIB among the multivalent anion group, has a modest reversible capacity of 100 mA h g ^-1 with remarkable rate performance at an operating potential of 3.3-3.4 V operated by V ⁴⁺ /V ³⁺ redox pair. generates Although Na ₃ V ₂ (PO ₄ ) ₃ (NVP) is the most preferred for SIB electrode applications, the use of expensive and hazardous vanadium components limits large-scale applications. To address these issues, the Goodenough group introduced the new NASICON series of structured NVMP electrodes for SIB. The introduction of inexpensive and environmentally friendly Mn into the NASICON framework was advantageous in that the inexpensive Mn metal reduces the production cost of cathode materials. Introducing Mn into NVP generates additional Na ⁺ . Due to the site of the framework, the capacity is improved (110 mA hg ^-1 ); Additionally, the high redox voltage of Mn ²⁺ /Mn ³⁺ (3.6 V) increases the operating voltage of the NVMP electrode. However, like most polyanionic materials, NVMP exhibits poor electronic properties, which hinders the full utilization of NVMP capacity. This problem can be solved by following general strategies used for general polyanionic anodes, such as carbon composite development, transition metal ion exchange, particle reduction, and morphology control through various preparation techniques. Most of the prior art mainly focused on the NVMP/conductive carbon composite aspect to improve electrochemical reactivity.

Therefore, not only can the production cost be lowered by not using or reducing the amount of vanadium acetyl acetonate, which is a vanadium-based electrode but is expensive, but it is also possible to fully utilize the NMVP electrode through changes in crystal arrangement and customized shape design with limited particle size. There was a need to develop new anode materials that could develop power.

Domestic Patent Registration No. 10-1192355

As a result of numerous studies, the present inventors completed the present invention by developing a technology that can improve the electrochemical properties of NVMP, a cathode material for sodium ion batteries.

Therefore, the purpose of the present invention is to improve the poor electronic properties of NVMP, which prevents full utilization of NVMP capacity, by providing three-dimensional porosity through controlling crystal growth, and to provide nanoparticles with improved conductivity compared to conventionally known nanoparticle NVMP. A method for manufacturing a dimensionally porous NVMP anode material is provided.

Another object of the present invention is to provide an NVMP anode material with a new composition that can improve the poor electronic properties of NVMP that prevent full utilization of NVMP capacity by doping with other metal salts.

Another object of the present invention is to provide a positive electrode having excellent electrochemical properties by including an NVMP positive electrode material with three-dimensional porosity and/or a new composition, 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, preparing reaction solution 1 by dissolving sodium salt and manganese salt 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 precipitate in an inert gas atmosphere.

In a preferred embodiment, the reaction solution 1 contains sodium:manganese at a molar ratio of 4:1 to 4.1:1.

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. It is one.

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 a preferred embodiment, a metal salt for doping is further added in the step of preparing reaction solution 1.

In a preferred embodiment, the metal salt for doping is at least one selected from the group consisting of chromium salt, cobalt salt, nickel salt, and copper salt.

Additionally, the present invention provides an NVMP anode material containing a compound represented by the following formula.

Na ₄ VMn _(1-x) A _x (PO ₄ ) ₃

Here, 0 < x < 0.3 is a real number, and A is any one selected from the group consisting of Cr, Fe, Co, Ni, and Cu.

In a preferred embodiment, the NVMP anode material 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 ₄ VMn _0.9 Cu _0.1 (PO ₄ ) ₃ .

In addition, the present invention provides an anode comprising an NVM P anode material manufactured by any of the above-described manufacturing methods or any of the above-described NVMP anode materials.

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

In a preferred embodiment, 90% capacity is maintained with a capacity of 79 mA hg ^-1 after 450 cycles at a rate of 1.5C.

In a preferred embodiment, 86% capacity is maintained with a capacity of 68 mA hg ^-1 after 3000 cycles at a rate of 30C.

The method for manufacturing the 3D porous NVMP anode material of the present invention described above can improve the poor electronic properties of NVMP, which prevents full utilization of NVMP capacity, by providing 3D porosity through controlling crystal growth.

In addition, the NVMP anode material of the present invention can improve the poor electronic properties of NVMP, which prevents full utilization of NVMP capacity, by doping with other metal salts.

In addition, the positive electrode of the present invention and the sodium ion battery including the positive electrode have excellent electrochemical properties by including NVMP positive electrode material with three-dimensional porosity and/or a new composition.

