The present invention relates to the field of advance material for energy storage devices. The present invention in particular relates to a method of synthesis of nitrogen doped carbon coated sodium vanadium fluorophosphates (NVPF) used as a cathode material for Sodium ion batteries.
DESCRIPTION OF THE RELATED ART:
[002] Therefore, sodium ion batteries (SIBs) have been generally considered as one of the most promising candidates for the large-scale grid energy storage applications. Furthermore, the components and working mechanisms of SIBs are basically similar to LIBs with the rock-chair type redox phenomenon, which operates through insertion/extraction of lithium or sodium ions between cathode and anode through a suitable electrolyte.
[003] However, it is found that the SIBs present lower energy density, poor structural stability, and inferior electrochemical performance than LIBs, making it less attractive for portable electronics and automotive applications. The reason for inferior electrochemical performances and rate kinetics in SIBs can be possible linked to following factor: (i) high molecular weight (23 for Na vs. 7 for Li) (ii) the lower redox potential of Na (-2.71 V vs.-3.04 V for Li), and (iii) the larger ionic radius (1.02 A for Na-ion and 0.67 A for Li-ion).
[004] The low redox potential of sodium makes it difficult for electrochemical reaction to proceed smoothly. Additionally, the larger radius creates unstable cathodes with structural variation during charge/discharge process, leading to a substandard cycling performance. Furthermore, existence of a larger energy barrier for insertion of Na-ion into many electrode materials leads to limited

capacity and poor rate capability. For instance, graphite with a layer spacing of 3.35 A, delivers low irreversible capacity in SIBs while it is well-established anode for LIBs.
[005] Reference may be made to the following:
[006] Publication No. CN107819115 relates to a doped and modified sodium vanadium fluorophosphate cathode material and a preparation method thereof. According to the cathode material, the problems that an existing sodium vanadium fluorophosphate cathode material is poor in rate capability and instable in cycle performance are solved. The nominal molecular formula of the cathode material is Na3V2-xCax(P04)2F3, wherein x is more than 0 and less than or equal to 0.2. The preparation method comprises the following steps: uniformly dissolving a sodium source, a calcium source, a vanadium source, phosphate and a carbon source into a deionized water medium in a stoichiometric ratio so as to obtain a mixed solution, and drying, so as to obtain a sodium vanadium fluorophosphate precursor, wherein the carbon source is used for controlling a V valence state in a compound; and carrying out thermal treatment on the precursor at 300-400 DEG C in an inert atmosphere, and sintering at 600-700 DEG C, so as to obtain the doped and modified sodium vanadium fluorophosphate cathode material. The material has relatively high ionic conductivity and electronic conductivity and therefore has excellent rate capability; the cycling stability of the material in electrochemical charging and discharging processes are enhanced; and the preparation craft process is simple. 9643268916
[007] The above invention describes the energy storage performance of Ca doped sodium vanadium fluorophosphate cathode material. Here, material synthesis involves sol-gel process followed by

sintering in an inert atmosphere whereas present invention synthesizes N-doped carbon coated sodium vanadium fluorophosphate cathode material and the synthesis involve hydrothermal process followed by sintering in a reducing-inert atmosphere.
[008] Publication No. MY 157833 relates to a lithium vanadium fluorophosphate or a carbon-containing lithium vanadium fluorophosphate. Such processes include forming a solution-suspension of precursors having v5+ that is to be reduced to v3+. The solution-suspension is heated in an inert environment to drive synthesis of livpo4f such that carbon-residue-forming material is also oxidized to precipitate in and on the livpo4f forming carbon-containing livpo4f or clvpf. Liquids are separated from solids and a resulting dry powder is heated to a second higher temperature to drive crystallization of a product. The product includes carbon for conductivity, is created with low cost precursors and retains a small particle size without need for milling or other processing to reduce the product to a particle size suitthle for use in batteries. Furthermore, the process does not rely on addition of carbon black, graphite or other form of carbon to provide the conductivity required for use in batteries.
[009] The above invention describes the synthesis of lithium vanadium fluorophosphate (LiVP04F) which is a cathode material for Li-ion battery whereas present invention synthesizes N-doped carbon coated sodium vanadium fluorophosphate (Na3V2(P04)2F3) which is a cathode material for Na-ion batteries.
[010] Publication No. CNl 11943161 relates to a preparation method and application of a sodium vanadium fluorophosphate and carbon composite secondary battery positive electrode material, and relates