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 shows a traditional thermal synthesis process and a modified thermal synthesis process according to the present invention.
Figures 2a and 2b show the corresponding XRD patterns and SEM images of the MnS2 cathode.
Figure 3a is an SEM image of the combustion precipitate obtained from the manufacturing method of the present invention, and Figure 3b shows a PXRD pattern.
Figure 4 is a graph comparing the XRD patterns of the cathode material obtained by the manufacturing method of the present invention and the cathode material obtained by the conventional method.
Figure 5 shows the XRD Rietveld refinement pattern: (a) Na4VMn(PO4)3 (inset: goodness-of-fit value), (b) Na4VMn0.9Cu0.1(PO4)3 (inset: goodness-of-fit value) (c) NVMP Structural model of /NVMCP, (d) facile Na-ion diffusion viewed from the enlarged C-axis.
In Figure 6, a is the SEM image of NVMP/C/NPs, and b is the SEM image of NVMP/C/CC.
In Figure 7, a is the SEM image of NVMCP/C/CC, b and c are HRTEM images of NVMCP/C/CC at low and high magnification, respectively, and d is the corresponding elements (Na, Mn, V, P, O, Cu). ), and e and f are the XPS profiles of the V 2p line and Mn 2p line of NVMCP/C/CC after deconvolution of peak values, respectively.
In Figure 8, a and b are additional TEM images and low-resolution images of Na ₄ VMn _0.9 Cu _0.1 0(PO ₄ ) ₃ cotton candy, and c is a local enlarged image.
Figure 9 is an XPS profile for NVMCP/C/CC with (a) C 1s; (b) O1s; (c) Na 1s; (d) P 2p; (e) Cu 2p and (f) survey spectra.
In Figure 10, a is a graph of the thermogravimetric analysis results of NVMCP/C/CC, and b is the Raman spectrum of NVMCP/C/CC.
Figure 11 is the nitrogen adsorption/desorption isotherm of NVMP/C/NP, NVMP/C/CC, and NVMCP/C/CC.
Figure 12a is a comparative graph of charge/discharge profiles for NVMP/C/NPs, NVMP/C/CC and NVMCP/C/CC anodes, and Figure 12b is a graph for NVMP/C/NPs, NVMP/C/CC and NVMCP/C/ This is a ratio performance comparison graph for CC anode.
In Figure 13, (a) is the PXRD profile of the Na ₄ VMn _0.85 Cu _0.15 (PO ₄ ) ₃ and Na ₄ VMn _0.8 Cu _0.2 (PO ₄ ) ₃ electrodes, (b) is the PXRD profile of the Na ₄ VMn _0.85 Cu _0.15 (PO ₄ ) ₃ electrodes. Charge/discharge profiles _of Na ₄ VMn _{0.8 Cu 0.2 ( PO 4} ) ₃ at _0.25 C rate _, ( _c ) at various current rates _{. 4} ) Velocity profile plot for ₃ electrodes.
Figure 14a is the periodicity pattern for the NVMCP/C/CC anode obtained at a rate of 1.5C, Figure 14b is the charge/discharge profile of the NVMCP/C/CC anode at a rate of 1.5C, and Figure 14c is the NVMCP/C/CC anode obtained at a rate of 30C. The periodicity curve of the CC anode and Figure 14d is the Ragone-diagram of the NVMCP/C/CC anode along different layered electrodes available in the literature.
Figure 15a is the discharge profile for the NVMCP/C/CC anode at 30C rate, Figure 15b is the external XRD profile, and Figure 15c is the external SEM image for the NVMCP/C/CC anode at 30C rate after 3000 cycles.
In Figure 16, (a) is the CV profile for NVMCP/C/CC at 0.1 mV s ^-1 , (b) is the multiple injection CV profile at various injection rates, (c) is the diffusion limiting capacity at 0.2 mV s ^-1 Contribution ratio of capacitive dose, (d) is a bar plot of dose contribution ratio at different injection rates.
(a) in Figure 17 is an electrochemical charge/discharge profile of the configured NVMCP/C/CC spectrum electrochemical cell tested within 3.8-2.4 V at 20 mA g ^-1 . The resulting profile kickoff from scan number 1 (OCV) to scan number 450 (2.4 V, end of first discharge). In situ XRD profiles at selected scanning angles (2) of (b) 18-26 ^o , (c) 27-36 ^o , and (d) 41-66 ^o .
In Figure 18a, (a) and (b) are the voltage profiles during GITT at a current density of 20 mA g ^-1 for the NVMCP/C/CC and NVMP/C/CC anodes, respectively, and (c) and (d) are the voltage profiles for the NVMCP/C/CC anodes. This is an enlargement of a single titration curve during the charging process for C/CC and NVMP/C/CC anodes, and (e and f) and (g and h) in Figure 18b are for NVMCP/C/CC and NVMP/C/CC anodes, respectively. It is the sodium ion chemical diffusion coefficient calculated from the GITT curve from the charge/discharge process during the electrochemical reaction.
Figure 19a is the total DOS of the NVMP sample, Figure 19b is the total DOS of the NVMCP sample, Figure 19c is the expected DOS of the NVMP sample, and Figure 19d is the expected DOS of the NVMCP sample.
Figure 20a (a) is the galvanostatic charge/discharge profile, (b) is the cyclicity plot of the Na ₄ VMn _0.9 Cu _0.1 (PO ₄ ) ₃ anode at 1.5 C in 1.0 M NaPF ₆ in the DGM electrolyte; In Fig. 20b, (c) is the galvanostatic charge/discharge profile and (d) is the cyclicity plot of the MnS ₂ cathode at 200 mA g ^-1 in 1.0 M NaPF ₆ in the DGM electrolyte. Figure 20c shows electrochemical performance of NVMCP/C/CC/MnS ₂ full cell in 1.0 M NaPF ₆ in DGM electrolyte; (e) Galvanostatic charge/discharge profile and (f) cyclicity plot in the potential range of 0-3.6 V at 40 mA g-1.

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 control crystal growth by using a modified thermal synthesis process rather than a traditional thermal synthesis process, and to provide three-dimensional porosity through this, thereby improving the poor electronic properties of NVMP, which prevents full utilization of NVMP capacity. A method of manufacturing a three-dimensional porous NVMP anode material with improved conductivity compared to the conventionally known nanoparticle NVMP, as well as a new composition of NVMP anode material that can improve the poor electronic properties of NVMP by doping with other metal salts manufactured by the method or by doping with other metal salts. It is in

That is, as shown in Figure 1, the traditional thermal synthesis process immediately prepares a comparative example reaction solution by simultaneously dissolving phosphoric acid, acetate (Na, Mn) metal precursor, and V acetylacetonate precursor in tetraethylene glycol (TTEG) solvent. However, the method of manufacturing the 3D porous anode material of the present invention uses the modified thermal synthesis process shown in FIG. 1 to prepare reaction solution 1 by dissolving only nitrate Na and Mn metal precursors in TTEG solvent, and using organic acid as a vanadium source. That is, reaction solution 2 was prepared separately by dissolving ammonium vanadate in deionized (DI) water using oxalic acid as a reducing agent to induce reduction of V ⁵⁺ to V ³⁺ , and then adding reaction solution 1 to reaction solution 1. This is because reaction solution 3 was created by adding 2 and then homogenizing it by adding phosphoric acid.

Due to the difference in the dissolution order of these components, as described later, the comparative example reaction solution and reaction solution 3 were ignited with a torch to induce a self-extinguishing combustion reaction to obtain combustion precipitate, and the obtained combustion precipitate was heat treated in an inert atmosphere. Even if the same method is performed, the structure of the finally obtained cathode material is as shown in Figure 1. The cathode material obtained through the traditional thermal synthesis process was aggregated NVMP/C nanoparticles, but with the modified thermal synthesis process of the present invention, the anode material obtained through the traditional thermal synthesis process was aggregated. This is because the obtained anode material was a three-dimensional porous structure with a uniform carbon coating in the shape of cotton candy, that is, NVMCP/C/CC.

Therefore, the method for manufacturing a three-dimensional porous NVMP anode material of the present invention includes preparing reaction solution 1 by dissolving sodium salt and manganese salt 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, reaction solution 1 contains sodium:manganese at a molar ratio of 4:1 to 4.1:1. The molar ratio of sodium:manganese was determined experimentally, and if it exceeds 4.1:1 or is less than 4:1, there is a problem of impurities forming. Because. The solvent used in reaction solution 1 is not limited as long as it can dissolve sodium salt and manganese salt, but in one embodiment, polyol may be used, such as ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, and tetraethylene. It may be any one selected from the group consisting of 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. .

If necessary, additional metal salt for doping can be added in the step of preparing reaction solution 1. The metal salt for doping may be any one selected from the group consisting of chromium salt, cobalt salt, nickel salt, and copper salt. The metal salt for doping may also be any salt that contains a metal ion for doping and is soluble in the solvent used in reaction solution 1. The metal salt for doping is added to replace the manganese ion that replaces the vanadium ion in the positive electrode material, and its content was experimentally determined as described later. Can be used in a range.

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 NVMP anode material of the present invention includes a compound represented by the following formula.

Na ₄ VMn _(1-x) A _x (PO ₄ ) ₃

Here, 0 < x < 0.3 is a real number, and A is any one selected from the group consisting of Cr, Fe, Co, Ni, and Cu.

In particular, the NVMP anode material made of a compound has three-dimensional porosity. As shown, as an example, it has an overall shape like cotton candy and its surface is formed by overlapping a number of fine and thin petals, so the specific surface area is high. The 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, when the anode material in which the compound is Na ₄ VMn _0.9 Cu _0.1 (PO ₄ ) ₃ has three-dimensional porosity, the electrochemical properties can be improved because the specific surface area is 141.87 m ² g ^-1 .

Next, the positive electrode containing the NVMP positive electrode material of the present invention has excellent electrochemical properties because it is composed of the positive electrode material containing the compound represented by the above-mentioned chemical formula, and the Na-ion battery containing the same is composed of the known nanostructured 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 1

The cathode material was synthesized as follows using the modified thermal synthesis method according to the present invention shown in Figure 1.

1. Steps to prepare reaction solution 1

Reaction solution 1 was prepared by dissolving 4 mmol sodium nitrate (Sigma-Aldrich, 99%) and 1 mmol manganese nitrate (Sigma-Aldrich, 97%) in 100 mL of tetraethylene glycol (TTEG) solvent.

2. Steps to prepare reaction solution 2

1 mmol ammonium vanadate (JUNSEI, 99%) was added to 5 mL DI water and 2 mmol oxalic acid (DAEJUNG, 99.5%) was added as a reducing agent and stirred well for 20 minutes to obtain reaction solution 2 (blue).

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

The combustion deposits were collected and heat-treated at 800°C for 12 hours in Ar atmosphere to obtain carbon-coated NVMP with a cotton candy-like microstructure, that is, three-dimensional porous NVMP anode material 1 (NVMP/C/CC).

Example 2

The same procedure as in Example 1 except that 0.9 mmol manganese nitrate and 0.1 mmol copper nitrate (Sigma-Aldrich, 99%) were used instead of 1 mmol manganese nitrate (Sigma-Aldrich, 97%) in the step of preparing reaction solution 1. By performing the method, 3D porous NVMP anode material 2 (NVMCP/C/CC) was obtained.

Comparative example

NVMP/C nanoparticles were prepared using the traditional thermal synthesis process shown in Figure 1 as follows.

Na and Mn metal acetate precursors (Sigma-Aldrich, 99%) and V acetylacetonate precursor (Sigma-Aldrich, 97%) were decomposed in tetraethylene glycol (TTEG) solvent and then phosphoric acid (Daejung, 85%) was added to stoichiometry. After adding it to the boiler, igniting the flammable reaction solvent with a torch to obtain combustion precipitates were collected and heated at 800°C for 12 hours in Ar atmosphere to obtain agglomerated NVMP/C, that is, comparative cathode material (NVMP/C/NP). .

Experiment method

1. Structure and shape characteristics

Powder X-ray diffraction (PXRD, Cu Ka radiation, l = 1.5406 Å) patterns of the electrode material were obtained using a Shimadzu X-ray diffractometer. Thermogravimetric analysis (TGA) was performed using an SDT Q600 thermobalance with a temperature gradient of 5 °C min ^-1 . High-resolution XRD patterns of the samples were achieved using a 3D high-resolution X-ray diffractometer (Empyrean, PANalytical, Netherlands). The surface morphology of the anode material was characterized by field-emission scanning electron microscopy (FE-SEM) using a Hitachi S-4700 equipped with an energy-dispersive X-ray spectroscopy (EDS) detector, and the lattice pattern was selected. Calculations were made by field emission scanning electron microscopy (FE-TEM) using a Philips Tecnai F20 (200 kV; KBSI Chonnam National University) equipped with a selected area electron diffraction system (SAED). Raman spectra were recorded using a JASCO Laser Raman Spectrometer NRS-5100 series to confirm the presence of carbon in the sample. The composition of the sample was characterized through ICP-OES (inductively coupled plasma-optical emission spectroscopy) using a PerkinElmer 4300 DV analyzer. The surface area of the sample was calculated using nitrogen adsorption and desorption isotherms using a Brunauer-Emmett-Teller surface analyzer (BET, Micromeritics ASAP 2010).

2. In situ synchrotron XRD characterization

In situ synchrotron The High-resolution formation in the incident space was confirmed by employing a Si (111) monochromator and a Si (111) crystal detector. The pattern was established using a wavelength of 0.9997 Å. Additionally, the XRD peak value was plotted after recalculating the 2θ value using Cu Ka radiation (l = 1.5414 Å). For field cell construction, 70% active material was mixed with 20% carbon black (Lion Corporation, Japan) and 10% TAB binder (Hohsen Corporation, Japan). Then, the slurry was spread on an aluminum mesh and maintained in a spectroelectrochemical cell. The battery was cycled to a fully charged/discharged state using a portable electrostatic potentiometer at a fixed current density of 20 mA g ^-1 . Kapton tape was attached to the outer case opening of the test cell.