to a preparation method and application of a battery positive electrode material. The invention aims to solve the problems of poor electronic conductivity, capacity and rate capability of the NVOPF improved by the existing method. The method comprises the following steps: preparing a low-valence vanadium solution; preparing hydrothermal mother liquor; preparing a three-dimensional carbon skeleton material; and conducting compounding to obtain the sodium vanadium fluorophosphate and carbon composite cathode material for the secondary battery. The first-circle specific charge capacity of the sodium ion battery prepared by the composite cathode material can reach 138 mAh.g<-1>, and the first-circle specific discharge capacity can reach 113 mAh.g<-l>. According to the prepared sodium ion battery, the first-circle specific charge capacity can reach 138 mAh.g <-l>, the first-circle specific discharge capacity can reach 113 mAh.g, and the coulombic efficiency can be kept at 95%. According to the invention, the sodium vanadium fluorophosphate and carbon composite cathode material for the secondary battery can be obtained.
[Oil] The above invention describes the synthesis of sodium vanadium oxy-fluorophosphate (Na3V202(P04)2F in short NVOPF) carbon composite whereas present invention synthesizes N-doped carbon coated sodium vanadium fluorophosphate (Na3V2(P04)2F3 in short NVPF).
[012] Publication No. CN107482180 relates to an intermediate liquid phase method for the preparation of a sodium vanadium fluorophosphate/ carbon composite cathode material. The invention describes the synthesis procedure of sodium vanadium fluorophosphate/carbon composite cathode material. The above method includes multiple synthesis steps; first a sodium source, a vanadium source, and a fluorine source completely dissolve in a

small beaker and heated in a hydrothermal liner at 100-180 °C for 12-48h to get an intermediate phase liquid. After that phosphorus source and an organic carbon source were complete dissolved in the deionized water, then cooled intermediate phase liquid was slowly added into the above solution drop wise and stirring for 20min until the solution turns into orange yellow. The obtained solution was dried at 60 °C in an oven for 48h then grind and pre-sintering in a nitrogen atmosphere at 350 °C for 2-6h, also conducting calcinations at 650-850 °C for 6-12h, and obtain the NaVP04F/C composite material.
[013] This present method involves in-situ synthesis of N-doped carbon coated sodium vanadium fluorophosphate cathode material and includes one-step hydrothermal process followed by sintering in a reducing-inert atmosphere.
[014] The article entitled "Na3V2(PC>4)3 coated by N-doped carbon from ionic liquid as cathode materials for high rate and long-life Na-ion batteries" by Yu Yao, Yu Jiang, Hai Yang, Xizhen Sun and Yan Yu; Nanoscale, Issue 30, 2017 talks about the facile and simple hydrothermal assisted sol-gel route to prepare nitrogen doped carbon coated Na3V2(P04)3 nanocomposites (denoted as NVP@C-N) as cathodes for sodium ion batteries (NIBs). An ionic liquid (EMIm-dca) was used as the nitrogen doped carbon source. The optimized N-doped carbon coated Na3V2(P04)3 (denoted as NVP@C-N150) displays a very thin and uniform N-doped carbon coating layer (thickness: ~2 nm), showing an excellent sodium storage performance (85 mA h g-1 at 20 C after 5000 cycles) and high rate capability (71 mA h g-1 at 80 C). Such a superior sodium storage performance derives from the nitrogen doping carbon coating, triggering amounts of extrinsical defects and active sites in the N-doped amorphous carbon layer, which highly accelerates Na-ion and