3. Electrochemical characterization

For electrochemical characterization, a homogeneous slurry was created from the active material (70%), Denta black conductive carbon (15%), and a polyacrylic acid binder (15%) of N -methyl-2-pyrrolidone. The slurry was uniformly coated on an Al-thin film current collector using a doctor blade technique, dried at 80 °C in a vacuum oven, compressed between a pair of stainless steel rollers (maintained at 120 °C), and taped into a circular disk. Sodium metal was employed as the counter electrode. NaPF6 (1 M) in an ethylene carbonate/propylene carbonate (EC/PC) electrolyte with 2% fluoro-ethylene carbonate (FEC) addition was collected in an argon-filled glovebox and stored overnight. After aging, 2.4 to 3.8 V vs. Electrochemical charge/discharge characterization was performed using a BTS2004H (NAGANO KEIKI Co., LTD., Ohta-ku Tokyo, Japan) battery tester at different current densities between Na+/Na. Cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT) measurements were performed using BIO-Logic Science Instruments. To assemble a sodium complete battery, MnS2 was used as the cathode and NVMCP/C/CC, a three-dimensional porous anode material, was used as the anode in the CR2032 coin battery configuration. The MnS2 anode material was prepared by a known method. The MnS2 cathode (80%) was mixed with Super P carbon (10%) in the presence of a 5 wt% aqueous solution of sodium carboxylmethyl cellulose (4%) and a 50 wt% aqueous solution of styrene butadiene rubber (6 wt%). The corresponding XRD patterns of the MnS2 cathode (slurry coating) are presented in Figures 2a and 2b along with SEM images. The calculated and normalized weight ratio of the two electrodes is 1:7 (cathode:anode). The electrolyte was 1.0 M NaPF6 in DGM. The anode and cathode materials were tested in half cells in the same electrolyte and the results are presented in ESI. The sodium full cell was tested in a potential window of 0-3.6 V at a current density of 40 mA g ^-1 .

4. Specific power and specific energy calculation

The specific energy was calculated as E (W h kg ^-1 ) = specific capacity ㅧ potential (average operating potential).

The specific power was calculated as P (W kg ^-1 ) = I ㅧ V /2 m .

Here, I is the applied current (A), V is the average operating potential (V), and m is the active mass on the anode side.

5. First principles calculations

First-principles calculations based on density functional theory (DFT) are performed using the Quantum-Espresso package with projector augmented wave (PAW) pseudopotentials and Perdew-Burke-Ernzerhof (PBE) functional exchange correlation. carried out. A plane wave reference set to a cutoff energy of 30 Ry (408 eV) was used. The positions of atoms in the pristine NVMP and NVMCP structures were relaxed using Broyden-Fletcher-Gold-farb-Shanno (BFGS) and the Brillouin zone was resolved using k -point meshes of 2 ㅧ 2 ㅧ 2 and 2 ㅧ 2 ㅧ 1, respectively. Samples were collected.

To study the density of states (DOS), the DFT + U method was applied using field potentials U of 3.9 and 3.25 eV, respectively.

Experimental Example 1

The structure of the combustion deposit obtained in Example 1 was observed by applying scanning electron microscopy (SEM), and the powder X-ray diffraction (PXRD) pattern of the combustion deposit was observed, and the results are shown in Figures 3A and 3, respectively. Shown in 3b.

As shown, the combustion sediment for NVMCP/C/CC was found to have a three-dimensional porous structure in the shape of cotton candy even before heat treatment (see Figure 3a). Meanwhile, as shown in Figure 3b, the powder X-ray diffraction (PXRD) pattern of the combustion deposit revealed amorphous features and the corresponding peak values were classified as triangular NVMP. However, a number of peaks were not completely grown in the samples obtained, indicating that the combustion deposits require heat treatment to obtain a desirable product with high crystallinity.

Experimental Example 2

PXRD patterns were observed for anode materials 1 and 2, namely NVMP/C/CC and NVMCP/C/CC obtained in Examples 1 and 2, and NVMP/C/NP obtained in Comparative Example, and the results are shown in FIG. indicated.

As shown in Figure 4, the comparative PXRD patterns of the obtained NVMP/C/NP, NVMP/C/CC and NVMCP/C/CC anode materials revealed similarities and all Bragg diffraction peak values were achieved from the X'Pert Highscore Plus program. It can be seen that it can be well classified as a monoclinic NVMP sample.

Experimental Example 3

To obtain a complete understanding of the crystalline configuration of the material, Rietveld refinement was performed on the obtained high-resolution patterns of NVMP/C/CC and NVMCP/C/CC samples with the help of the 5, Table 1, Table 2, and Figures 3a and 3b.

Table 1 shows crystallographic data of Na ₄ VMn(PO ₄ ) ₃ powder obtained from Rietveld tablets.

As shown in Figure 5 a and b, both samples It is well classified by the characteristics of the trigonal system. The lattice parameters of the NVMP structure are calculated as a = b = 8.9649 Å and c = 21.47864 Å. In particular, the “suitability” values for the two samples (inset figures in a and b of Figure 5) further confirm the refinement process. Structural models of these samples were constructed using the VESTA package and are illustrated in Figure 5c.

Table 2 shows crystallographic data of Na ₄ VMn _0.9 Cu _0.1 (PO ₄ ) ₃ powder obtained from Rietveld purification.

The NVMP/NVMCP structure was composed of edge-sharing MO ₆ (M = V, Mn, Cu) octahedral and PO ₄ tetrahedral units forming a [M ₂ (PO ₄ ) ₃ ] ₄ anionic framework. The M element is located at a similar Wyckoff site, i.e., the 12 c site, while the Na element is located at two different sites, i.e., 6b and 18e. After Cu-doping (NVMCP), slightly different crystallographic parameters can be observed, namely a = b = 8.96072 Å and c = 21.48843 Å (Table 2). Enlargement of the c -axis is also expected to provide easy Na-ion diffusion, as shown in Figure 5d.

Experimental Example 4

Field emission scanning electron microscopy (FE-SEM) was performed to observe the size and shape of all prepared anode materials, and the resulting photograph is shown in FIG. 6.

The NVMP anode, i.e., NVMP/C/NPs, manufactured using a conventional thermal synthesis process showed the form of nanoparticles with an average particle size in the range of 50-100 nm (Figure 6a). In contrast, the NVMP/C/CC anode prepared by the modified thermal synthesis process of the present invention exhibited a unique cotton candy-like morphology, that is, three-dimensional porosity, consisting of several nanoarrays with an average particle size in the range of 1-2 mm.

Experimental Example 5

Targeting NVMCP/C/CC anode materials, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), including FE-SEM, and energy dispersive X-ray (energy) Additional investigations such as dispersive X-ray (EDX) experimental mapping studies were conducted, and the results are shown in Figures 7 to 9.

As shown in Figure 7 a, the Cu-doped NVMCP/C/CC anode maintained a unique cotton candy-like morphology (consisting of multiple nanoarrays) and the same particle size.

A transmission electron microscopy (TEM) study investigating comprehensive crystallographic features confirmed that the NVMCP/C/CC anode was composed of a cotton candy-like microstructure assembled from nanoarrays (Figure 7b). High-resolution transmission electron microscopy (HRTEM) aimed at the edge of NVMCP/C/CC revealed a rich crystal lattice with a layer spacing of 0.28 nm due to the (116) plane of the trigonal NVMP (Figure 7). c). More importantly, the NVMCP cotton candy-like anode is clearly visible wrapped in a 5-7 nm thick layer of smooth amorphous carbon. Additionally, an energy dispersive X-ray (EDX) experimental mapping investigation was conducted to ensure the propagation of all elements in the synthesized product. The resulting experimental mapping image (Figure 7d) shows a uniform distribution of Na, V, Mn, P, O, and Cu elements in the NVMCP sample. Additional TEM images (FIG. 8a and b) further confirm the cotton candy-like three-dimensional porous structure.

The finite lattice fringes of the Na ₄ -VMn _0.9 Cu _0.10 (PO ₄ ) ₃ material can be clearly seen from the high-magnification image with a calculated band gap value of 0.61 nm, indicating the (012) plane of the trigonal NVMP structure (Fig. c) of 8.

To determine the fundamental structural properties, X-ray photoelectron spectroscopy (XPS) was performed on the NVMCP/C/CC materials. Figure 7e illustrates the V 2p spectrum, which shows two dominant peaks at 517.1 eV and 522.2 eV, corresponding to the 2p3/2 and 2p1/2 orbital energy states, respectively, and demonstrating the trivalent nature of V in the material. define. The Mn 2p profile in Figure 7f shows two dominant peaks with binding energies of 641.2 eV and 653.5 eV, which individually represent the 2p3/2 and 2p1/2 orbital energy states with energy separation of 12.3 eV, respectively. It indicates that the energy state of Mn is divalent. Moreover, the reorganization satellites present at 645.3 eV and 658.1 eV are due to the paramagnetic metallic state.

The high-resolution Likewise, the high-resolution Additionally, XPS peak values at 1070 and 132 eV confirm Na (1s) and P (2p) elements in the sample (c and d in Figure 9). More importantly, a small peak value can be seen for Cu 2p at 935 eV, confirming that Cu ²⁺ is doped into the crystal network of the NVMP/C product (e in Figure 9). Overall, the presence of multi-element characteristics of NVMCP/C/CC materials can be confirmed from the XPS survey results (f in Figure 9).

Experimental Example 6

Thermogravimetric analysis and Raman spectroscopy were performed on NVMCP/C/CC anode materials, and the results are shown in Figure 10.

In order to determine the exact carbon content as shown in a in Figure 10, thermogravimetric analysis was performed, and the precise carbon content present in the NVMCP/C/CC anode was found to be 3.5%. In addition, Raman spectroscopy was performed to identify the properties of carbon in the NVMCP/C/CC material, and the resulting profiles are 1346.3 cm ^-1 (D-band) and 1587.4 cm ^-1 through Figure 10b, which shows the results. Two characteristic Raman footprints were presented in (G-band). The I D/ I G ratio ( i.e. , sp3 - sp2 ratio) was calculated to be 0.85, showing the amorphous nature of carbon.

Experimental Example 7

To investigate the specific surface area and porous structure, nitrogen adsorption-desorption investigation was performed on NVMP/C/CC, NVMCP/C/CC, and NVMP/C/NP, and the results are shown in Figure 11.

As shown in Figure 11, the specific surface area of the NVMCP/C/CC material was measured to be 141 m ² g ^-1 , which is higher than 38.2 m ² g ^-1 for NVMCP/C/NP and lower than that for NVMP/C/CC. Close to 119.3 m ² g ^-1

Experimental Example 8

Na ₄ -VMn _0.9 Cu _0.10 (PO ₄ ) ₃ To obtain accurate elemental contribution information of the electrode material, inductively coupled plasmaoptical emission spectroscopy (ICP-OES) and elemental analysis were performed and the results are shown in the table. It is shown in 3.

The element ratios of NVMCP powder are in the order of (Na = 0.556; V = 0.16; Mn = 0.14; Cu = _0.015 ), which is equivalent to the stoichiometric composition of Na _3.97 V _1.1 Mn _0.994 Cu _0.106 0(PO ₄ ) 3 .

Experimental Example 9

Galvanostatic cycling tests were performed to characterize the electrochemical behavior of three electrode materials, namely NVMP/C/CC and NVMCP/C/CC, and NVMP/C/NP prepared by traditional and modified thermal synthesis methods, as follows. The investigation was conducted and the results are shown in Figures 12a and 12b.