electron transport. This approach is simple, facile and shows easy scale-up. It could be extended to other materials for energy storage.
[015] The above invention describes the synthesis of N-doped carbon coated sodium vanadium phosphate (Na3V2(P04)3 in short NVP) whereas present invention synthesizes N-doped carbon coated sodium vanadium fluorophosphate (Na3V2(P04)2F3 in short NVPF).
[016] The article entitled "Nitrogen-doped carbon coated Na3V2(P04)3 with superior sodium storage capability" by LaiqiangXu; Jiayang Li; Yitong Li; Peng Cai; Cheng Liu; Guoqiang Zou; Hongshuai Hou; Lanping Huang; Xiaobo Ji; Chemical Research in Chinese Universities 36(3); February 2020 talks about solvent evaporation method presented to obtain the nitrogen-doped carbon coated Na3V2(P04)3 cathode material, delivering enhanced electrochemical performances. N-Doped carbon layer coating serves as a highly conducting pathway, and creates numerous extrinsic defects and active sites, which can facilitate the storage and diffusion of Na+, Moreover, the N-doped carbon layer can provide a stable framework to accommodate the agglomeration of the electrode upon electrode cycling. N-Doped carbon coated Na3V2(PC>4)3(NC-NVP) exhibits excellent long cycling life and superior rate performances than bare Na3V2(P04)3 without carbon coating, NC-NVP delivers a stable capacity of 95.9 mA»h/g after 500 cycles at 1 C rate, which corresponds to high capacity retention(94.6%) with respect to the initial capacity(101.4 mA»h/g). Over 91.3% of the initial capacity is retained after 500 cycles at 5 C, and the capacity can reach 85 mA*h/g at 30 C rate.
[017] The above invention describes the synthesis of N-doped carbon coated sodium vanadium phosphate (Na3V2(P04)3 in short NVP)
*Tkf\i

whereas present invention synthesizes N-doped carbon coated sodium vanadium fluorophosphate (Na3V2(P04)2F3 in short NVPF).
[0181 The article entitled "Controlled synthesis of Na3(VOP04)2F cathodes with an ultralong cycling performance" by Xing Shen; Junmei Zhao; Yuqi Li; Xiaohong Sun; Chao Yang; H. Liu; Yong-Sheng Hu; ACS Applied Energy Materials 2(10); September 2019 talks about significant increase in rate capability and cycling stability for Na3(VOP04)2F (NVOPF). The improvement is achieved by mixing Ketjenblack (KB) with NVOPF combined with the addition of sodiation agent Na2C404, based on a controllable synthesis of NVOPF hollow microspheres. It is a green and facile hydrothermal synthetic method by simply adjusting phosphorus source and temperature. The nanosized [email protected] framework formed by high-energy ball milling provides better rate capability, which exhibits a capacity of 118.3 mA h g-1 at 20 C, while it is only 69.2 mA h g-1 for NVOPF at 20 C. To be interested, the addition of Na2C404 could greatly prolong the cycling stability. As an example, the capacity retention of [email protected] with Na2C404 increases to 99.41% from 33.42% compared with that without Na2C404 at 30 C. To our best knowledge, [email protected] shows the best Na-storage performance in terms of both superior rate capability (up to 150 C rate) and outstanding long cycling stability over 12000 cycles with a capacity retention of 71% reported so far. The current developed microsphere synthetic method and the improved strategies for cathodes would boost the development of positive electrode materials for ion batteries
[019] The above invention describes the synthesis of sodium vanadium oxy-fluorophosphate (Na3V202(P04)2F in short NVOPF) carbon composite whereas present invention synthesizes N-doped

carbon coated sodium vanadium fluorophosphate (Na3V2(P04)2F3 in short NVPF).
[020] However, exploring cathode materials with abundant active sites, high reaction potential, unimpeded ionic channels, and stable structures is the major challenge to construct high-energy density sodium ion batteries (SIBs). Hence there needed a low cost high performance cathode material for improving the sodium ion battery performance.
[021] In order to overcome above listed prior art, the present invention aims to provide a method of synthesis of nitrogen doped carbon coated Sodium Vanadium Fluorophosphates (N-doped carbon coated NVPF) material which is a low cost and high performance cathode material for Sodium ion batteries..
OBJECTS OF THE INVENTION:
[022] The principal object of the present invention is to provide a method of in-situ synthesis of nitrogen doped carbon coated Sodium Vanadium Fluorophosphates (N-doped carbon coated NVPF) material.
[023] Another object of the present invention is to provide nitrogen doped carbon coated Sodium Vanadium Fluorophosphates (N-doped carbon coated NVPF) material which is used as a cathode material for Sodium ion batteries.
[024] Yet another object of the present invention is to provide high yield and high performance cathode material for Sodium ion batteries.
[025] Still another object of the present invention is to provide low cost and high performance cathode material for Sodium ion batteries (SIBs).

SUMMARY OF THE INVENTION:
[026] The present invention relates to a method of in-situ synthesis of nitrogen doped carbon coated Sodium Vanadium Fluorophosphates (N-doped carbon coated NVPF) material. The invention provides high yield and high performance cathode material for Sodium ion batteries. The invention provides the crystal structure of NVPF and their electrochemical performance for application in Na-ion energy storage devices.
[027] The method of in-situ synthesis of N-doped carbon coated NVPF is novel, facile and industrially scalable. The process involves following synthesis steps;
[028] Preparation of Solution A: Mixing of citric acid and Polyvinylpyrrolidone (PVP) in certain weight ratio in de-ionized water. Addition of Ammonium metavanadate (NH4V03) in stoichiometric amount to the solution and stirred for some time.
[029] Preparation of Solution B: Stoichiometric mixing of Sodium fluoride (NaF) and Ammonium dihydrogen phosphate(NH4H2P04) in de-ionized water.
[030] Solution B is added into Solution A drop by drop and stirred for some time. Certain amount of ethanol is added to the mixture and adjustment of solution pH using oxalic acid solution. The whole solution is poured into Teflon lined hydrothermal autoclave and heating it at certain temperature for some time in an oven. Cool it naturally of the solution after heating. Wash the obtained material using ethanol and de-ionized water followed by drying in vacuum oven. After drying, the material was heated in a tube furnace at certain temperature under (Ar+H2) gas environment. The obtained material was then grounded and agedn heated in a tube furnace at certain temperature under (Ar+H2) gas environment. The obtained

green material is in-situ synthesized N-doped carbon coated NVPF. The obtained material delivered HOmAh/g capacity vs Sodium metal (Na/Na+) in the voltage range of 2.5 V - 4.2 V.
[031] The specific volume ratio of water and ethanol, heating environment, and citric acid and Polyvinylpyrrolidone (PVP) weight ratios achieves high performance nitrogen doped carbon coated Sodium Vanadium Fluorophosphates cathode material for Sodium ion batteries (SIBs). Thus, the invention provides low cost and high performance cathode material for SIBs.
BREIF DESCRIPTION OF THE INVENTION
[032] It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments.
[033] Figure 1 shows XRD pattern of Synthesized NVPF
[034] Figure 2 shows FESEM of the synthesized NVPF sample
[035] Figure 3 shows voltage vs Specific Capacity curve of NVPF
[036] Figure 4 shows XRD pattern of Synthesized NVOPF.
[037] Figure 5 shows voltage vs Specific Capacity curve of NVOPF
[038] Figure 6 shows XRD pattern of the sample Synthesized without using ethanol.
DETAILED DESCRIPTION OF THE INVENTION:
[039] The present invention provides a method of in-situ synthesis
of nitrogen doped carbon coated Sodium Vanadium

Fluorophosphates (N-doped carbon coated NVPF) material. The invention provides high yield and high performance cathode material for Sodium ion batteries. The invention provides the crystal structure of NVPF and their electrochemical performance for application in Na-ion energy storage devices.
[040] The method of in-situ synthesis of N-doped carbon coated NVPF is novel, facile and industrially scalable. The process involves following synthesis steps;
[041] Preparation of Solution A: Mixing of citric acid and Polyvinylpyrrolidone (PVP) in certain weight ratio in de-ionized water. Addition of Ammonium metavanadate (NH4V03) in stoichiometric amount to the solution and stirred for some time.
[042] Preparation of Solution B: Stoichiometric mixing of Sodium fluoride (NaF) and Ammonium dihydrogen phosphate(NH4H2P04) in de-ionized water.
[043] Solution B is added into Solution A drop by drop and stirred for some time. Certain amount of ethanol is added to the mixture and adjustment of solution pH using oxalic acid solution. The whole solution is poured into Teflon lined hydrothermal autoclave and heating it at certain temperature for some time in an oven. Cool it naturally of the solution after heating. Wash the obtained material using ethanol and de-ionized water followed by drying in vacuum oven. After drying, the material was heated in a tube furnace at certain temperature under (Ar+H2) gas environment. The obtained material was then grounded and again heated in a tube furnace at certain temperature under (Ar+H2) gas environment. The obtained green material is in-situ synthesized N-doped carbon coated NVPF. The obtained material delivered HOmAh/g capacity vs Sodium metal (Na/Na+) in the voltage range of 2.5 V - 4.2 V.