Comparative galvanostatic charge/discharge patterns at operating voltages of 3.8-2.4 V at 0.25 C are shown in Figure 12a. All electrodes show two pairs of voltage plateaus close to 3.45/3.35 V and 3.63/3.51 V resulting from the V ³⁺ /V ⁴⁺ and Mn ²⁺ /Mn ³⁺ redox pairs, and the reversible Na with NVMP NASICON structure. ⁺ Indicates (de)insertion. Compared to NVMP/C/CC (112 mA hg ^-1 ) and NVMP/C/NPs (108 mA hg ^-1 ) composite electrodes, NVMCP/C/CC composite electrodes due to the combined effects of copper doping and unique cotton candy-like morphology. It provided excellent discharge capacity (117 mA hg ^-1 ) in the first cycle. Additionally, among the three electrodes, NVMCP/C/NP shows the least electrochemical polarization, indicating improved conductivity due to Cu doping. To further clarify the morphological and doping influence on rate performance, all anodes were subjected to rate tests at different current inrush from 0.25C to 40C. The resulting rate performance is compared in Figure 12b. Among the three anode materials, it is clear that NVMP/C/CC anode material has better ratio capability than NVMP/C/CC and NVMP/C/NP anode materials. When run at low C-ratios of 0.25C and 0.5C, the differences between the electrodes are small. However, with C-rate acceleration, the reversible capacity and periodicity of NVMCP/C/CC are greatly improved over NVMP/C/CC and NVMP/C/NP. For example, the reversible capacity (68 mA hg ^-1 ) realized for the NVMCP/C/CC anode at a very high rate (40 C) is lower than that for NVMP/C/CC and NVMP/C/NP at the same rate (40 C). (45 and 20 mA hg ^-1 , respectively).

These results show that the NVMP/C/CC anode, which has a cotton candy-like structure, shows superior electrochemical performance compared to NVMP/C/NPs due to the 3D electron passage system.

Experimental Example 10

The improved electrochemical stability realized in Cu-doped NVMCP/C/CC with cotton candy-like nanostructures may originate from the combined conductivity enhancement due to the presence of Cu in the crystal structure and the 3D electron channel arrangement arising from the unique 3D morphology of Cu. will be.

In addition to the unique cotton candy structure, Cu doping was found to enhance the electrochemical activity of the NVMP anode. Therefore, to understand the effect of Cu content on sodium storage properties, two NVMP samples prepared by the same modified thermosynthesis technique and with 0.15 and 0.2 mol Cu content, respectively, namely Na ₄ VMn _0.85 Cu _0.15 (PO ₄ ) ₃ and Na ₄ VMn _0.8 Cu _0.2 (PO ₄ ) ₃ were investigated and the results are shown in FIG. 13.

The XRD patterns obtained for the samples were clearly classified into the standard triangular NVMP state calculated with the Xpert highscore plus program (see Figure 13a). As a result of applying this to the rate performance evaluation (see Figure 13 b and c), NVMP samples with 0.15 and 0.2 mol Cu concentrations were found to have a higher temperature at 40C compared to NVMP samples with Cu content of 0.1 mol or less (68 mA hg ^-1 at 40C). showed low rate capacities of 40 and 35 mA hg ^-1 . This clearly indicates that NVMCP anodes with optimized Cu content (i.e. Na ₄ VMn _0.9 Cu _0.10 (PO ₄ ) ₃ ), in addition to their unique morphology, are essential for realizing outstanding sodium storage properties.

Experimental Example 11

Since NVMCP/C/CC showed the best rate performance, the electrochemical properties of the NVMCP/C/CC anode were observed in depth by cycling at a rate of 1.5C. The corresponding periodicity results are shown in Figure 14a. The NVMCP/C/CC anode provides an initial discharge capacity of 87 mA hg ^-1 , and the anode material undergoes a slight capacity drop in the first cycle (81.26 mA hg ^-1 , 25th cycle). Later, the anode material underwent a slow activation process until the 103rd cycle, obtaining an activation capacity of 84 mA hg ^-1 due to the improved Na+ diffusion effect resulting from the increased availability of reversible sites along the electrode periphery during the repeated cycling process. . As the number of cycles increases, capacity decreases slowly. For example, the specific capacity provided after the 200th cycle was ∼82.87 mAhg ^-1 , after which it was clear that the electrode exhibited adequate cycling stability. After 450 cycles, a stable reversible capacity of 79 mA hg ^-1 and 90% capacity retention were observed. Good cycling stability at low current rates determines the stability of the electrode. Therefore, the excellent cycling stability at low current rates confirms the long-term cycle life of NVMCP/C/CC anodes. The corresponding charge-discharge patterns selected for the 50th, 100th, 200th, and 300th cycles, respectively, are presented in Figure 14b. The shape of the charge-discharge pattern and the position of the redox pair for V ³⁺ /V ⁴⁺ and Mn ²⁺ /Mn ³⁺ are maintained throughout the entire cycle, which is also due to the stable NASICON framework structure of the anode. Indicates that the extraction/insertion mechanism is maintained. This is further enhanced by the unique cotton candy-like morphology and conductivity support from Cu-doping.

Additionally, the fast Na + insertion and deinsertion properties of NVMCP/C/CC were evaluated even at high current rates (30 C), and the resulting long-term cyclic stability curves are shown in Figure 14c. The anode shows unusual cycle stability and a stable reversible capacity of 68 mA hg ^-1 even after 3000 repetitive cycles. During periodicity analysis, the coulombic efficiency is preserved almost entirely. After providing an overall high initial discharge capacity (79 mA hg ^-1 ), the anode showed a small capacity drop (70 mA hg ^-1 ) until the 40th cycle. Afterwards, the electrode achieved a stabilization process and proceeded with a stable circulation path. Therefore, after 3000 cycles, 86% capacity maintenance was achieved while providing almost constant cycling performance and a fixed discharge capacity of 68 mA hg ^-1 . This demonstrates the fast sodium ion reversibility of this NVMCP/C/CC anode.

More remarkable features of the NVMCP/C/CC composite anode were identified by comparing the Ragone diagram of the anode with many other recently recorded multivalent anion type anodes (Figure 14d). The specific energy and power values were different from 0.25 C to 40 C. It was calculated based on the total active mass of the anode obtained from the charge/discharge diagram at the time of current inrush. This diagram shows that the NVMCP/C/CC anode maintains a high specific energy of 398 W h kg ^-1 at a specific power of 90 W h kg ^-1 and stands out among the multivalent anion-based anodes previously reported for SIB. indicates that Therefore, the NVMCP/C/CC composite anodes reported so far include NMVP/C/rGO, NMVP/C/GA, NMVP/C/CNTs, Na ₃ MnTi(PO ₄ ) ₃ /C, Na ₂ FeP ₂ -O ₇ /C/rGO, NaVTi(PO ₄ ) ₃ /C, Na _3.32 Fe _2.34 (P ₂ O ₇ ) ₂ /C, Na ₇ V ₄ (P ₂ -O ₇ ) ₄ (PO ₄ )/C/graphene, and Outperforms other anodes such as NaVPO ₄ /C. It is noteworthy to compare the electrochemical results of this NVMCP/C/CC anode with the available registration reports for Na ₄ MnV(PO ₄ ) ₃ . The statistical correlation between different Na ₄ MnV(PO ₄ ) ₃ materials is shown in Table 4.