[044] The volume ratio of water and ethanol and volume % of the Teflon cell utilized, heating environment, and citric acid and Polyvinylpyrrolidone (PVP) weight ratios have been tuned to achieve high performance nitrogen doped carbon coated Sodium Vanadium Fluorophosphates cathode material for Sodium ion batteries (SIBs). Thus, the invention could lead to the manufacturing of low cost and high performance cathode material for SIBs,
[045] The invention will be more fully understood from the following examples. These examples are to be constructed as illustrative of the invention and not limitative thereof:
[046] Examples
[047] This section will provide the information regarding the In-situ synthesis of Nitrogen Doped Carbon Coated pure phase Sodium Vanadium Fluorophosphates (NVPF) via Hydrothermal approach.
[048] Example 1; Hydrothermal synthesis of NVPF, with -25% ethanol in the total solution
[049] Synthesis Method;
[050] First citric acid (0.52 gm) and (0.52 gm) PVP were mixed in 100 mL D.I Water. After that stoichiometric amount of NH4V03 (1.04 gm) was added into it and stirrer for 30 minutes. This was labeled as solution "A". Then solution "B" was prepared by mixing 3.3 moles of NaF (0.616 gm) and stoichiometric amount of NH4H2P04 (1.0224 gm) in 51 mL D.I water. After that solution "B" was added into solution "A" drop by drop and stirrer. 15 minutes later 50 mL ethanol (Merck) was added into this mixture and pH was adjusted to ~ 3.5 using 2 M oxalic acid solution. This solution was transferred into the Teflon lined hydrothermal autoclave of 240 mL and heated at 190C for 12h in an oven. Here total volume of the

Teflon cell filled with the solution was ~85%. After reaction oven with hydrothermal autoclave was allowed to cool down naturally. The obtained greenish material was washed with D.I water and ethanol, and dried in a vacuum oven at 100 C for 8 h. The obtained material is pure phase NVPF. To increase its crystallinity material was further heated at 350 C in Ar/H2 environment for 5 h, and again grinded and heated at 500 C for 5h. The energy storage performance of thus obtained well crystallized nearly pure phase N-doped carbon coated NVPF (Figure 1) cathode material was investigated. The synthesized sample consists of micro cuboids type morphology.
[051] For sodium storage properties assessment, the composite electrode is made-up of synthesized cathode material, binder and activated carbon in mass ratio of 80:10:10. A thick ink was made using the solvent i.e. 1-methyl 2-pyrro-lidone (NMP, Sigma Aldrich, 98%) and then it was coated onto a current collector (thickness ~ 10-15 pm) before allowing it to get dried at 80°C in air to evaporate the used solvent. The material coated substrate was then pressed between twin roller and cut in to discs of the diameter of 16 mm diameter followed by de-moisturizing at 70°C before transferring to an Ar-filled glove box (M-Braun, Germany).
[052] For making a Sodium ion (Na-ion) battery coin cell, standard stainless-steel cups and lids fitted with a plastic O-ring were used for the casing. Coin cells (CR2016; 20 mm diameter and 1.6 mm thick) comprised composite electrodes, Na metal as counter electrode, glass filter fiber as the separator and 1 M NaC104, dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume) as the Na-ion conducting electrolyte. The electrode was placed at the center of the cup which forms the positive terminal of the cell and wetted with a few drops of the electrolyte. This was covered with the separator that is permeable to the Na-ions but