Table 4 highlights the sodium storage properties of this NVMCP/C/CC anode synthesized through ultrafast thermal synthesis technique. The documented sodium storage properties of NVMCP/C/CC anodes realized through the ultrafast thermal synthesis process of the present invention are comparable to NVMP anodes prepared by time-consuming sol-gel and spray-dry methods.

Experimental Example 12

The corresponding sodium discharge patterns selected for the 100th, 1000th, 1500th, 2000th and 3000th cycles at the applied current rates are shown in Figure 15a. Similar discharge shapes are realized over long cycle operations, indicating an extremely stable framework of NVMCP/C/CC anodes. Additionally, the trigonal Na ₄ VMn _0.9 Cu _0.10 (PO ₄ ) ₃ /C structure was maintained after the deep cycle process (ex situ XRD in Figure 15b), similarly indicating that the lattice structure is easily preserved.

More importantly, ex situ SEM analysis was used to investigate the morphological changes in NVMCP anodes after 3000 cycles at 30 C and the corresponding SEM images are presented in Figure 15c.

It is clear that the NVMCP/CC/C anode retains almost all of its cotton candy structure even after harsh cycling conditions. This clearly shows that the structure of the NVMCP/C/CC anode itself, with its unique cotton candy microstructure, supports the overall electrochemical properties in addition to Cu doping. Therefore, adopting a dual strategy, i.e. , fabricating Cu-doped NVMP anodes with unique structures, is very advantageous to achieve excellent electrochemical properties as revealed by NVMCP/C/CC composite anodes.

Therefore, ex situ

Experimental Example 13

Cyclic voltammetry (CV) was performed at 0.1 mV s ^-1 for the NVMCP/C/CC anode in a potential window of 3.9 to 2.4 ^V for Na + /Na, and the results are shown in a in Figure 16. It was. The CV pattern shows two pairs at ∼3.48/3.28 and 3.66/3.45 V resulting from the reversible transformation of V ³⁺ /V 4+ and Mn ²⁺ /Mn ³⁺ , respectively, during Na + (de)insertion into the NVMCP/C/CC framework. It consists of the dominant redox peak values of To determine the capacitive effect on the NVMCP/C/CC anode, CV analysis was performed at various scanning rates (0.1-1 mV s ^-1 , Figure 16b). The current at a fixed potential is found to consist of two influences: surface ( k 1 v ) and diffusion ( k 2 v ) limited processes, which can be written by the formula:

Here, i and v are the peak current and scan rate of the CV result, respectively, and k 1 v and k 2 v 1/2 are the surface and diffusion controlled redox reactions, respectively. From this formula we can determine the ratio of the two contributions at each injection rate. At 0.2 mV s ^-1 the diagnostic result (c in Figure 16) is close to the 17.12% rate of the surface center contribution. Additionally, the histogram shown in Figure 16d shows two different contributions to the multiple scan ratio. The rate of surface-driven responses increases with increasing scanning rate. In particular, the contribution of the surface-driven reaction to the specific capacity increases from 17.12% (0.2 mV s ^-1 ) to 72% (1 mV s ^-1 ), indicating the dominant role of the surface reaction at high scanning rates.

Experimental Example 14

The reaction mode behind the NVMCP/C/CC anode material was analyzed using an in situ synchrotron XRD technique, and the results are shown in Figure 17. Figure 17 ^illustrates the in situ The resulting charge/discharge profiles along selected field scan numbers are listed in a of Figure 17 and selected XRD 2q regions are shown in the remaining panels from b to d. After the start of Na+ extraction, a clear shift is recognized for the (211), (116), (030), (223), (226), (140) and (146) reflections. During the charging process, all planes showed a change in peak position towards a high 2q angle (after the 220th scan number), and after the discharge process (scanning number 450, indicating cycle completion), they gradually returned to the original point, indicating a controlled electrochemical actuation agenda. Lattice respiration is emphasized during the Na+ (de)insertion process. Meanwhile, the (104) and (113) reflections split into two peak values ( i.e. from 3.5 to 3.8 V) from scan numbers 120-220 during the charging reaction. This clearly suggests that the initial Na+ extraction was a coupled V ⁴⁺ /V ³⁺ redox-driven single-state reaction and the remaining Na+ extraction was associated with a two-state reaction resulting from the Mn ³⁺ /Mn ²⁺ redox pair. . More importantly, after the end of the discharge process, the two peaks were fused into a single peak as recognized by the open-circuit voltage (OCV) at scan number 450 (end of discharge, 2.4 V). This confirms that the original structure is restored at the end of the discharge process. The NVMCP/C/CC anode underwent continuous two-state transitions during the reversible Na+ (de)insertion process recognizing the complete framework, exhibiting two well-established charge/discharge plateaus.

Experimental Example 15

To fully understand the two-state reaction mode and diffusion rate of Na+ ions in Cu-doped and undoped NASICON frameworks, 20 mA at the specified operating voltage zero (OCV - 3.8 V) for the first cycle with the same pulse interval (10 min). Galvanostatic intermittent titration technique (GITT) analysis was performed on the NVMCP/C/CC and NVMP/C/CC anodes at a ratio of g ^-1 , and the results are shown in Figures 18a and 18b.

The remaining time was fixed at 1 h to reach a quasi-electrochemical equilibrium voltage. This was preserved across action potential space. The resulting single titration results during the cycling process for NVMCP/C/CC and NVMP/C/CC anodes are shown in Figure 18c and d, respectively. Significant variations in the shape of the equilibrium voltage for the two anodes (a and b in Figure 18a) indicate the presence of a two-state reaction during Na + deinsertion between 3.5 and 3.8 V. Using the transient potential response evidenced from the GITT results, the sodium ion diffusion coefficient can be calculated according to Fick's second law:

Here, D _Na (cm s ^-1 ) represents the _diffusion coefficient, m _B (g) represents the total mass loading _of the active material, VM (cm 3 mol ^-1 ) is the molar volume, MW (g mol ^{- 1} ) is the molecular weight, A (cm2) is the sum of the surface areas of the anode, and τ (s) is the current pulse time; ΔE _s and ΔE _τ represent the steady-state voltage and the total variation in cell voltage that occurs during constant pulses of single-state GITT analysis.