impermeable to particles of the composite electrode and electrons. Circular Na-metal disc of the size ~16 mm diameter was placed centrally on the separator. Then, a steel spring was placed on Na-metal disc and formed the negative terminal of the cell. Finally, hermetic sealing was done with a mechanical press. The weight of active material in the working electrode was kept in the range of 5.0-8.0 mg.
[053] Electrochemical characterization (Sodium ion battery):
[054] Galvanostatic charge-discharge cycling (GCD) study was performed at room temperature by battery analyzer (Neware).
[055] The data shown in Figure 3 derived from the constant current cycling data for the cathode material (NVPF) in a Na-ion cell. The constant current data were collected at an approximate current density of 10 mAg-1 between voltage limits of 2.0 and 4.2 V. Figure 3 shows the cell voltage profile (V) versus specific capacity (mAhg-1) for the initial cycles. These data demonstrate a good initial capacity with very low level of voltage hysteresis. It further justifies this material could be a prospective high energy density cathode material in sodium ion battery.
[056] Example 2; Hydrothermal synthesis of NVOPF, with -50% ethanol in the total solution
[057] Synthesis Method;
[056] First citric acid (0.52 gm) and (0.52 gm) PVP were mixed in 50 mL D.I Water. After that stoichiometric amount of NH4V03 (1.04 gm) was added into it and stirrer for 30 minutes. This was labeled as solution "A". Then solution "B" was prepared by mixing 3.3 moles of NaF (0.616 gm) and stoichiometric amount of NH4H2P04 (1.0224 gm) in 51 mL D.I water. After that solution "B" was added into

solution "A" drop by drop and stirrer. 15 minutes later 100 mL ethanol (Merck) was added into this mixture and pH was adjusted to - 3.5 using 2 M oxalic acid solution. This solution was transferred into the Teflon lined hydrothermal autoclave of 240 mL and heated at 190C for 12h in an oven. Here total volume of the Teflon cell filled with the solution was ~85%. After reaction oven with hydrothermal autoclave was allowed to cool down naturally. The obtained greenish material was washed with D.I water and ethanol, and dried in a vacuum oven at 100 C for 8 h. The obtained material is pure phase NVPF. To increase its crystallinity material was further heated at 350 C in Ar/H2 environment for 5 h and again grinded and heated at 500 C for 5h. The energy storage performance of thus obtained well crystallized NVOPF (Figure 4) cathode material was investigated.
[059] The data shown in Figure 5 derived from the constant current cycling data for the cathode material (NVOPF) in a Na-ion cell. The constant current data were collected at an approximate current density of 10 mAg-1 between voltage limits of 2.5 and 4.2 V. Figure 5 shows the cell voltage profile (V) versus specific capacity (mAhg-1) for the initial cycles. The first charge/discharge cycle suggests the amount of sodium reversibly/irreversibly liberated from the cathode material in initial cycles, it is observed that there is a huge difference in the above mentioned both the materials (see Figure 3 and 5),
[060] Example 3; Hydrothermal synthesis of NVPF, with 0% ethanol in the total solution
[061] Synthesis Method;

[062] First citric acid (0.52 gm) and (0.52 gm) PVP were mixed in 150 mL D.I Water. After that stoichiometric amount of NH4V03 (1.04 gm) was added into it and stirrer for 30 minutes. This was labeled as solution "A". Then solution "B" was prepared by mixing 3.3 moles of NaF (0.616 gm) and stoichiometric amount of NH4H2P04 (1.0224 gm) in 51 mL D.I water. After that solution "B" was added into solution "A" drop by drop and stirrer. 15 minutes later pH of this solution was adjusted to - 3.5 using 2 M oxalic acid solution. This solution was transferred into the Teflon lined hydrothermal autoclave of 240 mL and heated at 190C for 12h in an oven. Here total volume of the Teflon cell filled with the solution was ~85%. After reaction oven with hydrothermal autoclave was allowed to cool down naturally. The obtained greenish material was washed with D.I water and ethanol, and dried in a vacuum oven at 100 C for 8 h. The obtained material is highly impure material (Figure 6) which cannot be converted to pure phase NVPF even after heating at different temperatures in different environments.
[063] Numerous modifications and adaptations of the system of the present invention will be apparent to those skilled in the arty and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the true spirit and scope of this invention.
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WE CLAIM:

1. A method of ethanol assisted in-situ synthesis of nitrogen doped
carbon coated Sodium Vanadium Fluorophosphates (N-doped
carbon coated NVPF) material includes following steps;
- Preparation of Solution A by mixing of citric acid and Polyvinylpyrrolidone (PVP) in certain weight ratio in de-ionized water. Addition of Ammonium metavanadate (NH4V03) in stoichiometric amount to the solution and stirred for some time.
- Preparation of Solution B by mixing Stoichiometric mixing of Sodium fluoride (NaF) and Ammonium dihydrogen phosphate(NH4H2P04) in de-ionized water.
- Adding solution B is added into Solution A drop by drop and stirred for some time.
- Adding ethanol to the mixture and adjustment of solution pH using oxalic acid solution.
- Pouring whole solution into Teflon lined hydrothermal autoclave and heating it at certain temperature for some time in an oven
- Cooling it naturally of the solution after heating.
- Washing the obtained material using ethanol and de-ionized water followed by drying in vacuum oven.
- After drying, the material was heated in a tube furnace at certain temperature under (Ar+H2) gas environment.
- The obtained material was then grounded and again heated in a tube furnace at certain temperature under (Ar+H2) gas environment and the obtained green material is in-situ synthesized N-doped carbon coated NVPF.
2. The method of ethanol assisted in-situ synthesis of nitrogen
doped carbon coated Sodium Vanadium Fluorophosphates (N-
doped carbon coated NVPF), as claimed in claim 1, wherein the

volume ratio of water and ethanol, heating environment, and citric acid and Polyvinylpyrrolidone (PVP) in certain weight ratios provides high performance nitrogen doped carbon coated Sodium Vanadium Fluorophosphates cathode material for Sodium ion batteries (SIBs).
3. The method of ethanol assisted in-situ synthesis of nitrogen doped carbon coated Sodium Vanadium Fluorophosphates (N-doped carbon coated NVPF), as claimed in claim 1, wherein the synthesized N-doped carbon coated NVPF has HOmAh/g capacity vs Sodium metal (Na/Na+) in the voltage range of 2.5 V - 4.2 V.
4. The method of n-situ synthesis of nitrogen doped carbon coated Sodium Vanadium Fluorophosphates (N-doped carbon coated NVPF), as claimed in claim 1, wherein the method of ethanol assisted in-situ synthesis of nitrogen doped carbon coated Sodium Vanadium Fluorophosphates (N-doped carbon coated NVPF), led to the high yield and high-performance cathode material for Sodium ion batteries.
5. The method of ethanol assisted in-situ synthesis of nitrogen doped carbon coated Sodium Vanadium Fluorophosphates (N-doped carbon coated NVPF), as claimed in claim 1, wherein method of preparing ethanol concentration (%) in the water-ethanol solution for pure phase NVPF synthesis via hydrothermal approach is > 0, < 50%.
6. The method of ethanol assisted in-situ synthesis of nitrogen doped carbon coated Sodium Vanadium Fluorophosphates (N-doped carbon coated NVPF), as claimed in claim 1, wherein method of preparing the weight ratio of citric acid and Polyvinylpyrrolidone (PVP) in the solution during synthesis has been tuned to achieve high performance nitrogen doped carbon

coated Sodium Vanadium Fluorophosphates cathode material for Sodium ion batteries (SIBs).
7. The method of ethanol assisted in-situ synthesis of nitrogen doped carbon coated Sodium Vanadium Fluorophosphates (N-doped carbon coated NVPF), as claimed in claim 1, wherein method of preparing oxalic acid is used to adjust the pH of the final solution and the pH of the final solution for pure phase NVPF synthesis via hydrothermal approach is > 0, < 6.
8. The method of ethanol assisted in-situ synthesis of nitrogen doped carbon coated Sodium Vanadium Fluorophosphates (N-doped carbon coated NVPF), as claimed in claim 1, wherein method of preparing volume of the Teflon cell filled with the water-ethanol solution for the pure phase NVPF synthesis via hydrothermal approach is between 50% to 95%.
9. The method of ethanol assisted in-situ synthesis of nitrogen doped carbon coated Sodium Vanadium Fluorophosphates (N-doped carbon coated NVPF), as claimed in claim 1, wherein method of preparing the Teflon lined hydrothermal autoclave containing final solution is heated at temp >100 C for more than lOh.
10. The method of ethanol assisted in-situ synthesis of nitrogen
doped carbon coated Sodium Vanadium Fluorophosphates (N-
doped carbon coated NVPF), as claimed in claim 1, wherein
method of preparing the pure phase NVPF material obtained after
hydrothermal reaction and drying in a vacuum oven is further
heated in an inert environment (N2/Ar/Ar+H2) at 350 C and
again at 500 C for 5h to improve crystallinity of the material.