Chemical diffusion line diagrams for NVMCP/C/CC and NVMP/C/CC anodes in charge and discharge reactions are shown in Figure 18b, e, f and g, h, respectively. The GITT results clearly demonstrated that the NVMCP/C/CC anode achieved improved sodium diffusion than the NVMCP/C/CC anode. For example, the diffusion coefficient ^value calculated for the NVMCP/ ^C /CC sample ( ^{2.46 -9} ~ 9.14 ㅧ 10 ^-10 cm 2 s ^-1 ), which is comparable to the value for NASICON-based anode materials, explaining the excellent mobility of Na + ions in the NVMCP/C/CC NASICON framework. This clearly shows that Cu doping significantly promotes the mobility of Na + ions, which helps in the rapid sodium storage properties of NVMCP/C/CC anodes. Therefore, it is clear that Cu-doped samples exhibit unusual rate performance and significantly faster current test capabilities compared to undoped samples due to their excellent Na+ diffusion properties.

Experimental Example 16

First-principles calculations based on density functional theory were performed to obtain much intuition regarding the electronic structure evolution of pure (NVMP/C/CC) and Cu-doped (NVMCP/C/CC) samples. The total density of states (DOS) of the NVMP and NVMCP samples are shown in Figures 19a and 19b, respectively. It can be seen that the NVMP sample showed semiconductor properties with a band gap of 0.83 eV. Notably, after including Cu in the structure, impurity appeared within the Fermi energy, showing an increase in conductivity of the NVMCP sample. Additionally, the partial DOS of the NVMP sample showed that the conduction and valence bands were dominated by transition metal-3d electrons with some P and O 2p electrons (Figure 19c). After adding Cu, similar trends could still be observed, but with some impurity (Figure 19d). Based on the DOS investigation, it is noteworthy that the Cu-doped sample showed an essentially metallic behavior, which actually had a much larger conductivity than the pure sample, promoting fast electron transfer and also consistent with the experimental results.

Experimental Example 17

To evaluate various applications of the NVMCP/C/CC anode, a complete cell with a MnS ₂ cathode was assembled. Before investigating the full cell, the MnS ₂ cathode showed stable electrochemical properties in diglyme-based electrolyte (1 M NaPF ₆ in DGM), so the cathode and anode were optimized for sodium half-cell setup using diglyme-based electrolyte, and the comprehensive The typical electrochemical results were illustrated in ESI (a in Figure 20a and d in Figure 20b). This provides an opportunity to test the durability of the NVMCP/C/CC anode even in highly adhesive diglyme-based electrolytes, and the stable electrochemical properties demonstrate the performance of the anodes even in highly adhesive electrolytes. More importantly, the desired gain was achieved by employing a mass ratio of 1:7 for MnS ₂ :NVMCP/C/CC. MnS ₂ : NVMCP/C/CC According to the periodicity pattern of the full cell, the corresponding constant current charge/discharge profile is shown in e and f of Figure 20c, and the circulation pattern shows that the full cell maintains 73% of the initial value after 50 cycles. It clearly shows that it has a stable cycle life. It is noteworthy that the high anode mass ratio and slow diffusion kinetics of Na+ ions in the high-viscosity diglyme electrolyte absolutely affected the final productivity of the full cell. However, the reasonably good performance of the NVMCP/C/CC anode along with the MnS ₂ cathode in full cells even in high viscosity electrolytes demonstrates their high suitability for practical applications. The research results explored for NVMP anodes primarily use cured carbon anodes in full cell configurations. However, the present invention demonstrates the versatility of NVCMP/C/CC anodes in sulfide-based cathodes.

As can be seen from the above experimental results, the three-dimensional porous anode material of the present invention showed excellent sodium storage ability due to the following characteristics. It is reasonable to suggest that the high electrochemical performance derives from the unique cotton candy-like morphology with a thin and uniform carbon covering, i.e. 3D porosity, which results in high electrochemical active sites thanks to the 3D cotton candy-like shape and good porosity due to the uniform carbon covering. Improves the interfacial redox reaction due to reduced diffusion paths for Na+ ions originating from the conductive support. Additionally, for the anode material of the present invention, Cu doping played an important role in providing conductive channels that enable conduction.

Therefore, according to the present invention, it is possible to successfully synthesize a carbon-coated NASICON type NVMCP/C anode doped with other metals such as Cu, which has a unique cotton candy-like three-dimensional porosity, and this unique three-dimensional porous structure and improved High-performance anode materials with electrochemical consistency could accelerate the development of stable electrochemical energy storage devices.

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 (18)

delete delete delete delete delete delete delete delete delete delete NVMP anode material containing a compound represented by the following chemical formula.
Na ₄ VMn _(1-x) A _x (PO ₄ ) ₃
Here, 0 < x < 0.3, and A is any one selected from the group consisting of Fe, Co, Ni, and Cu.
According to claim 11,
NVMP anode material composed of the above compound is characterized in that it has three-dimensional porosity.
According to claim 12,
NVMP anode material, characterized in that when it has the three-dimensional porosity, its specific surface area is 100 m ² g ^-1 or more.
According to claim 11,
The NVMP anode material is characterized in that the compound is Na ₄ VMn _0.9 Cu _0.1 (PO ₄ ) ₃ .
An anode comprising the NVMP anode material of any one of claims 11 to 14.
A Na-ion battery comprising the positive electrode of claim 15.
According to claim 16,
A Na-ion battery characterized in that 90% capacity is maintained with a capacity of 79 mA hg ^-1 after 450 cycles at a rate of 1.5C.
According to claim 16,
A Na-ion battery characterized in that 86% capacity is maintained with a capacity of 68 mA hg ^-1 after 3000 cycles at a rate of 30C.